Studies in Surface Science and Catalysis 91 PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts
This Page Intentionally Left Blank
Studies in Surface Science and Catalysis Advisory Editors: Bo Delmon and J.T. Yates Vol. 91
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts Proceedings of the Sixth International Symposium, Louvain-La-Neuve, September 5-8,1994 Editors
G. Poncelet Universit6 Catholique de Louvain, Unit~ de Catalyse et Chimie des Mat~riaux Divis~s, Louvain-La-Neuve, Belgium J. Martens Katholieke Universiteit, Centrum voor Oppervlaktechemie en Katalyse, Heverlee (Leuven), Belgium B. Delmon Universit~ Catholique de Louvain, Unit& de Catalyse et Chimie des Mat~riaux Divis6s, Louvain-La-Neuve, Belgium RA. Jacobs Katholieke Universiteit, Centrum voor Oppervlaktechemie en Katalyse, Heverlee (Leuven), Belgium R Grange Universit6 Catholique de Louvain, Unit~ de Catalyse et Chimie des Mat6riaux Divis~s, Louvain-La-Neuve, Belgium
ELSEVIER A m s t e r d a m ~ L a u s a n n e - - N e w Y o r k - - Oxford - - Shannon ~ Tokyo
1995
ELSEVIER SCIENCE B.V. Sara Burgerhartstraat 25 P.O. Box 211, 1000 AE Amsterdam, The Netherlands
ISBN 0-444-82078-7 © 1995 Elsevier Science B.V. 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 B.V., Copyright & Permissions Department, P.O. Box 521, 1000 AM Amsterdam, The Netherlands. Special regulations for readers in the U . S . A . - 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 photocopies of parts of this publication may be made in the U.S.A. All other copyright questions, including photocopying outside of the U.S.A., should be referred to the copyright owner, Elsevier Science B.V., unless otherwise specified. 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, instructions or ideas contained in the material herein. This book is printed on acid-free paper. Printed in The Netherlands
ORGANIZING COMMI'ITEE
XV
OPENING ADDRESS
XVII
AKNOWLEDGEMENTS
XXI
CONTENTS
Vanadium phosphorus mixed oxide from the precursor to the active phase: catalystfor the oxidation of n-butane to maleic anhydride F. Cavani and F. Tdf'Lr6
Use of 31p NMR by spin echo mapping to prepare precursors of vanadium phosphate catalystsfor n-butane oxidation to maleic anhydride M.T. Sanan6s, A. Tuel, G.J. Hutchings, J.C. Volta
27
The role of aging on the formation of porous silica T.P.M. Beelen, W.H. Dokter, H.F. van Garderen, R.A. van Santen, E. Pantos
33
In situ techniquesfor the investigation of phase transformations in copper catalyst co-precipitates G.C. Chinchen, L. Davies, R.J. Oldman, S.J. Andrews
•
49
Influence of preparation method on the properties of V-Sb-O catalysts for the ammoxidation of propane G. Centi and S. Perathoner
59
Novel procedure for the preparation of highly active platinum-titania and palladium-titania aerogel catalysts withfavourable texturalproperties M. Schneider, M. Wildberger, D.G. Duff, T. Mallat, M. Maciejewski, A. Baiker
75
Preparation of combustion catalysts by washcoating alumina whiskers-covered metal monoliths using a sol-gel method M.F.M. Zwinkels, S.G. Jar~ts, P.G. Menon
85
Preparation of supported catalysts by equilibrium deposition-filtration A. Lycourghiotis
95
Preparation of K-C-Fe/Al203 catalystsfor ammonia synthesis at mild conditions K. Kalucki, A.W. Morawski, W. Arabczyk
131
A novel [PtMo6]/MgO catalystfor alkane-to-alkene conversion D.I. Kondarides, K. Tomishige, Y. Nagasawa, Y. Iwasawa
141
Spectroscopic characterization of supported Cr and Cr, Ti catalysts: Interaction with probe molecules B.M. Weckhuysen, I.E. Wachs, R.A. Schoonheydt
151
A new supported dehydrogenation catalyst: influence of the support and preparation variables L.A. Boot, A.J. van Dillen, J.W. Geus, F.R. van Buren, J.E. Bongaarts
159
Alumina~water interfacial phenomena during impregnation J.B. d'Espinose de la Caillerie, C. Bobin, B. Rebours, O. Clause
169
Nanometals and colloids as catalyst precursors H. B~Snnemann
185
Preparation of nanometer size of Cu-ZrdAl203 catalyst by phase transfer. Part 3. Sol preparation and phase transfer conditions Ze-Shan Hu, Song-Ying Chen, Shao-Yi Peng
197
Flame synthesis of nanostructured vanadium oxide based catalysts Ph.F. Miquel, J.L. Katz
207
The preparation of stable Ru metal clusters in zeolite Y used as catalyst for ammonia synthesis U. Guntow, F. Rosowski, M. Muhler, G. Ertl, R. Schl6gl
217
Preparation of nanometer gold strongly interacted with Ti02 and the structure sensitivity in low-temperature oxidation of CO S. Tsubota, D.A.H. Cunningham, Y. Bando, M. Haruta
227
Proton affinity distributions: a scientific basis for the design and construction of supported metal catalysts Cr. Contescu, J. Jagiello, J.A. Schwarz
237
~Aluminas-supported Pd-Mo mixed systems: effect of Mo deposition procedure on dispersion and catalytic activity of Pd F. Devisse, J.-F. Lambert, M. Che, J.-P. Boitiaux, B. Didillon
253
Metal catalysts supported on a novel carbon support M.S. Hoogenraad, R.A.G.M.M. van Leeuwarden, G.J.B. van Breda Vriesman, A. Broersma, A.J. van Dillen, J.W. Geus
263
Soft chemistry route for the preparation of highly dispersed transition metals on zirconia C. Geantet, P. Afanasiev, M. Breysse, T. des Couri~res
273
Influence of titania loading on tungsten adsorption capacity, dispersion, acidic and zero point of charge properties of W/TiO2-AI203 catalysts R. Prada Silvy, F. Lopez, Y. Romero, E. Reyes, V. Le6n, R. Galiasso
281
Preparation of titania supported on silica catalyst: study of the dispersion and the texture of titania R. Castillo, B. Koch, P. Ruiz, B. Delmon
291
Preparation of catalytic polymer~ceramic ion exchange packings for reactive distillation columns U. Kunz, U. Hoffmann
299
Synthesis of MCM-41 mesoporous molecular sieves O. Franke, J. Rathousk~, G. Schulz-Ekloff, A. Zukal
309
Preparation of spherical and porous Si02 particles by fume pyrolysis N. Kakuta, T. Tanabe, K. Nishida, T. Mizusima, A. Ueno
319
Sol-gel zirconia spheres for catalytic applications M. Marella, M. Tomaselli, L. Meregalli, M. Battagliarin, P. Gerontopoulos, F. Pinna, M. Signoretto, G. Strukul
327
vii
Surfactant based synthesis of oxidic catalysts and catalyst supports U. Ciesla, D. Demuth, R. Leon, P. Petroff, G.D. Stucky, K. Unger, F. Schtith
337
Preparation and properties of ceramic foam catalyst supports M.V. Twigg, J.T. Richardson
345
A new methodfor the preparation of metal-carbon catalysts P.A. Barnes, E.A. Dawson
361
Conversion of activated carbon into porous silicon carbide by fluidized bed chemical vapour deposition R. Moene, L.F. Kramer, J. Schoonman, M. Makkee, J.A. Moulijn
371
A new strong basic high surface area catalyst: the nitrided aluminophosphate: AIPON and Ni-AIPON P. Grange, Ph. Bastians, R. Conanec, R. Marchand, Y. Laurent, L. Gandia, M. Montes, J. Fernandez, J.A. Odriozola
381
Preparation of silica or alumina pillared crystalline titanates S. Udomsak, R. Nge, D.C. Dufner, S.E. Lott, R.G. Anthony
391
Silica preparation via sol-gel method: a comparison with ammoximation activity D. Collina, G. Fornasari, A. Rinaldo, F. Trifir6, G. Leofanti, G. Paparatto, G. Petrini
401
Control of porosity and surface area in TiO2-Al203 mixed oxides supports by means of tmvnonium carbonate T. Klimova, Y. Huerta, M.L. Rojas Cervantes, R.M. Martin Aranda, J. Ramfrez
411
Preparation of metallo-silicate solid catalysts by sol-gel method with regulation of activity and selectivity I.M. Kolesnikov, A.V. Yablonsky, S.I. Kolesnikov, A. Busenna, M.Yu. Kiljanov
421
A new procedure for preparing aerogel catalyst Chi-Ming Zhang, Song-Ying Chen, Shao-Yi Peng
427
Preparation of single and binary inorganic oxide aerogels and their use as supports for automotive pallach'um catalysts C. Hoang-Van, R. Harivololona, B. Pommier
435
Synthesis and characterization of sintering resistant aerogel complex oxide powders D.M. Lowe, M.I. Gusman, J.G. McCarty
445
Effect of reactant mixing mode on silica-alumina texture J.P. Reymond, G. Dessalces, F, Kolenda
453
Synthesis, characterization and performance of sol-gel prepared Ti02-Si02 catalysts and supports S. Bemal, J.J. Calvino, M.A. Cauqui, J.M. Rodrfguez-Izquierdo, H. Vidal
461
Preparation of CaO-, La203- and Ce02- doped Zr02 aerogels by sol-gel methods Y. Sun, P.A. Sermon
471
viii
Preparation of nanometer size of Cu-Zn/AI203 catalyst by phase transfer. Part 1. Study of basic preparation conditions Ze-Shan Hu, Song-Ying Chen, Shao-Yi Peng
479
The preparation of ultrafine Sn02 by the supercritical fluid drying technique (SCFDT) Fan Lu, Song-Ying Chen
489
Plasma preparation of a dispersed catalyst for hydroconversion of heavy oils L. Rouleau, R. Bacaud, M. Breysse
495
Preparation and structural properties of ultrafine gold colloids for oxidation catalysis D.G. Duff, A. Baiker
505
Synthesis, characterization and catalytic activity of manganese oxidic nanoparticles C.S. Skordilis, P.J. Pomonis
513
Development of LaxMOy nanocatalysts dispersed in a layered silicate matrix S. Moreau, S. Pessaud, F. Beguin
523
Nanometer size copper particles in copper chromite catalysts T.M. Yur'eva, L.M. Plyasova, O.V. Makarova, T.A. Krieger, V.I. Zaikovskii
533
Silica immobilized Ru complexes with a different nuclear number as catalysts of the hydrode halog enation reaction V. Isaeva, Y. Smirnova, V. Sharf
539
Colloidal routes to Pt-Au catalysts K. Keryou, P.A. Sermon
545
Catalysts by solid-state ion exchange: iron in zeolite K. L~iz~, G. P~il-Borb61y, H.K. Beyer, H.G. Karge
551
Modified ruthenium exchanged zeolites for enantioselective hydrogenation V.I. Parvulescu, V. Parvulescu, S. Coman, C. Radu, D. Macovei, E. Angelescu, R. Russu
561
Preparation of conjugated polymer supported heteropolyanions - New efficient catalysts for ethyl alcohol conversion M. Hasik, I. Kulszewicz-Bajer, J. Pozniczek, Z. Piwowarska, A. Pron, A. Bielanski, R. Dziembaj
571
Regenerable sorbent for high-temperature desulfurization based on ironmolybdenum mixed oxides .............. R. van Yperen, A.J. van DiUen, J.W. Geus, E. Boellaard, A , A van der Horst, A.M. van der Kraan
579
A comparative study of the photocatalytic activities on iron-titanium (IV) oxide photocatalysts prepared by various methods:! spray pyrolysis, impreghation and .... co-precipitation R.I. Bickley, L.T. Hogg, T. Gonzalez-Carren0iL, Palmisano' • ' ~~ ~
589
Coordination compounds of metals incorporated in polyorganosiloxane matrices. XIIL (Co)(ll) complexes with salen, salophen and molecular oxygen T.N. Yakubovich, V.V. Teslenko, K.A. Kolesnikova, Yu.L. Zub, R. Leboda
597
Preparation and characterization of ASn03 (A = Ca, Sr or Ba) tin compoundsfor methane oxidative coupling C. Petit, M. Teymouri, A.C. Roger, J.L. Rehspringer, L. Hilaire, A. Kiennemann
607
Lao.sSro.2MnO3+x supported on LaAl03 and LaA111018 prepared by different methods: Influence of preparation method on morphological and catalytic properties in methane combustion P.E. Marti, M. Maciejewski, A. Baiker
617
Properties of Lao.6Sro.4Co03 prepared by complexing agent-assisted sol-gel method Y. Muto, F. Mizukami
627
Monolith perovskite catalysts of honeycomb structure f or fuel combustion L.A. Isupova, V.A. Sadykov, L.P. Solovyova, M.P. Andrianova, V.P. Ivanov, G.N. Kryukova, V.N. Kolomiichuk, E.G. Avvakumov, I.A. Pauli, O.V. Andryushkova, V.A. Poluboyarov, A.Ya. Rozovskii, V.F. Tretyakov
637
Study on the preparation of nanometer perovskite-type complex oxide LaFe03 by sol-gel method Zi-Yi Zhong, Li-Gang Chen, Qi-Jie Yan, Xian-Cai Fu, Jian-Min Hong
647
Preparation of perovskite-type catalysts containing cobalt for post-combustion reactions L. Simonot, F. Garin, G. Maire, P. Poix
657
Characterization and reactivity of MgxFe2..2x03-2x .and MgyZnl _yFe204 solid solution spinels prepared through the supercnncal drying metliod G. Busca, M. Daturi, E. Kotur, G. Oliveri, R.J. Willey
667
Effect of the iron catalyst mechanical treatment on the activity in ammonia synthesis reaction W. Arabczyk, R. Drzymala, U. Narkiewicz, K. Kalucki, W. Morawski
677
Cobalt catalystfor ammonia oxidation modified by heat treatment K. Krawczyk, J. Petryk, K. Schmidt-Szalowski
683
Characterization and catalyticproperties of MgO prepared by different approaches Kai-Ji Zhen, Sen-Zi Li, Ying-Li Bi, Xiang-Guong Yang, Quan Wei
691
Permanganic acid: a novel precursor for the preparation of manganese oxide catalysts C. Kappenstein, T. Wahdan, D. Duprez, M.I. Zaki, D. Brands, E. Poels, A. Bliek
699
Systematic control of crystal morphology during preparation of selective vanadyl pyrophosphate E. Kesteman, M. Merzouki, B. Taouk, E. Bordes, R. Contractor
707
Vanadium exchanged titanium phosphates as catalysts for the selective reduction of nitrogen oxide with ammonia M.A. Massucci, P. Patrono, G. Russo, M. Turco, S. Vecchio, P. Ciambelli
717
Influence of the precursorformation stage in the preparation of VPO catalysts for selective oxidation of n-pentane Z. Sobalik, S. Gonzalez, P. Ruiz
727
Role of segregation phenomena information of active surface of V-Sb-O catalysts for selective oxidation of propylene to acrolein M. Najbar, E. Bielanska
737
Preparation, physico-chemical characterization and catalytic properties of vanadium-doped alumina- and titania-pillared montmorillonites K. Bahranowski, R. Dula, J. Komorek, T. Romotowski, E.M. Serwicka
747
The use of sepiolite in the preparation of titania monoliths for the manufacture of industrial catalysts J. Blanco, P. Avila, M. Yates, A. Bahamonde
755
Design of monolith catalysts for strongly exothermic reactions under nonadz'abatic conditions E. Tronconi, M. Bassini, P. Forzatti, D. Carmello
765
Some aspects of extrusion procedure for monolittu'c SCR catalyst based on Ti02 V. Lyakhova, G. Barannyk, Z.R. Ismagilov
775
Preparation and characterization of catalytic supports with variable composition in the system Si02-AI203-AIP04 F. Wijzen, A. Rulmont, B. Koch
783
New mo&fication of alumina: preparation procedure and existence conditions B.P. Zolotovskii, R.A. Buyanov
793
Preparation and characterization of silica-pillared layered titanate Wen-Hua Hou, Qi-Jie Yan, Yi Chen, Xian-Cai Fu
799
Alumina support modified by Zr and Ti. Synthesis and characterization T. Viveros, A. ZArate, M.A. L6pez, J. Ascenci6n Montoya, R. Ruiz, M. Portilla
807
Synthesis, characterization and applications of new supports for heterogeneous Ziegler-Nana type catalysts L. Pavanello, S. Bresadola
817
Catalyticfilamentous carbon as adsorbent and catalyst support V.B. Fenelonov, L.B. Avdeeva, O.V. Goncharova, L.G. Okkel, P.A. Simonov, A. Yu. Derevyankin, V.A. Likholobov
825
Preparation of boron-containing alumina supports by kneading J.L. Dubois, S. Fujieda
833
Characterization of alumina paste by cryo-microscopy E. Rosenberg, F. Kolenda, R. Szymanski, M. Walter
843
xi
Preparation of cation-substituted hexaaluminates with large surface area using mechanical activation methods O.A. Kin'chenko, O.V. Andrushkova, V.A. Ushakov, V.A. Poluboyarov
851
A new approach to catalyst preparation using sample controlled temperature programme techniques P.A. Barnes, G.M.B. Parkes
859
Preparation of fine particles as catalysts and catalyst precursors by the use of ultrasound during precipitation U. Kunz, C. Binder, U. Hoffmann
869
Scientific bases for the preparation of new cement-containing catalysts V.I. Yakerson, E.Z. Golosman
879
Nucleation and growth of ceria from cerium III hydroxycarbonate M. Pijolat, J.P. ViriceUe, M. Soustelle
885
Hydrotalcite-type anionic clays as precursors of high-surface-area Ni/MglAI mixed oxides A. Vaccari, M. Gazzano
893
Preparation and characterisation of cobalt containing layered double hydroxides S. Kannan, C.S. Swamy
903
Synthesis of silver supported catalysts with narrow particle size distribution S.N. Goncharova, B.S. Barzhinimaev, S.V. Tsybulya, V.I. Zaikovskii, A.F. Danilyuk
915
Preparation of supported platinum catalysts by liquid-phase reduction of adsorbed metal precursors M. Arai, K. Usui, M. Shirai, Y. Nishiyama
923
Preparation of supported mono- and bimetallic catalysts by depositionprecipitation of metal cyanide complexes E. Boellaard, A.M. van der Kraan, J.W. Geus
931
Clusters and thin films prepared by DC-sputtering: morphology and catalytic properties D. Duprez, O. Enea
941
Preparation and characterization of a platimun containing catalytic membrane Xiu-Ren Zhao, Jun-Hang Jing
949
The utilization of saturated gas-solid reactions in the preparation of heterogeneous catalysts S. Haukka, A. Kyttikivi, E.-L. Lakomaa, U. Lehtovirta, M. Lindblad, V. Lujala, T. Suntola
957
Identification of supported phases produced in the preparation of silica-supported Ni catalysts by competitive cationic exchange M. Kermarec, A. Decarreau, M. Che, J.Y. Carriat
967
xii
Influence of an interaction of PdCI2 with carbon support on state and catalytic properties of Pd/C catalysts P.A. Simonov, E.M. Moroz, A.L. Chuvilin, V,N. Kolomiichuk, A.J. Boronin, V.A. Likholobov
977
Synthesis of eggshell cobalt catalysts by molten salt impregnation techniques S.L. Soled, J.E. Baumgartner, S.C. Reyes, E. Iglesia
989
Bismuth(Ill) and molybdenum(ll) acetates as mono- and homopolynuclear precursors of silica-supported bismuth molybdate catalysts O. Tirions, M. Devillers, P. Ruiz, B. Delmon
999
Preparation of catalysts by chemical vapor-phase deposition and decomposition on support materials in a fluidized-bed reactor S. Ktihler, M. Reiche, C. Frobel, M. Baerns
1009
Preparation of highly loaded nickel~silica catalysts by a deposition-precipitation method. Effect of the aging time on the reducibility of nickel and on the textural properties of the catalyst V.M.M. Salim, D.V. Cesar, M. Schmal, M.A.I. Duarte, R. Frety
1017
Preparation of small metal nickel particles supported on silica using nickel ethylenech'anu'neprecursors Zheng-Xing Cheng, C. Louis, M. Che
1027
Preparation and characterization of CoMo/AI203 HDS catalysts: effects of a complexing agent P. Blanchard, C. Mauchausse, E. Payen, J. Grimblot, O. Poulet, N. Boisdron, R. Loutaty
1037
Impregnation during gelation and its influence on the dispersion of the impregnant A.E. Duisterwinkel, G. Frens
1051
Synthesis and characterization of titanium oxide monolayer N.S. de Resende, M. Schmal, J.-G. Eon
1059
Alumina washcoating and metal deposition of ceramic monoliths Xiao-Ding Xu, H. Vonk, A. Cybulski, J.A. Moulijn
1069
Cr-free iron-catalysts for water-gas slu'ft reaction J. Ladebeck, K. Kochloefl
1079
Preparation of Rh-Co/Al203 heterogeneous catalysts using a diisocyano-ligand as an integral design component M.S.W. Vong, P.A. Sermon
1085
Preparation of highly ch'spersedsupported catalysts by ultrasound C.L. Bianchi, R. Carli, C. Fontaneto, V. Ragaini
1095
Regularities of Pt precursors and modifying dopes sorption during the preparation of bimetal catalysts supported on spinels N.A. Pakhomov, R.A. Buyanov
1101
xiii
Tin(W) oxide supported noble metal catalysts for the carbon monoxide oxidation at low temperatures K. Grass, H.-G. Lintz
1111
Preparation of PMoNi/~AI203 catalysts from solutions of phosphomolybdates in water, ethanol-water and dimethylformamide P.G. V~tzquez, M.G. Gonz,41ez, M.N. Blanco, C.V. C~iceres
1121
Impregnation design for preparing bimetallic catalysts A.K. Aboul-Gheit, S.M. Abdel-Hamid
1131
Comparative study on low-temperature Cu/activated carbon catalysts prepared by impregnationfrom aqueous and organic media D. Mehandjiev, R. Nickolov, E. Bekyarova, V. Krastev
1137
Thermostability of copper-chromium oxide catalysts on alumina support promoted by lanthanum and cerium R.A. Shkrabina, N.A. Koryabkina, O.A. Kirichenko, V.A. Ushakov, F. Kapteijn, Z.R. Ismagilov
1145
Non-hydrothermal synthesis, characterisation and catalytic properties of saponite clays R.J.M.J. Vogels, M.J.H.V. Kerkhoffs, J.W. Geus
1153
Composite catalysts of supported zeolites N. van der Puil, E.W. Kuipers, H. van Bekkum, J.C. Jansen
1163
AUTHOR INDEX
1173
Studies in Surface Science and Catalysis (Other volumes in the series)
1177
This Page Intentionally Left Blank
XV
ORGANIZING COMMITTEE President
Prof. B. DELMON
Universit6 Catholique de Louvain
Executive Chairmen
ProL P. GRANGE
Universit6 Catholique de Louvain
Prof. P.A. JACOBS
Katholieke Universiteit Leuven
Prof. J. MARTENS
Katholieke Universiteit Leuven
Dr G. PONCELET Dr P. RUIZ
Universit6 Catholique de Louvain Universit6 Catholique de Louvain
SCIENTIFIC COMMITTEE Dr J. BOUSQUET,
Elf Aquitaine, France
Dr K. DELLER, Prof. B. DELMON,
Degussa AG, Germany Universit6 catholique de Louvain, Belgium
Prof. E.G. DEROUANE,
Facult6s Universitaires N.-D. de la Paix, Belgium
Dr J. DETHY,
Catalysts and Chemicals Europe, Belgium
Dr A. DI MARIO, Dr T. FUGLERUD,
Montecatini Tecnologia, Italy
Prof. P. GRANGE,
Universit6 catholique de Louvain, Belgium
Dr A. HAAS,
Grace GmbH, Germany
Dr H. HINNEKENS,
Labofma, Belgium
Dr JACKSON,
ICI Catalysts, England
Norsk Hydro, Norway
Prof. P.A. JACOBS,
Katholieke Universiteit Leuven, Belgium
Prof. J. MARTENS,
Katholieke Universiteit Leuven, Belgium
Dr M. NOJIRI,
Mitsubishi Petroleum Co. Ltd., Japan
Dr K. NOWECK,
Condea Chemie GmbH, Germany
Dr G. PONCELET,
Universit6 Catholique de Louvain, Belgium
Dr P. RUIZ,
Universit6 Catholique de Louvain, Belgium
Dr J.P. SCHOEBRECHTS, Solvay et Cie., Belgium Dr E. VOGT,
AKZO Chemicals B V, The Netherlands
Dr G. SZABO,
Total CFR, France
Dr M.V. TWIGG,
Johnson Matthey, England
Prof. J.A.R. VAN VEEN,
Koninklijke Shell Laboratorium, The Netherlands
Prof. E. VANSANT,
Universitaire Instelling Antwerpen, Belgium
Mr S. VIC BELLON,
Empresa Nacional del Petroleo SA, Spain
xvi FINANCIAL SUPPORT The following companies have accepted to provide financial support to the Vlth Symposium. The Organizers gratefully acknowledge them for their generosity.
AKZO Chemicals, B.V., The Netherlands BAYER AG, Germany CATALYSTS AND CHEMICALS EUROPE, Belgium DEGUSSA AG, Germany DOW BENELUX N.V., The Netherlands DSM RESEARCH B.V., The Netherlands EKA NOBEL, Sweden ENGELHARD DE MEERN, The Netherlands GRACE GmbH, Germany HALDOR TOPSOE, Denmark HOECHST AG, Germany JOHNSON M A ~ Y , Catalysts Systems Division, England MONTECATINI TECNOLOGIE SRL, Italy NORSK HYDRO, Norway PROCATALYSE, France REPSOL PETROLEO, Spain SOLVAY S.A., Belgium SUD-CHEMIE AG, Germany UNICAT S.A., Belgium
xvii
Opening address Professor P. Rouxhet, Pro-Rector
Dear Colleagues, ladies and gentlemen,
It is my great pleasure to welcome you in the Universit6 Catholique de Louvain and in Louvain-la-Neuve. I hope you will feel at home here and appreciate the environment. Let me first say a few words about our University. This is quite an old institution; it was founded in 1425. It is composed of 10 faculties, from theology and philosophy to engineering and agricultural sciences. It counts a bit more than 20.000 students, 20% of which are foreign students. We have the status of free university like a few other institutions in Belgium. According to this status, most of the financial support is coming from the state and the university m u s t be managed according to certain rules imposed by the state. Beside that, the institution is run independently of the public authority and of the political p o w e r in terms of n o m i n a t i o n , p r o g r a m m e s , internal allocation of ressources, etc. This is why it is called free; however it is not a private university in the sense used in some countries.
The term catholic deserves also some explanation.
The Catholic Church, through
local bishops, is involved at the highest level for basic decisions. not mean that this is a religious university.
However it does
The institution does not of course
interfere at all with the personal convictions of its members, either students, professors or other staff. However there is an important aspect inherited from our catholic tradition, a sort of spirit which is spread in a diffuse way through the institution. This spirit is a certain vision of man, of the world, of life. It involves a deep respect for the h u m a n person and his freedom, w h a t I could call the freedom of the children of God. One consequence is a deep sense of h u m a n fraternity. This creates a certain type of relationship between colleagues, between the professors and the
xviii students. It also creates a style of management. Another component of that spirit is the recognition of the limitation of man.
It is a p e r m a n e n t invitation to be
concerned, in complete freedom, by a level of thought which is beyond the material aspects of life. We believe that such invitation is more important than ever. Except the Faculty of Medecine, the University is located in Louvain-la-Neuve.
As
a matter of fact the decision taken 26 years ago to move the French speaking section of the former bilingual university of Louvain-Leuven appears as a key step in the evolution of Belgium to a federal state. From the way of life we had in the old city of Leuven came the will not to build a campus but to develop a new city, a real city in which academic and non academic activities would mix together, a real city with families, children and elder persons, with shops, services and business. In addition, the option was taken to organize the city in such a way that contacts between people be made easier. Therefore it was designed with a great attention to pedestrians, the car being considered as a way to reach Louvain-la-Neuve, but not the best way to circulate in Louvain-la-Neuve. If you walk a few minutes, you will pass through academic zones and residential areas; you will appreciate h o w modern architecture can fit with beauty, quieteness and conviviality. At the center you will find all kinds of shops and facilities. You may also have a pleasant walk around the lake. Many of you have already been here before; some have spent months or years at our University. So they may have followed the evolution of the city. They may be interested to know that the big building in construction near the Grand Place will be occupied by the Faculty of Philosophy and by the Faculty of Psychology and Education.
Important developments are also taking place in the residential areas;
residing in Louvain-la-Neuve has indeed become very attractive.
The zone
Bruy~res, dominating the southern rim of the lake, is at present the place of a very intensive construction programme. By the way, this week is the end of the session of examinations, so the atmosphere may be a bit special; in particular the caf6s should be well attended. This may also be a sweet memory to some of you.
xix Part of the challenge in developing Louvain-la-Neuve, was to stimulate the development of private business. This is required to make a real city and this has been quite successful indeed. An area of 160 ha has been reserved for a science and industry park; at present it accomodates a bit less than 100 companies, providing work to about 3000 people. Priority is given -
to research and development companies or engineering activities,
-
to production centers based on advanced technology,
-
-
to companies which have activities complementary to research, and, of course, to spin offs of the university.
The firms installed in Louvain-la-Neuve cover a wide range of fields, from international research centers of big companies to small, family size entreprises. They create a s t i m u l a t i n g e n v i r o n m e n t for the university, a n d all together, L o u v a i n - l a - N e u v e has t u r n e d development in the country.
out
to be an i m p o r t a n t
pole
of economic
Economic development and scientific research !This summarizes challenges which are addressed to scientists and to universities : - combine the search of the truth and the concern for the whole society, prepare the future while being imbedded in the present,
-
- develop a strong and sharp expertise while being ready to enter n e w fields of science. Concerning catalysis, the expectation of the society may be the d e v e l o p m e n t of new processes which are more respectful of the environment or help to restaure its quality. On the other hand, the discovery of a new catalyst involves trials and errors, requires intuition but is also based on scientific concepts, on rational approaches which are relevant of molecular engineering. Networking is the key word to take up such challenges, and the preparation of heterogeneous catalysts, as worked out in our institution, provides an illustrative example. - It is strongly anchored in basic science : solid state chemistry, surface and colloid chemistry, a d v a n c e d methods of analysis. On the other h a n d it has direct implications in projects with big and small companies.
XX
It involves sharing an impressive equipment and expertise between laboratories of the Faculty of Sciences, the Faculty of Engineering and the Faculty of Agricultural Science and Engineering. - It covers collaborations between colleagues who are specialized in solid state chemistry, chemistry of organo-metallic complexes, process engineering. It benefits from progress but also stimulates progress in the areas of materials and surfaces; it represents a significant part of the activity of the Research Center for Advanced Materials of our University.
-
-
Such networking has also been very active at the international scale, as demonstrated by the title of the conference : "Scientific bases for the preparation of heterogeneous catalysts " and by the fact that this is the sixth edition. You are here to contribute to such exchanges and to take benefit from them... I wish you a pleasant stay and a fruitful conference.
xxi ACKNOWLEDGEMENTS The Organizing Committee is obliged to Professor P. Macq, Rector of the Universit6 Catholique de Louvain, who allowed the Sixth International Symposium to be held in Louvain-la-Neuve. The organizers thank Professor P. Rouxhet, Prorector, for his welcome address to the participants. We also gratefully acknowledge the University Authorities for providing us with the lecture room where the Poster Sessions were organized. The members of the Scientific Committee of this symposium, who were once again faced with the difficult task of selecting the communications, are all most sincerely thanked for their outstanding dedication. The organizers express special thanks to the industrial companies for their financial support. Their contribution allowed us to rearrange our budget so that several participants were able to attend the symposium and present their communication. The Organizing Committee is grateful to the authors of the 240 submitted abstracts: those who contributed an oral or a poster presentation, as well as those whose contribution, mainly because of the limitations of time and space, could not be selected. The organizers are pleased to thank the authors of the stimulating plenary lectures and extended communications and, in particular, Prof. H. B6nnemann, Prof. D. Hilvert, Prof. J.T. Richardson, Prof. F. Trifir6, Prof. J.A. Schwarz, Prof. G. Centi, Professor A. Lycourghiotis, Dr. O. Clause and Dr. T.P.M. Beelen, for their excellent oral presentations. Twenty-three persons deserve special praise for their performance as session chairmen during the symposium: Prof. R.I. Bickley, Prof. E. Bordes, Dr. A. Di Mario, Prof. J.W. Geus, Dr. J. Johansen, Dr. K. Kochloefl, Dr. F. Kolenda, Prof. I.M. Kolesnikov, Dr. Z. Kricsfalussy, Prof. H.G. Lintz, Dr. L. Martens, Prof. P.J. Menon, Dr. F. Mizukami, Prof. A. Pentenero, Dr. N. Pernicone, Prof. J. Ramirez, Prof. J.T. Richardson, Dr. J.P. Schoebrechts, Prof. J.A. Schwarz, Dr. M.V. Twigg, Prof. A. Vaccari, Prof. J.A.R. van Veen and Dr. E. Vogt. The hostesses of the REUL (Relations Ext6rieures de l'Universit6 de Louvain), and particularly Mrs. D. Pelegrin, are congratulated on their perfect achievement. We also wish to extend our gratitude to Mr. H. Bourgeois and Mr. L. Peeters, of the "Service Logement", for their dedication to the symposium. We owe particular credit to the secretaries, F. Somers, and especially M. Saenen, 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 students from the "Unit6 de Catalyse et Chimie des Mat6riaux Divis6s" and the "Centrum voor Oppervlaktechemie en Katalyse, K.U. Leuven", who contributed to the success of the symposium, in particular : Ph. Bastians, N. Blangenois, A. Bernier, A. Centeno, F. Collignon, T. Curtin, P. Espeel, N. Fripiat, Fu Li-Jun, E. Gaigneaux, A. Gil Bravo, S. Gonzalez, G. Guiu, B. Kartheuser, P.P. Knops-Gerrits, S. Korili, C. Lahousse, R. Loenders, N. Mariano, A. Massinon, R. Molina, S. Moreno, P. Oelker, M. Remy, R. Reynerds, M. Ruwet, W. Souvereyns, A. Stumbo, R. Sun Kou, X. Vanhaeren, YangLiang Xiong, Xiao Yan, Mo-Hua Yang.
This Page Intentionally Left Blank
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
V a n a d i u m / p h o s p h o r u s m i x e d oxide from the precursor to the active phase: Catalyst for the oxidation of n-butane to maleic anhydride F. Cavani and F. Trifirb Dipartimento di Chimica Industriale e dei Materiali, Viale Risorgimento 4, 40136 Bologna Tel. +39-51-6443682, Fax +39-51-6443680
This review examines the recent scientific and patent literature dealing with V/P/O-based catalysts for the synthesis of maleic anhydride by n-butane oxidation. Attention is focused on the different methods of preparation claimed by each company, as well as on the main parameters in precursor preparation and thermal treatment affecting the final catalytic performance. The role of the various promoters reported in the literature is also discussed.
1. INTRODUCTION Several industrial processes exist for the production of maleic anhydride from n-butane, which differ regarding the type of reactor and the method employed for malcic anhydride recovery and purification (1-3). All processes employ the same kind of catalyst, based on a vanadium/phosphorus mixed oxide (4-8). Different methods of preparation for the V/P/O catalysts have been reported in the scientific and patent literature. All of them achieve the ultimate active phase via the following stages: 1) Initial preparation of the active phase precursor, (VO)HPO40.5H20. 2) Thermal decomposition of the hemihydrate vanadyl orthophosphate, with partial or total loss of the hydration water, formation of new phases, and elimination of precursor impurities (chlorine ions, organic compounds) as well as of additives employed for powder tabletting in the case that the tablets are prepared before the dehydration stage. 3) Formation of the catalysts in such a way as to achieve the best mechanical resistance for use in fixed-, fluid- or transported-bed reactors. 4) Activation or aging inside the reactor; phase and morphological transformations, recrystallization, creation or elimination of structural defects, selective poisoning by high boiling compounds, migrations of vanadium and phosphorous species occur at this stage. This stage can last for periods ranging from a few days to one month, and it is a necessary step to achieve catalysts with optimum catalytic performances. Industrial V/P/O-based catalysts can differ in the type of chemistry involved in the different stages, in the nature of the promoters added, and in the type of reactor technology employed for maleic anhydride synthesis. This review is divided into two parts: 1) In the first part we report examples taken from patents issued by different companies
(most of which are involved in the commercial production of maleic anhydride) about the different types of procedures employed for the preparation. 2) In the second part we draw a chemical picture or a comprehensive model of the several stages of the preparation, based essentially on data from the scientific literature; this model can explain, even though not completely, the chemistry involved in each stage of the preparations claimed in the patents. The examples that we have selected from the patent literature neither necessarily correspond to the induslrial preparation actually employed nor are they necessarily the best preparations described by each company. However, many of the conditions reported in the selected examples are repeated in the claims of the several patents issued over the years, indicating that they can be considered as "the finger print" or the innovative feature developed by each company.
2. EXAMPLES LITERATURE
OF
PREPARATIONS
SELECTED
FROM
THE
PATENT
In this chapter we report only indications about the stages of preparation of the precursor and its thermal activation proposed by the different companies. The key-features which characterize the method of preparation are the nature of the raw materials, reducing agents for vanadium and the solvent, the temperature of precipitation and of digestion, the choice of either dry or wet milling of V205 and the precursor, the P/V ratio, the presence of promoters (metal ions) and the presence of additives (organic compounds). Two main methods of preparation of the precursgr can be singled out: 1) Reduction of V 5+ compounds (V205) to V4+ in water by either HC1 or hydrazine, followed by addition of phosphoric acid and separation of the solid by either evaporation of water or by crystallization. 2) Reduction of V 5+ compounds in a substantially anhydrous medium with either an inorganic or an organic reducing agent, addition of dry phosphoric acid and separation of the solid obtained either by filtration, by solvent evaporation or by centrifugation. The addition of phosphorous compounds before V 5+ reduction has also been claimed for both methods of preparation, but it does not seem to be the preferred procedure. A substantially anhydrous medium means the use of a dry organic solvent, of dry metal salts and components, as well as the use of phosphoric acid containing more than 98 % H3PO4; moreover, the water formed by vanadium reduction and by digestion is removed by azeotropic distillation during the preparation. The organic solvent must possess the properties to dissolve, but not to react with, the phosphoric acid, eventually to reduce the vanadium species and not dissolve the precursor. In the preparation in organic solvent, intercalated or occluded organic materials may represent 25 % by weight of the precursor (9). The aqueous solvent must be capable of dissolving the components of the precursor and the reducing agent but unfortunately, at the same time, it also dissolves the precursor. In the preparation in aqueous medium the anions of the metal, i.e. sulfates or chlorines, can be incorporated into the structure of the precursor. Sohio (BP America) has issued patents (10,11) in which the catalyst precursor is prepared in an anhydrous medium, and removed continuously by azeotropic distillation of (i) the
organic liquid which contains the water produced during vanadium reduction as well as (ii) the oxidation products of the organic solvent as soon as the reduction of vanadium occurs. This procedure allows the preparation of catalysts with higher surface area and with higher activity than those prepared under total reflux. The stages of preparation are summarized in Scheme 1. Mitsui has developed a catalyst for fluid-bed operation claimed to possess high density (1.1 g/ml), high surface area (40-50 m2/g), and higher attrition resistance and to be active at lower temperature than other catalysts (12-14). The steps in catalyst preparation are reported in Scheme 2. It has been suggested that the role of polyols is to increase the surface area and decrease the crystallinity of the precursor. In order to use the polyols it is necessary to decouple the stage of formation of V204 from that of formation of the precursor, because the presence of polyols can create problems during the stage of reduction of V205. Scheme 1. BP preparation in an organic solvent.
Addition of the V5+ compound to an organic solvent selected from alcohols and glycols (isobutanol and ethylene glycols)
Addition of p~sphoric acid Reduction of the vanadium by heating the solution under distillation and by removing 1.5 moles of organic liquid (including organic ~y-products) per mole of vanadium reduced
Recovery of the orecursor, drvine and calcination in ~ at 400"C Scheme 2. Mitsui decoupled anhydrous preparation with polyol additives
,
Preparation of V204 by reduction of V205 in isobutanol and benzyl alcohol (1/1)
I Addition of phosphoric acid and of Mg,Zr promoters to the preformed V204 in an organic solvent, together with polyols (ethylene ~lycol:preferred), and heating under reflux Separation of the resulting precursor by filtration, washing with isopropanol
J,
Calcination of the catalyst in an oxygen-poor atmosphere (nitrogen/air 4/1), in order to achieve partial oxidation of vanadium at about 500"C Amoco has issued several patents for methods claimed to increase the lifetime and the productivity of the catalyst for the fixed-bed process (15-19). The main innovative features of the catalyst preparation are reported in Scheme 3. A key feature of the Mitsubishiprocess, the first company to build a fluid-bed reactor for this reaction, is the preparation of the catalyst under hydrothermal conditions, thus avoiding corrosive reaction conditions and the problems of flammable waste treatment encountered in organic preparation (20); the main steps of the preparation are reported in Scheme 4. Scheme 5 summarizes the main features of theprep~ation described by Chevron (21).
Scheme 3. Amoco anhydrous preparation with in-situ generation of the reducing agent
Introduction of V205 and of salt promoters (Mo, Zn, Li and POCI3) in an organic solvent based on ethers (tetrahydrofuran is the most preferred) in the presence of hydrogen donor compounds (ethanol or water are the most preferred) Hydrolysis of POCl3 by the H donor (temperature is raised), with formation of anhydro_us phosphoric acid and HCI which dissolves all the metal compounds and reduces the V 5+ Addition of organic modifiers; aromatic acids or anhydrides, or aromatic hydrocarbons, such as benzoic acid, phthalic anhydride or xyl~ne, are added during reflux of the solvent 4, Recovery of the catalyst precursor ; the thick syrup obtained by solvent evaporation is dried under vacuum at 130-200"C
Calcination of the precursor in air at 300"C (below 350"C) and then grinding and forming in geometrical shapes with lubricants such as grafite or stearic acid, and with binders such as polyviny1 alcohol Scheme 4. Mitsubishi hydrothermal preparation
Reduction of V205 with hydrazine in an aqueous solution, under reflux Addition of iron salt into the V4+-con~ni~g solution, together with chelating agents possessing two ligand groups (ethylene glycol or oxalic acid), heating under reflux 4, Addition of the phosphorous compound, and introduction of the solution together with seed crystals of the precursor in a closed vessel at 120-200"C under steam pressure
Filtration of the slurry, drvin~ r
activation in N~ at 500"C and then in air
Scheme 5. Chevron anhydrous preparation with HCI as the reducing agent.
Suspension of V205 and Mo, Zn, Li salt promoters in anhydrous alcohol (isobutanol is the most preferred, which acts also as a mild reducing agent) Dissolution and reduction of VS+by bubbling gaseous HC1 through the solution at temperatures lower than 60"C Addition of phosphoric acid and digestion under reflux Stripping of the alcohol under vacuum at temperatures lower than 170"C Calcination at 260 ~C(or in any case at temperatures lower than 300"t2) and then tabletting
Alusuisse has issued patents for both f'Lxed-bed and fluid-bed operation, in either aqueous or organic media (22-25). Reported in Scheme 6 is the preparation in an aqueous medium, which is significant for the method employed to crystallize the precursor (36). Scheme 6. Alusuisse aqueous preparation
Suspension of v205 in a concentrated solution of HC1 and heating under reflux at 100*C Addition of oxalic acid and of phosphoric acid
,L ,L
Concentragon of the solution until a viscous solution is obtained Addition of excess water m the viscous solution; a bright blue crystalline compound is obtained (the precursor), filtered, washed and dried 4, Addition of hydroxyethylceUulose m the fdtrate and shaping in cylinders
Acgvqt~Ol~qt 450 ~Cin nitrogen flow Recent patents by Monsanto (9,26) involve a peculiar procedure of activation of the compound (VO)MmHPO4.aH20.b(P2Os).n(organics), precursor of the active phase, (VO)2MmP207.b(P2Os), in many stages. This procedure gives an active and selective catalyst in a short time (Scheme 7). The precursor is prepared in an anhydrous medium by reduction of V205 in isobutanol and oxalic acid after addition of phosphoric acid and of promoters, followed by digestion under reflux, separation of the precursor and drying in a nitrogen atmosphere.
Scheme 7. Monsanto multistage thermal trasformation of the precursor in the presence of steam
Roasting the dried precursor by calcination at 250~ in air to eliminate the occluded organic compounds, and then formation into geometric shapes Initial heat-up stage to 275*C in air (with no control of heat up) Rapid heat-~q) stage in a molar 50/50 air/steam stream with a heating rate of 1 degree/rain to 425"12 m dehydrate the catalyst Maintenance at 425 ~ in the air/steam flow for 1 h to oxidize the catalyst to a valence of 4.5 Finishing stage by flowing a 50/50 steam/nitrogen stream for 6 h to avoid overoxidation of the catalyst, and allowing time for complete transformation of the catalyst precursor to the activ~ phase
3. PREPARATIONS OF CATALYSTS SUITABLE FOR FLUID-BED TECHNOLOGY
In order to increase the attrition resistance of catalysts for fluid-bed reactors, four preparation techniques can be envisaged: 1) impregnation of active components onto a support with optimal fluidization properties; 2) embedding of the active component in an inert material with high attrition resistance; 3) addition of small amounts of additives to the precursor; 4) encapsulation of the active component in a thin shell of silica. Only the last two techniques are used commercially. The silica and alumina used in the first two techniques are not sufficiently inert towards the active components and also towards maleic anhydride, with a global effect of decreasing the selectivity. Optimum properties of a catalyst for fluid-bed operation are as follows: -density higher than 0.75 g/cm3; -spheroidal particles ranging in size from 20 to 300 I.tm (with preferably 80% in the range 30 to 80 lain); -most preferably 25% to 45% of the particles with an average diameter less than 45 lain. A fluid-bed catalyst has been jointly developed by Lummus Crest and Alusuisse Italia (27). The catalyst can be prepared by a double spray-drying technique. The preferred procedure is reported in Scheme 8 (22,24,25). Microspheres ranging from 40 to 200 Ixm in diameter, with high attrition resistance and a surface area of 26 m2/g are obtained with this procedure. The relative amounts of the two components determine both the attrition resistance and the activity. Increasing the amount of uncalcined catalyst increases the activity but decreases the attrition resistance. A1, B, Zr and phosphoric acid act as binders in order to increase the attrition resistance. Scheme 8. Alusuisse-Lummus preparation of unsupported fluid bed catalysts
Comminution of dried V/P/O precursor together with Zr hydroxide in a water slurry with a ball mill until less than 0.5 lain particles are obtained Spray drying of this slurry to obtain 40-200 gm particles, and then calcination of the particles in air at about 400~ Mixing of this first component with the dried precursor (1/1 by weight the preferred ratio) in an aqueous slurry, addition of phosphoric acid, boron and aluminum salts as promoters, followed by comminution in a ball mill and spray drying of the slurry Activation of the particles at about 470~ in a nitrogen atmosphere
A patent from BP America (28) has claimed the preparation of catalysts for fluid-bed application by staged impregnation of preformed supports (suitable for fluid-bed operation) with a solution of metal alkoxide. The preparation is summarized in Scheme 9.
Scheme 9. BP preparation of fluid-bed catalysts by impregnation of a fluidizable support
Impregnation of the support (fluidizable alumina or silica with particles ranging in size from 20 to 300 ~tm) by a wet impregnation technique with a solution of ter-butoxyvanadium in ter-butanol (non-reducing alcohol) Decomposition of the alkoxy compound in order to obtain deposition of vanadium oxides inside the pores of the support Further impregnation with a solution of phosphoric acid in isobutanol (a reducing alcohol) .. to reduce vanadium and to form the catalyst orecursor in-situ Another patent by BP America (29) deals with the preparation of an attrition-resistant fluid-bed catalyst based on unsupported vanadium/phosphorus mixed oxide. The mechanical properties are given to the catalyst only by the special preparation procedure. The procedure is summarized in Scheme 10. Key-features of this procedure are i) densification of the catalyst precursor by tabletting or pelleting, followed by dry ball milling, ii) preparation of an aqueous slurry with the uncalcined comminuted catalyst, because the calcined catalyst (and the oxidized precursor, also) may be altered by water, and iii) activation of the particles in the fluid-bed which gives higher attrition resistance than static calcination. Scheme 10. BP preparation of unsupported fluid-bed catalysts i
Comminun'on of the particles of the precursor prepared in an organic medium to 1 ~rn particles by densification and dry milling, ~nd introduction into water to form a slurry. 4, Addition of small amounts of silica to the water slurry (maximum 10 wt. %), to improve the attrition resistance properties
Spray drying of the water slurry to microspheroidal particles ranging from 20 to 300 gm Calcination of the catalyst and activation under fluidization initially at 300-325"12 in an air stream; then the temperature is raised to 400-425"12 at about 2~
Du Pont has developed a process for the production of tetrahydrofuran through the synthesis of maleic anhydride from n-butane with a transport-bed technology (30). The main features of the preparation are summarized in Scheme 11. The silica coats the active components and forms a very strong shell which gives high mechanical resistance and does not cause loss in selectivity. Preparation of catalysts with optimal properties for fluid-bed operation (31) have been claimed by Mitsubishi to be obtained by spray drying an aqueous slurry of the components, according to the procedure described in Scheme 12. It has been proposed that the second component may act as a binder and the silica as the carrier; the second component also
contributes to improve the fluidizability properties and optimize the density of the bed. Scheme 11. Du Pont preparation of transport-bed catalysts.
Grinding of the precursor into 2 ~m particles and formation of a slurry with freshly prepared silicic acid to produce a sample containing 10 % silica by weight
Spray drying of the slurry. During drying, silicic acid migrates to the surface of the particles and ultimately polymerizes on the surface of the particles
Calcination of the spray dried catalyst in the regenerator zone at 390~
then the active catalyst is produced by running the n-butane oxidation in air for several hours in the
regenerator
Scheme 12. Mitsubishi preparation of fluid-bed catalysts.
Preparation of an aqueous slurry containing the following three components: 1) an already activated catalyst (activation realized in nitrogen at about 500"C), prepared under hydrothermal conditions, and also containing iron as a dopant 2) a solution containing dissolved V205, oxalic acid and phosphoric acid; the oxide content (expressed as V204 + P205) in the concentrated solution typically is 30 wt.% 3) a colloidal solution of silica (silica content 20 wt.%) Spray drying of the water slurry and activation of the solid at 500"C in nitrogen.
4. OPTIMUM PHYSICAL PROPERTIES OF FIXED-BED CATALYSTS
4.1 Preparation of pellets with minimum thermal expansion Amoco has issued patents claiming a procedure to minimize the thermal expansion of the catalyst pellets (17). Thermal espansion of the pellets inside the reactor causes crushing of the particles with the formation of f'mes which increase the pressure drop and consequently decrease the lifetime and productivity of the catalyst. Three procedures have been proposed which it seems can be used contemporaneously when superior results are required: 1) control of the H20/P ratio during the preparation stage (the optimum water-to-phosphoril compound molar ratio is around 3). Both insufficient and excess water can create higher thermal espansion. 2) calcination of the tablets (before their exposure to the oxygen-containing stream) in nitrogen at a temperature around 400"C; minimal expansion of the pellets occurs with such treatment. In Figure 1 the thermal espansion of the pellets is plotted versus the calcination temperature, for treatments carried out in nitrogen and in air.
tablet volume change, % 6
4 2 O
.
.
.
.
.
-4 ,,,6
...................................................................... I
3~;0
375
I..,
400
I
425
..
I
I
450
475
.,.
500
temperature of calcination, "C
Figure 1. Thermal expansion of precursor pellets for calcination in air ( 9 and in N2 ( 9 3) use of a small amount of oxygen (0.1% in an inert atmosphere) during the step of preparation of the precursor (during reflux of the solvent, evaporation and drying). In addition, the use of a small amount of oxygen also results in a considerable decrease in the amount of chlorine ions present in the catalyst.
4.2 Optimum shape of the pellets The shape of a catalyst for fLxeA-beA operation is an important factor which can affect the activity, productivity and lifetime. Indeed, by giving a particular shape to the catalyst it is possible to decrease the pressure drop along the bed, and hence to increase the lifetime and flow rate. In addition, a better removal of the heat from the catalyst and therefore an increase in productivity can be achieved by operating at higher inlet concentration and conversion, or using less catalyst. For istance, Denka describes cylindrically shaped catalysts with an axial hole for fixed-bed reactors (32). V/P/O catalysts shaped to obtain enhanced activity (weight of product per unit weight of catalyst) have recently been patented (33). Such structures are characterized by i) void spaces in the external surface; ii) a geometric volume ranging from 30 to 67 % of that exhibited by the void space-free solid geometric form; iii) an external geometric surface area-to-geometric volume ratio of at least 20 cm'l; iv) a bulk density ranging from 0.4 g/cm3 to 1.4 g/cm3, and v) good mechanical resistance to maintain integrity under handling. Figure 2 shows several of these shaped structures.
l0
@
@ @
Figure 2. Shaped structures for fixed-bed V/P/O catalysts (33).
5. ACTIVATION AND REGENERATION PROCEDURES After the first thermal treatment, consisting of a calcination carried out in air at low temperature before tabletfing in the case of fixed-bed catalysts, in a flow of nitrogen at higher temperature (usually higher than 400"C) for fluid-bed operation, it is necessary to activate the catalyst. Some of the activation procedures can also be employed for regeneration of deactivated catalysts. Several procedures have been proposed to activate and reactivate the catalysts, both batchwise or continuous, in order to increase performance and therefore lifetime: -activation at low hydrocarbon concentration in air; -reduction at high temperature; -treatment with phosphorous compounds; -treatment with chloride compounds; -treatment with H202; -addition of scavengers for V and P; -treatment with steam.
11
5.1 Activation in lean hydrocarbon atmospheres Scientific Design (34) has proposed activation of the catalyst by slowly bringing the catalyst up to operating temperature (heating rate 10 degrees per hour) and adjusting the concentration of the n-butane from 0.5 to 1.0 mol % at an initial gas flow of 900 h "l up to the final value of 2500 h "1. According to Blum et al. (35) conditioning or activating a catalyst in a flow of n-butane under usual operating conditions has too little a beneficial effect in catalytic performances for a fluid-bed where no hot spot exists. Therefore preliminary conditioning of the catalyst inside the reactor in nitrogen at temperatures higher than the reaction temperature is proposed before the introduction of the lean hydrocarbon gas mixture.
5.2 Reduction at high temperature Stefani and Fontana (36) have proposed activation of either the precursor after tabletting or a deactivated catalyst by a reducing treatment in a hydrogen or n-butane flow (n-butane 50 % in nitrogen is the most preferred gas composition), at a reaction temperature of 400-450"C for less than one hour. It is suggested that this activation is necessary to reduce the valence of vanadium to an average degree value than four. Others patents claim the reduction of a deactivated catalyst with methane, H2S or CO at 500"C (37). According to Blum et al. (35) overreduction of the catalyst has an immediate effect on the catalyst performance since it reduces an overoxidized catalyst, but it may have a detrimental effect on the catalyst life. These authors propose that activation in the presence of an excess of hydrocarbon with respect to oxygen (i.e., with an amount of oxygen lower than the stoichiometric one necessary to reach complete combustion: n-butane/oxygen/nitrogen 1/0.2/3.8) at a temperature about 30"C higher than the optimal reaction temperature, is a more effective procedure. Indeed, the reduction of a catalyst in the presence of oxygen is dynamic in nature, in contrast with the static removal of lattice oxygen carried out via reduction in an oxygen-free atmosphere. The authors suggest that during this type of reduction the active sites and microcrystaUine structure of the catalyst undergo dynamic reorientation. This results in localized crystalline changes which optimize the catalytic activity. In addition, the presence of oxygen during the reactivation (or activation) furnishes the heat necessary to carry out the activation procedure inside the fluid-bed. Blum et al. (35,38) have suggested that in order to activate the fresh catalyst which must be added as make-up to the fluid-bed from time to time, an activation procedure with a poor oxygen-containing stream at high temperature must be carried out, or, alternatively, a slip stream of catalyst must be withdrawn from the reactor continuously, reactivated, and later-on reintroduced. It is a peculiar property of V/P/O based catalysts that deactivation occurs with an increase of the activity and a decrease in the selectivity to maleic anhydride (18,39). Deactivation phenomena are not well explained in the patent and scientific literature, but very likely are due to overoxidation of the catalyst and/or migration of phosphorus.
5.3 Addition of phosphorous compounds Organic phosphorous compounds can be added continuously or batchwise in order to maintain the catalyst performance constant or to reactivate the catalyst. Therefore, phosphorous compounds can be considered either as stabilizers or as reactivators for the
12 catalyst. Although there is not full agreement, it seems that the addition of phosphorus restores the surface P/V ratio to the optimum value for selectivity, expecially in the hot-spot zone. Deficiency of phosphorus with respect to the optimum P/V ratio increases the activity and decreases selectivity, while excess phosphorus decreases the activity and increases the selectivity. The preferred organic compounds are lower alkyl phosphates or phosphites (trimethyl or triethyl). Phosphorus compounds can be added alone (40) or with water (19), or alternating their introduction in the reactor with injection of water (41,42). It has been suggested (41) that the role of water is to redistribute the phosphorus evenly through the bed avoiding its accumulation in the zone close to the inlet part of the tubular reactor. The addition of phosphorus compounds has the following effects: 1) The hot-spot temperature is decreased, thus avoiding run-away conditions and degradation of the catalyst; therefore, it does increase the lifetime. 2) The selectivity increases and therefore the yield also increases. 3) The average temperature inside the reactor increases, but with a more isothermal profile along the bed, thus allowing better heat exchange with the salt bath. For these reasons the addition of phosphorus also makes it possible to operate with higher hydrocarbon concentration which results in increased productivity. Taheri (19) has reported the values of the temperature along the catalytic bed during operation before and after the addition of phosphorus (Figure 3). It can be seen that the addition of phosphorus does not change the position of the hot-spot in the bed, but only
temperature,~
5001
,
480
460
440~- . . . . . . . / ~ ~ " . . . . . ~ ' " . . . . . . . . . . . . . . . . . . . . . . . . . . .
20
.
400 0
.
.
.
.
.
.
.
.
.
.
.
, 40
.
.
.
.
.
,
.
.
.
.
.
.
t 80
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
, 120 reactor
.
.
.
.
, 160 length,
.
.
.
"9~ " 99"-'~"'~" 999 9....
.
.
.
.
.
.
.
.
I 200
.
.
.
.
.
.
.
.
.
.
.
.
.
, , 240
cm
Figure 3. Temperature profile along the catalytic bed before (A) and after ( o ) addition of phosphorus in the feed stream (19).
13 decreases the value of the maximum temperature. In addition, the temperature prof'lle is more isothermal, thus indicating that a larger fraction of the catalytic bed is working. On the contrary, when no phosphorus is added, the major part of the reaction occurs in the hot-spot zone. Several patents have been issued claiming an optimum procedure to introduce phosphorus compounds. Optimum amounts of phosporus additions range from 0.1 to 6 mg hr "1 kg "1 of catalyst (43), or 1 to 6 ppm by weight of the total feed flow. According to Ebner (26), the optimum amount of phosphorus to be injected must be related to the amount of water entering the reactor, to the level of conversion, to the air flow, and to the pressure and amount of entering hydrocarbon. Ebner proposed the following relationship to control the optimum addition of phosphorus compounds: N = 5 * C4 + 6 * (H20 - 2.4)+ 0.75 *(conv - c) + SV/(25 * Pin) where N = amount of trimethylphosphate in ppm to be added in the feed stream C4 = the mole % of n-butane in the gas entering the reactor H20 = the mole % of moisture in the gas entering the reactor conv = the % n-butane conversion in the reactor SV = hourly space velocity of the gas at the inlet, reduced to normal conditions Pin =pressure at the inlet of the reactor in psig c = 84 * 0.05 (SV * C4/Pin)
5.4 Vanadium elimination Deleterious vanadium species (most likely V205) can be eliminated via batchwise injection of organic and inorganic halogen compounds (CC14, from 0.01 to 0.1 g/g of catalyst in a nitrogen flow for less than 30 minutes at a temperature ranging from 375 to 475~ in order to increase the P/V ratio (15). Successively, if necessary, the optimum P/V ratio is reached by the addition of phosphorus compounds. 5.5 Addition of H202 Addition of H202, or of other peroxides (5-100 pprn) (44) to the feed stream has been claimed to be effective in lowering the reaction temperature at a given conversion and thus in prolonging catalyst lifetime. The optimum value of hydrogen peroxide is around 500 wt. pprn of the total reactor feed gas stream. 5.6 Addition of scavengers The addition of a scavenger for V and P has also been proposed for the purpose of increasing catalyst lifetime. Phosphorus and vanadium sublimate from the active component in the hot spot in fixed-bed reactors, or during the high temperature activation in fluid-bed reactors (during start-up or during regeneration), and condensate on colder parts produce deactivation of the catalyst. The proposed scavengers for n-butane oxidation are inert materials based on Mg, Sb, and Bi oxides supported on silica (39).
14 6. PREPARATION OF THE PRECURSOR
There is general consensus (6,45-51) that the necessary conditions to obtain an optimum catalyst are the following: -synthesis in organic solvent of microcrystalline (VO)HPO4.0.5H20 with a preferential exposure of the (001) crystallographic plane; -presence of stacking defects in the platelets; -slight excess of phosphorus with respect to the stoichiometric amount; for istance, an atomic P/V ratio of 1.1 (the excess phosphorus remains strongly bound with the vanadyl acid phosphate). Preparation in an anhydrous solvent is considered the best one to obtain active and selective catalysts. In all preparations essentially a single phase has been obtained, (VO)HPO40.5H20. Only when a considerable excess of phosphorus (P/V>2) is used, may another phase appear, VO(H2PO4)2, (46). When reduction of the V3+compound is not complete, small amounts of V205 or VOPO4 are also present which affect the nature of the products obtained by calcination (52). The main differences observed between the several precursors regard the morphology of the (VO)HPO40.5H20 crystallites obtained. Figure 4 shows the X-ray diffraction patterns of the precursors prepared in organic and in aqueous media (49). The spectra confirm the results formerly published that the precursors prepared in an aqueous medium are more crystalline and exposure of the crystallographic plane (001) is less pronounced, since no preferential line broadening of the corresponding reflection is observed (6,46,53).
A
B
10
30
50
70 2O
Figure 4. X-ray diffraction patterns of (VO)HPO40.5H20 prepared in organic (A) and aqueous (B) media.
15 The following steps for the formation of the precursor in organic media (schemes 1 and 2) can be proposed: -formation of colloidal V205 at the water-alcohol interface; this has been proposed by some authors (52), but according to others (49) it is not an important step; -solubilization of V 5+ through the formation of vanadium alcoholates (49) or of VOCI3 in the case HC1 is used as reductant; -reduction of the alcoholate in the liquid phase to solid V204 by the organic compound (the solvent itself or another more reactive alcohol such as benzyl alcohol) or by an inorganic reducing agent, such as HC1; -reaction at the surface of V204 with H3PO4 to form (VO)HPO4.0.5H20 at the solid-liquid interface; -separation of the precursor by filtration, centrifugation, decantation, and evaporation or by extraction of the solvent with a more volatile solvent followed by distillation under vacuum; alternatively, the precursor is washed with water to allow an organic layer to separate from an aqueous layer, followed by recovery of the precursor by drying. An alternative route that might occur in the Amoco and Chevron preparations where HC1 is used as the reductant (schemes 3 and 5), is the formation of V4+ chloride or oxychloride species soluble in organic media which react with the H3PO4 and form the precursor. A less likely alternative or parallel route is the solubilization of V4+ in an aqueous emulsion (water formed by vanadium reduction is not easily removed) and formation of (VO)HPO40.5H20 in water droplets (49,52). The type of aliphatic alcohol used modifies the temperature at which vanadium is reduced; the reduction is kinetically controlled and complete only when benzyl alcohol is present (forming benzaldehyde and benzoic acid), when a long reduction time is used and after the addition of phosphoric acid (49). It has also been observed that the type of alcohol may affect the morphology of (VO)HPO40.5H20 (46). In dry milling of the precursor, the rosette-like crystallites (formed when the preparation is carded out in isobutanol) can be broken and the effect is to decrease the (001) crystallographic plane exposure, while in wet milling shear forces allow the platelets to slide away, thus increasing the (001) exposure (46). In the preparation in the presence of benzyl alcohol many authors report the formation of platelets with stacking faults (deduced from the preferential line broadening of the (001) reflection) attributed to the trapping of the alcohol between the layers of the precursor and its release during activation (6,46,49). In the preparation in an aqueous medium (scheme 6) the following steps for the formation of the precursor can be proposed: -reduction of V205 to soluble V4+; -after addition of H3PO4 no precipitation occurs, due to the strong acid conditions (6,54); -development of (VO)HPO40.5H20 with another spurious amorphous phase only after complete evaporation of the solvent (55); -alternatively, crystallization of pure (VO)HPO40.5H20 by addition of water when the solution is highly concentrated (when it is very viscous) (6,54), or by seeding under hydrothermal conditions (high temperature and steam pressure).
16 7. THERMAL DEHYDRATION OF THE PRECURSOR
Thermal dehydration of the precursor is usually realized with a multistage procedure. The first stage is roasting at temperatures lower than 300"C in order to eliminate the organic impurities or chlorine ions from the precursor without however causing dehydration to occur. After this treatment, different types of thermal dehydration have been proposed: 1) Dehydration inside the reactor starting from a low temperature (280"C) in a flow of a lean reactant mixture and at low flow rate until standard operating conditions are reached in approximately one day. 2) Dehydration in an oxygen-free atmosphere at temperatures higher than 400"C, followed by introduction of the reactant mixture (n-butane in air). With this procedure, after the first step, crystalline (VO)2P207 is obtained which, after the introduction of the reactant mixture can remain substantially unmodified or be partially or totally reoxidized to a vS+-containing phase (46,54,56). 3) Single or multistep calcination in air until a temperature lower than 400"C is reached, and then introduction of the reactant mixture (46,55,57). Controversial results are found in the literature, regarding expecially the transformation of precursors to the active phase, because many different phases can form depending on: -temperature, time and atmosphere of treatment; -morphology of the precursor; -P/V ratio; -presence of additives; -presence of defects in the structure. After calcination at 280"C, the precursor is still present during release of the trapped benzyl alcohol, and this release leads to disruption of the structure (55), causing an increase in the surface area. Figure 5 shows the evolution of the X-ray diffraction patterns of the precursor prepared in an organic medium when it is treated in air at high temperature (58). When the precursor is maintained at 380~ in air, the reflections typical of vanadyl orthophosphate progressively decrease in intensity, while evident amorphization occurs (55). When the diffraction lines of the precursor have disappeared completely, only an amorphous material remains. After 3-6 hours at 380~ in air, the sample is highly amorphous, and weak reflections relative to the vanadyl pyrophosphate and to a V3+phase are observed. Transformation to the well-crystallized (VO)2P207 occurs in the reactor, after several hundreds of hours of time-on-stream. Different types of VOPO4, more or less reducible to (VO)2P207, have been identified such as the a (59), 13or ~' (60), [~* (52), 8 and 1~II (61,62), and YVOPO4 (59). Key factors in catalyst preparation to avoid the oxidation of (VO)2P207 and/or of the intermediate amorphous phase to a V 5+ phosphate, the formation of which is known to be deleterious for activity and selectivity (59-65), are the following: -The P/V ratio. P/V ratios in the precursor higher than the stoichiometric one stabilize the (VO)2P207 not only in the reactant atmosphere but also for calcination in air at high temperature. -Minimization of impurities. The presence of free V205 (52), even in traces, or of additives such as Mn 2+ (63) facilitates the oxidation of the pyrophosphate in the reactant
17
10
20
3O
40
50
60 2e
Figure 5. Ex-situ evolution of (VO)HPO40.5H20 at 380"C in air. A: precursor; B,C and D: samples at increasing times of calcination; E: equilibrated catalyst: (VO)2P2OT. atmosphere.
-Morphology. It has been proposed that oxidation of (VO)2P207 starts at the side fo~es of the (100) plane (60). Catalysts with an higher exposure of this plane, such as those prepared in an organic medium, are therefore less oxidized. -Low temperature of treatment in oxygen-containing atmosphere. Precursors prepared in an organic medium and which contain defects transform at lower temperatures than those prepared in an aqueous medium, that are more crystalline. -Additives. The presence of Zn2+ as a promoter avoids overoxidation of the catalyst at high temperature (49). By means of electron microscopy (46,49) it has been observed that the dehydrated phases maintain the morphology of the precursor. Moreover, X-ray diffraction analysis has shown that broadening of reflections relative to the basal plane of the precursor also occurs for
18 the reflections relative to (hO0) crystallographic planes (parallel to the basal plane) of (VO)2P2OT; the reflection relative to the (200) plane looks particularly broadened. These findings, together with analogies of the two structures, allowed Bordes et al. (66) to propose that the transformation from (VO)HPO40.5H20 to (VO)2P207 is topotactic. Recently Thompson et al. (67) have suggested, on the basis of different symmetries of the two structures, that this transformation is not a topotactic one. In addition, it is difficult to assume a topotactic reaction for the formation of an active phase when an intermediate amorphous phase has been identified which transforms very slowly to vanadyl pyrophosphate. It has been found (68) that different phases present in calcined catalysts can cooperate to improve the catalytic behaviour. Very likely these findings are less important for the most active and selective catalysts, where only one phase has been detected, but they can be important in the stage of formation and as regards the catalytic properties of vanadyl pyrophosphate during the activation procedure. Scheme 13 summarizes the possible evolution of (VO)HPO40.5H20 with temperature.
(VO)HPO40.5H20_.. , amorphous phase
, (VO)2P207---, V s§ phases T , _l
Scheme 13. Evolution of the precursor with temperature.
8. ACTIVATION/AGING PROCEDURES After the stage of dehydration the catalyst has to be activated; this stage can be carded out either in the presence of or without an n-butane/air atmosphere. During prolonged exposure to the reactant atmosphere changes occur with time-on-stream both in catalytic behavior and in the physico-chemical properties of the catalyst. In catalysts calcined in air the transformation from a partially amorphous, possibly oxidized compound to an almost completely crystalline vanadyl pyrophosphate inside the reactor and in the presence of the reactant mixture requires more than 100 hours (69), depending on the features of the fresh catalyst, i.e. the calcination conditions employed. If the fresh catalyst is highly oxidized (sample A in Figure 6), after 80 h time-on-stream the (VO)2P207 has in part.crystallized, but the catalyst is yet oxidized. More than 500 h are necessary to reduce V 3+ completely and obtain well crystallized vanadyl pyrophosphate. When the fresh catalyst is only slightly oxidized (i.e., after a milder calcination treatment, sample B), a period of 80 h time-on-stream leads to an increase in crystallinity. In this case the final crystalline compound is obtained in a shorter period of time (200-300 h), because vanadium is already in the reduced state. During aging the activity usually decreases, but the global effect in catalytic behavior is an increase in the yield of maleic anhydride, owing to the fact that the catalyst can operate at higher temperature and conversion while maintaining high selectivity (values as high as 56 %
19
v
A V O P O 4 l====~VOPO4.nH=O v
A
9
A
/
| --'+
v
9
9
9
j '
I
20
9
--,
" -
10
,,
I
30
I
40
-~lW-r-v- -
I
50
9
60
I" IIp"
1
I
20
I"
O ~ .
I
30
~ j ~ . ,
I
40
~1,
i
l
50
60 2e
Figure 6. XRD patterns illustrating the structural evolution of fresh catalyst in the reaction environment. Samples were obtained by static calcination in air at 380"C for 30 h (A) and 2 h (B). The precursor contained 5 wt.% organic binder (58). molar yield to maleic anhydride have been reached). Also in the case that the precursor has been treated in nitrogen (the fresh catalyst obtained is a vanadyl pyrophosphate (70)), modifications in activity (which is progressively increased) occur during the first 100 h. This makes it possible to operate at lower temperature, while the yield to maleic anhydride is maintained high (or even increased), due to a progressive increase in the selectivity. During this activation the vanadyl pyrophosphate structure remains unaltered. A fresh catalyst has be~n designated as a "non-equilibrated one" (6), and a catalyst after prolonged time-on-stream (i.e., after activation) as an "equilibrated one". Warning has also been given against extrapolating the initial activity of the catalyst to its behavior in industrial-like conditions. A "non-equilibrated" catalyst is more active and has lower selectivity to maleic anhydride, especially at high conversion, owing to the easier oxidizability of vanadium in the last part of the reactor; the reactant mixture here becomes more oxidizing due to the considerable decrease in n-butane concentration, while the oxygen concentration is still in excess with respect to the stoichiometric ratio. A more precise definition of an "equilibrated" catalyst has been recently given by Ebner and Thompson (71) (Table 1). According to these authors, an "equilibrated" catalyst is one which has been kept in a flow of n-butane with a concentration of 1.4-2 % in air and at least GHSV 1000 h "l, for approximately 200-1000 h. Table 1. Features of the "equilibrated" catalyst 71) Average degree of oxidation of vanadium 4.00-4.04 Bulk P/V ratio 1.000-1.025 XPS surface atomic P/V ratio 1.5-3.0 16-25 m2g-1 BET surface area X-ray diffraction pattern vanadyl pyrophosphate Morphology rectangular p!atelets and rod-like structures
20 According to Sola et al. (69) the conditions required can be less severe. In particular, an "equilibrated" catalyst is one that maintains a constant catalytic behavior for at least 50 hours. One of the main properties of an "equilibrated" catalyst is the formation of stable V 4+ (average valence state 4.00-4.03) (70-72). "Equilibrated" catalysts can no longer be reoxidized in air at 400~ whereas freshly prepared (VO)2P207 or "non-equilibrated" catalysts can be oxidized at this temperature.
9. NATURE AND ROLE OF PROMOTERS 9.1 Analysis of the patent literature An empirical formula which can represent all the catalyst formulations described in patents is the following: VPaMebOx/y inert. Wide variations in the value of a (0.8 to 1.5) and a wide spectrum of Me promoters (prafically all the elements of the Periodic Table) have been claimed in the patents; a non-exhaustive list of promoters comprises ions of the following metals: Li, Zn, Mg, In, B, A1, Bi, Sb, Ta, Co, Fe, Ni, Cr, Ti, Mo, W, U, Zr, rare earths. The most preferred catalyst compositions are the following: a from 1.03 to 1.25; b from 0 to 0.1; x balances the positive charge of all the other elements; y (colloidal silica) from 0 to 20 % by weight with respect to the active component. All companies claiming the use of promoters report that the latter have to be added before the precursor is formed. Only aluminum and boron in an Alusuisse catalyst for fluid-bed reactors are introduced after the precursor has been formed; they have been chimed to increase the mechanical resistence through the formation of phosphate binders. For all the catalysts phosphorus is the main promoter. In fact in all compositions an excess of phosphorus with respect to the stoichiometfic ratio of the (VO)2P207 is claimed. Moreover, control of the amount of phosphorus during preparation and time-on-stream is the most important factor to control activity and selectivity. As the amount of phosphorus increases, the activity decreases and the selectivity increases; however, the optimum amount also depends on the type and amount of the other promoters. Phosphorus affects the redox properties of vanadium in (VO)2P2OT. Plotted in Figure 7 are the indexes of reducibility and oxidizability of the catalyst as functions of the P/V ratio (6). The amount of V5+observed in the catalyst after calcination in air at 400~ for half an hour has been taken as the index of (VO)2P207 oxidizability. The amount of V 3+ formed after reduction in diluted H2 has been taken as the index of reducibility. Catalysts with excess P with respect to the stoichiometric ratio are more difficult to both oxidize and reduce. A low reducibility corresponds to a lower catalytic activity, while an higher amount of V 5+ (for catalysts with a P deficiency) can be responsible for maleic anhydride overoxidation. The best compromise is achieved with a slight excess of phosphorus (P/V 1.05-1.1), which does not penalize activity too much and stabilizes vanadium against overoxidation. The promoters listed in Table 2 are those reported in the examples of many patents issued by each company, but clearly are not the only ones claimed. The reported compositions are not necessarily the optimum ones, even though they are likely to be close to the preferred compositions.
21
25
V s+ I at.%
W* ! ~
J
at.%
10
(VO),,P,,O,
20-
V3*
15-
10-
-4
5-
-2 defect of P i
0 0.9
excess of P
I
i
I
I
I
0.95
1
1.05
1.1
1.15
0 1.2
PN, atomic ratio Figure 7. Effect of the P/V atomic ratio on (VO)2P207 reducibility and ease of oxidation. The tests were carried out in a thermobalance. Table 2. Main promoters repo accl in patents Promoter, Me Company Mg, Zr Mitsui Toatsu Chem Inc. Fe Mitsubishi Kasei Co. Zr, B Alusuisse/The Lummus Co. Mo Amoco Co. In, Ta, Sb, Si E.I. Du Pont de Nemours Mo, Zn, Li Denka Chem. Co. Fe (Zn)l Li Monsanto
Me/V, atomic ratio 0.05,0.05 0.026 0.05-0.15,0.05-0.15 0.031 0.014,0.037,0.014,0.11-0.24 0.013,0.01,0.01 0.0016,0.003
Reference 13 73 25,74 16,19 75 32 76
Scientific Design (34,77) has suggested that the role of molybdenum as a promoter is to produce a more stable and active catalyst with a longer lifetime and to atlow the use of lower amounts of phosphorus. Zn and Li also enhance the stability of the catalyst and Li also improves the activity. In Mitsui patents (13) promoters have been claimed to decrease the activity, but nevertheless they allow high selectivity to be maintained, although higher temperatures are ne~ed. It seems that the promoters decrease the decomposition of the maleic anhydride. In the case of the Mitsubishi catalyst (73), it~has been reported that Fe increases the activity, and thus makes it possible allows to operate at lower temperature and with higher selectivity.
22
9.2 Analysis of the scientific literature A complete review dealing with the effect of promoters on the catalytic activity of V/P/O catalysts was published by Hutchings in 1991 (78); therefore, we shall examine here only recently published papers. A comparative study of the role of the addition of Zn, Ti and Zr on V/P/O catalysts has been carded out by Sananes et al. (79). They found a promoter effect for activity, with a maximum in activity as a function of the amount of promoter. These authors did not observe any structural modifications in the (VO)2P207 after the addition of promoters, and no correlation between activity and surface area was found. XPS analysis showed only a surface enrichment of promoters. Ye et al. (51) have investigated the role of several additives on the surface and structural modifications as well as on the catalytic activity of (VO)2P2OT. These authors also reporteA, for comparison, the modifications observed in unpromoted catalysts as a result of changes in preparation procedure. The variations in activity and selectivity caused by either adding promoters or by changing the preparation method were attributed to modifications in the following properties of the (VO)2P2OT: -surface area; -exposure of the (200) plane; -defects in the (200) planet -number of surface (V---O)~+ sites. The increase in activity was attributed to the increased exposure of the (200) plane, to the greater number of (V--O) 3+ sites and to the increased defectivity; the increase in selectivity was attributed to the increased (200) plane exposure. Meisel et al. (54) have recently prepared catalysts in an aqueous solvent and studied the influence of the addition of sulfuric acid on the crystallization of (VO)HPO40.5H20. It was found that up to 10% sulfur can enter the structure of the precursor by substituting for phosphorus anions. Calcination resulted in the removal of a considerable part of the sulfur from the structure of the (VO)2P207 formed. Line broadening of the (200) reflection in relation to the (024) reflection, and also a shifting of its .position corresponding to a lattice expansion, were observed only at low sulfur content (SO4 '~'/V 0.007). Therefore it seems that sulfur ions are able to modify the catalyst obtained by the preparation in an aqueous medium, leading to catalysts which are more similar to those prepared in an organic solvent. Sulfur-promoted catalysts have shown very high activity and selectivity (53% yield to maleic anhydride, similar to the values obtained with catalysts prepared in an organic solvent). Bey and Rao (80) have investigated the catalytic behavior of cerium and molybdenum-promoted V/P/O catalysts. They found that the promoted catalysts are less active than unpromoted ones, reaching the same level of conversion of the latter at higher temperature or longer residence time. At low conversion the promoted catalysts exhibit the same selectivity as unpromoted ones, but the promoted catalysts are more selective at high n-butane conversion. Therefore, it seems that the role of the couple Ce/Mo is to avoid decomposition of the maleic anhydride formed when operation is carried out so as to achieve high reactant conversion. In conclusion the additives (or promoters) can be divided into the following three groups: 1) Basic ions, such as Zn, Mg, Li, A1, B and Zr, which interact with free phosphoric acid acting as a tool for f'me tuning of the optimum surface P/V ratio and acidity, and forming
23 phosphate binders which increase the mechanical resistance and avoid phosphorus migration. 2) Ions which can substitute for phosphorus, such as S and Si, in the precursor. The partial or total elimination of these ions by calcination influences the morphology and defects of the (VO)2P207. Indeed, they may have the same role as that proposed for benzyl alcohol trapped inside the structure of the precursor. 3) Transition element ions which substitute for vanadium and act as real modifiers of the reaction pattern, forming a stable solid solution and possibly being directly involved in the reaction. These promoters act either to poison activity, allowing the operation to be carded out at higher temperature without decomposition of the maleic anhydride (this is the case of molybdenum-doped catalysts), or to promote of activity (such as iron), allowing the reaction to be carded out at lower temperature with higher selectivity. ACKNOWLEDGEMENTS The Ministero dell'Universi~ e della Ricerca Scientifica e Tecnologica (MURST, 60%) is gratefully acknowledged for financial support. REFERENCES
1) F. Cavani and F. Tdfirb, Appl. Catal. A:General, 88 (1992) 115 2) J.C. BurneR, R.A. Keppel and W.D. Robinson, Catal. Today, 1 (1987) 537 3) R.M. Contractor and A.W. Sleight, Catal. Today, 1 (1987) 587 4) G. Centi, "Vanadyl Pyrophosphate Catalysts", Catal. Today, 16 (1993) 5) G. Centi in "Elementary Reaction Steps in Heterogeneous Catalysis", R.W. Joyner and R.A. van Santen (Eds.), Kluwer Academy Publisher, Netherlands, 1993, p. 93. 6) G. Centi, F. Trifirb, J.R. Ebner and V.M. Franchetti, Chem. Rev., 88 (1988) 55. 7) F. Cavani and F. Trifirb, Chemtech, 24 (1994) 18. 8) F. Cavani and F. Trifirb, in "Catalysis Volume 11", Royal Society of Chemistry, Cambridge, 1994, p. 246. 9) J.R. Ebner and W.J. Andrews, US Patent 5,137,860 (1992), assigned to Monsanto Co. 10) N.J. Bremer, D.E. Dria and A.M. Weber, US Patent 4,365,069 (1982), assigned to The Standard Oil Co. 11) N.J. Bremer, D.E. Dria and A.M. Weber, US Patent 4,448,893 (1984), assigned to The Standard Oil Co. 12) J. Takashi, Y. Kogure, T. Kiyoura and K. Kanaya, Eur. Patent 466,480 A1 (1991), assigned to Mitsui Toatsu Chem. Inc. 13) J. Takashi, T. Kiyoura, Y. Kogure and K. Kanaya, US Patent 5,155,235 (1992), assigned to Mitsui Toatsu Chem. Inc. 14) T. Kiyoura, J. Takashi, Y. Kogure and K. Kanaya, Eur. Patent 384,749 B1 (1990), assigned to Mitsui Toatsu Chem. Inc. 15) R.C. Edwards, US Patent 4,918,201 (1990), assigned to Amoco Co. 16) R.C. Edwards and W.S. Eryman, US Patent 4,845,241 (1989), assigned to Amoco Co. 17) M,S. Haddad and W.S. Eryman, US Patent 5,134,106 (1992) assigned to Amoco Co. 18) H. Taheri, US Patent 5,011,945 (1991), assigned to Amoco Co. 19) H. Taheri, US Patent 5,117,007 (1992), assigned to Amoco Co. 20) M. Hatano, M. Masayoshi, K. Shima and M. Ito, US Patent 5,128,299 (1992), assigned to Mitsubishi Kasei Co.
24 21) K. Katsumoto and D.M. Marquis, US Patent 4,132,670 (1979) assigned to Chevron Res. 22) C. FumagaUi and G. Stefani, US Patent 4,713,464 (1987), assigned to Alusuisse Italia 23) C. Fumagalli, G. Golinelli, G. Mazzoni, M. Messori, G. Stefani and F. Trifiri5, in Preprints II World Congress & IV European Workshop Meeting on New Developments in Selective Oxidation, V. Cort6s Corberan and S. Vic Bellon (Eds.), Benalmadena Spain 1993, p. C2. 24) G.D. Suciu, G. Stefani and C. Fumagalli, US Patent 4,511,670 (1985), assigned to The Lummus Crest and Alusuisse Italia 25) G.D. Suciu, G. Stefani and C. Fumagalli, US Patent 4,654,425 (1987), assigned to The Lummus Crest and Alusuisse Italia 26) J.R. Ebner, US Patent 5,185,455 (1993), assigned to Monsanto Co. 27) S.C. Arnold, G.D. Suciu, L. Verde and A. Neri, Hydroc. Process., 64(9) (1985) 123. 28) D.E. Dria and N.J. Bremer, US Patent 4,400,306 (1983), assigned to The Standard Oil Co. 29) N.J. Bremer, D.E. Dria, P.R. Blum, E.C. Milberger and M.L. Nicholas, US Patent 4,525,471 (1985), assigned to The Standard Oil Co. 30) R.M. Contractor, H.E. Bergna, U. Chowdhry and A.W. Sleight, in "Fluidization VI", J.R. Grace, L.W. Shemilt and M.A. Bergougnou (Eds.), Engineering Foundation, New York, 1989; p. 589. 31) M. Otake, M. Murayama and Y. Kurawagi, US Patent 4,520,127 (1985), assigned to Mitsubishi Chem. Co. 32) B.J. Barone and G.T. Click, US Patent 4,283,307 (1981), assigned to Denka Chem. Co. 33) J.R. Ebner and R.A. Keppel, US Patent 5,168,090 (1992), assigned to Monsanto Co. 34) B.J. Barone, Eur. Patent 458,541 A1 (1991), assigned to Scientific Design Co. 35) P.R. Blum, E.C. Milberger and M.L. Nicholas, US Patent 4,748,140 (1988), assigned to The Standard Oil Co. 36) G. Stefani and P. Fontana, US Patent 4,178,298 (1979), assigned to Lonza Ltd. 37) UK Patent 1,439,489 (1976) 38) P.R. Blum, E.C. Milberger and M.L. Nicholas, US Patent 4,518,523 (1985), assigned to The Standard Oil Co. 39) M.J. Desmond and M.A. Pepera, US Patent 4,801,569 (1989), assigned to Standard Oil Co. 40) R.O. Kerr, US Patent 3,474,041 (1969), assigned to Petro-Tex Chem. Co. 41) T.C. Click and B.J. Barone, US Patent 4,515,899 (1985), assigned to Denka Chem. Co. 42) R.C. Edwards, US Patent 4,810,803 (1989), assigned to Amoco Co. 43) M. Becker and J. Walden, Eur. Patent 174,173 B1 (1985), assigned to Scientific Design Co. 44) H.A. Mc Candless, J.L. Ceraly and H. Taheri, US Patent 4,950,769 (1990), assigned to Amoco Co. 45) J.R. Ebner and M.R. Thompson, in "Structure-Activity and Selectivity Relationships in Heterogenous Catalysis", R.K. GrasseUi and A.W. Sleight (Eds.), Elsevier Science Publ., Amsterdam, 1991, p. 31. 46) H.S. Horowitz, C.M. Blackstone, A.W. Sleight and G. Teufer, Appl. Catal., 38 (1988) 193 47) I. Matsuura, Catal. Today, 16 (1993) 123 48) T. Okuhara and M. Misono, Catal. Today, 16 (1993) 61 49) L.M. Comaglia, C.A. Sanchez and E.A. Lombardo, Appl. Catal. A:General, 95 (1993) 117 50) E. Bordes, Catal. Today, 16 (1993) 27 51) D. Ye, A. Satsuma, A. Hattori, T. Hattori and Y. Murakami, Catal. Today, 16 (1993) 113 52) M. O'Connor, F. Dason and B.K. Hodnett, Appl. Catal., 64 (1990) 16; 42 (1988) 91 53) S. Irvin-Monshaw and A. Klein, Chem. Eng., 96 (1989) 35
25 54) M. Meisel, G.U. Wolf and A. Bruckner, in Proceed. DGMK Conference on "Selective Oxidations in Petrochemistry", M. Baerns and J. Weitkamp (Eds.), Tagungsbericht, 1992, p. 27 55) L.M. Comaglia, C. Caspani and E.A. Lombardo, Appl. Catal., 74 (1991) 15 56) G. Bergeret, M. David, J.P. Broyer, J.C. Volta and G. Hecquet, Catal. Today, 1 (1987) 37 57) R.M. Contractor, J.R. Ebner and M.J. Mummey, in "New Developments in Selective Oxidations", G. Centi and F. Trifir6 (Eds.), Elsevier Science, Amsterdam, 1990, p. 553 58) G. Calestani, F. Cavani and F. Trifirb, unpublished results 59) E. Bordes, in "Structure-Activity and Selectivity Relationships in Heterogeneous Catalysis", R.K. Grasselli and A.W. Sleight, Elsevier Science, Amsterdam, 1991, p. 21 60) I. Matsuura and M. Yamazaki, in "New Developments in Selective oxidation", G. Centi and F. Trifirb (Eds.), Elsevier Science, Amsterdam, 1990, p. 563 61) F.B. Abdelouahab, R. Olier, N. Guilhaume, F. Lefebvre and J.C. Volta, J. Catal., 134 (1992) 151 62) M. Guilhoume, M. Roullet, G. Pajonk, B. Grzybowska and J.C. Volta, in "New Developments in Selective Oxidations by Heterogeneous Catalysis", P. Ruiz and B. Delmon (Eds.), Elsevier Science, Amsterdam, 1992, p. 255. 63) B. Kubias, G. Ladwig and B. Lucke, in ProceeAings DGMK Conference on "Selective Oxidations in Petrochemistry", M. Baems and J. Weitkamp (Eds.), Tagungsbericht, 1992, p. 287 64) V.A. Zazhigalov, G.A. Komashko, A.I. Pyatnitskaya, V.M. Belousov, J. Stoch and J. Haber, in "Preparation of Catalysts V", G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Eds.), Elsevier Science, Amsterdam, 1991, p. 497. 65) F. Cavani, G. Centi, F. Trifirb and R.K. GrasseUi, Catal. Today, 3 (1988) 185 66) E. Bordes, J.W. Johnson and P. Courtine, J. Solid State Chem., 55 (1984) 270 67) M.R. Thompson, A.C. Hess, J.C. White, J. Anchell, J.B. Nicholas, M.I. McCarthy, J.R. Ebner and F.W. Lytle, in Preprints II World Congress and IV European Meeting on "New Developments in Selective Oxidation", V. Cortes Corberan and S. Vic Bellon (Eds.), 1993, p. C1 68) P. Ruiz, Ph. Bastians, L. Caussin, R. Reuse, L. Daza, D. Acosta and B. Delmon, Catal. Today, 16 (1993) 99 69) G.A. Sola, B.T. Pierini and J.O. Petunchi, Catal. Today, 15 (1992) 537 70) B. Kubias, U. Rodemerck, G.U. Wolf, M. Meisel and W. Schaller, in ProceeM. DGMK Conf. on "Selective Oxidations in Petrochemistry", M. Baems and J. Weitkamp (Eds.), Tagungsbericht, 1992, p. 303 71) J.R. Ebner and M.R. Thompson, Catal. Today, 16 (1993) 51 72) F. Trifirb, Catal. Today, 16 (1993) 91 73) M. Hatano, M. Murayama, K. Shima and M. Ito, Eur. Patent 362,817 A1 (1989), assigned to Mitsubishi Kasei Co. 74) G.D. Suciu, G. Stefani and C. Fumagalli, US Patent 4,594,433 (1986), assigned to Lummus Crest Inc. and Alusuisse Italia 75) R.M. Contractor, US Patent 4,668,802 (1987), assigned to E.I. Du Pont de Ncmours and Co. 76) R.A. Xcppcl and V.M. Franchctti,US Patent 4,632,915 (1986), assigned to Monsanto Co. 77) B.J. Barone, US Patent 5,158,923 (1992), assigned to ScientificDesign Co. 78) G.J. Hutchings, Appl. Catal.,72 (1991) I 79) M.T. Sananes, J.O. Pctunchi and E.A. Lombardo, Catal.Today, 15 (1992) 527 80) S.K. Bcj and M.S. Rao, Appl. Catal.A:Gencral, 83 (1992) 149
This Page Intentionally Left Blank
PRLPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
27
Use of 31p NMR by Spin Echo Mapping to prepare precursors of Vanadium Phosphate catalysts for n-Butane oxidation to Maleic Anhydride M.T. Sanan6s 1, A. Tuel 1, G.J. Hutchings 2 and J.C Volta 1 1. Institut de Recherches sur la Catalyse, CNRS, 2 Avenue A. Einstein, 69626, Villeurbanne, C6dex, France. 2. Leverhulme Centre for Innovative Catalysis, Department of Chemistry, University of Liverpool, PO Box 147, Liverpool, L69 3BX, United Kingdom.
1. INTRODUCTION Vanadium Phosphate Oxide catalysts are well.known to perform the mild oxidation of n-butane to maleic anhydride. The preparation of the precursor of this catalyst, the vanadyl phosphate hemihydrate VOHPO4, 0.5 H20 appears to be very important to control the final properties of the VPO catalyst since the t r a n s f o r m a t i o n p r e c u r s o r / v a n a d y l p y r o p h o s p h a t e (VO)2P207 which corresponds to the final active phase is topotactic . It thus appears that it is possible to control the morphology of the final catalyst by the control of the morphology of its precursor (1-4). We developed 31p NMR by spin echo mapping to study VPO catalysts. With this technique, it is possible to discriminate between the phases of this system with v a n a d i u m in different valencies and corresponding to different environments (5-6). In this paper, we use the possibilities of 31p NMR by spin echo mapping to study the conditions of the formation of the hemihydrate VOHPO4, 0.5 H20 by reduction of VOPO4, 2 H 2 0 with isobutanol. This method of preparation provides a new route for obtaining a precursor with a different morphology with a high development of the (220) X-rays line as compared to the (001) one
(7).
28 The advantage of 31p NMR by spin echo m a p p i n g originates from the possibility to follow the v S + / v 4+ reduction from the dihydrate (V 5+) to the h e m i h y d r a t e (V 4+) and thus to follow the process of preparation of this last phase. 2. EXPERIMENTAL
VOPO4, 2 H 2 0 was prepared by refluxing V205 (12.0g) and H3PO4 85% (115.5 g) in water (24 ml H 2 0 / g solid) for 8 hours. The resulting VOPO4, 2 H 2 0 was recovered by filtration and washed with water. To prepare the h e m i h y d r a t e V O H P O 4 , 0.5 H 2 0 , the dihydrate VOPO4, 2 H 2 0 (4 g) was refluxed with isobutanol (80 ml) for 23 hours. Samples were recovered after 2, 4, 8, 16 and 23 hours and analyzed by X-Ray Diffraction and 31p NMR by spin echo mapping. X Ray diffraction patterns of the materials were recorded with a SIEMENS diffractometer using Cu K~ radiation. All 31p NMR experiments were performed in a BRUKER MSL 300 NMR spectrometer. Conventional spectra were obtained at 121.5 MHz using a 90~ (acquire) sequence. The 90 ~ pulse was 4.2 ms and the delay time between two consecutive scans was 10 s. Samples were typically spun at 4 kHz in zirconia rotors using a double bearing probehead. The 31p spin echo spectra were recorded under static conditions, using a 90~ 180~ - (acquire sequence). The 90 ~ pulse was 4.2 ms and t was 20 ~ts. For each sample, the irradiation frequency was varied in increments of 100 k H z above and below the 31p resonance of H3PO4. The number of spectra thus recorded was dictated by the frequency limits beyond which no spectral intensity was visible. The 31p NMR Spin Echo Mapping
information was then obtained by
superposition of all spectra. 3. RESULTS AND DISCUSSION Figure 1 shows the XRD pattern of VOPO4 2 H20, and Figure 2 shows the XRD spectra of the samples obtained after different time of refluxing of the dihydrate with isobutanol. The characteristic lines of VOPO4 2 H20, oberved at 11.8 and 24 ~ are progressively displaced which is indicative of a regular transformation of the dihydrate. The (001) line (11.8 ~ 20) appears at higher angle which corresponds to a decrease of the d spacing of the 001 interlayers associated to the basal planes of the dihydrate structure This observation indicates the progressive loss of water from the dihydrate structure. After 4
29 hours new lines appear at 15.65 and 30.4 ~ which correspond to the VOHPO4, 0.5 H 2 0 structure (001 and 220 lines, respectively). The transient existence of an intermediate VOPO4 hydrated phase cannot be excluded with the weak signals observed at 22.5 o 2@ (2 hours) and 21.5 ~ 2@ (4 hours). The VOHo.16PO4. 1.9H20 phase presents characteristic lines at 12.6 (020 line), 24.8 (002 line) and 25.1 (040 line)~
which can be also considered.
As previously observed (7), this
preparation gives a hemihydrate with a special morphology.
6000
!
d
l
.
|
.
10
4
9
20
'"
.
20 (o)
|
9
"
!
''
m,
.30
40
Figure I 9 XRD pattern of VOPO4, 2 H20.
1600
,.
.
.
.
.
.
.
b---.
10
20
2o(o)
30
d~-40
Figure 2 9XRD patterns of the material after refluxing isobutanol for 2 hours (a), 4 hours (b), 8 hours (c) and 16 hours (d).
30 Figure 3 shows the 31p NMR spectrum by spin echo mapping of VOPO4, 2 H20. It presents a typical signal at 0 ppm indicative of P atoms bonded to V 5+ atoms of this structure (5). Figure 4 shows the spectra of the samples depending on time of refluxing with isobutanol.
40"00
'
20"00
"
6 ppm
Figure 3" 31p NMR by spin echo mapping of VOPO4, 2 H20.
Fgure 4" 31p NMR by spin echo mapping of the material after refluxing isobutanol for 2 hours (a), 4 hours (b), 8 hours (c), 16 hours (d) and 23 hours (e).
b a ~
4ooo'
2ooo
'
6
ppm
31 After 2 hours, we mainly observe the characteristic signal of VOPO4, 2 H 2 0 at 0 ppm. However, a signal is observed at 100 p p m and a very weak shoulder at 1625 ppm. This last observation is indicative of the beginning of the appearance of VOHPO4, 0.5 H20, while the signal at 100 p p m should correspond to the structure of an intermediate hydrated VOPO4 phase as previously postulated. This is confirmed :by a 31p NMR examination in static conditions of the corresponding material (Figure 5). Two signals were observed which provide evidence for the existence of two e n v i r o n m e n t s
of p h o s p h o r u s atoms,
characteristic of two VOPO4 structures.
o~ e~
I
Figure 5 9 31p NMR in static conditions of the material after 2 hours isobutanol refluxing
'
'
260
'
6
'
-_2 a_a_ " ppm
After 4 hours, the signal at 100 p p m has been displaced to 600 ppm, showing that the intermediate hydrated phase has been modified, while the signal at 1625 p p m has increased. After 8 hours, the two signals at 0 and 1625 p p m are only observed, and signal at 0 p p m has almost disappeared at 16 and 23 hours. This shows that the transformation of VOPO4 2 H 2 0 into VOHPO4, 0.5 H 2 0 is almost total after 16 hours of refluxing with isobutanol. Another interesting remark concerns the small contribution in the 300-1400 p p m range
of the
spectra, which has been previously attributed to V 4+ atoms in a disorganized structure (6). It clearly appears that this contribution decreases progressively with time on refluxing, which is indicative of an increase of the crystallization of the hemihydrate.
32 4. CONCLUSIONS 9 31p NMR by spin echo mapping is a powerful technique that enables an improved understanding to be obtained for the preparation of the VOHPO4, 0.5 H 2 0 phase, the precursor of the VPO catalyst for n-butane oxidation to maleic anhydride. It also permits the detection of the transient appearance of intermediate hydrated VOPO4 phases which is difficult to be observed by X-Ray diffraction. The hemihydrate is more easily detected by 31p NMR by spin echo mapping as compared to XRD. It is presently applied to the study of the synthesis of other VPO precursors when changing the nature of the reducing alcohol reagent. Indeed, we have recently shown that it is possible to control both the nature of the obtained precursors and their morphology (8).
ACKNOWLEDGMENTS We thank the European Community (Human Mobility Capital P r o g r a m m e Contract N ~ CHRX-CT92 0065) for financial support.
REFERENCES
1."Vanadyl Pyrophosphate Catalysts" Catal. Today, Vol 16, n~ (1993), G. Centi (ed), Elsevier, Amsterdam, 1993. 2. G. Centi, F. Trifiro, J.R. Ebner, and V.M. Franchetti, Chem. Rev., 88 (1988) 55. 3. E. Bordes, E. Courtine, J. Catal., 57 (1979) 236. 4. N. Guilhaume, M. Roullet, G. Pajonk, B. Grzybowska, and J.C. Volta, Studies in Surface Science and Catalysis, Vol. 72, Elsevier, Amsterdam, (1992) 255. 5. Sananes, M.T., Tuel, A. and Volta, J.C., J. Catal., 145 (1994) 251. 6. Sananes, M.T., Tuel, A., Hutchings, G.J. and Volta, J.C., J. Catal., accepted for publication. 7. G.J. Hutchings, R. Olier, M.T. Sananes and Volta, J.C., Preprints II World Congress "New Developments in Selective Oxidation", Benalmadena, September 1993, P 41. 8. I.J. Ellison, G.J. Hutchings, M.T. Sanan6s and J.C. Volta, J. Chem. Soc., Chem. Comm., accepted for publication.
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
33
The Role of Aging on the Formation of Porous Silica T.P.M. Beelen, W.H. Dokter, H.F. van Garderen, R.A. van Santen and E. Pantos a Schuit Institute of Catalysis, Eindhoven University of Technology, P.O. Box 513, 5600 MB EINDHOVEN, The Netherlands "Daresbury Laboratory, Daresbury, Warrington WA4 4AD, U.K.
Abstract. Porous silica gel has been prepared by acidification of water glass. To study aggregation, gelation and aging use has been made of 29Si-NMR to investigate silica transformations on molecular scale. Q* ratios were used to define distribution of silica in particles and gels. On colloidal scale both IH relaxation of water and small angle scattering (SAXS, SANS) was very informative, especially because changes in fractal dimension could be used to describe silica transformations during aggregation and aging. Interpretation of fractal dimension in terms of aging mechanisms is performed by computer simulations of both aggregation/aging processes and calculation of the corresponding scattering spectra. Comparison of simulated spectra of aged silica, based on hypothetical aging mechanisms, with experimental spectra emphasized the important role of formation of rings on local scale. In freeze-dried silicas pore distributions were investigated with both neutron scattering (SANS) and physisorption (BET), revealing different pore structures, both in growth and form, after aging at 80~ and aging catalyzed by fluorine. 1. INTRODUCTION. Amorphous silica gels exhibit a large diversity in structural properties. To be used as a supporting agent in heterogeneous catalysis, high specific Surface and high stability is necessary. Moreover, for many applications in selective catalysis a tailormade porous structure is necessary or very desirable. When silica is prepared by acidification of water glass (alkali solution of silica), polycondensation reactions occur between dissolved oligomeric silica species, resulting in (sub)colloidal particles [1]. These primary particles combine to very ramified aggregates, a process described by diffusion or reaction limited cluster-cluster aggregation with power-law dependent density (fractals) [2,3]. After gelation the fractal structure is still preserved at sub-micrometer scale, while at l a r g e r scale Euclidean behaviour is observed. After drying, these systems often appear to be microporous because in general the fractal structures are too weak to resist capillary forces or even gravity and the fragile aggregates collapse during the drying process. Therefore, reinforcement of the weak and teneous structures aging processes is necessary [1,4]. During aging silica is redistributed in the gel. Although this redistribution is based on hydrolysis/recondensation reactions of silica monomers, oligomers or particles, depending on process parameters (temperature, concentration, pH, catalysts) many transformations and structures may be formed, resulting in a wide selection of porous
34 structures. The investigation of aging mechanisms is quite challenging. Although understanding aging reactions is necessary to prepare tailor-made porous silicas on a scientific basis, the choice of proper methods is difficult, especially because of the vulnerability of the fragile system only very few methods may be used, while the extended length scale (more than 4 decades: from sub-nanometer to a few microns) requirs the combination of several techniques. At molecular scale we have chosen NMR because recent developments in both 295iNMR and spin-spin relaxation on hydrogen atoms made applications for silicas in colloidal systems possible. For the colloidal scale both the availability of synchrotron radiation providing a high-brilliance source for x-rays and the development of highintensity neutron sources made scattering methods with x-rays (SAXS) and neutrons (SANS) very suitable and opened new possibilities for studying mass density distributions during aggregation and aging processes, using fractal concepts to quantify these transformations and mass distributions. Extremely helpful in interpretation of scattering results proved to be computer simulations, forming a bridge between experimental results and explanations based on simulations of transformations of silica during aging. With the combination of computer programs (GRASP and DALAI) both aggregation and aging and the corresponding scattering spectra can be simulated, allowing an immediate check of mechanistical hypotheses with experimental spectra. As will be shown in this paper, combination of scattering methods and simulation is a new and very promising tool to study transformations in colloidal systems and may be applied succesfully in investigations in the preparation of porous materials.
2. EXPERIMENTAL SECTION. Water glass solutions were prepared by dissolution of amorphous silica (Aerosil 200 and 380, gracefully obtained from Degussa AG) in sodium hydroxide (Merck p.a. using teflon or polyethylene beakers. In a typical experiment the overall molar composition was chosen to be SiO2 : NaOH : H20 = 3 : 2 : 125. Silica gels were prepared by acidification of the alkaline silica solutions. The water glass solution was dosed drop by drop with a Pasteur pipette to a solution of 1.0 M HC1 while stirring vigorously, until pH = 4.0 was reached and the stirring was stopped. Gelation time was definied by the period of time lapsed between the end of acidification and the moment when no meniscus deformation could be observed on twisting the beaker. To avoid evaporation, aging was performed in closed containers. When the catalytic influence of fluorine was investigated, before acidification appropriate quantities of NaF were added to the HC1 solution. To avoid collapse of the fragile structure during drying, freeze drying was applied. After precooling to -40~ for at least 3 hours, small samples of the frozen gel (typically 5 g wet gel) were connected to a Labconco Lab-top freeze dryer operating at 900 Pa and -75~ condensor temperature during 24-48 hours. To remove NaC1 the gels were washed with circa 250 ml doubly distilled water. Dried gels were characterized by physical adsorption/desorption of nitrogen (BET) after outgassing under vacuum for 16 hours at 180~ Sorption measurements were performed on a Carlo Erba Strumentazione Sorptomatic 1900 using liquid nitrogen as sorbent. Typical adsorption/desorption runs demanded 8 hours of analysis. Care was taken that the equilibrium pressure was reached before introduction or withdrawal of
35 a calibrated volume of nitrogen. 29Si-NMR experiments were performed on a Bruker CXP-300 FT-NMR instrument operating at 7.05 T at room temperature. Magic angle sample spinning (MAS) was applied to average any chemical shift anisotropy arising in gelated and freeze dried samples. Spin-spin relaxation times have been determined by the Carr-Purcell-Meiboom-Gill 90~176 sequenze at a frequenzy of 20 MHz using a Bruker Minispec pcl00 at a field strength of 0.47 T. The operational temperature was 40~ in order to prevent temperature changes due to external influences. All samples were stored at 40~ The magnetization curve was analyzed using a monoexponential fit. A multiexponential technique yielded no perceivable improvement of the quality of the fit. SAXS experiments were performed at the Synchrotron Radiation Source at Daresbury Laboratories (United Kingdom) using NCD beamline 8.2. Wet gels and solutions were measured in closed cells with mylar windows (spacing 0.2 - 0.5 mm), dried gels were fixed at cellotape. With wavelength fixed at 0.15 nm, variation of camera length (sample to detector distance) between 1.0 and 4.0 m and appropriate positioning of the beamstop, the Q-range between 0.05 and 2.5 nm ~ could be covered. To enhance sensitivity at low Q a quadrant detector was used. Satisfactory signal to noise ratios were obtained with acquisition times between 1 and 5 minutes. Subtraction of parasitic (slits) and background scattering (water solution, mylar windows, cellotape) was applied using the procedure introduced by Vonk [5], adapted to fractal systems. SANS experiments were performed at the Rutherford Appleton Laboratory, ISIS facility, Abingdom, U.K. Pulsed neutrons with wavelengths between 0.22 - 1.0 nm were used in the LOQ diffractometer (time-of-flight) and were recorded on a 64 cm diameter position sensitive detector at 4.3 m from the sample. Scattering vectors between 0.05 and 2.3 nm ~ were obtained, providing information on distance scales from roughly 2 to 100 nm in a single measurement. Wavelength dependent corrections for sample transmission and detector efficiency have been included in the data reduction procedure to obtain a composite cross section in absolute units. In the cases where contrast variation was used, dried silica samples were impregnated with a H 2 0 / D 2 0 mixture (63 vol% D20) to obtain matching conditions.
3. RESULTS AND DISCUSSION.
3.1 Polymerization. Silica is prepared by acidification of water glass, a concentrated solution of silica in water at high pH (pH = 12-14). In water glass, monomeric silica is present as a mixture of ions of silicic acid [1]: Si(OH) 4
~
Si(OH)30-
+ H §
,~
....
~
Si
44- +
4H §
Due to condensation/hydrolysis reactions also dimers and oligomers are present:
36 -Si
-
O-
+
HO
-
Si-
'~
=Si
-
0
-
Si-
+
OH-
with three horizontal bars at Si representing bonds with OH, O or -O-Si-- groups. Because the Si-O-Si angle can very easily be varied between 90 ~ and 150 ~ [6] also 3, 4, 5, or higher membered rings are formed, being precursors for three-dimensional structures as the prismatic hexamer or the cubic octamers. Investigation of the composition of water glass by 29Si NMR is a very appropriate method, because the electron distribution near the silicon nucleus in different surroundings may be easily discerned, especially the chemical shift for the different Q* types of Si atoms. The Q~ nomenclature [7] is based on the number of siloxane bridges Si-(O-Si -- )~ : n = 0, 1, 2, 3 or 4) with Q0, representing monosilicic acid, up to Q4 for fully condensed siloxanes. More than 20 different silica anions have been identified by 29Si-NMR in water glass [8,9]. This situation is even further complicated by the products of condensation/hydrolysis reactions between the numerous species and the dynamic equilibria between them. Because both the pI~ values of the many kinds of --SiOH groups [1] and the reaction rates between the oligomeric species show appreciable differences, small changes in pH, concentration, temperature or the addition of small amounts of cations result in different compositions of water glass. A typical example is the influence of quaternary ammonium ions: tetramethylammoniumhydroxide strongly favours the presence of cubic octamers [ 10,11]. Decreasing the pH of a water glass solution favours condensation over hydrolysis, resulting in bigger oligomers or polymers. Due to the flexible Si-O-Si angles and the resulting tendency for ring formation and cross-linking, three-dimensional polymers are formed. Moreover, because activation energy of hydrolysis or solvolysis reactions is much lower for single bonded groups (Q~) compared to fully condensed species (Q4), the dynamic condensation/hydrolysis equilibria favour ultimately the formation of three-dimensional networks of Q4-type silica atoms. This results in roughly spherical particles with - S i - O H and --Si-O only at the surface, with pH and to a lesser extent also concentration and temperature controlling the -=Si-O/~-Si-OH ratio and therefore reactivity. Also growth of these primary particles depends strongly on surface charge and the catalytic influence of hydroxyl anions (pH) on condensation/hydrolysis reactions, resulting in a maximum radius ranging from 1-2 nm at pH = 2 to 100/zm at pH = 8 [1]. Due to stabilisation by surface charge, in the pH range 710 even stable sols may be formed if the concentration of electrolyte is less than 0.1 M and at low silica concentrations [1].
3.2 Aggregation. At low pH or after screening by electrolytes of negative surface charge, silica particles may form interparticle bonds due to condensation reactions between -SiOH and -SiO- groups on different primary particles. Because the distribution of reactive groups on the surface of the particles is stochastic, directions of particle-particle bonds are rather arbitrary and therefore particle-particle interactions result in teneous aggregates with an amorphous structure. Interactions are not restricted to particleparticle or particle-aggregate bonds, but also interactions between aggregates or clusters of particles are possible, leading to highly ramified and extended aggregates. Polymerization or particle formation and aggregation, however, are no strict sequential processes: both are based on the same (condensation) reaction and
37 therefore are to be considered as competitive processes. Moreover, because OH- is both a reagent and a catalyst for the condensation/hydrolysis reaction, reaction rates at low pH are much lower than at high pH and are comparable to diffusion rates of small particles. Therefore, before elementary particles have been grown to maximum size, aggregates may be formed by either reaction limited or diffusion limited clustercluster aggregation, depending on the ratio between reaction and diffusion rates [2]. If the acidified water glass solution contains sufficient silica, the growing aggregates ultimately contact each other forming a percolating system: the gel. Especially at low pH (small particles) the gel can be visualized as a teneous network of interconnected aggregates with the silica density mainly concentrated in the centra of the aggregates. The branches of the aggregates are relatively thin threads composed of chains of silica particles [12]. Voids between the aggregates or within the branches of the aggregates are still filled with a solution containing silica as monomers, oligomers, elementary particles and small aggregates. After the gelation point this silica is added gradually to the thin threads, reinforcing the weak gelatinous system (to be discussed in 3.6). 3.3 Fractals. To characterize stochastic processes as aggregation and gelation and subsequent transformations during aging, fractal concepts are almost indispensible. Introduced to the scientific community rather recently (Mandelbrot's "The Fractal Geometry of Nature" was published in 1977 [13]), many phenomena in physics, chemistry and biology can be described using fractal principles, including aggregation [2,14]. To explain basic principles of fractal analysis we will use 2-dimensional models of aggregates depicted in figure 1. Figure la. Fractal aggregate, constructed by computer simulated diffusion limited aggregation. Fractal dimension ,~ . D = 1.44
x.X
...:.~: :.:.:.:
:.:. :~ ~:.:
:.:.p:"" ~.,....:
~p:" 9~,.~:
.•.:~-': oO%.SC- -1,..
:.:'--:.: :r
.:.-:..: 9 .f 9 :':.3": :':...:'." * "~::",..:': :':.2: ;9
9
o %,.~-o
,~
i~
9149 o
.-..-'~: :,:'.~.:
;5
~o
~%hoo
:,:'~,:
, o- 9 o-~
.j%~oo
..,:"...:
Figure lb. Deterministic Vicsek fractal constructed of 1, 5, 25 and 125 basic units respectively. Fractal dimension D = 1.465
38 In figure la an aggregate is shown with a mass density gradient: mass distribution in the center is distinctly different (higher average density) compared with the mass density in the periphery. As will be shown in the next paragraphs this mass distribution and its gradient is determined by the physics of the aggregation process and therefore related with the process parameters. Moreover, the aggregate is (in statistical sense) self-similar: the same gradient in density distribution is observed on different length scales, resulting in a characteristic quantity or variable for the density gradient: the fractal dimension. This concept is more easy to understand with the growth of an "artificial aggregate" in figure lb, known as Vicsek's 2d deterministic fractal [14,15]. As shown, this can been constructed by adding repeatedly the figure to its corner points, each iteration resulting in a threefold increase in size R. The "mass" M (= number of points), however, is increasing only fivefold at each step (instead of ninefold according to Euclidean geometry), resulting in the relation M --- R D with D = log5/log3 = 1.465 as can be shown easily [14]. Contrary to mass-size relations in (two-dimensional) Euclidian geometry with M -- R 2, in Vicsek's fractal one is dealing with a non-integer ("fractar') dimension (D = 1.465) for this relation. Concerning the mass distribution fractal systems show a typical behaviour: mass density is not a constant, but is depending on R or any other representative length scale. So, contrary to Euclidian systems as both non-fractal porous and non-porous materials, one can observe a non-zero mass density gradient, described by the fractal dimension D. In non-deterministic fractal systems like the aggregate in figure la, the mass density also decreases with increasing R, but now D (determined with statistical methods [2]) is 1.44 (the corresponding aggregate in 3-dimensional space has D = 1.81). Although shape and morphology are quite different, the fractal dimension D and therefore the decrease in mass density are almost the same in figure la and lb. With the concept of fractal dimension, differences or similarities in mass density distributions between aggregates may be quantified, such as changes due to growth and aging. Therefore the fractal dimension can be considered to be an important parameter to describe aggregation, gelation and aging phenomena in silicas, comparable to other system parameters as, for example, density or porosity.
3.4 Small Angle Scattering. To study growth of primary particles and subsequent aggregation and gelation of these particles in wet systems, most techniques can not be applied. Methods based on NMR or other spectroscopic techniques give information only at atomic or molecular scale and not at (sub)colloidal scale (1 - 100 nm). Moreover, methods requiring the removal of fluids prior to analysis can be discarded because the ramified and fragile structure may be changed or even destroyed during sample preparation. Finally, as shown in the preceding section on fractals, methods have to be found providing information on mass distributions and fractal dimension. Methods based on scattering of radiation satify these requirements. Although scattering of visible light can only be used for transparant systems, scattering of x-rays and neutrons can be applied both for transparent and opaque systems allowing the in situ study of silicas from acidification of water glass up to the dried systems. Because constructive interference between scattered radiation is only possible at interfaces between phases with different electron density (x-rays) or different nuclei (neutrons), it is possible with scattering of radiation to obtain information concerning both the size of primary particles or clusters of particles and the size and fractal mass density of aggregates. To measure aggregates or particles at colloidal scale, however, due to
39 the Bragg relation the interference of x-rays or neutrons can only be observed at very small angles (typically less than 1~ and therefore use has been made of SAXS (Small Angle X-ray Scattering) and SANS (Small Angle Neutron Scattering). High intensity sources, necessary for in situ dynamical experiments, are available at the Synchrotron Radiation Source 'Daresbury Laboratory, UK) and the pulsed neutron source at ISIS (Rutherford Appleton Laboratory, UK) respectively. quadrant or SAXS detector
incoming beam
beam
Figure 2. Schematic picture of set-up of SAXS measurements.
stop
In figure 2 the set-up of a SAXS measurement is sketched. The available 20 -range is determined by the choice of the sample-detector distance (camera length), the height of the beam stop of the primary beam and the height and sensitivity of the detector. To eliminate the dependence on wavelength, the intensity I of the scattered radiation is expressed as function of the scattering vector Q with magnitude IQI = Q = (21r/X)sin20. Because Q -- 1/d (Bragg's Law), the Q-range and therefore also the d-range are determined by the same parameters determining the 20-range, with a long camera length corresponding with measurements at relatively low Q and high d (big particles) values and the other way round for short camera lengths. With variation of camera length between 1 and 4 meters colloidal systems between 1 and 50 nm may be studied. With SANS the camera length is fixed, but the pulsed time-of-flight system provides measurements in roughly the same colloidal range. A very important feature of SAXS and SANS is the direct information concerning fractal properties. Because the number of elementary particles N(R) in a fractal aggregrate is given by N(R) -- (R/R0) D with R0 = radius of primary particle, it can be proved [18,19,20] that I(Q) -- Q-D resulting in a straight line with slope = -D in a log(I)-log(Q) plot. See figure 3. (,,)
(b)
:
log
(c)
- RQ
I
Figure 3. (a) Logaritmic scaling of an aggregate, part of an aggregate, primary particle and part of a primary particle. (b) Log(I)-Log(Q) plot on the same scale and corresponding with (a). R~ and Ro in (a) are approximations for the radius of gyration of the aggregate and the primary particle as measured in (b).
(d)
"-" Ro
-Di
(a)
(d)' ( e ) ~ ~' IlRG
i
llRo
log
" Q
40 Because in fractal aggregates the fractal region is restricted both by the size of the aggregate (upper size Rg) and the size of the primary particle (lower size R0) also the straight line in the log(I)-log(Q) plot has a limited length. The cross-over at the low-Q limit is representative of the size of the fractal aggregate Rg (more accurately: radius of gyration) and the cross-over at the high-Q limit is representative of the radius of the primary particle. The region Q > Q(R0) is the Porod region with slope = -4 in the case of monodisperse and non-fractal primary particles [20,21,22]. As can be concluded from this discussion, log(I)-log(Q) plots of SAXS or SANS spectra may produce inmediately the most important parameters describing growth and development of (fractal) aggregates. The width of the "Q-window", however, in many silica systems is too small to show the full fractal curve. This is often due to the large extension of the fractal range. For example, with the combination of SAXS, USAXS (Ultra Small X-Ray Scattering) and STXM (Scanning Transmission X-Ray Microscopy) we could show that the scale of the fractal range could be observed from approximately 0.5 nm up to 10/~m [23], more than 4 decades! 3.5 Simulation
Since the pioneering work by Jullien et al and Meakin in 1983, computer simulations of fractal growth have given an extremely important contribution to the development and understanding of fractal concepts in growing aggregates (for a review see [2] and [24]). Although the calculation of fractal dimensions or the position of the high- and low-q limits of the fractal region can not be performed using first principles, large scale computer simulations have proven to be very useful in studying the transformation of a "solution" (sol) of particles into a continuous threedimensional network (gel) and to find relations between physical parameters and fractal properties of aggregating, gelating and aging systems. For example, it is rather difficult to predict a priori the change in fractal dimension due to hydrolysis and recondensation of primary particles or small clusters in aggregates or due to growth of primary particles by ring formation [2,25,26]. The concept of diffusion-limited cluster-cluster aggregation (DLCA) is very useful and applied in many simulations. In this type of simulation process, particles are placed in a box and subjected to Brownian (random walk) movements. Aggregation (clustering) may occur when two or more particles/clusters come within the vicinity of each other and the combined cluster continues the random walk. The simulation is stopped at the gelation point (percolating system) or when all particles are combined in one final aggregate. The fractal dimension of the DLCA aggregates is approximately 1.8. In the case when the reactivity is not limited by diffusion, but by the rate of reaction between colliding particles or clusters of particles, the aggregation process becomes reaction limited (RLCA). Although the ramified aggregates appear to be rather similar to the DLCA aggregates, the fractal dimension is increased to 2.1. This can be explained by the observation that during growth the majority of particles, approaching the aggregate, collide with the outermost particles or branches. In this way the inner part of the aggregate is screened rather effectively. However, if only a small percentage of the collisions is successful and results in the formation of a bond, some particles may pass the screening outermost branches and react with branches in the core of the aggregate, resulting in a more compact structure with a smaller mass gradient and higher fractal dimension. Recent calculations [3] have shown that by using differences in reactivity in relation to local coordination a continuous array of D
41 values may be found, limited by D --, 3 (dense structures). Because during aggregation, gelation and aging of silica also a great variety of D values has been found using SAXS or SANS, computer simulation might be an important technique for the interpretation these data. For this reason we developed GRASP, an off-lattice box program for formation of aggregates using cluster-cluster aggregation, combining DLCA and RLCA. The aggregates obtained were introduced to DALAI, a program to calculate SAXS or SANS spectra from the coordinates of scattering particles. The spectra produced by the G R A S P / D ~ combination can be used not only to test the influence of physical parameters upon the SAXS spectrum and to compare the simulated with the experimental spectra, but also to measure the fractal properties dimension D and the radius of gyration Rg of a simulated aggregate easily and reliably [15]. 3.6 Aging. After acidification, aggregation and gelation silica gel is still far from thermodynamic equilibrium. By the dynamic condensation/hydrolysis equilibria a continuous process of dissolving and recondensation of monomers or oligomers of silica will change the network. Due to the difference in surface energy, silica at highly curved surfaces (convex surfaces) will dissolve relatively easy and recondensate preferentially in the "necks" between particles or in the crevices in the centre of the aggregates (concave surfaces). This effect (Ostwald ripening) decreases the number of small particles and smoothens the chains or surfaces of the gel network and is the main contribution to the aging process [1]. During aging the gel network is reinforced considerably and will be stronger in withstanding better the capillary forces during drying resulting in a porous structure of the dried silica. Without sufficient aging the weak gel structure shall collapse during drying and only a microporous silica would be produced [4,27,28]. The influence of aging on local (atomic) scale can be studied by NMR. In 295iNMR the Q3/Q4 ratio, indicative for the ratio between surface and bulk Si atoms, decreases considerably during aging [9] and is in agreement with the model of transfer of Si from convex surfaces to gaps or necks between particles. The decrease in surface area is also observed by the change in spin-spin relaxation ('1"2) of hydrogen 3.50
3.50
regation 3.10
3.10
E
2.70
2.70
~
2.30
2.30
1.90
1.90
F/Si > 0
0
FISi > 0 FISi - 0
Gelation point 1.50 -1.50
. -0.80
.
. -0.10
. 0.60
1.30
2.00
log (Time (hrs))
Figure 4a. Spin-spin relaxation time T2 during aggregation, gelation and aging. pH = 4, conc.(SiO2) = 0.73 M, F/Si - 0.0
1.50 -1.50
. -0.80
.
. -0.10
. 0.60
1.30
2.q
log (Time (hrs))
Figure 4b. As 4a except for F/Si ratios circles: F/Si=0.00; triangles: F/Si=0.01 diamonds: F/Si=0.03; squares: F/Si=0.10
42 atoms of water influenced by the silica surface [29,30,31]. This method is based on the decrease of "['2 when free water is influenced or weakly bonded to a silica surface. As shown in figure 4a, after the steep decrease of I"2 due to the formation of aggregates the curve increases during aging, indicating a (slow) decrease of surface area. If aging is accelerated by addition of fluorine (fluorine ions are a catalyst for hydrolysis/ condensation reactions [1]), the decrease of silica surface is also enhanced (figure 4b). See reference [29] for more details concerning use of relaxation methods to study aging or pore formation. 5 4-1.4- h r s . -
" ..........
~
:3" a
5.0
hrs.
~ -1.9
v
v 0
2 0.5 hrs.
0 -0.80 ,
I
-0.52
I
I
-0.24-
0.04-
=
,
l
0.52
0.60
log ( Q (nm-~)) Figure 5. SAXS spectra of silica with fluorine during aggregation and aging at various times. Gelation point at 0.8 hrs. Conc(SiO2) = 0.73 M, pH = 4.0, F/Si = 0.01. SAXS spectra also confirm Ostwald ripening during aging. In figure 5 three SAXS curves in a log(I)-log(Q) plot are shown at various aging times. The spectra after 0.5 and 5 hours show only the fractal region, the Q values related with aggregate size and size of the primary particles being outside the Q-window of the SAXS apparatus. Therefore, the radius of the primary particles has to be smaller than 1 nm. After 41.4 hrs aging, however, the cross-over between fractal region and Porod region is observed at approximately Q = 0.5 nm 4, corresponding with R0 -~ 5 nm. Ostwald ripening has increased the radius of the building blocks of the aggregates (the primary particles) by at least a factor of 5 [29]. These figures are confirmed by experiments in aggregates from acidified potassium water glass, resulting in Ro ~ 4.5 nm after 1 month aging and also catalysed by fluorine [9]. Aging at pH = 7-8 even shows R0 = 10 nm [9], but probably before aging primary particles have been much larger at this pH compared to pH = 4 [29]. Both aging experiments and SAXS spectra, however, strongly indicate that aging is much more complicated and can not be described using Ostwald ripening alone. For example, in wet gels a considerable shrinking and discharge of water during aging is also observed. To explain this phenomenon one has to assume changes in the structure at a relatively big scale compared to smoothing of branches by hydrolysis/recondensation equilibria. Moreover, in many experiments SAXS spectra
43 show a decrease of the fractal dimension during aging although both from intuition and from simulations [3,25,26] an increase had to be expected during restructuring. To relate hypothetical aging mechanisms with information obtained from SAXS spectra computer simulations proved to be extremely informative. To explain the growth of primary particles and the (slight) decrease of the fractal dimension during aging, an aging mechanism was postulated. This was based on the solvolysis of primary particles at the periphery of the.aggregates (dissolution of the outermost branches), migration by diffusion towards the center of the aggregates and recombination in the inner crevices. Simulations with the GRASP/DALAI combination, however, showed very clearly that solvolysed particles will probably will never arrive at the center of the aggregates: during the random walk they will stick on the ramified branches. Even worse, if no preference for hydrolysis is given to the outermost particles, the aggregates become more ramified during aging. Much more succesfull were recent simulations based upon ring formation [32]. In this model single bonded particles were allowed to perform small movements with respect to each other, resulting in reinforcement of the thin branches by the formation of rings. In figure 6 fractal dimensions are shown, extracted from the SAXS spectrum which was calculated from the simulated aggregates before and after aging according to local ring formation. The fractal dimension D = 1.45 before aging has decreased to 1.26 at low Q (large scale effects) and increased to 1.87 at high Q (local effect). Recently we confirmed these results with SAXS experiments on aging silica [33]. In figure 7 the experimental log(I)-log(Q) plots show the same pattern: the fractal dimension D = 2.2 (corresponding with D = 1.45 in 2 dimensions) after short aging, shows a decrease at low Q and an increase at high Q after prolonged aging. These results can be explained assuming different effects of local reorganizations at small and large length scales. See figure 8. At small scale (scale a) the density of silica has increased resulting in a lower density gradient and therefore an increase in le+08
"
-
i
-
-
-
w
-
-
-
/i
-
J
-
!
.
.
I
,
,
1e+07
le+06
%
N~\
o,-,~
t000o0 N
",
Op = i.87
10000 Ro
1 0 0 0
0.0(31
9
9
I
0.01
9
,
,
I
-
.
9
0.1
9
I
t
9
.
10
10{3
q
Figure 6. Simulated SAXS curves before and after aging by local ring formation.
44 1.00
2.05
-q.,= "-I
0.40 v
>., 09 t,.(11 t,-
-0.20
L..,
"~*-t Q4 =o~.o ~~"="'~o,..~.o,,..
/
-0.80
_..,.
r 0
2.35
2.2
o
-1.40
--,,,, . -2.00 -1.50
. . . . . . . . .
I
- 1.30
1
_
- 1.10
I
I
-0.90
-0.70
.*
-0.50
log Q (~-1)
Figure 7. SAXS curves of silica gels aged for various times" (a) 1 week, (b) 2.5 months, (c) 5 months. Conc.(SiO2) = 0.73 M (4 wt%), pH = 4.0.
non-aged
aged
C
Figure 8. Pictural view of aging by local increase in density. fractal dimension. At large scale (scale c), however, mass is even more concentrated in the "linear" branches without changing the overall morphology and therefore resulting in an increase of the mass gradient and corresponding decrease of D. Although we do not believe ring formation is the only aging mechanism, the agreement with fractal properties of aged systems indicates that ring formation problably makes an important contribution to aging. 3.7 Pore formation.
Although during aging reorganizations of silica reinforce the ramified and teneous network, it is difficult to show formation of pores during aging with SAXS or SANS. According to the Babinet principle [17], the dispersed component in a two-phase system is scattering and in wet gels therefore always scattering by silica is recorded.
45 On the contrary, in dried gels the pores are the dispersed component and therefore pores had to be investigated by SAXS and SANS only in freeze-dried gels. As shown [4,29], freeze-drying was necessary to avoid collapse of the ramified system by capillary forces during drying, especially in gels with short aging times. Because with SANS contrast variation using HzO/D20 mixtures could be applied, with this method we were also able to prove scattering was caused by pores and not by silica [22]. Log(I)-log(Q) plots of SANS spectra of aged and freeze-dried silicas showed rather low fractal dimensions (D = 1.4 - 1.5). See figure 9 for a typical spectrum. The low fractal dimension corresponds very well to an aging mechanisms according to figure 8, because the decreasing volume density of pores from the core to the outer parts of the aggregates may be expected if the main branches are preferentially reinforced, creating many pores near the centre [22]. In figure 10 the relation between D and the distribution of pores is explained in 2-dimensional examples.
5.00 >.,
1.,_50
09 c (1.) c-
o-~ o
-1.4
0.00
-1.50
-2.40
-1.80
-1.20
-0.60
Jog e Figure 9. SANS spectrum of dried gel after aging for 1 hour at 80~ D=
1
D=
1.5
in wet gel state. D=2
.' . ) r ~ %
.. \
/ 9.,,~ ~ . y . . . /
\ \' . "o' . "' o. ' ' _ ' :/ . ' 7 Figure 10. 2-dimensional models of pore distribution depending on D. D = 1: high pore density gradient and highest density at the center. D = 1.5: moderate pore density gradient. D = 2 (Euclidean geometry): no pore density gradient.
46 As already published [1,4,12,29] accelaration of gelation and aging by fluorine results both in different SAXS spectra of wet systems and in different surface area and pore structure of dried gels. In figure 11 we have compared systematically aging with fluorine and aging at 80 ~ Pore radii have been measured both with SANS (using
90 o<( oo D
=..-
60
-8 C) 9
5O
o rl
0
1100
2200
Aging time
5500
(rain.)
Figure 11. Pore radii determined by SANS (squares) and BET (diamonds) after aging at 80~ (open symbols) and with fluorine (closed symbols). 500 z',-
300
(o)
375
(b)
200
250 >
100
125 0
0.00
0.25
0.50
PlPo (')
0.75
1.00
0 0.00
0.25
0.50
O. 75
1.00
P/Po (')
Figure 12. Adsorption/desorption hysteresis curves for gel aged at 80~ aged with fluorine (right).
(left) and gel
the low-Q cross-over point in the log(I)-log(Q) plot) and with physisorption (BET) as a function of aging. The gels at 80 ~ showed reasonable agreement between the pore sizes obtained from scattering data and from BET. The deviations may be explained by assuming the presence of non-interconnected pores. Clearly the deviations are much bigger in the fluorine catalysed system. This bad conformity can probably be assigned to the presence of slit-like pores as can be deducted from the
47 adsorption/desorption hysteresis curve (figure 12) according to the classification of de Boer (type B) [35]. The radius determined with BET is the distance between the walls of the slit, but with SANS we are measuring the radius of gyration with contributions also of the depth of the slit. Therefore SANS-radius > BET-radius, in accordance with results presented in figure 11. 4. Conclusions.
To study aggregation, gelation and aging of silica, prepared by acidification of water glass, both NMR and scattering spectroscopy proved to be very efficient methods. With 29Si-NMR reactions at molecular scale could be studied using shifts in Qn ratios. Especially Q3/Q4 (ratio between surface- and bulk-silica) was very useful describing aging reactions. On colloidal scale 'H-relaxation time (T2) of water could be used to characterize silica conversions, with T2 decreasing during aggregation and increasing again by loss of silica area during aging. Studying aggregation and aging on colloidal scale SAXS was very informative, with the fractal dimension D representing silica transformations. To interpretate changes of D during aging, computer simulations were indispensable providing calculations of simulated spectra corresponding to various possible aging mechanisms. In dried and porous silicas care has to be taken in comparing SANS and BET results because radius of gyration (SANS) and pore radius (BET) might be different, depending on the shape of the pores. Acknowledgements.
Financial support (W.H.D. and H.F.v.G.) was given by the Dutch Department of Economic Affairs, as part of the "IOP-Katalyse" programme. Beam time both at Daresbury Laboratory and Rutherford Appleton Laboratory were provided by the SERC/NWO agreement on use of synchrotron radiation and pulsed neutron source respectively. We thank Drs. Wim Bras (NWO/SERC) for his assistance at SAXS and dr. Richard Heenan (SERC) for his assistance at SANS. Minispec experiments were performed at Bruker Spectrospin N.V., Wormer, The Netherlands and we appreciate the assistance of Piet Ruigrok.
REFERENCES.
[1] [2] [3] [4]
[5] [6] [7]
R.K. Iler, "The Chemistry of Silica", John Wiley & Sons Inc., New York, 1979. R. Jullien and R. Botet, "Aggregation and Fractal Aggregates", World Scientific, Singapore, 1987. M. Kalalla, R.Jullien and B. Cabane, J.Phys.II(France), 2 (1992) 7 P.W.J.G. Wijnen, T.P.M. Beelen, C.P.J. Rummens, H.C.P.L. Saeijs, J.W. de Haan, L.J.M. van de Ven and R.A. van Santen, J.Coll.Interf.Sci., 145 (1991) 17. C.G. Vonk, J.Appl.Cryst., 6 (1973) 81 B.W.H. van Beest, J. Verbeek and R.A. van Santen, Catal.Lett.,_l (1988) 147 G. Engelhardt, H. Jancke, M. M~igi, T. Pehk and E. Lippmaa, J.Organo-Met. Chem., 28 (1971) 293
48
[81 [91 [10] [111 [121 [13] [141 [15] [16] [171
[~8] [191 [20] [21] [22] [23] [24]
[251 [26] [271 [281 [291 [30] [31]
[32] [34]
[35]
C.T.G. Knight, J. Chem. Soc., Dalton Trans. (1988) 1457. C.T.G. Knight, R.J. Kirkpatrick and E. Oldfield, J. Chem. Soc., Chem. Comm. (1989) 919. P.W.J.G. Wijnen, T.P.M. Beelen, J.W. de Haan, L.J.M. van de Ven and R.A. van Santen, Coll. Surf., 4__55(1990) 255. I. Hasegawa, S. Sakka, Y. Sugahara, K. Kuroda and C. Kato, J. Chem. Soc., Chem. Comm. (1989) 208. T.P.M. Beelen, P.W.J.G. Wijnen, C.P.J. Rummens and R.A. van Santen, "Better Ceramics through Chemistry IV", MRS Symp. Proc. 180, (1990) 273. B.B. Mandelbrot, "The Fractal Geometry of Nature", Freeman, New York, 1977. T. Vicsek, "Fractal Growth Phenomena" (2e Ed.), World Scientific, Singapore, 1992 H.F. van Garderen, W.H. Dokter, T.P.M. Beelen, R.A. van Santen and E.Pantos, to be published in Modell.Simul.Mater.Sci.Eng., 2 (3) May 1994 A. Guinier and G. Fournet, "Small-Angle Scattering of X-rays", J.Wiley and Sons, New York (1955) O. Glatter and O. Kratky, "Small Angle X-ray Scattering", Academic Press, London (1982) T. Freltoft, J.K. Kjems and S.K. Sinha, Phys.Rev.B, 33 269 J. Teixeira in "On Growth and Form", H.E. Stanley and N. Ostrowski, Eds., M. Nijhoff, Dordrecht (1986), 145. J.E. Martin and A.J. Hurd, J. Appl. Cryst., 2__9_0(1987) 61. J.D.F. Ramsay, Chem. Soc. Rev., 15 (1986) 335. W.H. Dokter, T.P.M. Beelen, H.F. van Garderen and R.A. van Santen, accepted for publication Symp.Proc. COPS III, Marseille 1993 T.P.M. Beelen, W.H. Dokter, H.F. van Garderen, R.A. van Santen, M.T. Browne and G.R. Morrison, "Better Ceramics through Chemistry V", MRS Symp. Proc. 271 (1992) 263. P. Meakin, Physica Scripta 46 (1992) 295 P. Meakin, J.Chem.Phys., 83 (1985) 3645 P. Meakin, J.Coll.Interf.Sci., 112 (1986) 187 L.L. Hench and J.K. West, Chem. Rev. 90 (1990) 33. C.J. Brinker and G.W. Scherer, "Sol-gel Science", Academic Press, San Diego, 1990. W.H. Dokter, H.F. van Garderen, T.P.M. Beelen, J.W. de Haan, L.J.M. van de Ven and R.A. van Santen, Coll. & Surf.A, 72 (1993) 165 D.P. Gallegos, D.M. Smith and C.J. Brinker, J.Coll.Interf.Sci., 124 (1988) 186 and references cited therein. C.L. Glaves, C.J. Brinker, D.M. Smith and P.J. Davis, Chem.Mat., 1 (1989) 34. H.F. van Garderen, E. Pantos, W.H. Dokter, T.P.M. Beelen, M.A.J. Michels, P.A.J. Hilbers and R.A. van Santen, submitted J.Chem.Phys. W.H. Dokter, T.P.M. Beelen, H.F. van Garderen and R.A. van Santen, submitted to "Better Ceramics through Chemistry vr', MRS Symp.Proc. (1994) J.C.P. Broekhof and R.H. van Dongen in "Physical and chemical Aspects of Adsorbents and Catalysts", B.G. Linssen (Ed.), Academic Press, London (1970), 1-59
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
49
In situ techniques for the investigation of phase transformations in copper catalyst co-precipitates G.C. Chinchen and L. Davies ICI Katalco, Research, Technology & Engineering Department, P O Box 1, Billingham, Cleveland TS23 1LB, United Kingdom R.J. Oldman and S.J. Andrews ICI C&P Ltd., Analytical and Physical Sciences Group, R&T Department, The Heath, Runcom, Cheshire WA7 4QF
SUMMARY The detailed chemistry of the developing mineralogy in copper/zinc catalyst coprecipitates is complex and is affected both by the method of precipitation employed and by a large number of processing variables in both the initial precipitation and the subsequent ageing of the coprecipitates. To probe this chemistry completely requires the application of a wide range of physical characterisation techniques, preferably used in situ, in order to access accurately the developing mineralogy. 1. INTRODUCTION Co-precipitated copper/zinc catalysts with and without alumina have been well studied because of their industrial importance in methanol synthesis, water gas shift, and hydrogenation reactions. A fairly comprehensive literature has arisen over the past 15 years dealing with the structural characterisation of these catalysts, particularly of the precipitated copper/zinc basic carbonate precursor mineralogy and the effect of such mineralogy on the ultimate catalytic efficacy of the final reduced catalysts [ 1-11 ]. Relatively few investigators however have attempted to chart the detailed course of the chemistry involved throughout the catalyst preparation steps, particularly between the initial precipitation and the subsequent crystallisation of the relevant phases E12-16 ~. These previous investigations of the developing phase chemistry during precipitation and the subsequent ageing of the precipitates in the mother liquor have two major features. First, only ex situ methods of characterisation have been used; samples have been extracted, filtered, washed and dried, leaving to doubt how accurately the samples reflect the actual wet precipitate chemistry. Second, the detailed observations and chemistry show little general agreement; there is great variability in the progress of the mineralogy from apparently similar precipitation experiments.
50 Waller et al [ 12 ] found that the initial precipitate from mixed nitrate solution (Cu/Zn molar ratio 2:1) at 60~ and pH 7.0 consisted of zineian malachite and auriehaleite. Ageing this precipitate in its mother liquor caused the rapid disappearance of the auriehalcite with growth of a zinr malachite phase with higher zinc content. Joyner et al [ 14 ] under similar conditions, found the initial precipitate contained gerhardite (copper hydroxynitrate) and zineian malachite. After ageing, the gerhardite deeomposexl to eop~r oxide resulting in a mixture of copper oxide and zincian malachite. Waller L13 ] suggested that as a copper hydroxynitrate phase is only generally observed when precipitation occurs under acidic conditions, Joyner's study is consistent with a situation where the initial eoprecipitation occurred under acidic conditions and the slurry pH then rose, leading to the formation of malachite and ultimately tenorite (copper oxide). The problems were linked with poor initial mixing and very vigorous mixing during ageing leading to loss of CO2 and an increasing pH. The higher temperature of Joyner's study (80~ may also have contributed to the differences. Pollard et al L15 J modify Wallcr's conclusions by claiming that the initial precipitate is a zincian version of a recently discovered blue copper mineral "georgeite" which is XRD amorphous and eharaeterised only by its IR spectrum. During ageing zincian georgeite re-erystallises to a mixture of low zineian malachite and aurichaleite; further ageing progressing as per Wallet's original work, with the disappearance of the aurichalcite. Finally, Shen et al [ 16 ] studied the effect of precipitation conditions on the mineralogy of Cu/Zn precipitates over a range of Cu/Zn molar ratios. In well aged precipitates, they identified only zincian malachite in the copper-rich materials and hydrozincite also in the zinc-rich materials, when the precipitates were prepared by slow addition. The crystallinity of the malachite was lower with increasing zinc content. Aurichalcite was only found when precipitates were prepared by faster addition in materials with copper contents of 30%-70% molar. In a further attempt to chart the progress of the chemistry, 30/70 Cu/Zn precipitates were then sampled both during addition and ageing. The slow addition precipitates contained initially only malachite and sodium zinc carbonate, the latter converting during ageing to hydrozincite. The faster addition precipitates also contained sodium zinc carbonate but only XRD amorphous copper species shown to be hydroxycarbonates with strong IR absorptions at 1480, 1370 and 835 cm-~. With ageing, the amorphous material and sodium zinc carbonate appeared to react to form aurichalcite. It is interesting that there are noticeable similarities between the IR spectra of Shen's amorphous copper hydroxycarbonate and Pollard's georgeite. From these somewhat discordant investigations, it would seem that both the detailed chemistry and kinetics involved are very dependent on both precipitation and ageing conditions. At the temperatures used in most of the studies (60-80~ the time for malachite to crystallise from the initial amorphous precipitate is less than 9 minutes [ 17 ]. Most of the samples examined were consequently well "aged"; only Pollard and Shen sampling early in a precipitation of rapid addition have detected amorphous phases which transform or react to produce crystalline copper/zinc minerals.
51 We can now report, in a very preliminary way, an on-going attempt to chart in detail the chemistry involved in the developing crystalline mineralogy of some binary Cu/Zn and ternary Cu/Zn/AI systems using both a wide range of in situ techniques as well as conventional ex situ characterisation methods on extracted samples. The characterisation requirements placed on the in situ techniques e.g. analysis/description/fingerprinting of both crystalline and relatively amorphous materials in the presence of aqueous mother liquor in a timescale of only a few minutes, led to the use of a wide range of techniques such as UV-VIS, infra red ATR and Raman spectroscopy and synchrotron energy dispersive XRD and XAS. 2. EXPERIMENTAL
Material preparation All the precipitates described were made from copper-rich mixed Cu/Zn nitrate solutions and sodium carbonate solutions, precipitatexl by various standard techniques (e.g. reverse batch, continuous) and then further aged whilst stirrred in the mother liquor for periods of up to several hours. Some samples also contained A1203 and typically the range of temperatures used for precipitation and ageing was 35 ~ to 60~ with the pH generally essentially neutral. Where samples were withdrawn from the ageing precipitate for ex situ characterisation, they were immediately filtered and quench washed with cold water before being dried at 100~ Characterisation techniques DRIFFS spectra were obtained on a Nicolet Magna 550 spectrometer with Spectratech diffuse reflectance attachment. Samples were diluted 1:10 in KBr and intimately ground to a fine powder. The spectrometer was purged with nitrogen and the samples were scanned 400 times with a resolution of 4 cm-~. ATR spectroscopy was carded out in a Spectratech thermal horizontal attenuated total internal reflectance attachment using an aluminium edged zinc selenide crystal. Raman spectra were taken using a Dilor XY Raman spectrometer, with an argon ion laser oscillating at 488nm as the excitation source. All samples were examined using backscattering excitation geometry; dry powders were illuminated directly as lightly pressed pellets, while slurries were examined either in glass vials or as damp pastes held directly in the laser beam. The samples were photosensitive so only a few mW of power was focused onto them (ca 50~tm diameter spot). Data acquisition times varied but were usually of the order of a few minutes; all spectra were recorded at ca 4 cm-1 resolution. Energy dispersive XRD and XAS were carded out at the SERC Daresbury Synchrotron Radiation Source. XAS measurements have been made both real time in energy dispersive mode on precipitate slurries, which have also been subjected to energy dispersive XRD measurements, and also in normal transmission mode on extracted, dried, powder samples diluted with the appropriate level of polypropylene powder to give approximately 20% X-ray transmission after the absorption edge in question and then pressed to form 12mm diameter pin-hole free discs.
52
The XRD measurements were made on the white beam station 9.7 where the usable energy range was 15 to 60keV, equating to d-spacings between 3 and 12J,. The samples were heated (temperatures of 35-55~ magnetically stirred precipitate slurries in a large vessel from which an X-ray path length of 5mm was selected and bounded by polyester film of 231x thickness (Figure 1). With highly crystalline specimens the energy dispersive technique allows an entire diffraction pattern to be collected in seconds; in the current work a 2 minute dine resolution gave the best compromise between speed and data quality.
Glass beaker
~
Film windows
7
~,
.~"
Incident Polychromatic X-ray beam i
. ~,-=~
I
~
Movable Glass plunger J
Stirrer/hotplate
\
Figure 1. Schematic of the cell used for in situ XRD analysis.
53
3. RESULTS AND DISCUSSION Raman spectroscopy proved capable of indentifying most of the pertinent copper zinc hydroxycarbonate minerals (Figure 2) e.g. malachite, zincian malachite, aurichalcite, hvdrotalcite. The free carbonate anion should exhibit three Raman active modes [18 ]; near 1070 cm-I (symmetric stretch), 1490 cm-I (assymetric stretch) and 720 cm-~ (in-plane deformation). The lowering of symmetry with the ion in a crystal lattice causes formerly degenerate modes to split and modes due to metal-hydroxyl vibrations and lattice vibrations also appear giving complicated spectra. The carbonate ion vibrations vary greatly in intensity in the pertinent minerals, indicating that the ion interacts strongly with the lattice. Fortunately, each phase possesses a unique identifier; the strong band progression at low frequency for the malachite, the strong carbonate symmetric stretch at 1073 cm-~ for the aurichalcite and the broad mode at 507 cm-~ in the hydrotalcite spectrum. Thus, differences in the lattice vibrations (<600 cm-~) and changes in the carbonate vibrations allow "finger-printing" of the major mineral phases. The inclusion of zinc disrupts the malachite lattice significantly with broadening of the lattice modes; thus, variations in Cu/Zn ratio perturb the spectra of these phases.
3000
2500-
u)
9 ~
y
-w-
Z
~2000rn
r~
z w I-z_
i
1500-
I~
Io'oo
~o
W A V E N U M B E R SHIFT / C M -1
Figure 2. Raman spectra of synthetic Audchalcite and Malachite.
54 Raman spectroscopy was not only able to finger-print extracted dried powder samples, but also wet slurries; however in the latter case removal of excess liquor and examination of a wet paste gave improved signal to noise ratios. Consequently, it was possible to demonstrate for the first time that drying samples did not influence the Raman spectra of the major ultimate pertinent copper/zinc hydroxycarbonate mineral phases already described, implying that these minerals at least are not perturbed by the drying process. This important result validates any previous results obtained by characterisation, for instance, by XRD or IR spectroscopy on extracted, washed and dried samples, at least as far as the major ultimate crystalline minerals are concerned. ATR was investigated as a method of carrying out infrared spectroscopy on aqueous slurries without further processing. It was possible to obtain a series of spectra of a basic copper carbonate precipitated slurry through its ageing period (Figure 3, spectra A-G). The spectrum from the initial precipitate had similarities to that claimed for "georgeite" [ 15 ], whilst ultimately after ageing further peak~ developed in @e subsequent spectra corresponding to the major peaks of malachite L12, 13, 15 J. Unfortunately, irreversible damage occurred to the ATR crystal during the experiment, presumably due to contact with the slurry, so further work would be required in selecting crystal material and/or edging materials to develop this analytical method further.
70
A
, ,a,
t900
t01~
171111 t600
1500 t400 t300 t200 ImvlnmlMn~ { 9
c
1t1~
,0 ,E.F
i~
900
s
01~
Figure 3. ATR spectra of an ageing basic copper carbonate
The energy dispersive in situ XRD analysis proved to be very revealing when compared to ex situ characterisations by XRD and DRIFFS on extracted, dried samples. Whereas extracted, dried samples taken immediately after precipitation and early in the ageing period had shown much amorphous material and only sodium zinc carbonate (3ZnCO3Na2CO33H20) and possibl X traces of basic copper nitrates as crystalline phases (very similar to Shen's results L16J); the in situ
55
XRD analysis of such ageing precipitates (Figure 4) showed the presence of other crystalline phases such as chalconatronite (Na2CO3CuCO33H20) as well as sodium zinc carbonate. The absence of chalconatronite in ex situ samples is readily explained by its low thermal stability, since it readily d~omposes at normal drying temperatures and is unstable under aqueous conditions l 19 J. In general, soon after precipitation, the ageing precipitate contains chalconatronite and sodium zinc
Energy Time Figure 4. In situ XRD analysis of an ageing precipitate
carbonate as well as amorphous material and after an induction period determined by both the composition of the precipitate and the ageing conditions, the chalconatronite gradually disappears and phases such as poorly crystalline zincian malachite and possibly aurichalcitr appear and grow. The sodium zinc carbonate also fades, but more slowly than the chalconatronitc. Considerable quantitative variation around this general theme occurs depending on the precise method and conditions of precipitation, and whether alumina is present or not. In the extremes the initial precipitate may be entirely amorphous, chalconatronitr is not always present but sodium zinc carbonate is usually observed, and aurichalcite may or may not be formed. Preliminary XAS studies have observed noticeable changes to the copper and zinc co-ordination spheres as precipitate ageing progresses. Initially, precipitates have no crystalline order beyond the first co-ordination shells of copper (4-fold in oxygen) and zinc (3-4-fold in oxygen). After the induction period, long range order around copper and zinc is observed and structural rearrangement continues with further ageing; the copper co-ordination number falling slowly whilst the zinc co-ordination gradually rises. There is also evidence that initial precipitates of nominally similar composition but made with small changes to precipitation conditions can show striking differences in their first coordination shell geometry which probably defines the course of subsequent ageing and the resultant major phases. '
56 The concept that differences in initial precipitate structure resulting from even small changes in precipitation method and conditions, can have an overwhelming influence on the subsequent course of ageing and the consequent mineralogy has also been shown by some DRIFTS analysis of extracted, dried, samples taken from ageing precipitates made by differing precipitation methods to the same overall recipe (Figures 5 and 6).
20 l0
40 -~ : 0 RI~
1700
1600
1500
1400
t300 1200 1100 ~/avenu~ers (c~:)
I000
900
800
Figure 5. DRIFTS spectra of three types of precipitate.
Of the three precipitates studied, two initially have ave similar IR spectrum (A and B) which is very similar to Pollard's georgeite [ 15~. The other precipitate (C) has a totally different IR spectrum which has not been completely identifiexl, but suggests the initial precipitate is quite unlike A and B (Figure 5). Upon ageing under similar conditions, mineralogical developments occurred at different rates for these three precipitates (Figure 6); A was the slowest to transform into an apparently stable mixture of zincian malachite and aurichalcite, whereas B, initially apparently similar to A, took less time to transform into just zincian malachite. There was some evidence (from shifts in the 8OH peak at 879 cm -1) that further ageing of B after the initial transformation resulted-fit further zinc enrichment of the zincian malachite. C, from its different initial precipitate structure, most rapidly of all developed into a mixture of zincian malachite and aurichalcite which was then also like A, apparently stable under further ageing.
57
40
.
.
.
.
.
.
.
3O Z0 t0 40
i00
5O
.+
i700
t600
isoO
t400
1300
:I~0
l:lO0
'.-000
900
800
Wavenumers (c~-i) Figure 6. DRIFTS spectra of corresponding aged precipitate.
$. CONCLUSIONS The preliminary conclusions of this on-going investigation are that these systems are complex and both the precise nature and rates of the precipitate ageing chemistry are very dependent not only on the ageing conditions but also perhaps even more on the method and conditions of precipitation. Since precipitation is a difficult process to control, this is a possible reason for the somewhat discordant nature of the existing literature in this field and shows the importance of having standard, well-defined/characterised unit processes for both precipitation and precipitate ageing, both when studying the chemistry of these systems and for catalyst manufacture of consistent quality. To probe this chemistry in detail also clearly requires the application of a wide range of physical characterisation techniques, preferably applied in situ, particularly in studying the earlier stages of precipitate ageing, before the major ultimate copper/zinc hydroxycarbonate minerals have crystallised, if the whole chemical picture is to be obtained. ACKNOWLEDGEMENTS The authors gratefully and warmly acknowledge the skilled, expert contributions made by D A Creaser, ICI C&P Ltd, A and PS Group for the execution and detailed interpretation of the XAS work, and by N J Everall, ICI C&P Ltd, CAPS Group for the execution and detailed interpretation of the Raman spectroscopy work.
58 REFERENCES
1. R.G. Herman, K. Klicr, G.W. Simmons, B.P. Finn, J.B. Bulko and T.P. Kobylinski, J.Catal., 56 (1979) 407. 2. G. Petrini, F. Montino, A. Bossi and F. Garbassi in "Preparation of Catalysts III", G. Poncelet, P. Grange and P.A. Jacobs Ed., Elsevier Amsterdam, 1983, Vol 16, 735. 3. Y. Okamoto, K. Fukino, T. Imanaka and S. Teranishi, J.Phys.Chem., 87 (1983) 3740. 4. C. Busctto, G. Del Piero, G. Manara, F. Trifiro and A. Vaccari, J.Catal., 85 (1984) 260. 5. P.B. Himelfarb, G.W. Simmons, K. Klier and R.G. Herman, J.Catal., 93 (1985) 442. 6. R.H. Hoppcner, E.B.M Doesburg and J.J.F. Scholten, Appl.Catal., 25 (1986) 109. 7. E.B.M. Doesburg, R.H. Hoppener, B. de Koning, Xu Xiaoding and J.J.F. Scholtcn in "Preparation of Catalysts IV", B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet Eds., Elsevier Amsterdam, 1987, Vol 31,767. 8. P. Porta, S. De Rossi, G. Ferraris, M. Lo Jacono, G. Minelli, J.Catal., 109 (1988) 367. 9. P. Porta, G. Fierro, M. Lo Jacono and G. Moretti, Catal.Today, 2 (1988) 675. 10. G. Ghiotti and F. Boccuzzi, Cata.Rev.Sci.Eng., 29 (1987) 151. 11. G. Sengupta, D.P. Das, M.L. Kundu, S. Dutta, S.K. Roy, R.N. Sahay, K.K. Mishra and S.V. Ketchik, Appl.Catal., 55 (1989) 165. 12. D. Waller, D. Stifling, F.S. Stone and M.S. Spencer, Faraday.Disc.Chem.Soc., 87 (1989) 107. 12. D Waller, PhD Thesis, University of Bath (1992). 14. R.W. Joyner, F. King, M.A. Thomas and G. Roberts, Catal.Today, 10 (3) (1991) 417. 15. A.M. Pollard, M.S. Spencer, R.G. Thomas, P.A. Williams, J. Holt and J.R. Jennings, Appl.Catal.A:General, 85 (1992) 1. 16. G Cheng Shen, S. Fujita and N. Takezawa, J.Catal. 138 (1992) 754. 17. H.S. Parekh and A.C.T. Hsu, l.and E.C.Prod.Res.andDev. 7 (3) (1968) 222. 18. K. Nakamoto "Infrared and Raman Spectra of Inorganic and Coordination Compounds", Wiley Interscience, New York, 1986. 19. M.P. Appleby and K.W. Lane, J.Chem.Soc. 113 (1918) 609.
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
59
Influence of Preparation Method on the Properties of V-Sb-O Catalysts for the Ammoxidation of Propane Gabriele CENTI and Siglinda PERATHONER
Dept. of lnd. Chem. and Materials, V.le Risorgimento 4, 40136 Bologna (Italy), Fax:+39-51-644-3680, e-mail:
[email protected] The characteristics of V-Sb-O catalysts for the ammoxidation of propane prepared according to the different procedures reported in patents and open literature (solid state reaction, redox reaction in solution, sol-gel preparation and coprecipitation methods) as a function of the Sb:V ratio, heat trealment and changes during catalytic reaction are discussed. Results show the influence of the preparation method on the final characteristics of the catalyst due to the competition between the formation of the VSbO4 mtile-like phase and oxidation of vanadium to form vS+-micro domains. During the catalytic reaction the vS+-oxide reduces to form an intergrown ruffle-like V204 phase, but depending on the preparation method the V-oxide may not be accessible to the gaseous reactants. It is also shown that sbS+-oxide on the surface of vanaditun-antimonate is active in the synthesis of acrylonitrile and that vanadium catalyzes the reoxidation of antimony, as well as plays other roles in the mechanism of oxidative dehydrogenation of propane to propylene and in the side reaction of ammonia oxidation to N2. INTRODUCTION The one-stage synthesis of acrylonitrile from propane is a new chemical oxidation process [1] and commercialization of the process is forecast to be possible in a few years [2]. A survey of patent and literature data [1] shows that the best performances in this reaction are achieved with V-Sb-O based catalysts. The first patents on these catalysts, issued to Power Gas-I.C.I., report a selectivity in acrylonitrile of around 60%, with a very low productivity [3]. For around 15 years no more patents were published on this reaction and catalyst, then in 1988 Standard Oil (now BP America) published a series of five key patents on V-Sb-O based catalysts reporting much better yields and productivities in acrylonitrile from propane. The catalysts were prepared by redox reaction of Sb203 and V205 in an aqueous medium, followed by the addition of alumina or silica-alumina as the support and a series of doping elements (W, in particular) [4a,4b]. The use of V205 or of NH4VO3 as the source for vanadium and the nature of the heats treatment were found to have a considerable effect on the formation o f acrylonitrile. Good catalytic results were obtained only for Sb:V ratios higher than 1.0. An alternative patented method [4c] consists of reacting Sb203 with a monoperovanadium ion VO(O2) +, obtained by reaction of V205 with H202. The claimed advantage of the method is the possi-
60 bility to obtain catalysts with good hardness and greater resistance to attrition. Selectivities in acrylonitrile above 50% were reported for low propane conversions (10-15%) and high propane concentrations in the feed (around 60%) [4c]. In a second later patent [4d] the aqueous solution of the monoperoxovanadium ion was maintained under reflux for longer times (up to 16 hours) before adding the antimony oxide, in order to form first a sol and then a gel of vanadium. Several other patents have been published by this company i) on the modification of the catalyst by various doping elements, ii) on the use of a dual-bed catalyst in order to improve the conversion of intermediate propylene to acrylonitrile and iii) on various technological aspects of the process, but new specific aspects of the preparation method were not reported. Lynch et al. [4e] recently found, however, that the selectivity to acrylonitrile can be increased by putting the calcined catalyst into contact with a liquid hydroxy compound such as isobutanol. The increase in selectivity is considerable for VSbl.65Ox (from 10% to about 60%), whereas the effect is less remarkable for modified samples such as VSbl.sSn0.2Ox (from 52% to 58%). The presence of tin promots formation of the VSbO4 rutile phase, because SnO2 also has a mille structure and Sb2Ox-SnO2 is a known catalyst for propylene ammoxidation [5]. Thus, tentatively it appears that treatment with the alcohol eliminates the formation of unselecfive side phases [4e]. The role of the heat treatment is also emphatized indicating that an activation temperature of at least 780"C is necessary [4e]. The transformation during the heat treatment as a function of the temperature and atmosphere of reaction has been extensively studied by Berry et al. [6]. Their results were based only on samples with a V:Sb ratio of 1.0, prepared by solid state reaction between V205 and Sb203. Comparisons between the structural results reported by Berry et al. [6] and the patented results are thus difficult. Berry et al. [6], however, showed that the structural characteristics of the ruffle-like VSbO4 phase (presence of non-stoichiometry, composition, cell parameters) and the phase composition (mono- or bi-phasic system containing ~- or ~l-Sb204 together with the rude-like phase) depend on both the atmosphere of the heat treatment (O2-free nitrogen, N2 flow or air) and temperature. A VSbl-yO4-3/2y (0
61 relationship with their surface and bulk properties have been reported by Centi et al. [9] and Andersson et al. [10]. These studies showed that i) a Sb:V ratio greater than 1 is necessary for the selectivity [9a-9c,9f,10], ii) transformation of the catalyst takes place during the catalytic reaction [9b,9c,9g], iii) both vanadium and antimony are reduced during the catalytic reaction [9b,9c,10], iv) a mixed-valence ruffle vanadium-antimonate phase forms with a composition that depends on the preparation method and treatment during the catalytic reaction [9d], and v) amorphous vS+-oxide patches may be present on the surface of the catalyst and negatively influence the selectivity to acrylonitrile because the presence of V 5+ oxide promotes the side reaction of ammonia oxidation to N2. The kinetic aspects of the reaction of propane ammoxidation were also reported [9a,9b,9e] and it was shown that i) propylene is the reaction intermediate from propane to acrylonitrile, ii) propylene is oxidized to carbon oxides easier than acrylonitrile and iii) the composition of the feedstock has a considerable influence on the selectivity. The results reported in the literature show the considerable dependence in V-Sb-O samples of catalytic properties on the preparation procedure and in particular i) the role of the modality of reaction between vanadium and antimony sources, i/) the influence of the temperature and atmosphere of the heat treatment, and iii) the modification of the surface reactivity during the catalytic reaction. The data obtained in these studies, however, do not allow clear conclusions to be drawn regarding these aspects. The scope of the work reported here was to compare the characteristics of a series of samples prepared according to the methods reported in patent and open literature.
EXPERIMENTAL
Preparation of Catalysts Solid State Reaction : Samples were prepared by mechanical mixing and grinding of commercial V205 and Sb203 samples in such amounts as to obtain a Sb:V ratio in the 1-3 range. The samples were calcined at 400~ for 6 hours and then at 600~ for 3 hours. During calcination the samples were periodically taken out from the oven and ground and mixed in order to enhance the occurrence of the solid state reaction. Part of the samples were then further calcined at 750 and 850~ for 3 hours. Redox Reaction in Solution: A slurry method analogous to that reported in patents was used [4a,4b]. The preparation involves refluxing for at least 8 hours (usually 24 hours) an aqueous solution containing NH4VO3 and Sb203 (in such a ratio as to obtain an Sb/V atomic ratio in the 1-3 range) followed by solvent evaporation in a rotavapour and drying at 100~ overnight. The solid is then heat treated at 350~ for 4 hours followed by a further step at 500~ (6 hours) in the presence of air or under vacuum (10 -3 Torr). After this heat treatment the samples are ground and mixed and then calcined at 600~ (2 hour) or 850~ (2 hours). Sol-gel: The method is analogous to that reported in patents [4c-4e]. To an aqueous solution containing V205, a 30% aqueous solution of H202 (H202]V205 g/g ratio about 7) is added in three aliquots to form the monoperoxovanadium ion. Stirring is continued for 2 hours forming a sol or for about 16 hours to form a gel. Sb203, in a such an amount as to have the desired Sb/V ratio, is then added to the sol or gel of vanadium and after addition of some water to re-
62 duce the viscosity, the solution is maintained under reflux and stirring for 3 hours. Water is then evaporated on a hot plate and after drying overnight at 110~ the solid was crushed and calcined as in the former method.
Coprecipitation: sbS+-hydroxide is prepared by dropping SbC15 into an aqueous solution of 10% H2Q2 maintained at about 0~ The filtered solid is then dried at 110~ An aqueous solution of V4+ is prepared by reduction of V205 with a slight excess of oxalic acid at about 100*C and then the Sb5+-oxide is added in a such an amount as to have a Sb/V ratio in the 1-3 range. The slurry is maintained under stirring and reflux for 3 hours, then the water is removed by evaporation. The resulting solid is dried at 110*C, and then calcined as in the former methods.
Characterization of the Samples X-ray diffraction (XRD) analysis was carried out using the powder method and a Philips PW1840 diffractometer with CuKa radiation. Infrared (IR) analysis was carried out with a Perkin-Elmer FT-IR 1750 instrument using the KBr disc technique and calibrated amounts of the samples. Scanning electron micrographs were recorded with a JEOL 32 instrument. Differential scanning calorimetric tests were carried out with a Perkin Elmer 2C instrument in a flow of nitrogen and at a heating rate of 20*C/min. Surface area was measured by the BET method using N 2 adsorption at 77K. The chemical analysis method was based on potentiometric redox titrations with Fe 2+ and KMnO4 combined with extraction of V 5+ in an ammoniacal solution. Details of the method have been reported previously [9d]. Catalytic tests were carried out in a conventional plug-flow-type reactor with on-line gaschromatographic or mass quadmpole analysis, the latter system was used when the change in surface reactivity with time-on-stream was monitored. Further details on the apparatus for the catalytic tests have been previously reported [9]. Tests were made using 1 g of sample with particle dimensions in the 0.1-0.2 mm range, 3% propane and a O2/C3 and NH3/C3 ratio of 2.0 and 1.0, respectively, a reaction temperature of 500"C, and a total flow rate of 8 L/h.
RESULTS AND DISCUSSION Different procedures for the preparation of unsupported V-Sb-O catalysts have been reported in patent and open literature and can be summarized in four main groups: 1) Solid state reaction between V205 and Sb203 [3b,6,8] (called solid state reaction method). 2) Redox reaction in an aqueous ammoniacal medium between a V5+-compound (NH4VO3 or V205) and Sb203 (under reflux and mixing usually for more than 12 hours) [4a,4b,9b-9e] (briefly called redox reaction in solution method). 3) Reaction as in point 2), but in water and in the presence of H202 [4c-4e]. The hydrogen peroxide is added to the solution containing V 5+ forming a sol or a gel before adding Sb203 or alternatively is dropped into the slurry containing Sb203 and then V 5+ is added. Toft et al. [4c,4d] claim that the first method is preferable. This procedure is briefly called the sol-gel preparation method. 4) Solid state reaction between coprecipitated Sb5+- and V4+-hydroxide [9a,9f] (briefly called coprecipitation method).
63
1. Samples prepared by the solid-state reaction method Differential scanning calorimetry (DSC) analysis in a flow of N2 (heating rate 20~ of an equimolar mechanical mixture of V205 and Sb203 shows an exothermic peak centred at 370~ (around -2 ca!/g) and a second very broad peak centred at around 500~ The infrared spectra of the sample after these two stages are similar and both indicate a considerable decrease in the intensity of the bands characteristic of crystalline V205 (1020 and 820 cm-!) and the appearance of the bands characteristic of the VSbO4 rutile phase (675 and 545 cm "1) [9,11]. This suggests that probably both DSC peaks must be attributed to the formation of the rutile-like phase, but the first is related to the reaction at the grain boundaries between V205 and Sb203 and the second broad peak is related to the reaction controlled by ion diffusion. The formation of the VSbO4 rude-like phase thus occurs in the 400-500~ temperature range. Up to temperatures of about 600"C the reaction is not complete and the presence of small amounts of crystalline V205 together with VSbO4 is detected by X-ray diffraction (XRD) analysis in the sample with a V:Sb=I.0 ratio (Fig. la). When excess antimony is present (Sb:V=3.0), crystalline V205 is not detected by XRD analysis which shows the presence of only VSbO4 and ~-Sb204 (Fig. lb). The values observed for the rutile phase correspond to those attributed by Berry et al. [6a] to VSbl.yO4.2y (oxidized non-stoichiometric vanadium-antimonate). Infrared characterization of the samples, however, suggests a more complex phase composition than that indicated by the XRD results. Compared in Figure 2 are the spectra of the same samples shown in Fig. 1. In the sample with an equimolar Sb:V ratio (spectrum a) a series of additional bands to those of pure VSbO4 (675 and 545 cm "1 [9b-9d,11]) indicate the presence of V205 (bands at 1020 and 820 cm "1) and o~-Sb204 (shoulders at 740, 680 and 450 cm "l) [9b9d,12]. A clear shoulder also is pre9 9V S b O , sent at 890 cm -1 o a-Sb,04 which cannot be as"#-Sb,04 9V , O , signed to Sb203 or Sb204, but reasonably indicate the presence of Sb 5+J! a~ 9 9 oxide. Sb203 is oxidized directly to II Ili o, ,~ ~ Sb204 by calcinao O O o b' o tion at temperatures in the 500-700"C range in the absence of vanadium 20 3o 40 so a~ so [12] and therefore the presence of Fig. 1 X-ray diffraction patterns of samples prepared by the solid state Sb5+-oxide may be reaction method with a Sb:V ratio of 1.0 (a,a') and 3.0 (b,b'). Calcination attributed to the reat 600"C (a,b) or 850"C (a',b'). 9
6
64
,"
\,,~
I/ ,3 ,-,
;I
,'.
I1,1 I'--"\
!,,/
8bo" " c.-'1 Fig. 2 Infrared spectra of samples prepared by the solid state reaction method. Symbols as in Fig. 1.
1:oo
action with vanadium. The reoxidation of the reduced vanadium formed in the redox reaction between V 5+ and Sb 3+ is thus faster than the reaction with Sb 5+oxide to form the vanadium-antimonate rutile phase leading to incomplete formation of the latter phase in an equimolar V205-.Sb203 mixture. At temperatures above 600-700~ Sb3+-oxide reduces to a-Sb204 [12]. In the Sb:V=I.0 sample, together with the VSbO4 ruffle phase, unreacted V205, o~-Sb204 and Sb3+-oxide are thus present, the latter two phases being however XRD amorphous and detected only by infrared characterization. When excess antimony is present with respect to an Sb:V ratio of 1.0, the infrared characterization of the sample calcined at 650~ clearly shows a considerable decrease in the relative intensity of the bands due to unreacted V205 (spectra a and b in Fig. 2) and a considerable increase in the relative intensity of the bands due to t~-Sb'204 (745, 650, 605, 530, 450 cm -1) [12], in agreement with XRD data. Even though weaker than in the Sb:V=l.0 sample, the shoulder at 890 cm "1 indicates the partial presence of sbS+-oxide also in this sample. The relative amount of unreacted V205 thus decreases with increasing Sb:V ratio in the sample, but the amount of the Sb5+-oxide phase also decreases.
Calcination of these samples at 750~ does not modify the infrared spectra and XRD patterns, but further calcination at 850"C leads to a considerable change. XRD patterns of the samples show the nearly complete t~ --~ 13transformation of Sb204 in the sample with Sb:V=3.0 (Fig. lb') and the presence only of the rutile phase in the sample with Sb:V=I.0 (Fig. la'). In the latter case also a slight change in the relative intensifies of the various reflections is noted (in Fig. 1 compare a with a'). The presence of VS+-oxide in the samples can be, however, clearly detected by infrared characterization (spectra a' and b' in Fig. 2) which suggests also that the amount of this species does not change considerably with respect to that present in the samples calcined at 650~ However, the Sb3+-oxide phase nearly completely disappears (comp~e spectra a with a' and b with b' in Fig. 2). The change in the infrared spectra below 800 crn"l is consistent, on the contrary, to the t~ ~ 13Sb204 transformation observed by XRD. It should be noted that the shoulder at 820 cm "1 (Sv-o-v) clearly shows the presence of V5+-oxide microdomains with oxygens bridging two vanadium atoms, even though crystalline V205 cannot be detected by XRD. This evidence sheds doubt on the statement of Berry et al. [6] that for an equimolar Sb:V=I.0 sample prepared by solid state reaction above 800*C, V-doped ~-Sb204 crystals form due to transformation of o~-Sb204 catalyzed by vanadium. This attribution was mainly based on the absence of the detection of the V 5+-oxide side phase by XRD analysis, but IR data show that vanadium is probably present as microdomains in the vanadium-antimonate phase. If this is the case, the composition of the
65 non-stoichiometric rutile-like vanadium-antimonate phase given by Berry et al. [6] and BirchaU and Sleight [8] (see introduction) based on the same considerations also is doubtful.
2. Samples prepared by the redox reaction in solution method This method of preparation shows analogies with that by solid state reaction, but the redox reaction between V5+ and Sb3+ occurs in an ammoniacal solution rather than directly between the two crystalline oxides. Several key patents on propane ammoxidation use this preparation method for the synthesis of V-Sb-O catalysts [4a,4b]. The two-electron redox reaction in aqueous solution between V 5+ and Sb 3+ to form V 3+ and Sb 5+ is fast [13], but the rate of reaction is governed by the low solubility of Sb203 in aqueous solution. On the other hand, other useful commercial sources for Sb3+ are not available. Figure 3 reports the amount of V 5+ in solution determined by chemical analysis as a function of the time under stirring and mixing for an ammoniacal aqueous slurry of NH4VO3 and S.b203. The amount of V 5+ declines to zero in about 8 hours, but the redox reaction between V 3 + and V 3 + to form two V4 + ions competes with the reduction by Sb 3+. In fact, the chemical analysis of the mixed V-Sb hydroxide obtained after solvent evaporation indicates that all the vanadium is present in the valence four state and about half of the antimony is present as Sb 3+, the remaining being Sb 5+, for a Sb:V ratio of 1.0. Similar results were obtained when excess antimony is present, but the excess antimony is not oxidized. For a Sb:V ratio of 2.0, in the dried mixed hydroxide there is 100% V 4+, about 25% Sb 5+ and 75% Sb3+. Therefore, the composition of the starting precursor significantly differs in terms of valence state from samples prepared by solid state reaction. Calcination in air of the Sb:V=I.0 sample leads to a complex mixture. XRD analysis of the sample calcined at 500"C shows the presence of Sb203 (valentinite) and o~-Str204 together with the vanadium-antimonate rutile-like phase 100 [9d]. The in~ared spectrum shows V3+-oxide in 8O greater amounts than that observed for the sample prepared by solid state re> 60 action (Fig. la), but relatively lower amounts of "u 9~ 4 0 sbS+-oxide and higher amounts of ot-Sb204. The presence of the amorphous 20 vS+-oxide was confirmed by the chemical analysis of the sample. Indeed, when 0 0 5 10 15 20 the sample was treated with an aqueous ammoniaTime, hours cal solution, 32% of the Fig. 3 Change of the amount of V5+ in solution during the redox vanadium was extracted as reaction with Sb203 in the preparation by the redox reaction in the soluble V5+-salt. solution method. Sb:V ratio of 2.0. Chemical analysis of the insoluble residue showed !._.
66
|
a
I
100/~m
L
b
|
10/~m
Fig. 4 Scanning electron micrographs of the sample prepared b3~the redox reaction in solution method and calcined in air before (a) and after (b) extraction of V~+ with an aqueous anunoniacal solution (3N) at room temperature. that 8% of the vanadium is still present as V 5+, and the remaining vanadium is present as V 3+ and V4+ in a ratio of about 2:1. The scanning electrom micrographs of the sample before and after the extraction with ammoniacal solution (Fig.s 4a and 4b, respectively) show the presence of macrocrystals of t~-Sb204 and microcrystals of V-antimonate, the latter characterized by a spongy microstructure. EDS microprobe analysis indicates a relatively uniform V:Sb ratio centred around 1.0 without apparent segregation of V-oxide phases. However, the SEM micrographs of the sample after extraction of V 5+ (Fig. 4b) show the presence of new microholes, reasonably due to the extracted vanadium. This confirms the hypothesis that the V 5+ is present as microdomains embedded in the vanadium-antimonate or antimony-oxide matrix. In the Sb:V=I.0 sample prepared by redox reaction in solution with respect to that prepared by solid state reaction, i) some unreacted Sb903 is present, i/) the amounts of vS+-oxide and t~-Sb204 are higher, and iii) the amount of Sb'5+-oxide is lower. The Sb3+-oxide transforms to ct-Sb204 by calcination above 650~ The difference observed in the calcined samples, however, suggests that probably the V4+-hydroxide deposits over the Sb 3+-Sb 5+-oxide aggregates explaining the easier oxidation of vanadium, but also the formation of smaller crystallites of the vanadium-antimonate phase. In fact, the diffraction lines of the rutile phase in the sample prepared by redox reaction in solution are significantly wider than those of the sample prepared by solid state reaction. The 20 width at half height of the more intense reflection of the rutile phase at 27.35* (110 reflection) is 0.50 and 0.21, respectively, due to the presence of smaller crystallites of vanadium-antimonate (about 165 A for the sample prepared by redox reaction in solution and about 450 A for the sample prepared by solid state reaction).The surface areas are 9.6 m2/g and below 2 m2/g, respectively. When the heat treatment is carded out in the absence of oxygen (dynamic vacuum), the VS+-oxide side phase is absent [9d] as well as the SbS+-oxide phase. XRD patterns show the
67 presence of o~-Sb204 together with the vanadium-antimonate phase. A small change in the position of the XRD reflections for the rude-like phase as well as a slight change in the relative intensities of the reflections for the various crystallographic planes also are noted [9d]. This change is particularly evident for the 101 reflection of the ruffle phase at 20 about 35.6*. Accordingly, the cell parameters of the ruffle phase are slightly different from those observed for the sample calcined in air: for the tetragonal cell a=b=4.61/~ and c=3.01/~ for the oxidized sample and a=b--4.60 ,~ and c=3.07/~ for the sample treated under vacuum. The corresponding cell volume increases from 64.0/~3 to 64.9/1, 3. This difference is similar to that observed by Berry et al. [6]. In the Sb:V=I.0 sample treated under vacuum, XRD and IR analyses show the presence of only the rutile-like phase and a-Sb204. The missing vanadium (corresponding to the equivalent part of t~-Sb204) may be present in the form of a non-stochiometric ruffle phase, according to that suggested by Birchall and Sleight [8] (V1.05Sb0.9504). However, it is more reasonable to consider that, by analogy with the calcined sample, microdomains of reduced vanadium-oxide are present. The absence of 02 during the heat treatment leads to the intergrowth of the ruffle V204 with the ruffle vanadium-antimonate. The presence of the intergrown V204-VSbO4 ruffle phase explains the expansion of the cell volume (see above). In agreement with this hypothesis, it should be pointed out that calcination of this sample at 600"C for 6 hours leads to the appearence of a band at 1005 cm "1 related to the presence of a vS+-oxide phase, even though V ~+ cannot be extracted with the aqueous ammoniacal treatment. The heat treatment in the absence of oxygen avoids the oxidation of vanadium, but even so the formation of the vanadium-antimonate phase is not complete in the Sb:V=I.0 sample prepared by redox reaction in solution and some a-Sb204 is present. After the catalytic tests in propane ammoxidation, the IR spectra of V:Sb=I.0 samples calcined in air or under vacuum are similar and characterized by two main bands at 675 and 545 cm "1 [vSbO in V-antimonate] and a weaker band centred at 995 crn1 suggesting the presence of a small amount of V 5+ possibly spread on the surface according to the analogy of the position of the band with that observed for V5+ on TiO2 [ 14]. XRD data in both cases indicate the presence of VSbO4 and t~-Sb204. The cell dimensions and parameters of the ruffle phase correspond to those found for the sample activated in vacuum (see above). Chemical analysis of the sample treated under vacuum after propane ammoxidation shows the presence of 8% V 5+ and similar results were found for the calcined sample. Clearly, the two samples (V:Sb=I, calcined in air or under vacuum) undergo changes during the catalytic reaction. A considerable reduction of vanadium occurs in the sample calcined in air, whereas in the sample treated under vacuum, partial reoxidation of the vanadium occurs. In both cases, a change in the surface reactivity during the first 4-5 hours of time-on-stream during the catalytic ammoxidation of propane is observed (Fig. 5A for the sample treated under vacuum and B for the calcined sample), but with a different time dependence. In the sample treated under vacuum, the propane conversion increases with a slight decrease in the formation of the various products, whereas in the calcined sample the conversion of propane decreases and that of acrylonitrile and HCN increases. The change in surface reactivity of the calcined sample after removal of surface V 5+ by extraction with the aqueous ammoniacal solution is also reported in Figure 5 (graph C). Similarly to the sample activated in vacuum, the conver-
68 30 __.
"-.
*--
--"
sion of propane increases, but the selectivity to propylene decreases with a corresponding increase in that to acrylonitrile. These results indicate that the reduction of surface V 5+ leads to a decrease in the rate of propane depletion, but to an increase in the selectivity to acrylonitrile. A corresponding decrease in the rate of oxidation of ammonia to N2 was observed and thus reasonably the increase in the selectivity to acrylonitrile is correlated with the decrease in the rate of the ,_. side reaction of ammonia depletion. How4 ever, the results reported in Fig. 5 also indicate that partial oxidation of the surface vanadium is necessary probably both for the selective activation of the alkane via an oxidative dehydrogenation mechanism similar to that suggested, for example, for V-MgO samples [15], and for the synthesis of acrylonitrile from the intermediate propylene. Reasonably, in the latter case the role of vanadium is to reoxidize the reduced antimony ions as will be discussed later. Accordingly, the difference in the time 4 dependence of acrylonitrile formation for the sample treated under vacuum ~Fig. 5A) and for the calcined sample after V ~+ extraction (Fig. 5C) is probably due to the presence of some surface SbS+-oxide in the sample prepared under vacuum that is absent in the calcined sample after treatment with the aqueous ammoniacal solution.
4.
W
v
j
20
, 9
s,II"""""..ll-
rj e
....
9....
~10,"
9 ,
0
9....
.41
s S~
:
"
A
i' 0
1
2
3
~
~e''~
30 A
~10
~ =_
i
.~
.
.
.
.
I
0
.
.
.
.
i
1
.
.
.
.
J
2
.
.
A
3
30
L--
3. Samples prepared by the sol-gel preparation method
o
A 10 (.1 =; I= o (.1 .
0
.
.
.
i
1
.
.
,
.
!
.
.
2
,
.
i
3
9
.
.
,
4
Time, hours
Fig. 5 Change of the catalytic behavior with the time-on-stream of Sb:V=I samples prepared by redox reaction in solution and heat treated at 500"C in vacuum (A) or air (B). Satnple ((2) is as B ,b tu after xe traction of soluble V~+.
This preparation method is analogous to that reported in various patents [4b-4d] for the synthesis of catalysts with a high hardness and good selectivities to acrylonitrile using a high propane concentration. V205 reacts with an aqueous solution of H202 to form the red-orange peroxovanadium ions, that gradually form a sol (in around 1-2 hours) and then a gel (in
69 around 16-20 hours) with a considerable increase in the viscosity of the solution. Sb20 3 can be added at the sol or gel stage. Similar results in terms of phase composition were found for both the sol and gel procedures, but while the surface area of the calcined sample prepared by addition of Sb203 at the sol stage was higher (18.2 mZ/g), the mean crystal size of the vanadiumantimonate active phase was larger (around 340 A in comparison to 175 A for the sample prepared by addition of Sb203 at the gel stage). The procedure involving the addition of Sb203 at the gel stage will be discussed in more detail. After addition of Sb203 to the solution containing the gel of V 5+ and heating at the reflux temperature, the solution becomes dark-green in around 1-2 hours clearly indicating the reduction of V 5+ by reaction with Sb 3+. Notwithstanding the initial addition of H202, the results are similar to those obtained by direct reaction of V 5+ and Sb3+. H202 allows mainly the solubilization of V205, whereas the effect on the change of the valence state of antimony and vanadium is secondary. The infrared spectra of calcined samples prepared using this method l '~ ~ ii I1 (Fig. 6) are similar to those prepared by solid state re~." i action, for example, when the Sb:V ratio is both 1.0 or '~ !tl" 3.0 (see spectra a and b in Fig. 6 and in Fig. 2).
w b'
i/r/ !; i ~
; e
I~0
"
cr~ T
400
Fig. 6 Infrared spectra of samples
prepared by the sol-gel method (Sb203 addition at the gel stage of vanadium). Sb:V ratio of 1.0 (a,a', al) and 3.0 (b,b',bl). Calcination at 600"C (a,b) or 850"C (a',b'). Samples after the catalytic tests in propane ammoxidation: (al,bl)
Some differences, however, exist. XRD diffraction patterns (Fig. 7) show that the diffraction peaks for the various crystal planes of the rutile-like phase are significantly broader than those observed for the preparation by solid state reaction due to the smaller crystallites (175/~ and 480 A, respectively). Furthermore, when the Sb:V ratio is increased from 1.0 to 3.0 the diffraction peaks for the rutile phase further broaden (dimensions of crystallites around 140/~), whereas no similar effect is observed for the preparation by solid state reaction. Calcination at 850"C, however, leads to sintering of the rutile phase and a corresponding narrowing of the diffraction peaks of the ruffle phase (compare diffractograms b and b' in Fig. 7). It is worth noting that after calcination at 850~ and even more so at lower calcination temperatures, the mean crystallite size of the vanadium-antimonate phase (around 350/~) is still significantly smaller than that of the Sb204 phases (both tz and 13) (mean crystallite size 600-800/~ which does not considerably change in the ~ ---> 13 transformation). A further difference observed in the comparison with the corresponding samples prepared by solid state reaction is that in the samples calcined at 850*C the relative ratio of the 13-Sb204~SbO4 crystalline phases is lower for samples prepared by the solgel preparation method, suggesting an higher amount of the vanadium-antimonate phase, at least in the crystaUine form.
70
t
A
~
.
VSbO,
o =-Sb, O,
fl ; 7|
,
Other aspects are relatively similar in the samples prepared by the solgel preparation and
,~-Sb, O,
_
,
.~
.
A .
*
solid state reaction
-
I !1
~
oo
A
,
methods. Also in the former samples with a Sb:V ratio of 1.0, the formation of V SbO4 is not complete, even
A
'2 9 9. . . . . . . . .
9 , ......... 30
20
, ......... 40
, ......... 50
j 2~
6O
~-Sb204 is also observed by XRD in the sample calcined at 650~ together
X-ray diffraclion patterns of samples prepared by the sol-gel with small reflections due to V205. The presence of V205 is clearly indicated by infrared spectroscopy (Fig. 6). Furthermore, also in this sample the presence of Sb3+-oxide is indicated by a shoulder at 890 cm "1. Calcination at 850"C leads to the disappearence of the SbS+-oxide and to the (x -~ 13 Sb204 transition. No significant change in the transition temperature was noted. Fig. 7
preparation method. Symbols as in Fig. 6.
After the catalytic reaction of propane ammoxidation, some changes are observed and in particular, a small reduction in the relative amount of the vS+-phase and a shift in the maximum from 1020 to 995 cm "1 (compare spectra a with al and b with bl in Fig. 6; samples with a Sb:V ratio of 1.0 and 3.0, respectively), due reasonably to a spreading of vanadium on the surface of the vanadium-antimonate phase during the catalytic reaction. Similar results have been observed by XPS spectroscopy [9b,9c]. However, in comparison with the analogous samples prepared by redox reaction in solution or by coprecipitation, the reduction of V 3+oxide (IR band at around 1000 crn"1) and of Sb 5+ -oxide (shoulder at 890 c m 1) after the catalytic tests is not complete (Fig. 6). The XRD patterns (Fig. 7) do not suggest significant changes in the composition of the crystalline phases, but do indicate a decrease in the (xSb204/VSbO4 ratio (compare diffract, grams b and bl in Fig. 7). This suggests that partial crystallization of the vanadium-antimonate phase occurs during the catalytic tests, but the reaction is not complete differently from other preparations, possibly due to the presence of an ovedayer of the other phases which limits the accessibility of the gaseous reactants.
4. Samples prepared by the coprecipitation method Differently from the other preparation methods, in this method Sb 5+ does not derive from the redox reaction of Sb3+ with V5+, but rather is added already in the form of sbS+-hydroxide. V 5+ is thus reduced to V4+ by adding a separate reducing agent (oxalic acid). The advantage of this method is that it limits to limit the presence of Sb ~+ deriving from the incomplete reac-
71 tion with V 5+, caused by the presence of the side reaction of disproportion between V 5+ and V 3+ to form 2V 4+ . The formation of crystalline VSbO4 using this method of preparation occurs at low temperature (300"C) (Fig. 8, diffractogram a300). Calcination at 600"C of the Sb:V=I.0 sample (diffractogram a in Fig. 8) leads only to the appearence of crystalline V205~ the presence of which, is also detected by infrared spectroscopy (Fig. 9a, band at 1020 cm" ). In this sample, the Sba+-oxide (shoulder at 890 crn'l) and small amounts of o~-Sb204 (weak shoulders at 740 and 600 cm "1) are also detected by infrared spectroscopy, whereas XRD analysis shows the presence of only VSbO4 and V205 (Fig. 8a). V205 and sbS+-oxide disappear nearly completely after the catalytic tests in propane ammoxidation (Fig. 9, spectrum al) and the diffractograms show only the presence of the rutile phase (Fig. 8 al). Small amounts of o~-Sb204 (weak shoulders at 740 and 600 cm "1) are, however, detected by infrared spectroscopy. These results indicate that using this method of preparation, similarly to the other methods, the formation of the vanadium-antimonate phase for a Sb:V=I.0 sample is not complete by calcination in air or after the consecutive transformation during the catalytic reaction. It is interesting to note, however, that the mean crystal size of the rutile phase with this method, estimated from the XRD line broadening, is about 360 ,/k and thus slightly smaller than that with the sol-gel preparation method. The crystal size does not change significantly after the catalytic tests. When excess antimony is present, XRD patterns (Fig. 8, samples b) show the presence of Sb6013 (sbS+-oxide) up to calcination temperatures of 650~ (diffractograms b300, b550 and b in Fig. 8, corresponding to calcination temperatures of 300 ~ 550* and 650~ respectively). The Sb6013 progressively transforms to 9 j " VSb04 9V,O. o~-Sb204 and accord| l] ,*-Sb,04 z SbzOz(valent/n/te) ingly the infrared ; II 11: .: .Sb.O,. .. spectrum (Fig. 9) A II III, A shows a progressive bl decrease in the intensity of the shoulder at 890 cm 1. XRD data (Fig. 8b) show the absence of crystalline Sb5+-oxide in 9, 9 the sample calcined at 650"C, but the IR spectra indicate that Sb5+-oxidr (shoulder ao 3o 4a so a~ eo at 890 crnl ) compleo
.......
J / l _ .
e
o
J4
.
X-ray diffraction patterns of samples prepared by the coprecipitation method. Sb:V ratio of 1.0 (a3OO,a, al) and 5.0 [b300,b550,b,b1(550),B1(650)]. Calcination at 300"C (a300,b300), 550"C [b550, b1(550)] or 650"C [a,al,b,b1(650)]. Samples after the catalytic tests in propane ammoxidation: [al ,b1(550),b1(650)]. Fig. 8
tely disappears only by calcination above 750-800 ~ Infrared spectra also show that VS+-oxide is present in small
72 amounts in these samples (band at 1020 crn'l), but is not completely absent as suggested by the XRD data (Fig. 8).
....
After the catalytic tests, both IR (Fig. 9) and XRD (Fig. 8) data show that Sba+-oxide completely disappears and the relative amount of o~-Sb204 increases. The crystal size of t~-Sb204, estimated from the line broadening in XRD patterns, is around 300 /~ and thus significantly smaller than for the method of preparation by solid state reaction (around 600-700 tl,) and lower also than in the samples prepared by redox reaction in solution and the sol-gel preparation method (around 400/~). The crystal size does not change after the catalytic tests.
It is interesting to note that in the case of the sample calcined at 550~ after the catalytic tests low-intense diffraction peaks due to Sb203 (valentinite form) are observed. In addition, the sublimation of antimony oxide, which can be collected in the outlet stream from the reacI I tion by deposition on a cold plate, is observed. In fact, I thermobalance tests clearly show that Sb6013 reduces directly to Sb203 in the presence of ammonia or propane, when oxygen is absent. At temperatures of around 500~ Sb203 sublimes and thus is loss from the catalyst. In the presence of oxygen together with NH 3 and propane, 10OO Sb6013 transforms to ot-Sb204 at about 500~ but the parFig. 9 Infrared spectra of tial transformation to Sb203, especially when a high prosamples prepared by the coprecipitation method. pane to oxygen ratio in the feed is used according to patent indications [3,4b-4d], is reasonably possible. On the contrary, o~-Sb204 is rather stable and does not reduce to Sb203 or sublime, but only transforms to 15-Sb204 above 800~ These results show that calcination at temperatures above 750~ is necessary for the complete transformation of sbS+-oxide to ct-Sb204 which avoids the possibility of loss of antimony during the catalytic reaction. sbS+-oxide is the active component for the synthesis of acrylonitrile from propylene. In fact, transient catalytic tests (Fig. 10) demonstrate that the initial formation of acrylonitrile both from propane and from propylene is about two times higher than that in the steady-state. The infrared characterization of the samples after these tests shows the disappearence of the band of sbS+-oxide and it is thus reasonable that the change in the surface reactivity must be attributed to the progressive reduction of surface Sb5+-oxide, which at the steady-state, is only partially reoxidized. In-situ infrared studies [9g] further support this indication, showing that the band at 890 crn"1 disappears upon interaction with the hydrocarbon, but reforms by interaction with gaseous oxygen at high temperature. In addition, if a feed of only oxygen is sent to the catalyst after the transient catalytic tests of Fig. 10, the initial formation of acrylonitrile is higher, even though not twice as high as that for the fresh catalyst, but then rapidly reaches the steady-state value. It is also worth noting the much higher reactivity of propylene on this cata-
73 lyst in comparison with propane and thus the detection of propylene in the ammoxidation of prorpane [9] is due to the q) 0 40 limited number of E sites of ammoxidas 0 tion more than to the I limited difference in E 0 the rate of transfor~ 20 marion of the two hydrocarbons on this o catalyst (Fig. 10). It >,, (,) should also be < pointed out that tran0 1 2 3 4 sient catalytic tests with pure Sb5+-oxide Time, sec (. 10') show a high initial reFig. 10 Formation of acrylonitrile from propane at 500"C (solid line) and activity, but a rapid from propylene at 420~ (dotted line) as a function of time- on-stream irreversible deactivaduring transient catalytic tests on a fresh Sb:V=3.0 sample prepared by tion due to the transthe coprecipitation method and calcined at 650~ formation to o~-Sb204. Vanadium, therefore, catalyzes the reoxidation of antimony, reasonably that antimony situated in interface sites between Sb-oxide and vanadium-antimonate phases. 60
e
-l
_ Ik
l
-,,.
.m
i__
q~
,lw om
L
These results clearly indicate that sbS+-oxide, stabilized at the surface of vanadium-antimonate, is responsible for the selective synthesis of acrylonitrile from the intermediate propylene in agreement with previous suggestions [ 16], and that vanadium catalyzes the reoxidation of reduced antimony, as well as plays other roles in the mechanism of oxidative dehydrogenation of propane to propylene and in the side reaction of NH 3 oxidation to N 2 (see above). The design of active V-Sb-O catalysts for the synthesis of acrylonitrile from propane must thus realize an optimal distribution of vanadium and antimony-oxide surface species on the vanadium-antimonate crystals in order to maximize the rate of alkane activation and reoxidation of antimony, but limit the occurrence of the side reactions. The preparation method especially influences the relative amount of the various phases and of vanadium-antimonate, but also the mean dimensions of the crystallites and their modality of contact (surface architecture) which was shown to be of great importance for the catalytic behavior in propane ammoxidation. The non-stoichiometric characteristics of the rutile vanadium-antimonate phase depend on the nature and atmosphere of the heat treatment, whereas the calcination at temperatures above 800~ avoids the reduction during the catalytic reaction of surface Sb5+-oxide to Sb203 which causes the loss of antimony from the catalyst. The ct ~ 13 Sb204 transition, however, is observed above 800~ being the transformation catalyzed from the presence of vanadium. Finally, it was evidenced that various changes occur during the catalytic reaction, which, in tum, considerably affects the surface reactivity of the catalysts.
74
REFERENCES [1] (a) G. Centi, R.K. Grasselli, F. Trifirb, Catal. Today 13 (1992) 661. (b) Ibidem Cin'm. Ind. (Milan) 72 (1990) 617. [2] The Chemical Engineer, Sept. 13 (1990) 8. [31 (a) N. Harris, W.L. Wood, German Often 2,058,004 (1971); (b) N. Harris, F.J. Flinton, German Often, 2,224,214 ( 1973); assigned to Power Gas-I.C.I. [4] (a) A.T. Gutmann, R.K. Grasselli, J.F. Brazdil, U.S. Patent 4,746,641 and 4,788,317 (1988). (b) L.C. Glaeser, J.F. Brazdil, D.D. Suresh, D.A. Omdoff, R.K. GrasselliU.S. Patent 4,767,739 and 4,788,173 (1988). (c) M.A. Tort, J.F. Brazdil, L.C. Glaeser, U.S. Patent 4,784,979 (1988). (d) M.A. Tort, J.F. Brazdil, L.C. Glaeser, U.S. Patent 4,879,264 (1989). (e) C.S. Lynch, L.C. Glaeser, J.F. B r ~ , M.A. Tort, U.S. Patent 5,094,989 (1992). All patents assigned to Standard Oil Co. [51 G. Centi, F. Trifirb, Catal. Rev.-Sci. Eng., 28 (1986) 165. [61 (a) F.J. Berry, M.E. Brett, W.R. Patterson, J. Chem. Soc. Dalton (1983) 9 and 13. (b) F.J. Berry, M.E. Brett, lnorg. Chim. Acta, 76 (1983) L205. (e) F.J. Berry, M.E. Brett, R.A. Marbrow, W.R. Patterson, J. Chem. Soc. Dalton (1984) 985. (fl) F.J. Berry, M.E. Brett, J. Catal. 88 (1984) 232. (e) F.J. Berry, M.E. Brett, lnorg. Chim. Acta 76 (1983) L205. [7] R.G. Teller, M.R. Antonio, J.F. Brazdil, M. Mehicic, R.K. Grasselli, Inorg. Chem., 24 (1985) 3370. [8] T. BirchaU, A.E. Sleight, Inorg. Chem. 15 (1976) 868. [9] (a) G. Centi, R.K. Grasselli, E. Patan~, F. Trifir~, in New Developments in Selective Oxidation, G. Centi and F. Trifirb Eds., Elsevier Pub.: Amsterdam 1990, p. 515. (b) A. Andersson, S.L.T. Andersson, G. Centi, R.K. GrasseUi, M. Sanati, F. Trifirb, in New Frontiers in Catalysis (proc. 10th Int. Congress on Catalysis, Budapest 1992), L. Guczi et al. Eds., Elsevier Pub: Amsterdam 1993, p. 691. (c) A. Andersson, S.L.T. Andersson, G. Centi, R.K. Grasselli, M. Sanati, F. Trifirb, Appl. Catal., (1994) in press. (d) G. Centi, E. Foresti, F. Guarneri, in Proceedings, 4th Congress on New Developments in Selective Oxidation, Benalmadena (Spain) Sept. 1993, V. Cortes Corberan and S. Vic Eds., Elsevier Pub.: Amsterdam 1994, in press. (e) R. Catani, G. Centi, F. Trifirb, R.K. Grasselli, Ind. Eng. Chem. Research 31 (1992) 107. (f) G. Centi, D. Pesheva, F. Trifirb, Appl. Catal. 33 (1987) 343. (g) G. Centi, S. Perathoner, J. Catal. (1994) submitted. [~o] K. Nilsson, T. Lindblad, A. Andersson, C. Song, S. Hausen, Proceedings, 4th Congress on New Developments in Selective Oxidation, Benalmadena (Spain) Sept. 1993, V. Cortes Corberan and S. Vie Eds., Elsevier Pub.: Amsterdam 1994, in press. [11] C. Rocchiccioli-Deltcheff, T. Dupuis, R. Frank, M. Harrnelin, C. Wadier, C.R. Acad. Sc. Paris B 270 (1970) 541. [12] (a) D.J. Stewart, O. Knop, C. Ayasse, F.W.D. Woodhams, Canad. J. Chem., 50 (1972) 690. (b) C.A. Cody, L. DiCarlo, R.K. Darlington, lnorg. Chem., 18 (1979) 1572. (c) F.J. Berry, M.E. Brett, Inorg. Chim. Acta, 83 (1984) 167. [13] B.B. Pal, K.K. Sen Gupta, Inorg. Chem. 14 (1975) 2268. [14] G. Centi, D. PineUi, F. Trifirb, D. Ghoussoub, M. Guelton, L. Gengembre, J. Catal., 130 (1991)238. [151 M.A. Char, D. Patel, M.C. Kung, H.H. Kung, J. Catal., 105 (1987) 483. [16] R.K. Grasselli, J.F. Brazdil, J.D. Burrington, in Proceedings, 8th International Congress on Catalysis, Dechema Pub.: Frankfurth AM 1984, Vol. V, p. 369.
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparationof HeterogeneousCatalysts G. Ponceletet al. (Editors) 9 1995 ElsevierScienceB.V. All rights reserved.
75
N o v e l p r o c e d u r e for t h e p r e p a r a t i o n of h i g h l y a c t i v e p l a t i n u m - t i t a n i a and palladium-titania aerogel catalysts with favourable textural properties M. Schneider, M. Wildberger, D.G. Duff, T. Mall~it, M. Maciejewski and A. Baiker* Department of Chemical Engineering and Industrial Chemistry, Swiss Federal Institute of Technology, ETH-Zentrum, CH-8092 Ziirich, Switzerland Meso- to macroporous platinum or palladium containing titania aerogels have been prepared by a two-stage sol-gel process with ensuing hightemperature supercritical drying. They possess BET surface areas of 150-190 m2 g-1 and specific nitrogen pore volumes of 0.7-1.1 cm 3 g-1 after thermal treatment at < 673 K in hydrogen or < 773 K in air. Thermal analysis showed t h a t untreated aerogels contain a considerable amount of organic residues (0.5-1.9 wt% carbon), which is not completely removable by any t h e r m a l treatments at < 773 K. The titania matrix consists of anatase crystallites with 7-9 nm mean size. The volume-weighted-mean particle size, derived from TEM, varies significantly depending on the metal precursor-solution used. The liquid-phase hydrogenations of trans-stilbene and benzophenone revealed high activity and accessibility of the metal particles. Moreover, Pd-titania aerogels showed good selectivity in the hydrogenation of 4-methylbenzaldehyde. 1. INTRODUCTION The potential of aerogels for catalysis resides in their unique morphological and chemical properties [1]. These properties originate from their wetchemical preparation by the sol-gel method and the subsequent removal of the solvent via supercritical drying (SCD). Due to the 'structure-preserving' ability of supercritical drying, aerogels are usually solids of high porosity and specific surface area. The reductive alcoholic atmosphere during high-temperature SCD further enables the simultaneous reduction of various ionic metal precursors to corresponding metallic particles [2-6]. Moreover, co-gels of group VIII metals, highly dispersed in different metal oxide matrices, are readily prepared by sol-gel technology, ensuring uniform distribution of the metal throughout the solid [2,4-8]. This property often results in more intimately developed metal-support interaction compared to conventionally impregnated catalysts [7,8]. Generally, the preparation of such metal-metal oxide sol-gel products involved initimate molecular mixing of both constituents to whom correspondence should be addressed
76 [4,5,7,8]. Consequently, one should also face the fact t h a t such molecularly mixed preparation routes facilitate the incorporation of at least p a r t of the precious metal component within the oxide matrix [7,8], this being no longer accessible for catalytic processes. Recently it was demonstrated t h a t meso- to macroporous t i t a n i a aerogels with high surface area can be synthesized by applying h i g h - t e m p e r a t u r e SCD [9]. Based on this knowledge, a novel preparation procedure was developed, enabling the direct synthesis of binary m e t a l - t i t a n i a aerogels with high accessibility by a sol-gel-aerogel process. In the present work 2 or 5 wt% platinum or palladium containing titania aerogel catalysts have been synthesized by a two-stage sol-gel route and subsequent SCD, using different noble metal precursor-solutions and thermal t r e a t m e n t s of the r e s u l t i n g aerogels. Morphological p r o p e r t i e s were characterized by means of nitrogen physisorption, X-ray diffration (XRD), transmission electron microscopy (TEM), thermal analysis (TG, DTA) coupled with m a s s s p e c t r o m e t r y a n d h y d r o g e n c h e m i s o r p t i o n . A c t i v i t y a n d accessibility of the metal particles for the liquid-phase hydrogenation of transstilbene, benzophenone and 4-methylbenzaldehyde were studied. 2. EXPERIMENTAL Throughout this work a scheme of designations is u s e d - Pd5PAc as an example. The numeral following 'Pd' designates the nominal Pd content in weight percent and the subsequent letters represent the Pd precursor used (Table 1). These acronyms describe the untreated aerogel materials. The synthesis of the metal-titania aerogels involved the following noble metal precursors in solution: PtC14, (NH4)2PtC16, Pt(acac)2; Na2PdC14, (NH4)2PdC14, Pd(acac)2, Pd(OAc)2. The preparation is described in more detail in [10,11]. In brief, the sol-gel process was carried out in an anti-adhesive, closed teflon beaker, under nitrogen atmosphere and at ambient temperature (297• K). Two solutions were p r e p a r e d . The f i r s t s o l u t i o n c o n s i s t e d of 32.0 g tetrabutoxytitanium(IV) (TBOT) dissolved in 120 ml methanol and the second of 6.78 ml doubly distilled w a t e r and 0.52 ml nitric acid (65 wt%) in 30 ml methanol. The latter was added to the TBOT solution under vigorous stirring (ca. 1000 rpm). The resulting t i t a n i a gels were aged for 4 h and t h e n redispersed with different amounts of methanol (Table 1). The noble metal precursor-solution (Table 1) was added to the non-viscous titania solution and a second ageing-step for 19 h under vigorous stirring followed. The as-prepared sol-gel product was transferred in a Pyrex-glass liner into an autoclave with a net volume of 1.09 1 together with the appropriate amount of additional methanol (outside of the liner) (Table 1), t h u s exceeding the critical volume of the mixed solvent (solvent volume ca. 375 ml in all cases). The corresponding critical data for methanol, as the dominating component of all sol-gel solvents, are: Vc = 118 ml tool-l, Tc = 513 K and Pc = 8.1 MPa. Supercritical drying was performed in a batch operation and the appropriate conditions were set as follows: nitrogen prepressure of 5 MPa, heating rate of 1 K min-1 to final SCD temperatures of 533-553 K, 30 min t h e r m a l equilibration
77 (final pressure ca. 19 MPa) and isothermal depressurization with 0.1 MPa min-1. Finally, portions of the u n t r e a t e d (raw) aerogel powders were thermally treated in a U-tube reactor. The media applied were air, air followed by hydrogen, or hydrogen. The temperatures ranged from 473-773 K.
Table 1 Designations of aerogels and important preparation p a r a m e t e r s Aerogel
Precursor [rag]
Solvent composition [ml]
MeOH for Extra MeOH redispersion for SCD [ml] [ml]
Pt2PC Pt2PA Pt2NP PtSPC
L~tC14(265) PtQ2cac)2 (309) ~ ) 2 _ . ~ C 1 6 (349) _PtC14 (683)
H20 (1.8)/MeOH (24) MeOH (95) H20 (50) H20 (7)/MeOH (24)
71 50 50+100 71
130 80 25 130
Pd2PC
PdC12 (256)
H20 (1.0) /
71
130
Pd2NP Pd2PA Pd2PAc Pd5PAc
NaC1 (168) ~ ) 2 ~ d C 1 4 (410) Pd(~cac)2 (440) Pd(OAc)2 (324) Pd(OAc)2 (835)
MeOH (24) H20 (2) / MeOH (24) Benzene (30) Acetone (24) (warm) Acetone (40) (warm)
71 71 71 55
130 124 130 130
The t e x t u r a l p r o p e r t i e s were derived from n i t r o g e n p h y s i s o r p t i o n measurements at 77 K using a Micromeritics ASAP 2000 instrument. X-ray powder diffraction (XRD) patterns were m e a s u r e d on a Siemens 0 / 0 D5000 powder X-ray diffractometer. The diffractograms were recorded with CuKa radiation and a position sensitive detector with Ni-filter. The m e a n crystallite sizes were determined using the Scherrer equation and the {101}-reflection for anatase, the {lll}-reflection for Pd and Pt, and the {ll0}-reflection for PdO. Metal particle size distributions were derived from diffraction-contrast TEM using a Hitachi H-600 operated at 100 kV, with a point resolution of ca. 0.5 nm. High-resolution TEM was carried out on a Philips CM30ST at 300 kV, with a point resolution of 0.19 nm. TG and DTA investigations were performed on a Netzsch STA 409 i n s t r u m e n t coupled with a Balzers QMG 420/QMA 125 quadrupole mass spectrometer. Total carbon- and hydrogen-contents were determined with a LECO CHN-900 elemental microanalysis apparatus. Trans-stilbene, 4-methylbenzaldehyde and benzophenone hydrogenation were performed at 303, 333 or 343 K, respectively, and at atmospheric hydrogen pressure. Iso-propylacetate (trans-stilbene), ethanol (4-methyl-benzaldehyde) or butylacetate (benzophenone) were used as solvents. With trans-stilbene and
78 benzophenone, 15-20 mg catalyst powder (< 300 ~m) were suspended in 30 ml solvent, prehydrogenated, and 2 mmol reactant in 10 ml solvent were injected. In the case of 4-methylbenzaldehyde, a 1 wt% ethanolic solution and 10-30 mg catalyst powder (< 300 ~m), yielding a constant Pd:reactant weight ratio of 1:40, were used. The semi-batch apparatus, experimental procedure and analysis of the product mixtures were described in detail in [10,11]. To minimize the influence of side reactions, the initial rates were determined from r e a c t a n t consumptions at below 5 % conversion. Preliminary tests with the most active aerogels indicated t h a t i n t e r - p a r t i c l e a n d i n t r a - p a r t i c l e m a s s t r a n s f e r limitation could be ruled out. 3. RESULTS AND DISCUSSION 1.8
After t h e r m a l t r e a t m e n t in hydrogen at t e m p e r a t u r e s ~ 673 1.5K and in air at _< 773 K, all 1.2t i t a n i a - b a s e d aerogel s a m p l e s r~ 0.9 showed a type-IV isotherm with '7, a type-H1 desorption-hysteresis. 0.6Figure l a depicts the pore size ~ 0.3distribution of Pt5PC calcined in air at 573 K, which is charac0.0 . . . . . . . I . . . . . . . . I teristic for all p l a t i n u m - t i t a n i a 0 (b) (excluding P t 2 N P ) a n d palla3.Odium-titania aerogels. With Pt2NP calcined at 573 K, the 50 2.0ml extra water, added via the p r e c u r s o r - s o l u t i o n (Table 1), 1.0caused a narrower and virtually symmetric pore size distribution 0.0 . . . . . . . I . . . . . . . . | (Figure lb). This is likely a result 1 10 100 200 of the increased water-reactivity u n d e r the conditions applied Pore diameter(D) / nm during h i g h - t e m p e r a t u r e supercritical drying (SCD). All aeroFigure 1. Differential pore size distrigels s t u d i e d p o s s e s s specific butions derived from the desorption surface areas of 150-190 m 2 g-l, branch of nitrogen physisorption at 77 K meso- to macroporosity and only (STP; 273.15 K, 1 atm). (a) Pt5PC, (b) little m i c r o p o r o s i t y , y i e l d i n g Pt2NP; both calcined in air at 573 K. pore-size m a x i m a of ca. 30-50 nm, specific nitrogen adsorption pore-volumes of 0.7-1.1 cm 3 g-1 and specific micropore surface areas of <_ 14 m 2 g-l, as estimated from the corresponding t-plots. The crystalline fraction of the t i t a n i a m a t r i x is made up of a n a t a s e crystallites of ca. 7-9 nm mean size in all samples, almost independent of the different thermal treatments applied. The platinum-titania aerogels contained
79 crystalline, metallic Pt. With palladium containing titania aerogels, however, depending on the precursor-solution employed, either metallic Pd (Pd2PC, Pd2NP) or an interstitial solution of carbon in Pd, Pd-C, (Pd2PA, Pd2PAc, Pd5PAc) was formed [12]. This solid solution decomposed both in air and in hydrogen at > 473 K, i. e. the lowest temperature applied. Oxidative treatments in the temperature range 473-773 K generally lead to an increasing fraction of PdO. Note that the smaller the mean particle size was, the higher was the proportion of PdO formed. Consequently, the palladium-titania catalysts can contain 'palladium' in the form of metallic Pd, Pd-C and/or PdO. With regard to the genesis of the noble metal component, the observed colours of the sol-gel products (yellow to orange) prior to SCD indicated that the metal ions remained unreduced upon mixing the metal salts with the methanolic t i t a n i a suspensions. Thus, the metal reduction and formation of metal particles occurred mainly during high-temperature SCD. Only with Pd(OAc)2-derived aerogels (Table 1; Pd2PAc, Pd5PAc), the colour of the wet-chemical stage turned to black within minutes after the addition of the Pd precursor-solution. It is suggested that in this case the formation of the Pd-particles occurred predominantly via homogeneous nucleation. Depending on the metal precursor-solution used, the metal particle size distributions of the raw aerogels, determined by TEM, vary significantly, as illustrated in Figure 2. This behaviour is in good agreement with XRD linebroadening results for the metal component. The metal dispersions are generally lower with the 'palladium'-titania compared to the P t - t i t a n i a aerogels. The former demonstrated a distinct heterogeneity with wide and possibly multi-modal particle size distributions. With P t - t i t a n i a , the monomodal preparations derived from chloridic Pt-precursors are strikingly monodisperse and homogeneous. The corresponding Pt-particles are mostly monocrystalline, as determined by high-resolution TEM. With 'palladium'titania the best dispersed aerogels are achieved with the organometallic precursors and in the case of Pt-titania with the chloridic Pt-precursors. The morphological and textural stability of the titania matrix on the one hand, and the stability of the metal particles on the other, under all calcination (< 773 K) and reduction (< 673 K) conditions investigated, are essential properties of these materials. Applications in liquid-phase catalysis cause a refilling of the porous titania structure, creating differential capillary forces. The resulting shearing forces have to be sustained by the tenuous aerogel skeleton. With future high-temperature applications in mind, we have obviously succeeded in preparing aerogel catalysts t h e r m a l l y stable at temperatures < 673 K in hydrogen and at < 773 K in air. This stability justifies the creation of high internal surface areas with good accessibility. Thermal analysis coupled with mass spectrometry indicated t h a t the raw aerogels contain a considerable amount of organic impurities after the hightemperature SCD. The carbon-contents range from 0.5-0.7 wt% with the Pttitania aerogels and from 1.1-1.9 wt% with the 'palladium'-titania aerogels, as determined by elemental microanalysis. This is mainly attributed to the realkoxylation of titanium-bound hydroxyl-groups during SCD. To a lower extent, some organic oxidation products (precursor reduction) as well as
80 5040-
-30
30-
-20
20-
n=44
10-
,
0 40-
(c)
i
100
n=82
,t-.F'l
~
2010040-
A___~
200
300
400 , 50 (b) 4 0 ~
n = 30
3O20-
t
[
,
I
,
80
'
120
I
10L 1
2
50 ~" 4 0 - ( b )
]
30-
I]
0
' 0
30 20 ~
1_.. n = 407
40
~~ 2 0 -
~
~"
J
(d)
0
-10
,
30-
~
40
(a)
3
4
5
6
["--
7 ,
I
I
,
I
,
I
J
I
I
n=77 _j-
~'~
.2 ,1_ 20 40 60 80 100 120 Particle diameter / nm
,
0
,
I
9
0
160
200 60 (c) -50 -40 -30 ~ n = 25 - 2 0 - 1 0 {D~ [-~"! J , o g (d) -4O ~ -30 -2O n=46-10 ,
I
,
I
,
0
10 20 30 40 50 60 70 80 Particle diameter / n m
Figure 2. Particle size distributions derived from TEM for the r a w platinumtitania aerogels on the left side: (a) Pt2PC, (b) Pt2PA, (c) Pt2NP, (d) Pt5PC; and the raw palladium-titania aerogels on the right side: (a) Pd2PC, (b) Pd2PA, (c) Pd2PAc, (d) Pd5PAc. n represents n u m b e r of particles m e a s u r e d . Aerogel designations are explained in Table 1.
unhydrolysed incorporated alkoxide ligands must also be t a k e n into account. Thermal analysis coupled with mass spectrometry showed t h a t the organic residues generally evolved in two steps with maxima at ca. 540 and 970 K, as exemplified in Figure 3 for the Pd5PAc-series t r e a t e d u n d e r different conditions. Consequently, we can conclude t h a t these organic residues are not
81 completely removable neither in oxidative atmosphere at temperatures < 773 K nor reductive atmosphere at < 673 K. Prior to catalytic runs, the aerogel catalysts were p r e t r e a t e d in a h y d r o g e n m/z 44 (f) flow at t h e a p p r o p r i a t e reaction t e m p e r a t u r e (303343 K). In the case of the (partly) oxidized palladium c o n t a i n i n g a e r o g e l cata/ ' ~ ' ~ ~ _ (c) lysts, XRD i n v e s t i g a t i o n s a f t e r the c a t a l y t i c t e s t s (b) revealed t h a t the reductive conditions applied resulted in the formation of metallic 613 Pd. In contrast, the hydrogenation did not influence the Pd-C solid solution of 973 the catalysts derived from (a) o r g a n o m e t a l l i c Pd-precursors (Tablel; Pd2PA, 300 500 700 900 1100 1300 1500 Pd2PAc, Pd5PAc). W h e n Temperature / K compared to a commercial 5 wt% Pt on alumina catalyst, Figure 3. CO2-evolution during t h e r m o a n a it is interesting to note that lytical runs from Pd5PAc catalysts, treated in even the raw, well disair and/or h y d r o g e n at d i f f e r e n t temp e r s e d P t - t i t a n i a aerogel peratures. (a) raw, (b) air 473 K, (c) air 573 K, catalysts (Figure 2; Pt2PC, (d) air 673 K, (e) air 573 K followed by P t 2 N P , P t 5 P C ) exhibited hydrogen 673 K. Heating rate 10 K min-1; air m a r k e d hydrogenation actiflow: 25 ml rain -1. vities, which were ca. 1.5 times higher. Consequently, a remarkable fraction of active sites must already be available in the raw aerogels, based on a reasonable accessibility of the m e t a l particles. The influence of the different pretreatments is represented by the Pt2PC-series in Figure 4. In most cases the calcination of the aerogels yielded significantly increased initial activities. In contrast, hydrogen t r e a t m e n t mainly decreased the initial rates. Considering the thermal stability of the metal particle sizes, it emerges that the catalytic effects within a series of differently treated catalysts must be dominated by some varied coverage of the active noble metal surface. Two possible explanations are the oxidative removal of a fraction of the organic impurities from the metal surface or the partial coverage by TiOx overlayers under reductive conditions. This behavior can be interpreted from the reaction rates [10,11], chemisorption results [10] and literature data [13]. A comparison of the initial rates for trans-stilbene and benzophenone hydrogenation, studied both with Pd-titania and Pt-titania aerogel catalysts, shows a m a r k e d difference. The chloride derived Pt-titania catalysts do not r,D
~ i..,,i
82
i trans-stilbene(TS)
~
benzophenone (BP)
pretreatment in air 12
35
10
30 25 20 15
v=d !
10
t~ i
v-4 i
t~
v==4 ! C~
5
o
0 raw
o
v=d
]
air 573 K
I
air 673 K
pretreatment in H 2 30 c~
25 20 15 10
raw
lair 573 K / l a i r 573 K/] H2573K
H2473K
]
H2 6 7 3 K
H2673K
Figure 4. Initial rates for trans-stilbene (303 K) and benzophenone (343 K) liquid-phase hydrogenation at atmospheric hydrogen pressure, represented by the Pt2PC-series both raw and treated in air and/or hydrogen at different temperatures. specifically favour either benzophenone or trans-stilbene hydrogenation, whereas both the chloridic and organometallic derived Pd-titania catalysts prefer unequivocally trans-stilbene hydrogenation. Despite the significantly larger mean particle sizes of 'palladium'-titania compared to P t - t i t a n i a aerogels, higher initial rates are measured for trans-stilbene hydrogenation over Pd-titania. Note that the best dispersed Pd-titania catalyst shows an up to 7 times higher initial rate. This favourable catalytic performance is unlikely due to beneficial metal support interaction. It is well known that the nature of the
8] substituents on a reactant molecule can considerably affect the reducibility of a functional group [14]. Kazanskii and T a t e v o s y a n [15] showed t h a t the promoting effects of aryl groups on the C=C double bond hydrogenation are most prominent with Pd and less significant with Pt and Ni.
Table 2 Catalytic activity of Pd-titania catalysts in liquid-phase hydrogenation of 4methylbenzaldehyde (A) to 4-methylbenzyl alcohol (B), p-xylene (C) and ethyl 4methylbenzyl ether (D)
Catalyst
Time [hi
Temp. [K]
Pressure Product composition [%] IMPala A B C D
Pd2PAc (calcined in air at 673 K) Pd5PAc (calcined in air at 673 K)
1 4 0.5 4
333 333 333 333
0.05 0.05 0.05 0.05
3 2 3 3
80 12 95 18
17 86 2 79
0 0 0 0
0.5 wt% Pd/TiO2 (Pd 6)b
4
423
1
5
15
71
7
0.5 wt% Pd/TiO2 (Pd 1C)b
4
423
1
26
0
0
73
a hydrogen partial pressure b from ref. [16]: their sampie Pd 6 is a weakly acidic and Pd IC a strongly acidic catalyst
Table 2 shows the catalytic data for 4-methylbenzaldehyde hydrogenation over Pd2PAc and Pd5PAc catalysts, both calcined in air at 673 K. Literature data for a low acidity and high acidity 0.5 wt% titania-supported Pd catalyst are included for comparison [16]. These catalysts were prepared by impregnation of titania extrudates (phase ratio anatase:rutile, 3:1), subsequent calcination in air and reduction under a H2-N2 atmosphere. The tests were carried out using the same Pd:reactant ratio as in [16]. The results indicate that the aerogel catalysts possess m u c h higher hydrogenation activity. Almost full conversion to 4-methylbenzyl alcohol could be achieved in 0.5-1 h at considerably lower temperature and hydrogen pressure. The hydroxymethyl group of the benzyl alcohol derivative is subsequently converted to the corresponding methyl group. Note that the reduction of aromatic aldehydes on Pd is generally faster than the further reduction of the intermediate aromatic alcohols [14]. Another interesting point is the negligible formation of ethyl 4-methylbenzyl ether after 4 h (Table 2). The catalytic tests indicate that the Pd-titania aerogel catalysts are very active in the reduction of the carbonyl group in 4-methylbenzaldehyde to the corresponding hydroxyl group and in the hydrogenolysis of the C-O bond
84 to form hydrocarbons, but ethyl 4-methylbenzyl ether formation is negligible due to the absence of strong acidic sites. 4. CONCLUSIONS The novel, two-stage sol-gel process combined with s u b s e q u e n t hight e m p e r a t u r e supercritical drying proved to be an attractive route for the synthesis of meso- to macroporous Pt-titania and 'palladium'-titania aerogel catalysts with high specific surface areas. Both the metal particles and the titania matrix possess a remarkable structural stability up to 673 K in hydrogen and even up to 773 K in air. By varying the noble metal precursor, different metal particle size distributions can be obtained. Catalytic studies on trans-stilbene, benzophenone and 4-methylbenzaldehyde generally revealed good accessiblity and activity of the metal particles. The negligible ether formation in the hydrogenation of 4-methylbenzaldehyde over P d - t i t a n i a aerogel catalysts implies the absence of strong acidic sites in these materials. R ~ ~ C ~ ~
,
3. 4. 5. 6. 7. 8. .
10. 11. 12. 13. 14. 15. 16.
M. Schneider and A. Baiker, In Encyclopedia of Advanced Materials, D. Bloor, R.J. Brook, M.C. Flemings and S. Mahajan (eds.), Vol. 1, Pergamon Press, Oxford, 1994. G.M., Pajonk, Appl. Catal., 72 (1991) 217. J.N. Armor, E.J. Carlson and G. Carrasquillo, Mater. Lett., 4 (1986) 373. J.N. Armor, E.J. Carlson and P.M. Z~mbri, Appl. Catal., 19 (1985) 339. K. Balakrishnan and R.D. Gonzalez, J. Catal., 144 (1993) 395. Y. Mizushima and H. Makoto, Eur. Mater. Res. Soc. Monogr., 5 (1993) 195. P. Bosch, T. LSpez, V.-H. Lara and R. GSmez, J. Mol. Catal., 80 (1993) 299. T. LSpez, R. GSmez, E. Romero and I. Schifter, React. Kin. Catal. Lett., 49 (1993) 95. M. Schneider and A. Baiker, J. Mater. Chem., 2 (1992) 587. M. Schneider, D.G. Duff, T. Mall~it, M. Wildberger and A. Baiker, J. Catal., 146 (1994). M. Schneider, M. Wildberger, M. Maciejewski, D.G. Duff, T. Mall~it and A. Baiker, J. Catal., in press. M. Maciejewski and A. Balker, J. Phys. Chem., 98 (1994) 285. S.J. Tauster, Acc. Chem. Res., 20 (1987) 389. M. Freifelder (ed.), Practical Catalytic Hydrogenation Techniques and Application, J. Wiley, New York, 1971. B.A. Kazanskii and G.T. Tatevosyan, J. Gen. Chem. USSR, 9 (1939) 2256. M. Bankmann, R. Brand, A. Freund and T. Tacke, In Preprints 3rd
International Symposium Fine Chemistry Catalysis, Heterogeneous Catalysis and Fine Chemicals, p. C 75, Poitiers, 1993.
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
85
Preparation of c o m b u s t i o n catalysts by w a s h c o a t i n g alumina w h i s k e r s - c o v e r e d metal m o n o l i t h s using a sol-gel m e t h o d Marcus F.M. Zwinkels, Sven G. J ~ s , and P. Govind Menon Royal Institute of Technology, Department of Chemical Engineering and Technology, Chemical Technology, S-100 44 Stockholm, Sweden, Tel. +46 8 790 8254, Fax +46 8 108 579 ABSTRACT Monolithic catalyst supports were prepared by washcoating whiskers-covered metal monoliths with silica. Dip-coating with slurries consisting of silica powder mixed with a colloidal silica sol resulted in washcoats with controllable thickness in a single-step preparation. The use of colloidal sols with varying particle size distributions provided a way to vary the specific surface area and pore size distribution of the washcoat. Increasing thickness and greater surface area lead to higher activity for methane combustion over palladium supported on the thus prepared monolithic supports. The preparation procedure strongly influences the activity of the prepared catalysts.
1. INTRODUCTION Monolithic catalysts (or honeycombs) have received much attention ever since they were first applied in automotive catalytic converters [1]. An increasing interest in the use of monolithic reactors for other applications has also been noticed during recent years [2]. One application which particularly profits from the opportunities offered by the honeycomb structure is catalytic combustion for use in advanced gas turbines [3]. In a catalytic combustor, a premixed lean fuel-air mixture is ignited by the catalyst which results in complete combustion at maximum temperatures far lower than possible in conventional gas-phase combustors. Hence, the thermal formation of nitrogen oxides can almost completely be circumvented. This fact has promoted large R&D programs in catalytic combustion during recent years. Currently, two approaches for the design of catalytic combustors are being tested. The first approach, the multi-monolith catalytic combustor, is based on a very active catalyst at the combustor inlet, followed by less active but more thermostable catalyst segments [4]. Complete combustion is to be achieved within the monolithic catalyst in this case. The second approach, a hybrid combustor, is based on a partial combustion of the fuel in the catalyst, while the remainder of the fuel is converted in a homogeneous combustion zone downstream of the catalyst [5,6]. The advantage of the multi-monolith is its simplicity whereas the hybrid combustor provides a way to limit the temperature of the catalyst, thereby decreasing the demands placed on the catalyst materials. There are two major aspects that need further investigation before catalytic combustors can be fully developed. These are (i) the development of materials with sufficient thermal and chemical stability and (ii) the optimization of the monolithic catalyst in order to obtain ignition
86 ignition of fuel-air mixtures at as low inlet temperatures as possible. Low inlet temperatures are desirable in order to avoid air preheaters or pilot flames, which can produce NO x. Various aspects can influence the performance of a combustion catalyst at low temperatures. The most obvious are the design and composition of the honeycomb structure, the porous structure and the thickness of the porous washcoat, and the amount and type of active material. The most common honeycomb structures today are made of cordierite. Other ceramics, such as mullite and different composite materials, have also received certain attention. Metal monoliths are now gaining ground, since they have certain advantages over ceramics [6]. Metals have higher heat conductivity and higher resistance to mechanical loads, such as vibrations. Besides, metal monoliths can be produced with thinner cell walls than their ceramic counterparts, which results in reduced pressure drop. On the other hand, ceramics have higher maximum working temperatures and the deposition of washcoats on ceramic substrates is well developed. In this paper, we present a study in which combustion catalysts based on silica-coated metal monoliths were prepared. The aim of this study was to prepare washcoated metal monoliths with controlled properties. The properties varied are specific surface area of the washcoat and washcoat thickness or washcoat loading. Furthermore, we discuss how the preparation procedure affects the resulting catalyst properties and related performance. We deposited washcoats based on colloidal silica sols. Colloidal silica sols give porous materials with rather narrow pore size distributions when dried and calcined. This gives us excellent control over the pore size distribution of the washcoat, as will be discussed. The technique presented here, allows deposition of washcoats with controllable thickness in one step, unlike techniques based on pure silica sols, reported elsewhere [7,8]. Washcoats were impregnated with palladium salts to make active catalysts that were tested in methane combustion. The effects of the preparation procedure of the silica and of the impregnation procedure were studied using particulate catalysts.
2. E X P E R I M E N T A L 2.1 Materials The precursor materials in all experiments were Bindzil colloidal silica sols from Eka Nobel (Sweden). Three different grades were used, their significant properties are shown in Table 1. Potassium water glass (40 %w aqueous potassium silicate solution, K20:SiO2 = 1:3) from Eka Nobel was used as a dispersion agent. The addition of water glass to colloidal sols also has a documented effect on gelation of the sols by improving the strength of the resulting gel [9].
Table 1 Properties of Bindzil colloidal sols Grade
Measured dry substance
Average particle size
(wt% as SiO2) (nm)* 30/80 40/130 30/220
30.1 42.5 31.7
40 25 15
Specific surface area
Titratable alkali (wt% as Na20)*
(m2/g)*t
In Sol
In Silica+
0.13 0.19 0.30
0.43 0.45 0.95
80 130 220
* Manufacturer's specifications; t Sol particles; + Calculated from specifications
87 Metal monoliths were obtained from Emitec (Germany). They were subjected to hightemperature treatment by the supplier. The cell density of the monoliths used is approximately 400 cpsi. The monoliths consist of an iron-chrome-aluminum alloy which provides the surface with a textured whisker structure after suitable treatment. These whiskers, shown in Figure 8, act as anchors for the washcoat when deposited onto the substrate. The whiskers consist of aluminum oxide, completely covering the metal surface. This is shown by the data in Table 2, giving the results of EDX and XPS analyses of the whiskers-covered metal surface. Palladium nitrate (Johnson Matthey, England) and palladium chloride (Degussa, Germany) were used for depositing palladium on the supports. Table 2 EDX analysis (1 pm depth) of the bulk and surface composition of the monolith and XPS analysis (4 - 5 atomic layers) of the monolith surface. Element
AI Fe Cr Ti Si Mn Na Ca Mg C1 F O
Materials composition (wt%) EDX inside
EDX outside
3.7 74 21 -0.6 0.3 . . . . . . . . . . . .
53 36 36 0.36 11 -0.4 --4.5 . . . . 0.99 1.17 0.23 0.63 0.54 55.5
. . . . . .
. . . . . .
XPS
9
@
41~ ~,
>
Figure 1. Electron micrograph of the metal monolith surface, showing the alumina whiskers. The bar: 'represents 1 ktm. 2.2 C a t a l y s t p r e p a r a t i o n
Both monolith and particulate catalysts were prepared from the silica sols. The first step in both cases was drying of a sol at 75 ~C. The gels, thus prepared, were washed in order to remove the sodium present, following a procedure described by Cao et al. [9]. Samples were immersed twice consecutively in each of the following solutions in the order of 1 M NH4NO3, 0.1 M HNO3, and distilled water at about 70 ~ This was followed by drying at 75 ~ and calcination in air during 4 hours at temperatures between 500 - 900 ~ with heating and cooling rates of 5 ~ Grinding and sieving of the calcined material yielded particles between 250 and 425 I.tm. Incipient wetness impregnation with an aqueous palladium salt solution yielded 1 wt% Pd/SiO2 catalysts. These were used to study the influence of three parameters in the catalyst preparation
88 procedure on the catalyst activity: (i) alkali content of the support, (ii) type of palladium salt used in the impregnation step, and (iii) the calcination temperature. For catalysts based on sol 40/130, a factorial experimental design was used to show the effect of all three parameters and their interactions on the activity for methane combustion. The alkali content was varied by omitting the washing step in the preparation of the support in half of the experiments. The palladium salts tested were nitrate and chloride. The calcination temperatures chosen were 500 and 900 ~ The factorial design is shown with the results in Table 4 in Results and Discussion. Monolith catalysts were prepared according to the procedure shown in Figure 2. Slurries were prepared by mixing very fine silica powder with silica sol. The silica powder was obtained by the same procedure as for the pellets, described above, except for the sieving step. The sieve fraction under 63 ktm was calcined at 500 ~ in air during 4 hours prior to use in the washcoating slurry. In all experiments, the silica was washed according to the procedure described above, prior to calcination. In all cases, the powder particle size distribution showed nearly all particles to be between 1 and 40 I.tm, when measured using a sedimentation/ centrifugation technique with a Brookhaven Instruments XDC particle sizer. No significant differences were noticed between powders prepared from different sols.
slurry H dippingH washingH drying ~
preparation
calcination
H drying ~
impregnation
calcination
Figure 2. Procedure for preparation of monolithic catalysts. The sols were diluted to 30 wt% dry substance after which water glass was added, 2.5 wt% of the silica sol, based on dry material of both. Silica powder was mixed with the same sol from which it was prepared, in order to give washcoats with a simple pore size distribution. After slowly adding the powder to the sol, the slurry was stirred vigourously during at least 30 minutes. The particle size distribution of the powder in the slurry showed over 90 % of the silica particles to be between 0.5 and 7 [.tm, without significant differences between the different sols. The dispersion and mixing thus decreased the particle size of the silica powder. Metal monoliths and flat samples of the same material were immersed in the slurries. This was followed by withdrawal at the constant speed of 5 crn/min. After withdrawal of the monoliths from the slurry, the excess slurry was removed by either blowing with air or centrifugation for 30 seconds. Part of the dipping slurry from each experiment was saved for determination of alkali content and surface area. The samples were dried at 75 ~ and subsequently washed according to the procedure described above. After calcination in air at 600 ~ during 4 hours, the monolithic supports were impregnated with an aqueous palladium nitrate solution. The samples were dried at 75 ~ and again calcined in air at 600 ~ during 4 hours. The amount of palladium in all experiments was 4 wt%, based on the weight of the washcoat. The flat washcoated samples were used for characterization of the washcoat.
2.3 Catalyst characterization The surface area, pore volume, and pore size distributions of supports and catalysts were determined using a Micromeritics ASAP 2000 unit. Scanning electron microscopy (Zeiss DSM 940) was used for characterization of the whiskers-covered surface and washcoated samples. The thicknesses of the washcoats on flat samples were determined by an electro-magnetic method (Fischer Deltascope MP 3). The alkali content of the prepared supports was
89 determined by atomic emission spectrophotometry. Concentrated hydrofluoric acid was used to dissolve the silica samples, which were then diluted and analyzed using a Perk.in Elmer 1100 B spectrophotometer.
2.4 Activity testing The catalytic activity of the prepared catalysts for methane combustion was tested in a flow reactor unit. Bottled methane (99.995 % purity from AGA, Sweden) and air were fed to the system using mass flow controllers, giving a methane concentration of 2 vol%. The space velocity in all experiments was 50,000 h -1. The catalysts were placed in a vertical tubular Inconel reactor situated in a tubular furnace. The exiting gases were analyzed by gas chromatography using a Packard model 427 GC, equiped with a Poraplot Q fused silica capillary column and a thermal conductivity detector. The temperature in the furnace was controlled to give a linear temperature ramp of 2 ~ in all experiments. Hence, the conversion of methane to carbon dioxide and water was determined as a function of the gas inlet temperature.
3. RESULTS AND DISCUSSION 3.1 Washcoat preparation Washcoats with varying pore size distributions were prepared from the different colloidal sols. Sols with larger particles (See Table 1) yield silica with large pores and lower surface area, as is shown in Figure 3 and Table 3. The surface areas of the washcoats varied between 60 and 143 m2/g. On the other hand, the pore volume did not vary more than 15 % between the different samples. This allowed us to study the influence of the surface on the catalytic performance, without much disturbance from variations in porosity. It was calculated from the alkali contents of the samples that the washing procedure removed approximately 70 % of the sodium and 70 - 80 % of the potassium from the washcoat.
0,4 0,3 v
0
E
xh'
/ ~ \ m/ ,! x~ ~ ]- ,I ~ , ~ o O O ,
0,2 0 > 0 0'-0,1 13_
0,0 10
=
220
--z~-- 130 -~.-8o ~ -
"0 0
100 Pore diameter (A)
1000
Figure 3. Pore size distributions of washcoats from different sols after calcination at 600 ~
90 Table 3 Properties of silica washcoats after calcination at 600 ~ Silica-sol base for washcoat
30/80 40/130 30/220
Washcoat Washcoat surface pore area volume
Washcoat alkali content (wt%)
Theoretical K20 content without wash
(m2/g)
(mUg)
Na20
K20
(wt%)
60 109 143
0.23 0.26 0.27
0.099 0.103 0.118
0.063 0.056 0.080
0.23 0.28 0.28
The importance of the removal of alkali for the thermal stability of the silica is clear from Figure 4. The specific surface area for washed samples is partially maintained up to 900 ~ whereas the sample, for which the washing step was omitted, lost most of its surface area already at 750 ~ and virtually all of it at 900 ~ Alkali ions are well known for their destabilizing effect in silica [ 10]. 140
138 125
120
Washed silica
110
IM Non-washed silica
E ~00 80 60
58
51
to
1::: co
40 20 0,1 0
I
500
I
750
I
900
C a l c i n a t i o n t e m p e r a t u r e (~
Figure 4. Surface areas for silica samples from colloidal sol 40/130 after calcination in air during 4 hours at various temperatures. Well-adhering washcoats can be applied by the presented one-step dipping method. Figure 5 shows a scanning electron micrograph of the surface of a typical washcoat. The washcoat thicknesses for our samples are between 20 and 50 pm, whereas dipping with pure colloidal sols results in coatings only a few micrometers thick. The washcoat loading is one of the most important parameters studied here. The loading depends on various aspects in the preparation procedure, as can be seen in Figure 6. This graph shows the washcoat loading in weight per unit volume of the monolith versus the amount of solid material in the dipping slurry. The fraction of solid material in the slurry was varied by varying the amount of silica powder that was added to the colloidal sol.
91
Figure 5. Electron micrograph of the silica washcoat surface. The bar l 5 ~tm.
~ represents
It is clear from Figure 6 that increasing the amount of solid material in the slurry results in thicker washcoats. Experiments with higher fractions of solid material resulted in excessive viscosity of the slurries, which made intrusion of the slurry into the monolith channels impossible. Figure 6 also shows the significance of the method for removal of excess slurry from the monolith channels. Blowing with air results in the highest washcoat loadings, for all conditions. Removal of the excess slurry by centrifugation resulted in lower washcoat loadings. Higher centrifuge speeds lead to lower loadings. This effect is most significant at low speeds. The fact that blowing with air gives thicker coatings may be a consequence of the air flow causing rapid evaporation of the water in the slurry on the channel walls. This causes a rapid increase in film viscosity, followed by solidification. This effect is not seen when centrifugal forces are applied. The latter method results in more even coatings, and better reproducibility. 250 200-
[]
Air blower
o
200 rpm 500 rpm 800 rpm
zx 0
"o 150-
[] o
0
t~ 1000
.....Q.__ _._
0
__.
---.--
---
-I~
""
----
m 50...,.-
0 52
"
5'6
"
5'e
'
do
"
6'2
"
Slurry dry substance (wt%)
Figure 6. Amount of washcoat deposited versus fraction of solid material in the dipping slurry for different methods for removal of excess slurry. The figure legend rpm denotes the centrifuge speed used.
92
3.2 Activity testing The prepared particulate catalysts were tested for methane combustion in order to reveal the influence of (i) the washing step, (ii) the impregnation salt, and (iii) the calcination temperature on the catalytic activity. These three parameters were varied simultaneously between two levels giving 8 experiments. The experimental set-up, as well as the surface areas and temperatures required for 10 % conversion of methane (T10) over these catalysts are given in Table 4. Table 4 Experimental parameters, surface areas and values for T10 for 1 wt% Pd/SiO2 catalysts: space velocity 50,000 h -1, 2 vol% methane in air, heating rate 2 ~ Pd salt Nitrate Nitrate Nitrate Nitrate Chloride Chloride Chloride Chloride
Calcination
Alkali wash
Surface area
temperature (~
(Y/N)
(m2/g)
600 600 900 900 600 600 900 900
Y N Y N Y N Y N
128 105 72 0.1 124 117 72 0.6
T10 (~ 373 617 755 796 527 649 750 793
It is clear from Table 4 that all three studied parameters strongly influence the activity of the combustion catalysts. Calcination at 900 ~ destroyed almost all catalytic activity for all catalysts. The ignition temperatures for the washed samples, calcined at 900 ~ were still about 40 ~ lower than for the unwashed samples, but ignition occurred at such high temperatures that homogeneous ignition can not be excluded. The effect of type of palladium salt is strong, but only at low calcination temperatures. The chloride-based, washed catalyst ignited at about 150 ~ higher temperature than the nitrate-based sample. This confirms the results presented by Simone et al. [11] who found the presence of chlorine on the catalyst surface to be the main cause for inferior activity of chloride-based Pd/alumina catalysts for methane combustion. The effect of the alkali wash was also most pronounced at low temperatures, the non-washed samples having an ignition temperature about 120 ~ higher than the washed catalysts. This fact can not only be explained by the differences in surface area, since these are too small. On the other hand, the alkali ions may have accelerated the sintering of the metal particles during calcination [ 12]. These results incited us to use washed silica, washed washcoats, and palladium nitrate in the following experiments with monolithic catalysts. All samples in this series were calcined in air at 600 ~ during 4 hours before the activity tests. The tests results of the catalysts based on colloidal sol 30/220 are shown in Figure 7. It is clearly seen that, for a given surface area, the washcoat loading, and hence the palladium loading per unit volume of monolith, has a strong positive influence on the activity of the combustion catalysts. The influence of washcoat load is also seen for the catalyst samples with lower washcoat specific surface areas. These results are depicted in Figure 8, showing the temperatures required for 10 % conversion in methane combustion versus the washcoat load for different catalysts.
93
70 60 C
o
50
[]
150 g/I
z~
89 g/I
0
59 g/I
f-
40 o 30 20, 100-
T
400
300
500
600
G a s inlet t e m p e r a t u r e (~
Figure 7. Methane conversion versus gas inlet temperature for catalysts with varying washcoat loadings. Catalysts: SA = 143 m2/g, 4 %wt Pd (washcoat basis), sv = 50.000 h -1, methane concentration 2 vol% in air, heating rate 2 ~ It is obvious from Figure 8 that a larger surface area of the washcoat strongly promotes ignition at lower temperatures. Lower washcoat loadings, and hence lower palladium loadings, are needed for washcoats with greater surface areas. It can also be seen from Figure 8 that for the large surface-area sample, the increase of the washcoat loading over 100 g/1 does not decrease T]o significantly, indicating a less efficient use of the active material in this case. It is unclear from our current results if this is due to mass transfer limitations. On the other hand, the improvement in catalyst performance with increasing washcoat loading is much more pronounced for the samples with lower surface area. It is not clear at this moment, whether differences in palladium dispersion for the different supports also contribute to the differences in activity. 700 ~o~ 600
% O._
ro v O 500
~ ~
~
60 m2/g
--ZX-- 109 m2/g ----O--- 143 m2/g
I--
o
400
300 50
2()0
Washcoat
loading (g/I)
Figure 8. Temperatures required for 10 % conversion of methane (conditions as in Figure 7) for catalysts with various washcoat loadings and washcoat surface areas.
94 No conversion data are presented for gas inlet temperatures over 550 - 600 ~ since this was the calcination temperature for all monolith catalysts. The catalysts presented here are therefore not particularly suitable as high-temperature combustion catalysts for hydrocarbons. However, they may have interesting properties for removal of volatile organic compounds at lower temperatures, which is now under investigation.
4. CONCLUSIONS Well-adhering and even washcoats are obtained by washcoating whiskers-covered metal monoliths. The combination of a colloidal silica sol and fine silica powder in the washcoating slurry allows strict control over the pore size distribution of the washcoat, as well as the thickness or loading. Washcoats with single pore size distributions were prepared in this study It is shown that the washcoat with the highest specific surface area had superior properties as a support for combustion catalysts. Furthermore, the thickness could easily be varied in this onestep dip-coating process, thus allowing for optimization of the washcoat design. This method also allows for preparation of washcoats with bimodal pore size distributions, by using other powder materials. This may give interesting opportunities for other applications, in which mass and heat transfer play an important role.
ACKNOWLEDGMENTS
The financial support to this work, given by the Swedish National Board for Industrial and Technical Development (NUTEK) is gratefully acknowledged.
REFERENCES
1. K.C. Taylor, in J.R. Anderson and M. Boudart (Eds.), Catalysis - Science and Technology, Vol. 5, Springer-Verlag, Berlin, 1984, 119 - 170. 2. S. Irandoust and B. Andersson, Catal. Rev. - Sci. & Eng., 30(3), 341-392 (1988). 3. M.F.M. Zwinkels, S.G. J~is, P.G. Menon, and T.A. Griffin, Catal. Rev. - Sci. & Eng., 35(3), 319-358 (1993). 4. H. Sadamori, in 10th Symp. on Catal. Comb., Japan, November 1, 1990, In Japanese. 5. T. Furuya, T. Hayata, S. Yamanaka, J. Koezuka, T. Yoshine, and A. Ohkoshi, ASME Paper 87-GT-99 (1987). 6. R.A. Dalla Betta, N. Ezawa, K. Tsurumi, J. Schlatter, and S.G. Nickolas, US Patent No. 5,183,401 (1993). 7. I.-M. Axelsson, L. Ltiwendahl, and J.-E. Otterstedt, Appl. Catal., 44, 251-260 (1988). 8. R.L. Nelson, J.D.F. Ramsay, J.L. Woodhead, J.A. Cairns, and J.A.A. Crossley, Thin Solid Films, 81,329-337 (1981). 9. W. Cao, R. Gerhardt, and J.B. Wachtman, J. Am. Ceram. Soc., 71(12), 1108-1113 (1988). 10. R.K. Iler, The chemistry of silica, Wiley, New York, 1979. ll.D.O. Simone, T. Kennelly, N.L. Brungard, and R.J. Farrauto, Appl. Catal., 70, 87-100 (1991). 12. C.N. Satterfield, Heterogeneous catalysis in industrial practice, 2nd ed., McGraw-Hill, New York, 1991.
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
95
Preparation of Supported Catalysts by Equilibrium Deposition -Filtration A. Lycourghiotis Department of Chemistry - Institute of Chemical Engineering and Chemical Processes of High Temperatures, University of Patras, P.O. Box 1414, University Campus, GR 26 500 Patras, Greece. The subject of the present lecture is the preparation of supported catalysts using the method of "Equilibrium Deposition - Filtration" for which we propose the term EDF. We propose the following structure for the lecture: - First, to present the problems encountered when we use the usual impregnation techniques for preparing supported catalysts and to describe EDF. - Second, to present the methodologies developed in order to increase the concentration of the deposition sites located on the surface of the industrial supports. This is necessary in order to prepare supported catalysts with high active surface using EDF. - Third, to compare some surface and catalytic properties for catalysts prepared by dry or non-dry impregnation with corresponding catalysts prepared by EDF. - Finally, to present the very recent developments concerning the mechanisms of deposition of species containing catalytically active elements from aqueous suspensions to the surface of the industrial, oxidic, supports. 1.
IMPREGNATION
SUPPORTED
TECHNIQUES
USED
FOR
PREPARING
CATALYSTS.
It is known that the catalytically active species rarely have more than one of the following properties which are necessary in order to be useful in industry. - sufficiently high specific surface area. - convenient pore size and particle size distribution.
96 - high mechanical strength. -
sufficient resistance to sintering, fouling and poisoning.
It is therefore necessary to disperse these active species on the surface of a support exhibiting more than one of the above properties. The most important oxidic supports used in industry are y-AI203, SiO2 and TiO2. The deposition of active species on the surface of these supports is the most critical step in the preparation of supported catalysts. This deposition is usually performed by impregnating powder or pellets of the carrier in an aqueous solution containing one or more inorganic species of the element to be deposited. In fig. 1 we schematize the various techniques of impregnation used. " Dry impregnation ---> drying ---> calcination (DI). Successive 9 dry impregnations ---> drying ---> calcination (SDI).
9
Non-dry impregnation ---> slow vaporization ---> calcination (NDI). and drying Impregnation 9 in the presence of a prercipitating agent
---> drying ---> calcination (DP).
( D ep 0 sitio n-p r ec ip ita tio n) Impregnation using organometallic or corbonyl compounds (Anchoring)
---> filtration---> drying ---> calcination (An)
Non-dry 9 impregnation ---> long time ---> filtration ---> drying ---> equilibration
---> calcination (EDF)
Figure 1. Illustrates the various types of impregnation used for preparing supported catalysts It can be observed that in all cases the preparation involves drying and finally calcination for stabilizing the supported crystallites on the support surface.
9? SDI and NDI are followed when the amount of the active element which must be deposited cannot be dissolved in a volume of the impregnating solution equal to the pore volume of the impregnating support. It is obvious that a non-empirical development of the impregnation techniques presupposes a good understanding of the mechanisms followed for the deposition of the catalytically active species on the support surface. These are illustrated in table 1. Table 1 Compiles the mechanisms followed for the deposition of the catalytically active species on the support surface Impregnation technique
Prevailing deposition mechanism
Size of the supported crystallites
Simplicity of the technique
DI, SDI, NDI
Uncontrolled precipitation, mainly inside the pores of the support, in the step of drying
relatively large
very simple
DP
controlled precipitation in the step of impregnation
relatively small
quite simple
An
reaction with the support surface
very small
quite complicated (usually)
EDF
adsorption on the support surface or reaction with the support surface
very small
very simple
Although the classical types of impregnation, DI, SDI and NDI, result to the formation of relatively large supported crystallites they are used extensively in practice. There are two reasons for this. First because they are very simple. Second because using these techniques there is no practically limitation concerning the amount of the active element which can be deposited. The second
98 characteristic allows the achievement of sufficiently high active surface, though dispersity is low, by simply increasing the amount of the deposited phase. These techniques are therefore suitable in the cases where the element to be deposited is inexpensive. Controlled precipitation in the step of impregnation could be achieved by adding several substances in the impregnating suspension. Relatively small supported crystallites are obtained in this case and the so called "deposition precipitation" is quite promising for preparing supported catalysts [1-5] Extremely small supported crystallites are obtained when the reaction with the support surface is the predominant deposition process. The deposition by reaction may be maximized by using organometallic or carbonyl species to deposit the active element [e.g. 6-10]. However, the procedure required for anchoring is usually quite complicated. Moreover, the actually high active surface achieved is rather unstable. Therefore it usually decreases considerably during calcination. Presumably, for these reasons the grafted catalysts have not yet found industrial applications. Extremely small supported crystallites are also obtained when adsorption on the support surface or reaction with the support surface is the predominant deposition process. A very simple way to increase their contribution on the whole deposition is to use EDF. This technique involves the following steps. Impregnation 9 of the support in a relatively large volume of an electrolyte solution containing the species to be deposited. Equilibration, 9 under stirring, for many hours at given values of impregnation parameters (concentration, pH, temperature, ionic strength). 9 "drying "calcination Following EDF we may, in fact, increase the relative contribution of adsorption and reaction, which take place in the step of the long-time equilibration, with respect to the uncontrolled precipitation taking place in the step of drying [1141]. However, although EDF results to small supported crystallites and it is therefore
99 suitable for depositing expensive elements, it has the following weaknesses. Some times provides catalysts with low active surface. This is because the surface concentration of the depositing sites is frequently low limiting thus the amount of the active species deposited by adsorption or reaction. It is, therefore, obvious that a successful application of EDF for preparing supported catalysts with high active surface presupposes the following: - First, the elucidation of the nature of the depositing sites. - Second, the development of methodologies for regulating the surface concentration of these sites. 2. D E P O S I T I O N
SITES
(NATURE
AND
REGULATION).
Let's start with the nature of the deposition sites. It is known that the surface of the oxidic supports is fully hydroxylated in electrolyte suspensions. According to the surface ionization model [42] the surface hydroxyls may be protonated or deprotonated. The following acid-base equilibria describe the charging the surface mechanism. Ki~t
SOH~2 ~
4-
SOH + Hs
int
K2
SOH ~
4-
SO-+ Hs
(1)
H~ ~, H~ By SOH, SOH2 + and SO-we denote, respectively, the neutral, protonated and deprotonated surface hydroxyls. Hs+ and Hb+ represent the hydrogen ions on the surface and in the impregnating suspension, respectively. K1 int and K2int represent the equilibrium constants for the surface protonation-deprotonation reactions. The suspension pH at which the concentration of the SOH2 + groups is equal to the concentration of the SO- groups is defined as the point of zero charge (pzc). At pH's higher than pzc the deprotonated surface hydroxyls predominate, whereas at pH values lower than pzc the protonated surface hydroxyls are in excess. At pH equal to pzc the neutral surface hydroxyls usually predominate. It may be easily proved [43] that for simple oxides pzc is equal to (pK 1int + pK2int )/2. It is, therefore, a surface property depending exclussively on the nature of the support surface.
100
As we shall see later the study of the mechanisms of deposition of negative and positive species on the surface of the oxidic supports demonstrated the following. - The SOH2 § groups are responsible for the creation of adsorption sites for negative species (e.g. CrO42-, M070246-, ...). - The SOH groups are involved in the deposition process by reacting with negative species (e.g. MoO42-, 0r2072-, HCrO4-). - The SO- groups are responsible for the creation of adsorption sites for positive ions (e.g. Co 2+, Ni2+). The concentration of the various types of surface hydroxyls, the surface acidity constants (K1int and K2int ), the pzc and the surface charge ao= F ([SOH2 +] - [SO-]) are determined using potentiometric titrations [44-54]. In the case of titania which is a mixture of simple oxides (anatase and rutile) only the values of pzc and ao may be determined [55]. Now we shall try to present the methodologies developed in order to regulate the pzc and the concentration of the SOH2 +, SOH and SO- groups responsible for the creation of the deposition sites. The following methods will be discussed: - The change of pH of the impregnating solution. - The change of temperature of the impregnating solution. - The doping of the support. From equilibria ( 1 ) i t may be seen that an easy way to regulate the concentration of the various types of surface hydroxyls is to change the pH of the impregnating solution. Fig. 2 shows the variation with pH of the concentration of SOH2+ and SO- groups for y-AI203 and SiO2 as well as the variation of ao for TiO2.
.1.o 0.5
Figure 2. Variation with
'~" E 0.0
0
o
0.5
r
~9
-15
b@@~ 1.0 1.5
~
"g
g
-~ pH
~
~
,30
pH of the SOH2 § and SO- groups for y-AI203 (13) and SiO2 (A) and of the surface charge for TiO2 (O) as well.
101
This variation is in agreement with what it is anticipated from equilibria (1) and shows that decrease (increase)in pH should increase the deposition capacity of these supports for negative (positive) species. The traditional method of changing the pH may be rendered problematic in some cases. For instance, at the pH value where deposition by adsorption or reaction is enhanced, the species to be deposited may be unstable and the support may be partially dissolved. Moreover, deposition by spontaneous precipitation may take place in considerable extent and this results to large supported crystallites. From these it is obvious the necessity to develop alternative methodologies for regulating the concentrations of the various types of surface hydroxyls. This has been realized by us some years ago. From equilibria (1)it may be seen that the only way to change the concentration of the various types of surface hydroxyls at constant pH is to change the values of the surface acidity constants. The idea to change the values of these constants and therefore the value of pzc by altering the temperature of the impregnating suspension comes quite easily. This very simple idea has been tested for a first time on y-AI203 [47] and then on SiO2 [49] and TiO2 [55]. It was concluded that, indeed, the change in the impregnating temperature is an attractive methodology in order to change the values of the surface acidity constants and therefore of pzc as well as of the concentration of the various types of surface hydroxyls and ao at constant pH. Specifically, it was found that increase in the impregnating temperature of the y- alumina suspensions causes a decrease in the values of the surface acidity constants and therefore an increase in the pzc, [SOH2 +] and [SOH] whereas it decreases the value of [SO-]. Similar trends were obtained in the case of titania while the opposite trends were obtained for silica. Typical results which show the regulation of the acid-base behaviour of the industrial supports achieved by changing the impregnating temperature are illustrated in fig. 3. An alternative way to change the acid-base behaviour of industrial supports is to modify their surface by doping it with various amounts of Na +, Li+ and F-ions [44-46,49]. It was found that Na + and Li+ doping causes a decrease in the values of the surface acidity constants and therefore an increase in the pzc and [SOH2+], whereas it decreases the value of [SO-]. The opposite trends were achieved after doping with F-ions. Typical results are illustrated in fig. 4. From the previous considerations the following conclusions may be drawn. Decrease in pH, increase in impregnating temperature and doping with Na + and Li+ ions should increase the surface concentration of the deposition sites -
102
.
2.0
-r
o
Figure 3. Variation of the concentration of the charged groups (AIOH2 + and AIO-) with the temperature of the yAI203 suspension at ionic strength corresponding to 0.1 mol dm -3 KNO3. Numbers 1, 2, 3, 4, 5, 6 and 7 on the curves correspond to 10, 15, 20, 25, 35, 40 and 50 oC.
1.5
,<
~'- 1.0 I
E 0.5
r
~0.0 03
0.5
M I
o
~
1.0
1.5 6
pH
+
r
-r-
Figure 4. Dependence of the concentration of the charged groups on suspension pH in
4
o
N
I
2
E E
03 03
E~ v 2 .<
!
3
I
pH
;
12
Na-doped y-AI203, 0.1 M KNO3, 25 oC. Numbers 1,2, 3, 4, 5 and 6 on the curves correspond to 0.226, 0.309, 0.392, 0.621, 0.984 and 1.560 mmoles of Na per gram of the carrier.
of y-alumina and titania for negative species. Increase in pH, decrease in the impregnating temperature and doping with F-ions should increase the surface concentration of the deposition sites of yalumina and titania for positive species. -
- With the exception of the effect of temperature for which the opposite trend has been observed similar predictions may be done for silica. Extensive work done by our group in the last years on various catalytic systems confirmed the mentioned predictions in the measure which these have been tested so far. This allowed us to change properly the impregnation parameters in order to increase considerably the amount of the active element deposited on a support surface. Table 2 illustrates the catalytic systems examined.
103
It may be observed that in the most of cases the change in pH was employed in order to increase the extent of deposition. Therefore, much work should be done in order to be completed the study of the remainder variables of impregnation concerning the systems examined so far. Table 2 Deposition of active elements on industrial supports using Equilibrium DepositionFiltration (EDF) Support
Active Element
Importantspecies
Direction of change
in the solution
Optimum
Maximum
of impregnation
value of the
amountof
parameters for
impregna-
active
tion variable
elementM.
Increasing deposition y-AI20 3
Mo
MoO42-,Mo70246-
Decrease in pH
4.1
12.77
Increase in the amount of Na+
2.47 mmol g-1
33.67
Increase in the amount of Li+
2.47 mmol g" 1
11.60
55 0C 3.56
14.28 17.60
6.8
y-AI20 3
W
W O 42-
Increase of impregnation temperature decrease in pH
y-AI20 3
Co
Co2+
increase in pH
Ni2+
increase in the amount of Fincrease in pH
y-AI20 3
y-AI203
Ni
Cr
SiO2
Cr
TiO2*
Cr
TiO2*
Mo
TiO2**
V
HCrO4-, CrO42-, Cr2072-
increase in the amount of F" decrease in pH
HCrO4-, CrO42-, Cr2072HCrO4-, CrO42-, Cr2072MoO42-, Mo70246VO43-, V 100286-
*mixture of anatase and rutile **anatase
weight % M
3.818 mmol g" 1 6.3
0.45 0.73 0.43
3.818 mmol g 1 4.0
0.85 1.99
decrease in pH
3.0
0.30
decrease in pH
4.0
0.28
decrease in pH
4.6
4.10
decrease in pH
4.5
2.37
104
However, the methods of changing the impregnating temperature and the doping tested in several systems based on y-alumina have resulted to a considerable increase of the extent of deposition. Table 3 Compiles important physicochemical characteristics for Mo(vi)/y-AI203 catalysts prepared by EDF and SDI. [calcination at 500 oC for 5 hrs]. Characterization
Physicochemical characteristic
Samples
method
9.3 %MoO3(EDF) 10.1%MoO3(SDI) 17.0 %MoO3(EDF) 19.9 %MoO3(SDI)
Temperature H2 consumed at 500 ~ for bilayer species [O]* 0.71 programmed reduction H2 consumed at 850 ~ for monolayer species [O], [T]** 0.73
Thermogravimetric
1.94
Temperature of sublimation of the supported phase/~
analysis Laser Raman
1.29
absorption band/cm-1
930 895
750
945 [T],[O]
953 [T],[O]
spectroscopy
(360, 215 and 377)*** 954 [T],[O]
953 [T], [O]
(215, 995, 821,380)*** NO chemi-
number of active centers
16.3
10.8
sorption
(pmol NO per g of catalyst)
37.8
17.3
measure of the uncovered
0.0 0.0
0.5 0.1
002 chemisorption
support surface (pmol CO2 per g of catalyst)
* Indicates Mo(vi) species in octahedral symmetry. ** Indicates Mo(vi) species in tetrahedral symmetry. *** Indicates polymeric Mo species and supported MoO3 crystallites.
105
Table 4 Compiles important physicochemical characteristics of Co-Mo/y-AI20 3 catalysts in which the Mo(vi) phase has been deposited by EDF and SDI* Characterization Physicochemicalcharacteristic
Samples
method
9.1%MoO3(EDF) 9.9 %MoO3(SDI) 2.2 %CoO(DI)
2.2 %CoO(DI)
16.3 %MoO3(EDF) 19.1%MoO3(SDI) 3.4 %CoO(DI)
4.3 %CoO(DI)
Temperature H2 consumed at 500 ~ for bilayer Mo species [O]** 0.60 programmed reduction H2 consumed at 850 ~ for monolayer Mo species [O], [1]*** 0.75
1.20 1.44
NO chemi-
numberof active centers
74.9
63.2
sorption
(IJmol NO per g of catalyst)
89.1
75.2
* In all cases the Co(II) ions have been deposited by DI. Final calcination temperature: 500 oC for 5 hrs. ** Indicates Mo(vi) species in octahedral symmetry. *** Indicates Mo(vi) species in tetrahedral symmetry. 3. C O M P A R I S O N
OF CATALYSTS PREPARED
CORRESPONDING CATALYSTS IMPREGNATION TECHNIQUES.
PREPARED
USING USING
EDF WITH THE THE
CLASSICAL
Having prepared supported catalysts by EDF with relatively high loading in active element, the next reasonable step is to compare these catalysts with corresponding ones prepared using the classical impregnation techniques, namely DI, SD! and NDI. The comparison should be directed first to the physicochemical properties, mainly to those related with the magnitude of active surface, and secondly to the catalytic properties. In the frame of the present lecture we limit ourselves to present few but typical results drawn from our systematic studies on the catalytic systems: Mo(vi)/y-AI203, W(vi)/y-AI203 and V(vi)/TiO2.
106
Let's start with the first system. Important physicochemical characteristics of the Mo(vi)/u catalysts prepared by EDF and SDI are illustrated in table 3. TGA results indicate that the Mo(vi) phase formed in the samples prepared by EDF is strongly bounded on the support surface compared with that formed on the corresponding samples prepared by SDI. Moreover, the TPR and LRS results show an increasing Mo polymerization on the support surface leading to the formation of supported MoO3 for the specimens prepared by SDI. Finally, the NO and CO2 chemisorption clearly shows that the active surface is larger in the specimen prepared by EDF as it compared with the active surface obtained for the corresponding specimens prepared by SDI. As the Mo(Vi)/y-AI203 specimens are frequently used as precursor solids for preparing Co-Mo/y-AI20 3 hydrotreatment catalysts the critical question raised in this point is whether the benefit to use EDF is maintained after the Co(ll) deposition, by DI, on the Mo(Vi)/y-AI203 samples. Table 4 shows that this is, in effect, the case. Table 5. Rates, R, of hydrodesulfurization of thiophene at three different temperatures, over Co-Mo/y-AI203 catalysts in which the Mo(vi) phase has been deposited by EDF and SDI*. Catalyst
R x 105/mol min-1 g-l(MoO3 +COO) 250 0C 275 0C 300 OC
9.1%MoO3(EDF) 2.2 %CoO(Ol)
17.5
40.7
91.1
9.9 %MoO3(SDI)
13.0
30.5
68.5
16.3 %MoO3(EDF) 3.4 %CoO (DI)
10.9
28.4
62.4
19.1%MoO3(SDI) 4.3 %CoO (DI)
6.8
19.4
44.5
2.2 %CoO(Ol)
* In all cases the Co(II) ions have been deposited by DI. Final calcination temperature: 500 oC for 5 hrs.
107
The results illustrated in table 4 explain why the catalysts in which the Mo(v+) phase has been deposited by EDF are more active corresponding
compared
ones in which the Mo(vi) phase has been deposited
with
the
by SDI.
(Table 5). Let's now examine the second catalytic system, namely W(vi)/y-AI203 This has been studied extensively in the last years. A sufficient number of specimens has been prepared by EDF, characterized using various techniques and tested on Table 6. Compiles important physicochemical characteristics for W(Vi)/y-AI203catalyst s prepared by EDF and NDI. [calcination at 600 ~ Characterization
for 6 hrs]
Physicochemicalcharacteristic
method
Samples 11%WO3 (EDF) 11%WO3(NDI)
Thermogravimetric
Loss of weight due to the sublimation of
analysis
the supported phase. %
B.E.T.
Specific surface area m2 g-1
0.7
1.5
114"
87
55
85
Temperature p r o Reducibilitya.u. grammed reduction H2 consumed at 740 0C for bilayer species [O]**
0.21
0.64
H2 consumed at 900 0C for monolayer species [O], [T]*** Diffuse reflectance spectroscopy
F (R| at 210 nm due to the [1"] species 35
24
56
49
F (R| at 320 nm due to the [O] species X-ray photoelectron
(Iw/IAI)XPS x 103
spectroscopy NO chemisorption
Numberof active sites (pmol NO per g of catalyst)
* The specific surface area of the support is equal to 120 m 2 g-1 **Indicates W(vi) species in octahedral symmetry. ***Indicates W(vi) species in tetrahedral symmetry.
1.44
0.20
108
several reactions. Here we present only certain representative results which allow us to compare directly EDF with the classical impregnation technique of NDI. Important physicochemical properties for W(vi)/y-AI203 catalysts prepared by EDF and NDI are compiled in table 6. Inspection of this table clearly shows that the application of EDF leads to supported w(vi)/y-AI203 catalysts with relatively high active surface (XPS and NO chemisorption). Moreover, the supported phase achieved is mainly in the form of a monolayer strongly bounded on the support surface (TGA, TPR, DRS). Finally, it should be pointed out that the application of EDF does not disturb the texture of the support (BET). In the contrast to that a drastic decrease in the SSA is observed for the sample prepared by NDI. This is rather expectable as the very thin pores of y-alumina may be closed during precipitation and formation of relatively large supported crystallites. In view of these results one may expect that the "EDF catalyst" should be more active compared with the corresponding "NDI catalyst". This is, in effect, the case as it may be observed in table 7. Table 7. Rates, R, of hydrogenation of cyclohexene at three different temperatures over W(Vi)/y-AI203 catalysts in which the Mo(vi) phase has been deposited by EDF and NDI. Catalyst
R x 105 / mol min -1 g-l(w03 ) 2750C 3250C 3750C
11% WO3 (EDF) 11% WO3 (NDI)
35 15
105 51
240 100
In the case of the hydrodesulfurization (h.d.s.) processes the W(Vi)/y-AI203 specimens are mainly used as precursor solids for preparing Ni-W/y-AI203 catalysts. It is therefore necessary for us to examine whether the better physicochemical characteristics achieved by the use of EDF are maintained after the Ni(ll) deposition, by Di, on the W(vi)/y-AI203 samples. Table 8 shows that this is, in effect, the case.
109
Table 8. Compiles important physicochemical characteristics of Ni-W/y-AI203 catalysts in which the W(vi) phase has been deposited by EDF and NDI*. Characterization
Physicochemical characteristic
method
Samples
10.8 %WO3(EDF) 10.8%WO3(NDI) 1.3 % NiO (DI)
B.E.T.
Specific surface area
Temperature proReducibilitya.u. grammed reduction H2 consumed at 740 0C for bilayer species [O]**
1.3 % NiO (DI)
130
110
50
80
H2 consumed at 900 0C for monolayer species [O], IT]*** 0.17 Diffuse reflectance spectroscopy
F (R=) at 210 nm due to the [T] species
29
0.78
18
F (R| at 320 nm due to the [O] species NO chemisorption
Numberof active sites
12.6
2.9
(l~mol No per g of catalyst) * In all cases the Ni(II) ions have been deposited by DI. Final calcination temperature: 5500C for 6hrs. ** Indicates W(vi) species in octahedral symmetry. *** Indicates W(vi) species in tetrahedral symmetry. The results illustrated in table 8 explain why the catalysts in which the W(vi) phase has been deposited by EDF are more active
compared
with the
corresponding ones in which the W(vi) phase has been deposited by NDI (table 9). The last example that I would like to present refers to the V(vi)/'l'iO2 (anatase) catalysts. From the ratio of the XPS intensities of V2p3/2 to Ti2p photoelectrons and from the AEM results (table 10, Fig. 5) it may be inferred that the active surface of the catalyst prepared by EDF is higher compared with that obtained for the catalyst prepared by NDI. Moreover, the ratio of the XPS intensities of V(v) to V(iv) photoelectrons indicate that relatively stronger
"supported
phase-support"
interactions are exerted in the EDF catalyst. Both observations explain why in the EDF sample is not formed crystalline V205 (FT-IR).
llO
Table 9. Rates, R, of hydrodesulfurization of thiophene at three different temperatures, over Ni-W/y-AI203 catalysts in which the W(vi) phase has been deposited by EDF and NDI*. Catalyst
R x 105/m mol min-1 g-l(wo3 +NiO) 225 0C 250 0C 275 oC 300 0C
10.8 %WO3(EDF) 1.3 %NiO(DI)
35
108
258
640
10.8%WO3(NDI) 1.3%NiO(Ol)
15
53
111
232
* In all cases the Ni(II) ions have been deposited by DI. Final calcination temperature: 550 oC for 6 hrs.
Table 10. Compiles important physicochemical characteristics for V(V)TiO2 (anatase) catalysts prepared by EDF and NDI [calcination at 500 oC for 5 hr]. Characterization method B.E.T.
Physicochemical characteristic
Specific surface area m2 g-1
Samples 3.6 % V205 (EDF)
3.6 % V205 (NDI)
53
49
XPS
(I)V2p3,.2/(1)Ti2p
165
155
XPS
V(v)/v(iv) *
0.63
0.78
FT-IR
1023 cm- 1
__
crystalline V205
* Surface atomic ratio.
111
150
i
mm -
" ..
Figure 5. V/Ti ratio calculated after AEM data for samples containing 3.6 wt% V205/TiO2 prepared by EDF and NDI.
!!,i!i |1
1
I
6
I
!
11
16
! l llt il,, ll,,,,lll I I I I / I Y F
21
26
FF
31
FFV
36
No of particle
4. M E C H A N I S M Oualitative
OF
DEPOSITION
OF ACTIVE
SPECIES
USING
EDF:
approach.
The general conclusion that may be drawn from the previous results is that
Equilibrium Deposition - Filtration is a simple and attractive method for preparing supported catalysts with relatively high active surface and quite good physicochemical and catalytic properties. The critical question raised in this point is whether we may further develop this methodology in order to prepare more efficient catalysts. We hope that it will be possible in the future provided that we shall be able to control more precisely the deposition process rendering it selective as much as possible. In fact, it is known that the catalytic activity depends on both: on the magnitude of active surface and on the quality of the active sites. The later is possible to be related with the particular species deposited on the support surface during preparation. Thus, the achievement of a selective deposition is presumably the key factor for preparing catalysts with high activity per active site. But the achievement of a selective deposition in a given catalytic system requires the knowledge of the deposition mechanism. In the remainder of the lecture we present the recent developments concerning this subject. Specifically we describe the methodology followed for investigating the deposition mechanisms as well as the mechanisms established in some important catalytic systems. Describing the methodology we approach the deposition mechanism first qualitatively and then quantitatively. Let's start with the qualitative approach. In table 11 are illustrated the most important mechanistic points, the methodology followed to investigate each one of
112
that points and the conclusions drawn. The study of variations of the extent of deposition with the concentration of the SOH2 +, SOH and SO- groups allows to investigate the nature of the deposition sites. A typical example of these variations for negative ions (MOO42-, M070246-) is illustrated in fig. 6. Table 11 Compiles the most important mechanistic points of the deposition process, the methodologies followed to investigate these points and the qualitative conclusions drawn. mechanistic p o i n t nature of the deposition sites
methodology study of the variations
conclusion SOH2+, SOH: responsible for the creation
of extend of depositions
of deposition sites for negative species.
with the concentration
SO-: responsible for the creation of the
of the SOH2*, SOH and
deposition sites for positive species.
SO- groups. plane of the double
microelectrophoresis,
the adsorption of the active species takes
layer in which the acti-
potentiometric
place on the Inner Helmholtz Plane of the
ve species are depo-
titrations
double layer.
sited. Kinetic characteristics
study of the deposition
Langmuir type localized deposition on al-
of the deposition
isotherms
most energetically equivalent deposition sites. Lateral interactions are exerted between the adsorbed species.
It may be observed that the extent of deposition increases with the concentration of the protonated surface hydroxyls of y-alumina. Moreover, it may be observed that the extent of this deposition decreases with the concentration of the neutral surface hydroxyls. It should be noted that there is no practically deposition in the case where the negative groups predominate. The above could suggest that the deposition of the negative species takes place only on the protonated surface hydroxyls but careful inspection of the figure shows that
113
AIOH / sites nm-2 6,0 9 55
6.4
6.8
i
7.2
i
7.6
i
8.0
i
55
E
E
o
7
E 0
5 30 10
510 25 ~
15
I
31
0.05
0.55
1.65
AIOH2 + /
1.;5
sites
2.05
Figure 6. Saturation surface Mo(vi) concentration achieved at various temperatures as a function of the concentration of the protonated (curve a) and neutral (curve b) surface hydroxyls regulated by varying the temperature of the impregnating suspension of y-alumina. Temperature values are indicated in degrees centigrade.
n m -z
A[OH /
sites nm -z
0
2
4
6
8
0.0 0
2
4
6
8
0.8 'i'
4,23o
?
E s ,
E
~" 0.6
oo..
w
'-'=sY
.
E 0
j P.la6
,
,
t.2
o~
,
7.7 AIOH + AIOH2+ /
0.4
8.2
sites nm -z
Figure 7. Saturation surface W(vi) concentration obtained at various pH values as a function of the sum of concentration of the protonated and neutral surface hydroxyls regulated by varying the pH of the impregnating suspension of y-alumina. The values of pH are indicated.
AiO- /
sites nm -2
Figure 8. Saturation surface concentration of the 002+ (curves a and c) and Ni2+ (curves b and d)ions obtained for the doped carrier (F-x-y-AI203) as a function of the concentration of the deprotonated (curves a and b) and neutral (curves c and d) surface hydroxyls regulated by doping y-alumina with various amounts of fluoride ions.
]14 considerable deposition takes place even in the case where the surface of yalumina is fully covered by neutral surface hydroxyls ( total concentration of surface hydroxyls: 8 hydroxyls/nm2). Therefore, the participation of the neutral surface hydroxyls on the deposition of negative species can not be excluded. The participation of both SOH2 + and SOH groups in the deposition of negative species is more clear in other systems, for example in the deposition of the w(vi) negative species on y-alumina (fig. 7). In the contrast to that a few studies on the deposition of positive ions [Co 2+ and Ni 2+ ions on y-alumina] have suggested that this deposition takes place almost exclusively on the deprotonated surface hydroxyls (fig. 8). Let's now examine the second important mechanistic point. As the surface of the oxidic supports is charged in electrolytic solutions, an electrical double layer is formed between the support surface and the solution. Various models have been developed to describe the oxide/solution interface [43, 56-63]. It has been widely accepted that the triple layer model describes better this interface in the most of cases [33-39, 41]. A simplified picture of this model is illustrated in fig. 9. It should be noted that the SOH2 +, SOH and SO- groups are considered to be localized on the surface of the support (zero plane). On the other hand the centers of the water molecules surrounding the surface of the support particles constitute the so called Inner Helmholtz Plane (IHP). Moreover, the counter ions (of the indifferent electrolyte) are located on the Outer Helmholtz Plane (OHP). Very near to this plane is the shear plane and then the diffuse part of the double layer and the bulk
SOLID
Model plant
SOLUTION IHP OHP Shear
pl?ne I I I
!
PotenUal= I I
1! Charge denoitlee r
Col~oltle=
=m
==
1I
1I
I I I I
I I I
C,',
I
I I
Figure 9. Schematic representation of the triple layer model
115
solution. The charge and potential from the surface up to shear plane may be determined using microelectrophoresis. It is now understandable why the study of the microelectrophoretic curves achieved in the presence and absence of the species to be deposited offers an easy way to investigate the plane on which these species are located. Let's take an example, namely the deposition of HxWyOz x species on y-alumina. Fig. 10 illustrates the variation of the electrokinetic charge of y- alumina with the pH of the suspension both in the presence and absence of the tungstate species. You may observe that in the presence of these species the electrokinetic charge is always negative even at pH's below pzc where the surface should be positive. This precludes the location of the HxWyOz x- species on the diffuse part of the double layer. In fact, in such a case the charge from the surface up to shear place, namely the electrokinetic charge should be positive. Therefore, results similar to those illustrated in fig. 10 strongly suggested that the catalytically active species are located in the IHP. However, in some cases instead of these results it has been obtained a simple shift in the value of the isoelectric point to lower pH. A typical example is illustrated in fig. 11. It concerns the deposition of the HCrO4, CrO42- and Cr2072 ions on y-AI203. This clearly shows
1.5
12 (=)
1.0 Eo 0.5
T
6
0.0 -
o
.
5
-1.0 -1"52
(
4
6
I~
1()
12
pH Figure 10. Variation of the electrokinetic charge of y-alumina with the pH of the suspension at 25 oC: (a) y-alumina, 0.16 mol dm -3 NH4NO3 solution and (b) y-alumina, ammonium tungstate solution Co= 1x 10-3 W(vi) mol dm-3, I=0.16 mol dm -3 NH4NO3.
-123
4
5
6
-/
8
9
10
pH
Figure 11. Variation of the ~-potential of y-alumina with the pH of the suspension at 25 oC: (a) y-alumina, 0.01 mol dm -3 NH4NO3 solution and (b) y-alumina, ammonium dichromate solution Co= 3x 10-3 Cr(vi) mol dm -3, 1=0.01 mol dm -3 NH4NO3.
116
that the Cr(vi) negative species should be adsorbed in the IHP. In fact, as in this case the charge from the zero to the shear plane decreases, more acidic pH is required to increase the concentration of the SOH2 + groups (see equilibria 1) and therefore to compensate the additional negative charge. As already mentioned the deposition of the Cr(vi) negative species in the 1HP is expected to disturb the equilibria (1). In fact, the decrease in the concentration of the free SOH2 + groups due to the specific adsorption in the IHP and the decrease in the concentration of the free SOH groups due to the reaction with the Cr(vi) species is expected to cause a shift of the equilibria (1) to the left. It is, therefore, anticipated that the hydrogen ions consumed in these equilibria, determined by potentiometric titrations, will be greater in the presence than in the absence of the Cr(vi) species in the impregnating suspension. This is, in effect, the case as it is illustrated in fig. 12 .Microelectrophoretic and potentiometric results similar to those described above have shown that in all cases the adsorbed active species are located in the IHP.
7.0 6.5
Figure 12. Hydrogen ions consumed for the protonation of surface hydroxyls. (a) y-AI203/ NH4NO3 and (b) y-AI203/ NH4NO3/HxCryOz ~ ions.
% 6.o X +u
T 5.5 !
4.s
s.'s
6.'s
7.s
pH
Let's now investigate the last mechanistic point, namely the kinetic features of deposition. The S-type of the deposition isotherm achieved in all cases suggested Langmuir deposition with lateral attractive interactions between the deposited species. A typical example is illustrated in fig. 13.
117
50 ="
13
Figure 13. Surface concentration of Mo(vi) as a function of the equilibrium Mo(vi) concentration for the sodium doped carriers [Na-xy-AI203], pH = 5.0, T= 25 oC, I=0.1M NH4NO3. The values of x are indicated, x: mmol
2.470
40 E -'~ 3 0
~
E ~
20
~e-
m~ e
L--
______
I0
~~ o~-- -
9
o.o~ 9
p~--o~ --I?..
0.621
o.~2 o.3o9 0.226
0. 00 0.005 0.010 0.015 0.020 C~ / mol dm -3 5. M E C H A N I S M Ouantitative
OF
DEPOSITION
Na+/gcatalyst.
OF
ACTIVE
SPECIES
USING
EDF:
approach.
The most important steps of the quantitative approach to the deposition mechanisms are the following. -On the basis of the qualitative approach already mentioned and the species being in the suspension it may be written a tentative mechanism for the deposition. This mechanism involves several deposition equilibria. - Based on these equilibria a set of isotherms are derived ei 1 - 8 - Ki [chemical speciesi]bexp(Er i / rm,iRT),
(2)
where el, e, Ki, E, I-i and rm,i represent, respectively, the fraction of the sites covered by the species i, the fraction of the sites covered by all deposited species, the deposition constant for the species i, the energy of the lateral attractive interactions between the deposited species, the surface concentration of the deposited species i and the saturation surface concentration of the species i, corresponding to the plateau of the isotherms. The quantity in the brackets represents the concentration of the species i in the solution at equilibrium. - By combining the above equations a general equation for the deposition may be obtained under a certain number of assumptions, where I-, I-m and K
118
1
1
1
--= + , F Fm Fml~Ceqexp(EF/FmRT)
(3)
represent, respectively, the total surface concentration of the deposited species, the corresponding saturation surface concentration and deposition constant. By Ceq we symbolize the total concentration of the species to be deposited being in the solution at equilibrium. The values of I-, r'm and Ceq are expressed in terms of the concentration of the element to be deposited. Among them r and Ceq are experimentally determined. Fitting of the experimental deposition data by the above equation confirms the postulated deposition mechanism from the viewpoint of only two mechanistic points. First from the view point that the deposition takes place on energetically equivalent sites and second that attractive lateral interactions are exerted between the deposited species. A typical example is illustrated in figure 14. In this point it should be stressed that by combining the qualitative and quantitative procedure described before we have investigated the deposition mechanism for the catalytic systems illustrated in table 2 by answering the questions stated in table 11.
Figure 14. Reciprocal surface concentration of Cr(vi) as a function of 1/Ceqexp(EI-/r'mRT). The solid lines represent the values calculated using eq. (3). (O) pH=4.0, (A)pH=5.7, (O) pH-6.1
I_.
E 0
00
500 1000 1500 2000 tool dm-:5 / [ C,,Iexp(EF/I"mRT)
However, the establishment of a detailed deposition mechanism allowing to be achieved a selective deposition needs much more computation work. So far this work has been done for a few systems among those illustrated in table 2. Let's now present the most important steps of the procedure followed in order to establish a detailed deposition mechanism.
119
- Following a rather complicated procedure and applying a computer program called SURFEQL we may calculate the value ri, for a given kind of the deposited species involved in the deposition and for various values of the impregnating parameters (concentration, pH, temperature, ...). From the so calculated values of I-i are determined the corresponding values for the total surface concentration of the deposited species, I-. Agreement between the calculated and experimental values of I- is a strong evidence that the detailed mechanism proposed for the deposition is correct. A typical example is illustrated in fig. 15. Figure 15. Variation of the surface concentration of Cr(vi) with equilibrium Cr(vi) concentration: experimental (D) and calculated (~k)isotherm for E
"
the total Cr(vi) deposition. Symbols (S) (&), (O) and (A) correspond to the calculated isotherms for the Cr(vi)
j*
I,,_
o
II-./ ~,.~~'o~O..__o__o I~ / o . O ~ 0
0.00
~
"
--e.--------e--e--e----e
0.01
0.02 C,,q /
0.03
tool dm -:5
deposition through adsorption of one HCrO4-ion per site created by one ,, 0 AIOH2 + group, of one Cr2072- ion per site created by one AIOH2 + 0.04 group, through reaction of one HCrO4-ion with one AIOH group and of one 0r2072- ion with one AIOH group, respectively, pH=4.0, T=25 oC, I=0.1 M NH4NO3.
It refers to the deposition of the Cr(vi) species on the y-AI203 surface. You may observe the excellent agreement between the experimental and calculated values of I- which allowed the establishment of the deposition mechanism and the calculation of the surface concentration for each of the deposited species under a given set of impregnation parameters. - The presence of the deposited species generally disturbs the deposition of the hydrogen ions on the support surface taking place through the equilibria 1. The magnitude and the direction of the disturbance depends on the deposition mechanism. This observation offers an additional assessment of the proposed mechanism. Specifically, on the basis of the proposed mechanism it may be
120
calculated the variation, with pH, in the difference (in the presence and absence of the deposited species) of the total protonated minus total deprotonated surface hydroxyls. It may be easily demonstrated that this variation should be very similar with the corresponding variation in the difference of the hydrogen ions consumed on the surface in the presence and absence of the deposited species. The latter variation is determined by potentiometric titrations. A typical test concerning the deposition of the Cr(vi) species on the y-AI203 surface is illustrated in fig. 16. 12
6
6 -g r x
§
2
0 a.
I
-r-
,+
-""".'-,~'~-T
+.5
6.5
1.5
+ ~r
8.5
pH
Figure 16. Illustrates the variations, with pH, in the differences (in the presence and absence of Cr(vi) species) of the hydrogen consumed on the surface, AH+c, (curve a) as well as of the "total protonated minus total deprotonated surface hydroxyls", A(AIOH2+ - AIO-)t, (curve b). AH+c was determined experimentally whereas A(AIOH2+- AIO-)t was calculated using the proposed model. T-25 OC, 1=0.1 M NH4NO3.
0 --6
-123
4
5
6
7
8
9
10
pH
Figure 17. Variation of the ~-potential, with pH, in the presence of Cr(vi) species: (a) experimental values, (b) calculated values. T=25 oC, 1=0.01 M NH4NO3.
- A completely independent test of a mechanistic model proposed for a deposition is the comparison of the variation, with pH, of the ~;-potential determined experimentally using microelectrophoretic mobility measurements with the corresponding variation of the ~-potential calculated by SURFEQL on the basis of the postulated deposition mechanism. Fig. 17 illustrates a typical example. The very good agreement observed supports the postulated mechanism.
121
6. M E C H A N I S M
OF
DEPOSITION
OF ACTIVE
SPECIES
USING
EDF"
Case studies,
Let' s now present a few catalytic systems for which a complete elucidation of the deposition mechanism has been achieved. Let's start with the mechanism of deposition for the Mo(vi) species on the y-AI203 surface. The old problem whether AIOH2+ + MoO42",,Hp ~
AIOH2+...MoO42-
AIOH2+ + Mo70246-,,Hp .-
AIOH2+...M070246AI- O\
2AIOH
+
MOU42-,,Hp
O I / AI-- 0
-.
,,,>O Mo \\ 0
+
2OH-
12 E o
9
(b)
o
E
~L
6 3 0~
4
5
--
6
7
"
8
m
9
Figure 18. Variation with pH of the maximum amount of Mo deposited through adsorption on a site created by one AIOH2+ group of one MoO42- (curve a) and M070246(curve b) ion and through reaction of one MoO42- ion with two AIOH groups (curve c).
pH the deposition takes place by adsorption or reaction has been now solved. Both processes contribute to the whole deposition. That is adsorption of the MoO42- and M070246- species on sites in the IHP created by the protonated surface hydroxyls and reaction of the MoO42- ions, but not of the M070246- species, with the neutral surface hydroxyls of the support. However, as you may observe, the extent of each of the processes depends on pH (fig. 18). The variation of the surface concentration with pH for each of the deposited species allows a regulation not only of the total amount of Mo(vi) deposited on the surface using EDF but in addition of the relative concentration for each one of the supported Mo(vi) species. This
122
achievement is expected to help very much the designing of the preparation of supported Mo(vi)/y- AI203 catalysts. Another more complicated mechanism recently elucidated refers to the preparation of the Cr(vi)/y-AI203 catalysts using EDF. Again it should be stressed that the achievement of the variation, with pH, of the concentration for each one of the deposited species allows to be obtained a very severe regulation of the magnitude and quality of the supported Cr(vi) phase (fig. 19). AIOH2 + + CrO42",lHp ~
-~
AIOH2 +. .CrO42-
AIOH2+ + Cr2072-,iHp :
~': AIOH2 +...Cr2072-
AIOH2+ + HCrO4-,IHp :
:
2AIOH + CrO42-,~Hp
AIOH2+...HCrO4 AI-- O O zO / Cr AI-- O %0
~
+ 2OH-
OH AI-O--Cr-- O I
AIOH + HCrO4-,~Hp
II
O O AI
AIOH2 + + Cr2072-,iHp
0
2.0 E
1.5
',(d) I
E 1.0 ~L 0.5'
oo.t
"4.0
o
4.5
5.0
5.5
6.0 pH
6.5
7.0
7.5
~
O + OH" ~Cr\~ 0O--C/r=O O O Figure 19. Variation with pH of the maximum amount of Cr deposited through adsorption on a site created by one AIOH2 + group of one Cr2072- (curve a), HCrO4- (curve b) and CrO4- (curve c) ion and through reaction of one Cr2072ion with one AIOH group (curve d), of one HCrO4-ion with one AIOH group (curve e) and of one CrO42ion with two AIOH groups (curve f)
123
In the contrast to the above the deposition mechanisms of the Co 2+ and Ni2+ ions were proved to be very simple. AI O'... C 0 2 +,, Hp(N i2 +,iNP)
AIO- + Co2+,iHe(Ni2+,lHe)
The deposition in this case takes place exclusively by adsorption of the 002+ and Ni2+ ions on sites created in the IHP by the deprotonated surface hydroxyls. In this point it should be noted that the concentration of the Co 2+ and Ni2+ ions deposited following EDF still remains low though the concentration of the deprotonated surface hydroxyls has increased markedly by increasing pH or doping the support surface with F-ions (see table 2 and fig. 20).
0.6 Figure 20. Variation with pH of the maximum amount of Co (curve a) and Ni (curve b) deposited through adsorption of one Co 2+ or Ni2+ ion on a site created by one AIO- group.
? E "B 0.4E ~L
0.2
0"04.0
4. 's
5. 'o
' 5.5 pH
' 6.0
' 6.5
7.0
Moreover, it was found that the amount of the deposited by EDF, Co 2+ or Ni2+ ions on the surface of the Mo(vi)/y-AI203 specimens is also too low. Therefore, it is not possible to obtain Co-Mo/y-AI203 or Ni-Mo/y-AI203 hydrotreatment catalysts with acceptable concentration of Co 2+ or Ni2+ ions using EDF to deposit these ions on the Mo(vi)/y-AI203 specimens. This is the main reason for which very recently we have developed the co-deposition methodology in order to prepare the aforementioned catalysts using EDF. Specifically, it has been found a mutual promotion in the deposition of the Mo(vi) negative species and the positive Co 2+ or Ni2+ ions. Typical examples are illustrated in the fig's 21 and 22. You may observe the actual increase of the deposition of the Mo(vi) species in the presence of the Co 2+ ions (fig. 21) and of the Co 2+ ions in the presence of Mo(vi) species (fig. 22). Recent studies devoted to the elucidation of the co-deposition mechanisms of
124
?
E ~o
15
12 o~..__.
1.5
o o
*o
. %--(b) , (c)
E 1.2
.
L. I
0.3 ~o
~ ~ - - - - - ~ 1
,-*------*
9 (o)~
I
0.0000 0 . ~ 5 0.010 0.015 0.020 0.025 C.~ / mol Mo oR dm-=
Figure 21. Variation in the surface concentration of Mo(vi) with the Mo(vi) concentration in the impregnating solu-, tion, Ceq, (pH--4.9, T-25 0C, I=0.1 N NH4NO3): (a)in the absence of Co(II) ions, (b) in the presence of varying concentrations of Co(II) ions and (c)in the presence of a constant concentration of Co(II) ions (1 x l0 -2 mol dm-3).
0.000 0.005 0.010 0.015 0.020 0.025 Cq / tool Co==+ dm "=
Figure 22. Variation in the surface centration of Co(ll) with the Co(II) concentration in the impregnating solution, Ceq, (pH=4.9, T=25 0C, I=0.1 N NH4NO3): (a)in the absence of Mo(vi) species, (b)in the presence of varying concentrations of Mo(vi) species and (c) in the presence of a constant concentration of Mo(vi) species (1 xl0 -2 mol Mo(vi) dm-3).
Mo(vi) negative species with Co 2+ or Ni2+ ions revealed that the main reason for the observed mutual promotion in the deposition on y-AI203 is the development of very strong lateral interactions exerted between the Mo(iv) species as well as between the Co 2+ or Ni2+ ions in the presence of species with opposite charge. In fact, the values of the energy of lateral interactions determined in the presence of ions with opposite charges are much more higher than in the absence of these ions. As to the mechanism of co-deposition it was proved that it may be described by the deposition equilibria established for the separate deposition of the Mo(vi) species and [Co 2+ or Ni2+] ions. However, there is an important difference. Now the deposition of the Mo(vi) species takes place exclusively by adsorption. The contribution of the reaction mentioned before was demonstrated to be negligible (fig. 23). Finally, it was found that the intensity of the surface bond AIO-... Co 2+ (Ni2+), as it is expressed by the values of the adsorption constant, decreased
125
AIOH2 + + MoO42-,tHp <
~
AIOH2 + + MO70246",~Hp ~
AIOH2+...MoO42AIOH2+...M070246" AI-- O \
2AIOH + MoO42",lHe
AIO- + Co 2+,HP(Ni2+,,HP ) ,
//O
O Mo \ \ / / AI- O O ~
+ 2OH-
AIO...C02+,,Hp(Ni2+,IHp)
dramatically due to the co-deposition of the Mo(vi) negative species. This effect as well as the increase in the lateral interaction between the Co 2+ or Ni2+ ions mentioned before is expected to inhibit the formation of the catalytically inactive cobalt or nickel aluminate during calcination. This prediction as well as the increase of the adsorption capacity of u for Co 2+ or Ni2+ ions renders the codeposition of the Mo(vi) species with Co 2+ or Ni 2+ ions using EDF a promising methodology for preparing hydrotreatment catalysts. 14
2.0
? 12 E o
10 ~
-6 E
8-
Figure 23. Variation with pH '7 E o
Mo and Co deposited
"5 E
through adsorption of one MoO42- (curve a) and
1.5 o
6 E.
1.0 ~E
4 2 0
4.0
4.5
5.0
5.5
6.0
6.E)'5-
of the maximum amount of
Mo70246- (curve b)ion on a site created by one AIOH2 + group and of one Co 2+ ion on a site created by one AIOgroup (curve c)
pH 6. C O N C L U S I O N S The main conclusions drawn from the present lecture may be summarized as follows.
126
(i) Equilibrium Deposition-Filtration is a simple and attractive methodology for preparing supported catalysts with better physicochemical and catalytic properties than those achieved using the classical impregnation techniques. (ii) The physicochemical characteristics, mainly the active surface, and therefore the catalytic properties of the supported catalysts prepared following EDF may be regulated by controlling the concentration of the deposition sites of the industrial, oxidic, supports. This control may be achieved by changing the suspension pH, the suspension temperature or doping the carriers with various dopants. (iii) Study of the deposition mechanism for a number of supported catalytic systems has shown that the deposition of the species containing the active element on the support surface takes place through two routes: First through adsorption of these species on energetically equivalent sites located in the IHP. Second by reaction of negative species with the neutral surface hydroxyls of the supports. The contribution of each process to the whole deposition depends on the nature of the support, the species to be deposited and the suspension pH. In all cases studied the protonated (deprotonated) surface hydroxyls are responsible for the creation of the adsorption sites for negative (positive) species. In the most of cases lateral attractive interactions are exerted between the adsorbed species. (iv) The elucidation of the detailed mechanism for few catalytic systems allowed to be achieved the variation of the surface concentration with the suspension pH for each one of the deposited species. This, in turn, allows selective deposition which it is expected to help the control of the quality of the active sites of the resulting supported catalysts. (v) The mutual promotion in the deposition of the Mo(vi) or W(vi) negative species with the Co 2+ or Ni2+ ions on the y-AI203 surface renders the codeposition of these species using EDF an attractive methodology to prepare CoMo/y-AI203, Ni-Mo/y-AI203 and Ni-W/y-AI203 hydrotreatment catalysts.
REFERENCES 1. J. W. Gews, in "Preparation of Catalysts II1" (G. Poncelet, P. Grange and P.A. Jacobs, eds.), Elsevier, Amsterdam, 1983, vol. 16, pages 1-33. 2. K.P. de Joyg, in "Preparation of Catalysts V" (G. Poncelet, P. A. Jacobs, P. Grange and B. Delmon, eds.), Elsevier, Amsterdam, 1991, Vol. 63, pages 19-36.
127
3. L.M. Knijff, P. H. Bolt, R. Van Yperen, A.J. Van Dillen and J. W. Gews, in "Preparation of Catalysts V" (G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon, eds.), Elsevier, Amsterdam, 1991, vol. 63, pages 165-174. 4. European Patent Specification 258, 942 (1988) to S.I.R.M-B.V. 5. Netherlands Patent Application 68, 1677 (1970) to Stamicarbon 6. YU. I. Yermakov: Catal. Rev. Sci. Eng., 13 (1976) 77. 7. YU. I. Yermakov: Adv. in Catalysis, 24 (1975).173. 8. D. H. Ballard: Adv. in Catalysis, 23 (1973) 263. 9. J. P. Candlin et a.I Adv. Chem. Ser., 132 (1974) 212. 10. M.S. Scurrell, "Catalysis" Vol. 2, specialist periodical reports, the Chemical Society, Burlington House, London W 1VOBN, 1978, p.215. 11. L. Wang, W.K. Hall, J. Catal., 77 (1982) 232. 12. L. Wang, W.K. Hall, J. Catal., 83 (1983) 242. 13. L. Wang, W.K. Hall, J. Catal., 66 (1980) 251. 14. S. Kasztelan, J. Grimblot, J. P. Bonnelle, E. Payen, H. Toulhoat, Y. Jacquin, Applied Catalysis, 7 (1983) 91. 15. H. S. Thomas, M. N. Blanco, C.V. Caceres, N. Firpo, F.J. Gil Llambias, J.L.G. Fierro, A.L. Agudo, J. Chem. Soc., Faraday Trans., 86 (1990) 2765. 16. C. V. Caceres, J.L.G. Fierro, A.L. Agudo, M.N. Blanco, H.J. Thomas, J. Catal., 95 (1985) 501. 17. J.A.R. van Veen, H. De Wit, C.A. Emeis, P.A.J.M. Hendriks, J. Catal., 107 (1987) 579. 18. L.P. Milova, N.M. Zaidman, L.M. Plyasova, S.V. Ketchik, K. G. Rikhter, Kinetic Katalysis, 23 (1982) 123. 19. J. N. Fiedor, A. Proctor, M. Houalla, P.M.A. Sherwood, F.M. Mulcahy, D.M. Hercules, J. Phys. Chem., 96 (1992) 10967. 20. M.J. Fay, A. Proctor, D.P. Hoffman, M. Houalla, D.M. Hercules, Microchimica Acta, 109 (1992) 281. 21. J. Sonnemans, P.J. Mars, J. Catal., 31 (1973) 209. 22. C. C. Williams, J. G. Ekerdt, J.M. Jehng, F.D. Hardcastle, I.E. Wachs, J. Phys. Chem., 95 (1991) 8791. 23. T. Machej, J. Haber, A.M. Turek, I.E. Wachs, Applied Catalysis, 70 (1991) 115. 24. L. Wang, W.K. Hall, J. Catal., 82 (1983) 177. 25. J.A.R. van Veen, P.A.J.M. Hendriks, Polyhedron, 5 (1986) 75. 26. J. P. Brunelle, Pure Appl. Chem., 50 (1978) 1211. 27. K.Y.S. Ng, E. Gulari, J. Catal., 92 (1985) 340.
128
28. Cr. Contescu, M.I. Vass, Applied Catalysis, 33 (1987) 259. 29. D.S. Kim, Y. Kurusu, I.E.Wachs, F.D. Hardcastle, K. Segawa, J. Catal., 120 (1989) 325. 30. D.C. Vermaire, P.C. van Berge, J. Catal., 116 (1989) 309. 31. P. H. Tewari, W. Lee, J. Colloid Interface Sci., 52 (1975) 77. 32. F.M. Mulcahy, M.G. Fay, A. Proctor, M. Houalla, D.M. Hercules, J. Catal., 124 (1990) 231. 33. N. Spanos, L. Vordonis, Ch. Kordulis, A. Lycourghiotis, J. Catal., 124 (1990) 301. 34. N. Spanos, L. Vordonis, Ch. Kordulis, P.G. Koutsoukos, A. Lycourghiotis, J. Catal., 124 (1990) 315. 35. L. Vordonis, P.G. Koutsoukos, a. Lycourghiotis, Colloids and Surfaces, 50 (1990) 353. 36. N. Spanos, H.K. Matralis, Ch. Kordulis, a. Lycourghiotis, J. Catal., 136 (1992) 432. 37. L. Vordonis, N. Spanos, P.G. Koutsoukos, A. Lycourghiotis, Langmuir, 8 (1992) 1736. 38. L. Karakonstantis, Ch. Kordulis, A. Lycourghiotis, Langmuir, 8 (!992) 1318. 39. N. Spanos, S. Slavov, Ch. Kordulis and A. Lycourghiotis, submitted for publication. 40. N. Spanos, Ch. Kordulis and A. Lycourghiotis, submitted for publication. 41. N. Spanos, S. Slavov, Ch. Kordulis and A. Lycourghiotis, submitted for publication. 42. T.W. Healy, and L.R. White, Adv. Colloid Interface Sci., 9 (1978) 303. 43. C.P. Huang and W. Stumm, J. Colloid Interface Sci. 43 (1973) 409. 44. L. Vordonis, P.G. Koutsoukos and A. Lycourghiotis, J. Chem. Soc., Chem. Commun., (1984) 1309. 45. L. Vordonis, P.G. Koutsoukos and A. Lycourghiotis, J. Catal., 98 (1986) 296. 46. L. Vordonis, P. G. Koutsoukos and A. Lycourghiotis, J. Catal., 101 (1986) 186. 47. K. Akratopulu, L. Vordonis and A. Lycourghiotis, J. Chem. Soc., Faraday Trans 1, 82 (1990)3437. 48. L. Vordonis, P.G. Koutsoukos and A. Lycourghiotis, Langmuir, 2 (1986) 281. 49. K. Akratopulu, L. Vordonis and A. Lycourghiotis, J. Catal., 109 (1988) 41. 50. L. Vordonis, K. Akratopulu, P.G. Koutsoukos and A. Lycourghiotis, in "Preparation of Catalysts IV" (B. Delmon, P. Grange, P.A. Jacobs, and G. Poncelet, eds.), Elsevier, Amsterdam, 1987, Vol. 31, pages 309-321.
129
51. K. Akratopulu, L. Vordonis and A. Lycourghiotis "Heterogeneous Catalysis" Proc. of the Sixth Inter. Sym. (Bulgarian Academy of Sciences), Sofia, 1987, pages 412-417. 52. K. Akratopulu, L. Vordonis and A. Lycourghiotis "in proc. of the 10th Panhellenic Conference of Chemistry" Greek Chemists Assoc., Patras, 1985, p. 700. 53. L. Vordonis PhD thesis University of Patras, Chemistry Department, Patras, Greece, 1988. 54. K. Akratopulu PhD thesis University of Patras, Chemistry Department, Patras, Greece, 1989. 55. K. Akratopulu, Ch. Kordulis and A. Lycourghiotis, J. Chem. Soc., Faraday Trans 1, 82 (1986)3697. 56. W. Stumm, H. Hohl and F. Dalang, Croat. Chim. Acta, 48 (1976) 491. 57. H. Hohl and W. Stumm, J. Colloid Interface Sci., 55 (1976) 281. 58. W. Stumm, R. Kummert, and L. Sigg, Croat. Chim. Acta, 53 (1980) 291. 59. W. Stumm, C.P. Huang, and S.R. Jenkins, Croat. Chim. Acta, 42 (1970) 223. 60. J. Westall, and H. Hohl, Adv. Colloid Interface Sce., 12 (1980) 265. 61. D. Yates, S. Levine, and T.W. Healy, J. Chem. Soc., Faraday Trans 1,70 (1974) 1087. 62. J.A. Davis, R.O. James and J.O. Leckie, J. Colloid Interface Sci., 63 (1978) 480. 63. R.O. James, J.A. Davis and J.O. Leckie, J. Colloid Interface Sci., 65 (1978) 331.
This Page Intentionally Left Blank
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
131
Preparation of K-C-Fe/AI203 catalysts for ammonia synthesis at mild conditions K. Katucki, A.W. Morawski and W. Arabczyk Institute of Inorganic Chemical Technology, ul.Putaskiego 10, 70-322 Szczecin, Poland
Technical University of Szczecin,
1. I N T R O D U C T I O N K. Aika eta/. [1] studied alkali metal/transition metal/active carbon catalysts in ammonia synthesis. Authors postulated that carbon support enables electron transport from alkali metal towards transition metal. It is also well known that properties of K-graphite-transition metal systems based on so called "graphite intercalation compounds- GICs" are interested with respect to catalytic application in ammonia synthesis[2-4]. The studies carried out by Volpin's group [3] at atmospheric pressure suggest that the interaction of intercalated species with the graphite network is likely to cause not only the activation of these species, but also the activation of the carbon atoms net of graphite. K. Katucki and A.W. Morawski [4-7] studied the K-graphite-Fe catalysts derived from FeCl3-graphite intercalation compounds and further activation with vapour of metallic potassium. The high activity of K-graphite-Fe catalysts at pressure of 10 MPa we attributed to presence of K-C-Fe sites where 2s electron is taken by graphite n-electron system and is transported also towards iron. Such "electronegatively enriched" graphite framework simultaneously plays a role as an "electronic" and "structural" promoter of iron. Carbons prepared by decomposition of hydrocarbons have also variable physical and chemical properties, e.g. possibility of electrons transport and/or electrons accumulation. Therefore they are useful to application in new areas, including catalysis. K.S. Rama Rao et al. [8] lately described new type of carbon coated A1203 support for the preparation of Ru/C-AI203 catalyst for ammonia synthesis promoted by impregnation with CsNO 3. Above used carbon-coated aluminium oxides, were prepared by pyrolysis of an a!kene on AI203. In the present work we have changed the sequence of operation during preparation. The electron ability of thin carbon coating on Fe/AI203 was utilized for C-Fe/A1203 precursor and for further promotion by the reaction in a vapour of metallic potassium. These combinations produced novel K-C-Fe/A1203 catalyst of ammonia synthesis reaction.
132 2. EXPERIMENTAL The Fe/A1203 samples were prepared by coprecipitation procedure from solution of calculated mounts of inorganic salts. The AI(NO3)39H20 and Fe(NO3)39H20 (Fluka AG) were used. In Table 1 are given detailed quantities of weighted salts for coprecipitation.
Table 1. Used quantities of inorganic salts for preparation catalysts (g). Sample code
AI(NO3)39H20
Fe(1)/AI203 Fe(2)/A1203 Fe(3)/AI203 Fe(4)/AI203 Fe(5)/AI203 Fe(6)/AI203 Fe(7)/AI203
73.6736 73.63 77 73.6480 73.6704 75.6352 75.6435 75.63 79
Fe(NO3)39H20
1.0330 3.0576 6.03 80 10.0326 20.0190 40.0046 75.0348
The coprecipitation was carried out with water solution of ammonia. The dosage of NH4OH was finished when the pH of solution reached value of 7.4 - 7.8. Then samples were dried for 24 hrs at temperature of 115 ~ The dried samples were calcined for 4 hrs at temperature of 450 ~ The reduced samples of Fe/AI203 were obtained by reduction with a mixture of nitrogen and hydrogen (1:3) under atmospheric pressure at temperature of 475 ~ for 24 hrs using a flow reactor presented in Figure 1. To obtain C-Fe/AI203 precursors the feeder (2) of reactor (Fig.l) was tankaged with 7 ml of n-hexane. The nitrogen-hydrogen mixture was bubbled through of n-hexane to complete of alkene pyrolysis. Both the activation of C-Fe/AI203 precursors and the activity measurements of K-C-Fe/AI203 catalysts were performed in the same flow reactor (Fig.l). The metallic potassium was introduced into the carbon coated precursors by vapour deposition at a temperature of 3 50 ~ under a pressure of about 6 Pa. The potassium deposition process was carried out for 3 hrs. The activity of K-C-Fe/A1203 catalysts were studied at atmospheric pressure and at temperature of 350 ~ with space velocity (s.v.) of 5000 h -1. The each test duration was about three days. Occasionally same of them were tested at room temperature. The control activity test was also done on typical industrial iron catalyst. The samples were characterized by X-ray fluorescence spectroscopy (XRFS), X-ray powder diffraction (XRD), scanning electron microscopy(SEM) and Mrssbauer effect spectroscopy(MES). For MES measurements the 57Co in chromium matrix was served as the source. Isomeric shift values were given in relation to metallic iron. Spectra were computer-fitted and M6ssbauer parameters were calculated.
133 3/
1/
4/
t
]
1) inlet gas; 2) feeder of n-hexane; 3) three-way cock; 4) outlet to vacuum; 5) precursor (or catalysO; 6) grate; 7) potassium container; 8) outlet gas; 9) thermocouple wall
i
Figure 1. Flow reactor for the precursors preparation (reduction and carbon coating), precursors activation with vapour of metallic potassium and for activity measurements at atmospheric pressure.
5/ 6/
7/
S/
9/
3. RESULTS AND DISCUSSION
The results of both chemical composition and activity of tested samples are listed in Table 2. The activity of catalysts increases with iron content from very weak for K-C/AIzO 3 and K-C-Fe(1)/AI203 to most active catalyst of K-C-Fe(7)/A1203 with some tendency to reach a plateau The same iron influence was observed in the case of carbon deposition production atter pyrolysis of n-hexane. The obtained in work AI203 support exhibits low ability to carbon deposition and it results in low concentration of carbon in sample without of iron (Tab. 2.). The same behaviour of A1203 support was also described by Vrlter et al. [9], when n-hexane conversion at temperature of 500 ~ on AI203 catalyst was studied. Only small amount of active coke deposition was noticed. The activities of the following catalysts :K-C-Fe(4)/AI203, K-C-Fe(5)/AI203, K-C-Fe(6)/AI203 and K-C-Fe(7)/A1203 were higher than activity of typical industrial iron catalyst tested at the same conditions, as it was compared in Table 2. Above mentioned catalysts exhibit also weak activity ( from ca. 0.005 % to 0.02 % NI-I3) at ambient condition. The calculated composition of active phase of most active catalyst (K-C-Fe(7)/A1203) was Kz.03FeCo.42.
134 Table 2. Composition and activity in ammonia synthesis of used K-C-Fe/AI20 s catalysts. t = 350 C, s.v. 5000 h-1 ; atmospheric pressure. o
.
catalyst K-C/Al203
% Fe "
'
% K
% Al20s
% C *)
%NH 3
28.12
71.54
0.34
0.02
K-C-Fe(1)/AI203
1.72
52.5
45.19
0.59
0.07
K-C-Fe(2)/Ai203
5.72
54.3
38.67
1.31
0.11
K-C-Fe(3)/Ai203
11.09
48.2
38.46
2.25
0.18
K-C-Fe(4)/AI20 a
14.81
43.4
39.76
2.03
0.26
K-C-Fe(5)/AI203
23.36
38.2
36.63
1.81
0.36
K-C-Fe(6)/AI203
27.61
41.3
28.34
2.75
0.4
K-C-Fe(7)/AI203
33.51
47.57
15.9
3.02
0.48
industrial iron catalyst
0.18
i
*) Complementary value, to reach 100 %.
In both Figure 1 and Figure 2 presented are diffractograms of catalysts with extreme iron contents and during each step preparation. In the series of"2" catalyst with small iron content (Fig. 2) there is clear Al203 peaks existence ( 2 t9 ca. 52 ~ and 79 ~ ). The both broadening and intensity indicate on poorly crystallographic character of AI203 and of different forms. The very weak peaks of A1FeO3 and FeA1204 (hercynite) may be stated. After carbon coatings the low intensity peaks of carbons (and graphite-like) and carbides (mainly Fe3C - cohenite) was indicated by computer-fitting of diffractograms. In the diffi'actograms of the series "2" the phases of metallic iron were absent because of low concentration of one. The phase analysis of precursor activated with metallic potassium supposes disappearance of graphite-like carbons and simultaneous appearance of some intercalated in graphite forms of potassium like KC 8 (JCPDS card No 27-378 ) and C24K (JCPDS card No 19-945). Also some forms of Al203 with included potassium can not be excluded. The Fe(7)/AI203 sample (Fig. 3.) is formed mainly with Al203, Fe203 , F e 3 0 4 and A12Fe206. After reduction of Fe(7)/A1203 besides above listed phases the metallic iron appeared. Diffractogram of carbon coated C-Fe(7)/AI203 is enriched by carbides and graphite-like carbons (reflections at 219 near of 31~ Activation with metallic potassium
135 results in absence of support and hercynite reflections. Also rests of peaks are of lower intensity. There are peaks that can be attribute to intercalated phases of KC 8, KC9, and KC24. The comparison of selected regions (2 19 = 50 - 55 o ) of catalysts diffractograms is presented in Figure 4. The small intensity of iron peak for d = 202.8 pm ( 219 = 52.372 o ) is noticed only to start with catalyst K-C-Fe(5)/Ai203. The intensive peak of iron was found in the sample K-C-Fe(7)/Ai203 with highest of both the activity and the iron concentration. But proportionality of activity and iron concentration was not found.
I. . . .
i
i
Figure 4. Comparison of selected regions of catalysts diffractograms with different iron content. CoK= radiation. d) K-C-Fe(7)/AI203
=) K-C-Fe(S)/A~O,
'
b) K-C-Fe(a)/AI203 a) K-C-Fe(2)/AI20 a
!
b)
]
'
L
I
]" v "
v-
!
v,, ~ i ~ "-'
4
:-
','!(",.,,-v vl
I
I
I
5O
52
~
I
54
2e
136
Figure 2. Diffractograms of samples series "2" during each step preparation. CoKa radiation. c) K-C=Fe(2)/AI203 catalyst b) C=Fe(2)/AI203 reduced + + carbonized a) Fe(2)/AI203 calcinated
I
I L
1
'I
'
!
~o
2O
'
6o
~o
2~
80
2e
Figure 3. Diffractograms of samples series "7" during each step preparation. d) K=C=Fe(7)/AI203 catalyst c) C=Fe(7)/AI203 carbonized b) Fe(7)/AI203 reduced a) Fe(7)/AI203calcinated
'
1
m ,~
. . . .
t
,
1
'
ii
"
{
] 20
40
6O
137 Table 3.
Mtssbauer parameters for both the C-Fe(7)/AI~O3 precursor and K-C-Fe(7)/AI203 most active catalyst. The number in the brackets inform about accuracy of value, for example the value 0.33(2) is the same as 0.33+0.02 sample
~5
A
H
F/2
contents
mm/s
mm/s
kOe
mrn/s
%
330.6 (1)
0.17 (1)
38.6_+1.2 Femetallic
0.00
C-Fe(7)/A1203
0.41
0.94
0.28 (1)
12.6 (4)
0.85
2.24
0.28 (1)
6.3
AI-Fe-O
FeAI204
0.33 (2)
458.0 (2)
0.36 (1)
4.6 (3)
0.52 (1)
419.0 (1)
0.71 (2)
20.3+1.0
} Fe304
0.36
492.0
0.36 (1)
4.6 (3)
y Fe203
0.25 (1)
12.0 (5)
0.16 (1)
42.5+_2.8 Fe metallic
0.41 (1) 0.76 (2)
0,36 (1)
14.4+1.1
1.06
0,43
3.8 (4) 5.9 (5)
0.43 (3)
2.49
0.00
K-C-Fe(7)/AI203
(3)
remarks
330.8 (1)
1.00 (6)
x)
A1-Fe-O x.x)
0.31 (1)
474,0 (1)
0,25 (1)
0.45(1)
430,0(1)
0,50
0.36
492,0 (1)
0,25 (1)
5.9 (5)
0.19
208,0 (1)
0,50
8.4_+1.7 Fe3Cc~t~t~
0.18 (1)
5.8 (4)
-0.07 (1)
13.4+_2.7 }Fe304 y Fe203
~)
x) computer-fitted as austenite; xx) parameters similar to Fe 2+site, but QS a tittle lower [10], xxx) non-typical parameters, probably deformed by potassium presence. The three Mtssbauer parameters, isomer shift, quadrupole splitting and magnetic hyperfine field are ususally specific for a compound. These parameters of C-Fe(7)/A1203 precursor and K-C-Fe(7)/AI203 most active catalyst presented are in Table 3 . The dominant phase in carbonized precursor is the metallic iron (38.6 %). The reduction process was not finished because of Fe203 and Fe304 phases presence. After reduction and carbonization also bulk spinel phase of FeAl204 (6.3 %) and compound containing Fe-A1-O elements were formed. These parameters are in accordance with data reported by Vaishnava et al. [10]. The phase marked as x) was computer-fitted as austenite. Activation by vapour of metallic potassium produces higher concentration of metallic iron (42.5 %) in sample of K-C-Fe(7)/A1203 . The disappearance of FeA1204 phase is observed. The total percent of iron oxides ( Fe203 + Fe304 ) remains the same as before activation. Created is also a new phase (marked as xx) - Tab. 3. ) with parameters similar to Fe 2+site,
138
a)
b)
Figure 5. Scanning electron micrographs of catalyst precursors after reduction and carbon deposition: a) C-Fe(3)/AI203 b) C-Fe(7)/AI203
139
Figure 6. Scanning electron micrographs of most active catalyst K-C-Fe(7)/A1203 with different magnification.
140 but QS is a little lower as given in literature [ 10]. Most interesting is phase marked in Tab. 3. as xxx) which possess non-typical parameters, probably defected by potassium presence. Probably above phase would be responsible for activity. The MES measurements will be subject of separated further studies. Scanning electron micrographs of two different precursors (Figure 5.) shown the increase of carbon deposition on support with iron concentration in precursors. It is worthwhile to point out that produced AI203 without of iron was clear and white and not coated by carbon. The catalytic growth of carbon coatings is caused by presence of iron. The produced carbon is rather filamentous, as presented by Baker [11]. Scanning electron micrographs showing growth of carbon filaments on the surface of K-C-Fe(7)/AI203 catalyst is shown in Figure 6. 4. C O N C L U S I O N S The following preparation method of novel catalyst of K-C-Fe/AI203 for ammonia synthesis at mild conditions is proposed: 1) coprecipitation of iron+aluminium hydroxides, 2) calcination, 3) reduction of iron oxides/Al203 to Fe/AI203, 4) carbon coatings of Fe/AI203, 5) activation of C-Fe/AI203 precursor by metallic potassium. The influence of iron on activity was found. When the well-crystallized metallic iron detected by XRD was stated in catalyst then the maximum of activity was reached. The creation of thin film of carbon intercalated-like with metallic potassium on Fe/AI203 is postulated. The high activity of catalyst was attributed to presence of K-C-Fe sites where 2s electrons of metallic potassium are transported by carbon electron system towards supported iron metal.
REFERENCES 1. K. Aika, H. Hori and A. Ozaki, J. Catal., 27 (1972) 424. 2. M.Ichikawa et al. J.C.S. Chem. Comm., (1972) 176. 3. Ju.N. Novikov and M.E. Volpin, Physica B+C, 105 (1981) 471. 4. K.Katucld, and A.W. Morawsld, Stud. Surf. Sci. Catal., 7 (1981) 1496. 5. K. Ka~cki and A.W. Morawski, Synth. Metals, 34 (1989) 713. 6. K.Ka~ucki and A.W. Morawski, J. Chem. Technol. and Biotechnology, 47 (1990) 357. 7. K.Katucki and A.W. Morawski, Preparation of Catalysts V, eds: G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon, Elsevier Science Publishers B.V., Amsterdam - Primed in The Netherlands (1991) ; in: Stud. Surf. Sci. Catal., 63 (1991) 487. 8. K.S. Rama Rao, P. K. Rao, S.K. Masthan, L. Kaluschnaya and V.B. Shur, Appl. Catalysis, 62 (1990) El9. 9. J.V61ter, H.D. Lanh, B.Parlitz, E.Schreier and K.Ulbricht, Proc. of the 10th International Congress on Catalysis, July, 1992, Budapest, Hungary; eds. L. Guczi et al., Elsevier Publishers B.V., The Netherlands (1993). 10.P.P.Vaishnava, P.I.Ktorides, P.A.Montano, K.J.Mbadcam and G.A.Melson, J.Catalysis, 96(1985)301. 11. R.T. Baker, Carbon, 27 (1989) 315.
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
141
A Novel [PtMo6]/MgO Catalyst for Alkane-to-Alkene Conversion D.I. K o n d a r i d e s a,b K. Tomishige, Y. N a g a s a w a , and Y. I w a s a w a * D e p a r t m e n t of Chemistry, Graduate School of Science, T h e University of T o k y o , Hongo, Bunkyo-ku, T o k y o 113, Japan
A novel [PtMo6]/MgO ensemble catalyst was prepared using a [PtMo6024] 8heteropolyanion precursor and characterized by EXAFS. Analysis of the EXAFS spectra showed that after calcination at temperatures above 673 K platinum and molybdenum atoms interact with the support. Platinum ions (Pt4+) replace surface Mg2+ ions of the MgO carrier, while Mo 6+ ions locate on the magnesia surface in a distorted octahedrally coordinated framework. Catalytic tests using i-butane, n-butane and propane dehydrogenation as probe reactions showed that the novel ensemble catalyst exhibits a much better catalytic performance compared to conventionally prepared catalysts. I. I N T R O D U C T I O N The dehydrogenation / oxidative dehydrogenation of hydrocarbons is a matter of considerable importance in todays petrochemical industry and also serves as a target reaction for catalyst design in fundamental research. Dehydrogenation reactions are often sU~cture sensitive and the activity and selectivity may depend on the surface geometry and composition of the employed catalyst. It is, therefore, evident that the development of catalysts with the desired geometry and composition of surface multimetallic sites is required to improve the catalytic performance. In the present work a novel [PtMo6]/MgO ensemble catalyst was prepared using an Anderson-type heteropolyanion precursor. The specially prepared [PtMo6] supported catalyst, with a well defined surface geometry, was applied to the n-butane, i-butane and propane dehydrogenation reactions and its catalytic performance was compared with that of conventionally prepared bimetallic and monometallic catalysts. The results show that the novel ensemble catalyst has high activity and selectivity towards olefin formation and increased resistivity against carbon deposition. a Permanent address: Institute of Chemical Engineering and High Temperature Chemical Processes (ICE/HT), and Department of Chemical Engineering, University of Patras, P.O. Box 1414, GR 26500 Patras, Greece. b Acknowledgment: D.I.K. is grateful to the Japanese-German Center Berlin (JGCB) for financial support through a Special Exchange Program (SEP) fellowship.
142
2. EXPERIMENTAL 2.1. Catalyst preparation The Pt-Mo bimetallic ensemble catalyst (denoted as [PtMo6]/MgO ) was prepared by impregnating MgO with an aqueous solution of (NH4)4NH4PtMo6024 ] polyanion, a well defined organized ensemble with plane structure composed of a central Pt (IV) ion and six octahedral molybdates [1]. A catalyst with the same Pt-Mo composition (denoted as PtMo/MgO) was also prepared with a coimpregnation method using an aqueous solution of chloroplatinic acid and ammonium heptamolybdate. For comparison, monometallic Pt/AI20 3, Mo/MgO and Pt/MgO catalysts were also prepared. The metal loading was the same for all catalysts (1 wt % for Pt and 3 wt % for Mo). MgO formed by calcination of Mg(OH) 2 at 773 K for 2 h was employed as support.
2.2. Catalyst characterization The structures of the bimetallic catalysts at all stages of preparation, pretreatment and reaction were examined using EXAFS. Spectra of the [PtMo6024] 8- precursor and the Pt/MgO catalyst were also obtained. EXAFS spectra of the fresh [PtMo6]/MgO and Pt-Mo/MgO samples after impregnation as well as after calcination at various temperatures between 423 and 773 K were obtained in order to examine the extent of the interaction between the precursor and the support upon increasing the calcination temperature. The samples were calcined in a closed circulating system under an oxygen pressure of 13.3 kPa. Samples of the [PtMo6]/MgO and Pt-Mo/MgO catalysts were also examined after reaction (propane dehydrogenation at 723 K for 4 h). After the corresponding treatment the samples were transferred to glass EXAFS-celIs with Kapton windows without contacting air. The Pt LiIi-edge and the Mo K-edge EXAFS spectra were obtained in a transmission mode at the BL-7C and BL-10B working stations of the Photon Factory in the National Laboratory for High Energy Physics (Proposal No 92001). The optical length of the EXAFS cells was 10 mm for the Pt Liii-edge and 5 mm for the Mo K-edge measurements. All EXAFS spectra were obtained at 298 K. Data were analysed using the EXAFS analysis program "EXAFSH" [2]. The amplitude and phase shift functions for the Pt-Pt and Mo-O bonds were extracted from EXAFS spectra obtained from Pt foil and K2MoO 4, respectively, at 298 K. The corresponding functions for the Pt-O, Pt-Mg and Mo-Mg bonds were theoretically calculated using the FEFF5 program [3].
2.3. Apparatus-Procedure The experimental apparatus employed to study the catalytic performance of the samples consists of a flow measuring and control system, the reactor and an on line analytical system. The reactor is a 30 cm long pyrex tube with an expanded 2 cm long section in the middle (8 mm I.D.) in which the catalyst sample is placed. The catalyst powder is held in place by means of quartz-wool pieces. The furnace temperature is controlled by means of a temperature controller using a K-type thermocouple placed between the reactor and the walls of the furnace. The temperature inside the catalyst bed is measured by means of a K-type thermocouple (0.5 mm O.D.) placed in a 1/16" O.D. ss well which runs through the center of the cell. Alkane dehydrogenation reactions were studied at reaction temperatures between 573 and
143 773 K at atmospheric pressure. In all experiments reported here the pure alkane was used in the feed. The flow rates used were 20 cc/min for i-butane, 15 cc/min for n-butane and 18 cc/min for propane. In a typical experiment 300 mg of a catalyst in powder form was placed in the pyrex reactor. The sample was then heated to 773 K under flowing nitrogen and calcined at the same temperature for 2 h under a flowing O2-N 2 mixture. After flushing with nitrogen to remove gas phase oxygen from the tubing and the reactor the sample was cooled down to the lowest reaction temperature examined. A flow of the reactant alkane was then measured through a loop bypass the reactor and at t=0 the alkane was introduced to the catalyst by means of two 3-way valves. The product distribution at the effluent of the reactor was recorded every 20-30 minutes for approximately 5 h. After the completion of a reaction run the catalyst was again heated at 773 K under an O2-N 2 mixture to remove the carbonaceous deposits and prepared for the next reaction run at a higher reaction temperature. "Blank" experiments showed that the pyrex reactor, the quartz wool pieces and the stainless steel thermocouple well did not affect at any measurable extent the kinetic measurements. Representative experiments were repeated to confirm the reproducibility of the results. The initial yield of alkenes was calculated by fitting the corresponding data of yield versus time-on-stream with exponential curves and extrapolating to t=0.
3. RESULTS AND DISCUSSION 3.1. EXAFS Analysis The EXAFS analysis of the spectra obtained from the fresh [PtMo6]/MgO sample after impregnating the MgO support with an aqueous solution of the heteropolyanion showed that the framework of [PtMo6024 ]8- was broken upon impregnation. No Pt-Mo bond was observable and the analysis of the F't Liii-edge and Mo K-edge EXAFS spectra only showed the existence of Pt-O (0.202 nm) and Mo-O (0.173 nm) bonds. Examination of the [PtMo6]/MgO samples after calcination at temperatures between 423 and 773 K showed that at temperatures lower than 573 K there is no observable interaction between the precursor and the support. On the contrary, calcination above 573 K led to the appearance of Pt-Mg (0.302 nm) and 'long' Pt-O (0.360 nm) bonds, while above 673 K a Mo-Mg bond (0.282 nm) was observed. The intensity of the corresponding k3-weighted Fourier transformed EXAFS peaks was found to increase upon increasing calcination temperature. It is apparent that upon calcination above 673 K both Pt and Mo atoms interact with the support. In Fig. 1 are shown the F't LIiI-edge and Mo K-edge EXAFS spectra of the [PtMo6]/MgO catalyst after calcination at 773 K. The corresponding curve fitting results are listed in Table 1. After calcination at 773 K, the bond lengths of Pt-O, Pt-Mg and Pt-O (long) for the [PtMo6]/MgO catalyst are 0.202, 0.302 and 0.360 nm, respectively (Table 1). These values are similar to the bond lengths between Mg-O (0.210 nm), Mg-Mg (0.297 nm) and Mg-O (0.364 nm) observed for MgO crystal, respectively [4]. This coincidence of the bond lengths indicates that after calcination at 773 K platinum atoms substitute Mg atoms of the MgO carrier. This is schematically shown in Fig. 2(a), where the local structure of the platinum atoms of the [PtMo6]/MgO catalyst after calcination at 773 K is drawn.
144
4i-,
-"-T"-'-"
(a)
10
~'
(~,
.
6
8
.
.
.
5
0
-5 6
4
8
Wavenumber
"
~
~.
i
.....l_._-
.....--.L.
,
10
k / 10
"fi ,
s
10
Wavenumber
n m "1
i
.
!
4
12
J
I
8
~S
~"
lO
"~
12
14
k / 10 n_m-1
1
I
I
i
1
1
2
3
4
5
R
/10-1nm
" (d)
E" 2
s --
1
2
Distance
f
~
3
R
4
/10
r
5
6
0
0
Distance
"lnm T
/
6
t
3 2 1
Zo -2
i-
-3 .z._
4
6
8
Wavenumber
10
k /10
12
n m -1
10
Wavenumber
12
k / 10 nm "1
Figure 1.
Pt LiIi-edge [(a), (c), (e)] and Mo K-edge [(b), (d), (f)] EXAFS spectra for [PtMo6]/MgO catalysts after calcination at 773 K. (a) and (b): k3-weighted x(k); (c) and (d): Fourier transform of k3-weighted x(k); (e) and (f): Curve fitting by (e) Pt-O + Pt-Mg + Pt-O waves and (f) Mo-O + Mo-Mg waves (solid line: observed and broken line: calculated); Fourier transform range: 30-130 nm ~ for Pt LIiI-edge and 35-145 nm -1 for Mo K-edge; Fourier filtering range: 0.10-0.35 nm -1 for Pt Liii-edge and 0.10-0.30 nm- 1 for Mo K-edge.
145
T a b l e 1. The curve fitting results of Pt LIII-edge and Mo K-edge EXAFS for [PtMo6]/MgO catalysts after calcination at 773 K.
R~ nm b
Na
AEo/eV c
o/nm d
Pt-O Pt-Mg Pt-O
6.2+0.9 8.1+1.2 4.8 •
0.202• 0.302+0.001 O.3 60• O01
10.6-~.0 7.4• - 2.1 •
0.0064• 0.0093• O.0064•
Mo-O Mo-Mg
1.6+0.4 3.0•
0.174+0.001 0.282•
-7.1• 15.0•
0.0059• 0.0102•
.
.
.
.
R factor e / %
0.6 O010
2.5
.
a: coordination number; b: bond distance; c: the energy difference between the origins of the photoelectron wave vector; d: Debye Waller factor; e: residual factor; Fourier transform range: 30-130 nm -1 for Pt Liii-edge and 35-145 nm -1 for Mo K-edge; Fourier filtering range: 0.100.35 nm -! for Pt Liii-edge and 0.10-0.30 nm -1 for Mo K-edge.
(a)
C) Mg2+ Opt4+ (b) Mo
Figure 2.
Models for the local structure around Pt atoms (a) and Mo atoms (b) for [PtMo6]/MgO based on the results of EXAFS analysis.
146
The oxidation state of the platinum ion in the calcined [PtMo6]/MgO and Pt-Mo/MgO catalysts was estimated from the 'white line' intensity of the corresponding Pt Liii-edge XANES spectra which is related to the d-electron vacancies of the absorbing atom [5]. The Pt LII I 'white line' intensity of the two samples was almost the same as that observed for the [PtMo6024] 8- heteropolyanion precursor, indicating that platinum atoms in these catalysts are in a Pt 4+ state. The Mo K-edge XANES spectra obtained from the [PtMo6]/MgO catalyst exhibited a preedge peak which is attributed to a normally forbidden ls-->4d bound-state transition [6]. It has been suggested that the intensity of this peak, which is more intense for tetrahedral symmetry, can be used as a measure of the number and shortness of the Mo-O bonds [6]. In our experiments, the intensity of the pre-edge peak of the samples after calcination at 773 K was found to be much smaller than that of MgMoO 4, in which Mo is tetrahedrally coordinated, suggesting that in the calcined samples Mo is octahedrally coordinated. Further evidence for an octahedral structure of Mo supported on MgO comes from detailed studies of a series of MoO3/MgO samples using XANES [Ta] and Laser Raman spectroscopy [Tb] which have shown that for catalysts with a Mo loading below 3.3 wt %, isolated distorted octahedral species are present on the MgO surface after calcination at temperatures above 673 K. This seems to be the case in our experiments, too. The coordination number of the Mo-O bond (Table 1) is much smaller than the expected from an octahedrally coordinated Mo 6+ species, which may be due to distortion of the structure. A model structure of the Mo 6+ species present on the calcined [PtMo6]/MgO catalyst is shown in Fig. 2 (b). The EXAFS spectra obtained from the Pt-Mo/MgO catalyst after calcination at 773 K revealed similar features to the ensemble catalyst except from the values of the coordination numbers, indicating that high temperature calcination results to the same local structure around the Pt and Mo atoms for both catalysts. The fact that the framework of the [PtMo6024] 8precursor breaks upon impregnation, something proven by the absence of Pt-Mo bonds, does not necessarily mean that the surface geometry around the Pt atoms is the same for both catalysts. It is possible that in the case of [PtMo6]/MgO platinum atoms are surrounded by six molybdates in a way similar to the precursor, while in the case of the conventionally prepared Pt-Mo/MgO catalyst Mo and Pt atoms are randomly distributed. After propane dehydrogenation reaction at 723 K for 4 h, only a Pt-Pt bond is observed in the EXAFS spectra of the [PtMo6]/MgO catalyst, indicating that under reaction conditions platinum atoms are reduced ( pt4+-->pt0 ). The size of the platinum particles formed after propane dehydrogenation reaction can be estimated using the correlation between the coordination numbers obtained from the EXAFS analysis and the morphology of metal particles [8]. Assuming spherical shape, calculations give particles of 1.0• nm in diameter (dispersion: 0.7• In the case of the Pt-Mo/MgO catalyst not only Pt-Pt bonds were observed after propane dehydrogenation reaction but Pt-O, Pt-Mg and Pt-O bonds as well, indicating that a part of the Pt atoms maintained their structure after calcination. From the results of the coordination numbers of the corresponding EXAFS data, the ratio of the unreduced Pt atoms was estimated to be 0.3• It is very probable that these unreduced Pt atoms are located at the Mg 2§ sites in bulk MgO. In the case of the [PtMo6]/MgO catalyst the results indicate that platinum atoms are located on the first surface layer. Assuming that MgO has (100) surface,
147 which is the most stable surface of magnesia [4], the coordination number of Pt-O, Pt-Mg and Pt-O is 5, 8 and 4, respectively, in very good agreement with the curve fitting results (Table 1).
3.2.
i-butane
dehydrogenation
Dehydrogenation of i-butane was studied in the temperature range of 573 to 773 K. Catalyst activity and selectivity were measured for all samples versus time-on-stream for about 5 h. In Fig. 3 (a) is shown the yield of i-butene versus time-on-stream for the ensemble [PtMo6]fMgO catalyst at different reaction temperatures. It is observed that the yield to i-butene remains constant with time-on-stream at reaction temperatures up to 723 K and only at 773 K a gradual decrease with time takes place. Selectivity to i-butene initially increases with time-onstream and reaches an almost constant value, typically above 96% with an increasing trend with time, after about 2 h in operation. The initial increase in selectivity is more pronounced at high reaction temperatures. Similar experiments have been conducted for all catalysts. The conventionally prepared Pt-Mo/MgO catalyst exhibited much lower activity and decreased selectivity compared to the ensemble [PtMo6]/MgO catalyst and was found to be deactivated much faster. Pt/AI20 3 exhibited a comparable catalytic performance with the Pt-Mo/MgO sample, while the monometallic Pt/MgO and Mo/MgO catalysts were found to be almost inactive at the experimental conditions used. The superior catalytic performance of the novel ensemble catalyst becomes pronounced if one plots the conversion or yield of the examined catalysts versus timeon-stream at any reaction temperature. In Fig. 3 (b) is shown such a plot for the reaction tempe-
15
i
12
Reaction Temp. (K)
(a) ~
9
10
m
"7, 6
._
573
o 623 A 673 o 723
v
.~ 10
O
0
03
~
G
:1: 100
e. --I e. 200
~3 ~
I 300
Reaction time (min)
Figure 3.
Pt/A.1203
9 Pt-MofMgO 9 [PtMo~/MgO
"9, 4 3
9
400
0
100 200 300 Reaction time (min)
400
(a) Isobutene yield versus time-on-stream for [PtMo6]/MgO in the temperature range of 573 to 773 K. (b) Isobutene yield versus time-on-stream at 723 K for the examined catalysts.
148 rature of 723 K. It is observed that the difference in yield to i-butene between the [PtMo6]/MgO and the rest of the examined catalysts increases with time on stream since, unlike the ensemble catalyst, the Pt-Mo/MgO and Pt/AI20 3 catalysts rapidly deactivate. As mentioned in paragraph 3.1 platinum was found to be reduced on the ensemble catalyst after reaction at 723 K. In order to examine the effect of this factor in the catalytic performance of [PtMo6]/MgO, the catalyst was exposed to flowing H 2 (40 cc/min) at 773 K for 2 h. After this treatment the activity of the catalyst was significantly lowered. The initial yield of i-butene was found to be, less than two thirds of the unreduced catalyst and, most importantly, deactivation was much faster, comparable to that of the Pt-Mo/MgO catalyst. Calcination of the sample at 773 K for 2 h did restore neither the activity nor the prior deactivation rate of the unreduced catalyst.
3.3. n - b u t a n e
dehydrogenation
Catalytic tests were also conducted using n-butane dehydrogenation as a probe reaction. The initial yield of n-butenes for all catalysts as a function of the reaction temperature is shown in Fig. 4 (a). As in the case of i-butane dehydrogenation, the ensemble catalyst is more active and its initial yield to n-butenes reaches the upper limits predicted by thermodynamics [9]. Selectivity towards n-butenes (1-butene, cis- and trans-2-butenes) was typically above 98% for the [PtMo6]/MgO ensemble catalyst, the rest being small amounts of C1-C 3 hydrocarbons. No butadiene was formed over the ensemble catalyst, even at 773 K. At reaction temperatures up to 688 K selectivity to n-butenes for the Pt/AI203 and Pt-MoflvlgO catalysts was slightly lower
1014 ~.~ 12 o = ION
-.... ~ ..r -A-
theoretical [9] [PtMo6]/MgO Pt/A]203 Pt-Mo/MgO
",t (03
* ~o6]/MgO 41,
8
.
Pt/Al20
3
o=~
.:/
a 8-
-
O
'~4
-~ 6 .~ 4 -
2 ~
2
-.,
0 ~ ....... i
500
Figure 4.
~
I
l
l
,
600 700 Reaction Temperature (K)
I 800
OH0
I ~.~ = I r-': I O0
200
~ ~300
I 400
Reaction time (min)
(a) Initial yield of n-butenes as a function of the reaction temperature for the examined catalysts. (b) Yield of n-butenes versus time-on-stream at 723 K for the examined catalysts.
149 but dropped to much lower values at higher reaction temperatures when deactivation became significant. A small amount of butadiene was produced over the latter two catalysts at 773 K. The deactivation rate is higher in this reaction compared to i-butane dehydrogenation and for the ensemble catalyst becomes significant even at 723 K. The same is also true for the rest of the examined catalysts which deactivate even more rapidly as observed in Fig. 4 (b) where the yield of n-butenes versus time-on-stream at 723 K is plotted.
3.4.
Propane
dehydrogenation
In the case of propane dehydrogenation too, the [PtMo6]/MgO catalyst exhibited a very good catalytic performance. In Fig. 5 (a) is shown the percentage yield of propene versus timeon-stream for the examined reaction temperatures. It is observed that the ensemble catalyst is very stable at reaction temperatures up to 723 K and that even at 773 K deactivation is slow compared to the i- and n-butane dehydrogenation reactions discussed above. Selectivity to propane initially increases at the first I-2 h on stream and reaches constant values typically above 97% in the continue. The Pt/AI20 3 and Pt-Mo/MgO catalysts also gave good results in this reaction. In Fig. 5 (b) is shown the yield of propene versus time-on-stream at 723 K for the three catalysts. As in the other reactions examined, the ensemble catalyst exhibited higher activity and was found to deactivate more slowly, although differences between the catalysts are not so striking in this case [Fig. 5 ( b)]. Selectivity to propene is almost the same for all catalysts at reaction temperatures above 673 K (97-98%) and only at lower temperatures higher selectivity values are observed for the ensemble catalyst.
Reaction (a.)
12
Temp. (K)
10 "~ 8
(tO
A m
.~ -
~".,...
9 o 9 zx 9
573 623 673 723 773
4 "~ 3
*~ 6 2
~.4
9 ta'tMo6]~gO
_
/x Pt-Mo/MgO II
.1~._--=1~ 0
Figure 5.
e. p
:=1
100 200 300 Reaction time (min)
! 400
0-1-
0
Pt/A1203
I
I
!
100 200 300 Reaction time (rain)
I 400
(a) Propene yield versus time-on-stream for [PtMo6]/MgO in the temperature range of 573 to 773 K. (b) Propene yield versus time-on-stream at 723 K for the examined catalysts.
150 4. C O N C L U S I O N S A novel [PtMo6]/MgO catalyst was prepared by using a [PtMo6024] 8- heteropolyanion precursor followed by calcination to increase the precursor-support interaction by chemical bonding. After calcination at temperatures above 673 K, where the precursor interacts with the support, Pt 4+ ions replace Mg2+ of the carrier surface while Mo6+ ions locate over the MgO surface in a distorted octahedrally coordinated framework. The [PtMo6] catalyst has unique ensemble sites composed of a Pt atom and six surrounding molybdates, where the Pt atom is bonded to surface oxygen and Mg atoms. The catalytic performance of the novel [PtMo6]/MgO catalyst during the alkane dehydrogenation reactions is much better compared to that of the conventionally prepared bimetallic and monometallic catalysts. The activity is higher, reaching the maximum limits indicated from thermodynamics, and the selectivity to the corresponding alkenes is typically above 97%. Coke formation seems to be inhibited by the specific surface geometry of the active sites.
REFERENCES 1. 2. 3.
4. 5.
6. 7.
8. 9.
U. Lee and Y. Sasaki, Chem. Lett., (1984) 1297. EXAFS analysis program "EXAFSH", coded by T. Yokoyama and T.Ohta, The University of Tokyo (1993). a) J.J. Rehr and R.C. Albers, Phys. Rev., B41 (1990) 8139 ; b) J.J. Rehr, J. Mustre de Leon, S.I. Zabinsky and R.C. Albers, J. Am. Chem. Soc., 113 (1991) 5135; c)J. Mustre de Leon, J.J. Rehr, S.I. Zabinsky and R.C. Albers, Phys. Rev., B44 (1991) 4146. V.E. Henrich and P.A. Cox in "The Surface Science of Metal Oxides", Cambridge University Press, 1994. a) F. W. Lytle, P.S.P. Wei and R. B. Greegor, J. Chem. Phys., 70 (1979) 4849; b) J.A. Horsley, J. Chem. Phys., 76 (1982) 1451; c) A.N. Mansour, J.W. Cook and D.E. Sayers, J. Phys. Chem., 88 (1984) 2330. C.T.J. Mensch, J.A.R. van Veen, B. van Wingerden and M.P. van Dijk, J. Phys. Chem., 92 (1988) 4961. a) S. R. Bare, G. E. Mitchell, J.J. Maj, G. E. Vrieland and J. L. Gland, J. Phys. Chem., 97 (1993) 6048. b) S. -C. Chang, M. A. Leugers and S. R. Bare, J. Phys. Chem., 96 (1992) 10358 R.B. Greegor and F. W. Lytle, J. Catal., 63 (1980) 476. S. Carra and L. Forni, in "Catalysis Reviews", Vol. 5, Edited by H. Heineman, Marcel Dekker, Inc., New York, 1972.
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
SPECTROSCOPIC Cr,Ti CATALYSTS
151
CHARACTERIZATION OF SUPPORTED Cr and : INTERACTION WITH PROBE MOLECULES
Bert M. Weckhuysen a, Israel E. Wachs b and Robert A. SchoonheydP a Centrum voor Oppervlaktechemie en Katalyse, K.U.Leuven, Kardinaal Mercierlaan 92, B3001 Heverlee, Belgium Zettlemoyer Center for Surface Studies, Department of Chemical Engineering, Lehigh University, Bethlehem, PA 18015, USA
b
Cr and Cr,Ti supported catalysts, with different support composition were investigated by Diffuse Reflectance Spectroscopy (DRS) and Electron Spin Resonance (ESR). The interaction between a series of molecules (HC1, CH2C12, CH3CH2CH2OH, CH3CH2OH, H20, CO, H 2, CH2CH2 and NH3) and supported Cr was evaluated. Interaction with acids results in hydrolysis and reduction, while bases give only reduction of the supported Cr. With ethylene, CO and H2, Cr 2§ is the dominant species on silica, while on alumina C r 3§ is preferentially formed. The redox behavior of Cr is also dependent on the sequence of impregnation. Impregnation of Cr followed by impregnation with Ti retards the reduction of Cr, while the reverse order facilitates reduction. These results supports the idea of a support controlled redox behavior.
1.INTRODUCTION Cr supported catalysts are known to be active in the polymerization of olefins [1,2]; hydrogenation and dehydrogenation reactions of respectively alkenes and alkanes [3], dehydrocyclisation reactions [3,4] and oxidation processes [5,6,7]. The catalysis is sensitive to minor changes in composition, preparation and treatment conditions [8]. For example, Ti is often incorporated in the silica support to control the molecular weight of the polyethylene polymer (lower average molecular weight and a broader molecular weight distribution) [9-12]. However, up to now the influence of these parameters on the properties of supported Cr are scarce. Diffuse Reflectance Spectroscopy (DRS) and Electron Spin Resonance (ESR) are excellent techniques in this regard because they allow the detection of Cr 6§ Cr 5§ C r 3§ and Cr 2.. In previous studies [13-15], we have shown that the molecular structure and redox behavior of Cr is dependent on the support composition, i.e. the SiO2-content. An increasing clustering of Cr is observed with increasing silica-content of the support, while a higher silicacontent facilitates the reduction of Cr to Cr 2§ Thus, the dichromate:chromate ratio and
152
Cr2+:Cr3+ ratio after reduction increase with increasing SiO2-content of the support, at least for low Cr-loadings. For higher Cr loadings, chromium clusters are detected (polychromates, Cr203) even on an alumina support. The purpose of this work is to use probe molecules in combination with DRS and ESR for studying the influence of the support composition (Si, AI) on the chemistry of supported Cr. Furthermore, the influence of Ti on the redox behavior of Cr is investigated.
2 . E X P E R I M E N T A L SECTION
2.1. Sample preparation The Cr catalysts were prepared by the incipient-wetness method with chromium(VI)oxide (CrO3) (UCB) onto alumina and silica. The chromium loading was 0.2 wt%. Silica and alumina were prepared by the sol-gel method starting from respectively TEOS and AI(iP)3 and H20, followed by titration. The obtained gels were dried, calcined and crushed. Details about the preparation method and the characteristics of the materials were published elsewhere [13,14] (TEOS = tetraethyl orthosilicate, AI(iP)3 = aluminium triisopropoxide). The Cr, Ti catalysts were prepared by impregnation with Cr('NO3)3.9H20 (Allied Chemical Co.) onto silica (Cab-O-Sil) followed by impregnation with Ti(iP)4 (iP = isopropyl, Aldrich) (preparation method 1) or in the reverse order onto the same support (preparation method 2). Ti impregnation was done in a toluene solution under nitrogen to avoid decomposition of the air-sensitive precursor The Cr- and Ti-contents were respectively 0.5 and 1.5 wt%.
2.2. Pretreatment and Experimental Techniques
Pretreatments. The Cr-catalysts were dried at 50 ~ for 8 h and granulated. The size fraction of 0.160.40 mm was loaded in a quartz flow cell with Suprasil window for DRS and a side arm for ESR. The samples were subsequently dried at 90 ~ during 16 h followed by calcination at 550 ~ during 6 h in an oxygen stream. DRS spectra were recorded on these calcined samples. The samples were then contacted during 30 min. with a N2-stream saturated with hydrochloric acid, ethanol, propanol, water, ammonia at room temperature and with carbon monoxide, hydrogen, dichloromethane at 400 ~ and ethylene at 100 ~ After each treatment, DRS spectra were taken. The Cr, Ti-catalysts were pretreated in the same way and DRS and ESR spectra were taken of the calcined samples. The samples were then reduced with CO at 400 ~ during 30 min.. After reduction ESR and DRS spectra were recorded. An oxygen flow of 3600 ml/h and a N2/probe molecule flow of 1800 ml/h were used for all the treatments.
Experimental Techniques. DRS spectra were taken with a Varian Cary 5 UV-Vis-NIR spectrophotometer at room temperature. The spectra were recorded against a halon white standard in the range 2200 200 nm, The computer processing of the spectra consisted of the following steps : (1) subtraction of the baseline; (2) conversion to wavenumber and (3) calculation of the KubelkaMunk (KM) function. ESR spectra were measured with a Bruker ESP300E instrument in the
153 X band (-~ 9.5 GHz) between 120 and 370 K. Absolute spin concentrations were determined by using Cu(acetylacetonate)/KC1 mixtures as standard for spin determinations (number of spins : 10 ~6 - 1019/g) after double integration of the recorded spectra.
3.RESULTS AND DISCUSSION
3.1. Interaction of probe molecules with supported Cr Dependent on the pretreatment and probe molecule, three oxidation states of Cr can be detected on the surface of the amorphous supports by DRS. Cr 6§ can be either chromate or polychromate and is detected by its charge transfer bands O ~ Cr 6+ (dO). Chromates have two strong bands at 27,000 - 30,000 and 36,000 - 40,000 cm ~, while dichromates (polychromates) possess an additional band at around 22,000 cm 1. Cr 3+ (d 3) and Cr 2§ (d 4) possess typical d-d absorption bands : Octahedral Cr 3§ 15,000- 18,000 cm 1 (4A2g ~ 4T2g), 22,000 - 25,000 (4Azg ~ 4Y~g(F)) and 30,000 - 36,000 (4Azg ~ 4T~g(P)); (pseudo) octahedral Cr 2 , 12,500 cm ~ (SEg --~ 5T2g) and (pseudo) tetrahedral Cr 2§ 7,500 - 10,000 cm -~ (ST2 ~ 5E) [13,14,16].
A
.= ==
I
40000
20()00 CM -1
Figure 1. DRS spectra of supported Cr/SiO 2 (A) and Cr/A1203 (B) after interaction with propanol at room temperature.
154 After calcination at 550 ~ the colours of the 0.2 wt% Cr/A1203 and Cr/SiO2 catalysts are respectively yellow and light orange. The spectrum of Cr/AI203 is dominated by two bands at 27,500 and 41,000 cm ~, typical for chromate. For Cr/SiO2, four bands at 15,500, 22,000, 30,500 and 40,500 cm R and a shoulder at 27,000 cm l are visible. They are due to the presence of chromate, dichromate and traces Cr 3+. After interaction with different probe molecule_ (H20, CH3C_I--I2OH,CHsCH.,CH2OH, HC1, CO, H2, CH2CH 2 and CH2C12) the colour of the samples and the related DRS spectra are drastically changed. The colour and observed Cr-species are summarized in table 1. Upon hydration of the Cr-catalysts, the colours are yellow for Cr/AI203 and yelloworange for Cr/SiO v The spectrum of Cr/A1203 consists of two bands at 26,900 and 36,500 cm ~, while the spectrum of Cr/SiO2 is dominated by bands at 22,500, 28,400 and 37,100 cm -~. The shift of the absorption at 38,000 cm ~ with respect to the same band after calcination (around 40,000 cm ~) is due to the transformation of an anchored Cr-species to a non-anchored Cr. Thus, the interaction of supported Cr with water vapour at room temperature results in a hydrolysis of the X-O-Cr bondings (with X = Si or A1). The molecular structure of the non-anchored Cr depends on the iso-electric point (IEP) of the support 9on alumina (IEP = 7-8) only chromate is observed, while on silica (IEP = 1 - 2) dichromate is the dominant species [ 14,15].
Table 1. Summary of the colour and DRS observable Cr-species on supported Cr-catalysts after interaction with small probe molecules. Cr catalyst -~
Cr/SiO2
Probe molecule ,1,
Colour
Cr-species
Colour
Cr-species
water
yellow-orange
dichromate, chromate
yellow
chromate
ethanol
yellow-green
Cr 6+, Cr 3+
yellow-green
Cr 6+, Cr 3+
propanol
yellow-green
Cr 6+, Cr 3+
yellow-green
Cr 6+, Cr 3+
hydrochloric acid
orange-red
Cr 6+ (Cr 3+)
deep orange
Cr 6+ (Cr 3+)
dichloromethane
green
Cr203
green
Cr203
carbon monoxide
blue
Cr 2+ (Cr 3+)
blue-green
Cr 3+ (Cr 2+)
hydrogen
blue
Cr 2§ (Cr 3+)
green-blue
Cr 3§ (Cr 2+)
ethylene
blue
Cr ~+ (C? +)
blue
Cr 3§ (Cr 2+)
ammonia
yellow-green
Cr 6+, Cr 3+
green-yellow
Cr 6+, Cr 3+
Cr/AI203
155 The DRS spectra of supported Cr catalysts after interaction with propanol at room temperature are shown in figure 1, and are identical to those of ethanol. Ethanol and propanol can hydrolyse the X-O-Cr bondings at room temperature resulting in the formation of nonanchored Cr on silica and alumina, with typical spectral features of respectively dichromate and chromate. However, this non-anchored C r 6. oxidizes alcohols and consequently C r 6isreduced to the green C r 3§ This is evidenced by a decrease in intensity of the O ~ C r 6. CTbands and the formation of an additional band around 16,000 cm I. Increasing of the contact time results in an almost complete reduction of C r 6+ to C r 3+. By addition of hydrochloric acid vapour onto calcined Cr/SiO2 and Cr/A1203, the colour is drastically changed to respectively orange-red and deep orange. On alumina three intense bands are observed at 24,300; 33,700 and 41,000 cm 1 and one shoulder around 19,000 cm ~. Treated Cr/SiO2 is dominated by three intense bands at 26,800; 33,700 and 40,300 cm !, one weak band at 14,500 cm I and one shoulder at 19,500 cm ~. These spectral features suggest the formation of traces of C r 3+ and polymeric C r 6§ Reaction of dichloromethane and oxygen at 400 ~ over Cr-catalysts results in a degradation to hydrochloric acid and CO/CO2 [17,18,19]. During reaction the color of the catalyst turns from yellow/orange to green. The spectra on silica and alumina are almost the same and consists of three bands at 15,100; 23,300 and around 30,000 cm ~, typical for
B A
==
i _,1 I.U
rn
I
40'000
C M -1
20000
Figure 2. DRS spectra of Cr/SiO 2 (A) and Cr/A1203 (B) after interaction at 400 ~ with CO.
156 octahedral Cr 3§ in an oxidic phase. The oxidation of dichloromethane with oxygen results in the formation of hydrochloric acid, which hydrolyses the X-O-Cr bondings. This free Cr 6§ at 400 ~ readily tranforms to Cr203, which cannot be reoxidized to Cr 6. by oxygen. The DRS spectra of supported Cr catalysts after interaction with CO and H2 are shown in respectively figure 2 and 3. Interaction at 400 ~ results in a total removal of the CT bands of Cr 6§ with the formation of d-d bands in the visible region, typically for reduced Cr (Cr 3§ and/or Cr2+). Blue Cr-silica catalysts are formed, while Cr/AI203 turns to blue-green and green-blue with respectively CO and H2. On silica, mainly octahedral Cr 2§ was formed with hydrogen, while with CO both tetrahedral and octahedral Cr 2§ is present. On alumina, mainly Cr 3§ is formed but traces of octahedral Cr 2§ are detected after reduction with H2. Thus, after reduction with CO and H 2 mainly Cr 3§ is formed on alumina, while on silica Cr 2+ is the dominant species. Reduction with ethylene at 100 ~ gives blue catalysts. On alumina the spectrum is dominated by bands at 17,200; 24,900 and 38,600 with a shoulder at 12,800 cm ~, while on silica the spectrum consists of bands at 16,400; 33,800 and a weak band at 10,000 cm". In any case, the amount of Cr 2+ is higher on silica than on alumina. The better catalytic performances of Cr/SiO 2 for ethylene polymerization [8] can be explained by the higher amount of Cr 2+ on the silica surface. The DRS spectra of ammonia treated supported Cr catalysts are totally different from
B
,
,,,m
,,
i
I
I
40000
20000 CM -1
Figure 3. DRS spectra of Cr/SiO2 (A) and Cr/A1203 (B) after interaction at 400 ~ with H 2.
157
those of the calcined samples. The CT- bands of C r 6+ a r e drastically decreased in intensity at the expense of absorption bands in the visible region of the spectrum. On alumina absorption bands at 14,900; 17,700; 28,100 and 38,300 cm ~ are observed. For Cr/SiO 2, bands are resolved at 14,900; 18,300; 27,000 and 36,900 cm l. The two latter bands are typical O ~ C r 6+ bands, while the two former absorptions are the first allowed d-d transitions of surface Cr3+-complexes. The high value of 17,700 and 18,300 cm l for the first transition suggests the presence of at least one NH 3 molecule in the coordination sphere around Cr. In summary, the interaction between a series of molecules with increasing basicity (HC1 < CH2C12 < CH3CH~CH2OH ~ CH3CH2OH < H 2 0 < C O ~ H 2 ~ CH2CH 2 < NH3) and Cr supported silica and alumina was spectroscopically evaluated. Two phenomena can occur : (1) hydrolysis of the Cr-O-Support bonding and (2) reduction of C r 6§ t o a lower oxidation state. It is concluded that interaction with (weak) acids results in hydrolysis and reduction, while (weak) bases give only reduction. Reduction of Cr with ethylene, CO and H2 goes mainly to Cr 3§ on Cr/alumina, while on Cr/silica a mixture of Cr 2§ and Cr 3§ is formed. Interaction with ethanol and propanol gives reduction of non-anchored C r 6§ t o C r 3+ and consequently an oxidation of the alcohols. The observation that silica facilitates reduction, while on alumina reduction of Cr is more difficult, supports the idea of a support controlled redox behavior. Thus, the supports take actively part in the redox chemistry of Cr. This can be explained with the hardnesssoftness concepts, first introduced by Pearson : the higher the Al-content of the support, the harder the support, the less polarizable or susceptible for electron fluctuations, the more difficult reduction [ 13,14].
3.2. Influence of Ti on the interaction of CO as probe molecule with supported Cr
After calcination the supported Cr, Ti catalysts are orange, independently of the preparation method. The related ESR spectra show an axially symmetric signal with ga_ = 1.96 and gll - 1.92, which is due to isolated Cr 5+ (7-signal) [8]. With DRS, the typical O C r 6+ CT bands are detected. After reduction with CO at 400 ~ the colour turns to bluegreen and green for respectively the first and second preparation method. The obtained DRS and ESR spectra are dependent on the preparation method. In ESR, a weak Cr 5§ signal is observed on a broad isotropic signal with g around 2 (13-signal, due to clustered C r 3§ [8]). Quantitative ESR show that more than 98 % of the CrS+-species is reduced away for the Cr, Ti supported catalyst prepared by the first method, while for the other catalyst this amounts to 80 %. With DRS, octahedral and tetrahedral Cr 2§ are observed on the catalyst surface prepared by the first method, while C r 3§ is the dominant species for the catalyst prepared by the second method. In summary, the redox behavior of Cr on amorphous supports is dependent on the order of impregnation of Ti and Cr. When Cr is impregnated before Ti, the reduction of Cr is more difficult (more C r 3§ and CrS§ while the reverse order facilitates the reduction of Cr (more Cr2+). This can be explained by a covering Cr by TiOx, resulting in a redardation of the reduction.
158 4.CONCLUSIONS
Diffuse Reflectance Spectroscopy (DRS) and Electron Spin Resonance (ESR) are valuable spectroscopic techniques for the evaluation of the interaction of probe molecules with supported Cr and Cr, Ti catalysts. Two phenomena can take place upon interaction of supported C r 6§ with various probe molecules : (1) hydrolysis of the support-Cr bond and (2) reduction of supported Cr. The latter process is influenced by the support composition, the presence of additional elements (Ti, AI) and the sequence of impregnation. The supports take actively part in the redox chemistry of Cr and therefore one can suggest a support controlled redox behavior.
ACKNOWLEDGMENTS
B.M.W. acknowledges the Belgian National Fund for Scientific Research (N.F.W.O.) for a grant as research assistant. This work was financially supported by the Fonds voor Kollectief Fundamenteel Onderzoek (FKFO) under grant N ~ 2.0050.93.
REFERENCES
1. Hogan, J.P.; Banks, R.L. Belg. Pat. 530617, 1955. 2. Hogan, J.P.; Norwood, D.D.; Ayres, C.A.J. Appl. Polym. Sci. 36 (1981) 49. 3. Grunert, W.; Saffe:., W.; Feldhuas, R., Anders, H. J. Catal. 99 (1986) 149. 4. Kn6zinger, H.; Ratnasamy, P. Catal. Rev.-Sci. Eng. 17 (1978) 31. 5. Richter, M.; Ohlmann, G. React. Kinet. Catal. Lett. 29 (1985) 211. 6. Parlitz, B.; Hanke, W.; Fricke, R.; Richter, M.; Roost, V.; Ohlmann, G. J. Catal. 94 (1985) 24. 7. Kim, D.S.; Tatibouet, J.M.; Wachs, I.E.J. Catal. 1992, 136, 209. 8. McDaniel, M.P. Adv. Catal. 33 (1985) 47. 9. Pullukat, T.J.; Hoff, R.E.; Shida, M. J. Polym. Sci., Polym. Chem. Ed. 18 (1980) 2857. 10. Conway, S.J.; Falconer, J.W.; Rochester, C.H.J. Chem. Soc., Faraday Trans. 1 85(1) (1989) 71. 11. Conway, S.J.; Falconer, J.W.; Rochester, C.H.J. Chem. Soc., Faraday Trans. 1 85(7) (1989) 1841. 12. Daniel, M.P.; Welch, M.B. Dreiling, M.J.J. Catal. 82 (1983) 118. 13. Weckhuysen, B.M.; De Ridder, L.M.; Schoonheydt, R.A.J.Phys. Chem. 97 (1993) 4756. 14. Weckhuysen, B.M.; Verberckmoes, A.A.; Buttiens, A.L.; Schoonheydt, R.A.J.Phys. Chem. 98 (1994) 579. 15. Weckhuysen, B.M.; Schoonheydt, R.A.; Jehng, J.-M.; Wachs, I.E.J.Phys. Chem. (submitted for publication) 16. Szabo, Z.G.; Kamaras, K.; Szebini, S.; Ruff, I. Spectrochim. Acta 34a (1978) 607. 17. Petrosius, S.C.; Drago, R.S.J. Chem. Soc., Chem. Comm. (1992) 344. 18. Chatterjee, S.; Greene, H.L.J. Catal. 130 (1991) 76. 19. Van Loco, J. Engineer Thesis, K.U.Leuven (1993).
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
159
A New Supported Dehydrogenation Catalyst: Influence of the Support and Preparation Variables L.A. Boot a, A.J. van Dillen a, J.W. Geus a, F.R. van Buren b and J.E. Bongaarts b
a:
Department of Inorganic Chemistry, Debye Institute, Utrecht University, P.O. Box 80083, 3508 TB Utrecht, the Netherlands b: Dow Benelux N.V., P.O. Box 48, 4530 AA Terneuzen, the Netherlands The preparation of titania- and zirconia-supported dehydrogenation catalysts using pre-shaped support bodies was investigated. Catalysts containing only iron and catalysts containing a potassium additive were prepared using simple salt and chelating agent precursors. The effect of the drying time on the distribution of the active components was studied. Catalysts were characterized using microscopic techniques, XRD, TPR, magnetic analysis, DRIbTS, XPS, M6ssbauer Spectroscopy, and by the catalytic dehydrogenation of 1-butene. It was possible to prepare catalysts showing a good interaction of the active components with the support, with a uniform distribution over the support pellets. This interaction proved detrimental with titania-supported catalysts, which deactivate rapidly due to the formation of mixed compounds of iron and/or potassium with the support. Zirconia-supported catalysts, however, proved to be very stable in the dehydrogenation reaction, which was attributed to the formation of a finely divided supported phase. 1.
INTRODUCTION
Bulk iron oxide based catalysts used in non-oxidative dehydrogenation reactions are known to deteriorate under the reaction conditions imposed in the presently applied processes [ 1,2]. To solve problems related to iron oxide phase transformation and potassium migration a supported catalyst system has been developed that provides a matrix supporting and stabilizing both the iron phase and the potassium promoter. Stobbe et al. [3] used magnesia as a support material which displayed a superior performance in butene dehydrogenation as compared with a commercial reference catalyst. This was attributed to the advantageous formation of supported potassium ferrite in the preparation stage, which yields potassium carbonate entities which are finely divided over the iron oxide containing phase upon decomposition. Thus, by applying the potassium and iron containing phases onto the support in intimate contact, the preparation route to a catalyst displaying this desirable molecular scale design became feasible. However, magnesia, the support used by Stobbe et al., may react in the presence of water vapour at low temperatures (<250~ to the hydroxide (brucite). This could diminish the mechanical strength of the support bodies. Therefore, in the present work research on the behaviour and applicability of other support materials is described. Selection criteria for other supports include sufficient thermostability and stability towards steam, availability as pre-shaped supports, proper acid-base characteristics and an adequate interaction towards the components to be applied. The latter two demands are not immediately obvious. The first of these is connected to the negative influence of acidic materials on hydro-
160 carbon cracking and carbon deposition. More basic oxides might display a smaller tendency to catalyze these reactions. The final requirement relates to the effects observed more often in supported catalysts: formation of bulk phase mixed oxides might result in surface depletion of the elements responsible for either dehydrogenation or gasification of carbonaceous deposits, whereas an insufficient interaction causes sintering of the applied components. In view of one or more of the aspects mentioned above, silica and alumina were considered less suitable for application in non-oxidative dehydrogenation. In this paper, the use of titania and zirconia is investigated. Titania and zirconia are both placed among the amphoteric oxides, having both acidic and basic reactivity. However, they do not display any strong (intrinsic) acidic properties [4]. Therefore, no excessive carbon deposition is anticipated. Also, several examples of titaniaand zirconia-supported catalysts have been reported [5-7]. However, none of these are employed in dehydrogenation reactions. So, whether the interaction between active component, additive and the present supports will fulfill the demands of the dehydrogenation process with its rather severe conditions (600~ 30% steam) is not known. It is not expected that zirconia will form mixed compounds with iron or potassium [8, 9], which are the catalytically active components, but several compounds containing both titanium and iron and/or potassium are known [e.g. 10]. Consequently, the interaction of the two oxides with iron and potassium during catalyst preparation and testing will have to be investigated. No examples or recipes of catalysts prepared using commercially available pre-shaped titania or zirconia supports have been found in literature. Therefore, the result of the use of preparation procedures, such as impregnation using chelating compounds, will be of interest as well. 2.
EXPERIMENTAL
2.1.
Preparation of the catalysts
Catalysts were prepared by incipient wetness impregnation. Daiichi RSC-H zirconia (100% monoclinic, pellets O 3 mm) and Degussa 7701 titania (mainly futile, ca. 85%, pellets O 4.5 mm), specified to be >97% and >99% chemically pure, respectively, were treated at 850~ and 700~ respectively, in air for 16 h to obtain a stable specific surface area of about 20 mZ/g for both supports and pore volumes of about 0.25 and 0.15 ml/g. Also Engelhard L6132 zirconia (mixture of monoclinic and tetragonal, ca. 60%-40%, pellets O 3 mm) was used to study the influence of the presence of a different crystal modification. To investigate the effect of support pre-treatment on zirconia-supported catalysts, also catalysts were prepared using fresh, as-received supports. Ammonium Fe(IIl)citrate (Merck, 28% Fe), ammonium Fe(III) EDTA (prepared according to Stobbe et al. [ 11 ]) or iron nitrate nonahydrate (Merck) were used as precursor compounds; precursor solutions were added to evacuated support bodies to establish a loading of 3 wt% Fe. Potassium containing samples (typically 3 wt% K) were prepared by co-impregnation with ammonium Fe(III)citrate and potassium carbonate solutions. Catalysts were dried either rapidly in flowing air at room temperature for 2 h, or slowly in static air for 16 h. Finally, calcination of the samples took place in flowing air, by the following procedure: 150~ for 2 h; 500~ for 2 h; 750~ for 16 h (ramp between stages: 5~ A physical mixture of pretreated zirconium or titanium dioxide and iron ([!I) oxide (3 wt% Fe) was prepared by manually grinding the components in a mortar. The iron oxide was prepared by calcining ammonium Fe(III)citrate (Merck, 28% Fe) in air at 750~ for 16 h.
161
2.2.
Characterization
Inspection of samples by microscopy. Samples were examined in a Leitz light microscope and a Philips EM420 transmission electron microscope (120kV), mainly by bright-field techniques. Element analysis (EDAX) was performed in a Philips CM-20 (200kV) equipped with a FEM/STEM field-emission gun. X-ray Diffraction. Powder XRD was carried out in a Philips powder diffractometer mounted on a Philips PW1140 X-ray generator with Fe K~,2 radiation (1.93735 ~). Temperature-Programmed Reduction. Reduction experiments were carried out in an atmospheric flow reactor using a thermal conductivity detector to monitor hydrogen consumption. Water produced in the reduction reaction was frozen out using a CO 2 (s/g) cold trap. A fractured catalyst sample (particle size: 0.5-0.85 mm) was reduced in a 10 vol% H2/Ar gas flow (50 ml/min), while the temperature was raised from RT to 900~ with a linear heating rate of 5~ Thermo-Magnetic Analysis. High-field magnetic measurements to study the reduction behaviour were performed using a modified Weiss-extraction technique as described by Stobbe et al. [11]. The apparatus allowed in situ magnetization measurements during reduction of fractured catalyst samples (0.5-0.85 mm) in a 10 vol% H2/Ar flow, while the temperature was raised up to 525~ with a heating rate of 0.5~ Analyses were performed at a field strength of 7000 Oe. Diffuse Reflectance Infra-Red Fourier Transform Spectroscopy. Spectra were acquired in situ using a DRIFT accessory (Collector) equipped with an environmental cell (Spectratech). DRIFT spectra were recorded in nitrogen up to about 500~ (to avoid absorption bands due to physisorbed or hydrogen-bridged water) on a Perkin Elmer 1600 FTIR spectrometer (64 scans, resolution 8 cm-1).
X-ray Photoelectron Spectroscopy. Spectra of powdered samples were recorded on a VG Microtech XP Spectrometer equipped with a Clam II hemispherical analyzer, using a Mgsource (1253.6 eV) operated at 10 mA. To calculate peak areas, backgrounds were subtracted according to a procedure suggested by Shirley [ 12]. MOssbauer Absorption Spectroscopy. Spectra were acquired at room temperature in a constant acceleration spectrometer using a 57Co in Rh source. Isomer shifts are relative to the NBS standard sodium nitroprusside. Magnetic hyperfine fields were calibrated with the 515 kOe field of ot-Fe203 at RT. MiSssbauer parameters were determined by fitting the collected spectra with reference sub-spectra consisting of Lorentzian-shaped lines using a non-linear iterative minimization routine. Test reaction. Non-oxidative dehydrogenation experiments were carried out in an automated apparatus as described by Stobbe et al. [ 13]. Calculations of conversion, selectivity to product i and yield of product i were carried out in the same manner as was done earlier [ 13]. A gas mixture (atmospheric pressure) of 5 vol% 1-butene (Hoek: Loos, 2.5), 30 vol% steam (added by leading the gases through a saturator kept at 70~ in N 2 (Hoek Loos, 5.0) yielding a molar ratio of water/1-butene = 6, was passed upstream through the catalyst bed which had been previously heated up to 600~ Typically, about 1g of catalyst was used at a gas flow rate of 50 ml/min, resulting in a 1-butene weight hourly space velocity of 0.35 g/g.h.
162 3.
RESULTS AND DISCUSSION
3.1.
Catalyst preparation
Catalyst preparation and inspection by microscopy. By light microscopy the macroscopic distributions of the iron compound over the support could be examined after impregnation, drying and calcination. It was established that on both supports the redistribution effects described below occurred exclusively during the drying step. This step is known to greatly influence the macroscopic distribution of an active phase through the support pellets [ 14]. It was observed that on pre-treated titania, a homogeneous distribution was obtained using the ammonium citrate or nitrate precursors. The use of EDTA as a chelating agent resulted in an egg shell distribution. The obtained results confirm that the application of a badly crystallizing, viscous complex precursor does not automatically yield a homogeneous distribution throughout the catalyst pellet, whereas employment of a simple salt can bring about the aimed uniform distribution [ 15]. This behaviour is caused by the acidic properties of the titania support pellets. The eggshell obtained with EDTA might be a result of the acid-base behaviour of the Fe-EDTA complex, which is present as the easily crystallizing NH4FeEDTA species at low pH values. During the drying step capillary forces give rise to transport of the dissolved precursor to crystals formed at the outer edge of the titania pellet. All transported material then crystallizes in this outer edge region. In a more basic environment or with a basic support (as MgO) badly crystallizing anions are present, which leads to a more homogeneous distribution [ 11 ]. In the case of nitrate titania presumably reduces the iron hydroxide oligomer concentration in the solution due to its lower surface pH, which can be derived from the colour change from brown to yellow upon impregnation. Magnesia displays the opposite effect: immediate hydroxylation of the dissolved iron (III) ions takes place upon contact with the solution [ 11]. With pre-treated Daiichi zirconia, different results were obtained. When a high drying rate was applied, using nitrate and EDTA an egg shell distribution was obtained. Even using ammonium Fe(III)citrate, an iron concentration gradient was always visible after drying at high rates. Therefore, a low drying rate was applied also, which resulted in homogeneously coloured pellets for the ammonium citrate precursor. Using nitrate, a slight gradient remained visible, whereas with EDTA an effect of the impregnation pH could be observed, in a similar way as has been reported for silica-supported catalysts [ 15]. However, on zirconia best results were obtained with EDTA solutions just above the uncorrected pH after preparation of the complex (pH = 5.5), viz., at pH = 6 or pH = 7. Increasing the pH of the solution to higher values did not improve the distribution, but, on the contrary, yielded egg shell distributions. These results did not differ when catalysts were prepared using pre-treated Engelhard material, or when as-received supports were impregnated. Finally, by co-impregnation catalysts containing both iron and potassium were prepared using ammonium iron citrate and potassium carbonate as precursors. Judging from light microscopy, it was possible to obtain catalysts with a homogeneous distribution on both supports. Again, best results were obtained using a low drying rate. However, the influence of the drying rate was less pronounced than with catalysts containing iron only. Possibly the formation of a complex in which the ammonium group is replaced by a potassium ion is playing a role here. Little is known in literature about both the solid state or solution chemistry of these complexes, especially at concentrations approaching the maximum concentration as in the
163 solutions used here. However, the formation of gaseous ammonia can be observed clearly when preparing the impregnation solution (indicating the progress of the substitution reaction) and although the precise function of the ammonium citrate complex is not known as yet, it is conceivable that this replacement may induce different crystallization or adsorption properties. One or both of these altered properties might account for the better results of co-impregnation. On the other hand, the mere presence of a higher amount of dissolved precursors probably increases the viscosity, which could also improve the desired solution characteristics for obtaining a homogeneous distribution. However, the viscosity of the original solution containing only the iron complex is already quite high, which makes this explanation not very likely. On the two oxides, a different phase was formed. With zirconia, homogeneously coloured, olive green pellets were obtained, which changed to light brown very rapidly upon exposure to atmospheric air. This indicates the formation of a potassium ferrite phase. On titania, this phenomenon was not observed, as the brownish colour was present already immediately after calcination. In TEM, the inspected calcined samples containing only iron did not show large differences for the two supports. Besides the support crystallites hardly any other material is visible in the catalysts displaying good macroscopic distributions. With EDAX, it was established that on zirconia finely divided iron species were present. This indicates that also a microscopically well dispersed catalyst is obtained. In co-impregnated sampies, the active components ~~i." 9 . ~ , "~. , ,,. ~ ~, ...~ .~, can be located with TEM. A representative micrograph of a sample of 3% iron and 3% potassium on zirconia is presented in Fig. 1. In this sample, it appears that the support is covered completely with a layer of deposited material.
Fig.1. TEM Micrograph of 3% iron and 3% potassium on zirconia (Daiichi) after calcination.
3.2.
Characterization
X-Ray Diffraction. Diffractograms of the catalysts containing iron only show that crystalline phases containing iron are formed upon calcination at 750~ In catalysts supported on titania as well as on zirconia hematite (tx-Fe203) is formed. Only in the catalyst samples with good iron distributions, the most intense diffraction peak (hematite (104)) is hardly discernible
164 from the noise in the diffractogram, indicating the presence of only a small amount of larger crystalline iron oxide particles. The particle sizes calculated from line broadening range from 200-300/~. As stated earlier, no large iron particles were observed in TEM, but comparison of the diffractograms of catalysts with that of the physical mixture suggests that only a small amount of larger iron oxide particles was formed. Moreover, the crystallinity of the support can obscure the presence of scarcely present hematite crystallites in TEM, so these results do not necessarily disagree. When potassium is applied also, a different behaviour is observed for the two oxides. In titania-supported catalysts a mixed compound containing titanium, iron and potassium is formed, probably the non-stoichiometric oxide Ko.aFe0.aTil.204 [16]. Since the colour of the samples supported on zirconia after calcination indicated the formation of potassium ferrite (KFeO2), which is known to decompose readily in atmospheric air, diffractograms were recorded excluding air. In these catalysts, however, only excess potassium carbonate was observed, and no diffraction lines emanating from other phases than zirconia were detected. In combination with the results from TEM, it is concluded that a finely distributed phase, most probably potassium ferrite, was formed, in which iron and potassium are intimately mixed.
40 a
30 o r~
b
20
C
10 rj [.., 0
d 300
600
Fig. 2. TPR of zirconiasupported catalysts: a: 3%Fe, 3%K; b: ex citrate; c: on Engelhard; d: ex EDTA
900
Temperature (~
Temperature-Programmed Reduction and Thermo-Magnetic Analysis. TPR profiles of zirconia-supported samples are displayed in Fig. 2. Titania-supported samples have been reported on elsewhere [ 17]; they will not be treated in detail here, in view of their bad catalytic performance (vide infra). It can be derived that the supported phases display an interaction with the support which causes reduction of the iron phase to take place over a larger temperature range than is found with bulk catalysts and physical mixtures [6, 17]. To ascribe the reduction peaks to the transformations of distinct iron oxides by analyzing the reduction patterns only is difficult. E.g., in the pattern of the catalyst ex citrate, at least four reduction stages can be seen; it is obvious that the pattern cannot be explained by stating that it represents the reduction of hematite to magnetite, wustite and t~-iron, as is done commonly [e.g., 6]. Despite the dynamic nature of these investigations additional information could be obtained from magnetic measurements. High-field measurements have been performed to follow the magnetic properties during TPR. The general course of the processes occurring during reduction is confirmed: in the beginning formation of a magnetic phase, magnetite, is observed, after which the magnetic signal slightly decreases, to increase eventually to a high level, indicating the formation of magnetic iron. This is in line with TPR, which also indicates paral-
165 lel reduction of iron(HI) and iron(H) taking place in the sample. Moreover, the calculated final extent of reduction is about 90%, showing that a large portion of Fe(0) is present in the end of the reduction experiments. Some conclusions can be drawn about the interaction of the iron-on-zirconia catalysts. In catalysts containing only iron, it is observed that while the first onset of reduction is equal in all samples, complete reduction is retarded in the catalysts prepared using the Engelhard support and with EDTA. Addition of potassium shifts the onset temperature of the initial reduction to temperatures about 100~ higher than in catalysts without potassium. This effect has been observed before [3]. It can be stated that catalysts of differing interaction with the zirconia support have been prepared. The possible effect of these differences will be investigated using the test reaction.
DRIFTS, XPS and MOssbauer Spectroscopy. Further characterizations are reported for zirconia-supported catalysts only, again in view of the bad catalytic performance of the titaniasupported catalysts (vide infra). The region which incorporates the characteristic hydroxyl frequencies of zirconia was monitored in situ at various temperatures. The two characteristic frequencies found to be present up to at least 500~ for the zirconia support (3725 and 3643 cm -1) changed into one band (3680 cm -1) for the catalyst containing iron, which might be attributed to an iron hydroxyl group. This absorption band had completely disappeared at about 400~ which supports this assignment. The samples containing potassium did not show any hydroxyl frequencies, which is a result of the anchoring of iron and potassium to the surface of the support, yielding a complete coverage of the zirconia surface. XPS was used to determine the iron oxidation state and the dispersion of the applied iron phase of the iron-on-zirconia catalysts. As expected, iron was found to be in the trivalent state. Dispersion calculations (performed as published elsewhere [ 17]) indicated that no significant difference in dispersion was obtained when using either different precursors, or different supports, and whether or not applying a thermal pre-treatment. The iron oxide particle sizes calculated according to the procedure of Kuipers et al. [18] range from 100 to 150]k, which is smaller than found with XRD. This can be explained, however, when taking the specific sensitivities of XPS and XRD into account. Also from analysis of the M6ssbauer spectra it is apparent that iron is in the oxidation state (Ill), which is in line with the results obtained with other techniques. It was found that on zirconia a bi-modal size distribution might be present. This observation relates the results of XRD, which demonstrated the presence of larger particles, to those of TEM, DRIFTS and XPS, which indicated the existence of small particles. It also provides a possible explanation for the complicated TPR patterns, which might result from the reduction of differently sized iron oxide particles. Test reaction. The results of non-oxidative butene dehydrogenation experiments are represented in Figs. 3-5. As expected, catalysts without potassium deactivate rapidly during dehydrogenation, since no compounds capable of gasifying carbonaceous deposits are present in these systems (Fig. 3) [3,11]. However, the deactivation of iron-on-titania proceeds relatively more rapidly. This is attributed to the formation of inactive ilmenite (FeTiO3) in this catalyst under reaction conditions [17]. When potassium is added to avoid deactivation (Fig. 4), it can be seen that deactivation is effectively suppressed in the system supported on zirconia, but that the system
166
based on the titania support deactivates as rapidly as the system containing only iron as the active component. The formation of the mixed compound (as detected with XRD) is detrimental to the activity of the iron-and-potassium-on-titania catalyst. The small initial butene conversion probably results from some free iron oxide still present in the catalyst after calcination.
50 = 40
~30 o= 20 ~
10
ffi
•ffi
•ffi
-
0
0
3
6
9
Time (h) Fig. 3.
Butene conversion of iron-on-zirconia (El) and iron-on-titania (11) catalysts versus time-on-stream
~. 50 = 40 O
~ 30 >
o 20 10 0 0
3
6
9
12
15
Time (h) Fig. 4.
Butene conversion of iron/potassium-on-zirconia (El) and iron/potassium-ontitania (11) catalysts versus time-on-stream
In Fig. 5, the influence of the variation in interaction of the iron with the support as seen in TPR seems to be reflected in the rate of deactivation displayed by the various catalysts. This can be understood if the two deactivation regimes (0-2 h and 2.5-4 h) are interpreted as being dominated by reduction and carbon deposition, respectively. Butene or other hydrocarbon molecules are able to reduce the active phase faster and more efficiently if the dispersion of the iron phase is higher. As is seen in titania-supported catalysts, irreversible reduction from Fe(III) to Fe(II) can be disastrous for the catalytic activity. If potassium is added to the catalyst, gasification of carbonaceous species from the surface can keep the surface accessible for
167 the feed. If a continuous layer of intimately mixed iron and potassium oxides is applied onto the support, as in the zirconia-supported catalysts, a high activity is maintained. ~ 60 ~ 50 0
"'~ 40 30 o
20 N 10 m
0
0
I
I
t
2
4
6
8
Time (h) Fig. 5.
4.
Butene conversion of iron-on-zirconia catalysts ex citrate (El), ex EDTA ( . ) and on Engelhard support (11)
FURTHER DISCUSSION AND CONCLUSIONS
It was possible to prepare titania or zirconia-supported iron-potassium catalysts displaying a macroscopically as well as microscopically homogeneous distribution. The characterization results, however showed an important difference between the two oxidic supports: In titania-supported catalysts, both unpromoted and promoted with potassium, the supportapplied phase interaction is too strong, causing the formation of various mixed oxides. These solid phase reactions are observed both during preparation and during the test reaction. This is reflected in the catalytic performance, which displays a fatal deactivation within a short period of time. Considering impregnated zirconia-supported catalysts, however, the expected solid state reactions take place in a similar, advantageous, way as found with the previously investigated magnesia-supported system: The formation and subsequent decomposition of a finely divided supported potassium ferrite phase can be put to use to obtain a supported catalyst system that does not display any deactivation during the observed period of time. Next to this absence of deactivation, the zirconia-supported catalysts display a butadiene yield which is equally high as has been reported for the magnesia-supported system [3]. While no bulk compounds with the support oxide are formed, it is deduced from the applied surface analysis techniques that anchoring and spreading of the applied components takes place, which stabilizes the applied iron oxide phase. Industrial application of zirconia-supported dehydrogenation catalysts therefore seems to be viable. In summary, the concept of using the interaction between the support and the applied active phases proves to be applicable to zirconia, but not to titania. This evidences clear that a careful selection and characterization of candidate supports is necessary, emphasizing the importance of the possible formation of mixed compounds of the applied components and the support material. The employment of a mixed potassium-iron compound, probably potassium ferrite, KFeO 2, to obtain a well-defined supported catalyst able to dehydrogenate and to prevent carbon deposition, was shown to be also applicable to zirconia, yielding a catalyst exhibiting a more stable activity than titania- or magnesia-supported catalysts.
168
Acknowledgments The authors wish to thank H.J. Vermeer and E.K. de Wit for additional experimental work and discussions. A.J.M. Mens and O.L.J. Gijzeman of the Surface Science Department (Debye Institute, Utrecht University) are acknowledged for performing the XPS work. A.M. van der Kraan and A.A. van der Horst of the Interfacultair Reactor Instituut (Delft University of Technology) have performed the M6ssbauer experiments and analysis of the presented data, for which they are gratefully acknowledged. The HR-TEM and EDAX experiments were performed at the laboratories of Philips N.V., Eindhoven (Netherlands).
References 1. 2. 3. 4. 5. 6.
7. 8. 9. 10. 11. 12. 13. 14. 15.
16. 17. 18.
B.D Herzog and H.F. Rase, Ind. Eng. Chem. Prod. Res. Dev., 1984, 23, 187. P.G. Menon, Catal. Today, 1991, 11, 161. D.E. Stobbe, F.R. van Buren, A.J. van Dillen, J.W. Geus, J. Catal., 1992, 125, 548. K. Tanabe, Solid Acid and Base Catalysts, in Catal.- Sci. Tech., vol. 2, eds. J.R. Anderson and M. Boudart, Springer, Berlin, 1981, p.231. R.I. Bickley, T. Gonzalez-Carreno and L. Palmisano, Mater. Chem. Phys., 1991, 29, 475. J.G. van Ommen, H. Bosch, P.J. Gellings and J.R.H. Ross, in Stud. Surf. Sci. Catal., vol. 31 (Prep. Catal. IV), eds. B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet, Elsevier, Amsterdam, 1987, p. 151. W. Ji, S. Shen, S. Li and H. Wang,in Stud. Surf. Sci. Catal., vol. 63 (Prep. Catal. V), eds. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon, Elsevier, Amsterdam, 1991, p. 517. M. Tournoux, Rev. Hautes Temp. R~fract., 1964, 1 343. G. Marest, C. Donnet and J.A. Sawicki, Hyperfine Interact., 1990, 56, 1605. Gmelins Handbuch der Anorganischen Chemie, Vol. 8, Syst. Nr. 41 (Ti), Verlag Chemie, Weinheim, 1951 D.E. Stobbe, F.R. van Buren, A.W. Stobbe-Kreemers, J.J. Schokker, A.J. van Dillen and J.W. Geus, J. Chem. Soc., Faraday Trans., 1991, 87, 1623. D.A. Shirley, Phys. Rev. B, 1972, 5, 4709. D.E. Stobbe, F.R. van Buren, M.S. Hoogenraad, A.J. van Dillen and J.W. Geus, J. Chem. Soc., Faraday Trans., 1991, 87, 1639. L.M. Knijff, Ph.D. thesis, Utrecht University (1994) P.J. van den Brink, A. Scholten, A. van Wageningen, M.D.A. Lamers, A.J. van Dillen and J.W. Geus, in Stud. Surf. Sci. Catal., vol. 63 (Prep. Catal. V), eds. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon, Elsevier, Amsterdam, 1991, p. 527. D. Groult, C. Mercey and B. Raveau, J. Solid State Chem., 1980, 32, 289. L.A. Boot, S.C. van der Linde, A.J. van Dillen, J.W. Geus, F.R. van Buren and J.E. Bongaarts, in: Proc. 6'h symp. 'Catalyst Deactivation', Oostende, 1994, to be published H.P.C.E. Kuipers, H.C.E. van Leuven and W.M. Visser, Surf. Interface Anal., 1986, 8, 235; the method was modified by O.L.J. Gijzeman (1994)
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparationof HeterogeneousCatalysts G. Ponceletet al. (Editors) 9 1995 Elsevier ScienceB.V. All rights reserved.
169
Alumina/Water InterracialPhenomena During Impregnation J.-B. d'Espinose de la Caillerie+ ,C. Bobin, B. Rebours and O. Clause* Kinetics and Catalysis Division, Institut Fran~ais du Pdtrole, 1&4 Avenue de Bois Prdau, B P 311 F-92506 Rueil Malmaison Cedex, France
The adsorption of Co(II), Ni(II) and Zn(II) - m m i n e complexes from aqueous solutions onto T-alumina at near neutral pH (7
1. I N T R O D U C T I O N A large class of oxide supported metal catalysts are prepared by the deposition of ionic species from aqueous solutions onto oxides followed by a suitable activation procedure. The nature of the ion-support interactions at the oxide/water interface during the first steps of the preparation, i.e., impregnation, washing, aging or ripening, and drying, has a large influence of the fundamental properties of the final catalysts such as metal particle dispersion or supported oxide stability and reducibility. Depending on experimental parameters during impregnation such as pH or the presence of complexing or chelating ligands, a weak, electrostatic-type adsorption, the graf~ng of isolated or polynuclear ions, or the formation of layered silicates have been found to take place onto the silica surface before calcination [1-5]. Recently, the formation of surface compounds, i.e., hydrotalcite-type coprecipitates including AI(III) ions from the support has been demonstrated to occur during impregnation of Co(II), Ni(II) and Zn(II) ions onto alumina and + Present address: Laboratoire de Physique Quantique, URA 1428 CNRS, ESPCI, 10 rue Vauquelin, 75231 Paris Cedex 05, France
170 this even for pH in the vicinity of the isoelectric point of alumina [6]. The formation of supported hydrotalcite-type coprecipitates in the presence of the impregnating solution can be viewed as an early strong ion-support interaction. This strong ion-support interaction leads to thermally stable systems upon calcination. For ex-mple, the calcination of nickel-aluminum hydrotalcite-type structures produces NiO particles "decorated" with alumlnate-type phases, which are highly resistant to thermal sintering [7]. In this work the formation of nickel, cobalt or zinc a l u m i n , m hydrotalcitetype coprecipitates upon impregnation of T-alumlna at near neutral pH and ambient temperature was confirmed by EXAFS and X-ray diffraction. The role of the metal ion concentration in solution on the composition of the supported coprecipitate was studied as well as the influence of the specific surface area of the T-alumina. The deposition of Co(II), Ni(II) and Zn(II) ions onto a commercial alumina was first investigated. Since the coprecipitation mechanism is likely to be affected both by the impurity level and the thermal pretreatment of the carriers before impregnation, supports of high purity were prepared by hydrolysis of aluminum alkoxides and were submitted to identical pretratments immediately before impregnation. The deposition of Ni(II) onto these supports was then examined.
2. EXPERIMENTAL
2.1 Sample P r e p a r a t i o n Preparation of aluminas. The aluminas were prepared by hydrolysis of aluminum isopropoxide (AIP, Aldrich) dissolved in anhydrous isopropanol [8,9]. Variable AIP/isopropanol and AIP/water ratios were used so as to obtain specific surface areas in the 300-500 m2/g range. The gels were dried at 393K overnight then calcined at 873K during 12 h. The XRD diffraction patterns revealed a poorly crystallized T-alumina structure, see discussion. Transmission electron micrographs showed platelets along with block-like alumina morphologies. A commercially available alumina was also used. Textural characteristics of the prepared and commercial aluminas are gathered in Table 1 and the impurity levels in Table 2. The aluminas were calcined under flowing oxygen at 800K for 2 hr before impregnation. Co/A120 3, Ni/A120 3, Zn/AI20 3 sample preparation. The samples were prepared by putting in contact 5 g of alumina in powder form with 200 mL of Co(II) or Ni(II) nitrate solutions of M(II) concentrations ranging from 0.01 to 0.1M containing also 1M of ammonium nitrate. The pH was adjusted by bubbling gaseous ammonia. The suspensions were stirred at 298K during a fixed time, referred to as contact time. The Co(II) solutions and suspensions were kept under Ar in order to prevent the Co(II) to Co(III) oxidation, which is favored by the ammonia complexation. After centrifugation and washing with a solution containing 1M ammonium nitrate
171 at the some pH as the final pH of the impregnating solution, the wet samples were kept in air (or under Ar for the Co(II) containing samples) during a fixed time, hereafter referred to as the aging time. The samples were dried in an oven at 373K during 12 h. The amounts of adsorbed ions were derived from the concentration differences in the impregnation solutions before and after impregnation. The preparation of hydrotalcite-like materials [Ni 10.66~0.33(OH)2](CO3)0.16 9nH20, [Co0.66A10.33(OH)2](CO3)0.16 .nH20 and [Zn0.66~0.33(OH)2](CO3)0.16 .nH20 has been reported previously [10-13]. These reference compounds will be referred to as NiA12, CoAl2 and ZnA12 (2 stands for the value of the atomic ratio M(II)/AI(III)). The X-ray diffraction pattern of NiAI2, CoAl2, ZnA12 showed a well-crystallized, hydrotalcite-like structure, a-Ni(OH) 2 was prepared by precipitation from 1M Ni(NO3) 2 solution at pH 7.2 with ammonia at 313K. ~-Co(OH) 2 hydroxide was precipitated from Co(II) nitrate at pH 9.0 and 333K using NaOH as a precipitating agent. The cobalt nitrate solution was decarbonated and the precipitation was performed under argon atmosphere to avoid any oxidation of Co(II) to Co(III) ions. The precipitate was aged in the mother liquor at 333K under argon during 48h and was dried at 100~ without exposure to air. X-ray diffraction revealed a well crystallized ~-Co(OH) 2 structure. Table 1 Textural characteristics of aluminas used for impregnation experiments Sample
Specific Surface Area (m2/g)
Pore Volume (cc/g)
Dubinin Volume (cc/g)
A B
314
0.448
0.128
399 480 195
0.636 0.710 0.640
0.151 0.191 not determined
C D (commercial)
Table 2 Impurities of aluminas used for impregnation experiments (ppm) Somple A-C D (commercial)
Na20 <20 <20
CaO 20 <5
Fe203 20 140
TiO2 <10 1795
ZrO2 <15 400
2.2 E X A F S a n d XRD m e a s u r e m e n t s ~
EXAFS measurements at the Co, Ni and Zn K edges were performed at the LURE radiation synchrotron facility (Orsay, France) using the D44 X-ray beamline emitted by the DCI storage ring (positron energy: 1.85 GeV; ring current 300 mA). The spectra were recorded at liquid nitrogen temperature in
172 the transmission mode using two air filled ionization chambers. A Si (331) double crystal monochromator was used. Higher harmonics in the X-ray beam were minimized by using a double mirror system. The energies were scanned with 2 eV steps for EXAFS analysis. The resolution AE/E at the Ni, Co and Zn K-edges was 2 . 10 -4. EXAFS measurements were carried out three times for each sample. The dried samples were finely ground and homogeneously dispersed in cellulose pellets. The amounts of nickel, cobalt or zinc in the pellets were calculated so that the absorption variations A(~x) through the edges ranged between 0.8 and 1.2. The wet samples were pressed between two kapton windows. All spectra were taken at the liquid nitrogen temperature except for the spectra at the Zn K edge which were taken at 298I~ Data analysis was performed using the University of Washington XAFS analysis package [14]. X-ray powder diffraction analysis was carried out with a Phillips PW1820 diffractometer using a cobalt target and a secondary beam monochromator.
3. R E S U L T S AND D I S C U S S I O N The Co(II), Ni(II) and Zn(II) adsorptions as a function of pH have been presented in a previous paper [6]. The main result was the presence of adsorption maxima for pH in the 6.5-8.5 range, i.e., for pH values close to the isoelectric point of alumina. This feature suggested that the interaction of Co(II), Ni(II) and Zn(II) ammine ions with alumina over the entire pH range was not simply electrostatic. Several different adsorption modes are probably involved. This behaviour has also been encountered during impregnation of Ni(II) ammine ions on silica as a function of pH [5]. The ion-support interaction could be monitored by varying the pH of the impregnating solution. Layered silicates were formed when the pH was around 8.3 and hexaamminenickel(II) complexes were electrostatically adsorbed when the pH was higher than 9.5. In this study we will focus on the adsorption mode at near neutral pH onto alumina.
3.1 Ion c o m p l e x a t i o n in the i m p r e g n a t i n g solutions The metal ions were complexed with ammonia to avoid the precipitation of hydroxides in the solutions [5,15]. 1 mole/1 ammonium nitrate was added to the impregnating solutions to ensure a sufficient ammonia complexation and thus a concentration of hydrolyzed species lower than required for hydroxide precipitation at near neutral pH [16]. Another beneficial effect of am rn_onium nitrate is to buffer the solutions so that pH variations were negligible during ion deposition. On the other hand, the high ionic strength can play a significant role on a possible dissolution/reprecipitation process. The impregnating solutions were found to be stable with time at any pH. We checked by small angle X-ray scattering that the concentration of condensed particles with sizes higher t h a n 2 n m was negligible. Thus the precipitation of hydroxides or of basic salts in the absence of alumina was prevented.
173 3.2 Rate o f Co(II) a n d Ni(II) d e p o s i t i o n o n t o c o m m e r c i a l a l u m i n a D The Ni and Co uptakes from the impregnating solutions during the first 24 h of contact and for a 0.01M initial metal concentration are presented in Figure 1. The pH was adjusted to 7.2 and 8.1 for Ni(II) and Co(II) respectively and did not vary significantly during impregnation. Provided sufficient contact time, namely two weeks, the ions were quantitatively removed from the solutions. In other words, the equilibrium concentrations are very low, i.e., typically lower than 5 ppm, which is reminiscent of hydroxide or insoluble salt solubility products rather than of "equilibrium adsorptions" as described by site binding models. The Co(II) were deposited more quickly than the Ni(II) ions, the relative deposition rate order being Co(II) > Ni(II) >>Zn(II). This order was independent of the 7c alumina tested.
100 ,~o
80
o2
60
9
40
o
9.0 0
0
5
10 time
15
20
(h)
Figure 1. Ni (1) and Co (A) uptake from the impregnating solutions 3.3 Rate of Ni(II) d e p o s i t i o n onto a l u m i n a s A.D The Ni fraction removed from solution after 1.5 h impregnation at pH 7.2 is shown in Figure 2 as a function of the s~ecific surface area of alumina. The initial Ni(II) concentration was 5 . 10 -z M and the impregnations were performed at 298K. As expected, the yield of Ni(II) deposition is higher when the specific surface area of alumina is enhanced. 3.4 EXAFS study of the a l u m i n a / m e t a l ion i n t e r a c t i o n s d u r i n g i m p r e g n a t i o n a t n e u t r a l pH Even though no precipitation was observed in the solutions before impregnation, the support might act as heterogeneous nucleus of precipitation for some hydroxides or basic salts. Indeed, the maximum concentrations of hydrolyzed species are located in the 7-8 pH range for Ni(II), Co(II) and Zn(II) ions, i.e., in the vicinity of the adsorption maxima as a function of pH [5,15,16]. The presence of defect-rich surfaces such as those of T-alumina may induce the
174
precipitation of hydroxides selectively onto the surface. It is interesting, however, to note that the adsorption ma~rlmum of Ni(II) onto silica is shifted towards higher pH values (8.5 instead of 7.2 in the case of alumina). Thus, the carrier does not act simply as a physical precipitation nucleus. A chemical interaction probably develops between the support and the deposited or precipitated ions. Actually, this was confirmed on silica since the formation of layered silicate structures during impregnation was evidenced by EXAFS and IR. In this case, the ion/support interaction is even more than a chemical interaction since chemical bonds at the silica surface had to be broken for the formation of layered silicates to occur [5]. 100 no
9O 8070
" 100 200 B. E . T .
300 400 500 surface area
(m 2
/
g)
Figure 2. Ni fraction removed from the solution as a function of SBE T of alumina after 1.5 h impregnation. We performed EXAFS at the Co, Ni and Zn K edges in order to gain more insight into the local environment around the deposited ions. More specifically, we attempted to distinguish clearly by EXAFS hydroxides from coprecipitates including aluminum ions possibly formed during impregnation [6]. Let us briefly review the structure of the hydroxides and hydrotalcite-like phases, a and ~-Ni(OH) 2 have the C6-type layered structure with a hexagonal unit cell of dimensions a=0.3126 nm and c=0.4605 nm in the case of ~-Ni(OH) 2 and approximately a=0.308 nm and c=0.8 nm in the case of a-Ni(OH) 2 [17,18]. This structure is built of stacked layers, each layer consisting of sharing edge NiO 6 octahedra. Nickel-aluminum hydrotalcite-type coprecipitates have a structure similar to that of nickel hydroxide except that some Ni(II) are replaced for Al(III) ions in the octahedral layers. This results in positively charged octahedral layers so that the presence of anions, such as carbonate, nitrate or choride ions is required for the electrical balance of the structure. The anions are located between the cation containing sheets along with water hydration molecules [12]. The cobalt hydroxide and cobalt- and zinc-aluminum
175
X(k)*k^3
cj1
r163
b-A
Co
b-L
Figure 3. Normalized EXAFS spectra after multiplication by a k 3 factor of a: a-Ni(OH) 2 at the Ni K edge; b: ~-Co(OH) 2 at the Co K edge; c: NiA12 at the Ni Kedge; d: CoAl2 at the Co K edge; e: ZnA12 at the Zn K edge.
176 hydrotalcite-type structures are isomorphous to the nickel containing structures. An a form of zinc hydroxide with the C6-type layered structure is also known [19]. The nearest shell composition around the metal ions in hydroxide and hydrotalcite-type compounds cannot be distinguished, since in both cases the cations are octahedrally coordinated to six OH groups at comparable distances The next nearest shell consists of 6 Ni (or Co or Zn) backscatterers in the hydroxide structures, wheareas, in the hydrotalcite structure, the next nearest backscatterers are Ni (or Co or Zn) along with AI ions, with NM(II)+NAl(III ) = 6 [13,20]. a-Ni(OH) 2, ~-Co(OH) 2 and hydrotalcite-like materials NiA12, CoAl2 and ZnA12 were synthesized and used as reference compounds for the EXAFS analysis. The normalized EXAFS of a-Ni(OH) 2 and NiA]2 at the Ni K edge, Co(OH) 2 and CoAl2 at the Co K edge, and ZnA12 at the Zn K edge are shown in figure 3. It can be seen that the signals of hydroxides were very similar, see figures 3a and 3b, which is predictible since the structure of nickel and cobalt hydroxide is the same and since the Co and Ni atomic numbers are very close. Conversely, the signal of hydrotalcite compounds can be easily distinguished from t h a t of hydroxides, in particular in the 4-6, 8-9 and 11-13 A -1 regions. This shows that EXAFS is very sensitive to the presence of aluminum in the precipitates.
b I 1
3
5
7
9
11
13
15
k (A-l) Figure 4. Normalized EXAFS spectra at the Ni K edge after multiplication by a k 3 factor of a: NiA12 and b: a Ni(II)/A1203 sample. The spectra were taken at the liquid nitrogen temperature. Full information on EXAFS analysis is given elsewhere [20]. Fourier transforms were performed over the [3.2-15.4] k range ([3.0-11] at the Zn K
1 '/7
Table 3 Structural Parameters of Reference Compounds and Samples as Determined by E X A F S at the Ni tCo and Zn K Edges Ssmple
Back-
N
R(A)
oc-Ni(OH) 2 O Ni
5.5 5.4
2.03 3.08
6.4 8.7
3.9 3.9
O Ni A1
5.5 3.4 2.0
2.04 3.03 3.03
4.5 4.2 4.2
1.8 1.8 1.8
~-Co(OH) 2 0 Co
5.3 6.4
2.09 3.18
5.5 6.1
2.0 2.0
O Co A1
5.7 3.9 1.8
2.08 3.08 3.08
6.8 7.1 7.1
2.3 2.3 2.3
2.0
O Zn A1
5.8 4.0 2.0
2.09 3.11 3.11
11.8 8.8 8.8
-2.5 -2.5 -2.5
2.0
O Ni A1
6.0 3.7 2.2
2.04 3.04 3.04
5.2 6.2 6.2
2.2 2.2 2.2
1.7
O Ni A1
5.9 4.8 1.2
2.05 3.06 3.06
7.8 7.0 7.0
1.7 1.7 1.7
4.0
O Co A1
5.1 3.6 2.8
2.08 3.08 3.08
6.5 7.4 7.4
1.9 1.9 1.9
1.3
O Zn A1
5.6 3.6 2.4
2.05 3.11 3.11
16.5 7.8 7.8
-4.1 -4.1 -4.1
1.5
scatterer
NiAl2
CoAl2
ZnA12
E
F
G
H
o(A).103 AE0(eV) M(II)/M(III) initial M(II) conc.
1.7
0.05M
0.1M
0.01M
0.05M
Concentration of the M(II) cation in solution before impregnation (mole/l) The inaccuracy on the M(II)/AI(III) ratio as determined by E X A F S is • The inaccuracies on the distances, M-O, and M-M(AI) coordination numbers are • 2%, • 20% and • respectively.
)'/~
edge) after multiplication by a k 3 factor using a Hanning window. Fourier backtransforms were performed over 1.2
o~ < 4t
b
l" 1
I
I
I
I
I
I
3
5
7
9
11
13
15
k (A-l) Figure 5. Normalized EXAFS spectra at the Co K edge after multiplication by a k 3 factor of A: CoAl2 and B: a Co(II)/A120 3 sample. The spectra were taken at the liquid nitrogen temperature. The normalized EXAFS of a Ni(II)/AI20 3 sample (sample E) prepared at pH 7.2 on alumina D (initial Ni(II) concentration, 0.05M; contact time, 1.5h, no aging, washing with distilled water, no drying, Ni loading, 7.2 wt%) is presented in Figure 4 along with the spectra of the NiA12 hydrotalcite. Prior to any mathematical treatment, a simple examination of the spectra indicates that
179 a NiAI hydrotalcite was present in the sample. The EXAFS parameters are given in Table 3, see sample E. The contribution of AI backscatterers is necessary to fit correctly the data [20]. The Ni environment in the sample was very close to that of Ni in the NiA12 standard compound. As EXAFS averages over all Ni containing species, the Ni species preponderant in s~mple E was a NiA1 hydrotalcite-type coprecipitate with a Ni/AI atomic ratio close to 2. Since in hydrotalcites the AI(III) are homogeneously distributed among the M(H) cations in octahedral sheets, this result showed that alumiDa reacted at neutral pH and ambient temperature in the presence of Ni(II) ions.
o~ 4t
b
1
3
5
7 k (A-l)
9
11
Figure 6. Normalized EXAFS spectra at the Zn K edge after multiplication by a k3 factor of a: ZnA12 and b: a Zn(II)/AI20 3 sample. The spectra were taken at 298 K. The normalized EXAFS of a Co(II)/AI20 3 sample (sample G) prepared at pH 8.1 on alumina D (initial Co(II) concentration, 0.01M; contact time, 1.5h, no aging, washing with distilled water, no drying, Co loading, 2.4 Co wt%) is presented in Figure 5 along with the spectra of the CoAl2 hydrotalcite. The normalized EXAFS of sample H prepared at pH 7.5 on alumina D (initial Zn(II) concentration, 0.05M; contact time, 2h, no aging, washing with distilled water, no drying; Zn loading, 2,0 Zn wt%) is presented in Figure 6 along with the spectra of the ZnA12 hydrotalcite. As in the case of sample E, the formation of hydrotalcite in these samples is clearly visible. The M(II)/AI(III) atomic ratios in the coprecipitates are indicated in Table 3. It appears that the CoAl2 and ZnA12 reference compounds are structurally close to the observed supported hydrotalcite-type structures. This strengthens the EXAFS analysis. Two
180 15
a
10
-5 -10 -15
15
b
10
'9
|
|
!
1
~ -5
r
-10 -15 -
R(A)
Figure 7. Imaginary part and magnitude of Fourier transform (solid lines; k 3 weighted; without phase correction) of a: the CoAl2 standard compound and b: sample E. The results of EXAFS analysis obtained with the best calculated coordination parameters are shown with triangles. The fits were performed over the [1.2-3.2] r range.
181 exemples of best fits along with the experimental data are shown in Figure 7, for the CoAl2 reference compound and for sample E. 3.5 Effect of the M(H) c o n c e n t r a t i o n in t h e i m p r e g n a t i n g s o l u t i o n s o n the c o p r e c i p i t a t e c o m p o s i t i o n The composition of the supported hydrotalcites was dependant on the metal ion concentration in the impregnating solution. As a rule, the M(II)/AI(III) atomic ratio increased with increasing M(II) concentration. An exemple was given in Table 3, see samples E and F. 3.6 Effect of the specific s u r f a c e a r e a o f a l u m i n a The X-ray diffraction patterns of aluminas A-D impregnated with Ni(II) at pH 7.2 (Ni loading ranging from 7.2 to 10 Ni wt %) are shown in Figure 8. The impregnation parameters were those used for section 3.3, i.e., 1.5 h contact time for an initial Ni(II) concentration of 0.05M. The s~mples were washed and dried. The patterns of aluminas A-D before impregnation is reported in dashed lines. The formation of poorly crystallized hydrotalcite was likely in all samples. However, it must be mentioned that the reflections of a-Ni(OH) 2 are close to those of the NiA] hydrotalcites. In the absence of precise data relative to the reflection intensities, such as is the case for poorly crystallized materials, it is difficult to distinguish accurately hydrotalcites from hydrated hydroxides by XRD. This makes the EXAFS analysis particularly usefid. While the effect of surface area is obvious on the ion deposition kinetics, see section 3.3, the effect on the coprecipitate composition is more subtle. The EXAFS analysis indicates a Ni/A1 atomic ratio close to 1 in samples prepared from aluminas A and B. In the same conditions using the commercial alumina D (195 m2/g only), a ratio close to 1.7 was obtained, see Table 3. Thus, it would seem that the Ni/AI ratio decreases with increasing the alumina surface. However, the impurity levels are very different in aluminas A-C and alumina D, see table 2. Unfortunately, the hydrotalcite structure formed on alumina C was very disordered so that the Ni/A1 ratio in the coprecipitate could not be estimated.
4. C O N C L U S I O N S Alumina has long been known to pass into solution for acidic (pH<4) or basic pH's (pH>10), then to give rise to reprecipitation reactions with transition metal ions. For example, this phenomenon has been reported when alumina is brought into contact with aqueous solutions containing strong acids such as H2PtC16 or palladium nitrate in the presence of nitric acid [21-25]. In this work, we present evidences for similar dissolution-reprecipitation phenomena in the presence of Co(II), Ni(II) or Zn(II) ions even for non aggressive pH's, i.e., neutral or near neutral pH's. This result is quite intriguing since the alumina dissolution rate is minimal for such pH's in the vicinity of the isoelectric point. It opens a wide range of questions and interests relative, first, to the role of
182
v Bayerite t Alpha alumina
HT o
v v
I
Iv
.~
v I
v
9
v
I
b
k I
10
'
I
20
'
i
30
'
I
40
'
I
5J
'
I
'
60
I
70
'
I
80
'
I
90
2 Theta(( o K alpha)
Figure 8. Full lines: XRD patterns of ~he aluminas a-d impregnated with Ni(II) at pH 7.2 and of the NiA]2 reference compound (sample HT at the top); dotted lines: aluminas a-d before impregnation.
183 experimental parameters, such as alumina surface, impurities, pretreatment, the solution composition, ionic strength, temperature etc.; second, to the mechanism of coprecipitate formation, i.e. via a dissolution-reprecipitation process or via a gel-mediated path; third, to the interactions, grafting, possibly epitaxy, between the coprecipitates and alumina. Obviously, much additional work is needed to suggest answers to these numerous unresolved matters. The purpose of this article was to underline the former point, i.e., the influence of experimental variables on the formation rate and composition of coprecipitates. The formation of coprecipitates has been observed onto all T-alumina samples which were tested independently of their purity, thermal pretreatment and specific surface area. Aluminas of high purity were prepared in this work and were found to behave similarly as commercial samples. Thus, the formation of coprecipitates is not related to the presence of impurities in alumina. Conversely, the rate of ion deposition was found to be very dependant on the alumina characteristics, in particular the specific surface area, possibly the impurity level. The time dependence of the amount of adsorbed ions revealed fast adsorptions during the first minutes, then slower processes which took place until quantitative consumption of the ions from the solutions. Coprecipitate formation was evidenced during the slow processes, i.e., aider contact times higher than 1 h at ambient temperature. Clearly, some aging may take place during the slow processes. Unfortunately, very few information is available on the species formed immediately aider the fast adsorption process, since the systems were found to be highly disordered and only the nearest oxygen backscatterers were revealed by EXAFS. Thus, it is not possible to specify whether coprecipitates are formed after a few minutes of impregnation. However, a chemical interaction with the support is likely to take place already during the fast process, since washing with distilled water did not remove significant amounts of adsorbed ions. The origin of the observed differences between the deposition rates of Co(II), Ni(II) and Zn(II) is not clear. We have observed that the Co(II) deposition was the fastest whatever the T-alumina sample tested. This behaviour may be related to the concentration of hydrolyzed species, which is higher for Co(II) than for Ni(II) or Zn(II) at neutral pH in 1M ammonium nitrate solutions [16]. The mechanism of coprecipitate formation is likely to involve a dissolutionreprecipitation process, since a long range A1 (III) diffusion is needed for hydrotalcite crystallites to be revealed by EXAFS or XRD. However, the presence of a gel consisting mainly of hydrated alumina at the carrier/coprecipitate interface cannot be excluded. We did not observe any epitaxy between the hydrotalcite crystallites and the alumina lattice by electron diffraction. We suspect that the adsorption of the metal ions during the fast process lowers the strength of the bonds in alumina layers underneath, tasking easier the further dissolution of the oxide. Work is in progress to confirm this hypothesis.
184 REFERENCES
I. G.C.A. Schuit and L.L. van Reijen, Adv. Catal., N e w York, Academic Press, 1958, p. 245. 2. J.A. van Dillen, J.W. Geus, L.A.M. Hermans and J. van der Mejden, Proc. 6th Int. Cong. on Catal., London, 1976, p. 677. 3. J.W. Geus, Preparation of Catalysts III,Elsevier, AmsterdAm, 1983, p.1. 4. C. Marcilly and J.P. Franck, Rev. Inst. Fr. Pet., 3 (1984) 337. 5. O. Clause, M. Kermarec, L. Bonneviot, F. Villain and M. Che, J. Am. Chem. Soc., 114 (1992) 4709. 6. J.L. Paulhiac and O. Clause, J. Am. Chem. Soc., 115 (1993) 11602. 7. B. Rebours, J.-B. d'Espinose de la Caillerie and O. Clause, J. Am. Chem. Soc., 116 (1994) 1707. 8. H. Adldns and S.H. Watldns, J. Am. Chem. Soc, 73 (1951) 2184. 9. K. Ishiguro, T. Ishikawa, N. Kakuta, A. Ueno, Y. Mitarai and T. Kamo, J. Catal., 123 (1990) 523. 10. P. Courty, D. Durand, E. Freund and A. Sugier, J. Mol. Catal., 17 (1982) 241. 11. E.C. Kndssink, L.L. van Reijen and J.R.H. Ross, J. Chem. Soc., Faraday Trans. 1, 77 (1981) 649. 12. F. Cavani, F. Trifiro' and A. Vaccari, Catal. Today, 11 (1991), 201. 13. O. Clause, B. Rebours, E. Merlen, F. Trifiro' and A. Vaccari, J. Catal., 133 (1992) 231. 14. J. J. Rehr, J. Mustre de Leon, S.I. Zabinsky and R.C. Albers, J. Am. Chem. Soc., 113 (1991) 5135. 15. D.W. Fuerstenau and K.J. Osseo-Asare, J. Colloid Interface Sci., 118 (1987) 524. 16. J. Kragten, Arias of Metal-Ligand Equilibria in Aqueous Solution, Wiley, New York, 1978, p. 175. 17 R.S. McEwen, J. Phys. Chem., 75 (1971) 1782. 18 K.I. Pandya, W.E. O'Grady, D.A. Corrigan, J. McBreen and R.W. Hoffman, J. Phys. Chem., 94 (1990) 21. 19 Gmelins Handbuch der Anorganischen Chemie, Vol. 32; Verlag Chemie: Weinheim, 1979, p. 827. 20 J.-B. d'Espinose de la Caillerie, C. Bobin, M. Kermarec and O. Clause, J. Am. Chem. Soc, submitted for publication. 21 R.W. Maatman, P. Mahaffy, P. Hoekstra and C. Addink, J. Catal., 23 (1971) 105. 22 E. Santacesaria, S. CarrOt and I. Adami, Ind. Eng. Chem., Prod. Res. Dev., 16 (1977) 41. 23 E. Santacesaria, S. Carr~ and I. Adami, Ind. Eng. Chem., Prod. Res. Dev., 16 (1977) 45. 24 J.P. Brunelle, Pure Appl. Chem., 50 (1978) 1211. 25 S. Subramanian, D.D. Obrigkeit, C.R. Peters and M.S. Chattha, J. Catal., 138 (1992) 400.
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
185
N a n o m e t a l s a n d c o l l o i d s as c a t a l y s t p r e c u r s o r s
Helmut B6nnemann Max-Planck-lnstitut fer Kohlenforschung 45466 M~lheim an der Ruhr, Germany The reduction of transition metal salts and oxides using hydrotriorganoborates in organic media allows the production of X-ray amorphous nanopowders of metals and alloys under mild conditions. The reduction of TiCI 4 with K[BEt3H ] gives an ether-soluble [Ti(0) 0.5 9 THF]x which serves as a catalyst for the hydrogenation of titanium or zirconium sponges and related systems and as a powerful activator for heterogeneous hydrogenation catalysts. The use of tetraalkylammonium hydrotriorganoborates as reducing agents leads to colloidal transition metals in organic phases. These colloids may also be obtained using conventional reducing agents after first reacting the metal salts with the stabilizing tetraalkylammonium halide. Colloidal metals prepared in this way serve as sources for heterogeneous metal catalysts. 1. S U R V E Y
The chemical reduction of transition metal salts and oxides allows the production of X-ray amorphous nanopowders (I) or colloidal metals protected by THF (11) or NR4+- or PR4+-groups (111). Cr
~Co@ NNiW
Mo
t~Rh~ NPd ~ .
Hf
Ta
W
Re
Os
.
.
.
.
.
.
.
.
.
.
Ag
F
Cd
.
Mlrm
~ P t Nt
nanopowder-metals (I) ether- and thioether-stabilized nanometal-colloids (11) ~)
Zn
NR4+- und PR4+-stabilized nanometal-colloids (111)
Au
Hg
186
2. X-RAY A M O R P H O U S METAL N A N O P O W D E R S Metal salts of the goups 6 - 12 and SnCI 2 may be reduced using alkalihydrotriorganoborates in hydrocarbons between -20~ und 80~ to give boron-free powder metals (I). According to X-ray diffraction the particles are nearly amorphous. The particle sizes were found by TEM to lie between 1 and 100 nm. By simple coreduction of suspended metal salts binary or ternary alloys and intermetallic compounds were obtained.
U MX v + v M'(BEt3H )u
THF
uMS + v M ' X u +
uvBEt 3 + uv/2H21"
X = Halogen
Cr
Mn
Fe
Co 3 - 5 nm
Ru
Rh 1 - 4 nm
Re
Os
Ir
Ni 5 - 15 nm
Pd
Cu
Zn
25 - 90 nm
Ag
Cd
J,,, Sn
12 - 28 nm
Pt
Au
2-5nm
X-ray amorphous nanopowders and alloys (I)
3. ORGANOSOLS OF EARLY TRANSITION METALS STABILIZED BY THF An ether soluble Ti(0), stabilized by complexed THF, was isolated by reducing [TiCI 4 92 THF] with K[BEt3H ] at 40~ A large variation of THF-stabilized early transition metal colloids is accessible using this method. The isolated, dry organosols (11) are very soluble in THF and hydrocarbons.
THF x.[TiCI 4 . 2 T H F ]
+x.4
2 h, 40~
~ K[BEt3H ]
1 [Ti. 0,5 THF]x + 4 BEt 3 + x. 4 KCI + x. (2 - m/2) H21" 2
187
Coo
,o o
,E
Ti
V
Cr
Zr
Nb
I~1o
Hf
Ta
W
I~ln
THF-stabilized organosols of early transition metals (11)
The IR- and NMR-spectra show intact THF coordinated to the metal core. The XPS-spectrum of [Ti. 0.5 THF]x allows the assumption of Ti(O). The results of an EXAFS-analysis support the description as colloidal Ti having three Ti-shells. From the observed Ti-Ti-distances a hexagonal structure similar to m-titanium may be derived. The isolated metal colloids are very stable under argon and useful precursors for catalytic applications.
Shell
2
ct-Ti (a = 2.952, c = 4.689 A [4])
~-TiHI.971 (a = 4.440 A [4])
1
2.96A
2.90 ~.1 .o.~ 2.925 A 2.95 AJ
3.14A
2.95 A 2
4.21 A
4.14 A
4.44 A
3
5.68 A
5.85 A
5.44 A
The corresponding Zr-colloid was isolated by adding the THF-solution after filtration from KCI slowly to pentane, where the Zr-colloid precipitates. The workup of the Vand Nb-colloids was performed similarly. The reduction of the THF-adduct to MnBr 2 at 40~ yielded a stable, isolated [Mn 0.3 9 THF] x colloid containing typically 60% Mn.
188
THF
x . [MnBr 2 29 THF] + x . 2 K[BEt3H ] 2 h, 40~
[Mn
1
0.3 9 THF]x + x . 2 BEt 3 + x . 2 KBr
+ x . H2?
NR4 +- STABILIZED METAL COLLOIDS
The reduction of metal salts using tetraalkylammonium-hydrotriorganoborates in organic solvents yields metal colloids stabilized by NR4+(III ). The metal particles are well protected by the long-chain alkylgroups which make the colloids very soluble in lipophilic organic phases.
MX v + vNR4(BEt3H )
THF Iv
(M = metals of groups 6 - 11;
MColloid + v NR4X + v BEt 3 + v/2 H2"I" X = CI, Br;
v= 1,2, 3;
R = alkyl, C 6 - 020
The stabilization of metal particles may be achieved easily by coupling the NR4X agent to the metal salt prior to reduction. The reduction itself may now be performed conveniently using a large variation of inorganic or organic reducing agents.
(NR4) w M x v Y w + vRed
~ MColloid + vRedX + w N R 4 Y
M = metals (111) Red = H 2, HCOOH, BEt 3, K, Zn, LiH, LiBEt3H, NaBEt3H, KBEt3H X, Y = CI, Br;
v, w = 1 - 3;
R = alkyl, 0 6 - 012
189
~x-
~NvN
+
§j ~ ~
Mn
+ x-~j
Cu
Ni
Fe
Co
3,0 nm
2,8 nm
Ru
Rh
Pd
1,3 nm
2,1 n m
2.5 nm
2,8 nm
.
Pt
Ir N+
+
-
+N~q"
1,5 nm
2,8 nm
NR4+-stabilized Organosols (111)
5. PARTICLE SIZE OF NR4X-STABILIZED METAL COLLOIDS Since our synthesis supports a high local concentration of the protecting agent at the reduction center, consequently the resulting colloid particles are rather small (55 - 300 metal atoms). Electron micrographs also show generally a very narrow particle size distribution.
35-
30
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
25 . . . . . . . . . . .
20
.
.
i
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
15 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10 . . . . . . . . .
5
.
.
.
.
.
.
.
.
-
. . . . . . . . . . . . . . . . . . . . . . . . .
.
|
,
Ii
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
0
0,5
1,0
1,5
2,0
2,5
3,0
3,5
4,0
4,5
[nm] Particle size distribution of the NR4+-stabilized Ru-colloid.
5,0
190
6. NANOMETAL POWDERS FROM O R G A N O S O L S After extraction using e.g. ethanol the protecting shell of the colloid may be detached from the metal core giving nanometal powders under full conservation of the particle size of the colloidal starting material. For example the extraction of colloidal Pt (average particle size = 2.8 nm) with ethanol yields a grey Pt-powder of again 2.8 nm size according to TEM.
7. PREPARATION OF ORGANOSOLS FROM METAL P O W D E R S Precipitated nanopowders obtained by chemical reduction may be transferred into soluble metal colloids by subsequent reaction with NR4X. For example a sample of magnetic cobalt powder (particle size 4 nm), precipitated from CoBr 2 by reduction in THF was reacted with an excess of N(Octyl)4Br to give a clear, dark red solution of the corresponding cobalt organosol.
8. C O L L O I D A L M E T A L A L L O Y S The coreduction of a mixture of metal-tetra-alkylammoniummetallates yields colloidal metal alloys. A high resolution TEM of the coreduction of NR4RhCI 4 and (NR4)2PtCI 4 (magnification: 6.3.105) showed particles of an average size of 2.8 nm and a net-plane-distance of 0.25 nm. The EDX-Analysis of 30 particles (point resolution: 0.8 nm) has confirmed that Pt und Rh are both present in the particles. An electrochemical investigation of the NR4+-Rh/Pt-colloid has shown that a homogeneous Pt/Rh-alloy is present in the bimetallic colloid. These findings have been confirmed by EXAFS. Bimetallic Pt/Rh-systems on ceramic supports have been claimed to be the active component of the automotive exhaust catalysts.
9. APPLICATIONS
9.1. Amorphous Metals X-ray powder diffraction measurements have revealed that the metal powders obtained by the deoxygenation of metal oxides are generally nanocrystalline. The grain size of the nickel and cobalt powders was determined by TEM to range between 6- 10 nm and 1 - 5 nm. An interesting application of the deoxygenation reaction described above consists in a process for the transformation of finely divided, needle-shaped iron oxides into acicular iron magnet pigments for magnetic signal recording. The conventional
191
industrial process for the production of acicular iron pigments from the corresponding iron oxides or oxide-hydroxides by reduction with excess gaseous hydrogen, generally affords temperatures above 250 ~ in the fluid bed reactor. Using hydrotriorganoborates in the presence of compressed hydrogen (20 - 100 bar), we were able for the first time to reduce iron oxides to yield well shaped acicular iron particles at only 80 ~ The metal oxide is suspended in organic solvents and treated with the combination of molecular hydrogen and the soluble hydrotriorganoborates. Surprisingly, under these mild reaction conditions the oxygen may be removed from the iron oxide lattice quantitatively. The pyrophoric iron pigments may be isolated from the clear organic solution by filtration. The trialkylborane acts as a complex carrier in organic solution both for the metal hydride and the resulting metal hydroxide and may be reused after hydrolysis of the triorganohydroxoborate. The regeneration amount of the carrier exceeds 97 %. The product consists of an acicular iron pigment of ca. 250 nm length and a diameter of ca. 20 nm, and does not exhibit any sintering. The acicular iron pigments prepared were found to have superior magnetic properties. The crucial characteristics of magnetic materials for recording purposes are saturation magnetization M s, remanence MR, and coercive field strength He. In practice an adjustment of certain coercive force ranges He of the magnet pigment to be used is desirable. Whereas the magnetic properties of iron pigments from the high temperatur gas phase reduction process, can be varied only via the particle size and shape of the iron oxide starting compounds, the low temperature deoxygenation allows to prepare acicular iron pigments which exhibit high values of the saturation magnetization M s and remanence MR along with a coercive force value which can be adjusted for the special sector of application simply by thermal aftertreatment without causing undesirable sintering of the metal needles.
9.2. Colloidal metals
9.2.1. General aspects 9The reductive synthesis of organosols is easy to scale up. 9The resulting metal colloids are very stable. 9By the coreduction pathway plurimetallic colloids are easily accessible. 9The metal colloids are active homogeneous catalysts in organic phases.
192
9.2.2. Heterogeneous catalysis The preformed metal colloids offer the possibility to obtain small monodisperse particles of discrete particle sizes on commercially available supports. The activity, lifetime, and selectivity of supported mono- and plurimetallic colloids may be studied in relation to: 9Particle size 9Protecting shell 9Composition and structure 9Doping with further components
9.2.3. THF-stabilized organosols of the early transition metals Besides the application as dopant for noble metal hydrogenation catalyst, the colloidal [Ti(0) 9 0.5 THF]• has been found to be a very efficient catalyst for the hydrogenation of titanium and zirconium sponges as well as for a nickel hydride battery alloy. The uncatalyzed hydrogenation of titanium or zirconium sponges using compressed hydrogen affords pressures above 100 bar and minimum reaction temperatures of 150 ~ The hydrogenation of these metals under such drastic conditions, however, is associated with unwanted sintering of the materials, so that the products can only be used after additional grinding. After the addition of 1 % Ti to the metal sponges a smooth hydrogenation of titanium and zirconium is observed at low temperatures (60 o. 90 ~ The reaction may be carried out in THF solution, toluene suspension or in the dry state after depositing the catalyst on the surface of the samples by evaporation of the solvent in vacuo. Using a special device, the mass specific uptake of hydrogen depending on pressure, temperature and time was monitored automatically. After a certain latent period with only negligible uptake of H2 an abrupt start of the catalytic hydrogenation occurs; followed by a period of rather constant H2 uptake (denoted as the hydrogenation period). Towards the end of the H 2 uptake the reaction turns into a slow decay.
9.2.4. NR4+-stabilized metal colloids may be adsorbed from the solution on supports without agglomeration of the particles. The advantage of the supported colloid-catalyst is demonstrated by comparison of the activity of three Rh-catalysts (5 % on charcoal) in the butyronitrilehydrogenation-test:
193 Activity 300 --/~ 250
i
[Nml/(g rain)] 263
r-1 oxigenated 230
B 0 , 2 % Ti; oxigenated
160 150 -- ,
124
100 50
o
IIndustrial catalyst 5% Rh on charcoal
Cluster catalyst 5% Rh on charcoal
Colloidal catalyst 5% Rh on charcoal
x"
,
.... /.. ~':,,, . . . .
./
x
The conventional precipitation catalyst shows large metal agglomerations on the surface besides a minor part of particles of 1 - 5 nm size. The Rh4-cluster collapses on the support giving particles of 2- 6 nm size, which are very uniformly distributed. The cluster preparation, however, is rather complicated. The optimum activity is observed with the preformed Rh-colloid of 1.2 - 2.2 nm size, which may be adsorbed without any agglomeration. All three catalyst types show a significant increase of activity when doped with 0.2 % Ti(0)-colloid. In addition, also the lifetime of the colloid-catalyst is superior to the conventional precipitation systems. Whereas the activity of the conventional Pd/C-catalyst expires completely after the performance of 38x103 catalytic cycles per Pd atom, the colloidal Pd/C-catalyst still shows a residual activity after 96x103 catalytic turnovers.
194
Activity in [Nml/g min] 250 -o- Colloidal Pd/C,.catalyst 200
150
100
f
0
~
20000
I
r
I
40000
60000
800OO
1001 KX)
TON 9.2.5. Selectivity control by doping The doping of Rh-colloid-catalysts with Sn has a strong effect on the selective C=O-group-hydrogenation of cx,j3-unsaturated aldehydes. A Rh-colloid/C-catalysts doped with Sn (Rh/Sn = 1.5/1) exhibits 86 % selectivity in the hydrogenation of cinnamic acid to cinnamic alcohol.
9.2.6. Synergistic effects of alloys in catalysis The catalysts prepared by mixing Pt/C and Rh/C powders or by consecutive adsorption of Pt- and Rh-colloid on charcoal show a linear increase of activity with increasing content of Rh (additive effect). The corresponding activity plot of the bimetallic Pt/Rh systems clearly show two maxima at Pt70Rh30 and Pt20Rhso. Since the second maximum exceeds the activity found for Rh alone, this finding is a strong indication for a synergetic effect. Activity [Nml/g min]
5000
- 5000
--- Rh/Pt-colloid on C (curve 1) .....( Rh colloid + Pt colloid ) on C (curve 2) 4000 m Hn colloid on C + l-'t colloid on ( c u r v e y
/
3O00
J
......
2000
1000 0
- 4000
~"
,111~i~~ i
oo mol % Pt
20OO
~
1000
-'''~ i
i
70/30
3000
i
1
50/50
i
i
26 r80
0 100 mol % Rh
195
REFERENCES 1. B6nnemann, W. Brijoux and Th. Joussen, Angew. Chem. Int. Ed. Engl., 29 (1990) 273. 2. B6nnemann, W. Brijoux and R. Brinkmann, Studiengesellschaft Kohle mbH, US Patent No. 5 053 075 (1991). 3. B6nnemann and B. Korall, Angew. Chem., 104 (1992) 1506, Angew. Chem. Int. Ed. Engl. 31 (1992) 1490. 4. B6nnemann, W. Brijoux, R. Brinkmann, E. Dinjus, Th. Joussen and B. Korall, Angew. Chem. Int. Ed. Engl., 30 (1991) 1312. 5. B6nnemann, R. Brinkmann, R. KSppler, P. Neiteler and J. Richter, Adv. Mater., 4 (1992) 804. 6. H. B6nnemann, W. Brijoux and Th. Joussen, Studiengesellschaft Kohle mbH, DE OS 3 934 351 (1991); (b) H. B6nnemann, W. Brijoux and Th. Joussen, Studiengesellschaft Kohle mbH, EP OS 0 423 627 (1991). 7. B6nnemann et al., J. Mol. Catal. 86 (1994) 129-177. 8. B6nnemann, W. Brijoux and R. Brinkmann, Studiengesellschaft Kohle mbH, US Patent 5 053 075 (1991 ).
This Page Intentionally Left Blank
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparationof HeterogeneousCatalysts G. Ponceletet al. (Editors) 9 1995 Elsevier ScienceB.V. All rights reserved.
197
P r e p a r a t i o n of N a n o m e t e r Size Cu-Zn/Al203 Catalyst by P h a s e Tr~rtsfer P a r t 3. Sol P r e p a r a t i o n and P h a s e T r a n s f e r Conditions Z.-S. Hu, S.-Y. Chen and S.-Y. Peng State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, P.O. Box 165, Taiyuan 030001 P.R. China 1. INTRODUCTION In our previous p a p e r [1], m u l t i c o m p o n e n t n a n o m e t e r particle CuZn/A1203 catalyst has been prepared by phase transfer with cationic surfactant cethyltrimethyl a m m o n i u m chloride and some basic p r e p a r a t i o n conditions have been studied. The objective of the present paper is to study in detail the effect of sol preparation and phase transfer conditions on the particle size of the solid. 2. EXPERIMENTAL Details of the experimental conditions are given in the previous paper [1]. 3. RESULTS AND DISCUSSION &l. Main factors affecting particle size It can be learned from a previous paper that the preparation procedure for nanometer size particlecatalysts by phase transfer with cationic surfactant includes sol preparation and phase transfer, removal of w a t e r and solvent, drying and calcination. The previous results [1] showed also t h a t the way, time and t e m p e r a t u r e of drying were not important factors affecting particle size. These facts mean t h a t the main factor affecting particle size could not be the drying process. Figures 1 and 2 are TEM micrographs of sample BTX-73-5 before and after calcination at 350~ respectively. Figure 1 shows t h a t the border of the particles before calcination is not clear, a possible reason being the adsorption of the surfactant, and that the particle size seems to be bigger t h a n after calcination. A minor reduction of the particle size after calcination could come from the decomposition of hydroxide and surfactant. This result shows that the calcination at 350~ could not change distinctly the particle size of the product, which means that the main factor affecting particle size also could not be calcination at 350~ In the previous paper, it was shown t h a t the aggregation and recrystallization of the particles occurred in the step of removing water and solvent. For the explanation of the experimental facts
198 mentioned above, a microscopic process for the preparation of nanometer particles by phase transfer was proposed which was diagrammed as follows 9 water pool
~.~C.,~,~ .
aging or stilling
--
.
i ~=
calcination '"
9 9 9
9 crystalline
sol preparation
~
and
S 9
9
recrystallization
phase transfer
larger particle aggregation ._:.
~
9
9
9
9
initial particles in oil phase
,.'
U
~: 5
~b3985 Figure 1 9TEM micrograph of sample Figure 2. 9TEM micrograph of BTX-73-5 before calcination sample BTX-73-5 after calcination Although it was found that the phenomena of particle aggregation and recrystallization existed in the step of water and solvent removal, it could also come to a greater extent from the step of sol preparation and phase transfer,
199 especially in the adsorption of the surfactant and the damage of emulsion. Therefore, the main steps affecting particle size could be the sol preparation and the phase transfer. 3.2. A m o u n t of s u r f a c t a n t u s e d In the preparation of n a n o m e t e r size particles by phase transfer, the amount of surfactant used depends on several factors. 1. The charge density at the surface of the sol particle (for ionic surfactant). 2. The area of the adsorbed s u r f a c t a n t molecule, which depends on the e l e m e n t c o m p o n e n t a n d constitution of the surfactant molecule, and could vary with the solution component [2]. 3. The Critical Micelle Concentration (CMC), which could slightly vary with additive [3] (the CMC value in w a t e r can be found in handbooks). If the concentration of the surfactant is lower t h a n the CMC, the surface of the sol particles is only partly covered by the adsorbed surfactant. If the concentration of the surfactant is l a r g e r t h a n the CMC, similarly to emulsion, some patches could be formed on the surface of the particle. When the c o n c e n t r a t i o n of the s u r f a c t a n t f u r t h e r increases, double l a y e r of adsorption of surfactant would take place on the surface of the particle, the constitution of which is similar to hemimicelle [4]. Hemimicelle is bigger t h a n micelle [5] and has no advantage on the phase transfer. The diagram of the different cases mentioned above is given as follows 9
surfactant interface
@@@|174
eeeee|
sol particle concentration
concentration>CMC
concentration>>OMC
Obviously, the optimal amount of surfactant used should be to ensure that the concentration of the surfactant in the organic solvent approaches the Critical Micelle Concentration after adsorption of the surfactant. Because of the complexity of the affecting factors, it is very difficult to estimate the optimal amount of surfactant by theory. A suitable procedure for determining the optimal a m o u n t is the m e a s u r e m e n t of the Zeta potential change of sol particles with the amount of surfactant during the adsorption of the surfactant on the particles. However, it is known t h a t the a m o u n t of surfactant used influences the sol particle size and stability, which determine the size and surface a r e a of the final product. Therefore, the simplest m e t h o d for determining the optimal amount of surfactant is the experimental observation. Figure 3 shows the surface area change with the amount of surfactant, all the other conditions being fixed. It can be seen from Figure 3 that the optimal mole
200 ratio, i.e. the optimal amount of surfactant used, is about 2:10 for the system studied in this paper.
3.3. Effect of pH or amount of precipitation agent It is essential for phase transfer preparation with cationic surfactant to make sol particles carry negative charge. This can be achieved by use of excess of precipitation agent, which leads to sol particles which are negatively charged by the adsorption of OH, on the basis of Fajan's rule [6]. Obviously, there would be an equilibrium between the charge density at the surface of the sol particles and the concentration of OH. Denoyel et al [2] pointed out t h a t OH could change the density of adsorption sites but not the adsorption heat of the surfactant on the surface of SiO2 and Al203. The relation between the surface area of the product and the final pH of the sol preparation is shown in Figure 4, which indicates that there exists a critical pH value above which the surface area of the product increases rapidly as pH rises. However, too much ammonia naturally results in a loss of copper by the formation of copperammonia complex ion. 140
100
o0
20
1 iO ....
3i0
5:10
7:10
9:10 10 10
Mole ratio of surfactant to nitrate Figure 3. 9Effect of the amount surfactant used on surface area of the product
201
140
100
60
20~ 6.0
7.0
8.O
9.0
f'mal pH value Figure 4. 9Effect of final pH value on the surface area of the product 3.4. Effect of concentration of nitrate a n d precipitation agent on surface a r e a of
meproduct
The lowest limit of the particle size of the product is decided by the size of the single sol particles, i.e. it is mostly desired that sol particles prepared in the process of sol preparation neither crystallize nor aggregate into large particles. The utilization of s u r f a c t a n t can prevent particles from aggregating. Recrystallization may be weakened by immediate filtering after aging for a short time and letting the water out. Therefore, it is important for the size of the particle how to make the sol particles as small as possible. In order to reduce the size of sol particles, the nucleation rate must be raised and the orientation rate must be lowered according to Weimarn theory [6]. For the single component systems, it is possible to control the concentration so t h a t only nucleation takes place and orientation does not [7]. For m u l t i c o m p o n e n t systems, however, the lowest c o n c e n t r a t i o n limits of nucleation and orientation vary with the components. Therefore, it is very difficult for multicomponent systems to use only concentration control to depress orientation. Even if it were possible, there would be no industrial value because of a too low concentration. When the concentration goes up,on the one hand, both rate of nucleation and orientation will rise as the supersolubility increases. On the other hand, the aggregation of the sol particles will also be accelerated because the particle density in solution is larger, i.e. the distance between the particles decreases, which results in the increase of their attractive force (the attractive force, which comes from their long range interaction, is inversely proportional to the third power of the distance between the particles [6]). Therefore, a high concentration will result in not only an increase of orientation rate, but also an acceleration of the aggregation. Both cause growing up of product particle.
202 Figure 5.a. is a plot of product surface area versus concentration of the precipitating agent, ammonia, with a fixed nitrate concentration. It can be seen t h a t the surface area of the product varies with the concentration of ammonia and there is an optimal concentration range. ammoniaconcentration (mole/L) 2.0
4.0
6.0
80
0.5
1.0
1.5
2
140
Q
100
!
~176 20
0
nitrateconcentration(mole/L) Figure 5. Effect of concentration of precipitation agent and n i t r a t e on the surface area of the product. 140
100 at}
60
20 A
0
0.!
0
Nitrate concentration (mole/l) Figure 6. Effect of concentration of precipitating agent and nitrate (in a fixed ratio) on surface area of the product. The molar concentration ration of ammonia to nitrate is 4:1
203 Figure 5.b. shows the change of the product surface area vs. the change of nitrate concentration with a fixed ammonia concentration. Although the rule is similar in Figure 5. a. and b., the effect of nitrate concentration is more remarkable. One of the possible reasons for this is that the ionic migration rate of OH is much larger than t h a t of the metal ions, so t h a t OH can migrate rapidly in water solution by means of hydrogen bond. Therefore, for this case, local overthickness is not too serious during the sol preparation. Another possible reason could be that the concentration of OH does not vary proportionally with t h a t of ammonia, i.e. OH is released through the dissociation equilibrium of ammonia. Figure 6. shows the relation between the surface area of the product and both concentration of ammonia and nitrate (in a fixed ratio). All these figures have a similar pattern. The extent for the effect on the surface area of the product, however, is different from each other. It should be reminded that the molar concentration of the precipitation agent (ammonia) decreases in the process of sol preparation by reverse coprecipitation. So does the molar concentration of the surfactant. In addition, as mentioned above, the concentration of OH does not change proportionally with t h a t of ammonia. All these factors lead to the fact t h a t the relation between the product surface area and the concentration of ammonia and nitrate cannot be simply studied by changing the original concentration. However, although the results above are a global result of m a n y complicated factors, they could be more useful than the study of a single factor. Figures 7 and 8 are TEM micrographs of samples p r e p a r e d from different concentrations of ammonia and nitrate (in a fixed ratio). Figure 9 and 10 are for different nitrate concentrations (in a fixed ammonia concentration). Their surface areas and concentrations used are listed in Table 1. The phenomenon of particle aggregation appears in Figures 8 and 10, because of the larger concentrations of the precipitating agent. However, e n l a r g e m e n t of single particles cannot be observed in these micrographs, indicating that neither the change of the concentration results in the change of orientation rate nor t h a t the effect of orientation rate on single particle size is obvious during sol preparation and adsorption of the surfactant. Table 1. Effect of concentration on surface area of the product Change method ammonia nitrate concentration** concentration* Catalyst concentration (mole/L) surface area (m2/~) * in fixing ratio
BTX-57 0.1 136.9
BTX-74-7 BTX-69-1 BTX-69-3 0.2 0.2 0.8 93..9 117.7 62.1 ** in fixing ammonia concentration
3,5.EITect of ~Iditives The results above show t h a t the first of the main steps affecting the product particle size is the sol preparation and phase transfer, and the second one is the removal of water and solvent. In order to improve the preparation, some special additives can be used. The use of an additive for breaking emulsion is for the fast removal of water in sol, and its negative effect is the decrease of the surfactant concentration at the surface of the sol particles
204
because of competitive adsorption. The purpose of adding Span 80 (a non-ionic surfactant) is to increase the solubility of the cationic surfactant in organic solvent. The addition of synergist [8] aims at reducing more effectively the interface tension of the particles. .,: .. 9 S ,
"~ . '
iiiiiii:iii: :!,:J:~i~!!;~!i!
Figure 7. TEM micrograph of sample BTX-57
Figure 8. TEM micrograph of sample BTX-74--7
The results in Table 2 show that both additives (for breaking emulsion and Span 80) decrease the surface area of the product. The reason for Span 80 found experimentally could be t h a t filtering becomes more difficult. The Synergist, however, obviously increases the surface area of the product. The reason, as mentioned above, could be t h a t it increases the potency of the surfactant so t h a t the sol particles become more stable and not easy to aggregate to each other. Table 2 Effect of additives on surface area Catalyst BTX-78 BTX-76 Additives Reference * Amount** 50 ml Surface area (m2/g) 103.6 52.3 9 additive for breaking emulsion 9* amount per mole of surfactant
BTX-77
BTX-79
BTX-81
Span 80 250 g
Span 80 50 g
Synergist 89.5 ml
46.3
75.0
123.2
205 The results represented in the series of papers indicate that the particle aggregation, to some extent, always takes place for all samples prepared. Therefore, it could be necessary to select cationic surfactant and synergist more extensively and to study their compatibility to increase the potency of the surfactant.
"
..
Figure 9. TEM micrograph of sample Figure 10. TEM micrograph of BTX-69-1 sample BTX-69-3
4. CONCLUSIONS The main steps affecting the particle size of the product is the sol preparation and the phase transfer for the preparation of nanometer particles Cu-ZnA1203 catalysts with cationic surfactant by phase transfer in benzenic solvent. The optimal mole ratio of surfactant to nitrate is about 2:10. The optimal amount of precipitation agent ammonia is to ensure final pH higher than 8.5. There exists an optimal concentration range of both ammonia and nitrate for obtaining the highest surface area of the product. The effect of nitrate concentration is more marked. The future research for phase transfer preparation with cationic surfactant could be the selection of surfactant and synergist in larger range and the study of their compatibility to increase the potency of the surfactant, i.e. to more effectively prevent particles aggregation.
206 R~'~CES 1. Hu Ze-shan, Chen Song-ying and Peng Shao-yi, "Preparation of nanometric size of CU-ZN/AI203 catalyst by phase transfer. Part 1: study of basic preparation conditions", Sixth International Symposium on Scientific Bases for the Preparation of Heterogeneous Catalysts", September 5-8, 1994, Louvain-la-Neuve (Belgium). 2. R. Denoyel and J. Rouquerol, J. Colloid Inter. Sci., 143,2 (1991). 3. Salila Panda, B.K. Sinha, J. Indian Chem. Soc., 67,199-201 (1990). 4. T. Imae, K. Muto, S. Ikeda, Colloid Polym. Sci., 269, 43-48 (1991). 5. P. Chandar, P. Somasundaran and N.J. Turro, J. Colloid. Inter. Sci., 117, 31 (1988) 6. Shen Zhong and Wang Guo-ting, Colloid and Surface Chemistry, Industrial Chemistry Press (Beijing, China) 1991, 9. 7. Tadao Sugimoto, Advance in Colloid and Interface Sci., 28, 65-108 (1987). 8. Pan Heng-guo, Xiao An-ruing, Hua Xi-yan, Industrial Chemistry for Daily Use, 1, 1-7 (1990).
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
207
Flame synthesis of nanostructured vanadium oxide based catalysts P.F. Miquel and J.L Katz Department of Chemical Engineering, The Johns Hopkins University, Baltimore, Maryland 21218, U.S.A. t
Nanostructured V205-TiO2, V205-A1203, and V-P-O powders were synthesized in hydrogen-oxygen flames using a counterflow diffusion flame burner. This flame synthesis technique allows one to select a specific powder morphology or crystalline structure by selecting the appropriate flame temperature, precursor concentration ratio, and precursor. The operating conditions appropriate to produce vanadium oxide "monolayer" on TiO 2 and A1203, VOPO4.2H20, f l - V O P O 4, ' y - V O P O 4, r 4, and (VO)2P207 are presented and discussed.
1. INTRODUCTION Vanadium oxide based catalysts are widely used for the selective oxidation of hydrocarbons [1]. For example, V205-TiO2 and V205-A1203 are used for the selective oxidation of o-xylene to phthalic anhydride, and vanadium-phosphorus oxides (VPO) are used for the selective oxidation of butene and n-butane to maleic anhydride. These mild oxidation reactions have been shown to be structure-sensitive, i.e., the selectivity for a given product is related to a specific morphology of the catalyst or to a specific crystallographic plane exposed at the surface of the catalyst [2]. In the case of V205-TiO2 and V205-A1203, high selectivity and activity is achieved when the vanadia forms a bidimensional layer, called "monolayer", on the surface of the support [3]. In the case of VPO catalysts, the selectivity towards maleic anhydride is related to the presence of specific and known crystalline VPO phases, and in particular the presence of (VO)2P207 [4][5]. To obtain these specific structures, "traditional" methods of preparation typically require a very high number of steps, such as pretreatment, mixing, chemical reaction, filtration, purification, drying, and calcination. These methods thus can be expensive and can cause waste treatment difficulties. Nanostructured materials are a subject of growing interest in catalysis since they are expected to exhibit enhanced catalytic reactivity when compared to conventional materials [6]. We have previously used a counterflow diffusion flame burner to produce and study the formation of nanostructured oxides and mixed oxides [7][8].
*This research was supported by the Division of Materials Sciences, Office of Basic Energy Sciences, United States Department of Energy via Grant DE-FG02-88ER45356.
208 More recently, we have applied this technique to produce vanadium oxide based powders for use as catalysts [9][10]. This novel technique offers significant advantages over traditional methods. One can form powders of desired particle size, morphology and crystalline structure, by selecting the appropriate flame temperature, precursor concentration ratio, and precursor. Flame synthesis also reduces the powder production process to a single step operation, drastically reducing the processing time and the number of environmentally detrimental side streams. Finally, it allows one to obtain powders with a high degree of purity and high surface area.
2. EXPERIMENTAL SETUP 2.1. The counterfiow diffusion flame burner
We are using a rectangular version [11] of the counterflow diffusion flame burner (see Figure 1). It consists of two vertically opposed tubes of rectangular crosssection. Each tube is subdivided into three channels, a large central channel and two end channels. The oxidizer (0 2 diluted in argon) flows downward from the top tube, while the fuel (H 2 diluted by argon) flows upward from the bottom tube. This rectangular geometry aided, by fused silica plates on the burner's sides, causes the combustion gases to flow out its front and back. Flanges minimize entrainment of surrounding air. A flame is visible in the region where the two opposed gas streams collide. The horizontal plane where this occurs is called the gas stagnation plane. The location of the stagnation plane can be changed by changing the relative flow rates of the gas streams. The flame generated is very flat and uniform in the horizontal plane; temperature and concentration distributions are also very uniform in the horizontal plane. The main advantage of this geometry is that the flow along the vertical direction (i.e., the Z axis in Figure 1) is essentially one dimensional. Thus, optical methods, such as spectroscopy or light scattering measurements, can be used to measure accurately temperature, particle
ed J
Perforat Plates ~
"/////zI~------"
02 + Ar
! ~'~Top Main VN~ ++1~, 9 +Channel 9 9
~
~
rtanges
4 o'
++
..,
........................
Silica Windows~ ::::::::5:::
~.~~~III] /
,u
J-7 s
~2000
Z imm)
:::::::::::::::::::::::::::::::::::::::::: ~ "
Glass Beads~-
~
~
Bottom Main ~.a..~!.
~ ~
"--,o'-~o' ;
.....
5000
",~ ~2~00 | ~-
r 1500
.............................
_J ir=,~ Flome 1 ~ J "m'm'u. g" ..o..~, " i o#++~,~ ~ ~
j ~"
,,~
'~
',, "
,.geFlam e 2
End Channels
r 1000
I--
H2+Ar
'+~o '+~o"
X (mm)
Figure 1. Front view of the Counterflow Diffusion Flame Burner.
500
........................... -6 -4 -2 0 2
4
6
z
Figure 2. Temperature as a function of vertical position (see Fig. 1). Filled symbols are OH Rotational Temperatures and open symbols are thermocouple measurements.
209 size, or number density. In this study, all measurements were made as a function of the vertical direction. The oxides (and the mixed oxides) are produced by bubbling a small part of the fuel or the oxidizer stream through one (or several) gas washing bottles containing the appropriate liquid precursors, e.g., TiC14 or VOC13. (The concentration of precursor is expressed as mole of precursor per mole of all gases in that feed stream). The precursors react when entering the flame with the oxygen present as oxygen molecules (O2), oxygen radicals (O.), or hydroxyl radicals (OH.). The oxides thus formed nucleate and grow as they flow towards the gas stagnation plane. Near the gas stagnation plane, the oxide particles flow out of the burner with the outflowing gases. In this study, five precursors were used: VOC13, VC14, TIC14, AI(CH3) 3, and PC13. Four systems (VOC13-TiC14, VOC13-AI(CH3)3, VOC13-PC13, and VC14-PC13) forming three mixed oxides (V2Os-TiO2, V205-A1203, and VPO) were studied.
2.2. Spectroscopy and light scattering This burner geometry enables one to study oxide formation and growth in the flames using optical (therefore nonintrusive) techniques, i.e., light scattering, and absorption or emission spectroscopy. Two sets of flow rates, producing the temperature profiles shown in Figure 2, were used in this study. They will be referred to as the high temperature flame (or Flame 1), and the low temperature flame (or Flame 2). These temperature profiles were measured using two techniques with overlapping range [12]. Above 1500 K, the rotational fine structure in the UV absorption spectra of OH was measured using a 75 W xenon arc lamp (the light beam traverses the burner along the X direction). From the measured spectra, the distribution of the ground state population of hydroxyl radicals was determined, and from it their rotational temperature. Below 2000 K, silica coated Pt-Pt 10% Rh thermocouples were used, and the temperatures measured were corrected for radiation losses. In Figure 2, temperatures measured using the optical method are shown as solid symbols and those measured by thermocouples are shown as open symbols. (Note that they agree well in the regions where both can be used). The same optical setup used to measure the OH spectra was used also to detected the presence of suboxides in the flames. The presence of phosphorus oxide (PO) was detected and quantified by measuring the absorption of light at the 324.62 nm peak in the PO/3 band system (X2II-A2~;+) [13]. An argon ion laser operating at 514.5 nm was used to measure light scattering along the vertical direction (the laser beam traverses the burner along the X direction). Light scattering was measured at 90 ~ to the laser beam with a photomultiplier. Further details about the experimental setup and the underlying light scattering theory is available in our earlier publications [7][12]. 2.3. Particle collection and analysis Particles were collected directly in the flame using a TEM sampling probe technique [14]. This technique consists of placing a T r a n s m i s s i o n Electron Microscope (TEM) grid directly onto the tip of a thin probe, which then is rapidly driven into and out of the burner. Particles are driven onto the grid by thermophoresis. The location of the tip of the probe is adjusted such that when it is inserted into the burner, the surface of the TEM grid lays exactly in the middle of the laser beam. This enables
210 us to correlate the morphology of the particles collected on the grids with the light scattering measurements. To collect the larger amounts needed for x-ray diffraction, FT-IR spectroscopy, or for surface area measurements, particles were collected on two auxiliary stainless steel strip located on the front and back side of the flame (at Z = 0 mm). An electric field of up to 1300 V was applied between them. TEM analysis of particles collected with the strips showed that neither the presence nor the strength of the electric field affected their size or morphology. The particles collected on the strips were indistinguishable in size from particles collected directly onto the TEM grids inserted into the flame at elevation near the stagnation plane (the plane where the gases impinge and flow out of the burner). However, their degree of aggregation or sintering was somewhat higher. Particles were also collected onto the strips when the strips were located at different elevations in the burner. In every case, the particles collected were indistinguishable from those collected at Z = 0 mm, indicating that the particles which actually deposit onto the strips come from the gases flowing out near the stagnation plane. The crystalline forms of the powders collected on the strips were determined using a Philips APD 3720 X-ray Diffractometer. CuKc~ radiation was used and the diffractometer was run over a 20 angular range of 10 to 50 ~ FT-IR spectra were recorded at 2 cm -1 resolution, using 32 scan averaging, on a Mattson (Polaris) Fourier transform infrared spectrometer using the KBr disk technique [9][10]. Surface areas were determined by a single point BET measurement of nitrogen desorption using a Micromeritics Flowsorb II 2300 apparatus. Subsequent thermal decomposition of the p o w d e r s was accomplished by heating them at 390~ in an inert a t m o s p h e r e (30% N2/He), followed by heating at up to 750~ in flowing helium in a tube furnace.
3. RESULTS When precursors are added to a gas stream, oxide particles nucleate and grow as they flow vertically towards the stagnation plane. They flow out of the burner at the stagnation plane, where they are collected on stainless steel strips for post-treatment and characterization. The optical techniques used provide insight into the particles formation mechanisms in the flame, as a function of their vertical position. Two vanadium-based systems (VCla-PC13 and VOC13-AI(CH3)3) will be described here, correlating light scattering and absorption measurements with information obtained by observing the oxide particles collected on TEM grids. The crystalline structures of the particles collected on the stainless steel strips strongly depend upon the operating conditions and their growth history in the flame. We here show how, by selecting appropriate flame temperature, precursors, and precursor concentration ratio, one can produce vanadium oxide particles whose morphologies and crystalline structures match those of the known active catalysts: V2Os-TiO2, V2Os-A1203, ~ - V O P O 4 , ~ - V O P O 4 , ~-VOPO4, and (VO)2P207. 3.1. Particle formation process The VCI4-PCI 3 system: Figure 3 shows the light scattering intensity profile when a mixture of 0.03% V C I 4 and 0.03% PC13 (u) is added to the fuel stream in Flame 2. For comparison, the scattering intensity when only 0.03 % VCI 4 (e) was added to the fuel stream is also shown. No scattered light was detected when only PC13 was added to the
211 Temperature (K) I >,, 5 . 0
1010 650 I I Mixture
1400 I
1780 2 0 7 0 I I
Temperoture (K) 2240
650
0.4
125.
1010 ,
,
1 780
,
224-10" 25 U'I
r- 4.0 Z 3.0 cn E92 0
k.. G9 o o O0
.0.3 0q3
"~ 100
cr 0.2 o
c
tr
"
E ~
1.00
! 75
0.75
",.
tO
.E 5O.
0.50
0
--
VOCl 5
o 25
1.0
o o
0.25
o ,
0.0__7 "6
1,
!
-5
"4
,
|
,
3 ...... Z (mm)
Figure 3. Scattering using a mixture of 0.03% VC14 and 0.03% Pel3(m), or VC14 alone (o). Absorbance by PO using 0.03 % PC13(o) or together with 0.03% VC14 (D).
0
-7
-6
-5
-4
-3
Z (mm)
-2
-1
0
0
Figure 4. Scattering intensity using a mixture of 0.03 % VOCI3 and 0.03 % AI(CH3)3, or using VOC13 or AI(CH3) 3 alone.
fuel stream [10]. The presence of phosphorus oxide was detected by measuring light absorption by PO at 324.62 nm [13], and is shown as open symbols (using the Y axis on the right hand side). When the mixture of precursors is used, the sharp increase in light scattering intensity at Z = -6.0 mm is due to the particle nucleation and aggregation. Samples collected on carbon coated TEM grids at the first scattering intensity peak, i.e., Z = -4.0 mm, showed that the particles are mainly chain-like structures composed of spherical primary particles 15 to 20 nm in diameter (see Figure 5A). Analytical Electron Microscopy (AEM) of these particles showed that they are a homogeneous mixture of vanadium and phosphorus oxide. Between Z = - 4 . 3 mm and Z = - 3 . 7 mm, the increasing temperature encountered by these chain-like structures as they flow upwards, causes them to collapse into spherical particles. Particles collected on TEM grids at Z = -3.7 mm are spherical and have a diameter of 30 to 50 nm (see Figure 5B). Since the overall diameter of a spherical particle is much smaller than that of the chain-like structure from which it formed, the scattering intensity decreases. (Light scattering intensity is proportional to the 6th power of the particle diameter, for spherical particles whose diameter is much smaller than the wavelength of the light being scattered [7]). Between Z = - 3 . 7 mm and Z = - 2 . 9 mm, the particles continue to grow by further surface condensation or by collision-coalescence, giving rise to a second scattering intensity peak. Finally, above Z = - 2 . 9 mm, the light scattering intensity decreases sharply. This is due to the presence of the stagnation plane. It is informative to compare the scattering intensity of this mixture (m) to that observed when VC14 is used alone (o), and also to compare the absorption of light by PO when the mixture is used (D) to the absorption which occurs with only PC13 (o). One sees that the absorbance by PO is much weaker and does not begin until the gases are h i g h e r in the b u r n e r when VC14 is used with the PC13 than in its a b s e n c e . Correspondingly, the light scattering by the mixture is much stronger in that region (-5 mm to -4.5 mm) where PO absorbance was not detected (with the mixture), i.e., PO is not detected because the phosphorus is condensing together with the vanadium. This
212
~
.......:~i!!~:i:i:i!i!~i3i!i!i'I,~ :~ :~!'ili:i:'i~!il
Figure 5. Particles collected on TEM grids in the burner (all bars represent 50 nm). is consistent with the AEM observation that these particles are an homogeneous mixture of vanadium and phosphorus oxide. The VOCI3-AI(CH3) ~ system: Figure 4 shows the scattering intensity profile (m) when a mixture of 0.03% VOC13 and 0.03% AI(CH3) 3 is added to the fuel stream of Flame 2 [9]. Also shown are the scattering intensity profiles using only VOC13 ( 9 or only AI(CH3) 3 (o) at the same concentrations. The measured scattering intensities at elevations where the edges of the burner partially block the laser beam, are represented by open symbols and dashed lines. Note that the onset of particle formation for the mixtures and for AI(CH3) 3 occurs at or inside the burner mouth. The scattering intensity profile with the mixture of precursors is much stronger but otherwise similar to that obtained when AI(CH3) 3 is used alone. For both there is a sharp increase in light scattering intensity followed by a decrease. Figure 5C is a TEM micrograph of particles collected at Z = -6.9 mm. One sees two types of chain-like structures: one composed of small, nearly spherical particles (--- 10 nm in diameter) and a second composed of larger somewhat oblong particles ( - 5 0 nm in diameter). AEM analysis showed that the small particles are composed mainly of A1203 and that the large ones are composed mainly of vanadium oxide. As the chain-like structures flow upwards and encounter higher temperatures, the vanadium oxide rich particles melt and encapsulate the A1203 chain-like structures. Figure 5D shows a TEM micrograph of particles sampled at Z = -3.0 mm. One sees that the particles have a core-mantle structure, a core composed mainly of A1203 particles, and a mantle of vanadium oxide. 3.2. Formation of vanadium oxide "monolayer" on TiOz and AI203 V205-TiO2 and V205-AI203 mixed oxides are well-known catalysts for the selective oxidation of hydrocarbons, particularly the oxidation of o-xylene to phthalic anhydride [1], and for the selective reduction of nitrogen oxide with NH 3 [15]. High activity and high selectivity are achieved when the vanadium oxide is present in the form of an amorphous bidimensional layer (called a "monolayer") on the surface of TiO 2 or A1203. Such powders were obtained in our burner using Flame 2 [9]. AI(CH3)3, TiC14, and
213 VOC13 were used as precursors in the fuel stream and the concentration of VOC13 was adjusted so as to collect powders having a monolayer-like structure of vanadium oxide on TiO 2 and on A1203. The x-ray diffraction and FT-IR spectra of the V205-TiO2 mixtures collected on the stainless steel strips, indicated that vanadium oxide is crystalline V205 when VOCI3:TiC14 ratios of 1:3 or larger are used, whereas it is amorphous for smaller ratios. The spectra of the V205-A1203 mixtures indicated that vanadium oxide is amorphous for VOC13:AI(CH3) 3 ratios of 1:3 or lower. Moreover, at very low VOC13 loadings, the FT-IR spectra of both mixed oxides showed a sharp peak between 1020 and 1030 cm -1. We attributed this peak [9] to the presence of monomeric vanadyl species. At higher VOC13 loadings, the FT-IR spectra showed a broad band at 980 cm -1 for the V:Ti mixed oxides, and at 995 cm -1 for the V:A1 mixed oxides. We attributed these bands to polyvanadate species. At even higher loadings, crystalline V205 was observed. The amorphous vanadium oxide "monolayer" obtained by traditional methods has been shown by others [16] to be composed of: monomeric vanadyl species at very low vanadium loadings, polyvanadate octahedraUy coordinated species at higher vanadium loading, and crystalline V205 at even higher loadings. The FT-IR spectra of our powders showed identical peaks and band systems, suggesting the presence of similar vanadium oxide species. Moreover, these peaks and band systems depend upon the VOCI 3 concentration used, in agreement with their dependence on vanadium loading shown by others [17][18]. These results show that the vanadium oxide structure formed on TiO 2 and A1203 in our burner matches that of vanadium oxide-based catalysts.
3.3. Formation of/~-VOPO4, 3,-VOPO4, and/i-VOPO 4 Vanadium-phosphorus oxides (VPO) are the most widely used catalysts for the selective oxidation of butene and n-butane to maleic anhydride [19]. ~ - V O P O 4 is the most active VPO phase for the selective oxidation of butene [5], whereas (VO)2P207 in the presence of 7 - V O P O 4 o r / 3 - V O P O 4 is the active phase for the selective oxidation of n-butane [19][20]. Furthermore, ~ - V O P O 4 , ~ , - V O P O 4 , and t ~ - V O P O 4 are all easily reduced in an inert atmosphere to (VO)2P207 [5]. PC13 and VOCI 3 were added to the fuel stream in a 1:1 ratio to produce VPO powders [10]. Flame 1 and Flame 2 were used to investigate the effect of temperature on the powders' crystalline structure. When Flame 1 (the high temperature flame) was used, a mixture of VOPO 4.2H20 and tS-VOPO 4 was obtained on the stainless steel strips. This mixture was heated at 390~ in an inert atmosphere and a pure t 3 - V O P O 4 phase was obtained. This t S - V O P O 4 phase was then heated at 750~ in flowing helium, and 7 - V O P O 4 was obtained. When Flame 2 (the low temperature flame) was used, the x-ray diffraction pattern of the powder obtained on the stainless steel strips did not match any known VPO phases. We have assigned the FT-IR spectrum of this powder to a VOHxPO 4 9YH20 phase [10]. This powder formed an amorphous e t - V O P O 4 phase when heated at 390~ in an inert atmosphere, and converted to /3-VOPO 4 on subsequent reheating at 750~ in flowing helium. The VOHxPO 4. yH20 phase obtained in Flame 2 was characterized by x-ray diffraction pattern at d-spacings 7.05 (vs), 6.84 (w), 4.21 (w), 3.54 (m), 3.04 (vs), 2.60 (w), 1.95 (w), and 1.56 (m) [10]. The same phase was also obtained when the t S - V O P O 4 phase, obtained using Flame 1, was left in open atmosphere for several days. This diffraction pattern does not match that of VPO powders formed by traditional methods. However, it does match that of a VPO powder also produced at high-temperature by Moser[21], using a high-temperature aerosol reactor. These
214 results show that one can obtain specific crystalline phase of VPO powders by varying the temperature. Moreover, the surface area of the powders are very high, often much higher than those obtained by traditional methods (see Table 1). Unfortunately, the posttreatment required to obtain the desired phases, in some cases, lowered these surface areas to values more typical of those presently obtained by others [10]. 3.4. Formation of (VO)2P20 7 The vanadium phosphorus oxide phase which is active in the selective oxidation of n-butane to maleic anhydride is in dispute in the literature. However, there is general agreement that the presence of vanadyl pyrophosphate, (VO2)P207 is necessary for the reaction to proceed. The best catalysts are believed to be related to the presence of (VO)2P207 when they preferentially exhibit (100) faces [4], or to the presence of "y-VOPO4/(VO2)P207 or of/~-VOPO4/(VO2)P207 mixtures [19][20]. (VO2)P207 is traditionally obtained by topotactic transformation of the hemihydrate of vanadyl acid phosphate ( V O H P O 4 . 0 . 5 H 2 0 ) in the temperature range 500-750~ in an inert atmosphere [ 19]. We currently are investigating the possibility of producing either (VO)2P207 or VOHPO 4. 0.5H20 directly in the burner. Since all the anhydrous VPO powders obtained in the burner using PCI 3 and VOCI 3 as precursors were in V 5 + oxidation state, and since (VO)EP207 and VOHPO4"0.5H20 require V 4+, we are investigating the possibility of forming these phases using PC13 and VCl 4. Initial results are presented here. PCI 3 and VC14 were added in a 1:1 ratio to the fuel stream of Flame 2 (the low temperature flame). The x-ray diffraction pattern of the powder collected on the stainless steel strips is shown in Figure 6a. Five broad lines can be observed at d-spacings 7.1, 4.1, 3.6, 3.12, and 3.0 A. The lines are very broad because the particles that compose this powder are very small in size. We attribute the 4.1, 3.6, and 3.12 A lines to the presence of t~-VOPO 4 (A) and the 7.1 and 3.0 A to the presence of the same VOHxPO 4. YH20 phase (o) obtained previously. FT-IR measurements on this powder
D-spacing (A) 7.0 I
I
I
I
5.0 I
I
4.0 5.5 I
I
5.0 I
2.5
2.0
I
I
[]
o
AA
o
c--
10
1'5
2'0
2'5
5'0 2|
5'5
4'0
4'5
50
Figure 6. X-ray diffraction pattern of powders produced using PCI 3 and VC14 in the fuel stream (a), after heating at 390~ (b), and after heating at 700~ (c).
215 confirm this assignment. Furthermore, the diffraction lines attributed to the VOHxPO 4. yH20 phase do not match the diffraction pattern of VOHPO 4.0.5H20. The powders collected on the strips were heated at 390~ in an inert atmosphere (Figure 6b). This heat treatment caused a sharpening of the diffraction lines corresponding to ~-VOPO 4 (A), i.e., 4.03, 3.67 and 3.12 A, and the disappearance of the lines attributed to the VOHxPO 4.yH20 phase. The presence of (VO)2P20 7 is not detected. This powder was then heated at 700~ in a tube furnace in flowing helium. The diffraction pattern (Figure 6c) shows the presence of three VPO phases: /~--VOPO 4 ((~x) d-spacings 3.05, 3.39, 5.17, 4.58, and 2.97 ,/k), ~I-VOPO4 ((El) d-spacings 3.05, 2.99, 3.55, 4.41, and 2.21 A), and (VO)2P207 (( 9 d-spacings 3.86, 3.13, and 2.99/~). (Note the low intensity of the (00/) lines in the diffraction pattern of t~II-VOPO4, i.e., 4.41 and 2.21 /~, indicating some disorder along the c axis.) The diffraction pattern of the powder collected in Flame 2 using PC13 and VOC13 also showed the presence of the VOHxPO4 9yH20 phase. However, in this case, when the powder was heated in the tube furnace at 750~ (VO)2P207 was not detected. It is the use of V 4+ in the precursor (as VCl4) which leads to its presence in VPO powders. VPO powders were collected also on the stainless strips using PC13 and VC14 at the same concentration ratio and in the same flame as described above, but bubbling the two precursors through the oxidizer stream. The x-ray diffraction pattern of this powder was identical to that obtained when feeding the precursors through the fuel stream (i.e., tS-VOPO 4 and VOHxPO4 9yH20 ). However, when this powder was heated in the tube furnace at 700~ its x-ray diffraction pattern showed only the presence of ~ - V O P O 4. Note that when the precursors are added to the flame through the fuel stream they nucleate in an oxygen poor region of the flame. Thus, they probably nucleate as suboxides and become fully oxidized only as they near the stagnation plane. The lack of (VO)2P207 in this case suggests that its formation would be favored by the use of fuel rich flames. Investigation of this is currently under way in our laboratories and will be reported on at the conference. Table 1 Surface area of powders collected on the auxiliary stainless steel strips. Powders VPO
Precursors (ratio) VOC13 :PCI 3 (I:1)
Flame
Surface area (m2/g)
Flame 1
45.7
Flame 2
20.2
VC14 : PC13 (1:1)
Flame 2
22.9
V2Os-TiO 2
VOC13 : TiC14 (0.5:3)
Flame 2
47.4
V205-A1203
VOCI 3 : AI(CH3) 3 (1:3)
Flame 2
79.3
216 4. S U M M A R Y
The flame synthesis technique used here presents a novel route for the formation of vanadium oxide catalysts, with significant advantages over traditional methods. The powders are produced in a single step operation, with very low processing times, a high degree of purity, and particle sizes in the nanometer range. The main advantage of this technique is its ability to produce powders of very different morphologies or crystalline structures by varying the process variables: vanadium oxide "monolayer" onto TiO 2 or A120 3 particles were obtained by varying the vanadium precursor concentration; /3-VOPO4 , ' y - V O P O 4 , and tS-VOPO 4 were obtained by using different flame temperatures; and (VO)2P207 was obtained by using a V 4+ vanadium precursor. The possibility of producing a pure (VO)2P20 7 phase directly in the burner is currently under investigation
REFERENCES
.
3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
D.J. Hucknall, Selective Oxidation of Hydrocarbons, Academic Press, New York, 1974. J.C. Volta and J.L. Portefaix, Appl. Catal., 18 (1985) 1. G.C. Bond and S.F. Tahir, Appl. Catal., 71 (1991) 1. G. Centi, F. Trifiro, J.R. Ebner, and V.M. Franchetti, Chem. Rev., 88 (1988) 55. E. Bordes, Catal. Today, 16 (1993) 27. D.D. Beck and R.W. Siegel, J. Mater. Res., 7 (1992) 2840. C-H. Hung and J.L. Katz, J. Mater. Res., 7 (1992) 1861. C-H. Hung, P.F. Miquel and J.L. Katz, J. Mater. Res., 7 (1992) 1870. P.F. Miquel, C-H. Hung, and J.L. Katz, J. Mater. Res., 8 (1993) 2404. P.F. Miquel and J.L. Katz, J. Mater. Res., 9 (1994) 746. S.L. Chung and J.L. Katz, Combustion and Flame, 61 (1985) 271. J.L. Katz and C-H. Hung, Combust. Sci. Technol., 82 (1992) 169. A.G. Gaydon, The Spectroscopy of Flame, Chapman and Hall, London, 1974. R.A. Dobbins and C.M. Megaridis, Langmuir, 3 (1987) 254. H. Bosch and F. Janssen, Catal. Today, 2 369 (1988). H. Eckert and I.E. Wachs, J. Phys. Chem., 93 (1989) 6796. M. Inomata, K. Mori, A. Miyamoto, T. Ui, and Y. Murakami, J. Phys. Chem., 87 (1983) 754. M. Inomata, K. Mori, A. Miyamoto, T. Ui, and Y. Murakami, J. Phys. Chem., 87 (1983) 761. E. Bordes, Catal. Today, 1 (1987)499. N. Harrouch Batis, H. Batis, A. Ghorbel, J.C. Vedrine, and J.C. Volta, J. Catal., 128 (191) 248. W.R. Moser, in Catalytic Selective Oxidation, S.T. Oyama and J.W. Hightower (eds.), ACS Symposium Series, Washington (1993), p. 244.
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
217
T h e p r e p a r a t i o n o f s t a b l e R u m e t a l c l u s t e r s in z e o l i t e Y u s e d as c a t a l y s t for a m m o n i a s y n t h e s i s u. Guntow, F. Rosowski, M. Muhler, G.Ertl and R. Schl5gl ~ ~Fritz-Haber-Institut der Max-Planck-Gesellschaft Faradayweg 4-6, D-14195 Berlin (Dahlem), Germany
Ru-exchanged zeolite NaY is an active catalyst sytem for ammonia synthesis in which the Ru clusters are prevented from sintering by the zeolite framework. The influence of the precursor synthesis conditions was studied by in-situ and ex-situ UV/VIS spectroscopy, AAS, and XRD. The intrinsic lability of [Ru(NH3)6]C13 against hydrolysis and oligomerisation was found to be increased in the presence of zeolite Y. The resulting free ammonium ions led to a cation-exchange of the zeolite and allowed insertion of the modified monomeric Ru complex. Oligomerisation at a single site within the zeolite formed a molecular precursor to a metallic Ru cluster. The activation procedure of the precursor yielding metallic Ru particles was studied by T P D / T P R experiments. Heating in Ar allowed to monitor dehydration, deammination, autoreduction and catalytic ammonia decomposition. The optimum catalytic NH3 synthesis activity was observed after heating the oligomeric precursor in a N2/H2 - 1/3 synthesis gas mixture. The catalytic activity was found to increase with increasing particle size providing evidence for the structure sensitivity of NH3 synthesis on Ru.
1. I n t r o d u c t i o n Ammonia synthesis is carried out in industrial practice over a promoted metallic iron catalyst with special morphological properties. This catalyst has been optimized to its limits but exhibits as one major draw-back a high sensitivity to poisoning by oxygenic compounds. The application of Ru in a suitable highly disperse form may represent a less oxygen-sensitive alternative. We are interested in finding strategies for the development of such a catalyst by merging surface analytical efforts with controlled synthetic procedures. One facet in this catalyst development is the generation of Ru clusters enclosed in a zeolite Y matrix allowing to investigate analytical and kinetic properties of well-defined small Ru particles. We have presented surface analytical and kinetic data on Ru enclosed in zeolite Y and supported on zeolite Y and A [1-3]. The present communication focusses on the preparation of the precursor intercalation compound and its activation into the active state for ammonia synthesis.
218 2. E x p e r i m e n t a l Zeolite NaY was obtained from DEGUSSA (KM-390). Its bulk elemental analysis was confirmed by RFA in wt % for Na (9.23%), A1 (11.64%) and Si (30.65%) after dehydration at 1353 K (water loss 9.27 wt%). It agrees well with the theoretical formula Na56[(A102)56(Si02)136] * 250H20 allowing to refer exchange reactions on a relative scale to a sodium content of 56 units per formula unit zeolite. A potassium form (KY) was prepared in a one-step exchange at 333K for lh with KC1. These two starting zeolites were reacted with [Ru(NH3)6]C13 either obtained from HERAEUS or made from RuC13 in an ammoxidation reaction in bi-destilled water at 333 K. The identity of the two products was checked with elemental analysis, IR and UV/VIS data. We here discuss the following four samples out of 64 preparations which were reproducible in their final catalytic behaviour and which could be scaled in batch size from 0.1 g to 50 g: KY precursor, Ru/KY, Ru/NaY, Ru/NaY(295K) prepared by reacting NaY with [Ru(NH3)6]C13 at 295 K . A typical recipe for a sample of the type of Ru/NaY is given as follows: 137 ml water and 1.73 g [Ru(NH3)6]C13 were warmed to 333 K. 5.03 g NaY were washed with 25 ml cold water into the solution which gradually turned from pale yellow to light purple. After 1 h at 333K the reaction mix was allowed to cool to room temperature for 45 min and filtered off in air. The product was dried at ambient temperature over silica gel in a dessicator. During drying the prodtict turned deep-purple. Drying in vacuum accelerated the colour change. Elemental analysis of the reaction solutions was carried out by UV/VIS for Ru and AAS for Na and K. The exchanged zeolites were analysed by RFA for Ru. Control data were obtained from AAS analysis after dissolution in concentrated HC1 up to 425 K. Powder X-ray diffraction was carried out in focussing Bragg-Brentano geometry with transmission samples with internal Si standard. Catalytic testing was carried out in an all stainlesssteel microreactor setup using an on-line IR NH3 detector (BINOS). The gases used had a purity of 99.9993% and were further purified by a self-designed purification unit [4]. The usual synthesis gas flow was 40Nml/min using 137mg sample. Temperature-programmed desorption and reduction (TPD, TPR) experiments were carried out with a conventional glass set-up with cold trap and thermal conductivity detector and an all stainless steel set-up equipped with a calibrated mass spectrometer. 3. Results and Discussion 3.1. Ru exchange and oligomerisation in zeolite Y The Ru ammine complex was chosen due to its relative high stability in water against hydrolysis and oxidative oligomerisation to ruthenium red [5]. This reaction is assumed to take place during ion exchange into the zeolite giving rise to the colour change of the zeolites from pale yellow to purple. The process was already studied in the context of Ru chemistry in zeolites intended to be used as catalysts in CO hydrogenation [6,7]. It is pointed out that the details of the preparation exert a significant influence on the stability of the final activated catalyst, in particular on the tendency of the final Ru particles to remain under reducing conditions inside the zeolite framework [2,8]. This final stability may be pre-formed already during the preparation of the precursor taking into consideration the possible cation exchange sites within the framework. The nature
219 of this exchange reaction in which one trivalent cation formally exchanges 3 monovalent ions is in this context of particular significance. The constitution of the Ru complex in the exchange solution needs also some consideration as possible hydrolysis products may exchange with different sites or with different kinetics as the starting Ru complex. Characteristic data of the intercalated zeolites are reported in table 1. Table 1 Characteristic data of Ru-exchanged zeolites. The ion content in solution is given as % of the total ion exchange capacity assuming an exchange ratio of Ru/Na = 3/1.
Sample prepared from % Na in solution % K in solution % Ru in solution lattice constant / ppm BET surface /
m2/g
Ru/NaY NaY 47 -
Ru/KY KY i0 33
Ru/NaY(295 NaY 26
K)
-
50
48
25
2467
2467
2467
549
517
546
The data show that there is good numerical agreement between the exchanged amount of alkali and the exchanged amount of Ru (actual content divided by 3). The absolute content of a 50% exchanged sample in Ru is about 5.6 wt%. The deliberately partially exchanged Na/K sample reacted with the Ru complex under simultaneous exchange of Na and K indicating that the kinetic preference for ion exchange is different for K and Ru. This is the first hint to the inequivalence of the exchange reactions between monovalent and polyvalent ions. The second hint arises from the purple colour pointing strongly to the presence of [(NH3)sRu+3-O-Ru+4(NH3)4-O-Ru+3(NH3)5] C16 (Ru red) in the zeolite. The complex ion is formally 6 fold positively charged. These findings are in apparent contradiction to the data in table 1 suggesting a Na / Ru exchange ratio of 3/1. In this situation the exchange isotherms determined simultaneously for the alkali ion and the polyvalent ion should indicate the nature of the exchange process. For a true ion exchange mechanism the two isotherms must exhibit the identical shapes. Exchange of Na by K satisfies this condition very accurately as shown in fig.1 (upper left isotherm). The Ru intercalation reaction exhibits, however, very different isotherms which are given in fig.1 for three reaction temperatures. The massive deviation in shapes is not due to a kinetic effect. We have determined the exchange kinetics and found complete reaction after 20 min exchange time with 85 % exchange after 2 min. The data in fig.1 indicate independent processes for the loss of Na and the incorporation of Ru into the zeolite. We assume that the exchange occurs actually between Na + and NH + liberated from the Ru complex by hydrolysis. The primary Ru ammine hydroxy chloride complex polymerises to oligonuclear compound independently from the cation exchange process. Low reaction temperatures and high Ru complex concentrations favour the formation of fewer larger Ru oligomers inside the zeolite, high temperatures polymerise the Ru compound prior to intercalation and reduce the total amount of intercalated Ru.
220 1.4
1.3 1.2
333 K
-
room
tempe
I,I 1,0 ),9 ._
o.s
r.9
0,6
"~
0,4
),8 3.7
).4
--O'--
K C on to n to f the F i Itrate
9
0.0
. . . .
!
0
. . . .
|
5
. . . .
I0
!
. . . .
|
15
. . . .
20
|
. . . .
25
!
,
),1
0
1,o
. . . .
30
"
35
!.4
. . . . |
. . . .
0
5
w . . . .
I0
!
. . . .
15
!
u on n o e Na Content o f the Filtrate . . . .
!
20
. . . .
25
!
. . . .
30
35
1,4
1,3
333 K
1,3
1,2
363 K
1.2
I,I
I.!
I.O
!.0
0.9
0,9
0,8
0.8
0,7
0,7 O ~
0.6 0.5
ly "
1,3
/'~'ll
O
0
O
9
0.6
Q
4
o
0.4
0.4
0.3
0.3 I
0.2
i
9
O
O
O
0.5
Ru Con~.,nt o f the Zeolite
0,2
0.1
9 Ru Content o f the Zeolite
0.1 0
0.0
.... 0
I .... 5
! .... I0
I .... 15
I .... 20
! .... 25
Concentrations [mmol Ru/l]
0,0
I .... 30
35
. . . .
0
w . . . .
5
!
I0
. . . .
!
15
. . . .
i
20
Na Cont~at o f the Fiitrat~ . . . .
w . . . .
25
|
30
. . . .
35
Concentrations [mmol Ru/i]
Figure 1. Potassium and Ruthenium exchange isotherms of zeolite NaY in aqueous solution.
The following observations support this interpretation of the "ion exchange" which is better refered to as intercalation process. We tried several Ru compounds which form monomeric complexes in water without ammine ligands as exchange reagents and all failed to incorporate into the zeolite. In-situ observation of the exchange solution by UV/VIS spectroscopy showed that the reaction solution underwent a chemical reaction after the addition of the zeolite optically visible by a pale purple colouring. In fig.2 spectra of Ru red, [Ru(NH3)6]C13 and the reaction solution of Ru/NaY are compared. The optical colour change is not indicative of oligomerisation to Ru red but to the formation of a new mono-nuclear complex in solution. The shifted absorption maximum from 265 nm to 294 nm is consistent with [Ru+3(NH3)5OH] 2+ (see data in ref. [5] and references therein) in full agreement with the interpretation of the isotherm data. The broad shape of the 265 nm absorption of the pure Ru compound in solution implies that the compound undergoes hydrolysis already without the zeolite present. This observation gave rise to a systematic study of the starting complex in solution as function of time, temperature and pH. At 333 K the compound is stable in acidic and neutral media for 24 hours. The compound hydrolyses to a small extent above pH of 2.5 but the product which is not identifiable from
221
0.4
"~ Ru red
0.3
e0.2
i/
"
Ru/NaY
/ :
.Q
o
Ru(NH3)6CI3
/
.:,'"
(/)
<
9 \
-
..
0.1
"... ":%..
|
m
,
|
,
i
i
,
300
,
,
,
=
,
,
,
m
1
,
400
,
,
,
,
,
,
,
,
i
,
500
,
,
i
,
,
,
,
,
i
,
600
,
,... ...................
,
,
,
,
,
,
,
I
,
700
,
,
,
,
,
,
,
,
I
800
Wavelength / nm
Figure 2. UV/VIS spectra in aqueous solution of [Ru(NH3)6]C13, Ru red and of the reaction solution of Ru/NaY
literature spectra remains at a steady concentration of below 10% of the starting solution assuming comparable molar extinctions. If the starting complex is intercalated into NaY at pH 3 or below, a pale yellow solid is obtained which converts, however, after drying either in air or under highly anaerobic conditions (Ar, less than 1 ppm oxygenates) within a period of days into the usual purple compounds. The oligomerisation reaction occurs thus as consequence of the drying process which seems to remove additional ligands from the primary intercalation product. The time and temperature evolution of the stability of the Ru complex at pH 6 was further investigated by in-situ UV/VIS spectroscopy yielding a rational limit for the reaction conditions if the presence of a mononuclear starting complex is required for the desired zeolite product. At 300 K the solution is stable within the definition given above. At 323 K significant reaction sets in at about 8 h in solution, at 333 K this is the case after 1.5 hours. These data led us to 333K and 1 hour reaction time. At longer times and higher temperatures the Ru complex hydrolyzes to an unidentified intermediate in equilibrium with Ru red as final product under these conditions. The reaction sequence is independent from the concentration of the solution within our limits of observation (upper limit about 0.05 mol/1) Powder X-ray diffraction gave the following valuable diagnostic information for the successful preparation of a stable catalytic precursor material. Stable final catalysts proved to exhibit a characteristic change in the intensity distribution of the diffraction pattern. Typical data are displayed in fig.3. The exchange of Na by K leads to no significant changes in the intensity profile. We note a systematic loss in crystallinity of a fraction of the material. The Ru exchanged samples show a reproducible and characteristic modifi-
222 cation of the intensity distribution. Most sensitive are the reflections (220), (311), (333), (440) which give an interchange in their relative intensities easily seen in fig.3. All other intensities are also changed in a way not to be accounted for by the changed X-ray absorption coefficient of the sample caused by the presence of the heavy scatterer Ru. The data imply a defined location of the Ru inside the zeolite such that all structure factors of the framework are affected.
NaY
KY
5
ffl e-
Ru/NaY i
0
e-
-
Ru/NY
.
9
.
10
20
30
40
20
Figure 3. XRD transmission patterns of zeolite NaY, K-exchanged zeolite Y, Ru/NaY and Ru/KY
Other intensity distributions were not reproducible for identical preparations and always resulted in instable final catalysts. The observation indicate that under conditions of the presence of a defined starting complex intercalation of Ru into an ammonium exchanged zeolite Y occurs in one prefered site. As only these single-site intercalated samples yield stable catalysts it is suggested that this site is the supercage of the zeolite out of which the resulting Ru cluster cannot migrate during controlled activation. The oligomerisation reaction of a mixed Ru ammine hydroxy complex inside the zeolite during either drying or thermal activation is the key to the formation of a Ru cluster at an exchange state where
223 less than all possible exchange sites carry intercalated Ru species: would they be present as mononuclear metal species then diffusion of mobile Ru species and agglomeration inside the zeolite during activation would have to occur which may be difficult to discriminate from diffusion of the Ru species out of the zeolite crystal and formation of a large metallic particle on the outside of the zeolitic support (see for discrimination of the two situations ref. [8,2,9]). 3.2. A c t i v a t i o n of t h e R u - e x c h a n g e d z e o l i t e Y p r e c u r s o r The controlled activation of the precursor into a metallic Ru cluster species inside the zeolite framework is of vital importance to maintain the stability of the activated catalyst. Two pathways leading to metallic Ru clusters within the zeolite matrix are further investigated in the following. The Ru-exchanged precursor may either be heated in an inert carrier gas or in vacuum leading to autoreduction of the Ru ions by the ammine ligands, or the precursor is reduced by heating in a N2/H2 - 1/3 synthesis gas mixture.
2000
Ru/NaY
I
TPD
NH3 E
1500
to .m t.-
H2
1000
r O
500
I
300
I
400
I
i
i
I
i
I
500
i
I
600
i
700
Temperature / K
Figure 4. TPD profile of Ru/NaY obtained by heating in Ar with 10 K/min.
Temperature-programmed heating in Ar as shown in fig.4 allows to monitor dehydration, deammination, autoreduction and catalytic ammonia decomposition. The formation of N2 starting at 330 K unambiguously monitors the autoreduction of the Ru complex which passes through a maximum at 590 K. At about 600 K a drop in the concentration of NH3 and a simultaneous increase in the concentration of H2 occurs pointing to the presence of Ru metal particles since Ru metal is known to catalyse NH3 decomposition. During the subsequent TPR experiment no consumption of H2 is observed within the experimental error proving essentially complete autoreduction to Ru metal [10].
224
TPR RuO2
5 tO .==.
Ru/NaY
o. E
:3 r
o 0 oJ "I-
j,
I
Ru(NH3)6CI3
Ru red =, i
300
,
4110
,
I
;
I
I
i
500
i
600
,
i
I
',
i
700
Temperature / K
Figure 5. TPR profile of RuO2, Ru/NaY, [Ru(NH3)6]C13 and Ru red obtained by heating in 20% H2 in Ar with 10 K/min.
Heating RuO2 in H2 gives rise to a single, rather symmetric TPR peak at about 400K. The TPR profile of the Ru/NaY precursor is significantly more complex consisting of roughly three peaks at 400 K, 440 K and 500 K with an onset of reduction at about 370 K. Contrary to the Ru/NaY precursor the TPR profiles of the reference compounds [Ru(NH3)~]C13 and Ru red exhibit a single peak at about 530 K and 510 K, respectively. The TPR results obtained with the reference compounds indicate that the Ru-O bond is obviously easier to reduce than the Ru-NH3 bond. Hence the complex shape of the Ru/NaY TPR profile ranging from 370 K to 540 K points to the presence of various hydrolyzed Ru ammine complex compounds within the zeolite matrix. In the search for optimum catalytic activity the two different activation procedures were applied to Ru/NaY and Ru/NaY(295K). The on-line monitored NH3 concentration in the reactor exit gas is displayed in fig.6 while cycling the temperature between 664 K, 724 K, and 784K. Trace A was obtained after heating Ru/NaY with 10 K/min in synthesis gas to 844 K. The activity was found to increase during the first three cycles, and already during the fourth cycle steady state NH3 production was achieved. At 784 K the NH3 concentration is limited by thermodynamic equilibrium. When applying the autoreduction procedure to Ru/NaY following the recipe given by Cisneros and Lunsford [8], the catalyst was much less active as shown in fig.6B demonstrating a peculiar transient behaviour as response to temperature changes. Trace C resulted after heating the Ru/NaY(295K) precursor prepared at 295 K to 844 K in synthesis gas. The catalytic activity was observed to increase over a period of 80 h finally reaching the same activity as Ru/NaY.
225
1250
A
1000 750
E
Q.
500 250
~
tO t._
0 500
r (D 0 tO 0
250
x
1000
03
-1-z
0
750 500 250 0 _
0
10
20
30
40
50
60
70
80
784 724 664
E
{D
90
T i m e on s t r e a m / h
Figure 6. NH3 exit concentration as a function of time cycling between 664 K, 724 K, and 784 K. A: Ru/NaY after heating with 10 K/min to 844 K in synthesis gas. B: Ru/NaY after stepwise heating in Ar (lh at 373 K, lh at 473 K, lh at 573 K) to 724 K followed by heating in H2 from 300 K to 724 K. C: Ru/NaY(295K) after heating with 10 K/min to 844 K in synthesis gas.
TEM investigations after NH3 synthesis [11] revealed that the autoreduction procedure resulted in average particles sizes of about 1 nm in agreement with Cisneros and Lunsfords [8] observations. The reduction in synthesis gas, however, produced Ru clusters in the range from 2 nm to 3 nm. These observations support the postulated structure sensitivity of NH3 synthesis on Ru yielding increasing catalytic activity with increasing particle size [12,8]. Obviously, the Ru/NaY precursor prepared at 333 K provided larger oligomers resulting in larger clusters during the initial reduction compared with Ru/NaY(295K) prepared at 295 K. The increasing catalytic activity with time as shown in fig.6 is therefore attributed to the growth of the Ru clusters within the zeolite matrix reaching the maximum size of 3 nm after reduction in synthesis gas.
226 4. Conclusions The intrinsic lability of [Ru(NH3)s]C13 against hydrolysis and oligomerisation is increased in the presence of a zeolite. The resulting free ammonium ions are used to cation-exchange the zeolite and allow insertion of the modified monomeric Ru complex. Oligomerisation at a single site within the zeolite forms a molecular precursor to a Ru cluster large enough that under controlled activation which removes all ligands and reduces the Ru to the formal oxidation state zero the Ru is retained in the supercages of the zeolite. Furthermore, the zeolite matrix offers the opportunity to prepare Ru clusters with well-defined particle size distributions yielding supporting evidence for the structure sensitivity of NH3 synthesis on Ru. The present study shows that it is useful to accompany all steps of a synthetic route to a catalyst by analytical data in order to find for each reaction step chemically motivated reaction parameters and to reduce the number of experiments in the multi-dimensinal parameter space characteristic of even such a simple reaction as the "cation exchange" process of a zeolite. REFERENCES
1. J. Wellenb/ischer, U. Sauerlandt, W. Mahdi, G. Ertl and R. SchlSgl, Surf. Interf. Anal. 18 (1992) 650 2. W. Mahdi, U. Sauerlandt, J. Wellenb/ischer, J. Sch/itze, M. Muhler, G. Ertl, and R. SchlSgl, Catal. Lett. 14 (1992) 339 3. J. Wellenbiischer, M. Muhler, W. Mahdi, U. Sauerlandt, J. Sch/itze, G. Ertl and R. SchlSgl, Catal. Lett. 25 (1994) 61 4. B. Fastrup and H.N. Nielsen, Catal. Lett. 14 (1992) 233 5. J.N. Armor, H.A. Scheidegg'er, H. Taube, J. Am. Chem. Soc. 90 (1968) 5928 6. J.J. Verdonk, R.A. Schoonheydt, and P.A. Jacobs, J. Phys. Chem. 85 (1981) 2393 7. J.J. Verdonk, R.A. Schoonheydt, and P.A. Jacobs, J. Phys. Chem. 87 (1983) 683 8. M.D. Cisneros and J.H. Lunsford, J. Catal. 141 (1993) 191 9. J. Wellenbiischer, F. Rosowski, U. Klengler, M. Muhler, G. Ertl, U. Guntow and R. SchlSgl, Proc. 10th Int. Zeolite Conf. (1994) 10. J.J. Verdonk, P.A. Jacobs, M. Genet, and G.J. Poncelet, J. Chem. Soc. Faraday Trans. 1, 76 (1980)403 11. B.Tesche, to be published 12. S.R. Tennison, in: Catalytic Ammonia Synthesis, ed. J.R. Jennings (Plenum Press, NY, 1.Edition 1991) p. 303
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
Preparation the structure
of nanometer
gold strongly
227
interacted
w i t h T i O 2 and
sensitivity in low-temperature oxidation of C O
S. Tsubota, D.A.H. Cunningham, Y. Bando* and M. Haruta Osaka National Research Institute, AIST, Midorigaoka 1, Ikeda, Osaka 563, Japan *National Institute for Research in Inorganic Materials, Namiki 1-1, Tsukuba 305, Japan
Abstract Gold can be deposited on TiO2 as hemispherical fine particles with diameters smaller than 4rim by deposition precipitation. Gold hydroxide precipitates with high and homogeneous dispersion on TiO2, most probably on specific surface sites, in the pH range 6 to 10. The calcination of TiO2 with Au(OH) 3 in reducing gas atmospheres, such as H 2 and CO led to smaller gold particles than in air. In the case of Au/I'iO2 samples prepared by deposition precipitation, the catalytic activity for low-temperature CO oxidation was very high and could be observed even at temperatures below 0~ Physically mixed AufI'iO2 samples though of much lower catalytic activity, showed gradual improvement with increasing calcination temperature. An increase in calcination temperature not only caused particle coagulation but also brought about a stronger interaction with the TiO2 support. The above results therefore indicate that the catalytic activity of AufFiO2 is sensitive to the structure of the perimeter interface between Au and TiO2. 1. I N T R O D U C T I O N Gold has long been believed to be inactive as a catalyst. However, our recent w o r k [ I , 2 ] and the work followed by other g r o u p s [ 3 , 4 , 5 ] have clearly s h o w n that Au exhibits extraordinarily high catalytic activity for the low temperature CO oxidation when it is deposited on a selected group of metal oxides as small particles. We have reported that Au can be deposited on TiO 2 with particle sizes controlled in the range of 3 to 20nm by depositionprecipitation[6] as well as coprecipitation[2]. The p u r p o s e of the present paper is to clarify the role of structure of the interface between Au and TiO 2 for the genesis of high catalytic activity. To do this, we have investigated the effect of preparation conditions for depositionprecipitation on the size of gold particles. A simple physical mixture of Au colloid and TiO 2 powder was also prepared for comparison.
228 2. EX P E R I M E N TAL 2. l . P r e p a r a t i o n o f A u / T i O 2 C a t a l y s t s As a s u p p o r t was used powder TiO 2 JRC-TIO4(the reference catalyst provided by the Catalyst Society of Japan, Degussa P-25; primarily anatase with a surface area of approximately 50m2/g). For H A u C 1 4 ' 4 H 2 0 , reagent grade (Kishida Chemicals Co. Ltd.) with a purity of 9 9 . 5 % was used. The impurity levels of other noble metals in HAuC14 were approximately 2,7 and l l p p m for Pt, Ir and Pd, respectively, as determined by atomic emission spectroscopy. Ultra fine particles of Au with a mean diameter of 5nm were supplied by Vacuum Metallurgical Co.Ltd., Japan and were prepared by a vacuum evaporation and suspended in a-Terpineol at a Au concentration of 10wt%. All other chemicals used were the reagent grade and were used w i t h o u t further purification. The deposition-precipitation of gold onto TiO 2 was carried out as follows. Chloroauric acid with a Au loading of 13wt% was d i s s o l v e d in distilled water. After the pH of the aqueous solution was adjusted to a fixed point by adding 1M NaOH, TiO 2 powder was suspended and aged at 70~ for lh. The s u s p e n s i o n was washed with distilled water several times, dried under vacuum, and then calcined at different temperatures and atmospheres. The most frequently used standard calcination condition was 400~ in air for a period of 4h. In physical mixing preparation, Au colloid solution with an amount c o r r e s p o n d i n g to a Au loading of 3wt% was diluted by i s o p r o p y l ether and then TiO 2 p o w d e r as added to the organic suspension. After stirring for 1 h, the organic solvent was vacuum evaporated at 100~ for 4h, and the obtained AuTiO 2 mixture calcined in air at a variety of temperatures for 4h.
2.2.Characterization
o f prepared A u / T i O 2 c a t a l y s t s
The amount of Au deposited on TiO 2 was determined by X-ray fluorescence, using a Rigaku 3370 analyzer. The particle size of Au was determined based on the observation of more than 200 particles (TEM Hitachi H-9000 p h o t o g r a p h s ) . The fine atomic scale structure of the catalysts were observed by means of high-resolution TEM (JEOL J E M - 2 0 0 0 E X ) . XANES measurements were performed in the National Laboratory for High Energy Physics (KEK, Tsukuba). UV-VIS spectra of Au aqueous solution were measured by using Shimadzu UV-3100PC. IR measurements were carried out on a Nicolet 2 0 S X C to determine the presence or absence of organic solvents on catalysts. Catalytic activity measurements used a fixed bed reactor, with a standard gas containing 1 vol% CO in air passed through the catalysts bed at SV = 2 0 , 0 0 0 h- 1 ml/g-cat.
229 3.RESULTS AND DISCUSSION 3. l. Preparation of A u / T i O 2 by D e p o s i t i o n - P r e c i p i t a t i o n 3 . 1 . 1 E f f e c t of pH of starting s o l u t i o n ; Figures 1 and 2 s h o w the mean particle diameter of Au and the amount of Au deposited as a function of the pH of the starting solution. In general acidic solutions below pH 6 resulted in Au particles larger than 10nm in diameter. Alkaline solutions led to diameters smaller than 5nm. The amount of Au deposited also appreciably depends on the pH of the solution and reaches a m a x i m u m at pH 6. It corresponds to 60% of the total amount of Au contained in the solution. Since the Au particles were hemispherical as observed in TEM p h o t o g r a p h ( F i g . 3), it is possible to calculate the numbers of Au particles per unit gram of catalyst from the equation (1). N = A/
(Q
x
V)
( 1 )
where, N is the number of Au particles, A is the wt% of Au, Q the density, V the volume of a hemispherical particle, which equals 1/2x4/3~(DAu/2)3, and DAu is the diameter of the Au particle. Figure 4 shows that the number of Au particles is almost constant within the pH range 6 to 8. This results suggests that gold is deposited on specific sites of TiO 2 surface. In Fig. 5, HRTEM photograph of the precursor obtained at pH 7 before calcination show uniformly dispersed Au deposits with a diameter of about 2rim. In contrast, at pH 4 Au deposits are mostly h e t e r o g e n e o u s l y dispersed large particles having diameters greater than 10nm. X A N E S measurements of these precursors reveals that the Au is not metallic but bound to oxygen, most probably as a Au hydroxide. This is supported by UV-VIS spectroscopic analysis of the starting solutions which indicates that AuCl 4anion is transformed into different species (ie Au(OH)3C1- ) between pH 6 and 1017,8]. The above results suggest that Au hydroxide is deposited from Au(OH)3C1- on a specific surface sites of TiO 2, which may act as a nucleation site for Au(OH)3. The increasing amount of Au deposition with increase pH until 6 can be accounted for by a decrease in the ion exchange capacity and/or a decrease in ion interaction between TiO 2 and the Au species due to the decrease in the positive charge. The decreasing trend with increase pH above 6 is explained by the increase in solubility of Au(OH) 3 with pH.
3 . 1 . 2 . E f f e c t of c a l c i n a t i o n condition; In Table 1, the mean diameters and standard deviations for samples prepared under a variety of calcination conditions are summarized. The particle size of Au g r a d u a l l y increases with increasing temperatures for calcination in air.
230 E 20
~
C
10
O
~0
15
L.
C ,m qO
O.
O
=10 0 ~-
E N
5 -
::3
O 5
~-
t-~--O-
O
O
E
0 2
I
I
I
I
4
6
8
10
,~
I
I
I
O I
4
6
8
10
0
12
2
pH of Au solution
Figure 1 The mean particle diameters of Au as a function of pH of HAuCI 4 solution.Calcination conditions" 400~ in air for 4h.
12
pH of Au solution
Figure 2 Amount of Au loading as a function of pH of HAuCI 4 solution. Calcination conditions: 400~ in air for 4h.
--
30
o
,...
25-
~;
20-
X
(,,1 ...,. '-
15-
!
r
=
10-
<:
5
I i i i -
i i
0
~
9
v.L
0 2
4
~l
6
I
I
8
10
12
pH of Au solution
Figure 3 TEM photograph of Au/I'iO 2 prepared by deposition precipitation at pH7 after calcination at 400~C in air for 4h.
Figure 4 Number of deposited particles of Au as a function of pH of HAuC14 solution.
231
10 nm
Figure 5 a)
b)
TEM photograph of precursor of AuffiO2 prepared at pH7 by deposition precipitation before calcination. TEM photographs of precursor AudiO 2 prepared at pH4 by deposition precipitation before calcination.
232 Table 1 Mean diameters and standard deviation determined from TEM observation of samples prepared by deposition precipitation calcined under variety of conditions. Calcination conditions
Diameter of Au particles
standard deviation ,.,
,
Temperature/~
Medium
DAu /nm
200
air
2.95
•
300
air
3.08
-----0.75
400
air
3.58
•
500
air
4.33
+---0.96
600
air
6.68
-I- 1.35
250
air *
3.27
-----0.59
250
N2 *
2.72
-----0.63
250
10%CO/He *
2.64
-----0.54
250
10%H2/He *
2.13
•
1
o/nm
Prepared at pH 7; calcined for 4h.; Au loading = 1.4 atom%. *:flowing gases rate at 20ml/m. The effect of atmosphere for calcination on the mean particle diameters was appreciably large, proving that reducing gases result in smaller Au particles. Since hydrogen can reduce Au hydroxide more rapidly than CO, it might be assumed that faster reduction leads to a stronger interaction with TiO 2 preventing Au particles from coagulation.
3.2 Physical mixing of colloidal gold particles with TiO 2 powder Figure 6 shows TEM photographs of samples prepared by the mixing of colloidal Au particles with TiO 2 powder calcined in air at 200~ and 600~ Figure 7 shows particles size distribution of Au for different calcination temperatures. Colloidal Au particles keep their original particle size and are well dispersed over the TiO 2 support when calcined at 200~ At higher calcination temperatures, Au particles grow with increasing calcination temperature. The tendency of increasing particle size with temperature is greater than that of deposition precipitation. This result s h o w s , as expected, that samples prepared by deposition precipitation have a stronger interaction between the Au particles and the TiO 2 supports and are thus more resistive agains t coagulation.
3.3 Catalytic activity for l o w - t e m p e r a t u r e
CO o x i d a t i o n Figure 8 shows the catalytic activities of Au/TiO 2 prepared in this study. Catalyst(DAu=3.5nm) prepared by deposition precipitation exhibits high
233
~-.~ ~.
.......
~.'~i
. .,.J~.:.~. ....
~.~,:,~~
.....
.......... ....
~ii~ii/i~..i~..... .....
r ~.. . . . . . . .
t
'i
!r
--
Figure 6
lOnm
TF_aMphotograph of AuffiO 2 prepared by Au colloid mixing with TiO2 powder, a) calcination in air at 200~ b) calcination in air at 600~
234
co o
~
'~176
,
a)
80
40
8
ao
2oo'c
,t
DAu-5.08:I:1.26
60
~
[]
0 50
e,
20
40
~
~6
t~
L -,oo l t .
co
20
co
8
0
10
8
4
0
.
0 I
09
,
I
,
I
I
,
I
,
I
5
10
15
20
0
5
10
15
20
D i a m e t e r of Au p a r t i c l e s ,
Figure 7
nm
Diameter
of Au p a r t i c l e s ,
nm
Particle size distribution of Au deposited on TiO 2 prepared by Au colloid mixing with TiO2 powder after calcination in air at, a) 200~, b) 400~, c) 600~C.
100
r
C-'-
0 o 0
t-0
50
. ..,,,
h.,
> C 0
o
0 -100 -50
0
I . . ~ l t 5 0 100 150200
Temperature,
Figure 8
250
~
Catalytic activity of AuffiO 2 for CO oxidation as a function of catalyst temperature. CO 9 lvol% in air, SV=20,000 ml h-l/g-cat. ( 9 ): by deposition precipitation; 3.3wt%, DAu=3.5nm ( &): by Au colloid mixing with TiO2; 3wt%, calcined at 200~ ( A ): by Au colloid mixing with TiO2; 3wt%, calcined at 400~ ( O ): by Au colloid mixing with TiO2; 3wt%, calcined at 600~
DAu= 5.1nm DAu= 7.5nm DAu= 12nm
235 catalytic activity for CO oxidation even at temperatures below 0~ On the other hand, catalyst (DAu-5nm)prepared by mixing of colloidal Au with T i 0 2 powder calcined at 200~ exhibits catalytic activity only at temperature above 200~ This is partly because organic solvents still remaine as detected by IR spectroscopy and probably because the contact of Au with TiO 2 is weak. The catalytic activity increased with increasing calcination temperatures from 200 to 600~ although the agglomeration of Au particles noticeably took place. Figure 6 shows that not only the size of Au particles but also shape of particles are changed. The sample calcined at 200~ has spherical shape Au particles. The sample calcined at 600~ is transformed close to hemispherical shape, indicating that a strong contact between Au particles and TiO 2 is generated by calcination at 600~ We have proposed a reaction mechanism for the oxidation of CO on supported Au catalysts, in which the interfacial perimeter between Au particles and metal oxide support play the main role for the genesis of catalytic activity[i]. The above experimental results s u p p o r t our proposed model and strongly indicate that a strong contact between Au and TiO 2 is indispensable for the synergy between Au and TiO 2 for the lowtemperature oxidation of CO. 4. C O N C L U S I O N 1. Nanometer size of Au particles can be deposited on TiO 2 with a relatively narrow size distribution by deposition-precipitation. A reducing gas atmosphere is preferable for smaller gold particles. 2. In deposition precipitation at p H ' s above 6, the AuC14- anion is transformed into Au(OH)3CI- which deposits as a Au hydroxide precipitate at specific surface sites on the TiO 2 support. 3. A strong contact between Au particles and TiO 2 is indispensable for the synergetic enhancement of catalytic activity. REFEREN
C ES
1. M. Haruta, S. Tsubota, T. Kobayashi, H. Kageyama, M. Genet, and .B. Delmon, J. Catal. 144, 175(1993). 2. M. Haruta, N. Yamada, T. Kobayashi, and S. Iijima, J. Catal. 115, 301(1989). 3. S. D. Gardner, G. B. Hoflund, B. T. Upchurch, D.R. Schryer, E.J. Kielin, and J. Schryer, J. Catal. 129(191). 4. S. D. Lin, M. Bollinger, and M. A. Vannice, Catal. Lett. 17__,245(1993). 5. A. Baiker, M. Kilo, M. Maciejewski, S. Menzi, and A. Wokaun, in "Proceedings, 10th International Congress on Catalysis, Budapest, 1992",L. Guczi, et al.eds., p.1257, Elsevier, Amsterdam, 1993. 6. S. Tsubota, M. Haruta, T. Kobayashi, A. Ueda, and Y. Nakahara,.Stud. Surf. Sci. Catal. 6_33,G. Poncelet, et al. eds., p.695, Elsevier, Amsterdam, 1991. 7. J.A.Peck, C. D. Tait, B. I. Swanson, and G. E. Brown Jr., Geochimica et Cosmochimica Acta 5__55,671(1991). 8. C.E Baes,Jr. and R.E. Mesmer, eds., The Hydrolysis of Cations, p.279, Robert E. Krieger, Malabar, 1986.
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
237
Proton Affinity Distributions: A Scientific Basis for the Design and Construction of Supported Metal Catalysts Cr. Contescu 1, J. Jagiello 2 and J.A. Schwarz 3 Department of Chemical Engineering and Materials Science, Syracuse University, 320 Hinds Hall, Syracuse, New York, 13244-1190, USA SUMMARY We present a method for characterization of the proton affinity distribution (PAD) for binding sites at the oxide/aqueous solution interface. When applied to alumina, a commonly used catalyst support, the results revealed the heterogeneous distribution of acidic/basic surface hydroxyls and possible correlations with their structure. The pH-dependent charging behavior of alumina and the resulting adsorption properties for anions or cations are discussed in relation to the measured proton affinity distribution. Finally, we propose that previous reports on anomolous ion adsorption can be explained on the basis of the existing heterogeneity of proton binding sites at the oxide/solution surface. 1.
INTRODUCTION
An accurate description of the processes that result in the binding of ionic ligands to substrates used for catalyst supports is a requisite step to develop a basis for catalyst preparation. Few will dispute that strong binding of catalytic precursor ions to catalyst supports will result in a resistance to sintering and a higher dispersion of the active phase. If the view of electrostatic attraction between the catalytic precursor ion and the charge present on the surface is accepted, then the chemically induced/spatially defined architecture present on the support becomes the "traffic cop" which directs the potential fate of the structure of the finished catalyst. Our approach to this problem is to consider an interface comprised of an arbitrary heterogeneous distribution of binding sites and use experimental data directly to reveal any extent of heterogeneity. We apply this to proton transfer processes at the solid/aqueous solution interface of metal (hydr)oxides that are commonly used as catalyst supports. The results of our analysis yield the charging behavior of the substrate as a function of the proton concentration in the bulk solution. We use protons as probe
1 Institute of Physical Chemistry, Romanian Academy, Spl. Independentei 202, Bucharest, Romania 2 Institute of Energochemistry of Coal and Physicochemistry of Sorbents, University of Mining and Metallurgy, 30-059 Krakow, Poland 3 To whom correspondence should be addressed
238 ligands for several reasons. The classical concept of acidity is based on their concentrations, and thus our results can be in terms of an established index. They interact with hydroxyl groups present on oxide surfaces, which are receptor sites for catalytic precursors. Thus, the net consumption of protons is a measure of the charging template that would be created on the oxide during catalyst preparation. In addition, their concentration can be easily measured. Our objective is to provide a framework for the analysis of the heterogeneity of proton binding sites at the oxide/solution interface. To test the merit of our results, we turn to the classical catalytic literature for supporting evidence. In the case of binding of small ions (Na+, F-) on a conventional support, y-AI20 3, we should be able to account for the changes that have been measured in the IR bands of surface hydroxyls which serve as receptor sites for these species. The binding of more complex ions, such as PdC14 2" and Pd(NH3)42+, onto y-Al203 surfaces has been studied by a number of techniques due to the importance of the finished catalyst in such reactions as hydrogenation/dehydrogenation and in methane oxidation. Here we might expect the adsorption might be conditioned by geometrical constraints in that only preferred arrangements of charge centers at the interface can serve as the optimal receptors of these binding species. Composite interfaces formed by the mounting of a second-phase oxide onto a carrier such a s WO3/m120 3 are important catalytically due to their enhanced acidity and robust performance under demanding conditions. The structures of materials that fall into this class of catalytic supports have been studied by a variety of techniques such as EXAFS, XANES, and Raman spectroscopy. What had been considered a structurally stable support has been found to be, indeed, very dynamic, responding to the conditions of its surrounding environment. A further test of our assertion that proton binding isotherms are sensitive to the heterogeneity existing at oxide surfaces would be a comparison of our conclusions regarding these composite oxides with those in the recent literature. Finally, the "gray areas" that appear frequently in the catalytic literature, where phenomena occur that cannot be directly rationalized on the basis of accepted thinking, provide, perhaps, the'most challenging test to our proposals. Cationic and/or anionic binding under conditions when the apparent net surface charge would predict that binding should not occur is one example. If such results can be explained as a natural consequence of surface heterogeneity as probed by protons, then the methods described herein could provide a substantial step toward the development of a scientific basis for catalyst preparation. 2.
BACKGROUND
Potentiometric titration has been used to assess the acid/base properties of a variety of solid surfaces ranging from carbon blacks to metal (hydr)oxides. It is the experimental vehicle we use to determine proton affinity, but its application does present some limitations. Our determination of the charging behavior of the substrate is limited by a practical consideration: the concentration of protons must fall in the range equivalent to 3 < pH < 11 or otherwise the so-called buffering effect of water will introduce significant error in the measurements [1]. Cognizant of this restriction, and without any loss of generality, we must be able to establish some reasonable relationship between our experimental results and the
239 inventory of receptor sites that are within the pH "window" of measurement. Any test of our proposal is substrate-dependent, and here we rely on established literature to ascertain consistency. For example, in the case of v-A120 3, Kn6zinger and Ratnasamy (KR) rationalized with the aid of idealized models the configuration o f - O H groups on the dry surface. The potential receptor sites for protons are five different configurations [2]. Their occurrence depends on the relative contribution of the most densely packed crystal planes on the surface of alumina particles. Triply coordinated -OH groups are the most acidic sites (type III in KR model), terminal -OH groups are the most basic (type I sites), bridging hydroxyls (type II) are intermediate in character. The sites belonging to types I and II are further subdivided according to the type of surrounding AI3+ ions (octahedral or tetrahedral). Assuming that the rehydration of calcined samples does not disturb the ionic structure of the surface, it follows that at the alumina/solution interface there develops the same five types of-OH groups found on a partially dehydroxylated surface. It is also a logical extension to assume that the hydroxyls at the alumina/solution interface retain the same trend of acid/base properties as the original hydroxyls on the "dry" surface. Calculations based on the electrostatic valence rule predict the following order of increase in basicity of surface hydroxyls at the alumina/solution interface: (III) < (II-a) < (II-b) < (I-a) < (I-b) [2]. This is also the order of-OH stretching frequencies in IR spectra of isolated hydroxyls [2,3]; the same order was also found by quantum-chemical calculations [4,5]. Our analysis proceeds by first considering discrete receptor sites [6]. For a single population of binding sites, the degree of association or the "binding curve", with the meaning of an adsorption isotherm, can be expressed as
/~.[H]
Oi =
(1)
1 +/~.[H]
where 0 i represents the fraction of the total population which has bound the offered species (protons), [H] represents the molar concentration of unbound species at equilibrium, and ~ is the affinity constant which relates the equilibrium between the bound and unbound species: Si
+ H
..
H-S~
(2a)
For a heterogeneous population of sites with a discrete affinity distribution, the overall degree of protonation, o, is the weighted sum of the degrees of protonation of the different categories of sites; for a continuous affinity spectrum, the summation is replaced by an integral Kmax
0 = [
0(K,[H]) f(logK) d (log K)
(2b)
Kmin
where 0(K,[H]) is the local adsorption isotherm corresponding to eq. 1 and f(log K) is the normalized distribution function of intrinsic proton affinity constants, K, which characterizes the population of proton acceptor sites. Our approach to calculate the distribution function, f(log K), is based on the local
240 solution of the adsorption integral equation. The advantages of local solutions were recognized for a long time [7] due to their applicability to data measured over a limited experimental window of concentrations. We apply the approximate method proposed by Rudzinski and Jagiello (RJ) [8] for the calculation of adsorption energy distributions from gas-solid adsorption isotherms. This method is a special case of the exact local solution derived by Jagiello et al. [9] for the case of a Langmuir local isotherm. The local solution is given by the following series: f(log K)=
1/"2
O~
O log[/-/]
030
3!1n2(10) 0 log[H] 3
~r4
050
5!1n4(10)0 log[H] 5
]
(3)
logInl--logK
To approximate experimental adsorption isotherms and to calculate appropriate derivatives, we apply a procedure of smoothing splines, described by Reinsch [10]. In this approach, the experimental data are approximated by a cubic spline function g(x), which minimizes the following functional 1 i=N XN ~ . i ~ 1 [g(xi)-yi]2 + Xl
xf[g"(x)]2ax=minimum,
(4)
where N is the number of experimental points, xi and Yi are their coordinates, and ~. is a Lagrangian parameter. The first term in this equation represents the average square difference between data points and the corresponding function values, and the integral is a measure of the "roughness" of the function, g(x). We find by numerical simulation that plots of the log of the "roughness" vs the first term in Equation (4) yields tell-tale shapes [6]. They start as convex for low values of the abscissa; they pass through an inflection point for a certain value; then they decline slowly and almost linearly for higher values of the abscissa. This shape reveals the mechanism of smoothing, namely the first part of the curve responds to the process of smoothing out fluctuations caused by experimental error while the second part, when the curves decline linearly, corresponds to over smoothing. Thus, the distinct kink in such plots indicates the transition between under and over smoothing. Practically speaking, we have found it best to smooth to a value that approaches the kink in these characteristic plots. In Fig. 1, we show the normalized proton affinity distribution (PAD) derived from standard acid/base titration of mellitic acid [11]. This compound has six carboxylic groups attached to a benzene ring, and thus we would expect to find six "end points" in the raw data. These were invisible in the raw data set; the proton binding isotherm was also structureless. The first two ionization constants of mellitic acid (with pK~ about 1.39 and 2.19) are below the pH range of our experiments and could not be titrated because of increased water buffering below pH 3. The four peaks shown with maxima at log K 3.25, 4.60, 5.65 and 6.45, correspond to the other four acidic groups, and their positions are in very good agreement (within 3%) with literature data [12]. These results show that the method presented is capable to readily distinguish peaks separated by less than 1 pK~ unit (pK~ = log K). Fig. 2 shows the titration curve for a y-A1203 at an ionic strength of 0.1 mol dm 3. Over the range of ionic strengths from 10-4 to 101 mol dm 3 there are small differences
241
3
Y v
,,.--
2
0
I
2
3
4
5
6
7
8
pKa Fig. 1 Distribution of acidity constants for mellitic acid
1.5
i
i
i
o
1.5
i
Q (mm01/g)
"T, ..-.-.------9 f (log K)
I
0
IE E
0.5
0.5
"Iv
-"~~176 ~~176. ~ i ~ ,
CL
0
v
-0.5
-0.5
2 pH
I
I
I
I
4
6
8
10
12 log K
Fig. 2 Proton affinity distribution for 7-A1203 superimposed onto proton binding curve from potentiometric titration. Notation in boxes assigns peaks on basis of local coordination of OH groups. in the titration curves for alumina; nevertheless, the deconvoluted isotherms, which yield the distributions of apparent proton affinities, were quite insensitive to ionic strength. In Fig. 2 the PAD, f(log K), is superposed onto the proton binding curve shown. It is seen that three or four categories of surface sites participate in proton reaction between pH 3 and 11. The distribution obtained at lower ionic strengths had the main peaks in the same positions as those shown in Fig. 2 for I = 0.1 mol dm-3; the peaks became sharper as the ionic strength decreased. This
242 finding suggests t h a t the effects of ionic s t r e n g t h are smaller t h a n the effects of local configurations of-O(H) and -OH(H) groups on the oxide surface, which d e t e r m i n e s the heterogeneity of proton-binding properties. Thus, it m a y not be necessary to consider the incorporation of electrostatics into modeling the charge development at the oxide/solution interface to a first approximation. The PAD obtained experimentally from potentiometric titration of a l u m i n a shows four p e a k s from pH 3 - 11. T h e y correspond to the following p r o t o n adsorption/desorption equilibria which take place specifically on four of the five types of surface hydroxyls predicted by the structural models: [(Aloh)3-O]-0"5 + Hs+ r [(Aloh)3-OH]+0'5 [(A1oh)-O-(A1oh)]-0"75 + Hs+ r [(A1oh)-OH-(A1oh)]+0"25 [(A1Td)-OH]-0"25 + Hs+ r [(AITd}-OH2]+0"75
log KIII< 2.5 log KII-a = 4.0 - 4.2
[(A1oh)-OH]-~ + H~+ r
log KI-b = 9.6 - 10.0
[(A1oh)-OH2]+~
log KI-a = 6.5 - 7.0
The bridging hydroxyls of the II-a type, w i t h a perfect c o m p e n s a t e d electrostatic charge, are inactive to proton t r a n s f e r reaction within the accessible pH window [13,14]: [(Aloh)2-OH]~ + Hs+ r [(Aloh)2-OH2]+1 [(Aloh)2-O]-1 + Hs+ r [(Aloh)2-OH]0 0
log KII-b(+) < - 2 log KII-b(-) > 12
PROTON AFFINITY DISTRIBWrION TO ASCERTAIN THE CHEMICAL AND S T R U C T U R A L A R C H I T E C T U R E OF CATALYTIC MATERIALS
A general method for preparing catalytic materials using aqueous procedures is to contact a substrate with an electrolyte containing ions which include either precursors of an active metal, a modifier, or a dopant. W h a t e v e r the case, the properties of the electrolyte, such as pH a n d ionic strength, are affected by these ingredients. In addition, the substrate itself can also "buffer" the electrolyte. All these factors can influence the proton affinity of the s u b s t r a t e compared to the case when the i n g r e d i e n t s are absent. S u b s e q u e n t processing might include drying, calcination, and/or reduction. The effects of these steps on the final architecture of the catalytic material will be conditioned by the t e m p l a t e established d u r i n g the initial contacting of the s u b s t r a t e w i t h the electrolyte. W i t h i n the context of our proposed assertion t h a t PADs can e s t a b l i s h a frame-work for analysis of processes occurring during c a t a l y s t p r e p a r a t i o n , we present three examples wherein proton affinity in conjunction with the results from other analytical techniques offer a rational basis for extension to scientifically prepare other catalytic systems.
3.1
Binding of Small Ions
Electropositive and electronegative ions such as Na + and F- h a v e a wellestablished effect on the acidity o f c a t a l y s t supports. In the case of y-AI203, fluoride addition increases its acidity while sodium is expected to decrease the
243 acidity. To examine the effects of these modifiers on the PAD of our reference 7-
A1203 (see Fig. 2), samples containing various concentrations of the modifier were prepared by the incipient wetness procedure using NH4F and NaNO3 solutions [15]. The m a t h e m a t i c a l procedures for sm oot hi ng and deconvoluting the experimental proton binding isotherms to obtain PADs were applied to the potentiometric titration data, and the results are shown in Figs. 3a and b. Diffuse reflectance FTIR spectra of each sample were also measured after dehydration at 873 K and are shown in Figs. 4a and b. For the fluoride modified aluminas, the most significant changes in the PAD occur in the acidic range, where addition of up to 1% F considerably affects acidic hydroxyls (bridging type II-a sites in the KR nomenclature). The IR spectra show a drastic variation of the band at 3772 cm -1 due to basic (terminal, type I) hydroxyls, which first shifts to 3757 cm -1 and reduces its intensity (for 0.2% F) and then disappears (for 1% F). At pH 4.5 - 4.7, where our samples have been impregnated in NH4F solutions, the types I-a and I-b hydroxyls carry positive charge [6]. This favors the electrostatic adsorption of F - i o n s on these sites. During subsequent thermal t r e a t m e n t s adsorbed F- ions replace the underlying exposed hydroxyls. This causes the "leveling" of the peak at pH 6 - 6.5 in the PAD, assigned to type I-a hydroxyls, for 0.2 and 1% F. On the (111) A plane, the I-a and II-a hydroxyls are close neighbors. The inductive effect of fluoride could cause the enhancement of the II-a peak observed in the PADs of these samples. This would also be in agreement with the increase of Bronsted acidity reported for F/AI203. The PADs for sodium doped samples show as complex a picture as of the fluoride doped ones. The peak close to pH 4 assigned to bridged II-a groups is sharply enhanced by addition of only 0.1% Na; at sodium contents larger t h a n 0.5%, it gradually decreases and simultaneously moves to lower pH. The peak in the basic range (type I-b hydroxyls) is also affected, though by a lesser extent, when the a m o u n t of sodium is increased. The asymmetric shape of this peak becomes more pronounced at 0.25% Na and a new, more basic peak emerges for 1% Na. In the intermediate pH range, the effect of sodium is opposite to t h a t of fluoride for comparable concentrations. Here a new structure centered at pH 7 develops from the rather flat peak seen on pure alumina, especially for 0.5 and 1% Na. These modifications are related to the changes observed in the IR spectra. Introduction of only 0.1% Na causes the extinction of the most basic band at 3772 cm -1 and gives rise to a new, comparatively more acidic band (3755 cm -1) which grows in intensity and dominates the spectrum as the sodium content increases. The band of type II hydroxyls (3728 cm -1) has an opposite progression: its position shifts to higher wave numbers (3734 cm -1) and its intensity is gradually diminished by increasing amounts of sodium until its complete extinction for 1% Na. The other bands (3677 and 3597 cm -1) are affected less. It is, indeed, difficult to explain why a basic modifier (sodium) produces very clearly the acidification of the most basic hydroxyls in the resulting IR spectra. The corresponding change in the PAD consists in the enhancement of the sharp peak at pH 4.5 and its shift towards lower pH at higher doping levels. We propose t h a t during calcination Na § ions are incorporated into vacant cationic positions in the near surface layer of the A1203 lattice to form a type of surface aluminate [16]. While it may be argued that from size considerations (ionic radius of Na § is twice t h a t of A13+, about 0.98 vs. 0.45 A) it would appear t h a t Na § could not fit in the spinel lattice, the near surface region enjoys the freedom to accommodate distortions that would otherwise be unlikely in the bulk.
244 5.5
5.5
,,,,,
u
2.5
0
o', 2.5
!
v
0
___A,0
_2"-b
-0.5
-0.5 2
4
6 8 10g K
10 12
2
4
6 8 10g K
10 12
Fig. 3a Proton affinity distribution for F/AI203 samples: (a) alumina, (b) 0.2% F, (c) 1% F, (d) 2% F, (e) 4 % F. The curves were arbitrarily shifted for clarity. Fig. 3b Proton affinity distribution for Sa/Al203 samples: (a) alumina, (b) 0.1% Na, (c) 0.25% Na, (d) 0.5% Na, (e) 1% Na. The curves were arbitrarily shifted. tn .,m
r
.f: V
a
3800
3600
u (cm-I)
3400
3800
3600 3400 ( c m I)
Fig.4a IR spectra of surface hydroxyls for F/AI203 samples: (a) alumina, (b) 0.2% F, (c) 1% F, (d) 2% F, (e) 4 % F. Fig.4b IR spectra of surface hydroxyls for Na/A1203 samples: (a) alumina, (b) 0.1 % Na, (c) 0.25% Na, (d) 0.5% Na, (e) 1% Na.
245 Our results show t hat 0.5% Na is the highest concentration where the IR band at 3734 cm -1 is still observable, and the peak at pH 4 is still sharp in the PAD. Addition of sodium perturbs mostly the I-a and II-a groups, which are responsible for the peaks around pH 4.5 and 6.7, respectively, in the PAD. The conclusion is in agreement with the experimental observations in Fig. 3b which show the development of peaks in the PAD in the corresponding pH range. Fluoride and sodium are the most acidic and most basic in terms of any modifying ions. Fluoride is also distinguished by its property to create Bronsted acidic sites (< 2% F). This effect could be seen in the PAD of F/AI203 samples as an intensification of the peak around pH 4. However, a much sharper modification of the same peak was observed for Na]AI20 3 samples, which evidently contradicts the expectation t h a t sodium would decrease the acidity of alumina. It is known, however, t h a t poisoning of alumina by alkali results in transformation of strong acid sites to weak acid sites [17]. On the sodium doped samples, more surface sites are seen which dissociate protons in the pH range of 4 - 4.5 and 6 - 8 in aqueous solutions. These are weak acidic sites, if compared with the very strong acidic sites normally present on alumina under conditions of severe dehydration.
3.2
Binding of Large Ions Containing Palladium
The anchoring mechanism and the chemical nature of supported species on catalysts dried after impregnation step depend both on the support and the precursor compound. The chemical state of Pd(II) in aqueous media containing chloride and/or ammonium ions depends on the pH and on other solution variables. At pH values below 3 and above 8, PdC142- and Pd(NH3)42§ are the dominant stable species [18]. We studied the adsorption of these large ions on our reference T-A1203 (Fig. 2) at both low and high pH values [ 19,20]. The anionic and cationic precursors were prepared from palladium chloride. For the cationic precursor adsorption isotherms were obtained at pH values of 8 and 10. If binding is electrostatic then the manifold of charged sites available is different. We should, therefore, with supporting data using other analytical techniques, be able to elucidate details of the anchoring mechanism. Diffuse Reflectance (DR) spectra in the UV-visible range of the dried PdC142catalyst were recorded, and we found the DR spectrum of the dried sample is very similar to t h a t of the impregnation solution. From the n u m b e r and position of bands, it is inferred t h a t adsorbed species retain much of the s q u a r e - p l a n a r symmetry characteristic of Pd 2§ ion. The spectrum suggests predominant electrostatic adsorption of PdC142" , the main component of the impregnation solution at pH 2.2. However, there are few differences with respect to the spectrum of PdC142- species. An increase in intensity of d-d transitions is attributed to support interaction. Small deviations from the square-planar symmetry of the four C1 ligands around Pd 2§ would, in principle, enhance the d-d transitions without altering their positions. In addition to the electrostatically adsorbed PdC142- species, part of palladium is probably present as PdC12(O)2 species; we estimate this to be 30 - 40 % of the total [20]. In the case of the Pd(NH3)42§ catalysts, the adsorption isotherms were fit to Langmuir-type functions. Data measured at pH 8 could be fitted by a single pair of adsorption parameters. However, at pH 10 the quality of the fit could be improved by adding a second Langmuir-type isotherm. From these results, it appears that new sites become available for adsorption at pH 10; they are in a larger amount that those available at pH 8.
246 A comparison between the amount of palladium complex ions adsorbed and the number of oppositely charged sites shows, however, a serious discrepancy; the amount adsorbed represents ca. one tenth of the total number of available charged sites. The electrostatic condition alone determines that palladium cations are attracted by negatively charged alumina surface sites, but not all charged sites that develop on the surface by proton reactions need be effective in the adsorption of complex ions. An effectiveness factor is needed to express the probability that complex ions approaching oppositely charged sites will stick in a bound adsorption state. This factor might well be regarded as a geometrical constraint [ 19]. Consequently, we re-examined the charge development on o u r ~/-AI203 surface assuming there are present different crystallographic planes. Fig. 5 shows ideal structures of low-index planes comprising the surface of alumina. There are two types of layers parallel to the (111) and (110) planes, designated as A, B and C, D layers, respectively. A random distribution of occupied octahedral positions on the (100) face was assumed. Upon rehydration, the five types of hydroxyl groups which are exposed on the surface are distributed as follows: I-a and II-a groups on the (111) A plane, II-b and III groups on the (111) B plane, II-b and I-a on the (110) C plane and only I-b groups on (110) D and (100) planes.
pH 2.5
..........
II-b
pH 5
(IIIIII) pH 8
pH 11
(I 11 ) A
(I 11 ) B
(I 10) C
(I 10) 0
(100)
Fig. 5 Model of charge distribution as a function of pH on crystal planes representative of an alumina surface (black circles- positive charges; g r a y neutral sites; open circles - negative charges). Note the variation in charge and configuration of charged sites as fimction of pH on each plane.
247 The rest of Fig. 5 shows the configuration of surface groups and their charge on various surface planes at different pHs. Those values of pH were chosen which corresponded to minima in the PAD (Fig. 2). If the assumption of localized proton adsorption is valid, the picture shown in Fig. 5 indicates that development of surface charge as a function of pH has completely different characteristics on various surface planes. Based on Fig. 5 we will examine the geometric configuration of the most probable adsorption sites for palladium complexes. The geometry of [Pd(NH3)4] 2+ ions is square planar with Pd-N and N-N distances of 2.04 and 2.88 A. The best fit for this particular geometry on the alumina surface is on the (100) face, where the Ib type of sites are arranged in squares, with O-O distances of 2.80 A. Above pH 9.7, these sites are charged negatively, so that they offer the perfect configurations which satisfy both the condition of electrostatic attraction and that of geometrical fit. Sites of Ib type can be also found on the (110) D surface, where they exist as parallel rows with O-O distances of 2.80 A within the rows and 3.96 A between neighboring rows. Again, at pH > 9.7, the electrostatic and geometric conditions are both satisfactorily fulfilled for retention of the square-planar [Pd(NH3)4]2§ cations by a purely electrostatic attraction. On the rest of the alumina planes, though negative charge develops at lower pH values, the geometrical arrangement of charged sites does not match the geometry of adsorbing complexes. Therefore, at pHs lower than 9.7, adsorption may take place on these planes by a different mechanism, such as replacement of NH 3 ligands by basic surface hydroxy groups. This is in perfect agreement with the recent results by Knfzinger and coworkers [21] who showed by UV-VIS spectroscopy that palladium amine complexes are held unchanged on alumina (electrostatic adsorption) when the impregnation pH was higher than 9.5; a bathochromic shift in the electronic spectra for samples prepared at lower pH indicated ligand substitution during adsorption. The results in Fig. 5 can be analyzed in terms of anion binding. The square planar PdC142 ion is characterized by Pd-C1 and C1-C1 distances of 2.30 and 3.25 A. The best fit to this geometry that exists on the alumina surface at pH = 2.2 is on the (100) face, where the I-b sites are arranged in squares, with O-O distances of 2.80 and 3.96 A. Sites of I-b type are also found on the (ll0)-D surface. The PdC142" ions fit the underlying charged sites only on these two alumina planes. We propose that the electrostatically attached species evident in the DRS spectrum of the dried catalyst are located on the I-b sites of (100) and (110) alumina planes. On the other planes where the geometrical fit is imperfect, the attached complexes result from the surface reactions in the ligand sphere, and species such as PdC12(O)2 as identified in DRS are formed. 3.3
Binding of Protons to Composite Oxides
Recent studies demonstrated the occurrence of two-dimensional oxide overlayers in catalysts prepared by dispersing metal oxides like CrO 3, MoO 3, WO 3, Re20 7 or V20 5 over a primary oxide substrate which has a high specific surface area. The conclusions of structural characterization of these materials were that the nature of the surface metal oxide species is dependent on specific oxide support, surface coverage, extent of surface hydration and calcination temperature. Recently, Wachs' group reported on the transformation of several transition metal composite oxides exposed to ambient conditions [22,23]. The dehydrated support appears to undergo a type of hydrolysis whereby the second phase reverts to a structure reminiscent of its configuration if it were suspended
248 in an aqueous environment. These findings are extremely interesting and could have some very important implications related to supporting metals in that the second phase oxide would exist as large polyanionic clusters on the primary phase during aqueous phase impregnation. In light of our interest in the WO3/A1203 system, we examined a series of WO3/ml203catalysts prepared by the incipient wetness method [24]. Fig. 6 shows PADs for ammonium metatungstate (AMT) salt, alumina, bulk WO3, and two composite WOa/AI203 samples. The stable composites (no loss of WO3) have affinity spectra that quantitatively resemble that of AMT. Analysis of the curve for AMT solution shows the pH domains of maximum buffering power, which correspond to different solution equilibria leading to the hydrolytic polymerization of WO42" species on acidification. For alumina, the pH of maximum surface buffering capacitance corresponds with the log K values already assigned to specific surface hydroxyls. In the case of WO3/AI203composite oxides, the surface buffering capacitance in the neutral and acidic range follows closely the behavior of AMT solution rather than that of bulk WO 3. This shows that the pH-dependent speciation of surface tungsten in WO3/A1203 composites is similar to that resulting from chemical equilibria in AMT solutions and differs from that of bulk WO 3.
2
~
v _
O v NI,=,
m
-1
2
1
I
I
I
4
6
8
10
12
log K
Fig. 6 Distribution of apparent acidity constants for (a) A1203, (b) AMT, (c) 12% WO3/h1203, (d) 30% WO3/h1203, (e) crystalline WO 3 It follows from the previous discussion that the development of the negative charge on the surface of WO3/A1203composites in aqueous solutions depends on solution pH. From the point of view of catalyst preparation, this has major consequences on the adsorption of a second catalyst component such as cationic Ni or Co. Recent results from our laboratory [25] have shown, indeed, that during the
249 impregnation process, nickel formed stable association compounds with surface tungsten moieties. These structures were reproducible after continuous reduction/oxidation cycles and were evident after different pretreatment procedures such as drying, calcination or reduction/passivation. The tendency to form such nickel-tungsten surface structures was a strong function of both the impregnation pH and the WO 3 loading. The pH effect was seen not only on the amount of nickel loaded by equilibrium adsorption (which was nearly constant at pH 4 and 5 but increased two times at pH 6), but also on the amount which could be reduced under the most severe conditions. Separate XRD results indicated that more NiWO 4 has formed for catalysts prepared at pH 6 than at'pH 4, i.e., under conditions where surface aggregation was less expected. A cobalt-tungsten interaction species was also detected when cobalt was supported at pH 5 over a 12% WO3/m1203composite [26]. However, a portion of cobalt could not be reduced to the metallic state, although Co in CoWO4 was totally reducible to metallic Co. It was shown that the Co-W interaction species were also present on Co/WO 3 catalysts (impregnation pH 2) and on a commercial cobalt/tungsten oxide compound and that these species were the active catalytic centers in the conversion of H 2 and CO to methane. 0
APPARENT ANOMALIES IN ION ADSORPTION DURING CATALYST PREPARATION
The model introduced by Brunelle [27] has served as a practical guide to rationalize the interfacial chemistry during adsorption of catalytic precursors on oxides. The model assumes electrostatic attraction of adsorbing ions by a homogeneous oxide surface. Experimental evidence, indeed, has shown that adsorption of cations/anions takes place from solutions with pH higher/lower than a certain pH, called PZC, where the net charge of the surface is assumed to be zero. When experimental data have contradicted the concept of ion attachment at the interface by electrostatic forces only the effect of some "specific" or "chemical" interaction has been invoked. Below we revisit several earlier literature reports of adsorption of cations/anions that apparently contradict either the principle of electrostatic adsorption or the assumption of uniform adsorbing surfaces. We will show, however, that at least part of the apparent deviations from the electrostatic mechanism can be accounted for by the existence of distinct sites with characteristic proton affinity on the hydroxylated surface of commonly used supports, without reference to specific binding of any "mysterious" nature. We will limit our discussion to alumina as a prototype oxide, the most extensively used support for catalysts. Adsorption of various cations (such as Pb 2§ [28], Ni 2§ [29], Cu 2§ [30]) was reported on alumina from solutions with pH < pHpz c. Under these conditions, the surface would be positively charged if it consisted of homogeneous amphoteric sites. We plot in Figure 7 the pH dependence reported by Vordonis and Lycourghiotis [31] for adsorption of Co 2§ and Ni 2§ These data were explained by the contribution of deprotonated surface hydroxyls which could presumably exist on the alumina surface below pHpz c, but no evidence was shown for that. A comparison of their adsorption data with the speciation mechanism revealed by PAD shows that sites II-a acquire a negative charge at pH 4.5 where adsorption of cations is enhanced. Similarly, the increase in adsorption of Pd(NH3)42§ above pH 9.5 - 10 was accounted for by the contribution of negative charge developed by I-b sites above this pH [19]. A complex
250
0.8 0.7
- 4-.
0.8
- .-=- - N i ( 2 + ) - R e f . 31
t,-
cn cD
C o ( 2 + ) - R e f . 31
0.6
- .~--
~ (1.5
_
'~ 0.4 ~ 0.3 ~
0.2
~
0.1
Pd(NH3)4(2+)-
- .-,=- - P d ( N H 3 ) 4 ( 2 + )
Ref. 19 0.6
- Ref. 40
-
0.4
-
0.2
; ,; 9
#
-
.,,
2
r
I
I
I
4
6
8
pH Fig. 7 Proton affinity distribution for reference cations
~
-
.,-
"
0
j i
10
-0.2 12
log K
u
and binding curves for several
pH effect reported by other authors in the adsorption of Cu, Ni and Co ammine complexes on alumina [32,33], which eventually results in formation of new surface species [33], may involve as a first step the electrostatic attraction of these complex cations by specific site geometries on the alumina surface which become negatively charged at characteristic pH values [19]. Mulcahy and Hercules [34] presented evidence that tungstate and molybdate anions adsorb on two types of surface sites at pH 4.5; Spanos and Lycourghiotis [35,36] assumed that these are protonated and neutral hydroxyls below pHpz c. We collected in Figure 8 equilibrium loadings data as a function of pH reported by Wang and Hall [37] and by Vermaire and van Berge [38] for oxyanions derived from W, Mo, V and Cr, and data for PdC142 ions reported by Contescu and Vass [18]. The loading curves show distinct breaks. They were assigned to crystal planes with different local PZCs [34] or to a change from electrostatic adsorption to chemisorption at certain pH values [35]. However, Figure 8 shows that breaks in the loading curves coincide fairly well with maxima in the PAD and could be explained by step-wise contributions of electrostatic adsorption of anions on positively charged II-a sites (below pH 4 - 4.5), I-a sites (below pH 7 - 7.7) and I-b sites (below pH 9 - 9.5). This explanation is supported by Okamoto and Imanaka [39] who reported that molybdenum ions impregnated at pH 5.8 consume basic I-b groups. 5.
CONCLUSION
A definitive model for adsorption of catalyst precursors on oxides has yet to be established. However, a picture that emerges from the recent literature in this field increasingly emphasizes the importance of the intrinsic heterogeneity of adsorption sites
251
-- e--- =--- =--- =---*--- ,L--
D,,= I . ,
E
5-
t"
r
4B |
03 ._o r
W(VI) -Ref. 37 W ( V l ) - Ref. 3 8 M o ( V I ) - Ref. 3 7 V (V) - Ref. 3 7 P d C I 4 ( 2 - ) ( x 3 ) - Ref. 1 8 C r (VI) (x3) - Ref. 3 7
0.8
0.6
t
3-
O
/r'N,.
.~-~.~%...
D
t-
0.4
<:
0.2 t,-
o
U~
"O ,<
.L/
1-
" '::..
.... * 0 0
~- ~
~'e
I
I
I
I
I
2
4
6
8
10
pH
Fig. 8 Proton affinity distribution for reference u anions.
-0.2 12
k~:j K
and binding curves for several
(surface hydroxyls) on oxidic supports. Although the adsorbed species may undergo more profound chemical transformations during drying and calcination, the step of electrostatic attraction probably precedes any chemical process. Due to the existing heterogeneity of proton binding sites on oxides, we propose that at all pH values used during preparation of catalysts the oxide/solution interfaces exposes well-defined configurations of sites (surface hydroxyls) that are able to interact electrostatically with either positive or negative species from the impregnating solution. Changing the pH results in variations in both the number and the surface geometry of sites with positive and negative charges. ACKNOWLEDGEMENT The Basic Energy Sciences, Chemical Sciences Division of the U.S. Department of Energy supported this work under Grant No. DE-FG02-92EER14268. REFERENCES .
2. 3.
4. 5.
6.
W. Stumm, J.J. Morgan, Aquatic Chemistry, Wiley, New York, 1981. H. Knfzinger, R. Ratnasamy, Catal. Rev. Sci. Eng., 17 (1978) 31. J.B. Peri, J. Phys. Chem., 69 (1965) 220. H. Kawakami, S. Yoshida, J. Chem. Soc. Faraday Trans. 2, 81 (1985) 1117. G.M. Zhidomirov, V.B. Kazansky, Adv. Catal., 34 (1986) 131. Cr. Contescu, J. Jagiello, J.A. Schwarz, Langmuir, 9 (1993) 1754.
252
o
8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40.
G.F. Cerofolini, Chem. Phys., 33 (1978) 423. W. Rudzinski, J. Jagiello, Y. Grillet, J. Colloid Interface Sci., 87 (1982) 478. J. Jagiello, G. Ligner, E. Papirer, J. Colloid Interface Sci., 137 (1990) 128. C.H. Reinsch, Numer. Math., 10 (1967) 177. T.J. Bandosz, J. Jagiello, C. Contescu, J.A. Schwarz, Carbon 31 (1993) 1193. G. Kortum, W. Vogel, K. Andrussow, in Dissociation Constants of Organic Acids in Aqueous Solution, Butterworth, London, 1961. T. Hiemstra, W.H. van Riemsdijk, G.H. Bolt, J. Colloid Interface Sci., 133 (1989) 91. T. Hiemstra, J.C.M. DeWit, W.H. van Riemsdijk, J. Colloid Interface Sci., 133 (1989) 105. Cr. Contescu, A. Contescu, C. Schramm, R. Sato, J.A. Schwarz, J. Colloid Interface Sci., 164 (1994) in press. Y. Okamoto, M. Oh-Hara, A. Maezawa, T. Imanaka, S. Teranishi, J. Phys. Chem., 90 (1986) 2396. J.M. Parera, N.S. Figoli, J. Catal., 14 (1969) 303. Cr. Contescu, M.I. Vass, Appl. Catal., 33 (1987) 259. Cr. Contescu, J. Hu, J.A. Schwarz, J. Chem. Soc. Faraday Trans., 89 (1993) 4091. Cr. Contescu, D. Macovei, C. Craiu, J.A. Schwarz, Langmuir (1994) submitted. D. Spielbauer, H. Zelinger, H. Kn6zinger, Langmuir, 9 (1993) 460. G. Deo, I.E. Wachs, J. Phys. Chem., 95(1991) 5889. I.E. Wachs, K. Segawa, in Characterization of Catalytic Materials, I.E. Wachs, L.E. Fitzpatrick (eds.), Butterworth-Heinemann, Boston, 1992, p. 69. Cr. Contescu, J. Jagiello, J.A. Schwarz, J. Phys. Chem., 97 (1993) 10152. D.W. Southmayd, Cr. Contescu, J.A. Schwarz, J. Chem. Soc. Faraday Trans., 89 (1993) 2075. R. Zhang, J.A. Schwarz, A. Dayte, J.P. Baltrus, J. Catal., 35 (1992) 200. J.A. Brunelle, Pure Appl. Chem., 50 (1978) 1211. H. Hohl, W. Stumm, J. Colloid Interface Sci., 55 (1976) 281. P. Chu, E.E. Petersen, C.J. Radke, J. Catal., 117 (1989) 52. J.-M. Dumas, A. Kribii, J.C. Menezo, J. Barbier, Bull Soc. Chim. France, 1988, 937. L. Vordonis, N. Spanos, P.G. Koutsoukos, A. Lycourghiotis, Langmuir, 8 (1992) 1736. D.W. Fuerstenau, K. Osseo-Asare, J. Colloid Interface Sci., 118 (1987) 524. J.L. Paulhiac, O. Clause, J. Am. Chem. Soc., 115 (1993) 11602. F.M. Mulcahy, M.J. Fay, A. Proctor, M. Houalla, D.M. Hercules, J. Catal., 124 (1990) 231. N. Spanos, L. Vordonis, Ch. Kordulis, A. Lycourghiotis, J. Catal., 124 (1990) 301. N. Spanos, L. Vordonis, Ch. Kordulis, P.G. Koutsoukos, A. Lycourghiotis, J. Catal., 124 (1990) 315. L. Wang, W.K. Hall, J. Catal., 77 (1982) 232. D.C. Vermaire, P.C. van Berge, J. Catal., 116 (1989) 309. Y. Okamoto, T. Imanaka, J. Phys. Chem., 92 (1988) 7102. Ch. Sivaraj, Cr. Contescu, J.A. Schwarz, J. Catal., 132 (1991) 422.
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
253
y A l u m i n a s u p p o r t e d P d - M o m i x e d s y s t e m s : E f f e c t of M o d e p o s i t i o n p r o c e d u r e o n d i s p e r s i o n a n d c a t a l y t i c a c t i v i t y of P d F. Devisse a, J-F. Lambert a, M.Che a, j . p . Boitiaux
b,c,
B. Didillon
b
a Laboratoire de R~activit~ de Surface (URA 1106), Tour 54-55, 2~me ~tage Universit~ Pierre et Marie Curie, 4, Place Jussieu, 75252, Paris Cedex 05 France
Institut Franqais du P~trole, 1-4, Avenue du Bois-Pr~au, BP311 92506 Rueil-Malmaison Cedex- France
b
c C u r r e n t address: Procatalyse, 212-216, Ave. Paul Doumer, 92500, RueilMalmaison- France ABSTRACT Pd-Mo/yA1203 systems were prepared by sequential deposition onto alumina of molybdates, and of Pd II complexes. The mutual influences of both supported metals could be evidenced: Deposition of palladium caused a modification of the UV-visible spectra and TPR reduction peaks of surface molybdates that suggested a specific interaction of square planar Pd II complexes with polymeric octahedral surface molybdates (as opposed to monomeric tetrahedral forms). The presence of surface molybdates caused modifications of the average size of Pd 0 particles after reduction (as compared to Pd/A1203 monometallics). These modifications can be interpreted in a nucleation-growth model if monomeric tetrahedral molybdates selectively consume Pd nucleation sites, while polymeric octahedral forms create new nucleation sites. Catalytic activities for butyne hydrogenation evidence effects due to particle size modifications, but also other effects necessitating a finer study of Mo Pd intaeractions. The modification of alumina by controlled molybdates deposition allows significant improvements in hydrogenation activities. -
-
1. I N T R O D U C T I O N Supported bimetallic systems containing molybdenum and a modifier such as Co, Ni... are f r e q u e n t l y e n c o u n t e r e d in the l i t e r a t u r e , especially as desulfurisation catalysts. On the other hand, little attention has been devoted to Pd-Mo/oxide systems so far, despite their use for the CO + NO reaction by Halasz et al. [1] and several mentions as selective hydrogenation catalysts in the patent literature.
254 Our initial interest in these systems arose from studies of Ni/SiO2 prepared by a two-step method: initial deposition of Ni 2+ nuclei in strong interaction with the support by selective ion exchange, followedby deposition of a "reservoir" of Ni species in weak interaction with the support t h r o u g h incipient w e t n e s s impregnation. It was shown that the Ni 2+ nuclei could act as anchoring sites for the more mobile species from the reservoir in a heterogeneous nucleation-growth process. It was then possible to modify the average size of reduced Ni particles by controlling the number of nuclei at constant total Ni loading [2]. We wanted to check if the role of nucleus and reservoir could be played by two different metals. The Pd-Mo/TA1203 system was chosen because of preliminary results indicating synergy effects in selective hydrogenation reactions known to be sensitive to metal particle size. The general scheme of preparation procedures was then: 1. to modify the surface structure of alumina by deposition of different Mo VI containing species (avoiding the formation of bulk Mo phases) - this gave Mo/?A1203 precursors. 2. to deposit a Pd II salt on the precursor, leading to Mo-Pd/?A1203 systems, reduce the palladium to Pd 0, and compare metal dispersion and catalytic activity with those of a Pd/TA1203 monometallic. It was thought that in this scheme, Mo species might play the role of germs for the nucleation of metal Pd particles from a palladium salt reservoir. 2. E X P E R I M E N T A L
2.1 Preparation of the catalysts The support was a T (t) A1203 from Rh6ne-Poulenc, with surface area 73 m2/g and porous volume 0.71 cm3/g. It was ground to grains of 150-500 ~m prior to use. The point of zero charge was measured to be approximately pH=8.2. Mo VI deposition was accomplished either by incipient wetness impregnation from a solution of (NH4)6 Mo7024 (procedure denoted M o I ) or by selective adsorption: the pH of a 0.1M (NH4)6 Mo7024 aqueous solution was adjusted either to 2 by addition of concentrated HNO3 (procedure denoted MoE2), or to 8 by addition of concentrated NH3 (MoEs). T A1203 was then added (6.5 g/l) and the suspension was left to equilibrate for 100 h under strirring. The solid phase was separated by centrifugation and washed several times with distilled water. All Mo/A1203 precursors were dried at 353K for 24 h; eventually, they were calcined under flowing 02 (24 l/h) up to 673K (temperature ramp: 10K/min). The use of dry 02 avoids possible reduction of MoVI by NH3, and formation of volatile Mo species in the presence of water. Mo loadings for procedures M o E were determined by atomic absorption spectroscopy (Service Central d'Analyse du CNRS - Vernaison). Pd II deposition was effected by incipient wetness impregnation of either the M o/A1203 precursors or the bare support with aqueous solutions of either [Pd(H20)4] 2+ nitrate (prepared by dilution of a commercial solution immediately before use to avoid Pd II hydrolysis - procedure denoted P d N ) or of [Pd(NH3)4] 2+ chloride (PdA, for Palladium tetraAmmine). Samples mentioned in this study
255 were not calcined after Pd deposition. Final reduction was effected under flowing H2 (3.6 l/h; 5K/min) or by TPR (vide infra). The full n a m e for a catalyst t h e n consists of the successive deposition procedures applied, together with the resulting metal loading(s) in weight% of the metal with respect to A1203: e.g., M o E 2 3%-PdA0.5%. F u r t h e r details are given if needed.
2.2 C h a r a c t e r i s a t i o n t e c h n i q u e s UV-visible diffuse reflectance spectra were recorded on a Beckman UV 5270 spectrometer in the 700-230 nm range. T e m p e r a t u r e Programmed Reductions (TPR) were followed by c a t h a r o m e t r y in a flow of 5%H2/Ar (1.5 lfn), with a t e m p e r a t u r e ramp of 7.5K/min and sample weights of 0.1 to 0.3 g. Transmission Electron Microscopy (TEM) micrographs were taken in a J E M 100 CXl electron microscope, with an optimal resolution of 6 /~. P a l l a d i u m particle size statistics were estimated on at least 200 to 300 particles for P d N samples; for P d A , visible particles were scarce and the statistics had to be limited to less t h a n 100 particles. Catalytic activity for butyne hydrogenation was measured in a Grignard type reactor, at 293K and under a constant H2 pressure of 106 Pa. The reactant was a 10% solution of butyne in n-heptane; the reaction was followed both t h r o u g h H2 c o n s u m p t i o n a n d t h r o u g h a n a l y s i s of t h e r e a c t i o n m i x t u r e by gas chromatography. The catalysts were reduced to 423K immediately before use.
3. R E S U L T S
AND DISCUSSION.
3.1 Mo/TA1203 p r e c u r s o r s 3.1.1 A b s e n c e of bulk Mo p h a s e s Selective adsorption experiments indicate t h a t the a m o u n t of Mo species retained by the support is strongly dependent on the pH of the aqueous solution: a plateau of 3.0% Mo is observed between pH=2.5 and 5, followed by quick decrease at higher pHs (1.0% at pH=8). The evolution of molybdate retention is correlated with, but not directly proportional to, the surface negative charge of the 7A1203 support. The value at the plateau corresponds to a surface density of 2.5 Mo/nm 2, i.e., close to the values given for a "monolayer" by Giordano et al. [3] and Okamoto and I m a n a k a [4]. At the same time, in incipient w e t n e s s i m p r e g n a t i o n procedures, bulk Mo phases (MOO3, A12(MoO4)3) could only be observed for loadings >3% Mo in our systems (as inferred from XRD, TEM, microanalysis, UV-vis and TPR). We are then led to believe t h a t Mo is present, in all samples with loadings <=3%, as isolated groups or subnanometric clusters. Only these submonolayer samples were selected as precursors for bimetallics.
256
3.1.2 M o n o m e r i c molybdates.
tetrahedral
vs. p o l y m e r i c
octahedral
surface
This p a r a g r a p h introduces a basic distinction between surface molybdates t h a t will prove helpful in the characterisation of our systems. In w 3.1.3, we will discuss how this distinction t r a n s l a t e s into observable results in UV-vis and TPR. Once we have established t h a t Mo is not present as a bulk phase, f u r t h e r characterisation would include determining the coordination n u m b e r and the connectivity (presence of Mo-O-Mo or Mo-O-A1 bonds) of surface m o l y b d a t e species. In our conditions, the predominent species in aqueous solution was the monomeric tetrahedral complex [MOO4]2" for procedure MoEs, and the h e p t a m e r [Mo7024] 6", with all Mo's in octahedral coordination, for procedures M o E 2 and MoI [5]. However, it is known that the surface basic OH groups of alumina are able to hydrolyse Mo oligomers in solution [6], and the Mo species deposited on the surface m a y then be different from those initially present in solution. This effect is most pronounced for low Mo loadings, while at higher Mo loadings the "basic effect" of the alumina surface seems to be exhausted. Thus, in good agreement with the literature [7-11], we found t h a t Mo loading was the main factor in determining the coordinence of surface deposited Mo's. On average, under 1% Mo, the monomeric tetrahedral form predominates while for the h i g h e s t submonolayer loadings, polymeric species of octahedral Mo are predominent. This distinction between monomeric t e t r a h e d r a l and polymeric octahedral Mo will prove important later. It is easy to visualise monomeric tetrahedral Mo. In first approximation, it may consist either in [MOO4]2" ion-exchanged over protonated basic OH groups, or in grafted species formally derived from the latter by the following equation: _
[ 1 o''o
H+ HH+ H
-o
-o
Ii
0
0
\\ Mo//
o"
+ 2 H20
(1)
21
On the other hand, the polymeric octahedral form may simply consist of ionexchanged heptamolybdate; however, there are some indications of the existence of another, grafted species, presenting both Mo-O-Mo and Mo-O-A1 bonds, which we m a y term "high t e m p e r a t u r e polymeric form" and for which no satisfactory model exists so far.
3.1.3 UV and T P R of different s u r f a c e m o l y b d a t e s p e c i e s Mo/TA1203 precursors all present an intense band at ~<=230 nm, i.e. in the charge transfer region. In addition, high Mo loading samples show a s o m e w h a t less intense feature in the 250-280 nm region (see Figure la). KnSzinger and Jeziorowski [11] have proposed t h a t the band at 230 nm is due to a (terminal O =) -~ Mo VI electronic transition, while t h a t at 250-280 nm would correspond to a
257 (bridging O =) ~ Mo VI transition. While other factors m a y complicate the assignment [12,13], the band at 250-280 nm m a y thus be t a k e n as an indicator of bridging oxides, and thus of polymeric Mo species. TPR spectra of submonolayer Mo/TA1203 always present two distinct reduction peaks: one at low t e m p e r a t u r e s (Tmax<600~ denoted LT), and one at high t e m p e r a t u r e s (Tmax>750~ denoted H T - see Fig. 2a). For Mo loadings <1%, the LT peak has a m a x i m u m at 550-600~ and the ratio of i n t e g r a t e d intensities HT/LT is >3. For Mo loading=3%, the LT peak has its m a x i m u m in the 450-500~ r a n g e and HT/LT--2. At i n t e r m e d i a t e loadings (e.g. M o E 2 2%), a complex LT peak with two m a x i m a can be observed. Similar differences in the behaviour of low and h i g h e r loading submonolayer Mo/A1203 can be found in the l i t e r a t u r e [14,15]. There seems to be a clear correlation with UV-vis spectroscopy. The polymeric, octahedral Mo forms would be characterised by HT/LT--2 and Tmax(LT)=450500~ We m a y compare this behaviour with a very complete study of TPRs of bulk MoO3 due to Arnoldy et al. [16], where two separate stages were identified: Mo VI -~ Mo IV reduction (kinetic control, 2 electrons process), followed by Mo IV Mo O reduction at h i g h e r t e m p e r a t u r e s ( t h e r m o d y n a m i c control, 4 electrons process). The same two stages are probably observed here, accounting for the HT/LT ratio of 2 for 3% Mo loading. Monomeric, t e t r a h e d r a l Mo forms, on the o t h e r h a n d (in low l o a d i n g Mo/TA1203), would be characterised by HT/LT>3 and Tmax(LT) >550~ It was thought t h a t the first stage might correspond here to Mo VI ~ Mo V, but E P R of the precursors after the LT peak revealed only 10% of total Mo as Mo V. The situation is then certainly more complicated, but TPR spectra m a y still be used as fingerprints to discriminate polymeric octahedral from monomeric t e t r a h e d r a l Mo. Thus, a combination of UV-vis and TPR seems suitable to d i s c r i m i n a t e between monomeric t e t r a h e d r a l and polymeric octahedral forms of surface Mo's. However, other i m p o r t a n t distinctions may be overlooked, especially between grafted (covalent bonding, presence of Mo-O-A1) and ion exchanged (electrostatic bonding, no Mo-O-A1) forms of surface molybdates.
3.2 P d - M o / T A I 2 0 3 : E f f e c t o f p a l l a d i u m molybdates.
on the properties
of surface
F i g u r e 1 shows the effect of Pd t e t r a a m m i n e deposition on the UV-vis spectrum of precursor MoI 3% (calcined). Two differences can be observed: the a p p e a r a n c e of a band at 400 nm, due to d-d t r a n s i t i o n s in Pd II complexes (probably anchored to surface oxides, [17]), and a significant decrease in the relative intensity of the band at 250-280 nm t h a t was assigned to bridging oxides in Mo-O-Mo. The l a t t e r phenomenon is often observed, both on [Pd(NH3)4] 2+ ( P d A ) and on [Pd(H20)4] 2+ ( P d N ) d e p o s i t i o n , although in the l a t t e r case the charge t r a n s f e r band overlaps s o m e w h a t with broad Pd II d-d transitions. A decrease in the i n t e n s i t y of the (bridging O =) -~ Mo VI b a n d i n d i c a t e s a modification in the geometry or local environment of polymeric Mo's, and it m a y suggest an intimate interaction between these polymeric octahedral forms and the Pd II complex. Analog systems in homogeneous chemistry would help assess
258 the validity of such a hypothesis, but they are h a r d to find; Matveev et al. [18] have observed interactions between p a l l a d i u m and h e t e r o p o l y m o l y b d a t e s in solution, but apparently the interaction was strong only with Pd 0 (and not PdII). F i g u r e 2 i l l u s t r a t e s typical modifications in the TPRs upon p a l l a d i u m deposition ( P d N ) . The new features at Tmax <200~ are m a i n l y due to Pd II nitrate reduction, but the Mo reduction peaks are also strongly modified. The LT peak, supposed to be under kinetic control, starts at much lower t e m p e r a t u r e s in the presence of Pd 0 which can act as a source of spillover H; it even seems to occur entirely before 250~ in the case of M o I 3% - P d N 5%. The same t r e n d is observed for the P d A series. However, quantification reveals t h a t p a r t of the H T peak m u s t also be displaced to low t e m p e r a t u r e s . If we are right in a s s u m i n g t h a t the HT peak is under thermodynamic control, this would m e a n a change in the reducibility of molybdates and constitute a n o t h e r a r g u m e n t in favour of a chemical interaction between palladium and surface molybdates. F u r t h e r m o r e , the decrease in HT peak i n t e n s i t y is more pronounced for high m o l y b d e n u m loadings, i.e., for those p r e c u r s o r s t h a t show evidence of P d - m o l y b d a t e interactions in UV-vis.
t
3oo n m Figure 1: UV-vis spectra of." a. M o I 3%, b. M o I 3% P d A 1%
Figure 2" TPR spectra of: a. M o I 3%, b. M o i 3 % P d N 3% c. M o i 3 % P d N 5%
3.3 Mo-Pd/TAI203 (Pd II nitrate deposition): Effect of s u r f a c e m o l y b d a t e s on Pd d i s p e r s i o n after reduction. Monometallic Pd/TA1203 catalysts p r e p a r e d by procedure P d N show monomodal particle size d i s t r i b u t i o n s after r e d u c t i o n at 673 K, w i t h a v e r a g e diameters ranging between 23 ( P d N 0.5%) and 60/~ ( P d N 5%). The effect of modifying the alumina surface by molybdates deposition is shown in Figure 3, where the average d i a m e t e r of palladium particles is shown as a
259 function of molybdenum loading (x) in the series MoI x% (calcined). P d N 1% and MoI x% (calcined) - P d N 2.5%. A similar evolution is observed in both cases: low molybdenum loadings cause an increase in dpd, while higher Mo loadings have an opposite effect and actually give higher dispersions than on bare alumina.
A
20~
.....
_ _ 9.............
Figure 3; Evolution of average palladium particle diameter after reduction at 400~ for the series MoI x% (calcined) - P d N 1% (a) and MoI x% (calcined) P d N 2.5% (b) as a function of Mo loading x. The effect of low Mo loadings could be explained in a nucleation-growth model if the monomeric tetrahedral Mo's, predominent under these conditions, "wiped out" the nucleation sites for Pd particles. This could be the case if those nucleation sites consisted of Pd complexes bound to the same type of OHs t hat also bind monomolybdates (equation (1) above). A more precise formulation is hampered by the question of why complexes bearing charges of opposite sign should bind to the same surface sites; however, this hypothesis is in agreement with observations in mid IR [19] showing that deposition of both [MoO4] 2- and palladium complexes causes the disappearance of the same OH groups, assigned to terminal, basic OHs. In contrast, the deposition of polymeric octahedral Mos (predominent at high loadings) would cause the appearance of new nucleation sites. It is interesting to correlate this with the results discussed above (w 3.2) hinting at a preferred interaction of Pd complexes with polymeric surface molybdates. Some of these Pd complexes chemically bound to polymeric molybdates might constitute the nucleation sites for Pd particles. It remains an open question w h e t h e r the particle growth stage occurs during drying (as for Ni/SiO2,[2b]) or duri ng subsequent reduction.
3.3 Pd-Mo/TAI203 from p a l l a d i u m t e t r a a m m i n e : e x i s t e n c e of ultrad i s p e r s e d Pd? Micrographs of reduced monometallic catalysts from the P d A series generally show a very small number of visible particles in comparison with catalysts of the same loading from the P d N series. Increasing the reduction t e m p e r a t u r e from 673 to 1073K did not cause a significant modification in the n u m b e r of Pd
260 particles, but it did induce a large increase in their average diameter (from 40 to 80 A for P d A 0.5%). These observations would s u g g e s t the existence of a population of high dispersion palladium, undetectable by conventional TEM (and thus consisting in entities of subnanometric size). A s i m i l a r observation was also made for bimetallic Mo-Pd/TA1203 w h e n palladium was deposited as t e t r a a m m i n e complexes. This prompted us to try and detect the presence of ultra-high dispersion palladium: we selected by TEM regions of the support showing no visible Pd particles and s u b m i t t e d t h e m to microprobe analysis. In MoE2 1% - P d A 0.5% reduced at 150~ no significant Pd signal was observed in most cases; in contrast, for M o E 2 3% - P d A 0.5%, a significant signal for Pd emerged from the noise in 25% of the analysed regions. Since this result was confirmed by m e a s u r e m e n t s in two different electron microscopes, we believe t h a t they constitute valid evidence for the existence of subnanometric Pd-containing entities. It is interesting t h a t this observation was made specifically in a case where a separate body of evidence led us to expect a specific interaction between the Pd complexes and the surface m o l y b d a t e s ( r e m e m b e r t h a t for 3% Mo, the p r e d o m i n e n t form is polymeric o c t a h e d r a l molybdate - vide supra).
3.4. Catalytic p r o p e r t i e s for b u t y n e h y d r o g e n a t i o n In our conditions, selectivities very close to 100% for butene-1 were observed in all cases, and therefore no meaningful effects on selectivity can be evidenced. Activities, on the other hand, were very different, a n d estimates for several catalysts are given in Table I. The values provided, denoted t70, correspond to the times needed to convert 70% of the initial butyne. Table I: Values of t70 for butyne hydrogenation for several palladium catalysts. Catalyst P d N 1% MoI 3% (calcined) P d N 1% MoE2 3% (calcined) P d N 1% MoE2 3% P d N 1% P d A 0.5% MoE2 0.4% PdA 0.5% MoE2 1% P d A 0.5% MoE2 3% PdA0.5% MoE8 0.4% P d A 0.5%
Average particle diameter (/k) 40 35 31 36 (a) (a) (a) (a) (a)
t70 (min) 17 22 20 16 21 13.5 14.2 21.2 16.9
(a): Too few visible particles for reliable statistics. Some of the results obtained can be explained purely on the basis of a size effect: it is known t h a t an a n t i p a t h e t i c size effect is observed for selective hydrogenations on supported Pd, i.e., under a certain critical diameter (40/~ for Pd/TA120 3) the activity decreases with decreasing particle size [20]. For example, comparing the monometallic P d N 1% with corresponding bimetallics (calcined
261
precursor) showing a better dispersion (lines 2 and 3 of Table I) show t h a t the latter are indeed less active. Still, the correlation breaks down for bimetallics from uncalcined precursors: catalysts M o i 3 % (calcined) P d N I % and MoE2 3% P d N 1% have approximately the same average particle diameter, but different activities. Two conclusions may be drawn: * Factors other than particle size play a role in determining the activity of MoPd/TA1203 bimetallics - possibly effects on metal electronic density. * Simply classifying surface molybdates between polymeric octahedral and monomeric t e t r a h e d r a l may be sufficient to explain effects on Pd nucleation, but overlooks some important distinction since polymeric octahedral molybdates are supposed to be predominent in both precursors. This could be the distinction between ion-exchanged and grafted species. The second part of Table I shows sample results for the P d A procedure and is in agreement with the above. The trend in activity in the MoE2 x% P d A 0.5% is the negative of the expected trend on Pd particle size, since particle size is expected to increase for 0.4% Mo and then to decrease again for >l%Mo (see w 3.3). On the other hand, samples MoE2 0.4% P d A 0.5% and M o E s 0 . 4 % P d A 0.5%, where monomeric tetrahedral molybdates are expected to predominate, show different activities: once again, we probably see a combination of size effects and electronic effects. The low t70 values observed in this series indicate the possible practical interest of Pd-Mo/TA1203 systems: indeed, it is possible to obtain significantly higher activities for bimetallics with 0.5% Pd than for monometallics with 1% Pd, if the preparation procedure is well controlled. 4. C O N C L U S I O N S The present study demonstrates t hat the deposition of surface molybdates may be used as a tool to control the dispersion of noble metal catalysts. Different Mo containing species may have adverse effects: while the monomeric, 4coordinated forms predominent at very low Mo loadings seem to consume Pd nucleation sites and thus decrease Pd dispersion, the polymeric, 6-coordinated molybdates th at appear at higher loadings create new nucleation sites. At the same time, there is evidence of specific interactions between Pd II complexes in aqueous solution and polymeric molybdates containing Mo-O-Mo linkages. The precise n a t u r e of these interactions is still unclear due to the lack of clear analogs in homogeneous systems: however, preliminary results indicate t h a t platinum (II) complexes have a behaviour very similar to palladium. Catalytic results for a structure-sensitive reaction show t hat this control of Pd dispersion may be used to tune the activity of Pd-supported systems. However, they also show that the presence of surface molybdates has other effects on Pd particles t h a n simple size control. They also point at the need of a still finer characterisation of Mo/TA1203 precursors, especially concerning the presence of Mo-O-A1 bonds. Altogether, this work presents encouraging prospects for finetuning the properties of supported metal catalysts by prior surface engineering of the support.
262
REFERENCES 1. I. Halasz, A. Brenner, M. Shelef, K.H.S. Ng, Appl. Catal. 82 (1992) 51. 2. a. Z.X. Cheng, C. Louis and M. Che,Z. Phys. D, Atoms, Molecules and Clusters, 20 (1991) 445. b. Z.X. Cheng, Ph. D. Thesis, Paris, 1991. 3. N. Giordano, J.C. Bart, A. Vaghi, A. Castellan, and G. Martinotti, J. Catal., 36 (1975) 81. 4. Y. Okamoto and T. Imanaka, J. Phys. Chem., 92 (1988) 7102. 5. C.F. Baes, Jr., and R.E. Mesmer, The Hydrolysis of Cations, R.E. Krieger, Malabar (Florida), 1986, p. 253. 6. P. Sarrazin, Ph. D. Thesis, Lille, 1989. 7 J. Medema, C. van Stam, V.H. De Beer, A.J.A. Konigs, and D.C. Koningsberger, J. Catal., 53 (1978) 386. 8. C.C. Williams, J.C. Ekerdt, J.M. Jehng, F.D. Hardcastle, and I.E. Wachs, J. Phys. Chem., 95 (1991) 8791. 9. H. KnSzinger, Proc. Int. Cong. Catal., 9th, 5 (1988) 20. 10.D.S. Zingg, L.E. Makowski, R.E. Tischer, F.R. Brown, and D.M. Hercules, J. Phys. Chem., 84 (1980) 2898. 11. H. Jeziorowski, and H. KnSzinger, J. Phys. Chem., 83 (1979) 1166. 12. Iannibello, S. Morengo, P. Tittarelli, G. Morelli, A. Zecchina, J. Chem. Soc., Faraday Trans., 80 (1984), 2209. 13. M. Fournier, C. Louis, M. Che, P. Chaquin, and D. Masure, J. Catal., 119 (1989) 400. 14. R.Lopez Cordero, F.J. Gil Llambias, and A.Lopez Agudo, Appl. Catal., 74 (1991) 125. 15. K.S. Chung, and F.E. Massoth, J. Catal., 64 (1980) 320. 16. P.Arnoldy, J.C.M. De Jonge, and J.A. Moulijn, J. Phys. Chem., 89 (1985) 4517. 17. J-F. Lambert, M. Che, and F. Bozon, to be published. 18. K.I. Matveev, Kinet. Katal., Int. Ed., 18 (1978) 716. 19. X. Carrier, and J-F. Lambert, unpublished results. 20. S. Vasudevan, Ph.D. Thesis, Paris, 1982.
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 1995 Elsevier Science B.V.
263
Metal Catalysts supported on a Novel Carbon Support M.S. Hoogenraad, R.A.G.M.M. van Leeuwarden, G.J.B. van Breda Vriesman, A. Broersma, A.J. van Dillen and J.W. Geus
Debye Institute, Department of Inorganic Chemistry, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands ABSTRACT Carbon fibrils can be produced smoothly by exposing finely dispersed iron or nickel particles to reducing carbon containing gas flows. Alumina-supported iron particles were used to grow carbon fibrils with cylindrical graphite layers parallel to the fibril axis. The carbon fibrils interweave during growth, resulting in the formation of tangled skeins. Owing to the size of the skeins (about 3 ~tm), the pore volume, the pore size distribution, the surface area, the filterability and the mechanical strength, the skeins are very appropriate as supports for, e.g., noble metal particles, in liquid phase catalytic processes. Therefore, we examined the properties of the bare carbon fibrils, as well as the properties of catalysts supported on carbon fibrils. To obtain carbon fibril-supported palladium catalysts, a homogeneous deposition-precipitation procedure was utilized. Varying preparation conditions, such as, the atmosphere during drying and the pretreatment of the carbon fibrils, affect the activity in the liquid-phase hydrogenation of nitrobenzene significantly. 1. INTRODUCTION Activated carbon is widely used as an adsorbent and as a support material for active components in liquid-phase processes. The high resistance towards strong acidic and basic solutions and the high specific surface area are advantageous. Another reason for application of carbon as a support in liquid-phase processes is the possibility to recover the active component after the catalytic process, e.g., by oxidation or hydrogenation of the carbon support. The recovery is especially essential with noble metals. However, the use of activated carbons also leads to severe problems. First of all, the presence of micropores can cause (appreciable) transport limitations. To minimize transport limitations small support bodies, just sufficiently large to be separated from the liquid, can be used within a vigorously agitated liquid. Prevention of attrition to bodies of a size, that separation from the liquid cannot be performed efficiently, asks for catalysts of a high mechanical strength. Most active carbons, do not exhibit a high mechanical strength. Attrition can therefore lead to the formation of fines and thus to the loss of active components. Moreover, fines may severely hamper the separation of the catalyst from the liquid phase. Carbon fibrils, on the other hand, do not bear these constraints and a study on the application of this material as a support in liquid phase processes was estimated to be worthwhile.
264
EXPERIMENTAL 2.1. The growth of carbon fibrils. Carbon fibrils can be produced rather easily, e.g., by exposing supported, finely dispersed iron or nickel particles to reducing carbon containing gas flows. To this end, one has to produce first finely dispersed iron or nickel particles on a support material, such as alumina or silica. The desired catalyst can be prepared, e.g., by incipient wetness impregnation of the support material with a suitable metal salt solution or by means of homogeneous deposition-precipitation of the metal ions onto the carrier. The catalyst used in this work to grow carbon fibrils (with cylindrical graphite layers) has been an alumina (Alon-C Degussa) supported iron (20-wt.% Fe) catalyst, prepared by homogeneous deposition-precipitation. Into a vigorously stirred suspension of the support kept at a constant pH level of 5, an iron nitrate solution was slowly injected. After washing, filtering and drying the catalysts precursor was calcined at 600~ for two hours. A weighed sample of this iron oxide-on-alumina catalyst was placed into a specially designed reactor and carefully reduced in a 10% hydrogen in argon flow by heating from room temperature to 700~ within two hours. Subsequently, a mixture of carbon monoxide and hydrogen in balance argon (40, 13 and 47 ml/min, respectively) was passed through the reactor that previously was brought at 570~ This temperature was measured to be the optimum temperature for the fibril growth with this type of catalyst. The growing process was stopped as soon as the volume of the carbon fibrils had completely filled the reactor. Generally, the initial weight then had increased by at least a factor of two.
2.2. Testing the filterability. Experiments have been executed in order to compare the filterability of the carbon fibrils with that of commercially available activated carbon products. One gram of each support material contained in an equal amount of water was filtered in the same filter-device. The time required for filtering was measured. 2.3. The preparation of palladium catalysts supported on carbon fibrils. In order to study the properties of the carbon fibrils as a support material for an active component in a catalytic reaction, the liquid-phase hydrogenation of nitrobenzene was chosen as a test reaction and palladium as the active component. The inertness of the carbon fibrils, advantageous with respect to aggressive environments, makes it difficult to apply small particles of the active component that exhibit an interaction sufficiently high with the carbon surface to resist sintering or mechanical separation. To raise the reactivity, the surface of the carbon fibrils has first to be 'activated'. For deposition of the active component, palladium, onto the carbon fibrils a precipitation method is utilized 1. The method consists of four steps, viz., pretreatment of the carbon surface in boiling nitric acid, suspension of the treated fibrils in an aqueous solution and addition of the solvated palladium precursor, injection of a formaldehyde solution to reduce the palladium ions, and, finally, filtering and drying. Firstly the carbon fibrils were kept in refluxing 65%, nitric acid for 10 or 30 minutes. Surface groups are thus formed that, subsequently, are capable to anchor the precursor of the active components. After thoroughly washing of the carbon fibrils, they were dried at 120~ for one hour in air. After this pretreatment the carbon fibrils were suspended in water under a nitrogen
265 atmosphere which was maintained during the precipitation of the palladium, and the pH was brought at a level of 6. Then, the solvated palladium complex, Pd(NH4)C12, was added under vigorously stirring. To reduce the palladium-(2+)-ions an excess of a formaldehyde solution was injected. The suspension was filtered and dried in an inert atmosphere at room temperature. Next to this, the loaded carbon fibrils were heated in 12 h up to 80~ and cooled down to room temperature, still under inert conditions. This procedure of slowly heating has been chosen to avoid sintering. 2.4. Characterization. The specific surface area (BET) and pore volume were measured using a Micromeretics ASAP 2400. Prior to the physisorption experiment, the sample was outgassed in nitrogen at 250~ X-ray diffraction experiments were performed in a Guinier camera with a Johansson monochromator using Cu K~I radiation (~ = 1.5406). Thermogravimetric analysis measurements were performed with a Netzsch STA 429. Flows of H2 or 02 (10 ml/min) in argon (50 ml/min) were used and a heating rate was chosen of 5~ / minute. Samples were examined in a Philips EM420 transmission electron microscope with an accelerating voltage of 120 kV. Samples were prepared by suspension in ethanol under ultrasonic vibration. Some of the thus produced suspension was brought on a copper grid with holey carbon film and the solvent was evaporated.
2.5. Testing of carbon fibril-supported palladium catalysts. The catalytic properties were measured using the hydrogenation of nitrobenzene as a test reaction. The procedure can be summarized as follows. First a catalyst sample was suspended into 125 ml isopropylalcohol (IPA) and brought into a reaction vessel kept at 25~ For two hours hydrogen was fed into the reactor under vigorous stirring (1900 rpm) to reduce possibly oxidized palladium particles. In all experiments a gas recirculating stirrer was used. Subsequently, a solution of 5 % nitrobenzene in isopropylalcohol was added. The hydrogen uptake was recorded every ten minutes. When no more hydrogen was taken up, the experiment was stopped.
3. RESULTS AND DISCUSSION 3.1. The growth of carbon fibrils In situ TPR measurements revealed that, under conditions of the growth experiments, the degree of reduction of the iron phase measured at least 85%. Immediately after the TPR experiment, the temperature was brought at 570~ and the CO/H 2 mixture was fed into the reactor. The concentrations of CO and H2 beyond the reactor were measured as a function of time. The results of a representative experiment are represented in figure 1. From the difference between the concentrations in the flow before and beyond the reactor, viz. 40 and about 26 ml/min, it can be concluded that carbon deposition proceeded. In figure 2 a TEM micrograph is shown of the carbon deposits. At lower magnifications it can be observed that fibrils have grown that had interweaved to clusters, tangled skeins. In the TEM micrograph we can observe that the carbon fibrils all exhibit approximately the same diameter. This is caused by the uniform size of the supported metal particles. It turned out that the size distribution of the supported iron particles not only determines the distribution of the diameter of the fibrils, but also the texture of the tangled skins. For example, the skein
266 structure is less pronounced when bigger iron particles are used, i.e., the interweaving is less, which makes the material as a support less attractive. 30 25
0- ........ 0
.....
0
......
- 0 " ......... 9 ........ ' ~
. . . .
"I~"'~'-~
......
-O
H2
A
co._
.E 20
E15
CH4
U
oU 1 0
o
o
o
o
e
o
~:
CO2 O
0
=-=
4.5
.
,
.
_i
.
.
.
.
5
i
55
_
_
_
I
6
time (hours)
input 40 9 ml/minCO and 13 ml/min H2 total flow 100 ml/min, T=570~ Figure 1. Growth of carbon fibrils out of supported iron particles. Course of the concentrations of H2, CO, CH4 and CO2 measured beyond the reactor as a function of time
Figure 2. TEM micrograph of carbon fibrils out of supported iron particles; typical result of growth experiment as in figure 1
267
3.2. Characterization of the carbon fibrils (CF) and the carbon fibril supported palladium catalyst. The structure of the fibrils depends on the nature of the metal from which the fibrils were grown. Small supported iron particles yield fibrils with cylindrical graphite layers parallel to the axis (see figure 3), whereas small supported nickel particles yield fibrils consisting of cone-shaped graphite layers. The mechanical strength is related to the structure of the fibrils. In this work we will focus on carbon fibrils obtained from supported iron particles. This is a type of carbon fibrils exhibiting a high mechanical strength. As dealt with above, mechanical strength is a highly important feature with suspended solid catalysts. An other important feature is the size and texture of the skeins. These tangled skeins do form an open network of pores, thus providing fast transport of reactants and products. No micropores are measured to be present. The specific surface area (BET) is approximately 230 m2 per gram, while the pore volume turned out to be 1.6 ml/gram. The pore size distribution is shown in figure 4. Another important feature of the carbon fibrils is the fact that they are readily wetted by organic solvents. The size of the skeins, i.e., the clustered fibrils, is important in view of the transport of reactants and products, on the one, and the filterability of the clusters, on the other hand. Generally, sizes between 1 lam and 10 lam are required. The clusters we prepared measured diameters of approximately 3 to 4 ~tm, which fulfills the requirements nicely.
i
2.4-
I
axis I
2 29 -
O}
0 0
v
1.6-
E
1.2-
==.,,
o
~)
0.8-
L_
o
ca.
0.40.0
Figure 3. Structure of carbon fibrils out of iron particles
Figure 4.
............... I
...... IO pore radius (nm)
.,,;-o
'
'
Pore volume distribution plot as determined using
nitrogen desorption, of carbon fibrils grown out of iron particles.
As mentioned above, an important feature is a good filterability. The results of representative filtration experiments are shown in figure 5. In this figure the relative filtration times of equal weights of a commercial, activated, carbon support sample and a sample of the carbon fibrils contained in equal amounts of water have been plotted. From this figure it can be concluded
268 that filtration of the fibrils can be executed significantly faster. The filtration experiments were carried out with fresh samples. Also, the tendency of a support material to produce fines during vigorous stirring is important. To examine the attrition, the carbon fibrils were vigorously stirred at 2200 rpm for 48 hours in a vessel provided with baffles. After this experiment the sample was studied with TEM. No fines could be observed, and the filterability as compared with that of the fresh sample had not changed significantly. Earlier we emphasized the advantageous features of 1.2 carbon with respect to its resistance towards strong acidic and basic environments. Equally important is its behavior 1 under oxidizing and reducing conditions. Therefore ~'~ thermogravimetric (TG) experiments were carried out in ._~0.a which samples of carbon fibrils were exposed to flows of o= = 0.6 oxygen or hydrogen. Representative results of TG measurements are .~ 0.4 shown in figure 6. From this figure we can conclude that the carbon fibrils are stable in reducing atmospheres up to high 0.2 temperatures. In an oxidizing environment, on the other 0 hand, the carbon fibrils are completely oxidized beyond about Act. Carbon CarbonFibrils 300~ under formation of presumably CO and CO2. In liquid Figure 5. Relative filtration times of phase processes, however, temperatures above 300~ are activated carbon and carbon fibrils most unusual. Active species anchored or deposited onto the fibrils may decrease the stability in reducing or oxidizing atmospheres. Also experiments with Pd/CF catalysts were therefore executed in reducing and oxidizing environments. In hydrogen, a TPR experiment was performed. In this experiment no measurable consumption of H2 was observed after the reduction of the palladium oxide. This leads to the conclusion that no methane was formed under reducing conditions. The reactivity of a Pd-on-carbon fibrils sample towards oxygen was measured therrnogravimetrically. No decrease of the onset temperature of oxidation could be detected. lira
120 100 .
..........................................
atmosPhere H2
m
"= 60 ~-" 40 2O 00
atmosphere 0 2 200
400
600
temperature(*C)
800
11000
Figure 6. TG experiment of carbon fibrils in reducing and oxidizing environment The weight leit ai'ter a TG experiment in oxidizing environment is due to Fe203, originating from the supported iron catalyst. The original iron catalyst does not interfere in catalytic processes, such as liquid phase hydrogenations, since the iron crystallites, located mostly on top of the fibrils are shielded from the reactants by at least a few monolayers of carbon. The
269 iron surface only becomes exposed when these monolayers are removed by treatments, such as, oxidation. ...........................................................
XRD patterns of a 2.5 wt.% Pdon-carbon fibrils catalyst lacked diffraction peaks of palladium, which is due to small
Pd particles present in the sample. Obviously no sintering had taken place. TEM micrographs of the fresh 2.5 wt.% Pd-on-carbon fibrils catalyst showed no palladium species on the carbon fibrils. Only after reduction in liquid- or gasphase, palladium particles could be observed. A TEM micrograph of the 2.5 wt.% Pd-on-carbon fibrils catalyst reduced in a hydrogen flow at a temperature of 250~ is shown in figure 7. In this micrograph we can observe small palladium particles of a mean size of approximately 4 nm. It can be concluded Figure 7. TEM micrograph of a 2.5 wt.% Pd-on'carbon that the size of the palladium particles fibrils catalyst after TPR up to 250~ increased during reduction due to sintering. Therefore, the metal surface area decreases somewhat with temperature 2.
3.3. Hydrogenation activity of carbon fibril supported palladium catalysts. For testing the carbon fibril supported catalysts, the liquid phase hydrogenation of nitrobenzene was studied The test reaction was also performed using a commercially available catalyst, i e palladium supported on activated carbon (Engelhard ESCATI 0) Object of the experiments was to determine the effects of the preparation conditions on the catalytic performance The reaction proceeds according to equation ( I )
~NO2
~NH2 + 3 H2
~
+2 H20
(l)
Some preparation parameters and their effect on the catalytic activity in the hydrogenation of nitrobenzene were thoroughly investigated. First of all, the effect of the atmosphere during the distinct preparation steps was investigated. Samples, prepared under inert conditions during all preparation steps, as described earlier, were tested. In figure 8 the conversion as a function of time is plotted. It can be observed that, right from the start, the reaction proceeded at a constant rate up to high conversions. Also a sample was measured that was prepared under slightly different conditions, i.e., after deposition of palladium the batch was dried not in a stream of argon, but in a stream of air. Using this catalyst, the rate of reaction turned out to be significantly lower, the rate of hydrogenation had roughly decreased by a factor of two. The exclusion of air during the preparation procedure seems to have a substantial impact on the activity of the carbon fibrilsupported palladium catalyst. When the TEM micrographs of these two catalysts are studied, one
270 can observe that sintering of the palladium particles had taken place with the air dried sample. The catalyst prepared under inert conditions still showed small palladium particles only, homogeneously distributed over the carbon fibrils. From these findings it can be concluded that sintering had been responsible for the decreased catalytic activity. Probably, when the sample is dried in air, palladium particles are reoxidized. The palladium(2+) ions can become mobile over the carbon fibril surface, and thus cluster. Another possibility is that paUadium(2+)-ions formed are solvated in the remaining solvent. Further evaporation of the solvent leads to an inhomogeneous distribution of large palladium crystallites. 1.2
= 0.8 ._
r
~0.6
ir
C
~ 0.4 0.2 0
i
I
0
J
I
1O0
,
200
I
a
1
300 time (minutes)
,
I
400
500
,
600
Figure 8. Conversion-versus-time curves of the hydrogenation of nitrobenzene with a catalyst prepared under (1) totally inert atmosphere, (2) inert atmosphere and dried in air
Another parameter studied has been the effect of the pretreatment of the carbon fibrils in nitric acid. Oxidation of the carbon support appears to increase the number of surface oxygen groups and to enhance the dispersion of palladium 3'4. A number of samples were tested of which the bare support had been pretreated for different periods of time in refluxing nitric acid. Figure 9 contains the results of these tests. 1.2
0 minutes =
10 minutes 30 minutes
= 0.8 ._.
o
9 0.6 o~ 0.4 0.2 O -
--
0
--':
-
.
50
.
.
.
.
.
1O0
.
.
.
.
-r
-
150 200 time (minutes)
-,-
-
"r
-
250
-I-
-
"r
300
Figure 9. Conversion-time curves of the hydrogenation of nitrobenzene after pretreatment of support in boiling HNO 3 for O, 10 and 30 minutes
Obviously, no or hardly any palladium had been deposited onto the carbon fibrils that were not pretreated in boiling nitric acid. TEM micrographs confirmed the absence of palladium. A
271 pretreatment of the support of only 10 minutes appeared to be sufficient to obtain the highest loading attainable. Larger periods of time of pretreatment turned out to result in only a small increase of the performance, especially up to a conversion of 50%. Finally, the activity of a commercial palladium-on-activated carbon catalyst was measured and compared with the activity of a Pd-on-carbon fibrils catalyst. To enable us to compare the activity with that of the Pd-on-carbon fibrils catalyst the amount of the commercial sample was adapted so as to have an equal amount of palladium within the reactor. The carbon fibril-supported catalyst exhibited a comparable activity as the commercial, activated carbon supported catalyst.
4. CONCLUSIONS Carbon fibrils are suitable as a support material in liquid-phase catalytic processes. The production of these fibrils is fairly easy. The thickness of the fibrils and the interweaving during growth can be assessed by variation of the growth conditions. Under certain conditions, the growth will result in clustered carbon fibrils. The cluster size of 3 to 4 lam displays a good filterability, a suitable pore volume, and a high specific surface area. Furthermore, no attrition is observed under extreme conditions. Next to this, the fibrils display a good stability in reducing environment, whereas in oxidizing atmosphere the oxidation starts beyond 300~ Application of small palladium particles on the carbon fibrils is viable. The homogeneous deposition-precipitation method used gives best results when air (oxygen) is excluded. Anchoring of palladium on the carbon fibrils is possible after activation of the surface of the carbon fibrils with boiling nitric acid. XRD patterns do not exhibit diffraction maxima of palladium. Only after reduction up to 250~ of the palladium (oxide), it could be detected by TEM due to sintering. The palladium particle size was small; approximately 4 nm. The activity of the carbon fibril-supported palladium catalyst obtained was comparable to that of a commercial, activated carbon supported palladium catalyst.
REFERENCES K.P. de Jong, Ph.D. thesis, University of Utrecht, 1982 I.L. Dodgson, D.E. Webster, Preparation of Catalysts (B. Delmon, P.A. Jacobs, G. Poncelet (eds.), Elsevier, Amsterdam 1976, 279-292 Dong Jin Suh, Tae-Jin Park, Son-Ki Ihm, Ind. Eng. Chem. Res. 1992, 31, 1849-1856 4.
Dong Jin Suh, Tae-Jin Park, Son-Ki Ihm, Carbon, Vol. 31, pp.427-435,1993
This Page Intentionally Left Blank
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 1995 Elsevier Science B.V.
273
Soft.chemistry route tbr the preparation o f highly dispersed transition m e t a l s on zlrconla. C. Geantet a, P. Afanasiev a, M. Breyssea., T. des Couri6res b. Institut de Recherches sur la Catalyse 2 Av. A. Einstien, 69626 Villeurbanne Cedex, FRANCE.
a
b EIf-ANTAR France, Centre de Recherches Elf Solaize, BP22, 69360 Solaize, FRANCE. Abstract.
Molten nitrates were used to prepare in one step dispersed oxoanions at the surface of zirconia. The simultaneous reaction of zirconium oxychoride with another precursor salt of V, Nb, Cr, Mo or W generates interactions between the growing zirconia particles and the oxoanions of the other transition metal present in the melt. As a consequence highly dispersed oxoanions at the surface of zirconia with enhanced textural properties are easily produced. The products were characterized by XRD, BET measurements, SEM, UV and XPS spectroscopies. 1. INTRODUCTION The classical way to prepare supported catalysts consists of impregnating a support with an aqueous solution of a precursor salt. Being the simplest and the cheapest, it has certain drawbacks and limitations. Particular attention has to be paid to the isoelectric point of the support which influences the primary interaction of the support surface with the ionic precursor of the deposited compound. This factor governs the dispersion of the active phase but the mechanism by which ionic species are deposited on the support surface remains unclear and is subject to investigations [1-2]. Another limitation is due to the surface area of the support itself and its thermal stability. One challenging aspect of the preparation of catalysts is to overpass these limitations by using unconventional techniques involving new types of interactions between the support and the active phase. Molten salt method may provide an original chemistry for preparing catalysts. Molten nitrates have many advantages as a nonaqueous reaction media: low melting points, unusual dissolving properties, and excellent heat transfers are some of these advantages. The reactivity of a precursor salt in molten nitrates can be often classified as a Lux-Flood acid-base reaction, i.e. as a transfer of the oxide anion 0 2In this concept a donor of the oxide ion is a base while an acceptor of 0 2- is an acid. In fact the chemistry of most elements in molten nitrates is now relatively well understood [3].
274 Depending on the nature of the reacting element, it is possible either to precipitate an oxide (ZnO, NiO, TiO2,... ) or to stabilize oxoanions dissolved in the melt (CrO42-, PO43-,...). Recently, molten nitrates were used to prepare supports like zirconia alone or stabilized by yttria [4]. The textural properties (specific surface areas, thermal stability) were found to be better than those obtained by conventional methods. This encouraging result incited us to establish the possible uses of molten nitrates in order to prepare supported catalysts [5]. Our purpose, in this work, was to study the simultaneous reactivity in the nitrate melt of a zirconium salt which precipitates zirconia and an element which provides oxoanions (V, Nb, Cr, Mo, W). 2. EXPERIMENTAL. Hydrated zirconium oxychloride was employed as zirconia precursor; various salts of a second d- element M (vanadyl acetylacetonate, niobium oxalate, ammonium heptamolybdate or paratungstate, chromium chloride) were used as admixtures. Different M/Zr atomic ratio were studied: 0.01<M/Zr<0.4. d-metal precursors were tightly mixed with 10-fold molar excess of NaNO3-KNO 3 eutectic (mp:220~
Then, reaction was carried out in the Pyrex
reactor under nitrogen atmosphere. All the samples were prepared by using the following sequence of treatment: dehydration at 150~ for 2 h, reaction in the range 300-500~ for 2 h, extraction by distiled water and drying at 100~
in air. The solids prepared were
characterized by XRD, BET technique, SEM, UV-visible and XPS spectroscopy. Their chemical composition was determined by atomic emission method. 3. RESULTS
3.1 Chemical analysis. Large amounts of the doping elements were found in the solid products, but the atomic ratio M/Zr in the solid was always less than its initial value in the mixture of parent compounds. The part of doping element missing in the solid was found in the washing solution. The exemple of V dopant is given in Fig. 1. At low loadings, the dopant content in the solid is close to the initial value but diminishes with higher loadings. Na, K were present in a percentage lower than 0.2%. Residual nitrate (and/or chloride) were found to be less than 200ppm.
275 12 % V 2 0 5 in the solid product. 10
4
0
2
4
6
8
10
12
Initial content of V 2 0 5 w%
Figure 1. V20 5 w% in the solid product versus the initial loading. 3.2. Characterization of the doping element. XPS spectroscopy was performed on a series of M doped zirconia (M = Mo, V, Nb). The Binding Energies of Mo 3d5/2, Nb 3d5/2, V 2p3/2, W 4d5/2 are given in Table 1. Binding energy of Zr 3d5/2 was taken as a reference at 182.0 eV. These BE values are in agreement with oxidation states of Mo (VI), Nb (V), V (V) and W (VI). From the integrated peak areas, surthce stoechiometries have been determined and compared with the chemical analysis data (see Table 1). The molten salt catalysts manifested superficial atomic M/Zr ratios close to the data of chemical analysis; this apparently indicates a good dispersion of the doping element. Table 1. XPS Binding Energies of the doping elements and stoechiometries. Comparison between XPS intensities and chemical analysis Mo 3d5/2
Nb 3d5/2
V2p3/2
W4d5/2
Binding Energy (eV).
232.6
207.2
517.3
247.2
Atomic ratio.
Mo/Zr
Nb/Zr
XPS
0.07
O. 11
O. 10
0.061
Chemical analysis
0.062
0.07
O. 10
0.05
V/Zr
W/Zr
276 UV-vis spectroscopy shows that the doping elements present in the form of oxoanions. The absorption bands are due to ligand-metal charge transition. In all cases UV bands were found to correspond to polyoxoanions MxOy z-. Fig. 4 gives the exemple of Nb doped samples which demonstrate a band characteristic for a niobate compound compared with the bulk oxide.
3
1,5
1 F(R) 0,5
0
|
200
|
|
|
260
|
|
|
|
|
"~
320 380 Wavelength nm
440
|
500
Figure 2. UV-vis spectra of Nb doped samples and Nb oxide (reference - pure ZrO 2 ) : 1) non calcined sample, 2) sample calcined at 800~ 3) Nb20 5
10
3 F(R)
0 ,
200
250
300
350
400
450
500
550
Wavelength nm
Figure 3. UV-vis spectra of V doped zirconia with an increasing loading: V205 w% = 1, 3, 5, 8, 10. (Reference- ZrO 2 ).
277 Depending on the nature of the M atoms, more or less pronounced evolution of the UV bands with a thermal treatments at 800~
are observed. In the case of Mo, Cr and V, a
broadening corresponding to a higher polymerization was observed. By contrast, practically no effects are noticable in the case of Nb (see Fig.2), and W. The effect of the loading on the nature of the grained species corresponds to an increase of the polymerisation of the oxoanions (Fig.3). In the case of V, a UV absorption band with a maximum at 290 nm characteristic for polyvanadate ions [6] is observed. With an increasing content of V, a broadening of the UV spectra occured, characteristic tbr the condensation of vanadate species. The similar effect was observed with the other doping elements. No crystalline phases of doping elements was observed in the XRD patterns of the samples, either non-calcined or atter the calcination at 800~
Therefore, stable dispersed oxospecies of
V, Nb, Cr, Mo, W are present in the solids prepared by the molten salt method. 3.3. Effect of the doping element on the textural properties.
XRD patterns of the solids demonstrate only the presence of monoclinc or tetragonal zirconia, even at relatively high loading of the dopant. After calcination at 800~
no extra
phases can be detected. Crystallite sizes were deduced from the broadening of the diffraction lines of the (11-1) and (111) reflections of the monoclinic phase and for the (111) reflection of the tetragonal phase by using the Scherrer relationship. These data obtained before and after calcination at 800~
are presented in table 2. Small particles of zirconia are obtained; they
present also a high stability toward
XRD patterns of the solids demonstrate only the presence
of monoclinic or tetragonal thermal treatment since only a two-fold increase in the particles size was observed after calcination at 800~ Table 2 Sizes of zirconia particles (nm) doped with M elements after preparation at 500~ calcination at 800~ M
V
Nb
Cr
Mo
W
non doped
500~
6.5
6.3
6.4
5.1
6.0
8.0
800~
12.0
16.0
9.0
10.3
8.5
>50
and
278 Such small particles may provide high surthce area. In fact, BET specific surthce area are higher than those obtained after classical preparations involving aqueous media. It can be seen in fig. 4 that the presence of the doping element increase the surface area of the zirconia compound as well as its textural stability. Specific surface areas above 200 m2/g can be obtained., twice higher than those from conventional preparations under similar heat treatments (500~
The values of surface areas determined by nitrogen adsorption are close to
those calculated fi'om the crystallite sizes of XRD patterns and according to the assumptions made in ref [7]. Under thermal treatment crystallite growth is not intense but inter-crystallite sintering occurs in the case of V, Nb and Cr dopants and consequently a strong decrease in the specific surface area is observed.
250 i 200 150
Surface area m2/g
I
/
.
,oo/i// V
Cr
/
m
li
Nb Mo W Doping element.
l I A f t e r preparation
*
~Calcinationi800~ J
Figure 4. Specific surface areas of zirconia doped and undoped (*) samples after preparation and calcination at 800~
3.4 Effect of the loading of the dopant on the textural properties of the zirconia compound. With an increase of M/Zr ratio, it was observed that only one part of the dopant is fixed at the surface of the solid. The amount of dopant in the solid product notably influenced the texture of the zirconia compound. As it can be seen in Fig.5, the specific surface area versus the V loading goes through a maximum. A similar effect was observed with the other dopants.
279 250
S m21g 200
150
100
50
0
1,7
3,6
4,1
5,6
6,5
V 2 0 5 w% loading
Figure 5. Irtfluence of the loading of the dopant on the texture of the solid product.
4. DISCUSSION. Although the use of zirconia is small in comparison to alumina, interest in this new support is growing since original acidobasic and redox properties are observed. However, as compared to alumina, zirconia exhibits smaller specific surface area which decrease markedly with an increase of temperature (up to 850~
Incorporation of metal cations such as Mg 2+, Ca 2+,
y3+, La3+ forms solid solutions which stabilize the texture of the tetragonal zireonia. Oxoanions of metals of group V and VI can also have a similar effect but the mechanism of stabilization is different than that of the metal cations mentioned. Due to the formation of superficial compounds, oxoanions seem to modify the superficial energy of zirconia and prevent its sintering [8, 9]. Molten salt method was previously applied for the synthesis of zirconia stabilized with yttrium and solid solutions were obtained [4]. The surface area of the product was found to be about 130 m2/g. Such an yttria-stabilized zirconia was used as a catalytic support for hydrotreating catalysts. In the present work, we demonstrate that, in molten nitrates media, oxoanions of metals of group V and VI stabilize tetragonal zirconia. All the effects presented here were representative for the following dopant elements: V, Nb, Mo, Cr, W. It can be emphasized that during the
280 nucleation and growth of zirconia particles, interactions occur with the oxoanions favouring the formation of small crystallites. The dopant element is found to be present at the surface of the zirconia in the tbrm of well-dispersed monomeric or polymeric oxospecies. As a consequence, it is possible to optimize the surface area of the solid product up to 200 m2/g. As compared to classical preparations of zirconia, a substantial gain in specific surface area is obtained. Oxoanions improve also the thermal stability of the product. Surface areas as high as 60 m2/g.can be kept after calcination up to 800~ 5. CONCLUSION Molten salt chemistry can be a source of new kind of interactions between an active phase and a support. We have shown here that the reactivity of a mixture of precursor salts in molten nitrates leads in a one-step process to the synthesis of new solids with excellent textural properties. These solids present all the characteristics for catalytic applications [8], they can be used either as catalysts by themselves or as supports since free space remains at the surface of the solid [9]. Moreover, the preparation method may involve several species and mixed phases may be grafted at the surface of zirconia [ 10]. This work gives an exemple of the fascinating potential of molten salt medium for the preparation of catalysts. 6. REFERENCES
1. M. Che, in: Proc. 10th Int. Cong. on Catalysis, ed. L. Guczi, F. Solymosi, P. Trteniy, Elsevier, 1993, 31. 2. N. Spanos, L. Vordonis, C. Kordulis, and A. Lycourghiotis, J. Catal., 136 (1992) 432. 3. D.H. Kerridge in "Chemistry of Non-aqueous Solvents" Ed J.J. Lagowski, Vol VB, Acad. Press, New York 1978. Chap 5. 4. D. Hamon, M. Vrinat, Breysse M., Durand B., Jebrouni M., Roubin M., P. Magnoux and T des Courirres, Catal. Today, 10 (1991) 613. 5. P. Afanasiev, C. Geantet, and M. Breysse, in "Proceedings, Int. Symp. Chimie douce, Soft chemistry routes to new materials" 6-10 Sept 1993, Nantes, France. 6. A.B.P. Lever "Inorganic electronic spectroscopy" Elsevier 1984. 7. P.D. L; Mercera, J.G. van Ommen, E.B.M. Doesburg, A.J. Burggraaf, and J.R.H. Ross, Applied Catal., 57 (1990) 127. 8. J.R. Sohn, S.G. Ryu, M.Y. Park, J. Mater. Sience, 28 (1993) 4651. 9. P. Afanasiev, C. Geantet, M. Breysse, submitted to J. Mater. Science. 10. P. Afanasiev, M. Boulinguez, M. Breysse, C. Geantet, T. des Courieres, French Patent N ~ 9310535, assigned to Societe Nationale Elf Aquitaine. 11. P. Afanasiev, C. Geantet, M. Breysse, submitted to J. Catal. 12. P. Afanasiev, C. Geantet, M. Breysse, T. des Courieres, ACS meeting, Washington, DC, August 21-26, 1994.
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
281
INFLUENCE OF TITANIA LOADING ON TUNGSTEN ADSORFHON CAPACITY, DISPERSION, ACIDIC AND ZERO POINT OF CHARGE PROPERTIES OF W/TiO2-A1203 CATALYSTS R. Prada Silvy, F. Lopez, Y. Romero, E. Reyes, V. Le6n, R. Galiasso. INTEVEP S.A, Refining Department, Applied Catalysis Section, P.O. Box 76343, Caracas 1070-A, Venezuela. SUMMARY The influence of the titania loading in Ti-AI mixed oxides on the tungsten adsorption capacity, surface dispersion, acidic and zero point of charge has been investigated by several physicochemical techniques (XRD, ZPC, XPS, AES and pyridine adsorption). The results indicated that titanium induces the following changes on alumina; i) increases specific surface area, ii) alters pore sizes distribution, iii) ZPC shifts toward lower pH values, iv) increases surface acidity and v) modify the adsorption capacity of tungsten. For the Ti-AI support containing a titanium loading of 15 wt% TiO 2, a tungsten amount of monolayer is reached for WO 3 content higher than 27 wt%. Auger spectroscopy analysis of the Ti-A1 mixed oxides indicated that titanium atoms in this support is in chemical environment different from that present in anatase, probably in a transition region between anatase and alumina phase. XPS and ZPC results of W/Ti-AI and W/~,AI20 3 catalysts suggested that tungsten is preferentially adsorbed on alumina surface. This results are explained based on the colloidal chemistry of exchanging solutions and tungsten surface adsorption properties of anatase and alumina. The activity properties of NiW/Ti-A1 and NiWtyAI20 3 catalysts are compared in the mild hydrocracking and aromatic saturation reaction of cracked feedstocks.
INTRODUCTION Binary metal oxides, such as; TiO2-AI20 3, SiO2-A120 3, ZrO2-A120 3, TiO2-SiO2, etc, have been found in the past few years to be of increasing interest as catalyst supports. This is motivated by the observation that transition metals supported on this type of carriers developed higher activities than when supported on unmodified alumina or silica (1-3). In addition, the need for developing catalysts for new petroleum refining processes has promoted research on the use of supports other than conventional ones based on alumina. It has been reported that titania, used as an additive in the formulation of alumina, produces several effects, such as, modification of the metal-support interaction improving the degree of dispersion of active phase, altering acid strength distribution as well as zero point of charge of the alumina, enhancement of the reducibility and sulfidability properties of Ni, Co and/or Mo oxidic phases in alumina supported catalysts, enhanced mechanical strength and
282 specific surface area of alumina and strikingly improving the intrinsic activity for the I-IDS, HDN, HDA and MHC reactions (3-9). As surface and catalytic properties are strongly influenced by the nature of the carder, we decided to examine further the effect of the titania on tungsten adsorption properties, dispersion, acidity and zero point of charge for W/TiO2-AI203 catalysts. Although, most of the published literature report the effect of titanium on the behavior of Mo/TiO2-AI203 catalysts, very little is known on the W/TiO2-A1203 system. This communication is part of a research program aimed at a systematic investigation of the preparation of TiO2-A1203 binary oxides and their surface properties as catalyst supports. Essentially, we study the influence of the titania loading on metal dispersion, acidic and zero point of charge and tungsten adsorption capacity. For this purpose, a TiO2-AI203 support series, containing different TiO 2 loading (0-15 TiO 2 wt%), was prepared by eoprecipitation of TiOC12, A12(SO4) 3 and Na2AI204 aqueous solutions followed by consecutive washing, drying and calcination steps. The solids were characterized by means of the following physieochemical techniques: BET surface area, X-ray photoelectron spectroscopy (XPS), Auger spectroscopy (AES), X-ray diffraction (XRD), zero point of charge (ZPC), acidity measured by pyridine adsorption. The tungsten adsorption capacity of the different TiO2-A1203 supports was measured using the equilibrium method..
EXPERIMENTAL A TiO2-A120 3 series of supports, with different TiO 2 loading (0, 5, 15 and 100 wt% TiO2), were prepared by coprecipitation of a mixed solution of TiOCl 2, AI2(SO4) 3 and Na2Al20 4 at a final pH of about 7.5. The solids were washed with hot de ionized water until SO42"and Na + ions concentration was lower than 500 ppm. Subsequently, they were dried at 120 ~2 for 16h and calcined in air at 250 ~C for 2h and at 550 ~C for 2h. The samples will be referred as (X) Ti-A1, where x represent the percentage of weight of TiO 2. Tungsten supported Ti-A1 catalyst series were prepared by equilibrium adsorption method, using aqueous solutions containing ammonium metatungstate, followed by drying at 120 9(2 for 2h and calcination at 550 ~2 for 2h. Chemical composition, as determined by atomic absorption, of the different catalysts vary in the 5-30 wt% WO 3 range. The irreversibly adsorbed tungsten on the samples was determined after 24h of contact time. In a set of experiments, weighed amount of solids, c.a 10g was placed in contact with a known volume of ammonium metatungstate (50 ml) solutions of concentrations in the range 1.5 to 25 g/l, at 25 ~2. The corresponding amounts of tungsten in the supematant liquid and the solids was determined after 24h; in this way the adsorption isotherm was constructed. Specific surface area and pore size distribution of the different supports were determined by the BET method using a Micromeritics 2400 instrument.. X-ray diffraction patterns of the different Ti-A1 supports were performed with a Phillips 1730/10 powder diffractometer using Nickel filtered Cu K0t-radiation ( X=1.5418 Ao). The nature and surface composition of tungsten species as a function of the TiO2 loading in the support was determined by use of XPS and Auger techniques. The spectra were recorded using a Leybold Heraeus LHS-11 apparatus equipped with a computer which allowed the determination of the peak areas. The excitation source employed was the AI ktz line (E= 1486 eV). The C ls energy level (284.5 eV) was taken as a reference. Atomic surface concentration of tungsten was obtained from the peak integrated areas and the sensitivity factors provided by the equipment manufacturer.
283 The total acidity has been determined gravimetrically using pyridine as probe molecule adsorbed on the surface. The total irreversibly adsorbed pyridine amounts was measured at 100 9(2. Results was expressed as mmol of adsorbed pyridine per surface area of support. ZPC of the solids were determined using the mass titration technique. The following procedure was adopted from the literature (10). Different amounts of solid were added to a fixed volume of 0.1M NaC1 solution contained in plastic bottles and then sealed, thus obtaining samples containing solid fractions ranging from 0.1 to 5 wt/v %. The samples were left equilibrating for 24h, and pH of the supernatant liquids were measured. The ZPC was determined from the asymptotic value of equilibrium pH vs solid fraction curves.
RESULTS Specific surface area, ZPC and surface acidity results corresponding to ~,AI203, anatase and the different Ti-A1 supports are presented, respectively, in Table 1. It can be observed several effects when increasing titanium loading in the alumina carrier in the 0-15 wt% TiO2 range: i) BET surface area increases, ii) both pore volume and average pore diameter of the samples shift toward lower values, iii) ZPC is modified from 8.0 to 7.1, and iv) the irreversible pyridine adsorbed amount per surface area of catalyst support increases progressively. XRD patterns of the various catalyst supports are shown in Figure 1. The spectra corresponding to Ti-A1 mixed oxides did not show clear evidences of peaks attributed to anatase-like phase. The XRD patterns correspond to a typical high surface area yalumina phase. For tungsten supported catalysts, X-ray diffraction measurements indicated that no tungsten oxide peaks appeared for W/Ti-A1 catalyst series when WO 3 loading was lower that 20 wt%. However, for the WHAI20 3 catalyst, whose tungsten loading was 19.4 wt%, we have observed some diffraction patterns corresponding to well crystallized tungsten ~ e s in oxidic form. Figure 2 represent the adsorption isotherm of tunsgten on the 15 Ti-A1 support, which has been obtained from the measurements of the tungsten content on the solid at equilibrium conditions. A tungsten monolayer value is reached at about 27 wt% WO 3.
Table 1 Physical properties, ZPC and surface acidity corresponding to the different supports Property Surface Area (m2/g) Pore Volume (cc/g). Average Pore Diameter (nm) ZPC (pH units) Total Acidity (mmol py./m2) 106
~,AI203 264 1.70 17.6 8.0 152
5Ti-A1
15Ti-A1
TiO 2
280 1.52 13.6 7.6 568
330 0.93 7.7 7.1 852
58 0.19 6.4 6.8 102
284
30
10
20
30
(g/l)
10
20
30 40 2-0-
50
60
Figure 1. XRD patterns corresponding to the different catalyst supports
Figure 2. Adsorption isotherm of tungsten on 15Ti-AI support
The variation of the ZPC as a function of the WO 3 loading is presented for W~AI20 3 and W/15Ti-A1 catalyst series in Table 2. As it can be seen, ZPC decreases continuously with increasing metal content upto 10 wt% WO 3 and then, an asymptotic value of ZPC is reached at about 5.7 pH units at higher tungsten loading. The same behavior is observed for 7A1203 as a function of WO 3 loading. In the latter case, the asymptotic value is reached at 4.9 pH units. The A12o, Ti2D3/2 and Ols XPS energy levels and the Ti(LMM) and O(KLL) X-ray Auger energy trans~%ons c'orresponding to the different supports are shown in Table 3. For all mixed oxides, practically there is not significant differences between both Al2p and Ti2p3/2 binding energies with respect the peak positions observed for either 7A1203 or anatase. However, the Ti2p3/2 width line increased from 2.4 eV for anatase to 3.4 eV for the Ti-A1 mixed oxide samples. The Ols binding energy value is slightly lower ( about 0.6 eV) for the Ti-A1 mixed oxide when comparing the binding position obtained for yalumina. Auger analysis show more sensitive changes with respect to those observed effects by XPS measurements. For the Ti-A1 mixed oxide, there is greater differences in binding energy values for the Ti(LMM) level (above 1.0 eV) with respect the position obtained for anatase. The O(KLL) level also is modified in the Ti-A1 (about 0.6 eV with respect alumina) but in a lower extent than that observed for Ti(LMM ) level.
285 Table 2 Variation of ZPC vs tungsten loading for "tA1203 and 15Ti-A1 supports ZPC wt% WO 3
~,AI203
0 5 10 15 20 25 30
15 Ti-A1
8.0 7.0 6.0 5.4 5.0 4.9 4.9
7.2 6.2 6.0 5.9 5.8 5.7 5.7
Table 3 Binding energies corresponding to the different supports
Support
A12p
XPS Ti2p3/2
yAl20 3 5Ti-AI 15Ti-AI TiO2
74.0 74.0 74.2 ....
.... 458.9 458.6 458.7
Ols 531.3 530.7 530.6 529.5
Auger Ti(LMM) O(KLL) --379.8 380.0 381.3
507.6 508.4 508.8 512.0
Table 4 Binding energy, and T12p/Al2p, W4f/Ti2p and W4f/Al2p relative intensities corresponding to the different Ti-A1 supports and W/Ti-A1 catalysts.
Catalyst
W4f7/2
19.4W/~,A1203 14.7W/5Ti-A1 11.5W/15Ti-A1 3.6W/TiO 2
36.0 35.8 35.6 35.5
Relative Intensity (Ti/A1)c W/Ti
W/AI
. . . . . . . . . . . . 0.026 0.025 1.69 0.049 0.061 1.33 ........ 0.17
0.055 0.040 0.066 ....
(Ti/A1)s
where; s= support, c= catalyst
Table 4 reports W4f7/2 energy levels and the Ti2p/A12p, W4f/A12p, W4f/Ti 2 relative intensifies corresponding to a W-supported catalyst series which was prepared by e~uilibrium
286 adsorption. The final adsorbed tungsten amount in the samples, as determined by atomic absorption, was 19.4, 14.7, 11.5 and 3.6, expressed as wt% of metal, fo yalumina, 5Ti-Al, 15Ti-Al and anatase, respectively. This results indicates that titanium affect the adsorption properties of "/alumina. The atomic surface concentration of the different elements on surface was estimated using the Wagner atomic sensitivity factor (11 ). The W4fT/2 energy level corresponding to the different Ti-AI catalysts indicates smaller differences in peak position with respect to tungsten supported cat,o',~sts on pure ~ahanina or anatase. The signal at about 36.0 eV can be assigned to W +6 oxidic ~eciesin both tetrahedral or octahedral coordination (12). The Ti 2p/Al2p intensity ratio practi lly does not change after tungsten incorporation into the 5Ti-AI carrier. Although there are ~ rked differences in metal loading in the samples, the Ti2p/A12p and W4f/Ti2p intensities : io increases significantly while the W/Ti decreases with the increase titanium loading. Thi~ indicates that tungsten is preferentially adsorbed on alumina.
DISCUSSION Our results indicated that the presence of titanium into alum~ t induce changes in texture and surface properties of the carrier. Results of Table 1 indicato hat coprccipitation of both titania and aluminium hydroxides produce solids with higher sm ce area than pure yalumina synthesized under the same conditions. This increase in surface area with increase in TiO2 loading in the 0-15 wt% TiO2 is accompanied by: i) a continuous decrease in pore size distribution, ii) modification of ZPC toward lower pH values and iii) a simultaneous increase in surface acidity. The fact that surface area and acidic properties increase while ZPC decrease with the increase titania loading in the alumina support agree with those results obtained by other researchers (13,14). For instance, Zhaobin et al (13) studied the effect of titania on alumina on a serie of supports, prepared following different procedures (grafting, impregnation or precipitation). The authors observed that for samples prepared by precipitation of titanium tetrachloride on ~AI203, the specific surface area increase progressively but the surface acidity practically remained unchanged with increasing TiO2 loading. A study of characterization by electrophoretic measurements of a retie of TiO2-A1203 supports carried out by Gil Lliambas et al (9) indicated similar ZPC variations than that observed in this work with respect the titania loading and the preparation procedure. Note that "monolayer" of titanium is reached at lower metal loading than in the present work which is explained by an effect of preparative method and initial surface area of the alumina employed. Rodenas et al (8), observed an optimal surface acidity, for samples prepared by co-precipitation of A12(SO4)3 and TiOSO 4 with ammonia or urea, for a Ti/AI atomic ratio of about 1/9. However, in a previous work (15), we have observed that other parameters can strongly affect the final properties of the Ti-A1 mixed oxide such as; the nature of the aluminium and titanium salts employed for the co-precipitation, the stirring rate, temperature of co-precipitation, pH conditions and aging time, the sequence of precipitating reagent solution, percentage of solid in solution, the sodium or sulfate lavels in the samples, type of drying and calcination conditions of the powders. The XRD patterns, showed in Figure 1, did not reveal presence of either anatase or rutile like phases. The diffraction patterns corresponded to highly dispersed pure yalumina crystallites. The broad diffraction peak in the 20-50 20 range for the 15 Ti-A1 sample may
287 suggests the presence of small titanium crystaUitcshighly dispersed ( crystal sizes smaller than 4 nm). This results are consistentwith previously published work (13,14). For the W-Ti-AI catalyst series, the X R D spectra did not show signals corresponding to any form of the oxidic supported metal. Precisely, because of the tungsten loading in these samples is below the amount of monolaycr, indicating that tungsten species are highly dispersed on surface exhibiting an amorphous character. This explanation is supported by the results of Figure 2 which clearly showed that a monolaycr of W O 3 is reached at about 27 w t % W O 3. Similar behavior was observed by Rarnirez et al (14) who studied by X R D a series of Mo/TAI203 and Mofri-Al catalysts. In the latter work, the molybdenum loading in the samples was about 12 w t % M o O 3, which is equivalent to a tungsten loading of about 19.3 w t % W O 3. For the 19.4 W/TAI203 catalyst (24.5 W O 3 wt%), X R D revealed diffraction Peaks corresponding to tungsten in the oxidic phase. In this case, the amount of tungsten in this catalystsis above the monolaycr value. Results of Table 2 indicated that the Z P C of both AI203 and 15 Ti-AI supports decreased progressively with increasing tungsten loading. This behavior is explained by the fact that tungsten ions modify the surface charge of the alumina. The Z P C decreased much more for W/yAI203 catalyst than for W/15Ti-AI catalyst. This is probably due to differences in tungsten adsorbed amount and surface structure.The surface mngstatcs on alumina or on TiAI mixed phase respond to the Z P C of the solid as well as the p H and concentration of the tungsten aqueous impregnating solution. In order to explain more in detail our results,let us to refer to some fundamental studies dealing with the colloidal chemistry of the exchanging solutions and the adsorption of tungsten on alumina and titania supports. The hydroxyl groups of both titania and alumina surface in solution tend to be either positively or negatively charged below or above the ZPC of these compounds. Table 1 indicated that the ZPC for pure alumina was 8.0 units while, anatase showed a ZPC value of 6.8. According to the oxide charging mechanism proposed by Akratopulu et al (16), two chemical equilibria are stablished for TiO 2 as a function of the pH. xl r,2 TiOH2 + <=> TiOH + Hs + <=> TiO-+ Hs+ z3 Hs + <=> Hb+ where; s and b respresent the hydrogen ions on surface and in bulk solution, respectively. Above the ZPC of titania, the hydroxyls groups of this compound tend to be negatively charged by formation of the deprotonated titania (TiO-) species. So that it is relatively difficult for de tungsten ions to be adsorbed on the titania surface, due to the electrostatic repulsions in the solution. Below the ZPC, on the alumina the surface is charged positively, therefore, tungsten anions tend to be adsorbed on the alumina surface. The adsorption of tungsten on u has also been extensively studied by researchers using different physico-chemical techniques (9,16-18). These studies indicated that pH and metal concentration of the solution, and ZPC of the support controls the amount and surface structure of the metal oxide. At pH values above 7.8 and 1 M in tunsgten, tetrahedral WO42- species were observed. At pH values near to 7.8 and 1 M in total tungsten, the octahedral W1204212" polyanion forms and it persist until a pH of about 5.7. Below a pH of 5.7, octaheAraUy coordinated W12 polyanion, such as W120396", were characterizexL Both
288 tetrahedral WO42- and octahedral W1204212- anions coexist in a solution at a pH near to 7.8. Similarly, a solution at a pH near 5.7 will contain both octahedml W12 polyanions. The following chemical equilibrium between the tungsten ions is established as a function of the pH's of the exchanging solution. pH_<7.8 pH_~.7 12W042- <-> W1204212- <=> W120396-
In this work, at low tungsten loading, < 5 wt% WO 3, the pH's of the exchanging solution and the ZPC of the 15Ti-AI mixed oxide were 6.8 and 7.2, respectively. So, the pH of solution is the same of that corresponding to the ZPC of the TiO2 and below the ZPC of alumina. The adsorption mechanism of tungsten on both oxides is differents because the surface is negatively charged for TiO2 and positively charged for alumina. It should be expected that tungsten ions are adsorbed preferentially on the alumina surface than on titania surface. After 24 hours, the solution pH increased to 8.8 mainly due to the neutralization reaction of the hydroxyl groups by the acidic tungsten ions during adsorption process. This pH conditions corresponds to a slightly alkaline aqueous solution, therefore tetrahedral coordinated tungsten species are expected to form on the support. For WO 3 loading higher than 5 wt%, the acidity of the impregnating solution increased continuously upto 4.7 pH units with the increase of metal loading. After 24 hours, the final pH of the exchanging solution was 7.4 for those samples prepared with high metal loading (20-30 wt% WO3). Under these conditions, both the tetmhedral mngstate and octahedml polymngstate structures could be formed, from both monomeric and polymeric adsorbed ions on surface after calcination. In our opinion, the adsorption properties of the support depend on the differencesbetween Z P C of the solid and the p H of the tungsten exchanging solution (ApH). The initialA p H values for alumina, 5Ti-AI and 15 Ti-AI were 3.3, 2.9 and 2.4, respectively.The above reported tungsten content for the differentsupports are in agreement with thishypothesis. The X P S and Auger results presented in Tables 3 and 4 respectively, indicated the following effects with increasing titania loading into alumina in the 0-15wt% TiO 2 composition range; i) Ti2p3/2 line increase~l about leV for the Ti-AI mixed oxides with respect to that corresponomg to anatase, ii) the T i ( L M M ) level shift toward lower energy values with respect to that observed for pure anatase, iii)the O(KIj~) levelis about 0.6 eV higher than that observed for pure alumina and about 3.2 eV lower than that determined for anatase, iv) the Ti/Al intensityratio increases after tungsten incorporation into the 15Ti-AI carrier, and v) the w f r i intensity ratio decreases while W/A1 increases with increasing titania loading. The fact that there are differences in the width of Ti2p3/2 XPS signals and in Ti(LMM ) and O(KI~) Auger peaks position for the Ti-AI mixed oxides, with respect to that observed for pure alumina and anatase, suggests the presence of various types of titania and oxygen groups, in different chemical environments (probably in a transition region between anatase nuclei and alumina bulk). From the above effects, a possible explanation could be advanced. This is that we may consider that co-precipitation of titanium and aluminum salts leads to the formation of an aluminium-titanate like phase. In this case, colloidal particles of both elements are formed simultaneously at pH values near to 4.0. In concentrated suspensions, the colloidal hydroxides could act as nucleation centers and then coagulates easily to gels in the 4.0-9.0 pH range. Interactions between both titanium and aluminium hydroxides may occurs under this pH
289 conditions. An aluminium titanate-like phase (AlxTiyOz) containing a non well defined stoichiometry may crystallize by a solid state reaction after calcination of the eoprecipitated hydroxides. The aluminium atoms in ~/alumina are in both tetrahedral and octahedral coordination while, titanium atoms in anatase is only present in octahexlral geometry. Previous studies carried out by Ramirez et al (14) on Ti-AI mixed oxides prepared by co-precipitation of aluminium and titanium isopropoxide indicated by diffuse reflectance spectroscopy (DRS), that for low titania loading, titanium atoms enter into the alumina lattice adopting both tetrahedral and octahedml geometry. In the aluminium titanate compound AI2TiO5 titanium is present in a random distribution of tetraheclral and octahedral sites. The theoretical formulation of AI2TiO5 correspond to a TiO2ffiO2+Al203 mole ratio of 0.5. One may speculate that the formation of an aluminium titanate-like phase might be responsible of the observed differences in surface area, acidic properties, ZPC and tungsten adsorption capacity properties of the Ti-Al mixed oxides. Although in the literature, it has been well established that A12TiO5 formation occurs by a solid state reaction between TiO2 and ctA120 3 powder mixtures at high temperatures (T> 1000 9(2 )(19), we cannot discard the possibility that a similar phase may be formed from the coprecipitated hydroxides. Experiments to clarify this point are in progress. Returning to the above discussion, for the W/Ti-AI samples the Ti/AI and W/A1 intensities ratio increased while the W/Ti intensity ratio decreased after tungsten incorporation into the 15 Ti-AI support. This results support the above explanations which suggested that tungsten in preferentially adsort~ on alumina than on titania. In order to obtain more information about the use of Ti-Al mixed oxides as catalyst supports, we have studied the catalytic properties of two NiW catalysts, one supported on alumina and the other supported on 15 Ti-AI mixed oxide. The activity test were carried out in the presence of a hydrotreated cracked feedstock under typical mild hydroeracking operating conditions ( T= 380 9(2, PH2= 800 psig, LHSV= 0.55 h -1, H 2 ~ C ratio= 1000 Nm3/m 3. The feedstock had the following properties: sulfur= 0.394 wt%, nitrogen= 460 ppm, aromatic content= 48 wt%, A.P.I gravity = 26.0, 370 9(2+ fraction= 42 v%. The catalysts contained 20 wt % WO 3 and 6 wt% NiO, respectively, and they were prepared using the pore volume technique. The activity results of the NiW/TAI20 3 and NiW/15Ti-A1 catalysts is shown in Table 6. The activity data clearly demonstrate the positive effect of titanium on the I-IDS, HDN, MHC and aromatic saturation activities. This behavior can be related to the surface properties of the Ti-Al support.
Table 6 Activity of the NiW supported catalysts Catalyst NiW/TAI20 3 NiW/15Ti-Al
%HDS 84 93
%HDN 89 96
%MHC 14 26
%HDA 13 24
290 CONCLUSIONS From the foregoing discussion, the following conclusions may be drawn. Incorporation of titania to alumina by coprecipitation of both gels has the effect of: increasing the specific surface area of the support, the surface acidity increases with titanium content, pH of ZPC of the supports decrease with increasing titanium content and tends asymptotically to the ZPC of pure titania, The tungsten adsorbed amounts in the support dec'reams with titanium loading, tungsten ions tend to adsorb preferentially onto alumina sites on the Ti-AI mixed oxides supports. Auger spectroscopy seems to be more sensitive than XPS technique to detect changes in metal ion surface environment in Ti-AI mixed oxides system.
REFERENCES .2.3..5.6.7..9.-
10.11.12.13.14.15.-
16.17.-
18.19.-
K. Foger, J.R. Anderson., Appl. Catal 23 (1986) 139. J. Rieck, AQ. Bell., J. Catal, 99 (1986), 262. R. Prada Silvy, R. Galiasso, Y. Romero, E. Reyes, R. Mufioz., U.S. Patent 5,229,347, July 1993. M.A. Stranik, M. Houalla and D. Hercules., J. Catal. 106 (1987) 362. Z. Wei, S. Jiang, Q. Xin, S. Sheng, G. Xieng., Catal. Letters., 11 (1991) 365. Z. Wei, Q. xin, G. Xiong., Catal. Letters., 15 (1992) 255. J. Ramirez, S. Fuentes, G. Diaz, M. Vrinat, M. Breysse, M. Lacroix., Appl. Catal. 52 (1989) 211. E. Rodenas, T. Yamaguchi, H. Hattori, K. Tanabe., J. Catal. 63 (1981) 434. F.J. Gil Llambias, L. Bouyssieres, A. L6pez Agudo., Appl. Catal. 65 (1990) 45. J.S. Nob, J. Schwarz, J. Colloid Interface Sci. 130, 157-164 (1989). C.D. Wagner, L.E. Davis, M.V. Zeller, J. Taylor, R.H. Ray Mond, L.H Gale., Surf. Interface Analysis., 3 (1981) 211. C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Muilenberg., Handbook of X-Ray Photoelectron Spectroscopy, Physical Electronics Industries (1979). W. Zhaobin, X. Qin, G. Xiexian, E.L. Sham, P. Grange, B. Dclmon., Appl. Catal. 63.(1990) 305. J. Ramirez, L. Ruiz-Ramirez, L. Cedeno, V. Harle, M. Vrinat, M. Breysse., Appl. Catal. A General, 93 (1993) 163. R. Prada Silvy, E. Reyes, Y. Romero., INTEVEP S.A, Confidential Report 1993. K. Akratopulu, Ch. Kordulis, A. Licourghiotis., A. J .Chem. Soc. Faraday. Trans. 86 (1990) 3437. S.D. Kohler, J.C. Ekerdt, D.S. Kim, I.E. Wachs., Catal. Letters, 16 (1992) 231. N. Spanos, H.K. Matralis, Ch. Kordulis, A. Licourghiotis., J. Catal. 136 (1992) 432. B. Freudenberg, A. Mecellin., J. Am. Ceram. Soc. 70 (1987) 33.
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
291
Preparation of titania supported on silica catalyst" study of the dispersion and the texture of titania. R. Castillo, B. Koch 1, P. Ruiz and B, Delmon. Unit6 de Catalyse et Chimie des Mattriaux Divists. Universit6 Catholique de Louvain, Place Croix du Sud 2/17, 1348 Louvain-la-Neuve, Belgium. 1 Solvay S.A. Laboratoire Central, Rue de Ransbeek 310, 1120 Bruxelles, Belgium. Deposition of TiO2 on silica was realized by three differents methods: 1) precipitation from TIC14, 2) grafting with TIC14 in inorganic medium and 3) grafting with titanium isopropoxide in organic medium. The third method was used to study the influence of the amount of titania (between 2.0 and 20%wt). Samples were characterized by BET surface area mesurements, XPS, Zeta Potential, DRS, XRD and Electron Microscopy. Results show that titania is deposited at the external surface of the silica in the sample prepared by the precipitation method. Grafting with inorganic titania gives the best internal TiO2 dispersion, although the formation of large crystallites at the external surface is observed. Grafting with organic titanium isopropile provides the best external superficial dispersion of titania. A very good dispersion is obtained for contents lower than 15 % in wt. of TiO2.
Introduction Titania oxide is of increasing interest as catalyst or support (1-5). A great disadvantage of titania as a support is its low surface area. Inert oxides like silica have been used as supports to obtain higher surface area dispersed titania (6, 7). The nature of the support has a dramatic influence on the surface structure and catalytic activity of supported metal catalyst (8). The morphology as well as the surface properties of the TiO2 coated supports depend on the preparation procedure (1-3). In this work, titania supported on silica catalysts were prepared by three different methods: precipitation and grafting in liquid phase with inorganic or organic titania compound. The influence of the amount of titania was studied using this last method. The supported samples were characterized by BET surface area, XRD, XPS, DRS, EPMA, TEM and Zeta Potential measurements. The objective of this paper is to draw attention in particular to four points: a) the dispersion of titania on silica, b) the morphology of the supported titania, c) the interaction between titania and the silica and d) the influence of the titania content.
Experimental a) Sample Preparation A silica support from Grace (BET surface area of 320 m2/g) was used. The silica was first calcined at 600~ for 12 h before deposition of titania.
292 Precipitation from TiC14(PTi method) 16.76 gr pure TIC14 (Merck, pure grade) was added to a diluted solution of HC1 (pH-0.5-1.0). 40 g of silica support were then added and ammonium hydroxide (Merck, pure grade) was added under agitation until a final pH of 7.5 was reached. The resulting solid was filtered and washed with distilled water, dried at 120 ~ overnight and finally calcined in air at 500~ for 20 h. Grafting with TiCM_.in inorganic medium(GTi method) .30 g of the support-were added, under nitrogen atmosphere, to a solution containing 12.56 g of TIC14 in 200 cc of n-hexane(Merck, pure grade). After stirring for 4 h, the solvent was removed by evaporation in a Rotavapor at 50~ and then at 80~ for 1 h. The resulting solid was dried at 120 ~ for 12 h and then calcined in air at 500~ for 20h. Grafting with titanium IV isoorot~oxide in organic medium fGIP method) 740 g of the support-were added to a solution containing 26.11 g of titanium IV isopropoxide (Janssens, pure grade) in 350 cc of isopropyl alcohol (Janssens, pure grade). After stirring, the alcohol was removed by evaporation in a Rotavapor at 70~ The resulting solid was dried at 120~ for 12 h and calcined in air at 500~ during 20 h. Samvles with different amount of Titania The influence of the amount of titania was studied with samples prepared by the GIP method. Samples containing between 2.0 and 20 % wt of TiO2 over silica were prepared. Recalcination of samvles. A portion of ~ e samples were recalcined at 815~ during 8h. Samoles used for comvarison For comparative purposes a sample was prepared as a physical mixture (TiO2 + SiO2) containing also 15 % wt TiO2.
b) Characterization of samples Samples were characterized by the following methods: 3.1- Specific surface area BET BET surface areas were measured with a Micromeritics Asap 2000 equipment using nitrogen at 770K, after evacuation for 2 hr in vacuum at 373~ Theoretical surface area (TSA) was calculated as the sum of BET surface area of each individual oxide, assuming TiO2 has the same dispersion as the commercial TiO2 sample. 3.2- X-ray photoelectron spectroscopy (XPS) The X-ray photoelectron spectra were obtained with a Surface Science Instruments SSX100 model 206 with a monochromatised A1Kot source, operating at 10 kV and 12 mA. The Cls, O ls, Si2p, and Ti2p lines were investigated and their binding energies were referenced to the Cls line at 284.8 eV. The normalized intensities of the Ti2p and Si2p photoelectrons divided by their sensitivity factors, proposed by Wagner (9), were calculated in order to obtain the (Ti/Si)ESCA atomic ratios. The pressure inside the analysis chamber was less than 5x10 -9 torr. The theoretical XPS intensity ratio Ti2p/Si2p was calculated by using the sheet model for dispersion of the catalyst particles (10), using the photoionization cross sections (11 ) and the ratio of mean free path for electrons (12). A linear relationship representing the theoretical monolayer model at constant crystallite size can be expected for a series of catalysts with increasing titania content (10,13)
293 3.3- Zeta Potential Zeta potential measurements were carded out in a Zeta Meter 500, using 25 mg of sample ultrasonically dispersed in 200 ml of a solution containing 10-3 M KCI. The pH was adjusted with 10 -3 M solutions of KOH or HCI. The zeta potential values were obtained from electrophoretic migration rates using Smoluchowski's equation (14). The isoelectric point (IEP) was taken at zero rate of migration. The apparent surface coverage (ASC) of the TiO2 over the silica was calculated by using the following equation (7): %ASC = MTi 0EPsi- IEPTi/Si)/(Msi(IEPTi/Si-IEPTi)- MTi(IEPTi/Si-IEPsi)) where MTi and Msi are the molecular weights of titania and silica respectively, and the subscripts Si and Ti/Si refer to the silica and titania-silica samples, respectively. The surface area developed by the supported TiO2 was calculated as the product of ASC value and BET value of the sample. 3.4- UV-Vis Diffuse Reflectance Spectra (DRS). UV-Vis diffuse reflectance spectra were obtained with the reflectance attachment of a CARY 1756 spectrometer connected to a Hewlett Packard computer. The spectra were recorded between 210-700 nm using BaSO4 as reference. 3.5- X- Ray Diffraction X-Ray diffraction patterns were taken on a Philips PW 1050 diffractometer. The spectra were taken using the CuKa line. 3.6- Electron Microscopy. Scanning Electron Microscopy (SEM) and Electron Probe X-ray Micro Analyses (EPMA) were performed. The morphology of the samples was analysed on a Cambridge instrument 250 M I ~ I I Scanning Microscope. Electron Probe X-ray Micro Analyses were performed on a Link E 5431 instrument. Average particle size of the samples was obtained from SEM images. EPMA analyses were realized on the surface during SEM analyses and through the diameter, at the border, in the middle and in the center of the particles.
Results 4.1- BET Analyses. Specific surface area BET, theoretical surface area (TSA) and % wt TiO2 are presented in Table 1. Table 1. Results of BET, Zeta potential(TSA, ASC) and XPS( Ti2p3/2 binding energies, (ITi2p/ISi2p)ESCA) of samples prepared by precipitation (PTi), grafting in inorganic medium (GTi) and grafting in organic medium (GIP).
Sample
% wt TiO2
BET m2/g
TSA
IEP
294 314 290
3.7 3.3 3.8
ASC (%) Binding energy
ITi2p/ ISi2p
(eV) PTi 16.3 293 GTi 9.2 288 GIP 17.8 309 TiO2+SiO2 mech. 15 mixtures SBET of silica: 341 m2/g SBETof TiO2:55.4 m2/g
80 62 84
459.0 458.9 458.8
0.070 0.034 0.104
458.0
0.016
294 All samples show a decrease of surface area after deposition of titanium dioxide, compared to the original silica. The samples prepared by PTi and GTi methods showed the highest loss in surface area (approx. 15%). The sample prepared by GIP method has a decrease of only 5%. The samples prepared by PTi and GTi give a BET surface area equal to and lower than the TSA value, respectively. The sample prepared by GIP gives a BET surface area above the TSA value. For samples prepared by GIP containing different amounts of TiO2, the BET surface area diminish with the increase in the TiO2 content. For samples with TiO2 content between 1020 % wt, the decrease is less drastic. 4.2- X-ray photoelectron spectroscopy (XPS) XPS results obtained for samples prepared by PTi, GTi and GIP methods are shown in Table 1. All the binding energies of titanium (Ti2p3/2=458-459 eV) are in agreement with the reported binding energy of Ti +4 in TiO2 (1, 3, 15). The sample prepared by the GIP method presents a high (Ti/Si)~cA atomic ratio. On the contrary, the sample prepared by GTi presents a low (Ti/Si)ESCA atomic ratio. XPS results obtained for samples prepared by GIP method with different TiO2 content are presented in Table 2. Theoretical (Ti/Si)ESCA atomic ratios calculated by the method indicated in section 3.2, as function of the measured XPS intensities for samples prepared by GIP method with different TiO2 content, are correlated by a straight line for contents below 15% wt of TiO2. For higher TiO2 contents, the points are under the straight line (15) The XPS analysis for samples prepared by GIP with different TiO2 content shows that all the binding energies of Ti2p3/2 correspond to Ti +4 (1,3,15) The binding energy of titanium increases when the content of TiO2 on the support decreases. When the samples containing 5.2 and 17.8 %wt TiO2 were subjected to calcination at 815~ the shift of the Ti2p3/2 binding energy increases to 0.5 and 0.6 respectively. Table 2. XPS ( Ti2p3/2 binding energies and (ITi2p/ISi2p)ESCA)results for samples prepared by GIP method with different TiO2 contents
Sample
% wt TiO2
i 20.9 2 17.8 2* 17.8 3 10.3 4 5.2 4* 5.2 5 2.0 TiO2 * Samples recalcined at 815 ~ during 8 hours.
Bindingenergies (eV) 458.9 458.8 459.3 459.3 459.3 459.9 459.3 458.0
ITi2p/ISi2p 0.0916 0.1048 0.0836 0.0641 0.0370 0.0356 0.018
The Ols line spectra for the mechanical mixture (15% wt TiO2) shows that the difference between the binding energies of the Ols line of the two components (TiO2 and SiO2) is 3.6 eV (namely 533.0 - 529.4 eV ). The largest peak coincides with the Ols line of pure SiO2 peak position (533.0 eV), and the small peak (529.4 eV) with the Ols line spectra of Ti-O bond in TiO2. For the sample containing 17.8% of TiO2 calcined at 815~ the same two components can be observed, but the differences in the binding energy between the two peaks is only 2.4 eV (533.0 - 530.6 eV). The largest peak corresponds to Ols line of pure SiO2 (533 eV). The Ols
295 line for the small peak does not correspond to the binding energy of the Ti-O bond (530.6 against 529.4 eV). 4.3- Zeta Potential The IEP points for pure SiO2 and TiO2 were 2.05 and 4.25, respectively. The IEP values for samples prepared by the three methods are also presented in Table 2. From the ACS values, it can be observed that for sample prepared by PTi, GTi and GIP ( 17.8% wt), the fraction of the surface covered by TiO2 is about 80%, 60% and 84%, respectively (15). 4.4 UV-Vis Diffuse Reflectance Spectra(DRS) The absorption edge of the Ti-O charge transfer band in the mechanical mixture was at 320 nm. The samples prepared by PTi, GTi and GIP showed absorption edges at about 300nm. The samples prepared by GIP with 5.2 and 17.8 % wt TiO2 showed absorption edges near 290 nm for the fwst and both at 290 and 320 nm for the second sample, respectively. 4.5- X-ray Diffraction All the samples showed weak diffraction peaks corresponding to anatase. The sample prepared by GTi, that had the lower amount of TiO2 (9.0 % wt approx.), nevertheless showed diffraction peaks with the highest intensity. For GIP samples, anatase crystallites are well detected only for a TiO2 content of 17.8 % wt or higher. 4.6- Electron Microscopy Micrographs obtained by SEM analyses are shown in Figure 1. The size of the particles of pure silica was around 94tt. The size of the particles in the sample prepared by GIP is about 130tt. For the sample prepared by PTi method the particle size is twice that of the original silica (about 209tt). In the sample prepared by GTi, the particle size is about 100~. The results of some EPMA analyses are are presented in Table 3. In GTi sample, TiO2 is uniformly distributed across the particles. In the PTi sample, almost all the titanium dioxide is deposited at the surface of the particle. In the GIP sample, the distribution is better but a substantial enrichment is also observed near the periphery of the particles. Table 3. EPMA results.Semi-quantitative analysis of Ti (in atomic %). Average values on several parts of the particles. Analyses were realized on the surface during SEM analyses and through the diameter: at the border (B), in the middle (R) and in the center (C) of the particles. Method of preparation
During SEM surface analysis
Through the particles border
middle
center
PTi
15
2
1.7
1.7
GIP
12
8.5
7.7
7.3
GTi
4
4
3.7
3.7
Discussion The method of preparation of titania supported on silica strongly influences the morphology and dispersion of TiO2 on support. 5.1 External dispersion of titania A good dispersion at the external surface of the particles is obtained for samples prepared by GIP and PTi methods (high (Ti/Si)ESCAatomic ratio and high ASC). GIP samples
296
(a)
(b)
(c)
(d)
Figure 1. Micrographs obtained by SEM analyses for: silica 2.5 cm=400~tm (a), and samples preparate by GIP 1 cm=200rtm (b), PTi l cm=200tma (c) and GTi lcm=200ttm (d) methods. The results of some EPMA analyses are presented in Table 3. containing less than 15.0% wt follow the proportionality indicated by the (Ti/Si)ESCA for the theoretical monolayer model. This shows that the dispersion at the external surface of the particles seems to be very good. At higher TiO2 contents (> 15% wt), the formation of agglomerates explains the decrease in the (Ti/Si)ESCA atomic ratio (1,10). The good external dispersion is also confirmed by the XRD analysis, where samples with less than 15% wt of TiO2 do not show the diffraction peaks of anatase. These samples present the lowest loss in surface area, which confimas that the GIP method provides the best external dispersion of titania over silica, thus allowing the formation of small cristallites at its external surface. Samples prepared by GTi method show a low (Ti/Si)EscA atomic ratio and a low ASC value, which indicates a low external dispersion.
297 5.2 Morphology and concentration profiles of TiO2 inside the silica. According to SEM analyses, the sample prepared by PTi method is constituted of particles of size twice that of the original silica. This indicates that there is agglomeration of the particles. The particles of silica are "pasted" by the TiO2 particles. EPMA analyses indicate that TiO2 is deposited preferentially at the external surface of the support particle. The BET surface area values of samples prepared by PTi method suggest that both oxides constitute a system similar to mechanical mixtures (SBET=TSA). The sample prepared by GTi shows the same particle size as the original silica, even though it contains 9 % wt TiO2. EPMA analyses show a flat profile of TiO2 across the particle. The GTi method presents the highest loss in surface area due to the formation of crystallites of TiO2 within the pores. The low ASC and (Ti/Si)ESCA atomic ratio values indicate that TiO2 is poorly dispersed and forms large crystallites. The particle size of the sample prepared by GIP method is slightly higher than that of the original silica, suggesting that the amount of TiO2 deposited in these particles is between the amounts deposited with the PTi and GTi methods. EPMA analyses confirm these results, indicating some enrichment at the surface of the TiO2 particles, but less important than with the PTi samples. These samples present the lowest loss in surface area. A model of the concentration profile of TiO2 in the different samples is presented elsewhere (18). 5.4 Interaction between titania and silica Compared with the TiO2 in the mechanical mixtures, all the samples showed a shift of the binding energy of the Ti2p photoelectrons corresponding to 0.8 to 1.3 eV. This shift can be explained principally by a change in the coordination number of titanium by the formation of Ti-O-Si bond (15,16). The UV-visible reflectance spectra of samples prepared by the GIP method support this conclusion. The sample containing 5.2% wt of TiO2 shows the absorption edge at 290 nm (titanium in tetrahedral coordination in Ti-O-Si) and the sample containing 17.8% wt shows an adsorption edges at 290 and another at 320 (titanium in octahedral coordination as in anatase). The shift in the binding energy is produced when titanium is associated to silica in Ti-O-Si bond (19, 20). Recalcination of some samples (Table 2) showed an increase in the shift of binding energy of TiO2. On the other hand, the change in the coordination of titanium also induces changes on the O1 s photoelectron peak associated to Ti. The results lead us to conclude that the changes in binding energies observed are principally attributed to the different coordination number of titanium.
Conclusions The main conclusions obtained in this work are summarized in the the following: Comparison of the preparation met.hod The method of preparation strongly influences the morphology and dispersion of TiO2 on the support. The TiO2 is deposited at the external surface of the silica in the sample prepared by the precipitation method. The grafting with organic titania method provides the best external superficial dispersion of titania. The grafting with inorganic titania method gives the best internal TiO2 dispersion, although there is formation of relatively large crystallites at the external surface. Influence of the amount of titani~ Good dispersion of TiO2 over silica and a homogeneous crystaUite size were observed for TiO2 contents lower than 15 % wt. At higher TiO2 contents, large TiO2 crystallites were formed.
298 Interaction b~veen titania and silica There is a interaction between the titania and silica with formation of Ti-O-Si bonds. The intensity of this interaction depends strongly on the amount of TiO2 deposited on the support. It is favored by an increase of the temperature of calcination. References 1. 2. 3. 4. 5. 6. 7. 8. 9 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.
Z. Wei, Q. Xin, X. Guo, E. Sham, P. Grange, and B. Delmon, Appl. Catal., 1990, 63, 305317. A. Mufioz, and G. Munera, in Studies in Surface Science and Catalysis. Preparation of Catalyst V, eds. G. Poncelet., P.A. Jacobs., P. Grange and B. Delmon, Elsevier, 1991, p 627. P. Wauthoz, M. Ruwet, T. Machej, and P. Grange, Appl. Catal., 1991, 69, 149. A. Fernandez, J. Leyrer, A. Gonzalez, G. Munera and H. Knozinger, J. Catal., 1988, 112, 489. K. Foger, and J.R. Anderson, Appl.Catal., 1986, 23, 139. F. Gil Llambas and L.Bouyssieres, Appl.Catal., 1990, 65, 45. K. Foger, Catalysis Science and Technol. 6, 1984, 227. C. Bartholomew and C. Vance, J. Catal., 1985, 91, 78. C.D. Wagner, L. E. Davis, H.V. Taylor, R.H. Raymond, L.H. Gale, SurfJnterfce. Anal. 3, 211981. F.B. Kerkhof and J.A. Moulijn, J.Phys.Chem., 83, 1612 (1979). J.H. Scofield, J.Elec.Spectrosc.Relat.Phenom. 8, 129 (1976). D.R. Penn, J.Elec. Spectrosc. RelatPhenom. 9, 29 (1976). R. Siuda, Surface Science, 177 ( 1976) L 1011-L 1014, North-Holland, Amsterdam. M. Smoluchowski, in Hanbuch der Elektrizitat und der Magnetism, vol 2, ed. B. Gractz, Leipzig, 1914, p.366. R. Castillo, B. Koch, P.Ruiz and B. Delmon, (Submitted for publication) S. Mukhopadhyay and S. Garofalini, J2Von-Cryst.Solids, 1990, 126, 202. M. Mohai, I. Bert6ti and M. R6v6sz, Surf. and Inter. Anal. Vol 15, 364-368 (1990). R. Castillo, B. Koch, P.Ruiz and B. Delmon, Journal of Materials Chemistry, Royal Society of Chemistry. (Accepted for publication) D. Sandstrom, F. Lytle, P. Wei, B. Greegor, J. Wong and P. Shultz, J. Non-Cryst.Solids, 41, 201-207 (1980) R. Greegor, F. Lytle, D. Sandstrom, J. Wong, P. Shultz, J. Non-Cryst.Solids, 55, 27-43 (1983)
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
Preparatlon ionexchange
columns
of
catalytlc
packings
Tor
299
po!Emer,/" cera.m, lc
reactive
oistlllatlon
Ulrich Kunz, Ulrich Hoffmann Institut fur Chemlsche Verfahrenstechnik, Technlsche UnlversittR ClausthaJ Lelbnlzstr. 17, 38678 ClaustheJ (Germany) Phone: 0-5323-72-2187 Fax: 0-5323-72-2182 Summary Reactive distillation Is a method with Increasing importance for the chemical industry. Very often the reaction is catalyzed by an ionexchange resin. Ionexchange resins are only available as small beads which can not be used directly as packings for reactive distillation columns. The conventional technique is to sew commercially available ionexchange beads into glassfiber cloth or into a wire net. This is neccessary, because it is essential to .have a low pressure drop and a high surface area inside the reactive distillation column. Instead of using one of these techniques a method is presented how to produce Raschlg-rings, ordered packings or monoliths directly from polymer and ceramic as catalysts for reactive distillation processes. One way to produce the packings is to Incorporate the polymer into the porous structure of a ceramic carrier. The advantage of the last method is, that a well suited ceramic packing, a Raschig-ring or on ordered packing can be produced with well known methods. Then the step of polymerization follows. The polymer can be bonded chemically to the carrier to avoid removal under reaction conditions. The activation of the polymer/ceramic fillings is done by sulfonation. For the catalytic reaction it is Important, that the polymer is in a macroporous state. The aim is achieved by controlled polymerization inside the macroporous carrier using an organic pore forming compound. The result is a macroporous catalytic polymer encased in a porous ceramic carrier. Several ceramic materials and polymers were investigated to produce Raschig-rings, ordered packings and honeycombs with acidic ionexchange polymers inside the porous structure. One polymer is a styrene/divinylbenzene copolymer which is sulfonated with chlorosulfonic acid. Catalytic activities are about 1 meq H+/g. Swelling behaviour and catalytic performance were tested for the synthesis of MTBE (methyl-tertiary-butyl-ether) and compared to commercial ionexchange catalysts. Introduction Ionexchange resins are suitable catalysts for acid catalyzed reactions. An application of ionexchange catalysis Is the process of catalytic distillation in a reaction column. Catalytic distillation means to perform a chemical reaction and simultaneously distill the reaction mixture. The benefits of this process are the recover of heat of reaction for the separation process and the fact, that the continuous removal of product shifts the posltion of the equilibrium towards the product. Mechanical stable polymer based catalytic packings for reactive distillation columns are not available up to date. An intermediate solution Is to fill small beads of ionexchange resin into wire nets or glassfiber cloth bags. These bags or bales are introduced into the
300
reactive distillation column. Examples for this route are the packings of CDTECH /CDT (1990)/, Sulzer, Koch Englneerlng and BASF. The main application of acidic ionexchange catalysts in this regard ts the production of ethers for unleaded fuel. Examples of importance are methyl-tertiary-butyl-ether (MTBE), tertlary-amyl-methyl-ether (TAME) and ethyl-tertiary-butyl-ether (ETBE) synthesis. A catalytic packing has not only to satisfy the demands of a good catalyst. It has also to fulfil the requirements of a distillation packing, that means the catalyst has to be in the form of a dlstlllation packing. Examples are Raschig-Rings, honeycombs or corrugated packings. In Table 1 the main requirements for a catalytic reactive distillation column packlng are summarized.
Table1: Requirements for a catslytlc reactive distillation column packing ,
,.
.
.
.
.
.
.
Requirement .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
For catalysts .
.
.
.
Form and Structure
.
.
.
.
.
.
.
.
.
_
_
- -
....
=.,
For distillation packings .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
Favourable porestructure for the acces of the reactants
.
.
.
.
.
.
.
.
.
.
.
.
High surface area for good separatlon Avolding of bypassing Good wetting
Fluid' flow . . . . . . . . . . . . . . . Mechanical stability
Chemlcal behaviour
Temperature resistance Low cost
and the active sites
, . .
__
Low wessure drop . . . . . . . . . . . Unlform dispersion of the fluid Long lasting and resistance against swelllng ............... forces Low attrition Chemical resistance of the carrier Chemical resistance of the carrier Chemical resistance of the active sites High activity and selectivity Reproducibllity . . . . Temperature resistance of the carrier Temperature resistance of the c a r r i e r
Siml;le and cheap production . . . . . . . . . . . . . . -
~ .
.
.
.
.
.
.
.
.
.
.
.
.
.
_.
_
,
.
.
.
.
.
.
.
.
.
.
.
In the past there were several efforts to prepare catalytic ionexchange packings to fulfil the requirements of reactive distillation processes. Spes /Spe (1966)/ was the first who described polymer ionexchange packings. His samples with several centimeters in diameter were made by embeding commercial lonexchange resins Into a matrix of an Inert polymer. The mechanical stability was reached by sintering. A problem was the temperature control during the slntering process. The additional polymer must not plug the pores of the ionexchange resin. Consequently a strong sintering is not possible with the result that the mechanical stability is low. A similar attempt was done by Fuchigami /Fuc (1990)/. He sintered ionexchange resin in powder form with polyethylene. As a result he prepared pellets with a size of 7 ram. Compared to commercial ionexchange beads this was not a great improvement in regard to the pressure drop in a reactive distillation column. Chaplits /Cha (1976)/ prepared catalytic ionexchange pellets extrudating a mixture of ionexchange powder in combination with polyethylene or polyvinylchlorlde. Yoshioka /Yos (1983)/ described fibers made by melting ionexchange resins with an additional polymer. Woven cloth of this material is suitable as cataJysts. An other method is to dissolve a polymer in a solvent and to coat a carrier CRL /CRL
301
(1981)/. After evaporation of the solvent a film of polymer stays on the carrier. After sulfonation this film can catalyze chemlcal reactlons. The preparation of polymer Raschig,rings was done by Rehfinger /Reh (1988)/. These rings were made by polymerization of a monomer mixture diluted with a pore forming agent to produce a macroporous ionexchange catalyst. The above mentioned methods lead not to catalytic packings which can withstand the severe conditions In a reactive distillatlon column. Their mechanical stability is weak or their catalytic activity is low. Packings made solely of polymer are damaged by swelling and shrinking caused by phase changes of the fluid in the column. Coated carrlers can also not be stable because the different expansion coefficients of the swelling or skrinking polymer and the Inert carrier will lead to crackling of the polymer fllm. Our solution is the combination of the properties of the catalytic polymer and a mechanical strong inorganic support in the form of a distillatlon packlng. Experimental Regularly macroporous lonexchange resins are prepared by suspension polymerization using water as supporting fluid. In addition to the monomer mixture a pore forming agent is included In the suspended droplets. Polymerlzatlon occurs Inside the droplets. At the end of the polymerization reaction the resin beads are removed from the water and sulfonated to creat acid groups on the polymer surface. This method is not suited to prepare an ionexchange resin inslde a ceramic carrier. The dlmenslons of the monomer droplets and the pore dlameter of the carrler are very different. Stirring Inside the carrier is not possible. Our method uses precipitation polymerization. That means a monomer mixture is mixed with a precipitating agent. Properly chosen this substance can also fulfil the task of a pore forming compound. At the start of the polymerization reaction a homogeneous fluid is soaked Into the pores of the carrier. During chaingrowth the solubility of the polymer in the reaction mixture changes. At a certain molecular mass the polymer is Insoluble In the mixture. A solid polymer precipitates inside the pores. The polymer itself is porous because the pore forming compound is added to the initial reaction mixture. After polymerization the carrier filled with polymer Is removed and the pore forming substance is washed of with a solvent. The polymer is crosslinked during polymerization to reach insolubility in the solvent and more important to reach insolubility in the reaction mixture of the reactive distillation process. Then the carrier is dried and sulfonated to create acid groups which are the active sites for the catalytic reaction. The result is a finely dispersed polymer lonexchange catalyst Inside the pores of a mechanical strong carrier. Optional a pretreatment of the carrier Is possible. The polymer can be bonded to the carrier chemicaly by silanes. Example Preparation of a macroporous ionexchange resin based on styrene and divinylbenzene" 98.7 g of styrene is mixed with 12.8 g dtvlnylbenzene. 0.22 g azobisisobutyronttrile (AIBN)is added. After dissolution at room temperature 59.8 g of the pore formlng compound is added. A mixture of n-C14 - Cle aliphatic hydrocarbons is well suited for this purpose. Thls modified baslc mixture for the preparatlon of macroporous lonexchange resins was diluted with additional pore forming agent according to the factorial design
302 (see the result section). After 10 min of stirring the ceramic carrier (Raschig-rings) ts poured into the monomer mixture. The carrier should be covered completly with the monomer mixture. For a time of 10 min a vacuum of 50 mbar is set up in the polymerization reactor to remove the air in the pores of the carrier. Then the reactor is closed and polymerization is started by heating to a temperature of 65 o C. After a polymerization time of 24 h the solidified reaction mass Is removed from the reactor. A sieve is used to separate the rings from the adhering polymer. Then the Raschig-rings are filled Into a soxhlet extractor. During 24 h the pore forming agent is removed with chloroform. After drying the rings at 65 o C they are shaked to remove the polymer in the center hole of the rings. Drying causes a shrinking of the polymer which makes it e a s y to remove the excess polymer in the center hole. The polymer contents of Raschig-rings prepared according to this method are 9 %, assuming a pore volume of the carrier of 55 - 60 % and a dilution of 4. If higher polymer contents are desirable the polymerization procedure may be repeated. Activation by sulfonation 1000 ml of dried chloroform is mixed with 100 ml chlorosulfonlc acid. At room temperature 1000 ml of polymerized carrier is added to the sulfonation solution. Under exclusion of water the reaction mixture Is kept at room temperature for 24 h. Then the mixture is poured into cold water to destroy excess acid. The Raschig-rings are rinsed with water and then washed wlth chloroform. After thls they are filled Into a soxhlet extractor and treated with methanol for 2 0 h. Then they are removed and stored in methanol. Catalysts prepared according to this procedure have ionexchange capacities of 0.5 to 1.5 meq H+/g, depending on the pore volume of the carrier. Optional pretreatment of the carrier Porous Raschig-rings were heated under reflux in a mixture of dry toluene and trichlorovinylsilane or trimethoxyvinylsilane during 5 h. Then the rings were washed with dried toluene and the toluene was removed under reduced pressure. The pretreated rings were used the same way as described above. The flow sheet of the preparation steps Is depicted in Figure 1. The structure of the final catalyst is shown in Figure 2.
[Conditioned , carri'er I
Ceramic Carrier as 1 delivered by supplier I i f Pretreatment: ] Etching, washing, drying j ......
I I
(-Pretreatment: -~'~ ~,~ Silanisation, drying J
-Precipitati0n PolymerizaUon ~'~ Inside the porous structure J
... ,,
I_
[Carrier filled With polymer]
lPretreated carrier I
I
~
'1
!
~Extraction of pore forming agent Macroporous polymer Inside1 ceramic carrier .......
i
Sulfonation, washing, drying Catalytic polymer/ceramic lonexchange packing _
,.
Rgure 1: Preparation steps for catalytic polymer Ionexchange packings
,,,
303
~
Active
~
Acid
Site
~
\
SOH ~S03H~ 3 _~
Sllan~ LIID~ P r i m e r ~
~J
~
~ F'-~(
so3H
.,r"---
I ~
Figure 2: Structure of polymer/ceramic Ionexchange catalyst. 3 scales and different steps of preparation: Surface bonded primer, sulfonated crossllnked chain, bonded macroporous polymer flakes To avoid the difficulty to remove the polymer from the center hole of the Raschig-rings the polymerization reaction can be controlled by the use of an on-line vlscometer. At the moment when the polymer is solidified enough not to leave the pores - but still of low viscosity to leave the hole In the center of the ring - the excess polymer mixture may be poured of. For this purpose a recently developed commercial on-line quartz viscometer was tested /ICV (1993)/. In Figure 3 the increase In viscosity over the reaction tlme is depicted. :1.8
9
,
.
,
.
,
.
,
.
,
.
Figure
....~
3:
Course of viscosity during preclpltation polymerization a t 6 5 ~
C
Composition of the monomere mixture: styrene 11,4 % divinylbenzene 1,5 7. AIBN 0,1% n C14- Cle aliphatic hydrocarbon 8 6,9 7. 0
9
I 4
i
I 8
time of
9
I 12
,
I la
polymeHsatlon
.
I 20
rh3
, 24
304
Results According to the procedure described In the experimental section fife catalyst carriers were tested. The tested carriers were porous glass-ceramlc Raschig-rlngs, a foam glass, a siliconcarbide based foam ceramic, a porous corrugated Sulzer packing and a porous alumina carrier. In addition the Influence of crossllnking compound content (dlvinylbenzene DVB) and contents of pore forming agent in the monomer mixture were Investigated using factorial design. The influence of sulfonation was also tested. In Table 2 the parameter values of the factorial design are summarized.
Table 2: Parameter values of the 3 In various cerarnic carriers Dilution $ 1 2 4
93
94 factorial design of preclpltatlon polymerization
crosslinking compound (DVB) 5 11 7, 20 7.
Polymerization steps 1 2 3 4 ,
Dilutlon $=1 means 1 volume Dilution $=2 means 1 volume Dilution $=4 means 1 volume DVB contents are related to 9
.
of the basic of the basic of the basic the mass of .
.
.
.
.
,
i
,
mixture + 1 volume of pore forming compound. mixture + 2 volumes of pore forming compound. mixture + 4 volumes of pore forming compound. DVB in styrene. .
.
.
.
-
That means 3 93 I 4 = 36 experiments were performed for each carrier. Summarized thls are 36 t 5 = 180 experlments. To save preparation time different carriers were treated in the same polymerization mixture at the same time. In addition this ensures comparability. In Table 3 the ceramic supports, thelr pore diameters and pore volumes are listed.
Table 2: Pore dlsrneter and pore volume of the tested ceramic supports Support Form Mulllt ceramic corrugated packing Raschig-ring Ring, 9mmi9mm Kerapor (AI203) X-form, 2cmI2cmIlcm Glass foam Irregular pieces, 2 cm Siliconcarbid foam Irregular pieces, 2 cm
Pore diameter 0.15 Ilm 60 - 300 wn 0.1 mm 0.2 mm 1.5 mm
Pore Volume 35 ~. 5 5 - 60 X 85 7, 75 ~. 80 ~
Price 1400 DM/I 100 DM/I ? ? ?
In Figure 4 the Influence of divinylbenzene contents, the dilutuion ratio and the steps of polymerization on polymer contents, ionexchange capacity and stability is depicted. As expected the polymer contents rises with the steps of polymerization. The highest polymer contents are reached at low dilution ratios. The course of lonexchange capacity follows this trend. At four steps of polymerization only the samples with a high degree of crosslinklng and high or medium dilution ratios are mechanical stable. Samples with low crosslinking and low dilution ratio have polymer contents that cause high forces when swelling occurs. They are not stable. The test for stability agalnst swelling was treatment with hot methanol for several hours.
305
X
40
4-DV8 b'~; $ - 1 +DVB b'%;$-2
35
~-DVas~,;$-4 e O v a 1 1 ~ $ - 1 ~DVa t1%;$-2 § 11~$-4
== 3 0 * m ~ = o ~ ; = - I
''
'
'
'"
'
2 ;al va ~ ; $-1
'
+ o v a s~; $-2
I-~PDVB5%; $=,4 e'Ova 11%;$-1
,.
~
-)(-OVB11%;$,,2 §
1,5 !.or. $ova - , .~~%; ; $-,
~DVa~o'x.~S-=
11%;$,-4
"~
2~
~ ' ; ="
25
:~
0,5
0
0
1
-
2
Steps
3
4
-
0
1
Of Polymerization
2
3
Steps O f Polymerization
Figure 4: Influence of crossllnklng compound contents ( DVB ), dilution ratio ( $ ) and steps of polymerization on polymer contents, Ionexchange capacity and stablllty for the Raschlg-rings Figure 5 shows the Influence of different carriers on polymer contents, ionexchange capacity and stability at low DVB content and at low dilution ratios. Stable catalysts can only be prepared with carriers of low and medium pore volume and with narrow or medium pores. Supports with narrow pores have limited polymer contents at fewer polymerization steps than those with wider pores. High polymer contents lead not always to high ionexchange capacities. Actlvatron by sulfonatlon may become masstransport limited. Only the core of the polymer contains sulfur. This was confirmed by microprobe measurements in a REM and with staining experiments using methyl-red as acid indicator. This effect is most pronounced for the carrier with the smallest pores. 60
I /
,'~
50
~
/
|
40
o
20: ----/_:: .... y /.o
="
"
9R a m c h l g - R l n g & Foeu~n Glass
1,5
. .+
/11 . ' "
./.."
2
I
X Corrugated Packing
,, /
II
,•
Ra$chlg:-Ring- .....
& Foam Glass
L~
+ Sillconcarblde
-'
o Ke,',,,oo,-
~..L~. ~'-'
..
_
Iti.../ la.-
~ 0,5
."
~.~;,, _~
#
.~'s
0
0
........
-
-
1
2
.....
Steps O f Polymerization
3
4
0
1
Steps
2
3
4
Of Polymerization
Figure 5: Influence of carrier and steps of polymerization on polymer contents, Ionexchange capaclty and stability at low degree of" crossllnklng ( 5 ~ DVB ) and at low dilution ratios ( $ = 1 ) for 5 carriers Stable catalysts with high polymer contents can only be prepared with high degree of crossllnklng at high dilution ratios. This Is deplcted in Flgure 6. With an increase of polymerlzation steps the polymer becomes denser and the sulfonation reaction becomes more and more masstransport limited.
306
50
-
~= 14|'.R=sch,o-R,ng, ~: 1 21&F~ G,us
' '
S ~ _ n S . S r_.Ln,,_U _ n , n~ u, 9 Foam
40 . X
Glass
, -"
Corrugate<J
Packing
--
/-"
I X Co~'rugat~l
/
IrA
~-SIIIconcarblde
~,~ --u
.A"
/
~
/-I
.
9~
..-
_ .... . . . . . . . . .
~
0
i
:2..
( ol
i
1
,
St~ps
P a c k i n g / . . . . - " . ' ~ . ' - ' -
"
-{-SIIIooncaJ'blde
-
- -
OKerspor
"
. . . . . . .
..:~."
n _`'~ -"
"
.... ........
-"
. ---" ~
.~.-
,-/....;.~/
0.
_ .- r
~ . . . .
/
11
J
~ ,~.."...+
.....
2
Of
3
//:"-'"
r- ";-"1.#'. i/.
o
0~"
4
. . . . . . . . . . . -.. _~............ x............ ~ ............
......
'.
.
-0
.
.
.
.
1
Polymerization
.
.
.
.
2
Steps
Of
.
3
Polymerization
Rgure 6: Influence of carrier and steps of p o l y m e r i z a t i o n on polymer contents, Ionexchange capaclty and stabillty at hlgh degree of crossllnklng ( 20 ~ DVB ) and at hlgh dilution ratios ( $ = 4 ) for 5 cerrlers Catalytic testing In addition to the measurement of lonexchange capacities selected catalyst samples were tested for their activity in the etherification reaction of isobutylene with methanol to methyl-tertiary-butyl-ether (MTBE). Two laboratory apparatuses were used. In a first step the activity was investigated in a continous stirred tank reactor. The equipment for these tests was described earlier /Reh (1990)/. Raschig-rlngs (DVB = 11 7., dilutlon $ = 4, polymerlzed to a polymer content of 207.) were used as catalysts at two ratios of methanol 9isobutylene and at two temperatures. Their activity was compared to the commercial ionexchange catalyst Amberlyst 15 (Rhone Poulenc). In Table 4 the results of these experlments are Ilsted. _
_
...... :
. . . . . . . . . . . . . . . . . . . . . . . . .
~.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
. . . . . . . . . . . .
Table 4: Comparison of the catslytlc activlty of polymer/ceramic Ionexchange packings with commercla! Ionexchange resln in a contlnous stirred tank reactor Reaction conditions: 30 ~ isobutyiene-in n'butylene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ionexchange capacity in the reactor volume. 4.1 meq H + Volumetric feed rate: 10 ml/min Particle diameter of commercial catalyst Amberlyst A 15: 0.63-0.8mm Dimensions of polymer/ceramlc Raschig-Ring CVT. 9 t 9 mm ,,
Catalyst
.
nMeOH nlsobutylen
.
.
rMTBE [mmol/soeq]
CVT 1 21 at A 15 1 42.5 at CVT 0.2 12.5 at A 15 0.2 18.4 at - means product was not observed empty spaces mean experiment was
90oc 90oc 90oc 90oc
6.7 at 60oc 7.1 at 60oc
.
.
.
.
.
.
.
.
~
. . . . . . .
~. . . . .
rDime r [mmol/steq]
-
at 90oc
-
at 90oc
1.3 at 90oc 12.6at 90oc
-
at 60~
0.3 at 60oc
not performed
The same catalyst was tested in a laboratory reactive distillation column. The catalyst which was stable under the conditions of the continuous stirred tank reactor had some attrition under the more severe conditions of the column. But this can be avoided by a
307
presintering of the ceramic carrier and/or lower polymer contents. Lower polymer contents are favourable for the MTBE synthesls. At high catalytic actlvlty the back reaction which forms isobutylene will occur in distinct parts of the column. Isobutylene has a lower boiling point than the surrounding liquld phase. The evaluated gasfilm may cause a worse wetting of the catalyst, resulting in lower rates. Compared to a commercial ionexchange resin whlch has 5 tlmes the ionexchange capacity than polymer filled Raschlg-rings the residence time in the reactive distillation column needs not to be 5 tlmes higher. Using this catalyst clean MTBE could be produced, that means selectivity was not changed. Swelling and moving of the catalyst bed was eliminated with this catalyst.
Conclusions The presented new method of precipitation polymerization inside the porous structure of inorganic carriers Is well sulted to prepare stable polymer/ceramlc ion exchange packings for reactive distillation processes. Raschig-rings for disordered packings or corrugated materials for ordered packings can be prepared as well. The polymer contents of the carrier can easily be controlled by successive polymerization steps. The increase in polymer contents is less from step to step, caused by the decreasing free pore volume. High polymer contents do not always mean that the ionexchange capacitiy is high. At high polymer contents the sulfonatlon reaction can become masstransport limited.This was especially observed for supports with small pores. The result is a shell like distribution of active acid sites. High polymer contents are also not favourable in regard to the reaction. High polymer contents only mean high ionexchange capacity. But that does not mean that the reaction rate in the catalytic process is high too /Gat (1992)/. Depending on the swelling behaviour of the reaction mixture the pores of the carrier can be filled completly with polymer, hindering the access of the reactants. The highest ionexchange capacities will be reached by different treatment depending on the pore structure of the carrier. Carriers with large pores, like glass foam, should be treated with a monomer mixture with high contents of crosslinking compound and high contents of pore forming agent. The result Is a polymer with great permanet pores, a high lonexchange capacity and good access for the reactants. Carriers with small pores, like the tested Sulzer packing, should be treated with a reaction mixture of low crosslinking compound contents and low contents of pore forming agent. This results in small permanent pores, at very low contents of crosslinklng compound a macroporous structure can not be formed. Sulfonation and the catalytic reaction will become masstransport limited. In the authors experiments carriers with mean pore diameters of 60 - 300 t~m were a good compromise. The swelling of the polymer inside the carrier Is greatly influenced by the reaction mixture. The swelling forces can crack the Inorganic material. This effect is more pronounced in liquids like alcohols or water. In hydrophobic solvents the polymer contents may be higher, because swelling is less. Catalytic activities are lower than those of commercial Ion exchange resins caused by the inert ceramic carrier. For the MTBE synthesis this is an advantage as explained above. To satisfy the requirements of a special process an optimization has to be done with regard to polymer content, carrier material and swelling behaviour in the reaction fluid. Ackm~edgement The authors thank VEBA OEL AG for the financial support of a part of this work. Thanks
308
to H. Langer and Y. S. Chen who prepared the catalyst samples, to H. KiJnne who performed the catalytic tests in the continuous stirred tank reactor and to K. Sundmacher who tested the catalyst under the conditions of the reactive distillation process. References
/CD (1990)/ = CDTECH, Houston, Texas AIChE Summer Meeting, San Diego, Calofornia, (1990), Aug. 21 /Cha (1976)/ = Chaplits D. N. et al: "Ion Exchange Molded Catalyst and Method of its Preparation", US Patent 3965039 (1976) /CRL (1981)/ = Chemical Research and Licensing Company, US Patent 250052 (1981) /Fuc (1990)/ = Fuchigami Y.: "Hydrolysis of Methyl Acetate in Distillation Column Packed with Reactive Packing of Ion Exchange Resin", J. Chem. Eng. Japan 23 (3), (1990) 35-359 /Gat (1992)/ = Gates B. C.: "Catalytic Chemistry" John Wiley and Sons 1992, ISBN 0-471-51761-5 /ICV (1994)/ = Institut fLir Chemische Verfahrenstechnik and Flucon Fluid Control, 38678 ClausthaI-Zellerfeld, Germany, (1994) /Reh (1988)/ = Rehfinger A.: "Reaktionstechnische Untersuchungen zur FIUsslgphasensynthese yon Methyl-t-Butylether MTBE an einem sauren makropor6sen Ionenaustauscher" Promotion TU Clausthal, 38678 Clausthal, Germany, (1988)
stark
/Reh (1990)/ = Rehfinger A., Hoffmann U.: "Kinetics of Methyl Tertiary Butyl Ether Liquid Phase Synthesis Catalyzed by Ion Exchange Resin: I. Intrinsic Rate Expressions in Liquid Phase Activities", Chemical Engineering Science 45 No. 4 (1990) /Spe (1966)/ = Spes H.: "Verfahren zur Herstellung katalytisch wirksamer offenporiger Formk6rper aus Ionentauscherharzen" Deutsches Patent 1285170, (1966) /Yos (1983)/ = Yoshioka T.: "Studies of Polystyrene-based Ion Exchange Fiber. Novel Fiberform Chelating Exchanger and its adsorption Properties for Heavy Metal Ions", Bull. Chem. Soc. Japan 58, (1985) 2618-2625
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
309
Synthesis of MCM-41 mesoporous molecular sieves O.Franke a, J.Rathousk~b, G.Schulz-Ekloff a and A.Zukal b alnstitute of Applied and Physical Chemistry, University of Bremen, 28359 Bremen, Germany bJ.Heyrovsk~ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolej~kova 3, 18223 Prague, Czech Republic MCM-41 molecular sieves with different chemical composition (silicates, aluminosilicates, titanosilicates) can be prepared by a liquid-crystal templating method with good reproducibility. When an auxiliary organic compound is used in order to enlarge the pore size a tuning of the synthesis procedure is needed.
1. INTRODUCTION
In 1992 the synthesis of mesoporous molecular sieves using a liquid-crystal templating mechanism (LCT) was reported [1,2]. In this mechanism~ the structure of the porous solid is defined by the organisation of surfactant molecules into micelles which serve as templates for the formation of the pores. These silicate and aluminosilicate sieves, designated as MCM-41, have been synthesized with regular, hexagonal array of uniform channels with pore sizes varying from 2 nm to about 10 nm. They seem to be very promising as sorbents for both fundamental adsorption studies [3-5] and separations of large molecules [6,7], and as supports of colloidal dispersions [7]. What is also attractive is the possibility to modify their chemical composition in order to obtain new catalysts [8]. Recent results on silicate MCM-41 have revealed that the liquid crystalline phase is not present in the synthesis mixture during the formation of MCM-41 [9,10]. More likely randomly oriented rod-like micelles interact with silicate species to yield two or three monolayers of silica encapsulating the external surface of micelles. Subsequently, these composite species spontaneously assemble into the longrange ordered structure characteristic of MCM-41. As all factors influencing the LCT mechanism have not been explained fully yet, a further investigation is needed. It may be expected that the nature of the surfactant (including its counterion), its concentration and reaction conditions, such as the way of mixing individual components (especially the way of addition of the auxiliary organic) will be of importance. Our contribution is aimed at the assessment of the synthesis conditions, the determination of critical parameters and their relation with the texture characteristics of prepared materials. Further the limits of the
310
possibilities of tailoring the pore size are explored and simple and cheap procedures for the reproducible preparation of MCM-41 materials are suggested.
2. EXPERIMENTAL 2.1. Description of synthesis procedures As the synthesis of materials with pores considerably varying in their size and differing in their chemical composition (silicates, aluminosilicates, titanosilicate) was aimed at, three procedures were used. Tab.1 and 2 present an overview of samples prepared with procedures used. As concerns the chemical composition of sieves, SIMS, AIMS and TiMS stand for a silicate, aluminosilicate and titanosilicate, respectively. The number after the hyphen corresponds to the serial number of the sample in laboratory records. All aluminosilicate sieves contained 3.1 tool % of AI203. The content of Ti in titanosilicates is given in Tab. 1 (determined by AAS). Procedure I The reaction mixture was prepared as follows: 0.31 g of AI(OH)3 (98%, J.T.Baker), 0.3 g of sodium hydroxide (p.a., Janssen) and 1 g of deionized water were put into a 60 ml glass beaker and brought to the boil under stirring until a clear solution resulted. Then 9.26 g of tetraethylammonium hydroxide solution (TEAOH, Merck, 20 wt % solution in water) were added and the solution was cooled. In a separate 400 ml polypropylene beaker 9.26 g of Ludox AS-40 (Du Pont, 40 wt % colloidal silica in water) were stirred with a magnetic stirrer (at ca 600 rpm). These two mixtures (sodium aluminate and silica suspension) were combined at room temperature by adding the aluminate solution to the silica suspension. The gel was stirred for 5 min (to achieve good homogeneity the agitation rate of up to 1000 rpm may be needed), mixed with ca 11 g of the 25%:water solution of surfactant (see Tab.l) or, with AIMS-63, ca 22 g of the 25% water solution of octyltrimethylammonium bromide (all surfactants Fluka), and stirred for another 5 min (500 rpm). Then the gel was reacted with stirring (150 rpm) in a 250 ml polypropylene autoclave at 104oc for 24 hours. The resulting solid product was recovered by filtration, washed with water, extracted with ethanol for 4 hours in a Soxhlet apparatus and finally calcined in air at 600oc for 22 hours. The reaction mixture of samples corresponded to an oxide molar ratio of 1 AI20 3 :31.45 SiO2 : 3.21 ('rEA)20 : 1.66 Na20 : 661 H20. The chemical nature and the oxide molar ration of surfactant used are given in Tab.1. With the synthesis of silicate samples the sodium aluminate solution was omitted and the solution of tetraethylammonium hydroxide was added directly to the silica suspension. Except for the absence of the aluminate component, all other steps of the procedure and component ratios were preserved. Procedure II The reaction mixture was prepared as follows: 19.26 g of Ludox AS-40 (Du Pont, 40 wt % colloidal silica in water) were put into a 400 ml polypropylene beaker and stirred by a magnetic stirrer at the agitation speed of ca 1200 rpm. Afterwards,
311
18.52 g of tetraethylammonium hydroxide solution (TEAOH, Merck, 20 wt % solution in water) and subsequently 16 g of hexadecyltrimethylammonium chloride (HDTMACI, 25% water solution, Fluka, 1/3 of the total amount) were added. To the gel formed the other 2/3 of the surfactant and an appropriate amount of tetrabutyl orthotitanate (TBOT, p.a., Merck, 3.6, 1.8 and 0.9 g with TiMS-3, TiMS-6 and TiMS--5, respectively), diluted with 0.9 g of 2-propanol (p.a., Merck), were added simultaneously. All vessels, except that with TBOT, were washed with 10 g of deionized water to achieve a quantitative transfer of reagents. As the vessel with TBOT could not been washed with water because of the hydrolysis, extra 0.05 g of TBOT were always added to make up for the losses. All components were cooled down to 10oc in an ice bath and also the gel was formed at the same temperature. The gel was agitated for 1 min and then reacted with stirring (150 rpm) in a 250 ml polypropylene autoclave at 104oc for 24 hours. The resulting solid product was recovered by filtration and treated as in the procedure I. The reaction mixture corresponded to an oxide molar ratio of 32.94 SiO 2 3.3 9 (TEA)20 4.72 9 (HDTMA)20 1021 9 H20. The molar ratio of (TBO'I')2 was 1.27, 0.63 and 0.32 with TiMS-3, TiMS-6 and TiMS-5, respectively. Procedure III
The reaction mixture was prepared as follows: 0.62 g of AI(OH)3 (98%, J.T.Baker), 0.6 g of sodium hydroxide (p.a., Janssen) and 1.5 g of deionized water were put into a 250 ml glass beaker and brought to the boil. After a clear solution resulted, 18.52 g of tetraethylammonium hydroxide solution (Merck, 20 wt % solution in water) were added and the solution was cooled. Then 22 or ca 50 g of the 25% water solution of hexadecyltrimethylammonium chloride (Fluka, as regards the amount of surfactant see Tab.2) were added. Aluminate, which precipitated at first, dissolved again on warming. In a separate 400 ml polypropylene beaker 18.42 g of Ludox AS-40 (Du Pont, 40 wt % colloidal silica in water) were agitated with a magnetic stirrer (at ca 500 rpm). The two mixtures (sodium aluminate and silica suspension) were combined at room temperature by adding the aluminate solution to the silica suspension. Immediately a gel formed which was stirred for 30 min (at 500 rpm). Then an appropriate amount of auxiliary organic additive (hexadecane, toluene, mesitylene, all Merck) was added and homogenized for 1 min at the agitation rate up to 1000 rpm. Later on the gel was reacted with stirring (150 rpm) in a 250 ml polypropylene autoclave at 104oc for 4 hours. The resulting solid product was recovered by filtration and treated as in the procedure I. The reaction mixture of samples corresponded to an oxide molar ratio of 1 AI20 3 : 31.45 SiO 2 : 3.21 (TEA)20 : 1.66 Na20 : 661 H20. The molar ratios of (HEX)2, (TOL)2, (MES)2 and (HDTMA)20 are given in Tab.2. 2.2. Methods Powder X-ray diffraction data were obtained on a Seifert 3000 P diffractometer in the Bragg-Brentano geometry arrangement using CoK(z radiation with a graphite monochromator and a scintillation detector. The adsorption isotherms of nitrogen (Linde, purity of 5.6) at -196oC were meas-
312
Tab.1 Effect of synthesis conditions on the porous structure of MCM-41 samples Sample (Surfactant)20
x
Procedure I SIMS-60 2.21(C12Br ) SIMS-48 2.21(C16CI)
-
AIMS-63 AIMS-49 AIMS-1 AIMS-13 AIMS-3 AIMS-9
-
4.85(C8Br) 2.17(C12Br) 2.17(C16OH) 2.17(C16CI) 2.17(C16Br) 2.17(C18OH)
-
-
Procedure II TiMS-3 4.72(C16CI) 0.089 TiMS-6 4.72(C16CI) 0.043 TiMS-5 4.72(C17~CI) 0.022
S 131ET (m2/g)
Vme (cm3/g)
Dme (nm)
d 1oo (nm)
ao (nm)
832 858
0.409 0.452
1.96 2.14
2.95 3.05
3.40 3.52
411 879 1072 902 971 1059
0.139 0.452 0.807 0.706 0.779 0.780
(1.36) 2.06 3.02 3.14 3.22 2.94
(3.13) 3.18 3.58 3.95 4.11 3.68
(3.61) 3.67 4.13 4.56 4.76 4.25
880 827 770
0.482 0.712 0.621
2.18 3.44 3.22
3.51 3.80 3.86
4.05 4.39 4.46
x=TiO2/(TiO2+SiO2), molar fraction of TiO 2
Tab.2 Effect of the addition of an auxiliary organic compound (AOC) on the porous structure of MCM-41 samples Sample (Surfactant)20 (AOC)2 Mole AOC/ SET Vme mole surf. (mE~/g) (cm3/g)
Dme (nm)
dl00 (nm)
(nm)
Procedure III AIMS-31 2.21(C16CI) AIMS-30 2.21 (C16CI) AIMS-29 2.21(C16CI) AIMS-20 2.21(C16CI)
3.06 3.52 3.18 4.00
4.42 4.60
5.10 5.32
AIMS-39 AIMS-36 AIMS-23 AIMS-22
5.01(C16CI) 5.01(C16CI) 4.91(C16CI) 4.91(C1~CI)
2.20(HEX) 2.20(TOL) 2.21(MES) 3.41(MES)
1.00 1.00 1.00 1.54
859 821 932 1015
0.657 0.696 0.742 0.987
1.70(MES) 4.11(MES) 6.82(MES) 10.23(MES)
0.34 0.82 1.36 2.04
667 510 295 117
0.527 0.439 0.299 0.119
no no no no
ao
MCM-41 MCM-41 MCM-41 MCM-41
CsBr = octyltrimethylammonium bromide, C12Br = dodecyltrimethylammonium bromide, C16OH = hexadecyltrimethylammonium hydroxide, C16CI = hexadecyltrimethylammonium chloride, C16Br = hexadecyltrimethylammonium bromide, C18OH = octadecyltrimethylammonium hydroxide, HEX = hexadecane, TOL = toluene, MES = mesitylene
313
(a)
(b)
or} Z LU
z
=.,..
0
4
8 1"2 2 THETA
1"6
20
6
4
12 2 THETA
..8,
1"6
20
Fig.1 X-ray diffractograms of AIMS-1 (a) and TiMS-6 (b)
ured with an Accusorb 2100E instrument (Micromeritics). The temperature of the liquid nitrogen bath was checked by a thermistor probe. Each sample was degassed at 330oc for at least 20 hours until a pressure of 10 -4 Pa was attained. The content of titanium was determined by AAS (PU 9200, Philips). Prior to analysis samples were dissolved in a mixture containing 1/3 of HF and 2/3 of HNO 3.
3. RESULTS
Powder X-ray diffraction patterns of samples AIMS-1 and TiMS-6 are shown in Fig.l. The distinctive characteristic of these diffractograms is that they exhibit reflections only at small angles 20 (<6o). From the comparison with published data [2] it follows that they can be indexed on a hexagonal lattice typical of MCM-41. Broadness of the (100) reflection might be attributed to a certain distribution of pore sizes in the MCM-41 materials. The XRD dlo 0 spacing and the lattice constant a o = = 2.d100 / q3 of samples prepared are given in Tab.1 and 2. Fig.2 shows the adsorption isotherms of nitrogen on representative samples AIMS-63, TiMS-6 and AIMS-20. The part of isotherm up to the relative pressure of 0.2 with AIMS-63, 0.5 with TiMS-6 and 0.7 with AIMS-20 corresponds to the filling of the MCM-41 porous system proper. At higher relative pressures the capillary condensation in larger pores (cracks in crystals, intercrystallite spaces in aggregates) occurs, which causes the adsorption hysteresis. The detailed examination of the MCM-41 porous structure was based on analysis of adsorption branch of isotherms using the method of comparison plots [11]. In this method the amount adsorbed on the solid under investigation (a) is plotted against that adsorbed on a reference adsorbent (a*) at the same equilibrium pressure. In this study the adsorption in the pores of MCM-41 was compared with that on the open flat surface of the reference solid material. It was prepared by the thermal destruction of corresponding MCM-41 samples at 1000oc for 2 hours. As these reference adsorbents had different surface area the adsorption on them was
314 16
16. (a)
12
12
'! 8~
f.O0
012
014
016
018"
I
0
4
9
6
~2
x 25
25.
20
20.
15
15
a*
12
"
20
18
2.
3o
32
40
10 5 O0
0'12
014
016
018
1
x 40. 30
a*
40 (c)
30
1 j.
/...
'01 .......-""
110
0.2
0.4
0.6 x
0.8
1
O0
8
1"6
24 a*
Fig.2 Adsorption isotherms of nitrogen at -196oc on AIMS-63 (a), TiMS-6 (b) and AIMS-20 (c) and corresponding comparison plots solid points, desorption; x=p/p o, relative pressure; a, adsorption on the sample under investigation (mmol.g-1); a*, adsorption on the reference solid (l~mol.m -2)
315
related to unit surface area. From the comparison plots obtained distinctive characteristics of adsorption on MCM-41 materials can be derived. The initial direct proportionality between a and a* can be ascribed to the monolayer-multilayer adsorption on the pore walls. The small pores of AIMS-63 are filled completely by this process, which manifests itself by a knee in the comparison plot (Fig.2a). With larger pore samples, such as TiMS-6 and AIMS-20, capillary condensation occurs, which causes a steep upward swing passing gradually into a plateau. Depending on the pore size this upward swing may occur in the reversible (Fig.2b) or irreversible (Fig.2c) parts of isotherm. The porous structure of MCM-41 materials was characterized by their BET surface area, mesopore volume and diameter (Tab.1 and 2). The mesopore volume Vme was obtained as the adsorption (converted to liquid volume) corresponding to the filling of the MCM-41 porous system proper. The external surface area of crystals was estimated from the desorption branch of the hysteresis loop as the surface area of pores larger than the MCM-41 pores proper. The size (diameter) of mesopores was calculated using a tubular model as 4Vme/S where S stands for the BET surface area minus the external surface area. Apart from AIMS-63, the surface area of all samples prepared varies from ca 800 to ca 1000 m2/g, being typical for MCM-41 materials. The exceptionally smalll surface area of AIMS-63 is probably caused by the lower crystallinity, which is confirmed by the lower intensity of the (100) peak. Together with the pore diameter Dme calculated from the adsorption data, the pore size was also characterized by the lattice constant ao. The difference between ao and Dine should give the mean thickness of the pore walls. From Tab.1 and 2 it is clear that it varies between 1 and 2 nm, which is a reasonable value. 4. DISCUSSION
The following discussion is based both on syntheses presented in Tab.1 and 2 and a number of partly unsuccessful attempts, which have shown the limits of the formation and variation of the MCM-41 structure. t
4.1. The effect of the nature and concentration of surfactants The experiments have proved that the length of the alkyl chain of the surfactant plays a decisive role in determining the pore size but only up to C16, namely 1.36 nm with AIMS-63 (C8) vs 3.22 nm with AIMS-3 (C16). However, a further increase in the alkyl chain length to 18 carbon atoms has not caused any additional increase in the pore size. The porous system of MCM-41 products was slightly influenced by the nature of the counterion (OH-, CI-, Br-). The differences in the critical micelle concentration, counterion binding and aggregation number with systems using hydroxide or halide counterions are subtle and probably overshadowed by the effect of large concentrations of electrolyte. Because of the slight influence of the nature of the counterion there is no worth using the mixture of halide and hydroxide as is suggested in the original patent (cf. [1]), which simplifies the synthesis procedure.
316
As concerns the concentration of the surfactant, the optimum value equals ca 4.4 molecules per one molecule of AI203. With higher concentrations of surfactant (such as 10 molecules per one molecule of AI203) the channel structure forms but some problems may arise. With AIMS-63 a greater concentration of the surfactant (octyltrimethylammonium bromide) was needed because of its much higher solubility in comparison with surfactants containing longer alkyl chains.
4.2. The role of other parameters Other changes in the composition of the reaction mixture have only a very slight influence. For example, in contrast with most zeolites, the change of KOH for NaOH or TMAOH for TEAOH did not effect the synthesis. Of importance is probably only the total concentration of OH- anions. Other requirements on the synthesis procedure are not especially strict. Only a too intensive stirring can interfere with the LCT mechanism. 4.3. The chemical composition of the mesoporous product MCM-41 structures can be synthesized with differing chemical compositions. A substantial restriction, however, has been found, viz the content of silica in the material of which the walls are composed must not decrease under a certain limit (SiO2/AI20 3 _> 18). The pure silicate can be also prepared ('lab.1). The attempts to synthesise aluminophophates have failed. The pore size was found to be influenced by the MCM-41 chemical composition (2.1 nm with silicate SIMS-48 vs. 3.1 nm with aluminosilicate AIMS-13). This phenomenon has not been explained yet. What is very promising is the possibility to synthesize large-pore titaniumcontaining molecular sieves. The crux of the synthesis of these mesoporous sieves is to avoid the hydrolysis of the titanium component (such as tetrabutyl orthotitanate) to large titania particles. The performed syntheses of mesoporous titanosilicates have proved that the hydrolysis of TBOT to larger titania particles can be prevented. What is a striking feature of titanosilicates is the similarity of their structure parameters with either aluminosilicates AIMS-l, -13 and -3 or silicate SIMS-48. From the similarity between structure characteristics of samples TiMS-5 and TiMS-6 and, e.g., AIMS-3 it follows that these titanosilicates must contain the major part of titanium built in the pore walls. The similarity between structure characteristics of TiMS-3 and SIMS-48 clearly shows that the composition of the pore walls in these materials must be practically identical, i.e. the pore walls of TiMS-3 are titanium-free. Therefore titanium must be present as highly dispersed extra-wall oxide species. As the reaction mixture of TiMS-3 contains a substantially higher concentration of TBOT at the beginning of the synthesis than those of other two samples the formation of extra-wall oxide species is more likely. These conclusions about the state of Ti are also supported by the UVNIS spectra [8]. 4.4. Widening of pores using auxiliary organic compounds This procedure is much more complicated and problematic than the synthesis without AOC. First the requirements which a suitable compound must meet are
317
rather restrictive. It should be low volatile and stable under the demanding conditions of the synthesis (temperature of 100-110oc, pH of ca 13). Further it must be sufficiently soluble in the micellar core. Based on these requirements and on the known dependencies of the solubility on their chemical nature (especially the reduction in the solubility with increasing the chain length of an n-alkane or n-alkylsubstituted benzene, their increase with compounds containing unsaturated bonds or cyclic structures), four compounds have been chosen, i.e. mesitylene, toluene, hexadecane and dibutylphtalate. In the first three rows of Tab.2 the influence of the nature of the AOC is shown. Mesitylene, toluene and hexadecane were found to swell surfactant micelles effectively and are therefore usable as AOC. With dibutylphtalate no MCM-41 formed. The decisive factor in the synthesis is the order of individual steps of the procedure. The AOC must reach the micellar core during the hydrothermal synthesis. If it is mixed with the surfactant solution before the hydrothermal synthesis starts, a substantial widening of micelles has been observed (accompanied by an increase in viscosity). During the hydrothermal synthesis, however, the widening of micelles disappears and no increase in the pore size of MCM-41 materials has been observed. Therefore the AOC must be added to the reaction mixture as the last component. Another difficulty is connected with the very low solubility of AOCs used in water. In order to increase the transport of their molecules, an intensive stirring is unavoidable. Without stirring the widening of MCM-41 pores cannot occur. On the other hand, a too intensive stirring (>400 rpm) will prevent the formation of the MCM-41 structure at all. From the comparison of two series of samples prepared using different surfactant concentrations it follows that also this parameter plays an important role. The samples prepared using a higher concentration of surfactant than optimum (ca 10 molecules vs 4.4 molecules per one molecule of AI20 3) are characterized by a decreasing surface area with the increasing content of AOC and no MCM-41 structure forms.
5. CONCLUSIONS 1. Silicate and aluminosilicate MCM-41 molecular sieves can be prepared by a liquid-crystal templating method with good reproducibility. 2. The performed syntheses of mesoporous titanosilicates have proved that the hydrolysis of the titanium component to larger titania particles can be prevented. 3. When an auxiliary organic compound is used in order to enlarge the pore size a number of different parameters must be optimized.
Acknowledgement The authors are grateful to the Volkswagen Foundation for financial support (Grant 1/69 159).
318
REFERENCES
1. C.T.Kresge, M.E.Leonowicz, W.J.Roth, J.C.Vartuli and J.S.Beck, Nature, 359 (1992) 710 2. J.S.Beck, J.C.Vartuli, W.J.Roth, M.E.Leonowicz, C.T.Kresge, K.D.Schmitt,C.T.W.Chu, D.H.Olson, E.W.Scheppard, S.B.McCullen, J.B.Higgins and J.L. Schlenker, J.Am.Chem.Soc., 114 (1992) 10834 3. O.Franke, G.Schulz-Ekloff, J.Rathousk~, J.St&rek and A.Zukal, J.Chem.Soc., Chem. Commun., (1993) 724 4. P.J.Branton, P.G.Hall and K.S.W.Sing, J.Chem.Soc., Chem.Commun., (1993) 1257 5. D.Akporiaye,E.W.Hansen, R.Schmidt and M.StScker, J.Phys.Chem., 98 (1994) 1926 6. P.Behrens,Adv.Mater., 5 (1993) 127 7. P.Behrens, G.D.Stucky, Angew.Chem., 105 (1993) 729 8. O.Franke,J.Rathousk~, G.Schulz-Ekloff, J.St~rek and A.Zukal, in J.Weitkamp, H.G.Karge, H.Pfeifer and W.H5lderich (eds.), Proc.10th Int.Zeol.Conf., Garmisch-Partenkirchen, 1994, Elsevier, Amsterdam 9. C.-Y.Chen, H.-X.Li and M.E.Davis, Microporous Mater., 2 (1993) 17 10. C.-Y.Chen, S.L.Burkett, H.-X.Li and M.E.Davis, Microporous Mater., 2 (1993) 27 11. Gregg, S.J. and Sing, K.S.W. Adsorption, Surface Area and Porosity, Academic Press, London,1982, p. 94
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
319
Preparation of Spherical and Porous SiO2 Particles by F u m e Pyrolysis N. Kakuta, T. Tanabe, K. Nishida, T. Mizusima, and A. Ueno a Department of Materials Science, Toyohashi University of Technology, Tempaku, Toyohashi, Aichi Pref. 441, Japan
aFaculty of Engineering, Shizuoka University, Johoku, Hamamatsu, Shizuoka Pref. 432, Japan
ABSTRACT Porous SiO 2 spheres were prepared successfully by means of fume pyrolysis using gel solutions derived from Si-alkoxides. Particle sizes in all samples were about 1.5 IJm large but surface areas varied from 20 m2.g -1 to about 500 m2.g -1 with viscosities of source solution. Pores in SiO 2 spheres were formed with almost the same size and the mean pore sizes were controlled in the range of about 2.2 nm to 0.7 nm in radii. The pore size and surface area were closely connected with the preparation condition and it was proved that the properties of formed SiO 2 were associated with the gel structures in the fumed droplets and the combustion conditions under 0 2 stream.
INTRODUCTION A preparation of designed catalyst is one of the interest subjects to understand the catalysis. Efforts have been paid for the development of unique preparation method[I]; those are metal cluster catalysts derived from metal carbonyls, tailored metal catalysts through organometallic processor and ultra-fine metal particle catalysts prepared by metal alkoxides, etc. These preparation methods are mainly concentrated to design the active sites on support surfaces. However, the property of support itself is also a dominant factor in order to conduct smoothly the catalytic reaction. It is known that some supports are valuable for the improvement of selectivity. For example, zeolites are often used as catalysts and supports for their regular pore structures which act effectively for the shape selective reaction[2]. In order to understand the property of support, the following factors can be pointed out besides the pore structure: structure, shape, surface area, pore size, acidity, defect, etc. Since these are strongly correlated to the preparation procedure, lots of preparation techniques, therefore, have been proposed, too. Studies have been still continued to discover the preparation method of novel materials as well as zeolites[3]. A sol-gel process is one of the unique preparation methods for inorganic materials.
320 We have employed this method to prepare a highly dispersed metal support catalyst and ultra-fine particle support catalysts were obtained[4]. Recently, a fume pyrolysis with the sol-gel technique was examined to prepare aluminum oxide powders from boehmite sol suspension derived from AI-alkoxide. Aluminum oxide was spherical shape and crude structure. In addition, the aluminum oxide possessed a high thermal resistance at 1473K as high as 50 m2g -1 in surface area[5]. The fume pyrolysis method was also an effective preparation for thin films composed of mixed oxides because one of the advantages is that the atomic composition in source solution is the same as the formed oxide particles. A Y1Ba2Cu307-x film was successfully prepared[6]. These suggest that the combination of the fume pyrolysis and the sol-gel method seem to be an interesting preparation method. In order to confirm the possibility as the new preparation method in designed catalyst supports, the fume pyrolysis with the sol-gel method was attempted to prepare several SiO 2 spheres under deferent conditions. The effects of hydrolysis process of Si-alkoxide and the dependence of fume temperatures were also investigated. Deferent Si-alkoxides were employed to study role of functional group with respect to the generation of pores.
EXPERIMENTAL Solution Preparation A gel solution was prepared by a hydrolysis of tetraethyl orthosilicate(TEOS; Si(OC2H5)4). The solution, consisting of distilled water, TEOS and HNO 3 which was added water I I o ~ / ~ ~ 0.02 times of TEOS concentration, oollo ~ ~"X'~ was heated up to 80~ and stirring under atmosphere. With stirring time, the viscosity of the solution increased, measured at 25~ by a rotating viscometer(Tokyo Seiki Co., Visconic E.L.D.). The concentration of TEOS in the solution was kept to 0.4 mol.liter -1 in all experiments. It may be noted that these 3 components have to be mixed at room temperature and be heated simultaneously up to 80~ Otherwise, if other combinations (TEOS and HNO 3 are 02gas added at 80~ to distilled water or HNO 3 is poured at 80~ to the mixed solution of TEOS and distilled Figure 1. Apparatus used for fume pyrolysis water) are chosen, the properties
! 0(~o~_.~ ~ gl~stShteUrbm e ocouple
I! r
mos,.,
II
e'='!ceL
quartztube1 flowmeter
solution
Ziontroller
vibrator
321
of SiO 2 sphere are not reproducible, even though the viscosity is the same. This means that the gel structures are correlated sensitively with the gelation process, indicating that the preparation process in the sol-gel method shpuld be handled carefully.
SiO2 formation The apparatus employed for a fume pyrolysis has been described previously[5]. Briefly, it consists of two parts; a reservoir for the gel solutions at room temperature, equipped with a supersonic vibrator(1.5 MHz) at the bottom to generate fumes into a reactor with flowing 0 2 at the rate of 15 liter.min -1 . Another part is the reactor made of quartz tube, equipped with three heaters at the top, middle, and bottom to give an appropriate temperature distribution. The electric furnace is 600 mm in length and 45 mm in diameter. Fumes generated were instantly( passing time was less 2sec) burned out and shrunken in the reactor to yield silica particles, which were collected in a distilled water. These collected silica particles were dried and calcined at 500 ~ for 4h. The diagram of theapparatus is illustrated in Fig. 1. Characterization of SiO2 sphere The calcined sample were submitted to a transmission electron microscope(TEM, Hitachi, H-800), operated at an accelerating voltage of 200 KV and a magnification of xl05. Silica particles were suspended in ethanol with the aid of a supersonication. Some of finest part of the suspension was pipetted on to a microgrid for TEM. The change in the pore size distributions of the micropores was examined by the isothermal desorption of N2 in adsorption/desorption measurements at its liquid temperature. The pore diameter and pore volume were calculated on the basis of the Barrett-Joyner-Halenda(BJH) method. The surface areas were also measured by BET method.
Results and Discussion
Change in viscosity of solution Figure 2 shows the change in the viscosity of ~ 5 . 0 the solution at 80~ with the stirring time. The vis>, cosity of solution increases slightly after the ~ 2.5 stirring for 4 h and then ino creases rapidly just after ~ , prolonged stirring for 8h. Several viscous solutions 0 1 2 3 4 5 6 7 8 9 in the range of about 1 c.P. to 3 c.P. were used for the Time(h) fume pyrolysis. Since visFigure 2. Change in viscosity of solution cosities in all source solu-
322
tions were maintained before and after the fume pyrolysis, it can be presumed that the gel structures are held during experiments. Highly viscous solutions were eliminated in this experiment because of the poor fume generation. TEM observation
A transmission electron micrograph of a typical eO SiO 2 sample calcined at 60 500 ~ for 4 h is given in v 40 Fig. 3 together with its 8 particle size distribution 3o calculated by the collec2O tion over 200 points in TEM pictures. This indicates that all SiO 2 parti2.0 4.0 6=.0 1pro -~ 0 8.0 Particle size (pro) cles are almost spherical in shape. And their sizes seem to be uniform. The Figure 3. TEM photograph and particle size distribution mean particle size was estimated to be 1.5 pm in diameter. It was observed that the viscosities had no effect upon the particle sizes, indicating that the size of SiO 2 sphere might be related to the droplet size generated. In addition, when metal alkoxides were employed as source materials, all SiO2 particles formed were always spherical in shape. The polymerization structures produced by hydrolysis presumably play important roles in the formation of spherical shape. L
"T
a'l.2c.P. b'l 7c P c ' 2 l c P.
E
d ' 2 7 c.P
400 !
_
L
04"T
"T 4 0 0
.,...
d
E
"o (!.) j:~
(0 L
O "{3
O
'~ 2 0 0
co 1::
0~
E
=
200
0
1.0
PIP0 Figure 4a. Nitrogen adsorption isotherm of SiO2
0
//
0
0.5
v
E
<E
0
E
A
v
-0.2
I
1.0
1
I
2.0 Viscosity ( c P )
_
0 Q.
O
3.0
Figure 4b. Changes in BET surface area and pore volume with viscosity of solution
323
Characterization by N2 adsorption isotherm Figure 4a depicts the result of N2 adsorption isotherms a:l.2c.P. of SiO 2 prepared from various b:l.7c.P. viscous solutions of 1.2, 1.7, 4.0 c:2.1c.P. 2.1, and 2.7 c.P., respectively. d:2.7c.P. BET surface areas and pore volumes of these SiO 2 are given also in Fig. 4b. The isotherms ascribed to Type IV are > obtained in all samples and the 2.0 hysteresis loops are clearly realized with the increase of viscosity. BET surface area and pore volume vary in proportion to the viscosity. Since the theoretical surface area based on the mean particle size is es0 2.5 5.0 7.5 10.0 timated to about 3 m2.g -1, the Rp (nm) porous structure of SiO 2 sphere might be enhanced with Figure 5. Pore size distribution of SiO 2 change in the viscosity. obtained at 850 ~ Relation between the pore size distribution and the viscosity is shown in Fig. 5. The distribution profiles in all sama:l.9c.P. ples are almost the same; two peaks at about 2.0 and 2.2 nm b:2.6c.P. 8.0 b in radii. The pore with radii of c'3.0c.P. 2.2 nm preferentially generates a in the relatively low viscosities corresponding to 1.2, 1.7, and 2.1 c.P., respectively. On the other hand, when the viscosity increases to 2.7 c.P., the small pore corresponding to the first peak is predominant, indicating that the pore size in SiO 2 2O sphere is correlated with the viscosity of source solution if the same fume temperatures are the same. 0 2.5 5.0 7.5 Rp (nm) Figure 6.
Pore size distribution of SiO 2 obtained at 750 ~
324
Effect of fume temperature The fume temperature was changed to 750 ~ from 850 ~ The pore size distribution curves, prepared from three viscous solutions of 1.9, 2.6, and 3.0 c.P., respectively, are illustrated in Fig. 6. The profiles in all samples, which is one sharp peak with small shoulder, are almost the same but the shape is clearly different from the result shown in Fig. 5. The mean pore size is estimated to about 0.7 nm in radii and is 2.8 times smaller than that obtained at 850 ~ Both results are summarized in Table 1.
Table 1 Properties of SiO 2 sphere Fume Viscosity Temperature(~ (c.P.) 1.2 850 1.6 2.0 2.7 1.9 750 2.6 3.0
Surface area (m2.g -1 ) 68 163 229 350 378 448 490
Pore volume (ml.g -1 ) 0.12 0.21 0.33 0.44 0.21 0.23 0.28
Mean pore radii (nm) 2.0, 2.2 2.0, 2.2 2.0, 2.2 2.0, 2.2 0.7 0.7 0.7
In comparison with the results obtained at 850 ~ the surface area increases but the pore volume decreases at the similar viscosities. These might be due to the difference of pore size. The enhancement of surface area at 750 ~ can be explained that a large number of small pores are distributed into the SiO 2 sphere and the new exposed surfaces, therefore, increase with increasing the number of pores. On the other hand, the pore volume can not be expected to increase drastically such as the surface area because the pore radii is too small to enhance the pore volume, even though numerous pores are present as compared with those of the SiO 2 sphere prepared at 850 ~ The reason of the small pore formation is that the transformation to SiO 2 sphere from fumes containing Si-O-Si polymers presumably is performed mildly. In other words, the evaporation of liquid phase during pyrolysis might not occur explosively but relatively smooth at 750 ~ resulting that the pore becomes small in size. In order to confirm this assumption, fume pyrolysis was carried out at 500 ~ The mean pore size was estimated to be less than 0.5 nm in radii and the enhancement of surface area was realized in the relatively low viscosity region of 2 c.P. to 1 c.P.[7]. These results supports above assumption, indicating that the fume temperature is closely correlated to the generation of pores.
325
Effect of Si alkoxides In this part, other two Si-alkoxides, tetramethyl orthosilicate (TMOS; Si(OCH3) 4) and tetrapropyl orthosilicate(TPOS; Si(OC3H7)4), were employed for fume pyrolysis to study the effect of the functional groups(methyl, ethyl, and propyl). The source solutions were prepared as described in experimental part and both viscosities were adjusted to about 2.0 c.P. The fume pyrolysis was performed at 850 ~ The pore distribution curves are given in Fig. 7. The result of TEOS also is listed, for comparison. No significant differences are observed c with respect to the mean a: TMOS pore sizes in these SiO 2 spheres. This seems that the b" TEOS sizes of free alcohols do not C" TPOS effectively act as a template on the pore generation. This 4.0 suggests that it is necessary to pay attention to water molecules which are pre5 a dominant in the source solution. The concentration of > Si-alkoxide is very thin, the mole ratio of H20/TEOS is 2O 120/1, in comparison with common sol-gel process for powder production, resulting that the solution density can , be assumed to be the same as that of H20. 1 i I I As mentioned above, the 2.0 4.0 small change in viscosity Rp (nm) provides the large change in Figure 7. Pore size distribution of SiO 2 surface area under the same obtained at 850 ~ fume temperature as described previously. The surface area seems to be related with the number of pores and the pore generation might be dependent on the micro polymerization structure of Si-alkoxide in solution. The micro structures can be assumed that alcohol and water clusters is presumably incorporated in the polymer structure made of (Si-O-Si)n networks. The amounts of theses encapsulated clusters might be proportional to the viscosity because the (SiO-Si)n network sizes become greater with the degree of polymerization and the capacity for incorporation also increases. It is presumed that these macro structures corresponding to the viscosities effectively act to the generation of pores through the evaporation of liquid phase under several fume temperatures as described in previous part. Consequently, it can be pointed out that the elucidation of the macro structure containing liquid phases and the instant evaporation mechanism at high temperatures is the next subject to design successfully the SiO 2 sphere by means of
326
fume pyrolysis method. Further investigations are continuing and results will be published in the future.
REFERENCES
1. Y. Iwasawa, Advances in Catalysis, 35(1987)187; M. Che and C. O. Bennett, ibid., 36(1989)55; M. Ichikawa, ibid., 38(1992)283. 2. G. Ohlmann, H. Pfeifer and R. Fricke(eds.), Catalysis and Adsorption by Zeolite, Studies in surface science and catalysis vol. 65, Elsevier, Amsterdam, 1991. 3. T. Inui, S. Namba and T. Tsumi(eds.), Chemistry of Microporous Crystals, Studies in surface science and catalysis vol. 60, Elsevier, Amsterdam, 1991; K. Ishizaki, L. Sheppard, S. Okada, H. Hamasaki and B. Huybrechts(eds.), Porous Materials, Ceramic Transactions vol. 31, The American Ceramic Society, Ohio, 1993. 4. A. Ueno, H. Suzuki and Y. Kotera, J. Chem. Soc. Faraday Trans 1, 79(1983)127; K. Tohji, Y. Udagawa, S. Tanabe and A. Ueno, J. Am. Chem. Soc., 106(1984)612 ; K. Ishiguro, T. Ishikawa, N. Kakuta, A. Ueno, Y. Mitarai and T. Kamo, J. Catal., 123(1990)523. 5. T. Ishikawa, R. Ohashi, H. Nakabayashi, N. Kakuta, A. Ueno and A. Furuta, J. Catal., 134(1992)87. 6. T. Miki, S. Yamada, T. Ogawa, N. Kakuta, A. Ueno, Y. Suzuki, K, Koyama, S. Noguchi and M. Yamada, Chem. Lett., (1989)201; T. Ogawa, S. Yamada, T. Miki, H. Tsuiki, N. Kakuta, A. Ueno, Y. Suzuki, K. Koyama, S. Noguchi and M. Yamada, Bull. Chem. Soc. Jpn., 62(1989)1844. 7. T. Mizusima, T. Tanabe, N. Kakuta and A. Ueno, manuscript in preparation.
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparationof HeterogeneousCatalysts G. Ponceletet al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
327
Sol-gel zirconia spheres for catalytic applications M. M a r e l l a a, M. Tomaselli a, L. Meregalli a, M. Gerontopoulos a, F. Pinna b, M. Signoretto b and G. Strukulb
B a t t a g l i a r i n a, P.
a TEMAV- Centro Ricerche Venezia Via delle Industrie 39, 30175 P.to Marghera, Venice, Italy b Chemistry Department, University of Venice Calle Larga S. Marta 2137, 30123 Venice, Italy
ABSTRACT Zirconia spheres have been p r e p a r e d by gel s u p p o r t e d p r e c i p i t a t i o n procedure, yielding high surface area materials retaining good mechanical resistance, proper shape, and narrow distribution dimensions in the range 0.01-2 ram. Spheres of 1.7 mm d i a m e t e r have been c h a r a c t e r i z e d a f t e r calcination at four different temperatures (573-1073K). One of these samples has been used as support for the preparation of a 0.98% Pt catalyst showing good activity in the low temperature methane catalytic combustion. 1. I N T R O D U C T I O N Zirconia is recently attracting considerable interest both as a catalyst and as a support material. In fact, due to its acid-base bifunctional properties, it catalyses the hydrogenation of different substrates such as for example benzoic acid [1], carboxylic acids [2], and carbon monoxide [3]. However, the main use of zirconia is as an efficient support for a variety of catalytic systems [4, 5]. The c u r r e n t synthetic technologies for the p r e p a r a t i o n of zirconia are generally based on precipitation with suitable bases from soluble forms of Zr4+ ion. The traditional method starting from aqueous solutions of zirconyl salts (nitrate, chloride, sulfate) leads to very small p r i m a r y particles t h a t upon drying coalesce into cakes with poor mechanical stability, eventually leading to very fine powders (< 100 ~m) [6]. The major drawback of this procedure is the n e a r impossibility of obtaining the final particles with proper shapes and diameters in the 0.5-2 mm range. Alternatively, sol-gel techniques for the preparation of ZrO2 starting from alkoxides are becoming increasingly popular [7, 8]. This method, while leading again to fine powders, can allow in some cases [9] to prepare monodisperse spherical zirconia particles with diameters t h a t can be t u n e d from 0.1 to 2.5 ~m. The drawbacks of these methods are the cost of reagents, their stability and
328 again the fine grain size of the powders, reflecting the fact t h a t these technologies have historically been developed for structural ceramics r a t h e r than for catalyst carriers. For commercial applications of ZrO2 based catalysts, the pelletization of powders retaining a large surface area and a proper shape is a very i m p o r t a n t point. This appears to be a hard task to be overcome, for which very little information is available in the literature [2]. In this work we wish to report an application of the gel supported precipitation procedure, originally developed for the disposal of nuclear waste [10], to the preparation of zirconia spheres (0.01-2 mm). In addition to the controlled geometric form and the relatively large dimensions, this process allows to obtain large surface area materials starting from inexpensive salt precursors. As a preliminary test of their properties, these materials have been checked in the preparation of Pt catalysts for the low t e m p e r a t u r e catalytic combustion of hydrocarbons. This modified sol-gel technique is quite general and suitable for the preparation of a variety of materials of interest for catalytic applications. 2. EXPERIMENTAL
2.1. Preparation A ZrO(NO3)2.2H20 solution (2.4 M) was denitrated by solvent extraction (1/3 v/v) with an alkyl tertiary amine diluted in an organic solvent (1/10 v/v) (Solvesso 100@). The final Zr 4+ content was 2.3 M, the [NO3-]/[Zr 4+] molar ratio was 0.95 and the viscosity (~) was about 40 mPa.s. The e x t r a c t a n t was regenerated by a continuous process. The solution was diluted to 1.0 M, then an organic additive, e.g. methyl cellulose (1%) and a s u r f a c t a n t e.g. iso-octylphenoxy-polyethoxy-ethanol, were added. For the preparation of mm-size spheres the solution was dripped from either a vibrating or a non vibrating capillary into a concentrated NH4OH solution containing the surfactant. After ageing, the spheres were filtered and washed with H20 down to pH=9. A final azeotropic dehydration with toluene was preferentially performed. The samples were calcined in air (60 l/h) for 3h at four different temperatures (573, 723, 823 and 1073K) with a heating procedure optimized on the basis of thermal analysis. In order to obtain the desired final particle diameters, a n u m b e r of p a r a m e t e r s were varied, such as the capillary diameter (from 0.5 to 3.5 mm) and the type, molecular weight and concentration of the organic additive. Alternatively, for the preparation of fine ZrO2 microspheres (20-60 ~m), the feed solution was atomized using a rotating (5,000 rpm) cup atomiser of 60 m m diameter. The droplets were dripped in a gelation bath as reported above. After ageing in a concentrated N H 4 0 H solution and w a s h i n g with H 2 0 , the microspheres were azeotropically dried and calcined at 1073K for 3 h. The P t ~ r 0 2 catalyst was prepared by incipient wetness impregnation of the 823K calcined sample, with an aqueous solution of H 2 P t C 1 6 . After impregnation, the catalyst was dried in air for 18 h at 383K and then calcined in flowing air at 773K for 4 h. The Pt metal loading was 0.98 wt%.
329 2.2. C h a r a c t e r i z a t i o n Simultaneous differential thermal and gravimetric analysis (DTA/TG) were carried out in a Netzsch STA 409 i n s t r u m e n t on the azeotropically dried samples. The measurements were performed either in air or in nitrogen (18 l/h), from room temperature to 1073K at a heating rate of 2K/rain. The morphology was studied both by scanning electron microscope (SEM) on a Philips 505 i n s t r u m e n t and on an optical Reichert Polyvar 2 microscope. X-ray diffraction (XRD) patterns were obtained by a Siemens D 500 powder diffractometer equipped with a graphite crystal m o n o c h r o m a t o r u s i n g a Copper Ka X-ray radiation source. Experiments were r u n in step-scan mode with a step interval of 0.02 ~ 20 and a count rate of I second per step over the range 5 ~ to 90 ~ 20. The fraction of t e t r a g o n a l to monoclinic form was determined by Rietveld method [11,12] while the crystallite size was determined by Warren-Averbach X-ray broadening method [13]. Nitrogen a d s o r p t i o n and desorption were m e a s u r e d at 77K w i t h an automatic Carlo Erba Sorptomatic 1800 adsorptiometer. The samples were pretreated at 573K under vacuum (1.33 Pa). C r u s h s t r e n g t h m e a s u r e m e n t s were carried out on single spheres by a Wolpert equipment, following ASTM D 4179-88. Attrition resistance measurements (A.I.F.) on the spheres were performed on 10 g samples by an equipment assuring a displacement of 40 m m to an ampulla (72 mm length and 34 mm i.d.). The speed was 700 rpm and the test lasted 5 min. H2 chemisorption measurements were performed at 298K using a pulse flow system. Before measurements, catalysts were heated (10K/min) in a H2 stream (40 ml/min) at 673K for 2 h. After reduction the surface was cleaned for 2 h with an Ar stream, then cooled at 298K. The chemisorption m e a s u r e m e n t s were performed with 5% diluted H2 in Ar. In the T e m p e r a t u r e Programmed Reduction (TPR) experiment the sample was calcined at 773K for 1 h in an oxidizing flow (5% 02 in Ar), cooled to 298K, treated with pure Ar and then reduced by heating (10K/min) to 1023K in a 5% H2 in Ar mixture. 2~3. Catalytic m e a s u r e m e n t s Catalytic investigations were carried out in a quartz flow microreactor (i.d. 8 mm, length 300 mm) immersed in a fluidized sand bath heated by an external oven. The catalys t was diluted with quartz beads to avoid t e m p e r a t u r e gradients a n d the temperature was measured with a thermocouple immersed in the middle of the catalyst bed. The experiments were performed using 4 ml of catalyst and a space velocity of 1,800 h -1. The feed stream contained 2.5 vol% CH4 and 13 vol% 02 in an Ar background. Before testing, the catalysts were activated in flowing air for 2 h at 773K. The reaction mixture was analyzed by an on-line gas c h r o m a t o g r a p h and calibration curves of the i n d i v i d u a l components were used for quantitative analysis. 3, RESULTS AND DISCUSSION DTA carried out in air (Figure 1) shows a n u m b e r of features: i. an endothermic peak occurring between 353 and 473K, related to water evolution
330
A
O
X u.I
O a z 1.1.1
V I
I
I
I
273
473
673
873
T(K)
Figure 1. DTAfFG plot in air on an azeotropically dried sample. from hydrated intermediate species; ii. a broad exotermic peak between 473 and 593K, due to the burning of the organic additive; iii. a sharp crystallization peak of ZrO2 centered at 688K. Weight loss is no longer observed above 693K. The total weight loss is 28%. DTA in nitrogen shows similar features with the exception of the broad peak between 473 and 593K and the shift of the crystallization peak towards higher temperatures (723K). Weight loss is no longer observed above 773K. A SEM micrograph (Figure 2) of an azeotropically dried sample, shows near perfect spheres with a constant diameter of 1.7_+0.1 mm. The volume reduction is 39% after calcining at 573 and 723K and 58% after 823 and 1073K.
Figure 2. SEM micrograph of an azeotropically dried sample.
331 The crystalline phases evolution was determined by XRD. Two broad peaks centered around 20 = 31.5 ~ and 52.2 ~ appear in the X-ray diffractogram of the sample calcined at 573K indicating a poor crystallinity. In the diffraction p a t t e r n s of the samples calcined at 723 and 823K both t e t r a g o n a l and monoclinic phases are present, while at 1073K almost pure monoclinic zirconia is obtained. Table 1 shows the amount of tetragonal and monoclinic phases, as determined by Rietveld method, together with the average crystallite sizes, as determined by Warren-Averbach X-ray broadening method. Table 1 Crystallographic data as a function of calcination temperature
Calcination Temperature (K)
723 20 5.0 80 6.0
Tetragonal phase (%) Average crystallite size (nm) Monoclinic phase (%) Average crystallite size (nm)
1073 1
823
15 8.0 85 10.0
-
99 16.0
The nitrogen adsorption-desorption isotherms of the samples calcined at four different temperatures are shown in Figure 3. All isotherms are of type IV and H2 hysteresis for (a) and (b) samples and H1 hysteresis for (c) and (d) samples according to IUPAC convention [14]. 300 600
250 C
a ~' ~
400
E
300
._i
150
100
200
50
loo
0
0.2 0.4 0.6 0.8 RELATIVE PRESSURE P/Po
0 1.0
200
i
0
i
i
i
0
1.0 0.2 0.4 0.6 0.8 RELATIVE PRESSURE P/Po
Figure 3. Nitrogen adsorption-desorption isotherms at 77K of samples calcined at (a) 573K, (b) 723K, (c) 823K and (d) 1073I~ A s u m m a r y of the morphological p a r a m e t e r s of the ZrO2 spheres is presented in Table 2. As can be seen the initially high BET surface area drops rapidly with increasing calcination temperature. The total pore volume (Vp) determined from the adsorption at P/Po=0.99 decreases from 0.57 to 0.11 ml/g.
332 Table 2 Morphological properties as a function of calcination temperature Calcination Temperature (K) SBET (m2/g) Vp (ml/g) St (m2/g) rp max (nm)
573 381 0.57 398 3.0
723 133 0.35 136 3.7
823 82 0.36 85 4.8
1073 39 0.11 31 23.1
The statistical thickness t of the adsorbed layer was calculated by Duchet equation [15] which takes into account the adsorbate-adsorbent interaction of pure zirconia samples. The plot of the adsorbed volume (ml STP/g) vs. t (nm) is given in Figure 4. 200 13_ 1-O3 1so
E q)
E 0
100
>
9 J
"10 q)
.e o
5o
t/) "10
<
0
0.5
1
1.5
t(nm)
Figure 4. Adsorbed volume (ml STP/g) vs. t (nm) of samples calcined at (squares) 573K, (circles) 723K, (diamonds) 823K and (triangles) 1073K. In almost all cases the first part of the curve is a straight line passing through the origin, from which St (m2/g) can be calculated according to Lippens et al. [16]. The data are reported in Table 2 and are in good agreement with the values of SBET, except for the sample calcined at 1073K. Furthermore, the y-intercepts are extremely small, again with the exception of the latter sample. This leads to the conclusion that the samples calcined at 573, 723 and 823K have negligible microporosity, while the sample calcined at 1073K has a rather small microporosity (5-10-3 mug). The sample calcined at 723K shows an upwards deviation for t>0.8 nm (P/Po>0.6) indicative of the occurrence of capillary condensation.
333 The pore size distribution was obtained from the desorption branch of the isotherms following Barrett et al. method [17]. The plots are shown in Figure 5. 1.2
E c
o
0.8
I
0.4
xlO
"o
0
10
100
Pore Radius (nm)
Figure 5. Pore size distribution (dV/dlogR) vs. the average pore radius of samples calcined at (squares) 573K, (circles) 723K, (diamonds) 823K and (triangles) 1073K. The mesopore size distribution is essentially unimodal with the pore radius centered at the rp max values shown in Table 2. Crush strength data are reported in Table 3. Table 3 Single pellet crush strength Calcination Temperature (K) Average crush strength (N) Standard deviation (N) 80% spread (N) 95% reliabilit$ (N)
573
723
823
31.6 9.8 19.0-44.1 28.0-35.1
27.5 11.0 13.4-41.5 24.6-30.5
25.1 5.5 19.6-30.6 23.6-26.6
The attrition resistance was carried out on the sample calcined at 723K. The % A.I.F., expressed as the ratio between the weight of material remained above the sieve after the test and the original weight of the sample, was 99.9% indicating a good attrition resistance of the spheres. The Pt/ZrO2 catalyst, prepared according to the procedure described in the Experimental, was reduced at 673K on the basis of a prior TPR experiment
334 showing a m a x i m u m H2 consumption at about 504K. A H2 c h e m i s o r p t i o n m e a s u r e m e n t indicated a 27% metal dispersion. The CH4 oxidation activity of the catalyst was characterized by t e m p e r a t u r e run-up experiments with fixed feed composition. Carbon dioxide was the only carbon-containing reaction product detected at t e m p e r a t u r e s above 500K, indicating the complete oxidation of CH4 in the feed. Figure 6 shows the conversion of CH4 as a function of temperature: the onset of the reaction was followed by a gradual increase in methane conversion covering a t e m p e r a t u r e range of about 200K, indicative of absence of light-off of the catalyst.
100
I
A
o~
v
tO
o..
(1) r
50
O eD ,or
"T"
o
I~
0 400
500
I
I
I
600
700
800
900
Temperature (K) Figure 6. Conversion of CH4 as a function of t e m p e r a t u r e over the Pt/ZrO2 catalyst. U n d e r the experimental conditions used, the catalyst appears to be quite active, since a 12% conversion is observed even at 573K and at 773K full conversion to carbon dioxide can be obtained. Although a comparison with previous l i t e r a t u r e d a t a is complicated by the different choice of the experimental parameters, a general comment seems in order. There seems to be a general a g r e e m e n t that t e m p e r a t u r e s higher t h a n 773K are normally required for 50% conversion in CH4 oxidation [18-21]. Interestingly, the catalyst here reported reaches a 50% conversion at a t e m p e r a t u r e about 150K lower; moreover, the Turnover Number (0.042 moles CO2 produced/sec.moles surface Pt) is comparable with the values found by Yao [18] and by Oh et al. [21] but at a temperature (673K) about 100K and 200K lower respectively. 4. CONCLUSIONS The gel supported precipitation method employed in the p r e s e n t study
335 provides a general synthetic procedure for the preparation of mechanically resistant oxides with proper shape, controlled dimensions, narrow distribution and large surface area. As an example, in this preliminary work the method has been described for the preparation of spheres of zirconia, however it can be applied to a wide variety of inorganic oxides starting from suitable water soluble precursors. As exemplified by the present study, these materials are very promising for industrial catalytic applications. ACKNOWLEE~MENTS Thanks are due to Messrs. F. Gerolin and F. Danieli and Miss T. Fantinel for skillful technical assistance. R~'~CI~ 1
2. 1
1
1
6.
t
8. ,,
10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
E. J. Strongny; U.S. Patent 130755 (1980). T. Yokoyama, T. Setoyama, N. Fujita, M. Nakajima, T. Maki and K. Fuji, Appl. Catal. A, 88 (1992) 149. K. Maruya, A. Takasawa, T. Haraoka, M. Aikawa, T. Arai, K. Domen and T. Onishi, Stud. Surf. Sci. Catal., 75 (1993) 2733. P.D.L. Mercera, J.G. Van Ommen, E.B.M. Doesburg, A.J. Burggraaf and J.R.H. Ross, J. Catal. 57 (1990) 127. J.R.H. Ross, XIII NAM of Catalysis Society, Pittsburgh, (1993) D40. R.A. Koeppel, A. Baiker, Ch. Schild and A. Wokaun, in G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (eds.), P r e p a r a t i o n of Catalysts V Scientific Bases for the Preparation of Heterogeneous Catalysts, Elsevier, Amsterdam (1991) 59. S.J. Teichner, ChemTech, (1991) 372. M.A. Cauqui and J.M. Rodriguez-Izquierdo, J. Non-Cryst. Solids, 147&148 (1992) 724, and references therein. L. Lerot, F. Legrand and P. De Bruycker, J. Mater. Sci. Lett., 26 (1991) 2353. G. Brambilla, P. Gerontopoulos and D. Neri, Symposium on Sol Gel Processes and Reactor Fuel Cycles, Gatlingsburg, TN, USA, May 4-7 (1970) Conf-700502. H.M. Rietveld, J. Appl. Cryst., 2 (1969) 65. C.J. Howard and R.J. Hill, J. Mater. Sci, 26 (1991) 127. R.J. Matyi, L.H. Schwartz and J.B. Butt, Catal. Rev., 29 (1987) 41. K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol and T. Siemieniewska, Pure & Appl. Chem. 57 (1985) 603. J.C. Duchet, M.J. Tilliette and D. Cornet, Catal. Today, 10 (1991) 507. B.C. Lippens and J.H. De Boer, J. Catal., 4 (1965) 319. E.P. Barrett, L.G. Joyner and P.P. Halenda, J. Am. Chem. Soc., 73 (1951) 373. Y.-F. Yu Yao, Ind. Eng. Chem. Prod. Res. Dev., 19 (1980) 293. M. Niva, K. Awano and Y. Murakami, Appl. Catal., 7 (1983) 317. K. Otto, Langmuir, 5 (1989) 1364. S. H. Oh, P. J. Mitchell and R. M. Siewert, J. Catal., 132 (1991) 287.
This Page Intentionally Left Blank
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
337
Surfactant Based Synthesis of Oxidic Catalysts and Catalyst Supports U. Ciesla a, D. Demuth a, R. Leon b, P. Petroffb, G.D. Stuckyc, K. Unger a, F. Schiith ~ a Institut fiir Anorganische und Analytische Chemie, Johannes Gutenberg-Universit~t Mainz, 55099 Mainz, Germany b Materials Department, University of California, Santa Barbara, California, 93106 USA c Department of Chemistry, University of California, Santa Barbara, California, 93106 USA The preparation of mesostructured metal oxides based on the surfactant controlled synthesis of MCM-41 is presented. It can be expected that the new metal oxide materials will exhibit a high surface area of nearly 1000 m2/g and could have a possible use as catalysts, especially in partial oxidation reactions and as hosts for quantum sized material, if the surfactant can be removed without destroying the structure.
1. Introduction In 1992 scientists of the Mobil Oil Corporation published the surfactant controlled synthesis of mesoporous silicate material, designated MCM-41 [1]. Previous studies concerning the preparation and characterization [2-4] and the mechanism of MCM-41 formation [5] have been published. Based on the synthesis mechanism the idea was further developed to design mesostructured transition metal oxides analogous to the MCM-41 structure. The mechanism is based on ionic interactions of the cationic surfactant headgroups with the anionic silicate species. In the synthesis of MCM-41 related materials the charged silicate species are substituted by metal oxides which are able to form polyanions. The metal oxide polyanions perform an analogous function to the silicate species in the synthesis of MCM-41. If it is possible to create mesostructured phases from an anionic metal oxide/cationic surfactant system, as in the case of MCM-41, it should also be feasible using the reversed system: the combination of metal oxide polycations with anionic surfactants should lead to similar materials. In both systems the ionic interactions of the metal oxide and the surfactant should be comparable and the mechanism in the reversed system should be analogous to the synthesis of MCM-41. In this communication we present examples of both pathways, which were successful in the design of mesostructured metal oxides: 9 Metal oxide polyanion / cationic surfactant system: tungstate and molybdate were used as polyanionic compounds and combined with quatenary ammonium surfactants. The polyanions of tungsten and molybdenum are the most thoroughly investigated polyanion forming compounds. They are formed over a wide pH range from pH 1-10 with different compositions. 9 Metal oxide polycation / anionic surfactant system: for polycation forming oxides we used lead and iron. The formation of polycations is more sensitive to the pH changes
338 than the polyanions of tungsten and molybdenum, l_e.ad oxide forms polycations in the pH range of 6-12, whilst iron oxide polycations are formed in the pH range of 3-5. Hexadecylsulfonic acid was chosen as the anionic surfactant. With the exception of the lead oxide the metal oxides used are very interesting materials with respect to catalytic applications. Tungstates and molybdates are widely used in partial oxidation reaction [6], iron oxides are the basis of many important industrial catalysts, which are for instance, used for the dehydrogenation of ethyl benzene to styrene. If the surfactant template could be removed from the structures, very high surface area catalysts could be accessible.
2. Experimental 2.1. Synthesis 2.1.1 Metal oxide polyanion / cationic surfactant system The synthesis of metal oxide surfactant compounds is analogous to MCM-41, substituting the silicate for tungstate or molybdate polyanions. The surfactant used are alkyltrimethylammonium salts with alkyl - C12H25 , C14H29, C16H33, C18H37 . The synthesis procedure was as follows: A solution of commercial ammoniumtungstate ((NI-I4)6H2WI2040) or ammoniumheptamolybdate tetrahydrate ( ( N H 4 ) 6 M o 7 0 2 4 , 4 H 2 0 ) w a s combined with the surfactant solution. The mixture was stirred at room temperature for 30 min and then heated at 90~ for three days. The obtained white product was filtered, washed with water and dried at 90 ~
2.1.2 Metal oxide polycation / anionic surfactant system The different conditions for the formation of polycations are the reason for varying the synthesis procedures of lead oxide surfactant and iron oxide surfactant. The synthesis of the lead oxide surfactant compound was carried out as follows: An aqueous solution of Pb(NO3)2 was adjusted to pH 7-8. This solution was combined with a suspension containing the sodium salt of hexadecylsulfonic acid in H20 and stirred for 30 min. The mixture was heated at 90~ for 3 days. The obtained product was filtered, washed with water and airdried. The synthesis procedure for the iron oxide surfactant differed: A freshly prepared solution of FeC12 with a pH of 3 was added to a surfactant suspension of sodium hexadecylsulfonic acid. At this point the pH was adjusted to 6 by addition of aqueous NH3. After standing for 2 days at room temperature, the brown yellow product was filtered, washed with water and air-dried. The synthesis had to be carried out at room temperature to prevent precipitation of amorphous material.
2.2 Characterisation techniques used The powder X-ray diffraction (XRD) patterns were obtained on a Seifert 3000TT instrument with an automatic divergence slit (ADS) using copper K~ radiation (0.154 nm). This allows spectra starting with 20 values of around 0.5 ~. For the representation all data collected with ADS were transformed to a fixed slit configuration. TEM analysis of the sample was done using a JEOL 200FX at the University of Santa Barbara, USA. The electron beam accelerating voltage was 200kV.
339 3. Results and Discussion
The best results synthesizing a MCM-41 analogous hexagonal structure were obtained using tungstate as the polyanion.
= ~ ~ ~J~
hk0100110 d3.882.24[nml
2
200 1.94 210 1.47 300 1.28
.~
220 1.12
. . . .
" ' " ' s
. . . . . . . . . .
10
" . . . . . . . . .
15
2 theta
'
....
'
. . . .
2"o
. . . . . . . . . .
25
Figure 1. X-ray powder diffraction pattern of tungsten oxide surfactant indexed on a hexagonal unit cell with a=b=4.5 nm. Fig.1 shows the XRD pattern of the tungstate surfactant compound with an almost pure hexagonal phase which was synthesized using hexadecyltrimethylammoniumchloride as surfactant and, deviating from the procedure given above, at 140~ The XRD pattern shows reflections only in the narrow angle range from a 20 angle of 2 ~ to 8 ~. The first intense reflection corresponds to a d-spacing of 3.9 nm, which agrees with that of MCM-41 using the same surfactant. Most of the peaks below 5 ~ (20) can be indexed assuming a hexagonal phase (P6). However, there is a difference in the total number of reflections in comparison to the silicate species. The XRD pattern of MCM-41 shows only four reflections, whereas there are usually more in the case of the tungsten oxide surfactant. This may be due to either the presence of a lamellar phase, another impurity phase, or to a periodic structure of the walls which Figure 2" Transmission electron micrograph of are amorphous in the silicate case. The the hexagonal phase of tungstate surfactant.
340 fact that the amorphous peak is missing in the tungstate compound, which is always present in the XRD pattern of MCM-41 at 20~ o (20), may be an indication of wall crystallinity in the tungstate compound. The hexagonal phase is also identified by TEM analysis (fig.2). The micrograph shows the regular hexagonal array of uniform channels. The d-spacing of 4.0 nm is in excellent agreement with the results from the XRD pattern. Recording of the micrographs proved to be relatively difficult, since the tungstate compound forms (in contrast to the silicate) needle like crystals with the channels along the needle axis. Thus, in most cases the tubes can only be seen sideways, appearing like a layer. The optimal pH for the synthesis of the hexagonal phase is 4-8, but mostly these products also contain a lamellar phase. A typical XRD pattern containing a hexagonal as well as the lamellar phase with a d-spacing of 3.1 nm for the (001) peak can be seen in fig 3 (region B). At the extremes of the pH scale, mainly layers were obtained. At high pH (> 9) two layers can be identified with d-spacings of 2.8 nm and 3.0 nm together with another phase exhibiting a larger d-spacing.
---w
--w---
I'1 0
I 2
4
--,
"i 20
.... i'"
"
II 8
B
pH
.....
;,---:----
m
c 10
m
20
I 12
14
Figure 3: Dependence of the product composition on the pH with corresponding typical XRD patterns of the tungsten oxide surfactant using alkyltrimethylammoniumchloride as surfactant.
So far template removal was not successful. In contrast to MCM-41, it is not possible to calcine the tungsten oxide surfactant by thermal treatment at 500~ Degradation of the mesostructure occurs, and the thermodynamically more stable yellow WO3 is obtained. Removal of the template by extraction in different solvents (diethylether/HC1, ethanol, ethanol/HC1, dimethylsulfoxide) was also unsuccessful. In all cases the TG/DTA analysis showed that the surfactant was completely retained by the pore system. Template removal by oxygen plasma calcination are under investigation and seem to be very promising. Nevertheless, even by the calcination of the products without special precautions it semms
341 to be possible to produce high surface area oxides, even if the regular pore structure is lost. While WO3-surfactant calcined at 250~ and at 500~ exhibited only low surface areas (0.2 m2/g and 3.75 m2/g), a sample calcined at 350~ for 4h had a specific surface area of appr. 100 m2/g. It seems to be possible to optimize the calcination protocol in order to produce even higher surface areas. Using molybdenum polyanions only a layer structure was obtained, despite varying the pH of the synthesis solution over a pH range from 1 to 13. The d-spacings of the layers can be controlled by variation of the surfactant alkyl chain length. A shorter chain length was shown to give a lower d-spacing and vice-versa (fig.4 a+b).
~ -ff ~.
/ 002 2.44 1.23
2.6
2.4
C D
w "6
2.2
Q.
-6 I
. . . .
|
. . . .
|
. . . .
1,
,'~',
|
. . . .
s
i
. . . .
1
, " - . . .
, , , . |
15
,,
2.0
..................................................
, .... , ....
9
2 theta
12
14
16
18
alkyl chain length
Fig.4a: XRD pattern of Mo-oxide surfactant synthesised with hexadecyltrimethylammoniumchloride
Fig.4b: Dependence of the (001) d-spacing on the surfactant alkyl chain length
In the metal oxide polycation / anionic surfactant system the use of iron and lead polycations proved to be successful in the formation of mesostructured materials. Fig. 5 shows three XRD patterns of Fe-oxide surfactant samples synthesized with different Feprecursors: FeC12 (A), FeSO4 (B) and Fe(NO3)3 (C).
j B
-_.,__
A
F '
'
I
. . . .
I
5
'
'
'
'
I
'
'
'
'
I
10
'
'
'
'
I
'
'
'
'
I
15
'
'
'
'
I
'
'
'
'
I
'
'
'
'
I
'
'
20
'
'
25
2 thelta
Figure 5: XRD patterns of Fe-oxide surfactant sythesised with various Fe-precursors
342 All XRD patterns show layer phases with large d-spacings indicative of mesoporous material. The samples of B and C appear to be pure layer phases with two or three coexisting layers. Also sample A shows the presence of a layered phase with a (001) d-spacing of 3.75 nm. However this phase is not pure and a second hexagonal phase could possibly exist, indicated by the shoulder of the (001) reflex. Template removal by calcination and by solvent extraction have so far proved to be unsuccessful, but was possible by oxygen plasma calcination. Unexpectedly, the layer structure did not collapse after calcination as determined by X-ray diffraction (fig. 6).
|
2
4
6
8 10 2 theta
12
14
Fig 6: XRD pattern of the iron oxide after oxygen plasma calcination.
The successful removal of the template is proven by the very strong decrease of the CH- or CC-vibration in the Raman-spectra of the samples as compared to the iron oxide surfactant. From fig.6 it can be infered that probably two layered materials have formed in the iron oxide surfactant. Since the layers would not be stabile without any "spacer", probably some kind of pillaring has occured during the plasma calcination. The color change from light brown to a darker brown, moreover, indicates that oxidation of the iron, which is predominantly Fe 2§ in the iron oxide surfactant, to Fe 3+ has taken place. The XRD pattern of the Pb-oxide surfactant shows two co-existing phases (fig. 7). The first d-spacing of 4.58 nm results from the hexagonal phase, which can be indexed on a hexagonal unit cell with a=5.29 nm (2d~0o/~/3). In addition, a layer with a d-spacing of 2.92 nm is noticeable.
343
hex. phase* layer hkl d[nm] hkl d[nm] 100 4.58 001 2.92 110 2.63 002 1.46 220 2.27 300 1.53 (/)
r0
.r
'
'
'
1
. . . .
I
5
. . . .
I
. . . .
I
10
. . . .
I
. . . .
1
15
. . . .
I
. . . .
/
. . . .
I
. . . .
20
25
2 theta
Fig.7: XRD pattern of Pb-oxide surfactant
Preliminary experiments with partial oxidation reactions (propene to acrolein) yielded promising results when using catalysts derived from the surfactant tungstates and molybdates. Moreover, it seems be to improve performance of the catalysts by including promoting metals like bismuth in the synthesis. According to the mechanism presented in the introduction, such metals should easily be incorporated into the structure. 4. Conclusion The experiments reported here prove that it is possible to generalise the mechanism described in (5) to other oxides than silica. This is highlighted by the formation of the hexagonal phase of the tungsten oxide surfactant as determined by TEM. Detailed characterisation of the materials described here are under way. It can be expected that also other metal oxides and combinations of various metal oxides will be synthesised following the procedure lined out here. The application of the mesostructured transition metal oxides might lead to a new generation of highly active redox catalysts.
Acknowledgements This work was supported by the FCI, the DFG (Schu 744/6-1), the Office of Naval Research, and the MRL Program of the NSF.
References
1 C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992), 710.
344 2 J.S. Beck, J.C. Vartuli, W.J. Roth, M.E.I.e~nowicz, C.T. Kresge, K.D. Schmitt, C.T.-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J.Am.Chem.Soc. 114, (1992), 10834. 3 0 . Franke, G. Schulz-Ekloff, J. Rathousky, J. Starek and A. Zukal, J. Chem. Soc. Chem. Commun. 1993, 724. 4 P. Branton, P.G. Hall, K.S.W. Sing, J. Chem.Soc. Chem. Commun. 1993, 1257. 5 A. Monnier, F. Schiith, Q. Huo, D. Kumar, D. Margolese, R.S. Maxwell, G.D. Stucky, M. Krishnamurty, P. Petroff, A. Firouzi, M. Janicke, B.F. Chmelka, Science 261, (1993), 1299. 6 G.E. Keulks, L.D. Krenzke, T.M. Notermann, Advanced in Catalysis, Vol.27, Academic Press, New York, 1978.
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of HeterogeneousCatalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
345
Preparation and properties of ceramic foam catalyst supports Martyn V. Twigga and James T. Richardson b aJohnson Matthey, Catalytic Systems Division, Royston, Herts. SG8 5HE, United Kingdom bDepartment of Chemical Engineering, University of Houston, Houston, TX, 772044792, USA Ceramic foams are preformed materials used extensively as filters, advanced burners, diffusers and mixers, but catalytic applications are now beginning to appear. These include catalytic solar receivers, partial oxidation, catalytic combustion, and diesel exhaust catalytic filters.The foams are sponge-like porous structures and are made by filling flexible open-cell organic polymer foams with slurries of ceramic particles such as alpha alumina, zirconia, silicon nitride, etc.. The plastic is burned off and the ceramic sintered to yield either a positive or negative replica of the original foam depending on exact loading procedures. Mega-pore opemngs range from 0.02 to 1.5 mm, apparent densities from 0.5 to 1.5 g c m ~, porosities from 40 to 85%, and the number ofpores per cm from 4 to 30. With appropriate moulding or machining of the plastic foam, the ceramic may be fabricated ln any shape or size. Megaporosity enhances turbulence in gases flowing through the foam and produces better mixing with lower pressure drop. The foam may be impregnated with catalytic agents, with or without an appropriate washcoat. Although pressure drop correlations follow the Ergun equation, the high porosity gives much lower pressure drop than equivalent beds of particles. Mass transfer follows standard correlations but turbulent flow is seen at much lower flow rates. Heat transfer is enhanced by the superior conductivity of the web structure. These feature combine to make ceramic foams attractive possibilities for many applications. The only disadvantage is the relatively low strength, a feature which may be controlled in some cases. 1. INTRODUCTION Successful catalysts fulfill different requirements simultaneously. In addition to prolonged good activity and appropriate selectivity, a range of physical properties are important. For example, particles in a large fixed-bed reactor must not crumble, and attrition resistance of a fluidized bed catalyst must be sufficient to minimize loss of fines from the reactor but without being so abrasive that plant equipment erodes. Many catalytic and physical properties are related; an illustration is crush strength, which depends on porosity and pore structure and in turn impacts activity and selectivity through diffusion effects. These and other considerations are important when catalyst powder is agglomerated into particles by pelleting and extrusion, and many difficulties encountered are alleviated in the alternative approach of impregnating intrinsically high activity species onto preformed supports with suitable material properties. Impregnating platinum group metals on alumina is an example, but pressure drop, heat transfer, and diffusion effects are not dramatically improved because particle sizes are usually comparable and micromeritics of agglomerated powder is similar to that of most preformed supports. In fixed-bed duties, high geometric area provided by small particles xmproves low catalyst effectiveness, but the associated high pressure drop ~s
346 unacceptable, whether the catalyst is an agglomerated powder or impregnated preformed support. It therefore appears the development of new high performance catalyst systems requires complete decoupling of catalytic and physical functions to achieve maximum effectiveness, together with the use of reactors designed to take advantage of improved activity, heat and mass transfer, etc. To a considerable extent, coated monoliths achieve this, most notably with autocatalysts where strength, vibration resistance, flow patterns and low pressure drop are provided by a monolithic structure and optimized catalysis is afforded by special coatings. Since their introduction, they have been developed into highly reliable systems whose overall performance far exceeds that of conventional systems based on pellets. A related approach could involve open cell, high porosity ceramic foams, similar to those used for filtering molten metals. When fabricated in shapes and coated with suitable ceramic formulations, they should have physical advantages similar to monoliths, with additional benefits from internal radial transfer. The resulting high effectiveness could be exploited in reactors designed for high transfer coefficient conditions and high catalyst effectiveness. Although these potential benefits have been known for some time, relatively little has been published on ceramic foams in catalytic roles. Here we review the pertinent information available and detail some recent work on fundamental properties that demonstrate specific advantages of foambased catalysts. 2. PREPARATION AND PROPERTIES
2.1 Ceramic foam morphology Open-pore ceramic foams are materials with high temperature resistance, low bulk density, and tortuous flow patterns, together with high open porosity [1]. This porosity, which varies from 40 to 85%, is formed from megapores .04 to 1.5 mm in diameter. Characteristic parameters include cell size, window size and surface area, all correlated with the number of pores per centimeter. Each cell connects with adjacent cells through the windows. The megaporosity provides a tortuous path for internal gas flow, and turbulence is much enhanced. This results in forced convective flow within the structure, a feature known to produce beneficial performance over conventional pellets that allow only diffusional transport through meso- and macro-pores [2-7]. The high porosity also provides much lower pressure drop. Higher thermal conductivity is expected from the continuous web-like structure of the foam, thereby providing improved heat transfer into and throughout the foam. These features were first applied in the development of molten metal filters [8,9], and later to catalytic combustion devices [11-13]. Catalytic applications are now appearing as the combined advantages of these structures become apparent. Specifically, these advantages are: (1) Preformed fabrication that provides shapes matching the application and allowing easy reactor loading. (2) High surface to volume ratios, simulating very small particle diameters and giving high activity with low diffusion resistance. /43/High porosity, leading to low pressure drop at high flow velocities. Increased thermal conductivity and better convective heat transfer. 2.2 Preparation techniques Commercial preparation of ceramic foam starts with a foamed organic precursor having the same porosity as the desired final product [14-19]. The most common organic precursor is polyurethane, which is available in the form of flexible, open cell foams with pore sizes ranging from 4 to 30 pores cm "1 (10 to 80 pores inch-l). However, other organic plastics, such as polyolefins, are equally suitable. The pores of the organic precursor are then filled with an aqueous slurry of the desired ceramic. This typically
347 comprises a 20 wt % mixture of ceramic particles (0.1 to 10 ~ m in diameter) in water, together with appropriate amounts of wetting agents, dispersion stabilizers and viscosity modifiers. A list of ceramics successfully formed into foams is given in Table 1. Table 1 Suitable ceramics for foam production alpha-alumina alumina silicate zirconia toughened alumina stabilized zirconia (Ca, Mg, La) mullite calcium aluminate titania kaolin haematite magnetite The original plastic foam can be fabricated in a wide variety of geometric shapes and dimensions, e.g. cylinders, rings, rods, or custom-designed configurations. These are produced either during fabrication of the plastic foam or by appropriate machining or pressing from sheets of the material. These structures are immersed or soaked in the ceramic slurry, if necessary with agitation, to ensure proper filling of theplastic ~ores. Alternatively, blocks of the foam material may be filled with the slurry and then s aped to give the required geometry. Figure 1 shows examples of commercially available shapes. "lWo variations in the procedure are possible at this point, leading to ceramic foams with different characteristics. In the first, the organic foam is impregnated with a relatively low viscosity slurry and the excess slurry removed by blowing air through the foam or by compress~'ng the foam in one or more stages. The impregnated foam is then dried at 100~ or below, leaving a coating of ceramic on the plastic, and calcined at temperatures above 1000~ This last treatment removes the organic precursor through vaporization and combustion and causes the ceramic to sinter. The resulting cerarmc foam is virtually a ceramic copy or positive image of the plastic foam skeleton, with filaments havin~ a hollow core. Bulk densities are low, porosities high, but mechanical strength is relatwely low. In the second procedure, a negative image of the plastic foam is created by using thixiotropic ceramic slurries with increased viscosity, e.g. through addition of thickening agents [19]. The foam is shaken or vibrated to remove excess slurry at the external surface of the structure, but no air blowing or compression is used so that the organic pores remain substantially filled with the ceramic. Upon calcination, the plastic is removed, leaving pores that correspond to the original organic material. Pore diameters are smaller than the previous method, bulk densities are higher and porosities lower, but mechanical strength is higher. For applications where high surface area is not important, the ceramic foam may be loaded with catalytic agents through single or multiple impregnation of suitable salts, followed by heat-treatment at moderate temperatures. Alternatively, a washcoat may be added with the same procedures used for monolithic substrates [20]. Surface areas increase from less than 1 m 2g 1 to above 30 m 2g-,1 depending on the amount of washcoat added. In this way, washcoated foams have been loaded with metals and oxides [18,19], zeolites [20] and carbon [21]. Other methods, e.g. chemical vapor deposition, have also been suggested [23,24].
348
Figure 1. Examples of available shapes in preformed ceramic foams (courtesy Hi-Tech Ceramics, Inc., Alfrea, N.Y., U.S.A.) 2.3 Properties of the ceramic foams Typical properties of positive-image, non-washcoated foams are given in Table 2. Pore diameters were determined by scanning optical micrographs. Bulk densities are low and porosities remarkably high, even with a wide variation of pore size. A comparison between a positive- and negative-image foam is given in Table 3. The positive image foam is more porous and has half the bulk density of the negative-image, but the crushing strength is lower by a factor of twenty. The effect of adding a washcoat is demonstrated in Table 4. The pore diameter decreases slightly and the porosity is sufficiently lower, suggesting some of the smallest pores may be totally blocked with washcoat. As expected, the surface area is drastically increased. The stability of the washcoat is demonstrated by the surface area after a thermal treatment at 1000~ for 4 h, showing only a 13% decrease. In another test, the washcoated ceramic foam was loaded with 0.7 wt% Rh by impregnation with RhC13. 3H20 solution. The catalyst was sintered at various temperatures for 2 h and ttie results are given in Table 5. Dispersion of the metal is fairly constant until above 600~ dropping considerably at 1000~ These results demonstrate that ceramic foams may be effectively washcoated with efficient stabilization of supported catalysts. 2.4 Characteristic length Particles in packed beds are usually characterized in terms of the equivalent diameter, dp, i.e. the diameter of a sphere with an equal surface to volume ratio as the
349 Table 2 Typical properties of ceramic foams a Ceramic: 92% ~-A1203, 18% mullite Positive-image No washeoat Pores cm 1
Pore diameter mm
Bulk density g c m "3
Porosity %
4
1.52
0.51
87
8
0.94
0.61
85
12
0.75
0.66
83
18
0.42
0.65
84
26
0.29
0.70
81
31
0.21
0.67
83
a Courtesy Hi-Tech Ceramics, Inc.
Table 3 Comparison of positive- and negative-image ceramic foams a Form: cylinders, 1.12 cm diameter, 1.30 cm length Ceramic: 0t -A]203 Original pore size: 7 pores cm 1 Property
Positive-image
Negative-image
Bulk density, g cm3:
0.75
1.54
Helium density, g cm3:
3.97
3.98
Porosity, %:
81
61
Horizontal crushing strength, kg:
11
230
a Reference 19
350 Table 4 Effect of adding the washcoat a Form: cylinders, 1.27 cm diameter, 2.54 cm length Ceramic: 92% a-AL,O,, 18% mullite Original pore size: 1~2~ores cm "1 Washcoat: 6 wt% hydrated alumina Property
No washcoat
With washcoat
Pore diameter, mm
0.759
0.734
Bulk density, g r
0.66
0.70
Helium density, g cm "3
3.45
3.45
Porosity, %
83
78
Surface area, m2g-1 Fresh 1000~ 4 h
1.0 -
4.6 4.0
"3
a Reference 25 particle. This term is then used in calculations of the Reynolds number, pressure drop, mass transfer coefficient, heat transfer parameters, and effectiveness factor. The most obvious length to use for ceramic foams is the average pore or cell size, d c, which is usually determined by examining enlarged photographs of cross-sections of foam pellets. The pores have a distribution of sizes and shapes, are interconnected and tortuous. Nevertheless, it is convenient to assume that d c represents the cylindrical form of the hydraulic diameter: de = 4 x wetted surface/wetted perimeter
(1)
which gives dc = 4 e / a
(2)
where e is the porosity and a the surface area per unit volume. Typical values of a based on the data in "I~able 2 ~re given in Table 6. The inherent appeal of the foam i~ apparent. For example, the 12 pore cm 1 foam has an equivalent diameter of 1.4 mm, yet it would be impossible to pack a bed ofparticles this small with a void fraction of 0.83. Particles this small typically exhibit a voidfraction of about 0.30-0.35 rather than the 0.83 for the foams. Pressure drop, which is dependent on porosity, will be higher. Thus the foam offers high porosity with all the inherent advantages of small diameter particles.
351 Table 5 Stability of the ceramic foam loaded with Rh a Form: cylinders, 1.27 cm diameter, 2.54 cm length Ceramic: 92% a-A1,O,, 18% mullite Original pore size: ~2 ~ores cm "1 Washcoat: 6 wt% hydrated alumina Wt% Rh: 0.7 b Calcination Temperature,~
Dispersion c %
400
26.3
600
24.5
800
15.5
1000
7.2
a Reference 25 b Measured by oxygen uptake of the reduced catalyst at 500~ c Measured by hydrogen uptake at 25~ Table 6 Surface area per unit volume for typical ceramic foams Ceramic: 92% a-ALOe, 18% mullite Positive-image, no ffas~acoat Pores cm 1
Pore diameter
a
mm
cm 2 cm 3
4
1.52
22.8
8
0.94
36.2
12
0.75
44.2
18
0.42
80.0
26
0.28
112
31
0.21
158
352 3. PROVEN APPLICATIONS From the above discussion, foam catalysts are expected to display maximum advantages in reactions which are chemically fast but suffer significant diffusion limitations. Compared with conventional porous catalysts in such situations, selectivity is also likely to be markedly improved with foam catalysts that minimize secondary reactions leading to by-products. In the following section, most of the published information on foam catalysts is reviewed. The common theme that emerges vindicates these suggestions; the only exception concerns catalyzed diesel particulate traps, which use the filtering properties of foams. 3.1 Ammonia oxidation
Selective oxidation of ammonia to nitric oxide, a key step in nitric acid manufacture, is conventionally carried out over a pad of Pt/Rh wire gauze from 800-1100~ with extremely short contact times to give high selectivity. This situation should be well suited for foam-based catalysts, and such novel systems have recently been reported [26]. Examples include Pt (about 10 wt%) on mullite foam. As predicted, the foam-based catalyst works well with the claimed advantages of using less than 15% of the amount of t~ineCOn~entiona~gau~es and emler~afiOneOf'~hot SPrOotS~ttho~t~o behaerCOnSeesqUee~egf ry P P g 9 p ' g Y "g p P and higher thermal mass, which helps dampen out hot spots. It will be interesting to see how this approach develops, one practical limitation might be migration of catalyst poisons (impurities) from the foam material caused by high temperature operation. 3.2 Catalytic combustion
Foam-based catalytic combustion was one of the earliest applications, and a number of novel designs have been suggested [27-30]. These include combustion of hydrogen [27], light hydrocarbons [28, 29], and natural gas [30]. Ceramic foams are attractive for combustion devices because of improved turbulence and mixing, together with preformed shapes configured to suit specific equipment. 3.3 Partial oxidation
An important application has recently appeared for foam catalysts in partial oxidation, both for synthesis gas production and oxydehydrogenation [30-34]. Here the emphasis is on very short contact times that allow highly selective reactions with good temperature control and efficient mixing. These examples demonstrate the selectivity provided by foam catalysts in partial oxidation. There should be many more similar applications in the future. 3.4 Steam reforming Steam reforming of natural gas and light hydrocarbons to synthesis gas is another highly diffusion-lima'ted reaction operating up to 1000~ In addition, the process is endothermic and heat transfer limited, with consequential pressure drop problems in the large array of parallel tube reactors. It is expected that foam-based catalysts will have decided advantages over conventional large-diameter particles. This has been shown to be the case in an application in which foam catalyst particles were of comparable size to conventional catal),sts, unlike the larger foam structures previously discussed [19]. This takes advantage otthe forced convection in the foamed pellets to provide higher activity and lower pressure drop. For example, overall heat transfer was increased by about 10% and pressure drop reduced by 25%. With diffusion limitations removed, the catalyst was
353 better heat transfer through the ceramic solid structure. These features are still being explored. Nevertheless, it is clear ceramic foam supports are superior for reactions with high activity and low effectiveness factors. I
1500
1
I
1000 r
o
E I-tv"
500
-
(K 0 -500
600
Z
Pellets
700
800
TEMPERATURE,
900
1000
~
Figure 2. Comparison of catalytic CO2-CH 4 reforming on Rh-loaded ceramic foams and pellets 4.2 Pressure drop measurements
Pressure drop through the catalyst bed is an important factor in reactor design. This is especially important for heat-transfer limited reactions, such as steam reforming, for which long, narrow reactor tubes are required. Ceramic foams, with their large porosities, promise substantially lower pressure drop. Although this has in fact been shown, very little fundamental investigation on pressure drop correlations has been reported. As part of the University of Houston program, pressure drop-flow rate measurements were made for a large number of ceramic foams with varying pore size. Typical results are shown in Figure 3. Pressure drop was measured with a water manometer across the length of a single pellet 2.5 cm in length and 1.25 cm in diameter, packed tightly into a quartz tube. Measurements were made using air at 25~ The curvature of the data in Figure 2 shows that flow is non-Darcian and most probably turbulent. The Forcheimer dependence found by Philipse and Schram is confirmed [52]. We fitted the data with the Ergun equation [53] expressed in terms of the hydraulic or pore diameter: DP/L = [6.667(1-e )vU/d c + 0.1167dgU2]/e dc
(3)
where DP/L is the pressure drop (Pa cml), e the porosity, v the viscosity,, d the gas density de the pore size and U the gas velocity. The best fit to Equation 3 was fobnd with constants of 7.227 and 0.1378 respectively instead of the Ergun values. This may have been due to uncertainties in the values of ~ and d. These studies reveal the classical Ergun equation is an adequate representation for pressure drop estimations through bulk ceramic foam structures. However, it should be
354 more active at higher temperatures, but this full potential may not be realized due to heat delivery limitations of conventional reformers. 3.5 Auto and diesel exhaust Among the first reports (more than 30 years agol) of foam catalysts was the oxidation of residual hydrocarbons in vehicle engine exhaust gas by a vanadized ceramic foam catalyst [14]. The autocatalyst area has developed remarkably since those pioneering days of simple oxidation catalysts, and work on foam catalysts have appeared [35,36]. Three-way autocatalyst formulations have been applied to ceramic foams with apparently acceptable results [15, 37-39], and it is not obvious from published work why foams have not competed successfully with monolithic catalysts. Perhaps this is due to physical considerations such as strength and vibration resistance. Catalyzed ceramic foam as a catalytic diesel particulate filter is an application different from other systems discussed in this paper. Here high temperature resistance and low bulk density are important, but tortuous flow paths together with open porosity are key features. As with conventional monolithic catalysts, cordierite foams have been used because of the very low thermal coefficient of expansion [40-45] 3.6 Solar processes An interesting application was a recent test in the solar CAESAR project, a joint U.S.-German program [46-50]. A parabolic foam volumetric receiver was fabricated, 65 cm in diameter and 5 cm thick and mounted in a quartz reactor situated at the focal point of a solar furnace. The foam, loaded with 0.2 wt% Rh, was then used to absorb solar energy to drive the CH.-CO. reaction as a means of storing solar energy. The foam was a very good absorber" of ~olar energy, a fact consistent with excellent foam performance in radiant heaters, and satisfactory results were obtained proving the feasibility of solar applications. This feature could be important in conventional uses in which a foam-based catalyst structure is heated by radiant infra-red energy.
4. RESEARCH AT THE UNIVERSITY OF HOUSTON Research at the University of Houston has been devoted for a number of years to catalytic applications of ceramic foams. These include solar receivers, steam- and CO 2reforming of methane, and catalytic conversion of potentially hazardous wastes to useftil products. The following examples have been selected to demonstrate the relevent properties of ceramic foams as catalyst supports. 4.1 CH4-CO 2 reforming experiments Rhodium has been shown to be an effective catalyst for CH.-CO. reforming without carbon formation [51]. This is similar in many ways to ste~am ~eforming and low effectiveness factors, and heat-transfer limitations are usually encountered. A comparison between a Rh-loaded ceramic foam and a conventional pellet catalyst with the same amount of metal was made and the results are shown in Figure 2. The ceramic foam described in Table 5 was cut into small segments and used in a differential reactor to measure the rate of reforming at different temperatures. The same procedures were carried out with a 3-mm "egg-shell" pellet containing 0.5 wt% Rh/A120 3. Both catalysts were heated at 1000~ before the measurements. Dispersions were almost identical at 7.2% and 11.0% respectively. The CO./CH. ratio was one. Figure 2 shows a large difference between the two supports. Fo~ example, at 900~ the foam has a rate of 12~0 mol h 1g R h ,1 whereas the perle.ted catalyst is 100 mol h-1g R h .1 Expressed as turnover numbers, this corresponds to 497 and 27 molecules s 1 site-1 respectively, a factor of 18 different! An obvious explanation for this difference is the foam has a very high effectiveness factor compared to the pellet, but there could also be an enhancement in rate due to
355 remembered that particles with the same equivalent diameter would not pack into a reactor bed at the same voidage as the foam, so that the pressure drop is substantially higher. 1400
1200 E o
1000
o ra
o. 0
o~ =) o3 o~
'"
o~ o.
800
600
400
200
IV"
,•r"•lwllr'•l
0
-
0
100
I
200
I
300
I
400
I
500
I,
600
700
VELOCITY, c r n / s
Figure 3. Pressure drop in a 12 pores cm1 ceramic foam 4.3 Mass transfer correlations
Predicting mass transfer coefficients is important in designing ceramic foam applications, yet no systematic investigation of these parameters has been reported. We adopted the procedure of measuring catalytic conversions under conditions deliberately selected to ensure mass transfer limitations [54]. The reaction used was the oxidation of carbon monoxide over platinum catalysts. The washcoated foam described in Table 4 was loaded with 5-10 wt% Pt using chloroplatinic acid impregnation procedures. A thin section (0.318 cm) was wrapped in quartz wool, tightly fitted into a quartz tube and surrounded on each side with quartz wool packing. The reactor was operated differentially with 5 vol% CO in oxygen in the temperature range 200-600~ Tests confirmed the system was operating in an external diffusion-controlled regime. Rate data were taken at 550 C for increasing velocities, and the mass transfer coefficient, kc, calculated assuming first order dependence. Fluid properties were used to find the mass transfer factor, Jd, which was then correlated with the Reynolds number, Re., based on the pore diameter. Results are shown in Figure 4. The data correlated" well with Equation (4), which is within accepted range of precision for the hydraulic equivalent of e Jd = 0"326R% "~
(4)
the popular Satterfield equation (0.487R%~ This agreement indicates that standard correlations for mass transfer coefficients are acceptable for ceramic foams.
356 4.4 Heat transfer correlations Heat transfer into foams is expected to be higher than packed particles because of added conduction through the struts and forced convection into the pores due to their larger size [55, 56]. This was tested in a series of experiments in which four pellets (each 10
~
10
0
|
i
i
|
|.
-1
-2
I
10 0
!
n
I
I
I
!
t
I
10 1
~
t
~
t
t
t
n
I
10 2
Re h Figure 4. Mass transfer factor correlation for a 12 pores cm "1 ceramic foam 12 pores cm 1 maintained at temperatures transfer into coefficient, h and flow rate h
and 2.54 cm in length) were loaded into a 1.25 cm diameter quartz tube a constant temperature between 500~ and 850~ and inlet and outlet were measured at increasing flow rates. Using a 1-D model for heat the foam, the functional form of the wall convective heat transfer , was adjusted until the best overall fit was obtained over the temperature range. This relationship is given by Equation (5) where kg is the thermal
= 0.755kgReh~176
(5)
conductivity of the gas. Comparison of Equation (5) with similar expressions for packed beds is difficult since particles with similar diameters to the foams have much lower porosities. This was done more effectively in simulation calculations described in the next section. 4.5 Model comparisons Precise comparisons between packed beds and ceramic foam structures are complex since many factors - activity, effectiveness factors, mass and heat transfer, and pressure drop - are all interdependent. We have simulated the performance of a conventional steam reformer and compared it to one containing a ceramic foam cartridge loaded to achieve equivalent intrinsic activity per gram of catalyst. A 1-D model developed and tested previously for heat-pipe reformers with isothermal walls was used [57]. Pressure drop, mass transfer and heat transfer correlations for the packed bed were known to be accurate for commercial catalysts; those used for the foam were determined in the studies described above. Process conditions and results are given in Table 7. The most dramatic result is a decrease in the required length of the reformer tube by about a factor of two. This is a consequence of the higher effectiveness factor and heat transfer properties of the foam. The higher porosity gives a decrease in the pressure drop
357 for the same tube length of a factor of three. The smaller bed required for the foam is an added advantage, decreasing pressure drop by almost a factor of ten. These advantages promise substantial reduction in reformer size, capital costs, and operating conditions. It must be emphasized, however, that these benefits may only be realized with reformers having higher heat transfer coefficients than conventional radiant or convective systems. The impact of lower mechanical strength for the foam remains to be addressed.
Table 7 Simulation comparison between a conventional packed bed reformer and a ceramic foam cartridge. Tube diameter, cm: 10 CH. flow, mol hX: 1500 H.t)/CH. ratio: 3 W~all temperature, ~ 800 Inlet temperature, ~ 550 Pressure, atm: 20 Convention catalyst: multi-hole cylinders Ceramic foam: 1:~pores cm"1 Property
Conventional
Foam
Tube length for reaction, cm:
916
439
Effectiveness factor at outlet:
0.05
1.00
Average heat flux kW m':
31.2
67.1
Pressure drop atm:
1.21
0.14 (0.40) a
a for the same tube length as the Conventional Reformer 5. FUTURE DIRECTIONS The potential benefits of foam-based catalysts have been adequately demonstrated. The most important attributes are decreased diffusion limitations, lower pressure drop, increased heat transfer, improved mixing, and prefabrication of special shapes. Ideal processes are highly exo- and endothermic reactions and those requiring good selectivity control. Other novel applications, e.g. in trickle bed reactors, will no doubt appear. The main disadvantage is the relative weakness of ceramic foams. Although some work on the elastic and mechanical properties has appeared [58-60] very little attention has been given to improving these properties. Possible alternatives are negative-image foams [19] and incorporation of additives into the ceramic [61].
358 6. ACKNOWLEDGMENTS Research at the University of Houston reported in this paper was supported by Sandia National Laboratories, Albuquerque, N.M., U.S.A. under contract No: 55-4032 and by the Texas Higher Educational Coordinating Board ATP Program, Grant No: 003652121 ATP. We are grateful for the contributions of M. Garrait, D. Remue, and J-K Hung. REFERENCES
1. L.J. Gibson and M. F. Ashby, Cellular solids, structures and properties, Pergamon Press, Oxford .(1988). 2. A. Nir and L. Pxsmen, Chem. Eng. Sci., 32 (1977) 35. 3. A. Rodriques, B. Ahn, and A. Zoulanian, J. AIChE, 28 (1982) 541. 4. D. Cresswell, Appl. Catal., 15 (1985) 103. 5. A. Rodriques and R. Quinta Ferreira, AIChE Symp. Ser. 84 (1988) 80. 6. A. Rodriques and R. Quinta Ferreira, Chem. Eng. Sci., 45 (1990) 2653. 7. R. M. Quinta Ferreira, M. M. Marques, M. F. Babo, and A. E. Rodrigues, Chem. Eng. Sci., 47 (1992) 2909. 8. J. W. Brockmeyer and L. S. Aubrey, Ceram. Eng. Sci. Proc., 8 (1987) 63. 9. P. K. Serville, R. Clift, C. J. Withers, and W. Keidel, Filtr. Sep. 26 (1989) 265. 10. K. Mangold, W. Taetzner, Ger. Often. DE 3,731,888 (1989). 11 V.A. Maiorov, L. L. Vasirev and V. M. Polyaev, J. Eng. Phys. 47 (1984) 1110. 12. R. Viskanta, in (J. R. Lloyd and Y. Kurosaki, Eds.), Proceeding s of the third ASME/JSME Joint Thermal Engineering Conference, ASME/JSME, New York (1991) 163. 13. F. Anderson, Prog. Energy Combust. Sci. 18 (1991) 12. 14. K. Schwartzwalder and A. Somers, U.S. Patent 3,090,094 (1963). 15. F. Druche, Ger. Often. DE 3,510,176 (1986). 16. H. Kondo, H. Yoshida, Y. Takeuchi, S. Nakagawa, JP 62 61,645 (1987). 17. F. F. Lange and K. T. Miller, Adv. Ceram. Mater., 2 (1987) 827. 18. M. V. Twigg and W. M. Sengelow, U. S. Patent 4,810,685 (1989). 19. M. V. Twigg and W. M. Sengelow, U. S. Patent 4,863,712 (1989). 20. I. Satoyuki and S. Nonaka, Jpn. Kokai Tokkyo Koho JP 03 123,640 (1991 / 21. I. Satoyuki and S. Nonaka, Jpn. Kokai Tokkyo Koho JP 03 123,641 (1991ji 22. I. Satoyuki and M. Inoe, Jpn. Kokai Tokkyo Koho JP 03 122,070 (1991). 23. R. A. Clyde, U.S. Patent 3,998,758 (1976). 24. R. A. Clyde, U.S. Patent 3,900,646 (1975). 25. M. Garrait, A ceramic matrix catalyst for solar reforming, M.S.ChE. Thesis, Department of Chemical Engineering, University of Houston, (1989). 26. L. E. Campbell, U.S. Patent 5,256,387 (1993); 5217939 (1993). 27. M. Haruta, Y. Souma, and H. Sano, J. Hydrogen Energy, 7 (1982) 729. 28. T. Inui, T. Kuroda, and T. Otowa, J. Fuel Soc. Jap., 64(1985) 270. 29. T. Inui, Y. Adach, T. Kuroda, M. Hanya and A. Miyamoto, Chem. Express 1 (1986) 255. 30. K. Mangold, G. Foerster and W. Taetaner, Ger. Often. DE 3,732,653 (1989). 31. D. A. Hickman and L. D. Schmidt, Science 259 (1993) 343. 32. M. Huff and L. D. Schmidt, J. Phys. Chem., 97 (1993) 11815. 33. K. A. Vonkeman and L. V. Jacobs, Eur. Pat. Appl. EP 576,096 (1993). 34. P. M. Torniainen, X. Chu and L. D. Schmidt, J. Catal., 146 (1994) 1. 35. G. Weldenbach, K. H. Koepernik and H. Brautigam, U.S. Patent 4,088,607 (1978). 36. T. Narumiya and S. Izuhara, U.S. Patent 4,308,233 (1981).
359 37. H. Hondo, H. Yoshida, Y. Miura, Y. Takeuchi and S. Nagagawa, JP 63 883,049 (1988). 38. A. Muramatsu and K. Yoshida, Jpn. Kokai Tokkyo Koho JP 04 04,237 (1992). 39. K.Tabata, I. Matsumoto, T. Matsumoto, J. Fukuda, Jpn. Kokai Tokkyo Koho JV 04 04,019 (1992) 40. Y. Watabe, K. Irako, T. Miyajima, T. Yoshimoto and Y. Murakami, SAE Technical Paper 830082 (1983). 41. J. J. Tutko, S. S. Lestz, J. W. Brokmeyer and J. E. Dore, SAE Technical Paper 840073 (1984). 42. T. Inui and T. Otowa, Appl. Catal. 14 (1985) 83. 43. M. Kawabata, S. Matsumoto, K. Kito, H., Yoshida, JP 01 143,645 (1989). 44. T. Mizrah, A. Maurer, L Gauchler and J-P Gabathuler, SAE Technical Paper 890172 (1989). 45. M. Nitsuta and M. Ito, Jpn. Kokai Tokkyo Koho JP 02 173,310 (1990). 46. R. E. Hogan, Jr., R. D. Skocypec, R. B. Diver, J. D. Fish, M. Garrait, and J. T. Richardson, Chem. Eng. Sci., 45 (1990), 2751. 47. R. Buck, J. F. Muir, R. E. Hogan, Jr., and R. D. Skocypec, Solar energy materials, proceedings of the 5th symposmm on solar high-temperature technologies, Davos, Switzerland, August 1990, 24 (1991) 449. 48. R. E. Hogan, Jr., and R. D. Skocypec, J. Solar Eng. Eng., 114 (1992) 106. 49. R. D. Skocypec and R. E. Hogan, J. Solar Eng. Eng., 114 (1992) 112. 50. J. F. Muir, R. E. Hogan, Jr., R. D. Skocypec and R. Buck, The CAESAR project, Sandia Report SAND92-2131 (1993). 51. J. T. Richardson and S. A. Paripatyadar, Appl. Catal. 61 (1990) 293. 52. A. P. Philipse and H. L. Schram, J. Am. Ceram. Soc. 74 (1991) 728. 53. S. Ergun, Chem. Eng. Prog. (1952) 89. 54. D. Remue, Properties of ceramic foam catalyst supports, M.S.ChE. Thesis, Department of Chemical Engineering, University of Houston, (1993). 55. L. B. Younis and R. Viskanta, Int. J. Heat Mass Transfer, 6 (1993) 1425. 56. W. H. Meng, C. McCordic, J. P. Gore and K. E Herold, ASME/JSME Thermal Engineering Proceedings, 5 (1991) 181. 57. J. T. Richardson, S. A. Paripatyadar, and J. C. Shen, AICHE J., 34 (1988) 743. 58. H. Hagiwara and D. J. Green, J. Am. Cer. Soc., 70 (1987) 811. 59. R. Brezny and D. J. Green, J. Am. Ceram. Soc., 72 (1989) 1145. 60. H. Hagiwara and D. J. Green, J. Am. Ceram. Soc., 70 (1987) 811. 61. M. Usiu and, O. Yonemochi, JP 62 212,282 (1987).
This Page Intentionally Left Blank
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
361
A new method for the preparation of metal-carbon catalysts
P. A. Barnes and E. A. Dawson Catalysis Research Unit, Leeds Metropolitan University, Calverley Street, Leeds LS1 3HE, UK.
SUMMARY Metal carbon catalysts were prepared by exchange of transition metal ions with cellulose ion exchange resins. Thermal decomposition of these materials, followed by activation of the carbon by water vapour, yielded small metal particles supported within the carbon matrix. The materials were characterised by gas adsorption techniques for total and metal surface areas, by X-ray line broadening for crystallite size measurement and by transmission electron microscopy. The method gave similar products to those produced by thermal decomposition of metal cellulose composite materials, but smaller crystaUites and higher dispersions were produced. The metal loadings can be controlled easily. The presence of chloride ions during the preparation did not appear to affect crystallite growth.
1. I N T R O D U C T I O N Metal carbon catalysts are conventionally prepared by impregnation of an active carbon with a solution of the metal salt, followed by drying, calcination and reduction to yield small metal particles supported on a porous carbon. This method produces active catalysts, eg for hydrogenation in fine chemical manufacture, but can suffer from variations in the active carbon, sintering of the metal particles and limited metal loadings. In an attempt to overcome some of these limitations, a new approach involving the thermal degradation of some metal-cellulose precursors was adopted [1 ]. This involves the preparation of a copper cellulose complex by dissolving cellulose in an aqueous solution of a copper(II) complex with 1,3 propanediamine. Elimination of the amine in alkaline conditions produces a highly cross-linked structure (fig. 1) with a stoichiometry of 1 copper ion to 2 glucose units. The copper(II) ions may then be reduced in situ with hydrazine to give small copper particles trapped in the cellulose film. Other metals such as palladium or silver which are less electropositive than copper can be exchanged after reduction of the copper(II) ions with hydrazine, providing a route to a range of metal carbons. It was hoped the precisely defined
362
OCH=
0
0u
0
0
0
0
~0
ci%o
Figure 1. Repeating unit of the copper(II) cellulose complex ratio of metal to carbon would yield reproducible metal-carbons after heating (charring) to break down the glucose to form carbon. Following the charring in an inert atmosphere at 400 ~ the carbon in the material was activated by heating to 600 ~ under a flow of nitrogen and water vapour to develop a pore structure to expose the metal particles to the gas ph~e. It was shown [2,3] that the metal particles catalysed the activation, giving a network of interconnecting pores. The materials produced by this method were all microporous carbons, typically of surface area 250 m 2 g-l, with a mean copper crystallite size of 20 nm and a metal-carbon ratio of about 50%. These properties were directly due to the amount of copper necessary to solubilise the cellulose (ca 16% w/w) which, after activation (and loss of carbonaceous matter) increased to ca 50%. The relatively low surface area for an active carbon and the relatively large copper particle size could therefore be accounted for by the high metal content. A series of experiments, following the statistical method of Taguchi [4,5], was carried out in order to maximise the total surface area and minimise copper crystallite size. The charring and activation conditions which produced the best combination of these properties were adopted in subsequent experiments, although it was recognised that different metals may affect the course and extent of the thermal degradation and activation of the cellulose. However, this route suffered from two disadvantages: the relatively large metal particle size and the limited number of metals which could be exchanged for copper. A more direct synthesis was then devised in which the required metal, as a suitable salt or complex in solution, is reacted with either an anionic or cationic cellulose ion exchange resin. The metal-cellulose material was then charred and activated as before to give a dispersion of metal particles in a porous carbon matrix. The method has the immediate advantage of extending the range of metals which can be incorporated to include those more electropositive than copper eg nickel.
363 2. E X P E R I M E N T A L
2.1. Metal ion exchange The cation exchange material was carboxymethyl cellulose, (CMC, Whatman) as the sodium salt (fig. 2). Hydroxyl groups on the cellulose chain are modified to -OCH2COONa, where sodium is the exchangeable cation. The maximum exchange capacity is 0.6 meq/g. Since the resin is the salt of a weak acid, at low pH the ion exchange capability is less, due to the predominance of the unionised acid form. The anion exchange cellulose was diethylaminoethylcellulose (fig. 3, DEAEC, Whatman) as the hydrochloride. The modifying group is -OCH2CH2NH(Et)2C1 with chloride as the exchangeable anion. The maximum exchange capacity is 1.0 meq/g, under conditions where pH is less than 10, since the free amine is a moderate base.
HOCI-~
HO
o
OCI-hCOONa
Figure 2. Carboxymethylcellulose, sodium salt.
HOC I-IO
o
OC
C
N
CICI%
Figure 3. Diethylaminoethylcellulose.
Various transition metals were used at various concentrations corresponding to 10%, 20%, 50% and 100% of the theoretical exchange capacity of the cellulose. The exchange simply involved stirring the ion exchange material (2g) with a solution (total volume 200 cm 3) of the metal ions at ambient temperature until decolorisation of the original solution occurred. The effect of pH on the exchange was not studied in these initial experiments. Finally, the solid was filtered off, washed thoroughly and dried. The compounds used for ion exchange were K2PtC16, K2PtC14, Pt(NH3)4C12, K2PdC14, Pd(NH3)4C12, NiC12, Ni(NO)3, RhC13, Fe(NO3) 3 and CuSO4. Stock solutions containing 10 meq/dm 3 were prepared and aliquots taken as necessary.
2.2. Charring The dried materials (2g) were placed in a vertical silica glass tube (od 2 cm), between quartz wool plugs and charred in a furnace at 400 ~ for one hour under flowing nitrogen (40 cm 3 mint). After cooling under nitrogen, the products were lightly crushed before activation.
364
2.3. Activation All samples were activated under the same conditions which had been found previously to give the highest surface area for samples containing copper [5], ie heated at 600 ~ for 1 hour under flowing nitrogen saturated with water vapour at 25 ~ The concentration of water vapour in the gas stream was measured using GC and found to be about 5-5.5x10 "2 mg cm "3, depending on the exact flow rate of the carrier gas. Sample numbers with suffix A denote unactivated products, B denotes activated products.
2.4. Surface area Total surface areas were measured by nitrogen adsorption at -196 ~ using an automated instrument (Omnisorp 100CX, Coulter Electronics Limited). The cross sectional area of the nitrogen molecule was assumed to be 16.2 x 10.20 m 2. Pore type and volume data were also obtained by this method, using t-plot analysis. Metal areas were measured by selective chemisorption of hydrogen at 30 ~ in the same instrument. Copper surface areas were measured in a flow system by nitrous oxide chemisorption at 60 ~ Samples were outgassed at 250 ~ ovemight under vacuum before nitrogen adsorption and reduced under flowing hydrogen at 300 - 400 ~ depending on the metal, for 45 min before chemisorption.
2.5. Metal particle size Metal crystallite size was measured by X-ray diffraction line broadening using a Philips goniometer (PW 1050) and Hiltonbrooks generator. Copper K~ radiation (wavelength 0.154 nm) was used with a monochromator to remove the I~ component. The samples were scanned in increments of 0.01 ~ counting for 5 sec per increment. The usual corrections were made for line broadening due to the K~ doublet and instrumental factors. Finally, the Scherrer equation was used to calculate the mean crystallite size. As the charring and activation stages involve relatively high temperatures, we believe it is possible to equate the mean crystallite and particle sizes.
2.6. Metal content Metal content was measured by atomic absorption spectrophotometry using a Pye-Unicam SP9 spectrophotometer. Samples (0.05 g) were ashed for 6 hours at 600 ~ and the residue dissolved in aqua regia. Any remaining carbon residue was filtered off and the volume made up to 100 cm 3 with hydrochloric acid (0.1 mol dm 3) containing lanthanum chloride (equivalent to 1% w/v La) to control ionisation interference. For comparison purposes, two commercially available metal carbon catalysts (5% Pt on carbon and 5% Rh on carbon, Aldrich) were also characterised by the above methods.
365
3. R E S U L T S
AND DISCUSSION
3.1. S u r f a c e area Two samples were measured before activation (226A, containing 17% Pt and 249A containing 0.5% rhodium). The nitrogen adsorption isotherms were classed as type 2 and hence BET total surface areas were calculated for these samples (5 and 33 m 2 g-t respectively). They showed no micropore volume on inspection of the t-plots and the whole surface area for each sample was accounted for by the combined area of the meso and macropores. These t-plot values are in good agreement with the total surface areas (table 1) and show that the subsequent activation process develops a microporous structure not present in the chars.
Table 1. Total surface area properties Sample
Pd Pt
% Metal content
231B 1 223B 12 224B 7 225B 1 226A 17 226B 17 241B 3.5 243B 1 Cu 211B 18 213B 14 Rh 245B 0.5 249A 0.5 249B 1 Ni 352B 354B Fe 351B 353B RhPd 250B 0.5/0.5 RhPt 251B 0.5/0.5 Pt/C Aldrich 5 Carbon Cu 200B 50 *BET area
Micropore volume/cm 3g-I Surface area/m 2 g-I L--angmuir Meso+macropore ,
497 563 233 494 5' 434 650 303 496 508 687 33" 719 522 459 552 413 702 736 1122 678 230
,
48 91 68 52 0 46 82 67 52 52 139 35 141 26 34 43 49 127 112 604 110 21
0.148 0.149 0.041 0.145 0 0.128 0.182 0.070 0.147 0.151 0.167 0 0.180 0.170 0.142 0.171 0.117 0.176 0.200 0.121 0.183 0.067
366 In general the Langmuir total surface areas (table 1) of the activated metal-carbons were in the range 400-500 m 2 g.t, which is low compared to commercially activated carbons used as catalyst supports. This was probably due to the time allowed for activation, and it is possible that longer times would increase the total surface area. However, this may be at the expense of loss of metal area due to sintering. The activation time of one hour was a result of the original work [ 1] on samples containing only 50% carbon, where this time was adequate for maximum activation. There appeared to be slight differences between the metals regarding development of surface area, with rhodium containing carbons having the highest activated areas. The blank carbon, activated at 200 ~ higher temperature under otherwise identical conditions had a slightly higher surface area (678 m 2 g-i) than many of the samples, except those containing rhodium. It has been shown previously [2,3] that copper catalyses both the thermal degradation of cellulose and carbon activation. Sample 200B, prepared according to reference [1] had a low area compared to the other activated materials, but this can be accounted for by the presence of 50% w/w of non-carbon material, ie copper. The relatively low temperature (600 ~ of activation of the metal carbons and the resulting increases in surface areas would appear to confirm that all the metals used catalyse carbon gasification. Other workers have also noted the catalytic effect of some of these metals on carbon activation [6,7].
3.2. Pore size distribution With the exception of the unactivated samples, all the metal-carbons were microporous. As can be seen from table 1, the meso and macropore areas as calculated from t-plots were ca 10% of the total surface areas as calculated from the Langmuir plots. Although it is recognised that Langmuir plots represent only the equivalent surface area of a monolayer calculated from the volume of gas adsorbed, it can be seen that most of the adsorptive capacity of these materials is in the micropores.
3.3. Metal content Analysis of the samples by atomic absorption spectrophotometry showed that less than the theoretical degree of ion exchange occurred, ie even at initial amounts below the theoretical maximum for CMC and DEAEC, some metal ions remained in the filtrate and were not taken up by the cellulose. In these experiments, the effect of pH on exchange capability was not investigated, but it is probable that maximum exchange cannot be achieved with CMC and some of the platinum complex cations, since these are highly acidic in aqueous solution.
3.4. Metal areas Dispersion figures (table 2) were with few exceptions at or above 20%. These compared well with the commercial Pt/C catalyst and indicate that the metal crystallites had not sintered excessively during the thermal processes involved in preparation. The lowest dispersion of 10% (sample 224B) was associated with a relatively low total surface area in the sample. It is possible that for this sample, activation did not fully develop the pore
367 network to enable the internal metal particles to be exposed to the gas phase. Materials prepared containing rhodium exhibited dispersion figures comparable to platinum and certainly higher than the commercial Rh/C catalyst. However, the mixed metal-carbon (251B) containing both Rh and Pt appeared to have a dispersion figure of 90%, calculated by assuming equal numbers of surface Rh and Pt atoms. This may not be exactly the case, as the dispersion could be between 117% (100% Pt on surface) and 62% (100% Rh on surface). A further consideration, given these high dispersion figures is the possibility of hydrogen spillover occurring on the carbon. This has been reported to occur in platinum carbons [8], and may intrude in determinations of metal areas by chemisorption of hydrogen [9].
Table 2 Metal area properties Sample
Pd Pt
Ni Rh
RhPd RhPt Pt/C Rh/C Cu
231B 223B 224B 241B 242B 243B 352B 354B 245B 249A 249B 250B 251B Aldrich Aldrich 200B
Metal area/ m 2 g~ catalyst
% Dispersion
% Metal content
2.4 6.1 1.8 2.4 1.6 0.6 2.2 0.7 0.5 0 1.3 2.8
54 20 10 28 22 23
1 12 7 3.5 3 1
22 0 28 90
2.0 1.1 25
16 5 5
0.5 0.5 1 0.5/0.5 0.5/0.5 5 5 50
The metal areas of the unactivated metal-carbons (226A and 249A) measured by H2 chemisorption appeared to be zero ie the reversible and irreversible chemisorption isotherms were co-incident. Therefore, although a small amount of reversible chemisorption did take place, this was shown to be due to chemisorption on the carbon as chemisorption isotherms for the carbon containing no metal and sample 249A (Rh/C) were virtually superimposable.
368 Sample 200B, prepared according to reference [1 ] had the highest metal area per gram of catalyst, but this was not unexpected since it contained the most metal. However, the dispersion figure was relatively low. Results for palladium were less reliable because of the possibility of hydride formation. For this reason, chemisorption was conducted at a higher temperature.(100 ~ to minimise the dissolution of H 2 in the metal [10]. 3.5. M e t a l c r y s t a l l i t e size Generally, before activation, all the metals showed crystallite (particle) sizes near the limit of measurability of the XRD line broadening method, (table 3). Materials with more than 3.5% Pt showed an increase of mean crystallite size on activation which was rather high e,g from 3 to 19 nm or 4.5 to 23 nm (samples 223 and 224). However, materials with less metal showed little or no increase in size which remained in the 2-3 nm range. Presumably the particles were too far apart for sintering to occur. Palladium and nickel showed similar crystallite size increases on activation, irrespective of the amount of metal present. The increase for palladium appeared slightly greater, ie from 2-3 nm rising to 10-15 nm, whereas the nickel crystallites increased in size from smaller than 2-3 nm to 11 nm maximum. Since the difference in melting temperature between the two metals is only ca 100 ~ in 1500 ~ the similarities are not unexpected. Some correlation between the crystallite size and the measured metal areas was found, although exact agreement cannot be expected, as the line broadening method encompasses a range of sizes, and does not detect very small particles. Samples with low loadings of platinum (241B-243B, 1-3.5% Pt) had measured mean crystallite sizes of 2-3 nm. Calculation of the metal area from these figures yields an area of 3-4 m 2 g-1 of catalyst compared to a value measured by chemisorption of 0.5-2.5 m 2 g-l. Alternatively, the measured metal areas of these samples would correspond to a mean crystallite size of 4-5 nm. There was similar agreement between measured values of crystallite size and metal area for sample 224B, with a higher platinum loading of 7%. The measured mean crystallite size of 23 nm would yield a calculated metal area of ca 0.8 m 2 g'l compared to the measured value of 1.8 m 2 g~. In the case of the low loading samples, the measured area is lower than that calculated from the crystallite size figures. It is unlikely therefore that spillover of hydrogen is occurring, as this would tend to make the measured metal areas unrealistically large. Sample 200B was prepared from copper-cellulose according to reference [1] and accordingly showed different characteristics because of the much higher copper loading. The crystallites grew by 50% on activation and were 5 times larger than the ion exchange copper crystallites. It is probable that shorter inter-crystallite distances and the relatively low melting temperature (1083 ~ of copper account for this. The XRD line broadening particle size measurements were confirmed by TEM, the new method of preparation giving a fine uniform dispersion of metal particles.
369 Table 3 Crystallite size measurements ,
.,
Pd
Pt
Ni
Fe Rh Cu RhPd RhPt Pt/C RtgC Cu
Sample
Mean crystallite size/nrn,, Unactivated Activated (A) (B)
231 232 233 234 223 224 226 241 242 243 227 228 352 354 351 353 245 249 211 213 250 251 Aldrich Aldrich 200B
3 3 2 2 3 4 19 2 3 2 2 * 3 * * * * 2 4 4 2 2 20
15 8 9 14 19 23 31 2 3 3 4 10 11 7 2 2 2 3 6 7 9 2 4 2 30
Metal ion source
Pd(NH3)4C12
K2PdC14 " " K2PtC16 " " K2PtC14 K2PtC16 Pt(NH3)4CI z NiC12 " Ni(NO3) 2 "
Fe(NO3)3 "
RhC13 " CuSO 4 " RhC13/K2PdC14 RhC13/K2PtC14
* Peaks too broad to measure accurately (crystallites < 2 nm diameter).
3.6. Influence of ligand atoms It was expected that the presence of chloride ions in the preparation may affect the resulting crystallite sizes and/or act as poisons to chemisorption sites. However, sample 243B, prepared by cation exchange of Pt(NH3)42§ with CMC did not appear to be better with respect to crystallite size or metal surface area compared to the anion exchange materials. The samples were not analysed for chloride ion, but it is probable that it was lost during thermal degradation and subsequent sample preparation procedures. If this is so then the presence of such a catalyst poison is not critical during this method of preparation.
370 4. C O N C L U S I O N S The method for making metal carbons using an ion exchange process was similar in many ways to the technique previously used [1]. Both produced microporous carbons with dispersions of metal crystallites which were accessible to the gas phase. The main difference arising from the use of ion exchangers as the source of cellulose appears to be the production of much smaller metal crystallites in the ion exchange materials. In the ion exchange process there is the opportunity to control the metal loading at low levels with a corresponding increase in dispersion. The discrepancy between the measured metal surface areas and those calculated from the X-ray measurements may be due to the presence of very small particles (not detected by the X-ray method) or particles not accessible to the gas phase (not detected by chemisorption), depending on which surface area figure is the larger. It is possible that the platinum carbons with low metal content yielded activated products which did not have a fully developed pore system. This could account for the lower measured surface area than that calculated from particle size measurements. These preliminary results are sufficiently encouraging to warrant a more detailed study.
ACKNOWLEDGEMENTS The authors would like to thank Johnson Matthey PLC for the loan of the precious metals.
REFERENCES
.
4. 5. 6. .
8. 9. 10.
W. Airey, S.I. Ajiboye, P.A. Barnes, D.R. Brown, S.C.J. Buckley, E.A. Dawson, K.F. Gadd and G. Midgley, Catal. Today, 7 (1990) 179. P.A. Barnes, E.A. Dawson and G. Midgley, J. Chem. Soc. Faraday Trans., 88(3) (1992) 349. P.A. Barnes and E.A. Dawson, J. Thermal Anal., (1994) in press. P.J. Ross, Taguchi Methods for Quality Engineering, McGraw Hill, (1988). E.A. Dawson and P.A. Barnes, Appl. Catal. A, 90 (1992) 217. R.R. Adair, E.H. Boult, E.M. Freeman, S. Jasienko and H. Marsh, Carbon, 9 (1971) 763. H. Marsh and B. Rand, Carbon, 9 (1971) 63. A.J. Robell, E.V. Ballou and M. Boudart, J. Phys. Chem., 68(10) (1964) 2748. P.A. Sermon and G.C. Bond, Catalysis Rev., 8 (1974) 211. BS 4359, Part 4, (draft) 1994.
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
371
C o n v e r s i o n of activated carbon into porous silicon carbide by fluidized bed chemical v a p o u r deposition* R. Moene a, L.F. Kramer a, J. Schoonman b, M. Makkee a, and J.A. Moulijn a aDepartment of Chemical Process Technology, bLaboratory of Applied Inorganic Chemistry, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands
ABSTRACT A new preparation method is described to synthesize porous silicon carbide. It comprises the catalytic conversion of preformed activated carbon (extrudates or granulates) by reacting it with hydrogen and silicon tetrachloride. The influence of crucial conversion parameters on support properties is discussed for the SiC synthesis in a fixed bed and fluidized bed chemical vapour deposition reactor. The surface area of the obtained SiC ranges from 30 to 80 m2/g. The metal support interaction (MSI) and metal support stability (MSS) of Ni/SiC catalysts are compared with that of conventional catalyst supports by temperature programmed reduction. It is shown that a Ni/SiC catalyst shows a considerable lower MSI than Ni/SiO 2- and Ni/A1203-catalysts. A substantially improved MSS is observed; an easily reducible nickel species is retained on the SiC surface after calcination at 1273 K.
1. INTRODUCTION During the last two decades much effort has been devoted to the development of ceramic, non-oxidic, and non-metallic catalysts [1-3]. This class of materials consists mainly of carbides, nitrides, and boddes of transition metals, and possesses interesting properties regarding their catalytic activity and thermal stability. However, the difficulty in controlling the surface composition during synthesis and application of these materials have limited their use at commercial scale. Research for the development of ceramic non-oxidic catalyst supports has mainly been focused on silicon carbide. The physical properties of bulk SiC (high thermal stability, resistance against oxidation, hardness, and inertness of its surface) suggest that it is a promising candidate for catalytic operations at high temperatures or liquid phase reactions at demanding pH conditions. Several ways are reported to synthesize high surface area SiC powder. Examples are gas phase decomposition of Si(CH3)4 at 1773 K to obtain SiC powder with surface areas near 50 m2/g [4], reaction of gaseous SiO and activated carbon to form SiC of 59 m2/g [5], and pyrolysis of organosilicon gels to arrive at SiC of
This research was part of the Innovative Research Programme on Catalysis (IOP-Katalyse, project 90017b) and was financially supported by the Ministry of Economic Affairs of the Netherlands
372 179 m2/g [6]. A limited amount of data is reported which compares the properties of SiC based catalysts with those of conventional catalyst supports. Because all reported preparation procedures of porous SiC are expected to be much more expensive than those of SiO 2 and A1203, utilization of SiC has to provide substantial advantages over conventional supports to make its production economically attractive. Properties of SiC, which are expected to allow an improved performance compared with SiO2 and A1203, are high thermal stability, stability under demanding pH conditions, and weak metal support interaction. In practice, this means that application of SiC as catalyst support will be in processes which benefit considerably from these particular properties. This paper reports on the results of the conversion of preformed activated carbon such as extrudates and granulates, utilizing a fixed bed and fluidized bed reactor for reacting activated carbon with hydrogen and silicon tetrachloride (SIC14). To achieve a substantial carbon conversion the use of an additional catalyst is essential [7]. Thus, the reactivity of the carbon is enhanced owing to the gasification activity of the catalyst. Research has shown that nickel is appropriate for catalyzing both the gasification and SiC formation. The overall reaction of carbon, hydrogen, and silicon tetrachloride to silicon carbide and hydrogen chloride can be separated into reaction 1 and 2. Ni
C(s) + 21-I2(g) ~. CH4(g) Nl
SiCl4(g) + CH4(g) • SiC(s) + 4HCl(g)
(1)
(2)
The influence of crucial synthesis parameters on the conversion and characteristics of this support material has been investigated. Especially the stability of the catalyst at elevated temperatures is of primary importance. To discriminate between differences in catalytic behaviour originating from a pure metal support interaction and reaction of the support with the active phase, the expression "metal support stability" (MSS) is introducexi. Both the MSI and MSS are investigated for the Ni/SiC catalyst and are compared to nickel catalysts based on conventional supports (SiO2 and A1203).
2. EXPERIMENTAL This section deals with the methods of preparation of porous SiC and SiO2, A1203 and SiC based catalysts. Physical properties of the applied supports are shown in Table 1.
2.1. Preparation of high surface area SiC Activated carbon extrudates (Norit RW08) are impregnated with nickel by the incipient wetness method (Ni(NO3) 2. 6H20 in water) to arrive at nickel contents of 2, 5, or 8 w%. After drying overnight at 385 K the extrudates are placed as a fixed bed (length 1 mm) in a tubular quartz reactor (internal diameter 42 mm). The reactor is heated (0.167 K/s) under flowing hydrogen at 100 kPa to 1400 K and maintained at this temperature for 5 minutes. Subsequently, the reactor was cooled down to the desired reaction temperature. The concomitant weight decrease for 2w% Ni/C due to gasification amounts to 18 %. The hydrogen flow is subsequently increased to 3.69 mol/h, the pressure is adjusted to 10 kPa and
373 Table 1 Physical properties of catalyst supports
Code
Support
SBI~T
Vpore
(m~/g)
(ml/g)
Geometry
Activated carbon extr.
Norit RW08
947
1
3 mm x 0.8 mm
Activated carbon gran.
Norit Elorit
655
0.6
ds0:450/~m
SiO 2
Engelhard Si- 162-1
30
0.6
grinded extr.
A1203
Engelhard A1-4196
8
0.6
grinded extr.
extr.: extrudate, gran. :granulate gaseous silicon tetrachloride (SIC14) is introduced (flow rate 0.14 mol/h). After reaction the reactor is pressurized to 100 kPa and cooled down under flowing hydrogen to room temperature. Nickel loaded activated carbon granulates (Norit Elorit) are prepared using the incipient wetness method (Ni(NO3)2.6H20 in water) to arrive at 5 w% and followed by drying overnight at 385 K. The conversion is carded out in a fluidized bed chemical vapour deposition (FB-CVD) reactor of which the set-up is displayed in Figure 1.
I
3
I oven t'. . . . . . . . . . . . . . . .
Ar/H2/SiCI4
product outlet
I
J' c o l d
I
trap
--
v a c u u m
pump
Figure 1. Schematic drawing of the Fluidized Bed CVD reactor The enlarged part represents the cone shaped quartz reactor (cone angle 7 ~ length 0.15 m) inside an alumina tube. The gas flow rate at the cone entrance (internal diameter 2.7 mm) exceeds the velocity of entrainment of the particles. Low pressure operations are frequently necessary for CVD reactions. Sub-atmospheric pressure control is achieved by incorporation of a vacuum pump, cold trap, and a butterfly valve downstream of the reactor. The conversion procedure starts by filling the reactor with 3 g activated carbon (5w% nickel) under flowing argon (2.46 mol/h). Air is removed by decreasing the pressure and flow rate in steps to 10 kPa and 0.246 mol/h, respectively, followed by restoring the initial pressure and flow rate. This procedure is repeated three times, followed by heating to 1380 K. At this
374 point the reaction is started by replacing part of the argon by hydrogen and SiC14. 2.2. Characterization of porous SiC
X-ray Diffraction (Philips Powder Diffractometer PW1840, CuKc0 has been used to detect the crystalline products in the material. The morphology of the porous SiC has been investigated by Scanning Electron Microscopy (JEOL JSM-35, 15 to 20 kV); the substrates were coated with gold or platinum to suppress charging. Thermal Gravimetric Analysis (Stanton Redcraft STA-1500) has been used to determine the carbon conversion. A sample of 20 mg is heated by 0.167 K/s in air to 1273 K, the weight decrease and heat flux are simultaneously recorded. 2.3. Testing of SiC based catalysts
Removal of the residual carbon present in the SiC/C mixture after conversion is carded out by oxidation in dry air (1023 K, 4h). The SiC resulting from the 5w% Ni/C will be referred to as SIC-5. Catalyst preparation for TPR consisted of loading Ni on SiO2, A1203, and SIC-5 by the incipient wetness method to arrive at a 5w% metal content. After impregnation the catalysts were dried at 353 K overnight and calcined at 773 K and 1273 K for 8 hours. Temperature programmed reduction was carded out with a 2:1 hydrogen-argon mixture (total flow rate 0.5 ml/s) and a heating rate of 0.167 K/s. The hydrogen consumption and hydrocarbon production was analyzed with a TCD and FID respectively. Typically 100 to 400 mg sample was used. The thermal and hydrothermal stability were evaluated according to a method described by Lextnor and de Ruiter [8]; SIC-5 was aged at 1273 K under flowing nitrogen; the hydrothermal stability was determined at 1023 K and 1273 K under flowing nitrogen containing 2v% H20.
3. RESULTS AND DISCUSSION 3.1. Conversion of activated carbon extrudates
The optimal temperature for SiC formation has been determined by performing experiments at 1075, 1250, and 1378 K and utilizing 2w% Ni on carbon. The results of XRD analysis of the products are displayed in Figure 2. At low temperatures (1075 K) mainly silicon is formed by reaction 3. SIC1, (g) * 2 H2 (g) a Si(s) + 4 HCl (g)
t3)
Silicon deposition is encountered in similar experiments in the absence of a catalyst at 800 to 1400 K. Increasing the temperature leads to silicon carbide formation, which is most pronounced at the highest temperature, i.e. 1378 K. The enhancement in SiC formation originates from at least one catalytic effect of nickel, i.e. gasification of the carbon according to reaction 1. Research by Moene et al. [9] showed that the reaction between activated carbon and SiC14 is limited below 1450 K. Vincent et al. [10] reported that a considerable noncatalytic conversion of graphite powder into SiC according to reaction 4 necessitates temperatures above 1600 K. SiC14(g) + 21-I2(g) + C(s) - SiC(s) + 4HCI(g)
(4)
375 Conventional CVD of SiC usually comprises decomposition of CH3SiCI3, although numerous reports display the possibilities of using separate carbon and silicon sources [11], e.g. SiH4/C3H8 and SiC14/CH 4. Utilizing CH3SiC13 the deposition temperature can be as low as 1173 K, which is the result of the formation of gas phase radicals by decomposition of the Si-C bond in the CH3SiC13 molecule. The temperature required for stoichiometric SiC formation from separate C and Si sources is generally higher (above 1373 K). Extensive SiC deposition at 1378 K (Figure 2) points to the presence of gaseous carbon precursors during the synthesis reaction. 60
m
~
0
v
0
.o_
100
5o
g
4o
~
U3
L.
0
"-'
95
co
30
>
~9 _.e
90 20 85
o ,
10
i
20
10
:ff
1075 K ,
I
30
,
i
40
,
i
50
,
i
60
,
i
70
u
g ~ o
....,
0
80
o
,
80
2 nickel
5 content
8 (w%)
2 theta
Figure 2. XRD profiles of converted extrudates 2w% Ni/C, 1 h reaction 9 :SIC, II'Si
Figure 3. CH 4 selectivity to SiC formation (+) and total carbon conversion (A) as a function of nickel content (1 h reaction, extrudates)
Of course, it is essential to deposit SiC without Si formation. This means that sufficient methane has to be formed by gasification in order to eliminate Si formation. The amount of CH 4 and SiC formed during reaction can be determined utilizing TGA in combination with a silicon and carbon mass balance. The results of these calculations are shown in Figure 3. Identical reaction rates for CH 4 and SiC formation are found for a nickel content of 2w%, which is shown in Figure 3 as a 100% CH 4 utilization for SiC formation. The corresponding total carbon conversion equals 22 %. Increasing the amount of nickel results in higher carbon conversions and a concomitant decrease in CH 4 selectivity. Mass transfer calculations show that this decline originates from diffusion limitations of SiCI4 inside the carbon extrudate.
3.2. Conversion of activated carbon granulates Activated carbon granulates are converted in the FB-CVD reactor. TGA analysis has been used to calculate the composition of the granulates after conversion and carbon removal. These compositions and the specific surface areas are shown in Table 2. The amount of ash originates from the activated carbon and the nickel applied. Owing to a concomitant increase in carbon conversion by utilizing increased amounts of nickel, the relative nickel content remains almost constant. Prior to oxidation the surface area is determined by a combination of the original carbon and the SiC. Removal of the carbon discloses the textural properties of the SiC; the surface areas vary between 30 and 80 m2/g, which is sufficient for catalytic purposes, especially at high temperatures reactions ( > 1000 K).
376 Table 2 Composition and specific surface area of converted activated carbon extrudates 2w% Ni
5w% Ni
8w% Ni
SiC (w %)
36
(82)
54
(85)
57
(82)
C (w%)
56
(0)
36
(0)
32
(0)
8
(18)
10
(15)
11
(18)
300
(80)
206
(31)
137
(34)
ash (w%)
SBET (m2/g)
Values in brackets" properties of the extrudates after removal of residual carbon by oxidation 3.3. Testing of SiC based catalysts The TPR profiles of nickel catalysts are depicted in Figures 4 to 6.
co
r0
E} -IO I1)
to -i -o I1)
~
a
t~ LL
k..
i,
200 600 1000 1400
200 600 1000 1400
T (K)
T (K)
Figure 4. TPR profiles of Ni/SiO 2 calcined at 773 K (C). Both the TCD and displayed (upper and respectively)
SiO2 (A), 5w% (B) and 1273 K FID signal are lower curve,
Figure 5. TPR profiles of A1203 (A), 5w% Ni/A1203, calcined at 773 K (B) and 1273 K (C). Both the TCD and FID signal are displayed (upper and lower curve, respectively)
The silica contains sulfate which produces a broad peak from 600 to 1200 K (Fig. 4A). The TPR profile of a 5w% Ni/SiO 2 catalyst calcined at 773 K for 8 hours is in agreement with literature [12,13]. Calcination at 1273 K leads to a considerable increase of the temperature
377 at the maximum reduction rate (Tmax), viz. from 655 K to 1110 K. A similar trend is observed for the alumina catalysts; an increase of calcination temperature from 773 K to 1273 K corresponds to a shift in the Tmax from 630 K to 1120 K. Both observations can be explained by solid state reactions of NiO with the support resulting in silicates and aluminates, respectively [14,15]. The TPR profiles of the Ni/SiC-5 catalysts differ considerably compared to those of the conventional supported NiO catalysts.
C c O .m o ;D "D ~)
1300 a
I_
b_ 1100
O Y
•
900
700
A ,
200
500 I
I
600
I
I
m
R
i|
773 273 SiO 2
7 7 3 1273 AI203
7 7 3 1273 SIC-5
1 0 0 0 1400
T (K) Figure 6. TPR profiles of SIC-5 (A), 5w% Ni/SiC-5 calcined at 773 K (B), and 1273 K (C). Both the TCD and FID signal are displayed (upper and lower curve respectively)
Figure 7. Temperature at the maximum rate of reduction for nickel catalysts (5w% Ni on SiO2, A1203, and SIC-5) calcined at 773 K and 1273 K
It is shown in Figure 6 that part of the nickel oxide, applied in the carbon conversion, reduces around 600 K. Calcination of the Ni/SiC-5 catalyst at 773 K results in an easily reducible nickel oxide species as is shown by the low Tmax (575 K). After the formation of metallic nickel a second, sharp, peak arises at elevated temperatures (955 K) and the FID signal increases. This probably originates from the formation of Ni3Si and a carbon phase according to reaction 5. 3 Ni(s) + SiC (s) ,~ Ni3Si (s) + C (s)
(5)
Similar solid state reactions are reported by Chou et al. [16], who reported extensive
378 decomposition of SiC at temperatures exceexling 1370 K. The peak at 955 K can be rationalized by considering the intimate contact of the catalyst system which decreases the temperature at which the first SiC layer reacts with nickel. The solid carbon gasifies into methane at 955 K, at 1220 K this reaction accelerates and starts to consume the SiC considerably. The most remarkable feature of the Ni/SiC-5 system is, however, its behaviour after calcination at 1273 K for 8 hours. Nickel oxide reduction occurs in a broad region of which the Tmax (640 K) is similar to that of Ni/SiO2 and Ni/A1203 catalysts calcined at 773 K. This points to a remarkably low metal support interaction and a high metal support stability. Surface oxidation of SiC is to be expected during calcination at 1273 K. This Si-O layer, however, does not correspond to bulk silica as is shown by the differences in reduction temperatures of NiO. The broad shoulder appearing in the reduction peak of profile 6 c in the temperature range of 700 to 900 K suggests the presence of nickel silicates [12]. The oxidic layer probably prevents reaction of metallic nickel with the underlying silicon carbide up to temperatures of 1373 K. Figure 8 shows that a complete reduction of NiO is achieved below 700 K for all catalysts after calcination at 773 K. 1.5
m~llow temperature peak ~ h i g h temperature peak
I 0
._
_o
1.0
Ii
Z C 0
~
L_ @
> 0.5
cO
0.0 773 1273 Si02
773 1273 AI203
773 1273 SIC-5
Figure 8. Conversion of NiO on SiO2, A1203, and SIC-5 (5w% metal content) calcined at 773 and 1273 K, calculated from the total hydrogen consumption, 1000 K is used to distinguish a low and high temperature region The amount of nickel oxide reduced in the high temperature region (above 1000 K) relative to the total reduced amount NiO, increases to over 90 % for the silica and alumina based catalysts calcined at 1273 K. Complete reduction of NiO is achieved in the low temperature region for the SiC based catalyst calcined at 1273 K. The presence of Ni20 3 can rationalize the more than stoichiometric hydrogen consumption of the Ni/SiC catalyst. High temperature calcination decreases the maximum reducible NiO amount for the SiO2 catalyst to 72%. Although some difficulties are encountered in integrating the reduction peak owing to the presence of sulfur in the SiO2, this may point once more to a significant difference between
379 the interaction of NiO on partially oxidized SiC and on SiO2. Finally, the stability of SIC-5 at 1273 K in nitrogen and at 1023 K in a nitrogen-steam mixture is very good, no sintering of the porous structure is observed. However, exposing SiC at 1273 K to 2v% steam in nitrogen results in a considerable decrease in specific surface area, i.e. from 31 to 19 mE/g. Part of this decrease (from 31 to 26 m2/g) originates from a weight increase owing to surface oxidation of the SIC-5.
4. CONCLUSIONS Preformed activated carbon, such as extrudates and granulates, can be converted into porous SiC. The presence of nickel on the activated carbon is essential for catalyzing the gasification by hydrogen and subsequent reaction of methane with silicon tetrachloride into SiC. This procedure results in SiC with surface areas of 30 to 80 m2/g. TPR analyses of 5w% Ni/SiC catalysts disclose a remarkably low metal support interaction compared to nickel catalysts based on conventional supports. The SiC based catalysts aged at 1273 K in air show a metal support stability which is substantially higher than that of Ni/SiO2 and Ni/A1203 catalysts. The thermal stability in non-oxidizing environments is very good, which permits utilization of this catalyst at elevated temperatures. The areas, in which SiC is applicable, are restricted; complete oxidation of SiC has to be avoided. It can be concluded that this Ni/SiC system can be exposed to high temperatures (above 1100 K) in reducing environments, in which metallic nickel maintains its catalytic activity. Application of this system at these conditions will probably provide substantial advantages over nickel catalysts based on SiO2 and A1203. REFERENCES 1. 2. 3. 4. 5.
6. 7. 8.
9.
R.B. Levy, in J.J. Burton and R.L. Garton (eds.), Advanced Materials in Catalysis, Academic Press, New York, 1977, p. 101. L.I.e, clerq, in J.P. Bonnelle, B. Delmon, and E.G Derouane (eds.), Surface Properties and Catalysis by Non-Metals, Reidel, Dordrecht, 1983, p. 433. S.T. Oyama, Catal. Today, 15 (1992) 179. M.A. Vannice, Y-L Chao, and R. M. Friedman, Appl. Catal., 20 (1986) 91. M.J. Ledoux, S. Hantzer, C. Pham-Huu, J. Guille, M.-P. Desaneaux, J. Catal., 114, (1988) 176; M.J. Ledoux, C. Pham-Huu, S. Matin, and J. Guille, Eur. Patent No 89-04433. D.A. White, S.M. Oleff, and J.R. Fox, Adv. Ceram. Mater., 2 (1987) 53; J.R Fox, D.A White, US Patent 4818732 (1989). R. Moene, F.W. Tazelaar, M. Makkee, and J.A. Moulijn, Dutch Patent Application No. 930017 (1993). P . W . Lednor and R. de Ruiter, in Inorganic and Metal-Containing Polymeric Materials, J. E. Sheats, C. E. Carraher, C. U. Pittman, M. Zeldin, and B. Currel (eds.), Plenum, New York, 1990, p. 187. R. Moene, M. Makkee, J. Schoonman, and J.A. Moulijn, Carbon '92, Proceedings of
380
10. 11. 12. 13. 14. 15. 16.
the 5th International Conference, Essen, German Carbon Group, 1992, p. 474. H. Vincent, J.L. Ponthenier, L. Porte, C. Vincent, and J. Bioux, J. Less-Commen Met., 157 (1990) 1. J. Schichtling, Powder Metall. Int., 12 (1980) 141 and 196. E.E. Unmuth, L.H. Schwartz, and J.B. Butt, J. Catal., 61 (1980) 242. B. Mile, D. Stifling, M.A. Zammitt, A. Lovell, and M. Webb, J. Catal., 114 (1988) 217. O. Clause, L. Bonneviot, and M. Che, J. Catal., 138 (1992) 195. B. Scheffer, P. Molhoek, and J.A. Moulijn, Appl. Catal., 46 (1989) 11. T.C. Chou, A. Joshi, and J. Wadsworth, J. Mater. Res., 6 (1991) 796.
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
381
A New Strong Basic High Surface Area Catalyst : The Nitrided Aluminophosphate: AIPON and Ni-AIPON Paul GRANGE 1, Philippe BASTIANS 1, Roland CONANEC 2, Roger MARCHAND 2, Yves LAURENT 2, Luis GANDIA, 3 Mario MONTES 3, Javier FERNANDEZ 4, Jos6 Antonio ODRIOZOLA 4 1 Universit6 Catholique de Louvain,, Unit6 de Catalyse et Chimie des Mat6riaux Divis6s, Place Croix du Sud, 2/17, 1348 Louvain-la-Neuve, Belgium 2 Universit6 de Rennes 1, Laboratoire Verres et C6ramiques, UA CNRS 1496 CNRS, Rennes, France 3 Universidad del Pafs Vasco, Grupo de Ingenierfa Qufmica, Apto 1072, 20080, San Sebasti(m, Spain Universidad de Sevilla, Instituto de Ciencias de Materiales, Apto 1115, 41071 Sevilla, Spain
4
ABSTRACT Preparation of new aluminophosphate oxynitride (A1PON) and nickel modified A1PON are presented. Such basic and polyfunctional catalysts are tested in Knoevenagel condensation and one step synthesis of methylisobutylketone (MIBK) from acetone. It is evidenced that the nitrogen content of the A1PON, which controls the rate of benzaldehyde-ethylcyanoacetate condensation depends on the activation procedure. 90% selectivity in the MIBK synthesis is obtained on Ni formate impregnating A1PON activated at 400 ~ for 4 hours. TPD of CO2 and NH3, DRIFT analysis and quantum mechanical calculation evidence that the O/N substitution creates strong basic sites. INTRODUCTION Solid base catalysts have been much less studied than acid catalysts (1-8). However, they can be used in a large number of chemical processes. Modified oxides (5), zeolites (8,9), alkaline substituted clays (10) and hydrotalcites (11) have demonstrated interesting activities. On the other hand, non-oxide catalysts such as molybdenum nitrides or silicon oxynitride have been recently proposed as basic catalysts (12-23). Recently, we reported that the substitution of oxygen by nitrogen in aluminophosphate (A1PO4), at high temperature, leads to the synthesis of high surface area catalysts (24-28). This article evidences that (i) a careful control of both the temperature and time of nitridation of the aluminophosphate precursor allows to modify the O/N ratio of the solid and to tune the acid-base properties of the aluminophosphate oxynitride (A1PON), (ii) these A1PON solids are active in Knoevenagel condensation,
382 (iii) after impregnation with nickel salt, the Ni-A1PON can be used as bifunctional catalyst in one-step synthesis of methylisobutylketone (MIBK) from acetone, (iv) physico-chemical characterization and SCF MO ab initio Hartree-Fock quantum mechanical calculation on model cluster highlight the reasons of the basic character of these new solids. EXPERIMENTAL
Preparation of the precursors (AIPO4) In order to prepare high surface area amorphous oxide, the sol-gel method developed by Kearby (29) was used. At low temperature, 0~ at most, 3 moles of propylene oxide per mole of aluminium are slowly added to a solution of A1C13, 6H20 and H3PO4. The AI/P ratio was fixed at 1. At the end of the propylene oxide addition, the pH of the solution increased to a value close to 3. After standing overnight at room temperature, the gel obtained is carefully washed with isopropanol, dried and calcined at different temperatures between 650 and 800 ~ This method allows to prepare amorphous high surface area precursors (table 1).
Preparation of the aluminophosphate oxynitride (AIPON) Nitridafion of the oxide precursor was performed with flowing pure ammonia. Both time and temperature of nitridafion have been changed. At the end of the activation process, the samples were cooled down under pure and dry nitrogen flow. The temperature and time of nitridation are reported in table 1.
Preparation of the nickel aluminophosphate oxynitride (Ni-AIPON) The aluminophosphate oxynitride containing 13.7% nitrogen has been used as support for the preparation of the nickel supported AIPON. Pore volume impregnation with an aqueous solution of nickel formate was used. After impregnation, the solid was dried at 170 ~ under a residual pressure of approximately 5 mbar for 12 h.
Catalytic reaction The A1PON series was tested in Knoevenagel reaction, namely condensation of benzaldehyde with ethylcyanoacetate. 4mmole of each reactant and 30 ml of toluene as solvent were introduced in a stirred reactor at 50 ~ Then 0.2 g of catalyst were added. The liquid samples were regularly withdrawn with a filtering syringe and analyzed by gas chromatography using a capillary column (CP Sil 8CB-25m). For the Ni-A1PON catalyst, acetone reaction with hydrogen was carded out in a fixed bed reactor operating at 1 atm and 200 ~ with a feed stream containing 20 mol% acetone in H2. Prior to catalytic activity measurements, samples were H2 (100 cm3min -1) reduced at 400 ~ After heating to 400 ~ at a rate of 8 ~ the time on stream of the isothermal part of the treatment was varied between 0 and 12 h. The W/FAo ratio was kept constant for all experiments, and was equal to 0.647 gcat. h mol acetone -1. Selectivities were defined as the molar fraction of the reacted acetone which was converted into a given product.
Physico-chemical characterization Nitrogen content of the solids The principle of the chemical analysis of nitrogen is based on the reaction of the nitride ions N 3- with a strong base and the formation of ammonia which is then titrated. In the case of refractory oxynitrides, the difficulties of the alkaline attack in solution has been solved by using another procedure (30). The oxynitrides are reacted at 400 ~ with melted potassium hydroxide under inert atmosphere.
383
X-ray diffraction Both the A1PO4 and A1PON catalysts have been analyzed using a Siemens D5000 X-ray diffractometer.
Acid-base properties The acid base properties were evaluated through TPD of NH 3 and CO2. After flushing the sample under inert (N2) atmosphere, the samples are cooled down to 130 ~ before NH 3 adsorption or 25 ~ for CO2 adsorption. Then the solid contacted with the probe molecule for 30 min and then with N2 in order to remove the physisorbed molecules. The heating rate during the TPD measurements is 10 ~ min -1. In order to control the amount of desorbed molecules and to check the values obtained by evaluation of the area of the TPD curves, one line chemical titration of desorbed ammonia is also performed.
DRIFT analysis The DRIFT spectra were taken in a Nicolet 510P instrument in which a diffuse reflectance cell (Spectra-Tech) was fitted. For obtaining a reasonable signal-to-noise ratio, 200 interferograms were collected with a resolution of 4 cm -1. All the spectra are presented without manipulation and only Kubelka-Munk transformations are employed for ensuring quantitivity. RESULTS AND DISCUSSION In addition to the method of preparation of the precursor described in this paper, both precipitation described by Kehl (31) and Campelo et al (32) or the citrate methods have been checked (33) and give almost the same results. The first important point deals with the precursor form. For the experiments reported, the AI/P ratio =1 was constant for all the samples. In these conditions, the, X-ray diffraction patterns evidence that after calcination the samples are amorphous. The second important parameter deals with the nature and composition of the oxynitride. Table 1 evidences that for one A1PO4 composition, the O/N ratio may be changed in two different w a y s , by acting either on the temperature or on the time of nitridation. It has been previously reported (24) that the AI/P ratio may also influence this ratio, under the same nitridation conditions. Let us mention here that the bulk nitrogen enrichment of the solid is lower for higher A1/P ratios. The temperature of the nitridation reaction plays an important role and two main domains have been defined. Between 650 and 800 ~ nitrogen substitutes to oxygen in the anionic network and the global composition of the solid may be represented as A1PO4-3/2xNx. At temperatures above 800 ~ NH 3 may reduce the oxidation state of phosphorus. The global composition of such solid may be represented as A1PI-a O4-3/2x-5/2aNx. In these conditions, time and temperature of reaction decreases the phosphorus content up to obtaining pure aluminium nitride A1N. One also observes in this range of temperature a progressive increase of the crystallinity of solids. For these reasons the A1PON catalysts have always been prepared in the low temperature domain, namely up to 800 ~ adjusting mainly the time of nitridation procedure. Table 1 evidences that the longer the nitridation time at 800 ~ the higher the N content obtained. The highest nitrogen content obtained in these conditions is 20%. After nitridation at 800 ~ whatever the nitrogen content, all the samples are almost completely amorphous, as indicated by X-ray diffraction. Although no detailed investigation of the structure has been performed, we may however suggest that nitrogen substitutes to oxygen in the PO4 tetrahedra. Based on chemical analysis and evaluation of the nitrogen content and assuming this oxygen to nitrogen substitution, we proposed the following global formulas for this solid (table 1). Starting from amorphous high surface area precursors, the A1PON solids also present high surface area (table 1).
384 Oxynitride Nitridation Nitridation Surface N (%) area area temperature time (h) temperature (m2g-1) (m2g-I) (~ (~ 1.5 220 3.3 8OO 8OO 225 AIPO3.58N0.28 3 210 5.8 225 8OO 8OO A1PO3.28N0.48 195 8 8OO 225 750 A1PO3.01N0.66 8 225 11 225 8OO 8OO AIPO4 A1PO2.68N0.88 10 205 13 8OO 225 8OO A1PO2.44N1.04 11 210 13.7 8OO 8OO 225 A1PO2.38N1.08 36 210 20 8OO 8OO 225 A1PO1.72N1.52 290 20 740 650 39O AIPO1.72N 1.52 Table 1. ('omposition, ca cinauon temperature and surface area of the oxides. Composition, nitridation temperature, surface area and nitrogen content of the oxynitrides. Oxide Composition Calcination Surface
Composition
The third point to be mentioned is the evaluation of the acid-base properties. The TPD of NH3 (figure 1) and TPD of CO2 (figure 2) allows to give a first indication on the acid-base properties of these solids. Increasing the nitrogen content decreases the total acidity. The temperature at which the maximum amount of NH3 is desorbed does not change. However, the A1PON solid with the highest nitrogen content presents very small amount of acid centers. Figure 2 indicates that CO2 is strongly adsorbed on the solid and, even at 700 ~ a large amount of CO2 is not yet desorbed. It should be noted that the total amount of desorbed CO2 is not proportional to the N content of the solids. Taking into account both TPD experiments, the following conclusion can be stated: the nitridation treatment with NH3 linearly increases the basic/acid sites ratio. 150
3OO 9 * 9 "
%
z 200
%1~=20% %N=13% %N=8% %N--.3.3%
%N=20% [ %N=13% %Nr.8% %N=3.3%
.""''*"'""
9 " 9176
100
9 %-
:i.~. "-,. .~.".<:~.,~~.... ...,z~
%
100
l
9~
***%. * . =,l-.=** .
ae iI
..__.:.." ~. 0
0
200
400
600
800
Temperature (C ~
0
...~
-..~.
0
200
,
!
400
|
i
600
800
Temperature (C ~)
Fig. 1. TPD NH3
Fig. 2.
TPD CO2
The A1PO4 oxide as well as two A1PON solids containing respectively 5.8 and 11% N have been characterized by DRIFT. The DRIFT spectrum of all the samples in the 4000 - 3000 cm -1 region, once stabilized in air, is dominated by a broad, intense and featureless band centered at around 3300 cm -1, indicative of hydrogen-bridged hydroxyl groups. On degassing the solids under N2 flow, water desorption occurs and peaks corresponding to isolated OH and NH groups appear. Figure 3 shows the DRIFT spectra of the solids at 773 K in a N2 atmosphere 9 In this case, sharp peaks at 3787 and 3673 cm -1 appear in all the samples and a broader band peaking at 3360 cm -1 is evident for the samples containing nitrogen. According to previous ascriptions for these bands (34), the presence of tetrahedral aluminium cations (3787 cm -1) and phosphorous atoms (3673 cm -1) bonded to hydroxyl groups can be stated 9The broad band at 3360 cm -1 has to be
385 ascribed to the stretching frequency of NHx species. At higher frequencies, 3456 cm-1, a shoulder of this later band is observed which, according to Moffat et al (35), is indicative of the presence of P-NH2 groups. On increasing the nitrogen content of the catalysts, the relative intensity of the A1-OH to P-OH signals increase, indicating that during the nitrading process the NH3 species interact with POH species on the A1PO surface, resulting in PE r NHx species. !
7
t,r
2
7 7
Based on chemical analysis showing nitrogen substitution, TPD evaluation DRIFT analysis indicating the presence of P-NH2, P-OH t~ ~ and A1-OH binding, it was necessary to try to 9 A understand the development of the basic sites of these solids. In a first attempt we try to evaluate the variation of the basicity when the P/A1/O/N ratios are modified on model compounds. This analysis is based on SCF MO ab initio HartreeFock quantum mechanical calculations carded out using the standard 6-3 lg basis set. With this aim, a model based on metaphosphinic acid, compound I, has been used (fig. 4). Starting from compound i i ,,i I, ring substitution of an oxygen by a NH group 3600 3300 3000 3900 gives compound II, which can be understood as a PON prototype. Successive ring substitutions of WAVENU/dBERS ( c m -~) one or two PO2H groups by A1OH gives compounds III and IV, corresponding to P/A1 Fig. 3. DRIb'T analysis. ratios of 2 and 0.5, respectively. Finally, full substitution of phosphorous by aluminium gives compound V, which would depict an A1ON type structure. Obviously, these models are far from the exact I II description of the real catalysts. It does not take into account the exact composition of the surface and is too simplified. It just brings some orientation for further development. Compounds I to V have been optimized. Atomic Ulll Rill changes obtained from Mulliken population analysis are reported in table 2. As can be seen, both P and A1 centers are positive, with small variations in their charge upon substitution. The negative sites correspond to oxygen and nitrogen species, the latest being more negatively charged in Ira I~ agreement with its higher basic character. The electron density of the nitrogen atom increases sensitively on going from II to V; although a strong structural dependence can be observed. Thus, in compound IIIa, nitrogen is 0.1 more negative than in its parent II. This situation corresponds to N binding A1 and P. In compound IIIb, although the ratio Fig. 4. Schematic structure P/A1 is the same, the charge on the nitrogen is almost the of compounds I-V. N stands same as that of compound II since this structure corresponds for NH, P for PO2H and A1 to a N having only phosphorous as direct neighbor. In for A1OH. compound IVa, the charge of N lowers again by 0.1 e when the substitution is made from a phosphorous bound to the nitrogen. The possibility IVb gives the pattern P-N-A1 and the charge of this N is closed to that found in compound IIIa. Finally, in compound V, where N is bonded only to A1, a similar charge as that of compound IVa is observed. These indications clearly show that one may expect an increase of basicity in the A1PON as compared to the A1PO4 solid and, in addition, it is reasonable to understand why the basic (acid) strength may be modulated as function of the composition.
386 II qN qo
IIIb
IIIa
- 1.33 - 1.08
- 1.42
- 1.34
- 1.09
- 1.18
IVb
IVa 1.52 - 1.18
- 1.17
2.10 2.14
qAl
- 1.52
- 1.19
- 1.30
(P-O-A1)
(P-O-P)
qP
V
- 1.42
- 1.28
(P-O-A1) 2.08 2.14 2.03
2.10 2.10 2.09
(A1-O-A1) 2.05
2.13 1.96
2.05 (O-A1-O) 1.98
(O-A1-N)
1.90 (N-AI-O) 1.99 (O-AI-O)
Table 2. Mulliken atomic charges for compounds II to V. The Knoevenagel condensation is illustrated in figure 5. For comparison, commercial MgO catalyst is also reported. This figure evidences that the conversion depends on the nitrogen content of the catalyst. Comparable behavior has been observed in the malononitrile-benzaldehyde condensation (28). 60 "~
%N'-3,3% i MgO %N--8% %N-13% %N=20%
50 4o
3O 20 ,~.---.A
30
" 6'0
90
120
180 240 Time (min)
Fig. 5. Condensation of benzaldehyde with ethylcyanoacetate. The strength of the basic sites is not yet evaluated. However, a very simple evaluation could be done by comparing the reactivity of the solid base catalyst prepared with results obtained using liquid base as catalyst. Using the same experimental conditions, but replacing the A1PON by pyridine or pyperidine as catalyst in the Knoevenagel condensation, the following results have been obtained: there is no reaction with morpholine and comparable rate of conversion with pyrrolidine. Assuming then that pKa are respectively 8.33 and 11.27, one could assume that basic strength of the prepared A1PON ranges between these two values. The study of the results of the Knoevenagel condensation may confirm the evaluation. Whatever the catalysts, only the first condensation has been observed. This indicates that the strength of the basic site developed by the surface of the A1PON structure does not allow further reaction like Michael condensation which needs higher basic strength. The high-temperature reaction between acetone and H2 can be represented by an overall reaction scheme which contains two main parallel processes (36), as shown in the following scheme.
387
basic ~ r centy
H20 DAA ~
+H2 MSO
1,.-
MIBK
Metal
Acetone " Metal ~ 2-Propanol Firstly, we have the acetone aldol self-condensation reaction over basic sites to give diacetone alcohol (DAA). Dehydration of this alcohol yeilds mesityl oxide (MSO) which, in turn, can be selectively hydrogenated over reduced metal sites to finally give methyl isobutyl ketone (MIBK). In addition to the aldol condensation route, the acetone carbonyl functional group can also be directly hydrogenated over reduced metal sites yielding 2propanol. Other reaction by-products such as methane, propane, diisopropyl ether and diisobutyl ketone have been detected in some experiments, but in very low amounts, lower than 2% of the total reaction products. Figure 6 shows the overall 10 acetone conversion for the Ni-A1PON catalysts as a function of the O ~9 isothermal reduction treatment time at 8 ~~...............~ 400 ~ It can be seen that the acetone / c o n v e r s i o n i n c r e a s e s with the 6 reduction time between 0 and 4 h, but r/ a longer treatment, 12h, produced a 4 slight decrease of the catalyst activity. When the reduction treatments were o 2 carried out at 300 ~ only DAA and MSO were detected, showing that 0 , ~ ~ ~ , ~ 300 ~ is too low as the reduction 0 2 4 6 8 10 12 14 temperature for this catalyst to obtain Reductiontime(h) metallic nickel, and only products related with the catalyst basicity were Fig. 6. MIBK conversion in function of obtained. Figure 2 shows the reduction time of the Ni-A1PON. selectivity towards the different reaction products obtained after ,--, 1oo reduction at 400 ~ as a function of _.o o the reduction time. At this reduction temperature, MIBK is formed in i_~ 80 addition to DAA and MSO, thus revealing the presence of reduced 60 metallic sites. When the reduction time increases from 0.5 to 2h, the 40 selectivity to M I B K increases ~ ~ proportionally to the decrease in the 20 selectivity of both DAA and MSO. An additional increase of the 0 .. ~. . t , , ~ reduction time to 4 h produces an 0 2 4 6 8 10 12 14 almost complete disappearance of Reductiontime(h) DAA and MSO, an increase in the production of M I B K and the appearance of 2-propanol. No Fig. 7. Selectivity in the acetone to MIBK significant changes in the selectivity can be synthesis in function of reduction time of the observed when the reduction time was Ni-A1PON. increased from 4 to 12h. These results are in agreement with an increasing amount of
388 metallic sites when the reduction time at 400 ~ is increased. These metallic sites are able to hydrogenate MSO, improving the selectivity to MIBK. It is interesting to note that the results obtained in this work for the hightemperature reaction between acetone and H2 show that hydrogenation of MSO to MIBK by nickel is easier to carry out than that of acetone to 2-propanol, since 2-propanol is produced only when a high excess of reduced nickel is present with respect to the nickel required to hydrogenate all the MSO found. These results suggest that the selective production of MIBK, without by-products (2-propanol) formation, will be achieved by a fine balance between basic and metallic sites. CONCLUSIONS 1. 2. 3. 4.
5. 6.
From these results, the following conclusions may be made. Nitridation of aluminium phosphate with pure NH3 at temperature up to 800 ~ allows the preparation of aluminium phosphate oxynitride A1PON. Careful control of both the temperature and the time of reaction allows to tune the O/N ratio of this solid and hence the basic/acid site ratio. The impregnation of nickel, followed by a precise activation procedure (reduction), leads to bifunctional (basic-metal) catalysts. The nature of the basic site, conformed by DRIFT and quantum mechanical calculation, allows to propose that they correspond to negative sites corresponding to oxygen and nitrogen species and that it is possible to tune this effect by modification of the O/P/N ratio. The benzaldehyde and ethylcyanoacetate condensation rate is directly linked to the basic character of this solid. One step synthesis of MIBK from acetone at 200 ~ on Ni-A1PON leads to 90% selectivity.
ACKNOWLEDGMENTS This research is supported by COST (EEC) programme. We also thank the "R6gion Wallonne", Belgium, for financial support. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
S. Malinowski, M. Marczewski, in Catalysis: A review of recent literature, Royal Chemical Society, 8, 1987, 107. K. Tanabe, in Catalysis by Acids and Bases (B. Imelik et al., eds.), Elsevier, Amsterdam, 1985, p. 1. K. Tanabe, M. Misono, Y. Ono, H. Hattori, in New Solid Acids and Bases, Their Catalytic Properties, Studies in Surface Science and Catalysis, Elsevier, Amsterdam, 1989. B. Barthomeuf, G. Coudurier, J.C. V6drine, Mat. Chem. Phys., 18, 1988, 553. H. Pines, W.M. Stalick, Academic Press, N.Y., London 1977. W. H61derich, M. Hesse, F. Naiimann, Angew. Chem. Int. Ed. Engl., 27, 1988, 226. W.T. Riechle, J. Catal., 94, 1985, 547. A. Corma, R.M. Martin-Aranda, F. Sanchez, J. Catal., 126, 1990, 192. L.R.M. Martens, P.J. Grobet, P.A. Jacobs, Nature London 315, 1985, 568. A. Corma, R.M. Martin-Aranda, J. Catal., 130, 1991, 130. A. Corma, V. Fornes, R.M. Martin-Aranda, F. Rey, J. Catal., 134, 1992, 58. R.W. Lednor, R. de Ruiter, in Inorganic and Metal-containing Polymeric Materials (J.E. Sheats, C.E. Carraher, C.U. Pittman, M. Zeldin, B. Currel, eds.), Plenum, New York, 1990, p. 187. P.W.I.extnor, R. de Ruiter, J. Chem. Soc., Chem. Commun., 1989, 320. P.W. Lednor, R. de Ruiter, J. Chem. Soc., Chem. Comm., 1991, 1625.
389 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36.
P.W. Lednor, Catal. Today, 15, 1992, 243. L. Volpe, M. Boudart, J. Solid State Chem., 59, 1985, 332. L. Volpe, M. Boudart, J. Phys. Chem., 90, 1986, 4878. G.S. Ranhotra, A.T. Bell, J.A. Reimer, J. Catal., 108, 1987, 40. J.C. Schlatter, S.T. Oyama, J.E. Metcalf, J.M. Lambert, Ind. Eng. Chem. Res., 27, 1988, 1648. S.T. Oyama, D.J. Sajkowski, Prep. Am. Chem. Soc. Div. Pet. Chem., 35(2), 1990, 233. E.J. Markel, J.W. Van Zee, J. Catal., 126, 1990, 643. H. Abe, A.T. Bell, J. Catal., 142, 1993, 430. R.S. Wise, E.J. Markel, J. Catal., 145, 1994, 344. R. Marchand, R. Conanec, Y. Laurent, Ph. Bastians, P. Grange, L.M. Gandia, M. Montes, J. Fernandes, I. Odriozola, patent application FR 9401081. R. Conanec, R. Marchand, Y. Laurent, Ph. Bastians, P. Grange, in Soft Chemistry Routes to New Materials, Nantes, France, in press. P. Grange, Ph. Bastians, R. Conanec, R. Marchand, Y. Laurent, submitted, Chem. Comm. R. Conanec, R. Marchand, Y. Laurent, High Temp. Chem. Processes, 1, 1992, 157. P. Grange, Ph. Bastians, R. Conanec, R. Marchand, Y. Laurent, Appl. Catal., submitted. K. Kearby, Proc. 2nd Int. Cong. Catal., Paris, Technip, 1961, 2567. R. Marchand, Y. Laurent, J. Guyader, P. L'Haridon, P. Verdier, J. Europ. Ceram. Soc., 8, 1991, 197. W.L. Kehl, U.S. Patent 4,080,311, 1978. J.M. Campelo, A. Garcia, D. Luna, J.M. Marinas, J. Catal., 111, 1988, 160. C. Marcilly, Ph. Courty, B. Delmon, J. Amer. Chem. Soc., 53, 1970, 56. B. Rebenstorf, T. Liusblad, S.L.T. Anderson, J. Catal., 128, 1991, 293. J.B. Moffat, Catal. Rev., 18, 1978, 199. L.M. Gandia, M. Montes, Appl. Catal. A General, 101, 1993, L1.
This Page Intentionally Left Blank
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
PREPARATION TITANATES*
OF SILICA OR ALUMINA
391
PILLARED
CRYSTALLINE
S. Udomsak a, R. Nge a, D.C. Dufner b, S.E. Lottc, and R.G. Anthony a'** aKinetics, Catalysis, and Reaction Engineering Laboratory, Department of Chemical Engineering, Texas A&M University, College Station, TX 77843-3122, United States bElectron Microscopy Center, Texas A&M University, College Station, TX 77843-2257, United States cSandia National Laboratories, Albuquerque, NM 87185, United States ABSTRACT Layered crystalline titanates (CT) [Anthony and Dosch, U.S. Patent 5 177 045 (1993)] are pillared with tetraethyl orthosilicate, 3-aminopropyltrimethoxysilane, and aluminum(III) acetylacetonate to prepare porous and high surface area supports for sulfided NiMo catalyst. Tetraethyl orthosilicate or aluminum(III) acetylacetonate intercalated CT are prepared by stepwise intercalation. First, the basal distance is increased by n-alkylammonium ions prior to intercalation with inorganic compounds. However, an aqueous solution of 3-aminopropyltrimethoxysilane can directly pillar CT without first swelling the titanate with n-alkylamine. The catalytic activities for hydrogenation of pyrene of sulfided NiMo supported silica or alumina pillared CT are higher than those of commercial catalysts (She11324 and AmocatlC). The silicon and aluminum contents of the pillared CT, used as supports, have considerable effects on the catalytic activities and physical properties of the supports. 1. I N T R O D U C T I O N Anthony and Dosch [1, 2] prepared a series of new crystalline titanates (CT) by modifying the procedure used to prepare hydrous titanium oxide. CT with basal spacings of 1.0, 1.17, and 1.6 nm were prepared. The catalytic activities of Pd supported catalysts were varied depending on the type of CT. One labelled as type 2 titanate, when used as a precursor in the preparation of a supported Pd catalyst, had the highest activity for hydrogenation of pyrene and 1-hexene among these Pd supported CT. However, CT were not thermally stable as * This work was performed at Texas A&M Universityand SandiaNational Laboratories. The work at Texas A&M was funded by Sandia National Laboratories under Texas A&M Research Foundation contract numbers 6806 and 8346. Sandia National Laboratories is supported by the U.S.Department of Energy under contract number DE-AC04-94AL85000. Corresponding author.
392 reported by Anthony et al.[3]. For example, after calcination, the BET surface area of type 2 crystalline titanate (T2CT) decreased significantly, and crystalline Na0.23TiO2 phase formed. There has been a great deal of interest to prepare layered compounds such as clays, silicates, double hydroxides, phosphates, oxides, and perovskite oxides with high thermal stability by using inorganic compounds to pillar the layered materials. These pillared materials have a high potential as catalyst supports. Recently, Clearfield and Kuchenmeister [4] and Clearfield et al. [5] published reviews on pillared materials. Aluminum Keggin ions [A11304(OH24)(H20)12] 7+ and tetraethyl orthosilicate were usually used to prepare silica or alumina pillared materials by pillaring them into n-alkylammonium swollen materials. Cheng and Wang [6] used this technique to prepare alumina pillared tetratitanate, while silica pillared layered oxides were prepared by Landis et a1.[7,8] using a similar technique. However, Li et al. [9] also showed that silicon-amine compounds could directly pillar layered material without first expanding the layers by n-alkylamine. Udomsak and Anthony [10] showed that tetraethyl orthosilicate could be used to pillar type 2 titanate by a procedure similar to Landis's. In this manuscript, the preparation, characterization, and catalytic activity of silica or alumina pillared CT are shown. The catalytic activity of sulfided NiMo supported catalysts was evaluated by using the model reaction, hydrogenation of pyrene at 573 K. 2.
EXPERIMENTAL
2.1. C~_alyst Prelmrafion Titanium(IV) isopropoxide, an aqueous solution of sodium hydroxide, a solution of 25 weight% tetramethylammonium hydroxide in methanol, aluminum nitrate nonahydrate and tetrapropylammonium bromide were used in the preparation of sodium CT. These chemicals were mixed to produce a white slurry. The slurry was loaded into a closed reactor and heated in an oven set at a temperature in the range of 423-473 K. The crystallization time was varied between twelve hours to two days. The synthesis details were presented by Anthony and Dosch [2]. Sodium CT was acidified by ion exchanging with an aqueous solution of HC1 at pH of 2.0 for several times to remove Na+. In order to intercalate tetraethyl orthosilicate (TEOS) or aluminum(III) acetylacetonate (A1Ac) into CT, the acidified CT was first refluxed with an aqueous solution of n-alkylamine for one day. The product was washed with hot water and dried. The n-alkylamine swollen CT was mixed with an ethanol solution of TEOS or A1Ac for three days at room condition. The product was filtered and dried. The product was then mixed with water for one day. Finally, the product was washed and dried. Silicon-amine was used to pillar CT by refluxing acidified CT with an aqueous solution of 3-aminopropyltrimethoxysilane for one day. The product was filtered, washed with acetone, and dried. In addition, acidified CT was directly mixed with a solution of TEOS or A1Ac. These samples were hydrolyzed, filtered, washed, and dried by the same procedure used for pillaring n-alkylamine swollen CT. These samples (H-CT+ TEOS) and (H-CT + A1Ac) were used for a comparison with silica or alumina pillared CT (SiCT or A1-CT). Si-CT and A1-CT after calcination at 723 K were used as supports for Mo and Ni. Catalytic activities were evaluated by using hydrogenation of pyrene. Mo was loaded into SiCT by impregnation with an aqueous solution of ammonium molybdate(VI) tetrahydrate, and Ni from nickel(II) nitrate hexahydrate was then loaded by impregnation. The NiMo catalysts
393 were calcined at 773 K and sulfided at 698 K, or sulfided at 698 K only prior to catalytic evaluations. 2.2. Catalyst Characterization The concentrations of silicon and aluminum in pillared CT were determined by atomic absorption spectroscopy using a Varian AA-30 spectrophotometer. BET surface area and pore size distribution were determined by nitrogen sorption isotherms using a Micromeritics Digisorb 2600. Powder X-Ray diffraction patterns (XRD) were collected on an X-Ray diffractometer system, Scintag Inc., model XDS2000 with copper radiation (KtXl, Z, = 0.15405 nm) at a scanning rate of 4~ The TEM micrographs of the titanates were made from a TEM microscopy, JEOL JEM-2010 200 KV TEM, equipped with an ultrahigh resolution polepiece. The samples were prepared by dispersing the crystals in acetone and mounting them onto holey carbon filmed TEM grids. Conventional bright field imaging with electron diffraction techniques were used to characterize the samples. The acidity of pillared CT was determined by temperature programmed desorption of ammonia using a heating rate of 12 K/minute. The catalytic activities of sulfided NiMo/Si-CT or AI-CT were studied by pyrene hydrogenation at 573 K. The reaction was performed in batch microreactors. The catalytic activity was evaluated by using the rate constant. The microreactor set-up and operation details were presented by Dosch and McLaughlin [11]. 3.
RF~ULTS AND DISCUSSION
3.1. Characterization of Si-CT and AI-CT Type 2, 3, and 4 titanates (T2CT, T3CT, and T4CT) were used in this study. T2CT and T3CT have similar XRD patterns and basal spacings. However, there were significant differences in the preparations of T2CT and T3CT. Titanium(IV) isopropoxide was used as a titanium precursor for T2CT preparation, while hydrous titanium oxide was used for T3CT preparation. Aluminum nitrate nonahydrate was only used in the preparation of T2CT. The crystallization of T4CT was performed in a methanol solution, whereas an aqueous solution was used for T2CT and T3CT preparations. The basal spacings of T2CT and T3CT were 1.0 nm, and the basal spacing of T4CT was 1.17 nm. In order to swell CT with n-alkylamine, it is necessary to replace Na + located between the layers by ion exchanging with an acidic solution. This phenomenon is very common for the swelling of layered titanate by n-alkylamine, even though n-alkylamine can be ionized in an aqueous solution to form n-alkylammonium ion due to its high pK a. Izawa et al. [ 12] reported that sodium trititanate and potassium tetratitanate could not be directly swollen by nalkylamine, unless they were acidified prior to the swelling. The basal spacing of T2CT could be increased to 3.29 nm using n-dodecylamine [10]. The basal spacings of CT after being swollen by n-hexylamine were increased to 2.45, 2.06, and 2.26 nm for T2CT, T3CT, and T4CT, respectively. The basal spacings were increased to 2.51 and 2.65 nm for T2CT and T4CT, respectively, using n-octylamine. Nine weight% of Mo was loaded into n-octylamine swollen T3CT by anion exchanging at a pH of 3.5. However, only 3.8 weight % of Mo could be loaded into T3CT prior to swelling. In this pH range, the prevalent specie of ammonium molybdate in solution is Mo7023(OH) 5-, a bulky anion. The increase of the ion exchange capacity after swelling indicates the layered structure of CT has been opened after being
394 swollen by n-alkylamine. Tetraethyl orthosilicate (TEOS) was loaded into n-octylamine swollen T2CT, n-hexylamine swollen T3CT and T4CT. Different TEOS concentrations in ethanol solutions were used to prepare silica pillared CT with Si:Ti molar ratios of 0.4 to 5.0. The sample identifications are Si(etx)-TXCT-y, where x = type of titanate and y = Si:Ti molar ratio in the solid. The XRD patterns of the Si(etx)-CT were similar to those of n-alkylamine swollen CT. A comparison between the TEM micrographs of acidified T2CT and Si(etx)-T2CT-0.4 shown in Figure 1 clearly shows that the layers of Si(etx)'T2CT-0.4 were more separated than those of acidified T2CT, and the basal spacing of Si(etx)-T2CT-0.4 determined by XRD was also confirmed by the TEM micrograph.
9, ~ l P . , kJ
(a)
c~'~'m = 4.90 nm
(b)
Figure 1. The TEM micrographs of (a) acidified T2CT and (b) Si(etx)-T2CT-0.4. 3-Aminopropyltrimethoxysilane was used to intercalate acidified T2CT. This sample Si(am)-T2CT-0.47 has a Si:Ti molar ratio of 0.47. The basal spacing was increased to 1.90 nm after being swollen by the silicon-amine. The amount of intercalated silicon-amine was approximately equal to the ion exchange capacity of the solid. Aluminum(III) acetylacetonate (A1Ac) was loaded into n-octylamine swollen T2CT and T3CT. The concentration of A1Ac in the pillaring solution was varied in such a way that alumina pillared T2CT with AI:Ti molar ratios of 0.4 and 0.7 and alumina pillared T3CT with AI:Ti molar ratio of 0.5 were prepared. The XRD patterns of alumina pillared CT were similar to those of n-octylamine swollen CT. The basal spacings of A1-T2CT and A1-T3CT were 2.52 and 2.38 nm, respectively. Two heating procedures were used to study the thermal stability of Si-CT. First, the sample was degassed in a vacuum (P < 0.025 torr) at a temperature range of 373 to 723 K. The temperature was raised to the desired temperature with a rate of 10 K/minute. For the second procedure, sample was loaded into an oven preset at 723 K. The heating period was four hours. The XRD patterns of Si(etx)-T2CT-0.4 after being heated by these two procedures, illustrated in Figure 2, show that the basal spacing decreased from 2.51 nm to
395 1.07 nm after degassing at 723 K. However, after calcination at 723 K by the second procedure, the XRD pattern of Si(etx)-T2CT-0.4 indicates an amorphous material. The thermal stresses introduced by the abrupt change of temperature in the second heating procedure may result in the collapse of the layers. The layers formed disoriented stacks of cards after being abruptly heated as illustrated by the TEM micrograph in Figure 3. The collapse of the layers was also observed for Si(etx)-T3CT heated by the second procedure. The XRD pattern of silicon-amine intercalated T2CT after calcination by the second procedure also shows the collapse of the layers. The calcination procedure was also applied to A1-T2CT-0.4, A1-T2CT-0.7, and A1-T3CT-0.5. The XRD patterns of these alumina pillared CT also indicate the collapse of the layered structures. IPS 11o0.0 i
8. 838 /
4. 436 I
2. 976 /
2. 252 1
1. B23 I
1. ~41 l
990.01
.0.0~
-.o]n
(C) .
.
.
.
.
.
.
.
.
.
.
.
.
.
_
........
"ooli I 440.0"~
~
9
3 3 o o J ~ ~ ~ - ; - ' , - ~ ' - ~ , . ~ = - - " -
.... ~-:'---'~
~o
Figure 2. The XRD patterns of (a) as prepared Si(etx)-T2CT-0.4, (b) Si(etx)-T2CT-0.4 degassed at 723 K, and (c) Si(etx)-T2CT-0.4 calcined in air at 723 K.
lcm
=
117.6nm
Figure 3. The TEM micrograph of calcined Si(etx)-T2CT-0.4. BET surface areas and pore structures of AI-CT and Si-CT were determined from isotherms of nitrogen sorption. The samples after calcination at high temperature were used, instead of the samples degassed at high temperature, since the basal spacing of the degassed
396 sample (about 1.0 nm) may be too small for pyrene, a reactant used in the activity experiments. As the silicon content increases, the surface area of Si-CT increases, illustrated in Table 1. The same trend was also observed for A1-CT. Using TEOS as a pillaring agent instead of silicon-amine yielded the most porous material among the samples with similar Si:Ti molar ratios. The surface areas of calcined A1-CT and Si(am)-T2CT-0.47 were in the same range. However, the surface areas of calcined A1-T2CT-0.4 and Si(am)-T2CT-0.47 were significantly higher than that of calcined T2CT (about 70 m2/g). The surface areas of calcined (H-T2CT + TEOS) and (H-T2CT +A1Ac), which were prepared without using nalkylamine as a swelling agent, were 22 and 53 m2/g, respectively. These surface areas were significantly lower than those of pillared CT. Table 1.
The surface areas, pore volumes, and silicon or aluminum contents of Si-CT and A1-CT after shock calcination at 723 K for four hours.
Sample
Si or AI : Ti (molar ratio)
Surface area (m2/g)
Pore Volume (cc/g)
T2CT
0.00
70
-
T3CT
0.00
31 a
-
T4CT
0.00
19a
-
Si(am)-T2CT-0.47
0.47
168
0.55
Si(etx)-T2CT-0.4
0.50
341
0.76
Si(etx)-T2C T-5.0
5.00
974
1.76
Si(etx)-T3CT-0.5
0.50
188
0.53
S i(etx)-T3 CT-5.0
5.00
308
0.38
S i (etx)-T4C T-0.5
0.50
226
0.43
S i(etx)-T4C T-5.0
5.00
243
0.32
A1-T2CT-0.4
0.40
131
0.53
AI-T2CT-0.7
0.70
193
1.12
A1-T3CT-0.5 0.50 aThe samples were calcined at 773 K.
175
As illustrated in Figure 4, the pore size distributions of the pillared CT after calcination are unimodal with the average pore diameters in the range of mesopores, defined in IUPAC classification as 2 to 50 nm. The average pore diameters of calcined Si(etx)-T2CT-0.4 and Si(am)-T2CT-0.47 are in the range of 5 to 7 nm. However, the surface areas and pore volumes of calcined Si(etx)-T2CT-0.4 are significantly greater than those of calcined Si(am)T2CT-0.47. The pore volume and pore size distribution of calcined AI-T2CT-0.7 indicate that the sample is much more porous than calcined A1-T2CT-0.4, even though the difference between the surface areas of these two samples is small.
397
t
........
I
Si(etx)-T2CT-0.4
0.025
........
I
Si(am)-T2CT-0.47
0.015
g
0.010
0.005
~5 0.000 1
10
100
Pore Diameter (nm)
Figure 4. The pore size distributions of silica and alumina pillared CT after calcination at 723 K for four hours. The acidities of Si(etx)-T2CT-0.4, Si(etx)-T2CT-5.0, and Si(am)-T2CT-0.47 were determined by temperature programmed desorption (TPD) of ammonia and compared with the profile of commercially available ), alumina (Strem Chemical, Inc.). The acid densities of these samples are 4.4"1013, 2.9"1013, 3.4"1013, and 1.2"1012 molecules of NH3 desorbed/cm 2 for ~ alumina, Si(am)-T2CT-0.47, Si(etx)-T2CT-0.4, and Si(etx)-T2CT-5.0, respectively. The acid site distribution of Si(etx)-T2CT-0.4 is bimodal with peaks at 523 K and 623 K, while the site distributions of Si(am)-T2CT-0.47 and Si(etx)-T2CT-5.0 are unimodal with a peak at 503 K. In general, as the silicon content increases, the acid site density decreases. Using silicon-amine as pillaring agent yielded a sample with less acid site density than that of a sample prepared by TEOS, for the samples with similar Si:Ti molar ratios. However, the acid site distribution of a sample prepared by silicon-amine is much more uniform than that of a sample prepared with TEOS. In general, Si-T2CT are less acidic than ), alumina.
398
0.7
I
o.e
"
I
'
I
'
I
'
I
'
I
'
I
"
I
'
I
'
I
"
Si(arn)-~-.O.47
0.4
4 0.3
o.1 0.0
350
400
4,qO
500
,qSO T
~
6IX) (
650
700
7,50
800
850
~
Figure 5. The comparison of the TPD profiles of ammonia on Si-T2CT and )' alumina. 3.2. C~alytic activities The rate constant for pyrene hydrogenation was used to evaluate the activities of sulfided NiMo catalyst. The activities for this model reaction have been shown to correlate with the ultimate activity for coal liquefaction [13-15]. A comparison of pyrene hydrogenation activities of sulfided NiMo/Si-CT and A1-CT with SheU324 and AmocatlC is presented in Table 2. The data in Table 2 illustrate the effect of the Si or AI:Ti molar ratio in the pillared CT on the rate constant. For the samples prepared with TEOS (Si(etx)-T2CT, Si(etx)-T3CT, and Si(etx)-T4CT) and with similar Mo and Ni loadings, the activities were inversely proportional to the Si:Ti molar ratios in the supports. When comparing A1-T2CT and A1T3CT, the trend was opposite. As the AI:Ti molar ratios in the supports increased, the catalysts became more active. NiMo/Si(etx)-T2CT was the most active catalyst, for the supports prepared by TEOS with the Si:Ti molar ratios in the range of 0.4 to 0.5, while NiMo supported Si(etx)-T4CT was the least active. The support prepared by TEOS (Si(etx)T2CT-0.4) was a more active catalyst than the sample prepared by silicon-amine (Si(am)T2CT-0.47), even though they were prepared from the same type of titanate and have similar Si:Ti molar ratios. In general, calcination and sulfiding was a better way to activate the catalyst than sulfiding only. However, this phenomenon was opposite for NiMo/Si(etx)-T2CT-
399
0.4 which was the most active NiMo/Si-CT catalyst. The activity of NiMo/Si(etx)-T2CT-0.4 was more active than commercial catalysts, while using a lesser amount of Mo. The activity of NiMo/AI-T2CT-0.7 was comparable to commercial catalysts, but the Mo loading was only half of those of commercial catalysts. The catalytic activities of NiMo/Si-T2CT correlate with the acid site densities of the supports. For example, Si(etx)-T2CT-0.4 which was the most active catalyst has the highest acid site density, while Si(etx)-T2CT-5.0 which was the least active has the lowest acid site density. Table 2.
The comparison of catalytic activities among NiMo/Si-CT and A1-CT, NiMo/ hydrous metal oxide, and commercial catalysts for pyrene hydrogenation.
Support
Si or AI:Ti molar ratio
Mo (wt %)
Ni (wt%)
She11324
-
13.2
2.7
0.158 c ~
1.20 c~s
AmocatlC
-
10.7
2.4
0.155 c~s
1.45 c ~
Nao.sTiSio.25a
0.25
9.6
3.2
0.207 c ~
2.16 c~s
Si(am)-T2CT-G.47
0.47
10.4
3.5
0.124 s 0.146 c ~
1.19 s 1.40 c~s
Si(etx)-T2CT-0.4
0.40
8.7
3.4
0.206 s 0.180 c ~
2.37 s 2.07 c ~
Si(etx)-T2CT-5.0
5.00
7.5
2.9
0.110 s 0.115 c~s
1.47 s 1.54 c ~
Si(etx)-T3CT-O.5
0.50
9.6
3.2
0.101 s 0.131 c ~
1.05 s 1.45 c~s
Si(etx)-T3CT-5.0
5.00
9.6
3.2
0.080 s 0.093 c ~
0.83 s 0.97 c ~
Si(etx)-T4CT-O.5
0.50
9.6
3.2
0.093 s 0.122 c ~
0.97 s 1.27 ca's
Si(etx)-T4CT-5.O
5.00
9.6
3.2
0.092 s 0.106 c~s
0.96 s 1.10 c ~
A1-T2CT-G.4
0.40
7.3
2.0
0.094 s 0.027 c ~
1.28 s 0.37 cS,s
A1-T2CT-G.7
0.70
6.3
2.2
0.118 s 0.126 c~s
1.87 s 2.00 c ~
A1-T3CT-G.5
0.50
8.6
2.8
0.064 s 0.128 c~s
0.74 s 1.49 c ~
aNao.sTiSio.25 S C&S
kpyrt~ (g cat.sec) -x
= an amorphous material prepared by sol-gel method [11]. = The catalyst was sulfided only. = The catalyst was calcined and then sulfided.
kpyrM~ (g Mo.sec) -1
400
4. CONCLUSIONS High surface area and mesoporous materials can be prepared from calcined silica or alumina pillared crystalline titanates (Si-CT and A1-CT). Tetraethyl orthosilicate and aluminum(III) acetylacetonate can be used to pillar CT by first swelling the CT with nalkylamine prior to pillaring, while 3-aminopropyltrimethoxysilane can directly pillar CT without pre-swelling it by n-alkylamine. The pillared CT, when used as supports for Mo and Ni, show considerable activities for pyrene hydrogenation. The activity and the acidity of the support are very well correlated to the Si or AI:Ti molar ratios in the solids. The most active catalyst, NiMo/Si(etx)-T2CT, is more active than commercial catalysts, while using less Mo. Moreover, the activity of NiMo/A1-T2CT-0.7 is comparable to commercial catalysts, but the amount of Mo was only half of those of commercial catalysts. Therefore, Si-CT and A1-CT have a high potential as supports for coal liquefaction.
REFERENCES 1. 2. 3. 4.
5.
6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
R.G. Anthony and R.G. Dosch, in G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (eds.), Preparation of Catalysis V., Stud. Surf. Sci. Catal., 63 (1991) 637. R.G. Anthony and R.G. Dosch, U.S. Patent 5 177 045 (1993). R.G. Anthony, E. Gonzalez, C.V. Philip and R.G. Dosch, Catal.Today., 14 (1992) 253. A. Clearfield and M.E. Kuchenmeister, in T. Bein (ed.), Supramolecular Architecture: Synthetic Control in Thin Films and Solids, ACS Symposium Series 499, American Chemical Society, Washington D.C., 1992, p. 128. A. Clearfield, M.E. Kuchenmeister, K. Wade, R. Cahill and P. Sylvester, in M.L. Occelli and H.E. Robinson (eds.), Expanded Clays and Other Microporous Solids, Van Nostrand Reinhold, New York, 1992, p. 245. S. Cheng and T.C. Wang, Inorg. Chem., 28 (1989) 1283. M.E. Landis, P.Chu, I.D. Johnson, G.W. Kirker, M.K. Rubis and B. Cynwyd, U.S. Patent 4 859 648 (1989). M.E. Landis, B.A. Aufdenbrink, P. Chu, I.D. Johnson, G.W. Kirker and M.K. Rubin, J. Am. Chem. Soc., 113 (1991) 3189. L. Li, X. Liu, Y. Ge, L. Li and J. Klinowski, J. Phys. Chem., 95 (1991) 5910. S. Udomsak and R.G. Anthony, Catal. Today., 21 (1) (1994) 197. R.G. Dosch and L.I. McLaughlin, SAND92-0388, Sandia National Laboratories, 1992. H. Izawa, S. Kikkawa and M. Koizumi, Polyhedron, 2 (8) (1983) 741. H.P. Stephens and R.N. Chapman, Preprints, Fuel. Div., ACS, 28 (5) (1983) 161. H.P. Stephens and F.V. Stohl, Preprints, Fuel. Div., ACS, 29 (6) (1984) 79. H.P. Stephens and R.J. Kottenstette, Preprints, Fuel. Div., ACS, 30 (2) (1985) 345.
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
401
Silica preparation via sol-gel method: a comparison with ammoximation activity D. Collina ~, G. FomasarP, A. Rinaldo ~, E Trifir/5", G. Leofanti b, G. Paparatto b and G. Petrini b aDip. Chimica Industriale e Materiali, Viale Risorgimento 4, 40136 BOLOGNA (I) bENICHEM s.r.l. Research Center, via S. Pietro 50, 20021 BOLLATE MI (I)
Abstract The influence of the synthesis parameters over the physico-chemical features of porous silica gels derived from tetraethyl orthosilicate has been studied using nitrogen adsorption and FT-IR. Under acid conditions long shaped siloxane particles were produced and microporous amorphous silicas with high surface area were obtained. These samples showed a relatively high concentration of H-bonded and/or "internal" hydroxy groups. The base-catalyzed samples exhibited features of colloidal gel and mesoporous amorphous materials were obtained. The structure is characterized by relatively low surface area with a monomodal porosity. These silicas have an high concentration of the free surface silanol groups. The acid-catalyzed samples resulted completely inactive in the ammoximation reaction, while the base-catalyzed silicas were active in the same reaction.
1. INTRODUCTION Research groups have expressed a new interest in the preparation of amorphous oxides by a process known as sol-gel (1-4). The advantages are compositional homogeneity and the low processing temperature. The process consists of the hydrolysis of a silicon alkoxide, which under appropriate conditions, is polymerized. Normally the alkoxide is dissolved in alcohol and hydrolyzed by the addition of water under acidic, neutral or basic conditions. The final result is SiO2: Si(OC2Hs) 4 + H20 .... > Si-O-Si + C2HsOH The reverse of hydrolysis and condensation reactions, siloxane bond alcoholysis and siloxane bond hydrolysiK promote bond breaking and reformation processes that, if extensive, permit complete restructuring of the growing polymer (5). The synthesized gel contains residual organic groups that can be eliminated by thermal treatment. A high purity oxide fzee of possible poisons is obtained. The preparation parameters strongly influences the structural and textural properties of these materials (5,6). In all the cases particles with controlled size, size distribution and shape can be formed in the sol, which is essential for subsequent processing, and formation of various shapes in the course of sol-to-gel transition. The challenge is to develop control over wet chemical
402 processing so that gels with uniform and proper size pores can be produced. Silica is used in general as metal catalyst support, but it is unexpected active in some oxidation reactions, such as methanol oxidation (7), methane to formaldehyde (8) and ammoximation (9). The ammoximation of cyclohexanone in vapottr phase produces the oxyme with air in presence of excess ammonia. The best performances are obtained with an amorphous high surface silica, such as AKZO F-7, other commercial silica samples give extremely variable results (10,11). However in addition to its practical importance, the ammoximation is a convenient reaction for probing the "defective state" and the silanols concentration. Much work has been done by our research group to study this reaction. In particular, the ammoximation catalysts must present a bifunctional nature (12). The catalyst must exhibit surface silanols on which cyclohexanone can react giving two adsorbed species, an enamine and a ketimine form of the cyclohexanone imine, bonded to the silica surface by a Si-N bond (13). The second site must present a good capability to activate the molecular oxygen in order to make it available for imine oxidation to oxime (12). This paper describes several strategies we have explored to control the pore volume, surface area and pore size distributions of dried gels (xerogels) and the number and the nature of the oxidative sites and surface reactivity. Some samples were tested in ammoximation reaction with the aim to find active silicas and to correlate the catalytic performances with the preparation parameters.
2. EXPERIMENTAL Several silica samples were prepared in acidic or in basic conditions, starting from tetraethyl orthosilicate (TEOS), water and nitric acid (65%) or ammonia (30%). The gels were all prepared mixing in the right molar ratios the quantity of constituent of solution and stirring and heating at the desidered temperature till the gelation happened. After that the gel was dried and calcined. The preparation parameters were changed in a wide range. The changed parameters were the following: H20/TEOS ratio, HNO3/TEOS or NH3/TEOS ratio, aging time, drying temperature and time, calcination temperature and time. The catalytic tests were performed in a glass tubular fixed bed plug flow microreactor (maximum capacity 4.0 ml, 1.0 g of catalyst, i.d 8 mm). A thermocouple, placed in the middle of the catalyst bed, was used to verify the real reaction temperature. The reaction products were accumulated in a solvent and the analysis was carried out by gas-chromatography using an internal standard method. A complete description of the whole apparatus has been reported elsewhere (14). Standard conditions for the catalytic tests were the following: reactant concentration in the reaction gas cyclohexanone=2.8 mol%, NI-I3=35 mol%, O2=10 mol%, T=220 ~ W/F=175 g.h/mol cyclohexanone. The surface areas of all the samples were measured using the B.E.T. method with nitrogen adsorption at 77 K and a Micromeritics AS AP 2000 for the determination of the pore size distribution for the most interesting ones. Mesopore size distributions were calculated using the Barrett, Joyner and Halenda (BJI~ method, assuming a cylindrical pore model (15). In the analysis of micropore volume and area, the t-plot method is used in conjunction with the Harkins-Jura thickness equation (16). The IR spectra were recorded at room temperature using a Perkin Elmer 1750 Fourier
403 Surf, area (m2/g)
Surf. area (m2/g)
900
800 700
o~ ot ~176 9
650
45O I o9- - -
+ base cat. 55O
500
,~
i o o
9 "'. 4-
s
4-
+/
- 300 450
" ."-'~
+ |
. 0 6
0:4
' 0.8
200 1
.o . . . . . . ~, . . -
350
fo.O.~
." t
300
o~
- 250
.+....~."
~ ~ 1 7 .6. . . . . . . . . . . . . . . .
0.2
400
." !
"''"......
4000
......
e
,__.,,
35O
1~ ~176176 4----/
--0-
/
J
-.,
600
Surf. area (m2/g)
9acid cat.
+ base cat. 400
. - e-'" .q.. 9
Surf. area (m2/g) 450
9acid cat.
350
0
-
|
I
I
5
10
15
H N O 3 r r E O S or NH3rrEOS (mol/mol)
/
I
20
250 30
25
H2OISi(OEt)4 (mol/mol)
Figure 1. Influence of HNO3/Si(OEt)4 or NH3/Si(OEt)4 o n surface area.
Figure 2. Influence of H20/Si(OEt)4 on surface area.
Transform instrument. The catalyst was pressed into self-supporting disks and activated in vacuum at different temperatures in the conventional IR cell (NaC1 windows).
3. RESULTS The effects of the synthesis parameters on the surface area are reported in Figures 1-4. In Figure 1 and 2 the influence of the nitric acid or ammonia to TEOS ratio and water to TEOS ratio respectively on surface area is shown. In the acidic conditions the synthesis parameters show a relevant influence over the physico-chemical features. The surface area was taken as preliminary parameter of characterization. For the preparation in the acidic conditions the increase of the amount of the nitric acid increases strongly the surface area
700
Surf, area (m2/g) . . . .
Surf, area (m2/g) _ 350 9acid cat.
+ base
cat. 300
600
t
"
+ i
..'~
Surface area (m2/g) 800
9a c i d cat.
700 600
...........
250 500 -
~o~176176 .,~." " , ,
500 -
200 o
4050
' 1 O0
' 150
' 200
150 250
Temperature ('C)
Figure
3. I n f l u e n c e
of the drying
tomperamrr on surface area.
400 30O 20O
I
I
300
400
I
500
I
I
600
700
800
Temperature ('C)
Figure 4. Influence of the c a l c i n a t i o n temperature on surface area.
404 Table 1. Surface area and pore size distribution of silicas prepared in acidic and basic conditions. Notation ' HNO3/TEOS NH3/TEOS 'Surface area Micropore Mesopore Mean pore vol. vol. diam. (mol/mol) .. (mol/mol) (m2/g) ( c m 3 / j z / (cm3/z) (nm/ ,
Acidic I Acidic 2 Basic 1 Basic 2
0.1 0.3 ---
--0.52 0.31
582 708 305 240
0.21 0.25 < .01 < .01
< .01 < .01 1.66 1.13
,.,
1.43 1.42 21.6 19.3
from 500 to 800 m2/g, on the contrary the "basic" samples show a decrease in the surface area from 370 to 230 m2/g, increasing the quantity of ammonia. The increasing of hydrolysis ratio, H20/TEOS, up to 10 increases strongly the silica surface area from 100 to 600 m2/g, for values higher than 10 the surface area remains almost constant. A similar behaviour is shown by the "basic" samples, however the effect is lower and the values of the surface area are lower too. In Figure 3 the effect of drying temperature on surface area is reported. The increase of drying temperature up to 150 "C, in the case of acidic preparation, decreases the surface area, while in the case of basic preparation, it increases slightly the area. At 150 *C, drying time modifies in a relevant way the surface area, in the case of acidic preparations increasing from 1 to 5 hours it increases from 100 to 700 m2/g. The calcination temperature shows relevant effects upon morphology and various properties. The heat treatment decreases silica surface area, much more rapidly at temperature higher than 600 "C, see Figure 4. At 700 *C the area is lower than 400 m2/g. In Table 1 the data of the surface area and pore size distribution for some samples calcined at 300 "C are reported. The two types of synthesis show completely different results. The silica obtained under acid conditions has a monomodal porosity in the micropore region and a large surface area. The isotherm is shown in Figure 5a. The total pore volume is about 0.2-0.3 ml/g and no volume due to the mesoporosity is shown. The pores are within micropore region with a mean diameter of the order of 1.5 nm and with a narrow size distribution. The pores with a diameter exceeding 2 nm are absent. On the contrary the silicas obtained under basic conditions have a monomodal porosity in the mesopore region and a surface area in the range of 200-400 m2/g. The total porosity is about 1.0-2.0 ml/g and is due to the mesoporosity only. The pores are within the mesopore region with a mean diameter of the order of 20 nm and with a narrow size distribution (see figure 5b). Analysis of the t-plots does not show evidence of microporosity in either sample. The IR spectra reported in Figures 6-7 show the OH stretching bands of the silica samples prepared in acidic and basic conditions after outgassing in the IR cell at different temperatures. As usual for silicas, the "basic" sample shows a sharp band near 3745 cm 1 due to free surface silanol groups, see Figure 6a and b. This absorption is almost "isolated" after outgassing at 530 ~ (Figure 6b). After evacuation at lower temperatures, broad absorptions are also evident at the lower frequency side of this band, assigned in the literature to H-bonded and/or "internal" hydroxy groups (17,18). In particular a broad shoulder is still
405
Pore volume (cc/g nm)
Volume adsorbed (cc/g STP) 200
-'- adsorb. desorb.
160 _-
F 0.04[ 0.03~
. . . . . . . . . . . . . . . . . . . . . . . . . . .
f
120 80
0"02 I
40
0"01 t
0
0
. . . . . . . . .
' . . . . . . . . .
' . . . . . . . . .
* . . . . . . . . .
' .
0.2 0.4 0.6 0.8 Relative pressure (P/Po)
.
.
.
.
1
Figure 5a. Isotherm of a silica prepared in acidic conditions.
.
.
.
.
0~ 1
.
,1 : I
1"
'
-"--
10 100 Pore diameter (nm)
:
1000
Figure 5b. Pore size distribution for the "basic 1" silica.
evident centered near 3560 cm "l, while other shoulders can be seen near 3680 and 3710 cm ~, see Figure 6a. As compared the "acidic" silica has a much lower concentration of free OH groups (3740 cm "l band) and this band presents a broad shoulder at lower wave numbers (see Figure 7a and b). Besides a broad, very strong band is evident at 3530 cm l (Figure 7a). Also after Absorbance
Absorbance
i
. . . . . . . . .
3500
3000
Wavenumber (cm-1)
2500
I
3500
-
J
. . . . . . . . . .
3000
2500
Wavenumber (cm-1)
Figure 6. FT-IR spectra of the silica prepared in acidic conditions after evacuation at: (a) 200 C, (b) 530 "C. 9
.
406
Absorbance
Absorbance
a
\ J , , i , , , , , , J , , , ,
3500
3000
3500
2500
3000
2500
Wavenumber (cm-1)
Wavenumber (cm-1)
Figure 7. FT-IR spectra of the silica prepared in basic conditions after evacuation at: (a) 200 o
c, (b) 530 "C. .
evacuation at 530 "C, a very broad shoulder is strongly evident centered between 3600 and 3700 cm "l. The catalytic tests carried out to investigate the activity of these samples in ammoximation reaction show a different behaviour for the two types of sample. The catalytic behaviour was followed until deactivation of the catalyst. The data obtained are reported in Figure 8a and b. The "acidic" sample (see Figure 8a) show the cyclohexanone immine Conversion (%)
Yield (%) 9 Conv.
30
Cony., yield (%) 6
+ Imine y.
100
g tar/g c a t
9Cony. + Oxyme y. ~( g tar/g cat.
80
,.41. 4._..._~t
9
20
9
9
9
lP
.........
1.5
~.
-4 60
-1 40 10
+
-I-4.
0 0
' 2
" 4
+ ' 6
Time on stream (h)
20 "
4-__
' 8
0 10
0 0
..§ .... §
+
9.-I-.....+....j:.. 9
,
3
;
~--.e_.e. "'+ ..... .+,. t
9
12
-0.5
0 15
Time on stream (h)
Figure 8. Catalytic behaviour in gas phase ammoximation of silica prepared: (a) in acidic conditions; (b) in basic conditions.
407 formation only. The oxime was absent in all the tests. Only the long time gelified samples evidenced the presence of heavy products which remain irreversibly adsorbed on the catalyst as tars, but after few hours the rate of formation decreased to zero. On the contrary the "basic" silicas show a good conversion. The oxime yield and selectivity are interesting, but not as high as the performance of the commercial amorphous silica A K Z O F-7, showed in a previous work (11). Other reaction products are tars and organic volatile compounds. However as compared with commercial AKZO F-7, the rate of tar deposition is higher.
4. DISCUSSION
The preparation parameters modify the sol-gel process and determines the structure of the gel: - under acidic conditions hydrolisis is fast and has ended before condensation starts; in this case polymer-like silica chains develop, which are weakly cross-linked and thus are prone to shrinkage; - under basic conditions hydrolisis is slower than condensation and hydrolyzed species condense immediately. Comparatively dense colloidal particles grow, which form branching and interconnected chains. By this way the structure of the final material is completely different. In the acidic preparation long shaped siloxane particles are obtained, while using an ammonia catalyst round-shape~ particles are produced. Then the choice of the type of catalyst (acid or base) determines the structure of the material. The silicas that we prepared are amorphous materials. In the acidic condition, microporous amorphous gels are obtained. The formed gel is clear and trasparent. The similarity in form of the isotherms would indicate similar structure. This structure is characterize~ by a high bulk density and high surface area. The form of isotherms suggests the presence of rather regular channels having a very small diameter (about 1.5 nm). The base-catalyzed samples exhibit features of colloidal gel. The gel is mesoporous with a mean pore diameter of the order of 20 nm. The structure is characterized by low bulk density and relatively low surface area. The pore structure (pore volume, surface area and pore size) of a xerogel is also a consequence of the sequential (or overlapping) gelation, aging and drying processes. The acid or base/alkoxide ratio influences the rates of condensation and hydrolisis. The increasing of the acid amount favours the hydrolisis reactions and as a consequence the formation of weakly cross-linked polymeric chains. These phenomena can explain the increase of the surface area, due to the microporosity. An opposite behaviour is shown by the base-catalyzed samples. Under basic conditions, condensation reactions are favoured and the resulting sol species are most r (5).The surface areas decrease with increasing the base amount and the area is due to the mesoporosity only. Using conditions near the neuWality, low amount of acid or base, intermediate structuresare formed with surface area of about 400 rn~g. The effect of the water/alkoxide ratio (hydrolysis ratio) on the surface area is almost the same in the two conditions. At low hydrolysis ratio,about 4, the hydrolisis is not favoured, however the particles association is favoured. As a consequence cross-linking is favoured and the surface area decreases. At hydrolysis ratio higher than 10 the diluition could lead to a
408 lower association, but the effect over the surface area is very low. The drying temperature shows a noteworthy effect. When a gel is dried, stresses develop which lead to fracture the body. These stresses result in particular from capillary forces. Upon solvent removal, the capillary forces from the receding liquid collapse the weak structure. Shoup (19) has pointed out the importance of large pores in obtaining crack-free sintered bodies (because of the smaller capillary forces associated with such pores and the larger thickness of interpore material which generally accompanies larger pore sizes). For this reason in the case of "acid" silica the increasing of drying temperature implies a strong decrease of the surface area. The small pore size and the thickness of interpore material enhance the capillary forces and it is easily compacted at the final stage of drying. In this case the drying stresses can be reduced by slow drying. For the "basic" samples the influence is very slight and opposite. Under basic condition, the cluster-cluster conctact is also more mechanically rigid. May be at temperatures lower than the water boiling point the obtained gel was not completely dried (and that could produce some stresses during the calcination stage). In the case of acid-catalyzed samples the increase of calcination temperature produces a decrease of the surface area and above the 600 *C the area drops. From room temperature to above 600 ~ the silica particle grows larger and the weakly cross-linked network collapses due to dehydratation and further cross-linking. Kondo et al. (20) attributed this phenomena to the mild surface diffusion of silicic ion. At temperature higher than 700 ~ a strong change of the structure starts, which leads or to a crystalline alfa-cristobalite or to a fight foamy solid. The FT-IR characterization of the two types of silica show an unusual intensity of the absorption in the OH region, in particular with regard to the component at lower frequencies, evidencing an anomalous concentration of surface Si-OH groups. In particular the "basic" sample has an high concentration of the free surface silanol groups. This absorption is almost "isolated" at 800 K. While the acid-catalyzed sample show a relatively high concentration of H-bonded and/or "internal" hydroxy groups. This phenomenon can be explained by the different structure. The acid-catalyzed structure has micropores or submicropores with hydroxylated surface. In micropores the surface OH groups are brought closer together by the curvature and can form more stable hydrogen-bonded pairs (21). Besides the "acid" samples seems loss hydroxyl groups during heat treatment more easily than the "basic" ones (22). In the ammoximation reaction the presence of the free surface silanol groups is really relevant, as the fzrst step of the process is the production of the Si-bonded immine (13). The change of the preparation media completely modifies the surface reactivity and, as a consequence, the activity of pure silica catalysts. Only the catalysts obtained by base-catalyzed silicas are active in the ammoximation reaction. In a previous work (23) for the cyclohexanone oxirne formation the following scheme was suggested: cyclohexanone ...... > imine ...... > oxime The ketone reacts with ammonia on silanols to produce the corresponding immine, which is oxidixed to the oxime by some activated oxygen species. Togheter with the oxime, tars and other condensation products are formed. In view of the preparation method the activity cannot be due to the presence of impurities in the catalyst. The previous results suggested that the catalyst exhibit a real bifunctional nature (12). The presence of free silanols is necessary to activate the ketone but the catalyst must also be able to activate the molecular oxygen. Hence the surface reactivity of silicas is
409 dependent on the number and the distribution of their isolated silanol groups. On the nature of oxydant species there is still a lack of knowledge. Two hypothesis can be advanced: 1) activation of molecular oxygen at the surface of SiO2 by organic adsorbed radicals (tars); 2) activation of imine with formation of radical (C-N') by surface defects (SiO', SiO2") which can directly interact with molecular oxygen, forming peroxides intermediate. In both hypothesis the pore structure, besides the presence of silanol groups, could assume a paramount importance in order to allow, in the first hypothesis, the formation of tars inside the pores or the accumulation of radicals inside the pores to let start a radical chain reaction.
5. CONCLUSIONS Silica gels of different pore structure were prepared by the sol-gel route, using tetraethoxy orthosilicate as a precursor. Two clearly different structural unit types were obtained on the base of the preparation media. Under acidic conditions long shaped siloxane particles were produced and microporous amorphous silicas with high surface area were obtained. The form of isotherms suggests the presence of rather regular channels having a very small diameter (about 1.5 nm). The base-catalyzed samples exhibited features of colloidal gel. Round-shaped particles were produced and mesoporous amorphous materials were obtained. The structure is characterized by low bulk density and relatively low surface area with a mean pore diameter of the order of 20nm. The pore structure (pore volume, surface area and pore size) of these unit types could be also modified as a consequence of the modification of the sequential gelation, aging and drying processes. The silicas showed an anomalous concentration of surface Si-OH groups. The base-catalyzed samples had an high concentration of the free surface silanol groups and were active in the ammoximation reaction. While the acid-catalyzed samples showed a relatively high concentration of H-bonded and/or "intemar' hydroxy groups and were inactive in the ammoximation at all. The change of the preparation media completely modifies the surface reactivity of the silica. The ammoximation results a convenient reaction for probing the "defective state" and the silanol concentration.
Acknowledgments. The financial support from C.N.R.-"Progetto Finalizzato-CHIMICA FINE 2" (Rome) is gratefully acknowledged. REFERENCES 1. 2. 3. 4. 5.
C. Sanchez and J. Livage, New J. Chem. 14 (1990) 513. D. Ulrich, J. Noncrystalline Solids, 100(1-3) (1988) 174. B.E. Yoldas, J. Mater. Sci., 14 (1979) 1843. P.A. Haas, Chem. Engr. Prog., April 1989. C.J. Brinker, J. Noncrystalline Solids, 100(1-3) (1988) 31.
410 6. M. Guglielmi and G. Carturan, J. Noncrystalline Solids, 100(1-3) (1988) 16. 7. L. Cairati and E Trif'triS, J. Catal., 80 (1983) 25. 8. G.N. Kastanas, G.A. Tsigdinos, J. Schwank, Appl. Catal., 44 (1988) 33. 9. J.N. Armor, J. Catal., 20 (1981) 72. 10. D.P. Dreoni, D. Pinelli, E Trifir6, in proceeztings "12 Simposio Ibero Americano de Catalise", Rio de Janeiro, Inst. Brasileiro de Petroleo (1990) vol.2, pp. 305-312. 11. D.P. Dreoni, D. Pinelli, E Trifir/5, H. Habersberger, Z. Tvaruzkova, P. Jiru, in "New Frontiers in Catalysis, Proceedings of the 10th International Congress on Catalysis, Part C", L. Guczi, E Solymosi, P. Tetenyi Eds., Elsevier Science Pub., Amsterdam, 1993, pp. 2011-2014. 12. D.P. Dreoni, D. Pinelli, E Trifir6, in "New Developments in Selective Oxidation by Heterogeneous Catalysis Ill vol. 72", P. Ruiz and B. Delmon Eds., Elsevier Science Pub., Amsterdam, 1992, pp. 109-116. 13. D.R Dreoni, D. PineUi, E Trifir/5, J. Mol. Catal., 7"-1(1992) 111. 14. D.P. Dreoni, D. Pinelli, E Trifir/5, J. Mol. Catal., 69 (1991) 171. 15. E.P. Barrett, L.G. Joyner, P.P. Halenda, J. Am. Chem. Soc., 73 (1951) 373. 16. 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. 17. G. Ghiotti, E. Garrone, C. Morterra, E Boccuzzi, J. Phys. Chem., 83 (1979) 2863. 18. S. Kondo, H. Yamagouchi, Y. Kajiyama, T. Ishikawa, J. Chem. Soc., Faraday Trans., 1-80 (1984) 2033. 19. R.D. Shoup in "Colloid and Interface Science", Academic Press, New York, 1976, vol. 3. 20. S. Kondo, E Fujiwara, M. Muroya, J. of Colloid and Interface Sci., 55-2 (1976) 421. 21. R.K. Iler, in "The Chemistry of Silica", John Wiley, New York, 1979. 22. B. Handy, K.L. Walther, A. Wokaun, A. Baicher, in "Preparation of Catalysts V vol. 63", G. Poncelet, P.A. Jacobs, P. Grange, B. Delmon (Eds.), Elsevier Science Pub., Amsterdam, 1991, pp. 239-246. 23. D. CoUina, E. Pieri, D. Pinelli, E Trifir/5, G. Peu'ini, G. Paparatto, in "Heterogeneous Catalysis and Fine Chemicals HI vol. 78", M. Guisnet et al. Eds., Elsevier Science Pub., Amsterdam, 1993, pp. 479-486.
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
411
Control of porosity and surface area in T i O 2 - A 1 2 0 3 mixed oxides supports by means of a m m o n i u m carbonate T. Klimovaa, Y. Huerta a, M.L. Rojas Cervantes b, R.M. Martin Aranda b and J. Ramirez a aDepartamento de Ingenieria Quimica, Facultad de Quimica, UNAM, Cd. Universitaria, 04510 M6xico D.F., M6xico, Fax 525-6225367 bDepartamento de Quimica Inorg/tnica y T6cnica, Facultad de Ciencias, LINED, 28040 Madrid, Espafia
1. INTRODUCTION In many catalytic systems, the performance is known to depend not only on the inherent catalytic activity of the active phase, but also on the textural and physieoehemieal properties of the support. In the attempt to obtain better catalysts the use of some new supports, such as TiAl mixed oxides, has been tried with promising results in some reaction systems such as hydrodesulfurization reactions. In this ease, greater catalytic activities have been found due to the role of the support [ 1,2]. In general, sol-gel methods have been preferred to produce this mixed oxide system. However, the control of the surface area and porosity of the catalysts remains a problem, since small diameter pores of the order of20-30A are normally obtained [3]. Numerous methods are used to create materials with specific surface properties and most commercial supports are already available in a variety of surface area-pore size combinations which are somewhat interrelated in such a way that large pore supports typically have a reduced surface area. In heterogeneous catalytic reactions, it is important to have control over both the surface area and the pore size in order to achieve the highest possible catalytic activity. Several methods have been reported in the literature for preparing refractory support materials with variable surface characteristics. Careful control of the specific gelation and ageing conditions of pseudo-boehmite has been found to have an effect on the surface properties of the final alumina support [4]. The control of pore structure in alumina supports has also been realised by rapidly swinging the solution pH during the gelation process [5]. The mean pore size of the support can be further increased by adding a pore-regulating agent (or additive) to the stock solutions from which the gel is prepared. In the preparation of porous oxides (Al203, SiO 2, Al203-SiO2) various types of additives have been used. In general, they are organic materials which can be evaporated or eombusted during calcination of the gel [6]. Also, the use of template molecules (usually quaternary ammonium ions) has been described in the literature [7-9]. The effect of ammonium carbonate and bicarbonate as additives upon the physical properties of AI203 and Al203-SiO2 mixed oxide, has been reported previously [7,8], however,
412 from the results of these studies it is clear that the mechanism of action of this additives is still not well known and that more detailed studies are required to gain a deeper insight into the behaviour of these technically important materials. In the present work, we test the effectiveness of the use of the ammonium carbonate (AC) in controlling the porosity and surface area of a TiO2-AI203 mixed oxide, prepared by the sol-gel method. In order to inquire on the mechanism of action of this additive, an IR study of the intermediate entities (hydroxide precipitates) was realised. During the study, the amount of AC and water and method of addition of the reactants was varied.
2. E X P E R I M E N T A L The samples of the mixed oxide were prepared with variable TiO2/(TiO2+AI203) molar ratios (R), using Ti and Al isopropoxides as precursors and n-propyl alcohol as a solvent. In the experiments, a solution with known amounts of AC in water (30 and 112 times the stequiometric required amount) was used to produce the formation of the metallic hydroxides. The resulting precipitates were aged, with slow stirring, for 24 hours, filtered under vacuum and washed three times with water ( 100 ml per gram of hydroxides) and filtered under vacuum once more. The solids were then dried at 373K during 24 hours and calcined 24 hours at 773K. Also, some samples of pure TiO 2 and Al203 were prepared and characterised in order to observe the changes produced in the single oxides systems. The mixed oxides will be referred here after as Ti-Ai-(R), were R is the TiO2/(TiO2+Al203) molar ratio. Two methods of precipitating the isopropoxides were used, method A consisting of a slow addition of the ammonium carbonate solution to the titanium and aluminium isopropoxides n-propyl alcohol solution, and method B which consisted in the addition of the isopropoxides dissolved in npropyl alcohol to the ammonium carbonate aqueous solution. Also, in order to explore the effect of pH in a wider range, experiments at pH=8 using ammonium bicarbonate were performed in addition to the experiments at pH of 9 and 10 made with ammonium carbonate. All the solids were characterised by surface area and porosity using a BET physisorption commercial apparatus. FTIR spectra were taken from the dried solids made into KBr pastilles using a Nicolet 510 FTIR spectrometer. Measurements of the zero point charge of the solids was obtained using a Z-meter apparatus. For these measurements the ionic strength was adjusted to 1 x 10 -3 using LiC1 and the pH was varied using solutions of LiOH and HCI.
3. RESULTS The results show that in general, the use of ammonium carbonate as additive allows to increase significantly surface area, cumulative pore volume and mean pore diameter. Table 1 presents the results obtained for the Ti-AI-(0.5) samples. In this table, it is observed that an increase in the amount of AC, keeping other variables constant, leads to an increase in surface area, pore volume and pore diameter (experiments 1.1 to 1.3). In contrast, increasing the amount of water used to hydrolyse the alkoxides increases surface area but decreases pore volume and mean pore diameter (compare experiments 1.2 and 1.4, 1.3 and 1.5, 1.6 and 1.8, and 1.7 and 1.9 ). Regarding the method of preparation, the addition of alkoxides to the water (method
413 B), leads to greater pore volume and mean pore diameter than when water is added to the alkoxides solution (method A). Table 1 Samples prepared using ammonium carbonate as additive at pH =9. No. Oxides ratio (N/-I4)2CO3 n20 Method Surface Cumulative (R) (g) (ml) area pore volume, (m2/g) (~cm3/g) 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9
0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5
0.00 2.88 7.50 2.88 7.50 2.88 7.50 2.88 7.50
40 40 40 150 150 40 40 150 150
A A A A A B B B B
233 248 343 293 345 226 326 343 394
0.243 0.373 0.874 0.355 0.765 0.642 0.999 0.514 0.955
Mean pore diameter
39 41 76 34 64 78 90 40 70
The use of different amounts of ammonium carbonate led to solids with mono-modal or bidu D modal pore size distribution. Figure 1 shows the results obtained from the preparations using 0.0, 2.88 and 7.5 g. of ammonium carbonate 2.4 during the preparation of the Ti-AI-(0.5) mixed oxide using the method A ( experiments 1.1, 1.2 and 1.3 in table 1). 1.8 In this figure, it is possible to see that the sample prepared without AC, which here is considered as the standard preparation, led to a I1 unimodal pore size distribution with pore sizes 1.2 of the order of 39 A in diameter. On the other ,/! hand, when 2.88 g. of AC are used in the preparation (experiment 1.2 in table 1), pores of larger diameter, around 130 A, begin to appear. 0.6 and then, with even more AC, the pore size distribution turns to be clearly bi-modal with two maximums in the pore size distribution curve around 40 and 130 A. To study the effect of pH, experiments Figure 1. Pore size distributions of the Ti-AI- were conducted at pH values of 8, 9 and 10, (0.5) sample prepared with: no AC (1), 2.88 using ammonium bicarbonate (ABC), instead g. of AC (2) and 7.5 g. of AC. of AC to perform the experiments at pH=8.
12) /(3)
/'
1o
.
.
.
.
.
.
iOO
,
,
,
,
,
,
,,,
olAI
Table 2 shows the results of these experiments for two different levels of either AC or
414 ABC. In this table, it is observed that for the smaller amounts of AC or ABC, were the influence of the pH appears more clear, increasing the pH value from 8 to 10, keeping other variables constant, the surface area, cumulative pore volume and mean pore diameter are increased. With larger amounts of the additive, AC or ABC, the surface area is increased significantly only when the pH is changed from 8 to 9. However a change of pH from 9 to 10 leads to surface areas which are similar or slightly lower than at pH=9. The response of cumulative pore volume and mean pore diameter to a change of pH from 9 to 10 also follows the same trend as that shown by the surface area. In this case, the cumulative pore volume is increased 23 % when the pH is changed from 8 to 9 and only 14 % when the pH is changed from 8 to 10. With these larger amounts of additive, the effect on mean pore diameter is almost nil although, as has been said before, the amount of additive alters the form of the pore size distribution curve leading to mono or bi-modal pore size distributions. In this case, experiments 2.2, 2.4, 2.5 and 2.6 led to bi-modal pore size distributions plots. Table 2 Textural effects induced by variations of pH and amount of additive ( AC or ABC) in the Ti-A1(0.5) mixed oxide, using method B of preparation and 150 ml. of water to precipitate the Ti and A1 isopropoxides. No Additive Amount of pH Surface Cumulative pore Average pore additive g.(moles) area (m2/~) volume (cm3/g) diameter (A) 2.1 2.2 2.3 2.4 2.5 2.6
NH4HCO 3 (NH4)2CO 3 (NH4)2CO 3
2.37 6.17 2.88 7.50 2.88 7.50
(0.030) (0.078) (0.030) (0.078) (0.030) (0.078)
8 8 9 9 10 10
260 284 343 394 344 361
0.371 0.767 0.505 0.947 1.106 0.878
57 108 59 96 90 97
Additional information on the behaviour of these systems was obtained by FTIR. The spectra, taken atter drying the solids at 373 K during 24 hours, indicated the presence of a different compound when the mixed oxide was prepared with AC or ABC instead of only water. The formation of this compound was not evident when low pH and small amounts of either AC or ABC were used in the preparation (see figure 2a and 2b). Previously reported infrared vibration frequencies for NH4 +, -OH, CO32" and M-O-CO 2- [10,11], were used to assign the IR bands found in our spectra. Figure 2a shows that when the molar amount of either AC or ABC is kept constant at its lower value of 0.03 moles (AC concentration = 0.2 M.), a change in the IR bands is observed when the pH is raised from 8 to 9 or 10. The IR spectrum of the sample at pH = 8 shows a wide band with the maximum at 3445 cm -1, due mainly to the stretching vibration contribution of surface -OH groups in the mixed oxide and a band at 1632 cm -1, assigned to the -OH bending vibration. Only a very small evidence of the NH4 + bands in the region of 30303300 cm -1 and 1451 cm -1 gave indication of traces ofNH4 + groups. Finally, the bands at 1541 and 1393 cm -1 assigned respectively to the antisymetric and symetric contributions of monodentate carbonate were also present. When the pH is raised to 9 and 10, clear bands appear in the region of 3000 to 3200 cm -1 and in 1451 cm -1, which correspond respectively to the
415
stretching and bending vibrations of NH4 +. Furthermore, in figure 2b, where a higher amount of carbonate was used, all spectra show the bands corresponding to NH4 + and those of monodentate carbonate. The presence of small amounts of free ionic carbonate and bicarbonate is also possible but the overlapping of the bands with the mono-dentate carbonate ~nd -OH species made difficult to ascertain its presence. These results indicate that at high pH and high ammonium carbonate concentrations, the formation of an interaction compound between NH4 +, -O-CO 2- and the titanium and/or aluminium hydroxides is promoted. At low pH this mixed compound is formed only when a substantial amount of ammonium bicarbonate is used (ABC concentration of about 0.52 M.). The formation of the proposed "mixed compound" appears to be related to the increase in surface area, cumulative pore volume and pore size as can be observed in table 2, where the presence of the "mixed compound" was only evident in experiments 2.2 to 2.6.
~la]/ ~
.
t (bl z
i
. ,
. .
.
.
.
4000
.
.
.
.
.
I
. . . . . . . . .
I
.
.
.
.
.
.
.
.
3000 2000 1000 WAVENUIdBER (cm-1)
4000
.
.
.
.
.
.
.
i
3000 2000 1000 WAVENUMBER (cm-1)
Figure 2. (a). IR spectra of the Ti-AI-(0.5) samples prepared with 0.03 moles of AC and at different pH values: pH=8 (1), pH=9 (2) and pH=10 (3); (b). IR spectra of the Ti-AI-(0.5) samples prepared with 0.078 moles of AC at pH=8 (1), pH=9 (2) and pH=10 (3). Table 3 presents the results obtained when the TiO2/(TiO2+AI203) molar ratio (R) is varied from 0.0 to 1.0. The experiments in this table were performed by the method B of preparation using a 0.52 M. ammonium carbonate concentration, 150 ml of water to produce the hydrolysis of the titanium and aluminium isopropoxides and at a pH of 9. It is possible to observe in table 3 that when the ratio R is increased, the surface area is increased initially from 319 m2/g (R= 0.0) to 394 m2/g (R= 0.5) and then decreased continuously to 42 m2/g at R = 1.0. On the other hand, the cumulative pore volume did not change much between R= 0.0 to R= 0.5 and then decreased sharply at R = 0.75 and R= 1.0. The mean pore diameter dropped from 142 )k (R= 0.0) to around 96 A for all other values of R.
416 Table 3 Textural effect produced by ammonium carbonate (7.5 g.) on the TiO2-AI20 3 mixed oxide supports with different molar TiO2/(TiO2+AI203) ratios, pH=9 and using method B of preparation. No. Molar ratio R Surface area Cumulative pore Average pore ~m2/~) volume (cm3/~) diameter (A) 3.1 3.2 3.3 3.4 3.5
0.00 0.25 0.50 0.75 1.00
319 329 394 198 42
1.128 0.779 0.947 0.469 0.102
142 95 96 95 96
Regarding the shapes of the pore size distribution plots, figure 3 shows that in all samples, with the exception of the pure titania sample, exist a small population of pores of around 40 A in diameter and a main pore population of around 130 A in diameter. The pore size distribution curve of the pure titania sample showed in contrast, the first maximum at 60 A and the indication of larger pores with the maximum beyond 300 A.
dVIdlog R
151 2.4
141
1.6
//All31 (2)
0,8
lo
.......
ioo
....
iaii)
Figure 3. Pore size distributions of the Ti-A1(R) samples; R=I.0 (1), R=0.75 (2), R=0.50 (3), R=0.25 (4), R=0.0 (5). See table 3.
4000 .
.
.
.
.
.
.
.
.
50o0
.
.
.
.
.
~,ooo .
.
.
.
.
-iooo
WAVENUIdBER (cm-l)
Figure 4. IR spectra of the Ti-AI-(R) samples" R=I.0 (1), R=0.75 (2), R= 0.5 (3), R=0.25 (4), and R=0.0 (5).
Figure 4 shows the FTIR spectra of the oxide samples with different R values. In this figure, it can be seen that as the value of R decreases in the sample (the amount of aluminium
417 increases), new IR bands appear due to the presence of mono-dentate carbonate (1393 and 1541 cm-1) and ammonia (1452 and 3000 to 3200 cm-1). On the other hand, the pure titanium hydroxide sample shows a band which could correspond to free ionic or mono-dentate carbonate ( 1393 cm-1), and no band giving evidence of the presence of ammonia. Table 4 presents the textural effects produced on the pure Ti and AI oxides when increasing amounts of AC are used in the preparation (method B). The results in this table show that the changes induced by the ammonium carbonate are mainly due to an interaction with the aluminium and not with the titanium. In the case of the pure alumina sample, significant positive changes in surface area, cumulative pore volume and mean pore size are achieved when the amount of AC is increased. On the contrary, for the titania sample, surface area, cumulative pore volume and mean pore diameter, are little affected by the presence of the AC additive. Table 4 Textural effect produced on AI203 and TiO 2 oxide supports by ammonium carbonate (pH=9, 150 ml of water and using method A of preparation). No. Sample Amount of ammonium Surface Cumulative pore Average pore carbonate (g) area (m2/g) volume (cm3/~) diameter (A) 4.1 4.2 4.3 4.4 4.5 4.6
Al203 AI203 Al203 TiO 2 TiO 2 TiO 2
0.00 2 88 7.50 0.00 2.88 7.50
210 185 319 35.0 41.7 41.8
0.301 0.478 1.128 0.100 0.103 0.102
50 104 142 50 71 97
The analysis of the IR spectra of the samples included in table 4 indicates that in the case of the pure titanium hydroxide sample, no IR bands which could be assigned to ammonia vibrations were present and only one intensive band at 1400 cm -1 , assigned to free ionic carbonate, was clearly evidenced. On the other hand, the spectra of pure aluminium hydroxide with different amounts of AC, clearly shows the bands corresponding to ammonia in the region of 3000 to 3200 cm -1 and 1452 cm -1 and the bands corresponding to mono-dentate carbonate (1392 and 1541 cm-1), see figures 5a and 5b. These results, together with those in table 4, indicate that it is mainly through an interaction with the aluminium and not with the titanium that the ammonium carbonate is capable of producing large increases in surface area, cumulative pore volume and pore size due to the formation of a "mixed compound", in which the aluminium hydroxide interacts with both, the ammonium cation and the carbonate or bicarbonate anion. In order to confirm that for the formation of the "mixed compound" it was necessary the concurrence of the aluminium hydroxide and the ammonium and carbonate ions, additional experiments were made to compare the results in surface area, cumulative pore volume and pore size in the Ti-AI-(0.5) samples using as additive only NH4OH , (NH4)2CO3, K2CO 3 and no additive (only water). The results, shown in table 5, indicate that it is only when the ammonium carbonate is used that significant changes in the three responses, surface area, cumulative pore volume and pore size are obtained. When potassium carbonate is used, solids with large pore diameters
418
~131 4000
3000
...... 2000
1000
4000
WAVENUMBER (cm-1)
3000
2000
WAVENUMBER (cm-ll
1000
Figure 5. (a). IR spectra of the Ti-AI-(0) sample prepared with 0.0 g. of AC (1), 2.88 g. of AC, and 7.5 g. ofAC. (b). IR spectra ofthe Ti-AI-(1.0)sample prepared with 0.0 g. of AC (1), 2.88 g. of AC and 7.5 g. of AC. Table 5 Textural effect produced by different additives on the Ti-AI-(0.5) sample, using method B of preparation and 150 ml of water to precipitate the metallic isopropoxides No. Additive Amount of pH Surface Cumulative pore Average pore additive ~mole) area (m2/g) volume (cm3/g) diameter ~A) 5.1 5.2 5.3 5.4 5.5
(H20) NH4OH
(NH4)2CO3 K2CO 3.
(NH4)2CO3
--(*) 0.03 0.03 0.03
7 9 9 10 10
233 242 343 11 327
0.234 0.278 0.505 0.021 0.886
30 46 59 357 108
(*) required amount of NH4OH to give pH=9 (357 A) but very small surface area and cumulative pore volume are obtained. This effect has not yet been fully elucidated. The sample treated with only ammonium hydroxide had similar values of the three responses to those in the sample with no additive, indicating that it is not the ammonium ion alone that produces the textural changes. Clearly, it is necessary the presence of both, the ammonium and carbonate ions to produce the desired textural changes. Further proof of the interaction between the Ti-AI mixed hydroxide and the carbonate and ammonium ions was given by the changes observed in the zero point charge (ZPC) value for the mixed Ti-AI-(0.5) sample when the amount of AC was increased from 0 to 15 g. and the
419 amount of water was also varied from 40 to 150 ml. The ZPC of the standard sample (no AC additive and 40 ml of water) was found to be 7.3. This value is increased to 8.6 when the amount of AC is increased to 7.5 g. If for the same amount of AC (7.5 g.), the amount of water is increased from 40 to 150 ml, the ZPC drops from 8.6 to 7.95 due to the reduction in the AC concentration when the amount of water is increased. An increase in the pH value from 9 to 10, using 2.88 g. of AC and 150 ml of water, leads to an increase in the ZPC from 7.6 to 8.0 due to a greater interaction of the carbonate and ammonium ions with the Ti-A1 mixed hydroxide.
4. DISCUSSION The use of ammonium carbonate has been shown here to produce substantial changes in the surface area, cumulative pore volume and pore diameter of Ti-Al-(R) mixed oxides prepared by the sol-gel method. The evidence from this study indicates that to produce substantial increases in the surface area and porosity of the Ti-AI-(R) mixed oxides, it is necessary the formation of an interaction compound between the NH4 +, CO3 = and the Al ions. As the table 2 shows, the level of change in the three said responses depends also on the pH of the solution and the concentration of (NH4)2CO 3. The IR results confirm that it is only when a "mixed compound" NH4+-CO3=-AI is formed, that the surface area, cumulative pore volume and pore diameter of the samples are significantly increased. The analysis of the IR spectra also indicate that in the case of samples with only Ti, the "mixed compound" is not formed since no IR bands indicate the presence of NH4 + and mono-dentate carbonate in the solid sample. Also, the experiments with only NH4OH as additive indicate that it is necessary in addition, the concurrence of the CO3 = or HCO 3- ions to increase the surface area and the porosity of the TiAl-(R) samples. Furthermore, when K2CO 3 is used instead of (NH4)2CO3, the results are completely different (see table 5), which indicates that it is necessary the participation of the NH4 + ions to produce the desired textural changes. The variations in surface area, cumulative pore volume and mean pore size, shown in table 3, also indicate that when the ratio R in the Ti-AI-(R) mixed oxide is increased beyond 0.5, the surface area and pore volume drop almost linearly with the increase of the amount of Ti in the formulation of the mixed oxide. This fact also points out that it is through the interaction of the (NH4)2CO 3 with the AI and not the Ti, that the surface area and the porosity are increased. With respect to the shape of the pore size distribution curves, figure 3 shows two types of pores: the pores around 30-45A in diameter which are the normal size of pores when no additive is used, and the pores around 120A in diameter, which are the result of the change in structure promoted by the interaction of the NH4 + and CO3 = ions with the aluminium in the mixed oxide. The higher diameter pores found in the pure titanium sample may be the result of the generation of CO 2 gas from the decomposition, during calcination, of the free ionic CO3 = found in this sample, which promotes the formation of a few large pores of more than 200A in diameter and which contribute little to the total surface area (42 m2/g) and the cumulative pore volume (0,102 cm3/g). The results from the figure 1 show that increasing the amount of AC, changes the shape of the pore size distribution curve from mono to bi-modal. This change can be explained by a change in the structure of the mixed oxide due to the formation of the NH4+-CO3=-AI interaction compound, evidenced by the IR observations.
420 Finally, regarding the method of preparation, method B, in which the alkoxides are added to the ammonium carbonate solution, leads to surface areas, cumulative pore volumes and pore sizes higher than in the method A, due to an effect of increased local (NH4)2CO 3 concentration during the hydroxides precipitation step.
5. CONCLUSIONS From the present study, the following conclusions can be drawn: The use of (]N~4)2CO 3 as textural modifier leads to enhanced surface areas, cumulative pore volume and pore sizes in the Ti-AI mixed oxide. The increase in surface area, cumulative pore volume and pore size are related to the formation of a "mixed compound" NH4+-CO3=-AI which was evidenced by IR observations. The "mixed compound" is only formed with aluminium and not with titanium leading to decreased surface areas and porosities when the proportion of Ti is increased in the mixed oxide. Mono or bi-modal pore size distributions can be achieved depending on the (NH4)2CO 3 concentration and the pH used during the preparation of the Ti-AI mixed oxide.
ACKNOWLEDGMENTS We acknowledge financial support from DGAPA-UNAM (Mex.) and the EEC.
REFERENCES
1. M. Breysse, J.L. Portefaix and M. Vrinat, Catal. Today, 10 (1991) 489. 2. W. Zhaobin, X. Qin, G. Xiexian, E.L. Sham, P. Grange and B. Delmon, Appl. Catal., 63 (1990) 305. 3. J. Ramirez, L. Ruiz-Ramirez, L. Cedefio, V, Harle, M. Vrinat and M. Breysse, Appl. Catal., 93 (1993) 163. 4. B.E. Yoldas, J. Appl. Chem. Biotechnol., 23 (1973) 803. 5. D.L. Trimm and A. Stanislaus, Appl. Catal., 21 (1986) 215. 6. D. Basmadjian, G.N. Fulford, B.I. Parsons and D.S. Montgomery, J. Catal., 1 (1962) 547. 7. R.F. Vogel, G. Marcelin and W.L. Kehl, Appl. Catal., 12 (1984) 237. 8. R. Snel, Appl. Catal., 12 (1984) 347. 9. M.R.S. Manton and J.C. Davidtz, J. Catal., 60 (1979) 156. 10. K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, Wiley, New York, 1980. 11. L.H. Little, Infrared Spectra of Adsorbed Species, Academic Press, New York, 1966.
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 1995 Elsevier Science B.V.
421
P r e p a r a t i o n of metallo-silicate solid catalysts by sol-gel method with regulation of activity and selectivity I.M. Kolesnikov, A.V. Yuablonsky, S.I. Kolesnikov, A. Busenna, M.Y. Kiljanov State Academy of Oil and Gas named after Goubkin I.M., 117296 Moscow, Leninsky Prospekt 65, Russia The petroleum industry uses metallo-silicates as catalysts for cracking processes, alkylation of benzene with propylene and ethylene, and as adsorbents. Activity and selectivity of those catalysts increase with the help of zeolites with different structures and compositions. We worked out a method of synthesis based on the application of a thermal blow upon heating of sol or gel metallo-silicates (MS). The physical and catalytical properties of the solid MS depend on the pH of the sol and gel, time of syneresis, nature of the metal in the composition of the MS, acidity or basicity of the sol and gel, and time of initial coagulation of the sol into the gel.
I n f l u e n c e of pH of the sol on the t i m e of b e g i n n i n g o b v i o u s c o a g u l a t i o n of the sol We studied the properties of the sol under the influence of different pH values at T = 298 IL The experimental data are presented in figure 1.
1GQ
q
=,
120
qo
8
40
i
~a pH
Figure 1. Influence of pH on time of beginning obvious coagulation of sol into gel; a: alumino-silicate; b: zircono-silicate. It can be seen from Fig. I that the sol has a maximum rate of coagulation at pH values between 6 and 10. Similar results were obtained for the following
422 metallo-silicates: Be-, Mg-, Zn-, Sr-, Al-, Zr-, Fe-, Mn-, Co- and Ni-Si. For those MS, the pH regions with maximum rate of coagulations are:
MS
.pH
MS
pH
MS
pH
Be-Si Sr-Si Fe-Si Ni-Si
4.6-10.0 8.0-10.0 4.0-10.4 4.0-9.2
Mg-Si A1-Si Mn-Si
5.1-10.0 6.0-10.0 5.8-9.8
Zn-Si Zr-Si Co-Si
6.1-10.0 6.0-10.0 5.0-9.2
.....
The sol stability increases in the following sequence: Be-Si > Mg-Si > ZnSi, namely in the sequence of increasing ion radius: rBe2+ < rMg2+ > rZn2+ of decreasing ~ potential: ~Be-Si < ~Mg-Si < ~Zn-Si
The kinetic of changing hydrosol properties One of the steps in the catalysts preparation is the syneresis of sol and gel. During this step, several processes occur: creation and decomposition of the micelles, formation of polyhedral structure from micelle nuclei, dissolution of the small micelles and enlargement of the large ones, solvatation and change of the thickness of the solvate shell, and so on. The process passes at time and is characterized by the kinetics of syneresis. The rate of syneresis depends on the concentration of the initial salt solution, time of syneresis, temperature and nature of Me-Si. To investigate the role of the parameters mentioned above, we used colorimetry and the formula of Lambert-Beer: J = Jo exp (- e.c.1) where
J, Jo - intensity of passing a n d initial b e a m
of light
e - m o l a r coefficient of light absorption c - concentration I - thickness of the solution - sol or gel.
To study the law ruling the change of the properties, the sol and gel were prepared using a middle pH = 3.3 or pH = 10.7. We synthesized sols at pH = 3.3 and pH = 10.7. Those two types of sols were aged at T = 298 K for 18 h. The experimental data concerning the light transmission by sol or gel aluminosilicates and zircono-silicates are shown in Fig. 2 and Fig. 3.
423
lOO 9Q
gO
T
. Q
So
~
7o-
~
-
,,
I
I
G[I"
!
:..I
~so T .,,,,~
3o
9
7o
~ 1
!
I
1
?.
3
,
~0
L
I
I
q
5"
6
-
\
I
2,
'~
3
tl
--
5-
Figure 2. Dependence of light transmitted by alumino-silicate hydrosol-1 and hydrogel-2 vs. ln(l+~), z-time, rain.; a) pU 3.3; b) pH 10.7. 10U ~
1
~
90E_____~ q o
'
"~
2,
I
3
4
t
5,
t
L
~ "t.,n.C~+'c)
/ t
t
~,
3
zt
.5"
""
--
G r.~.~ ~."c )
Figure 3. Dependence of light transmitted by zircono-silieate hydrosol-1 and hydrogen-2 vs. In (1+~), ~-time, rain.; a) pH = 3.3; b) pH = 10.7. CA12(S04)3 g/l: 1/20; 2/60; 3/80; 4/100; 5/120; 6/140; 7/150.
424 It can be seen t h a t the time at which obvious coagulation begins changes with the pH value. The dotted curve indicates the time of beginning of obvious coagulation of hydrosol into hydrogel. The type of curve depends on the pH value, but z-coag - varies according to the concentration of the A12(804)3 solution. Influence of the preparation conditions on the surface area
Figure 2 presents the results of changing properties of sol and gel upon action of syneresis time, and concentration of the salt into water. Using the method of thermic blow, we prepared a series of catalysts at determined times of sol or gel aging. The surface area of this series of samples was m e a s u r e d by adsorption methods. The data are presented in Table 1. Table 1. Specific surface areas. Catalyst Aging time (rain) alumino120 silicate 240 410 600 100 200 245 600 42
pH of hydrosol
CA12(SO4)3 g/] 20 20 20 20 140 140 140 140 10
3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 10.7
S m2/g 122 128 419 337 162 110 161 115 142
84
10.7
10
101
128 170
10.7 10.7
10 10
107 107
From this table, the following conclusions can be drawn: - increasing the pH of the sol from 3.3 to 10.7 decreases S; - the value of S passes through a m a x i m u m with increasing ~ aging (for pH = 3.3); - increasing the concentration of A12(804)3 solutions decreases S. Infrared spectra
The alumino-silicate catalysts with different compositions were examined by IR spectroscopy in the following spectral domains: valence vibration" 1000 - 1200 cm -1 for ~ Si-O-Si 800,877, 750 cm -1 for ~ Si-O-AI~ 960 cm -1 for ~ AI-O-H 700, 668 cm -1 for ~ Si-O-AI~ isolated type I
560, 550, 535 cm -1 for ~ Si-O-AI~ condensed type I
425 473, 450, 430 cm -1 for -~ Si-O- condensed type. The q u a n t i t y of (SiO4.AlO4)-tetrahedra of "isolated" type increases for one of the series of A1-Si and decreases for the other one. The activity of the catalysts of the first type are higher t h a n for the second one.
Activity of the catalysts synthesized from sol and gel The activity of the catalysts synthesized from sol or gel w i t h different times of aging was studied. The d a t a are presented in Fig. 4 for the solids obtained from hydrosols or hydrogels at pH = 3.3, by the method of t h e r m a l blow. It can be seen t h a t the activity of the alumino-silicate catalysts passes t h r o u g h a m a x i m u m . The catalyst with the m a x i m u m activity was synthesized at a syneresis time of 300 rain, namely at the border of the sol into gel transformation.
. QQ
1
-
"1
il'l
2, I
5
FQ
/
--
I
,I . . . .
l
,I
l
n
Figure 4. Influence of aging time of hydrosol (1') and hydrogen (2') at pH = 3.3 for A1-Si catalysts on the conversion of cumene at 673 K (1), 723 K (2) and 773 K (3); CA12(S04)3 - 40 gfl. The conversion data obtained on the series of A1-Si synthesized from sols and gels at pH = 10.7 are presented in Fig. 5. The same results were obtained with zirconium silicates synthesized from sol a n d gel a n d for other metallo-silicate catalysts. This new m e t h o d of preparation allows to increase the catalytic activity by a factor of 1.8 - 2.0 in the cracking reaction of individual hydrocarbons. We studied the cracking of kerosene-gasoil fraction in the presence of synthetic alumino-silicates at T = 723 K and space velocity v=0.7 h. We used the catalysts points a, b, c in Fig. 4. The yields of benzene were 41.5, 44.6, 47.8 % mass., respectively and the coke yields were 2.6, 2.4 a n d 2.25 % mass., respectively.
426
I00
-
I
x,% 50
s
P.,{J
~o
6Q
Figure 5. Dependence of cumene conversion in A1-Si catalysts synthesized a t pH = 10.7 from sols (1') and gels (2') at 723 K (1) and 773 K (2).
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparationof HeterogeneousCatalysts G. Ponceletet al. (Editors) 1995 ElsevierScience B.V.
427
A n e w p r o c e d u r e f o r p r e p a r i n g aerogel catalyst C.-M. Zhang, S.-Y. Chen and S.-Y. Peng State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Science, Box 165 Tai Yuan Shanxi 030001 P.R. China. 1. INTRODUCTION Aerogel, a new kind of catalytic materials, exhibits several advantages [1], such as very high porosity, i.e. big pore volume, high specific surface area and very good textural stability during h e a t t r e a t m e n t at high t e m p e r a t u r e s . Aerogel can be p r e p a r e d by the removal of solvent from a wet gel at a t e m p e r a t u r e and pressure above the critical t e m p e r a t u r e and pressure of the solvent. Some aerogel may have unusual catalytic activities, selectivities and stabilities for several reactions. But the large number of variables involved in the synthesis of the gel and the process of supercritical drying makes it very difficult to understand clearly how to prepare special catalysts [2]. The process of supercritical fluid drying is not a suitable method for the p r e p a r a t i o n of some catalytic m a t e r i a l s , which are of low m e l t i n g points a n d easily crystallized, such as molybdenum oxide. In order to expand the use of this technique, a new procedure, i.e. gel-like method, is presently proposed for the preparation of aerogel. In the gel-like method, the gel is filled by solvent and a modifying agent(such as detergent). For the preparation of hydrogel with this procedure, a modifying agent should be used. The gel-like can be dried with supercritical fluid. The influence of SCF drying and calcination on aerogel are discussed. Two kinds of aerogels were p r e p a r e d by the gel-like method : alumina, which is easily prepared gel-like and molybdenum oxide, which is more difficult to prepare. 2. EXPERIMENTAL 2.1. Supercritical d r y i n g eqm'pment The scheme of the supercritical drying equipment is shown in Figure 1. 2.2. C h a r a c t e r i z a t i o n techniques Surface areas were calculated by the BET equation from the nitrogen adsorption isotherms at 77K. The N2 isotherm curves were obtained with a Carlo-Erba-1800 adsorption equipment. X-ray diffraction (XRD) patterns were recorded with a Rigaku Dmax-rA diffractometer operated at 40KV and 33mA with nickel filtered CuK radiation. Morphological study was carried out by T r a n s m i s s i o n Electron Microscopy (TEM) using a Philips EM400, a n d
428 S c a n n i n g E l e c t r o n Microscopy (SEM) u s i n g a H i t a c h i H-600. Thermogravimetric data were obtained with STA-780 Thermogravimetric Analyser. aldnooouJJaql
-lUa^lO S la9
aoou~n..I
5U!lOOO
Dllno
,.
^
^
aBno6 adNSSadd
Figure 1. Autoclave equipment for the evacuation of the solvent under hypercritical conditions.
3. RESULTS AND DISCUSSION 3.1. Alumina aerogel p r e p a r e d from gel-like The preparation scheme is shown in Figure 2. A1Cl3 water solution HC1 Polyethyl alcohol r acidic Change the pH ~lumina solution [gel-like
Mumina aerogel
SCFDT Wash with alcohol
I
Alumina alcogel
Figure 2 : Preparation scheme of alumina aerogel by gel-like process All the chemical reagents were chemically pure. The content of the modifying agent in the gel was much less than 10 percent. The alcogel was vigorously stirred for 15 minutes and then poured into a cylindrical stainless tube (30cm diameter and 250cm high) before hypercritical evolution. To evacuate the solvent under hypercritical condition, the tube containing the alumina gel was placed inside the autoclave with a capacity of 1 liter. The
429 autoclave was flushed with nitrogen several times : ethanol was used as supercritical solvent (for pure ethanol T=520K, P=6.7MPa) and was pumped into the autoclave, and the rise of t e m p e r a t u r e was programmed and the pressure was controlled automatically. The supercritical state was kept for 30 minutes. Thereafter, the pressure was decreased by venting off the solvent vapor. When the pressure reached atmospheric pressure, the heating was turned off, the autoclave was flushed overnight with dry nitrogen and the product was removed after cooling the autoclave to room temperature. Using the above described operation procedure, a series of samples was obtained. Their texture and properties are discussed below : a. Texture comparison of the aerogel prepared with this procedure and the conventional aerogel. The TEM and SEM observations indicate that there are large differences between the two kinds of aerogel. The alumina aerogel is laminar, whereas conventional alumina aerogel (TEM) is agglomerated.
a
b
Figure 3.: TEM (a) and SEM (b) micrographs of the alumina aerogel XRD results indicated t h a t A1203 aerogel was amorphous. After calcination at 773K for 4 hours, the alumina aerogel was transformed into a-A12 03. This is the same as the conventional alumina aerogel. The distribution of particle diameter before and after calcination are shown in Figure 4. b. Surface areas and pore volumes The surface areas and pore volumes of the products obtained at different t e m p e r a t u r e s and pressures (all the samples had the same content of modifying agent) are given in Table 1.
430 For the supercritical p r e p a r a t i o n s , the surface a r e a of the products decreases w i t h the i n c r e a s e before calcinetion of p r e s s u r e a n d t e m p e r a t u r e . This could be e x p l a i n e d by t h e h i g h e r pressure and temperature which after c ~ I c i n a ti.on m a y p a r t i a l l y destroy the s t r u c u r e of 0 > gel before r e a c h i n g the s u p e r c r i t i c a l condition. The modifying a g e n t h a s a major influence on the p r e p a r a t i o n of 0 a l u m i n a aerogel. A suitable c o n t e n t of modifying a g e n t h a s a f a v o r a b l e i% o effect on the surface area. T h i s is V" -- d 'k shown by the difference of the surface a r e a before a n d a f t e r c a l c i n a t i o n of the s a m p l e s as c o m p a r e d w i t h t h e surface a r e a of the original aerogel powder. W h e n the alcogel is dried in U~ 9 SCF, the modifying a g e n t is p a r t i a l l y removed and the gel s t r u c t u r e is kept. is | I It_ - r If t h e s a m p l e is c a l c i n e d , t h e 2 4 6 8 m o d i f y i n g a g e n t is b u r n t off. PORE DIAMETER (nm) Therefore, the modifying agent p r e v e n t s t h e collapse of t h e gel structure and enlarges the surface Figure 4. 9Distribution of pore particle a r e a a n d pore volume. F r o m S E M d i a m e t e r of a l u m i n a aerogel. photograph, it m a y be observed t h a t the modifying a g e n t leads to a change of the surface morphology a n d pore volume; the l a m i n a r p r o d u c t s a r e v e r y thin. As a c o m p a r i s o n , a few characteristics of the aerogels p r e p a r e d with and without modifying a g e n t are given in Table 2.
4
,,
Table 1. NO
1 2 3 4 5 6
T(K)
547 547 547 551 561 571
P(MPa)
6.7 7.8 9.2 8.2 8.2 8.2
original powder
S(m2/~) 525 300 263 217 177 156
V(ml/g) 1.19 -
calcined (673K) samples 4h. S(m2/g) V ( m l / g ) 438 256 236 598 1.67 223 218 -
8h S(m2/g) 360 -
The aerogel with the modifying a g e n t has h i g h e r pore volume. Although the a l u m i n a gel in acid solution cannot form the h y d r a t e d skeletal inorganic compound, addition of the modifying a g e n t r e s u l t s in bigger pore volume.
431 After b u r n i n g off the modifying agent, the pore volume reached 1.67 ml/g, which is larger t h a n t h a t of the common aerogel. TG results indicate t h a t the modifying agent is the gel could not be completely removed during the SCF drying operation, as shown in Figure 5.
'~.6
g
I
I
J
I
200 400 600 800 TEMPERATURE (*C) Figure 5. 9Thermogravimetric curve of A1203 aerogel with modifying agent Table 2 Comparison of the aerogels prepared from the two processes
S(m2/g) Vp(ml/g) ,
common alumina aerogel aerogel from this procedure
, .....
600
598
, L
1.30 1.67
.
.
R(A) .
.
app.
state
den.
.
30-50 50
0.02-0.04 amorphous 0.05 amorphous
The IR spectra of pyridine adsorbed show bands at 1450 cm -1 a n d 1490 cm -1, indicating t h a t there exist only Lewis acid centers on the surface of the alumina aerogel. This is the same result as for the conventional alumina aerogel.
3.2. MoOx aerogel The m a t e r i a l s used were a m m o n i u m h e p t a m o l y b d a t e (abbr.A.H.M.) deionised w a t e r and agents A and B (A.R.). P r e p a r a t i o n : the p r e p a r a t i o n procedure is shown in Figure 6. The t e x t u r a l c h a r a c t e r i s t i c s of the conventional and the MoO3 aerogel are shown in Table 3.
432 Table 3 Textural characteristics of conventional and MoO3 surface area (m2/~) 1.6 19.13
C-MoO3 A-MoO3
Pore volume (ml/g) 0 0.12
~NH4)6Mo7024
apparent density (ml/g) 3.0 0.33
MoOx aerogel
1
+water
I
A agent
!
.solution
SCFDT B agent
gel product transparent
] alcogel
]
methanol
Figure 6. : Preparation of MoOx hydrogel and aerogel The influence of SCF condition and calcination on the surface area and pore volume are shown in Table 4. a. Influence of supercritical conditions on MoO3 aerogel. Table 4 Influence of supercritical conditions on MoO3 aerogel N~ T(K) P(MPa) S(m2/g)
711
712
713
716
726
712
727
573 14 22.15
573 11.5 19.13
573 10 16.8
533 11.5 4.9
552 11.5 17.18
573 11.5 19.13
593 11.5 21.79
With the increase of pressure and t e m p e r a t u r e , the surface a r e a increases. The modifying agent is easily completely destroyed (decomposition temperature less t h a n 523K as shown in Figure 7) at supercritical condition. The modifying agent and MoO3 at high pressure and t e m p e r a t u r e have a s t r o n g interaction. The gel structure could be kept. At the supercritical condition, the modifying agent is completely destroyed. So, the modifying agent could be the main reason for the increase of the surface area. b. Influence of the calcination temperature on MoO3 The sample has been calcined at the different temperatures. The results are shown in Table 5.
433 Table 5 Influence of calcination temperature on MoO3 aerogel T(K)
non calcined
473
523
573
673
19
9.28
8.18
8.37
7.3
,|
S(m2/g)
From the above discussion, it is surface area of the aerogel prepared modifying agent. This experimental widely used in the preparation of other
possible that the large influence on the by the gel-like method is due to the procedure to prepare aerogel may be catalytic components.
8
~ I
0"
2(J0
4(J0 6()0 8(J0 TEMPERATURE (*C)
Figure 7. 9Thermogravimetric curve of MoO3 aerogel R~ERENCI~ 1. G.M. Pajonk, Appl. Catal. 72 (1991), 217 2. L.L. Hench, Chem. Rev. 90 (1990), 33
This Page Intentionally Left Blank
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
435
P r e p a r a t i o n of single a n d b i n a r y i n o r g a n i c oxide a e r o g e l s a n d t h e i r u s e as s u p p o r t s for a u t o m o t i v e p a l l a d i u m c a t a l y s t s C. Hoang-Van a, R. Harivololona a and B. Pommier b aURA au CNRS '~Photocatalyse, Catalyse et Environnement", Ecole Centrale de Lyon, B.P. 163, 69131 Ecully C~dex, France. bInstitut de Recherches sur la Catalyse, CNRS, 2, avenue A. Einstein, 69626 Villeurbanne C~dex, France.
ABSTRACT Different methods based on the sol-gel process combined with the supercritical drying technique have been applied to the preparation of highly divided single or binary oxide aerogels. The surface areas of single oxide aerogels were generally lower than those of binary alumina - based oxide-oxide aerogels which always exceeded 600 m2.g-1 when a co-hydrolysis preparation was used. This procedure allowed us to produce a well dispersed mixture of the oxide phases. In contrast, the counterparts of these binary aerogels synthesized by a two-step procedure probably consisted of a layer or of agglomerates of the oxide additive deposited on alumina. Oxide aerogel supported Pd catalysts were efficient in the reaction CO + NO + 02 (stoichiometric mixture) for both CO and NO conversions. 1. I N T R O D U C T I O N Inorganic oxide aerogels exhibit very high porosities and specific surface areas as well as very good textural stabilities d u r i n g h e a t t r e a t m e n t at high t e m p e r a t u r e s . These properties m a k e t h e m interesting either as c a t a l y s t s or supports of catalysts [1]. In this paper, we describe methods for the p r e p a r a t i o n of single or b i n a r y inorganic oxide aerogels t h a t could be used as carriers for high performance supported Pd automotive catalysts. Indeed we have shown t h a t a l u m i n a - based aerogel supported palladium catalysts were very active in propane oxidation [2]. Here we present the catalytic performances, in the reaction CO + NO + 02, of p a l l a d i u m c a t a l y s t s s u p p o r t e d on some of the p r e p a r e d aerogels a n d compare their performances with those of Pd or Pt-Rh catalysts supported on a commercial a l u m i n a .
436 2. EXPERIMENTAL 2.1. Preparation of aerogels The preparation of the aerogels was based on the sol-gel process combined with the supercritical drying method [3]. Starting from molecular organic precursors (alkoxides or acetylacetonates in this work) dissolved in an alcohol, corresponding alcogels were obtained by hydrolyzing with stoichiometric amounts of added water. These alcogels were dried in an autoclave above the critical temperature and pressure of the solvent. Table 1 shows the precursors used as well as the solvents and their critical temperature (Tc) and pressure (Pc). Table 1 Precursors and alcohols used for the preparation of aerogels Aerogel
Precursor Alcohol
Tc(~
P c (bar)
A1203 Ti02 Zr02 Ce02
Al-sec-butoxide Ti-isopropoxide Zr-propoxide Ce-acetylacetonate
2-butanol 2-propanol 1-propanol 2-butanol
263 235 ~:~ 263
41 47 51 41
La203
La-acetylacetonate
or methanol 2-butanol
240 263
80 41
or methanol
240
80
2-butanol or methanol
263 240
41 80
BaO
Ba-acetylacetonate
Solvent
Binary alumina-based oxide-oxide aerogels were synthesized either by a coor by a t w o - s t e p hydrolysis procedure. In the co-hydrolysis procedure using alkoxide precursors, the co-gelling of both precursors in an appropriate solvent was prepared by adding water in stoichiometric amounts and the alcogels were then supercritically dried. When acetylacetonate precursors were used for the oxide additives, 2-butanol was employed as a solvent. Their hydrolysis started ca. 15h before a solution of A1sec-butoxide in 2-butanol was added to the first solution c o n t a i n i n g stoichiometric amounts of water required for the hydrolysis of both precursors. This was because of the much lower rates of hydrolysis of acetylacetonate precursors as compared with those of the alkoxide ones [4]. The solvent was finally supercritically evacuated. In the two-step procedure, the oxide additive precursors were hydrolyzed in an a p p r o p r i a t e solvent ( m e t h a n o l was used in this p r o c e d u r e for acetylacetonate precursors). A1203 aerogel, already prepared, was introduced into this solution which was then supercritically dried. hydrolysis
2.2. Preparation of catalysts Palladium catalysts (ca. lwt%) were obtained by impregnation of the
437 supports with a methanolic solution of palladium acetylacetonate. The solids were then dried at ll0~ for about 15h. 2.3. Characterization Surface areas were determined from N2 adsorption at - 196~ Prior to the adsorption measurements the samples were treated in a He flow at 250~ for 2h. The structure of solids was examined by X-ray diffraction (Siemens Kristalloflex D500, CuKa radiation). The dispersion of palladium was measured by CO chemisorption, following the procedure described earlier [5]. The state of palladium and the structure of supports were investigated, in some cases, by grazing-incidence X-ray diffraction (GIXD) as previously reported [6].
2.4. Activity measurements Catalytic experiments were carried out using a flow system at atmospheric pressure. The procedure was the same as t hat described earlier [5]. The gas mixture used [0.75% CO + 0.1% NO + 0.35% 02 + N2 (diluent)] was almost stoichiometric, since the stoichiometry number was s = (202 + NO)/CO = 1.07. Activation of the catalyst consisted in a t r e a t m e n t at 500~ in the flowing reaction mixture (20L/h) for 3h. The conversions of CO and NO were measured at increasing temperatures in the range 150-500~ at a program m ed rate of 2~ 3. RESULTS AND DISCUSSION
3.1. Single oxide aerogels Table 1 gives the surface areas and the state of crystallization of the single aerogels prepared. The surface area of aerogels decreased in the order 9 A1203 > ZrO2 > TiO2 > CeO2 A1203 and CeO2 aerogels issued from the autoclave were amorphous whereas TiO2 and ZrO2 aerogels were obtained in crystalline forms (anatase and cubic ZrO2). Table 1 Surface areas and crystalline state of single aerogel s oxide Aerogel
Surface area (m2.g -1)
Crystalline state
A1203 ZrO2 TiO2
490 370 100
amorphous Cubic ZrO2 anatase
CeO2
65
amorphous
3.2. Binary alumina-based oxide-oxide aerogels For these mixed aerogels, a l u m i n a was always the major component
438 (ca. 90 wt%). The oxide additives (CeO2, BaO, La203 and ZrO2) are known to be good promotors for automotive catalysts [7,8]. Table 2 gives the surface areas and crystalline state of binary oxide aerogels prepared by co-hydrolysis or by the two-step procedure. Table 2 Surface areas and crystalline state of binary alumina-based oxide-oxide aerogels Aerogel ZrO2-A1203 La203-A1203 C eO2-A1203 BaO-A1203 Ti O2-Al2O3 Aerogel ZrO 2/A1203 La203/A1203 C eO2/A1203 BaO/A1203 TiO2/A1203
Binary aerogels (Co-hydr01ysis) Surface area (m2.g -1) Crystalline state 790 745 654 630 615
am o rp h o u s amorp ho us amorp ho us amorphous am o rp h o us
Binary aerogels (Two-steps procedure) Surface area (m2.g -1) Crystalline state 530 430 490 490 510
am o rp h o us amorphous am o rp ho us boehmite anatase and boehmite
Binary aerogels prepared by co-hydrolysis exhibited much higher surface areas than those of single aerogels and of binary aerogels synthesized by the two-step procedure. The following order of surface areas was observed for co-hydrolysis : ZrO2-Al203 > LaO3-Al203 > CeO2-A1203 > BaO-AI203 > TiO2-AI203 whereas the two-step procedure led to the sequence : ZrO2/Al203 > TiO2/AI203 > CeO2/AI203 _~ BaO/A1203 > La203/A1203 Binary aerogels prepared by co-hydrolysis were amorphous. This preparation procedure led very likely to a well dispersed mixture of the two oxide phases which allowed the obtention of very high surface area binary aerogels in the amorphous state (Table 2), including TiO2-A1203 and ZrO2A1203 which contained oxide additives (TiO2 or ZrO2) whose single aerogels exhibited a crystalline structure (Table 1). In the two-step procedure, already prepared alumina aerogel was introduced into a solution c o n t a i n i n g the additive precursor which underwent hydrolysis. The resulting binary aerogel probably consisted of a
439 layer or of agglomerates of the oxide additive deposited on alumina. The surface area of these binary aerogels might reflect that of the alumina aerogel modified by a second drying in the autoclave and, to a lesser extent, by the presence of the oxide additive. Therefore, the surface areas exhibited by these binary aerogels (Table 2, lower part) were not very different from that of alumina aerogel (Table 1, 490 m2.g-1). On the other hand, the absence of a thorough interaction between the two oxide phases in these aerogels was suggested by the formation of TiO2 anatase in the TiO2/A1203 aerogel (Table 2, lower part), in contrast to the amorphous TiO2-A1203 aerogel (Table 2, upper part). Nevertheless, oxide additives could induce a modification of the state of the initially amorphous a l u m i n a aerogel d u r i n g the d r y i n g u n d e r supercritical conditions of the two-step synthesized aerogels, since the A10(OH) boehmite phase was detected for BaO/A1203 and TiO2/A1203 (Table 2).
3.3. Pd/aerogel catalysts Pd catalysts supported on A1203, CeO2 and CeO2/A1203 aerogels were characterized by BET measurements, CO chemisorption, XRD and GIXD determinations. The results obtained are summarized in Table 3. Prior to all determinations, the samples were reduced in a H2 flow at 500~ for 3h. For comparison purposes, a Pd/A1203-Degussa and a conventional Pt-Rh/A1203 three-way catalyst were also studied. Table 3 Textural and structural characteristics of catalysts catalyst a) SBET Metal dispersion Support structure (m2.g -1) Pd/A1203-A
280
(%) 22 c)
(DRX) amorphous
Metal state (GIXD) Pd ~
Pd/A1203-D 110 25 c) y-Al203 Pd ~ Pd/CeO2/A1203-A 396 38 cubic CeO2;y-A1203 Pd ~ Pd/CeO2-A 21 26 d) cubic CeO2 Pd ~ Pt-Rh/A1203 b) 100 55 y-A1203+5-A1203 _ e) a) Metal loading: ~ 1 wt% - A: aerogel - D : Degussa b) conventional dechlorinated three-way catalyst : 1% Pt - 0.2% Rh/A1203 [9]. c) Pd particle diameters in the range 3-9 nm as observed by TEM. d) given only as an indication because CO can adsorb on CeO2 e) - : not measured.
3.4. Catalytic performance in the reaction CO + NO + 02 The light-off curves for CO a n d NO conversions over Pd/A1203-A, Pd/A1203-D, Pd/CeO2-A and Pd/CeO2/A1203-A are shown in Figures 1, 2, 3 and 4, respectively. N20 was not observed in this work.
440
~ ~, o ~
100 90 80 70
o
60 50 40
0 r,.)
30 20
(D
9 100
m-m--m
_
90
--
80
_
70
""~-.D...Q__~"" --
10
150
200
250
300
350
400
450
60 50 40 30 20 10 0
O r.D
O
9 Z
500
temperature (~ Figure 1. P d / A l 2 0 3 - a e r o g e l - CO an d NO conversions as functions of the r e a c t i o n t e m p e r a t u r e for a s to ich i ome tric r e a c t i o n m i x t u r e (0.75%C0 + 0.1%NO + 0.35%02).
O r~
100 908070-
9
100 90 80 70
m-m--
~ e.,.!
O
o
(D
6050403020100150
60 50 40 30 20 10 0 200
250
300
350
400
450
500
temperature (~ Figure 2. P d / A l 2 0 3 - D e g u s s a - CO an d NO conversions as functions of the reaction t e m p e r a t u r e for a stoichiometric r e a c t i o n m i x t u r e .
O o0
,F-4
O {9
9
441
00 90 80 70
100 -~90--
~
0
80-70-, 60-
0
0
r..?
50403020 - /
60 50 40 30
0
20
Z
0
10 0
10,
150
~. "~ . Io "-I
200
250
300
350
400
450
500
temperature (~ Figure 3. P d / C e O 2 - a e r o g e l - CO and NO conversions as functions of the reaction t e m p e r a t u r e for a stoichiometric reaction m i x t u r e .
~'~
,-,~
1 O0 90
.
.
.
.
=-,.-.. 1 O0 90
80
80
70
70
60
60
50 40
50 40
30 20
30 20
9
10
,~
.~ = 0 0
10
0 150
I 200
I 250
I 300
I 350
I 400
I 450
0 500
temperature (~ Figure 4. P d / C e O 2 / A 1 2 0 3 - a e r o g e l - CO and NO conversions as functions of the reaction t e m p e r a t u r e for a stoichiometric reaction mixture.
442 For aerogel supports (A1203-A, CeO2-A a n d CeO2/A1203-A), the conversion of NO started at a t e m p e r a t u r e consistently higher t h a n t h a t observed for CO conversion (Figures 1,3 and 4) whereas for the Pd/A1203-D catalyst, CO and NO conversions were similar between 150 and ca. 230~ (Figure 2). Therefore, oxide aerogels used as cariers for Pd c a t a l y s t s markedly improved their activities for CO conversion as compared with that of NO. Furthermore, if the activities of the studied catalysts were represented by their light-off temperatures (at 50% of CO and NO conversions, table 4), the activity sequence for CO conversion was 9 Pd/CeO2/A1203-A >Pd/CeO2- A > Pd/A1203-D > Pt-RhA1203 and that for NO conversion was 9 Pd/CeO2-A > Pt.Rh/A1203 - Pd/CeO2- A1203-A > Pd/A1203-D : Pd/A1203-A Table 4 Light-off t e m p e r a t u r e s (at 50% conversion) of catalysts. Stoichiometric reaction mixture 90.75%CO + 0.1% NO + 0.35% 02 + N2 (diluent). catalyst* Pd/A1203-A Pd/A1203-D Pd/CeO2/A1203-A Pd/CeO2-A Pt-Rh/A1203 * See Table 3
CO 0 T50%(C)
221 243 182 192 2(}9
T~500(oC) 250 248 236 225 232
Obviously, Pd supported on CeO2/A1203 and CeO2 aerogels led to the highest activities for CO conversion under stoichiometric reaction mixture. Also, aerogel-based supported Pd catalysts were much more active than conventional catalysts for CO conversion, especially the Pt-Rh/A1203 conventional three-way catalyst. Similar conclusions were already reported in a previous study for CO + NO + 02 reaction under markedly oxidizing conditions (s _~ 2) [5]. The light-off t e m p e r a t u r e s for NO conversion were s i m i l a r for the Pd/CeO2/A1203 - aerogel and the three-way catalysts. Pd/CeO2-aerogel exhibited the highest activity whereas both alumina-aerogel and a l u m i n a - x e r o g e l (Degussa) supports led to almost the same light-off t e m p e r a t u r e for NO conversion. 4. CONCLUSION The surface areas and the crystalline state of single oxide aerogels strongly depended on the chemical nature of the aerogels. This however, was not true for binary alumina-based aerogels prepared by co-hydrolysis which always exhibited very high surface area (S > 600 m2.g-1). Furthermore,these binary aerogels were always amorphous, suggesting that the co-hydrolysis procedure led to a well dispersed mixture of the two oxide phases. In contrast, the twostep procedure allowed the production of binary alumina-based aerogels which
443 probably consisted of the oxide additive deposited on alumina either in the form of a layer or as distinct agglomerates. Under a stoichiometric mixture of the reactants, the use of oxide aerogels as carriers for Pd catalysts markedly improved the activity for CO conversion in each case and the activity for NO conversion in the cases where the aerogel supports contained Ce02. ACKNOWLEDGEMENT We wish to thank Mr. R. Vera (Universit~ Lyon I) for the X-ray diffraction measurements. REFEICENCF~ 1. 2. 3. 4. 5. 6. 7. 8. 9.
G.M. Pajonk, Appl. Catal., 72 (1991) 217. C. Hoang-Van, R. Harivololona and P. Pichat, Europacat - 1, Montpellier (France), Sept. 1993, Book of Abstracts, vol. 2, p. 921. S.J. Teichner, Rev. Phys. Appl., 24 (1989) 1. J. Livage, M. Henry and C. Sanchez, Prog. Solid State Chem., 18 (1988) 259. C. Hoang-van, B. Pommier, R. Harivololona and P. Pichat, J. Non-Cryst. Solids, 145 (1992) 250. C. Hoang-van, R. Harivololona and S. Fayeulle, Third Intern. Cong. on Catal. and Autom. Pollut. Control (CAPoC3), Brussels, Belgium, April 1994. K. Masuda, M. Kawai, K. Kuno, N. Kachi and F. Mizukami, Studies in Surf. Sci. and Catal., "Preparation of Catalysts V", eds. B. Delmon et al., Elsevier, 63 ( 1991) 229. J.E. Kubsh, J.S. Rieck and N.D. Spencer, Studies in Surf. Sci. and Catal., Second Intern. Cong. on Catal. and Autom. Pollut. Control (CAPoC2), ed. A Crucq, Elsevier, 71 (1991) 125. J.L. Duplan, thesis n ~ 91-91, University of Lyon (1991).
This Page Intentionally Left Blank
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
Synthesis and Characterization Complex O x i d e P o w d e r s
of Sintering
445
Resistant
Aerogel
D. M. Lowe, M. I. Gusman, and J. G. McCarty Materials Research and Chemical Engineering Laboratory, SRI International, Menlo Park, CA 94025-3493 USA
ABSTRACT Aerogel powders of binary aluminate and zirconate oxides were prepared by a sol-gel technique and super-critical drying. The synthesis conditions are described and initial properties. Changes in the surface area and phases present in these powders and several reference materials are followed as they are heated at various temperatures in air containing 10 vol.% water vapor. In addition, a time series was done on the most sintering resistant material (LaAI11018) in order to estimate its sintering rate. INTRODUCTION Support materials for noble metals and complex oxides used as combustion catalysts require superior sintering resistance in the presence of humid air at elevated temperatures [ 1]. Interlocking one- and two-dimensional oxide grains with high aspect ratio could give high sintering resistance. Using the simple intuitive concept that the extreme porosity of amorphous aerogels would lead to formation of high aspect ratio particles, we prepared a series of binary oxide aerogel powders. Sol-gel synthesis and super-critical drying were used to make several binary oxide aerogel powders that retain BET surface areas greater than 10 m2/g after extended heat treatment above 1200~ in air containing 10 vol.% water vapor.
PREPARATION OF POWDERS Aerogel powders were prepared using a sol-gel technique and super-critical drying. Solutions containing alkoxides and metal salts such as aluminum isopropoxide and lanthanum nitrate dissolved in alcohol were mixed with distilled water or a base to form viscous gels. In general, the metal salts were dissolved in alcohol and/or distilled water with heat and stirring. The alkoxides, which were also dissolved in alcohol, were kept under dry inert conditions until the salt/water solution was added to the vigorously stirring solution. Thick gels that formed in less than 30 seconds were thoroughly mixed and then aged overnight. Super-critical drying was done in an autoclave pressurized with nitrogen gas. The temperature was increased to 300~ with a corresponding increase in pressure then the alcohol vapor was vented from the system above the critical point to produce very high surface area aerogel powders. Finally, the powders were calcined in dry flowing air in steps to 1000~ to decompose any residual organics, salts, and hydroxides.
446
Synthesis and Calcination of Aerogel Binary Oxide Powders Table 1 shows the target phase, calculated and measured molar ratios, the starting chemicals, hydrolysis ratio, and initial surface area for each of the aerogel powders. The target phase is the major crystalline phase expected to be found in the samples when they are used as catalyst support materials in a combustion environment. The molar ratio of the two metal elements in the binary oxide powders was calculated using the known weights of the starting materials. However, since the composition of some of the commercial solutions and chemicals Table 1 Chemical composition, synthesis conditions, and initial BET surface areas of aerogel and reference powders Calculated Measured
Sample
Target
Molar
Molar
Starting Chemicals
Ratio 2.83
Ratio 3.47
0.97
1.00
MN in NBA/AIPO in IBA
Hydrolysis Ratio 1.73
SA, m2/~ 291
LN in IBA/AIPO in IBA
6.00
155
#4
Phase MgA1204
#6
LaA103
#8
I.aAl11018
10.69
10.05
LN and H20 in IPA/ AIPO in IBA and IPA
2.61
615
#9
MgA1204
1.93
1.69
MN and H20 in IPA/ AIPO in IBA and IPA
4.58
212
#12
SrZtO 3
1.00
-
SN in H20/ZNPO in NPA
20.00
31
#13
BaAll2019
18.71
BA in H20/ AIPO in IBA and IPA
1.87
698
#18
LaAlllOl8
13.52
LN and H 2 0 in IPA/ AIPO and Ball Milled
2.50
566
#21
SrZrO 3
1.00
H20 in IPA/ SrZr(OR) 6 in IPA/
3.50-7.50
19
#22
MgAl204
2.00
H20 in IPA/ Mg~2(OR) 8 in IPA/ TMAH in H20
2.00-3.50
468
#25
CeAlllO18 10.98
ACN and H20 in IPA/ AIPO in IBA and IPA
1.51
470
19.15
~ 1 1 0 1 8 in IPA
ReL#1
y-A1203
-
T-A1203 from Harshaw Chemical Company
91
Ref. #2
MgO
-
LN and T-A1203 MgO from UBE Industries
38
Ref. #3
O-AI203
-
Ball Milled in H20/
38
18.99
TMAH in n 2 0 MN = Mg(NO3) 2 96 H20 LN = La(NO3) 3 ~ 6 H20 BA = Ba(C2H302)2 NBA = Butanol
SN = Sr(NO3) 2 ACN = (NH4)2Ce(NO3) 6 TMAH = N(CH3)4OH IBA - Isobutanol
AIPO = AI(O-i-C3H7) 3 ZNPO = Zr(O-n-C3H7) 4 IPA = Isopropanol NPA = Propanol
447 can vary by as much as 3% (e.g., the concentration of aluminum isopropoxide in isobutyl alcohol and the water content of the salts), it was expected that the actual molar ratios might differ significantly from stoichiometric. Elemental analysis of several samples by the Inductively Coupled Plasma (ICP) method proved this point. The measured molar ratios for the four powders analyzed are based on these results. The initial specific surface area of the various powders, as measured by nitrogen gas adsorption using the BET method, fell between 19 and 698 m2/g. The surface area was mainly dependent on the hydrolysis ratio, which is the number of moles of water and base per mole of alkoxide during the formation of the gel. Low hydrolysis ratios produced aerogel powders with a very high specific surface area. In the first two samples, Aerogels #4 and #6, the water molecules attached to the nitrate salts were used to react with the alkoxide and form the gel. This was possible since both of the nitrate salts have 6 water molecules attached and the ratio of alkoxide to salt was low. In many of the other aerogel powders additional water had to be added to the salt solutions to increase the hydrolysis ratio or dissolve the salts. Aerogels #21 and #22 were made using double metal alkoxides and #18 was seeded with crystalline powder. Both methods were attempts to initiate the formation of the target phase at a lower temperature without going through all the phase transitions.
Other Sintering Resistant Oxide Powders The ~,-A1203 and MgO powders (References #1 and #2) shown in Table 1 were obtained from Harshaw Chemical Company and UBE Industries, respectively. The 3'-alumina powder had moderately high specific surface area with presumably filamentous grains and the magnesia powder consisted of agglomerates of submicron single-crystalline cubic particles. Reference powder #3 is La203-stabilized ~'-AI20 3 that was prepared in our laboratory by ball milling lanthanum nitrate with the oxide in water, then precipitating with a very strong base while monitoring the pH of the mixing slurry. This material was then centrifuged and the excess liquid decanted. Finally, the La203-stabilized 3,-A1203 powder was vacuum dried and calcined in dry air at 1000~
Sintering and Characterization All of the powders were sintered for 4 hours in flowing air containing 10 vol.% water vapor from 1000 to 1600~ in a furnace consisting of an AI20 3 tube heated with MoSi 2 elements. X-Ray Diffraction (XRD) analysis of the samples for determination of phase and crystal structure was performed using a Ni-filtered Cu Kcx X-ray source scanning 20 from 15~ to 80 ~ In addition, the microstructure of the powders was examined using Scanning Electron Microscopy (SEM) and the surface area was measured by nitrogen gas adsorption using the BET method. RESULTS AND DISCUSSION
Aerogei Powders Two competing processes are taking place during heat treatment which decrease the surface area of the aerogel powders. The first and most significant is the nucleation and growth of crystalline phases from the amorphous oxides. The second is the sintering and densification of these phases with time. The aerogel powders go through several phase changes during heat treatment as shown in Table 2. With the exception of SrZrO3 (Aerogels #12 and #21), all of the powders were amorphous before calcination (when observed by XRD and SEM) and had very high surface areas as shown in Table 1. Aerogel #12 began to crystallize during the supercritical drying step forming tetragonal ZrO 2, SrCO 3, and a trace amount of monoclinic ZrO 2.
448 This is not surprising since the formation of SrZrO 3 in alkoxide derived powders has been observed to begin at 250~ [2]. Since Aerogel #21 was made from a similar double metal alkoxide we assume that crystallization of this powder also began at 250~ leading to a comparatively low surface area. All of the other higher surface area powders began to crystallize only during calcination at 1000~ or at higher temperatures, so the values measured after the 1000~ treatment in air and water vapor are much lower. The solid-state reactions taking place in Aerogel #8 (LaAlllO18) during heat treatment and their effect on surface area followed the results of Arai, et al. [3,4]. Although the initial surface area of #8 was 615 m2/g, it consisted of separate amorphous lanthana and alumina particles. The reaction between La20 3 and A1203 to form LaA103, which occurred between 800 and 1000~ decreases the surface area to 247 m2/g. The perovsldte phase, LaAIO 3, then reacts with AI20 3 to form LaAlllO18 between 1000 and 1100~ with another large drop in surface area to 46 m2/g. Above 1100~ the surface area drops more gradually as the solid-state reaction nears completion and bulk sintering begins. Similar results have been observed for the formation of BaA112019 from high surface area powders [5,6]. Table 2 XRD phases present in samples after sinterin$ for 4 hrs. in air and 10 vol.% water vapor Sample Initial 10(OC 1100~ 1200~ 130(PC 1400~ 1500~ 161~C ,,,
#4
A
MA, (T) MgO
-
MA, a
-
#6
A
LO, LA
#8
A
y, LA
#9
A
MA
-
MA, a
-
MA, (T)a
-
LA, LO
LA, LHA
LA, LHA
-
LA, LO
-
LA, LO
-
LHA, LA
-
LHA, LA
MA,
-
MA,
-
MA, MgO
sz
MgO
MgO
#12
TZO, SC, (T) MT_O
SZ
sz
sz
sz
sz
sz
#13
A
T, BA
T, 13, BA
T, 13, BA, BHA
BHA, a, BA
-
BHA, a
#18
A
T
0, T
0
a, LHA
a, LHA
a , LHA
a, LHA
-
-
#21
-
SZ
SZ
SZ
SZ
SZ
#22
A
MA
MA
MA
-
MA
#25
A
CO, T
CO, y
CO, T
Ref. #1
T
Ref. #2 Ref. #3
MgO T
a,
K, 0
MgO LA, T
a,
K
LA, 0
CO, a
CO, a
a
a
a
MgO LA, 0, a
LA, a , LHA
MgO a , LA, LHA
MgO
,L
A = Amorphous MA = MgA120 4 a = a-A120 3 = ~-AI203 MT_D = Monoclinic ZrO 2
0 = 0-A1203 y = T-A1203 K = K-A1203 BHA = BaA112019 TZO = Tetras. ZrO 2
LHA = LaAI11018 LA = LaA103 LO = La203 BA = BaA1204 (T) = Trace
449 Samples of Aerogel #8 (LaAlllO18) heated to 1400 and 1600~ for 4 hours in flowing air and water vapor were examined with the SEM. The microstructure of these powders consisted of thin hexagonal platelets of LaAlllO18 and very small crystallites of LaA10 3. This is consistent with the XRD results shown in Table 2. Although the hexagonal platelets mainly experience two dimension growth [7] during heat treatment thereby forming a stable interlocking porous structure, the perovskite sinters in three dimensions and loses its porosity. Therefore, the decrease in the surface area of Aerogel #8 (LaAIllO18) between 1100~ and 1200~ shown in Figure 1 is mainly attributed to the sintering enhanced by the LaAIO 3 perovskite phase.
1000
O
E
A
100
m A
<
o
O
m
tn,, r
O A
10_
D
= =
E3
O
I
0
=
1 800
9
i
1000
.
I
1200
,
I
1400
,
I
1600
,
1800
Temperature, ~
Figure 1. BET surface area as a function of sintering treatments for 4 hrs. in air and 10 vol.% water vapor: O, Aerogel #8 LaAlllO18; A, Aerogel #13 BaAll2019; , , Aerogel #18 LaA1110]8; D, Aerogel #25 CeAl11018. Semi-empirical sintering rate laws for the aerogel-derived binary oxides were determined by sintering for time periods up to 72 hoursat temperatures ranging from 1200 to 1600~ Sintering was taken to follow the expression, S = S0[1 + ]~(k(Ti) ti)]-1/n
(1)
where S is the specific surface area after a series of ramp and hold time intervals ti at temperature Ti and n is the time reciprocal power exponent. Least squares analysis of a series of extended sintering experiments with Aerogel #8 (LaAlllO18) showed that the power law exponent was =- 4.93. The hexa-aluminate powders, which consisted of an agglomeration of high aspect ratio platelets, showed the best sintering resistance. Extrapolated sintering rates
450 (Figure 2) showed that powders such as Aerogel #15 (LaAlllO18) should retain a surface area greater than 10 m2/g for exposure periods of more than 8000 hours at 1250~ in a combustion environment. Reference
Powders
As with the aerogel powders, the loss of surface area in the reference powders was caused by phase changes and by sintering and densification (see Figures 3 and 4). Reference #1, initially T-A1203, goes through successive phase transitions during heat treatment in air and water vapor. At 1000~ the 7 phase transforms to ix, Ic, and 0. The later two phases were unstable with respect to alpha alumina which becomes the dominant phase at 1100~ and higher temperatures. Once (x-AI203 is formed, the surface area of the oxide powder drops quickly from sintering. The addition of La203 to the 7-A1203 powder has been shown to have a stabilizing effect, delaying the appearance of the 0t phase until 1200~ [8]. Although our La203-stabilized T-A1203 powder (Reference #3) had a lower initial surface area than the original 7-A1203 powder, its surface area only dropped from 38 to 27 m2/g at 1100~ whereas the T-A120 3 dropped from 91 to 9 m2/g under the same conditions. The stabilized powder forms a surface layer of LaA10 3 that prevents the panicles from sintering and leads to the formation of a stable 0-A1203 transition phase instead of (x-A1203. Finally, although no phase changes occurred in the MgO powder (Reference #2), major sintering and reduction in the surface area takes place which is perhaps caused by the formation of a hydroxide surface layer.
100
1250oc
4)
10
.
O.
.
.
.
.
.
.
.
I
1
.
.
.
.
.
.
.
I
.
.
10
.
.
. . . .
I
100
,
=
.
9
9 . , .
1000
Sintering Time, hrs.
Figure 2. Calculated and measured BET surface area versus sintering time: #, Aerogel #8 l.aAlllO18 measured; e, Aerogel #15 I.aAll]Ol8 measured; lines are calculated.
451 1 O0
o 9 9
E
0
9 o
<
10 A
9 O
e A
4~
o O
m
+ A
1
i
.
800
,
9
1000
i
9
1200
9
1400
, 1600
. 1800
Temperature, ~
Figure 3. BET surface area as a function of sintering treatments for 4 hrs. in air and 10 vol.% water vapor: +, Aerogel #6 LaA103; A, Aerogel #12 SrZrO3; e, Aerogel #21 SrZK)3; e , Reference #1 7-A1203; O, Reference #3 La203 Stabilized 7-A1203.
1000
c~ E
A 100
,=C r
r'l
El
0 n A 1
,
800
'
I
1000
i
I
1200
,
I
1400
a
!
1600
1800
Temperature, *C
Figure 4. BET surface area as a function of sintering treatments for 4 hrs. in air and 10 vol.% water vapor: #, Aerogel #4 MgA1204; A, Aerogel #9 MgA1204; O, Aerogel #22 MgA1204; E], Reference #2 MgO.
452 CONCLUSIONS Several conclusions are drawn from this work about the preparation of high surface area sintering resistant complex oxides: 9 Our binary oxide aerogels regardless of approach to preparation consisted of very highly dispersed amorphous aluminate and zirconate, gels with larger separated phases of counter-ion oxides and hydroxides after calcination. 9 Calcination and sintering of the heterogeneous gels caused the growth of initial oxide and nucleation and growth of less sintering resistant intermediate complex oxides e.g.,
LaA103, BaA1204. ~ Phase transformation of alumina (T-, ~r and 0- to a-alumina) or formation of intermediate phases during the reaction of aluminate gels with stabilizing oxides provides the opportunity for the nucleation and growth of large grains and the collapse of the very highly porous gel structure. 9 Preparation of high surface area layered aluminates is ultimately limited by the natural contradiction between high cation mobility needed for the formation of the complex oxide and sintering rates. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.
M . F . M . Zwinkels, S. G. Jaras, and P. G. Menon, Catal. Rev. Sci. Eng., 35 [3] (1993) 319-358. J.S. Smith, R. T. Dolloff, and K. S. Mazdiyasni, J. Am. Ceram. Soc., 53 [2] (1970) 91-95. R.C. Ropp and B. Carroll, J. Am. Ceram. Soc., 63 [7-8] (1980) 416-419. A. Kato, H. Yamashita, H. Kawagoshi, and S. Matsuda, J. Am. Ceram. Soc., 70 [7] (1987) C-157-159. M. Machida, K. Eguchi, and H. Arai, J. Am. Ceram. Soc., 71 [12] (1970) 1142-1147. J.C. Debsikdar, J. Mater. Sci., 24 [10] (1989) 3565-3572. M. Machida, K. Eguchi, and H. Arai, Bull. Chem. Soc. Jpn., 61 (1988) 3659-3665. B. Beguin, E. Garbowski, and M. Primet, Appl. Catal., 75 (1991) 119-132.
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
453
E f f e c t of r e a c t a n t m i x i n g m o d e on s i l i c a - a l u m i n a texture J.P. Reymond, G. Dessalces and F. Kolenda UMR CNRS-IFP n ~ 36, IFP-CEDI, B.P. 3, 69390 Vernaison, France
In order to control the porous texture of silica-alumina usable as catalyst matrices the influence of the reactant mixing mode on the solid characteristics has been studied. The preparation mode used includes the formation of a silica hydrogel (partial neutralisation of sodium silicate by sulfuric acid) followed by the alumina precipitation inside this gel. The silica-alumina texture depends on the silica hydrogel structure and the binding mode of alumina in/on the silica framework. The silica-alumina texture is modified by changing the gelation conditions of silica without changing the precipitation conditions of alumina. Two preparation processes are examined (same rate of sodium silicate neutralisation by sulfuric acid). In the batch process one reactant (acid or silicate) is added to the other (silicate or acid) maintained in a stirred reactor. In this case the silica-alumina texture is related to the silica gelation pH. In the semi-continuous process the two reactants are simultaneously introduced into the reactor. The silica-alumina texture is mainly related to the duration of the reactant addition. 1. INTRODUCTION: Due to theirs poor physical characteristics some catalytically active solids (non-cohesive particles for example) cannot be used in a catalytic reactor. They must be mixed to another material called the matrix. This latter material, which can have catalytic properties, allows the shaping of the active phase under the form of spheres, extrudates, pellets.., and brings the required physical and mechanical properties. As they present many advantages, silicaaluminas are widely used as matrices, in cracking catalysts for example. One of the more important characteristics of a matrix is its porous texture which must allows good reactant and product fluxes in the catalyst bulk. According to its use, it is necessary to give to a catalyst a tailor-made texture (pore size and size distribution, pore volume and surface). Obtention of silica-alumina matrices with controlled texture is the aim of the present work. In the case of precipitated silica-aluminas the texture depends on the structure of the silica when silica results from a sol-gel transition. The condensation of silicic acids leads to the formation of primary spherical particles (sol) which aggregate in defined conditions, forming the tridimensional network of the gel (1,2). In the gel structure each primary spherical particles of silica adheres to two or three other particles (1) and the pores are the cavities between the globules (3). The size of the globules determines the specific area, the pore volume and diameter of the gel. So, the silica texture can be controlled by mastering the dimensions of the globules and their packing (4) which depend on the conditions of preparation of silica sol and gel. A change in the porous texture of silica can be obtained by varying the operating parameters of the preparation process (5). Several ways have been described: control of the effects of sol-gel transition pH and temperature (6), aging of the gel (7); mixing of reactants (8); addition of organic polymers (9) or change of the intermicellar solvent (10); use of hydrothermal treatment of
454 the wet particles (11); etc... However, when texture modification of a material is due to a change in the preparation mode undesirable surface property modifications can occur, which can alter the catalytic properties of the solid. So, to avoid such alterations, we have studied the effects of the mixing mode of reactants on silica-alumina texture, all other operating parameters remaining constant. To control the silica-alumina texture it is necessary to control the texture of the silica hydrogel, but two steps of the preparation process can strongly modify its texture (12): precipitation of alumina and drying of the silica-alumina gel. The drying of a gel causes a shrinkage of the material and consequently a drastic decrease of pore diameter, volume and surface (13). However the textural characteristics of the xerogel remains related to those of the silica-hydrogel network. The drying mode and conditions must be carefuly choosen to minimize the drying effects (14,15). Alumina precipitation in the silica hydrogel results in: i. an isomorphic substitution of A104- tetrahedrons between Si04 tetrahedrons which does not induce textural change but creates surface acid sites. ii. formation of free alumina which can cover partially the silica gel network or precipitate in the cavities (macropores) of the gel network. Free alumina formation induces textural changes leading to an increase of pore volume and mean diameter and a decrease of pore surface (16). All operating parameters governing the alumina precipitation must be controlled to avoid variations in its effects on the hydrogel texture. 2. EXPERIMENTAL" 2.1. Silica-alumina preparation: hydrogels are prepared in an aqueous media, following a two steps process: a. preparation of a silica hydrogel (sol-gel transition) with partial neutralisation of a sodium silicate solution (Si02/Na20 = 3,44; 6 % wt silica) by a sulfuric acid solution (35 or 20 % by weight) at given pH (5.5 or 9.5) b. after silica gel aging (30 mn), addition of an aluminium sulfate solution (A12(S04) 3, 18 H20; 33 % wt sulfate) in the silica gel. The precipitation of alumina occurs by adding an ammonia solution (20 % wt NH 3) up to pH 6. Preparations, conducted at 40~ take place in a stirred glass vessel equipped with sensors (pH, temperature, torque motor measurement of the stirrer shaft) allowing the control of the different steps of the preparation. Reactants pretreated at the preparation temperature, are introduced in the reactor at defined mass flow rates by means of calibrated peristaltic pumps. Reactants can be introduced in the reactor, successively (batch process) or simultaneously (semi-continuous process). In the later case reactants are mixed in the feeding pipe. Silica-alumina hydrogels are then filtered under vacuum and washed several times to remove impurities (S042-, NH4 + and Na + ions). The suspension (6-8 % wt silica alumina) obtained by repulping the last filtration cake is dried in a spray-dryer (12,15) in well defined conditions. This drying mode gives solid spherical particles having reproductible physical characteristics, in particular water content, size distribution and porous texture
(15). 2.2. Silica-alumina characterization: Hydrogel texture is evaluated by thermoporometry, which is a calorimetric method (17) giving pore diameter distribution (4 nm to 150 nm), pore surface and volume of wet or dry materials.
455 Xerogel texture is evaluated by thermoporomotry and adsorption-desorption nitrogen isotherms (B.E.T. specific area and mesopore diameter distribution). 3 . RESULTS AND DISCUSSION: Modification of silica-alumina xerogel texture by changing the mixing mode of reactants which give the silica hydrogel implies that: a. textural effects are really obtained on silica hydrogels b. these textural effects still exist on silica-alumina xerogels after alumina precipitation and drying of the material. Thermoporometry measurements on wet materials prepared as described above, evidence the textural changes of silica hydrogels. Figure 1a presents the evolution of pore volume (cumulative volume) versus pore radius for silica hydrogels obtained from batch mixing (curve 1) and semi-continuous way (curve 2:210 s; curve 3; 600 s). Figure lb is relative to silica xerogels obtained by spray-drying of silica hydrogels (15). Similarly, figures 2a and 2b are devoted to the silica-alumina hydrogels and their corresponding xerogels. During the experiments reported here, the gelation time is 250 s and the acid solution contains 35 % wt H2S04.
600
600
400
4oo g
200
r 200 i
o
s
io R ('rim.)
0
2
' R (rim.)
c,
Figure 1" Effect of the mixing mode on the texture of silica hydrogels (a) and xerogels (b) batch: curves 1; semi-continuous: curves 2 (210 s) and curves 3 (600 s) The silica hydrogels prepared with semi-continous addition of reactants have pore diameter distribution larger than the one from batch preparation. When the duration of reactant addition increases, the pore mean diameter increases, the pore surface decreases and the pore volume remains almost constant. It appears also from figures la and lb that the textural variations observed on silica hydrogels still exist on silica xerogels. Though they were minimized, the textural modifications realised during the preparation of silica hydrogels are preserved during the drying.
456 Comparison of figures la and 2a shows also that the textural effects observed on silica hydrogels are found for silica-alumina hydrogels and then on silica-alumina xerogels (fig. 2b). As the main objective of this work is to control the texture of silica-alumina xerogels this last observation is essential: the texture of silica-alumina xerogels can be modified by changing the mixing mode of the reactants generating silica hydrogels.
1400-
1600
120~ 1200
'00~ 800
800
~~. 600 40O
400
20O 0
o
20
40 R (nm.)
60'
0
5
o
1o
i5
20
R (nm.)
Figure 2" Effect of the mixing mode on the texture of silica-alumina hydrogels (a) and xerogels (b) batch" curves 1" semi-continous: curves 2 (210 s) and curves 3 (600 s) Tables 1 and 2 give more details on the textural characteristics of silica and silicaalumina hydrogels (table 1), and of the corresponding xerogels (table 2). The main effect of the semi-continuous addition of the reactants is an increase of the mean pore radius of the silica-alumina xerogels when duration of the reactant addition increases. Table 1 Textural characteristics of silica and silica alumina hydrogels for each mixing mode (gelation time: 150s) Mixing mode Batch
S i02 Si02- A1203
220s
Si02 Si02 - A1203
600 s
Si02 Si02 - A1203
pore volume (mm3/g) 516
mean pore radius (nm) 2,9
1380
5,5
2,4- 14
567
3,8
2,7 - 5,2
1443
8
4,4 - 47,4
525
5,4
3,5- 13,7
1230
10
5,5 - 68,2
pore radius range (rim) 2,3 - 3,7
457 Table 2 Textural characteristics of silica and silica-alumina xerogels Mixing mode Batch
Si02 Si02- a1203
220 s 600 s
pore volume (mm3/g) 176
mean pore radius (nm) 2,3
pore radius range (nm) 1,8 -2,6
425
2,9
1,9- 7,5
Si02
287
2,8
2,1 - 4,3
Si02 - A1203
625
4,1
2,2 - 9,3
555
4
2,2- 7,3
904
5,3
2,9- 33,5
Si02 Si02 - A1203
From the results presented above, it can be assumed that the textural changes observed are for the main part, caused by silica network modifications (alumina precipitation is assumed to lead to the same effects in each case). Figures la,b and 2a,b higlight the textural effects of the drying step (constriction of the gel network) and of alumina precipitation (formarion of larges pores as demonstrate by curves 1). As the gelation pH of silica has a drastic effect on the size of the elementary particles of silica (1) its influence has been evaluated for different addition durations. In the case of the semi-continuous process the gelation pH is easily modified by changing the ratio of the reactant fluxes. For the batch process the order of reactant introduction in the reactor must be changed to have an acid pH (silicate poured in acid) or a base pH (acid poured in silicate). As complex modifications can arise in the reaction mechanisms when the order of reactant addition is changed (18) this latter case has not been studied in this work. Two different pH, corresponding to the same gelation time (60 s), has been selected: 5.5 and 9.5. In order to control as well as possible the acid flux, its concentration has been fixed to 20 % wt H2S04. Table 3 presents the textural characteristics of the silica-alumina hydrogels as evaluated from thermoporometry measurements. Table 3 Textural characteristics of silica-alumina hydrogels (thermoporometry) gelation pH
9,5
5,5
addition duration (s) 30 (batch) 30 60 600 1200 1800 30 300 600 1200 1800
pore volume (nm3/h) 1965 2247 2486 2405 2089 2309 2371 1975 1302 1521 1738
mean pore radius (nm) 5,7 8,7 20,8 20,1 16,5 19,4 12,3 25,4 16 19,8 13,2
pore radius pore range (nm) surface (m2/g) 2,3 - 11,5 814 2,8 - 21,4 583 3,4- 50,4 327 3,2- 50,6 349 3,2 - 51,9 328 3,7 - 61 318 3,2 - 25,3 494 4,7 - 62,4 202 3,1 - 32,3 226 3,5 - 43,6 230 2,9 - 33,9 356
458 Table 4 is devoted to xerogel characteristics. The semi-continuous mixing of silica reactants favors the formation of large pores for addition durations below 600 s. Beyond this value the effects are weak. Materials prepared at pH value 5.5 have smaller pore volume and surface than those prepared at pH values 9.5. For the two pH values the mean pore radii are nearly similar. The characteristics of silica-alumina xerogels evaluated from nitrogen adsorptiondesorption isotherms (B.E.T. - surface area; B.J.H. calculations for pore radius and volume evaluations) are presented in table 5. The effects evidenced in this table are almost similar to those detected by thermoporometry: opening of the texture for simultaneous introduction of silica reactants. This effect exists for addition durations no longer than 600 s. Table 4 Textural characteristics of silica-alumina xerogels (thermoporometry) gelation pH
9,5
5,5
addition duration (s) 30 (batch) 30 300 600 1200 1800 30 300 600 1200 1800
pore volume (mm3/h) 417 692 1133 1229 1284 1274 845 1153 996 1049 852
mean pore radius (nm) 3,2 4 7,2 11,1 9,2 10,9 4,6 7,4 13 7,5 5,2
pore radius (nm) 2,1-5,1 2,1- 8,1 2,7- 14,5 2,9- 17,1 2,8- 18,1 29-20,9 2,3 - 9,2 2,5 - 15,9 2,9 - 18,1 2,4- 13,9 2,2 - 11,1
pore surface (m2/g) 318 419 360 279 333 270 436 383 206 357 387
Table 5 Textural characteristics of silica-alumina xerogels from nitrogen adsorption-desorption isotherms silica gelation
reactant
S BET
pore
pore
pH
addition duration (s) 30(batch) 30 300 600 1200 1800 30 300 600 1200 1800
(m2/g)
volume (P/Po- 0,95) (mm3/g) 552 681 1008 845 985 970 889 900 936 903 858
radius (nm) 4,7 5,7 10,6 11,4 12,2 12,3 6,6 9,6 12,1 9,6 6,8
9,5
5,5
448 487 479 461 462 411 597 599 565 539 612
459 The isotherms are of type IV with hybrid hysteresis loops (by reference to de Boer's classification (19). They would be attributed to mesopores with heterogeneous pore diameter distributions; these pores having narrow necks of varying radii. Cavities between spheres of different radii could explain such a pore network). Silica hydrogels prepared in acid conditions conduct to silica-alumina xerogels having pores smaller than those obtain in basic conditions. The batch mixing of silicate and acid leads to the formation of silica hydrogels whose pores have a narrow size distribution (from 2.3 to 3.7 nm). Excepted local over-concentrations du to the introduction of one reactant into the other one, the silica sol obtained, using this mixing mode, is homogeneous. The gelation occurs at the same time in all the reactor. The gel is built by the aggregation of primary silica particles having almost the same diameter and the same lifetime. Such conditions favor the formation of a gel network having regular meshes and characterized by a narrow pore diameter distribution. Particle and network dimensions are governed by the pH value of gelation (1). The simultaneous introduction of reactants avoid the local over-concentrations of one reactant; the pH value of the rectant mixture is constant and is the same in the reactor and the introduction pipe. If the duration of the reactant pouring is longer than the gelation time the elementary silica particles wich compose the sol does not aggregate each other at the same time. The content of the reactor is a gelatineous substance composed by pieces of gel at every step of its reticulation immersed in the sol. But, as pH value, salt concentration and temperature are constant, the elementary particles of silica would have roughly the same diameter (1). The building of the gel network is continuous and is spread over a long time. This would lead to a silica gel framework with meshes of irregular dimensions characterized by a large pore size distribution. However the textural effects reach a maximum for addition duration value near 10 nan (tables 3,4 and 5). In the batch process there is formation of a single gel network following a fast percolation like mechanism which concerns the whole reactor content. The stirring breaks the gel in pieces keeping the original gel framework. In the semi-continuous process the gel network building is slow. It would result from the disordered agglomeration of silica particles on numerous gel embryos dispersed in the reactor. 4. CONCLUSIONS: The preparation of silica-alumina xerogels of controlled porous texture has been studied. The process includes the formation of a silica hydrogel from a sol-gel transition, followed by incorporation of alumina in the silica gel network. The structure of the silica gel network can be modify by changing only the mixing mode of reactants, all others operating parameters keeping constant. The modifications operated on silica still exist in silicaalumina xerogels. The process used combines semi-continuous addition and mixing of reactants generating silica. As compared to conventional batch process, it favors, in a limited way, the formation of large pores. Differences in the building of the gel network would explain the observed texture modifications.
BIBLIOGRAPHY: 1.
R.K. Iler in "The chemistry of Silica" (John Wiley and Son, New-York 1979)
2.
L.L. Hench and J.K. West, Chem. Rev. 90 (1990), 33-72
3.
C.L. Planck and L.C. Drake, J. Colloid Sci. 2 (1947), 399
4.
S.A. Mitchell, Chem. Ind. (1966), 924-933
460 5. A.G. Foster and J.M. Thorp in "The structure and properties of porous materials" (D.H. Everett and F.S. Stone Eds., Butterworths, London 1958), 227-235 6.
W. Vogelsberger, D. Schutze and G. Rudakoff, Silikattechnik 38, (1987), 3-5
7.
V.M. Chertov and V.V. Tsyrina, Koll. Zh. 4"/(1985), 922-926
8.
M.E. Winyall, Applied Industrial Catalysis 3 (1984), 43-61
9. D. Basmadjian, G.N. Fulford, B.I. Parsons and D.S. Montgomery, J. Catal., 1 (1962), 547-563 10. O.P. Stas', R. Yu Sheifain and I.E. Neimark, Koll. Zh. 29 (1967), 256-259 11. C.J.G. van der Grift, J.W. Geus, H. Barten, R.G.I. Leferink, J.C. van Miltenburg and A.T. den Ouden in "Characterization of Porous Solids" (K.K. Unger et al Eds., Elsevier Science Publisher, Amsterdam 1988), 619-628 12. I. Biay, G. Dessalces, C. Hypolite, F. Kolenda and J.P. Reymond in "Preparation of Catalysts V" (G. Poncelet et al. Eds., Elsevier Science Publishers, Amsterdam 1991), 1-17 13. G.W. Scherer, J. Amer. Ceram. Soc. 73 (1990), 3 14. C.J. Brinker and G.W. Scherer in "Sol-gel-Science" (Academic Press Inc.,Boston1990), pp 453-513 15. J. Andrieu, G. Dessalces, C. Joly-Vuillemin, J.P. Reymond and F. Kolenda in "Drying 92" (A.S. Mujumdar ed., Elsevier Science Publishers, Amsterdam 1992), pp 533-542 16. G. Dessalces, I. Biay, F. Kolenda, J.F. Quinson and J.P. Reymond, J. Non-Crystal Solids 147-148 (1992), 141-145 17. M. Brun, A. Lallemand, J.F. Quinson and Ch. Eyraud, Thermochimica Acta 2__! (1977), 59-88 18. J.R. Bourne, Rev. Inst. Fr. Petr. 48 (1993), 615-630 19. J.H. de Boer in "The Structure and Properties of Porous Materials" (D.H. Everett and F.S. Stone Eds, Butterworth London 1958) p 68
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
Synthesis, characterization catalysts and supports
and p e r f o r m a n c e
461
o f sol-gel p r e p a r e d
TiO2-SiO 2
S. Bernal, J.J. Calvino, M.A. Cauqui, J.M. R o d r i g u e z - I z q u i e r d o and H. Vidal Departamento de Ciencia de los Materiales e Ingenieria MetalOrgica y Quimica Inorgfinica. Universidad de Cfi.diz, Apdo.40, 11510 Puerto Real, SPAIN. SUMMARY
Tetraethoxysilane and tetrabutylorthotitanate were used as precursors to obtain a series of TiO2-SiO2 gels. The use of ultrasound, and the type of drying treatment followed to obtain catalytic materials, play a significant role in the dispersion of Ti atoms. The structural properties of these gels, studied by XRD, XANES and FTIR spectroscopy can be related to their acidity. Thus, the trends of activities for tert-butanol decomposition to isobutene are in ht connection both with the T i O 2 c o n t e n t and with the observed levels of Ti dispersions. and Pt supported catalysts, prepared from several TiO2-SiO 2 gels, present higher activities and selectivities for n-butane hydrogenolysis than other preparations used as references. I. INTRODUCFION
Sol-Gel methods offer interesting routes to prepare catalysts and supports [1,2]. An advantage of these soft chemical procedures is the possibility of obtention of solid materials with high surface areas and controlled pore volumes and pore size distributions. Other relevant feature of sol-gel synthesis is the feasibility of mixing different molecular precursors to obtain mixed compounds, such as mixed oxides. The proper choice of such precursors and the adequate selection of the gelling conditions can lead to the obtention of high purity homogeneous materials. The processing steps from just formed wet gels to dried solids are of crucial importance. Transformations taking place during drying can induce textural and chemical rearrangements, resulting in the loss of some potential virtues of the products [3-5]. In preparations in which the emphasis is stressed on good development of the textural properties it is advisable to follow supercritical extraction methods in an autoclave for the removal of the residual solvent. In such way, we avoid the capillary stress due to differential strain between the pore liquid and the gel network, which results in a collapse of the porosity [5-7]. The materials obtained in this way are called aerogels. The supercritical fluid can be the solvent used for mixing the molecular precursors (methanol, ethanol, etc.), or a different one, such as CO2. Supercritical extraction with CO2 presents the advantage of requiring lower operation temperatures (Tc = 304 K) [8,9], thus preventing the possibility of some undesirable reactions. Alternatively, gels can be dried in air, thus leading to xerogels. In this case the treatment requires longer times and results in a poorer textural development by the shrinkage effect of capillary forces [10]. In this work we prepare TiO2-SiO2 gels starting from metallo organic precursors. The TiO2 contents ranged from 0 to 10 mole % . The use of ultrasound to promote gelling, and the effect of different drying treatments on the structure and texture of the products are the
462 objects of our attention. TiO2-SiO 2 materials are interesting from the point of view of their use as catalysts and supports [11-13]. Thus, the dispersion of titania on, or in, silica generates Lewis acid sites [14] and opens the possibility to modulate the redox properties of the Ti atoms. In this work we check the behavior of these samples for an acid catalyzed reaction: the dehydration of tertbutanol [15], looking for correlations between structure and activity. Sofne recent publications focus the attention on the performance of titania modified metal/silica catalysts in an attempt to get a better understanding of the metal-promoter-support interactions [11,16]. We include some data on the preparation of Rh and Pt dispersed on TiO2SiO2 reporting information on the catalytic behavior for the n-butane hydrogenolysis reaction.
2. EXPERIMENTAL 2.1. Preparation of samples The classic procedure for the sol-gel synthesis of high purity glasses and ceramics starts from a mixture of alkoxides and water in a common solvent, usually an alcohol. Tarasevich [ 17] proposed an alternative route to obtain silica gels, in the absence of a solvent, by means of ultrasonic activation. This procedure has been extended to the preparation of multicomponent systems [18]. Materials obtained in this unconventional way are called sonogels. Tetraethoxysilane (TEOS) and tetrabutylorthotitanate (TBOT) were used as precursors for the preparation of gels. Because TBOT is very reactive with water, it was modified by addition of acetic acid (HAt) as described elsewhere [19]. The selected HAc/TBOT molar ratio was 5.5. The H20/TEOS ratio was 4 for all the prepared gels. Hydrochloric acid, at pH=l.5, was present as a catalyst in the hydrolysis water. Sonogels (S) with 0, 1, 5 and 10 mole % of TiO2, were prepared as follows: 40 mm diameter glass beakers containing 20 ml TEOS-H20 mixtures were exposed to the action of ultrasound produced by a sonifier (Kontec,Eurocomercial) operating at 20 kHz. The tip of the sonifier was introduced into the reactive mixture. After 10 min a foaming effect was observed, the liquid turned homogeneous, and ethanol vapor evolution could be detected. At this moment, irradiation was stopped, the beaker was cooled to 273 K, and appropriate amounts of the BuOH-HAc-TBOT solution were added under vigorous stirring. The liquids were transferred to hermetic glass containers and stored in an oven at 323 K. Gelling times decreased for increasing TiO2 loadings, ranging from 20 to 90 min. Classic gels (C) were obtained mixing the TEOS-H20 mixtures with the solution containing the modified TBOT precursor, in the presence of 50% vol. ethanol, at 298 K. Gelling took place in hermetic glass containers, at 323 K, after periods of time ranging between 1 and 5 days. The sono and classic gels were aged for one week at 328 K before drying. Drying was carried out by three different methods: a) In a 0.5 1 autoclave, with addition of the required amount of ethanol to reach a pressure of 190 bars at 600 K, to obtain aerogels (A), b) By exchange in two steps with isoamylacetate for 15 min and further supercritical drying with CO2 at 315 K and 75 bars, to obtain carbogels (C), and c) In air at 300 K for 10 days to obtain xerogels (X). As examples of the nomenclature followed in this work, AS 10 represents an aerogel obtained from a sonogel with 10 mole % TiO2, and CC5 would represent a carbogel derived from a classic gel with 5 mole % TiO2. The preparation of a TiO2-SiO2 reference sample (TS) obtained by grafting a
463 commercial silica support with a TBOT solution in hexane has been described elsewhere [20]. Some of the gels were used as supports to obtain dispersed metal catalysts (Rh and Pt). Thus, 2.5wt% Rh catalysts have been prepared by impregnation of the AS and AC series with aqueous solution of rhodium nitrate. A Rh/TS reference catalyst has been prepared in a similar way. Two 0.9wt% Pt catalysts were prepared by impregnation with hexachloroplatinic acid solutions of the XS 10 and XC 10 gels.
2.2. Characterization Techniques and Catalytic Test Reactions Fourier transform infrared spectra were obtained from pressed pellets of each sample diluted with KBr, in a Mattson 5000 equipment, operating at 4 cm ~ resolution The X-ray diffraction diagrams (XRD) have been recorded in a Siemens D-500 instrument with Cu Kcx radiation The N 2 and H 2 adsorption experiments were carried out in a conventional glass vacuum system equipped with a MKS-Baratron BHS 1000 pressure transducer Titanium Kedge X-ray absorption spectra (XANES) were recorded at LURE (Orsay, France) The operating conditions in the storage ring DCI were the following: energy of 185 GeV and intensities of about 230 m A Dehydration of tert-butanol has been run in a microcatalytic flow reactor working at atmospheric pressure, with an Ar/t-BuOH ratio of 9 The samples were previously calcined in a flow of Ar for one hour at 773 K The reaction temperature was 373 K, and the alcohol conversions into isobutene were always lower than 10% The n-butane hydrogenolysis tests were carried out at atmospheric pressure, with an H2/n-butane ratio of 9 The catalysts were previously reduced in a flow of hydrogen for 1 hour The reaction was run between 400 and 475 K, reaching conversions lower than 10% 3. RESULTS
3.1. Textural Characterization B.E.T. specific surface areas of the dried T i O 2 - S i O 2 gels are always higher than 450 m2-g~. If we compare the sonogels with the classic gels, the effect of the different drying treatments (Aerogels, Carbogels, Xerogels), and the influence of the TiO 2 content, some general trends appear. Thus, the B.E.T. surfaces for the sonogels are slightly higher than those of the classic gels. As a few examples of such behaviour the AS 10, CS 10 and XS 10 samples (870, 570 and 499 m2-g1) show specific surfaces of about 50 m2.g"~ in excess to their classic counterparts AC10, CC10 and XC10 (832, 515 and 459 m2-g~). In the above data we can check that the method of drying lead to a sequence in B.E.T. surface areas as follows: Aerogel>Carbogel>Xerogel. Concerning the effect of the TiO2 we just observe a slight increase in surface areas followed by a decrease for the 10% samples. The study of the De'Boers t plots reveals, as expected, that the drying method plays an important role in the type of porosity exhibited by the samples, Figure 1. The aerogels, A, did not exhibit micropores for any of the preparations and the St surfaces derived from the linear part of the t curves are in an acceptable agreement with the B.E.T. specific surface areas. On the contrary, it is evident that the xerogels show a very significant contribution of micropores and the St surface areas represent only a small fraction of the B.E.T. areas. Carbogels have intermediate behavior, both in microporosity and in the St parameters. It is necessary to remark that subtle differences in the preparation conditions (concentration of the molecular precursors, amount and pH of the hydrolysis water, mixing and gelling temperature or dose of ultrasound) induce significant variations in the textural properties of the products. Whereas, the trends commented on above remain unchanged, and
464
,
|
1
600 13. I.z Z"
I
1
750
A 400
500
200
250
-
E (3
"~"
..
"10 0
.. 1
|
I
5
10
15
0
s
0 ....
.
ca
z "E
l
10
.
|
,
l
15 !
200
XClO
13_ z
l
5
......
....---
..
CClO
300
150 .......
200
1 O0
U
o;
100
50
""
"10 0
>,
0
0
I
l
I
5
10
15
t (nrn)
0
0
i
I
I
5
10
15
t(nrn)
Figure 1.- De'Boers t plots of the referred dried gels calcined at 773 K.
the B.E.T. surface areas of the samples are always higher than 400 m2-g"~. An interesting point to consider for catalytic applications is the textural stability of the TiO2-SiO2 gels. To check this aspect we studied the effect of thermal treatments under a flow of hydrogen at several temperatures ranging between 623 K and 1173 K. The samples were treated for 3 h. at each of the selected temperatures. Our data show that up to 773 K the B.E.T. areas remain unchanged, and in the range 773 K- 1073K there is a moderate loss of about 20% of the starting values, Figure 2. The aerogels, A, are easily obtained as monoliths. This allows to estimate their apparent densities by means of mercury picnometry. The values
for the AS series are all between 0.5 and 0.65 g.cm 3, and for the AC series between 0.30 and 0.35 g-cm 3. Such values are high if compared with other aerogel preparations reported in the literature 0.1-0.25 [ l 0]. 1ooo It can be an advantage for catalytic purposes because of the better mechanical properties of our gels, thus ,~ avoiding problems of handling materials with very ~'c~ g o 0 poor attrition resistance. ~E
I
I
I
0 -
I
$5 9 ClO
-
800
3.2. Structural
Characterization
O3
FTIR spectroscopy, XRD and XANES 70O allowed a structural characterization of the TiO2-SiO2 gels. Several authors report on the use of the 950 I I I I cm -] IR absorption bands to discuss the formation of 623 773 923 1073 Si-O-Ti bridges in TiO2-SiO 2 samples [3,21,22]. (K) Some of them remark that Si-OH groups show an absorption band at the same wavenumber [21]. Figure 2.- Effect of the calcination Thermal treatment of the samples in air at 773 K for temperature in a flow of hydrogen on 2h eliminates the absorption bands in the 2500-4000 the B.E.T. surface areas of some of the cm ~ region, thus showing that the samples are free studied TiO2-SiO2 gels. of hydroxyls as well as from residual organic contaminants. Figure 3 presents the spectra of several TiO2-SiO2 gels after calcination at 773 K.
465
o v v
o
o
c o .13 L. 0
to ..o t... 0
.0
<
rt .<
1300
1100
900
700
500
1300
1100
900
-1
700
500
-1
cm
cm
Figure 3.- FTIR spectra of 10% mole TiO2-SiO 2 gels calcined at 773 K. Although the 950 cm ~ bands are rather broad and overlap with other silica bands at higher wavenumber, after the referred calcination treatment they can be considered as a fingerprint for the formation of Si-O-Ti bonds [22]. It is apparent from Figure 3 that the XC10 sample presents the highest absorbance and the AS10 the lowest one. The intensity decreases from the xerogels to the aerogels and the use of ultrasound in our preparations does not seem to be helpful to get better dispersion of Ti atoms into the silica network.
J
~
ZI 0 cO .! .i.a t~
ta)
4-a
0 r~
XC10 15
20
25
30 35
4.0
(20)
Figure 4.- XRD patterns of gels calcined at 773K. AS10 ~: sono aerogel calcined at 1173 K. CC10*" non calcined classic carbogel.
4960
5000
eV
Figure 5.- Ti K-edge XANES spectra of the gels calcined at 773 K.
466 XRD data are in line with the FTIR results commented above, Figure 4. The diffraction peak at 219=25.3 ~ is characteristic of anatase crystals. This peak is an indication of phase segregation and should grow in an opposite way to the presence of Si-O-Ti bonds. It is necessary to remark that the calcination treatment in air at 773 K does not modify the XRD diagrams except for the carbogels. For the series of classic gels there is no evidence of anatase crystallization for the xerogel, and on the opposite a neat diffraction peak appears for the aerogel. As the starting wet gel is the same, these results allow to conclude that the structural differences among XC10, CC10 and AC10 are due to the different drying treatments. The comparison between the series of sonogels and the classic gels reveals that the degree of dispersion of Ti atoms is better for the classic series. The XANES results confirm the same trends of behavior observed by FTIR spectroscopy and DRX, Figure 5. For the classic series, the XC10 sample shows Ti K-edge features characteristic of Ti atoms in a non-centrosymmetric environment. Similar spectra have been interpreted in the literature as indicative of Ti atoms with tetrahedral coordination [2325]. On the opposite, the spectrum of the AC10 sample resembles that of anatase, although the contribution of a small fraction of Ti atoms in tetrahedral or amorphous environment cannot be ruled out. The sono samples present absorption features that approach to Ti anataselike coordination more than the corresponding classic gels. 3.3. Dehydration of tertobutanoi TiO2-SiO: gels calcined at 773 K were tested in the catalytic dehydration of tertbutanol to isobutene. After calcination such reaction must proceed on Lewis acid sites associated to the existence of Si-O-Ti bridges [ 15]. Pure silica and titania reference samples show activities several orders of magnitude lower than TiO2-SiO2 samples. The 10 mole % TiO2-SiO2"gel catalysts show higher activities than the titania on silica sample, TS, prepared by grafting a silica support with a metallo organic titanium precursor (TBOT), Table 1. If we compare gel samples with different drying treatments we can conclude that the higher activities are obtained for the xerogels, and the lower for the aerogels, Table 1. This agrees with the structural results who show that the degree of Si-O-Ti linking varies according to the following sequence: X>C>A. With regards to the comparison of sono and classic gels, the latter behave as more active catalysts. These results are also in line with the reported structural characterization data supporting the hypothesis that the Ti atoms are better dispersed for the classic gels. Finally, it is also clear that in the range of 0-10 mole % TiO2 the activities for alcohol dehydration increase with the loading.
Table 1 Textural data of studied samples and catalytic properties for the dehydration of tert-butanol Samples
St (m 2. g-l)
Vm (cm 3. g-l)
XC10
77
CC10
Catalytic Activity (xl0 2) mmol/g.s
Bmol/SBET- s
pmol/St-s
0.17
23
50
298
290
0.12
21
41.3
74
AC10
700
0
11
13.8
16.5
TS
260
0.15
3.0
11.2
12.4
467 3.4. TiOz-SiO2 Aerogeis as suppom of metal catalysts The AS and AC series of gels with different TiO: loadings were tested as supports to disperse rhodium catalysts. A method used for the incorporation of the metal was the conventional incipient wetness impregnation. This preparation technique can lead to a significant~loss of surface area because drying of the impregnating solution is not carried out under hypercritical conditions; the capillary stresses generated during drying can produce a collapse of the porous structure of the gel [6,7]. Whereas, in this case the loss of surface area was moderate, probably because the relatively high density and mechanical resistance of the aerogels. Thus, B.E.T. surface areas after impregnation, drying and reduction of the catalysts are all in the range 500-675 m:.g ~. Hydrogen chemisorption data for Rh catalysts reduced at 623 K lead to H/Rh apparent ratios between 1.10 and 1.45 and after increasing the reduction temperature up to 773 K they never decreased by more than 15%. This suggests that following this preparation method we obtain high metallic dispersions. TEM studies confirm this conclusion. Figure 6 shows the activity results for the sono and classic series after reduction treatments at 623 K. It is clear that the addition of titania to the gels promotes an increase of the activities. The sonogel-supported catalysts are more active than the classics. The increase in the reduction temperature induces a depletion in the rate of hydrogenolysis, but such inhibition effect is lower in magnitude to that observed for a Rh/TS reference catalyst. Rh/gel catalysts showed selectivities toward ethane higher than 80%. We also tried to disperse the metal by adding a rhodium precursor (nitrate or acetylacetonate) to the hydrolysis water before the gelling process takes place. In such preparations we did not succeed in getting catalysts with acceptable dispersions, as checked by TEM, 1.2 hydrogen chemisorption and catalytic 0 AS activity measurements. Whereas, 9 AC further work in this field can be of n,,. interest because, in agreement with T ol 0 . 8 the observations of other authors m [26,27], the metal is reduced in the m o autoclave drying treatment. The tight E v metal-support contact would eventually induce singular interaction N o.4 I-phenomena. (J .( Pt catalysts dispersed on XS 10 and XC10 gels present similar o.o I I I I dispersions to EUROPt- 1 0 1 5 10 (6.3%Pt/SiO:), as shown by H2 chemisorption and TEM. Whereas, TiO2 their activities are about 12 times higher and the selectivities toward the Figure 6.- Influence of the TiO 2 loading on the nbreakdown of the C-C terminal bonds butane hydrogenolysis activities of Rh catalysts of n-butane are also improved with reduced at 623 K. respect to the EUROPt-1, Table 2. P I f= =C
468 Table 2 Hydrogenolysis of n-butane over Pt catalysts reduced at 623 K SAMPLE
ACTIVITY ~
EUROPt-1
117.3
Pt~S.10
1530
SMct(%)
SEt(%)
20
27
26
27
0
40
20
40
Sxso(%)
Sprop(%)
Pt/XC10 1300 0 41 19 40 * Activity in mmoles n-butane converted 9glpt- hl; Reaction Temperature: 573 K. 4. DISCUSSION If we pay attention to the structural characterization results of the XC10 sample it is clear that in such case the dispersion of the Ti atoms in the silica matrix is fairly good. The CC10 and AC10 samples come from the same wet gel as the XC10. This suggests that following the classic route we succeeded in the aim of obtaining an intimate mixing and linking of the molecular precursors in the sol-gel synthesis. The segregation of crystalline titania in the dried CC and AC samples must be related to transformations taking place during the different steps of the drying processes. For the case of the sonogels the dispersion of Ti atoms was always lower than for the classics. Nevertheless, in this point we must shed light on the potential advantages of ultrasound. Thus, the gelling times are shortened by a factor of 100. The sonogel route allows to diminish the content of organic residues in the wet gel. The possibility of avoiding the use of a solvent lead to gels with much higher apparent densities. Of course, the solvent retained by the wet gel prepared by the classic method generates large pores during drying leading to products with lower densities and poorer mechanical resistance. These aims are achieved without a loss of surface area. On the contrary the areas of the sonogels are higher than those obtained for the classic gels. As we have fine tunned the synthesis conditions to obtain good Ti dispersion throughout the classic route it is reasonable to loose dispersion when we activate in a selective way the hydrolysis of TEeS by ultrasound. Nevertheless it would be also possible to look for new synthesis conditions in which the ultrasound can accelerate the gelling process, lead to high surface area materials, and allow the obtention of mixed oxides with better levels of homogeneity. The control of the dose of ultrasound and alternative modifications of the TBOT precursor can help to get such objective. As pointed out above one aspect who deserves further comment is the effect of the drying treatment on the structural and textural properties of the gels. Some recent works discuss the effect of the autoclave drying treatment on the structure of silica gels [4,5,7]. Their conclusions disagree with the proposals of other authors who assume that the supercritical drying in an alcohol atmosphere leaves the pores intact [28]. Thus, the dissolution of silica in alcohol under the effect of pressure and temperature, followed by redeposition on the narrower pores, can modify the structure and the textural properties of the material. This mechanism resembles the Ostwald ripening phenomena taking place in precipitates, in which the driving force is the tendency to decrease the surface free energy. In the particular case of TiO:-SiO2 gels, another problem to consider is the reactivity of Si-O-Ti bonds toward water. TiO2-SiO2 homogeneous materials become inhomogeneous after a few hours in contact with a water vapor pressure of 0.9 bar at 380 K [21 ]. After such
469 tests, TEM allowed to detect anatase particles of 10-30 nm in diameter. If we assume that in the autoclave treatment at 600 K there is as low as 0.2 ml of residual water it can lead to similar water vapor pressures and the above referred segregation of anatase can occur. If the two commented effects take place during the drying process to obtain our aerogels we can understand why the observed dispersion of Ti is far from that of the xerogels. The autoclave conditions would allow a selective leaching and mobilization of titanium species that would preferentially redeposit into the narrower pores. This allows to understand why AC10 and AS10 do not show microporosity at all, Figure 1. The catalytic activity for tert-butanol decomposition, and the fact that the intensity of the anatase XRD peak can grow by thermal treatment at very high temperature (Figure 4, AS 10#), suggests that a small fraction of the Ti atoms in the AS10 sample remain in a dispersed state. Concerning the carbogels, their structural and textural behavior is intermediate between aerogels and xerogels. The exchange of ethanol by another solvent miscible with CO2 can be a crucial step to get a proper low temperature supercritical drying. In this sense, the poor permeability of the alcogel can explain the loss of a significant fraction of the initial porosity. In [3], the authors propose that solvent exchange operations before drying induce a depletion of the Ti content in TiO2-SiO2 carbogels. EDX analysis suggests that in our case there would be no loss of titanium in the carbogels, but the exchange solvent can probably produce a certain degree of dissolution and segregation of Ti to form an incipient anatase phase, growing in crystallinity with the calcination treatment at 773 K. We will focus the discussion of the results for the tert-butanol decomposition reaction on the comparison of XC 10, CC 10 and AC 10 samples. Table 1 shows the activity results referred to gram of catalyst, B.E.T. surface area and t surface area. The sequence of activities does not change with the reference parameter, following the trend XC10>CC10>AC10>TS. Whereas, the differences observed when St is the reference are in the best agreement with the relative dispersion of Ti atoms into the silica matrix suggested by the structural results (XRD, XANES, FTIR). Thus, large differences are observed among the three samples becaming damped for other references (g or SB~.T). This can be understood if we assume that the level of dispersion of Ti is related to the number of Lewis acid sites exhibited by the gels, and that the catalytic reaction is severely restrained by diffusion into the micropores. Under these assumptions the St referred values can give a good indication of the degree of Si-O-Ti linking. The lower activities of the sono series when compared to the classic one and the increase in activities with TiO2 loading are also in line with the above hypothesis and with the structural characterization results. Regarding the use of TiO2-SiO2 gels as supports of metal catalysts the textural properties of the aerogels are particularly favorable. The lack of microporosity joints to the very high values of specific surface areas and, as pointed out in the results, their porous structure can resist a conventional impregnation treatment without severe changes in texture. Incorporation of TiO2 to the gels promotes the activities of Rh supported catalysts in n-butane tiydrogenolysis, Figure 6. Apparently the presence of TiO2 produce a favorable modification of the surface properties of the supports for this reaction. This result agrees with the conclusions of Ebitani et al.[12] for Pt/TiO2-SiO2 catalysts. The promotion effect is more important for the AS series than for the AC. If we remember that the segregation of anatase is more significant for the AS series we can suggest that both anatase precipitates and dispersed Ti atoms are responsible of this effect, with anatase probably playing a major role. The XC10 and XSl0 gels have been used as supports for Pt catalysts. The high activities and the modification of selectivity with respect to the EUROPt-1 catalyst must also
470 be related to the favorable surface chemical properties of the support. If we consider that these XC 10 and XS 10 samples present microporosity, a significant fraction of the deposited Pt must be present into the micropores. Then, the high activity values observed are even more remarkable. In summary, sol-gel preparations of TiO2-SiO2 allow to obtain materials with a broad range of structural properties, which lead to different surface chemical properties. Such gels can also be useful as supports of dispersed metals. At the present stage of this work, we can report that the Pt, Rh/TiO2-SiO2 gel catalysts present quite interesting behavior both from the perspectives of activity and selectivity. ACKNOWLEGMENTS: We thank the DGICYT, Project PB90-0671, for financial support. REFERENCES 1. G.M.Pajonk, Appl.Catal., 72 (1991) 217 2. M.A.Cauqui and J.M.Rodriguez-Izquierdo, J.Non-Cryst.Solids, 147&148 (1992) 724 3. M.Begui, P.Chiurlo, L.Costa, M.Palladino, M.Pirini, J.Non-Cryst.Solids, 145 (1992) 175 4. P.Wang, A.Emmerling, W.Tappert, O.Spormann, J.Fricke and H.Haubold, J.Appl.Cryst., 24 (1991) 777 5. T.Woignier, J.Phalippou, J.F.Quinson, M.Pauthe and F.Laveissiere, J.Non-Cryst.Solids, 145 (1992) 25 6. H.D.Gesser and P.C.Goswami, Chem.Rev., 8_29(1989) 765 7. J.Fricke and A.Emmerling, J.Am.Ceram.Soc., 75 [8] (1992) 2027 8. G.Dagan and M.Tomkiewicz, J.Phys.Chem., 9_7_7(1993) 12651 9. S.M.Maurer and E.I.Ko, Catal.Lett., 12 (1992) 231 10. C.J. Brinker and G.W.Sherrer, Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing, Academic Press, New York, 1990 11. J.S.Rieck and A.T.Bell, J.Catal., 99 (1986) 262 12. K.Ebitani, T.M.Salama and H.Ha~n, J.Catal., !34 (1992) 751 13. C.I.Odenbrand, S.L.Anderson, L.A.Anderson, J.G.Brandin and G.Busca, J.Catal., 12__~5 (1990) 541 14. H.H.Kung, Transition Metal Oxides, Elsevier, Amsterdam, 1989 15. F.Figueras, A.Nohl, L.de Mourgues and Y.Trambouze,Trans.Faraday Soc., 6_7_7(1971) 1147 16. G.C.Bond and L.Hui, J.Catal., 142 (1993) 512 17. M.Tarasevich, Am.Ceram.Soc.Bull., 6_33(1984) 500 lg. L.Esquivias, C.Fernb.ndez-Lorenzo and J.M.Rodriguez-Izquierdo, Riv.Staz.Sper.Vetro, 20 (1990) 262 19. D.Doeuff, M.Henry, C.Sknchez and J.Livage, J.Non-Cryst.Solids, 89 (1987) 206 20. M.A.Cauqui, J.J.Calvino, G.A.Cifredo, L.Esquivias and J.M.Rodriguez-Izquierdo, J. Non- Cryst.Solids, 147&148 (1992) 758 21. A.Matsuda, T.Kogure, Y.Matsuno, S.Katayama, T.Tsuno, N.Tohge and T.Minami, J.Am.Ceram.Soc., 76 [11] (1993) 2899 22. M.R.Boccuti, K.M.Rao, A.Zecchina, G.Leofanti and G.Petrini, Structure and Reactivity of Surfaces,C.Morterra, A.Zecchina and G.Costa(eds.), Elsevier, Amsterdam, 1989 23. A.Mufioz-Paez and G.Munuera, Preparation of Catalysts V, G.Poncelet, P.A.Jacobs, P.Grange and B.Delmon (eds.), Elsevier, Amsterdam, 1991 24. D.R.Sandstrom, F.Lytle, P.S.Wei, R.B.Greegor, J.Wong, P.Schultz, J.Non-Cryst.Solids, 4_! (1980) 201 25. F.Babonneau, S.Doeuff, A.Leaustic, C.Sknchez, C.Cartier, M.Verdaguer,Inorg. Chem., 2_7_7 (1988) 3166 26. M.Astier, A.Bertraad, D.Bianchi, A.Chenard, G.G.Gardes, G.M.Pajonk, M.Taghavi, S.J.Tei chn er, B.Viii emin, Preparation of Catalysts, B.Del mon,P. A.Jacobs, G.Pon eel et( eds. ), Elsevier, Amsterdam, 1976 27. J.N.Armor, E.J.Carlson and P.M.Zambri, Appl.Catal., 19(1985) 339 28. S. Komarmeni, R.Roy, U.Selvaraj, P.B.Malla and E.Breval, J.Mater.Res., _8 (1993) 3163
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
471
Preparation of CaO-, La203- and CeO2-doped Zr02 aerogels by Sol-gel Methods Y.Sun* and P.A.Sermon Solids and Surfaces Research Group, Department of Chemistry, Brunel University, Uxbridge, Middlesex UB8 3PH, U.K.
Zirconia aerogels of high surface area can, it is shown, be synthesised with and without doping by a sol-gel route involving supercritical drying; products have been characterized by XRD, TG/DTA, SEM/TEM and nitrogen adsorption with particular attention given to changes caused by the addition of CaO, La203 and CeO2. The primary aerogels had high surface area (e.g. 350 m2g1) and were amorphous with the cross-linked clusters of particles smaller than 5 nm. The additives were found to improve the thermal stability of the aerogels and as a consequence, the predominant phase of the doped zirconia aerogels was tetragonal.The additives increased to some extent both the surface area and mesoporosity exhibited.
1.INTRODUCTION ZrO2-based materials have attracted considerable interest in recent decades. In a catalytic sense, they appear to have some advantages in areas of practical application over traditional oxides, such as SiO 2 and A1203 [1-4]. More significant is the fact that additives can bring about a strong modification of the surface structure of zirconia [5-8], in which case substitution of Zr +4 with dopant cations results in a rise in anion vacancy concentrations and conductivity. Indeed this is the basis of its redox properties and the catalytic use of stabilized zirconia. Doped zirconia has been shown to be active at relatively low temperature as acid/base and redox catalysts for isomerisation, (de)hydration, aldol-condensation, hydrogenation and oxidative coupling etc [9-11]. However, their lower surface area has undermined their practical application. One method of overcoming this limitation, the sol-gel route, uses supercritical drying to produce aerogels of high surface area [3,12,13]. Doped zirconia aerogels so produced have been shown by the present authors to be active and selective towards methanol synthesis via CO/CO2 hydrogenation [11,14,15]. In the present work, our attention has focused on the microstructure of doped zirconia of high surface area.
* Permanent address: Institute of Coal Chemistry, Chinese Academy of Sciences, P.O.Box 165, Taiyuan, 030001 P.R.China
472 2.EXPERIMENTAL
2.1.Preparation Aerogels of pure and doped zirconia were prepared by drying the corresponding hydrogels supercritically. Zirconyl chloride was f'trst dissolved in doubly-distilled water, the pH of which was adjusted to 1.4-2.0 by a solution of HCI, and the solution was then titrated by a NH4OH solution to a pH of 9-11 to form a hydrogel. Ceria-, lanthania- and calcia-doped zirconia gels were produced by the addition of NH4OH to the aqueous mixture of their nitrate and zirconyl chloride. The precipitate was aged in the mother liquor for 40 min; after filtration, it was washed by doubly-distilled water until the wash-water gave a negative test for chloride ions and was then washed by ethanol to partially remove water. This gel was redispersed in ethanol and evacuated under supercritical conditions with respect to ethanol (To=516K, Pc=6.38 MPa) at 531K. As a result, pure and doped ZrO2 aerogels were obtained. To investigate the effect of additives on their microstructure, the resulting aerogels were treated by calcining (at 723K in flowing air (100 cm 3 rain ~) in a tube furnace for 2 h and at 723K in the air for 2 h). In the former case, the temperature was increased at 2K rain ~ to the final temperature, while in the later case, the rate was 10K rain ~.
2.2.Characterization All the samples were characterized by thermogravimetry (TG/DTG/DTA), nitrogen physical adsorption, X-ray powder diffraction (XRD) and electron microscopy (SEM and TEM). The thermal analyses were all performed by raising the temperature linearly at 20K min ~ in air and a STA-780. X-ray diffraction patterns were recorded using a Philips PW 1710 diffractometer with nickelfiltered CuKaradiation using both continuous and step-scan (over the range 25 ~ < 20< 35 ~ in steps of 0.02* with data collected 10s each step) techniques. Data were analysed according to known procedures and relationships [22] with A1 powder used as standard to define instrumental line broadening. A JEOL 100CX transmission electron microscope (TEM) was used to observe the shape and size of sample particles and the corresponding scanning electron microscopy (SEM) for the aggregate morphology. Single point nitrogen gas adsorption (Sorpty 1750, Carlo Erba) was used to determine specific surface area of the samples. Full nitrogen adsorption-desorption isotherms at 77K were measured using a Carlo Erba 1800 after outgassing for 4h at 523K.
3.RESULTS AND DISCUSSION
3.1.Structure Characteristics of doped zirconia aerogels The thermal stability of the primary zirconia aerogel in the air is illustrated by TG/DTA (see Fig.l). The process of loss of weight due to the volatilization of ethanol and water occurred in two stages (at about 358K and 593K) and continued until 853K. The final weight loss was about 22% (see Fig.la); in addition to an endothermic peak at about 358K, two exothermic peaks at 598K and 728K were seen. The former peak corresponded to the loss of weight at
473 593K was oxidative decomposition of chemisorbed ethanolic species [ 16] as confirmed by insitu FTIR. The latter one was similar to that of the xerogel [17,18], and could be attributed to the 'glow-exotherm' which is characteristic of the crystallization of initially X-ray amorphous zirconia. This indicated that supercritical drying with ethanol had little influence on the process of crystallization of hydrous zirconia. In the case of doped ZrO 2, a typical decomposition of the primary aerogels is shown (in Fig.lb) which is similar to that of pure zirconia with a crystallization temperature of 782K, but a tetragonal structure was produced rather than monoclinic on cooling down to the room temperature (as illustrated below by
XRD).
100
~
100 598K
728K
90-
595K
177 ---I
782K
0
_n
80-
~
17"
,
i
.~
I
em ,
N 7o
60
w
--b
'
273
I
I
I
I
I
I
373
473
573
673
773
873
I
I
I
973 1 0 7 3 1 1 7 3 1 2 7 3
Temperature (K) Figure 1. Thermal analysis of aerogels: (a) ZrO2, (b) 5.2 mol% CeO2-ZrO 2
XRD revealed that under the same conditions, doped zirconia was mainly tetragonal. The results in Fig.2 illustrate the phase change due to the addition of CeO 2, La~O3 and CaO. In the case of the primary aerogels, the broad bands in the range 25* < 20 < 35 ~ could be typical of XRD patterns for both pure and doped zirconia (i.e. was indicative of no crystallinity [19]). The calcination of pure zirconia at 723K in flowing air gave rise to a mixture of tetragonal and monoclinic phases. However, the addition of CeO2, La~O3 and CaO led to the majority being tetragonal (i.e. similar to xerogels [20]). Data in Table 1 give the phase composition of the aerogels, which clearly indicate that the doped zirconia was predominantly tetragonal.
474 100 T(111) t.-
A f 0,m.
G)
tO
o L-i i
I
I
I
i
I
25 26 27 28 29 30
I
I
31
32
I
I
33 34
35
2e (o1 Figure 2. X-ray diffraction patterns of ZrO2 aerogels (pure and doped): fresh aerogel of ZrO 2 (a) and CeO2-ZrO2 (b), and ZrO 2 (c), 5.2 mol% CeO2-ZrO2 (d), 5.1 mol% La~O3-ZrO2 (e) and 8.6 mol% CaO-ZrO2 (f) calcined at 723K in flowing air
Table 1 Effect of additives on the tetragonal volume fraction and the aerogel particle size (d) of ZaO2
Tetragonal Fraction (Vol.%) Samples
,
Primary
Zr02 CaO-ZrO 2 La203-Zr02 CeO2-ZrO2
d (nm) .
-
9
,
,
.
,.
,
Calcined air, 723K
Primary (TEM)
Calcined (TEM/XRD)
16 94 96 98
<5 <5 <5 <5
19.6/18.8 17.8/17.4 16.6/16.1 16.4/15.9
475
3.2.Morphological and textural structure The morphology of pure and CeO2-doped ZrO2 aerogels is shown in Fig.3. As illustrated in Fig.3a, the primary aerogels of pure and doped zirconia consisted of clusters of primary particles, showing similar morphologies. TEM (see Fig.3b) revealed that doped zirconia, similar to pure zirconia, consisted of cross-linked clusters of particles smaller than 5 nm; namely, it had essentially the same structure as the hydrogel. This kind of structure could be maintained on calcination to 723K in flowing (or static) air. After calcination at 732K in the air, they transformed into three-dimensional aggregates smaller than 25 nm. TEM showed detailed differences in their structure. Samples of doped zirconia calcined at 723K in the air (see Fig.4) showed features characteristic of the tetragonal phase and they were smaller than pure zirconia areogel. This demonstrated the appreciable influence of the additives on the structure of the aerogel and an improvement in its thermal stability. Average particle sizes as deduced from both TEM and XRD are shown to be in good agreement in Table 1.
Figure 3. Morphology of prirnary particles in ZrO2 aerogels by SEM (a) and TEM (b)
476
Figure 4. Transmission electron micrographs (1 cm = 20 nm) of pure and doped zirconia calcined at 723K in air: (a) ZrOz, (b) 5.2 mol% CeO2/ZrOz, (c) 5.1 mol% La203/ZrOz, (d) 8.6 mol% CaO/ZrO2 500 450 400
"'350
'~oo ~250 ~r200 >
150 100 50 0
't
0.0
I
I
I
I
l
I
I
I
i
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
P/Po
Figure 5. Physical adsorption-desorption isotherm at 77K of nitrogen over aerogels produced at pH 9.5 and calcined at 723K in flowing air: (a) ZrOz and (b) 5.2 mol% CeOz/ZrOz
477 400 350
'~oo .~.E250 ,-200 ,,~
oe 150 -I NI.-,
,- 100 -
50 0 300
I
I
I
I
400
500
600
700
800
Temperature {K} Figure 6. Effect of additives on the total surface area of ZrO 2 aerogels: (a) 5.2 mol% CeO2]ZrO 2 and (b) ZrO 2 calcined at 723K in flowing air, (c) 5.2 mo1% CeO~ZrO 2 and (d) ZrO 2 calcined at 723K in air
Fig.5 shows an additive effect on the observed texture. The isotherms are seen to be of type IV (BDDT classification) and so characteristic of well-developed mesoporous powders [21]. Compared with pure ZrO2, the addition of ceria improves isotherm hysteresis even if adsorption at P/Po above 0.5 was decreased in extent, and therefore the additive may promote the mesoporosity and increase or sustain surface area. The values of SBzr as a function of calcination temperature (see Fig.6) confirm that the additives (typically CeO2 here) prevent or retard a decrease in the surface area of the aerogels on calcination. This may be attributed to an improvement of the thermal stability of the tetragonal phase produced by the additives.
4.CONCLUSIONS Clearly, the addition of CeO 2, La203 and CaO into the ZrO 2 aerogel shows the same effect On its structure as that seen for ZrO2 xerogels; there is an improvement in their thermal stability of the aerogels as a consequence of the predominant phase of doped zirconia aerogels being tetragonal. Supercritical sol-gel chemistry and the route to ZrO 2 based on this clearly produced highly dispersed zirconia aerogels which are relatively stable against sintering. Such preparative methods are now being developed for other oxides of catalytic interest.
478 REFERENCES
1. Ronald G.Silver, C.J.Hou and John G.Ekerdt, J.Catal., 118 (1989) 400. 2. Nancy B.Jackson and John G.Ekerdt, J.Catal., 126 (1990) 31. 3. G.M.Pajonk and A.E1.Tanany, React.Kinet.Catal.Lett., 47 (1992) 167. 4. A.Benedetti, J.Catal., 122 (1990) 330. 5. K.Tanabe, Mater.Chem.Phys. 13 (1987) 347. 6. E.V.Prokhorenko, Kinet.Catal., 29 (1988) 702. 7. L.A.Bruce, G.J.Hojze and J.F.Mathews, Appl.Catal., 8 (1983) 349. 8. G.R.Gavalas, J.Catal., 88 (1984) 54. 9. T.Lizuka, Y.Tanaka, and K.Tanabe, J.Mol.Catal., 17, 381(1982). 10. A.Baiker, M.Kilo, M.Maciejewski, S.Menzi and A.Wokaun, Proc.10th Int.Congr.Catal., Budapest, 1992, p. 1257. 11. Y.Sun and P.A.Sermon, J.Chem.Soc., Chem.Commun., 1993, p.1242. 12. Z.Congshen, et al., J.Non-Cryst.Solids, 63 (1984)105. 13. H.D.Gesser and P.C.Goswani, Chem.Rev., 89 (1989) 765. 14. P.A.Sermon, Y.Sun and K.M.Keryou, Catal.Today, 17 (1993) 391 15. Y.Sun and P.A.Sermon, Catal.Lett., 1994, in press. 16. P.D.L.Mercera, J.G.VanOmmen, E.B.M.Doesburg, A.J.Burggraaf, J.R.H.Ross J.Mater.Sci., 27 (1992) 4890. 17. R.Srinivasan, M.B.Harris, S.F.Simpson, R.J.De Angelis and B.H.Davis, J.Mater.Res., 3 (1988) 787. 18. C.J.Norman, P.A.Goulding, I.McAlpine and P.J.Moles, EuroCat-1, Vol.1 (1993) 437. 19. M.J.Ready, J.Am.Ceram.Soc., 73 (1990) 1499. 20. R.Stevens, Zirconia and Zirconia Ceramics, Mangnesium Elecktron LtD, 1986. 21. F.Blanchard, J.Mol.Catal., 17 (1982) 171.
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
479
Preparation of Nanometer Size of Cu-ZrdAI203 C a t a l y s t b y P h a s e T r a n s f e r P a r t 1 : S t u d y of Basic P r e p a r a t i o n Conditions Z.-S. Hu, S.-Y. C h e n a n d S.-Y P e n g S t a t e Key L a b o r a t o r y of Coal Conversion, I n s t i t u t e of Coal C h e m i s t r y , C h i n e s e A c a d e m y of Science, P.O. Box 165, T a i y u a n 030001 P.R. C h i n a 1. I N T R O D U C T I O N
Recently, m u c h attention has been paid to the preparation of nanometer particle catalysts. Several methods have been developed to prepare nanometer particles. The liquid phase method could be better for the preparation of catalyst with nanometer size [1]. A main problem for this preparation process is how the aggregation of fine particles can be avoided, especially at the drying step. The key is to reduce surface tension. Alcoholate method was developed for it in which alcoholate hydrolysis instead of aqueous solution to m a k e the sol(gel) was used to avoid surface tension, i.e. avoid vapor-liquid interface; freeze drying method, in which the solution is first frozen and then sublimated to remove it, and supercritical fluid drying method, in which the solvent is solved in a supercritical fluid to remove it, were also developed. Although these methods have successfully reduced or avoided the effect of surface tension, the preparation cost including raw material, special equipment and time is increased. A n alternative way to decrease surface and interface tension is the use of a surfactant which can weaken the aggregation of the fine particles, and leads to propose a microemulsion method for the preparation of nanometer particle catalysts [2-4]. In the preparation process, the solution is first emulsified into micelles [4] (size about 30-40 A), then the other reactants are added into the system. However, there exist interactions between the water pools of each micelle [5], which result in aggregation and even recrystallization of particles. One problem of the method is that the amount of organic solvent and surfactant required is high. Moreover, there is a "cancer" in this method, i.e. the easier the emulsification is, the more difficult the damaging of the emulsion and the removal of water is. Phase transfer method was firstly applied by Ito et al. [6] in 1984 to prepare nanometer particles of Fe203, TiO2 and A1203. In this method, sol particles were first transfered into organic solvent by use of an anionic surfactant D B S (docecyl benzene sodium sulfonate) then water was removed by decompression stilling. Shi Su-hua et al. [7], and W a n g Xiao-hui et al. [8] also prepared Co203 nanometer particles by phase transfer method. The greatest defect for the use of D B S is that its sulphonate cannot be removed. Although single component nanometer particles have been successfully prepared by phase transfer method, it is very difficult to prepare multicomponent oxides because of the different precipitation conditions. Therefore, it is necessary to improve the phase transfer to m a k e multicomponent oxide with nanometer size. Cu-Zn/Al203 is widely used as
480 methanol synthesis catalyst. To increase its activity and reduce the operating pressure, nanometer size Cu-Zn]A1203 catalysts may have to be prepared. 2. EXPERIMENTAL R e a g e n t " A.R. aqueous ammonia, metal nitrates, benzene, commercial cetyltrimethyl ammonium chloride and kerosine. A typical preparation procedure was 9(1) Sol preparation and phase transfer 9dissolve cetyltrimethyl ammonium chloride into ammonia solution, add benzene into the system, then add dropwise the nitrate (copper nitrate, zinc nitrate and aluminum nitrate) solution into the system under stirring. (2) RemQv~l of water 9after finishing the dropping, the sample is aged for a short time to reduce water, then let the water out and filter. (3) Drying at 120~ for 2 hours. (4) Calcination at 350~ for 2 hours. Typical amounts of agents used are shown in Table 1. C h a r a c t e r i z a t i o n " the thermal stability of cetyltrimethyl a m m o n i u m chloride was tested with a Du Pont 99 Thermal Analyzer. The morphology of the catalyst was examined by a HITACHI H-600 Electron Microscope. The surface area of the samples was measured with an ASAP200 of Micromeritics Co. Table 1. Amounts of agent used in the catalyst preparation. Agent Concentration Amount
Ammonium ca. 0.8M 50ml
Nitrate 0.2M 50ml
Surfactant
Benzene
0.002mol.
50ml
3. R E S I S T S AND DISCUSSION 3.1 Selection of the surfactant In phase transfer process, the sol particles leave the water phase and are transfered into the organic solvent by the adsorption of a surfactant. If an anionic surfactant is employed, as reported in the literature on phase transfer, sol particles should carry positive charges which requires that the metallic salt is in excess, i.e. there is excessive metal cation adsorbed on the sol particles. In this case, it is difficult to prepare multicomponent nanometer particles. If a normal sequence of co-precipitation was used, successive precipitation would be inevitable in the preparation of multicomponent oxides. If reverse-sequence of co-precipitation was used, the particles produced would be unstable and aggregate to each other, because they first carry a negative charge (OH is adsorbed on the particles), then become neutral, and finally carry a positive charge. Therefore, anion surfactant is not suitable for the p r e p a r a t i o n of multicomponent oxides with nanometer size. The adsorption of a cationic surfactant, however, requires sol particles carrying a negative charge, which can be achieved naturally by reverse-sequence co-precipitation and excess of precipitation agent (ammonia). There are other requirements for the selection of the surfactant. For example, it should be stable in the phase transfer process
481 and removed at lower calcination temperature. Cetyltrimethyl a m m o n i u m chloride could be a better cationic surfactant. The result of TG measurement, shown in Figure 1, indicates that its initial temperature of decomposition is about 175~ showing its stability in the p r e p a r a t i o n process, and the temperature of complete decomposition is about 250~ which points out t h a t it is easy to remove the surfactant at lower temperature and therefore the final product is not polluted.
'50
100
150
200
250
300
350
400 ~
3.2. Way of a d d i n g t h e surfactant. The formation of the precipitate m u s t go t h r o u g h different states, i.e. : first sol, then gel and finally p r e c i p i t a t e . If the gel or precipitate particles are phase transfered, the produced particles are certainly big. The time and way of adding the s u r f a c t a n t , of course, affect the size of the product particles. As shown in Table 2, the preparation procedure for sample BTX is the typical procedure described in the e x p e r i m e n t a l section, a n d leads to s i m u l t a n e o u s sol formation, surfactant adsorption and phase transfer.
Figure 1. TG curve of cetyltrimethyl ammonium chloride. Table 2. Effect of the way of adding the surfactant on the surface area of the product. Catalyst
Surface area (m2/g)
Part I
Part 2
BTX
BHX
BTX
BHH
BTX-741
BHX-2
35.84
27.91
31.86
30.58
136.9
128.9
For sample BTH, the procedure is the same as that of BTX except t h a t the surfactant was dissolved in nitrate solution instead of ammonia solution. For BHX, the phase transfer process was separated from sol formation and surfactant adsorption, i.e. the organic solvent was added after r a t h e r t h a n simultaneously. Sample BHH was prepared as follows : t h e sol particles were prepared by reverse-sequence, namely first co-precipitation then phase transfer by adding the emulsion of surfactant, benzene and water (because the surfactant cannot be dissolved in benzene), which means t h a t the sol
482 preparation process was separated from the surfactant adsorption and phase transfer process. In Table 2, all the conditions were the same, except for the preparation procedure, and the final pH of the sol preparation for the samples of part 1 was higher t ha n t h a t of part 2. The m e a s u r e m e n t of the surface area of the products indicates t h a t the procedure for BTX is obviously the best. The next is for BHX.
3.3. Effect of solvent 3.3.1. Effect of the nature of the solvent It can be seen from the results given in Table 3 t h a t benzene is a better solvent for the system to be studied. The results of surface area m e a s u r e m e n t show t h a t the amount of benzene used in the operation should be about 50ml for the special conditions in Table 1. The reference sample is the one prepared by the co-precipitation method in similar conditions. It is interesting to note t h a t with the addition of alcohol or ether in benzene, phase t r a n s f e r was not successful. Table 3 Effect of the nature and amount of solvent on phase transfer. Solvent
Amount (ml)
xylene
ligarine 60-90~
50
50
50
50
50
25
fail
transfer
fail
*
29.9 41.8
Surface transfer area (m2/~)
benzene benzene kerosene benzene and ether an d alcohol
reference
50 24.55
* Stilling failed so there is no surface area value of this product. The reason could be t h a t alcohol and ether are extracted into the water phase when the organic phase is mixed with water, which results in a competitive adsorption with the surfactant and a decrease of the transfer ability of the sol particles into benzene. When xylene or ligarine is used as solvent, phase transfer also failed, although the properties of the former is similar to benzene. The phase tr an s fer is successful for kerosene, but the filtering to remove kerosene and residual water would be very difficult. In addition, the formation of colloidal m a t t e r and oxidation-reduction reaction between copper ion and kerosene is possible when continuously heating. Thus, kerosene is not a suitable solvent for the system to be studied. 3.3.2. Effect of solvent density. As m e n t i o n e d above, there were different results for ligarine and kerosene solvent, which probably come from the difference of solvent density. In order to confirm the assumption, a series of solvents with different densities was p r e p a r e d by mixing benzene and carbon t e t r a c h l o r i d e in different proportions, and used in phase transfer process for the p r e p a r a t i o n of CuZn/A1203 catalyst with n a n o m e t e r size. The results are given in Figure 2. There exists, indeed, an optimal density of the solvent, as indicated by Figure 2. An explanation could be t h a t if the density difference between solvent and
483 water solution is too large, the particles produced could easily aggregate to each other, at least to some extent, and if they are too close, the removal of water becomes very difficult. Our results showed t h a t the most suitable density of the solvent is about 0.900g/cm3 or 1.170g/cm3.
130[
120[
3.4. Effect of c h a r g e t y p e
carried by parades
Type of c h a r g e on a particle could be d e t e r m i n e d by the m e a s u r e m e n t of Zeta .. 100/ 00 potential [9] or a t i t r a t i o n curve. According to F a j a n ' s rule [10], the ions forming a J 90" crystalline p h a s e should be 80 adsorbed prior to other ions. 1.0 1.2 1.4 1.6 For the system studied in this p a p e r , t h e sol p a r t i c l e s ,density of solvent (g/cm 3) produced m u s t carry positive c h a r g e to m a k e it s t a b l y Figure 2. Effect of solvent density on surface adsorb the cationic surfactant. area of the product. In order to elucidate the effect of charge type on the particle prepared, the titration curve of ammonia solution by a nitrate solution of Cu-Zn-A1 was established (given in Figure 3). The pH value of the equivalent point is about 6.5, which indicates the pH value of the isoelectric point of sol particles. Obviously, with an excess of nitrate, the sol particles will adsorb a metal cation and carry a positive charge, and if an excess of ammonia is supplied, OH ion will be adsorbed and the sol particles will carry a negative charge. The samples prepared from sol particles with neutral, positive and negative charge are given in Table 4.
0.8
It can be seen from Table 4 t h a t the / surface area is larger I / for the sample of sol particles w i t h nega8.0 ive charge t h a n t h a t with positive charge, and that there exists a big difference between BTX-39 a n d BTX-72, 6.0 a l t h o u g h b o t h were p r e p a r e d f r o m sol particles w i t h negative charge. This 4.0 indicates that the Amount of ammonia product particles depend not only on the Figure 3. Titration curve of reserve co-precipitation of type of charge but also nitrate and ammonia solution. on the final pH value
/
484 of the sol preparation, i.e. the charge density of the sol particles. Table 4 Effect of charge type carried by sol particles Catalyst
BTX-42
BTX
BTX-39
BTX-72
Final pH Charge type Transfer Surface area (m2/g)
6.3 + partly 34.58
6.5 0 partly 35.84
7.5 completely 41.76
8.6 completely 114.9
Fig. 4-6 are TEM micrographs (amplified 105 times) of samples BTX-42, BTX-39 and BTX-72, which indicate that the particle size of the product from a sol with positive charge is non-uniform (BTX-42), and uniform for t h a t with negative charge (BTX-39 and BTX-72). However, the particle size is much smaller for BTX-72 (about 3 nm) than for BTX-39. It is easy to understand that if the charge density is too small, cationic surfactant adsorbed is not enough and the particles aggregation cannot be effectively prevented. 3.5. Effect of the way of water removal Some amount of water is still involved in the sol particles transfered into the organic phase, and aging for a short time is necessary for the removal of most water. The residual water can be removed by two methods. One is vaporization by heating in atmosphere, which will results inevitably in the weakness of surfactant adsorption, i.e. aggregation of particles will occur to some extent, because its adsorption on the sol particles is exothermic. Another one is aging for a long time and then filtering. Obviously, water cannot be removed completely by this method. For comparison, the results given in Table 5 show that the second method is better. TEM micrographs of samples 3-31-a and 4-3-a, shown in Figures 7 and 8, respectively, indicate that the particle size of the former is smaller than for the latter and the aggregation of particles has taken place for the first method. Table 5 Effect of the way of removing remanent water on surface area of the product Catalyst
part1 3-31-a
Way Surface area (m2/g)
stilling 57.6
part 2 4-3-a
filtering 111.4
3-27-6 stilling 34.7
3-31-c filtering 100.1
485
Figure 4. TEM micrograph of s_ample Figure 5. TEM micrograph of sample BTX-39 BTX-42
: ~ ~ . . .
?. ~ , ~ . ~ . ,
, ::~i'o
.~ ~,~,~,~ ~ ~ . .
o.. ,.... .9 j . 9
~..
Figure 6. TEM micrograph of sample BTX-72
Figure 7. TEM micrograph of sample 3-31-a
486 calcination time (hour) 2.0
6.0
4.0
8.0
E 12o
t~lO0
8O 3O
20 300
350
400
450
calcination temperature (~
Figure 8. TEM micrograph of sample 4-3-a
Figure 9. Effect of calcination time and temperature on the surface area of the product.
3.6. Effect of calcination In general, the thermal stability of nanometer particles is worse t h a n the one of normal particles because of large surface energy and high proportion of surface atoms. The effect of calcination time at 350~ shown in Figure 9 for nanometer particles of CuO-ZnO/A1203 indicates that the surface area slightly increases in a short initial time and then keeps constant for 8 hours. Figure 9 also shows the effect of calcination temperature, which indicates t h a t the surface does not change up to 400~ These results demonstrate t h a t the nanometer particles of CuO-ZnO/A1203 prepared by the phase transfer method described in this paper are stable at 400~ and at 350~ for more than 8 hours. 4. CONCLUSIONS 1. Nanometer particle CuO-ZnO/A1203 can be prepared by phase transfer with cationic surfactant cetyltrimethyl ammonium chloride. The product is stable at 400~ and can be calcined at 350~ for a long time. Cetyltrimethyl ammonium chloride possesses a universal suitability. 2. The best preparation procedure is the one where the surfactant and organic solvent are added into the precipitation agent (ammonia solution) and then titrated by the nitrate solution. 3. The organic solvent has a vital influence for the success of the phase transfer method. Polar organic solvent could destroy the phase t r a n s f e r
500
487 process. There exists an optimal density of solvent, i.e. about 1.170 g/cm 3 or 0.900 g/cm3 for this system. 4. It is essential that the precipitation agent (ammonia) is in sufficient excess so that the sol particles carry enough negative charges. 5. It is better to remove the residual water by filtering instead of stilling. REFELtENCES 1 2. 3. 4. 5. 6. 7. 8.
9. 10.
Feng Li-juan, Chen Song-Ying, Peng Shao-yi, "Proceedings of the Second National Symposium on Science and Technology of Ultrafine Particles and Surfaces" 5 1991, Wuhan China. Kazue Kurihare et al., J. Am. Chem. Soc. 105, 2574-79 (1983). Kijiro Kon-no, Misao Koide and Ayao Kitahhara, The Chemical Society of Japan, 9 (6), 815-22 (1984). Masao Gobe et al, J. Colloid. Interf. Sci. 93, 1 (1983). Christopher Oldfield, J. Chem. Soc. Faraday Trans. 87 (16) 2607-12 (1991). Seisoro Ito, Shizai Kyokaishi 57 (7) 394-402 (1984). Shi Su-hua at al., "Proceedings of the Second National Symposium on Science and Technology of Ultrafine Particles and Surfaces", P 259, 5. 1991 Wuhan China. Wang Xiao-Hui, Wang Zi-Chen, Xiao Liang-Zhi, Xu Bao-Kun, "Proceedings of the Second National Symposium on Science and Technology of Ultrafine Particles and Surfaces", P 264, 5. 1991 W u h a n China. T. Smae, K. Muto, S. Ikeda, Colloid Polym. Sci. 264 43-48 (1991). Shen Zhong, Wang Guo-ting, "Colloid and Surface Chemistry", Industrial Chemistry Press, (Beijing, China).
This Page Intentionally Left Blank
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
The p r e p a r a t i o n of u l t r a f i n e drying
technique
SnO2 by the supercritical
489
fluid
(SCFDT)
F. Lu and S.-Y. Chen State Key Laboratory for Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, P.O. Box 165, Taiyuan 030001, P.R. China. Nanometer size Sn02 was prepared with a sol-gel process followed by supercritical fluid drying. Their structure, particle size and specific surface area were characterized by XRD, TEM and nitrogen adsorption method. The effect of the preparation parameters is also discussed. 1. INTRODUCTION Ultrafine particles (UFP) are one of the most prospective materials for the twenty-first century, because they show many unusual physical and chemical properties. Much a t t e n t i o n has been paid to t h e i r p r e p a r a t i o n and characterization and many methods have been developed for the synthesis of UFP. Among these methods, the sol-gel technique is very effective and has been widely used in the recent years, because of its low cost and simplicity. In the solgel process, one of the key steps is the drying, in which water and solvent are removed. In general, as for the drying process with heating, it is difficult to avoid the aggregation of fine particles because of the existence of gas-liquid interface leading to large surface tension. In order to avoid the appearance of gas-liquid interface in the drying process, two methods could be used: supercritical fluid drying and freezing vacuum drying. In the present paper, supercritical fluid drying (SCFD) technique is applied to prepare nanometer size tin oxide particles. Under supercritical conditions, solvent is of large density and able to dissolve water and other materials. Therefore, removal of water does not need the appearance of gasliquid interface, and the aggregation of fine particles could be decreased to a great extent. It is well known that tin oxide is widely used for sensing material, since it is sensitive to gas composition at surface, and is a partial oxidation catalyst. To improve its sensing and catalytic performance, it could be necessary to prepare nanometer size Sn02 with sol-gel process followed by SCFD technique. Obviously, smaller particle size and higher surface area (SA) might give better performance. Another objective in this paper is to study the effect of the preparation parameters on SnO2 particle size.
490 2. EXPERIMENTAL 2.1. M a t e r i a l s Inorganic salt SnC14.5H20(AR), NH4OH(AR) and organic solvent, absolute ethyl alcohol (AR), were used to prepare sol-gel and as supercritical drying solvent. 2.2. C h a r a c t e r i z a t i o n XRD (Dmax-A,Japan), TEM (Hitachi H-600,Japan), N2 adsorption (ASAP 2000, Micromeritics, Co.USA) were applied to characterize the structure, particle size, and SA of the Sn02 powder.
2.3. Preparation procedures Sn02 powder was prepared by a sol-gel process followed by SCFD technique and then Sn02 was calcined. There are several steps : (1) gel preparation : SnC14 in solution was hydrolyzed and gelatinized with the adding of NH40H at different acidity, SnC14 concentration and temperature; (2) solvent replacement : water was replaced by ethanol several times after the ageing, washing and filtering of the gel, and the so-called alcogel was prepared; (3) supercritical drying 9alcogel was put into an autoclave, filling with ethanol and raising the temperature until the pressure in the autoclave exceeded the critical value of the solvent, keeping the supercritical state for a given time, then slowly venting the vapors in the autoclave. At this stage, N2 should be used to flush the autoclave in order to prevent ethanol from condensing onto the gel during cooling. When ambient pressure and temperature are reached, a porous dried aerogel filled with air is obtained; (4) calcination : the dried aerogel Sn02 was calcined at 773K for 2-4 hours, and nanometer size Sn02 was obtained. 3. R E S U L T S AND DISCUSSION According to the experimental procedures mentioned above, SnO2 powders were prepared at different preparation conditions. Their structure, particle size and surface area were measured. It was found that nanometer size SnO2 could be prepared with the sol-gel process followed by supercritical fluid drying, and the preparation parameters had a large effect on the properties of SnO2. Figure 1 gives the diffractogram of a typical SnO2 sample, which indicates that the powder is a kind of polycrystalline material; the crystal structure is tetragonal. TEM micrograph of a typical sample is given in Figure 2, which shows that the surface of the sample is homogeneous. The size of the SnO2 particle is about 5-9 nm from the estimation of the spot in Fig. 2 (xl00,000) which is an evidence that the SnO2 particles prepared by SCFD are nanometer size particles, indeed. A s s u m i n g the SnO2 p a r t i c l e is Figure 1. XRD of a typical Sn02 sample, spherical, an estimation of its external area indicated that its value was about the surface area given by N2 adsorption measurement, implying that the Sn02
491 particles prepared could be non-porous. So, the surface area of SnO2 powder could be used to represent its particle size. The results reported in literature showed that SnO2 particle size depended largely on the preparation method. SnO2 powder can be prepared by different methods. As early as 1940, Kistler et al. [4] applied hydrolysis of SnC14 in a cellophane dialyzer followed by hypercritical drying to prepare SnO2 powder. Other methods include hydrolysis of Sn(OC4H9)4 followed by heat drying [5], thermal decomposition with SnSO4 [6], reaction with tin powder and concentrated nitric acid [7]. The sol-gel process followed by heat drying was also widely used [8-9]. For comparison, the surface area of SnO2 samples prepared by different methods is given in Table 1. It can be seen from Table 1 that SA of the sample prepared by SCFD is 3-5 times larger than that of other samples. Table I. Surface area of SnO2 powders prepared with differentmethods
Method
sol-gel process with heating
References .
SA (m2/g)
.
.
.
.
.
(9)
(8)
thermal decomposition with SnSO 4 (6)
16
21
23
.
our experimental conditions this work 50-100
Even for SCFD, the SA of SnO2 is influenced by the preparation conditions, including acidity and concentration of SnC14 solution, temperature of hydrolysis, gelatinization and ageing conditions (during gel preparation process), solvent, pressure and temperature, etc... (during supercritical drying process). They are investigated experimentally and the results are given hereafter.
-too
70 trt
~o
Figure 2. TEM micrograph of SnO2 sample.
Figure 3. S A vs. p H of the gel.
492
3.1. Effect of acidity of SnCI4 solution The pH value of the SnC14 solution was adjusted by adding 0.1 N hydrochloric acid. The result is shown in Fig 3, which indicates that the acidity is one of the most important parameters. The formation of the two-humped curve could be explained in this way : at lower pH, there exists an acid protective hydratation film around each colloidal particle which ensures the stability of the sol (gel), resulting in a larger surface area and small particle size. As the pH increases, the concentration of OH ion increases. The protective film could be destroyed and the coagulation of particles could be catalyzed, the result, of course, being the growth of the particles and sharp decrease of SA. With the gradual increase of OH ions, another basic protective film could be formed and the second hump curve appears. 3.2. Effect of SnCI4 concentration From Table 2, it can be seen that there exists an optimum range of concentrations (the concentration is expressed by the amount of SnC14 in 100 ml distilled water), and when the concentration is too low (1.5g SnC14 in 100 ml distilled water), no gelation occurs, and if the concentration is too high, the aggregation of fine particles would take place. Table 2 Relation between SnC14 concentration and surface area of SnO2 SnC14 (g)
1.5
Surface Area
3.7
7.4
14.9
26
31
66
59
54
(m2/g) conditions 9pH=7, gel temperature = 298, drying pressure = 8.0 M P a , drying temperature = 353 K
3.3. Effect of gel temperature The effect of temperature is shown in Table 3. It indicates that the surface area of the particles decreases with the rising of the gel temperature. The best condition for hydrolysis of SnC14 is ice-water temperature, i.e. about 273 If, Table 3. Relation between gel temperature and surface area of Sn02 Temperature (K)
273
298
323
353
Surface area (m2/g)
54
49
40
23
preparation conditions 9p H = 6-7, C S n C I 8.5 M P a , drying temperature = 358 K
4 -
15 g/100 ml water, drying pressure =
3.4. Effect of ageing c o n d i t i o n s Ageing is another important and complex factor, because in the process m a n y p h e n o m e n a may occur, singly or s i m u l t a n e o u s l y , i n c l u d i n g polycondensation, syneresis, coarsening, etc... Time, temperature and pH are all
493
parameters that can effectively alter the ageing process. In this paper, only time and temperature of ageing were studied. The effect of time is shown in Figure 4, indicating that there is an optimum time range for our results, i.e. 2-3 hours. The reason could be that the gel particle could be unstable for too short ageing times, whereas for too long times the aggregation of fine particles could occur to some extent. The effect of ageing temperature is shown in Figure 5, pointing out that lower temperature favors larger surface area of the product particles. It is not difficult to explain the results : higher temperature accelerates the rate of particle aggregation.
20 Ot
~sNr
nM~,hr
Figure 4. Effect of ageing time
AeEV~ ~~TURE~ K Figure 5. Effect of ageing temperature.
3.5. Effect of supercritical extraction solvents Table 4 shows the results of the use of some solvents for supercritical fluid drying. It can be seen from Table 4 that: a) the best solvent for the system studied is methanol, b) polar solvents are b e t t e r t h a n non-polar solvents, c) materials with pentagonal, rhombohedral and hexagonal shapes can be observed by TEM when isobutanol is used as solvent, but the reason is not clear.
Table 4 Results of different supercritical drying solvents Solvent
Drying pressure
Drying temperature
Surface area (m2/g)
Particlesize (nm)
(MPa)
(K)
Methanol Ethanol
8.5 8.0
548 553
66 52
3 -4 3 -4
Isobutanol Benzene
6.0 6.0
548 593
55 50
6- 7 7-8
494
3.6. Effect of the s u p e r c r i t i c a l i t y Drying pressure and temperature for SCFD have a marked effect on the surface area of the product. Our results show that the increase of the two parameters causes the decrease of SA. For example, at 558K, pH=7, for pressures between 7.5MPa and 10.0MPa, the SA of the powder decreases from 56 m2/g to 23 m2/g. The suitable temperature range is 20 - 40 K higher than the critical point. If ethanol is used as solvent, the condition for SCFD operation should be selected as follows : Pressure : 7.5 - 9.0 MPa
Temperature : 536- 556K
Both the rate of temperature increase and the water content of the alcogel can also affect the particle size of the final product. The stability of the gel could become bad for too high and too low rates. If too much water is lei~ in the alcogel, the condition for SFCD operation would not easily be controlled, and sometimes some glassy blocks material, not the aerogel, would be formed. 4. CONCLUSIONS An organic solvent-inorganic salt system has been successfully applied to prepare nanometer size SnO2 particles with a sol-gel process followed by supercritical fluid drying. The studies on the effect of the preparation parameters show that the particle size of SnO2 is controlled by many factors. Under the best preparation conditions, the preparation of Sn02 with particle size as small as 3 nm and surface area as large as 100 m2/g is possible. REFERENCES 1 2 3 4 5 6 7 8 9
L.L. Hench, Chem. Rev., 90 (1990) 33 H.D. Gesser, Chem. Rev.., 89 (1989) 765 Hiroshi Yagita, Appl. Catal., 53 (1985) L5 Kistler, US Patent N ~ 2188007 (1940) Hatiboku Shuumei, J. Ceram. Soc. Jap., 100 (1992) 1158 H. Tovvela, J. Electron. Mater., 1 (1986) 7 S.Z. Zhang, J. Chin. Univ. Sci. Tech., 9 (1984) 436 Sensing Group, Chin. J. Appl. Chem., 28 (1974) H.L. Chen, J. Nanjing Univ., 1 (1993) 72
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
495
P l a s m a preparation of a dispersed catalyst for h y d r o c o n v e r s i o n of heavy oils L. Rouleau, R. Bacaud and M. Breysse Institut de Recherches sur la Catalyse, C.N.R.S. 2, Avenue Albert Einstein 69626 ViUeurbanne Cedex, France
A plasma-based process for the production of a dispersed supported catalyst is proposed. The catalyst is obtained by a high-voltage discharge sparking between two metallic electrodes immersed in a liquid hydrocarbon. At the interface of the electrodes, a local evaporation of metal occurs; metal vapors are quenched and a highly dispersed metallic phase is produced. Simultaneously, pyrolysis of the hydrocarbon generates a high surface area carbon-based solid. The metal particles are trapped within the fibrous structure of the solid. The respective yields of carbonaceous solid, metal and gases have been determined as a function of the nature of the liquid hydrocarbon medium, nature and geometry of the electrodes. The solids thus produced contain a highly dispersed metallic phase. At low metal level, plasma-produced catalysts exhibit a significant activity during hydroconversion of a residual oil, in terms of hydrogen transfer and reduction of gas production.
1. INTRODUCTION Dispersed catalysts are defined as heterogeneous catalysts flowing along with the reactants in a reactor system. The residence time of catalyst and reactants are thus equivalent and consequently, a continuous renewal of the catalyst-reactant interface is afforded. They demonstrate their usefulness in case of competitive fast deactivation or when accessibility of reactants to catalyst surface is hampered by feed characteristics. This kind of situation typically corresponds to heavy feeds processing, and effectively, the use of dispersed catalysts is practically limited to coal liquefaction and petroleum residues conversion. Preparing a dispersed catalyst means elaborating a large surface area material and, in this respect, the objectives and problems are exactly the same heterogeneous catalysts manufacturers must deal with. However, some specifics must be pointed out. They concern the preparation, activation step and exacting properties required for convenient operation of multi-phase reactors. An ex-situ elaboration of dispersed catalysts may obviously be performed applying known protocols of preparation of heterogeneous catalysts. Precipitated oxides or sulfides are effectively proposed as base materials. The porous solids thus obtained do not exhibit
496 the textural properties expected for a dispersed catalyst. In contrast with supported heterogeneous catalysts, for which the catalyst-reactant interface is essentially composed by a porous structure, preparation of dispersed catalysts attempts to develop a large external surface. In order to reach this objective, original metods have been explored and welldefined, nanometer-sized particles have been generated by flame hydrolysis (1), laser pyrolysis (2) or electro-erosion (3). The field of investigation for new methods is ample, but cost considerations considerably limit the realistic range. An ex-situ prepared solid, or an in-situ added precursor may seldom be considered as the actual active phase, which will result from the interaction of the precursor with the feed in the conditions of the considered reaction. The activation step is probably one of the most distinguishable features of dispersed catalysts, as well as the source of misunderstanding concerning their evaluation, since it is hardly performed in adequate conditions. The lack of control in the generation of the active phase is particularly glaring when oil-soluble precursors are introduced in petroleum residues for hydroprocessing (4). The final state of the catalyst, identified as a sulfide, is defined, but nothing is known concerning the process of generation of this active phase. In contrast, activation of supported catalysts operated in fixed bed reactors, be it conducted in-situ or ex-situ, is performed in well controlled conditions of temperature gradient, partial pressure of activating reagent, flow rate. The impact of these parameters upon the properties and activity of hydrotreatment sulfide catalysts have been studied by Breysse et al (5) and justify the special attention paid to this critical stage of catalysts generation. Finally, dispersed catalysts must comply some specific requirements underlined hereafter, which make their elaboration more arduous. The recovery of the catalytic substance in unconverted residue can hardly be contemplated, though the cost of this disposable material must be low. This primary consideration has considerably limited the range of investigated candidates. The solid catalyst is flowing along with the reactants. It is therefore necessary to reduce the amount of displaced substance, for avoiding the transport of useless material and for limiting the problems associated with sedimentation and abrasion of reactor and equipments. Additionally, even when a solid is considered as disposable, the management of solid wastes prescribes a drastic reduction of their volume. The method of preparation described in the present paper aims at producing, in a simultaneous operation, an active phase and a textural support. It is based on the observation that iron oxides, used as dispersed disposable catalysts, suffer a fast, severe sintering during sulfidation in liquid phase; but the presence of a carbonaceous substance acting as a support, limits particles agglomeration (6). Considering that the essential property of a dispersed disposable catalyst is a high intrinsic activity, the objectives of the proposed method are the production of a highly dispersed active phase and the simultaneous generation of a support. The method consists in volatilizing a transition metal by means of a high-voltage discharge sparking between two metallic electrodes immersed in a liquid hydrocarbon. Two simultaneous phenomena occur: a volatilization of metal at the electrodes interface and a pyrolysis of the hydrocarbon. As a result, metal vapors are quenched in the liquid medium and deposit on the carbonaceous product resulting from pyrolysis. In contact with a sulfur-containing feed, metal particles are sulfided. This production process, experimented a laboratory scale in a batch reactor, may easily be
497 designed for continuous in-situ generation in petroleum residues hydroconversion processes (7). An evaluation of preparation parameters and their impact on catalyst production is presented along with an electron microscopy characterization of the actual active phase generated during hydroconversion of a deasphalted vacuum residue. Finally, the potential interest of this catalytic system is illustrated by an evaluation of its performance in hydroconversion of a petroleum residue.
2. E X P E R I M E N T A L 2.1. Catalyst preparation
The experimental arrangement consists of a heavy-walled vessel containing a hydrocarbon; two opposing electrodes - one of which is insulated - are fixed through the wall of the autoclave. Inter-electrodes spacing can be adjusted by means of a micrometric thread. Before starting the electrical discharge, the vessel is purged several times with nitrogen to eliminate residual air. The generator can supply a voltage up to 20 kV. The current is limited to 3 mA. Once the DC discharge is established, pyrolysis of the hydrocarbon occurs, producing a gas phase, and a solid phase comprising a carbonaceous material which contains the metal vaporized from the electrodes. Each batch of solid is characterized by the following parameters: - composition of the hydrocarbon, - dissipated power, - yield and composition of the gas phase, - yields of metal and of total carbonaceous solid phase. The yields are expressed in gram mole per hour, one mole of solid being defined from the elemental analysis of the solids, giving a mean H/C ratio of 0.57. The BET surface area of the solid is 400 m2.g -1. The pore volume is 0.5 cm3.g -1 consisting essentially of meso-pores (85 % of the pore volume is in the range 2-20nm). 2.2. Hydroconversion reaction
Experimental arrangement and evaluation procedures are described in reference (8). The charge is a butane-deasphalted 450+ vacuum residue (DVR). Its elemental analysis (weight %): C = 86.2; H = 11.9; O = 0.78; N = 0.36; S = 0.88; Ni = 1.5 ppm; V = 3.7 ppm. Experiments were carried out in a 250 cm 3 autoclave equipped with a magnetically driven impeller, a cooling coil and a pressure transducer. Induction heating of the reactor allows heating rate up to 1.5 K.s -1 and isothermal stability better than +_0.5 K. Gases were analyzed by gas chromatography after the addition of a known amount of nitrogen as internal standard. The results include C 1 to C 4 hydrocarbons, CO 2, and H2S. The liquid products were characterized by simulated distillation in a gas chromatograph. Aromaticity was evaluated by UV absorption spectroscopy. From the material balance of hydrogen and the GC analysis of the gases, hydrogen incorporated to, or released by, the liquid fraction of the products was evaluated.
498 3. RESULTS AND DISCUSSION From the preparation experiments which were performed, significant parameters influencing the yields as well as properties of plasma produced catalysts could be identified. They concern the nature of the hydrocarbon medium, electrodes material and electrodes geometry. 3.1. Liquid hydrocarbon medium One of the objectives of the present investigation was to demonstrate the ability of this method of catalyst elaboration to be integrated in a hydroconversion process. In this perspective, introduction of the electrodes in the feeding pipe of the reactor must be envisaged. Thus, evaluating the impact of the properties of the liquid feed on the preparation parameters was one of the first concern of the program. Hydrocarbons ranging from atmospheric residue to pentane were investigated. The corresponding yields of gases, carbonaceous solids and metal obtained with nickel electrodes are presented in table 1. Gas production increases slightly as the volatility of the liquid medium augments. But there is not a direct correlation. The gas is essentially composed of hydrogen (60%) and acetylene (22%), the remaining gases being C 1 to C 4 hydrocarbons. This composition is constant and is not affected by changes in the composition of the liquid hydrocarbon. A dramatic effect is observed concerning the production of solid carbonaceous material which is considerably decreased when a light hydrocarbon is added, even at low concentration. The presence of as low as 10% pentane in an atmospheric residue (A.R.) causes the production of solids to decrease from 40 to 3.4 mmole/hour. Pure pentane gives the same rate of production as a mixture containing 10% pentane in A.R.. In contrast, addition of decane to the same A.R. does not cause a noticeable effect. The production of metal obtained with nickel electrodes in various hydrocarbons is apparently not related with the properties of the liquid, presumably because the process of volatilization at the electrodes surface is essentially controlled by metal volatility.
Table 1 Yields of gas, solid and metal obtained with various hydrocarbons expressed in mmole/hour (Nickel electrodes, spacing 0.5mm) Hydrocarbons
Gas
Solid
Metal
Atm. Residue (AR) Vacuum distillate AR + decane AR + pentane Pentane
44.6 56.7 58 76.3 84.4
39.8 49 44.6 3.4 3.4
0.14 0.11 0.11 0.19 0.18
Metal content (w %) 1.65 1.07 1.17 25.8 25.3
499 The very high yields of carbonaceous solids and the low metal contents obtained when the electrical discharge is established into a heavy hydrocarbon exclude the generation of a catalyst precursor directly in the feeding line of a hydroconversion reactor; it would produce an excessive amount of carbonaceous solids. It must be kept in mind that the catalyst is destined to conversion of residues, therefore the introduction of excessive amount of useless material must be kept to a minimum. 3.2. Influence of electrodes material Table 2 presents the material balance obtained with three different combinations of electrodes material: two nickel electrodes, two molybdenum electrodes, and a nickel facing a molybdenum electrode.
Table 2 Yields of gas, solid an metal (mmol/hour) as a function of electrodes material (electrodes spacing 0.5ram) Electrodes
Gas
Solid
Metal
Ni-Ni Mo-Mo Ni-Mo
55.4 71.9 75.9
6.9 3.48 5.71
0.28 0.09 0.18
Metal content, % 19.53 20.7 19.7
The low volatility of molybdenum causes the yield of metal to decrease strongly when this metal is used as electrode material. Using a constant electrodes geometry, the presence of molybdenum results in an increased gas yield and a reduction of solid production. The ratio of metal to solid production is constant, as indicated by the invariable metal content of the prepared catalysts. Concerning the catalytic activity, introducing molybdenum in the catalysts results in some modifications of the performance in hydroconversion of a deasphalted vacuum residue (DVR). Figure 1 summarizes data activity of catalysts comprising pure nickel or molybdenum and a mixed Ni-Mo catalyst, according to several analytical criteria evaluating the hydrogen transfer activity and hydrocracking properties. Non-catalytic conversion results in hydrogen depletion of the liquid products. Hydrogenation activity is improved by the presence of Mo, as reflected by the increased hydrogen incorporation and reduced aromaticity of the products. In parallel, hydrocracking activity is limited as indicated by the reduction in gas production during hydroconversion. 3.3. Influence of electrodes geometry Variations in inter-electrodes volume affect material production. The data plotted in Figure 2 indicate that metal production remains unaffected and that gas formation correlates with inter-electrodes volume. The yield of solid is slightly affected and parallels metal production.
500 120 100
200loo. .---4 O
O" -100.
,:~
80
~
60
I~
-200.
0,4 0,3 0,2
40
0,1
20 0
0
Ni- Mo- NiCat. Ni Mo Mo
5
[] H2 trans.fer 9 Aromatics I Figure 1. Hydroeracking of influence of electrodes material.
DVR;
_~ -~
Gas
20 40 80 Volume(mm3) ~
(X10)S~ --.O-=-,Metal
Figure 2. Yields of gas, solids and metal as a function of inter-electrodes volume.
The metal content of the resulting solids is constant, ranging between 19.5 and 21%. The constancy of metal content, whatever the nature and geometry of the electrodes, would indicate that carbonaceous solid production, like metal volatilization are essentially surface phenomena occuring at the hydrocarbon electrodes interface. Energy dissipation is related with electrodes geometry as illustrated by figure 3. Owing to the appearent parallelism of energy consumption and gas formation, both increasing with inter-electrodes volume, a primary conclusion is that increasing this parameter only gives rise to excess gas production. The energy balance expressed in kJ per gram mole of products, is effectively constant as indicated in table 3. However, electrodes geometry has some influence upon activity of the generated catalysts in hydroconversion of DVR.
Table 3 Energy consumption as a fuction of inter-electrodes volume. Volume (mm 3) Energy (kJ/mole)
4.9 970
19.6 990
39.3 950
78.5 930
The performance of several nickel-based catalysts, obtained at various inter-electrodes volumes, are compared in figure 4. As inter-electrodes volume is increased, hydrogenation activity is improved and gas formation is reduced.
501
30
y
25
20 t.., cD
,,---4r
15
O
I
O
10 cat.
0
20
40
60
/-.,U
40
80
80
Volume (ram3)
IGas Figure 3. Dissipated power as a function of inter-electrodes volume
El H2 transfer II Aromatics ]
Figure 4. Hydrocracking of DVR; influence of inter-electrodes volume.
3.4. Characterization of plasma prepared catalysts Electron microscope (EM) examination of plasma solids evidences a very wide distribution of nickel particle size. In the greater part of the examined areas, nickel particle diameter lies within the range of the detection limit of EM (0.3 nm). EDX analysis performed in selected areas where apparently no nickel particles are visualized indicates the presence of nickel. On the other hand, some very large particles exist in some areas; their apparent diameter can be as large as 500 nm. After sulfidation, no change in the distribution of the particle size is evideneexl: some large particles coexist along with a population of evenly spread-over clusters. EDX analysis was performed over several zones of variable area in order to determine the sulfur-to-nickel ratio as a function of the localization. The resulting data are summarized in figure 5. When the analysis concerns the carbonaceous support with no detected nickel, sulfur is absent. Very large particles are not sulfided and sulfidation proceeds as the nickel particles are smaller. Particles behind the detection limit of EM exhibit excess sulfur, compared to the expected stoichiometry NiS2. Then, sulfidafion does not fundamentally modify the characteristics of the plasma precursors. The catalytic system issued from the sulfiding treatment consists of a mixture of phases, existing in a wide range of particle size, the chemical structure of which depends on their dimensions. The main part of this population is constituted by very small aggregates which are easily sulfide& As the particles become larger, the difficulty of sulfidafion increases; very large particles (some hundred nm in diameter) are not sulfided. However, the contribution of these ensembles to the total population of nickel species is small, as evidenced by the mean chemical composition determined by bulk analysis which
502 corroborates the mean stoichiometry NiS2.
1000 .~ CO g~ o,-.i
~o
100 10
CD
0,1 0
0,5
1
1,5
2
2,5
S/Ni
Figure 5. Stoichiometry of nickel sulfide as a function of particle size.
4. CONCLUSION A synthetic catalytic system designed as a dispersed disposable catalyst for hydroconversion of petroleum residues has been prepared. It is obtained by a plasma discharge sparking between two metal electrodes immersed in a light hydrocarbon. This operation generates a metal vapor that condenses and is trapped onto the carbonaceous substance resulting from the pyrolysis of the hydrocarbon. The material obtained by this method, using nickel as the electrodes metal and pentane as the hydrocarbon, has been characterized. Although some large particles are stripped from the electrodes, their contribution to the distribution of particle size is probably small, as evidenced by electron microscopy and EDX analysis. The main part of the metallic species is present as very small particles whose size is in the range of the detection limit of EM (0.3 rim). These particles are easily sulfided, during hydroconversion of a vacuum residue, and their state of dispersion is retained. This solid possesses the properties required for a dispersed catalyst intended to be used in hydroconversion of heavy petroleum feeds: the active phase is highly dispersed and its dispersion state is preserved from sintering by a high surface area support; its production could be easily integrated to the conversion process, without the need for a separate preparation step. A variety of active phases can be generated changing the electrodes material.
503 AKNOWLEDGEMENT This work was supported by SHELL France. We particularly aknowledge the fruitfull collaboration of J. Dufour and P. Moureaux.
REFERENCES
1. M. AndrOs, H. Charcosset, L. Davignon, G. Djega-Mariadassou, J.P. Joly and S. Pr6germain, Fuel, 62 (1983) 69. 2. P.C. Eklund, J.M. Stencel, Xiang Xin Bi, R.A. Keogh and F.J. Derbyshire, Preprints Am. Chem. Soc., Div. Fuel Chemistry, 36 (1991) 551. 3. A. Coteron and C.N. Kenney, Appl. Catal. A, 95 (1993) 237. 4. R. Bearden and C.L. Aldridge, Energy Progress, 1 (1981) 44. 5. M. Breysse and al, Catalysis Today, 4 (1988) 39. 6. G. Djega-Mariadassou, M. Besson, D. Brodzki, H. Charcosset and Tran Huu Vinh, Fuel Process. Technol., 12 (1986) 143. 7. J.J. Dufour, Eur. Pat. Appl. EP 0 429 132 (1991) to Shell Int. 8. L. Rouleau, R. Bacaud, M. Breysse, and J. Dufour, Appl. Catal. A, 104 (1993) 149.
This Page Intentionally Left Blank
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparationof HeterogeneousCatalysts G. Ponceletet al. (Editors) 9 1995 ElsevierScienceB.V. All rights reserved.
505
P r e p a r a t i o n a n d S t r u c t u r a l P r o p e r t i e s of U l t r a f i n e G o l d C o l l o i d s for Oxidation Catalysis Daniel G. Duff,* Alfons Baiker
Department of Chemical Engineering and Industrial Chemistry, Swiss Federal Institute of Technology, ETH-Zentrum, CH-8092 Ziirich, Switzerland. The preparation of noble metal catalysts via colloidal solutions (sols) of the metals has been investigated in recent years, because the method promises enhanced control over particle size and morphology, with direct preparation of the metal component by reduction of the metal ions in solution, independent of the nature of the support. Low-temperature oxidation catalysis by gold particles, however, is reported [1] to rely on the particles being smaller than usually achieved by preparations of colloidal sols of gold. We have conceived a synthesis employing a highly effective nucleating (reducing) agent, tetrakis(hydroxymethyl)phosphonium chloride (THPC), which produces exceptionally fine gold particles of about 1.5 nm mean diameter [2]. These nanoparticles can then be used as nuclei for the preparation of coarser catalytic gold colloids. 1. GOLD NUCLEI The preparation of the standard hydrosol involves reduction by partially hydrolysed TtlPC [chemical formula P(CH2OH)4+C1 -] of chloroaurate (III) ions in aqueous solution, resulting in the rapid formation of a clear dark orange-brown solution [2]. Following extensive dialysis to remove true solution species from this sol, chemical analysis of the colloid following dehydration showed that 7.0% by weight P, 6.3% C and 1.2% H remained in the colloid. Estimation of particle size was carried out using a variety of techniques, including transmission electron microscopy (TEM), analytical tfltracentrifugation, X-ray diffraction line broadening and crystal growth experiments. TEM images of particles deposited from the standard sol yielded an (number) average diameter
of 1.5 nm (mass-weighted mean 2.2 nm), and a coefficient of variation (c/) of 38 %, but some cluster coalescence within the specimen was observed. This caused bands of much coarser particles (<10 nm) at locally high concentrations to be present in the specimen. Only the well separated clusters were measured for the distribution presented here. The addition of the colloid-stabilising agent gelatin to the hydrosol prevented the formation of these bands of presumably * present address: Zentrale Forschung-TTP1, Grenzfl~ichenphysik, Geb. E41, Bayer AG, D-51368 Leverkusen, Germany.
506 coalesced species, but still a much slower cluster coalescence process could be imaged, occurring on a timescale of tens of seconds electron beam irradiation at small particle separations. In agreement with this the cluster size distribution measured from micrographs from a sol treated with gelatin (0.1 m g / m l sol) showed a tail to high particle diameters not present when only the well separated particles from an untreated sol were measured. . . . .
without gelatin 30
~9 ,
with gelatin -
60
A
o~ 25
-50 m ,..,,,= ~
0
o 20
-40 ~"
~15
-30 .~ e-
t,.(D :D
10
- 2 0 0~
r
13.
5 "-O
1
2
3
4
5
6
7
8
Particle diameter (nm) Figure 1: Particle size distribution from TEM for the sol with and without l m g / m l gelatin stabiliser present. Because of the uncertainty surrounding TEM images of these smallest nanoclusters, other methods for obtaining particle size information were applied to the sol. The optical extinction in the ultraviolet-visible region of the spectrum (UVvis) shows a trace of the plasmon feature typical of gold colloids, strongly suppressed due to finite-size effects [3] (Fig. 2, curve A). The spectrum resembles that of other gold cluster systems estimated at about 2 nm mean diameter [4], 1 - 2 nm [5] and about 1.2 nm [6]. Sedimentation velocity measurements of the standard sol in the ultracentrifuge gave a minimum value of (weighted) average diameter of 1.4 nm. Clusters of larger average diameter than 1.4 nm could also possess the same sedimentation velocity if a compensating surface-adsorbed layer of non-dense species (e.g. solvent, reducing agent) were present m e.g. 2 nm gold clusters with 1 nm surface-adsorbed layer would also be compatible with the measured velocity [2]. Powder X-ray diffraction of the gold particles dispersed on alumina support produced a very broad peak at 20 = 40.0 ~ after subtraction of the background pattern from the alumina (see Fig. 3). Recent Debye Function Analysis (DFA) shows this pattern to be consistent with decahedra (decahedral multiply twinned particles, MTPs) of about 2.2 nm m a s s - m e a n diameter [7]. X-ray diffraction from a
507
Extinction coefficient (dm 3 g-atom "1 cm -1) 5O00
-
C
A
'
200
I
'
300
I
'
400
I
'
I
500
'
600
I
'
I
700
800
Wavelength (nm) Figure 2. The optical extinction spectrum of three sols: A. The ultrafine dispersion of gold clusters (nuclei for catalyst preparation) = 1.5 nm B. The coarser sol of gold catalyst particles -- 3-5 nm arising from coalescence of gold clusters in A, induced by boiling the sol. C. A gold sol prepared by the traditional citrate method, = 15 nm k9
.k
0r~ t-:3
co
v
,m O0 r
(D t"
. . . .
20
I
30
. . . .
I
40
. . . .
I
. . . .
50 2-theta
I
60
. . . .
I
70
. . . .
I
'
'
80
Figure 3. Powder XRD pattern of A. alumina 'C', B. alumina with 20 wt% Au clusters, C. subtraction to yield the contribution of the gold nanoparticles alone. concentrated sol sample in transmission mode gave a similar broad feature to the pattern for the supported clusters.
508 Growth of the clusters to a size more easily measurable by TEM, employing a growth medium containing gold (III) ions and the reducing agent hydroxylamine hydrochloride, allowed a limiting maximum value of mean diameter of 2.4 nm for the original THPC gold sol to be deduced from TEM measurements of the grown colloids. Coalescence of the primary nucleus particles during this chemical development process is indicated by the observation that the calculated nucleus diameter is larger for higher concentration of these 'seed' clusters (Fig. 4). A
E6 r
,-
5
O
$
E4 (I:1
/
~( / J 3
2 r-
_Ib
1
t"
~o
o
0 <
f
I
I
I
2000
4000
6000
8000
[Au] present as nuclei (nM)
Figure 4. Graph of calculated nucleus size against concentration of gold in the added "nuclei" The results of the various particle size measurements are compared in the Table. They are in agreement as showing systems of gold clusters of between I and 2.5 nm mean diameter. The recent DFA interpretation of the XRD powder pattern of the clusters on support additionally rules out that the gold lattice is to any significant extent penetrated by, say, phosphorus atoms - - i.e. they are essentially clusters of elemental gold [7]. Table Comparison of sizing data for the sol of gold clusters formed by the THPC-method Method
Parameter measured
Weighting
Mean metal-core diameter (nm)
TEM chemical development ultracentrifugation powder XRD [7] W-visible
metal core diameter number of particles metal core and coat metal core (simulated) metal core (qualitative)
none none (vol. avged) ~-volume= volumestrong
--- 1.5 < 2.4 > 1.4 -- 2.2 1.0 - 2.0
509 2. GOLD CATALYST PARTICLES Coarsening of the system is easily accomplished by boiling the sol of gold clusters in air (for e.g. 2 h) after addition of trisodium citrate to I mM concentration. One thus prepared red sol was measured at = 4.4 nm, c.v. = 20 % (see Fig. 5). 30
-'1
o 25-
I
t"
___.1
o- 2 0 -
I I
L_
~15c~ t-- 10_ to L._
13.
50 0
I I I
I I I "1 I
I
i
I
I
I
I
l
1
1
2
3
4
5
6
7
8
9
Particle diameter (nm) Figure 5. Particle size distribution of a boiled (coarsened) sol. The good fit to a lognormal is typical for a system formed by coalescence of primary particles (nuclei) [8]. The dashed histogram shows for comparison the size distribution of an asprepared sol of the primary gold clusters before boiling, as in Fig. 1. High resolution (transmission) electron microscopy (HREM) of the particles from this sol showed lattice detail ({111} and {200} fringes) and particle structures (including, rather rarely, MTPs [9]) typical of colloidal gold microcrystallites. The UV-vis extinction spectrum of this boiled (coarsened) sol showed the expected strengthening of the plasmon feature due to the increase in particle size and concomitant decrease in finite-size effects [3] (see Fig. 2, curve B). These systems were loaded onto ultrafine support powders such as alumina or titania using the principle of heterocoagulation- aqueous dispersions of the support were prepared and, after any necessary pH adjustment, the gold sol was added resulting in adhesion of the negatively charged gold particles separately to the positive support matrix. This procedure is in general useful for supports of isoelectric point (IEP) at high and medium values of pH, for example A1203 (IEP = pH8) and TiO2 (IEP = pH5). At pH-values below the IEP the oxide will be positively charged in water (assuming only indifferent, non-adsorbing electrolyte to be present) and adhesion of negatively charged colloids/clusters to high concentration with good dispersion can occur. HREM images showed that this took place without significant change in the gold particle size, a good separation between the gold clusters being maintained (Fig. 6).
510
Figure 6. HREM of 5-wt% gold on TiO2 P25 catalyst (calcined, 300~ oxygen, 30 min); micrograph courtesy of R. Wessicken.
flowing
The effect on the gold particle size characteristics of calcination in air could be followed with a combination of UV-visible diffuse reflectance spectroscopy, monitoring the evolution of the surface plasmon absorbance, and TEM. The well established Kubelka-Munk relation for the diffuse reflectance of non-translucent films of fine powders gives the following for each wavelength: F(R) = (1 - R) 2 / 2R = K / S, provided a number of conditions are established [10], where R is the diffuse reflectance spectrum, K is the absorption coefficient and S the scattering coefficient of the sample. In the case of gold nanoparticles on alumina, where S(Au) = 0 and K(A1203) -- 0, the Kubelka-Munk spectrum, F(R), is essentially the gold surfaceplasmon absorbance, sensitive to changes in particle size, shape and environment [9], divided by the scattering spectrum of the alumina powder (which can be assumed to be monotonic over 400-700 nm for such fine particles). In Figure 7 we see the change in K/S upon calcination of a particular catalyst (prepared from a gold sol treated with ion-exchange resin). The increase in sharpness of the plasmon peak to that resembling a classical gold colloid of 15 nm is in agreement with TEM observations which show the mean particle size to have increased to about 20 nm. In many other cases it was possible to calcine the catalysts with only a very small change in plasmon-peak shape. This technique of diffuse reflectance provides a quick and qualitative check of the particle size characteristics of catalysts containing gold (or other group 1B metals) on white support powders, and can be used as a complement to TEM observations. The advantages are that it can be used on systems too dilute or
511
ultrafine for simple Scherrer analysis of X-ray powder patterns, and is also possible on systems where the metal clusters are susceptible to electron beam damage in TEM.
F (R) 9 8 7 6 5 4
~
3 21-
0-400
450
500
550
600
650
700
Wavelength (nm) Figure 7. Kubelka-Munk transformed diffuse reflectance spectrum of (A) untreated and (B) calcined catalyst 10-wt% Au on aluminium oxide "C"; calcination conditions 400~ air, 4 h. The change in the plasmon absorbance of the supported gold colloid due to change in particle size can be clearly seen; c.f. Figure 2 curves B and C. (the Kubelka-Munk function F(R) is not the simple absorbance spectrum but is divided by the scattering spectrum of the white alumina support, which is normally assumed to be monotonic: F(R) = K/S) Preliminary catalytic tests indicated that the titania-supported gold particles show an interesting potential for low-temperature CO-oxidation. The activity was found to depend strongly on pretreatment conditions. Note that a strong dependence of the CO-oxidation activity on the pretreatment conditions was found for titania-supported gold catalysts prepared by the incipient wetness method [11] . Hence this behaviour should not be considered characteristic for the gold-sol derived catalyst. Studies are in progress to optimise the pretreatment conditions and to compare the performance of the Au/TiO2 catalyst prepared from the gold hydrosol with similar catalysts prepared by impregnation and coprecipitation. REFERENCES
1. H. Haruta, T. Kobayashi, H. Sano and N. Yamada, Chem. Lett., (1987) 405. 2. D.G. Duff, A. Baiker and P.P. Edwards, Langmuir, 9 (1993) 2301; D. G. Duff, A. Baiker, I. Gameson and P.P. Edwards, Langmuir, 9 (1993) 2310. 3. U. Kreibig and L. Genzel, Surf. Sci., 156 (1985) 678.
512 4. K. Fauth, U. Kreibig and G. Schmid, Z. Phys. D, 12 (1989) 515; U. Kreibig, J. Physique, 38 (1977) C2-97. 5. E.B. Zuckerman, K. J. Klabunde, B. J. Olivier and C. M. Sorensen, Chem. Mater., 1 (1989) 12. 6. G. Schmid, R. Pfeil, R. Boese, F. Bandermann, S. Meyer, G. H. M. Calis and J. W. A. van der Velden, Chem. Ber., 114 (1981) 3634. 7. W. Vogel, D. G. Duff and A. Baiker, submitted to Langmuir. 8. C.G. Granqvist and R. A. Buhrman, ]. Appl. Phys., 47 (1976) 2200. 9. A.I. Kirkland, P. P. Edwards, D. A. Jefferson and D. G. Duff, Ann. Rep. Prog. Chem. C, 87 (1990) 247. 10. G. Kort/~l, Reflektanzspektroskopie, Springer, Berlin, 1969. 11. S.D. Lin, M. BoUinger and M. A. Vannice, Catal. Lett., 17 (1993) 245.
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
Synthesis, c h a r a c t e r i z a t i o n
and
catalytic
513
activity
of
manganese
oxidic
nano-particles Costas S. Skordilis and Philip J. Pomonis. Department of Chemistry, University of Ioannina, Ioannina 45332, Greece. Manganese oxidic nano-particles have been prepared via hydrolysis of a trinuclear manganese
complex
[Mn30(CH3COO)6(pyr)3]C104 . The
amorphous and crystallize to Mn20 3 at T,500~ 150-200m2g -1 at 200~176 at 650-700~
obtained
particles
are
Their BET specific surface area is
and diminishes to 50m2g -1 at 500~
and 10m2g-1
The pore volume distribution of the particles heated at 200~
shows a
maximum at 5.Snm while almost 95% of the pores lies between 4 and 12nnm In SEM it seems that the obtained particles are almost spherical with diameter 0.1-0.21m~ The obtained particles are active for the N20 decomposition, which was used as a probe reaction, above 300~
Activation energies calculated assuming first order kinetics
are around 100 kJ/moL Comparison with other catalysts containing manganese in oxidic
form,
precipitation
like
perovskites,
conventionally
prepared
manganese
oxide
by
of Mn(OH)2 and manganese oxidic nano-particles supported on clay,
shows that the activity of the nano-particles, calculated either per unit mass or unit area of active phase, is higher as referred to the other systems.
1. INTRODUCTION The fabrication of small particles, of oxidic or metallic nature, having uniform composition and shape, has attracted considerable interest for several reasons [1]. They perform unusual properties because there is an extensive number of atoms located on the surface of them. Their technological importance is well recognised because of their wide use in magnetic recording media, diagnostics in medicine, in ceramic manufacture, in the preparation of paints and as catalysts [2]. As far as catalytic nano-particles concern one of their more important characteristics is the high dispersion and the increased surface area which favours heterogeneous catalytic reactions carried out on them [3-5].
514 A survey of the literature relevant to preparation of nano-particles indicates that the main element examined is iron [L7-10] a fact which is related with the fabrication of magnetic devices [11-13]. Manganese based nano-particles appear scarcely in the literature and usually in combination with iron [3, 8]. In other cases high temperature and pressure hydrothermal conditions lead to manganese oxides with tunnel structures having high surface area [6]. Such porous manganese based structures attracted considerable interest for use in batteries technology [14,15]. The present work deals with manganese oxidic nano-particles prepared by a novel technique, namely the hydrolysis of a multinuclear manganese complex [16] as well as their catalytic activity v i s a vis other manganese containing compounds like perovskites and conventionally prepared manganese oxide.
2. EXPERIMENTAL 2.1 Preparation of the nano-particles. The preparation of manganese based nano-particles took place by controlled hydrolysis of a trinuclear manganese complex [Mn30(CH3COO)6(pyr)3]CIO 4 [171. The preparation of the trinuclear complex took place along the lines suggested in ref. 17 and is briefly as follows: 2.00g of Mn(CH3COO)2.4H20 (6.15mmoles) were dissolved in a mixture of 20ml EtOH, 3ml pyridine and 12ml glacial acetic acid. In this mixture 1.14g (3.15mmoles) of N-nBu4MnO 4 were added under intense stirring. Then addition of 0.69gr (5.65mmoles) of NaCIO4 results in precipitation of a brown mass of the complex which was subsequently filtered and washed by ethanol Recrystallization was followed
from
acetone.
The
obtained
brown
solid was
examined for its IR spectra which was found similar to that reported in [17]. The prepared complex was dissolved slowly in water and as a result of hydrolysis a brown precipitate was obtained consisted of small particles. They were separated by filtration, dried at ll0~ temperatures (200-700~
for 24 h and heated subsequently at different
Portions of the precipitate were left for digestion up to
25 days to examine the possible effect of aging on the obtained particles. In a different experiment hydrolysis took place in a bath containing 10-4M sodium hexameta-phosphate as a de-coagulation factor, but its effect proved negligible. Small manganese oxidic particles supported
on montmorillonite 45% w/w were also
prepared, by performing the hydrolysis procedure in 1% dispersion of clay in water. The particles
prepared
as described
above
were
characterized
as
next.
For
515 comparison, a sample of Mn(OH)2 was conventionally prepared through precipitation by NH 3 from a Mn(NO3) 2 solution and heated thereafter at various temperatures. 2.2 Characterization of the particles. Thermogravimetry. The dried precipitate (110oc) was tested for its gravimetric
behaviour in a thermobalance TRDA3H of the Chyo Balance Co., from RT to 750~ The rate of heating was 5deg.min -1 and the experiment took place under a flow of 30mLmin -1 air. The obtained signals of TG and DTA are shown in Figure 1. X-Rays. The XRD patterns of the nano-particles heated at 350~
and 550oc are
shown in Figure 2. The system used was a Siemens diffractometer with Cu anode (L=0.154nm).
L
i
I
i
I
i
I
|
-1
i
1 _ "
eight loss
"
0 .~176 __
e~0
~0.9 r~
550~ -0.7~
0.8 DTA I
0
350~
" I
200
I
I
400
,
I
600
,
0.6 v 800
!
10
Temperature (q2)
!
!
30
i
50
!
!
70
20
Figure 1. DTA and TG results of the precipi-
Figure 2. X-rays patterns of the hy-
tate obtained after hydrolysis of trinuclear
drolysis products heated at 350~
complex.
and 550~
Surface area and pore structur~ The BET specific surface areas (m2g -1) of the nano-
particles left for digestion at various times (0.Sh, 8h, 24h and 25d) and dried thereafter at ll0~
200oc, 300oc, 400oc, 500oc and 700~
were determined by the
single-point method of N2 adsorption at T=77K using a Carlo Erba 1750 Sorpty apparatus. The results are shown in Figure 3.
516 The pore size distribution of a sample heated at 200~
was calculated f r o m the
desorption branch of N 2 at 77K The apparatus employed was a Coulter Omnisorb system and the results are shown in Figure 4. S E M m e a s u r e m e n t ~ In order to measure the size of the prepared nano-particles we
took SEM photographs. Two of t h e m are shown in Figure 5.
250 ..... 2007 ~0 150 -
'
I
'
I
'
I
'
,.~. 400
9igestiol :imes
f'h'
0
"-'200
-o- o.5
--
8.0h
~'
"~
~
e,,l
24h 5d
#
0
05 P/Po
1
= lO0--
ra~
500 0
i
I
200
i
I
400
i
I
600
~'
:~ :3 ~; 1'0 2'0
800
T e m p e r a t u r e (~
10'0
Rp (nm)
Figure 3. Variation of the specific surface
Figure 4. Pore size distribution of the
areas of the hydrolysis products a f t e r se-
nano-particles heated at 200~
The
veral digestion times and heated at the
adsorption and desorption isotherms of
indicated temperatures.
N 2 at 77K are also shown.
2.2. Catalytic experiments The prepared particles in pure form, as well as supported on a clay surface, were tested
for
their
catalytic
activity,
using
decomposition. The samples were heated at 450~
as a probe
reaction
the
N20
for 4 hours before the catalytic
tests. The experiments took place in a bench scale plug flow reactor (PFR) connected with a gas chromatograph Shimadzu CR6A for the analyses of reactants
and
products. Similar systems have been described elsewhere [18-22]. The total flow of the gases through the reactor was 100 ml.min -1 in a ratio N20:He = 0.25. The catalyst mass put into the bed was 0.200g. The variation of the reaction rate R(moles of N 2 0 reacted per sec or per m 2 of the catalyst) with t e m p e r a t u r e is shown in Figure 6.
517
Figure 5. SEM photographs of the manganese oxidic particles. Magnifications xl0000 and x35000
518 In Figure 7 the Arrhenious plots are shown which were obtained assuming first order kinetics R = kPN20 and using the design equation of the plug flow reactor Fdx = Rds (F=feeding rate, mols.sec -1, S-surface area). After the relevant manipulation the following equation is easily produced [20-22] from which the activation energies of the reaction are easily calculated. ln[-ll ln(1-x)-x] = C - E a / RT
(1)
where x is the conversion, E a the activation energy, C constant depending on the experimental conditions. 8 ,
'
,
'
,
-0- Mn3 Mn-c MoMn3
~ 6 "~ ~
'
, |
I ,
(a) q I -~
LaMn03
t
~'
'
,
'
,
'
' I
)1
H_l_ Mn_c 0.4 [1"- MoMn3
~~
-]
~"o" , LaMa03 T
d~t
~ 4
i ~
i 2
0.
~
". 300
0 400 500 Temperature ( o C)
600
300
400 500 Temperature (oC)
600
Figure 6. Catalytic activity data expressed as rate of reaction R (a) per unit mass (g) or (b) per unit surface (m 2) of the noticed catalysts, as a function of temperature. Figures 6 and 7 contain for comparison the catalytic activity data of the following catalysts: - Mn2
Small manganese nano-particles prepared as described above, by hydrolysis of the trinuclear complex
- Mnc
Manganese oxide prepared by precipitation of Mn(OH)2 from Mn(NO)3 with NH 3 and heated at 450~ for 4h to form MnO x.
-
-
MoMn2 LaMn03.
Small manganese oxidic nano-particles supported on montmorillonite. Perovskite LaMnO 3 containing manganese [23].
A comparison of the catalytic activity expressed as the temperature needed to achieve the came conversion for the above catalysts is shown in Figure 8. For the
319 MoMn3 catalyst the activity has been calculated per mass of s u p p o r t e d m a n g a n e s e oxidic nano-particles and their corresponding surface area. 15
, A
Ma3
I0
v
Mnc
o
MoMa3
i
I
'
I
,
I
,
300
I
I
,
I
I
I
. . (a)
400 i rj 500
0
600
5
I (b)
500 ,
-5
12
13
I
t
1.4
I
t5
t
600
I
t6
I Mn3
t Mnc
t
LaMn03 MoMn3
Catalyst
IO00/T Figure 7. Arrhenius plots for catalysts
Figure 8. T e m p e r a t u r e necessary (a) for
Mn3, Mnc and MoMn3.
the same conversion (x=5%) and unit mass (g) of the catalysts and (b) for the same conversion (x=20%) and unit surface (m 2) of the catalysts.
Table 1 Some characteristics of the catalysts tested. Catalyst
ssa(m2/g)
Ea (kJ/mol)
Mn3
75 (450~
90
Mnc
24 (450~
88
MoMn3
65 (600~
92
LaMnO3
11 (650~
106
3. DISCUSSION F r o m Figure 3 it is clear that the obtained particles possess a very high surface area 150-200m2g -1, in
the
temperature
range
200~176
if
compared
to
the
520 conventionally prepared MnO x which does not exceeds 40m2g -1 at 300~
Above
400~
the surface area decreases to 50m2g -1 at 500~
650~
The DTA curve in Figure 1 of the nano-particles shows an endothermic effect
at 450-500~
and further to 10m2g -1 at
which is relevant to the drop of the surface area. The XRD patterns
(Figure 2) indicate that the hydrolysis products appear amorphous at T,500oc and crystalline
above
550~
corresponding
to
the
formula
Mn20 3.
observations mean probably that the endothermic effect at 450-500~
The
above
is provoked
by the crystallization of the particles to Mn20 3. This crystallization is accompanied by an abrupt drop of the surface area. In the adsorption-desorption isotherm (Figure 4) an hysteresis
loop is apparent
and
characterized
as type-IV
of
BDDT
classification corresponding to mesoporous solids [24]. We also notice that the BET N 2 surface area calculated using the data of Figure 4 is found 195m2g -1 in perfect agreement with the BET single point measurements. The pore size distribution curve show a peak at 5.5nm and a total pore volume 0.65 mug. The SEM pictures of the particles at xl0.000 and x35.000 magnification in Figure 5 shows that the primary particles possess a size 0.1-0.21xm and exhibit a more or less spherical shape. Nevertheless it is also apparent that such primary particles aggregate to secondary particles containing several of them. Such aggregates should almost certainly possess an open-pore structure although it remains the problem of the closeness of the packing and therefore the pore diameter of the aggregates and/or agglomerates. The results described above, indicate that primary particles, which consist the aggregates, are bounded in such a way that the solid appears mesoporous with maximum pore radius 5.5nm. On the other hand if the measured specific surface area (-200 m2g -1) was due only to the external surface of the particles, then assuming a density value of 2.25gr.cm -3, the mean particle diameter should be d=10nm, or d=10-21xm. This value is one order of magnitude smaller than the observed by SEM (Figure 4) diameter of particles (d=0.1-0.2~tm). These observations imply, that with the intervention of the organic fragments, small particles are produced and stack together forming porous of radius from 4.0 to 10.0nm with a maximum at 5.Snm. In Figure 9 a model that is fitted on the above calculations is illustrated. The material remains porous and amorphous up to 500~
but after heating at 550~
the
individual particles suffer sintering and loose their surface area and porosity transformed thus to crystalline solid. The obtained manganese nano-particles show higher catalytic activity compared to the other catalysts tested for the N20 decomposition.
521
dnenoperticle = 1Onto
(calculated by surface area) . . = .1. lam
~ O
.
.
.
.
.
.
(measured in SEM ~ - ) ~ v / ' photographs)
dpore = 5.5 nm ( calculated by adserptiondeserption measurements)
Figure 9. Model describing the packing of nano-particles. Especially if the catalytic activity is expressed per gram of catalyst (Figures 6a and 8a) they are obviously better catalysts as compared to the conventionally prepared MnO x, the manganese oxidic nano-particles supported on clay and the manganese perovskite LaMnO 3. This is due mainly to their high surface area. If their catalytic activity is expressed per surface of catalyst (Figures 6b and 8b) the difference from the other samples is decreased but the manganese oxidic nano-particles are still more effective. Probably the procedure of particle growing during hydrolysis is responsible for the creation of more capable active centres on the surface of the nano-particles for the N20 decomposition. The values of activation energy are close to each other in a range of 88-106 kJ/mol. This novel method of fabrication of manganese oxidic nano-particles seems to be very promising, especially for applications that need high surface areas such as adsorption
and catalytic applications. The mechanism of hydrolysis and growing of
the particles is not still completely understood and to this point future work is already on the way in order to reveal the factors that influence the formation and the properties of the nano-particles. Aknowledgements: We wish to thank Professor P.Koutsoukos for the SEM photographs and the STRIDE HELLAS-33 program of the E.U. for financial assistance.
REFERENCES 1. NATO
ASI
"Nanophase
Materials:
Synthesis-Properties-Applications",
Greece, 20 June - 2 July 1993. 2. E.Matijevic, Mater.Res.Bul., XIV, 19(1989).
Corfu,
522 3. H.Itoh, S.Nagano, T.Urata and E.Kikuchi, Applied Catalysis 77, 37, (1991). 4. H.Itoh, H.Hosaka, T.Ono, and E.Kikuchi, Applied Catalysis 40, 53 (1988). 5. W.P.Hettinger, Intern. Patent. AppL 9207044, (1992). 6. Chi-Lin O'Young "Symposium on Advances in Zeolites and Pillared Clay Structures", AnlChenlSoc. New York, August 25-30, 1991, pp 348-355. 7. E.Matijevic, Acc.ChenlRes., 14, 22, (1981). 8. Z.X.Tang, C.M.Sorensen, K.J.Klabunde and G.C.Hadjipanayis, J. Colloid Interface Sci, 146, 38, (1991). 9. G.M.Sutariya, R.V.Upadhyay, R.V.Mehta, J.Colloid Interface Sci., 155, 152, (1993). 10. M.Suzuki, M.Kagawa, Y.Syono, and T.Hirai, J.Mater Sci., 27, 679, (1992). 11. S.Mann, and F.C.~VIeldrum, Adv. Mater. 3, 103, (1991). 13. A.J.Kramer, J.J.M.Janssen, and J.A.A.J.Perenboom, IEEE Trans Magn, 26, 1858, (1990). 14. C&EN, Jan. 4, 34, (1993) 15. M.A.G.Avanda, J.P.Ahfield and S.Bruque, Angew.Chem.Int.Ed.EngL 31, 1090, (1992). 16. C.S.Skordilis and P.J.Pomonis, J. Colloid Interface Science, (accepted for publication) (1994). 17. J.B.Vincent, H.R.Chang, K.Folting, J.C.Huffman, G.Christou, D.N.Hendrikson, J. Am. Chen~ Soc. 109, 5703, (1987). 18. A.K.Ladavos and P.J.Pomonis, J.ChenlSoc Faraday Trans. 87(1991)3291. 19. A.K.Ladavos and P.J.Pomonis, Applied CataL B: Enviromental, 1(1992)101. 20. D.E.Petrakis, P.J.Pomonis and A.T.Sdoukos, J.ChertLSoc Faraday Trans. 1,85(1989)3173. 21. Ch.Kordulis, L. Vordonis, A.Lykourgiotis and P.J.Pomonis, J.Chem.Soc. Farada Trans. 1,83(1987)627. 22. Ch.Kordulis, H.Latsios, A.Lykourgiotis and P.J.Pomonis, J.Chem~Soc Faraday Trans. 86(1990)185. 23. S.P.Skaribas, Ph.D. Thesis, University of Ioannina (1992). 24. S.J.Gregg and K.S.W.Sing, "Adsorption, Surface Area and Porosity", 2nd ed., Academic Press, London, 1982.
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
523
D e v e l o p m e n t o f L a x M O y nanocatalysts dispersed in a l a y e r e d silicate matrix. s. Moreau, S. Pessaud and F. Beguin C.R.M.D., C.N.R.S.-Universit6, 1B rue de la F6rollerie 45071 ORLEANS CEDEX 2, FRANCE.
ABSTRACT Using a sot~ chemistry process, we succeded in controlling the stoichiometry of LaxM(fsa)2en,NO3.H20 heterobinuclear complexes (M = Ni, Cu; (fsa)2en = N,N'- bis (3carboxysalicydene)ethylene diamine). The thermal treatment of these complexes led to perovskite type LaNiO 3 or La2CuO 4 type structures, which parameters have been computed and found in good agreement with the data given in the litterature. A large interlayer distance expand was induced by exchanging the sodium cations of montmorillonite with LaxM(fsa)2en cations. Lamellar silicate/oxide nanocomposites were formed by heating the Mont/LaxM(fsa)2en compounds at temperatures lower than the one necessary to prepare the oxide phases from the heterobinuclear complexes.
1. INTRODUCTION Perovskite type oxides (LaxMOy) have proved to be of great interest in catalysis during the last decades. At the beginning, they were synthesized by grinding a stoichiometric mixture of a rare earth oxide and a metal (M) oxide and subsequent heating at 800~ [1]. Unfortunately, this method generates highly heterogeneous solids with a non-controlled stoichiometry. An improvment is the thermal decomposition, either of oxalates formed by adding concentrated oxalic acid to a mixture of lanthanum nitrate and metal M nitrate, or of cyanides formed by the reaction of lanthanum nitrate with K3M(CN)6 [2]. The most recent method is the thermal decomposition of heterobinuclear metallic complexes with a Schiff base as a ligand [6], but the homogeneity of these solids has not been controlled carefully. The catalytic performances of LaxMOy oxides could be considerably improved if they were incorporated in a support allowing to enh-ance selectivity and dispersion. Indeed, previous attempts showed a noticeable increase of the surface area and catalytic activity of perovskite pillared montmorillonite in comparison with pure perovskite [3]. Therefore, we were interested in elaborating nanocomposites made of LaxMOy oxides dispersed in a layered silicate matrix. For that, we used a process based on the Cationic Exchange Capacity (CEC) of NaMontmorillonite : the sodium cations are exchanged with heterobinuclear complex cations and subsequent heat treatment leads to the nanocomposite [4]. In this paper, we will report new data on the preparation and characterization of oxides from heterobinuclear metallic complexes, and on the formation of silicate/oxide nanocomposites after the intercalation of these complexes in montmorillonite.
524 2. EXPERIMENTAL The La-M heterobinuclear complexes noted LaxM(fsa)2en,NO3.H20 were synthesized according to the methods previously described using nickel or copper acetates (Aldrich, 98%), lanthanum nitrate (Janssen, 98%) and N,N'-Bis(3-carboxysalicydene)ethylene diamine (noted (fsa)2en) as a ligand ([5], [6], [7]). M (Ni 2+ or Cu 2+) was introduced at first in sodium carbonate medium, whereas La was complexed in a second step in LiOH/CH3OH medium. For the intercalation into Na montmorillonite (Na-Mont), 0.328 g of LaxM(fsa)2en,NO3.H20 complex (1.25 CEC) were soaked in 50 ml of a 1% aqueous suspension of Na-Mont, and stirred for several days at 40~ After repeated washing with water and filtration, the solids were freeze dried. The LaxM(fsa)2en,NO3.H20 complexes (M = Ni, Cu) and some of their pyrolysis products were characterized by Elemental Analysis (Laboratoire Central d'Analyse, C.N.R.S., Vernaison, France) in order to control their stoichiometry. Thermal treatments of the complexes and ofMont/LaxM(fsa)2en, H20 were carried out in a ThermoGravimetric Analysis (T.G.A.) apparatus coupled with a Balzers QMG 420 C mass spectrometer, under O2/Ar (1/1, 15 ml/min) atmosphere with a 100~ temperature ramp. KBr pellets (0.5% dispersion in 0. l g KBr) were analyzed by Infra-Red spectrometry (Nicolet 710 FT-IR spectrometer). X-Ray diffractograms of powdered solids spread on a glass plate were recorded on a Siemens D500 diffractometer (Cu Kot radiation, ~, = 1.54056 A) in the reflection mode.
3. RESULTS AND DISCUSSION Elemental Analyses showed important differences in the stoichiometry of the heterobinuclear complexes depending at first on the metal used, nickel or copper (table I). Table I. Elemental Analyses of the heterobinuclear metallic complexes. Weight Percentages LaxCu(fsa)2en,NO 3. H20 Sample 1 Sample 2 LaxNi(fsa)2en,NO 3 . H20 Sample 3 Sample 4 Sample 5
Chemical Composition
La 25.90 25.49
Cu "8.09 8.05
C 27.41 28.55
La 1.46 1.45
Cu 1 1
C 17.92 18.76
La 30.21 22.38 22.16
Ni 7.09 8.70 9.16
C 30.89 30.03
La 1.80 1.09 1.02
Ni 1 1 1
..... C 17.37 ..16.02
These differences may be attributed to the nature of the first cation complexed (Ni 2+ or Cu 2+) which can indeed influence the formation of the mononuclear complex, and consequently the stoichiometry of the heterobinuclear complex, as shown by the ratios 9La/Cu = 1.46; LaJNi- 1.80 (Samples 1, 2, 3).
525 Taking the example of nickel, L a ~ i = 1.80 ratio in the case of sample 3 indicates that some La 3+ cations have taken the place of the Ni 2+ ions in the N 2 0 2 cavity. Since we wanted to prepare LaNiO3, we decided to change the pH of the reacting mixture during the formation of the nickel mononuclear complex. Using a too strong basic medium, as in the method described in the litterature [5], probably induces to almost complete ionization of phenolic and carboxylic groups. In sodium carbonate solution, the presence of Ni 2+ cations in stoichiometric amount leads to the precipitation of a mixture of Ni2(fsa)2en,x H 2 0 homobinuclear complex, NiH2(fsa)2en,x H20 mononuclear complex and unreacted ligand. In LiOH/CH3OH medium, only the mononuclear complex and the free ligand are dissolved, leading, in the presence of lanthanum to a mixture of LalNi(fsa)2en,NO3.xH20 and La2(fsa)2en,(NO3)2.xH20, explaining then a La/Ni ratio larger than one. To better control the formation of the mononuclear complex, we decided to lower the pH using NaHCO 3 instead of Na2CO 3. Such a way, we succeded in obtaining a stoichiometric heterobinuclear La 1Ni(fsa)2en,NO3.H20 complex (samples 4, 5). The mono- and binuclear complexes have been analyzed by FT-IR spectrometry (figure 1). The typical bands of each kind of complex are given in table II with their interpretation.
a
c
f
f
(O m
E d
[9
4000
v
t
.
2500
1000
(cm "1) 4000
2500
1000
Figure 1. FT-IR spectra of (a) Cu, (b) Ni mononuclear complexes, and (c) La-Cu (d) La-Ni binuclear complexes. The difference of electronic structure between Ni 2+ and Cu 2+ is still responsible for the positions of the stretching vibration of the carboxylic group 9about 1710 cm -1 for the mononuclear complex and close to 1560 cm-1 when it is coordinated (figure 1). Table II. IR bands of mono- and binuclear complexes (cm-1).
Mononuclear complexes CuH2(fsa)2en NiH2 (fsa)2en 1705 1724 1640 1632 1590 1595 2300-2800 2300-2800
Binuclear complexes LaxCu(fsa)2en La 1Ni(fsa)2en 1562 1562 1643 1630 1597 1597 -
Interpretation C=O C=N C=C OH (COOH)
526 In the case of nickel, using Na2CO 3 basic medium [5], the OH and C = O bands of the nickel mononuclear complex were not clearly observed, in good agreement with the formation of Ni2(fsa)2en, x H20 mentionned before. Indeed, using a strong base, both OH and COOH protons are taken off" the OH band thus disappears and the C = O band is shifted at the position for the carboxylate group, i.e. about 1555 cm- 1. The LaxM(fsa)2en,NO3.H20 complexes were heat-treated under oxygen in the T.G.A. apparatus and the evolved gases were analyzed by mass spectrometry (figures 2, 3, 4). For the Lal.45Cu(fsa)2en,NO3.H20 complex, the total 54.5% weight loss (figure 2) is in good agreement with the theoritical value (58%). Since the equipment was vacuum evacuated before heating the sample, no important loss of solvating water was obs.erved at c a . 50 to 150~ The decomposition of the complex occurs in the range 200-700~ with the evolution of H 2 0 (m/e = 18), CO 2 (m/e = 44), CO and N 2 (m/e = 28) and H 2 (m/e = 2) (figure 3). The gases appear to be formed together at every step of the decomposition. G0
O. 35 ot
I,
~50
0.3 O. 25
40 0.2
r~ r~
o
t--
30
0.15
J= .1~1) 20
0.1 . . . .
," " ' , o . ,
., '
"O
O. 0 5
lO
~:
0
0
1O0
200
300
400
500
600
-0. 05 700
Temperature ( ~ ) Figure 2. Thermogram ofLal.45Cu(fsa)2en,NO3.H20 (the dashed line is for the derivative curve).
m / e = 18
~ t/j
1r r
m / e = 2 8 , 44 m / e ffi 2 I
I
100
200
' I
300
I
t'
I
I
400
500
600
700
Temperature ( ~ ) Figure 3. Mass Spectrometry ofLal.45Cu(fsa)2en,NO3.H20
527
,
6O
~__.._
o.4
o~ 5o r4~
0.3
40
[..,
0.2
o
~., 30
o. 1 ,~
~0 "~ 20
lO ! 1O0
o
!
!
200
300
9
!
!
!
400
SO0
600
-o.
1
700
Temperature ( *C ) Figure 4. Thermogram of LalNi(fsa)2en,NO3.H20 (the dashed line is for the derivative CUrVe).
The weight loss of the La 1Ni(fsa)2en,NO3-H2 O complex (figure 4) occurs on a smaller temperature range than with the Cu derivative. The total loss measured is still in agreement with the theoritical value (58.5% as compared to 61%). The small step below 100~ is indicating of residual solvation water, not removed by vacuum outgassing. Samples resulting from the thermal treatment at 680~ were then characterized by XRD. As expected, the LaNi-complex with a c a . 1"1 La:Ni ratio leads, aider heating, to a perfect hexagonal LaNiO 3 perovskite type structure (figure 5 (a)) : a = b = 5.450 A; c = 6.571 A. These values are in good agreement with the data given in the litterature, i.e. a = b = 5.457 A; c = 6.572 tit (reference : JCPDS file n~ In contrast, the LaxNi-complex with a 1.8:1 La:Ni ratio gives a mixture of LaNiO 3 and La2NiO 4 (figure 5 (b)), as expected in the presence of excess La. .
2O
40
a
60
80
2e
20
.
.
40
.
'
b
.
.
.
60
.
.
.
/
80
Figure 5. X-Ray diffraction patterns of the decomposition products formed from the LalNi(fsa)2en,NO3.H20 (a) and Lal.SNi(fsa)2en,NO3.H20 (b) at 680~ (the bars are for reference LaNiO 3 (a) and La2NiO 4 (b) samples). ~. = 1.54056 A.
528 The decomposition product of the Lal.45Cu(fsa)2en,NO3.H20 complex appears as a La2CuO 4 type structure (figure 6) (both positions and intensities are in agreement with the reference spectrum) although the starting La:Cu ratio is only 1.45:1. From the elemental analysis on the oxide, we found a ratio La/Cu = 1.45 (Table II) in good agreement with the value in the starting heterobinuclear complex. The following parameters calculated from the spectrum : Orthorhombic cell, a = 5.356 A, b = 5.375 A, c = 13.139 A, are in agreement with the data given in the litterature for La2CuO 4 i.e. a = 5.356 A, b = 5.401 A, c = 13.149 A (reference JCPDS file n~ Apparently, this process leads to a lanthanum deficient La2CuO 4 type structure. Therefore, we think that the La/Cu ratio should be adjusted at 2 in the LaxCu(fsa)2en,NO3.H20 complex to obtain a "perfect" La2CuO 4 phase which could be compared with the one presented here. Table III. Elemental Analyses of Lal.45Cu(fsa)2en,NO3.xH2 O complex. "
Sample 1 Sample 2
the
LaxCuOy
~,'k'eight Percentages ' La Cu 0 62.90 19.89 17.59 62.20 19.42 16.04
Ii 20
I
oxide
formed
Chemical La 1.45 1.46
from
the
Composition Cu 0 1 3.51 1 3.28
,!l I dT,! ,...... 40
60
80
20
Figure 6. X-Ray diffraction pattern of the decomposition product formed from the Lal.45Cu(fsa)2en,NO3.xH20 complex ( the bars are for a reference La2CuO 4 sample). Z, = 1.54056/~. The compensating cation of Na-Montmorillonite was exchanged by the heterobinuclear cations and the FT-IR spectra of LaxM(fsa)2en,NO3.xH20 and of Mont/LaxM(fsa)2en were compared (figures 1 and 7). On the spectrum 7(b), all the lines corresponding both to the LalNi(fsa)2en cation and to montmorillonite are easily observable. However, in comparison with the spectrum of the complex (figure 1 d),one can see that the line corresponding to the stretching vibration of NO 3- at 1384 cm -1 considerably decreased, whereas it is enhanced in the filtrate (figure 7 c). This is an indication of the exchange, i.e. that the negatively charged montmorillonite layers in the Mont/LaNi(fsa)2en phase plays the same role as NO 3- in the LaNi(fsa)2en,NO3.H2Ocomplex. The remaining small line of NO 3- at c a 1380 cm "1 in
529 Mont/LaNi(fsa)2en could reveal the presence of residual LaNi(fsa)2en,NO3.H20 complex which has not been completely removed in spite of repeated washing.
<-r
tjj
gg t..
[-
4000
'
'
2200
'
400
(cm-1)
Figure 7. FT-IR spectra of (a) Na-Mont, (b)Mont~aNi(fsa)2en. and (c) filtrate after evaporating the solvent (the arrows are for the NO 3" stretching vibration). As compared to Na montmorillonite, the XRD spectra of the Mont/LaxM(fsa)2en samples show an increase of the Identity Period I c from 12.5 A (Na-Mont) to 23.2 A (Mont~aNi(fsa)2en) and 23.9 A (Mont/La~Cu(fsa)2en) (figure 8). Taking into account the thickness of the montmorillonite layer (9.6 A), we deduced that the interlamellar spacings are respectively 13.6 A for Mont/LaNi(fsa)2en and 14.3 A for Mont/La 1 45Cu(fsa)2en. By comparison with the computed dimensions of the complex (length 16 ~ wi'dth 9.75 A), it can be concluded that the complex is in the standing up position, but tilted. eq
,-~
12 ~
-
_
i
,
b
~
a
i
I
.
5
.
.
.
.
.
-=--_
15
.
25
.
_
-..
35
.
_A
.
45
..
.
,
.
55
20
Figure 8. XRD diffraction patterns of (a) Na-Mont, (b) Mont/LaxCu(fsa)2en and (c)Mont/LaNi(fsa)2en. L = 1.54056 A.
530 Taking into account the results of the TGA observations shown before, Mont/LaNi(fsa)2en was heat treated up to 680~ to allow the complete decomposition of the complex. On the TGA curve (figure 9), the total weight loss reaches 32%, whereas it should be only 26% from the computation. We consequently conclude that the amount of complex in the solid was above 1 CEC. Since the thermograms of LaNi(fsa)2en,NO3.H20 and Mont/LaNi(fsa)2en are very similar, we conclude that this difference could be due to excess complex adsorbed at the surface, as already stated from the IR spectra, showing a residual NO 3- line. For the LaNi(fsa)2en,NO3.H20 binuclear complex, the last important weight loss occurs at c a . 350~ against 310~ for Mont/LaNi(fsa)2en, as seen on the derivative curves. This temperature shift is attributed to the intercalation which weakens the thermal stability of the complex by changing the nature of the counter anion. 35
0.2
3O ~9
',
2s
r~
=,
0.16
f~
.
o
o:
o
I t
o. 12
o :20 ~J ,.= 15
[-.,
o.os~
E
,,..,
0.04
~
5
0
. I
I
I
I
I
I
I00
200
300
400
500
600
-0.04 700
Temperature ( ~ ) Figure 9. TGA on Mont/LaNi(fsa)2en (the dashed line is for the derivative curve). 14,
O. 25
12
.f.
10
..
= 8
i' ;
,.= tNO
6
~:
4
0.2
e
0.15~ ,
0.1
E
o
. -.v,,,
o
0
,
~
100
200
,
,
,
,
300
400
500
600
..
-0.
05
700
Temperature ( ~ ) Figure 10. TGA on Montmoriilonite (the dashed line is for the derivative curve).
531 It is also remarkable that the exchanged montmorillonite does not exhibit any important weight loss, attributable to OH groups, in the range 500-700~ as it occurs with Na Montmorillonite (figure 10). The montmorillonite structure appears to be stabilized by the presence of intercalated complex which prevents sintering of the layers, therefore avoiding the elimination of OH groups [8].
5
20
40
60
80
20
Figure 11. XRD diffraction patterns of Mont/LaNi(fsa)2en heated at 200~ (a), 300~ 400~ (c), 425~ (d), 450~ (e), 660~ (f) and Na-Mont heated at 660~ (g).
(b),
In order to better understand the structural changes occuring during the thermal treatment, the samples were heated at given temperatures for 15 hours and subsequently analyzed by XRD (figure 11). The corresponding values of d001 are ranging from 23.2 A at room temperature to 22.9 A (200~ 15.5 A (300~ 14.9 A (400~ 14.2 A (450~ and 13.61 A (660~ One can ascertain the stability of the lameilar structure at 200~ which is only slightly modified by the loss of a small amount of solvating water between 20 and 100~ The most important transformation occurs between 200~ and 300~ in good agreement with TGA (figure 9) showing the destruction of the complex. The small residual weight lOss appearing between 425 and 660~ on the TGA curve does not induce any major change in the structure of the intercalation compound as shown by the diffractograms at 425, 450~ and 660~ However, the diffractogram of Na-Mont heated two hours at 660~ gives evidence of a partial destruction of its lamellar structure. Therefore, it would be better to keep HTT around 425450~ for the formation of these composites. The bad quality of the 001 reflections above 400~ only allows assumption on the size of the perovskite clusters formed by thermal degradation. Taking into account the average value of d001 for the nanocomposite, ca. 14 A, and the layers thickness, ca. 9.6 A, the size of the intercalated layer can be estimated to about 4 to 5 A, if the composite can be described as a regular sequence of silicate layers and oxide layers.
532 4. CONCLUSION Using a soft chemistry process, the conditions for the synthesis of LaxM(fsa)2en,NO3.H20 complexes (M = Ni, Cu) with a controlled stoichiometry were determined. By the thermal treatment under oxidative atmosphere these complexes were transformed either into perovskite LaNiO 3 or La2CuO 4 type structures. The complex cations are readily exchanged with Na in montmorillonite thus inducing an important I c expand. The thermal treatment transforms these exchanged clays into lamellar silicate/oxide nanocomposites. Intercalated, the complexes appeared thermally less stable than free, and the nanocomposites are formed at lower temperatures (425~ than the one necessary to form the oxide from the starting heterobinuclear complex. Hence, we succeded in preparing catalytic active compounds dispersed between the layers of a silicate matrix. The oxide layer has now to be identified, in order to know exactly its structure : for such analysis EXAFS could be helpful. Further information on the texture of the nanocomposite will be given by Transmission Electron Microscopy and compared to the data obtained by porosity measurements. Particularly, due to the important volume contraction caused by the thermal treatment, perovskite clusters can segregate in given intervals during their formation, leading to an alternance of empty and filled intervalls. Therefore the value of d001 cannot simply be taken for determining the thickness of the perovskite layers.
REFERENCES [1] Wold A., Post B.and Banks E. (1957), J. Amer. Chem. Soc.,79, 4911-4913. [2] Tascon J.M.D., Mendioroz S. and Gonzalez Tejuca L. (1981), Zeitschrii~ ~ r Physikalische Chemie Neue Folge, 124, 109-127. [3] Lavados A.K., Pomonis P.3. and Skaribas S.P. (1992), Materials Science Forum, 91-93, 799-804. [4] Skaribas S.P., Pomonis P.J., Grange P. and Delmon B. (1992), J. Chem. Soc. Faraday Trans.,88 (21), 3217-3223. [5] Tanaka M., Kitaoka M., Okawa H. and Kida S. (1976), Bull. Chem. Soc. Jpn., 49 (9), 24692473. [6] Skafibas S.P., Pomonis P.J., and Skoudos A.T. (1991), J. Mater. Chem., 1 (5), 781-784. [7] Duff J.C. and Bills E.J. (1932), J. Chem. Soc., 1987. [8] Oya A., Omata Y. and Otani S. (1985), J. Mater. Science, 20, 255-260.
PREPARATION OF CATALYSTSVl Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 1995 Elsevier Science B.V.
533
N a n o m e t e r size c o p p e r p a r t i c l e s in c o p p e r c h r o m i t e c a t a l y s t s T.M. Yur'eva, L. Plyasova, O.V. Makarova, T.A.Krieger, V.I.Zaikovskii Boreskov Institute of Catalysis, Russian Academy of Sciences, Siberian Division, Prospect Ak. Lavrentieva 5, Novosibirsk 630090, Russia. Fax: 35-56-50. E-mail: [email protected]. INTRODUCTION It is hardly possible to obtain copper in atomic state on the surface of an oxide catalyst. So, a challenging task to achieving metallic copper with large surface area becomes of crucial importance. It is known that highly dispersed copper can be obtained on the surface via reduction of copper-containing oxides. We have shown by in situ investigation that the nature of hydrogen interaction with copper chromite depends on the method of reduction. The treatment of the sample preheated at 450-500~ under H2 leads to the formation of cuprous chromite which decomposes to Cu ~ and -Cr203 at higher temperatures. If the temperature is slowly raised, a change in the n a t u r e of the hydrogen interaction with copper chromite is observed. Reduction at 450~ results in copper and chromia and, in this case, CuCrO2 is not observed. We have observed that at 200-350~ in hydrogen, copper chromite contains nanometer size metallic copper particles epitaxially bonded to the spinel surface. This state of catalyst exhibits high activity in hydrogenation reactions [1]. These copper particles were studied in detail in this work.
2. EXPERIMENTAL Copper chromite, CuCr204, was prepared by thermal decomposition of copper-chromium hydroxycarbonate in air at 900~ The hydroxycarbonate was coprecipitated from a solution of the metal nitrates at constant pH and temperature. The chemical composition of the sample was confirmed by AAS method. X-ray spectra were obtained with a D500 Siemens diffractometer with CuK radiation (graphite monochromator with a reflected beam). HTK-10 Anton PAAR cell was used for the high-temperature recording of the spectra. Samples were heated (2 K/rain) in pure H2, calcined at a fixed temperature for 2-4 h., and then the spectra were recorded. In some cases, the samples preheated under H2 flow were treated by He stream. The content of copper was estimated by internal standard method. The mean particle size was calculated using the Selyakov-
534 S h e a r e r formula [2]. Precise s t r u c t u r a l analysis was made by a profile refinement method of X-ray powder diffraction [3]. A JEM-100CX electron microscope was used to examine the samples. The phase composition and peculiarities of the particle structure were determined by electron microdiffraction. T h e r m a l analysis profiles were recorded with a STA 409 Netzsch derivatograph in a reductive mixture and then in a He flow at a heating rate of 5 K/rain using 100-mg samples in a Pt crucible. 3. R E S U L T S AND D I S C U S S I O N
Characterization of the parent sample. X-ray diffraction analysis shows that the parent copper chromite is a spinel phase with a tetragonal distortion of the lattice (a = 8.532A, c = 7.779A) and a standard cation distribution (Fig. 1, curve 1). The mean particle size is 1000 A. The X-ray diffraction pattern of the spinel coincides well with the one given in [4]. Electron microscopy (EM) confirms the formation of well-crystallized spinel particles with crystal sizes from 500 to 1000 A. The well-faceted crystals have no noticeable shape anisotropy (Fig. 3a). 440 511,333 422 I
I
400 222 311
I
i
I I
220 I
111 I
+
3
0
+
+
0
L
+
I
I
65
55
I
2|
rpa~
I
I
35
25
Figure 1. X-ray diffraction patterns of copper chromite: 1. p a r e n t sample; 2. reduced in H2 at 320~ 3. sample 2 calcined at 320~ for 10 h. The reflections of metal Cu (o), silicon (+) are given in Figure 1. Positions of the reflections of a cubic type CuCr204 spinel are marked on the top.
Phase composition of the samples treated in H2. No noticeable change in the X-ray pattern of copper chromite is observed upon in situ reduction with H2 up to ~200~ The rise in T is accompanied by the appearance of the Cu metal phase, with particle size of ~100/~. The quantity of copper increases with rising T, and at T = 320~ the degree of reduction reaches 50% (taken per total
535 quantity of copper in chromite) (Fig. 1, curve 2). At the same time, the tetragonal distortion of the spinel lattice disappears, and a broadening of the reflections is observed. A f u r t h e r rise in T to 350~ is not accompanied by a noticeable increase in the Cu metal content and particle size. B u t at T = 450~ the degree of reduction practically reaches 100% and the size of copper particles increases to 300-500/~. The change of the spinel structure becomes deeper upon the rise of T value until complete disordering and transformation to a chromia structure [1].
Phase composition of the reduced samples treated in inert atmosphere. Replacement of H2 by He at 320~ in situ results in a noticeable change of the Xray p a t t e r n : reflections of Cu metal fall practically to zero and the (220), (422) and (333) reflections of a cubic type spinel reappear. Calcination of the sample in He at 320~ for 10 h leads to the restoration of the spinel structure close to the parent one (Fig. 1, curve 3). DTA in He of the sample prereduced at 320~ shows the weight loss with Tmax = 278~ Repeated admittance of hydrogen restores copper evolution and its disappearance in helium. The lattice p a r a m e t e r of copper determined in H2 cooling from 320~ to room t e m p e r a t u r e is equal to the s t a n d a r d one, n a m e l y a = 3.615 A. The distribution of the reflection intensities of the copper points to the defectness of the phase. Values of i n t e g r a l reflection i n t e n s i t i e s for the copper p h a s e represented in Table 1 were found from in situ X-ray p a t t e r n obtained at 320~ in H2. The experimental data are seen to differ from those calculated for a model with ideal Cu o structure (discrepancy factor R = 0.14). The results obtained for a model with defect structure [Cu0.875Ho.125]Cuo.125 with a copper atom in the center of hydrogen atom octant (Fig. 2) are closer to the experimental date (R = 0.07). This s t r u c t u r e seems to be stabilized by h y d r o g e n atoms. The IRspectrum of the sample [5] contains the absorption band in the region 2000-1900 cm -1 corresponding to the Me-H bond vibration [6]. Table 1 Integral reflection intensities hkl Itcu e~ I
I
111
100
100
100
200 220 311
46 20 17
30 24 25
33 32 25
R
0.14
-
0.07
536 y
0 @~ I @
I
0
Q O
[e"
O" I @ 0
I
|
I
--
Cu (000)
--
Cu(01/21/2)
--
Cu (1/4 1/4 1/4)
dl
T i i @~0~@ I I [
@ 0~0
O--
H
O
Figure 2. Projection of the Copper defect structure on (001) plane. The samples c h a r a c t e r i z i n g the different stages of the chromite transformation (reduced in H2 at 320 and 450~ were quickly cooled down in H2 and exposed to air. X-ray data showed no essential difference in the phase composition of the samples studied both in situ and after the brief contact with air. These samples were further studied by EM. EM images of the samples reduced at 320~ show the formation of wellfaceted flat particles, with 50 x 100 x 100 A sizes, in close contact with spinel facets. The copper particles are bonded epitaxially to the oxide surface : the electron diffraction p a t t e r n (Fig. 3b) confirms the correpondence between the (111) reciprocal lattice planes of copper and spinel, with a superposition of Cu(220) chromite (440) reflections. EM date confirm the disappearance of the Cu metal particles from the spinel surface after calcination of these samples in He. These particles have a r a t h e r long life time (about two weeks) in air at room temperature. The copper transforms to copper oxide when kept long in air. The copper particles are stable to segregation due to bonding to the oxide surface. In the sample reduced at 450~ the size of the Cu particles increases to 500A, a noticeable part being spherical and weakly bonded to the oxide surface (Fig. 3c). Metal Cu in the sample reduced at 450~ is not capable of phase transformation upon H2 removal. The samples of copper chromite treated in H2 at 200-350~ demonstrate the unique property of segregating copper to particles of nanometer size at 200350~ in H2 atmosphere. Copper goes back to the spinel s t r u c t u r e upon replacement of H2 by He." As a result of substituting hydrogen by an inert gas, the 5 x 10 x 10 n a n o m e t e r size copper p a r t i c l e s are oxidized a n d t r a n s f e r r e d into crystallographic positions of the spinel. Apparently, protons are generated by hydrogen, and absorbed over the spinel bulk, which reduce copper ions to metal and thus provide the formation of
537 nanometer size copper particles bonded epitaxially to the surface of copper chromite spinel. Removal of the protons destroys the small copper particles. A similar phenomenon has been observed for CuO-ZnO solid solutions [7]. b
c
figure ~. Electron micrographs (x 400000) of: parent CuCr204 (a); reduced in H2 at 320~ (b) and reduced in H2 at 450~ (c). REFERENCES 1. O.V. Makarova, T.M. Yur'eva, G.N. Kustova, A.V. Ziborov, L.M. Plyasova, T.P. Minyukova, L.P. Davydova and V.I. Zaikovskii, Kinet. Katal., 34, No 4 (1993) 683. 2. A. Guinier, Thdorie et technique de la radiocristallographie, Dunod, Paris, 1956, 458. 3. M.M. Rietveld, J. Appl. Cryst., 2 (1969) 62. 4. V.M. Ust'yantsev and V.I. Mar'evich, Izv. Acad. Nauk SSSR, Neorg. Mater., 9, N~ (1973) 336. 5. Private communication from Dr. G.N. Kustova. 6. Chatt and Shaw, J. Chem. Soc., 12 (1962) 5755. 7. T.M. Yur'eva, L.P. Plyasova, T.A. Krieger, V.I. Zaikovskii, O.V. Makarova and T.P. Minyukova, React. Kinet. Catal. Lett., 51, N ~ 2 (1993) 495.
This Page Intentionally Left Blank
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
539
Silica immobilized R u c o m p l e x e s with a d i f f e r e n t c a t a l y s t s of the h y d r o d e h a l - o g e n a t i o n r e a c t i o n
nuclear
number
as
v. Isaeva, Y. Smirnova, and V. Shaft N.D. Zelinskii Institute of Organic Chemistry, Leninsky Prospekt 47, Moscow 117913, Russia
Russian Academy of Sciences,
As we showed earlier [1], the ligand surroundings of binuclear Rh(II) complexes to be immobilized determine the structure and properties of the resulting catalytic system: in the case of acetate ligands, the binuclear structure with the M-M bond is retained. Fixed binuclear complexes of Rh(II) differ from mononuclear complexes by higher activity as to the reduction of ketones and halogen derivatives of p-benzene or of substituted phenylcyclopropanes [1,2]. This study considers Ru complexes with different nuclear numbers fixed on modified silica gels and analyzes how the structure of these complexes may influence the activity of the resulting catalyst as to reductive hydrodehalogenation of p-bromotoluene (p-BT) by hydrogen transfer from NaBH 4 in propanol-2. 1. E X P E R I M E N T A L The following Ru compounds were selected to be fixed at the surface of modified silica gels: mononuclear RuC1 a, binuclear Ru(II,III) tetraacetate and tetrasulfate with the M-M bond, and trinuclear hexaacetate where Ru atoms are connected only by bridging ligands. We also prepared immobilized dirhodium(II) tetraacetate for comparison by [3]. Table 1 Hydrodehalogenation of p-BT in the presence of IRu2( ~ "SO4) 4(H20) 2I immobilized on modified silica gels 2.5 x 10 -6 mole Ru, 4.45 x 10 -s mole p-BT, 10 ml propanol-2, 7.7 x 10 .4 mole NaBH 4, 3.8 x 10 -4 mole CaO, 82o M0d'ified silica gel Si-O-Si- (CH2) a-NH 2 Si-O-Si-CH2-S- (CH 2) 2-CN Si-O-Si- (CH2) 2-SH Si-O-Si- (CH 2) 3-NHCOH
.
.
.
.
.
Wo
.
.
.
.
0.28 0.27 0.29 0.31
The synthesis of IRu2 (~-O2CMe) 4(H20) 2I, IRu2(M-SO4) 4(H20) 2I, and IRu3~-O a) (~-O2CMe)6I was carried out according to the p"rocedures described in [4], [5], and [6], respectively. Ruthenium black and Ru/SiO 2 were prepared by sodium borohydride treatment of RuCl a in methanol and RuC1a on $iO 2,
540 respectively (RuC1 a was deposited on SiO 2 by impregnation). The carriers were Silochrome silica gels (Sso=120 m2) modified by qC-aminopropyl (~'-AMPS), formamide, sulfide, cyano, hnd mercapto groups (the ~ontent of modifying groups is 7 x 10 -4 mole/g) (Table 1). The complexes were heterogenized from their aqueous and methanol solutions in the atmosphere of Ar, as described in [2]. The experiments were performed in a reaction vessel with a magnetic stirrer, equipped by a water jacket, reflux condenser, and sampling device [1]. We loaded 0.05 - 0.1 g of the catalyst (4.95 x 10 -6 g-atom of Ru) and 0.025 g of NaBH 4 into the reaction vessel, filled the system with Ar (or air), added 5 ml of propanol-2, stirred the reaction mixture at 82.5 ~ for some definite time period to activate the catalyst, and then added p - B T (9.9 x 10 -5 - 4.95 x 10 -4 mole). The composition of the catalyzate was analyzed by gas-liquid chromatography. The activity of the catalysts was characterized by the initial specific rate of toluene formation (W o, mole/g-atom Ru.min), which was found by the graphic curve. Infrared spectra of the synthesized metal complexes were obtained by Specord M-80, and XPE spectroscopic studies were conducted with the use of the ES-200B device [7]. 2. R E S U L T S AND D I S C U S S I O N 2.1. The activity of the catalysts As is seen in Table 1, catalysts on the basis of Ru tetrasulfate immobilized on modified silica gels catalyze p-BT hydrogenolysis with close rates (0.27-0.31 mole/g-atom Ru.min). Evidently, the activity of the resulting metal-complex systems does not depend on the nature of the group that is grafted to silica gel. The experimental data show that the structure of the complex to be immobilized plays a significant role in the reaction (Table 2). Table 2 Hydrodehalogenation of p-BT in the presence of [-AMPS-fixed Ru complexes 4.9 x 10 -6 mole Ru (2.4 x 10 -6 mole Rh), 9.9 x 1 0 -5 (4.45 x 10 -5) mole p-BT, 10 ml propanol-2, 7.7 x 10 -4 (3.35 x 10 -4) mole NaBH 4, 3.8 x 10 -4 mole CaO, 82 ~ No. of catalyst
1 2 3 4 5
Complex to be immobilized
RuC1 a [Ru2 (1~1-SO4)4(H20) 2] [Ru2 (~-O2CMe) 4] [Rua (~-O a) ~-O2CMe) 6] [Rh2 (O2CMe) 4]
Initial specific rate of toluene formation W o, mole/min.mole Ru(Rh), in the atmosphere of Ar
air
air bubbling
0.17 0.54 1.37 0.17 6.98
0.15 0.16 1.17 1.15 0.51
0.08 1.31 -
Hydrogenolysis of the C-Br bond proceeds at lower rate in the presence of mononuclear RuC13 deposited on y'-AMPS than in the presence of heterogenized binuelear Ru sulfate. Imrnobilizedbinuelear Ru tetraaeetate displays the highest
~41 activity among all the studied catalytic systems. However, heterogenized Ru hexaacetate without M-M bonds has low activity (equal to that of RuCla). Thus, the catalytic properties of the system to be immobilized depend both on the nature of the ligand surroundings and on the structure of the initial complex. The dependence of W o on the time of the catalyst's preliminary activation by NaBH 4 has an extremum: W o reaches its maximum value in 5-10 min. Hydride ions, which are formed during the reaction of sodium borohydride with alcohol [8], both activate the catalyst and recombine with the formation of molecular hydrogen, which is not active in the studied reaction. The increase of the initial specific rate is probably due to some increase in the number of metal-complex panicles containing Ru in active hydride forms [9]; at the same time, the quantity of non-yet-decomposed NaBH 4 remains high enough for the hydrodehalogenation reaction. The fall of activity is probably due to the decreasing quantity of not-yet-decomposed NaBH 4. The initial quantity of NaBH 4 (the hydrogen donor) also significantly affects the reaction rate. The dependence of W o on the quantity of NaBH 4 has an extremum. This fact is also explained by the dual role of NaBH 4 as the hydrogen donor and the metal complex activator. The increase of activity may be explained by the increasing number of active Ru-complex panicles, and the decrease corresponds to Ru "overreduction" with the formation of Ru(0), which is not active in the reaction. Indeed, it was shown that metallic Ru, both in the form of black and deposited on silica gel, is not active in this reaction. Further studies show that the activity of the catalytic systems in question depends on the amount of the deposited complex. The curve that represents such a dependence has a maximum both for the sulfate and for the acetate complex fixed on Ir-AMPS. This form is very typical for fixed metal complexes [10]. As follows from results of XPE studies (Table 3), the Ru/Si ratio for the sulfate complex on ~-AMPS is smaller by a factor of 2 for the sample with 0.5% of Ru on the carrier as compared to the sample with 1% of Ru. Table 3 The state of Ru in ~r-AMPS-fixed complexes Complex to be immobilized
IRu 2 I~1-SO4) 4(H20) 2I IRu 2 Id-SO4) 4(H20) 21 IRu 2 ~1-S04) 4(H20) 21 IRu 2 ~-SO4) 4(H20) 21 IRu 2 M-OECCH3)41
content of Ru,
Ebouad3ds/2, eV
Ru/Si
Ru(II)
Ru(III)
281.8(40%) 282.0(18%) 282.0(25%) 281.7(48%) 282.0(49%)
283.2(60%) 283.4(82%) 283.0(75%) 283.1 (52%) 283.2(51%)
%
0.5 1.0 2.0 1.0
0.010 0.021 0.018 0.015
However, this ratio is one and the same for the samples with 1% and 2~o of Ru. This fact may be explained by the penetration of Ru complexes into deep pores, which are inaccessible for XPE measurements. The catalytic data confirm this assumption: the initial specific rate is smaller by a factor of 2 for the sample with 2~o of Ru as compared to the sample with 1% of Ru. Apparently, part of the metal complexes are placed in deep pores and do not participate in the catalysis.
542 2.2. The effect of oxygen As is generally known [11], many metal-complex systems are easily deactivated by oxygen. As is seen in Table 2, the activity of the catalytic system in the air atmosphere depends on the ligand surroundings of the complex to be immobilized. When deposited RuC1 a is used as the catalyst, the reaction rate in the air atmosphere is constant. The catalytic activity decreases only when the reaction mixture is bubbled with air. A similar phenomenon is also observed for immobilized trinuclear Ru hexaacetate (catalyst 4). In the case of Ru tetrasulfate on ~-AMPS (catalyst 2), the rate of hydrodehalogenation in the air atmosphere decreases by a factor of 4. Immobilized Ru tetraacetate (catalyst 3) is the most oxygen-resistant: the reaction rate in its presence does not decrease even when the reaction mixture is bubbled with air. As compared to Ru-containing catalytic systems, immobilized dirhodium (II) tetraacetate (catalyst 5) is more sensitive to atmospheric oxygen: W o decreases by a factor of about 15 in the air atmosphere. Thus, the studied Ru catalysts are characterized by high resistance to the influence of air oxygen under the conditions of the hydrodehalogenation reaction. We believe that this stability is an important advantage of Ru systems as compared to Rh-based catalytic systems, especially those containing Rh(I). 2.3. IR and XPE spectroscopic studies
After the acetate complex is deposited on ~'-AMPS, some changes are observed in the IR spectral range that is characteristic for the carrier's stretching vibrations: instead of two absorption bands in the initial ]r-AMPS spectrum, which are characteristic for primary amines, two additional absorption bands appear at 3280 and 3140 cm -~. These bands may refer to Ru-coordinated amino groups, since coordination-bound amines are characterized by a short-wave shift of the N-H stretching vibrations [12]. Thus, Ru is coordinated by the NH2-group; at the same time, part of the amino groups remain free. The absorption bands in the range of carboxyl group vibrations are 9u(COO) 1585 cm -I and 9 s ( C O 0 ) 1420 cm -~. The same absorption bands are observed in the initial complex (1585 and 1425 cm-~). The high intensity of these absorption bands and the absence of shifts for the vibrations of COO-groups indicate that the complexes that are fixed at the carrier surface retain four acetate bridges. The situation is different for the deposited trinuclear Ru hexaacetate complex without M-M bonds. The IR spectrum of this complex contains no characteristic absorption bands of carboxyl groups. In the range of the carrier's stretching vibrations, we observe the same changes as described above for immobilized binuclear Ru tetraacetate. Thus, in contrast to binuclear complexes with the M-M bond, we may suppose that heterogenization of the trinuclear acetate complex without Ru-Ru bonds is accompanied by decomposition: the initial complex turns into a mononuclear one, and the acetate groups are substituted by the carrier's amino groups. For the sulfate complex on ~r-AMPS, our IR spectroscopic studies show that Ru is coordinated by the carrier's amino groups, but the overlapping between the absorption bands of the carrier and the possible bands of sulfate groups does not let us judge whether the initial ligand surroundings are retained. Using XPE spectroscopy, we proved the absence of the 2p line that is produced by sulfur; apparently, t h i s fact indicates the substitution of sulfate groups by the amino groups of the carrier. The XPE spectrum of the ~"-AMPS-immobilized Ru acetate complex (Table 3) contains lines with Ebound3ds/2 = 283.0 and with Ebouna3ds/2 = 281.0, which correspond to Ru(III) and Ru(II), respectively. The R u ( I I ) : Ru(III) particle ratio is 1 : 1. Together with the data of IR spectroscopy, this fact may mean that the binuclear structure of the Ru(II)-Ru(III) tetraacetate complex is retained
543 at the surface after immobilization. A similar phenomenon was observed earlier [2] for ~-AMPS-immobilized dirhodium(II) tetraacetate, where the presence of bridging acetate ligands favors the conservation of the binuclear structure. As is seen in Table 3 for the heterogenized sulfate complex, the sample with 2% of Ru is the most similar to the initial complex: the intensity ratio of the Ru(II) and Ru(III) lines is 1 : 1. As the amount of the complex at the carrier decreases, the XPE spectrum displays some changes as compared to the nondeposited compound: the intensity of the Ru(II) line decreases; accordingly, the content of the Ru(III) particles increases and reaches 85% for the sample with 0.5% of Ru (Table 3). Probably, the binuclear Ru(II,III) sulfate complex may undergo partial decomposition during deposition; the decomposition product is a mononuclear complex, and Ru(III) particles are formed at the carrier surface. The authors thank Zh.L. Dykh and A.N. Baeva (Institute of Organic Chemistry, Russian Academy of Sciences) for recording and discussing the IR and XPE spectra. 2.4. The structure of complexes and their activity Our spectral data enable us to consider the question about the relationship between the structure of a metal complex and its catalytic properties. As is mentioned above, the heterogenized Ru acetate complex displays the highest activity. According to IR and XPE spectra, the binuclear structure of the complex is likely to be retained after immobilization. In the case of the sulfate complex, which is less active than the acetate complex, we believe that both mono- and binuclear structures are present at the carrier. The activity of immobilized trinuclear Ru hexaacetate, where metal atoms (nuclei) are interconnected by bridging acetate groups and by central oxygen, is by an order of magnitude smaller as compared to the catalyst on the basis of binuclear Ru tetraacetate with the M-M bond and is equal to the activity of heterogenized mononuclear RuC1a. Basing on the spectral data, we may suppose that this trinuclear complex is decomposed at the carrier surface, and the mononuclear complex is formed. Thus, we showed that immobilized Ru complexes with the preserved binuclear structure of the initial compound have higher catalytic activity in the reaction of p-BT hydrogenolysis than heterogenized mononuclear systems, just as it was noted before [1] for heterogenized binuclear complexes of Rh. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9.
V.I. Isaeva, Zh.L. Dykh, L.I. Lafer, V.I. Yakerson, and V.Z. Shaft, Izv. Akad. Nauk SSSR, Ser. Khim., No. 1 (1992) 65. V.I. Isaeva, V.Z. Shaft, and A.N. Zhilyaev, Izv. Akad. Nauk SSSR, Ser. Khim., No.2 (1991) 311. B.C. Hui and G.L. Rempel, J. Chem. Soc. Chem. Commun., No. 8 (1970) 1195. F.A. Cotton, Inorg. Chem., 27, No. 24 (1988) 43. A.N. Zhilyaev, T.A. Fomina, I.V. Kuz'menko, I.V. Krotov, and I.B. Baranovskii, Zh. Neorg. Khim., 34, No. 4 (1989) 948. A. Spencer and G. Wilkinson, J. Chem. Soc. Dalton Trans., No. 1 (1971) 15. G.V. Antoshin, E.S. Shpiro, V.N. Belyatskii, O.P. Tkachenko, and Kh.M. Minachev, Izv. Akad. Nauk SSSR, No. 8 (1983) 1983. A. Haiosh, Kompleksnye Gidridy v Organicheskoi Khimii (Complex Hydrides in Organic Chemistry), Mir, Moscow, 1974, p. 60. D. Evans, G. Yagupski, and G. Wilkinson, J. Chem. Soc. A, No. 11 (1968) 2660.
544 10. 11. 12.
A.D. Pomogailo, Kataliz Immobilizovannymi Metallokompleksami (Catalysis by Immobilized Metal Complexes), Nauka, Moscow, 1992, pp. 43-54. B.R. James, Homogeneous Hydrogenation, John Wiley and Sons, New York, 1973. J. Chatt, L.A. Duncanson, and L.H. Vanazi, J. Chem. Soc., No. 8 (1956) 2712.
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
Colloidal
545
Routes to Pt-Au Catalysts
K. Keryou and P.A. Sermon Solids and Surfaces Research Group, Department of Chemistry, Brunel University, Uxbridge, Middlesex UB8 3PH, UK
Citrate-derived monometaUic Pt and bimetallic PtrAul00.x colloidal particles have been prepared and adsorbed onto graphitic supports; their adsorptive and catalytic properties are considered as is the wider applicability of this preparative route to heterogeneous catalysts. It appears that PtrAu~00.x dispersed bimetallic particles have properties which are very close to those of bulk materials and therefore that colloidal preparative routes to mono- and bi-metallic catalysts are very promising. However, the cleanliness of surfaces needs to be considered as does the extent of surface enrichment caused by the method of preparation.
1. INTRODUCTION Two basic approaches have been used in the preparation of metallic colloidal dispersions: disintegration of the macroscopic metallic elements or synthesis of particles from metal salts using appropriate reducing agents, ultrasonics, pulse and laser radiolysis [11. The general characteristics of metal colloidal dispersions are well understood even though the mechanisms by which such metallic particles are formed in the colloidal state are not. This is easily understood if one recognises the complexity of the processes [1] involved in nucleation, growth and coagulation. In order to understand fully and control the preparation, these three steps of course have to be studied individually. Water purity, trace impurities, H + concentration, cleanliness of the reaction vessel and other factors all have a significant effect on the size and morphology of the particles [1,2] and therefore need to be controlled. Many reducing agents have been tried in an attempt to synthesise uniform and stable sols of Pt, Au and Pt-Au (e.g. H2Oe, CO, H e, CzHe, HCOOH, hydrazine, NHzOH.HCI, CzH 4, C4H10, C6H12, Na3C6HsO7.2H20 ), but most of these resulted in precipitation of the Pt despite producing stable gold sols. It seems then that there may be a mismatch between the needs of the Pt sol and the Au sol if one intends to prepare PtrAul0o.x. Over the years separate stable colloids of Pt and Au have been prepared [3,4] with trisodium citrate: Na
546 which presumably complexes, reduces and stabilises [1]. One problem with tri-sodium citrate is than an organic bi-product of the reduction will remain to act as a protective agent for the sol minimising particle growth and stabilising the sol. Nevertheless mono-dispersed colloidal Pt, Au and Pt-Au [3,4] can be prepared by the reaction of tri-sodium citrate with hexachloroplatinic(IV) acid and tetraehloroauric(III) acid at 373IL It was against this background that the present work was undertaken.
2. EXPERIMENTAL 2.1 Preparation of Sols For all present preparations HPLC-grade (Romil Chemical) water was used. All glassware used was cleaned with lipsol/water mixture, water, and doubly-distilled water, and was then steamed for several hours and finally rinsed with the HPLC-grade water before use. 100era 3 of an H2PtCI 6 (Johnson Matthey Specpure) aqueous solution containing the appropriate metal content of Pt was added to l dm 3 pre-boiled HPLC-grade water in a 1.5din ~ round-bottomed flask heated to 373K using an electrothermal heater. After the reaction temperature was re-established, 200em 3 of a 1% w/v aqueous solution of trisodium citrate (BDH, AnalaR) was added and the reaction mixture was maintained at the reaction temperature for 2-4h. Continuous stirring was maintained throughout the reaction time by means of a motor-driven glass propeller. On completion of the reaction time, the sol produced was cooled rapidly in an ice-water slurry and stored in a refrigerator at which temperature any on-going reduction was stopped. Table 1 gives the full details of the preparative method for Pt and PtrAul00.x sols.
Table 1 Pt and PtrAul00.x colloids prepared from HzPtC16 and/or HAuC14 at 373K using 1% Na3C6HsO 7 over 240 rain and then characterised before and after purification (for 60
rain) Conductivity ~,/cm x
mg Pt
mg Au
Initial Final
pH
100
150
0
731
10
4.34
91
150
15
663
55
3.82
75
150
50
1052
148
3.30
50
150
150
1116
33
4.32
547 2.2. Purification of Sols Ion-exchange was used to remove excess citrate and unreacted metal salt in preference to dialysis. A mixed-bed ion-exchange (A.R. Amberlite MBI) resin was mixed with the sol and stirred continuously while the sol conductivity was noted to decrease with time (see Table 1) to a constant value. The final sol was then separated by filtration and stored in a refrigerator. 2.3. Sol adsorption onto graphite Graphite (Fluka; l lmZ/g)-supported Pt and PtrAux00.x catalysts, were prepared (see Table 2) by adsorption and drying at 373IC In these preparations 10g of Fluka graphite (previously washed with doubly-distilled water to remove the finer particles) was dried in air at 373K. The bulk of the colloid containing the appropriate metal content was contacted with graphite whose surface had been pre-wetted with a 1% v/v solution of absolute ethanol. The suspension was stirred for lh and then allowed to settle over one day. A contact time of between 1-4 days for the suspension was maintained with agitation. The slurry was then filtered and dried in air at 373K for 1-2 days. 2.4. Methods of Characterisation and Catalysis The authors have previously [4] shown how the size of Pt~ul00. x sols increased with decreasing x in line with expectation and that such sols were catalytically active in hydrogenolysis of alkanes. The extent of O2-H titration was estimated volumetrically on the graphite-supported sol-derived bimetallic PtrAul00.x catalysts corresponding to Pt91Au9, PtT5 Au25, and Pts0Aus0. H 2 (99.99%) and n-butane (99.5%) were supplied by Air Products Ltd. In hydrogenolysis measurements the reactants (100cm ~ Hz/min, 10era 3 n-butane/min, and 30cm 3 Nz/min ) were passed through the catalyst sample (0.2-0.5g) in a Pyre reactor at 568-688K. Products were analysed chromatographically. A Jeol micro-Auger was used to probe the Pts0Aus0 (x= 50) sol after it was dried onto an A1 surface using an IR lamp. Table 2 Description of graphite-supported catalysts prepared using Pt and PtrAux00_x
x
Catalysts
Support Wt.(g)
Purified Sol(era 3)
Adsorbed Sol(era 3)
Contact Drying(h) a Time(day)
Expectedloading %Pt %Au
100 Pt/C
10
1150
1050
4
48
1.4
91
Pt91Aug/C
10
102
920
4
48
1.4
0.14
75
Pt75Au25/C
10
740
640
4
48
1.4
0.43
50
PtsoAuso/C
10
1140
1340
4
48
1.4
1.4
548 3. C H A R A ~ R I S A T I O N The ability of the bimetallics to titrate chemisorbed H with O 2 was only observable on Pt91Au9, but not on Pt75 Auz5 and PtsoAu50 (see Table 3). It was not surprising that PtTsAuz5 and Pts0Aus0 did not show evidence of O2-H titration and hence presumably hydrogen chemisorption H2, since Au is known not to chemisorb H z at ambient temperature. Table 3 Extents of adsorption a of graphite-supported catalysts
x
Catalyst %Pt %Au
100 91 75 50
1.4 1.4 1.4 1.4
0.00 0.14 0.43 1.40
/anol Hz/g cat O z Titration of H
% Dispersion
2700 900 0 0
14.0 (3.5) (-) (-)
Activity % Conv /anol/g cat/h S H
2.100 0.020 0.004 -
1000 10 2 -
39 81 100 -
a from the linear Langmuir gradient, b activity measured at 622K_+3K
,,
m
0
e- -2
i n ,
m
1.4
1.6
1.8
IO00/T(K) Figure 1. 'Arrhenius' plot for hydrogenolysis and isomerisation of n-butane over colloidally-prepared Pt/C (0) and Pt-Au(gI.9)/C (O)
Sz
61 19 0
Z oF..~
0
I I
"i."
I
I
Na
as prepared
after lOs sputtering
A1
i!l
iii
tl
I
~i I ii ii I iI I ii I il .I
I, 'iAu I II
P~ 'loI
tI
l II l II
I' Ii,, I IIi I
l
2000
ii I
I
, after 30s sputtering :',, ', I
IIi
I I
I
II l
I
!
I
I i I I
i
Sn,
'
i l ,I till I i II I I II 'I
,, I, ',',
!I +Au i,I
I I I I I I I
I' I 'l II I I I I I
II I
l l
,. ~ ~
1000 Kinetic Energy (eV)
Figure 2. Micro-Auger analysis of PtsoAu5o (x=50) sol particles
549
550 Equally Table 3 and Figure 1 show that the activity of Pt/C and Pt-Au/C catalysts in butane hydrogenolysis and isomerisation at 622K and as a function of temperature. The activation energies were 174 and 158 kJ/mol respectively and this again is in keeping with catalytic expectation for samples in which rather inert gold is added to the more active Pt. Figure 2 shows the results of micro-Auger analysis of the PtsoAus0 (x= 50) sol after it had been dried onto an aluminium surface. First when analysed it reveals the presence of Sn, O, Na on the surface of the sol particles. However, after 10s and 3OS sputtering these decrease substantially while Pt, Au Auger responses increase; clearly Au-NVV (70eV) overlaps with the Pt-NVV peak. Such results suggest the need to be very careful about the surface purity of colloidal particles. The O and Na may arise from the citrate reduetant-stabiliser (already illustrated). Micro-Auger does not yet allow the surface Pt:Au ratio of individual particles to be deduced; it may be that one metal will be reduced before the other causing some surface enrichment. 4. CONCLUSIONS It appears that Pt~ul00. x particles prepared by colloidal routes have properties close to those of bulk materials, but care needs to be exercised in ensuring the cleanliness of such surfaces. REFERENCES
11
G. Bredig, Z. Ang. Chem. 11 (1898) 952; The Svedberg, Zeit. Elektrochem. 12 (1906) 853, 909; "Die Existenz der Molekule", Lecp~p, (1912); Turkevich, P.C. Stevenson, and J. Hiller, Discuss. Faraday Soc. 11 (1951) 55. R.H. Morris, and W.O. Milligan, J. Electron Microscop, 8 (1960) 17. J.Turkevich, R.S. Miner, and L. Babenkova, J. Phys. Chem. 90 (1986)4765; J. Turkevich, R.S. Miner, I. Okura, S. Namba, and N. Zakharima, "Perspective in Catal." ed. Larsen (Lun, Sweden), (1980), p.lll; R.S. Miner, S. Namba, and J. Turkevich, Proc. 7th Inter. Cong. Catal., 160 (1981), eds. T. Seiyama, and Tanabe, Elsevier, New York; J. Turkevich, and G. Kim, Science 169 (1973) 873; B.V. Enustun, and J. Turkevich, J. Am. Chem. Soc. 85 (1963) 3317. P.A. Sermon, J.M. Thomas, K. Keryou and G.R. Millward. Angew. Chem. 26. 918 (1987); P.A. Sermon, K. Keryou, J.M. Thomas and G.R. Millward. Mat. Res. Soc. 111. 13 (1988). Faraday Discussions, 92 (1991)
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
551
Catalysts by solid-state ion exchange: Iron in zeolite K. Ldzdra, G. PaI-Borb~lyb, H.K. Beyerb and H.G. KargeC a Insitute of Isotopes, Budapest, P.O.B. 77, H-1525, Hungary b Central Research Institute for Chemistry, Budapest, P.O.B. 17, H-1525, Hungary c Fritz-Haber-lnstitute of the Max-Planck-Gesellschaft, Berlin, D-14195, Germany
Abstract
The method of solid-state ion exhange has been studied for preparing iron containing catalysts. Mechanical mixtures of certain iron salts (ferrous chloride, -oxalate and -acetate) and NH4-Y zeolites were heated and investigated by X-ray diffraction, thermogravimetry and MSssbauer spectroscopy. It is demonstrated that ion exchange proceeds in various extents in the studied systems. Single ions are exchanged in a great extent from chloride. The extent of ion exchange is rather limited with oxalate, the prevailing processes are the solid state transformation and decomposition regardless to the presence of zeolite. Finally, the exchange is modest with acetate and probably associated forms of ions take also part in the process, and nanometer-size magnetite particles are formed at a moderate temperature. Beside ion exchange, redox processes took also place and the adsorbed water present in the zeolite plays also a certain role. Although the transformations and products formed are different in the studied cases, each system might be considered for catalytic application.
1. INTRODUCTION
Preparation and stabilization of highly dispersed particles exerting catalytic activity can be generally attained by procedures starting with incorporation of metal cation species onto exchange sites in the structure of microporous systems (e.g. zeolites) by conventional as well as solid-state ion exchange [1]. The incorporation of transition metal cations is of special interest since valency changes may easily occur and transition metal species are known to exert unique catalytic features. The present publication reports on attemps made to prepare Y zeolites containing iron species via solid-state ion exchange between some iron(ll) salts (chloride, oxalate, and acetate, resp.) and NH4-Y zeolites. The formation of ammonium salts easily removable from the product after the exchange by thermal decomposition was the main criterion for the selection of the particular iron salts as reactants. Emphasis is laid on the description of processes taking place during the preparation. Formation and stabilization of various iron species are demonstrated. Pieces of information on oxidation and coordination states of iron species in zeolites
552
deduced from in-situ MOssbauer spectra are correlated with results obtained by other methods.
2. EXPERIMENTAL Mechanical mixtures of NH4-Y zeolites and iron salts were prepared by intensive grinding in an agate mortar at ambient temperature. The zeolite used for the solid-state ion-exchange experiments with FeCI2-4H20 and Fe(COO)2-2H20 was prepared from a parent Na-Y zeolite (provided by Union Carbide) by 15-times repeated ion exchange with 1N ammonium chloride solution. Its idealized unit cell composition is H10.3(NH4)s4.1Nao.s[AI64.9Si127.10384] 2.4 9 H20. In this experiments no precautions were taken to exclude atmospheric moisture and oxygen (air) during grinding. The amounts of iron salt applied were equivalent to the aluminium content in the zeolitic component of the mixture. Solid-state ion exchange with Fe(CH3COO)2 was performed applying an NH4,NaY zeolite prepared by conventional ion exchange of Na-Y (provided by W.R. Grace & Comp.) without repetition of the exchange step. Its unit cell composition corresponds to H7.7(NH4)35.0Na14.4[Ai57Si1350384] 1.9 9 H20. In this case hydrated as well as dehydrated NH4,Na-Y zeolite and iron(ll) acetate were applied in mass ratios of 1:5, 1 "12 and 1:20. The iron(ll) acetate (obtained from Aldrich) contained 9 % of the total iron in the trivalent state (determined by iodometric titration). The iron acetate/NH4,Na-Y mixtures were ground under argon atmosphere in a glove box in order to avoid or minimize hydrolysis and/or further oxydation of the iron(ll) acetate. Samples were characterized by X-ray diffractometry (Philips PW 1000, graphite monochromator and CuK~ radiation). Thermogravimetric measurements were carded out with a MOM derivatograph under nitrogen atmosphere at a sample heating rate of 10 K min -1. M6ssbauer spectra were recorded at ambient temperature and at 80 K. Treatments for these measurements were carried out in an in situ cell (2 - 4 h treatments at 420, 520, 620 and 720 K at ca. 10 Pa). Preliminary catalytic studies were carded out by studying the conversion of carbon monoxide in 1:2 mixture with hydrogen in a flow-through reactor.
3. RESULTS and DISCUSSION 3.1. Iron(ll) chloride Upon grinding the mixture of iron(ll) chloride and NH4-Y (Fe/AI = 0.5) at ambient temperature, partial contact-induced ion exchange proceeds according to FeCI2 + NH4-Y ---> Fe-Y + NH4CI This is evidenced by the intensity decrease of the XRD reflections of iron(ll) chloride and the appearence of the reflections typical of crystalline ammonium chloride in the diffractogram of the ground mixture. As recently reported [2], part of the incorporated iron ions are transformed to hydroxy-cations by hydrolysis involving
553
water adsorbed in the zeolite coordinatively bound in the iron salt: Fe2+ + H20
~
or
Fe(OH) + + H +
and oxidized by atmospheric oxygen during the grinding according to 2Fe 2+ + 1/2 02 + H20
fVELOCITY (ram/s)
Figure 1. Sequential 300 K MSssbauer spectra of FeCI 2 .4H20 / NH4-Y (a: after grinding, then evacuated at b: 420 K, c: 520 K, d: 620 K, e: 720 K)
2Fe(OH) 2+
As shown by mass-spectrometric determination of the volatile products evolved during temperature-programmed heating of the ground mixture, gaseous HCI formed during the thermal decomposition of ammonium chloride, i.e. one of the products of the preceding contact-induced ion exchange reacts with the cationic hydroxyspecies according to Fe(OH)(n-1) + + HCI ~
23
~
FeCl(n-1)+ + H20
and, finally, interaction between the formed cationic species and the lattice protons formed during the preceding hydrolysis results at somewhat higher temperatures in "dehydrochlorination": FeCl(n-1)+ + H+ --> Fe n+ + HCI.
These observations are in line with the interpretation of MSssbauer spectra (shown in Fig. 1) of the ground iron (11)chloride/NH 4Y mixture heat treated in high vacuum at different temperatures. Typical iron species (listed in Table 1) could be identified in FeCI2/NH4-Y heat-treated at different temperatures by comparison of the isomeric shift (IS - position of the centre of pairs of corresponding lines) and quadrupole splitting (QS - line separation) values of the doublets obtained by spectra decomposition to Lorentzian curves with literature data of compounds of known oxidation and coordination states. After grinding, most of the incorporated iron is present in form of an octahedrally coordinated trivalent species (Fe(lll)oct), obviously as hydrated hydroxy-cation. The Fe(ll)tet r species are probably Fe(OH) + ions coordinatively bound to 3 oxygen atoms of the 6-membered rings (sites SII, SlI' or SI"). At higher temperatures, upon dehydration, trigonal Fe2+ and Fe3+ ions are observed, probably located in the same sites. Autoreduction of the iron (111)species occurs to some degree already at about 420 K and is almost completed above 620 K.
554
Table 1. Data obtained from 300 K M(~ssbauer spectra after sequential 4 h evacuations of mechanical mixture of FeCI 2 .4H20 + NH4-Y (IS: isomer shift, relative to (z-iron, mm/s, QS: quadrupole splitting, mm/s, RI relative spectral area %) Temp. (K) Component 300
420
520
620
720
Fe(lll)oct Fe(ll)tet r Fe(ll)oct-1 FeCI.xH20 Fe(lll)oct Fe(ll)tet r Fe(ll)oct-1 FeCI.xH20 Fe(lll)oct Fe(lll)trig Fe(ll)tng .. Fe(ll)oct-3 FeCI.xH20 Fe(lll)oct Fe(lll)trig Fe(ll)trig ^ Fe(ll)oct-Z Fe(I I)oct-3 Fe(lll)oct Fe(lll)trig Fe(ll)trig ^ Fe(I I)oct-~' Fe (I I)oct-3
IS
QS
RI
0.37 0.69 0.83 1.13 0.35 0.72 0.79 1.04 0.31 0.25 1.08 1.06 1.06 0.30 0.23 0.88 0.95 1.23 0.33 0.22 0.92 0.99 1.27
0.63 0.35 1.95 1.83 0.72 0.36 2.00 1.79 0.75 1.53 0.78 2.60 1.90 0.72 1.71 0.62 2.20 2.19 0.58 1.73 0.68 2.13 2.16
57 13 9 21 44 11 22 23 38 11 21 9 21 5 11 21 37 26 2 2 30 45 20
This process is probably accompanied by a partial release of framework aluminium resulting in minor local lattice damages. Interaction of the so-formed oxidic extraframework aluminium with Fe(ll) results in species characterized by the M(~ssbauer parameters of Fe(ll)oct-2. Nevertheless, it could be shown by XRD that the crystal structure of the zeolitic component is retained even at 720 K. Catalytic properties of Y zeolite containing iron species were studied in a separate series of combined catalytic and in-situ M(Sssbauer measurements [3]. Even at 720 K the extra-framework iron species could not be reduced with hydrogen to the metallic state. Nevertheless, in presence of hydrogen a significant catalytic conversion of carbon monoxide was observed at 670 K. It could be evidenced that :x-iron carbide directly formed from Fe(ll) ions under the experimental conditions of this catalytic reactions was the catalytically active component.
555
3.2. Iron oxalate
Grinding of iron oxalate with NH4-Y zeolite should result in the Fe(ll) and, if oxydation by oxygen occurs, in the Fe(lll) form of Y zeolite and ammonium oxalate. Since CO is formed as decomposition product of the latter compound, secondary reactions between this strong reducing agent and iron species may also occur. Contrary to expectations, neither M6ssbauer spectroscopy nor X-ray diffractometry provided evidence that, up to 720 K, solid-state ion exchange proceeds to a significant degree. The M6ssbauer spectra registered after heat-treatment of the ground mixture exhibit, depending on the temperature, doublets typical of iron oxalate and/or carbonate (Fig. 2 and Table 2). Maximum 10 % of the total spectral area can be attributed to iron species introduced into the zeolite structure by solid-state ion exchange. The iron carbonate phase is thermostable up to 700 K. In the spectrum registered after heat treatment at 820 K magnetite (61%, displaying two magnetic sublattices with characteristic intrinsic magnetic fields of 48.5 and 45.5 Tesla for the A and B sites, respectively), e~-Fe (11 % exhibiting a characteristic sextet with 33 T magnetic field) and ~-Fe2.2C (13 % - with sextets of different positions of iron) can be detected. Magnetite and (~-Fe can be regarded as products of the disproportionation of primarily formed FeO while the iron carbide might have been formed by a carbidization mechanism involving metallic Fe and CO. However, only 85 % of the spectral area are due to these decomposition products of FeCO3, the remaining 15 % has to be attributed to iron -' 23''21 ' I ' J3 ' ions occupying lattice cation positions in the zeolite structure. It may be suggested that water adsorbed in the pore space of zeolites is involved in the e mechanism of contact-induced and solidstate ion exchange in such a way that it -~;-'a"s"#'# ~' ~' +' ;' ~ '~o mediates the transport of the applied salt into VELOCITY (ram/s) the pores by solvation. Thus, solubility of the salt applied should be a prerequisite for the Figure 2. Sequential 300 K progress of this process. This would explain M6ssbauer spectra of why iron oxalate unsoluble in water failed to Fe(COO) 2 .2H20 / NH4-Y take part in solid-state ion exchange at lower (a: after grinding, then evacuated temperatures. at b: 520 K, c: 620 K, d: 720 K, = ~
~ ,
=
.
.
=
.
.
_..L,==,j,
.
.
..-r
J
V
e:820 K)
556
Table 2. Data obtained from 300 K MSssbauer spectra after sequential 4 h evacuations of mechanical mixture of Fe(COO)2 + NH4-Y (IS: isomer shift, relative to o~-iron, mm/s, QS: quadrupole splitting, mm/s, MHF: magnetic hyperfine field, Tesla, RI relative spectral area %) Temp. (K) Component 300 420 520 620 720 820
Fe(COO)2 Fe(COO)2 Fe(CO)3 Fe(COO)2 Fe(CO)3 Fe(CO)3 Fe(CO)3 Fe(ll)tet r Fe(ll)tet r o~-Fe Fe30 4 (A) Fe30 4 (B) E-Fe2.2C (I) E-Fe2.2C (11
IS
QS
1.18 1.21 1.13 1.18 1.17 1.16 1.16 0.88 0.87 -0.03 0.26 0.67 0.28 0.28
1.70 1.68 1.51 1.30 1.65 1.58 1.58 0.63 0.62
MHF
33.0 48.5 45.5 23.6 13.1
RI 100 50 50 17 82 100 90 10 15 15 20 41 8 5
Nevertheless, the iron oxalate/NH4-Y system heat-treated at high temperatures might be considered for catalytic applications since the decomposition products (metallic iron, iron carbide and magnetite) may be highly dispersed and, hence, may exert catalytic activity. Studies are in progress to prove this suggestion. 3.3. Iron acetate For various reasons iron acetate seems to be a promising reactant for solid-state incorporation of iron into zeolites: i/ammonium acetate, to be expected as reaction product, decomposes at relatively low temperatures to volatile compounds: ii/even if acedic acid is formed by hydrolysis of the iron salt no substantial damage of the zeolite lattice may occur because of the low acid strength of this compound: iii/iron acetate is soluble in water that may favour the contact-induced ion exchange during grinding. In the MSssbauer spectrum of iron(ll) acetate ground with NH4,Na-Y the doublets typical of iron(ll) salts of short-chain monocarboxylic acids [4] dominate and only a small part of the spectral area (11.9 %) may be attributed to the impurity of trivalent iron in the salt used in this study. Upon heat treatment at 420 K the greater part (80 %) of the iron(ll) acetate is decomposed as reflected by the strong decrease of the respective spectral area and the weight loss measured by thermogravimetry. In contrast, iron(ll) acetate itself is completely thermostable up to at least 520 K as proved by MSssbauer spectroscopy, thermal analysis and XRD. Thus, iron(ll) acetate
557
'
23
'
21
'
1
'
~
' ~~
_ (~_J8' 16' ~ 4 ' _ ~ 2 ' 6 ' ~ ' ~ ' ~ ' VELOCITY (ram/s)
W
~ '110
Figure 3. Sequential 300 K M6ssbauer spectra of Fe(CH3COO)2/NH4,Na-Y (a: after grinding, then evacuated at b: 420 K, c: 520 K, d: 620 K)
V-'LOCITY (ram/s)
Figure 4. M0ssbauer spectra of Fe(CH3COO)2/NH4,Na-Y (1:12 mass ratio, 1h at 420 K and lh at 520 K, a" 300 K, b: 80 K)
must interact in some way with the zeolite though the M0ssbauer parameters of the formed iron species are not identical with those of the lattice cations species found after solid-state ion exchange with iron(ll) chloride [2]. In line with these findings is the strong intensity decrease of the X-ray reflections of crystalline iron(ll) acetate upon heat treatment at 420 K, i.e. at a temperature at which the salt itself is still stable. The decomposition observed at such low temperatures can not be due to hydrolysis of the salt since it is not accompanied by evolution of acetic acid. Surprisingly, most of the iron is represented by an octahedrally coordinated iron(Ill) species. Thus, part of the iron(ll) must have been oxidized during the heat treatment. The decomposition of iron(ll) acetate ground with NH4-Y, seems to be favoured in presence of water since a less pronounced intensity decrease (60 %) of the doublet typical of this salt was observed when dehydrated zeolite was applied. After heat treatment at 520 K the spectrum is composed of two characteristic doublets (Fig. 3 and Table 3) exhibiting isomer shift values typical of magnetite. Thus, both doublets may be ascribed to a precursor species of magnetite.
558
Table 3. Data obtained from 300 K MSssbauer spectra after sequential 2 h evacuations of Fe(CH3COO)2/NH4Na-Y (1:5 mass ratio) (IS: isomer shift, relative to r ram/s, QS: quadrupole splitting, mm/s, MHF: magnetic hyperfine field, Tesla, RI relative spectral area, %, Fe(ac): Fe(CH3COO)2 ) Temp. (K) Component 300
420
520 620
Fe(ac) (A) Fe(ac) (B) Fe(ll)tetr Fe(ac) (A) Fe(ac) (B) Fe(ll)tet r Fe(lll)oct Fe304(A ) Fe304(B ) Fe(ll) Fe304(A ) Fe304(B ) (Fe,AI)30 4
IS
QS
1.09 1.26 0.53 0.99 1.20 0.59 0.31 0.28 0.44 0.77 0.28 0.65 0.51
2.34 2.19 0.62 2.59 2.19 0.79 0.80 0.74 0.73 2.05
MHF
48.3 45.4 42.6
RI 38.3 49.8 11.9 7.6 12.7 17.7 62.0 41.5 58.4 12.0 21.1 38.8 26.9
After heat treatment at 620 K the spectral features typical of magnetite appear also in the 300 K M~ssbauer spectrum. Beside magnetite an additional phase of a mixed aluminium/iron oxide (27 %) was found. Additionally, even the "particle size" of the mentioned precursor state of magnetite (attained by the 520 K treatment) can be estimatated. From the comparison of the 300 and 80 K MSssbauer spectra of an iron acetate/NH4,Na-Y sample (mass ratio 1:12), the superparamagnetic behaviour of the developed magnetite particles is clearly demonstrated. As it is shown in Fig. 4 the extent of magnetic ordering depends strongly on the temperature of measurement. From the ratio of spectral area of the magnetically split sextets and non-split doublets a characteristic size for the particles can be estimated. This characteristic size for the presented particular spectra (32 % spectral area for the magnetically split part of magnetite spectrum at room temperature) is ca. 4 nm [5]. Thus, these particles might be confined to 2 - 3 supercages. For catalytic applications, the associated iron ions present inside the zeolite framework may provide better precursors for catalytic centres than single ions [6]. Thus, the present iron acetate - Y zeolite system seems to be a promising candidate for catalytic application. In preliminary experiments the 620 K treated sample did not exerted the expected activity in conversion of CO - now the characterization of samples obtained at lower temperature treatments is in progress.
559
4. CONCLUSIONS In the system FeCI2/NH4-Y contact-induced ion exchange takes place to a considerable extent already upon mere grinding. Ion exchange is completed at higher temperatures. Incorporated iron ions oxidized during the grinding procedure by atmospheric oxygen undergo autoreduction when the mixtures are heated up to about 620 K. The product obtained by heat treatment at 720 K is an iron(ll)-Y zeolite with tri- and tetracoordinated Fe(ll) lattice cations probaly located at the 6-membered rings (Sl', SII', SII). In the system iron oxalate/NH4-Y, solid-state ion exchange proceeds, even at higher temperatures, to only a minor degree. Most of the salt applied decomposes via iron carbonate and is unaffected by the zeolite. At high temperatures metallic iron is formed as a product of the disproportional decomposition of FeO. In the iron acetate/NH4,Na-Y system formation of magnetite or magnetite-like cationic clusters can be suggested at temperatures as low as 520 K. In at least one dimension size of the clusters may be around 4 nm, in the other ones they might be smaller. Thus, the clusters may extend for 2-3 supercages. Although each of the studied systems exhibit particular behaviour and different transformations occur in them, from various reasons each system can be considered for catalytic applications.
Acknowledgements The supports provided by the National Scientific Research Grant (OTKA - 7364) and by the PHARE-ACCORD (H9112-0338) fund are gratefully appreciated.
REFERENCES 1. H.G. Karge and H.K. Beyer, Stud. Surf. Sci. Catal., 69 (1991) 43. 2. K. Ldz.&r, G. P&l-Borb~ly, H.K. Beyer and H.G. Karge, J. Chem. Soc., Faraday Transactions, in press. 3. K. Ldz.&r and L. Guczi, Solid State Ionics, 32/33 (1989) 1000. 4. N.N. Greenwood and T.C. Gibb, M6ssbauer Spectroscopy, Chapman and Hall, London, 1971. 5. S. Morup and H. Topsoe, J. Magn. Magn. Mater., 31-34 (1983) 953. 6. K. L&z&r, I. Manninger and B.M. Choudary, Hyperf. Interact., 69 (1991) 747.
This Page Intentionally Left Blank
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
561
M O D I F I E D R U T H E N I U M E X C H A N G E D ZEOLITES FOR ENANTIOSELECTIVE H Y D R O G E N A T I O N V.I. Parvulescu, ^ a V. P~rvulescu b, S.Coman a, C. Radua, D. Macovef, Em.Angelescu a and R.Russu d a University of Bucharest, Faculty of Chemistry, Department of Chemical Technology and Catalysis, B-dul Carol I, 13, Bucharest 70031, Romania b Institute of Physical Chemistry, Splaiul Independentei 202, Bucharest, Romania r Institute of Physics of Materials, M~gurele-Bucharest, Romania d Institute of Refinery and Petrochemistry, B-dul C~mpinei 176, Ploie~i, Romania
Abstract Preparation of modified ruthenium molecular sieves has been investigated in two steps deposition of ruthenium and modifying of ruthenium molecular sieves in presence of ligands. As support sieve there were used two molecular sieves with large pore apertures and low acidity strength (zeolite L and APO-34). Correlation of in situ UV-VIS ruthenium deposition measurements with catalyst characterisation revealed that ruthenium deposition takes place not only through ionic exchange but also through adsorption of ruthenium hydrolysed species. Modifying of the ruthenium molecular sieves catalysts with ligands has as effect a diminution of electronic charge on metal. The experimental data indicate that the presence of ligand favours an enantioselective hydrogenation of D-fructose to D-mannitol even if the yields are not very high.
Introduction Enantioselective synthesis focused in the past several years a great interest because of the need to develop more efficient and safe drugs and agrochemicals. In such idea enantioselective hydrogenations have been much investigated. A very important number of the non-enzyme enantioselective catalysts are soluble metal complexes containing some type of chiral ligand [I-3 ]. Generally, such catalysts are effective because of the chiral environment created around metal centre by the chiral ligands.The chirality of the product is due to the steric environment of the complex and not necessarily to any specific ligandsubstrate interactions. For eliminatethe inconvenientof the chiral homogeneous catalysts (the most important being destrudJon of the catalystcomplex and the loss of the metal and chiral iigand during the separation of the reaction product) chiral heterogeneous catalysts have been used. The most investigated chiral heterogeneous catalysts are Ni/tartaric acid [4-6] and cinchona alkaloid modified I~ or Pd supported on AI203 [7-9]. Comparatively with these metals, ruthenium is less investigated, even some data about the asymmetric hydrogenationofoximes, allyticalcohols or [3-keto esters using BINAP, DIOP or BIPHEM
562
ligands have been published [I 0-12]. Because the great majority of the ruthenium catalysts used in enantioselective synthesis are homogeneous catalysts almost all of the studies concerned on the preparation of these take into account only the interaction between the ligand and metal [I 3-14]. The aim of our investigations was to prepare and characterize some supported modified ruthenium catalysts for enantioselective hydrogenation of D-fructose to D-mannitol. Two different molecular sieves' zeolite L and APO-34 were used as support and as ligand high molecules without an optical active carbon. There are some information in the literature about the ruthenium-based zeolite catalysts mainly linked to R,u-Y and P,u-X systems used in CO hydrogenation or ammonia synthesis [I S- 17]. All of these consider that ruthenium deposition takes place only through an ionic exchange. Experimental l.Catalyst preparation. l.a.ruthenium deposition' High purity L zeolite and APO-34 obtained in Na form according to [18-19]were used as supports. The L zeolite had a silicon-to-aluminium ratio 6.4 as characterized by X-ray dilfraction and elemental analysis. The APO-34 sieve had an aluminium-to-phosphorus ratio of 1.26, as it was characterized by the same methods. In H-form NH3-TPD indicated 0.391 mmol/g for zeolite L (3796 strong sites and 26% weak sites) and 0.192 for APO-34(3196 strong sites and 35% weak sites).As ruthenium precursor RuCl3.3H20 (Riedel-de-Haen AG) was used. Prior to ruthenium deposition, the zeolites were evacuated at 300 ~ to a few mm Hg to avoid occlusion of air or maintained in water for 6 hours. Ruthenium deposition was ~ out at different temperatures in the range 30-80 ~ using different conoentralJonsof ruthenium chloride and O. I g support. The experiments were performed in a UV-VIS quartz spectrophotometer cell for following the process in situ. Electronic spectra were recorded with a PYE UNICAM UV-VlS spectrophotometer model SP8- I00. In parallel, in the some conditions but with I g quantity of support, deposition experiments under strong agitation were performed. From time to time, the solution was separated and spectrophotometer analysed.The variation of pH during the preparation was automatically monitored using a Radelkis device. After deposition, the solid was separated through centrifugation and dried under vacuum at room temperature, and then characterized. l.b.coordination of ligand' Coordination of ligand has been carried out through refluxation of mixture of ligand with ruthenium molecular sieves for 48 hours in an argon atmosphere. The same process was also followed in situ in the UV-VIS quarlz spectrophotometer cell at one maximum wavelength of the complexes, respectively at 359 nm. II. Catalyst characterization ll.a. FTIR spectra were recorded with a Brucker IF 388 spectrometer for samples compressed with cesium jodide. Spectra were recorded at 298 K at a resolution of I cm~ using 30 scans fo~ each measurement. Before the determinations sloping background spectrum was made, then after substraction only the sample spectrum was registered. ll.b. WAXS measurements of the samples were made with a SIEMENS D-S000 (0/0) diffractometer of power of 40-S0 mA, and equipped with a variable slit, diffracted beam monochromator and sdnl~llation counter. The diffractograms were registered in the range 20:(0-80 ~ with a speed of 0.5 ~ using Cul~ radiation (~. = 1.5418 A~
563
II.c. XPS spectraw e r e recorded usinga SSI-X probe (SSX- 100-206) spectrometer of FISONS with monochromatised AIKa radiation.The spectrometer energy scale was calibrated using the Au 4f7/2 peak ( binding energy 93.98 eV). VV'r the analyzer resolution used (50 eV) for recording individual peaks the full width at half maximum of Au 4f7/2 peak was of l.0 eV. III. Hydrogenation of D-fructose Hydrogenation of fructose was carried out in a 250 ml stainless steel stirred autoclave under 15-40 bar hydrogen pressureat 30-80 ~ using 25 wt.% aqueous solutions of fructose. The reaction products were analysed by liquid chromatography using a Varian model 5000 chromatograph. Results
I.Catalyst preparation I.a.Ruthenium deposition UV-V1SspeclTaof HCI solution of RuCI3 showed the presence of 598 nm and 372 nm bands due to a charge transfer ~-'Y5 and 267 nm due to 7t-*Ts typical for a low spin d complexes. Ionic exchange of RuCI3 determines the elimination of NaCl and OH zeolitic groups that favours a hydrolysis of the precursor. In these conditions the change of the shape of the UV-VIS speclTatakes place very quickly (in maximum 3 minutes) (Figure I). However, these determinations showed that the ruthenium exchange in the case of L zeolite is controlled by a mass transfer mechanism becausethe d ~ in the signal intensity at room temperature is a slow process (Figure 3). The increase of the temperature at 80 ~ favours both an increase of hydrolysis of ruthenium chbride and of the exchangingprocess(Figure 5). Kinetic of ruthenium deposition has the same failure for the other concentrations of ruthenium. 200
z >... IZ
.<: I--
s
rY
100
/
/J 0
"
i
i
O0
~00
i
200
WAVE"LENGTH,NH Figure I. UV-VIS spectra of RuCI3-zeolite L
U
,
i
600
I
~00
ii
200
WAVE LENGTH,NH
Figure2. UV-VIS spectra of RuCI3-APO-34
564
Co~ dat~for the deposition of ruthenium on APO-34 are presented in Figures 2,4 and S). The decrease of the 372 nm signal intensity is smaller than in the case of L zeolite. The increase of the 372 nm signal intensity for molecular sieves maintained for 6 hours in water before ruthenium deposition, after contacting with ruthenium solution, could be due to the quick hydrolysis process of ruthenium by OH groups eliminated from zeolitic surface .The variation of the pH shows an important increase in the case of L zeolite whereas in the case of APO-34 this increase is not important (Figures 3 and 4). The experimentscarded out at pH constant by continuous introduction of HCI showed that the shapeof'UV-VIS spectra of RuCI3 solutions were modified justvery little whereas the diminution 2_5 I--,,/. D R I E D ~ L ~ ' "12 r ~.2.0
I.--
~lj)
.,(
======---\WETL
8"1'0 ~
~,,W ETL OL ~
0.g /+i0.6 "
~
o.s
A P O ' 3 4 j
~
~
/'~"
WETAPO-3r
--1{---"--'------WETAPO'3/,
3
0! 0
500 1000 TIHE,MI1NS00 0 Figure 3. Variation of 372 nm intensity signal as a function of time for Ru-zeolite L ~stem,
500 1000TIME.MINi500 Figure 4. Variation of 372 nm intensity signal as a function of time for Ru-APO-34 system.
ql ,-. r l f
. APO'3r
I
,UAN0 A
m
~r~
LISAND B ZEOLITEL 0
30o /+00 100 200 TIME,MI~0 TIME.MIN Figure S. Variaton of 372 nm intensity Figur 6. Variation of 359 nm intensity signalas a function of time for signal as a function of time 80 ~ for ligand A-Ru-zeolite L. of signal intensity was roughly the same. I.b.coordination of liRand Figure 6 shows the variation of 359 nm signal intensityin time for ligand A and both molecularsieves.As one can observe the deposition of the ligand takes place as a very slow process. The variationsmonitored for the ligand B are consistent with those of ligand A. As ligands there were used' CHs CHs /2N-N,~\ ,t/N-.. N~ § H~CN -- C C - S H (BF4)'" Ph-N -. C / C - - S H (CIO4 ) 0
lOO
200
v
CHs
CHs
565 ll.catalyst characterization Catalyst characterization was carried out for catalysts with I 51 or 2.O7 vvt.% Ru and ligand-to-ruthenium w ratio of 1:4. ~3.0~. ll.a.Textural properties -~ Structural composition and textural ~" properties are presented in Table I II.b.WAXS. < The ditfractograms of 2.~5 ruthenium and ligand modified ruthenium molecular sieves are presented in Figures 8 (for zeolite L) and 9 (for APO-34). In the case of 1J}6 metal molecular sieves only very small lines of Ru-O species were identified, whereas no Ru-CI species were determinated. II.c.FTIR. Spectra of ruthenium 1.26 ~00 exchanged with and without the ligand '
I
Z
ZEOLITE L
~]
...
Ru-L
:]
--
A-Ru-L
',I
I,i
,
---
,
o
9
nn
.",I
# i ;
'
~,
"
I
!
4
,
~
",.'.,;" 'dL: \.k " V -". ,, ""~ . "~
,
/,80
,
, %%,,e
",,
_ .
460 4~0 WAVE NUMBER, CM -I
[jgure 7. IR spectra of L and Ru-L catalysts
Table I. Structural composition and textural properties of ruthenium samples comparatively with molecular sieves. Metal content
~.% zeolite L Ru-L * Ru-L Ligand-Ru-L APO-34 Ru-APO-3 4 * Ru-APO-34 Ligand-Ru-APO-34
2.15 2.07 2.07 1.59 1.51 1.51
~-prepared at constant values of the pH
Surface area
Micropore volume
~/g
cra/g
376 296 234 192 I01 89 78 56
0. 122 0.091 0.077 0.052 0.025 0.021 0.018 0.012
566
,
,
,
,,,
c
B
|
6
'
J
10
!
1#
I
,,
,
18
i
22
2-6
Figure 8. WAXS diffractograms of zeolite L and P,u-Lsamples A: zeolite I~ B: Ru-zeolite I~ C' ligand A-Ru-zeolite L D: ligand B-Ru-zeolite L
I _L__., 6
10
1~
18
WAXS diffractograms of APO-34 and Ru-APO-34 samples A: APO-34, B: Ru-APO-34, C: ligand A-Ru-APO-34
22
26
567
are presented in Figures 7 (for zeolite I_) and I0 (for APO-34). In both cases introduction of ruthenium determines a strong reduction of the bands located in the regbn 450 - 490 a n l, because of the rigidization of the O-AI-O and maybe O-Si-O bonds. No soecific contribution of the ligands has been identified. In the same time a diminution of the band located at 36OO cm J was observed.
2.90 . v ~ "~.+
~9 ~,,'i ' t~:
~
- -- AP 0-3/+ "'" Ru-APO-3I~
"~
',P'," 'r'lL '
',,.I ,~!
t~
,e'
2.0/,
1.61
500
I
I
480
r 60
,
i
r
~ 20
WAVE NUMBER,CM-1
Figure 10. IR spectra of APO-34 and Ru-APO-34 catalysts.
II.d.XPS. The results of the investigated samples are presented in table 2.The values of 280.42 eV are consistent with Table 2. XPS resultsfor ruthenium molecular sieves the results of Lunsford in Ru-Y zeolites [I 7]. These values are consistent with the greater localization Catalyst Ru 3d~ O,, eV eV eV_ of charge, on the metal in Ru-L and RuRu-L 280.42 532.18 APO-34, comparatively with 103. I Ligand-Ru-L 281.46 532.19 ruthenium-ligand modified zeo103. I Ru-APO-34 281.29 532.27 lites. Also, it could be observed Ligand-Ru-APO-34 281.71 532.21 that the presence of ligand diminishes the charge on the metal. 9The peak positions are corrected with C~, peak adjusted to 284.8 eV.
III. Hydrogenation of D-fructose Figures II and 12 show the activity data obtained in hydrogenation of D-fructose to Dmannitol. As ligandstwo high molecules were used. No differences between the two ligands were evidenced but the reactions performed on APO-34 revealed high conversions. The yield to Dmannitol does not exceed 30 96.
568
I
o~
i\
?~
I
o
LlfiAND
40o~ ._.I l.zJ
_
,.,
E2o
~~'~-
"
0
_ I
&0
50 _ 60.
.,.~+o_~t
a
/
_. . . . P:/,O BAR
!0
/
0
0/
70
,r
80
.'.-~--~'~-
.
I 1 1 1.--
Oz z
. o P=I,0 BAR
TEMRC
10
0 ~0
I
! 201 I
0
:0 ""
~'~',/~
%"
~
o
/ 20
o
9 3o so, HYDROGEN PRESSURE
F~ure II. Hydrogenation of D-fructose on Ru-L zeolites.
0 o
2o
3o HYDROGEN PRESS
Figure 12. Hydrogenation of D-fructose on Ru-APO-3 4.
Discussion Between zeolite L and APO-34 there are important differences. Zeolite L typically exhibits high exchange site density comparatively with APO-34. This is consistent with the concentration of the OH groups that could easily interact during ruthenium deposition. The UV-VIS and pH valuesdatashowed that deposition of ruthenium on zeolite L or APO-34 mainly takes place through ionic exchange. The variation of the pH from 1.96 to 8 in the case of zeolite L is sudden. Such a variation should be due to a passage from the pH of the acid ruthenium solution to those of zeolite. The pH variation from 8 to 9.46 is due to ruthenium. A high ion like [Ru(1-120)2Cl4~rneedstime to diffusein the internal structure of zeolite where it finds a surface covered by H30m. This behaviour explains the fact that the differences between the wet and dried zeolite diminish in time. The pH increase determines a precipitation of ruthenium. In this way a reduction of the pore apertures and surface area could be explained. The textural measurements showed an important decrease of surface area, respectively of 38% in the case of L zeolite or 23% in the case of APO-34. Because of the superficial heterogeneity for ruthenium deposition, zeolite L offers four differentways : first, the strong acide sites which change Na + with H + (from HCI added) and hydrated RuCIx(3x)+ spedes; second, the weak acid sites that could be exchanged with hydrolysed [Ru(OH)xCI3.x] species; third, through adsorption as [Ru(H20)2CI47 species on Bronsted acid sites; and fourth, physical deposition of hydrated species followed by grafting. Hydrate species are formed during exchange process and are consistent with the pH increase. The data presented in the Table I showed that surface area of ruthenium molecular sieves prepared at constant pH have higher values than in the cases in which no HCI was added. In
569
condition of the ruthenium deposition at constant pH, the hydrolysis is avoided whereas the ionic exchange and adsorption on Bronsted sites are favourised. The increase of the 372 nm signal intensity after contacting with dried zeolite samples could be mainly associated with the adsorption of water and concentration of the solutions. However the increase of the 372 nm signal intensity even if not at the same values in the start of exchanging of molecular sieves maintained in water for 6 hours before metal deposition denotes a very quick hydrolyzation of ruthenium in the presence of zeolite even if UV-VIS spectra look like those of RuCI3 solution. Deposition of the hydrotysed species could be performed as an ionic exchange or adsorption process. The shape of dit[ractograms of the samples obtained using wet samples are similar with those presented in Figures 7 and 8 for dried samples. The diffractograms of ruthenium molecular sieves indicated the presence of Ru-O species, even when the metal content in these catalysts was of 2.07 or 1.5 wt. %. These data suggest that during the preparation takes also place a metal agglomeration process. The increaseof the temperature determines just an acceleration of the ruthenium deposition. The measurements of the catalyst characterization do not indicate special influences. Also, the experiments carried out using I g of catalysts do not indicate differences against the in situ UV-VIS measurements. APO-34 exhibits a smaller concentration of exchange sites than zeolite L and 35 percents of these are weak sites. In such conditions the ruthenium deposition mainly takes place through adsorption. Because the exchange is very slowly no important variation of the pH was observed. Also, the xrariatbnsof the surface area are smaller than in the case of zeolite L. In a first stage, on the surface ruthenium is probably linked through polar ion-dipol bonds. . The FTIR results revealed a strong reduction of the bands located in the region 450 - 490 cm ~ These data should be also a proof of the fact that ruthenium deposition takes place in part through ionic exchange. Ruthenium deposition could generate a rigidization of O-Si-O or O-AI-O bonds, as it was observed from these measurements. In the same time a small decrease of the band located at 3600 cm1 indicates an OH elimination. Grafting ofthe ligandstakes place with low rates as it was presented in Figure 6. From UV-VIS spectra no differences between the two ligands are observed. The presence of ligand in the mentioned ratio (Ru/iigand-4/I) does not influence FTIR spectra and gives not new lines in the dif[~ctograms of the ruthenium modified molecular sieves. In exchange, the textural measurements showed a supplementary decrease of the surface area. XPS measurements of ligand modified molecular sieves showed that the presence of the ligand determines a smaller localization of the charge on the metal comparatively with the parent ruthenium molecularsieves. These data are consistent with an aspirant effect of the ligand and clearly denote a direct metal-ligand interaction and an electronic effect of these. Modification of the ligand substituent are consistent with binding energies lower than 0.5 eV and therefore it is difficult to be associated with a real effect. Hydrogenation of D-fructose on these catalysts revealed conversions lower than 50% and excess in rapport with D-mannitol yield of maximum 30% (Figures I I and 12). The presence of phenyl substituentgenerates only a very small increase of the activity. The yield to D-mannitol is lower than those previously mentioned using chiral modifiers [20,21] but in this case it was obtained an enantioselective hydrogenation in presence of ligands without optical carbon and more, containing sulphur.
570
Conclusions De~ of ruthenium on molecular sieves with large pore apertures like zeolite L or APO34 takes place not only through ionic exchange but very probably through ruthenium hydrolysed spedesadsorpEon.In this way the superficial agglomeration of ruthenium is possible. The ruthenium aggregatesfavoursnot only the ligandchemisorption, but also the interaction with reactant molecules. Utilization of the heterocompounds with sulphur determines an enantioselective hydrogenation of D-~uctose to D-mannitol but only with low yields. These results are consistent with the published results about the ligands with sulphur. Akno.vvledgement The authorsthank very much Professor Paul Grange for helpful discussions and possibility to perform XPS measurements. References I. I.Ojima, N.Clos and C.Bastos, Tetrahedron, 45(1989)690 I. 2. J.C.Fraud, CataI.Met.Complexes, 12(199 I)107. 3. D.Arntz and A.Schaefer, CataI.Met.Complexes, 12(199 I)161. 4. Y.Izumi, Adv.Catal., 32(1983)215. 5. H.Brunner, M.Muschiol, T.Wischert and J.W'~hl, Tetrahedron:Asymmetry, 1(1990) 159. 6. M.A.Keane and G.Webb, J.Catal., 136(1992) 1. 7. H.U.Blaser, Tetrahedron:Asymmetry, 2(199 I)843. 8. H.U.Blaser, S.K.Boyer and U.Pettelkow, Tetrahedron:Asymmetry,2(1991)721. 9. H.U.Blaser, M.Garland and H.P.Jallet, J.Catal., 144(1993)569. 10.C.Botteghi, M.Bianchi, E.Benedetti and U.Matteoli, Chimia, 29(1975)256. I I.P.Krasik and H.Alper, Tetrahedron:Asymmet~,, 3(1992) 1283. 12.B.Heiser, E~.Brogerand Y.Crameri, Tetrahedron:Asymmet~,2(199 I)5 I. 13.M.M.Taqui Khan, R.I.Kureshy and N.H.Khan, Tetrahedron:Asymmet~, 2(199 I) 1015. 14.M.M.Taqui Khan, N.H.Khan and FLI.Kureshy, Tetrahedron~kwmmetry, 3(1992)307. 15.R.Oukad, A.Sayari and J.G.GOodwin, J.Catal., 102(1986) 126. 16.W.Mahdi, U.Sauerlandt, J.Wellenbuscher, J.Schulz, M.Muhler, G.Ertl and R.Schogl, CataI.Lett., 14(1992)339. 17. M.D.Cisneros and J.H.Lunsford, J.Catal., 141(1993)19 I. 18. L.E.Davis and S.Saldanaga, Zeolites, 8(1988)362. 19. K.J.Bombaugh, Nature, 197(1963) 1102. 20. M.Makk~e, ,&P.G.Kieboom and H.Bekkum,Carbohydr.Res., 128(1985)225. 2 I.M.Hegedus,S.Gobolos and J.L.MargitfaM in M.Guisnet et al.(editors),HeterogeneousCatalysis and Fine Chemicals III, Studies in Surface Science and Catalysis,vol.78, Elsevier, Amsterdam, London, New York, Tokyo, 1993, p. 187.
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
571
PREPARATION OF CONJUGATED POLYMER SUPPORTED HETEROPOLYANIONS - NEW EFFICIENT CATALYSTS FOR ETHYL ALCOHOL CONVERSION M. Hasik, 1 I. Kulszewicz-Bajer, 2 J.Polniczek, 3, Z. Piwowarska, 4 A. Prori, 1'2 A. Bielariski 3, R. Dziembaj 4
1 Department of Materials Science and Ceramics, Academy of Mining and Metallurgy, Al. Mickiewicza 30, 30-059 Kvakdw, Poland 2 Department of Chemistry, Technical University of Warsaw, ul. Noakowskiego 3, 00-664 Warszawa, Poland 3 Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, ul. Niezapominajek, 30-239 Krakdw, Poland 4 Department of Chemistry, Jagiellonian University, ul. Ingavdena 3, 30-060 Kvakdw, Poland 1. I N T R O D U C T I O N Heteropolyacids (HPA) are strong acids of a general formula: H + [X~MmOv] q- , where: M: Mo, W, V, Nb, T1 in their highest oxidation state; X: P(V), Si(IV), As(V), Ge(IV). The most characteristic feature of all heteropolyacids is the structure of their anions. Even though a variety of structures are known [1] compounds of the so-called Keggin anion structure (dodecaheteropolyacids, HqXM12040) are most widely studied. This is due to their easy preparation, high stability and to the fact that they are active catalysts for various chemical reactions. Their catalytic properties have been reviewed several times [2, 3]. One of the disadvantages of heteropolyacids as catalysts in heterogeneous systems is their rather low surface area (ca 10 m2/g). Therefore it is desirable to disperse them on a suitable support. Silica [4], heteropolyacids salts [5] and activated carbon [6] have been used for this purpose. These supports however do not provide uniform nor stable distribution of HPA on their surfaces. Quite recently it has been shown that HPA can serve as convenient supports for HPA. Heteropolyanions are introduced to these polymers via the so-called doping reaction either electrochemically [7, 8, 9] or chemically [10, 11]. In view of their potential applications as catalysts chemical preparation is more advantageous since it allows to prepare larger amounts of the materials. It should be pointed out that during doping polymer chain aquires positive charge (and the polymer becomes conducting) which is neutralized by the negative charge of heteropolyanions. Additionally doping induces dispersion of HPA at the molecular level. In the present work we describe the preparation and properties in ethanol conversion of conjugated polymer supported heteropolyanions of Keggin-type structure. Thus 12-molybdophosphoric acid (H3PMol2040) has been supported on polyacetylene (PAc)
572 and polypyrrole (PPy), 12-molybdophosphoric. 12-tungstosilicic (H4SiWl2040) and 12tungstophosphoric (HaPW12040) acids supported polyaniline (PANI) has been also obtained. The details of the catalytic tests have been published elsewhere [12, 13, 14].
2. EXPERIMENTAL 2.1. P r e p a r a t i o n of polymer supported catalysts
2.1.1. Synthesis of H3PMo12040 doped polyacetylene Neutral polyacetylene was prepared from acetylene using Ti(OC4H9)4/AI(C2H5)3 catalyst according to a modified Ito method [15]. The lustrous film obtained in such a way was then chemically doped with H3PMo12040 This method involved treatment of a PAc film with the saturated solution of H3PMo12040 in acetonitrile. It was necessary to add a small amount of a reducing agent, such as pyrolle, indole etc. in order to initiate the reaction. 2.1.2 S y n t h e s i s of HaPMol2040 doped polypyrrole Since pyrrole can be readily obtained in the doped form by chemical polymerization of pyrrole in the presence of a suitable oxidizing/doping agent, such as FeC13 [16] we have decided to use the analogous method in order to obtain PPy doped with H3PMo12040. It was prepared in the one-step reaction in which 0.3 g (5 mmole) of pyrrole was added to 50 ml of the 0.1M solution of HaPMo12040 in ethanol/water (1:1) mixture. The reaction was carried out for 2 hours. Then the product was filtered, washed with ethanol and dried.
2.1.3 Synthesis of HPA doped polyaniline It has been found that polyaniline doped with various HPA can be prepared either by protonation of the previously synthesized polyemeraldine base (two-step method) or by polymerization of aniline in the presence of the appropriate HPA (one-step method). Both procedures will be described briefly below.
Protonation of polyerneraldine base with HPA In this procedure polyemeraldine base was prepared first by polymerization of aniline with (NH4)2S~O8 in hydrochloric acid followed by its deprotonation with aqueous ammonia as recommended in [17]. Protonation with HPA was achieved by treating polyemeraldine base with the appropriate HPA in 0.1M or more diluted HPA solutions in acetonitrile for 6 hours. The product was washed several times with acetonitrile in order to remove the excess of HPA.
Polymerization of aniline in the presence of HPA According to this procedure 0.7 g (8 mmole) of aniline was added to 50 ml of 0.1M solution of the appropriate HPA in acetonitrile or acetone. Then it was polymerized using (NH4)2S208 as the oxidant (2.28 g, i.e. !0 mmole). The precipitate obtained was then thoroughly washed with acetonitrile or acetone, water and finally it was dried. 2.2. S t u d i e s of c o n j u g a t e d p o l y m e r supported HPA All the samples of polymer supported catalysts were subjected to elemental analysis.
573 X-ray diffraction patterns were taken using the CuK~ radiation. FTIR spectra were measured on a Digilab FTS-60V spectrometer. The samples studied were in the form of free-standing films (PAc) or KBr pressed pellets (other polymers). ESCA measurements were performed on ESCA 100 (USW Manchester) using MgK~ radiation of 1253.6 eV under pressure of 10-s bar. Cls=284.5 eV was applied as an internal standard. The samples were fixed to the sample holder using the double-sided adhesive tape. The original ESCA curves were numerically deconvoluted into components of mixed Gaussian-Lorentzian shape. Catalytic tests were performed using the pulse method at 230,240, 280, 300 or 320 ~ depending on the sample. Pulses of 0.5 #1, i.e. 3.68.10 -4 g of ethanol were introduced into helium carrier gas. Products were analyzed using gas chromatography. 3. R E S U L T S
AND
DISCUSSION
As it has been found by detailed studies the preparation procedures described in the previous section allow to obtain conjugated polymers doped by heteropolyanions of Keggin structure. Thus it has been established by X-ray diffraction that HPA do not form a separate phase in the systems. The XRD patterns exhibit only the features typical of the polymeric phase, i.e. a broad amorphous halo in the case of PPy/H3PMoI2040, one crystalline peak at d=12 ~ and a broad halo in the case of PANI/HPA and highly crystalline reflections of the polymer in the case of PAc/H3PMoa2040 system. Thus it can be concluded that the dispersion of heteropolyanions on polymer matrices occurs at the molecular level. The information on the kind of interactions between the polymer matrices and HPA and on the type of doping species has been derived from spectroscopic studies. FTIR and ESCA spectroscopies have proved especially useful. FTIR spectra of the synthesized materials are presented in Fig. 1.
400
!
I
i
I
I
I
600
800
1000
1200
1400
1600
Wavenumbers, cln -1 Figure 1. FTIR spectra of: PAc/H3PMo,204o (A), PANI/H3PMo1204o (2-step preparation, B, 1-step preparation, C), PPy/H3PMo1204o (D).
574 All the IR spectra show the bands originating from the doped polymer backbone as well as the ones corresponding to the Keggin anions vibrations. In the spectrum of PAc/H3PMox2040 two bands characteristic of the doped state are present: at 1370 cm -1 and a broad band at around 900 cm -1, i.e. in the spectral region in which the bands due to Keggin units are expected. Even though in this case the bands due to Keggin units are superimposed on this broad doping-induced band there are characteristic shoulders at 785, 970 and 1060 cm -1 arising from the dopant. Similar conclusions can be drawn basing on the spectra of PPy/H3PMoa2040 and PANI/HaPMoa2040. In the former one the peaks of Keggin units cause an increase in the polymeric bands intensities at 780, 870 and 960 cm -1. The characteristic feature of the IR spectra of polyaniline protonated with HPA in addition to appearance of the bands corresponding to Keggin unit vibrations is the disappearance of the band at 1164 cm -1 typical of quinone-imine vibrations and the appearance of the broad doping-induced band at around 1130 cm -1 originating from the vibrations of the protonated chain of the polymer [18]. Further corroboration of the doping of polymers and the preservance of the Keggin structure comes from ESCA measurements. The Mo3d spectra of 12-molybdophosphoric acid doped polymers show only the doublet which can be ascribed to two molybdenum (VI) levels, i.e. Mo3ds/2 and Mo3d3/2. Thus Mo is not reduced in the doping reaction. Similarly W4f spectra of PANI doped with 12-tungstophosphoric and 12-tungstosilicic acids show doublets of W4fT/2 and W4fs/2 proving the preservance of W(VI) in the dopants. -
-
A
.
i
!
. _ ,
. . . . . .
|
. . . . .
t
| ....
=L
. . . . . . .
B
.........
408
i .........
! .........
403
I .........
! .........
! .........
398
! .........
393
! .........
eV
Binding energy
Figure 2. Nls spectra of PANI protonated with HaPW1204o: A y=0.0013, B y=0.03.
575
Changes in the composition of the polymer backbone in the case of polyaniline can be easily followed by observing the Nls ESCA spectra. The Nls spectrum of polyaniline base can be deconvoluted into two main peaks at binding energy (B.E.) of 398.4 eV and 399.5 eV. The former one is ascribed to imine nitrogen atoms and the latter one to amine nitrogen atoms [16]. In the Nls ESCA spectra of polyaniline protonated with HPA new peaks at B.E. above 400 eV appear. They are due to protonated (charged) nitrogen atoms of the polymer as it has been reported for PANI protonated with typical inorganic acids, such as HC1 [19]. It is interesting to note the changes in the N ls ESCA spectra of PANI containing various amounts of HPA (Fig.2). As the protonation level increases the intensity of the peak corresponding to the imine nitrogen atoms decreases with the simultaneous increase in the intensity of the peaks originating from the protonated nitrogen atoms. Elemental analysis allowed to calculate the empirical formulae of the samples of conjugated polymers doped with heteropolyanions and to determine their doping level, y, defined as the number of moles of HPA per one mer of the polymer. Table 1 presents the results of these calculations. Table 1 Empirical formulae and the maximum doping level achieved in the doping of conjugated polymers with heteropolyanions. Support
Empirical formula
PAc PPy PANI, 1-step
-[CH(H2P Mo,20,0)~]-~ -[C, H2N(H2PMo,2040)y]-~ -[C6H4.sN(HPA)y]-,
PANI, 2-step
-[Cr
Y,~
HPA content, wt %
0.0045 0.11 0.10
38.7 75.5 66.9 76.1 37.7 48.8
0.03
1 2 1 2
1 Protonated with H3PM012040 2 Protonated with H4SiWl2040 and HaPWl2040 As it can be seen from Table 1 the method of preparation of polymer-supported HPA strongly influences the doping level that can be achieved in this process. Thus the polymers that are doped in a two-step procedure (PAc and PANI) contain smaller amount of the dopant than the polymers that were doped in a one-step reaction (PPy and PANI). This may be due to the fact that during the preparation by the former method HPA are dispersed mainly on the surface of the polymer while in the latter one they are introduced also into the bulk of the polymer. Indeed we have observed differences in the doping level in the case of PANI/HPA when polyemeraldine base of various surface area was used for protonation. It is worth noting that the two-step procedure allows to control easily the doping level of the materials by slightly changing the reaction conditions, such as the molar ratio of
576 HPA and the polymer or the concentration of the doping solution. Thus, for example, samples of PANI/HPA of doping level starting from 0.0014 have been obtained. The effect of the molar ratio of tungsten-containing HPA and PANI in the protonating solution on the resultant doping level is presented in Table 2. Table 2. Influence of the starting molar ratio of HPA and PANI in the protonating solution on the protonation level of PANI. 1 HPA/PANI ratio
0.0016:1 0.0030:1 0.0060 : 1 0.0090 : 1 0.0126 : 1 0.0160 : 1
Protonation level, y H4SiWl204o
H3PW12040
0.0015 0.0028 0.0059 0.0083 0.011 0.014
0.0014 0.0030 0.0057 0.0088 0.010 0.014
1 All the samples of PANI were protonated in 0.001M solution of HPA in acetonitrile. It should be also pointed out that due to the chemical nature of doping, which in the case of PAc and PPy involves partial oxidation of the polymer chain, only oxidizing acids, e.g. H3PMo12040 can be used for these polymers. PANI, whose doping is a simple acid-base reaction, can be doped with various HPA. All the systems obtained were subjected to catalytic tests in ethyl alcohol conversion. This reaction has been chosen since it reveals both acidic and redox properties of a catalyst. The main products of catalytic ethanol conversion are ethylene and diethyl ether which are formed on the acidic centers and acetic aldehyde which is formed on the redox centers of the catalysts. Table 3 shows the results of catalytic experiments carried out on polymer supported catalysts in comparison with the results obtained for the corresponding unsupported crystalline heteropolyacids. As it can be seen from Table 3 dispersion of HPA on the polymeric supports changes the total activity and selectivity of the catalysts. Generally the systems that have been prepared by a two-step method are more catalytically active than the ones obtained in the one-step reaction. Conversion of ethyl alcohol over PANI/HPA prepared by the latter method has to be carried out at higher temperatures and in spite of that the total activity of these catalysts constitutes only a small part of that observed for the samples synthesized by the 2-step method. The differences in the overall activities of ethanol conversion may be explained by better accessibility of the catalytic centers in the case of the catalysts prepared by the two-step method. They are located here at the surface of the polymers. One should also notice the changes in selectivity of HPA when they are supported on conjugated polymers with respect to crystalline unsupported HPA. The yield of acetaldehyde increases for all samples prepared by the two-step procedure. Significant enhance-
577 Table 3 Ratio of yields of the products of ethanol conversion over polymer-supported HPA and crystalline HPA (calculated per 1 g of HPA in the catalyst).
Catalyst
PAc/HaP Mo1204o PPy/HaPMo12040 PANI/HaPMo1204o, 2-step PANI/H4SiW1204o, 2-step PANI/HaPW1204o, 2-step
Reaction temperature, 0C
CHaCHO
(C~Hs)20
C2H4
Total
230 320 240 240 240
42.9 0.6 18.9 9 19.6
11.1 0.5 1.3 1.2 **
9.4 0.6 2.5 2.7 0.5
17.4 0.5 5.6 2.0 0.9
9 crystalline H4SiW1204o gives only traces of CHaCHO whereas it constitutes 4.76 % of the products of the catalytic reaction over PANI/H4SiW1204o; 9, crystalline H3PW12040 does not give diethyl ether whereas it constitutes 25 % of the reaction products over PANI/H3PW12040. ment of the redox functions over the acid-base ones observed in all conjugated polymers supported HPA catalysts can be explained by the chemical nature of the doping process which involves abstraction of one or more protons from the molecule of heteropolyacid lowering in this manner its acidity. CONCLUSIONS It has been shown that conjugated polymers, such as polyacetylene, polypyrrole and polyaniline can be doped with heteropolyanions of Keggin structure. They constitute a new type of catalytic support at which catalytically active species are dispersed on the molecular scale. Conjugated polymer-heteropolyacids systems can in principle be prepared using the one-step and the two-step methods. The products of the latter one are more active catalysts in ethyl alcohol conversion. Therefore from the catalytic point of view the second method of preparation is more advantageous. ACKNOWLEDGMENT This work was financially supported by the Committee for Scientific Research of Poland (KBN) grant number 20898 91/p01 and p 02. REFERENCES
1. M. Pope, Heteropoly and Isopoly Oxometalates, Springer-Verlag, Berlin, 1983 2. F. A. Chernyshkova, Petrol. Chem, 31,571 (1991)
578 3. M. Misono, New Frontiers in Catalysis (Proceedings of the 10th International Congress on Catalysis), ed. L. Guczi et al., 61, Elsevier Sci. Publ., 1993 4. C. Rocchiccioli-Deltcheff, M. Amirouche, M. Fournier, J. Catal., 138, 445 (1992) 5. K. Br/ickman, J. M. Tatibou~t, M. Che, E. Serwicka, J. Haber, J. Catal., 139, 455 (1993) 6. M. A. Schwegler, P. Vinke, M. van de Eijk, H. van Bekkum, Appl. Catal., 80, 41 (1992) 7. G. Bidan, E. M. Genies, M. Lapkowski, J. Chem. Soc., Chem. Commun., 533 (1988) 8. B. Keita, D. Bouaziz, L. Nadjo, J. Electroanal. Chem., 255, 303 (1988) 9. B. Keita, D. Bouaziz, L. Nadjo, A. Deronzier, J. Electroanal. Chem., 279, 187 (1990) 10. M. Zag6rska, I. Kulszewicz-Bajer, E. Lukomska-Godzisz, A. Profi, I. Gtowacki, J. Ulafiski, S. Lefrant, Synth. Met., 37, 99 (1990) 11. I. Kulszewicz-Bajer, M. Zag6rska, A. Proli, D. Billaud, J.J. Ehrhard, Mat. Res. Bull., 26, 163 (1991) 12. J. Polniczek, I. Kulszewicz-Bajer, M. Zag6rska, K. Kruczata, K. Dyrek, A. Bielafiski, A. Profi, J. Catal., 132, 311 (1991) 13. J. Poiniczek, A. Bielafiski, I. Kulszewicz-Bajer, M. Zag6rska, K. Kruczala, K. Dyrek, A. Profi, J. Mol. Catal., 69, 223 (1991) 14. M. Hasik, A. Profi, I. Kulszewicz-Bajer, J. Polniczek, A. Bielafiski, Z. Piwowarska, R. Dziembaj, Synth. Met., 55-57, 972 (1993) 15. T. Ito, H. Shirakawa, S. Ikeda, J. Polym. Sci., Polym. Chem. Ed., 13, 1943 (1975) 16. A. Proli, Z. Kucharski, C. Budrowski, M. Zag6rska, S. Krischene, J. Suwalski, S. Dehe, S. Lefrant, J. Chem. Phys., 83, 5923 (1985) 17. Y. Cao, A. Andreatta, A.J. Heeger, P. Smith, Polymer, 30, 2305 (1989) 18. J. Tang, X. Jing, B. Wang, F. Wang, Synth. Met., 24, 231 (1988) 19. E. T. Kang,, K. G. Neoh, S.H. Khor, K. L. Tan, B. T. G. Tan, J. Chem. Soc., Chem. Commun., 695 (1989)
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
579
Regenerable Sorbent for High-Temperature Desulfurization based on Iron-Molybdenum-Mixed Oxides R. van Yperen, A.J. van Dillen, J.W. Geus a) E. Boellaard, A.A. van der Horst, A.M. van der Kraan b) a) Department of Inorganic Chemistry, Debije Institute, Utrecht University P.O. Box 80083, 3508 TB Utrecht, The Netherlands b) Interfacultair Reactor Instituut, Delft University of Technology Mekelweg 15, 2629 JB Delft, The Netherlands Abstract Iron-molybdenum mixed oxide absorbents supported on amorphous A1PO4 can be very well adapted for the removal of H2S from coal gas at high temperatures. Although the reaction of iron(III) with phosphate decreases the capacity of the absorbent to remove H2S, the strong interaction with the support ensures a high thermal stability. During regeneration with oxygen of absorbents based on iron oxide at temperatures below 650~ Fe2(SO4) 3 is formed. Decomposition of Fe2(SO4) 3 in the subsequent absorption step leads to contamination of the purified coal gas with SO 2. With desulfurization processes based on iron-molybdenum mixed oxides regeneration can be performed at a temperature lower than 650~ while Fe2(SO4) 3 is not formed The required regeneration temperature decreases with lower Fe/Mo ratios. Instead of 0 2, also SO 2 can be used for the regeneration of sulfidized samples. Ironmolybdenum mixed oxide absorbents applied onto amorphous A1PO4 were regenerable with SO 2 at a temperature of 500~ However, the regeneration rate is lower than with oxygen.
1. I N T R O D U C T I O N The gasification of coal and heavy oil fractions is of high interest. Coal gas produced by gasification mainly consists of CO, H 2 and H20. It can be used for several chemical processes. In combination with a steam and gas turbine cycle, gasification provides an efficient and clean procedure for raising electricity [1]. Coal gas, however, contains several impurities, such as, dust, halides, hydrogen cyanide and sulfur compounds, mainly H2S and COS. These compounds are poisonous for catalysts used in subsequent processing of the coal gas and corrosive towards the equipment used. Therefore the impurities have to be removed before the coal gas can be used [2]. The removal of the sulfur compounds from gas streams is mainly achieved by scrubbing at temperatures below 100~ by means of a liquid absorbent. However, wet scrubbing and subsequent processing of the impurities separated lead to a low thermal efficiency. Before scrubbing the coal gas must be cooled and the steam must be condensed to prevent dilution of the absorbent. For further processing of the purified coal gas, e.g., to produce liquid fuels or for combustion to raise (electric) energy, the coal gas has to be reheated. Furthermore, the thermal regeneration of the liquid used in the scrubbing process and the processing of the formed impurities within, e.g., a Claus process, preferable with a tail gas treating unit, call for much thermal energy and equipment. The application of a solid absorbent which is capable of removing H2S from large coal gas streams at high temperatures and producing elemental sulfur upon regeneration would increase the overall efficiency of, e.g., electricity production. The
580 improvement is not only due to a higher overall thermal efficiency, but also to lower costs of investments [ 1]. Solid absorbents capable of removing H2S from coal gas mainly consist of metal oxides. The removal of the H2S is accomplished by the reaction of the metal oxides with H2S to the corresponding metal sulfides and water [3, 4]. The activity of the absorbent should be sufficiently high to reduce the H2S concentration in the coal gas to a level of about 20 ppm or less. Since the disposal of spent absorbent is not acceptable economically and from an environmental point of view, the absorbent should additionally be regenerable. The capacity should be as high as possible to minimize the number of absorption and regeneration cycles per hour. Furthermore, the absorbent particles should be chemically, thermally, and mechanically stable to limit the consumption of absorbent and to prevent dust problems. The product resulting from the regeneration is preferably gaseous elemental sulfur. Recent publications show that there is actually no solid absorbent based on bulk metal oxides available that meets the conditions formulated above for application in high-temperature desulfurization processes. Absorbents that have been developed all show one or more undesired characteristics. Disintegration due to a poor mechanical and chemical stability is commonly found. Metal sulfate formation during regeneration with 0 2 is a considerable problem [5]. Furthermore, absorbents based on bulk metal oxides display a relatively low activity towards the removal of H2S from coal gas. The above problems can be avoided by the application of an active compound onto a thermally and chemically stable support material. Since desulfurization has to be performed on a large scale, the absorbent must be cheap. Expensive supports or active materials are not acceptable. The only generally used support that is inert towards coal gas and SO 2 is silica. However, silica is not thermally stable at high temperatures. Especially in the presence of steam sintering proceeds rapidly and even volatilization may occur. Until now a supported absorbent appropriate for high-temperature desulfurization is therefore not available. Therefore our research was aimed at the development of an absorbent that fulfills all the specifications for employment in hot-gas clean up. 2. E X P E R I M E N T A L
2.1 Preparation of the absorbents The A1PO4 supports were prepared by the method developed by Kehl [6]. To this end a precipitation vessel as described by Van Dillen [7] was filled with 2 liter of doubly distilled water. Under vigorous stirring the pH was raised to the desired value by injection of a 5 vol% ammonia solution and kept constant during the precipitation process. As soon as the pH was stabilized, a solution of about 0.05 moles Al(NO3) 3 and, depending on the A1/P ratio desired in the A1PO4 support, an amount of (NH4)2HPO 4 in 600 ml doubly distilled water was injected below the surface of the suspension through a capillary with a maximum rate of 6 ml/min. Immediately, precipitation proceeded. In the same precipitation vessel the freshly precipitated A1PO4 supports were loaded with the active components by deposition-precipitation from homogeneous solutions [8] at a constant pH level of 5. When the precipitate had an AI/P ratio of unity or higher, the suspension of the precipitate was not removed out of the vessel. The precursors of the active material were injected below the surface of the suspension through a capillary at a rate of 1.5 ml/min. Only the precipitates with an A1/P ratio lower than one were first collected and washed with doubly distilled water and resuspended. This procedure had to be executed to remove excess phosphate. As the iron precursor a solution of Fe(NO3) 3 was used and as molybdenum precursor a solution of ammonium hepta-molybdate. After the completion of the precipitation the absorbents were filtrated and dried at 120~ overnight. After shaping to tablets, the bodies were broken and the sieve fraction between 150 and 500 ~m was collected. The obtained absorbents were calcined in air in a
581 flow reactor. The temperature was increased with 5~ to 500~ one hour. The following code for the prepared absorbents is used: Px/yFeaMob
wherein:
X-"
y= a-"
b=
and kept at this level for
the pH of precipitation of the support the A1/P ratio in the A13+ and PO43- precursor solution the loading of iron oxide (Fe203) and the loading of molybdenum oxide (MOO3) in wt% of the total weight of the absorbent
2.2 Characterization techniques
Specific surface areas were measured by nitrogen sorption with a Quantochrome Quantasorb at 77K. The samples were degassed at 300~ for 30 minutes, before the nitrogen uptake was recorded. Ex situ X-ray diffraction (XRD) was performed on a Philips Powderdiffractometer PW 1140, using FeKct radiation of a wavelength of 1.93735/~. In situ high-temperature X-ray diffraction (HTXRD) experiments were performed using an Enraf Nonius Lenn6 camera supplied with a high-temperature cell, using also F e I ~ radiation. The temperature-programmed reduction of the samples was performed in 10 vol% H 2. To this end the 10 vol% H 2 in helium flow was passed through the sample cell. The temperature was increased with 15~ X-ray photoelectron spectroscopy was performed using M g I ~ radiation with a Fisons instrument. Ex situ M6ssbauer spectra were recorded at 293, 77, and 4.2K with a constant acceleration spectrometer using a 57Co in Rh source. The source was kept at room temperature. The Mtissbauer parameters were resolved by fitting the collected spectra with Lorentzian shaped profiles. Isomer shifts are given relative to sodium nitroprusside at room temperature. Hyperfine fields (Heft) were calibrated against the 515 kOe field of o~-Fe203 at 300K. The accuracies of the parameters are: Isomer shift (IS) 0.02 mm s -1, quadrupole splitting (QS) 0.04 mm s -1, and the spectral contribution 5%. 2.3 Desulfurization test procedure
The absorbents prepared were tested for their behavior in the desulfurization of gas streams containing H2S. The gas flow was analyzed using a UV-VIS spectrophotometer, Varian Cary 1. The H2S concentration was measured at 232 nm and the SO 2 concentration at 262 nm. The experiments consisted of several absorption and regeneration cycles during which the H2S and SO 2 concentrations were monitored. The standard test conditions applied during the absorption of H2S and the regeneration of the absorbent are given in table I. The absorption step was preceded by a reduction step in 10 vol% H 2 to reduce the possibly formed Fe2(SO4) 3 at the same temperature as at which the absorption step had been performed. Between two consecutive experiments the reactor was flushed with argon for 10 minutes. Table I: Standard test conditions. absorption sample size 1.5 ml reaction mixture 1 vol% H2S 10 vol% H 2 balance Ar total flow 100 ml/min 4000 h- 1 space velocity temperature 400~
regeneration 1.5 ml 2 vol% 02 balance Ar 100 ml/min 4000 h- 1 400~
582 To make comparison of absorption and regeneration test results possible, the data are thus represented: The y-axis represents the ratio of the H2S concentration monitored beyond and at the inlet of the reactor. If all H2S had absorbed, the ratio equals zero, if no H2S had absorbed the ratio equals one. The x-axis represents the actual absorption time per gram of absorbent, divided by the calculated theoretical absorption time at 100% efficiency 9 The metal sulfides used to calculate this theoretical time are FeS and MoS 2. 3. R E S U L T S AND D I S C U S S I O N 3.1
Characterization
Figure 1 shows the specific surface areas of the absorbents as a function of the pH of precipitation of the A1PO 4. The A1/P ratio is unity and the iron loading 25 wt% F e 2 0 3. The curve has the same characteristic shape as the curve which had been obtained with the bare supports [3]. It is evident that the possibility to control the specific surface area of A1PO 4 supports is not impaired by the deposition of iron oxide.
350 300 ~t
250 -
2ooA w
= e~ 150
t--; ufi
9
100
-
500
0
2
4 6 pH level of precipitation
8
10
Figure 1" Specific surface area as a function of the precipitation pH of the AIPO 4 support. The Al/P ratio is unity and the iron oxide loading is 25 wt% for all samples.
The iron oxide absorbents were subjected to high-temperature treatments in air at temperatures of 800 and 1000~ for 1 to 16 hours. XRD patterns of the supports loaded with iron oxide all showed very broad peaks due to iron oxide, pointing to small iron oxide particles. Absorbents of which the supports were prepared at a pH of 5 showed some sintering of the iron oxide particles. This is due to the small specific surface area of this support. The consequently higher loading of iron per square meter of surface area of the support cannot be stabilized by the support at temperatures as high as 800~ However, taking the high iron oxide loading into account, the sintering was not severe. The stabilization of the iron oxide particles by supports prepared at pH levels of 3 and 8 seemed to be different. Bare A1PO 4 supports precipitated at a pH of 8 displayed a higher stability than A1PO 4 supports precipitated at a pH of 3. The decrease of the specific surface area of the support resulted in sintering of the iron oxide particles too. A higher thermal stability was also obtained by increasing the A1/P ratio. The absorbent with a support of an A1/P ratio of 1.9 showed no diffraction pattern, indicating very small iron oxide particles, in contrast to the use of supports with an A1/P ratio of 0.7.
583 The thermal stability of the iron oxide absorbent supported by AIPO 4 is mainly determined by the stability of the support. Although the A1PO4 supports are fairly thermally stable in 0 2 and H 2 atmospheres up to temperatures as high as 1000~ the thermal stability of the iron oxide absorbents is much lower in reducing atmospheres. This is probably caused by the reduction of the iron oxide to metallic iron at temperatures of about 800~ In addition, sintering of the iron particles most likely initiates the sintering of the support. By means of HTXRD the reduction of the P3/1Fe25 absorbent was followed. At 680~ iron oxide was transformed into Fe3P, which reacted at 870~ to Fe2P. The formation of iron phosphides suggests that during the reduction of the iron oxide a reaction with the support takes place. Then the thus formed iron phosphides initiate the sintering of the A1PO4 [3]. 3.2 A c t i v i t y Figure 2 shows the absorption and breakthrough of H2S with five different iron oxide absorbents. The absorbent encoded P8/1Fel4 shows a much lower capacity towards the removal of H2S than the absorbent with a alumina support. As soon as only 25% of the iron oxide of P8/1Fel4 has reacted to FeS, H2S starts to slip through the reactor. The absorbent with the alumina support has a capacity of about 90% of the theoretical level. Because both supports have the same iron oxide loading, the difference in capacity must be caused by the nature of the supports and the interaction of iron oxide with the support. If the decrease in capacity is due to surface phosphate groups, it may be possible to enhance the capacity by adaptation of the preparation conditions of the support. A very significant difference in capacity between P3/1.3Fe 14 and P5/1.3Fe 14 is evident. The main differences between both supports is the specific surface area. P3/1.3Fe14 has a substantially larger surface area than P5/1.3Fe14. It is therefore possible to accommodate more finely divided iron oxide. However, the difference in specific surface area cannot be the only reason for the remarkable difference in capacity, as the different capacity of P8/1Fel4 and P8/5Fe14 is not due to a difference in specific surface area. The specific surface areas of these absorbents are almost equal. A1PO4 supports with an A1/P ratio of one contained more and stronger acid sites, hence, the main difference is the acidity of the support. Since the acidity is related to the amount of phosphate groups on the surface, the decrease in capacity can likely be ascribed to the amount of surface phosphate groups. 1.2
100
I
P8/1Fe 14
"T ~ 0.8 0.6
?
.........,:.-.?~-:'-.'.-'.--..-":.'.,'.,2;z--~
~: ".........i / / . . . . . , P3/I.3FeI4 .."..... ,'" P8/SFeI4 ~
,,
80
60
'
.--.....
~ 40
0.4
i /
0.2
/ P5/I.3Fei4 f ~
/ alumina 20
/
0
0 0
0.2
0.4 0.6 0.8 time / theoretical time
1
Figure 2: The capacity to remove H2S at 400~ of several absorbents,
!
0
!
50 100 surface phosphate concentration / a.u.
150
Figure 3: The relation between the concentration of surface phosphate groups of the AIPO 4 support and the capacity of Fe203 supported by AIPO 4.
To investigate the influence of the amount of phosphate groups at the surface of the support on the capacity to remove H2S, several absorbents with different A1/P ratios were measured. The A1/P ratio on the surface of the support was assessed by means of XPS. To get an indication of the total amount of phosphate exposed at the surface, the surface area was
584 multiplied by the reciprocal value of the A1/P ratio as measured with XPS. As a standard the capacity of the iron oxide on alumina absorbent was defined to be 100%. Figure 3 represents the capacity as a function of the number of phosphate groups per surface unit area. The linear relationship between the concentration of surface phosphate groups of the support and the capacity of the absorbent proves that the amount of phosphate groups at the surface of the support determines the loss in capacity of the iron oxide absorbent. The deactivation of a part of the iron oxide is presumably caused by the formation of FePO 4. Tests with bulk FePO 4 as an absorbent for H2S show that FePO 4 is inactive in the removal of H2S. The formation of Fe3P during the above mentioned HTXRD experiment for the P3/1Fe25 absorbent in a reducing atmosphere is also an indication of formed FePO4 in the as prepared absorbent. A Fe203 absorbent on an A1PO4 support prepared at a pH level of 5 and with an A1/P ratio of 2 shows less FePO 4 formation. Indeed a higher capacity is found for this absorbent and no Fe3P formation can be observed during the HTXRD experiments. Although these results indicate that FePO 4 formation is the cause of the loss in capacity, it is not unequivocally proven by the above results. Mtissbauer spectroscopy was used to try to confirm the above interpretation. 3.3 Miissbauer Figure 4 shows the M/Sssbauer spectra of three absorbents measured at 300, 77 and 4.2K. As a reference FePO 4 was measured. The spectrum of the FePO 4 sample was analyzed with two doublets. The doublet with IS=0.65 and QS=I.08 was assigned to FePO 4 with a poorly defined composition due to a variable degree of hydration and/or surface effects [9]. Table II gives the isomer shift (IS), the quadrupole splitting (QS), the spectral contribution (SC), and the hyperfine field (Heft) of the samples. In the case of the P3/0.7Fe25 absorbent the support was not removed out of the precipitation vessel to be washed. Excess phosphate will therefore not have been removed, resulting in an absorbent that does not show any activity at all towards the removal of H2S. It therefore can be assumed that this absorbent mainly consists of FePO 4. The Mtissbauer spectrum of P3/0.7Fe25 recorded at 300K was analyzed by two doublets which have identical IS but different QS. The two doublets can be ascribed to FePO 4 with a bulk and a surface contribution [10]. The high percentage of the doublet contribution with the large QS of P3/0.7Fe25 implies a high surface contribution, which indicates small particles with a strong interaction with the A1PO4 support. The spectrum recorded at 77K exhibits a slightly asymmetric doublet, whereas the spectrum at 4.2K exhibits besides a doublet an extremely broadened sextuplet. The latter spectral component is indicative for a very highly dispersed Fe(III) phase with a low Debije temperature as is apparent from a strong increase of the overall resonant absorption area of the spectra at decreasing temperature. The absorbents P8/1Fel4 and P8/5Fe14 both show activity in the removal of H2S. However, the former has only a capacity of about 30%, whereas the latter shows a capacity of 70%. The activity experiments indicate a different amount of active component being present. At all temperatures the P8/1Fel4 sample gave rise to a doublet which can be assigned to FePO 4 and a well developed sextuplet of large t~-Fe20 3 particles. Since the spectrum at 4.2K shows no broadening of the central doublet the presence of highly dispersed Fe(III) oxide is not likely. Moreover, the P8/1Fel4 absorbent exhibited only a small temperature dependency of the overall resonant absorption area. The low capacity of this absorbent is therefore ascribed to the formation of inactive FePO4. The M6ssbauer spectrum recorded at 300K with absorbent P8/5Fe 14 exhibit a doublet and a sextuplet. The doublet is significantly different from the doublets exhibited by the other samples and may represent a dispersed Fe(III) oxide phase, and the sextuplet a small fraction of large ~-Fe20 3 particles. The spectrum recorded at 4.2K prove the assignment of the central doublet to a highly dispersed Fe20 3 phase as the doublet broadened and even split to a sextuplet. It cannot be excluded that the relatively broad doublet at 4.2K contains also a FePO 4 contribution. This FePO 4 may form the interlayer between support and active material.
585 The parameters of the sextuplet are difficult to qualify. From the temperature dependency of the doublet due to F e 2 0 3, it was concluded that the F e 2 0 3 particle size was about 2-4 nm. The results of the experiments together are a strong indication for the formation of FePO 4 on A1PO 4 supports. The amount of FePO 4 formed is related to the concentration of surface phosphate groups in the original support, as we indicated earlier. P3/0.7Fe25 .03
"~
P8/1Fe 14 3 0 0 K 7.27
P8/5Fe 14 300K
1.14
300K
.93
o
", "
77K
3.6
77K
"
.33
77K
2.5 4.2K
4.2K
4.2K
T
-10
5
0
5
doppler velocity mrn/s
Figure 4:
I
10-10
5
0
doppler velocity mm/s
I -10
5
0
5
doppler velocity mm/s
MOssbauer spectra of P3/O.7Fe25, P8/1Fe14, and P8/5Fe14 at 300, 77 and 4.2K.
Table II: M/issbauer parameters.
FePO4
' IS
QS
SC IS OS SC IS
QS
SC Heft IS
QS
SC Heft
10
300K 0.65 1.08 13 0.57 0.62 86
300K 0.65 1.11 41 0.65 0.55 59
177K 14.2K 0.75 1.11 42 0.76 0.64 58
0.77 0.88 <33
P8/5Fe 14
P8/1Fe 14
P3/0.7Fe25 300K 0.63 1.22 19 0.64 0.69 66 0.64 0.10 15 508
77K [ 4.2K 0.74 0.78 1.12 0.64 17 39 0.75 0.76 0.96 0.66 46 67 0.75 0.75 -0.07 -0.07 15 16 532 531
300K 0.57 1.49 45 0.59 0.87 49 0.63 0.08 5 514
77K I 4.2K 0.68 0.7 1.49 1.3 49 48 0.70 0.87 46 0.72 0.76 -0.22 -0.24 5 3 538 538 0.66 0.00 49 429
586
3.4 Regeneration During the regeneration with 2 vol% 0 2 of the loaded absorbent, all oxygen is consumed to produce SO 2. Although the rate of regeneration of FeS is high, problems arise during the following absorption step. Figure 5 shows an absorption curve as measured after a regeneration step. The H2S breakthrough curve is almost the same as that of a fresh absorbent. During the first part of the absorption process, however, formation of SO 2 is measured. A large amount of SO 2 is formed during the interaction of the regenerated absorbent with H2S as active oxygen present on the surface of the supported iron oxide species oxidizes hydrogen sulfide to SO 2. In addition SO 2 is formed by the decomposition of Fe2(SO4) 3 which is formed during the regeneration. Temperature-programmed reduction experiments show that Fe2(SO4) 3 is reduced to FeSO 4, which is thermally not stable at a temperature of 400~ the temperature at which the absorption step is carried out. Hence, formation of SO 2 consequently proceeds during the absorption step as shown in figure 5. In oxygen Fe2(SO4) 3 is decomposed at a temperature of about 650~ This is in good agreement with the results of our thermodynamic calculations. G.I. Chufarov et al. [ 11] and N.J. Kertamus [12], however, found higher decomposition temperatures. The design of the reactor, the precise location of the thermocouple, and the reaction conditions may cause the apparent difference in the decomposition temperature. Especially the severe temperature rise as a consequence of the exothermic reaction can result in the decomposition of F e 2 ( S O 4 ) 3 t o proceed at temperatures higher than the measured temperature of the reactor. According to thermodynamic calculations also the formation SO 3 during the regeneration of Fe2(SO4) 3 in 0 2 is possible. However, with the equipment used in this study it was impossible to detect SO 3. ! 0.8
~
"eductiol ,, 15000
SO2
'T
i
,,4-
0.6
! e i | |
9 0.4 0.2
H2S
10000 ~ 9
I I
fJ 2
5000
/ \ ~
0
12000
1
20000 ii
0.5
0
I
time / theoretical time
absorption
0.8
9000
"T r~
~~
0.6
i
& 0.4
SO2
H2S
6000
~lj
0.2
~ 9 ~
3000
~ ;,
|
0
"
-0.4
%
,
.
0.1
0.6
J
0
i.1
time / theoretical time
Figure 5: The absorption of hydrogen sulfide by a Figure 6: The absorption of hydrogen sulfide by a regenerated 25 wt% iron oxide on silica absor- regenerated 25 wt% iron oxide on silica absorbent at 400~ bent preceded by a reduction step at 400~
So, SO 2 formation during the absorption of H2S by regenerated iron oxide can be avoided by performing the regeneration at temperatures higher than about 650~ However, such an elevated temperature demands high investments. Alternatively a third step in the absorption regeneration cycle could be introduced, viz., the reduction and decomposition of F e 2 ( S O 4 ) 3 between every regeneration and absorption step. But, as we will show below, such an approach is unactractive for industrial processes Figure 6 shows an absorption curve preceded by the decomposition in H 2 of the Fe2(SO4) 3 formed during the regeneration. At the beginning of the absorption only 10 vol% H 2 and no H2S flows through the absorbent bed. During this period of time the formation of SO 2 is detected. At t=0, H2S is also passed through the reactor. The absorption curve has the
587 same characteristic shape as observed with a fresh absorbent. The capacity of the absorbent has not decreased substantially. Introduction of a third step in the high-temperature desulfurization process is therefore a solution to the sulfate problem. However, high SO 2 concentration in the off gas are desired for the technical processing. The regeneration of the absorbent with mixtures of SO 2 and 0 2 to achieve such a high SO 2, results in large amounts of Fe2(SO4) 3. The reaction to FeS after decomposition causes a significant decrease of the capacity. It is clear that together with the three product gases resulting from the three steps in the desulfurization cycle, make this approach less attractive for industrial purposes. The formation of Fe2(SO4) 3 can also be prohibited by the use of other metal oxides. Because molybdenum sulfates are not known, iron-molybdenum mixed oxide absorbents were prepared and tested. Figure 7 shows a H2S absorption curve of P5/2Fe 12Mo 10. The capacity is slightly smaller than observed with iron-molybdenum mixed oxide absorbents supported by silica due to the strong interaction of a part of the active compounds with the phosphate groups. The metal phosphates formed are not active in the absorption of H2S. The iron-molybdenum mixed oxide absorbents still showed release of some sulfur dioxide during the reduction step. This indicates the formation of Fe2(SO4) 3 during the preceding regeneration by oxygen. Experiments at several temperatures, however, revealed that the formed Fe2(SO4) 3 is less stable than the sulfate formed with pure iron oxide absorbents. The decomposition temperature in oxygen of Fe2(SO4) 3 in iron-molybdenum mixed oxide absorbents turned out to be related to the Fe/Mo ratio. With increasing Fe/Mo ratios the decomposition temperature of Fe2(SO4) 3 rises. The reason for this behavior is found in the Mo/Fe ratio at the surface of the absorbent. By means of XPS it was found that the Mo/Fe ratio at the surface is significantly higher than the ratio of the bulk. Although regeneration of loaded absorbents with mixtures of SO 2 yields gas flows of high SO 2 concentrations, the ultimate goal of many investigations is to produce elemental sulfur in the regeneration step. Preliminary tests revealed that iron, molybdenum, and iron-molybdenum mixed oxides show different activities during regeneration with SO 2. It was evident that the iron-molybdenum mixed oxide absorbents were regenerated to a higher extent during the period of time applied and at lower temperatures. The regeneration time, however, is much longer than with 0 2. With 2 vol% 0 2 it takes about 5 minutes in the experiments described here, while the regeneration time with SO 2 was set to one hour. Figure 8 shows the absorption of H2S by P5/2Fe 12Mo6 after regeneration of the absorbent with 100% SO 2 during one hour at 500~ The absorption behavior is the same as found after regeneration with 0 2. SO 2 formation during the reduction step is not observed, hence, no Fe2(SO4) 3 has left after the regeneration with SO 2. 1000
1
reduction .=
absorptionS
0.8
12000
800
9000
H2S [ 0.6
600
SO2
0.4
400
,
, I000
reductior 750
D. r./3 r
SO2 6000
500
3000 :I .-' " , - ' , 9: ' , , '
25O
C/2
~
0.2
2OO
i
4
0
...._, ~_s.....~' . . . . ,(~'r
0 0
-0.4
0.I
0.6
I.I
time/ theoreticaltime Figure 7: The absorption curve of H2S with P5/2Fel2MolO at 400~ preceded by a reduction step.
. . . .
-0.5
J 0 0.5 I time/ theoreticaltime
J0 1.5
Figure 8: The absorption curve of H2S with P5/2Fe12Mo6 at 400~ preceded by a reduction step. The absorbent had been regenerated with 100% sulfur dioxide at 500~ for one hour.
588 4. CONCLUSIONS The controlled preparation of A1PO4 leads to supports that can be very well adapted to be utilized in hot-gas cleaning. The application of iron oxide and molybdenum oxide, which are active in the absorption of H2S, onto these supports can be easily performed. The highly dispersed active oxide particles display a high stability towards thermal sintering. Keeping the absorbents for prolonged periods of time at 1000~ does not have a significant effect on the specific surface areas. The specific surface area of the support can be controlled b y the pH level during precipitation of the support, and the A1/P ratio of the A13+ and PO 4 solution from which the precipitation is performed. However, a certain fraction of the active material reacts with the surface phosphate groups of the support to inactive iron phosphate and molybdenum phosphate. The amount of metal phosphate depends on the specific surface area of the support and the surface concentration of phosphate groups. Both are influenced by the pH of precipitation of the support and by the AI/P ratio in the initial A13+ and PO43- solution. Although the formation of metal phosphate decreases the capacity of the absorbent towards the removal of H2S, the strong interaction with the support ensures a high thermal stability. The use of absorbents based on iron oxide for high-temperature desulfurization processes meets with some problems due to the formation of Fe2(SO4) 3 during the regeneration with oxygen at a temperature below 650~ The formed Fe2(SO4) 3 decomposes to iron oxide at a temperature of 650~ and SO 2 is formed. Under reducing conditions Fe2(SO4) 3 is reduced to FeS and SO 2. The reduction should therefore be performed in a separate step. The use of iron oxide as an absorbent leads therefore to a process operating either at temperatures above 650~ or to a three step process. Both demand high investments. The use of absorbents based on iron-molybdenum mixed oxide inhibit Fe2(SO4) 3 formation. The regeneration temperature has to be higher the higher the iron oxide content, to prevent Fe2(SO4) 3 formation during regeneration. The regeneration temperatures are, however, much lower than required with pure iron oxide absorbents. Iron-molybdenum mixed oxide absorbents applied onto amorphous AIPO 4 could be regenerated with SO 2 at temperatures of 500~ The regeneration rate is, however, lower than with oxygen.
Acknowledgment We would like to thank A.J.M. Mens of the Surface Science Division of the Debije Institute of the Utrecht University for performing the XPS experiments.
References [ 1] System study High-temperature cleaning at IGCC systems, NOVEM, the Netherlands (1990). [2] Coal gasification: direct applications and synthesis of chemicals and fuels; Energy: the international journal, 8/9 12 (1987). [3] R. van Yperen, PhD thesis Utrecht University, the Netherlands (1994). [4] P.R. Westmoreland, J.B. Gibson, D.P. Harrison, Environmental Science & Technology 11(5) (1977). [5] R. Clift and J.P.K. Seville (eds.), Gas cleaning at high temperatures, Glasgow (1993). [6] W.L. Kehl, U.S. Patent 4.080.311 (1978). [7] A.J. van Dillen, J.W. Geus, L.A.M. Hermans, J. van der Meyden, Proc. 6th Int. Conf. on Cat., London, G.C. Bond, P.B. Wells, F.C. Tomkins (eds.), 2 667 (1976). [8] J.W. Geus, Prep. of Cat.III, G. Poncelet, P. Grange, P.A. Jacobs (eds.) (1983)1 [9] W. Kerler, W. Neuwirth, Z. Physik 167 (1962) 176. [10] A.M.v.d. Kraan, Phys. Stat. Sol. (a) 18, (1973) 215. [ 11] G.I. Chufarov, B.D. Averbukh, Chem. Abst. 43, 7854g (1949). [12] N.J. Kertamus, Prep. Am. Chem. Soc-Div. Fuel Chem. 18, (1973) 131-140.
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
589
A Comparative Study of the Photocatalytic Activities of Iron-Titanium (IV) Oxide Photocatalysts prepared by Various Methods; Spray Pyrolysis, Impregnation and Co-Precipitation. Roger I. Bickleyl, * Laurence T. Hogg 1, Teresita Gonz~lez-Carre~o 2 a n d Leonardo Palmisano 3. 1Department of Chemical E n g i n e e r i n g / C h e m i s t r y and Chemical T e c h n o l o g y , University of Bradford, Bradford, West Yorkshire, BD7 1DP, U.K. 2Instituto de Ciencia de Materiales, C.S.I.C., Madrid, Espana. 3Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universita di Palermo, 90128 Palermo, Italia. * Author to whom correspondence should be addressed. 1. ABSTRACT. A series of iron/titanium oxide photocatalysts of varying Fe 3§ content have been prepared by a number of different methods and subjected to controlled thermal treatment. Specimens have been characterised in terms of their crystalline structure and morphology, and indicate solid solutions containing both the anatase and ruffle phases of TiO 2, together with the formation of pseudo-brookite in some specimens. Intrinsic photo-activity m e a s u r e m e n t s have been p e r f o r m e d using a s t a n d a r d liquid-phase photocatalytic reaction. Results indicate that the photocatalytic activity of the specimens differs according to the method of preparation, and that all solids containing iron (III) species were less active than the parent titania, irrespective of w h e t h e r or not they were true solid solutions; the solid s o l u t i o n s c o n t a i n i n g pseudo-brookite being less active than those which did not. 2. INTRODUCTION. Catalysts which are doped with foreign ions in order to modify their activity have been p r e p a r e d traditionally by i m p r e g n a t i o n of the solid s u p p o r t or by coprecipation followed by controlled thermal treatment. Similar methods have been adopted also in the preparation of iron-titanium (IV) oxide photocatalysts in which the introduction of iron as iron (III) species has resulted in the enhancement of the activity of the solids in the photocatalytic reduction of dinitrogen by water vapour [1,2,3]. Recently, a novel method for the preparation of highly dispersed solids using a spray-pyrolysis technique has been reported [4]. This technique has been applied to the synthesis of iron-titanium(W) oxide photocatalysts in the present study, in which are provided details of the method of spray pyrolysis used to prepare solids with a range of nominal iron contents from 0.1 - 10 atom %. Structural characterisation has
590 been achieved by X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM), Energy-Dispersive X-ray Analysis (EDX), and Fourier Transform infra-red Spectroscopy (FrlR). Correlations of the information obtained have been made with the extensive data already reported in previous studies of similar solids prepared by impregnation and coprecipitation [5,6,71 Photocatalytic activities of the solids have been d e t e r m i n e d in a batch photo-reactor under carefully controlled thermal and photometric conditions, details of which are described in the present study. A measurement of the zero order rate of the photo-oxidation of liquid propan-2-ol to propanone [8] has been adopted as a standard test reaction.
3. EXPERIMENTAL METHODS. Four series of iron-titanium oxide photocatalysts have been prepared; two series by impregnation of solid titanium dioxide (Tioxide CLDD 1625/2 {TOx}, and Degussa P25 {Dg]), with aqueous iron Fe 3+ ions, and two by chemical methods from titanium and iron salts (coprecipitation {CP}, and spray pyrolysis [SP]). The concentration of iron, expressed as percentages of the total cationic content of the TiO 2 matrix ranged from 0.1 to 10 atom percenL Specimens have been allocated a code n u m b e r in accordance with their method of preparation, nominal content of iron and the temperature at which the catalyst has been calcined e.g. T F / S P / 1 / 8 7 3 and T F / D g / 5 / 1 2 7 3 refer to specimens prepared by spray pyrolysis and impregnation of Degussa P25 solid pre-cursor, containing 1 and 5 atom% ofiron and which have been calcined at 873K and 1273K respectively.
3.1. Catalysts prepared by Spray Pyrolysis (TF/SP specimens). The preparation of fine TiO 2 and Fe-TiO 2 particles has been achieved using a spray pyrolysis method, details of which have been described elsewhere [4]. Liquid droplets generated by an atomiser spray are carried via an air flow system into two consecutive heated zones held at different temperatures. The initial furnace (473-573K), effected the rapid evaporation of the solvent producing solid pre-cursors, whilst the function of the second furnace was to decompose the solid at 773-823K p r o d u c i n g i r o n - t i t a n i u m oxides. E t h a n o l i c S o l u t i o n s (0.07M), of t i t a n i u m tetra-ethoxide and iron III nitrate, were atomised at a c o n s t a n t flow rate of 1.64cm3min -1. Subsequent to solvent evaporation and calcination in the first and second heated zones, specimens were collected and fired at higher temperatures in air in the range 823-1073K.
3.2. Catalysts prepared by Coprecipitation (TF/CP specimens). Iron and titanium hydroxides were coprecipitated in the required nominal amounts by reacting an aqueous solution of TiC13 (15 wt.%, Carlo Erba), containing the required quantity of Fe 3+ ions (ex Fe(NO3).9H20, Merck), with aqueous ammonia (25 wt.%, Merck). Subsequent to careful separation of the solid, the specimens were washed and dried before aliquots were calcined in air at fixed temperatures in the range 773K to 1273K for a period of 24 hours.
591
3.3. Catalysts prepared by Impregnation (]'F/Dg and TF/TOx specimens) Commercial titanium dioxide preparations were added, at constant temperature and with stirring, to aqueous solutions of iron III nitrate containing the required amounts of iron ions in the minimum volume of doubly-distilled water. Subsequent to separation and drying, the solid specimens were calcined in air at fixed temperatures in the range 773K to 1273K for a period of 24 hours. Experimental methods detailing the preparations of ii and iii above have been reported previously [5,71. 4. CATALYST CHARACTERISATION.
4.1. General Characterisation. a. Specimens prepared by coprecipitation and impregnation have been extensively characterised and the results have been reported previously [7]. b. Catalysts prepared by spray pyrolysis have been characterised by x-ray diffractometry (Phillips PW 171 diffractometer, Cu-Kct radiation source), infra-red spectrometry (Nicolet 20 SXC), and transmission electron microscopy (Phillips 300 microscope). The chemical composition of the specimens prepared was evaluated by TEM (Jeol 200 FX2), using an energy-dispersive x-ray analyser (Link QX 2000). Specimens have also been characterised in terms of an intrinsic photoactivity index.
4.2. Determination of Photoactivity. The photocatalytic activity of each specimen was determined in a small batch photoreactor (200cm 3 capacity), held at constant temperature (24+1~ and under constant focussed photon flux from a 500W medium pressure mercury arc lamp (Oriel Products Ltd.). The photoactivity index was measured as the zero order rate of the photocatalytic oxidation of liquid propan-2-ol to propanone [8], under oxygenated conditions for a fixed mass of catalyst (150mg, catalyst concentration = 0.75g.1-1). Photoactivities are expressed in terms of moles converted per gram of catalyst per hour of irradiation. Propanone analysis was conducted at regular intervals using gas chromatography (Perkin Elmer GC1830, 8m PEG/Chromosorb W column, Helium carrier gas). 5. DISCUSSION OF RESULTS.
5.1. Specimens prepared by Spray Pyrolysis. The diffraction patterns of the decomposed aerosol products ofTiO 2 and Fe-TiO 2 obtained by spray pyrolysis showed poor crystallinity. Specimens were therefore further calcined at temperatures of 823K, 873K, 973K and 1073K in air for a period of 2 hours in order to improve their crystalline structure. Fig.1. shows the x-ray diffraction patterns of titanium dioxide (anatase), and a series of iron-titanium oxide specimens of nominal Fe(IlI) concentration equivalent to 0.1, 1, 2, 5 and 10 atom%, prepared by the spray pyrolysis method and subsequently calcined at 823K in air for 2 hours.
592
II
[cotLnts] J.4BO
1200
I000
Anatase*~,~,~-.,~
8 El0
10% 6130 5% 100
2% 1%
2013
0.1%
8.8
0
~, I
I
I
20
i
|
48
GO
[ ': Z~? ]
Figure 1. XRD Data for TF/SP Specimens. Good crystallinity of the anatase phase is shown in both the pure specimen and the iron doped catalysts. Significant quantities of the rutile phase are also apparent at this calcination temperature in specimens of higher iron concentration (above ca. 2 atom%). This indicates an anatase-rutile transformation which occurs at some 200 ~ below the temperature at which it is reported to occur in pure anatase TiO 2 [9]. This transformation may be observed at lower iron concentrations as the firing temperature of the secondary calcination is progressively raised from 823K to 1073K according to Table 1. TABLE 1. % of anatase in TF/SP Specimens. T~/K
823
873
973
1073
% Fe 0 0.1 1 2 5 10
100 98 95 89 46 15
89 100 94 82 46 0
77 85 76 21 6
42.5 32 25.5 0 0(PB)
O(PB)
O(PB)
* (PB) indicates the presence of pseudo brookite in these specimens.
593 Percentages of thc crystal phases present were determined from integrated XRD peak heights according to the method of Criado et.al [10]. Additionally, the formation of pseudo-brookite is apparent in specimens of greater than 2 atom% nominal iron content calcined at 1073K. Evidence of pseudo brookite is also obtained in the specimen TF/SP/10/973. Results obtained by infra-red spectroscopy showing lattice vibrations over the range 850-200cm -1 also indicate the presence of rutile in selected specimens in accordance with data obtained by x-ray diffractometry. Chemical composition and morphology measurements obtained by TEM and EDX indicate actual iron concentrations in specimens prepared by spray pyrolysis to be in close agreement with the predicted values of 0.1, 1, 2, 5 and 10 atom%. TEM microanalysis of single particles also shows compositional surface homogeneity of Fe/Ti ions with no islands of iron oxide present in specimens of higher iron content. Microscopic studies, Fig.2., reveal that the solid particles of Fe-TiO 2 produced by spray pyrolysis are non-agglomerated fine spheres with a moderately narrow particle size distribution and similar morphology.
Figure 2. TEM of TF/SP/2 particles. Particle size analysis (TEM), indicates that the surface area of these specimens does not fall significantly below 10m2g1, and further that the average particle size is decreased from 163nm in the pure titania to 12 l nm in the specimen T F / S P / 2 / 8 7 3 and further decreased at higher iron contents/calcination temperatures.
594
5.2. Photoeatalytie activity of Fe-TiO2 catalysts. Standard photoactivity measurements were performed on all specimens prepared by spray pyrolysis subjected to secondary calcination at 823K, and a selection of impregnated and coprecipitated specimens. Results presented in Table 2. show a marked decrease in the photocatalytic activity of specimens prepared by the spray pyrolysis method with increasing iron content, Fig.3.
\ 25 r,I !
~.9 20 I
0 ~: 15
< 10
9
0
0
I
I
I
I
I
2
4
6
8
10
[Fe3+]/atom%
Figure 3. Photo-activity of TF/SP specimens, calcinced at 823K, for propan-2-ol photo-oxidation. TABLE 2. Photocatalytic activities (mol.g-lh-1). Specimen Code i,,,
1 1 104 Photoactivity/mol.g-h
ii,
TF/SP/0.1/823 TF/SP/1/823 TF/SP/2/823 TF/SP/5/823 TF/SP/10/823
28 26 15 9 7
TF/Dg/0.5/923 TF/Dg/0.5/1073 TF/Dg/0.5 / 1273
46 2.5 0
595 TABLE 2 continued. Photocatalytic activities (mol.g-~h~). Specimen Code
10 4
Photoactivity/mol.glh ~
TF/Dg/1/923 TF/Dg/2/1073
17 9
TF/CP/0.5/773 TF/CP/0.5/923 TF/CP/0.5/1073 TF/CP/0.5/1273 TF/CP/1/923 TF/CP/2/1073
51 29 0 0 45 34
TF/TOx/0.5/773 TF/TOx/0.5/923 TF/TOx/0.5 / 1073 TF/TOx/0.5/1273 TF/TOx/1/923
12 0 0 0 14
This trend is also observed for T F / D g specimens, whilst photoactivity data for TF/TOx and T F / C P specimens indicate an increased activity in catalysts of higher nominal iron contenL The effect of calcination temperature upon the photocatalytic activity of impregnated and coprecipitated specimens is also shown in Table 2., where a fall in activity is observed for Fe-TiO 2 specimens calcined at progressively higher temperatures. O n e o f the most active specimens prepared was T F / D g / 0 . 5 / 9 2 3 (46x10 "4 mol.g-lh-1), which may reflect the initially high activity of the pre-cursor material (Degussa P25 - 123xl 0 .4 mol.g-lh-1). All specimens containing a nominal concentration of iron were s u b s t a n t i a l l y less active than the pure titania for the s t a n d a r d photo-oxidation reaction. 6. CONCLUSIONS. i) All dilute solutions are either monophasic or biphasic as a result of the anatase-rutile transition for all forms of specimen. ii) Clear evidence of iron promoting the anatase-rutile transition, presumably through the creation of vacancies in the TiO 2lattice, is observed [111. iii) At higher Fe 3§ concentrations pseudo-brookite appears; its appearance in the SP series of specimens is detected at lower concentrations of Fe 3§ than in other series ie. 2% Fe 3§ rather than 5% Fe 3§ iv) The particle shape and size can be controlled more effectively through the use of the spray pyrolysis technique. v) The intrinsic photocatalytic activities of the T F / S P series of specimens decreased with increasing Fe 3§ content to reach an apparent threshold value at 10%
596 Fe 3§ The decrease is consistent with the corresponding effects in the T F / D g impregnated specimens but contrasts with previous observations using the N2/H20 reaction 12,31. vi) An apparent increase in photocatalytic activity is observed for the TF/TOx and TF/CP series of specimens which is contrary to those observed for TF/SP and TF/Dg specimens. These differences currently cannot be reconciled. 7. REFERENCES. 1. Schrauzer, G.N., Guth, T.D., J.4.C.S, 99, p7189, 1977. 2. Augugliaro, V., Conesa, J.C., Palmisano, L., Schiavello, M., Sclafani, A., Sofia, J., J. Phys. Chem., 95, p287, 1991. 3. Augugliaro, V., D'Alba, F., Rizzuti, L., Schiavello, M., Sclafani, A., InL J. Hydrogen Energy, 7, p851, 1982. 4. Gonzdlez-Carrefio, T., Mifsud, A., Palacious, J.M., Sema, C.J., J. Materials Chemistry and Physics, 27, p287, 1991. 5. Bickley, RI., Gonz~lez-Carrefio, T., Palmisano, L. in Preparation of Catalysts IV, B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet Eds., p297, 1987. 6. Bickley, RI., Gonzdlez-Carrefio, T., Gonzalez-Elipe, A~R, Munuera, G., Palmisano, L., J. Chem. Soc. Faraday Trans., (in press). 7. Bickley, RI., Lees, J.S., Palmisano, L., Schiavello, M., Tilley, R, J. Chem. Soc. Faraday Trans., 88, p377, 1992. 8. Green, K.J., Rudham, R, J. Chem. Soc. Faraday Trans., 85, p1867, 1993. 9. Rao, C.N.R, Can. J. Chem., 39, 1961. 10. Criado, J., Concha, R, J. Chem. Soc. Faraday Trans., 79, p2765, 1983. 11. Bickley, RI., Gonz~lez-Carrefio, T., Palmisano, L., J. Materials Chemistry and Physics, 29, p475, 1991.
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparationof HeterogeneousCatalysts G. Ponceletet al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
597
C o o r d i n a t i o n c o m p o u n d s of m e t a l s i n c o r p o r a t e d in polyorganosiloxane matrices. XIII. (Co)(II) complexes with salen salophen and molecular oxygen T.N. Yakubovich a, V.V. Teslenko a, K.A. Kolesnikova a, Yu.L. Zub a, and R. Leboda b a Institute of Surface Chemistry of Ukrainian Academy of Sciences, pr. Nauky 31, Kiev 252028, Ukraine b Faculty of Chemistry, Maria Curie-Sclodowska University, P1. Marii CurieSklodowskiej 3, Lublin 20-031, Poland ABSTRACT Using sol-gel method, samples of poly(3-aminopropyl) siloxane matrices containing incorporated complexes of Co(II) with N,N'-bis (salicylaldehyde) e t h y l e n e d i a m i n e and N,N'-bis (salicylaldehyde) o-phenylenediamine were obtained. On the basis of ESDR and EPR investigation of incorporated complexes, it is determined that a high proportion of cobalt belongs to a highspin t e t r a h e d r a l complex and a small part of it occurs in a low-spin oxygen complex. 1. I N T R O D U C T I O N Coordination compounds of metals displaying a structural and/or functional analogy with oxygen-containing centres of n a t u r a l carriers of molecular oxygen are of doubtless interest as oxidation catalysts of organic substrates [1,2]. A coordination centre in such systems is usually planar, and in most cases, it has a MN4 composition (more rarely MN202) where M- is a 3d-metal ion. An additional coordination in axial position of an electron-donor ligand favors the reversible O2 bonding by the sixth vacant position of the distorted octahedron [3]. Modeling a protein s u r r o u n d i n g the n a t u r a l carriers of molecular oxygen p r e s e n t s c e r t a i n difficulties [4]. The idea of u s i n g polyorganosiloxanes (POS) with incorporated metal complexes was expressed in 1985 [5]. Actually, POS matrices with functional electron-donor as well as alkyl groups produced by a sol-gel method [6] allow a hydrophobic surrounding to be formed around the coordination centers. In addition, in a process of a matrix synthesis, one can easily vary the nature of the electron-donor groups and the distance between these groups as well exert an additional influence on the coordination centre at the expense of the matrix effect [7]. Thus, studying POS matrices with incorporated metal complexes, it is possible to produce new heterogeneous catalysts for various oxidative processes. In this paper, we p r e s e n t the results of the p r e p a r a t i o n and investigation of new cobalt(II) oxygenated complexes with N,N'-bis (salicylaldehyde) ethylenediamine (salen)
598 and N,N'-bis (salicylaldehyde) o-phenylenediamine (salophen) incorporated in poly(3-aminopropyl) siloxane matrix. This problem was briefly discussed in [8]. 2. EXPERIMENTAL 2.1. S t a r t i n g r e a g e n t s The complexes of Co(II) with salen and salophen were p r e p a r e d according to [9,10]. The poly(3-aminopropyl) siloxane matrix (SAP) was obtained by the p r o c e d u r e [11] with some small modifications of this m e t h o d [12]. Tetraethoxysilane (TEOS) and 3-aminopropyltriethoxysilane (APTES) (Aldrich) were used without previous purification.
2 ~ Catalyst p r e p a r a t i o n The reaction of hydrolytic polycondensation of tetra- and threefunctional silanes in the presence of Co(II)-complexes was used for the preparation of the incorporated catalyst" +H20(DMF) (EtO)3Si(CH2)3NH2+(CoBS)202+Si(OEt)4 ................... > -EtOH
->SiO2 9 -O-Si(CH2)3NH2CoBS.O2 I
where BS is Schiff base. The complex of cobalt with Schiff base was dissolved in 24 mmol of APTES, then 48 mmol of TEOS were added to this system. The obtained solution was placed on an ice-bath and 0.26 tool of H20 was added while mixing. After a few minutes, the mixture was solidified. It was crushed, allowed to stand at room t e m p e r a t u r e for two hours, and dried for six hours at 100~ The solid was washed with water (600ml) and dried at 100~ once more. The obtained catalyst was a light-brown powder. The product yield was 6 g. For the p r e p a r a t i o n of incorporated Cosalophen complex with CCo=10.8.10 -5 tool/g, 10ml of DMFA was added to the systems. The content of the incorporated complex (Table 1) was determined by EDTA titration of Co 2+ in solution prepared after treating the samples with a mixture (1:1) of sulphuric and perchloric acids under heating. 2~. ~ t r a l studies Electronic absorption and diffuse reflectance spectra (ESDR) were obtained with a "Specord M-40" spectrophotometer. IR spectra were recorded with a "Perkin E l m e r FT-IR 1725X" s p e c t r o p h o t o m e t e r provided w i t h diffuse reflectance accessory for solid samples. EPR spectra were recorded with a SE/X-2543 spectrometer at 77 K and 300 K. Primary treating and simulation of EPR spectra were carried out by special algorithms using IBM PC/XT type computers. 2.4. M e a s u r e m e n t s of specific surface a r e a The specific surface area of samples was determined by the BET method ("Carlo E r b a Sorptomatic-1800") using nitrogen adsorption at 77 K. The samples were heated in vacuum at 100~ before measurement.
599 3. RESULTS AND DISCUSSION Using the method described above, SAP matrices were obtained which contained incorporated complexes : cobalt with salen - from 1.8.10 -5 to 15.3.10 -5 tool/g; cobalt with salophen - 1.2.10 -5 to 10.8.10 -5 mol/g (Table 1). The values of the specific surface area and average pore radius of the resulting samples are given in Table 1. As can be seen in this table, the specific surface area depends on the content of the incorporated complex. This was observed in previous studies [13,14], but Ssp decreased with the increase of the complex content (for example [Co(CO)3L)2, Rh(CO)C1L2 [13] or heroin [14]). However, in our case, the increase of the complex content leads to an inverse result : Ssp is increased. Under a given value of Coo, the specific surface area decreases. Obviously, the influence of the complex nature on the value Ssp for SAP matrices as well as the dependence of Ssp on Cco have a complicated character. As shown in Table 2, the change of CCo has an effect on the C/N molar ratio (theoretical value of C/N for SAP matrix is 3). Table 1 Specific surface area and average pore radius of SAP matrix at different contents of incorporated complex Complex in SAP
CCo.105,mol/g
Ssp,m2/g
R,/~
1.8 6.4 11.1 15.3 1.2 3.1 4.3 10.8
85 128 152 116 88 95 140 2
185 90 85 97 72 78 88 57
CO(II) with salen CO(II) with salophen
Table 2 Analytical data for SAP matrices with incorporated Cosalen
Cco. 105,mol/g
% Si
%C
%H
%N
C/N
6.4 11.1 15.3
31.93 32.30 27.82
15.52 15.57 15.84
3.98 3.88 5.18
5.24 4.93 4.82
3.3 3.4 3.5
CfN
-
%Cs %Ns ~ 12 " 14 ' where %Cs = %C -%Ccomplex ; %N s by analogy to %C s
As follows from the above mentioned method of p r e p a r a t i o n of the incorporated complex, the interaction between APTES and cobalt complex precedes hydrolytic polycondensation. It is known from [15] t h a t dioxygen complexes are formed by dissolving Cosalen in nitrogen-containing solvents
600 u n d e r oxygen atmosphere. The absorption s p e c t r u m of such complex in DMFA solution is c h a r a c t e r i z e d by a b a n d in the visible region with Vmax=25.8 103 cm -1. Heating the dioxygen complex (to approximately 100 ~ C) leads to deoxygenation. The process of deoxygenation is accompanied by changes of the absorption spectrum 9the band at 25.8.103 cm -1 disappears, but several new bands appear at Vmax=28.8.103, 24.6.103 and n e a r 20.6.103 cm -1. A similar picture is observed when dissolving Cosalen in APTES in air 9the band appears at ~max = 25.3.103 cm "1 (Fig.la) and disappears after heating (100 ~ C) under Ar atmosphere (Fig. lb). A further oxygenation leads to the formation of the oxygenated complex (02 molecule may be bonded again with the complex if oxygen is passed through the solution cooled at 4~ In this case a band at ~)max = 25.3.103 cm ' l is observed again in the spectrum (Fig. lc). Thus, the electronic spectra unambiguously confirms the formation of the dioxygen complex in dissolving Cosalen in APTES. However, it should be noted that this complex may be both a mononuclear (APTES Cosalen 02) a n d binuclear compound ([APTESCosalen]202). The absorption spectrum of Cosalophen in APTES (Fig. ld) is characterised by bands with absorption peaks at 27.5.103, 25.2.103, 21.4.103 and 19.0.103 cm -1. The deoxygenation of such complex solution at ~ 100 ~ C (Fig. le) is accompanied by the disappearance of the first and second bands and by a shift of the third band, from 21.4.103 cm -1 to 22.1.103 cm-1. Figure 2 shows the ESDR spectra of cobalt complexes incorporated in SAP matrices. The sample with Cosalen is c h a r a c t e r i z e d by a shoulder at 45.103 cm -1, absorption bands at 39.103, 26.103 and a shoulder at 19.103. After heating this sample at 115 ~ C, an additional band appears at 31.5. 103 cm "1. The ESDR spectra of Cosalophen incorporated in SAP matrix shows bands of v max at 39.103, 29.103, 27.5.103, 25.103 and 21.6.103 cm -1. Heating (- 115~ this sample does not lead to a change of the spectral characteristics. The behavior of the sample under heating indicates that most of the complex incorporated in SAP matrix is the cobalt compound exempt from molecular oxygen. Figure 3 shows the IR spectra of samples of cobalt complexes incorporated in SAP matrices and of the SAP matrix. Two strong absorption bands at 1060 and 1145 cm -1 are observed in the 900-1800 cm -1 region, indicating the formation of cross-linked polyorganosiloxanes [16]. Besides, a n u m b e r of additional weak bands are observed at 1632 cm -1 and 1548 cm -1 (for Cosalen), and at 1630 cm -1 and 1532 cm -1 (for Cosalophen), typical of the vibration of phenyl ring and conjugate C=N bond in Schiff bases. A very broad EPR line (Fig.4) for the SAP m a t r i x with incorporated complexes centered at 1500 G is due to paramagnetic centers with spin value S=3/2. A similar signal is observed for high-spin tetrahedral Schiff base cobalt complexes [17]. Besides, two additional superimposed EPR lines are observed in this spectrum.
601
~
t.5
a
d
d
1
r
t.0
0.5
r
"
a
,
25"
20
5O
Figure I. Absorption spectra of cobalt complexes in APTES : cobalt with salen (c=l.4.10-4mol/l)-(a) initial;(b) after heating (~ 100~ (c) sample (b) after oxygenation (~ 4~ 02); cobalt with salophen (c-7.4.105mol/l)-(d) initial;(e) after heating (- 100~
I
a,
..,
________.~-.-.-"-------.. .. ..... 2000
Y600
1 . . . . . . .
4,0
i
,
}) , /o303 c nZ # 2_~,o
Figure 2. Diffuse reflectance spectra of cobalt complexes in SAP : with salen-(a) initial; (b) after heating (~ 115~ and with salophen - (c) initial; (d) after heating (- 115~
L,'
Jli
t ~-..
2
,,~,Xx,.~..,.__..____,.,~,.
t20Q
g00
400
~), c ~ - t
Figure 3. IR spectra 9(1) cobalt with salen in SAP; (2) cobalt with salophen in SAP; (3) SAP
602 J)
t.5"
d
,t .,.o
t.0
0.'5
25"
20
L~
t5
~. (o ~,cm-:~---~
Figure 1. Absorption spectra of cobalt complexes in APTES :
cobalt with sa]en (c=l.4.10-4mo]/])-(a) initial; (b) aI~r heating (~ IO0~ (c) sample (b) aRer oxygenation (~ 4~ 02); cobalt with salophen (c=7.4.105mo]/l)-(d) initial; (e) after heating (- IO0~
Figure 2. Diffuse reflectance spectra of cobalt complexes in SAP : with salen-(a) initial; (b) after heating (- 115~ and with salophen - (c) initial; (d) aider heating (- 115~
f' ~d .........
2000
. ............
I
t600
....
=. . . . . . . .
1
t200
.......
t
. . . . .
1
4
. . . . . .
3#0
~
,1
L
400
Figure 3. IR spectra 9(1) cobalt with salen in SAP; (2) cobalt with salophen in SAP; (3) SAP
603
. . . . . . . . . . . . . . . . . . . . . . . . .
!
2000
. . . . . . . . . . . . . . . . . . . . . . . . . . . .
H, c,,
I
.............
L/O~O
Figure 4. EPR spectra (77 K) of cobalt with salen in SAP. The first one represents a narrow symmetrical line with a g-value typical of a free radical (Fig. 5a and 6a). This signal appears when washing the sample with water. The second one is a broader asymmetrical line superimposed on the first narrow line. This line begins to grow after heating (130~ the sample (Fig.5b and 6b), and disappears under vacuum t r e a t m e n t (0.13 Pa) of the heated sample (Fig.5c). But it appears again upon exposure to air (Fig.5d), and its amplitude is much larger t h a n the amplitude of the EPR line of the spectrum b (Fig.5). Such a behavior is an evidence for the formation of mononuclear dioxygen adduct of the low-spin CoBSO2 complexes in SAP matrices. The obtained results agree with those of previous works [17, 18], where it has been shown that, in solid state, the cobalt (II)-Salen complex consists of a mixture of oxygenated and deoxygenetad complexes. The latter exists both in mono- and binuclear forms. The new paramagnetic centers which appear after heating a crystalline complex are due to the destruction of the diamagnetic complexes [18]. It should be noted that the EPR signals are not observed for Cosalen an Cosalophen in APTES solutions (Fig.5e). This may be due to the creation of diamagnetic binuclear complexes [CoSB(APTES)]202. Such a behavior is inherent to CoSB in nitrogen-containing solvents [19]. Probably either complete or partial destruction of such binuclear compounds occurs at the incorporation of the complexes in a SAP matrix. Let us examine the behavior of the narrow intense EPR line located near g=2.00. First, the g-value and the half-width (DHPP) did not change with an increase of temperature. Secondly, these parameters did not change under vacuum treatment. Its EPR amplitude depends on the cobalt concentration : it decreases when the cobalt content decreases. Its g-value is close to the g-value of a free electron (g=2.0023). A similar narrow EPR line is observed for the crystalline Co(II)-Schiff base complex [17]. One may assume t h a t the appearance of this signal is due to the transfer of electron density from Co 2+ to give a free radical. A Lorentz form of this line should be noted. As it is known, the molecular motion is one of the origins of that form.
604
c
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3200
3300
H,a
3400 -
-
-
-
-
Figure 5. EPR spectra (77 K) of cobalt complex with salen in SAP 9(a) initial; (b) sample (a) after heating (~ 130~ (c) sample (b)in vacuum (0.13 Pa); (d) sample (c) exposed to air; and (Cosalen)2 O2 in APTES solution (c=l.8.10-2mol/1)
-(e). In this case, the temperature dependence of the half-width should be observed. The absence of such dependence indicates that the width of EPR line is determined by another mechanism, e.g. dipole-dipole interaction. To confirm the correct signal attribution of the incorporated complexes (in the 3200 G region) complete simulation of the spectrum has been carried out (Fig. 7). For this purpose, the g-value and half-width of the free radical signal are taken from the experimental spectrum. The parameters of the hyperfine interaction with the best agreement of experimental and simulated spectra are listed in Table 3.
60b
b
......
3200
H,6
3~00
|
............. i..........
J200
J~O0
H,G----~
- - - -
Figure 6. EPR spectra (77 K) of cobalt complex with salophen in SAP : (a) initial; (b) sample (a) after heating (130~
Figure 7. EPR spectra (77 K) of cobalt complex with salen in SAP 9(1) experimental; (2) simulated.
Table 3 EPR values of incorporated complexes Paramagnetic
A, G
T,K
DHPP,G
centre
g• High spin tetrahedral complex
gll
A•
All
Shape of line
DH• pp DHIIpp
77
4.5
Paramagnetic centre corresponding to narrow signal
77 300
2.003 2.003
Co salen 02, SAP
77
2.079
1.999
10
18
15
20
Gauss
Co salophen 02, SAP
77
2.079
1.997
10
18
15
20
Gauss
12 12
Lorentz Lorentz
606 A good agreement is observed between the parameters listed and those for CosalenPyO2 and CoSalophenPyO2 referred to in [20]. It is a strong evidence in favor that the wide EPR signal (Figs. 5b and 6b) located in the region of 3200 G belongs to oxygen-containing cobalt(II) complex. Thus, the low- and high-spin cobalt compounds are formed at the incorporation of CoSalen and CoSalophen complexes in a SAP matrix. Lowspin complexes exist as mononuclear oxygen compounds which are capable of reversible oxygenation. High-spin compounds are formed probably due to the distorsion of square-planar complexes by incorporation into a SAP matrix. From the EPR standpoint, high-spin compounds may be only t e t r a h e d r a l complexes of Co(II) [17]. The analysis of the EPR and ESDR spectra testifies that a lot of cobalt belongs to a high-spin tetrahedral complex and a small part of it occurs in a low-spin oxygen complex. The latter statement is important in view of the search for new heterogeneous catalysts for oxidation processes such as are metal complexes incorporated in polyorganosiloxane matrices. 4. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
N.B. Kozlova and Y.I. Skurlatov, Uspekhi Khimii, 58 (1989) 234 A.E. Martell and D.T. Sawyer (eds), Oxygen Complexes and Oxygen Activation by Transition Metals, N.Y., 1988. Yu. I. Bratushko, Coordination coumpounds of 3d-Transition Metals with Molecular Oxygen, Naukova Dumka, Kiev, 1987. D. Mansuy, Pure and Appl. Chem., 62 (1990) 741. K.B. Yatsimirskii, Yu. N.Shevchenko, N.I. Yashina, I.M. Samodumova, V.A. Nazarenko and L.I. Kiseleva, Zhurnal Obshchei Khimii, 55 (1985) 405. Yu. L. Zub, L.S. Drozd and A.A. Chuiko, Book of Abstr. of COPS-III, 1993, 95. T.N. Yakubovich, Yu.L. Zub and V.V. Teslenko, Abst. of 3rd Int. Work. Electron Mag. Res. of Disordered Systems, Bulgaria, Sofia, 1992, 23. T.N. Yakubovich and Yu.L. Zub, Reversible bounding of molecular oxygen by metal complexes, Book of Abst. on III Inter-State Workshop, Ukraine, Donetsk, 1993, 26. R.H. Bailes and M. Calvin, J. Amer. Chem. Soc., 69 (1947) 1886. D. Chen and A.E. Martell, Inorg. Chem., 26 (1987) 1026. J.S. Khatib and R.V. Parish, J. Organomet. Chem., 369 (1989) 9. Yu.L. Zub, L.S. Kovaleva, B.V. Zmud', S.N. Orlik and et. al., Proc. 7th Int. Syrup. Heterog. Catal., Bulgaria, Bourgas, part 1, 1991, 557. U. Schubert, K. Rose and H. Schmidt, J. Non-Cryst. Solid, 105 (1988) 165. T.N. Yakubovich, Yu. L. Zub and R. Leboda, Koordinatsionnaya Khimiya, 1994 (in press). I.B. Afanas'ev, N.G. Baranova, Zhurnal Obshei Khimii, 52 (1982) 972. L.P. Finn and I.B. Slinyakova, Koloid. Zhurnal, 37 (1975) 723. Yu.V. Yablokov, E.G. Rukchadze, V.K. Voronkova, V.F. Shishkov and G.P. Talizenkova, Teoret. i Exper. Khimiya, 9 (1973) 92. K.S. Murray, G. van der Bergen, B.J. Kennedy and B.O. West, Aust. J. Chem., 39 (1986) 1479. Ei Ichiro Ochiai, J. Inorg. Nucl. Chem., 35 (1973) 1727. K.D. Jones, D.A. Summerville and F. Basolo, Chem. Rev., 79 (1979) 139.
PREPARATION OF CATALYSTSVI Sciemific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
607
P r e p a r a t i o n a n d c h a r a c t e r i z a t i o n of A S n O 3 (A = C a , S r or B a ) t i n c o m p o u n d s for m e t h a n e o x i d a t i v e c o u p l i n g C. Petit1, M. Teymouril, A.C. Rogerl, J.L. Rehspringer2, L. Hilairei, A. Kiennemannl 1 LERCSI- EHICS, 15RA 1498, 1, rue Blaise Pascal 67000 Strasbourg- France 2 GMI-IPCMS, 23, rue du Loess, 67037 Strasbourg- France
ABSTRACT Tin containing perovskites A S n O 3 (A = Ca, Sr, Ba) are prepared by a solgel method starting from tin oxygenated precursor (SnO) or from chlorinated precursors (SnCI4). The respective reactivity tests in oxidative coupling show that perovskites prepared from chlorinated precursors have a much higher C2 hydrocarbons selectivity than that obtained from oxygenated ones (70% compared to 40% for BaSn03). This difference in reactivityis interpreted by the modification of the basicity of the system by bulk or surface chlorine (C02 thermodesorption). 1. INTRODUCTION
Perovskite A B O 3 catalysts have been widely studied in oxidation reactions (CO, NOx, hydrocarbons...) (1). More recently titanium (2-4), bismuth or lead (5) containing ones have been tested in methane oxidative coupling (OCM). Tin containing catalysts have been reported to have interesting properties. A S n 0 3 perovskites have been brieflymentioned (5) and one work on tin pyrochlores (6) has shown the interest of definite structure compounds. Most of the preparations have been performed by firing-milling although more and more sol-gel preparations are reported (7-10). It is now well known that addition of chlorinated compounds in O C M results in a clear increase of C2 hydrocarbons selectivity. In most cases, the chlorinated compounds are introduced as organic molecules (11,12). However, more recently, preparations (e.g. Li/MgO) start through a chlorinated inorganic derivative (13). In the present work, perovskites prepared by a sol-gel method from oxygenated (I) or chlorinated precursors (II) have been characterized. Methane oxydative coupling has been used as probe reaction to compare the catalysts.
608 2~ EXPERIMENTAL 2.1. Catalyst preparation The tested catalysts were perovskites of ASnO3 (A = Ca, Sr, Ba) general formula. They were obtained either from oxygenated (SnO, Ca, Sr or Ba acetate, carbonate or oxide) or from chlorinated (SnC14) precursors using a sol-gel method as described below. 2.1.1. Oxygenated precursors Barium (4.28 g) or strontium (4.08 g) acetates were dissolved in 50 ml of propionic acid at 70~ Calcium oxide (1.36 g) was rendered soluble by refluxing in 150 ml propionic acid. SnO (respectively 2.24 g, 2.48 g and 3.29 g in function of the alkaline-earth used) was dissolved like CaO in propionic acid. The solutions containing the alkaline-earth and tin were mixed at 70~ and the excess of propionic acid was evaporated until the formation of a translucent solid. This residue was treated by liquid nitrogen and the obtained solid was calcined by heating up to 750~ with a slope of 3~ min-1. For the formation of CaSn03, it is better to use butyric acid either than propionic acid. 2~1~. Chlorinated precursors 4.50 g SnCI4 were dissolved at 20~ in propionic acid. Barium (10.22 g), strontium (7.64 g) or calcium (5.18 g) carbonate were suspended in propionic acid at 20~ These two preparations were mixed under stirring. The solution was cooled to 0~ and the formed alkaline-earth chloride (BaCI2, CaCI2 ) w a s filtered off, SrCl2 doesn't precipitate. The propionic acid was evaporated until obtention of the translucent gel which was then calcined at 750~ (10 hrs, 3~ rain-l). 2~2 Catalyst charactexization BET surface areas were measured before and after reactivity tests in a Perkin Elmer Shell 212C Model apparatus. XRD were obtained by a Siemens D500TT diffractometer, using the K~I line of cobalt. Carbonates which were present before and after reactivity were observed on a Nicolet 5DC FT-IR spectrometer. X ray microprobe analyses were performed by Kevex. The XPS apparatus was a VG ESCA 3 model whose coupling with a calculator permits an automatic recording of the spectra. The basicity of the catalysts has been evaluated by measuring the CO2 adsorption at 20~ by injecting of calibrated pulses of C02 until saturation of the sample. The amount of adsorbed C02 was measured by difference with a calibrated TCD gas chromatograph. The standard treatment of the catalyst begins through heating the sample from 20~ to 800~ with a slope of 10~ min-1, and maintaining at 800~ for 15 hrs under 02 and cooling down again to 20~ Pulses of C02 were then admitted on the catalyst until saturation. After CO2 adsorption TPD was performed, under helium between 20 and 800~ increasing the temperature with a 40~ min-1 slope. 2~ Reactivity measurement device The different reactivity tests were performed in a quartz U shaped tube (6.6 mm of I.D.) The catalyst was maintained between quartz fibers and the
609 reactor was filled with crushed quartz. The reaction conditions were as follows: P = I arm, 400 < T < 800~ catalyst weight: 0.2 g., total gas flow: 15 1.h-1 g.cat-1 controlled by electronic mass flowmeters, CH4/O2 ratio : between 2 a n d 20. During the reactivity measurement, the catalyst was heated up to 400~ with a 0.5~ min-1 slope under the reaction mixture : 02 and CH4,with or without He. From 400~ and upwards all 50~ a step of a to 2 hrs was maintained to reach stationnary state. At the end of each step the oulet gas was analyzed by a T.C.D. gas chromatograph. This process was followed from 400 to 800~ 3. RESULTS AND DISCUSSION 3.1 P r e l m ~ t i o n of catalytic systems The sol-gel method has been used to prepare the ASnO3 perovskites as well from the oxygenated (I) as from the chlorinated p r e c u r s o r s (II). The preparations are very similar for Ca, Sr and Ba catalysts and are s~lmmarized in Scheme I.
(I)
(II)
Ca,Sr, Ba acetate or oxide
SnO
Ca,Sr,Ba carbonate
Solution into propionic acid
Solution into propionicacid
Suspension in propionic acid
Ca,Sr,Ba propionate
Sn propionate
mixing of the two solutions
evaporation of the excess of propionic acid
SnC14
Solution into propionic acid
mixing of the tin solution and alkaline-earth suspension formation of alkaline-earth chloride filtration evaporation of the excess of propionic acid
gel formation
gel formation
liquid nitrogen
liquid nitrogen
solid formation
solid formation
calci_nation(750~
calcination (750~
{Y {y
ASn03 formation (A = Ca,Sr,Ba) Scheme 1. Summarized ASn03 perovskite preparation (A = Ca,Sr,Ba) by solgel method. (I) Oxygenated precursor II) Chlorinated precursors.
610
Sol-gel methods have sometimes been used to prepare oxidative coupling catalysts (9-10) essentially to permit a better insertion of chlorine into the structure. However, all the preparations start from alkoxyde. Chlorine is added as an organic compound (CC14) and the gel is obtained by a controlled hydrolysis of the alkoxide. The preparations in the present work start from carboxylates (propionates). Their is no induced hydrolysis and more essential, chlorine is introduced through an inorganic compound (SnCI4)instead of an organic one. During the gel formation, mixed alkaline-earth and tin propionate is produced, and the introduction of chlorine by an inorganic compound should permit to have the chlorine linked to tin in solution and in the gel formation step. 32 C ~ ' o n The specific surface areas of the catalysts before and after reactivity tests are given in Table 1.
Table 1 9B E T surface area of A S n O 3 catalysts (I) Oxygenated precursors (II) Chlorinated precursors. "Surface area m2 g-I Before reactivity ARer reactivity
CaSn03 (I) 6.1 11.8
SrSnO'3 (H) 2 1
(1) 7.1 8.3
BaSnO 3 (ID
.
1 2
(I)
(I~
10.8 11.8
6 5
O n Table 1 it can be seen that the surface area of catalysts (I) are systematically higher than that of catalysts (H). It is also noteworthy that the surface areas of BaSnO3 (1) and BaSnO3 (If)are higher than that of BaSnO3 used elsewhere (5). For CaSnO3 (I),the B E T surface area is enhanced during reactivity.That seems to be a firstindication of the instabilityof the structure and of the possible formation of carbonates. As can be observed by FTIR, carbonates are initiallypresent in small amounts on the catalytic surfaces, the amounts are equivalent whatever A is and whatever preparation is used. After reactivity, this amount remains constant for Ba and Sr but highly increases for CaSnO3(1). This structure appears to be the most instable and the most difficultto obtain by sol-gelmethod (use of butyric instead of proponic acid). X R D spectra show that for catalysts (I) the ideal (cubic) perovskite structure is obtained for BaSnO3 and SrSnO3 (14) and the orthorhombic structure (14-15) for CaSnO3. The structures remain the same before and after reactivity tests.They are given in Figure 1. It can be observed that the distance between the corresponding lines for Ca, Sr and Ba containing structure increases with 2e. The best crystallizedsample at 750~ is BaSnO3 followed by SrSnO3 and finallyCaSn03. With the chlorinated precursors neither alkalineearth oxides nor carbonates are observed. The structure are identical to that obtained from the oxygenated precursors. Further the structures are preserved al~r the reactivitytests.
611
b a 9~
(
1
I
1
,
18.~00
• " 2theta 9 '
!
!
,
52. Linear
1
1(~(].(~0;)>
Figure 1 9 XRD spectra of ASnO3(I) perovskites. a) BaSnO3 b)SrSnO3 c) CaSnO3 3.3 Oxidative coupling ~ v i t y i) oxygenated presursors 9The m e t h a n e conversion versus t e m p e r a t u r e is given on Figure 2 for a CH4/02 ratio of 2. 100
m
m
--
--
80
30-t
d
60
20 40
b 10
20
/02
T~ 0 300
9
400
500
600
700
800
Figure 2 9Conversion of CH4 for ASnO3 versus t e m p e r a t u r e a) CaSn03(I) b) SrSnO3(I) c) BaSnO3(I)
!
9
10
Figure 3 :Activity of BaSnO3 versus CH4/O2 ratio a) m e t h a n e conversion (C) b) C2 selectivity (S) c) 02 conversion
d) (C + S)
!
20
612 The conversion increases for all the systems for temperatures higher t h a n 500~ and is higher than 25% from 700~ and up. Oxygen conversion is total as soon as 700~ In these conditions the C2 hydrocarbons selectivity is 20% for BaSnO3, and 0% for SrSnO3 and CaSnO3. For these two catalysts the selectivity is 100% into CO2 at 800~ For BaSnO3, the C2 hydrocarbons formation starts only when the oxygen conversion is almost total (98.9%). Changing the CH4/O2 ratio from 2 to 20 (Figure 3) the selectivity (S) can be increased : (41% for a ratio of 10 with a methane conversion of 8.5 ) leading to a C+S value of 50 which is following the classification of Maitra (17) a bad catalyst. It must however be noted that these conditions can be maintained up to 100 hrs without change in the catalytic properties. ii) Chlorinated precursors : the activity of the three catalysts versus temperature is given on Figures 4a and 4b (CH4/O2 = 2).
50 % 40
80
%
c a
0"
30 0"
20 t
10 0 ~00
20"
ot2 400
500
600 700
800
Figure 4a : Conversion of CH4 for ASnO3 versus temperature a) CaSnO3(II) b) SrSnO3 (II) c) BaSnO3 (II)
T~ 900
0 ~00
400
500
600
700
800
900
Figure 4b: C2 hydrocarbons selectivity versus temperature a) CaSnO3 (II) b) SrSnO3 (II) c) BaSnO3 (II)
The methane conversion is 38,15 and 41 for Ca, Sr and BaSnO3 (H) respectively and the selectivity 28, 66 and 42%. The low activity of SrSnO3 is noteworthy and related with the method of preparation. It must be recalled that SrCI2 is not eliminated (or at least part of it) during the gel precipitation. SrCI2 is therefore in excess in (or on) the catalytic system compare to BaSnO3 and CaSnO3. This addition modifies the catalytic behaviour of the SrSnO3 catalyst. At 800~ (CH4/O2 = 10, without helium), the results have been represented on Table 2.
613
Table 2 Activity of ASnO3 (I) and (II) catalysts. CI-Lt/O2 = 10, T = 800~ Gas flow : 15 1 h-1 g cat-]. (without He) Catalysts BaSnO3 SrSnO3 CaSnO3* I II I H I II CH4 conversion % 9 16 29 14 29 38 02 conversion % I00 95 100 100 100 28 C2 Selectivity % 39 68 0 73 0 23 C2H6/C2H4 2.32 1.33 1.23 0.92 *CH~O2---2 From this table, it can be seen : - starting from the oxygenated precursors only BaSnO3 forms C2 hydrocarbons whereas all tin perovskites obtained form SnCI4 produce C2 hydrocarbons. - the selectivity to C2 hydrocarbons is situated around 70% for Sr and BaSnO3 (II), it means that the C+S value is about 85, which places these catalysts amoung the interesting ones in oxidative methane coupling. - the addition of chlorinated organic derivatives (CH3CHC12, CC14) as probe molecules on BaSn03 (I) enables an increase of the selectivity from 39 to 56%. However, whereas BaSn03 (II) has a stable activity for more than 120 hrs (CH4 conversion maintained, lost of selectivity 6%), the selectivity of BaSnO3 (I) rapidly returns to its initial value when the addition of the organic chlorinated compound is stopped. This shows that chlorine doesn't remain on the surface when an organic molecule is added whereas on the one prepared from the chlorinated precursors the behaviour of the catalysts is more deeply modified. The succint characterizations (BET, XRD and FT-IR) do not permit to differentiate between catalyst I and II. One more complete characterization was performed by X.P.S. and by evaluation of the basicity by CO2 (TPD) to better understand the role of chlorine. 3.~ Additional charactexiT~tion 3.4.1.
~LP.S.
The BaSnO3 catalysts prepared from oxide or chlorinated precursors (SnCI4) have both been studied by XPS after their formation. The position of the peaks in the Ols, Cls, Ba3d, Sn3d are given in Table 3. Table 3 XPS results on BaSn03(I) and BaSnO3(II) catalysts. (reference 9contamination carbon at 284.8 ev) PEAK POSITION (eV) Ba3dS/2 Sn3d5/2 02- net O- carbonate BaSnO3(I) 778.6 485.8 529.0 531.1 BaSnO3(II) 778.9 486.0 529.1 531.1 INTENSITY RATIO O/Ba C/Ba Sn/Ba Cl/Ba BaSnO3(I) 7.76 0.53 1.92 BaSnO3(II) 3.80 0.37 1.10 0.28
Cls carbonate 289.2 289.2
614 The bonding energies have been adjusted per comparison w i t h t h e contamination carbon (Cls at 284.8 ev.). The C ls area contains two peaks, one for the contamination carbon (284.8 eV.) and one for carbon linking to oxygen in carbonates (289.2 eV). This last peak is a little more intense in the case of the chlorine preparation containing. The 3d peaks of barium and tin are symmetric and have the same mid-height (width) for the two preparations. The peak in the Ols area is more complex and contains at least two peaks, the 02- type oxygen of the perovskite net (529 eV) and a less negatively charged species (531.1 eV) noted O-. The intensity of the oxygen feature is two times higher for the catalysts prepared from an oxygenated precursors t h a n for t h a t prepared from SnC14. The surface composition is computed for the peak surface area in each domain using the corresponding sensibility factor (ESCA Handbook). For the preparation from oxygenated products the C ls peak attributed to carbonate indicate the presence of a big a m o u n t of b a r i u m carbonate on the catalytic surface. The Sn/Ba ratio of 1.92 and the O/Ba one of 7.76 suggest a strong migration of Sn02 onto the surface. The perovskite phase appears thus to be a minority part and the surface seems to be a mixture of B a S n O 3 (1/5), BaCO3 (1/5) and SnO2 (3/5). This mixture fits with the experimental ratios. It must be noted that the barium carbonate and tin oxide are not detected by XRD. For the catalysts from chlorine containing precursors the a m o u n t of residual chlorine is clearly lower t h a n the introduced one (C1/Ba = 4). The peak position are almost unchanged as compared to BaSnO3 prepared by the oxide pathway. However the Sn/Ba and O/Ba ratios, respectively 1.1 and 3.8 indicate a lower amount of SnO2 on the catalytic surface. The most striking effect, observed by XPS, is the lower oxygen coverage of the surface but the symmetry of the barium and tin (3d) peaks suggests no anion effect. The electronic distribution around Ba and Sn are identical on both catalysts. &4~ Adsorption of CO2 and TPD ~ CO2 adsorption The basicity has been proposed to be one of the fundamental criteria for a good reactivity in oxidative methane coupling (16, 17). Two methods have recently been described (3, 5, 18) for basicity measurements. One is through benzoic acid (Bronsted basicity), the other one through the adsorption and the desorption (TPD) of CO2 (Lewis basicity). The last one has been used by sending CO2 pulses on the surface at 20~ followed by TPD (see experimental). The amount of CO2 adsorbed at 20~ on the ASnO3 (I) and (II) perovskites are given Table 4. Table 4 Amount of adsorbed CO2 at 20~ on the ASh03 perovskites Catalysts Molecules of adsorbed C02 mole of CO2 adsorbed per per gram of catalyst (X1019) mole of ASn03 (in %) BaSh03 I 4.30 2.20 II 0.46 0.23 SrSnO3 I 0.50 0.21 II 0.18 0.08
615 The amount of adsorbed C02 is about ten times lower than that of benzoic acid (4.3 x 1019 compared to 50 x 1019 for BaSnO3 (I)). These results are explained by the nature of the probe BrSnsted or Lewis acid). It seems thus t h a t most of the basic sites of ASnO3 at 20~ are Br6nsted sites. The second important point is that 10 times more CO2 is adsorbed on BaSnO3 (I) t h a n on BaSn03 (II). This is in agreement with the results of the literature (5,9,10) on other catalysts. It must be noted that this difference is much higher t h a n the change of the specific surface area and can therefore not be directly a t t r i b u t e d to it. The shape of TPD curves after CO2 adsorption, on the perovskites treated at 20~ show also clear differences between the catalysts obtained through oxygenated or chlorinated precursors.
.;', i
i !
!
,
,. !
t
s
):
t!
t :
! .,' Ji
i!
;)
:'
t
; 9 t, , olij
t
:i l ~"/
.
9
:
[,
~
.
"\ / ' " ~ - /
i
"
~
,:".,.
:
:
; '
..... Y /
t
| 1
9
,/
"
t
'
~.
BaSnO~
::
',,
"~.:~.._ / .... "<7.
(I1)
..""~ :
"...~.y/
.-- B a S n O ~
(!)
!
Z$
250
l 475
I 700
i 925
T~
Figure 5 9TPD after C02 adsorption on BaSn03: (I) oxygenated precursors (II) chlorinated precursors. These spectra are more complicated t h a n t h a t reported by Ding et al (3) on titanium barium, strontium or calcium perovskites. A first body at T < 200~ will be attributed to weakly chemisorbed C02. These peaks are very intense for (I) but relatively week for (II). Peaks of the second area (250 to 480~ are ascribed to strongly adsorbed C02 by Ding et al (3). CO2 peaks at 310, 420 for (I) and 272~ for (II) would enter in this class. Peaks at 590 (I) and 640 (II) would correspond to the decomposition of surface carbonates and the last feature (T > 800~ to the decomposition of bulk carbonates. B a r i u m carbonates are reported to decompose at 1000~ (16). It can be noted t h a t the omount of C02
616 desorbed in the first area is much lower for preparation (II) than for (I). The bulk carbonates are equivalent or even higher. But more, no species (CO2) are desorbed between 300 and 600~ for catalyst (II). This area is typically that of the beginning of the activity (see Figure 2). It seems therefore that the role of the chlorine can be qualitatively be related to the fact that CO2 which is a poison in oxidative coupling is adsorbed differently on perovskites (H) and (I). More particularly in the temperature area where methane begins to be activated, no more CO2 other than surface and bulk carbonates is adsorbed. 4. CONCLUSION The results obtained in the present work permit to propose t h a t the selectivity value obtained by preparing the catalysts from chlorinated precursors can qualitatively be related to the TPD after CO2 adsorption. CO2 which is a poison for oxidative coupling is less adsorbed on the catalyst prepared from chlorine containing precursors. Thus chlorine is proposed to stabilize the sites which are active in oxidative methane coupling with respect to the poisoning by CO2. R~IRENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
L.G. Tejuca, J.L.G. Fiero and J.M.D. Tascon, Adv. Catal. 36 (1989) 237. C. Yu, W. Li, W. Feng, A. Qi and Y. Chen, Proc. 10th Int. Cong. Catal. Budapest 19-24 July (1992) (eds. L. Guczi, F. Solymosi, P. T~t~nyi) Vol. B p. 1119. W. Ding, Y. Chen and X. Fu, Appl. Catal. A. 104 (1993) 61. W.J.M. Vermeiren, I.D.M.L. Lenotte, J.A. Martens and P.A. Jacobs, Stud. Surf. Sci. Catal. 61 (1991) 33. D. Dissanayake, K.C.C. Kharas, J.H. Lunsford and M.P. Rosyneck, J. Catal. 139 (1993) 652. C. Petit, A. Kaddouri, S. Libs, A. Kiennemann, J.L. Rehspringer and P. Poix, J. Catal. 140 (1993) 328. T. Lopez, I. Garcia-Cruz and R. Gomez, J. Catal. 127 (1991) 75. A.Z. Zhan and E. Ruckenstein, J. Catal. 139 (1993) 304. P.G. Hinson, A. Clearfield, J.H. Lunsford, J. Chem. Soc. Chem. Comm. 1430 (~991). S.J. Conway and J.H. Lunsford, J. Catal. 131 (1991) 513. S. Ahmed and J.B. Moffat, Appl. Catal. 58 (1990) 83. R. Burch, S. Chalker, P. Loader, D.A. Rice and G. Webb, Appl. Catal. A 79 (1991) 265. R. Burch, S. Chalker, P. Loader, J.M. Thomas and W. Ueda, Appl. Catal. A 82 (1992) 77. Natl. Bur. Stand. (U.S.) Monogr. 25, 3 11 (1964). Natl. Bur. Stand. (U.S.) Monogr. 25, 8 80 (1970). A.M. Maitra, I. Campbell and R.J. Tyler, Appl. Catal. 85 (1992) 27 A.M. Maitra, Appl. Catal. A 104 (1993) 11. V.D. Sokolovskii, G.M. Alley, O.V. Buyevskaya and A.A. Davydov, Catal. Today, 4 (1989) 293
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
617
La0.sSr0.2MnO3+ x SUPPORTED ON LaA103 AND LaA111018 PREPARED BY DWFERENT METHODS: INFLUENCE OF PREPARATION METHOD ON MORPHOLOGICAL AND CATALYTIC PROPERTIES IN METHANE COMBUSTION
P.E. Marti 1, M. Maciejewski 2 and A. Baiker 2'* 1 Department 2 Department Technology, Fax.: (41-1)
of Combustion Technology, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland of Chemical Engineering and Industrial Chemistry, Swiss Federal Institute of ETH Zentrum, CH-8092 Ziirich, Switzerland. Tel.: (41-1) 632 31 53, 262 17 62.
ABSTRACT LaA10 3 and LaA111018 as supports for Lao.sSro.2MnO3+ x catalysts have been prepared by conventional coprecipitation and by a complexation method (citrate method). The supported Lao.8Sro.aMnO3+ x catalysts were prepared by impregnation of the supports with an adequate amount of an aqueous solution of the corresponding metal nitrates resulting in a loading of 20 wt% Lao.sSro.2MnO3+ x. The mean crystallite size as estimated by XRD line broadening was larger for the perovskite supported on LaA111018 than on LaA10 3. Lao.sSro.2MnO3+ x on both differently prepared LaA10 3 supports was much more stable towards thermal decomposition than on LaA111018, as conf'trrned by oxygen evolution and thermal analysis. Kinetic studies of methane combustion were carried out in a fixed-bed microreactor in the range 600 - 1220 K and at atmospheric pressure using a reactant mixture with a ratio CH4:O 2 = 1:4. For the LaA103-supported catalysts the reaction rates, referred to the weight of Lao.sSro.2MnO3+x, were about three times higher at 770 than corresponding rates of the unsupported Lao.sSro.aMnO3+ x. In contrast, the LaA111018-supported catalysts showed similar activities as unsupported Lao.8Sro.xMnO3+x.
1. INTRODUCTION Catalytic combustion devices with high heat throughput, such as gas turbines or industrial burners, require active and thermally stable catalysts. Lao.sSro.2MnO3+ x has been reported as one of the most suitable perovskite-type oxide for the methane oxidation [ 1]. The intrinsically low surface area of this catalyst resulting upon heating it at high temperatures can be increased by dispersing the material on a thermally stable support material such as LaA10 3 and LaA111018 [2]. Courty et al. [3] reported that highly homogeneous mixed-oxide catalysts can be prepared using complexation methods. They used citric acid, as complexing agent (citrate method), for the preparation of LaA10 3 and obtained a very homogeneous material. Zhang et
To whom correspondence should be addressed
618 al. [4] prepared by this method perovskite-type oxides with very large surface areas. In the present work we report on the preparation of LaA103 and LaA111018 and the application of these oxides as supports for La0.sSr0.2MnO3+x catalysts suitable for methane combustion. The supports have been prepared using a complexation method (citrate method) as well as conventional coprecipitation. The structural and catalytic properties of the supported La0.sSr0.2MnO3+ x catalysts are elucidated.
2. EXPERIMENTAL
2.1. Catalyst preparation Citrate method: The amorphous citrate precursors were prepared using a similar procedure as given by Courty et al. [5]. Adequate amounts of La(NO3)3-6H20 and Al(NO3)3.9H20, from FLUKA (puriss p.a.), were dissolved in deionized water to obtain a 1.25 M solution. Citric acid monohydrate (FLUKA, puriss p.a.) was added to the solution, with a molar ratio citric acid / total cations = 1. The solution was concentrated in a rotary evaporator at 330 K and 20 mbar for 3 h. The viscous and vitreous product was finally dried in a vacuum stove at 370 K and 90 mbar for 20 h. After drying, a foaming precursor was obtained, which was calcined in air at 1070 K for 8 h, at 1170 for 12 h, and finally at 1370 K for 8 h. The support materials were crushed and sieved. The size fraction between 100 and 300 lam was used for catalyst preparation. Coprecipitation: The supports were also prepared by calcination of water-insoluble hydroxides mixtures, employing a similar technique as described by Vidyasagar [6], however, the use of chlorine as oxidant was omitted. Adequate amounts of the metal nitrates were dissolved in deionized water (0.5 M total cation concentration) and added dropwise together with a 2.8 M aqueous-solution of tetramethyl ammonium hydroxide to 100 ml water kept under vigorous stirring at a constant pH of 8.5. The precipitated hydroxides were separated from the liquid by centrifugation and washed several times with deionized water. Barnard et al. [7] reported for LaCoO 3 that samples which were washed with acetone before dehydration had higher surface areas than samples which were directly dehydrated by air-drying. Hence the precipitates were washed with acetone before drying at 400 K in air. Subsequently they were calcined using the same procedure as described above. Supported perovskite: La203, Sr(NO3) 2, both from FLUKA (puriss), and Mn203, from Alfa Products (98%), were used as start materials. La203 was dissolved in diluted nitric acid while the dissolution of Mn203 in concentrated nitric acid was facilitated by dropwise addition of H202. Aqueous solutions with a cation ratio Mn:La:Sr of 1:0.8:0.2 were prepared. The supports were impregnated with the adequate amount of solution to reach a perovskite loading of 20 wt%. The samples were dried in a rotary evaporator, and finally calcined at 1220 K for 10h.
2.2. Physicochemical characterization Phase identification of the catalysts was carded out by powder x-ray diffractometry using a Siemens D5000 diffractometer. The position of the reflections was adjusted using a mechanical mixture of the samples and quartz. The patterns obtained were compared with
619 JCPDS data files. Physisorption measurements were performed with a Micromeritics ASAP 2000 instrument. The BET surface areas were determined by nitrogen adsorption at 77 K in the relative pressure range 0.05 < p/p0 < 0.20. Thermoanalytical investigations were carried out using a Netzsch simultaneous thermoanalyzer (STA 409). Temperature-programmed desorption (TPD) of oxygen was measured in a flow system. Each sample (0.100 g) was placed in a fused-quartz microreactor and pretreated in air (300 ml/min STP) at 1120 K for 1 h. The sample was then cooled to room temperature in the same atmosphere and subsequently heated in a He stream (300 ml/min STP) at a constant heating rate of 10 K/min. The evolving oxygen was monitored with an on-line quadrupole mass spectrometer (Balzers GAM 445). 2.3. M e t h a n e c o m b u s t i o n tests Kinetic studies were performed in a continuous fixed-bed microreactor operated at atmospheric pressure. The reactant feed rate was controlled by mass flow controllers (Brooks 5850E). Both inlet and outlet gas compositions were quantitatively analyzed using an on-line quadrupole mass spectrometer (Balzers GAM 445). Comparative activity tests were carried out under the following conditions: reactant feed, 1% CH 4 (99.995%), 4% O 2 (99.999%) and 95% He (99.998%); catalyst load, 0.100 g mixed with 0.100 g SiO 2 powder to reduce the heat release per unit volume; the gas flow rate was adjusted to achieve a gas hourly space velocity (GHSV) of 135'000 h 1. The temperature was increased with a constant heating rate of 10 K/min from 570 to 1220 K. The methane conversion to CO 2 was calculated from a carbon balance: PCO2/(PcHa+Pco2+Pco). 100, where PCH4, Pco2 and Pco are the partial pressures of CH 4, CO 2 and CO, respectively.
3. RESULTS AND DISCUSSION 3.1. B u l k structure Supports prepared by the citrate method are referred to as cit and supports prepared by coprecipitation as cop, respectively. 20 wt% Lao.sSro.2MnO3+x/LaAIO3" X-ray diffraction patterns of the LaA10 3 supports prepared by coprecipitation and by citrate method after calcination at 1370 K confirmed the formation of the perovskite phase. Sharp and well-defined peaks were observed for both samples, indicating that the preparation method had no influence on the final structure after calcination at 1370 K. The BET surface area of the samples are listed in Table 1. LaA10 3 cop had after calcination a slightly higher surface area than LaA10 3 cit, however, the surface areas were not significantly affected by the preparation method. Figure 1 shows the XRD patterns of the four supported Lao.sSr0.2MnO3+ x perovskites. The position of the reflections of the supported La0.8Sr0.2MnO3+x (not shown) was identical to that of the corresponding unsupported material. The reflections resembled those of LaMnO3.15, but were slightly shifted, as a comparison with the corresponding JCPDS file indicated. 20 wt% La0.sSr0.2MnO3+x/LaAlllOls: The LaA111018 phase in both supports was still X-ray
620 amorphous after calcination at 1370 K. LaAlllOz8 cit showed broad and low intensity peaks corresponding to LaA103. For LaA111018 cop only traces of poorly crystalline 13-A1203 were detected.
' Lao.sSro.2MnO3.x/LaAiO3 'Co~ *
,
=i
i
,
c.
,
Lao.sSro.2MnO3.x/LaAI11 O1 e Cop 4~
**
*
**
**
.
_1.~ 1 ~
.1==1
~
c
.
Cit
Lao.sSro.2MnO3+x/LaAIO 3 ,
Lao.8Sro.2MnO3+x/LaAI11, O18
1
r m
.
* *
20
30
40
50
*
60
70
*
**
0
*
80 20
*
30
40
Cit I
50
.1~ ,
60
70
80
20 / degrees Figure 1.
XRD patterns (CuKa) of 20 wt% Lao.sSro.2MnO3+x supported on differently prepared LaA103 and LaAl11018. The discernible phases are: ( ~ ) Lao.sSro.2MnO3+x' ( . ) LaAIO3' (-4~) A1203.
Nitrogen sorption measurements showed a distinctly different hysteresis for both LaA111O18samples and consequently different pore size distributions (mesopores). LaA111018 cit had a low surface area, similar to the surface area of LaA103 (Table 1). However, LaA111018 cop maintained a surface area of 40 m2/g after calcination at 1370 K. The structural collapse with concomitant loss of the surface area can be associated with the formation of crystalline LaA10 3 in LaAlllO18 cit. Kato et al. [8] investigated the crystal structure of LaaO3.nA1203. They observed for a molar ratio La/A1 of 8/92 the presence of both 13-A1203 and LaA103 crystalline phases. The retardation of sintering of T-alumina to m-alumina was attributed by these authors to the formation of 13-alumina and not to LaA10 3, as earlier proposed by Schaper [9]. However, B6guin et al. [ 10] reported recently that the stabilisation of alumina with lanthanum oxide is due to the formation of micro domains of LaA10 3 on the alumina surface, whilst the formation of the 13-alumina can be associated with the loss of the stabilizing effect. The Lao.8Sro.2MnO3+x phase was formed on LaA111018 cit, as XRD patterns revealed
621 (see Fig. 1). However, the width and the position of the peaks of the crystalline phase on LaA111018 cop hint to an overlapping of the reflections of the LaA10 3 and Lao.sSr0.2MnO3+ x phases, indicating that during the impregnation procedure lanthanum oxide reacted to form LaA10 3. The crystallite size of the active perovskite on the different supports, as determined by XRD line broadening, was larger on the LaA10 3 than on the LaAl11018 supports. 3.2. T h e r m a l b e h a v i o r The thermal behavior of the supports and the supported perovskites has been studied by
TABLE 1 BET surface areas Support
Calcination temperature(a) [K]
Surface area [m2/g] 20wt%(b)
1170 1370
8.3 4.1
4.1
LaAIO3 cop
1170 1370
13.4 5.2
4.6
LaAI11018 cit
1170 1370
21 4.6
3.2
1170 1370
140 51.3
41
1170
5.0
LaA103 cit
LaAII IO18 cop Lao.sSr0.2MnO3+x
_
_
_
.
_
_
(a): Calcined at 1070 for 8 h, at 1170 K for 12 h and at 1370 for 8 h. (b): Nominal loading of La0.sSr0.2MnO3+x.
temperature-programmed desorption of oxygen (TPD) and by thermal analysis (TA). Figure 2 depicts the oxygen evolution profiles of bulk and supported perovskites. The rate of oxygen desorption per mol Mn in the samples is plotted as a function of the catalyst temperature. Prior to the measurements, the samples were heated to 1220 K under a stream of air, and then cooled to room temperature under the same atmosphere. It should be noted that the pure supports did not exhibit any detectable oxygen desorption up to 1370 K. Marked differences were observed between the oxygen desorption behavior of the supported perovskites compared to the bulk Lao.sSro.2MnO3+ x. The thermal decomposition of bulk Lao.sSro.2MnO3+x occurred in two steps, as revealed by the two discernible peaks in the TPD curve, with maxima at 890 and 1350 K, respectively (Fig. 2A). The decomposition behavior of Lao.8Sro.2MnO3+x is similar to that of LaMnO3+ x. The first oxygen evolution event from LaMnO3+ x led to the formation of the stoichiometric LaMnO3.00 perovskite phase [ 10]. Tofield and Scott [ 12] reported that the oxidative non-stoichiometry of these manganites is related to the formation of cation vacancies instead of interstitial oxygen. The partial substitution of La with Sr simply decreases the cation vacancies and consequently the amount of oxygen which desorbs during the first step [13]. Heating Lao.8Sro.2MnO3+x to 1370 K led to its partial decomposition, with formation of a new perovskite phase and traces of La20 3. Interplanar spacings and intensities of XRD patterns of this new phase were very similar to the patterns of the oxygen deficient phase LaMnO2.875. Further heating to 1720 K decomposed completely the Lao.sSro.2MnO3+ x phase leading to the oxygen deficient perovskite phase and La20 3. XRD patterns of bulk Lao.8Sr0.2MnO3+x, Lao.8Sro.2MnO3+x/LaA103 cit and Lao.8Sro.2MnO3+x/LaA111O18 cit measured
622 after heating them to 1720 K under Ar, are shown in Fig. 3 A, B and C, respectively. The decomposition of the supported perovskites started around 800 K, with exception of Lao.sSro.2MnO3+x/LaA111018 cit, which started already at 600 K. Note that the decomposition /~ 1.6xl 0 -4 _ A La o.8 Sro .2 MnO3 + x process is different for the B Lao.eSro.2Mn03+x/L.~ll 101 e c~ differently prepared w
!
'
I
'
I
'
I
'
I
w
C Lao.e Sro .2 MnO3+x/LaAI1 ~O~ e cop
L ao.8 S ro.2MnO3+x/LaA111018 D Lao.BSro 2MnO3+x/LaAIO3 c/t ] =E catalysts above 1200 K (Fig. 2B E Lao.sSro 2M and C). Whilst the oxygen 1.2xl 0 -4 desorption-rate increases for E Lao.sSro.2MnO3+x/LaAlll O18 cit above this temperature, it begins "6 to decrease for E 8 . O x l o -5 Lao.sSro.2MnO3+x/LaAl11018 cop. la It is interesting to note that after heating Lao.sSr0.2MnOa+x/LaA103 tto 1720 K in Ar, the reflections ._O of the partially decomposed ~ 4.0xl 0s O Lao.sSr0.2MnO3+x phase on the o~ LaA103 supports were gradually r~ shifted to the reflections of O~ LaA103 (Fig. 3B). The oxygen0.0 deficient Lao.sSr0.2MnO3+x phase 400 600 800 1000 1200 1400 has similar XRD patterns to that of LaA103, as revealed by XRD T Temperattte / K. emperature-programmeo oxygen measurements with Figure 2. evolution from unsupported Lao.sSr0.2MnO3_x supported on Lao.sSro.2MnO3+x (A) and 20 wt% spinels. This behavior can be Lao.sSro.2MnO3+ x supported on attributed to the stabilization differently prepared LaA111018 and towards decomposition of LaA103. Sample weight: 0.1 g, carder Lao.sSro.2MnO3+x through the gas: He (300 ml/min), heating rate: support. Near the surface is the 10 K/min. formation of oxygen-deficient phases faster than in the interfacial regions, i.e., at the Lao.sSro.2MnO3+x - LaA103 interface, the strong interaction with the support stabilizes the manganite. La0.sSr0.2MnO3+x/LaA111018 samples exhibit a different thermal-decomposition behavior compared to corresponding LaA103 supported catalysts. After heating the samples to 1720 K the Lao.sSr0.2MnO3+x phase disappeared completely, while a new phase, with similar but slightly shifted (higher 20) reflections as that of LaA103 was formed. This heat treatment led also to crystallization of LaA111018. The amount of oxygen, which desorbed from the samples during the TPD I1.=,
'
I
'
I
'
I
'
I
'
I
'
I
623 measurements, was calculated by integrating the oxygen desorption rate with time, the results are summarized in Table 2. The amount of 0 2, which evolved from both Lao.sSro.2MnO3+x/LaA103 samples is about 2.5 times lower than the amount evolved from bulk Lao.sSro.2MnO3+x . This difference corroborates the results discussed above about the stabilizing effect of the support towards decomposition of Lao.sSro.2MnO3+x. In contrast, the
'
I
'
I
'
I
'
I
'
,
I
'
9 La2 0 3 LaAlO3 La,,N~ ~O~ 8
A
9
5 B
m m
C m
C
C
~
'
20
I
I
I
I
I
30
40
50
60
70
80
20 / degrees Figure 3.
XRD patterns of neat Lao.8Sro.2MnO3+ x (A), Lao.sSro.2MnO3+x/LaA10 3 cit (B), and Lao.8Sro.2MnO3+x/LaA111018 cit (C) after heating them to 1720 K under Ar.
624 amount of oxygen which evolved from the La0.sSr0.2MnO3+x/LaAl11018 catalysts is greater than the amount evolved from the unsupported perovskite. The reduction of Lao.sSr0.2MnO3+ x (LSM) with hydrogen was studied by means of TA. The TG results indicate that the reduction occurred in two steps, in the range 560 to 800 K (weight loss = 2 %) and 930 to 1060 K (weight loss = 2.5 %), respectively. XRD analysis of the reduction products revealed the presence of only La203 and MnO. The composition of the unsupported perovskite after preparation was calculated to be La0.sSr0.2MnO3.05, according to the weight loss after complete reduction. Both supports prepared by the citrate method contained traces of carbon, even after calcination at 1370 K for 8 h, as revealed by TA coupled with MS. Courty et al. [4] reported for YA10 3 that the carbon content in the sample remained practically constant after calcination above 1270 K.
3.3. Catalytic activity for methane combustion Methane conversions over the different catalysts were measured by temperature-programmed reaction (heating rate = 10 K/min, GHSV = 135'000 h-l). It is important to note that pure LaA10 3 exhibits significant activity for methane oxidation above 750 K. At 770 K, the methane conversion over LaA10 3 was 0.5% compared to 4% conversion over LSM/LaA10 3 cop. The relatively high oxidation activity of LaA10 3 was already reported by Quinlan et al. [2]. In contrast, the methane conversion over both LaA111018 supports was negligible at temperatures below 850 K. CO was produced over LaA103 at temperatures above 850 K with a maximal CO-yield of 15% at 1030 K, and over LaA111018 above 900 K with a maximal COyield of 21% at 1130 K. Table 2 summarizes the temperatures at which 50% of methane oxidation was achieved (Ts0~), the apparent activation energies and the reaction rates. The catalytic activities were compared at a GHSV of 135'000 h -1. The overall activity of the unsupported Lao.sSr0.2MnO3+ x was higher than the activity of the supported catalysts, probably due to the higher accessible active surface area of the unsupported perovskite. The apparent activation energies were determined at conversions below 10%. They were for the LaA10 3 supported catalysts about 10 - 30 kJ/mol lower than for the other samples. Figure 4 depicts a comparison of the reaction rates, per gram La0.8Sro.2MnO3+x (LSM), in form of Arrhenius plots. The reaction rates measured with LSM/LaA10 3 samples were about three times higher than that of the unsupported perovskite and that of the LaAlllO18 supported perovskite. The higher activity of the LaA103 supported samples is attributed to the higher dispersion of the La0.sSr0.2MnO3+x particles. Zhang et al [ 14] reported for methane oxidation over La0.sSr0.2MnO3/La203-9A120 3 that the reaction rate per unit weight of La0.sSr0.2MnO 3 was about three times higher at loadings of 10 and 20 wt% than that of the corresponding mechanical mixtures. However, in this work we observed no difference in the activity of La0.8Sr0.2MnO3+x/LaAlllO18 and unsupported La0.8Sr0.2MnO3+x.
625 TABLE 2 Characteristic data of oxygen desorption measurements and kinetic results Amount oxygen evolved(a) [mmol.mollMn]
Ea(b)
T5o%(c)
Reaction rate(d)
[kJ-moll]
[K]
[pmol.sl.g-lLSM]
LSM/LaA103 cit
36
91
991
3.8
LSM/LaA103 cop
31
93
949
5.8
LSM/LaAll]O]s cit
112
105
1067
1.4
LSM/LaAlzlOls cop
115
122
1001
1.0
Lao.sSr0.2MnO3+x (LSM)
81
104
896
1.4
Catalyst
(a): Oxygen which evolved from 0.1 g sample into 300 ml/min He at a heating rate of 10 K/min in the temperature range 600 - 1370 K, following treatment at 1120 K in air for 1 h. (b): Apparent activation energy. (c): Temperature at which 50 % methane conversion was attained. (d): Calculated at 770 K.
Temperature / K 850
9oo
800
I
750
700
w
LSM/LaAIO 3 cit ,,.;-, 8.
10.5
LSM/La~O~ cm
,,.;.,
LSM/La,aJl101e c/t
z_
tO
10 -6
L S M / L a ~ l l O l e cop
=m
o m (9
I:1::
Lao. eSr0.2 ~ 3 '
I .2 I
'
1.13
1000/'r /
Figure 4.
'
(bulk) 1.14
K "1
Arrhenius plots of pure and supported Lao.8Sr0.2MnO3+ x (LSM). Reactant gas composition: 1% CH 4, 4% 02, He (balance); sample weight: 0.1 g, GHSV" 135'000 h-l; heating rate: 10 K/min.
626 4. CONCLUSIONS Thermally stable mixed-oxides with the formula LaA103 and LaAl11018 have been used as supports for La0.sSr0.2MnO3+x methane combustion catalysts. The supports have been prepared by coprecipitation and by the citrate method. The preparation method had no marked influence on the structure of the supports after calcining them at 1370 K. Temperatures above 1500 K are required for crystallization of the LaA111018 phase. Major differences between the oxygen desorption behavior of the supported and the bulk Lao.sSr0.zMnO3+ x have been observed. La0.sSr0.zMnO3+x supported on LaA10 3 was found to be more stable towards thermal decomposition than the other catalysts investigated. The overall activity as well as the reaction rate per gram La0.sSr0.2MnO3+x perovskite showed a marked dependence on the kind of support material used, while the method of support preparation had no significant influence. Under the reaction conditions used (reactant gas composition: 1% CH4, 4% O z, He (balance); sample weight: 0.1 g, GHSV: 135'000 h l ; heating rate: 10 K/min), La0.sSro.2MnO3+x supported on LaA10 3 exhibits a considerably higher activity for methane combustion than La0.sSr0.2MnO3+x/LaAl11018.
ACKNOWLEDGMENT Financial support of this work by the "Schweizerisches Bundesamt fur Energiewirtschaft" is kindly acknowledged.
REFERENCES .
2.
.
.
5. 6. .
8. 9. 10. 11. 12. 13. 14.
Arai, H., Yamada, T., Eguchi, K., and Seiyama, T., Appl. Catal., 26, 265 (1986). Quinlan, M.A., Wise, H., and McCarty, J.G., Basic Research on Natural Gas Phenomena - Catalytic Combustion, SRI International, Menlo Park, CA, (1989) GRI89/0141. Courty, Ph., and Marcilly, Ch., in "Preparation of Catalysts III", (G. Poncelet, P. Grange and P.A. Jacobs, Eds), Elsevier, Amsterdam, 1983. Zhang, H.-M., Teraoka, Y., and Yamazoe, N., Chem. Letters, 665 (1987). Courty, Ph., Ajot, H., and Marcilly, Ch., Powder Technology 7, 21 (1973). Vidyasagar, K., Gopalakrishnan, J., and Rao, C.N.R., J. Solid State Chem.,58, 29, (1985). Barnard, K.R., Foger, K., Turney, T.W., and Williams, R.D., J. Catal. 125,265 (1990). Kato, A., Yamashita, H., and Matsuda, S., Stud. Surf. Sci. Catal. 44, 25 (1989). Schaper, H., Doesburg, E.B.M., and Van Reijen, L.L., Appl. Catal. 7, 211 (1983). B6guin, B., Garbowski, E., and Primet, M., Appl. Catal. 75, 119 (1991). Marti, P., and Baiker, A., Catal. Letters, in press. Tofield, B.C., and Scott, W.R., J. Solid Sate Chem. 10, 183 (1974). Yamazoe, N., and Teraoka, Y., Catal. Today 8, 175 (1990). Zhang, H.M., Teraoka, Y., and Yamazoe, N., Appl. Catal. 41, 137 (1988).
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
627
Properties of L a 0 . 6 S r 0 . 4 C o O 3 p r e p a r e d b y c o m p l e x i n g a g e n t - a s s i s t e d sol-gel method Yukihiro Muto a and Fujio Mizukami b aFukuoka Industrial Technology Center, 332-1, Kamikoga, Chikushino, Fukuoka 818 Japan bNational Institute of Materials and Chemical Research, 1-1, Higashi, Tsukuba, Ibaraki 305 Japan
Six types of La0.6Sr0.4CoO3 (LSCO) powders were prepared from two kinds of raw materials, metal nitrates and acetylacetonates, by the sol-gel method using organic polydentate ligands such as ethylene glycol (EG), diethylene glycol monomethyl ether (DEMM), and 2-methyl-2,4pentanediol (HG) as the solvent. The effects of the raw materials and ligands on the formation of provskite phase in the powders and on the catalytic activity of the powdrers in the CO oxidation were investigated. When the raw materials were the nitrates, the perovskite phase tended to be produced in the order of HG>DEMM>>EG. On the contrary, the order was EG>_DEMM=HG when the raw materials were the acetylacetonates. In the CO oxidation, the orders of the catalytic activities of the powders from the nitrates and acetylacetonates were DEMM>HG>>EG and EG>_DEMM=HG, respectively, being in considerably good harmony with the corresponding orders on the perovskite formation tendency. From the results, it was found that the complexing abilities of the counter anions of the raw materials and the ligands used in the powder preparation process have big influences on the formation of perovskite and the activity of LSCO powders for the CO oxidation.
1. INTRODUCTION La0.6Sr0.4CoO3 (LSCO) is one of typical perovskite oxides and expected to be applied to a semipermeable device and electrode for oxygen because of its high catalytic activity in oxygenation [ 1-9]. The mixed oxide is usually prepared by solid phase reaction from the respective oxide powders or pyrolysis of a mixture of the acetates [2-6], but it is difficult to obtain the
628 mixed oxide having homogeneous composition by the traditional methods. On the other hand, it is well known that sol-gel process is suitable to prepare homogeneous mixed oxides [10-13]. Here we prepared various LSCO by a sol-gel method, and investigated the effects of raw materials and organic polydentate ligands used in the sol-gel process on the formation of perovskite phase in the mixed oxides and on the catalytic activity of the mixed oxides in CO oxidation. 2. EXPERIMENTAL 2.1. Preparation of LSCO powder Metal nitrates of L a ( N O 3 ) 3 . 6 H 2 0 , Sr(NO3)2 and C o ( N O 3 ) 2 o 6 H 2 0 acetylacetonates of La(C5H702)3~
or metal
Sr(C5H702)2o2H20 and Co(C5H702)2~
were used as the raw materials, and ethylene glycol (EG), diethylene glycol monomethyl ether (DEMM), and hexylene glycol (2-methyl-2,4-pentanediol; HG) were used as the organic polydentate ligand in the sol-gel process. When the nitrates were the raw materials, the three nitrates were heated in a mixture of ethoxyethanol and an organic polydentate ligand at 120 ~ for 12 h to give a sol. During the heating, NOx gas was evolved. The sol was dried into a LSCO precursor gel at 150 ~ under reduced pressure. When the acetylacetonates were used, the three acetylacetonates were heated in a similar mixture in the presence of acetic acid at 120 ~ for 12 h, and the solution was cooled to 90 ~ After water was added to the solution and kept at the temperature for 3 h, the solution was distilled off at 150 ~ under reduced pressure to give a LSCO precursor gel. All the precursor gels were calcined at 500, 600, 800 and 1000 ~ for 1 h.
2.2. CO oxidation Activity of LSCO powders for CO oxidation was measured using a fixed-bed continuous flow microreactor with 0.2 g of LSCO calcined at 800 ~
A gas mixture containing CO (0.5%), 0 2
(0.25%) and helium was flowed through the reactor at the contact rate of 3.33 g/cm2osec and analyzed by gas chromatography. The CO conversion and CO2 selectivity were calculated on the basis of the concentration of helium used as the internal standard.
2.3. Characterization of LSCO powder The specific surface area was obtained from the nitrogen adsorption and desorption isotherms at 77 K, using a micro-BET apparatus (an AccuSorb 2100 of Micromeritics). The X-ray diffraction (XRD) patterns were recorded on a MAC Science MXP- 18 instrument using Cu-Ko~ radiation with a Ni filter. The thermal gravimetry and thermal differential analysis (TG-DTA) were
629 carried out on a MAC Science TG-DTA 2100 instrument with a heating rate 10 ~
under a
flow of 100 cm3/min dry air. The scanning electron microscopy (SEM) was measured by a JEOL FE-SEM instrument. 3. RESULT AND DISCUSSION 3.1. Formation of perovskite phase Six types of LSCO powders were prepared by the sol-gel method using two kinds of raw materials, metal nitrates and acetylacetonates, and three organic polydentate ligands, EG, DEMM and HG. Figs. 1 and 2 show the effect of the raw materials and ligands on the formation of perovskite phase in the powders. It is found that XRD patterns of LSCO powders vary with a combination of raw materials and a polydentate ligand. In the case of the nitrates, when the ligand was HG, the LSCO powder began to have perovskite phase by the calcination at 600~ and showed almost only perovskite phase by the calcination over 800~
But, when EG and
DEMM were the ligands, even the LSCO powders calcined at 1000~ showed XRD patterns clearly indicating the existence of another phase besides perovskite. Thus, perovskite tends to appear in the order of HG>DEMM>>EG, when the raw materials were the nitrates. On the other hand, in the case of the acetylacetonates, all the three types of LSCO powders calcined at 1000~ showed perovskite patterns with impure peaks, and the hight of the impure peaks
in-
creased in the order of EG
3.2. Specific surface area and surface observation Fig. 3 shows the specific surface area of LSCO powders prepared from the nitrates and acetylacetonates using the three organic iigands and calcined at different temperatures, together with that of LSCO prepared by the conventional acetate pyrolysis. Except the LSCO powder obtained by the combination of the nitrates and HG, when the calcination temperatures were below 800~
the powders prepared by the sol-gel process showed higher surface areas than the
powder prepared by the acetate pyrolysis, but there was almost no differece between the surface areas of five so-gel and one acetate pyrolysis LSCO powders when the calcination temperatures were over 800~
On the other hand, the surface area of LSCO powder prepared from the ni-
trates using HG was extremely low even when the calcination temperatures were below 800~
630
O O
o Jk..,.~_--~
20
O
O
,A_
40 20 (a) EG
30
-
50
60
0
0
20
0
30
0
40 20 (b) DEMM
50
O O ____L
20
. . . .
30
O _L
0
60
O
~_
40 50 60 20 (c) HG Figure 1. XRD pattern of LSCO powders prepared from nitrates using organic polydentate ligands [(a), EG; (b), DEMM; (c), HG] and calcined at 600 - 1000"(2. O:perovskite
631
O O
o
o
h_
20
_ L
30
__
h
40 20 (a) EG
o
_
_~
50
60
o 0 -
0 .~_~~
~ f
20
i
30
40 20 (b) DEMM
50
60
o
'~
0
20
o
30
40 28 (c) HG
o
o 0
50
60
Figure 2. XRD pattern of LSCO powders prepared from acetylacetonates using organic polydentate ligands [(a), EG; (b), DEMM; (c), HG] and calcined at 600 - 1000"C. O:perovskite
632 30-
"7, ell)
20-
-,,,:
L~
~
10
0 4( )0
X~
500
qlm Q ~
6;0
7;0
8;0
9;0
1(~00
1100
Calcination temperature / ~ Figure 3. Effect of organic polydentate iigands on surface area of LSCO powders. From nitrates:O, HG; A, DEMM; I'1, EG.
• Pylolysis of acetates.
From acetylacetonates: O, HG; A, DEMM; II, EG.
and was the lowest among the seven powders. This seems to concert with the XRD results or the easiness of the perovskite formation by the combination of the nitrates and HG. Figs. 4 and 5 show the observation of the surfaces of LSCO powders prepared by the sol-gel method. The particle size and shape of LSCO powders and the temperature profile of the surfaces change with the raw materials and organic polydentate ligands used. Furthermore, it is found from the samples calcined at 1000~ that the powders from the nitrates have higher sintered surfaces than those from the acetylacetonates, among the three samples from the nitrates the powder obtained with DEMM is composed of grains with relatively uniform size although the grains have many cracks, and as to the samples from the acetylacetonates, all the three powders are made up of big and small grains and the amount of the big grain increases in the order of EG
633
600~
800~
(a) HG
600~
I,
800~ (b) DEMM
lpm
I
800~ (c) EG
1000~
1000~
I
600~
llam
I
1000~
I
1lam
I
Figure 4. FE-SEM image of LSCO powders prepared from nitrates using organic polydentate ligands and calcined at 600- 1000~
634
.....~,~!
600~
800~
i
(a) HG
i,i~ ~ ...........~,~.~
~.! ............ 9 .~
1000~
llam
:~ 9 :T:-~84 ~
.,~.
600~
800~
i000~ llam
(b) DEMM
600~
800~
(c) EG
1000~
I,
1lam
I
Figure 5. FE-SEM image of LSCO powders prepared from acetylacetonates using organic polydentate ligands and calcined at 600- 1000~
635
3.3. CO oxidation Fig. 6 shows the effect of the raw materials and organic polydentate ligands used in the LSCO powder preparation process on the catalytic activity of the powders in the CO oxidation. The catalytic activities of the powders from the nitrates were stronger affected by the ligands used than those of the powders from the acetylacetonates. When the nitrates were the raw materials, the activity of the powders for the CO oxidation increased in the order of DEMM>HG>>EG. When acetylacetonates were the raw materials, the order was EG>DEMM=HG. These two orders are in considerably good harmony with the two orders on the perovskite formation tendency, although HG can assist stronger the perovskite formation than DEMM when the raw materials are the nitrates. The catalytic activities of the powders are of course influenced by their specific surface areas, but they are very close to each other. The higher catalytic activity of the powder by the combination of the nitrates and DEMM may be correlated to the result of the SEM observation that the grains in the powder have many cracks. From the above results, it is deduced that the complexing ability of the counter anions of the raw materials and the ligands used in the powder preparation process has a big influence on the formation of perovskite [ 14-17] and the activity of LSCO powders for the CO oxidation.
100
100
80
80
~ 9 60 r~
o 60 O
O
9 40 r~
9 40
20
20
i
100
200 Temperature/~ (a)from nitrates
300
16o
200 Temperature/~
300
(b)from acetylacetonates
Figure 6. Effect of raw materials and organic polydentate iigands on CO oxidation with LSCO powders calcined at 800 ~
O, HG" A, DEMM; t-l, EG; X, acetate.
636 REFERENCES
1. J.A. Marcos, R.H. Buitrago and E.A. Lombardo, J. Catal. 105 (1987) 95. 2. Y. Teraoka, H.-M. Zhang, K. Okamoto and N. Yamazoe, Mat. Res. Bull., 23 (1988) 51. 3. Y. Teraoka, M. Yoshimatsu, N. Yamazoe and T. Seiyama, Chem. Lett., (1984) 893. 4. Y. Teraoka, H.-M. Zhang and N. Yamazoe, Che. Lett., (1985) 1367. 5. H. Fujii, N. Mizuno and M. Misono, Chem. Lett., (1987) 2147. 6. K. Tabata, I. Matsumoto, S. Kohiki and M. Misono, J. Mater. Sci., 22 (1987) 4031. 7. T. Nitadori, T. Ichiki and M. Misono, Bull. Chem. Soc. Jpn., 61 (1988) 621. 8. T. Nitadori, M. Muramatsu and M. Misono, Bull. Chem. Soc. Jpn., 61 (1988) 3831. 9. P. Shuk, A. Vecher V. Kharton, L. Tichonova, H.D. Wiemh6fer, U. Guth and W6pel, Sensors and Actuators B, 15-16 (1993) 401. 10. F. Mizukami, S.Y. Matsuzaki, F. Furukori, S. Niwa, M. Toba and J. Imamura, J. Chem. Soc., Chem. Commun., (1986) 491. 11. F. Mizukami, S. Niwa, M. Toba, T. Tsuchiya, K. Shimizu, S. Imai and J. Imamura, Stud. Surf. Sci. Catal., 31 (1987) 45. 12. M. Toba, F. Mizukami, S. Niwa, Y. Kiyozumi, K. Maeda, A. Annila and V. Komppa, J. Mater. Chem., 4 (1994) in press. 13. M. Toba, F. Mizukami, S. Niwa, T. Sano and K. Maeda, J. Mater. Chem., 4 (1994) in press. 14. F. Mizukami, Y. Kobayashi, S. Niwa, M. Toba and K. Shimizu, J. Chem. Soc., Chem. Commun., (1988) 1540. 15. K. Maeda, F. Mizukami, S. Miyashita, S. Niwa and M. Toba, J. Chem. Soc., Chem. Commun., (1992) 1268. 16. K. Maeda, F. Mizukami, S. Niwa, M. Toba, M. Watanabe and K. Masuda, J. Chem. Soc., Faraday Trans., 88 (1992) 97. 17. K. Kojima~ F. Mizukami, M. Miyazaki and K. Maeda, J. Non-Cryst. Soilds, 147& 148 (1992) 442.
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
637
MONOLITH PEROVSKITE CATALYSTS OF HONEYCOMB STRUCTURE FOR FUEL COMBUSTION L.A.Isupova a, V.A.Sadykov a, L.P.Solovyovaa, M.P.Andrianova a, V.P.Ivanov a, G.N.Kryukova a, V.N.Kolomiichuk a, E.G.Avvakumov b, I.A.Pauli b, O.V.Andryushkova b, V.A.Poluboyarov b, A.Ya.Rozovskii c, V.F.Tretyakov c. aBoreskov Institute of Catalysts SD RAN, Pr.Lavrentieva,5, 630090 Novosibirsk, Russia bInstitute of Solid State Chemistry SD RAN, ul.Derzhavina,18, 630091 Novosibirsk, Russia CTopchiev Institute of Petrochemical Synthesis RAN, Leninskii pr.,29, 117912 Moscow, Russia
New method of dispersed perovskites synthesis based upon mechanochemical activation of the solid starting compounds is elaborated. The influence of defect structure of these compounds as well as surface segregation on their catalytic properties is discussed. Basic stages of the monolith perovskite catalysts preparation are optimized. The experimental samples of monolith catalysts of various shapes are obtained, possessing high activity, thermal stability and resistance to catalytic poisons.
1. INTRODUCTION Catalytic combustion is now considered as one of the promising ways to abate NO x in flue gases of power plants, turbines and other sources [1]. For this application monolith honeycomb catalysts capable of retaining high mechanical strength and activity at elevated (900-1300~ temperatures are required. Traditional oxide catalysts of complete oxidation including those supported upon ceramic monoliths (cordierite etc) are known to deactivate rapidly in these conditions due to sintering, interaction with support etc. Platinum group metals are expensive, and pronounced loss of these active components via evaporation at high temperatures [2] makes their wide-scale application unreasonable. At the same time, mixed oxides of transition and rare-earth metals possessing perovskite structure appear to be suitable for catalytic combustion due to their well-known stability at high temperatures and in a broad range of oxygen partial pressures [3].
638 Among them, the most active in complete oxidation are partially substituted manganites and cobaltites of lanthanum (Lal.xSrxMnO3,Lal_xSrxCoO3) [4], though to the present day any industrial production of these catalysts is absent. To elaborate technology of the perovskite monolith honeycomb catalysts production, methods of preparation of highly dispersed, chemically active powders with uniform phase composition and narrow particle size distribution should be invented. Such properties of the powders are necessary to minimize the content of an inorganic binder required for shaping the perovskites as monoliths thus ensuring mechanical strength, thermal shock resistance and stability to poisoning of the final catalysts. Moreover, any large-scale methods of the perovskite powders preparation should be more or less cheap and wasteless. These requirements are met by the method of mechanochemical activation (MA) of starting compounds (oxides, hydroxides, carbonates etc) in the high-powered mills with a subsequent thermal treatment [5]. However, perovskites were yet not attempted to synthesize by this method. The present work was undertaken to develop the methods of active powdered perovskites preparation via mechanical activation, as well as to shape monoliths by extrusion and to investigate their catalytic properties and stability.
2. METHODS OF THE SAMPLES PREPARATION A N D INVESTIGATION.
As s t a r t i n g compounds, oxides and carbonates of lanthanum, cobalt, manganese and strontium of ,chem. pure) or ,pure for analysis) grade were used. Mechanical activation of the starting compounds was carried out using highpowered EI or AGO planetary ball mills [5]. The ratio of the weights of milling balls and powders was equal to 10; time of activation -1-30 min; acceleration (achieved in the drums of the mills) - 40 and 60 , g , , respectively. After mechanical treatment the powders were air annealed for 2-4 hours at temperatures ranging from 550 to 1000~ Specific surface of the samples was determined by BET method using Ar thermal desorption data. Phase composition was analyzed with a URD-63 diffractometer using CuK~radiation (scanning region was 0 - 4-30~ X-Ray small-angle scattering (SAXS) experiments to obtain the relative densities of extended defects (dislocation, stacking faults etc) were carried out with Cu K -radiation with a nickel filter and by using an amplitude analyzer [6]. Catalytic activities in the reactions of CO and butane oxidation in the excess of air were determined in the batch-flow and/or microcatalytic systems equipped with the gas-chromatographical analysis of the reaction mixture components. The pore size distribution was investigated by the method of mercury porometry using an Auto-Pore 9200 machine. Electron microscopic data (TEM) were obtained using a JEM-100 CX machine operating at 100 kv (resolution 2.4 ~ ).
639
The surface concentrations of elements were determined by SIMS method using a mass-spectrometer MC-7201. Extrusion. The catalysts of simple (cylinders) or complex (rings, honeycomb monoliths) forms were prepared by extrusion of the plastic pastes composed of perovskites powders with addition of the binder based upon alumina, peptizers (acetic acid or HN03), and some surfactants.After optimizing theological properties of the pastes and the regimes of drying and calcination for rings and micromonoliths,the monoliths of rectangular shape (70•215 ram) with square channels 4• mm and walls ca 2 mm were formed using die (spinneret) designed and manufactured at the Institute of Catalysis and attached to a plunger forming machine. After drying the monoliths were calcined in the range of 400-1100~ for 2-4 hours.
3. RESULTS AND DISCUSSION 3.1. Mechanochemical synthesis of perovskites The general feature of the solid-state reaction between mechanically activated phases was found to be a considerable influence of the stoichiometry of transition metal oxide on the degree of interaction with lanthanum compounds. Thus, in the case of Co304 as starting phase, nearly 100% conversion into perovskite was achieved, while only 50% of CoO was converted into the product (Table 1). Some results of the experiments aimed at optimization of the synthesis conditions for LaMnO3, LaCoO3, Lal.xSrxCoO 3 (variation of the time of activation, a type of the mill, a temperature of sintering) are shown in Table 2. On the basis of the results obtained, some general trends in the mechanochemical synthesis of perovskites can be noted. Table 1 Mechanochemical synthesis of perovskites* Initial substances
Mechanochemical activation
Product yield,%
La203 ,CoO La2(CO3)3, CoO La203, CoO La2(CO3)3,Co2(OH)2CO3 La203, Co304
+ + +
0 0 50 100 100
La203, MnO 2 La203, Mn203 La203, Mn304
+ + +
100 60 40
*Temperature of synthesis - 700~ time of activation - 5 min.
time of sintering - 2-4 h,
640 Table 2 Influence of the preparation conditions of perovskites on their dispersion and phase composition. Initial substances La2(CO3) 3, C~
Mill Time of T~ activation, min. }
C0304, La203 vv
vv
La2(CO3)3,SrCO3, C02(0H)2CO 3 La203,SrO,C0304 vv
vv
La2(C03)3 } C02(OH)2CO 3 Sr(NO3) 3 La203,MnO 2
S,m2/g
Phases
EI EI EI EI AGO
5 15 30 5 3
700 600 550 700 700
7 9 12 7 7
EI
5
700
-
Lao.TSr0.3CoO3 (hex)
EI AGO AGO AGO AGO
5 3 1 3 3
900 900 700 700 1000
4.1 2.5 15 17 2
Lao.TSr0.3CoO3 (hex)
EI EI EI EI EI
1 1 3 5 5
900 1000 900 600 700
4 2.6 4.1 10
ImMnO3(rh ), LaMnO3(rh ), ImMnO3(rh ), LaMnO3(rh), LaMnO3(rh),
EI AGO
5 3
800 700
7 4
LaCoO3 (hex) LaCoO3 (hex) LaCoO3,adm.Co304,La(OH) 3 LaCoO3, adm. La(OH)3 LaCoO3, tr. La203 adm. La202CO3, SrCO 3 "-"(hex), "-"(hex), "'-"(hex), "-"(hex),
tr. C0304 adm.SrCO3,La(OH)3 adm. La203 tr. Co304 adm. La202CO3 tr. La203 tr. La203 MnOx,La202CO 3 adm. MnO x,
La202CO3 LaMnO3(rh ), tr. La(OH)3 LaMnO3(rh ), adm. MnOx,
La202CO3 (hex)- hexagonal, (rh)- rhombohedral, adm.- admixture, tr.- traces. 1.
2.
3.
4.
The increase of the time of activation allows to reduce the temperature of sintering. Thus, the lowest temperature of synthesis was achieved in the case of LaCoO 3 (550 ~ activation of the starting mixture for 30 min in EI). At the same sintering temperature, a time of activation depends upon the type of the mill. Hence, at 700 ~ LaMnO 3 was found to appear in the mixtures activated at least for 5 min in EI, while only 3 min treatment in AGO was sufficient to start reaction. The nature of the products of solid-state reaction (mainly if not exclusively perovskite phase) is independent upon the type of starting compounds (oxides or carbonates). The optimum conditions of mechanochemical synthesis appear to be 3 min activation in AGO with the subsequent annealing at 700 ~ for 2 h.
641 Therefore, our efforts allowed to decrease the temperature of the efficient solidstate interaction from 1100 ~ to 600-800 ~ while duration of the reaction was reduced to several hours from hundreds of hours typical to ceramic technology. The perovskite powders thus obtained have rather high specific surface values (ca 7-17 m2/g) approaching those for samples synthesized via other routes (precipitation etc) [4].
3.2. Catalytic properties of perovskites prepared via mechanical activation route: influence of defect structure and surface segregation The range of activities of the samples obtained was found to depend upon the chemical composition of the perovskites. Thus, for pure lanthanum cobaltites prepared by mechanochemical method a specific catalytic activity was higher than for ceramic samples [7]. However, it is not a general phenomena:in the case of the strontium-substituted cobaltites the trend is reversed- ceramic samples are more active. Table 3 Some properties of LaCoO 3 versus preparation method.
Preparation T~ method Precipit. Ceram. MA
650 1100 700
S,m2/g
16.1 0.4 9.1
Phase
Rate of CO oxid. at 140~ W o1 0 - 1 7 , mol. CO/m2.s
LaCoO3(hex ) LaCoO3(hex ) LaCoOa(hex )
1 0.3 1
I., Integral intensity of SAXS
Surface concent. of Co, rel. un.
3.5 17.6
0.053 0.104
In general, the influence of the method of preparation on the specific catalytic activity of perovskites can be caused by the change of the type of a crystal structure, variation of defect structure or by surface segregation of the basic components/impurities [8]. As follows from the data of Table 3, the first reason does not play any role for lanthanum cobaltite. At the same time, a considerable enrichment of the surface layer of MA samples by cobalt was revealed by SIMS (Table 3), while bulk composition including such impurities as sodium and potassium was identical with that for ceramic samples. According to TEM data, this enrichment could be assigned to surface segregation of a Co304 microphase having typical dimensions of crystallites ca. 40-50 ~ . Highly disordered boundaries between these crystallites and the perovskite matrix can generate extended defects found in MA sample by SAXS method ( their relative density is nearly 6 time higher than in ceramic sample). It seems reasonable to assign an increased activity of MA sample to high density of defect centers created and fixed (stabilized) by a surface microheterogeneity of such system. In the case of Sr-substituted cobaltites the most important role is played by defect structure. As we have found recently [9], in the reaction of CO oxidation a maximum of the steady-state activity of Lal.xSrxCoO 3 system at x-0,3-0,4
642 (observed also by Chan et al [4]) corresponds quite well to the highest density of extended defects in this series of samples. Simultaneously, no correlation w i t h the surface concentration of cobalt have been observed. From the practical point of view, a r a t h e r high specific activity of MA samples of perovskites comparable with t h a t of pure Co304 and Mn304 allows not only to maintain a process of high-temperature combustion, but also to ignite it at start-up conditions. 3.3. F o r m i n g of m o n o l i t h s To elaborate the techniques of monoliths extrusion, the batches of powdered perovskites (up to 100 kg) were synthesized via mechanochemical route. In contrary to ceramic perovskite powders, MA samples can be formed as rings without any addition of binder or peptizers into water-based pastes. However, after calcination at 500-900~ these rings have a r a t h e r low mechanical s t r e n g t h , which reaches an acceptable (ca 10 k g / c m 2) level only after annealing at 1000~ A n y a t t e m p t s to extrude micromonoliths without binders were unsuccessful. Table 4 Catalytic activity of monolith perovskites versus composition and the t e m p e r a t u r e of calcination. Perovskite
LaCoO 3
% of alumina added 8
Acid
CH3COOH
T,~
S,m2/g
W . 102,cm3C4Hlo/g 9s 300 ~
400 ~
500 ~
400 500 700 900 100
56 48 32 11 2.6
0.014
0.38 0.58 0.96 0.69 0.13
1.32 1.20 1.70 1.92 0.76
Lao.7Sro.3CoO s 13
HNO s
400 500 700 900 1100
102 79 59 25 4.2
0.083
0.89 0.85 0.74 0.93 0.89
1.74 1.95 1.35 1.74 0.53
~ O
HNO 3
400 500 700 900 1100
95 86 67 16 4.2
0.066
0.43 0.42 0.44 0.41 0.26
1.14 1.02 1.12 1.00 0.89
400 500 700 1100
31 20 14 1.5
0.21
0.63 1.07 0.96 0.17
1.15 1.99 1.55 0.71
s
LaMnO3
(rings)
13
643 A g r e a t deal of work has been carried out to i m p r o v e the r h e o l o g y of pastes and mechanical s t r e n g t h of the e x t r u d a t e s by using v a r i o u s binders, p e p t i z e r s a n d s u r f a c t a n t s . Some properties of the samples obtained are given in Tables 4 and 5. B i n d e r . As follows f r o m these results, a binder based u p o n a l u m h l a e n h a n c e s specific s u r f a c e of the samples and improves considerably mechanical s t r e n g t h . Thus, an acceptable s t r e n g t h is achieved even a f t e r calcination at 400~ t h a t can be explained both by an increase of the n u m b e r of contacts in e x t r u d a t e and s t r e n g t h e n i n g of such contacts. In this case an observed decrease of a c t i v i t y in kinetic r e g i o n was caused probably by blocking of a p a r t of t h e active c e n t e r s by the particles of a l u m i n a . The analysis of the pore s t r u c t u r e of the samples has revealed t h a t , all o t h e r factors being the same, a m e a n pore r a d i u s (Table 5) declines as a l u m i n a content increases. The invariance of t h e total pore v o l u m e observed t h e r e w i t h evidences more densely packed a r r a n g e m e n t of particles in e x t r u d a t e s h a v i n g h i g h e r content of alumina. This m e a n s t h a t indeed t h e increase of the mechanical s t r e n g t h is connected not only w i t h the s t r e n g t h e n i n g of t h e u n i t contact, b u t also with the increase of a n u m b e r of contacts. Table 5 Some p r o p e r t i e s of La-Mn-based monoliths calcined at 700~ composition. Content of A1203, %
Acid.
Content of aSur.% S,m2/g
Pore structure V,cm3/g bp,mkm
v e r s u s pastes
Rate of Bu oxidation W-102, cm 3 C4H10/g.s at 400~
13
HNO a HNO 8 HNO 3 CH3COOH CH3COOH
2 4 1.6 4
57 -
0.32 0.24 -
0.24 0.04 -
0.28 0.25 0.32 0.1 0.61
23
HNO 3 HNO 3 HNO 3 HNO 3 CHsCOOH HNO 3 CHaCOOH
1.2 2 4
108 115 95 102
0.32 0.31 0.32 0.28 0.38 -
0.035 0.032 0.18 0.16 0.074 -
0.19 0.19 0.1 0.3 0.39 0.18 0.32
2
0.5* 0.5*
-
102 -
S u r f a c t a n t s : ethylene glycol, *- carboxylmethyl h y d r o cellulosa. a . Sur., %- s u r f a c t a n t , % ; b~_ mean pore radius S u r f a c t a n t s . A d d i t i o n of some s u r f a c t a n t s such as ethylene glicol etc up to 1.8 weight % was f o u n d not to influence noticeably a pore s t r u c t u r e of t h e samples. However, at h i g h e r concentrations an increase of the m e a n pore r a d i u s was observed possibly leading to some decline in the n u m b e r of i n t e r p a r t i c l e contacts. Indeed, a s u b s t a n t i a l d e t e r i o r a t i o n of the mechanical s t r e n g t h was d e t e c t e d f o r
644 granules formed from the pastes with a high content of surfactants. Peptizers. Using of such peptizer as acetic acid instead of HNO 3 would be desirable to eliminate NO x emission at calcination stage. However, the samples obtained with addition of HAc into forming masses have a somewhat decreased mean radius and total pore volume (cf. lines 2 and 4, Table 5) t h a t seems to be connected with a lower optimum water content in these pastes. Though in kinetic region the catalysts thus prepared have higher activities, at elevated t e m p e r a t u r e s the diffusion limitations are expected to be more essential. Therefore, a choise between these two acids has to be determined also by conditions of exploitation. Drying. Optimizing of this stage was the most important to ensure the absence of cracks. Only slow drying in controlled humidity conditions was shown to be suitable for obtaining monoliths without cracks. Calcination stage. Variation of the temperature of calcination in the 400-900 ~ range was shown not to influence catalytic activity. Only after annealing at 1100 ~ some decrease of activity was observed, alumina containing samples being more stable. Thus, alumina appears to suppress sintering, possibly due to f o r m a t i o n of microphases of the hexaaluminate type having very high melting points [10].
3.4. Catalytic activity and stability Optimized catalysts in the form of honeycomb monoliths were tested in several pilot installations (table 6). High-temperature catalytic combustion of gas and liquid fuels (Table 6) even in catalyst-supported regime (flame is located within La-Mn- monolith) sharply decreases emissions of NO x and CO as compared with an open flame regime. Earlier [11], such results were achieved only with Pd catalysts s u p p o ~ on metal monolith carrier. Flameless combustion on our catalysts appears to be even more efficient to abate NO x. The catalysts worked for two months without loss of activity and monolith integrity withstanding everyday start-ups and shut-offs.
Table 6 Fuel combustion on monolith perovskites Fuel
Process
Emission, vol.% CO
propane-butane
noncatalytic combustion catalyst-supported combustion flameless catalytic combustion
3.3 0.35 0.01
0.34 <0.01 <0.01
gasoline
noncatalytic combustion catalyst-supported
-
0.04 <0.01
NO x
High tolerance of our catalysts to the action of chlorine-containing compounds was demonstrated by the results of the month-long working in the furnice for
645 combustion of photomaterials. While other oxide catalysts were deeply deactivated due to HC1 action (a fall of activity averages an order of magnitude), the activity of our catalyst remained rather high (ca 30 % of the initial level).
4. CONCLUSIONS. 1. The original technology of powdered perovskite synthesis based upon mechanochemical activation (MA) of solid starting compounds has been elaborated. It is distinguished by the simplicity, high productivity and yields highly dispersed perovskites of uniform phase composition. 2. Factors determining an enhanced activity of MA perovskites are elucidated; the most important appears to be an increased density of extended defects and surface segregation of the microphases of transition metal oxides. 3. Technology of preparation of the monolith perovskites with honeycomb structure has been developed. 4. High thermal stability of these catalysts and tolerance to catalytic poisons have been demonstrated.
REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9.
10. 11.
D.L.Trimm, Appl.Catal., 7 (1983) 249. L.D.Pfefferle, W.C.Pfefferle, Ibid., 29 (1987) 219. L.G.Tejuca, J.L.G.Fierro, J.M.D.Tascon, Adv. Catal., 36 (1989) 237. K.S.Chan, J.Ma, S.Jaenicke, G.K.Chuan, Appl.Catal., 107 (1994) 201. E.G.Awakumov, Mehanical methods of activation of chemical prosesses, Russia, Novosibirsk, 1986. V.A.Sadykov, S.F.Tikhov, G.N.Kryukova, N.N.Bulgakov, V.V.Popovskii, V.N.Kolomh'chuk, J.Solid State Chem., 74 (1988) 200. I.A.Pauli, E.G.Awakumov, L.A.Isupova, V.A.Poluboyarov, V.A.Sadykov, Sib. Khim Zhurn., 3 (1992) 133. J.Haber, New Developments in Selective Oxidation by Heterogeneous Catalysts, Elsevier, (1992) 279. L.A.Isupova, V.A.Sadykov, V.P.Ivanov, A.A.Rar, S.V.Tsybulya, M.P.Andrianova, V.N.Kolomh'chuk, A.N.Petrov, O.F.Kononchuk, React.Kinet. Catal. Lett., (1994) accepted. M.Machida, H.Kawasaki, T. Shiomitsu, K.Eguchi, H.Arai, Shokubai, No 3 (1989) 325. I.Stambler, Gas Turbine World, May-June (1993) 32.
This Page Intentionally Left Blank
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
647
Study on the preparation of nanometer perovskite-type complex oxide LaFeO3 by sol-gel method* Ziyi Zhong Ligang Chen Qijie Yan** Xiancai Fu Department of chemistry, Nanjing university Jianmin Hong Modem analysis center, Nanjing university Nanjing, 210008, China
ABSTRACT Nanometer Perovskite-type complex oxide LaFeO3 was prepared by sol-gel method using citrate ion as ligand. The influence of the amounts of citric acid, the types and amounts of dispersant, the calcining temperature and time on particle size have been studied. The formation process of LaFeO3 was also investigated by XRD, TEM, FT-IR and Raman spectroscopy. The most uniform particles with average size of 21.8nm after calcination at 650"C for 4h were found in the case of La:Fe:citric acid molar ratio--l:l:4. After adding suitable amounts of dispersant such as ethanol, propanol, isopropanol or acetic acid, the homogeneity was improved, and even smaller average size of the particles was obtained. After calcined at 800~ for 4h or 650"C for 9h, the particles were sintered partially to reach an average particle size of 26.4nm and 34nm respectively. The XRD, TEM, IR and Raman results reveal that the decomposition of semidecomposed precursor and crystallization of nanometer LaFeO3 are completed in the temperature range of 450~ 1.1NTRODUCTION Perovskite-type complex oxides (ABO3) have a well-defined structure, and it is possible to change the A or B site ion variously, without affecting the fundamental structure[l]. Therefore perovskite-type complex oxides are suitable materials for the study of the relationship between
* The project supported by NSFC and NSFJ **To whom the correspondence should be addressed
648 the solid-state chemistry and catalytic behavior of metal oxides. Perovskite-type oxide LaFeO3 and its partially cation-substituted oxide system have shown excellent catalytic activity on complete oxidation of CH412], CH3CH2CH3[3] and CO[4]. Since the catalytic properties of the ultrafine particles are unique, ultrafine LaFeO3 and its partially cation-substituted oxides were prepared in our lab. In this paper, we report the preparation of ultrafine LaFeO3 by sol-gel method. The influence of the types and amounts of complexing agent and dispersant, and the product formation process have been studied. Based on the study of using citric acid, acetylacetone and tartaric acid as complexing agent, citric acid system was chosen for study in detail. TEM, XRD, BET surface area measurement, IR and Raman spectroscopy were used to characterize the products calcined at different temperatures, the formation process of LaFeO3 was discussed. 2.EXPERIMENTAL Lanthanum oxide(99.9%) was dissolved in 1:1 HNO3 solution and mixed with equal mol of Fe(NO3)3 and then added complexing agent and dispersant( when needed) to make a transparent solution. This solution was kept at 60-70"C to evaporate the water, and the viscous gel was formed through sol process. Kept the gel on water bath for 3-4h and then dried at 120"C, finally calcined in air at 650"C for 4h (except the calcination temperature effect study). XRD studies were conducted using Shimadzu XD-3A diffractometer with Cu K(x radiation for crystalline phase detection. TEM studies using JEM-100s electromicroscope were conducted for the particle size determination. The IR spectra of the gel and the products calcined at different temperatures were also measured by using Nicolet 510p FT-IR spectrometer, Micromeritics ASAP 2000 was used for BET surface area and pore size distribution measurement. Raman spectra were recorded by using a Spex Ramanlog Model 1403 Spectrometer, with Ar ion laser turned to 488.0nm wavelength line for excitation was used. The samples were pelletized with KBr[5] into a wafer for mounting on a spinning sample holder. 3.RESULT AND DISCUSSION 3.1 Effect of preparation condition on the particle size of nanometer LaFeO3 3.1.1 Influence of the amount of citric acid
As the tool. ratio of La:Fe:citric acid changed from 1:1:2 to 1:1:6, TEM results showed that the particle size of LaFeO3 was changed obviously. The most uniform and smallest particles were found for La:Fe:citric acid=l: 1:4 without adding dispersant. These results are listed in Table 1. However, the particle diameter range is rather broad, a few particles close to 100nm
649 were observed. In order to improve the homogeneity of the panicle size, a series of dispersant was tested. Table 1 Effect of citric acid amounts on LaFeO3 panicle size No.
molar ratio of La:Fe:citric acid
particle size(nm)
average diameter(nm)
1 2 3
1:1:2 1:1:3 1:1:4
10-90 10-90 10-77
30.0 27.4 21.8
4 5
1:1:5 1:1:6
10-60 10-80
25.0 35.0
| m,
,,-~
.' _,
"1'
'+ 9 "'~"
+ .
"
',+'-'--','-'" ~
..++~
, + ~ +,, ++
",+ql~
++
,+
41
I --
:+++~
...., " .41 ++.,ab, m .-,.+
Fig.l TEM p h o t o g r a p h of LaFeO 3 c a c l c i n e d at 650"C a:.La:Fe:cit=l:l:4 b:La:Fe:cit:ethanol=l:l:4:8
3.1.2 Effect of dispersant
Ethanol, Propanol, isopropanol, acetic acid(36wt%) were chosen as dispersant to improve the homogeneity of the LaFeO3 particle size. While kept the La:Fe:citric acid molar ratio at 1:1:4, the effect of the dispersant amount on the product panicle size was studied. TEM results(Fig.I) show that adding dispersant leads to the decrease of the average diameter of the particles and the increase of the homogeneity. The relationship between average particle size
650 and dispersant adding amount is shown in Fig 2. It can be seen that for each system there is a best dispersant adding amount. At this point, the smallest average particle size can be obtained. Below or beyond this point, the average particle size is much larger. For various dispersant, the best adding amount is not the same. Tile role of the dispersant can be explained as follows: since the surface tension of ethanol, propanol and isopropanol are much smaller than that of water[6], it is expected to reduce the attraction between colloid particles and prevent the aggregation of the particles by adding such agent to the system. However, too much dispersant would lead to the particles growth, it is important to control the amount of dispersant for keeping the smallest particle size. Acetic acid is also a complexing agent, it can coordinate with Fe3+ and La 3§ its influence on the product size is more complicated and not understood yet. 3.1.3 Influence of the calcining temperature and time
on the
LaFeOa particle size.
890 T09C)690 45 22. P-, 20-
.~
E
A
E
r
V
18.
22,5, (p
E c35
N m
Q N M
o~
6 O1
t)
I) t~
P 18-
s6
14. o
12
--~
dl-
1~ ' ";1~8' 2b ....
Molar rotlo of dispersont/Fe a+ Fig,2 Effect of dlspersont on LoFe0~ pOrt]cle size x -isoproponou o --proDonol o -ethonol A-oc~Ic ocld(3kwt)
15(
_ 12
12
Fig.3 Effect of calcining time(650=C) and calcining temperoture(4h) on LoFe0~ particle size
To examine the effect of calcining temperature on the particle size of LaFeO3, the precursor was heated to different temperature using a heating rate of 10"C/min and maintained at different temperature for 4h. Fig.3 shows that while the calcination temperature increased from 650~ to 800"C the average particle size increased from 21.8nm to 26.5nm. The BET surface area decreased from 22 to 5m2/g confirmed the sintering of tile product. According to nitrogen
651 adsorption/desorption isotherm measured, the shape of the hysteresis loop is type HI[7] indicates the samples consist of agglomerates having narrow distribution of pore size. All the samples exhibited unimodal pore size distributions except the one with surface area of 39.7m2/g exhibited double humped pore size distributions. With increasing temperature the pore size distribution and consequently also the most frequently pore radii are seen to shift towards higher values. However, the pore size is similar to the particle diameter and the pore size distributions revealed that the pore originated from the inter-crystallite. Fig.3 also shows the variation of the particle size as a function of calcination time at 650"C. After a slight increase of the particle diameter within the initial 4h, the particle size of LaFeO3 increases rapidly and reached to 45nm after 12h calcination at 650~
3.2 Formation mechanism of nanometer LaFeO3 prepared by sol-gel method
h__LL d
0
.~
0.1
II
0.3 Relative
I
I
-
0.5 0.7 pressure(P/Po)
Fig.4 Isotherm plot c a l c i n e d at 650~ 4h,
of L a F e O 3 +ads *des
:
-
0.9 I
_
/
:
I
~
1
24 ~ ~ F i g . 5 X-ray d i f f r a c t i o n pattern of the samples calcined at different temperatures a:450~ b:480*C c:550~ d:650~
3.2.1 XRD results revealed that all the samples calcined at 450"C or below were amorphous(Fig.5). The TEM images showed that the particles were heterogeneous(Fig.6), some particles diameter was larger than 300nm consisting of very small particles suggesting that the particles were formed by breakup of lump precursor. The electron diffraction
652
200rim
q ~.~
Fig.6 TEM p h o t o g r a p h s temperature a :450 ~
of s a m p l e s c a l c i n e d b: 550 ~
at d i f f e r e n t
pattern confirmed the samples were amorphous. As the calcining temperature increases to 480~ 514~ XRD pattem show that LaFeO3 crystalline phase formed, TEM images showed a few large particles still coexisted with the ultrafine particles. After 550"C calcination for 4h, the TEM image showed that very small and rather uniform particles with average diameter of 13.6nm was formed and XRD results confirmed the complete crystallization of LaFeO3. The above results suggest that complete decomposition of the precursor and crystallization of LaFeO3 is finished in the temperature range of 450"C to 550~ This is also confirmed by the m6ssbauer study[8]. Since intermediate compound such as La203 or Fe203 have not been detected, we may conclude that the formation of LaFeO3 is directly from semidecomposed citrate complex precursor and its formation temperature is in the region of 450"C to 550"C, much lower than that of using nitrate decomposition method or solid state reaction method. The product particle size is ranged in nanometer region. 3.2.2 The IR results of the sol, the precursor and the LaFeO3 product are shown in Fig.7 and Fig.8. The peak at 1732cm ~ in the spectrum of the sol assigned to C=O stretching vibration of free -COOH group of citric acid shifted to 1713cm -~ for citrate complex precursor after dried at 120*C, and then disappeared after calcined at 300~ This fact indicates that above 300~ the free citric acid components are eliminated. Peak at 1640cm ~ in the IR spectrum of sol assigned to - C O 0 group vibration of citrate complex shifted to lower wavenumber with decreasing calcination temperature. Two strong peaks at 1471cm-~, 1400cm -~ assigned to monodantate carbonate[9] were distinct after calcined at 400"C and 450~ which were so strong that peak in the region of 1600cm] was superposed by it. After 550*C calcination, all these peaks disappeared confirmed the complete decomposition of the precursor occurred under 550~ calcined observed by XRD&TEM.
653
g
e
d
i
_ , ,
9
2(X)O
_
' -
'
'
~
.
.
.
.
.
" -
-
~ ' "
1600 1200 Wavenumber/crn'l
: - ~
~
~
-
'
11-
"
'
-
9
_ .
'"
'
9
'
- - -
800
Fig. 7 IR spectra of the samples calcined at different temperatures a:sol b:120"C c:300~ d:400*C e:450 f:514"C g:550~
WavenumberlcmFig. 8 IR spectra of the samples in the low wavenumber region a:480"C b:550"C c:650"C d:800*C
On the other hand, the peak at 1385cm ~ assigned to NO 3 vibration[10] still remained after calcined at 550*C although very weak. Raman spectra also showed that the broad peak at 1350cm ~ assigned to NO 3, existed after calcined at 550"C. Another broad peak at 1600cm -t disappeared after 550"C calcination revealed the elimination o f - C O O group, this is inconsistent with the IR result. It is suggested that the breakup of the citrate complex occurred around 300"C, the residual NO 3 was eliminated above 550~ A strong and broad peak at 563cm ~ assigned to LaFeO3[11] after calcined at 480"C appeared revealed the formation
654
._~
1060
-.
1220 1.380 1540 Wavenumber/cm-1
1700
Fig.9 Raman spectra of samples calcined at different temperatures, a:450~ b:480~ c:550~
of the framework of perovskite structure consistent with the result of XRD. This peak shifted to lower wave number slightly with increasing the calcining temperature. This "blue shift" may be due to the effect of particle size decreasing after lower temperature calcination. 4. C O N C L U S I O N 4.1 Sol-gel method using citric acid as complex is an effective method for preparing ultrafine perovskite-type complex oxide LaFeO3. 4.2 Using dispersant such as acetic acid, ethanol, propanol and isopropanol, can improve the homogeneity of ultrafine LaFeO3 particles, based on the control of the amount of dispersant at a suitable value. 4.3 Ultrafine LaFeO3 is formed directly in the temperature range of 450-550"C by the decomposition of the mixed citrate. Intermediates such as La203, Fe203 have not been detected in the decomposition process. ACKNOWLEDGMENT We thank Mr. Liu Zhihui for TEM measurements, and Dr. Fan Yining for helpful discussions.
655
REFERENCES 1. R.J.H. Voorhoeve, "Advanced materials in catalysis" P129 Academic Press, New York, 1977. 2. I.Arai, T.Yamada, K.Eguchi, and T.seiyama, Appl. catal., 26(1986) 265-276. 3. Taihei Nitadori, Makoto misono, J. catal., 93 (1985) 459-466. 4. J.M.D. Tasc6n, S. Mendioroz, and Tejuca L. Gonz~ilez, Z. phys. chem. 124 (1981) 109. 5. J.T. Rrichard, W.B. chris, H.H. Robert, Applied spectroscopy, 32 (1978) 532-535. 6. R.C. Weast, M.J.Astle, W.H. Beyer, CRC handbook of chemistry and physics,(66th edition), F-32-35, CRC press INC,1985. 7. K.S.W. Sing, D.H. Everett, R.A.W. Haul, L.Moscou, R.A. Piesotti, J.Rouquesol, T.siemieniewska, Pure &Appl. Chem. 57 (1985) 603. 8. Z.Zhong, Z.Hu, K.Chen, L.Chen, Q.Yan and X.Fu, to be published. 9. Hua-Min Zhang, Yastutake Teraoka and Noboru yamazoe, Chem. Lett., (1987). 665-668. 10. A.N. Richard, O.K. Ronald, Infrared Spectra of Inorganic Compounds, New York, Academic Press INC, (1971) 125-147. 11. M.Couzi et, P.V. Huong, Ann.Chim., T.9, (1974) p19~29.
This Page Intentionally Left Blank
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
657
P R E P A R A T I O N OF P E R O V S K I T E TYPE C A T A L Y S T S C O N T A I N I N G COBALT FOR POST COMBUSTION REACTIONS Laure SIMONOT, Francois GARIN, Gilbert MAIRE and Paul POIX Laboratoire d'Etudes de la R~activit~ Catalytique, des Surfaces et Interfaces, URA 1498 CNRS - U L P - EHICS, Institut Le Bel, 4, rue Blaise Pascal, 67070 Strasbourg cedex, France
ABSTRACT To evaluate differences in the reactivity of a series of cobalt containing perovskite type catalysts for the CO oxidation reaction, a "single path" preparation route is desirable. Two such routes were tested using three precursors of BaCoO3, LaCoO3 and Ba2CoWO6 : namely co-precipitation and sol-gel methods.
1. INTRODUCTION Since 1970, the perovskite type oxides, typically rare earth oxides with a (ABO3) formula, have been suggested as substitutes for noble metals in automotive exhaust catalysis (1). The most studied perovskites are LaMO3 ( M = first row transition metal ) (2,3,4), where M is considered as the active site of the catalyst. The cobaltites show good activity as oxidation catalysts, the reactivity seems to depend on the facility of cobalt to undergo the transition Co II (-~ Co III, which may be correlated to an oxygen non stoichiometry, and to the spin state of the cation (5). Furthermore, series of LaMO3 oxides revealed similar profiles for CO adsorption studies as for NO adsorption, with NO adsorption maxima for M = Mn and Co (6). The reactivity of these catalysts has been shown not only to depend on the surface area, but also on the preparation process (7). The aim of our work is to study, for automotive depollution, the reactivity of cobaltites with cobalt present in different oxidation states. In addition, a single preparation p a t h w a y is required, to minimize the differences in reactivity depending on the preparation process. Two different preparation-routes were tested for the three precursors of BaCoO3, LaCoO3 and Ba2CoWO6 : namely coprecipitation and sol-gel methods.
658 2. PREPARATION OF THE PRECURSORS VIA A CO-PRECIPITATION PROCESS 2. 1 Introduction We must keep in mind, that for the automotive exhaust reactions, which are oxydo-reduction type, the catalyst has to be chlorine free. Thus, we never use precursor salts containing chlorine. By examining the solubility of the different salts of barium, lanthanum and cobalt it appeared that for the three, the nitrate salts are very soluble as opposed to the carbonate salts which are insoluble in water. For tungsten, no salt was found to be very soluble in water, or any inorganic base except ammonia. However cobalt is known to form a large number of ammoniacal complexes, thus this base cannot be used; therefore no co-precipitation pathway was found to prepare tungsten containing perovskites. 2. 2 Experimental Each precursor salt was dissolved in distilled water to obtain 200 cm 3 of a solution of 1 mol.dm-3. The solutions of the two different cations were mixed together in the desired proportions. Under vigourous mixing, a stoichiometric quantity plus 10% of an aqueous solution of K2CO3, was rapidly added. Under basic conditions (pH>9) the water was evaporated, before filtering and washing the precipitate with ice=cooled distilled water, until the pH of the filtrate became neutral. For the precursor of Ba2CoWO6, the barium and the cobalt cations were coprecipitated and then mixed vigorously with tungstic acid, in such a way as to obtain a good dispersion of this sparingly soluble phase, before evaporating the water. The precursors were then dried in an oven at 385 K overnight and then ground prior to calcination (section 4 ).
3. PREPARATION OF THE PRECURSORS VIA THE SOL-GEL PROCESS 3. 1 Introduction In this method we have utilised the fact that cobalt, l a n t h a n u m and barium, readily form propionates. After the total evaporation of the solvent, these propionates form inorganic polymers, which have a sol-gel texture. This gel formation can be described by the following equation, from a general nucleophilic substitution 9
M(OR)n + mXOH
;[M(OR)n_m (OX)m ]+ mROH
(1)
659 were X can be:
H in the case of an hydrolysis M (metal) in the case of a condensation L (ligand) in the case of a complexation
With two different metals present, we can also have heteroalkoxydes, where the ratio M / M ' is determined by the imposed stoichiometry, with-M-O-M'- bonds. The cations are thus well dispersed in the gel and near to each other. In comparison to the coprecipitation precursors, the distances between the metals are reduced, implying less diffusion processes and less phase segregation during the calcination process. 3. 2 Experimental The solvent used was propionic acid and all the solutions were prepared with a concentration of 1 mol.dm-3. La203 was used to prepare the lanthanum propionate ; by heating under reflux for 3-4 hours, one iodine crystal was added to aid La203 dissolution. In the case of cobalt we used two different precursors : Co(CH3COCHCOCH3)3 or COCO3. Taking the former compound, the propionate is rapidly formed by reflux heating for one hour, however, the presence of the acetylacetonate chain means that this gel could not be "flash-heated" in the calcination process without inducing an explosion. Thus "flash calcination"was carried out using the COCO3 salt. For barium, BaCO3 was used. However, on addition to the propionic acid, only a very small part of the salt dissolved corresponding to a superficial layer of the grain. The barium propionate formed, covers the surface of the undissolved BaCO3 salt, stopping the dissolution. By adding a small quantity of water (10 cm3), this superficial layer was dissolved, and the formation of barium propionate can continue. Direct dissolution of tungsten precursors in the acid did not yield any propionates. We observed that by adding ammonium metatunsgtate to a cobalt propionate solution, under reflux for two hours followed by solvent evaporation, we obtained a very homogeneous gel. This effect was not observed when we put the tungsten salt in the barium propionate solution. This suggests that we could have a tungsten propionate formation in the presence of cobalt propionate, probably due to the formation of a mixed-propionate (W, Co). By X-Ray diffraction, we verified that we obtained an amorphous precursor for the gel formed with cobalt and tungsten, as opposed to that formed with barium and tungsten. The latter showed some diffraction peaks, which did not correspond to the a m m o n i u m metatungstate, however these peaks were not identified. The gel was dried overnight at room temperature under a small air flow and then cooled with liquid nitrogen to aid grinding.
660 4. CALCINATION OF THE PRECURSORS During calcination, the precursors are transformed to the desired oxide. The final temperature of the calcination depends on the formation temperature of the perovskite and on the nature of the precursor. The more h o m o g e n e o u s the precursor, the less harsh the calcination conditions are. In the case of sol-gel precursors, the temperature and time of calcination would normally have been reduced, since the diffusion distances are smaller. In our study, we have chosen, for the same sample prepared via different routes, the same t e m p e r a t u r e and calcination time, to enable the comparison of the different reactivities, w i t h o u t including too many parameters. 4. 1 Conventional Calcination
For the coprecipitation precursors, the furnace was heated at a rate of 3K.min-1. For the sol-gel precursors the furnace was first heated at a rate of 2K.min d up to 523 K , to remove the solvent, and then at a rate of 3 K.min d to the final temperature. Temperatures and times of calcination are listed in Table 1. All the calcinations were performed under air. Table 1 Calcination conditions Catalysts Temperature LaCoO3 1075 K BaCoO3_x 1125 K Ba2CoWO6 1225 K
Time 4 hours 3 hours 5 hours
4. 2 Flash Calcination
This type of calcination was used only for the sol-gel precursors, to try to minimize the formation of any transient species. The furnace was heated to the desired temperature, and stabilized, the precursor was then carefully introduced to the hot furnace. Upon introduction the precursor burnt for 2-3 minutes, and was then kept in the furnace for the same time as in the other types of calcination. 5. RESULTS AND CHARACTERISATION 5.1 Results for the samples obtained using conventional calcination All the samples were identified by X-Ray diffraction : i) in a Guinier c h a m b e r (K~ F e ) a n d ii) on a D5000 from Siemens (Kcx Cr). All the BET measurements were performed by the use of krypton at a temperature of 77 K with a P0 = 3.276 mBar, the cross sectional area of krypton was taken equal to 19.5 A 2. The microanalysis was performed at Vernaison in the CNRS laboratory.
Catalysts
LaCoQ
Coprecipitation precursors
Sol-gel precursors
XRay*
Analysis
BET
XRay*
Analysis
BET
Lace
K = 5%
2.2 m2g-1
LaCoQ
Co/La = 1.01
3.2 m2g-1
Cot La =1.01
Ba3C02W09 Data listed for the X-ray describe the phases identified on the diffractograms Table 2 : Characteristics of the catalysts
662 The results from the identification and characterizations are listed in Table 2. 5.2 Discussion For Ba2CoWO6, the coprecipitation process did not give satisfactory results : the dispersion of the tungsten salt did not produce a homogeneous precursor, demonstrated by the presence of two phases with different stoichiometry. For the other samples obtained by the same procedure, the major problem was the presence of potassium as an impurity. A more stringent washing procedure was employed, but, in the case of LaCoO3, we observed a partial redissolution of the cobalt, probably due to the difference of the precipitation constants between the hydrogenocarbonate and the carbonate of cobalt, which provoke a defect in the stoichiometry. This problem was not so important for BaCoO3. Other precipiting agents were tested under the same calcination conditions, w i t h o u t success .
With the sol-gel process, we obtained a good reproducibility over many tests and suppressed the problem of phase segregation and impurities. The BET surface areas were in the same range, as for the coprecipitation samples, the particle diameters measured by SEM as beeing between 0.1 to 0.2 , m in all cases. 5.3 Results for the samples obtained with a "Flash calcination" These tests were performed only for LaCoO3 and Ba2CoWO6. Using the same sol-gel precursor, one half was used for a c o n v e n t i o n a l calcination and the other half for the "flash-calcination" to enable a clear comparison between these two calcination methods. After X-ray profile analysis of the diffractograms, we did not observe any differences in the relative intensities, half-height widths or the peak positions. We have also performed ESCA and SEM experiments. All the results are listed in Table 3.
Table 3 ESCA, SEM and BET results i
Catalysts
Conventional calcination
Flash calcination
ESCA
SEM*
BET
LaCoO3
C_..~o. 0.69 La
Type I
--
Ba2CoWO6
C_..oo. 0.59 Ba
Type 2
0.5 m2g -1
ESCA
SEM*
CO - 0.75 La
Type 1
Co = 0.50 Ba
Type 1
W W --- = 0.35 --- - 0.32 Ba Ba * The types given are related to the following photos (Figures 1 and 2).
BET
1 m ~ -1
663
i-~~ure 4_ .
"rT~,~
~ SQ.mf:,e~
664 From ESCA, we notice that the surface composition differs from the bulk composition given by the micro and the X-ray analysis. We observe that a superficial segregation of lanthanum and barium occurred during the calcination processed. The importance of this phenomenon is identical for both flash and conventional calcination. 6. REACTIVITY 6.1 Experimental A sample of the catalyst powder (0.8 g) was placed in a tubular quartz reactor. The composition of the gas flow was obtained by mixing the components (CO, 02) diluted in N2 with the use of gas flowcontrollers. Before each reaction, the reactor was bypassed in such a way as to know precisely the composition of the gas flow before reaction. The analysing section is composed of : i) IR detectors for CO, C O 2 ii) paramagnetism detector for 0 2 . The experiments were carried out by increasing gradually the temperature at a rate of 4 K.min-1 to a maximum temperature and following the concentration of the gas flow. In all experiments, the catalyst was cooled under N2. All experiments were performed four times. The first one being considered as the activation process. The reaction studied was CO + ~ O 2 =~ CO 2 and we analysed the variation in gas composition for the reactants and products as a function of the temperature. Temperature Programmed Reduction (TPR) measurements were obtained u n d e r 1.5% CO/He. CO and CO2 were analysed by IR. Approximately 0.015 g of the catalyst was placed in a U-shaped reactor, and was heated at a constant rate of 5 K.min -1 under a gas flow of 40 cm3.min -1 from room temperature to 1075 K. 6.2 Results and discussion Two types of study were undertaken. Firstly, we compared the reactivity of the perovskites prepared following the two different processes ; secondly, a comparison was made for the sol-gel preparation between the flash and the conventionally calcined samples. The following points were noted : I st :the presence of potassium as an impurity in the compound was found to dramatically decrease reactivity ; a deviation of 150 K or more was observed in the temperature o f half conversion for the two catalysts both with and without potassium in the case of LaCoO3. For BaCoO3_x only 10 K separated the reactivity of the catalysts. 2 n d 9 for Ba2CoWO6, the "flash-calcined"sample showed a higher activity than for the sample classicaly calcined. The temperature of half conversion was 100 K lower. For LaCoO3, the reactivity followed the opposite order, but for the two samples the deviation was only of 10 K. The TPR results did not point out any differences between the Ba2CoWO6samples calcined u n d e r the different conditions.
665 The reactivity order for CO oxidation was found to be : LaCoO3 > BaCoO3_• > Ba2CoWO6 regardless of the preparation route. 7. CONCLUSION From the two different preparation processes the coprecipitation route does not give the best catalytic activity, due to the fact that these samples have alkaline impurities. The sol-gel method is preferable and leads to very active catalysts. For the CO + 02 reaction, the LaCoO3 catalyst prepared via the sol-gel route showed a temperature of half conversion 100 K lower than for a 2% platinum alumina catalyst (8).
AKNOWLEDGEMENTS
The authors would like to thank Dr C.Petit, Dr L.Hilaire, Dr J.L.Schmidt and Prof. M.H.Simonot-Grange for their contributions.
REFERENCES
(1) (2) (3) (4)
(5) (6) (7) (8)
W.F.Libby, Science. 171 (1971) 499 Yung-Fang Yu Yao, J.Catal. 36 (1975) 266 L.G.Tejuca, J.L.G.Fierro and J.M.D.Tascon, Advances in Catalysis. 36 (1989) 279 R.J.H.Voorhoeve, D.W.Johnson JR., J.P.Remeika and P . K . G a l l a g h e r , Science, 195 (1977) 827 K.Nag and A.Roy, Thermochimica Acta. 17 (1976) 247 J.M.D.Tascon, L.G.Tejuca and C.H.Rochester, J.Catal. 95 (1985) 558 A.K.Lavados and P.J.Pomonis, J.Chem.Soc.Faraday Trans. 87(19) (1991) 3291 L.Simonot, F.Garin and G.Maire, CAPoC III (1994), to be published
This Page Intentionally Left Blank
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
667
Characterization and reactivity of M g Fe2.2xO3.2x and MgyZnl.yFe20 4 solid solution spinels prepared through the supercritical drying method. G. Busca", M. Daturi", E. Koturb, G. OliverP and R. J. Willey b a IstitutO di Chimica, Facolti~ di Ingegneria, Universit/L 1-16129 Genova (Italy) b Department of Chemical Engineering, Northeastern University, Boston, Ma 02115 (USA)
Mixed oxides with the formula Mg Fe2.2xO3.2x (0 < x < 1) and MgyZnl.yFe204 (0 < y < 1) have been prepared as aerogels (i.e. gels obtained by hydrolysis of metal acetates or alkoxides in methanolic solution followed by drying under supercritical conditions). MgxFe2.2xO3.2xsamples with 0 < x < 0.5 are Mg-deficient spinels, giving rise to MgFe204 + tx-Fe203 mixtures when calcined at 773 K. With x = 0.5 the stable phase MgFe204 is obtained. With 0.5 < x < 0.66 a Mg-excess spinel phase is obtained that is substantially stable even at 1073 K. Surface areas of the spinel phases range around 100 mVg. MgyZnl.yFe204 are stable solid solution spinels in the entire range, and convert progressively from normal to partially inverted spinels by increasing y. The bulk and surface properties of these materials have been investigated by XRD, TG-DTA, FT-IR and FT-Raman spectroscopies, BET surface areas and SEM microscopy. Also FT-IR spectra of the surface OH groups and catalytic activity in the reduction of NO by NH 3 were investigated.
1. INTRODUCTION Materials belonging to the system MgO-Fe203 constitute industrial catalysts for the oxidative dehydrogenation of linear butenes to butadiene (1-3). The active phase is likely the spinel MgFe204 although excess of MgO can be present. Fe203 and other ferrite spinels like ZnFe204 have also been found to be active oxy-dehydrogenation catalysts (2,4). ZnFe204 powders find application in the chemical industry as regenerable sorbants of H2S in hightemperature desulphurization processes (5,6). Fe203-based materials are also excellent catalysts for the reduction of NO x by NH 3 at least in SO2-free atmospheres (7). MgFe204 and ZnFe204 and their solid solutions are applied in the pigment and magnetic materials industry (8), and have been considered as sensor active phases (9). For these applications, the availability of highly dispersed but structurally well characterized powders is useful, also in the case of successive sintering, in order to obtain bulk materials with superior microstructure. However, the production of high-area monophasic mixed oxides like ferrite spinels with controlled structure and stoichiometry and free from contaminants is not always an easy task.
668 In this paper the preparation and the characteristics of MgxFe2.2xO3.2x (0 < x < 1) and of MgyZnvyFe204 (0 < y < 1) high area solid solution spinels are reported. These materials are aerogels, i.e. powders derived from colloidal gels within which the liquid phase has been removed under supercritical conditions. This method, first reported by Kistler in the thirties (10), has been largely utilized after the work of Teichner and co-workers (11,12)mostly for the preparation of simple oxides like silicas, aluminas, etc. New emphasis has been given more recently to this procedure, as at least four review papers appeared in the recent literature (13-16). The present contribution will emphasize the usefulness of this method for preparing very homogeneous and well characterized mixed oxide solid solutions.
2. EXPERIMENTAL 2.1. Aerogei preparation. The preparation of the ferrite aerogels was as follows (example for the Mg0.25Zn0.75Fe204 sample): 87.95 g of methyl alcohol were placed in an Erlenmeyer flask. To this were added 9.07 g iron acetylacetonate, 2.169 g of zinc acetate dihydrate, 0.69 g of magnesium acetate tetrahydrate, and 1.6 g of bidistilled water (Zn : Mg atomic ratio 3 : 1; (Zn+Mg) : Fe atomic ratio 1 : 2). The amount of water added was the amount of water required to hydrolyze ferric acetylacetonatc over to the ferric hydroxide, taking also into account of crystallization water of Mg acetate. The solution was allowed to mix overnight. Then the solution was placed into a PyrexTM liner that was then sealed inside a 300 cc stainless steel autoclave. The heating rate was 3 K/min. Heating continued until the temperature reached 523 K and the pressure reached 117 bar. These conditions are well above the critical point of methyl alcohol (T=512.6 K, Pc=80.9 bar). Depressurization consisted of lowering the pressure to atmospheric conditions at constant temperature of 521 K over approximately 15 min time. Next, a slow flow of nitrogen was introduced into the autoclave while cooling overnight. The aerogels when removed were brown powders. All starting materials were from Aldrich (USA). 2.2. Aerogel characterization. The powders have been characterized by the following methods:XRD with a Philips 1130 (CoKo~ radiation) diffractometer, simultaneous TG-DTG-DTA analyses with a Setaram TG92 instrument, FT- IR and FT-FIR spectroscopies with a Nicolet Magna 750 spectrometer, FTRaman spectroscopy with a Bruker RFS100 instrument. Surface area and porosity were performed at 77 K by the BET method. The catalytic activity in the SCR of NO was determined in a continuous flow unit. NO conversion was measured with a Thermo Electron Series 44 chemiluminescent analyzer. 3. RESULTS AND DISCUSSION
3.1. Structural characterization by XRD and DTA-TG. M_.~. ~ v . ~ ~ " samples. XRD patterns show that the samples with x = 0 and x = 0.5 already without any treatment consist of pure well crystallized o~-Fe203 (hematite; JCPDS Table n. 33-664) and MgFe204 (magnesioferrite; JCPDS Table n.36-398), respectively. The samples with 0 < x < 0.5 show an XRD pattern similar to that of MgFe204, so they are constituted of disordered lacunar spinels containing an excess of Fe 3§ with respect to the spinel stoichio-
669
1.o 1.......
/8.48
~176 o.81
/ I
I ~ E*o
a
0.7
8.44
0.6"
oo
--i
8.42
0.5"
--2 0 -3
0.4"
\ \
8.40
\
0.3 9
0.21 0.1
8.38 /,'/ ,.,..
O.O.t-[, , 0.2 0.0
_
| ''r
|--
[
0.4 0.6 Y (Mg)
I
|
0.8
I
- ! 8.36 1.0
Fig. 1. Unit cell parameter a (samples calcined at 1073 K) and inversion parameter k of MgyZn,.y Fe204 samples as such (full line) and calcined at 773 (point line) and at 1073 K (broken line).
_iO0
...
i000
300
_ ,_
~'rURE
,400
tel"
Fig. 2. Simultaneously-recorded DTA (full line) and TG (broken line) curves of the sample Mgo.sZno.sFe204 in air, heating rate 10 K/min.
metry. DTA-TG data suggest that they could also contain small amounts of Fe ~+. They progressively segregate into MgFe204 and o~-Fe~O3 by calcination at 773-1073 K, showing that these non-stoichiometric spinel structures are metastable. In fact, the phase diagram relative to the MgO-Fe203 system shows that the solubility of Fe203 into MgFe204 is significant only at very high temperature (17). The sample with x = 0.66 is constituted by a mixture of a spinel phase and an hydrotalcite-like mixed Fe-Mg hydroxy compound containing methoxy- and acetate ions. By calcination at 773 K it converts into a monophasic powder whose XRD pattern is that of a Mg-excess spinel-type structure; it only segregates very small amounts of MgO at 1073 K. The sample with x = 0.86 is constituted by a mixture of a spinel-type phase and another mixed Mg-Fe compound. By calcination they give mixtures of MgO and MgFe~O4. Mg Z~_n~.~D,a samples. XRD studies show that the MgyZn,.yFe204 samples are all constituted by well crystallized cubic spinels (Fd3m space group), without any evidence of unit cell distortions or of superstructures arising from cation orderings. The end compounds are pure, well crystallized ZnFe204 (franklinite, JCPDS Table n. 22-1012) and MgFe204 (magnesioferrite). The inversion parameter k (the fraction of tetrahedral sites occupied by Fe3§ measured by comparison of the measured and calculated (18) XRD peak intensities, shows that our
670 ZnFe204 sample as prepared is partially inverse (k = 0.21) but it converts into normal spinel (k = 0) by calcination. The inversion of the spinel structure increases with increasing y, i.e. the Mg content (Fig.l), up to k = 0.7 for MgFe204 . This agrees with the normal spinel structure and with the predominantly inverse spinel structure usually taken by ZnFe204 and MgFe204 , respectively (19). The peak position undergoes also small shifts, evidence of a progressive contraction of the unit cell parameter by increasing y (Fig. 1). It seems interesting to remark that this cell contraction reflects the smaller ionic radius of tetrahedrally coordinated Fe 3§ (0.49 A), predominant in the partially inverted spinel MgFe204 , with respect to tetrahedrally coordinated Zn 2§ (0.60 A) (20), present in the normal spinel ZnFe204 while it contrasts the bigger size of octahedrally coordinated Mg 2§ (0.72 A) with respect to octahedrally coordinated Fe 3§ (0.55 A). Then, the cell volume is more sensitive to tetrahedral than to octahedral cations. All samples as prepared contain relatively small amounts of organic matter. As shown by simultaneously-recorded TG-DTA analyses, these residues burn in air near 573 K, giving raise to a weight loss not exceeding 5 % of the total catalyst mass, with a correspondent exothermic peak (Fig. 2 for the compound Mg0.sZn0.sFe204, as a typical example).
3.2. Structural characterization by vibrational spectroscopies. The skeletal IR spectra of MgFe204 and ZnFe204 (Fig. 3) agree with those reported in the literature (21,22). Their FT-Raman (Fig. 3) spectra are also similar to those of other ferrite spinel powders (23,24). The normal cubic spinels like ZnFe204 belong to the Fd3m = Oh7 space group. The factor group analysis (21) gives the following irreducible representation for the optical modes (R = Raman active; IR = IR active; ia = inactive): Fopt = Alg (R) + Eg (R) + Flg (ia) + 3 F~g (R) + 2 A2, (ia) + 2 E, (ia) + 4 F 1. (IR) + 2 F2, (ia) In the case of inverse spinels like MgFe204, the octahedral cations are generally distributed random, so they belong to the same space group and the same irreducible representation apply. Then, for both normal and inverse spinels, four IR-active and five Raman-active fundamental modes are expected. We clearly observe four IR bands and five Raman peaks for ZnFe204 but three IR bands and five Raman peaks for MgFe204. The position of the observed IR and Raman peaks for these materials are compared in Table 1 with those reported in the literature for spinel-type compounds in the form of monocrystals. However, we must mention that the IR spectra of powders are complex from the sensitivity of the IR-active modes to the crystal shape (shifting from the TO modes toward the corresponding LO modes) and to the possibility of superimposition of components arising from particles with different morphologies. The lower values of the most intense IR bands v 1 (IR) and v2 (IR) as well as of the highest frequency Raman modes vl (R), v2 (R) and v3 (R) for ZnFe204 with respect to MgFe204 agree with the transition from a normal (ZnFe204) to a predominantly inverse spinel (MgFe204). In fact, all of these modes, although sensitive to both tetrahedral and octahedral cations, are essentially associated to the stretchings of the tetrahedra, expected to be found at lower frequency for Z n O 4 tetrahedra with respect to F e O 4 tetrahedra, in agreement with the higher polarizing power of Fe 3§ with respect to Zn 2§ As discussed elsewhere (28), the IR and Raman spectra of the MgxFe2.2xO3.2xspinels with 0 < x < 0.66 are all closely similar. However, the v 1 (R) and v5 (R) modes clearly shift progressively although slightly up by increasing x. In parallel, v 1 (IR) and v2 (IR) shift also slightly
671 Table 1. Wavenumbers (cm -~) of fundamental skeletal modes of spinel-type compounds.
Ac. MgA1204 ZnCr204
Notation Sym.
single crystal ref.25 vl (R) vl (IR) v2 (R) v3 (R) v2 (IR) v4(R) v3 (IR) v5(R) v4 (IR)
A1, F,. F~, F~, F~. Eg Fz. F2, F,.
y
=
R IR R R IR R IR R IR
772 670 671 492 485 410 428 311 305
Fe304
MgFe204 ZnFe204
single crystal single Ref. 24 cryst, powder TO LO refs. 23,26,27
powder this work
powder this work
692 621 610 515 509 457 370 186 186
700 563 555 477 402 331 250 216 =
645 541 500 460 380 350 313 175 166
675
711
575 376 194
706 570 550 576 420 490 380 320 336 = 298 230 =
0
0.75
0.05
0.95
u
0.25
= m 0
0.50
,~o
11oo I ~
9oo
uoo
7~i
~avenolbevs
~o~
~
I c l - 11
-
4od'"3"od, "~
. . . . . . . . . . . . . . . . . . . . . . . . ........ 9 , ................... ,
, , T L
1 tO0
tO00
900
OO0
700
600
500
400
300
200
~avenu-~ers (ca- I!
Fig. 3. FT-IR/FT-FIR (full lines) and FT-Raman (broken fines) spectra of MgyZn~.yFe~O, powders.
672 up. This indicates that by increasing x the occupancy of tetrahedral sites by Fe 3§ (i.e. the inversion degree) increases. In all cases, in spite of the non-stoichiometry, the vibrational spectra show that cation vacancy ordering does not occur, in contrast to "y-Fe203 (29). The IR spectra of the MgyZn~.yFe204 are shown in Fig. 3.The skeletal spectrum progressively modifies with shifts up of v 1 (IR) and v2 (IR) by increasing y, as well as with a decrease in intensity of v3 (IR) that likely shifts also down. These spectra indicate that the change from normal to inverse spinel occurs progressively. IR and Raman spectra (that are frequently more sensitive to cation orderings than XRD) confirm that cation ordering in the inverse spinel MgFe204 as well as in the mixed spinels does not occur.
3.3. Morphological characterization. The crystal sizes of the fresh powders, as measured by the Scherrer method (19) range near 80-100 A and, correspondingly, the surface areas range near 100 m2/g (Table 2). Scanning electron micrographs show that all powders are very similar in nature. The primary particle sizes range from 80 to 200 A, according to the crystal sizes measured by XRD and deduced from the experimental surface areas and crystal densities, showing that amorphous material is substantially absent. These primary particles agglomerate into bigger particles roughly 8 to 40 I.tm. These morphologies are typical of mixed oxide aerogel powders prepared through the supercritical drying method. The apparent densities are very low, as listed in Table 2. The morphological properties of the Mg Fe2.2xO3.2xsamples have been discussed previously (30). It was shown that the surface areas retain relatively high values as far as the spinel structure is stable, but drop when precipitation of either MgO or ~-Fe203 occurs. The comparison of the measured surface areas of the MgyZn~.yFe204 aerogels with the theoretical ones calculated on the basis of the crystal sizes evidences that amorphous materials is present, if any, only in the case of samples with y approaching to 1, without any previous calcination. After calcination, the theoretical surface areas are greater than the experimental ones, showing that intraparticle porosity is negligible. The surface areas decrease as expected by calcination. After any thermal treatment the surface area roughly depend on y, so showing that Mg tends to increase it. This again agrees with the measured crystal sizes. Table 2. Crystal sizes (r) and surface areas (S) of MgyZn~.yFe204 aerogels
y 0.00 0.05 0.25 0.50 0.75 0.95 1.00
Tc ->
= 81 85 78 65 65 81 75
r (A) 773 1073 139 122 115 108 85 108 98
390 390 279 325 279 260 279
Tc = calcination temperature. * samples as such.
= 62 78 98 110 111 95 117
S (mVg) 773 1073 27 36 34 37 52 61 56
4 6 7 8 11 14 10
apparent density g/cm 3 * 0.17 0.11 0.12 0.11 0.24 0.12 0.18
673
3.4. Surface characterization by FT.IR spectroscopy. The IR spectra of the surface hydroxy- groups of the spinel-type samples Mgo.+Fe203.4, MgFe20 + and Mg2Fe205 after different outgassing treatments are reported in Fig. 4. The sample Mgo.4Fe203.4 shows spectra similar to those of the spinel-type polymorph of ferric oxide T-Fe203, discussed previously (31), although with small band shifts. The spectra consist of a weak band evident as a shoulder at 3715 cm t (with perhaps an even weaker component at 3735 cml), a very strong band at 3690 cm ~ and a medium-strong band at 3635 cm x. The stoichiometric spinel MgFe204 shows two strong bands whose intensifies invert by progressive heating upon evacuation, centered at 3688 cm t (stromzer first) and at 3708 cm t (stronger
ii, /~fl
~-~ 9" " - - . ~ . ~ . ' 7 "
I
i#~A
j
" - . ~"- . ...
,,~
Mg0 4Fe203
~)
"
m t~ k~ o
MgFe20
i"~ /
I
1.. ~
"
!I~,i
\
-
--.
4
~ .
~.. -~..~. - -
/t.
.,,,~/
.
. ~
~
"~
. ~
,
Mg2Fe205
ZnFe204
\ I~,.I<
",, \
.~,.~
:,~+,, 9
-~ . - ~ . ~ . ~ . . : ~ .
40
.....
Z
] "~
-
.~.~ .~,
~
J
3900 3800 3700 3600 3500 3400 3300
l~avenumbers (cm-1)
Fig. 4. FT-IR spectra of the surface hydroxy-groups on Mgo.4Fe203.4, MgFe204, Mg2Fe205 and ZnFe204,outgassed at 523 K (broken lines), 573 K (dashed lines) and 673 K (full lines).
674
later). Moreover, a shoulder can be found near 3745 cm ~ and another band at 3660 cm t. A strong broad band is observed centered at 3535 cm -~ but with components at 3605 and 3640 cm ~. The last bands disappear by outgassing near 673 K. After mild outgassing the Mg2Fe205 sample shows a main sharp band at 3700 cm -1, with shoulders at 3710 and 3745 cm ~ at its higher frequency side, and a broad band centered near 3580 cm ~ at lower frequencies. After outgassing at temperatures above 623 K a multiple absorption is found with unresolved maxima at 3740 and 3715 cm -~, and a tail at lower frequencies. These data can be interpreted on the basis of a comparison with the data concerning MgO, the normal spinels MgA1204 and MgCr204 and the ferric oxide polymorphs (31). This comparison is also based on the observation that MgO and the predominantly inverse spinel MgFe204 have related structures, both characterized by a cubic close-packed array of oxide ions, with Mg 2§ mainly in octahedral sites (19). MgA1204 and MgCr204 being normal spinels, have Mg ~§ in tetrahedral sites. The spectrum of surface hydroxyls on well outgassed MgO is dominated by a very sharp band at 3740-3750 cm -1, assigned to terminal hydroxy groups on highly uncoordinated cations placed on steps or on edges. At high coverages the spectrum can show another sharp band at 3700 cm -~ that could be due to terminal OH's on "regular" faces, so being bonded to octahedral sites. Taking into account that the spectra of both Mgml204 and MgCr204 show a component in the region 3750-3730 cm ~, assigned to OH's bonded to Mg 2§ ions, we can propose that terminal OH's on tetrahedral Mg 2§ should absorb in the region 37503730 cm 1 while terminal OH's on octahedral Mg 2§ are expected at lower frequencies. Accordingly, we can assign the bands near 3740 cm 1 to OH's on tetrahedral Fe 3§ superimposed to those bonded to tetrahedral Mg 2§ if any, while the bands observed near 3710 cm -~ and 3690 cm ~ should be due to OH's bonded octahedral Mg 2§ and octahedral Fe 3§ respectively. The bands near 3650 cm ~ should be due to bridging OH's and the broad ones in the 3600-3500 cm t region to triply-bridging OH's. The spectrum of ZnFe204 after outgassing at 673 K (Fig. 4) only shows one band with three components at 3680, 3650 and 3610 cm ~. These bands can be assigned to terminal OH's on tetrahedral Zn 2§ and on octahedral Fe 3+, and to bridging OH's, according also to the spectra of OH's of ZnO and of ZnA1204 and ZnCr204 (31). The absence of bands above 3700 cm -~ shows that tetrahedral Fe 3§ is not present on the surface as in the bulk of this normal spinel. According to this picture, the comparison of the spectra of the surface hydroxy-groups of the spinel-type MgxFe~O3§x mixed oxides shows that Mg 2§ ions essentially substitute Fe 3§ in octahedral sites at the surface as in the bulk. In fact the relative intensity of the band near 3710 cm ~ regularly grows by increasing the Mg content, while the band near 3680 cm ~ correspondingly decreases. In all samples the band of terminal surface OH's bonded to tetrahedrallycoordinated Fe 3§ is also evident. So Mg Fe203+~ compounds behave at the surface as inverted spinels, in contrast to ZnFe204 that is a normal spinel in the surface as in the bulk.
3.5. Catalytic behavior in the SCR of NO. The MgxFe2.2xO3.2x samples have been tested as catalysts of the reduction of NO by NH3 in the presence of oxygen, and found to be active. The NO conversion (2000 ppm NO and 2200 ppm NH 3 in air; gas flow rate = 28 cc/s at 293 K and 1 atm; W at = 100 mg) over the stoichiometric spinel MgFe204 shows a maximum conversion of near 30 % at 600 K. At higher temperatures, NO reduction by ammonia is balanced by its production by ammonia oxidation: so the NO conversion decreases. Higher conversion levels can be obtained in the
675 same conditions using both Fe-excess (x < 0.5) and Mg-excess (0.5 < x < 0.8) non-stoichiometric spinel-type phases. With the sample with x = 0.29 conversion exceeds 75 % at 600 K. This sample appears to be by far more active with respect to both 7-Fe203 and ot-Fe203. Table 3 shows the trend of the first order rate constants for this reaction measured in the same conditions for the different catalysts, and calculated on the bases of both their iron content and their surface area. It is clear that maximum activity is found for non-stoichiometric materials, while stable phases have lower activities. The catalytic activity of the MgyZnl.yFe204 catalysts is presently under study. Table 3. Intrinsic rate constants k' (m3/s kgFe) per kg of iron, and k"(m/s* 10"6) per m 2 of catalyst surface area, for NO SCR on Mg Fe2.~xO3.2x aerogels.
x
Tr 500
0.29 0.40 0.50 0.66 0.80 0.86
0.120 0.040 0 0.164 0.032 0.050
k' (ma/s kgFe) 550 600 650 0.642 0.160 0.077 0.364 0.063 0.086
1.231 0.616 0.331 0.704 0.072 0.126
1.648 0.521 0.115 0.469 ox ox
k"(m/s* 10"6) 700
500
550
600
1.001 0.393 0 0.230 ox ox
0.760 0.194 0 0.919 0.051 0.177
4.080 0.774 0.336 2.036 0.101 0.304
7.820 2.976 1.445 3.940 0.115 0.443
650 10.470 2.516 0.500 2.627 ox ox
700 6.360 1.895 0 1.289 ox ox
ox = negative NO conversion because of NH 3 oxidation to NO.
4. CONCLUSIONS The data reported above allow the following conclusions: i) spinel-type non stoichiometric spinels with the formula MgxFe2Oa§x (0 < x < 2) can be prepared in metastable forms (except for x = 1 that corresponds to the stable phase MgFe204) by the aerogel route. ii) they retain relatively high surface areas (near 100 m2/g) as far as the spinel structure is stable. When phase precipitation of MgO or of Fe203 occurs the surface area drops. iii) Mg 2§ occupy tetrahedral sites at the surface as in the bulk, and causes the formation of a typical surface hydroxy group characterized by a vOH band near 3710 cm a. iv) ferric ions both in tetrahedral and octahedral environments are evident, like in 'y-Fe203. v) the non-stoichiometric samples are more active as catalysts for the SCR of NO with respect to both the stoichiometric spinel MgFe204 and the ferric oxide polymorphs. vi) stoichiometric solid solution spinels with formula MgyZnl.yFe204 can also be prepared in a very homogenous form via the aerogel method. They are well characterized stable solid solutions with surface areas in the interval 50-100 m2/g. vii) the spinel ZnFe204 as prepared is partially inverse but after calcination does not show any inversion degree both in the bulk and at the surface, being tetrahedral ferric ions absent. viii) MgyZnl.yFe204 spinels convert from normal to inverted spinels progressively, without any evidence of cation ordering in the interval 0 < y < 1.
676 5. ACKNOWLEDGEMENTS This work has been supported in part by MURST (Rome, Italy) and by NATO (CRG n. 900463). We acknowledge assistance by the Northeastern University - National Science Foundation Young Scholar Mentors - Joseph Corkery, Gregory Smith, and Eric Wisnaskas. REFERENCES R.J. Rennard and W.L. Kehl, J. Catal. 21 (1971) 282. 2. H.H. Kung and M.C. Kung, Advan. Catal. 33 (1985) 159. 3. D.E. Stobbe, F.R. VanBuren, M.S. Hasgenrade, A.J. VanDillen and J.W. Geus, J. Chem. Soc. Faraday Trans. 87 (1991) 1623. H.H. Kung, M.C. Kung and B.L. Yang, J. Catal. 69 (1981) 506. 5 R.E. Ayala and D.W. Marsh, Ind. Eng. Chem. Res 29 (1991) 55. 6. M.C. Woods, S.K. Gangwal, D.P. Harrison and K. Jothimurgesan, Ind. Eng. Chem. Res. 29 (1991) 100. R.J. Willey, H. Lai and J.B. Peri, J. Catal. 130 (1991) 319. 8. W. Buchner, R. Schliebs, G. Winter and K.H. Buchel, Industrial Inorganic Chemistry, VCH, Weinheim, Germany, 1989. Y. Shimitzu, S. Kusano, H. Kuwayama, K. Tanaka and M. Egashira, J. Am. Ceram. Soc. 73 (1990) 818. 10. S.S. Kistler, Nature, 127 (1931) 731. 11. G.A. Nicolaon and S.J. Teichner, Bull. Soc. Chim. France., (1968) 1906. 12. S.J. Teichner, G.A. Nicolaon, M.A. Vicarini and G.E.E. Gardes, Adv. Colloid Interface Sci., 5 (1976) 245. 13. H.D. Gesser, and P.C. Goswami, Chem. Rev. 89 (1989) 765. 14. G.M. Pajonk, Appl. Catal. 72 (1991) 217. 15. J. Fricke and A. Emmerling, J. Am. Ceram. Soc. 75 (1992) 2027. 16. J. Fricke and A. Emmerling, in "Structure and Bonding", Vol. 73, Springer Verlag, Berlin, 1992, p. 37. 17. A.E. Paladino, J. Am. Ceram. Soc. 43 (1960) 183; R.L. Mozzi and A.E. Paladino, J. Chem. Phys. 39 (1963) 435. 18. K. Yvon, W. Jeitschko and E. Parth6, J. Appl. Cryst. 10 (1977) 73. 19. A.R. West, Solid State Chemistry and Its Applications, Wiley, New York, 1984. 20. R.D. Shannon and C.T. Prewitt, Acta Cryst. B25 (1969) 925. 21. W.B. White and B. DeAngelis, Spectrochim. Acta, 23A (1967) 985. 22. J. Preudhomme and P. Tarte, Spectrochim. Acta 27A (1971) 1817. 23. P.R. Graves, C. Johnston and J.J. Companiello, Mater. Res. Bull., 23 (1988) 1651. 24. H.D. Lutz, B. Muller and H.J. Steiner, J. Solid State Chem. 90 (1991) 54. 25. M.P. O'Horo, A.L. Frisillo and W.B. White, J. Phys. Chem. Solids, 34 (1973) 23. 26. J.L. Verble, Phys. Rev. B9 (1974) 5236. 27. A.J.M. Kuipers, and V.A.M. Brabers, Phys. Rev. Lea., 39 (1977) 488. 28. M.I. Baraton, G. Busca, V. Lorenzelli and R.J. Willey, J. Mater. Sci. Lett.,13 (1994) 275. 29. C. Greaves, J. Solid State Chem. 49 (1983) 325. 30. R.J. Willey, S.A. Oliver, G. Oliveri and G. Busca, J. Mater. Res. 8 (1993) 1418. 31. G. Busca, V. Lorenzelli, G. Ramis and R.J. Willey, Langmuir, 9 (1993) 1492. ~
.
.
.
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparationof HeterogeneousCatalysts G. Ponceletet al. (Editors) 9 1995 ElsevierScienceB.V. All rights reserved.
677
E F F E C T OF T H E I R O N CATALYST M E C H A N I C A L T R E A T M E N T ON THE ACTIVITY IN AMMONIA SYNTHESIS R E A C T I O N
W. Arabczyk, R. Drzymala, U. Narkiewicz, K. Kalucki and W. Morawski Institute of Inorganic Chemical Technology, Technical University of Szczecin, Pulaskiego 10, 70-322 Szczecin, Poland INTRODUCTION The active form of fused iron catalysts for ammonia synthesis is obtained by the reduction of the precatalyst composed mainly of magnetite and small amount of promoters, such as A1203 (2,5-3%), K20(O,5-1,1%), CaO (2,5-3,5%) and SiO2 (0,3-0,8%) [1, 2]. The industrially applied catalysts are obtained by melting, cooling and grinding. The catalysts prepared as above have irregular shape grains. From the technological point of view, it would be interesting to prepare fused catalysts with regular grain shape. Tableting of the prereduced catalyst could be a method for the preparation of regular grains. Trials have been u n d e r t a k e n to obtain catalysts with a r e g u l a r shape by sintering [3], pressing and violent cooling of drops of the fused catalyst lava [4]. Tableting causes the changes of the m i c r o s t r u c t u r e of the catalyst pores and the deformation of the crystal structure of the a-Fe. The aim of this work is to establish the influence of the tableting pressure on the catalyst activity, catalytic thermal resistance, pores' distribution and crystallite size. EXPERIlV~NTAL The industrial catalyst PS3-INS was reduced, ground up to the size of < 0.5 m m and tableted under the pressure of 600-1700 MPa. Pellets with 8 mm diameter and 0.3 g mass were obtained. The pellet height changes depending on the tableting pressure. For the activity tests, the pellets were ground and the size grain of 1-1,5 m m was taken. It was suggested t h a t crystal defects influence the growth of the active centers and increase the catalyst activity [5]. The activity of non-ground, prereduced, industrial catalyst with the same g r a n u l a t i o n was t a k e n as a reference. The activity tests in a six-channel integral reactor [6] under a pressure of 10 MPa and with a space velocity of N2/H2 mixture equal to 22000 h -1 and in the t e m p e r a t u r e range of 350-470~ were carried out. Before the activity tests, the catalysts were reduced in the polythermal m a n n e r in the range of 350-500~ The reduction process was carried out under atmospheric pressure except for the last step of the reduction where the applied pressure was 10 MPa. After completing the activity test, the catalysts were heated at 650~ for 15 hours and then the activity test was repeated for the estimation of the catalytic t h e r m a l resistance. The results were interpreted on the basis of Temkin-Pyzhev [7] equation. At the end of the
678 activity test, the catalyst samples were passivated and their microstructure was determined. The pore size and the surface area were calculated using the d a t a of the m e r c u r y porosimetry. The average crystallites' size of the investigated catalyst was measured by X-ray diffractometry. RESULTS AND DISCUSSION A logarithmic value of the reaction-rate constant t a k e n from TemkinPyzhev equation as a function of reciprocal t e m p e r a t u r e for the catalysts pressed at different pressures is presented in figure 1. This function is nonlinear, the reason for this being an increase of the diffusion effects together with increasing temperature. 10
-X-O MPa
Q.
-*- 620 MPa
-Ir r
"+" 1250 M P a ~ , ,
-o- 1700 M P a
"- 1 z
','~\.
"h\\ ',',~\ ','~k,\
":N
0.1
",-,~\\
0.01 1.3
1.35
1.4 1.45 1.5 1.55 1 / T * 103 , K "1
1.6
1.65
Fig. 1. Relationship of a reaction-rate constant logarithm of the a m m o n i a synthesis versus a temperature reciprocal. catalysts after reduction ........................ catalysts after overheating The slope of lines in figure 1 for the kinetic region of the reaction is constant for all catalysts. The r e s u l t a n t activation energy of the a m m o n i a synthesis process is independent from the compression ratio of the catalyst s a m p l e s a n d a m o u n t s to a b o u t 170 kJ*mo1-1. In the whole r a n g e of
679
temperatures, there is a drop of the catalyst activity with an increase of the tableting pressure. As a r e s u l t of overheating, the activity of the u n p r e s s e d catalysts decreases to a small extent. During the overheating process, the pressed catalysts undergo considerably g r e a t e r deactivation, w h e r e a s the a p p a r e n t activation energy (within the limits of accuracy) does not change. The stability of the resulting activation energy suggests t h a t in the process of catalyst overheating or pressing, there is no change in the m e c h a n i s m of a m m o n i a synthesis. For a more dinstinctive presentation of this relationship, the relative values of activity before and after overheating are shown in fig. 2, expressed as a ratio of reaction-rate constants on the pressed catalysts to the reaction-rate constants of the unpressed ones as a function of t e m p e r a t u r e (p = const. tableting pressure). Relative changes of the catalyst activities versus their tableting pressure can be approximately expressed by the linear function (fig. 3). By extrapolation of this dependence to the tableting pressure equal zero, the value (k/ko)p=O=0,85 was obtained. This value suggests t h a t in the grinding process, there is a local pressure increase in the catalyst micrograins, which is comparable in results with the tableting process.
1.1 0
1 m OMPa
0.9
~ 620 MPa
o 1250 MPa
0 1700 MPa
0.80.7 0.6 0.5
~V" -- 0 . . . . . . . . . . . . ..................
0.4-
~
...........
....
O .............
.......
C
0 .......
0"3340
I
390
Fig. 2. Relative dependence synthesis versus t e m p e r a t u r e sample) catalysts after ........................ catalysts after
I
440
T ,~
of a reaction r a t e c o n s t a n t of the a m m o n i a (at constant tableting pressure of a catalyst reduction overheating
Porosity m e a s u r e m e n t s by mercury porosimetry show a bimodal porous structure of the industrial catalyst, with predominant pores' radius 22 and 30 nm. During overheating of a catalyst, the bimodal pore s t r u c t u r e is still observed, whereas their average size decreases (fig. 4). As an example, for the unpressed catalysts, pore's fraction (measured by the mercury porosimetry) up from 50 n m covers 73% of total volume. After pressing the catalyst under a
680 pressure of 1700 MPa, its volume increases to 96%. In spite of increasing the small pores' fraction, the total area of a sample decreases proportionally to the tableting pressure (see fig. 5). In the range of tableting pressure investigated, the a r e a decrease of the catalyst is about 3 times smaller as compared to its activity loss (fig. 3, catalysts after overheating). This suggests t h a t the a r e a formed as a result of the recrystallization process does not take p a r t in the catalytic process (or is less active). In fig. 5, the average crystallites' size of the investigated catalysts is presented as well. At the medium tableting pressures (and for the overheated catalysts), a growth of the average size of crystallites is observed.
=:oj 0.8
... 9 ".'-.'.... ~
0.7
".? .*...
9
'. ",i'.".
9". "'*. "'. 'IL, "..
*~ '.. "..
':.. '.
9
*. ) ...
". ..
0.4 Temperature, "*" 4 7 0 0.3 0
1 200
-*- 4 3 0 I 400
I 600
Tableting
-~- 4 0 0
~ §
... I '.~
350
I , I I I I 800 1000 1 2 0 0 1 4 0 0 1 6 0 0 pressure,
MPa
Fig. 3. Relationship of the relative catalyst activities in the ammonia synthesis versus a catalyst tableting pressure ( .......... catalysts after overheating)
681 0
---
re'
"o -o
15
10
5 ~
10
R,nm
100
Fig. 4. Dominant radius of the pores in catalyst after overheating, unpressed catalyst, .................... pressed catalyst
This indicates that the deformation of the crystal lattice as a result of pressing accelerates the recrystallization effects between a-Fe crystallites with the formation of larger crystals. At the very high tableting pressures (1700 MPa), the average size of the crystallites is comparable with the size of iron crystallites in the unpressed catalysts. This suggests t h a t recrystallization process proceeding between differents crystallites is accompanied by the recrystallization inside a-Fe crystal with the formation of smaller crystals.
682 50
10
O9 ...o
i
-45
E
g~ 9
o m ~
r
U}
~
-40 s s s
,,
~ < as"~ 9
- 30 X
7
9
0
200
400
~
~
600 800 1 0 0 0 1 2 0 0 1 4 0 0 Tableting pressure, MPa
~
9~
1600
Fig. 5. Relationship of the specific area and crystallites' size versus a catalyst tableting pressure for overheated catalysts. CONCLUSION In the pressing process of the reduced form of an iron catalyst, a regular shape of grains can be obtained. The activity of these catalysts decreases with an increase of the tableting pressure. The pressing process leads to a reduction of the large pores' fraction and to the decrease of the specific area of the catalysts. As a result of pressing and overheating of the catalysts, a crystallization process occurs. The formed area has a smaller activity in the ammonia synthesis process.
R~'~~TC~ 1. P.H. Emmet, The Physical Basis for Heterogeneous Catalysis, ed. by Drauglis and R.I. Jaffe, Plenum Press, New York (1975). 2. A. Nielsen, An Investigation on Promoted Iron Catalysts for the Synthesis of Ammonia; Jul. Gjellerups Forlag, Copenhagen (1968). 3. S. Weyhert, Z. Rzanek-Boroch, Proceedings Second Symposium on Technology of Catalysts and Catalycal Processes, Szczecin (Poland), September 1989, ZPPS "ZAPOL" Szczecin (1989), pp. 58-59. 4. L.P. Kuzniecow, L.M. Dymitrenko, P.D. Rabina, Ju.A. Sokolinski, Sintez ammiaka, Chimija, Moskwa, 1982. 5. O.M. Poltorak, Zh. Fiz. Khim., 33 (1959) 2524. 6. R.J. Kalenczuk, K. Kalucki, Proceedings Second Symposium on Technology of Catalysts and Catalytical Processes, Szczecin (Poland), September 1989, ZPPS "ZAPOL" Szczecin (1989), pp. 72-75. 7. M.I. Temkin, W.M. Pyzhev, Zh. Fiz.
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
683
Cobalt catalyst for a m m o n i a oxidation modified by heat treatment ~ Krzysztof Krawczyk, Jan Petryk and Krzysztof Schmidt-Szalowski Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, PL 00-664 Warszawa, Poland. ABSTRACT Heat treatment of the cobalt oxide catalyst is a substantial condition for obtaining the product of high activity and of sufficient mechanical strength. It was shown that the porous structure of catalyst grains, produced in the course of heat treatment, is one of the parameters responsible for catalytic activity. The volume of the coarse pores (0.5-1/zm in radius) is of primary importance for the performance of the catalyst. I.INTRODUCTION The cobalt oxide catalyst for oxidation of ammonia, worked out in our laboratory, has the form of granules of high mechanical strength, owing to which it may be applied both in stationary and in fluidized beds. The yields of ammonia oxidation to NO measured during laboratory and large laboratory studies of that catalyst exceeded 95 %. Optimum temperature of ammonia oxidation process carded out on our catalyst (760-780"C) is lower than that needed for platinum-rhodium wire gauze currently applied in industrial reactors. The manufacture of the cobalt catalyst [1,2,3] proceeds in the following stages: - preparation of cobalt(II)-cobalt(III) oxide CoaO4 of appropriate purity and grain size (Table 1). blending the oxide with an organic binder and shaping into granules of diameter 1.6-2.0 mm, heat treatment for sintering the granules and burning out the organic binder. The essential condition for obtaining granules of sufficient mechanical strength is to follow an appropriate procedure in the heat treatment. At the sintering temperature (above 10(OC) Co304 is reduced to CoO, and partly even to Co metal. The latter appears in very small amounts so it presents some difficulties in the identification. Its presence is probably due to some products of decomposition of the organic binder, e.g. carbon, which is a reducing agent for cobalt oxides. It is interesting to note, that the reduction of cobalt oxides to cobalt metal may occur even in cases where the sintering is carded out in air atmosphere. This fact is probably due to slow rate of oxygen diffusion from the atmosphere inside the strongly -
-
This work was grantexl by the State Committee for Scientific Research in Poland: Project No. 3 P405 007 05.
684 sintered material of the granules. Former studies have shown that the cobalt catalyst active in oxidation of ammonia to NO consists mainly of Co304 [4]. In order to obtain an active catalyst the material passed through the heat treatment should be re-oxidized. The oxidation of CoO to Co304 appears to be a slow process, probably because of slow rate of oxygen penetration from the atmosphere inside the sintered granules. Table 1 Characteristics of cobalt(II)-cobalt(III) oxide (PA-0) for the catalysts manufacture Component
Contents, wt. %
Fe
3.4"10 .2
Mg
8.0"10 .3.
Mn
5 . 0 " 1 0 "2
Ni
5.0"10 -3
Pb
1.0"10 .2
Cu
5.7"10 .3
V
0.02
Na
2.2"10 .3
K
0.7"10 .3
Ca
2.8"10 .3
grain size
< 0.063mm mesh
A separate problem is to develop, in the course of the heat treatment, an appropriate internal structure of the sintered granular material. The porosity of th.e catalyst is relatively small, and the role of the inner surface of the granules in the ammonia oxidation process has not been sufficiently explained in hitherto studies. It is generally considered that the internal surface of the catalyst grains has no substantial meaning for the process of ammonia oxidation to NO. It is even supposed that in the case of strongly developed internal surface the selectivity of the catalyst may decrease, since the reagents retained for a long time inside the porous grain structure may react with one another up to a state near chemical equilibrium, in which, as it is generally known, the main products of ammonia oxidation are free nitrogen and water vapour. Up till now it was supposed, that the process of ammonia oxidation on selective cobalt catalyst is effected mainly on the outer surface of the catalyst grains. Reaction rate measurements showed that the process proceeded mainly in the region of external diffusion [5].
685 2.EXPERIMENTAL The heat treatment of the catalyst material granulated with addition of an organic binder was carried out at temperatures attaining 1250~ Thermogravimetric measurements have shown (Fig. 1.) that on heating with a rate of 450~ the decomposition of the binder proceeds at 300-400~ and the dissociation of Co304 to CoO is observed about 900~ The transformation of Co304 is a result of thermodynamic properties of the system CoO-Co304-O2. In air atmosphere under pressure of lat the boundary between the stability regions of CoO and Co304lies near 900~ [6]. The changes in sample mass during calcination suggested the possibility of formation of small amounts of cobalt metal as a result of reduction with products of the binder decomposition. Further heating above 1050~ did not influence the sample mass. Cooling the sample with a rate of 450~ at temperatures below 900~ resulted in partial oxidation of CoO to Co304. This process was much slower, however, than the dissociation of Co304, and it ceased after about 1/3 of the CoO mass had been oxidized. The product thus obtained contained then too less Co304 with respect to the amount needed for an active catalyst for oxidation of ammonia.
c~o~ o
0
.-e*-I -+-- "-2
c%o4
-3 -4 -5
E--s
<1-7 -8
-9 -10
F .
.
.
.
.
.
.
.
.
.
.
%
CoO 200
\
400
600
800
1 1000 1200
TEMPERATURE [~
CoO I
101313
01
1
i
1100
I
1150
I
1
1200
1250
TEMPERATURE[*C] Figure 2. Composition of the samples (oxygen wt. %) vs. temperature of calcination. In order to obtain a catalyst of appropriate mechanical strength and high content of Co~O4 a series of experiments has been carried out, in which the granulated catalyst was calcined during 5h at 1000-1250~ in air atmosphere, then it was kept for 24 h at 800~ for reoxidation of CoO to Co304. The composition of the catalysts obtained in such a procedure depended on the temperature of calcination (Fig. 2). It proved impossible to oxidize to Co3Oa the sample calcined at 1250~ even on prolonged heating at 800~ in air atmosphere. All the obtained catalyst samples had high compressive strength, and their specific surface determined by the BET method was within 1.6-4.1 m2/g. The graph presents also the composition of the KA catalyst formerly used in experiments in the large laboratory studies. Figure 1. The changes in sample mass during thermogravimetric measurement Am, wt. %.
686 We have measured the catalytic activity of four samples of catalysts prepared according to the above-presented procedure: PA-100, PA-105, PA-110, and PA-125, calcined at temperatures 1000, 1050, 1100, and 1250~ respectively (Table 2). The measurements were carded out in a quartz reactor of diameter 17 mm, with fluidized bed of the catalyst acting in the isothermal regime. The temperature range (720-800~ was chosen optimal for this catalyst. The air-ammonia mixture (10 % vol. NH3) was introduced into the reactor with the space velocity of 30000 h~. Table 2 Average composition of samples and size of crystallites. Sample "~
Max. temp. of sintering, ~
Average composition oxygen wt. %
Average crystallites size, nm
PA-0
raw CthO4
26.58
57
PA-100
1000
26.30
76
26.20
77
PB-100 PA-105
1050
PB-105 PA-110
26.20 1100
PB- 110 PA-125
26.30 26.20
1250
PB-125 KA
26.20
From large laborat, study
KB
24.95
87
26.20
92
25.50
85
26.20
95
") A before the activity measurement B after the activity measurement. The result of the ammonia oxidation process was determined by means of the following gas analyses: 1. NH3 content in the inlet to the reactor (by the gravimetric and titrimetric method) 2. NO content in the outlet from the reactor (by the gravimetric and titrimetric method) 3. N20 content in the outlet from the reactor (by a chromatographic method) The activity of every catalyst sample was measured at four temperatures within the region given above. At each temperature the process was carded out until the concentration of the reaction products became constant. At the end of a series of measurements for a given sample we have repeated the measurement at a temperature close to that applied in the first experiment. In this way we have checked if the catalyst had not been disactivated in the course of the measurements. The whole cycle of experiments for a given sample lasted for at least 12 h within two days (with a pause for the night).
687 The activity measurements (Fig. 3) have shown that the catalysts studied had high activity and high selectivity. The degree X~ of ammonia transformation to NO was 90-95 %, and the degree X2 of ammonia transformation to N20 did not exceeA 2 %. The amount of non-reacted ammonia was so small that it was impossible to determine it even by the sensitive colorimetric method with Nessler's reagent. The degree X3 of ammonia transformation to elemental nitrogen may thus be estimated to 5-10%. The temperature at which activity measurements were taken (720-800~ had, in general, very small effect on the transformation degrees given above.The catalysts had not been dis,activated in the course of the measurements except of the sample calcined at temperature 1250~ (PA-125). 100 o - PA-100 x - PA-105 o - PA-110 e - PA-125 = - KA ii KA 95
PA-IO5
(2)
~
1
x
0
9O t-X
(3
X
...__ .....-
-2
85 -125
e--
- c ' - - PA-IO0 I
720
7t.O
..,I
9
x-I--9 ''0-
760 780 TEMPERATURE [~ ]
~l,
800
Figure 3. Measurements of activity; degree of ammonia transformation to NO (X~) and N20 (X2) vs. temperature of catalyst bed. Sequence of measurements is denoted by numbers at the points. The activity of the catalysts measured by the degree of ammonia transformation to NO (X1) decreased with increasing calcination temperature from 10(OC to l l00~ Fig. 3 presents the results of analogous measurements taken for the KA catalyst prepared in a larger lot (50 kg) and used in experiments in a large laboratory and semi-technical scale. T h e results of X-ray diffraction studies have confirmed that the catalyst before the heat treatment (PA-0) consists of Co304. Similarly, no other phases besides to Co~O4 have been found in a catalyst calcined at 1000~ and then annealed at 800~ This is true for a catalyst subjected
688 to the heat treatment before (PA-100) and also after the activity measurement (PB-100). Catalyst calcined at 1250~ (PA-125) contains some amounts of CoO. CoO is present in that catalyst also after the activity measurement (PB-125).The composition of KA (and KB) was similar, however less CoO was observed. The X-ray measurements have shown that the process of sintering, which first results in dissociation of Co304 then in re-oxidation of CoO to Co304,is accompanied by a growth of crystallites in the catalyst samples (Table 2). The average size of the crystallites was 57 nm before the sintering (sample PA-0) and 76-86 nm after it. A further~ small increase of the crystallite grains up to 77-96 nm was observed in the course of activity measurements. Samples subjected to heat treatment under different temperatures differed greatly in their total porosity (Fig.4,5). The increase of temperature resulted in reduction of total porosity which is evidently due to the fact that the processes of sintering and recrystallization proceed more rapidly at increased temperatures. However, it should be pointed out that samples having high catalytic activity (PA-100, PA-105, and KA) have similar volumes of pores with radii exceeding 0.5/~m and they differ in this respect from the low activity sample PA-125 easily undergoing a disactivation. Porosity of the catalysts was reduced in the course of activity measurements, especially within a range of narrow pores.
160
14C
~ " '~A-IO0 120;
120
~oo"
I.
~ 8o
-'~,
i
m ~. 60 ....
~
I
,
60
40 ....
20 ....
V~-lZ~ 0 0.001
0.01
0.1 1 PORE RADIUSj )~rn
10
100
Figure 4. Measurement of the pores volume distribution, before the measurement of activity. PA-100, PA-110, PA-125 sintered at 1000,1100 and 1250~ KA-from large laboratory study,
0,001
001
,..____._~.__~
0.1 1 PORs RADIUS, ,urn
10
100
Figure 5. Measurement of the pore volume distribution, after the measurement of activity. PB-100, PB-110, PB-125 sintered at 1000, 1100 and 1250~ KB-from large laboratory study.
689 Microscopic observations have confirmed the differentiation of the porous structure of catalysts sintered at different temperatures. It is easily seen (Fig. 6), that the structure of PA-100 and PB-100 samples is distinguishable by high contents of fine pores (radii within 0.1 to 0.5 ~tm). No changes in the porous structure of the catalyst grains can be seen however, as resulting in the process of ammonia oxidation during the activity measurements. Another structure is observed in PA-125 and PB-125 samples, consisting of much coarser elements of weakly developed surface. Their porosity is much smaller. The structure of the PA-125 sample undergoes some reconstruction in the course of activity measurement. The PB-125 sample, which is its equivalent, contains in its structure more regular, rather coarser grains of several/~m in diameter. It should be noted that average composition of all the samples under examination was similar after the measurement of activity (Table 2).
.... lO,,~m
"~I
Figure 6. Porouse structure of the catalyst near the surface of granules. A.Catalyst PA-100 sintered at temp.l(XXY'C, before measurement of activity. B.Catalyst PB-100 sintered at temp.1000~ after measurement of activity. C.Catalyst PA-125 sintered at temp.1250~ before measurement of activity. D.Catalyst PB-125 sintered at temp.1250~ after measurement of activity.
690 3.DISCUSSION The above presented results confirm the previously formulated conclusion, that the heat treatment has a substantial effect on activity of a cobalt catalyst and on its selectivity in the process of ammonia oxidation, as well as on its mechanical strength. A condition necessary for obtaining granular material of sufficient mechanical strength, indispensable for application in a fluidized bed, is to carry out the sintering process at a temperature not less than 1000~ Under such conditions the primary component of the catalyst Co304 is dissociated to CoO and is partly reduced to free cobalt due to the presence of organic binder. Since a condition for obtaining a catalyst of good activity and high selectivity is a large content of Co304, the process of following heat treatment should provide a reoxidation of the CoO formed. This condition may be difficult, however, since the oxidation of CoO by oxygen from the atmosphere is slow at temperatures below 900~ and it can be completely inhibited despite of still considerable contents of CoO. This is mainly due to slow rate of oxygen diffusion from the atmosphere to the inside of the catalyst grains. The diffusion is particularly slow in the cases of grains strongly sintered at high temperatures, e.g. at 1250~ For this reason it is unadvisable to carry out the sintering process at excessively high temperatures, as this produces catalysts of low activity. The results of this study have shown, that an important condition for obtaining catalysts of adequate activity is to obtain a suitable porous structure of the catalyst grains. The primary problem is not the overall porosity, but the content of pores with radius exceeding 0.5/zm. For example, the catalyst sample KA had the total porosity five times as low as that of the PA-100 sample, but their contents of the pores exceeding 0.5/zm in the radius were similar, and the two catalysts have had similar and high activity in the process of ammonia oxidation. On the other hand, the catalyst samples KA and PA-125 had similar total porosities, but the PA-125 sample contained three times as little of coarse pores with the radius above 0.5/zm. The PA-125 catalyst was much less active in the initial state and it was rapidly desactivated in the course of the process of ammonia oxidation. The above conclusion suggests a need for revision of opinions on the factors that influence the rate of ammonia oxidation on oxide catalysts. It should be admitted that although the rate of ammonia oxidation is influenced by the external diffusion of gas reagents from the stream to the catalyst surface, the porous structure of the grains is of great importance. Our experiments have shown that the pores of radius within 0.5-1.0 #m are of primary importance for the performance of the catalyst. The above presented results confirm the possibility of obtaining cobalt catalysts for oxidation of ammonia suitable for being applied in fluidized bed reactors. REFERENCES 1. J.Petryk, K Krawczyk, Manufacturing of the precursor of cobalt catalyst, PL Patent No. 149619 (1990) 2. K.Schmidt-Szalowski, J.Petryk, K.Krawczyk, Przem.Chem. (in press) 3. H.Rembertowicz, J.Petryk, Reaserch Project No 2.0645.91.01 Final Report,1994. 4. J.Petryk, II Symposium on Technology of Catalysts and Catalytical Processes, Szczecin, 1989. 5. W.S.Beskow,D.W.Zarow,React.Kin.Cat.Letters.No.3 (1976) 4 6. I.K.Kazenas,Davlenie i sostav para nad okislami chimi6eskich elementov, Moskva, 1976.
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
691
Characterization and Catalytic Properties of MgO Prepared by Different Approaches Kai-Ji Zhen 1, Sen-Zi Li 2, Ying-Li Bi 1, Xiang-Guong Yang3 and Quan Wei 4 1 Department of Chemistry, Jilin University, Changchun, 130023, P.R. China 2 Department of Chemical Engineering, Pohang Institute of Sci. & Technol., Pohang, Korea, 790330 3 The Institute of Applied Chemistry, Changchun, 130021, P.R. China 4 The Testing Center, Jilin University, Changchun, 130023, P.R. China
The effect of the structure of M g O on its catalytic properties for oxidative coupling of methane was investigated. A series of M g O catalysts were prepared by various methods. X R D , IR, S E M and X P S techniques were adopted to measure the bulk and surface structure. The structural data and the catalytic properties of the catalysts were correlated. The structural factors influencing the catalytic performance for O C M reaction are considered and the difference in the catalytic properties of the samples is discussed in detail. 1. I N T R O D U C T I O N
Based on the fact that M g O is an important active component or support for selective oxidation, particularly oxidative coupling of methane [1], it is reasonable to seek some essential clues to explain the relationship between the structure and the catalyticperformance of M g O which is used either as an active component or as a support. As a first step, it should be m a d e clear how the structure affects the catalytic performance. Aiming at the above-mentioned purpose, the present authors used different methods to prepare a series of magnesia catalysts and characterize their bulk and surface structure, followed by evaluation of the corresponding catalyticproperties for O C M reaction. 2. E X P E R I M E N T A L
2.1. Catalyst preparation Sample A : pure MgO produced by E. Merck. Sample B : prepared by decomposing MgC03 at 1173 K for 8 h. Sample C : prepared by decomposing Mg(NO3)2.6H20 at 1173 K for 8 h.
692 Sample D : prepared by p r e t r e a t m e n t of m a g n e s i u m ribbon in dilute hydrochloric acid so as to get black oxide film on the surface prior to burning in pure oxygen, followed by collecting the white powder formed. Sample E : prepared by precipitation of magnesium hydroxide, which was formed by adding an excess of ammonium hydroxide to a solution of m a g n e s i u m nitrate under intensive stirring at 343 K. The resulting slurry was washed and filtered, followed by drying at 383 K for 8 h. Sample F : prepared by precipitation of magnesium carbonate, which was produced by adding an ammonium carbonate solution to a solution of magnesium nitrate u n d e r stirring. The obtained white precipitate was the h e a t e d u n d e r stirring, washed, dried and filtered at 383 K for 8h, and calcined at 1173 K for 8h.
2.2. Catalyst c h a r a c t e r i z a t i o n 2.2.1. Bulk phase detection of catalysts All the synthesized catalysts underwent X-ray diffraction m e a s u r e m e n t s to give their bulk phase composition. A Shimadzu XD-3A X-ray diffractometer was employed. CttKa was used as radiation source u n d e r 30 KW and 20 mA. Qualitative phase analysis of the samples was carried out at a scanning speed of 4 ~ 2e/min and the determination of the lattice parameters and particle size of the crystalline phases was conducted at a scanning speed of 1~ 20/4 rain. S t a n d a r d peak of single crystal CaF2 was used to do calibration and spectra pure KC1 was used as internal standard. Particle size of the catalysts were calculated according to the formula: kX D 200 = ( fl~. fl O)cosO (1) where k = 0.89, ~, = 1.5418/k, ~ = FWHM of (200) peak for MgO, ~0 = FWHM of peak for CaF2. Lattice parameters were calculated by using the following formula: X~]h2 + K 2 + 12 a = (2) 2sin0 Lattice distorsion (e)2 was estimated according to equation: (fl- fl~
=
~
+ 32 (e2)sin220
where (e 2) denotes the mean square lattice distortion of the crystal faced to the direction perpendicular to the (h*k*l*) plane. 2.2.2. Morphological analysis of the catalysts A Hitachi X-650 scanning electron microscope was used to obtain the shape information of the samples which had been vacuum gold-plated before scanning. 2.2.3. Surface composition of the catalysts A VG ESCA LAB MK II X-ray photoelectron spectrometer was employed. MgKa X-ray source (BE = 1253.6 eV) was adopted. All the obtained binding energies of the corresponding photoelectrons of the samples were calibrated by using C Is(BE=286.4 eV).
693 2.2.4. Specific surface area of the catalysts Low-temperature air physical adsorption method was used for surface area measurements. The surface areas were calculated from equation: AP S = m (A + B P ) (3) where P = equilibrium pressure, A and B = apparatus constants, AP = pressure difference measured before and after adsorption under equilibrium pressure, m = weight of sample. 2.2.5. Catalytic properties of the catalysts Evaluation of the catalysts in the oxidative coupling of m e t h a n e was carried out in a fixed bed quartz reactor (i.d. 9 mm). The reaction was conducted u n d e r the following conditions: total flow rate of feed: 80 ml/min with methane:oxygen:nitrogen = 2:1:5. Reaction temperature: 723-1073 K, catalyst weight: 0.2 g with a particle size of 20-60 mesh. Reaction products, after cooling for separating water and oxygen containing hydrocarbons, were analysed by means of a 2305 gas-chromatograph equipped with a ionization detector and a porapak Q column. The catalytic parameters were calculated according to the traditional definitions. 3. R E S U L T S AND D I S C U S S I O N
3.1. Structural properties of the samples As it is known, MgO belongs to a crystal system with a face-centered cubic structure. In the range of t e m p e r a t u r e between 35 ~ and 65 ~, three main diffraction peaks of MgO appear. The lattice parameters, crystalline sizes and degree of distortion of the samples are listed in Table 1. From Table 1, one can see that the stronger the intensity of the corresponding diffraction peak, the smaller the lattice distortion, and vice-versa. For instance, the relative intensity of the main peaks in different samples increases in the sequence of A, F, B, E, D, C; in addition, their lattice distortion degree decreases in the same sequence. In order to gain information on the crystalline particle size of the samples, a comparison of the SEM profiles was also made. As shown in Fig. 1, it can be seen that the difference in morphology of the samples prepared by the different methods is quite obvious. For example, samples A, C, D and F. However, there is no parallel relationship between the lattice parameter and the particle size, as also shown in Table 1. This is because the accommodation of the crystal cells to form so-called secondary accommodated phases may be quite different in samples synthesized by different approaches [2,3]. It shows that some crystalline samples with the same lattice parameter can manage to form either larger crystalline particles or smaller secondary particles, depending on the preparation conditions. Table 1 also shows that for sample D, the crystalline size is minimum and the corresponding lattice distorsion degree is maximum.
694 Table 1. Structural parameters of different M60 oxides. Sample A B ....... C
a(k) D2oo (/~) (e)Y2Xl04
4.2155 422 6.0
4.3153 750 3.0
4.2147 1126 1.0
Is/IA 1.0 2.4 3.1 Is and IA: intensity of (200) peak of the samples
E
F
4.2137 1502 0.8
4.2165 676 1.7
3.1
2.7
4.2198 376 3.2 2.1
D.....
H
3.2. Surface composition of the samples The XPS measurements of the samples give the characteristic peaks and the related binding energies of Ols, Mg2p and Cls. The calculated data are given in Table 2. 3.2.1. O is spectra It is obvious that all the samples, except A, give assymmetrical Ols peaks. The peak fittings show a main peak and a shoulder near 529.8 eV and near 531.5 eV, respectively. The low binding energy is attributed to lattice oxygen on MgO surface (represented as Ols, 1) and the high binding energy peak, to the adsorbed or coordinatively unsaturated oxygen (denoted by Ols, u). A r --
9 ..... i
84184184 ~ ~i!~:!~iiiill
:
:.:.
,,
,
Figure 1. SEM profiles of MgO samples.
: ..... ::: ~: ! '/ i~i:i ~i~ii :i~~!i
~
695 3.2.2. Cls spectra For all the samples measured, the C ls peak from surface CO:- ions appears beside the peak of the contamination carbon at 286.4 eV. From Table 2, one can find that for all the samples, the contents of the surface CO~- are almost similar; however, both the amounts of the lattice oxygen and the coordinatively unsaturated oxygen sharply change from sample to sample. This fact might give a clue that the coordinatively unsaturated oxygen species cannot be attributed to CO~-, but to surface adsorbed oxygen.
3.3. I n f r a r e d s p e c t r a Samples prepared by different methods show IR spectra with quite different peaks, peak shift and number of peaks as displayed in Fig. 2. A possible explanation of the changes lies, perhaps, in that the transmission spectra of MgO are contributed, in fact, by the statistics of the Mg-O bonds, resulting in the formation of a band. As indicated in the foregoing section of the paper, samples B and D possess larger particle size and the rest has particle size of less than 1000/k, belonging to ultra-fine particles.
F
I 13()0 1120 940 760 580 400 W a v e n u m b e r (cm -1 )
Figure 2. Infrared spectra of the samples.
696 Table 2. XPS data of MgO before reaction Sample.
C is (eV)
A 289.9 B 289.7 C 289.4 D 289.7 E 289.4 F 289.4 1 9lattice oxygen
Ols %
l(eV)
%
u(eV)
1.5 529.4 1.5 531.2 2.4 529.5 16.1 531.5 1.9 529.7 28.8 531.7 2.3 529.5 23.0 531.7 0.9 529.4 20.6 531.4 2.3 529.4 13.1 531.4 ; u 9coordinatively unsaturated
Mg2p %
Total
(eV)
%
53.0 31.4 18.6 24.1 25.5 31.3 oxygen
54.5 47.5 47.4 47.1 46.1 44.4
49.3 49.1 49.1 49.1 49.2 49.2
44.0 50.1 50.7 50.6 53.0 53.2
=,
Due also to the electrical charge, the coordination numbers in the bulk and the surface are different. In addition, the surface and bulk Mg-O bonds undergo different vibrations. For MgO particles with large dimensions, the vibration of Mg-O bonds is mainly attributed to the bulk composition, giving a symmetric band. On the other hand, for crystalline particles with smaller dimensions, the statistical weight of the surface Mg-O vibrations increases, leading to an obvious change in the vibration of the two types of Mg-O bands and, as a result, the shape, wave-numbers and the number of peaks vary very strongly.
3.4. Evaluation of the catalytic properties The catalytic performance of the series of magnesia for OCM was established in two sets of tests: 1. Measuring methane conversion and C2 hydrocarbon selectivity (of course, giving their yield) at same temperature. 2. Recording difference of temperature at which the same methane conversion or C2 selectivity are reached over different catalysts. Figures 3 and 4 show the dependence of methane conversion and C2 selectivity, respectively, of this series of catalysts on temperature. It is very apparent that catalysts prepared by different methods exhibit quite different methane conversions and C2 selectivities at the same temperature. For instance, sample A shows the highest methane conversion and C2 selectivity (at 873 K, methane conversion is 38.5%). Sample B gives high methane conversion, sample F shows lower methane and sample C possesses the lowest catalytic activity for the above mentioned reaction. As shown in Fig. 4, at the given temperature, the C2 selectivity on these catalysts decreases in the same sequence.
697 C
4o
4O
--
'
9
oA 3O
3o
D F
4-J
.2 r
20
20
o
d
t~
rj
lO
lO
0 773 823 873 923 973 1023 1073
T(K) Fig. 3. Dependence of CH4 conversion on temperature
823 873 923 973 1023 1073 T(K) Fig. 4. Dependence of C2 selectivity on temperature
3.5. S p e c i f i c s u r f a c e a r e a As displayed in Fig. 5, the specific surface area of the sample also declines in the order A, B, D, C, which is parallel to the sequence of methane conversion. Two aspects concerning the effect of the specific surface area on the catalytic properties have to be considered. (l) The larger the specific surface area, the higher the re, tuber of active centers by unit weight of the catalyst and, of course, the faster the rate of methyl radical formation. (2) Large specific surface areas may lead to an increase of the rate of formation of COx [4], due to the collision of methyl radicals with the surface. In case of catalysts with smaller specific surface areas, the formation rate of methyl radical is larger than the rate of deep oxidation. Thus, an increase in the specific surface area is beneficial to the enhancement of methane conversion. At the same time, the C2 selectivity increases as well. However, if the formation rate of methane radical is smaller t h a n t h a t of its deep oxidation, the increase in the specific surface area might enhance the methane conversion with a decrease in C2 selectivity. The only case where the highest selectivity and yield of C2 hydrocarbons are reached is when the two rates are equalized, namely when the catalyst has an optimum specific surface area.
698
20
8
40 o
30
o~ zo
cY
lO o A B C DE
F
Figure 5. Effect of specific surface area on CH4 conversion and C2 selectivity at 923 K. In addition, as it is well known, t e m p e r a t u r e is a very i m p o r t a n t factor which influences the catalytic properties. On the one hand, the increase of t e m p e r a t u r e may result in an increase of both the rate of formation and of desorption of methyl radical, which can obviously lead to an increase in the m e t h a n e conversion and C2 selectivity; on the other hand, the increase of temperature may also give rise to an increase of the rate of complete oxidation of methyl radicals and of the produced ethane and ethylene. As a result, there should be an optimum temperature at which the rate of methyl radical formation and its deep oxidation are equal to each other, leading to an optimum of m e t h a n e conversion and of the selectivity of C2 formation. 4. R E F E R E N C E S 1. K. Aika, T. Moriyama, N. Takashki and E. Iwamatsu, J.C.S. Chem. Comm. (1986), 1210. 2. J.A.H. Pol, S. Badyal, H.K. Zhang and R.M. Lambert, Surf. Sci. Lett., 225 (1990), L1. 3. J.H. Lunsford, J.X. Wang and D.J. Driscoll, J. Amer. Acta, 107 (1985), 58. 4. K. Otsuka and K. Jinno, Inorg. Chem. Acta, 121 (1986), 237.
5. A C K N O W L E D G M E N T The NNSFC is gratefully acknowledged for financial support.
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
699
PERMANGANIC ACID : A NOVEL PRECURSOR FOR THE PREPARATION OF MANGANESE OXIDE CATALYSTS. C. Kappenstein a, T. Wahdan a*, D. Duprez a, M. I. Zaki b, D. Brands c, E. Poels c and A. Bliek c. a LACCO, Laboratoire de Catalyse, Facultd des Sciences, F-86022 Poitiers, France. b Chemistry Department, Faculty of Science, Kuwait University, 13060 Kuwait. c Department of Chemical Engineering, University of Amsterdam, Nieuwe Achtergracht 166, 1016 Amsterdam, The Netherlands. Unsupported and y-alumina supported MnO x catalysts (1-10 wt-% Mn) were prepared from aqueous solutions of HMnO 4 and compared with nitrate based samples. They were characterized by XRD, XPS, BET area, oxygen storage capacity and by their catalytic behaviour versus ammonia DeNOx reaction. The unsupported oxide is amorphous at 300" C with a high surface area (45 m2.g -1) and transforms to c~-Mn20 3 below 600"C. The supported samples show high dispersion, which correlates well with high catalytic activity and the absence of detectable crystalline phase. The difference between HMnO 4 and manganese nitrate precursors vanishes after calcination at 1000~ with the formation of Mn30 4 and the transformation of the carrier into ,:~-alumina ; the DeNOx activity remains high but the selectivity towards N 2 decreases as the temperature increases.
1. INTRODUCTION The use of unsupported and supported manganese oxides as catalysts remains limited despite their potential activity in redox reactions. Two main procedures are generally proposed for the preparation of such catalysts : via manganese nitrate [1] or from in situ precipitation of manganese hydroxide [2]. In previous papers, [3, 4], we have demonstrated that ammonium permanganate was an adequate precursor for unsupported catalysts, but lost * Present address : Chemistry Department, Faculty of Science, Mansoura University (Egypt).
700 a great deal of its activity when supported [4]. We propose here the use of permanganic acid as a novel precursor for the preparation of highly dispersed, stable MnO x or MnOx/Al20 3 catalysts. The catalytic properties were determined from oxygen storage capacity measurements [5] and selective reduction of nitrogen monoxide by ammonia [6] ; a comparison with nitrate based sample is also presented.
2. EXPERIMENTAL
2.1. Preparation The precursor was obtained by cationic exchange of a 0.2 mol.1-1 KMnO4 solution through Amberlite 120 resin in the H + form. The resulting permanganic acid solution was heated at 80 *C during 3 days until complete evaporation and the solid residue thus obtained was used as unsupported sample. Impregnation onto commercial alumina (Degussa, oxid C, 10 g, 55 m 2 g-l) with HMnO 4 solutions of adequate concentration (100 ml) led to supported samples with loading level between 1 and 10 weight % of Mn. After evaporation and drying at 100"C, air calcinations were carded out at 150, 300, 600 and 1000*C for 5 h. The nitrate precursor Mn(NO3)2, 4H20 (A.R. grade, LABOSI, France) was calcined at the same temperatures to obtain the unsupported samples. The supported samples were prepared with the same procedure as above. 2.2. Characterization The samples were subjected to extensive bulk and surface examinations : - X-ray diffraction (XRD) : Siemens D500 diffractometer, Cu Ko~ radiation [7] - Temperature programmed reduction (TPR) " pulses of H2 (0.268 cm 3) were injected every other minute from ambient temperature to 8000C (4*C rain-1) in a quartz reactor [8]. The final stage of the reduction was always MnO. - Surface area measurements (BET) by low temperature - one point - nitrogen adsorption method by using a Micromeretics Flowsorb II apparatus (P/Po = 0.30). - X-ray photoelectron spectroscopy (XPS) : Riber-Cameca (France) spectrometer, A1 K s source (1486.6 eV), Cls reference at 284.6 eV [7]. 2.3. Activity - Oxygen storage capacity (OSC) : CO oxidation in transient flow, leads to determination of OSC of the sample. Pulses of CO were injected every three minutes at 3000C on a sample saturated with 0 2 pulses at 300 ~C [5] " CO + "O" (stored oxygen) --* CO 2.
701 Selective NO reduction with NH 3 : a mixture of NO (500 ppm), NH 3 (550 ppm), 0 2 (2 vol %) and He (balance) flowed through the sample (corresponding to 1.2 mg Mn in the catalyst bed) at 105 Pa with a flow rate of 50 cm 3 min -1, a space velocity of 24 000 h-1 -
and in a temperature range 110 - 300 ~ The product stream is analyzed on line by using a computer-monitored mass spectrometer allowing automated data acquisition and process control [6].
3. PHASE IDENTIFICATION AND XPS RESULTS 3.1. Unsupported samples
After calcination at 300~ the HMnO 4 based sample shows only amorphous or very poorly crystallized phase (Fig. 1) which transforms before 600~ into ~ - M n 2 0 3 phase ; the more stable Mn30 4 phase appears below 1000~ with traces of ,~-Mn20 3. These samples display a surface area which is one order of magnitude higher than the corresponding nitrate based catalysts (Table 1). The mean oxidation states, obtained from the H 2 consumption (TPR data), slightly differ at 300~ but are the same at 600~ in agreement with the detected phases. Table 1. Characterization of unsupported samples and OSC activity. Precursor
Nitrate
calc. temp. (~
300
phases (XRD)
600
HMnO4 1000
300
600
~-MnO 2 a-Mn20 3 a-Mn20 3 a-Mn20 3 Mn30 4
amorph,
a-Mn20 3 Mn30 4 ~-Mn20 3
SBET(m2/g)
10
2
0.4
45
21
4.5
Mean oxidation state (TPR)
3.2
3.1
-
3.4
3.3
-
OSC 1a OSC5 b
118 103
9.2 5.9
11.6
817 741
146 89
9.6 4.3
3.5
1000
a CO consumption for the first pulse (/zmol/g Mn) ; b mean value for the first five pulses. 3.2. Supported samples
Table 2 summarizes the results concerning the highest loaded catalysts. The nitrate precursor leads to identified phases even at loading levels as low as 2 % (not given in table 2), whereas no information could be obtained from XRD for the HMnO 4 based samples at
702 300 and 600" C. This is in agreement with the surface Mn/AI atomic ratio determined from XPS measurements : the good dispersion correlates well with the absence of well crystallized MnO x phase. Another important point is the transformation of the carrier into ~ - A I 2 0 3 and the subsequent decrease of surface area for both precursors ; this transformation takes place at a rather low temperature (below 1000"C) and is due to the presence of manganese oxides [9] ; it was also observed at lower loading levels. This transformation could be associated with the formation of Mn30 4 which displays a spinel structure close to the structure of transition alumina, thus helping the atom displacements needed to form ~ - A I 2 0 3. This formation is also accompanied by a very good dispersion of manganese (table 2). I
I
I
i
i
J
I
I
I
I
I
I
I
,
J
300~
Mn203
~.~
'
600~
Mn304 j t B
0
20
40
60
2theta
80
Figure 1. X-ray diffractograms of the HMnO 4 based samples at the calcination temperature indicated.
703 Table 2. Characterization of gamma-alumina supported catalysts and OSC activity. Precursor
Nitrate
HMnO 4
calc. temp. (~
300
600
1000
300
600
1000
% Mn
7.1
7.2
7.6
7.4
7.7
7.8
phase (XRD)
#-MnO 2
a-Mn20 3
Mn30 4 a-Al20 3
none
none
Mn30 4 a-A120 3
SBE T (m2/g)
50
47
14
53
53
13
mean oxidation state (TPR)
4.5
3.4
2.8
3.8
3.4
2.8
Mn/A1 (XPS)
-
0.05
-
-
0.14
0.28
0.08
0.08
1120 (549)
585 (161)
Mn/A1 (volume) OSC1 a OSC5 b
0.09 475 (438)
167 (134)
714 (194)
2440 (1410)
a CO consumption for the first pulse (#mol/g Mn) ; b mean value for the first five pulses.
4. TEMPERATURE PROGRAMMED REDUCTION Figure 2 displays the TPR profiles for the supported catalysts for both series. The atomic H/Mn ratios obtained from the integration of the curves are transformed into initial mean oxidation state of manganese (tables 1 and 2). The calcination at 150 and 300~ leads to similar profiles and the transformation between 300 and 600 ~ is clearly evidenced for the nitrate based samples by an increase of the profile maximum by about 200~ A further maximum increase and a much broader profile is displayed by the sample calcined at 10000C (i.e. Mn304/c~-Al203). The same qualitative variations can be observed for the catalysts prepared with HMnO 4, but the maxima appear at lower temperatures and the peaks are broader. In agreement with the nature of the phases, the calcination causes a decrease of the mean oxidation number for both precursors (Table 2) in relation with the increase of the profile maximum.
704 lOO
60
~300"C
:'~
:" ~ 1 5 0
0
50
C
E 60
;,-,~600"C
e-
.2 4o e~
//
E
..,-"
",
,ooo-c
!30 20 10
e-
~
40
o 0
200
400
600 Temperature (~
nitrate precursor
800
0
0
200
400 600 Temperature (*C)
800
HMnO 4 precursor
Figure 2. TPR profiles of supported samples after calcination at the temperature indicated.
5. CATALYTIC ACTIVITY 5.1. Oxygen storage Capacity The results are given on tables 1 and 2 : OSC1 represents the CO consumption for the first pulse and OSC5 the mean value for the first five pulses. The CO consumption of the first pulse is due to the fast oxygen and to the reduction of the sample as well whereas the reduction effect predominates for the next pulses ; thus the difference between these two values represents a titration of the fast oxygen which can be used reversibly for catalytic redox reactions. The OSC1 are much greater for unsupported and supported HMnO 4 based catalysts after calcination at 300 and 600 ~
The main parameter which explains this difference is the
surface area for the unsupported samples and the manganese oxide dispersion for the supported ones. The transformation into c~-Mn20 3 induces a supplementary drop of activity which is clearly evidenced after calcination at 6000 C for both series of catalysts. After calcination at 1000~ the results are similar for both precursors, in relation with the formation of the Mn30 4 phase. For the nitrate based supported series the sample calcined at 1000~ does not follow the order we can derive from the H 2 consumption (TPR curves) at 300~
(i.e. the temperature of the OSC measurements), but presents a higher
OSC 1 value despite the decrease of the surface area (Table 1) ; this reveals a higher amount of fast oxygen, in agreement with the great difference between the OSC1 and OSC5 values (714 versus 194).
705 The same behaviour is exhibited by the HMnO 4 based sample calcined at the same temperature, although both values are slighly lower. Thus the presence of the fast oxygen could be associated with the formation of the Mn30 4 and ,:~-AI203 phases ; this point can be related to the results of Tsyrulnikov et al [9] who observed a sharp increase in hydrocarbon oxidation activity at 900" C for the same type of catalysts. 5.2. NO reduction
The catalytic activity and selectivity in NO reduction are shown in Figure 3 for the supported samples. The selectivity concerns the formation of N 2 or N20 according to the following equations : 4NO + 4NH 3 + 0 2 --* 4 N 2 + 6 H 2 0 4 N O + 4 N H 3 + 3 0 2 --~ 4 N 2 0 + 6 H 2 0
The selectivity t o w a r d s N 2 decreases as the temperature increases for all the samples (figure 3b). After calcination at 300"C, the sample prepared via HMnO 4 exhibits an exceptionally high activity with T l/2 around 140"C ; nevertheless the selectivity is very similar to the selectivity of the nitrate based sample, despite the high activity difference. The samples calcined at 1000*C display very similar activities and selectivities in accordance with the previous results. At low temperature the activity of the nitrate based sample is slightly higher, and thus follows the OSC data, but becomes less active as the temperature increases. 100 18000~ 0 1 3 0 A ._~) 0 0z
60
z"
HHnO~IO01 /
/
~
Ni,r. 1000
60
. 2
40
co
20
~
\
~N,,.
300
HHnO~IOI~""~" ~
40
0 100
140
180
220
260
300
0 100
| 140
| 180
i 220
i 260
Temperature(*C)
Temperature(*C)
Activity
Selectivity in N 2
Figure 3. Activity and selectivity towards N 2 for the supported samples.
300
706 6. CONCLUSIONS The use of HMnO 4 as a precursor leads to high surface area for unsupported samples (amorphous phase evidenced after 300~ calcination), or to high dispersion for alumina supported catalysts. By comparison manganese nitrate based samples display the presence of crystalline phases in unsupported and supported states (MnO2, Mn203). These data correlates well with the catalytic activity for NO reduction or with the oxygen storage capacity. Calcination at 1000~ diminishes strongly the difference between both precursors, due to the formation of Mn30 4 as the active phase and to the transformation of the carrier into ~-alumina for the supported samples. Despite the formation of these crystalline phases, the OSC measurements reveal the presence of fast oxygen onto the surface of the supported samples. The sample which display the highest activity for NO reduction keeps one of the highest selectivity towards N2 (i.e. HMnO4 based, calcined at 3000C), but the decrease of selectivity with temperature remains a strong limitation for the use of such catalysts.
Acknowledgement T. Wahdan thanks the Egyptian Government for the grant given to him and M.I. Zaki thanks the CNRS of France for a research associateship.
REFERENCES 1. P.W. Selwood, T.E. Moore, M. Ellis, J. Amer. Chem. Soc., 73 (1949) 693. 2. M.A. Baltan,~, A.B. Stiles and J.R. Katzer, Appl. Catal., 28 (1986) 13. 3. A.K.H. Nohman, M.I. Zaki, S.A.A. Mansour, R.B. Fahim and C. Kappenstein, Thermochim. Acta, 210 (1992) 103. 4. A.K.H. Nohman, D. Duprez, C. Kappenstein, S.A.A. Mansour and M.I. Zaki, Preparation of Catalysts V, G. Poncelet et al (eds.), Elsevier Sci. Publ. B. V., Amsterdam (1991) 617. 5. S. Kacimi, J. Barbier Jr, R. Taha and D. Duprez, Catal. Let., 22 (1993) 343. 6. L. Singoredjo and F. Kapteijn, 10th International Congress on Catalysis, L. Guczi et al (eds.), Akad6miai Kiado, Budapest (1993) 2705. 7. M.I. Zaki and C. Kappenstein, Z. Phys. Chem., 176 (1992) 97. 8. D. Duprez, J. Chim. Phys., 80 (1983) 487. 9. P.G. Tsyrulnikov, V.A. Drozdov, V.S. Salnikov, S.A. Stuken, A.V. Kalinkin, Yu. A. Kachurovsldi, P. Ye. Kolosov and Ye. I. Grigorov, 9th National Congress of Catalysis of India (1985) POS 5.
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparationof HeterogeneousCatalysts G. Ponceletet al. (Editors) 1995 ElsevierScience B.V.
707
Systematic Control o f C r y s t a l Morphology during Preparation o f Selective Vanadyl Pyrophosphate. E. Kesteman a, M. Merzoukia, B. Taouk a, E. Bordes a and R. Contractorb, a D6partement de G6nie Chimique, Universit6 de Technologic de Compi~gne, B.P. 649, 60206 Compi~gne C6dex, France. b E. I. du Pont de Nemours, Experimental Station Bldg 262, P.O. Box 80262, Wilmington, DE 19880-0262, USA.
The crystal morphology of (VO)2P207 can be determined by control of the pseudomorphic precursor VOHPO4.0.5H20. The standard method of preparation and calcination shows that the primary factor is the nature of alcohol used for reflux, which influences crystallinity and shape of secondary particles of precursor and catalyst. Surface area depends mainly on thickness and width of plates, the later being related to selectivity in oxidation of butane to maleic anhydride.
1. INTRODUCTION When pseudomorphic relations exist between a solid catalyst and its precursor, one can expect that a control of its properties can be exerted through the control of those of precursor. In the case of vanadyl pyrophosphate used for the selective oxidation of n-butane to maleic anhydride, it has been noticed since a long time that the best performance of (VO)2P207 is related to the presence of (100) planes [1], which is characteristic of samples obtained by calcination of VOHPO4.0.5H20 precursor. To account for {100} specificity we have used the Crystallochemical Model of Active Sites (C.M.A.S.), and shown that this face displays the right cluster of selective sites allowing the formation of one maleic anhydride and four water molecules, while perpendicular faces would lead to oxidized by-products [2, 3]. These findings have been recently confirmed by Inumaru et al., [4] who have shown that poisoning {100} faces by orthosilicate results in making more CO2, which is produced therefore on perpendicular planes. According to the equilibrium shape of (VO)2P207 crystals, {100} faces would be only 32 % of the external surface area of the crystal [3], the prismatic shape of which is like that for samples prepared by reduction of IB-VOPO4 at 760~ in N2 [5]. To optimize the area of selective {100} comparatively to other faces, the best way is to prepare VOHPO4.0.5H20 precursor because (001) VOHPO4.0.5H20 is structurally
708 related to (100) (VO)2P2OT, so that pseudomorphic crystals of the catalyst exhibiting {100} faces are obtained (Fig. 1) [6, 7]. If the numerous parameters involved during preparation and calcinationactivation and their effect on the catalytic properties of (VO)2P207 have extensively been studied [1, 8-10, 15], relatively little work [4, 9, 12, 13] is devoted to the effect of preparation on crystal morphology of the catalyst although it finally rules selectivity. We have decided to focus on the effect of the alcoholic m e d i u m from which the precursor is precipitated after reflux, since this parameter decides on the temperature, on the amount of water and on the reduction power, that is on the final state of v a n a d i u m phosphorus compound. A l t h o u g h it is k n o w n that performance is greater when P / V is close to 1.1-1.2:1, we have deliberately chosen the ratio 1:1 in order to favor the crystallization of the precursor. The influence of some other parameters, like addition of an acid or of tetraethylorthosilicate (TEOS) [11], dilution of H3PO4, duration of reflux, etc., has also been examined. The calcination of the precursor has always been performed in the same way, that is in a flow of inert gas supposed to induce the least changes in v a n a d i u m valence. To summarize the question to be answered in the present paper: is controlling the crystal morphology of precursor controls the crystal morphology of the catalyst?
2. EXPERIMENTAL V205 (0.26 mol.1 "1) and H3PO4 (85%) (P/V = 1:1) were mixed and refluxed (6 hrs) in various primary and secondary C2-C8 alcohols (Table 1), other parameters being kept constant. In the case of 2-Ethyl,l-Hexanol, VOPO4.2H20 was first prepared [6] and refluxed. The precipitate was filtered, washed three times with 80 ml of the same alcohol and dried at 120~ (12 hrs) in an oven. All samples of VOHPO4. 0.5H20 (P) were calcined in N2 at 500~ during 6 hrs (heating rate 30~ Other parameters were made to vary: TEOS was added to raw materials (@ 0.1 mol.1-1) in isobutanol and benzyl alcohol mixture (IBA/BA = 91/9) before reflux. Oxalic acid (0.26 mol.1-1) was added to raw materials in 2-butanol. H3PO4 was used at 99% and 50%. A longer time of reflux (24 hrs) was also experimented with most of alcohols. Several methods (XRD, FTIRS, SEM, TEM, BET) were used to characterize the morphology of crystallites of P and resulting catalyst (C). XRD data were obtained with the same amount and same display of the powder on the sample holder. The mean thickness of {001}, {100} and "length" {220}, {042} of plates of P and C respectively (Fig. 1) was calculated using Scherrer formula. The ratios Rp = (220)/(001) and RC = (042)/(200) between the Full Width at Height M e d i u m (FWHM) of (hkl) planes for P and C respectively were computed to have an evaluation of the relative dimensions of crystal plates. The angles between planes perpendicular to (001)P and (100)C were measured on TEM and SEM micrographs and compared to theoretical values computed from cell parameters [14]. In the case of C, the best fit was obtained for planes (043) and (092) (Fig. la) although these values are not in accordance with the principle of low index faces. This could be due to a lack of precision of parameters (structure solved at Rw - 9 %) [14].
709
04
11o
I VOHP04.0.5 H z0 (Precursor)
iT<,oo, (VO) 2 P zO 7 (Catalyst)
Figure 1. P s e u d o m o r p h i s m between V O H P O 4 . 0 . 5 H 2 0 precursor (left) and (VO)2P207 catalyst (right): a) crystal shapes (angles and indexation of faces), b) TEM showing growing dislocations on EK14 (2-ethyl-l,hexanol), and catalyst.
3. RESULTS AND DISCUSSION The powder pattern and FTIR spectrum of all precursors and catalysts prepared in alcoholic m e d i u m are consistent with those found in literature for VOHPO4. 0.5H20 (P) and (VO)2P207 (C) respectively, except in the cases of ethanol (presence of remaining V205, even after 24 hrs of reflux) and of H3PO4 50% (presence of VOPO4.2H20 and other lines corresponding to VOHPO4.nH20, n > 2) [13]. 3.1. Influence of alcohol.
Data on the temperature of reflux (6 hrs) and on surface area (SA), ratio R and thickness of crystals of P and C are reported Table 1 and Figs. 2-4. The measured temperature of reflux can be very different from the theoretical point of ebullition, as for C9 and C62 (120 and 140 instead of 185 and 205~ for pure alcohols). When surface areas of P and C are plotted against temperature of reflux the high-
710 Table 1 Effects of solvent on some characteristics of precursors (reflux 6 hrs) and catalysts Precursor Solvent
T (~
C8 2-But.ol C61 Isobut.ol C2 IBA/BA C41w ibid. C14 ibid+TEOS C9 @ Eth.Hex.ol C62 Benz. alc.
Catalyst
S.A. (m2/~)
Rp*
Thickness # (001) (220)
5 10 16 27 31 2 23
1.0 1.2 1.4 2.1 2.8 1.0 2.0
545 445 365 240 170 545 250
100 105 110 110 110 120 140
560 530 540 530 515 580 530
S.A. (m2/g)
RC*
Thickness # (200) (042)
6 15 23 32 31 5 28
2.3 5.2 2.1 3.0 3.4 2.2 1.0
185 130 185 130 125 175 350
425 510 375 385 425 390 510
*Rpand Rc = Ratio of FWHM of XRD lines; VOHPO4.0.5H20: Rp = (220)/(001); 20(001) = 15.5 ~ 20(220) = 30.5~ (VO)2P207 " RC = (042)/(200); 20(200)= 22.8 ~ 20(042) = 28.5 ~ (CuK00; #Plate thickness (~) by means of Scherrer formula: T (.~) = 0.89 Z/FWHM.cos0. IBA/BA = 91/9; Eth.Hex.ol = 2-Ethyl-l-Hexanol; TEOS = TetraEthylOrthoSilicate. wIBA/BA = 91/9 and P/V = 1.2:1; @ Prepared by reduction of VOPO4.2H20. ~ P/V1.2 Teos
~30
0 P/V1.2
,m
/ /
0 "0
/ /
q9
c20 Lq
I
~
I
o.
"6
I \
/f~
M
,(
/
9 10 M I::
I
~
/ \
sS
~
/ s"
~
t
/ ~
\
/
~ 9~
i
ss ~ sS
//s S/
99~ s ~ 9 sS D
O)
0
9o
160
lio
sS
/ D
s So 9
1L,o
[--~-~sAIP-sA 1~o
1~o
is0
Reflux Temperature (~
Figure 2. Effect of reflux temperature of alcohol on surface area of precursor and catalyst (6 hrs) (symbols: open for precursor, black for catalyst) est values are observed with i s o b u t a n o l / b e n z y l alcohol (C2) and pure benzyl alcohol (C62) (Fig. 2). They increase w h e n P / V is increased up to 1.2:1 in I B A / B A (C41) and also w h e n TEOS is added [11]. SA of catalysts are remarkably parallel to those of precursors and always greater. This is probably due to the high heating rate and use of nitrogen during calcination of the p o w d e r e d precursor. N o t only
711 water molecules escape as the result of dehydration, but also some molecules of alcohol, which were trapped inside the particles, escape. Thicknesses and values of R characterize the morphology: the greater the values of Rp and RC the thinner the plates. T(220) and T(042) are only indicative of the mean "length" between {220}P and {042}C faces, while T(001) and T(200) are more representative of the actual thickness. Owing to the physical principle of XRay diffraction these values are valid for coherent crystals, so that thicknesses measured from SEM pictures are always greater because of stacking of plates (vide infra). When plotted against reflux temperature, the thickness of P plates follows the inverse trend of SA (Fig. 3): the thinner the plates, the higher the surface area. One could think that the shape of primary particles (agglomeration of plates, eventually as rosettes) would rule surface area, but it seems that the primary factor is the thickness of secondary particles. The low value of SA found with 2ethyl,l-hexanol is probably related to the high crystallinity of the raw material (plates of VOPO4.2H20) which yields in turn large plates of VOHPO4.0.5H20. Catalyst plates are always (except C62) thinner than those of the corresponding precursor (mean value 180 instead of 365 ~), but there is no direct relation between thickness of {100}C or {001}P and surface area of C or P (Fig. 4). If it seems normal to consider that dehydration of P yields thinner plates of C because crystals are delaminated, the size of alcohol molecules trapped can influence the final thickness, as for C62 (benzyl alcohol). Data in Table 1 and Figs. 2-4 show that increasing P / V to 1.2:1 or adding TEOS are two methods to obtain the thinnest plates of P and C. Since TEOS is a nucleating agent the effect is better seen on P than on C, as {001}P makes only 170 ~ compared to the mean thickness 365 ~. In the case of P / V = 1.2:1 the excess of phosphoric acid adsorbed on plates could play the role of trapped alcohol, so that thickness of C is decreased.
TEOSO
30 84
.,
,...
pq1.=o / t !
I.--o-- P:,~, ] i--=-- P-O01
<...)
'550
".. 30"
/ 9
"~"~ t
\I
"" " .- .,,,.,.
' 550
""I
9
it///
'450 "~
"450
V
G,,
:.:.A-" P-24.,SA I P-24- 001 (24 hrs)
20" '350
,e,-
8 m o
&
|
o
10"
'~ 10
8
"250
a PN~.= \ / TEOS,
ol,go
lio
t
25O ~'~'~.
I
1~o
Reflux Temperature (~
I, o 8o
15o
I
"A
~
16o
~o
',y."
,,~
,o
,
150
14o
Reflux Temperature (~
Figure 3. Influence of temperature and time of reflux on surface area (large symbols) and thickness (small symbols) of precursor; a) 6hrs; b) 24 hrs.
712 6O0
0
/2.
--'[~--,~
P-O01 P-001-24 C-200-24
-.
o "u
a. 0 0
'%, "%,%
"6 IJ
J s 9
""rl
0 P N 1.2
8
..j..,
,-200 O
....,.....li
0 P/~
T ~ 001.2
I.-
IO0
0 Surface Area of P or C (m2/g)
Figure 4. Influence of surface area on thickness of plates for precursor and catalyst (large symbols: 6 hrs; small symbols" 24 hrs). Plates 1, 2 present TEM and SEM pictures of precursors and catalysts. The characteristic shape of VOHPO4.0.5H20 is retained after calcination in (VO)2P207 and faces exhibited are {001}P and {100}C [6, 7, 13]. Dislocations are evidenced on some crystals of precursor as they grow spirally, but also on the catalyst (Fig. lb). Plates are generally well-shaped and more or less wide (mean size 2-8 x 1-2 x 0.10.3 I~m). In the case of benzyl alcohol (Plate ld) primary particles have a shape rounder than generally observed ( 0 4 I~m). SEM pictures of primary and secondary particles of P with TEOS and resulting C are very different from others as already observed by Horowitz et al., [11]. Very thin petals (1-3 I~m) with thickness = 50 nm are displayed as well developed rosettes of same size for P and C (Plates If and 2a, b). 3.2. I n f l u e n c e of other parameters.
The concentration of H3PO4 as a raw material has some influence on the characteristics of the solids (Table 2). Using butan-2,ol (C4, C8, C5) the highest surface area is obtained with H3PO4 85%. There is no effect on the thickness of P plates but catalyst plates are thicker when H3PO4 99% is used. Dilution at 50% yields hydrates other than VOHPO4.0.5H20, as evidenced on Plate 2e exhibiting square crystals characteristic of VOPO4.2H20 [14]. XRD pattern shows that a mixture of (VO)2P207 and other phases is obtained by calcination. With the same alcohol, addition of oxalic acid which is also a reductor leads to comparable surface area and slightly thinner plates (C7), but primary particles look like thick rosettes (O 4 I~m) (Plate 2f). With IBA/BA the trend is similar to that in butan2,ol, that is higher surface area and thinner plates with use of H3PO4 85% (C2-C3, Table 2). Addition of TEOS again reinforces this trend (C14-C13). An increased time of reflux (24 hrs) corresponds to a lower surface area and thicker plates for all alcohols (0.2-0.6 l~m) (Tables 1, 3, Figs. 3b, 4). E2 is an exception
713
Plate 1. Examples of crystal morphology (SEM). a) 2-Propanol, x5K; b) 2-Butanol, x5K (24hrs); c) Isobutanol, x5K; d) Benzyl alcohol, xl0K; e) 2-Ethyl-lHexanol, x5K (24 hrs); f) TEOS in IBA/BA, xl0K (catalyst).
714
w
: i:= ~iiiiil :~2:1~;=:i ::ili!i!!!ii~,~,~!!~ii:ii~.......... !!~!'~'!~::iiii:~::~ ::i~'!:~,~::,~,
Plate 2. Addition of TEOS: a) Precursor, xl0K et b) Catalyst, xl0K. Dilution of phosphoric acid in 2-Butanol: c) 99%, x5K; d) 85%, x5K; e) 50%, x5K. Addition of oxalic acid: f) x5K.
715 (benzyl alcohol) because the catalyst particles are thinner and smaller after 24 hrs than after 6 hrs. Table 2 Dilution of phosphoric acid (Ph.A.) and effect of oxalic acid or TEOS Precursor Solvent
C4 2-But.ol C8 ibid. C5 ibid. C7 ibid.+Ox. Ac. C2 IBA/BA C3 ibid. C14 IBA/BA+TEOS C13 ibid.
Ph.A.S.A. Rp % (m2/~) 99 85 5O 85 85 99 85 99
Catalyst
Thickness S.A. (001) (220) (m2/g)
1.0 547 560 1.0 545 560 higher hydrates* 1.1 510 560 1.4 365 540 1.3 385 525 2.8 170 515 1.4 360 510
2
5 2 6 16 10 31 26
RC
Thickness (200) (042)
5 2.0 217 6 2.3 185 (VO)2P207 + X 12 2.3 165 23 2.1 125 10 1.2 320 31 3.4 125 22 2.5 130
438 425 385 425 395 425 340
* higher hydrates: VOPO4.2H20 + VOHPO4.nH20 (n > 2); X: extra lines (XRD). Table 3 Influence of reflux time (24 hrs). Precursor Solvent
E5 E7 E3 E14 E2
2-Propanol 2-But.ol Isobutanol Eth.Hex.ol Benzyl alc.
S.A. Rp (m2/[~) 7 3.5 13 6 8
0.9 1.0 1.2 1.0 0.9
Catalyst
Thickness (001) (220) 525 540 465 565 465
570 560 560 580 560
S.A. (m2/~)
RC
Thickness (200) (042)
8 6 20 15 11
2.2 2.3 2.9 2.0 2.5
195 200 160 210 180
435 450 400 425 450
4. CONCLUSION By using standard conditions of preparation and calcination while keeping several parameters constant we have shown that the nature of alcohol with its properties (boiling point modified by H3PO4, acidity, reducing power, molecule size) influences surface areas, crystal shapes, thickness and areas of the exposed faces, that is crystal morphology. The primary factor on surface area is the thickness of plates and not the way plates are displayed. Increasing P / V or adding
716 TEOS results in higher surface areas, thinner plates being obtained with TEOS, while adding oxalic acid has little effect. It is seen that the conditions claimed to get the best performing catalyst (H3PO4 85%, P / V = 1.1-1.2, isobutanol/benzyl alcohol and TEOS) [11] are indeed optimum for morphology. Our study shows that, when topotaxy and pseudomorphism have been evidenced, controlling the morphology of precursor is indeed controlling the morphology of catalyst, in the sense that wide (and thin) particles of {001}VOHPO4.0.5H20 yield wide (and thin) particles of {100}(VO)2P207. Consequently the faces known to be selective are large enough while unselective perpendicular faces have a m i n i m u m area. A high heating rate seems to be a means to get higher surface area for the catalyst. However, as we performed calcination in nitrogen, the presence of V5+ on the surface, which seems to be necessary for better activity [5], cannot be favored. Atmosphere and temperature of calcination-activation are very i m p o r t a n t parameters and will have to be studied. REFERENCES
1- Catal. Today, 1987, 1 and 1993, 16 and refs. therein. 2- J. Ziolkowski, E. Bordes and P. Courtine, J. Catal., 1990, 122, 126-150. 3- J. Ziolkowski, E. Bordes P. Courtine, Stud. Surf. Sci. Catal., 1990, 55, 625-633; ibid., J. Molec. Catal., 84 (1993) 307-326. 4- K. Inumaru, T. Okuhara and M. Misono, Chem. Lett., 10 (1992) 1955-58. 5- E. Bordes, Catal. Today, 3 (1988) 163-174; ibid., Catal. Today, 1993, 16 (1), 27-38. 6- E. Bordes, J.J. Johnson and P. Courtine, J. Sol. State Chem., 55 (1984) 270. 7- J.J. Johnson, D.C. Johnston, A.J. Jacobson and J.F. Brady, J. Amer. Chem. Soc., 106 (1984) 8123-8128. 8- M. O'Connor and B.K. Hodnett, Appl. Catal., 42 (1988) 91-104; ibid., 64 (199) 161-171. 9- G.A. Sola, B.T. Pierini and J.O. Petunchi, Catal. Today, 15 (1992) 537-545. 10- L.M.Cornaglia, C.A. Sanchez and E.A. Lombardo, Appl. Catal., 95 (1993) 117. 11- H.S. Horowitz, C.M. Blackstone, A.W. Sleight and G. Teufer, Appl. Catal., 38 (1988) 193-210. 12- A. Datta, A.R. Saple and R.Y. Kelkar, J. Mater. Sci., 11 (1992) 930-933. 13- E. Bordes, Materials Research Society, Ann. Meet., Boston, 2-6/12/91, U4.2. 14- Yu. E. Gorbunova and S.A. Linde, Dokl. Akad. Nauk SSSR, 245 (1979) 584-588.
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
717
V A N A D I U M E X C H A N G E D T I T A N I U M P H O S P H A T E S AS C A T A L Y S T S FOR THE SELECTIVE REDUCTION OF N I T R O G E N OXIDE WITH AMMONIA M. A. Massucci 1, p. Patrono 2, G. Russo 3, M. Turco 3, S. Vecchio 1 and P. Ciambelli 4. 1 Dipartimento di Chimica, Universitb. di Roma "La Sapienza", Roma, Italy. 2 I.M.A.I.-CNR, Area Ricerca di Roma, Monterotondo Scalo, Roma, Italy. 3 Dipartimento di Ingegneria Chimica, Universith "Federico II" di Napoli, Napoli, Italy. 4 Dipartimento di Ingegneria Chimica ed Alimentare, UniversitY. di Salerno, Fisciano (SA), Italy. The ion-exchange technique was employed for the preparation of VO 2+ vanadiummodified titanium phosphates as catalysts for the selective reduction of NO with NH 3. The samples were prepared by contacting different precursor materials such as amorphous or crystalline titanium phosphate or sodium half exchanged titanium phosphate. Different vanadium contents (0.08-2.3 wt %) were achieved by operating at different temperatures (20, 60~ solution concentrations (2.5 10-3-2.5 10-2 mol dm'3), and volume to solid ratios (150540 ml g-l). The precursor salt was vanadyl sulphate. XRD and thermal analysis TG/DTA showed that vanadium loading does not cause structural modifications in hydrogen titanium phosphate. A vanadyl containing phase was obtained when half sodium titanium phosphate was employed. Catalytic activity measurements were performed under dilute conditions. The reaction temperatures ranged from 200 to 400~ the space velocity was 80 000 h -1. The catalysts were pretreated at 380 and 600~ either in helium or air flow. It was found that vanadium content affects NO conversion. Treatment in He flow resulted in an increased activity with respect to that in air. Values of about 90% NO conversion were obtained with vanadium richest catalysts. 1. INTRODUCTION Most of the catalysts employed in the selective reduction of NO by N H 3 in the presence of 0 2 (SCR process) 4NO + 4NH 3 + 0 2 ---,4N 2 + 6H20
(1)
are based on vanadium oxides as the active component (1). The temperature of reaction generally ranges between 250 and 400~ Several supports have been tested to improve the dispersion of the active phase, titanium dioxide being resulted the most effective. The V20 5 content in vanadia/titania systems generally corresponds to a submonolayer coverage (1), and the presence of different vanadium oxide species has been supposed (2, 3). The activity of V 4+ vanadyl containing species has been hypothesized by some authors (3, 4). Layered tetravalent metal phosphates Me(HPO4)2.nH20, (Me = Zr, Sn, Ge, Ti), have ion-exchange properties (5). Half sodium exchanged zirconium phosphate ZrHNa(PO4) 2 5H20
718 was found to exchange oxocations (5). Massucci et al. reported in (6) the preparation and characterization of VO 2+ exchanged ZrHNa(PO4)2.5H20 with different vanadium loadings. In this paper we have employed the ion-exchange technique to prepare VO 2+ exchanged titanium phosphates as catalysts for the SCR process. The influence of precursor phase (crystalline or amorphous hydrogen titanium phosphate or half sodium exchanged titanium phosphate) was also investigated.
2. EXPERIMENTAL 2.1. Catalyst preparation. Three types of support were prepared: a)amorphous titanium phosphate u-Ti(HPO4) 2 nH20 (specific surface area=55 m 2 g-l) synthesized by adding to a 1.25 mol dm -3 H3PO 4 solution, TiCI 4 dissolved in 2 mol dm -3 HCI, by filtering, washing the solid with distilled water up to pH=3-3.5 and drying on P4OIo (7); b)crystalline titanium phosphate u-Ti(HPO4) 2 1-/20 (specific surface area=10 m 2 g-l), obtained by boiling the amorphous precipitate for 100 hours in 10 mol dm "3 H3PO 4 at PO4/Ti ratio of two (7), c) sodium half exchanged phosphate uTiHNa(PO4) 2 4H20 (specific surface area=16 m 2 g-l) by contacting the crystalline phase with a 0.05 mol dm -3 solution of sodium hydroxide under stirring, filtering and washing with distilled water. The typical preparations of the vanadium-exchanged phosphates were performed according to the following procedure: aliquots of vanadyl sulphate solutions were added to lg of cristallyne u-Ti(HPO4) 2 H20 (a-TiP), or amorphous u-Ti(HPO4) 2 nH20 (am-TiP) or uTiHNa(PO4) 2 4H20 (u-TiPHNa). The suspensions obtained were stirred for 4 days at 25~ except for the sample VTiP5 (Tab.l) for which the temperature was 60~ The solids were filtered off, washed with distilled water and air dried. The solutions were analysed for vanadium, pH and occasionally Na. The preparation conditions of the various materials and their vanadium content expressed either in moles per mole of titanium phosphate or in wt % with respect to the anhydrous materials are summarized in Tab. 1. 2.2. Chemical analysis and physical characterization. Vanadium uptakes were calculated from the concentration changes of the supernatant solutions before and after ion-exchange. Vanadium was first all oxidized to V 5+ and then determined potentiometrically by employing a 0.01 mol dm -3 (NH4)2Fe(SO4) 2 solution as titrant. In the preparation of VNaTiP, sodium released to the solution was determined by atomic absorption method on a Varian Techtron model 1100 spectrophotometer. Thermal analyses were performed on a Stanton Redcroft TG/DTA model 801 instrument (Pt crucibles Pt-Rh thermocouples, heating rate 8~ min -1) under air flow. X-ray powder diffraction patterns were taken on a Philips diffractometer using Ni-filtered Cu Ku radiation. The BET surface areas were obtained on a Quantachrome-Chembet 3000 instrument. 2.3 Catalytic activity measurements. Catalytic activity measurements were carried out in a continuous laboratory plant (3). The reactor was loaded with 500 mg of catalysts (grain dimensions = 212-300 ttm) and fed with a stream containing 700 ppm of NO and NH3, 27000 ppm of 0 2 and balance Ar. The total flow rate was 40 NI h -1. The temperature ranged from 200 to 400~
719 The analysis of NO and NO 2 was effected by a continuous chemiluminescence apparatus (Beckmann model 955), NH 3 was detected by a continuous IR spectrophotometer (Hartmann & Braun model Uras 3G). N 2 and N20 concentrations were measured by gaschromatographic analysis on a double packed 5A molecular sieves-Porapak Q by a Hewlett & Packard instrument (model 5890) with a TCD detector. The catalytic activity tests were performed on samples treated at 380 (300~ in the case of sample VNaTiP) or 600~ for 3 hours (heating rate 10~ min "1) in helium or air flow. In all runs the nitrogen mass balance was verified within 5%. 3. RESULTS AND DISCUSSION. 3.1. Chemical analysis In Tab. 1 the vanadium content of catalysts obtained at different temperatures, vanadyl solution concentrations and solution volumes is reported. For catalysts obtained from crystalline a-TiP, the increase of temperature from 20 to 60~ gives rise to an increase of vanadium loading. The same effect was achieved by increasing the solution concentration from 1.0 10-3 to 2.5 10-3, whilst a further increase to 2.5 10-2 resulted in lower vanadium loading. This latter effect could be due to the lower pH value of the mother solution that hinders the VO 2+ exchange. By operating with the same concentration and temperature, a decrease of solution volumes leads to a decrease of vanadium loading, probably due to the lower pH values occurring during the contact. Higher vanadium content with respect to previous samples is obtained by employing am-TiP. Finally a marked improvement is obtained with a-TiPNaH. The VO 2+ exchange process involves the hydrogen ions of the PO3-OH groups of the exchangers. By assuming a surface-OH concentration of 3.9 nm -2 (8), a surface vanadium exchange capacity of 0.165 and 0.825 wt % could be expected for catalysts obtained from crystalline a-TiP and am-TiP respectively. Therefore data of column 7 of the Tab. 1 indicate for VTiP and am-VTiP samples a partial or total external surface coverage and, in some cases, a slight vanadium interlayer exchange. In the case of TiPHNa precursor, the VO 2+ uptake occurs via a VO2+/Na + ion exchange, as suggested by the amount of Na + ions released to the contact solution and the moles of vanadium taken up. Therefore the exchange process gives rise to the presence of bulk vanadium species. Table 1. Operating conditions for the preparation of the different vanadium-containing materials (columns 2-5) and compositions in moles of VO+2/mole Ti or in V wt % (columns 6, 7). Samples
T
(oc) VTiP 1 VTiP2 VTiP3 VTiP4 VTiP5 am-VTiP VNaTiP
Starting materials
20 a-TiP . . . . . . . . . . . . 60 " 20 am. TiP " TiPHNa
Solution concentration (moidm -3)
Solution volume (ml g-l)
2.5 10-2 1 0 10-3 25 10-3 25 10-3 25 10-3 25 10-3 2.5 10-3
360 360 170 360 170 150 540
Vanadium content mol mol-1 0.0037 0.006 0.0068 0.010 0.011 0.040 0.120
wt%
0.0784 0 127 0 148 0.212 0.228 0.839 2.310
720 3.2. X-ray and thermal analysis. The X-ray diffraction patterns of as prepared a-TiP exchanged materials show the same signals of the parent compound, indicating that the ion-exchange process causes no significant structure modifications. In Fig. 1 the XRD spectra of u-TiP, VTiP 1 and VTiP4 after treatment at 380 and 600~ are shown. At 380~ the dehydrated phases (d=7.4 A) and after 600~ the layered pyorophosphates (d=6.98 A) are formed. The other crystalline vanadium modified phosphates show similar spectra. No changes of thermal behaviour were observed in all aVTiP samples with respect to a - T i P precursor. This confirms that the vanadium is mainly present on the external surface of a-TiP exchanged materials. TG/DTA curves (Fig. 3) show that the dehydration process occurs between 40 and 280~ with a weight loss corresponding to about 1 mole of water/mole of exchanger, while the condensation to pyrophsphate occurs in the range 400-600 ~ All samples undergo the transformation to cubic pyrophosphates (a-P207) at ca. 880~ as shown by the exothermic peak in DTA curves. In the case of am-VTiP, the dehydration process leads to a weight loss of about 1.5 mole/mole of a-am VTiP. The condensation to layered pyrophosphates starts from temperature lower than previous samples (ca. 300~ due to structural disorder, and the transformation to cubic pyrophosphate occurs at ca. 850~ XRD spectra of the as prepared VNaTiP, beside the peaks of the starting material, also show a small peak at 20=9.05 ~ (d=9.75 A) probably corresponding to the interlayer distance of a vanadium titanium phosphate phase of the type a-TiPH(VO)0.5 formed as a consequence of VO2+/Na + exchange. TG and DTA curves show that the dehydration process occurs in the temperature range extending from the r.t. to ca. 350~ the weight loss corresponding to about. 4 moles of water/mole of exchanger.The condensation process is completed at 600~ The X-ray patterns of TiPHNa, after treatment at 300~ (Fig. 2) show the signals of the layered dehydrated phase (d=7.13 A); after treatment at 600~ signals of a phase of the type monosodium dititanium triphosphate [NaTiz(PO4)3] and cubic titanium pyrophosphate (aTiP207) are present (9). As shown in Fig. 2, in the XRD pattern of VNaTiP sample treated at 300~ signals of layered phase (d-6.86 A) are still present. The XRD pattern of the material treated at 600~ seems to refer to a phase very similar to that of the parent material treated at 600~ (see Fig. 2) in which the vanadium could have partially substituted the titanium, forming a solid solution. At present we are investigating on this possibility.
721
!',."-.-.-.',~'~ ,,
I~...~ !
VTiP4 380~
---__._____.,' ~..__..____._._.,......~. ~
i
....... __~.,'L.~. I VTiP1 3 8 0 ~
~'~
"-""---"-~-';"~"""'-~ " ~ : ~ ' ' " ' ~ .
,
~,_,., 9
, ~
.; il .~,,
"---'------"
"
~.
vmiel
,
I
VTiP4 600 o C I 1
"'------'---"-'--t
,
I
600~
a-TiP
,~
t
600~ _.J
.
22.5 20 40 Fig. !. XRD spectra ofcz-TiP, VTiP! and VTiP4 samples treated at 380 and 600~ 5
t cx-TiPHNa 300~
Jl ....
.
. . . . . . . . .
.-~_.~,\...
VNaTiP 300~
~.
":',__,' - . ~ . ~ , ~ . .
,~ A
o~-TiPHNa 6 0 0 ~ C
ti
VNaTiP 6 0 0 o C
,h,
~.-,.,._._~,J\__,,'; I
5
.I
I
I
k....__,,-,,...~: ~...__,~__ I
22.5
I
I
;
I
I
40
20 Fig. 2. XRD spectra of e-TiPHNa and VNaTiP samples treated at 300 and 600~
722
DTA VNaTiP 0 x
VTiP4 0 "13 r-
am-VTiP 100
TG
A
VNaTiP
v
O co..
90
VTiP4 am-VTiP
t-
0
,
,
,
,
,
500
'
,
T (~
1000
Fig. 3. TG/DTA curves of vanadium VTiP4, am-VTiP and VNaTiP samples.
3.3. Catalytic activity measurements The catalytic activity measurements were effected on samples treated at 380 (300~ for sample VNaTiP) and 600~ in He or air flow, at different reaction temperatures. The results are reported in Fig. 4-6. The parent material c~-TiP exhibits low activity, NO conversions being lower than 5% up to 300~ either for sample treated at 380~ or at 600~ and reaching 15% at 400~ for sample treated at 600~ By contrast high activity is shown by vanadium modified phosphates even with low vanadium content. NO conversion increases with vanadium loading whatever the atmosphere and temperature of pretreatment. All samples, after treatment either at 380 or 600~ were found very selective towards the N 2 formation. The conversion to N20 was negligible at low temperature, and reached values of about 1-3% at 300-400~ suggesting the occurrence of ammonia oxidation reactions (3, 10) in low extent. However the treatment in helium flow gives rise to an enhanced activity, this effect being remarkable for VTiP 1 and VTiP3 samples (Fig. 4). Vanadium richest catalysts, am-VTiP and VNaTiP samples, treated at 380 (Fig. 5)and 600~ (Fig. 6 ) i n He flow, give almost complete NO conversion (85'90 %) at the highest temperature investigated. After treatment at 600~ catalysts obtained by modification of crystalline c~-TiP, with a low vanadium content (VTiP1 and VTiP2) show slightly higher activity then after the treatment at 380~ VTiP3 and
723 VTiP4 do not show significant changed activity and the same behaviour was observed on amVTiP sample. An improvement of the surface catalytic properties of the layered pyrophosphates compared to the corresponding metal acid phosphate precursor was reported in (11) and related to the enhancement of surface acidic properties induced by the condensation of interlayer P-OH groups. This hypothesis could be extended to the present case, by taking into account that the catalytic activity of metal oxide based systems for NO reduction is also affected by the acidic properties (1, 3). Such a promoting effect appears to vanish when vanadium content is close or slightly exceeding surface capacity. A different behaviour was observed on sample VNaTiP, which gives higher NO conversions after treatment at 600~ at all temperatures. A deeper characterization of the no longer layered phase formed at this temperature could allow to obtain a better understanding of its catalytic behaviour. In order to compare the various catalysts on the basis of vanadium specific activity, the rate constants referred to the vanadium unit weight were evaluated from the conversion data obtained at 300~ on samples treated at 380 (300~ in the case of VNaTiP) and 600~ On the basis of literature informations, a first order rate equation was assumed (1, 12). The computed values are reported as a function of vanadium content in Fig. 7. These are much higher than those evaluated from data reported by Czarnecki (13) for a V20 5 supported on pillared titanium phosphate, suggesting a higher activity of vanadyl exchanged titanium phosphates. The plots show the same trend for both the pretreatment temperatures. High specific activity is shown by low vanadium exchanged samples. By increasing the vanadium content, the rate constant values decrease up to an almost constant value for vanadium percentages close to the external surface capacity. This effect suggests that the external surface vanadyl species have different catalytic properties, the most active, probably isolated vanadyl groups, being present in low exchanged materials. The rate constants of high loaded phosphates, VTiP4, am-VTiP and VNaTiP samples, have similar values. Thus the nature of precursor material seems scarcely affect the specific activity. Lower rate constant values are obtained on samples treated in air flow. Massucci et al. reported in (6) that VO 2+ exchanged u-ZrNaHP show redox properties in the temperature range extending from 150 to above 400~ The treatment in He flow resulted in oxidation of V 4+ at lower extent in respect to air flow. On the base of these results, and literature informations dealing with vanadia/titania systems (3, 4, 12) the activity of V 4+ containing species towards NO reduction could be hypothesized.
724 10o
80
z tl 1
40
20
150
200
250
300
350
~TLRE
400
460
('c)
Fig 6. NO conversion as a function of temperature for catalysts treated at 600~ in He flow: (El) VTiPI, (A) VTiP2, ( e ) VTiP3, (O) VTiP4, (z~) VNaTiP, (m)am-VTiP.
0 * to ' ~ 8ooo
>=
I"1
E
I
C
O I
El
V
r c 4ooo o 0 (Dr rr" 20OO
r-! 0
.5
1
1.5
2
2,5
Vanadium content (wt ,~ Fig. 7. Rate constant as a function of vanadium content. Samples treated at 380~ in He (I-1) or air flow(m), at 600~ in He flow (O).
725 100
2O
0 150
200
250
300
350
400
460
TEMPERATtRE ('C) Fig 6. NO conversion as a function of temperature for catalysts treated at 600~ in He flow: ([:1) VTiPI. (a) VTiP2, (o) VTiP3, (O) VTiP4, (A) VNaTiP, (m)am-VTiP.
0 r-1
1I"1
E
o6OOO V
O
c ~J c 4000
0 0 (11
0 0
.5
1
1.5
2
2.5
Vanadium content (wt ,~ Fig. 7. Rate constant as a function of vanadium content. Samples treated at 380~ in He (1"1) or air flow(B), at 600~ in He flow (O).
726 4. Conclusions The ion-exchange technique allows to prepare VO 2+ modified titanium phosphates. Different vanadium loadings can be obtained by properly controlling the operating exchange conditions and precursor phase. Vanadyl modified titanium phosphates catalysts were found active and selective towards SCR reaction, either as hydrogen or pyrophosphate phase. The results obtained in this paper indicate that the activity of the materials can be relate to vanadyl species whose redox properties affect the catalytic behaviour. REFERENCES 1) H. Bosch and F. Janssen, Catal. Today, 2 (1988) 369. 2) G. C. Bond and S. Flamerz Tahir, Appl. Catai. 71 (1991) 1. 3) P. Ciambelli, G. Bagnasco, L. Lisi, M. Turco, G. Chiarello, M. Musci, M. Notaro, D. Robba and P. Ghetti, Appl. Catal. B: Env., 1 (1992) 61. 4) G. Ramis, G. Busca, F. Bregani and P. Forzatti, Appl. Catal., 64 (1990) 259. 5) A. Clearfield, Chapter 1 and G. Alberti and U. Costantino, Chapter 2, in "Inorganic Ion Exchange Materials", A. Clearfiled Ed. CRC Press, Boca Raton Fla, 1982. 6) C. Ferragina, A. La Ginestra, M. A. Massucci and A. A. G. Thomlinson, J. Phys. Chem. 88 (1984) 3134. 7)G. Alberti, P. Cardini-Galli, U. Costantino and E. Torracca, J. Inorg. Nucl. Chem., 29 (1967) 571. 8) G. Bagnasco, P. Ciambelli, A. La Ginestra and M. Turco, Thermochimica Acta, 162 (1990) 91. 9)A. La Ginestra and M. A. Massucci, Thermochimica Acta 32 (1979) 241. 10) M. Kotter, H. G. Lintz, T. Turek and D. L. Timm, Appl. Catal. 52 (1989) 225. 11) G. Bagnasco, P. Ciambelli, M. Turco, A. La Ginestra, and P. Patrono, Appl. Catal. 68 (1991)69. 12)V. Tufano and M. Turco, Appl. Catal. B: Env., 2 (1993) 9. 13)L. J. Czarnecki and R. G. Anthony, A. I. Ch. E. J., 36 (1990) 794.
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
727
I N F L U E N C E OF T H E P R E C U R S O R F O R M A T I O N S T A G E I N T H E P R E P A R A T I O N OF VPO CATALYSTS F O R SELECTIVE OXIDATION OF n-PENTANE. Z. Sobalik 2, S. Gonzalez 3, and P. Ruiz 1 Unit~ de Catalyse et Chimie des Mat~riaux Divis6s, Universit6 Catholique de Louvain, Louvain-la-Neuve, Belgium. 2 On leave from: Institute of Inorganic Chemistry, ASCR, Prague, Czech Republic. 3 On leave from Instituto de Quimica. Universidad de Salamanca, Salamanca, Espana ABSTRACT Vanadium phosphate catalysts were obtained from precursors prepared by two different methods i) by immediate precipitation of a solution containing vanadia in isobutanol and H3P04 and ii) by facilitating, before precipitation, the conditions for the intercalation of the isobutanol in the VOP04 hydrated phase. Catalysts were obtained from the precursors by "in situ" treatment under reaction conditions for the selective oxidation of n-pentane. Results show that the control of the stage of formation of the precursor is crucial for obtaining a selective catalyst for PA formation. The preparation of VOHPO4.1/2H20 via a full development of the VOPO4.2H20 phase, containing intercalated isobutanol, seems to favour the adequate structure of the precursor which promotes the formation of PA. The present results and those presented in the literature show that by careful control of the preparation of the VPO precursor, e.g., controlling the isobutanol/water ratio, the final catalyst could be tuned to the desired PA/MA ratio. Low XPS superficial P N atomic ratio and low vanadium oxidation state probably also promote the formation of the PA. INTRODUCTION Oxidation of paraffins is one of the most exciting subjects of research nowadays. Oxidation of n-pentane produces simultaneously maleic (MA) and phthalic (PA) anhydrides. The efficient use of n-pentane as a new material demands the control of the selectivity of the reaction, particularly in order to increase the phthalic anhydride selectivity (1). Despite,several attempts at using other catalysts, vanadyl pyrophosphate (VPO) is still the only industrially applied catalytic system for C4 and
728 potentially the most active for C5 paraffin oxidation (2,3). However, no information exists concerning the role of the chemistry of the preparation of the VPO catalysts to be used to obtain selectively PA from n-pentane oxidation. The importance of the role of the method of preparation of the VPO catalyst for the butane oxidation has been analyzed extensively in the literature (4-7). Usually the catalyst precursor to be used in this reaction, VOHPO4.1/2H20, is prepared by precipitating a solution containing vanadium (obtained generally by dissolving V205 either in water with a reducing agent or in an alcoholic solution) and phosphorous (generally using H3PO4). The active VPO catalyst is then obtained by activating the precursor under the reaction conditions of oxidation. It has been well established that, for butane selective oxidation, both the precursor formation step and its activation are of crucial importance for the final performance of the catalyst. The type of solvent and the kind of reducing agent used have been correlated with the catalytic activity. Apparently the use of organic media is regarded as more convenient than other methods. However, very limited information is obtained from the literature .with respect to the influence of these two steps in the oxidation of n-pentane. Only the surface topology of the catalyst was discussed as controlling the PA/MA ratio at npentane oxidation (8). The aim of this work is to study the role of the method of preparation of VPO catalysts in the selectivity of the oxidation of n-pentane to MA and PA. In particular, our objective is to study the role of the stage of the formation of the VPO precursor in organic medium in the performance of the resulting catalyst. Two methods of preparation were employed i) precipitating immediately the solution containing vanadia and phosphorous and ii) facilitating, before the precipitation, the conditions for the intercalation of the isobutanol in the precursor. Intermediate stages of the preparation methods were also studied. EXPERIMENTAL Materials: V205 (Janssen Chimica, purity > 99.9%), isobutanol (Aldrich, content of water < 0.1% (Karl Fischer titration)), H3P04 (Fluka, purity > 99%) were used as obtained. I.- Precursor preparation: In Diagram 1 the steps followed for the obtention of precursors and catalysts are indicated. A solution containing vanadium (vanadium solution) was prepared first: 40 g of V 2 0 5 were refluxed in isobutanol for 24 hours. The remaining undissolved vanadium oxide was separated by filtration. The resulting clear filtratecontained 0.0105 g of V/ml. A solution which we shall call solution P was prepared as follows: 5.51 g of H 3 P 0 4 dissolved in ca 60 ml of isobutanol, were slowly added to 250 ml of the vanadium solution, at room temperature under vigorous stirring. The amounts of reactants were such that the theoretical molar P/V ratio of the mixture was about 1.1.
729 V20s
[
Isobuumol
!
I
l
' '
] --[
Undissolved
wo5
l.bP04in bobutanol--I Solution P (PIV=I) -_{
Sarr,~48h Roomtetn~,t,ra:urd
s#,,x
(
...) ,
( Filw~'on ) Ist~twnol
D~fin#
water
( (
J
3 )
('
Diagram 1 Experimental steps followed for the obtention of the: solution of vanadium, solution P, precursor S, precursor A, precursor A1 and precursor N. The following VPO catalyst precursors were prepared from solution P : i) Precursor S : immediately after mixing, the solution P was heated to a boiling point and refluxed for 16 hours. During the first hours the colour of the solution turned from red to dark green and then after about four hours a blue solid started to form. The solvent was evaporated under vacuum at 45 ~ and then the solid was dried at 80 ~ for 16 hours. ii ) Precursor A" An "intermediate" precursor was prepared as follows: The solution P was stirred for 48 hours at room temperature. A yellow solid was slowly formed principally during the first 8 hours. The solid was isolated by filtration and dried under vacuum at 45 ~ for 40 hours.
730 iii) Precursor AI: The precursor A was dissolved in water, the solvent was evaporated under vacuum and the solid dried at 80~ for 16 hours. iii) Precursor N : About 6.5 g of precursor A was mixed with about 50 ml of the fresh isobutanol and refluxed for 16 hours. It was observed that after the first two hours, the solid turned from yellow to pale blue and then its colour was stable during the rest of the procedure. The solvent was then evaporated under vacuum at 45 ~ and the solid was dried at 80 ~ for 16 hours.
2- Catalysts preparation: The catalytically active form was produced from the precursors N or S by in situ treatment, in a tubular reactor, according to the following procedure: 0.2 g of the precursor was introduced in the reactor. The t e m p e r a t u r e of the reactor containing the precursor was slowly increased (at about 6 ~ to about 390 ~ under a reactant mixture containing 0.6 vol.% of n-pentane and 5.0 vol. % of oxygen and balanced with He. Total pressure was the atmospheric. The total flow was 30 ml/min. The temperature was then kept constant for 8 hours. The catalysts produced are referred to as catalyst S or N, according to the parent precursor used.
3- C a t a l y t i c test: Conversion of n-pentane is defined as the number of moles of n-pentane converted by the number of moles of n-pentane feed to the reactor (in % tool). Selectivities to maleic and phthalic anhydrides are expressed as the fraction of moles of n-pentane converted into AM and PM (in % tool.) respectively. Conversion and selectivity were measured after the time of activation of the precursor as described above (8 hours). The specific a m o u n t of n-pentane converted was calculated as the total moles of n-pentane converted by second divided by the BET surface area and the weight of the catalyst. Specific amounts of n-pentane converted into MA or PA were calculated in similar forms. 4- C h a r a c t e r i z a t i o n m e t h o d s : i) The P and V content in the samples were determined by atomic absorption spectroscopy after dissolution of the catalysts in 0.1 M of nitric acid. ii) BET specific surface area m e a s u r e m e n t s were carried out in a Micromeritics Asap 2000 with nitrogen as the adsorption gas at liquid nitrogen temperature. iii) X-ray diffraction analyses were made with a high-resolution X-ray diffractometer in a Siemens D-500 using CuKa radiation. iv) Fourier Transform Infrared analysis were realized in a Bruker IFS 88 spectrometer at resolution of 4 cm -1 using a sample of about 100 mg (pellets, 12 mm in diameter) containing about 0.7 % of KBr. v) XPS analyses were performed on a Surface Science Instruments, SSX100 Model 206 ESCA spectrometer equipped with a flood gun. The excitation radiation was A1Ka (1486.6 eV). Atomic concentration ratios were calculated by correcting the intensity ratios with theoretical sensitivity factors proposed by the manufacturer: 8.33 for V2p, 1.295 for P2p and 2.492 for 01s. An external reference, SiO2 was used. The C ls peak at 284.8 eV was used as reference for the binding energies. Two fitting constraints were used for V2p: the area ratio between V2p3/2 and V2pl/2 equal to 2.0 and the difference of binding energies between V2pl/2 and V2p3/2 equal to 7.5 eV.
731 vi) Average oxidation state of vanadium analysis were realized by titration of the sample dissolved in 200 ml of a 2 M H 2 S 0 4 solution using a 0.1 N K M n 0 4 solution. RESULTS
Characterization i ) Chemical analysis: The P/Vbulk atomic ratio of precursor S and N was determined to be 1.39 and 1.26, respectively. ii) BET surface area : BET surface area of precursor S and N are 24 and 33 m2/g, respectively. The BET surface area of catalyst S decreased to 18 m 2 / g and for catalysts N increased to 44 m2/g. iii) XRD" -~ 40o0 "|
200.0
320.0
"~ 160.0 I s
240.0
120.0
150.0
80.0
$0.0
400
0.0 /0~
O0
/5
20
2'$
Jo
35
40 45 2# (#egrees/
Figure I XRD spectra of the precursor A and precursor A1 (obtained by recrystalization of precursor A from water). The spectrum of the pure VOPO4.2H20 is presented for comparison.
15
2O
25
3O
~J
40 n
45
b'~s)
Figure 2. XRD spectra of precursors S and N and the catalysts .S and N
Results are p r e s e n t e d in Figures 1 and 2. The precursor A shows principally reflections similar to the VOPO4.2H20 phase b u t with some distortions. Precursor A1 gives an exact fit with the pure V O P O 4 . 2 H 2 0 (Figure 1).
732 Precursor N presents only lines corresponding to the VOHPO4.1/2H20 phase and catalyst N only the lines characteristic of the (VO)2P207 phase (2). The presence of a small amount corresponding to an additional amorphous phase is also observed. No peaks corresponding to other crystalline phase were observed. The same observation can be made for precursor and catalyst S, but in this case the spectra are more diffuse, more distorted and the presence of the amorphous phase more important (Figure 2). Both the precursor and the used catalyst of the type N show a higher degree of crystallinity than precursor and catalyst S. The absence of the (010) reflection at the spectrum of the precursor S is clearly evidenced. iv) F T m " Results are presented in Figures 3 and 4. The precursor S and N and the used S and N catalysts display spectra characteristic of V O H P O 4 . 1 / 2 H 2 0 and (VO)2P207 phases respectively (9). However, the spectra of both the precursor and catalyst S were clearly more diffuse than those of the corresponding N precursor and catalysts.
,
..
i
CatolystN /
l IZ00
1000
800 GO0 400 wave ~ /cm'I/
Figure 3. FTIR spectrtun of the precursor S and N.
.
" ~[~
, 1200
1000
800 600 400 were ~ w " Icm'lJ
Figure 4. FTIR spectrum of the catalyst S and N.
733 v) XPS: The photoelectron spectra for catalysts S and N are presented in Figure 5. From this figure, it is observed t h a t the spectra for both catalysts are different. Two different binding energies are observed in catalysts N. The corresponding values are presented in Table 1. From this table it is also observed t h a t the P/V atomic ratio is lower for catalyst N.
] Catalyst S
J
529.1
~
525.1
L
I
1
521.1
" -
517.1
.
.
.
.
513.1
509.1
Binding energy (eVJ
Figure 5. XPS results. XPS surface atomic ratios and V2p3/2 and V2pl/2 lines of Catalysts S and N. Table 1. XPS surface atomic ratios and binding energies for catalysts S and N.
Catalyst S
C/Si 0.42
XPS atomic ratio V/Si P/Si 0.23 0.51
V/P 2.22
Binding energies (eV) V2p3/2 V2p1/2 517.5 525.0
N
0.42
0.31
1.63
517.0 517.3
0.51
524.5 524.8
vi) Average oxidation state of vanadium: The average oxidation states for catalyst S is 4.5 and for catalyst N is 4.1.
Catalytic test The results of the catalytic tests are presented in Figure 6. From this figure, the superiority of the catalyst prepared from precursor N compared to
734 catalyst S is clearly shown both in the n-pentane conversion and in the selectivity to maleic and phthalic anhydrides. The specific total amount of npentane converted and the specific amount of n-pentane converted into PA, compared to the amount converted into MA, are significantly higher for precursor N. ~" 8O r Q
Catalyst S
Q cotol , t N Q" 60
[2.11 1~\\\\"
i
,~0
r3.o;
Io.9;
20
n -pentane conversion t~/
Maleic selectivity l%1
Ohtalic selectivity l%1
Figure 6. Catalytic activity results. Conversion of n-pentane and selectivitiesto maleic and phthalic anhydrides. The values of specifictotal amount of n-pentane converted times 109 (mol/m2-s) and the specificamount of n-pentane converted into P A or M A times 10 9 (mol/m2.s) are shown in ( ) and in [ ] brackets, respectively. DISCUSSION
In the following we shall discuss the reasons which could explain the better performances of catalysts N with respect to S. i) Surface area of the catalysts The activation of the alkane molecule is generally regarded as the rate limiting step at the M A or P A formation. The detail mechanism of this process is stillunsolved. The abstraction of one (10) or simultaneous abstraction of two (11) or four (12) H-atoms from the terminal methyl groups have been suggested. A correlation of the alkane conversion with the specific surface area has been identified. The difference in the n-pentane conversion could be related to the higher specificsurface area of catalyst N compared with catalyst S. However the observed increase of total conversion (20% compared to 5 % for catalyst S) is higher compared to the surface area increase (44 against 18 m2/g for catalyst S). The increase in the surface area could explain the increase in the specificyield of M A (1.0 to 2.1 xl0 -9 mol/(m2.s)) but not the increase in the specificyield of P A (from about 0.15 to 0.9xl0-9mol/(m2-s) namely 6 times) that was obtained (Figure 5). Then superior B E T surface area of catalyst N does not explain its high performances.
735 ii) P/V atomic ratio and oxidation state of vanadium It was established that the bulk P/V ratio plays a significant role in the MA yield in butane oxidation (13). Some role due to the lower bulk P/V value observed in catalysts N (1.26 compared to 1.39 for catalyst S) could explain the difference in the performance of both catalysts. However, results obtained by varying the bulk P/V atomic ratio (14) do not show the same change in conversion and selectivity as in the present work, thus excluding this explanation. Meanwhile, the XPS surface atomic ratio is also lower for catalyst N (1.63 compared with 2.22 for catalyst S) this could provide one explanation for the observed change in the PA/MA molar ratio. Catalyst prepared via precursor N also shows lower average oxidation state and lower surface vanadium oxidation state than catalyst S. These observations could indicate that conversion of n-pentane and simultaneously the selectivity in PA is favoured by a low oxidation state of vanadium. iii)structural features Reason of the changes in the selectivitytowards P A could be related to the structural characteristics of the precursor and its topotactic transformation into the final catalyst (15). In fact, X R D and FTIR results show that, contrary to catalyst S, catalyst N displays spectra characteristic of a well cristallized (VO)2P207 phase with a minor formation of amorphous phase. Our results suggest that the preparation of catalysts N, using a higher isobutanol/water ratio, permits the intercalation of more isobutanol in the structure of the precursor N and more precisely in the structure of the "intermediate" precursor A, which could be associated to VOPO4.(2-n)H20 containing "n" molecules of intercalated isobutanol into the layer structure. This suggestion is supported by the X R D analysis. The distortion observed in the X R D analysis of precursor A could be interpreted as due to this partial intercalation, as has been reported (16). This interpretation is also supported by the fact that, when the intermediate precursor A1 is recristallized from water instead of undergoing the treatment leading to precursor N, gives a yellow solid which corresponds to pure V O P O 4 . 2 H 2 0 due to the elimination of intercalated isobutanol during the dissolution in water and cristallization. Our results correlate with observations from the literature which show that the reduction of hydrate V O P O 4 dispersed in the isobutanol provides a V P O with higher surface area and higher catalytic performance in the selective oxidation of butane to M A (7).However, it is necessary to emphasize that in such work the high M A formation was explained only by the high B E T area of the catalyst. In the case of n-pentane oxidation, even if the preparation via the precursor N promotes higher surface area catalysts, our results show that such preparation method favours the adequate solid state transformation of the precursor into the catalyst which is optimal for the formation of PA. It is not easy to identify directly the individual surface structure responsible for such effects, nevertheless the more pronounced amorphous character of both the precursor and the catalyst S, and in particular the interlayer disorder observed by both the X R D and IR spectra, suggests that a higher level of structural disorder could be better tolerated during the process of the conversion of the n-pentane into M A and less by the structurally more demanding process of P A formation.
736 CONCLUSIONS
The control of the stage of formation of the precursor A is promising for obtaining a selective catalyst for PA formation. The p r e p a r a t i o n of VOHPO4.1/2H20 via a full development of the VOPO4.2H20 phase, containing intercalated isobutanol, seems to favour the adequate structure of the precursor which promote the formation of PA. The present results and those presented in the literature show that, by careful control of the preparation of the VPO precursor, e.g.controlling the isobutanol/water ratio, the final catalyst could be tuned to the desired PA/MA ratio. Low XPS superficial P/V atomic ratio and low vanadium oxidation state probably also promote the formation of the PA. ACKNOWLEDGMENT The Service de Programmation de la Politique Scientifique (Belgium) is gratefully acknowledged for its Concerted Action grant, especially for the support of Ing. Z. Sobalik and Dr. P. Ruiz. The stay of Mrs. S. R. G. Carraz~in was supported by the Direccidn General de Investigacidn Cientffica y Tdcnica del Ministerio de Educacidn y Ciencia de Espafia (Programa FPU). REFERENCES I. 2.
3. 4. 5.
6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
G.Centi, J .T. Gleaves, G. Golinelli and F. Trifiro. Ill European Workshop Meeting. " N e w Developments in Selective Oxidation" (P.Ruiz and B.Delmon, Eds.) Studies in Surface Science and Catalysis.N ~ 72 (1992), 231 G. Centi, F. Trifiro,J.R. Ebner, and V.M. Franchetti, Chem. Rev., 88 (1988), 55-80. G. Centi, J.L. Nieto, D. Pinelli and F. Trifiro, Ind. Eng., Chem. Res. 28 (1989) 400. B.K. Hodnett, Catal. Rev.-Sci. Eng., 27 (3) (1985) 373. N. Guilhaume, M. Roullet, G. Pajonk, B. Grzybowska, and J.C. Volta. III European Workshop Meeting, "New Developments New Developments in Selective Oxidation by Heterogeneous Catalysis," Studies in Surface Science and Catalysis, Vol. 72, pp. 255-265, Elsevier, 1992. N.H. Batis and H. Bags, J. Chimie Physique, 90 (1993) 491. G.J. Hutchings, R. Olier, M.T. Sananes, and J.-C. Volta, II World Congress and IV European Workshop Meeting, New Developments in Selective Oxidation, C. Corberan and S.V. Bellon(Eds.), G.E.C. 1993; P.41-1. L.M. Cornaglia, C.A. Sanchez, and E.A. Lombardo, Appl. Catal. A: General, 95 (1993) 117. M. Lopez Granados, J.C. Conesa, M. Fernandez-Garcia, J. Catal. 141 (1993) 671. J.S. Buchanan and S. Sundaresan, Appl. Catal. 26 (1986), 211 G. Centi, F. Trifiro, G. Busca, J. R. Ebner and J. T. Gleaves. Proc., 9 th. Int. Cong. Catal., (M. J. Phillips, M. Ternan Eds.), Ottawa (1988),1538 J. Ziolkowski, E. Bordes and P. Courtine, J. Catal. 122 (1990), 126 F. Cavani, G. Centi and F. Trifiro, Appl. Catal., 15 (1985) 151. Text under preparation E. Bordes, P. Courtine and J.W. Johnson. J. Solid State Chem., 55(1984)270. L. Benes, J. Votinsky, J. Kalousova, and J. Klikorka, Inorg. Chim. Acta, 114 (1986) 47-50.
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
737
ROLE OF SEGREGATION PHENOMENA IN FORMATION OF ACTIVE SURFACE OF V-Sb-O CATALYSTS FOR SELECTIVE OXIDATION OF PROPYLENE TO ACROLEIN
M. Najbar
E.
Bielallska*
Department o f C h e m i s t r y and * R e g i o n a l L a b o r a t o r y o f P h y s i c o - C h e m i c a l Analyses and S t r u c t u r a l Research, 3 a g i e l l o n i a n U n i v e r s i t y , u l . I n g a r d e n a 3, 30 060 Krak.~.w, Poland
ABSTRACT The segregation phenomena in V - S b - 0 c a t a l y s t for s e l e c t i v e propylene oxidation to a c r o l e i n w e r e s t u d i e d by m e a n s of X-ray photoelectron spectroscopy, scanning electron microscopy with EDS, as w e l l as e l e c t r o n and X - r a y d i f f r a c t i o n . The vanadium antimonate c r y s t a l s w e r e f o u n d to be a m a i n c o m p o n e n t of the c a t a l y s t . It was s t a t e d t h a t e p i t a x i a l l a y e r s of a n t i m o n y t e t r o x i d e on the b a s e f a c e s of v a n a d i u m antimonate crystals, exposing (010) p l a n e , w e r e r e s p o n s i b l e for h i g h c a t a l y s t selectivity.
INTRODUCTION A biphasic V-Sb-0 catalyst, containing vanadium antimonate and a n t i m o n y t e t r o x i d e , is w e l l k n o w n as a s e l e c t i v e c a t a l y s t for p r o p y l e n e o x i d a t i o n to a c r o l e i n (i-4). It was f o u n d by B e r r y and B r e t t (2) t h a t c a t a l y s t containing ca. 11 t Sb 0 o b t a i n e d in o x i d i z i n g atmosphere-showed the h i g h e s t s e l e c t i v i t y . As n e i t h e r V S b 0 p h a s e nor Sb 0 one are s e l e c t i v e c a t a l y s t s for this r e a c t i n it is c o m m o n l t h o u g h t t h a t some i n t e r a c t i o n s b e t w e e n the p h a s e s are r e s p o n s i b l e for h i g h c a t a l y s t selectivity. T h e n a t u r e of the p h a s e i n t e r a c t i o n s and the r o l e of o x y g e n in t h e i r f o r m a t i o n w e r e the s u b j e c t of i n v e s t i g a t i o n s p r e s e n t e d in t h i s p a p e r . T h e c a t a l y s t containing ca. 10 t of a n t i m o n y t e t r o x i d e was o b t a i n e d by a n n e a l i n g the e q u i m o l a r m i x t u r e of v a n a d i u m pentoxide and a n t i m o n y t r i o x i d e . The
738
a n n e a l i n g was p e r f o r m e d in a m o u n t of air. The c h a n g e s s u r f a c e of c a t a l y s t c a u s e d
sealed ampoule containing small in the c h e m i c a l c o m p o s i t i o n of the by redox p r o c e s s e s w e r e i n v e s t i g a t e d .
EXPERIMENTAL Catalyst For the c a t a l y s t p r e p a r a t i o n REACHIM vanadium p e n t o x i d e , pure f o r a n a l y s i s , and POCH antimony t r i o x i d e , o f the same grade o f p u r i t y , were used. The grounded m i x t u r e o f vanadia and antimony t r i o x i d e i n q u a r t z ampoule sealed w i t h o u t any e v a c u a t i o n was placed i n a cool oven, then i t was heated s l o w l y up to 973 K f o r 7 h o u r s , annealed a t t h i s t e m p e r a t u r e f o r 30 hours and s l o w l y cooled down to room t e m p e r a t u r e f o r 10 h o u r s . One p a r t o f b l a c k powder o b t a i n e d by t h i s p r o c e d u r e was x i d i z e d at 673 K i n a i r atmosphere f o r 30 hours a n o t h e r onewas reduced -d at the same t e m p e r a t u r e i n vacuum (10 T o r t ) f o r 4 hours. Methods E l e c t r o n scanning microscope PHILIPS XL 30 equipped w i t h EDS system LINK-ISIS was u s e d f o r the d e t e r m i n a t i o n o f the c a t a l y s t morphology and chemical c o m p o s i t i o n . DRON - 3 X - r a y d i f f r a c t o meter w i t h Cu K r a d i a t i o n f i l t e r e d by n i c k e l ) and t r a n s m i s s i o n e l e c t r o n microscope PHILIPS EM-301 were used f o r the phase a n a l y s i s . The s u r f a c e chemical c o m p o s i t i o n was s t u d i e d by means of XPS equipment manufactured by VSW S c i e n t i f i c I n s t r u m e n t s L t d .
RESULTS
AND
DISCUSSION
In Figs i - 3 secondary e l e c t r o n images (SEI) o f the powdered samples o f antimony t r i o x i d e , vanadium p e n t o x i d e and f r e s h l y prepared c a t a l y s t are p r e s e n t e d . As can be n o t i c e d the c a t a l y s t contains mainly small r e c t a n g u l a r c r y s t a l s but the b i g g e r elongated c r y s t a l s are a l s o p r e s e n t i n i t (Fig l c ) . Similar l a r g e e l o n g a t e d c r y s t a l s are observed a l s o i n powdered p r e p a r a t i o n o f antimony t r i o x i d e . I t suggests t h a t e l o n g a t e d c r y s t a l s i n c a t a l y s t o r i g i n a t e from antimony t r i o x i d e ones. In F i g . 4 c h a r a c t e r i s t i c X - r a y s p e c t r a r e c o r d e d from areas of s m a l l (b) and l a r g e (c) c r y s t a l s , marked i n m i c r o g r a p h ( a ) , are shown. I t i s easy to n o t i c e t h a t s m a l l c r y s t a l s c o n t a i n comparable amount o f vanadium and a n t i m o n y , w h i l e the b i g elongated c r y s t a l s c o n t a i n antimony w i t h s m a l l a d d i t i o n o f
739
Figure
1.
SEI
of
Sb
0
.
9
Figure
2.
SEI
of
V a.
Figure
3.
SEI
0
. .a
of c a t a l y s t .
740
Q.
Sb
b~
C.
Sb
V
Sb Sb V
E
Figure
4. a. SEI of c a t a l y s t . b. X-ray s p e c t r u m f r o m c. X-ray s p e c t r u m f r o m
small large
crystals. crystal.
741
vanadium. In Fig. 5 X-ray d i f f r a c t i o n p a t t e r n s (XRDPs) of the used a n t i m o n y t r i o x i d e (a), of a n t i m o n y t r i o x i d e s l o w l y h e a t e d f r o m room t e m p e r a t u r e to 973 K for 5 h o u r s (b), and of the f r e s h l y p r e p a r e d c a t a l y s t are p r e s e n t e d . As it is seen, s l o w h e a t i n g of of a n t i m o n y t r i o x i d e f r o m r o o m t e m p e r a t u r e to 973 K c a u s e s o x i d a t i v e t r a n s f o r m a t i o n of the a n t i m o n y t r i o x i d e i n t o q-antimony t e t r o x i d e c o n t a i n i n g t r a c e s of ~ - a n t i m o n y t e t r o x i d e . N h i l e , the s i m i l a r h e a t i n g of the m i x t u r e of v a n a d i a and a n t i m o n y t r i o x i d e f o l l o w e d by its a n n e a l i n g at 9 7 3 K r e s u l t s in f o r m a t i o n of the m i x t u r e of c r y s t a l s of v a n a d i u m a n t i m o n a t e , q-antimony t e t r o x i d e , and ~ - a n t i m o n y tetroxide. Vanadium a n t i m o n a t e is the base p h a s e of t h i s m i x t u r e . T h e r e are not l i n e s of the s t a r t i n g o x i d e s in X R D P of the c a t a l y s t . C o m p a r i n g the i n f o r m a t i o n o b t a i n e d f r o m the r e s u l t s p r e s e n t e d in F i g u r e s i-5 one can s t a t e t h a t the c a t a l y s t is c o m p o s e d of small c r y s t a l s of v a n a d i u m a n t i m o n a t e and l a r g e e l o n g a t e d c r y s t a l s c o n t a i n i n g ~ a n d / o r ~ p o l y m o r p h s of a n t i m o n y t e t r o x i d e . The o x i d a t i v e t r a n s f o r m a t i o n of the a n t i m o n y t r i o x i d e into a n t i m o n y t e t r o x i d e d u r i n g the s l o w h e a t i n g o c c u r s due to easy ion d i f f u s i o n in l o o s e l y p a c k e d layer s t r u c t u r e of a n t i m o n y t r i o x i d e (5). The p r e s e n c e of v a n a d i u m ions in h e a t e d s y s t e m f a v o u r s the f o r m a t i o n of ~ a n t i m o n y t e t r o x i d e (Fig. 5c), w h i c h p r a c t i c a l l y does not f o r m e d if pure Sb 0 is 2 g is h e a t e d (Fig 5b). The t r a n s f o r m a t i o n of pure ~ a n t i m o n y t e t r o x i d e into ~ p o l y m o r p h is k n o w n to o c c u r at t e m p e r a t u r e s a b o v e 1373 K (6). The l o w e r i n g of the t r a n s f o r m a t i o n temperature was s h o w n by B e r r y and B r e t t to be a r e s u l t of the d i s s o l u t i o n of p e n t a v a l e n t v a n a d i u m ions in the l a t t i c e of a n t i m o n y t e t r o x i d e (3). The p r e s e n c e of the v a n a d i u m in the l a t t i c e of a n t i m o n y t e t r o x i d e c r y s t a l s is d i s t i n c t l y v i s i b l e in characteristic X-ray s p e c t r u m r e c o r d e d f r o m e l o n g a t e d l a r g e c r y s t a l s of c a t a l y s t (Fig. 4c). T h e s o l i d s o l u t i o n of p e n t a v a l e n t v a n a d i u m ions in l a r g e a n t i m o n y t e t r o x i d e c r y s t a l s is f o r m e d due to d i f f u s i o n of v a n a d i u m ions f r o m the s m a l l n e i g h b o u r i n g v a n a d i a c r y s t a l s . T h e large d i m e n s i o n s of c r y s t a l s w i t h r e l a t i v e l y l o o s e l y p a c k e d l a t t i c e did not f a v o u r the f o r m a t i o n of the o v e r s a t u r a t e d s o l i d s o l u t i o n in w h i c h n u c l e a t i o n of the v a n a d i u m a n t i m o n a t e p h a s e c o u l d s t a r t . The g r a d i e n t of the c o n c e n t r a t i o n of v a n a d i u m ions b e t w e e n t h e s u r f a c e and c e n t r a l p a r t s of big c r y s t a l s may s t i l l exist. It may r e s u l t in the f o r m a t i o n of ~ p h a s e in o u t e r and ~ ones in i n n e r p a r t s of these c r y s t a l s . The p r e s e n c e of a n t i m o n y tetroxide crystals with simultaneous lack of v a n a d i u m o x i d e
742
ao
T A
eft
..
t
..t
,t
,.[ bO
oooo o~ Cq
TzO Figure
5.
3'0 XRDPs
c/ ~
of:
a/
catalyst. Sb
0 2
~ Sb
0
,
b/
5b Sb
~/S~O-0
-Q.
4
0
after
, ;~b30-& 2
28
heating,
9~
Sb
0
g
-0
,
24
4
f l ~ , ,==
b Figure
6.
a.
Transmission
b.
SADP
from
elektron bigger
VSbO
micrograph crystal
of -
VSbO
zone
crystals
ax~zs
[110].
743 p h a s e s in c a t a l y s t s h o w s t h a t v a n a d i u m antimonate nonstoichiometric w i t h r e s p e c t to a n t i m o n y .
is
If t a k e a f t e r B i r c h a l l and S l e i g h t (7) the o x i d a t i o n s t a t e s for a n t i m o n y and v a n a d i u m f i v e and t h r e e , respectively, the f o r m u l a for non s t o i c h i o m e t r i c vanadium antimonate c o u l d be w r i t t e n as VSb . . . . 0 . Nonstoichiometric vanadium antimonate p h a ~ e ' w i h- h • e x c e s s of v a n a d i u m may n u c l e a t e and g r o w f r o m the e n v i r o n m e n t h a v i n g the e n h a n c e d c o n c e n t r a t i o n of vanadium. It is, thus, r e a s o n a b l e to t h i n k t h a t m a i n l y V 0 2 5 k r y s t a l s are the e n v i r o n m e n t of v a n a d i u m antimonate phase nucleation and g r o w t h . The c o e x i s t e n c e of the s e p a r a t e c r y s t a l s of the s o l i d s o l u t i o n of a n t i m o n y tetroxide and of nonstoichiometric vanadium antimonate s h o u l d not c a u s e the enhancement of c a t a l y s t s e l e c t i v i t y . The small vanadium antimonate c r y s t a l s m a i n l y f o r m the s u r f a c e of c a t a l y s t and they s h o u l d be c o n s i d e r e d as r e s p o n s i b l e for its p r o p e r t i e s . The c l o s e s t p a c k e d (ii0) p l a n e s of v a n a d i u m antimonate lattice f o r m t h e b e s t d e v e l o p e d f a c e s of v a n a d i u m antimonate crystals. The s e l e c t e d area diffraction pattern (SADP) w i t h Ell03 z o n e axis t a k e n f r o m v a n a d i u m a n t i m o n a t e crystal is p r e s e n t e d in Fig. 6. T h e h i g h c a t a l y s t s e l e c t i v i t y of the o x i d i z e d c a t a l y s t s h o u l d be m a i n l y c o n n e c t e d w i t h e v o l u t i o n of (II0) s u r f a c e of the v a n a d i u m antimonate crystals occurring in o x i d i z i n g atmosphere. XPS w a s u s e d for the i n v e s t i g a t i o n of t h i s e v o l u t i o n . T h e surface composition of the f r e s h l y p r e p a r e d o x i d i z e d and r e d u c e d c a t a l y s t was i n v e s t i g a t e d . T h e r e s u l t s , p r e s e n t e d as
r a t i o s of i n t e n s i t i e s o f Sb 3 ds/2 and V 2p lines (l~5/Iv) and r a t i o s o f the numbers o f the" a n t i m o n y ~n~ vanadium atoms (N /N ) , are shown i n Table I . $5 v Table
I
The r a t i o s of the peak i n t e n s i t i e s , I Sb 3d x / I V 2p9/2 , and of the n u m b e r of a t o m s , N s b / N V ( c a l c u l a t e d ~"rom p h o t o e l e c t r o n spectra) for f r e s h l y p r e p a r e d , oxidized and r e d u c e d c a t a l y s t s . I
F. Ox. Red.
~k
/
I
x7
N
/
7372/1583.
2.8
( 3 4 7 8 + 3 8 9 7 ) /1078
4.0
6838/21ii
1.9
N
744
The
large
catalyst w i t h the
broadening
of
w a s the r e a s o n widths similar
the
Sb
of to
its the
3d l i n e for the 5/2 deconvolution into w i d t h of t h i s p e a k
p r e p a r e d and r e d u c e d c a t a l y s t s . T h e s u m both p e a k s was u s e d for the c a l c u l a t i o n is in
peaks the f r e s h
the i n t e n s i t i e s of N /N r a t i o . As it v s e e n , the r a t i o of the n u m b e r of a n t i m o n y "~b and v a n a d i u m atoms the s u r f a c e l a y e r of the c r y s t a l s of f r e s h l y p r e p a r e d
catalyst
is
2.8
times
higher
than
this
of of
in
stoichiometric
v a n a d i u m a n t i m o n a t e . T h i s s u g g e s t s t h a t the a n t i m o n a t e c r y s t a l s is s t r o n g l y e n r i c h e d in enrichment The
oxidized two for
might
increase
rise
antimonate
on
the
during
the
preparation
procedure.
N /N r a t i o up to the v a l u e 4 d u r i n g .sb v the o x i d a t i o n and i t s d e c r e a s e to the v a l u e 1.9 d u r i n g the r e d u c t i o n s h o w s t h a t c a t i o n s e g r e g a t i o n w i t h the d i r e c t i o n
depending
of
s u r f a c e of v a n a d i u m antimony. This
redox
grains.
of
potential Similar
of
gas
cation
phase
occurs
segregation
was
in
vanadium
observed
in V-Mo-0 ( 8 - 1 6 ) and V-N-0 systems ( 1 7 ) . The a n n e a l i n g o f t h e vanadium a n t i m o n a t e c r y s t a l s i n a i r atmosphere w i t h oxygen o f p a r t i a l p r e s s u r e h i g h e r than e q u i i i b r i u m oxygen p r e s s u r e causes t h e i r o x i d a t i o n . I n o x i d a t i o n process a d s o r p t i o n o f oxygen a t the s u r f a c e i s f o l l o w e d by i t s d i f f u s i o n t o w a r d the bulk o r / a n d c a t i o n d i f f u s i o n from the b u l k toward t h e s u r f a c e . (5Decreasing o f the oxygen p r e s s u r e down to I0 T o r r causes drop o f oxygen p r e s s u r e beIow e q u i l i b r i u m v a l u e t h a t r e s u l t s i n the l o s t o f oxygen from s u r f a c e I a y e r s . The a p p e a r i n g s u r f a c e d e f i c i e n c y o f oxygen i s e q u a l i z e d by i t s d i f f u s i o n from the bulk or by c a t i o n d i f f u s i o n from the s u r f a c e toward the b u l k . I n c r e a s e o f the r a t i o o f a n t i m o n y to vanadium i n s u r f a c e i a y e r s o f vanadium a n t i m o n a t e c r y s t a l s d u r i n g o x i d a t i o n and i t s decrease d u r i n g the r e d u c t i o n shows t h a t c a t i o n d i f f u s i o n takes p a r t i n the e q u a l i z a t i o n o f the g r a d i e n t s oxygen c o n c e n t r a t i o n i n vanadium a n t i m o n a t e c r y s t a l s and t h a t the r a t e o f d i f f u s i o n o f antimony i o n s i s h i g h e r than o f vanadium ones. Thus, the r a t e o f d i f f u s i o n i n w e l l packed r u t i l e s t r u c t u r e seems t o be d e t e r m i n e d by the c a t i o n r a d i u s , s m a l l e r f o r SbS, ( 0 . 5 9 o A ) than 3, o for V ( 0 . 7 3 A) i o n s . The f o u r t i m e s h i g h e r a n t i m o n y c o n c e n t r a t i o n t h a n o f vanadium i n s u r f a c e l a y e r s o f the o x i d i z e d c a t a l y s t g r a i n s
suggests the f o r m a t i o n o f a V-Sb-0 phase w i t h high a n t i m o n y c o n t e n t . The s o l i d s o l u t i o n s o f vanadium i n a n t i m o n y o x i d e s could be the phases o f t h i s k i n d . The e x i s t e n c e o f two peaks o f antimony o f s i m i l a r i n t e n s i t y , in p h o t o e l e c t r o n spectrum of o x i d i z e d c a t a l y s t (Table I ) , shows the presence o f t h e a n t i m o n y oxide c o n t a i n i n g a n t i m o n y i o n s i n two d i f f e r e n t o x i d a t i o n s t a t e
745
w i t h the s i m i l a r c o n c e n t r a t i o n . T h u s , it can be c l a i m e d t h a t oxidation of v a n a d i u m a n t i m o n a t e crystals l e a d s to the formation of the l a y e r s of the s o l i d s o l u t i o n of v a n a d i u m in a n t i m o n y t e t r o x i d e on t h e i r s u r f a c e . T h e c o m p l e t e r e s u l t s of the XPS i n v e s t i g a t i o n s w i l l be d i s c u s s e d elsewhere (18). The g r o w i n g of the a n t i m o n y t e t r o x i d e l a y e r s in the e n v i r o n m e n t of v a n a d i u m antimonate phase ensures the maximal concentration of V 5+ i o n s in Sb 0 t attice. The ~ antimony t e t r o x i d e is, d thus, e x p e c t e d t o = b e p r e s e n t on the s u r f a c e of o x i d i z e d vanadium antimonate crystals T h e h i g h v a l u e of N / N ratio s u g g e s t s t h a t the l a y e r s of V 5+ /Sb 0 c o v e r m o s t So~ vaVnadium antimonate c r y s t a l s . The s i m i l a r i t ~ f the the s t r u c t u r e of both p h a s e s s u g g e s t s e x i s t e n c e of the e p i t a x i a t relations between them. Considerations of the c r y s t a l l o g r a p h i c fit b e t w e e n the m o s t d e v e l o p e d (110) f a c e of v a n a d i u m antimonate and d i f f e r e n t p l a n e s of ~ Sb 0 l e a d to the c o n c l u s i o n that epitaxial l a y e r s of ~ Sb 0 ~ i ~ h (0k0) l a y e r s p a r a l l e l to (110) VSb 0 can be f o r m ~ d 4 on t h e m . T h e d i s t r i b u t i o n of the ~-x 4 - 2 . 5Y. antimony cations on ( 0 1 0 ) Sb 0 face is demonstrated in Fig. 7.
~
2
4
f--'\
0
\_../
0
,0 t'-'~ k /
Figure
7.
Distribution Sb 0 (OkO)
of
o+ Sb" ( ~ ) ,
~+ Sb ~ ( O ) ,
and
02-
~,~)
ions
on
plane.
The p o s i t i o n s of the o x y g e n i o n s w i t h the c e n t e r s of s y m m e t r y o l o c a t e d 0 . 5 8 A b e l o w this p l a n e are a l s o m a r k e d in the f i g u r e . Due to l a r g e r d i m e n s i o n of t h e s e i o n s t h e y a c h i e v e the s a m e l e v e l a b o v e (010) s u r f a c e as Sb g+ i o n s b e l o n g i n g to t h i s s u r f a c e . To d i s c u s s the s e l e c t i v e acrolein oxidation over Sb 0 epitaxial l a y e r s it s h o u l d be r e c a l l e d t h a t the formation the a l l i l r a d i c a l s on the c a t a l y s t s u r f a c e is
746
commonly
considered
trivalent ions
of
cations
higher
disturbing
in
connected
valency
c a t i o n s on the c o n d i t i o n s for the s u r f a c e of favours
as
(19),
while (20).
with
the The
the
oxygen
presence
of
insertion
presence
of
both
the
with kinds
the of
(010) Sb 0 plane c r e a t e s , thus, f a v o r a b l e z 4 selective propylene oxidation. The presence the o x y g e n i o n s a c c e s s i b l e for r e a g e n t s b u t the
selective
reagents
interactions
propylene
oxidation.
with Some
metal
ions
misfits
on
on not
also the
p h a s e b o u n d a r y s h o u l d r e s u l t in i n c r e a s e of the l i a b i l i t y of the o x y g e n i o n s , t h a t m a k e s e a s i e r the i n s e r t i o n p r o c e s s .
REFERENCES 1. 2.
Brit. Pat. 13336136. F. B e r r y a n d M. B r e t t ,
3.
F. B e r r y , M. B r e t t Trans., (1983) 9.
4.
3.
Catal.,
W.
Patterson,
3.
Chem.
Soc.
Dalton
Patterson,
3.
Chem.
Soc.
Dalton
5.
F. B e r r y , M. B r e t t a n d W. Trans., (1983) 13. C. S v e n s s o n , Acta Cryst.,
830
458.
6. 7.
D. T.
Proc. Chem. S o c . , (1964) 400. Inorg. C h e m . , 15 ( 1 9 7 6 ) 868.
Rogers Birchall
and
a n d A. S k ~ p s k i , a n d A. S l e i g h t ,
88
(1974)
(1984)
232.
8 . M. N a j b a r , S. N i z i o ! , 3.Solid State C h e m . , 26 ( 1 9 7 8 ) 339. 9 . A. B i e l a f i s k i , 3 . C a m r a , M. N a j b a r 3. Catal., 57 ( 1 9 7 9 ) 326. 1 0 . E. B i e l a f i s k a , J. ~agan, M. N a j b a r 2nd Intern. Conf. Applied Electron Microscopy, Zakopane, 1978. 1 1 . M. N a j b a r , E. B i e l a f i s k a , Proc. 9 t h Syrup. S o l i d State Reactivity, Krak~w, 1980, p. 465. 1 2 . M. N a j b a r , K. S t a d n i c k a , 3.Chem. Soc., Faraday Trans. 79 (1983) 27. 1 3 . M. N a j b a r , Proco 5th Intern. Syrup. H e t e r o g e n e o u s Catalysis, Varna, 1983 part II, p. 340. 1 4 . M. N a j b a r , Proc. 8th Intern. Congress on C a t a l y s i s , 15.
M.
16.
M. on
West Berlin Najbar, 3.
5 (1984) 323. Chem S o c . , Faraday
Najbar, 3. Camra, CatAlysis Calgary,
17.
M. N a j b a r
18.
M. N a j b a r ,
19.
W i t k o w s k i t o be p u b l i s h e d . B. Grzybowska, 3 . Haber and 3.
20.
150. T. S e i j a m a , Catal.,
and 3.
Proc. 9th 1988.
E.
24
M.
Trans.82 Intern.
Camra ,
to
be p u b l i s h e d .
Bielafiska,
S.
Nizio!,
Egashira,
(1972)
76.
T.
R.
3anas,
Sakamoto
(1986)
1673.
Congress
Dziembaj, J.
and
Catal. I.
Aso,
S. 49 3.
(1977)
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
747
Preparation, physicochemical characterization and catalytic properties v a n a d i u m - d o p e d alumina- and titania-pillaredmontmorillonites
of
K. Bahranowskia, R.Dula b, J. Komorek b , T. Romotowski b and E. M. Serwicka b a Faculty of Geology, Geophysics and Environmental Protection, Academy of Mining and Metallurgy, al. Mickiewicza 30, 30-059 Krakow, Poland b Institute of Catalysis and Surface Chemistry, ul. Niezapominajek 1, 30-239 Krakow, Poland
Polish
Academy
of
Sciences,
SUMMARY
Procedures leading to preparation of vanadium-doped alumina-and/or titania-pillared montmorillonites are described and physicochemical characterization (chemical analysis, XRD, BET, ESR) of the products is provided. Results show that introduction of vanadium into the pillared montmorillonites leads to a rigid association of the dopant with pillars, irrespective of the method of preparation. The mode of vanadyl attachment in aluminapillared samples does not depend on the mode of preparation, while in titania-pillared montmorillonite it does. Certain degree of delocalization of the unpaired electron into ligands and increased in-plane re-covalent bonding is observed for vanadyl ions present in the co-pillared (V-Ti)-PILC samples which also show particularly high activity in catalytic ammoxidation of m-xylene to nitrile product, as monitored by IR. A hypothesis is advanced that this effect is due to the unique character of vanadyl species present in these catalysts. 1. INTRODUCTION Pillared clays (PILC) are characterized by high surface area, accessibility of the interlayer space to gases and vapours, and significant concentration of acid centres [ 1]. These qualities have led to numerous attempts at application of PILC materials in catalysis, mainly of acid-base type. Potential application in catalytic processes of redox nature would require PILC structure to accommodate transition metal ions known to change easily their oxidation state. Among transition metal elements of interest only few form oligocationic species suitable for pillaring. Unfortunately, vanadium, the most popular component of the redox catalysts, exists in aqueous medium mainly as polyanionic species and therefore the standard ion exchange procedure used for PILC preparation cannot be applied. However, industrial vanadia catalysts are usually deposited on oxidic supports [2-4]. Luckily, the materials most frequently used in this capacity, i.e. alumina, silica, titania and zirconia, are also known to form stable PILC structures. In view of this, the intention of the present authors was to design a new class of supported vanadia catalysts where alumina and/or titania pillars would serve as support particles. One may expect such systems to display unique properties since small dimensions of individual pillars offer surface-to-bulk
748 ratio unattainable with conventional supports, while their separation and firm bonding to the silicate layers should improve resistance to sintering. The montmorillonite-transition metal oxide systems may also combine both the acid-base and the redox catalytic functions, a necessary requirement in many catalytic processes. This paper describes preparation, physicochemical characterization and catalytic properties of a series of vanadium-doped alumina- and titania-pillared montmorillonites obtained by various methods. The aim of this work was to investigate the influence of the preparation procedure and pretreatment on location of vanadium dopant within the PILC structure and to correlate the physicochemical characteristics of the samples with their catalytic activity in ammoxidation of m-xylene. 2. EXPERIMENTAL
Materials and syntheses Starting material The montmorillonite used in this study was the less than 2~tm particle-size fraction of bentonite from Milowice, Poland. The cation exchange capacity (CEC) of the clay is 76 meq per 100 g. The clay was subjected to exchange with Na + ions by stirring in 1 N NaCI solution for 24 h followed by repeated washing with 1 N NaCI. The resulting suspension was washed several times with distilled water until free of CI- ions as indicated by lack of reaction of the supernatant with silver nitrate solution. The solid separated by centrifugation was dried in air at 353 K. This material is henceforth referred to as Na-mt.
Alumina-pillared montmorillonite Alumina-pillared clay was prepared according to the procedure described by Vaughan [5]. Commercial aluminium chlorhydroxide known as Chlorhydrol (Reheis Chemical Company) was added with vigorous stirring to a water suspension containing about 10 g1-1 of Na-mt, in an amount corresponding to 0.17 g AI 3+ per gram of clay. After thoroughly homogenizing the slurry (about 30 min), the pH was adjusted to 2.0 with dilute hydrochloric acid, the mixture allowed to age for 30 min at 343 K, centrifuged, washed free of C1- and dried in air at 353 K. This product is referred to as Al-mt. Part of the material was calcined at 673 K for 20 h and designated AI-PILC.
Titania-pillared montmorillonite Titania-pillared clay was prepared according to the procedure proposed by Sterte [6]. The pillaring agent was obtained by adding TiCI4 (Fluka, pract.) to 6 M HC1. This mixture was then diluted to reach a final titanium concentration of 0.82 M. The amount of HCI solution used corresponded to a final concentration of 0.11 M. The solution was allowed to age for 3 h prior to use. The pillaring agent was added dropwise to a vigorously stirred water suspension containing 4 g1-1 of clay, in an amount corresponding to 10 mmol Ti per gram of montmorillonite. The resulting product was stirred for 3 h at room temperature, washed with distilled water till supernatant was free of C1- and dried in air at 353 K. This product is referred to as Ti-mt, and after calcination at 673 K for 3 h as Ti-PILC.
749
V-alumina-pillared montmorillonites Cationic exchange of A1-PILC and/or Al-mt with vanadyl VO 2+ ions was used to introduce vanadium. (a) AI-PILC was treated with VOSO4 (Merck, pure) solution (0.2 - 1.0 N), centrifuged and washed with distilled water till the supernatant was free of sulphate ions as indicated by lack of reaction with barium chloride. Samples of different V content, referred further to as V-(AI-PILC)-IA and V-(AI-PILC)-IB, were obtained this way. (b) In order to increase the vanadium content the V-(AI-PILC)-IA sample was subjected to a repeated exchange with 0.2 N VOSO4 solution. Samples V-(A1-PILC)-II and V-(AI'-PILC)-III were obtained this way. (c) Al-mt was treated with 0.02 N VOSO4 solution, centrifuged, washed with distilled water till the supernatant was free of sulphate ions, dried at 353 K and subjected to calcination in air at 673 K for 20 h. This product was further referred to as V(A1)-PILC.
V-titania-pillared montmorillonites Three different procedures were used to introduce vanadium into titania-pillared clays. Two of them consisted, as in the case of Al-pillared samples, in cationic exchange of Ti-PILC and/or Ti-mt with vanadyl ions while one, referred to as "co-pillaring", employed a VO2+-containing pillaring solution. (a) Ti-PILC was treated with a 0.002-0.2 N VOSO4 solution, centrifuged and washed with distilled water till the supernatant was free of sulphate ions. Samples V-(Ti-PILC)-IA and V-(Ti PILC)-IB and V(Ti-PILC)-IC were obtained this way. (b) Ti-mt was treated with 0.5 N VOSO4 solution, centrifuged, washed with distilled water till the supernatant was free of sulphate ions, dried at 353 K and calcined in air at 673 K for 3 h. This product is further referred to as V(Ti)-PILC. (c) A pillaring agent containing titanium and vanadyl ions was obtained by adding TiCI 4 to 6 M HCI containing dissolved VOSO4. After dilution with water the final concentrations were 0.82 M Ti, 0.082 M VO 2+ and 0.1-0.4 M HCI. Further treatment followed the procedure of Ti-PILC preparation. Samples (V-Ti)-PILC-A, (V-Ti)-PILC-B and (V-Ti)-PILC-C were obtained this way.
Physicochemical characterization X-ray diffraction X-ray diffraction (XRD) analyses were performed on oriented samples prepared on a glass slide. The XRD patterns were obtained with a DRON-3.0 diffractometer using Ni-filtered CuKcz radiation.
BET measurement The BET surface area of the samples was determined from argon adsorption at 77 K, after outgassing at 473 K for 2 h.
Chemical analysis Chemical analysis was carried out on an ICP-AES Plasma 40 Perkin-Elmer spectrometer.
750
ESR measurement The ESR spectra were recorded at room temperature and at 77 K with an X-band SE/X (Technical University Wroclaw) spectrometer. DPPH sample and NMR field marker were used for determination of g factors. Catalysis Catalytic ammoxidation of m-xylene was studied with aid of IR spectroscopy. The samples were pressed into self-supported plates of 5 mg cm -2 thickness and placed in a high temperature vacuum IR cell connected to the vacuum line. Prior to catalytic experiments the samples were activated at 473 K under dynamic vacuum of 1.33x10 -3 Pa. Then the mixture of m-xylene (133-670 Pa) with NH 3 (670-930 Pa) and air (3330-4000 Pa) was introduced into the cell and heated for 1 h at 573 K. Appearance in the IR spectrum of the nitrile group band at ~2240 cm -1 was taken as indicative of catalytic transformation. The IR spectra of adsorbed species were recorded at room temperature on a UR-20 double-beam spectrophotometer (Zeiss, Jena). 3. R E S U L T S A N D D I S C U S S I O N
XRD patterns and surface area measurements provide the basic test of the efficiency of the pillaring process (Table 1). The changes observed in the d001 basal spacing and the increase in the surface area confirm that pillaring procedures were effective. Table 1 Basal spacing d001, specific surface S and vanadium content of the investigated clay samples
Catalyst
A~-(PILC) V-(A1-PILC)-IA V-(A1-PILC)-IB V-(A1-PILC)-II V-(AI-PILC)-III V(AI)-PILC Ti-PILC V-(Ti-PILC)-IA
V-(Ti-PILC)-IB V-(Ti-PILC)-IC V(Ti)-PILC (V-Ti)-PILC-A (V-Ti)-PILC-B (V-Ti)-PILC-C
doo 1 (A)
18.4 177 184 164 16.4 184 26 0 26.0 26.0 26.0 25.0 25.0 25.9 25.0
S (mEg-1)
320.7 2299 272.0 161 7 183.4 160.5 345.6 314.2 381.2 315.0 362.4 285 7 225.3 285.9
VEO 5 (wt%)
0.59 0.90 1.16 1.36
1.21 0.96 2.23 2.43 2.29 0.06 0.10 0.13
751 Chemical analysis shows that the amount of introduced vanadium depends on the type of pillared matrix and choice of preparative conditions. In alumina-pillared samples the maximum V content corresponds to ca. 50% of the original CEC of the clay. This means that protons released on calcination of oligocationic Al species are, at least partly, available for further exchange. Moreover, the data for samples V-(AI-PILC)-I, V-(AI-PILC)-II and V-(A1- PILC)-III show that increase of V content can be achieved by repeated calcinationexchange cycles. It is interesting to note that also the V(A1)-PILC sample obtained by treatment of uncalcined Al-mt with vanadyl sulphate solution absorbs significant amount of vanadium. Elemental analysis shows that A1 oligocations remain in the sample, thus the exchange probably involves protons released on dissociation of water molecules coordinated to Al oligocations. Addition of vanadium causes a decrease in the specific surface area indicating that the dopant facilitates sintering processes. In the case of titania-pillared samples the efficiency of doping depends on the applied preparative procedure. The cationic exchange of vanadyl ions with Ti-PILC and/or Ti-mt matrices results in a relatively high level of doping. The maximum amount of introduced vanadium corresponds to ca. 90% of the original CEC of the parent clay. On the other hand, the procedure of"co-pillaring" produces samples of low vanadium content although, if the V:Ti ratio of the pillaring solution were maintained, one would expect the V content to be at least an order of magnitude higher. This may be due to the fact that in the highly acidic pillaring solution protons compete successfully with the vanadyl ions in the cationic exchange processes. ESR data presented in detail elsewhere [7] show that all vanadium-doped aluminaand/or titania-pillared montmorillonite samples contain immobilized vanadyl ions bound to the pillars. b) Q) g, = 1.9/,0 g, = 1.9/.2
I |
'
1006 1 'I
I
I
....
I
........
I I
,
' ....
g~=1.903 g.L=1.984
_ i
A~=69.1G
i
I I
A. :68.7G LJ
I
~
I I
l
=j
9
g. = 1.987
!
A.=68.0
I
I
V(Ti-PILC)
100 6
I
I
1
IA." 187.2G
(V-Ti)-PILC
I V(At)-PILC I I
A,=190 9G !
g. = I.939
I
tA.-rn.5a t
J
I
!
!
1
1
9
J
I
g II = 1.9]7
Figure 1. ESR spectra at 77 K of vanadium-doped (a) alumina- and (b) titania-pillared samples. Vertical lines are drawn to facilitate comparison of parallel features of the spectra.
752 Fig. l a shows typical ESR spectra of alumina-pillared samples obtained by different methods. Differences in the ESR parameters of vanadyl species introduced by exchange with unealcined or calcined pillars are very small indicating that similar vanadium species are formed irrespective of the preparative procedure. Also in the case of titania-pillared montmorillonite the ESR parameters of vanadyl ions deposited onto uncalcined (V(Ti)-PILC) and calcined (V-(Ti-PILC)) pillars are similar. In the co-pillared (V-Ti)-PILC sample, however, a vanadyl spectrum with different parameters is observed (e.g. smaller value of /~i, Fig. l b) pointing to a different character of Vanadyl-pillar bonding in this case. Analysis of the ESR parameters [7] shows that in the co-pillared samples the unpaired electron is partially delocalized into ligands and the vanadyl species posses increased inplane ~-covalent bonding. The results of IR experiments with ammoxidation of m-xylene to give nitrile product evidenced by the appearance of the characteristic CN band at 2240 cm -1 are presented in Figs 2 and 3.
-
r-
3
-
V-(AI-PILC)-IB
,~
V-(A|-PILC)~_ _"/___ ____.7 - / ' - " ~ \ x /I I "1t
//
./
/././
I..
V-(AI)-PILC
/a
_
-fff-
'
2 I.,.
I=I l-,-
mm
IX
Z Ill
I
2100
2200
1
1
0,5
1,0
2300
cm -1 1,5
wt % V2 05 Figure 2. Dependence of the IR nitrile band content in the alumina-pillared samples.
intensity
(arbitrary units) on the vanadium
753 The data show that both the undoped alumina- and titania-pillared matrices show certain activity in m-xylene ammoxidation. This effect is probably due to the presence of surface iron oxide/oxyhydroxide impurities, since pillared Texas montmorillonite, containing less Fe impurity, tested in the same conditions shows virtually no catalytic activity. Doping with vanadium strongly enhances the formation of nitrile species. For singly vanadyl-exchanged alumina-pillared samples V-(A1-PILC)-IA and V-(AI-PILC)-IB the intensity of CN band increases with amount of V (Fig. 2). Further increase of V content by means of multiple exchange (V-(A1-PILC)-II, V-(A1-PILC)-III) has no significant influence on CN band intensity although the vanadium content of the samples increases. It is possible that repeated calcination- exchange cycles cause partial loss of activity of incorporated vanadium species. Sample obtained by doping of uncalcined Al-mt matrix (V-(AI)-PILC) shows activity similar to the catalysts containing V introduced onto calcined pillars. This result is in accordance with the ESR data which show that similar vanadium species are formed irrespective of the method of preparation.
~ (V-Ti)-PILC-C t,,0
-
I
/ (V-Ti)- PILC -B I ! I
(/1 .4-
r-"
3,0 - I
V-(Ti )-PILC
I
J
V-(Ti-PILC)-IC
t_
t:l
I
21oo
22oo
"
d) (V-Ti)- PILC-A
t.. -I-o~.,.
V- (Ti - PILC) - IB
I
d::l
t.. 2,0 -I cl ,
in w at~
I
tr)
~
.
LCI-IA
~
.. "1,1 1,0 u
0,0
0
1
1
I
1
1
0,5
1,0
1,5
2,0
2,5
wt % V2 05 Figure 3. Dependence of the IR nitrile band content in the titania-pillared samples.
intensity (arbitrary units) on the vanadium
In the case of titania-pillared samples containing vanadium deposited onto calcined pillars (V-(Ti-PILC)-IA, V-(Ti-PILC)-IB, V-(Ti-PILC)-IC) the activity of nitrile formation increases with V content (Fig. 3). Comparison with the behaviour of the V-(Ti)-PILC sample containing vanadium introduced on uncalcined pillars shows that CN bands of similar intensity are formed for similar levels of doping, in agreement with the ESR data showing the same type of spectra
754 in both cases. On the other hand, ammoxidation of m-xylene over the co-pillared samples (V-Ti)-PILC-A, (V-Ti)-PILC-B and (V-Ti)-PILC-C gives CN bands of high intensity despite the level of doping by order of magnitude lower, with efficiency of CN formation increasing with V content. In this case ESR points to a different character of vanadyl bonding to titania pillars. The unpaired electron is partially delocalized into ligands and the vanadyl species show increased in-plane r~-covalent bonding. It is suggested that these properties of vanadium centres are responsible for the high efficiency of nitrile formation on the co-pillared samples, despite the low level of doping with V. 4. CONCLUSIONS All alumina- and titania-pillared montmorillonite samples doped with vanadium contain immobilized vanadyl ions bound to the pillars. The mode of vanadyl attachment in alumina-pillared samples does not depend on the mode of preparation, while in titaniapillared samples it does. Certain degree of delocalization of the unpaired electron into ligands and increased in-plane zc-covalent bonding is observed for vanadyl ions present in the co-pillared (V-Ti)-PILC samples. Catalytic testing in the reaction of m-xylene ammoxidation shows that doping with vanadium enhances nitrile formation on both series of catalysts. Catalytic activity depends on the mode of sample preparation. It is particularly high or the (V-Ti)-PILC co-pillared samples, despite the low level of doping with vanadium. A hypothesis is advanced that this effect is due to the unique character of vanadyl species present in these samples. REFERENCES
1. D.E.W. Vaughan, Catal. Today, 2 (1988) 187, and the references therein. 2. G.C. Bond and P. Konig, J. Catal., 77 (1982) 309. 3. M. Ga,sior, I. Ga,sior and B. Grzybowska, Appl. Catal., 10 (1984) 87. 4. I. Wachs, R.Y. Saleh, S.S. Chan and C.C. Cherish, Appl. Catal., 15 (1985) 339. 5. D.E.W. Vaughan, in L.V.C. Rees (Ed.), Proc. 5th Int. Conf. on Zeolites, Heyden, London, 1980, pp. 94-101. 6. J. Sterte, Clays Clay Miner., 34 (1986) 658. 7. K. Bahranowski and E. M. Serwicka, Colloids Surfaces, 72 (1993) 153.
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
755
THE USE OF SEPIOLITE IN THE PREPARATION OF TITANIA MONOLITHS FOR THE MANUFACTURE OF INDUSTRIAL CATALYSTS. J. Blanco, P. Avila, M. Yates and A. Bahamonde. Instituto de Catfilisis y Petroleoquimica (CSIC), Campus UAM, 28049 Madrid, SPAIN. FAX (34-1) 585 26 14
ABSTRACT In this work the merits of the use of a natural fibrous mineral, sepiolite, as a binder to produce titania based monoliths of high mechanical strength and abrasion resistance is discussed. The monoliths of square channels were conformed with an initial 7.5 channels cm -2 and 1 mm wall thickness. The textural characterization was made by mercury intrusion porosimetry (MIP), nitrogen adsorption/desorption (BET), and X-ray diffraction (XRD). The mechanical resistance, dimensional changes and weight losses at each stage of heat treatment were also determined. The thermal expansion coefficients (TEC) of the monoliths were determined between 200 ~ and 400~ since in practice the usual working temperature of DENOX catalysts lies between 250~176
1. INTRODUCTION Some of the most active catalysts used in the Selective Catalytic Reduction (SCR) processes to remove nitrogen oxides (NOx) from exhaust gas streams are those based on vanadia supported on titania [ 1]. In order to avoid the problems associated with pressure drop and diffusional limitations, encountered with conventional peletted catalysts in forms of cylinders and spheres etc., the supports should ideally be configured as honeycomb monoliths for these reactions which normally take place with high space velocities due to the large volumes of gas to be treated [2]. However, the difficulties encountered in the preparation of monoliths based solely on titania makes the inclusion of binders to both improve the rheological properties of the paste prior to extruding and the soundness of the monolith with subsequent thermal treatment a necessity [3]. In this work the use of a natural fibrous mineral, sepiolite, as a binder to produce titania based monoliths of high mechanical strength and abrasion resistance is discussed. The a-sepiolite (Si12MgsOao(OH)4(H20)4.8H20) [4], is a fibrous material whose fibres range from 0.2-2 #m in length and 0.1-0.3 #m [5] in diameter for the bundles of fibres. The selection of c~-sepiolite as a possible admixture was made due to the relatively low cost and high *This work was sponsored by grams from the Spanish GovernmentCICYT projects AMB 92-0190and 93-0244
756 abundance of the raw material and the enhanced handling characteristics of the paste produced during the early fabrication processes along with the good mechanical strength development of the finished products after thermal treatment.
2. EXPERIMENTAL.
2.1 Monolith preparation The raw materials used in this study were a hydroxylated gel of titania of about 50% water content with an average particle size of 90% < 40 ~m, supplied by Tioxide (UK) and a natural ct-sepiolite of >80% purity supplied by Tolsa S.A. (Spain). Monoliths were prepared to seven titania:sepiolite compositions: 100:0, 80:20, 65:35, 50:50, 35:65, 20:80 and 0:100 wt % respectively. The production method used for all of these monoliths has been reported elsewhere [6]. In this study the monoliths were configured with 7.5 square cells cm -2 and a 1 mm wall thickness on initial extrusion. The monoliths were heat treated at 110 ~ 500 ~ 800 ~ 1000 ~ or 1200~ respectively in an air atmosphere. The heat treatment for all of the samples followed the same general programme: the samples were first heated from ambient to l l0~ at 3~ min 1, maintained at that temperature for 4 hours then allowed to cool to ambient. These dried samples then underwent a further heat treatment programme: heating from ambient to the desired temperature at 3 ~ min -1, maintained at that temperature for 4 hours then allowed to cool to ambient.
2.2 Characterization techniques Mercury intrusion porosimetry (MIP) analyses were performed on a Micromeritics Poresizer 9320 after drying the samples in an oven at 100~ overnight. Using the nonintersecting cylindrical pore model of Washburn [7] with a mercury contact angle of 140 ~ and surface tension of 480 mNm -1 starting from vacuum and increasing the applied pressure to 2000bar, gave a range of c a . 150/zm-4 nm pore radius. Nitrogen adsorption/desorption isotherms at 77 K were determined using a Micromeritics 1310 ASAP. The samples were outgassed overnight at 100~ to a vacuum of < 10-4 torr to ensure a dry clean surface, free from any loosely held adsorbed species. Surface area determinations were made by application of the BET equation [8], taking the area of the nitrogen molecule [9] as 0.162 nm 2. Powder X-ray diffraction (XRD) patterns were recorded on a Philips PW 1710 powder diffractometer in the 5-75 ~ (20) region using CuKc~ radiation: h = 0.1518 nm. The axial strengths of the monoliths were determined using a Chatillon LTCM Universal Tensile Compression and Spring Tester with a test head of 1 mm 2. The dimensional changes due to heat treatment were determined on representative monolith samples. Their dimensions were measured to an accuracy of 0.01 mm at each stage of heat treatment. The thermal expansion coefficients (TEC) of samples pretreated at 500 ~ or 800~ for 4 hours were measured using a Netzsch 402EP Dilatometer. Samples of between 25 and 50 mm were heated at a rate of 5~ min-1 from ambient to 500~ and the expansion measured by the displacement of a strain gauge held against the sample.
757
3. RESULTS 3.1. General appearance On extrusion and after each stage of heat treatment the general appearance of the monoliths was recorded. From these observations it was noted that, in samples of > 80 wt% titania that cracks were formed immediately after extrusion, due to a rapid loss of water. As the amount of sepiolite used in the mixtures was increased the appearance of these flaws was greatly reduced. The pure titania monolith had a shiny surface texture when first extruded which was maintained at all the heat treatments. Conversely, samples containing sepiolite were always matt in appearance. All of the monoliths underwent colour changes during heat treatment, the most notable of which being that observed for the pure titania sample. On extrusion and heat treatment at 110~ the material was off white. However, as the samples were heated to 500 ~ 800 ~ and 1000~ the monolith gradually became darker. Between 1000~ and 1200~ although no weight losses were recorded the monolith colour changed from a light beige to a dark brown probably due to a change in the christalinity of the sample with extended treatment at this temperature. 3.2. Mercury porosimetry From the mercury porosimetry results, presented in Table 1, it should be noted that samples heated to 110~ had lower Hg pore volumes than the corresponding material heated to 500 ~ or 800~ except for the pure titania monolith which underwent a severe reduction in the pore volume at the latter temperature. This underestimation of the pore volume was due to the presence of micropores and narrow mesopores of less than 3.5 nm pore radii which remained undetected by the porosimetry technique. As the treatment temperature was increased the pore size distributions shifted to wider pores, bringing the pore volumes measured by mercury closer to the total pore volumes calculated by the summation of the volume of pores up to 3.5 nm obtained from the nitrogen isotherms and that of pores greater than 3.5 nm from the porosimetry curves, are presented in column 6 of Table 1. The surface areas calculated from the porosimetry intrusion curves (Hg Area), presented in column 4 were calculated using a cylindrical nonintersecting pore model, and only represent the surface area of pores down to 3.5 nm pore radius [10]. Discrepancies between the surface area results obtained from this method and that of gas adsorption, presented in column 5, were due to the presence of pores of less than 3.5 nm which remained undetected. These differencies were greatest with the lower heat treatments but as the thermal treatment was increased the shifts in the pore size distributions to wider mesopores and eventually to macropores, brought the two measurements into closer agreement. After treatment at 800 ~ or 1000~ the areas calculated from the porosimetry curves were higher than those from gas adsorption. This was due to the porosimetry curve being a measurement of the pore neck size distribution rather than the pore body distribution, which can lead to an under estimation of the volume associated with the wider pores and an overestimation of that associated with the narrower pores in materials with complex pore geometries in which a fraction of the wider pores may not be directly accessible, with a subsequent overestimation of the calculated area.
Table 1 Textural charaterization of monoliths Composition Ti0,:SEP wt%
100: 0
80: 20 65: 35 50: 50 35: 65 20: 80 0: 100
Heat Treatment "C 110
Hg Pore Volume cm3g-'
N, BET Hg Area Area m2g-' m2g-'
Total Pore Volume Hg+N, cm3g-'
Axial Strength kgcni2
Length Change % 85.2 85.0 85.7 86.7 85.7 88.6 81.9 84.0 83.3 85.5 86.0 85.5 88.1 81.7 71.2 80.0 81.9 82.9 82.9 85.2 80.0 63.1 61.4 61.9 63.3 64.8 69.8 7 1.2
TEC dL/"C E-6 200"-400"C
759 Using the results obtained with titania and sepiolite as standards the expected total pore volumes for purely physical mixtures of the two "?0.5 could be calculated and thus, any deviations in the pore volumes found experimentally for the intermediate composition monoliths could be judged. Using this procedure the total pore volumes ~0.3 . . . . . . ........ were found to be higher than expected in samples where the titania content was greater than 20 wt% nO0.2 ............. "'"' .... with heat treatments up to 800~ presented in Figure 1. The increase in the pore volumes over ~0.1 that expected was probably due to interference in the normal packing behaviour of the two materials 0,0 when mixed together. This was to be expected 0 10 20 30 40 64) 60 79 80 90 100 since the titania was composed of largely sphereical 11CONTENT~(,} particles or agglomerates while the sepiolite was a fibrous material, composed of bundles of needle Figure 1. Total Pore Volume v e r s u s Ti Content like materials. after pretreatment at # 110 ~ 9 500 ~ Of note was the low total pore volume of * 800~ and 9 IO00~ the pure titania monolith after treatment at 800~ in comparison to the materials containing sepiolite which maintained high total pore volumes after treatment at this temperature. After treatment at 1000~ the pore volumes were severely reduced due to the phase changes of anatase to rutile above 1000~ [11] and sepiolite to enstatite above 830~ [12], which caused a collapse of the original pore structure and a substancial shrinking of the monolith. After treatment at 1000~ the pore volumes of the mixed composition monoliths were all much lower than expected, especially for the 20:80 wt% material. 0.30 The changes in the pore size distributions of ,., the various materials with heat treatment may be Q ~0.t~ appreciated from the incremental intrusion curves shown in Figures 2, 3 and 4 for titania, sepiolite m and the 50:50 wt% material respectively. For the 0.2Otitania monoliths, presented in Figure 2, a shift in the distribution to wider pores with increasing heat .j0.15treatment temperature was clearly demonstrated. The average pore radii at the peak maxima shifting t, 0.10""~ from 0.006 #m after pretreatment at l l0~ to ' 'i. 0.012 #m and 0.014 /zm at 500 ~ and 800~ W n. 0.05 / ~I : t.,."'' ~ respectively. _ 0.. From the results obtained with the sepiolite 0.00 ' ~ : . . , . "; ~ . _ , monoliths, shown in Figure 3, it may be noted that O.001 0.01 0.1 1 although the pore volumes varied with different PORE RADIUS~m) heat treatments the pore size distributions remained constant. These monoliths displayed a major peak Figure 2. Pore size distribution curves for at 0.014/zm and two minor peaks at 0.050 #m and titania monoliths pretreated at l l0~ ~ , 0.092 /zm respectively. Of note was the broader 500~ and 800~ .-.. pore size distribution displayed by these sepiolite monoliths, which was due to both the wider range of particle sizes in the parent material and the different packing behaviour of the needle like particles in comparison with the spherical particles of the titania. 0,6
760 0,31
0,30
~
Q
~" 0,25
0.~, -
g W
U.I
3
3
>00.15
>00.15
_i
.1
IE 0.20
0.20 t
ol :l ."..
z 0.10-
~0.10
Ul
ul E
m
o z 0,05
o O,O5-
'" I
Z
m
0.00 0.001
0.01
0.00
0.001
0.1
PORE RADIUS ~m)
Figure 3. Pore size distribution curves for sepiolite monoliths pretreated at l l0~ ~ , 500~ and 800~ ..-.
I i I I I
0.01 0.1 PORERADILI6(pm)
Figure 4. Pore size distribution curves for 50:50 wt% monoliths pretreated at 110~ - - , 500~ - - and 800~ ---.
The results obtained with the mixed composition monoliths, presented in Figure 4, demonstrated a narrower pore size distribution than that found with either the titania or sepiolite materials, leading to much sharper peaks. As with the titania monoliths a pronounced shift in the pore size distribution to wider pores with increasing pretreatment temperature was demonstrated. Thus at l l0~ the peak maxima was located at 0.095 /zm, shifted to 0.016 /~m and 0.018 #m after pretreatment at 500 ~ and 800~ respectively. 3.2. Nitrogen adsorption
200 Q
uJ 150
o
0" 0.0
0.2
0.4
0.6
"";:::::: 0,8
1.0
RELATIVEPRBSURE( i ~ Figure 5. Nitrogen isotherms for titania monoliths pretreated at 110 oC ~ , 500 oc - and 800~ -...
The pure titania monolith gave a mixed TypeI/II adsorption isotherm with a narrow desorption hysteresis after treatment at l l0~ indicating that at this temperature some microporosity was present. On treatment at 500~ or higher this microporosity disappeared as the hydroxide species present in the original titania gel were decomposed. The narrow desorption hysteresis present in all of the materials studied, designated as Type lib [13], indicated the presence of slit shaped mesoporosity which extended into the macropore range, shown in Figure 5. Determination of the pore size distributions from the desorption branches of the isotherms by the BJH [14] method gave results which were in good agreement with those obtained from MIP. The narrowing of the hysteresis loops and their displacement to higher relative pressures with increasing heat treatment temperatures were indicative of the general shift to wider pores mesopores.
761
Although t~-sepiolite possessed microporosity after heating to 110~ treatment at 500~ was enough to cause a folding of the talc ribbons making up the fibres, which sealed off these micropores [15]. The absence of microporosity after heat treatment at 500~ or higher was confirmed by using a t-plot analysis [ 16], comparing the shape of the isotherm with that of a nonporous standard. The initially high surface areas of the samples after pretreatment at 110~ were thus mainly due to the presence of microporosity. The loss of microporosity on heating to 500~ halved the surface areas in samples containing sepiolite and reduced the titania monolith to less than a third. Heat treatment at 800 ~ further reduced the surface areas, especially in the titania monolith. After treatment at 1000~ the surface areas were very low for all of the materials. Thus, although the surface areas of all the materials were reduced with heat treatment, the inclusion of sepiolite even at only 20 wt% was enough to retard the loss of surface area.
3.3. X-Ray diffraction analysis Powder XRD studies on ground samples of the monoliths were determined to follow any phase changes and solid state reactions which resulted from heating the materials from ambient to 1200~ It should be noted that for crystaline phases to be detected by XRD the crystalites need 1200~ to be greater than 3 nm. Thus, the presence of certain phases may not be detected due to the small 1000~ size of the crystalites which would appear amorphous. 8~ C The XRD patterns obtained from samples of the titania monolith after treatment at various SI~~ temperatures are shown in Figure 6. After an initial treatment at 110~ the peaks for anatase could be clearly distinguished although with further 10 20 30 40 50 80 70 treatment at 500 ~ and 800 ~ the greater A N G I _ E (2e) chrystalinity of this phase was shown by the Figure 6. XRD patterns for titania monoliths narrowing of these peaks and the increase in their pretreated at various temperatures showing the relative intensities. On treatment at IO00~ the principle peaks for anatase (A) and rutile , (R). spectra changed dramatically as all the anatase was transformed into rutile. Heat treatment at 1200~ caused a slight loss in the chrystalinity of the rutile phase, shown by the decrease in the relative intensity of the peaks. The XRD patterns obtained with the sepiolite monolith after pretreatment between 110~ and 1200~ are presented in Figure 7. The bands had lower relative intensities than those found with the titania monolith because the sepiolite used only had a purity of c a . 80%. After treatment at 110 ~ 500 ~ and 800~ gave the peaks expected for sepiolite. Above this temperature the characteristic peaks for enstatite could be found since the sepiolite underwent a phase change on heat treatments higher than 830~ The band present at low angle in the sample treated at 110~ was due to the microporous nature of this material. The disappearance of this band with higher temperatures was due to the folding of the talc ribbons and partial collapse of this microporous structure. In monoliths of mixed composition no solid state reactions between the titania and sepiolite were observed. However, from the results, presented in Figure 8 for the 50:50 wt% sample, important differencies in the behaviour of the titania were observed. Up to pretreatment at 800~ the phases present were as expected from the results obtained with the two parent materials. However, after heating at 1000~ the phase change: anatase --, rutile was not completed. From calculation of the relative intensities of the principal peaks for anatase and rutile the conversion under these conditions
762 was found to be only conversion to rutile.
30% [ 17]. Even heat treatment at 1200~ was only sufficient to cause a 50%
ca.
~--------
.j
R
12oo'c
i
1 ~Do"c
lOOO-c
5
8
~ c
1 lo-c ....
O
,,,,I
....
,,,,,I,
"10
....
,,,,I
20
....
,,,,,
80
I,,,,,,,,,|,,,,,
40
~Gt.n
....
BO
I,,,,
....
60
i
"70
(20)
.
. 10
.
. 20
. 30 ANQLE
Figure 7. XRD patterns for sepiolite monoliths pretreated at various temperatures showing the principle peaks for sepiolite (S) and enstantite (E).
.
. 40
,11,,q7~, ~0
e4)
70
(2 e)
Figure 8. XRD patterns for 50:50 wt% monoliths treated at various temperatures showing the principal peaks for anatase (A), rutile (R), sepiolite (S) and enstantite (E).
From these results it may be observed that the inclusion of sepiolite in the monolith composition stabilised the anatase phase. This was important since in practice vanadia supported on titania is more active when the titania is anatase than when it is rutile. Thus, the stabilising effect of the inclusion of sepiolite on the anatase was beneficial for the activity of any vanadia catalyst supported on these mixed composition materials.
3.4. Axial crushing strength 1000
0.e .. 4b....
m
800 0.3
~.
.
.
.
.
,,
40O
20O
0.1 &
,-dr
0.0
7 ....
250
500
, .... 750
1000
TEMPERATURE *G
Figure 9. Axial Crushing Strengths (Full lines) and Total Pore Volumes (Dotted Lines) v e r s u s pretreatment Temperature. * 100:0 wt%, [] 0:100 wt%, 9 50:50 wt%.
The results of the fracture strength testing are presented in column 7 of Table 1. The increase in the overall strength brought about through pretreatment at successively higher temperatures was due to the reduction in the total porosities of the monoliths with increasing temperature. From the results presented in Figure 9 it can be seen that the crushing strength of this brittle ceramic material was inversely proportional to the total pore volume. Although the samples displayed a steady progression in strength development with increasing heat treatment, it should be noted that none of the materials showed any great improvement until treated at 1000~ at which temperature the titania undergoes a partial phase transition from anatase to rutile, and the sepiolite undergoes a phase change to enstatite, causing a significant reduction in the porosities of the monoliths.
763
3.5. Dimensional variations caused by heat treatment. The dimensional changes which the monoliths underwent on heat treatment were determined in all three planes: horizontally and vertically across the face of the monolith and along 9O its length respectively. The shrinkage due to heat treatments were found to be the same along each axis. All of the materials underwent similar trends k, . . . . . . . . . . . . . . . . . . A., with heat treatment, the magnitude of the changes observed not depending on the initial compositions of the monoliths, as seen from the results presented in column 8 of Table 1. The shrinkage observed on heating from ambient to l l0~ was due to the loss of water from the structure, allowing the solid phases to i I I 60 come closer together, presented in Figure 10. Since 280 500 750 1000 0 the samples had all been initially produced as T E M P E R A T U R E (~ monoliths from a dough which included water, in Figure 10. Percentage Length changes against order to achieve a workable paste, this had pretreatment Temperature for * 100:0 wt%, invariably led to the incorporation of water into the 9 0:100 wt%, and 9 50:50 wt%. structure which was not chemically bound and thus easily eliminated on heating to 110~ The monoliths displayed a long interval of relative dimensional stability between 110 ~ to 800~ A further shrinkage was observed on heat treatments at 1000~ due to the phase changes in both the titania and sepiolite caused at this temperature. 100
3.6. Thermal expansion coefficients The thermal expansion coefficient (TEC) of a monolithic support is an important aspect in determining its usefulness in practice since large volume changes in the usual working temperature range of the catalysts are undesireable. Determination of this property required that the measurement was that of a purely reversible thermal expansion and not contain any elements due to loss of free and bound water, hydroxyl groups or any phase changes and solid state reactions. Thus, the TEC results presented in the last column of Table 1 were measured in the temperature range of 200 ~ to 400~ since the usual working temperature for the catalysts lies between 250~176 Only samples which had been previously heated to either 500 ~ or 800~ for 4 hours were studied since pretreatment at higher temperatures led to a reduction in the pore volumes and surface areas, and also caused the conversion of anatase to rutile which prejudiced their use as good catalyst supports. From the results presented in the final column of Table 1 it should be noted that for materials pretreated at the same temperature the TEC's were similar. No data were obtained for the monolith based solely on titania since without the inclusion of sepiolite it was not possible to produce a length of monolith greater than 2 cm without flaws, necessary for the measurement. In general as the content of sepiolite was increased the expansion coefficients were reduced. Of note was that pretreatment at 800~ led to higher expansion coefficients, although the same general trends were maintained.
4. CONCLUSIONS The results obtained from this study show that although no significant enhancement in the strength development of the titania monoliths was obtained through the inclusion of sepiolite the workability of the dough before extrusion and the production of a monolith without flaws was greatly
764 improved. The inclusion of sepiolite especially after treatment at higher temperatures led to increased total pore volumes and surface areas without loss of the mechanical strength of the monoliths. Another advantage of the inclusion of sepiolite in the composition of the monoliths was the retarding effect on the phase change of anatase to rutile. The reduction in the TEC of the materials of mixed composition was also important, although the values still remained relatively high. Although heat treatment at 1000~ gave materials of much greater mechanical strength the loss in both the surface area and pore volume were undesireable.
REFERENCES
1. S. Matsuda and A. Kato, Appl. Catalysis 8, (1983) 149. 2. R.K. Shah, A.L. London, Tech. Rep. No. 75 (1971) Dept. Mech. Eng. Stanford University. 3. P. Avila, J. Blanco, A. Bahamonde, J.M. Palacios and C. Barthelemy, J. Mater. Sci. 28 (1993) 4113. 4. K. Brunauer and A. Preisinger, Tschermarks Miner. Petr. Mitt. 6 (1956) 120. 5. A. Alvarez, Developments in Sedimentology, 37, (1984) 253. 6. A. Bahamonde, PhD. Thesis, Universidad Complutense, Madrid (1992). 7. E.W. Washburn, Proc. Nat. Acad. Sci. U.S.A. 7 (1921) 115. 8. S. Brunauer, P.H. Emmett and E. Teller, J. Amer. Chem. Soc. 60 (1938) 309. 9. K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Mouscou, R.A. Pieroti, J. Rouquerol and T. Siemieniewska, Pure and Appl. Chem. 60 (1985) 603. 10. H.M. Rootare and C.F. Prenzlow, J. Phys. Chem. 71 (1967) 2733. 11. G.F. Hiittig and K. Kosterhon, Trans. Faraday Soc. (1925) 560. 12. U. Shuali, S. Yariv, M. Steinberg, M. Muller Vonmoos, G. Kahr and A. Rub, Thermal Analysis Proc. Ninth ICTA Congress (1988) 291. 13. K.S.W. Sing, Third International Conference on Fundamentals of Adsorption, Engineering Foundation, New York (1991) 67. 14. E.P. Barrett, L.G. Joyner and P.H. Halenda, J. Amer. Chem. Soc. 73 (1951) 373. 15. Y. Grillet, J.M. Cases, M. Francois, J. Rouquerol and J.E. Poirier, Clay Miner. 36 (1988) 233. 16. B.C. Lippens and J.H. de Boer, J. Catalysis 4 (1965) 319. 17. J. Criado and C. Real, J. Chem. Soc. Faraday Trans. 1, 79 (1983) 2765.
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
Design of reactions
E
9
monolith catalysts under nonadiabatic
Tronconi a
,
M
9
Bassini a
,
P
9
765
for strongly conditions
Forzatti a and D
9
exothermic
Carmello b
a D i p a r t i m e n t o di Chimica Industriale e Ingegneria Chimica "G. Natta", Politecnico di Milano, Italy b European Vynils Corporation, R&D Centre, P. Marghera, Italy 1.
INTRODUCTION Monolithic catalyst supports have found extensive applications in catalytic converters designed to control emissions from stationary sources (as e.g. in the DeNO x SCR process or in the oxidation of organic compounds produced by industrial processes) and from motor vehicles (catalytic mufflers). Use of monolithic supports in catalytic combustors for gas turbines is also a topic of current research. Typical ceramic or metallic monoliths consist of a matrix with a large number of parallel channels of regular shape (circular, square, triangular, sinusoidal), the catalytic material being deposited on the channel walls. Well established advantages of such structures include high geometric surface areas per unit volume and very low pressure drops as compared to conventional p a c k e d - b e d reactors. In the above applications the monolithic reactors operate under essentially isothermal or adiabatic conditions, so that not only material but also thermal interactions between the channels are negligible. There have been some sparse indications in the literature that monolith supports can be applied advantageously also in the chemical process industry. Flytzani-Stephanopoulos and Voecks (1981) measured lower radial temperature gradients and higher conversions in steam-reforming of n-hexane over a metal monolith catalyst than over catalyst pellets. Use of ceramic monolithic catalyst supports in ethylene oxychlorination to DCE has been patented by Degussa and Wacker (1989), claiming significant improvements over the conventional fixed-bed process. Heynderickx et al. (1991) have demonstrated efficient operation of a steam cracking pilot reactor based on ceramic monolithic substrates with high thermal conductivity. Since such applications involve non-adiabatic operation, it becomes necessary to analyze thermal interactions between all of the monolith channels in order to fully rationalize the potentials associated with monolithic supports. However, very few papers on m o d e l l i n g of non-adiabatic monolithic structures have appeared in the literature so far (Flytzani-Stephanopoulos et al., 1986; Kolaczkowski et al., 1988). Furthermore, in such models the axial conduction of heat in the monolith matrix is neglected, which seems critical for metallic supports. In this work we first develop our own analysis of heat
766
transport p h e n o m e n a in m o n o l i t h i c supports and compare it with previous models and with a v a i l a b l e data. The influence of the m o n o l i t h geometric p a r a m e t e r s (channel size, channel density) is then investigated for both ceramic and m e t a l l i c h o n e y c o m b matrices, the goal being to define guidelines for an o p t i m u m design with respect to heat transfer efficiency. The results are then applied to assess on a p r e l i m i n a r y basis the b e n e f i t s of non-adiabatic monolithic reactors versus traditional p a c k e d - b e d reactors, a n a l y z i n g d i f f e r e n t design c o n f i g u r a t i o n s for the example case of a m u l t i t u b u l a r reactor for e t h y l e n e oxychlorination. 2. MODEL OF HEAT TRANSFER IN A MONOLITHIC CATALYST 2.1 Assumptions and equations P r e l i m i n a r y r e c o n f i g u r a t i o n of the actual system into an equal number of square channels arranged in N c o n c e n t r i c rings to secure circular symmetry is assumed according to the approach suggested by Kolaczkowski et al. (1988). The c o n s i d e r e d heat transport m e c h a n i s m s include: i) c o n v e c t i o n inside the honeycomb channels; ii) interphase heat transfer between flowing gas and solid matrix; iii) radial and axial heat c o n d u c t i o n inside the solid matrix. T r a n s p o r t by axial d i f f u s i o n in the gas and by radiation are neglected. R e f e r to Figure 1 for the notation. A s s u m i n g a o n e - d i m e n s i o n a l treatment for a single channel in the i-th ring, (i=l, ... N), Gas-phase energy balance dTb(i) w i Lch 2 Cp = Gl(i) dz
(Tw(i)
(i)
- Tb(i) )
Energy balance at the gas-solid interface Gl(i)
(Tw(i) - Tb(i))
(2)
= G 2 (Tr(i) - Tw(i) )
Solid-phase energy balance d2Tr(i ) k I n(i)(l-E)A
+ n(i+l)
G 3 (Tr(i+l)
-Tr(i) ) =
d2z = n(i)
G3(Tr(i)
-Tr(i_l) ) + n(i)
G 2 (Tr(i+l)
-Tw(i) ) (3)
dTr(i) with
i = 2,
..- N-I
and
= 0 at
z = 0, z = L.
dz Suitable m o d i f i c a t i o n s of (3) apply for i=l and i=N, the latter case depending on the boundary condition at the m o n o l i t h wall. In the above equations, the unknowns Tb, T w and T r are temperatures of the gas, w a s h c o a t and matrix in the i-
767
th channel ring; n(i) is the number of channels in the i-th ring; A is the cross sectional area of the monolith; 6 the m o n o l i t h v o i d fraction; G k are thermal c o n d u c t a n c e s based on gas and solid p r o p e r t i e s and on geometrical characteristics.
r--4 .--4 ~3
-r-4 r--4 O
l'T.(i-
~-T
I)--
( I )----" r--4
[ Gz(i.1)
]
.
::
Tb(i - z )
:
(k 2 )
Q)
Tb( I ) (k 1)
( ..... 1/2 6 ; :(--)
)
, :
:
( ............. ) (--)
82
~1
52
( ................. )
( .............
)
81
Lch
Figure 1 - Schematic d i a g r a m of the m o n o l i t h i c matrix. Momentum balance The p r e s s u r e drop in the channels of the i-th ring is 28.4 ~m(i) ~p
n
=
W(i)2 w(i ) +
Pm(i)
Lch2
W(i)2 +
2
p IN(i)
with the individual mass flows W(i ) summing mass flow rate Mto t a c c o r d i n g to
(4)
2
P OUT(i) up to the overall
N Z n(i) A W(i) = Mtot (5) i=l N u m e r i c a l solution of the a l g e b r a i c - d i f f e r e n t i a l system (i) - (5) by orthogonal c o l l o c a t i o n techniques provides the axial profiles of T b, Tr and Tw, the mass flow W(i ) c o r r e s p o n d i n g to each one of the N channel rings along a monolith radius, and the overall pressure drop. 2.2 C o m p a r i s o n w i t h data and p r e v i o u s models Figure 2 contrasts air temperature data m e a s u r e d at the exit of a heated ceramic m o n o l i t h (Kolaczkowski et al., 1989) with p r e d i c t i o n s g e n e r a t e d by Eqs. (1)-(5). Results o b t a i n e d by a s s u m i n g uniform flow d i s t r i b u t i o n in the channels, and by adopting the model by K o l a c z k o w s k i et al. (1988) are also
768
displayed. The f o l l o w i n g c o n s i d e r a t i o n s are in order: i) our model seems to reproduce satisfactorily the experimental radial T-profiles; ii) the a g r e e m e n t w i t h data is b e t t e r w h e n a c c o u n t i n g for the influence of n o n u n i f o r m flow d i s t r i b u t i o n ; iii) our model appears e q u i v a l e n t or s u p e r i o r to K o l a c z k o w s k i et al. (1988), in spite of the g r e a t e r c o m p l e x i t y and g r e a t e r n u m b e r of v a r i a b l e s of the latter. The same c o n c l u s i o n s are confirmed by simulation of other data under different o p e r a t i n g conditions. Dimensionless 1.80 -
A s --Previous
gas
temperature ,,
]
data model
'] .'~
C.60
~.40
:1.20
9
1.00
24
I 26
^
I
A
~
28 '30 Channel
~
" ,
I
32 rings
,
I 34
36
Figure 2 - Comparison w i t h data from Kolaczkowski et al. (1989) - s i m u l a t i o n i. Previous model = K o l a c z k o w s k i et al. (1988). Model A = this model, u n i f o r m flow. Model B = this model, n o n u n i f o r m flow.
2.3
I n f l u e n c e of a x i a l c o n d u c t i o n in t h e m o n o l i t h A p a r a m e t r i c study on the effects of axial heat c o n d u c t i o n in the solid m a t r i x has shown that: i) such effects are n e g l i g i b l e in ceramic m o n o l i t h s (cordierite, kl = 1.4 W/m/K) but e x p e c t e d l y s i g n i f i c a n t in m e t a l l i c m o n o l i t h s (Fecralloy, k I = 35 W/m/K) when a c o n s t a n t heat flux is imposed at the external m a t r i x wall; ii) however, the influence of axial c o n d u c t i o n in m e t a l l i c m o n o l i t h s is much less a p p a r e n t if a c o n s t a n t wall temperature condition is applied, since the m o n o l i t h tends to an isothermal behavior. Metallic matrices e x h i b i t very flat axial and radial t e m p e r a t u r e profiles, w h i c h seems p r o m i s i n g for their use as c a t a l y s t supports in nona d i a b a t i c chemical reactors.
2.4 I n f l u e n c e of m o n o l i t h g e o m e t r y For c e r a m i c and m e t a l l i c matrices, respectively, Figures 3 and 4 p r e s e n t c a l c u l a t e d values of e x c h a n g e d heat power as
769
f u n c t i o n s of the cell d e n s i t y and of the channel size Lch. For a fixed Lch, i n c r e m e n t i n g the cell d e n s i t y brings about an increased gas-solid interfacial area which improves heat removal. On the other hand, the t h i c k n e s s of the m a t r i x wall 81 is reduced, which increases the resistances to heat c o n d u c t i o n in the solid phase. The m a x i m a o b s e r v e d in the curves at c o n s t a n t Lch result from a b a l a n c e of such two c o n t r a s t i n g effects. For a given cell density, a r e d u c t i o n of Lch e n h a n c e s the heat exchange due to an i n c r e a s e d wall t h i c k n e s s S 1 . In the c o n d i t i o n s of the Figures, this f a v o r a b l e effect dominates the associated decrease of gas-solid interfacial area. Notice that, in a d d i t i o n to a g r e a t e r e x c h a n g e d thermal power, metallic monoliths e x h i b i t also a much g r e a t e r o p t i m u m cell density, both effects b e i n g r e l a t e d to their h i g h e r conductivity. qemoved heat power,
Watt
550 Lch 4 m
450
-
Lch
-
6 Im
350 \
250
..~.
.....
\
...
I
15%.00 Channel density,
I
I
I
I
I
I
I
I
130.00 channels/m^2 * I.E-3
Figure 3 - E f f e c t of cell d e n s i t y and of channel size on heat transfer e f f i c i e n c y of a m o n o l i t h w i t h c o n s t a n t external wall t e m p e r a t u r e = 500 K. Case of c e r a m i c m o n o l i t h (Cordierite). Gas=air. 3. S T U D Y OF N O N A D I A B A T I C OXYCHLORINATION 3.1
Process
MONOLITHIC
REACTORS
FOR
ETHYLENE
considerations
In this Section we shall t e n t a t i v e l y apply the results of the previous study to investigate the feasibility of a n o n a d i a b a t i c reactor loaded w i t h ceramic m o n o l i t h c a t a l y s t s for the r e a c t i o n of e t h y l e n e o x y c h l o r i n a t i o n to DCE. Such a reaction is the heart of m o d e r n b a l a n c e d p r o c e s s e s for the p r o d u c t i o n of m o n o m e r vynil c h l o r i d e (Naworski & Velez, 1983). The r e a c t i o n 2 C2H 4 + 02 + 4 HCl .... > 2 C H 2 C I C H 2 C l
+ 2 H20
(6)
770
is carried out over CuCl2-based catalysts supported on AI20 3 in a cascade of three multitubular packed-bed reactors. The considerable heat of reaction is removed by boiling water circulating in the shell side of the reactors. The axial temperature profiles along the catalyst beds exhibit maxima (hot spots) which must be carefully controlled to prevent loss of selectivity, catalyst damage and incremented pressure drops. In industrial reactors the hot spot temperatures is limited below 280 - 300 ~ by diluting the catalyst pellets with inert particles and by varying the concentration of the active catalyst constituents along the reactor.
Removed heat power, Watt ~800 ~.600
i-- I ~
I:.:.
L c h 2 IW Lch 4 Im ,Lch @mm
I_400 ~.200
~000 800
// .
/.~.,."
" "
i../.'"
I
60%.00 Channel density,
I
I
I
,
l
i
channels/m^2,
I
30.00 I.E-3
Figure 4 - Effect of cell density and of channel size on heat transfer in a monolith with constant external temperature = 500 K. Case of metallic monolith (Fecralloy). Gas = air.
3.2 E x t e n s i o n of the m o n o l i t h chemical reaction
reactor
model
to i n c l u d e
a
The model described in the previous paragraphs representing heat transfer in monolith structures has been extended by including material balances for the key reacting species (02) in the gas phase and at the gas-solid interface, and by modifying the energy balance to incorporate the generation term associated with the oxychlorination reaction. Based on literature kinetic data (Carrubba & Spencer,1970) and considering that in the first stage reactor of an oxygenbased oxychlorination process oxygen is the limiting reactant (Markeloff,1984) the reaction rate RDC E can be represented by: RDC E = A exp(-Eatt/RT)
Po2 n
(7)
Typical values of the kinetic parameters in line with literature indications assumed in the calculations are listed in Table 1 (Carrubba and Spencer,1970; Zhernosek, 1971). To
771
account for intraporous d i f f u s i o n a l resistances, a g e n e r a l i z e d Thiele m o d u l u s ~ has been defined, a s s u m i n g an indefinite slab geometry for the catalytic washcoat, with thickness 52, where the r e a c t i o n is confined. The catalyst e f f e c t i v e n e s s factor is then estimated assuming pseudo-first order kinetics and isothermal conditions in the washcoat. Table 1 reaction
- Kinetic (6).
parameters
for
ethylene
oxychlorination,
R e a c t i o n o r d e r with respect to oxygen: n = 0.5 P r e e x p o n e n t i a l factor: A = 250 mole/(s m3cat Pa 0"5) A c t i v a t i o n energy: Eat t = 70 000 J/mole 3.3 A n a l y s i s of monolithic catalysts for ethylene oxychlorination. Table 2 provides characteristic values of the m o n o l i t h p a r a m e t e r s used in the calculations. In all of the simulations d i s c u s s e d b e l o w o p e r a t i n g conditions have been assumed typical of the first stage reactor in o x y g e n - b a s e d processes for ethylene oxychlorination, including e.g inlet pressure = 6 ata, c o o l a n t temperature = feed t e m p e r a t u r e = 200 ~ feed c o m p o s i t i o n (% molar): C2H4: 70; HCI: 25; 02: 5. P r e l i m i n a r y s i m u l a t i o n s d e m o n s t r a t i n g the influence of the catalyst design p a r a m e t e r s have p o i n t e d out that: i) For an assigned pitch, i n c r e m e n t i n g the m o n o l i t h void fraction brings about a reduced catalytic activity, which results in lower 02 conversions and c o n s e q u e n t l y in lower t e m p e r a t u r e s due to the reduced heat of reaction. Such effects are e x p l a i n e d by noting that the gassolid interfacial surface area decreases with growing pitch. ii) On i n c r e a s i n g the void fraction with a constant pitch, the location of the hot spot moves towards the reactor inlet and runaway c o n d i t i o n s are e v e n t u a l l y approached. This results from three d i s t i n c t factors, namely a less efficient heat exchange due to the lower linear gas velocity, a more d i f f i c u l t heat c o n d u c t i o n in the solid m a t r i x because of the smaller thickness of the monolith walls, and a ~ greater catalytic activity owing to an incremented gas-solid interfacial area. Table 2 - Parameters assumed in the s i m u l a t i o n of the m o n o l i t h i c reactor for o x y g e n - b a s e d ethylene oxychlorination. Reactor d i a m e t e r Reactor length M o n o l i t h void fraction Pitch W a s h c o a t thickness, 82 M o n o l i t h thermal c o n d u c t i v i t y
3 cm 5 m 0.25 3 mm 0.25 mm
1.4 W/(m K)
i
Calculation results indicate also that strong radial t e m p e r a t u r e gradients prevail in the m o n o l i t h matrix, so that
772
the critical hot spots are located in the channels at the monolith centerline, while the conversion of oxygen is significantly reduced in the peripheral channels where the washcoat temperature is low, being close to the coolant temperature Tcool Furthermore, the final portion of the reactor is not fully exploited, because most of the limiting reactant 0 2 has been consumed and the washcoat temperature approaches Tcool, so that the reaction kinetics slow down to a significant extent. Based on the above points, the following modifications to the design of the monolithic catalysts and of the reactor configuration are suggested. TW (~ 320 300
--
l----Length
~
I--Length
260
!'
24o
,,,
SO c. 25
cl
I
/
J,/7
" , vi-.-~y ~
v v v VVr/~A~,~
220
208.00
i
0.20
1
0.40
Dimensionless
i
0.60 Axial
I
0.80
.oo
Coordinate
Figure 5 - Axial temperature profiles of the catalytic washcoat at the monolith centerline versus length of the monolith sections loaded in the reactor. Pitch = 2.5 mm, other conditions as in Table 2. i) Monoliths with low void fractions must be employed, since heat exchange is limited by conduction in the solid matrix, ii) The catalyst loaded in each reactor tube should be partitioned in several monolith segments separated by mixing regions in order to favor radial transfer of heat and reactants, iii) The catalyst segments must exhibit an increasing catalytic activity along the reactor tubes in order to secure a high rate of reaction even in the final part of the reactor. Figure 5 shows the results of loading a single reactor tube with several monolithic matrices of different lengths, assuming a complete mixing between two consecutive sections. The Figure presents the calculated axial thermal profiles of the catalytic washcoat in the centre of the monolith, with the length of the monolithic sections as a parameter and neglecting the mixing length. It is apparent that the hot spot
773
t e m p e r a t u r e d e c r e a s e s on d e c r e a s i n g the length of the m o n o l i t h segments. On the other hand, the overall o x y g e n c o n v e r s i o n d e c r e a s e s only slightly (from 80% with 500 cm segments to 75% for 12.5 cm), though the r e a c t o r o p e r a t e s on the average at lower temperatures, due to the improved supply of reactants from the p e r i p h e r a l channels to the more r e a c t i v e central ones. Finally, in Figure 8 we show the axial w a s h c o a t Tprofile calculated assuming monoliths with three different c a t a l y t i c a c t i v i t y levels a l o n g the reactor tubes a c c o r d i n g to the loading p a t t e r n of Table 3. Three d i s t i n c t m a x i m a are a p p a r e n t c o r r e s p o n d i n g to the three loading zones. Tw (~ 300 280 260 240 220
206.n O0
I I I 1 o. 20 o. 40 O. 60 o. so Dimensionless Axial Coordinate
~. o0
Figure 6 - Same as Figure 5, u s i n g h o n e y c o m b c a t a l y s t segments w i t h three d i f f e r e n t a c t i v i t y levels s p e c i f i e d in Table 3.
Table 3 - C a t a l y s t of Figure 6. Length 0.30 0.15 0.55
loading p a t t e r n
Catalytic * L * L * L
assumed
activity
in the
relative
calculations
to Eq.
(7)
1.15/1 1.50/1 2.50/1
For the c o n d i t i o n s of Figure 6 the t e m p e r a t u r e levels of the w a s h c o a t are acceptable, w h i l e the 02 c o n v e r s i o n after a reactor length of 3.5 m (over 90%) is c o m p a r a b l e w i t h that reported for a c o n v e n t i o n a l p a c k e d - b e d first stage reactor in an o x y g e n - b a s e d ethylene o x y c h l o r i n a t i o n process. However, the c a l c u l a t e d p r e s s u r e drop in the m o n o l i t h i c reactor is b e l o w 0.i atm, w h i c h is about five times less than in the p a c k e d bed reactor.
774
4. C O N C L U S I O N S
i. A simple model has been developed to describe the temperature fields inside honeycomb monolithic matrices suitable for use as catalyst supports in non-adiabatic reactors. The model includes realistic features such as axial heat conduction and non-uniform flow distribution in the channels, securing a satisfactory match with available data. 2. Simulation results have d e m o n s t r a t e d that in metallic monoliths heat exchange properties are not downgraded significantly by introduction of a ceramic catalytic washcoat, and isothermal conditions tend to prevail. 3. Indications have been obtained for an appropriate selection of the channel size and of the void fraction in monolithic supports in order to optimize their overall heat transfer properties. 4. A preliminary analysis of the use of ceramic honeycomb catalysts for ethylene o x y c h l o r i n a t i o n reactors points out that monoliths with a low void fraction (25%) and small pitch (2.5 mm) should be selected in order to moderate the hot spots and to achieve satisfactory yields. Important improvements can be obtained by adopting a catalyst loading scheme where short monolith segments exhibiting a growing activity in the direction of flow are separated by mixing zones which promote radial transfer of heat and mass in the reactor tubes. Simulation results suggest that such a configuration may secure overall conversions similar to those of industrial packed-bed reactors with a significantly lower pressure drop. REFERENCES
Carrubba, R.V., J.L. Spencer, IEC Proc.Des.Dev.9,414 (1970). Degussa Aktlengesellschaft, Wacker Chemie GMBH, Europ. Pat. 0369439A2 (16.11.1989). Flytzani-Stephanopoulos, M., G.E. Voecks, DOE/ET-II326, Jet Propulsion Laboratory Publ.82-37, Pasadena (CA), 1981. Flytzani-Stephanopoulos, M., G.E. Voecks, T. Charng, Chem. Eng. Sci. 41, 1203 (1986). Heynderickx, G.J., G.F. Froment, P.S. Broutin, C.R. Busson, J.E. Weill, AIChE J. 37, 1354 (1991). Kolaczkowski, S.T., P. Crumpton, A. Spence, Chem. Eng. Sci. 43, 227 (1988). Kolaczkowski, S.T., P. Crumpton, R.P.J. Lee, Chem. Eng. J. 42, 167 (1989). Markeloff, R.G., Hydr. Processing, November 1984, p.91. Naworski, J.S., E.S. Velez, Applied Industrial Catalysis i, 239 (1983), Academic Press Inc. Zhernosek, V.M., Kinetica i Kataliz 12, 407 (1971).
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
775
Some aspects of e x t r u s i o n p r o c e d u r e for monolithic SCR catalyst based on TiO 2 V.Lyakhova, G.Barannyk, Z.Ismagilov Boreskov Institute of Catalysis, Novosibirsk, 630090, Russia
INTRODUCTION An increasing interest has been shown in the past few years to the monolithic honeycomb catalysts due to a high porosity of the contact layer formed (50-80%) and to a large hydraulic channel diameter (2-30 mm) [1]. Monolithic honeycomb catalysts are widely apply in catalytic purification of was exhaust gases containing ecologically harmful compounds: H2S, NOx, HCN, CO, etc [2,3]. Purification degree of the gas flows in a contact layer at 250-350~ is 80-85% [4]. Qualitative and quantitative composition of the catalysts for selective catalytic reduction (SCR) of nitrogen oxides with ammonia is often determined by the composition of a purified gas. Oxide vanadium-titanium compositions doped with MoO 3, WO 3 have gained the widest recognition [5,6]. Mixing oxides (or their precursors) with the proper inorganic and organic plasticizers it as possible to produce the catalyst composition suitable for the extrusive formation in form of the honeycomb monolith of various geometry. Inorganic plasticizers give the required mechanical strength to the catalyst monoliths, while organic plasticizers impart the necessary rheological properties to the catalyst mass which provide its successful extrusion. In the present work we study the effect of the organic plasticizer on the catalytic batch formation by extrusion and the catalytic properties of oxide compositions.
EXPERIMENTAL
1. Catalyst preparation Titanium dioxide used as a support was prepared in different manners such as vapour-phase hydrolysis of titanium tetrachloride, hydrolysis of titanium sulfate and butylate in an aqueous medium. After thermal treatment at 550~ the samples of TiO 2 represented anatase with a specific surface area from 20 to 100 m2/g. The active component ingredients were introduced in a form of vanadyl sulfate and wolfram oxide. The scheme of catalyst preparation is shown in Fig.1. As plasticizers of catalyst batch VOSO4 + WO 3 + TIC:)2 , we used organic and inorganic substances: polyethylene oxide (PEO), polyvinyl alcohol (PVA),
7'/'6
Preparation of ---~lFractionation ~ - ~ Mixing of porous oxide granular components materials
Preparation of plastic forming mass with desired rheological properties
I Organic plasticizer
Preparation of organic binder
Extrusion of mass in form of pipes and monoliths of honeycomb structure
Thermal treatment Fig.1. Block-scheme: Preparation of the monolithic DeNo x catalyst. carboxymethyl cellulose (CMC), polyakryl amide (PAA), natural resin (NR) combined with glycerin, ethylene glycol, polypropylene glycol hexatetramine, clay, alumina and zeolite. The catalyst mixture with the moisture content of 23-27% was extruded through the dies with a square side of 24,75 and 150 mm using a vertical hydraulic press (Russia), extruders VP-100 (Russia) and PVP-250 (Germany). After conventional thermal treatment, the catalyst monoliths a with specific surface area of 15-90 m2/g were obtained.
2. STUDY OF CATALYSTS
2.1. Rheological properties of catalyst pastes As shnplified characteristics of extruded catalyst pastes, we have chosen the plastic strength Pro, plastic viscosity Tim and the dynamic limit of catalyst paste fluidity Pk^MechaniCal strength of the catalyst paste structure was determined using the cone plastomer method with the cone angle of 45 ~ and the load from 100 to 1000 g by the formula: Pm = Ka P/H2, where K a m is a cone constant equal to 0,685; p m is a load on the cone, g; H m is the depth of cone immersion, mm.
(1)
777 Plastic viscosity of the paste was d e t e r m i n e d with a capillary type viscosimeter and calculated via the formula: Tim
=
nr4[p - 4 / 3 P o ] / 8 1Q ,
(2)
where r and 1 - - are a radius and a length of the viscosimeter capillary equal to 2,3 lO'3m a n d 11,5 lO'3m, respectively; Q - - is a mass consumption per second, m3/s; P, P0 - - are a c u r r e n t pressure and the limiting one, i.e. the pressure which provides the mass movement. Conventional dynamic fluidity limit, Pk-, serving as a dynamic characteristic of Z the catalyst paste, was defined f r o m the catalysts paste consumption dependence on pressure Pk2 -- 3r PQ/81
(3)
Thermal analysis of the catalyst pastes was performed in derivatograph Q 1500D. Support (200g) of the extruded mixture was heated at a rate of 10~ up to 700~ From the obtained DTA, T T and D T G curves the type of changes in the catalyst mixture procecding during treatment was defined. The X-ray data of the prepared catalyst were obtained on a D R O N installation using a monochromated Cu-K a irradiation at room temperature. To reveal the structure and morphology of the catalyst samples, electron microscopy analysis was performed on a R E M - 1 0 0 y installation.After the samples were prepared by the replica method, they were studied directly under electron microscope. Specific surface area of the catalyst support ( t i t a n i u m dioxide) was d e t e r m i n e d by a r g o n t h e r m a l desorption via BET. The sample porous s t r u c t u r e was defined using the m e r c u r y p o r o s i m e t r y method. The catalyst mechanical s t r e n g t h was defined as mechanical c r u s h i n g s t r e n g h t of the individual e x t r u d a t e between two parallel plates device MP-9C and calculated v i a the formula: P 0 - A N / ( d I - d2) 1
(4)
where A - - is an i n s t r u m e n t calibration coefficient, N - - is a load index at which an e x t r u d a t e destroys; d I - - is an e x t r u d a t e external diameter, mm; d 2 - - is a n e x t r u d a t e internal diameter, ram; 1 - - is an e x t r u d a t e length, ram. Catalytic activity was determined in a flow reactor, i n t r o d u c i n g 0,4 g of the catalyst grains, 0,6-1ram in size, in a glass reactor, 1 4 m m in diameter. S t a n d a r d gas m i x t u r e consisting of 0,005 w t % of NO; 0,005 w t % of NH3; 0,5 w t % of 02 in helium was supplied on the catalyst at a rate of 27000 h "1. Gas m i x t u r e was analyzed via c h r o m a t o g r a p h y . Catalytic activity was defined by the n i t r o g e n oxide conversion degree: X -- C(NO) start " C(NO) end / C(NO) start
(5)
778 R E S U L T S A N D DISCUSSION
Plasticizers play an important role in extrusive formation of the catalyst mass. They form the medium that allows the catalyst batch particles to move in respect to one another without deformation and collapsing. As a rule, the choice of plasticizers is random and is determined by their inertness in with respect to the catalyst batch constituents and the ability not to hinder the catalyst mass sintering and not to introduce the undesirable admixtures. To determine the effect of a plasticizer on catalytic properties at the initial stage, we analyzed the behavior of catalyst pastes under heating. Analysis of the thermographic data (Table 1) has shown that the introduced organic additives are susceptible to destruction under thermal treatment of catalyst pastes. Under such conditions, inorganic plasticizers (clay, A1203, zeolite) only lose water. Table 1 The thermographic data. Plasticizer
Effect
Intensity
T, ~
Notes
1. Polyethylene oxide ethylene glycol
endoexo-
low medium
100 190
H20 removal decomposition
2. Polyethylene oxide glycerine
endoexo
low medium
105 265
H20 removal decomposition
3. Polyethylene oxide propylene glycol
endoexo-
low medium
95 220
H20 removal decomposition
4. Polyethylene oxide hexamethylene tetraamine
endoexoexo
very low medium high
95 225 420
H20 removal decomposition decomposition
Derivation patterns of the catalyst pastes taken upon complicating the composition allowed to analyze the behavior of the introduced admixtures and to interpret the thermal effects on the heating curves (Fig.2). Endoeffect within 7095~ was found to involve the removal of moisture from the catalyst paste. Endoeffect at 195-210~ occurs upon destruction of organic additives. Exoeffect accompanies the VOSO4 ~ V205 transformation at 445-500~ Thus, in spite of the complex composition of the catalyst paste, its thermogram has a simple form. The X-ray phase analysis data of the initial catalyst batch and the prepared catalyst have showed that for this series of samples, among all the catalyst components, TiO 2 (anatase) and WO S are usually well registered. Diffractogram of the prepared catalyst is similar to that of the initial catalyst batch. This indicates the absence of any phase changes in the catalyst mass at the stage of catalyst preparation.
779 E
Comparative study of the structural characteristics (Ssp, Vz) of the oxide vanadium-titanium catalysts has shown D 90 145 ~ that the texture starts to form even at the stage of catalyst batch formation. When the plasticizer nature is varied, c catalyst specific surface area (Ssp) and 445 ~~.. 80~ _ 2 0 0 ~ , .. pore volume (Vz) remain practically unchanged (Ssn = 1 4 - 1 8 m 2 / g ; 445~ Vz - 0,36-0,4 cm~/g). 75 ~ When the content of glycerine, ethylene glycol, p r o p y l e n e glycol a n d Fig.2. DTA curves of initial masses: hexatetra-amine increases from 1,6 to A - VOSO4+TiO2 20 wt%, Ssp and Vr. do not change B - ["--"+TiO2+WO3] significantly either. Hence, upon therC - ["--"]+PEO mal treatment of the composite forms, D - ["--"]+PEO+ethylene glycol whose structure is determined by the E - ["--"]+PEO+ethylene glycol+latic acid initial components. As the electron microscopy photographs of the obtained catalysts with different organic plasticizers hav shown their morphology peculiarities are almost identical and remind the TiO 2 microstructure (Fig.3, Fig.4). So, the morphology of the catalyst does not depend on the nature and composition of plasticizer. It is known that the catalyst paste formation by extrusion and production of high-quality catalyst monoliths depend on the plasticizer composition, nature and the way of introduction. Mixing catalyst batch with a plasticizer it is possible to produce a dispersed structure consisting of the batch particles connected by plasticizer. This structure can be destroyed at a definite load. The plastic strength 5000
Fig.3. Microstructure of the catalyst paste based on polyethylene oxide x3600
Fig.4. Microstructure of the catalyst paste based on carboxymethyl cellulose •
780 value allows to define the maximum static tension in the mixture, typical for each extruded mass. Table 2 compares the plastic strength values, Pm, for the catalyst masses different in the nature of plasticizer. The data suggest a certain similarity of the thixotropic structure of catalyst pastes. Table 2. Effect of plasticizer nature on plastic strength of extruded catalyst mass Sample
Plasticizer
1. 2. 3. 4. 5
2% PEO + 10% 2% PEO + 10% 2% PVA + 10% 2% CMC + 10% 2% PEO + 10% + 10% of lactic
Pm 10"5 dyne/cm2 of glycerine ethylene glycol of glycerine of glycerine of glycerine + acid
1,1 1,5 1,4 1,7 1,1
The nature of plasticizer has a greater impact on the conventional dynamic fluidity limit and the plastic viscosity of catalyst pastes. Fig.5 presents the dependence of the catalyst paste consumption versus the pressure, which shows that in the presence of glycerine the pastes have a better structure. The catalyst paste based on ethylene glycol is more rigid and its dynamic strength limit is higher by a factor of 1,75 than t h a t of Q.IO-6 m3/c the glycerine-based one. The lower dynamic limit of strength and Bingam 1,2viscosity while glycerine is, probably, due to the nature of solvate shells of the catalyst batch particles. Perhaps, 0,8introduction of glycerine promotes the thickening of solvate shells thus affecting the adherence between particles 0,4and, hence, decreasing the s t r u c t u r e strength. .: . The data on the catalyst mechanical strength (P o) are shown in Fig.6. The 1;.10 -4, Pa functions of P o vs plasticizer (glycerine and ethylene glycol) are of a very simple form. Small amounts of addiFig. 5. Dependence of catalyst paste tives (< 3 wt %) increase the strength, consumption on pressure while further enhancement of the plasA [VOSO4+TiO2+wO~+PvA+glycerine; ticizer amount does not change the B -- ["--"]+CMC+glycerine; strength. This is typical for propylene C -- ["--"]+PEO+glycerine+latic acid; glycol and hexatetra-amine. Glycerine D -- ["--"]+PEO+glycerine+reinforcing as a plasticizer contributes the most to fibre; E - - ["--"] +PEO+glycerine; I -- ["--"]+PEO+ethylene glycol. the catalyst strength (1~o~9-12 kg/cm2). -
-
781
kg am 2
D
A
Po, kg am 2
1
..
O
4
12
C, ~
20
Fig. 6. Effect of the plasticizer nature on the catalyst mechanical strength A m glycerine; B ethylene glycol.
CpEO, ~ Fig. 7. Dependence of e x t r u d a t e mechanical s t r e n g t h on the content of polyethylene oxide.
In the absence of the second plasticizer (PEO, PVA, PAA, CMC, natural resin), the catalyst batch particles aggregate into sufficiently large secondary particles whose total boundary surface appears to be large and the solvate shells around the particles do not isolate the batch aggregates. Such compounds as PEO, PVA, PAA, CMC and natural resin, localize on the particle surface to make the m i x t u r e more plastic and to provide a closer cohesion between the batch particles. As seen from Fig. 7, the function of strength versus PEO content has a m a x i m u m at 1-2 wt %. If the content of PEO is no less than 4 wt %, the P o - PEO dependence is close to the linear one. The catalysts with plasticizers do not differ greatly in the manner of changing the mechanical strength. W h e n we compare the values of the catalyst mechanical strength, Po upon the introduction of 2 wt % of one_of the plasticizers (PEO, CMC, PVA, PAA, natural resin), it is apparent t h a t P o increases in the series PEO ~ PAA ~ CMC ~ PVA ~ natural resin. Catalytic activity values for the catalysts differing in the nature of organic plasticizer are compared in Table 3. Evidently, the increase of plasticizer a m o u n t does not affect significantly the NO x conversion degree. A more considerable influence on XNOX is provided by changing one plasticizer to another. The samples with P E O and CIqIC are comparable in activity, while on the samples with P A A and natural resin, XNO decreases to 58-76%. The sample with PVA has a m e d i u m value. The influence of plastmlzer on XNOx is not yet explicable. Perhaps, the final products of destruction of these plasticizers affect the catalyst active centers. It is also possible t h a t CMC and PEO decompose to form the m i n i m u m a m o u n t of carbon in the catalyst and natural resin causes carbonization of catalyst surface upon thermal treatment. X
~
~
782 Thus, the obtained experimental data allow to choose the plasticizer for regulation of the extrusion and catalytic properties of the formed masses. Using of PEO enables the production of highly active and sufficiently strong SCR catalyst. Table 3 Influence of the nature of organic plasticizer on the nitrogen oxide conversion degree Plasticizer
Content, wt %
Conversion of NOx, %
PAA PAA PAA PAA PAA
1,0 2,0 5,0 10,0 30,0
64,0 68,0 71,0 66,0 76,0
Resin
1,0 2,0 10,0
58,0 67,0 68,0
CMC
1,0 2,0
100,0 95,0
PVA
1,0 2,0
86,3 84,2
PEO
1,0 1,5 2,0 10,0
95,8 96,0 85,0 80,0
REFERENCES 1. 2. 3. 4. 5.
6.
E. Weber, K. Hubner, Energie, (1986), B.38, No.4, S.10 H. Bosch, F. Janssen, Catalysis Today, (1988), v.2, No.4, P.369 Z. Ismagilov, M. Kerzhentsev, Zh.Vses.Khim.Ob, (1990), v.35, No.l, p.43 L.Hamanu, P. Tieman, Energie, (1986), B.38, No.9, s.28 Japan Application No.58-45887, (1983) S. Matsuda, A. Kabo, Appl.Catalysis, (1983), v.8, p.149.
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
783
Preparation and characterization of catalytic supports with variable c o m p o s i t i o n in the system S i O 2 - A I 2 0 3 - A I P O 4. F. Wijzen a, A. Rulmont a and B. Koch b. aLaboratoire de Chimie Inorganique Structurale, D6partement de Chimie G6n6rale, Universit6 de Liege, Institut de Chimie B6, B-4000 Liege, Belgium. bSolvay S.A., Laboratoire Central, Rue de Ransbeek 310, B-1120 Bruxelles, Belgium.
A single method to synthesize supports of any composition in the system SiO2-AI20 3AIPO 4 is developped. Changing the chemical composition and controlling the preparation parameters allow to obtain supports with very different porosimetric and catalytic properties. The synthesis method has been designed to avoid phase segregation which generally gives materials which are poorly resistant to thermal recrystallization. Most supports are characterized by a high value of both the specific surface and the pore volume with a good thermal recrystallization stability. We analyzed the influence of the main experimental parameters on the properties of the supports.
1. INTRODUCTION Amorphous silica ,alumina and alumina-silica are commonly used in supported catalysis as they can be obtained with large specific surface values [1]. Aluminophosphates are also suitable catalyst support, mainly used as cracking catalyst [2-5] , for polymerization of ethylene [6,7], for oxidation or isomerization reactions [2]. The phosphate incorporation in silica supports is often achieved by impregnation with phosphoric acid [8] but in that case, the phosphate ions are only located at the pore surface. In the present work, supports with variable composition in the system SiO 2 - AIPO 4 - AI20 3 are prepared by coprecipitation of all the support constituant. 2. EXPERIMENTAL
2.1. Catalyst preparation There are several methods to prepare amorphous silica by precipitation : acidification of an aqueous solution of sodium silicate [9] or gelification of a silicon alkoxide solution [10]. As we want to use the same method to get the simultaneous coprecipitation of all the ions in the whole composition range, we choose to neutralize, by ammonia, a clear acid solution containing aluminum, phosphate and partially hydrolyzed tetraethylorthosilicate (TEOS).
784 The samples were prepared from tetraethylorthosilicate, phosphoric acid (85%) and hydrated aluminum chloride or nitrate. TEOS is first hydrolyzed by water in ethanol solution, with HCI as a catalyst. An aqueous solution of the aluminum salt and phosphoric acid is then carefully added. That clear acidic solution is injected at the base of a reactor containing an ammonia solution kept at 0*C and pH 8 by a pump coupled to a pH electrode. The gel is left for ageing in the mother solution at pH = 8 under slight stirring, washed with water and isopropanol and finally dried by water exchange in isopropanol. During the drying process, first, the azeotrope is continuously distilled, and than the isopropanol is distilled to recover a very fine free flowing powder. The dry powder is sieved (50 - 200 tam), calcined at 500"C before the impregnation of catalyst species.
2.2. Support Characterization. The composition of the final support was determined by X-ray fluorescence on the calcined samples. Mercury porosity has been measured with a Carlo Erba PORO 2000 device and surface area by the usual BET method. X-Ray diffraction patterns have been recorded on a Siemens D5000 X-ray diffractometer using CuKa monochromatized radiation ( ~, = 1.5406/~). 3. RESULTS AND DISCUSSION We investigated several chemical compositions described in the ternary diagram of figure 1 with the same synthesis parameters for the hydrolysis of TEOS, the precipitation and the ageing step.
AI203 13
1
2
3
4
5
6
7
8
Figure 1. Chemical composition of supports
9
785 3.1. Precipitate composition The chemical composition of the samples has been compared with the composition of the initial solution. The precipitate contains the same relative amounts of aluminum and silicon as in the solution before precipitation. But the phosphate content shows significant variations which can be correlated with the overall composition of the powder. Vogel and Marcelin [11] reported that pure aluminum phosphate can be prepared from aqueous media only at pH less than 4. At high pH (pH=8.5), they observed a mixed precipitate of alumina-aluminum phosphate. Approximately 6% of the phosphate was not precipitated. In our experiments, we also found a slight lack of phosphate (approximately 3%) in the formation of AIPO 4. Cheung [12] synthesized some aluminophosphates by the addition of ammonia to a solution containing aluminum and phosphate ions in less than 30 s without controling the pI-L For any ratio P/AI lower than 0,8, he observed the same composition for the precipitate and the precipitating solution. Above that value, the composition of the solid varies asymptotically towards AIPO 4 when the ratio P/AI in the initial solution increases. In our ease, we only observed a slight lack of aluminum in our aluminumphosphates which might be explained by the solubility of alumina at pH 8. It is interesting to note that although we worked at basic pH, we found less composition variation than Marcelin and Vogel who worked at a constant pH of 8.5 and still much less variation than Cheung who didn't control the pH. In the binary system SiO2-AIPO 4, a lack of phosphate appears in the solid and this lack increases with the proportion of silicon. The P/AI molar ratio in the solid phase decreases when the silica content increases whereas this ratio is kept equal to one in the solution. Figure 2 shows how the silica content modifies the P/AI ratio in the precipitate. When the silica content increases, all the phosphate ions cannot link n ~ a r i l y to a aluminum ff the coprecipitation is homogenous, and the probability of loosing phosphate by hydrolysis of the Si-O-P [13] bond increases.
1.20
0.80_ ~
-
0
~ r~
-
0.40
-
0
I
0
I
L
I
20
t
I
I
I
I
I
I
I
I
I
I
I
40 60 80 Silica content ( % )
~
i
l
100
Figure 2. Evolution of the P/AI ratio in the powder with silica content, in the binary system SiO2-AIPO4. The ratio P/AI in the solution is always equal to one.
786
3.2. Porous structure
Table 1 Effect of the chemical composition on specific surface area and porosity of supports. The numbers correspond to the chemical composition of figure 1.
n~1 n~ n~ n~ no5 n~ no7 n~ n~ n ~ 10 n ~ 11 n~ n~ n ~ 14 n ~ 15 n ~ 16 n ~ 17 n ~ 18 n ~ 19
SBE T (m2/g)
VBE T (cm3/g)
96 111 132 161 204 240 364 601 727 383 422 371 213 283 147 95 358 244 197
1.98 2.10 1.68 1.88 0.97 3.00 3.75 3.54 1.23 2.23 2.95 1.90 1.18 2.99 1.50 1.23 3.06 1.76 1.82
Vi-ig (cm3/g) 1.92 1.74 1.44 1.72 2.04 3.40 3.21 2.31 0.24 1.87 2.16 1.15 1.24 2.56 1.36 1.19 2.60 1.36 1.51
The three top compositions in thc ternary diagram (fig 1), synthesized by our method, have the following properties : silica contains mainly small (r<10nm) and spherical pores with large surface area (S= 727 m2/g). Alumina and aluminum phosphate have mainly open cylindrical mesopores of radius in the range of 8nm --- 20nm and 20nm --* 60nm respectively. They are characterized by a smaller specific surface area (S= 213 and 96 m2/g) than silica. All three compounds exhibit monomodal pore size distribution (fig 3). Table 1 and figures 3 and 4, show how the chemical composition strongly influences the porous structure of the materials in the system SiO2-AI203-AIPO 4. The specific surface area progressively decreases when the AIPO4 content increases in silica. In the system Al203-SiO2 , the coprecipitation of alumina into silica causes first a decrease of the surface area which seems to stabilize between 25% and 75% of alumina. AI 3+ and PO43" in silica reduce the micropores number and consequently the surface area.
On the contrary, in the systems x SiO 2- (100-2x) AI20 3- x AIPO 4 and Al203 - AIPO4, the surface area and pore volume go through a maximum for 75% alumina. From that maximum,
787 they decrease with the PO43" content. This trend has also been reported in that binary system by Marcelin and al [14] and Cheung [12] although the S e t V values and the preparation method are very different. 3.5 2.8 2.1
c s
1.4
9
s
9
9/
/
b
f~ I
0.7
, / , #
~ n
~
,
i
I
.,-"
t
! i
a '
~
t|
9~
1
lo
, J
t
lo 2
-I ~ . .. 9
I
|
i
lo3
9 i l.t
lo 4
Pore radius (nm)
a ! b A I
: : g
ii ii
!
~
z !
9
1
.
9 9 9 9 d,""
10
t,"*,
. . . . . . . . . . . . . . . . . .
102
"-"
103
104
Pore radius (nm) Figure 3. Vcu m and pore volume distribution by mercury porosity. Samples calcined at 700 ~C, a) SiO 2, b) AI20 3, c) AIPO 4. The presence of AI 3+ and PO43" in silica also changes the pore shape. Supports of intermediate composition in the binary systems SiO2-AI20 3 and SiO2-AIPO 4 both content open cylindrical and spherical pores while the pure compounds show only one pore type.
788 3.5
ff
2.8
_n
2.1 f"
1.4 0.7
""
b
o~, .~
/,! /
9
///
.L4:/"
0
10
1
102
103
10+
Pores radius (nm).
,-k
, i ,
.
.
.
.
.
.
Ill# i V i
10
102
103
104
Pores radius (nm).
Figure 4. Vcu m and pore volume distribution by mercury porosity. Samples calcined at 700"C, a) 50 SiO 2 50 AIPO4, b) 25 AI20 3 37.5 AIPO 4 37.5 SiO2, c) 50 AI20 3 25 AIPO 4 25 SiO 2.
There is also a gradual evolution of the pore size distribution as a function of the chemical composition and we do not observ a mixture of pores characteristic of the pure compounds which is some homogeneity criterion. On the contrary of the pure compounds, the pore size distribution is generally bimodal with the higher maximum lying between 10rim and 40nm
789 followed by a rather broad distribution of larger pores (fig 4). These pores (r>40nm ; with limiting value depending on composition) appears for most intermediate compositions and are responsible for the large pore volume measured by mercury porosimetry. The aggregation of the secondary particles seems thus deeply influenced by the presence of foreign ions in the silica network and our drying method allows the preparation of very open porous structure. Nearly the same evolution have been reported by P.A. Sermon at al [15] for the binary system SiO2-AI20 3. They also found that the introduction of AI 3+ in silica reduces the extent of microporosity and creates meso and macroporosity into the samples but they observed smaller increase of the pore volume for the intermediate compositions, probably because their drying method was less efficient. The variation in chemical composition can be used to monitor the porosity of the support but alters at the same time the properties of the surface sites. The porosity of the catalyst support is also strongly influenced by the ageing process. The behavior of the precipitated gel during ageing has been studied in the system SiO 2 - AIPO 4 and closely depends on the chemical composition. As expected, the specific surface area strongly decreases with ageing time and temperature by a coarsening process but the effect is more pronounced in the support containing 87.5% of silica (table 2). This can be explained by two effects : first, in basic solutions, the solubility of AIPO 4 is very small, second, the presence of AIPO 4 decreases the number of small pores in the original gel. Table 2 Effect of time and ageing temperature on the whole surface area (m2/g), and the surface area (m2/g) corresponding to pores with radius less than 8nm. Composition . 50% SiO 2 - 50% AIPO 4 20.C 2ia " 193, 105 13 h 218, 125 24 h -
.
.
. . 87.5% SiO 2 - 12.5% AIPO 4
45.C
70-C
20-C
45.C
70-C
215, 130' 199, 127 -
204, 114 184, 70
664, 390' 692, 417 551, 328
635, 401 642, 365 -
587, 303 304, 121
The pore volume evolution with ageing is less predictable. In fact, we haven't observed a regular increase of the BET pore volume with ageing (table 3). Moreover, the pore volume corresponding to radii lying between 7,5 nm and 105 nm does not vary a lot with ageing. This might be due to the drying method which is efficient enough to preserve the meso and macroporous structure even with a short ageing period. The main modification which occurs during ageing is the filling of small pores by coarsening which causes a decrease of the specific surface but does not change very much the pore volume. The BET pore volume variation is likely due to a lack of reproducibility in the macroporous range. This drawback could probably be avoided by using a spray drying method.
790 Table 3 Effect of time and ageing temperature on the pore volume BET (cm3/g) and the pore volume associated with pores of radius less than 105 nm (measured by mercury porosimetry (em3/g). Composition 50% SiO 2 - 50% AIPO 4
2h 13h 24h
87.5% SiO 2 - 12.5% AIPO 4 20"C
20"C
45"C
70"C
2.30, 1.37 1.94, 1.30 1.43, 1.29
1.47, i.38 1.71, 1.26 -
2'48, i.61 '" 3.71, 1~52 3.55, 1.55 2.33, 1.34 3.07, 1.16
45"C
70"C
2.99, 1.52 3.47, 1.38 -
3.64, 1.39 3.05, 1.21
3.3.XRD
C
f
a
10
20
30
40
Diffraction angle ('20) (Z. = 1.5406/~).
10
20
30
40
Diffraction angle (* 20) (~ = 1.5406/~).
Figure 5. X-ray diffraction patterns of support 50 SiO 2 - 50 AIPO 4 prepared at constant pH, calcined a) 4h at 1000*C, b) 4h at 1050.C, c) 4h at ll00*C and support 50 SiO 2 - 50 AIPO 4 prepared at variable pH, calcined d) 4h at 1000*C, e) 4h at 1050"C, f) 4h at ll00*C.
791 All supports keep their amorphous state on thermal treatment up to 700~ samples, some u or r I alumina structure appears at 700"C.
In alumina-rich
In the system SiO2-AIPO 4, three peaks characteristic of crystobalite and trydimyte phases appear at higher temperatures but most of the product remains amorphous. The resistance to crystallization of the amorphous support is influenced by different synthethic parameters as the precipitation method or the prehydrolysis of TEOS. Two samples of the same composition (50 SiO 2- 50 AIPO4) have been prepared by the same route except the coprecipitation step. In one case, the ammonia solution is injected in the acid solution until the pH is equal to 8 (pH variable) and in the other case, the acidic solution is injected at the base of a reactor containing an ammonia solution kept at 0*C and pH 8 (constant pH). The second sample is more resistant (constant pH) to reerystallisation (fig 5). That behaviour can be understood in terms of a better homogeneneity of the second sample. ~
I=
b
d
a ,,-
10
,
,,
,
|
.
.
.
.
|
.
.
.
.
20 30 z0 Diffraction angle (* 20) (k = 1.5406 ~).
10
20
30
40
Diffraction angle (*20) (k = 1.5406/~).
Figure 6. X-ray diffraction patterns of support 50 SiO 2 - 50 AIPO 4 prepared with hydrolysed TEOS, calcined a) 4h at 1000*C, b) 4h at 1050"C, c) 4h at 1075"C and support 50 SiO 2 50 AIPO 4 prepared with non hydrolysed TEOS, calcined d) 4h at 1000*C, e) 4h at 1050-C, f) 4h at 1075-C.
792 The prehydrolysis of the TEOS can also improve the resistance to crystallization. A sample of chemical composition 50 SiO 2- 50 AIPO 4 crystallizes at a higher temperature if the TEOS is hydrolysed during two hours at 60 ~ under acid catalysis than if no hydrolysis is made before the coprecipitation step (fig 6). Moreover, without the hydrolysis step, the precipitation of TEOS is not quantitative and the silica precipitation partly occurs after the precipitation of alumina or aluminophosphate. This causes inhomogeneity in the amorphous phase which eases the thermal recrystallization and decreases the useful temperature range.
4. CONCLUSIONS. Controled basic coprecipitation gives homogeneous porous and amorphous materials with a composition varying continously in the ternary system SiO2-AI203-AIPO4. The porous structure of mixed amorphous network is very different from that of the pure compounds. In some composition range, it is possible to get free flowing powders with a large value of both the specific surface and the porous volume. Furthermore, they can be activated in a large temperature range as they are very stable towards thermal re,crystallization. Due to the combination of all these properties, these materials are very suitable as catalysts supports.
REFERENCES
1. M.P. McDaniel, EU Patent No. 0 040 362 (1981). 2. J. B. Moffat, Catal. Rev -Sci. Eng., 18(2) (1978) 199-258. 3. K.K. Kearby, US Patent No. 3 342 750 (1967). 4. K.K. Kearby, Actes du 2* congr6s de catalyse (Paris 1960), Technip (1960) 2567-2578. 5. Rimantas, Glemza, EU Patent No. 0 215 336 (1987). 6. R. W. Hill, W.L. Kehl, T. J. Lynch, US Patent No. 4 219 444 (1980). 7. M.P. McDaniel, US Patent No. 4 364 854 (1982). 8. M.P. McDaniel, EU Patent No. 0 055 866 (1981). 9. D.R. Witt, US Patent No. 3 900 457 (1975). 10. M.P. Me Daniel, EU Patent, No. 0 040 362 (1981). 11. R.F. Vogel, G. Marcelin, J. Catal., 80 (1983) 492-493. 12. T.T.P. Cheung, K.W. Willcox, M.P. Me Daniel, M.M. Johnson, J. Catal., 102 (1986) 1020. 13. Iler, The chemistry of silica, Wiley, New York, 1979. 14. G. Marcelin, R. F. Vogel, H.E. Swift, J. Catal., 83 (1983) 42-49. 15. P.A. Sermon, T.J. Walton, M.A. Martin Luengo, M. Yates, Characterization of porous solids II, Elsivier Sciences Publishers, 1991.
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
NEW MODIFICATION OF ALUMINA: PROCEDURE AND EXISTENCE CONDITIONS
793
PREPARATION
B.P. Zolotovskii and R.A. Buyanov Boreskov I n s t i t u t e of Catalysis, Novosibirsk, 630090, Russia A l u m i n a is widely used in various industries as supports, catalysts and sorbents. The l a t t e r ' s properties depend to a great extent on a l u m i n a modification which is defined by the s t r u c t u r e of a preceding a l u m i n a / I , 2 , 3 / . All the known low-temperature modifications of alumina contain AI(III) ions 4- and 6-coordinated with respect to o x y g e n / 4 / . Recently, a 5-coordinated ion of A I ( I I I ) h a s been also found using 27A1 NMR to o c c u r beside 4- and 6coordinated AI(III) ions bonded to oxygen / 5 - 7 / . A new modification of alumina having AI(III) cations t h a t are 4-, 5- and 6-coordinated with respect to oxygen is called ~ - A 1 2 0 8 / 5 , 6 / . Here, we report on the preparation procedure and existence conditions of ~-A1208. For the first time, ~-A1208 was obtained by thermal t r e a t m e n t of X-ray amorphous a l u m i n i u m hydroxide produced via the mechanochemical activation of a l u m i n i u m trihydroxide. Mechanochemical activation of the layered AI(III) t r i h y d r o x i d e involves: (i) crushing of granules and crystals; (ii) shift and azimuthal t u r n of hydroxyl packets with cleavage of hydrogen bonds binding the packets; (iii) splitting of crystals into the plates with a minimal thickness up to I0 A. The plates in aggregates remain parallel to each other. In the course of mechanochemical activation the b r u t t o composition of t r i h y d r o x i d e remains AI(OH) s. However, IR data point to the changes in the state of s t r u c t u r a l "water". Some OH-groups interact with one a n o t h e r to produce the molecular water which is shown by 27A1 NMR data to stay in the first coordination sphere of AI(III) ion / 7 , 8 , 9 / . As a result, the first endothermal effect on the heating curves shifts to the low-temperature region by I20 ~ and a new exothermal effect appears at I083 K. A sample is calcined until the effect appears. Such calcination controlled by DTA and the X-ray phase analysis indicate an X-ray amorphous alumina in the range of calcination t e m p e r a t u r e s 523-I000K. The exothermal effect at I083 K is due to the transition of X-ray amorphous oxide (~-A1208) to ~-A1208. Thus, the new ~-A1208 exists in the range of 500-I000 K i.e. between the endothermal and exothermal effects. 27A1 NMR s p e c t r u m of a non-calcined X-ray amorphous hydroxide exhibits a singlet with a chemical shift 5 ppm t h a t corresponds to the 6coordinated AI(III) ion / 4 / (see the Scheme). As the product is heated, 27A1
gibbsite
amorphous phase
Al(OH)3
Al(OH)3
--
displacement
and turn around of packets
packets- I
MCA
-H20
A B
B
T
7
A
OH; H20,0m2 OH, H20,0-2
0"; OHo'~, OH
OH; H ~ O , O - ~ OH; H 2 0 , o 2 -
0'; OH0'; OH-
dehydration 7
OH
300 OC
180 OC
region of existence
- A1208
-
7) -A120a
795 NMR spectra exhibit two additional signals with the chemical shifts 63 and 36 ppm corresponding to the 4- and 5-coordinated AI(III) ions. All the aforesaid is illustrated in the Scheme. Cationic distribution in ~-Al203 is not constant t r e a t m e n t t e m p e r a t u r e (see Table I).
and depends
on the
Table I. Phase composition and parameters of 27A1 NMR spectra for the p r o d u c t of thermal treatment of mechanochemically activated AI(OH)3 under non-isothermal conditions. Phase composition
Chem. shift, ppm
Ratio of peak intensities
T oK Initial
523 633 823 998
1203 1323
4
5
6
I4/I 6
-
-
5
-
X-ray amorphous hydroxide
~-A1203 ~-A1203 ~-A1203 ~-A1203 ~-A1203 ~'A1203
I5/I 6
6
35
5
0.47
63
36
3
0.54
0.4
63
34
3
0.63
0.57
66
36
6
0.76
0.68
66
-
5
66
-
10
-
-
0.33
To elucidate the mechanism of formation of 5-coordinated AI(III), we have performed the quantum-chemical calculations / 6 / . Calculations have shown t h a t the most energetically preferable mechanism of dehydroxylation involves the interaction of OH-groups located on the edge common for two alumohydroxide octahedrons. The water formed stays in the first coordination sphere of A I ( I I I ) / 8 / . Upon thermal t r e a t m e n t , the molecular water is removed and two 5-coordinated AI(III) ions adjacent to each other are formed. According to our method of thermochemical activation / 1 0 / , one can also use a l u m i n i u m trihydroxide to obtain the product with 4-,5- and 6-coordinated AI(III) ions, close in its properties to ~-A1203. Dehydration via thermochemical activation is performed with the rates significantly greater t h a n t h a t of t r a n s f o r m a t i o n of trihydroxide with a layered crystalline frame into the oxide of a crystal cubic structure. The latter is provided by the rapid heating of trihydroxide, mild temperature conditions of activation for a definite period of time, by the maintenance of the given partial pressure of water vapors, rapid cooling and calcination of the activation product. Thus, remo~'al of the molecular water from the layered crystalline lattice of trihydroxide without its considerable t r a n s f o r m a t i o n gives the intermediate with 4-,5- and 6-coordinated AI(III) ions.
796 Understanding of the structure of AI(OH)3 fresh precipitates / 1 1 , 1 2 / allowed us to use these as a source of n-A1203 as w e l l / 1 2 / . Subsequently, it was reported /13/, that the aluminium oxide with 4-, 5and 6- coordinated A1 cations was synthesized via the thermal treatment of aluminium hydroxides produced by hydrolysis of various AI(III) alcoholates. All the above results allowed to define the existence conditions for the 5coordinated aluminium i o n / 1 4 / . a) The initial compound contains aluminium ions in oxygen octahedrons involving OH-groups. b) The layered structure is retained upon thermal, mechanochemical and other treatments leading to dehydroxylation. c) Initial molecular water or that provided by dehydroxylation leaves the first coordination sphere of AI(III) ion.
~
a 2
0
1
0
1200
1800
1'100 15'00 1600
17"00 sm'l
H2 / /
N f
"\
/
\
\
b
H
Figure 2. IR-spectrums NH3 adsorbed on TI-AI203 (1), n-A1203 (2) and dissociative adsorption NH3 on "coupled" Lewis center. Reprinted from: B.Zolotovskii, S.Paramzin et all. Kinet and Catal. 30 (1989) 1439.
797 So, we have considered the existence and preparation conditions for the new modification n-A1203. Note, t h a t here we do not give an e x h a u s t i v e account of n-A1203 preparation techniques. At the I n s t i t u t e of Catalysis ~ - A 1 2 0 3 has been produced in form of granules of various shapes: grafts, balls etc. The samples of n-A1203 are obtained, involving up to 45% of 5coordinated AI(III) ions. In conclusion, we would like to say a few words about the properties of the new modification. Polymorphous transformations upon heating of ~-A1203 may be presented as f o l l o w s / 1 5 / : ~-A1203
1000-1015 K
> T1-A1203
1133-1283 K
~ ~-A1203
As seen from the series presented, the phase t r a n s f o r m a t i o n s do not involve the Q-A1203 stage, what occurs when the initial oxide does not contain 4-,5- and 6-coordinated AI(III) ions. Adsorption properties of alumina with 4-, 5- and 6-coordinated ions should differ from those of the oxide with only 4- and 6-coordinated AI(III). This is mostly due to the presence of two 5-coordinated AI(III) located side by side in n-A1203. These ions may be considered as "coupled" Lewis centers. The IR-spectra of NH 3 adsorbed on T1-A1203 and n-A1203 are different (Fig. 2). The l a t t e r is assumed to result from the dissociative adsorption of NH 3 on the "coupled" Lewis centers in n-A1203. Such centers are likely to possess the p a r t i c u l a r adsorption properties and, hence, to find application in catalysis. Thus, n-A1203, as well as low-temperature transition forms T-, T1- and ~-A1203, are typical for the H20-A1203 system.
References
1. 2. 3. 4. 5. 6. 7.
B.Wippens,, J.Steggerda: Active A l u m i n a in : Physical and Chemical Aspects of Adsorbents and Catalysts, Ed B.Wincen, P a r t IV. Academic Press, London and New-York 1970. T.Sato, J of Thermal Anal., 32, (1987), 61. C.Misra, Industrial Alumina Chemicals, ACS Monograph, 184, W a s h i n g t o n , D.C., 1986. V.Mastikhin, O.Krivoruchko, B.Zolotovskii, R.Buyanov, React. Kinet. Catal. Lett, V.18 (1981) 117. V.Paramzin, B.Zolotovskii, O.Krivoruchko, R.Buyanov, Proc. VI I n t e r n . Symp. Heterogeneous Catalysis, Sofia, (1987) P.2, 369. B.Zolotovskii, S.Paramzin et al, Kinet. and Katal.,(Rus), 30 (1989) 1439. O.Krivoruchko, V.Mastikhin, B.Zolotovskii et al, Kinet. and Katal.(Rus), 26 (1985) 763.
798
Q
t
10. 11. 12. 13.
14. 15.
S.Paramzin, L.Plyasova, O.Krivoruchko et al., Izv. Akad. Nauk SSSR, Ser.khim. (Rus), (1988) 1209. S.Paramzin, B.Zolotovskii et al., Izv. Sib. Otd. Akad. Nauk S S S R (1989) Ser.Khim.Nauk, N2, 33, (Rus). R.Buyanov, O.Krivoruchko, B.Zolotovskii. Izv.Sib.Otd.Akad.nauk SSSR (1986), Ser.Khim.Nauk, No.6, 39 (Rus). J.Y.Bottero, M.Axelos, D.Tchonbar. J.M.Cases. J.J.Fripiat, J.Coll.Interface Scin., 117 (1987) 47. S.Paramzin, B.Zolotovskii, R.Buyanov et al. Sib.Khim.Zh., (Rus), (1992) 130. T.Wood, A.Siedle, J.Hill, R. Skarjune, J.Coodbrake, Mat.Res.Soc. Syrup. Proc, 180 (1990) 97. D.Klevtsov, O.Krivoruchko et al., Dokl. Akad. Nauk SSSR, 295 (1987) 381, (Rus). B.Zolotovskii, W.Loiko et al, Kinet and Katal., (Rus), 31 (1990) 1014.
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
Preparation layered
and
799
characterization
of
silica-pillared
titanate
Wenhua Hou*, Oijie Yah, Yi Chen and Xiancai Fu Department
of
Chemistry,
Nanjing
University,
Nanjing
210008,
People's Republic of China
A silica-pillared layered titanate was prepared by
reacting
H2Ti409 with pure NH2(CH2)3Si(OC2Hs)3(abbreviated hereafter as APS), washing the precipitate with water, which leads to the hydrolysis and polymerization of the intercalated APS with the formation of APS 01igomers, remove
the
and calcinatin~ the product
interlayered
organics.
The
in air at 500~
silica-pillared
to
layered
titanate thus obtained has an interlayer distance of 14.7~ and a specific surface area of 45.9m2/g.
1 .
INTRODUCTION
Layered metal oxides(e.g, layered titanates, titanoniobates and niobates)
intercalated
extensively sorption,
with
organic
compounds
studied because of their potential
catalysis and conduction[i-3].
have
applications
However,
oxides,
layered metal compounds[4-6]. titanoniobate
which
have
higher
oxides prepared We can
have react
thermal
by the
reported with
stability layered
layered
than
intercalation
that
in
there are few
studies on the preparation of inorganic oxide-pillared metal
been
those
of organic niobate
and
NH2(CH2)3Si(OC2Hs)3(abbreviated
hereafter as APS) aqueous solution to obtain porous, thermostable silica-pillared
layered
trialkoxysilane
compound,
metal
oxides[7,8].
APS
is
a
its trialkoxy groups quickly undergo
800
h y d r o l y s i s to form trihydroxyl groups and then p o l y m e r i z e to form a
polysilane
Contrary
network
to
structure
layered
titanate(H2Ti409.nH20)
niobate can
not
synthesize
silica-pillared
relatively
acidity.
successful
preparation
w h i c h was p r o d u c e d APS aqueous
of
calcinated
2.
of
in
the
titanate
due
present
a
work,
silica-pillared
and w a s h i n g
and p o l y m e r i z e
interlayered
APS
solution[9-11].
titanoniobate,
treated
layered
the
aqueous
by r e a c t i n g H2Ti409
solution,
w a t e r to h y d r o l y z e formation
In
be
in and
same to
we
layered
intercalated
the i n t e r c a l a t e d
oligomers,
the
in air to remove the i n t e r l a y e r e d
way its
to low
discuss
the
titanate,
w i t h pure APS
the
layered
instead
H2Ti409
of
with
APS w i t h the
product
was
then
organics.
EXPERIMENTAL
2.1. P r e p a r a t i o n p r o c e d u r e K2Ti409 was p r e p a r e d a c c o r d i n g to literature[I]. Ion e x c h a n g e of K2Ti409 was carried out with
1N HCI at 70~
to afford H2Ti409.
H2Ti409 was added to 33g pure APS with stirring. stirred at room t e m p e r a t u r e separated by c e n t r i f u g i n g
2.2. C h a r a c t e r i s a t i o n
for 120h,
The m i x t u r e
5g was
and the solid p r o d u c t was
and w a s h e d with d i s t i l l e d water.
techniques
P o w d e r X-ray d i f f r a c t i o n patterns were c o l l e c t e d on a S h i m a d z u XD-3A d i f f r a c t o m e t e r
operated with Cu Ku r a d i a t i o n
(A=1.5418~).
Infrared spectra were recorded on a N i c o l e t 510P FT-IR i n s t r u m e n t using
pore
size
d i s t r i b u t i o n of the sample were m e a s u r e d on a M i c r o m e r i t i c s
ASAP
2000
KBr
wafer
techniques.
instrument.
The
sample
The
was
surface
first
area
degassed
and
at
350~
followed by the m e a s u r e m e n t of N 2 a d s o r p t i o n - d e s o r p t i o n at liquid n i t r o g e n
3.
RESULTS
The
AND
layer
and
isotherm
temperature.
DISCUSSION
structures
of
K2Ti409 and
H2Ti409 have
been
well
801
documented[12].
The
interlayer
distances
of
KzTi409 and
HzTi409
were 8.7~ and 9.0~ respectively. It is interesting to find that although H2Ti409 does not react with APS aqueous solution,
it can react with pure APS to form an
intercalated titanate with a relatively high interlayer distance. Figure
1 shows
HzTi409 and
XRD
patterns
intercalated
temperatures.
Upon
of
HzTi409, pure
HzTi409 calcinated
treatment
with
pure
APS
in air APS,
intercalated at
the
distance(doo 2) of H2Ti409 increases
from 9.0~ to 17.0~,
seen
peak
from the
shift
of the
(002)
different interlayer as can be
from 20=9.8 ~ to 5.2 ~ in
Figure 1. The increase of the interlayer distance by as much as 8.0~ upon treatment clearly shows that intercalation has indeed taken place. and
reacts
It seems that HzTi409 might work as a BrSnsted acid with
organic
BrSnsted(APS)
to
form
intercalated
compound. A decrease in interlayer distance and c r y s t a l l i n i t y is found
when
500~ organics
the
intercalated
lc). within
Therefore,
DTA-TG
the
layers
product analysis begin
was
calcinated
results
to decompose
reveal
in
air
that
at about
at the
300~
after the intercalated product was calcinated in air
d w
c~
Ob.
I
c
2
"
e
Figure 1. XRD patterns of (a)H2Ti409, (c) (b) calcinated at 500~
and
(b)APS intercalated H2Ti409,
(d)(c) calcinated
at 600~
802
at
500~
the
silica-like a
interlayered
clusters
silica-pillared
interlayer Ti4092"
which prop
layered
distance
APS
oligomers
decomposed
open the Ti4092
titanate
is
of 14.7~(20=6.0~
from
the
interlayer
value of 5.7~ is obtained. silica-pillared
layered
Therefore,
collapse
as
the
600 ~ (Figure
and
has
an
of b a s a l
the d i a m e t e r
of K §
of K2Ti409(8.7~ ), and
the p i l l a r
tetratitanate
structure of this s i l i c a - p i l l a r e d
which
The t h i c k n e s s
distance
form
layers[13],
obtained,
layers can be estimated by s u b t r a c t i n g
ions(3.0~)
to
is
height
9.0~.
a
of this
The
layer
layered t e t r a t i t a n a t e b e g i n s to
calcination
temperature
was
raised
to
Id).
The above
intercalation
reaction
of pure APS
into H2Ti409 is
further confirmed by the formation of the NH3 + groups as d e t e c t e d by the p r e s e n c e solid product.
of NH3 § vibrations
in the FT-IR
spectrum
As shown in Figure 2, the i n t e r c a l a t e d
of the
H2Ti409 has
c h a r a c t e r i s t i c absorptions of the i n t e r c a l a t e d amines in a d d i t i o n to that of the host material[4]. 1168
and 958cm "I
indicates
The absence of Si-OC2H 5 bands at
that the
interlayered
APS
was
G
4000
3000
2000
1000
wave nurnber(cm -1) Figure
2. IR spectra and
(c)(b)
of
(a)HzTi409,
calcinated
(b)APS
at 500~
intercalated
HzTi409,
803
completely
hydrolyzed
distilled water[11]. can
be
attributed
hydrolyzed after
APS was
after
In addition, to
Si-O-Si
furtherly
washing[10,11].
after
heating
the
washing
It
the
can
disappearance
of the vibration
1528cm "I, etc)
evidenced
product
also
be
indicating to form APS
seen
from
product
in
air
modes
of C-H,
that
the
oligomers
Figure at
2
500~
that the
N-H and C-C(2934,
the removal of interlayer
and further polymerization
with
the bands near 1118 and 10240m "I linkages,
polymerized
reaction
solid
of the intercalated
organics
species.
The starting material K2Ti409 is nonporous, and has a fairly low BET
surface
area
silica-pillared area
of
45.9
intracrystal
as
layered
mZ/g,
titanate
In has
the
contrast,
the
a relatively
existence
of
high
an
resulted surface
appreciable
surface area. The N 2 a d s o r p t i o n - d e s o r p t i o n
isotherm
layered titanate is shown in Figure 3. The
isotherm of the pillared sample is between type I and
II at low pressures[14]. of
m2/g.
indicating
of the silica-pillared adsorption
3.3
This type of isotherm is characteristic
materials in which both micropores and mesopores are present.
The hysteresis is porous.
loop of the curve demonstrates
Moreover,
the loop is somewhat
that this material
like type H3 and H4
loops as classified by K.S.W. Sing, et a1.[14].
This type of loop
D.
,--adsorption + - - d eso rptio n
-,~.r o V
0
E20
relative Figure
pressure(P/P,)
3. A d s o r p t i o n - d e s o r p t i o n silica-pillared
isotherm of N 2 on
layered titanate (500~
804
is often observed with aggregates of p l a t e - l i k e p a r t i c l e s g i v i n g rise to slit-shaped pores.
4. C O N C L U S I O N S
A new method pillared
successfully
layered titanate.
titanate
is
(45.9m2/g), (>500~
has been
porous,
to p r e p a r e
silica-
The resulted s i l i c a - p i l l a r e d
layered
with
interlayer
a
used
relatively
distance(14.7~)
high
and
surface
thermal
area
stability
.
The support of the National Nature Science F o u n d a t i o n of China is greatly acknowledged.
REFERENCES
1. H.Izawa,S.
Kikkawa and M. Koizumi,
2. A. Grandin,
M.M.
Polyhedron,
Borel and B. Raveau,
2(1983)74.
J. Solid State Chem.,
60(1985)366.
3. R. Nedjar,
M.M.
Borel and B. Raveau,
Z. anorg,
allg.
Chem.,
S40/541(1986)198. 4. S. Cheng and T. Wang, 5. M.W.
Anderson
6. M.E.
Landis,
Chem.
Chem.,
B.A. Aufdembrink,
Kirker and M.K. Rubin, 7. W. Hou,
Inorg.
and J. Klinowski,
B. Peng,
Commun.,
P. Chu,
J. Am. Chem.
Q. Yah,
28(1989)1283.
Inorg.
Chem.,
29 (1990) 3260.
I.D. Johnson,
Soc.,
x. Fu and G. Shi,
G.W.
113(1991)3189. J. Chem.
Sot.,
(1993) 253.
8. W. Hou, J. Ma, Q. Yan and X. Fu, J. Chem. Soc., Chem. Commun., (1993)1144. 9. M.G. V o r o n k o v and Lavrent'yev, 102(1982)199.
Top.
Curr.
Chem.,
805
10. H. Ishida,
C. Chiang and J.L.
11. C. Chiang,
H. Ishida and J. Koenig,
Sci.,
Koenig,
Polymer,
J. C o l l o i d
23(1982)251. Interface
74(1980)396.
12. M. Dion,
Y. P i f f a r d and M. Tournoux,
J. Inorg.
Nucl.
Chem.,
4o(1978)917. 13. L. Li, X. Liu, Y. Ge, L. LI and J. Klinowski,
J. Phys. Chem.,
95 (1991) 5910. 14. K.S.W.
Sing,
R.A.Pierotti, Chem.,
D.H.
Everett,
J. Rouquerol,
57 (1985) 603.
R.A.W.
Haul,
L.Moscou,
T. Siemieniewska,
Pure & Appl.
This Page Intentionally Left Blank
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
807
Alumina support modified by Zr and Ti. Synthesis and Characterization Tonfis Viveros 1. , Alberto Z~irate1, Miguel A. L6pez 1, J. Ascenci6n Montoya 2, Richard Ruiz 1, Margarita Portilla a 1Area de Ingenieria Quimica, Universidad Autonoma Metropolitana Iztapalapa, Apdo. Postal 55-534, M6xico D.F. 09340 2Instituto Mexicano de1Petroleo, Apdo. Postal 14-805, M6xico D.F. 07730 aFacultad de Quiml'ca, UNAM, M6xico D.F. 04510
ABSTRACT Alumina-zirconia and alumina-titania mixed oxides were prepared by coprecipitation of their corresponding chlorides. Samples were synthezised at several concentrations, dried and calcined at temperatures between 400~ and 800~ All dried samples showed the bayerite structure, which upon calcination became first amorphous and later AI203. Anatase was observed only on the high content, high calcination temperature sample, but no crystalline phase of zirconia was detected. Surface areas of mixed oxides were higher than for alumina single oxide, and decreased with temperature. Carbon tetrachloride adsorption increased with TiO 2 content, but the opposite effect was observed on alumina-zirconia samples. 1. INTRODUCTION Alumina is one of the most employed catalytic materials, either as a support, a catalyst or a cocatalyst. Transition aluminas, especially y and 11, posses high specific surface areas, (100 300 m2/g), and amphoteric nature of the surface. ),-At203 has lower surface acidity than 11A1203 but is thermally more stable. Less common oxide supports such as TiO 2 and ZrO 2 posses interesting properties still unexploited. However they have some disadvantages: they are less stable to temperature and offer lower surface areas than alumina. Reports in the literature show that the addition of La, Zr and Si modify the alumina surface areas and its thermal stability (1.2). A1203-TiO 2 supports have been reported to offer higher acidity than alumina, and improved performance in hydrotreating reactions (3). On the other hand the combination ZrO2-A1203 provides greater mechanical strength which results in improved resistance to attrition (4,5), however the interaction does not modify the acidity of alumina, but decreases the Br~nsted basicity of zirconia (6). An important feature to consider is the preparation method of the mixed oxides as Tanabe et al. (7) have pointed out in relation to the evaluation of the acid-base characteristics. The majority of the papers dealing with mixed oxide supports have been concerned with the preparation of a metal oxide-coated alumina, so that the surface characteristics are different from those of the bulk. The interesting point about the preparation of mixed oxides by *To whom correspondence should be addressed
808 precipitation or gelation derives from the possibility of producing materials with surface areas larger than the individual components, and greater mechanical strength. In this work we report the preparation of alumina modified by zirconia or titania using a precipitation technique. The objective was to obtain mixed oxide supports for which we could asses the resulting structural and textural properties, and their thermal stability. AI203-TiO 2 and A1203 -ZrO 2 at several compositions were characterized by TGA, DTA, XRD, N 2 adsorption and CC14 adsorption. 2. E X P E R I M E N T A L
2.1. Support preparation The binary oxides were prepared from their corresponding chlorides, following the procedure outlined in the preparation o f single oxide supports (8). Typically 0.5 M solutions of the corresponding chlorides (TIC14, ZrOC12, AiC13) were prepared and stored. Appropiate amounts of the respective solutions to obtain a desired concentration o f the material were mixed, and added simultaneously with an ammonia solution to a stirred vessel containing an ammonia solution at a constant pH = 10. Once the precipitate was obtained, it was aged for 24 hr, filtered and washed with ammonia solution to eliminate the chlorides. The precipitate was then dried under an air atmosphere at 100~ for 24 hr. Calcination was performed at several temperatures in the range 400~ to 800~ The concentration of samples prepared are given in Table 1 below. Table 1. Supports composition Sample A1 AT05 AT10 AT40 AZ02 AZ05 AZ10 AZ20
wt % Al203 100 94.1 86.2 56.1 98 95 90 80.6
wt % TiO 2 . . . 5.9 13.8 43.9 ---------
.
.
wt % ZrO 2 . ------02 05 10 19.4
2.2 Materials Aluminium chloride and concentrated NH4OH were from Baker. Titanium tetrachloride, and zirconium chloride were from Aldrich. Deionized water was obtained using a Milli-Q water purification system (Millipore Corp). 2.3 Characterization Thermal gravimetric and differential thermal analyses of dried samples were performed on a DuPont 990 Thermal Analyser. X-ray diffraction patterns were obtained on a Siemens D500 di~actometer coupled to a Cu x-ray tube. A nickel filter selected the Kct radiation. Surface areas were obtained by nitrogen physisorption at 75K using a Micromeritics Accusorb 2100E apparatus. CC14 adsorption was performed on a TGA-7 Perkin Elmer Thermal analyser, for
809 which the samples were treated at 500~ in N 2 flow, cooled to the adsorption temperature (200~ then a CC14/N2 stream passed through the sample for approximately 3 hr to obtain a constant weight. 3. RESULTS AND DISCUSSION 3.1 Thermal behaviour TGA.- Thermal gravimetric results for titania-alumina and zirconia-alumina are shown in Figures 1 and 2. It can be seen that two main losses are present: loosely-bound water removal up to 150~ and a second process starting at --250~ which is due to loss of chemically bound water. The latter represents the transformation of the hydroxide to the oxide fornl The final weight attained in all samples is similar and is about 55-70 % of the original weight, suggesting that the parent hydroxide is of the type AI(OH)3 , bayerite or gibbsite. Above 600~ there is almost no weight variation. This result is consistent with results reported in the literature for the preparation of alumina single oxide. It is known that the precipitation of aluminium salts at pH=10, and room temperature produces the trihydroxide form (9). It is obvious from our results that even for high Ti or Zr content there is not an effect on the thermogravimetric behaviour.
(o) +55% (b)
50
2~
3 570 TEMPERATURE, *C
458% 0
930
Figure 1. Thermal gravimetric analysis of alumina-titania supports. Air atmosphere, 10~ Samples: a) AT05; b) AT10
810
~
. 100%
-~ae-c,~----~oo%\ ~ ~ 8 - c , 95 % 4~k.c~,~oo-z. ,
219"C191% 3:: P
239~
839"C,69% 4--
925"C~%
-
r(
925~ 4-
Id)
925"C,72~% w
o
"
2oo
"
800
I000
Figure 2. Thermal gravimetric analysis of alumina-zirconia supports. Air atmosphere, 10~ Samples: a) A1, b) AZ02; c) AZ10; d) AZ20.
DTA.- The differential thermal analyses are given in Figures 3 and 4 for alumina-titania and alumina-zirconia. Again the behaviour is similar for all samples irrespective of the concentration of the second metal oxide present. Two important endothermic peaks are observed: the first around 100 - 150~ which corresponds to the loosely-bound water elimination, and a second at-~300~ which is related to the removal of chemically-bound water, and transformation of the hydroxide form. The shape of the thermogram and the behaviour already seen in Figures 1 and 2 point to the formation of bayerite. It is important to note that no other important peaks are observed beyond 500~ so that the samples do not suffer timber structural transformations up to 900~ or that the transformations are athermal.
3.2 X-ray Diffraction X-ray dit~action spectra were obtained for dried uncalcined samples, and for calcined samples at several temperatures. Figure 5 shows the diffraction patterns for two samples of dried alumina-zirconia. The observed patterns correspond to the bayerite structure. This pattern was obtained for all the alumina-zirconia and alumina-titania samples, and agrees completely with the thermal behaviour described above. Thermal treatment at 400~ produced amorphous compounds. Calcination at higher temperatures revealed the formation of TI-AI203 at 500~ and the appearance of anatase at 700~ for the high concentration samples. In the case of zirconia-alumina no other compound was detected besides A1203, even for high concentration samples calcined at 800~ Comparing these results to single oxide samples it is possible to see that the structural behaviour of alumina-zirconia or alumina-titania is similar to what is found
811
(o)
~
133"C 133"(::
(b)
I ~
,12"<:
~
(C)
305~
go :z;o
,20
r;o
600
960
TEMPERATURE,"C
Figure 3. Differential thermal analysis of alumina-titania supports. Air atmosphere, 10~ Samples: a) AT05; b) AT10; c) AT40.
163"C
5 58'1:
788"C
502% _
658"C
'~(o)
'(b)
306"I;
(d)"~ 915~:
301"(:
-V-315"C o
2~o
"
*~
"
~x~
~
ooo
TEMPERATURE. "C
Figure 4. Differential thermal analysis of alumina-zirconia supports. Air atmosphere, 10~ Samples: a) A1; b) AZ02; c) AZ10; d) AZ20
812 in alumina alone (8). The behaviour of titania in the mixed oxide samples, is rather different, in the sense that no crystalline phase is observed even for the high Ti content mixed oxide sample, when calcined at 500~ whereas for titania single oxide, rutile is already present at 500~ coexisting with anatase (8). The case of zirconia is similar to that of titania. Single oxide ZrO2, obtained by the process outlined here, has a tetragonal structure at 500~ whereas in the case of the mixed oxide alumina-zirconia it is not detected by x-ray diffraction in samples treated at the same conditions. In fact the zirconia crystallization produces a very sharp exothermic peak at around 470~ and this characteristic feature was not observed in any of the samples prepared, whose thermal behaviour was shown above. It can be thought that at these conditions (500~ mixed oxides) both ZrO 2 and TiO 2 are well dispersed crystallites or amorphous compunds. B; Boyerite
B
B )..
B
I--
~ L _
-
4
i
17.2
i
i
i
30.4
43.6
56. 8
70
2e
Figure 5. X-ray diffraction spectra of alumina-zirconia supports. Dry tmcalcined samples: a) AZ02; b) AZ10.
3.3. S u r f a c e a r e a s
Specific surface areas for all samples are given in Figures 6 and 7. It is noticeable that both ZrO 2 and TiO 2 increase the surface area of alumina. The increase in promoter concentration produces an increase in surface area, although this is more noticeable at lower concentrations. The values of surface areas are comparable for both series of supports; and an increase in temperature of calcination brings about a reduction in surface area. It is interesting to note that the modification of AI203 by Zr or Ti produces a support more stable to thermal treatments, as can be seen from the comparison of surface areas at 800~ in Figure 6, in which the addition of 2% of ZrO 2 produces a material with a surface area about 50% higher than the alumina single oxide, and this being a difference not observed for the samples treated at lower temperatures. The reduction of surface areas with temperature appears to be more important for aluminatitania than for alumina-zirconia. This might be due to the fact that wheras alumina and titania form a solid solution, alumina and zirconia do not; therefore the structural arrangement should be different for both systems. Hence as the temperature increases the A1-Ti interaction would produce eventually AI2TiOs, whereas A1-Zr gives way to A1203 and ZrO 2 (10).
813
400
400
or
(a)
o
E 500
E300
w
w
@
(c)
JOOo
/
2O0
200
5
@
@
,o
~
~
(b)
"
38
Wt % TiOz Figure 6. Effect of concentration and Figure 7. Effect of concentration and calcination temperature on surface areas of calcination temperature on surface areas of alumina-zirconia supports. Samples calcined alumina-titania supports. Samples calcined at: a) 400~ b) 600~ c) 800~ at: a) 500~ b) 700~ W t % Z r02
3.4. Adsorption of C C ! 4. The adsorption of CC14 on the supports prepared was studied to get an indication of the acidity of the surface. The amounts of CC14 adsorbed are plotted in Figures 8 and 9. The results show two different trends for the two systems: in alumina-titania as the amount of titania increases the amount adsorbed increases; while for the alumina-zirconia samples the result is the opposite. In fact the chlorine retained by the samples with titania are higher than that observed on an alumina single oxide support used as reference. In the case of the Zrmodified aluminas the weight increase in the alumina reference lies between the 5% and 10% ZrO2-A1203 samples. The carbon tetrachloride adsorption is a process in which upon adsorption, the compound reacts leaving C1- adsorbed on the surface, releasing COC12 and CO 2 as products. The gaseous product distribution is a function of the surface state. Basset et al (11) established that the CC14 adsorption and reaction is kept only at the surface at temperatures below 220~ therefore the present results are indicative of a surface process. They also suggested that the surface reaction stoichiometry is affected by the hydroxilation state of the sample. C1- is adsorbed onto the A1 Lewis acid site, and the OH- and O = neighbouring sites react with the adsorbed molecule to produce COC12 first, followed by CO 2 formation. The reaction could then be considered as indicative of the acidity of the sample, but it is also indicative of the OHcontent of the surface. Our results suggest that upon Ti addition alumina supports increase in acidity, in other words, the number of possible acid sites for chlorine adsorption increases with the TiO 2 content. The case of ZrO2-A1203 samples is the opposite, an increase in ZrO 2 content implies a decrease in acid sites available for chlorine adsorption. The results on the former system (AI/Ti) could be explained in terms of the structural modifications that the inclusion ofTiO 2 brings about, which reflects the affinity of alumina and titania to form solid solutions. In the case of ZrO2-AI203 samples the solid mixture could be envisaged as if the ZrO 2 was onto the AI203 surface
814 hindering the adsorption of CC14. Lahousse et al. (6) reported recently that strong Lewis acid sites determined by pyridine desorption decrease linearly with the increase in zirconia content; although the total number of Lewis acid sites per square meter, slightly increase with zirconia content in the composition range used in the present work. Therefore from the agreement of Lahousse et al results and our results it would appear that CC14 adsorption and reaction occurs on strong Lewis acid sites.
4.5
(o)_~
(c)
(b) if)
( c )
o -r
_o 1.5
(o) (d)
O.OQO
60.0 I~00 T I M E , min
180.0
O.
0
I00 200 TIM E, min
300
Figure 8. Amount of CC14 adsorbed on Figure 9. Amount of CC14 adsorbed on alumina-titania supports. Samples calcined at alumina-zirconia supports. Samples calcined 500~ adsorption temperature: 200~ at 600~ adsorption temperature: 200~ Samples: a) AT05; b) AT10; c) AT40; Samples: a) AZ02; b) AZ05; c) AZ10. d)A2 (a commercial alumina)
4. CONCLUSIONS The modification of alumina supports by the addition of ZrO 2 or TiO 2 by coprecipitation produces mixed oxides with higher surface areas than alumina single oxide, and appear to be more stable to thermal treatment. All solid supports prepared showed after drying at 100~ the bayerite structure, independently of the ZrO 2 or TiO 2 concentration. The amount of CC14 adsorbed at 200~ increases with TiO 2 content in alumina-titania samples, but decreases with ZrO 2 concentration in alumina-zirconia samples. These results suggest that Lewis acidity increases with TiO 2 and decreases with ZrO 2 content.
815 Acknowledgements We thank Conacyt (M6xico) for their financial support through the contract 400200-1847-A. 5. REFERENCES
1. 2. 3. 4. 5. 6.
H. Schaper, E.B.M. Doesburg, L.L. Van geijen, Appl. Catal., 7 (1983) 211. R.M. Levy, D.J. Baueer, J. Catal., 9 (1967) 76. W. Zhaobin, X. Qin, G. Xiexian, P. Grange, B. Delmon, Appl. Catal., 75 (1991) 179. H.G. Shikata, Funtai Oyobi Funmatsu Yakin, 38 (1991) 369. A.C.Q. Meijers, A. Jong, Appl. Catal., 70, (1991) 53. C. Lahousse, A. Aboulayt, F. Maug6, J. Bachelier, J.C. LavaUey, Appl. Catal., 84 (1993) 283. 7. K. Tanabe, M. Misono, Y. Ono, H. Hattori, New Solids Acids and Bases, Stud.SurE. Sci. Catal., 51, Elsevier, Amsterdam (1989). 8. O. Ro .driguez, F. Gonz~lez, P. Bosch, M. Portilla, T. Viveros, Catal. Today, 14 (1992) 243. 9. R. Poisson, J.P. Brunelle, P. Nortier, in A. Stiles, Catalyst Supports and Supported Catalysts, Butterworths, Boston (1987). 10. E.M. Levin, H.F. McMurdie, Phase Diagrams for Ceramists, (1975) Supplement, Figs.4376, 4377. 11. J. Basset, M.V. Mathieu, M. Prettre, Revue de Chimie Min., 5 (1968) 879.
This Page Intentionally Left Blank
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 1995 Elsevier Science B.V.
817
Synthesis, characterization and applications of new supports for heterogeneous Ziegler-Natta type catalysts Lauro Pavanello, Silvano Bresadola* Dipartimento C.I.M.A.v. Marzolo 1, 35131 Padova (Italy) and Centro di Studio su Stabilit& e Reattivit& dei Composti di Coordinazione (C.N.R).
1. INTRODUCTION The interaction between (~-MgCI2 and electron donor compounds are nowadays subject to several investigations in order to prepare activated 8-MgCI2 as support for heterogeneous Ziegler-Natta type catalysts. [1]. In this context we have investigated the reaction between (z-MgCI2 and several Lewis bases, such as ethyl formate [2], benzyl alcohol [3], ethyl acetate [4], ethyl alcohol [5,6], formamide [7] and dimethyl acetamide [8,9]. We were able to isolate and structurally characterize some adducts of MgCI2 with these donors from which the highly disordered 8-MgCI2 form was obtained by thermal elimination of the coordinated bases [10]. We wish now to report here a preliminary study on the preparation of a MgCI2 support obtained following a new ready method that appears very promising. The synthesis is carried out by radicalic reactions between metallic magnesium and chloro-alkyl compounds, mostly 1-chlorobutane, initiated by UV radiation, without the use of Lewis bases. The products so obtained were characterized by means of elemental analysis and powder XRD diffraction spectra. The XRD patterns appear to be in
*To whom correspondence should be addressed
818
agreement with a highly disordered material. By treatment of these materials with titanium tetrachloride valuable supported catalysts for the propene Ziegler-Natta type polymerization were obtained. These catalysts were tested by slurry polymerization using triethylaluminium as cocatalyst and showed an interesting activity compared with that exhibited by a commercial catalysts. The polymer products were also characterized by measuring the molecular weight distribution by gel permeation chromatography technique. 2. EXPERIMENTAL 2.1
Materials
Magnesium metallic (powder), reagent grade 1-chlorobutane, hexane, heptane and titanium tetrachloride were supplied by Aldrich Chemicals. Propylene (polymerization grade) triethylaluminium (reagent grade) and the commercial reference catalyst were supplied by HIMONT Italia S.p.A. The reagents were purified by standard methods in order to obtain reproduceable polymerization tests. In particular with the purpose to remove the oxygen and water, the propene was treated with molecular sieves 4A and BASF catalyst R3-11. Titanium tetrachloride and triethylaluminium were used without further purification. 2.2 Apparatus
The reaction were carried out inside a four-neck glass reactor, equipped with a reflux condenser, a graduated funnel, a valve and a refrigerated quartz UV lamp. The reaction apparatus is represented in figure 1. The reagents were weighted and introduced into the reactor inside a Braun MB150 1/11dry box under strictly controlled inert atmosphere. The powder X ray diffraction patterns were scanned in transmission technique with a GD-2000 diffractometer (Ital Structures, Riva del Garda, Italy) operating in the Seemann-Bohlin geometry and equipped with a quartz-curved crystal monochromator of the Johansson type aligned on the primary beam. The Cu-Ko~l radiation (~ = 1.5406 A) was employed, and an instrumental 2e step of 0.1~ every 10 s was selected. The molecular weight distribution of the polymers were measured by gel permeation chromatography (WATERS mod. 200 apparatus) using four Spherosil
819
columns (103 - 107 A, 37- 75 I~m), 1,2-dichlorobenzene as solvent at 135 ~ and a refractive index detector.
2.3 Synthesis of the support material. It is known that the magnesium reacts with organic halide compounds, such as 1-chloroalkyl under UV radiation through a radicalic mechanism [12] yielding a mixture of chloroalkyl magnesium compounds.
?
(
F
( a UV lamp with quartz refrigerator b glass reactor c graduated funnel d reflux condenser
vacu
)
\j Figure 1. Reaction apparatus.
820
Thus we have treated 3 g Mg powder with an excess of 1-chlorobutane (80 ml) in the reactor of Fig. 1. The mixture was heated up to 80 ~ the UV lamp was then switch on and the heating was carried on for 4 h. Finally the liquid phase was removed and the solid product was washed three times with 50 cc of hexane and then dried under vacuum, for 8 hours at 50 ~ At the end we obtained 13 g of product. The results of the elemental analysis are: C-7.95 %, Mg-17.53 %, Mg/R=4.35 mol/mol.
2.4 Preparation of the supported catalyst. 3 g of the support material prepared as above reported were dispersed in 60 ml of hexane and treated in a glass flask equipped with a magnetic stirrer with 60 ml of TiCI4 for 2h at 115 ~ The mixture was cooled at room temperature, the liquid was eliminated and the treatment repeated without hexane. At the end, the solid (FOT catalyst) was washed three times with 40 c c of hexane and dried at 40 ~ under vacuum for 12 h. At the end of these processes the atomic absorption analysis showed a content of titanium of 1.935 %.
2.5 Polymerization tests The slurry polymerizations of propene were carried out under constant pressure of propene (P=4 atm) in a BUCHI 2 I stainless steel reactor equipped with a turbine stirrer using heptane as solvent of the monomer. A scheme of the polymerization reactor is reported in figure 2. The reactor, cleaned and dried, has been heated under vacuum at 105 ~ for 12 h and then in turn evacuated and filled with nitrogen several times. The reagents feed procedure was the following: 1.2 I of heptane were put inside in the reactor under nitrogen atmosphere and then a weighted amount of catalyst (0.05-0.1 g) was added. Successively, pressure and temperature were raised up to the selected reaction values (4 atm and 70 ~ and the polymerization started when the cocatalyst triethylaluminium (7-15 ml of solution 10 % w/w in hexane) was added (AI/Ti molar ratio 200 tool/tool). The reactor temperature and the pressure were maintained constant within _+0.5~ and _+0.02 atm, respectively. The stirring speed was 1200 rpm in order to avoid effects of the monomer diffusion through the gasliquid interface. The polymerization rate was determined by measuring with a mass
821
flow meter the rate of monomer consumption. At the end, the polymerization reaction was quenched by rapid decreasing of the temperature and pressure. By following this procedure, the entire reaction course (activation and deactivation of the catalyst) could be monitored.
nilTogen~ - - -
solvent cocatalyst
I
monomer- ~
I~
!
~
D<3-[>
vacuum
Figure 2. Polymerization reactor: PC pressure controller, MFM mass flow meter, T thermocouple and temperature controller.
3.
RESULTS
AND
DISCUSSION
The catalyst obtained has been characterized by powder XRD diffraction. Fig. 3 shows a comparison between the XRD pattern of o~-MgCi2 (a) and that of the activated MgCI2 form (b) obtained following the above reported method. The latter
822
species appears to be characterized by high structural disorder and then may be regarded as an effective support for the preparation of titanium based catalysts for the polymerization of ~-olefin. It is to be noted that the characteristic peaks shown by the ~-MgCI2 XRD spectrum (fig. 3a) are totally absent in the spectrum of the products obtained with the proposed method. Particularly, it should be pointed out the considerable modification of the 003 peak centered at 2e = 15 ~ This peak, shown by e~-MgCI2, may be associated with the stacking of the CI-Mg-CI triple layers along the c crystallographic direction [13].
100
100
80
80
60
60
40
40
a)
20
20
0
0 5
15
25
35 20
45
55
5
15
25
35
45
55
20
Figure 3. XRD patterns of a) ~-MgCI2; b) support obtained with the proposed method.
From the obtained polymerization data we have evaluate the activity of the catalysts employed. Table 1 lists the activity of the catalyst by us prepared (FOT cat.) compared with that exhibit by the commercial catalyst and the molecular weights of the obtained polymers. It is to be outlined that both these polymerizations were carried out in absence of hydrogen and external Lewis bases. Furthermore the polymerization promoted by the FOT catalyst was carried out also in absence of the internal base. As the presence of these components influences both the activity and stereospecificity of the catalysts, a comparison with the features shown by the commercially produced polypropylene it is not possible.
823
Table 1. Polymerization results Polymerization catalyzed by
Activity Kg PP/g Ti
commercial cat FOT cat
27.4 19.8
mol wt Mn
Mw
Mw/Mn
41500 19700
375000 239000
9.0 12.1
Finally, on the bases of the recorded polymerization data we have drawn the kinetic curves of the slurry polymerizations by us investigated (Fig. 4). It is remarkable the initial high activity exhibited by the FOT catalyst. However, this activity decays rapidly and in the course of ten minutes decrease below that shown by the commercial catalyst. It is to be pointed out that the procedure by us reported for the preparation of the active support is easy and rapid, and that the catalysts from them derived appear very promising on condition that the high activity can be stabilized. Therefore, studies are now in progress in order to improve the performance of these photochemicaUy obtained catalysts.
1.2 1.0 0.8 C .m
E
0.6
o
0.4 0.2 0.0 0
2000
4000
6000
time s Figure 4. Kinetic curves of slurry polymerization of propene with: a) commercial catalyst, b) FOT catalyst. Ratio AI/Ti = 200 mol/mol.
824
REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11 12. 13.
P.C. Barb~, G. Cecchin, L. Noristi, Adv. Polym. Sci., 81, (1987) 1. V. Di Noto, S. Bresadola, R. Zannetti, M. Viviani, G. Valle, G. Bandoli Z. Kristallogr., 201, (1992) 161. V. Di Noto, S. Bresadola, R. Zannetti, M. Viviani, G. Bandoli Z. Kristallogr., 204, (1993) 263. V. Di Noto, R. Zannetti, S. Bresadola, A. Marigo, C. Marega, G. Valle, Inorg. Chim. Acta, 190, (1991) 279. G. Valle, G. Baruzzi, G. Paganetto, G. Depaoli, R. Zannetti, A. Marigo, Inorg. Chim. Acta, 156, (1989) 157. V. Di Noto, R. Zannetti, M. Viviani, C. Marega, A. Marigo, S. Bresadola, Makromol. Chem. 193, (1992) 1653. L. Pavanello, P. Vison&, A. Marigo, S. Bresadola, G. Valle, Inorg. Chim. Acta, 216 (1994)261. L. Pavanello, P. Vison&, S. Bresadola, G. Bandoli, Z. Kristallogr., in press. L. Pavanello, P. Vison&, S. Bresadola, submitted to J. Mol. Catal. V. Di Noto, L. Pavanello, M. Viviani, G. Storti, S. Bresadola, Thermochimica Acta, 189 (1991 ) 223. ASTM Method D 3417-82. G. Wilkinson, F. Gordon, A. Stone, E. W. Abel (eds), Comprehensive Organometallic Chemistry, vol.1, Pergamon Press, 1982. V. Di Noto, R. Zannetti, M. Viviani, C. Marega, A. Marigo, S. Bresadola, Makromol. Chem., 193 (1992) 1653.
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
825
CATALYTIC F I L A M E N T O U S CARBON AS A D S O R B E N T AND CATALYST SUPPORT
V.B. Fenelonov, L.B. Avdeeva, O.V. Goncharova, L.G. Okkel, P.A. Simonov, A.Yu. Derevyankin and V.A. Likholobov Boreskov Institute of Catalysis, Russian Ac.Sci., Pr. Akademika Lavrentieva 5, Novosibirsk 630090, Russia Until recently, catalyst coking has been generally considered as a harmful side process t h a t causes catalyst deactivation and, in some cases, catalyst destruction [1, 2]. However, catalyst coking (carbonization) is reported in many studies as intentionally performed in order to obtain new systems with useful properties. Thus, alumina was covered by a carbon layer to prepare more inert carriers for desulfurization catalysts [3]. R. Leboda et al. (see, e.g.,[4]) showed a good performance of silica with partially carbonized surface as chromatographical adsorbent. Carrott and Sing [5] used carbonized silica to obtain standard isotherms of nitrogen adsorption. Let us note that the carbon deposition onto oxide catalysts results in complete or mosaic coverage of their surface. The deposition on metal catalysts such as Fe, Ni, Co, and others, generates various filamentous carbon species in the bulk of the catalyst followed by either retention of the initial morphology or its complete dispersion and formation of a tangle of carbon filaments [6-8]. The latter can be referred to as catalytic filamentous carbon (CFC) [9]. CFC are proposed to be used for the production of carbon composites, filters. They are of practical interest as adsorbents and catalyst supports as well [9]. Up to now, most of published work has been devoted to studies of m e c h a n i s m s of carbon filament growth d u r i n g conversion of carboncontaining compounds over metal catalysts [6,7]. Apparently, the properties of the filaments as adsorbents and supports have been neglected. Only a few studies on this matter may be mentioned [8,9]. The aim of this work is to bridge partially this gap and draw more attention to this promising new type of carbon materials. A number of high-percentage metal catalysts based on Ni and its alloys were developed at the Boreskov Institute of Catalysis; they allow conversion of a wide range of hydrocarbons, including industrial waste, yielding no less than 150-300 g of carbon per 1 gram of catalyst at 770-870 K. Finally, such a catalyst leads to CFC 's, the starting catalyst involved being an impurity. TEXTURE
O
A g r a i n of CFC consists of globules formed by interlaced carbon filaments. Typical structural and textural data of the CFC's are summarized in Tables 1 and 2. CFC' were prepared via cracking of CH4 at 823 K : CFC-1 -
826 over N i - c o n t a i n i n g c a t a l ys t s ; CFC-2,3,4 - over Ni/Cu alloy c a t a l y s t s . Comparative data for the carbon material S-36P of the Sibunit family, which is produced by depositing pyrocarbon onto soot followed by steam activation [10], are presented in Table 1. Table 1 Typical textural parameters of CFC CarboniSurface Sample zation Carbon ar e a rate content ABETN2 g/g cat.h % m2/g CFC-1 CFC-2 CFC-3 CFC-4 S-36P
9.2 8.0 4.7 2.1 -
99.3 99.6 99.3 99.0 99.5
103 119 226 301 300
Pore volume, V cm3/g
Average pore diameter d, nm
Density
0.25 0.37 0.73 0.63 0.62
10-15 13 13 13 20
2.17 2.10 2.06 2.00 2.02
pile g/cm 3
CFC pore volume d i s t r i b u t i o n for c h a r a c t e r i s t i c d i m e n s i o n s were calculated from the desorption branch of N2 adsorption isotherms at 77 K (Fig. 1). All the CFC samples are seen to possess mesoporous s t r u c t u r e s with r a t h e r n a r r o w pore size distributions but different total porosities V and specific surface areas A. The porosity of a filament tangle e = V p(l+Vp) is determined by the ratio of kinetic and diffusion factors of the cracking process. An e x t r u s i v e m e c h a n i s m of the filament growth is observed with carbon formed and segregated on different metal planes (and with carbon diffusing t h r o u g h a metal particle) [6,11]. A single filament grows on a Ni particle and several filaments on Ni/Cu ("squid" morphology of carbon).
2.01o -03
-
A aJ =2 A i 1.01o -03-
~
-
.......
i;o
......
i;'oo
PORE DL4J~L-'r~R, ~
Fig. 1. Pore volume distribution" 1 - CFC-1; 2- CFC-2; 3 - CFC-3; 4- CFC-4
The a p p r o a c h of the "head edge" of a f i l a m e n t w i t h a m e t a l c a t a l y s t p a r t i c l e t h e r e i n to a n y hindrance influences the uniformity of supplying gaseous r e a c t a n t s and is followed by d i s t o r t i o n of t he filament and, finally, t u r n i n g around the hindrance. The conditions of the r e a c t a n t supply change as the local packing density i n c r e a s e s . As a r e s u l t , u n i f o r m dense globules of i n t e r w i n e d filaments are formed, porosity e~0.3 for CFC-1, e - 0 . 4 1 for CFC-2 and CFC-3, and e~0.52 for CFC-4 being observed
827
S T R U ~ CFC are produced via epitaxial deposition of carbon on the s u r f a c e of m e t a l c a t a l y s t . T h e f i l a m e n t s are s h a p e d by g r a p h i t e doo2 b a s a l p l a n e s ; a f i l a m e n t can be i m a g i n e d as e i t h e r a pile of t r u n c a t e d cones p u t one o n t o a n o t h e r , each inside the next one, or, if flat, as a card pack [6,7]. Generally, a pile of cones with angle a m a d e by the g e n e r a t r i x a n d the Fig. 2. Schematic representation of the cones axes, v a r y i n g from 0 to 90 ~ structures 9(a) graphitized carbon black; m a y be t a k e n as a model of the CFC (b) activated charcoal; (c) CFC structure (Fig.2). The main s t r u c t u r a l p a r a m e t e r s of the CFC's are shown in Table 2.
b
Table 2 Typical s t r u c t u r a l p a r a m e t e r s of CFC's Sample
CFC-1 CFC-2 CFC-3 CFC-4 PM-105 S-36P
X-ray
Electron spectroscopy
d002 A
Lc nm
La nm
3.42 3.42 3.41 3.42 3.62 3.51
6.7 7.0 6.5 5.2 1.7 4.0
6.0 5.0 6.3 1.9 4.0
Filament diameter nm 50;100 120;10 120;10
Filament length ~m 15 63
5
45o;90 ~ 90 ~ 90 ~
Some comparative data for the carbon black PM1051 with ABET=110 m2/g and Sibunit 36P are also given. I n t e r l a y e r d i s t a n c e s are 0.340-0.343 n m w h i c h c o r r e s p o n d s to a t u r b o s t r a t e d structure [12]. Because of different lengths of the basal planes, the exposed filament surface is covered with atomic-sized roughnesses (Fig. 2); the depth of this r o u g h e n e d surface layers was analyzed by etching surface with Ar ions followed by detection of the generated fragments using SIMS technique [13]. The depth was found to be no more t h a n 1-1.5 nm.
1The sampleof carbon blackPM105 was providedthroughthe courtesyof Prof. H.P. Boehm,Inst. fur AnorganischeChemie,Munchen.
828 Therefore, one can suppose the exposed CFC surface to be roughened with shallow "surface micropores" whose widths are close to the interlayer distance, i.e., 0.34-0.343 nm. Thus, the structure of the exposed surface CFC essentially differs from those of conventional carbon blacks (CB), activated charcoals (AC), carbon m o l e c u l a r sieves ( C M S ) , etc. (Fig.2). This r e s u l t s in t h e i r different adsorbabilities. ADSORBABILITY.
Some adsorptive properties of CFC-1 obtained using gas chromatography with m i n i m u m fillings are presented in Table 3. Additional details on the procedure are given elsewhere [9]. Values of H e n r y coefficients (G, cm3STP/m 2) and initial adsorption heats (-AU, kJ/ml) at 298 K are reported; comparative data are presented for the graphitized carbon black (GCB) and the fine-porous carbon adsorbent Ambersorb XE-340. CFC's are seen to be remarkably superior to conventional adsorbents c o n t a i n i n g four and more carbon a t o m s as r e g a r d s a d s o r p t i o n of hydrocarbons. Additionally, adsorbabilities of the CFC's regress in the sequence C6H6>>C6H5CH3>>C6H5OH. These peculiarities are accounted for by a specific surface structure of the CFC's. A C6H6 molecule is flat, its "thickness" t is 0.34 nm, namely close to the interlayer distance do02 in the CFC and, therefore, to the dimensions of surface micropores. Accordingly, the benzene molecule can fill exactly in the pore by adsorbing on two sides; the increase of the adsorptive potential, Henry coefficient and adsorption h e a t results from it. If adsorbed on basal planes, benzene orients with the only side, which leads to considerably lower values of G and AU. Table 3 Adsorptive properties of CFC-1 observed using gas chromatography at infinite dilution. Adsorbat CFC-1 GCB Ambersorb XE 34O G -AU G -AU G cm3(STP)/m2 kJ/mol cm3(STP)/m2 kJ/mol cm3(STP)/m2 Benzene 1.8.1010 131.1 5.9.101 42.3 6.5.101 Toluene 5.4.106 76.6 1.6.101 48.3 3.25.103 Phenol 1.4.104 38.2 8.1.102 52.0 C-hexane 9.7.105 82.3 1.9.101 37.6 Hexane 9.0.1010 139.3 1.0.102 43.4 1.9.105 Heptane 8.0.1014 178.0 5.26.102 48.1 1.85.106
829 Table 4 Analysis of adsorption isotherms of heptane and H20 Sample
Adsorption of heptane ABET m2/g
amBET cm3/g
Vs cm3/g
W cm3/g
Eo kJ/mol
Vg cm3/g
Aa m2/g
AH20 m2/g
102 119 295
0.042 0.049 0.122
0.250 0.474 0.627
0.052 0.040 0.115
16.16 17.38 17.68
0.005 0.005 0.015
92.5 100 245
14.2 8.6 33.7
CFC-1 CFC-2 CFC-4
From benzene to toluene, the molecule "thickness" t increases by ca.0.06 nm. A f u r t h e r growth of t from phenol to cyclohexane r e s u l t s in lower adsorbabilities of the CFC's. Note t h a t a reverse sequence is observed with the same aromatic molecules adsorbed on the G C B and AC. The reported results allow to suggest t h a t flat aromatic molecules are adsorbed on the CFC t h r o u g h t h e i r "cut edges" (by filling, p a r t i a l l y or completely, surface micropores) but not "planes", which is characteristic of t h e i r adsorption on g r a p h i t i z e d carbon blacks a n d o t h e r k n o w n carbon adsorbents. Such spectacular values of G a n d - A U of the C F C ' s which differ from other carbon adsorbents, are i n h e r e n t to the r a n g e of very low concentrations (10-4-10 -6 vol % in our testing). CFC adsorbabilities close to those of conventional adsorbents are observed at high concentrations. 0.300'-
a, cm3/g 0.250-
E
o,,1 ~,2
0.200-
lt/[
0.150-
0.100-
0.050-
"Q
--v
":--u
0.2
. . . . . .~ . . .
0,4
0,6
t
1
0.8 1.0 P/Po
Fig. 3. Isotherms of adsorption on CFC-I:I - C6H6; 2 - heptane; 3 - H20
Indeed, if measured at 293 K, the adsorption isotherms of C7H16, C6H6 and H 2 0 vapors look like those of typical carbon adsorbents (Fig. 3). At the b e g i n n i n g , the i s o t h e r m s are s t r a i g h t e n e d in BET c o o r d i n a t e s . Surface area ABET of, e.g., h e p t a n e is found to be close to that calculated from N2 adsorption at the molecular landing co= 0.591 nm 2 in size (Table 4). With v a r i o u s CFC's, a l m o s t c o i n c i d i n g CTH 16 a d s o r p t i o n i s o t h e r m s w i t h respect to the calculated a r e a ABET (Fig. 4) a r e observed. An e x t r a isotherm of C7H16 adsorption on a nonporous carbon black calculated using the equation 7=70 exp(A/~Eo), suggested in [14], where 70 = 8.1 cm3/m 2, A=RT ln(P/Po), Eo=6.35 kJ/mol, ]]=1.482 (with C6H6 used as a s t a n d a r d reference) is
830 plotted in Fig. 4. The isotherm is similar to the isotherms obtained for the CFC~ within the range of low P/Po values. A formal analysis of the heptane adsorption isotherms referring to the D R equation a = W exp[-A/~Eo) 2] [16] with the calculated microporous volume W and energy constant Eo was carried out (Table 4, too). This equation leads to linear isotherm plots at P/Po<2.6.10 -2. Since the W values are close to a monolayer capacity am found from the BET equation (am,BET), they cannot be reliably identified as the real micropore volumes. It was only the application of a comparative method [17] (a modification of as method by Sing [18]) which allowed a reliable determination of the micropore volume. In Fig. 5, C7H16 adsorption isotherm plots are compared with the adsorption isotherm of carbon black (calculated from the above equation) used as a standard reference. These curves look like those typical for microporous systems; the linear fragment slope corresponds to the mesopore surface A a , and the intercept of the ordinate axis, to the micropore volume V~. 7-
0
(z, ~M/m 2
&
oV oV
0
0
5" oVo
49
9 o
DV
~o~~
3-
2f
o o
1.
o12 o13 o'.4 o:s 0'.8 o:z
Similar results were also obtained from the analysis of C6H6 a d s o r p t i o n isotherms. In a comparative analysis of water vapor adsorption isotherms, the adsorption isotherm referred to the active surface (as suggested in [19]) was used as a standard reference. Comparative linear plots which may be extrapolated to the origin of the coordinates were obtained, their slopes being used to find the AH20 values(Table 4); these values may be identified as a surface built by fragments of "prism" faces and covered by functional oxygen-containing species.
P/Po Fig. 4. Isotherms (297K) of heptane CFC as catalyst supports. adsorption as assigned to specific surface area ABET on(v)-CFC-1; The CFC's were used as supports to (0)- CFC-2; (O)- CFC-4; prepare Pd/C catalysts. The catalysts were (o)- calculated according to [14]. produced by i m p r e g n a t i n g CFC with aqueous solutions of H2PdC14 followed by a reduction under H2 at 623 K. Adsorption of H2PdC16 was studied using stepwise elution technique. Sites of weak (A1), strong (A2) and very strong (A3) adsorption of PdC12 according to [20, 21] were identified. The concentration of the sites was determined as follows : A3 - by the amount of Pd(II) not desorbed with 17% HCL; (A2+A3) - by the amount of Pd(II) not desorbed with acetone; (AI+A2+A3) by the maximum adsorption from aqueous solutions. It was shown in [20] that the sites of A1 type correspond to adsorption on basal graphite planes; A2 to adsorption on edge planes; and A3 - in micropores. The data are presented in Table 5. -
831 Table 5 Adsorption of H2PdC16 on CFC and dispersity of Pd on the catalysts containing 1% Pd Sample Concentration of adsorption sites K2 Dispersity }~mol/m2 CO/Pd
CFC-1 PM-105
A1
A2
A3
1.00 0.91
0.886 0.89
0.007 0.045
a, cm3/g 0.20-
9
o/
Y
0.15-
9 1
,32 0"10 i .
/
0.05
I
1
I
2
I
3
i
4
I
5
a, }~M / m 2 Fig. 5. Comparative isotherms of heptane adsorption (293 K) on: 1 - CFC-4; 2 - CFC-2; 3 - CFC-1
550 240
0.39 0.24
The dispersity of Pd on the p r e p a r e d c a t a l y s t s is influenced by K2 - a constant which characterizes the i n t e r a c t i o n of a c a t a l y s t precursor and the support; the d i s p e r s i t y of m e t a l palladium increases with the increase of K2 [21]. Therefore, Pd dispersity on, e.g., a CFC-1 based catalyst is higher than that on a carbon black PM-105 based one, though similar surface coverage and nearly equal total concentrations of sorption sites (i.e., A i ) a r e observed. Thus, supports such as CFC are c a p a b l e of stabilizing the finely dispersed Pd phase. In fact, with the obtained data, the CFC can be t h o u g h t as a specific and r a t h e r promising family of new adsorbents and catalyst supports.
R~'ERENC]~ 1. 2. 3. 4. 5.
J.H. Butt, EE. Petersen, Activation, Deactivation and Poisoning of Catalysts, Acad. Press, New York, 1988. Deactivation and Poisoning of Catalysts, Dekker Ink., New York, 1985. T.P.R. Vissers, F.P.M. Merck, S.M.A. Bouwens, J. Catal., 114 (1988) 292. A. Gierak, R. Leboda, J. Chromatography, 483 (1989) 197. P.J.M. Carrott, K.S.W. Sing, J.H. Raistrick, Colloid and Surface Sci., 21 (1986) 9.
832 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
R.T.K. Baker, Carbon Fibers Filaments and Composites, Kluwer Acad. Publ., Dordrecht, 1990. M. Audier, A. Oberlin, M. Coulon, J. Cryst. Growth, 55 (1981) 549. D.S. McIver, P.H. Emmett, J. F'~ys. Chem., 59 (1955) 1109. V.B. Fenelonov, L.B. Avdeew V.I. Zheivot et al., Kinetika I Kataliz, 34 (1993) 545.. Yu.I.Yermakov, V.F. Surovil l, G.V. Plaksin et al., React. Kin. Catal. Lett., 33 (1987)435. J. Guinot, M. Audier, M. Coul L,L. Bonnetain, Carbon, 19 (1981) 95. J. Biskos, B.E. Warren, J. Ap~ Phys., 13 (1942) 364. V.P. Ivanov, V.B. Fenelonov, ~.B. Avdeeva, O.V. Goncharova, React. Kin. Catal. Lett. (in press). T.V. Baikova, M.L. Gubkina, K.M. Nikolaev, N.S. Polyakov, Isv. Akad. Nauk SSSR, Ser. Khim., 8 (1993) 1381. N.N. Avgul, A.V. Kiselev, I.A. Ligina, D.P. Poshcus, Izv. Akad. Nauk SSSR, Ser. Khim., 1314 (1957). M.M. Dubinin, E.D. Zaverina, L.V. Raduchkevitch, J. Phys. Chem. (rus.), 21 (1947) 1351. A.P. Karnaukhov, V.B. Fenelonov, Pure Appl. Chem., 61 (1989) 1913. S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, Acad. Press, London, 1982. P.L. Walker, J. Janov, J. Colloid. Interf. Sci., 28 (1968) 499. P.A. Simonov, V.A. Semikolenov, V.A. Likholobov, A.I. Boronin, Yu.I. Yermakov, Izv. Akad. Nauk SSSR, Ser. Khim., 12 (1988) 2719. A.S. Lisitsyn, P.A. Simonov, A.A. Ketterling, V.A. Likholobov, Stud. Surf. Sci. Catal., 63 (1991) 449.
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
833
Preparation of boron-containing alumina supports by kneading. Jean-Luc DUBOIS* and Shigeaki FUJIEDA. Japan Energy Corporation, Central Research Laboratory, Research & Development Division, Petroleum Laboratory, 3-17-35 Niizo-Minami, Toda, Saitama 335, Japan. 1. INTRODUCTION Boron-containing hydrotreating catalysts have been far less studied than phosphorous-containing catalysts, although they are commercially available and claimed in several patents [1]. The literature on B-containing supports gives conflicting results, as for example for the effect of boron on the Side Crushing Strength (SCS) [2]. B-containing supports or catalysts can be prepared by various methods: impregnation [1a,1d,1e,3], coprecipitation [lb, lk,2,4], hydrolysis of alkoxides [lf, 1k] or by kneading [5]. In this study, B-containing supports were prepared by kneading of a pseudoboehmite with boric acid and ammonia. We have attempted to determine the effect of boric acid on the formation of mesopores and macropores, on the surface area and on the crystal structure.
2. EXPERIMENTAL 2.1. Support preparation method Alumina pastes (dough) were prepared by kneading of a commercial pseudoboehmite (2 kg) with a peptizing agent (nitric acid, 3 %, 1 I) for a given time before addition of H3BO3 and ammonia, with a 2.3 mole ratio: e.g 363 g of H3BO3 for 160 ml of a 29 wt % NH3 solution. The torque is monitored during kneading, and the temperature of the dough is kept below 60 ~ by flowing cooling water in the double casing of the kneader. In a typical experiment, boric acid and ammonia are added after 20 minutes of kneading and the total kneading time is 60 minutes. The dough was extruded as 1.6 mm cylinders, then dried at 120 ~ overnight and finally calcined at 600 ~ for 1 hr in a rotary kiln, under a flow of dry air. 2.2. Analysis Supports were analyzed using X-ray Diffraction (XRD) and X-Ray line broadening of the (020) and (120) planes of the pseudoboehmite (Rigaku Geigerflex, CuKcz); Thermo-Gravimetry and Differential Thermo-Analysis (Seiko TGDTA300); Differential Scanning Calorimetry (Seiko DSC200); Hg porosimetry (Micromeritics Autopore 9200); BET surface area, pore volume and pore size distribution measurements (Micromeritics Digisorb 2600, using N2); and Transmission Electron Microscopy (TEM, Hitachi H-9000 UHR). H3BO3 (99.5 %) t
Visiting ResearchScientist, Delegatedfrom Elf Antar France,CRES,BP 22, F-69360.
834
._.
-o ~
E
c"
t"
F~ F: F: s
C
e-
"~- .~
'
I
o
''''1
'
'
'
I
~
..
. . . .
,
,,, ~
.~,
....
....
__--",'-~"~
O ,--
I''''
m
.|
N
o
O
(V'6/F l) OP/AP ';~N - ewnlOA eJ0d
e-
"~- r: O 0 I::: ~I" t'~l O
..
o." . . . . .
m ,--
~~-~..
O N
(V'B/pI)0P/AP 'EN - ewnIoA eJod
O ~
~
,,,,,,,,,,
',
;'i
ooo i
t-:
_
E
o
g::"
~o_
~~-~
~..0"~0
"'0
C: r,,. ~
o
~ co o ~o ~ ~
-
~o~_~176 ._~ ~ ~
c~
I~ "~--
o
-{:3 e=}
9
m N
o,=~ ~e-" ~ o 9- 1 ~ ,__~ ~_~u
~
EZ
"
"~- =-
o
o
E
"
o
E
,
,
E
.(
GPlAP
F: F: ~ F:
E
=:~
"-
No
c~
~~
":
~= ~ ,m_ .-~
o~
.9
o~ .N "~
e-
mo~ '-(.3
8..~.2
or t , , . ~ > &i ~ N
o
.~
2
~-
N-e
.
lOAe.~
o~
""
O
0
.m
~
~.
~:
0
o
o
O ~I"
O
o
o =
~o~
"~~/~ ,
(V'6/I~) OP/AP 'gN - ewnl0A e j 0 d
_
~~o~ o (~,,~
~
=co~. ' o
Eb"
<~ D oo ' ~
oo
-~e,~ I
:E
=0
~
o
o
'~" ,--
o
O o |
o~
O
~
._~--~ u__~m
835
and Li2B407 (98 %) from Kanto Chemical Co, BPO4 (99.995 %) from Aldrich were used, as received, as references for FTIR (Jeol JIR-100) and 11B NMR (11B NMR, Jeol GSX-270 Widebore) analysis. The B203 reference was prepared by dehydration at 350 ~ of H3BO3. 3.
RESULTS
and
DISCUSSION
3.1. Effect of Boron content. The pore size distribution of a B-containing support prepared by kneading as described above is bimodal in the mesopore range, for boron contents between 1.2 and 6.2 wt % studied here, Figures 1-4. The surface area of B-containing supports (up to 3.7 % B: about 300 m2/g) is larger than for B-free supports (250 m2/g). Simultaneously TG and DTA analyses, on dried extrudates, revealed that boron shifts the formation of 1'alumina to higher temperatures, Table 1. The latter observation was confirmed by XRD, as the 1' alumina peak intensities decreased with an increase of boron content, Figure 5. DSC analysis carried out on the supports revealed an exothermal peak at high temperature. This exotherm is assigned to the formation of the aluminum borate 9AI203.2B203 in all cases, but also to the formation of the borate 2AI203.B203 for the 6.2 % B-containing support [6]. This was confirmed by XRD, after calcination at 900 ~ for 1 hr in a muffle furnace, of the 3.7 and 6.2 % B-containing supports.
Table 1 Effect of boron content Boron Surface Pore DTG DTA content Area Volume weight loss endotherm (wt %) { m 2 / g ) {ml/g) {oc) {oc) 0 249 0.653 383 415 1.2 295 0.621 409 452 2.5 293 0.576 420 470 3.7 297 0.552 425 488 6.2 236 0.480 450 520 a) Lower estimate due to overlapping with an other peak.
DSC exotherm (o.c}. 1116 986 920 887
Amorphous Content {wt % ) 10-20 20-30 50 60 60+ a)
The interesting feature in these B-containing supports is their partial amorphous character after calcination at 600 ~ Figure 5. Based on an other study reported elsewhere [7], in which the amorphous phase is turned into a boehmite phase, we found that our B-free support also contains 10 to 20 % of amorphous phase, in agreement with similar values obtained by other researchers [8]. The amount of amorphous phase reported in Table 1 is estimated assuming a linear relationship between the (440) peak area of 1' alumina and the crystallized phase content. The value obtained for the 3.7 % B-containing support is similar to what was obtained in our previous study [7]. In addition, TEM photographs obtained on a dried only and on a calcined 3.7 % B-containing support, clearly show the presence of amorphous aggregates in addition of crystallized aggregates, Figure 6. 3.2. Effect of H3BO3 and Ammonia addition. The kneading of the alumina paste (dough) is monitored by the torque required. When boric acid is added the torque decreases drastically but increases again
836
0%B
r
C 0
o
9
2.5%B
3.7%B
6.2%B
3
2.0
40
60
80
2 theta
Figure 5: X-Ray Diffraction spectra of B-containing supports calcined at 600 ~ 1 hr.
for
837
Figure 6: TEM picture of a 3.7 wt % B-containing support r .~d only. Microdiffraction patterns indicate the coexistence of amorphous and crystallized phases. Magnification: X 80000.
v
o "2
:3
o o
Figure 7" Variation of the macropore volume during kneading and after extrusion. 0.1 ~ ---o-- B-free support --~ -t3.7 % B-containing support } _ _ ~z~-- B-free support __~ 0.08 ~ ---A-- 3.7 % B containing suppo.~~.~
oo4O0G
-"'-
0.02 0
-
~ _. _s L
I
I
L
I
I
I
I A
0 20 40 Kneading time (min) Dough calcined at 550 ~
60 I hr.
After extrusion
838
when ammonia is added, however only for a short time. Ammonia is added to improve the quality of the dough (to reduce its viscosity) and allow extrusion. Due to its layered structure, it is believed that boric acid acts as a lubricant then producing a thick paste, and ammonia breaks this paste. It was observed, in 3.7 wt % B-containing supports, that the dough is 'trapped' at the time of addition of boric acid and that ammonia has only a limited effect. In order to distinguish the separate effect of boric acid and ammonia, 4 different supports were prepared. In all cases, the total kneading time was 60 min. Except for the Bfree support (A), boric acid was always added after 20 min, but ammonia was added at different times: 20 (B), 40 (C) or 55 min (D). Some samples were taken during the kneading, dried and calcined in a muffle furnace at 550 ~ (this temperature is used to reproduce a 600 ~ calcination in a rotary kiln). Surface area, pore size distribution and pore volume were measured on each sample, Table 2 and Figures 1-4. Table 2 Effect of boric acid and ammonia addition on Surface Area (SA), Pore Volume and average pore diameter (d5o). Sampling A B C D time a) SA PV dso SA PV d5o SA PV dso SA PV m2/~l ml/g A m2/g ml/g A m 2 / g ml/~l A m2/~l ml/g (rain) 246 0.47 54 0 287 0.54 62 20 270 0.62 80 326 0.54 67 316 0.52 63 40 330 0.50 55 253 0.67 90 316 0.55 68 321 0.56 71 319 0.55 60 a) Sampling before H3BO3 and/or ammonia addition.
(PV) dso A
59 69
From those data, it can be seen that from a monomodal pore size distribution of the pseudoboehmite, the B-free dough evolves through a bimodal pore size distribution at 20 min, but finally reaches a monomodal distribution after extrusion, Figure 1. Throughout the kneading the average pore size distribution increases. As boric acid is added after 20 min, and because it decreases drastically the torque of the kneader, the dough remains the same as long as ammonia is not added. Ammonia addition, through the increase of the torque, provides a further kneading of the dough, leading to an increase of the average pore diameter. A further increase of the average pore diameter is usually observed after extrusion, as it also provides and additional kneading. The time at which ammonia is added has nearly no effect on the final support, as can be seen by comparison of the final properties of the three B-containing supports. In an other set of experiments, to show that the dough is trapped by the addition of boric acid, the macropore volume was carefully monitored. During the kneading of a B-free support, it was observed that the macropore volume decreases, Figure 7. The size of those macropores corresponds to the free space between pseudoboehmite aggregates. Depending on the kneading conditions, the macropore volume may be varied. Two sets of supports were prepared, with in each case a B-free and a 3.7 % B-containing supports. The macropore volume, after calcination, was plotted versus the kneading time for all supports, Figure 7. It was observed that if macropores were present in the dough when boric acid was
839
added, they were likely to survive in the final support, suggesting once again that the dough was trapped at the time of boric acid addition. For methods of preparation of macropore-free and macropore containing supports see for example reference [9]. The bimodal pore size distribution which is observed in B-containing supports is due to the heterogeneity of the dough at the time of boric acid addition. It was observed by TEM that after only 20 minutes of kneading, the dough, dried at 130 ~ contained at least two types of aggregates distinguished by their state of ,~ispersion. In addition, if boric acid is added after 40 or 60 minutes, the mode at 50 decreases and the mode at higher pore diameter increases while shifted to larger pores. However, as one of the effect of boron is to stabilize the alumina, even if boric acid is added at 60 minutes, it is not possible to obtain the same pore size distribution than for a B-free support. During calcination, the B-containing support sinters less and the pore size distribution remains bimodal.
3.3. Short range structure of B-containing supports. In the preparation of a B-free support, it was observed that the crystallite size of the pseudoboehmite, measured on dried extrudates, increases by kneading, see Table 3. However, when boric acid is added, the average size of the crystallites decreases. In B-containing supports, the crystallite size of the boehmite increased with the kneading time before addition of boric acid, see Table 3, as did the average pore diameter. Table 3 Crystallite size measured by X-ray line broadening, after drying overnight. Support Kneading time Cn/stallite size based on the a Iwt %) Imin) 120 peak 020 peak .... PSB a) 0 (Raw material) 50-52 A 33-37 A 3.7 % B 20 b) 56 Ad) 28-35 A d) 3.7 % B 40 b) 62 A d) 43 Ad) 3.7 % B 60 b) 69 Ad) 46 Ad) 0%B Sampled at 20 min 70 A 52 A 0%B Sampled at 40 min 76 A 57 A 0%B Total: 60 min c) 73-76 Ad) 58-61 A d) a) PSB: Pseudoboehmite; b)boric acid and ammonia addition time, kneading is then maintained for 40 min; c)total kneading time: 60 min, d) measured after extrusion. 11B NMR and FTIR were used to characterize B-containing supports, Figures 8-9. The only B species present in B203 (glass) and H3BO3 is a triangular-planar BO3 [10] giving rise to a broad signal doublet-like in NMR and to a band in the 15001200 cm -1 in FTIR [4,11]. Tetrahedral borate species BO4 are characterized by a sharp peak in NMR and by a band in the 1200-900 cm-1 region in FTIR. BPO4 is known to contain only the tetrahedral type and Li2B407 contains both type in a 1:1 ratio, and were used as references here [10]. The IR spectrum of a 3.7 % B-containing support shows a broad band corresponding to the BO3 species and a very small shoulder peak in the 1200-900 cm -1 region corresponding to the BO4 species. It is much clearer from the NMR spectra that both species coexist in the support. In fact, it seems that the coexistence of both species does not depend much on the preparation method as it
840
B-free support
BPO4 3,7 % B-containing support Li2B407 .__~j.-/k,,_.__._/ BPO4 H3BO3
IL ) ~ ' " - M . . ~ "~'- ~
H3BO3
......
B203 "~~ .
B203 .. ~ , , , _ / x . f
/
---
3,7 % B-Containing support 100
I
50
I
0 ppm
I
-50
I
-100
Figure 8: 11B NMR spectra of reference compounds and of a support calcined at 600 ~
16(10 ' 12~)0 ' 80'0 ' 40() Wavenumbers (cm-1) Figure 9: FTIR spectra of reference compounds and of supports calcined at 600 ~ (KBr pressed disks).
841
has also been reported for supports prepared by coprecipitation and by decomposition of mixtures [4,11]. It is likely that the B04 species formed in our supports was due to dissolution of boron in alumina.
3.4 Effect of boron on the Side Crushing Strength. Using the B-containing supports described above, 3 wt % Co-10 wt % Mo catalysts were prepared. It was observed that 3 wt % B-containing catalysts maintained a high SCS after impregnation and calcination, although it sharply decreased for a B-free catalyst. The decrease of the SCS in B-free catalysts is explained by the formation of cracks in the support during the impregnation step, due to capillary forces [12]. Three different explanations are given for the beneficial effect of boron: (1) due to the presence of amorphous aggregates in the supports, cracks remain small; (2) because boron can be easily extracted with water [7], surface tensions were measured and found to decrease with an increase of boron content in impregnation solutions, thereby reducing the capillary forces; (3) boric acid in ammonia solution was shown to have a repairing effect. In the latter case, two B-free supports were impregnated with water only, dried and calcined. In order to reduce significantly their SCS, this cycle was repeated twice. Then they were impregnated with an ammoniacal boric acid solution, dried and once more calcined. The final SCS was found to increase again after the boron impregnation. The same results were observed when a B-free support was impregnated with a mixture of boric acid-molybdate solution, dried then impregnated with a Co solution, dried and calcined. The same mechanism is believed to occur during the impregnation of B-containing supports, as we have already shown that boron is easily extracted by hot water [7]. However, in case of an impregnation boron is simply moved to an other location inside of the pellet. These results suggest that boron containing supports might be able to repair themselves, making them intelligent materials. The effect can be explained by the melting effect of boron oxide, allowing it to repair the damaged pellets. Table 4 Repairing effect of boron on a B-free support. SCS values are given in kg. 3 wt % Co-10 wt % Mo catalys!s Water impregnated Final B content (wt %) 0 1 2 3 3 3 SCS of the B-free support 4.9 4.9 4,9 419 4'~'g 4.9 After 1st impregnation 4.1 4.0 After 2nd impregnation 3.3 3.1 Final SCS 2.9 3.4 3.6 4.3 4.4 4.8 ,,,,,,
3.5 Catalytic properties Using the B-containing supports described above, 3 % Co-10 % Mo catalysts were prepared and tested for the hydrotreatment of a VGO feedstock at 80 bars, 360-420 ~ LHSV-1-2, and were found to be more active than B-free catalysts. 3 % B-containing Co-Mo catalysts were confirmed to be more active in HDS, HDN and cracking, of a VGO feedstock, than B-free catalysts, in agreement with many previously published results [1,2]. This latter observation confirms the interest of the present preparation procedure to produce B-containing catalysts.
842
4. CONCLUSION Several effects due to boron addition in supports were observed in this study: Boron raises the temperature of formation of ~ alumina; Boron addition, up to 3.7 % increases the surface area of the support; The bimodal pore size distribution, observed on the supports, reflects the pore size distribution, in the dough, at the time of boric acid addition; Boron containing supports (macropore-free) retain a high SCS after impregnation. ACKNOWLEDGEMENTS
The authors would like to thank Misses Abe and Ono as well as Messrs Araki, Ikuta, Fujiwara, Ishidoya, Makishima and Saito, for their technical assistance. REFERENCES
1. (a) A.J. de Rosset, US Patent 2938001, (b) M.J. O'Hara, US Patent 3453219, (c) M.J. O'Hara, US Patent 3525684, (d) M.J. O'Hara, US Patent 3666685, (e) R.A. Plundo, US Patent 3617532, (f) L.A. Pine, US Patent 3954670, (g) L.A. Pine, US Patent 3993557, (h) R.J. Bolard and J.D. Voorhies, US Patent 4139492; (k) P. Dufresne and C. Marcilly, French Patent 2561945; (I) H. Toulhoat, European Patent 0297949. 2. a) M.-C. Tsai, Y.-W. Chen, B.-C. Kang, J.-C. Wu and L.J. Leu, Ind. Eng. Chem. Res., 30 (1991) 1801; b) C. Li, Y.-W. Chen, S.-J. Yang and J.-C. Wu, Ind. Eng. Chem. Res., 32 (1993) 1573. 3. H.Lafitau, E. Neel and J.C. Clement, in B. Delmon, P.A, Jacobs and G. Poncelet. (Eds), 'Preparation of Catalysts', Elsevier, Amsterdam, (1976) 393. 4. K.P. Peil, L.G. Galya and G. Marcelin, J. Catal., 115 (1989) 441. 5. M.L. Occelli and T.P. Debies, J. Catal., 97 (1986) 357. 6. a) O. Yamaguchi, S. Nakamura and K. Shimizu, Nippon Kagaku Kaishi, 1 (1979) 5; b) O. Yamaguchi, M. Tada, K. Takeoka and K. Shimizu, Bull. Chem. Soc. Japan, 52 (1979) 2153; c) P.J.M. Gielisse and W.R. Foster, Nature, 195 (1962) 69. 7. J.-L. Dubois and S. Fujieda, Preprints of the 37th Congress of the Japan Petroleum Institute, Spring meeting, 18th may 1994, Tokyo. 8. J.P. Franck, E. Freund and E. Qu~m~r~, J. Chem. Soc., Chem. Commun., 10 (1984) 629. 9. W. Stoepler and K.K. Unger, in B. Delmon, P. Grange and P.A. Jacobs, 'Preparation of Catalysts: Scientific Bases for the Preparation of Heterogeneous Catalysts', volume III, Elsevier, Amsterdam, (1983), 643. 10. V.F. Ross and J.O. Edwards, in E.L. Muetterties, 'The Chemistry of Boron and its Compounds', John Willey & Sons, New York, (1967), 155. 11. A. Delmastro, G. Gozzelino, D. Mazza, M. Vallino, G. Busca and V. Lorenzelli, J. Chem. Soc. Faraday Trans., 88 (1992) 2065. 12. a) C. Marcilly and J.-P. Franck, Revue de I'lnstitut Frangais du P~tmle, 39 (3) (1984) 337; b) R. Poisson, J.-P. Brunelle and P. Nortier, in A.B. Stiles (Ed.), Catalyst Supports and Supported Catalysts, Butterworth Publishers, 1987, 11; c) J.-F. Le Page et al., 'Catalyse de Contact, Conception, preparation et mise en oeuvre des catalyseurs industriels', Editions Technip, Paris, 1978.
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of HeterogeneousCatalysts G. Ponceletet al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
843
Characterization of a l u m i n a paste b y cryo-microscopy E. Rosenberg*, F. Kolenda, R. Szymanski and M. Walter *IFP - BP 311, 92506 Rueil-Malmaison Cedex, France IFP - CEDI, BP 3,69390 Vernaison, France
1. INTRODUCTION The development of new catalytic active phases is conditionned by the ability to put into shape these products (extrudate, bead, pellet,...) consistently with their use in an industrial process (fixed, moving, fluidized,..., beds). Specific constraints due to the forming process can be limiting factors in the manufacture of industrial catalysts. Most forming processes involve the use of suspensions, sols or gels (pastes). In order to solve handling problems or to tailor the rheological properties of these media to suit a particular forming process, one can act on mechanical parameters (choice and operation of equipments), modify the surface chemistry of the suspended or gelled particles (effect of pH, additives) or use a binder (colloidal silica, alumina,...). Such actions generally affect the final properties that the catalyst has to meet (textural, mechanical and catalytic requirements). A better understanding of the microtextural and chemical changes occuring at the sol-gel stages is highly desirable for the optimization of catalyst forming. It is however limited by the difficulty to characterize such media at a microscopic level. The texture of sols and pastes is studied at IFP by a number of techniques: thermoporosimetry (1), proton NMR relaxation and X-ray and neutrons diffusion (2,3). One major drawback of these indirect techniques is the need for a model (generally assumed as homogeneous model) to extract a computed textural information. Accordingly, cryogenic techniques in electron microscopy were developped in order to get a direct visualization of gel texture. The so-called cryo-microscopy are successfully applied to characterize the texture of polymers and clay cakes in relation with permeability properties (4). In this paper, the first application of cryo-microscopy to the characterization of alumina pastes in the extrusion process is presented.
844 2. EXPERIMENTAL 2.1. P a s t e P r e p a r a t i o n s :
Processes involved in kneading alumina p o w d e r into a paste suitable for extrusion are mainly of two types: * A process associating a batch kneading step, typically in a Z-blade mixer to an extruding step in a screw extruder * A process using a twin-screw mixer for kneading the p o w d e r and the liquid reactants as well as extruding on line the paste. In this work, a continuous process of type 2, as illustrated on figure 1, elaborates homogeneous alumina pastes.
II"mm'l'm
I
m
I
~I / twinscrew kneader
!
I HN~
I--I
I I
' nnnn
~L I I I I'"'
:
i
i I
I I i I
IIII
I
i
I
9
I',
II II
I I I I
// ' ~
// Ii //
I I I1
I I i Ia
I
I
nnnn I IIII llll IIII j
'
I
!
Ii
'
!
,
( ~
i
! ......
i
I
9
Figure 1- alumina preparation in a twin screw kneader The alumina paste is p r e p a r e d in a twin screw kneader designed by AOUSTIN, FRANCE, by introducing aluminium h y d r o x y d e p o w d e r (Pural SB from CONDEA CHEMIE GmbH) at a regulated mass flowrate and nitric acid solution (flow control). The two products are fed in the first mixing zone of the machine and react instantaneously. This peptization process can partially be modified by a d d i n g a m m o n i a solution at a controlled rate, which precipitates part of the colloidal peptized suspension. The paste is extruded in a ram extruder at a given velocity. Paste preparation is affected by a large amount of parameters g r o u p e d as follows: a) Formulation variables: These variables determine the chemical composition of the paste. * H N O 3 / A 1 2 0 3 and H2Oa/A1203 weight ratios in the peptization zone
845 * N H 3 / H N O 3 molar ratio and H2Ob/A1203 weight ratio in the neutralization zone b) Process variables: These parameters affect paste formulation by acting on the paste flow, rheology and residence time in the kneader. * Solid flowrate D kg/hr * Speed rotation of the co-rotating screws rpm * geometry of the screw * Temperature of the kneader jackets ~ This multi variable paste preparation is investigated by experimental design theory(5). Three samples are selected and observed at a gel state, and, after drying at 110~ and calcining at 500~ For each paste, values of each operating variables are reproduced in table 1. Table 1 Sample 1 2 3
HNO3/ A1203 0.008 0.025 0.008
H20 /A1203 1.25 1.25 1.075
NH3 /HNO3 0.7 0.1 0.7
H20 /A1203 0.15 0.15 0.075
D kg/hr 20 5 5
rpm 220 120 220
T
~ 20 50 50
2.2. Cryo preparation of samples When an hydrated sample is put at ambient temperature in the high vacuum of a scanning electron microscope (SEM), water is rapidly lost, causing shrinkage and distortion of the sample. By rapidly cooling hydrated specimen and maintaining them below-160~ they can be examined for extended periods in vacuum without water loss. The internal structure of cooled specimen can be revealed by freeze-fracture and greater details are often revealed by carefully raising the temperature to a point at which water begins to sublime at a controlled rate. The etching process is stopped by rapidly lowering the temperature once more.
2.3. Experimental procedure for studying hydrated pastes by electron microscopy The Oxford/Hexland cryotrans system fitted to a JEOL JSM6300F SEM is used in our experiments. Small pieces of gel (about 1.5mm large and 5mm long) are mounted on copper stubs and rapidly frozen by plunging them in nitrogen slush at about 63K. The stub is then transferred to a cryo-preparation chamber under vacuum which consists of a cold stage cooled at-165~ The sample is fractured under vacuum in order to expose a fresh fracture plane to observation. The sample is then transferred onto the cold stage of the microscope initially at-190~ A heater is incorporated in the stage enabling the temperature to be raised to any preset temperature from-190~ to 50~ The sublimation process is controlled by observation under the beam.
846 3. RESULTS
Typical SEM pictures of the hydrated pastes obtained with the cryoprocedure are shown on plate 1. The polished sections of the initial boehmite powder and of the dried and calcined pastes are shown on plate 2. Paste 1 appears as a packing of spherical particles with a size distribution very close to the initial boehmite particle size distribution. A dispersed gel of elementary aggregates appears lining the largest grains and w r a p p i n g the smaller ones. Corresponding dried and calcined paste appears as a sintered material in which spherical grains are no more individualised. Particles are bridged and the resulting system consolidated. The resulting porosity is very close to the porosity created by the rough stacking of boehmite grains. The presence of a large macroporous volume is confirmed by Hg porosimetry measurement (Vmacro=0.18 cm3/g). Paste 2 is composed of a dense gel, compartimented into large blocks, with no, or very few, residual boehmite grains. The polished section of the corresponding dried and calcined paste shows that most of the initial boehmite grains have disappeared except for very few larger ones (50 microns). These are dispersed in a dense and homogeneous gel. Non connected voids are observed in the calcined paste, while no voids at all are observed on the extruded support. No macroporous volume is detected by Hg porosimetry. Paste 3 is an intermediate case. Initial boehmite grains are partially eroded and dispersed in a gel phase. On backscattered electron images, a chemical contrast is observed mainly due to density differences between the residual boehmite grains and the binding gel (polished sections). This contrast is likely to be due to mesoporosity differences. The three types of paste texture can be schematically illustrated as follows. ........................... ...,.-..-.....-,.-,.)~. 9 t~.,. ~ . ~ , ~ . r ~ ~~,~,~,.,.,~,~;~
~ . : " .
Paste 1
Paste 3
t~l gen
.:,"
interstitial liquid
~,~-~.~.
boehmite grains
-},",.'~:;~_~~ ~ ~ . ~ t .%.%.%.
Paste 2
i
847 Plate I
,. d. i~.,
~ii~,~<~ ?~::
.... :
x~!~T" ........................ ~
~,
:
:: ......
'~ ,,~o t:::~ ,.~.& f:~ t -.:~n
Cryo-SEM secondary electrons images of hydrated pastes
848 Plate 2
,:::
:
::
::::::,:~;, ::~ ...........
===========================
bar =100 microns
SEM backscattered electrons images of dried pastes (polished sections)
849 4. DISCUSSION The results reported in this paper show that the parameters that enable a boehmite paste to be extruded have a drastic influence on its microtexture as illustrated for samples 1 and 2. Moreover, the textural features observed in the pastes are shown to be kept in the dried and calcined pastes. Accordingly, the tailoring of the final properties of extrudates should be improved by a better understanding of the relation between preparation parameters and pastes texture and cryo-microscopy is likely to be a promising tool to achieve this goal: In order to understand the observed pictures of paste texture, it is necessary to list the sequence of events arising in the kneader-extruder and generally known as peptization: 9 in the initial boehmite powder, elementary boehmite cristallites (platelets of 10 to 30 nm) are agglomerated into large spheroidal particles (mean radius: 40 micrometers); 9when the powder is mixed with the nitric acid solution in the kneader, the mechanical action comminutes the large particles into smaller units; 9 these units rearrange themselves in the solution according to interactions governed by Van der Waals and electrostatic repulsion forces. The electrostatic repulsion forces are due to positive charges created at the boehmite surface in acidic medium (isoelectric point close to 9). In very acidic media dissolved aluminic species could be involved in these interactions. So, in the kneader-extruder, peptization can be described as a continuous sequence of breakdown-reconstruction phenomena, the ultimate state of a successfull peptization being to allow the above rearrangements to take place between all the elementary boehmite cristallites. According to this description, it can be stated that when a peptization has been unsuccessfull or is just under progress, the paste shall consist of large agglomerates (cf. initial powder) with large interagglomerate macrovoids. These macroscopic voids will be filled with the acid solution poorly enriched in smaller boehmite aggregates or elementary cristallites. After drying and calcination, a large interagglomerate macroporosity will be created in the final solid. The meso/microporosity will essentially originate from the intraagglomerate microscopic voids present in the initial boehmite powder. This situation could correspond to that of sample 1. Conversely, at an advanced state of peptization, most of the large agglomerates shall be broken and thus, large macrovoids no longer be present. In an ideal homogeneous paste, the only observed porosity would be due to microvoids between elementary aggregates of the dispersed gel medium. Such a case can be illustrated by sample 2. Obviously, according to the advancement state of the peptization, intermediate situations can occur, where partially desagglomerated initial particles are included in a dense gel phase medium. The pastes look like a composite material (sample 3) with few macrovoids. The meso/microporosity
850 is then a combination of that of the initial powder and that created by the microvoids in the dispersed gel medium. It has been argued here above that direct observation of pastes by cryomicroscopy gives pictures of their microtexture that can be related, at least qualitatively, to the peptization state of boehmite powders in the extrusion process. It should be now questionned whether the peptization state observed in different samples can be related to the operating parameters of the kneaderextruder. Indeed, samples 2 and 1 correspond to extreme situations of peptization which are consistent with the operating parameters used: high acidity and long residence time, both favouring peptization in the case of sample 2 versus weak acidity and low residence time for sample 1. The pictures of sample 3, which differs from sample 1 by an increased residence time, are also consistent with an increased mechanical action. It is clear h o w e v e r that the comparison of intermediate situations between themselves will be more difficult, unless quantitative data are derived from the pictures. Work is still under progress to quantify the particles desagglomeration level (use of digitized image processing) and to estimate the interagglomerate gel phase density (use of Z contrast imaging mode).
5. CONCLUSION Direct observation of boehmite paste texture by cryomicroscopy is shown to be a very promising approach for the understanding of phenomena occuring in a continuous kneading process. This technique makes possible the characterization of a state of boehmite peptization in relation with the operating parameters. The porosity of the final extrudate can be related to the peptization state and could be tailored through a better control of this latter. Present studies are carried out to achieve a more quantitative description of peptized pastes.
REFERENCES
1. JF Quinson, F Kolenda, G Dessalces, JP Reymond, J. of non cryst, solids,147148,1992,pp141-145 2. M. Morvan, D. Espinat, R. Szymanski & R. Ober, Proc. 6th IFP Exploration and Production Research Conf., St Raphael, Sept. 1991 3. M. Morvan, PhD Thesis Universit6 Paris VI, May 1993 4. L. Loeber, PhD Thesis Universit6 Paris VI, December 1992 5. M. Walter, IFP internal report
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
Preparation of Cation-Substituted Hexaaluminates Surface Area using Mechanical Activation Methods
851
with
Large
0 . A . K i r i c h e n k o a, 0 . V . A n d r u s h k o v a a, V.A.Ushakov b, V.A.Poluboyarov a aInstitute of Solid State Chemistry, SB of RAS, Derzhavina, 18, Novosibirsk 630091, Russia bBoreskov I n s t i t u t e of Catalysis, SB of RAS, Lavrentieva, 5, Novosibirsk 630090, Russia
Catalytic combustion has long been in use ~md wide set of catalysts has been developed for this purpose [1,2]. However, the problem of catalytic materials for h i g h - t e m p e r a t u r e combustion is yet of considerable i n t e r e s t [3]. The catalyst m u s t be able to ignite at 350-400~ combustion of n a t u r e gas passing with flow rate of 1-20m/s, be stable at t e m p e r a t u r e s h i g h e r t h a n 1250~ for significant performance time, at least one year. Considerable recent attention has been focused on preparation of ceramics based on cation-substituted hexaaluminates for catalytic combustion at 7001300~ S u b s t i t u t e d barium hexaaluminate generally retained the larger surface area as compared with various oxide catalysts s u p p o r t e d on u n s u b s t i t u t e d one [3-5]. One of the main subjects is how to fabricate a heatr e s i s t a n t material with large surface area. The well known methods based on solid-solid reactions are extremely simple, but produced inhomogeneity and low surface area values. Hexaaluminate-like ceramic support and catalysts with large surface area have been prepared from alkoxides only [3-6] due to the u n i f o r m mixing of the components and decrease in t e m p e r a t u r e of the product formation. The complete formation of BaMnAlllO19 was observed a f t e r calcination at 1300oC. This catalyst showed the surface area of 13.7 m 2 / g and catalytic activity in methane oxidation comparable to t h a t of Pt/A1203 and LaCoO 3, yet not satisfactory to be used as a high t e m p e r a t u r e combustion catalyst. Both surface area and catalytic activity were enhanced by combination of m i r r o r plane cations, namely, Ba0.8K0.2MnAlllO19 and Sro.8Lao.2MnAlllO19. The surface area of these materials a f t e r calcination at 1300~ was about 23 m2/g, being effective in a t t a i n i n g high activity, especially u n d e r the high flow rate conditions as in a gas turbine combustor. However, it is known, t h a t coexistence of the distinct charge cations in the same sublattice brings about an increase in diffusion coefficients and, as a roule, excelerates isothermal sintering of oxide materials [7,8]. Really,
852 prolong t r e a t m e n t at 1300~ reduced the surface area of Sro.8Lao.2MrLA111019 from 18 to 4 m2/g [3]. Hence the problem of preparation of catalitically active materials for a high temperature combastor is still unresolved. Mechanochemical synthesis is a new way for preparation of complex oxides under mechanical stress at relatively low temperatures [9,10]. The reactions can take place under a mild conditions (so calle~i "soft-mechanochemistry"). Even if a desired materials cannot be obtained only by mechanical stressing, solid-state reactions are often promoted on the subsequent heating after g r i n d i n g of an initial reactant mixture. Selection of appropriate precursors, apparatus used as mechanical activators, milling and heating regimes are of vital importance in order to enhance the rate of mechanochemical reaction, to decrease the temperature of product formation, to develop ceramic material with large enough surface area, porosity and mechanical s t r e n g t h . In the present work cation-substituted barium hexaaluminates have been prepared using mechanical activation methods. The effect of precursors, milling regimes and calcination temperature on their synthesis has been investigated. 1.EXPERIMANTAL Reagent grade ~ - A1208, A10(OH), AI(OH)3, BaO 2, BaO, Ba(OH)2, MnO, Mn208, MnO2, MnO(OH)2 were used as precursors. The samples of ~- and - AI208 were obtained by calcination of gibbsite at 500oC and 1200~ respectively. The samples were treated in planetary mills, designed at I n s t i t u t e of Solid State Chemistry, with metal thimbles and balls, in air for 1 - 10 rain, and then were calcinated. X-ray diffraction and specific surface area measurements (argon desorbtion) were used to characterize them. The catalytic activity was measured by a flow-circulation technique and caracterized by the rate (W) of butane oxidation at its content of 0.2 vol.% and 400~ and by the temperature dependence of CO conversion (X). The initial content of butane and CO in the air mixtures was 0.5 vol.% and 1 vol.% respectively. The weight of the sample tested was 1 g. 2. R E S U L T S AND D I S C U S S I O N S
2.1. BaMnAIl1019 synthesis In the case of ordinary mechanical m i x t u r e of p r e c u r s o r s a multycomponent system was observed even after calcination at 1300~ The intermediate phases BaA1204 and BaMnO 8 are formed at 900oC, the former
853 one coexisting with ~-Al203 upto 1300~ That agree with the results obtained in [4] for BaCOs/7-AI208 mixtures. The preliminary mechanical treatment of the starting precursor mixtures for a short time of 1-2 rain in the planetary mills decreases the temperature, the hexaaluminate phase begins to form, from 1300oC to 1000oC. The T, ~ - ' ~ yield of BaMnAlllOl9 (the ratio I/Im 800 1000v 1200__. of the line d/n-3.70 intensities of I/Ira given sample and hexaaluminate 0.8 calcined at 1300~ increases with mechanical treatment time and with calcination temperature as shown in Figure 1 for one of the system 0.4 studied. Phase composition after calcination depends on precursor used (Table 1). Composition and structure of Ai- and Ba-precursor have profound effect on 0 4 8 12 the hexaaluminate phase formation, x, rain ~ whereas the nature of M n oxide does not have any considerable 'one. Figure 1. The yield of BaMnA111019 surface area is BaMnAI11019 at 1000~ vs M T independent on precursor one and is 8 time and vs temperature in - 12 after 1 0 0 0 - 1100~ and 6 x-A1203 - BaO 2- MnO 2. Calcination 8 m2/g after 1300~ time 1 h Table 1 Phase composition calcination for 1 h
of mechanically
Precursors
treated (for 5
900~
rain) samples
1000~
AI(OH)3, Ba(OH)2, MnO(OH)2 AIO(OH), Ba(OH)2, MnO 2 AIO(OH), BaO, MnO2
BA, BM,HDP HDP BA, HDP
BA,BA6, a a,BA6 BA, BA6,
~(-AI203, Ba(OH)2, MnO2 x-AI20s, BaO, Mn02 x-AI203, BaO2, MnO2
BA, HDP BA, BM, HDP
BA6, BA6, BA BA, BA8
BA: BaAI204, BM: B a M n O 3, H D P : highly dispersed phase
BA6:
after
II00~
BA6, cc ~, BA6 BA6, BA BA6, BA6, B A BA6, BA, cz
hexaaluminate-based
phase,
The use of hydroxides as precursors was believed to facilitate final complex oxide formation under mechanical treatment or subsequent
854 calcination [10]. The findings of the present studies show, t h a t this hypothesis break down for synthesis of substituted hexaaluminates. When hydrooxides were used, the by-product a-Al20 s arised (Table 1). Only a little a-A120 s formed reacts with another intermediate BaA1204 at the elevated t e m p e r a t u r e s up to 1300~ because of the weak reactivity of a-A1203. W i t h Ba(OH)2 as a precursor, a-A1203 formed at 1000oC, therewith t h e higher OH-content in a precursor m i x t u r e the more a-A1203 observed and the lower t e m p e r a t u r e of its appearance. On the contrary, no a-A120 s after calcination at the same condition was observed when oxides or peroxides were used, even w i t h o u t mechanical t r e a t m e n t before calcination. According to the previous reports [4, 11], phase transition t o a-A1203 did not occur during Ba-hexaaluminate synthesis by cooprecipitation or powder mixing of BaCOs/7-A1208 with subsequent calcination. All t h a t make it possible to suggest that a-A120 s formation is caused by a h y d r o t h e r m a l condition arising under heating of the mechanically activated powder, which tends to form the close packed aggregates from the precursors u n d e r mechanical treatment. The aggregate formation is sustained by the fact, t h a t the surface area of the powders after mechanical t r e a t m e n t is less one calculated assuming ordinary mechanical m i x t u r e (Table 2). Table 2 Masured/calculated surface area (m2/g) of powders mechanically treated for 5 rain Precursors
Ba(OH) 2
BaO
BaO 2
73/183 25/93
74/183 29/92
150/183 -
x-A1203, MnO 2 A1OOH, MnO 2
The most complete formation of BaMnAlllO19 at 1000oc were observed in the systems A1203 (X- or 7-) - B a O - MnO 2. The increase in activation time from 1 to 10 min results in BaMnA111019 formation at 900oC and in absence of any by-product at 1000oC for these systems (Table 3). Table 3 Effect of Al-precursor on BaMnA111019 formation in the system A1208 - B a O - MnO 2 Al-precursor (S) I/Ira at 900~ T,~ as I/Im--1 S T, m 2 / g S180o, m 2 / g
AIOOH(ll0)
7-A1203(220 )
~-A1208(270 )
a-A12Os(10 )
0
0.70
0.05
0
1100 10.5
1000 12
1000 9.7
>1300 -
4.8
7.8
7.7
-
855 a
@
@
9
9 B'aCOs
@
9
A
b
900~
f
x
X
V
V V 900~
d
_V_
V lO00~ V iV
,-,
I'V 1350~
h
MnO 2
x B a ~ 2 04 v
c
BaO
BA 6
V V1350~
I// 20
I
30
4'0 2|
deg --~
I
I
20
3'0
'
2|
deg
Figure 2. X-Ray diffraction patterns of systems x-A12| 8 - BaO - MnO 2 (a-d,h) and y-A12| s - B a O - MnO 2 (e-j) after MT and calcination at various temperatures. X-ray diffraction patterns during the course of BaMnAIllO19 p r e p a r a t i o n is shown in Figure 2. A f t e r mechanical t r e a t m e n t in the laboratory mill powders exhibit broaded lines of BaO, MnO 2 and BaCO 8 on the HDP background. The patterns of the samples treated in more powerfull mill are quite different from those of the precursors and too complex to identify any individual phase. So, the interaction between precursors and the changes in their physical and chemical properties begin upon the mechanical t r e a t m e n t . X-ray diffraction p a t t e r n of the sample with ~- a f t e r 900~ exhibits the blurred lines, t h a t can be attributed to the main h e x a a l u m i n a t e lines, whereas for the sample with ?- the hexaaluminate lines are well identified and high intensive with ratio somewhat differed from those of BaMnA111019. No change in X-ray p a t t e r n s was observed as heating at 900~ was continued. The t e m p e r a t u r e raise to 1000~ leades to the increase in h e x a a l u m i n a t e line intenSities and to the decrease in BaA1204 ones, especially when the calcination time is above 10 hs. The highest reactivity of ?-A1203 at 900~ in h e x a a l u m i n a t e synthesis may be due to its proto-spinel s t r u c t u r e [12]. BaO.6A1208 is known to possess magnetoplumbite-type s t r u c t u r e including spinel blocks [13].
856 2.2. Bal.xKxMyAll2.yO19. a synthesis
Bao.8Ko.2MnAlllO19 is formed at the same conditions and possessed the same magnitude surface area values if KMnO 4 is used as a precursor. It should be noted, that substitution of Ba +2 for K + in the mirror plane results in a-A1203 formation at 1000~ and increases the temperature of complete reaction. Therefore, the interaction of Ba compounds with transition aluminas to 1000~ is of great importance for the preparation of substituted hexaaluminates. The same procedure my be used for preparation of some substituted hexaaluminates (Table 4), the temperature of complete formation being 1300~ The substitution of A18+ for Mn 8+, Fe 8+ was confirmed by the shifts of diffraction lines to low angles and by the variation in their intensities compared with those of ASTM data for unsubstituted BaO.6A1203. Table 4 Effect of composition on the hexaaluminate synthesis Hexaaluminate
Precursors
Phase composition after 1100 ~
$1 aoo, m2/g
BaMn2All0019
x-A1203, BaO2, MnO 2
BA, BA 6
8.2
BaFeA111019
x-A1203, BaO, FeOOH
BA, BA 6
7.2
BaFe2All0019
x-A1203, BaO2, a-Fe208
BA, a-Fe203, FeA1204
4.5
Bao.8K0.2MnAlllO19
x-A1203, BaO2, MnO2, KMnO 4 BA, BA 6,
6.4
a-A1203 2.3. Catalytic activity The substituted hexaaluminates prepared with preliminary mechanical activation of precursor mixture possess relatively high catalytic activity in butane oxidation comperable with that of the well-known catalyst for fuel combustion - MgCr204 (Table 5), while their surface areas are less than previously obtained during preparation by the hydrolysis of alcoxides [4-7].
857
S'm~g0
._.----------O-
W,pmol/gs 0.4
25
0.2
0
Figure
l
25 CH2O,
3.
Effect
i
50
0
wt~
of
H20
addition
(grinding for 5rain) on surface area and butane oxidation at 400~ for BaMnAlllO19.
The additional mechanical treatment of powdered oxides in planetary mill after their synthesis is the way to develop the surface area. Usually, dry or wet grinding is emploied as well as desintegration in special medium [9,10]. After the dry grinding of BaMnAlllO19 with S - 6 . 7 m2/g the surface area is doubled. The surface area achieves 54 m2/g, as the moisture conyent in powder treated rises to 20 wt.%. The catalytic activity increases too, yet not as much as surface area (Figure 3).
Table 5 Catalytic activity in butane oxidation
Sample BaMnAI11019
Calcination temperature, ~
Surface area, m2/g
W, ~mol/g s
1300
6.7
0.12
1350
5.9
0.12
1350, 400
65
0.54
BaMn2AlloO19
1300
4.1
0.18
BaFe2A110019
1300
3.2
0.06
MgCr204
1200
3.4
0.10
MgCr204/7-A1203
700
140
0.50
The input of special detergent medium in mill thimble has resulted in the surface area of 65-70 m2/g. The samples with thus developed large surface area exhibit the hexaaluminate phase lines slitely differing in intensities from those of initial hexaaluminate (Figure 2h). The catalytic activity of these
858 samples is higher that of MgCr204 and comparable with MgCr204/YA1208 one (Table 5, Figure 4). The activation energy obtained for butane oxidation on these hexaalumi'nates is 27 kcal/mol, promising high activity at elevated temperatures.
x, % 80
f
l~
iI
40
P i
n/
I
I
I
200
400
600
T,~
-
w
Figure 4. Dependence of CO conversion on the reaction temperature" 1 - BaMnAlllO19, S-- 65 m2/g; 2 - Mg-Cr-O/y-AI203; 3 - MgCr204.
-
Conclusion remarks: new method of preparation of cation-substituted hexaaluminates with large surface area has developed the optimum conditions for mechanochemical synthesis of Mnand Fe-loading Ba-hexaaluminates are viewed
REFERENCES Io
2. 3. .
5. 6. .
8.
.
10. 11. 12. 13.
D.L.Trimm, Appl. Catal., 7 (1984) 249. R.Prasad, L.A.Kennedy and E.Ruckenstein, Catal. Rev., 26 (1984) 1. M.F.M.Zwinkels, S.G.Jaras, P.J.Menon and T.A.Griffin, Catal. Rev., 35 (1993) 319. M.Machida, K.Eguchi and H.Arai, Chem. Lett., (1986) 1993; (1987) 767. M.Machida, K.Eguchi and H.Arai, J.Cat., 103 (1987) 385; 123 (1990) 477 M.Machida, H.Kawasaki, K.Eguchi and H.Arai, Nippon Kagaku Kaishi, (1988) 2010. W.J.Johnson and R.L.Coble, J. Am. Ceram. Soc., No 3-4 (1978) 110. E.S.Lukin, T.V.Efimovskaya, A.V.Beliakov, V.P.Tarasovsky and P.A.Pshechenkov, Chim. and Chim. Tecnol. Silicate Materials, 128 (1983) 47. W.W.Boldyrev, Z. Phys. Chem. (Leipzig), 256 (1975) 342. E.G.Avvackumov. Mechanical Methods of Activation of Chemical Processes .- Novosibirsk: Nauka, 1986 G.Groppi, M.Bellotto, C.Crisfiani, P.Forzatti, Book of Abstracts EuropaCat-1, V.2, Montpellier, September 12-17. (1993) p.925. V.A. Ushakov, E.M. Moroz, React. Kinet. Catal. Lett, 24, Nol-2 (1984) 47. J.M.P.J.Verstegen and A.L.N.Steve!s, J. Lumin., 9 (1974) 406.
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
859
A new approach to catalyst preparation using rate controlled temperature p r o g r a m m e techniques P. A. Barnes and G. M. B. Parkes Catalysis Research Unit, Leeds Metropolitan University, Calverley Street, Leeds LS1 3HE, UK.
SUMMARY It is shown that the use of rate controlled temperature programme techniques in calcination at atmospheric pressure can determine the outcome of competing decomposition reactions and optimise surface areas, pore sizes and the homogeneity of the final catalyst.
1. I N T R O D U C T I O N A key stage in the preparation of many catalysts is the thermal decomposition of a precursor. Normally, such calcinations are carried out isothermally for a given time at a predetermined temperature, which is reached using a linear heating rate. Unfortunately, the rate of an isothermal thermal decomposition reaction varies widely during the experiment, so that the material produced at the beginning and end of the calcination, when the rate is low, is made under very different conditions from those prevailing when the reaction rate is at its highest level. Furthermore, it has long been known that the rates of many thermal decomposition reactions are influenced by the partial pressure of product gases [1]. Less attention has been paid to the variation of the reaction rate and the partial pressure of product gases throughout the material. These change considerably during the course of an isothermal calcination in a complex manner, depending on the kinetics of the decomposition reaction(s) and the balance between the (varying) rates of generation and removal of the gas. There is evidence to suggest that, as a result, conventional methods of isothermal calcination may not yield the optimum surface area [2], reproducibility or homogeneity of the product
[3]. One of the approaches described in this paper entails keeping the reaction rate and partial pressure of product gas constant during calcination by changing the temperature appropriately by means of a feedback loop. This technique has its origins in Controlled Rate Thermal Analysis (CRTA), which was developed by Rouquerol [4] to provide improved kinetic data and higher resolution in thermal analysis. He showed that constant reaction rate conditions could be of benefit also in preparing materials with specifiable surface areas.
860 CRTA is but one of a family of techniques, for which we use the generic term Rate Controlled Temperature Programme (RCTP) methods, in which the temperature of the sample is determined by the reaction (or transformation) rate. Another variant is Stepwise Isothermal Thermal Analysis (SITA)which was developed by Sorensen [5]. In this technique, the sample is subjected to a linear rising heating rate until the reaction rate exceeds a pre-set threshold, at which point the sample is held isothermally until the rate falls below the threshold, whereupon linear heating is resumed. When applied to the preparation of materials, this allows the reaction rate to determine the isothermal temperature(s) employed in the calcination. We will show that this method offers certain advantages in studying catalyst preparation as it allows overlapping events to be separated more effectively than conventional techniques. When used as the basis of preparative methods, rather than thermal analysis techniques, we use the terms CR to describe constant (reaction) rate and SI for stepwise isothermal methods. The thermal decomposition of gibbsite was chosen to illustrate the application of modem instrumental and computing techniques to preparative controlled reaction rate methods for making reproducible catalysts or catalyst supports of high surface area at atmospheric pressure. Gibbsite is one form of aluminium trihydroxide, AI(OI-I)3, the other being the mineral bayerite [6]. The thermal decomposition of gibbsite is complex, and is influenced both by crystallite size and water vapour pressure. Two processes, which may overlap under certain conditions, have been identified [7]: AI(OI-I)3 ---> A10(OH) + H20 gibbsite boehmite 2AI(OI-I)3 ---> A1203 + 3H20 gibbsite alumina During the gibbsite decomposition slit-shaped micropores are produced whose size is dependent on the pressure of the water vapour above the material [8]. At higher temperatures the boehmite itself decomposes to form alumina.
1.1.
Previous work on gibbsite decomposition techniques
using non-isothermal
Rouquerol [9,10] studied the effect of water vapour pressure on the decomposition, at low pressure, of 1 gm crystallites of gibbsite using a vacuum thermobalance and constant reaction rate regimes. He showed that, at pressures of less than 1.33 Pa, the formation of boehmite is minimised and the gibbsite decomposes directly to produce a highly microporous alumina. He found that the BET surface areas could be varied from 40 to 430 mz g'~ by changing the pressure of water vapour over the solid from 5 to 130 Pa. In a study of the kinetics of the thermal decomposition of gibbsite and boehmite, Stacey [8] also employed constant reaction rate conditions but using a fluidised bed system operating
861 at atmospheric pressure and large (20 g) samples. His work focused on larger grain sizes (> 50 ~m) and higher water vapour pressures, from 160 to 2660 Pa. Calculations based on his results show broadly similar surface areas to those found by Rouquerol. Analysis of adsorption data revealed the formation of complex structures containing macro, meso and micropores.
2. E X P E R I M E N T A L 2.1. R C T P A p p a r a t u s A diagram of the apparatus used is shown in Fig. 1. It consists of a low thermal mass, aircooled furnace in which is placed a quartz U-tube containing the sample. Helium carrier gas flows through the sample tube at a rate of typically 20 cm 3 min 4 and is controlled by means of a Brooks mass flow controller. The carrier gas and any evolved gases from the decomposition reaction then pass to a thermistor-based katharometer. Any change in the concentration of the gases entering the katharometer produces a change in the output voltage which is monitored by a 16-bit, 16-channel, analogue-to-digital-convertor (Comark Electronics). The ADC has type K thermocouple inputs, one of which is used to monitor the sample temperature. The fumace temperature is controlled, via a Eurotherm 818P temperature programmer, by a 486DX PC computer. The versatility of the equipment is attributable to the in-house software which can support a range of RCTP methods including SI and CR, as well as linear heating [ 11].
I
1
inlet MassFIow Controller
Katharometer
Furnace 'U' tube + sample
o o o
Compuface ADC
Eurotherm Tern peratu re Programmer
486 DX Corn puter
Fig. 1. Schematic diagram of RCTP apparatus.
862
2.2. Materials The gibbsite (BACO FRF 5 grade) was obtained from BA Chemicals Ltd. UK. This material has a mean grain size of 75 lxm and a given chemical composition of 65.1% A1203, 34.6% H20, with impurities being present at less than 0.2%. Approximately 750 mg of the trihydroxide was used for experiments 1 to 9 and cA. 50 mg for experiment 10. In all cases the sample was accurately weighed before and after calcination.
2.3. Heating Regimes Each sample of gibbsite was weighed into the U-tube and then placed in the furnace at an initial temperature of 60 ~ A helium gas flow of 20 cm 3 min "~ was set using the mass flow controller. The output signal from the katharometer was then monitored until a steady baseline value was achieved. Two main sets of decomposition experiments under CR conditions were undertaken, using heating rates in the range -1 to +1 ~ min "t" to a temperature of 370 ~ ie after the main decomposition stage but prior to the decomposition of any boehmite formed, to a temperature of 520 ~ ie after all dehydroxylation stages.
For both sets of experiments, a linear heating rate run of 1 ~ min "1, to the same maximum temperature, was performed to provide a basis for comparison. In addition, a SI experiment up to a temperature of 370 ~ was carried out. This had a heating rate of 1 ~ rain 1 and a threshold equal to a normalised reaction rate of 5 x 104 g min "t.
2.4. Surface area measurements The surface area of each sample was measured using an Omnisorp 100 CX, manufactured by Coulter Electronics Ltd. Approximately 50 points were measured across the full isotherm. The surface areas were calculated using the BET and Langmuir techniques and the t-plot method developed by Lippens and deBoer. The shape of the t-plot can give valuable information concerning micro and mesoporous materials. It is generally held to be a more appropriate method of interpreting adsorption on solids containing both micro and meso pores than the former two techniques. Pore slit widths were estimated using the method of Dubinin and Radushkevich. 2.5. X - r a y diffraction measurements X-ray diffraction patterns for each sample were obtained using a Philips X-ray diffractometer and computerised goniometer equipped with a Siemens secondary monochromator. The scans covered the range 10~ to 90 ~ in 0.1 o steps with a count of 3 seconds.
863
3. R E S U L T S 3.1. T h e r m a l
decomposition
Fig. 2 shows the evolved gas decomposition profile of a 0.737 g sample of gibbsite heated at a linear rate of 1 ~ min "~ to 520 ~ and monitored by the katharometer. The first peak (A) corresponds to the formation of the boehmite phase. The major peak (B) corresponds to the decomposition of the gibbsite, whilst the final peak (C) at cA. 470 ~ is due to further water loss as the previously formed boehmite decomposes. Note that the sample temperature shows a linear heating rate except for a small deflection corresponding to the endothermic second peak. The katharometer signal does not return to the baseline after the main decomposition, indicating that some further dehydroxylation process is still occurring prior to the boehmite decomposition. The size of the detector signal is a measure of both the reaction rate and the partial pressure of the product gas, which in this case is water vapour. It is clear that both vary considerably throughout the run.
B
600 *
75000
450
50000 !
O
300 O
25000
~/X,./
150 0
,
IKath.
.
,
,
100
200
300
C
-
~
400
.~ 0
500
600
Time / minute
Fig. 2. Gibbsite decomposition to 520 ~ using a linear heating rate of 1 ~ min "1.
Fig. 3 shows the decomposition of a 0.715 g sample of gibbsite under SI conditions. This curve shows that the first two processes can be resolved more effectively using this technique. The unusual appearance of the peaks is typical of the method and reflects the kinetics of the processes involved in the reactions. Two isothermal periods occur using the experimental parameters selected. The first is largely due to the formation of boehmite (A), whilst the second is the main decomposition stage 03). As in the case of the linear heating experiment, the reaction rate and the partial pressure of water vapour vary significantly, even although the two processes are now taking place at constant temperature.
864 12000
400 r..) ~ 300
9000
2oo
6000
100
3000
0 0
200
400
600
800
0 1000
Time / minute
Fig. 3. Gibbsite decomposition to 510 ~ under SI conditions.
Fig. 4 shows the decomposition profile of a 0.738 g sample of gibbsite under CR conditions with a reaction rate of 9.24 x 10.5 g min 1 normalised for a mass of one gram. It is the sample temperature profile which now delineates the consecutive processes. After an initial period of linear heating until the decomposition commences, there is a brief overshoot (A), which can be interpreted in terms of a nucleation process [11], followed by a period of slowly rising temperature during which boehmite formation occurs 03). This is followed by a long period of nearly isothermal conditions during the main decomposition stage (C). After the completion of this stage the temperature then rises quickly again slowing only through the decomposition of the boehmite (D). The main calcination now occurs at a constant reaction rate, in marked contrast to the results portrayed in Figs. 2 and 3. As in the case of the linear heating rate experiment, there is evidence of further dehydroxylation after the main decomposition stage but prior to the boehmite decomposition. 600
6OOO
r,.) ~ 450
A
4500
~-
Te 300
3000
150
1500 "
0
500
!
'
1000
'
!
!
1500
2000
0
2500
Time / minute
Fig. 4. Gibbsite decomposition to 520 ~ under CR conditions.
P
865 Table 1 shows the normalised reaction rates, water vapour partial pressures, experimental % mass losses, maximum temperatures and experiment duration. The reaction rates were calculated from the mass loss (due to evolved water) and the duration of the constant reaction rate stage, and were normalised with respect to a 1 g sample mass. The reaction rate and partial pressure values given for the 1 ~ min q experiments (expt. 1 and expt. 6) and the SI experiment (expt. 9) are the maximum values during these decompositions. Although, as mentioned elsewhere in this paper, the reaction rate and the partial pressure vary considerably during these experiments, a significant part of each process will occur at or near the maximum values. In so far as this is true, it seems reasonable to use the maximum values for the purpose o f comparing the results with those of the CR experiments.
Table 1 Experimental parameters and results for 750 mg samples Experiment no./type
Reaction *rate/rain "1
Pw,~ /Pa
Total mass loss/%
Maximum temp./~
Run time /min
1/linear
4.92 x 10 .3
**18543
29.4
370
367
2/CR
7.75 x 10-4
3382
29.7
370
649
3/CR
2.53 x 10-4
1147
30.7
370
1444
4/CR
1.65 x 104
745
30.4
370
2140
5/CR
6.25 x 10.5
289
30.0
370
5049
6/linear
4.96 x 10 .3
**18795
35.2
520
525
7/CR
1.67 x 10-4
770
34.6
520
2250
8/CR
2.44 x 10-4
431
35.9
520
1382
9/SI
9.79 x 10-3
*'7959
29.9
370
980
***10/CR
3.69 x 10 .3
131
29.0
370
1000
,,,
*normalised to a sample mass of 1 g. **These pressures are maximum values as explained in the text. **'50 mg sample mass.
866
3.2. S u r f a c e area a n d X - r a y r e s u l t s Nitrogen adsorption was used to investigate the surface areas and pore structures of the solids produced by the various processes described above. As might be expected from previous work, the full BET plots for all runs which finished at 370 ~ exhibited a slight curve, consistent with the presence of micropores, while the Langmuir graphs were linear. The converse was found for the solids which had been heated to 520 ~ when mesopores would be expected to predominate. BET surface areas were estimated, therefore, over the relative pressure range of 0.05 to 0.1 only. The t-plot method, which is held to apply to solids containing a mixture of pore types, was used (Fig. 5) to show the relationship between the development of mesopores, via their surface area, and the partial pressure of water vapour. The method of Dubinin and Radushkevich was used to estimate the micropore width as a function of water vapour pressure (Fig. 6). This is an appropriate technique as the pores are thought to be slit shaped [8]. The micropore surface area was calculated as the difference between the (limited range) BET value and the t-plot area and plotted against the partial pressure of water vapour (Fig.7). The samples heated to 370 ~ (expts. 1 to 5 and 10) have X-ray patterns giving diffraction peaks corresponding to d-spacings of 0.619 nm, 0.316 nm and 0.234 nm which are characteristic of boehmite. In addition there are weaker peaks with d-spacings of 0.139 nm and 0.238 nm consistent with the presence of ~-alumina. There are appears to be no difference in either the X-ray pattern or the relative peak intensities between the samples prepared at different rates. The SI experiment (expt. 9) shows a similar X-ray pattern. The samples heated to 520 ~ show two types of feature. Firstly there are very broad, weak peaks upon which are superimposed sharper, but still weak, peaks corresponding to 7~alumina. As with the 370 ~ experiments, there are appears to be no difference in the X-ray patterns in terms of d-spacing and the relative peak intensities between the samples prepared at different rates.
400 et0
< r,/3 o
,,.r
300
........---------i
9 200
"~
100
2
I
I
3
4
Log (water vapour pressure / Pa) Fig. 5. Meso and macropore surface area (t-plot) as a function of water vapour pressure.
867 2.1 ~=~
1.9
~-
1.7
o
f
o,-i
1.5 2
I
I
3
4
Log (water vapour pressure / Pa)
Fig. 6. Width of slit pores as a function of water vapour pressure.
300 250 200 150 100 50 2
3
4
Log (water vapour pressure / Pa) Fig. 7. Micropore surface area as a function of water vapour pressure.
4. C O N C L U S I O N S The results demonstrate that using linear heating rates, which are often employed to study calcination, and under typical catalyst preparation conditions, i.e. isothermal regimes, the reaction rate varies considerably. This is reflected in the properties (e.g. surface area and pore size) of the solid products which we have shown are highly dependent on the preparative conditions. Our results confirm Stacey's observation that the advantages of CR conditions apply equally at atmospheric pressure as well as low pressure. However, we have extended the work using smaller samples and relatively faster reaction rates over a wide range of partial pressures.
868 Changes in the set reaction rate under CR conditions give very different surface areas and pore sizes. As the reaction rate alters markedly throughout linear heating, and even isothermal, experiments there is considerable inhomogeneity in the products. There is every reason to believe that calcination under constant reaction rate conditions, will produce materials of greater uniformity. It appears that it is more difficult, at least in this case, to separate the two partially overlapping processes using the CR method, so that there is less influence over the outcome of competitive reactions. In such situations it may be more appropriate to employ isothermal methods of preparation. However, by using the SI approach, the isothermal temperatures can be determined by the threshold reaction rate and a few simple experiments will reveal the values required to effect the maximum possible separation of the events. We conclude that RCTP methods of preparation may offer valuable alternatives to conventional techniques, but that care must be exercised in the approach selected.
REFERENCES .
2. 3. 4. 5. 6.,
.
8. 9. 10. 11.
P.D. Gain and J.E. Kessler, Anal. Chem., 32 (1960) 1563. J. Rouquerol and M. Ganteaume, J. Therm. Anal., 11 (1987) 201. M.H. Stacey Anal. Proc., 22 (1985) 242. J. Rouquerol, J. Therm. Anal., 5 (1973) 203. O.T. Sorensen, J. Therm. Anal., 13 (1978) 429. N. Greenwood and AI Eamshaw, Chemistry of the Elements, Pergammon Press, 1984. J.T. Richardson, Principles of Catalyst Development, Plenum, 1989. M.H. Stacey, Langmuir, 3 (1987) 681. J. Rouquerol, F. Rouquerol and M. Ganteaume, J. Catal., 36 (1975) 99. J. Rouquerol and M. Ganteaume, J. Therm. Anal., 11 (1977) 201. P.A. Barnes, G.M.B. Parkes and E.L. Charsley, Anal. Chem., in press, 1994.
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
869
Preparation of fine particles as catalysts and catalyst precursors b y the use o f ultrasound during precipitation U. Kunz, C. Binder, U. Hoffmann Institut ~ r Chemische Verfahrenstechnik der TU Clausthal Leibnizstr. 17, 38678 Clausthal-Zellerfeld, Federal Republic of Germany SUMMARY
The formation of agglomerates in stirred suspensions of semi batch precipitated (Cu/Zn)2(OH)2CO 3 with different ultrasound intensities was investigated measuring the particle size distribution (PSD) and the Brunauer-Emmet-Teller adsorption isotherm (BET). Ultrasound (US) affects the size and the internal surface area of the agglomerates fundamentally. Precursors coprecipitated with ultrasound of low intensity have in general low surface areas and consist of small agglomerates. The precursors prepared with ultrasound of high intensity have higher surface areas and bigger agglomerates. A mathematical description of agglomeration follows the model from Hoyt. In addition a MnO2-catalyst was prepared under US influence and tested by the formation of oxygen from hydrogen peroxide. MnO2-catalysts prepared under US of high intensity show in general higher activities than those prepared by common methods without US. INTRODUCTION The preparation of fine particles is an important method for the production of special ceramics or catalysts. One method to produce fine particles is precipitation. The size of the formed particles is dependent on the degree of supersaturation and the rate of nucleation. The fine particles have a great tendency to form agglomerates. To prepare fine particles by coprecipitation it is important to achieve a high supersaturation at a very short time. A good mixing device is essential. Very fast mixing can be reached by the use of ultrasound during coprecipitation. The influence of ultrasound during coprecipitation of (Cu/Zn)2(OH)2CO 3 was investigated in a semi-batch ultrasound reactor and the precipitation of MnO 2 was investigated in a continuously driven ultrasound nozzle reactor. Copper/zinc hydroxycarbonates and the more stable copper/zinc/aluminium hydroxycarbonates have been used for the low pressure methanol synthesis for a long time. In the literature many preparative works exist for this type of catalyst [2,3,4,10]. As was shown by Klier in 1979 and by others the best catalyst contains Cu/ZnO in a molecular ratio of 30/70 [9]. According to this we choose this well known system for our US precipitation experiments. Popov investigated the influence of ultrasound on the preparation of Cr/MoO catalysts for the
870 oxidation of methanol to formaldehyde. His work showed a decrease of specific surface area with time due to sonification [ 1]. On the other hand Martsev discussed an increase in surface area for alumina gels obtained by the hydrolysis of aluminium salt with ammonia [8].Greguss observed an increase of activity of MnO 2 gel precipitated in an ultrasonic field of 875 kHz and 150 W/cm 2 [7, 8]. So far the influence of US during precipitation on the size of the particles and their catalytic activities is contradictious. The Intention of this work is to clear the influence of the intensity of ultrasound on the properties of precipitated catalysts. EXPERIMENTAL I. The copper/zinc hydroxycarbonate precursors were prepared using the common way of mixing a solution of carbonate ( 1 M ) with nitrate (1M) controlling the pH. To achieve a high supersaturation the two reactants where mixed using two solid jet nozzles .The mixed beam of the two nozzles where brought into a 21 stirred tank reactor with external circulation of the fluid. The scheme of this semi-batch reactor is shown in Figure 1. The use of two impinging streams is a useful method to contact two fluids. Intense mixing can be achieved by this method [ 12]. The use of this method for precipitation reactions was not tested before. At two locations in the reactor an ultrasound field can be brought into the system. The stirred tank is located in an ultrasonic bath with a field of low intensity (1 Watt/cm 2 , 35 kHz). In the external circulation a ultrasound probe horn with a tip diameter of 13 mm is located ( 90 Watt/cm 2 maximum energy output, 20 kHz). Three series of precursors of copper/zinc hydroxycarbonate where prepared in the semi-batch reactor (preparation under the influence of US of low intensity (1), without US (2) and with US of high intensity (3)). 1
2
NITIL RATE
1,2
feed tank
3,4
teed pump
5
solid jet nozzle
6
stirred tank m ultrasomc bath
7
recycle pump
CARBONATE 8
ultrasonic probe hom
9
pH meter
10
micro controller
I
g
I I I T=c
.
micro controller
'
'
I
product
V
Figure 1. Flow sheet of the semi-batch reactor: This reactor was employed to produce three series of (Cu/Zn)2(OH)2CO 3 precursors with different ultrasonic intensities ( ( 1 ) low intensity = 1 W/cm 2 (2): no ultrasound (3) high intensity = 90 W/cm2).
871 II. The manganese dioxide catalysts were prepared using a newly developed ultrasonic nozzle reactor (Figure 2). The reactor behaves like a continuous stirred tank reactor. The product flows into a beaker glass of 500 ml volume. It is characterized directly without further steps of preparation by chemical reaction with a solution of hydrochloric-acid and hydrogen peroxide in water. Two series of MnO 2 were prepared in the ultrasonic nozzle reactor (preparation under the influence of US of high intensity (4) and without US (5)). i
i
sound generator 20 kHz
~iii!i!i!!!ijliii!ii!ili!iii!i!iliiiiii~!i!iiii!i!~ feed solution 1
feedsolution2
in probe
"""
'!
titanium alloy probes
product
~ i
![
[_
l~i~lliil :i: :!
inannularslot ~
!~
"
~
N
volumetric feed flow V = 1.3, 3.3, 5.3 ml/s reactor volume VR = 6.5 , 10ml ultrasound power output I = 40 Watt/cm 2 (4), no US (5)
Figure 2. Schematic representation of the ultrasonic nozzle reactor. The experiments to produce the manganese dioxide catalysts were carried out in this reactor I. The route of preparation for the Cu/Zn-catalysts is the following : 500 ml of deionizated water was brought into the tank. The two feed pumps where started and the pH was controlled by the use of a micro controller which regulates the carbonate pump. In the reactor the following reaction takes place:
CuZ+r
+ Zn2+(,,q) + 20H-r
+C032-(~q)
vs )(Cu, Zn)z(OH)zCO3(s),I "
(1)
After well-defined times the circulating pump was stopped and various volumes of the gel were taken from the reactor. The gel was washed with water and ammonium nitrate solution to minimise the potassium contents and dried over night at 333 K. The resulting powder was divided into portions using a laboratory-sampler (0.2 g for the particle size measurements and 0.5 g for the surface area measurements). Some of the prepared precursors were calcined for 14 h increasing the temperature by 0.5 K per minute up to a temperature of 543 K. A fraction between 45 gm and 180 gm of these samples was reduced with H2/Ar (5/95 Vol.%) at a maximum temperature of 523 K for 21 h. The reaction rate for the methanol synthesis with a synthesis gas of CO/CO2/H 2 = 49/2/49 Vol.% was measured in a tubular-reactor at 5 MPa and 553 K at a GHSV of 12000 h -1, following the reactions (2a and 2b) 9
872
CO + 2H z
CO 2 + 3H z
(2a)
c,,,z,o >C H 3 0 H
(2b)
cu/z,,o >C H 3 0 H + H 2 0
The particle size was analyzed in aqueous medium with a solution of sodium pyrophosphate as dispersing agent using the diffraction pattern analyser HELOS (manufacturer Sympatec). The BET surface areas of the precursors and of some calcined catalysts were measured with a StrOhlein Areameter using the 1-point method. II. Preparation of manganese dioxide 10 litre of 0.05M KMnO 4 and 10 litre of 0.079M Na2SO3-solution were prepared and pumped with a tube pump through the ultrasonic nozzle reactor precipitating the manganese dioxide following reaction 3
2MnO4-(aq) + 3S032-(aq) + 1-120
>2 M n O 2 (~) ,1, +3S042-(aq)
--I-20H-(,,q)
(3)
In total 12 experiments were carried out. Three volumetric feed flows were used (1.3, 3.3 and 5.3 ml/s) with two reactor volumes (10 ml and 6.5 ml). Each experiment was realized without and with an ultrasonic field of 40 Watt/cm2.The precipitate was filtrated at room temperature and washed several times with deionizated water. The filtrate was dried for 12 h in a furnace at 65 ~ As a test for the catalytic activity of the MnO 2 particles evolution of 0 2 from H20 2 was used. For these tests a sieve fraction between 63 lam and 80 gm of dried MnO 2 was used. An amount of 0.05 g of manganese dioxide was given into a beaker with a solution of hydrochloric acid and hydrogen peroxide in water. The solid acts as a catalyst for the formation of oxygen (4a) and dissolutes simultaneously in the reaction (4b).
g~ o~ ~a~~
MnO 2
(4a)
> g~ o + ~ 02 "t
M n O z (s~ + H z O 2
>MnC12 ~aq)+ 2 H z O
+02
(4b)
The volume of the developed oxygen was measured and the time until the MnO 2particles were dissoluted was stopped. The results are shown in table 4. Results
In table 1 to 3 the results for the experiments in the semi batch reactor are shown. For each precursor the time t after that the gel was taken from the reactor and the temperature T in the reactor is indicated. The cumulative particle size distribution (PSD) is shown for the dried precursor and for some calcined catalysts. The x5, x50 and x90 values are presented. For example x50 = 3 gm means 50 % of the particles have a volume based size smaller than 3 gm (table 1 catalyst RF 4/89). The next column demonstrates the BET surface area of the precursors and of some calcined catalysts (values in brackets). In addition the densities for some precursors and the rates for the formation of methanol for some catalysts are shown. The precursors prepared with ultrasound of low intensity (ultrasonic bath) in general have low BET surface areas. The precursors prepared with ultrasound of high intensity in general show high BET surface areas. The calcination affects the surface area dramatically.
873 But moderate calcination conditions result in an increase of surface area. Only the precursor No 14 CB 11/92 lost surface area due to calcination. Precursors prepared by different methods exhibit particles of distinctive size. The x50 value of the low intensity prepared precursors is lower 10 Jam except for one precursor. Table 1
No
Precursor
T I~
t [mini
PSD [~tm] x 5 xs~) xg~)
BET Im2/g]
density [g/cm3]
rMEOH [mmol/h'gI
1
RF
4/89")
50
19
1 3 20
13
-
1.5
2
RF
6/89")
50
20
1 5 70
23
-
0.7
50
75
1 12 41
17
2.86
-
2.88
-
3
|
Precursors and calcined catalysts of (Cu,Zn)2(OH)2CO 3 prepared with US of low intensity (1 W/cm 2)
SK 5/93
4
DK
6/93
50
75
-
22-+1
5
SK 10/93
50
75
1 9 28
-
ii
Table 2
Precursors of (Cu,Zn)2(OH)2CO 3 prepared without ultrasound
No
Precursor
T [~
t [mini
PSD [~tm] x 5 Xso Xgo
BET [m2/g]
6
CB 1/92
25
9
3 26 51
31-+1 (43_+3)
CB2/92
25
29
1 17 43
42_+4(52_+3)
CB3/92
25
59
1 16 42
40+_6
CB 5/92
50
45
1 16 57
SK 4/93
50
75
1 16 50
10 Table 3
density [g/cm 3]
rMEOH [mmol/h.g]
40___1 (73)
3.62
23.5
32___1
3.12
Precursors of (Cu,Zn)2(OH)2CO 3 prepared with US of high intensity (45 Watt/cm 2) i
i
No
Precursor
T [~
t [mini
PSD [~tm] x 5 Xso Xgo
BET [m2/gI
11
CB 7/92
25
15
7 242 488
58+3
12
CB 8/92
25
42
3 200 570
50+_1(64)
13
CB 9/92
25
85
2 31 300
57+2
14
CB 11/92
50
30
1 20 240
57+_2(53)
3.41
15
SK 8/93 +)
50
75
1 17 49
44+1
3.55
+) prepared with 90 Watt/cm 2 US intensity *) calcined catalysts
density [g/cm3I
874 Precursors prepared with high intensity US show large particles (precursors 11, 12). With increasing time of sonification the particles become smaller (13). The influence of temperature seems to enhance this effect (15). 1
-
.i: i++;=
1 --D--
5/93 US bath (low intensity)
---El--- 10/93 US bath (low intensity) ! --o--9/92 US horn+bath (high intensity) O.B ---A--1192 no US
El'--,;+--
],
El' ,.13~
..O
6' 4t
~ ,~,, t
+
--
. . . . . . . .
r'n 0.6
/
E
/
,
o,' /
," . . . . . .
9
~
71#
'
[•I/:_ _
T-t
.I~I" A'
C3o.+
i"
0,IZl
O_
] /i
t
"
I"
0.2
....
,,+~,i~ "~-.'~+-''.+';~"
0
i
0.1
i
i
i
I
i
I
i
i
i
i
i
100
i0
l
1000
particle size [IJm] -,,Figure 3. Particle size distributions of (Cu,Zn)2(OH)2CO 3 precursors. The precursors prepared with US of low intensity consist of much smaller particles than precursors prepared conventional. Precursors prepared with US of high intensity are bigger. 80
70
high ultrasound intensity ~o
8 3o
low ultrasound
u~
.
.
.
.
.
tt~
lO
j
.
.... o
~
intensity
---
1
2
:3
5
6
7
number
.
x_ X
l:
X .
,;w
X
,X
X
8
g
x
4
.
>,_ "X --~-
,,
.
~<
.
.
5a
.
.
.
.
•
/
~_
].,
_~__
;,
_~__
z] 10
11
12
13
14.
15
of p r e c u r s o r
Figure 4. Internal surface areas of (Cu,Zn)2(OH)2CO 3 precursors. The precursors prepared in the US bath show a much smaller surface area than the precursors prepared conventional. Precursors prepared with the US horn have higher surface areas.
1(5
875 In table 4 the preparation parameters and the properties of the MnO 2 catalysts prepared in the ultrasonic nozzle reactor are shown. In this table the experiments with ultrasound and without ultrasound are compared. Three different volumetric feed flows are used and experiments with small (6.5 ml) and large (10 ml) reactor volume are performed. The mean residence time Zmit was calculated from the reactor volume and the volumetric feed flow. The time until the MnO 2 was completely dissoluted for experiments without US (tl) and for experiments with ultrasound (t2) was standardized on the production of 10 ml oxygen (index: nor). The MnO 2 catalysts produced with ultrasound dissolute in general faster than those produced without ultrasound. As can be seen in the last column this increase can reach values of 50% to 100% of improvement. Dividing the standardized dissolution times t lnor / t2nor gives the conclusion that ultrasound leads to a dramatic increase in reaction rate. Table 4
Preparation parameters and properties of MnO 2 catalysts prepared with and without ultrasound in the ultrasonic nozzle reactor with three volumetric feed flows (1.3, 3.3 and 5.3 ml/s) with US with US t, nor V R [ml] z mit[s] V102 [ml] tl nor [s] V202[ml] t2 nor [s] t2.o' i
7.5 3.0 1.9 4.9 2.0 1.2
10
6.5
9.7 9.8 10.2 9.9 9.5 9.8
i
212 184 246 181 228 210
10.0 10.1 10.1 10.3 9.8 10.4
270
] m
,,,,,
A w
240
El-
142 100 119 124 152. 130
1.5 1.9 2.1 1.5 1.5 1.6
I
,
I
i0 ml reactor volume, no ultrasound 5.5 ml reactor volume, no ultrasound
I0 ml reactor volume, with ultrasound 6.5 ml reactor volume, with ultrasound --..-.-ll
~210
E 180 N
15o
/ /
L_
-O c" t~ -.~
o9
.... _._ . . - I = I
/ /
-
120
[3
~
-.
_u l i r a
sotgr'rd--
'-t'3 --- 2
--
..... ---, ,--- "-7
---I
"-
[ i
3
s.-
I--"
..... ----
"'O
|
1
I
.....
[
~'-~.
"
go
---.........._
|
4
mean residence t i m e
5 6 Tmi t [S] - - - ~
7
8
Figure 5. Catalytic activities of precipitated manganese dioxide with ~and without ultrasound of high intensity. The graph shows the time to develop 10 ml of oxygen and to dissolute the catalyst. Under ultrasound influence prepared catalysts develop the oxygen generally in shorter time. They are more active.
876 THEORETICAL If the precipitant is an sparingly soluble salt, the precipitation process is determinated mainly by the agglomeration of the very small primary crystallites with little or no particle fracture from shearing forces or impact as a result of stirring. The basic assumption is that little error is introduced by representing the agglomerating mechanism as continuous growth in the same sense as crystal growth. It is assumed that three types of particles can be distinguished, namely : (1) crystallites of primary particles ( < 1 pm) (2) clusters of aggregated crystallites (1 to 6 pm), and (3) agglomerates that mainly arise from the agglomeration of clusters (> 6 pm). Each class of particles is modelled separately, with the
r ~
I~
sum of the three distributions
~ 1059 ~I~
'
' ' J ~ ' ~ 9 precursor1/g2noultrasound
~
precursor51g3ullrasound low intensily precursor glg2 ullrasoundhigh inlensily
representing the PSD of t h e E::)-IO18 1 to 6 pm cluster > 6 pm agglomerates I suspension. The crystallites are ., ~. ~up tohereclusters produced by nucleation and growth at a rate G x. They are -~ 1016 removed continuously by the c product flow stream, but they ._o 101s als6 disappear by forming 1014 clusters and aggregating with ~.. "t3 agglomerates. The growth of C 1013 0 20 30 lO 50 clusters and agglomerates is particle size [pm] -~represented by the rate G c and G a. Solving the populations balance equation for steady Figure 6. Semi log plot of the population density state and assuming McCabes's distribution (the two linear regions of the plot can be AL law of crystal growth gives used to determine the growth rates) (The rate of change of size, G = dL/dt, is often observed to depend only on the supersaturation rather than the size) the size distribution of the composite n T which is following Hoyt [5,11 ]:
~
/~T -- n ox " e
+
n co .e
+%o .e
(5)
Assumption: the solid jet nozzle is regarded as the precipitation reactor. The crystallites are to small ( < 1 lam ) to be detected with the method of laser diffraction pattern measurement. So here only G c and G a can be calculated. Plotting the natural logarithm of the population density against the particle size gives a characteristic curve with two linear regions (figure 6). The slopes of these regions can be determined by linear regression. Assuming a residence time of x = 1 second in the nozzle, the growth rates can be calculated (G = - 1/mz, m = calculated slope ). The calculated values for some dried precursors of (Cu,Zn)2(OH)2CO 3 are shown in table 5. The' exact values of the growth rates depend on the residence time in the nozzle. The significance of the assumed time is not really high, but to compare the growth rates of distinct prepared precursors it is fairly good.
877 The growth rates increase in the line Ge,c < Ge,a. The growth of clusters is generally slower than the growth of agglomerates, because agglomerates can grow by aggregating with other agglomerates. The growth rates of the different prepared precursors increase in the l!ne Gow intensi~ < Gno US < Ghigh intensity 9 These results confirm the data ot the particle size measurements. Comparing the values of table 5 with literature data one must consider that the process of drying is involved in these data. Coming to terms it can be stated that the proposed method of determining growth rates following the model of Hoyt is suitable to describe systems where two or more classes of particles are formed simultaneously. Table 5
Growth rates of (Cu,Zn)2(OH)2CO 3 particle classes for different preparation conditions
precursor( US intensity)
srowth rates clusters 1 to 6 lam
G~,~[~nVs]
|11
SK 5/93 ( 1 W/cm 2) CB 1/92 (0 W/cm 2) CB 9/92 (45 W/cm 2)
0.8 (r 2 = 0.97) 1.0 (r 2 = 0.97) 1.8 (r 2 = 0.97) ,,
~rowth rates agglomerates > 6 ~tm
G~. [~m/s] 5.0 (r 2 = 0.93) 6.8 (r 2 = 0.96) 15.2 (r 2 = 0.95)
r2 = correlation coeffizient
CONCLUSIONS The preparation of catalysts in an ultrasonic field (kHz range) affects the internal surface area and the size of the particles fundamentally. Precipitation in an ultrasonic bath (low intensities) leads to small particles with low surfaces areas. The preparation under the influence of the ultrasonic probe horn (high intensities) on the other hand leads to catalysts of large particles with high surface areas. Conventional preparation technique leads to particles with intermediate particle size distributions. Two catalyst systems were used as model systems The test of the MnO 2 catalysts have shown a dramatic increase of catalytic activity due to sonification with high intensity US. The methanol synthesis catalysts show a decrease in catalytic activity due to low intensity US. This effect is probably caused by the lower surface areas. Our more detailed investigations using ultrasound of different intensities can be an explanation for the contradictious results of the literature. The valuation of under ultrasound prepared catalysts has to be made considering the ultrasonic intensities. The model proposed by Hoyt is applicable to describe the formation of particles prepared under the influence of ultrasound.
ACKNOWLEDGEMENTS The Authors thank the Deutsche Forschungsgemeinschaf~ DFG for the financial support of a part this work.
878 NOTATION /2 G L n n~ rMEOH t t l nor, t2 nor Xmit V102, V202
[ml/s] [m/s] [m] [m -4 ] [m -4] [mmol/g.h] [s] [s] [s] [ml]
volumetric feed flow linear particle growth rate particle size population density at size L population density of nuclei reaction rate for methanol time standardized dissolution time mean residence time released oxygen volume
VR
[ml]
reactor volume
GHVS
[h-1]
gas hourly space velocity
Indices: x c a e T nor mit
nuclei cluster agglomerates effective total standardized mean
REFERENCES
[1]
T.S. Popov, D.G. Klissurski, K.I. Ivanov, J.Pesheva. in Studies in surface science and catalysis 31 elsevier (1986) Preperationof Catalysts IV, 191 [2] S. Gusi, F. Pizolli, F. Trifiro, A. Vaccari, G. Del Piero, Prep.of Catalysts IV, 753 [3] E.B.M. Doesburg, R.H. H6ppner, B. de Koning, Xu Xiaoding and J.J.F. Scholten Prep. of Catalysts IV, 767 [4] B.S. Rasmussen, P.E. Hojlund Nielsen, J. Villadsen and J.B. Hansen. Prep.of Catalysts IV, 785 [5] R.C.Hoyt,Ph.D.dissertation, Iowa State University Library, Ames Iowa (1978) [6] E. Plasari, L.Vincinguerra, R. David, J. Villermaux paper presented at the CHISA'93 Congress in Prague [7] A. Greguss and P. Greguss, Akust. Zhur 6 441 (1960) [8] A.N. Mal'tsev, Russ. J. Phys.Chem. 50 (7) 995 (1976) [9] J.B. Bulko, R.G. Herman, K. Klier, G.W. Simmons J.Phys. Chem. 83 (1979) 3118-3122 [ 10] G. Ghiotti, F. Boccuzzi ,Catal.Rev.-Sci.Eng, 29 (2+3) (1987) 151-182 [ 11 ] A.D. Randolf, M.A. Larson, Theory of Particulate Processes (Sec.Ed.) Academic Press Inc. San Diego (1988) [12] A. Tamir, A. Kitron, Chem.Eng.Comm. 50 (1987) 241-330
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparationof HeterogeneousCatalysts G. Ponceletet al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
879
SCIEN'r~-IC BASES F O R T I ~ PREPARATION OF NEW CEMENT. C O N T A I N I N G CATALYSTS
V.I. Yakersona and E.Z. Golosmanb a N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky Prospect 47, Moscow 117913, Russia b Institute of Nitrogen Industry, Novomoskovsk 301670, Russia The aspects of the preparation of different cement-containing catalysts are considered. The general trends in the mechanism of formation for nickel-, copper-, and zinc-containing catalysts based on calcivm a l u m i n a t e and high alumina cements are presented. The processes of cement hydration as well as the i n t e r a c t i o n of cements with hydroxocarbonates of active m e t a l s are outlined. It is shown t h a t the cement-containing catalysts combine a high activity with an n c r e a s e d t h e r m a l stability and e n h a n c e d m e c h a n i c a l strength. The high efficiency of cement-like compositions containing metal oxides in hydrogenation of CO, CO2 and 02, in decomposition of ammonia and hydrogenation of butyric aldehyde is clearly established. 1. I N T R O D U C T I O N Progress in modern chemical industry calls for the availability of active and selective catalysts, having favourable t i m e - o n - s t r e a m p a r a m e t e r s , an increased mechanical strength and a reasonable durability. Very significant for the design of the new commercial processes is the application of waste-free or low waste technologies for the synthesis and formation of catalysts. New promising practical possibilities in this area offer the development of catalysts containing either cements or calcium aluminates, which are known to be i m p o r t a n t cement components [1]. These t h e r m o s t a b l e s u b s t a n c e s with
excellent mechanical characteristics can be successfully applied as catalysts for low-and high temperature reactions, in particular for exothermic conversion. Moreover, the cement based catalysts have low coking tendency. Finally, high hydration activity and reactivity of cement and their ingredients provide the basis for the development of novel waste-free and ecologicallypure technologies for the preparation of cement-containing catalysts. METHODS The catalysts were prepared by mixing hydroxocarbonates of metals (MHC) with calcium m o n o a l u m i n a t e (CaA1204), calcium d i a l u m i n a t e (CaA1407) or high a l u m i n a cement (CaA1204 + CaA1407) in aqueous or ammonia-aqueous medium. Phase composition and dispersity of the catalysts at different steps of the p r e p a r a t i o n were determined by X-ray diffraction
880 technique (XRD). Specific surface area was estimated by using the adsorption of benzene and nitrogen and the pore size distribution was evaluated from the benzene hysteresis loop. Adsorption of oxygen was applied to determine the surface area of the metallic phase. IR spectra in the range of 400-4000 cm -1 were recorded on KBr pellets. The coordination state of a l u m i n i u m with respect to the nearest oxygen atoms was identified with 27A1 NMR spectroscopy. For electron microscope examination, u l t r a - t h i n slides or suspension of the catalysts were used. The mechanical s t r e n g t h of the catalysts was determined by the crushing pellet technique (P). Catalytic activity was measured in a flow circulation unit. 3, RESULTS AND DISCUSSION Cements based on calcium aluminates are normally used as hydraulic additives to enhance the mechanical strength and the stability of the catalysts. We succeeded to prepare calcium aluminate with well developed surface area which favourably combines many important properties required from the supports for effective catalysts. Hydration changes the mechanical and structural properties of calcium aluminates and high alumina cements. The range of the variations depends on temperature and duration of the treatment, dispersity of the components, liquid/solid ratio and phase composition of the cements. The t r e a t m e n t in w a t e r or aqueous a m m o n i a solution with subsequent calcination at 400-600~ results in an increase of the surface area (S) from 2 to 200 m2.g-1. After additional acid treatments, the surface area can reach the value of 300-400 m2.g -1. The first step of this process is hydration followed by the formation of species with a l u m i n i u m in octahedral coordination to oxygen. As evidenced by XRD-, IR-, and 27A1 NMR-data [2], the driving force of cement hydration is the unusual tetrahedral position of a l u m i n i u m and constrained Ca-O structures. The h e a t t r e a t m e n t and decomposition of calcium h y d r o a l u m i n a t e s favour the t r a n s f e r from octahedral to tetrahedral aluminium with respect to oxygen. An essentially linear relationship was established between the value of S and the degree of hydration for the starting calcium aluminates and cements. The mechanical strength of the cement based supports increases with increasing hydration degree and reaches values of 30-50 MPa. Another way to prepare calcium aluminate supports and sorbents is based on the interaction of Ca(OH)2 with AI(OH)3 followed by heat treatment of the resulting Ca3[Al(OH)6]2. This last compound can be thermolyzed to yield Ca12Al14033, in which the zeolite type structure was clearly established. IR spectroscopic investigation provided evidence for the bifunctional nature of the active sites on the surface of calcium aluminate based supports and sorbents. The presence of acidic and basic sites on the surface makes the use of these solids as catalysts for acid-base tranformations of organic substances promising. Nickel cement-containing catalysts were prepared by mixing nickel hydroxycarbonate (NiHC) with cements. In the course of mixing, exchange reaction proceeds to form hydroxyaluminate and hydroxycarboaluminate of nickel, CaCO3, AI(OH)3, and Ni(OH)2. The phase composition of the catalyst depends on the ratio of the starting components. The precursor of the active component exists primarily as nickel hydroxyaluminate (NiHA). Thermolysis of this precursor produces firsta poorly crystalline,disordered NiO-Al203 solid solution and then NiO. Heat treatment of nickel calcium aluminate catalysts
881 at 400-1000~ is accompanied by the interaction of A1203 with CaO rather than with NiO. This suppresses the formation of NiAI204 spinel. The presence of hardly reducible substances in nickel cement-like catalysts (NiO-A1203 solid solutions) preserves the high dispersity of the metallic nickel phase. This, in turn, makes nickel cement-like materials valuable catalysts for hydrogenation of CO and CO2 as well as for ammonia decomposition. The interaction of nickel salts with cements is associated with the formation of a mechanically stable polyphase system and concomitant enhancement of the surface area, from 2 to 200 m2.g -1. The activity of these catalysts in the purification of industrial gases from CO and CO2 [3] was found to be 160-170~ when expressed in terms of temperature for break-through of CO and CO2 to the level of <10 ppm. A more complete reduction of the solid solution increases the activity of the catalyst. Nickel catalysts show very high activity in the decomposition of ammonia : the remaining content of NH3 does not exceed 0.05-0.1% at 650-950~ Copper cement-like compositions are promising active catalysts for water shift of CO [4], synthesis of methanol, and oxygen hydrogenation [5]. Interaction of cements with copper hydroxycarbonate (CuHC) involves, as expected, the formation of copper hydroxyaluminate (CuriA). The activity of the catalyst in the water shift of CO correlates with the amounts of CuriA. The samples with the highest activity show an increased amount of CuriA, an enhanced concentration of strongly bound CuO, and a high dispersity of both CuO and Cu. The thermostable porous structure of disordered Cu-A1 spinel exerts a stabilising effect of the particles of CuO and Cu. The formation of CuO-ZnO solid solutions favours the catalytic activity of Cu, Zn-calcium aluminate system. The consequence of the introduction of copper and zinc components in the cements is also of importance. Cu, Zn-cement catalysts are very efficient in the hydrogenation of butyl aldehyde, whereas Cu, Ni-catalysts are excellent in oxygen hydrogenation. Thus, cement-containing catalysts can be used for a variety of reactions. The high activity of these catalysts is fortunately associated with high mechanical strength and thermostability. Another advantage of the cement-containing catalysts lies in the possibility to control their properties by varying the conditions of formation. 4. APPENDIX I Mechanism of formation of cemen~contalning catalysts. The reaction between cements (calcium aluminates) and the active component in aqueous medium takes place at room t e m p e r a t u r e and is accompanied by marked changes in texture and phase composition. This leads to reexamine the view t h a t calcium aluminates and cements are merely hydraulic binding agents. Metal hydroxycarbonate (MHC) reacts with calcium a l u m i n a t e s in aqueous or ammonia-aqueous medium to give a multiphasic system. The composition of the reaction products indicates both hydration of cements and exchange reaction between the calcium aluminates and MHC. Depending on the conditions, the reaction gives the metal hydroxyaluminates (MHA), C a C 0 3 , AI(OH)3, M(OH)2, Ca3[AI(OH)6]2, and Ca(OH)2. Since calcium aluminates, on dissolving in water, give Ca 2+ and AI(OH)4-, the reaction of MHC with alumina cements can be represented as follows : Ca 2+ ions react with CO~ ions in the labile structure of MHC to give insoluble CaC03, whose
882
formation displaces the reaction to the right. To preserve the electrical neutrality of M H C , the place of one 0032 ion carrying two negative charges must be taken by two AI(OH)4- ions, each one carrying only one negative charge: mMCO3.nM(OH)2 (MCH) + Ca 2+ + AI(OH)4- --~ m'M(OH)2.n'AI(OH)3 (MHA) + CaC03 + AI(OH)3 + Ca3[AI(OH)6]2 + Ca(OH)2 (1)
Since M H A has a layered structure, insertion of M(OH)2 into it is possible, so that the ratio M/AI will increase. In addition to metal hydroxyaluminates, hydroxycarboaluminates m a y also be formed. The reaction of M H C with calcium mono- and dialuminates is described by the equation : 3 CaAI204 + 12 H 20 + a[mMCO3.nM(OH)2] (MHC) = am CaC03 + (3-am)/3 Ca3[AI(OH)6]2 + (an+am)/3 M3~2(OH)12(MHA) + (12-2an)/3 AI(OH)3 (2) 3 CRA1407 + 21 H20 + a[mMCO3.nM(OH)2] -- (an+am)/3 M3A12(OH)2 + amCaC03 + (3-am)/3 Ca3[AI(OH)6]2 + (30-2an)/3 AI(OH)3
(3)
The composition of the reaction products depends on the numerical values of the coefficients"a", "m", and "n" (if M=Ni, re=l, and n=l; if M=Zn, m = l ou 2, and n=3; if M=Cu, m=n=l). The phase composition of cement-containing catalysts is described by general equations, based on the reaction between M H C and CaA1204 (CaAl407). The thermal decomposition of the compound formed by the chemical reaction and containing the active components can be described by the equation
m~ ~ MO-A1203 (solid solution) M3A12(OH)12 --~ 3MO-AI203 (solid solution) --, MO/MO-AI203
MO/MO-A120 3
(4)
If two metal hydroxocarbonates take part simultaneously in the reaction (copper and zinc, cobalt and copper, nickel and copper), the first stage of the reaction gives a mixed hydroxocarbonate, from which mixed hydroxoaluminates are formed, and converted into mixed solid solutions : m'M'CO3.n'M'(OH)2 + m"M"CO3.n"M"(OH)2 + CaAI204(CaA1407) + aq --~ M'M"A12(OH)12 --# M'O-M"O-A1203 (solid solution) --, M'O-M"O/M'O-M"O-A1203
(5)
Solid solutions of m i x e d metal hydroxoaluminates and hydroxycarboaluminates give anion-modified solid solutions. The reaction of metal hydroxocarbonate with Ca 2+ and AI(OH)4- ions can be represented as : mMCO3.nM(OH)2 + mCa 2+ + 2reAl(OH)4- --~ m{M[AI(OH)4]2}.nM(OH)2 + mC aC 0 3
(6)
883 The hydroxoaluminates, m{M[AI(OH)4]2}.nM(OH)2, can be represented as Mm+nAl2m(OH)8m+2n. Thus, depending on the composition of the MHC, the products are different hydroxoaluminates with different MO/AI203 ratios, which determine the composition of the products resulting from thermal decomposition (see Table 1):
Table 1. Composition of metal-hydroxocarbonates ' ( M H C ) and metalhydroxoaluminates (MHA), MO/AI203 ratio in the metal-hydroxoaluminates and composition of the products after thermal decomposition Of metalhydroxoaluminates.
mMCO3.nM(OH)2 Mm+nA12m(OH)Sm+ (MHC) 2n (MHA)
MO -4-1203
MHA~nMO + mMA1204 n/m
M = Ni, m=l, n=2 M=Co, m=1, n=2 M=Cu, m=l, n=l M=Zri; m=2, n=3 M=Zn, m=l, n=3
Ni3A12(OH)12 Co3~2(0H)12 Cu2AI2(OH)12 Zn5A14(OH)22 Zn4A12(OH)14
3:1 3:1 2:1 5:2 4:1
2:1
2:1 1:1 3:2 3:1
Table I shows that: a) the composition of the MHA depends markedly on the composition of MHC; b) the MO/AI203 ratio in the MHA is always greater than 1; c) the MO/MA]204 molar ratio of the thermally decomposed products of the MHA, that is the ratio of the free metal oxide to spinel, is exactly equal to n/re. 5. APPENDIX H Carriers and adsorbents based on cements. A source of calcium-aluminium carriers and adsorbents is provided by calcium mono- and di-aluminates, CaAI204 (CA) and CaA1407 (C.~2), which a r e the principal phases in high-alumina cements. The first stage in the preparation of these carriers is hydration which, at elevated temperatures, gives mainly Ca3[AI(OH)6]2 (C3AH6) 9 3CaA1204(CA) + 12 H20 ffi Ca3[AI(OH)6]2 + 4AI(OH)3 3CAA1407 (CA2) + 21 H20 = Ca3[AI(OH)6]2 + 10 AI(OH)3
(7) (8) l-
At lower temperatures, the less stable products C2AH8 and CAHlo are formed (CfCaO, A=AI203, H=H20). The thermal decomposition of hydrated calcium aluminates at different temperatures gives C12A7, A1203, CaO, (CaO)a.(Al203b.(H20)c. A method has been developed for obtaining carriers and adsorbents ("galyumin") with high surface areas based on calcium aluminates (cements). The specific surface area increases from 2-4 m2.g-1 to 400 m2.g-1, i.e., by two orders of magnitude. A study of the infrared spectra of a number of calcium aluminates (CA, CA2, C12A7) and catalysts and adsorbents based on them, with different S (30400 m2.g-1) showed that the bands in the range 400-900 cm -1 are due to the
884 vibrations of aluminium-oxygen tetrahedra in the framework of zeolite-like structure. The calcium ions apparently act as compensation cations. Depending on the preparation method, the value of S and the concentration of CaO, the aluminium-calcium adsorbents and catalysts can be divided into two groups : a) those with a zeolite-likestructure; b) those consisting chiefly of 7A1203 modified by calcium. The water in the aluminium-calcium adsorbents and catalysts is present in different states : it m a y be adsorbed, form surface O H groups, and be present as Ca(OH)2, AI(OH)3, and hydrated calcium aluminates. The fact that the hydration product of A1203 is formed in the hydration of aluminium-calci,m cements provides the starting point for the production and study of two-component systems, namely T-AI203-CA(CaAI204) and 7AI203-CA2(CaAI407). Their mechanical strength reaches 1450 kg.cm -2 and 1000 kg.cm -2 for the composition 40% CA-AI203 and 20% CA-AI203, respectively. The processes taking place in the production of catalysts based on calcium aluminates have been studied by high-resolution 27AI N M R . The structure of anhydrous calci~_!m aluminates (CA, CA2, C3A, C12A7) contains aluminium-oxygen tetrahedra. The alumini~m atoms in the tetrahedra are of different types and their non-equivalence increases, going from highly basic to weakly basic calcium aluminates. Upon hydration, which is one of the steps of the production of aluminium-calcium catalysts and carriers, the process [A104]--, [A106] takes place, and the process [A106]-, [A104] takes place on thermal decomposition. The presence of framework aluminium-oxygen [AIO4] tetrahedra in anhydrous calcium alumlnates makes their structure similar to that of zeolites. For zeolites, Loewenstein's rule applies : according to this rule, two aluminium atoms in tetrahedral coordination should not have a shared oxygen atom. The structure of calcium aluminates clearly indicates that Loewenstein's rule is not universal. R~'~CI~ 1. V.I. Yakerson and E.Z, Golosman, Catalysts and Cements, Khimiya, Moscow (1992). 2. V.I. Yakerson, V.D. Nissenvaum, E.Z. Golosman, V.M. Mastikhin, Kinetics and Catalysis, 27 (1986), 1231. 3. Z.A. Ibraeva, N.V. Nekrasov, V.I. Yakerson et al., Kinetics and Catalysis, 28 (1987), 339. 4. A.L. Turcheninov, N.V. Nekrasov, N.A. Gaidai et al., Kinetics and Catalysis, 28 (1987), 322. 5. B.N. Kuznetsov, V.N. Efremov, M.G. Chudinov et al., Kinetics and Catalysis, 33 (1992), 118.
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
885
Nucleation and growth of ceria from cerium III h y d r o x y c a r b o n a t e M. Pijolat, JP. Viricelle and M. Soustelle CRESA D6partement de Chimie-Physique des Processus Industriels Ecole Nationale Sup6rieure des Mines 158, tours Fauriel 42023 Saint-Etienne C6dex 2 (France)
ABSTRACT The kinetic study of the transformation of cerium III hydroxycarbonate into ceria has been achieved by means of thermogravimetry experiments and using a methodology based on the concepts of nucleation and growth. Two methods were used to determine the specific rates of nucleation and growth 9a geometrical modeling which allowed to determine their variations against the partial pressures of gases involved in the transformation; an experimental method which gave the variations of the specific rate of growth. The comparison of the results obtained by these two methods allowed to validate the geometrical modeling and, thus, the values of the nucleation specific rate. INTRODUCTION Mineral oxide powders have various applications in glasses industry or ceramics or catalysis. Especially, ceria is widely used for the automotive post-combustion catalysis [1]. These applications require particular textural properties. Thus, preparation processes must be controlled. The knowledge of the specific rates of growth and nucleation is of great importance for the control of textural properties of the produced powders. Cerium dioxide can be obtained by the thermal transformation of cerium III hydroxycarbonate (CeOHCO3). Thermal decompositions of rare earth carbonates, hydroxycarbonates, oxalates have been extensively studied. However, the conversion of cerium III hydroxycarbonate by reaction with oxygen has been scarcely published [2]. The kinetic study of this transformation has been achieved in order to determine the specific rates of nucleation and growth of the oxide against the partial pressures of gases involved in the reaction such as carbon dioxide and oxygen. Two methodologies have been used : a geometrical modeling based on the Mampers method [3], which was then developed by Delmon [4], and an experimental method based on the method of "insulation" [5]. 1. EXPERIMENTAL Samples of cerium III hydroxycarbonate were prepared from precipitation of cerium nitrate by ammonium hydrogenocarbonate. The mean radius of the individual particles was approximated to about 0.55 10-6 m from scanning electron microscopy which showed a regular spherical shape. The extent of conversion (~ : ceria mole number) versus time was
886 obtained by means of isothermal thermogravimetry. Samples (approximately 25 mg) were heated at 220~ in a thermobalance SETARAM MTB 10-8 under a controlled atmosphere. The partial pressures of water vapour (PH20) ' oxygen (Po2) and carbone dioxide (Pco2) were fixed. Thermogravimetric curves were converted into kinetic curves : the conversion (~) and the fractionnal conversion (~,) are defined by the two following relations : Z,(t) = Am(t), Amf ~(t) = N 0.Z,(t), in which Am(t) is the mass loss at the time t, Amf is the final mass loss and NO is the initial mole number of cerium hydroxycarbonate. As can be seen in figure 1, the experimental kinetic curves present a sigmoid shape.
d~
Consequendy, the corresponding rate curves -~t- ~.) that can be seen as broken lines in figure 2 have a maximum.
1
PO2=6666 Pa
tOexpet tOmod 1.2-
0.8
PO2= 6666 Pa, A=600
~
....
Experiment
0.6 0.6 0.4
0.4 0.2 0
] PH20=pCO2=666 Pal
0
36b0 ~/2'00 i0800 14zi00 time (s)
Figure 1. Example of kinetic curves for two oxygen partial pressures
0"20
",, 0
012
014
016
018
1
Figure 2. Reduced rate curves corresponding to the kinetic curves in figure 1
2. NUCLEATION AND GROWTH PROCESSES The kinetic study was based on the concepts of nucleation and growth of the new phase (CeO2) which nucleates at the surface of a cerium III hydroxycarbonate particle. The interfacial area between the two phases was noted SI, and the free surface of cerium hydroxycarbonate (that is not yet transformed in ceria) was noted SL (figure 3).
887
~ S
L
--H-H-- S I
CeO 2 Figure 3. Representative pattern of nucleation and growth phenomena
The absolute rate of transformation (-a~-)results from the two contributions of nucleation and growth phenomena and may be written as : d~ = Nod~ -~-~-= V S I + g SL,
(1)
in which g and v are respectively the specific rates of nucleation and growth. They are expressed in moles .m -2 .s" 1. The relation (1) also points out that the absolute rate is the product of a specific rate and of the corresponding reaction area, which is SI for growth and SL for nucleation. These areas depend on the intensive variables (temperature, partial pressures) but also on the time of reaction. If the transformation proceeds with a rate-limiting interfacial reaction, the specific rates g and v do not depend on the time, but only on the intensive variables. Thus, for an isothermal experiment under controlled atmosphere, these specific rates remain constant. In practise, nucleation phenomena give very small nuclei and the nucleation absolute rate (g SL) can be neglected in equation (1). The experimental rate of the transformation can therefore be expressed by the following equation :
d;
-di- = v. S].
(2)
However, the nucleation process is not neglected because the interfacial area SI depends on it. 3. GEOMETRICAL MODELING The theoretical rate law versus time of transformation can be established by means of a geometrical modeling which allows to calculate the areas SI and SL, the specific rates being fixed by the values of the partial pressures during an experiment. Various hypothesis must be made to build a geometrical modeling. As previously mentionned, the shape of the kinetic curves is sigrnoid 9this indicates that nor the nucleation, nor the growth can be considered as spontaneous phenomena, otherwise the kinetic curves have no inflexion point. The most general case of nucleation and growth has been described by a model based on the Mampel's method [3]. This modeling was then developped by Delmon [4] and was based on the following assumptions, the initial solid is composed of spherical homodispersed particles; all these particles have the same behaviour; the spherical nuclei which are formed on the surface grow according to an internal
888 development ; the transformation proceeds with a rate-limiting internal interfacial reaction. It 4~t
is thus possible to calculate the fractionnal conversion (~,) and the theoretical kinetic rate (aa,) dO versus a reduced time 0, using a dimensionless parameter A, through which are taken into account the specific rates of nucleation and growth. The values of 0 and A are defined by the following equations 9 0
=
Vmv
. t
Y0
3
4nro A
~
~
.
Vm
(3)
Y v,
(4)
in which Vm is the molar volume of cerium III hydroxycarbonate (Vm - 59 10-6 m 3 / mole), and r0 is the radius of the particles (r0 - 0.55 10-6 m); y is the nucleation frequency, that is to say the number of nuclei which appear per meter squared and per second. This rate is proportionnal to the specific rate of nucleation g (g = y Nr where Nc is the number of ceria moles per nuclei). The expressions of ~.(0) and -~-(0)obtained by this model are very complex and dO computering is absolutely necessary to calculate the related kinetic curves. To compare the
d~
d~
experimental rate -~-(~)and the theoretical rate ~ (~,), the reduced rates COexp(X) and dO r have been defined for the experiment and the model respectively :
d•--(t ~.)
%~p0,) =
,
(5)
.
(6)
(d~) (Z, = 0, 5)
dg (k) dO (OmoaO0 =
( ~ ) O. = o, 5) dO
For a given experiment, the values of v and T are fixed by the partial pressures and the temperature. Thus, the reduced time 0 is proportionnal to the experimental time t (equation 3)
889 dX d~ and the rate ~-~ ~,) is proportionnal to the theoretical rate . q dO dX dX dO dX Vmv dt dO dt dO ro Moreover, the parameter A remains constant during an experiment (equation 4). As a consequence, if the model correctly describes the transformation, there must exist a value of the parameter A such that the experimental and theoretical reduced rates defined by the equations (5) and (6) are equal. This value may thus be determined by the comparison of the reduced rate curves as shown in figure 2 9with an oxygen partial pressure of 0.67 kPa, the fitted value of A is 350, and with 6.67 kPa in oxygen, its value is 600. With the knowledge of A for a given experiment, it is then possible to determine the 0 relation between 0 and t by means of the Z,(t) 0.7 PO2=6666 Pa, A=600 (experiment) and ~(0) (model) kinetic curves. 0.6 As it can be seen in figure 4, this relation is linear and the slope of the corresponding line is 0.5 Vmv r0 according to equation (3). It is thus 0.4 0.3 / Pa, A=350 possible to calculate the value of the growth specific rate v, and the value of nucleation 0.2 ~ [ P H 2 0 = p C O 2 = 6 6 6 Pa I frequency T with equation (4). By modifying 0.1 the experimental conditions, the variations of v 1000 3000 5000 7000 9000 and T against the partial pressures in oxygen time (s) and carbon dioxide have been obtained and are shown from figures 5 to 8. The specific rates of Figure 4. Relation between the reduced nucleation and growth increase with oxygen time (0) and the experimental time (t) pressure increase (figures 5 and 6) and decrease with carbon dioxide pressure increase (figures 7 and 8).
v (moles.m'2.s "1)
3.5 1010
T
(nuclei.m-2.s"1)
1.4 10-6
9 .
s~ s So 9
1 10-6
I PH20?pCO2~ 667
6 10-7 5000
Pal
10000 15000 20000 pO2 (Pa) Figure 5. Variations of the specific rate of growth against oxygen partial pressure 0
O,.
~
9
"
s
I'| I ~f
2.5 1010.
O
O
1.5 1010.
o
It
.11" 5109 o,. 0
[PH20=pCO2= 667 Pa] 9
!
.
5000
.
.
.
i
.
.
.
.
i
.
.
.
.
!
10000 15000 20000 pO2 (Pa)
Figure 6. Variations of the specific rate of nucleation against oxygen partial pressure
890
..410_ 6
7 (nuclei.m'2.s'l)
(moles.m'2.s "1
v
11010 1 10 -6
PH20=PO2= 667 Pa I
I PH20--PO2= 667 Pa I
4D
6 109.
e
6 10-7
,e o o" . o 0-
.
. ~
9
~
210 -7 . . . * 0 2000 4000 6000 pCO2 (Pa) Figure 7. Variations of the specific rate of growth against carbon dioxide partial pressure ~
2 109.
9
"L "
0
8
"
"
"
O.
2()00 4000 pCO2 (Pa)
6()00
Figure 8. Variationsof the specificrateof nucleation 9 againstcarbon dioxidepartial pressure
4. EXPERIMENTAL METHOD OF DETERMINATION OF THE GROWTH SPECIFIC RATE VARIATIONS The experimental method, based on the insulation method [5] consists of modifying during an experiment and at a time tO, the partial pressure of one of the gases (Pi~
~), the other
ones being kept at the same value (pO). The absolute rate at the time tO can be written according to equations (1) and (2)"
(d~)(t
=to)
= (p~,p0 v
SI (p0,pO j .... )x~00 j .... t0).
By doing this experiment for several values of Pi, since SI is fixed both by the initial conditions of the experiment (Pi~ and the time tO, the variations of v versus Pi can be obtained. In the figures 9 and 10, are represented the kinetic curves obtained by this method. In figure 9, the partial pressure in oxygen was changed at time tO (1 hour) from 0.67 kPa to various values, those of water vapour and carbon dioxide being fixed at the same value (0.67 kPa) during all the experiments. In figure 10, the partial pressures in oxygen and water vapour have been kept constant and the partial pressure of carbon dioxide was changed (tO = 28mn) from 0.27 kPa to various values.
891
~, 1-
] PH20=pCO2= 666 Pa] 0.8-
0.8-
0.6-
0.6-
[ PH20= 667 Pa, PO2= 400 Pa I
~.
0.4- t~ 3 0.2-
~
0
-,-"
~
pO2= 666 Pa o pO2= 1333 Pa : pO2= 5333 Pa ---
0.40.20 ~
N
to =1680 s !
0
36'00 7200 10800 time (s) Figure 9.Experimental method of determination of the growth specific rate variations with oxygen partial pressure
pCO2= 267 Pa pCO2-- 667 Pa o--- pCO2- 1333 Pa -" pCO2- 2666 Pa ~ pCO2= 5333 Pa
~
0
i
i
!
3600
7200 10800 14400 time (s) Figure 10. Experimental method of determination of the growth specific rate variations with carbon dioxide partial pressure
The variations of v obtained by this experimental method against the partial pressures of oxygen and carbon dioxide are then used to confirm those deduced from the geometrical model. dg In figure 11 and 12, we have reported the values of the absolute rate (-d-i-) ( t - t 0) determined by the experimental method versus the values issued from the geometrical model when the partial pressures of respectively oxygen and carbon dioxide were varied. The straight line which can be drawn between the dots allows to validate the geometrical model and thus the values of the nucleation specific rate, as well as its variation against the partial pressures in oxygen and carbon dioxide.
(dk/dt)(t=lh) "experimental method
(d~/dt) (t=28 mn)" experimental method
3.5 10 -4 s
,
510 4 .
s
s s
9
3 10"4
s
J
J s
s Qs s
s 9
sO
,"
2.5 10 -4
3 10-4-
-d
s
s
OlD s
2 10-4 6 1(-7
d
f !
i
1 10-6 1.4 10 "6 v: geometrical modeling Figure 11. Comparison of results obtained by the experimental and the geometrical methods as oxygen partial pressure varied
1 10-4
2 10-7
6 10-7 ' 1 1'0-6 ' 1 . 4 i 0 -6 v" geometrical modeling Figure 12. Comparison of results obtained by the experimental and the geometrical methods as carbon dioxide partial pressure varied
892 CONCLUSION The specific rate of growth of ceria from cerium III hydroxycarbonate versus the partial pressures of oxygen and carbon dioxide have been determined by an experimental method and by a geometrical modeling. The comparison of the results obtained by these two methods allows to validate the values and the variations of the specific rate of nucleation which can only be obtained from the geometrical modeling. The methodology presented in this paper is general and can be used for many types of heterogeneous reactions.
REFERENCES 1. B. Harrison, A.F. Diwell and C. Hallett, Platinum Metals Rev, 32(2) (1988) 73-83 2. M. Akinc, D. Sordelet, Adv. Ceram. Mater, 2 (1987) 232-38 3. K.L. Mampel, Z. Phys. Chem., A 187 (1940) 235-249 4. B. Delmon, in "Introduction ~ la cin6tique h6t6rog~ne", Technip Paris (1969) 403 5. B. Delmon, in "Introduction ii la cin6tique h6t6rog~ne", Technip Paris (1969) 263
ACKNOWLEDGEMENTS The authors wish to acknowledge Rh6ne-Poulenc for financial support.
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
Hydrotalcite-type
anionic clays
as
precursors
893
of high-surface-area
Ni/Mg/AI mixed oxides A. Vaccari a and M. Gazzano b a Dipartimento di Chimica Industfiale e dei Materiali, Universit~ di Bologna, Viale del Risorgimento 4, 40136 Bologna, Italy. b CSFM/CNR, Via Selmi 2, 40126 Bologna, Italy.
Anionic clays with a hydrotalcite-type (HT) structure may be useful precursors of multicomponent catalysts, such as, for example, Ni/A1 and Ni/Mg/A1 catalysts. The flexibility of the HT structure is clearly demonstrated by the fact that pure Ni/Mg/A1 precursors may be obtained by coprecipitation regardless of the Ni/Mg ratio. The thermal decomposition of all the precipitates involves an initial loss of interlayer water followed by the elimination of hydroxide and carbonate, with some differences related to sample composition. Regardless of composition, all samples obtained by calcination up to 1073K exhibit a high surface area and stability, and are characterized by a low reducibility of the Ni 2+ ions. However, for Ni-rich samples both the lattice parameter of the oxide phases and Rietveld' s powder pattern fitting procedure indicate that the low reducibility is not due to the presence of foreign ions, but rather may be attributed to the presence of a surface spinel-type phase. However, with increasing Mg-content, the reduction of the Ni 2+ ions is further hindered by the presence of Mg 2+ and AI3§ ions in the oxide phases. At high temperatures, stoichiometric spinel phases form, with segregation of oxide phases and a considerable decrease in surface area. For the Ni-rich samples, this takes place with an increase in the reducibility of the main fraction of Ni 2+ ions, which show a behaviour similar to that of free NiO. On the contrary, the Mg-rich samples show a further decrease in reducibility, due the formation of NiO/MgO solid solutions.
1. INTRODUCTION High-surface-area homogeneous mixed oxides are widely required by industry for use as adsorbents, catalysts, and pigments, and in sensor and magnetic technologies [1-7]. These materials require preparation techniques that ensure an intimate mixture of the components, without high-temperature treatments, and the properties of the materials obtained are very different from those of the same solids synthetized using ceramic methods [8]. The synthesis of hydrotalcite-type (HT) anionic clays by coprecipitation, followed by thermal decomposition at moderate temperature, fulfills these requirements [7,9,10]. Synthetic HT
894 anionic clays have the general formula [M(II)l.x M(III)x (OH)2] x+ [A n- x/n] 9mH20 (where M= metal and A-- anion, usually carbonate) and many parameters may be modified in the preparation in order to produce materials with specific properties [7,10]. High-surface-area Ni/A1 mixed oxides resulting from the thermal decomposition of HT coprecipitates are among the most widely investigated catalyst precursors on account of the remarkable properties of the final catalysts, such as high metallic dispersion and particle stability against sintering under extreme conditions for nickel contents up to 75wt% [7,11-19]. However, the reasons for this interesting thermal stability are still under debate and different models have been proposed to explain the stability of the nickel particles during reduction [12,13]. Furthermore, the phases formed as a function of calcination temperature are not fully understood, mainly due to the poor crystallinity of these samples. Although several papers have been published describing the preparation and reducibility of NiO/MgO mixed oxides [20-24], little is known about ternary NiO/MgO/A1203 mixed oxides and, in particular, about the modifications of the properties induced by increasing amounts of MgO [25-27]. This topic is of particular interest, if one considers that MgO is often present as a promoter in commercial nickel catalysts and that the progressive dilution of the transition metal may also shed light on the properties of Ni/A1 systems. For all these reasons, the reactions which take place during the preparation of Ni/Mg/A1 catalysts deserve more thorough scrutiny. This study deals with the preparation, properties and reactivity of high-surface-area Ni/Mg/A1 mixed oxides featuring different N ~ g ratios obtained from HT anionic clays. In HT precipitates all cations are present inside the brucite-type layers, therefore the specific properties of each element may be evidenced without any interference due to phase segregation and/or physical dishomogeneity. 2. EXPERIMENTAL
The precursors were prepared by coprecipitation at pH 8.0 under energetic stirring of a solution containing the nitrates of the elements and a solution containing a slight excess of NaHCO 3. The precipitates were kept in suspension at 333K for 30min under stirring, then filtered and washed with distilled water until a Na20 content lower than 0.02wt% was obtained. The precipitates were dried overnight at 363K and calcined for 14h at various temperatures. XRD powder analysis was carried out using a Philips PW1050/81 diffractometer and Ni-filtered CuKa radiation (~.= 0.15418nm) (40kV, 40mA). A 2 0 range from 10~ to 80 ~ was investigated at a scanning speed of 60~ The lattice constants were determined by least square refinements, from the well defined position of the five most intense peaks. The crystallite sizes of the HT, oxide and spinel phases were determined by the Scherrer equation using the average values of [(003) and (006)], [(200) and (220)] and [(220) and (400)] line widths, respectively. The warren correction was used for instrumental line broadening, while the possible contribution of disorder effects and/or lattice strains was not taken into account. The quantitative compositions of NiO/MgO solid solutions obtained by calcination of the Ni/Mg/A1 precursors were determined on the basis of the lattice parameter a using Vegard's law [28], with a linear interpolation between the values reported in the ICDD files (MgO
895 Table 1 Composition and physical characterization of the precursors dried at 363K Sample
Ni:Mg:A1 (at. ratio %)
1 2 3 4 5
71.0:0.0:29.0 61.0:10.0:29.0 34.0:37.0:29.0 10.0:61.0:29.0 0.0:71.0:29.0
S.a. Phase identified (m2 g-l) by XRD 100 74 73 97 96
HT HT HT HT HT
C.s. (nm) 4.5 7.0 7.5 9.0 7.5
Crystallographic parameters a (nm) c (nm) c/a 0.3035(7) 0.3037(4) 0.3039(7) 0.3042(7) 0.3035(2)
2.293(7) 2.291(4) 2.295(9) 2.285(7) 2.272(2)
7.56 7.55 7.55 7.51 7.49
S. a. = surface area. HT = hydrotalcite-type anionic clay. C. s. = crystallite size.
4-829; Mg0.sNi0.50 24-712; Mg0.4Ni0.60 34-410; NiO 4-835). For the Rietveld's powder pattern fitting, the data were collected in the 2 0 range 20-140 ~ using a step of 0.05 ~ and a counting time of 0.5min. The refinement of the profile and structural parameters was carried out with the DBW 9006 program [29], release Wyriet 3.0 for P.C. The fractional coordinates of pure NiO (space group Fm3m) were used first to ref'me the nonstructural parameters (zero, scale factor, background intensity, half width, asimmetry, gaussianity) and then the cell parameters, thermal parameters and metal atom occupancy factors. IR spectra were recorded using the KBr disk technique and a Perkin-Elmer 1750 FTIR spectrometer. Surface area and pore volume were determined on a C. Erba Sorptomatic model 1700 apparatus, by means of N2 adsorption. Thermogravimetric (TG) analyses were carried out using a Perkin-Elmer TGS-2 thermobalance with a He-flow of 3dm~/h and a heating rate of 600K~. Temperature-programmed-reduction (TPR) profiles were obtained usin~ the previously described TG apparatus. A H2-He mixture (37:63 v/v) at a flow rate of 3dm~/h and a heating rate of 600K/h were employed. The weight losses due to the NiO + H 2 ~ Ni + H20 reaction were recorded as a function of the temperature. Preliminary tests of water adsorption and desorption on calcined materials showed that water was not adsorbed (or fully desorbed) at temperatures higher than 473K. The apparent activation energy values were calculated by analysing the reduction curves according to the empirical rate constant method [30,31]; the reduction runs were carried out isothermally within the various temperature ranges, increasing the temperature in steps of 10K.
3. RESULTS AND DISCUSSION The XRD spectra of the precipitates dried at 363K show the presence, for all samples, of only a well crystallized hydrotalcite-type phase (Table 1), in agreement with the M(II)/M(III) ratio and nature of the ions [7,9,10]. The crystallographic parameters a and c, calculated for an hexagonal cell on the basis of rhombohedral symmetry, show that the structure of Mg/A1
896
B
A
C
A / 1173K
.~
I 9 el
I ''73K 9
A 0 523,.,
Ivv,,l,,
,,i11
~o ~
III 5 2 3 ~
o f r I, r i , r 1 6 2
vlr
10 20 .30 40 50 60 70 80 2Thete (degrees)
9
il,
oo
ivvw,lVVV,lVVVWlVVWVl
_
/
A
9
0_ vvvvllvvvl
"
1173K
A 1
9 A'
,-,o
_
9
~ 523,., rvvv,
10 20 30 40 50 60 70 80 2Theto (degrees)
~blrvw I V l
WVl l i
VVlV
r
o 9
o w I llw'l
Ilvl
I | ~ vu i |
10 20 30 40 50 60 70 80 2Theta (degrees)
Figure 1. XRD powder patterns of the samples: (A) Ni/AI= 71.0:29.0; (B) N ~ g / A 1 = 34.0:37.0:29.0, (C) Mg/AI= 71.0:29.0 (atomic ratio percent) calcined at different temperatures. ( 0 ) HT phase; ( 9 ) oxide phase; ( 9 ) spinel phase. HT anionic clay exhibits the best packing and that the partial substitution with Ni2+ ions increases the disorder more than the partial substitution of Mg2+ ions inside the Ni/A1 structure. However, in all cases the HT anionic clays feature an elastic structure, that is compatible with the slight alterations related to the different compositions. TG analyses confh'm the absence of side phases [ 18,26] and show the presence of only two weight losses: the fLrst, at about 503K, is attributable to the elimination of the water molecules from the intedayers, while the second, at about 673K, is due to the dehydroxylation of the brucite-type layers and to the elimination of the carbonate anions from the interlayers [7]. As magnesium content increases, this latter loss is displaced towards the higher temperatures and is only a partial one, on account of the higher affinity of the Mg 2§ ions for CO2 [32-34]. This phenomenon is also observed for the samples calcined at 923K for 14h; the FFIR spectra of these samples confh-m the presence of residual carbonates. Up to 523K, XRD analysis of the calcined samples (Fig. 1) shows the presence of only a HT phase, with changes in the pattern being ascribable to the loss of water molecules from the interlayer region. When temperature is increased up to 1023K, the only XRD pattern observed is that of poorly crystallized oxide phase, which cannot, however, be identified with absolute certainty on account of the similarity of the patterns for MgO and NiO (ICDD 4-829 and 4-835, respectively). These data are in good agreement with the formation of rock-salt-type mixed oxides previously reported for Mg/A1 clays calcined at 773-1073K [35]. A further increase in calcination temperature leads to a structure rearrangement, accompanied by the segregation of oxide and stoichiometric spinel phases.
897 Table 2. Values of lattice parameter a for the oxide and spinel phases as a function of composition and calcination temperature (compositions as in Table 1) Sample
723K
923K
Oxide phase 1023K
1173K
Spinel phase 1173K 1273K
1273K
0.414(2) 0.4167(4) 0.4172(1) 0.4177(2) n.d. 0.415(1) 0.4173(6) 0.4173(3) 0.4178(4) a n.d. 0.4165(7) 0.4172(6) 0.4173(2) 0.4184(4)0.4195(3) b 0.417(1) 0.417(1) 0.4184(9) 0.4207(4)0.4205(2) c 0.418(1) 0.4179(8) 0.418(1) 0.4216(4) n.d.
sample
0.8051(3) n.d. 0.8048(3) n.d. 0.8056(5) 0.8076(6) 0.8080(5) 0.8079(7) 0.8086(7) n.d.
MgO/NiOexperimental = (a) 82:18; (b) 60:40; (c) 18:82 (atomic ratio %). MgO/NiOcalculated = (a) 92:8 ; (b) 61:39; (c) 18:82 (atomic ratio %), for spinel= (a) NiA1204 and (b and c) MgA1204. NiO= 0.4177nm (ICDD 4-835); MgO= 0.4213nm (ICDD 4-829); NiA1204= 0.8048nm (ICDD 10-339); MgA1204= 0.8083nm (ICDD 21-1152).
The values of lattice parameter a for both the oxide and spinel phases as a function of calcination temperature and composition are reported in Table 2. The values of a for the oxide phases in all samples calcined at 723K are smaller than those reported for pure oxides, a fact which may be attributed to the presence of AI(III) ions inside the oxide lattice [13,15]. Lattice distorsion for Ni-rich samples becomes negligible at the higher calcination temperatures, while, with increasing Mg-content these distortions are found also in the samples calcined at 1023K, which demonstrates that these samples are characterized by a higher tendency to retain the AI(III) ions inside the oxide lattice. NiA1204 and NiO were present in the Ni/A1 sample calcined at T~_ 1173K, while the a values of the oxide phases in the Ni/Mg/A1 samples clearly indicate the formation of NiO/MgO solid solutions, the quantitative composition of some of which were determined by means of Vegard's law, Table 3. Crystallite size (nm) of the oxide and spinel phases as a function of sample composition and calcination temperature (compositions as in Table 1) Sample
1
2 3 4 5
723K 3.5 2.5 3.0 3.5 3.5
Oxide phase 923K 1023K 4.0 4.0 4.0 4.0 3.5
5.5 5.0 4.0 4.0 3.5
1173K
Spinel phase 1173K
14.5 12.0 15.5 14.5 7.0
16.0 19.0 16.0 18.5 10.0
898
Fit indicators Rp 3.02 Rwp 3.80 RE 3.01 S !.26 DDW 1.20 ! ~ I[ I[
~ II I[ I[ IAI
g ~ "
Non-structural parameters scale 0.0174(2) zero 20 -0.022(9X*) ,, u 4.40(5) v -3.39(5) w 2.99(2) asymm. 0.023,2(6) NA 0.9~(3) NB 0.0013(6) backg. PI 902(3)
Structural parameters a 0.41683(7Xnm) B(Ni) 0.36(3) (~,2) B(AI) 0.43(4) (~2) B(O) 0.~4(5) (,~2) OF0sli) 0.0201(2) OF(AI) 0.0007(2)
|o
Reflections
position
I
I
I
I
20
40
l I I
60
80 2 Theta (degrees)
I
I fl [
' 100
II
I (C)
I
I
120
i 40
Figure 2. Observed (A), calculated (B) and difference (C) XRD powder patterns for the sample Ni/AI= 71.0:29.0 (atomic ratio percent) calcined at 1023K for 14h. The refined parameters as well as the fit indicators are also reported. using a linear interpolation between the a values reported in the ICDD data file (Table 2). Table 3 reports the crystallite sizes for the oxide and spinel phases as a function of composition and calcination temperature. Up to 1023K, no significant differences are observed as a function of the composition, with only a slight increase in crystallite size as calcination temperature increases. On the contrary, the segregation of the spinel phase gives rise to a remarkable sintering of all the samples calcined at 1173K, with the exception of sample 5 (Mg/AI= 71.0:29.0), for which the sintering was less. It is worth noting that the Rietveld's powder pattern fitting carried out for the Ni/A1 sample calcined at 1023K (Fig. 2) confhTns that the cationic sites of the oxide phase are almost entirely occupied by nickel atoms. In fact, the normalized occupancy factors for nickel and aluminum are 0.966(10) and 0.034(10), respectively. Therefore, this powerfulrnethod makes it possible to exclude a significant degree of substitution of A13§ ions for Ni 2+ ions in the oxide phase, in contrast with that previously reported in the literature [ 13,15]. It was not possible to apply the same refinement procedure to the Mg-containing samples, since Mg 2+ and A13+ ions are isoelectronic and have essentially the same X-ray scattering factors. Nevertheless, the presence of A13+ ions inside the oxide lattice may be hypothesized for these samples based on the XRD powder pattern of the samples calcined in the 723-1023K range (Fig. 1B and C). When the calcination temperature is increased from 1023K to to 1173K, the three (111), (200) and (220) peak s of the oxide phase shift towards higher 2 0 values, simultaneously at the beginning of the spinel segregation. This fact may be explained based on the segregation of significant amounts of smaller A13+ ions which induces an enlargement of the oxide interplanar distances, in agreement with the lattice parameters reported in Table 1. Furthermore, the XRD powder patterns of samples 4 and 5 calcined at 1023K show at 2 0 = 35.4* an additional shoulder to the (111) oxide reflection. A
899 similar finding has been recently reported by 300 Clause and eoworkers [36] and attributed to the presence of a non-stoichiometric spinel-type phase. 2S0 .......................................... Calcination temperature affects surface area to a greater extent than composition (Fig. 3). A two- to ~ 200 . . . . . . . . . . . . . . . . . . . . . . . . . . . . three-fold increase in the surface area is generally observed for the samples obtained by calcination at ~ /., ......... ,~. ................... about 723-823K, while a considerable decrease is , lS0 ..... (i'. associated with spinel formation at higher ~ ~' \Q temperatures. The extent of this increase seems to ~am100-~.. .................................. .~ ' -.-. suggest that exiting steam and CO2 escape through r holes in the crystal surface without any extensive change in crystal morphology [37]. This ............................. i so I .......................................... mechanism may also explain the pore size -, distribution pattern observed for these samples, with a narrow peak centred~ around the most 0 , i , ~ , 9 frequently occurring pore radius [17]. 300 700 1100 1.~ Calcination temperature (K) The reactivity of the samples obtained by calcination of HT precursors was investigated by Figure 3. Surface area as a function of TPR analysis up to 1273K (Fig. 4). For Ni-rich calcination temperature for the samples calcined at T~_ 923K, the weight losses Ni/Mg/AI= (e) 61.0:10.0:29.0 and ( 9) correspond to the total reduction of Ni 2+ ions with 10.0:61.0:29.0 (atomic ratio percent) an accuracy of 5%, while in the precursors HT precursors. ( 9 NiA1204 calcined at 723K higher weight losses were observed, due to the presence of small amounts of Ni 3+ ions [22] and, mainly, the incomplete decomposition of the carbonates in the interlayer regions. On the contrary, for the Mg-rich systems higher weight losses were detected in the samples calcined up to 1023K, on account of the greater affinity of Mg2§ ions towards CO2 [32,33]. However, for the samples calcined at T> 1023K, the weight losses correspond to the total reduction of Ni 2+ ions, with a slightly lower accuracy due to the low nickel content. For all the precipitates calcined up to 1023K, only a broad weight loss is found in the TPR profiles, which can be attributed to the reduction of Ni 2§ ions in the oxide phase. However, the maximum of this peak shifts towards higher temperatures as calcination temperature and Mg-content increase. For the samples calcined at higher temperatures, for which segregation of spinel phases (likely NiA1204 or MgA1204) was in the XRD patterns, two different behaviours may be identified. The Ni-rich samples show two distinct weight losses attributable to the reduction of NiO and NiA1204, respectively, with the first peak displaced towards a lower temperature typical of free NiO [13]. On the contrary, for high Mg-contents the reduction temperature increases regularly also for the samples calcined at T> 1023K. For these samples, the reduction temperatures are very similar to those reported in the literature for nickel-magnesia catalysts, which were attributed to the presence of Ni 2+ ions beneath the MgO surface [22]. This behaviour may be explained considering the formation in these samples of NiO/MgO solid solutions, taking also into account that the cubic rock-salt structure of MgO favours the dispersion and diffusion of Ni 2+ ions into the underlying
J
900 ''
[
I
,
.
1173K
J
i'
C
'1173K . . . . . . . . . . . . . . . . . .
,,. . . . . .
..~
o
i 523
J 773
I , I 1023 1273 5 2 3
773 1023 1273 Temperature (K)
523
773
1023
1273
Figure 4. Temperamre-progranmaed-reduction profiles and relative derivative curves of Ni/Mg/A1 HT precursors calcined at different temperatures. (A) Ni/AI= 71.0:29.0; (B) Ni/Mg/AI= 34.0:37.0:29.0; (C) Ni~g/Al= 10.0:61.0:29.0 (atomic ratio percent). lattice. The values of the apparent activation energy for the reduction of the different samples are reported in Table 4. For NiO two different apparent activation energies were determined, as a function of the temperature range investigated, in agreement with data reported in the literature on the influence of the Curie transition on the reduction rate [30,31,38]. On the other hand, the higher value detected for the stoichiometric spinel NiA1204 clearly shows a change in the reduction mechanism, due to the homogeneous presence of foreign ions. The Ni/A1 sample exhibits the same value of the apparent activation as NiO, notwithstanding the lower reducibility, while higher values were observed for the Mg-rich samples. Therefore, the delay in the reduction can be attributed in the first case case to a decrease in the accessibility of the NiO surface to the reducing mixture, while for the Mg-rich samples the presence of Mg 2+ and/or A13+ ions also has to be considered. 4. CONCLUSIONS In previous papers [18,19,39], a model was proposed for Ni/A1 mixed oxides obtained from HT precursors, involving the formation of NiO (containing a very small amount of A13+ ions) and Ni-dope d aluminaphases, which strongly interact with a spinel-type phase present at their interface. The spinel-type phase, probably a non-stoichiometric-type, is responsible for the thermal properties of the calcined precursors, hindering the growth and sinte~ng of the NiO crystallites. Likewise, the reduction of the NiO phase is hindered by the presence of the
901 Table 4. Values of the apparent activation energy and of the pre-exponential factor for the reduction of the calcined HT precursors and some reference compounds. (composition as in Table 1) Sample
Calc. temp. (K)
Reduc. temp. (K)
App. activation energy (kJ tool "1)
1 1 1 3 4 NiO NiO NiA1204
723 923 1023 923 923 723 723 1373
653-713 713-793 778-978 883-923 993-1023 483-513 513-553 1023-1123
70 + 6 79 + 6 75 + 6 136 + 6 211+ 18 110 + 6 66 + 6 130+ 6
Pre-expon. factor (In e A) 11 + 13 + 11 + 14 + 23 + 27 + 17 + 14 +
1 1 1 1 3 1 1 1
spinel-type phase, making the generation of nickel nuclei at the surface difficult [40] and/or decreasing the accessibility of the surface to the reducing gas. This model may be considered of a more general validity, on account of the behaviours of the Ni/Mg/A1 mixed oxides obtained from HT precursors, reported in this paper. However, some adaptations are required depending on composition; in particular, for the Mg-rich samples the reduction of the Ni 2+ ions inside the oxide phases is further hindered by the presence of Mg 2+ and A13+ ions, as clearly evidenced by the increase in the apparent activation energy. In all cases, the evolution of the spinel-type phases towards the stoichiometric forms, which takes place with a consistent segregation of oxide phases, destroys this system and leads to considerable modifications in the properties of the oxides obtained. In particular, for the Ni-rich samples an increase in the reducibility of the main fraction of the Ni 2+ ions was observed, with a behaviour similar to that of free NiO, while for Mg-rich samples the reduction of the Ni 2+ ions is further hindered by the formation of NiO/MgO solid solutions, with reduction temperatures very similar to those reported for nickel-magnesia catalysts.
REFERENCES 1. J.J. Burton and R.L. Garten (eds.), Advanced Materials in Catalysis, Academic, New York, 1977. 2. D.D. Dadyburior, S.S. Jewur and E. Ruckenstein, Catal. Rev. -Sci. Eng., 19 (1979) 293. 3. O. T. Sorensen (ed.), Non-Stoichiometric Oxides, Academic, New York, 1981 4. H. Yonagida, Angew. Chem. (Int. Ed. Engl.), 27 (1988) 1389. 5. S. Lew, K. Jothimurugesan and M. Flytzani-Stephanopulos, I.E.&C. Research, 28 (1989) 535. 6. K. Suresh, N.R.S. Kumar and K.C. Patil, Adv. Mater., 3 (1991) 148. 7. F. Cavani, F. Trifirb and A. Vaccari, Catal. Today, 11 (1991) 173 and references therein. 8. I. Aldinger and H.J. Kalz, Angew. Chem. (Int. Ed. Engl.), 26 (1987) 371.
902 9. W.T. Reichle, CHEMTECH, 16 (1986) 58. 10. A. Vaccari, Chim. Ind. (Milan), 74 (1992) 174. 11. J. R. Rostrup-Nielsen, Steam Reforming Catalysts, Teknish Forlag, Copenhagen, 1975. 12. L. Alzamora, J.R.H. Ross, E.C. Kruissink and L.L. van Reijen, J. Chem. Soc., Faraday Trans. 1, 77 (1981 ) 665. 13. P.C. Puxley, I.C. Kitchener, C. Komodromos and N.D. Parkins, in Preparation of Catalysts 1II (G. Poncelet, P. Grange and P.A. Jacobs, eds.), Elsevier, Amsterdam, 1983, p. 227. 14. M.J. Hemandez, M.A. Ulibarri, J.L. Rendon and C.J. Sema, Thermochim. Acta, 81 (1984) 311. 15. J.R.H. Ross, in Catalysis Specialist Periodical Reports (G.C. Bond and G. Webb, eds.), vol. 7, Royal Society of Chemistry, London, 1985, p. 1. 16. H.G.J. Lansink Rotgerink, H. Bosch, J.G. van Ommen and J.R.H. Ross, Appl. Catal., 27 (1986)41. 17. O. Clause, M. Gazzano, F. Tfifirb, A. Vaccari and L. Zatorski, Appl. Catal., 73 (1991) 217. 18. O. Clause, B. Rebours, E. Merlen, F. Trif'lrb and A. Vaccari, J. Catal., 133 (1992) 231. 19. P. Beccat, J.C. Roussel, O. Clause, A. Vaccari and F. Trif'lrb, in Catalysis and Surface Characterisation (T.J. Dines, C.H. Rochester and J. Thomson, eds.), Royal Society of Chemistry, Cambridge, 1992, p. 32. 20. J.G. Highfield, A. Bossi and F.S. Stone, in Preparation of Catalyst III (G. Poncelet, P. Grange and P.A. Jacobs, eds.), Elsevier, Amsterdam, 1983, p. 181. 21. A. Zecchina, G. Spoto, S. Coluccia and E. Guglielminotti, J. Chem. Soc., Faraday Trans. 1, 80 (1984) 1891. 22. G.C. Bond and S.P. Sarsam, Appl. Catal, 38 (1988) 365. 23. J. Hu, J.A. Schwarz and Y.J. Huang, Appl. Catal., 51 (1989) 223. 24. A. Parmaliana, F. Arena, F. Frusteri and N. Giordano, J. Chem. Soc., Faraday Trans. 1, 86 (1990) 2663. 25. A.M. Gadalla and M.E. Sommer, J. Am. Ceram. Soc., 72 (1989) 683. 26. O. Clause, M. Goncalves Coelho, M. Gazzano, D. Matteuzzi, F. Trifirb and A. Vaccari, Appl. Clay Sci., 8 (1993) 1. 27. D. Matteuzzi, F. Trifirb, A. Vaccari, M. Gazzano and O. Clause, in Proc. Intern. Symposium Soft Chemistry Routes to New Materials, Nantes (F), september 1993, in press. 28. A.R. West, Solid State Chemistry and its Applications, Wiley, Chichester, 1984, ch. 10. 29. D.B. Wries and R.A. Young, J. Appl. CrystaUogr., 14 (1981) 149. 30. B. Delmon, Introduction ~ la Cin6tique H6t6rog~ne, Technip, Paris, 1969. 31. V.V. Boldyrev, M. Bulens and B. Delmon, The Control of the Reactivity of Solids, Elsevier, Amsterdam, 1979. 32. G.J Ross and H. Kodama, Amer. Miner., 52 (1967) 1036. 33. W.F.N.M. Vleesschauwer, in Physical and Chemical Aspects of Adsorbents and Catalysts, Academic, New York, 1970, p. 265. 33. G.W. Brindley and S. Kikkawa, Amer. Miner., 64 (1979) 836. 35. T. Sato, T. Wakabayashi and M. Shimada, I.E.&C. Prod. Res. Dev., 25 (1986) 1. 36. B. Rebours, J.B. Espinose CaiUerie and O. Clause, J. Am. Chem. Soc., 116 (1994) 1707. 37. W.T. Reichle, S.Y. Kang and D.S. Everhardt, J. Catal., 101 (1986) 352. 38. F. Chiesa a~fd M. Rigaud, Can. J. Chem. Eng., 49 (1971) 617. 39. F. Trifirb, ,4,. Vaccari and O. Clause, Catal. Today, in press. 40. J.H.R. Ross, M.C.F. Steel and A. Zeini-Isfahani, J. Catal., 52 (1978) 280.
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
PREPARATION AND CHARACTERISATION CONTAINING LAYERED DOUBLE HYDROXIDES
903
OF
COBALT
S.Kannan and C.S.Swamy Department of Chemistry, Indian Institute of Technology, Madras - 600 036, INDIA. SUMMARY A series of hydrotalcites of general formula Co2+-M3+-COs-HT (M s+ = A1,Fe and Cr) are prepared by coprecipitation technique. The influence of parameters such as preparation method, atomic ratio, supersaturation levels, aging and hydrothermal treatments are investigated to study their effect on the structure and texture of these materials. The obtained materials are characterised by X-ray diffraction, FT-IR studies, thermogravimetry-differential scanning calorimetry, transmission electron microscopy and BET surface area measurements. Thermal calcination of these materials resulted in the formation of high surface area non-stoichiometric spinels whose catalytic activity is studied using N20 decomposition reaction as the test reaction. The order of activity observed is Co-A1-COs-HT>Co-Fe-COs-HT>Co-Cr-CO 3HT. INTRODUCTION Layered double hydroxides commonly referred as hydrotalcite-like (HT-like) materials, consists of brucite-like (Mg(OH) 2) network, wherein the divalent ion is substituted by trivalent ion whose excess positive charge is compensated by anions, usually carbonate, which occupy the interlayer positions [1-5]. They are represented by the general formula
[M(II)I.xM(III)x(OH)2]x+Ax/nn'.mH20 where M(II) and M(III) are divalent and trivalent ions and A is the interlayer anion where water of crystallisation also finds a place. The physico-chemical properties of these materials are mainly characterised by the nature of metal ions and their composition [6,7]. Although a wide spectrum of metal ions have been incorporated into the network [8], reports available on cobalt containing hydrotalcites are scarce [9]. Thermal calcination of these materials resulted in the formation of stable, high surface area and non-stoichiometric mixed metal oxides employed in many catalytic transformations like steam reforming, methanol synthesis, higher alcohol synthesis
904 and N20 decomposition [10-12]. The physicochemical properties of these unusual solids are entirely different from the solids obtained by conventional ceramic routes. The synthesis of such materials for a desired catalytic reaction is the prime objective of solid state chemistry and catalysis [13]. The objective of the present investigation is to study the change in structure, stability and reactivity of cobalt containing hydrotalcites with various trivalent metal ions as a function of preparation methods and composition and characterising their thermally calcined products. EXPERIMENTAL The HT-like compounds are prepared by sequential precipitation wherein NaOH/Na2CO 3 mixture is added to the metal nitrate solutions at room temperature with increasing pH. These compounds are also prepared by coprecipitation under low supersaturation conditions wherein both the precipitants and metal nitrates are added simultaneously holding the pH between 9-10. The final pH of the solution was kept at 10 in both the cases. The slurry obtained is aged at 65~ for 24h, filtered, washed thoroughly with distilled water and dried at 80~ overnight. Hydrothermal treatments are performed at 110~ for 2 days in a teflon autoclave under autogenous conditions. The chemical compositions of these materials are determined by inductively coupled plasma emission spectrometry (Model 3410, ARL7) by dissolving the compounds in minimum amount of hydrochloric acid. X-ray diffraction of these samples are taken in Philips X-ray generator (Model PW 1330) using CoK a radiation (k = 1.7902A). The lattice parameters are calculated using least square fitting of the peaks mainly considering the peaks whose 2{}>40~ IR absorption spectra are recorded using FT-IR spectrometer (Perkin-Elmer Model 1760) in the form of KBr discs. TGDSC studies are carried out in Perkin-Elmer TG-DSC/7 at the heating rate of 10~ under nitrogen atmosphere. Surface area measurements, using BET method of adsorption of N 2 at 77K, are carried out in Carlo Erba Model 1800 automatic sorptometer. Catalytic tests are carried out in an all glass recirculatory static reactor. About lg of the precursor namely the hydrotalcite is employed for the catalytic studies. Thermal calcination of the material was done in vacuum to generate "in situ" mixed metal oxides which are active catalysts. The decomposition of N20 was carried out at 50 torr initial pressure of the gas in the temperature range 150~176 The details regarding the activation procedure is mentioned elsewhere [14]. R E S U L T S AND D I S C U S S I O N Table-1 shows the composition and the phase obtained for the various samples synthesised. The closeness in the values between calculated and measured composition indicates the completion of precipitation. XRD of the samples showed the single phase formation of liT-like phase exhibiting sharp and symmetric reflections
905
Table 1 Composition and phase obtained of the samples synthesised Sample Code
Composition
Preparation Method
Cat A Co-A1 Sequential Cat B Co-A1 Sequential Cat C Co-A1 Sequential Cat Dd Co-A1 Sequential Cat E Co-A1 Low super a Cat F Co-Fe Sequential Cat G Co-Fe Sequential Cat H Co-Fe Low super Cat I Co-Fe Low super Cat J Co-Cr Sequential Cat K Co-Cr Sequential Cat L Co-Cr Low super Cat M Co-Cr Low super a - Low supersaturation preparation method b - Calculated c - Observed d - Hydrothermally treated e - not determined f - HT + possibly hexagonal Co(OH) 2
M2+/M 3+ atomic ratio 2.0 b 2.5 3.0 n. e 3.0 2.0 3.0 2.0 3.0 2.0 3.0 2.0 3.0
2.0 c 2.5 3.0 3.0 1.6 2.8 2.3 3.6 1.9 2.8 1.9 3.4
Phase obtained HT HT HT HT HT HT f HT HT HT amorphous amorphous vw HT HT
for (003), (006), (110) and (113) planes and broad and asymmetric reflections for (102), (105) and (108) planes characteristic of clay minerals possessing layered structure. These materials have a rhombohedral 3Rm symmetry with a and c unit cell parameters calculated by least square fitting of the peaks. The lattice parameters and surface area of some of the hydrotalcites are given in Table-2. It can be inferred that the difference in the values of unit cell volume of Co-A1, Co-Fe and Co-Cr-COs-HTlcs are in good agreement with the ionic radii of the trivalent element [15]. Comparison of Cat A and Cat C indicated that as the composition increases (Co/A1 atomic ratio) both the lattice parameters increases with consequent increase in the unit cell volume. The increase in the lattice parameter 'a' can be attributed to higher ionic radii of Co2§ (0.74A) in comparison with A18+ (0.50A) and increase in 'c' parameter is due to reduced electrostatic interaction between the layer and the interlayer network. The preparation method significantly influence the crystallinity of the materials synthesised. Compounds synthesised under low supersaturation (LS) conditions are more crystalline than by sequential precipitation (SP). Furthermore, the crystallinity also s with increase in atomic r a t i o . Hydrothermal treatments increased the crystallinity of the material. This result is corroborated with the reduction in the surface area of the hydrothermally treated samples. However, hydrothermal treatments performed on Co-Fe-CO3-HTlcs resulted in the
906 ii
i
,ll
i
I
Cat M
o >,
Cat
D
_=
(003) I Cat A
8O
_
_ (006)
I
I
60
40
.
J
I
I
,,
20
0
40oo
I
2oo0
I
iooo ~oo
Wavenumber (cm-1)
29 (degrees) Fig. 1 XRD patterns of some of the hydrotalcites prepared. ..
Fig.2
F T - I R spectra of the samples synthesised.
907 spinel formation indicating its thermal instability under these conditions. shows the XRD patterns of various samples synthesised.
Fig.1
Table 2 Unit cell parameters and surface area of the samples synthesised Sample Code Cat Cat Cat Cat Cat Cat Cat Cat Cat Cat
A B C D E G H I L M
Unit cell parameters a (A)
c (A)
V (A 3)
3.073 3.077 3.080 3.084 3.078 3.129 3.108 3.111 3.107 3.117
22.782 22.955 23.091 23.227 23.304 22.811 22.523 22.682 22.923 22.885
186.3 188.2 189.7 191.3 191.2 193.4 188.3 190.1 191.6 192.5
Surface area (m2/g) 64.8 27.7 69.4 42.5 35.0 88.7 60.7 72.2 250.7 155.6
The crystallinity of the material is also dependent on the nature of the trivalent metal ion present in the network. Co-AI-CO3-HT are more crystalline in comparison with Fe and Cr containing samples. In the case of Co-Cr-CO3-HT, preparation by sequential precipitation yielded amorphous material whereas preparation under low supersaturation resulted in a better crystalline material. In our Co-Fe containing samples, it is not completely possible to exclude the presence of hexagonal Co(OH) 2 prep_ared under sequential precipitation. These results indicated that presence of Al 3+ favours the formation of crystalline HTlc phase which is in accordance with the results observed by Clause et al for nickel containing hydrotalcites [7]. T E M results showed spherical to hexagonal platelets of thin and wide nature characteristic of these materials [3]. FT-IR absorption spectra of these materials, given in Fig.2, showed prominent bands around 3400cm "1,1630cm 1 and 1370cm" 1 corresponding to vOH stretching, vOH bending and v 3 carbonate stretching respectively, confirming the presence ofhydroxycarbonates. Absence of band around 3650cm'~indicates that all O H groups in the structure are hydrogen bonded and no free hydroxyl group is present [2]. Bands v 2 (out of plane deformation) and v 4 (in plane bending) of carbonate are observed around 870 and 680cm "I respectively. Differences noticed for all bands between observed vibrations of carbonate and free carbonate anion indicates even perturbation of anion in the interlayer. Bands observed at less than 1000cm "I are attributed to the lattice vibrations like M - O stretching and M - O - M bending vibrations [16]. A sharp band around 1600cm "I is observed for Fe and Cr containing samples suggests that it is not only due to water bending vibrations and but also due to the presence of bicarbonate anions [17].
908
----
113
....
DTG
Cot M ,
.~~/f-
I
,
'
Cat K
Cat I
,j ~
.. ...........
tla
tl -r"
D
\ ,~.s
c~ D
I!
,
l
Cat C C
v !_ 60
I.
200
I
1
_._
/.10 620 Temperature (~
Fig. 3 TG-DTG patterns of the samples prepared.
I
800
I 50
Fig. 4
i
,,
_':'-T--
160 310 /,60 Temperature ('C) DSC traces of some of the hydrotalcites.
909 Cat C exhibited a doublet at 1380 and 1365cm "1 for v 3 stretching of carbonate, which can be attributed to lower symmetry of carbonate present in the interlayer (D3h symmetry distorted to C2v). This can also cause activation in v I mode observed around 1020cm "1 [18]. However, upon hydrothermal treatment (Cat D) a singl_e sharp band around 1365cm "1 is observed, indicating the enhanced ordering of CO82" ion in the interlayer. Nevertheless, this value is very much lower than that of free CO82" species [19] (1415cm'1), indicating that a strong electrostatic interaction exists between hydroxyl group and H20 molecules in the interlayer with carbonate species. In the case of Co-Fe and Co-Cr containing samples such doublets are observed even ai%er h~drothermal treatments, which clearly shows that a large degree of disorder of CO8~" species in the interlayer. This result is corroborated with X-ray results exhibiting low crystallinity of these samples. A closer examination of vOH band for aged samples indicated that as the M2+]M3+ ratio decreases (compare Cat A and Cat C) the band is shifted to lower wave numbers. This shift could be due to depletion of electron density around OH group bonded to A13+ ion by polarisation. Such shifts were not observed for Co-Fe and CoCr containing samples indicating the weak polaris~bility of Fe 8+ and Cr 8+ in comparison with A1~+. Fig.3 shows TG and its differential curve for some of the hydrotalcites. Most of the samples showed two stages of weight loss wherein the first weight loss occurring in the temperature range 150-250~ is attributed to the removal of physisorbed and interlayer water molecules and the second weight loss occurring in the temperature range 250-350~ ascribed for dehydroxylation between the sheets and decarbonation (loss of CO 2) leading to the destruction of the layered structure [20]. However, Cat C showed the second weight loss occurring in two stages. The first can be assigned for partial dehydroxylation within the layer and second is due to complete dehydroxylation and decarbonation[21]. In the case of Fe containing HTs irrespective of the preparation conditions, the release of interlayer water, structural water and CO 2 occur simultaneously in the temperature range 150-200~ However, for Co-Cr-HTs a better thermal stability is achieved for the samples prepared under LS conditions in comparison with SP conditions, owing to the better crystallinity of the former sample. For hydrothermally treated materials, the second weight loss splits into two peaks without affecting T 1. This could be due to better ordering in the interlayer space leading to step wise losses. Marchi et al [22] proposed that the presence of new peaks in the hydrothermally treated sample can be attributed to heterogeneity of the precipitate obtained. However, our X-ray results showed that the samples are more crystalline and single phase in nature. The net weight loss and transition temperatures for some of the samples are reported in Table-3. It can be clearly seen from the Table that as M 2+/M~+ ratio increases the transition temperatures T~ and To decreases. This can be explained by considering that upon M 3+ substitution ~by M 2~ in the network, the positive charge density of the layer increases and thus enhancing the electrostatic interaction between the layer and interlayer. DSC results, given in Fig.4, substantiated TG results showing two endotherms corresponding to two weight losses. The DSC transition temperature, although slightly higher than TG temperatures, showed a
>r
I0
Q.
p,
R
8
o~
-rl uQ
~'$
0
"9
..
w O
v
O
8
~,
~N
-".~~.
5"~" ,=1
:3
n_.
o
lID
~o
910
o
o
O
~>.
!
(arb. units)
8
Intensity (orb. units )
Intensity
v
O
-.__ 9 I~
0
O
w ._= ._= v
911 Table-3 TG transition temperature and net weight loss of the samples
Sample
Cat Cat Cat Cat Cat Cat Cat Cat Cat Cat Cat
A B C D E F G H I L M
Transition temperature (~ T1
T2
198 190 171 179 179 171 167 173 163 185 153
274 252,300 245 232,290 235 222 201 244 206 268 256
Net weight loss (%)
33.0 30.9 29.7 29.2 29.8 28.7 23.3 24.0 26.7 31.3 26.2
similar trend in the temperature which decreases with increase in M2+/M 3+ atomic ratio. Comparison of DSC curves of aged and hydrothermally treated samples showed an interesting observation that the curves are more intense for hydrothermally treated samples which is indicative of the higher crystallinity.
Characterisation of thermally calcined catalysts: Thermal calcination of these materials at 400~ in air are given in Fig.5. All the compounds showed non-stoichiometric spinel phase independent of M 3+ ion. The non-stoichiometry can be explained by the differences in the values obtained in lattice parameters and IR band positions between calcined catalysts and stoichiometric compounds. In the case of Co-A1-HTs', as the atomic ratio increases, more amount of Co304 is formed (by oxidation of Co2+). The band position shifts to higher frequency as the Co/A1 ratio increases indicative of the formation of solid solution. This result is substantiated by X-ray results showing that the lattice parameters calculated are intermediate between CoA120 4 and Co30 4 (8.105A and 8.084A). Similar behaviour is observed for Co-Cr and Co-Fe systems on variation with elemental composition. These results are in accordance with the results obtained by Busca et al [23] and Uzunova et al [24] respectively. However, detailed study on thermally calcined materials will be published elsewhere. Fig.6 shows the variation of X-ray pattern of Cat B with calcination temperature. As the temperature increases, the crystallinity of the obtained spinel increases as evidenced from the increase in the intensity and sharpness of the peaks. The lattice parameter calculated for these materials showed that as the temperature increases, approach of the stoichiometry is achieved. This result can be substantiated with surface area measurements (Fig.7), which decreases with increase in the
912
O
o
L_ O
I!
O
~-' [3' <~'
~
~NNN~ ~
lo
I
O
I
0
.r
o
c~
O
o o o
~NNNNNNNN ~NNNNN -o
O
uo!sJe^UOD %
C~
/
I
/
LO
l!Jil
O O
=
r E-
%
0
~
~.
Vt (..)
~
>og~ u ~ ~~~._ g
O
m
r,:.= ~ ~
913 calcination temperature. However, an initialincrease in the surface area is observed which could be due to 'crater'formation as observed by T E M [25],through which C O 2 exit.
Catalytic activity of N20 decomposition: Catalytic activity of N20 decomposition on various t h e m a l l y calcined catalysts are reported in Fig.8. The activity followed the order Co-A1-HT>Co-Fe-HT>>Co-CrHT. The low activity of Co-Cr catalysts can be attributed to low activity of n-type Cr20 3. Generally for N20 decomposition p-type oxides are more active t h a n n-type oxides owing to the cumulative adsorption in the former [26]. We presume that Cr ~+ segregates on the surface in Co-Cr catalysts, and thereby covering the active cobalt sites. Such segregation is also observed for Co-Fe-HT, although the extent of is small [24]. In the synthesis of hydrocarbons on Co-Cr catalysts, it is reported that even with high Co contents, a low activity is observed indicating that Co alone is not responsible for the activity [27]. With increase in the Co/A1 atomic ratio (compare CatA and Cat C), an increase in the activity is observed which could be explained by the fact that the number of adsorption centers increases on the surface. Our XPS results also confirmed that the Co/A1 surface composition increases with increase in the bulk composition although the former is lower in value. The activity of the present samples are compared with some of the most active catalysts reported in the literature [28]. Under our experimental conditions, Co-A1-HTs' showed appreciable conversion even at 150~ and Cat C showed 100% conversion at 250~ which is 100~ less than the most active catalyst reported so far. Analysis of the spent catalysts by XRD showed nonstoichiometric spinel type oxides. The high activity for Co-A1-HTs' can be explained based on the specific interaction between Co 2+ and Co 3+ ion with the support responsible for the generation of the active sites. However, a detailed study is required in understanding the nature of active centers. CONCLUSIONS The effect of preparation procedure, nature of trivalent metal ion and elemental composition on various physicochemical properties are summarised as follows: a. Compounds prepared under the composition range 2.0<M2+/M 3+<3.0 yielded single phase hydrotalcite. Better crystalline material is obtained by preparing under LS conditions with higher atomic ratio. b. Nature of the trivalent metal ion and M2+/M 3+ atomic ratio modifies the thermal stability. Co-A1-HTs' are more stable in comparison with Co-Fe and Co-Cr samples. Increase in the atomic ratio decreases the thermal stability due to electrostatic interaction. c. Thermal calcination of these materials resulted in non-stoichiometric spinel whose stability and reactivity are strongly influenced by nature of trivalent metal ion and by the composition. d. Catalytic activity of N20 decomposition showed that Co-A1-HTs' are the most active followed by Co-Fe and Co-Cr catalysts respectively.
914 ACKNOWLEDGEMENTS
The authors thank Air Products Chemicals Inc.,U.S.A for the research grant REFERENCES I,
2. 3. 4. 5. ,
7. ,
9. 10.
11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28.
R.Allmann, Acta Crysallogr. 24 (1968) 972. S.Miyata, Clays Clay Miner., 23 (1975) 369. W.T.Reichle, Solid State Ionics, 22 (1986) 135. K.A.Corrado, A.Kostapapas and S.L.Suib, Solid State Ionics 26 (1988) 77. T.J.Pinnavaia, NATO ASI Ser., Ser.C (Zeolite Microporous Solids: Synthesis, Structure and Reactivity, 1992) p.91. W.T.Reichle, J. Catal, 94 (1985) 547. O.Clause, M.Gazzano, F.Trifiro,A.Vaccari and L.Zotorski, Appl. Catal., 73 (1991) 217. F.Cavani, F.Trifiro and A.Vaccari, Catal. Today, 11 (1991) 173. T.Sato, H.Fujita, T.Endo and M.Shimada, React. Solids, 5 (1988) 219. E.C.Kruissink, L.L.Van Reijen and J.R.H.Ross, J.Chem.Soc., Faraday Trans. I, 77 (1981)665. M.Piemontese, F.Trifiro, A.Vaccari, E.Foresti and M.Gazzano, in G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Prep. Catalysts V, Elsevier, Amsterdam, 1991 p.49. S.Kannan and C.S.Swamy, Appl. Catal.B., 3 (1994) 109. H.Yanagida, Angew. Chem. Int.(Eng) Edn., 27 (1988) 1389. J.Christopher and C.S.Swamy, J.Mol.Catal., 62 (1990) 69. R.D.Shannon and C.T.Prewitt, Acta CrystaUogr., B25 (1969) 925. M.J.Hernandez-Moreno, M_A.Ulibarri,J.L.Rendon and C.J. Serna, Phys. Chem. Miner., 12 (1985) 34. L.Pesic, S.Salipurovic, V.Markovic, D.Vucelic, W.Kagunya and W.Jones, J. Mater. Chem., 2 (1992) 1069. F.M.Labojos, V.Rives and M.A.Ulibarri, J.Mat.Sci., 27 (1992) 1546. K.Nakamoto, Infrared and R a m a n Spectra of Inorganic and Coordination Compounds, 3rd ed.,Wiley, N e w York, 1978. S.Kannan and C.S.Swamy, J.Mat.Sci.Lett., 11 (1992) 1585. F.Rey and V.Fornes, J.Chem.Soc., Faraday Trans. 88 (1992) 2233. A.J.Marchi, J.I.Di Cosimo and C.R.Apesteguia, in M.J. Phillips and M. Ternan (Editors), Proc. 9th Int. Congress on Catalysis, Ottawa, 1988, Vol.2, p.529. G.Busca, F.Trifiro and A.Vaccari, Langrnuir, 6 (1990) 1440 E.Uzunova, D.Klissurski, I.Mitov and P.Stefanov, Chem. Mater., 5 (1993) 576. W.T.Reichle, S.Y.Kang and D.S.Everhardt, J.Catal., 101 (1986) 352. P.Pomonis and J.C.Vickerman, J.Catal., 90 (1984) 305. F.Trifiro and A.Vaccari, in R.K. Grasselli and A.W. Sleight (Editors), Structure-ACtivity and Selectivity Relationships in Heterogenous Catalysis, Elsevier, Amsterdam, 1991, p.157. Y.Li and J.N.Armor, Appl. Catal.B., 1 (1992) L21.
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparationof HeterogeneousCatalysts G. Ponceletet al. (Editors) 1995 Elsevier Science B.V.
915
S y n t h e s i s of silver s u p p o r t e d c a t a l y s t s w i t h n a r r o w p a r t i c l e size distribution S.N. Goncharova, B.S. Bal'zhinimaev, S.V. Tsybulya, V.I. Zaikovskii, A.F.
Danilyuk Boreskov Institute of Catalysis, Siberian Branch of Russian Academy of Sciences, 630090 Novosibirsk, Russia 1.
INTRODUCTION
Silver is the only efficient catalyst for ethylene epoxidation providing the minimum of by-products. Why is it so unique? The question still remains open. Supported silver catalysts have one peculiar property: the influence of the size of the silver particles on the reaction rate is observed within particle sizes of 1001000 ~ [1-4]. This effect is explained by the change of surface structure, silver particle morphology, influence of support, etc. Probably, the effect of all these factors as well as a r a t h e r wide Ag particle size distribution causes the spread in the values of size effect as well as of crystal sizes, so t h a t the drastic change of the catalytic activity itself takes place. Supports characterized by a large surface, such as SiO2, WiO2 and TI-AI203 [1,4-5] are the most popular model supports for Ag catalysts. These supports allow to obtain highly dispersed silver catalysts with average particle diameter of 30-100 /~ [4-6]. Catalysts with a higher average size of Ag particles are quite polydisperse, since it is very hard to get rid of small particles [6,7]. It is even h a r d e r to prepare catalysts with a narrow Ag particle size distribution supported on a l u m i n a with a small surface. In all cases, the preparation of Ag catalysts on a-A1203 via conventional deposition and drying produces a polydisperse Ag particle size distribution. Probably, this results from a weak metal-support interaction. Indeed, Ag particles are known to migrate over alumina surface even under rather mild conditions [8]. Iri this paper, we report on a new synthesis procedure to produce catalysts with a r a t h e r uniform Ag particle size distribution. We prepared a series of such samples with particle sizes ranging from 100 to 1000/~. As a support, we used aA1203 with a quite high specific area (7m2/g) and SiO2 (150 m2/g). We have characterized the catalysts with X-ray, TEM, HREM techniques and oxygen chemisorption. Within a narrow range of particle diameters D = 300 - 500 A, we observed a considerable size effect providing a 20-fold increase of reaction rate. We discuss here the possible reasons for the observed phenomenon.
916 2. ~ E R I M E N T A L
2.1. Catalyst synthesis To avoid undesired effects of admixtures (alkaline metals a n d C1 in particular), all initial reagents for the synthesis of catalysts and supports were of the special pure grade or synthesized u n d e r conditions excluding sample contamination. The same demands towards the equipment used for the synthesis were followed. Amorphous silica was prepared by mixing the silica gel powder Aerosil A175 with a m m o n i u m acetate solution. The obtained paste was molded via extrusion and dried in air first at room temperature, then at 100 ~ for 10-15h. The samples were calcined under air flow at 550 ~ for 10-15h. a-Alumina was obtained via thermal decomposition of T-AI203 for 12h at T = 1250 ~ T-Alumina was obtained via thermal decomposition of pseudoboehmite at 500 ~ for 5 h. The latter was synthesized via aluminium nitrate solution treatment with ammonia. To prepare Ag/SiO2 and Ag/a-Al203 catalysts, we used the incipient wetness procedure [9]. Silver was deposited via impregnation of the support with the silver amine complex in ethanolamine with molar ratio Ag: NH2C2H4OH = 1 94. To obtain catalysts with a uniform Ag particle size distribution, 2 regimes of drying were used: a) ordinary heating at 90 ~ in vacuum; b) adsorptioncontact drying at room temperature [10]. In adsorption-contact drying, the wet sample is mixed with the precalcined and very small SiO2 granules. The size of the SiO2 granules was 10 times less t h a n that of the catalyst granules, while the amount of silica gel exceeded the catalyst mass by a factor of 10. Due to intensive agitation, the catalyst wetness quickly passed to the adsorbent. The latter was separated from the catalyst using sieves. The samples were reduced at 90 ~ in vacuum. Then the samples were washed with bidistilled water and dried in air at 100 ~ At the final stage, the catalysts were calcined in air at 240 ~ for 2h. To prepare the catalysts with various silver contents and dispersivities, we varied the concentration of the impregnating solution or repeated the impregnation procedure several times. To obtain the Ag/SiO2 catalyst, we used the sol-gel method. We synthesized silica alkogels via alkaline hydrolysis and condensation of tetramethylsilane [Si(OCH3)4] in water-alcohol solution in the presence of silver amine complexes. Then we removed methanol at 290 ~ and 120 arm and thus obtained SiO2 aerogels containing the metal silver particles.
2.2. Catalyst c h a r a c t e r i z a t i o n Ag content of the samples was determined by atomic absorption method with ASS1N (Carl Zeiss Jena). The content of alkaline metal admixtures in supports and catalysts was controlled with flame photometry (Flaphokol, Carl Zeiss Jena) and SIMS techniques. The total content of alkaline (K, Na, mainly) and alkaline earth metals (Ca, Mg, mainly) did not exceed 0.03 wt%. The porous structure of the sample was studied with mercury porosimetry (PoreSizer, Micromeritics); the specific surface area was determined with BET method using nitrogen. The phase composition of a - a l u m i n a was determined by X-ray analysis with URD-63 Freiberg (CuKa irradiation). The position of the diffraction maxima
917 and the interplanar distances were compared with reference data [1]. The synthesized support with specific surface of 7 m2/g was found to correspond to aA1203 phase. A g particle size distribution and average size of the crystals was determined for some samples from T E M and oxygen chemisorption data. T E M was performed with J E M - 1 0 0 C (resolution limit 4 ~) and J E M 400 E X (resolution limit 1.4 ~k).X-ray studies of the catalysts were performed in air in a URD-63 (Freiberg, Germany) diffractometer. The samples were powdered in an agate mortar and pressed into the holder (diameter 20 ram, thickness 2 ram). The scanning rate was 1~ (monochromatized C u K a source). Five main diffraction m a x i m a of silver with indices 1.1.1, 2.0.0, 2.2.0, 3.1.1, 2.2.2 were recorded. The silver crystal sizes were calculated with the Scherrer equation D = 0.9MBcos~) with the Warren correction B = (Bm2-Bi2) 1/2,where B m is the m a x i m u m width at half m a x i m u m intensity, Bi is the correction for instrumental broadening. Chemisorption was measured via the flow pulse technique reported elsewhere [12]. The catalytic activity of the supported silver catalysts was established via a flow circulation technique at T = 230 ~ and atmospheric pressure. The set up has been described elsewhere [13].At first,the catalyst placed in a quartz reactor was treated with a O2+N2 (7% of 02) flow for 2 h at 230 ~ The reaction rate was measured at the same temperature after the steady state had been attained (usually 3-6 h). The initial mixture composition was the following: 2 % C2H4, 7 % 02, balance N2. The catalyst sample weight (0.5 - 2 g) and flow rate of the reaction mixture feed (2-20 I/h) were chosen for the ethylene conversion under the steady state not to exceed 10-15%.
3. R E S U L T S AND D I S C U S S I O N 3.1. S a m p l e s w i t h a n a r r o w A g p a r t i c l e s i z e d i s t r i b u t i o n Table 1 lists such properties of catalysts as: average particle size, surface, silver content. On the whole, we observe a satisfactory coincidence of the average sizes determined by TEM and oxygen chemisorption techniques. Note t h a t the silver lattice parameters found with the X-ray analysis correspond with a good accuracy to the reference values for the bulk silver. Thus, the silver atoms in the bulk of the particles (hundreds of Angstroms in size) are identical to those in the structure of the bulk metal. Moreover, according to HREM, there are no data proving metal-support interactions. The incipient wetness and adsorption drying allow to narrow the particle size distribution (see Fig. 1). Most probably, this results from a very fast water removal from the porous space. A slow crystallization from solution, occurring upon heating, leads, as a rule, to a wider distribution due to the continuous formation and growth of nuclei. It seems likely t h a t with a d s o r p t i o n - c o n t a c t drying, the new crystallization nuclei have not managed to form and grow. We have managed to obtain a narrow Ag particle size distribution on SiO2 using the sol-gel method (Fig. 2a) without adsorption-contact drying.
918
20
a
0
75
150
225
300
375
20"
25-
b 2G
uJ .-I C.)
r
| i
~10a. 5 ,
6O
120
,
180
I
I
I
240
Figure 1. Histograms of the silver particles size distribution for 1% Ag/SiO2 catalysts p r e p a r e d with: (a): impregnation of the support; (b): p r e l i m i n a r y support wetting; (c): (b) combined with a low-temperature adsorption-contact
dr~ng.
Silver particles deposited on oc-A1203 (unlike those on SiO2) are not stable at elevated t e m p e r a t u r e s . Thus, at the calcination step, the particle size distribution changes and becomes polydisperse (a typical range is 50 - 1000/~). Table 1. Structural characteristics of supported silver catalysts Specific Ag content DTEM Catalyst surface of Ag (%, wt) (A)
(m2/g) Ag+oc-A1203
0.15 0.16 0.75 0.38 0.50 0.53 0.93 0.52 0.62
0.4 0.6 2.4 1.5 2.7 3.7 6.5 5.2 13.8
160 200 184 243 306 400 450 56O 1000
Dchem
cA) 130 200 300 250 560 1260
919
Ag/SiO2
0.38 1.7 4.6 5.2 2.6
0.67 5.0 16.0 25.6 18.4
100 165 189 280 400
330
Apparently, this is due to the absence of m e t a l - s u p p o r t interaction, so Ag particles are highly mobile. To decrease the particle mobility, we modified the aAlumina surface. The interaction of alkaline metals with silica [14] and a l u m i n a [15] is known to change significantly the acid-base properties of the supports, due to exchange of protons by ions of the alkaline elements. In p a r t i c u l a r , Na addition increases the electron density on the neighbouring oxygen atoms. Note t h a t the effect of Na is not local, it extends over considerable distances (7-8/~). The increase of oxygen anion radius due to the transfer of electron density from the alkaline m e t a l can cause the t e n s i o n of the lattice, especially in the subsurface layers. The tension relaxation can provide a considerable surface disordering and defects. We can expect the Cs effect to be even more pronounced.
ib
100nm
i
Figure 2. Typical TEM micrographs of Ag/SiO2 and Ag/a-A1203. a. 1% Ag/SiO2 ; b. 6.5 % Ag/a-A1203.
920 We modified the surface of the initial a-alumina support by treating with CsOH solution followed by drying and calcination at 240 ~ in air. To remove Cs, we boiled the support for 5h and washed it with bidistilled water. To control the process, the Cs concentration was analyzed with SIMS (sensitivity towards Cs < 10-3%). Using the modified support we managed to narrow considerably the Ag particle size distribution (Fig. 2b, 3) and increase the stability of the catalyst. The particle size deviation from the average did not exceed 30-40 %. TEM studies showed t h a t the particle size distribution did not change after the catalyst calcination and treatment with reaction mixture. Probably, the restriction of the Ag particle mobility on the modified support results from the "point" interactions of silver with a-A1203, which are hard to evidence by t r a n s m i s s i o n electron microscopy.
32
32
24
24
m
16
~8 -~ I I I-n r-n 4OO D, A i
200
i
400
|
|
1000
1600
F1
D,A
Figure 3. TEM crystallite size distributions of Ag/a-A1203 catalysts with silver content of 0.4% wt (a) and 13.8% wt (b).
3.2. X-ray analysis Table 2 presents the region of coherent scattering (r.c.s.), determined from the broadening of the diffraction maxima 1.1.1, 2.0.0, 2.2.0, as well as the Ag particle sizes determined from the TEM data for silver catalysts. For the small silver particles (DTEM < 200/~) the diffraction m a x i m u m 2.0.0 is so broadened that we failed to measure its width at half maximum intensity. The TEM data give no evidence of any dominating orientation or anisotropy of the silver particle size or shape. Therefore, we assume t h a t the difference in the r.c.s, for the different crystal directions in the silver particles results from the stacking faults, which appear at the catalyst preparation stage. When the particle size increases to 250-300 A, metal particles gain a regular structure, since effective size along all the crystal directions is approximately the same. When the particle size exceeds 300 A, the particles possess a domain structure because the TEM size is higher t h a n the X-ray one. DTEM/DX-ray ratio characterizes the average number of i n t e r g r a i n boundaries for silver particle of average size. For the small particles, this ratio is close to 1, which corresponds to imperfect silver particles consisting of two domains separated by intergrain boundary. The r.c.s, increases simultaneously with the increase of DTEM, but to a lesser extent. Thus, the stacking faults and intergrain boundaries are the most typical extended defects in the samples studied. The in situ X-ray studies of these
921 samples show that stacking faults disappear under the reaction conditions, thus narrowing the width at half maximum intensity of the 2.0.0, 2.2.0 peaks [16]. This process appears to be irreversible, since the width of these peaks does not change after cooling. At the same time, the r.c.s, does not grow considerably during the experiment. Thus, we may consider that the presence or absence of domains separated intergrain boundaries is the main factor determining the bulk structure of Ag particles. Table 2. X-ray and TEM parameters of supported silver catalysts Catalyst DT~M, D ~ I , D~0, D~0, DTEM/DX-ray ....Ag/a'-A1203
Ag/Si02
3.3. The size
185 225 250 310 400 400 780 1000 1400
200 180 280 300 250 280 470 600 720
125 162 400
135 170 .
-
250 270 150 235 290 450 430 50 90 .
.
REt~ mol. C2H4/m2s
170 160 230 320 180 200 390 450 520
0.9 1.2 0.9 1.0 1.55 1.4 1.65 1.7 2.0
0.2 0.65 0.1 0.6 1.9 2.3 4.9 3.8 5.4
90 150 .
0.9 0.95
0.1 0.2 4.09
effect
The r a t e of ethylene epoxidation increases by 20-fold within a r a t h e r narrow particle size range AD - 300-500/~ (see Table 2). Such a pronounced size effect was not observed earlier. We think t h a t this effect results from a narrow Ag particle size distribution and from the absence of noticeable admixture amounts as well. For a wider particles size distribution, the size curve of the reaction rate should smear. So, the size effect value (AD values) will vary within a wide range depending on the relative portions of small or large particles in a catalyst. Within 500 - 1000/~, the catalytic activity practically does not change. Note that the size effect does not depend on the nature of the support. This fact tells t h a t the silver properties determine the catalyst activity towards ethylene epoxidation, while the contribution of side reactions involving ethylene oxide and support is not essential. In our previous study [17], we analyzed in detail the reasons providing the size effect observed. We found that the structural and electronic properties of silver surface layers are changed considerably with the growth of the Ag particles. This greatly affects the catalytic and adsorption behaviour of silver. In particular, we have assumed that the size effect results from the change in the ratio of regular and defect surface regions of Ag. Ethylene epoxidation is likely to proceed at the boundaries between such surface regions. The data obtained in this study agree well with our assumptions on the origin of the size effect.
922 Indeed (see Table 2), we observe a good correlation between the rates of ethylene epoxidation and DTEM/DX-ray ratio, characterizing the n u m b e r of intergrain boundaries in the silver particles of average size. For the:particles with D < 300/~, Ag particle is a monolith, while for those with D > 400 A, Ag has a domain structure. Appearance of domains can change significantly the structure of silver surface and the ratio of regular and defect regions in particular. Thus, the size effect seems to result from the increase of the concentration of the active sites at the vicinity of intergrain boundaries. REFERENCES 1. J.C. Wu and P. Harriott, J. Catal., 39 (1975), 395. 2. M. Jarjoui, B. Moraveck, P.C. Gravelle, S.J. Teichner, J. Chim. Phys., 75 (1978), 1061. 3. X.E. Verykios, F.P. Stein, R.W. Couglin, J. Catal., 66 (1980), 368. 4. S.R. Seyedmonir, J.K. Plischke, M.A. Vannice, H.W. Young, J. Catal., 123 (1990), 534. 5. S.R. Seyedmonir, D.E. Strohmayer, G.J. Guskey, G.L. Geoffroy, M.A. Vannice, J. Catal., 93 (1985), 288. 6. J.K. Plischke, M.A. Vanice, Appl. Catal., 42 (1988), 255. 7. J.K. Lee, X.E. Verykios, R. Pitchai, Appl. Catal., 50 (1989), 171. 8. E. Ruckenstein and S.H. Lee, J. Catal., 109 (1988), 100. 9. M.B. Palmer, M.A. Vannice, J. Chem. Technol. Biotech., 30 (1980), 205. 10. L.I. Keifets, A.V. Neimark, Multiphase processes in porous media, Khimiya, Moscow, 1982, p. 279 (in Russian). 11. JCPDS DATA FILE No 10-173. 12. V.Yu. Gavrilov, in R.A. Byanov, N.N. Bobrov (Editors), Standardization of methods, apparatusses and devices for the control over commercial catalysts quality, Proc. USSR Symp., Novosibirsk, September 25-27, 1991, p. 128 (in Russian). 13. S.N. Goncharova, A.V. Khasin, S.N. Filimonova, D.A. Bulushev, Kinet. Katal., 32 (1991), 852. 14. A.P. Kouznetsova, E.A. Paukshtis, N.U. Zhanpeisov and G.M. Zhidomirov, React. Kinet. Catal. Lett., in press. 15. E.A. Paukshtis, K. Jiratova, R.I. Soltanov and E.N. Yurchenko, Collect. Czech. Chem. Comm., 47 (1983), 2044. 16. S.V. Tsybulya, G.N. Kryukova, S.N. Goncharova, A.N. Shmakov, B.S. Barzhinimaev, submitted for publication. 17. S.N. Goncharova, E.A. Paukshtis, B.S. Bal'zhinimaev, submitted for publication.
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
923
Preparation of supported platinum catalysts by liquid-phase reduction of adsorbed metal precursors M.Arai, K.Usui, M.Shirai and Y.Nishiyama Institute for Chemical Reaction Science, Tohoku University, Katahira, Aoba-ku, Sendai 980, Japan Supported platinum catalysts were prepared at room temperature by the adsorption of metal precursors followed by the reduction with sodium tetrahydroborate solution. It was shown that the alumina-supported catalysts so prepared were not only highly active for liquid-phase hydrogenation of cinnamaldehyde but also highly selective for the formation of cinnamyl alcohol at atmospheric pressure of hydrogen and 308 K. The prepared catalysts seemed to be different in the state of dispersion of platinum particles as compared to those prepared by usual hydrogen reduction at 773 K. 1. INTRODUCTION Supported metal catalysts are prepared in a variety of ways and the preparation procedures mostly include thermal treatments like activation and reduction following the loading of metal precursors [1]. Those treatments usually require high temperatures above 600 K and they often give a crucial impact on the activities of the finished catalysts. So, non-heat-treated catalysts are expected to indicate different catalytic properties, due to some physical/chemical difference in the state of dispersion of metal particles. The present work was undertaken to examine this possibility by trying a new method of low-temperature catalyst preparation. The method studied involves the adsorption of metal precursors on supports and the reduction by sodium tetrahydroborate solution for the preparation of supported platinum catalysts. The adsorption and reduction of platinum precursors are carried out at room temperature and the highest temperature during the preparation is 390 K for the removal of solvent. The activities of the catalysts prepared were examined for liquid-phase hydrogenation of cinnamaldehyde under mild conditions. Our attention was directed to not only total activity but also selectivity to cinnamyl alcohol, since it is difficult for platinum to hydrogenate the C=O bond of this ct,13-unsaturated aldehyde compared to the C=C bond [2]. We examined the dependence of the catalytic activity and selectivity on preparation variables including metal precursor species, support materials and reduction conditions. In addition, the prepared catalysts were characterized by several techniques to clarify their catalytic features. The activity of the alumina-supported platinum catalyst prepared by the present method was briefly reported in a recent communication [3].
924 2. EXPERIMENTAL 2.1. Catalyst preparation An AI203 gel, Neobead C (Mizusawa Industrial Chemicals, Ltd.) with a surface area of 126 m 2 g-X and an SiO 2 gel, Davisil grade 646 (Aldrich Chemical Company, Inc.) with 240 m 2 g-1 were used as supports. Platinum precursors were adsorbed by immersing the supports in an aqueous solution of H2PtC16 (pH 1.75) or Pt(NH3)4C12 (pH 11.8) in the dark at room temperature for 5 days. Then, the remaining solution was removed by filtering and drying in vacuum at 390 K for 3 h. The amounts of Pt adsorbed were determined from the amounts of the precursors remaining in the solutions after the adsorption measured by atomic absorption spectroscopy (AAS). The amounts of Pt (metal) loaded were adjusted to about 1 wt%. The precursor/support samples obtained were reduced by a 0.1 M NaBH 4 solution (pH 11.6) at 308 K. The mole ratio of NaBH 4 to Pt and the time of reaction were changed by 200 to 700 and by 30 min to 3 h, respectively. In most cases, the reduction was conducted at an NaBH4/Pt ratio of 400 and for a reaction time of 30 min. For the results obtained under such conditions, they are not specified in the following. The reduced samples were washed with distilled water several times and vacuum dried at 333 K. Control catalysts were prepared by reducing the same precursor/support samples with 1-12at 773 K for 3 h. The catalysts prepared through the reductions with NaBH 4 and 1-12are hereinafter referred to as LR (Low-temperature Reduction) and HR (High-temperature Reduction) catalysts, respectively. 2.2. Activity measurement and characterization The catalytic activity was tested with liquid-phase hydrogenation of cinnamaldehyde (CAL). Using a 0.75 g of catalyst, a 0.5 ml of CAL in a 5 ml of C2HsOH was hydrogenated with 1-12at the atmospheric pressure in a well-stirred glass flask at 308 K. The reaction products were analyzed repetitively by gas chromatography. The catalysts were handled carefully to avoid exposure to the atmosphere during the preparation and on the introduction into the reaction mixture. The surface properties of supported Pt particles were examined by gas adsorption and X-ray photoelectron spectroscopy (XPS) with Shimadzu ESCA-750. The uptake of 1-12and CO was measured by a static volumetric method at 292 K and an equilibrium pressure of about 25 kPa. In XPS, the catalysts were exposed to air on the introduction into the system, and the spectra for Pt and other species were collected after Ar + sputtering. The bulk structure of the particles was examined by extended X-ray absorption fine structure (EXAFS). Pt Lm-edge EXAFS spectra were measured at room temperature at the BL-10B station of Photon Factory in the National Laboratory for High Energy Physics (Proposal No.93G147). In addition, the supported Pt was extracted by immersing the catalysts in a mixed acid of HC1 and HNO 3 at room temperature for 24 h and the amounts of Pt extracted were measured by AAS.
925 3. RESULTS 3.1. Catalytic activity In the present CAL hydrogenation, the main products were cinnamyl alcohol (COL) and hydrocinnamaldehyde (HCAL) along with a few minor products. Figure 1 shows typical results over A1203-supported Lit and HR catalysts from Pt(NH3)4CI2, indicating that the activity and selectivity strongly depend on the catalyst used. Figure 2 gives the total activities and selectivities averaged at 50 % conversion, where the reactions proceeded at approximately constant rates for the catalysts examined. The 100
....
I ....
i .... '
8 0 !~ Q ~ , ~ ' ~AL X~
( a ) t t
60-
~
'
'
ibl
v
to
.m
i
oQ. 4O
Eo
L) 2O 00t
_
"
, , '~ ;..; ~ , , , ~,
5
10
Reaction
time
15
0
20
(h)
40
60
Reaction
80
time
100
(h)
Figure 1. Liquid-phase CAL hydrogenation over 1 wt% Pt-loaded LR (a) and HR (b) catalysts of Pt(NH3)nC12/A1203 system. COL H2PtCI6 AI203
Pt(NH3)4CI2 AI203
Pt(NH~),CI2 SiO 2
HCAL
LR HR
1~\\\\\\\\\\\\",,",,",,",,",,\'~10th
LR
e rs
M
HR LR
Z
HR i
0
I
0.5
Activity
i
I
1 (mmol
i
I
1.5
i
I
2
gpt -1 min -1)
I
20
,
I
,
I
40 60 Selectivity
,
I
80 (%)
,
I
100
Figure 2. Total activity and selectivity for 1 wt% Pt-loaded LR and HR catalysts of the three systems.
926 Al2Oa-supported LR catalysts are highly active and highly selective to COL as compared with the HR catalysts. This is a noteworthy result because Pt is known to be a poor catalyst for the selective production of COL [2]. In contrast, the SiO2-supported LR and HR catalysts are comparable in the activity and selectivity to COL, which are smaller than those of the Al2Oa-supported LR catalysts. In addition, it was found that the highly active Al2Oa-supported LR catalysts became inactive when they were treated at 773 K in 1-12. Figure 3 shows the influence of NaBHa/Pt ratio on the activity for Pt(NHa)aC12/AI203 system. As the ratio was changed from 200 to 400, the activity became increased by a factor of 2 while the selectivity remained almost unchanged. Further increasing of the ratio to 700 did not affect the activity and selectivity. For the other two systems, the activities were little influenced by the NaBHn/Pt ratio. It was further found that the activities of LR catalysts changed little with the length of reduction from 30 min to 3 h for the three catalyst systems studied. HCAL
Others
"
o
COL
200
89%
10%
1%
L-
n
400 ,q.
-r" II1 Z
93%
7%
94%
6%
W
7'O0
n 0
I 0.5
n
Activity (mmol
I 1 g p t -1
,
I
1.5
min -1)
Figure 3. Influence of NaBH4/Pt ratio on the activity for 1 wt% Pt-loaded LR catalysts of Pt(NH3)4C12/AI203 system. 3.2. Characterization of catalysts Table 1 shows the results of H 2 and CO adsorption, indicating a marked difference between the A12Oa-supported LR and HR catalysts in the ratio of CO and H 2 uptakes. The CO/H 2 ratios of the LR catalysts are much smaller than those of the HR catalysts. When the LR catalysts were treated at 773 K in H 2, the uptake of CO decreased only slightly but that of H 2 decreased significantly, increasing the CO/H 2 ratios to be comparable to those of the HR catalysts. Structural parameters obtained from EXAFS measurements are also given in Table 1. The coordination number for the A1203-supported LR catalysts is larger than 8 although it is somewhat smaller compared to that for the HR catalysts. The Pt-Pt bond distances are similar between the LR and HR catalysts. These results imply that Pt atoms dispersed in the LR catalysts are present as three-dimensional particles and the bulk structure of the particles are similar to those of the HR catalysts. For SiO 2-
927 Table 1 Results of I-I2 and CO adsorption and EXAFS measurements for 1 wt% Pt-loaded catalysts Uptake a)
EXAFS b)
Catalyst H 2 CO CO/H 2 (mol molpt-x) (-) HEPtCI6/A1203
N (-)
R (nm)
TOFO (10 -2 s -1)
LR
0.21
0.14
0.7
8.0
0.271
1.4
HR
0.14
0.58
4.1
9.9
0.273
0.13
Pt(NH3)4C12/A1203 LR
0.15
0.18
1.2
8.5
0.273
1.2
HR
0.10
0.44
4.4
10
0.274
0.13
LR
0.25
0.33
1.3
5.5
0.274
0.12
HR
0.16
0.22
1.4
11
0.276
0.19
Pt(NH3)4CI2/SiO2
a) CO/H2: the ratio of the CO and H 2 uptakes, b) N: coordination number, R: Pt-Pt bond distance. These were determined by a curve-fitting method using Pt foil as a reference where N=12 and R-0.276 nm. c) From the number of exposed Pt atoms from the H 2 uptake with HfPt=l.
Table 2 Amounts of Pt extracted by mixed acid (wt% relative to the support) Extracted Catalyst
Initially present
Before reduction
After reduction LR(308 K, 3 h)
HR(773 K, 3 h)
H2PtC16/A1203
0.96
0.88
0.84
~0
Pt(NHa)4C12/A1203
0.98
0.90
0.82
~0
Pt(NH3)4C12/SiO2
1.10
0.97
0.93
0.92
928
supported ones, the size of Pt particles in the LR catalyst may be smaller than that in the HR catalyst. The results of the acid-extraction of Pt are shown in Table 2. Pt was extracted in more than 85 % from the Al203-supported LR catalysts, while it was not extracted from the HR catalysts. So, the state of supported Pt is significantly different between these LR and HR catalysts. In contrast, no difference was observed between the SiO 2supported LR and HR catalysts. XPS measurements were conducted for the A1203-supported catalysts. Figure 4 shows the Pt 4d spectra for Pt(NH3)nC1JAI203 system, which are very similar between LR and HR catalysts. The heat treatment of the LR catalyst at 773 K in I-~ little affected the spectrum. For the LR and HR catalysts, C1 was detected in comparable levels and for the former, B and Na were detected in low levels.
(a)LR
(b)LR-,HR (c)HR I
340
=
=
=
j
I
=
i
330 Binding
=
J
I
=
=
320 energy
I
I
I
310 (eV)
I
I
I
I
J
300
Figure 4. XPS Pt 4d spectra for 1 wt% Pt-loaded LR catalyst (a), LR catalyst after heat treatment at 773 K in 1-12(b) and HR catalyst (c) of Pt(NH3)nC12/A1203system. 3.3. Turnover frequency (TOF) Table 1 includes TOF determined from the degree of Pt dispersion, D(Hz) , estimated by the Ha uptake for the 1 wt% Pt-loaded catalysts. The TOF for the A1203supported LR catalysts is larger by a factor of 10 than that for the HR catalysts. Figure 5 shows TOF for the catalysts in which the Pt loading ranges from 0.5 to 1.3 wt%. Similar plots are also obtained when the CO uptake is used instead of D(H2). It is worth noting that the A1203-supported LR catalysts have larger TOF's compared with the corresponding HR catalysts at the same D(H2) value, indicating a significant difference in the nature of active sites between those catalysts. Such a difference was not observed for the SiO2-supported catalysts. The TOF tends to decrease with increasing D(H2) and the extent of this decrease seems to depend on the Pt precursor used for the case of A1203.
929 1 0 -1
'
I
'
I
'
I
'
l
'
AO zxzx
--. 1 0 - a =o0 mm
9
0 I-1
q)O 0
zxA
0_a
1 0 -4
,
0
9mm
~& 0 9
I 0.2
I
zx 6~
~
~176
oo
I 0.4
I
I 0.6
I
I 0.8
1
D(H2) Figure 5. The relationship between TOF and D(H2) for LR and HR catalysts of the three systems. Open and closed marks are for LR and HR catalysts, respectively. Circles: H2PtC16/A120a, Triangles: Pt(NH3)4C12/A1203, Squares" Pt(NH3)4C12/SiO 2. 4. DISCUSSION The present results demonstrate that the catalysis of A1203-supported LR catalysts is clearly distinguished from the HR catalysts for CAL hydrogenation. The LR and HR catalysts are different in the surface state of Pt particles and in the state of dispersion of Pt as evidenced by H 2 and CO adsorption and acid-extraction, respectively. Several factors are suspected to contribute to those differences, including (i) the size and surface geometry of Pt particles, (ii) the Pt-support interactions, and (iii) the effects of residual C1, B, and Na. The XPS results indicate that the electronic state of exposed Pt atoms is similar between the LR and HR catalysts. The average size of Pt particles in the LR catalysts seems to be somewhat smaller than that of the HR catalysts fxom EXAFS results. However, this difference is so small that it cannot explain the great difference in the catalytic activity and selectivity. Judging from the results of acid-extraction, the Pt-support interaction would be weak for the LR catalysts and be strong for the HR catalysts. For the HR catalysts reduced 773 K, however, the Ptsupport interaction is not so strong as to markedly suppress the H 2 adsorption [4,5]. The XPS and EXAFS results do not indicate any interaction of Pt with other species. In addition, the LR catalysts lost the activities on the heat-treatment at 773 K. This treatment little altered the XPS Pt 4d spectrum and it was not assumed to cause a serious sintering of Pt from our previous study on a model Pt/A1203 catalyst [6]. Thus, the surface geometry of Pt particles is likely to be the most important factor, and in the LR catalysts it could be well tailored for CAL hydrogenation. Richard et al. suggested the importance of surface geometry of Pt particles in determining the selectivity in CAL hydrogenation [7]. The influence of thermal treatments on the morphology of Pt particles was recently reported by Rochefort et al. [8]. As shown in Fig. 5, the TOF tends to decrease with the increase in D(H2). The
930 Al2Oa-supported LR catalysts hold such a high selectivity to COL as 90 % for the range of D(H2) examined. This is contrast to the reported result that decreasing metal particle size resulted in a decrease in the selectivity to COL for supported Pt catalysts prepared through usual H2 reduction [7,9]. For our LR catalysts, the increase of D(H2) would cause a decrease in the specific activity of catalytic sites; however, these sites remain highly selective to the activation of the C=O bond with respect to the C=C bond. In the case of SiO 2, the LR and HR catalysts indicated similar activities, selectivities and TOF. The Pt precursor was ascertained from EXAFS to be in monomolecular dispersion before reduction as in the case of A1203. Elementary reactions involved in the reduction with NaBH 4 solution should be studied to account the effects of support as well as the structure of Pt particles. 5. CONCLUSION The liquid-phase reduction with NaBH 4 at ambient temperature can produce Al2Oa-supported Pt catalysts showing interesting catalytic properties that are not expected through usual high-temperature H 2 reduction. The difference in the catalytic activity may be explained by that the surface geometry of Pt particles formed would be different depending on treatment temperature during the preparation. The present results will open new possibilities of preparation and catalysis of supported metal catalysts.
REFERENCES 1. For example, papers in Stud. Surf. Sci. Catal., 1 (1976); 3 (1979); 16 (1983); 31 (1987); 63 (1991) 2. P.Gallezot, A.Giroir-Fendler and D.Richard, in "Catalysis of Organic Reactions", W.Pascoe (ed.), Marcel Dekker, New York, 1991, pp.l-17. 3. M.Arai, K.Usui and Y.Nishiyama, J. Chem. Soc. Chem. Commun. (1993), 1853. 4. G.J.Den Otter and EM.Dautzenberg, J. Catal., 53 (1978) 116. 5. K.Kunimori, T.Okouchi and T.Uchijima, Chem. Lett. (1980) 1513. 6. M.Arai, T.Ishikawa, T.Nakayama and Y.Nishiyama, J. Colloid Interface Sci., 97 (1984) 254. 7. D.Richard, P.Fouilloux and P.Gallezot, in M.J.Phillips and M.Ternan (eds.), Proceedings of 9th International Congress on Catalysis, Calgary, 1988, Chemical Institute of Canada, Ottawa, 1988, p.1074. 8. A.Rochefort, ELe Peltier and J.P.Boitiaux, J. Catal., 145 (1994) 409. 9. A.Giroir-Fendler, D.Richard and P.Gallezot, Catal. Lett., 5 (1990) 175.
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
931
Preparation of Supported Mono- and Bimetallic Catalysts by Deposition-Precipitation of Metal Cyanide Complexes E. Boellaard a, A.M. van der Kraan a, and J.W. Geus b aInterfacultair Reactor Instituut, Delft University of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands bDepartment of Inorganic Chemistry, Debije Institute, Utrecht University, P.O. Box 80083, 3508 TB, Utrecht, The Netherlands ABSTRACT To assess the suitability of metal cyanide complexes as active precursors for supported catalysts, a series of homo- and heteronuclear cyanide complexes has been precipitated in the presence of alumina, titania, and silica supports. To establish the distribution of the insoluble cyanide complexes on the support, the catalyst precursors were investigated by transmission electron microscopy. Conversion of the cyanide precursors into oxidic or metallic catalysts can be performed by thermal treatments in oxygen, argon, and hydrogen, respectively. Detailed results of the thermal treatment of a copper-iron cyanide precursor on alumina are presented. Oxidation of the cyanide precursors to highly dispersed oxides calls for treatment at relatively low temperatures, viz., about 573 K. The resulting oxide can subsequently be reduced smoothly to the corresponding (bi)metallic supported catalyst. Besides by electron microscopy, the catalyst (precursor) systems were characterized by M6ssbauer spectroscopy, XRD, and thermal analysis. It was demonstrated that the proportion of the metals within the stoichiometric cyanide precursors was retained in the reduced bimetallic catalysts. Heteronuclear cyanide complexes are therefore very well appropriate to produce bimetallic catalysts of a uniform chemical composition of the individual supported alloy particles. 1. I N T R O D U C T I O N With multi-component catalysts it is essential that the usually (very) small individual particles of the active component do not exhibit significant variations in chemical composition. When the active supported particles have a varying chemical composition, the experimentally observed activity and selectivity of the catalyst will display an average of the catalytic behaviour of active species of a different chemical composition. Generally supported bimetallic catalysts are being prepared using the same procedures as for the production of monometallic supported catalysts, viz., impregnation, depositionprecipitation, and ion exchange. These procedures, however, usually result in supported catalyst precursors of a non-uniform chemical composition of the individual active particles. The variation of the chemical composition is mainly due to a lack of interaction between the two metals to be alloyed during the various steps of the preparation procedure. Metal carbonyls, such as Fe(CO)5 and other organometallic complexes have been reported to react preferentially with metallic rather than with oxidic surfaces to the corresponding metals. Though the thus performed decomposition of suitable organometallic complexes ensures the formation of exclusively bimetallic particles, the composition of the individual particles may still vary. Preparation procedures based on the deposition of heteronuclear complexes are therefore much more attractive. The fixed stoichiometry of the complexes favours the desired uniform composition of the individual supported particles. Polynuclear metal carbonyls have proved to be very suitable for the preparation of bimetallic
932 catalysts with active particles of a uniform chemical composition. However, the small number of available polynuclear carbonyl complexes and the highly toxic character of the complexes is limiting the applicability of binuclear metal carbonyls. Another group of suitable complexes are the metal cyanides. Precipitates of a stoichiometrically fixed composition can be readily obtained by addition of a solution of a simple salt of the first metal to a complex cyanide anion of the second metal. A wide range of binuclear complexes can thus be prepared. Since homonuclear complexes can be precipitated too, the effects of introduction of other metal atoms can be unambiguously elucidated. An important demand on the preparation procedure based on the precipitation of cyanides is that the proportion of the metals in the precipitated stoichiometric binuclear complex is retained in the eventual bimetallic catalyst. In order to prevent unintentional promoting or poisoning effects of the cyanide ligands, it must be possible to remove the ligands completely at temperatures not too elevated to avoid sintering of the catalysts. In this paper a study is presented on the preparation of a series of supported catalysts by precipitation of metal cyanide complexes in the presence of suspended supports. As supports alumina, titania, and silica, have been used. The metals studied comprise iron, cobalt, nickel, copper, manganese, palladium, and molybdenum. Both monometallic, bimetallic and even trimetallic cyanides were precipitated. The stoichiometry of the precipitated complexes was controlled by the valency of the metal ions and by using both nitroprusside and cyanide complexes. Electron microscopy was used to evaluate the distribution of the deposited complex cyanides on the supports. 57Fe-M6ssbauer spectra were measured on the dried precipitated complexes to gain information on the chemical composition of the iron containing complexes. Since the prepared supported cyanide complexes are intended for use as catalysts, the cyanide precursors have to be converted to the corresponding oxides, metals or alloys, depending on the desired catalytic application. A detailed study of the thermal treatment in an inert, reducing, or oxidizing gas flow is presented for the copper-iron cyanides. The processes proceeding during pretreatment were studied by temperature-programmed reaction with mass spectrometric analysis of the evolved gases, thermogravimetry, and X-ray diffraction. The differently pretreated catalysts were investigated by M6ssbauer spectroscopy. In addition, some results on monometaUic iron and bimetallic nickel-iron catalysts are reported to illustrate the general suitability of the presented cyanide method.
2. EXPERIMENTAL 2.1 Catalyst Preparation The catalyst precursors were prepared by deposition-precipitation of complex cyanides onto oxidic supports. Precipitation was effected by addition of a solution of a soluble complex cyanide to a suspension of the powdered support in a solution of a simple metal salt. Iron cyanide complexes, K3Fe(CN)6, K4Fe(CN)6.3H20, and Na2Fe(CN)5NO.2H20, and simple metal salts FeCI2.4H20, Cu(NO3)2.3H20, Ni(NO3)2.6H20, Co(NO3)2.6H20 and Mn(NO3)2.4H20 were supplied by Merck. K4Mo(CN)8.2H20 was prepared by electrolysis of an acidified aqueous solution of MoO3.xH20 and subsequent complexation and precipitation with NH4SCN, CsHsN and KCN, while K2Pd(CN)4.H20 was prepared by precipitation and subsequent complexation of PdC12 with KCN. The support materials ~,-A1203 (Aluminium Oxid C, surface area 100 m2/g), TiO2 (P25, surface area 50 m2/g) and SiO2 (Aerosil 200, surface area 200 m2/g), were purchased from Degussa. The precipitations were carried out in a thermo-statted double-walled Pyrex vessel equipped with a stirrer and baffles to ensure thoroughly mixing of the contents. The inlets for blanket gas and liquids, and the pH electrode could be introduced via joints in the cover. The quantity of chemicals used was chosen to provide a loading of 20 wt.% of metal or alloy in the reduced catalysts. An amount of 4.00 g of the support material was suspended in 900 ml of demineralized water. The appropriate quantity of the simple metal salt was added to the suspension after dissolution in 100 ml of water.The initial pH was adjusted to 5.0 by addition of nitric acid or, when iron chloride was processed, with hydrochloric acid. The stoichiometric amount of the soluble cyanide complexes, dissolved in 100 ml water, was slowly injected into
933 the suspension through a capillary injection tube ending below the level of the vigorously stirred suspension using a peristaltic pump. The temperature of the vessel was kept at 295 K. When chemicals liable for oxidation were processed, the water was previously deoxygenated by boiling and kept under a nitrogen flow during and after cooling. The subsequent precipitation was performed under a nitrogen flow too. 57 ks after the start of the injection, the suspension was allowed to settle and subsequently decanted, washed twice with 500 ml water, and dried for 173 ks at room temperature in a vacuum of about 1.33 Pa. The yields of several batches prepared were mixed and subsequently ground, pelletized, and crushed. For further experiments a sieve fraction between 425 and 730 ttm was selected.
2.2 Catalyst Characterization In order to evaluate the distribution of the precipitated cyanide complexes over the support, samples of the dried cyanide catalyst precursors were investigated within a Philips EM 420 transmission electron microscope. The deposited cyanide precursors of some selected complexes were decomposed by thermal treatment in either an inert, an oxidizing, or a reducing gas flow. The temperatureprogrammed reaction processes were monitored by differential thermogravimetric (DTG) and thermal analyses (DTA) in a thermobalance, and by measuring the thermal conductivity of the gas flow before and after being passed through a conventional flow reactor. In some experiments the evolved gases were analyzed by a quadrupole mass spectrometer. Ex situ X-ray diffraction (XRD) was performed with a Philips diffractometer using FeKcxl,2 radiation (Z. = 0.193735 nm). M6ssbauer spectra were recorded with a constant acceleration spectrometer using a 57Co in Rh source. The spectrometer was operated with a symmetric, triangular velocity wave form and the obtained set of mirrored spectra was matched by folding in order to eliminate the curved background due to the varying distance between source and detector. For some experiments a saw tooth velocity mode was used, which gave rise to a curved background. The M6ssbauer parameters were determined by fitting the spectra with subspectra consisting of Lorentzianshaped lines using a non-linear iterative minimisation routine. With quadrupole doublets the line widths as well as the areas were constrained to be equal. Isomer shifts are reported with respect to the NBS standard sodium nitroprusside. 3. RESULTS AND DISCUSSION
3.1. Evaluation of the catalyst preparation procedure involving complex cyanides Since complex cyanides can be used to prepare a large range of industrially interesting supported catalysts, we studied a number of different combinations of metal ions capable of precipitation as complex cyanides. Accordingly active precursors were precipitated by addition of different iron, molybdenum, and palladium cyanide complexes to suspensions of a support in a solution of simple metal salts. The simple salts were iron(II) chloride, copper(II) nitrate, nickel(II) nitrate, cobalt(II) nitrate, and manganese(H) nitrate. A prerequisite for the successful utilization of deposition-precipitation for the production of supported catalysts is a sufficient interaction of the precipitating or flocculating active precursor with the suspended support. With deposition-precipitation of complex cyanides the interaction of the cyanides with the suspended support has thus to be established. Investigation of the dried supports within the transmission electron microscope provided information about the dispersion of the cyanide complexes over the support. The different combinations studied are summarized in table I. From the table it can be seen that combinations of a divalent metal ion, M(II), and iron cyanide complexes are possible in proportions M(II)/Fe of 1.0, 1.5, and 2.0, by using iron(II) nitroprusside, iron(III) cyanide, and iron(H) cyanide, respectively. Except iron-palladium cyanide and nickel-nitroprusside, all complex cyanides are coloured. Therefore the colour of the solid settled after completion of the injection will indicate whether a reasonable distribution over the support has been achieved. Precipitation of cyanide
934 complexes in the presence of suspended alumina, the most frequently used support, resulted in a homogeneously coloured solid. In an experiment with Fe2Fe(CN)6, the loaded titania support also exhibited a uniform colour. Silica, on the other hand, did not display a significant interaction with the cyanide complexes containing copper and iron. Settling of the silica support and the precipitated copper-iron cyanide led to deposition of two differently coloured layers; a white bottom layer, which appeared to be pure silica, and a top layer with copper-iron cyanide. Table I Precipitatedmetal cyanide complexes ' Catalyst code Stoichiometric Support 9 Complex FeFe FeFe(CN)5NO A1203 Fe3Fe2 Fe3[Fe(CN)6]2 A1203 Fe2Fe Fe2Fe(CN)6 A1203, TiO2 CuFe C u F e ( C N ) 5 N O A1203,SiO2 Cu3Fe2 Cu3[Fe(CN)6]2 A1203,SiO2 Cu2Fe Cu2Fe(CN)6 A1203, SiO2 NiFe NiFe(CN)sNO A1203 Ni3Fe2 Ni3[Fe(CN)6]2 A1203 Ni2Fe Ni2Fe(CN)6 A1203 CoFe C o F e ( C N ) 5 N O A1203 Co3Fe2 Co3[Fe(CN)6]2 A1203 Co2Fe Co2Fe(CN)6 A1203 Mn3Fe2* Mn3[Fe(CN)6]2 A1203 Mn2Fe Mn2Fe(CN)6 A1203 CuNiFe CuNiFe(CN)6 A1203 FePd FePd(CN)4 A1203 Fe2Mo Fe2Mo(CN)8 A1203 Cu2Mo Cu2Mo(CN)8 AI203 Ni2Mo Ni2Mo(CN)8 A1203 Co2Mo Co2Mo(CN)8 A1203 9Filtrate contains Mn-ions
Colour - wet
Colour - dry
salmon pink dark blue pale blue blue-green green purplebrown white dark yellow pale brown salmon pink brown red pastel green brown pale grey purple brown white green yellow purple pastel green orange
salmon pink dark blue dark blue blue-green green purple brown white green yellow pale brown salmon pink purple steel blue brown pale grey purple brown orange brown blue grey purple pale green sand
The rapid nucleation of insoluble complex cyanidesbrings about that very small particles of the complex cyanides are almost instantaneously formed when the cyanide solution enters the suspension of the support. The electrostatic charges on the resulting colloidal complex cyanide particles and on the particles of the suspended support determine the interaction between the cyanide particles and the support material. The Cu2Fe(CN)6 complex is reported to exhibit a negative charge. It can be expected that the other insoluble complex cyanides are negatively charged too. At the pH level at which the precipitation was performed, viz., 5.0, silica has a negative charge (iso-electric point is 2.0). Alumina, on the other hand, displays an iso-electric point of 9.0. At a pH level of 5.0, alumina is therefore positively charged and interacts strongly with negatively charged colloidal particles of complex cyanides. The iso-electric point of titania (anatase) is 6.6. Consequently, titania will also interact attractively with colloidal complex cyanide particles. The electron micrographs of the loaded supports indicate that whereas the samples FeFe, Fe3Fe2, CoFe, Mn3Fe2, and Co2Mo of Table I exhibited a good distribution of very small particles over the surface of the alumina support, the samples Fe2Fe, Cu2Fe, CuNiFe, and Cu2Mo showed some small platelets well distributed in between the elementary particles of the alumina support. It is remarkable that deposition-precipitations involving Fe(CN)64-, which are Fe2Fe, Cu2Fe, and CuNiFe, are apparently liable to the formation of platelets. Also with titania as a support, the Fe2Fe samples displayed platelets. In contrast to Co2Mo and Cu2Mo, the molybdenum containing specimens Fe2Mo and Ni2Mo showed large thin plates. To prevent with molybdenum precursors formation of large platelets, the injection must be performed carefully. Especially a continuous flow of the cyanide complex is important to avoid formation of large thin platelets.
935 i
j
l
I
,
j
,
i
J
M2Fe
'
IMF e
'
'
M3Fe2
6.61
6. O0 --
5.9~
5.40
S.3e
6.33
11.4
5.12
Fe
$
3.67
3.49
tO.g
4.8C
5.83
12.0
6.3~
11.6
6.
6.06
6.14
5.79
5.81
";~
-
V
Nt
o
%
5.45
-~
4.15
>. ~
3.87
~ILl
5.e2
-
4.08
3.95
5.27
3.38
OC
--
3.17 -I
I
I
I
I
I
I
I
I
t
i
-I
0
1
2
-!
0
1
2
-1
0
!
i
6.46 --
i
FeFe~~
j
i
~.~
i
l
DOPPLER VELOCITY (m~. e "! )
i
~ [ ~ ' ~
m o
6.18
w
6.79
6.67
-3
t
,
-2
-I
i , 0
1
t
,
|
2
3
4
DOPPLER VELOCITY (M~.e - ! )
Figure 1 Mi#ssbauerspectra of the dried ironcontaining cyanide complexes.
Figure 1 shows the M6ssbauer spectra of the dried iron-containing cyanide complexes. The parameters resulting from the deconvolution of the experimental spectra are collected in table II. The isomer shift, IS, the quadrupole splitting, QS, the line width, F, and the spectral contribution, SC, are indicated. M6ssbauer spectra of the dried supported cyanide complexes show close resemblance to the spectra of the corresponding alkali iron cyanide complexes that were injected into the suspensions of the supports. Substitution of the alkali metal ions by transition metal ions causes only a small shift, which indicates that the initial complex remains relatively unaffected. However, complexes precipitated from K3Fe(CN)6, which are the M3Fe2 complex cyanides, exhibited a singlet, besides the expected doublet. The singlet points to the presence of an Fe(CN)64- complex, which implies reduction of the iron(III) present in the initial cyanide complex. As can be seen from the data of table II, the contribution of the singlet is only about 10% with Ni3Fe2, Co3Fe2, and Mn3Fe2 but 31% with Cu3Fe2. With the Cu3Fe2 cyanide complex reduction of the iron(III) proceeds relatively fast after the complex has been precipitated and dried. With the Fe3Fe2 cyanide complex, electron transfer proceeds between the Fe(II) ion and the Fe(III) which is complexed by cyanide ions resulting in the formation of an Fe(III)-Fe(II) cyanide complex, With the iron(II) hexacyanoferrate(II) complex (Fe2Fe),
936
oxidation of the iron(II) ions was found to occur mainly during drying of the freshly precipitated complex. The recorded M6ssbauer spectrum consequently does not correspond to the initially precipitated complex. For the other cyanide complexes precipitated with iron(H) chloride, the M6ssbauer spectra indicate that the iron ions were slightly oxidized to a different extent. However, reduction / oxidation of the cyanide complexes after the precipitation did not affect the overall chemical composition of the precipitated particles. Table II M6ssbauer parameters of precipitated cyanide complexes Catalyst IS QS F SC code mm/s mm/s mm/s % FeFe 0.00 1.93 0.28 46 1.38 1.47 0.38 32 0.50 0.73 0.37 15 2.13 1.21 0.37 7 Fe3Fe2 0.65 0.56 0.43 57 0.11 0.31 43 Fe2Fe 0.13 0.31 44 0.69 0.34 0.58 56 CuFe -0.01 1.82 0.26 100 Cu3Fe2 0.10 0.56 0.35 69 0.18 0.49 31 Cu2Fe 0.17 0.37 100 NiFe -0.01 1.90 0.25 100 Ni3Fe2 0.11 0.47 0.32 92 0.18 0.36 8 Ni2Fe 0.17 0.36 100 CoFe -0.01 1.91 0.25 100 Co3Fe2 0.11 0.43 0.32 90 0.17 0.34 10 Co2Fe 0.17 0.41 100 Mn3Fe2 0.11 0.34 0.30 90 0.13 0.29 10 Mn2Fe 0.16 0.29 100 CuNiFe 0.17 0.37 100 FePd 1.41 1.22 0.26 52 0.59 0.68 0.52 48 Fe2Mo 1.42 1.97 0.37 73 0.67 1.13 0.48 27
3.2. Thermal treatment of the supported cyanide complexes Although cyanide complexes can be used as prepared and become in situ activated during the catalytic process, many catalysts are used as metals or alloys or as (mixed) oxides. The supported complex cyanides must therefore generally be pretreated to remove the cyanide groups and to react subsequently to either metals or alloys or to oxides. For a reliable evaluation of the catalytic activity and selectivity, it is essential that the cyanide groups are virtually completely removed. The thermal decomposition of the copper pentacyanonitrosylferrate(II) complex, which was present as very small particles on the alumina support, was studied in inert, reducing, and oxidizing gas flows. The behaviour of this complex is representative for the supported complexes prepared in this study. Figure 2 shows the change in weight and the evolved gases during thermal treatment of the supported complex in a hydrogen, an argon, and an oxygen containing gas flow. Although the evolved gas analysis was performed in a different apparatus as the thermogravimetric experiment, the temperatures at which the different reactions proceeded did not differ significantly. From figure 2 it can be seen that the composition of the gas flow during the thermal treatment strongly affects the reactions proceeding. With all gas
937
+
t
(CN)2 V H20
dM dT
NO
L H20
02
C02 NOx I
3O0
50O
I
I
7OO
Temperature
I
I
1100
/K
Figure 2 Thermogram of the thermal decomposition of the CuFe(CN)5NO catalyst precursor in hydrogen, argon, and oxygen.
flows used physisorbed and/or crystal water is released before the main decomposition reaction sets on. Removal of the cyanide groups in an argon flow calls for elevated temperatures, viz., about 940 K. At about 505-560 K desorption of NO and (CN)2 proceeded, after which the temperature had to be raised to about 910-940 Kto bring about desorption of more (CN)2 and N2. The chemical composition of the gaseous products set free during the entire decomposition indicates that the overall carbon-to-nitrogen ratio is lower than unity, which corresponds to that of the CN-ligands. Accordingly a significant amount of carbon was retained in the catalyst, presumably as badly crystallized or amorphous carbon. XRD proved that thermal treatment in a flow of argon leads to large metallic copper and iron particles. X-ray line broadening indicated a size for the copper particles of 116 nm and for the iron particles of 33 nm. Apparently copper is reduced to the metallic state at relatively low temperatures without being significantly oxidized. Usually metallic copper particles are anchored to oxidic supports, such as alumina, by copper ions present at the interface with the support. When the copper in the cyanide complex reacts rapidly and completely to metallic copper, the small interaction of metallic copper with the alumina surface cannot prevent very severe sintering. Whereas oxidation of copper by water vapour is thermodynamically prohibited, water vapour, which is transported relatively slowly out of the pores of the support, will (slightly) oxidize the metallic iron particles. As a result, some iron(U) aluminate will be present at the interface with the support, which will stabilize the iron particles. Experiments with the thermal conductance detector provided analogous profiles as that shown in figure 2 for the thermogravimetric experiments. Due to the relatively large difference in thermal conductivity, much higher peaks revealed during the reactions in which hydrogen was consumed. In a hydrogen-containing gas flow the onset of the decomposition does not differ from that in an argon flow. However, the decomposition is completed at lower temperatures. At about 675 K evolution of HCN is recorded and at about 750 K release of N2 and NH3. Some carbon remaining in the catalyst reacts to methane at temperatures above 800 K in spite of the unfavourable thermodynamic equilibrium. It tums out that the metallic copper particles are now significantly smaller than the iron particles, viz., 26 and 78 nm, respectively. As indicated by the curve at the bottom of figure 2, thermal treatment in oxygen requires a
938 fairly low temperature to complete the decomposition of the supported complex. At about 520 K CO2 and NOx were released and the reaction had completed below 600 K. Treatment of the supported complex in oxygen did not lead to a diffraction pattern different from that of the alumina support. As mentioned above, the interaction of oxidic phases with the surface of oxidic supports is much stronger than that of metallic phases. Consequently, reaction of the finely divided complex cyanide particles to oxides consequently does not lead to significant sintering. To prev.ent excessively high temperatures during the exothermic oxidation of the cyanide precursors, especially with larger catalyst batches, the oxygen concentration was decreased from 10 to 1 vol.%. To establish the most favourable temperature for the reaction with oxygen, the alumina-supported CuFe cyanide precursor was heated in a flow of 1 vol.% oxygen in helium at a rate of 83 mK/s to 543 K and kept at this temperature for 82 ks. In two other experiments, the catalyst was additionally further heated to temperatures of 655 and 850 K, respectively, and subsequently cooled down. The phases thus obtained were characterized by M6ssbauer spectroscopy performed at 300, 77, and 4.2 K. The spectra are presented in figure 3 and the corresponding parameters in table HI. After oxidation at eventually 543 and 655 K the spectra recorded at 300 K showed a set of doublets, while the sample oxidized finally at 850 K exhibited a set of both doublets and sextuplets. Spectra recorded at 4.2 K with samples pretreated at 543, 655, and 850 K are dominated by a distribution of sextuplets.The M6ssbauer spectra of figure 3 indicate that the cyanide complex was fully destroyed already by oxidation at 543 K at low oxygen partial pressures. The M6ssbauer parameters of the samples oxidized at 543 and 655 K are not significantly different. The two doublets present in the spectra measured at 300 K can be assigned to iron atoms in the surface and bulk of a superparamagnetic iron oxide phase. The state of the iron oxide can be established more precisely from the spectra measured at 4.2 K. At this temperature most of the iron oxide has become magnetically ordered, as is evident from the splitting of the doublets. The M6ssbauer sextuplet points to a distribution of hyperfine fields with a mean value of 460 kOe. Since the spectral contribution of the central set of doublets is still significant, part of the iron oxide was still superparamagnetic at 4.2 K. The superparamagnetic fraction may be higher within the sample oxidized at 543 K than for that oxidized at 655 K. Based on the almost symmetric profile of the spectrum together with the large line width and a hyperfine field well below the level of 550 kOe characteristic for bulk etFe203, the iron oxide may be identified as extremely finely divided Fe203 crystallites. Although copper cannot be investigated by M6ssbauer spectroscopy, the catalysts also comprised a phase containing copper, which is most likely highly dispersed copper(II) oxide. The spectra measured on the sample oxidized at 850 K indicate that a fraction of the small oxide particles had lost superparamagnetism and had become magnetically ordered at room temperature. Decreasing the temperature at which the spectra were measured from 300 K over 77 K to 4.2 K raised the spectral contribution of the magnetically ordered phase at the expense of the superparamagnetic constituents. The resolution of the spectra also improved at lower temperatures. It is obvious that the spectra are different from those recorded with the samples oxidized at 543 and 655 K. The effect on the spectra may be due to sintering of the oxidic particles or to a change in the chemical composition. The spectra measured with the sample pretreated at 850 K closely resemble spectra measured with copper ferrite, CuFe204[1]. The spectrum measured at 300 K exhibits a hyperfine field of 431 kOe, which differs from the hyperfine field of 475 kOe reported in the literature for CuFe204. The lower hyperfine field observed with our sample is presumably due to the superparamagnetic relaxation of the highly dispersed phase. Formation of CuFe204 implies a solid-state reaction between Fe203 and CuO, which proceeded during 3.6 ks, the period of time in which the temperature was raised to 850 K. This rapid reaction suggests that the iron oxide and copper oxide were intimately mixed. Reduction of the oxidic precursors resulting from the oxidation of the supported complex cyanides leads to metal or alloy particles. Figure 4 shows temperature-programmed reduction (TPR) profiles measured with the thermal conductivity detector for the oxidic precursors of iron, copper-iron and nickel-iron catalysts. With the pure iron catalyst profiles for the alumina and for the titania supported ones are presented. The reduction profiles of the iron-copper and
939
i
'
'
I
'
'
I
I
I
I
I
!
I '
'
I
i
,
-I0
-5
0
5
I0
[
0X655 ! .55 --
1.90
4.53 300K
1.47
--
4.42
3.87
0.65
--
3.11
~
3.82
0.62
--
3.04
~
3.77
2.30
3.05
3 74
2.26
3. O0
b-
o~
%
-IO
-5
o
5
1o
-tO
-S
0
5
10
OOPPLER VELOCITY ( m m . t " | )
Figure 3
M6ssbauer spectra recorded at 300, 77, and 4.2 K of the CuFe cyanide precursor after oxidation at 543, 655, and 850 K.
Table III M6ssbauer parameters Catalyst Temperature code K 300 OX543 77 4.2 OX655
300
OX850
77 4.2 300 77 4.2
of the CuFe precursor aider different oxidation treatments IS QS HF F SC mm/s mm/s kOe mm/s % 0.57 0.76 0.44 70 0.57 1.29 0.36 30 0.68 1.00 0.69 100 0.72 0.00 457 1.52 91 0.62 0.70 1.17 9 0.59 0.71 0.44 57 0.59 1.21 0.44 43 0.70 0.98 0.68 100 0.74 -0.01 463 1.20 100 0.57 0.86 0.67 48 0.61 0.04 431 2.00 52 0.68 0.93 0.67 40 0.78 0.00 523 0.45 14 0.67 0.00 486 0.79 46 0.58 0.61 0.67 9 0.79 0.00 529 0.45 18 0.69 0.00 493 0.85 73
nickel-iron catalysts are clearly different from that of the monometallic iron catalysts. Reduction of the copper or nickel oxide to the metal presumably proceeded initially, since the water vapour pressure does not affect significantly the thermodynamic equilibrium in contrast to the reduction of iron. Reduction of the initially formed Fe304 or FeO to metallic iron is thermodynamically possible when the water vapour pressure had dropped sufficiently and the temperature had increased. Spill-over of hydrogen atoms from the metallic copper or nickel surfaces may have assisted in the reduction of the iron oxide phase. The final temperature of the temperature-programmed reduction is about 1123 K. The XRD pattern of the thus reduced copper-iron catalyst exhibits b.c.c, iron crystallites and f.c.c. copper crystallites both of a size of 21 nm. Presumably copper-iron particles have segregated into a copper-rich phase and an iron-rich phase at the final elevated temperature of the T P R experiment. The XRD results indicates the size of the crystallites, but not whether the two crystallographic phases are present within a single particle.
940 With iron-nickel segregation is much less likely. Accordingly the XRD results of the nickel-iron catalysts point to a homogeneous phase, in which iron and nickel atoms have been randomly distributed over f.c.c, lattice positions. The experimental lattice parameter of the supported Ni-Fe catalysts allowed us to calculate the chemical composition of the alloy particles. Using data collected by Pearson [2], we calculated for the nickel content of the three nickel-iron alloy catalysts 51.9, 59.9, and 64.5 at.%, which agrees very well with the contents of 50.0, 60.0, and 66.7 at.% calculated from the stoichiometry of the nickel-iron cyanide complexes.
. . .:. -.
S~
;
\
i x.,, ',X,.,._. , ...,,,'' \ u: r
.........
- At
m
I
500
t~ / /
t
"~i ____-_:-_-,.. - ~.~_.---
300
:
~
t
I
\
i
I
700 900 TEMPERATURE/ K
\
I
.
\~Fe2Fe-A
I
1100
Figure 4 TPR profiles of the oxidic Fe2Fe-T, Fe2Fe-A, NiFe, and CuFe catalysts.
CONCLUSIONS Homo- and heteronuclear cyanide complexes are attractive precursors for the preparation of supported catalysts. First of all since the reaction with the support to an inactive compound does not proceed appreciably, and secondly, since the fixed stoichiometry of the complex cyanides results in active particles of a uniform chemical composition. Decomposition of the cyanide complexes in a hydrogen containing or in an inert gas flow is liable to lead to severely sintered metal particles, since the interaction of the resulting metal particles with the support is extremely low. Most attractive is oxidation of the cyanides to (mixed) oxides that can subsequently be readily reduced. Though the active components of the resulting metal or alloy particles will be intimately mixed, the crystallographic phases present in the reduced catalysts will depend upon the miscibility of the metals. Copper-iron, an unstable alloy, will lead to separate copper and iron phases after a high-temperature treatment, whereas nickel-iron, which is more stable, results in f.c.c, nickel-iron alloy phases. REFERENCES
1 2
B.J. Evans and S.S. Hafner, J. Phys. Chem. Solids, 29 (1968) 1573. W.B. Pearson, A Handbook of Lattice Spacings and Structures of Metals and Alloys, Pergamon Press, London, 1956.
PREPARATION OF CATALYSTSVI Sciemific Bases for the Preparation of HeterogeneousCatalysts G. Poncelet et al. (Editors) 9 1995 Elsevier ScienceB.V. All rights reserved.
Clusters and thin films prepared morphology and catalytic properties.
941
by
DC-sputtering:
D. Duprez and O. Enea Laboratoire de Catalyse en Chimie Organique. URA CNRS 350. 40, Avenue du Recteur Pineau, 86022 Poitiers Cedex, France.
The morphology of clusters and thin films of Pt, Pd, Au and Ni deposited on model substrates (Si, pyrolytic graphite, glass... ) by direct-current sputtering (DCS) was studied by scanning tunneling microscopy and X-ray reflectometry. Noble metals were also deposited by DCS on conventional supports (SiO 2, TiO 2, A120 3, CeO 2) and their properties in various catalytic reactions (benzene hydrogenation, cyclopentane hydrogenolysis, transient CO oxidation, photooxidation of alcohols) were investigated. 1. INTRODUCTION Physical vapor deposition methods (PVD) offer the possibility of preparing catalysts in which no foreign ions or molecules are introduced as is the case in the conventional "wet" impregnation methods. In evaporation methods however, the contact between metal and substrate produced by the deposition of metallic vapors is too weak to favor strong interactions and to enhance the catalytic activity and stability. By contrast, when a high-energy method like ion implantation is used, the metal is buried too deeply in the substrate and only a limited number of sites are available for the catalytic reactions. So far, directcurrent sputtering has been the only PVD method whereby reasonable amounts of active catalysts could be prepared [ 1]. In this study the morphology of clusters and of thin films of Pt, Pd, Au, Ni deposited on silicon, conducting glass or highly oriented pyrolitic graphite (HOPG) was studied by scanning tunneling microscopy (STM) and X-ray reflectometry. Noble metals (Pt, Pd, Pt-Rh .... ) were also deposited on powders like TiO 2, A120 3, SiO 2 or CeO 2, by using appropriate sputtering parameters (current intensity, voltage and time). The physico-chemical properties of these catalysts were examined by hydrogen chemisorption and TEM while their catalytic activity and selectivity were tested in various model reactions like the conversion of hydrocarbons (cyclopentane dehydrogenation and hydrogenolysis, benzene hydrogenation, transient CO oxidation for oxygen storage capacity, etc...), or the photooxidation of alcohols.
942
EXPERIMENTAL The sputtering device used in the present work (Fig. 1) ensured the uniform deposition of metallic d u s t e r s (Pt, Pd, Au, Ni .... ) on powdered or p l a n a r substrates [1]. The samples were placed in an A1 container used as an anode while the cathode was a 80 mm diameter foil of Pt, Pd, Au, Ni .... A vacuum of 10 -6 m b a r was created by means of a turbomolecular pump in the glass cylindrical vessel, flushed several times with pure Ar. The sputtering medium was Ar (high purity) introduced at a 1 mbar pressure and which flowed continuously during the whole deposition time. When the metallic target (Pt, Pd, Au, Ni .... ) used as a cathode was submitted to a DC potential of 500V, clusters of Pt, Pd, Au, Ni... were formed by an Ar ion bombardment (ionization energy = 13.5 eV); a plasma current of 20 to 25 mA can thus be m a i n t a i n e d constant during the whole deposition process. HOP Graphite samples (10 x 10 m m 2) were used to deposit small clusters and 10 x 20 mm 2 sheets of Si(100), glass or evaporated gold were used to prepare DC-sputtered films of Pt, Pd, Ni or Au [2][5]. In the case of powdered substrates like SiO2, TiO2, A1203 or CeO2, an uniform exposure was achieved by the mechanical vibration of the powder. Such vibration ensured a quasi-fluidization of the substrate and thus a rapid turnover of the powder surfaces exposed to the flux of metal clusters. Photocatalytic experiments were performed in a 80 ml Pyrex flask filled with 50 ml of PtfriO 2 suspension containing 0.5 M alcohol and illuminated at ~>350 nm with a 900 W Xenon l_~_mp. Gas aliquots were analyzed by gas chromatography with a 5m Porapak Q column.
b
i
I 9
.
:.......,:..,,
:ii
..-.
I
~r
r
E
Figure 1. DC-sputtering set-up, a: cathode (Pt, Pd,..), b: screen, c: sample
943 Hydrocarbon catalytic reactions were carried out in a dynamic reactor under the following conditions : benzene hydrogenation at 120~ H2/benzene molar ration of 20 ; cyclopentane dehydrogenation and hydrogenolysis at 460~ H2/cyclopentane molar ratio of 20. The main products (cyclohexane in benzene hydrogenation, cyclopentene, cyclopentadiene and C1-C 4 hydrocarbons in cyclopentane conversion) were analyzed by GC. A pulse chromatograph apparatus was used for H 2 chemisorption and oxygen storage capacity [6, 7]. 3. T H I N FILMS D E P O S I T E D ON M O D E L S U B S T R A T E S
3.1. Morphology of films Thicknesses and densities of thin films of Pt, Pd, Ni,.Au.. sputtered for 15 to 120 minutes on glass or Si(100) were obtained from the reflected profiles recorded by X-ray reflectometry. In the case of Pt films, density ranged from 5.0 to 7.7 g cm "3 and was always much lower than that of bulk Pt (21.4 g cm-3). The presence of amorphous Pt oxides, containing only 20 to 30% platinum, was confirmed by X-ray diffraction measurements under a grazing incidence. The thickness of Pt films, calculated from the interference fringes of the reflected profiles (Fig. 2), increases from 20 nm to 45 nm when the sputtering time increases from 15 to 30 minutes. The deposition rate of Pt was = 1.5 nm min.-1 in the experimental conditions used in this work for DC-sputtering. In the case of Pd, the density of DC-sputtered films (7 to 8.7 g cm "3) was closer to that of bulk Pd (12 g cm "3) and the Pd content reached up to 70%. Cristalline PdO having the 0.266 nm interatomic distance was detected by X-ray diffraction measurements.
a)
4.a ot..~
b)
I
.d$'4
.6
! I
63
ov-.~
o,-~
4-a
4.a r
/ /!
cD
I
0
!
0~
O(~
l
/
cD
09
l 1 t
I l
i
!
0.75
0
J
,k,,",.
0~ 0~
O(o)
9
Figure 2. X-ray reflection profiles of some DC-sputtered films, a: Ni/glass 60 rain., b: Au/Si sputtered for 3 rain. (dotted curve) or for 30 rain. (full curve).
944 Pd films sputtered for 30 to 60 minutes were only 13 to 27 nm thick respectively : the deposition rate of Pd (0.4 nm min -1) was found to be, under the same experimental conditions, about four times lower than for platinum. The density of DC-sputtered Ni films deposited at a rate of 0.6 nm min -1 was significantly lower (4.8 to 5.6 g cm "3) than the density of bulk nickel (8.9 g cm'3). DC-sputtered gold films had densities ranging between 9.86 g cm -3 and 12.65 g cm "3, i.e. significantly lower than the density of bulk gold (19.3 g cm-3). The examination of DC-sputtered films by STM shows surfaces with increasing roughness for longer deposition times (Fig. 3a,b), presumably due to a columnar type of growth. On the STM images of Pt films are seen large formations (700-1000 nm in size, 150 nm high) of small clusters between 7 and 20 nm large by 2 nm high. They are surrounded by holes 500 nm deep and 2000 3500 nm large. With this low density voided structure, DC-sputtered Pt films are not conductive enough (less than 10 -6 S) and show poor mechanical properties. A less rough topography is observed in the case of Pd films, probably due to a lower deposition rate, favoring the formation of both a crystalline PdO and a smoother film (Fig. 3c). The roughness of DC-sputtered Ni films imaged by STM is always greater than that of films prepared by other deposition methods, such as radio frequency. Depth concentration profiles determined by XPS-SIMS show that close to the silicon substrate there is more metallic Ni [4]. On the STM images recorded for gold films the increase of roughness with sputtering time is also observed. Large formations (2000 nm) are formed by coalescent, elongated gold particles of 10 nm.
Figure 3. STM images of Pt films at different sputtering times. X,Y = 2000 nm. a : 60 min., Az = 293 rim; b: 120 min., Az = 352 nm. STM conditions: It = 1 nA, U = 0.5 V.
945 3.2. T h e r m a l - r e d u c t i o n of DC-sputtered films under hydrogen The r o u g h n e s s of Pt a n d Pd reduced films ( l h at 300~ u n d e r H2 flow) decreases (Fig. 4) while Pt density values a n d m e t a l content increase from 5.5 g cm -3 a n d 25%Pt to 8.8 g cm "3 a n d 38% Pt after 30 m i n u t e s sputtering. Clusters, a p p r o x i m a t e l y 50 x 100 n m 2 in size a n d 6 to 10 n m h i g h are formed by the reduction a n d sintering of Pt or P d oxides. Moreover, the electrical conductivity of Pt a n d P d layers s p u t t e r e d on glass or silicon increases up to 5 S.
Figure 4. STM images of films reduced at 300~ u n d e r H 2. a: Pt, X,Y = 4000 nm, Az = 33 nm; b: Pt, X,Y = 500 nm, Az = 30 n m c: Pd, X,Y = 4000 nm, Az = 33 nm; d: Pd, X,Y = 500 nm, Az = 30 n m
946
4 . C L U S T E R S D E P O S I T E D ON H O P G A N D ON P O W D E R S 4.l.Morphology of clusters The conventional planar diode DC sputtering device allows a satisfactory mass-transfer rate so that highly uniform deposits are obtained, as shown by TEM micrographs (Fig. 5). The size distribution of Pt clusters ranges between 2.0 and 3.5 nm as seen in Fig. 5 where Ti planes 0.35 nm apart can be also observed. The HOP Graphite surface bombardment with Pt, Pd or Au clusters having a significant kinetic energy upsets the cristal structure (Fig. 5b). The density charge waves observed around the clusters on the STM images could be due to the strong interactions created between metal and substrate (SMSI).
t
i
Ilm
10 nm Figure 5. Pt clusters a: TEM micrograph of Pt clusters sputtered 80 min. on TiO 2 powder (P25 Degussa) ; b: STM image of Pt clusters sputtered on HOPG.
4.2.Low-temperature catalytic a c t i v i t y Dispersion measurements (D%) deduced from hydrogen chemisorption (H/Pt s = H/Rh s = 1) and catalytic activity in benzene hydrogenation (molec. at "1. h "1) are given in Table 1. The performance of DCS catalysts (60min., 500V, 20mA) are compared in some cases with those of catalysts prepared by "wet" impregnation. DCS catalysts are very active in BH even though they are not prereduced at high temperature (300-5000C) : they are "ready to use" while conventional catalysts require a HT reduction pretreatment to develop a good hydrogenation activity. Bimetallic clusters of Rh/Pt prepared by using a
947 Rh(10%)-Pt alloy as a cathode p r e s e n t strong synergy effects : the activity .of s i l i c a - a n d alumina- supported Rh-Pt catalysts is 5 to 10 times higher t h a n t h a t of Pt catalysts. Moreover we can notice t h a t the b u l k composition of bimetallic catalysts is very close to t h a t of the alloy used as a cathode : there is no preferential sputtering of one of the metals. ~ Table 1 Benzene hydrogenation at 120~ The catalysts were reduced at 120~ their use in reaction (except Pt/TiO2"B "wet" 300, reduced at 300~ Catalyst
wt.%Pt
Pt/A120 3 Pt/AI20 3''wet'' Pt/SiO 2 Ptfrio 2 Pt/TiO 2-B"wet" PtfriO2-B"wet"300 Pt-Rh/AI20 3 Pt-Rh/SiO 2
0.56 0.98 0.32 0.30 0.67 0.67 0.56 0.32
wt%Rh
D%
rHS
0.040 0.035
45% 63% 39% 28% 24 % 24% 49% 56%
2~480 540 1 470 1 560 15 840 8 :~90 15 200
, before
T.O.N. (h- 1) 5 500 860 3 800 5 600 60 3 500 17 700 27 100
The photocatalytic conversion of alcohols was investigated on "sputtered" a n d on "wet" PtfriO 2 catalysts. The reaction involves several steps : (i) chemisorption and dissociation of alcohol molecules on the surface; (ii) creation of electron-hole pairs under illumination; ('di) hole consumption by O H - and alkoxide ions; (iv)reactions between the radicals formed at the surface; (v) photocatalytic decarboxylation of the corresponding acids into hydrocarbons; (vi) hydrogen formation by H + or water reduction on the surface of cathodically charged Pt deposits which act as microelectrodes. The rates of formation of the different products, given in Table 2, show t h a t the sputtered catalyst is a l w a y s more active t h a n the catalyst prepared by wet impregnation. Table 2 Rates of production of H 2, CO 2 and hydrocarbons (L h -1 gPt -1) on i l l u m i n a t e d suspensions of "sputtered" and "wet" photocatalysts. 0.1g Pt/TiO 2, 0.5M alcohol, pHinit.=5, ~>350 nm.
Alcohol
Ptfrio 2 H 2 CO 2
Methanol Ethanol n-Propanol
20.8 23.1 21.5
4.2 0.8 1.4
"sputtered" CH 4 C2H 6 1.6 -
4.1
H2 5.5 6.0 3.8
Pt/TiO 2 "wet" CO 2 CH 4 C2H 6 0.7 0.7 0.3
, 0.1 -
0.5
948
4.3. High-temperature catalytic activity The conversion of cyclopentane (dehydrogenation and hydrogenolysis) was investigated at 460~ (Table 3). No significant difference is found between impregnated and sputtered catalysts, which seems to show that the benefits of the sputtering method are cancelled at elevated temperature. An exception however is observed with Pt/TiO2-B which exhibit a very high selectivity in dehydrogenation. Table 3 Conversion of cyclopentane at 460~ SelectiviW % Catalyst
Pt/CeO 2 Pt/CeO 2''wet'' Pt/TiO2-B Ptfrio 2 PtfriO2"wet"
% Pt
0.32 0.92 0.30 o. 18 0.45
Activity molec.at Pt" l h ' l 4200 5100 3400 3000 3100
C 1- C4 7 14 1 5 4
n-C 5 16 13 1 18 20
CPE+ CPD* 77 73 98 77 76
*CPE : cyclopentene ; CPD : cyclopentadiene Oxygen storage capacity of Pt/CeO 2 was measured at 350-500~ by titration with CO of the active oxygen available at the preoxidized surface. OSC values are about four times higher on the sputtered catalyst than on the s~mple prepared by wet impregnation (at 450~ : 600 instead of 150~mol CO 2 gPt-1). This proves that the mobility and the availability of surface oxygen ions of ceria are better when the catalyst is prepared by soft methods without solvent or foreign ions (C1 in this case). In conclusion, DC-sputtering appears as a convenient method of preparing active and selective catalysts, especially designed for low-temperature processes. Extremely clean model catalysts can also be prepared by this technique.
REFERENCES 1. P. Albers, K. Seibold, A.J. McEvoy and J. Kiwi, J. Phys. Chem., 93 (1989) 1510 2. O. Enea, M. Rafai and A. Naudon, Ultramicroscopy, 42-44 (1992) 572 3. O. Enea and A. Naudon, in A. Davenport and J. G. Gorden II, X-ray Methods in Corrosion and Interfacial Electrochemistry, EC. Set., PV 92-1, Electrochem. Soc., New-York, 1992, p. 194. 4. O. Enea, M. Rafai, A. Naudon, M. Cahoreau, and A.J.McEvoy, ISE Abstracts 43(1992)403 5. O. Enea and M. Rafai, Ultramicroscopy, submitted. 6. D. Duprez, J. Chim. Phys., 80 (1983) 487 7. S.Kacimi, J. Barbier Jr, R.Taha and D.Duprez, Catal. Lett., 22 (1993) 343.
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
949
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 a P l a t i n u m C o n t a i n i n g Catalytic Membrane
i
Xiuren Zhao and J u n h a n g Jing Chemical Engineering Department, School of Chemical Engineering, Dalian University of Technology, Dalian 116012, China. ABSTRACT P l a t i n u m was deposited by impregnation into the framework of Ta l u m i n a m e m b r a n e tubes with an a s y m m e t r i c configuration, u s i n g ammoniac-hexachloroplatinic solutions at different pH values and dipping times. Metallic platinum was obtained after calcination and reduction. The microstructure of the membranes was studied by SEM and BET; their gas permeabilities were m e a s u r e d as well. The h e a t delivered during the formation of PtO on membranes p r e p a r e d in different conditions were measured in order to compare their activities. Cyclohexane dehydrogenation reaction was carried out on these membranes. The effect of the preparation conditions on the catalytic activities is discussed.
1. INTRODUCTION Catalytic membrane reactor is one of the various membrane reactor configurations which combines separation and reaction processes. The characteristic of this kind of reactor is t h a t the membrane is catalytic and permselective. The idea of catalytic membrane was first suggested by Sun [1]. Thereafter, B u r g g r a a f [2] pointed out t h a t the modification of ~ - a l u m i n a membrane into catalytic membrane was prospective. Recently, Champagnie et al. [3] carried out ethane dehydrogenation reaction over platinum modified T -alumina m e m b r a n e . Zaspalis et al. [4] carried out oxydehydrogenation reaction of methanol over silver modified alumina membrane. CO catalytic oxidation reaction over platinum modified alumina membrane was studied by Veldsink et al. [5] and toluene hydrogenation reaction was investigated on the same kind of membrane by Uzio et al.[6]. Platinum is one of the most frequently used catalysts and since we have some experience in the p r e p a r a t i o n of a l u m i n a m e m b r a n e , we have p r e p a r e d p l a t i n u m containing catalytic m e m b r a n e on self made asymmetric y - a l u m i n a m e m b r a n e tubes. The morphology and permeability of the membranes before and after platinum deposition are compared. The effect of the preparation conditions, such as dipping time and pH value of the i m p r e g n a t i n g solutions, on the 1 This workis sponsoredby SINOPECof China.
%0 microstructure of membranes and their catalytic activity for cyclohexane dehydrogenation are studied. 2. EXPEREWENTAL The starting material was a one end closed 12cm long, lcm outer d i a m e t e r and 1.5ram thick a - a l u m i n a porous tube with a s y m m e t r i c configuration. Its support and outer layer had an average pore size of ca 2 and 0.15ttm, respectively. T-alumina membrane was coated on the periphery of the tube by sol-gel method according to Yoldas [7]. The porous a-alumina tube was dipcoated in a sol prepared by hydrolysis of aluminum isopropoxyde, using nitric acid as catalyst and dried in a fridge for 24 hours in order to form the gel. The gel was calcined in a furnace at a heating rate of 10 degrees per hour until the temperature reached 773 K. The dipcoating and calcinating steps were sometimes repeated several times in order to form a crackless membrane. A non-supported membrane on a glass plate was prepared in similar conditions in order to know the top layer pore size distribution of the supported membrane tube. Platinum containing catalytic membrane was prepared in the following way: aqueous ammonia was added to a 0.01 M hexachloroplatinic acid solution until the pH of the solution reaches a definite value. The closed vessel was kept in a cool place for one day until the complex ion of platinum and ammonia was formed. The membrane tube was dipped into this solution for at least 0.5 h, taken out and the surface was carefully washed with deionized water. It was dried at room temperature for one night and air calcined at 723 K. In order to study the effects of pH and dipping time on the microstructure and activity of the m e m b r a n e s , p l a t i n u m containing m e m b r a n e s modified at different conditions were also prepared. Non-supported modified m e m b r a n e s were prepared as well. Pore size distributions before and after modification of the membranes were compared using classical nitrogen adsorption-desorption (BJH) method on an ASAP 2400 apparatus. Permeability of pure nitrogen across these membranes were measured by an ordinary method. Electron microscopy (JEOL 100 CX microscope) was used to study the morphology and structure of the cross section and periphery of the membrane tubes. According to Stone et al. [8], a Shimadzu DT-30B DTA was used to measure the heat delivered during the formation of PtO on Pt modified membranes prepared under different conditions, using oxygen as strongly chemisorbed gas and pure nitrogen as sweep gas. By means of the total heat of adsorption and heat of physical desorption measured, the relative value of the heat delivered during the formation of PtO on various m e m b r a n e samples could be calculated.
951
Fec~in
I
i
'
"I
FeccJ ~oJt
i
~rare
Figure 1. Scheme of catalytic membrane reactor A catalytic m e m b r a n e reactor was designed and cyclohexane dehydrogenation reaction was carried out in order to compare the catalytic activities of the membranes prepared under different conditions. The reactor was 1 5 5 m m in length and 16ram inner diameter, the m e m b r a n e tube was located at the center of the reactor and the periphery of its open end was sealed on the neck of the reactor, in such a way that the tube divided the reactor into two zones, i.e.reaction and permeation sides. Cyclohexane vapor was brought into the reaction zone by nitrogen, and argon was used as sweep gas. The reactor was heated in a temperature controlled furnace and the reaction product was analyzed with a Shimadzu G C - 8 A chromatograph and C R 3 A chromatopac.
3. R E S I S T S AND DISCUSSION The SEM micrograph of the m e m b r a n e tube cross section (Figure 2) shows t h a t the thickness of m e m b r a n e layer is about 3ttm, and t h a t of the intermediate layer is 50pro. Figure 3 gives the pore size distribution of the m e m b r a n e s before and after platinum modification. The average pore diameters are 5.25 and 6.39 nm and the specific surface areas are 275 and 303 m2/g, respectively. It is clear t h a t the average pore diameter of the platinum containing membrane is about l n m less t h a n t h a t of the unmodified membrane. The results of pure nitrogen permeation test over these two kinds of membrane tubes are given in Figure 4. The dipping time of tube A is 0.5 h and tube B is 12 h. In this diagram all the lines are nearly parallel to the X axis, which means t h a t the flow p a t t e r n of nitrogen across these m e m b r a n e tubes are mainly Knudsen diffusion [9]. The permeability of the platinum containing m e m b r a n e is always lower t h a n t h a t of the unmodified membrane, and the longer the dipping time, the lower the permeability. According to the results of the pore size distribution and permeability m e a s u r e m e n t s (tube A), we find that the slopes of permeation lines vary a little after Pt deposition as well as the value of their intercepts. It shows t h a t the fine metallic P t particles have entered the micropores of the m e m b r a n e and slightly blocked the path. We assume t h a t very fine Pt particles are adhering on the wall of the T-alumina micropores, so the pore distribution of the m e m b r a n e s before a n d after modification only changes a little.
952
Figure 2. SEM of cross section
2.00 ~ .
v
o o
E m
1.50
1.00
J
0
>
~0
0
0.50
EL 0.00 100
10
Pore Diameter 9
With
Figure 3. Pore size distribution
Pt
0
(nm) None
Pt
953
(3
4
s
E I
0
K
9 (1)
o
2
v
>,
tv
. ~
.(3
E 0_
0 120
160
200
Mean pressure (Kpa)
Figure 4. Permeability of membranes before and after platinum deposition The permeability of the unloaded tube B obviously increases compared with that of platinum loaded. In figure 4, the intercept value of the line II is nearly twice as large as that of line IV (1.373 and 0.78, respectively), but the slope of line II is half that of line IV (0.00679 and 0.00344, respectively), which shows that with prolonged dipping time, the Pt loading on the membrane also increases. The pore blocking effect of Pt particles become also more serious, so that the pore size distribution of the membrane is improved. Table 1. Relative values of the heat of PtO formation on the membranes pH of impregnating solution Dipping time h 4 7 0.5 1.06 1.00 3 1.11 1.03 4 1.29 1.20
12 1.27 1.34 1.51
By means of DTA measurements, the relative heat delivered during the formation of PtO on non-supported platinum containing membranes prepared in various conditions are listed in Table 1. It is clear that with an impregnating solution of definite pH value, the longer the dipping time, the higher the heat delivered. The effect of pH is obvious. Either acidic or alcaline solution is better than neutral solution. Brunelle [10] pointed out that the adsorption of metallic complex ions on oxides is determined by two factors, i.e. the pH value the impregnating solution and the character of the metallic complex ion. In our experiment, [Pt C16]2- is adsorbed on the surface of T-alumina if the solution is acidic. On the other hand, [Pt(NH3)4] 2+ is adsorbed on the surface of Talumina. If the solution is neutral, the adsorbed species could be H2Pt C16. Different pH values cause different adsorption mechanisms. From the DTA
954 data, the best result is obtained if the pH value of the impregnating solution is 12. 100
80 ~
60
o
4o
:~
20
II
o 1.0
Flow
rate pH=4 ~
0.5
of
cyclolqexane~ql/h pH=7 ~
pH=12
Figure 5. Comparison of the catalytic activities over Pt containing membranes Three platinum containing membrane tubes with the same size as mentioned above were prepared using impregnating solutions of different pH values (pH = 4, 7, 12 respectively). Cyclohexane dehydrogenation reaction was carried out on these membranes in the catalytic membrane reactor. In order to compare their reaction activities, the relative conversion rates of cyclohexane over these membranes at different flow rates are shown in Figure 5. The relations obtained are similar to that of the DTA measurements. The best result is obtained when the platinum containing catalytic m e m b r a n e is prepared by impregnating in an ammoniac hexachloroplatinic solution of pH 12. 4. CONCLUSION 0
Q
0
1
Tubular asymmetric T-alumina ceramic membranes have been prepared. The flow pattern of N2 across these membranes are mainly Knudsen diffusion. P l a t i n u m m o d i f i e d m e m b r a n e s are also p r e p a r e d by m e a n s of impregnating the above mentioned membrane tubes into an ammoniac hexachloroplatinic solution. The average pore size of the platinum modified membrane is slightly smaller than that of the unmodified membrane. The heat delivered by the formation of platinum oxide measured by DTA and the dehydrogenation rate of cyclohexane over these platinum modified catalytic membranes show that the pH value of impregnating solutions
955 affects the activity of the catalytic membranes. In this work, the best pH value is 12. The effect of dipping time was also studied. R~'ERENCIgS 1. Y.M. Sun and S.J. Khang, Ind. Eng. Chem. Res., 29 (1990) 231. 2. A.J. Burggraaf and K. Keizer, ICIM (1) Proceeding (1989) 311. 3. A.M. Champagnie, T.T. Tsotsis, R.G. Minet and I.A. Webster, Chem. Eng. Sci., 45(1990) 2423. 4. V.T. Zaspalis, W. van Praag, K. Keizer, J.G. van Ommen, J.B.H. Ross and A.J. Burggraaf, Appl. Catal., 74 (1991) 235. 5. J.W. Veldsink, R.M.J. van Damme, G.F. Versteeg, W.P.M. van Swaaij, Chem. Eng. Sci., 47 (1992) 2939. 6. D. Uzio, A. Giroir-Fendler, J. Lieto and J.A. Dalmon, Key Eng. Mat., 61 & 62 (1991) 111. 7. B.E. Yoldas, Ceram. Bull., 54 (1975) 285. 8. R.L. Stone and H.F. Rase, Anal. Chem., 29 (1957) 1273. 9. K. Keizer, R.J.R. Uhlhorn, R.J. van Vuren and A.J. Burggraaf, J. Memb. Sd., 39 (1988) 285. 10. J.P. Brunelle, Pure Appl. Chem., 50 (1978) 1211.
This Page Intentionally Left Blank
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
The utilization of satm'ated heterogeneous catalysts
gas-solid
reactions
957
in
the
preparation
of
S.Haukka, A.KytOkivi, E-L.Lakomaa, U.Lehtovirta, M.Lindblad, V.Lujala, T.Suntola Microchemistry Ltd., P.O.Box 45, 02151 Espoo, Finland
ABSTRACT Saturated gas-solid reactions known from Atomic Layer Epitaxy (ALE) were used to process various catalysts. Good homogeneity of metal species was verified both along the entire catalyst bed and inside the particles. A variety of volatile metal compounds including metal chlorides, alkoxides and 13-diketonates were successfully used as reactants. The ALE processing is described with reference to examples demonstrating the achievement of surface saturation, reproducibility of processes, selection of process parameters, growth of oxides to modify the support and the binding of two metal compounds.
1. INTRODUCTION Gas-solid reactions are being widely explored for their potential in the manufacture of structurally well-defined catalytic surfaces. According to 1UPAC recommendations [1], deposition taking place by adsorption or reaction from the gas phase is called chemical vapour deposition (CVD). Examples of the use of CVD in tailoring support surfaces and in binding active components to the support can be found in references 2 - 7. Interesting features have been introduced to catalysts by these methods. Molecular level control and uniformity through the particles have not always been achieved, however. We describe here means to a better controlled preparation of catalysts by making use of gas-solid reactions and the property of the surface to saturate itself with the reactant as suggested in the growth of layer by layer structures in Atomic Layer Epitaxy (ALE) [8]. In the ALE technique, a better control of the build-up of surface structures is achieved by the sequential introduction of the active components, and other surface-modifying agents, in saturating gas-solid reactions. ALE can be classified as a special mode of CVD, since strict demands are made upon the conditions under which the gas-solid reactions are carried out. Uncontrolled deposition through condensation of the reactants or their decomposition products is prevented by the choice of reaction temperature. Not only must the solid surface be saturated with the chemisorbed species, but it must be stabilized before each reaction sequence. This means that physisorbed molecules such as water must be removed from the starting support surface by heating, and after each reaction any unreacted reactant must be removed, normally by inert gas purge.
958 For well-defined structures to be produced on the support surface chemisorption is required. The surface species then occupy their final bonding sites at the outset and there is no need for alter-treatment at elevated temperature. Through saturation of the surface, the surface density obtained during each reactant sequence is controlled by the surface itself. The capacity of the surface to chemisorb the reactant, not the dosing of the reactant, determines the amount bonded. The processing of the catalyst is not sensitive to a precise dosing of the reactant; all that is required is that the dose be sufficient for the reaction of all binding sites. This selfcontrolling feature of ALE allows a homogeneous (uniform) distribution of surface species throughout the porous support and gives good reproducibility in obtaining a desired saturation level. The self-control is also a good feature for scale-up. The saturation density obtained in a reaction sequence depends on the number of the bonding sites, the size and chemical form of the reactant molecule and the reaction temperature. If part of the original ligands of the reactants remain present in the surface complex the saturation density will usually be less than the full monolayer coverage of the corresponding oxide. Various means to regulate the saturation level have been reviewed in [9]. Active catalysts for a variety of reactions have been processed by the ALE method [ 10-13 ]. We shall present some examples of how the surface saturation proceeds using 5 - 1000 g of alumina or silica supports and of the reproducibility of ALE in binding a single metal compound on the surface. As well, the selection of reaction conditions will be described, and examples will be given of modifying the surface with a sublayer of a metal oxide and of binding two different metal species. 2. EXPERIMENTAL
2.1. Equipment The catalysts were processed in flow-type reactors with heated zones for the reaction vessel and for vaporizing solid reagents [9, 14]. Processing was done at a pressure of 6-10 kPa or at ambient pressure in nitrogen atmosphere. Reaction vessels of 0.1 - 2 1 were used. The heating and gas valving were computer controlled. 2.2. Reagents The support materials were 7-A1203 (AKZO Alumina 000-1.5E) and silica (Grace 955) with surface areas of 200 and 270 m2/g, respectively. Alumina was used as extrudates with a particle diameter of 1.5 mm and a length between 2 and 20 mm and as crushed and sieved to a particle size of 0.15 - 0.35 mm. The particle size of the silica was 40-80 ~tm. ZrCI 4 (Fluka), TiC14 (Merck), WOCI4 (Aldrich), Cr(acac)3 (Riedel-de-Haen,), Ni(acac)2 (Merck), Mg(thd)2 and Ti(OC3H7) 4 (Merck) were used as reactants without further purification. Mg(thd)2 was synthetized according to [ 15]. 2.3. Procedure The number of bonding sites was stabilized to a selected level by preheating the support at temperatures of 200-850 ~ The reactants were then volatilized from liquids or solids and led to the top of the solid support bed held at a selected temperature. The reaction temperature
959 was selected so that the activation energy for chemisorption was exceeded and the decomposition or condensation of the reactant was prohibited. The dose of the reactant was kept high enough to exceed the number of bonding sites available. The reaction time required was calculated so that an overdose of the reactant as compared with the number of bonding sites was brought into the reaction vessel. A purge with inert gas followed the chemisorption, at the same temperature. The efficiency of transport of the reactant vapour into the reactor is determined by the vapour pressure of the reactant at the vaporization temperature selected and the rate of the nitrogen flow. The process can consist of one chemisorption stage or of several stages each followed by an inert gas purge to avoid the presence of two reactants in the reaction chamber at the same time.
2.4. Characterization Inert sampling could be done when desired. Zr, W and Ni were determined by XRF, Ti and Cr by neutron activation analysis (NAA), Mg by AAS, C with a Leco carbon analyzer and CI by potentiometric titration. FTIR in diffuse reflectance mode was used to follow the chemisorption and to detect possible decomposition of the reactant. Scanning electron microscopy with an energy dispersive spectrometer (SEM/EDS) was used to determine element concentrations through the particles. The specific surface area and pore volume were determined by means of nitrogen adsorption and condensation with lk,ficromeritics ASAP 2400 equipment. Detailed experimental conditions used in the characterization are in Ref. 16.
3. RESULTS
3.1. Surface saturation and catalyst homogeneity The saturation of the support surface with the reactant was followed by taking samples from the top and bottom parts of the support bed. Figure 1 shows the achievement of surface saturation as a function of reactant dose. Saturation of bonding sites proceeds from the top of the support bed towards the bottom, i.e. in the direction of the reactant flow.
0
E E tO
0.6 0
o
0.4
Top
t__ t| 0
Bottom
0.2
0 0
|
0.0
0.2
0.4
0.6
0.8
1.0
Reactant dose (mmol/g)
Figure 1. Surface saturation (mmol/g) as a function of the reactant dose (mmol/g). Metal determinations in samples taken from the top and the bottom of the fixed bed were made.
960 The homogeneity of saturated samples was also verified for a larger scale processing with 300 -1000 g of support. Figure 2 shows the variation in Zr saturation density in different parts of a
Zr/SiO 2 catalyst bed containing 300 g of the catalyst.
l
120 o~ >,, e-.
~
t-
100 80 60
O
~
I
q/
6 cm
=
40
=
20
0
1 ~
15 cm >
2
3
4
5
Sampling point
Figure 2. Macroscopic homogeneity of Zr in larger scale processing from ZrC14 on SiO 2.
Macroscopic homogeneity does not rule out the existence of a concentration gradient within the particles. Uniformity within the particles was therefore investigated by embedding the catalyst in epoxy resin, cutting cross-sections of particles with a microtome and analysing by SEM/EDS. Macroscopically homogeneous W/A1203extrudate samples were found to contain an uneven distribution of W because of a too fast flow rate of the reactant in processing. The combination of extrudate support and a fast WOCI4 reactant flow rate resulted in the loss of the reactant through the bed. Table 1 shows the tungsten concentration of W/A1203 samples with two different flow rates of the reactant through powder and extrudate beds. Reaction time was constant. The lower tungsten content for the catalyst processed from extrudates at the higher reactant flow rate was due to an eggshell distribution, as revealed in SEM/EDS analysis.
961
Table 1 Tungsten and chloride concentration of extrudate and powder samples with fast and slow WOC14 flow rate. The preheating of alumina and the reaction with WOC14 were carried out at 200~ Particle size Flow rate W (wt-%) CI (wt-%) powder powder extrudates extrudates
fast slow fast slow
11 11 6 11
4.8 4.8 2.3 5.1
3.2 Reproducibility The reproducibility from run to run was investigated by determining the metal concentration in samples taken from the top and bottom of the fixed catalyst bed. Table 2 shows the reproducibility of binding a single metal compound to the support surface for different metal compound/support pairs. The reproducibility was within the accuracy of the element determination methods used ( XRF, NAA, AAS).
Table 2 Reproducibility of saturated metal concentrations (mean value _+ standard deviation) in different runs. Samples were taken from the top and bottom of the catalyst bed. Metal compound/ Preheating/ Metal (wt-%) Metal (wt-%) Number support pair reaction top bottom of temperature process (~ runs ZrCI4/SiO 2 300 / 300 6.6 + 0.2 6.6 + 0.2 5 ZrCI4 / SiO2 600 / 450 2.7 + 0.1 2.7 _+ 0.1 6 WOCI4 / AI20 3 200 / 200 10.6 + 0.4 10.6 + 0.3 10 Ni(acac)2/AI20 3 200 / 200 4.7 + 0.5 4.2 + 0.2 5 Cr(acac)3/SiO 2 820 / 200 0.75+ 0.03 0.70+ 0.0 3 Mg(thd)2/SiO 2 600 / 250 1.2 + 0.1 1.0 + 0.1 6
3.3. Process parameters Several ZffSiO 2 catalysts were processed by using three different reactors: two small-scale reactors operating under vacuum and at ambient pressure and a bench-scale reactor operating at ambient pressure. Figure 3 describes the control of the Zr concentration in Zr/SiO 2 as a function of the preheating temperature of SiO2. The results for processes carried out in the different reactors are in good agreement, demonstrating that surface saturation was achieved
962
o-.O. v
tO Ira. 4-o
C:
ID O C: o o
0 200
t___
N
400
600
800
Preheating temperature (~ Figure 3. The Zr concentration as a function of the preheating temperature Of SiO 2 in ZrCl4/SiO 2 processed with different reactors. Reaction temperature 300 ~ - V - 6 kPa, (5-10 g) + ambient pressure, 1 kg and 10 g , reaction temperature 450 ~ --o-- 10 kPa, (5-10 g) 9 ambient pressure in two different equipment (5-10 g).
A
5
10
C/Ti ratio
Ti (wt-%)
10
~o O~O
~
0
_ _
100 |
*
. . . .
3900
|
.
3400
9 ,
,
i
r3~O
150
o
0
200
R e a c t i o n t e m p e r a t u r e (~
2900
Wavenumber (cm "1) Figure 4. (a) FTIR-spectra, in the O-H- and C-H-vibration regions, of Ti(OPr)4/),-A1203 prepared at 100, 110, 150, 170 and 190~ (from the top to the bottom of the picture) compared with 800~ alumina, and (b) the corresponding C/Ti ratios and Ti concentrations.
963 Volatility and stability of the reactants at the chemisorption temperature is a prerequisite for ALE processing. Metal chlorides can often withstand high temperatures, whereas metals with organic ligands often need milder reaction conditions. Even though the reactant as such can withstand elevated temperatures, the support surface may catalyse decomposition of the reactant already at somewhat lower temperatures. The decomposition of titanium isopropoxide, Ti(OPr)4, on alumina is an example of this. The reaction of Ti(OPr)4 was followed by FTIR and element determinations at different temperatures as shown in Figure 4. The decomposition of the reactant is seen as the gradual decrease in the intensity of the C-H vibration bands (2800-3000 cm -1) of the ligand and in the atomic C/Ti ratio of the samples with increasing reaction temperature. At 190~ all carbon was removed from the sample already during the binding of Ti(OPr) 4 to alumina.
3.4 Modification of the surface by growing oxides Reaction sequences of a metal compound and a ligand-removing reactant can be used to modify the support with oxides, sulfides and so on. Useful metal compounds are chlorides, alkoxides or 13-diketonates and the reactant for ligand removal can be water vapour, H2S or 02. We have grown TiO2 on silica [14, 17], WO 3 on alumina [18] and ZrO 2 on silica and alumina [19] by the ALE method. The number of reaction cycles is selected according to the desired modification. Each reaction sequence is led to surface saturation. One requirement of such growth is that suitable bonding sites be available for the next chemisorption reaction of the metal compound. A lack or decrease of bonding sites halts the layer growth. 3.5 Binding of two metal compounds Two or several metal compounds can be bound selectively as long as bonding sites are available. The second metal compound can be brought onto a modified surface, for example Cr onto silica modified with TiO 2, or two different metal compounds can simply be brought onto a support alternately, with no ligand removing reaction in between. The pulsing order of the reactants to the surface may change the surface saturation density, as shown in Table 3.
Table 3 Saturation densities of Zr and Ti on SiO2 preheated at 450~ Chemisorption temperature of 300~ was used for ZrCI4 and 200~ for Ti-isopropoxide. Reactant and pulsing order Zr saturation density Ti saturation density ( at/nm2) (at/nm 2) ZrCI 4 + Ti-isopropoxide Ti-isopropoxide + ZrC14 ZrCI 4 alone Ti-isopropoxide alone
0.8 1.3 1.1 -
0.4 0.4 1.4
964 4. DISCUSSION
The progress of the surface saturation was followed for each reactant by determining the metal concentration of the samples taken from the top and bottom of the fixed bed. When the metal concentrations of the two samples are the same, the macroscopic surface saturation is assured. To confirm the penetration of the metal compound into the pores requires a determination by SEM/EDS of the metal in particle cross-sections. The use of fixed bed thus provides a means to check that the surface saturation is complete. Although a fluidized bed can be used, the information on the surface saturation cannot then be obtained by element determinations alone. In a fixed bed the unsaturated situation can easily be demonstrated by using low dosing of the reactant, either by vaporizing an underdose or by keeping the reaction time too short to reach the saturation. An unsaturated situation may also occur due to diffusion limitations, which might happen, when using a fast reactant flow in combination with extrudates. Gas phase methods relying on dosing are a common means of processing catalysts [5-7]. Many of the papers describing experiments in which the reactant is dosed note the difficulty of achieving good homogeneity even at macroscopic level. The macroscopic homogeneity can be achieved by using fluidized bed [5] or by rotating the whole catalyst bed [6]. However, whether the reactant penetrates into all possible bonding sites has not been carefully studied alter the mixing of the samples. Dosing most o~en results in inhomogeneous metal distribution, and the method cannot be considered as ready for scale-up. The use of surface saturation conditions offers advantages in this respect due to the selfcontrolling feature. Good homogeneity of metal content even in 1 kg scale, as well as, good reproducibility from run to run is obtained by ALE. The use of various reactant/support pairs shows that a wide variety of catalytic surfaces can be processed. A prerequisite for good reproducibility is that the number of bonding sites is stabilized to a selected level, the reactant is stable at the reaction temperature used and the reactant dose is high enough for surface saturation. A suitable dose for achieving surface saturation can be calculated once the chemisorption mechanism and the number of bonding sites are known. Regulation of the metal content, however, demands other means [9] than those commonly used in impregnation. In routine use, element determinations can be used to check the surface saturation and scaleup without the need for strict dose control makes the process facile. The pressure of the reaction chamber had no effect on the surface saturation, which is as expected since the saturation density is determined by the number of bonding sites and the energy available to produce chemisorption to these sites. The transport of the reactant into the reaction chamber is determined by the vapour pressure, and the flow rate of the vapour to the support bed. Once the reactant is inside a pore it will continue to react so long as bonding sites are still available. The same surface saturation was achieved by using three different reactor set-ups and either a lower pressure of 6-10 kPa or ambient pressure in nitrogen flow. In ALE processing the reaction conditions are selected to lead to chemisorption. Differing from many CVD processes, in which thermal decomposition of the reactant ot~en is a desired part of the reaction, in ALE processing decomposition of the reactant is prohibited. The
965 reaction temperature in ALE must nevertheless be high enough to avoid condensation of the reactant. The decomposition of many reactants, for example alkoxides and carbonyls, makes them in some cases unsuitable for ALE processes. In fixed bed, the first sign of decomposition was often an increase in the metal content in the top of the bed as compared with the bottom. Mixing of the bed by fluidization or stirring would thus destroy the first signs of decomposition of the reactant. Chlorides and oxychlorides are not very sensitive to decomposition, but a check of the decomposition temperature should be made for metal compounds with organic ligands. An example of the selection of the reaction temperature is the deposition of Tiisopropoxide on alumina. FTIR revealed a partial decomposition of the ligand, and this decomposition increased with the reaction temperature. The commencement of decomposition may remain undetected if samples are not thoroughly analysed. The reaction temperature is not only determined by the thermal behaviour of the reactant but also by the decomposition catalysed by the support. Thus the reaction temperature range within which Ti-isopropoxide can be bonded to an alumina support is narrower than the one usable for silica. The binding of one metal compound may be followed by an oxidation or reduction to change the oxidation state of the metal species. The support can also be modified by treating the surface with several cycles of metal compound and air/water. Modifying the surface often has important advantages. For example, the favourable mechanical properties of a support like alumina can be combined with the favourable chemical nature of the new surface species created, to obtain a catalyst that does not cause cracking or other undesired side reactions in catalysis. The surface areas of the support is not significantly reduced when modification is done by ALE. Thus materials that are difficult to produce with large surface area can be grown on various supports by ALE. Other surface species can be synthesised after the first metal compound has been bound. The new surface with a single metal compound serves as a support for the second reactant, which may be another metal or some other compound promoting the catalytic function. The second reactant may react with sites energetically unfavourable to the first reactant. It may also replace part of the sites already occupied by the first reactant or bind straight to the first metal or to its ligands. The bonding mode depends on the type of reactant/support pair. The pulsing order of the reactant was seen to have an effect on the saturation densities of the metals. When ZrC14 and Ti-isopropoxide were used as reactants and Ti-isopropoxide pulsed first, the saturation density of Zr was greater than that obtained with ZrC14 alone. When ZrC14 is subsequently pulsed to the Ti/silica surface, it replaces the main part of the Ti species and the volatile Ti compounds are vaporized. The tailoring of the catalytic surfaces becomes possible when more than one reactant is used, and the reactants and their pulsing order is selected so that desired surface density and proximity of the different metals is achieved. 5. SUMMARY Some basic features of the application of ALE to the processing of catalysts have been described. We successfully processed several types of active catalysts by the method, and easily achieved good homogeneity for several metal compounds. The controllability of the catalyst preparation is good so long as the proper reaction conditions are maintained. The advantages
966 of ALE reactions are even more obvious when it is desirable to have more than one metal compound bound to the surface or more complex surface structures.
REFERENCES
1. 2. 3. 4. 5.
J. Haber, Pure and Appl. Chem. 63 (1991) 1227. K. Asakura, M. Aoki and Y. Iwasawa, Catal. Lett. 1 (1988) 395. M. Niwa, N. Katada and Y. Murakami, J. Phys. Chem. 94 (1990) 6441. D. Mehandjiev, S. Angelov and D. Damyanov, Stud. Surf. Sci. Catal. 3 (1979) 605. M.P. McDaniel and P.M. Stricklen, CO reduced chromylhalide on silica catalyst. US Patent 4 439 543 (1984). 6. S. Sato, M. Toita, T. Sodesawa and F. Nozaki, Appl. Catal. 62 (1990) 73. 7. J. NicE, D. Dutoit, A. Baiker, U. Scharf and A. Wokaun, Appl. Catal. A: General 98 (1993) 173. 8. T. Suntola, Mater. Sci. Rep. 4 (1989) 261. 9. E-L. Lakomaa, Appl. Surf. Sci. 75 (1994) 185. 10. L-P. Lindfors, E. Rautiainen and E-L. Lakomaa, Catalyst for Aromatization of Light Hydrocarbons, US Patent 5 124 293 (1992). 11. H. Knuuttila and E-L. Lakomaa, Method for Preparing a Catalyst for Polymerization of Olefins. US Patent 5 290 748 (1994). 12. J. Hietala, P. Knuuttila and A. Kyt6kivi, Metathesis Catalyst for Olefins, FI Patent 87891 (1993). 13. L.P. Lindfors, M. Lindblad and U. Lehtovirta, Method for Manufacturing a Catalyst Suited for Hydrogenation of Aromatics, FI Patent 90632 (1994). 14. S. Haukka, E-L. Lakomaa and T. Suntola, Thin Solid Films 225 (1993) 280. 15. G. S. Hammond, D.C. Nonhebel and C-H. S. Wu, Inorg. Chem. 2 (1963)73. 16. S. Haukka, Characterization of Surface Species Generated in Atomic Layer Epitaxy on Silica, Diss. Helsinki Univ., J-Paino Ky, Helsinki, 1993.46 p + 8 App. 17. E-L. Lakomaa, S. Haukka and T. Suntola, Appl. Surf. Sci. 60/61 (1992) 742. 18. M. Lindblad and L.P. Lindfors, Proc. 10th Int. Conf. on Catalysis, July 19-24, 1992, Budapest. Hungary, L. Guczi, F. Solymosi and P. Tetenyi (Eds.), Akademiai Kiado, Budapest, 1993, p. 1763. 19. A. Kyt6kivi and E-L. Lakomaa, Proc. Europa-CAT-l, Sep. 12-17, 1993, Montpellier, France, Book of Abstracts 1, p. 499. ACKNOWLEDGEMENTS Mirja Rissanen is thanked for her contribution to the processing experiments.
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
967
I d e n t i f i c a t i o n o f s u p p o r t e d p h a s e s p r o d u c e d in the p r e p a r a t i o n of silicasupported Ni catalysts by competitive cationic e x c h a n g e M. Kermarec a, A. Decarreau b, M. Che a and J. Y. Carriat a a Laboratoire de R6activit6 de Surface, URA 1106, CNRS Universit6 Pierre et Marie Curie, 4 place Jussieu, 75252 Paris Cedex 05, France b Laboratoire de P6trologie de la Surface, URA 721, CNRS Universit6 de Poitiers, Avenue du Recteur Pineau, 86022 Poitiers Cedex, France Among the techniques used to characterize silica-supported Ni phases, FTIR spectroscopy is shown to be well adapted to identify ill-crystallized phases generated during the preparation by the competitive cationic exchange method. FTIR spectroscopy permits to discriminate a phyllosilicate of talc-like or serpentine-like structure from a hydroxide-like phase. Samples submitted to hydrothermal treatments have also been characterized by other techniques such as EXAFS and DRS spectroscopies. The pH and the specific surface area strongly influence the nature of the deposited phase, since they control the solubility and the rate of dissolution of silica. The results are discussed in terms of the respective amounts of soluble Si(OH)4 monomers and Ni2§ complexes at the interface. The relevant parameter as the Ni/Si ratio at the solid-liquid interface is assumed to control the routes to Ni-Si (Ni-Ni) copolymerization (polymerization) reactions leading to supported Ni phyllosilicates (Ni hydroxide). 1. I N T R O D U C T I O N In the preparation of supported metal catalysts where transition metal ions are deposited on an oxide support, the choice of the method of preparation and of the precursor complex controls the formation of isolated supported ions or the deposition of supported intermediate phases [1, 2]. In the former case corresponding to interfacial coordination chemistry, the support acts as a supramolecular tridentate ligand whereas in the latter case it acts as a reactant to produce intermediate phases. Depending on the experimental conditions, the preparation of Ni/SiO2 materials by exchange, impregnation and deposition-precipitation methods may give rise to layered silicates of talc-like, serpentine-like structure (named phyllosilicates) and/or nickel hydroxide phase [3-9]. When the competitive cationic exchange method (CCE) is employed, the deposition of [Ni(NH3)6] 2+ complexes was shown to give rise after drying to supported phyllosilicates whereas [Ni(en)3] 2+ complexes (where en = ethane-diamine) inhibit their formation and lead to isolated Ni 2§ ions, i. e., ions without Ni 2+ neighbors [8]. Although the presence of phyllosilicates has been postulated from TPR studies [7, 10, 11] and proven by EXAFS results [8, 9, 11, 12], the identification of the nature of the supported phases in the case of a mixture of phases is not straightforward. The possibility of precipitation of a Ni hydroxide-like phase adds to this difficulty. The objective of this work is to investigate the role of parameters such as the pH of the impregnation solution and the specific surface area on the nature of the supported phase. Since, for the characterization of a bulk phyllosilicate, the sensitivity of most of the techniques of characterization, depends on its degree of crystallinity [ 13, 14], the identification of supported phases was made by comparison with reference bulk compounds of various degrees of crystallinity. Spectroscopic techniques such as UV-vis diffuse reflectance (DRS), FTIR and
968 EXAFS were used and their relevance to the problem of characterization discussed. The results depicted hereafter have been restricted to the CCE method using the [Ni(NH3)6] 2+ complex. 2. E X P E R I M E N T A L Natural clays of TOT (2:1) or TO (1:1) structure are constituted of layers of SiO4 tetrahedra (T) and NiO6 octahedra (O) [15]. Synthetic clays corresponding to Si4Ni3010 (OH)2 (Ni talc, TOT phyllosilicate) and Si2Ni305(OH)4 (Ni serpentine referred to as nepouite, TO phyllosilicate) respectively, were hydrothermally synthesized in the 25-250~ range according to a procedure already described [ 16, 17]. These samples are referred to as Ta-x and Ne-x where Ta, Ne and x stand for talc, nepouite and the temperature of synthesis respectively. An ill-crystallized Ni(OH)2 sample was prepared by adding 40 ml of a 1M ammoniacal solution to 50 ml of a 0.4 M Ni(NO3)2 solution. The precipitate was then washed and centrifuged. A well-crystallized sample was obtained after hydrothermal treatment at 190~ during 14 days. Silica-supported Ni samples were prepared by the CCE method in ammoniacal solutions at various pH. Aerosil 380 and Aerosil OX50 silicas (specific surface areas: 380 and 50 m 2 g-1 respectively) supplied by Degussa were used as supports. Exchanged samples are referred to as E-pH-A380(50). Sample E-9.8-A380 (3.9% Ni) is prepared according to the following procedure: 2.5 g of silica were added to 50 ml of the exchange solution containing 0.1 M Ni (NO3)2 and 0.4 M NH4NO3. The pH was adjusted by bubbling gaseous NH3 before adding silica. The suspension was stirred in a thermostated vessel at 298K during 24 h. The sample was successively centrifuged and washed until the supernatant solution became transparent. After a last centrifugation, the sample was dried overnight at 80~ Sample E-8.3-A380 (9% Ni) refers to a sample prepared with the same exchange solution at pH 8.3. Samples mentioned above, submitted to hydrothermal treatment in water at 190~ are labelled as E-pH-A380(50)/h-z, where z stands for the number of days of hydrothermal treatment, denoted by the letter h. For comparison purpose, the supports were conditioned in the same way (medium, pH and duration of the impregnation treatment) as the supported Ni samples. DRS spectra were recorded at room temperature on a Beckman 5270 spectrophotometer in the 230-2500 nm range. For the IR study, the samples were dispersed in KBr pellets with a ratio of about lmg/100 mg KBr. The IR spectra were scanned at room temperature using a Bruker FTIR IFS 66V spectrophotometer equipped with a DTGS detector. The spectral conditions consisted in a resolution of 4 cm -1, with 30 scans. EXAFS experiments were performed at the LURE (Orsay) synchrotron radiation facilities using the D 44 beam line. The data were collected in the transmission mode. The samples were finely ground and homogeneously dispersed in cellulose pellets. The analysis of the EXAFS spectra was performed according to standard procedure for background removal, extraction of the EXAFS signal and normalization to the edge absorption. The EXAFS signal was simulated by using experimental phase and amplitude parameters for oxygen and Ni backscatterers (reference compounds are NiO and Ni(OH)2 respectively), while theoretical Mc Kale phase and amplitude backscattering functions were used for Si backscatterers. The energy threshold shift z ~ and "f parameters have been determined using crystalline samples Ta-500, and used as fixed parameters to calculate the number of backscatterers, the distances and the Debye-Waller factors (N, R,and ~ respectively) for ill-crystallized phyllosilicates synthesized at 25~
969 3. R E S U L T S
AND
DISCUSSION
3.1. Characterization of Ni phyllosilicates and Ni hydroxide as a function of the synthesis temperature. Previous studies by X-Ray diffraction have shown that the higher the temperature of synthesis, the better the crystallinity [ 13, 14]. We have characterized bulk Ni phyllosilicates and Ni hydroxide using EXAFS, DRS and FTIR spectroscopies.
3.1.1. Characterization by EXAFS For crystalline Ni talc and nepouite (Ni serpentine structure), the nearest coordination shell corresponds to oxygen atoms at 2.07 A while the next nearest Shell is constituted of 6 Ni and 4 (2) Si backscatterers for talc (nepouite) at very close distances (3.06 and 3.25-31 ,/k respectively) whose contributions cannot be resolved. For Ni hydroxide, the next nearest shell is constituted of 6 Ni atoms only. Accordingly, the intensity of the second peak of the Fourier transforms (FT) of the EXAFS signals grows in the series: Ni(OH2) < nepouite < talc. By contrast, the FT for the ill-crystallized samples synthesized at 25~ are very similar (not shown here). Table 1 reports the simulation parameters for the next nearest backscatterers as a function of the temperature of synthesis. The results show a decrease of the number of Ni and Si backscatterers when the degree of crystallinity decreases. The difference between the number of silicon atoms for talc and nepouite both synthesized at 150~ (ANsi = 1.5) is significant and agrees with the structure of phyllosilicates. For ill-crystallized samples synthesized at 25 ~ the number of next nearest backscatterers decreases markedly, owing to increased disorder in the tetrahedral and octahedral sheets. The number of Ni atoms is similar for talc and nepouite whereas the number of Si atoms (1.9 and 1.2) reaches values far from the theoretical ones (4 and 2 respectively). Table 1 Simulation parameters for Ni and Si backscatterers for phyllosilicates synthesized in the 25500~ range Sample
backscatterers N
o (A)
~(A)
R(A)
AE
Qa
Ta-500
Ni Si
b6 4
0.082 0.092
1.00 0.85
3.06 3.25
-0.6 -9.0
2.0. 10-3
Ta-150
Ni Si
5.6 3.6
0.086 0.104
1.00 0.85
3.07 3.25
-0.5 -8.9
1.8. 10 -3
Ta-25
Ni Si
4.8 1.9
0.096 0.113
1.00 0.85
3.10 3.24
-0.7 rg,1
8.0. 10-4
Ne-150
Ni Si
5.6 2.1
0.089 0.106
1.00 0.85
3.08 3.31
-0.6 -9.0
1.3. 10 -3
Ne-25
Ni
4.7
0.094
100
3.10
-0.7
1.3. 10-3
Si
1.2
0.115
0.85
3.26
-9.1
a agreement factor b underlined values correspond to fixed parameters.
970 The uncertainty on the number of Si atoms is larger than that on the number of Ni atoms. The value of the difference ANsi equals 0.7 for talc and nepouite synthesized at 25~ and seems too low (as compared to the theoretical value A NSi = 2), to permit the discrimination of of a phyllosilicate of talc structure from a phyllosilicate of nepouite structure.
3.1.2. Characterization by DRS The spectra of natural and synthetic phyllosilicates indicate an octahedral environment for Ni 2+ ions [ 18]. The deformation of this environment leads to a broadening of the Vl transition which corresponds to the intensity of the crystal field (A0 or 10 Dq). The results show that the crystal field increases with the cristallinity of the material. However, the ranges for Vl values found for minerals of talc, nepouite and Ni hydroxide structure overlap, and this precludes the use of this technique to clearly identify ill-crystallized supported Ni phases. 3.1.3. Characterization by FTIR Phyllosilicates of 2:1 and 1:1 type and Ni hydroxide exhibit characteristic OH and SiO vibrations [19-21]. Table 2 displays the frequencies for these modes. The 5OH mode allows to distinguish phyllosilicates of talc and nepouite structure, only if they are well-crystallized. Crystalline talc exhibit a 8OH mode around 711 c m 1, while this mode is observed around 668 cm -1, together with a tetrahedral SiO mode for nepouite. For ill-crystallized samples, the SiO band permits to identify the type of phyllosilicate. On the other hand, for Ni(OH)2, the 5OH vibration was shown to be very sensitive to the degree of cristallinity [22-23], and this in contrast to phyllosilicates. In any case, this band is always observed at lower frequencies than those observed for phyllosilicates. Table 2 SiO and structural OH frequencies for reference bulk compounds sample
VOH (cm "I)
8OH (cm -I)
8OH,SiO (cm -I)
SiO (era-I)
Ta-25 Ta- 150 Ta-250
3627 3627
712 711
669 665 665
1027 1031 1032
Ne-25 Ne- 150
3645
664 671
1048, 1007 1078, 977
Ni(OH)2-25 Ni(OH)2-190
3640
654 520
The analysis of the results obtained from these techniques applied to characterize bulk phyllosilicates, suggests that FTIR spectroscopy is the most reliable technique to identify the illcrystallized phases, generally produced in the preparation of Ni/Si02 materials.
971
3.2. Characterization of ill-crystallized Ni supported phases The experimental preparation conditions may influence the nature of the supported phases. Medium pH values around 8.5 are already well known to produce larger amounts of supported phyUosilicates [9, 24]. We have investigated the role of the pH and of the specific surface area of the support, by using EXAFS and FTIR spectroscopies only. 3.2.1. Influence of the pH and the specific surface area 3.2.1.1. EXAFS result Let us first examine the results concerning supports of high specific surface areas (A 380). The FT EXAFS signals concerning the next nearest Ni and Si backscatterers for Ni/SiO2 samples prepared at pH 9.8 and 8.3 are slightly more intense than those observed for bulk phyllosilicates synthesized at 25~ The number of next nearest backscatterers is higher for Ni/SiO2 samples than for bulk phyllosilicates, for comparable values of the Debye-Waller factor (Table 3). This result seems to indicate a higher degree of organization for the supported phyUosilicates. The existence of isolated Ni 2§ ions has been postulated for samples prepared at high pH (>9.8) in previous studies [9]. The presence of a significant amount of such entities (> 10%) would decrease the average number of Ni backscatterers. Hence, the higher value found for the number of Ni backscatterers for Ni/SiO2 samples in comparison to that observed for bulk phyllosilicates, suggests that no significant amount of isolated Ni 2+ ions (NNi < 1) is produced during the preparation. Table 3 Simulation parameters for Ni and Si backscatterers for phyllosilicates and Ni/SiO2 materials prepared at 25~ Sample
backscatterers N
o (A)
T(A)
R(A)
AE
Q
Ne-25
Ni Si
4.7 1.2
0.094 0.115
1,00 0.85
3.10 3.26
-0.7 -9.1
1.3. 10-3
Ta-25
Ni Si
4.8 1.9
0.096 0.113
1.00 0.85
3.10 3.24
-0.7 -9.1
8.0. 10-4
E-9.8-A380. Ni Si
5.1 2.5
0.095 0.115
1.00 0.85
3.09 3.30
-0.5 -9.0
1.4. 10-3
E-8.3-A380
5.3 2.3
0.096 0.111
1.00 0.85
3.09 3.29
-0.5 -9.0
2.0. 10-3
Ni Si
3.2.2.2. IR results Figure 1 a, b, c displays the spectra of samples E-9.8-A380 (3.9% Ni), E-8.3-A380 (9% Ni) and E-8.3-A50 (7.9% Ni) in the 1300-550cm -1 range. Besides the SiO vibrations of the silica support (around 1105 and 800 cm -1) the spectra exhibit a band around 668 cm "1 in the range of bending vibrations of structural OH groups. The intensity of this band increases with the Ni amount. The position of this band suggests the presence of an ill-crystallized phyllosilicate, but as discussed above, does not allow to identify its type (TOT or TO). Since the SiO band of the aerosil support overlaps the SiO band of the phyllosilicate, we have subtracted the spectrum of the support which has been conditioned in the same way as the
972 Ni/SiO2 sample. Figure 2 reports the subtracted spectra (spectra a, d, e) together with the spectra of Ta-25 and Ne-25 (spectra b and c respectively). The spectrum corresponding to sample prepared at pH 9.8 (spectrum a) resembles the spectrum of Ta-25 (spectrum b) and the SiO band is observed at proximate values (1030 and 1027 cm -1 respectively).
3
_•
1.00 -
eq ,q.
0.75
a
b
e~ e~
. .
~ 0.50-
r162
L,,
L.
@
0
.0
<
1-
0
<
I
1300
I
1000
0.00
I
750
0.25-
550
W a v e n u m b e r cm -1 Figure 1. Spectra of E-9.8-A380 (a), E-8.3-A380 (b) and E-8.3-A50 (c)
-I
1300
I
lO00
I
750
550
W a v e n u m b e r cm -1 Figure 2. Spectra of Ta-25 (b) and Ne-25 (c), Subtracted spectra for E-9.8-A380 (a), E-8.3-A50 (d) and E-8.3-A380 (e).
The asymmetric form of the SiO band for sample E-8.3-A380 (spectrum e), suggests the contribution of a high frequency component. Its maximum observed at 1012 cm -1, is close to the value of the maximum for nepouite (1007 cm -1). The position of this mode seems to indicate that the supported phase is composed of a mixture of phyllosilicates. Since the VOH frequencies are also characteristic of the type of phyllosilicate, KBr pellets have been calcined at 150~ for 80 hours in order to eliminate the contribution of water molecules which prevent the observation of VOH. For this latter sample, the position of VOH (3637 cm -1) also corresponds to an intermediate value between that observed for talc and nepouite (3627 and 3645 cm-1 respectively), which agrees with the presence of a mixture of talc and nepouite-like phases. When the exchange is carried out with a support with low specific surface area (A50), the subtracted spectrum for sample E-8.3-A50 which exhibits 2 maxima at 1042 and 999 cm -l, resembles that of Ne-25 (Figure 2, spectra d, c). Since the water content decreases with the specific surface area, the vOH vibration may directly be observed on uncalcined pellets. The frequency of this vibration observed at 3641 cm -1 corresponds to nepouite as well as to a hydroxide phase. The magnified figures in the 900-550 cm -1 range (not shown here), reveal a broad maximum for the ~5OHmaximum for sample E-8.3-A50 as compared to sample E-8.3A380. After self-deconvolution, 2 peaks at 668 and 624 cm -1 are observed (the procedures of subtraction and self-deconvolution are explained in [24]). The first one may be assigned to a nepouite-like phase whereas the second one is assigned to an ill-crystallized hydroxide phase.
973
These results show that the formation of a talc-like phase requires a high pH (9.8) of the impregnation solution and a support of high specific surface area, whereas the formation of a nepouite-like phase requires to operate at medium pH (8.3). In this case a mixture of phyllosilicates is produced. Ni hydroxide is observed only with the low specific surface area support. Previous work on the synthesis of bulk phyllosilicates suggests copolymerization reactions in solution to produce Si-O-Ni-OH2 and Si-O-Ni-O-Si intermediates which act as nuclei for the edification of talc and/or nepouite [ 13, 25, 27]. The type of phyllosilicate (2:1 or 1:1) is controlled by the Ni/Si ratio of species in the solution [25, 27] and corresponds to the stoichiometric ratio of the clay: Ni/Si = 0.75 for talc and Ni/Si = 1.5 for nepouite. If the Ni/Si ratio is intermediate between these values, a mixture of phyllosilicates is obtained. If the Ni/Si is higher than 1.5, the precipitation of Ni hydroxide may occur. The formation of silica supported phases is more complex since it first implies the dissolution of silica to form Si(OH)4 monomers which react with the Ni 2+ complexes, and a rearrangrnent of the geometry of silicon tetrahedra of the support (which differs from that of the tetrahedral sheet in a bulk phyllosilicate) to accomodate the supported clays. The dissolution of silica is catalysed by OH- ions and the rate of dissolution increases with the specific surface area [28, 29]. The Ni/Si ratio of soluble species available near the interface controls the nature of the deposited phase (Ni hydroxide and/or phyllosilicate) which grows via polymerization of Ni-O-Ni (Ni-O-Si) monomers. For high pH (> 8.3), and high specific surface areas, the solubility and the rate of dissolution of silica ensure a high level of Si(OH)4 monomers. During the washing steps, condensation reactions between Si(OH)4 monomers and [Ni (NH3)6_n (H20)n] 2+ complexes occur, leading to the formation of phyllosilicates. At pH 9.8, the amount of Si(OH)4 is high enough to permit the formation of a talc-like silicate, whereas at pH 8.3, the solubility of silica decreases and the Ni/Si ratio increases, which leads to the formation of a mixture of phyUosilicates. When the exchange is performed with aerosil OX50, the rate of dissolution of silica decreases. The coverage of the silica surface by the precipitated silicate increases, hence contributing to lower the rate of dissolution of the residual available surface. As a consequence, the amount of Si(OH)4 monomers in the exchange solution may reach such a low level as compared to that of Ni complexes, that it permits the precipitation of a Ni hydroxide-like phase.
3.3. Characterization of well-crystallized Ni supported phases Hydrothermal treatments performed at 190~ increase the crystallinity degree for all samples. The results from EXAFS and FTIR measurements, show that whatever the initial supported phase, the resulting product is the thermodynamically stable phase, i. e., talc.
3.3.1. EXAFS results The number of Ni and Si backscatterers increases with the temperature and duration of the hydrothermal treatment. Table 4 shows that the number of Si backscatterers reaches a value (Nsi = 3.4) slightly inferior to that found for talc synthesized at 150~ (NNi = 3.6), while the number of Ni backscatterers increases up to 5.9. EXAFS spectroscopy clearly shows that an ill-crystallized supported phase of talc-like structure is transformed into a cristalline talc.
974 Table 4. Simulation parameters for Ni and Si backscatterers for Ni/SiO2 materials: influence of the hydrothermal treatment Sample
backscatterers N
o (A)
7(A)
R(A)
AE
Q
E-9.8-A380.
Ni Si
5.1 2.5
0.095 0.115
1.00 0.85
3.09 3.30
-0.5 -9.0
1.4. 10-3
E-9.8-A380/h- 14.
Ni Si
5.9 3.4
0.088 0.108
1.00 0.85
3.07 3.26
-0.5 -8.9
1.7. 10-3
Ta-150
Ni Si
5.6 3.6
0.086 0.104
1.00 0.85
3.07 3.25
-0..5 -8.9
1.8. 10-3
3.3.2.
FTIR
results
Table 5 reports the nature of the supported phases before and after hydrothermal treatment. Table 5 Nature of the supported phases after hydrothermal treatment Sample
supported phase
Sample
supported phase
E-9.8-A380
ill-crystallized talc
E-9.8-A380/h- 14
crystalline talc
E-8.3-A380
ill-crystallized talc + nepouite
E-8.3-A380/h-8
crystalline talc
E-8.3-A50
ill-crystallized nepouite + hydroxide
E-8.3-A50/h-14
crystalline talc
0.75 -
Figure 3. Spectra of samples: E-9.8-A380/h, 14 (a) E-8.~3-A380/h-8 (b) and E-8.3-A50/h- 14 (c)
= r
iI
v.~v-
c
b
0"
< 0.25
0.00 900
800
700
550
W a v e n u m b e r cm -1
975 Whatever the pH of the preparation and the specific surface area of the support, the IR spectra exhibit in the 8OH range the presence of a doublet around 710-668 cm -1, for samples submitted to hydrothermal treatment (Figure 3). The higher intensity of the high frequency component (711 cm -1) as compared to that of the low frequency component (668 cm-1), indicates the presence of a well-crystallized talc phase (Figure 3 a, c). For sample E-8.3-A380/h-8, the 711 and 668 cm -1 components have similar intensities (Figure 3 b). This feature may be interpreted in terms of the presence of a talc phase with a lower degree of crystallinity or a mixture of talc and nepouite phases. However, the presence of only one VOH band at 3627 cm -1 (instead of the two expected for a mixture of phases), together with the presence of a SiO vibration at 1027 cm -1 in the spectrum obtained after subtraction of the support, confirm the assignment of the supported phase to a cristalline talc. 4. C O N C L U S I O N This work was carried out with the objective of identifying supported phases generated in the preparation of Ni/SiO2 materials prepared by the competitive cationic exchange method with [Ni(NH3)6]2+complexes. The characterization was done by comparison with bulk reference compounds of various degrees of crystallinity. Among the three techniques employed, FTIR spectroscopy is shown to be particularly relevant to characterize poorly-crystallized supported phases which are generally produced during the preparation. This technique only, permits to diffenciate a phyllosilicate of talc or serpentine structure from a phase of hydroxide structure. DRS spectroscopy cannot be recommended to identify supported phases whereas EXAFS is better adapted to characterize crystalline phases. However, although this technique fails to identify ill-crystallized phases, it shows that isolated Ni 2§ ions cannot be produced using ammine Ni 2§ complexes. The nature of the supported phase (phyllosilicate or hydroxide) emphasizes the role of the support as a reactant. The pH and the specific surface area of the support influence the solubility and the rate of dissolution of silica and the nature of the deposited phase appears to be controlled by the Ni/Si ratio of soluble species at the interface.
REFERENCES M. Che, Stud. Surf. Sci. Catal., L. Guzci, F. Solymosi and P. Tetenyi (eds.), Elsevier, Amsterdam, 1993, 75 A, p 31. M. Che and L. Bonneviot, Successful Design of Catalysts. Future Requirements and Development, T. Inui (eds.), Elsevier, Amsterdam, 1988, p 147. J. A. Van Dillen, J. W. Geus, L. A. M. Hermans and J. Van der Meuden, Proc. 6th. Int. Cong. Catal., G. C. Bond, P. B. Wells and F. C. Tomkins (eds.), The Chemical Society, London, 1977, p 677. L. A. M. Hermans and J. W. Geus, Preparation of Catalysts II, G. Poncelet, P. Grange and P. A. Jacobs (eds.), Elsevier, Amsterdam, 1979, p 113. J. T. Richardson, R. J. Dubus, J. G. Crump,P. Desai, U. Osterwalder and T. S. Cale, Preparation of Catalysts II, G. Poncelet, P. Grange and P. A. Jacobs (eds.), Elsevier, Amsterdam, 1979, p 131. J. W. E. Coenen, Preparation of Catalysts III, G. Poncelet, P. Grange and P. A. Jacobs (eds.), Elsevier, Amsterdam, 1979, p 89. R. Burch and A. R. Flambard, Preparation of Catalysts III, G. Poncelet, P. Grange and P. A. Jacobs (eds.), Elsevier, Amsterdam, 1983, p 331. L. Bonneviot, O. Clause, M. Che, A. Manceau and H. Dexpert, Catal. Today., 6 (1989) 39.
976 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
O. Clause, M. Kermarec, L. Bonneviot, F. Villain and M. Che, J. Amer. Chem. Soc., 114 (1992) 4709. B. Mile, D. Stirling, M. A. Zammit, A. Lowell and M. Webb, J. Catal., 114 (1988) 217. O. Clause, L. Bonneviot, M. Che and H. Dexpert, J. Catal., 130 (1991) 21. K. Tohji, Y. Udagawa, S. Tanabe and A. Ueno, J. Amer. Chem. Soc.,106 (1984) 612. H. Mond6sir and A. Decarreau, Bull. Miner., 110 (1987) 409. J.Y. Carriat, M. Che, A. Decarreau and M. Kermarec., Catal. Letts., 25 (1994) 127. Crystal Structure of Clay Minerals and their X-Ray Identification, G. W. Brindley and G. Brown (eds.), Mineralogical Society, London, 1980, p 2. A. Decarreau, Bull. Miner., 103 (1980) 579. A. Decarreau, Geochim. Cosmochim. Acta., 49 (1985) 1537. A. Manceau, G. Calas and A. Decarreau, Clay. Miner., 20 (1985) 367. The Infrared Spectra of Minerals, V. C. Farmer (eds.), Mineralogical Society, London, 1974, p 344. R . W . T . Wilkins and I. Ito, Am. Mineral., 52 (1967) 1649. P. G6rard and A. J. Herbillon, Clays and Clay Miner., 31 (1983) 143. M. Figlarz and S. Le Bihan, C. R. Acad. Sci. Paris., 272 (1971) 580. S. Le Bihan and M. Figlarz. Thermochim. Acta., 6 (1973) 319. M. Kermarec, J. Y. Carriat, P. Burattin, M. Che and A. Decarreau, J. Phys. Chem., in press C. Tchoubar, Bull. Soc.Fr. Miner., 88 (1965) 483 G.A. Martin, B. Imelik and M. Prettre, J. Phys. Chem., 66 (1969) 1682. T. Mizutani, Y. Fukushima, A. Okada and O. Kamigaito., Bull. Chem. Soc. Jpn, 63 (1990) 2094. R. Iler, The Chemistry of Silica, J. Willey (eds.), 1979, p 47. G.S. Wirth and J.M. Gieskes, J. Coll. Interf. Sci., 68 (1979) 492.
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
977
I n f l u e n c e o f a n i n t e r a c t i o n o f PdCI2 w i t h c a r b o n s u p p o r t o n state and catalytic properties of Pd/C catalysts P.A. Simonov, E.M. Moroz, A.L. Chuvilin, V.N. Kolomiichuk, A.I. Boronin and V.A. Likholobov
Boreskov Institute of Catalysis, Novosibirsk 630090 (Russia) Influence of substructure, texture and chemical properties of carbon supports at each step of the formation of Pd/C catalysts was studied for one of the conventional methods of catalyst manufacture involving adsorption of H2PdC14 on the support followed by drying and reduction procedures. The adsorption was found to be accompanied by the formation of mononuclear ~-complexes of PdC12 with >C=C< fragments of the support surface. Drying step gives rise to PdC12 clusters that remain the coordination with these fragments. The consequence of this strong interaction and carbon matrix imperfection is that the surface of the metallic particles formed at the reduction step appears to be partially or completely blocked by other carbon networks. 1. INTRODUCTION The interaction of a metal precursor with carbon supports is known to affect the physical and chemical properties of the supported metal catalysts. To study the interaction,it is necessary to answer the following questions: (i) what is the chemical nature of this interaction,(ii)what role do physical and chemical properties of the support play in this case, (iii)which peculiaritiesof the process determine the state and catalytic behaviour of the active component of the catalyst. This paper describes an attempt to answer these questions in detail for one of the commonly used methods for the preparation of Pd/C cataysts consisting in the adsorption of PdC12 on carbon supports from aqueous solutions of H2PdCl4 followed by drying and reduction procedures. 2. E X P E R I M E N T A L Commercial active carbons (Eponit I I3H, PN), carbon blacks (PME-800, Vulcan CX-72, PM-105) and Sibunit carbon [1] washed with solutions of 15% HCI + 5% H F were used in a powder form (fraction0.04/0.09 ram) as supports (table 1).
978 Table I. Texture and substructure properties of the supports Substructure parameters* no Support Surface area Micropore SBET* Sphen# volume, La Lc d002 Ioo2 Iool % % * m2/g m2/~ cm3/g HIS Din Bin
1 2 3 4 5 6 7
Eponit 113H PN PME-800 Vulcan CX-72 Sibunit PM-105 Corax 3 graph.
850 950 770 200 400 110 80
610 490 440 180 240 105 72
0.15 0.38 0.09 0.01 0.05 -
-
amorphous amorphous 2.2 1.8 0.356 1.6 1.8 0.366 3.4 3.8 0.350 1.9 1.7 0.362 7.5 6.3 O.346
0 0 27 18 60 45 100
0 0 5 20 1 2 0
* WAXS data. La, Lc are the dimensions of quasi-graphitic crystallites; d002 is the interlayer dspacing, I002, I001 are relative integral intensities of X-ray diffraction signals from the (002) and (001) reflections respectively, the integral intensity of the signal from the (002) reflection for graphitized Corax 3 (no. 7) being taken as a standard (a value of I002 can be considered as the sepcific content of quasi-graphitic crystallites; the value of I002 for Sibunit was also confirmed by the RDF method). ** Single point measurement. # Calculated from data on the adsorption of phenol from water solutions.
Three types of chemically modified samples of Sibunit carbon were prepared: 1) oxidized with aqueous H202 or KMnO4 [2]; 2) chlorinated with C12 at 100~ (lh); 3) hydrogenated with H2 at 300 ~ (5h) as well as at 250 ~ (3h) but in the presence of 2% Pd adsorbed as PdC12 from HC1 solutions (after the experiment, Pd was removed by 15% HC1 saturated with air, the rest of the Pd content being determined as 0.1%). Information about textural and substructural characteristics of the supports (surface area, micropore volume, crystallographic data) was derived from nitrogen and phenol adsorption data measurements, wide angle X-ray scattering (WAXS) spectra and high resolution electron microscopy (HREM). Concentrations of the surface oxides were determined using the data on t h e adsorption of Na2C03, NaOH, NaOEt and HC1 in accordance with Boehm's method [3]. Experiments on H2PdC14 adsorption were performed at room temperature during 20 h [2]. To prepare PdC12/C samples, the carbon slurry was filtered and dried in vacuum at 100 ~ The state of the catalyst precursor was studied by commonly used methods of X-ray photoelectron spectroscopy (XPS), WAXS, small angle X-ray scattering (SAXS) and atomic radial distribUtion (RDF). Pd/C catalysts were prepared from PdC12/C samples via reduction in a flow of hydrogen at 250 ~ [4]. Size distribution of the metallic particles in the Pd/C catalysts was studied using HREM and SAXS; the metal dispersion was also calculated from CO chemisorption data [4]. Catalytic testing of preliminary m o r t a r e d (to avoid diffusional limitations) catalysts was performed in hydrogenation of cyclohexene (ethanol solution, 0 C, I bar [4]).
979 3. RESULTS AND DISCUSSION 3,1. I m p r e g n a t i o n 3.1.1. F o r m a t i o n of s u r f a c e complexes of p a l l a d i u m c h l o r i d e Impregnation of carbon supports with H2PdC14 solutions is accompanied by both adsorption and reduction of Pd(II) species [2,5-6]. The first process appears, from XPS and WAXS data, to become dominant (that is, more than 90% of palladium adsorbed turn to Pd(II) state) when carbon powders (fraction < 0.1 ram) pre-washed with HC1 and HF are used [2]. The second one is typical for granulated carbons [6]. The equilibrium of the Pd(II) adsorption is not influenced by the pH of the solutions at pH = -1/2 and can be described schematically as follows P d C I 2- + A ." z
. p~l~12-,),,,.,<4-,)A + n C l -
(I)
whereA is an adsorption site and K is the equilibrium constant. The value of n depends on the concentration of C1-ions: n=l at [C1]>0.8 M, where n~2 at [C1-]<0.08 M [2,7]. At least three adsorption states of Pd(II) have been distinguished by eluent analysis: weak (A1), strong (A2) and very strong (A3) [2]. Palladi~lm(II), when adsorbed on A1 sites, may be removed from the support by washing with acetone or water, while Pd(II) adsorbed on A2 sites is not removed either by acetone or by aqueous solutions of HF, HC104, H2S04 and HNO3; it can, however, be desorbed by the action of HC1 or its salts with alkali metals. Palladium(II) adsorbed on A3 sites cannot be removed from the surface by any of the eluents. With mathematical simulation of the adsorption equilibria (1), mononuclear complexes of palladium chloride have been found to arise by the reaction of H2PdC14 with the surface sites [7]. On the basis of Langmuir's assumptions, constants of the adsorption equilibria with A1 and A2 sites (K1 and K2, respectively) were calculated. We did not succeed in any description of the equilibrium with A3 sites, since the adsorption was always shifted completely towards the formation of Pd(II) surface compounds [5,7]. 3.1.2. N a t u r e of the sites for H2PdCI4 a d s o r p t i o n Unmodified carbon supports. For carbons nos. 1-6 (Table 1), which possess heterogeneous surface (composed of both basal and edge planes of quasi-graphitic crystals, as can be seen from electron micrographs of the carbons), A1 and A2 sites are present in approximately equal quantities, which appear to be proportional to the specific surface area, while the content of A3 sites is smaller by an order of magnitude and strongly depends on the micropore volume (fig. 1). A graphitized carbon black (no. 7) exposing mostly basal planes (homogeneous surface) holds mainly A1 sites (fig. 1). According to XPS data, Eb (Pd 3d5/2) value for PdC12 adsorbed on A1 and A2 sites is equal to 336.8/337.2 eV, while that for PdC12 occupying A3 sites is equal to 339.5 eV [2] (for individual PdC12, Eb(Pd 3d5/2)= 337.5/337.9 eV). The former Eb values for the Pd 3d5/2 level are typical for ~-comp|exes of PdC12 with unsaturated C-C fragments of organic molecules [8]. As for the equilibrium constants of the surface complexes formation, the value of K1 was found to be independent of the support substructure parameters, while
980 K2 value is a function of them (fig. 2). Thermodynamical deduction of this relationship was presented in [9]. Oxidized carbon support. Modification of the support via oxidation of its surface with H202, KMnO4 or C12 leads to a decrease of the content of A1/A3 sites, the texture and substructure characteristics of the modified samples remaining close to those of the initial support (fig. 3). In all cases, A1 sites are more stable to action of the oxidants than A2 or A3 sites. However, aRer thermal decomposition of most of the surface oxides at 600 ~ in vacuum, concentrations of the adsorption sites tend to increase up to the initial values [10]. t~ 4)
0
0.2 !
~s
I
cm3/g
Vm;
I
I
I
I
#
s
9 s
K~'~ t03
I
Alt
q
G) .ka
o o 0.q
2 A8
oo
pben, m2/.g
500
0 0
doo2"Ioo2
10
L~ Figure 1. Relationship between the content of A1/A3 sites and the values of surface area and micropore volume of the support under study.
Figure 2. Correlation between the equilibrium constant (K2) of H2PdCl4 adsorption and the substructural characteristics of the supports. Numeration of the supports corresponds to t h a t set up in table 1.
XPS spectra of adsorbed PdC12 contain no lines that could be attributed to Pd(II) complexes with Pd-O bonds. Although it points out t h a t PdC12 does not interact directly with the surface oxides, the adsorption equilibria constants still increase after the carbon surface has been covered by oxygen groups [7]. K1 and K2 were found to depend on the type and n u m b e r of the surface oxides in accordance with an equation derived with some assumptions from the equation of Hammet and Taft (fig. 4). In Ki = In Ki 0 + 0.093n-COOH + 0.087n>c=0- 0.029n>C.OH
(2)
where Ki and Ki 0 are the equilibrium constants (i=1,2) for the oxidized and the initial supports, respectively; n-COOH, n>c=o and n>_C-OH are the n u m b e r or carboxyl, quinone or henolic groups per one Ai site (these data will be published in detail in [10]). According to Eq. 2, the ability of A1 or A2 sites to adsorb H2PdC14 is governed by a distribution of ~-electronic density between the sites and the surface groups through a system of conjugated >C=C< bonds, acceptor groups (>C=O, -COOH and -C1) enhancing, but donor ones (_>C-OH) diminishing
981 the stability of the surface complexes of PdC12. Similar peculiarities are inherent to ~-complexes of PdC12 with olefins [11].
6.5A1
.,2
el
1
o
0.~" 02-
,,.,-i r~
A3
o~
<
6
-1 -
,
-
'
0:5
[>C=O] + [-COOH], meq/g Figure 3. Plots of ALIA3sites content versus surface oxides content for the supports prepared by an oxidation of Sibunit sample.
5.5
o/
0
.
,
.
,05
t,
,~ 0
0.093n.COOH + 0.087n>c=o- 0.029n>C.OH Figure 4. Dependence of Hammet-Taft's type for the adsorption equilibria constants (K1 and K2) on the nature of the surface oxides, according to Eq. 2.
Hydrogenated carbon supports. Treatment of the support with H2 (see the experimental part) that would lead to the saturation of some amount of the surface >C=C< bonds decreases the number of A:/A3 sites, A1 sites proving to be more stable. The e x p e r i m e n t a l data presented above allow to conclude t h a t the adsorption of H2PdC14 leads to the formation of ~-complexes of PdC12 with the surface donor (A1 and A2) and acceptor (A3) sites containing u n s a t u r a t e d C-C bonds as ligands. A1 and A2 sites are located on the surface of meso- and macropores, while A3 sites are in micropores. An A1 site seems to be a hexagon of the basal plane of a carbon crystallite. An A2 site is a few >C=C< fragments which take up two neighbouring carbon networks on the edge plane [9]; in this case, some disordering of the networks will somewhat change the interaction of Pd(II) with the surface and, therefore, the stability of the complexes with A2 sites (K2). 3.2. D r y i n g 3.2.1. F o r m a t i o n of (PdCl2)n c l u s t e r s on c a r b o n s u r f a c e Drying (vacuum, 100 ~ lh) of supports loaded with adsorbed complexes of PdC12 was shown by XPS not to give rise to an essential reduction of Pd(II). HREM, SAXS, WAXS and RDF studies of the dried samples led us to the conclusion about the formation of (PdC12)n clusters measuring 1.6/1.8 nm in average diameter (n~70). The size of the clusters is slightly affected by both a fractional coverage of A1/A2 sites with PdC12 and the nature of the support, as it was found for supports no. 3, 5-6.
982 The RDF study of PdC12/PME-800 (0.51 mmol/g) showed that the clusters possess rhomboidal structure with interatomic distances (r) equal to 0.336, 0.381, 0.521 nm (Pd-Pd) and 0.381, 0.428, 0.482 (Pd-C1) which are in close agreement with those for bulk PdC12. Interatomic distances r=0.244 and 0.290 nm were also detected that seemed to be due to the definite position of Pd atoms with respect to hexagons of carbon networks in the (PdC12)n-Support interface. RDF data show the total amount of such clusters to be present in the supported PdC12 to the extent of about 60%; the other part exists either as particles with an extremely distorted structure or as individual molecules of PdC12.
3.2.2. I n t e r a c t i o n of (PdCl2)n clusters w i t h c a r b o n s u r f a c e Along with the above-listed interatomic distances, those with r=0.152 and 0.198 nm were also detected. Probably, they result from the chemical interaction of PdC12 with the support, namely from the transformation of ~-olefinic complex of PdC12 to ~-allylic via the transfer of a chlorine atom from the palladium to the diene-like fragment of the carbon network. The existence of C1-C bond in PdC12/C samples was confirmed by XPS (Eb(C1 2p3/2) = 199.8 eV). The small dimension of the supported clusters testifies t h a t carbons possessing heterogeneous surface contain a huge amount of fixation sites for (PdC12)n clusters. The surface steps may constitute these fixation sites. In this case, PdC12 chains which belong to the cluster interacting with the steps will keep such orientation towards the support surface as if they tended to prolong the edges of the carbon networks (fig. 5). At the same time, WAXS study of (PdC12)n/C samples revealed a modification of the structural organization of the support matrix resulting from the interaction of (PdC12)n clusters with the support. It is manifested as decay or the disappearance of the X-ray diffraction signal of the (001) reflection, those of the other reflections looking unchanged. However, the intensity of the signal from the (001) reflection increases as the main amount of PdC12 has been desorbed with the solution of HC1. ~.
....~-.--:::-
.... 9. "~-.:.:;:- "P.i:::::'~::::". '~:':::; .-..§ ....... ~::~~-~:~..::~i~,.,..~:~:.:'~:~i~.;,.~:~A~!~;S"..~'~,:::': .'.~- ..........
-.~.~";:::~ ~;#.~:~-i ~,~i ................'-.'............... . i:iii:::!:i:i.i::::./...: ! i i i i':~i ~!:-.!.i:...' i ! ,:.~:..~ ............ . i "w:!:i:i:i' ~ i ~'is::::::::isi:i::~->>~i:ii!;i~:ii~;;"si:!' ~ % ~ ~ ::i" C l
...~`~;z~i!!!~ii!iii~.~!$/...~!~!i~i:i~;~!.~.i~!~i~i~i~i~$.~!~i:i~:~.~2~.`.~r
-,:,-.;~_~;.' ~:V:~:. ~t~ :~!~:.-.,~,,.-.;.-:.~.-s:::.~..~ '.... .~.,',~.-#.::~$'~@.:v;.~...'. .........============================ ..'...i.'....' ..~:~.~.................... :::::::::::::::::::::::::::::::::::
b
"
~
Figure 5. Models of the structure and disposition of PdC12 clusters on the surface steps of carbon support: (a) balls and sticks model, (b) space fill model. The X-ray diffraction signal from the (001) reflection which is usually absent in the spectrum of perfect graphitic crystals may exist in the spectra of carbons composed of quasi-graphitic domains and those with high concentration of two-dimensional defects. The latter (let us mark it for brevity as F-component
983
of the support substructure) appears to be responsible for the origin of the signal of the (001) reflection;its content depends on the type of carbon support (table 1). It should be mentioned that the interaction of H2PdC14 with the support at the impregnation step is not influenced by the F-component (at least, its effet is negligible in comparison with the other factors).One can find the effect of the Fcomponent of carbon in the (PdC12)n -support interaction, which is retained even after the reduction of PdCl2 to Pd(O). 3.3. Reduction From HREM data, the reduction of (PdC12)n/C samples with H2 give rise to the target Pd/C catalysts possessing, as a rule, a log-norm size distribution of metallic particles. This implies that the metal particles growth proceeds through the mechanism of migration and coalescence of the particles [12]. Unmodified supports. According to SAXS and HREM data on particles size distribution for supported palladium, the average diameter and the width of the distribution slightly depends on the loading of A1-A2 sites of a support with the catalyst precursor, but they do decrease with an increase in the strength of A2 sites, defined as the value of K2 (fig. 6). With the proviso that K2-~ oo, uniform particles of Pd(O) of 1.1/1.3 nna size would be formed. Thus these data suggest that A2 sites influence the mobility of the former Pd(O) clusters generated from (PdC12)n clusters.
'1.0
.
, II
~
I
-
5
k 0
!
0 .,,-4
J
6 10~ 0/ J
0
4
E
Oo t i i J J
US
-c:s
i
,oai
0 r~
J
J i
0
K2_ I
'
5
Figure 6. Average diameter of Pd particlesin Pd/C catalysts as a function of the strength of A2 sites(K2 value) as derived from S A X S (B) and H R E M (O,Q) data. Pd/C samples were prepared via reduction in H2 (250~ 3h) of the catalyst precursor (PdCl2) adsorbed on A2+A3 (Ore) and AI+A2+A3 (Q)sites.
0
i
d S , ~m
"( H R E M " ) ~
Figure 7. Comparison of the values of surface average diameter of Pd particles derived from H R E M data and C O chemisorption measurements for the catalysts described in fig.6. Pd(OH)2/C (a) and PdCI2/Si02 (b) were used as precursors. Numeration of the supports corresponds to that set up in table 1.
984 The exception to the rule demonstrated in fig. 6 is provided by Pd catalysts supported onto Vulcan CX-72. From HREM data, palladium particles of the catalysts possess very narrow Gaussian distribution on sizes with a maximum at 1.2 nm, as if the particles were formed from (PdC12)n clusters with no sintering, though the K2 value for the support is not great enough to allow the latter. Vulcan CX-72 differs from the other studied carbons (table 1) in t h a t there is a high concentration of the F-component capable of strong interaction with particles of PdC12 or Pd(O). Because of this, one can conclude that the structural F-component takes a great part in preventing sintering of the palladium particles. For the catalyst support on carbon by the studied method, the surface average diameter of the Pd particles calculated from the CO chemisorption data is somewhat higher than that derived from the HREM data (fig. 7). The effect is not found after hydrolysis of adsorbed PdC12 before the drying step [4] or when silica has been chosen as a support (fig. 7). Previously [4], we explained the effect as a blockade of the surface of Pd particles by micropore walls for the access of reagents. However, the blocking effect peaks for the catalysts supported on Vulcan CX-72 being poor of micropores (table 1); moreover, it does not disappear even under the hydrolysis procedure. Electron micrographs of Pd/Vulcan CX-72 catalysts show palladium particles to be encapsulated in the carbon matrix; some ordering of the support structure takes place near the palladium-carbon interface which is clearly detected for the largest Pd particles. Similar micrographs were reported by K. Kinoshita [13] for Pt/Vulcan CX-72 catalysts. Consequently, carbon networks composing the F-component are very flexible and mobile, owing to that, when interacting with the metal, they become able to cover the surface with Pd particles t h u s preventing both sintering of p a l l a d i u m and its accessibility to reagents. The same properties are also characteristic of an amorphous component of carbon (so-called amorphous carbon) [14] whose content is the highest for active carbons (no. 1-12, table 1). Thus the nature of what could cause the blocking effect seems to be rather unambiguous. Oxidized carbon supports. As follows from HREM and CO chemisorption study of palladium catalysts supported on modified Sibunit carbons, the dimension of the Pd particles depends only slightly, if at all, on the content of the surface oxides, the blocking effect of the modified supports being retained the same as the one of the initial carbon. This may be explained in view of the fact that, under the reduction conditions, palladium can catalytically decompose the surface oxides [15] so that the chemical state of some part of the carbon surface around the Pd particles for a modified support and for the initial one becomes similar, which gives rise to similar conditions of sintering as the former Pd(O) clusters on these supports.
985
A3
A2
A1
T
3
q t
i
T
i
o.~
G i
3
I
"t
.
*p-4
r
*" 0.2,,r.,.l
.~,,4
r
~l
*v..,l
~
-
N r~
o
palladium content, %
5
Figure 8. The catalytic activity v e r s u s Pd content for Pd catalysts supported on sibunit carbon (O) and carbon black Vulcan CX-72 (4)) (the additional scale gives an information about occupation of the adsorption sites by the catalysts precursor).
0
Iool,
%
20
Figure 9. Correlation between the specific activity of the Pd/C catalysts and the content of the structural F-component in the support. Pd/C catalysts were prepared from PdC12/C samples with loaded A3 (I), A2+A3 (O) and AI+A2+A3 (4)) sites.
3.3.1. C a t a l y t i c p r o p e r t i e s Typically, the rate of cyclohexene hydrogenation is directly proportional to the Pd content in the catalyst [4]. A deviation on the linear dependence may occur for the catalysts with a low Pd content (fig. 8,a). An explanation of this is that the catalyst precursor has been mainly adsorbed on A3 sites; this results subsequently in Pd particles inside the support micropores. Such catalysts display the lowest specific activity although the reaction is known to be a structure-unsensitive one [16]. Moreover, the specific activity of Pd/C catalysts prepared from PdC12 adsorbed on A1-A2 sites of carbons with high micropore volume was found to be somewhat lowered [4]. The specific activity, defined as the ratio of the reaction velocity per gram of palladium and the specific number of Pd atoms capable of adsorbing CO, does not necessarily have to be constant, because (i) the process may be hindered by diffusion of the reagents through the small pores to the metal particles or (ii) the number of active sites may not vary in direct proportion with the number of the surface Pd atoms detected by CO chemisorption, since the difference in sizes of CO and cyclohexene molecules may lead to differences in the blocking effects for these adsorbates. The latter seems to explain the absence of any activity for cyclohexene hydrogenation of the 2% Pd/Vulcan CX-72 catalyst (fig. 8,b), although 28% of the Pd atoms are exposed to CO chemisorption. As it is seen in fig. 9, a drop in the specific activity of the Pd/C catalysts should be better described in terms of the blocking effect caused by the Fcomponent of the supports, insofar as these effects of micropores or amorphous carbon turn out to influence the specific activity to a lesser extent. Upon an increase of the palladium content in a catalyst, its specific activity tends to increase (fig. 9), although, according to fig. 6, the average diameter of Pd particles is not changed. This is possibly due to the drop in the relative number of blocked Pd particles with an increase of the Pd content in the catalyst.
986 4.
CONCLUSION
The results of the recent work shows that the formation and catalytic behaviour of Pd/C catalysts are affected by the properties of both the surface and the bulk of the carbon supports. A particular place among the factors determining characteristics of supported palladium such as the particle size distribution and the accessibility of its surface to the reagents is occupied by the ability of the carbon matrix to become deformed when interacting with the palladium species at all the stages of the catalyst manufacturing. Therefore, having broken the myth about carbon being an inert support, one should shelve its consequences concerning hardness and incapacity of a carbon matrix to change its conformation under a reaction with an adsorbate.
987 REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
Yu.I. Yermakov, V.F. Surovikin, G.V. Plaksin, V.A. Semikolenov, V.A. Likholobov, A.L. Chuvilin and S.V. Bogdanov, React. Kinet. Catal. Lett., 33 (1987), 435-440. P.A. Simonov, V.A. Semikolenov, V.A. Likholobov, A.I. Boronin and Yu.I. Yermakov, Izv.Acad. Nauk SSSR, Ser. Khim. No. 12 (1988), 2719-2724. H.-P. Boehm, in: Advances in Catalysis, Academic Press, New York and London, 16 (1966), 124. A.S. Lisitsyn, P.A. Simonov, A.A. Ketterling and V.A. Likholobov, Stud. Surf. Sci. Catal., 63 (1991), 449-458. Yu.A. Ryndin, O.S. Alekseev, P.A. Simonov and V.A. Likholobov, J. Molec. Catal., 55 (1989), 109-125. G. Reznic, Yu. Tarasenko, A. Bagreev and N. Yerushenko, Ext. Abstr., Carbon 92, 5th Internat. Carbon Conf., Essen, 1992, S.316. P.A. Simonov, A.L. Chuvilin and V.A. Likholobov, Azv. Acad. Nauk SSSR, Ser. Khim. No 9 (1989), 1952-1956. C.D. Wagner, W.M. Riggs, L.E. Davies, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer corp. Eden Praise, 1979. P.A. Simonov, E.M. Moroz and V.A. Likholobov, Izv. Acad. Nauk SSSR, Ser. Khim. No 7 (1990), 1478-1483. P.A. Simonov and V.A. Likholobov, Bull. Russ. Acad. Sci., Div. Chem. Sci. (1994), in press. M. Herberhold, Metal ~-complexes, Elsevier Publishing Company, Amsterdam, London, New York, 1972. C.G. Granqvist and R.A. Buhrman, J. Appl. Phys., 47 (1976), 2200. K. Kinoshita, Carbon, Electrochem. and Physicochem. properties, WileyInterscience Publication, New York, 1988, 28. R. Lamber, N. Jaeger and G. Schultz-Ekloff, Surf. Sci., 227 (1990), 15-23. T. Kuretzky, Dissertation, Univ. Munchen (1993). E.E. Gonzo and M. Boudart, J. Catal., 52 (1979), 462.
This Page Intentionally Left Blank
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
989
Synthesis of eggshell cobalt catalysts by molten salt impregnation techniques Stuart L. Soled a, Joseph E. Baumgartnera Sebastian C. Reyesa, and Enrique Iglesiab aCorporate Research Laboratories, Exxon Research and Engineering Co. Route 22 East, Annandale, NJ 08801 bDepartment of Chemical Engineering, University of California, Berkeley, CA 94720
Fischer-Tropsch synthesis catalysts with the active cobalt component preferentially located near the outer surface of support pellets were prepared by impregnation with molten cobalt nitrate. This synthesis procedure and the slow reduction of the impregnated nitrate to Co metal led to relatively high metal dispersions (0.05-0.1) at the high Co concentrations (4050% wt.) present within the shell region. The eggshell thickness is determined by the melt viscosity and by the contact time between the melt and the porous pellet and agrees well with values predicted by imbibition models using measurements of liquid and support properties. The resulting eggshell catalysts introduce intermediate levels of transport restrictions, which lead to optimum C5+ yields in the Fischer-Tropsch synthesis. I. INTRODUCTION Whereas changes in Co or Ru dispersion or in the type of metal oxide support (e.g., SiO2, A1203, etc.) have only a weak effect on Fischer-Tropsch (FT) synthesis turnover rates, diffusional constraints can dramatically alter apparent turnover rates and selectivities [1-6]. Transport restrictions become increasingly important when large catalyst pellets (1-3mm) are used in packed-bed reactors in order to avoid substantial pressure drops. As previously shown by Iglesia et al. [4-6], at typical FT synthesis conditions, two types of reaction-diffusion couplings occur: (a) diffusion-limited product removal from catalyst pellets and (b) diffusion-limited reactant arrival at catalytic sites. In the first regime, diffusion-enhanced readsorption of o~-olefins leads to higher product molecular weight and paraffin content as pellet size or active site density increase. In the second regime, catalyst pellets become depleted of CO, which favors formation of lighter products and decreases the desirable C5+ selectivity. Growing chains that desorb as olefins can readsorb and initiate chain growth, leading to desirable higher molecular weight products. As pellet size, site density or olefin carbon number increase, the probability of readsorption increases. With increasing transport restrictions, the catalytic sites are exposed to higher effective H2/CO ratios, which produce undesirable lower molecular weight products. Intermediate levels of transport restrictions lead to optimum product distributions.
990 We can modify the extent of transport limitations by manipulating the thickness of the active layer and the volumetric density of active sites during catalyst synthesis. The benefits of non-uniform intrapellet site distributions have been previously described for many catalytic reactions, including the FischeroTropsch synthesis [7-15]. Here, we report on the preparation of large SiO 2 pellets with uniform and eggshell Co distributions. The eggshell catalysts contain Co near the outer support surface when prepared by impregnating 2 mm silica spheres with molten cobalt nitrate. The local cobalt content approaches 50% wt. in a 0. l mm external shell; yet, we can obtain relatively high Co dispersions (0.05-0.10) by directly reducing the nitrate precursor at a slow heating rate.
2. EXPERIMENTAL In order to study the catalyst preparation process, we measured capillary imbibition rates of four different liquids on individual SiO2 spheres (Shell $980B, 260 m2g -1, calcined at 873K) that were glued with an epoxy resin to the ends of wooden applicator sticks. A standard aqueous solution (A, Table 1) contained 0.5 g Co nitrate/cm 3 H20. A melt of cobalt nitrate held at temperatures between (333 and 348K) provided a higher viscosity (31-48 cp) liquid. We prepared an aqueous cobalt nitrate solution (13) with similar viscosity to the cobalt nitrate melt by adding 1.0% wt. hydroxyethylcellulose to solution (A). A fourth solution (C), with low cobalt content, contained 0.01 g Co nitrate/cm 3 of H20. In order to characterize solution properties, viscosity and surface tensions were measured with a Nametre vibrating sphere viscometer and a Kruss K-10 tensiometer (ring method), respectively. The individual silica spheres were immersed in the four liquids for periods of 2,4,8,16,32, and 48 s. We then removed any excess liquid from the spheres, dried them for 0.5 h, and then calcined them in air at 623 K for 0.25 h. This treatment converted the nitrate to Co30 4, which provided sharper contrast in optical microscopy measurements. These immersion experiments were also repeated using heated (383K) and cooled (263K) silica spheres. Each data point was averaged from measurements on 10 individual spheres. Several cobalt catalysts were prepared on both powder and large particle carriers for testing in FT reactors. SiO2 powders (Davison 62, W. R. Grace Co., 280 m2g - 1, calcined 873 K, 0.143 mm average pellet diameter) were slurried with a cobalt nitrate (Co(NO3) 2 .6H20 , Alfa)/acetone solution and the excess solvent was evaporated. Both uniformly impregnated and eggshell pellets were prepared. The uniform pellets were ,prepared by incipient wetness impregnation of silica spheres (either Shell $980G: 115 m2g -~, 2.2 mm pellet diameter, or Shell $980B: 1.7 mm pellet diameter, 260 m2g-1; both calcined at 873 K for 16 h) with aqueous Co nitrate solutions. Ground samples of these pellets were obtained by crushing and separated into different size ranges (0.13 to 0.86 mm). The eggshell catalysts were prepared by imbibition with high viscosity (~ 40cp) cobalt nitrate melts. Molten Co nitrate (50 g, melting point --323 K) at 348-363 K was poured uniformly over a 2-3 cm bed of SiO 2 spheres (2-3 cm bed height, Shell $980G, 12.5g) that in turn was placed on top of a 15-20 mm layer of 6 mm non-porous glass beads held in a glass funnel (5.5 cm diameter). As the cobalt nitrate melt was added, the bed was stirred with a glass rod, and the molten nitrate was removed by vacuum filtration in order to limit contact times to 2-4 s. For comparison, samples were also prepared using this controlled contact time technique but with a room temperature aqueous Co nitrate solution (100 cm 3 Solution A,
991 Table 1) instead of the nitrate melt. All catalysts were directly reduced in flowing hydrogen. The samples were heated at 6-12 K h-1 from room temperature to 693-723 K and held at this temperature for 4-16 h. The samples were then passivated with a dilute oxygen stream (1% O2/He ) at room temperature before use. All catalysts were characterized by x-ray diffraction, hydrogen chemisorption, nitrogen physisorption, and optical microscopy. Co was analyzed by atomic absorption or by gravimetric measurements during reduction and oxidation cycles. Cobalt dispersion was measured by hydrogen chemisorption at 373 K assuming a 1:1 H:Co surface stoichiometry (1). The reduced and passivated catalysts were mixed with fine quartz powder (0.1-0.2 mm diameter) to insure isothermal operation and avoid bypassing, introduced into packed-bed reactors, and re-reduced in flowing H 2 at 623-723 K for 2-4 h. After cooling to 473 K, the catalyst was exposed to H 2 and CO reactants (H2/C0=2/1), temperatures were maintained at 473 K and pressures at 2000 kPa. Data were collected after reaching steady state (24-36 hours), and the products were analyzed with gas chromatography and mass spectroscopy [5]. Selectivities are reported as the percentage of reacted CO that appears as a given product. 3. RESULTS AND DISCUSSION 3.1. Liquid imbibition into porous SiO 2 pellets The imbibition of a liquid into a sphere depends on both solution properties (viscosity and surface tension) and solid properties (pore radius, pore tortuosity, and contact angle). Washburn [ 16] showed for a liquid partially penetrating a sphere through cylindrical capillaries, the fractional penetration depth was equal to:
~=~.t 1/2
(1)
where = [ 1/(~.Ro2) 9y.rp. cos(O)/(2.B)] 1/2 and p is the liquid viscosity, y the surface tension, rp the pore radius, ~ the tortuosity of the pore structure, R o the pellet radius, and 0 the contact angle between the liquid and the support surface. In the initial experiments, a given support pellet was immersed into molten cobalt nitrate and Solutions A, B and C for varying periods of time. The fractional penetration depth should depend on both viscosity and surface tension (assuming no change in contact angle). Table 1 shows that the surface tension of nitrate solutions, of solutions viscosified with hydroxyethylcellulose (HEC), and of nitrate melts are similar, so that penetration depth should de~end only on solution viscosity. Figure 2 shows the liquid penetration depths plotted against tl/2. As suggested by Washbum's Eqn. (1), a plot of ~ vs. t 1/2 gives a straight line with slope fl. We assumed perfect wetting of solid surfaces by the liquid (0=0) and a pellet tortuosity value of 1.8. Table 2 shows the slopes from Figure 2 and those calculated from Eqn. (2) using measured pore structure and solution properties. Surprisingly, nitrate melts (at 333 K) penetrate into silica spheres (2.7 mm diameter, 210 m2g-l) more slowly than nitrate solutions of similar viscosity and surface tension at room
992 Table 1 Properties of impre~natin8 nitrate solutions and melts. Impregnating Liquid Hydroxy Viscosity ethylcellulose (~t; cp)
Surface Tension (y, dynes cm- 1)
(wt%) Solution A(1) 0 Solution B(1) 1.0 melt 0 melt 0 Solution C(2) 0 Water 0 (1) 0.5 g Co nitrate/cm 3 H20 (2) 0.01 g Co nitrate/era 3 H20
3.2 (298K) 45 (298K) 48 (333K) 31 (348K) 0.93 (298K) 0.92 (295K)
65.6 66.4 -66.2 66.9 70.6
temperature (Solution B, Figure 1,ii and iii; curves C and B, Figure 2). As expected, both liquids penetrate silica spheres slower than aqueous nitrate solutions without HEC (Figure 1,i and curve A in Figure 2). In order to determine if the slow penetration of the melt resulted from cooling and solidification as it contacted the spheres, we measured imbibition rates into silica spheres heated (383K; curve C" in Figure 2) or cooled (263K, curve C' in Figure 2) before immersion. In both cases the behavior resembles that of spheres held at room temperature. The unexpected rapid penetration of the l%HEC/cobalt nitrate solution suggests that the solution viscosity within a support pore is much lower than in the bulk liquid. Pyrolyzing 1% HEC impregnated (cobalt-free) pellets at 673K in N 2 and examining the pellets via EDS showed that most of the HEC additive remains on the external pellet surfaces and is therefore ineffective in retarding imbibition within intrapellet pores. The agreement between theoretical and experimental penetration values is excellent for nitrate melts and solutions; experimental penetration rates differ significantly from theoretical predictions only for viscosified nitrate solutions, where surface retention of HEC renders the use of bulk viscosity in the Washburn model inappropriate. In order to produce large quantities of eggshell pellets for catalytic testing, we developed a vacuum filtration technique in which an excess of molten cobalt nitrate was poured over silica pellets held in a vacuum funnel and then filtered quickly. The vacuum filtration technique controls the liquid-pellet contact time while the high nitrate melt viscosity slows pellet imbibition during the contacting. Fig. 3,ii shows silica spheres (2.2 mm, 115 m2g -1) impregnated with molten Co nitrate (348-363 K) for 2-4 s using vacuum filtration to remove the excess melt. The pellets contain 10-13% wt. cobalt and with 75% of the SiO2 pellet volume void of cobalt, the local Co content in the shell is close to 50% wt. When the same technique was applied to aqueous Co nitrate solutions (Solution A, Table 1) in place of melts, the impregnating solution completely penetrated the silica spheres (Figure 3,i)
3.2. Cobalt dispersion and crystallite size Cobalt tends to form large crystallites on metal oxide supports Cobalt dispersion, defined as the fraction of the metal atoms residing at crystallite surfaces, rarely exceeds 0 1, except when using organometallic precursors [5] These precursor often leave carbonaceous residues and the resulting small crystallites tend to re-oxidize during FT synthesis
993
i,i ~ ~i:~i~, ~:i~ i",
m Co(nit)/c~c& 1%
(ii
.. ".:I:~:!, .. :....~;.,?~.~..........
:
.
.
.
.
!:li!:::.:
.
Figure 1. Optical micrographs of silica pellets individually immersed in solutions or melt: (i) solution A, 0.5 g Co nitrate/cm 3 H20 (ii) solution B, 0.5 g Co nitrate/cm 3 H20 & 1% wt. hydroxyethylcellulose (iii) cobalt nitrate melt (333 K)
994 1 e~
0.75
A.
0.5
~
0.25
O. 2
4
6
time ~ (sec)
Figure 2. Effect of Impregnating Solution on Liquid Penetration Depth (SiO2:210 m2/g; 2.7mm diameter); A: solution A; 0.5gm Co nitrate/cm 3 H20 B: solution B; 0.5gm Co nitrate/cm 3 H20 & 1% HEC C: nitrate melt, SiO2 at 298K; C': nitrate melt, SiO2 at 273K; C" nitrate melt, SiO2 at 383K; D: solution C; 0.01gm Co nitrate/cm3 H20 Table 2 Liquid penetration rates. Comparison of experimental values and model predictions. Impregnating Liquid Sphere Average Slope Slope Liquid/Solution Temperature Diameter Pore Radius (from Fig. 2) (from eqn 2) (K) (2.Ro, cm) (rp/nm) A 298 0.27 8.5 0.18 0.16 B 298 0.27 8.5 0.12 0.045 B 298 0.22 16.0 0.12 0.075 melt 333 0.27 8.5 0.028 0.042 melt 333 0.22 16.0 0.085 0.074 melt 348 0.27 8.5 0.035 0.052 (silica at 383K) melt 348 0.27 8.5 0.041 0.052 9 (silica at 263K) C 298 0.27 8.5 0.29 0.31
995
(i)!
i
.-lmm-
lmJl,,li ...................
. .
o )i Figure 3. Optical Micrographs of silica pellets impregnated using the vacuum filtration procedure: (i)solution A, 0.5 g Co nitrate/cm 3 H20 (ii) impregnation of melt. Sintering occurs more readily at high metal loadings. As a result, we expect cobalt dispersions to be very low at the locally high Co concentrations in our eggshell catalysts. We have found, however, that slow direct reduction of nitrate precursors leads to Co dispersions in eggshell catalysts that are similar to those obtained by conventional pretreatments in uniformly impregnated pellets, where local Co levels are significantly lower. High dispersions in eggshell catalysts require that we reduce the nitrate salts directly in flowing dihydrogen while ramping the temperature slowly (<12 K h"l) and that we avoid any intermediate calcination step. When we calcine a sample at 623 K for 3h and then reduce in hydrogen by heating the sample to 723 K at 240 K h -1, the cobalt dispersion is 2.8% (33 nm average crystallite size). Reducing nitrate precursors directly in hydrogen at similar conditions without an intermediate calcination increases the metal dispersion to 4.2 % (23 nm average diameter). When we reduce the nitrate directly but at a heating rate of 12 K h-1, the cobalt dispersion increases to 5.5% (17 nm average crystallite diameter). These results suggest that extensive sintering occurs during uncontrolled calcination of nitrate precursors leading to agglomerated oxide particles. Sintering also occurs during uncontrolled reduction of these nitrate precursors, possibly as a result of local exotherms or of the presence of high concentration of reduction products (H20, NO x, and nitric acid). The combination of melt impregnation synthesis techniques with slow reduction of nitrate precursors leads to well-defined eggshell regions within which small cobalt crystallites (10-20 nm) are formed at high local Co concentrations (40-50 % wt.). The resulting catalysts retain the high volumetric productivity of uniformly impregnated small pellets, while introducing transport restrictions that lead to optimum yields of desired C5+ products.
996
3.3. Optimum C5+ selectivity on eggshell Co catalysts Extensive catalytic tests on small and large uniformly impregnated pellets and on the eggshell catalysts of this study show that maximum C5+ selectivities are obtained at intermediate levels of transport restrictions (Figure 4) The parameter ~, and is proportional to:
oc (characteristic diffusion length) 2. (volumetric active site density) describes how catalyst structural properties influence the intensity of transport restrictions. Increasing values of g reflect more severe transport limitations for both reactant (CO) arrival and product (olefin) removal. Eggshell catalysts can be placed near the maximum of this C5+ vs. ~ curve independent of the pellet diameter required to avoid pressure drop restrictions. In effect, eggshell catalysts decouple the pellet diameter from the characteristic diffusion distance, which is then controlled independently by varying the eggshell thickness. Within these eggshell catalysts, moderate transport restrictions retard the removal of reactive olefins, which then readsorb and initiate surface chains leading to higher molecular weight products. Transport restrictions, however, are not sufficiently severe to introduce CO concentration gradients that reduce catalyst effectiveness and lead to undesirable lighter hydrocarbons.
100
I
I
I
I
Cs+ 95A
.....
~2"%" "' . . . . . . .
~.*,
90,
U
sl t
OrJ
o
be
85s
80-
"0,. 9 ..... 9 2
9
O--
/
,e /
75 - t,
/
7O 10~
l
10 2
I
I
10 3
10 4
X (m-l)
l
10S
x 10-1s
Fig. 4. C5 + Selectivity as a function of increasing transport limitations at 473 K, H2/CO=2.1, 2000 KPa and 50-60% CO conversion. O represent eggshell catalysts while 9 are powder catalysts and & are even pellets of different diameter.
997 4. ACKNOWLEDGMENTS We thank Dr. Rocco A. Fiato for many helpful discussions and Ms. Hilda Vroman and Mr. Bruce DeRites for the synthesis, characterization, and catalytic evaluation of some of these materials. We also thank Dr. Eric Herbolzheimer and Ms. Dee Redd for the viscosity and surface tension measurements. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12 13. 14. 15. 16.
E. Iglesia, S. L. Soled, and R. A. Fiato, J. Catal., 137 (1992) 212 E. Iglesia, S. C. Reyes, and R. J. Madon, J. Catal., 129 (1991) 238. R.J. Madon, S. C. Reyes, and E. Iglesia., J. Phys. Chem., 95 (1991) 7795. E. Iglesia, S. C. Reyes, and S. L. Soled, in "Computer-Aided Design of Catalysts and Reactors" (E. R. Becker and C. J. Pereira, eds.), p. 199,Marcel Dekker, New York, 1993. E. Iglesia, S. C. Reyes, R. J. Madon, and S. L. Soled, in "Advances in Catalysis and Related Subjects" (D. D. Eley, H. Pines, and P. B. Weisz, eds.) vol 39, p. 239. Academic Press, New York, 1993. R.J. Madon, S. C. geyes, S.C., and E. Iglesia, in "Selectivity in Catalysis", ACS Symposium Series (S. L. Suib, and M. E. Davis, eds.) 1992. R.W. Maatman and C. D. Prater, Ind. Eng. Chem., 49 (1957) 253. W.E. Corbett and D. Luss, Chem. Eng. Sci,. 29 (1974)1473. C.J. Pereira, G. Kim, and L. L. Hegedus, Catal. Rev. Sci. Eng., 26 (1984) 583 ; J. E. Summers and L. L. Hegedus, J. Catal., 51, (1978) 185. R.S. Dixit and L. L. Tavlarides, Chem. Eng. Sci., 37, (1982) 59, Ind. Eng. Chem. Proc.Des. Dev., 22 (1983) 1 R.C. Everson, E. T. Woodburn, and A. R. M. Kirk, J. Catal., 53 (1978) 186. A. Niemark, A. Khelfez, and V. Fenelonov, Ind. Eng. Chem. Prod. Res. Dev., 20 (1981) 439. M.F.M. Post and S. T. Sie, Eur. Pat. Appl. 174,696 (1985). W.A. van Erp, J. M. Nanne, and M. F. M. Post, Eur. Pat. Appl. 178,008 (1985). R.S. Sapienza, M. J. Sansone, W. Anthony, and R. Slegeir, GB Patent 2,104,405A (1983) E. Washburn, Phys. Rev., 17 (1921) 273.
This Page Intentionally Left Blank
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
999
Bismuth(llI) and m o l y b d e n u m ( l l ) acetates as mono- and homopolynuclear precursors of silica-supported bismuth molybdate catalysts O. Tirions a, M. Devillers a,', p. Ruiz b and B. Delmon b. Universit6 Catholique de Louvain, aLaboratoire de Chimie Inorganique et Analytique, place Louis Pasteur, 1. bUnit6 de Catalyse et de Chimie des Mat6riaux Divis6s, place Croix du Sud 2/17, B- 1348 Louvain-la-Neuve, Belgium.
1. I N T R O D U C T I O N Great interest is continuously devoted to the preparation of tailored solid catalysts by simple but reliable methods that ensure an appropriate surface architecture [ 1]. Supported catalysts are nowadays classically prepared by precipitation-deposition, impregnation, or grafting from solutions containing inorganic salts. Although these methods can lead to efficient catalysts, further improvements are still needed when complex multicomponent or multiphasic systems have to be generated, particularly from the point of view of controlling the type of phases formed and their dispersion. This was shown to be particularly critical in silica-supported catalysts for selective oxidation [2]. As far as bismuth and molybdenum are concerned, preparation methods are usually based on an impregnation procedure from highly acidic aqueous solutions of bismuth(IIl) nitrate and ammonium heptamolybdate. Under these circumstances, the achievement of high dispersion levels is severely restricted by the differences in precursor-support interactions during the impregnation stage. These are the consequence of complex hydrolysis equilibria which give rise to various stoichiometries in the parent solution. As shown in the frame of investigations on the hydrolysis products of bismuth(III) nitrate [3], species like Bi 3§ BiO § Bi(OH) 2§ and Bi(OH)2 § are accompanied by polycationic moieties such as [Bi604(OH)4] 6+, which seem to be the predominant species at pH values between 0.8 and 1.0. These species are formed according the following equilibria" 6 Bi 3+ + 8 H20 ~ 2 [Bi604(OH)4] 6+ . ,
[Bi604(OH)4] 6+
+
12 H +
(1)
~_~2 [Bi605(OH)3] 5+
+
2 H§
(2)
Similar phenomena may be invoked in the case of molybdenum, for which polyanionic species like Mo70246- and Mo80264- were shown to coexist with numerous other species of lower nuclearity, depending on the pH, the molybdenum concentration and the temperature [46]. Differences in the ionic charges carded by these entities and in their molecular sizes modify the electrostatic interaction with the support and the accessibility of the potential adsorption sites
* Corresponding author
1000 for the precursor species. In addition, the support itself is known to play an essential role in controlling the nature of the surface-attached species, as shown previously in aluminasupported MoO3 hydrodesulfurization catalysts, in which oligomerization of molybdate ions was found to proceed above a certain catalyst loading [7]. This precludes the adequate control of metal dispersion and was shown to be extremely critical in the case of carbon-supported molybdenum catalysts [8,9]. These drawbacks can be avoided by taking advantage of coordination compounds in which metal atoms are surrounded by well-defined ligand configurations. When this strategy is applied today to the preparation of multimetallic heterogeneous catalysts, its interest is, however, generally restricted to the possibility of anchoring mono- or polynuclear complexes onto a functionalized (usually hydroxylated) inorganic support. This procedure has been successfully applied to the preparation of silica- or alumina-supported Mo-based catalysts for propene oxidation or metathesis, by using metal complex precursors of different nuclearity and Mo oxidation states, like Mo2(TI3-C3H5)4 [10-13], Mo2(OC2H5)4 [14], the oxalate complexes [Mo204(C204)2(H20)] 2- or [Mo304(C204)3(H20)3] 2- [14-16], or other Mo clusters of higher nuclearity [ 16]. An interesting alternative seems to be the simultaneous immobilization of mono- or homopolynuclear compounds of different metals as precursors for the "one-step" incorporation of these elements in the active phase, according to an appropriate stoichiometry. The present work illustrates this new approach, that seeks to take advantage of constitutive and structural analogies between the precursors selected for incorporating the active metals, in order to achieve a better control of the surface stoichiometries of multimetallic supported catalysts. The envisioned objective can be reached by selecting, for both concerned metals, precursors containing the same or very similar ligands 9it seems therefore reasonable to expect more uniform van der Waals interactions, resulting in similar affinities for the support during the adsorption step. In addition to the geometrical features of the coordination sphere, both the nuclearity of these compounds and the occurrence of metal-metal bonding are assumed to play an important role in controlling the nature and dispersion of the obtained solid phases. We recently initiated a research programme aimed at providing milestones for the adequate control of the architecture of multimetallic oxidic catalysts for selective oxidation reactions. Its main purpose is to carry out a systematic study of the influence exerted by the nature and structure of the metallic precursors on the surface stoichiometry and the dispersion level of the active phase. One of the biggest challenges is however to preserve properly the structure and nuclearity of the selected precursors on the support up to the calcination stage ; this can be achieved by the judicious choice of the solvent, provided adequate handling and sampling conditions are strictly respected to warrant their stabilization. The present report deals with preliminary results obtained in the frame of experiments on silica-supported bimetallic bismuthmolybdenum catalysts. Bismuth(III)acetate, Bi(O2CCH3)3, and the dinuclear quadruply-bonded tetraacetatodimolybdenum(II) compound, Mo2(O2CCH3)4, were selected as precursors. This choice results from preliminary experiments concerning the preparation of biphasic Bi-Mo and Bi-W catalysts supported on MoO3 or WO3, suggesting that carboxylate-type bismuth precursors were promising starting materials for the design of such polymetallic systems [ 17]. Up to now, molybdenum(II) acetate has been only occasionnally used as starting material for the incorporation of molybdenum in supported catalysts [18,19]. In the frame of detailed XPS and chemisorption studies on calcined, reduced, and NO or CO-treated silica-supported Mo catalysts, tetracarboxylatodimolybdenum (II) compounds were shown to give rise to metalsupport interactions which are definitely different from those in conventionally prepared Mobased catalysts [ 19].
1001
2. EXPERIMENTAL 2.1. Preparation of precursors Bi(III)acetate, noted Bi(OAc)3, is a highly moisture-sensitive compound obtained in a dry nitrogen atmosphere from bismuth(III) oxide (Janssen, p.a., 99%) refluxed in a 1:1 volume mixture of acetic acid and acetic anhydride [20]. Thermal degradation in air at 598 K produces a finely divided form of ~t-Bi203 (mean BET specific surface area : ca. 0.6 m2/g). MoOD acetate, noted M o 2 ( O A c ) 4 , is prepared according to the literature [21] from refluxing hexacarbonylmolybdenum (Janssen) in the presence of an acetic acid/anhydride mixture in odichlorobenzene (Janssen, 99%) under inert atmosphere for 20 hours. Slow cooling overnight gave rise to bright yellow crystals which are filtered off and washed with absolute ethanol and ether. Thermal degradation into MoO3 occurs above 548 K.
2.2. Preparation of catalysts Dehydroxylated silica (BASF D11-10) preheated in air at 1073 K for 24 h (BET specific surface area of 125 m2/g) was used as support. Catalysts prepared from acetate precursors were obtained according to a deposition procedure in which the solids were put in contact with the silica support in dry n-heptane under ultrasonic agitation; these experiments were carried out under strictly air- and water-free conditions, using standard Schlenk techniques. Prior to its use, n-heptane was distilled under nitrogen over Na-benzophenone. The influence of various experimental parameters was considered, like the Bi-to-Mo ratio, the chronology of incorporation of both precursors (successively or simultaneously), and the final calcination temperature (673 or 773 K). Three different values of the Bi-to-Mo ratio were selected, that correspond to the stoichiometries of the various bismuth molybdate phases, i.e. Bi/Mo = 2.0 (as in 7-Bi2MoO6), Bi/Mo = 1.0 (as in 13-Bi2Mo209) and Bi/Mo = 0.66 (as in ~tBi2Mo3012). The total metal loading (Bi + Mo) was fixed at 2, 5 or 10 mol.% with respect to the silica content ((Bi+Mo/SiO2) = 0.02, 0.05 or 0.1). The amount of active phase corresponding formally, i.e. assuming perfect dispersion, to one monolayer of Bi2MoO6 (taken as "Bi203 + MOO3") can be calculated from the literature to be 3.7 mol. %. For comparison, catalysts were synthesized from usual inorganic precursors (bismuth nitrate and ammonium heptamolybdate) with the same total loading and relative amount of Bi and Mo, according to an impregnation method in aqueous medium at pH = 0. After addition of the silica support to the starting solution, the major part of the solvent was eliminated in a rotatory evaporator, then the temperature was raised to 363 K. Afterwards, the powder samples were calcined at 773 K for 20 hours under continuous air flow.
2.3. Physical characterization techniques Catalysts and precursors were characterized by XRD, FTIR and XPS measurements. Infrared spectra were registered with a Fourier Transform spectrometer PERKIN-ELMER type 1710, in the range 4000-450 cm -1 , in the form of KBr disks or Nujol mulls. Powder X-ray diffractometry was carried out on a SIEMENS 500 diffractometer equipped with a copper anode. X-ray photoelectron spectroscopy was performed on a spectrometer from SURFACE SCIENCE INSTRUMENT SSI 100, model 206. For the analysis of the precursor photoelectron spectra, the binding energy scale was calibrated with reference to the Cls
1002 photopeak of residual hydrocarbon taken at 284.8 eV. The XPS data of silica-supported catalysts were calibrated with respect to the Si2p binding energy at 103.5 eV. Specific surface areas were measured according to the BET method with a MICROMERITICS ASAP 2000 instrument, using nitrogen adsorption at 77 K. The thermal behaviour of the selected precursors was investigated by thermogravimetric analysis on a SETARAM TGC 85 analyzer. 3. R E S U L T S AND DISCUSSION
3.1. Uncalcined catalysts The uncalcined catalysts were characterized by FTIR and XPS measurements. In the IR spectrum of an uncalcined silica sample loaded with Mo2(OAc)4 only, the presence of typical absorption bands of this dinuclear molybdenum(H) precursor at 629, 676 937, 1353 and 1445 cm -1 indicates that the molybdenum acetate can be deposited on silica without any damage for its structure. When Bi(OAc)3 is added to the same sample, new bands that are characteristic of this compound appear at 614 and 957 cm-1 (Fig. 1). The XPS results collected on the pure starting materials and the uncalcined silica-supported catalysts are summarized in tables 1 and 2. As indicated in table 2, the absolute content of residual carbon (C(I)) on the uncalcined catalysts prepared from inorganic precursors is very low and comparable with the one observed on pure silica (C(I)/Si = 0.25). This precludes the traditionnal use of this signal as internal standard for the calibration of the binding energy scale in our silica-supported catalysts and justifies its replacement by the well-defined Si2p photopeak.
-==
*
~
--~=
#
[.-,
1500
/3i
1-
,_.
1200
800
~" ( c m . 1 ) 4 0 0
Figure 1. IR spectrum of uncalcined silica catalyst loaded with Bi(OAc)3 (#) and Mo2(OAc)4 (*) (Molar ratios: Bi+Mo/SiO2 = 0.1; Bi/Mo = 2.0)
1000
" ' " " 8()d
" " ' " " 6 1 ) 0 ' 4' 0"( c 'm0 ' l") ~ '
Figure 2. IR spectra of calcined silica catalysts prepared from acetate precursors Calcination temperature: 773 K Molar ratio Bi+Mo/SiO2 = 0.1 2a.(fuU line): Bi/Mo = 2.0 2b.(dashed line): Bi/Mo = 0.66
1003 Table 1 XPS characterization of precursors and catalysts : selected bindin$ energy values. Bi+Mo (a) Bi (a) SiO2 ~
Tcalc
(K)
(b)
Eb ( c V )
C1s
01s
SiO2 calcined at 1073 K~Oh
284.8 (Ia) 532.7 (I) 286.8 (Ib)
Bi(O2CCH3)3
284.8 (Ia) 530.9 (II) 288.0 (II) 529.4
(c)
Mo3d5/2 Bi4f'//2
159.0
284.8 (Ia) 531.8 (II) 228.7 287.5 (II)
Mo2(O2CCH3)4
Catalystsprepared from Bi(O2CCH3)3 +
Mo2(O2CCH3)4
0.05
1.0
-
284.4 (Ia) 532.7 (I) 228.2 286.8 (Ib) 531.0 (II) 288.2 (II)
159.2
0.05
1.0
773
284.7 (Ia) 532.6 (I) 232.3 286.7 (Ib) 530.2 (II)
159.2
0.02
2.0
-
284.5 (Ia) 532.7 (I) 228.0 286.7 (Ib) 530.8 (II) 288.1 (II)
159.0
0.02
2.0
773
284.6 (Ia) 532.7 (I) 232.2 286.0 (Ib) 530.4 (II)
159.0
0.02
0.66
-
284.3 (Ia) 532.7 (I) 228.0 285.8 (Ib) 531.0 (II) 286.9 (II)
158.9
0.02
0.66
673
284.7 (Ia) 532.7 (I) 232.6 286.6 (Ib) 530.5 (II)
159.2
0.05
1.0
-
284.8 (la) 532.7 (I) 233.0 286.7 (Ib) 530.7 (II)
160.1
0.05
1.0
773
284.8 (Ia) 532.7 (I) 232.6 286.3 (Ib) 530.4 (II)
159.7
0.02
2.0
773
284.6 (Ia) 532.7 (I) 286.1 (Ib)
159.8
Catalystsprepared from Bi(NO3)3.5H20 + (NH4)6Mo7024.4H20
232.1
(a) Molarratio. (b) Calcinationtemperature. (c) Bindingenergy;reference samples are calibrated with respect to Cls at 284.8 eV; catalysts, with respect to Si2p at 103.5 eV.
1004 Table 2 .Intensit.y ratios observed by XPS in uncalcined and calcined catalysts prepared from acetates or inorganic precursors.
Bi+Mo (a) Bi (a)
sio2
~
(c)
(b)
Acetate precursors
Tr162 Bi Mo
O(I) Si
C(I) Si
-
0.9 2.3 1.4 1.2 0.4
1.9 2.0 2.0 2.1 1.9
0.82 0.50 0.32 0.34 0.35
773 673 673 773 773 673 773 773 673 673
1.8 2.8 0.6
2.1 2.0 2.3
0.09 0.06 0.13
1.8 3.2 1.7 4.9 3.9 2.4
0.7 1.0 1.3 0.7 0.4 0.3
1.9 1.9 1.9 1.8 1.8 1.9
0.13 0.17 0.07 0.07 0.06 0.06
3.1 3.4 2.8 6.7 6.7 8.0
(K)
O(ll) O(I 0 Bi Mo
Inorganic precursors
CI(I) O(H) Si Bi
(c)
O(II) Mo
Bi Mo
O(I) Si
0.3
1.9
0.10
1.0
1.9
0.19
0.3
2.0
0.45
7.1
2.3
0.2 0.6 0.5 1.2 0.4 0.5 0.3
2.0 1.9 1.8 1.8 1.9 1.9 1.9
0.36 0.10 0.08 0.05 0.07 0.07 0.06
5.5 1.8 3.3
1.1 1.2 1.8
Uncalcined catalysts 0.10 0.05 0.02
0.66 1.0 2.0 1.0 0.66
Calcined catalysts 0.10
2.0
0.05 0.05
0.66 2.0 1.0
0.02
2.0 1.0 0.66
2.0 3.3 3.9 4.6 3.1 2.7
(a) Molarratio. (b) Calcinationtemperature (c) Analyticalphotopeaksare Cls, Ols, Si2p, Mo3d5/2and Bi4f7/2.
In the acetate-based catalysts, the observed Mo3d5/2 binding energy values (Eb (Mo(II)) = 228.1 + 0.2 eV) are close to those of Mo(II) contained in Mo2(OAc)4 (228.7 eV); when converted with respect to the present reference binding energy scale, literature values for dimolybdenum(II) tetracarboxylates are reported to lie in the range 228.6-229.1 eV [ 19]. The first C ls component (C(Ia)) includes the residual hydrocarbon and the methylic carbon belonging to the acetate groups; the component at higher energy (C(II)) corresponds to the carboxylate group. This explains the regular increase of the C(I)/Si ratios with the absolute precursor loading on silica in the case of the acetate-based catalysts. In summary, the characterizations of the supported catalysts prepared from the acetate precursors before their calcination confirmed unequivocally the presence of both the bismuth and the dinuclear MoOD compounds on the surface, indicating the suitability of this preparation method for preserving these well-def'lned structural moieties up to the calcination stage.
1005
3.2. Calcined catalysts The calcined catalysts were characterized by XRD, FTIR and XPS techniques.
3.2.1. Infrared spectroscopic studies Because most IR bands characteristic of the various molybdate phases are masked by two broad absorption bands generated by silica at 472 and 802 cm-l,the interpretation of the IR spectra in the range 1000-400 cm -1 is not an easy task. The infrared investigations were therefore restricted to the silica samples loaded with 10 mol. % of active metals. Typical IR spectra of catalysts prepared by the acetate route are presented in fig. 2. Those of catalysts made from inorganic precursors were very poorly resolved. The main comments are as follows. When the Bi-to-Mo ratio is equal to 2 (Fig. 2a), a characteristic band appears systematically at about 740 cm -1, together with a shoulder at 835 cm -1, whatever the calcination temperature. According to the literature [22], this band and shoulder can be assigned either to the v-bismuth molybdate phase (v = 735 (vs), 842 (m) cm -1 ) or to the fi-phase (v = 740 (m), 840 (sh) cm -1 ). When the Bi-to-Mo ratio equals 0.66, the IR spectra exhibit quite different shapes and vary slightly with the calcination temperature. The main relevant bands are: - after calcination at 673 K: 902 (vw) and 936 cm -1 (vw); - after calcination at 773 K: 670 (w), 721 (w), 903 (vw), 935 (vw) and 951 cm -1 (vw) (Fig. 2b). All these values are in line with those mentioned for a-bismuth molybdate [22].
3.2.2. X.ray diffraction results The following comments can be formulated on the XRD results. It should be mentioned first that, if the interpretation of the powder X-ray diffractograms is unambiguous in the case of the 10% loaded silica samples, it becomes harder for the samples loaded with 5 mol.% and sometimes really hazardous for those loaded with 2 mol.% only of active metals. In general, the quality of the diffraction patterns is significantly higher in the samples obtained by the acetate route than with the classical inorganic precursors. The silica support itself is amorphous. (i) Whatever the amount of active phase and the nature of the precursors (acetates or inorganics), experimental conditions can be found that generate at the surface of the support the Bi-Mo-O phase whose stoichiometry corresponds to the Bi-to-Mo ratio as determined by the relative amounts of the complexes introduced, sometimes in the pure form, otherwise accompanied by other bimetallic phases. Calcination at 773 K is however required to promote the formation of the v-molybdate phase, while the temperature of 673 K is in any case sufficient to induce the formation of a-bismuth molybdate. The latter phase is obtained free from any other molybdate phase when Bi and Mo acetate precursors are introduced in the molar ratio 2:3. (ii) The presence of the fl-phase is clearly favoured at the lower temperature. Only in the catalysts made from acetate precursors with Bi/Mo = 1.0, this phase is found to appear as practically pure. In all the other combinations, it is accompanied by a mixture of the a and ~,phases. This behaviour is in line with the instability of the fi-phase above 673 K according to the following equation: 2 Bi2Mo209 ~ Bi2MoO6 + Bi2Mo3012 The occurrence of the fi-phase under these conditions is unexpected if one refers to the phase diagram of the system Bi203-MoO3 [23], in which the stability range of this phase is mentioned to be 813-938 K. (iii) The preferential formation of the v-phase on the 2% loaded support with Bi/Mo = 2.0 or 1.0 seems easier when acetates are used instead of inorganic precursors.
1006 (iv) For the catalysts loaded with 5 or 10 mol % active metals, the presence of small amounts of the monometallic phases tx-Bi203 or MoO3 can not be ruled out. The presence of B i ( I ~ oxide in the samples characterized by Bi/Mo = 2.0 is in line with the fact that the 13-phase was found to accompany the expected v-phase in these catalysts. (v) Although bismuth silicates like Bil2SiO20 or Bi4(SiO4)3 are reported in the literature to form when mixtures of Bi203 and SiO2 are heated above 1123 K [24], the occurrence of these phases in our calcined catalysts was carefully considered. While the peaks that are typical of the Bi 12SIO20 compound can be scarcely differentiated from those of the molybdate phases, the presence of Bi4(SiO4)3 can not be merely excluded. 3.2.3.
X-ray
photoelectron
spectroscopy
This technique was used to collect informations about the nature of the phases formed, their dispersion on the surface of the support and the valence state of a given element. The most relevant data are listed in tables 1 and 2, presenting selected values of the binding energy (Table 1) and the intensity ratios of the various elements on the surface (Table 2). These data suggest the following comments: (i) Oxygen ls spectra appear in our catalysts as a superposition of two species whose binding energies are respectively (532.7:k-0.1) eV (O(I)) and (530.4_+0.5) eV (O(II)). These values match those of oxygen contained in silica (O(I)) and bismuth molybdates (O(II)), respectively [25]. Let us note that the second species could not be detected in the 2% loaded catalysts made from inorganic precursors. (ii) Data relative to silica prove that the support is not affected either by the thermal treatment nor by the deposition procedure itself (the values are not shifted after treatment); the O(I)/Si atomic ratios are furthermore in any case close to 2.0. (iii) The Mo 3d binding energy values observed in the calcined catalysts are close to those issued from the literature [25] for Mo contained in bismuth molybdates (Eb (Mo 3d5/2) = 232.0 eV) with AEb (Mo3dS/2 - Mo3d3/2) = 3.2 + 0.1 eV. (iv) The Bi 4f binding energies are comparable to those corresponding to the various bismuth molybdates (Eb = 158.5 to 159.0 eV [25]). (v) The fact that these C(I)/Si ratios are not increased in the acetate-based calcined catalysts prove that the thermal degradation of the carboxylate precursors proceeds without leaving additional carbon on the support. (vi) The comparison between the intensity ratios in the different loaded samples shows that: - for the 5 and 10 mol.% loaded silica, the Bi-to-Mo ratio observed by XPS on the surface is close to the expected atomic ratio when acetate precursors are used (1.8 for 2.0 at 773 K; 0.6 for 0.66 ; 1.0 for 1.0 at 673 K). On the contrary, we are quite far from this value in the case of inorganic precursors, particularly at high loading (0.2 and 0.3 instead of 2.0). In the latter catalysts, the Mo-to-Si ratio is furthermore by far larger than the one expected from the involved Mo amount (9.3 and 6.5 % instead of 3.3 and 1.7 %, respectively). - for the 2% loaded samples, the significantly lower counting rate renders these results much more approximative, whatever the nature of the precursor.
1007 4. C O N C L U S I O N S Silica-supported bismuth molybdates were obtained under mild temperature conditions as majority phases from the thermal degradation of acetate precursors previously deposited on the surface of the support in an organic solvent. When the Bi/Mo ratio was equal to 0.66, the pure a-phase Bi2Mo3O12 was systematically observed, independently from the calcination temperature. For a Bi/Mo ratio of 2.0, a mixture of ~-Bi2Mo209 and ~,-Bi2MoO6 was obtained, the 13-phase being the most abundant one at 673 K and the ,t-phase, at 773 K. The pure 13-phase was obtained for a Bi/Mo ratio of 1.0 after calcination at 673 K. Although the presence of any other monometallic oxide cannot be ruled out, this route is apparently a valuable alternative to generate these catalytically active phases on silica. At high bismuth and molybdenum loading, the Bi/Mo ratio observed on the surface of the catalysts made by the acetate precursor method is identical or, at least, similar to that one selected for the incorporation of the two metals, while this is clearly not the case for catalysts prepared in aqueous medium from Bi nitrate and ammonium heptamolybdate, suggesting that, in the former case, the relative dispersion of the two metals on the surface can be modulated more appropriately. Although significant fluctuations are still observed in the characteristics of the obtained catalysts, this first series of experiments indicates that the use of analogous Bi(III) and Mo(II) carboxylates as precursors for silica-supported bismuth molybdate catalysts allows the appropriate control of the nature of the active phases and of the relative dispersion of the metals on the surface, provided the choice of the Bi/Mo ratio and the calcination temperature are adequate. The present results therefore suggest that methods based on the use of mono- or polymetallic complexes in organic medium can provide interesting alternative routes for preparing heterogeneous catalysts. Complementary work on the use of structurally related precursors is under progress to provide a deeper insight into the possibilities of optimizing the catalytic properties of these catalysts by changing the nuclearity of the selected precursors and the nature of the surrounding ligands. It should however be remembered that the surface architecture of these supported catalysts might be strongly affected by any additional carbon deposit on the active phase. The fact that residual carbon was not generated in detectable amounts by the thermal decomposition of the short-chain carboxylate ligands used in the present work can not be merely generalized to longer carboxylate chains. This could result in significant changes in the thermal stability of the catalyst, which might be appreciably improved if suitable experimental conditions are found out to optimize the relative dispersion of the two active metals and carbon.
Acknowledgments The authors greatly acknowledge financial assistance from the Belgian National Fund for Scientific Research, Brussels (Research convention with the "Fonds de la Recherche Fondamentale et Collective"), and from the "Minist~re de la Region Wallonne", Belgium, in the frame of a Concerted Action. This work was supported by a fellowship allotted to O. Tirions by the "Institut pour rEncouragement de la Recherche Scientifique dans rlndustrie et rAgriculture" (I.R.S.I.A.), Belgium.
1008
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25.
J.T. Wrobelski and M. Boudart, Catal. Today 15 (1992) 349. F. Delannay, Characterization of heterogeneous catalysts (F. Delannay, ed.), Marcel Dekker, New York, 1984, pp. 113. F. Lazarini, Bull. Bismuth Inst. (Brussels) 32 (1981) 3. J.J. Cruywagen and H.F. De Wet, Polyhedron 7 (1988) 547. P. Sarrazin, B. Mouchel and S. Kasztelan, J. Phys. Chem. 93 (1989) 904. N.P. Luthra and W.-C. Cheng, J. Catal. 107 (1987) 154. H. Weingold, J. Catal. 83 (1983) 85. J.M. Solar, F.J. Derbyshire, V.H.J. de Beer and L.R. Radovic, J. Catal. 129 (1991) 330. J.M. Solar, C.A. Leon y Leon, K. Osseo-Asare and L.R. Radovic, Carbon 28 (1990) 369. Y. Iwasawa, Catal. Today, 6 (1989) 27. Y. Iwasawa and M. Yamagishi, J. Catal. 82 (1983) 373. Y. Iwasawa, Y. Sato and H. Kuroda, J. Catal. 82 (1983) 289. S.M. Aliev, V.D. Sokolovskii, A.N. Startsev and B.N. Kuznetsov, React. Kinet. Catal. Lett. 22 (1983) 133. A.N. Startsev, O.V. Klimov and E.A. Khomyakova, J. Catal. 139 (1993) 134. O. V. Klimov, E.A. Krivoshchekova and A.N. Startsev, React. Kinet. Catal. Lett. 42 (1990) 95 P. M. Boorman, C. Fairbridge, K. S. Jasim and R.A. Kydd, J. Molec. Catal. 53 (1989) 371 M. Devillers, O. Tirions, L. Cadus, P. Ruiz and B. Delmon, Abstracts of the First European congress on Catalysis (Montpellier, France, sept. 1993), p. 476. J. Smith, W. Mowat, D.A. Whan and E.A.V. Ebsworth, J. Chem. Soc., Dalton Trans. (1974) 1742. S.A. Best, R.G. Squires and R.A. Walton, J. Catal. 60 (1979) 171. A.P. Pisarevskii, L.I. Martynenko and N.G. Dzyubenko, Russ. J. Inorg. Chem. 35 (1990) 843. D.S. Martin, R.A. Newmann and P. E. Fanwick, Inorg. Chem. 18 (1979) 2511. I. Matsuura, R. Schuit and K. Hirakawa, J. Catal. 63 (1980) 152. M. Egashira, K. Matsuo, S. Kagawa and T. Seiyama, J. Catal. 58 (1979) 409. G. Gattow and H. Fricke, Z. allg. anorg. Chem. 324 (1963) 287. I. Matsuura and M.W.J. Wolfs, J. Catal. 37 (1975) 174.
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
1009
Preparation o f Catalysts by Chemical Vapor-Phase Deposition and D e c o m p o sition on Support Materials in a Fluidized-Bed Reactor S. KOhler, M. Reiche, C. Frobel and M. Baerns Lehrstuhl ~ r Technische Chemic, Ruhr-Universitat Bochum, D-44780 Bochum, Germany Catalysts consisting ofPt, Cr203 and V205 supported by SiO2 and Al203 were prepared by vapor-phase adsorption of metal acetyl acetonates (Pt(acac)2, Cr(acac)3 and V(acac)3 ) on supports and subsequent transformation of the adsorbates to the catalytic compound using a fluidized-bed technique. Surface processes occurring during adsorption and decomposition on the support were investigated by DRIFTS and MS analysis of the volatile decomposition products. The influence of preparation conditions on dispersion of the catalytic compound was studied. The metal acetyl acetonate adsorption mode as well as the decomposition pathways and finally, the dispersion is affected by properties of the support surface. Pt and Cr203 particles agglomerate on silica, this is enhanced by increasing the loading on the support material; on alumina the particles are stabilized resulting in a higher degree of dispersion. Catalysts prepared by vapor-phase adsorption and decomposition of metal acetyl acetonates are contaminated by carbonaceous surface compounds and deposited carbon which can be removed by an additional air treatment of the samples above 570 K.
1. INTRODUCTION Catalyst preparation by means of chemical vapor phase decomposition (CVD) is carried out by vaporizing a suitable precursor and adsorbing the gaseous compound on the support material. Subsequently, the adsorbate is transformed to the catalytic compound [ 1-4]. In the present paper the preparation of supported Pt, Cr203 and V205 catalysts by vaporphase adsorption of metal acetyl acetonates (Pt(acac)2 , Cr(acac)3 and V(acac)3 ) on SiO 2 and y-Al203 and the subsequent decomposition of the adsorbates to the catalytic compound is described. The process was carried out in a fluidized-bed reactor by which all particles were evenly exposed to the vapor ascertaining homogeneous loading of the catalytic compound on the support material. To study the adsorption of metal acetyl acetonates on the supports and the subsequent transformation, in-situ DRIFT spectroscopy supplemented by mass-spectrometric analysis of volatile decomposition products was applied. To elucidate the influence of preparation conditions on dispersion of the catalytic compound, series of silica- and aluminasupported Pt, Cr203 and V205 catalysts were prepared by varying the amount of adsorbed metal acetyl acetonate. The Pt(acac)2 adsorbates were decomposed in N 2 as well as in air, while the adsorbates of Cr(acac)3 and V(acac)3 were decomposed in air only. Also supported Pd catalysts were prepared by decomposition of adsorbed Pd(acac)2 on the support material.
1010 However, the application of the method was less successful because Pd(acac)2 tends already to decompose in the gas phase before adsorption is accomplished [4]. 2. EXPERIMENTAL Catalyst preparation The quartz-made fluidized-bed reactor used for catalyst preparation was electrically heated; a cyclone was incorporated into the freeboard of the reactor to prevent elutriation of fines. The support, which was fluidized by N 2 was first thermally pretreated to remove physisorbed water. Then the vaporized metal acetyl acetonate was adsorbed on the support at 400 K at constant partial pressure in a flow of N 2 for a given period of time. The subsequent decomposition of the adsorbate was carried out by increasing the fluidized-bed temperature with a rate of 4 K/min. Pt was deposited in either N 2 or air at 573 K; Cr20 3 and V20 5 were deposited in air at 673 K. Pt catalysts which were decomposed in N 2 were additionally treated in air at 573 K (Pt/SiO 2 catalysts) respectively at 773 K (Pt/AI203 catalysts) [4, 5]. DRIFT and mass-spectrometric analysis of thermal decomposition products Diffuse reflectance IR spectra of supported metal acetyl acetonates were recorded with a Perkin-Elmer 1710 spectrometer using a diffuse reflectance cell (Spectra Tech 003-102). lnsitu decomposition experiments for supported adsorbates were carried out in the temperature range from 295 K to 773 K in N 2 (supported Pt(acac)2 ) and in air (supported Cr(acac) 3 and V(acac)3), respectively. Volatile decomposition products evolved during heating of the sample in a flow of He (supported Pt(acac)2 ) and in a flow of 7 % 0 2 in He (supported V(acac)3), respectively, were detected by means of a quadrupole mass spectrometer (Micromass PC, VG Quadrupoles) [4, 5]. Elemental analysis and dispersion of the catalytic compound The Pt content of the catalysts was determined by ICP after melting with Na202, the Cr and the V content was determined by XRF. Dispersions were determined by gas chemisorption. The chemisorption conditions for the different catalysts are listed in Table 1. Table 1 Experimental conditions for gas chemisorption on Pt, Cr203 and V20 5 catalysts Pt catalysts
Cr20 3 catalysts
V20 5 catalysts
probe molecule
CO
02
assumed stochiometry
Pt : CO = 1 : 1
Cr
catalyst pretreatment adsorption temperature method
H2, 473 K, 2 h 298 K GC-Pulse
H2, 623 K, 2 h 298 K micro-balance
02 V:02=2:1 H2, 623 K, 6 h
: 0 2 =
2:1
623 K, [61 GC-Pulse
1011 3. RESULTS Results on adsorption and decomposition of Pt(acac)2, Cr(acac)3 and V(acac)3 are presented first. Then, the influence of preparation conditions on the dispersion of the catalytic compound is discussed. 3.1. Adsorption of Pt(acac)2, Cr(acac)3 and V(acac)3 on SiO 2 and A120 3
The DRIFT spectra of adsorbed metal acetyl acetonates show different band intensities indicative for different metal acetyl acetonate loadings, while different band positions and relative intensities are indicative for different metal acetyl acetonate adsorption states on both supports Furthermore, the spectra show a band at ca 1635-1640 cm-1 which is due to water physically adsorbed on the support surface at room temperature
V(acac)3
Vco Vcc
Pt(acac)2 0.5
.
5~
Pt(acac)2 (a)
Hacac Ill
coA\ / I
I
I
Vco
II
II
\:
'A' '
'
1700 1600 15'00 14'00 Wave n ~ n b e r / c m " 1 Figure 1. DRIFT spectrum of Pt(acac)2 adsorbed (a) on SiO2 (4.6 wt.-% Pt) and (b) on AI20 3 (1.6 wt.-% Pt) at 298 K.
'
|
I
0'
I
1700 1600 1500 1400 Wave number / cm-1 Figure 2. DRIFT spectrum of V(acac)3 adsorbed (a) on SiO 2 (0.3 wt.-% V) and (b) on A1203 (0.3 wt.-% V) at 298 K.
The DRIFT spectrum of silica-supported Pt(acac)2 (see Figure 1) agrees with that of the unsupported compound [4, 5], when band shi~s due to adsorbate formation are taken into account. The spectrum of alumina-supported Pt(acac)2 (see Figure 1) shows a change in relative band intensities of the acetyl acetonate ligands and moreover, additional vibrations at 1609 cm -] and 1575 cm -1 which are due to acetyl acetone adsorbed as an enolate [5, 7, 8], and a vibration at 1460 cm -1 which is characteristic for surface carbonate on alumina [9].
1012 The DRIFT spectrum of silica-supported Cr(acac)3 is in agreement with that of the unsupported Cr(acac)3 [4] apart from an additional acetyl acetonate ligand vibration at 1556 em -1 which was also identified for alumina supported Cr(acac)3. The latter adsorbate showed further vibrations at 1601 cm -1 and 1460 cm -1 due to adsorbed acetyl acetone and surface carbonate on alumina which is comparable to the spectrum of adsorbed Pt(acac)2 on alumina. For V(acac)3 adsorbed on silica (see Figure 2) a band at 1583 cm -1 (shoulder) was identified, besides the characteristic precursor vibrations which was also observed for unsupported solid VO(acac)2 [8]. Furthermore, the spectrum of silica-supported V(acac)3 shows a broad vibration at 1713 cm -1 characteristic for compounds containing carbonyl groups like acetyl acetone, acetone or acetic acid weakly adsorbed on silica [8, 10]. The spectrum of adsorbed V(acac)3 on alumina corresponds more to the spectrum of acetyl acetone adsorbed as an enolate on alumina (bands at 1605 cm -1, 1576 cm -1, 1535 cm -1, 1400 cm -1 and 1375 cm -1 [8]) than to the spectrum of the unsupported solid. Also for alumina-supported V(acac)3 a band at 1462 cm -1 due to surface carbonate was identified. On the basis of these observations partly different adsorbate structures of the metal acetyl acetonates on silica and alumina are proposed: On silica Pt(acac)2 as well as Cr(acac)3 adsorb undecomposed. The fourfold coordinated Pt(II) as well as the sixfold coordinated Cr(III) cannot form additional coordinations to the surface; only the delocalized n-electrons of acetyl acetonate ligands and free electron pairs of the ligand oxygenes may form hydrogen bridges with surface silanol groups. Therefore, it is assumed that the metal acetyl acetonates interact via hydrogen bridges with silanol groups of the silica surface (see Figure 3). Because of the octahedral geometry of Cr(acac)3 only a part of ligands is attached to the surface while all ligands of the square planar Pt(acac)2 may form interactions via hydrogen bridges. In contrast to Cr(acac)3 the octahedral V(acac)3 undergoes partial decomposition when adsorbing at 400 K on silica. The partial decomposition involves presumably the hydration and split of one acetyl acetonate ligand and may involve the coordination of the remaining V(acac)2 + to surface oxygen. Acetyl acetone may be further decomposed to acetone and acetic acid according to: CH3 g - CH2 g - CH3 a d +
H2Oads"-'> CH3"g-CH3ads +
CH \ 3 / CH 3 ,C--'=-Q..N Z Q--:---c ..r U1-13
o /o ;
H
,
H
\
CH3
Figure 3. Proposed adsorbate structure of Pt(acac)2 on SiO2: H-bridges between surface silanoles and the delocalized n-electrons and oxygen of the acac-ligands.
C H 3 - C ~ H ads
H a cxC-c.x, cca I( )1 0', /0 Figure 4. Acetyl acetone adsorbed on surface Al3+sites as an enolate.
1013 On alumina the metal acetyl acetonates are partly decomposed by catalytic action of the acidic and basic sites of the support surface. The surface reaction involves the split of acetyl acetonate ligands which may adsorb on Al3+-sites in their enolate form (see Figure 4), and possibly the further conversion to acetone and acetic acid strongly adsorbed on AP+-sites. In consecutive reaction steps carbon oxides originating from decomposition form surface carbonates. V(acac)3 adsorbed on alumina is, in contrast to Pt(acac)2, and Cr(acac)3, nearly wholly decomposed to acetyl acetone and consecutive products; this is attributed to the low stability of this metal acetyl acetonate. 3.2. Decomposition of Pt(acac)2, Cr(acac)3 and V(acac)3 on SiO 2 and Ai20 3 Supported Pt(acac)2 From the in-situ DRIFT spectra (see Figure 5) it is apparent that adsorbed Pt(acac)2 decomposes during heating in N 2 between 420 K and 570 K.
2038 i 573 K C~"~
~"q'
0.5
. A~
--
Hacac / rl acetone/,I
PtO
498 K 473 I~ Pt(acac~/Al203
/~|
493 K
1.0 I CIO
2000
473 K
1800 1600 1400 Wave n u m b e r / c m "1
Only the decomposition on silica goes along with the intermediate weak adsorption of the gaseous decomposition products acetyl acetone and acetone (adsorbate vibrations in the wave number range from 1740 cm -1 to 1700 cm-l). For the reaction on silica as well as on alumina CO is the main gaseous decomposition product which was derived from the mass-spectrometric product analysis. CO adsorbs at least partly on Pt particles already formed which is reflected by respective vibrations in the DR/FT spectra in the wave number range from 2038 cm -1 to 2103 cm -1. From analyzing the IR frequency of these bands, it is concluded that Pt is first deposited in its oxidized state (PtO-COvibration > 2100 cm -1 [ 11 ]) but is then reduced (Pt-CO-vibration < 2100 cm -1 [11]) by the reductive gas atmosphere formed during decomposition. On alumina surface carbonates and carboxylates were present at~er final Pt deposition.
Figure 5. ln-situ DRIFT spectra of d e composition of Pt(acac)2 adsorbed on SiO 2 (4.6 wt.-% Pt) and on AI20 3 (1.6 wt.-% Pt) in N 2.
Supported Cr(acac)3. The adsorbates on silica as well as on alumina were completely decomposed when heating in air to 670 K. From the respective in-situ DRIFTS [4] it was furthermore derived that on SiO 2 water is adsorbed on the sample surface at ca. 570 K. This is supposed to originate from condensation of OH groups of the deposit going along with
1014 crystallisation of the initially amorphous Cr203; final XRD analysis showed that Cr203 is crystallized on silica [4]. In contrast, on alumina amorphous chromium oxide was deposited. Supported V(acac)3. As indicated by the
in-situ DRIFT spectra (see Figure 6) de-
0.5]
V(acac)3/S
1 323 K K
V(acac)3/A1203 I
373 K
composition on silica is complete at 470 K during heating in air. The adsorbate is continuously transformed to V205 accompanied by the evolution of diacetyl, acetic acid and small amounts of acetone. V(acac)3 decomposition on alumina is complete at ca. 570 K. In contrast to the reaction on silica acetyl acetone, acetone, acetic acid and traces of CO and CO 2 are evolved. Surface carbonates and carboxylates formed during complete V(acac)3 transformation were present up to 673 K.
0.5] 1800
Figure 6. In-situ DRIFT spectra of decomposition of V(acac)3 on SiO2 (0.3 wt.-% V) 1700 1600 1500 1400 Wave number / cm "1 and on AI203 (0.3 wt.-% V) in air.
Metal acetyl acetonate decomposition on alumina goes along with the formation of carbonaceous surface compounds, Pt particles which were deposited in N 2 are masked by adsorbed CO. Furthermore, detectable amounts of carbon are deposited on the catalyst surface when decomposing the adsorbed metal acetyl acetonate in N 2 [4, 5]. The contaminations are removable by an additional air treatment of the samples as applied in catalyst preparation (see above). 3.3. Dispersion of Pt, Cr203 and V205 on SiO 2 and Ai203 For silica- and alumina- supported Pt and Cr203 catalysts which were prepared by the fluidized-bed technique the amount of adsorbed probe molecules and calculated dispersions of the catalytic compound are listed in Table 2 as a function of loading. Dispersion of the catalytic compound on silica and on alumina was affected by the amount of Pt and Cr203, respectively, on the support. Furthermore, Pt dispersion was affected by the atmosphere in which Pt(acac)2 decomposition took place. These effects are described and discussed below for the support materials. For supported V205 catalysts an average dispersion of about 44 % was determined on SiO2 as well as on AI203 although the V205 loading varied between 0.5 wt.-% and 14.8 wt.-%. On the other hand, investigation of the samples by means of TPR gave rise to the assumption that the V205 particle size is affected by the loading [8]. The phenomenon is not understood yet.
1015 On silica Pt loadings between 0.1 wt-% and 8.5 wt-% and Cr203 loadings between 1.2 wt-% and 16.9 wt.-% were obtained. Whether the adsorbate was decomposed in N 2 or in air, the dispersion of Pt decreased steadily with increasing loading from 39 % to 14 %. The same effect was observed for Cr203 dispersions which decreased from 53 % to 4 % with increasing Cr203 loading. By increasing the amount of the catalytic compound on the support surface agglomeration of the primary small Pt or Cr203 clusters was promoted on the rather inert silica surface; it may be also assumed that the primary clusters act as seeds for further crystallite growth. Thus, with increasing metal coverage of the surface larger crystallites are formed. TABLE 2 Pt and Cr203 dispersion on silica and alumina support On SiO2 loading*
On AI203 D*
loading*
wt-%
adsorbed gas pmol g-1
%
Pt deposited in N 2
0.6 2.6 4.8 6.0 8.5
11 35 65 61 62
Pt deposited in air
0.1 0.2 0.4 0.6 0.9
2 3 5 7 9
1.2
28 38 24 30
Cr203 deposited in air
3.8 9.1 16.9
_ .
D*
wt-%
adsorbed gas ~tmol g- 1
36 26 26 20 14
0.3 0.7 1.2 1.6 2.0
4 11 21 33 33
26 30 34 40 32
39 29 27 21 19
0.2 0.6 1.2 1.4 2.0
4 8 14 21 34
39 26 23 29 33
53 22 6 4
0.9 3.5 6.8 11.8
30 77 96 118
77 49 32 22
%
* referred to Pt in the case of Pt catalysts and referred to Cr203 in the case of Cr203 catalysts. **CO for Pt catalysts and 02 for Cr203 catalysts. The dispersion of Pt depended significantly on the atmosphere in which the adsorbate was decomposed. At comparable metal loading (e.g. 0.6 wt-% Pt/SiO2) the Pt dispersion amounted to 2 1 % when decomposing the adsorbate in air, but to 36 % when using nitrogen instead. This effect can be explained by a higher mobility of small PtO clusters during Pt(acac)2 decomposition in air which do not exist in an inert atmosphere [5].
1016 Although on alumina the Pt loading varied between 0.3 wt-% and 2.0 wt.-%, the Pt dispersion on alumina changed randomly between 23 % and 40 %, i.e., no dependency on loading was observed. The average dispersion amounted to about 30 % for nitrogen-decomposed as well as for air-decomposed samples. The preferred existence of dispersed, alumina-stabilized Pt was confirmed by CO adsorption states as observed by DR/FTS [4]. The stabilization of dispersed Pt on AI203 may be tentatively ascribed to the Lewis acidity of the support by which the mobility of surface metal species is limited. Since donor-acceptor interactions between deposited Pt and AI3+ sites may exist and result in support stabilisation of Pt, all the other preparation conditions are obviously overruled [5]. The Cr203 dispersion on alumina decreased with increasing Cr203 loading between 0.9 wt.-% and 11.8 wt.-% from 77 % to 22 %. In comparison to silica the less marked decrease of Cr20 3 dispersion with increasing loading on alumina is ascribed to the absence of crystalline particles (see above). Amorphous Cr203 on alumina exhibits a higher catalytic surface than the compact korund-type crystallites on silica. 4. CONCLUSIONS Surface properties of the support material influence the adsorption states of Pt(acac)2, Cr(acac)3 and V(acac)3 as well as the decomposition pathways of the adsorbates and finally the dispersion of the catalytic compound. The deposited particles are more mobile on silica and hence, more capable to agglomerate. Alumina-supported Pt may be stabilized by coordinatively unsaturated Al3+ surface ions; similar arguments may apply for the stabilization of amorphous Cr203 on alumina. Because the metal acetyl acetonate decomposition is accompanied by deposition of carbonaceous compounds an additional air treatment of the samples is required. Finally, the fluidized-bed technique has been proven to be applicable for preparation of catalyst particles of uniform dispersion of the catalytic compound throughout the whole bed of particles.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
J. NicE, D. Dutoit, A. Baiker, Ber. Bunsenges. Phys. Chem., 97 (1993) 217. T. Hattori, M. Matsuda, K. Suzuki, A. Miyamoto, Y. Murakami, Proc.-Int. Congr. Catal., 9th Ottawa, Ont. 1988, vol 4, pp. 1640. S. Sato, K. Urabe, Y. Izumi, J. Catal., 102 (1986) 99. S. KOhler, Dissertation, Ruhr-Universit~t Bochum, 1994. S. K6hler, S.Trautmann, H. Dropsch, M. Baems, to be submitted to Appl. Catal. F. Majunke, M. Baerns, Characterization of Titania Supported V20 5 Catalysts by Reduction and Subsequent Oxygen Adsorption, Catalysis Today, 20( 1994)53. A. Rakai, D. Tessier, F. Bozon-Verduraz, New. J. Chem., 16 (.1992) 869. M. Reiche, Diploma Thesis, Ruhr-Universitat Bochum, 1994. A.A. Davydov, Infrared Spectroscopy of Adsorbed Species on the Surface of Transition Metal Oxides, John Wiley & Sons, New York, 1990, p. 38, 151. J.A. Anderson, C.H. Rochester, J. Chem. Faraday Trans. I, 85 (1989) 1117. D. Tessier, A. Rakai, F. Bozon-Verduraz, J. Chem. Soc. Faraday Trans., 88 (1992) 741.
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
1017
P r e p a r a t i o n o f highly loaded nickel/silica catalysts 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 m e t h o d . E f f e c t of the aging time on the reducibility o f nickel a n d on the textural properties o f the catalyst. V.M.M. Salim a *, D.V. Cesar a , M. Schmal a , M.A.I. Duarte b and R. Frety c. a Programa de Engenharia Qufmica, NUCAT/COPPE, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brasil, Fax Number 55 21 290.6626 b Centro de Pesquisa Leopoldo Miguez - CENPES, PETROBRAS, Rio de Janeiro, Brasil, Fax Number 55 21 598.6626 c Institut de Recherches sur la Catalyse, 2 avenue Einstein, 69626, Villeurbanne Cedex, France, Fax Number: 33 72445399 9 to whom correspondence should be addressed
High loading of nickel supported on Celite FC, a diatomaceous earth with a surface area of 45 m2/g, were prepared by the deposition-precipitation method. After precipitation at 298, 343 and 363 K the slurry was allowed to age up to 10h, at pH = 7.6, at the precipitation temperature. After washing and calcining, the catalysts were characterized by Temperature Programmed Reduction (TPR) and dynamic hydrogen adsorption. Increasing aging time, after precipitation at 343 and 363 K, resulted in an increase in the metallic area, although the reduction of the catalysts was strongly inhibited. Together with the increased metallic area, the BET surface area of the catalysts also increased due to the formation of mesopores in the 40 A range. Precise control of the aging conditions permits one to optimize the interaction between silica and the nickel precursor. This seems to be a very effective method to providing both an enhanced metallic surface area and a good accessibility of the metallic particles to large molecules.
1. INTRODUCTION Silica supported nickel catalysts, with very high metal loading, are frequently used in industrial processes of edible oil hydrogenation (1). Recently, such catalysts have also been considered in mild severity conditions for the hydrogenation of aromatics in distillate fuels as an alternative to less active sulfide catalyst (2) (3) (4). All these catalysts require a large metallic area and a limited microporosity to increase the accessibility of large molecules to the active sites. The diffusion problems in the hydrogenation of fatty oils and the mass transport limitations within the pore structure of Ni/SiO 2 catalysts have been studied by Coenen and coworkers (1) (5) (6). They demonstrated that edible oil hydrogenation is a structure insensitive reaction only when the active phase is located in pores with diameter wider than 50 A. Wilson et al. (3) also discussed the necessity of optimization of pore structure in order to hydroprocess synfuels, in liquid phase, with nickel catalysts.
1018 For the preparation of highly loaded supported metal catalysts, the depositionprecipitation (DP) method developed by Geus et al. (7) (8) seems very convenient. They showed that, without a large surface area carrier, the precipitation from a homogeneous solution mostly led to a limited nucleation, followed by nuclei growth, i.e., formation of poorly dispersed catalysts. They also demonstrated that the specific surface area of the carrier, the temperature of the precipitation and the mode of addition of the hydroxyl ions affected the interaction of nickel ions with the carrier and the general structure of the catalyst. Richardson et al. (9) (10), Montes et al. (11) (12) and Uchiyama et al. (13) also described some benefits linked with the use of (DP) method. Recently Echeverria and Andres (14) discussed the importance of the method of preparation on the interaction of 10 wt-% nickel with the surface sites of a low surface area diatomite. They showed that the formation of different nickel-hydrosilicate compounds depended markedly on the pH and the temperature of preparation. They also studied the influence of the support in the selectivity of Ni/Clay catalyst for vegetable oil hydrogenation and attributed the significant differences in selectivity to the difference in morphology of the support (15). In a former study, we have prepared very highly loaded nickel catalysts, efficient for the hydrogenation of edible oil, by a (DP) method and examined the influence of the carrier surface area on the metallic dispersion and on the metal interaction with the silica. The best activity in soybean oil hydrogenation was obtained using a macroporous support with an intermediate surface area, the FC Celite (Manville) (16). We also observed that precipitate aging, at the end of the (DP) method, induced important modifications in the textural properties of the catalysts. In the present work, we have investigated further the preparation of very high loading nickel catalysts, using FC Celite as a support and carbonate ions as precipitating agent. The influence of the temperature of the precipitation and of the aging time was studied to optimize a satisfactory metal surface area and sufficient accessibility to the catalyst sites.
2. EXPERIMENTAL 2.1. Preparation of the catalysts. As described previously, the silica carrier was the FC Celite (Manville), a macroporous diatomaceous earth, with 1 wt-% alumina. The total surface area of the Celite was 45 m2/g and the porous volume (essentially macroporous) was 1.0 cm3/g, with a mesopore volume contribution of only 0.1 cm/g. The catalyst precursors were prepared using the (DP) method, varying the temperature of precipitation (298, 343 and 363 K) and the time of aging of the precipitate. The precipitation of nickel was induced by slowly adding NaHCO 3 (100 g NaHCO 3 in 1,2 1 water) to a slurry containing 14 g of Celite and nickel nitrate solution (125 g of nickel nitrate in 0,5 1 of distilled water). The agitation of the reacting medium was maintained by bubbling CO 2 in the slurry during all the preparation. At the end of the addition (pH = 7,6) the solids were maintained in the mother solution, at the precipitation temperature, for different periods of time called thereafter "aging time". The precipitate was filtered and washed several times by slurring in hot water up to alkalinity elimination. The washed material was dried at 373 K (10h), calcined at 573 K (3h) and 723 K (2h). The corresponding materials are the calcined precursors (CP).
1019
2.2. Characterization of the catalysts. After dissolution of the (CP) in HF and HNO 3 solution, the nickel content was determined by complexometry with EDTA. The reducibility was estimated through temperature programmed reduction (TPR). The reduction was performed in an argon-hydrogen (1.5%) mixture up to 823 K, with a heating rate of 8 K/min. The hydrogen consumption during the TPR was expressed as a reduction degree (%red), with the assumption of a transformation of Ni +2 to Ni 0. At the end of the TPR experiment, the metallic dispersion was measured as in (17) from the adsorption and desorption peaks generated by quickly cooling (823 K down to room temperature) and heating (room temperature up to 823K) the reduced sample in the argon-hydrogen mixture (heating/cooling rates around 100 K/min.). Nickel metallic areas were calculated assuming a stoichiometry of 1 H/surface nickel at room temperature. The geometric area of a surface nickel atom was taken as 6.3/~ (5). The nickel area (S) and the nickel dispersion (%D) are refereed to reduced nickel. Nitrogen adsorption measurements at liquid nitrogen temperature (77 K) were performed with a commercial apparatus (ASAP 2400, Micromeritics) after sample outgassing at 473 K for 2h, in vacuum. The BJH method in the desorption branch of the isotherms was applied to calculate the pore size distribution. Pore shape was inferred from the shape of hystheresis loop (18). Mercury porosimetry measurements were performed with an Autopore II apparatus (Micromeritics), after drying the sample overnight at 423 K.
3. RESULTS AND DISCUSSION
3.1. Catalysts preparation. Three series of catalysts were prepared varying the (DP) temperature and the aging time. The catalysts are designated NiDx_y where x is the (DP) temperature, in K, and y is the aging time, in hours. The (DP) temperature of 343 K was recommended by Styles (19) who indicated that at this preparation temperature, a digestion at pH = 7.5 leads to a catalyst with an optimum precursor-silica interaction and therefore to optimum catalyst properties. The (DP) temperatures of 298 and 363 K were chosen according to Hermans and Geus (8), who showed that these temperatures favored quite different reactions between high surface area silica and hydrolyzed nickel ions. The nickel content of each CP is reported in Table 1. The evolution of pH is recorded after dosewise addition of basic sodium carbonate to nickel nitrate in the presence or absence of Celite. By contrast to results presented previously (7) (8) (10) the presence of the carrier does not affect the conditions of precipitation. This means that, in our conditions, the precipitation occurs in a heterogeneous way, and must lead to a non uniform distribution of the nickel precursor onto the carrier, together with a poor dispersion of the final metallic phase. However, in our previous work, we have shown that, at a temperature of (DP) of 363 K, variations of the aging time induced significant changes in the properties of very highly loaded nickel silica catalysts (16). We now consider different temperatures of (DP) and a wider aging time scale to complete our previous results.
3.2. TPR analyses. Results obtained by TPR and dynamic adsorption/desorption following TPR are given in Table 1. Characteristic reduction profiles are shown in Figure 1 (a, b and c). In the
1020 majority of cases, the reduction profile presented only one peak, suggesting that the CP in the solid is rather homogeneously localized in the support. In fact, various reduction peaks appearing at quite different temperatures in the reduction profile are often analyzed as due to both an inhomogeneous distribution of precursor in the carrier and (or) a difference in precursor particle size depending on its localization (20). Table 1 Properties of the metallic phase, after TPR experiment, in the reduced nickel catalysts. Catalyst
wt (% Ni )
% Red.
%D
SNi (m2/g Ni)
NiD29~_o NiD298_l.5 NiD298_5 NiD29~_ 1o
48.2 49.4 49.9 49.2
99 100 90 100
2.0 2.2 2.5 2.3
14.6 16.9 16.3 15.4
NiDa4~_o NiD343_ 1.5 NiD~4~_s NiDa4a_lo
49.0 51.9 51.5 49.0
100 87 80 58
2.2 2.3 3.3 8.4
14.8 15.1 21.6 56.1
NiD363_o NiDa63_l.s NiD~6~_5 NiD363_ 1o
49.0 49.3 50.4 48.9
95 66 56 41
3.0 6.3 7.5 11.8
20.4 41.9 50.0 78.6
Whereas the aging time does not affect the reduction profile for a (DP) temperature of 298 K, an increase in the aging time shifts the reduction profiles to higher temperatures when the aging has been performed at temperatures of 343 and 363 K. Further, together with the shift towards higher temperatures of reduction, we observe that the area under the reduction peak decreases. This inhibition of the reduction is more intense for the preparation at 363 K. In parallel with an increased difficulty of reduction, Table 1 shows that there is an increase in the nickel area. Therefore, in agreement with previous work with less concentrated nickel catalysts (8) (14) (17), it appears possible to obtain different degrees of nickel-silica interaction, in the precursor state, even with very high nickel loading and a silica of medium surface area. The results of Table 1 also show that without aging of the precipitate the differences brought about by the different temperatures of precipitation are limited.
1021
~
0) Z Q.
b) 98-0
I
98-5
ill
- .
~=
I I
i
Z 0 I.I
I
I i
N
I I
i 574
3?3
i 823
..
T (K}
373
574
823
TIK|
This implies that the major modifications are produced during the aging, a parameter not I clearly isolated up to now for such I 0 preparations. I I-,O. 3 -0 I Coming back to the TPR i profiles, after (DP) at 298 K, or at zero aging time, the reduction 3r - 1.5 N profile is quite similar to the one -r obtained with pure unsupported nickel oxide. 1 In that case, the major part of 373 574 823 T(KI the active phase has no or only very mild interaction with the Figure 1. TPR profiles for the calcined precursor silica, in agreement with the preparedat different temperatures. (a) 298 K, (b) 343 K analysis of the pH during and (c) 363 K. precipitation. As consequence, the metallic area is low. On the contrary, for precipitation at higher temperature, and for longer aging times, a more important interaction between the nickel precursor and the silica is occurring, probably through the formation of "nickel silicates", quite documented in the literature (6) (14) (21) (22) (23). Although these "silicate" compounds are more difficult to reduce, they favor a high dispersion of the nickel precursor phase within the silica matrix and lead, after reduction, to relatively small metallic particles, and consequently to larger metallic surface. The differences of interaction degree between the nickel and the silica suggest modification on both sides during the aging. Therefore, we have studied the textural modifications induced by the variation of the preparation parameters.
c)
3.3. Textural
analysis
The effect of (DP) temperature and aging time can be qualitatively analyzed from nitrogen adsorption-desorption isotherms shown in Figures 2 for NiD343. After (DP) at 298 K irrespective of the aging time, the shape of the nitrogen adsorptiondesorption isotherm is not modified. The H3 type hystheresis loop may correspond to
"-
1022 agglomerates of plate-like particles giving rise to slit shaped pores. There is no increase in total pore volume. When the (DP) occurs at 343 K, an increase in the aging time produces a progressive change in the nitrogen adsorption isotherm. Figure 2 shows that the total surface area and the total pore volume are increased whereas the hystheresis loop tends to an H2 type. However according to IUPAC recommendations, such a hystheresis is rather difficult to interpret. The increase of the aging time after (DP) at 363 K induces an increase in the BET area, but the pore volume passes through a maximum: after an aging time of 1.5h, there is an increase in the specific area and a more definite adsorbed volume at pip0 = 1. For longer aging times, there is a progressive decrease in the pore volume, indicating a loss in mesoporosity. Table 2 summarizes the L textural parameters obtained 0 '~ from mathematical treatment ,'~ 200 . . . . . 1.5 of the nitrogen adsorption-.-~ .,:~ ...... lo ...... desorption isotherms. Data obtained with pure Celite FC 343 - 10 ~ . . . ........ -:":::::'" .//'/ 13. . / l /ff are also reported for I-O1 comparison, although in that ZOO E . . . . . . \ ,, .,//,,' case the variations induced by treatment at different O 1.5 temperatures, for 5h aging time, are not really 1 I 1 I 1 l I I significant. 0 015 PIP0 An increase in the Figure 2. Nitrogen isotherms for the calcined precursor (343K). aging time changes the surface area more quickly, ] ' when the temperature of the l ', . ; 0 aging is increased: in fact, for ' , 1.5 ' : 5 a precipitation at 298 K, 10 : : 343-10 ........ 10 aging time alteration does not E affect the surface area. For a / *l t , precipitation at 343 K, there o is a limited increase in o, i Wad surface area up to an aging O a. 5 time of 5h: further increase in 1 I the aging time strongly develops the BET surface I ., area. For a precipitation at _ ~q ~ 363 K, the major , modifications in the BET - T ~ -r-l--~=~----:.~ i 50 100 zz0 . area are obtained for small PORE DIAMETER, ( A ) aging times. Figure 3. Differential pore surface area vs. pore size (343K). It is important to note that whatever the preparation conditions, the texture of the solid is increased after introduction of nickel. However, at zero aging time, the differences in surface area are limited. _
1023 Figure 3 presents one example of the contribution of pore size to the increase in surface area, for the (DP) at 343 K. Up to 5h aging time, surface area is due to pores with diameter around 80/~, a situation similar is obtained after (DP) at 298 K. But after 10h aging, the large increase in surface area is due to pores with diameter close to 40/~, implying a severe textural modification. This later situation is also occurring after (DP) at 363 K, although in that case, longer aging times minimize the contribution of pores with diameter around 40 ,/k whereas a major fraction of the surface increase arises from pores with smaller diameters. Table 2 Textural properties of the carrier and the catalysts. Catalyst
SA (m2/g)
Pv (0.98) (cm3/g)
Pv meso a (cm3/g)
Pv micro (cm3/g)
Psize b
Ce2o~_o Ce~4s_s Ce~63_5
45 30 30
0.1 0.1 0.06
0.09 0.09 0.06
0.005 0.001 0.004
93 126 108
NiD29g_o NiD298-1..s NiD20~_s NiD29~_ Io
62 67 68 61
0.23 0.21 0.22 0.22
0.21 0.20 0.21 0.20
0.000 0.001 0.000 0.000
112 102 102 109
NiDa4~_0 NiDa4a_ 1.5 NiD~4~_s NiD~4~_ 1o
53 53 92 218
0.20 0.20 0.29 0.31
0.20 0.20 0.28 0.29
0.000 0.000 0.000 0.000
129 127 97 46
NiDa6a_o NiD~6a_ 1.5 NiD~6~_5 NiD~6a_lo
67 229 267 253
0.20 0.37 0.34 0.32
0.20 0.38 0.33 0.31
0.001 0.005 0.000 0.008
99 40 44 45
(,3,)
a BJH cumulative desorption pore volume of pores between 20 and 600/~ diameter. b BJH desorption average pore diameter 4P/S
The effect of both the temperature of (DP) and the aging time in the generation of the total pore volume (calculated at piP0 = 0,98) are the same as described for surface areas. There is no appreciable change in pore volume for (DP) at 298 K, whatever the aging time. When (DP) occurs at 343 K, the increase in the aging progressively leads to a larger pore volume. At 363 K, increasing the aging time produces a rapid increase, followed by a
1024 contraction of the pore volume. In parallel, as seen in Figure 4, presenting the pore volume as a function of the pore diameter, it appears that the preceding modifications are linked initially to a large decrease, followed by a small increase in the mean pore size. Richardson et al. (10) precipitated 25 to 40 wt-% nickel on silica with 291 m2/g, by adding urea at 363 K. They showed that increasing precipitation time from 4 to 20h induced pore neck constriction, that became more severe as precipitation continued. The same (DP) method at 363 K was applied in (11) to low and high loaded nickel catalysts supported on silica with 200 m2/g surface area. A decrease in average pore diameter measured by mercury porosimetry was observed in the case of a highly loaded catalyst. In a recent publication, Van der Grift et al. (24) using urea as precipitating agent for the preparation of copper silica catalysts, discussed the formation of an extra surface area and the changes in pore size distribution, by comparison with the original support. The formation of the extra surface area was attributed to a progressive increase in the copper content in their catalysts; decrease in surface areas was, by contrast, attributed to long aging times. In these works, a clear separation of the contribution of the precipitation-deposition method and the aging time was not obtained since precipitation of highly loaded catalysts needed longer precipitation times. In our case, a better separation between the precipitation-deposition and the digestion process made clear that a high metal content by itself is not sufficient to explain huge modifications in textural properties of the final catalysts. The preceding results f, I ~ 0 indicate that in the present 5 . . . . l.s experimental conditions (high "~ O.OZ2 /'; ' t ---5 nickel loading, silica with mid ,o range surface area, basic sodium carbonate as a 0.008 / "l !~ precipitating agent) the .J 363 - I 0 o "I/ !~ temperature of the precipitation > ,I: \ / 3 6 3 - 1 . S w '..'. I =: 0.004 ...... t is not a very critical parameter: o / I~ a. / on the contrary, the conditions of slurry aging are of great 1 ! I I~ importance. 50 tOO 120, It is worth mentioning that PORE DIAMETER ( A ) Figure 4. Differential pore volume vs. pore diameter. the observations presented here are due to the simultaneous presence of nickel and Celite, as Celite alone, submitted to the same aging conditions, does not present significant textural alterations. The increase of the textural properties with the aging time is therefore due to the association zeo of silica and nickel in the precipitating m e d i u m , _ suggesting a partial dissolution of silica during,,'~ z2o the aging followed by the deposition of a "nickel ao -343-0 silica" precursor in high dispersion state, on the o remaining silica. > 40 Finally, "t-plot" curves using the Harkins & I 1 1 I Jura expression were also constructed. They are 0 4 8 12 16 presented in Figure 5. t p,ot (~) Figure 5. "t-plot curves"- 343 K.
~/.-363
-
.
.:
.
.
.
.
.
1025 Their common feature is the absence of microporosity, for all temperatures and aging time used, an important point to maintain the accessibility of the metallic phase to the edible oil molecules. Further, the shape of these "t-plots", when the isotherm approaches saturation, points to the preservation of a large fraction of the meso and probably macroporous structure in the catalysts. Table 3 summarizes total intrusion volume as seem by mercury porosimetry between 200 and 55000 psi, for the Celite FC and some typical catalysts in their calcined form. Whatever the catalyst precursor, the intrusion volume is 40 to 50% that of the pure support indicating that mercury porosimetry Table 3 also detects some texture modifications in the Results of mercury porosimetry (cm3/g) catalyst, as compared to the original carrier. However, if we take into account that 60% of Catalyst Volume the mass in the calcined catalyst is due to Celite - FC 1.01 nickel, we reach the conclusion that the total NiD998_0 0.54 pore volume of the carrier in the supported NiD29~_10 0.55 precursor is not different from that in the NiD-~6,~_o 0.62 pure carrier. NiD363_10 0.56 Whereas DP temperatures and aging time have a huge influence on the mesopore structure of the catalysts, the preceding results suggest that the limited variations observed for the micro and macroporosities are insufficient to greatly decrease the accessibility of the catalytic sites to reactant molecules. 4. CONCLUSIONS The precipitation-deposition method applied to very highly loaded nickel silica catalysts, using Celite as a carder and basic sodium carbonate as precipitating compound, can generate quite different catalysts, when modifying the conditions of aging of the slurry. The increase in aging temperature and aging time led always to catalysts with a higher surface area due to an increase in the mesoporosity without important changes in micro and macroporosity. The increase in the aging time and temperature also induces more important interaction between the precursor of the active phase and the carrier. As a consequence, the catalysts are more difficult to reduce but, in compensation, present higher metallic area at the end of the reduction. Slurry aging in defined conditions seems a possible way to control, at the same time, active phase dispersion and active phase accessibility. Acknowledgments V.M.M.S. thanks CNPq (Conselho Nacional de Pesquisa, Brasil) for financial support. R.F. is grateful to CNRS (Centre National de la Recherche Scientifique, France) for granting a 15 months stay in Brazil. REFERENCES 1. J.W.E. Coenen, Ind. Eng. Chem. Fundam., 25 (1986) 43. 2. M.F. Wilson, I.P. Fischer and J.F. Kriz, J. Catal., 95, (1985) 155. 3. M.F. Wilson, P.R. Mainwaring, J.R. Brown and J.F. Kriz, Appl. Catal., 41 (1988) 177.
1026 4. M.F. Wilson, O. Antilunoma and J.R. Brown, in "Preprint of the Preparation and Characterization of Catalysts", Div. Petrol. Chem., Amer. Chem. Soc., Los Angeles Meeting, 33 (1988) 669. 5. J.W.E. Coenen and B.G. Linsen, in "Physical and Chemical Aspects of Adsorbents and Catalysts", B.G. Linsen, Ed., Academic Press, London, 1970, p. 495. 6. J.W.E. Coenen, in "Preparation of Catalysts II", B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet, Eds., Elsevier, Amsterdam, Stud. Surf. Sci. Catal., 3 (1979) 89. 7. J.A. van Dillen, J.W. Geus, L.A.M. Hermans and J. van der Meijden, in "Proc. 6 Intern. Cong. Catal.", London, 1976, G.C. Bond, P.B. Wells, F.C. Tompkins, Eds., The Chemical Society, London, 1977, p. 677. 8. L.A.M. Hermans and J.W. Geus, in "Preparation of Catalysts II", B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet, Eds., Elsevier, Amsterdam, Stud. Surf. Sci. Catal., 3 (1979) 113. 9. J.T. Richardson and R.J. Dubus, J. Catal., 54 (1978) 207. 10. J.T. Richardson, R.J. Dubus, J.G. Grump, P. Desai, U. Osterwalder and T.S. Cale, in "Preparation of Catalysts II", B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet, Eds., Elsevier, Amsterdam, Stud. Surf. Sci. Catal., 3 (1979) 131. 11. M. Montes, Ch. Penneman de Bosscheyde, B.K. Hodnett, F. Delannay, P.Grange and B. Delmon, Appl. Catal., 12 (1984). 309. 12. M. Montes, J. Soupart, M. de Saedeler, B.K. Hodnett and B. Delmon, J. Chem. Soc., Faraday Trans. I, 80 (1984) 3209. 13. S. Uchiyama, Y. Obayashi, T. Hayasaka and N. Kawata, Appl. Catal., 47 (1989) 155. 14. S.M. Echeverria and V.M. Andres, Appl. Catal., 66 (1990) 73. 15. J.A. Anderson, M.T. Rodrigo, L. Daza and S. Mendioroz, Langmuir 9 (1993) 2485-2490. 16. V.M.M. Salim, M. Schmal, R. Frety, M.M. Rodrigues and M.C. Silveira, in Reprint "5 Brazilian Seminar on Catalysis", Guaruj~i, sept. 1989, Instituto Brasileiro de Petr61eo, Ed., p. 93. 17. A.F. da Silva Jr., V.M.M. Salim, M. Schmal and R. Frety, in "preparation of Catalysts V", G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon, Eds., Elsevier, Amsterdam, Stud. Surf. Sci. Catal., 63 (1991) 123. 18. C.P. Barrett, L.G. Joyner and P.P. Hallenda, J. Amer. Chem. Soc., 75 (1951) 373. 19. A.B. Styles, in "Catalysts supports and supported catalysts. Theoretical and applied concepts", A.B. Styles, Ed., Butterworth, Boston, 1987, p. 63. 20. B. Mile, D. Stirling, M.A. Zammitt, A. Lovell and M. Webb, J. Catal., 114 (1988) 217. 21. Y. Trambouze, C.R. Acad. Sci., 228 (1949) 1432. 22. Van Eijk van Voorthuysen and P. Franzen, Rec. Trav. Chim., 70 (1951) 793. 23. G.A. Martin, B. Imelik and M. Prettre, C.R. Acad. Sci., Ser. C, 264 (1967) 1536. 24. C.J.G. van der Grift, P.A. Elberse, A. Muller and J. W. Geus, Appl. Catal., 59 (1990) 275.
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
1027
P r e p a r a t i o n o f small metal nickel particles s u p p o r t e d on silica u s i n g nickel e t h y l e n e d i a m i n e precursors Zheng Xing Cheng, Catherine Louis*, and Michel Che Laboratoire de R&tctivit6 de Surface - URA 1106 CNRS Universit6 P. et M. Curie, 4 place Jussieu, 75252 Paris Cedex 05, France The use of ethylenediamine chelate ligands in the preparation of silica supported nickel catalysts appears to be a prerequisite for obtaining highly dispersed metal particles (about 20/~) particularly at high nickel loadings (20 wt. %). 1. I N T R O D U C T I O N A mechanism of growth of nickel particles onto nickel nuclei was shown to occur during the preparation of supported Ni/SiO2 catalysts with a two-step procedure [1-3]. This preparation consists first of the deposition of nickel ions strongly interacting with silica, and then of the impregnation of nickel ions in weak interaction with the support. Nickel ions in strong interaction act as nuclei for the growth of nickel particles arising from the weakly interacting nickel ions. The main advantage of this preparation method is that, depending on the respective amounts of nickel deposited at each step of the preparation, it is possible to control the average size of the metal particles obtained after reduction. The cation exchange of nickel ethylenediamine complex followed by calcination (600~ is one method to produce the Ni nuclei. The goal of the calcination is to graft, i.e., to chemically bond, nickel to silica and get strongly interacting nickel. In the same time, the ethylenediamine ligands are decomposed. During this study, we noticed that if the exchanged samples were not calcined before the second step of impregnation, small metal particles of constant size (20 A) were obtained regardless of the amounts of exchanged and impregnated nickel [2, 4]. These results will be presented here.
2. EXPERIMENTAL 2.1. Catalyst preparation
In order to better understand the reasons for the formation of small metal particles in such a case, other types of Ni/SiO2 catalysts containing ethylenediamine have been prepared as well as standard impregnated samples (Table 1). a. incipient wetness impregnation with nickel nitrate (INi samples): aqueous solutions of nickel nitrate were put into contact with silica at room temperature for two hours in a close vessel (1.5 ml/g). The samples were dried overnight at 90~
b. cation exchange with nickel ethylenediamine [Ni(en)3] 2+ (ENien samples):
solutions of [Ni(en)3](NO3)2 (0.13, and 0.5 M) were obtained from mixtures of nickel nitrate and ethylenediamine solutions. The concentration ratio [en]/[Ni Ix] was higher than 3 in order to form the [Ni(en)3] 2+ complex (pH=12). 80 cm 3 of solution were added to 5g of silica. The suspension was continuously stirred at 25~ in a thermostated vessel for 48 hr. Then, it was filtered and washed with an ethylenediamine solution (0.24 M) at pH 12. The samples were dried at 90~ for 24 hr.
1028
c. impregnation with nickel nitrate of silica containing exchanged [Ni(en)3] 2+ complexes (ENien+INi): these samples were prepared by a two-step procedure. After drying at 90~ the ENien samples (preparation b) were impregnated with Ni(NO3)2.6H20 as described in a. d. impregnation with nickel nitrate of silica containing adsorbed ethylenediamine (en-SiO2+INi): these samples were also prepared in two steps. Aqueous solutions of different concentrations of ethylenediamine (1 to 10 M of en) were put into contact with silica during 48 hours under continuous stirring. After filtration, the modified silica was dried at 90~ Then, 1 to 20 wt. % of nickel were deposited by incipient wetness impregnation with nickel nitrate solution as described in a. e. impregnation with [ N i ( e n ) n ( H 2 0 ) 6 - 2 n ] ( N O 3 ) 2 complexes (0
impregnation with nickel nitrate cation exchange with nickel ethylenediamine [Ni(en)3] 2+ impregnation with nickel nitrate of silica containing exchanged [Ni(en)3] 2+ impregnation with nickel nitrate of silica containing adsorbed ethylenediamine impregnation with [Ni(en)n(H20)6.2n](NO~) ~
enccx~ Ni no yes yes
enads. Ni no yes yes
yes
yes
~,es
no
All the samples were prepared from a silica spherosil XOA400 (Rhtne Poulenc, France, SBET = 356 m2/g, pore volume = 1.25 cm3/g, average pore size = 80 A). Chemical analyses of nickel and nitrogen elements were performed in the center of chemical analysis of the CNRS (Vernaison, France). The sample Ni loadings are expressed in wt. % of Ni per g of dehydrated silica. The ethylenediamine amount was deduced from the nitrogen titration. In the following, encoord./Ni expresses the average number of ethylenediamine ligands in the Ni coordination sphere (see w 3.3.), and enads./Ni expresses the number of ethylenediamine moles adsorbed onto silica per Ni (see Table 3). The ENien, ENien+INi and en-SiO2+INi contain ethylenediamine both in the Ni coordination sphere and adsorbed onto silica. The INi(en)n samples only contain ethylenediamine coordinated to Ni since the amount of ethylenediamine added was adjusted to partially or fully coordinate the nickel (Table 1).
2.2. Techniques Diffuse reflectance spectra in the UV-visible-near IR range were recorded on a Beckman 5270 spectrometer equipped with an integration sphere and a double monochromator. BaSO4 was used as reference. EXAFS measurements at the absorption edge of Ni were performed at the LURE radiation synchrotron facilities (EXAFS I) using the X-ray beam emitted by the DCI storage ring, according to the conditions described earlier [2, 3, 6]. The samples were reduced by Temperature Programmed Reduction (TPR), from room temperature to 700~ with a heating rate of 7.5~ under 5% H2 in argon (25 cm3/min) at atmospheric pressure. The intensities of the TPR profiles are expressed in arbitrary units and
1029 are not directly comparable since both the sample weight and attenuation of the catharometer detector may be different. The nickel particle sizes were measured from electron micrographs obtained with a Transmission Electron Microscope (TEM, JEOL 100CXII UHR). The average particle diameter d was calculated from the following formula: d =~nidi/~ni where ni is the number of particles of diameter di and ~ni=300. The detection limit is about 10 and 15/~ for metal and oxide nickel particles supported on silica, respectively. A measurement of the surface concentration of particles Ns was obtained from the number of particles per cm 2 of silica on the electron micrographs. It may be noted that the measured surface area does not correspond to the real silica surface since ground particles of silica always possess a given thickness. However, the Ns values can permit the comparison of the particle concentrations between the different samples. 3. RESULTS AND DISCUSSION
3.1. Comparison of the metal particle size in the samples prepared in two steps, calcined (ENien/600+INi) or not (ENien+INi) before reduction As described in the introduction, after TPR up to 700~ the average metal particle size in the ENien+INi samples is smaller (20 A) than that in the ENield600+INi samples, i.e., in the samples calcined at 600~ under oxygen before the second preparation step of impregnation (Table 2). For the latter series, the average metal particle size depends on the respective amounts of nickel deposited at each step of the preparation [1, 3] while the particle concentration Ns remains unchanged for samples containing the same amount of exchanged Ni. In contrast, the reduction of the ENien+INi samples leads to the formation of small metal particles whose average size, 20 A, does not seem to depend on the Ni loading. Table 2 Metal particle size in the ENi~u+INi samples, calcined or not before TPR samples ENienl.3+INi3 ENicn2.4+INi3 ENienl.3/600+INii.5 ENien2.4/600+INil.5 ENien2.4/600+INi3 ENicn2.4/6.00+INi6
overall Ni loading (wt. %) 4.3 5.4 2.8 3.9 5.4 8.4
d(Ni 0) . (A) 20 20 31 19 30 41
'
Ns (1011/cm2) 16 20 6.7 11.5 11 10.4
The differences in metal particle size between samples, calcined or not before impregnation, may be related to the presence or absence of ethylenediamine. In order to check this point, we have studied three other types of samples containing ethylenediamine: - silica exchanged with [Ni(en)3] 2+ (ENien samples) - silica containing ethylenediamine impregnated with nickel nitrate (en-SiO2+INi samples) - silica impregnated with Ni ethylenediamine complexes (INi(en)n samples).
3.2. Comparison of the metal particle sizes in the samples containing ethylenediamine After TPR, the ENien, en-SiO2+INi and INi(en)n s,amples exhibit metal nickel particles between 10 and 40 A with an average diameter of 18-23 A for nickel loadings between 2 and 20 wt. % (Table 3). For Ni loading of about 1 wt. %, the nickel particles are not visible by TEM. For each series, the particle concentration Ns increases with the Ni loading.
1030 These three types of catalysts exhibit particles as small as those obtained in the ENien+INi catalysts, and smaller than those obtained in the ENien/600+INi catalysts (Table 2). Hence, these results confirm that the formation of small metal particles is related to the presence of ethylenediamine in the samples.
3.3. Characterization of the nickel species in the en-SiO2+INi, ENien and ENien+INi samples by diffuse reflectance spectroscopy The UV-visible-near IR diffuse reflectance spectra of the en-SiO2+INi, ENien and ENien+INi samples exhibit three d-d transitions, characteristic of nickel complexes in octahedral symetry. In the following, we will only consider the V1 band due to the 3A2g m > 3T2g transition, because the v2 and V3 ones are less resolved. For the samples considered, the V1 band position varies between 890 nm which is characteristic of the [Ni(en)3] 2+ complex, e.g. present in the ENien samples, and 1160 nm which is characteristic of [Ni(H20)6] 2+ complex, e.g. present in standard impregnated INi samples (Table 3) [2]. It may be deduced that in the en-SiO2+INi and ENien+INi samples, impregnated nickel initially as [Ni(H20)6] 2+ in the impregnation solution, is coordinated by ethylenediamine when silica contains adsorbed ethylenediamine or exchanged [Ni(en)3] 2+. Using the rule of the average environment [7], the number of ethylenediamine ligands replacing H20 in the nickel coordination sphere, i.e., the encoord./Ni ratio, may be deduced from the frequency shift of the V 1 band [2, 4]. As shown in Table 3, for the en-SiO2+INi samples, this ratio depends on the amount of ethylenediamine present onto silica before impregnation: the larger the enads./Ni ratio, the larger the encoord./Ni ratio. For the ENien+INi samples, the eneoord./Ni ratio is smaller than that in the ENien samples. It is known that the coordination of Ni II by ethylenediamine in aqueous medium is thermodynamically favored because of the chelate effect [8]. This fact allows us to conclude that during the preparation of the en-SiO2+INi samples, ethylenediamine adsorbed onto silica, desorbs in the impregnation solution and replaces the water ligands in the coordination sphere of the nickel complexes [2, 4]. The same phenomenon probably also occurs in the ENien+INi samples since the exchange solution used for the first preparation step, also contained excess of ethylenediamine which could adsorb onto silica and then desorb during the impregnation. 3.4. TPR profiles Regardless of the preparation method, but whenever the catalyst contains ethylenediamine, the TPR profile consists of only one reduction peak (Figure l a-d). The temperature corresponding to the maximum of the peak depends on the preparation method (Table 3). This temperature varies between 330 and 550~ for the ENien+ INi samples, ENien and en-SiO2+INi samples whereas it is 330~ for the INi(en)n samples. For catalysts which do not contain ethylenediamine, i.e., the standard impregnated INi samples, the TPR profile exhibits three peaks (Figure l e): i) at 300~ attributed to the decomposition of nickel nitrate into NiO, ii) at 380~ due to the reduction of NiO into Ni 0, iii) at 490~ due to the reduction of 1:1 nickel phyllosilicates formed during the catalyst preparation [6]. In order to understand the reason for the presence of a unique peak in the TPR profile of the samples containing ethylenediamine, it may be noticed first that for all these samples, the hydrogen consumption during TPR, expressed by the molar H 2 ~ i ratio, is larger than the expected value of 1 to reduce Ni II into Ni 0. It is also larger than the value of 2 obtained with the standard impregnated INi samples; in the latter, hydrogen also participates in the decomposition of nitrate into N2 (first TPR peak at 300~ [2, 6]. The overconsumption of 1-12is therefore related to the presence of ethylenediamine. Sabatier and Gaudion [9] have shown that metal nickel catalyses the reaction of hydrogenolysis of amines at temperature lower than 200~ It can be deduced that the hydrogenolysis of
Table 3 Characteristics of the different Ni/Si@ catalysts overan Ni d(~iq lodink! (wt (A) ENLnl.3 a ENkn2.4 18 ENin1.3+INi3 4.3 20 ENien2.4+INi3 5.4 20 en(l0M)-Si&+INil 1.O a en(l.5M)-Si@+INi2 2.0 22 en(1M)-Si&+INi3.2 3.2 22 en(3M)-Si@+INi8 8.0 21 en(5M)-Si@+INil3 13.0 21 en(5M)-Si@+INi20 20.0 21 INi(en)g 1.1 1.1 a INi(en)g7 7.0 22 INi(en)g20 20 22 INi(en)29.5 9.5 21 INi(en)l9.6 9.6 22 INi(en)0,49.7 9.4 22 INi 1.5 to 37 62 to 70 a: particles non visible on electron micrographs b: molar ratio c: ethylenedhnkadsorbed onto silica aftex d o n exchange d: ethylenediamineadsorbed onto silica before impregnation -: not meastlred or calculated
:::
v1
d(NiOj, range ( )
Ns (101llcm2)
10-30 10-40 10-40
11 16 20
10-40 10-40 10-30 10-30 10-30
9 14 21 35 47
(nm) 890 910 950 970 900 970 1010 1100 1160 1160
10-40 10-40 10-40 10-40 10-40 20 to 150
19 46 27 28 26 0.7 to 8
1160
e
wb Ni 3 3 2
enah. b Ni 5.2C 0.9 18.6 5.8 2.8 1.4 1.2 0.9 0 0 0 0 0 0 0
*
Tmax ("c)
H2/Ni b
460 16 395 12 395 7 365 7 550 16 465 14 430 10 362 7 350 5 330 3 330 12 330 12 330 12 330 9 330 6 330 4 300,380,490 -2
1032 ethylenediamine probably takes place simultaneously with the nickel reduction. This hypothesis is reinforced by the fact that: i) the larger the enads./Ni ratio, the larger the H2 overconsumption (Table 3), ii) there is no hydrogen consumption during the TPR of silica containing adsorbed ethylenediamine, in the absence of nickel [2], iii) when the samples containing ethylenediamine are calcined at 600~ before TPR, they do not exhibit anymore H2 overconsumption during TPR (see w
2E ~e
(d)
200
300
400
500
600
700
T(*C)
Figure 1. TPR profiles of : a) en(10M)-SiO2+INil, b) en(1.5M)-SiO2+INi2, c) en(3M)-SiO2+INiS, d) en(5M)-SiO2+INi20, e) INi6 The TPR peak shift observed for the en-SiO2+INi, ENien and ENien+INi catalysts may be related to the presence of ethylenediamine adsorbed onto the silica surface. Indeed, the INi(en)n samples whose encoord./Ni varies, but which do not contain ethylenediamine adsorbed onto the silica surface (Tables 1 and 3), exhibit a fixed TPR peak at 330~ For the en-SiO2+INi catalysts, the TPR peak shifts from 330 to 550~ with decreasing Ni content which also corresponds to an increasing enads./Ni ratio. For the ENien samples, the TPR peak is at higher temperature for the sample containing the highest enads./Ni ratio. In addition, when the ENien samples are impregnated in a second step (ENien+INi), the enads./Ni ratio decreases since the overall Ni loading increases, and the maximum temperature of the peak also decreases. The TPR peak shift may be explained as follows: the larger the amount of ethylenediamine adsorbed onto silica, the longer the delay for the hydrogenolysis, and the higher the temperature of the TPR peak. Another interesting point observed with catalysts containing ethylenediamine is that even if they are calcined before TPR, the metal particle size obtained after TPR, remains small. 3.5. Comparison of the metal particle size in the Ni/SiO2 samples calcined or not before reduction
As shown in Table 4, when the samples containing ethylenediamine are calcined at 600~ before TPR, the metal particles are almost as small as those obtained by direct reduction. The average metal particle sizes varies from 22 to 30 A with increasing Ni loading. For each series,
1033 the particle concentration Ns increases with the Ni loading. Compared to the samples reduced without previous calcination, calcination leads to an increase of 10 to 30 % in the metal particle size and to a slight decrease in Ns (Table 4). In contrast, the electron micrographs of the impregnated INi6 sample reveal the presence of metal aggregates of 300 to 800 A. These aggregates are constituted by metal particles of about 100-200 A which are much larger than the particles obtained after direct TPR (67 A). Therefore, calcination of the impregnated INi samples, i. e., samples which never contained ethylenediamine, leads to an increase in the metal particle size and to particle aggregation. Hence, the presence of ethylenediamine in the samples favors the formation of small metal particles whether the samples have been calcined or not before reduction. Table 4 Comparison of the metal particle size in Ni/SiO2 samples calcined or not before reduction metal particles metal particles after calcination and TPR after direct TPR samples ~(~A~0) dfNi0) Ns d(Ni0) d(Ni0i ' Ns range (A) (1011/cm2) (A) range (A) (1011/era2) V x] ,,, ENienl.3 a a ENien2.4 18 10-30 11 18 10-30 11 en(10M)-SiO2+INil a a en(1.5M)-SiO2+INi2 22 10-40 5 22 10-40 9 en(1.5M)-SiO2+INi3.2 23 10-40 11 22 10-40 14 en(1.5M)-SiO2+INi5.2 25 10-60 14 23 10-40 17 en(5M)-SiO2+INil3 31 10-70 22 21 10-30 35 en(5M)-SiO2+INi20 30 10-80 33 21 10-30 47 INi(en)37 22 10-50 14 22 10-40 19 INi(en)9,49.4 26 10-60 18 22 10-40 26 INi6 100-200 67 20-150 2 a, -: see comments on Table 3
3.6. Characterization of the calcined catalysts In a previous study [3], it was shown that the UV-visible-near IR diffuse reflectance spectra of the ENien samples calcined in air at 600~ then maintained in air at 25~ exhibited absorption bands at 410, 660 and 1160 nm attributed to isolated hexacoordinated grafted nickel species, [(-SiO)2NiII(H20)4]. An additional band at 330 nm was attributed to dimers of grafted nickel (NiII-O-NiII). The absence of bands of N-H vibrations of ethylenediamine in the near IR range, a t 1530 and 2030 nm, indicates that ethylenediamine adsorbed onto silica is decomposed. An EXAFS study confirmed this interpretation [2, 3]. After calcination, the en(1.5M)-SiO2+INi2 (2<encoord./Ni<3) and en(5M)-SiO2+INi20 samples (0<encoord./Ni<
1034
F(Rl
~"r
m[ 9
:,,
0
2
..
9
,,
4
-..
6
8
]o
~(~,)
Figure 2. k3-weighted Fourier transform (k3x(k)) of EXAFS spectra of: a) en(1.5M)-SiO2+INi2, b) en(5M)-SiO2+INi20, c) NiO 3.7. Nickel oxide particle size in the calcined catalysts
We have attempted to observe the nickel oxide particles in the different samples by TEM (Table 5). However, for Ni loading lower than 4 wt. %, the oxide particles are not visible because of the too weak contrast between oxide particles and silica. Above 4 wt. % of Ni, the oxide particle sizes never exceed 80 .~ and their average size is always lower than 35 A even for the highest loadings (Table 5). The particle concentration Ns in the en-SiO2+INi and INi(en)n samples seems to increase with the Ni loading. In standard impregnated INi catalystsa the oxide particles axe much larger, 100 to 200/~ (Table 5), and form aggregates (300-900 A). Hence, the particles of nickel oxide arising from samples prepared from ethylenediamine, are also drastically smaller than those of impregnated INi catalysts which never involved ethylenediamine. Table 5 Characteristics of the calcined samples reduced samples calcined samples TPR samples ~(NIII) d~NiII) Ns peak ~(Ni 0) d(Ni0) Ns range (A) (1011/cm2) (~ (A) range (A) (1011/cm 2) ENi~al a 550 a 10-30 11 ENien2 a 550 18 en(10M)-SiO2+INil a 380, 560 a 10-40 5 en(1.5M)-SiO2+INi2 a 380, 560 22 10-40 11 en(1.5M)-SiO2+INi3 a 390, 590 23 10-60 14 en(1.5M)-SiO2+INi5 33 15-70 380, 550 25 10-70 22 en(5M)-SiO2+INil3 30 15-50 3.2 390, 630 31 10-80 33 en(5M)-SiO2+INi20 34 15-80 4.6 370, 570 30 10-50 14 INi(en)37 27 15-60 1.7 380, 650 22 10-60 18 INi(en)o,49 32 15-60 2.4 390, 630 26 I00-200 INi6 e 100-200 370, 450 a, -: see comments on Table 3 e: N s could not be delexminedsince the particles in calcined and reduced samplesare agglomerated 3.8. T P R o f the calcined s a m p l e s
The TPR profile of the calcined INi catalysts exhibits two temperature peaks at 370 and 450~ (Figure 3g, Table 5), the lower temperature peak being more intense. According to a previous study [2, 6], the low T peak corresponds to the reduction of NiO while the high T
1035 peak is attributed to the reduction of phyllosilicates. The nickel is totally reduced into Ni 0 at 700~ The TPR profile of the calcined ENien samples is composed of a unique broad peak with a maximum temperature at 550~ (Figure 3a). It has been attributed to the reduction of grafted Ni II species, i.e., species in strong interaction with silica and therefore reducible at high temperature [2, 3]. The TPR profiles of the calcined en-SiO2+INi and INi(en)n samples exhibit two poorly resolved peaks at 370-390~ and 550-650~ (Figure 3b-f). The low T peak occurs at the same temperature as the reduction peak of NiO observed in the calcined INi samples (Figure 3g). The broad high T peak whose maximum varies between 550 and 650~ looks like the unique peak at 550~ of the calcined ENien catalysts. Hence, it is probably related also to the reduction of grafted nickel formed during calcination at 600~ These TPR results, in agreement with the EXAFS results which showed that NiO was not the unique nickel phase present in the enSiO2+INi samples, indicate that in the en-SiO2+INi and INi(en)n samples, NiO coexists with nickel in strong interaction with silica, probably grafted nickel. The fact that for each sample, the low temperature peak is less intense than the high T peak, also indicates that NiO is not the main species. It may be noted that regardless of the preparation method, there is no more overconsumption of hydrogen during TPR when the samples are previously calcined. Indeed, the H 2 ~ i ratio is equal to 0.9-1.1, indicating that H2 only participates in the reduction of Ni II and that ethylenediamine has been fully decomposed during calcination.
(b)
(c)
i
200
JO0
_.
i
_
i
400 ~0 T(*C)
..
i
6OO
7OO
Figure 3. TPR profdes of calcined samples: a) ENien2.4, b) en(1.5M)-SiO2+INi3, c) en(1.5M)-SiO2+INi5, d) en(5M)-SiO2+INil3, e) en(5M)-SiO2+INi20, f) INi(en)37, g) INi6 Table 5 also shows that after TPR of the calcined catalysts, Ns of metal particles is larger by a factor of 7 to 10 than Ns of oxide particles, and the average size of metal particles is smaller than that of oxide particles. The size distribution of metal particles is close to that of oxide particles. However, it is difficult to compare these two types of particles because oxide particles are not observed for Ni loading lower than 4 wt. % whereas metal particles are visible for 1 wt. % of Ni. In addition, oxide particles smaller than 15/k cannot be observed whereas metal particles of 10 A can be detected. The smaller Ns value for the oxide particles is therefore probably due to the lower contrast between oxide particles and silica on the micrographs than between metal particles and silica. However, since the larger particles of oxide and metal nickel have almost the same size, it may be deduced that the particles are probably reduced without any important nickel migration. The same conclusion can be drawn for the standard impregnated INi samples: aggregates of nickel oxide are reduced without significant Ni migration, in agreement with previous results [2, 61.
1036 3.9. D i s c u s s i o n The nickel oxide particles in the en-SiO2+INi and INi(en)n samples are much smaller than those in standard impregnated INi catalysts (Table 5) which in addition, are not isolated, but form aggregates. These differences probably arise from the different nature of the nickel species present before calcination: the complexation of nickel by ethylenediamine inhibits the formation of aggregates and large oxide particles. It leads to the formation of small oxide particles, then, after TPR, of small metal particles. This is probably due to the fact that most of the Ni complexes are transformed into species strongly interacting with silica during calcination, as attested by the presence of the high TPR peak at 550-650~ This transformation would occur through the formation of intermediate isolated grafted Ni II species, [(-SiO)2NilI(en)2], which is known to be formed in exchanged ENien catalysts above 300~ in air [ 10, 11]. According to the same studies, this species is then transformed into [(-SiO)2Ni II] which is stable at 500~ in air [10, 11]. Hence, the grafting reaction can take place before the ethylenediamine decomposition and prevent nickel migration and formation of large oxide particles. In the en-SiO2+INi and INi(en)n samples, the same transformations probably occur. Metal particles in the en-SiO2+INi and INi(en)n samples have almost the same size whether the samples are calcined or not before reduction in contrast to the impregnated INi samples. That suggests that the grafting reaction of Ni species probably also occurs during TPR when the samples are not previously calcined, i.e., before nickel reduction and ethylenediamine hydrogenolysis, and inhibits afterward the formation of large metal particles.
4. C O N C L U S I O N S This study proposes a new way for improving the dispersion of metal particles on an oxide support. The complexation of Ni II ions by ethylenediamine which is a strong chelating ligand, leads after reduction, whether the samples are calcined or not before reduction, to the formation of small metal nickel particles of about 20 A even for high nickel loadings (20 wt. %). The formation of small metal particles is related to the formation of grafted nickel, i.e., of nickel strongly interacting with the support, during calcination or reduction. The grafting reaction inhibits nickel migration, and formation of large particles. The two preparation methods involving ethylenediamine (en-SiO2+INi and INi(en)n samples) described here combine the advantages of the preparations by impregnation (ease of preparation and deposition of high nickel loading), ion exchange (small metal particles), and deposition-precipitation (high nickel loading) that each of them cannot provide simultaneously. REFERENCES
1 2 3 4
Z.X. Cheng, C. Louis and M. Che, Z. Phys. D, 20 (1991) 445 Z.X. Cheng, thesis Pads (1992) M. Che, Z.X. Cheng, and C. Louis, J. Am. Chem. Soc., submitted Z.X. Cheng, C. Louis and M. Che, Stud. Surf. Sci. Catal., L. Guczi, F. Solymosi, P. Tdttnyi. (eds.), Elsevier, Amsterdam, 75 (1993) 1785 5 N.L. Swink and M. Atoli, Acta Cryst. 13 (1960) 639 6 C. Louis, Z.X. Cheng and M. Che, J. Phys. Chem. 97 (1993) 5703 7 C.K. JCrgensen, Acta Chem. Stand., 10 (1955) 887 8 F.A. Cotton and G. Wilkinson, in "Advanced Inorganic Chemistry", 3rd Ed., Interscience Publishers, John Wiley & Sons, New York, (1972) 650 9 P. Sabatier and G. Gaudion, C. R. Acad. Sci. France, 9 (1915) 165 10 L. Bonneviot, O. Clause, M. Che, A. Manceau and H. Dexpert, Catal. Today 6 (1989) 39 11 O. Clause, L. Bonneviot and M. Che, J. Catal., 138 (1992) 195
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
1037
PREPARATION AND CHARACTERIZATION OF CoMo/AI20 3 HDS CATALYSTS: EFFECTS OF A COMPLEXING AGENT P. Blanchard, C. Mauchausse, E. Payen, J. Grimblot Laboratoire de catalyse h6t6rog6ne et homog6ne, URA CNRS n~ Universit6 des Sciences et technologies de LILLE, Bat. C3, 59655 Villeneuve D'Ascq, Cedex, France O. Poulet, N. Boisdron, R. Loutaty Centre de recherches TOTAL -FRANCE, Gonfreville rOrcher, 76700 Harfleur, France ABSTRACT Alumina supported CoMo catalysts have been synthetized by dry impregnation using complexing agents in the impregnating solution. The effect of the preparation on the state and dispersion of molybdenum and cobalt in the oxidic and sulfided form was studied by various characterization techniques (UV, XPS, Raman spectroscopy, HREM). Molybdenum appears to be present as well dispersed oxomolybdenum species in the oxide form even at high loading (30wt% MOO3). This good dispersion of molybdenum is preserved after sulfidation which induces the formation of MoS 2 crystallites whose morphology has been determined by H R M . This preparation method allowed a better dispersion of the Co atom in interaction with the MoS2 crystallites. The catalytic activities were evaluated in thiophene hydrodesulfurization and the relation between the activity and the dispersion is discussed. INTRODUCTION In hydrotreating catalysis of crude petroleum, lowering the sulfur content of refined products (in particular gas oils) is of great importance. To achieve this goal, searching new formulations for producing more active catalysts seems promising instead of using more drastic conditions in existing refinery plants. It is now generally (1) well accepted that the so-called "CoMoS (NiMoS)" type structures supported on 3' alumina are among the most active species in CoMo industrial catalysts commonly used to hydrotreat sulfur containing feed stocks. These actives phases, in which the Co-atoms are located at certain edge positions of the MoS 2 slabs, are obtained by sulfidation of the corresponding CoMo/AI203 oxidic precursors. The existence of optimal structures implies that the catalytic activity is determined to a large extent by the state of the calcined oxidic precursors. Hence, the catalyst morphology after sulfidation is expected to be highly dependent on the preparation method. The conventional dry impregnation method followed by calcination has been employed extensively in preparation of these supported oxides. For molybdenum supported catalyst, inorganics acid or their salts, like ammonium heptamolybdate (AHM) in aqueous solution are generally used . The nature of the
1038 oxomolybdenum species present in the impregnating solution is dependant on the pH, temperature and concentration of the solution (2). Ammonia or HNO 3 is generally used as a pH controlling agent. Although preparation has a great influence on the catalyst properties, there are very few studies concerning the use of selective complexing agents in the impregnating solution (3). For industrial applications, the promotor is generally introduced in the impregnating solution with the molybdenum entities. Apart from its presence in the CoMoS structure, the cobalt atoms may also be in other phases like in the alumina lattice and as Co9S8.. The use of dopants have sometimes been used to manage the repartition of the deposited entities in these catalysts (4,2b). In order to avoid formation of unwanted phases and to manage the repartition of the promotor to give catalysts with maximum concentration of CoMoS active entities, we have investigated new preparations of CoMo catalysts using Co solutions in which a complexing agent was used. In the present report, therefore, we have investigated the preparation of both Mo and CoMo series of catalysts in order to optimize the deposition of Co. More generally, the aim was to control the preparation procedure and find correlations between the structure of the oxidic precursor(s), the state of the catalysts after sulfidation and finally with selected catalytic results. Here, we report only data on thiophene HDS but in some way, the method of preparation chosen has a beneficial effect on gas-oil desulfurization (5). EXPERIMENTAL: Preparation of the impregnating solution and catalysts. The catalysts were prepared by incipient wetness impregnation of an Akzo 3' A1203 (pore volume: 1.1 cm3g-1; specific surface area: 350 m2g -1) with solutions containing the appropriate amounts of the active metals components. The impregnated extrudates were dried at 393K overnight and then calcined at 773K for 3h. The nomenclature of the studied samples recalls the preparation parameter. A sample will be designated as aCoXMo(Y), where a is the ratio NCo/(NCo+NMo), X the Me loading in wt% of MoO 3 and Y the complexant used in the synthesis ie. ethylenediamine-EDA-. Co or Y are omitted respectively for sample prepared without cobalt or without complexing agent. Three main series of samples were prepared as follows: -Me and CoMe series: The impregnating solution was an aqueous solution of AHM at its natural PH ( 5.5). A one step impregnation at 323K was used for the preparation of the 30Me sample in order to avoid precipitation. For the CoMe samples the cobalt nitrate was dissolved in the AHM solution. -aCoXMo(EDA): The impregnating solution contained AHM (with or without cobalt nitrate dissolved in a 3 M aqueous EDA solution. -XMo(EDA)/aCo(Y) and a4:~o(Y)/XMo(EDA): Some preparations were performed in which the sequence of introduction of the Co was varied i.e. impregnation by a Co solution with or without EDA was performed prior to or after the Me impregnation. An intermediate calcination at 773K during 4h was performed between each impregnation step. Some cobalt supported on alumina solids were prepared with aqueous cobalt nitrate solution containing or not the complexing agent. They will be referred hereafter as XCo(Y) where X is the Co loading in wt% of CoO and Y the EDA or the monoethanol amine-MEA-. For purposes of comparison a Mo(EDA) catalyst was prepared via equilibrium adsorption. An aqueous Mo-EDA solution (200ml) was added to 10g of alumina and the mixture was vigourously shaken during 4h; then the extrudates were washed. This catalyst will be referred hereafter Mo*EDA.
1039 The calcined extrudates were sulfided with a 10% vol. H2S in H 2 at 673K for 2h. The Mo content was determined X-Ray fluorescence by the "Service Central d'analyses" of the CNRS. Characterizations Laser Raman spectroscopy (LRS) was performed using a Raman microprobe (XY from Dilor), equipped with a photodiode array. The exciting light source was an Ar + laser emitting the 488 nrn line with a power at the sample of 10 mW except when otherwise specified. The X-ray photoelectron spectra (XPS) were obtained by using an AEI 200 EB spectrometer equipped with an aluminium X-ray source working at 300W. The binding energies have been measured by reference to Al2p peak of the support at 74.8 eV., a value generally encountered for 3" A1203. Variations of the integrated intensity ratio of typical core levels OVlo3d3/2_5/2/A12p, Co2Pl/2_3/2/A12p) provides information on the surface composition and the surface atomic ratio Co/AI have been deduced from the XPS intensity ratio. Since different oxidation states may exist on the sulfided catalysts, the experimental spectra are rather complicated and, in order to obtain reliable quantitative information, we will use the decomposition procedure described elsewhere (6) to calculate the extent of molybdenum sulfidation. Ultra-violet diffuse reflectance spectra were recorded, in the 250-900 nm spectral range, with a diffuse reflectance accessory. Samples were placed in quartz cuvettes and were scanned against a pure alumina background. High resolution electron microscopy ~ M ) analysis were performed on a philips EM30 electron microscope. The powder, after sulfidation, was dispersed in alcohol for the preparation of the electron microscope grid. Catalytic activities for thiophene HDS were measured at atmospheric pressure in a flowtype reactor packed with 0.2g of catalyst. The solids were first sulfided with a H2S/H 2 (10/90) mixture at a flow rate of 100ml/min at 673 K for 2h and then cooled down to 573 K. After purification by vacuum distillation, thiophene was introduced in the reactor at constant pressure (50 torr) in a flow of purified hydrogen (20 ml/min.). The reaction products were analyzed by gaz chromatography. RESULTS AND D I S C U S S I O N The specific surface area of some samples is reported in tables I. Table I: Specific surface area evolution after solution impregnation SAMPLES
~,o~
EDA/AI~O~
,.,22Mo(EDA)
.2SCoMo(~A)
Specific surface area (m2g -1 ) 375 348 275 256
It can be noted first that the pore volumes of the samples are not very different from that of the alumina support. The specific surface area only decrease with addition of Mo, which is normal to its high atomic weight. Therefore, in the following, we will assume that a
1040 complexing agents like EDA have no detectable influence on the textural properties of the support. OXIDIC PRECURSOR Mo/Al203 The XPS Mo3d doublet is broad and corresponds to Mo VI species in interaction with the support as is currently reported for Mo/A120 3 systems (7). The XPS characteristics of Mo and CoMo catalysts in their oxidic form are reported in table II and the evolution of the representative intensity ratios as a function of the Mo loading is reported in fig 1. Table II: XPS characteristics of Mo and CoMo catalysts in their oxidic form; Nomenclature
Mo at.rim-2
Co/AI 9102
(a)
A
~kE*
14Mo 1.8 22Mo 3.1 4.8 30Mo(Timp. =323k) 1.8 14Mo(EDA) 3.1 22Mo(EDA) 4.8 30Mo(EDA) 5.0 5.7 4.3 (2.8) 4Co 5.0 5.5 4 (2.8) 4Co(EDA) 5.4 5.6 3 (2.5) 1.9 .28Co14Mo 5.2 5.6 3.6 (2.5) 1.9 .28Co14Mo(EDA) 5.8 5.8 4.9 (3.6) 2.9 .28Co20Mo(EDA) 6.8 6.2 6.4 (6.4) .28Co30Mo0EDA) 3.1 .35Co22Mo0EDA) 2.9 .28Co/20Mo(EDA) 2.9 .28Co(EDA)/20Mo(EDA) 2.9 20Mo(EDA)/.28Co 2.9 20Mo(EDA)/.28Co(EDA) 6.2 6.4 7.7 (6.4) 5.2 .28Co(EDA)/30Mo (EDA) 6.7 6.4 a CoMoO4 b CoMoO4 6.3 6.3 a: XPS atomic ratio; In parentheses are reported the bulk atomic ratios. AE *= Separation between the main peak and its satellite structure; A: 2p3/2; B: 2p 1/2 For the Mo series, with our preparation method, a linear relationship is observed for all the samples studied which indicates that molybdenum is well dispersed up to 4 . ' 8 M o nm -2. By studying the adsorption of different isopolyanions on 3,A1203, Meunier et al (8~ observed that the number of adsorption sites of the carriers is quite similar (.35 sites.nm "~ or 2.45 Mo at.rim -2 when heptamers are adsorbed) whatever the impregnating species involved. It appears here that this monolayer like coverage of Mo may be improved by a dry impregnation under controlled temperature (323 K instead of room temperature). It is thus possible to prepare solids with higher Mo content as well dispersed oxomolybdate entities in spite of the high
1041
/ c] iI
el/
Pl O
15-
Q ii /
m A O
o/
~o-
~Y
m
:~ MoF..DX/AJ=O3
0 Mo~O/,U203 D CoMor..OX/~J2Oa
A C=MoH=O/AI=O3
"i
'
=(Mo)/nmZ Fig l : n ( M o ) / n ( A l ) surface ratio ,r
the molybdenum surface densit7
9)~).qO
950
d
e
I 19.40
II
996
' __JL--< 0
,,,.-'"'At/"?/ 90o
~(,=-b f
9.~b
:> ~(==-~
Ti= 2: R = , , , ~ Spectra or .Mo Cata}.~r162 z: l ~ o , b: ~Mo, c 30Mo, d: 30MoEDA r -sol u t; o,, MoEDA, f: 20MoEDA ~,~t, IF 20MoZDA dried at I20~ It, h 20MoEDA r ,=t :~SOand $00"C
1042 AHM concentration ([AHM]= .43M). No bulk precipitation occurs during the impregnation. However the Raman spectra of this 30Mo solid, reported in fig2, show the lines of MoO 3 (main lines at 996 and 820 cm-1). In contrast, when the impregnation was performed in presence of EDA no deviation from the straight line is observed up to 5.2 Mo.nm "2 and the Raman lines of MoO 3 are not evidenced on the 30Mo(EDA) sample. Raman analysis of all these samples, examined under ambient conditions and in an hydrated state with Mo loading varying from 14 to 30 wt%, revealed the classical polymeric coordinated molybdate (9) (main Raman line at 950 em-1). The evolution of the Rarnan spectrum of the 20Mo(EDA) catalyst at different steps of the preparation is reported in fig2. The Raman spectrum of the impregnating Mo-EDA solution is characteristic of tetrahedral molybdate (9) (line at 900 cm-1). These Rarnan features which are also observed on the wet samples, whatever the Mo loading should correspond to the species in solution in the pore. In contrast the Rarnan spectrum of the dried extrudate is characteristic of the heptamolybdate entity 0~--940 era" 1) which main line shiRs to 970 cm-1 upon calcination. No bulk MoO 3 nor the polymeric precipitate, identified by VanVeen & al. (10), have been detected by LRS when EDA is used. A great number of publications (2,10,11 and refs herein) discussed the possible mechanism of adsorption of the oxomolybdate entities present in the impregnating solution. Most of them concerned samples prepared under neutral or acidic conditions and it was deduced that the oxomolybdenum entities are bound to the surface by electrostatic forces. Van Veen et al. (10) suggest that adsorption of AHM takes place via three processes: i) irreversible adsorption with the basic surface OH group to form monomolybdate, ii) largely reversible physisorption on coordinatively unsaturated sites (CUS) AI3+ sites iii) precipitation well characterized by LRS (presumably presence of [(NH4)6MosO27]n). In a basic medium the surface of the carrier may be considered as negatively charged and no CUS ALl3+ sites would exist. So the amount of adsorbed oxomolybdenum species on samples prepared under equilibrium adsorption with ammonia is generally low (2) (el tablelI). As a result the Mo loading of the Mo*(EDA) sample is also low ( 1.8 wt% MOO3). In contrast, the use of EDA in a dry impregnation process allows the deposition of higher molybdenum loading as well dispersed entities. With the interpretation of our Raman data, several important observations earl be made about this Mo/AI20 3 system. In line with the conclusions of I.E. Wachs et al. (11), the structure of the calcined precursor is only minimally dependent on the impregnating solution for the dry impregnation procedure of preparation. All the monomerie entities are in the pore of the carrier and surface polymolybdates are obtained aRer calcination. Even if some molybdate/surface OH condensation reactions may occur, the majority of the ions in solution do not really interact with the carrier during the impregnation step. We previously stated (12) that the drying step should induce the formation of heptamolybdate ions through the equation 7 MoO42- + 8H +
v_.2" Mo70246- + 4H20
The molybdate entities transform into AHM ions upon drying and calcination as the PH in the pore decreases with the evaporation of EDA and then are bound to the carrier. CoMo/AI203 precursor The (Co/AI) XPS atomic ratios, reported in table II, show that the Co is well dispersed on the carder. As shown in fig 1, the presence of Cobalt nitrate in the AHM-EDA impregnating solution does not influence the dispersion of the Mo entity. Unformnatly, due to
1043 precipitation phenomena, such simultaneous impregnation with high Mo loadings is not possible without EDA. 990 __ 84e
o
95o
"V (era-l)
1000
~
b
500
Fig 3: Raman Spectra of CoMo Catalysts: a: 0.2,SCoMo20EDA, b: Sample as in (a) with P laser = 600 mW c: 0.28CoMa30EDA, d: Sample as in (c) with P laser = 600 mW
The Raman spectrum of the .28Co20Mo(EDA) sample shows a broad band at 950 cm-1
(fig 3a) characteristic of a well dispersed oxomolybdate entities. In contrast the Raman spectrum of the .28Co30MoEDA show a line at 940 cm-1 on the broad underlying band of the oxomolybdate entities (fig3c). This former one may be attributed to CoMoO4 by reference to litterature data (13), however in view of the very high diffusion cross section of CoMoO 4 compared to supported oxoanions, this line should not be interpreted as the presence of large amount of this compound. In fact no well defined CoMoO 4 has been characterized by XRD on the calcined precursor, as did A.D. Van der Kraan (14) or Yokoyama et al. (15), when EDA is used in the impregnating solution. This implies that if it exists, it could only be amorphous, microcrystallir~ or present in a non detectable amount. The Co XPS BE difference (AE*) between the main peaks and their satellite structure may also give informations on the Co environment. The values, calculated for some of the catalysts and for the cobalt molybdate (a or b form), reported in table]], show that AE* increases upon increasing the loading. The composition of the .28Co30MoEDA sample seems to be the lower limit of formation of compound like defined CoMoO 4.
1044
Whatever the loading, upon increasing the power of the laser beam (fig 3b and d) a modification of the Raman spectrum occurs which has been previously ascribed (16) to a dehydration phenomenon oeeuring more easily on CoMo catalysts. This strongly suggests the existence of an interaction between Co and the oxomolybdate entities in the oxidie precursor. Therefore it can be deduced that presence of EDA allows simultaneous impregnation of the Mo and the Co at high Mo loading without formation of crystalline CoMoO 4. UV-Visible spectra, reported in fig 4, give some informations on the localisation of the Co atom in these catalysts;
o 480
510
305
6"-~o ~
9s ~
~o
'
'
j
'" 700
a
b rrl
i =
~
400
638
.
580
d I~)o"
~
/.~
_
obo '
_
~
"
6bo
"
9oo
~ (nm)
A,65
,
"
~
,
,
Fiz 4: UV Spectn or CoMoCats~ts: cc CoEDAsolution,~ CoH20 soluUou, a, b, c: Co/AI2C~ wet, dried at 120~ and calcined f 6, e., f: CoEDA/AJ203wet, dr4ed, and calcined, g, h, i: CoMoEDA/AI203wet, drled and calcined, J: 20MoEDA/0.2$CoEDA,k: 0.?.$CoEDA/20MoEDA, ~ u O~::o/ZOMoEDA,uc ZOMoEDX/O~SCc~ 9OO
(rim)
let us discuss the effect of a complexing agent on the adsorption of the Co on the alumina. The UV spectra of the cobalt solutions without (or with) Mo exhibit the features of the cobalt complexes Co(EDA)32+ (main bands at 277 and 360 nm, spectrum (x) when EDA is
1045 used in the impregnating solution. These main bands are still observed on the spectra of the wet Co(EDA) sample with a non identified band at 470 nm. Upon drying no modification of this spectrum is observed (spectra d,e). In contrast, the bands characteristic of the Co(H20)62+ complexes (main bands at 305,476 & 510 nm) still present on the spectrum of the Co wet sample disappear upon drying (spectrum a,b) and the main features of the tetrahedrally coordinated Co 2+ ion (540, 570, 623nm) are observed (spectrum b). These features are only observed after calcination on the Co(EDA) solids (spectrum f). This is in agreement with ATG measurements which show beginning of decomposition at 200~ A shoulder at 750 nm is seen on the UV spectrum of the calcined Co sample (spectrum v) which could be assigned to the existence ofCo30 4 (17). Thus the use of EDA avoids the formation of bulk Co304. The same behaviour have been observed with the MEA. The UV spectra of the calcined CoMo(EDA) samples exhibit the same evolution upon drying and ealeination.(speetra 4 g,h,i). The large absorption band shown for wavelengths shorter than 450 nm is due to the molybdenum. The UV spectra of the CoMo (not reported here) or the CoMoEDA solids characterize mainly the tetrahedrally coordinated Co 2+ ion. No bulk Co30 4. ( nor CoMoO4) is evidenced. The spectra reported in fig.4(j,k,l,m) show that, on samples prepared by sequential impregnation, the formation of the bulk cobalt oxide only occurs if the cobalt is introduced first without EDA (spectrum m). From this UV study it appears that the use of a complexing agent of cobalt avoided the formation of the CoA120 4 surface entity during the drying step and Co30 4 upon calcination for solids with or without Mo. This is probably due to the higher thermal stability of the Co~ eomplexeompared detected_ to the Co(H20)6 2+~ The tetrahedrally coordinated it should be pointed out that, due to the difference in the extinction coefficient (18), the bands for the octahedral Co 2+ ions could not be observed. SULFIDED CATALYSTS The Mo 3d5/2 BE of all samples show nearly the same value, 229.2eV., as reported previously (6). The dispersion of the Mo in the Mo and CoMo based catalysts is not modified, as shown in fig 1 and the sulfidation rate of the Mo is about the same for all the samples studied ( ae 80%). As this preparation method allow the deposition of higher loading of Mo as well dispersed oxomolybdate entities, it would be interesting to compare the morphology of the disulfide crystallites with those obtained on conventionally prepared systems. Therefore some of these samples have been observed by transmission electron microscopy (see typical mierograph in fig. 5).
FIG 5" HREM picture of sulfided 30Mo(EDA) catalyst
1046 Based on the detection of more than 400 crystallites on several micrographs the distribution of lengths L of the elemental layers as well as their stacking N can be obtained. The result reported in table III concern the mean N values. For all the samples studied in this work the single layer crystallites are dominant. Upon increasing the Mo loading from 14 to 30 %wt of molybdenum oxide (with or without EDA), N increases slightly. At high Mo loading longer and wrapped crystallites are observed. Upon introduction of the promotor, no drastic change of the morphology is observed whereas stacking (N= 2.4) is generally observed for CoMo catalysts prepared by coimpregnation (19). A great variety of techniques have been applied to characterize the location of the promotor ions in the sulfided catalysts (see for example ref 20) including mossbauer emission spectroscopy, XANES, EXAFS or XPS. In sulfided CoMo/AI20 3 catalysts, depending on the loading, three different types of cobalt species could exist, ie. cobalt in the alumina lattice, cobalt as separate Co9S 8, and cobalt in the so-called CoMoS (I) or (II) phases. It has been shown that information could be obtained concerning the nature of the nickel (21) or the cobalt (22) from XPS analysis. The main XPS characteristics of the Co in these sulfided catalysts are reported in table III. Table III: characteristics of sulfided samples; sample
Co/~ 9102
N 0a)
Thiophene Activity
(r
AE (d)
i
FWH M (e)
14Mo 1.27 5.6 22Mo 1.54 6 5 30Mo(Timp. = 50~ 14Mo(EDA) 1.41 5.6 --1.63 22Mo(EDA) 6 1.36 30Mo(EDA) 5.6 3.7 616.5 Co 6.7 616.5 3.5 Co(EDA) 2.4 617.0 25 .28Co14Mo 2.5 33 617.1 3.4 .28Co14Mo(EDA) 5.2 1.65 38 2.6 .28Co20Mo(EDA) 8.3 1.54 41.5 2.4 .28Co30Mo(EDA) 37 617.2 2.4 .35Co22Mo(EDA) 31 617.1 2.7 .28Co/20Mo(EDA) 36 617.3 2.4 .28Co(EDA)/20(MoEDA) 4.3 29 617.2 20Mo(EDA)/.28Co 3.8 34 617.1 20Mo(EDA)/.28Co(EDA) 8.5 44 617.1 2.3 .28Co(EDA)/a0Mo(EDA) a: XPS atomic ratios; b: mean layer stacking of MoS 2 crystallites; c: thiophene activity in % conversion; d: Binding energy difference (eV.) between the main Co2p3/2 peak and the $2p3/2 peak; e: FWHM (eV.) of the Co2p3/2 peak. Alstrup et al suggest that the XPS BE difference between the Co 2p3/2 and the S 2p peak, 617.1 and 616.5, may be used to characterize respectively the Co in a "Co-Mo-S" phase or in Co9S 8. However Van-Veen et al.(23) show that this BE difference does not allow one to make an unequivocal distinction between these entities when they are not supported on alumina. Here the BE difference observed for the sulfided Co and Co(EDA) samples characterizes bulk Co9S 8. In fact a decrease of the (Co/AI)XPS atomic ratio is observed for
1047 the Co, the Co(EDA) and the CoMo catalysts which may be correlated to the formation of bulk Co9S 8. In contrast a better dispersion is obtained upon sulfidation of the CoMo(EDA) catalysts and the BE difference corresponds to the value observed by Topsoe for their "CoMoS" phases. The full width at half maximum-FWHM-of the Co peak could also be used to characterize the existence of several Co species namely the Co9S8, the CoMoS phase and the surface CoAI20 4 as this latter entity is generally not sulfided. The existence of Co9S 8 and CoAI20 4 may explain the high FWttM value observed for the Co, the CoEDA samples and when the Co is impregnated before the molybdenum. In contrast the lowest FWHM value is observed if the cobalt is introduced as an EDA complex after the molybdenum species. Whatever the Mo loading, even at high Mo content, this low value is obtained for sample prepared by coimpregnation; this is in agreement with the non existence in the oxide form of Co30 4 or bulk cobalt molybdate; This procedure of preparation avoids, at high Mo loading, the formation of (bulk) cobalt molybdate which is not a good precursor for the active phase, sulfidation leading mainly to Co9S $ (24). The increase of the dispersion of the Co and the decrease of the FWHM of the XPS Co peak of the sulfided catalysts when EDA is used are strongly in favor of the existence of a well dispersed cobalt entity. This latter may correspond to a CoMoS phase (I or II) or to Co9S8 nanocrystals. CATALYTIC ACTIVITY Table III also shows the activity in HDS conversion of the thiophene. The decrease of the activity of the 30Mo catalyst is due to the existence of bulk MoO 3 on the oxidic precursor. This activity slightly increases on increasing the Mo loading for the Mo(EDA) series. This correspond in fact to a decrease of the activity per Mo atom which may be correlated to the morphology of the MoS 2 like crystallites, taking into account the geometrical model previously developed by Kasztelan et a1.(25). Stacking does not affect the thiophene activity but in contrast a small variation of the distribution of the length (some longer crystallites are detected) may induce a great variation of the fraction of molybdenum atoms at edge or comer positions. Upon introduction of the cobalt, the well known promoting factor is observed. One might speculate that the value of a is optimum in the .28Co20Mo(EDA) sample. In fact the activity of the .35Co22Mo(EDA) catalyst is lower than that of the .28Co20Mo(EDA) one. The more important fact is the net increase of this promoting factor when EDA is used. The catalysts on which Co has been introduced after the molybdenum as a Co(EDA) complex show the higher activity and the lowest FWHM value of the XPS Co peak. When Co is introduced first, lower activities and broader Co XPS peaks are obtained. This confirms the fact that EDA induces a better dispersion of the cobalt in interaction with the MoS2 crystallites. We may also consider that we have well dispersed Co9S8 nanocrystals interacting with the molybdenum disulfide crystallites according the synergy model (26). This may also correspond to a better decoration of this crystallites according the Topsoe's model. Thus the use of EDA allow the optimization of the synthesis of the active sulfided phase. This have to be related to the non existence in the oxidic precursor of bulk CoMoO4 or Co30 4 and to the existence of a strong interaction between the cobalt and the molybdate supported species. Unfortunately no information has been obtained as to whether the octahedral configuration of the Co atom -widely reported as being necessary for a good catalyst- was present or not.
1048 CONCLUSION The following conclusions can be drawn about the use of a complexing agent of Co in the preparation of CoMo/AI20 3 catalysts by dry impregnation: -1) The use of EDA improves the dispersion of the molybdenum as well dispersed polymolybdate entities in spite of the presence of isolated molybdate in the impregnating solution. A more complete study of the mechanism of adsorption in presence of complexing agent should be undertaken -2) This method gives, whatever the Mo loading, a CoMo oxidic precursor where the Co and the Mo are well dispersed and strongly interacting; They can be considered as being the precursor of the sulfided active phase. -3) The use of EDA avoids the formation of unwanted large crystallites of Co9S $ which arise from the sulfldation of Co304 or CoMoO4. It also induces a better dispersion of the Co atom in interaction with the MoS 2 crystallites whatever the mode of preparation, successive or simultaneous impregnation. -4) An increase of the thiophene HDS activity per gram of catalyst is obtained. REFERENCES: 1-H. Topsoe, B.S. Clausen, Appl. Catal. 25 (1986) 273 2- a: E. Payen, S. Kasztelan, J. Grimblot, H.Toulhoat, Appl. Catal. 7 (1983) 91. b:N. Spanos, L. Vordonis, Ch. Kordulis, P.G. Koutsoukos, A. Lycourghiotis, J. Catal. 124 (1990) 301 3- a: Van Veen J A 1L Gerdema E, Van der Kraan A M, Knoester A, J.Chem. Soc. Chem. Comm. 1684 (1987). b: Y.Yoshimura, N. Matsubayashi, T. Sato, H. Shimada, A. Nishijima, Appl. Catal. 79 (1991) 145. c: L. Bonneviot, O. Clause, M. Che, A. Manceau, H. Dexpert, Cata. Today 6 (1989) 39 4- S. Houssenbay, E. Payen, S. Kasztelan, J. Grimblot, cata today, 10 (1991) 541. 5- French Patent (1993) 6- J. Grimblot, P. Dufresne, L. Gengembre, J.P. Bonnelle, Bull. Soc. Chim. Belg., 90 (1981) 1261. 7- P. Dufresne, E. Payen, J. Grimblot, J.P. Bonnelle, J. Phys. Chem., 85 (1981) 2344. 8-G. Meunier, B. Mocaer, S. Kasztelan, L.R. Le Coustumer, J. Grimblot, J.P. Bonnelle, Appl. Catal.,21 (1986) 329. 9-E.Payen, S. Kasztelan, J. Grimblot J. Phys. Chem. 91 (1987) 6642. 10- a: J.A.1L Van Veen, P.A.J.M. Hendriks, Polyhedron, 5,1/2, (1986) 75. b: J.A.1L Van Veen, H. de Wit, C.A. Emeis, P.A.J.M. Hendriks, J. catal. 107 (1986) 583. c: J.A.1L Van Veen, P.A.J.M. hendriks, E. J. G. M. Romers, R.R. Andrea, J. Phys. Chem. 94,13 (1990) 5275,. 11-D. Soung Kim, K. Segawa, T. Soeya, I.E. Wachs, J. Catal. 136 (1992) 539.
1049 12- S. Kasztelan, E. Payen, H.Toulhoat, J. Grimblot, J.P. Bonnelle, Polyhedron 5, (1986) 157. 13-E; Payen, M.C. Dhamelincourt, P.Dhamelineourt, J. Grimblot, Appl. Spect. 36,1, (1982) 30. 14-J.A.R. Van Veen, E. Gerkema, A, M. Van der Kraan, P.A.J.M. Hendriks, H. Beens, J. Catal; 133 (1992) 112. 15-Y. Yokoyama, K. Teranishi, A. Nishijima, M; Matsubayashi, H. Shimada, M. Nomura, Jpn. J. Appl. Phys. 32, 2, (1993) 466. 16- a: J.M. Stencel, L.E. Mokovsky, T.A. Sarkus, J. de Vries, R. Thomas, J.A. Moulijn, J. Catal. 90,314, 1984 b: E. Payen, S. Kasztelan, J. Grimblot, J.P. Bonnelle, J. Raman Speetr.17, 233, 1986 17-M. Lo Joeano, A. Cimino, G.C.A. Sehuit, Gazetta Chim. Italiano 103 (1973) 1281 18-J.H. Ashley, PCH Mitchell J. Chem. Soe. A (1969) 2730 19-J. Grimblot, E. Payen, R. Hubaut, O. Poulet, S. Kasztelart, J. Catal. in press 20-S.M.A.M. Bouwens, F.B.M. Van Zon, J. Catal. 146 (1994) 375. 21-S. Kasztelart, S. Houssenbay, J. grimblot, J. Phys. Chem. 22-1. Alstrup, I; Chorkendorfl', tL Candia, B.S. Clausen, H. Topspoe, J. Catal. 77,397,1982 23-S.M.A.M. Bouwens, J.A.tL Van Veen, D.C. Koningsberger, V.H.J. de Beer, J. Prins, J. Phys. Chem. 95,1,123,1992 24-V.H.J. De Beer, C.G.A. Sehuit, "in preparation of catalysts I" (13. Delmon, P.A. Jaeobs, G. Poneelet, Eds. ) p 343, Elsevier. Amsterdam, 1976 25- S. Kasztelan, H. Toulhoat, J. Grimblot, J.P. Bormelle, Appl. Catal. 13 (1984) 127 26-13. Delmon, Bull. Soe. Chim. Belg. 88 (1979) 979
This Page Intentionally Left Blank
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
1051
I m p r e g n a t i o n during gelation and its influence on the dispersion o f the i m p r e g n a n t A.E.
Duisterwinkel*
G.
Frens
Delft University of Technology, Laboratory of Physical Chemistry, Julianalaan 136, 2628 BL Delft, The Netherlands
I. INTRODUCTION New technologies need new materials and new materials need new preparation methods. An example of this thesis is regenerable desulphurization in fluidized bed combustion (FBC) of coal, the principle of which is shown in figure 1. A sorbent material is needed which [ 1]: - can be fluidized at FBC conditions; - reduces the SO2-concentration below 100 ppm at 850~ at sufficient rate; - releases more than 4v% SO 2 at 850~ under reducing conditions at sufficient rate; - withstands continuous wear during fluidization and transport between reactors; - withstands thermal degradation at 850~ and high levels of water (up to 15v%); - can be separated easily from the fly ash; - is available in sufficient amounts; - can be produced cheaply.
Figure 1. Diagram of regenerative desulphurization in fluidized bed combustion of coal. The route followed by the sorbent is shown by fat lines. Fresh sorbent is added to the combustor (C). When sulphated, it is separated from the coal ash and conveyed to the regenerator (R). There, reducing conditions are applied to remove the sulphur from the sorbent. The sorbent is fed again to the combustor and the regenerator off gas is used for producing, say, H2SO 4.
[TURBN i ESI -GAS
~
SO;? H SOt.or 'S] PLANT
COAL~ ~ O A L
AIR
or CH~,
ASH AIR
Calcium aluminates (CaO.xAl203) with x < 1 appear to be the compounds most likely to meet these demands [1]. As AI203 is very strong, it is preferred as a support. If CaO is dispersed very well on the internal surface of the AI203, stable calcium aluminates are expected to be formed at 850~
~t
present adress: TNO-PML, P.O. Box 45, 2280 AA Rijswijk, The Netherlands
1052 In short, sorbent particles consisting of calcium aluminates should be porous, not very dense (1-2 g/cm3), spherical, ca. 3 mm in diameter, have a smooth outer surface and be strong. A preparation method had to be developed for this purpose. Here, we present this method, in situ impregnation during sol-gel preparation, with emphasis on the dispersion of the impregnated CaO on the alumina surface. Figure 2 shows the principle of this method. As an extension of the sol-gel process of CONDEA Chemie GmbH, FRG [2], we add calcium nitrate to the ammonia solution used for gelating the acidified boehmite sol [3]. Given sufficient residence time in this medium (ca. 10 min), large amounts of calcium enter the gelating particle. The boehmite surface is charged negatively in the alkaline medium (pH i 7-8) and the Ca-ions 'titrate' the negatively charged surface sites. If an excess Ca is not added, no more than a monolayer of calcium aluminates can be formed in this way. Figure 2. Diagram of a set-up for continuous sol-gel preparation with m situ impregnation, where 1. sol preparation 2. dropping of sol into layer of light oil 3. gelation reactor filled with an aqueous solution of ammonia and calcium nitrate 4. pump circulating the ammonia solution 5. supply vessel for the ammonia solution 6. particle separation 7. overflow for maintaining a specified level in the gelation reactor (3) 8. addition of fresh ammonia and calcium nitrate 9. purge 10. pump for dropping the sol
2. PREPARATION
Here, we outline the general preparation procedure. Details are given elsewhere [1,3]. Pseudoboehmite powder (AIOOH.xH20, kindly provided by CONDEA Chemie GmbH) is dispersed in an aqueous solution of urea and HNO 3 a high-shear mixer. Aged sol is dropped into the reactor (fig. 2). This solid laden sol remains liquid through the action of the acid which adsorbs on the AIOOH, giving all particles a positive surface charge. Almost perfect spheres are formed by dropping the sol into light oil ((2) in fig. 2; the oil layer is shaded). Some surfactant is added to this oil. This facilitates the formation of a thin oil layer of 5-25 ~tm around the droplets when the drops fall further into the aqueous solution of calcium nitrate and ammonia (3). The oil film remains in place for a few seconds, after which it breaks up and rises to the oil layer. It is calculated that in these few seconds sufficient NH 3 diffuses through the oil layer in
1053 order to neutralize the acid in the sol. Thus, the droplet gelates and survives as a solid, though weak body. It is essential that the oil film remains in place until the droplets have fully gelated. On the other hand, the film must be removed before the next step in the preparation can take place: the in situ impregnation. The aqueous solution of calcium nitrate and ammonia (3) is circulated through the reactor such that a residence time of more than 5 minutes is achieved for the gelating drops. In this time, sufficient calcium diffuses into the particles (the in situ impregnation). The calcium ions are adsorbed by the now negatively charged boehmite surface. A simple penetration model appears to predict the weight content of CaO in calcined particles, XCaO, as a function of calcium content of the ammonia solution, [Ca2+] and the residence time x (fig. 3). This model, XCa O = 7.31 104 [Ca 2+] z0.35 / (d 3 Db)
(1)
where d and D b are the calcined particle diameter and bulk density, respectively, enables a simple control of the calcium content of the sorbent. J --
Figure 3. CaO-mass per calcined particle (A) determined experimentally, compared to the prediction of the semi-empirical model (drawn line), based on the calcium concentration [Ca 2+] and residence time x in the ammonia solution (equation (1))
6
I 0 0
&j
J
z~1
Z, ,- AI 2 8 ~.l.---
I 10
1 12
1 1L,
[ Ca 2" ] x -r 0.35 (m0l it.s 0.35 )
Gelated and m s/tu impregnated particles are carefully sieved from the ammonia solution ((6) in fig. 2), briefly dried in a microwave oven and subsequently in a ventilator oven at 80~ (> 5h). During drying, the particles shrink considerably, ca. 75v% and ca. 55w%. Fast drying causes cracking of the particles and must be avoided. Calcination is performed by heating to 500~ at l~ This slow rate prevents the particles from explosion due to the decomposition products of calcination. Further heating to 850~ was performed at ca. 3~ After 10 h at 850~ samples were cooled to room temperature in ca. 3 h. 2.1 The influence of CaO content on sorbent texture
The pore volume of calcined particles decreases with increasing CaO content (fig. 4), more than expected from the solid density of CaO. This is thought to be due to the osmotic imbalance between the inner part of the gelating particles and the ammonia solution. The larger this is, the more water is expelled from the gelating particles to compensate the imbalance. This causes shrinking of the particles, which can be counteracted by increasing the osmotic pressure of the sol, for instance by adding more urea (fig. 4). The BET-surface area (N 2, 77K) also decreases with calcium content, probably due to some sintering during calcination at 850~ Prolonged heating at 850~ does not cause further sintering, but heating at higher temperatures dramatically decreases surface area. The experimental data have been compared to estimates of the surface area S needed to disperse a certain calcium oxide content in exactly one monolayer:
1054 S = A XCa O
(2)
The factor A has been estimated from the molar densities and lattice parameters of several calcium aluminates to be 12.8• m2/g.w%. Also, using the molecular area of CaO obtained from well-known methods [4], A = 10.8• m2/g.w%. Using these two data we find that a monolayer of calcium aluminates can exist only for particles containing less than 9.5• w% CaO for the samples in this research (figure 5).
o.'I lSO~"--.........
Vp,O b
"-§
o 0.3-
d .
(: b o
-~ 5
~001/ !
0 ~.aD-,
J 0.2
X CoO (gig)
Figure 4. The pore volume Vn as a function of the CaO weight content XCaO for in situ impregnated sol-gel spheres (a) made with 0.25 M urea in the sol (+) Co) linear regression on all data (c) made with 0.75 M or 1 M urea (o) (d) pore filling model
/~7
= ! /
I 0.1
, C / ~ ~", o"" ~ ".o, ,,
/
~= 'oF 0.1-
"o: AT'_'o o',,o ~ Oo oo~,_o
/
,
/C/ C /
\
/
0/ 0 ~~'-"
",,.o0
oo;,
~
o
",
I 10 Xcoo
\
o o ,, , o o
I 20
(w%)
Figure 5. BET-surface area SBE T versus CaO weight content XCa O for m situ impregnated sol-gel spheres; batchwise preparation (o); continuous preparation (+) (a) the CaO does not fiH a monolayer (b, c) theoretical CaO monolayer capacity (d) the monolayer capacity for CaO is exceeded
3. EVIDENCE FOR THE MONOLAYER CONCEPT A number of arguments exist that the CaO is indeed dispersed in a monolayer for XCa O < 9.5• w%. First, direct observation with EPMA shows that the local variation in calcium signal is much smaller for an in situ impregnated sol-gel particle than for impregnated pellets of alumina pellets. In X-ray diffraction no peaks other than that of the support are found. This proves that no crystallites of CaO or any calcium aluminate are present to any appreciable amount. Third, regeneration efficiency (i.e., the conversion of CaSO4 into CaO) as determined by the Sulphation-Regeneration-Oxidation-test in a thermobalance [5] depends strongly on the
1055 CaO content (fig. 6). In general, regeneration efficiency is high for the sol-gel samples as compared to other sorbents [ 1]. This is thought to be due to the following solid-solid reaction
(3)
CaSO 4 + x AI203 + H 2 = CaO.xAl203 + SO 2 + H 2 0 which competes with CaS formation according to
(4)
CaSO 4 + 4 H 2 = C a S + 4 H 2 0
For H 2 and H20, CO and CO 2 can be read in equations (3) and (4), respectively. Although reaction (3) is thermodynamically favoured over reaction (4), its rate will be insufficient when solid-solid diffusion has to take place. In that case, CaS is formed and the regeneration efficiency decreases. For samples containing more than a monolayer of CaO, not all CaSO 4 can be in direct contact with the alumina, and part of it is converted into CaS. Samples containing less than 9.5w% show a regeneration efficiency ~,2 close to 1 (fig. 6). Dispersion in these samples must be good.
1.0-
---o--
§ 9 § o., ,- -*--o 9 9 o + \_ § 2 4 7
o o. "~ooOo
~ 0
~176176
0.8
-
Figure 6. Regeneration efficiency ~,2 (conversion of CaSO 4 into CaO) versus CaO weight content XCa O for sol-gel samples, impregnated: o in situ, batchwise preparation + in situ, continuous preparation 9 after calcination of non impregnated, continuously prepared sol-gel samples dotted line is drawn to guide the eye
~
~ "}5
oae \
o
~
Oo
\\
i
\
\ \\
N
\
l 0.6-
o o
I
~
~
Xca 0
'\ \
1
0.1
0
o
0.2
\
\
I
0.3
(g/g)
The final argument for the existence of a monolayer of CaO stems from the sulphation model. The sulphation reaction has been described by a shrinking core model. The calcium aluminate dispersed on the pore walls is assumed to react immediately when SO 3 arrives*. Due to this fast chemical reaction a skin of reacted sorbent is formed, while the inner core remains unreacted. Taking external mass transfer into account, we obtained for the inverse of the rate of change in sulphation conversion ~ 1 [6]
* It has been proven that the oxidation of SO2 is not rate determining by impregnating sorbent particles with Pt, a known catalyst for this oxidation. No significant influence on reaction rate was found.
1056
(5)
(d~ 1/dt)- 1 = k f(~ 1)/Dexp + k/Rkg where f(~,l) - (1-r
-1
(6) (7)
k = (W~-Wo)/[4rmMRCo]
Here, Dex p is the effective pore diffusion coefficient, R is the universal gas constant, kg is the external mass transfer coefficient in thermogravimetric experiment, (Woo-Wo) is the maximum weight change during sulphation, n is the number of sorbent particles tested, M is the molar weight of SO 3 and C o is the SO 2 inlet concentration. 7-
10 8 c
O
O
6
I0
--
3 X
O
l
t~
1 0 0
I 1
~.~D--
f (~1)
I 2
I 3
(-)
Figure 7. Test of the shrinking core model" (d~,l/dt)-1 versus f(~l) with linear regression for all points with f(~l) < 1.5 or ~1 < 0.93. o: experimental data points
0 ~a~---
0.3
0.6 r
0.9
1.2
1.5
(ram)
Figure 8. Calcium (fat line) and sulphur (thin line) profiles for an in situ impregnated sol-gel sphere, sulphated (0.5v% SO 2 in dried air) at 850~ for 10 minutes
The model fits experimental results excellently for ~ 1 < 0.93 (cf. for instance fig. 7). The values obtained for Dex p (0.5-1 10-7 m2/s) and kg (1-2 cm/s) are realistic. Core diameters have been predicted and showed satisfactory agreement with sulphur profiles of partly sulphated sorbent particles as determined with Electron Probe Micro Analysis (EPMA). The latter also showed that the original assumption of a shrinking core is correct (figure 8). Dex p has been compared to a theoretical estimate of the effective pore diffusion coefficient Dth from the Knudsen diffusion coefficient multiplied by the particle porosity and divided by the theoretical pore tortuosity of ~/2. The quotient of these two A - D e x ~ t h has been plotted versus CaO content (fig. 9). Although the experimental results show considerable error, good agreement is found between theory and experiment for XCa O < 9.5w%. For higher CaO contents, however, the theoretical estimate is too large.
1057
-
b
!
Figure 9. Inverse quotient of experimental and theoretical pore diffusion coefficient A-1 versus calcium oxide weight content XCa O for in situ impregnated sol-gel samples. Theoretically it should hold that A-1 < 1 for XCaO < XCaO, c where the latter denotes the critical calcium oxide weight content (9.5w%) for a monolayer dispersion. + Slowly dried samples; linear regression for points at XCa O > 8w% o Fast dried samples; linear regression for all data points
i
%
.
I.
I I I I II 0.1
,, 0
I
0.2
XCo 0 ( g l g l
- - ~ " -
This is thought to be due to solid-solid diffusion, which is necessary at that point because the dispersion is not monomoleeular anymore. Any solid state diffusion will considerably slow down the reaction, disabling the 'shrinking core' assumption and causing discrepancy between theoretical and experimental di~sion coefficients. This effect should be stronger for samples with worse dispersion. Indeed, for samples that have been dried at a high rate, the effect is larger. These samples have a less good distribution of the CaO over the particle as determined by EPMA (fig. 10) than slowly dried samples, confirming above explanation.
250r-
§
§
I%...
I _
4F~
Figure 10. Calcium distribution profile as determined by EPMA for in situ impregnated sol-gel samples o dried at 27~ in a ventilator oven (slow drying) + dried for 15 min in a microwave oven and then at 82~ in a ventilator oven (fast drying)
150 -4
a
, I
s~I 0
I
0
~.e~--.
I
500
1000 L
(pm)
I
1500
I
2000
2500
1058 The fact that A = 1 for XCa O < 9.5 w% once again shows that the dispersion of the CaO on these samples is very good and to a first approximate may be referred to as monomolecular. In general, m situ impregnation during sol-gel preparation enables well-controlled production of porous materials. As two process steps that consume considerable amounts of energy (drying and calcination of unimpregnated gel particles) are eliminated from the production process, this well-controlled product is expected to be economically attractive. EPILOGUE From the above discussion, it is clear that the m situ impregnated sol-gel sorbent, due to the good dispersion of the calcium, shows good sulphation and regeneration efficiency. Also, the sorbents resists attrition very well [1]. It is therefore very suitable for regenerative desulphurization during fluidized bed combustion (FBC). Korbee et al. [7] have shown that this process is an economically interesting alternative to once-through desulphurization in FBC with natural limestone. The solid waste stream is reduced roughly by a factor 35 by using the m s/tu impregnated sol-gel sorbent while useful products like sulphur or sulphuric acid are produced. ACKNOWLEDGEMENTS This research was financed by the EC (contract number EN3F-0014-NL(GDF)), the NOVEM (contract no. 20.35-016.30) and the STW (contract no. DST77.1386). Materials were kindly provided by Dr. Noweck of CONDEA Chemie, Brunsbtittel, Germany. REFERENCES [ 1] [2] [3] [4]
A.E. Duisterwinkel, Ph.D. thesis, Delft University of Technology, The Netherlands, 1991 A. Meyer, K. Noweck, European patent 0090994 (September 1987) A.E. Duisterwinkel, Dutch patent application 90.01039 (May 1990) S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, 2nd edition, Academic Press, London, 1982 [5] A.E. Duisterwinkel, E.B.M. Doesburg, G. Hakvoort, Thermochim. Acta 141 (1989) 51 [6] A.E. Duisterwinkel, G. Hakvoort, Bull. Soc. Chim. Beiges 98 (1989) 51 [7] R. Korbee, J. Grievink, J.C. Schouten, C.M. van den Bleek in Proc. 1993 Intern. Conf. on Fluid. Bed Comb., L. Rubow and G. Commonwealth (Eds), The American Society of Mechanical Engineers, Book No. IO344B (1993) 1143
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
1059
Synthesis and characterization of titanium oxide monolayer. Neuman S. de Resende, Martin Schmal and Jean-G. Eon NUCAT/COPPE - Federal University of Rio de Janeiro C.P.- 68.502, 21945.970 Rio de Janeiro - Brasil. FAX- 55 (021) 290.66.26 The structural evolution of titanium oxy-hydroxide overlayers was studied for 3% and 6% TiO2/AI203, prepared by grafting the y-AI203 support at different temperature of calcination. Results showed that during calcination the oxy-hydroxide is dehydrated at around 550 K; the y -AI203 support hinders the crystallization of TiO 2 anatase structure. The titanium oxide is well dispersed on the alumina surface, as confirmed by TEM and XPS data. UV-visible results showed that dried materials have small polymerization degree. A linear correlation is proposed between the position of Ti charge transfer band and the number of titanium atoms in the second coordination sfhere of titanium. This leads to the model that polymeric units [Ti(OR)4]n are grafted without modification on the support; but after calcination, linear or cyclic chains of edge-sharing TiO6 octahedra are linked to the support. 1. INTRODUCTION Titanium oxide monolayer on y-Al20 3 is a potential support for noble metals [1-4]. Many studies have shown that two-dimensional transition metal oxide overlayers are formed when one metal oxide (V205, Nb205,, MOO3, etc.) is deposited on an oxide support (A1203, TiO 2, etc.) [5-7]. The influence of the molecular structures of surface metal oxide species on the catalytic properties of supported metal oxide catalyst has been examined [8-9]. It has been demonstrated that the formation and location of the surface metal oxide species are controlled by the surface hydroxyl chemistry. Moreover, thin-layer oxide catalysts have been synthesized on alumina by impregnation technique with alkoxide precursor [10]. It has been found for titanium oxide, by using Raman spectroscopy, that a monolayer structure is formed for titanium contents below 17%; and that polymeric titanium oxide surface species only posses Ti-O-Ti bonds and not Ti=O bonds. Titanium is typically ionic in its oxy-compounds, and while it can exist in lower oxidation states, the ionic form Ti4+ is generally observed in octahedral coordination [11-12]. However, there is no information available about the Ti coordination and structure of this oxide in a supported monolayer. In this work we have studied the structural evolution of the titanium oxy-hydroxide overlayer obtained from alkoxide precursor, during calcination. We have applied thermal analysis (DTA-TGA), diffuse reflectance spectroscopy (DRS), transmission electronic microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) techniques to obtain structural information about the nature of the titanium environment in TiO2/Al203.
1060 2. EXPERIMENTAL
2.1 Catalyst preparation Pure titanium dioxide was obtained by hydrolyzing the titanium isopropoxide. Thin-layer oxide supports were prepared by grafting the y-alumina (Harshaw, AI 3996) with a non aqueous solution of Ti[OCH(CH3)2] 4 in THF in Ar flow, followed by hydrolyzing, drying at 393 K and calcining at 623 K and 773 K in air (5 h). Two "/-alumina supports, calcined at 873K and 1073 K, were used to prepare 6% and 3% TiO2/AI203, respectively.
2.2 Characterization The samples were characterized by UV-visible reflectance spectroscopy using a VarianCarry-5 spectrometer equipped with a double monochromator. Diffuse reflectance spectra were recorded in air at room temperature in the range 200-800 nm against alumina as reference. Spectra are presented indicating the frequency of the Schulz-Munk-Kubelka equation as function of the wavelength. Textural properties were obtained by ASAP-2000 Micromeritics. Metal content was determined by atomic absorption on a Perkin-Elmer-1100B. A Jeol JEM-2000FX electron microscope with a point resolution of 3.0 A was used for sample examination. Elemental analysis was performed using a Tracor Northern 2000 energy dispersive spectroscopy system. X-ray diffraction was recorded using Jeol GBX-8P diffractometer with a Cu-Kot radiation. The samples were analyzed at room temperature. Differential temperature analysis was carried out using a Rigaku - TAS 100 with a thermogravimetric support. The analyses were performed with nitrogen flow at 40 ml/min, in the temperature range of 296-1573 K. X-ray photoelectron spectra were recorded using a Perkin-Elmer 1257. The samples were analyzed in the oxide form, without pre-treatment. Mg anode was operated at 200 W. The spectra were calibrated with respect to the C 1s line with binding energy of 284.6 eV.
3. RESULTS 3.1. Thermal Analysis (DTA/TGA) DTA and TGA results for TiO2, TiOx/Al203, and A1203 are shown in Figure 01. Hydrolyzed titanium oxide exhibits two main transformations at around 533K and 703K. Both are exothermic and only the first one gives rise to 10% weight loss. X-ray diffraction analysis has been performed on the hydrolyzed and calcined materials. The results (not reported here) show that the hydrolyzed oxide is amorphous. At intermediate temperature of calcination (623K), the material presents anatase structure. Rutile structure is observed at 773K. Hence, the first transformation at 553K is attributed to the dehydration of oxy-hydroxide and crystallization to anatase structure. The weight loss measured by TGA corresponds to the overall composition TiOx.(OH)4.2x (x ~ 1,5). The second one, at 773K, corresponds to anatase-rutile phase transition. Figure 01 shows also that TiOx/Al203 samples present one transformation at the titanium oxy-hydr0xide decomposition temperature, as well as the characteristic features of A1203. It is noteworthy that the y - c~ transition peak is shit~ed from 1573 to 1488 K.
1061 The anatase-rutile transition is not observed in supported samples. This transition is generally catalyzed by oxide mixture; we conclude that y-alumina support hinders the crystallization of the TiO 2 anatase structure, in agreement with DRX data.
o•' 0
-~'
'
'
.....
'
'
'
'
'
(a)
~ -10 ~-20
~
. . . .
_ Ti___O x
,mal
I
] i
I
I
I
I
I
I
I
I
I
I
I
I
(b)
Ti~ 9
\/
TiO/AIOA 03
,
200
I
400
i
I
600
,
I
800
,
I
,
I
,
I
,
I
,
1000 1200 1400 1600 1800
Temperature (K) Figure 1. a) TGA curve for TiOx, from 303 K to 1600 K; b) DTA curves for TiOx; TiOx/Al20 3 and AI203.
1062
3.2. Textural Properties The textural properties of A1203, TiO 2 and of the grafted TiO 2 on AI203 are presented in Table 1. As shown, the area surface of the pure support decreases with increasing the temperature of calcination, and as consequence the pore volume decreases. However, titanium calcined at 623 K presents an increase of pore volume. In the case of TiO2/A1203, Table 1 shows that grafting process does not affect textural properties of alumina. Table 1 Textural properties of titanium and alumina material foUowin 8 the calcination. Calcination BET Vp dp Temperature (K) (m2/g) (cm3/g) (A)
Al203
TiO 2
3% TiO2/Al203 6% TiO2/Al20
-
247.20
0.69
111
873
187.56
0.57
121
1073
156.50
0.48
124
hydrolyzed
102.71
0.13
51
623
81.70
0.22
107
823
23.30
0.06
109
dried
156.86
0.51
150
823
159.06
0.58
146
dried
194.85
-
114
623
150.56
-
121
Vp = pore volume, dp= pore diameter
3.3. Diffuse Reflectance Spectroscopy (DRS) DRS spectra performed on TiO x are shown in Figure 2. A single peak is observed at 306 nm for dried compound and is shifted to 316 and 333 nm, respectively after calcination at 623 and 773 K. This peak is attributed to ligand-to-metal charge transfer (CT) band of the Ti (IV) (d ~ cation. It has been shown [13] that the shift to higher wavelengths of the CT band corresponds to an increase of polymerization degree of six-coordinated titanium (TiO6). This agrees with the dehydration and crystallization process of oxy-hydroxide. Figure 3 shows that the titanium CT band observed in TiOx/Al20 3 sample occurs at lower wavelength (270 nm) and is shifted to 260 nm after calcination at 773 K. We attribute this shift to a decrease of the polymerization degree of titanium on ~/-alumina after dehydroxilation, indicating a better dispersion of titanium dioxide at the surface. Moreover, we note that a coordination number of titanium must be six; as it is known, the tetrahedral titanium is characterized by a CT band at 205 nm[ 13 ].
1063 I"
.
.
.
.
15
.
.I
t'
.
.
15
a- hydrolyzed
10
.
.
.
6% - hydrolyzed
10~
./~~.3% - hydrolyzed
~/~I~
6%"c'773K 3 -c. 73K
5
e- 773
5
0
0 u
,
I
I
I
I
I
9
200 250 300 350 400 450 500
Wavelength (nm)
200 250 300 350 400 450 500
Wavelength (nm)
Figure 2. DRS spectra of TiO 2. a) hydrolyzed b) calcined at 623 K and c) at 773 K.
Figure 3. DRS spectra of 3% and 6%TiOx/A120 3, hydrolyzed and calcined.
3.4. X-ray Photoelectron Spectroscopy (XPS) Figure 5 shows the photoelectron spectra of O l s level for TiO x following temperature of calcination. The parameters of these spectra are given in Table 2. The asymmetrical O ls XPS peak of the dried material indicates the presence of different oxygen species in this sample, respectively at 530.1 and 531.8 eV. The peak at 531.8 eV is attributed to OH group, and the ratio of the areas of these peaks ([O]/[OH] = 5,8) is compatible with the value estimated by TGA.
Table 2 XPS spectral parameters of the TiO• system. TiO 2
hydrolyzed calc. 623 K calc. 773 K
Atomic Ratio
Binding Energy (fwhm) eV O l s (a)
O ls (b)
Ti 2p 33
Ti 2p 1/2
Ti / O
530.1 (1.37) 530.2 (1.32) 530.2 (1.32)
531.8 (1.37) 531.8 (1.32) 531.7 (1.32)
459.1 (1.36) 459.0 (1.27) 458.8 (1.23)
464.7 (2.33) 464.7 (2.12) 464.7 (2.14)
0.435 0.472 0.471
(a) and (b) = different species of oxygen.
1064 Table 2 shows that Ti 2p 3/2 level is shifted from 459.1 eV, for the hydrolyzed TiO x sample, to 459.0 eV and 458.8 eV, after calcination at 623 K and 773 K, respectively. A shift in the opposite direction is observed for the supported sample. This might indicate a decrease in the number of titanium atoms in the second coordination sphere of titanium, following dehydroxilation of supported samples. Moreover, Table 3 shows that the atomic ratio Ti/A1 increase from 1.3% to 4.0% after calcination. This observation confirms the re-dispersion of titanium on the alumina surface after decomposition of the oxy-hydroxide.
Table 3 XPS spectral parameters of the TiOx/Al20 3 system.
O ls (a)
AI 2p
Ti 2p 3/2
Ti 2p ~/2
Surface Content % Ti
531.4 (2.61) 531.9 (2.65)
74.5 (2.22) 7 5 . 1 (2.13)
459.0 (2.38) 459.2 (2.20)
464.5 (2.22) 464.6 (2.97)
1.3 4.0
Binding Energy (fwhm) eV
TiO2/Al203
hydrolyzed calc. 773 K
i
i
50
i
i
O is !"
9
,no 2
25
-
'~" 40
i
9
Ti 2p 3/: ~
!
"1]O2/AI2 03
20 O
~
30
~
~
2o
w~
15 Ti 2p 1/2
"~ 10
M
10
=
5
I~
(b)
"
)
0 i
.
524
I
528
,
I
532
9
I
.
536
Binding Energy (eV) Figure 5. XPS spectra of titanium (O ls ): a) TiO x hydrolyzed, b) calcined at 623K, c) calcined at 773K.
s
450
I
460
~
I
470
Binding Enemy (eV) Figure 6. XPS spectra of 3% TiOx/Al203, (Ti 2p 3/2)" a) hydrolyzed, b) calcined at 773K.
1065 3.5. Transmission
Electronic
Microscopy
(TEM)
Figure 7a presents TEM micrograph for TiOx/Al203. It shows that TiO2, as an amorphous phase, is highly dispersed on y-Al203. This was confirmed by the elemental analysis, using EDS, in different regions (Figure 7b).
(a)
(Sui'.so~: 0.5,.9~l(eV : 0
R0I
(t6) 0.(~0: I ~ . ~
1 ! I i ....... 9
... .. ::
! @,....................
! ..........
EI_G,..II:) _ _
t
:
!
. . . . . . .
~ . ~ w ~ - . . ~ .
Lt ~_m 9
_~.._
i 9
i
-~-: .....:......~ .....!...........:............. i......................... :...................... i-....
_ ....................~_ .............. :.......... :.................... :................
r~ '. ! ~ : !
'
i'
~..-..t
'
'.
t .......
.......
;
:. . . .
i ~
........ |
.
: ~ .......
i
.
II
~
L
B-
c:
9
9
.B.St~
'~............
, !
: j .....
e.-.
i-......
I
5
"
:
I ....
...... . . . . . . . . . . .
k.
:
.. . . . . . .
i,:
i i:-u
:. . . . . . . . . . . . .
~S
" i28
9 iB
Figure 7.3% TiO2/AI203 after hydrogen flow at 773 K. a) Transmission electron micrograph; b) Elemental analysis by EDS (it was the same for different regions).
4. DISCUSSION By comparing thermal analysis data for grafted and bulk dried materials, we observe that the first transformation at around 550 K, previously attributed to dehydroxilation of titanium oxyhydroxide, occurs in both samples. However, the anatase-rutile transition is not observed in supported samples. Indeed, UV-visible and photoelectron spectra show that titanium is redispersed on the support after decomposition of the oxy-hydroxide. Thus a stable amorphous titanium oxide layer is formed at the surface of y-Al203 , in agreement with TEM data.
1066 UV-visible spectra have shown that titanium is six-coordinated and partially polymerized. We have attempted to evaluate the polymerization degree of superficial titanium by plotting the position of the CT band as a function of the number of titanium atoms in the second coordination sphere of titanium (CN2) in known compounds (benitoite (CN 2 - 0) [13], anatase (CN 2 = 8), and rutile (CN 2 = 10), at respectively 225 nm, 316 and 330 nm). These points are located on a straight line, shown in Figure 8. 340 320 '~
/
300
/
Y
280
2402200
2
4
6
8
I0
CN 2 (Ti) Figure 8. Position of the ligand-to-metal charge transfer (CT) band as a function of the number of titanium atoms in the second coordination sphere of titanium (CN2)
(a)
(b)
Figure 9. Structural model of octahedral titanium oxide monolayer; a) and b) cyclic chain, c) infinite chain.
1067 Applying this correlation (Figure 8) to the UV-visible data, it is possible to predict a small polymerization degree (CN2 ~ 4) for dried TiOx/Al203 samples. We propose that the polymeric units [Ti(OR)4]n in equilibrium in solution are grafted and hydrolyzed without strong modification. As regards the UV-visible data performed on supported samples after calcination at 773 K, Figure 8 indicates a possible value of 2 for CN2. This number supports a linear infinite or cyclic chain of edge-sharing TiO 6 octahedra, as shown in Figure 9.
REFERENCES
1. M.J. D'Aniello Jr., D.R. Monroe, C.J. Carr, and M.H. Krueger, J. Catal. 109 (1988) 407422. 2. D. Kalakkad, A.K. Datye, and H. Robota, Appl. Catal. B: Environmental, 1 (1992) 191219. 3. R.T.K. Baker, E.B. Prestridge, and R.L. Garten, J. Catal., 59 (1979) 293-302 and 56 (1979) 390-406. 4. N.S. Resende, L.A.H. Terrones, M. Schmal, and J.-G. Eon, "Proceedings, 2th Interamerican Congress of Electron Microscopy, Cancun, Mexico, 1993". 5. H.H. Kung, in "Transition Metal Oxide: Surface Chemistry and Catalysis", Elsevier, Amsterdam, 1989. 6. L. Dixit, D.L. Gerrard, and H. Bowley, Appl. Spectrosc. Rev., 22 (1986) 189. 7. K. Asakura and Y. Iwasawa, J. Phys. Chem., 95 (1991) 1711-1716. 8. I.E. Wachs, G. Deo, D.S. Kim, M.A. Vuurman, and H. Hu, in "Proceedings, 10th International Congress on Catalysis, Budapest., 1992", 72-74. 9. M. Foumier, C. Louis, M. Che, P. Chaquin, and D. Masure, J. Catal., 119, 400-414 (1989) and J. Catal., 119 (1989) 415-425. 10. M.A. Vuurman, and I.E.Wachs, J.Phys.Chem. 96 (1992) 5008-5016. 11. A.F. Wells, in "Structural Inorganic Chemistry", 5th ed., chap. 12, Oxford Univ. Press, London, 1984. 12. W.R. Russo and W.H. Nelson, J.A.C.S., 92:6 (1970) 1521-1526. 13. T. Blasco, M.A. Camblor, A. Corma, and J. P&ez-Pariente, J.A.C.S., 115:25 (1993) 11806-11813.
This Page Intentionally Left Blank
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
1069
A l u m i n a w a s h c o a t i n g and metal deposition of ceramic monoliths Xu Xiaoding, H. Vonk, A. Cybulski and J.A. Moulijn Delft University of Technology, STM/CPT, Julianalaan 136, 2628 BL Delft, The Netherlands, fax. 31 15-784452 1. INTRODUCTION Monolithic catalysts are interesting and promising catalysts [1-3]. They are widely applied in exhaust gas catalysis, e.g. three-way catalysts for the treatment of exhaust gases from gasoline (or diesel) engines and more recently also in hydrogenation processes [3-5]. A fascinating development are rotating monolith reactors e.g. for heatexchange or cleaning of flue gases [3]. Monolithic reactors can be advantageous, compared with the conventional catalysts. The main advantages are a lower pressure drop, possibility for a high flow rate of the reactants, high reproducibility of pressure drop and mass/heat transfer characteristics, high surface-to-volume ratio, short diffusion length, and associated with them, a potentially high selectivity, and easy scaling-up [3]. There are two types of monoliths: metallic and ceramic ones. In general, monolithic catalysts are prepared by washcoating of a low specific surface-area ceramic monolithic carder by the sol-gel method, followed by impregnation of the washcoated monolith with a solution containing the precursor of the active phase and calcination [1]. Among the monolithic carriers, cordierite is one of the most commonly used and can be used at high temperatures. Alumina is most often used as material for the washcoating layer. Therefore, alumina washcoating on a cordierite monolith has been chosen as the subject of the present study. Since the washcoating layer must be able to resist thermal shock in many of its applications, the attachment of the washcoating layer is an important quality criterion. Moreover, the washcoating layer should be porous with a suitable pore structure. Furthermore, the amount of the washcoated material should be substantial in order to be able to load sufficient amount of active phase to the support. We studied the washcoating of alumina on cordierite monoliths using the sol-gel method. Three types of different aluminium sols were used in the washcoating; the first starting from pseudoboehmite, A10(OH).xH20, and an aqueous solution of urea and HNO3 according to Duisterwinkel ('SOL-1') [6], the second from the hydrolysis of A1C13 and aluminium powder ('SOL-2') [5] and the third from the hydrolysis of tri-secbutoxide aluminium ('SOL-3') [7-9]. Various parameters for the preparation of the sols, for the calcination of the washcoated monoliths and the methods of washcoating were studied. Moreover, for SOL-2, an aqueous solution of hexamethylenetetramine (HMT) or urea was added to accelerate the gelation of the sol and/or to increase the porosity of the alumina. These two sols are called SOL-2H and SOL-2U.
1070 Loading procedures of metal to the alumina-washcoated cordierite monoliths was studied by using nickel as the metal. It was concluded that simple adsorption method using aqueous solutions of nickel acetate or nitrate leads to unsatisfactory distribution of nickel on the monolith. Homogeneous nickel distribution on the washcoated alumina layer can be" achieved by using the deposition precipitation method developed by Geus and his coworkers [ 10]. 2. EXPERIMENTAL
2.1. Preparation of sols Various aluminium sols were prepared for the washcoating of cordierite monolith. SOL-1 was made according to Duisterwinkel [6] except for that gelation by NH3 was omitted. HNOa (0.195 M, p.a. Baker) and distilled water were mixed with 13.5 g urea (p.a. Baker) in a Janke & Kunkel mixer (500 ml) (RW20-DZM-p4 at 140 rpm). The total volume of the solution was 250 ml. Pseudoboehmite (A10(OH).xH20, x = 0.37, PURAL SB-70 from CONDEA Chemic) was added slowly to the solution at a high shear rate (Ultra-Turrax T 25 shear mixer fitted with a dispersing head S25N10G at 135 rpm). After all the pseudoboehmite was added the suspension was stirred for another 10 min at 13500 rpm. Then, it was aged by stirring at 140 rpm for different period. The amounts of pseudoboehmite were respectively 75, 100 and 125 g. The sols obtained are called SOL-1A, SOL-1B and SOL-1C, respectively. In some cases, 5 % aqueous sol/ltion of ammonium hydroxide (Baker) was used to gelate the A1 sol. SOL-2 was prepared as described in [5]. To 0.25 mole of A1C13.6H20 (Baker Anal.), 150 ml distilled water was added. After the salt was dissolved, 30 g aluminium granulars (Aldrich Chemic, +99 %, 40 mesh) was added in two portions slowly under vigorous stirring. It was heated to 363 K and maintained at that temperature until all the aluminium was dissolved. Afterwards it was filtered and the resulting light yellow sol is denoted as SOL-2. SOL-2 was used without any additive or by adding an aqueous solution of hexamethyltetramine (HMT, Merck, for synthesis) or urea (Baker, Analysed) for washcoating. These two sols are called SOL-2H or SOL-2U, respectively. SOL-3 was prepared by hydrolysis of tri-sec-butoxide aluminium (Merck, for synthesis, - 97 %) in water and HNO3 at 353-368 K (Molar ratio of AI:H20:HNO3 = 1:100:0.07) as described by Bugosh and Yoldas [7-9]. The sol after hydrolysis was concentrated to various degrees and used for washcoating.
2.2. Washcoating of monoliths The washcoating was carried out using the dipping method. The monolith block (cordierite monolith, 400 cells/in 2, from Degussa) was dried at 353 K before washcoating and dipped into the aluminium sol so that the monolith block was covered completely. After one hour, the remaining sol in the channels was removed and the washcoated monolith block was dried at room temperature until gelation had taken place. Then, it was calcined in an oven at a heating rate of 0.6 K/min up to 823 K and maintained at 823 K for 1 h, if it is not otherwise mentioned.
1071 2.3. Ni loading on alumina and alumina washcoated monolith Aqueous solutions of NiAc2.4H20 (Aldrich Chemie, 98 %) and Ni(NO3)2.6H20 (Merck, pro Analyse) were used to load nickel on an alumina-washcoated monolith (Degussa). Aqueous solution of 0.617 or 8.2 M nickel nitrate or 0.617 M nickel acetate were used in the adsorption method. It appears that all the adsorption methods result in an inhomogeneous distribution of nickel species on the alumina. The weight increases were, respectively, 1.49 %, 8.29 % and 2.45 % (means of two experiments) for catalysts using the three salt solutions. Later on, deposition precipitation methods were applied [10,11]. Dried monolith block was dipped in an aqueous solution of nickel nitrate (0.88 M) and urea (0.95 M). After heating at 363 K with air bubbling through the solution for ca. 5 h, the pH value increased from 5.52 to 6.09 and the block was drained of the solution. After drying and calcination at 10 K/rain up to 723 K for 2 h, an averaged weight increase of 8.69 % was recorded. The monolithic catalyst was homogeneously coloured. Deposition methods using the decomposition of nickel-ammonia complex by CO2 with or without the addition of 2 M NaOH solution as described in [11] resulted in a weight increase of respectively 1.81 (without NaOH) and 8.83 % (with NaOH). This method was not optimal since the precipitation of Ni species outside the monolith was observed and the deposited nickel species tends to separate from the monolith. 3. CHARACTERIZATIONS The viscosity was measured using a Contraves concentric cylindric coneviscometer at 293 K. The shear rate increases from 0 to 780 rpm in 10 rain. The viscosity at the highest shear rate (~| at 780 rpm) and at the lowest shear rate (T0, at 0 rpm) were calculated from the slope of the curve recorded (shear stress divided by shear rate). Specific surface areas of the washcoated monoliths were determined by nitrogen or krypton physisorption at 77 K using a Digisorb 2600 equipment from Micromeritics and the BET method. The weight increase due to washcoating was obtained by weighing the monolith before washcoating after drying and after washcoating and calcination. The value of SB~T for washcoated alumina (S~) was calculated from the amount of alumina washcoated and the SaET of the washcoated monolith (Smo~o.). The amount of nickel loading was calculated from the weight of the washcoated monolith before and after nickel loading and calcination, assuming that the weight increase is caused by that of NiO. Powder X-ray diffraction (XRD) patterns were measured in a Guinier-De Wolff camera (Enraf-Nonius, Mark II). Powder High Temperature-XRD (HT-XRD) measurements were performed on sol-derived aluminium-gels in a Nonius GuinierLenne camera in a flow of nitrogen (10 cm3/min) using a temperature programme. The sample was heated at 10 K/rain to 673 K and kept at 673 K for 2 h. Then it was heated at 1 K/rain to 1173 K and kept at the temperature for 2 h before cooling to room temperature. Transmission Electron Microscopy (TEM) measurements were performed using
1072 a Philips E400 electron microscope. The ground samples were dispersed in methanol in an ultrasonic bath. Drops of the suspension were dosed on a copper grid. The dried grid was measured in the TEM.
4. RESULTS AND DISCUSSION
4.1. Properties of aluminium-sols and aluminas thereof The viscosities of various aluminium-sols were measured which are a function of time, aluminium concentration, temperature and type of sol. Table 1 shows results of the sols and aluminas derived from them. Table 1 Properties of aluminium-sols and washcoated monoliths using them SOL
To max
~/o min
'q, max
~/, min
[AI] max
g/l min
S,no
m2/g
S,~
m2/g
-
1A
9.85
-
34.86
-
-
-
16.3
-
226.3
1B"
20.16
-
54.66
-
-
-
39.3
43.1
295.4
150.6
1C"
242.8
-
139.5
-
-
-
34.0
-
172.0
-
2
11.94
5.66
24.79
11.94
118.1
47.02
0.6
1.6
3.9
19.9
2H
12.11
7.92
16.64
11.61
93.01
66.44
8.34
9.76
159.7
154.7
2U
7.75
9.25
17.66
22.47
58.96
58.96
8.1
-
103.2
-
3
10.19
5.21
20.83
7.24
38.57
13.5
9.38
-
268.2
-
7/0 and ~/** in mPa.s. Smono ans Salu values are specific surface area of washcoated monolith and washcoated alumina, respectively. The monolith used was unwashcoated cordierite. " Inhomogeneous washcoating layer resulted.
The viscosities of the sols vary with the aluminium concentration in the sol and the time of ageing of the sol. This is illustrated in Figs. 1.a (SOL-3) and 1.b (SOL-1A). It can be seen that the viscosity increases with the ageing time (due to the on-going process of condensation) and concentration of aluminium ([All) in the sol. At too high an [All, the viscosity becomes too high; the sol is unsuited to use in the washcoating. 4.2. Aiumina-washeoating All the sols mentioned above were used in washcoating the monolith. The conditions for drying and calcination were studied. It was found that in order to obtain a washcoat layer with high thermal stability and good attachment to the surface of the monolith, it is better to dry the washcoated monolith slowly, e.g. under ambient conditions. Moreover, a high heating rate during calcination usually leads to fractured washcoat layer. Therefore, a low heating rate, typically 0.6 or 1 K/min, was used later in our work.
1073 42o
y
40 o
/
Q
/ !
M
/ /
20O
//
210 /
140
/
B
o
o
0
o
m
.
,'
i+ te
/
v
12
o
V
~
.
m 4 0
I~
0.0
0.8
'-'v'-
"1
1,0
- I
1.0
I
I
2.0
|.0
0
0
AI mol/I
I
I
'
I
J
go
1110
270
880
460
Ageing time/min
Figure 1. The viscosity of sols. a. as a function of aluminium content (SOL-3) and b. as a function of ageing time (SOL-1A). 70 (v) and 7** (El) are the viscosity at shear rate of 0 and 780 rpm, respectively. Table 2 shows the results of alumina washcoating using various sols. Table 2 Comparison of various aluminium sols and washcoated aluminas thereof A1 Sol
1A
2
2U
2H"
2H b
3
AW/Wo % Wm % Sm,,o. m2/g Sin. m2/g quality~ stability d
7.07 6.56 16.4 226.4 + hrs
11.94 10.67 1.6 10.9 +++ months
8.52 7.85 8.1 103.2 ++ month
5.6 5.3 9.1 171.6 ++ hrs
5.5 5.2 6.9 139.6 ++ hrs
3.37 3.26 9.4 268.3 + min/hrs
9Own results. b From literature [5]. ~ The adhesion of the washcoat layer to the monolith. a The time that the sol is not gelated.
1074 Here, results with SOL-1B and -1C are not presented since they lead to either a lower weight percentage of alumina washcoated or to a worse quality of the washcoat layer. Fig. 2 shows the BET surface areas of alumina, calcined at various temperatures and washcoated aluminas derived from SOL-1A. It appears that the SBpr values of washcoated aluminas are not substantially different from those of sol-derived bulk alumina. m
2OO
13
160
0 13 13
100
O IlO0
i
I 7'00
=
i ~
,
| 1100
rl ,
i 1800
T=,h,/~
Figure 2. The BET surface area of alumina derived from SOL-1A as a function of calcination temperature. O" alumina. I1" washcoated aluminas.
From Table 2, it is obvious that SOL-2 leads to a high percentage of alumina washcoated and a good quality of the washcoated alumina layer on the monolith. There was no cracking observed in the washcoating layer, whereas this is not the case for washcoated "alumina layer derived from using SOL-1A or SOL-3. Unfortunately, the alumina washcoat layer from SOL-2 is hardly porous, which excluded it from using as a good catalyst carder. On the other hand, washcoated aluminas from SOL-2H or SOL2U represent a compromise. Though the adhesion of the alumina to the monolith is slightly worse than that using SOL-2, the aluminas are porous and the weight increases due to washcoated alumina are reasonable. Table 3 compares the results of multiple washcoating using SOL-2U or SOL-2H. ' It appears that our own results using SOL-2H are comparable to those of literature [5]. The amount of alumina washcoated can be further increased by multiple washcoating or by using a more concentrated A1 sol. Moreover, washcoating using SOL-2U appears to be a good choice, lit leads to a higher weight of washcoated alumina after one or three times of washcoating (8.40 % versus 5.5 and 17.5 % versus 14.2 %,
1075 respectively). Furthermore, due to the high stability of SOL-2U (it stayed as a sol even after one month of preparation), it can be conveniently used in washcoating. Table 3 Results of weight increase by multiple washcoating AI sol
2U ~
2U b
2U ~
2U a
2U ~
2H r
2H
2.74 2.94 3.04 8.9 3.48 3.60
3.74 2.76 3.06 9.39 3.67 3.14
6.19 5.31 5.08 17.50 3.60 3.60
6.92 8.40 5.47 6.04 4.58 17.453.51 3.24 3.51 3.24
5.5 4.0 3.0 14.2 3.00 -
5.6 -
zx W/Wo~ lsttime 2nd time 3rd time overall pH g pH h
Own data which were averaged from at least two samples. ' Volume ratio of 4.8 M urea solution/SOL-2 = 1 : 1 (1.00). b Volume ratio of 4.8 M urea solution/SOL-2 = 5 : 7 (0.714). ~ Volume ratio of 6.9 M urea solution/SOL-2 = 1 : 3 (0.333). d Volume ratio of 9.8 M urea solution/SOL-2 = 1 : 3.1 (0.323). Volume ratio of 9.8 M urea solution/SOL-2 = 1 : 6.14 (0.163). f Data from [5]. z. Before washcoating, h. After washcoating.
From XRD and HT-XRD data, it appears that the gels of SOL-1A and SOL-3 are 3,-bochmite or pseudoboehmite (JCPDS file no. 21-1307) and possibly a small amount of tohdite, 5A1203.H20 (JCPDS file no. 22-1119). After calcination the solids from SOL-1, SOL-2H and SOL-3 all contain 3~- and/or 8-A1203 (JCPDS file no. 10-425 and 16-394). The alumina (ex-SOL-3) is less crystallized in accordance with its higher surface area per gram of alumina and with the observation in Table 4.
1076 TEM photos of six samples were taken. The results are summarized in Table 4. Table 4 TEM results of aluminas from various sols SOL
Dp/nm
D.w/nm
Comments
1A~ 1A 1B
0.8-1.0 3.6-12.2
2.3-9.5 3.6-7.9 25.6-38.9
1C 2H
1.8-7.1 2.9-6.5
29.4-84.1 23.5-124
3
1.8-4.1
Homogeneous, layered structure Aggregates of particles Layered structure with needles of 1.1-4.1 nm thickness and length of 5.9-20 nm Aggregates of particles. Needle or layered structure Aggregates of particles. Needles (thickness 1.5-3 nm) No aggregates. Homogeneous
All aluminas were calcined at 823 K for lh. Dp is the diameters of the primary particles. D~r is the diameter of the aggregates. "5 % NH3 gelated. It appears that the sizes of alumina aggregates increases from SOL-1A to SOL1C, viz. with the concentration of aluminium in the sol. The gelation by 5 % NI-I4OH interrupted the condensation process, leading to small aggregates. 4.3. Ni-ioading The nickel catalysts were calcined at 723 K for 2 h. It is supposed that the nickel salt (either acetate or nitrate) is decomposed into NiO. It appears that the NiO loading decreases in the following order of the salt solutions: 8.2 M Ni(NO3)2 > > 0.617 M NiAca > 0.617 M Ni(NO3)2 for catalysts prepared by the adsorption method. Moreover, it was observed that after dipping the alumina-washcoated monolith block into nickel salt solution and draining, the colour of the monolith blocks or alumina pellets was homogeneous. However, after calcination, the nickel distribution became inhomogeneous. This inhomogeneity of nickel distribution is tentatively attributed to either the gravity of the solution leading to a concentration of the nickel species at the bottom of the monolith; or to the migration of the nickel salts which eventually leads to a non-homogeneous distribution of nickel oxide. Note that the melting point and boiling point of nickel nitrate are 56.7 ~ and 136.7 ~ respectively and the boiling point of nickel acetate is only 16.6 ~ [12]. The low boiling points of these salts certainly facilitated the migration of nickel species during the calcination. Moreover, if the monolith blocks were allowed to dry vertically, the colour at the bottom of the monolith was, as a rule, darker than at the top. It was found that when the Ni-loaded monolith was dried in a microwave oven this phemonenon of inhomogeneity of nickel can be circumvented or improved.
1077 Deposition precipitation [10,11] was later used to deposit nickel onto the alumina-washcoated monolith blocks. It was found that the method as described by [10] using aqueous solution of nickel nitrate and urea was suitable to obtain a homogeneous deposition of nickel on the washcoated alumina layer of monolith. However, due to the slow rate of urea decomposition under heating a long reaction time, typically, at least 5 h, is necessary. The equation is as follows: NH2CONH2 + 3 H20--> 2 NI-I4+ + CO2 + 2 O H
[1]
There are several possible explanations for the fact that the deposition precipitation method leads to a homogeneous distribution of nickel. In the deposition precipitation the nickel is deposited as nickel hydroxide, which is anchored to the surface via reaction with surface OH groups: Ni(OH) + + A1OH + O H - - - > A1ONi(OH) + 1-120
[2]
Due to the relative high melting and boiling points of nickel hydroxide and/or oxide they are not volatile and not redistributed during the calcination as nickel nitrate or acetate does. Moreover, due to the decomposition of urea and the equilibria of NH3 and CO2 with water, the pH value of the solution is raised to a high and fixed value (pH = 6.1), the pH value of the solution is buffered which may facilitate the homogeneous anchoring of nickel hydroxide. 5. CONCLUSIONS Various aluminium sols were prepared and their properties studied. They were used in the washcoating of cordierite monolith. It was found that the viscosity of the sol increases with the aluminium content in the sol and the ageing time of the sol when the temperature and type of sols are the same. The amount of alumina washcoated increases, up to a certain extent, with the aluminium concentration in the aluminium sol. At too high an aluminium concentration or viscosity the sol is no longer suitable for use in washcoating. Although SOL-2 (from hydrolysis of A1C13 and aluminium) is the most stable sol studied and it leads to a high alumina loading on the monolith, it is not a suitable sol for washcoating due to the low porosity of the alumina washcoated. SOL-2H and SOL-2U (SOL-2 with the addition of HMT or urea solution, respectively) are both suitable for washcoating the monolith. Considering the high stability of the SOL-2U its use in washcoating is preferred. Deposition precipitation method was found suitable to obtain a homogeneous distribution of nickel on monolith washcoated with alumina. Drying in a microwave oven is helpful in maintaining a homogeneous metal dispersion over the monolith.
10'/8 REFERENCES 1. J.P. DeLuca and L.E. Campbell, in J.J. Burton and R.L. Garten (eds.), Advanced Materials in Catalysis, Academic Press, London, 1977, pp. 293-324. 2. S. Irandonst and B. Anderson, Catal. Rev. - Sci. Eng., 30(3) (1988) 341. 3. A. Cybulski and J.A. Moulijn, Monoliths in Heterogeneous Catalysis, Catal. Rev. Sci. Eng., in press. 4. C.J. Pereira, J.E. Kubsh and L.L. Hegedus, Chem. Eng. Sci., 43 (8) (1988) 2087. 5. R.J. Peterson, "Hydrogenation Catalysis", in Chemical Technology Review, 1st ed., 94 (1977) 27. 6. A. Duisterwinkel, Clean Coal Combustion with In Situ Impregnated Sol-Gel Sorbent, Ph.D. Thesis, TU Delft, 1991. 7. J. Bugosh et al., Ind. Eng. Chem., Prod. Res. Devel., 1(3) (1962) 157. 8. B.E. Joldas, J. Mat. Sci., 10 (1985) 1856. 9. C.J. Brinker and G.W. Scherer (eds.), Sol-Gel Science, The Physics and Chemistry Sol-Gel Processing, 2nd edi., Academic Press, New York, 1990. 10. L.M. Knijff, R.H. Bolt, R. van Yperen, A.J. van Dillen and J.W. Geus, in B.Delmon, P. Grange, P. Jacobs and G. Poncelet (eds.), Preparation of Catalysts V, Elsevier, Amsterdam, 1991, pp. 166-174. 11. H. Schaper, Delevopment and Charaterization of a Thermostable Nickel-Alumina Methanation Catalyst, Ph.D. Thesis, TU Delft, 1984. 12. R.C. Weast (edi.), Handbook of Chemistry and Physics, 54th Edi., CRC Press, B113-4.
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
1079
Cr-free iron-catalysts for water-gas shift reaction J. Ladebeck and K.Kochloefl Catalytic Laboratory, SOD-CHEMIE A.G., Waldheimer Str. 13, 83052 Bruckmiihl, Germany
Fresh as well as spent chrome promoted iron catalysts used in the industrial high temperature water-gas shift reaction are containing usually 1-2 wt.-% of Cr 6+ which makes their handing difficult with respect to its hazardous effect on human organism. Therefore it was tried to replace Cr203 b y the combination of A1203 and oxides like ZrO2 or MnO2 or La203 or CeO2. The highest activity and a sufficient thermoresistance provided iron catalyst containing A1203 and CeO2, which was found to be superior in activity to a commercial high temperature shift catalyst.
1. INTRODUCTION
The water-gas shift reaction (eq.1) represents a very important step in some industrial catalytic processes like production of hydrogen, ammonia and other bulk chemicals. The basic informations concerning water-gas shift reaction and catalysts used, were reviewed by D.S.Newsome [1] and M.V. Twigg [2]. CO + HaO = COE + HE
(1)
In the HTS (high temperature water-gas shift) chromium promoted iron catalysts are applied in the industry [3]. The Cr203-content in commercial catalysts amounts to 8 - 12 wt.-%. However, fresh as well as spent catalysts usually contain 1 2 wt.-% of Cr 6+ which makes their handling and finally their deposition quite difficult with respect to its dangerous effect on human organism. Therefore attemps were initiated in the past [4,5] to replace Cr in the iron HTS-catalysts. As the results of these studies indicate Cr-free iron catalysts did not fulfill all placed requirements regarding activity, selectivity and thermoresistance. The effect of chrome in iron water-gas shift catalysts was investigated in the past by various authors e.g. [6,7]. As well known C r 3§ possesses ionic radius of 0.63 A which is very close to Fe 3+ (0.64) and therefore its incorporation in the Fe203-1attice is very easy. From this point of view e.g Mn 3+ (0,66A) could substitute chrome.
1080 Moreover the addition of ZrO2 or La203 or CeO2 was proposed to improve the thermoresistance of Cr-free iron catalysts. To achieve sufficient BET-surface area and to decrease the bulk density of designed catalyst, A1203 was incorporated in the system. Finally Cu was added like an additional activity enhancer as in the case of some commercial HTS-catalyst. However, we did not consider in our design such elements which will make the price of the catalyst to high that it could not compete with existent commercial HTS-catalysts. The composition and the preparation of newly developed catalysts is covered by the patent application [8]. 2. EXPERIMENTAL
2.1 Catalyst preparation The corresponding catalyst precursors (oxidic state) were prepared by coprecipitation of watery metal sulfates or nitrates with sodium hydroxides at 60~ in such a way that the sodium hydroxide was placed at first in the precipitation vessel and the metal sulfates or nitrates were pumped in under vigorous agitation. The precipitation was finished when pH reached value of 7,5. During 4h aging at 60~ the oxygen stream was introduced in the slurry. After filtration and washing (Na-content <1000 ppm, S-content < 30 ppm) the filtercake was dried at 180~ during 12h and finally calcined at 600~ (12h) and shaped after addition of 2 wt.-% of graphite in to 4.5x4.5 mm pellets.
2.2 Catalyst characterization Prepared catalyst precursors were analyzed and characterized by conventional physical methods. Their nominal chemical composition before shaping is presented in Table 1. Table 2 is showing some characteristic physical data of catalyst precursors in the powder as well as in the pellet form. It has been found that an important characteristic is the total crystallinity and the ratio between ct-Fe203 (hematite; d = 1,84 A) and y-Fe203 ( maghemite ; d = 2,95 A), determined from the XRD-pattern. Table 1 Nominal chemical composition of catalyst precursors (powder) Catalyst Chem.composition
C-1
C-2
C-3
C-4
Fe203
90
90
90
90
Al203 ZrO2 MllO2
5
5
5
5
(wt.-%)
2,5
La203 ~CeO2
CuO
2,5
2,5 2,5
2,5 2,5
2,5 2,5
1081 Table 2 Some physical data of catalyst precursors (oxidic state) Catalyst Physical data (powder) BET-s.a. (m2/g) Total crystaninity (%) Hemat./Maghemratio (%/%) (pellets) Bulk density (g/l) Pore volume (cm3/g) Pore size distribution
C-0
C- 1
C-2
C-3
C-4
36 97
46 94
38 98
40 95
38 98
100/0
93/7
100/0
95/5
100/0
950 0,40
1300 0,22
1250 0,25
1360 0,21
1300 0,24
0,6 5,0 92,7 1,7
0,1 2,4 86,4 11,1
0,8 4,9 91,5 2,8
0,1 2,7 90,4 6,8
0,0 4,9
(%)
> 1750 nm 1750- 80 80 - 14 14 - 7,5
86,6 8,5
Catalyst C-0 represents a commercial Cr-promoted iron catalyst for HTS produced by SUD-CHEMIE A.G., Munich, Germany, denoted G-3 C. Pore volume and pore size distribution were measured by mercury intrusion.
2.3 Activity measurements Catalyst precursors were submitted to a standard activity and thermal aging test using bench scale laboratory apparatus equiped with electrically heated tubular reactor. Gaseous reaction products were analyzed by gas chromatograph connected directly with the testing apparatus. The reaction conditions were as follows: total pressure 20 bar, temperature 3 50 and 370 ~ GHSV = 7000 1/hlcat., ( dry feed). H20/CO-mol.ratio = 10, feed composition - CO (10 vol.-%), H2 (90 vol.-%). Catalyst precursors were at first activated at atmospheric pressure with mentioned feed at 370~ during 1 h. The activity measurement lasted 12h, afterwards the tested catalyst was submitted to 12h thermal aging at 500~ (total pressure, GHSV and feed composition as during the activity measurement) and the activity test was repeated again. Usuall 4 aging cycles were carried out for the evaluation of the catalyst thermoresistance. The catalyst activity was expressed by the reaction rate constant k of the pseudo first order rate equation (2). The commercial catalyst denoted G-3C (C-0) served as a standard. Obtained data are given in Table 3. k = GHSV In ( Xeq / ( Xeq - X)), where Xeq is the equilibrium CO conversion, x is the measured conversion (mol).
(2)
1082 Table 3 Relative activities (k~l.) of developed catalysts in the water-gas shift reaction Catalyst
C-O
C-1
C-2
C-3
C-4
T = 350~ Initial activity Aging cycle 1 2 3 4
1,00 0,91 0,88 0,86 0,85
3,5 2,3 1,6 1,2 1,1
3,6 2,0 1,4 1,0 0,9
2,6 1,6 1,1 0,7 0,5
3,8 3,2 2,6 2,2 2,1
T = 370~ Initial activity Aging cycle 1 2 3 4
1,00 0,94 0,90 0,87 0,86
3,2 1,9 1,3 1,0 0,9
3,3 1,5 1,1 0,9 0,8
2,4 1,5 1,0 0,4 0,5
3,5 2,7 2,1 1,8 1,8
For the standard catalyst C-0, k (350oC) = 7230 h 1 and k(370oC) found.These figures were taken as the basis for the krel. calculation.
=
10420 h "1 were
3. RESULTS AND DISCUSSION As the data summarized in the Table 3 indicate, all designed Cr-free iron catalysts exhibit higher initial activity at 350~ as well as 370~ than a standard catalyst denoted C-0. Iron catalyst containing CeO2, however, was found to be most active, followed by Mn- and Zr-containing systems. Unexpected, catalyst promoted by La203 showed the lowest activity from the developed catalysts. Table 4 Catalysts thermoresistance expressed as activity retention (%) Catalyst
C-O
C- 1
C-2
C-3
C-4
T = 350~ Activity retention (%)
85
31
25
15
55
T = 370~ Activity retention (%)
86
28
24
17
51
1083 After four aging cycles Cr-free catalysts, with exception of C-3 have been still more active than the standard (C-0) at both testing temperatures. The activity figures, especially for Ce-promoted catalyst (C-4) are showing the stabilization of the activity level after four aging cycles. The thermoresistance of individual catalysts was expressed by the activity retention (%) after four aging cycles as can be seen from Table 4. As this table demonstrates the Mn-promoted iron catalyst provided quite a poor thermoresistence. The idea that Mn 3§ which possess the ionic radius close to Fe 3+ could easy incorporate in the iron lattice as Cr 3§ and take over its function, failed. Also catalysts containing ZrO2 and La203 were found to be less thermoresistant than with Ce-promoted system which exhibited the highest thermoresistance from designed Cr-free iron catalysts. REFERENCES
1. D.S. Newsome, Catal.Rev.-Sci.Eng., 21 (1980) 275. 2. M.V. Twigg in Catalyst Handbook, 2nd edit., Wolfe, London, 1989. 3. C.L. Thomas, Catalytic Processes and Proven Catalysts, Academic Press, New York, 1970. 4. G.C. Chinchen, Euro Patent A-0 062410 (1982). 5. D.G. Rethwisch and J.A. Dumesic, Appl. Catal., 21 (1986) 97. 6. G.C. Chinchen, R.H. Logan and M.S. Spencer, Appl.Catal., 12 (1984) 89. 7. J.C. Gonzales, M.G. Gonzales, M.A. Laborde and N. Moreno, Appl. Catal., 20 (1986) 3. 8. M. Schneider, K.Kochloefl, G.Maletz, Ch.Heinisch and J.Ladebeck, Ger. pat. application P 4303715 (1993).
This Page Intentionally Left Blank
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
1085
Preparation of Rh-Co/AI203 heterogeneous catalysts using a diisocyanoligand as an integral design component M.S.W. Vong and P.A. Sermon Solids and Surfaces Research Group, Department of Chemistry, Brunel University, Uxbridge, Middlesex UB8 3PH, UK Here consideration has been given to the potential for diisocyanide ligands to complex a transition metal (TM) cation to produce a monolayer of a highly-porous polymer in which more than 95% of metal atoms are constrained in well defined and characterisable active states with stable and known metal-metal coupling, symmetry and oxidation state. Catalysts prepared in this way have shown activities and selectivity in alkene hydrogenation, whereby the molecular network provides the shape-selectivity and the guest metal ion provides, the chemical reactivity and selectivity. This will probably lead to a new range of highly active and selective TM mono- and bi-metallic catalysts and adsorbents which could be of immense technical and scientific importance. 1. I N T R O D U C T I O N In recent years much attention has focused on the production of novel catalysts, adsorbents and sensors with high reactivity and selectivity 9 Rh is a particularly interesting metal to consider in this context in that it is selective in alkene hydrogenation [1, 2], asymmetric hydrogenation [3] and CO/CO 2 hydrogenation [4]. In essence only a small fraction of Rh atoms in traditional supported catalysts are highly active [5] and their activity and selectivity are also affected by the fractal dimension of catalyst surfaces through the rates of reactants and products diffusion to and from the active sites [6]. Also immobilized and heterogenized transition metal complexes act as catalysts for homogeneous reactions and have become an area of intense catalytic interest. Others have used supported complexes, clusters [7] and polymers which are catalytically active. Molecular precursors may be one answer of particular relevance to highly dispersed Rh. The aim of this work was to use molecular receptors to produce, two- and three-dimensional networks into which a Rh reaction centre could be constrained so as to control their chemical nature and stereo environment to produce catalysts of high activity-selectivity in hydrogenation. In such an aproach to reactivity-control, bridging aryl-diisocyanide ligands (e.g. CNX-NC, where X is a benzene, diphenyl, etc group), are interesting in that (i) CN- groups chelate very effectively with metals via MdCxv)-to CN(2~) and CNfss* )to Md(x2_v2) overlap [8], producing net electron donation to Rh [9]. - (iO there is extended delocalisation in the x-y plane of the 'polymer' which is greatest when conjugation [10] is present and this produces substantial stability for the supported networks constraining the Rh +. (iii) metal-metal bonding is also allowable in the z-direction if multilayers are produced on a support (between the x-y laminae) since the sheets then stack up with metal centres aligned with weak dz2-dz2 a d Pz-Pz overlap. Thus in terms of metal-metal bonding and one-dimensional conductivity, ~ere are "similarities between the diisocyanocomplexes [11] and other one-dimensional conductors K2Pt(CN)4.Brx.3H20 [ 12].
1086 Diisocyano-Rh dimers photocatalytically decompose water [13] and diisocyano complexes catalyze hydrogenation and isomerisation of alkenes and alkynes (although they are far less active than the Wilkinsons catalyst [14]). Alkene hydrogenation is a probe reaction for such reaction centres, especially since the hydrogenation of alk-1-enes over Wilkinson's catalyst [hydrido-carbonyl tris(triphenylphosphine).Rh +] in benzene is quite selective (i.e. was 45 times faster than the cis-alk-2-ene [ 15]). Surprisingly, the active centres in such catalysts are still not entirely understood, despite extensive analysis [16]. Rh complexed with 4,4'-diisocyanobiphenyl and 1,4-diisocyanobenzene is active in hydrogenation and isomerization of 1-hexene [ 14], while Rh complexed with aliphatic amines is active in catalysis of hydrogenation of alkenes and cycloalkenes [16]. Ligands such as the 1,4-diisocyanobenzene or 1,4-phenyl-diisocyanide have been described as rigid collinear bridging ligands, which separate Rh ions by 1.2nm [ 17]. This has been used exclusively here and is denoted L (e.g. the unsupported polymer is given by [RhL2] n if we ignore any halogen and water components). The activity in hydrogenation of butenes of oxide-supported Rh/Co 1,4-diisocyanobenzene complexes is compared with that for simple impregnated catalysts. The unsupported Rh-diisocyanobiphenyl polymers are quite stable to 469K-617K in H 2 [9] but turnover frequencies in cyclohexene hydrogenation 293K were 3000 times lower than for silica-supported Rh and also decreased rapidly with use above 363K; hence the approach here to use the supported polymers. 2. EXPERIMENTAL 1,4-diisocyanobenzene (L) (Aldrich, 99% purity); [Rh(Co)2C1]2 (Aldrich; >97% purity), CoC12 (Fisons, >97% purity), A1203 (Degussa; 99% purity) and SiO 2 (Davison 923) were used.
2.1. Preparation 1,4-diisocyanobenzene undergoes a facile reaction with tetracarbonyl-bis-lxchloridedirhodium to produce a bulk tetragonal Rh-l,4-diisocyanobenzene chloride polymer with a [RhL2+.C1-.xH20] network. 2(CN-~-NC) + 1/2[Rh(CO)2C112
~. [Rh(CN-~-NC)2]+.CI'.x-H20 + 2CO
(1)
Here supported complexes were prepared by first impregnating the oxide with 5% w/w or 10% w/w ligand from a dichloromethane solution and then drying this under vacuum at room temperature. This was then allowed to exchange with a solution of [Rh(CO)2C1] 2 or a mixed solution of [Rh(CO)2C1]2 and CoC12 in ethanol (in which L was insoluble). From this the supported complex formed very quickly. After washing with fresh solvent (to remove unreacted chloride and ligand), the sample was then evacuated to dryness at room temperature.
2.2. Catalyst characterisation
AA spectroscopy was used to study the rate and extent of metal incorporation under the above conditions. Powder X-ray diffractometry (Philip PWl710), FTIR (Perkin Elmer 1710), diffuse reflectance spectroscopy (Perkin Elmer Lamda 9 uv-vis near ir photospectrometer), XPS (Kratos ES300 with A1 radiation at 1468.6 ev), ESR (Varian E3 calibrated with DPPH) and TEM (Joel Instruments CX100) were used to confirm the formation of this ordered polymeric Rh + network on the oxide surface. TPR and in-situ FTIR were also used to study the thermal stability of the supported and unsupported complexes in H 2, CO and air. The total surface area of the oxide-supported catalysts was measured by BET N2 adsorption (Carlo Erba Sorptomatic 1800) at 77K.
1087
2.3. Catalytic activity measurements
The activity of the catalysts in alkene hydrogenation was measured by passing a reactant stream (1.24kPa butene, cyclohexene or benzene, 76kPa H 2 and N 2 (balance to 101kPa)) through a catalyst sample (0.02-0.05 g) at 30 cm 3 rain "1 and 295-6K. Reaction products were analysed as a function of time by FID gas chromatography. Before reaction the sample were pretreated in hydrogen (393K) and then flushed with N 2 while cooling to the reaction temperature. 3. RESULTS
3.1. Characterisation
AA showed that when the metal to ligand ratio (M/L) added was 0.5, 99% of the Rh and 80% of the Co was taken up rapidly by the A1203-supported diisocyanobenzene (compared to 20% and 10%, respectively without L) and there was also lower extents of leaching of A13+ from the support when L was present:
Table 1 Metal concentrations in solution (ppm) measured by AA analysis .. L/Alumina
0 1 2 3 10
Alumin0t
[Rh] [Co] [AI]
[ma] [Co] [A1]
16.0 1.5 2.1 1.6 1.8
16.0 12.2 11.0 12.7 12.1
9.0 0.2 0.2 0.1 0.1
0.4 7.1 0.8 0.0 0.2
9.0 4.0 9.2 124.1 7.5 96.7 8.3 78.5 8.5 182.5
X-ray diffraction patterns obtained for the unsupported and oxide supported Rh complex corresponded to a crystalline network of tetragonal structure [14, 18] and FTIR spectroscopy indicated (see Figure 1) that the oxide supported complex is similar to the unsupported polymers [ 19] with absorption corresponding to the aromatic ring around 260 nm and the strong CN stretching frequencies at about 2120-2160 cm -1 being retained, although there is probably evidence of decrease conjugation in the oxide-supported state. Spectra of L/silica and L/alumina showed only a weaker CN absorption band indicating that the diisocyanobenzene ligand also complexed to a smaller extent with the A13+ or the Si 4+ ions on the surface of the catalyst supports. Interestly this absorption band was not observed for the 1,4-diisocyanobenzene ligand alone. The CN band of the diisocyano-network is relatively constant in oxidised or reduced atmosphere at 378K and when used in but-l-ene hydrogenation at 338K. UV-Vis diffuse reflectance spectra of the Rh/L polymer showed one intense absorption maximum at--400nm, close to that observed with Rh(CNPh)4BPh 4 monomer [20] and a board absorption band centred at 700nm-750nm, which is characteristic of weak intrachain Rh ....Rh interaction in polymers [18, 21] such as {Rh(diisocyanide)2+C1-]n. These electronic absorptions were asigned to the transition between energy levels in the A lz(dz2) bonding and A2u(p2p*) antibonding orbitals. The intensity of the above peaks decrease~l for
1088 d[H]/dt
% Transmission
b
(c) 3200
2400 1800 Wavenumber (cm -1)
Figure 1. FTIR spectroscopic properties of 3%Rh/0.6%Co/L/A120 3 (a), 10% L/A1203 (b) and A1203 (c). (*) denotes the CN stretching band of the diisocyano-network.
273
I
473
I
673
I
873
I
1073
T (K) Figure 2. TPR profile of 2%Rh/1.5%Co/ 10% L/A1203 (a), 2%Rh/L/A1203 (b) and unsupported Rh/L (c).
Figure 3. TEM micrograph of 4%Rh/L/A120 3 (x 893,000).
1273
1089 polymers in the supported state, which suggested a lower degree of metal-metal interaction (i.e. a decrease in the conjugation of the network and a resulting lower stability). The presence of Co in the complex increased the intensity of the absorption band and shifted it to longer wavbelength. Photoelectron spectroscopy revealed binding energies for Rh 3d5/2__(308.4eV) in the alumina-supported diisocyano network, which is well above the value (307.7eV) seen for Rh ~ The binding energies measured for Co 2pl/2 suggested that the Co was also in positive oxidation state. The constrained metal centres were stabilized by the extended conjugated ligand networks via electron delocalisation in the x-y plane. TG/DTA confirmed that Rh/Co/L/A120 3 was stable to at least 543K in H 2, with an exothermic reaction at only 581K. TPR profiles for the rhodium 1,4-diisocyanobenzene polymer are shown in Figure 2. The unsupported rhodium complex reduced at higher temperature (as much as 250K) than the Rh-Co/A120 3 impregnated catalyst. This was direct evidence of the stabilising effect of the polymeric network. The first reduction peak at low temperature was related to the liberation of chlorine ions as HC1 and the reduction of the rhodium centres (Rh:H = 1:4, after allowing for HC1 liberation). Hydrogen uptake at high temperature is believed to correspond to the degradation of the polymer matrix (L:H= 1:6). When the complex was supported on oxides, it became less stable and reduction occurred at lower temperature. Co incorporation increased the stability of the alumina-supported Rh network and raised the reduction temperature by 30K. The Rh-Co/L/A120 3 catalyst also showed an improved catalytic activity in alkene hydrogenation. Total surface areas of the catalysts were determined by BET N 2 adsorption at 77K and X-ray diffraction line broading (XRDLB). Table 2 Total surface areas (S) and average crystallite sizes (d) of the Rh complex catalysts* S (m2 g-l) AhO 3 (C) 100 L/A120 3 125 1%Rh/L/A120 3 144 4%Rh/L/A120 3 170 RhL 2 90 3 % Rh/0.575 % Co/L/A1203 109 2%Rh/1.15 %Co/L/A1203 107 (CoL was amorphous and showed no structure which was
d (nm)
47.9 43.3 41.2 14.9 detectable by XRD).
* deduced from BET and XRDLB at 7.8~ The Rh-1,4-diisocyanobenzene polymer was highly porous and revealed a Type II N 2 adsorption isotherm. It possessed an extensive rnesoporous structure and a zeolite-like microproous structure. The the total surface area of the non-porous A120 3 support was increased by L and metal-L complexation. Thus this increase in porosity suggested that these materials could be used as a selective adsorbents. Transmission electron micrograph (Figure 3) of the alumina supported Rh-diisocyano complex shows semicrystalline layer structure and unlike the impregnated catalyst, no Rh particles were observed.
3.2. Catalytic activity
In Figure 4 the activity in hydrogenation of butenes of this alumina-supported 1,4diisocyanobenzene-constrained Rh/Co catalyst is shown and is compared to the characteristic
1090 nmol/g cat./min.
O
60
O
O
(a)
O 40 20 A
ID (b) 800 6OO
-
~
O-," I v
9
o
~
o
""
200
0
1
I
20
I
!
40
I
I
I
60
t (min)
Figure 4. Activities of 3%Rh/0.6%Co/I.JA120 3 (a) and 3%Rh/0.6%Co/A1203(b ) in hydrogenation of but-1-ene (O), cis-but-2-ene (e) and trans-but-2-ene (0) at 295K.
b
P
Y Figure 5. Manner of adsorption of cis-but-2-ene ( ~ ) on Rh ( ~ ) (constrained by diisocyanobenzcne in the x-y plane) along the z-axis where steric control depends upon alignment of aromatic rings and this will depend upon interactionswith the oxide support. The structuregiven is also thatseen for unsupported networks.
1091 of those of the corresponding impregnated materials. F~st the rates of butene hydrogenation over both catalysts are reasonabUy stable with reaction time. Second, unlike the impregnated catalyst, the supported polymer did shows stereoselectivity with decreasing rates in the sequence: but-1-ene > cis-but-2-ene > trans-but-2-ene even though the apparent hydrogenation activity of the supported polymer was 104-105 folds lower than the conventional Rh-Co/A12Oy For but-l-ene hydrogenation there was no significant isomerisation to but-2-ene isomers for the alumina-supported polymer, but only for the impregnated catalyst (as seen previously [22]). Others have found that the cis:trans ratio in isomerisation products accompanying alkene hydrogenation was very variable [23]. 4. DISCUSSION Often homogeneous catalysts are more selective (although more subject to diffusionlimitation) than heterogeneous ones [15]. Attempts to induce higher selectivity on metal active centres in heterogeneous catalysts are important in that the products would not be subject to the diffusion limitation to the extent that homogeneous catalysts are. In addition, immobilization of homogeneous catalysts using ligands would eliminate the difficulties in separating catalysts from reactants and/or products and allow catalysts to be regenerated and in some cases, reduce rate of observed catalyst deactivation. It appears that one mode of steric and/or electronic control is via a stabilising network to constrain the metal ions [22] using pre-adsorbed bridging aryl-diisocyano ligands which (i) chelate very effectively, (ii) have a stability enhanced by extended delocalisation and conjugation in the x-y plane, and (iii) allow metal-metal bonding in bulk networks in the zdirection (between the x-y laminae). I n the unsupported state such polymers do not appear to show this selectivity. At 298K and 2 atmos H 2 the Pd 0 complex of 4,4'diisocyanobiphenyl catalysed the hydrogenation of hex-1-ene to n-hexane [9, 14] but also produced trans- and cis-hex-2-ene in the ratio 4 (as did a 10%Pd/C catalyst) by isomerisation. However, the precise product ratio varied with time in a manner reminiscent of an alkene titration [24] with isomerisation at intermediate times. It may be that with such complexes the H concentration at Pd centres changes with the approach used on but-l-ene titration was to assume that hydrogenation and isomerisation occurred on different types of site (i.e. 3M and 2M respectively) [25], where such sites have been defined [26]. Not only do such unsupported polymers show low selectivity with regard to hydrogenation over isomerisation, but they also deactivate. Thus the unsupported Rhdiisocyanobiphenyl polymers are quite stable to at least 469K in H 2 [27] but their meagre turnover frequencies in cyclohexene hydrogenation decreased rapidly with the use above 363K. It may be that while in the unsupported diisocyanobenzene polymers the benzene rings lie in the x-y plane and control the availability of the metal centre very little, in the supported polymer interactions with the support may cause the benzene tings to rotate and hinder access to the metals and induce some selectivity. Shape specificities of aromatic tings have been considered as one of the important factors governing the rate of hydrogenation of styrenebutadiene copolymers (AB & ABA block) using Wilkinson's catalyst [17] and the shape selectivity in the clathration of o-diborombenzene and o-dichlorobenzene over their meta- and para-isomers and catalysis of a two-dimensional square network material {[Cd(4,4'bipyridine) 2] (NO3)2}~ [28]. Figure 5 may hint at this with changes in conjugation at the benzene ring and changes in the CN bond strength. Here, additional work is required to increase selectivity by the introduction of alkyl groups at the 3-position on the aromatic ring to control the metal centres even more effectivitely (electronically and/or sterically)with Rh and with other transition metals, Thus
1092 one wonders what the steric control would be like in alkene hydrogenation when either (i) the alkene was larger or had a longer chain length or had bulky functional groups, or (ii) the diisocyano ligand contained a bulky side group (e.g. 2,4-diisocyanotoluene or a shorter bridge (e.g. 1,3-diisocyanopropene) [29]. These matrices can be applied to other (Group 1B and transition) metals and immobilized them upon a wide range of supports [30]. Alternatively, it may be that chemically-anchoring [31] will be valuable for incorporating an active molecular precursor (e.g. (MeO)3Si(CH2)2- 2-pyridyl..Rh) onto an oxide support. Catalyst preparations are said by some to be as much art as a science [32]. It is hoped that this approach allows more control over such critical area of catalysis. It may be that species once thought of as homogeneous catalysts (e.g. [15]) will eventually come to fruition in control of heterogeneous catalysts. 5. ACKNOWLEDGEMENTS The authors gratefully acknowledge the support of SERC (Grant No. GR~/58021) for MSWV. REFERENCES
.
3. .
"
6. .
.
9. 10. 11. 12. 13. 14. 15. 16.
J.H.A. Martens, R. Prins and D.C. Koningsberger, J. Phys. Chem. 93 (1989) 3179; R. Kieffer, A. Kiennemenn, M. Rodriguez, J.P. Hindermann and A. Deluzarche, Acta. Simp. Iberoam., Catal. 9th. 2 (1984) 1509; US patents 541,660, 4096164, 4101450, 4162262. D.E. Bergbreiter and R. Chandran, J. Amer. Chem. Sot., 109 (1987) 174. B. Bosnich (ed.) Nato ASI Series in Appld. Sci. 103E. 'Asymmetric Hydrogenation' (1986). M.M. Bhasin and G.L. O'Connor, Belgian Patent 824 (1975) 822; M.M. Bhasin, W.J. Bartley, P.C. Ellgen and T.P. Wilson, J. Catal. 54 (1978) 120; W. Liu etal., Sur. Sci. 180 (1987) 153; J.R. Shapely etal., Inorg. Chem. 21 (1982) 3295. J.B.F. Anderson, R. Burch and J.A. Cairns, J. Chem. Soc. Far. Trans I, 83 (1987) 913. P.A. Sermon, Y. Wang, M.S.W. Vong, Y. Sun and M.A.M. Luengo "Molecular Precursors and Probes for Designer Catalysts: The Quest for Complementarity" (1994). S.D. Jackson, R.B. Moyes, P.B. Wells and R. Whyman, J. Chem. Soc. Far. Trans I 83 (1987) 905; V.D. Alexiev, N. Binsted, J. Evans, G.N. Greaves and R.J. Proce, J. Chem. Soc. Chem. Comm. (1987) 396; M. Capka etal., RKCL, 31 (1986) 41; I.M. Saez etal, J. Chem. Soc. Chem. Comm. (1987) 361; P. Escaffre etal., J. Chem. Soc. Chem. Comm (1987) 146. K. Krogmann, Angew Chem. 8 (1969) 35. S.A. Lawrence, P.A. Sermon and I. Feinstein-Jaffe, J. Molec. Catal. 51 (1987) 117. I.L. Finar, Organic Chemistry, 6 Ed. Vol.1, Longman (1973), Ch. 20. S.A. Lawrence, K.A.K. Lott, P.A. Sermon. E.L. Short and I. Feinstein-Jaffe, Polyhedron 6 (1987) 2027. A.E. UnderhiU and D.M. Watkins, Chem. Soc. Rev., 9 (1980) 429; A.E. Underhill, Philo. Trans. Roy. Soc., 314A (1985) 125. K.R. Mann, N.S. Lewis, V.M. Miskowski, D.K. Erwin, G.S. Hammond and H.B. Gray, J. Amer. Chem. Soc. 99 (1977) 5525. A. Efraty and I. Feinstein-Jaffe, Inorg. Chem., 21 (1982) 3115; I' Feinstein-Jaffe and A. Efraty, J. Mol. Catal., 35 (1986) 285; 40 (1987) 1. C.O. Connor and G. Wilkinson, J.Chem. Soc. A (1968) 2665. K. K. Turisbekova, L.P. Shuikina, O.P. Parenago and V.M. Frolov, Kinetika i kataliz,
1093
17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32.
29, (1988) 1023. X. Guo, .J. Scott and G.L. Rempel, J. Mol. Catal., 72 193 (1992); M. Carvalho, L.F. Wieserman and D.M. Hercules, Appl. Spectros. 36 (1982) 290. A. Efraty, I. Feinstein, F. Frolow and L. Wackerle, J. Am. Chem. Soc., 102 (1980) 6343. A. Efraty, I. Feinstein, L. Wackerle and A. Goldman, J. Org. Chem., 45 (1980) 4059. K.R. Mann, N.S. Lewis, R.M. Williams, H.B. Gray and J.G. Gordon, Inorg. Chem., 17 (1978) 828. A. Efraty, I. Feinstein, F. Frolow and A. Goldman, J. Chem. Soc. Chem. Comm., (1980) 864. P.N. Rylander, H. Greenfield and R.L. Augustine, Marcel Dekker, Catalysis of Organic Reactions (1988); J.P. Boitiaux, J. Cosyns and E. Robert, Appl. Catal., 35 (1987) 193. R.L. Banks and G.C. Bailey, I. Eng. Chem. Prod. Res. Dev., 3 (1964) 170. P.A. Sermon and G.C. Bond, J. Chem. Soc. Fara. Trans. I, 72 (1976) 745. R.L. Augustine, M.M. Thompson and M.A. Doran, J. Chem. Soc. Chem. Comm. (1987) 1173. S. Siefel, J Outlaw and J. Garti, J. Catal., 52 (1978) 102. H. Bonnemann, W. Brijorix, R. Brinkmann, E. Dinjus, R. Fretzen, T. JouBen and B. Korall, J. Mol. Catal., 74 (1992) 323. M. Fujita, Y.J. Kwon, S. Washizu and K. Ogura, J. Am. Chem. Soc., 116 (1994) 1151. F. Bonati and L. Malatesta, Isocyanide Complexes of Metals, Wiley, Interscience. P.A. Sermon, M.S.W. Vong and A. Jones, Preparation of supported metal catalysts via complexation with oxide-supported diisocyano networks, (in preparation). M. Capka, M. Czakoova, J. Hjortkjaer and U. Schubert, React. Kinet. Catal. Lett., 50 (1-2) (1993) 71. TechAlert, Dept. Trade & Industry, Chem. Britain, (1989) 1192.
This Page Intentionally Left Blank
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
1095
Preparation of highly dispersed supported catalysts by ultrasound C.L.Bianchi, R.Carli, C.Fontaneto, V.Ragaini 1 Department of Physical Chemistry and Electrochemistry, University of Milan - Via Golgi, 19 - 20133 Milan (Italy)
Palladium supported on alumina or active carbon catalysts were prepared using ultrasound during the preparation steps. A large increase in the metal dispersion and in the catalytic activity of the samples, tested during the reduction Of acetophenone with flowing hydrogen, was found.
1. Introduction One of the most important target, preparing metal supported catalysts, is to obtain a very high dispersion of the metal on the support [1]. In the recent literature, there are few examples of the preparation of highly dispersed catalysts with vapor phase deposition or impregnation of organometallic precursors [2]. Studying non-classical preparation procedures, it has been already shown that ultrasound play a relevant role in preparing high dispersed pure amorphous iron [3]. Moreover, it has been recently proposed that ultrasound can improve the metal dispersion in alumina supported ruthenium catalyst [4, 5]. Concerning these methods, it is well known which are the effects of the implosion of the bubbles, created during a sonication run, on a solid surface; these effects are commonly used to clean or to erode the surface itself [6]. In fact, the implosion of the cavitation bubbles generate high energy shock waves with a pressures of several thousands of atmospheres, besides a very high local temperature; moreover, near the surface of a solid, the collapse of the liquid phase causes a microstreaming of a jet of solvents hitting the surface with a very high velocity, sometimes estimated to be as high as 100 ms -1 [7]. Taking advantage of the effects of the ultrasound (U.S.) on solid
1To whom correspondence should be addressed
1096
materials, alumina or carbon supported palladium catalysts were prepared using ultrasound during the reduction step. All the samples were characterized by several techniques and then tested following the reduction of acetophenone to assess the activity of the palladium catalysts.
2. Experimental
2.1 Samples preparation
The catalysts were prepared following the wet impregnation method described in [4, 5] starting from usual precursors: PdCI 2 and Pd(NO3)2• (all Fluka samples, 99% purity degree). The catalysts are supported on AI203 Akzo (surface area (BET): 159 m2/g) with a metal loading of 0.5, 1, 5%, (wt%) and on active carbon Supersorbon ASTV/420 (surface area 920 m2/g) with a metal loading of 5%. After the metal impregnation, all the samples were divided into two batches which were reduced following the same procedure [4, 5], but one batch was also sonicated in the meanwhile. In order to increase the ultrasound effects, the reduction of all the samples was performed with hydrazine at 80~ for 20 rain undertaking the solid-liquid suspension at a sonication of frequency 21 kHz and power 30 W, calculated with the calorimetric method [7]. This particular power was chosen in order to minimize the possible effects of destruction of the support; in fact the transformation of the alumina in fine powder, loosing the morphological structure was observed increasing the power just at 45 W. A change in the alumina phase (usually from n to = phase) was also detected in similar condition at room temprature [8]. The catalysts on carbon underwent the sonication run during the impregnation step as well.
2.2 Samples characterization
All the catalysts were fully characterized, both from the support and the metal point of view, by several techniques as BET analysis (Carlo Erba Strumentazioni), SEM (Cambridge Stereoscan 1 50), XRD (Rigaku) and XPS (MProbe, SSI). The metal loading on the catalyst was checked by elemental analysis ( A E S - ICP) and also the residual chlorine atoms, coming from the chlorinated precursors, were detected by ionic chromatography. All the samples were also characterized by hydrogen chemisorption, performed following a particular procedure to determine the metal dispersion (dispersion index, ID%) [9, 10].
2.3 Catalyst testing
The Pd catalysts, both on alumina and carbon, were tested in a bench scale reactor studying the reduction of 1 g of acetophenone in 10 cm 3 of absolute ethyl alcohol at atmospheric pressure. The runs were performed at 24~ for the Pd/C samples and at 34 and 44~ for the Pd/Ai203 samples. The activity values were measured by the consumed hydrogen in cm3/min.
1097 3.
Results
a n d Discussion
In Tab.1 and Tab.2 the results coming from the characterization by H2-chemisorption are reported. It is quite easy to observe an increase of the dispersion value in all the sonicated samples especially for low metal loading. This trend was already observed for the Ru catalysts [4, 5]. Tab.2 Samples supported on active carbon
Tab. 1 Samples supported on alumina -SAMPLE ed % " A 0.5 A' 0.5
PRECURSOR U.S. ID % PdCI2 yes 6216 ~PdCI2 no 31.2
B
1
PdCI 2
yes
B' C C'
1 5 5
PdCI2 PdCI2 PdCI2
no 27.8 yes 25.7 no 22.2
D D' E E' F F'
0.5 0.5 1 1 5 5
Pd(NO3)2x2H20 yes Pd(NO3)2x2H20 no Pd(NO3)2x2H20 yes Pd(NO3)2x2H20 no Pd(NO3)2x2H20 yes Pd(NO3)2x2H2O no
45.7
_
SAMPLE Pd% G 5 G" 5 G'
5
PRECURSOR U.S. ID% PdCI2 no 37.1 PdCI2 yes 43.1 PdCI 2 yes 38.6
64.2 44.1 48.3 17.7 18.7 14.6
Moreover, on alumina the non sonicated samples from PdCI 2 show an ID% value much lower than the catalysts, always non sonicated and with the same metal contents, but prepared from Pd(NO~)2x2H20. The chlorine atoms probably bind themselves to the metallic sites and prevent the hydrogen chemisorption on the metal [4, 5] affecting the metal dispersion measurements. On the contrary, it is interesting to notice that the sonicated samples s h o w a similar ID% value as if the chlorine atoms have no effects on the H 2 up-take and thus that the ultrasound had probably cleaned the samples surfaces from the chlorine atoms. This hypothesis was confirmed by ionic chromatography that recorded a chlorine loading of 0.71% (wt%) for the non sonicated sample, notwithstanding all the sample undergo several washings in comparison of a value of 0 . 0 2 % for the sonicated one. This result has a fundamental importance in the preparation of supported catalysts from chlorinated precursors, which are commonly used due for a low price and an easy solubility in water: by means of ultrasound, it is possible to prepare supported catalysts from chlorinated precursors without leaving chlorine atoms, which are able to change the properties of the catalysts, increasing the support acidity or poisoning the metal center. The same conclusion is correct for the catalysts supported on active carbon. In this case it is interesting to notice how the influence of the sonication run during the imbibition step in comparison to the reduction step. From a morphological point of view, both the SEM and the XRD analysis pointed out a perfect analogy between the sonicated and non sonicated samples: structure, phase and cristallinity of the support were not
1098
changed.
3.1 Catalytic testing The samples reduced with hydrazine and ultrasound always show an increase in the catalytic activity in comparison with the samples, with the same metal loading, but reduced without ultrasound. In Fig. 1 the comparison between the activity of A and A', at t w o different temperature (T = 34 ~ and 44~ is reported as time (rain) for a 85% reduction of acetophenone.
time (min) 85% acetophenone conversion 20
44~
34~ A' 15
AI
10
Fig.1 - Activity of samples A and A' (0.5% Pd from PdCI 2 on AI203).
For the samples supported on carbon: all the catalysts were tested at 24~ and in Fig.2 a comparison with a commercial sample is also reported. It is very interesting to observe that in this case the activity of the sample reduced with ultrasound (G') is not so different to the non sonicated sample (G), which show the same activity of a commercial sample. On the contrary the sample G " , sonicated during the impregnation step, shows a very high activity so that the time to obtain the 85% reduction of acetophenone is nearly halved. In fact, it was already observed (Tab.2) that on the carbon supported catalysts even the increase of the ID% value was substantial only for the sample treated with ultrasound during the impregnation and not during the reduction step.
1099
time
(min) 85% acetophenone
conversion
24~
COMMERCIAL SAMPLE
G
l:::;-.;;:: +'++++++
G
!:::'....'::::-.~;.s..~.%-- +,+*+*+§ !li +++++++++'++++++ : , " §* * ++.,t. § 2§4+7§
:.'.:-. " - ' . ' . ' : , ' : ; - "
...-...-.-...~,~ 9 -,'~ ~- , ; - +§247247247
G ::::""':':".:;S;"'"'; II
.9. , - . - O o . . o ~
- . ~,
~,-
§247247247247247
++++++++
i! i
++++++++
. ' . " ; ' ; ' ; ' ~ ' . , '!-.S- '' ;- ;, -" ;-;;!
+++++§ + + ++ ++++ § 2 4 7 2 4 7§224477
+.§247247247247247247
;.9".'.','.'. "',~; S ", ", "-.. +§ ++ § § 9 .'...
.;-
o'.-..~',,
.
.
j
.
. §247247247247247
;-y--:::--:.:.~:- :. ;:;;
++.,.+++.,.+
: : ' : : ' : - " : : : : s ~ : - ; S ; " +++§ +++* 9 ,
.
.
,,
-s
-
,'-s
"':': ".'.";"-" :'- ;. '- - ,, ~,,,S ,,",," +'§ § +'+ ,9' . - . ' . ' . , . ' . v , /, ~'§247247247247247 :.;...'-;'.-">, ~ ; ; ; ' ~ S *§247247247
Fig.2 - Activity of samples G, G', G " (5% Pd from PdCI 2 on active carbon) in comparison with a commercial sample.
4. Conclusion Sonicating the samples during a particular step of their preparation, very interesting results were obtained: 1) all the sonicated samples showed a very high dispersion value as revealed by H2-uptake measurements. Depending of the kind of support (alumina or carbon) the sonication step has to be carried out in a particular moment of the catalyst preparation to achieve the best results; 2) as far as the chlorinated precursors are concerned, it was observed a nearly complete disappearance of all the chlorine atoms after the sonication run. It is well known h o w it is difficult to eliminate all the traces of chlorine atoms on the alumina supported catalysts, even after a reduction in flowing hydrogen or with hydrazine or even after calcination at high temperature. The presence of the chlorine atoms may be a very important poison for all those reaction involving the hydrogen chemisorption as a fundamental step [11]. 3) all the sonicated samples showed an increase in the catalytic activity presented as a decrease in the time occurring to the reduction of acetophenone.
REFERENCES 1, J.T.Wrobleski and M.Boudart, Catal. Today, 15 (1992) 349 2. H.Abbrevaya and W.M.Targos, UOP U.S.Patent 4,714,693
1100
3. K.S.Suslick, S.B.Chol, A.A.Cichowlas, M.W.Grinstaff, Nature, 353 (1991) 414 4. C.L.Bianchi, R.Carli, S.Lanzani, D.Lorenzetti, G.Vergani, V.Ragaini, Catal. Lett., 22 (1993) 319 5. C.L.Bianchi, R.Carli, S.Lanzani, D.Lorenzetti, G.Vergani, V.Ragaini, Ultrasonics-Sonochemistry Vol.1, N.1 (1994) S47 6. T.J.Mason, J.P.Lorimer, Sonochemitry, ed. T.J.Kemp, Ellis Horwood Ltd., Chichester (1 988) 7. T.J. Mason, in: Practical Sonochemistry, J. Mellor (ed.), Ellis Horwood, Chichester, 1991 8. J. Gasgnier, private communication 9. R. Giannantonio, V. Ragaini, P. Magni, J. Catal., 146 (1 994) 103 10. V. Ragaini, R. Giannantonio, P. Magni, L. Lucarelli, G. Leofanti, J. Catal., 146 (1994) 116 11. C.L. Bianchi, M.G. Cattania, V. Ragaini, Mater. Chem. and Phys., 29 (1991) 297
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
1101
R e g u l a r i t i e s of P t p r e c u r s o r s and m o d i f y i n g dopes s o r p t i o n d u r i n g the p r e p a r a t i o n of bimetal catalysts s u p p o r t e d on spinels N.A.Pakhomov and R.A.Buyanov Boreskov Institute of Catalysis, 630090 Novosibirsk, Russia
ABSTRACT
[PtC16]2" and cations Cu 2+, Cd 2+, In 3+ and Sn 2+ have been found to sorb on the surface of spinel supports at the deposition of their precursors from the single component and binary solutions when the solution pH is lower than the isoelectric point of a support. Precursors of platinum and modifying cations affect mutually and in a complex manner the sorption from the binary impregnating solutions. Possible mechanisms of precursor deposition on the surface of spinels are discussed.
1. INTRODUCTION The stage of support impregnation with the solutions of active component precursor is the first and often the one determining the chemical composition, dispersity and active metal distribution over the support granule. The main factor affecting the state of supported compound at the impregnation stage is its ability to adsorb on the support surface. For example, supported platinum catalysts obtained via adsorption or ion exchange possess a higher dispersity of active metal than the catalysts synthesized with no strong interaction of precursor with the support surface [1]. We can find a detailed information on the regularities of H2PtC16 sorption on A1203 and SiO 2 generalized in a series of reviews [1-3]. Nowadays, supported bimetal catalysts are widely applied. However, the sorption of modifying dopes or active component at the simultaneous or stepwise deposition of their precursors is not studied in detail. When we synthesize bimetal catalysts we need to obtain a highly disperse active metal. Moreover, we should prepare the catalyst with desired optimal ratio of supported components and create conditions favorable for the formation of bimetal particles at the reduction stage. The majority of studies of a two-component impregnation concerns the sorption of [PtCI6] 2 assisted by the anions of inorganic or organic acids. These anions adsorb competitively and thus regulate the force of platinum precursor interaction with alumina and, thus, platinum distribution over the support granule [2-3].
1102 From the viewpoint of competitive adsorption we can consider as well the sorption of precursors upon the synthesis of Pt-Re/Al203 catalysts f r o m H2PtC16 and rhenium acid [4] and of Pt-Ru/SiO 2 catalysts from the ammonia complexes of P t and R u [5]. In both cases precursors were in the same ionic form and adsorbed on the same sites. The less studied and less predictable is the situation, when precursors are in the different ionic forms, which is quite frequent in practice: e.g., when P t enters as anion [PtC16] 2, while a modifying dope is a cation. In this paper we consider the sorption of [PtC16]2" and cations Cu 2+, Cd 2+, In 3+ and Sn 2+ on the surface of spinel supports like MeAl204 (where Me: Zn, Mg) and T-Ai203 . The components were s u p p o r t ~ either separately or simultaneously. The above dopes and supports belong to a large family of supported bimetal catalysts for the conversion of hydrocarbons, including the catalysts for the low paraffin dehydrogenation in steam [6-8].
2. E X P E R I M E N T A L
2.1. Supports Zn-A1 spinel was prepared via the wet mechanical mixing of ZnO powders with pseudoboehmite. In some cases SnO 2 powder was added to the mixed suspension. Mg-A1 spinel was obtained via coprecipitation from the solutions of their nitrates. The t e m p e r a t u r e and time of extrudate calcination for each support was chosen to provide a m a x i m u m spinel yield at the satisfying specific surface. Alumina was obtained via the calcination of aluminum hydroxide pseudoboehmite extrudates at 700 ~ Table 1 lists the phase and chemical composition, texture parameters and pH values at the isoelectric point (IEP) of supports. The content of free phases ZnO and MgO was determined by their selective dissolving in the 25% ammonia solution of ammonia chloride (2%). 2.2. Sorption equilibrium and kinetics. Components were adsorbed from the aqueous solutions of H2PtC16 and nitrates of copper, indium and cadmium. Tin was adsorbed from the SnC12 solution in isopropanol. Adsorption isotherms were measured on the small support granules (0.5-1.0 mm) at the contact with the corresponding solutions of various concentrations thermostated at 25 ~ (2 g of support/ 10 ml of solution) for 96 h. Kinetics was studied under the static conditions and solution circulation t h r o u g h the support bed. For platinum, some experiments were performed on extrudates with d=2.2 mm. The q u a n t i t y of adsorbed ion was determined from the decrease of its concentration in the impregnating solution. In all experiments we controlled the content of Mg (or Zn) and a l u m i n u m in the impregnating solution.
1103
2.3. Analysis Pt content was analyzed with photocolorimetry (using its colored complex with SnC12. Other elements were analyzed with atomic absorption. The X-ray phase analysis technique used is described elsewhere [7]. The temperature programmed reduction (TPR) was performed in the mixture H2(5%)-N 2 with the heating rate of 10 ~ at the 50 cm3/min consumption and the 250 mg sample weight. Table 1 Parameters of initial supports. Sample
Zn-Al-1 Zn-A1-2
Thermal MeO:AI203 Phase treatment compoconditions sition T time ~ h mol
Lattice parameter nm
900 1000
6 4
1.0:1 1.1:1
ZRA1204 0.8084 ZnA1204 0.8085
Mg-Al-I 1200 Mg-AI-2 1200 AI-O 700
6 6 4
1.0:1 0.9:1 --
MgAI204 0.8082 MgAI204 a) 0.8078 T-A]203 0.792
Free MeO phase content wt.% 2.0 4.2 0.9 0.8 0.2
ZnO ZnO SnO 2 MgO MgO --
S
V
IET
m2/g
cm3/g
24 14
0.30 0.22
8.0 8.2
50 48 170
0.51 0.54 0.70
9.5 9.1 7.8
a) defect Mg-A1 spinel 3. R E S U L T S AND DISCUSSION
3.1. Adsorption from the single component solutions pH of all aqueous precursors solutions is lower than the IEP pH of spinels and aluminum due to precursor hydrolysis. Hence, according to the widely used electrostatic model of cation and anion adsorption [1-3] on the oxide surface we could expect only [PtCI6]2 to adsorb predominantly on the surface of spinels, since cations usually adsorb when the solution pH is higher than the IEP pH. However, we have found, that not only [PtCI6] 2 but also cations Cu 2+, Cd 2+, In 3+ and Sn 2+ adsorb readily and strongly on spinels and alumina. Adsorption isotherms fit the Langmuir equation. Table 2 presents the calculated isotherms. From the equilibrium constant and Henry coefficients we know, that Pt adsorption on spinels is weaker than on alumina. At the same time, the maximum specific adsorption on spinels is somewhat higher (see Table 2). Fig. 1 shows the typical kinetics of Pt adsorption on spinels. The sorption attains equilibrium not less than in 4 days. The crashing of support granules and the circulation of impregnation solution do not change significantly the adsorption rate. Evidently, neither external nor inner diffusion limit the process. P t is known
1104 to adsorb on alumina readily and strongly [2-3]. The process is limited by the inner diffusion of chloroplatinate anion, which produces the egg shell metal distribution over the support granule.
0 $ 9 - static regime .m- circulation regime "0
o-d o-n-d
= 0.51.0 mm = 2.2 mm
9
0.4 1
.Q
1
Q
2
~.......-~
9
-
_~
_
o.2
t
2
4
fl
T i m e (hours)
I
24
I
I
96
:
Fig. 1. A m o u n t of Pt adosrbed on Zn-A1-2 as a function of wet impregnation time for various initial concentrations (Co), granule diamter (d) and impregnation regime. 1. CO = 0.02 mol/1; 2. Co-- 0.0085 mol/1 The t e x t u r e parameters of spinels and adsorption properties of their surface appear to be more favorable for the uniform metal distribution. The calculations according to the model described in the review by Lee and Aris [2] for Pt/ZnA1204 catalyst show, t h a t adsorption and diffusion occur with comparable rates. So, we can attain a uniform P t distribution in less than 2 h not using special sorption competitors. On all supports under study P t sorption is accompanied by the increase of pH in the impregnating solution. Thus, according to the idea of Duplyakin et al expressed in [9], we think, that [PtC16]2 adsorb on spinels via the anion exchange with OH-groups. Adsorption is complicated by the side desorption of the surface Zn 2+ and Mg 2+ cations combined in the spinels. For Mg-A1 supports we have found, t h a t atomic ratio Ptads/Mgde s is around 0.5 and does not depend on the overall q u a n t i t y of Pt adsorbed. Mg seems to desorb due to its exchange with the protons of H2PtC16: Mgsu2+ + 2H+ol
-
-
+ r + Mgso 2+ 1 2Hsu
(I)
W e did not observe the noticeable desorption of A1 cations into the solution. According to the X-ray data for support Zn-A1-2, phase ZnO dissolves partially in the concentrated solutions of Pt. Analysis of isotherms and kinetic data reveal some general and particular regularities of the sorption of cation modifying dopes depending on the nature of
ll05
Table 2 Calculated sorption isotherms for H2PtC16 and modifying cations on spinel supports. Cation desorbing from support
Sorption parameters of anion or cation Sample
Precursor element Amaxa) Kx10 .4 E wt.% mmol/m 2 ml/mol
Me2+ A13+ H E/Me 2+ ml/m 2 mmol/m 2 atom mmol/m 2
Zn-AI-1 Zn-A1-2 Mg-AI-1 Mg-A1-2 A1-O
Pt Pt Pt Pt Pt
0.61 0.46 2.01 2.10 2.86
1.2 1.7 2.1 2.2 0.9
88 71 10 10 600
1.1 1.2 0.2 0.2 5.2
Zn-AI-1 Mg-AI-1 Mg-A1-2 A1-O
Cu Cu Cu Cu
3.51 4.26 1.38 4.40
39.4 13.5 4.5 4.0
527 75 64 28
Mg-AI-1 A1-O
In In
5.85 6.90
10.2 3.5
Mg-AI-1 A1-O
Cd Cd
2.05 1.67
Zn-A1-1 A1-O
Sn Sn
A1-O
Mg
4.6 4.4
0.5 0.5
<0.1 <0.1 <0.1
207 10 3 0.1
44.5 9.0 3.8
0.9 1.5 1.2
0 0 0 0.8
36 63
3.6 2.2
9.8
1.0
7.8 3.4
3.7 0.9
74 12
2.7 0.1
3.2
1.2
0 <0.1
4.36 5.54
15.3 2.7
3 19
0.5 0.5
13.4
1.1
0 <0.2
0.22
0.5
2
0.01
a) Amax is the maximum specific sorption K is the equilibrium constant H-Area x K is the Henry coefficient adsorbingcation and support used. Figs. 2-5 present the curves of copper sorption on Mg-A1 spinels, which are similar to those of any dope under study. As a rule, cations sorb faster than Pt on spinels. Table 2 shows, that specific maximum adsorption and sorption strength of cations on spinels is also higher. At the low concentrations of the impregnating solutions cations sorb practically irreversibly (see Fig. 2A). In all cases, except for Cd 2+, the sorption is accompanied by the increase of solution pH to those exceeding the starting pH of the corresponding hydroxide precipitation (Fig. 2B). W i t h the help of a qualitative reaction with diphenylamine we have found, that the impregnation solutions contain no NO 3 ions at the irreversible sorption. For Cd 2+, pH increases at the irreversible sorption only, while at higher concentrations the solution pH decreases to the values lower than initial pH.
1106 Co x 104 (mol/g) 8
W i t h spinels, Mg 2+ or Zn 2+ desorb s y n c h r o n o u s l y w i t h the ~ 6 L A 6 ~~ sorption of a cation. Moreover, the O overall amount of Mg t r a n s f e r r i n g 4 ~ ~O 4 to the solution exceeds by several O 2 o~ , fold its amount in phase MgO not -~ 2 combined in the spinel. Note, t h a t atomic ratio of chemosorbed copper M 0 ~ 0 ~ O (determined after the sample thor,'o 4 ~ ough washing with water to remove o 2 + the weakly bonded copper species) to the overall a m o u n t of desorbed ~&A~_____~ 4 + 4 Mg is close to unit and does not depend on the composition of spinel B and adsorbed copper amount. W e 8 [ , ~ - ~ r ~ PHeq observe also the equication desorption of Mg 2+ (or Zn 2+) at the sorption of Cd 2+ and Sn 2+ (see Table 2). I I I I I Only In 3+ sorption on both spinels 0.1 0.2 and alumina causes a noticeable Co (mol/1) A13+ desorption into the solution. Fig. 2. A m o u n t of adsorbed Cu 2+ (1,2), Table 2 shows, t h a t ratio In3+ads/ desorbed Mg 2+ (3,4) (A) and impregnating A13+des for these supports is close solution pH (B) as the functions of initial to the equication one. concentration (Co) of copper in the soluEarlier we have shown with the tion and support composition. Supports: ESR method [10], t h a t a small 1 , 3 , B - Mg-AI-1 2,4 - Mg-A1-2 portion of copper supported on MgA1204 surface consists of the isolated Cu 2+ ions in a octahedron oxygen environment. Electron diffusion reflectance spectra tell, that copper, unobserved in the ESR spectra, stays in weakly magnetic associates Cu-OH-Cu, since the a.b. at 14000 cm "1 [10] was registered. X-ray phase analysis of the samples containing C u > l wt.% reveals the Cu2(OH)3NO3 :phase. The TPR spectra show two maxima: a low t e m p e r a t u r e m a x i m u m at 250-260 ~ typical for the reduced hydroxide and oxide copper phases and a high temperature one within 400-420 ~ corresponding to copper cations strongly bonded to the support [7]. Thus, we can suggest the following schemes of cation anchoring on the surface of spinels and alumina: (i) Cation exchange of copper, cadmium and tin with the subsurface cations of Mg 2+ or Zn 2+, cation exchange of In s+ with aluminum.
Mg 2+ + Cu2o+~ stir
2+ + Mg 2+ CU~ur ~ol
(2)
1107 (ii) ~Surface precipitation,. Here, at the first stage, OH group undergoes anion exchange with an anion of dissolved salt and then a cation precipitates on the surface as a hydroxide or the corresponding basic salt due to the increase of pH in the impregnating solution in the support pores: S-OH + NO, sol
-
-
S-NO 3 + OHio1
2 CuOH + + 2OH ~ + NO~
=~ Cu2(OH)3NO3sur
(3) (4)
On alumina Cu 2+ and Sn 2+ cations adsorb via the surface precipitation. Note, however, that specific maximum sorption and sorption strength are considerably lower, than those on Mg-A1 and Zn-A1 spinels. Table 2 shows, that like Mg 2+, Cd 2+ adsorbs in less amounts on alumina. The starting pH for Mg hydroxide precipitation is higher than the IEP pH of A1203. According to electrostatic theory, Mg 2+ adsorbs from the alkaline solutions via cation exchange with the protons of OH groups [11]. Apparently, such a scheme is realized for Cd 2+ as well, since the process is accompanied by the decrease of solution pH.
3.2. Simultaneous sorption of precursors We can split the binary solutions of Pt and modifying dopes into two groups: i) those, where the solution components can form complexes with each other, ii) solutions, where we do not register such an interaction of components. Solutions of H2PtC16 with SnC12, where various Pt-Sn complexes form, are the typical first group solutions. However, aqueous solutions of Pt-Sn complexes are not suitable for the synthesis of highly disperse supported Pt-Sn catalysts, because it is necessary to maintain a high concentration of CI" ions, which suppress the adsorption of complexes. It is possible to sorb Pt-Sn complexes on alumina from the non-aqueous solutions, but according to [12] the maximum adsorption and strength are by an order of magnitude lower than those for [PtCle]2". Since the surface of Zn, A1 spinels is smaller by several fold than that of alumina (see Table 1), the absolute adsorption of Pt supported from the isopropanol solutions of Pt-Sn complexes appears to be not high enough to prepare efficient catalysts. Due to this reason we used a stepwise adsorption of Pt onto the Sn-carrying supports to obtain Pt-Sn/ZnA1204 catalysts with a high Pt dispersion. For the solutions of the second group the situation is even more complex. Despite the fact, that chloroplatinate anion and cations anchor on the spinel surface via the different mechanisms, they affect mutually their adsorption from the binary solutions. For Pt-Cu solutions this mutual influence differs considerably from the ordinary competitive sorption, when a component with a higher affinity to the support suppresses the sorption of another weakly sorbed component. The data presented in Figs. 3,4 show, that Pt is a more active competitor, though copper adsorption on spinels is stronger. Pt reduces the rate of Cu sorption (Fig. 3A), decreases the maximum adsorption capacity and sorption strength (Fig. 4). The competitive behavior of Pt seems to result from the fact, that the protons of H2PtC1 e exchange with the surface Mg 2+ cations (reaction (1)) and
1108
A
0.8 Z
~
~,0.4
_.._...O-
j
O~
2
~O o.2
o 0.4
I
f r~
O
I
/
2 4 Time (hours)
I
,1
2 4 Time (hours)
Fig. 3. Kinetics of copper (A) and p l a t i n u m (B) sorption on s u p p o r t Mg-Al-1 from the single component (1) and binary (2) H2PtC16 + Cu(NO3) 2 solutions.
1 7o
O
e~
~ 30
ol
5" 1'0 Concentration Cu 2+ 9102 (tool/l) Fig. 4. I s o t h e r m s of copper soprtion on Mg-A1-2 at its co-impregnation w i t h [PtC16]2". C o n c e n t r a t i o n of H2PtC16xl03 (mol/1): 1 - 0, 2 - 0.78, 3 - 4.36, 4 - 8.5.
0
'
l'
'
2'
'
Concentration H2 PtCls" 102(tool/I)
Fig. 5. I s o t h e r m s of H2PtC16 s o r p t i o n on Mg-A1-2 at its c o - i m p r e g n a t i o n w i t h C u ( N O s ) 2. C o n c e n t r a t i o n of C u x l 0 2 (mol/1): 1 - O, 2 - 1.4, 3 - 2.3, 4 - 11.0.
suppress the Cu 2+ exchange via reaction (2). A t the high P t c o n c e n t r a t i o n s , reaction (4) is suppressed, since the equilibrium p H of b i n a r y solutions in this case are l e s s than those of CU2+ hydroxide precipitation. Copper, on its t u r n , can accelerate the sorption of [PtC16]2- at the low P t concentrations in the solution (Fig. 3B,5), t h u s increasing P t e x t r a c t i o n f r o m the
1109 solution. A t the higher initial concentration of Cu, Pt sorption from the concentrated solutions decreases progressively. The isotherms of Pt adsorption loose their Langmuir shape and pass through a maximum (Fig. 5). Obviously, two parallel processes occur upon the sorption form the binary solutions: i) increase of [PtC16] 2" adsorption, because the preadsorbed copper provides additional adsorption sites; ii) decrease of Pt adsorption due to a competitive effect of NO~ ions on the anion exchange of PtC182" with the surface OH-groups (reaction (3)). The increase of Pt sorption efficiency upon its stepwise deposition on the Cu containing Mg-A1 spinels confirms the first process 'to occur (Fig. 6).
00
80
I
I
I
1
I
2
Cu content, wt. % Fig. 6. Dependence of the equilibrium extraction of Pt from the impregnation solution H2PtC18 vs. the content of the preadsorbed Cu on Mg-A1-2 (calculated concentration Pt -- 0.5 wt.% ). The mutual influence of components in Pt-In solutions is less pronounced. It becomes noticeable only at the large excess of one component with respect to another and is an ordinary competitive adsorption of components with the similar adsorption properties. Note, that In has no effect on the initial Pt adsorption within a wide concentration range. It only rather decreases the equilibrium concentration of adsorbed Pt. Here, we also can explain In effect on Pt sorption by a competitive adsorption of NO a- ions, while Pt effect results from the decrease of equilibrium pH in the binary solutions. Thus, our results oppose the popular opinion, that it is not possible to sorb simultaneously in comparable amounts cMoroplatinate anion and dope cation on the surface of amphoteric and basic supports from the acid impregnating solutions at the synthesis of bimetal catalysts. Note, however, that a noticeable suppression of copper sorption by Pt limits the catalyst synthesis via a simultaneous deposition from the solutions with the known Pt content and C u / P t ratio. Our experiment results show, that only for Pt concentrations less than 0.5 wt.% we can obtain a wide range of C u / P t ratios. At the Pt concentration exceeding 1 w t . % , the obtained ratio shall be less than 2. Moreover, the registered acceleration of Pt
1110 sorption by copper requires additional studies on a possible non-uniform Pt distribution over the support granule. For Pt-In catalyst, the situation is more favorable. Due to this reason, we can obtain the supported catalysts with a sufficiently wide range components ratio via the simultaneous impregnation method.
REFERENCES
1. J.P.Brunelle, Pure Appl. Chem., 50 (1978) 1211. 2. S.Y.Lee and R.Aris, Catal. Rev.-Sci. Eng., 27 (1987) 207. 3. A.Gavriilidis, A,Varma and H.Morbidelli, Catal. Rev.-Sci- Eng., 35 (1993) 399. 4. B.K.Duplyakin, V.B.Fenelonov, K.Richter, A.V.Rodionov, A.Chelut, L.I.Kheifets, A.V.Veimsrik, V.V.Moskovtsev, in Scientific Foundations for Catalyst Technology, Nauka, Novosibirsk (1981) 137. 5. S.Alerasool and R.Gonzalez, I. Catalysis, 1254 (1990) 203. 6. N.A.Pakhomov and R.A.Buyanov, in Prasado Rao Ed., Advances in Catalysis, Sci. Technol., Wiley Estenu Limited, New Dehli (1985) 305. 7. N.A.Pakhomov, N.A.Zaytceva and E.M.Moroz, Kinetika i Kataliz 33 (1992) 426 (in Russian). 8. E.E.Davies, J.S.Elkins, R.C.Pitkethly, Improvements relating to platinumcontaining catalysts and their use, GB Patent No 1332361 (1973). 9. V.K.Dyplyakin, V.P.Doronin, T.V.Tsymbal, L.Ya.AIt, A.S.Belyi, Dokl. Akad. Nauk SSSR, 281 (1985) 89 (in Russian). 10. G.A.Dergaleva, N.A.Pakhomov, V.N.Bemdurin, V.F.Anufrienko, Kinetika i Kataliz, 32 (1991) 490 (in Russian). 11. C.-P.Huang and W.Stunm, J. Colloid Interface Sci., 43 (1973) 409. 12. V.K.Duplyakin, Doctor Degree Thesis, Institute of Catalysis, Novosibirsk, 1990.
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
1111
T i n ( I V ) o x i d e s u p p o r t e d n o b l e m e t a l c a t a l y s t s for t h e c a r b o n m o n o x i d e o x i d a t i o n a t low t e m p e r a t u r e s K. Grass and H.-G. Lintz a aInstitut ffir Chemische Verfahrenstechnik der Universits (TH) Karlsruhe, Kaiserstrafle 12, D-76128 Karlsruhe The successive loading of a ceramic carrier with tin(IV)oxide and a noble metal is an adequate method to prepare catalysts for carbon monoxide oxidation at low temperatures. The preparation includes several calcination steps. The influence of this temperature treatment on tin(IV)oxide and tin(IV)oxide containing catalysts has been investigated. Prolonging the calcining time at 350 ~ leads to a decrease of the surface area and a moderate dehydration, increasing the calcining temperature from 350 ~ upwards equally causes a decrease in surface area, but a complete dehydration. Additionally, an influence of the ceramic carrier on the variation of the surface area during the temperature treatment has been observed. Quantitative information about the activity of various catalysts is obtained by rate measurements in a recirculation system. The results indicate that the observed synergism between the noble metal and tin(IV)oxide is due to spillover of oxygen. The comparison of reaction rates, measured with catalysts mainly differing in the sorption capacity relative to oxygen, shows that the rate determining step of the carbon monoxide oxidation should be the migration of adsorbed oxygen. 1. I n t r o d u c t i o n The catalytic oxidation of carbon monoxide at temperatures between 20 ~ and 100 ~ is used in life rescue devices and for air purification. Moreover, it can be applied in sealed carbon dioxide lasers to recombine decomposed carbon dioxide. Catalysts which contain both tin(IV)oxide and a noble metal can be used for these purposes as they are active even at room temperature up to carbon monoxide contents of 1.5 vol.%. By contrast, catalysts containing only one of the two components are not active under practical conditions and at such low temperatures [1,2]. Different approaches were made to investigate the catalytic system noble m e t a l - tin(IV)oxide. Several studies were done by two collaborating groups (Prof. Hoflund of the University of Florida and Dr. Schryer at Langley Research Center, Virginia). They focused on the characterization of the catalyst surfaces using spectroscopic methods [3,4], the effect of pretreament on the catalytic activity [5,6], and the comparison of different catalysts for carbon monoxide oxidation at low temperatures
[7].
Intending to elucidate the synergism between the noble metal and tin(IV)dioxide, in previous works we examined the activity of differently structured catalysts at varying re-
1112 action conditions [8,9]. In this context the sorption capacities relative to carbon monoxide and oxygen turned out to be characteristic features of the catalyst. In [9] the impact of altering the carbon monoxide sorption capacity by varying the platinum dispersion has been investigated. In the present study we have aimed at the preparation and examination of catalysts with different oxygen sorption capacities. For that purpose the characteristics of the tin(IV)oxide had to be altered and thus, firstly, a more general investigation of the modification of tin(IV)oxide powder by an adequate temperature treatment was performed. Subsequently, reaction rates for the carbon monoxide oxidation were determined with catalysts of different oxygen sorption capacities in order to obtain additional information concerning the synergism between the noble metal and tin(IV)oxide. 2. E x p e r i m e n t a l 2.1. P r e p a r a t i o n Tin(IV)oxide powder is prepared by dissolving metallic tin granules in nitric acid at a temperature of 20 ~ (4- 5 ~ and drying the thoroughly washed precipitate overnight at 80 ~ The resulting powder is calcined at temperatures in the range from 350 ~ to 1000 ~ the calcining duration being varied from 1 to 60 hours. Starting point for the preparation of the catalysts is a ceramic carrier: Either leaflets of cordierite (0.425.15.30 mm3), cut from a commercially available monolith (Coming), or leaflets of a-AlcOa (0.81 911.5-30 mm3), are used. The latter are prepared by extrusion of a suitable mixture of 7-A1203, A10(OH), and ammonium alginate, followed by calcination at 1300 ~ The leaflets are loaded with 23 weight% tin(IV)oxide by repeated immersing into a tin(IV)oxide hydrate sol [10]. Briefly, this sol is prepared by dissolving metallic tin granules in nitric acid and peptising the thoroughly washed precipitate using cyclohexylamine. The loaded carriers are dried for 1 hour at 80~ and calcined at 350~ the calcining duration being varied from 1 to 80 hours. Subsequently, the noble metal is added by impregnation with a Pt(NHa)~ + - salt. Solutions with different concentrations of salt are used to control the noble metal content. Finally, the precursor is calcined for 1 hour at 350 ~ For comparison, a platinum catalyst without tin(IV)oxide is prepared by impregnation of a-A12Oa leaflets with the same platinum containing solution, followed by calcination for 1 hour at 350 ~ 2.2. C h a r a c t e r i z a t i o n of t i n ( I V ) o x i d e , loaded carriers and c a t a l y s t s The extent of dehydration during temperature treatment of tin(IV)oxide powder is ascertained by thermogravimetry (Simultaneous Thermal Analysis STA 409, Netzsch). Surface areas are determined by nitrogen adsorption using a standard volumetric BET apparatus. The tin(IV)oxide content of the loaded carriers and the catalysts is quantified gravimetrically. The platinum content of the catalysts is obtained by a spectrophotometric method which has been described elsewhere [11]. 2.3. K i n e t i c m e a s u r e m e n t s
The activity of the catalysts is quantified by the rate of carbon monoxide oxidation. Rate measurements are carried out in a gradientless recirculation system operated at
1113 steady state at a flow rate of 180 ml/min (STP) and a reflux ratio of 30. Using the following stoichiometry CO + 1/2 02 ~ C02 , the reaction rate, which is related to the total catalyst mass, is defined as
I rm
d~ 9
m=~
dt
The extent of reaction ~ is determined by mass balancing the open system in steady state. The concentrations of carbon monoxide and carbon dioxide are measured by nondispersive infrared spectroscopy (Binos, Rosemount), oxygen is determined by use of a magnetic device (Magnos 3, Hartmann & Braun).
2.4. D e t e r m i n a t i o n of sorption capacities Sorption capacities for carbon monoxide and oxygen are determined by a titration method. For that purpose the reactor is operated as a simple flow-through reactor without recirculation. It is successively passed by the following gas streams: Firstly, it is flushed with nitrogen containing one of the reacting components (1 vol.% carbon monoxide or 6 vol.% oxygen), then swept with pure nitrogen, and finally passed by the other of the two reactants. In the last step the preadsorbed component reacts with the added one to carbon dioxide. The quantity of the preadsorbed component is obtained by the measurement of the total amount of carbon dioxide evolved. 3. Results and discussion 3.1. Variation of the t i n ( I V ) o x i d e properties The tin(IV)oxide powder was either calcined for one hour at different temperatures, or for different times at 350 ~ the calcination temperature during catalyst preparation. Both the variation of the surface area and the remaining relative water content have been determined. The total amount of water was quantified by the weight loss during calcination at 1200 ~ As can be seen in figures 1 to 4, an increase in both the calcining time and the calcining temperature leads to a decrease in surface area and water content. Heightening the calcination temperature has a much stronger impact than prolonging the calcination duration. Calcining for 1 hour at 600 ~ leads to approximately the same surface area as calcining for 60 hours at 350 ~ but by contrast to complete dehydration. To examine the influence of the temperature treatment on tin(IV)oxide loaded carriers, they were calcined for different times at 350~ The calcining temperature was not varied, because former experiments had shown severe activity losses of the catalysts calcined at temperatures above that value. This may be due to the complete dehydration of tin(IV)oxide, which has been observed with tin(IV)oxide powder. To allow a comparison of tin(IV)oxide powder and tin(IV)oxide loaded carriers, the measured values of the surface area and the mass loss of the loaded carriers are related to their tin(IV)oxide content. The water content of the loaded carriers equals approximately the values for tin(IV)oxide powder (figure 4), but the carrier supported oxide has to be calcined for a longer time (figure 3) to lead to the same decrease of the surface area. Thus
1114 15
200 T
o I/1
v powder
o
150--
(/3
E
41, t~
E
10-
o N -r
100-
E
I-I.iJ
m h
50-
fo=lh
0
0
" ~
I
I
I
250
500
750
f 1000
I
I
250
500
v
Tc/~
200
15
v 0
powder loaded carrier
0 Ill
150 o
E
E
10-
o.
100 -4
~
Figure 2. The influence of the calcining temperature on the water content of tin(IV)oxide.
powder loaded carrier
o
1000
TJ~
Figure 1. The influence of the calcining temperature on the surface area of tin(IV)oxide.
I
I
750
-I-
E
kI,
5 50m
0
I
I
I
20
40
60
80
fc/h Figure 3. The influence of the calcining time on the surface area of tin(IV)oxide.
T c - ,350 ~
0 0
l 20
i 40
J 60
80
fc/h Figure 4. The influence of the calcining time on the water content of tin(IV)oxide.
1115 Table 1 Characteristics of catalysts with altered tin(IV)oxide surfaces dimension carrier cordierite a-A1203 calcining time h 1 1 surface area m2/gs=o2 101 151 Pt content weight% 0.76 0.98
cordierite 80 52 0.66
a-A1203 80 87 0.71
the tin dioxide particles seem to be stabilized within the pore structure of the ceramic carrier, The nature of the ceramic carrier itself seems to influence the temperature stability of the surface area as can be seen from the data given in table 1. Carriers with higher surface area lead to higher values of the tin(IV)oxide surface area, due to a retardation of the sintering process. The catalysts listed in table 1 had been prepared by use of solutions containing the same platinum salt concentration. The correlation between the obtained loading and the tin(IV)oxide surface area implies an interaction between the platinum salt and the oxide as it has been indicated earlier [11]. As the aim of the preparation is the sole variation of the surface area of tin(IV)oxide, leaving all other properties of the catalyst unchanged as far as possible, the following procedure is used: The calcination temperature is kept constant at 350 ~ in order to alter the water content as little as possible. The surface area of the oxide is varied by increasing the calcination time. To prepare catalysts with comparable platinum content, the influence of different surface areas, related above, is compensated by use of impregnation solutions of different concentrations [11]. 3.2. R e a c t i o n Kinetics Figure 5 represents typical results obtained at 60~ at a constant oxygen concentration of 6 vol.%. The reaction rate rm is plotted against the carbon monoxide content xco in the case of two catalysts, a platinum catalyst with 0.9 weight% platinum (catalyst 4 on table 2) and a p l a t i n u m - tin(IV)oxide catalyst with 0.98 weight% platinum and 23.5 weight% tin(IV)oxide (catalyst 2 on table 2). One observes the two regimes well known on platinum catalysts [12]. In the first regime, where the reaction is first order with respect to carbon monoxide, diffusional limitations restrict the reaction rate. In the second regime, only the tin(IV)oxide containing catalyst is active, the rate being zero order with respect to carbon monoxide. In this domain the reaction rates are not affected by mass transport limitations to the catalyst surface. Measurements with other platinum tin(IV)oxide catalysts show the same pattern, varying only in the magnitude of the reaction rate in the second regime. In order to compare different catalysts the average value of the reaction rate in the second regime (0.2 < xco/% < 1.6) is used. Characteristic data of different catalysts, including the sorption capacities for both oxygen and carbon monoxide, and the rates determined in the second regime at 60 ~ and an oxygen concentration of 6 vol.%, are resumed in table 2. In the case of the tin(IV)oxide containing catalysts (cat. 1 to cat. 3) both the platinum content and the carbon monoxide sorption capacity related to the catalyst mass vary slightly. However, since carbon
1116
25 V u o
0
20-
O0
I
0 PI/ot-AI20~ (4)
V 10 - ~
0
L
E
(2)
15-
E ,11,
Pf/SnO2/~-AI20s
V
V
5
0
~
0
1 0.2
V
V
V
V V
v
v V
V V
V
I
I
!
0.4
0.6
0.8
•162 (70) Figure 5. The dependence of the reaction rate on the carbon monoxide content at a temperature of 60 ~ and an oxygen content of 6 vol.%.
monoxide is known to adsorb exclusively on the noble metal and not on tin(IV)oxide, the carbon monoxide sorption capacity is a measure of the accessible platinum surface [13]. Relating the sorption capacity to the platinum mass gives a measure of the platinum dispersion [13]. We obtain nearly identical values for the catalysts 1 to 3. Therefore the size of the platinum crystallites on these catalysts should be approximately equal. The catalyst not containing tin(IV)oxide (cat. 4) has a platinum content of the same order of magnitude as the other ones but the carbon monoxide sorption capacity is one order of magnitude lower, indicating a very poor dispersion of the platinum on the carrier c~-A1203. The oxygen sorption capacities of the four catalysts vary in a broad range. They cannot be correlated with the carbon monoxide sorption capacities, independent of the fact whether the latter are related to the catalyst mass or to the platinum mass. No explanation could be found for the comparatively low oxygen sorption capacity of the catalyst prepared by use of the cordierite carrier (cat. 1). The results obtained with the two catalysts prepared using a-A1203 carriers (cat. 2 and cat. 3) clearly indicate that a prolonged calcination leads to both a diminuation of the tin(IV)oxide surface area and a lower oxygen sorption capacity while the platinum dispersion obtained is not markedly influenced. There is a strong correlation between the sorption capacity related to oxygen no and the reaction rate rm measured in the second regime, as can be seen in figure 6. A line-
lll7 Table 2 Characteristics of catalysts used for kinetic measurements dimension 1 2 carrier cordierite a-Al203 calcining time h 1 80 surface area m 2/gc~t 23 20 surface area m2/gsno2 103 87 Pt content weight% 0.81 1.2 tin(IV)oxide content weight% 22.3 23.5 CO sorption capacity 10-Smolco/gc~t 2.6 3.9 CO sorption capacity 10-3molco/gpt 3.1 3.2 O sorption capacity 10-Smolo/gc~t 1.5 5.3 reaction rate 10-Smol/(gcat " s) 1.9 6.4
3 a-Al203 1 36 151 0.98 23.5 3.2 3.3 11.6 15.1
4 a-Al203 4 0.90 0 0.3 0.4 0.3 0
ar dependence is clearly shown including the value obtained with the catalyst without tin(IV)oxide (cat. 4). As stated above, the dependence of the reaction rate on the carbon monoxide content shows the two kinetic regimes well known in the case of platinum catalysts. The synergism between platinum and tin(IV)oxide is operative in the domain where the rate is zero order with respect to carbon monoxide. It is often attributed to the spillover of reactive species, either carbon monoxide [14,15] or oxygen [16]. We have discussed elsewhere [8,9], why we consider that oxygen is moving from the oxide to the platinum surface or to the three phase boundary, where it immediately reacts with adsorbed carbon monoxide as it is known from pure platinum. The linear dependence of the reaction rate on the oxygen sorption capacity shown in figure 6 not only corroborates this image, but gives additional insight in the rate determining step. As the data are taken at conditions where the reaction is zero order with respect to both carbon monoxide and oxygen, the rate cannot be controlled by the rate of adsorption of the reactants. It is also independent of the carbon monoxide sorption capacity but strongly dependent on the oxygen sorption capacity, a measure of the number of oxygen adsorption sites. The order zero with respect to oxygen implies that at reaction conditions the saturation coverage is attained. Therefore an increase of the oxygen sorption capacity means a higher surface concentration of oxygen during reaction. Considering the oxygen flux to the location of reaction proportional to this surface concentration, the rate increase with raising oxygen sorption capacity is easily explained. The linear connection implies that the migration of oxygen from the location of adsorption to the place of reaction is a relatively slow process and under the reaction conditions set rate determining. In contrast, the reaction between the adsorbed species itself and the desorption of carbon dioxide must be fast processes, otherwise an influence of the sorption capacity with respect to oxygen should not be found. What can be said concerning the location of the oxygen adsorption sites? As there is no correlation between the sorption capacities of carbon monoxide and oxygen, and as we consider the former to be adsorbed exclusively on platinum, the oxygen adsorption sites cannot solely be located on platinum. Another possible place for the oxygen adsorption
1118 16 ,I
o (J
ED
12-
0
E oO I
8-
41'
0 m
E
0
I
I
4
8
12
no /(I0-S. mol/gr Figure 6. The dependence of the reaction rate in the second regime on the oxygen sorption capacity at a temperature of 60 ~ and an oxygen content of 6 vol.% (cat. 1 to cat. 4).
could be the boundary line platinum - tin(IV)oxide. The extension of this line is proportional to both the platinum content and the platinum dispersion. In the present case no correlation is found between these quantities and the oxygen sorption capacity. Thus this hypothesis can be excluded as well. However, the results obtained with catalysts 2 and 3 are consistent with the view that the tin(IV)oxide surface contains the oxygen adsorption sites, indicating the direction of further experiments: The preparation of catalysts with different surface areas by use of the same ceramic carrier,
4. C o n c l u s i o n s The variation of both the temperature pretreatment and the ceramic carrier of platin u m - tin(IV)oxide catalysts allowed the preparation of catalysts with nearly identical platinum content and dispersion, but broadly differing oxygen sorption capacity. Rate measurements on these catalysts imply the following conclusions: The synergism between the noble metal and the oxidic component is due to spillover of oxygen. The oxygen adsorption sites are neither exclusively located on platinum nor on the three phase boundary. The rate determining step of the carbon monoxide oxidation should be the migration of adsorbed oxygen.
1119 REFERENCES
1. J.P. Dauchot and J. P. Dath, J. Catal., 86 (1884) 373. 2. M.J. Fuller and M. E. Warwick, J. Catal., 29 (1973) 441. 3. Closed-Cycle, Frequency-Stable CO2 Laser Technology, NASA Conf. Publ. 2456 (1987). 4. S. D. Gardner, G. B. Hoflund, M. R. Davidson and D. R. Schryer, J. Catal., 115 (1989) 132. 5. J.E. Drawdy, G .B. Hoflund, S. D. Gardner, E. Yngvadottir and D. R. Schryer, Surf. Inteff. Anal., 16 (1990) 369. 6. S.D. Gardner, G.B. Hoflund, D. R. Schryer, and B. T. Upchurch, J. Phys. Chem., 95 (1991) 835. 7. S.D. Gardner, G. B. Hoflund, B. T. Upchurch, D. R. Schryer E. J. Kielin and J. Schryer, J. Catal., 129 (1991) ll4. 8. H.-G. Lintz, C. F. Sampson and N. Gudde, in K. H. Steinberg (Editor), Proc. 2nd Conf. on Spillover, Leipzig 1989, Karl Marx Universit~t, Leipzig, (1989) 47. 9. A. Boulahouache, G. Kons, H.-G. Lintz and P. Schulz, Appl. Catal. A, 91 (1992) ll5. 10. C. J. Wright and C. F. Sampson, Catalyst Preparation, UK Patent No GB2134004B (1986). II. H.-G. Lintz, Ind. Eng. Chem. Res., 30 (1991) 2012. 12. T. Engel and G. Ertl, Adv. Catal., 28 (1979) I. 13. T.Mallat and J. Petro, React. Kinet. Catal. Lett., II (1979) 307. 14. G. C. Bond, L. R. Molloy and M. J. Fuller, J. C. S. Chem. Comm., (1975) 796. 15. M. Sheintuch, J. Schmidt, Y. Lecthman and G. Yakov, Appl. Catal., 49 (1989) 55. 16. D. R. Schryer, B. T. Upchurch, B. D. Sidney, K. G. Brown, G. B. Hoflund and R. K. Herz, J. Catal., 130 (1991) 314.
This Page Intentionally Left Blank
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
1121
Preparation of PMoNi/3,-AI203 catalysts from solutions of phosphoinolybdates in water, ethanol-water and dimethylformamide P.G. V~quez, M.G. Gonz~ilez, M.N. Blanco and C.V. C~iceres Centro de Investigaci6n y Desarrollo en Procesos Cataliticos (CINDECA), UNLP, CONICET, 47 N~ - La Plata- ARGENTINA
ABSTRACT The equilibrium adsorption of phosphomolybdates on alumina was studied. Aqueous solutions of heptamolybdate and phosphoric acid at a P/Mo molar ratio of 0.4 and solutions of phosphomolybdic acid (H3PMo~2040) in water, ethanol-water and dimethylformamide, were used. The adsorption isotherm of molybdenum from P2M050236- had two plateau while that obtained with PMot203exhibited only one plateau, thus showing the adsorption of two and only one species, respectively. The study of solutions by NMR and of solids by DRS indicated decomposition of P2M050236- after contacting the alumina, while the PMo~203 undergoes partial degradation in water and maintains its structure in the other solvents. The presence of MoO 3 crystals was detected by XRD only for calcined samples from the adsorption isotherm of P2M050236. The PMoNi/AI203 catalysts were prepared by pore-filling impregnation of alumina spheres by adding both Mo and P first and then Ni. The Mo and P profiles showed that the concentration continuously diminishes from the surface to the center of the sphere whereas egg shell profiles were obtained for phosphomolybdic acid in dimethylformamide. The hydrodenitrogenation activity of catalysts was comparable to that of a commercial one. Catalysts obtained with the first two solutions were more active than a catalyst without phosphorus, while catalysts prepared with phosphomolybdic acid in organic solvents gave lower activities in the pyridine test reaction.
1. I N T R O D U C T I O N Phosphorus is considered as one of the most effective additives of MoNi/3"-A1203 catalysts used for hydrotreatments. Until now, preparation of the majority of these catalysts was carried out by impregnation of 3,-alumina with aqueous solutions of ammonium heptamolybdate and phosphoric acid [1-4]. Studies on the subject allowed to conclude that not only the characteristics and dispersion of chemical species of the active component and of the promotors present on the surface but also the macrodistribution along the pellet, have a considerable influence on properties of the finished catalyst and therefore on its performance. However, studies previously made did not cover other items such as the influence of the phosphomolybdates initial structure in the impregnant solutions on the above-mentioned aspects of catalysts nor the studies have covered the behaviour of phosphomolybdates during the impregnation stage. The adsorption of some phosphomolybdates has been described recently in the literature
1122 [5-6]. Cheng and Luthra [5] reported that 12-molybdophosphate, PMo1204o3-, is adsorbed intact on the alumina. However, van Veen et al. [6] observed that this phosphomolybdate interacts with the basic surface OH groups of alumina, which leads to its decomposition. It must be remarked that the polyoxoanions are not perfect complexes, because the dilution leads to their degradation to less condensed species [7]. This difficulty can be minimized by using a solvent mixture which allows for the stabilization of the initial species through kinetic or thermodynamical effect [8-9]. In this paper, adsorption kinetics and isotherms at 293 K of aqueous solutions of P2M050236and PMOl20403-(Keggin structure) on y-alumina are presented. On the other hand, in order to study the influence of organic solvents on the stability of Keggin structure, experiments were carried out with solutions of the Keggin structure in ethanol-water and dimethylformamide. Solutions were studied before and after contacting the alumina by using NMR. Diffuse reflectance spectra and X-ray diffraction patterns of solid samples were obtained. PMoNi/y-AI203 catalysts were prepared by pore filling two step impregnation: first Mo-P and then Ni. The y-alumina spheres were impregnated with the above mentioned P-Mo solutions. Molybdenum content was 10% Mo while nickel content was 3 % Ni. On the catalysts thus obtained and for comparison purposes, the molybdenum and phosphorus concentration profiles were determined along the sphere radius by SEM. The hydrodenitrogenation activity of catalysts was also determined by using pyridine as a test molecule, which allowed to observe the performance of the systems prepared from both aqueous and organic solutions. 2. EXPERIMENTAL Ammonium heptamolybdate (AHM) Merck p.a., Phosphoric acid 85 % (PA) Mallinckrodt p.a., Phosphomolybdic acid (FK) Aldrich p.a., Ethanol (E) Merck p.a., Dimethylformamide (DMF) Anedra a.c.s, and commercial ~,-alumina spheres (sphere diameter: 4mm; surface area: 280 m2/g; porosity 54 % and average pore diameter: 4.2 nm) were used. In order to perform kinetic and adsorption studies alumina, grounded at a particle size less than 0.147 mm, was shaken with a solution of known concentration during a certain time. More details on the technique employed here are given in a previous work [10]. The solutions were prepared in the 5-120 mgMo/ml molybdenum concentration range by solubilizing: AHM and PA at a P/Mo=0.4 molar ratio, in water (W); FK in W; FK in a 1" 1 (v/v) mixture of E and W; FK in DMF. The experiments were performed at 293 K. Once the contact time was over, each sample was centrifuged and the solids were dried at 293 K and calcined at 823 K, both during 24 h. The molybdenum content in the solutions was determined by atomic absorption spectrometry. The P content in solution was obtained by precipitating it with the addition of ammonia AHM solution and concentrated nitric acid. The precipitate thus obtained was dissolved with NaOH and the excess of it latter was titulated with HC1. Solutions were characterized by NMR while solids were studied both by DRS and by XRD. PMoNi/AI203 catalysts were prepared by means of pore filling two step impregnation: first, molybdenum and phosphorus were impregnated on the alumina by employing identical solutions as those used for the equilibrium adsorption. In the second step and after drying the samples, these were impregnated with an aqueous solution of nickel nitrate. Solids thus obtained were dried at room temperature during 24 h and then calcined at 723 K during 1 h.
1123 By means of acid dissolution of the solids, Mo and Ni contents were analyzed by atomic absorption spectrometry. The distribution of both Mo and P along the sphere radius was obtained by electron microprobe analysis according to a previously described technique [ 11]. The hydrodenitrogenation activity (HDN) was determined in a flow equipment with fixed bed reactor and 0.3 g of a catalyst, sulphurized in situ at 553 K with a benzene current containing 10000 ppm S during 2 h. The HDN reaction was determined by feeding the reactor with a benzene-pyridine mixture (2000 ppm N2) at 3 MPa, 553 K, at a H2/HC molar ratio of 5 and LHSV=0.75 min ~-. The reactants and reaction products were analized by gas chromatography with ionization flame detector in a column packed with Carbowax 20M + 0.5% KOH on Graphpac, at 283 K and N2 as carrier. The activity was determined by following the conversion degree of pyridine. 3. RESULTS AND DISCUSSION
3,1. Adsorption isotherms In order to determine adsorption isotherms it was necessary to know the time required to reach equilibrium between the solution and the support, so these kinetic measurements were made in advance. In all systems under study, equilibrium was achieved, in practice, in less than 5 h; nonetheless in the present experiments, contact times of 48 h were employed. The adsorption isotherm of Mo and P on alumina from aqueous solutions of AHM and PA with a P/Mo=0.4 molar ratio, which corresponds to the P2Mo50236-heteropolyanion, were determined in the first place. Figure la shows the Mo adsorption isotherm thus obtained. It can be observed that the adsorbed molybdenum concentration (Ca) increases with the final molybdenum concentration in the equilibrium solution (Cf). However, the isotherm exhibits two different regions: one characterized by a plateau at low concentrations and the other by a concentration increase until a second plateau is reached. This behaviour is similar to that obtained during the adsorption of molybdenum from AHM solutions [12], as it can also be observed on Figure l a. Differences between both isotherms lay in the fact that the first plateau extends itself and the molybdenum content of the second plateau is greater for adsorption from molybdenum and phosphorus solutions. In the case of adsorption from solutions containing molybdenum without phosphorus, it was proposed that, at low concentrations, monomeric molybdenum is adsorbed while, at high concentrations, polymeric molybdenum is adsorbed on other type of sites [13]. When solutions of both molybdenum and phosphorus are employed, phosphomolybdates and phosphate can be adsorbed in addition to molybdenum species. This makes the change of adsorbed molybdenum species with the solution concentration different from that of the adsorption of AHM alone. On Table 1, values of Ca and of adsorbed P concentration (CAP) from representative samples of both zones of the isotherm, are presented. For the first two samples corresponding to the first plateau zone, CaP values are similar (0.4-0.5 %P). The same situation takes place with the other two samples corresponding to the second plateau, though in this case, the CaP value is significantly higher (2.7 %P). This shows the adsorbed phosphorus also presents two different zones which are coincident with those of the molybdenum adsorption isotherm. On the other hand, even when the CaP/Ca molar ratio in the solid of the second plateau zone (0.25) is greater than that of the first plateau (0.12), it is still less than the value used in the original solution (0.4) which in turn corresponds to the stoichiometric ratio of the
1124 40 304.
~ 2O o
Table 1 Adsorption isotherms of Mo and P from solutions of AHM and PA
..--
10 .
.
.
.
~. 301, 20
Ca CaP Cf CaP/Ca CfP/Cf (%Mo) (%P) (mgMo/ml) (mol/rnol) (mol/mol)
o
..
--
10 .~.._,,----~ - - - - - - - = 9
,
..
,
,,
,
,
.-. 3 0 20-
10.16
0.40
21.54
0.12
1.03
12.25
0.50
32.30
0.12
1.33
32.93
2.78
40.43
0.25
0.71
31.22
2.63
44.06
0.25
0.81
~
, - - . ~
,-_.
=,
=
10.~ _
.
.
.
.
.
.
.
.
.
.
.
.
.
.
o
100
"
"
-
40
56
60 70
80
90
Of (mgMelml)
Figure 1. Mo adsorption isotherms from solutions of: a ~ A H M +W;mAHM + P A +W, b)FK + W,c)FK + E + W,d)FK + DMF. P2MosO236 heteropolyanion. In accordance with this fact, the P/Mo molar ratio in the equilibrium solution is greater than 0.4 which results in a value of 1-1.3 for the first plateau zone and of 0.7-0.8 for the second plateau. These results indicate that the molybdenum adsorption with respect to phosphorus is greater than that of P2Mo50236- and can evidence that phosphomolybdate undergoes decomposition. According to Cheng and Luthra [5] the following equilibrium exists in the solution of AHM and PA:
8 H + + 5 MoO4 2" + 2 HPO4 2" < . . . . . . . . . > P2Mo50236- + 5 H20
(1)
As a result of adsorption, it was observed that the pH of the solution increases, mainly due to the release of hydroxyl groups from the alumina surface by ion exchange mechanism. This mechanism has been proposed by Iannibello and Mitchell [14] for MoO42- adsorption on alumina. This pH increase shifts the equilibrium of equation (1) to the left, thus causing pentamolybdodiphosphate decomposition. When the amounts of both molybdenum and phosphorus adsorbed in the first plateau are substracted from the total amounts adsorbed in the second zone of the isotherm, the
1125
calculation of the CaP/Ca molar ratio leads to a value close to 0.4. This could indicate that, in the second zone, and besides the monomeric form, the other adsorbed species is
P2Mo50236. As a consequence it can be proposed that, at low concentrations, monomeric molybdenum and phosphate are adsorbed and that, as the concentration increases, phosphomolybdate is also adsorbed. Moreover, the presence of heptamolybdate cannot be set aside, since it can occurs as a consequence of the P2Mo50236-anion adsorption and according to a mechanism previously proposed by Van Veen et al. [6]: 7 P2Mo50236- + 16 OH- < ......... >
14 HPO42 + 5 M070246- + H20
(2)
With respect to the adsorption isotherms of phosphomolybdic acid solutions, Figures l b, l c and l d show the Mo adsorption isotherms in aqueous phase, ethanol-water and dimethylformamide, respectively. Unlike the isotherm already presented in this work, the latter isotherms possess only one plateau, a fact that indicate the adsorption of only one species which would probably be the PM0120403- heteropolyanion. Comparison of the three adsorption isotherms from FK solutions allow to observe that the concentration of adsorbed molybdenum from both aqueous solutions were similar and at the same time greater than that obtained from solutions using dimethylformamide as a solvent. This can be explained in terms of a weaker solvation of FK in water [15]. On the contrary, when the DMF solvent is used, the latter becomes protonated and interacts with the anion [7]. This implies that the heteropolyanion-solvent structure has an ionic radius greater than that in water, leading to a less amount of adsorbed molybdenum due to steric effect. 3.2. N M R
31p NMR spectra of the initial AHM and PA solutions (P/Mo=0.4) and of resultant solutions after the adsorption on ,g-alumina have indicated that the species present in the original solutions is P2M050236 while, in the final ones, and besides this species, the HPO 2 species would be present. Figure 2a shows, as an example, the original solution spectra (I) and that of a solution after the adsorption (II). According to literature [6,16] the + 2 ppm peak corresponds to P2M050236 while the -6 ppm peak represents the HPO42 species. This leads to deduce that P2M050236undergoes depolymerization to MO70246" or MoO42 and HPO42-. On Figure 2b, the NMR spectra of one of the FK aqueous solutions employed in the obtainment of the adsorption isotherm and that of the final solution are shown. From the chemical shifts observed in these spectra, it can be concluded that species present in both aqueous solutions, i.e., before and after adsorption, is mainly PMo110397- (-1.2 ppm) while the PM01204o3- (-3.1 ppm) species is found in a much smaller proportion [6,16]. This implies that, in aqueous media, FK only undergoes a partial depolymerization. On the other hand, from the spectra corresponding to FK solutions in both E-W and DMF, it is observed that the FK remains unaltered both in the original and in the final solutions (Figure 2c). The stabilizing effect of non-aqueous solvents was observed by other authors [8,9]. These results would confirm what was indicated in the previous item, i.e., that the P2MosO236- species dissociates as a consequence of adsorption and that, on the solid, the ratio of both adsorbed P and Mo is different from that corresponding to the stoichiometry of such phosphomolybdate, while in the assays that used FK solutions, the adsorbed species is a phosphomolybdate.
1126
C
I ~l
I I
t
I
I
I
I
I
!
i
I
I
n
I
I
I
I
I
I
I
I
I
I
II -
I
8
I
i
__
]I 9
I
I
n
I
t.
I
I
I
I
0 -~ ppm
I
I
-8
n
I
I
0 -2 -~
ppm
I
0
-3
ppm
,.
I~1
- -
-
I
-6
Figure 2.31p NMR spectra from solutions of: a) AHM + PA +W, b) FK + W, c) FK + E +W. I) original solution, II) final solution. 3.3. DRS and XRD Figure 3 shows the characteristic diffuse reflectance spectra of non-calcined solids from the adsorption of different molybdenum-phosphorus solutions on alumina. For comparison purposes, the figure also includes both AHM and FK spectra. The spectrum of a non-calcined solid impregnated with AHM and PA solution (P/Mo=0.4) is similar to that of AHM which indicates that, during adsorption, the P2MosO236 species depolymerizes. With respect to the non-calcined solids impregnated with FK solutions in different solvents, the spectra show that, for the adsorption from E-W and DMF, the FK species are present. However, in samples obtained from aqueous solutions, this phase appears to be degraded. It has to be pointed out that conclusions arrived at coincide with those obtained from both NMR and adsorption isotherm studies. According to the literature [12,17] and to the model compounds spectra, the tetrahedral molybdenum (Mo(t)) exhibits two absorption bands at 220 and 260 nm, while, in addition to both bands, the octahedral molybdenum (Mo(o)) presents a characteristic band at 350 nm. In the FK case, the latter band extends up to 500 nm. In the DRS spectra of calcined solid samples which were prepared from FK solutions, the 350 nm band does not continue. This indicates that, during calcination, the FK degraded. From the spectra of calcined solid samples, Schuster-Kubelka-Munk function was calculated at both 350 nm and 260 nm. The ratio of these functions versus Cf is displayed in Figure 4 with the purpose of showing the variation of the Mo(o) to Mo(t) ratio as a function of the solution concentration. Figure 4a shows that this ratio increases with molybdenum concentration in the AHM and PA solution, in a similar fashion to that of the increase of Ca. If this results are compared with data corresponding to samples prepared without PA, it can
1127
2ot 15
=
d
....._L-.-
"
,~
-~
N 20 t
i:k
200
300
400
500
,1. ( n m ) Figure 3. DRS spectra of FK, AHM and non-calcined solids from: a) AHM +PA +W, b) FK +W, c) F K + E + W , d) FK+DMF.
c
' !
-~ '
--J
o ~5 2o 30- ,~o so 60 7o ao 90 C I: ( rng Ho/rnl)
Figure 4.SKM function ratio from calcined solid spectra of: a)x,AHM +W~,AHM + P A + W , b)FK +W,c)FK + E +W, d)FK + DMF
be observed that the ratio of octahedral to tetrahedral molybdenum is similar for samples corresponding to the first and second plateau zones. In the case of samples prepared from FK solutions, the Mo(o)/Mo(t) ratio increases as Cf increases but, as in the corresponding isotherms, no two plateau zone but a continuous increment is observed (Figures 4b, 4c, 4d). This is basically related to the fact that the adsorbed species is the same, regardless of the solution concentration under use. In accordance with what it was indicated in previous works [10,12] during the calcination of solids obtained from adsorption of high AHM concentration solutions on "y-alumina, the molybdenum polymeric species decomposes, thus leading t o M o O 3 clusters. This crystalline compound has been observed by X-ray diffraction. The same fact can be expected from the results of samples prepared with AHM and PA solutions (P/Mo=0.4). Actually, MoO3 was detected in samples corresponding to the second plateau. However, the presence of MoO3 was not observed in samples prepared from FK solutions. It can therefore be said that, in this case, after calcination, the molybdenum would be as Mo(t) which interacts strongly with the alumina, and as Mo(o) probably better dispersed than in the previous systems.
1128
3.4. PNiMo/AI203 catalysts The precursors and solvents used for the preparation of the catalysts and their composition are shown in Table 2. Results obtained by means of both DRS and XRD of NiMo, PNiM01, PNiM02, PNiMo 3 Table 2 Catalyst composition and HDN activity Catalyst
Precursor
Solvent
%Mo w/w
%Ni w/w
Commercial NiMo PNiMol PNiMo2 PNiMo3 PNiMo4
AHM AHM +PA FK FK FK
W W W W-E DMF
10.00 10.00 8.24 8.20 9.93 8.90
3.98 3.41 3.20 3.03 2.89 3.92
Specific Conversion 2.94 2.31 3.01 2.83 1.48 1.91
and PNiM04 catalysts were similar to those of calcined solids from isotherms obtained from corresponding solutions. In the DRS spectra of such catalysts, bands imputed to Mo(t) and Mo(o) were observed in all cases, while, in corresponding samples prepared from FK solutions, FK species was not found. On the other hand, in XRD patterns of NiMo and PNiMo~ catalysts, MoO3 was detected. However in patterns of PNiM02, PNiM03 and PNiM04, the interplanar spacing lines of neither M o O 3 n o r any other crystalline species were detected. Concentrations of Mo and P along the sphere radius of catalysts prepared are shown in Figure 5. The Mo concentration profile of the NiMo catalyst is practically fiat, except near the boundaries where a higher Mo concentration value is observed (Figure 5e). On the other hand, for PNiMo~, PNiMo2 and PNiMo3, the Mo concentration diminishes continuously from the surface to the center of the sphere (Figures 5a, 5b, 5c). It must be pointed out that the catalyst prepared in DMF shows a profile different from the others; it is egg shell type, i.e., it has a high concentration on the surface and near-zero value in the rest of the pellet (Figure 5d). This is due to interactions between FK and DMF which result in a structure having a greater ionic radius, as it was previously mentioned, thus avoiding deep penetration into the pores. For all catalysts, the P concentration diminishes towards the sphere center. It can be pointed out that the P profile shape of PNiMo4 catalyst is similar to that of Mo. The specific conversion values of pyridine, i.e., pyridine conversion divided Mo concentration, for the series of catalysts prepared are shown on Table 2. From those values, it can be concluded that, even when this catalysts present a HDN activity of the same order of that corresponding to a commercial catalyst, there are slight differences between both. Catalysts prepared from AHM and PA and those obtained by impregnation of FK aqueous solutions were more active than the catalysts without P. However, according to the pyridine test those catalysts prepared from FK solutions in non-aqueous solvents were less active. This order does not appear to be directly related to the Mo dispersion because, through XRD, crystalline M o O 3 w a s not detected for PNiMo2, PNiMo3 and PNiMo4 catalysts.
1129
For both PNiM01 and PNiMo 2, and due to phosphomolybdates degrade totally or partially during adsorption, the active phase would probably be more easily formed between Ni and Mo than for the PNiM03 and PNiM04 cases, in which the FK remains intact. However, the latter catalyst could be more active than the rest for reactions where diffusion of reactants towards the pellet interior controls the reaction rate because egg shell profiles are present.
e
A
A A
d
9
,=.
~
c rib,
A
A
4k.
Figure 5. Mo and P concentration profiles of the catalysts: a)PNiM01, b)PNiM02, c)PNiM03, d)PNiM04, e)NiMo.
A
A
b A A
A
M
K
K
~
,A
,= o,.
,
--
a
A 9
-1,0
m
N
K
-0,6
-0,2
r/R
0;2
0~6
1
4. CONCLUSIONS Species present in AHM and PA solutions (P/Mo=0.4) used in this work is P2Mo50236which depolymerizes to either M070246" or MoO42 and HPO42- as a consequence of the adsorption on alumina. At low Mo concentrations, both monomeric Mo and phosphate are adsorbed. At higher concentrations, phosphomolybdate and eventually heptamolybdate are adsorbed in addition to the species previously mentioned. The presence of tetrahedrally coordinated Mo, octahedral Mo and MoO3 was observed in calcined solids. Partial degradation of the heteropolyanion was observed for FK aqueous solutions. On the
1130
contrary, it remains unaltered in E-W and DMF. Adsorption isotherms from FK solutions evidence the adsorption of only one species which would be a phosphomolybdate. As a result of calcination, FK underwent degradation and the presence of MoO3 was not detected. Even when the series of catalysts obtained show a HDN activity of the same order of the commercial ones, catalysts prepared here from both AHM and PA solutions and those prepared by impregnation with FK aqueous solutions were more active than the phosphorusless catalysts. However, catalysts impregnated with FK solutions in non-aqueous solvents showed less activity according to the pyridine test reaction. For catalysts prepared with P, the Mo concentration along the sphere radius diminishes continuously from the surface to the center. The catalyst prepared from dimethylformamide exhibits an egg shell profile. Then, the use of different precursors and solvents provides an adequate way for obtaining controlled profiles in these impregnated catalysts. ACKNOWLEDGEMENTS The authors thank L. Osiglio, D. Pefia and F. Iborra for their experimental contribution. REFERENCES 1. D. Chadwick, D.W. Aitchison, R. Badilla-Ohlbaum and L. Josefsson, Stud. Surf. Sci. Catal., 16 (1982) 323. 2. M.M. Ramirez de Agudelo and A. Morales, Proc. 9~ Int. Congress Catal., M.J. Phillips and M. Terman (eds.), Ottawa, 1 (1988)42. 3. P. Atanasova, T. Halachev, J. Uchytil and M. Kraus, Appl. Catal., 38 (1988) 235. 4. R. L6pez Cordero, N. Esquivel, J. IAzaro, J.L.G. Fierro and A. L6pez Agudo, Appl. Catal., 48 (1989) 341. 5. W.C. Cheng and N.P. Luthra, J. Catal., 109 (1988) 163. 6. J.A.R. van Veen, P.A.J.M. Hendriks, R.R. Andr6a, E.J.G.M. Romers and A.E. Wilson, J. Phys. Chem., 94 (1990) 5282. 7. M. Fournier, R. Thouvenot and C. Rocchiccioli-Deltcheff, J. Chem. Soc. Faraday Tram., 87(2) (1991) 349. 8. M. Frouchart and P. Souchay, C. R. Acad. Sci. C266 (1968) 1571. 9. P. Souchay and R. Contant, C. R. Acad. Sci., C265 (1967) 723. 10. C.V. Cficeres, J.L.G. Fierro, A. L6pez Agudo, M.N. Blanco and H.J. Thomas, J. Catal., 95 (1985) 501. 11. P.G. V~quez and J. Riveros, X-ray Spectr., 21 (19.92) 197. 12. M.A. Aulmann, G.J. Siri, M.N. Blanco, C.V. Cficeres and H.J. Thomas, Appl. Catal., 7 (1983) 139. 13. M.I. Morales, M.N. Blanco and H.J. Thomas, Adsorp. Sci. Tech., 5(1) (1988) 57. 14. A. Iannibello and P.C.H. Mitchell, in Preparation of Catalysts II, B. Delmon, P.A. Jacobs and G. Poncelet (eds.), Elsevier, Amsterdam (1978)469. 15. L.C.W. Baker and M.T. Pope, J. Am. Chem. Soc., 82 (1960) 4176. 16. G.B. Me Garvey and J. B. Moffat, J. Mol. Catal., 69 (1991) 137. 17. J.H. Ashley and P.C.H. Mitchell, J. Chem. Soc. A (1961) 2821; (1969) 2730.
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
Impregnation
Ahmed
Design
K. A b o u l - G h e i t
Egyptian
Petroleum
ABSTRACT
:
for Preparing
and
Sohair
Research
M.
1131
Bimetallic
Catalysts
Abdel-Hamid
Institute,
Nasr
City,
Cairo,
Egypt.
The data show how two metal precursors, possessing equal rates of adsorption on a Z-alumina support, be homogeneously dispersed through adding a proper additive (HCI) for preparing bimetallic catalysts, whereas two metal precursors, possessing different adsorption rates, could not be homogeneously dispersed or distributed in the support while using this additive. Chloroplatlnic and chloroiridic acids are examples of the first type whereby PtIr-containing catalysts are prepared 9 Chloroplatinic acid and ammonium paratungstate are examples of the second type whereby PtW-containing catalysts are prepared. For preparing PtIr-containing catalysts a single impregnation of the two precursors from one solution is thus possible, whereas for preparing PtW-containing catalysts, two impregnations from two separate precursor solutions are to be carried out. Key Words : Catalyst, persion.
Impregnation, platinum, iridium, Tungsten, metal dis-
INTRODUCTION Bimetallic catalysts constitute a very important category of industrial catalysts used in the petrochemicals and petroleum refining industries. Bimetallic catalysts, particularly, those possessing low metal contents of noble metals, require high metal dispersion in the support to give maximum catalytic activities. During impregnation of the metal precursors in the support, certain chemicals (additives) are added to modify the interaction mode of these precursors with the support For instance, HF T M HCI T M NH4CI T M and citric acid ~4~ have been used as additives but without disclosing their role. The author c5-'~ prepared bimetallic catalysts containing PtGe, PtMo, PtRe, PtRu, PtW, RuRe and PdRe to be used for aromatics hydrogenation and n-paraffins isomerisation, and found that an additive may be suitable for a given bimetallic combination but unsuitable for another. In the present work, the impregnation of chloroplatinic acid in ~-alumina in presence of chloroiridic acid or ammonium paratungstate is examined via tracing the metal dispersion (or distribution) to shed light on the possibility of using coimpregnation or two successive impregnations (more expensive) for preparing bimetallic catalysts containing PtIr and PtW on alumina.
1132
EXPERIMENTAL
:
Materials = Catalyst Support : performed Y-alumina in the form of 3 X 3.5 m m pellets was used as a support for all catalyst preparations. This alumina has the properties : surface area 210 mZg-1; total pore volume 0.46 cm3g-X; grain density 1.0 cm -3 and structural density 3.14 g cm -3. Metal precursors : Chloroplatinic acid, chloroiridic acid and a m m o n i u m paratungstate were AR reagents from Merck. Additive : Hydrochloric acid was used in all impregnations. The ratio of Cl-/PtCI~was used in this w o r k as the ratio of c o n c e n t r a t i o n of chloride ion in the additive to that of the c h l o r o p l a t i n a t e ion"'chloroplatinic acid. Similarly, the ratio of CI-/IrCI~- was used to represent chloride ion in HCI per chloroiridate ion in chloroiridic acid. Also, the ratio C I - / W was used to represent chloride ion in HCl added per tungsten atom in the paratungstate precursor.
Impregnation : In case of Pt and Ir containing catalysts, aqueous solution of each precursor, adjusted to obtain the desired w e i g h t percentage of m e t a l in the final catalyst (0.15, 0.35 or 0.70%) was used together with a calculated amount of the additive (HCI). The total volume of impregnation m i x t u r e used was three times as large as the volume of the desired w e i g h t of the support. The solution-alumina mixture was left w i t h intermittent shaking till complete depletion of the metal precursor. In case of Wand Pt-W-containing catalysts, H20~ was added in excess (20% by volume of the impregnating solution) to prevent precipitation of paratungstic acid. Metal D i s p e r s i o n Hydrogen Chemisorption was carried out using a pulse technique as described by Freel [9] to determine the dispersion of Pt and Pd on the support surface. Prereduction of the metal was carried out by h e a t i n g at 500~ in a current of hydrogen (50 cm 3 min -~) for 8 h [~~ followed by d e g a s s i n g for 2 h in N2 flow of 30 cm 3 min -~. A f t e r cooling down to room temperature, hydrogen was pulsed into the carrier gas till complete saturation. Hydrogen uptake was calculated as hydrogen atoms adsorbed per total metal atoms on basis of i : i stoichiometry [ ~ - ~ 2 ~ . For W containing catalysts, H2 uptake was not reproducible. Metal D i s t r i b u t i o n : i0 pellets were randomly picked out from the impregnating solution, cut perpendicular to the axis to expose a n e w sectional face then sprayed w i t h the proper indicator. The impregnated zone was then m e a s u r e d by visual inspection under magnification. Details of the procedure are given elsewhere c~e]. The indicators used are : Potassium Iodide : 2% KI in ethanol Bromine-Benzidine : The catalyst was exposed to Br2 for 15 min., heated for 30 min at IO0~ then sprayed w i t h benzidine solution (50 mg in i00 cm 3 of glacial acetic acid). Dithiol-TiCl3 : 5 cm 3 of a solution of 15% TiCI3 is m i x e d w i t h 20 cm 3 of 3 N HCI then with dithiol (2% in amylacetate).
1133
RESULTS
AND
DISCUSSION:
Impregnation of Chloroplat~ic and C h l o r o l r l d l c A c i d s : The i n t e r a c t i o n b e t w e e n c h l o r o p l a t i n i c acid or chloroiridic acid and the )'-alumina support used is so strong that poor dispersion (and distribution) of Pt and Ir is obtained. Fast adsorption of a metal precursor in the support (strong interaction) decreases the metal dispersion, whereas slow interaction gives uniform dispersion r133. However, the use of HCI as an additive in impregnating H~PtCle and H2IrCl6 in >'-alumina is found to increase Pt and Ir dispersions by increasing the concentration of HCI. In absence of HCl, the dispersions of Pt and Ir are found to be almost equal and amounting to 0.45, 0.35 and 0.25 H/M for metal contents of 0.15, 0.35 and 0.70 wtZ, respectively; i.e., dispersion decreases with increasing the metal content. Almost complete dispersion for a metal content of O.15Z has been attained at an HCl concentration ratio of 500 CI-IPtCI~-. Higher metal contents require higher HCl concentrations to be completely dispersed. Equations (i) to (3) may explain the role of HCI in improving Pt (or Ir) dispersion. In absence of HCI, the precursor reacts with alumina directly: I 2 - A1 - OH
fast + 2H+PtCI 6-
~
~
(1)
(AI)~ -PtCle + 2 H20
f This reaction is fast and gives poor dispersion of Pt. However, presence of HCI, competitive adsorption between both acids takes place :
I
fast - A1
- OH
+
HCl
Then chloroplatinic (-AI-CI) :
-,
acid
I 2 - A1 - C1
in
-
reacts
AI-CI
slowly
+
H20
with
the
(2)
chlorinated
alumina
slow + 2 H2PtCle ,
. -(Al)z- PtCle + 2 HCI
(3)
i This slow anion exchange is of prime importance sion of Pt (and Ir) in the support.
for ameliorating
the disper-
Since Cl- ion diffuses much easier than the voluminous PtCl2- ion, The AI-CI phase will thus be pre-established in the support. The PtCI~anion can then diffuse via its exchange with CI- ion (equation 3), resulting in deeper migration towards the pellet center. Data in Fig. 1 (a) show that the dispersion of Pt and Ir in the alumina support fall on the same curve for each metal content using HCI as an additive. Since metal dispersion is a function of the rate of metal precursor uptake, it is assumed that the rates of HzPtCle and H21rCle uptake are equal in presence of HCI and that the anion exchange reactions (equation 3) for the Pt and Ir precursors with the - AI-CI phase have almost equal rates. In commercial preparations, the logarithic form of Fig. 1 (a) may determine the critical additive concentration to be used. Fig. 1 (b) shows that the logarithm of the dispersion values is a linear function of the CI-/PtCIZratios up to i00. Beyond this value, limited dispersion increments are obtained by relatively large additions of HCI.
1134
1.0
3)
oZ 0.8 Or) (3_
o.6
_J
,,, :E
0/.
0.2 l
I
I
1
t
J
J
S
~ -0.2
Vl
~
9
9 0.35% I r
I-~ II ~,/ -0.6
0
NO ADDITIVE
~
0.35"/oPt+O35% Ir
~
0.70%
Pt
l
I
I
I
I
J
100
200
300
400
500
600
2-
C(/PtCL~-or Ct~/~rC t6
Fig. i: Dispersion of different contents of Pt and Ir in ~-alumina as a function of HCI concentration Impregnation
of Ammonium
Paratungstate:
It is found that ammonium paratungstate in water solution gives a precipitate of paratungstic acid. So, H202 has been added to prevent this precipitation through converting it to the soluble diperoxytungstic acid : (HW602x) 5- + 12 H202 paratungstate ion
~~"
3 (We011) 2- + H + + 12 Hz0 dipe roxytung s t ate (soluble)
(4)
When HCl is added to the impregnating solution, the -AI-CI phase is first formed as in equation (2), then reacts more slowly with diperoxytungstic acid to give (AI)2We0~I which has been found to be well distributed (well dispersed) in the support as shown by visual inspection using the dithiol-TiCls indicator. In absence of HCI, tungsten distribution is around 50% (Fig. 2, curve a), and on addition of HCI the distribution continually increases to reach as higher as 95Z at a CI-/W ratio of 175.
1135
cr o
5o
~00
150
1.0~
,
,
,
,100
(c)
oz0815s
18~
a. 0.6
- 60
to
~
to
~, ~ _
:
~,m,'"
(b)
m
0.4
'~
- 40
I
9 0.35)'oPt in presence of W ond H202
a.. 0.2F
o
l
o
0.35 %W in presence of P t a n d
o
0.35 % W in presence of H202onty
I lo0
0
~
l 200
,
H202
I , 300
~ ~
c3 20
0 400
C["/PtCl&-
Fig. 2: Dispersion of Pt and distribution of W as a function of HCl concentration. Impregnation of one metal precursor in presence or absence of the other. Dispersion
and
Distribution
of M e t a l s
in t h e
Support:
The relation between Pt and Ir dispersion as determined by H~ chemisorption and their distribution as determined by visual inspection in the alumina support under study has been found to be practically linear whereby a complete metal dispersion, i.e., 1.0 H/M, corresponds to a complete metal distribution, i.e. , IOOZ, which implies the application of the relationship: Distribution = Dispersion X 100 Hence, any one of these two parameters, in the present study, can be used to represent the other. In catalyst preparations where Pt or Ir dispersions in y- alumina can not be practically determined using the known techniques of chemisorption of gases, e.g., H~ or CO, the metal distribution data can be used instead. of A m m o n i u m P a r a t u n g s t a t e and Chloroplatinic Acid : When ammonium paratungstate and chloroplatinic acid are impregnated from one solution, using H202, tungsten distribution has only reached 27Z. However, addition of HCI has gradually increased W distribution to a maximum of about 47Z at an HCI concentration ratio (CI-/W) of 150, beyond which no improvement of the distribution takes place (Fig. 2, curve b). On the other hand, the dispersion of platinum has been improved even more than when impregnated in a separate solution (compare Figs. 1 and 2, curve c). Moreover, in absence of HC1, Pt dispersion reaches 0.55 H/M which is relatively high compared to that obtained during the impregnation of chloroplatinic acid separately. This may be attributed to the presence of hydrogen peroxide which may contribute to a slower uptake of the platinum precursor by the support. Coimpregnation
1136
It is to be pointed out that although Pt dispersion has been improved in presence of the W precursor and hydrogen peroxide, the distribution of W has been largely inhibited by the presence of Pt precursor. The latter may have limited the surface of alumina which is accessible for the W precursor to spread upon.
CONCLUSION
:
The data obtained indicate that precursors having similar natures such as H2PtCI6 and HuIrCl6 can be coimpregnated from one solution, which is commercially profitable. However, precursors of different natures such as HuPtCI6 and ammonium paratungestate should be impregnated in two stages, each stage followed by washing, drying and calcination, as normally carried out when the wet-impregnation technique is carried out. Nevertheless, it appears important to examine in all cases, when an active support is used, the dispersion of one metal precursor in presence of the second and vice versa for preparing bimetallic catalysts.
REFERENCES I. 2. 3. 4. 5. 6. 7. 8. 9. I0. ii. 12. 13.
E. Nichalko; US patents 3 259 454 and 3 259 589. A.K. Aboul-Gheit and J. Cosyns, J. Appl. Chem. Biotechnol., 26, 15 (1976). J.P. Brunelle, A. Sugier and J.F. LePage, J. Catal., 43. (1976) 273. J.C. Summers and L.L. Hegedus, J. Catal. 51, (1978) 185. A.K. Aboul-Gheit and J. Cosyns, J. Appl. Chem. Biotchnol., 26, (1976) 536. A.K. Aboul-Gheit and J. Cosyns, Rev. Inst. Mex. Petrol., 7 (3), (1975) 61. A.K. Aboul-Gheit and J. Cosyns, J. Appl. Chem. Biotchnol., 27, (1977) 121. A.K. Aboul-Gheit, M.F. menoufy and F.M. Ebeid, Appl. Catal., 4, (1982) 181. J. Freel, J. Catal., 25, 139 (1972). A.K. Aboul-Gheit, Aromatics Hydrogenation on supported Bimetallic Combinations, Inst. Franc. Petrole, Rep. Ref. No. 20 874 (1973). R.M. Fiedorow, B.S. Chahar and S.E. Wanke, J. Catal., 51, (1978) 193. A.K. Aboul-Gheit, J. Chem. Tech. Biotechnol., 29, 480 (1979). J.C. summers and S.A. Ausen, J. Catal., 52, (1978) 445.
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
1137
Comparative study on low-temperature Cu/activated carbon catalysts prepared by impregnation from aqueous and organic media D. Mehandjiev, R. Nickolov, E. Bekyarova and V. Krastev Institute of General and Inorganic Chemistry, Bulgarian Academy of Science, Sofia 1113, Bulgaria
1.1NTRODUCTION Activated carbon has an increasing application as a catalyst support. However, irrespective of the numerous studies on the preparation of carbon-supported catalysts, the publications on the active phase-support interaction during the active phase deposition and the role of the porous texture and chemical nature of the carbon surface with respect to the properties of these catalysts are not many in number. The results on the preparation and investigation of carbon-supported catalysts reported during the past years show that the chemical nature of the support surface is often of the same importance for the incorporation of the active phase into it as are the specific surface area and the porous structure. The chemical nature of the surface affects the active phase incorporation mostly through the media (aqueous or organic) of the catalytic precursor. The purpose of the present investigation was to elucidate the effect of oxidizing modification of the support (activated carbon) surface on the active phase during the preparation of the Cu/activated carbon catalysts, by depositing copper nitrate from aqueous and methanolic media using the incipient wetness technique.
2. EXPERIMENTAL 2.1. Samples Low ash content activated carbon from apricot shells [1] was used as support. The samples were oxidized with concentrated HNO 3 (17.5 ml per 1 g activated carbon) for 14 days at room temperature. After the end of the process the samples were washed with distilled water to constant pH. The initial activated carbon sample was denoted by AC, and the oxidized sample, by OAC. The catalysts were prepared by a modified incipient wetness technique [2]. The support (AC or OAC) was sprayed four times with aqueous or methanol solution of Cu(NO3)2.3H20 in amounts slightly less than the total pore volume of the samples and a concentration ensuring a final load towards Cu after the last deposition of about 4 wt%. After each deposition the samples were allowed to stand in a controlled closed volume for 24 h and dried as follows: 1 h under vacuum at room temperature, 1 h in a dry box at 313 K and finally 10 min under vacuum at room temperature. The samples thus prepared were heated at 476 K for 6 h and stored in a desiccator.
1138 Four types of samples were prepared in this way: ACM, obtained by deposition of the active phase on unoxidized AC from a methanolic solution; ACW, prepared by active phase deposition from an aqueous solution on unoxidized AC; OACM resulting from deposition of the active phase on OAC from a methanolic solution, and OACW, with which the active phase was deposited on OAC from an aqueous medium.
2. 2. Methods of investigation The characterization of the porous texture of the initial and oxidized activated carbon and the catalysts was achieved using nitrogen adsorption at 77.4 K in a standard volumetric apparatus. The nitrogen adsorption isotherms were used for determining the following texture parameters: specific surface area (SBET), by the BET method; micropore volume (VMI) and specific surface area of the mesopores (SMEs), by the ffF method; mesopore volume (VMEs) as the difference between the maximum nitrogen adsorption volume (Vs) and VMI, and pore distribution within the range of 2-150 A determined for the mesopores by the Pierce method using the adsorption branch of the N 2 isotherms and for the micropores by the simplified equation [3]. The samples were investigated by IR spectroscopy, XPS analysis and atomic absorption spectroscopy. The IR studies were carried out with a SPECORD MS0 spectrometer within the range of 1800-800 cm -1. The tablets consisted of 200 mg KBr and 0.5 mg OAC and were homogenized well. A tablet of KBr and AC obtained in the same way was used as a standard. XPS spectra were registered using an ESCALAB Mk/I (VG Scientific) X-ray spectrometer with a 1486.6 eV AIKoc source. The Cls peak (284.6 eV) was used as an internal standard for calibrating the binding energies. The Cu contents of the samples were determined by atomic absorption using a Pye Unicam SP 90B spectrophotometer.
3. R E S U L T S A N D D I S C U S S I O N 3.1. Supports The nitrogen adsorption isotherms (77.4 K) for the AC and OAC supports are presented in Fig. 1. They are close over the whole range of relative pressures. The texture parameters of the samples AC and OAC are given in Table 1 Table 1 Texture parameters of activated and oxidized activated carbon and Cu/AC catalysts Sample Cu content a, Impregnation SBET, VMI, S M E S , VMES, AC OAC ACM ACW OACM OACW
(wt%) 3.90 4.00 4.13 4.13
media CH3OH H20 CH3OH H20
(m2/g) 1060 1006 1040 960 955 985
a Determined by atomic absorption spectroscopy
(cm3/g) 0.40 0.37 0.41 0.36 0.36 0.34
(m2/~) 153 156 173 259 208 294
(cm3/g) 0.51 0.51 0.43 0.48 0.58 0.61
1139 The SBET and VMI values for both samples are very dose. A weak trend to decrease is observed with sample OAC with respect to the values for sample AC. Juxtaposition of the micro- and mesopore size distributions (Fig. 2) reveals a slight shif~ of the maxima for the micropores from 10 A for AC to 9 A for OAC, a simultaneous homogeneity increase in the mesopore region for the latter sample being observed.
o-OAC O-OACM
~
o
60 .
o -AC a -OAC
50
E15 E
10 00.0
0.2
0.4
0.6
P/Po
0.8
Figure 1. N 2 adsorption isotherms of AC (curve 1),OAC (curve 2) and OACM (curve 3) samples.
1.0
40
60 r, A
80
100
120
Figure 2. Pore size distribution curves over the 2-150 A range for samples AC (curve 1) and OAC (curve 2).
Regardless of the fact that the texture characteristics change negligibly, the changes themselves evidence that HNO 3 reacts with the organic matter of the initial activated carbon. The results from pH determination and acid/base neutralization according to Boehm's procedures (Table 2) indicate that oxidation with HNO 3 produces a larger amount of acid surface groups and leads to a drop in number of the basic groups, i.e. the chemical nature of the activated carbon surface has changed significantly due to the oxidation with HNO 3. Table 2 , Neutralization capacity for HCI and different bases and pH of AC and OAC Sample Amoumt neutralized, (meq/100 m2). 102 pH
,
AC OAC ,
.... ,
,,
,,
1,
HCI
NaOfi
Na2CO3 "' NafiCO3
7.7 2.7
1.4 18.5
. . . . . -' 13.0 7.0
1,,,,
,
,,,,=
'7.3 4.8
1111,
The XPS analysis data on AC and OAC agree with those in Table 2. The differences between the XPS spectra of OAC and AC show an increase in oxygen amount at the surface, the appearance of a more pronounced nitrogen peak (-407 eV) and a much more pronounced asynunetric character of the C ls peak which is shifted to the oxygen containing groups (at 285-289 eV). After the OAC sample is argon ion (3 kV) etched for 1 min, the asymmetric character of the C ls peak probably becomes equal to that of AC, i.e. the oxygen surface
1140 groups are destructed. The nitrogen peak also changes with respect to the binding energy (404 eV) which is characteristic of the reaction NO3--->NO2-. The results on the oxygen surface groups obtained by IR spectroscopy are scarce [4]. The spectra obtained contain absorption bands at 1650-1550 cm -1 and 1300-1150 cm -1.
,
800
I
1000
I
I
I
I
|
1200 1400 11SO0 wovenumber,cm-
I
1800
Figure 3. IR spectrum of the OAC sample. The bands within the 1650-1550 cm -1 range are interpreted differently by the different authors. The absorption at these frequencies is mostly attributed to C=C stretching vibrations and to chelate carboxyl groups. According to Daza et al [5], the bands at 1600-1580 cm -1 are due to the enolic form of the dicarbonyl group. Absorption at 1300-1150 cm -1 is established by many authors. It is ascribed to the C-O stretching vibration due to lactonic and phenolic groups. The IR studies and the Boehm analysis indicate that, as a result of oxidation with HNO3, a large number of acid oxygen complexes such as carboxylic, lactonicic and phenolic complexes are formed on the surface.
3.2. Cu/activated carbon catalysts. The negligible differences in the porous textures of the supports AC and OAC reduce the differences in composition and character of incorporation into the support of the active phase of samples ACM, ACW, OACM and OACW as a result of the effect of two factors: the media from which the active phase is deposited and the chemical nature of the carbon surface. Fig. 1 shows the N 2 isotherm of sample OACM. The isotherms of the remaining catalysts are similar to this type. The texture parameters of the above catalysts are given in Table 1. SBET exhibits a typical decrease which is not in agreement with the change of VM]. With SMES and VMEs there is a similar situation. In addition, the effect of the media (methanolic and aqueous) on the change of the above parameters depends on whether the carbon surface is oxidized or not. In this sense the active phase distribution of ACM is, due to the wettability of the hydrophobic carbon surface, much more uniform than is the case of OACM. The coinciding maxima of the distribution curves of the micropores (Fig.3) reflect the effect of the same media (methanol). The difference in the texture parameters of ACM and OACM is probably due to the high content of oxygen surface groups of OACM. Due to the high degree of dissociation of Cu(NO3)2.3H20 in methanol [6], the phase is incorporated preferentially into the above oxygen groups which, on their part, are located in the region of fine pores and at the entrances to micropores and very narrow pores. In this case local maxima such as that at 27 A (Fig.3) appear. The texture differences between OACM and OACW can be associated with quite the opposite reason: the presence of
1141 molecules undissiciated in methanol. The above assumptions also concern samples ACW and OACW (Fig.4). Contrary to sample ACM, where the homogeneity (according to the distribution curve) is associated with the presence of a minimum amount of oxygen surface groups, in the case of OACW the homogeneity is due to the high content and the relatively uniform distribution of the oxygen groups. OACW displays (Fig.4) a shift in the micropore distribution curve maximum (with respect to the maximum of ACW) in the direction to larger sizes. 70~
-
70
o -ACI~
60-
a -OACM
o -OACW
~, 50
0 50
0
)3o 20
20
10
10
0
20
40
60 r, A
80
100
120
Figure 4. Pore size distribution curves over the 2-150 ,~ range for samples ACM (curve 1) and OACM (curve 2).
20
40
60 80 100 r, A Figure 5. Pore size distribution curves
120
over the 2-150 ,~ range for samples ACW (curve 1) and OACW (curve 2).
The same maximum (of the curve of OACW) almost coincides with the maxima of the curves for ACM and OACM (Fig.3). Hence, the incorporation into the support of the metal phase from a methanolic medium (for AC and OAC) and from an aqueous medium (for OAC) at the same precursor concentration and constant conditions, proceeds within the region of the fine support pores. On unoxidized carbon (AC) and from aqueous media the metal phase is incorporated into the support within the larger mesopores region. XPS analysis was used for estimating the concentration and degree of Cu oxidation. Table 3 shows the binding energies (BE) with the more intense lines (Cu2p3/2 , O ls and N ls). Table 3 Binding energies of the Cu2p3/2, O l s and N ls peaks and kinetic energy of the Cu LMM peak
(ev)
Sample ACM ACW OACM OACW
Cu2p3n CuO Cu20 933.9 934.1 934.0 932.4 934.1 -
Cu LMM Auger CuO Cu20 917.9 917.9 917.9 917.1 917.8
CuO 529.8 529.8 529.8 529.8
O 1s N1 s Cu20 Cu(OH) 2 CuO Cu20 531.7 407.6 531.6 403.1 407.0 530.3 531.7 407.3 531.7 407.7 T,
1142 It has been established that with the same degree of Cu oxidation, the binding energy values of Cu2p3/2 (Table 3) exhibit a certain increase due to the electronegativity of the surroundings (NO3-, NO2- ). A drop in intensity of the satellite shake-up region (about 939 to 949 eV) is characteristic of the Cu(2+)~Cu(l+)transition and is observed with the OACW and OACM samples. However, only for OACM the presence of Cu20 has been established using a LMM Auger electron spectrum. May be Cu(l+) is present in the composition of the supported phase of the other samples, however in concentrations lower than the sensitivity of the instrument. The values of the binding energy for O ls in Table 3 show an additional maximum in the presence of OH-. Table 4 presents the surface composition of the catalysts. Table 4 Composition of Catalysts as ~iven by XPS Sample Composition, at% Cu O N C ACM 2.4 13.7 4.8 79.1 ACW 1.3 10.9 2.7 85.1 OACM 0.4 10.8 1.7 87.0 OACW 0.5 11.7 2.1 85.7 The binding energy (BE) of the NO 3- electrons which are directly connected with Cu, is higher than the BE for NO 3- in the outer sphere of the active phase compounds. The presence of NO 2- indicates reduction of NO 3" by the carbon surface. In spite of the complicated character of the active phase, which is difficult to the define, the probable existence of CuO, Cu20, Cu(OI-I)2 and Cu(NO3)2.3Cu(OH)2 can be assumed on the basis of the XPS results. Similarly to the case in ref. [7], the latter compound is also established by our thermogravimetric studies on Cu(NO3)2.3H20 deposited on oxidized and unoxidized activated carbon from an aqueous medium. Table 5 shows the Cu/C ratios from XPS and chemical analyses, as well as their ratio. Table 5 Elemental ratios as determined by XPS and chemical analyses Sample Cu/C Cu/C (Cu/C)chem.analysis (XPS) (chemical analysis) (Cu/C)xp s ACM 0.030 0.049 1.6 ACW 0.015 0.046 3.1 OACM 0.005 0.047 9.4 OACW 0.006 0.048 8.0 The ratios obtained by the two methods [8] allow elucidating the role of the carbon surface oxidation and the kind of the media (aqueous or methanol) with respect to the impregnation process, i.e. to the incorporation of Cu into the internal pore network or onto the external support surface. In our case (Table 5) there is a continuous increase of the concentration of Cu in the internal surface, which for the different samples varies between 3.1 and 9.4. The ACM
1143 sample alone is an exception. Here, uniform distribution of the phase in the whole volume may be accepted, which completely coincides with the results form the distribution curves of the same sample (Fig.3).
4. C O N C L U S I O N The composition and the character of incorporation of the copper phase into the activated carbon depends on the chemical nature of the carbon surface, the media of precursor deposition and their combination. Choosing oxidized or unoxidized carbon surfaces as well as aqueous or methanolic media, the incorporation of the active phase can be directed to the internal or external support surface. With the same precursor concentration under constant conditions, the Cu phase is distributed: (i) in the region of fine pores (about 20 A) of the support irrespective of its kind (AC or OAC) when methanolic medium is used, and (ii) around the fine pores again on an oxidized support (OAC) and in the mesopore region of an unoxidized support (AC), an aqueous medium being used in both cases. Samples with a uniform mesopore region are obtained when Cu(NO3)2.3H20 is deposited from an aqueous medium on oxidized activated carbon and from a methanolic medium on unoxidized activated carbon.
A CKNO W L E D G M E N T The work has been performed with the financial support of the Foundation for Scientific Investigation at the Ministry of Education and Science, Bulgaria.
REFERENCES 1. E. Bekyarova and D. Mehandjiev, J. Colloid Interface Sci., 161 (1993) 115. 2. H. Juntgen, Fuel, 65 (1986) 1436. 3. D. Mehandjiev, E. Bekyarova and R. Nickolov, Carbon, 32 (1994) - in press 4. C. Ishizaki and I. Marti, Carbon, 19 (1981) 409. 5. L. Daza, S. Mendoiroz and J. A. Pajares, Carbon, 24 (1986) 33. 6. M. D. Stoev, Thesis, IGIC, BAS, Sofia, Bulgaria, (1991). 7. J. Chose and A. Kanungo, J. Thermal Analysis, 20 (1981) 459. 8. J. A. Rossin, Carbon, 27 (1989) 611.
This Page Intentionally Left Blank
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
1145
THERMOSTABILITY OF COPPER-CHROMIUM OXIDE CATALYSTS ON ALUMINA SUPPORT PROMOTED BY LANTHANUM AND CERIUM. R . A . S h k r a b i n a , N . A . K o r y a b k i n a , O.A.Kirichenko, V.A. Ushakov, F.Kapteijn*, Z.R.Ismagilov. Boreskov I n s t i t u t e of Catalysis, Prosp. Akad. L a v r e n t i e v a , 630090, R u s s i a * D e p a r t m e n t of Chemical Engineering, Delft U n i v e r s i t y J u l i a n a l a a n 136, 2628 BL Delft, The Netherlands.
5, Novosibirsk of
Technology,
1. INTRODUCTION
Aluminium-copper-chromium oxide catalysts are widely used in highly exothermal catalytic combustion and V O C removal,though, the interaction of the active component with the support through catalytic process yields phase conversions and deactivation of the catalyst [1-5]. To increase the catalyst thermostability, which is primarily determined by that of the support, the latter is modified by various additives, decelerating phase conversion in the support [6-7]. X-ray method was recently used to study thermal stability of alumina supports promoted with La208 and CeO 2 [6,7], as well as phase composition and thermal stability of Cr/AI203, Cu/Al208 and Cr-Cu/Al208 [8-10]. Here we compare thermal stabilities of Cr-, Cu- and Cr-Cu systems on alumina support promoted by lanthanum and cerium. 2. E X P E R I M E N T A L
Pseudoboehmite hydroxide obtained via procedure developed in [II], was used to extrude alumina in rings. Before the moulding, hydroxide was modified via a direct introduction of nitric acid salts of lanthanum or cerium. The content of the modifying ion (2 wt.% MenO2n) was selected by data of [6,7]. The modified supports were consequently dried and thermally treated at room temperature (10 h), 383 K (4 h), 773 K (8 h) and 1173 K (8 h).The support was impregnated with solutions of the required content of Cu(NO3) 2, CrO 3 or [CuCr207+Cu(NO3)] by the incipient wetness impregnation method, then consequently dried at room temperature and under IR lamps for 4 hours. Catalysts were thermally treated in air at 773 K ( 4 h ) and at 973 and 1273 K (2 h). The catalysts contained 2, 7 and 15 wt.% of the active component; for [Cu+Cr] system, the Cu:Cr - 2:1 ratio was maintained.
1146 X-ray analysis of the samples was carried out in a HZG-4 d i f f r a c t o m e t e r with copper radiation applied like in [12]. The phases observed were identified in accordance with the data of ASTM [13]. The content of 5- and a-A1203 was determined from the calibration diagram for mechanical m i x t u r e s [14]. Specific surface areas of the samples were determined by N2 adsorption at 77K (BET method) and expressed in m 2 per gram alumina. Samples containing 7 wt.% of the active component were used for comparing the effect of additives. The mechanical crushing s t r e n g t h of the granules was determined by the s t a n d a r d method by means of an MP-gC apparatus. An average value of s t r e n g t h Pay was calculated from the volume of aggregate data for 30 granules, the accuracy of determination b e i n g - 10%. 3. R E S U L T S
3.1 X - R a y study. X-ray analysis data were systematized with respect to supported components, special attention was paid to variations in the phase composition of the samples in the t e m p e r a t u r e range of 773-1273 K. Alumina supports. Since supports used for the catalyst formation, were precalcined at 1173 K, their diffractogramm corresponded to the mixture of 8- and yforms of alumina (at a 90% content of 5-A1203 for the non-modified support). Cerium modified support along with Y- and ~ forms of alumina (45% of 5-A]203), contains a highly dispersed cerium oxide phase with a particle size of d=80 A. L a n t h a n u m modified support has a lower content of 5-A1203 (40%), either l a t h a n u m compounds are not exhibited in d i f f r a c t o g r a m m s . Supported copper-oxide catalysts. At 773 K, if 7% of Cu on the nonmodified support is supported, a roughly dispersed copper oxide (d=250/~) and a finely dispersed copper-containing spinel-type phase are formed. Thermal t r e a t m e n t at 973 K provides both a perfection of the spinel-type phase and growth of copper oxide particles. Two phases appear at 1273 K, i.e. a stoichiometric spinel CuA1204 and a-A1203, thus t e s t i f y i n g to the mineralizing effect of copper ions, which accelerates | -~ a-A1203 transition. The cerium promoted support exhibits similar changes in the phase composition upon thermal t r e a t m e n t of the samples containing 7 and 15% of copper. At low copper content (2% of Cu), CuO phase is not observed t h r o u g h the whole t e m p e r a t u r e range. At 1273 K, only 50% of alumina which does not convert into a stochiometric spinel CuA1204, t r a n s f o r m into a-form. The complete t r a n s f o r m a t i o n into a-A1203 is observed at higher concentrations of the supported copper. All the samples contain cerium only as the oxide phase. L a n t h a n u m promoted supports with 7% of copper and higher thermally t r e a t e d at 773 and 973 K contain a roughly-dispersed copper oxide, while the s p i n e l form is not found probably because of its high dispersity and disorder. At 1273 K, unlike the cerium system, l a n t h a n u m exhibits a stabilizing effect: only at 15% of Cu, all alumina appears to be in the form of a-A1203.
1147 %, a - A I 2 0 3
A
(~
100 ~
-1
% a -AIzO3
B
100
50
50 -3 -2
0
5
-5
10 C[Cu+Crl, %wt
o
5
1o
15 q c . + c ~ ] , %
F i g . I ( A , B ) . The effect of modified s u p p o r t on a-A1203 g e n e r a t i o n vs. the c o n t e n t of components in A1-Cu-Cr oxide c a t a l y s t upon its t h e r m a l t r e a t m e n t at 1273 K. A - L a n t h a n u m modified s u p p o r t upon i n t r o d u c t i o n of Cu (I), Cr (2) and (Cu+Cr) (3); B - C e r i u m modified s u p p o r t upon i n t r o d u c t i o n of Cu (4), Cr (5) and
(Cu+Cr) (6);
U n m o d i f i e d support: ( I ) (Cu+Cr) m i x t u r e ;
7% of Cu; ( I I ) -
7% of Cr; ( I I I ) -
7% of
F i g u r e I(A,B) shows a dependence of a-A1203 c o n t e n t of the t h r e e s t u d i e d s u p p o r t s a f t e r t h e i r t h e r m a l t r e a t m e n t at 1273 K on c o n c e n t r a t i o n of the active component. The m i n e r a l i z i n g effect of the copper ions decreases clearly, if the modified s u p p o r t s are used. The efficiency of l a n t h a n u m ions is considerably h i g h e r t h a n t h a t of c e r i u m ones, especially at a low (less t h a n 7% ) c o n t e n t of the s u p p o r t e d component. Supported chromium oxide catalysts. A t p r o m o t i o n of a n o n m o d i f i e d s u p p o r t w i t h 7% of Cr at 773 K, a solid c h r o m i u m solution f o r m s , which is proved by v a r i a t i o n s of the corresponding lines intensities. A f t e r the t h e r m a l t r e a t m e n t at 973 K, X - r a y p a t t e r n s of the samples change i n s i g n i f i c a n t l y , while at 1273 K, both | and a-A1203 f o r m s coexist. The increased p a r a m e t e r of a-A1203 e l e m e n t a r y cell testifies to the f o r m a t i o n of a solid c h r o m i u m solution w i t h the content not exceeding 1 0 % . W h e n a c e r i u m modified s u p p o r t is coated w i t h c h r o m i u m , w h a t e v e r the c h r o m i u m ion content, t h e r m a l t r e a t m e n t at 773 and 973 K provides s i m i l a r changes in t h e phase composition. At 1273 K, a weak r e l a t i o n s h i p is observed between t h e c o n t e n t of the f o r m e d a-A1203 and the c o n c e n t r a t i o n of the i n t r o d u c e d c h r o m i u m (Fig. 1).
1148 Contrary to cerium system, l a n t h a n u m modified support exhibits a limited c h r o m i u m solubility, since a-Cr208 appears already at the deposition of 7% of Cr. At 1273 K, the content of a-A1203 is practically as large as in the cerium system with 2 and 7% of chromium and significantly lower, if Cr content is 15% (Fig. 1). Support modifications with cerium and l a n t h a n u m ions does not significantly influence the a l u m i n a / c h r o m i u m system at low (less t h a n 7%) c h r o m i u m contents (Fig. 1). Supported copper-chromium oxide catalysts. The non-modified s u p p o r t a f t e r its thermal t r e a t m e n t at 773 K, if coated with a 7% (Cu+Cr) m i x t u r e , seems to contain a series of spinel-type phases on the base of the support and copper chromite s t r u c t u r e s as well. Then d i f f r a c t o g r a m m s of the sample are characterized by distorted lines of the support only. A f t e r thermal t r e a t m e n t at 1273 K, there coexist a-A1208 with the increased cell p a r a m e t e r and aluminium-copper-chromium spinel with a = 8.098 ~, which is typical for Cu(All.8Cr0.2)O 4 composition. No lines of copper (+1), i.e. Cu2Cr204 and Cu2A1204, are observed. W h e n 2 and 7% of (Cu+Cr) m i x t u r e are introduced, the cerium modified support pretreated at 773 K, exhibits only its own lines, while when 15% of (Cr+Cu) is supported, the lines of roughly dispersed copper oxide are observed. According to X-ray patterns, the lines intensity of the spinel-based solid solutions increases with the increase of the supported components concentration. At 1273 K, all the studied samples reveal the presence of a-A1203 with the increased cell p a r a m e t e r and the spinel-type solid solution with a p a r a m e t e r typical for Cu(AI1.sCr0.2)O 4. Concentration of components introduced determine a-Al203 content. The content of a-oxide is considerably lower in the sample promoted with a 7% (Cu+Cr) m i x t u r e than in the pure alumina. L a n t h a n u m modified support, coated by 2% of (Cu+Cr) m i x t u r e and then thermally treated at 773 K, exhibits lines of solid solutions only, while at 7% and higher contents of (Cu+Cr), there appear traces of a roughly dispersed copper oxide. At 973 K, the whole CuO reacts with a support. The increase of the active components concentration, yields the rise of lines i n t e n s i t y of solid solutions based on the spinel s t r u c t u r e . At 1273 K, a solid solution with p a r a m e t e r s typical for Cu(All.TCr0.8)O 4 and a-A1203 with an increased p a r a m e t e r are observed all t h r o u g h the concentration range of the supported components. La stabilizes a | --~ a-Al203 transition more efficiently t h a n Ce, since cerium promoted samples at all concentrations of the active component (Fig. 1) have a twice higher q u a n t i t y of a-A1203. It should be noted, that the catalyst coating by the active component with the subsequent thermal t r e a t m e n t at all t e m p e r a t u r e s under study, provide no variations of the promoter state: diffractogramms exhibit the presence of cerium oxide, while c r y s t a l l i n e - p h a s e s of l a n t h a n u m compounds are not observed as well as [6,7].
1149 3.2.
BET
Surface
Area
Measurements.
The BET s u r f a c e a r e a is a good p a r a m e t e r to assess t h e d e g r e e of s i n t e r i n g of a c a t a l y s t . In Table I t h e r e s u l t s of BET m e a s u r e m e n t s on s a m p l e s c a l s i n e d f o r 4 h o u r s at 7 7 3 K a n d 2 h o u r s at 1 2 7 3 K are s u m m a r i z e d . In o r d e r to d e m o n s t r a t e t h e t e r m o s t a b i l i t y of t h e stabilized c a t a l y s t , s a m p l e s of t h e c a t a l y s t , n o n m o d i f i e d also w e r e s i n t e r e d . In all cases t h e s u r f a c e a r e a of t h e d o p e d s u p p o r t was a b o u t 120 m 2 / g f o r Ce/A1203 and 110 m 2 / g f o r La203. The s u r f a c e a r e a of t h e c a t a l y s t s u p p o r t e d on t h e non m o d i f e d a l u m i n a was 90 m 2 / g . A f t e r 2 h o u r s at 1 2 7 3 K t h e s u r f a c e a r e a of t h e s e c a t a l y s t d e c r e a s e d to 5 m 2 / g . The B E T s u r f a c e areas of Table I d e m o n s t r a t e t h e p o s i t i v e e f f e c t of l a n t h a n u m a n d c e r i u m on t h e t h e r m o s t a b i l i t y of t h e c a t a l y s t . La203 i s t h e m o s t e f f i c i e n t i n h i b i t o r of t h e s i n t e r i n g of c o p p e r c a t a l y s t , t h e s e is a c c o r d a n c e w i t h X - r a y d a t a .
Table 1 S u r f a c e a r e a of t h e c a t a l y s t s number
Cu, w t . %
Cr, w t %
2 w t % of modificator
773 K m2/g
1273 K m2/g
1 2 3 4
2 7 15 2
-
Ce Ce Ce La
80 86 65 100
12 10 10 60
5
7
-
La
85
40
6
15
-
La
80
25
7 8
-
2 7
Ce Ce
120 85
16 12
9
-
15
Ce
75
10
10 11 12
-
2
-
7
-
15
La La La
120 100 95
60 45 30
13
1,4
0,6
Ce
110
1
14
5
2
Ce
80
10
15 16 17
10,6 1,4 5
4,4 0,6 2
Ce La La
75 100 90
10 60 40
18
10,6
4,4
La
80
30
1150 3.3. Mechanical strength. The mechanical s t r e n g t h is one of the most i m p o r t a n t characteristics of g r a n u l a t e d catalyst working at high temperatures. It is established t h a t the interaction of an active component with a support upon thermal t r e a t m e n t leads to a decrease in the t e m p e r a t u r e of polymorphic t r a n s f o r m a t i o n s of alumina supports, resulting in the change of phase composition and structural-mechanical properties of a catalyst. The results of the m e a s u r e m e n t s of mechanical s t r e n g t h of catalyst granules are presented irt Table 2. The similar changes are characteristic for chromium and copperchromium catalyst. As the active component in creases the s t r e n g t h of granules is observed to grow from 30 MPa to 60 MPa. However when the t e m p e r a t u r e of thermal t r e a t m e n t reaches 1273K the s t r e n g t h of the samples decreases to value c.a. 40 MPa. This is probably due to the f o r m a t i o n of r The phase inhomogeneity leads to the appearance of additional tensions in the s t r u c t u r e and consequently to the decrease of s t r e n g t h . Table 2 The mechanical s t r e n g t h of catalysts number 1 2 3 4 5 6 7
i
Content Cu, wt%
2wt% of modificator
Mechanichal 773K
strength, MPa 1273 K
2 7 15 2 7 15 7
Ce Ce Ce La La La -
25,0 30,0 61,0 30,0 40,0 66,0 38,0
21,0 22,0 40,0 37,0 35,0 37,0 20,0
4. D I S C U S S I O N The observed dependence of a-A1203 content on the concentration of the supported active components evidences indirectly for t h e i r interaction with the support. As noted in [8], thermal decomposition of solid solutions based on transition states of alumina, accelerates a-A1203 formation, accompanied by Cu2A]204 and Cu2Cr204 yield. The fact, t h a t these compounds were not exhibited in the X-ray patterns, apparently suggests another mechanism of a-A1203 f o r m a t i o n in La- and Ce-modified alumina. According to X-ray studies of the phase content, we consider copper ions to be the most strong mineralizing element for the a l u m i n i u m oxide system leading to a-A1203 formation while chromium is relatively hardly active. Figure 2 presents a dependence of a-A]203 content on the content of copper ions in the active component of three types of supports at 1273 K. Apparently, only at Cu contents below 10 w t . % , its mineralizing effect can be
1151
%, a-AI203
511
0
5
10
C Cu, %wt
Fig. 2. Effect of copper ions content on a-A1208 formation in A1-Cu and A1-Cu-Cr oxide catalysts upon thermal t r e a t m e n t at 1273 K (for notations see Fig. 1): 1,3 - La/A12Os; 4,6 - Ce/A1203. in this case the stabilizing ions of the support (La and Ce) as well as the second component of the catalyst - Cr, are involved. La208 is the most efficient inhibitor of the copper mineralizing effect. For Cr-containing catalysts, the stabilizing effect of Ce and La is not so evident, since Cr ions appeared to be weak mineralizations at their content less t h a n 10% and the t e m p e r a t u r e s below 1273 K [8]. W h e n Cr is supported on Ce-containing alumina, the stabilizing effect of cerium is enhanced by Cr. The q u a n t i t y of a-A1203 formed vs Cr concentration with respect to the absolute value becomes comparable with La effect ( F i g . l , A, B). L a n t h a n u m ions introduced into alumina s t r u c t u r e , decrease the solubility of copper and chromium ions, thus, providing the decrease of the mineralizing effect. Copper and chromium ions not involved in the support s t r u c t u r e , yield an e x t r a q u a n t i t y of copper chromite, which enriches correspondingly the solid solution of CuA1204 and CuCr204 spinels, providing the change of the lattice parameter. On the basis of the above results and recently obtained d a t a on the mineralizing effect of copper (in some cases ~ chromium) [1, 8 10], it is concluded, t h a t in order to suppress a~ formation in alumina catalysts, the introduced stabilizing ion should, first, r e s t r i c t the solubility of the catalyst components possessing a pronounced mineralizing effect and, second, form a compound with the alumina stable at high t e m p e r a t u r e s (which does not decompose producing a-A1203).
11~2 REFERENCES
1. 2. 3. 4. 5. 6.
7.
8. 9. 10. 11.
12. 13. 14.
Z.R.Ismagilov, D.A.Arendarskii, O.A.Kirichenko, G.B.Barannik, E.M.Moroz, V.A.Ushakov and V.V.Malakhov., Kinetika i Kataliz, 30 (1989) 918.(in Russian). E.Garbowski, L. Blanchard, M. Primer in Z.R.Ismagilov (Editor), Proc. 8th Soviet-French Seminar on Catalysis, Novosibirsk, June 18-21, Novosibirsk, 1990, p. 235. H.Schaper, E.B.M.Doesburg, L.L.van Reijen, Appl.Catal., 7 (1983), 211. Zhow Yu-Ming, Ding Ying Cu, J.Catalysis (Cuihua Xuebao), 12 (1991) 167. L. Guan-Zhong, Wang Ren, ibid, 261. Z.R.Ismagilov, F.Kapteijn, J.W.Bijsterbosch, N.A.Koryabkina, M.F.Lausberg, E.M.Moroz, R.A.Shkrabina, V.A.Ushakov. "The thermal behaviour of doped alumina studied by XRD. I. La203-A1203". Appl.Cat.,1994, (in press). Z.R.Ismagilov, F.Kapteijn, J.W.Bijsterbosch, N.A.Koryabkina, M.F.Lausberg, E.M.Moroz, R.A.Shkrabina, V.A.Ushakov. "The thermal behaviour of doped alumina studied by XRD. II. CeO2-A1203. Appl.Cat.,1994, (in press). E.M.Moroz, O.A.Kirichenko, V.A.Ushakov and E.A.Levitskii, React. Kinet. Catal. Lett., 28 (1985) 9. O.A.Kirichenko, V.A.Ushakov, E.M.Moroz and Z.R.Ismagilov, React. Kinet. Catal. Lett., 38 (198) 307. O.A. Kirichenko, V.A. Ushakov, E.M. Moroz, Kinetika i Kataliz, 34 (1993) 739 (in Russian). R.A.Shkrabina, Z.R.Ismagilov, M.N.Shepeleva S.R.Lohokari, M.C.Vaidya and D.R.Sane in B.Vistmanthan and C.N.Pillai (Editors), Recent Developments in Catalysis. Theory and Practica. Proc. l Oth Nat.Symp. on Catalysis, Madras, December 18-21 (1990), New Delhi, 1990, Part 11, p. 30. N.A.Koryabkina, Z.R.Ismagilov, R.A.Shkrabina, E.M. Moroz, V.A. Ushakov. Appl.Catal.,72 (1991) 63. Card Index of ASTM. R.A.Shkrabina, Yu.K.Vorobiev, E.M.Moroz, T.D.Kambabarova, and E.M. Levitskij, Kinetika i Kataliz, 21(1981) 1080 (in Russia).
Acknowledgement. Authors wish to thank I.P. Andreevskaya for supports preparation.
PREPARATION OF CATALYSTS VI
ScientificBases for the Preparation of Heterogeneous Catalysts
G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
1153
Non-Hydrothermal Synthesis, Characterisation and Catalytic Properties of Saponite Clays R.J.M.J. Vogels, M.J.H.V. Kerkhoffs, and J.W. Geus Department of Inorganic Chemistry, Debije Institute, Utrecht University, P.O. Box 80083, 3508 TB Utrecht, The Netherlands
Saponites were synthesised at 90~ and 1 atmosphere from a Si/Al-gel and a solution containing urea and M2+-nitrate (M2+= Zn, Mg, Ni and Co) in only a few hours. The products were characterised by XRD, TEM, BET, 27A1- and 29Si-MASNMR. Furthermore, the catalytic properties of the synthetic saponites in the Friedel-Crafts alkylation of benzene with propylene to cumene were tested. Incorporation of Zn, Mg, Ni, Co, or a combination of Zn and Mg, in the octahedral layer, as well as controlling the Si/Al-ratio in the tetrahedral layer between 7.9 and 39 could easily be established. The specific surface areas and the pore volumes of the saponites are extremely high, viz., 100-750 m2/g and 0.03-0.32 ml/g, respectively. Zn-saponite with A13+ in the interlayer exhibited a higher catalytic activity as compared to a commercial SPA-catalyst (Solid Phosphoric Acid).
1. INTRODUCTION With many manufacturing processes of fine and bulk chemicals, homogeneous acid catalysts are used in liquid- and gas-phase reactions. Friedel-Crafts alkylations, such as the alkylation of benzene with propylene to cumene, are well known industrially important examples of acid-catalysed reactions, usually executed with aluminium chloride as a homogeneous catalyst. Because of large unavoidable problems of the usual acid catalysts with respect to separation, selectivity, corrosion, environmental damage and recycling, the replacement of homogeneous catalysts by heterogeneous catalysts is highly relevant. The first heterogeneous catalyst used for the production of cumene was phosphoric acid supported on silica or kiezelguhr, generally denoted as "solid phosphoric acid" (SPA) [1]. Although SPA-catalysts are solid acids, the use of these catalysts has drawbacks, e.g., the production of undesirable by-products. Examples of such by-products with the production of cumene are dialkylated and trialkylated compounds, n-propylbenzene, heavy polyaromatic compounds, and oligomers of the alkylating agents. Furthermore, SPA-catalysts require the use of a water co-feed, which causes phosphoric acid leaching and the production of a corrosive sludge. Disposal of used SPA-catalysts, finally, leads to environmental pollution. Nowadays, much research is going on to develop 'green' solid acid catalysts of an elevated selectivity. A high selectivity usually calls for a rapid transport of reaction products from the active sites of the catalysts. Solid acid catalysts, however, mostly consisting of (highly)
1154 porous bodies having narrow pores, are displaying less favourable transport characteristics. Especially with liquid-phase reactions, where the diffusion coefficients are low, the transport properties of solid acid catalysts are decisively affecting the performance of the catalysts. Tl~ selectivity can be enhanced by a good design and engineering of the acidity and porous structure of the catalyst. Recently, some successful results relating to the cmmme prodtmfion with dealuminated mordenites have been obtained [2]. For a long period of time application of natural clays as acid catalysts has been investigated. When using clay minerals in catalytic processes, such as alkylation, previous dehydration of the clay is required for a good peffommaace irt view of the low solubility of the organic compounds in the adsorbed water layer. ~ e r , thermal dehydration of naturaI clay minerals often results in a collapse of the clay structme, and, consequently, "m a low catalytic performance. Other disadvantages of natural clay minerals with regard to catalytic activity and selectivity are the inhomogeneous and difficult to con~ol chemical composition, the large and non-uniform size of the bodies causing transport diffmulties, and the different stacking of the elementary platelets of the clays. Pillaring of natural clays with inorganic polyoxocations may raise the catalytic performance, but do not improve the selectivity sufficiently. To materialise the promising catalytic properties of clay mineralS,~ the textural characteristics which determine the transport properties, i.e., the dimensions of the porous: catalyst bodies and the pore-size distribution, must be controlled. The acidity and the density of the acid sites usually has to be adapted to the organic reaction to be carded out. Another important characteristic of solid acid catalysts is therefore the nature and the number of ac,id sites. Besides the transport properties, it is thus required to be able to control also the nature. and the density of the acid sites with the preparation procedure of the solid acid catalysts. Synthesis of smectite-type clay minerals, such as sat)onites, may eircum~rcnt the above mentioned difficulties. The usually demanding hydrothermal treatment [ ~ , the loag duration of the synthesis [5], and problmm with the scale-up of the ptepm'atioa procedur~ however, have thus far severely limited lie use of synthc-tic clays as solid acic~ catalysts. The work descn~z~ll in this article imvolves a rapid synthesis of sapo~tes under ma~ hydrothermal condi~on~ The procedure allows us to control well tl~ tzxture and the chemical composition of the octalledral sheet (Mg, Ni, Co or Zm) as well as el r the tetxalmdral sheet (a desired Si/Al-ratio)ot~ the resulting clay min.~ats. We are ~ W prepare clay mitmmls displaying a house-of-cards structure without pillaring. Both the sam-king and the size of the clay platelets is limited and tan be oantrolai~ resdting in cxcellem transport properties of organic compounds. Finally, the thermal stability of the new d a y minerals is high, which allows us to delaydrme the c l ~ s efficiently.
2. E X P E R I M E N T A L
2.1 Preparation of catalysts The starting material for all experiments was a gel, being the source for Si, AI and Na, prepared by mixing a Na2SiO3-solution with a solution containing NaOH and Al-nitrate at 20~ The Si/A1 atomic ratio was varied between 5.67 and 39.0. This gel was suspended in an excess of demineralised water at 90~ in a double-walled vessel equipped with baffles, and vigorously stirred at 1500 rpm [6]. The temperature was controlled by circulating
1155 thermostatted water between the inner and outer wall. After being mixed homogeneously, the saponite synthesis was started by addition to the gel suspension of a solution containing an excess of urea and the stoichiometric amount of M2+-nitrate, corresponding to the theoretical composition of the saponite [Nax+(M2+6){Sis_x,Alx}O20(OH)4]. Mg, Ni, Zn, Co or a combination of Zn and Mg was used as M2+-species. Saponites with, for example, Mg in the octahedral sheet and A13+ as interlayer cation are further denoted as Mg-saponite(A13+). Typical synthesis durations were 5 to 20 hours, after which the solid product was washed five times with demineralised water. To introduce additional acidity in the clay materials, ionexchange with solutions containing A13+ at 20~ followed by rewashing was executed. All solid products were dried overnight at 120~
2.2 Analytical techniques The pH of the suspension during the synthesis was measured. X-ray powder diffraction (XRD) patterns were recorded on randomly oriented samples with a Philips PW 1050/25 diffractometer using CuKtx radiation. Specific surface areas and pore volumes were obtained from nitrogen adsorption/desorption isotherms at -196~ using a Micromeritics ASAP 2400. Prior to the measurements, all powdered samples were degassed at 130~ under vacuum. The morphology of the samples was investigated with a Philips EM-420 transmission electron microscope (TEM) operated at 120 kV. High-resolution magic angle spinning nuclear magnetic resonance (MASNMR) measurements of 27A1 and 29Si were performed on a Bruker AM-500 spectrometer at 11.7 T. The 27A1 MASNMR experiments were done at 130.3 MHz, with a spinning rate of 7.0-7.7 kHz, a pulse length of llas, a pulse interval of Is, and a number of 1000 scans. 29Si MASNMR experiments were run at 99.3 MHz with a pulse length of 6.51.ts, a pulse interval of 40s, a spinning rate of 5.0 kHz and a number of 100 scans. Chemical shifts of 27A1 and 29Si are reported in ppm relative to [Al(H20)63+] and tetramethylsilane (TMS), respectively.
2.3. Catalytic measurements The synthetic clays and a commercial SPA-catalyst were tested for their ability to alkylate benzene with propylene to cumene. A sieve fraction between 0.1 and 0.4 mm of the catalysts was taken. To avoid introduction of water, the synthetic clays were calcined under a nitrogen flow for 3 hours at a desired temperature. After drying, the saponites were suspended in dry benzene and transferred into a stainless steel autoclave. During continuous mixing propylene was introduced into the autoclave after which the temperature was raised to the desired level. Excess of benzene was used to prevent multiple alkylation. The catalyst concentration amounted to 1.5 or 0.2 wt.%. The benzene/propylene molar ratios, the type of catalyst, the reaction temperature, and the reaction duration will be provided in the appropriate table. Analysis were performed on a Carlo Erba Instruments HRGC 5300 gas chromatograph using a capillary Chrompack CP-Sil-CB column.
1156
3. RESULTS AND DISCUSSION 3.1. Characterisation of the Si/Al-gel TEM investigation learned that the gel consisted of spheres with a slightly inhomogeneous size distribution of 5 to 40 nm. Neither by XRD nor by TEM crystalline phases could be detected. 27MASNMR revealed that all aluminium was tetrahedrally coordinated exhibiting a resonance at approximately 56 ppm. 29Si MASNMR showed that Si is randomly coordinated exhibiting a broad signal at -95 ppm. 3.2. Synthesis of saponite After addition of the M2+-nitrate and urea the pH of the suspension was 4.5 to 5.5. Subsequently the pH slowly increased to approximately 7 to 8 after 20 hours, depending on the type of M2+-ion used. The rise of pH is due to the homogeneous increase of the hydroxyl ion concentration during the hydrolysis of urea at 90~ The increase of the pH was very slow as compared to the pH rise of pure urea solutions at 90~ [7], due to the consumption of hydroxyls during the nucleation and growth of the solid phase. 8OO 700 600 500
Zn6
400 300
Zn4Mg2
200
Mg4Zn2
100 0 0
10
20
30
40
o 2 Theta
50
60
70
80
Figure 1. XRD patterns of saponites with Mg, Zn and a combination of Mg and Zn in the octahedral layer after 20 hours of synthesis. X-ray diffraction patterns of all synthesis products clearly show the formation of trioctahedral clay minerals within 5 hours, as indicated by the (060/-332) reflection at 1.54A, revealing that Mg, Ni, Zn or Co is incorporated into the octahedral sheet of the clay minerals. After a preparation time of 20 hours, only Zn-saponites clearly display the (001) reflection at 12.5-13.0A, indicating the formation of saponite instead of the non-swelling talc. XRD patterns of Mg-, Zn- and MgZn-saponites after 20 hours of preparation are presented in Figure 1.
1157 TEM shows the formation of Zn-saponite already after 1.5 hours by the presence of platelets (length 15 nm) in between the gel particles. The clay particles display hardly any stacking. After 12 hours, the gel had completely reacted to clay platelets having a length of 100-200 nm consisting of about 10 layers (Figure 2). Further increase of the synthesis duration to 20 hours resulted in a further growth and an increased stacking of the clay platelets.
Figure 2. TEM photograph of Zn-saponite after 20 hours of preparation. (
= 50 nm)
The growth of Mg-saponite is significantly slower than that of Zn-saponite. Within 3 hours, the reaction to Mg-saponite has proceeded to an substantial extent resulting in platelets (length 5-10 nm) with no stacking. Nevertheless after 20 hours some gel particles had still not reacted. Even after 20 and 47 hours (Figure 3), the Mg-saponite particles displayed small platelets (15-25 nm) with almost no stacking, which agrees with the absence of the (001) reflection as mentioned above.
Figure 3. TEM photograph of Mg-saponite after a synthesis of 47 hours. (
= 50 nm)
1158 For the use of clay minerals in acid-catalysed reactions, it is very important that the acid sites are situated at the surface of the clay layers. The acid sites at the surface are obtained by replacement of silicon by aluminium in the tetrahedral sheets. 64.3
64.1
64.2
.J ,I
I00
I
80
,,
I
60
I
40
ppm
Si/AI= 5.7
I
20
I
0
, I
,I
-20
100
i
80
,
I
60
I
,
40 pore
Si/AI= 12.3
I
20
, I
0
i
80
,,,
I
I
60 40 ppm
I
I
I
20
0
-20
Si/AI= 39
Figure 4. 27A1 MASNMR spectra of three Zn-saponites after a synthesis duration of 20 hours. Since aluminium ions can also be accommodated in the octahedral layer and at interlayer positions, the highest number of acid sites at the layer surfaces corresponds to the maximum amount of aluminium tetrahedrally coordinated within the clay sheets. To obtain information about the position of the aluminium in the samples, 27Al MASNMR measurements were performed on Zn- and Mg-saponites. Figure 4 represents three spectra of Zn-95.9 saponite(NH4+) with different Si/Al-ratios. All spectra exhibit two resonances at approximately 64 ppm and 10 ppm. The resonance peaks can be ascribed to aluminium -91.1. situated in the tetrahedral (Al4) and octahedral sheet (A16) of clay minerals, respectively [8]. From Figure 4 it can be seen that decreasing -86. the Si/A1 ratio from 39 to 5.7 is accompanied by an increase of the A16/Al4-ratio from approximately 2 to 30%. 29Si MASNMR measurements on Zn-saponite with Si/Al=5.7 ' ' ' exhibit three resonances at -96, -91 and -86 - 0 - 0 -100 -110 -120 ppm (Figure 5), originating from silicon ppm coordinated with zero, one and two aluminium ions, respectively, which agrees with literature Figure 5. 29Si MASNMR spectrum of Zndata for clay minerals [9]. saponite after 20 hours with Si/Al = 5. 7
1159
3.3. Effects of the saponite composition on the texture As mentioned, only the XRD patterns of Zn-saponites synthesized during 20 hours display a sharp (001) reflection at 12.5-13~, whereas Co-saponites show a broad shoulder around 13~, and Mg- and Ni-saponites do not exhibit a (001) reflection. The results indicate an increase of stacking in the order (Ni,Mg)
Specific surfaces and pore volumes of saponites after synthesis for 20 hours. Sample
Surface area (m2/g)
Pore volume (ml/g)
Zn-saponites Co-saponites Ni-saponites Mg- s aponite s Mg4Zn2-saponite Mg2Zn4-saponite
100-200 400-500 500-600 600-750 474 400
0.03-0.05 0.22-0.25 0.18-0.22 0.26-0.32 0.26 0.21
Because of the 'house-of-cards'-structure, and the limited size and stacking of the platelets of all saponites prepared in this work, the specific surface areas and pore volumes are extremely high, which is favourable for application in organic reactions. Except for Znsaponites, all surface areas and pore volumes are comparable (Co-saponite) or even higher (Ni- and Mg-saponite) than those reported thus far for pillared clays [10]. Calcination of the synthetic saponites at 450~ for 4 hours do not affect strongly the textural properties; the BET surface area and pore volume decrease by 1 to 10 percent. As can be seen from Table 1, the influence of the octahedral ion on the surface areas and pore volumes is striking. Mg- and Zn-saponites exhibit the largest differences in textural characteristics. Surprisingly a combination of both Mg and Zn in the octahedral sheet results in textural properties intermediate between that of pure Zn- and Mg-saponite, as evidenced in Figure 1 and Table 1. TEM investigations show an increase in the stacking and size of the saponites with an increase of the Zn/Mg-molar ratio of the saponite, which confirms the results presented in Figure 1 and Table 1. The above mentioned results provide us the ability to establish a desired surface area and pore volume by adjusting the chemical composition of the octahedral sheet of the saponites.
1160 3.4. Catalytic performance The results of the benzene alkylation are summarized in Table 2. For comparison a measurement with a 1.5 wt.% SPA-catalyst at 190~ after 2 hours of reaction has been included.
Table 2. Type and amount of catalyst, calcination temperature, reaction conditions, conversion and selectivity with the catalytic alkylation of benzene with propylene to cumene.
Sample (Interlayer)
Si/Alratio
Perc. catalyst
Calc. temp.
B/Pratio
Reaction temp. /duration
Conv.
Selec.
SPA-catalyst Zn-sap.(A1) Zn-sap.(A1) Zn-sap.(A1) Zn-sap.(A1) Zn-sap.(A1) Zn-sap.(A1) Zn-sap.(A1) Mg-sap.(Al)
39 39 39 39 39 39 7.9 7.9
1.5% 1.5% 1.5% 0.2% 0.2% 0.2% 0.2% 0.2% 0.2%
H20* 120~ 120~ H20* 25~ 120~ 300~ 120~ 120~
8.7 7.7 7.1 6.9 7.3 6.8 6.8 7.6 7.3
190~ 190~ 160~ 160~ 160~ 160~ 160~ 160~ 160~
85% 99% 99% 0% 63% 87% 60% 48% 32%
80% 83% 80% ..... 81% 74% 95% 77% 89%
hr hr hr hr hr hr hr hr hr
* Water added to the feed. It can be seen that with 1.5 wt.% saponite the conversion at 160~ is 99% after 0.25 hour. Decreasing the concentration of the catalyst used at 160~ from 1.5 to 0.2 wt.% results in a slight drop in conversion from 99 to 87%. Accordingly the activity of the Zn-saponite(A13+) is substantially higher, while the selectivity is comparable to that of the SPA. As was to be expected, addition of water to the feed completely destroys the activity, while a catalyst dried at only 25~ exhibits a lower activity than a catalyst treated at 120~ Treating the saponite thermally at 300~ instead of at 120~ brings about a drop in surface area, but raises the pore diameter. Hence the conversion decreases, whereas the selectivity rises to 90%. A higher aluminium content, which results from decreasing the Si/Al-ratio from 39 to 7.9, severely lowers the activity. The more open structure of the Mg-saponite(A13+) is reflected by the selectivity of 89% as compared to that of the corresponding Zn-Saponite of 77%. Removing the SPA-catalyst from the autoclave after the reaction was almost impossible. A thick layer of material was strongly attached to the reactor wall. Synthetic saponite, on the contrary, can be removed from the autoclave without problems.
4. CONCLUSIONS Saponites can easily be prepared at 90~ and 1 atmosphere within some hours. Adjustment of the chemical composition of the octahedral layer (Mg, Zn, Ni, Co, or a combination of Mg and Zn) and the tetrahedral layer (desired Si/Al-ratio) of the synthetic saponites is viable. The composition of the octahedral layer has a large effect on the texture of the synthetic saponites.
1161 It is possible to prepare saponites with specific surface areas between 100 and 750 m2/g and pore volumes of 0.03-0.32 ml/g, all displaying a 'house-of-cards' structure. The thermal stability of the synthetic saponites is high, which enables us to dehydrate the clay minerals efficiently, prior to the catalytic reaction. Friedel-Crafts alkylation of benzene with propylene to cumene on synthetic Znsaponite(A13+) is successful. At 160~ for 0.25 hours, the 0.2 wt.% Zn-saponite(A13+) exhibits a comparable conversion as 1.5 wt.% SPA-catalyst at 190~ after 2 hours. Removing the saponite from the autoclave after the reaction provides no problems.
ACKNOWLEDGEMENTS The technical assistance and the useful discussions provided by Engelhard de Meern, The Netherlands is greatly acknowledged. This study was financed by the IOP, project nr. 91054.
REFERENCES
1. 2.
V.N. Ipatieff, US Patent No. 2 382 318 (1945) G.R. Meima, M.J.M. van der Aalst, M.S.U. Samson, J.M. Garces and J.G. Lee, Proc. 9th International Zeolite Conf., Montreal, (1992), 327 J.T. Kloprogge, PhD Thesis, Utrecht University, The Netherlands (1992) 4. H. Suquet, J.T. Iiyama, H. Kodama, H., and H. Pezerat, Clays and Clay Min., 25 (1977) 231 A. Decarreau, Sci. G6ol. M6m., 74 (1983) 6 A.J. van Dillen, J.W. Geus, L.A.M. Hermans, J. van der Meyden, Proc. 6th Int. Conf. on Cat., 11 (5) (1977) A.C. Vermeulen, J.W. Geus, R.J. Stol, and P.L. de Bruyn, J. Coll. Interf. Sci., 51 (3). (1975) 449 D.E. Woessner, Amer. Mineral., 74, (1989) 203 9. M. Lipsicas, R.H. Raythatha, T.J. Pinnavaia, I.D. Johnson, R.F. Giese Jr., P.M. Costanzo, and J.-L. Roberts. Nature, 309, (1984) 604 10. S. Chevalier, R. Franck, H. Suquet, J.-H. Lambert, and D. Barthomeuf, J. Chem. Soc., Faraday Trans., 90, (1994) 667 .
.
.
.
This Page Intentionally Left Blank
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All fights reserved.
1163
Composite Catalysts of Supported Zeolites N. van der Puila, E.W. Kuipers b, H. van Bekkum a and J.C. Jansen a aLaboratory of Organic Chemistry and Catalysis, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands bKoninklijke/ShelI-Laboratorium, Amsterdam, Shell Research B.V., Badhuisweg 3, 1031 CM Amsterdam, The Netherlands ABSTRACT
Three-layer composite materials were synthesized consisting of lateraUy oriented silicalite-1 crystals, a catalytic phase and a [100] Si support. In particular chromium(Ill), manganese(Ill) and iron(lit) oxide particles were obtained by spin coating. HRSEM and AFM measurements proved that continuous layers of chromium, rnanganese and iron oxide are deposited, which are 0.5-2 nm thick. HRSEM observat~ns, the spin coating model and XPS analysis were in good agreement. TEM measurerner~ combined with X-ray elemental analysis prove~th e presence of Fe towards the interface ~ ~ 'the silicalite and the support. 11,. I N T R O D U C T I O N
The synthesis of supported zeolite crystals has been ,repc,'ted ipEeviously. Different supports such as stainless steel, aluminium foil, mullite and mica ,were combined with different types of zeolites [1-3]. Zeolite c o a t ~ can ~be .a~lied as membranes, catalysts and sensors [4]. In general, catalytic activity of the zeolite framework is obtained by ion exchange or isomorphous substitution of silicon by other elements. However, the !t~mited accessibility and bonding restrictions of the framework reduce the n,umb~ of modifications and hetero-atom stability. Supported zeolite systems can not only offer additional modifications, but can also improve process handling. In order to optimize the catalytic activity the supported crystals must obtain a specific orientation on the support, which is directly related to the particular crystal shape and the channel direction. Most catalytic sitesare present at the internal surface of the zeolite, thus the number of channel entrances at the crystal surface has to be maximized. An example of this configuration is an "end-of-pipe" de-NOx catalyst system, in which axially oriented Cu-exchanged crystals of MFI on a metal gauze are used [5]. In conventional catalyst systems zeolites are used in a fixed bed or fluid bed configuration. Advantages offered by the structured supported zeolite systems are i) low pressure drop, ii) attrition
1164
reduction, iii) dust free process handling and iv) heat transfer improvement. In the present study the catalytic site is introduced on the support at the interface between zeolite and support. The diffusion pathway through the framework to the catalytic site on the support must be minimized, while the channels must be oriented in the direction of the support. Advantages offered by these composites are i) combination of a catalytic site which can not be synthesized or stabilized in the zeolite lattice with framework shape selectivity and ii) bifunctional catalysis by addition of framework activity. A schematical drawing of the composite system design is given in Figure 1.
oriented zeolite layer catalytic phase support
2 0 0 - 3 0 0 nm 1-2 nm
T
ram-size
1 Figure 1: Schematical representation of the composite system. In case of laterally oriented sificalite-1 the straight channels are perpendicular to the support.
The shape selective capacity of the composite materials is depending on the continuity of the zeolite layer, although the presence of minor amounts of pin-holes is not expected to exclude shape selectivity. The performance and compatibility of the composite systems depends on the thickness of the zeolite layer and the silicon to metal ratio. The thickness of the zeolite layer as well as the continuity are dependent on the flatness of the support surface. For example, in order to maintain the flatness of a Si-wafer support, the catalytic phase must be as thin as possible. In the optimized case a monolayer of catalytic sites forms the interface between zeolite and support. In case of an oxidic substrate, bonding of the catalytic sites takes place at hydroxyl groups. If a model Si/SiO 2 support is used which contains 4 hydroxyl groups per nm , a monolayer of monomeric metal species consists of 6.64.10- 0 moles of metal atoms per cm2..A zeolite layer thickness of 100 nm thus leads to a silicon to metal ratio of 450, although closer packing of the catalytic sites should be considered possible. In case of a continuous monolayer of metal oxide, such as Fe203, in which the Fe-O distances are 1.96 and 2.10 ~, a silicalite coating of 100 nm results in a Si/Fe ratio of 287. The order of magnitude of this silicon to catalytic site ratio is to be compared with ion exchanged and isomorphously substituted zeolite catalysts. Depending on the synthesis conditions for the in situ growth of MFI-type crystals on [100] Si wafers, both axial and lateral orientations can be obtained [6]. The laterally oriented crystals form a continuous layer of 200 nm in thickness. In this orientation diffusion towards the support is possible, since the b-direction is perpendicular to the
1165
support and the 2-dimensional system will allow adsorption of reactants and desorption of products (molecular traffic). In case of the in situ growth of mordenite crystals, stacks of the needle shaped crystals are obtained, which are parallel to the support [7]. Since the channels in the one dimensional structure are in the direction of the crystal length, diffusion to the support is impossible. TEM results of the supported MFI layers showed that the crystals are attached to the support by chemical bonding, taking place by hydrolysis of surface OH-groups and Si-OH groups in the synthesis gel [8]. By this bonding the crystal symmetry after calcination remains orthorhombic, which influences adsorption and diffusion properties. The adsorption capacity for p-xylene molecules decreases, since the flexibility of the framework is reduced by the presence of surrounding crystals [9]. In composite materials coatings of laterally oriented silicalite-1 crystals are assumed to be a (shape) selective component in catalysis. A catalytic membrane is obtained if a noble metal coating is applied after growth of the crystal layer, which is self-supporting or bonded to a meso-porous support. Also the catalytic site can be applied onto the support before the in situ growth of the zeolite layer. In this paper preliminary results of the synthesis and characterization of model composites, consisting of Si wafers covered with thin layers of chromium, manganese and iron oxide and a silicalite-1 coating are presented. 2. EXPERIMENTAL 2.1: Synthesis As a support Si [100] wafers from a silicon single crystal, cut to 10x10 mm platelets (0.7 mm thick) were used. The wafers were cleaned by a special procedure [6]. Metal oxide coverage was obtained by spin coating of a metal salt solution [10]. The samples were mounted on a disk connected to a stirring motor, rotating at 2000 rpm in a nitrogen atmosphere at room temperature. The metal precursors were Cr(NO3)3.9H20 (p.A., Aldrich), Mn(OCOCH3)2.4H20 (p.A., Janssen Chimica) and FeCla.6H20 (p.A., Janssen Chimica). Of each precursor a 0.1 wt% solution in dry ethanol (p.A., J.T.Baker) was made. In each experiment 1 ml of the solution was passed onto the rotating wafer through a 0.45 #m FP-Vericel membrane filter (Gelman Sciences). Assuming that the radial flow and the evaporation of the liquid determine the film height, the amount of precursor material deposited was calculated with the use of the evaporation time [11]. The decrease of the film height h as a function of time is given by: dh dt
2*p*~2*h 3 3,TI
(1)
in which p is the density, ~ the radial velocity, 77the viscosity and ~0is the mass flux by evaporation. The mass flux can be obtained by solving the equation with h = 0 at t = tvaD, tvap being the evaporation time, and h = ht =0, which is set at 100/~m. At the equilibriu~n film height h e, the evaporation becomes dominant and precursor loss by radial flow becomes negligible. The weight amount of metal precursor deposited, mp, ~s given by"
1166
mp=Co,A,he=Co,312*P *~23.r1.r
(2)
in which c o is the precursor concentration in the spin coating solution, and A is the support area. The precursor materials were converted to the metal oxides by calcination at 450~ for 3 hours. The silicalite-1 coating was grown on top of the metal oxide layers. The platelets were cleaned in boiling toluene for 2 minutes before zeolite synthesis. Chemicals used for the layer growth were tetraethyl orthosilicate (98%, Aldrich), tetrapropylammonium hydroxide (25%, CFZ) and deionized water. The molar oxide ratio in the gel was 6.5 SiO2: 1 TPA20 : 800 H20. The gel was aged at room temperature for 1 hour. Crystallization took place at 150~ for 3 hours in teflon-lined 35 ml stainless steel autoclaves under static conditions. The platelets were positioned in the upper part of the synthesis mixture by suspension from the lid of the teflon insert. After cooling of the autoclave, the wafers were washed with distilled water. The template was removed by calcination at 450~ for 6 hours.
2.2: Analysis Atomic Force Microscopy measurements were carried out on a Topometrix 2010 TMX Microscope under ambient conditions. High Resolution Scanning Electron Microscopy was done with a Jeol JSM-6000F scanning microscope. Transmission Electron Microscopy was performed on a Philips CM-30 FEG microscope to prove the presence of metal oxide after crystallization of the silicalite-1 layer. X-ray Photoelectron Spectroscopy took place on a Phi 5400 spectrometer. The bonding electron peaks of the metals were corrected relative to the 2p C peak at 286.4 keV. The intensity ratios IM/Isi were used to estimate the metal oxide layer thickness, with the following formulas [12]:
I,=1;
011 -exp(-
t
)]
(a)
Is1 Is1 ( 1 - 0 ) [ 1 - e x p ( - ~ t ) ] ~"St,M,Oy in which IM~176 and Isi~176 are the intensities of the reference materials (MxO v and SiO2), 0 is the surface coverage, ,kM,MXUy ..... is the mean free path of the metal in ttie metal oxide phase, ,~.~I,MXUy .... is the silicon mean free path in the metal oxide phase, and t ~s the layer thickness of the metal oxide. The surface coverage =s e calculated by: e: mMxo~ A,t,p
(4)
in which is m Mxuy . . . - is the mass of the metal oxide precipitate, obtained with the use of Equation (2) and p is the metal oxide density.
ll6?
Figure 2: Chromium oxide coating on Si-wafer. The average particle size is 20 nm. The particles are part of a continuous layer of metal oxide. The support surface is visible between the metal coating and the large dust particle.
:x&
....! ~ i ~ .......~
~
..... ~ , ~ T
~J"
.....::~.~ ii~:~iil;;=.~I~'~" ......
------
KSLA i 5
10KU 2.0A
XIO.
l ~ m
808
W
Figure 3: Iron oxide coating on Si-wafer. The presence of the metal oxide coating becomes visible by scratching the surface.
1168
3. RESULTS AND DISCUSSION
XPS analysis of the spin coated iron samples showed a 2p3/2 peak at 710.8 eV, indicating the presence of Fe203 particles. The Mn 2p3/2 peak is positioned at 641.6 eV, which is identified as Mn203. The (Fe/Fe + Si) and the (Mn/Mn + Si) signal area ratio were 0.32 and 0.19 respectively, thus indicating a high coverage of the SiO 2 surface. The amount of deposited material was calculated using Equation (2), after which a layer thickness was deduced by assuming e = 1. From the intensity ratios measured with XPS, a layer thickness can also be calculated using Equation (3). The results of both calculations are compared in Table 1. The mean free paths of Fe in Fe203 and of Si in Fe203 are estimated at 2.030 and 2.693 nm, respectively, and the values of the mean free paths of Mn in Mn203 and of Si in Mn203 are 2.238 nm and 2.828 nm.
Table 1. Mn203 and Fe203 layer thickness t and mass of oxide deposited m, obtained by the spin coating model and XPS Sample
m spin (#g)
Mn203 Fe203
0.276 0.257
t spin (nm) 0.61 0.49
t xp s (nm) 0.44 0.60
m xPS (#g) 0.199 0.313
These data show that the modelling of the spin coating procedure gives a reasonable estimate of the amount of deposited material. AFM measurements of the Cr203 samples showed particles of 15-30 nm on top of a corrugated layer. No particles were observed for the Fe203 samples, although based on the XPS results a high coverage of the SiO2 surface was expected. HRSEM pictures demonstrate the presence of 20 nm particles in the Cr203 samples, see Figure 2, which however appear to be part of a thin continuous layer of metal oxide, that is probably formed between the hemispheres during calcination. The transition between support and metal oxide layer is demonstrated by flaws on the surface. The average number of hemispheres on the surface is lower than in figure 2. The chromium oxide layer thickness is estimated at 1-2 nm. HRSEM measurements again showed that the iron oxide coating does not consist of particles, but forms a continuous flat layer. This was demonstrated by scratching the surface, see Figure 3. AFM measurements of the Mn203 samples showed that the metal oxide coating consists of 50 nm hemispheres, forming a continuous layer, see Figure 4. According to light microscopy the silicalite-1 coating extends over 1 cm 2. XRD reveals that the crystals are laterally oriented, thus with the straight channels of the pore system perpendicular to the support. Figures 5 and 6 show the AFM and HRSEM pictures of the silicalite layer on top of the Fe203 coating. The zeolite crystals form a rather continuous layer leaving a small amount of pin holes. The average size of the laterally oriented intergrown crystals is 0.6 p,m. On top of the lateral crystal coating partly axially oriented and twin forms are present, which however do not affect the layer concept. The same silicalite-1 coatings were grown on the Mn203 and Cr203 layers. The average silicalite-1 layer thickness is estimated at 200 nm.
1169
'"~~"..i!i~iii~!!:ii":'i'!$i ...... ~,~'{" ...~,'!;i~'ir
i"ii.....:!:!~!i~
~i!.',:~=~'i'i.i !!i!}ii~i !i}
....i~i
9
Figure 4: AFM picture of manganese oxide coating on Si-wafer. Image size is 550 x 550 nm.
Figure 5: AFM picture of laterally oriented silicalite-1 crystals on a Fe203 layer. Image size is 5 x 5 #m.
Figure 6: HRSEM photograph of silicafite-1 crystals grown on a continuous layer of Fe20 3.
1170
Transmission Electron Microscopy measurements were made of wedge shaped slices of the composites, which were obtained from a cut. This preparation method leads to a random orientation of the sample on the holder, yielding a limited number of succesfull measurements. The silicalite crystals of the Fe203 composite materials are tightly attached to the support XES measurements show that the surface and the bulk of the zeolite layer, as well as the Si wafer do not contain iron. The presence of iron becomes evident only at the interface between the silicalite layer and the wafer. The thickness and continuity of the iron layer after zeolite synthesis however, remains unknown. Apparently the metal oxide layer has not dissolved drastically by exposure to the zeolite synthesis mixture at high pH. This phenomenon is also observed during synthesis of thin silicalite-1 layers on Si/SiO 2 surfaces, where no etching of the wafer takes place at relatively low temperatures and short synthesis times [6]. It appears that the kinetics and/or the equilibria which cause etching at high pH are shifted by the presence of a Si source. TEM micrographs and XES analysis of all composite materials using a crosssection technique, as well as preliminary catalytic tests will be published in a forthcoming paper. 4. CONCLUSIONS
The synthesis of composite materials consisting of a thin metal oxide layer such as Cr203, Mn203 or Fe203, and a thin oriented layer of silicalite-1 appears to be possible. Well defined thin layers of metal oxide are prepared by spin coating. The synthesis of laterally oriented zeolite layers on these metal oxide has succeeded, and in case of the Fe203/silicalite-1 composite the presence of iron towards the support/silicalite layer interface is proven by TEM/XES. The composites are expected to exhibit a high shape selectivity in future catalytic experiments. ACKNOWLEDGEMENT
The authors thank Ing. P. van Acker and E. Rodenburg for their help with the spin-coating procedure. We also thank Dr.A. Knoester and Ing. N. Groesbeek from the Royal/Dutch Shell Laboratory in Amsterdam for the HRSEM measurements. Dr. H. Zandbergen is acknowledged for the TEM measurements. LITERATURE
[1] I.M. Lachman and M.D. Patil, US Patent 4,800,187 (1989). [2] S.P. Davis, E.V.R. Borgstedt and S.L. Suib, Chem. Mater., 2 (1990), 712. [3] T. Sano, M. Kawamura, F. Mizukami, H. Takaya, T. Mouri, W. Inaoka, Y. Toida, M. Watanabe and K. Toyoda, Zeolites, 11 (1991), 842. [4] H. van Bekkum, E.R. Geus and H.W. Kouwenhoven, Proc. Summerschool of the 10th Int. Zeolite Conf., Garmisch-Partenkirchen, Elsevier, (1994), in print. [5] H.P. Calis, A.M. Gerritsen, C.M. van den Bleek, C.H. Legein, J.C. Jansen and H. van Bekkum, Can. J. Chem. Eng., (1994), accepted.
1171
[6] J.C. Jansen, W. Nugroho and H. van Bekkum, In: R. von Ballmoos, J.B. Higgins and M.M.J. Treacy (eds.), Proc. 9th Int. Zeolite Conf., Montreal, Bulterwoilh, (1993), 247-254. [7] J.C. Jansen, D. Kashiev ar~ A. Erdem-Senetalar, Proc. Summerschool o f t l ~ 10th Int. Zeolite Conf., Garmisch-Partenkirchen, Elsevier, (1994), in print [8] J.H. Koegler, J.C. Jansen, H. van Bekkum, Proc. 10th Int. Zeolite Co{ff., Garmisch-Partenkir~, (1994), accepted. [9] N. van der Pull, J.C. Jansen and H. van Bekkum, unpublished _dat~_: [10] E.W. Kuipers, C. Laszlo and W. Wieldraaijer, Cat. LetL, 17 (1993), 71-79. [11] R.M. van Hardeveld, P.LJ. Gunier, L.J. van Llzendoom, E.W. I(uipels and J.W. Niemantsverdriet, Appl. Surf. Sci., (1994), to be p u b l i C . [12] H.P.C.E. Kuipers, H.C.E. van Leuven and W.M. Visser, Surf. Interf. Analysis, 8 (1986), 235-242.
This Page Intentionally Left Blank
1173 AUTHOR INDEX Abdel-Hamid S.M. Aboul-Gheit A.K. Afanasiev P. Andrews S.J. Andrianova M.P. Andrushkova O.V. Andryushkova O.V. Angelescu Era. Anthony R.G. Arabczyk W. Arai M. Ascenci6n Montoya J. Avdeeva L.B. Avila P. Avvakumov E.G. Bacaud R. Baerns M. Bahamonde A. Bahranowski K. Baiker A. Barzhinimaev B.S. Bando Y. Barannyk G. Barnes P.A. Bassini M. Bastians Ph. Battagliarin M. Baumgarmer J.E. Beelen T.P.M. Beguin F. Bekyarova E. Bernal S. Beyer H.K. Bi Ying-Li Bianchi C.L. Bickley R.I. Bielanska E. Bielanski A. Binder C. Blanchard P. Blanco J. Blanco M.N. Bliek A. Bobin C. Boellaard E. Boisdron N. Boitiaux J.-P. Bongaarts J.E. B6nnemann H. Boot L.A. Bordes E. Boronin A.J. Brands D. Bresadola S. Breysse M. Broersma A. Busca G. Busenna A.
1131 1131 273 49 637 851 637 561 391 131,677 923 807 825 755 637 495 1009 755 747 75,505,617 915 227 775 361,859 765 381 327 989 33 523 1137 461 551 691 1095 589 737 571 869 1037 755 1121 699 169 579,931 1037 253 159 185 159 707 977 699 817 273,495 263 667 421
Buyanov R.A. 793,1101 C~iceres C.V. 1121 Calvino J.J. 461 Carli R. 1095 Carmello D. 765 Carriat J.Y. 967 Castillo R. 291 Cauqui M.A. 461 Cavani F. 1 Centi G. 59 Cesar D.V. 1017 Che M. 253,967,1027 Chen Li-Gang 647 Chen Song-Ying 197,427,479,489 Chen Yi 799 Cheng Zheng-Xing 1027 Chinchen G.C. 49 Chuvilin A.L. 977 Ciambelli P. 717 Ciesla U. 337 Clause O. 169 Collina D. 401 Coman S. 561 Conanec R. 381 Contescu Cr. 237 Contractor R. 707 Cunningham D.A.H. 227 Cybulski A. 1069 d~Espinose de la Caillerie J.B. 169 Danilyuk A.F. 915 Daturi M. 667 Davies L. 49 Dawson E.A. 361 de Resende N.S. 1059 Decarreau A. 967 Delmon B. 291,999 Demuth D. 337 Derevyankin A.Yu. 825 des Couri&es T. 273 Dessalces G. 453 Devillers M. 999 Devisse F. 253 Didillon B. 253 Dokter W.H. 33 Drzymala R. 677 Duarte M.A.I. 1017 Dubois J.L. 833 Duff D.G. 75,505 Dufner D.C. 391 Duisterwinkel A.E. 1051 Dula R. 747 Duprez D. 699,941 Dziembaj R. 571 Enea O. 941 Eon J.-G. 1059 Ertl G. 217 Fenelonov V.B. 825 Fernandez J. 381
1174
Fontaneto C. 1095 Fomasari G. 401 Forzatti P. 765 Frankc O. 309 Frens G. 1051 Frety R. 1017 Frobel C. 1009 Fu Xian-Cai 647,799 Fujieda S. 833 Galiasso R. 281 Gandia L. 381 Garin F. 657 Gazzano M. 893 Geantet G. 273 Gerontopoulos P. 327 Geus J.W. 159,263,579,931,1153 Golosman E.Z. 879 Goncharova O.V. 825 Goncharova S.N. 915 Gonzalez S. 727 Gonzgdez M.G. 1121 Gonzalez-Carreno T. 589 Grange P. 381 Grass K. 1111 Grimblot J. 1037 Guntow U. 217 Gusman M.I. 445 Harivololona R. 435 Haruta M. 227 Hasik M. 571 Haukka S. 957 Hilaire L. 607 Hoang-Van C. 435 Hoffmann U. 299,869 Hogg L.T. 589 Hong Jian-Min 647 Hoogenraad M.S. 263 Hou Wen-Hua 799 Hu Ze-Shan 197,479 Huerta Y. 411 Hutchings G.J. 27 Iglesia E. 989 Isaeva V. 539 Ismagilov Z.R. 775,1145 Isupova L.A. 637 Ivanov V.P. 637 Iwasawa Y. 141 Jagiello J. 237 Jansen J.C. 1163 Jlir~ts S.G. 85 Jing Jun-Hang 949 Kakuta N. 319 Kalucki K. 131,677 Kannan S. 903 Kappenstein C. 699 Kapteijn F. 1145 Karge H.G. 551 Katz J.L. 207 Kerkhoffs M.J.H.V. 1153 Kermarec M. 967 Keryou K. 545
Kesteman E. Kiennemann A. Kiljanov M.Yu. Kirichenko O.A. Klimova T. Koch B. Kochloefl K. K6hler S. Kolenda F. Kolesnikov I.M. Kolesnikov S.I. Kolesnikova K.A. Kolomiichuk V.N. Komorek J. Kondarides D.I. Koryabkina N.A. Kotur E. Kramer L.F. Krastev V. Krawczyk K. Krieger T.A. Kryukova G.N. Kuipers E.W. Kulszewicz-Bajer I. Kunz U. Kyt6kivi A. Ladebeck J. Lakomaa E.-L. Lambert J.-F. Laurent Y. Lltz,~ K. Leboda R. Lehtovirta U. Lcofanti G. Leon R. Le6n V. Li Sen-Zi Likholobov V.A. Lindblad M. Lintz H.-G. Lopez F. L6pez M.A. Lott S.E. Louis C. Loutaty R. Lowe D.M. Lu Fan Lujala V. Lyakhova V. Lycourghiotis A. Maciejewski M. Macovei D. Maire G. Makarova O.V. Makkee M. MaUat T. Marchand R. Marella M. Marti P.E. Martin Aranda R.M. Massucci M.A.
707 607 421 851,1145 411 291,783 1079 1009 453,843 421 421 597 637,977 747 141 1145 667 371 1137 683 533 637 1163 571 299,869 957 1079 957 253 381 551 597 957 401 337 281 691 825,977 957 1111 281 807 391 1027 1037 445 489 957 775 95 75,617 561 657 533 371 75 381 327 617 411 717
1175
Mauchausse C. McCarty J.G. Mchandjiev D. Menon P.G. Mcregalli L. Merzouki M. Miquel Ph.F. Mizukami F. Mizusima T. Moene R. Montes M. Morawski A.W. Moreau S. Moroz E.M. Moulijn J.A. MuhlCr M. Muto Y. Nagasawa Y. Najbar M. Narkiewicz U. Nge R. Nickolov R. Nishida K. Nishiyama Y. Odriozola J.A. Okkel L.G. Oldman R.J. Oliveri G. Pakhomov N.A. P61-Borb~ly G. Palmisano L. Pantos E. Paparatto G. Parkes G.M.B. Parvulescu V. Parvulescu V.I. Patrono P. Pauli I.A. Pavanello L. Payen E. Peng Shao-Yi Perathoner S. Pessaud S. Petit C. Petrini G. Petroff P. Petryk J. Pijolat M. Pinna F. Piwowarska Z. Plyasova L.M. Pools E. Poix P. Poluboyarov V.A. Pommicr B. Pomonis P.J. PortillaM. Poulet O. Pozniczek J. Prada S ilvy R. Pron A.
1037 445 1137 85 327 707 207 319,627 319 371 381 131,677 523 977 371,1069 217 627 141 737 677 391 1137 319 923 381 825 49 667 1101 551 589 33 401 859 561 561 717 637 817 1037 197,427,479 59 523 607 401 337 683 885 327 571 533 699 657 637,851 435 513 807 1037 571 281 571
Radu R. 561 Ragaini V. 1095 Ramfrcz J. 411 Rathousky J. 309 Rebours B. 169 Rehspringer J.L. 607 Reiche M. 1009 Reyes E. 281 Reyes S.C. 989 Reymond J.P. 453 Richardson J.T. 345 Rinaldo A. 401 Rodffguez-Izquierdo J.M. 461 Roger A.C. 607 Rojas Cervantes M.L. 411 Romero Y. 281 Romotowski T. 747 Rosenberg E. 843 Rosowsld F. 217 Rouleau L. 495 Rozovskii A. Ya. 637 Ruiz P. 291,727,999 Ruiz R. 807 Rulrnont A. 783 Russo G. 717 Russu R. 561 Sadykov V.A. 637 Salim V.M.M. 1017 Sanands M.T. 27 Schl6gl R. 217 Schmal M. 1017,1059 Schmidt-Szalowski K. 683 Schneider M. 75 Schoonheydt R.A. 151 Schoonman J. 371 Schulz-Eldoff G. 309 SchUth F. 337 Schwarz J.A. 237 Sermon P.A. 471,545,1085 Serwicka E.M. 747 Shaft V. 539 Shirai M. 923 S hkrabina R.A. 1145 Signoretto M. 327 Simonot L. 657 Simonov P.A. 825,977 Skordilis C.S. 513 Smirnova Y. 539 Sobalik Z. 727 Soled S.L. 989 Solovyova L.P. 637 Soustelle M. 885 Strukul G. 327 Stucky G.D. 337 Sun Y. 471 Suntola T. 957 Swamy C.S. 903 Szymansld R. 843 Tanabe T. 319 Taouk B. 707 Teslenko V.V. 597
1176
Teymouri M. 607 Tirions O. 999 Tomaselli M. 327 Tornishige K. 141 Tretyakov V.F. 637 Trifir6 F. 1,401 Tronconi E. 765 Tsubota S. 227 Tsybulya S.V. 915 Tuel A. 27 Turco M. 717 Twigg M.V. 345 Udomsak S. 391 Ueno A. 319 Unger K. 337 Ushakov V.A. 851,1145 Usui K. 923 Vaccari A. 893 van Bekkum H. 1163 van Breda Vriesman G.J.B. 263 van Buren F.R. 159 van der Horst A.A. 579 van der Kraan A.M. 579,931 van der Puil N. 1163 van Dillen A.J. 159,263,579 van Garderen H.F. 33 van Leeuwarden R.A.G.M.M. 263 van Santen R.A. 33 van Yperen R. 579 Vgtzquez P.G. 1121 Vecchio S. 717 Vidal H. 461 Viricelle J.P. 885
Viveros T. Vogels R.J.M.J. Volta J.C. Vong M.S.W. Vonk H. Wachs I.E. Wahdan T. Walter M. Weckhuyscn B.M. Wei Quan Wijzen F. Wildberger M. Willey R.J. Xu Xiao-Ding Yablonsky A.V. Yakerson V.I. Yakubovich T.N. Yah Qi-Jie Yang Xiang-Guong Yates M. Yur'eva T.M. Zaikovskii V.I. Zaki M.I. Zarate A. Zhang Chi-Ming Zhao Xiu-Ren Zhen Kai-Ji Zhong Zi-Yi Zolotovskii B.P. Zub Yu.L. Zukal A. Zwinkels M.F.M.
807 1153 27 1085 1069 151 699 843 151 691 783 75 667 1069 421 879 597 647,799 691 755 533 533,915 699 807 427 949 691 647 793 597 309 85
1177
STUDIES IN SURFACE SCIENCE A N D CATALYSIS Advisory Editors: B. Delmon, Universit~ Catholique de Louvain, Louvain-la-Neuve, Belgium J.T. Yates, University of Pittsburgh, Pittsburgh, PA, U.S.A.
Volume 1
Volume 2
Volume 3
Volume 4
Volume 5
Volume 6
Volume 7
Volume 8 Volume 9
Volume 10
Volume 11
Volume 12
Volume 13 Volume 14
Preparation of Catalysts I.Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings ofthe First International Symposium, Brussels, October 14-17,1975 edited by B. Delmon, P.A.Jacobs and G. Poncelet The Control ofthe 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 Preparation of Catalysts I1. 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 Growth and Properties of Metal Clusters. Applications to Catalysis and the Photographic Process. Proceedings of the 32nd International Meeting of the Soci~t~ de Chimie Physique, Villeurbanne, September 24-28, 1979 edited by J. Bourdon Catalysis by Zeolites. Proceedings of an International Symposium, Ecully (Lyon), September 9-11, 1980 edited by B. Imelik, C. Naccache, Y. Ben Taarit, J.C. Vedrine, G. Coudurier and H. Praliaud Catalyst Deactivation. Proceedings of an International Symposium, Antwerp, October 13-15,1980 edited by B. Delmon and G.F. Froment New Horizons in Catalysis. Proceedings of the 7th International Congress on Catalysis, Tokyo, June 30-July4, 1980. Parts A and B edited by T. Seiyama and K. Tanabe Catalysis by Supported Complexes by Yu.l. Yermakov, B.N. Kuznetsov and V.A. Zakharov Physics of Solid Surfaces. Proceedings of a Symposium, Bechyhe, September 29-October 3,1980 edited by M. Lazni~,ka Adsorption at the Gas-Solid and Liquid-Solid Interface. Proceedings of an International Symposium, Aix-en-Provence, September 21-23, 1981 edited by J. Rouquerol and K.S.W. Sing 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 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.Jin~ and G. Schulz-Ekloff Adsorption on Metal Surfaces. An Integrated Approach edited by J. B6nard Vibrations at Surfaces. Proceedings of the Third International Conference, Asilomar, CA, September 1-4, 1982 edited by C.R. Brundle and H. Morawitz
1~178
Volume 'F5 Volume 16
Volume 17 Vorume 18 Volume 19 Volume 20 Volume 21 Volume 22 Volume 23 Volume 24
Volume 25 Volume 26 Volume 27 Volume 28 Volume 29 Volume 30 Volume 31
Volume 32 Volume 33 Volume 34
Heterogeneous Catalytic Reactions involving Molecular Oxygen by G.I. Golodets Preparation of Catalysts Ill. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings ofthe Third international Symposium, Louvain-la-Neuve, September 6-9, 1982 edited by G. Poncelet, R Grange and P.A.Jacobs Spillover of Adsorbed Species. Proceedings of an International Symposium, Lyon-Villeurbanne, September 12-16,1983 edited by G.M. Pajonk, S.J. Teichner and J.E. Germain Structure and Reactivity of Modified Zeolites. Proceedings of an International Conference, Prague, July 9-13, 1984 edited by RA. Jacobs, N.I. Jaeger, R Ji~J, V.B. Kazansky and G. Schulz-Ekloff Catalysis on the Energy Scene. Proceedings of the 9th Canadian Symposium on Catalysis, Quebec, RQ., September 30-October 3, 1984 edited by S. Kaliaguine and A. Mahay Catalysis by Acids and Bases. Proceedings of an International Symposium, Villeurbanne (Lyon), September 25-27, 1984 edited by B. Imelik, C. Naccache, G. Coudurier, Y. Ben Taarit and J.C. Vedrine Adsorption and Catalysis on Oxide Surfaces. Proceedings of a Symposium, Uxbridge, June 28-29, 1984 edited by M. Che and G.C. Bond Unsteady Processes in Catalytic Reactors by Yu.Sh. Matros Physics of Solid Surfaces 1984 edited by J. Koukal Zeolites: Synthesis, Structure, Technology and Application. Proceedings of an International Symposium, Portoro~-Portorose, September 3-8, 1984 edited by B. Dr~aj, S. Ho~evar and S. Pejovnik 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 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 Catalytic Hydrogenation edited by L. Cerven~ New Developments in Zeolite Science and Technology. Proceedings of the 7th International Zeolite Conference, Tokyo, August 17-22, 1986 edited by Y. Murakami, A. lijima and J.W. Ward Metal Clusters in Catalysis edited by B.C. Gates, L. Guczi and H. Kn6zinger Catalysis and Automotive Pollution Control. Proceedings of the First International Symposium, Brussels, September 8-11, 1986 edited by A. Crucq and A. Frennet Preparation of Catalysts IV. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Fourth International Symposium, Louvain-la-Neuve, September 1-4, 1986 edited by B. Delmon, P.Grange, P.A. Jacobs and G. Poncelet Thin Metal Films and Gas Chemisorption edited by P.Wissmann Synthesis of High-silica Aluminosilicate Zeolites edited by P.A.Jacobs and J.A. Martens Catalyst Deactivation 1987. Proceedings of the 4th International Symposium, Antwerp, September 29-October 1, 1987 edited by B. Delmon and G.E Froment
1179
Volume 35 Volume 36 Volume 37
VoLume 38 Volume 39 Volume 40 Vo~iume 41
Vt~um e 42 Vt~ume 43 Vo;urrm 44.
VOlume 45
Volume 46
Volume 47 Volume 48 Volume 49 Volume 50
Volume 51 Volume 52 Volume 53
Keynotes in Enmgy-Related Catalysis edited by S. Kaliaguine Methane C o n v e ~ m . 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, RE Howe andFS.Yurchak Innovation in ZeoUte Materials Science. Proceedings of an International Symposium, Nieuwpoort, September 13-17,1987 edited by R3. ~ W ~ J . I~Bctier, E.F. Vansantaad G. Schulz-Ekloff Catalysis 1987. P r r ~ [ n g s of the 10th North American Meeting of the Catalysis Society, San Diego, CA, May 17-22, 1987 edited by J.W. Ward Characterization of. Porous Solids. Proceedings ofthe IUPAC Symposium (COPS I), Bad Soden a. Ts., April 26-29,1987 edited by K.K. Unger, J. Rouquerol, K.S.W. Sing anld H. Kral Ptwsics of.Sof~ S ~ a s 1987. Proceedings ofthe Fourth Symposium on Surface Physics, Bechyne Castle, September 7-11,1987 ed~.ed by J. Koukal' HC,tmogeneous Catalysis a~l Fine Cttemir162 Proceedings of an International Symposium, Poitiers, March 15-17, 1988 editc,d by M. ~ J. Bmrault, C. Bouclkoule, D. Duprez, C. Montassier and G. I~rot Laboratory Studies of Hetmogeneous Catalytic Processes by E:G. Christoff~, revised and edited by 7_ Pafil Catalytic Processes under Unsteady-State Conditions by Yu. Sh. Mottos Successful ~ of Catalysts. Future Requirements and Development. Proceedings ofthe Worldwide Catalysis Seminars, July, 1988, on the Occasion of the 30th Anniversary ofthe Catalysis Society of Japan edited by T. Inui Transition Metal Oxides. Surface Chemistry and Catalysis by I~LH.Kung Zeolites as Catalysts, Sorbents and Detergent Builders. Applications and Innovations. Proceedings of an International Symposium, W~rzburg, September 4-8,1988 edited by H.G. Karge and J. Weitlkamp Photochemistry on Solid Surfaces edited by M. Anpo and T. Matsuura Structure and Reactivity of Surfaces. Proceedings of a European Conference, Trieste, September 13-16, 1988 edited by C. Morterra, A. Zecchina and G. Costa Zeolites: Facts, Figures, Future. Proceedings of the 8th International Zeolite Conference, Amsterdam, July 10-14, 1989. Parts A and B edited by RA. Jacobs and R.A. van Santen Hydrotreating Catalysts. Preparation, Characterization and Performance. Proceedings of the Annual International AIChE Meeting, Washington, DC, November 27-December 2, 1988 edited by M.L. Occelli and R.G. Anthony New Solid Acids and Bases. Their Catalytic Properties by K. Tanabe, M. Misono, Y. Ono and H. Hattori Recent Advances in Zeolite Science. Proceedings of the 1989 Meeting of the British Zeolite Association, Cambridge, April 17-19, 1989 edited by J. Klinowsky and RJ. Barrie 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
1180
Future Opportunities in Catalytic and Separation Technology edited by M. Misono, Y. Moro-oka and S. Kimura New Developments in Selective Oxidation. Proceedings of an International Volume 55 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 Polymerization Catalysts, Tokyo, October 23-25, 1989 edited by T. Keii and K. Soga Volume 57A Spectroscopic Analysis of Heterogeneous Catalysts. Part A: Methods of Surface Analysis edited by J.L.G. Fierro Volume 57B Spectroscopic Analysis of Heterogeneous Catalysts. Part B: Chemisorption of Probe Molecules edited by J.L.G. Fierro Introduction to Zeolite Science and Practice Volume 58 edited by H. van Bekkum, E.M. Flanigen and J.C. Jansen Heterogeneous Catalysis and Fine Chemicals II. Proceedings of the 2nd Volume 59 International Symposium, Poitiers, October 2-6, 1990 edited by M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, G. Pdrot, R. Maurel and C. Montassier Volume 60 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 61 Natural Gas Conversion. Proceedings of the Symposium on Natural Gas Conversion, 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 Preparation of Catalysts V. Scientific Bases for the Preparation of Heterogeneous Volume 63 Catalysts. Proceedings of the Fifth International Symposium, Louvain-la-Neuve, September 3-6, 1990 edited by G. Poncelet, RA. Jacobs, P.Grange and B. Delmon Volume 64 New Trends in CO Activation edited by L. Guczi Volume 65 Catalysis and Adsorption by Zeolites. Proceedings of ZEOCAT 90, Leipzig, August 20-23, 1990 edited by G. (~hlmann, H. Pfeifer and R. Fricke Volume 66 Dioxygen Activation and Homogeneous Catalytic Oxidation. Proceedings of the Fourth International Symposium on Dioxygen Activation and Homogeneous Catalytic Oxidation, Balatonf~red, September 10-14, 1990 edited by L.I. Simandi Volume 67 Structure-Activity and Selectivity Relationships in Heterogeneous Catalysis. Proceedings of the ACS Symposium on Structure-Activity Relationships in Heterogeneous Catalysis, Boston, MA, April 22-27, 1990 edited by R.K. Grasselli and A.W. Sleight Volume 68 Catalyst Deactivation 1991. Proceedings of the Fifth International Symposium, Evanston, IL, June 24-26, 1991 edited by C.H. Bartholomew and J.B. Butt Volume 69 Zeolite Chemistry and Catalysis. Proceedings of an International Symposium, Prague, Czechoslovakia, September 8-13, 1991 edited by RA. Jacobs, N.I. Jaeger, L. Kubelkova and B. Wichterlova Volume 54
1181
Volume 70
Volume 71 Volume 72
Volume 73 Volume 74 Volume 75
Volume 76 Volume 77
Volume78
Volume79 Volume80 Volume81
Volume82
Volume 83 Volume 84
Volume 85
Poisoning and Promotion in Catalysis based on Surface Science Concepts and Experiments by M. Kiskinova Catalysis and Automotive Pollution Control II. Proceedings of the 2nd International Symposium (CAPoC 2), Brussels, Belgium, September 10-13, 1990 edited by A. Crucq New Developments in Selective Oxidation by Heterogeneous Catalysis. Proceedings of the 3rd European Workshop Meeting on New Developments in Selective Oxidation by Heterogeneous Catalysis, Louvain-la-Neuve, Belgium, April 8-10, 1991 edited by R Ruiz and B. Delmon Progress in Catalysis. Proceedings of the 12th Canadian Symposium on Catalysis, Banff, Alberta, Canada, May 25-28, 1992 edited by K.J. Smith and E.C. Sanford Angle-Resolved Photoemission. Theory and Current Applications edited by S.D. Kevan New Frontiers in Catalysis, Parts A-C. Proceedings of the 10th International Congress on Catalysis, Budapest, Hungary, 19-24 July, 1992 edited by L. Guczi, F. Solymosi and R Tdtdnyi Fluid Catalytic Cracking: Science and Technology edited by J.S. Magee and M.M. Mitchell, Jr. New Aspects of Spillover Effect in Catalysis. For Development of Highly Active Catalysts. Proceedings of the Third International Conference on Spillover, Kyoto, Japan, August 17-20, 1993 edited by T. Inui, K. Fujimoto, T. Uchijima and M. Masai Heterogeneous Catalysis and Fine Chemicals III. Proceedings ofthe 3rd International Symposium, Poitiers, April 5 - 8, 1993 edited by M. Guisnet, J. Barbier, J. Barrault, C. Bouchoule, D. Duprez, G. P6rot and C. Montassier Catalysis: An Integrated Approach to Homogeneous, Heterogeneous and Industrial Catalysis edited by J.A. Moulijn, P.W.N.M. van Leeuwen and R.A. van Santen Fundamentals of Adsorption. Proceedings of the Fourth International Conference on Fundamentals of Adsorption, Kyoto, Japan, May 17-22, 1992 edited by M. Suzuki Natural Gas Conversion II. Proceedings of the Third Natural Gas Conversion Symposium, Sydney, July 4-9; 1993 edited by H.E. Curry-Hyde and R.F. Howe New Developments in Selective Oxidation II. Proceedings of the Second World Congress and Fourth European Workshop Meeting, Benalmddena, Spain, September 20-24, 1993 edited by V. Cort6s Corberan and S. Vic Bellbn Zeolites and Microporous Crystals. Proceedings of the International Symposium on Zeolites and Microporous Crystals, Nagoya, Japan, August 22-25, 1993 edited by T. Hattori and T. Yashima Zeolites and Related Microporous Materials: State of the Art 1994. Proceedings of the 10th International Zeolite Conference, Garmisch-Partenkirchen, Germany, July 17-22, 1994 edited by J. Weitkamp, H.G. Karge, H. Pfeifer and W. H61derich Advanced Zeolite Science and Applications edited by J.C. Jansen, M. St6cker, H.G. Karge and J.Weitkamp
1182
Volume86 Volume 87 Volume 88 Volume89
Volume 90 Volume91
Oscillating Heterogeneous Catalytic Systems by M.M. Slin'ko and N.I. Jaeger Characterization of Porous Solids III. Proceedings of the IUPAC Symposium (COPS III), Marseille, France, May 9-12, 1993 edited by J.Rouquerol, F. Rodriguez-Reinoso, K.S.W. Sing and K.K. Unger Catalyst Deactivation 1994. Proceedings of the 6th International Symposium, Ostend, Belgium, October 3-5, 1994 edited by B. Delmon and G.F. Froment Catalyst Design for Tailor-made Polyolefins. Proceedings of the International Symposium on Catalyst Design for Tailor-made Polyolefins, Kanazawa, Japan, March 10-12, 1994 edited by K. Soga and M. Terano Acid-Base Catalysis II. Proceedings of the International Symposium on Acid-Base Catalysis II, Sapporo, Japan, December 2-4, 1993 edited by H. Hattori, M. Misono and Y. Ono Preparation of Catalysts VI. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Sixth International Symposium, Louvain-La-Neuve, September 5-8, 1994 edited by G. Poncelet, J. Martens, B. Delmon, RA. Jacobs and R Grange
