Studies in Surface Science and Catalysis 91

PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts

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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

Edi to rs

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 Amsterdam ~ Lausanne- - New Y o r k - - Oxford - - Shannon ~ Tokyo 1995

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1995 Elsevier Science B.V. All rights reserved.

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ORGANIZING COMMI'ITEE

OPENING ADDRESS

AKNOWLEDGEMENTS

CONTENTS

Vanadium phosphorus mixed oxide from the precursor to the active phase: catalyst for 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 catalysts for n-butane oxidation to maleic anhydride M.T. Sanan6s, A. Tuel, G.J. Hutchings, J.C. Volta

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

In situ techniques for the investigation of phase transformations in copper catalyst co-precipitates G.C. Chinchen, L. Davies, R.J. Oldman, S.J. Andrews

Influence of preparation method on the properties of V-Sb-O catalysts for the ammoxidation of propane G. Centi and S. Perathoner

Novel procedure for the preparation of highly active platinum-titania and palladium-titania aerogel catalysts with favourable textural properties M. Schneider, M. Wildberger, D.G. Duff, T. Mallat, M. Maciejewski, A. Baiker

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

Preparation of supported catalysts by equilibrium deposition-filtration A. Lycourghiotis

Preparation of K-C-Fe/Al203 catalysts for ammonia synthesis at mild conditions K. Kalucki, A.W. Morawski, W. Arabczyk

A novel [PtMo6]/MgO catalyst for alkane-to-alkene conversion D.I. Kondarides, K. Tomishige, Y. Nagasawa, Y. Iwasawa

Spectroscopic characterization of supported Cr and Cr, Ti catalysts: Interaction with probe molecules B.M. Weckhuysen, I.E. Wachs, R.A. Schoonheydt

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

Alumina~water interf acial phenomena during impregnation J.B. d'Espinose de la Caillerie, C. Bobin, B. Rebours, O. Clause

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XVII

XXI

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33

49

59

75

85

95

131

141

151

159

169

Nanometals and colloids as catalyst precursors H. B~Snnemann

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

Flame synthesis of nanostructured vanadium oxide based catalysts Ph.F. Miquel, J.L. Katz

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

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

Proton affinity distributions: a scientific basis for the design and construction of supported metal catalysts Cr. Contescu, J. Jagiello, J.A. Schwarz

~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

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

Soft chemistry route for the preparation of highly dispersed transition metals on zirconia C. Geantet, P. Afanasiev, M. Breysse, T. des Couri~res

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

Preparation of titania supported on silica catalyst: study of the dispersion and the texture of titania R. Castillo, B. Koch, P. Ruiz, B. Delmon

Preparation of catalytic polymer~ceramic ion exchange packings for reactive distillation columns U. Kunz, U. Hoffmann

Synthesis of MCM-41 mesoporous molecular sieves O. Franke, J. Rathousk~, G. Schulz-Ekloff, A. Zukal

Preparation of spherical and porous Si02 particles by fume pyrolysis N. Kakuta, T. Tanabe, K. Nishida, T. Mizusima, A. Ueno

Sol-gel zirconia spheres for catalytic applications M. Marella, M. Tomaselli, L. Meregalli, M. Battagliarin, P. Gerontopoulos, F. Pinna, M. Signoretto, G. Strukul

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237

253

263

273

281

291

299

309

319

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

Preparation and properties of ceramic foam catalyst supports M.V. Twigg, J.T. Richardson

A new method for the preparation of metal-carbon catalysts P.A. Barnes, E.A. Dawson

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

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

Preparation of silica or alumina pillared crystalline titanates S. Udomsak, R. Nge, D.C. Dufner, S.E. Lott, R.G. Anthony

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

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

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

A new procedure for preparing aerogel catalyst Chi-Ming Zhang, Song-Ying Chen, Shao-Yi Peng

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

Synthesis and characterization of sintering resistant aerogel complex oxide powders D.M. Lowe, M.I. Gusman, J.G. McCarty

Effect of reactant mixing mode on silica-alumina texture J.P. Reymond, G. Dessalces, F, Kolenda

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

Preparation of CaO-, La203- and Ce02- doped Zr02 aerogels by sol-gel methods Y. Sun, P.A. Sermon

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401

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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

The preparation of ultrafine Sn02 by the supercritical fluid drying technique (SCFDT) Fan Lu, Song-Ying Chen

Plasma preparation of a dispersed catalyst for hydroconversion of heavy oils L. Rouleau, R. Bacaud, M. Breysse

Preparation and structural properties of ultrafine gold colloids for oxidation catalysis D.G. Duff, A. Baiker

Synthesis, characterization and catalytic activity of manganese oxidic nano- particles C.S. Skordilis, P.J. Pomonis

Development of LaxMOy nanocatalysts dispersed in a layered silicate matrix S. Moreau, S. Pessaud, F. Beguin

Nanometer size copper particles in copper chromite catalysts T.M. Yur'eva, L.M. Plyasova, O.V. Makarova, T.A. Krieger, V.I. Zaikovskii

Silica immobilized Ru complexes with a different nuclear number as catalysts of the hydrode halo g e nation reaction V. Isaeva, Y. Smirnova, V. Sharf

Colloidal routes to Pt-Au catalysts K. Keryou, P.A. Sermon

Catalysts by solid-state ion exchange: iron in zeolite K. L~iz~, G. P~il-Borb61y, H.K. Beyer, H.G. Karge

Modified ruthenium exchanged zeolites for enantioselective hydrogenation V.I. Parvulescu, V. Parvulescu, S. Coman, C. Radu, D. Macovei, E. Angelescu, R. Russu

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

Regenerable sorbent for high-temperature desulfurization based on iron- molybdenum mixed oxides . . . . . . . . . . . . . . R. van Yperen, A.J. van DiUen, J.W. Geus, E. Boellaard, A , A van der Horst, A.M. van der Kraan

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' ' ~ ~ ~

479

489

495

505

513

523

533

539

545

551

561

571

579

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

Preparation and characterization of ASn03 (A = Ca, Sr or Ba) tin compounds for methane oxidative coupling C. Petit, M. Teymouri, A.C. Roger, J.L. Rehspringer, L. Hilaire, A. Kiennemann

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

Properties of Lao.6Sro.4Co03 prepared by complexing agent-assisted sol-gel method Y. Muto, F. Mizukami

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

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

Preparation of perovskite-type catalysts containing cobalt for post-combustion reactions L. Simonot, F. Garin, G. Maire, P. Poix

Characterization and reactivity of MgxFe2 ..2x03 - 2x .and MgyZ nl _yFe204 solid solution spinels prepared through the supercnncal drying metliod G. Busca, M. Daturi, E. Kotur, G. Oliveri, R.J. Willey

Effect of the iron catalyst mechanical treatment on the activity in ammonia synthesis reaction W. Arabczyk, R. Drzymala, U. Narkiewicz, K. Kalucki, W. Morawski

Cobalt catalyst for ammonia oxidation modified by heat treatment K. Krawczyk, J. Petryk, K. Schmidt-Szalowski

Characterization and catalytic properties of MgO prepared by different approaches Kai-Ji Zhen, Sen-Zi Li, Ying-Li Bi, Xiang-Guong Yang, Quan Wei

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

Systematic control of crystal morphology during preparation of selective vanadyl pyrophosphate E. Kesteman, M. Merzouki, B. Taouk, E. Bordes, R. Contractor

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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

Influence of the precursor formation stage in the preparation of VPO catalysts for selective oxidation of n-pentane Z. Sobalik, S. Gonzalez, P. Ruiz

Role of segregation phenomena information of active surface of V-Sb-O catalysts for selective oxidation of propylene to acrolein M. Najbar, E. Bielanska

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

The use of sepiolite in the preparation of titania monoliths for the manufacture of industrial catalysts J. Blanco, P. Avila, M. Yates, A. Bahamonde

Design of monolith catalysts for strongly exothermic reactions under non- adz'abatic conditions E. Tronconi, M. Bassini, P. Forzatti, D. Carmello

Some aspects of extrusion procedure for monolittu'c SCR catalyst based on Ti02 V. Lyakhova, G. Barannyk, Z.R. Ismagilov

Preparation and characterization of catalytic supports with variable composition in the system Si02-AI203-AIP04 F. Wijzen, A. Rulmont, B. Koch

New mo&fication of alumina: preparation procedure and existence conditions B.P. Zolotovskii, R.A. Buyanov

Preparation and characterization of silica-pillared layered titanate Wen-Hua Hou, Qi-Jie Yan, Yi Chen, Xian-Cai Fu

Alumina support modified by Zr and Ti. Synthesis and characterization T. Viveros, A. ZArate, M.A. L6pez, J. Ascenci6n Montoya, R. Ruiz, M. Portilla

Synthesis, characterization and applications of new supports for heterogeneous Zie g ler-Nana type catalysts L. Pavanello, S. Bresadola

Catalytic filamentous 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

Preparation of boron-containing alumina supports by kneading J.L. Dubois, S. Fujieda

Characterization of alumina paste by cryo-microscopy E. Rosenberg, F. Kolenda, R. Szymanski, M. Walter

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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

A new approach to catalyst preparation using sample controlled temperature programme techniques P.A. Barnes, G.M.B. Parkes

Preparation of fine particles as catalysts and catalyst precursors by the use of ultrasound during precipitation U. Kunz, C. Binder, U. Hoffmann

Scientific bases for the preparation of new cement-containing catalysts V.I. Yakerson, E.Z. Golosman

Nucleation and growth of ceria from cerium III hydroxycarbonate M. Pijolat, J.P. ViriceUe, M. Soustelle

Hydrotalcite-type anionic clays as precursors of high-surface-area Ni/MglAI mixed oxides A. Vaccari, M. Gazzano

Preparation and characterisation of cobalt containing layered double hydroxides S. Kannan, C.S. Swamy

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

Preparation of supported platinum catalysts by liquid-phase reduction of adsorbed metal precursors M. Arai, K. Usui, M. Shirai, Y. Nishiyama

Preparation of supported mono- and bimetallic catalysts by deposition- precipitation of metal cyanide complexes E. Boellaard, A.M. van der Kraan, J.W. Geus

Clusters and thin films prepared by DC-sputtering: morphology and catalytic properties D. Duprez, O. Enea

Preparation and characterization of a platimun containing catalytic membrane Xiu-Ren Zhao, Jun-Hang Jing

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

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

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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

Synthesis of eggshell cobalt catalysts by molten salt impregnation techniques S.L. Soled, J.E. Baumgartner, S.C. Reyes, E. Iglesia

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

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

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

Preparation of small metal nickel particles supported on silica using nickel ethylenech'anu'ne precursors Zheng-Xing Cheng, C. Louis, M. Che

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

Impregnation during gelation and its influence on the dispersion of the impregnant A.E. Duisterwinkel, G. Frens

Synthesis and characterization of titanium oxide monolayer N.S. de Resende, M. Schmal, J.-G. Eon

Alumina washcoating and metal deposition of ceramic monoliths Xiao-Ding Xu, H. Vonk, A. Cybulski, J.A. Moulijn

Cr-free iron-catalysts for water-gas slu'ft reaction J. Ladebeck, K. Kochloefl

Preparation of Rh-Co/Al203 heterogeneous catalysts using a diisocyano-ligand as an integral design component M.S.W. Vong, P.A. Sermon

Preparation of highly ch'spersed supported catalysts by ultrasound C.L. Bianchi, R. Carli, C. Fontaneto, V. Ragaini

Regularities of Pt precursors and modifying dopes sorption during the preparation of bimetal catalysts supported on spinels N.A. Pakhomov, R.A. Buyanov

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Tin(W) oxide supported noble metal catalysts for the carbon monoxide oxidation at low temperatures K. Grass, H.-G. Lintz

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

Impregnation design for preparing bimetallic catalysts A.K. Aboul-Gheit, S.M. Abdel-Hamid

Comparative study on low-temperature Cu/activated carbon catalysts prepared by impregnation from aqueous and organic media D. Mehandjiev, R. Nickolov, E. Bekyarova, V. Krastev

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

Non-hydrothermal synthesis, characterisation and catalytic properties of saponite clays R.J.M.J. Vogels, M.J.H.V. Kerkhoffs, J.W. Geus

Composite catalysts of supported zeolites N. van der Puil, E.W. Kuipers, H. van Bekkum, J.C. Jansen

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AUTHOR INDEX 1173

Studies in Surface Science and Catalysis (Other volumes in the series) 1177

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ORGANIZING COMMITTEE

President

Prof. B. DELMON Universit6 Catholique de Louvain

Executive Chairmen

ProL P. GRANGE

Prof. P.A. JACOBS

Prof. J. MARTENS

Dr G. PONCELET

Dr P. RUIZ

Universit6 Catholique de Louvain

Katholieke Universiteit Leuven

Katholieke Universiteit Leuven

Universit6 Catholique de Louvain

Universit6 Catholique de Louvain

XV

SCIENTIFIC COMMITTEE

Dr J. BOUSQUET,

Dr K. DELLER,

Prof. B. DELMON,

Prof. E.G. DEROUANE,

Dr J. DETHY,

Dr A. DI MARIO,

Dr T. FUGLERUD,

Prof. P. GRANGE,

Dr A. HAAS,

Dr H. HINNEKENS,

Dr JACKSON,

Prof. P.A. JACOBS,

Prof. J. MARTENS,

Dr M. NOJIRI,

Dr K. NOWECK,

Dr G. PONCELET,

Dr P. RUIZ,

Elf Aquitaine, France

Degussa AG, Germany

Universit6 catholique de Louvain, Belgium

Facult6s Universitaires N.-D. de la Paix, Belgium

Catalysts and Chemicals Europe, Belgium

Montecatini Tecnologia, Italy

Norsk Hydro, Norway

Universit6 catholique de Louvain, Belgium

Grace GmbH, Germany

Labofma, Belgium

ICI Catalysts, England

Katholieke Universiteit Leuven, Belgium

Katholieke Universiteit Leuven, Belgium

Mitsubishi Petroleum Co. Ltd., Japan

Condea Chemie GmbH, Germany

Universit6 Catholique de Louvain, Belgium

Universit6 Catholique de Louvain, Belgium

Dr J.P. SCHOEBRECHTS, Solvay et Cie., Belgium

Dr E. VOGT,

Dr G. SZABO,

Dr M.V. TWIGG,

Prof. J.A.R. VAN VEEN,

Prof. E. VANSANT,

Mr S. VIC BELLON,

AKZO Chemicals B V, The Netherlands

Total CFR, France

Johnson Matthey, England

Koninklijke Shell Laboratorium, The Netherlands

Universitaire Instelling Antwerpen, Belgium

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

Opening address

Professor P. Rouxhet, Pro-Rector

xvii

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 universi ty like a few other institutions in Belgium.

According to this status, most of the financial support is coming from the state and

the university must be managed according to certain rules imposed by the state.

Beside that, the institution is run independent ly of the public authority and of the

political power in terms of nominat ion , p rog rammes , 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. However it does

not mean that this is a religious university. 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 human person and his freedom, what I could call the freedom of the

children of God. One consequence is a deep sense of human 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 l imitation of man. It is a pe rmanen 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 how 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 s t imulate 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 imula t ing envi ronment for the universi ty , and all together,

Louvain- la-Neuve has tu rned out to be an impor t an t pole of economic

development in the country.

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 new fields of

science.

Concerning catalysis, the expectation of the society may be the development 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 intuit ion 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 preparat ion 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, advanced methods of analysis. On the other hand 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 internat ional 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, Yang- Liang Xiong, Xiao Yan, Mo-Hua Yang.

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.

Vanadium/phosphorus mixed oxide f rom 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 OF PREPARATIONS SELECTED FROM THE PATENT LITERATURE

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 V 4+ 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 V 5+ 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

Concentragon of the solution until a viscous solution is obtained ,L

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 ~C in 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 I . . , I . . I I . , .

3~ ;0 375 400 425 450 475 500

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/cm 3 to 1.4 g/cm 3, 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

t e m p e r a t u r e , ~

5001 ,

4 8 0

4 6 0

440~- . . . . . . . / ~ ~ " . . . . . ~ ' " . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 " ~ " 9 9 9 9 9 "-'~"'~" 9 . . . .

20 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4 0 0 , , t , , I , , 0 40 8 0 120 160 2 0 0 2 4 0

r e a c t o r l e n g t h , c m

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).

A

B

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).

14

10 3 0 5 0 7 0 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 V 4+ 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 V 4+ 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), 13 or ~' (60), [~* (52), 8 and 1~II (61,62), and Y VOPO4 (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 Zn 2+ 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 , (VO)2P207---, V s phases phase 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

A VOPO 4 l====~ VOPO4.nH=O v 9 A

/ |

--'+ ,,

v 9 9 9 9

j - - , " - -~lW-r-v- - 9 I" II p" I" O ~ . ~ j ~ . , ~ 1 ,

I I I I I I I i l

v

A

' 10 20 30 40 50 60 1 20 30 40 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

Average degree of oxidation of vanadium Bulk P/V ratio

XPS surface atomic P/V ratio BET surface area

X-ray diffraction pattern Morphology

71) 4.00-4.04

1.000-1.025 1.5-3.0

16-25 m2g -1

vanadyl pyrophosphate 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+ at.% I W * at.% J 10

2 0 -

1 5 -

1 0 -

5 -

0 0.9

! ~ (VO),,P,,O,

V3*

defect of P i excess of P

I i I I I 0.95 1 1.05 1.1 1.15

- 4

- 2

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

Company Mitsui Toatsu Chem Inc.

Mitsubishi Kasei Co. Alusuisse/The Lummus Co.

Amoco Co. E.I. Du Pont de Nemours

Denka Chem. Co. Monsanto

accl in patents

Promoter, Me Mg, Zr

Fe Zr, B Mo

In, Ta, Sb, Si Mo, Zn, Li Fe (Zn)l Li

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 Analy