Preparation of Catalysts III: Scientific Bases for the Preparation of Heterogeneous Catalysts (Studies in Surface Science and Catalysis)

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Studies in Surface Science and Catalysis 16 PREPARATION OF CATALYSTS III Scientific Bases for the Preparation of Heterogeneous Catalysts

Studies in Surface Science and Catalysis Volume

1

Preparation of Catalysts I. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the First International Symposium held at the Solvay Research Centre, Brussels, October 14-17, 1975 edited by B. Delmon, P.A. Jacobs and G. Poncelet

Volume 2

The Control of the Reactivity of Solids. A Critical Survey of the Factors that Influence the Reactivity of Solids, with Special Emphasis on the Control of the Chemical Processes in Relation to Practical Applications by V.V. Boldyrev, M. Bulens and B. Delmon

Volume 3

Preparation of Catalysts II. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Second International Symposium, Louvain-Ia-Neuve, September 4-7, 1978 edited by B. Delmon, P. Grange, P. Jacobs and G. Poncelet

Volume 4

Growth and Properties of Metal Clusters. Applications to Catalysis and the Photographic Process. Proceedings of the 32nd International Meeting of the Societe de Chimie physique, Villeurbanne, September 24-28, 1979 edited by J. Bourdon

Volume

Catalysis by Zeolites. Proceedings of an International Symposium organized by the Institut de Recherches sur la Catalyse - CNRS - Villeurbanne and sponsored by the Centre National de la Recherche Scientifique, Ecully (Lyon), September 9-11, 1980 edited by B. Imelik, C. Naccache, Y. Ben Taarit, J.C. Vedrine, G. Coudurier and H. Praliaud

5

Volume 6

Catalyst Deactivation. Proceedings of the International Symposium, Antwerp, October 13-15, 1980 edited by B. Delmon and G.F. Froment

Volume 7

New Horizons in Catalysis. Proceedings of the 7th International Congress on Catalysis, Tokyo, 30 June-4 July 1980 edited by T. Seiyama and K. Tanabe

Volume 8

Catalysis by Supported Complexes by Yu.1. Yermakov, B.N. Kuznetsov and V.A. Zakharov

Volume 9

Physics of Solid Surfaces. Proceedings of the Symposium held in Bechylle, Czechoslovakia, September 29-0ctober 3, 1980 edited by M. Laznil!ka

Volume 10

Adsorption at the Gas-Solid and Liquid-Solid Interface. Proceedings of an International Symposium held in Aix-en-Provence, September 21-23, 1981 edited by J. Rouquerol and K.S.W. Sing

Volume 11

Metal-Support and Metal-Additive Effects in Catalysis. Proceedings of an International Symposium organized by the Institut de Recherches sur la Catalvse - CNRS Villeurbanne and sponsored by the Centre National de la Recherche Scientifique, Ecully (Lyon), September 14-16, 1982 edited by B. Imelik, C. Naccache, G. Coudurier, H. Praliaud, P. Meriaudeau, P. Gallezot, G.A. Martin and J.C. Vedrine

Volume 12

Metal Microstructures in Zeolites. Preparation - Properties - Applications. Proceedings of a Workshop, Bremen, September 22-24, 1982 edited by P.A. Jacobs, N.1. Jager, P. Jiru and G. Schulz-Ekloff

Volume 13

Adsorption on Metal Surfaces. An Integrated Approach edited by J. Benard

Volume 14

Vibrations at Surfaces. Proceedings of the Third International Conference, Asilomar, California, U.S.A.,1-4 September 1982 edited by C.R. Brundle and H. Morawitz

Volume 15

Heterogeneous Catalytic Reactions Involving Molecular Oxygen by G.1. Golodets

Volume 16

Preparation of Catalysts III. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Third International Symposium, Louvain-Ia-Neuve, September 6-9, 1982 edited by G. Poncelet, P. Grange and P.A. Jacobs

Studies in Surface Science and Catalysis 16

PREPARATION OF CATALYSTSm Scientific Bases for the Preparation of Heterogeneous Catalysts Proceedings of the Third International Symposium, Louvain-Ia-Neuve, September 6-9,1982

Editors G. Poncelet and P. Grange Universite Catholique de Louvain, Groupe de Physico-Chimie Minerale et de Catalyse, Louvain-Ia-Neuve, Belgium

and

P.A. Jacobs Katholieke Universiteit Leuven, Centrum voor Oppervlaktescheikunde en Colloidale Scheikunde, Heverlee, Belgium

ELSEVIER Amsterdam - Oxford - New York 1983

ELSEVIER SCIENCE PUBLISHERS B.V. Molenwerf 1 P.O. Box 211, 1000 AE Amsterdam, The Netherlands

Distributors for the United States and Canada: ELSEVIER SCIENCE PUBLISHING COMPANY INC. 52, Vanderbilt Avenue New York, NY 10017

ISBN 0-44442184-X (Vo\' 16) ISBN 0444-41801-6 (Series)

© Elsevier Science Publishers B.V., 1983 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science Publishers BV., P.O. Box 330, 1000 AH Amsterdam, The Netherlands. Printed in The Netherlands

v CONTENTS

Organizing Committee Foreword Acknowledgements Financial Support

v IX XI XIII XV

Production and thermal pretreatment of supported catalysts (J.W. Geus) Theoretical and experimental aspects of catalyst impregnation (S. Y. Lee and R. Aris)

35

Competitive adsorption of H2PtC16 and HCl on A1203 in the preparation of naphtha reforming catalysts (A.A. Castro, O.A. Scelza, E.R. Benvenuto, G.T. Baronetti, S.R. De Miguel and J.M. Parera)

47

The role of competitive adsorbate in the impregnation of platinum in pelleted alumina support (Wang Jianguo, Zhang Jiayu and Pang Li)

57

The influence of solvent nature of chloroplatinic acid used for support impregnation on the distribution, dispersity and activity of platinum hydrogenation catalysts (V. Machek, J. Hanika, K. Sporka, V. Ruzicka, J. Kunz and L. Janacek)

69

Influence of the various activation steps on the dispersion and the catalytic properties of platinum supported on chlorinated alumina (J.P. Bournonville, J.P. Franck and G. Martino)

81

Preparation and properties of platinum crystallites supported on polycrystalline tin oxide (G.B. Hoflund)

91

Synthesis and properties of Pt-Sn/A1203 catalysts by the method of molecular deposition (D.P. Damyanov and L.T. Vlaev)

101

Production of silver bimetallic catalysts by liquid-phase reduction (K.P. de Jong and J.W. Geus)

111

Preparation and characterisation of highly dispersed palladium catalysts on low surface alumina. Their notable effects in hydrogenation (J.P. Boitiaux, J. Cosyns and S. Vasudevan)

123

Preparation of colloidal particles of small size and their catalytic effect in redox processes induced by light (J. Kiwi, E. Borgarello, D. Duonghong and M. Gratzel)

135

Synthesis, surface reactivity and catalytic activity of high specific surface area molybdenum nitride powders (L. Volpe, S.T. Oyama and M. Boudart)

147

The preparation of ceramic-coated metal-based catalysts (C.J. Wright and G. Butler)

159

The design of pores in catalytic supports. Magnesia-aluminaaluminum phosphate (G. Marcelin, R.F. Vogel and W.L. Kehl)

169

Dispersed-metal/oxide catalysts prepared by reduction of high surface area oxide solid solutions (J.G. Highfield, A. Bossi and F.S. Stone)

181

Preparation of monodispersed nickel boride catalysts using reversed micellar systems (J.B. Nagy, A. Gourgue and E.G. Derouane)

193

Thioresistant flammable gas sensing elements (S.J. Gentry and P.T. Walsh)

203

VI The formation of active component layer in coated catalysts (R. Haase, U. Illgen, G. Ohlmann, J. Richter-Mendau, J. Scheve and 1. Schulz)

213

Preparation of highly active composite oxides of silver for hydrogen and carbon monoxide oxidation (M. Haruta and H. Sano)

225

The effect of preparation method upon the structures, stability and metal/support interactions in nickel/alumina catalysts (D.C. Puxley, I.J. Kitchener, C. Komodromos and N.D. parkyns)

237

An assessment of the influence of the preparation method, the nature of the carrier and the use of additives on the state, dispersion and reducibility of a deposited "nickel oxide" phase (M. Houalla)

273

Thermally and mechanically stable catalysts for steam reforming and methanation. A new concept in catalyst design (K.B. Mok, J.R.H. Ross and R.M. Sambrook)

291

Synthesis of methanation catalysts by deposition-precipitation (H. Schaper, E.B.M. Doesburg, J.M.C. Quartel and L.L. van Reijen)

301

Preparation of titania-supported catalysts by ion exchange, impregnation and homogeneous precipitation (R. Burch and A.R. Flambard)

311

Influence of phosphorus on the HDS activity of Ni-Mo!y-AI203 catalysts (D. Chadwick, D.W. Aitchison, R. Badilla-Ohlbaum and L. Josefsson)

323

Study of the influence of the preparation conditions on the final properties of a HDS catalyst (C.V. Caceres, M.N. Blanco and H. J. Thomas)

333

The evolution of Co species on the surface of y-A1203 and Si0 2 modified with the pre-transition cations (A. Lycourghiotis)

343

Criteria for the evaluation of bauxite as carrier for low-cost hydrotreating catalysts (S. Marengo, A. Iannibello and A. Girelli)

359

Preparation and properties of supported liquid phase catalysts for the hydroformylation of alkenes (H.L. Pelt, L.A. Gerritsen, G. van der Lee and J.J.F. Scholten)

369

Role of the metal-support interaction in the preparation of Fe!MgO catalysts (H. Mousty, B.S. Clausen, E.G. Derouane and H. TopsiVe)

385

New Fischer-Tropsch catalysts of the aerogel type (F. Blanchard, B. Pommier, J.P. Reymond and S.J. Teichner)

395

Selective doping of a carbon substrate transition-metal ammonia catalyst (F.F. Gadallah, R.M. Elofson, P. Mohammed and T. Painter)

409

A study of the preparation and properties of precipitated iron catalysts for ammonia synthesis (D.G. Klissurski, loG. Mitov and T. Tomov)

421

Influence of the preparation technique of pd-silica catalysts on metal dispersion and catalytic activity (G. Gubitosa, A. Berton, M. Camia and N. Pernicone)

431

Preparation of non-pyrophoric metallic catalysts (A.V. Krylova, G.A. Ustimenko and N.S. Torocheshnikov)

441

Effect of metal-support interaction on the chemisorption and CO hydrogenation activity of FeRu catalysts (L. Guczi, Z. schay and I. Bogyay)

451

VII

The palladium alumina systenl: influence of the preparation procedures on the structurp of tbe metallic phase (S. Vasudevan, J. Cosyns, E Lesage, E. Freund and H. Dexpert)

463

Bimetallic supported catalysts prepared via metal adsorption. Preparation and catalytic activity of Pd-Pt/Al203 catalysts (J. Margitfalvi, S. Szab6, F. Nagy, S. G6bolos and M. Hegedus)

473

A scientific approach to the preparation of bulk mixed oxide catalysts (Ph. courty and Ch. Marcilly)

485

Deposition of ternary oxides as active components by impregnation of porous carriers (M. Kotter, L. Riekert and F. weyland)

521

Effect of support and preparation on structure of vanadium oxide catalysts (Y. Murakami, M. Inomata, K. Mori, T. Ui, K. Suzuki, A. Miyamoto and T. Hattori)

531

Structural modifications of V-P mixed oxides during calcination in air or in a mixture of butenes-air (G. Genti, C. Galassi, 1. Manenti, A. Riva and F. Trifiro)

543

Method of impregnation with transition metal alkoxides. Vanadium-alumina and vanadium-silica systems (M. Glinski and J. Kijenski)

553

The significance of the mullite phase in a silver catalyst for the oxidation of ethylene into ethylene oxide (Lin Bing-Xiong, Zhang Wan-Jing, Yan Qing-Xin, Pan Zuo-Hua, Gui Lin-Lin and Tang You-Chi)

563

Preparation of active carbon supported oxidation catalysts (J.L. Figueiredo, M.C.A. Ferraz and J.J.M. Orfao)

571

Influence of the preparation variables on the activity and on the mechanical properties of an industrial catalyst for the propylene oxidation to acrylic acid (R. Covini, C. D'Angeli and G. Petrini)

579

Design and preparation of hydrocracking catalysts

587

(J.W. Ward)

Influence of sodium chloride on the catalytic properties of tellurium-loaded Y-zeolites (B.E. Langner and J.H. Kagon)

619

Control of the pore structure of pcrous alumina (T. Ono, Y. Ohguchi and 0. Togari)

631

The properties of commercial alumina base materials and their effect on the manufacture of active porous alumina supports by means of extrusion (W. Stoepler and K.K. Unger)

643

Influence of aluminium hydroxide peptization on physical properties of alumina extrudates (K. Jiratova, L. Janacek and P. Schneider)

653

Formation of silica gel porous structure (V.A. Fenelonov, V.Yu. Gavrilov and L.G. Simonova)

665

Study of the preparation of iron catalysts for liquefaction of coal by hydrogenation under pressure (M. Andres, H. Charcosset, P. Chiche, G. Djega-Mariadassou, J.P. Joly and S. Pregermain)

675

Impregnation of y-alumina with copper chloride. Equilibrium behaviour, impregnation profiles and immobilization kinetics (R.J. Ott and A. Baiker)

685

Preparation of copper supported on metal oxides and methanol steam reforming reaction (H. Kobayashi, N. Takezawa, M. Shimokawabe and K. Takahashi)

697

VIII

Effect of preparation methods and promoters on activity and selectivity of Cu-ZnO-A1203-K catalysts in aliphatic alcohols synthesis from CO and H2 (C.E. Hofstadt, M. Schneider, 0. Bock and K. Kochloefl)

709

Preparation of Cu-Zn-Al mixed hydroxycarbonates precursors of catalysts for the synthesis of methanol at low pressure (P. Gherardi, 0. Ruggeri, F. Trifiro, A. Vaccari, G. Del Piero, G. Manara and B. Notari)

723

Preparation and characterization of very active Cu/ZnO and Cu/ZnO/Al203 LTS catalysts using a single phase CU-Zn precursor compound (G. Petrini, F. Montino, A. Bossi and F. Garbassi)

735

Cu/kieselguhr catalysts for hydration of acrylonitrile (E. Nino, A. Lapena, J. Martinez, J.M. Gutierrez, S. Mendioroz, J.L.G. Fierro and J.A. Pajares)

747

Phase-structural characteristics of the oxide systems in their first stage of preparation using one of the ingredients of the catalyst compounds as a precipitating reagent (D.S. Shishkov, N.A. Kassabova and K.N. Petkov)

757

MINI SYMPOSIUM ON CATALYST NORMALIZATION Standardization of catalyst test methods (R.J. Bertolacini and A. Neal)

767

Progress report on the Committee on Reference Catalyst, Catalysis Society of Japan (Y. Murakami)

775

The SCI/IUPAC/NPL standard nickel-silica catalyst (R. Burch, A.R. Flambard, M.A. Day, R.L. Moss, N.D. parkyns, A. Williams J.M. Winterbottom and A. White)

787

Research group on catalysis, Council of Europe. Standard catalyst projects (J.W.E. Coenen and P.B. Wells)

801

Progress report of BCR activity in surface area and pore size reference materials (N. Pernicone)

815

Standardization of procedures for determination of activity and selectivity of commercial catalysts by comparative kinetic investigations in different laboratory reactors (M. Baerns and H. Hofmann)

821

List of participants

833

Author index

853

IX ORGANIZING COMMITTEE

President

Prof. B. DELMON, Universite Catholique de Louvain

Executive Chairmen

Dr. P. GRANGE, Universite Catholique de Louvain Dr. P.A.JACOBS, Katholieke Universiteit Leuven Dr. G. PONCELET, Universite Catholique de Louvain

Secretaries

Dr. J.M. DIEZ TASCON, U.C.L., Belgium Dr. B.K. HODNETT, U.C.L., Belgium

Scientific Committee

Dr. J.P. BRUNELLE, Rhone Poulenc, France Dr. R. CANDIA, Haldor Tops¢e, Denmark Prof. B. DELMON, U.C.L., Belgium Dr. U. DETTMEIER, Hoechst A.G., Germany Dr. P. GRANGE, U.C.L., Belgium Dr. P.A. JACOBS, K.U.L., Belgium Dr. H. NEUKERMANS,Catalysts & Chemicals Europe,Belgium Dr. N. PERNICONE, 1st. G. Donegani (Montedison), Italy Dr. G. PONCELET, U.C.L., Belgium Dr. J. SONNEMANS, AKZO Chemie, The Netherlands Dr. S. VIC BELLON, Enpetrol, Spain

This page intentionally left blank

XI FOREWORD

There have now been three Symposia on the Scientific Bases for the Preparation of Heterogeneous Catalysts. The First Symposium, held in 1975, was organized in order to gather together experts from both Universities and Industry to discuss the scientific problems involved in the preparation of real, industrially used, heterogeneous catalysts. In spite of the heterogeneity of the subjects treated, the large international response demonstrated the great interest held by Universities as well as Industry in these topics. Although initially it was not the aim of the organizers to produce a series of symposia, it became clear that the First symposium had to be followed by a Second one, and that a smaller number of scientific domains concerned with the preparation of industrial catalysts should be considered. The Scientific Committee composed of representatives of Industry, Universities and Research Institutes decided, therefore, to focus the Second Symposium on two unit processes in catalyst preparation, namely impregnation and activation of supported catalysts. At the same time it appeared that normalization of catalyst test methods was a field of significant current concern. For the Second Symposium, 36 communications were selected (out of 102 submitted) that best fitted the proposed topics; 4 plenary lectures and 3 extended communications introduced the different sessions dealing with the preparation and activation of distinct groups of catalysts. In the concluding remarks of the Second symposium, the great majority of the

participants appeared to believe that the number of topics concerned with the preparation of real industrial catalysts that had not been dealt with justified a further symposium. Accordingly, when arranging the programme of the Third Symposium, the aim of the Scientific Committee was to organize self-consistent and related sessions. Therefore, the Third Symposium was devoted to reforming, hydrogenation, selective oxidation, HDS ... and also new trends in catalyst preparation. A special session on normalization of catalyst test methods was also scheduled. Out of 150 high quality papers originally proposed, 58 were selected by the Scientific Committee, keeping in mind the need for a balanced program centered on the chosen topics. The sessions were introduced by an invited lecturer. As for the previous symposia, 300 participants, representing 33 countries, attended the Third Symposium, 60% of the audience belonging to Industry, and 30% of the accepted papers coming from industrial laboratories.

XII

Twenty-one companies showed their interest in this symposium by sponsoring the social events. The increasing number of submitted papers, the steady number of participants, and the constant proportion of participants and accepted papers from Industry confirmed the view of the local organizers that fundamental research on the various and inter-disciplinary aspects involved in the preparation of industrial heterogeneous catalysts, as they have been treated by the three Symposia, corresponds to a real need and seems to be much appreciated by Industry. As a result of the demand for the continuation of such Symposia, as expressed by the participants to the organizers, there will hopefully be a Fourth Symposium organized for 1986.

P. GRANGE P.A. JACOBS G. PONCE LET

Xli ACKNOWLEDGEMENTS

The Organizing Committee is greatly indebted to Mgr. E. Massaux, Rector of the Universite Catholique de Louvain, who agreed that this Symposium could again be held in Louvain-la-Neuve, and for all facilities provided by the university. We thank very sincerely Prof.

B.

Delmon, who initiated this series of sym-

posia, for the major role he played in the assessment of the scientific content of this symposium. The organizers also greatly appreciate the constructive comments of several authorities in the field of catalyst preparation whose suggestions were largely responsible for establishing the base and scope of this Third Symposium. In this respect, we are particularly grateful to Dr. P. Andreu (INTEVEP), Dr. S.P.S. Andrew (I.C.I.), Dr. J.P. Brunelle (Rhone Poulenc), Dr. H. Charcosset (I.R.C.), Prof. J.W.E. Coenen (Unilever), Dr. G. Martino (I.F.P.), Prof. E. Matijevic (Clarkson Institute), Dr. L. Moscou (AKZO), Dr. N. Pernicone (Montedison), Prof. J. Petro (Techn. Univ. Budapest), Dr. S.T. Sie (Shell), Prof. D.L. Trimm (Univ. New South Wales), Dr. D.A. Whan

(Univ. Edinburgh).

The local organizers convey special thanks to the members of the Scientific Committee who not only selected the papers with outstanding care and conscientiousness but also, acting as chairmen, stimulated the discussion and enabled the sessions to run smoothly. The success of the minisymposium on Catalyst Normalization is due to Dr. N. Pernicone, who alone handled its organization.

The Organizing Committee is most

obliged to him and to those who contributed to this very fruitful session. The plenary lectures given by Prof. J.W. Geus, Dr. D.C. Puxley and Dr. J. Ward were most stimulating.

The Organizers gratefully acknowledge these authors

and congratulate them for achieving with such competence a difficult goal. Two excellent extended communications were given by Prof. J.J.F. Scholten and Dr. P. Courty.

Our thanks go to these authors.

The Organizing Committee acknowledges the authors of the 150 papers that were submitted for presentation at this Symposium.

Special thanks are due to

the authors of the papers included in the present Proceedings. Our congratulation and gratitude go, once more, to the team of hostesses of the REUL (Relations Exterieures de l'Universite de Louvain) , headed by Mrs F. Bex, and to Mr. J. Therrer, of the Service du Logement, for their devoted and enthusiastic assistance. The Organizing Committee wants to associate with the acknowledgements all those of the Groupe de Physico-Chimie Minerale et de Catalyse and of the Laboratorium voor Colloidale en Oppervlakte Scheikunde, K.U.L., who contributed in various degrees to the success of this symposium: C. Ancion, B. Arias, T. J.L. Dalons, M. Gennen, P. Jacques, J.P. Marcq, J. Martens, F. Melo-Faus,

Bein,

XIV M. Montes, N. Mazes, C. Pierard, D. Pirotte, M. Ruwet, A. Schutz, R. Sosa Hermandez, P. Struyf, M. Tielen, D. Van Wouwe, B. Vidick, K. Willemen, and WU Qin. Finally, we all owe our deepest appreciation to R.M. Torres and to M. O'Callghan, and more particularly to the secretaries, B.K. Hodnett and M. Diez-Tascon, who, from the very beginning right until the very end, took care of the unavoidable and least challenging parts of the organization of the symposium.

P. GRANGE

P.A. JACOBS G. PONCELET

xv FINANCIAL SUPPORT

The Organizing Committee gratefully acknowledges the financial guarantee of the "Fonds National de la Recherche Scientifique" and the "Ministere de l'Education Nationale et de la Culture Fran,t..s /

,"

-

-

COMPOSITON-----

Fig. 7.

"

,

"", /

particle in

Fig.B. Top: difference in free energy on

formation of a solid

particle in

the bulk of the solution as a func-

the suspension of the support as a

tion of the particle size for the 3

function of the particle size at one

regions indicated in the diagram at

concentration between L+ssupport and

the bottom.

L+s.

Bottom: equilibrium diagram for

Bottom: equilibrium diagram for a more stable bulk and surface compound

a pure solution.

with the carrier.

To

avoid clustering of active particles especially at high loadings of

carrier,

the support would be favourable.

To investigate the conditions leading to pre-

cipitation only onto the support, more

the

precipitation of the active precursor exclusively onto the surface of

closely.

The

we shall consider the precipitation

process

bottom of figure 7 shows an equilibrium diagram where

the

concentration of a saturated solution is given as a function of the temperature (solubility tween

a

curve).

At the top of figure 7 the difference in free energy

solution with a solid particle and a homogeneous

solution

of

be-

equal

overall composition is represented as a function of the particle size. When the concentration of the solution is below that of the solubility curve, energy grows on formation of a solid particle. particles

of the solubility curve,

free

Since the free energy of larger

increases linearly with the volume of the particle,

proportional to the third power of the particle size. that

the

the increase is

At concentrations

above

the free energy of a solid particle and a satu-

13 rated solution is lower than that of the homogeneous solution. ticles

with large par-

the decrease in free energy is proportional to the third power

particle size. ference

of

the

The decrease in free energy per unit volume grows with the dif-

between the concentration of the homogeneous solution and that of

solubility

curve.

the

At the concentration of the solubility curve the difference

in free energy is zero.

When the size of the precipitated particles is small, the

the surface energy of

solid can appreciably affect the change in free energy on formation

solid

particle.

of

a

At relatively elevated concentrations the drop in free energy

per unit volume is sufficiently large to cause the free energy to decrease even with

very small particles,

particles. curve,

When

though less rapidly with smaller than with

the concentration is not much beyond that of

the

larger

solubility

the surface energy brings about that the difference in free energy pas-

ses

through a (positive) maximum as the size of the solid particle

The

rate

strongly

of

nucleation

depends

of solid particles from

a

increases.

supersaturated

on the sign of the difference in free energy at

solution

very

small

particle sizes. When the difference in free energy steadily drops at increasing particle size, initially changes,

the rate of nucleation is much higher than when this difference

rises. roughly

The concentrations at which the rate of nucleation

starts to drop steadily; bility curve. energy

of

abruptly

coincides with those at which the difference in free these concentrations are indicated by the

energy

supersolu-

The position of the supersolubility curve depends on the surface

the solid in contact with the solution and on the decrease in

energy per unit volume of the bulk precipitate.

free

These quantities can vary con-

siderably with different solids and with the stoichiometry and the defect

con-

centration

with

the

of the precipitate.

The rapid increase in rate of nucleation

concentration brings about that we can distinguish the

perature range L+S in figure 7,

concentration-tem-

where large particles only are stable, whereas

very small particles are also stable in the range L+s.

When the concentration of the solution is raised homogeneously, crossing the concentration tate.

of the solubility curve does not lead to formation of a precipi-

Nucleation starts only when the concentraticn reaches that of the super-

solubility curve. With a relatively large difference between the concentrations of the solubility and the supersolubility curve the nuclei once generated

will

grow fast. Since in the first stage of the process the addition of the precipitating species usually will not keep up with the consumption of the precipitant by

the rapidly growing nuclei,

We

shall demonstrate that especially with basic Cu salts the concentration

the concentration will pass through a maximum.

the precipitating OH--ions sharply drops after the concentration of the

of

super-

14 solubility curve has been reached.

Since subsequently the concentration of the

solution remains below that of the supersolubility be

generated.

The

relatively large crystallites. locally number

curve, more nuclei will not

growth of a small number of nuclei consequently When,

on the other hand,

leads

raised considerably above that of the supersolubility curve, of nuclei results.

to

the concentration is a

large

Growth of these numerous nuclei leads to many small

crystallites.

In figure 8 the case of a finely divided carrier suspended in a solution the

active precursor is considered.

It is assumed that the ions of the active

species chemically interact with the surface of the carrier. teraction the

carrier lower than in the bulk of the solution.

able

to

figure

Owing to this in-

the concentration of the supersolubility curve is at the surface

L+ssupport in figure 8.

of

This is the curve

marked

We shall find that often the active precursor is

form a bulk compound with the carrier.

We therefore have assumed

8 that the solubility curve has also shifted to higher

of

also in

concentrations.

The top of figure 8 shows the difference in free energy for the same concentration which is between that of the two supersolubility curves. At the surface of the

support

the free energy steadily decreases.

whereas in the bulk

solution the free energy initially rises on formation of a solid concentrations active

of

particle.

between that of the curves L+ssupport and L+s of figure 8,

precursor will precipitate exclusively onto the surface of the

(DEPOSITION-PRECIPITATION).

Especially

a considerable

At the

carrier

when an active precursor is to be

plied onto a carrier having rather narrow pores,

the

ap-

concentration

difference is required to transport the precursor at a reasonable rate into the porous

support.

To be able to establish an appreciable concentration gradient

without inducing nucleation in the bulk of the solution,

the concentration

of

the two supersolubility curves must differ sufficiently. The difference in concentration is related to the bond strength to the surface of the support. erally

Gen-

a significant interaction with the carrier is required to carry out de-

position-precipitation at an acceptable rate.

Practical Aspects of Deposition-Precipitation Simple addition of a precipitating agent to a suspension of the carrier in a solution of the precursor does not lead to the homogeneous increase in· concentration required to get deposition-precipitation. When the solution of the precipitant locally

is poured into the suspension of the support,

the concentration

rise above that of the supersolubility of the bulk compound.

sult nucleation proceeds locally in the bulk of the solution. are

too

stable or have grown too large to redissolve when the

can

As a re-

Often the nuclei suspension

is

homogenized. SUbsequent growth of the nuclei in the solution cannot be avoided.

15 Local concentration differences in the suspension of the support can be minimized by the following two procedures.

The first procedure separates addition

An instance is the increase in hydroxyl

and reaction of a precipitating agent.

ion concentration by hydrolysis of urea (19). marked rate only above about 60 oC,

Since the hydrolysis occurs at a

the solution can be homogenized at a

lower

temperature and subsequently brought at a temperature where the reaction rapidly proceeds.

According to the second of the above procedures a solution of the

precipitant the

is injected into the suspension of the support below the level

liquid (20).

of

The injection tube must end below the surface of the liquid.

because no sufficiently large shear stresses can be established at a gas-liquid interface. Provisions are required to assure a without

interruption

steady flow of the

as well as high shear stresses at the

Since the suspension must thus be agitated vigorously, difficulties. volume which with

of

As indicated in figure 9,

injection

scaling up can

the

content

of

While the first procedure has been carried out

identical results in vessels of 1 and of 2000 liter,

straightforward with the second procedure. dure may take e.g.

point. present

it is possible to recirculate a large

the suspension through a relatively small vessel,

is intensively agitated.

precipitant

scaling up is

On the other hand,

24 hours to finish the precipitation,

less

the first proce-

while the

injection

procedure may be carried out more rapidly.

INJECTION into SUSPENSION of the SUPPORT kept at a constant pH leveL

DEPOSITION PRECIPITATION from HOMOGENEOUS SOlUTION

·Change In pH_level -INCREASE

N,ID! CulIIIFelIIJ CrlIJII

-DECREASE

V( V)

s-av:

.Change In Valency -OXIDATION Fe(II)--FerDI; Mn(II) -MnflVJ -REDUCTION Cr(V1)--CrfmJ

CulJII-CulJi PfllVl PdlIIJAglJi ~Mefal5

• DeComplexing REMOVAL of NH)

UREA CO(Nl-',), --NH:.CNOCNO-.3 H,O --NH:.HCQ.OHCNO-.2 H,O ~NH:.CO,.2 OHLower Temperotures More limitedreactionof SILICA CYANATE CNO-.3H,O ~NH;'+HCO;-.OH CNO-.2H,O ~NK.Co,.2 OW

No ComplexFormation of NH, NITRITE 3 NQ. H,O ~ 2 NO.NQ.20H-

Oxidation of £DTA

Fig.9.

Fig. 10

Injection procedure on a

Methods developed for deposition-

large scale.

precipitation onto suspended supports.

16 Precipitation the

method

according to the first of the above procedures corresponds to

of precipitation from a homogeneous solution used

analysis to prepare well crystallized, easy

to filter (21,22,23).

provide

extremely small particles.

number of other methods has been developed,

figure 10.

gravimetric

relatively large crystallites that

Deposition-precipitation,

urea

a

in

are

on the other hand,

can

Besides the well described utilization which are

summarized

Many active precursors can be precipitated by raising the

of in

pH-level

of a solution of the active component. Figure 10 gives some importnt instances. Cyanate is utilized when the precipitation has to be done at lower temperatures than about 70 oC, the temperature at which urea hydrolyzes rapidly. To avoid formation

of soluble ammine complexes,

nitrite can be favourably

paper will deal mainly with precipitation by raising the pH-level.

used.

This

Other meth-

ods that have also been used successfully, will be dealt with shortly.

Anionic creasing

species can be deposited onto surfaces of suspended carriers by dethe pH-level (24).

used with Mo{VI).

Besides with vanadium{V) this procedure has

Oxidation at a pH-level where the ions of the lower

been

valency

are soluble and the oxidized species insoluble, can also be utilized to precipitate active precursors.

Dissolved oxidation agents, such as nitrate ions, are

very suitable to precipitate from a homogeneous solution.

The interaction with

the surface or the bulk of the carrier can strongly depend on the pH-level. accurately

control the interaction,

can be fixed at a desired level, tion.

As

injection of alkali or e.g.

keep the pH at the chosen level.

tion

is favourable.

This is possible with

oxida-

subsequent reaction (hydrolysis) of the oxidized species can consume

hydroxyl ions,

soluble

Tb

precipitation at a constant pH-value that

The

hydrolysis of urea must be used to

oxidized ions can also react with

metal ions to insoluble compounds.

other

An important instance is the reac-

of Fe(III) with Fe(II) to insoluble magnetite;

to prevent

formation

of

more insoluble hydrated Fe(III) oxide, the Fe(III) ions must be generated homogeneously (25).

In the presence of other dissolved divalent metal ions, oxida-

tion of Fe(II) can lead to precipitation of ferrites.

Reduction With

to insoluble ionic compounds has been done with Cr,

copper hydroxy-acid complexes can be applied (26).

form a soluble complex,

can be obtained.

Since Cu(I) does

Especially with noble metals

be

reducing

not

good

Elsewhere in this volume the production of supported

alloy catalysts with extremely small particles is described. must

and Mo.

reduction brings about precipitation. Reduction to the

corresponding metal has been practiced too. results

Cu,

Generally

excluded during the reduction to prevent catalytic oxidation agent to proceed only.

Still more important is to avoid

oxygen of

the

reoxidation

17 and,

hence,

redissolution of finely divided reduced material during filtering

and washing of the loaded carrier.

Decomplexing onto

has

also been used to precipitate from

suspended carriers.

ammine

complexes,

redissolution

utilization of ammine complexes is attractive.

solution decompose

To

prevent

of the metal ions at the decreased pH obtained after removal

the ammonium ions, struction

homogeneous

Since raising the temperature suffices to

anions such as hydroxyl or

of complexing EDTA by e.g.

to produce supported catalysts (27).

carbonate have to be used.

of De-

hydrogen peroxide had also been employed With metal ions catalyzing the decomposi-

tion of hydrogen peroxide the reaction has to be carried out in a thin layer.

Evaporation of the solvent seems to be very obvious to gradually and homogeneously raise the concentration of a solution.

As dealt with above, however, a

very large fraction of the solvent has to be removed to increase the concentration sufficiently to induce crystallisation.

Consequently the impregnating so-

lution does not remain continuous during the evaporation. Transport of elements of the concentrated liquid leads to

an inhomogeneous

distribution of the

ac-

deposition-precipitation

the

tive material.

Deposition-Precipitation on Silica SUpports

In

the

production of supported catalysts by

interaction

of

the precipitating precursor with the carrier plays a

dominant

role. We will deal more in detail with the interaction with silica and alumina. Since the hydroxyl ion concentation can be monitored relatively easily, we will concentrate on precipitation by increase in hydroxyl ion concentration.

In and

an earlier volume of this series (28),

it had been shown how the extent

the strength of the interaction with the carrier can be inferred from

curves.

The pH-value continuously recorded during addition of hydroxyl ions to

solutions added.

of

the active precursor is plotted against the amount

active precursor, of

to a suspension of the carrier in pure water,

the carrier in a solution of the precursor are

experiments suspended

hydroxyl

the

of

without against

and to a sus-

measured.

concentration of the solution and the amount of

the

In

The consumption of hydroxyl ions by the

the carrier in pure water and that by the solution of a

suspended

carrier are mathematically added.

The

the sum

the

suspenprecursor

is

plotted

the amount of hydroxyl added and compared with the consumption of ions

the

carrier

as well as the volume of the liquid and the rate of addition of

hydroxyl ions are kept equal.

droxyl

of

pH-curves resulting from addition of hydroxyl ions to a solution of the

pension

sion

pH-

experimentally meaured with the suspension of the carrier in

hythe

18 solution of the precursor.

A marked interaction is evident from the suspension

taking up more hydroxyl ions at an equal pH-level than the calculated tion.

A marked interaction with the carrier brings about

libria

causing reaction of hydroxyl ions at a lower

ions

consump-

a shift in the equi-

pH-level.

When

hydroxyl

are made available by reactions of a rate difficult to assess accurately,

as e.g.

the reaction of urea or nitrite, the pH-value is plotted preferably as

a function of time. pH-versus-tirne curves can ison

of

be reproduced very well. Compar-

pH-versus-time curves recorded under identical

conditions

with

and

without a suspended carrier can also demonstrate quite clearly interaction with the carrier.

The above procedure had been used to investigate the interaction of precipitating Ni ions with fumed silica (surface area 200 and 380 m2g- 1)(28). The pHcurves

at 25 0C indicate reaction of the surface of the silica

recorded

initially

the pH-value of the suspension of the silica in the nickel

rises much more slowly than that calculated silica the

theoretically.

only;

solution

The surface of the

hence strongly reacts with precipitating nickel ions.

The point

where

pH-curve slowly approaches the calculated curve indicates that the

silica

does not markedly react beyond the surface layer. When the same experiments are carried out at 90 oC, the pH remains considerably below the calculated values, until the Ni ions (at lower loadings) or the silica (at elevated loadings) have completely from

reacted.

The different extent of reaction of the silica as evident

pH measurements has been corroborated by redissolution

experiments,

frared measurements (disappearance of the lattice vibration of silica), sis of the porous structure of the loaded carrier, tron

microscopy.

Besides the temperature,

analy-

thermal analysis and

the particle size and the

in-

electhermal

pretreatment strongly affect the extent of reaction of the silica. Silica of a smaller surface area (about 10 m2 g-1) reacts incompletely at 90 oC.

Ni(II) nickel

ions

can

hydrosilicate.

compound

whether

shows

Fe(III).

bulk nuclei

We now want to investigate

It is therefore interesting to investigate whether Fe(III)

the

to a

hydrosilicate, can be

deposited onto

increase in pH on addition of hydroxyl ions to

with ions,

silica. Figure a

solution

of

The consumption of OH--ions already at a pH of 2 indicates the stabi-

of hydrated Fe(III) oxide. be seen.

rises.

a

and

a stable bulk compound is required to get sufficient interaction

which do not react

can

compound,

Figure 7 thus applies to the Ni-silica system;

by interaction with the silica surface.

the carrier.

lity

bulk

more stable than the bulk hydroxide or hydroxy-carbonate

stabilized

11

react with hydroxyl ions and silica to a

As long as the pH is about 2 no

precipitate

The well known red-brown precipitate appears when the pH steeply

Polynuclear

complexes or

very small oxide particles that coagulate at

19

/

7

e 6 5

1

rr:: /'

5

pH

1·

/

1/

-- --T=90 ·C

4

pH 3

Water Aerosil FeCl2 FeCl2·Aerosil

3

2

2 1.0

OH/Fe

2.0

60

3.0

120

timeCmin)-

+

Fig.ll

Fig. 12

Precipitation of Fe(III) from ho-

pH-versus-time curves recorded for urea solutions at 90 oC. Curves of a pure

mogeneous solution.

Neither with

urea nor with alkali injection an

urea solution, of a suspension of silica

effect

in water, of a Fe(II) solution, and of a

of

suspended

silica was

apparent.

silica suspension in a

Fe (II)

solution

are represented.

higher pH-levels are formed at pH = 2.

The presence of suspended silica has no

any effect on the pH-curve. Apparently the species generated at pH

=

2 does not

interact with the silica surface. The absence of interaction has been confirmed by thermal analysis (dehydration), In

the

X-ray diffraction and electron

microscopy.

pores of the support large clusters (about 20 to 70 nm) of very

particles

(about 3 nm) could be observed.

Evidently precitation from

small homoge-

neous solution even with a very rapid nucleation, which leads to small crystallites, is not sufficient to apply an active component uniformly onto a carrier.

To produce supported Fe use Fe(II).

An Fe(II)

(oxide) catalysts it is therefore more attractive to

solution can be easily prepared by reacting an excess of

metallic iron with hydrochloric acid. prevents the

When air is excluded, the excess of iron

formation of Fe(III). Figure 12 shows the pH-curves recorded during of urea at 90 oC. It can be seen that the fumed silica alone

hydrolysis

consumes

a small amount of OH--ions above a pH of about 6.

react markedly above pH = 4.8.

Fe(II)

starts

to

without silica rapid nucleation sets in a pH of

6.7. The subsequent rapid growth of the nuclei leads to the drop in pH that was mentioned above. With suspended silica the maximum of the pH and the subsequent

20 level is appreciably below that of the curve without silica. the

nuclei

and

Accordingly, both

the bulk compound are mor-e stable and figure 7

applies

here

also.

7 /

6

/

/ T=45·C injection no silica injechon silica

pH

4

2

60

time(min) --------+

120

Fig.13. Precipitation of Fe(II) by injection of NaOH with and without suspended silica.

150

H2

uploke

1

UREA 900C

o.u

650

unsuppor t ed

U 90 (l)

NoOH

9O"C

190 (1)

145 (1)

1000

·c--

Fig. 14. Left:structures

resulting from deposition-precipitation of Fe(II) by different

procedures. Right:temperature-programmed reduction of Fe oxide deposited on silica by hydrolysis of urea (U 90(1)), injection at 90 0c (I 90(1)), and injection at 4SoC. For comparison bulk Fe oxide is included.

Comparison represented,

of figure 12 with 13, shows

that

where an injection experiment at 4S oC

the reaction with the support is less

extensive

is at

21 45°c. A slightly more elevated pH at the injection point brings about formation of

a less reactive Fe precipitate.

As a result the attack of the

support

is

more limited on injection at 90 o C. The structures obtained in the three differents 3xperiments are as indicated in figure 14.

The temperature-programmed re-

duction experiments of figure 14 demonstrate the different extent of hydrosilicate formation.

Previous air-drying partially oxidizes the Fe(II). As interac-

tion with silica stabilizes Fe(II), reduction step to Fe(II). hydrosilicate

Fe

the supported Fe specimens show a separate

This step is not displayed by the bulk Fe oxide. The

obtained in the urea precipitation at 90°C is fairly

and is reduced only above 650 oC.

stable

The Fe(II) precipitated at 45°C is less inti-

mately connected with the silica and consequently starts to be reduced at about

450°C. tion

Figure 14 shows that injection at 90°C leads to a less extensive to hydrosilicate.

reac-

The micrographs of figure 15 confirm the different ex-

tents of reaction of the support.

The carrier loaded at 45°C shows the

silica

particles covered by very small Fe oxide particles, while the carrier has reacted to thin hydro silicate platelets at 90°C.

Fig. IS.

Electronmicrographs of silica loaded by Fe(II). Precipitated by

a. injection of NaOH at 4SoC.

The

b. hydrolysis of urea at 90°c.

above experiments have shown that Fe(II) does and Fe(III) does not

in-

teract sufficiently with silica to precipitate exclusively onto the support. It may

be questioned whether the difference between Fe(II) and Fe(III) is due

to

the ability to form a bulk compound with hydroxyl ions and silica. Formation of

22 a surface or interfacial compound,

which must suffice,

easily than that of bulk compounds.

proceeds generaly more

The lack of interaction of Fe(III) may

be

due to the low pH at which Fe(III) precipitates. An electrostatic charge on the suspended silica particles may be instrumental in the transport of Fe(III) ions to the silica surface (29). face

with

As shown in figure 16,

reaction of an oxidic sur-

water leads to a hydroxylated surface onto which a layer

of

water

molecules is bound by hydrogen bridges. At increasing pH-levels the reaction of the surface OH-groups shifts from the left to the right-hand side in figure 16. The pH at which the surface charge of suspended oxides changes sign varies with the nature of the oxide. Silica has no charge at a pH of about 2, while alumina changes the sign of its charge at pH-values from 6 to 7 depending on the preparation and the pretreatment.

M/

~/ "0

M/ "0

M/

-,

/0

M'oH

~H

....-oH M

M/OH -,

> "0

+H20~

+H20~

M

M'oH

M -,

M/ -,

~H

-,

M/ -, 'PH

M'~

M/

"0

/0

M)'

'0 0 '0

/

M,OH /OH

"0 /

M/

~

-,

M"

..

M/ 'OH

~: ,.oH ~OH ....-oH M'OH ,.oH ""OH M!'H

M~tj! /OHO~

...

....... U)

0.3MOL /l H Cl

C3

n:

~

N

:x:

•

-I

0

4"/

0.3MOUl H Cl

2 o ;:[ O'--I

•

;:[

(~). ..I..-_..J

LO

'52

(d)

0.1 MOUl H Cl

0.1 MOUl H Cl

OL-_--I. 15 10 r-----=--=", 5

0.8

---l

• 0.6

0.4

0.2

0.8

0

r/R Fig. 4. HZPtCl6 surface concentration as a function of the radial position for different initial HCI concentrations. a-d: 0.38% Pt, e: 0.97% Pt

Fig. 5. HCI surface concentration as a function of the radial position for different initial HCI concentrations. a-d: 0.38% Pt, e: without Pt

10-6/CONC IN L1Q (MLlMOLl 0

(5

5

10

(5 100

..... -I

'-I

o;:[

~ 10

:> ":J 50 H Cl

o0'=-----±c

Z-----!4c-----'

10-4/CONC IN L1Q (MLlMOLl Fig. 6. Linearized plot of the adsorption isotherms for HCI and HZPtCl6

~ M

+

. CATALYST

•

Calcined at vorious temperatures

e Calcined at 500·C and reduced o

at various temperatures Dried and reduced at various tem eratures

2,0

e

•

o 1,0

"------:-":~----7------'D D,S

Fig. 4. The variation of the activity towards benzene hydrogenation versus metallic dispersion.

DISCUSSION-CONCLUSION The necessary sequence of the elementary operations for obtaining a well dispersed metallic phase may be conceived in the particular case of platinum supported on chlorinated alumina as follows - during the impregnation, the fixation of the metal can be represented by an ionic exchange at the surface of the support: after drying, platinum keeps the same halogenated octahedral surrounding

(ref. 27).

- during the calcination, the nature and the number of the ligands of platinum would be modified :removal of chlorine and exchange between chlorine of the complex and oxygen atoms of the support. This structure best fits the oxidation state of platinum (pt +4 ) and agrees with the work of ESCARD et al. (ref. 31). When the calcination is performed at high temperature (T)600°C) , the metal is badly dispersed : the oxychlorinated platinum complex decomposes into an oxije phase which is not stable when temperature rises and leads to

89 platinum metal weakly bound to the support. - after reduction of the superficial oxychloroplatinum complex in hydrogen at a sUfficiently high temperature, the metallic phase is made up of particles less than one nanometer in size, ie containing less than 20 atoms, in the case of a correctly processed catalyst. The results presented here have been obtained in connection with an applied research in progress on behalf of Procatalyse.

ACKNOWLEDGEMENTS The authors thank Drs H. DEXPERT and E. FREUND for the results obtained by B.X.A.F.S. and Electronic microscopy.

REFERENCES 1 J.F. Le Page and AL , "Catalyse de Contact", Technip Ed, Paris 1978. 2 M. Boudart,Aiche JournaT, Vol. 18 n03, p. 465 (1972). 3 R. Montarnal and G. Martino, Revue de l'Institut Fran~ais du Petrole XXXII n03, p. 367 (1977). 4 M. Boudart, Adv Catal 20, p. 153 (1969). 5 M. Poltorak and VS. Boronin, Ross J phys Chern, Vol. 40, nOll, p. 1436 (1966). 6 P. Marecot, Thesis University of Poitiers, (1979). 7 A.W. Aldag, L.D. Ptak, J.E. Benson and M. Boudart, J. catal. 11, p. 35 (1968). 8 Kraft and Spindler, Proceedings of the fourth International Congress on Catalysis, paper 69, Moscou (1968). 9 A. Morales, J. Barbier and R. Maurel, Rev. Port Quim 18, p. 158 (1976). 10 J.P. Boitiaux, G. Martino and R. Montarnal, Cras 281 C 48 (1975). 11 J. Barbier, P. Marecot, A. Morales and R. Maural, Bull Soc Chim Fr I 31 (1978). r 12 H.J. Maat and L. Moscou, Proceedings of the 3 International Congress on Catalysis, p. 1277 (1964). 13 M.R. Bursian, S.B. Kogan and Z.A. Davydova, Kinetika y Kataliz 8-123 (1968). 14 R.W. Joyner, B. Lang and G. A. Somorjai, J. Catal. 27-405 (1972). 15 G. Abolhamd, Thesis, Paris (1980). 16 W. Molina, Thesis, Poi tiers (1981). 17 A.Renouprez, C. Hoang Van and P.A. Compagnon, J. Catal 34, p. 411, (1974). 18 T.A. Dorling, B.W.J. Lynch and R.L. Moss, J. Catal 20, p. 190, (1971). 19 R.T.K. Baker, C. Thomas and R.B. Thomas, J. Catal 38, p. 510, (1975). 20 H. Spindler, Int Chern Eng 14, p. 725, (1974). 21 G.R. Wilson and W.K. Hall, J. Catal 17, p . 190, (1970). 22 J. Freel, J. Catal 25, p. 149, (1972). 23 R.M. Fiedorow and S.E. Wanke, J.Catal.43, p. 34, (1976). 24 Y.F. Chu and E. Ruckenstein, J. Catal 55, p. 348, (1978). 25 A.G. Graham and S.E. Wanke, J. Catal 68, p. 1, (1981). 26 J.P. Bournonville and G. Martino, in "Catalyst Desactivation" B. Delmon and G.F. Froment Ed. p. 159, (1980). 27 T. Murata, A. Fontaine, P. Lagarde, D. Raoux, J.P. Bournonville, H. Dexpert and E. Freund, unpublished results. 28 J.E. Benson and M. Boudart, J. Catal 4, p. 704, (1965). 29 J.P. Bournonville, Thesis, Paris, (1979). 30 S. E. Wanke, Conference on "Catalyst Desactivation and Poisoning", '!ay 1978 Berkeley, California. 31 J. Escard, S. Pontvianne, M.T. Chennebeaux and J. Cosyns, Bull Soc Chim 3, 349, (1976). 32 J.C. Schlatter, Mater. Sci. Res, Vol. 10, p , 141, (1975). 33 H. Blume, C. Szkibik, F. Pfeiffer, H. Klutzsche, E.R. Strich, K. Becher and G. Weindebach, Chern tech. 18, p. 449, (1966). 34 H. Scheifer and M. Frenkel, Z. Anorg. Allg. Chern., 414-437, (1975). 35 J.M. Deves, P. Dufresne and E. Freund, unpublished results.

90 DISCUSSION In the experimental part of your paper it is mentioned that K. KOCHLOEFL y-alumina extrudates were used as starting material of your Pt-catalyst. What grain size of the platinum catalyst did you use in the benzene hydrogenation ? J.P. BOURNONVILLE : The non-diffusional conditions for the measurements of the hydrogenation activity were checked according to two parameters: the pellet size and the activation energy. When pellets smaller than 1.4 millimeter in size were used, the limiting effect due to the diffusion of the reactants was suppressed; then an activation energy of 46 KJ/mole/oC was found. J. MARGITFALVI: with respect to your Fig. 1, what will be the influence of the initial concentration on the dispersity changes upon increasing the calcination temperature? Did you have a complete reduction if the reduction was carried out at 4~O°C? This concerns the catalysts with high metal loading. J.P. BOURNONVILLE: At low calcination temperature, the chlorine content will not have a detrimental effect on the metal dispersion. But at high temperature, lower is the chlorine concentration, higher is the sintering rate of the metal phase: the absence of chlorine during the oxidizing thermal treatment shifts the equilibrium between sintering ana redispersion towards the sintering. T.P.R. experiments showed that the rate of reduction did not depend on the metal loading up to 3 wt % of platinum, when the samples were reduced at sufficiently high temperature (T ) 450°C). When the metal loading increases, the overall metal-support interaction decreases; so, the reducibility of the supported metal increases but leads to a lower dispersion. H. CHARCOSSET You showed three ways for decreasing the dispersion of Pt/A1203: i) increasing the calcination temperature above a certain value; ii) increasing the reduction temperature above a certain value; iii) using wet hydrogen for reducing the catalyst. In each of these three cases, does or does not the sintering preserve the homodispersity of the Pt phase? J.P. BOURNONVILLE In accordance with other results not reported in the present paper, it appears that the nature of the atmosphere during the thermal treatments may influence the size distribution of the metal particles. In oxidizing atmosphere, a well dispersed phase remains in equilibrium with a sintered phase, due to the presence of chlorine, which leads to a bimodal particle size distribution; on the other hand, in reducing atmosphere, the size distribution remains narrower and monomodal during the sintering. H. CHARCOSSET: Is it true that the 700°C reduced catalyst keeps its normal Pt properties, as well in H2-02 titration as in benzene hydrogenation? This would indicate the absence of any metal-support interactions in your case, even at this high temperature of reduction. J.P. BOURNONVILLE: After reduction at high temperature, our samples were in contact with air at room temperature before being re-reduced at 450°c for platinum surface area and hydrogenating activity determination. According to your results and to those of Dautzenberg, the contact with oxygen destroys the interaction between platinum and partially reduced alumina. In consequence, we could not observe this phenomenon. Moreover, this phenomenon has been particularly described in the case of catalysts reduced at high temperature after drying without calcinatio~ and on which the metal phase was rather badly dispersed. R.J. BERTOLACINI: Your paper recognizes the importance of chloride on the dispersion of Pt on Cl-Al203 supported catalysts. Your paper does not indicate the same Cl- dependence for Pt-Ir/Al203 catalysts. Would you comment on the Cl- effect on Pt-Ir catalysts? What was the chloride content of your Pt-Ir catalyst ? J.P. BOURNONVILLE: After calcination at 500°C and reduction at 450°C in dried atmosphere, the Pt-Ir catalyst contains about 1.2 % chlorine in weight. As far as the activation and regeneration steps are concerned, we found that the chlorine effect was very similar for Pt and Pt-Ir catalysts.

G. Poncelet, P. Grange and P .A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands

91

PREPARATION AND PROPERTIES OF PLATINUM CRYSTALLITES SUPPORTED ON POLYCRYSTALLINE TIN OXIDE

GAR B. HOFLUND Department of Chemical Engineering, University of Florida, Gainesville, Florida 32611

ABSTRACT The preparation and characterization of highly dispersed platinum supported on polycrystalline, antimQny-doped tin oxide films is discussed. The usefulness of numerous surface characterization techniques (such as ESCA, AES, ESD, SIMS, etc.) in relating catalyst surface properties to the methods of preparation is demonstrated.

INTRODUCTION Tin oxide and its modified forms I'kiich are prepared by doping or pl atinization are active catalysts for numerous reactions. Chromia-doped tin oxide (1,2) is active in nitric oxide reduction, antimony-doped tin oxide is active toward the oxidative dehydration of butene to butadiene (3) and the selectlve oxidation of propene to acrolein (4) while platinized, antimony-doped tin oxide is as much as 100 times more active per surface platinum than metallic platinum toward the electrochemical oxidation of methanol (5-9). Also, the addition of tin to alumina-supported, platinum reforming catalysts has resulted in greatly improved stability (10-16). Tin oxide is ideal as a support material in fundamental studies for highly dispersed platinum because it is conductive, it has a high transmissivity to photons over a broad range and it is fairly inert to most chemical treatments in addition to its interesting catalytic properties. These properties permit novel methods of catalyst preparation and characterization l'kIich will be discussed in this paper. CATALYST PREPARATION For this study the tin oxide films were prepared by a high temperature spray hydrolysis. This consisted of multiple sprayings of a Solutlon of 3M SnCl' 5H20

92

0.03M SbC1 3 and 1.5M HCl onto a quartz plate held at 500·C I'kiich had been ultrasonically cleaned and heated in air at 800·C for 3 hours. This results in a polycrystalline film I'kiich is several thousand angstroms thick. There are other methods which could be used to prepare the fi lms such as vapor- or 1i quid-ph ase deposition of SnC14 followed by hydrolysis, but the film's properties could vary greatly depending upon the impurities present, the oxygen-to-tin ratio and the geometrical structure. Platinum deposition can be carried out using anyone of five different methods: (l) an electrochemical deposition from a chloropl atinic acid solution, (2) chemisorption of pl at inum from a chloroplatinate solution, (3) a spray hydrolysis deposition by incorporation of a platinum salt into the spray solution, (4) deposition from a molten salt containing platinum and (5) decomposition of an adsorbed metallo-organic compound such as platinum acetylacetonate. The characteristics of the platinum can be altered in each of the five deposition methods by changing numerous variables. Catalysts prepared by methods (1) and (2) will be discussed here. A thorough discussion of catalyst preparation by method (2) has been presented by Watanabe, Venkatesan and Laitinen (17) I'kiile Katayama (8,9) has discussed both the preparation and ESCA characterization of samp1es prepared by method (3). It has been found that a caustic pretreatment of the tin oxide film enhances the deposition of platinum by methods (1) and (2). This pretreatment consists of soaking the films in a 10M NaOH solution at 90·C for 1 hour. It is believed that this process hydroxylates the surface. Indirect evidence of this has been published by Morimoto et al , (18) and Ansell et al . (19) while evidence that is more direct than the above will be presented in the next section. CATALYST CHARACTERIZATION A large number of techniques have been used to characterize tin oxide surfaces. A few selected ones including transmission electron microscopy (TEM), particle-induced X-ray emission (PIXE), Rutherford backscattering (RBS), Auger electron spectroscopy (AES), electron spectroscopy for chemical analysis (ESCA), electron-stimulated desorption (ESD) and secondary ion mass spectrometry (SIMS) will be discussed here. TEM is useful in observing both the tin oxide structure and the platinum crystall ites. Figure 1 is a TEM photograph of an antimony-doped tin oxide sample showing the polycrystalline structure of the tin oxide. The tin oxide crystals are approximately 100-150A in diameter, and the light patches are thought to be a separate phase consisting primarily of SnO.

93

Fig. 1. TEM photograph showing the polycrystalline structure of the tin oxide support.

Bulk elemental analysis of the films is accomplished using RBS and PIXE. Typical spectra are shown in figure 2. RBS has been shown to be sensitive to 0.3ng of platinum, and the response is linear from 0.01 to 80\Jg Pt cm- 2 of substrate surface area (20). PIXE is particularly sensitive to intermediateatomic weight elements such as K, Ca, Fe and Zr, l'Itlich are film impurities. Antimony, which constitutes 1% of the film, is detectable with PIXE. The usefulness of RBS and PIXE in relating total platinum content of a catalyst to preparative variables of method (2) platinization of tin oxide is demonstrated in reference 17. The number of surface Pt atoms can be determined using electrochemical or chemisorption techniques so that dispersion can be cal cul ated. AES and ESCA are surface sensitive techniques used to characterize the composition and oxidation states of atoms near the surface. A typical AES scan of a platinized surface is reproduced (21) in figure 3. Impurities such as P, S, Cl, C and Ca are present. A portion of the oxygen feature at 515eV is due to contamination probably in the form of adsorbed CO. Antimony features are not observed because they are masked by the Sn peaks at 454 and 458eV. Also, the primary electron beam has been shown to modify the surface through electron stimulated desorption (22).

94

(a)

(b)

Sn KCa

Pt

E-

Fig. 2. oxide.

Spectra from (a) RBS and (b) PIXE of platinized, antimony-doped tin

~

Z

::J

> a:

> a:

«

a:

I-

iii

a:

« w Z

Fig. 4. High resolution ESCA spectrum of the platinum 4f core levels from a tin oxide sample which had been platinized electrochemically. Unfortunately the oxidation state of the tin cannot be determined in the same manner because the Sn 3d peaks are located at the same position for both SnO and Sn02. However a method suggested by Lau and Wertheim (23) can be used to distinguish between SnO and Sn02. In this method an ESCA scan is taken of the valence band as shown in figure 5.

The peak at 3eV

1S

characteristic of SnD

while the peak at 5eV is characteristic of Sn02- Curve (a) is an ESCA scan immediately after insertion of the sample into the vacuum system, and curve (b) is a scan after heating the sample to 500·C. Heating causes desorption of impurities and sharpens the features. This particular sample contains more SnD than Sn02 as shown in curves (a) and (b). Curve (c) is a spectrum after heati ng the sample at about 400·C in 9xl0 5L of oxygen. This has caused the Sn02 peak to grow with respect to the SnO peak. The UPS results of Powell and Spicer (24) show similar features during the oxidation of tin. Their interpretation was not correct as pointed out in a later AES and electron-energy loss spectroscopy (EELS) study by Powell (25).

96

ESD can be an especially powerful tool for studyinq surface species particu1 arly now that it is better understood (26). It is one of the very few techniques which can be used to study hydrogen on surfaces. Fiqure 6 shows the result of a positive-ion scan over the low AMU r anqe using a 1000eV electron beam, and table 1 gives possible identifications of the ions. The 7 peak is most likely N++ since CH;+ probably is not stable. This would indicate that the peaks at 14 and 28 have a nitroqen component and possibly other components such as CH~ and CO+ respectively. Nitroqen has been identified on these surfaces using AES and probably is in the form of a nitrate. The argon peaks at 20 and 40 are due to ionization of gas phase arqon, which was present after sputtering in the system. The peaks often cannot be identified conclusively because there is a cracking process occurring on the sample's surface as 1'.I:!11 as in the ionizer of the mass spectrometer. TABLE 1 Possible identification of ESD peaks shown in fiq ure 6

m/e 1 2 6 7

8 12 14

Possible Identity H+

H~ C++ N++ , CH++ 2 0++ C+ N++ N+ CH+ 2' , 2

m/e 16 19 20 23 24 28 40

Possible Ident ity 0+ + + F ,H 3O Ar++,Ne+ Na+ Mg+ N;, CO+ Ar+

ESD is a destructive technique because the electron beam alters the surface. This means that the relative peak heiqhts in figure 6 change and, thus, are not of particular siqnificance. However, it is interesting to compare the H+ and 0+ peak heights as a function of time for an untreated vs. a caustic-treated sample. For both the H+ and 0+ peaks from the caustic-treated sample, the peak rises to a higher initial value and decays slower than the untreated sample. This result is consistent with the concept of surface hydroxylation by means of a caustic pretreatment even though no OH+ ions are detected. SIMS is another technique which is useful for characterizing catalyst surfaces because it is very sensitive to trace quantities, it can detect hydrogen and it can be used to depth profile surfaces. Its drawbacks are that SIMS is a destructive technique, and it is difficult to quantify the results. A SIMS scan of tin oxide is shown in figure 7. Curve (b) is the same as curve (a) after

97

N(E)

10

5

BINDING ENERGY

Fiq. 5. ESeA spectra of the valence band (a) immediately after sample insert i on, (b) after heat i nq and (c) after heat i nq in oxygen.

ESD POSITIVE ION SPECTRUM

o

Fig. 6. ESD positive-ion spectrum obtained by striking a tin oxide sample with lOOOeVelectrons.

98

beinq maqnified by a factor of 11. SIMS spectra usually are complex due to lts sensitivity and a detailed discussion of the SIMS spectrum will not be given here. The total metals content other than tin in the SnC14·H20 is 0.02%. SIMS identifies these impurities as Li-7, Na-23, Mg-24, K-39, Ca-40, Mn-55, and Fe-56. (b) shows a peak at 17 AMU which is identified as OW. Even more information is contained in the 100-300AMU range. Sb is observed at 123, but its height very rapidly decreases to the baseline indicating that most of the Sb is located at the surface.

SIMS POSITIVE ION SPECTRUM

_

...................M-...........(b]

(a)

75

Fig. 7. tion.

85

95

SIMS posit ive ion spectrum usi ng a 1000eV argon-ion beam for exci t a-

CONCLUSION It has been demonstrated that numerous surface techniques can be used to understand catalysts in terms of the methods used to prepare them. A better fundamental understanding of catalysts and their preparation should result in improved catalysts.

99

ACKNOWLEDGMENTS I would like to thank John Hren, Henry Van Rinsvelt and Paul Holloway for use of the transmission electron microscope, Van de Graff generator and SIMS spectrometer respectively. REFERENCES 1 F. Solymosi and J. Kiss, J. Cat., 41(1976)202. 2 M. Niwa, T. Minami, H. Kodama, 1. Hattori and Y. Murakami, J. Cat., 53(1978)198. 3 H.H. Herniman, D.R. Pyke and R. Reid, J. Cat., 58(1979)68. 4 Y. Boudeville, F. Figueras, M. Forissier, J.L. Portefaix and J.C. Vedrine, J. Cat., 58(1979)52. 5 M.M.P. Janssen and J. Moolhuysen, J. Cat., 46(1977)289. 6 M.M.P. Janssen and J. Moolhuysen, Electrochimica Acta, 21(1976)869. 7 M.R. Andrew, J.S. Drury, B.D. McNicol, C. Pinnington and R.T. Short, J. App. Electrochem., 6(1976)99. 8 A. Katayama, Chern. Lett., (1978)1263. 9 A. Katayama, J. Phys ••Chem., 84(1980)376. 10 R. Burch, J. Cat., 71(1981)348. 11 R. Burch and L.C. Garla, J. Cat., 71(1981)360. 12 R. Bacaud, P. Bussiere and F. Figueras, J. Cat., 69(1981)399. 13 J. Volter, G. Lietz, M. Uhlemann and M. Hermann, J. Cat., 68(1981)42. 14 B.H. Davis, J. Cat., fl6(1977)348. 14 B.H. Davis, J. Cat , , 46( 1977)348. 15 A.C. Muller, P.A. Engelhard and J.E. Weisang, J. Cat., 56(1979)65. 16 F.M. Dautzenberg, J.N. Hells, P. Biloen and W.M.H. Sachtler, J. Cat., 63( 1980) 119. 17 M. Watanabe, S. Venkatesan and H.A. Latinen, to be published. 18 1. Morimoto, M. Kiriki, S. Kittaka, 1. Kadota and M. Nagoa, J. Phys. Chem., 83(1979)2768. 19 R.O. Ansell, T. Dickinson, A.F. Povey and P.M.A. Sherwood, J. Electrochem. Soc., 124(1977)1360. 20 J. Rosenfarb, H.A. Laitinen, J.T. Sanders and H.A. Van Rinevelt, Anal. Chim. Acta, 108(1979)119. 21 G.B. Hoflund, D.F. Cox and H.A. Laitinen, Thin Solid Films, 83(1981)261. 22 G.B. Hoflund, D.F. Cox, G.L. Woodson and H.A. Laltinen, Thin Solid Films, 78(1981)357. 23 C.L. Lau and G.K. Wertheim, J. Vac. Sci. Technol., 15(1978)622. 24 R.A. Powell and W.E. Spicer, Surf. Sci., 55(1976)681. 25 R.A. Powell, Appl. Surf. Sci., 2( 1979)397. 26 M.L. Knotek and Peter J. Feibelman, Phys. Rev. Lett., 40(1978)964.

100 DISCUSSION I appreciate your comments concerning the value of electron D. CHADWICK stimulated desorption. However, in view of your observation of AES induced effects and the high signal-to-noise ratio of your spectra, perhaps you could comment on the current densities used to obtain your AES spectra. The accepted limit for no interogation phenomena in AES is around 5 mA/cm 2. G.B. HOFLUND The estimated electron beam current used in the slides presented was about 200 mA/cm 2 which is very large. Smaller beam currents of 24 mA/cm 2 were used in the ESD experiments. We use very small beam currents of 0.008 mA/cm 2 and pulse counting techniques for AES when we wich to minimize beam damage. However, electron beam damage occurs at all beam current densities qnd even at very low beam voltages (~ 20 eV on these samples). Since different samples are damaged at different rates and the damage may depend upon the total beam exposure, accepted limits must be considered only as rough guidelines. D. CHADWICK : With respect to your observation of a decrease in the low energy Pt Auger peak intensity with running time, have you considered segregation or sintering of the ~t? Your failure to observe Pt ions in the mass spectrometer seems inconsistent with the desorption of Pt as oxide species, since Pt ions would be expected from fragmentation of the parent ions (of course, the Pt ions may be multiple charged). A possible method for detecting segragation or sintering is to study the chemisorption of carbon monoxide by XPS before and after running the AES spectra. G.B. HOFLUND: We have not investigated this observation carefully at this time. Both segregation and sintering of the Pt are possible explanations and the suggested detection method will be one technique used in studying this phenomenon. J.W. GEUS We are using RBS to investigated the reaction between NiO and Al203. In this research we have found that the RBS spectra contain much more information than the thichness of the Pt layer: the position of the loading edge of the tin indicates whether the tin oxide is completely covered by the platinum or not. The oxygen-to-tin ratio can be calculated from the heights of the tin and oxygen peaks in the spectra. Did you use also the RBS spectra to get this information ? G.B. HOFLUND: RBS and PIXE are promising techniques in many respects. Thus far we have only used RBS to analyze the total Pt content of our samples. B.E. LANGNER: You have used different UHV techniques for the physical characterization of your catalysts. Did you carry out any reaction on the sno2/Pt or what does give you the knowledge that you have investigated a catalyst system and not only a mixture of inorganic compounds? Furthermore, a quantitative correlation between physical properties in UHV and real catalytic activity would be very important, as the activity of catalyst may change under UHV conditions. G.B. HOFLUND : The first 16 references cited in the paper document the catalytic properties of tin oxide before and after platinization and various dopings. In addition we are relating the electrochemical catalytic reactivity for several reactions to the surface characterization. In the near future gas phase reactions at intermediate presures will be run in the vacuum system. Much effort is being expended to relate UHV surface characterization to catalytic activity under realistic reaction conditions.

G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts II! © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

SYNTHESIS AND PROPERTIES OF Pt-Sn/A1

20 J

101

CATALYSTS BY THE METHOD OF MOLECULAR

DEPOSITION

D.P. DAMYANOV

and L.T. VLAEV

Higher Institute of Chemical Technology, Bourgas 8010, Bulgaria

ABSTRACT It is shown that SnC1 Y-AI

vapours interact with the surface hydroxyls of 4 The quantity of tin deposited can be controlled by the packing density

20 3. of the hydroxyl groups. By hydrolysis of the modified samples with water vapour, followed by calcination and further treatment with SnCI

4,

complete formation of

a surface tin-containing'layer can be achieved. In synthesizing bi-metallic pt-sn/AI the

by the above method , 203-catalysts thermal stability and acidic properties for the same pore distribution of

the carrier were increased. The absence of catalytic activity with respect to hydrogenation of benzene, and high activity, stability and selectivity in dehydrocyclization of n-hexane are characteristic properties of this system.

INTRODUCTION Recently interest in bimetallic catalysts for reforming has increased considerably. Among various promotors of Pt/A1 systems,special attention has 203 been given to tin, The introduction of the latter in synthesizing Pt-sn/A1 20 3 catalysts was carried out by impregnation of the carrier with aqueous or nonaqueous solutions of its various compounds: chlorides (refs. 1, 2), tartrates (refs. 3, 4), organometallic compounds (refs. 5, 6)

and cluster compounds (refs.

7, 8). The difference observed in stability, activity and selectivity of these catalysts is directly related to their pore- and acidic properties and the state of the metal on the surface, conditioned naturally by the method employed for their formation. In literature there are no data available on the introduction of the promoting element by reaction between vapours of its volatile compounds and the surface of A1 Similar reactions have been successfully employed for the modifi203. cation of Si0 (refs. 9-11). Having in mind the potential possibilities and ad2 vantages of this method compared to conventional methods of impregnation, it is of interest to study the interactions between y-A1 ce the possibility of synthesizing pt-sn/A1

203

and SnC1

vapours and hen4 which is the aim of

20 3-catalysts, this work. Since these reactions involve deposition with atomic dispersion of the promoting ion, we define them as reactions of "molecular deposition".

102 EXPERIMENTAL The chosen support was y-A1 (Leuna, GDR) having a specific surface area 20 3 3/g. 2/g (with nitrogen) of 150 m and pore volume of 0.64 cm By calcination in air to constant weight of a fraction of 0.25-0.64 mm within the temperature range 250-750 °C, a series of samples labelled as: A-250, A-350, etc. were obtained having different contents of surface OH-groups determined thermogravimetrically. The calcined Al (or the sample of Pt/AI obtained on its basis by im20 3 20 3 pregnation with H was treated with a stream of nitrogen, saturated with 2PtCl 6) vapour of SnCI in a glass reactor at 150°C for 4 hours. After stopping the 4, flow of snCl and passing nitrogen through the samples, they were analysed for 4tin and chloride contents. Sample A-550 after modification with snCl was futher subjected to successi4 ve treatments including: hydrolysis with water vapour at 150°C for 1 hour, calcination for 6 hours at 550 °C and another treatment with SnCI

Such alternated 4. hydrolysis·-calcination operations made it possible to increase the quantity of the promoting agent deposited. Wafers

(10 x 30 mm) of thickness 12-15 mg/cm

2

°

fo fine Al powder were pre2 3 2 pared for IR spectroscopy by pressing at a pressure of 150 kg/cm . Studies were carried out in a portable quartz IR-cell with NaCI windows permitting calcination, outgassing, modification of the wafers with different quantities of snCl vapours and hydrolysis an UR-20

4 (in situ). Spectra were recorded at room temperature with

(Carl Zeiss, lena) apparatus. An optical attenuator was placed in the

reference beam. The catalytic activity of the samples thus obtained after suitable thermal activation in air and reduction with hydrogen (in situ) was studied in a flow reactor for the following reactions: hydrogenation of benzene, dehydrogenation of cyclohexane and dehydrocyclization of n-hexane

(Fluka AG). The analysis of

the reaction products was done by direct dosing of a gas sample by means of a six-way valve to a T.C.D. gas-chromatograph fitted with a 15 % squalane of chromosorb P AW column. RESULTS AND DISCUSSION Interaction between y-A1

20 3

and SnCl

4-vapours

Table 1 presents the hydroxyl group content of the samples and the quantities of CI

and tin fixed on the surface of Al

203

after its treatment with SnCl

vapours.

~Up

to 400°C coordination water is still present on the surface of y-A1 and 203 therefore, the data on OH-groups up to this temperature are approximate and

higher than the real ones.

~

4

103 TABLE 1 Dependence of the hydroxyl group ( IX ) content, the quantities of the "fixed" OR 4+ 4+ Cl and Sn ,and the Cl /Sn ratio, on the degree of dehydroxylation of

Parameters

Samples

' mgequ/g OR Cl-,mgat/g 4+,mgat/g sn 4+ Cl /Sn IX

A-250

A-350

A-450

A-550

A-650

A-750

3.17

2.64

2.00

1. 32

0.76

0.44

1. 10

1. 04

1. 01

0.97

0.91

0.84

0.38

0.36

0.31

0.28

0.25

0.22

2.89

2.89

3.36

3.46

3.64

3.82

It is obvious that with a decrease in packing density of the hydroxyl coverage, the quantity of Cl tio increase

and tin deposited regularly decreases,while their ra-

The relatively small quantities of Cl

and tin, as compared to the

total content of hydroxyl groups, can be accounted for by their incomplete inteteraction due to steric difficulties and by secondary reactions. More detailed data on the surface reactions were obtained by IR spectroscopy (Figs.

and 2).

In the spectrum of fresh Y-A1 0 dehydrated at 550°C (la-l), in conformi2 3 ty with the model of Peri (ref. 12), absorption bands were observed at 3780, 1 3744, 3733 and 3700 cmdue to stretching vibrations of isolated OR-groups. After contact of A1 0 with successively increasing quantities of snC1 these 2 3 4, bands changed and disappeared altogether (la-2, 3, 4). The most reactive OR1. groups are characterized by bands at 3780 and 3700 cmA new wide band with 1 a maximum at about 3580 cm- appeared, which in agreement with (refs. 13, 14), may be accounted for by hydrogen bonds between OR-groups and neighbouring chlorine ions. This requires a secondary reaction of RCl, evolved as a result of the chemisorption of SnC1

4:

'\ AI""

°-,/ ./'

-, AI-Cl /

+ AI-O

RCI

°\

(1)

AI-OR

/'

Further pretreatment decreased the intensity of this band and eliminated 1 partially reap-

it at 350°c. At the same time the bands at 3744 and 3733 cm-

peared, which confirms that not all the OR-groups reacted. These bands became much more pronounced in the spectrum obtained after calcination-hydrolysis-calcination (lb-3). This indicates the formation of "secondary" OR-groups which, as seen from spectrum (lb- 4), are reactive and interact with other portions of

104

----~:~l ~ 7

Q

0 1 ~5

e 0 'iii

'fUl c:

7 6

~

6

~

5 ~I

4 3

,,,,,

,

I

I

0

co

I~ I ~

gl

~'""

Lt'l

I

I I

0

~II

t!I

""

0

3800

2 ~I

I

~

~I ......

3400

3000

3400

1800 1400 Frequency, em'

pretreated at 550 DC (1) and after treatment with 20 3 20 DC increasing quantities of SnC1 2.6 ~mol (2), 5.2 umo I (3) at P 1 =23 4-vapours: SnC 4 Torr (4), and following pretreatment at :150 (5), 250 (6) and 350 DC (7). Fig. 1a. IR-spectra of Y-A1

lb. After additional successive treatments including : calcination and outgassing at 550 DC (1, 3, 6), hydrolysis with water vapour (2, 5) and treatment with vapours of SnC1

4

(4, 7).

snC1

The fact that the initial hydroxyl coverage after hydrolysis is not fully 4. recovered testifies to sufficient stability of the Al-O-Al bonds formed with

their participation. Under these conditions the identification of the Sn-OR 1, is impossible

groups which, according to (ref. 9), are to appear at 3665 cmas in this region the stretching vibrations of OR-groups hydrogen bonds

connected through

are observed. Repeated hydrolysis and calcination result in a

considerable decrease in the intensity of the bands of these OR-groups which is a proof that the building up of the surface layer is made at their expense. This is also verified by data from chemical analyses which point to an increase in the tin content at the second and the third cycles by 0.15 and 0.11 mgat/g, respectively, while the chloride content remains approximately constant and is within of the order

0.98 mgat/g.

After sample A-750 had been treated with snC1 and hydrolysed with 4-vapours water vapour, changes similar to those for sample A-SSO were observed (Fig. 2), the only difference being that its characteristic bands were less pronounced. Moreover, the difference between the thermal stability of the hydroxyl groups obtained by hydrolysis with water vapour and those obtained by modification must be

pointed out. With the latter (fig. 2-3) it was considerably smal-

ler because of the protonation resulting from neighbouring Cl- ions.

105

---./ 5

C

a

'iii

'

x o • 0 x 000 x ~ x

c

0

x

o o

d

Fig. 4. A diagram of reaction routes:a)partially dehydroxylated surface of Y-AI

(X-AI ion from a lower layer, O-oxygen ions, 0-0H-groups); b) after 203 reaction with SnCl (o-tin structures of type A,B and C, 0 -AI-CI, O-"secondary" 4 OH-groups, ... -Hydrogen bonds); c) after hydrolysis with water vapour (~coordi nation water ); d)

after calcination.

moval of part of the hydroxyl coverage and the formation of tin surface structures of configuration A, Band C (Fig. 4b). Part of the HCI, evolved during the reaction, interacted with the surface, forming more OH-groups and AI-CI. The rest of the HCI, because of a lack of suitable centres, did not react with the surface, which is proved by a Cl-/Sn 4 + ratio smaller than 4. The next stage of hydrolysis (Fig. 4c) results in a dissociation-adsorption of water, the

forma~

tion of additional quantities of OH-groups, and coordination filling-up of surface aluminium ions with water molecules. The calcination stage leads to the removal of coordination water

as well as water molecules and HCI formed from the

recombination of neighbouring OH and CI

ions. If dehydroxylation was purely ac-

cidental, then chlorine ions and newly formed OH-groups remaining on the surface would, upon the second treatment with SnCl

vapours, interact in a similar way 4 and increase the quantity of tin deposited, packing in this way the surface tin-

containing structures.

108 Properties of Pt-sn/A1

catalysts

20 3-

After calcination of Pt/A1 0 at 550 aC in air and treatment with 2 3 or impregnation with a solution of SnC1 two bimetallic Pt-Sn/ 4-vapours 2, A1 0 catalysts of platinum and tin content of 0.51 and 3.3 % by wt, respective2 3 ly, were obtained. Table Z gives some of their main characteristics compared to snC1

those of the initial Pt/A1 0

Z 3

sample.

TABLE 2 Influence of the method of preparation of pt-sn/A1 0 on their struc2 3-catalysts tural characteristics. Sample

Pt/A1 0 2 3 ~t-sn/Alt3 by lmpregna lon Pt-sn/A1 with 20 3 SnC1 pours 4-va

Parameter 2/g S, m

V

3 em /g

p'

Temp. of phase transition y-a) , °C

Chemisorbe

NH , mmol/g 3

170

0.59

950

0.246

140

0.48

1050

0.330

177

0.62

1150

0.362

The results presented in Table 2 show that the method of modification affects both pore- and acidic properties, and the temperature of phase transition (y-a A1 0 The advantage of modifying with snC1 manifests itself in 2 3). 4-vapours retaining the pore distribution, increasing the phase transition temperature and the acidic properties of the initial y-A1

Furthermore, IR spectroscopy 20 3. studies of CO chemisorption showed a considerable decrease in the band frequen-

cy of CO chemisorbed on Pt-sn/A1 0

2 3,

which indicates the presence of electronic

effects in the system. All these changes as a whole specifically affect the catalytic activity of the Pt-sn/A1 0 system. As an illustration, Table 3 summarizes the results of 2 3 1 cyclohexane dehydrogenation at mass velocity 2 hmol ratio H : C ~ 6 and 2 6H12 atmospheric pressure. TABLE 3 Dependence of the conversion of C on reaction temperature. 6H1 2 Composition of reaction products (wt; %)

C 6H12 C 6H6

Temperature, °C

390

420

450

480

530

36.0

19.8

2.0

traces

traces

64.0

80.2

98.0

ca.100

ca.100

109 The only product of dehydrogenation of cyclohexane is benzene, the bimetallic catalysts displaying lower activity at lower temperatures as compared to monometallic Pt!A1

catalyst. A sim'lar tendency in the Pt-Sn!A1 system has 20 3 203 been noted by other authors also (ref. 15,16) ,but, at temperatures characteristic of reforming, conversion is selective and complete. Table 4 contains the results from catalytic tests of n-hexane dehydrocyclization under the conditions cited above. TABLE 4 Dependence of the composition of the reaction products on the temperature of

n-hexane dehydrocyclization. Temperature, °C

Composition of reaction products, C

1-C 5

2MP+CP

3MP

(wt %)

n-C H 6 14

MCP

C H 6 6

450

0.9

2.7

2.7

90.0

1.8

1.0

520

7.4

6.6

7.3

48.8

6.2

23.7

540

10.1

4.8

4.7

26.0

6.5

46.9

550

9.3

3.6

3.5

19.1

6.7

56.6

1.2

570

14.6

1.9

1.5

11. 1

5.8

64.2

0.9

570 after 5-fold regeneration

16.3

2.0

1.6

10.1

4.1

64.8

1.1

1.0

2MP= 3-Methylpentane; CP= Cyclopentane; EMP= 3-Methylpentane; MCP=Methylcyclopentane. Because of the inhibition of the cracking reactions at the expense of the ensemble effect, catalysts rich in tin showed a weaker tendency to form coke and hence, an increased stability. This is confirmed also after a 5-fold regeneration with air, the initial activity of the catalyst being preserved. The increased dehydrocyclization ability of pt-Sn!A1

can be attributed 203-catalysts to the "ligand effect" of the promotor which leads to a change of the electronic

density of the active metallic component. As regards the benzene hydrogenation,bimetallic Pt-Sn!A1 catalysts exhi20 3 bited a total lack of hydrogenating activity within the whole thermodynamically suitable temperature range (100-180 °C) ,\-,hich is attributed to the absence at these temperatures

of hydrogen free platinum centres (17, 18).

In conclusion, chemisorption of SnC1 modifiying the Pt!A1

on A1 can be successfully used for 4 203 system as well as for the formation of bimetallic cata-

203 lysts with interesting properties. This offers practical prerequisites for the

production of stable and selective discrete Pt-Sn!A1

203

catalysts.

REFERENCES 1.

V.N. Selesnev, Yu. V. Fomichev, M.E. Levinter, Neftekhimiya, 14

(1974) 205-

110

2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

208. B. Davis, G. Westfall, J. Watkins and J. Pezzanite, J. Catal. ,42 (1976) 247-256. Eng. Patent 1356383 (1974). F.M. Dautsenberg, M.J. Helle and W.M.H. Sachtler, J. Catal., 63 (1980) 119128. A. Campero, M. Ruiz and R. Gomez, React. Kinet. Catal. Lett., 5 (1976) 177 182. B.N. Kuznehzov, V.K. Duplyakin, V.I. Kovalchuck, Yu.A. Ryndin and A.S. Bely, Kin. Katal., 22 (1981) 1484-1489. V. Keirn, H. Leuchs, B. Engler, Forschungsberichte den Landes Nordrhein-Westfalen, BRD, 2838 (1979) 1-36. V.I. Zaikovskii, V.I. Kovalchuck, YU. A. Ryndin, React. Kinet. Catal. Lett., 14 (1980) 99- 103. B. Camara, P. Fink, G. Pforr and B. Rackow, Z. Chern., 12 (1972) 451-455. W:Hnake, R. Bienert, H. Jerschkewitz, Z. anorg. allg. Chern, 414 (1975) 109 129. S.I. Koltzov, V.B. Aleskovskii, Silica gel, its structure and chemical properties, Goskirnizdat, Leningrad, 1963. J.B. Peri, J. Phys. Chern., 69 11965) 220·230. J.B. Peri, J. Phys. Chern., 70 (1966) 3168-3179. M. Tanaka, S. Ogasawara, J. Catal., 16(1970) 157-163. A.S. Bely, V.K. Duplykin et al. React. Kinet. Catal. Lett., 7 (1977) 461466. J/ V61ter, H. Lieske, G. Lietz, React. Kinet. Catal. Lett., 16 (1981) 87-91 A.S. Bely, V.K. Duplykin, YU. V. Fomichev et al., Sb. Kataliticheskaya konversiya uglevodorodov, Kiev, USSR, 4 (1979)29-33. J. V61ter, G. Lietz, M. Uhlemann, M. Hermann, J. Catal., 68 (1981) 42-51.

G. Poncelet, P. Grange and P .A. Jacobs (Editors), Preparation of Catalysts II! © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

111

PRODUCTION OF SILVER BIMETALLIC CATALYSTS BY LIQUID-PHASE REDUCTION K.P. DE JONG and;J.W. GEUS Department of Inorganic Chemistry, University of Utrecht, Croesestraat 77A, 3522 AD Utrecht, The Netherlands

ABSTRACT The possibilities of a liquid-phase reduction technique for the production of supported bimetallic catalysts are investigated. Although the method is more generally applicable, we here concentrate on one combination of metals, viz. Pt and Ag. The catalysts are-characterized by electron microscopy and infrared spectra of adsorbed CO. PtAg/Si02 catalysts are prepared using a 6 wt.% Pt/Si0 2 catalyst as the starting material. Silver was deposited selectively onto the Pt particles by reduction of Ag(NH 3); ions dissolved in an aqueous suspension of the platinum-loaded silica. Using formalin as a reducing agent bimetallic particles having sizes from 30 to 50 ~ result, whilst infrared spectroscopy indicates the platinum surface to be covered almost completely by silver. The selective deposition of Ag onto the Pt particles during preparation appears to be due to the suppression of nucleation of Ag particles in the liquid phase on the one hand, and the catalyzing effect of Pt or metallic Ag on the reduction, on the other hand. INTRODUCTION In the production of supported bimetallic catalysts, besides a high dispersion to provide a large active surface area, an intimate contact between the two metal phases is important to bring about alloy formation at low temperatures. Impregnation and drying procedures have been successfully utilized with the preparation of supported bimetallic catalysts of two group VIII metals [1,2]. Preparation of catalysts containing both a group VIII and a group IB metal appears to be much more difficult. Boudart and coworkers [3,4] succeeded to prepare highly dispersed PdAu/Si02 catalysts by carefully applying an ion-exchange method. To this end they used a silica with a very high surface area (700 m2/g). Ion-exchange at the silica surface called for positively charged metal complexes. Many examples in the literature exist, however, where the deposition of a group VIII and IB metal onto a support using simple impregnation and drying procedures led to two separate metal phases. Preparing RhAg/Si0 2 by co-impregnation, Anderson et al. [5] observed both very small Rh particles (50 ~) and silver crystallites of

112

500 ~. X-ray patterns of PdAg/SiO Z catalysts prepared by Soma-Noto and Sachtler [6J revealed the presence of considerably larger Ag crystallites besides small PdAg particles. O'Cinneide and Gault [7] had to calcine their PtAu/SiO Z catalysts at 770 0C in air to obtain alloy particles. These examples convincingly demonstrate the problems encountered with the production of bimetallic catalysts by impregnation and drying. Anderson [8J concludes that separation of the two metals already starts during the impregnation step. Usually, the group VIII metal ions will adsorb onto the support, whereas the IB metal ions remain dissolved. After drying and hydrogen reduction the result is obvious: the transition metal has formed very small particles due to the atomic dispersion after impregnation, whereas the IB metal has formed much larger crystallites in the pores of the support. of conventional methods, we will explore the possiBecause of the drawback~ bilities of a liquid-phase reduction technique for the production of supported bimetallic catalysts. This work deals with liquid-phase reduction of noble metal ions to the metallic state in an aqueous suspension of the support. Concerning monometallic catalysts we have reported previously on the preparation of silicaand alumina-supported silver catalysts by reduction of complex silver ions [9). Favourable preparation conditions to enhance the silver dispersion were observed to be: homogeneous addition of the reducing agent, the absence of micropores in the carrier and an appreciable interaction between the soluble metallic complex and the support. In this paper attention will be paid to the difficult task of preparing small alloy particles of a group VIII and a group 18 metal. The preparation method comprises selective deposition of the second metal onto particles of the first component already present on the support. In an aqueous suspension of the support covered by particles of the first metal, a compound of the second metal is dissolved and the corresponding metal is subsequently deposited by addition of a reducing agent, e.g. formalin, hydrazine or gaseous hydrogen. Since the particles of the first component catalyze the reduction reaction, the second metal is deposited exclusively onto the metal particles. As a result the two metal phases are now in an intimate contact after the reductive deposition and alloy formation can be easily achieved. With a Pt/SiO Z catalyst as the starting material, the preparation of PtAg/SiOZ catalysts will be described in detail. To demonstrate the general applicability of the method, the preparation of RuAg/SiOZ will also be reported. The catalysts produced will be compared with a sample prepared by impregnation and drying. The catalysts will be characterized by electron microscopy, infrared spectra of adsorbed CO, and X-ray diffraction. EXPERIMENTAL Platinum-silver alloy catalysts were prepared by using a 6 wt. % Pt/SiOZ

113

catalyst as the starting material. This catalyst has been prepared by Johnson Matthey and proposed as a common standard (Eurocat) by a research group sponsored by the Council of Europe. The Pt/SiO Z catalyst was powdered and suspended in an aqueous solution containing Ag(NH 3)z+ ions*. Unless stated otherwise, the reduction of the silver complex was performed at 50C by injection of a solution of formalin or hydrazine into the suspension through a capillary tube having its end below the level of the liquid. During the injection the suspension was vigorously agitated. After addition of the reducing agent the suspension was slowly heated to room temperature. This reduction procedure took two hours. The composition of the alloy catalyst produced was varied by depositing different q~antities of silver on an identical batch of Pt/SiOZ' A RuAg/SiOZ catalyst was produced by first preparing a Ru/SiOZ sample via impregnating silica (Aerosil ZOOV) with a RuC13 solution followed by drying and reduction with HZ at 400 0C. Next,silver was deposited onto this sample as described for Pt/SiOZ' The'reducing agent used was hydrazine. Because under the above experimental conditions oxygen is able to reoxidize the deposited metallic silver, air was excluded during the liquid-phase reduction by working in an NZ atmosphere. The concentration of silver ions in the solution duriRg precipitation was continuously measured by means of a combination of a silver-ion selective electrode (Philips IS 550-Ag+) and a reference electrode (Philips R44/Z-SD/l). Characterization of the samples dried at lZOoC by X-ray diffraction was done using a Debye ,-Scherrer camera. A Philips EM301 and a Jeol ZOOC microscope were used for detailed examination of the catalysts by electron microscopy. We estimated the surface composition of the PtAg/SiOZ catalysts from transmission infrared spectra of adsorbed CO. To that end a powdered catalyst was pressed into a self-supporting disk and transferred to an in situ infrared cell. As a standard treatment the catalyst was oxidized at 4000C in 1 atm of Oz to equilibrate the alloy particles. Subsequently the disk was reduced and evacuated at 4000C followed by oxygen adsorption (10 Torr OZ) at the same temperature. Short evacuation was followed by cooling down the catalyst to room temperature. The thus mildly oxidized catalyst was exposed to 100 Torr CO at room temperature and spectra were recorded using a Perkin-Elmer 580B spectrophotometer. Gas phase absorption was compensated for by an identical cell placed in the reference beam of the spectrophotometer. The spectra were corrected for background absorption of the SiOZ carrier.

*In the absence of stirring, silver-ammonia solutions spontaneously form extremely explosive silver-nitrogen compounds.

114

RESULTS The BET surface of the Pt/Si02 catalyst used as a starting material was established to be 189 m2/g. At room temperature the reduced platinum catalyst adsorbed 3.30 ml(STP)H2/9 catalyst (H2 pressure 10 Torr). Using a surface stoichiometry H/Pts = 1.8 [10] we arrived at a surface-mean particle size of 23 ~. An electron micrograph of the catalyst has been reproduced in fig.la. This representative micrograph ~hows a very homogeneous distribution of Pt particles of a fairly narroW size distribution, which displayed a maximum around 20 ~. Silver was deposited onto Pt/Si02 by reduction of Ag(NH3); with either formalin or hydrazine at 50C. A survey of PtAg/Si0 2 samples produced is found in table 1. Catalysts prepared by formalin or hydrazine are designated by F or H, respectively. followed by a serial number. Using formalin as a reducing agent. it is important to carry out the liquidphase reduction below room temperature. As dealt with before [9]. formalin does not react with A9(NH3); at 50C in a suspension of unloaded Si0 2. In the presence of Pt/Si02. however. the reduction takes place very fast at 50C as was inferred from a rapid decline of the silver concentration measured by the ion-selective electrode. It is obvious that the platinum particles do catalyze the reduction of the silver complex more effectively than does the pure carrier. It has to be expected that silver will be deposited very selectively on the Pt particles TABLE 1. Survey of PtAg/Si02 catalysts produced by liquid-phase reduction of Ag(NH 3); in the presence of Pt/Si0 2. One catalyst (15) has been prepared by impregnation of Aerosil 200V with a mixed solution of silver and platinum nitrate. Catalyst

Pt/Si0 2 F1* F2* F3 F6 H4 15

Loading (wt.%Ag)

Alloy composition At.%Pt At.%Ag

2.0 5.5 5.7 1.5 5.3 5.7

100 62 36 36 69 37 36

0 38 64 64 31 63 64

Particle size from TEM (~) 20 30 50 50-100 + 20-300++

t See text for explanation.

* Total amount of formalin added at once. + Some large clustered particles besides very small ones. ++ Bimodal particle size distribution.

Absorbancet at 2090 cm- 1 1.68 0.08 0.61 0.31

115

Fig.I. Transmission electron micrograph of the 6 wt.% Pt/Si0 2 catalyst (a); micrographs of the PtAg/Si02 catalysts FI (b), F2 (c), F3 (d). leaving the support uncovered. Fig.I.b shows the experimental facts supporting this reasoning. This electron micrograph of a freshly prepared and dried (I20 0C) PtAg/Si02 sample shows irregularly shaped metal particles, sometimes rod-like, whereas the original Pt particles (fig. I.a) definitely exhibit regular shapes. Obviously, metallic silver has been deposited exclusively adjacent to the Pt particles. Fig.I further reveals the absence of large silver particles (~ 300 ~) characteristic of the Ag/Si02 samples produced by liquid-phase reduction [9]. The alloy particles produced have a narrow size distribution exhibiting a maximum around 50 ~. PtAg/Si02 catalysts prepared by hydrazine reduction display a broad particle size distribution if compared with F-type catalysts (table 1). Hydrazine is so

116

Fig.2. Transmission electron micrograph of PtAg/Si02' sample 15. The catalyst has been reduced at 400 0C in H2. fast a reducing agent that it does produce Ag both on the Pt particles and in the liquid phase. TEM revealed clustered silver particles the number of which was scarce, however. For comparison, one PtAg/Si02 sample was prepared by impregnation and drying (table 1, catalyst 15). An electron micrograph of this catalyst (fig.2) revealed a bimodal particle size distribution. Both particles of 20 to 50 ~ and of 100 to 300 ~ can be seen. The X-ray pattern of catalyst 15 showed fairly sharp Ag lines and no Pt lines, while there was no evidence of alloy formation. These observations strongly suggest that the smallest particles mainly consist of Pt, whereas the larger ones consist of Ag. Adsorption of platinum ions and silver ions remaining dissolved during impregnation readily explain these phenomena. We have thus shown that the liquid-phase reduction technique is superior to the impregnation and drying method for the production of PtAg/Si02. A most elegant way to demonstrate the success of our preparation method is by infrared spectra of adsorbed CO. Besides results obtained with the PtAg/Si02 catalysts F2 and F6 results for Pt/Si0 2 have been added for comparison (fig.3 ). To appreciate the results shown it is necessary to know that spectra of CO adsorbed on reduced and mildly oxidized Pt/Si0 2 catalyst are almost identical [11,12]. Apparently, CO is able to remove adsorbed oxygen from a Pt surface already at room temperature. On the contrary, the PtAg catalysts show largely different spectra for reduced and oxidized samples [12], which demon-

117

10 _ PtAg36/64 ___ PtAg69f31 _._. Pt

•

o8

II II

jlx05

oe

I rl "

r.,

j:l

,: \ 1'1

u

oA

c:

i: 1

-" I-

': i

0

., \

0

'"

.o

\

I,"

0

I

'""

i'",

o2

I,

\,.,

I'

"

I

.: -,

I

~ ...

o.C _/ -,i 2200

~:"

.

'~ .,.",,"';...,.

wavenumber(c';;:'-

2100

2000

1900

Fig.3. Infrared spectra of CO adsorbed on oxidized catalysts. Samples F2 ( - ) , F6 (- - -) and Pt/Si0 2 (- . -). strates strongly held oxygen. This lower reducibility of PtAg compared with Pt has been used to estimate the surface composition of our alloy catalysts. The absorption band at 2090 cm- 1 shown in fig.3 is ascribed to CO adsorbed on reduced Pt sites [11,12] while the band at 2175 cm- 1 is due to CO bound to Ag+ ions [12,13]. The intensity of the 2090 cm- 1 band is positively correlated with the amount of Pt surface not covered by Ag; see fig.3 and table 1. From the intensity the almost complete covering of the Pt particles by silver in catalyst F2 can be inferred. At this point the reader should note that we start our preparation with pure Pt particles; every Pt particle not covered with Ag during the liquid-phase reduction will contribute to the band at 2090 em-I. Whereas catalyst F2 hardly exposes free Pt surface, catalyst F6 displays a significant absorption around 2090 cm- 1 (fig.3 ). Due to the low silver loading, not all of the Pt particles in catalyst F6 have been covered with Ag. Comparison of catalysts F2 and H4 (table 1) shows that hydrazine is less successful than is formalin to deposit uniformly Ag onto the Pt particles. To show that our preparation method is not limited to PtAg catalysts we produced a RuAg/Si0 2 sample by liquid-phase reduction. Transmission electron micros-

118

copy of both Ru/Si02 and RuAg/Si0 2 yielded results similar to those obtained with the PtAg catalysts. RuAg particles ranging in size from 50 to 100 ~ have been distributed homogeneously over the support. DISCUSSION Concerning the preparation of Ag/Si02 catalysts [9], l'~uid-phase reduction of Ag(NH3); with formalin at temperatures ranging from 20 to 500C typically produced a bimodal particle size distribution. Particles nucleated on the support displayed sizes of 35 to 70 ~, whereas particles formed in the liquid phase were considerably larger (200-400 ft). A more or less fundamental lower limit of the average particle size for the silver-silica system of about 60 ft was recognized and put together with the high mobility of silver particles over silica surfaces. This mobility, which is especially high in the presence of water [14], was though" to reflect a weak metal-support interaction. Clearly, a strong interaction between silver and the support is a prerequisite to keep the deposited Ag particles small. The work presented here shows that the presence of Pt (or Ru) particles effectively enhances the silver-silica interaction. The enhanced interaction can be inferred from the fast reduction of the silver ions at low temperatures (SoC) and from the very high dispersion of the silver metal deposited. We emphasize two factors contributing to this high dispersion. First of all the absence of large silver particles, which nucleate in the liquid phase, is important. The absence of the large particles (~ 300 ~) is due to the low reaction temperature utilized where the reaction proceeds exclusively on the catalytically active Pt particles. Another contribution to the silver dispersion of the Pt particles is the anchoring of metallic silver on the support. Apparently, Pt particles are strongly bound to silica and are therefore effective in stabilizing the deposit, silver. The reader will take for granted that the strong interaction between silver and the platinum-loaded support is due to strong intermetallic bonds between tr two metals. Intermetallic bonding is much stronger than the physical interacti< between Ag and Si0 2. Strong interaction between two metals leading to small nUl of the deposited metal was observed also by Wassermann and Sander [15]. These authors deposited iron onto rocksalt and gold substrates kept at 80 K. Whereas many isolated iron crystallites were observed on rocksalt, an almost continuou layer was obtained with gold as a substrate which points to a much higher dens of iron nuclei. Besides the anchoring of silver, it is worthwhile to examine more closely 1 reduction of the silver complex. From the transmission electron micrographs (cf. figs. l.a and l.b ) it appears to us that the deposition of metallic si

119

mainly occurs lateral on the support. This phenomenon can be explained by assuming adsorption of the complex silver ions on the support prior to the reduction step. An appreciable interaction between the silver complex and the support appeared to be favourable to enhance the dispersion of monometallic silver catalysts [9]. In a forthcoming paper we will show that the extent of metallic complex adsorption is an important parameter which controls the production of supported PtAu bimetallic particles [16]. The above results demonstrate that the concept of increasing the interaction of a group IB metal with a silica support by introduction of a second metal has been used to produce excellent bimetallic catalysts. PtAg and RuAg particles of 30 ~ on silica, homogeneously distributed over the support, can easily be produced using this technique. The method has been shown to be superior to impregnation and drying procedures. The reduction of a metal complex in the presence of a loaded supp~rt can be considered to be both a method to prepare bimetallic catalysts and a method to improve the dispersion and thermal stability of monometallic catalysts. The latter application calls for less expensive metals strongly adhering to the support. ACKNOWLEDGEMENTS The authors are indebted to Mr. R. Hendriks for preparing the greater part of the catalysts. The investigations were supported by the "Netherlands Foundation of Chemical Research" (SON) with financial aid from the "Netherlands Organi zat i on for the Advancement of Pure Research" (ZWO). REFERENCES 1 C.H. Bartholomew and M. Boudart, J. Catal., 25 (1973) 173-181. 2 J.H. Sinfelt and G.H. Via, J. Catal., 56 (1979) 1-11. 3 Y.L. Lam and M. Boudart, J. Catal., 50 (1977) 530-540. 4 E.L. Kugler and M. Boudart, J. Catal., 59 (1979) 201-210. 5 J.H. Anderson, P.J. Conn and S.G. Brandenberger, J. Catal., 16 (1970) 404-406. 6 Y. Soma-Noto and W.M.H. Sachtler, J. Catal., 32 (1974) 315-324. 7 A. O'Cinneide and F.G. Gault, J. Catal. 37 (1975) 311. 8 J.R. Anderson, Structure of Metallic Catalysts, Academic Press, London, 1975, p. 176. 9 K.P. de Jong and J.W. Geus, Applied Catalysis, submitted. 10 J.-P. Candy, P. Fouilloux and A.J. Renouprez, J. Chern. Soc., Faraday Trans. I, 76 (1980) 616-629. 11 H. Heyne and F.C. Tompkins, Trans. Faraday Soc., 63 (1967) 1274-1285. 12 K.P. de Jong, Ph. D. Thesis, Utrecht, 1982. 13 G.W. Keulks and A. Ravi, J. Phys. Chern., 74 (1970) 783-786. 14 L. Bachmann and H. Hilbrand, in R. Niedermeyer and H. Mayer (Eds.), Basic Problems in Thin Film Physics, Van den Hoeck and Rupprecht, Gottingen, 1966, p. 77. 15 E.F. Wassermann and W. Sander, J. Vac. Sci. Technol., 6 (1969) 537-539. 16 K.P. de Jong, R.C. Verkerk and J.W. Geus, in preparation.

120 DISCUSSION H. CHARCOSSET drag en at 400°C ?

What happens during heating your PtAg/Si02 catalyst in hyIs there some interdiffusion of Pt and Ag in these conditions?

K.P. de JONG A freshly prepared sample of PtAg/Si02 (catalyst F2) has been studied after reduction at 120°C. It was observed that adsorption of CO only led to a weak band in the IR spectrum. As Co hardly adsorbs on silver, we conclude that the Pt particles are still covered up by silver and that no interdiffusion of Pt and Ag has taken place at 120°C. However, after reduction at 400°C this sample adsorbed a considerable amount of CO, which shows the presence of Pt sites at the surface of the alloy particles. Apparently, the interdiffusian of the two elements within the alloy particles has been effected at 400°C. S. VASUDEVAN adsorption?

Why do you oxidize your catalysts before the IR study of CO Does the CO adsorb on the metal or on the metal oxide ?

K.P. de JONG 1. An extensive IR study of CO adsorption on the reduced samples showed a considerable shift of the vibrational frequency of adsorbed CO with the silver content of the alloy particles. Moreover, adsorbed CO caused a serious surface segregation of Pt. Both factors contribute to the difficulty of estimating the composition of the bimetallic particles after reduction. It was shown that oxidation caused segregation of silver (ions) to the surface. Under these conditions adsorbed CO hardly led to segregation of platinum to the surface, while the IR band of CO adsorbed on Pt sites did not shift considerably (fig. 3). Due to these three factors a fair impression of the bulk composition of the alloy particles could be obtained by studying the oxidized catalysts. 2. At room temperature CO was able to remove to a large extent adsorbed oxygen from the platinum surface. After this removal CO adsorbed on metallic Pt sites which led to the IR band at 2090 cm- 1. Co adsorption on platinum oxide would give rise to a band at 2120 cm- 1 (ref. 11). A. MIYAMOTO: On the basis of your method, the surface of Pt is covered with Ag. Then, is it possible to cover the Pt surface with Rh ? K.P. de JONG Whether a second metal, e.g. Rh, will be deposited selectively onto Pt particles already present on the support, depends on three factors: - Is there a considerable interaction between the metallic precursor and the support ? - What is the nature of the alloy produced, viz. exothermic or endothermic? Preliminary experiments with supported PtAu have shown that the preparation of this endothermic alloy is much more diffi~ult than of PtAg, which is an exothermic alloy. - Is the deposition reaction catalyzed by the metal itself? The catalytic action of Pt should be more pronounced than that of the second metal. If this is not true, the second metal will agglomerate and not be distributed homogeneously over the Pt particles. J. MARGITFALVI 1. what is the initial form of your Pt particles? Are they oxidized or are they covered with hydrogen ? 2. In what phase of the preparation will the contact between two metal phases be formed ? 3. What is the practical use of your PtAg/Si0 2 catalysts? K.P. de JONG: 1. Our starting material is been produced by ion-exchange and subsequent Because the catalyst has contacted air prior Pt particles will be covered with a layer of inferred from an induction period during the correlates with the removal of this adsorbed

the EUROPT-1 catalyst which has reduction with hydrogen at 400°C. to the use in our experiments, the adsorbed oxygen. This could be liquid-phase reduction, which oxygen.

121 2. As shown by electron microscopy, the two metal phases already are in contact after preparation and drying at 120°C (fig. 1). 3. Thepractical use of our PtAg/Si0 2 catalysts is twofold. In this study the catalysts served as a model system to elucidate general factors which control this liquid-phase reduction method. The preparation technique can now be used for other (practical) bimetallic systems. Secondly, silver is immobilized in the PtAg/Si02 catalysts by underlying platinum metal. With conventional supports, e.g. Si02 and a-A1203' silver metal is very mobile which leads to extensive sintering at elevated temperatures. Our preparation method offers the possibility to prepare thermostable, highly dispersed silver catalysts. L. GUCZI: On support not containing Pt, large silver particles are formed due to some additional migration on the surface. Bearing in mind this mechanism, some other can be proposed in addition to the migration. That is, on Pt/Si02 the nucleation rate is higher, due to the presence of Pt and the immediate formation of bimetallic particles. On the other hand, with silver alone, the nucleation rate is slower; thus, either particles can be grown prior to reduction, or, more simply, the already existing particles can be increased by being attached with non-reduced silver precursor. K.P. de JONG: Both mechanisms are indeed operative in the formation of particles in Ag/Si0 2 samples. From an extensive study of the preparation of supported monometallic Ag catalysts (1), the bimodal particle size distribution observed could be related to these two mechanisms. Large particles (~ 300 ~) are formed by reduction of silver ions from the liquid-phase at the surface of metallic silver particles present on the support and/or in the liquid-phases. The small particles (35-70 ~) have been formed from silver ions adsorbed on the support. Literature data show a similar particle size (40-80 ~) for catalysts produced by ion-exchange and H2 reduction (2). In the latter case an initial atomic distribution was obtained, but again a high mobility of the silver particles led to a particle size similar to that of particles produced by liquidphase reduction of adsorbed silver ions. (1) K.P. de JONG and J.W. GEUS, Applied Catalysis, in press. (2) M. JAF~OUI, B. MORAWECK, P.C. GRAVELLE and S.J. TEICHNER, J. Chim. Phys. Physicochim. Bioi. 75 (1978), 1060.


123

G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

PREPARATION AND CHARACTERISATION OF HIGHLY DISPERSED PALLADIUM CATALYSTS ON LOW SURFACE ALUMINA. THEIR NOTABLE EFFECTS IN HYDROGENATION

J.P. BOITIAUX, J. COSYNS and S. VASUDEVAN Institut Fran9ais du Petrole, Rueil-Malmaison

(FRANCE)

ABSTRACT Highly dispersed Pd catalysts (D:l00%) were prepared on low surface area aluminas 2g- 1) (9-100 m by wet impregnation technique, using palladium acetylacetonate as precursor. Characterisation of these catalysts by CO chemisorption confirms the stoichiometry of 1Pd-ICO. Electron Microscopy (EM) results confirm the presence of highly dispersed metallic particles «

lnm) and the diameter observed in EM corre-

lates with the diameter obtained by CO chemisorption. Large shifts

in I.R spectra

of adsorbed CO are observed over these catalysts, as dispersion increases. Turnover number for the hydrogenation of 1-butene is constant with dispersion but for 1,3-butadiene it diminishes sharply as dispersion increases. These results are explained by the

high

adsorption strength of small particles.

INTRODUCTION For selective hydrogenation of acetylenics in an olefin or diolefin cut, palladium based catalysts have by far proved to be the most active and selective ones (ref. 1). Literature studies indicate that on low surface aluminas the metallic dispersion (ratio of the atoms on the surface to the total number of atoms) obtained has been low (refs. 2,3). Thus only a fraction of the palladium impregnated participates in the reaction. High dispersion has been obtained over high surface

~aluminas,

(ref. 4), but these supports are avoided in selective hydrogenation reactions as they promote polymerisation reactions. Attempts made to anchor organometallic compounds in order to obtain high dispersion did not produce any significant improvements (ref. 5). Though high dispersion could be obtained over a high surface silica (ref. 6) by anchoring Pd (C attempts to impregnate low surface aluminas 3H5)2' using Pd acetylacetonates (Pd (AcAc) 2) yielded dispersions at best 15%. (ref. 7). In the present work we show that Pd

(ACAC)2 can be anchored to low surface

aluminas to yield high dispersion catalysts. The Pd catalysts thus prepared were fUlly characterised by

co

chemisorption, Electron Microscopy and I.R spectroscopy,

124 thus revealing the special properties of highly dispersed particles. These special properties are reflected in the abnormal catalytic behaviour during the liquid phase hydrogenation of 1,3 butadiene.

EXPERIMENTAL METHODS Materials Support : The aluminas used in our studies were supplied by Rhone-Poulenc (France). 2 An~Alumina of 9m g-l and 0.43 ml g-l pore volume and two ~ -aluminas of 69 and 2 -1 -1 t 104 m.g surface area and 0.56 and 1.1 ml g pore volume,respectivelY,were used. Precursor: Palladium acetylacetonate (Pd (C

5H702)2 J

was procured from Johnson

Mathey (Paris). Gases : All the gases were procured from Air Liquide (France). Hydrogen : Ultra pure

(> 99 :999%)

deoxo unit and Union Carbide CO : of purity

~99.995%

was further purified by passing through n

4A molecular sieves.

was used as such.

1 butene and 1,3 butadiene: )99.0% were purified by passing them through a bed of 4A+13 X molecular sieves, in order to eliminate traces of water and sulfur compounds. Solvents: Organic solvents for catalyst preparation (benzene,

dichloro-

ethane, carbon tetrachloride) were supplied by Merck and were of analytical grade. Heptane used as a solvent for catalytic tests was procured from Halterman (W.Germany). Its purity was )99.8% and was further passed through a 4A+13 X bed of molecular sieves. Apparatus and procedure CO chemisorption : The metal surface area was mesured by CO chemisorption under dynamic conditions at room temperature (ref. 8). The catalyst sample (1-2 g)

was

reduced with hydrogen at 250°C and desorbed at 300°C under inert gas (argon) for 2 hours respectively. The sample was then cooled under argon and known volumes (0.5 cc) of CO were injected.The volume of CO unadsorbed by the catalyst was detected by a catharometer; By supposing a stoichiometry of 1 CO-1 atom of Pd, the number of Pd atoms exposed was determined and thus the dispersion was calculated. I.R spectroscopy: The spectra of adsorbed species were recorded on a Beckmann 1

I.R 4240. In the condition of our study the resolution was better than 5 cm-

A special cell was used for the study of catalyst wafers, wherein the catalyst could be heated under H or inert gas, and gases like CO could be injected in 2 pulses under a stream of inert gas. Several pulses of CO were normally injected in order to ensure a complete saturation of the catalyst. Microscopy : CTEM : a JEOL 120 X was used to examine the catalyst particles. Under the conditions of our study

particles~1

nm could be identified.

STEM : A VG HB 5 microscope with provisions for high speed recording of X-ray

125 emission microanalysis of particles was used. In the case of metallic crystallites it was established that the signal could be resolved best when at least 50 atoms were present. Catalytic tests : Catalytic tests were done in liquid phase and in batch in a CSTR reactor at 2 I1pa (20 bars) pressure. The hydrogenation rate was followed by noting the pressure drop in the constant volume hydrogen ballast, upstream of the pressure regulator, as a function of time. Cooling water was circulated in the reactor jacket in order to maintain the reactor at 20°C.

RESULTS AND DISCUSSION I. Catalyst preparation (a)

Impregnation : The catalysts were prepared by the wet impregnation technique,

using benzene as a solvent (other solvents tried like CH or CCl gave identical 4 2Cl 2 results) . In order to establish the nature of impregnation, studies were performed on 2 two aluminas having different surface areas of 9 and 69 m g-1. The maximum Pd that could be retained by these two supports was determined as follows Solutions of Pd(ACAC)2 of varying concentration were added to known quantities of support. The solution was left in contact with the support for at least 48 hours after which the support and the liquid were separated and their Pd content analysed. Fig. 1 shows the Pd retained by the two supports as a function of Pd remaining in the solution. We can deduce from this figure that at low Pd loadings the support retains all the Pd that is present in the solution and that the maximum possible metal loading increases with the surface area of the support. Attempts were made to characterise, by I.R spectroscopy, the different species involved during the impregnation. Spectra of the support impregnated with Pd

(AcAc)2'

(fig. 2 B)

resembled more closely the spectra of the support saturated with acetylacetone, (Fig. 2 C), than the precursor Pd

(AcAc)2'

(Fig. 2 A), thus suggesting the liberation

of acetylacetone during the impregnation, which adsorbs on the alumina support. The influence of this adsorbed acetylacetone in the impregnation reaction is further demonstrated by the

fol~owing

experiments.

- A support which easily retains 0.3% Pd, is able to retain only 0.07% Pd when acetylacetone is preadsorbed on it. -When a support which has retained 0.3% Pd is immersed in a solution containing an excess of acetylacetone, more than 80% of the Pd is lost from the support to the solution. These observations along with certain other studies (refs. 5,9) lead write an equation as follows :

us to

126 J-OH

+

Pd (ACAC)2

AcAcH (ad)

--

~O-Pd

(AcAc) + AcAcH(ad)

: Acetylacetone (CSHSO) adsorbed on the support hence not detectable

in the solution after impregnation (ref. 10) .

• b't Alumina (69m 2 g-l) • ex Alumina (9m2 g-l)

1::

0 0. 0.

= 0.6

t

III

... ~

•

.= >.a

&: 0

'iii

III

's

..... III

"Q

&:

~

ftI

&:

....

'j; ~

"Q

a. ~

~

...--' ...---e-:

OL.-_-l..._ _...L--_~--"'"

o

1 2 3 4 Concentration of Pd in the solution hi/I)

Fig. 1. Pd retained by support as a function of Pd remaining in solution after impregnation.

1600

1400 1200 ~cm-1

Fig. 2. I.R spectra of (a) Pd (AcAC)2 salt, (b) support impregnated with Pd (AcAc)2 (c) support saturated with acetylacetone.

(b) Thermal treatment : The Pd impregnated aluminas were oven-dried at 120°C to eliminate the solvent. A series of catalysts were then calcined in air at temperatures varying from 200-600 oC and then reduced at 200°C. The curve representing CO chemisorbed as a function of calcination temperature shows a maximum at 300°C. Fig. 3 and Table 1 show

that the catalysts calcined at 300°C yield

disper-

sions of almost 100% when reduced under H at temperatures up to 300°C. At higher 2 temperatures of reduction or inert gas treatment there is a sharp fall in the dispersion due to metal sintering.

127

0.29% Pd/rtA1203 •, 0.33% PdNAI203 0.76% Pd/rtA12 •, 0.17% Pd/a.AI 2003

,

100 80 60

.1... w i:5

Alumina Alumina

..

1.0

d_ , CO- .

0.8

·i

/

u u

'1:1

\•

• 'e.

20

IX

-...

III III

)'t

•

~

1.2

-C Do

40

3

•

.t~

w

.D D III

/'

lJ

/'I'

0.6 c

•.. /

U

0.4 0.2

•

./"

/,/

~~

=1.5

./

/'

V

%Pd

Fig. 3. Dispersion as a function of re3uction temperature.

Fig. 4. Determination of stoichiometry of CO chemisorption over Pd catalysts.

catalyst Treatment Diameter of CO condition (OCl chemisor- Disper:- particle~.L Air H sian From CO From ption N 2 2 % chemi- electron Calc ina- reduc- desorp- cc/g N° tion tion tion sorption microsvalues cOPv 1 300 300 300 0.4 0.382 100 (1.0 2 O.17%Pd/ 300 500 500 0.268 71 0.8 3.5 3 300 700 700 0.057 15 6.0 4.9 4 'it A1 20 3 800 300 800 0.046 12 7.6 8.7 5 300 350 350 0.742 100 0.4 0.0 6 0.33%Pd/ 300 250 1.8 400 0.405 55 1.3 7 1'tA1 300 250 600 0.137 2.8 19 4.7 20 3 8 250 700 0.129 300 18 5.0 3.5 9 0.76%Pd/ 300 1.012 60 300 300 1.1 1.7 10 300 700 700 0.461 27 3.1 3.2 'tt A12 03 11 300 550* 550 49 3.3 0.835 1.6 12 0.278%Pd/ 300 40 200 0.497 80 0.7 1.7 A12 03 13 300 100 100 98 0.4 (.1.0 0.610 't 14 300 2.9 600 600 0.190 30 2.8 15 0.34%Pd/'t-A12 03 100 100 0.360 47 3.3 1.6 % Pd and support

~

SINTERED WITH WET HlrDROGEN

Table 1. Comparison electron microscopy.

of particle size as determined by CO chemisorption and

128 II. Characteriz ation (a) CO chemisorption : chemisorption of CO as a means of characterization of

pa

surface has been widely used and recommended (refs. 11,12,13). Two modes of adsorption of CO have been proposed over Pd , the linear form Pd-CO, and the bridged from "CO, and the stoichiometry depends on the ratio of these two forms. A Pd/CO Pd Pd ratio of 1 has often been used (ref. 4) to calculate the dispersion even though a ratio of 1.5 has also been suggested (refs. 12,14,15). In order to determine the ratio which is valid in our conditions of studY,we plotted on fig. 4 the volume of the CO chemisorbed by the highest dispersed catalysts for different metal loadings and for two alumina carriers

(0(,

and 'it)' From this figure we observe that

the experimental points align more closely on the straight line representing a Pd/CO ratio of 1.0 rather than 1.5. (b)

Electron microscopy:

CTEM: a certain number of catalysts,

(table 1.),

were examined in CTEM. From electron micrographs of these catalysts it was observed that the repartition of the particles was statistical without any preferential zone, and the particle size distribution was mono-modal for all the dispersions

studied,

(fig. 5.). The mean dimension based on the volume to surface ratio was calculated by the following formula :

where n is the number of particles having a diameter 0; The diameter thus obtained i was compared with the diameter determined from the CO chemisorption values, (fig. 6),by supposing a FCC structure,

(refs. 16,17);

the correlation appears to be quite

satisfactory. STEM: at dispersions close to 100%, no particles were detected in CTEM indicating that their average dimensions at these high dispersions WRre less than 1 nm.Further X-ray microanalysis of some zones in STEM reveals the spectra of Pd, thus confirming that the Pd particles were ultra dispersed.

129

o. 76%PdNA1 2 03 Dispersion: 60% IZ22Z:l Dispersion: 27%

c:n

(See tablel

100 80

-

N°9 810)

E 8 ..5 :IE

-~

loLl

6 &

III

4U

U

60 'f

•a.

-..

4

0

40

4U

.a

2

E

:::I

••

••

20 z

o

1.0

2.0 3.0 4.0 0i Particle diameter (nm)

Fig. 5. Particle size distribution for t",O catalysts.

(c)

Fig. 6. Correlation between particle size calculated from CO chemisorption and determined by electron microscopy (see also table 1 )

Infra-red spectroscopv : the IR spectra of CO adsorption over three Pd cata-

lysts of dispersion varying between 27 and 86% are presented in fig.7. The spectra sho\ 1 -1 a symmetrical band at frequencies >2000cm(HF band) and another band at 320 nm in a solution containing different concentrations of Ti0 2 colloidal per 25 ml solution at pH = 3.8. a) 130 mg Ti0 2/25 ml solution; b) 65 mg Ti02/25 ml solution; c) 32 mg Ti02/25 ml solution; d) 16 mg Ti0 2/25 ml solution. Figure 2 presents the H2 evolution of the colloidal particles used through out this work as a function of Pt loading of the catalyst. 0.33 cm

3/hour/25

The lower limit of

ml solution is given by H2 evolution in the Ti0 2 particles

which are free of noble metal loading.

The efficiency of H2 production begins

to decrease after 10 mg Pt/l loading due to the absorption of light by the Pt particles.

This result agrees with similar systems used in sustained H2 prodColloidal

uction via sacrificial systems,as reported by Kiwi and Gratzel [11].

suspensions are always in a better position than powders to adsorb ions and metals and do so more readily and uniformlY,as stated by Zsigmondy [12],and this is reflected in the good reproducibility of the H2 yield obtained during this work. Since a good contact between Pt and Ti0 2 may control the efficient evolution of H2, a

Ti0 2 suspension was irradiated in the UV for 18 hours to

138 reduce the Ti0 2 to Ti+

4

and only then H2PtC16was injected (0.4 cc in 25 ml) In this way, the cation Ti+ 4 already produced

under continuous irradiation.

allows the approach of pta and,as reported by Tauster et allow close contact between the Ti+

4

cation and platinum.

a I. [13],would not Results of Pt

(8 mg/l)-Ti0 2 (pre-irradiated sample) run under the same conditions,as shown in 1.2 cm 3 H2/h/25 ml solution, confirming that higher rates of H2 are observed when good contact between Pt and Ti0 2 exists.

Figure 2,give

I

0.9

c .2

-:::J

0

11l

E

It)

N

s:

N

J:

E

2.0

8.0mgPt/l-

Fig. 2 - Rate of hydrogen evolution evolved in irradiations with light> 320 nm in a solution containing 130 mg Ti0 2 colloidal per 25 ml solution at pH = 3.8, as function of Pt concentration in the solution In figure 3 spectrophotometric measurements for colloidal Ti0 2 are presented.

The Ti0 2 particles do not exhibit any absorption in the visible. The small baseline drift is due to scattering. The absorption rises sharply at

A < 380 nm.

This onset agrees well with the 3.2 eV band reported for anatase by

D. Duohghong et al. [5].

The size of the colloidal Ti0 2 particles was determined

by photon correlation spectroscopy according to the derivation proposed by Corti and Degiorgio [14].

Correlation functions were obtained with a Chromatix light

[15] scattering instrument.

They can be presented by a single exponential over

at least two correlation times ,indicating a low degree of polydispersity «0.20). Application of the Stokes-Einstein equation yields a

hydrodynamic radius of

a

Rh

= 200A o

215A.

for Ti0 2.

When Pt and Ru0 2 are loaded onto the particle the Rh is -

139

Ti O 2 Solution

\I

ABSORBANCE

1.5

0.320

360

400

440

),(nm)-

Fig. 3 - Visible and near UV absorption spectrum for Ti0 2 sol, at concentrations 100 mgl1 and 1 gil and pH 1.5Further information concerning the structure of the colloidal Ti0 2 particles was obtained from X-ray studies. of anatase cystals. X-ray amorphous.

The diffraction pattern shows the presence

A significant part of this material was also shown to be Figure 4 shows data obtained from the photolysis of aqueous

Ti0 2 sol loaded with Pt and simultaneously with Pt and Ru0 2 for these

meta~and

•

oxides are shown in the legend to Figure 4.

Loading conditions This result

shows the true cyclic nature of the hydrogen photogeneration under study.

After

a short induction period, the H2 generation in the case of the bifunctional catalyst becomes linear.

The process was stopped after seven hours, when pressure

had build up in the reaction vessel and the gas produced was flushed out with N2

•

Upon reillumination H2 generation resumes at the initial rate.

can be repeated many times. Pt and Ti0 2 were observed.

~(H2)

= 0.4 at 308 nm.

,

The quantum yield for H2 production was determined

by illuminating through a Balzers RUV 308 interference filter. was

This cycle

Turnover numbers of 1700, 500 and 8 for the Ru0 2

The value of H2

140 o PI, RuO,. TiO, • PI,TiO,

Fig. 4 - Water cleavage by near UV photolysis of TiO. dispersions in 25 ml samples. N. indicates that the solution has been flushed out with N. 6 Pt (1 mg). RuO. (0.2 mg) TiO. (25 mg) 0 Pt (1 mg), TiO. (25 mg) . These H. evolution results can be rationalized in terms of the model proposed by Nozik [16] presented in Figure 5.

Band gap excitation produces an ele-

tron-hole pair in the colloidal TiO. particle.

The electron is subsequently

channelled to Pt sites where hydrogen evolution occurs.

An ohmic contact be-

tween Pt and TiO. takes place as stated by Gerischer [17].

The role of RuO.

in

the water splitting process is to accelerate the hole transfer from the valence band of TiO. to the water as shown by Kiwi and Gratzel [18].

The low over-vol-

tage characteristic for oxygen evolution on RuO. renders hole capture by water highly efficient inhibiting electron-hole recombination as has been shown by Trasatti [19], Anatase has been used because its flat band potential is about 250 mV more negative than that of rutile,as determined by Rao et al.[20].

The

shift towards more negative potentials affords enough driving force to affect efficient hydrogen production from water. UV

Consistent with this interpretation

experiments were carried out with TiO. sols loaded with Pt in isopropanol.

The rate of H. generation was two times faster than in isopropanol-free solution, approaching a quantum yield of 100%.

In the presence of isopropanol ,current

doubling takes place,as measured by Dutoit et al. [21] ,from the isopropyl radical into the TiO. conduction band,increasing the observed current in TiO. crystal anodes by a factor of

~2.

141

Fig. 5 - Schematic illustration of the photoinduced events on Ti0 2 induced by UV light leading to water cleavage. In a recent study by Kiwi [6] electron injection due to electron transfer from excited RU(bipy);' (Ruthenium-tris-bipyridyl) to the semiconductor material has been demonstrated.

OscillBscope traces illustrating the time course of

events are shown in Figure 6.

Figure 6b shows that luminescence of

is strongly reduced by the TiO, colloids present.

RU(bipy)~'

This effect is more drastic

than the effect shown in Figure 6a, where only Ru(bipy);' was present.

The

faster decay is considered to be caused by the quenching process due to electron transfer from the excited

by the semiconductor material.

RU(bipy)~2

Loading

with Pt and Ru0 2 increased the quenching at room temperature since two species are involved in the electron transfer: Ti0 2 and the noble metals. Since Ti0 2 increased its Fermi level upon electron injection. and the energy flows from a higher to a lower level when equali~ation

takes place (of the levels involved).

the electron discharges to the electrolyte. forming Pt- as shown by Smith [22].

-1200I-n5

b

a

1

10mV

T

./

v

1L

if

V

/'r'"

V

V

V

J

~

/'

I

2V

V

/

J

./

V

V

V

2L

I

V

II

1/

If

Fig. 6 - Osc1lloscope traces obta1ned from the 602 nm laser photolysis of aqueous solutions a) RU(bipy);2 2 10- 4 at pH 3, luminescent signal 1 and 2 are taken at 75°C and 25°C.respectively. b) same as in a) using

142 Ti0 2 500 mg/l as quencher. 75°C as 25°C respectively. Besides colloidal

Luminescent signals 1 and 2 are taken at

Ti0 2 involved in hydrogen generation via light-in-

duced processes,Ti0 2 anatase has been prepared by thermal hydrolysis of titanium sulfate according to the Blumenfeld procedure as outlined in Barksdale's book [23].

Upon dilution a gel-like material is precipitated.

If Nb+

5

doping was

desired an appropiate amount of Nb20s was digested together with TiOS04 in sulfuric acid.

The powders obtained reveal a prolate shape for these particles,

° respectively. the short and long axis being 1500 and 300 A

Loading with Ru0 2

was achieved by adding.RuCl a.H 20 at pH6, drying at 100°C overnight in air. Higher yields of H2 are observed for these powders as compared with the Ti0 2 sols previously reported by J. Kiwi et al [2].

This is shown in Figure 7.

The H2

formation follows a monophotonic process involving one electron reduction of H+. Hydrogen evolution is proportional to the intensity of applied light.

Since the

yields of H2 are condiserably higher than at room temperature,this implies that the rate of formation of electron-hole pairs depends on two factors 1) the flux of incident photons 2) the temperature of the semiconductor which is a controlling factor of the Fermi band position of Ti0 2,as shown by E. Borgarello et al. [4].

t a

3

2 1

Fig. 7 - Effect of irradiation intensity on the amount of H2 evolved in solutions irradiated in 25 ml flask a) Ti0 2 500 mg/l. Loading 0.1% Ru0 2, 0.42% Nb20s and 60 mg Pt/l. A > 250 nm, pH 4.3 and temperature 75°C b) same conditions as in a) but irradiation was carried out with A > 400 nm and RU(bipy)t 2 2 10-4 M. Low scattering Ti0 2 colloids are also active in decomposing water by visible light induced processes.

Results obtained from visible light photolysis experi-

ments are shown in Figure 8. derivative of Ru(bipy>t 2

has

In Figure 8,Ru(bipy);2 lP as an isopropylester + 1.5V vs NHE redox potential.

This high redox

143 potential makes possible the occurrence of 02 evolution even at low pH's (though the oxygen produced is not observed since at the low concentration produced it is adsorbed on Ti0 2) and the concomitant H2 yields are shown in the lower line of Figure 8.

Using

having redox potentials at + 1.26V vs NHE there is

RU(bipy)~2

enough to drive 02 evolution at pH 3,the H2 yields are plotted in the second lower line in this figure. Rhodamin-B (Rh-B) with 1.3V vs NHE redox potential is also active in H2 evolution process at pH 3.

The hydrogen generation rate

increases significantly upon addition of methyloviologen (MV+2) to the solution. In this case electron transfer quenching takes place,as worked out by Gafney and Adamson [24], between

excited state and MV+2:

RU(bipy)~2

RU(bipy)t 2 + MV+2 ---+ RU(bipy)t 2 + MY+

(1)

that renders H2 subsequently in a dark reaction: My+2 + OH-+ 1 / 2 H2 and 02 (actually adsorbed on Ti0 2) by reaction:

(2)

RU(bipy)t 2 + H20

(3)

MV+ + H20 ~

~

2H+ +

RU(bipy)~2

:z: >

+

~

02

+

1.5 RU(bipy)~',MV

',PH=3

1.0

Fig. 3 - Water cleavage by visible light irradiation of aqueoup solutions of Ti0 2 colloidal. Sensitizer concentration 2 10- 4M and MV+ 2 = 5 10- 3M. When excited by light in the visible (400 nm cuttoff filter placed in the light beam direct band gap excitation of Ti0 2) ,excited state quenching of s~ kes place

ta-

by Ti0 2 as shown by Kiwi [6]. These electrons are channelled via the conduction band of Ti0 2 and charge the metal islands present on the surface of Ti02 mediating water reduction in a process:

144 (4) The excited sensitizer S* renders the ion S+ that has the necessary potential to inject charge through the Ru0 2 sites (in reality these are oxide islands on the Ti0 2 particle).

The rate of oxygen evolution will depend on the nature of

the sensitizer cation and the ground state redox potential of the sensitizer.

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.

K. Ka1yanasundaram, J. Kiwi and M. Gratzel, He1v.Chim.Acta, 61 (1978) 2702. J. Kiwi, E. Borgarello, E. Pelizzetti, M. Visca and M. Gratzel, Angew. Chem Int. Ed. Engl. 19 (1980) 647. M. Gratze1, Disc. Faraday. Soc. 70 (1980) 359. E. Borgarello, J. Kiwi, E. Pelizzetti, M. Visca and M. Gratzel, J. Amer. Chem. Soc. 102 (1981) 6324. D. Duonghong, E. Borgarello and M. Gratzel. J. Amer. Chem. Soc., 103 (1981) 4685. J. Kiwi, Chem. Phys. Letts., 83 (1981) 594. J.M. Hermann, J. Disdier and P. Pichat, J. Chem. Soc. Faraday Trans. 1. 77 (1981) 2815. G. Parfitt. Progr. Surf. Membr. Sci. 11 (1976) 181. H. Boehm, J. Catal. 22 (1971) 347. E. Borgarel10, J. Kiwi, E. Pelizzetti, M. Visca and M. Gratzel, J. Amer. Chem. Soc. in pres 1982. J. Kiwi and M. Gratzel, J. Amer. Chem. Soc., 101 (1979) 7214. R. Zsigmondy, Kolloidchemie. Otto Spamer Verlag, Leipzig 1927. S. Tauster, S. Fung and R. Garten, J. Amer. Chern. Soc. 100 (1978) 170. M. Corti and V. Digiorgio, Ann Phys. 3 (1978) 303. Chromatix Application Note LS-8 (1978) 560 Oaxmead Parkway, Sunnyvale California 94086 U.S.A. A. Nozik, Appl. Phys. Letts., 30 (1977) 567. H. Gerischer, Pure and Appl. Chern., 52 (1980) 2649. J. Kiwi and M. Gratzel. Chimia, 33 (1979) 289. S. Trasatti, Electrodes of Conductive Metallic Oxides. Elsevier, Amsterdam 1981. M. Rao, K. Rajeshwar, V. Vernecker and J. Dubov, J. Phys. Chem., 84 (1980). 1987. E. Dutoit, F. Cardon adn W. Gomes, Ber. Bunsen. Phys. Chem., 80 (1976) 1285. R.A. Smith, Semiconductors. Cambridge University Press, Cambridge, 1976. Jelks Barksdale, Titanium. Ronald Press New York. 1966. H. Gafney and A. Adamson, J. Amer. Chem. Soc. 94 (1972) 8238.

145 DISCUSSION B. NAGY: Did you try to prepare colloidal Ti02 particles either from normal or reversed micelles? If so, what is the difference between these two types of preparations as far as the size of the particles is concerned?

J. KIWI: No preparation of colloidal Ti0 particles either from normal or 2 reversed micelles has been atterr~ted so far. L. GUCZI Have you tried other transition metal oxides for the oxygen cycle beside ruthenium? On what basis was Ru02 chosen for this step ?

J. KIWI : Yes, other transition metal oxides besides ruthenium oxide, such as Pt02 and Ir02 have been tried for the oxygen cycle. This has already been reported by J. Kiwi and M. Gratzel in Angew. Chern. Int. Ed. Engl. 17, 860,1978. The basic reason for selecting Ru0 2 for this oxidation process is that,of all the metallic oxides, Ru02 was reported to have the lowest overvoltage for the oxygen evolution process in aqueous solutions (P. Lu and S. Srinivasan, Brookhaven National Laboratory, Report 24914, New York, 1979 ; M. Miles and M. Thomason, J. Electrochem. Soc., ~, 1459, 1976). Nevertheless, the overpotential for a given current density, in Ru02 depends on the aggregation state of Ru02 crystals and becomes progressively lower as the oxide is prepared at lower temperature attaining a higher state of oxidation. Hydrated oxides of Ru02 are effective in sacrificial water oxidation and this has recently been shown in our laboratory (J. Kiwi and M. Gratzel, Chimia, iI, 289, 1979). A. BAlKER: You have shown that oxygen adsorption on the colloidal Ti02 particles can severely affect their catalytic activity by depolarizing of Pt-. I am wondering how feasible it is to replace the Ti02 by other oxides which are active, but exhibit a lower affinity for oxygen adsorption. Furthermore, what do you think about poisoning the Ti0 2 surface in order to inhibit oxygen adsorption ? J. KIWI The depolarization of Pt- when enough 02 has accumulated in the system via the reaction: Pt- + 02 + 07 + Pt is a serious inhibitor for the reaction leading to water decomposition~of the type: Pt- + H20 -.- Pt + 1/2 H2 + OH-. Other semiconductor materials like SrTi03 and CdS2 have been used in water splitting processes but with significantly lower efficiencies and are also affected in their H2 generation capacity by the 02 present. We do not think that it is feasible to poison the Ti0 2 surface (as suggested) because electron injection from species excited by visible light having the adequate redox characteristics, e.g. Ru(bip)+2 takes place on the available surface of Ti02- If this surface is hindered by ~n additive in its receptivity to affect charge transfer from solution or to the solution (via Pt clusters) then its catalytic role would be expected to decrease considerably. K.S.W. SING: In your opinion what form of Ti02 has the highest potential activity for the photochemical cleavage of the H20 into H2 and 02? Colloidal particles of Ti0 2 are likely to have a high degree of surface heterogeneity. Do they have high intrinsic catalytic activity? If so, whu is this the case?

J. KIWI: The anatase form of Ti02 with -0.6 eV cb has the best activity ~or water splitting at moderate potentials, e.g. pH 5. At this pH,about 300 rnV are needed for H+/H 2 reduction and, therefore 300 mV are available to drive the reaction producing a fast kinetics favourable to this process. The relation between the catalyst structure and hydrogen production has been studied in detail and reported (E. Borgarello, J. Kiwi, E. Pelizzetti, M. Visca and M. Gratzel, J. Am. Chern. Soc. 103, 6324, 1981). For 02 production we have to reckon with the fact that Ti02-serves as an adsorbant for the 02 produced during the photolysis. High surface area Ti02 (> 200 m2/g) which is beneficial for H2 production affording a high contact surface area between the catalyst and 02 is

146 nevertheless detrimental for 02 generation in the same process. The 02 which is strongly attached to the Ti~2 surface is reduced by conduction band electrons to 02 and in the presence of H ,the radical H02 trapped on the surface has been reported (C. Jaeger and A.J. Bard, J. Phys. Chern., 24, 3146, 1979). Through this mechanism the amount of 02 in solution is kept very low. To evolve 02 efficiently a low surface area Ti0 2 having low microporosity as base material seems necessary. Flame-hydrolyzed Ti02 (p-25 Degussa) having a small number of surface hydroxyl groups and hence a low affinity for 02 binding has been proven to be useful in our laboratory'for efficient 02 production in dark and light induced processes. We have not investigated the surface heterogeneity of Ti0 2. In Degussa p-25 sizes and shapes as shown by electron microscopy reveal that they are very heterogeneous in size as well as in form.

G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

147

©

Syr4THESIS, SURFACE REACTIVITY, AND CATALYTIC ACTIVITY OF HIGH SPECIFIC SURFACE AREA MOL YBDENU~l NITRI DE PO~JDERS L. VOLPE, S.T. OYAMA+ and M. BOUDART Department of Chemical Engineering, Stanford University, Stanford, CA 94305,U.S.A.

ABSTRACT High specific surface area powders of t10 2N have been prepared by temperature-programmed reaction of 1100 3 and ammonia. This topotactic transformation produces small nitride particles that have an orientation relation with the original large trioxide platelets. Unsupported stable powders have thus been obtained with as much as 224 m2g-1 as measured by N~L BET adsorption. Bulk structure was studied by x-ray diffraction, and the surfacE was characterized by selective chemisorption of CO. The powders were active catalysts for NH 3 synthesis. Turnover rates based on titration of sites by CO indicate a pronounced effect of particle size on catalytic activity. This confirms the structure-sensitive character of ammonia synthesis. IJJTRODUCTION Molybdenum is known as one of the more active catalysts for ilmmonia synthesis [lJ; the pertinent literature was reviewed recently by Ozaki and Aika [2J. During synthesis on Mo powders at atmospheric pressure and temperatures up to 900 K, dimolybdenum nitride, y-~102N, is formed as the stable phase. To use a catalyst particle efficiently, its surface-to-volume ratio must be maximized. This can be accomplished either by dispersing small catalyst particles in a highly porous support or by using an unsupported powder with a high specific surface area, Sg (m 2g-1). Because it is difficult to reduce Mo compounds on various supports [3J, the former approach seems to be less promlslng. It becomes very desirable, therefore, to prepare an unsupported nitride powder with a high Sg value. Furthermore, structural properties of the surface are expected to depend on particle size below ca. 10 nm, which in the case of Mo 2N corresponds to 2 -1 60 mg. Hence, high-S g catalysts add the opportunity of investigating the structure sensitivity of NH 3 synthesis, a phenomenon observed on both small [4J and large [5J crystals of iron. +Present address:

Catalytica Associates, Inc., Santa Clara, CA 95051, U.S.A.

148

Previous studies of ammonia synthesis were performed on nitrided ~10 samples that had low or unspecified Sg and were often insufficiently reduced. Kiperman and Temkin [6J thoroughly analyzed the synthesis kinetics on MOZN prepared by reacting ammonium molybdate and NH at 873-9Z3 K. They did not measure the 3 Sg of their catalyst. Hillis ~~. [7J studied the reaction on molybdenu~ dioxide (MoO Z) partially reduced in HZ and nitrided with NZ at temperatures around 773 K. Aika and Ozaki [8J carried out isotopic tracer investigations of NH 3 synthesis on MOZN powder with 13 mZg- l They established that the reaction's rate-determining step was the same as on Fe catalysts: dissociative adsorption of NZ. Their sample was prepared by reduction of molybdenum trioxide (Mo0 3) in HZ followed by nitridation of Mo metal in NZ and NZ/H Z mixtures, all at a constant temperature of about 773 K. Most recently, two of us [9J studied NH 3 synthesis on three molybdenum compounds: Mo, MOZC, and MOOxC y. Despite profound differences in composition and crystallography, the reaction rate on these materials reached similar steady-state values, when referred to the number of sites as titrated by CO. Steady state was achieved only after the catalysts absorbed small amounts of nitrogen that corresponded to at most three atomic layers. Thus, the catalytic activity of these materials appeared to be determined by surface molybdenum nitride layers, regardless of the structure or composition of the bulk. Among these studies, the last was the first one that used selective chemisorption [lOJ to count the number of active sites for NH~ synthesis . This work describes new ways to synthesize unsupported t10 ZN powders with Zm-gl ) ,and low (lZ mZ-l) high ( 190-ZZ4 mZ-l g ), medium (50 g values of Sg. Comparison of their activity towards NH 3 synthesis confirms the structure sensitivity of this reaction. The paper also presents catalyst characterization by selective chemisorption, physisorption, microscopy, and x-ray diffraction (XRD). ..J

EXPERIMENTAL RESULTS Catalyst Preparation The three types of catalysts used in this study had low, medium, and high values of Sg and will, hereafter, be distinguished as MOZN-L, M, and H. All of them were synthesized at atmospheric pressure by downflow of reactive gases over packed beds of Mo0 3 powder. The powder bed was contained in a quartz or Pyrex cellon top of a coarse fritted disc made out of the same material. The cell was designed both to use its packed-bed portion as a plug-flow reactor and to perform volumetric adsorption measurements. Its entire volume could be isolated by stopcocks and removed from the gas-delivery system to conduct all experimentation in situ, without exposure of its

149

contents to air. The temperature of the cell's reactor section could be varied, in a furnace or otherwise, and monitored locally with a thermocouple. The preparation of MOZN-L and Mconsisted of sequential reduction and nitridation of Mo0 3 (MC&B, 99.5%) performed isothermally [9,11J. The trioxide had its normal orthorhombic crystal structure according to the XRD pattern. The reduction was carried out at 773 K by Pd-diffused HZ. In the case of MOZN-L, HZ was passed at the rate of 91 ~mol s -1 over 1 g of Mo0 3 for 30h, and in the case of MOZN-M, 18 g of Mo0 was treated for 300h with 3 -1 the gas flowing at the rate of 35 ~mol s Then the surface of the powders was passivated in a flowing 0Z/He mixture for XRD examination. The patterns of the reduction products had only Mo metal peaks. Subsequently, both powders were treated with a mixture of about 1% NH 3 in HZ. This produced MOZN-L as a result of Z4h of nitriding at 773 K, whereas 7Zh of nitriding at 803 K yielded the MOZN-M s~mple. The MOZN-H catalyst, in turn, was synthesized from ultra-high purity Mo0 3 powder (Johnson MattheY, Puratronic, 99.998%, batch 5.86897). The BET Sg of Mo0 3, measured after evacuation at 470 K to 10-3 Pa, was 0.86 mZg-1 The Mo0 3 crystals had an orthorhombic lattice, and the anomalously strong inte9rated intensities of all the (OkO) reflections in the diffractometric pattern indicated a morphology with extensive (OkO) planes. This morphology could be destroyed by grinding the powder in a mortar with a pestle. The unground Mo0 crystals seen through an optical microscope (Fig. 1) were 3 shaped as slabs, consistent with the layer structure of the solid, and were about ZO ~m in size.

Figure 1.

Optical Micrograph of Mo0

3

(Johnson Matthey).

150

The Mo 2N-H sample was prepared, as schematically shown in Fig. 2, by temperature-programmed reaction between 1 g of Mo0 3 and NH 3 (Matheson, Anhydrous) flowing at a rate of 70 ~mol s-l The ammonia was purified by passage through a sodium trap. The temperature-time program consisted of two consecutive linear increases followed by a brief isothermal portion. After rapid heating to 690 K, the reactor temperature was raised to 740 K at the rate of 0.01 K s-l and then further to 979 K at 0.05 K s-l. Subsequently, this fjnal temperature was maintained for 0.5h.

1000

atmospheric pressure

~

1. Hence the presence of phosphate affected the extent of Mg precipitation.

Because of

this, only data in which Mg/P " 1 are reported in order to be confident of the stoichiometry of the final material. a

selected

number

of

materials

The X-ray powder diffraction patterns of

expanding

the

range

of

compositions

were

examined and showed the materials to be completely amorphous, indicating the absence of any pure alumina phase.

Their amorphous nature over such a wide

range of compositions, confirms previous claims by Kehl (ref. 4) that alumina and aluminum phosphate form not merely physical mixtures, but rather composites in much the same silica-alumina.

manner as the more

familiar iso-structural analog,

The incorporation of a third component, in this case mag-

nesia, does not destroy this intimacy of mixing but serves to distort the order, causing variations in the physical properties and thermal stability (to be discussed later) of the composite.

172 TABLE Physical properties of magnesia-alumina-aluminum phosphate composites

Stoichiometry MgO Al 20 3 A1P0 4

Sample * 1 3 4 5 6 7 8 10 11 12 15 16 17 18 19 24 25 26 27 28 29

1 1 1 '3 9 3 1 2 3 5 5 3 1 8 3 1 4 1 2 4 3

* = All One

4 8 5 3 2 6 1 7 20 20 8 7 10 3 4 1 1 1 1 13 2

1 1 4 4 9 11 8 4 5 8 7 10 9 9 13 1 4 12 2 10 15

samples were prepared at pH of

the aims of

Median Pore Radius (A)

Pore Volume (cc/g)

81.8 40.8 151.9 190.3 199. f! 177.6 191.9 150.2 60.9 95.4 167.3 177.0 183.0 184.3 198.0 113.6 92.4 186.7 113.4 115.7 157.0

0.83 0.50 0.96 0.88 0.72 0.96 0.61 1.11 0.60 0.90 0.88 0.72 0.83 0.67 0.67 0.84 0.63 0.62 0.61 0.88 0.51

=9

this work was

Average Pore Radius

BET Surface Area (m 2/g)

(A)

54.4 34.0 93.1 134.2 135.3 112.6 101.4 106.4 44.4 67.7 121.8 117.5 131.4 130.2 131.9 80.6 66.9 108.4 69.3 98.4 79.0

304.5 292.7 207.2 130.4 105.7 169.8 120.9 208.0 272.0 267.2 144.2 122.5 126.5 102.7 102.1 209.0 187.1 115.1 174.9 179.6 130.1

followed by calcination at 500°C. to develop materials with predictable

surface properties which could be tailored to particular needs depending on their preparation procedure.

Because of the important role that pore sizes

play in reaction selectivity the tailoring of pore size distributions was of particular interest.

Figures

1 and

2 show the effect that varying the stoi-

chiometry can have on the pore size distributions of the finished material. Figure 1 is designed to show the effect of relative alumina concentration on pore size distribution.

It shows qualitatively that increasing the alumina

content

the

form.

in materials

of

type

under discussion causes

smaller pores

to

As the composition is changed from 1:8:1 to 4:1:4 (MgO:AI 20 3:AIP04 ) the A and a broader

median pore radius is found to increase from 40.8 A to 92.4 distribution with larger pores is obtained.

It is interesting to note that

the low alumina compositions exhibit a slightly bimodal pore size distribution as evidenced by the wave in the cummulative pore volume curve.

This feature

was reproducible for low alumina compositions and may be due to the breakup of the

composite,

thereby

forming discriminate

magnesia,

alumina,

or aluminum

phosphate entities, each with their characteristic pore size distribution.

173

UJ

%

p----------------,

~IOO --J o > w

UJ

a:

15 0..

o

CL

UJ

50

.~

>

I-

I-

:::> ~. :::> o

o

100

50

250

150

200

250

PORE RADIUS IAJ

Fig. 1. Cumulative pore size as a function of total pore volume for a series of composites of varying stoichiometry, showing the effect of alumina content on the pore size distribution. Molar ratios represented (MgO:Al203:AlP04) are: 0, 1:8:1;0,1:4:1; e, 1:1:1; &,2:1:2; , 4: 1: 4.

Fig. 2. Cumulative pore size as a function of total pore volume for a series of composi tes of varying stoichiometry, showing the effect of aluminum phosphate content on the pore size distribution. Molar ratios represented (MgO:Al203:AlP04) are: e, 1: 1: 1; . , 1: 1: 8; 0, 1: 1: 12; 0, 3:4:13.

An increase in the aluminum phosphate concentration caused an increase in

pore size only for a limited range of compositions.

Figure 2 shows that an

increase in pore sizes is indeed observed as the stoichiometry is changed from 1:1:1

to

content

3:4:13, does

but

not

further

cause

any

increase significant

in

the

relative

increase

aluminum phosphate

in either

the

pore

size

distribution or the median pore radius. In order to determine whether in fact any statistically significant correan

arbitrary

parameter which accounted for all three components was defined as

lation

exists

between

composition

and

surface

properties,

(Mg+Al) IP.

Pearson correlations between this parameter and the average physical properties

listed in Table 1--surface area,

median pore -0.73,

and

pore volume,

radius--were calculated and -0.80,

respectively.

With

found the

average pore radius,

to be equal to

exception of pore

0.83,

and

-0.16,

volume,

good

correlations exist between composition and physical properties for the range of catalysts studied. Sufficiently

good

correlations

were

obtained

between

composition

and

surface properties

(excepting pore volume) so that when fitted using linear

models

no

involving

higher

than

quadratic

terms

in

molar

fractions,

174

ZOOoc, but with Boudouard reaction; H2 at T > Z700C, ascribed to activated chemisorption). 5. Catalytic properties of reduced CoO-MgO. Reduced MCo was active for ethane hydrogenolysis and CO hydrogenation. Table 2 shows results on CO hydrogenation for MCo 10 taken through sequences of treatments. Thus in A a sample was first subjected to mild reduction in H2 at 500°C for 1 h, cooled to Z500C and tested

°

.

°

TABLE 2 Catalytic behaviour of reduced CoO.1M90.gO solid solutions in CO hydrogenation Gas preReaction Initial Conversion Products (mol %) treatment condit. rate % (time,h) :...CH....:...;.;;;.;..C:...H~..:C-H:....:...-C-H------T(oC)/t(h) (rel.) 4 24 2 6 HZ/5001l HZ/630/4 A vac/670/3 vac/7l01l6 } HZ/72512 HZ/730/5 B [H Z/610/5

~CO/515/4

C CO/630/4 } vac/660/Z

a b c a

0.11 0.07 0.09 0.30 0.05 0.09 0.30

18(lh) 15(5h) lZ(6h) 80(4h) 30(6h) 70(5h) 40(1.5h)

86 75 51 87 74 42 85

a

0.90

80(l.5h)

95

a

1.00

a a a

d

e

7 8 10 Z 2

Z

3 6 C3H8 zC 4 CO Z 4 7 0.5 0.5 6 1.5 1 8 4 3.5 Z.5 1.5

1 1.5 1

3

31 2 2 44 0.5

3.5

0.5 6 5 2 6.5

0.1

1.1

3.5

75(1h)

97.5 0.7

l.Z

0.5

0.10

40(3.5h)

55

0.01 0.70

9(Zh) 65(1 h)

~45

99.5

8

16

10

23

lZ.5 0.5

2

14

5 11.5 Z.O

a - Standard conditions [CO:H Z =1:3; T = Z50 0C; P(initial) = 30 Torr], b - CO:H Z = 1: 1; c - P(initial) = 140 Torr; d - T= ZOOoC; e - T= 300°C.

2.5

3.5

189

for CO hydrogenation in three different reaction conditions (a,b,c). The same solid was then given a stronger reducing treatment (630°C for 4 h), followed by further tests. The sequence in A is that of increasingly strong reduction treatments in hydrogen. B is an isolated experiment with a newly prepared sample aimed at confirming the result for corresponding conditions in the A sequence. C shows the effect on catalytic behaviour of reducing in CO. DISCUSSION The results show that incorporation of nickel and cobalt ions in solid solution in MgO is an effective way to control their reducibility. Ni 3+ and C0 3+ for.med superficially during oxygen contact are reduced below 300°C, but the divalent Ni 2+ and C0 2+ ions resist reduction strongly. Reduction is limited to only the first few layers even at 600-650 0C. Figs. 1 and 3 show that the actual amount is equivalent to less than two complete solute monolayers of Ni 2+ or C0 2+ being reduced to the z~rovalent state. These are indeed conditions which should favour preservation of the reduced ions in a finely-divided state. The catalytic activity developed for ethane hydrogenolysis and CO hydrogenation (which require ensembles of metal atoms) shows that metal particles have been produced, so the following processes must have occurred during the reduco( . t ion: (a) x Nl.3+(or Co 3+) -.- x N1o2+( or C02+) ; (b) y N·1 2+( or C0 2+) -.- Y N°1 or Co). 0 , (c) z Nio(or Coo) -rNi(or Co) particles. Fig. 5 illustrates a model for the initial and the reduced state. Taking MN being reduced by H2 as the example (the same will hold, mutatis mutandis, for MCo, and also for CO reduction if temperature is high), process (b) comprises: ... 0

2-

Mg

2+ 2- 02+ 2- 2+ 0 Nl 0 r~g ••• + H 2

- + .•• 0

2- Mg 2+OH - Ni 001-1 - Mg 2+

The model presumes that outermost Ni o will nucleate to form metal particles, but some (that which lies deeper) will remain isolated in the matrix, either as Ni o or as a charged species in a low oxidation state. The decrease in matrix volume I(surface metal atomsl II (interior metal atoms)

~\

.. 1.'. Initial state of solute ions

o •

Reduced state

Fig. 5. Model for the initial state (left) and reduced state (right) of NiO-MgO and CoO-MgO solid solutions. Only the Ni (or Co) species are indicated: ions of the solvent MgO are omitted. The reduced state illustrated is that after reduction of a few solute monolayers, with a metal particle supported on the oxide.

190

shown in Fig. 5 takes account of the fact that some actual extraction of oxygen 2- + CO as H20 (or CO 2) will also ensue [20H - + 2- + H20 (or C0 23 + 2)J. Reduction of divalent solute by CO has the special feature that the threshold temperature is.low enough (450-500°C) for processes (b) and (c) to be rapidly replaced by the Boudouard reaction (for the MCo case, see Fig. 4). Thus the reduction is self-regulating. The reduced species should accordingly remain highly dispersed, albeit with the complication that carbon is simultaneously deposited. By about 600 0C, however, the Soudouard reaction no longer dominates the action of CO: this is because Reaction (2) is an equilibrium which between 500° and 600°C becomes increasingly favourable for the back reaction, especially for carbon in contact with finely-divided metal [5J. Genuine reduction is then resumed. There is evidence, however, that the mechanism (at least near the 'take-over' temperature) is: 2CO + C + CO 2; C + 02- + CO + 2e; 2e + Ni 2+(or C0 2+) + Nio(or Coo). Direct reductive action (cf. MN and H2 above) will occur at higher temperature. The chemisorption results are notable for the 3-stage nature of the uptake of oxygen (Fig. 2). For MN, the 20°C stage (I) agrees in extent with that with N20 and with CO chemisorption (see data for MN 5), suggesting that only surface metal atoms are involved. The limiting uptake for oxygen on bulk nickel and cobalt at room temperature is generally held to be 2-3 monolayers [6J, but for systems (such as the present case) where heat dissipation is good the value could be lower [7J. The proposition that stage I measures surface metal atoms on particles (albeit with some possible attack beyond the outermost layer) leads to a ready explanation for plateaux II and III in Fig. 2, namely that II represents oxidation of the interior of the particles and III describes the more difficult oxidation of matrix-isolated species (Fig. 5). It follows that the ratio 1/(1 + II) is a measure of the metal dispersion in the reduced solid solution. Thus for D and E in the MN results, dispersions (maximum values) are 0.33 and 0.42, respectively. For hydrogen-reduced MCo 10, the dispersion (assuming I is one monolayer) is 0.4/0.75 =0.53. However, the lack of agreement between the N20 chemisorption and oxygen uptake suggests that there is incorporation with oxygen at 200C (i.e., I is more than 1 monolayer), so the true dispersion will be rather lower. The low I (200C) value for 10ngterm CO-reduced MCo 5 is thought to be due to carbon atoms produced by CO disproportionation having diffused into the oxide sub-surface, yielding Coo atoms at depths from which they do not easily exsolve to give Co particles. Reoxidation at 300-400 0C which yields oxide particles of NiO, CoO (or C0 304) supported on MgO can be readily distinguished from reoxidation at 6000C which re-forms the solid solution by the behaviour on re-reduction in H2: the former exhibits rapid reduction below 5000C, whilst the latter gives the same profile as Fig. 1 or Fig. 3.

°

°

191

The catalysis results (Tables 1 and 2) show that higher hydrocarbons than CH 4 are formed under certain conditions, in agreement with the results of Vannice for Ni/A1 203 and Co/A1 203 [8J, and especially Doesburg et a1. [9J for Ni/A1 203. The MgO solid solutions studied here show very clearly that the manner and extent of reduction have important consequences for the activity and selectivity in CO hydrogenation. The gentle self-regulating reduction at 500-550 0C in CO (see B in Table 1 and C in Table 2) yields a catalyst of low activity but high selectivity towards C2 and C hydrocarbons. Strong reduction, on the other hand, 3 favours methanation. The same trend can also be seen in the case of the hydrogen reductions. The less severe the reduction conditions, the greater the incidence of C2-C4 hydrocarbons in the subsequent CO hydrogenations. The chemisorption data confirm that the reduced catalysts have high dispersion, notwithstanding the uncertainties about the absolute significance of I (200C), and the gentler the reduction the higher is the dispersion [cf. I (200C) for A and C, respectively, in the MN 10 seriesJ. The catalytic results for CO hydrogenation are therefore consistent with greater departure from methanation behaviour the greater is the dispersion. However, the effect is not necessarily a particle size effect per se. The reduced solid solution provides a strongly ionic environment at the particle-support interface and even more so for any reduced species which are at the surface and not fully exso1ved. Partially-ionized (electron-deficient) Ni and Co may be the source of the departure from methanation behaviour. This is similar to the conclusion reached by Doesburg et al. [9] for CO hydrogenation on Ni/alumina. REFERENCES 1 A. Cimino, M. Schiavello and F.S. Stone, Discussions Faraday Soc., 41(1966)350. 2 J.C. Vickerman, in Catalysis, Vol. 2, p.107, Spec. Per. Rep., Chemical Society, London, 1978. 3 A.P. Hagan, C. Otero Arean and F.S. Stone, Proc. 8th Int. Symp. Reactivity of Solids, Gothenburg, 1976, Plenum Press, New York, 1977, p.69. 4 A.P. Hagan, M.G. Lofthouse, F.S. Stone and M.A. Trevethan, in Preparation of Catalysts, II, Elsevier, Amsterdam, 1979, p.417. 5 J.R. Rostrup-Nie1sen, Steam Reforming Catalysts, Teknisk For1ag A/S, Copenhagen, 1975. 6 M.W. Roberts and C.S. McKee, Chemistry of the Metal-Gas Interface, Oxford University Press, Oxford, 1978. 7 R.M. Dell, D.F. K1emperer and F.S. Stone, J. Phys. Chern., 60(1956)1586. 8 M.A. Vannice, J. Cata1., 37(1975)449. 9 E.8.M. Doesburg, S. Orr, J.R.H. Ross and L.L. van Reijen, J.C.S. Chern. Commun., (1977)734.

192 DISCUSSION J.H.R. ROSS What happens to the carbon deposited in the Boudouard reaction? Does it not have an effect on the catalytic behaviour of the reduced catalyst ? F.S. STONE .: The amount of carbon deposited by the Boudouard reaction under our conditions is very small, of the order of 10 18 atoms m- 2. As such, it is likely to be reactive towards hydrogen, so I am not inclined to think it impairs the activity of the reduced catalyst. J.W. GEUS: Could you digress on the mechanism of the reduction of the nickel or cobalt ions inside the support? Is the reduction proceeding by migration of hydrogen or carbon monoxide to the ions and carbon dioxide or water from the reduced specie? or are you considering an indirect reduction process ? F.S. STONE: I believe the reduction process for internal transition metal ions is indirect. It is not necessary for H2 or CO to migrate to the ions. It is sufficient that they convert surface oxide ions to OH- or CO~(H2 + 20 2- ~ 20H- + 2e; CO + 20 2- ~ CO~+ 2e); the electrons thereby freed then migrate by a hopping mechanism to the ions and reduce them. S.P.S. ANDREW: In the reduction of ammonia synthesis catalyst there is a pronounced effect of the partial pressure of water vapour during reduction with hydrogen on the structure. The catalyst becomes coarser as pH20/pH2 increases. Did you notice any such effect ? F.S. STONE: We do not observe such an effect. However, this could well be because we are carrying out so little reduction that the partial pressure of water vapour present in the hydrogen is always insignificant. Indeed, most of the reacted hydrogen can be contained as OH- on the magnesia surface in the case of our dilute solutions. If, on the other hand, one were to reduce a concentrated solid solution, then I would think it quite likely that the effect you describe would result. J.W.E. COENEN: In answer to a question from Professor Ross, you expressed the opinion that carbon left behind by the Boudouard reaction in CO-reduction was rather innocuous. However, I note that in table 1 of the paper, samples A, B, C, D, E of MN 10 show alternating activity behaviour and I find that the low activities do belong to samples reduced by CO. Doesn't that indicate that the carbon blocks sites which otherwise would have been active in methanation ? F.S. STONE: It is true that one could construe from Table that low methanation activity is associated with CO reduction (experiments B and D). However, one should note that gentle hydrogen reductions (experiments A dn E) also give low methanation activity relative to experiment C. Thus it does not follow that deposited carbon is responsible, and it will be necessary to do more experiments to decide the matter.

193

G. Poncelet, P. Grange and P.A. Jacob. (Editors), Preparation orCata/yot.III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

PREPARATION OF MONODISPERSED NICKEL BORIDE CATALYSTS USING REVERSED MICELLAR SYSTEMS

J.B. NAGY, A. GOURGUE and E.G. DEROUANE Facultes Universitaires de Namur, Laboratoire de Catalyse, 61, rue de Bruxelles, B-5000 Namur, Belgium

ABSTRACT The reduction by NaBH of Ni(II) cation dissolved in the aqueous disconti4 nuous phase of reversed micelles provides a convenient way to produce nickel boride particles of predetermined size and size distribution, given a welldefined micellar composition and concentration of the reducing agent. obtained

The so-

particles normally in the colloidal state, can be deposited on a sup-

port if needed.

Such catalysts are nearly monodispersed and show an activity

apparently superior to that of Raney nickel in the hydrogenation of n-1-heptene.

INTRODUCTION Nickel-based catalysts, due to their low cost and high activity, are widely applied in the field of organic reductions. Among these catalysts, the so-called "nickel borides" obtained by reduction of nickel salts by sodium or potassium borohydride have been known for a long time (refs. 1,Z).

With respect to Raney

nickel, they present some advantages like better selectivity and fatigue resistance (refs. 3-6).

Their initial preparation method has been improved by chan-

ging the reaction medium.

More active catalysts may be prepared using ethanol

rather than water as solvent (refs. 7,8).

Such new catalysts show an activity

in the hydrogenation of olefins which is close to that of Raney nickel.

More

recently colloidal nickel boride particles have been obtained in ethanol using polymers as stabilizing agents (refs. 9,10).

We describe in this paper an at-

tractive new preparation method, using a "cage effect" in reversed micellar systerns, thereby controlling the size and size distribution of the catalyst particles.

EXPERIMENTAL An excess of NaBH solution is added dropwise, under N flow and vigorous Z 4 stirring at QOC,to the degassed micellar solution containing the NiCI salt. Z

194 At the end of the reaction, the temperature is raised up to room temperature until complete hydrolysis of the excess NaBH occurs. 4 The composition of the nickel boride particles was determined by chemical analysis (complexometric titration for Ni (ref. 11) and proton induced y-ray emission for boron content (ref. 12». The size of the catalyst particles, after deposition on silica, was measured using a Philips EM 301 electron microscope in the transmission mode (ref. 13). The method of Peret was used to obtain the mean-diameter of the particles (ref. 14) • The size of the micellar droplets was calculated from the 19F-chemical shift variations (ref. 15).

The 19 p_ NMR measurements were carried out at room tempe-

rature with a Bruker CXP-200 spectrometer working in the Fourier-transform mode at 188 MHz. The n-l-heptene hydrogenation catalytic tests were performed under continuous stirring in a static system by following volumetrically the consumption of hydrogen.

RESULTS AND DISCUSSION We will first discuss the factors influencing the size of the catalyst particles, such as the micellar composition and the nickel (II) and NaBH concen4 trations. Then, we will focus on the catalytic activity of the nickel boride particles. Size of the nickel boride particles The catalysts systems were prepared by reduction of NiCl CTAB-n hexanol-water reversed micellar system.

2

with NaBH

in the 4 ratio was held

The NaBH 4/NiCl 2 equal to 3, because larger particles were obtained for a lower value, the particle size remaining constant above that ratio.

The final composition of the

nickel boride particles is NixB with x close to the literature value of 2 (ref. 16). The weight-percent compositions of the different reversed micellar systems used in this study are listed in Table 1.

TABLE 1 Composition of the reversed micellar systems (wt.%) component Water n-Hexanol CTAB

Composition (wt. %) 4.0 90.0 6.0

8.0 80.0 12.0

12.0 70.0 18.0

16.0 60.0 24.0

20.0 50.0 30.0

Figures la and Ib show the variation of the mean radius of the water core as a function of water content or of the Ni(II) ion concentration respectively. The higher the water content,the greater are the dimensions of the inner water core.

195

12

18

Water (weight %): 15.8 12.6

8

...:I 4

o

rM=3.6+0.52 (% water) (R-O.9997)

10

6;-..........--.-..,..-..........--.-..,..--r--. 4

8 12 Water (weight %)

16

o

0.2 [Ni++]

0.4

0.6

0.8

(molal/water>

Fig. 1. Variation of the average radius (r of the water core as a function M) (a) of water content and (b)'of the Ni(II) ion concentration.

Figure 2 shows the dependence of the nickel boride particles size on the water content in the reversed micelle and on the Ni(II) ion concentration.

The

average size of the particles decreases with decreasing size of the inner water core (decreasing water content), while a complex behaviour is observed as a function of the Ni(II) ion concentration: a minimum is detected at an approxi2 mately 5 x 10- molal concentration. All these observations can be rationalized if one analyzes the nucleation process.

The number of nuclei (N ) formed from the reversed micellar water con

res is proportional to the number of these water cores (N It depends also M). on the integral of the gaussian distribution function (G) describing the number of Ni(II) ions per micelle (average value of N with a lower limit equal to A), the critical number of Ni(II) ions per micelle (n necessary for the formation c) of a stable nucleus. Finally, it is also proportional to an efficiency factor (Fe)' taking into account the ability of NaBH diffusion in sOlution with res4 pect to the rate of rearrangement of the micellar droplets :

196

Fig. 2. Variation of the average diameter (d in A) of the nickel boride catalyst particles as a function of water content and Ni(II} ion molal concentration The number of nuclei (N is computed from the total weight of NiCl intro2 n) duced in the reaction mixture and from the average weight of nickel boride particles determined from their size (electron microscopy), density (ref. 17) and -2 composition. From about a 4 x 10 molal NiCl concentration, the number of 2 water cores (N is independent of either the micellar composition (water conM) tent) or the total NiCl concentration (Fig. 3). For a constant NiCl concen2 2 tration, the average number of Ni(II) ions per micelle (N is a constant, henA) ce also the integral of the gaussian function G (N Therefore, the only facA). tor to be considered in this particular state is the efficiency factor (Fe). The latter factor is determined by the rate of diffusion of the reducing agent NaBH ~ncreasing with NaBH concentration and with increasing lability of the 4 4 interface) and the rate of rearrangement of the micellar droplets (which also increases with increasing lability of the interface).

The lability of the in-

terface has two opposing effects on the nucleation process,cancelling each others as a first approximation. tion effects of

NaBH~.

Therefore, we can only retain the concentra-

o

4 8 12 [NiH] x1 0 2 (molal)

16

Fig. 3. Dependence of the number of micelles per gram of the reversed micellar system (N on the Ni(II) ion concentration at different water contents. M) The lower the water content iS,the higher the local concentration of the reducing agent is when reduction occurs in the micellar system.Hence, the linear decrease of the number of nuclei per micelle (Nn!N with the water content (or M) 2 = 4 x 10- molal (Fig. 4a).

size of the water core) at [Ni(II)] The influence of the NiCl sigmoid curve (Fig. 4b).

concentration on the Nn!N ratio appears as a Z M In this case, both the integral of the gaussian dis-

tribution function

G(N and the efficiency factor Fe vary. The critical conA) centration of Ni(II) ion per micelle (n remains probably constant, but the c) average number of Ni(II) ions per micelle (N increases quasi-linearly with the A) total NiCl concentration (due to the constant value of N in Fig. 3) displaM 2 cing the maximum of the gaussian function toward higher N values. Hence, the A integral of the gaussian function itself increases following a sigmoid curve. In addition, the efficiency factor increases with increasing NiCl tion, because of the constant NaBH

4!NiCI 2

ratio.

Z

concentra-

198

15 '

...

' ... ,

10 5 ' ...

.....

'

...

~

0

x

..... ~

25

z "c:: z 20

10

5

0

15

20

25

Water (weight %)

water

20%

15 10 5

o

5

10

15

[Ni++] X 10 2 (molal)

Fig. 4. variation of the number of nuclei (N per micelle (N as a function n) M) (a) of water content and (h) of Ni(II) ion concentration.

The factors influencing the nucleation process can therefore adequately plain the experimental

data.~e

larger the amount of the nucleation centers,the

smaller the average size of the particles is (Fig.2,~nfluence concentration).

At low

ex-

of the water

NiCl

concentration, the nucleation centers N (only n 2 in a small amount) lead to rather large particles. At the optimum concentra-

tion (inflexion point in the Nn/N

a Ni(II) curve), the size reaches a minimum. M At higher concentration, because of the lower rate of increase in the Nn/N M function, the average size of the particles increases again with increasing NiCl

2

concentration (Fig. 2).

199 Catalytic activity in the n-1-heptene hydrogenation The hydrogenating activity of the nickel boride catalysts was tested in a mixture of the reversed micellar system (22% v/v) with ethanol (78% v/v) at 20 .:l:. i

-c (Fig. 5).

100 c:

o

...

CIl

Q)

> c o

o

?fi 50

o

20

10

30

Time (mln.)

Fig. 5. Time dependence of the conversion of n-1-heptene on nickel boride cata2M lysts : Ni boride catalyst (2.5x10- 2M; PH = 760 Torr; 5x10alkene) . - in

2

ethanol (a)

- in an ethanol-micellar system (b) Ni boride catalysts obtained from reversed micellar systems (4.0, 12.0 and 20.0 water wt.%) - in an ethanol-micellar system (c). The results for the newly prepared catalysts (c) are compared to the activity of nickel boride particles obtained in ethanol (95% v/v) - water (5% v/v)

,

(cal-

led Ni P-2 catalyst (refs. 5-8») ,the activity of which is close to that of Raney nickel, tests being conducted either in ethanol-water (Fig. Sa) or in ethanolmicellar systems (Fig. 5b).

An enhancement factor of ca. 3 is found with res-

pect to the catalyst of (b) in the initial rate of n-1-heptene conversion using o

a nickel boride catalyst (particles of 30-50 A diameter) obtained in the CTAB-n hexanol-water micellar system.

200 CONCLUSIONS The reduction of NiCl by NaBH in the CTAB-n hexanol-water reversed micellar 2 4 o system leads to small nickel boride particles (30-60 A) with a narrow particle size distribution. duction. ched.

The micellar water cores act as reaction cages for the re-

Nucleation can only occur if a critical number of Ni(II) ions is rea-

The efficiency of the nucleation is essentially linked to the rate of

diffusion of the reducing agent.

The latter is increased with decreasing water

content, yielding consequently smaller particles.

The hydrogenation activity

of these catalysts seems higher than those of catalysts prepared in ethanol only solutions. The formation of small catalytic particles using reversed micelles thus opens new possibilities in the preparation of heterogeneous catalysts.

REFERENCES 1 H.I. Schlesinger and H.C. Brown, U.S. Patent, 2.461.661 (1949). 2 H.I. Schlesinger, H.C. Brown, A.E. Finholt, I.R. Gilbreath, H.R. Hoekstra and E.K. Hyde, J. Amer. Chern. Soc., 75(1953)215-219. 3 R.C. Wade, D.G. Holah, A.N. Hugues and B.C. Hui, Catal. Rev. Sci. Eng., 14(1976)211-246. 4 R. Paul, P. Buisson and N. Joseph, Ind. Eng. Chern., 44(1952)1006-1010. 5 C.A. Brown, J. Org. Chern., 35(1970)1900-1904. 6 M. Kajitani, Y. Sasaki, J. Okada, K. Ohmura, A. Sugimori and Y. Urushibara, Bull. Chern. Soc., Japn., 47(1974)1203-1206. 7 H.C. Brown and C.A. Brown, J. Amer. Chern. Soc., 85(1963)1005-1006. 8 C.A. Brown and V.K. Ahuja, J. Org. Chern., 38(1973)2226-2230. 9 Y. Nakao and S. Fujishige, Chern. Lett., (1979)995-996. 10 Y. Nakao and S. Fujishige, J. catal., 68(1981)406-410. 11 Methodes d'Analyses complexometriques pour les Titriplex, 3e ed., E. MERCK, Darmstadt, p. 48. 12 Measurements carried out in Laboratoire d'Analyse par Reactions Nucleaires, Facultes Universitaires de Namur, Namur. 13 Measurements carried out at unite Interfacultaire de Microscopie Electronique, Facultes Universitaires de Namur, Namur. 14 In J.R. Anderson, structure of Metallic Catalysts, Academic Press, London, 1975, p. 364. 15 T. Nguyen and H.H. Ghaffarie, .C.R. Acad. Sci. Paris, Ser. C, 290(1980) 113-115. 16 J.A. Schreifels, P.C. Maybury and W.E. Schwarts, Jr., J. Catal., 65(1980) 195-206. 17 G.V. Samsonov and I.M. Vinitskii, Handbook of Refractory Compounds, Plenum Press, New York, 1980, p.'96.

201 DISCUSSION R. CAHEN: As this catalyst is more active than the conventional Raney is it more or less resistant to poisons such as sulfur ?

nicke~

Poisoning experiments have not been carried out on our samples. B. NAGY : Nevertheless, the effects of n-butanethiol and of thiophene were already studied on the hydrogenation of 1-hexadecene and 1-octene, on nickel boride (P-3Ni) 1 and Raney nickel catalysts. The n-butanethiol affects drastically the hydrogenation activity of 1-hexadecene on Raney nickel,while the activity on nickel boride is only slightly decreased at low poison concentration. In both cases monotonous decrease of activity is observed with n-butanethiol concentration. The poisoning effects of thiophene on the hydrogenation of 1-octene on P-3Ni and Raney nickel are more complex, but in the presence of large amount of poison, both catalysts retain approximatively 25% of their original activity. S. KALIAGUINE

Should not the size distribution of micelles be time-dependent?

B. NAGY : From the 19F_NMR data we compute an average size of the inner water cores and therefore we do not have any information on an eventual time dependent size distribution. G.W.E. COENEN: 1. you create a colloidal dispersion of high surface energy. Does it not agglomerate/sinter very fast? 2. How do you seperate the catalyst from the reaction medium? Clearly you cannot filter. 3. Did you observe any special selectiVity effects ? B. NAGY 1. The synthesized colloidal nickel boride particles are immediately deposited on silica gel in order to impede possible agglomeration. The catalyst is filtered in a glove box under nitrogen atmosphere. 2. For catalytic tests, the nickel boride catalyst is prepared in situ, in a nitrogen atmosphere by the reduction of NiC12 with NaBH4 in the reversed micelle composed by CTAB-n hexanol-water. Ethanol is then added to the colloidal system and a hydrogen atmosphere is introduced in the static reactor. Finally, the solution of 1-heptene in ethanol is added to the catalyst suspension. The catalysts previously deposited on silica gel are not active in the hydrogenation reaction. 3. The selectivity of our colloidal nickel boride catalysts on the hydrogenation reactions was not investigated yet, as the emphasis was put on the preparation and characterization of these catalysts. The selectivity study will be an interesting and important part of a later catalytic study: the interaction of the different functional groups (e.g. olefine and carbonylic) with the surface can be characterized and a relation can be looked for with the selectivity. A recent work deals with the selectivity of p-1 nickel boride catalysts doped with 2% Cr in the hydrogenation of phenol to cyclohexanone and cyclohexanol. At 150°C, a 42.5% cyclohexanone selectivity was obtained at 48.5% total conversion~ C.J. WRIGHT: Presumably .the particles of nickel boride that are prepared by your technique have a surface which os covered with adsorbed surfactant molecules. Have you been able to estimate the fraction of the surface which is covered with surfactartt, and would you expect the nature of the surfactant used in the preparation to modify the activity and selectivity of the catalysts ? B. NAGY : The surfactant molecules, or even the other components of the reversed micellar system, have a definite influence on the hydrogenation rate of n-l-heptene. The hydrogenation activity of the Ni P-2 catalyst is higher in the reaction mixture composed by ethanol-micellar system than in ethanol (Fig. 5). The specific adsorption of the surfactant molecules was not studied yet but the surface covered by the adsorbed surfactant molecules should be determined for a detailed kinetic study in these reaction mixtures.

202 R. SIGG: At what temperature you have done the experiments? mum temperature for the nickel-boride catalyst ?

What is the maxi-

The catalytic tests have been carried out at 20·C. The maximum B. NAGY : available temperature was not searched for in this study. On similar catalysts, hydrogenation reactions were carried out at temperatures as high as 160·C 2. M.H. REI : Have you tried this catalyst in reaction other than hydrogenation of olefin ? will micelle isolate Ni2B from further reactions with NaBH4 to form NaB03? Is the particle size solely responsible for the activity? B. NAGY 1. Only the reaction of hydrogenation of n-1-heptene has been carried out as the catalytic test to characterize these colloidal nickel boride catalysts. 2. The analysis of our nickel boride catalysts deposited on silica gel, by nuclear reactions (PIGE or proton induced gamma ray emission method) indicate strongly the presence of NaB02 on the silica surface. It was previously reported that Ni2B catalyzes the decomposition of NaBH4 to NaB02 in the presence of water 3. The same reaction occurs in the inner water core of the reversed micellar system and it is quite rapid at.room temperature. 3. The particle size is probably important at low catalyst concentrations. At higher concentrations where our catalytic tests were carried out, the rate of n-1-heptene conversion is not dependent on the particle size, but only on the rate of transfer of hydrogen from the solution to the active sites of the catalyst 2. Little is known on the geometric-electronic effects of these colloidal particles. A high resolution electron microscopy study is presently carried out on these catalysts at the laboratory of Professor J.M. Thomas (University of Cambridge, England). REFERENCES 1. D.G. Holah, I.M. Hoodless, A.N. Hughes and L. Sedor, J. Catal., 60,148 (1979). 2. C.C. Chang, MS Thesis, National Taiwan University, Taipei, Taiwa~ 1982 (Prof. M.H. Rei). 3. H.I. Schlesinger, H.C. Brown, A.E. Finholt, J.R. Gilbreath, H.R. Hoekstra and E.K. Hyde, J. Amer. Chern. Soc., 75, 215 (1953).

203

G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III

e 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

THIORESISTANT FLAMMABLE GAS SENSING ELEMENTS S.J. GENTRY and P.T. WALSH Health and Safety Executive, Sheffield (U.K.)

ABSTRACT A number of alumina-supported noble metal catalysts (Pd, Rh, Pt and Ir), covering a wide range of intrinsic activities and porosities, have been studied.

The initial rate of poisoning by hydrogen sulphide has been shown to

depend on the porosity of the catalyst in relation to its intrinsic activity, while the equilibrium degree of poisoning is dependent on the oxide/metal ratio of the noble metal.

The role of thoria, as an additive, is to decrease

the porosity and increase the oxide/metal ratio.

INTRODUCTION The use of catalytic gas sensors for the detection and monitoring of flammable gas is widespread throughout industry (ref. 1).

However a severe

limitation on their use can be the poisoning of the catalyst by sulphur containing gases such as hydrogen sulphide.

This is particularly important in

such applications as monitoring flammable gas concentrations in coking ovens and off-shore oil rigs, where concentrations in excess of several parts per million (ppm) of sulphurous gases may be present. A typical catalytic element (ref. 2) consists of a coil of platinum wire (dia. 0.05 mm) encapsulated in a bead of low porosity, refractory oxide with a surface layer of noble metal catalyst (total dia. 1 mm).

The oxide is

commonly -AI20), deposited as a saturated solution of aluminium nitrate which is then decomposed by electrical heating of the coil.

The catalyst commonly

consists of a mixture of palladia and thoria, deposited from a solution of palladium and thorium salts and decomposed by electrical heating.

The coil

acts both as a heater and temperature sensor to detect the heat liberated during oxidation at the catalyst surface.

The element is mounted between two

posts which serve as electrical contacts to the sensor circuit. A catalytic element of this type is strongly inhibited by sulphurous gases. For example, after exposure to air contaminated by 100 ppm hydrogen sulphide the sensitivity of the catalytic element is reduced by about 70%.

Clearly

204 this performance is unsatisfactory and thus a thioresistant catalytic element suitable for applications described above is required.

This paper describes

the development of such an element.

PREPARATION OF ELEMENTS Standard elements were prepared as outlined above using«-Alz03 as the oxide and depositing a solution of 0.45 M (NH4)Z PdC14 and 1.1 M Th(N03)4.6 HZO to form the catalyst mixture.

The elements were pretreated by

heating in a 1Z% CH4 + air mixture to about 1100 K for 1 min and then in air to about 900 K for 1 min. Thioresistant catalytic elements were 'prepared (ref. 3) from slurries of a range of noble metal salt solutions and fine particulater-Alz03 (B.D.H. Ltd.) having an elementary particle size of about 0.05 pm and a specific surface area of 100-1Z0 mZg- l• The slurry was deposited dropwise onto a coil of alumina coated platinum wire (Pt wire dia. 0.05 mm, total dia. 0.088 mm) which was mounted between the posts of a T05 transistor header.

A porous, spherical

bead (dia. 1.6 mm) encapsulating the coil was formed on decomposition of the slurry by electrical heating. The noble metals investigated were Pd, Rh, Pt and Ir, prepared from the salts (NH4)2 PdC14, Rh(en)3 C13.3 HZO, Pt(NH3)4 ClZ.H20 and (NH4)3 IrC16.HzO (Johnson Matthey Ltd.), respectively. For Pd, Rh and Pt 0.11 and 0.45 M solutions were used to give noble metal contents of approximately 0.4 and 1.6 pmol.

However, due to the insolubility

of the Ir salt, only the lower loading Ir element could be produced.

The

metal loadings (% w/w) of the elements were: Pd 3.Z, 11.7; Rh 3.1, 11.4; Pt 5.6, 19.6; Ir 5.5. In order to investigate the role of thoria, elements were prepared from mixed solutions of (NH4)2 PdC14 and Th(N03)4.6HZO to achieve the same Pd content as in the elements above.

The Pd loadings were 2.7 and 6.9% w/w with

a mole ratio of Pd:Th of 0.41. the same as that in the standard element. Each element was pretreated by exposure at 1100 K to 12% CH4 + air for 5 min and then at 900 K to air for 10 min.

Four elements of each type were

manufactured. CHARACTERISATION BY TEMPERATURE PROGRAMMED REDUCTION Three samples of each type of element were characterised individually by temperature programmed reduction (t.p.r.). ref. 4.

The apparatus used is described in

A 10% HZ + NZ mixture was used as the reducing gas at a flow rate of

205

ZO ml min-I, with a heating rate of 14 K min- 1 from Z40-800 K.

Before

reduction each sample was pretreated in the reactor in flowing air at 470 K The temperature was then lowered to Z40 K before introduction of

for 10 min.

the reducing gas. The presence of noble metal in an oxidised state is indicated by positive peaks in the reductogram.

The area under the peaks is related to the amount

of HZ consumed in the reduction, and the positions of the peak maxima (Tm) are characteristic of the environment of the oxidised species. No reduction was observed for the Pt elements.

Rh and Ir elements produced

single peak reductograms, while Pd elements produced a large positive peak followed by a negative peak.

It is assumed that the negative peak arises from

the evolution of absorbed hydrogen.

Consequently the area of this peak is

subtracted from the positive peak to obtain the amount of hydrogen consumed by the reduction process. X-ray diffraction analysis showed that the oxidised noble metals in the samples were PdO, RhZ03, and IrOz, in agreement with other studies (refs. 5, 6 and 7).

Thus the amount of metal present as oxide could be derived from the

amount of hydrogen consumed.

The fraction of oxidised metal was obtained as

the ratio of the above to the total weight of metal in the element.

The

results of the t.p.r. measurements are shown in Table 1. TABLE I T.p.r. data for catalytic elements Element

% oxidised metal

Pd Rh Pt lr Pd-ThOZ Standard

31 91 0 59 69

± 9, 40 ± 8 ± 5, 77 ± 8

,

0

± 3, ± 4, 71 ± 5 70

Tm (K) 370, 360 390, 380

,

530, 360, 340 360

Values quoted are the mean of three measurements, low loading data reported first. It can be seen that the fraction of oxide increases in the order: Pt Am)' Eqn. (2) may be simplified to:

For various active phase loadings and assuming that no change of the surface area occurs during the preparation of the supported phase, taking Eqn. (1) into account, we may write (3)

Eqn. (3) shows that, in this case, the factor (Im/Is) (s/m) is directly proportional to the dispersion lid of the supported phase. When the active phase concentration is maintained constant, Eqn. (3) may be expressed as (4)

INFLUENCE OF THE PREPARATION ilETHOD 14ateria1s The influence of the preparation method has been examined in the case of two series of impregnated 1 and ion-exchanged ~ silica-supported nickel oxides. The silica carrier used (Ketjin, grade F2) had the following characteristics: surface area, 307 m2g-l, pore volume, 1.12 cm 3g-l. Series 1 was obtained by pore volume impregnation of the support with nickel nitrate solution of the concentrations required for obtaining the desired NiO content. Series ~ was obtained by 'adsorption' of the cation complex [Ni(NH 3)6]2+ onto the support. More details about the preparation method are given

277

e1sewhere [17,18]. Samples of both series were dried at 120°C and ca1cined for 4 h at 500°C. They are designated by i(or e) Ni:x:Si, where x indicates the nickel content expressed as wt. NiO per 100 g of the final supported metal oxides rounded to the nearest integer. The exact chemical composition of the samp1es and their surface area (S) are given in Tables 1 and 2. TABLE 1 Nickel content and specific surface areas of impregnated NiO/Si0 samples 2 i Ni:x:Si Ni:2:Si Ni:4:Si Ni:6:Si Ni:7:Si Ni:9:Si Wt% NiO 1.91 3.7 5.6 7.38 9.43 300 30"1 303 292 291 S m2g-1 TABLE 2 Nickel content and specific surface areas of ion-exchanged NiO/Si0 2 samples e Ni :x:Si e Ni:2:Si 3 Ni :4:Si e Ni:9:Si e Ni :6:Si Wt% NiO 2.29 3.56 5.53 9.16 S m2g-1 342 337 355 318

Results and discussion XRD. XRD patterns taken for i samples show only the presence of well crystallized NiO. Conversely, no evidence of any crystalline NiO can be deduced from XRD spectra of ion-exchanged specimens~. Only broad and asymmetrical bands attributable to an ill defined and badly crystallized nickel hydrosilicate have been observed [17]. AEM. In agreement with XRD data, TEM micrographs of impregnated samples indicate the presence of dark particles identified by electron diffraction as NiO. The mean size of NiO agglomerates (dNi) increases gradually from 15 to 70 nm as nickel content increases. The variation of the dispersion of NiO, defined as l/dNi, as a function of the Ni loading is shown in Fig. l a. EPr~A of "crystallites free" areas shows that no highly dispersed NiO phase (dNi < detection limit in TEM ~ 2.5 nm) or surface compound can be found in nickel rich samples (x = 6, 7 and 9). For i Ni :4:Si and i Ni :2:Si samples, the nickel signal which has been found corresponds to a maximum amount of dispersed or combined phase 12.6 13.7 17.7 27.0 33.9 37.2 17.9

0.75 0.61 0.96 0.89 0.74 0.62 0.71

0.27 0.29 0.47 0.38 0.30 0.26 0.65

1. 12 0.98 1. 51 1. 25 1.00 0.97 1.00

0.43 0.54 0.46 0.45 0.27 0.25 0.64

15.5 1.6 4.0

0.28 0.00 0.10

0.21

1. 08 0.00

0.39

69.7

0.64

0.13

0.58

0.32

(coprecipitate) *Methanation activity at 250

oC,

expressed as mol CO/g Ni, h.

The first thing we note is the large activity of uncalcined samples made by deposition-precipitation compared with calcined samples. This is not observed for a sample made by coprecipitation. However, after the sinter test these differences generally disappear. Comparing the results for gamma alumina (samples 1, 2), theta alumina (sample 8)

and alpha alumina (sample 9) we note

that the use of gamma alumina gives better results than theta alumina. The sample based on alpha alumina has no activity at all. The methanation activity, expressed per gram nickel, of samples made by deposition-precipitation on gamma alumina compare favourably to the activity of the sample made by coprecipitation. The thermostability of samples made by deposition-precipitation is distinctly better than of the sample made by coprecipitation. Successive deposition-precipitations (samples 3-6) show that the activity, expressed per gram catalyst, does not increase after the second step; the activity, expressed per gram nickel is highest for the sample which was treated once. scaling-up of the process from 10 to 400 grams gave no problems. The activity

306 of the latter sample is a little higher, but the thermostability is remarkably better.

DISCUSSION Deposition-precipitation It is shown that deposition-precipitation on gamma or theta alumina results in the formation of nickel aluminium hydroxycarbonate. This is surprising since no aluminium ions were introduced into the solution. Obviously the alumina support provides aluminium ions, which react with nickel ions. Since a high initial pH of the solution.(10.S) is necessary for this process we speculate that part of the support is dissolved by hydroxide ions. From the nickel content of the product and the nickel/aluminium ratio in the nickel aluminium hydroxycarbonate we calculate that 10% of the support has reacted. One monolayer of alumina corresponds to 20%. Furthermore, since the product can be detected by X-ray diffraction, it has to be several layers thick. Combining these facts leads to the conclusion that only a small part of the surface reacts. We may imagine this as pits etched into the alumina surface by hydroxide ions. These pits are filled with a AI(OH)~

solution which reacts with nickel ions, released

by decomposition of nickel ammino complexes. Further investigations (electron microscopy) are planned to test this model. For alpha alumina this reaction probably does not occur, because of the low surface area and the stability of the support against hydroxide ions. This explains the negative results obtained for the sample based on alpha alumina. Attempts to "activate" the alpha alumina surface by a five hour treatment at 100

0c

in a 1 M sodium hydroxide solution, followed by deposition-precipitation,

were not successful.

Catalyst activity and stability Although the reproducibility of the deposition-precipitation still gives some problems, mainly for scaling-up experiments, we can point out some important results. The activity of samples made by deposition-precipitation on gamma alumina, expressed per gram nickel, is approximately the same as that of coprecipitated samples. The activity corresponds to a nickel crystallite size of 10 nm, assuming that the activity is inversely proportional to crystallite size, as was found by Kruissink (ref. 5). The thermostability of depositionprecipitation samples is distinctly better. The difference in thermostability of samples made by deposition-precipitation and coprecipitation is unexpected since the same nickel aluminium compound is formed. Probably the large interaction with the gamma alumina support causes the better thermostability of samples made by deposition-precipitation.

307 The difference in activity of uncalcined and calcined samples after reduction, that was found only for samples made by deposition-precipitation, may be explained as follows: the nickel aluminium hydroxysalt that is formed during deposition-precipitation contains mainly carbonate ions but also a small amount of nitrate ions. Upon calcination the nitrate ions decompose, resulting in the presence of nitrogen oxide vapours in the pores of the extrudates. This may lead to sintering of nickel oxide crystallites (ref. 2). However, if this decomposition is carried out in a hydrogen flow, as is the case during reduction of uncalcined samples, these nitrogen oxide vapours are probably reduced to nitrogen or ammonia and water, catalyzed by nickel oxide or aluminium oxide (ref. 7). This would prevent sintering of nickel oxide, resulting in smaller nickel crystallites (4-7 nm) and higher activities. The sample made by coprecipitation contains practica1ly no nitrate ions and is calcined in the form of powder, allowing nitrogen oxide vapours to be removed very quickly. Therefore the difference between calcined and uncalcined samples after reduction should be very small for this sample, as confirmed by experimental results. Finally, from these results we conclude that deposition-precipitation on gamma alumina in the way described in this contribution is a very promising method to produce thermally stable nickel alumina methanation catalysts.

ACKNOWLEDGEMENTS The authors wish to thank G. Hakvoort for the development of the method of activity determination by D.S.C. and J.P. Koot from the Department of Analytical Chemistry for assistance with the sodium and nickel analyses. They also thank N.M. van der Pers and J.F. van Lent from the Department of Metallurgy for assistance with the X-ray diffraction measurements. Finally they thank the Dutch Organization for Pure Scientific Research (Z.W.O.) for financial support.

REFERENCES 1 B. Hohlein, Jul. Report No. 1433, KFA Julich GmbH, 1977. 2 E.C. Kruissink, L.E. Alzamora, S. Orr, E.B.M. Doesburg, L.L. van Reijen, J.R.H. Ross and G. van Veen, in B. Delman, P. Grange, P. Jacobs and G. Poncelet (Eds.), Preparation of Catalysts, II, Elsevier, Amsterdam, 1979, pp. 143-153. 3 J.A. van Dillen, J.W. Geus, L.A.M. Hermans and J. van der Meijden, in G.C. Bonds, P.B. Wells and F.C. Tompkins (Eds.), Proc. 6th Int. Congress on Catalysis, London, 1976, The Chemical Society, London, 1977, pp. 677-685. 4 D.P. McArthur, U.S. Patent 4,042,532 (Aug. 16, 1977). 5 E.C. Kruissink, Coprecipitated Nickel Alumina Methanation Catalysts, thesis, Delft University Press, Delft, 1981, p. 25. 6 E.C. Kruissink, H.L. Pelt, J.R.H. Ross and L.L. van Reijen, Appl. Catal., 1 (1981) 23-30. 7 Gmelins Handbuch der anorganischen Chemie, 8e AUflage, 4, Stickstoff, Verlag Chemie GmbH, Berlin, 1936.

308 DISCUSSION D.C. PUXLEY Is the nitrate not associated with a lower Ni/AI ratio in the precursor phase? Could the apparent increase in sintering be associated with the lower Ni/AI ratio rather than the presence of nitrate ? H. SCHAPER: In our previous work with coprecipitated nickel alumina catalysts we have never noticed an appreciable effect of Ni/AI ratio either on nitrate content or on the activity or stability at high temperatures. For higher nickel alumina ratios we expect similar differences between calcined and uncalcined samples after reduction as observed in this work. C.M. LOK: Upon calcination of your nickel-alumina catalysts sintering of nickel oxide crystallites occurs, which may be caused, as you suggest, by the presence of nitrogen oxide vapours in the pores of the extrudates. My question is whether the effects of calcination can also be attributed to the accumulation of water vapour in the pores of the extrudates from which water is more slowly removed than from coprecipitates. H. SCHAPER: During reduction of uncalcined samples water vapour is formed at two stages. The decomposition of hydroxide ions from the Feitknecht compound takes place at around 300·C. Upon reduction of nickel oxide water vapour is formed at temperatures in the range of 450·C to 600·C. Sintering of nickel crystallites is much more severe at the higher temperatures, so we do not think that water vapour formed at lower temperatures will increase the nickel crystallite size. Furthermore, when calcination is performed in an air flow, we do not observe any difference between calcined and uncalcined samples, after reduction. J.W. GEUS You mentioned that omitting the previous calcination with nitrate containing catalysts results in smaller nickel particles. The same observation has been made previously by Eischens and van Hardeveld (Int. Catalysis Congr. Amsterdam) for silica impregnated with nickel nitrate: calcination of impregnated nickel nitrate led to rather large nickel oxide particles and consequently to larger metallic nickel particles. Have you any suggestion about the mechanism of sintering of nickel oxide by nitrogen oxides during the calcination? Is there evidence of any volatile nickel-nitrogen-oxygen compound? H. SCHAPER: We think that formation of a volatile compound is the most likely explanation. However, for nickel there is no evidence of such a compound. For copper the formation of a volatile Cu(N02) is known (M. Pospisil & P. Taras, ColI. Czech. Chern. Commun. 42 (1977) 1266). J.W.E. COENEN: You mention nickel crystallite sizes derived from methanation activity. That presupposes a one to one relation between the two quantities. Can you tell us more about that? H. SCHAPER: For coprecipitated nickel-alumina catalysts, where nickel crystallite sizes can be easily determined from X-ray diffraction line broadening, we investigated samples with crystallite sizes in excess of 2 nm and found that the activity plotted versus the reciprocal crystallite size yields a straight line. For some samples made by deposition-precipitation nickel crystallite sizes could also be determined from X-ray line broadening and for those the same relationship was found. S.P.S. ANDREW: Could it be that it is undesirable to perform calcination of nitrate melts slowly, it being better to employ a higher calcination temperature ? Thus calcination of the calcined and subsequently reduced catalyst was at 450·C, whereas when both operations were performed together, the temperature used was 600°C, so that the calcination operation would be much quicker.

309 H. SCHAPER: The nitrate ions that we are talking about are present in the form of nickel aluminium hydroxynitrate. This compound does not melt before decomposition, so there are no nitrate melts present during calcination. Your suggestion that the time of calcination is responsible for the observed differences does not explain why we only find differences for samples made by deposition-precipitation (extrudates), and not for coprecipitated samples (powder). A.R. FLAMBARD: My question concerns your observation that the direct reduction of uncalcined Ni/Si0 2 catalysts can lead to.higher metal-free surface areas; a phenomenon which has been observed on a number of previous occasions (ref. 1-5). You have mentioned that the presence of nitrate and nitrogen oxides may playa role in determining the final free metal surface area. I agree with you that these de~omposition products can cause some sintering of the nickel oxide particles produced during calcination. However, are you also aware that the heating rate used during the reduction of uncalcined samples in the presence of nitrates can also dramatically affect the subsequent metal surface areas? In agreement with Bartholomew and Farrauto (ref. 3), I have shown that above ca. 563 K the strongly exothermic reaction

+ Ni(N03)2 + Ni + 2NH3 + 6H20 ~H = - 1115.9 kJ/mol 2 predominates (ref. 6). Under fast heating rates excessive reduction to ammonia occurs, accompanied by localized heating and metal sintering. However, with a slow heating rate such as you have, I have found little reduction of the decomposition pr~ducts. Have you any information concerning the reduction of your catalysts after using different heating rates? There is also the problem that the water produced in the above equation can also sinter the metal particles. What are your comments concerning this ? Secondly, what are your comments in relation to the proposition that the direct reduction of uncalcined catalysts can lead to particles of different morphology (surface structure) as to those obtained when the catalysts have been precalcined? For example, Montarnal (ref. 7) has reported that the direct reduction of silica supported Ni(NH 3)4 (HCOO)2 gives materials of higher metal surface areas which he has interpreted in terms of microporous nickel crystallites. I have some evidence for a similar conclusion from my work with impregnated Ni/Si02 catalysts (ref. 6). I am investigating the possibility that after calcination "smooth" metal particles may be obtained upon subsequent reduction, whereas after direct reduction the particles may be "rougher". I see that you carry out your reductions under quite severe conditions which may cause an annealing of the metal surface, but I would be interested in your comments on this possibility. 9H

H. SCHAPER: First of all I have to remind you that we presented results for nickel/alumina catalysts. The nitrate ions that we are talking about are not present in the form of nickel nitrate but in the form of nickel aluminium hydroxynitrates, so your interesting remarks on the reduction of nickel nitrate do not apply in our case. We have no information on the effect of the heating rate on the reduction of uncalcined samples. All samples were heated at 2°C/min, which was found to be the best heating rate for calcined coprecipitated samples. Your second question, concerning the morphology of nickel crystallites is very interesting. We have no information on the morphology of our nickel crystallites, but it will be a subject for future research. 1) 2) 3) 4) 5) 6) 7)

Schuit, G.C.A. and van Reijen, L.L., Adv. Catal., 10, 242 (1958). van Hardeveld, R. and Hertog, F., Adv. catal., 22,~5 (1972). Bartholomew, C.H. and Farrauto, R.J., J. Catal.~45,41 (1976). This conference, paper by R. Burch and A.R. Flambard, M.A. Day et al. Burch, R. and Flambard, A.R., submitted to J. Catal. (1982). Flambard, A.R., Ph.D.thesis, Univ. Reading (1982). Montarnal, R., Proc. Ith Int. Symp. Brussels 1975, General Discussion, p.471.


311

G. Poneelet, P. Grange and P.A. Jacobs (Editors), Preparation orCata/ysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

PREPARATION OF TITANIA-SUPPORI'ED CATALYSTS BY ION EXCHANGE, IMPREGNATION AND

HCMJGENEDUS PRECIPITATION R. BURCH and A. R. FLAMBARD

Olenistry Department, The university, Whiteknights, Reading RG6 2AD, England

ABSTRACl'

Titania and silica-supported

cata~ysts

have been prepared by the techniques

of wet :inpregnation and ion exchange, and by precipitation either by the addition of alkali or hanogeneously by the hydrolysis of urea.

After drying,

the uncalcined materials have been investigated by X-ray diffraction, surface area rreasurernant, and tarperature-prograrrrned reduction.

Titania-supported

catalysts prepared by wet :inpregnation or ion exchange shewed little evidence of interaction with the support.

For hanogeneously precipitated catalysts, there

was sare indication of the formation of titanate-type species.

INl'roDOCTION It

is nOlrl recognised that oxide supports used to disperse metal particles are

rarely, if ever, inert.

In addition to the sanetimes extensive interaction

betv.een the deposited phases and the support which can occur during the early stages of catalyst preparation - to fonn specific canpounds such as silicates or aluminates (1,2) - it has also been found recently that a support may influence the catalytic properties of the rretal after reduction.

The most,

striking effects are found when reducible transition rretal oxides, such as titania, are used as supports. (3)

Such strong metal-support interactions (SMSI)

w:rre first noted for platinum metal/titania catalysts whose chenisorption and catalytic properties were modified after reduction at high tarperatures. (4,5) In earlier w:>rk (6,7,8) we have shown that Ni/titania catalysts can exhibit

special properties under conditions where SMSI are absent, and it became apparent that there was a need for a thorough investigation of the influence of sample history on catalytic properties. The precise way in which the active metal or its precursors are first

brought into contact with the support can affect the structure, reducibility, dispersion, and even the rrorphology of the catalyst.

The three methods of

preparation rrost carroonly encountered are wet impregnation, deposition/

312 precipitation, and ion exchange.

Impregnation may not be silrple as other effects,

such as adsorption may occur simultaneously. (9)

Frequently, poor dispersions

and broad particle size distributions are obtained. (10,11)

Often, there appears

to be little interaction between support and deposited phase, although this will depend on metal loading. (12)

The deposition/precipitation of metal precursors

onto a support may also be accanpanied by adsorption. (13, 14)

In the usual methcd

where the precipitant is added directly to a suspension of the support, local supersaturation and inharogeneity can result in poor dispersions. (14)

However,

the hcnoqenecus generation of the precipitant can yield high dispersions and

unifonn particle size distributions, often as a direct result of the fonnation of compounds with the support. (13-16)

Adsorption, or ion exchange, usually leads

to catalysts with high metal surface areas. (10-12)

Depending on the conditions

and system under investigation this mayor may not be a consequence of direct canpound fonnation between deposited phase and support.

Catalysts prepared by

this method are frequently rnuch more difficult to reduce than ilrpregnated catalysts. (10-12) In this paper

'lie

describe the preparation, using these three techniques, of

titania-supported Ni catalysts, and canpare these with silica-supported catalysts prepared as reference materials. EXPERIMENTAL

Materials and reagents The titania (Degussa P25) consisted of 80% anatase and 20% rutile, and had a surface area of 50 m2g-1. The silica (Davison grade 57) had a surface area of 2g-1, 290 m and was ground up before use. The 35-60 mesh size fraction was used. The source of nickel for all the preparations was nickel nitrate (Fisons).

Preparation of catalysts Wet impregnation.

Suspensions of the supports were contacted with solutions

of the appropriate Ni concentration for 0.25 h at 298 K before excess water was removed by rotary evaporation at 343 K.

Catalyst precursors 'J'Jere dried in air at

393 K for 16 h, and stored under vacuum until required. Deposition/precipitation. (i) addition of NaCE at 298 K. 3 200 an of 0.0144 M Ni nitrate solution 'llere added to 1. 52 g of support, and the pH adjusted to 2.4.

Snall doses of 0.01 M NaoH solution were then added by means

of a graduated burette. was allowed to

After the addition of each dose of NaaI, the solution

cx:me to equilibrium, and the

(ii) hydrolysis of urea at 353 K.

pH noted.

The reaction vessel was charged with 1.52 g

of support in 200 an3 of 0.0144 M nickel nitrate solution, and heated to 353 K. The pH was adjusted to 2.4, 4.6 g of urea powder was added, and the change in pH with time recorded autanatically.

313 Ion exchange.

Solutions of the required nickel concentration were prepared

and arnronium hydroxide added to raise the pH to 11.

The adsorption of the hex-

anmino Ni(II) CCllplex (12,17) was performed by adding this solution to a suspension of the support, in water and leaving for a period of 500 h. materials 393 K.

~

After this time, the

filtered off, washed with dilute arnronium hydroxide, and dried at

The dried materials were stored under vacuum,

Techniques used in catalyst characterisation Samples were analysed for their Ni contents by atanic adsorption spectrophotanetry, after dissolution with HF. measured using a calm microbalance.

Nitrogen adsorption isotherms were

X-ray powder patterns were measured with a

Phillips horizontal diffractaneter using nickel filtered Cu radiation. Tar[lerature-prograrmed reduction profiles were measured in the usual way.

The

heating rate was 7 K minute-I, the gas mixture contained 5 or 25% H in argon, 2 3 -1 and the gas flow rate was 10 an minute • RESULTS AND DISCUSSION Our objective in this work has been to canpare the preparation of titania and

silica-supported Ni catalysts in order to gain insight into the nature of the interactions between titania and metals.

Three methods of catalyst preparation

have been used - impregnation, deposition/precipitation, and ion exchange - in order of increasing degree of interaction between the support and the metal precursor. Wet impregnation Little adsorption of Ni onto silica or titania surfaces is to be expected fran acidic solutions, and this has been confirmed under our conditions by a spectrophotanetric investigation of the adsorption equilibrium.

Therefore, the majority

of the Ni taken up by the support during wet impregnation consists initially of a deposit of nickel nitrate.

HClINeVer, when the materials are dried to rarove

excess solvent, dehydration of the nickel nitrate crystallites may lead to an interaction with the support as the Ni tries to recover its co-ordination sphere. Figure 1 shows the nitrogen adsorption isothenns for the supports and sane of the catalysts, and the pore size distribution (PSD) for the silica samples. sunmarises the relevant surface area and porosity data.

Table 1

The Ni/Si0

2 catalyst

has the sarre BEl' surface area as the original silica, but a snaller internal

surface area. pores.

The PSD shows that the catalyst has a larger fraction of narrow

This suggests that during drying the nickel nitrate tends to wet the

support and spread out.

In the case of titania, the support is non-porous, but

the introduction of nickel nitrate results in the fonnation of mesopores.

is accanpanied by a substantial reduction in the surface area.

This

These effects

are thought to occur because the deposited nickel nitrate CCllqJOUIlds act as an 'adhesive' to hold the primary titania particles together, as shown in Figure 2.

314

b.

Q

...

11) Q)

O'l

.... 0

-

~500

....0..0

"tJ

....

Q)

.Q

Q)

~250

~ ~

{l 0

~C\j

05

1·0

1-0

P/Po Fig. 1. Nitrogen adsorption isothenns for silica (a) and titania (c) catalysts, and pore size distributions for silica samples (b).

TABLE 1 surface areas of the supports and the Ni catalysts 2 SBmI -1 m (g support)

sal 2 I -1 m (g support)

Silica

286

254

Ni9.8Si

285

230

Sample

Titania

52

30

Nil.OTi

48

29

Ni4.7Ti

46

26

Ni9.8Ti

33

22

a interna l surface area.

Fig.

2. Fonnation of a secondary titania structure after liIpregnation.

315 Thennogravimetric analysis of these catalysts shows (Table 2) that after drying, the average number of water molecules retained by the deposited Ni nitrate is in

the range 1-3, Le. the hexahydrate has becane dehydrated. TABLE 2 Experimental and calculated weight loss of Ni catalysts during reduction Sample

calculated wt , lossa/rng

Measured

wt. loss/rng

3

2

1

0

NiO.94Si

2.39

2.37

2.13

1.89

1.65

Ni4.8Si

10.12

11.42

10.26

9.11

7.96 14.77

Ni9.8Si

17.06

21.20

19.06

16.91

Nil.OTi

2.46

2.49

2.24

1.99

1.74

Ni4.7Ti

6.50

11.24

10.10

8.97

7.83

Ni9.8Ti

17.18

20.45

18.38

16.32

14.25

~umbers

refer to number of water molecules retained by the nickel.

X-ray diffraction of the Ni9.8Si catalyst indicated the presence of NiCOH)2.Ni(N03)2.2H20 (making up about 70% of the deposited material, Ni(OO)2 (about 5%), and a third phase which resenbled NiO. X-ray diffraction indicated that Ni (00) 2.Ni (00

For the Ni9.8Ti catalyst,

2.2Hp made up about 40% of the

3) total nickel, Ni (00) 2 about 30%, and Ni (N03) 2' 4H the remainder. There was no 20 evidence of direct canpound formation between the titania and the nickel.

Even allowing for the fact that the metal loading for a given surface area of

support differs for the two sets of catalysts, we conclude that for impregnated catalysts there is little interaction between nickel and titania. Deposition/precipitation ceus and co-workers (13,14) have dem:mstrated that useful information on the degree of interaction between a metal ion and a support can be obtained by monitoring the pH of the solution as hydroxide ions are gradually introduced.

We

have used their method to ccmpare the properties of silica and titania. (a) addition of NaCH Figure 3 shows the changes in pH as NaOH is added to distilled water, a nickel nitrate solution, and to suspensions of the supports in distilled water or nickel nitrate solution.

I f the addition of hydroxide ions did not lead to

the interaction of Ni ions with the support, then the measured pH curves should be equivalent to the sum of the curves for the nitrate solution and the support alone.

Figure 3 shows that the experimental curves differ markedly fran the

calculated curves (shown as broken lines in Figure 3), especially in the case of

316

g

Q

H2O

9

Ni 2 + 7

P

I

Si 02+ Ni

0-

2 +

5

3 50 Fig.

75

100 50 5 OH X 10 Imoles

75

-

100

3. 'l'itration curves for the silica and titania systems at 298 K.

titania.

It is inferred that for both supporta there is an interaction which

ccmrences at pH 5.2 for silica and pH 4.2 for titania. of Geus and Hennans (14) on Ni/Si0

By analogy with the work

catalysts we conclude that the precipitation

2 of Ni hydroxide on the silica surface began at podnt; P on Figure 3 (a) . titania, there is a similar change in slope at a pH of 5.4.

For the

These experiments

show that the adsorption of Ni ions onto a titania surface occurs at a pH of 4.0,

and that this is most probably followed by a smooth change over to give a

precipitate attached to the surface of the support.

XRD measurerrents on the

products of these preparations failed to'identify the structure of the precipitate, no lines could be detected.

Chemical analysis showed that the Ni content of roth

the silica and titani.a-suppor'ted materials was 0.9%, indicating that for the

addition of a given arrount of hydroxide the amount of nickel deposd.ted is independent of the support.. (b) hydrolysis of urea Figure 4 shows the change in pH with time as hydroxyl ions are generated by the hydrolysis of urea at 353 K.

The pH curve for water corresponds to the hydrolysis

of urea to give amronium carbonate. curve is obtained.

When silica is present, a broadly similar

With titania, however, the pH rises above the value for

317

6

HD

9

Si0

2

Ni + SiD2 +Ni 2+

2

5 4 I

c.

0·5 Fig.

1·0 Time Ih

1·5

4. Urea hydrolysis curves for the silica and titania systems at 353 K.

distilled water.

'!'his is thought to be due to the adsorption of carbonate ions

on the titania, which is known to contain basic hydroxyl groups. When the experiments are perfonned in the presence of nickel nitrate solution, quite differen\ curves are obtained, especially in the case of titania.

For both

systems the pH curves exhibit transient max.ilI1a (not illustrated in the titania case) characteristic of a nucleation barrier.

Such maxima were absent for Ni

nitrate in the absence of a support, and are stringly indicative of a supporcNi ion interaction.

XRD again failed to show evidence for any crystalline phases,

even th)ugh in this case the Ni content was about 3%. We conclude that this methcrl of preparation gives w=ll dispersed Ni both for silica (13-16) and for titania supports. Ion exchange Under the oonditions used in the ion exchange experiments it was found that the silica had adsorbed 95% and the titania 40% of the available nickel.

If

318 allowance is made for the different surface areas of the supports, these values correspond to 25% and 60% coverage. XRD

~

However, although for the silica catalyst

only very broad lines oorresponding to Ni silicate (12), in the case

of titania well defined lines oorresponding to Ni hydroxide, Ni Oxide, and Ni nitrate were observed.

It is apparent that this method of preparation leads to

extensive interaction between Ni ions and silica, presumably because under alkaline conditions the silica has a tendency to dissolve.

In contrast, the

interaction with thil titania appears to be limited to creation of a surface on which Ni oxide and Ni hydroxide can deposit.

There is no evidence for the

formation of Ni titanates. Temperature-prograrrrred reduction The influence of the method of preparation on the reducibility of sane of these catalysts has been investigated by TPR.

Fiqure 5 shows TPR profiles for

uncalcined sarrples of impregnated and ion exchanged catalysts.

These results

daronstrate rather well the differences between the Ni corpounda formed in the case of silica and titania.

In the case of the impregnated catalysts, the sharp

peaks at about 590 K have been identified as being due to the reduction of Ni oxide, the remainder of the profiles being indicative of the reduction of supported Ni oxide.

The ion exchanged Ni/Si0

catalyst is very difficult to

2 reduce, and is canparable in reduction characteristics to Ni silicate.

Significantly, however, for the ion exchanged Ni/ri0

catalyst reduction is much

2

more facile, confinning the absence of strong interactions with the support. CDNCLUSIOOS These experiments have demonstrated that a titania surface is much less reactive towards metal ions than a silica surface, which is often itself considered to be rooderately inert.

In particular, it has been shown that

titania-supported nickel catalysts, whether prepared by impregnation, deposition/precipitation, or ion exchange have little tendency for reactive interaction between the netal Lens and the support.

Even under oonditions

where there is a strong adsorption of Ni ions by the titania surface there is little direct evidence for the formation of titanates.

319

s

c 0

~

a. .~

..

E ::J Ul

C 0

u

c

~

Q)

Cl 0

L.

-g, ::c

g

473

673

873

T/K Fig. 5. 'I'E!rperature-progranrned reduction profiles for uncal.ctned , :impregnated and ion exchanged catalysts. (a), 10.7%Ni/Si0 i.rrpregnated, (b), 9.2%Ni/Si0 2 ion exchanged, 2 (c), 13.8%Ni/I'i0 inpregnated, (d), 4.0%Ni/I'i02 ion exchanged. 2

ACKNCMLEDGEMENI

R.B. thanks Amax Inc., and A.R.F. thanks the States of the Island of Jersey for financial support.

we

are grateful to Degussa and W.R. Grace for supplying

samples of the catalyst supports.

320

1 2 3 4 5 6 7 8 9

G.C.A. Schuit and L.L. van Reijen, Advances in Catalysis, 10(1958)242. K. r.brikawa, T. Shirasaki and M. Okeda, Advances in Catalysis, 20(1969)97. S.J. Tauster, S.C. FUng, R.T.K. Baker and J.A. Horsley, Science, 211(1981)1121. S.J. Tauster, S.C. FUng and R.L. Garten, J. Amer. Chem. Soc., 100(1978)170. P. ~iaudeau, B. Parmier and S.J. Teichner, C.R. Acad. Sci., C289(1979)395. R. Burch and A.R. Flambard, J. Chern. Soc. Chern. Communications, (1981)123. R. Burch and A.R. Flambard, React. Kinet. Catal. Letts., 17(1981)23. R. Burch and A.R. Flambard, suhnitted to J. Catal. (1982). J.R. Anderson, 'Structure of Metallic Catalysts', Academic Press, London, (1975)17110 V.A. DZis'ko: Kinet Catal., 21(1980)207. 11 M.S. Borisova, B.N. Kuznetsov, V.A. Dzis'ko, V.I. Kulikov and S.P. Noskova, Kinet. Catal., 16(1975)888. 12 M. Houalla, F. Dellanney, I. Matsuura and B. DelIron, J. Chern. Soc. Faraday I, 76 (1980) 2128. 13 J.A. van Dillen, J.W. Geus, L.A.M. He:rrnans and J. van der Meijden, Proc. 6th Int. Congr. Catal., London, 1976. (Eds. G.C. Bond, P.B. wells and F.C. Tompkins), 2(1976)677. 14 L.A.M. Hermans and J.W. Geus, Proc. 2nd Int. Sympositml, IJJuvain-1a-Neuve, 1978. (Eds. B. Delmon, P. Grange, P. Jacobs and G. Poncelet), (1979)113. 15 J.T. Richardson and R.J. Dubus, J. Catal., 54(1978)207. 16 J.T. Richardson, R.J. Dubls, J.G. Crump, P. Desai, U. Osterwalder and T.S. Cale, Proc. 2nd Int. Sympositml, IJJuvain-la-Neuve, 1978. (Eds. B. DelIron, P. Grange, P. Jacobs and G. Poncelet), (1979)131. 17 M. Primet, J.A. Dalmon and G.A. Martin, J. Catal., 46(1977)25. 18 M. Primet, P. Pichet and M.V. Mathieu, J. Phys. Chern., 75(1971)1221. >

321 DISCUSSION J. KIWI: 1. To which temperature did you heat the Ni(N03)2 on Ti02 and how long, since you report that no NiTi03 has been formed in your systems? 2. In Fig. 1 you report nitrogen adsorption isotherms for silica that at the left hand side evidence the outside BET area and towards the right show evidence for inner pores in this material. How did you assess the reported curve for the inner pores you report? Did you try titration methods to determine the contribution of the internal surface area since H20 has a diameter of 2.8 ~ as compared with N2 having 14 A2 in particle surface? A.R. FLAMBARD 1. The catalyst precursors discussed in this paper were all uncalcined, that is to say they were only subjected to a mild drying stage (16 h at 393 K). However, I have carried out separate experiments (Flambard A.R., Ph. D. Thesis, University of Reading 1982) into the effects of calcination on titania - supported nickel catalyst precursors and have not observed the presence of any detectable nickel titanate - type phases after calcination at temperatures up to 873 K. 2. The N2 BET isotherm for the silica shown in Fig. 1 (a) shows many of the characteristics of a Type IV isotherm, which is not uncommon for xerogels. The pore-size distribution curves of Fig. 1(b) were determined by use of the Kelvin equation, assuming cylindrical pores and a liquid-solid contact angle of zero. We did not investigated the possibility of using titration methods in order to determine microporosity. R. SIGG What is the difference between you BET surface area and the internal surface area? (Table 1). A.R. FLAMBARD: The difference between the BET surface areas and the internal surface areas listed in Table 1 arises from the use of two different models and hence equations (the BET and the Kelvin equations) for their determination. Although the surface area of a porous material is largely composed of contribution from the surfaces of the pores, the BET model will estimate the external surface as well. The Kelvin model will only estimate the surface due to the walls of the pores. J.W. JENKINS It would seem as though you are comparing the different surface reactivities of titania prepared by flame hydrolysis and silica prepared by precipitation. Have you had a chance to look at a flame hydrolyzed silica or a precipitated titania gel ? A.R. FLAMBARD: It is true that the titania and silica samples used as catalyst supports in this investigation were prepared by different methods. We have not investigated a flame hydrolyzed silica and only briefly looked at some precipitated titania gels because of the following reasons. Firstly, the precipitated titania gels that were available to us were all of a low surface area « 10 m2g- 1) and prepared from titanium (IV) sulphate. As the results presented in this paper represent only a fraction of those assimilated during an intensive investigation which was primarily geared to look at the active catalysts (i.e. after reduction), then these precipitated titanias were, for obvious reasons, considered unsuitable. Scondly, the function of the silica supported materials were to act as references. Therefore, in order to be in line with most of the literature, a precipitated silica was employed. I would consider a material prepared by flam hydrolysis to be perhaps more reactive than a material prepared by precipitation, so that it may be speculated that the use of a silica prepared by the former method would show up the differences between silica and titania even more effectively. G.C. BOND: With reference to the TPR plot of the ion-exchanged Ni/Ti02 catalyst (Fig. 5(d» ,it appears as if there may be an uptake of H2 in excess of that required to reduce Ni 2 + to Ni o• Is this your opinion also? If so, what degree of reduction of the Ti0 2 does it correspond to ?

322 A.R. FLAMBARD: We have carried out quite a detailed investigation into the reducibility of titania - supported nickel catalyst precursors, both before and after calcination. I am of the opinion that these measurements indicate that there is srnne surface reduction of the support. However, this does not imply the formation of'such phase as Ti4C7 (the Magneli phases) and indeed our results indicate that the support surface is only reduced as far as TiOl.98' This may have important consequences as far as the SMSI effect is concerned (Burch, R. and Flambard, A.R., submitted to J. catal. 1982). For the uncalcined catalyst whose TPR profile is shown in Fig. 5(d), this support reduction is difficult to observe directly because of the presence of peaks due to the reduction of nitrate decomposition products. The reduction of this nitrate is believed to account largely for the ~cess hydrogen consumption in this case.

323


INFLUENCE OF PHOSPHORUS ON THE HDS ACTIVITY OF Ni-Mo/y-A1

Z03

CATALYSTS

D. CHADWICK, D.W. AITCHISON, R. BADILLA-OHLBAUM and L. JOSEFSSON Department of Chemical Engineering and Chemical Technology Imperial College, London, SW7 ZBY (U.K.)

ABSTRACT A series of coimpregnated

P~Ni-Mo/y-A1Z03

catalysts have been prepared with

various phosphorus loadings and their activities for thiophene HDS measured. HDS activity is found to increase with phosphorus content reaching a maximum at about 1% wt P.

The catalysts have been characterised by a number of techniques

including XPS.

XPS studies show that the phosphorus is in monolayer form and

that it influences the repartition of Mo in the catalysts.

No evidence was

found for the involvement of phosphorus in a sulphide species.

INTRODUCTION The properties of Co-Mo and Ni-Mo catalysts have been widely investigated with respect to hydrodesulphurisation (HDS) reactions.

Efforts have also been

made to find additional promoters which can increase further the HDS activity of these catalysts.

For the Co-Mo system, no such promoters have emerged, but

phosphorus has been reported to improve the performance of Ni-Mo catalysts in HDS and in hydroprocessing (refs. 1-5).

However, the promotion effect of phos-

phorus on HDS activity has not been properly charted and the structures of phosphorus containing catalysts have not been established. The present paper describes preliminary results from a study of P-Ni-Mo/y-A1 catalysts which was undertaken in order to clarify the influence of phosphorus on HDS activity.

The results presented here are for a series of coimpregnated

catalysts with a Ni and Mo loading common in industrial catalysts.

Catalyst

activities for thiophene HDS are reported and are discussed in relation to characterisation studies by physico-chemical techniques including X-ray photoelectron spectroscopy.

Z03

324 EXPERIMENTAL Catalyst preparation A series of P-Ni-Mo/y-Al

catalysts with various P loadings were prepared Z03 by coimpregnation using the dry soaking method with a solution containing Ni(N0

(NH Mo and H In each case the initial pH of 3P04• 3)Z.6HZO, 4)6 70Z4.4HZO the impregnating solution was Z. For the catalysts containing small P loadings,

a small amount of nitric acid was added to ensure pH was Norton SA-6l75 (surface area

=

particle size range of 30-50 mesh.

~

Z59 mZg, pore volume

Z.

The y-Al support Z03 3/g) cm with a

= 0.55

All catalysts contained a constant Ni:Mo:Al

molar ratio based on 4% wt Ni (as NiO) and 15% wt Mo (as Mo0 The catalysts 3). were dried in dry air at 383 K for 7Z hours and 'calcined at 673 K for 8 hours in dry air at a flow rate of 400

m~/min.

After calcination the catalysts were

analysed for phosphorus content.

Catalyst characterisation A microbalance system was used to measure N BET surface areas, pore-size Z distributions and surface acidity using pyridine adsorption (refs. 6,7). X-ray photoelectron spectra were obtained with a VG ESCA-3 using AlK radiation. a Binding energies were referenced to Al Zp = 74.5 eV.

Catalyst activities Activities for thiophene HDS were measured in a tubular reactor at atmospheric pressure and 6Z3 K using a total flow rate at SOD molar ratio of 13.

Catalyst charge was 3 g.

m~/min

and hydrogen/thiophene

Catalysts were pre-sulphided for

1 hour at 573 K in 10% v/v HZS in HZ at a flow rate of 100

m~/min.

The steady

state conversion was determined from the disappearance of thiophene.

RESULTS Catalyst activities In order to examine the effect of phosphorus on thiophene HDS, the catalysts were prepared with a constant Ni:Mo:Al molar ratio based on 4% wt Ni (as NiO) and 15% wt Mo (as M00 This loading and P loadings in the range 0-Z.4l% wt P. 3) corresponds to a P/Mo atomic ratio in the range 0-0.74. A consequence of the constant Ni:Mo:Al molar ratio is that there is a slight reduction in the amount of Ni and Mo per gram of catalyst as the phosphorus loading increases. The apparent kinetic constants for thiophene HDS at 6Z3 K and atmospheric pressure were calculated from the measured conversions assuming first order kinetics and plug flow.

The results are shown in Fig. 1.

The HDS activity

was observed to increase steadily with phosphorus content reaching a broad

325

0·6

~

,;:::

.

+'

0.&0

c

........ Q

I'll

'8

0.4 0

,5 ~

0.2:0

• • I

0.00

undoped Li

Na

K

Rb

Cs

Be

Mg

-

Ca

I Sr

Ba

(b> 1.50

E c

Q

.. :e

,....ftI 8

1.00

It:

'-' LL

undoped

Li

Na

K

Rb

Cs

1.50

(c)

E

c 1.0

.. ~

,....I I 8

It:

'-' LL

0.50

0.00

w1doped Be Mg Ca Sr Sa V'ariation of the F(Roc,)750 for the M-2.92-Si0 M-O.9842-600-Co-610(a), (M:a.c., b) and M-O.984-A1 (M:a.e.c., c). -A1 203-600-Co-620 203-600-Co-600 Fig. 4.

350

a given time and temperature of calcination, the amount of "cobalt oxide" located on the support surface

is minimized in those specimens with medium X values,

Only a few exceptions from this general trend have been observed.

However, the

exact X value at which the minimum amount of "cobalt oxide" is observed, varies with the kind of p.Lc,

The dependence of "cobalt oxide" concentration. on the

kind of the p.t.c. at relatively high temperatures was different for the specimens based on Si0

as compared with those based on y-A1 To discuss this de203, 2 pendence,let us consider typical examples. Fig. 4 illustrates the variation of

the F(Roo)750 for the M-2.92-Si0

M-O.984-AI (M:a.c., 203-600-Co-600 2-bOO-Co-6l0(a), (M:a,e.c., c). An inspection of this figure de-

b) and M-O.984-AI 203-600-Co-620 monstrates that concerning the Si0 a decrease of . qUlte wea k'In -t -t -t Na , K , Rb , sed catalysts K-f- , Rb-t , Cs-t- ,

based specimens, ~ the p.t.c. bring about 2 the "cobalt oxide"/ [Co-Si0 -tco~-t] critical ratio. This effect is 2 d d speclmens, . becomi t h e Be2-t , Mg 2-t , L,-t lope ecomlng conslLdera bel 'In t h e 2-t 2-t 2-t Ca , Sr and Ba modified samples. With regard to y-A1 ba203 2-t] 2-t one may observe that the doping by [Li-t, Be and Mg and [Na~ . an increase and a decrease of the Ca2-t Sr 2-t and Ba 2-t] results In

"cobalt oxide"/ [CoAI -tCo~-t] ratio, respecti vely These trends concerning the 204 dependence of the ratios on the kind of the p,t.c. hold at almost all temperatures studied in excess of 600 0C.

The values of the ratios at T>600

ned by the rate of two processes, namely

0C

are determi-

the direct "CO(N03)2-support" reaction

observed at low temperatures and the decomposition of the "cobalt oxide". The 2-t rate of the first process as already mentioned, decreased after Li-t, Be and 2-t Mg doping while following modification by the other p.t.c. increased. Investigation of the second process requires to' determine

the activation energies

of the "cobalt oxide" decomposition over the various supports.

Such determina-

tions were carried out for a large number of samples using a method previously described (refs. 8,18). based specimens

The activation energies determined for the dopedy-Al 203 were smaller than those obtained for the samples prepared using

pure y-A1

Thus, for instance, the activation energies determined for the 203, M-O.OOO-Al Be-O.984-A1 Mg-O.984-A1 Ca203-600-Co-Z, 203-600-Co-Z, 203-600-Co-Z, -O.984-A1 Sr-O.984-A1 and Ba-O.984-AI were 203-600-Co-Z, 203-600-Co-Z 203-600-Co-Z found to be 118, 79, 83, 63, 32, 70 KJ,mol-~ respectively On the contrary,the

activation energies determined for the modified specimens based on Si0

were 2 higher when compared with those obtained for samples synthesized using pure Si0 2• To take an example, the activation energies determined for M-O.000-Si0 -600-Co-Z, 2 Be-4.63-Si0 2-600-Co-Z, Mg-4.63-Si0 Ca-4.63-Si0 2-600-Co-Z, Sr-4.63- Si 02 2-600-Co-Z, l, -600-Co-Z and Ba-4.63-Si0 were 8,25,83,75,91,80 KJmolrespectively, 2-600-Co-Z

M-X-AI20iY-Co-500.

To study the influence of the calcination temperature

351 after p s t c , impregnation on the "cobalt i

oXide"/[coA12olf+co~+J

critical ratio,

Some specimens have been synthesized with various Y values. In all the cases examined,it was found that the action of the p.t.c. mentioned required heating of the doped carriers

before Co deposition, above a critical temperature.(usually oC). Otherwise,the p.t.c. were found to be almost inacti-

centered at about 500 ve.

To take some typical examples {M-O.OOO-A1 Rb203-600-Co-600:F(Roo)750=1.20, -0.984-A1 20S-SOO-Co-600:F(Roo)750=1.22 and Cs-0.984-A1 20S-SOO-Co-600:F(Roo)750 = 1.2S} but {Rb-O.984-A1 20S-500-Co-600:F(Roo)750=0.61 and Cs-O.98lf-A1 20 3-500-Co-600:F(Roo)750=O.82 (ref. 14). Na-CHSCOONa(aq)-0.98lf-A1203-600-Co-600, Na-CH3COONa(b)-0.984-A1 203-600-Co-600 Na-NaOH(aq)-O.984-A1 20 3-600-Co-600.In order to examine whether the anion of

the salt used for the introduction of the p.t.c. into the support participated to the mechanism

of the Co species evolution, we synthesized some specimens

using the title compounds and the F(Roo)750 values were measured.

The results

obtained are compiled in Table 1. TABLE 1 F(Roo)750 values obtained for some specimens prepared (ref. 15)

Sample

F(Roo )

M-0.000-A1203-600-Co-600 Na-O.98lf-A1203-600-Co-600 Na-CH3COONa(aq)-0.984-AI203-600-Co-600 Na-CH 3COONa(b)-O.984-A1203-600-Co-600 Na-NaOH(aq)-0.984-A120S-600-Co-600

1.20 0.52 0.50 0.51 0.45

An inspection of this table demonstrates that there is no

important ef-

feet caused either by the negative part of the salt used or the solvent employed.

by inverse and co-impregnation (refs. 10, 14)

Catalysts prepared

To investigate whether the order of the impregnation procedure determine the "cobalt oXide"/[coA120lf+co~+J

ratio, some specimens prepared by inverse or co-

impregnation have been synthesized. is a key factor: "cobalt

It was found that the order of impregnation

even with X values> 0.984, the disturbances caused on the

oXide"/[coA1204+Co~+]

ratio ar-e negligible.

se and co-impregnation as well.

This is true for the inver-

Thus for example, the F(Roo)750 values obtained

for the catalysts M-0.OOO-Al20 3-600-Co-600, Co-A1203-600-Rb-O.984-600, Co-A120 3-600-Cs-0.98lf-600, Rb-0.984-Co-AI 20S-600 and Cs-0.984-Co-A1 203-600 were 1.20,

352 1.20, 1.21, 1.26 and 1.30, respectivelY

(ref. 14).

DISCUSSION Novel mechanistic schemes The results presented demonstrate

that the mechanistic routes proposed ear-

lier [see fig. IJ fail to describe the real evolution of the Co species on the y-A1 203 and Si0 2 surfaces. Therefore, two novel mechanistic schemes are' proposed (Fig. 5), which take into account the most important observations, notably the direct formation of tetrahedral Co2+ species [step a]. nisms a discussion of the

phenom~na

Based on these mccra-

observed will be attempted.

The effect of modifiers on the rates of the various steps of the mechanistic routes Modification of the supports by Be 2+ ,Ll. + and Mg 2+ decreases the ratio

[ ra-

te of the step(a)/rate of the step(b)], whereas doping by the remainder p.t.c. increases it.

This explains the observed ratios ["cobalt oxide'Ytetrahedral co-

balt] at relatively low temperatures where the rate of cobalt oxide decomposition [steps (d) and/or (e)J is too low.

At relatively high temperatures the ra-

te of this decomposition becomes important thus contributing to the determination of the ratio mentioned above. In Si0 2 based specimens, all the p.t.c. 's increase the activation energy of "cobalt oxide" decomposition; therefore, in the temperature region above the isokinetic point, p.t.c. increase the rate

of

steps(d) and/or (e).

This, in

conjunction with the effect caused by p.t.c. on the rate of the step(a) explains + Be2+ ,Mg 2+J and [Na +, the slight and dramatic decrease brought about by [ Li, + + + 2+ 2+ 2 + ] . [ ] [ ,respectlvely, on the "cobalt oxide" / tetraheK , Cs , Rb ,Ca ,Sr ,Ba dral cobalt] (Fig. 4a). In the y-Al based specimens ,all the p. Lc. ',s decrease the activation energy 203 and consequently the rate of "cobalt oxide" decomposition. Thus, an increase 2 of the mentioned ratio of cobalt species is expected after doping by Be +, Li+ 2t and Mg [the rate of the step (a) is decreased] (Fig. 4b, c). On the other hand,the decrease of this ratio caused after modification by the other p. t.c. is the overall effect of two antagonistic processes ,namely the augmentation of the rate of step(a) and the decrease of the velocity of the steps(d) and/or (e) (Fig. 4b,c). An interpretation of the results using a tentative model The above considerations showed that the regulation of the ratio [rate of

353 direct reaction with 5i0 (a)

2

I----------~·~Co-Si:::r

(a) reaction with 5i0

2

(b)

decomposition ~------------~CoO/Si02

direct reaction with

(b)

(b)

decomposition

Fig. 5. Two novel mechanistic schemes proposed to describe the evolution of cobalt species on the surface of Si0 (a) and y-A1 (b) based specimens. 203 2

354 the step (a)/rate of the stap (b)J is a key factor for obtaining a final control on the Co species. to

On the other hand,the experimental results related

the influence of the mode of impregnation, calcination temperature before

Co deposition and kind of the p.t.c.-salt used cies, showed that p.t.c.-entities

on the evolution of the Co spe-

formed above a critical temperature on/with

support surface are responsible for the regulating capability of the p.t.c. To explain the influence of the various "dope parameters" on the ratio

of

the rates mentioned, we propose the following model based on the existence of these p.t.c.-entities. (i) P.t.c.-entitie~ can regulate the local surface pH of the support and . . 0 f Co2+ a d sor b e d to Co2+ preclpltate " d [h consequent I y t h e ratlo t e I atter preclpitates as Co(N03)2]' (ii) During the first or second impregnation the p.t.c.-entities hydrolyse slightly the surface of the carriers and promot~

the evolution of the surface

2-

2

species (for example A1204)' These species can react with Co + to form pounds in which C0 2+ has tetrahedral symmetry (for example CoA1204)' (iii) tion

com-

P.t.c.-entities serve as centers of nucleation or blocking the adsorp-

agents.

They favour the precipitation of CO(N0

If one takes in mind that Co(N0

agglomerates. 3)2 agglomerates are transformed into "cobalt

3)2 2+ oxide" aggregates during heating whereas the C0 adsorbed react with support 2+

species to form tetrahedral C0

surface compounds, one can understand the con-

tributionof the (i), (ii) and (iii) to the control of the ratio G:'ate of the step (a)/rate of the step (b)J:

the rates of the steps(a) and (b) increase due

to (ii) and (iii),respectively.

In most cases, the rate of step (a) increases

due to (i).

In fact, the regulation of the local pH by p.t.c.-entities usually

favours adsorption over precipitation. The experimental results obtained show that mechanisms (i) and (ii) prevail . t h e case 0f ' + Rb+, Cs+ ,Ca 2+ ,Sr 2+ an d Ba2+J In speclmens mod lLf ile d by [Na+, K, 2+

whereas mechanism (iii) is predominant in the samples doped with [Li+, Be Mg

2

ij .

and

Moreover, the results obtained demonstrate that mechanism (iii), at d i f ilcatlon, i on b ecomes lmportant . , sammOl In

• t h e case 0 f L'+ 1 east In l , Be2+ an d Mg 2+

i

ples containing relatively high p.t.c. content.

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355

8. A.Lycourghiotis,D~attis and Ph.Aroni, Z.Phys.Chem. (N.r.), 120(1980) 211. 9. A.Lycourghiotis, D.Vattis and Ph.Aroni, Z.Phys.Chem. (N.r.), 121(1980)257. 10. A.Lycourghiotis, C.Defosse and B.De1mon, Bu11.Soc.Chim. Be1g., 89(1980)929. 11. C.Defosse, M.Houa11a, A.Lycourghiotis and r.Delannay, in T.Seiyanna and K.Tanabe (Eds.) Proc. 7th Int. Cong.Catalysis, Tokyo, 1980, Elsevier, Amsterdam, 1981, p.l08. 12. r.Delannay, C.Defosse, M.Houalla, A.Lycourghiotis and B.De1mon, in R.Langer (Ed.), Proc. 12th Swedish Symp. on Catalysis. Perspectives in Catalysis, Lund, October 11th, 1979, p.85. 13. A.Lycourghiotis, D.Vattis and N.A.Katsanos, Z.Phys.Chem. (N.r.), 125(1981) 239. 14. A.Lycourghiotis, D.Vattis, Ph.Aroni and N.A.Katsanos, Acta Chimika (in press). 15. A.LJcourghiotis, React.Kinet.Catal.Lett., 17(1981) 165. 16. A.Lycourghiotis, A.Tsiatsios and N.A.Katsanos, Z.Phys.Chem. (N.r.), 126(1981) 95. 17. A.Lycourghiotis, A.Tsiatsios and N.A.Katsanos, Z.Phys.Chem. (N.r.), 126(1981) 85. 18. A.Lycourghiotis, M.Kotinopoulos, N.A.Katsanos and G.Karaiskakis, React. Kinet.Catal.Lett. (in press).

356 DISCUSSION B. GRIFFE DE MARTINEZ: Have you tried to correlate the doping parameters of the elements that you have studied with the activity and selectivity of the catalyst on any catalytic reaction ? A. LYCOURGHIOTIS: Of course our final purpose is the extension of the control obtained on the Co-phase by altering the doping parameters to the sorptive and catalytic properties of the catalysts. Preliminary results obtained on the CO oxidation show that such a control could be achieved. N.P. MARTINEZ : I want to comment on the use of the Kubelka-Munk function and its straight line correlation with the concentration of the absorbing species. As Beer's law, K-M law is followed by low concentrations of the adsorbing material, in the case of a CoMo/alumina, we have found that K-M function is followed for the oxidic state until you have no more than 3.5% of cobalt as CoO. Have you determined the Kubelka-Munk function vs concentration? How much cobalt do you have in your catalysts ? A. LYCOURGHIOTIS I agree with you that the K-M,relationship is followed at quite low concentrations of optically active species. In the case of Co species, you have found that this relationship is followed until 3.5% of Co as CoO. Since our catalysts contain lower amount of active phase than yours the K-M relation must be held in our case. D. CHADWICK: Could you explain in more detail what you mean by accelerate and rate with respect to decomposition of Co salts. For example, how did you quantify these effects ? A. LYCOURGHIOTIS: In our text the terms acceleration and rate have the usual meaning. To understand the quantification attempted on the change of the rate of the Co (N03)2 decomposition, we must bear in mind that the solid state processes are in general kinetically controlled. Specifically, in the decomposition of the solid salts, thermodynamic equilibrium is achieved only seldom. Therefore, the difference in the concentrations of an optically active product (C0304 and/or CoO in our case) simply reflects analogous difference in the rates of the salt decomposition (Co(N03)2 in our case. Under constant time and temperature, the higher the rate of the Co(N03)2 decomposition, the higher is the concentration of the C0304 and/or CoO on the surface of the modified support and, therefore, the higher the value of the K-M function determined at 750 nm. F.S. STONE It is interesting to see the effects of pre-transition metal ions reported in this paper. However, I have two comments in regard to the reflectance measurements. First, it is doubtful whether the shoulder at 750 nm is "due to octahedral C0 2 + in CoO and/or Co 304", as is stated in the paper. Absorption in this region is more pften regarded (1) as being due to octahedral Co(III). A matter which is more relevant, however, is that absorption at 750 nm is influenced very greatly by Co(II)-Co(III) intervalence charge transfer absorption, which is intense and broad in the visible. This being so, any changes which increase the amount of CO(III) will increase F(Roo)750' and vice versa. This could therefore be a basis for using F(Roo)750 as a measure of C0304' as the author has done. However, and this is my second point, it is probably not justified to assume that Co(III), and hence Co(II)-Co(III) intervalence absorption is present only in the pure cobalt oxide phase. For example, in an aluminabased system, Co(III) could be present as a minor component in a surface aluminate phase such as the spinel Co(II) CO(III)x AI2-x04. Thus F(~)750 may not be a very precise measure of "cobalt oxide". (1)

P. Gajardo, P. Grange and B. Delmon, J. Catal., 63, 201 (1980).

357 A. LYCOURGHIOTIS: I do not fully agree with Professor stone that the absorption at 750 nm must be attributed to the presence of CO(III) rather than to Co(II) in octahedral symmetry, because a quite high value of K-M function is obtained at 750 nm even after calcination at qUite low temperatures, for instance at 140°C (1), where the oxidation of co2+ to Co3+ seems to be rather difficult. On the other hand, considerable absorption is observed at 750 nm after calcination at extremely high temperatures, where the concentration of Co3+ is quite high. Therefore, I think that we can at~ribute the absorption shoulder at 750 nm to the presence of both Co2+ and Co + in octahedral symmetry. Moreover, although the formation of CO(I'I) Co(III)x Al 2_x0 4 phase on yAl203 cannot be excluded, I believe that its concentration is to low to disturb considerably the magnitude of, the signal at 750 nm. (1) A.

~ycourghiotis

et al., unpublished results.



359

CRITERIA FOR THE EVALUATION OF BAUXITE AS CARRIER FOR LOW-COST HYDROTREATING CATALYSTS S. MARENGO, A. IANNIBELLO and A. GIRELLI Stazione sperimentale per i Combustibili, San Donato Milanese (Italy)

ABSTRACT The determination of the porous structure after thermal activation, the measure of the anionic exchange capaci~y with respect to ~lo(VI) and W(VI) species and the study of the functionality of the catalytic systems in model reactions have proved to be efficient criteria for a preliminary evaluation of natural materials as carriers for low-cost hydrotreating catalysts. Experiments performed with two bauxite samples, from a USA ore and a Southern Italian ore respectively, have made it possible to ascertain the possibility of util i zi ng the sampl e with lower i ron content and hi gher surface area as a support for catalytic systems with properti es comparable to those of more expensive alumi na-based cata lysts.

INTRODUCTION According to the most recent forecasts, petroleum will continue to occupy a predominant role in the next 20 years with increasing production of heavy crudes (1, 2). This trend, together with the increasing demand for conversion of residuum to lighter products (2), explains the growing interest in residuum hydroprocessing and hydrodesulfurization catalysts. The irreversible poisoning of the catalyst due to metal contaminants (principally V and Ni), weigh heavily on the cost of heavy crudes hydroprocessing. In the development of more effective catalytic systems, particular attention is being devoted to the nature of the support: novel materials such as modified aluminas, manganese nodules and bauxite have been the object of recent studies (2-5). In order to define efficient criteria for a preliminary evaluation of natural materials as carriers for low-cost hydrotreating catalysts, two samples of bauxite were considered: sample A from a USA ore, and sample B - rich in iron - from an ore from South Italy. Data obtained with a commercial y-alumina are included for comparison.

360

The following properties were studied: i) surface area, pore volume and pore size distribution after thermal activation; ii) anionic exchange capacity with respect to Mo(VI) and W(VI) species to ascertain the limiting amount of catalytic components which can be dispersed on the bauxite surface after calcination at 550°C; iii) functionality of the catalytic system with respect to the content of Mo(VI), W(VI), Co(II), Ni(II) and to activation procedure. EXPERIMENTAL SECTION Materials y-alumina Ketjen 000-1.5 E; commercial bauxite from a USA ore (A); natural bauxite from a Southern Italian ore (B). Ammonium heptamolybdate, ammonium dodecatungstate, nickel nitrate and cobalt nitrate Carlo Erba, pure reagent grade. Analysis Surface area, pore volume and pore size distribution were determined with Carlo Erba Strumentazione Sorptomatic 1800 Series and Mercury Pressure Porosimeter 200 Series. Molybdenum and tungsten in the solid phase were determined by X-ray fluorescence using a Philips Model PW 1540 spectrophotometer. For gas chromatographic analysis of the products of cyclohexene and thiophene conversion, a column packed with 15% squalane on chromosorb was employed. Adsorption of Mo(VI) and W(VI) The support was impregnated with molybdenum(VI) and tungsten(VI) aqueous solution by the step addition technique (6). A glass column (500 mm high, 4 mm into diam.) filled with the test material (particle diameter 0.1-0.3 mm) was first washed with water and then fed with a 3.8 10-2 Maqueous solution of ammonium paramolybdate or ammonium paratungstate; the pH of the solution at the column outlet was recorded as a function of the amount of fed solution. Catalytic tests The catalytic activity of the systems investigated was determined in a pulse microreactor placed in the vaporization chamber of a 2350 Carlo Erba ~as chromatograph in series with a chromatographic column. The catalytic bed consisted of particles with a diameter of 0.1-0.3 mm held between two plugs of quartz wool inside a stainless steel tube (80 mm long, 4 mm into diam.). The amount of catalyst corresponded to a surface areaof20m 2; the sizeof the pulse of liquid reactant was 0.5 ~l. The tests were performed at 375°C and atmospheric pressure in a H2 flow of 30 ml/h. Activation was carried out at 375°C according to the following procedures: a) heating for 2 h in a flow of He and reducing for 4 h in H2: b) heating for 2 h in He and sulfiding for 2 h with 10% H2S in H2'

361

RESULTS AND DISCUSSION Physicochemica1 properties of bauxites The two samples of bauxite considered are characterized by quite different physicochemica1 properties (table 1). TABLE 1 Chemical composition of bauxites A 57.4 10.5

A1203 Si02 t4g0 CaO Fe003 Fe MnO Cr2 O3 NiO Ti02 * H20 ( )

2.5 0.2

1.1 28.3

B 57 1.1 0.2 0.2 28 0.2 0.2 0.1 13

(*) weight 10ss at 500°C Sample A, from a USA ore, is a commercia1 product containing gibbsite as the main component before activation. In the active form, y-alumina prevails and the surface area is comparable to that of a commercial y-alumina (Fig. 1). Sample B, 300

BAUXITE B

200

400

1000

Activation

Fig. 1. Surface area as a function of activation temperature.

362

from a Southern Ita 1i an ore, conta ins ma i nly boehmiteamong the a1umi na compounds and is characterized by a relevant content of iron oxides. The raw material is in the form of pisolytes with a diameter of 5-30 mm and very low surface area; after crushing, washing with water and heating at 500-600°C, the surface area increases considerably, but, compared with type A, remains at a lower level. The pore size distribution of bauxite A is significantly different from that of the commercial y-alumina (Fig. 2); in particular, a consistent fraction of 0.6 (-ALUMINA

0.5

O"--..L---"----...L.----~---

10

102

103

.....

105

Pore rddius,;'

Fig. 2. Pore size distribution of bauxite A and y-alumina macropores is evidenced in A, which is expected to exert a relevant influence on the performance of the finished catalyst in the hydrotreating of heavy feedstocks. Adsorption of Mo(VI) and W(VI) The step addition technique was previously used to investigate the interaction of Cr(VI), Mo(VI) and W(VI) with alumina (6). The shape of the pH profile obtained in such experiments recalls the frontal analysis and gives information on the chemical processes occurring on the solid surface. The adsorption of Cr(VI), Mo(VI) and W(VI) onto y-alumina was described through an ion exchange process between Me0 4 species and surface hydroxyl groups of alumina. In Fig. 3 the pH curves determined in the addition of tlo(VI) and W(VI) to the different supports are reported. The curve relative to high-purity y-alumina shows only one well defined step, as hydroxyl groups are the only species that take part in the ionic exchange process. The curves produced by the two bauxite samples show different steps, reflecting elution of different anionic species from the surface (7) (e.g. the presence of Cr04-- in the effluent from bauxite B was ascertained).

363

7 Addition of MoM) T=25°C

6

o

2

8

10

12

14

16

pH

9

8

7

6

Addition of W (VI) T=60oC

m

o 2 4 6 8 Q ~ lli Total Me (VI) in the Fed solution [gat/g suppJ·10 4 Fig. 3. pH of the effluent in the step addition experiments. The higher steps of the pH profile can be correlated with the exchange of Me(VI) oxospecies with the surface hydroxyl groups. The results of adsorption experiments are reported in Table 2. The correlation between the amount of Me(VI) adsorbed and the surface area of the support suggests that Mo(VI) and W(VI) oxospecies are presents as surface phases with an equivalent degree of dispersion. In accordance with recent EXAFS studies, the presence of isolated Me0 4 species can be hypothesized (8). More-

364

TABLE 2 Amount of Mo0 3 and W0 3 adsorbed in the step addition experiments Support y-alumina Bauxite A Bauxite B

f1003 adsorbed wt % mo 1ecules . nm -2 9.3 6.6 2.6

1.6 1.3 1.2

1-1°3 adsorbed wt % 21 11.6

5.9

molecules.nm-2 2.2 1.4 1.7

over, it must be observed that the specific surface of the support changes only slightly after addition of molybdenum and tungsten, as can be expected for a process involving only the surface of the porous material. Thus, step addition technique makes it possible, by an efficient and relatively simple procedure, to investigate and control the adsorption process of different chemical species on a potential catalyst support. The practical implications of this result are relevant, as the method can easily be scaled up for preparative purposes. Catalytic activity The conversion of cyclohexene and thiophene were employed as model reactions for the evaluation of the catalytic properties of the bauxite-based catalysts. In the experimental conditions considered, the following reactions of cyclohexene were evidenced: i) hydrogenation-dehydrogenation to cyclohexane and benzene; ii) isomerization to methylcyclopentenes; iii) isomerization plus hydrogenation to methylcyclopentane; iv) hydrogenolysis to lighter hydrocarbons. Selectivity to product X was defined as moles of cyclohexene converted to X .100. moles of cyclohexene converted The products of thiophene hydrodesulfurization were HZS, n-butane and butenes. The following catalytic systems were tested: bauxite A and B, the systems obtained by step addition of ~10(VI) and Iv(VI) to bauxite, and the ternary systems prepared by adding, according to the pore filling procedure (9), Z wt % CoO or NiO to the Mo0 3/bauxite and W0 3/bauxite systems. Bauxite A shows prevailing isomerization activity (Fig. 4). Incorporation of Mo(VI) and W(VI) on the surface promotes hydrogenation activity; nevertheless, the system maintains a marked bifunctional character (isomerization plus hydrogenation) more evident in the W0 3/A system. In the case of an analogous alumina-based system, this behaviour was explained by hypothesizing that acid centers are associated not only with the support, but also with molybdenum and especially with tungsten species (10). In the sulfided state, Mo(VI) and W(VI) containing systems exhibit enhanced isomerization properties,

365

Fig. 4. Catalytic properties of bauxite-based catalysts. Selectivity in cyclohexene conversion: 1 hydrocracking, 2 benzene + cyclohexane, 3 methylcyclopentane, 4 methylcyclopentenes. Desulfurization activity: 5 thiophene conversion. compared to the reduced state, and a moderate desulfurization activity. Addition of cobalt and nickel produces a further increase of the hydrogenating function; the marked difference in selectivity between molybdenum and tungsten is maintained also in the ternary systems. Hydrodesulfurization activity is significantly improved in the ternary molybdenum-containing catalysts, while the NiO-W0 3/A system results as being less active in tiophene conversion. In general, the catalytic properties of the systems obtained from bauxite A are similar to those exhibited by analogous alumina-based catalysts (10). In the reduced form bauxite B shows marked hydrogenation and hydrogenolysis activity, attributable to its relevant iron content. In the sulfided form, isomerization activity prevails. Molybdenum promotes hydrogenation and desulfurization activity of the sulfided system, while tungsten is more effective

366

in enhancing isomerization. Cobalt and nickel promote hydrogenolysis activity in the reduced form and hydrogenation and desulfurization properties in the sulfided state. Nevertheless, in all systems prepared from bauxite B, the nature of the support limits activity in cyclohexene and thiophene conversion to a remarkable extent. CONCLUSION The physicochemical properties determined for bauxite A make this material a promising support for low-cost hydrotreating catalysts. More relevant is the possib~lity as confirmed by the above described results, of establishing efficient and relatively simple criteria for screening potential catalyst supports. Step addition experiments made it possible to study the surface chemical properties of two natural materials and to prepare systems in which the active components are present as surface species with a controlled degree of dispersion. Catalytic tests indicated the possibility of preparing, from properly selected natural materials, catalytic systems with properties comparable with those of more expensive alumina-based catalysts. ACKNOWLEDGEMENTS This work was carried out with the financial support of the Consiglio Nazionale delle Ricerche. The Authors thank Carlo Erba Strumentazione - microstructure applications lab - for surface characterization. Mr. S. Scappatura is thanked for valuable contribution in performing the experiments. REFERENCES 1 G.R. Brown, CEP, Sept. (1981) 9-15. 2 D.C. Green and D.H.Broderick, CEP, Dec. (1981) 33-39. 3 C.D. Chang and A.J. Silvestri, Ind. Eng. Chem., Process Des. Develop., 13 (1974) 315-316. 4 V. Berti, A. Iannibello and S. Marengo, Riv. Combultibili, 29 (1975) 121-134. 5 A. Iannibello, S. Marengo and A. Girelli, Riv. Combustibili, 33 (1979) 373-383. 6 A. Iannibello, S. Marengo, F. TrifirO and P.L. Villa, in B. Delmon, P. Grange, P. Jacobs and G. Poncelet (Eds.), Preparation of Catalysts II, Elsevier, Amsterdam, 1979, pp. 65-76. 7 A. Iannibello and F. Trifiro, Z. Anorg. Allg. Chem., 413 (1975) 293-304. 8 B.S. Clausen, H. Topsoe, R. Candia, J. Villadsen, B. Lengeler, J. Als-Nielsen and F. Christensen, J. Phys. Chern., 85 (1981) 3868-3872. 9 A. Iannibello and P.C.H. Mitchell, in B. Delmon, P. Grange, P. Jacobs and G. Poncelet (Eds.), Preparation of Catalysts II, Elsevier, Amsterdam, 1979, pp. 469-478. 10 A. Iannibello, S. Marengo and P.L. Villa, in H.F. Barry and P.C.H. Mitchell (Eds.), Proc. 3rd Int. Conf. on The Chemistry and Uses of Molybdenum, Ann Arbor, August 19-23, 1979, Climax Molybdenum Co., Ann Arbor, 1980, pp. 92-98.

367

DISCUSSION H. CHARCOSSET

what about the particle size of your catalysts ?

S. MARENGO: Before the physico-chemical characterization, the samples of bauxite were ground and sieved to particles with diameter of 0.1-0.3 mm. Also in the catalytic tests the same size of the particles was utilized. H. CHARCOSSET : Have you any idea about the dispersion (crystallite size) of Fe203 in your sample B (in relation to theoretically possible use of that bauxite as a catalyst for direct hydro liquefaction of coal) ? S. MARENGO : We carried out no further investigation on sample B till now. At present, our interest is mainly centered on the hydrotreating of heavy petroleum ~actions. In this connection, bauxite A has been considered more suitable for evaluation in the hydroprocessing of a petroleum residuum in a micropilot reactor. R.J. BERTOLACINI: I would agree with Dr. Charcosset's comment, that should be a good catalyst ,for coal liquefaction. I also suggest that because of its high iron content (28%), may be a good Fischer-Tropsch I would further add that Hydrocarbon Research Inc., USA, has patented as a hydrodemetallation catalyst for reduced crude feedstocks.

bauxite B bauxite B, catalyst. Mo-bauxite

N.P. MARTINEZ: You mention the possibility of using these catalysts for hydrotreating heavy crudes. Most literature which has been published on catalysts for heavy crudes says that big pores are necessary. According to the pore size distribution presented in your paper, I assume your catalyst would not be good enough to hydrotreat heavy oils. Would you please comment on that? S. MARENGO: In our investigation we started with considering mainly the chemical properties of our materials. Bauxite A, after addition of Mo(VI) and Ni(II) or Co(II), exhibited good catalytic properties with model compounds. In view of the utilization of these systems in the hydrotreating of residua, a second phase of this study should undoubtedly include the improvement of the pore size distribution in the direction that you indicated. S. VASUDEVAN You have shown that bauxite B has low activity but higher selectivity (for certain reactions). Have you compared these selectivities at isoconversions? Or does selectivity remain constant with conversion in your study? S. MARENGO: The pulse technique was utilized in the catalytic tests with the aim of evaluating a large number of catalysts in comparable experimental conditions. Our data refer to a fixed temperature and flow rate of the carrier gas (H2); till now, we have not studied in a systematic way the change of selectivity with conversion.


369

G. Poneelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts /11 e 1983 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands

PREPARATION AND PROPERTIES OF SUPPORTED LIQUID PHASE CATALYSTS FOR THE HYDROFORMYLATION OF ALKENES

H.L. PELT, L.A. GERRITSEN, G. VAN DER LEE AND J.J.F. SCHOLTEN .X) Department of Chemical Technology, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands

SUMMARY A description is presented of the preparation and characterization of supported liquid phase (SLP) rhodium catalysts for the hydroformylation of alkenes. Special attention is given to those aspects which do not play a role in classical heterogeneous catalysts, viz. the degree of pore filling and the adsorptive withdrawal of rhodium complexes at the support/liquid interface.

INTRODUCTION When a molten or dissolved catalyst is dispersed in a porous support we are dealing with a supported liquid phase catalyst (abbreviation: SLP catalyst). In the chemical literature a number of examples may be found of the industrial application of catalysts in the SLP form. One such example is the molten mixture of V

and K supported on diatomaceous pellets, used for the oxidation of 20 S 2S04 sulfur dioxide, and another is phosphoric acid impregnated on Kieselguhr, for

the gas-phase oligomerization of low molecular weight alkenes, which is probably the oldest SLP application (ref. 1). An interesting application of the principle of supported liquid phase catalysis is heterogenizing homogeneous metal-organic catalysts. In such case, the advantages of the homogeneous catalyst, viz. a high selectivity, a high degree of utilization of the precious metal and a high resistance against poisoning, are combined with the advantages of heterogeneous catalysis, viz. a continuous separation between the catalyst and the products and a circumvention of eventual corrosion problems, as is the case in the Wacker oxidation of alkenes. The .first publication about the use of a supported liquid phase catalyst in the field of organometallic complex catalysis is due to Acres c.s.

(ref. 2),

whQ isomerized 1-pentene with a solution of rhodium trichloride in ethylene glycol dispersed in Kieselguhr. In 1969 Rony (ref. 3)

reported on the hydro-

X)Author to wnom all correspul1Uence should be addressed.

370 formylation of propylene with RhCOCl(PPh

3)3 and brought into the pores of silica gel.

dissolved in benzyl butyl phtalate

Hydroformylation with a special type of supported liquid phase rhodium catalyst

was described extensively by Gerritsen and coworkers (refs. 4-8).

These investigators dissolved a Wilkinson-type catalyst, hydridocarbonyltris (triphenylphosphine) rhodium (I) , in one of the ligands of the complex, PPh

3, called the solvent-ligand. The activity and selectivity of these catalysts are high compared with known analogues. Moreover these catalysts exhibit a very high stability; in -fact nQ sign of deactivation was observed in the hydroformylation of propylene, even after runs of more than 800 hrs (refs. 9-10). Various types of support were studied , and a number of tertiary phosphines related to PPh

were used as solvent ligands. 3 It is the aim of the present article to discuss the preparation of SLP

rhodium catalysts and their characterization. Special attention will be given to a number of aspects which do not play a role in classical supported heterogeneous catalysts.

EXPERIMENTAL Materials RhHCO(PPh was prepared by the method of Ahmad (ref. 11). Triphenylphosphine 3)3 (Fluka, Switzerland, 99.5%) was used as received. Benzene (Merck, Germany, 99.7%) and toluene (Merck, Germany, 99%) were dried over molecular sieve 3A (from Union Carbide, USA). Nitrogen (from Air Products, USA, 99.98%) was freed from oxygen and water over a BASF catalyst R3-11 and molecular sieve 3A, respectively. Hydrogen (99.99%) and carbon monoxide (99.5%) were obtained from Air Products, USA. Silica 000-3E, silica-alumina LA-3D, y-alumina 005-0.75E, 000-1.5E and 000-3p were all obtained from Akzo Chemie, Amersfoort, The Netherlands. a-Alumina, type 5A-5202, is obtained from Norton, England. Silica Dll-ll was from BASF, Arnhem, The Netherlands. KieselgUhr MP-99 was obtained

from

Eagle Pitcher, USA. Silica S and silica H were silica research-samples from DSM, Geleen, The Netherlands (refs. 13-13), both of low sodium content and hydrophobic. Amberlite XAD-2 was a macroreticular resin obtained from Serva, Germany. If necessary, the support materials were crushed and sieved to the desired size fraction.

Apparatus The catalyst preparation apparatus is shown in Fig. 1. The apparatus is constructed from Pyrex glass, and provided with a thermostated

371

A

t

.

--cooling water

c

Figure 1. Catalyst preparation apparatus. A = reflux cooler, B solution holder, C support holder, D = magnetic stirrer.

catalyst

mantle in order to prevent crystallization of the PPh

solution. 3!complex A special apparatus was constructed for studying the adsorption of rhodium

complexes at the PPh

interface under the conditions of a hydroformy3!support lation experiment, i.e. in the presence of a mixture of hydrogen and carbon monoxide (Fig. 2). For the same reasons as indicated for the catalyst preparation apparatus, this pyrex glass apparatus was provided with thermostated mantles (heating jackets). The PPh

distribution across a catalyst particle was measured by means of 3 X-ray microanalysis (RMA) using a Jeol JXA-50A apparatus with a lateral solution of about 1

~m.

The catalyst particles were embedded in Woods metal, after which

thin slices of thickness 0.2 mm were cut from the material. The specific phosphorus and silicon X-ray

emissions were measured, generated by bombard-

ment with a high-energetic electron beam of 25 kV. so sruall (1

~m)

The diameter of the beam is

that a relatively high resolution is arrived at.

372

C

_

oil 70·C

C

Fig. 2. Adsorption apparatus. A material, C = heating mantle.

filter; B

+

support

BET surface areas were determined from the physical adsorption of nitrogen at -196

°c, taking for the cross-sectional area of a nitrogen molecule 16.2 ~ 2

Use was made of a semi-automatic adsorption apparatus, a Sorptomatic type 1800, from Carlo Erba, Italy. Pore size distributions were measured with the same apparatus, and the analysis of the adsorption and desorption isotherms was performed according to the methods introduced by Broekhoff et. al. The

(~efs.

14-15).

before and after adsorption of RhHCO(PPh on 3, 3)3 the various support materials, was in some cases determined by neutron activa~hodiumcontent

of PPh

tion analysis, using the "single

comparator method", with zinc as reference

material (ref. 16). In other cases use was made of a Philips PW 1450 sequential wavelength-dispersive X-ray fluorescence spectrometer. A very important characteriseic ofa catalyst is, of course, its catalytic performance. We measured the activity and selectivity in the hydroformylation of ethylene, propylene or butylene-1: R-CH=CH + co + HZ .. R-CH (normal and iso-aldehydes). 2-CH2-CHO 2 A detailed description of these measurements is to be found in refs. 4-10.

373 RESULTS General description of the system In the preparation of supported liquid phase catalysts in general, and more specifically when preparing SLP catalysts for hydroformylation, we have to reckon with a nwnber of variables which don't playa role in classical heterogeneous catalysis. This may be elucidated from the schematic representation of a partly filled pore in an SLP catalyst (Fig. 3).

Fig. 3. Schematic representation of a partly filled pore in an SLP rhodium complex catalyst. S = the support, for instance silica; L = the solvent-ligand PPh 3 ; G = the gas-phase (reactant-gases CO + HZ + alkene); open circles are the Rh complex molecules; a is a complex in the meniscus; b is a complex adsorbed at the PPh interface; ~ is a dissolved co~plex; ~ is the 3/silica meniscus.

The function of the solvent-ligand, PPh is threefold. The support material 3, is chosen in such a way that the pore wall is wetted by the PPh in doing so 3; a suction force is created according to Kelvin's law, which, for pores with Z. diameters of a few nm, may become as low as - 50 kg/cm Hence, the solution is strongly fixed in the pores. A second, chemical, function of the PPh of carbon monoxide and an excess of PPh equilibria:

co

=

3

is the followin~. In the presence 3 one may imagine the following

RhHCO(PPh C

RhH(CO)Z(PPh 3)2 D

3'Z

=co

RhH(CO)Z(PPh D

3'Z

374 In this series of complexes the selectivity for the formation of normal (linear) aldehydes increases according as the number of ligands around the rhodium increases. Both sterical and electronic arguments are advanced for this. At the same time, however, the catalytic activity decreases proceeding to the left in the series, and it is very likely that complex

~

is no longer catalyti-

cally active. Strong arguments are presented by Gerritsen et al. the rhodium complexes in the SLP catalyst being mainly in

form~,

(ref. 5)

for

and hence

complexes in solution (c, Fig, 3) are inactive. Active complexes are found in the meniscus

and in the thin layer of adsorbed PPh

these last complexes only ar~

(complexes a, Fig. 3). As 3 in direct contact with the reactant-gases (their

solubility in PPh

is very low), and as the degree of surrounding with free 3 is lower in the meniscus than in the liquid, an equilibrium shift in the

PPh

3 meniscus will occur from complex

~

to complex

~.

Complex B is likely to be the

active center, and, due to its relatively high number of PPh

ligands, it 3 exhibits a high selectivity for n-aldehydes. It follows from the foregoing that the second function of the solvent-ligand is increasing the selectivity to the economically most favourable products. For instance, in the case of propene

hydroformylation (ref.B), selectivities S for n-butyraldehyde of 30 to 40 are arrived at (S is the ratio n-aldehyde/iso-aldehyde). In homogeneous hydroformylation S is of the order of only ten in most cases. A third important function of the solvent-ligand is its stabilizing action. By the excess of PPh

ligands around the rhodium centre 3 remains high, and an activity loss through the formation of dimeric complexes 3

the number of PPh

is avoided. In practice stability problems are totally absent; for instance, in the hydroformylation of propylene at 90 DC and 16 Atm., no change in activity or selectivity is observed after test periods of more than BOO hours (ref.?).

catalyst preparation In the preparation of the catalyst the support may be impregnated with a solution of the rhodium complex or a precursor thereof in the solvent-ligand PPh

without any other sol~ents. Just such an amount of solution is then used 3 that the required loading degree is reached immediately. However, it is easier to use an inert auxiliary solvent, which means impregnating the support with a solution of the complex in a mixture of the

solvent-li~land

and a volatile

solvent, and removing the volatile solvent thereafter. The ratio between the solvent-ligand and the volatile solvent is determined by the required loading degree of the catalyst. Except for XAD-2 and silica S, the supports were dried in vacuo, first at 150 DC for 3 hrs and then at 500 DC for 16 hrs. Only silica Sand XAD-2 were dried in air at 120 DC for 16 hrs. The dried supports were placed in the cata-

375 lyst preparation apparatus shown in Fig. 1. Calculated amounts of RhHCO(PPh

3)3 were dissolved in benzene or toluene at 70°C under flowing nitrogen. 3 The total volume of the catalyst was taken exactly equal to the total pore and PPh

volume of the support (dry impregnation). The catalyst solution was added dropwise to the stirred support, which was 1 kewise held at 70°C. Next, the benzene was slowly

evaporated under flowing nitrogen at room temperature for

3 hrs and then at 90°C for 16 hrs, during which period the PPh tribute in the pore system. By varying the PPh

3

could redis-

or PPh

volume 3!toluene 3!benzene ratio in the catalyst solution, several degrees of pore filling with catalyst solqtion could be realized after evaporation of the benzene or toluene. The dry and free-flowing catalyst particles were stored at -20°C. It turned

out that two batches of catalyst, prepared in the same way, showed the same catalytic performance; the catalyst preparation is fUlly reproducible.

Catalyst characterization The PPh

distribution across a catalyst particle with a diameter of 0.423 0.50 rom, as measured by RMA, is presented in Fig. 4.

10r---------------------------------~

Intensity

(Q.U)

1 8

6

100

300

200 Particle coordinate

400

(~m)

Fig. 4. PPh distribution across a catalyst pellet. Support XAD-2 (line C); 3 degree of pore filling with PPh3 is 65%. Support silica 000-3E (lines A and B); the pore filling is 56%. Line A: Si-signal. Line B: P-signal. Line C: P-signal.

The phosphorus line-scans prove that the catalyst is not a mantle catalyst; no PPh 3 enrichment is found at the outer surface of the particles. It is seen

376 from the figure that in silica 000-3E a decrease of the silicon -signal is generally accompanied

by an increase of the phosphorus-signal. This shows the

porosity of the silica to be non-uniform, the more porous regions being filled up with more PPh as 16

~m.

The dimensions of these PPh regions are as large 3-enriched 3. XAD-2 (linescan c) shows a somewhat uniform distribution, which has

to be attributed to the relatively regular framework of microspheres in this material. The influence of the degree of liquid loading on the distribution of the catalyst solution in the support was measured by nitrogen capillary condensation at -196 °C.

with some typical silica 000-3E SLPC's

Result~obtained

are given in Fig.5.

( c

3

gm

)

0·6

0·4 ......- - - - - -

0·2

o L -_ _...L-_---L......._ - ' -_ _~-~:'::";;" 2

5

10 1 -

2 rp (nrn)

Figure 5. Pore size distribution of a silica 000-3E SLPC at various degrees of liquid loading. The cumulative pore volume Vc um is plotted as a function of the pore radius. X = hare support; 0 = degree of pore filling. 0.038; A = degree of pore filling is 0.17; degree of pore filling is 0.56; X.= degree of pore filling is 0.65; V = degree of pore filling is 0.88.

.=

377 It is clearly seen from Fig.5 that the catalyst solution in silica 000-3E is distributed as predicted by the theory of capillary condensation (ref. 1415); at low liquid loadings the walls of the pores are covered with a thin layer of physically adsorbed PPh

whereas at higher liquid loadings the thickness 3, of this layer increases, and the smaller pores get completely filled up with

capillary-condensed PPh

The BET surface area, equivalent to. the surface area 3• will therefore decrease strongly with increasing degree of

exposed by PPh

3, liquid loading, as appears from Table 1.

TABLE Surface area of the gas/PPh interface on silica 000-3E, at various degrees 3 of liqUid loading. liquid loading 600(a)

a - Fe 223 { Fe304 : 320 260 Fe304

6.8 31.0

(a) the most abundant phase is underlined. Higher surface areas of silica supported aerogels are in good agreement with better dispersion of Fe

(determined by XRD and M.E.) on this carrier than on 304 alumina. The E.M. observation probably accounts for particles formed from many

crystallites whereas the XRD method gives the mean diameter of elementary particles. In the case of aerogels prepared in one step (not listed in the Tables) the dispersion observed by E.M. was much smaller. Unsupported iron oxides show (Tables I and 2) particles of higher diameter than for supported oxides and the resulting surface area is very low. They are not substantially different from conventional xerogel oxides. It has not been attempted to measure the dispersion of iron by titration or chemisorption of a gas because chemisorption of an adsorbate (like CO) was not found to be specific of ionic or zerovalent iron (ref. 14). The comparison of the diameter of particles in iron oxide supported aerogels with those in the conventional iron oxide supported xerogels (Table 2) shows that the dispersion of iron oxide in xerogels is smaller. Moreover, iron oxide in xerogels is under the form of hematite a-Fe

whereas practically only Z03, magnetite Fe 304 is found in aerogels. It is shown below that the crystalline form of the precursor plays a role in the catalytic activity. It may be also observed (Table 2) that supported iron oxide aerogels and xerogels exhibit higher dispersion of the precursor phase that unsupported iron oxide. Finally, all supported aerogels exhibit high macropore volume (6 to 10 cm3 g-I), as determined by mercury porosimetry, and show the absence of any microporosity which would be

401

detected by the t-plot of nitrogen adsorption isotherm. CATALYTIC ACTIVITY OF AEROGELS AND XEROGELS It is well known that the commercial fused iron catalyst has to be thoroughly reduced (60h) by H2 at 500°C in order to present the catalytic activity (ref. 15) in the Fischer-Tropsch reaction. On the contrary, unsupported aerogel catalysts and supported xerogel catalysts (Table 3) exhibit catalytic

(re-

~ctivity

corded after Zh or after ZOh time on stream and expressed as the number of micromoles of CH 4 per minute and per gram of iron, irrespective of the oxidation state of ~ron) even without a previous reduction (pretreatment in He at 250°C during Ih) or in a mildly reduced state (pretreatment in H2 at 250°C, during 15 h). The results in Table 3 give also the activities after a strong reduction (pretreatment in H at 500°C duri~g ISh) and the nature of phases in the solids, 2 detected by XRD analysis i) after the pretreatment ii) after ZOh of reaction. For a better comparison, the results for supported aerogel catalysts are shown separately in Table 4. TABLE 3 Catalytic activity and nature of solid phases for unsupported aerogels and supported xerogels Nature of the solid

AcAcFe(H)

Pretreatment

He,2s0°C,lh

Crystalline phase (XRD) after pretreatment

after 2h

a- Fe22J

170

Catalytic activity (~mole CH4 mn-lg-IFe) after 20h 64

Fe3Q4 ;

150

75

SiFeIO(X)

AlFeIO(X)

;

~Q4

H2,sOO°C,lsh

a-Fe

60

15

~O~g;

He,ZsO°C,Ih

Fe304

50

73

~Q4

160

74

5

4

a-Fe ; X-FezC

HZ,2s0°C,lsh

Fe304 ; a-Fe

HZ,sOO°C,Ish

a-Fe

He,2s0°C,lh

a-FeZ03

400

153

Fe3Q4 ; X-FeZ C

H2,Z50~C,Ish

Fe304

800

174

~Q4;

HZ'sOO°C, ISh

a-Fe

560

100

£-FezC ;

He,2s0°C,lh

a-FeZ03

660

125

Fe304 ; £- Fe2 C

(o-AlZ03) HZ,Z50°C,15h

Fe304

HZ,sOOoC,15h

a-Fe (o-Al Z03)

Fe304 ; Fe20C9 ~o~g

; a-Fe

£-FezC

(o-AlZ03) 65

60

1075

Z34

(o-Al203)

(a)

;

~Q4

X-FeZC

a-FeZ03

AcAcFe(D)

of

X-Fe2 C

Fe304 H2,ZsO°C,lsh

Crystalline phase (XRD) after 20h reaction (a)

Fe304 (o-AlZ03) FeZOC g (o-AlZ03)

the most abundant (supported in binary catalysts) phase is underlined.

402

These results show that for unsupported aerogels [AcAcFe(H) and AcAcFe(D») a previous reduction (HZ at 500°C) is rather detrimental to the catalytic activity which is higher after a non reducing pretreatment (He at 250°C) or a mild reducing pretreatment (HZ at 250°C). After ZOh time on stream a high activity always corresponds to the presence of magnetite Fe

and only to a partial carburiza304 tion of iron. This carburization is always limited (Table 3) if the catalyst is

initially formed by iron oxides (ref. 16) instead of reduced iron after a strong reducing pretreatment (HZ at 500°C). The influence of the

~rrier

is always beneficial to the catalytic activity

as shown by the results concerning supported xerogel catalysts, reduced initially or not. The decrease in the activity between Zh and ZOh on stream always corresponds to an increase of the content of iron carbides (ref. 16). With the exception of the alumina supported catalyst the highest activities are obtained with the

unreduced (He at Z50°C) or partially (HZ at Z50°C) reduced catalysts

and only iron oxide in the form of magnetite is found in the spent catalyst together with a small amount of carbide. On the contrary, only carbides are found in the highly reduced (HZ' 500°C) spent catalysts which initially contain a-Fe. Table 4 shows the same characteristics for supported aerogel catalysts. After their preparation by the autoclave method iron in all supported aerogels is under the form of magnetite Fe304 (Table Z). After a pretreatment in He at 250°C this phase is of course conserved but it is only moderately active in the Fischer-Tropsch reaction. The important point to notice is that these unreduced catalysts do not contain carbides after ZOh of reaction [with the exception of SiFeIO(A)(ZD) where only traces of FeZOC g are found]. This behaviour is different from that of catalysts of Table 3 which are pure aerogel oxides or supported xerogel oxides. Indeed, for these unreduced (He, Z500C) catalysts carbides are found after the reaction. The lack of formation of carbides is also recorded for aerogel supported catalysts (Table 4) after a mild reduction (H 2, Z50°C). A strong reduction (H2 at 500°C) which leads to a-Fe increases the catalytic activity of aerogels [except for AlFeIO(A) (2D») and simultaneously carbides are found in the solids after ZOh of reaction. But because the phase at the beginning of the reaction is now a-Fe this carburization is to be expected (ref. 16). It appears therefore that the aerogel precursors, unreduced or mildly reduced (He or HZ at 250°C), contain initially the phase Fe

which is practically not 304 modified by the reactants (CO + HZ) after ZOh of reaction, but which is only moderately active. After a strong reduction (HZ at 500°C) all catalysts of Tables 3 and 4 contain a-Fe which is transformed into carbide by the reactants. After 20h of reaction the carbide is in general the most abundant phase [except for SiFeIO(A) (ZD»). It has been found recently that iron carbides are not the active agents of the Fischer-Tropsch synthesis (ref. 17). Their formation is, on the

403 TABLE 4 Catalytic activity and nature of solid phases for supported aerogels Nature of the solid

SiFelO(A) (ZH)

Pretreatment

He,Z50·C,lh

Crystalline phase (XRD) after pretreatment (a)

Catalytic activit, (umo Ie CH4 mn-Ig- Fe) after Zh after ZOh

Crystalline phase (XRD) after ZOh of reaction (a)

Fe304; a-FeZ03

SiFelO (A) ZD)

HZ,Z50·C,15h

Fe304

50

40

HZ,500·C,15h

a-Fe

350

ZIO

He,Z50·C,lh

Fe304

40

35

HZ,500·C,15h

a-Fe

Z80

ISO

Z300

5000

Fe304

80

5Z

Fe304

°Z,500·C,Zh

AlFe6(A) (ZH)

He,Z50·C,Zh

~Q4

a-FeZ 03

Fe304 a-Fe; FezoC 9 Fe3Q4

FeZOC9 (traces' a-Fe; £-FeZC

(a-Alz03,HZO)

(a-AlZ0 3,HZO) HZ,Z50·C,15h

Fe304

HZ,500·C,15h

a-Fe

350

ZZ4

300

149

ZO

IS

(a-Alz03,H ZO) (y-AlZ03) AlFeIO(A) (ZD) He,Z50·C,lh

Fe304

HZ,Z50·C,15h

Fe304

Fe304 (ex-AIZ03. HZO)

(a-AlZ03,H ZO) 80

Z5

45

Z5

550

1900

(ex-AIZ03,HZO) HZ,500·C,15h

ex-Fe

0Z,500·C,Zh

ex-FeZ03

FeZoC g (y-AIZ03)

(y-AlZ03) y-FeZ03

Fe304 (y- AI Z0 3 )

(a) the most abundant supported phase is underlined. contrary. responsible of the deactivation of the catalyst with time on stream. This is well observed for catalysts of Table 3 which show the decrease of the catalytic activity between Zh and ZOh of time on stream. This deactivation is however less severe in the case of aerogel catalysts of Table 4 despite the formation of carbides. Also if for these catalysts the formation of carbides could be restricted or even suppressed, an unusual activity would be observed. This is the case for the aerogel precursors of Table 4 which have been oxidized (OZ' 500·C) into a-Fe

Z03

and y-Fez0

3

previously to the reaction. The catalyst then

404

acquires an exceptional activity which increases with time on stream. After ZOh of reaction the initial oxide Fe

is reduced to magnetite Fe only and no Z03 304 traces of carbide could be identified. These results are in agreement with the

chemical analysis and with hydrogen etching of spent catalysts. The formation of carbides does not

~eem

therefore to be required for a high catalytic activity of

iron catalysts in Fischer-Tropsch synthesis. The high catalytic activity could be correlated perhaps to a better dispersion of the active phase but it seems that the beneficial effect of the oxidation treatment is correlated with the oxidation degree of iron which is here maximum (Fe

after this treatment. Now Z03) the high catalytic activity is not encountered with xerogels SiFeIO(X) and

AlFeIO(X) of Table 3 which also contain a-Fe

after He pretreatment. Moreover, Z03 the oxidation pretreatment (OZ at 500°C) applied to xerogels of Table 3 does not increase their catalytic activity which is then almost the same as that recorded

after He pretreatment of these xerogels. Any interaction between iron oxides and the support (AI Z03 or SiOZ) which would lead to the formation of an aluminate or silicate has not been detected. Also, at the present time, the peculiar behaviour of preoxidized supported aerogel catalysts could be attributed to a bprter dispersion of the active phase. It is however intriguing to observe that the phase which is found on preoxidated spent aerogels is magnetite Fe304 (whereas the initial phase is Fe Z03) which is not active to the same extent when it forms the initial phase of aerogels pretreated in He or HZ at ZSO°C (Table 4). Therefore the initially present magnetite in aerogel catalysts cannot be activated during the Fischer-Tropsch synthesis (catalysts of Table 4, unreduced or moderately reduced by HZ at ZSO°C). On the contrary when magnetite is formed during the Fischer-Tropsch reaction from the initial phase Fe

(on preoxidized aerogel catalysts) the recorded Z03 activity is very high. Now, for catalysts of Table 3 which may also contain initially Fe Z03, and Fe after the reaction, the recorded activity is much lower 30 4 than for aerogel catalysts of Table 4 exhibiting the same sequence of iron oxidized phases (Fe

initially, Fe 304 after ZOh of reaction). Z03 Finally, it should be pointed out that preoxidized aerogel catalysts show an

activity

per unit mass of iron which is 300 times higher than the activity of

commercial fused iron catalyst (ref. IS). Concerning now the selectivities into various hydrocarbons of preoxidized supported aerogel catalysts, they are not very much different from those of unreduced (He, ZSO°C) supported xerogel catalysts, as it is shown in Table 5. In conclusion, a new type of supported iron oxide catalysts, in aerogel form, in which iron is active in the oxidized state, has been developed. If the precursor, instead of being reduced, is preoxidized into Fe

(the maximum oxidaZ03 tion state of iron) the reactants (CO + HZ) of the Fischer-Tropsch synthesis at ZSO°C transform this phase into Fe304 which then exhibits a particularly high

405

TABLE 5 Selectivities into various hydrocarbons Nature of the

Selectivities (%)

solid

CI

C 2

C 3

C 4

C 5

C 6

CO

siFeIO(A) (2D) preoxidated

44.5

22.5

16.5

7.5

4.5

1.5

3

52.5

AlFeIO(A) (2D) preoxidated

46

20

18.5

6.5

4.5

2.5

2

52

SiFe"]O(X) unreduced

44.5

25

14

6

2

8.5

47

AlFeIO(X) unreduced

55

25.5

11

2

7

38.5

2

EC>I

activity without any formation of iron carbides and the catalyst does not deactivate with time on stream. This high activity and the absence of deactivation are not found on the same aerogel catalysts if the precursor is directly the Fe 304 phase. For unsupported pure oxides or supported xerogel precursors the initial presence of the maximum oxidation state of iron (Fe 203) does not induce this very high activity, neither does it prevent the formation of carbides or the deactivation of the catalyst with time on stream. REFERENCES I 2 3 4 5 6 7 8 9 10 II 12 13 14 15 16 17

R.B. Anderson, Catalysis, 4 (1956) 29. A. Jones and B.D. McNicol, J. Catal., 47 (1977) 384. L.J. Hofer, Catalysis, 4 (1956), 373. H. Pichler, Advan. Catal., 4 (1952) 272 M.E. Dry, Ind. Eng. Chern. Prod. Res. Dev., IS (1976) 282 M.A. Vannice, J. Catal., 50 (1977) 228. M.A. Vannice, J. Catal., 37 (1975) 449. G.B. McVicker and M.A. Vannice, J. Catal., 63 (1980) 25. A. Brenner and D.A. Hucul , Inorg. Chern., 18 (1979) 2836. S.J. Teichner, G.A. Nicolaon, M.A. Vicarini and G.E.E. Gardes, Advan. Colloid Interface Sci., 5 (1976) 245. M. Astier, A. Bertrand,D. Bianchi, A. Chenard, G. Gardes, G. Pajonk, M.B. Taghavi, S.J. Teichner and B.L. Villemin, "Preparation of Catalysts", Elsevier Sc , Publ , Corp. Ed. Amsterdam, 1976, p . 315. B. Pommier, F. Juillet and S.J. Teichner, Bull. Soc. Chim. Fr. (1972) 1268. H. Matsumoto and C.O. Bennett, J. Catal., 53 (1978) 331. J.P. Reymond, B. Pommier, P. Meriaudeau and S.J. Teichner, Bull. Soc. Chim. Fr., (1981), I, 173. J.P. Reymond, P. Meriaudeau, B. Pommier and C.O. Bennett, J. Catal., 64 (1980) 163. J.P. Reymond, P. Meriaudeau and S.J. Teichner, J. Catal, in press. J.W. Niemantsverdriet and A.M. Van der Kraan, J. Catal., 72 (1981) 385.

406 DISCUSSION P.A. JACOBS 1. If you plot the data on your catalysts according to a SchulzFlory law (log molar distribution against carbon number of products) an almost perfect straight line is obtained for the data of Table 5" In contrast to what is usually found: - no high yield of C1 is found nor is a drop in the curve at the C2 level observed. Have you any idea whether this behaviour might be related to its unusual preparation procedure? 2. You also work with extremely high H2/CO ratios (~ 9) and nevertheless find a growth factor (whiCh can be derived from the slope of the semi-logarithmic plot) which is of the same order as usually found for more classical iron-based catalysts at much lower H2/CO ratios. I wonder whether you could comment on this. 3. Is there any difference in product distribution between your unreduced and/or oxidized catalyst and the same catalyst reduced in a classi~al way ? J.P. REYMOND: 1. We observed that the schulz-Flory law is observed in an usual way for the reduced catalysts. Your observation concerning the unreduced catalysts (or preoxidized) is very interesting and may be perhaps correlated to the oxidized state (initially) of these catalysts. 2. It would be of interest to examine the selectivity behaviour for normal H2/CO ratio (3/1) which will probably induce still higher selectivity into higher hydrocarbons and perhaps olefins. 3. We reported at the Bruges meeting that the length of hydrocarbon chains is increased after the oxidation pretreatment in comparison with that observed after the conventional reducing pretreatment. L. VOLPE: You have rather large catalyst particles. Catalysis is an interfacial phenomenon. XRD gives you information about the bulk. What is the state (composition and structure) of your surface? Could it be independent of the bulk under the reaction conditions ? J.P. REYMOND: The state of the surface of our supported catalysts is difficult to study in the experimental conditions of the reaction, taking into account our facilities in XPS equipment in Lyon. However, the peculiar surface composition of the unsupported oxidized catalyst in comparison with that of the reduced catalysts was well recorded and is described in J. Catal., 1982, ~, 39-48. L. GUCZI: If I understood you correctly, the exceptional activity of your aerogel supported catalyst is due to the formation of Fe304 phase. For F.T. reaction we need a dissociation of CO which - in turns- requires metallic sites. How can you accomodate your metallic sites in Fe304 ?

°

J.P. REYMOND The problem of the dissociation of CO into C and on iron and of the redox character of this reaction was envisaged also on Fe304 as it was described in the paper presented in Bruges and also in J. Catal., 1982, ~, 3948. The thermodynamic data for both equilibria are : Fe + CO t FeO + C 2 Fe304 + CO l' 3 Fe203 + C

~Go523 ~Go523

10.6 kcal 10.8 kcal

There is no restriction to have reoxidation of Fe304 by CO into Fe203' in the same way as to have oxidation of Fe by CO into FeO. M. BAERNS: Could you please comment on the activity of the aerogel type F.T.catalyst in comparison to conventional F.T.-catalysts as they are used in the SASOL plant. What would be the required space velocity to achieve conversion in the order of 70 to 80 % ? J.P. HEYMOND This comparison has been made in our laboratory. An industrial catalyst (fused iron C.C.I.) present an activity in the differential reactor conditions, described in this paper, which is 300 times smaller than that of the silica-iron oxide aerogel (preoxidized SiFel0(A) (2D». This first catalyst is described in J. Catal., 1980, 64, 163. We did not yet examine the behaviour of

407 aerogel catalysts in an integral reactor conditions.

J. KIWI

I missed the BET area for the most active aerogel-iron catalyst used to produce methane under your experimental conditions. Could you elaborate on it ?

J.P. HEYMOND: The total surface area of the silica supported aerogel iron catalyst is 690 m2g- 1• It is reduced by 10 % by the F.T. reaction and the diameter of iron oxide particles (XRD and TEM) is not changed (d ; llb A).


409

G. Poncelet, P. Granae and P.A. Jacoba (Editors), Preparation of Catalyst. III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

SELECTIVE DOPING OF A CARBON SUBSTRATE TRANSITION-METAL AMMONIA CATALYST F.F. GADALLAH,

R.M. ELOFSON,

P. MOHAMMED

and

T. PAINTER

Alberta Research Council, Edmonton, Alberta, Canada Alberta Research Council Contribution No. 1116

ABST~CT

Ammonia catalysts designed to operate at moderate temperatures and pressures were prepared by studying the effects of various techniques of impregnating carbonaceous materials with alkaline earth ions, ruthenium ions and alkali ions. The influence of the impregnation process on ruthenium dispersions is discussed. The degree of catalytic activity for the production of ammonia and the surface area studies were the two methods used to select the best catalyst. It was found that the nature of the carbon support and the order of impregnation have significant effect on catalytic activity. INTRODUCTION A high distribution of the active components of a supported catalyst does not necessarily qualify it as the best catalyst for a particular process.

The

appropriate dispersion of these active components for optimum catalytic activity for specific reactions can be controlled by the nature of the support, impregnation method, drying, order of impregnation for a multi-component catalyst, and by activation. It is understood that every step taken during the preparation of a catalyst influences its qualities.

Every step is a critical variable which controls

qualities of the catalyst such as activity and stability. The steps involved in the development of this new family of catalysts were: (1)

preparation of the carbonaceous support (charring temperature and

activation),

(2)

of impregnation

impregnation technique, impregnating salts and the sequence (ref. 1, 2, 3).

Active carbon was chosen as the catalyst support because its catalytic power (donor acceptor properties) is often quite specific and could be controlled by the method of its preparation (ref. 4, 5). EXPERIMENTAL Preparation of the Carbon Support Although commercially available forms of activated carbon (i.e. coconut charcoal) could be used as a support material, we prepared our own.

This

exercise enabled us to control parameters such as the starting material and

410

charring temperature.

Hardwood, either maple or birch, polyvinylidene chloride

and cellulose were some of the materials used for charring.

Carbonization was

carried out using known procedures (ref. 6, 7). Because charring temperature affects the size of the graphite crystallites, support carbon was prepared at two temperatures, 600°C and 800°C, to determine if the size of the crystallites has any effect on catalyst performance. The char was then ground to the desired mesh, and activated in a

fluidiz~d

bed by a mixture of steam and air, or by carbon dioxide, at 850° to 900°C. The effect of the charring temperature on catalyst performance was studied by the ESR propert.ies of the char and its activity towards nitrogen fixation. Impregnation and Drying Three methods were tried to prepare an active and homogeneous (as close as possible to achieve uniformity) catalyst: (b)

tumbling

and

(c)

(a)

soaking and drying,

impregnation under vacuum.

The most active catalysts

were obtained from the vacuum method. Typically, the activated carbon was degassed under vacuum

(70-l00~)

at a

temperature of 250°C for a period of four hours, then cooled under vacuum to room temperature.

Barium, an amount equal to 2% of the weight of the carbon,

was added under vacuum as an aqueous solution of the nitrate, in sufficient water to cover the char (5 ml per gram of carbon).

The slurry was then kept

under vacuum and the temperature increased gradually until all the water was driven out at about l50 aC.

The product was then baked under vacuum for four

hours at 250°C and cooled.

An aqueous solution of RuC1 3"3HzO, 4% of the weight of the carbon in ruthenium, was added under vacuum. Although chloride ion might exhibit a poisoning effect on the catalyst, ruthenium chloride trihydrate was used for its greater stability over other ruthenium salts. obtaining a uniform coating of the carbon support. and then cooled under vacuum.

This is important in The slurry was dried, baked

The process was repeated a third time by doping

and baking as previously described with an aqueous solution of potassium hydroxide, 12% of the weight of the carbon in potassium. In other runs, other metal ions were impregnated in the same manner, the order of the individual impregnations was altered and the concentration of each metal varied. The highest level of activities were obtained when each salt is added separately under vacuum and when the doped product is baked. In Figure I the reactivity of the new catalysts is compared to the commercial catalyst (Top Soe) operating at Nz/Hz 1/3. The surface of the resulting catalyst was black and lustrous and gave no

411

20

,,

Figure 1. Reactivity of the commercjal and a new catalyst

,/,'7 /

, ,,

15

..

,, '

#. :I:

z

,,/, /

,, , / " /'.,..,... »>:

10 ",~

~

-----

IV

,

.;/'

5

V 0

:7 20

40

60

80

BaLaRuK (4,1,4, 14) I Equil: values at 42()OC VI New Catalyst: 4200C, 6000 s.v . VII New Catalyst: 4200C, 9000 S.v. IV Top Soe: 42()OC, 5000 s.v,

100

Patm

visual indication of any precipitated salts; no attempt was made to study the surface microscopically. about 550 - 950 m2/g.

The various catalysts displayed a surface area of

They could be stored under ambient conditions prior to

activation or under nitrogen after activation.

Catalysts prepared by this

technique have reproducible activities. Since a low evaporation rate of the solvent at a low temperature causes a high salt concentration on the surface of the porous support, and also produces a non-uniform distribution of large crystallites

(ref. 8), it is reasonable to

suggest that the high activity of these catalysts is partly due to impregnation under vacuum.

Vacuum assists penetration by drawing the air from the pores on

the surface particularly at high temperature where surface tension and viscosity of the impregnating solutions are low.

Also rapid evaporation of the solvent at

high temperatures produces an even distribution of small crystallites. Baking acts to convert the salts of some doping solutions (i.e. nitrates) to their respective oxides and apparently produces a better distribution on the carbon support.

412 Activation The catalyst was placed in a stainless steel. double-walled laboratory reactor shown in Figure 2. degassed under vacuum for four hours at 400°C and then activated with hydrogen at 12 - 25 atm. for 12 hours.

Activation was

considered complete when 95% of the chloride ion (in RuCI 3) was recovered as AgCl.

8

Figure 2.

A. Cap IThr!'laded) 1. Outer Wall 2. Heating Tape 3. Annular Space 4. Inner Wall 5. Cavity (Catalyst Bed) 6. Catalyst 7. Perforated Cap 8. Thermocouple Well 9. Inlet Gas 10. Outlet Gas

7

10

RESULTS AND DISCUSSION Ideally the active components of an impregnation-type catalyst should cover uniformly the porous structure of the support. inert carrier material.

Also the support should be an

However. this is not the case in a support like active

carbon which has catalytic activity.of its own. Effect of Charring Temperatures Since ESR properties characterize the interactions of free radicals with each other and with their surroundings. we tried to establish a correlation between the number of spins in a char before and after activation and impregnation with activity.

The results are shown in Table 1.

413

TABLE 1 Effect of Charring Temperature on ESR Properties* and Catalytic Activity 800° Char

600° Char No. Spins/g x 10 18

Sample

l. NA, ND 2. A, ND 3. A, D(Ru) 4. A, D(Ba, Ru, K)

38.0 26.0 4.3 2.9

a

Cat. Act. Y .b Eff.%c "0 abs

No. Spins/g x 10 18

Cat. Act. a Y %b Eff.%c .·abs

0.07 2.4 9.6

15. 60.

20. 74.

3.1 11.7

* We thank Dr. Karla F. Schulz for the ESR measurement and helpful discussion. 1 Not activated, not doped 2 Activated, not doped 3 Activated, doped, Ru 4% 4 Activated, doped, Ba 2%, Ru 4%, K 12% a Cat. Act. at 400°C, 50 atm., 3000 space velocity and Nz/Hz b y % (moles NH 3/moles (Nz + Hz) x 100 abs c Eff. % (actual yield/equilibrium yield) x 100

1/3

As expected the number of free spins in 600° char is much higher than 800° char.

Since activation of 600° char occurred at 850 to 900°C, the expected

drop in the number of spins occurred, but the decrease was not as large as expected (compare activated not doped 600° char with a value of 26 x 10 18 to that of non-activated, not doped 800° char of 0.07 x 10 18). Whether the chars were activated or not, they had no catalytic activity in spite of their high population of free spins.

From these results, it appeared that there is no

relation between the density of free radicals and catalytic activity.

However,

the activity of 800° char was always higher than the 600° char (Fig. 2). But, for 600 0 the decrease in the number of spins from 26 to 4.3 to 2.9 x 10 18 is indicative of a strong interaction between the donor acceptor sites of the active carbon support and the doping species. Effect of Nz~ Ratio As early as 1974 'we found that the catalytic activity of ruthenium is inhibited by hydrogen adsorption (ref. 9a-d) and the efficiency of a ruthenium catalyst could be optimized by careful selection of the gas ratio.

In other

words, the gas feed composition is as important a factor as temperature, pressure and gas feed rate in assessing the efficiency of a catalyst for a particular process.

Table 2, Figures 3 and 4 show that the inhibiting effect

of hydrogen could be countered by decreasing the partial pressure in the gas feed mixture.

Later, Rambeau and Amariglio (ref. 10) suggested that the

performance of ruthenium powder in ammonia synthesis may be strongly enhanced

414 by application of a periodic feed of nitrogen then hydrogen to the catalyst. TABLE 2 Effect of N2/H 2 Ratio on Ammonia Yield Mole Ratio

Y % abs

Eff. %

1/3 1/1 3/2

8.5 11.1 7.3

42. 78. 67.

Catalyst: 600 0 char, Ba, La, Ru and K in 2,2,4, and 14% Reaction Conditions: 400°C, 68 atm., 9000 S.v.

Figure 3. Effect of gas feed ratio on efficiency. , Top Soe s.v. 3000 N/H = 1/3 " Top Soe S.v. 3000 NIH = 1/1 III BaLaRuK (6OO'C) NIH = 1/3 IV 14.1.4,14) s.v, 9000 NIH = 1/1 V BaLaRuK (8OO"C) NIH = 1/3 VI s.v, 6000 NIH = 1/1

Effect of Promoters Chemisorption data and catalytic activity of some ruthenium catalysts are shown in Table 3 (ref. 11).

The amounts of hydrogen or nitrogen chemisorbed on

the active carbon support are negligible (a). The decrease in N -BET surface area when alkali promoters are added (first column) suggests that the micro-

415

II

12

11 10

/1

9

*-

-£ z

/

I

/

"

BaRuK (2,4,121, s.v. 7500 I NIH II NIH

/

8 7

/1

6

/

5 4

/

/

Figure 4. Effect of gas feed ratio on ammonia yield.

1/

1/3, 1/1

/

380

420

400

TOC

TABLE 3 Surface Characteristics after Activation,* and Comparative Catalytic Activity of Ruthenium Catalysts

Cata1yst

a. b. c. d. e.

a

Comparative BET Hz Percent N2 Surface Area Chemisorbed Dispersion Chemisorbed Cata1ytic b Activity (mz/g) IlM/g llM/g

Active Carbon Ru/carbon Ru, K/carbon K, Ru/carbon Ba, Ru/carbon f. Ba, Ru, K/carbon g. Ru, Ba, K/carbon h. 5% Ru/carbon (comm) * a b

967 765 710 907 662 554 932

13.8 58.0 35.7 15.4 62.5 117.0 29.5

1.3 32.0 25.0 27.0 14.0 18.0 6.0 17.0

7.0 29.3 18.1 7.8 31.6 59.1 11. 9

We thank Dr. S. Parkash for making the surface area measurements. Metal concentrations are 4% Ru, 12% K, and 2% Ba. Reaction conditions, 400°C, 27 atm., 3000 s.v. and

N2/Hz

1/3.

1.0 2.0 1.4 9.5 2.4

416

pores of the supporting material have been occupied as expected.

The decrease

in surface area is more in the case of potassium than it is in the case of barium, and there is a significant increase in metal dispersion by the alkali addition (ref. 9a).

The increased dispersion is more than four-fold in the

case of potassium as in (c) and eight-fold for barium and potassium (g).

This

suggests either a reaction between the adsorbed ruthenium and the added potassium solution followed by a redistribution of the transition metal during impregnation, or a redistribution during the activation process when the anions were being removed.

The

is greater in the case of potassium than

~ispersion

barium (d and e) and very high when both metals are used as in (g). The chemisorption of nitrogen follows a different route than that of hydrogen.

Potassium has a stronger inhibiting effect than barium, and for the

doubly-promoted catalyst, the decrease in nitrogen chemisorption is additive. Barium, not only inhibits sintering (ref. 12), it also modifies the surface of the support, especially when it is added as the first impregnating metal. Unfortunately, we did not look at the nature of this modification. The results in Table 3 clearly illustrate the very important role played by potassium and barium in controlling the dispersion (the metal crystallite size) of the active species, ruthenium, to produce the exact environment for the chemisorption of the reactants.

We found that high alkalinity is still

necessary to develop a catalyst to produce ammonia up to the equilbrium yield. Effect of Order of Impregnation Changing the order in which barium and ruthenium were added (c and d, Table 4) changed the ratio of chemisorbed nitrogen and hydrogen.

This ratio

controls the reaction rate.

Catalyst (d), with a Nz/Hz ratio of 1/3.5 has higher activity (four-fold) than (c), with a ratio of 1/19.5 toward ammonia

production.

However, the Nz/H2 adsorption ratio were measured at 77°K, and it

is not at all certain that these measurements have significance in the 673 0 to 720 0K range in which we were working. It is critical to determine how and when to add the promoters to achieve the optimum conditions for a particular

r~action.

To obtain meaningful results and

to assess the role of order of impregnation, two more catalysts in this series RU,K,Ba and

K,Ru,Ba

will be prepared and the surface characteristics and

catalytic activities of the group will be studied and correlated. We found that these catalysts are also very active in hydrogenation of carbon monoxide under mild conditions.

417

TABLE 4 Effect of Order of Impregnation on Catalytic Activity Catalyst (600°) a.

Ba K Ru

b.

c.

K Ba Ru Ru Ba K

d.

Ba Ru K

Comparative Catalytic Activity

Nz/H z Chemisorbed

1.0 1.8

1/19.5 11 3.5

3.7 14.4

concentrations as in Table 3 Reaction conditions as in Table 3

Me~al

Effect of CO The commercial iron catalyst is very sensitive to poisoning by CO, especially at high pressure.

A poisoned iron catalyst regains its activity

only slowly after treatment with pure gas for several days.

The new carbon

catalysts are less sensitive to CO, (Table 5), and regain full activity quickly when CO is removed from the feed gas stream.

It appears that CO is a temporary

inhibitor for these catalysts rather than a poisoning agent. The data in Table 5 are not taken at comparable temperatures and pressures. However, since it is generally true that the poisoning effect of CO is greatest at low temperatures and high pressures, it seems reasonable to conclude that the effect of CO on the carbon catalyst is less than its effect on the iron catalyst. TABLE 5 Effect of CO Poisoning on Ammonia Yield CO % in Gas Stream

Conditions

Catalytic Activity Remaining % Top Soe

450°C, 450°C, 500°C, 500°C, 420°C, 420°C,

100 100 100 100 70 70

atm atm atm atm atm atm

0.08 0.04 0.08 0.04 1.0 0.1

Carbon Catalyst

16 34

55 85 32 78

CONCLUSION One of the most important factors influencing the reactivity of a carbon support catalyst impregnated with alkali and transition metal ions, is the order of impregnation of the metals.

More research in catalyst design and

spectral analysis is needed to clarify the relation between structure and reactivity.

418

REFERENCES 1 G. Berrebi and Ph. Bernusset, in "Preparation of Catalysts I" (B. Delmon, P. Jacobs and G. Poncelet, Eds.), Elsevier, Amsterdam, 1976. 2 G.H. van den Berg and H.Th. Rijnten, in "Preparation of Catalysts II" (B. Delmon, P. Grange, P. Jacobs and G. Poncelet, Eds.), Elsevier, Amsterdam, 1979. 3 J. R. Anderson, "Structure of Metallic Catalysts", Academic Press, New York, 1975. 4 P. Ehrburger, OvP, Mahagan and P.L. Walker, Jr., J. Cat a l . 43, 61 (1976). 5 T. Mahmood, J.a. Williams, R. Miles and B.D. McNicol, J. Catal. ~, 218 (1981). 6 John W. Hassler, "Activated Carbon", Chemical Publishing Co. Inc. New York, N.Y. 1963. 7 R.M. Elofson and F.F: Gadallah, Can. Pat. 1094532. 8 M. Kotter and L. Reibert, 2nd Int. Symp., "Scientific Bases for Preparation of Heterogeneous Catalysts", Louvain-la Neuve, Belgium, 1978. 9a C. ERTL Cat. Rev. Sc. Eng. 21, 201 (1980). b R.M. Elofson and F.F. Gada11ah, Am. Pat. 4142993 filed December 1977. c M. Boudart, Catal. Rev. Sci. Eng., 23, 1 (1981). d Anders Nielsen, ibid, 23, 17 (1981).-10 G. Rambeau and H. Amarig1io, App1. Catal., 1, 291 (1981). 11 S. Parkash, F.F. Gadallah and S.K. Chakrabartty, Carbon 17, 403 (1979). 12 G.B. McVicker, R.L. Garten and R.T.K. Baker, 5th North Amer. Catal. Symp. Abstracts 15-10 Pittsburgh (1977).

419 DISCUSSION L. VOLPE K. Aika and A. Ozaki in the mid 70's found that Ru/C catalysts, especially when promoted with alkali metals are far more active than the iron catalyst for NH synthesis. Nevertheless would you sincerely propose that an expen3 sive noble metal could compete with the rugged cheap Tops¢e catalyst ? F. GADALLAH: For the first part of the question,Ozaki et al. impregnated their carbon support with the metals (MO) which made the catalysts ,pyrophoric. OUr catalyst is impregnated with the metal ions. Concerning the composition between the new and commercial catalysts : economic evaluation made by ARC (Alberta Research Council) found that the cost/ton NH3 is very competitive due to the high reactivity of the new catalyst. Moreover, other engineering processes, besides the fixed bed used for Tops¢e catalyst, such as fluidized bed with space velocities up to 50,000 would be suitable for the new catalyst. The high yields of ammonia would offset the high cost of the noble metal Ru very comfortably. A. KORTBEEK: 1. I have difficulties to accept a comparison between the activity of a non-noble metal such as Fe with a rare and expensive metal such as Ru, which is known to exhibit a much higher activity than Fe. Have you tested Fe impregnated on your carbon support and if so how does its performance compare with the commercial catalyst ? 2. It is known that noble metals and alkali can catalyse hydrogasification of carbon into methane. Did you observe any volatilization of the carbon support during NH3 synthesis ? F. GADALLAH 1. The Fe/C catalyst prepared in our laboratories had poor performance at the low temperatures 420°C. However, the iron catalyst is at its best around 500°C and carbon would be hydrogenated at these temperatures. 2. Carbon support gasification did not occur at temperatures up to 435°C. However, it occurs above 450°C. B.E. LANGNER: It is known that the yield in ammonia synthesis is restricted not only by the intrinsic activity of the catalyst, but by transport phenomena like pore diffusion. Did you use always the same particle size for the comparison of your catalysts with the commercial catalyst? F. GADALLAH Pore size and pore diffusion are different than particle size. We did not study the pore size but we always used the same particle size (same mesh) in our comparisons. M.V. TWIGG How tolerant are your catalysts to water vapour; if there are effects, are they short or long term ? F. GADALLAH: We expect low tolerance at low temperature; however, the tolerance increases with increasing temperature. We did not study the effect of water vapour but we used commercial hydrogen and nitrogen which contain traces of water (ppm) and there ~ere no ill effects. BAlKER Do you have any experience about the thermal stability against sintering of your carbon supported Ru particles? In particular, have you observed an effect of the alkali promoters on this stability?

A.

F. GADALLAH: As the working temperatures of 420°C the catalysts were stable. Some of the catalysts were taken off stream after eight months of continuous production without loss of activity. However, higher than 450°C methanation occurs. With K at 14% by wt. the catalysts were stable. Above 16% alkali concentration, deterioration in the catalyst occurred. Whether deterioration is due mainly to the high concentration of the alkali remains to be verified.

420 D. CHADWICK:

In Table 3 of your paper you compare activity and hydrogen chemisorption data. In all the catalysts except one the activity seems to follow the dispersion. In the exceptional case, the activity differs markedly from the other catalysts. Are you sure that the chemisorption and activity data are entirely reproducible ? F. GADALLAH: The catalytic activity of this ammonia catalyst has to be related to the chemisorption of the two reactants H2 and N2. We believe that in the Case g in Table 3 that the chemisorbed H2 covers the catalyst surface and prevents N2 chemisorption. Besides, we are very sure of the reproducibility of these catalysts. NG CHING FAI: Did you de~ermine the pore size distribution of your carbon ? Do you know whether your Ru in inside the pores or not ? F. GADALLAH We did not determine the pore size distribution, but we are going to start this study. However, in the text, we discussed the effect of promoters and we mentioned that the decrease in N-BET surface area when alkali promoters are added (first column, Table 3) suggests that the micropores of the supporting material have been occupied. Since the highest values of surface areas are when Ru alone was added, we can conclude, inderectly, that Ru goes inside the pores.

G. Poncelet, P. Gran,e and P.A. Jacobs (Editora), Preparation of CataZ)'ItBIII

e 1983 Elanier Science Pub1isbera B.V., Amsterdam - Printed in The Netherlanda

A STIJDY OF 1HE

PREPARATH~

421

AND PROPERTIES OF PRECIPITATED IRON CATALYSTS FOR

AMMONIA SYN1HESIS D.G. KLISSURSKI, I.G. MITOV and T. TOMOV+ In;;titute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1040 (Bulgaria) +Institute of Nuclear Energetics, Bulgarian Academy of Sciences, Sofia 1040 (Bulgaria)

ABSTRACT 'I'h e prelmration,reduction,activi ty and therm8.l stability of ~-Fe203 based precipitated catalysts have been systematically studied. An Lncrease of both:a)the rate of reduction and b) the specific activity with the increase of the atomic number of the alkali metals used as promoters was established. A method for the preparation of catalysts with a large specific surface area,s~~able porosity and high therrrlal stability is proposed.The specimens promoted with 0,8 at.% Rb or Cs have shown a specific and total actiVity comparable with that of widely used industrial melted catalysts. INTRODUCTION The precipitated catalysts for arrunonia synthesis have not yet been used industrially ,but are considered as promising(I-5). Besides,theypermit a more uniform distribution of the promoters, a larger specific' surface area and a higher porosity to be obtained.This offers a principal possibility to achieve a higher total activity. The present communication contains results concerning the influence of the preparation conditions and of the different alkali prollioters on the final properties of this type of catalysts. EXPZRINiENTAL Catalysts

422

Type A. Different specimens were prepared by Impz-egne.t Lon with alkali hydroxide solutions of ol-Fe containing A120 (3 ! 0,2 wt. 20 3 3 %) in the form of a solid solution.Before impreenation this composition ~as heat-treated for 4 h at BOOoe.The mixed Fe 0 20 3-A1 2 3 oxides were pre~ared by thermal decomposition of the coprecipitated hydroxLdes at 20 0e and pH=9.The content of the alkali promoters \";as:O,84;I,68 and 3,36 at %,respectively.For specimens containing K as promoter this corresponds to 0,5;1,0 and 2,0 wt % of K20.

Type B. Specimens \'/i th the carne c ompos Ltions were prepared by addition of alkali metal hydroxides to the fresh coprecipitated hydroxides of Fe(III) and Al and subsequent calcination for 4 h at 800 0 e. kodel reduction measurements have been pcrf'ormed \'d th 0( -2e 20 3 aarn oLea containine 10 at.% of Le (1ja'~l..i,Na,K,Rb or Cs) and -prepared in the same manner. aethods.The activity of the catalysts was tested in a pilot-plant installatiop,in which the preliminary reduction of the catalysts waa also performed(8) .Fractions of the cate,lysts with grain size of 0,4-0,6 mm were used.The reduction was perfor1,ed in a stepwise regime for 36 h in the temperature range 300-500 0C.A s-pace velocity of the N2-H 2 mixture of 3.10 4 h- I and a pressure of 50 a tmvwez-e chosen.The activity was measured in the terrJperature range 0e 350-550 at a presBure of 100 atm and a space velocity of 3.10 4

h- I.

Mercury porosiraetry and a modified .i:l-C;T method wer-e ap o.Li.ed for determination of the pore distribution and the specific surface urea of the reduced and unreduced catalysts. The I\;ossbauer spectra were taken at room temperature, the source used being e0 57 in Pd.The conatant acceleration spectrometer vias calibruted with rJ...-ie and the centroid of its spectrum was used as a reference -point forthe isomer shift measurements. ~SULTS

AND DISCUSSION

To obtain a rraximun stabilizine effect of the textural proffioter-A1 20 conditions of its incorporation in the form of a 3,the solid aolution were stuJied.As cen be seen fro~ Jig.l .,the for~u tion of a solid solution of A120 in ol-le 20 proceeds at an ap3 3 propriate rate at BOOoe.This temperature also permits the formation of a large specific surface area of the final catalysts.

423

This indicates that in both-type A and type :a catalysts,i.1 2 0 is 3 practically completely in5,425 r------------, c or-porv t od in the f orr. of a solid solution. ~ossbauer spectra of Fe(III) 6el heated at 80, 330 and 400 0 C showed its couplete transformation to after 4 h heatine at 400 0C.The DTA curves showed the exothermal ef5,405 fects of o(-Fe 20 crystalli3 zation in the temperature 0C range 460-670 dependin to on the amount of the aluminium component in the 5,395 L--._~_.r--_.r-----' system. Obviously, this com5 10 15 20 ponent increases the stabimol% Al203 lity range of the X-rays Fig.I.Variation of the lattice ronorphous component in the system,i.e.it has a retarparameter of cl-Fe203 as a function of the A120 content after 4 h dation effect with respect 3 heat-treatment at different tempeto the crystallization. It has also been estabratures:I-5000C;2-6000C;3-800oC(and 950°C) . lished that this promoter (A1 20 has a measurable 3) effect on the rate of cGtalyst reduction. However the effect of the alkali promoters 1s more pronounced.As can be seen in Eig.2 the rate of reduction increases with increasing atomic number- of the alkali metals during the first stage of reduction.On the other hand,it was found that in the case of A type catalysts the alkali metals do not change the basic mechanism of d.. -Fe 20 reduction. 3 The Illossbauer spectra of specimens reduced up to 11,33 and 92 f. corresponded to the spectra of mugnetite,and maenetite and~-Fe, respectively( 5 ),i.e. to those observed during the reduction of ~-Fe203( 6 ).The main phase transitions durinG the reduction are: Fe203----~Fe304----~o(-Fe.Only at higher temperatures(above 500°C) the formation of an intermediate ~vstite phase was established. The kinetics and mechanism of the reduction of B-type preparations showed a detectable difference.In this case the formation ~-Fe203

a

424

of ferrites was established. In the case of LiOH additive,the fornmtion of 1iFe508 was confirmed;in the case of NaOH,NaFe0 2 and Nale50a;in the case of KOH,RbOH or CsoH-IIFe 20y1.e20,where i..e=K,Rb or ca, The kinetics of the reduction ofc(-Fe 20 impregnated with al3 kt'.li hydroxide sOlutions(IO at.% of alkali liIetal in the final ~reparation} and calcined 4 h at 800 00 is illustrated in Fig.2.

_0,9 c

e

.\~. :~3 •. : "',., ·w V " \. s~eeific surface areas : . . .',: :; \,II'~.:-:; '. c. , , of the catalysts to 048 -8 -4 different dee:reeD am1 in ooposite directions. V [mm/sl As can be scen froln the data in Table I these ?ig.3.fuossbnuer spectra of: I-11re 20 different influences 3.K 20 2-rH'e 20 reduced up to '? =11: ~~ arc .nos t pronounc e« in 3.K 20 3- ~ -Fe 20 + KOll, reduced UO) to =11: I~ the course of tile re3 duction. I ,

:.

'1

TABLE I surface area of the catalysts at different derrees of reduction,

~pecific

Composition of the specimens

S~ecific

'l = J. -Fe 20 3 ci. -Fe 20 3

o(-Fe 20

3

+ CsOH + esOH + Al 20 3

surface area(m 2/g) at

° % 9=5-6 %

0,26 1,05

0,83 0,9

4,6

5,7

1=

'7=IO-II % '7=98-100;' I,r 0,47 10,6

5

28,4

The difference between the promoters is more pronoWlced when comparing the total and the specific activity of tho catalysts. Data on the total and the specific rate constants for A type

426

catalysts are summarized in Table 2 Table 3.

and for B type catalysts in

TABLE 2 Data on the catalytic activity of A type catalysts(K 4oo-total rate constant at 400oC,K~OO-specific rate constant at 400 oC) Alkali promoters in at.%

Specific 'surface area, m2/g

K 400

K~OO

atmO,5/h

atmO,5/hm2

Li

0,84 1,68 3,36

23,4 18,1 13,1

84 312 472

2,6 12,3 25,3

Na

0,84 1,68 3,36

25,7 17,1 6,2

156 652 366

4,3 23,9 42,2

25,6 21,2 10,7 27,4 23,2 14,4 28,9 19,9 12,6

936 827 692 2519 2262 1787 2613 2177 1869

26,1 27,9 45,8 64,7 69,4 88,1 64,7 77 ,8 105,6

15,5

2008

62

K

0,84 1,68 3,36 Rb 0,84 1,68 3,36, Cs 0,84 1,68 3,36 Industrial catalyst CA-I

These data havG shown sGveral basic tendencies.First of all it is worth noting the increase of the promoting action with increasing atonic number of the alkali luetals.This tendency is nost pronounced when compar-Ing the specific r-a t e constants of the reaction,calculated according to the Tenkin-Pizhev equa.tion.A sharp increase of the values as a result of the transition from Ii to Cs is observed.A oecond tendency is the increase of the specific rate constants ve.Lues \'/i th the increase in the alkali metal contents.On the other hand the increase in this content leads to an unfavorable decrease in the specific surface areaS of the catalysts(Table 2 and 3).As a result the total activity of K,Rb or Cs promoted catalysts,which are i;nportant for the practice,c1ecreases with the increase of the alkali metal content from 0,84 to 3,4 at. %.In the case of Li promoted catalysts the total activity increases and in the case of Na promoted apec uaens it passes tihz-ough a

427

mnxiuum.1t should be notea,however,that the latter two catalysts do not seem promising. TABLE 3

Data on the catalytic activity of B type catalysts(K 400-total rate cons terrt at 400°C ,K:OO-specific rate constent ,at 400°C) Alkali promoters in at.%

K

Spo.cific surface area, m2/g

atmO,5/h

Ke400 E'.tm0 , 5/m2h

K400

0,84 1,68 3,36

23,0 15,4

II,I

472 430 312

12,8 18,9 19,9

Rb

0,84 1,68 3,36

24,8 16,6. 13,4

1225 II05 830

34,6 47,0 43,7

CS

0,84 1,68 3.36

28,4 17,7 10.6

2177 1787 1434

54,2 72,1 96.9

Th2 Dro~oting action of the alkali metals can be related to local changes of the \York function( 7 ).Thesc changes facilitate the electron transfer to the ni·tro~en molecules adsorbed on the catalyst.On the basis of more general considerations it can be expected that the change of the wQrk function will increase wi th the increase of the at onu c number- of the alkali aletals. Cs, which has the lowest value of this parameter has also the highest promoting effect. According to some other concepts the action of the alkali promoters should be related to a neutralization of the acidity of A120 this way the strong chemisorption of m~lonia on the ca3.1n talyst surface is avoided.?or the present this concept cannot be completely ignored.However the influence of the alkali pr-on.o teron the electron transfer in the rate liutiting stace of the reaction seems to be prevailing. The results summarized in Tables 2 ~nd 3 support the view that the content of the best prollioters,i.e. Hb and Cs,should be below 0,8 at.%.Otherwise the total activity of the catalysts decreases and the sintering processes are accelerated. It has been established that the partial sintering of the catalysts at BoOoc leads to e. significant improvement of their

428

mechanical properties.Their mechanical strength remains well over 150 ke/cm 2• of this type of catalysts is their A very Lnrpoz-t r.n t pro~)erty re3i~tivity to overheating and poisoning. For an estimation of these parameters, samples were treated for 2 hrs at 700°C or in the presence of water vapour (CH20 = 0.1 vol.%). The ratios of the specific rate constents with respect to the amillonia synthesis K' and K'400/ K 400 a s well as the spe400'IK '·400 s s cific surface arena 3~S before end after heating are given in Table 4.

TABLE 4 Date. on the resistance Alkali promoters at. %

of A type ca't a.Ly a t s to overheating

.r

_K_IOO,fa

S'

K

S

100,%

K'B

K

0,8 3,4

67 39

84 78

80 50

Hb

0,8 3,4

73 57

86 77

84 74

0,8 3,4

76 65

87 81

88 80

Cs

100,%

Ks

Table 5 contains the values of the same paror:eters before and after poisoning.

TABLE 5 Data on the resistance Alkali pr-ono t e rn , at. ,.f/0

of A type cetnlysts to poisoning .,/

K'

100,5~

-"-IOO,;~

K

s

K'

__ s_, 100,%

Ks

K

0,8 3,4

85 60

91 85

93 69

Rb

0,8 3,4

89 73

93 87

96 84

92

97

96

Industrial en tf!.l.Y.:Jt

429

The following more important tendencies have been observed: a) The chemical nature of the alkali pro;;:oter influences, although to a small extent,the resistance ae-aiDS~ overheating end poisoning. The catalysts promoted with RhOH or CsOIf showed a higher stability as com9ared to those 9romoted with KOB. b) The stability decreases with an increase i~ the content of the alkali promoter. The stability of A and 3 type catalysts, obtained by imprcgnation(A type) or impregnation and consequent sinlultaneous calcinatidn of the components are close,but B tYges are more stable. It has also been established that the resistivity of 3 type catalysts ~romoted with RbOH or CSOH(0,84 at.% alkali metals) is close to that of the industrial catalysts CA-I. As can be sup~osed,overh3ating and poisonine lead to a chaneo in the porous structure of the specimena.A cOlllplete or e. partial transfer from a bidisperse to a monodisperse porosity accompanied with a remarkable decrease of the total pore volume was observed.

600l

_50

o VfO,223cmtg .VTO,161 cmtg

->40 ';f.

30 20 10 0,01

o,os

0,3

Fig.4.Pore distribution for a doubly promoted catalyst(3,0 wt.% A1 20 + 0,64 at.

% K):

3

I-before overheating 2-after overheating at 700°C A comparison of the total and specific activity of the precipitated catalysts prepared by the proposed methods shows that

430

they are comparable with and even hither than those of some widely used industrial catalysts prepared by melting. A further improvement of the precipitated catalysts can be achieved by introduction of additional promoters. In 6eneral,the results of the present study show that the precipitated and partially sintered ammonia synthesis catdlysts are promising for industrial application. REFERENCES 1.V.S.Kornarov,E.• D.2ffros,G.S.lemeshonok,A.T.lozin,Veszi Akadernii Navuk BS8R,Serya Khim.navuk(Russian),I (1978) 15 2.A.T.Rozin,V.J.KoDarov,L.D.~fross,G.3.j~G~eshonok,J.I.~rcmenko,

Veszi Akademii Navuk 33SR,Serya Khim.naVUk,No5(IgCi),35. 3.A.T.Rozin,V.~.Komarov,k.D.Efro3s,G.J.lemeshonok,Veazi Akademii Havuk 13.3Sit,Jerya khim.navuk,No2 (1900) ,27 4. O.;~. Tihonova,~. I. Zubovd(NH 4] C1 2 Pd(C )Z

it

Hld21~

Z

catalysts Pd conc.% 0.88 3.47 0.95 4.26

T

s OK

416 411 3Z8 r.t.

Tm OK 453 465 373

Molar ratio H/Pd 1.13(1)

1.00

(1) Including a small additional peak with Tm Ts T m

starting reduction temperature temperature of maximum reduction rate

It may be seen that the reducibility decreases in the order (as precursors):

The catalyst having H as precursor consists mainly of PdO, which is ZPdCl 4 easily reduced by hydrogen even at room temperature. The highest reducibility of the Pd(C3HS)2 catalyst with respect to that of

434 the catalysts prepared by aminocomplexes is to be connected with the higher stability of the Pd-NH with respect to the Pd-allyl bond. According to Yermakov (ref. 4) the interacti~n of Pd(C with silica hydroxyls gives surface spe3H5)2 cies si-0-Pd'Pc H2';-CH. . ' H2 re d ' . .1S evolved from the am1nocomplex . Dur1ng the uct10n ammonia cata I ysts. Its amount agrees satisfactorily with that determined by chemical analysis. The molar ratios NH are reported in Table 2 for catalysts with different Pd con3/Pd tents prepared from Pd(NH )4(OH) • These values are slightly higher than 2,apart from some samples prepareJ with fiigh excess of NH , thus showing that the surface species obtained is probably 3

Si -

o

Si -

!+ has first to be decomposed to give easily reducible species. N. PERNICONE I agree with you that zero hydrocarbon order is more common in olefins hydrogenation. However your suggestion about our data cannot be accepted for the following reasons : -due to our particular hydrogenation procedure, impurities in hydrogen and/~r solvent,if any, cannot give an apparent first order, because enough time available for their interaction with pd before starting the reaction; -from the results of the experiments carried out at different initial concentrations of l-octene, the presence of poisons in the olefin can be excluded. Apparent orders lower than 1 can be found, in our opinion, when too high reaction rates are used in the kinetic experiments. We have not calcined our catalysts during their preparation. One of the results of our work is that it is possible to obtain high Pd dispersions starting from amino-complexes without any need for calcination.


441

l'nEPARATION OF NON-PYROPHORIC ME;-l'ALLIC CATALYSTS A.V. KRYLOVA, G.A. USTILffiNKO and N.S. TOROCHESHNIKOV D.I.Llendeleyv's Institute of Chemical Engineering, Moscow (U.S.S.R~

ABS'l'RACT The data on the mechanism of passivation and pyrophoric oxidation of ammonia synthesis catalys~obtained by various physico-chemical methods have been generalized. It has been shown that the participation of weakly-bonded adsorbed oxygen in passivation is responsible for a long duration of the process and instability of the protective layer. The temperatures have been determined (-195°- -85° and 450-550 0C) at which thin protective layers with an enhanced stability to oxidation are rapidly formed on the catalyst surface. Effective methods have been proposed for the elimination of the ?yrophoric nature of the ammonia synthesis catalyst and of other catalysts.

IN'rRODUC TION Many metallic catalysts are pyrophoric. Preparation of non-pyrophoric catalysts is very important for reduction of the catalysts outside the converter, for repair work inside the converters and for removal of spent catalyst. Non-pyrophoric catalysts are prepared by passivation in a nitrogen flow with a low oxygen concentration at 20-60 0C [1J • Passivation is a long and complicated process which is carried out under emnirically chosen conditions, requires an exact content of oxygen in the nitrogen flow, and does not provide for a stability of the protective layer. Passivated catalysts undergo further reoxidation and overheating when the storage conditions are violated. The aim of this study was to consider the mechanism of oxygen interaction with the ammonia synthesis catalyst under passivation and oxidation within a wide range of temperatures and partial oxygen pressures and to develop new effective methods of preparing non-pyrophoric catalysts.

442

The samples of the industrial aWfionia synthesis catalyst contained as promoters 3 mass % of A120 , 1% of K20, 2.5% of CaO, 3 and 0.7% of 5i02• The weighed samples from 5.0 to 1300 g, fractions 2-3 rom were used in the experiments. The details of studying the interaction of oxygen with the catalyst in the course of passivation and the properties of passivated catalysts with the aid of various physico-ehemical methods have been published earlier [2-8] • To study pyrophoric properties, use was made of a flow-type installation into the reactor of which (with a reduced sample) a flow of dry air was delivered at a preset velocity. 'I'her-mograms o f heating, oxygen consumption, and change in the catalyst weight were determined during oxidation. The pyrophoric properties were characterized by a maximum temperature of pyrophoric heating of the sample (Tmax) , time of oxidation (q;) determined by completion of oxygen consumption, and the oxidation degree of the catalyst (~) calculated by add-on weight of the sample.

RESULTS The passivation mechanism According to the pulse and microcalorimetric methods [2-3] , passivation of the catalyst at room temperature is caused by adsorption of 7-10 molecular layers of oxygen as calculated for metallic surface. The oxygen of the passivating layer is non-uniform. Approximately one oxygen layer is weakly bonded to the surface and is removed from the sorption centres in helium and hydrogen flows at temperatures up to 200°C; 2-4 layers are mobile and can be removed in a helium flow at 400-500°C; 4-6 layers can be removed only by hydrogenation and are, therefore, the oxygen of oxide. The initial oxygen adsorption heat is close to the known value of the formation heat of iron oxide and is 95±5 kcal/ /mole. In the process of filling the passivating layer the heat of oxygen adsorptiori was observed to be less than 20 kcal/rnole. The formation of different states of oxygen under passivation conditions was confirmed by the methods of isotopic exchange [4] . Fig.1a illustrates the kinetic curves of a homomolecular oxygen exchange on a passivated catalyst which proceeded at a high rate and low temperature s, was charac terized by an ac ti vation energy of about 2 kcal/mole and, consequently, pointed to the participation of weakly-bonded oxygen in passivation. The exchange reaction was

443

terninated when carbon oxide, hydrogen, or water was introduced into the system because of the replacement of this form of oxygen. Fig.1b shows that the exchange rate decreased rapidly when the catalyst was kept in oxygen for 24 hours; then the decrease was slower. After contacting the catalyst with oxygen for 8 days, however, the exchange rate exceeded the rate in blank experiments by an order of magnitude.

a)

0,6

b)

7)5 ~

c:u ~

~

~

0,+

.5,0

~.... ~

I ~

'-

..... 25 1::::)'

-

,

~O,2

-

~

1

2

3

4

5 cr,mln

2

6

8 ~dogs

Fig. 1. Hornomolecular oxygen exchange on passivated catalyst: a) the effect of the impurities on the exchange kinetics; b) the effect of the catalyst exposure to oxygen on the exchange rate. This points to a low rate of transition of weakly-bonded oxygen into strongly-bonded one. The number of the molecules participating in a heteroexchangeat low temperatures, calculated by a change in isotope content, was 0.1-0.5.10 19 molecules 02/m2Fe, i.e. about half of the molecular layer in accord with the data on desorption of weakly_bonded oxygen. On the catalysts in an oxide form and on partially oxidized with oxygen at 300-500 oC passivated catalysts no" exchange was observed at low temperatures and only within the temperature range from 300 to 500°C the exchange took place with an activation energy of 30 kcal/mole. Thus, as follows from the" data on the exchange reactions, oxygen on the surface of passivated catalyst is not identical to oxygen of iron oxide. Unlike the passivation at room temperature, passivation at 80-100 oC decreased the rate of low-temperature isotopic exchange and strengthened the bond of the oxygen with the catalyst.

444

The electron work function measured upon oxygen adsorption within the temperature range -30-180 oC on an iron catalyst [5J also indicated that oxygen is present in different states: adsorbed negative dipoles, formed at the initial moment, passed into dissolved or oxide oxygen (partially at low temperatures and completely at elevated temperatures). The study of industrial passivated catalyst by thermodesorptir on and deri vatographic methods [6J has shown that passivated catalysts contain a great amount of admixtures, including more than 1 mass % of wate~, and are stable to oxidation in air below 107°C. Adsorption and other methods in which deuterium and oxygen isotopes are used have demonstrated that water may remain in the catalysts after reduction, be formed in the process of passivation and adsorbed upon contact with air. The relationships for adsorption, nature of the oxygen bond in passivated catalysts, thermal stability, evolution of the products of destruction of t)le passivated layer and admixtures found for industrial and model ammonia synthesis catalysts with a modified composition of promoters proved to be similar. The data considered allow a conclusion to be drawn that the forrnationof weakly -bonded oxygen, its slow transition into a strongly-bonded state, and possibility of its replacement with admixtures (water, hydrogen, and others) are responsible for a long duration of the industrial process of passivation and for an instability of the protective layer. Pyrophoric properties of the catalysts It is known that the contact of catalyst with air under temperature conditions of passivation results in pyrophoric oxidation. The main specific feature of pyrophoric oxidation consists in that the reaction proceeds at an increasing temperature. Using in our experiments the ammonia synthesis catalysts, low-temperature and high-temperature shift catalysts, catalysts for methanation and others, we have established that pyrophoric properties depend mainly on the concentration, dispersion and type of the metal in the catalyst; for the same catalyst it also depends on the degree of reduction and space velocity of air. Tmax may vary by hundreds of degrees depending on these factors. Pyrophoric nature of the ammonia synthesis catalyst depended slightly on the composition of the promoters and on the presence

445

of the residual hydrogen. When the catalyst fraction decreased from 4-5 n~ to 0.5-1 rom, Tmax increased by 70°C.

-1,0 2,30 2.20 ~

+~

Q

-~ .~.

,":".: "

+1,0

"

2,-to

. . .;.:.

45'OO~90 '''.\

~;

~

~

-,

4:80

~ I ..

470

' .

I •

'J. mm/s

/ .•...... ..; 4

.:..{' r" 2

'-". :

:

::

:".:: I

' . ' '. , .

2~9~ .....: " ", ..-. .'._.'. ..... ..... . ......'......3 219 ~Og _ ~ ~ ~ .{. ': .: ::,:..,\: .~"

t

~~

I r ·:~·: :·-· · /·': '·\ ~~ 1~

I

4 Fe~O~

6

Fig. 2. M~ssbauer spectra of the catalyst after pyrophoric heating up to: 1-150°, 2-225°, 3-350°, 4-400°. Fig. 2 shows Mossbauer spectra of the ammonia synthesis catalyst subjected to pyrophoric oxidation to temperatures of 150-400 0C [7J The spectra points to the formation of the magnetite phase during pyrophoric heating. The spectrum of the samples after slight heating displayed only one, the most intensive, peak 6 of magnetite corresponding to the beginning of oxidation. At higher temperatures intensity of this peak increased and the peaks IA and IB appeared. A change in the spectrum parameter H (the effective ef f magnetic field) pointed to an increase in size of crystallites as temperature of oxidation increased. ThUS, pyrophoric oxidation results in a fast formation of the oxide phase in the catalysts. Short-time heating of the catalysts in air up to 400°C did not affect the activity. Heating in deriva~ograph in an air flow up to 900°C resulted in (according to the data obtained on a scanning microscope [8] ) sintering and in a redistribution of the ingradients in the catalyst (enrichment of the surface with iron). As a whole, the data on studying passivation and pyrophoric oxidation of the catalyst with air have shown that temperature and

446

pressure of the gas are the main factors determining the nature of the bond of oxygen with the catalyst. Preparation of non-pyrophoric catalysts One may assume that in the process of passivation oxidation of the bulk of the catalyst is slowed down because of a low oxygen pressure. The same factor determines slow oxidation of the sUJ;face and instability of the protective layer. The process of surface oxidation can be accelerated and bulk oxidation slowed down either when pr~ssures of oxygen are high and temperature low or at high temperatures when the rate of surface oxidation exceeds that of oxygen diffusion into the bulk of the catalyst. In connection with this the interaction of the ammonia synthesis catalyst with oxygen was studied at temperatures being varied from 20 to -195°C and from 20 to 550°C under different concentrations of oxygen from air to pure oxygen.

x,% 4D 30

-too -100

0

iOO 200 . . 48Q SOO

18,oe

Fig. 3. The oxidation degree of the ammonia synthesis catalyst as a function of temperature of contact with air. Fig. 3 shows the oxidation degree of the industrial ammonia synthesis catalyst as a function of the temperature of contact with air. Two temperature regions (-85- -195°C and 450-550°C) were revealed in which the contact of the catalyst with air leads

447

to a low oxidation degree (3-4%) typical for the process of preparing passivated catalysts. Within the intermediate temperature region the interaction of the catalyst with air results in considerable oxidation of the catalyst. Near~5°C the transition point is established from a low (3%) to a very high (36%) oxidation degree. The maximum value is observed near 225°C. With further increase in temperature the value of ~ gradually decreases to about 4% at 500-550°C. The temperature dependence found did not change its character when oxygen content in air increased or pure oxygen was used. The results obtained show that for obtaining protective oxygen layers on the surface,it is reasonable to use the temperature regions corresponding to the minimum oxidation degree of the catalysts in which the possibility of local overheating is excluded because of a fast termination of the oxidation reaction. The study of pyrophoric properties of the catalysts has confirmed the efficiency of the processes of preparing non-pyrophoric catalysts within the above temperature range. Table 1 lists the values characterizing low-temperature interaction of the catalyst with air. '1'ABL.I:!l 1

Pyrophoric properties of the catalyst at low temperatures Temperature of primary contact wi th air T ,oC

Maximum tem¥erature of heating , °C max

Time of oxidation q: , min

Degree of iron oxidation X. , %

0 -83 -85 -87 -136 -195

372 366 35 27 30 31

36 42 1.5-2 1.5-2 1.5-2 3

33 36 3.1 3.0 3.1 3.3

0

The treatment of the catalyst with air at temperatures below -85°C makes it possible to obtain non-pyrophoric catalysts instantaneously (for 1.5-2 minutes). Passivation of the same amount of the catalyst by the traditional method requires 2-3 hours. Stabilization with air of copper-containing shift catalyst took place in a wider temperature range from -195 to -40°C for several minutes and resulted in a degree of copper oxidation in thecatalyst of about 7%. At temperatures from -39 to 25°C the reaction

448

time increased up to 20 minutes and interaction with air led to pyrophoric heating up to 230-285°C and to almost complete oxidation of copper in the catalyst. 'rable 2 lists the values which characterize the process of high-temperature interaction of the ammonia synthesis catalyst with air. Heating of the catalyst which takes place" in air flow is almost completely ruled out when air is fed portion-wise. TABLE 2 Pyrophoric properties of the catalyst at high temperatures Temperature of the primary contact wi th air To' °C

Maximum temperature of' heating T" °C max,

Oxidation time ~ , min

Degree of iron oxidationX, %

flow 500 of air 500 portions 550 of air

511

5.5 32 25

6.0

569 505

400

2

555

3.2

4.8

5.9

The catalysts treated with air at 400-550 0 C did not exhibit pyrophoric properties and the formation of the protective layers proceeded at a high rate. Activity of the catalysts stabilized both at low and high temperatures, does not differ from that of industrial passivated catalysts.

500 200

100 iO

20

30

40 150 Torr).

The zeolite is thus cha-

racterized by a well organized structure, an important microporosity (C 6H6 2.g- 1) sorption = IS.4 wt %, S = 672 m and a high thermal stability (up to about SOO°C) . If NH is calcined from 150 to 500°C under high water partial pressure 4Y ( > 500 Torr) a limited collapse of the structure is observed (C sorption 6H6 2.g- 1) 12.2 wt %, S = 406 m (43, 44). All these results can be explained by the intervention of two competitive modification processes : one is aluminum extraction from the framework, the other is migration of silicon created by the removal of aluminum. on the pperating conditions. is much faster than silicon

toward holes

Relative rates of both processes depend

The structure collapses if aluminum extraction migration.

crystallinity is maintained if the

rates of both processes are of the same order of magnitude.

502

SAMPLE

nr

CALCINATION wt%

final

lTemp atmosphere adsorbed C6H6 °C

SAMPLE CHARACTERISTICS S m~g_l

X .ray Diffraction

. 1

450 very dry air

< 1 200torr H2O

4

1Jld II

air... water

500 vapor from 12.2 406 150°C >500tarr H2O)

.1.

".~J.

I

"J

Fig. 14 : X-ray diagrams of NH zeolites after air calcination under various 4Y conditions

11.3. Washing of the hydrated precursor This elementary step aims at removing compounds (by-products of coprecipitation) retained in the hydrated precursor and which are undesirable because of the various following risks : - inhibition or modification of activity and/or selectiVity of the final catalyst - bad evolution of the precursor during the next elementary steps. Compounds to be removed can be divided into two groups : - those dissolved in the mother liquor still present within the porosity ions, mineral or organic molecules. - those fixed at the surface of the hydrated precursor

mainly ions.

503 The first ones can easily be removed by simple washing with distilled water. But the ease of washing will depend

·on the nature of the precipitate.

Amorphous precipitates like hydrogels or coagulates are in general hydrophilic, voluminous and their sedimentation is slow.

They are difficult to wash out

because of diffusional limitations in their particular porous texture.

On the

contrary, well crystallized precipitates decant more easily and can be washed out without difficulty.

This case is well illustrated by Fig. 8 which shows

that the sodium content of washed Cu-Zn-Co-Al

mi~ed

oxides decreases when the

crystallinity of the precursors increases. Removal of ions of the second group requires special washing conditions which will be briefly described.

An amphoteric oxide placed in water acts either as an

anionic or cationic exchanger depending on pH and ionic strength conditions. Its surface, which is indeed essentially

positively or negatively charged,

is neutralized by ions from the solution (45 to 48).

The isoelectric point

(which corresponds to an overall neutral surface containing an equal number of positive and negative charges) is reached when the pH value is equal to pHi. If pH

>

pHi' the solid surface is negative and the solid acts as a cationic

exchanger.

If pH

1.0 calcined at 380°C, the a-

mount of vanadium (V) was lower (60 % and 20 % V(V)

for

P:V 1.16 and 1.25, res-

pectively). The XRD patterns (Table 1) showed the same lines of altered a-vopo

4,

548 but also those of the S-phase that in the patent literature (ref.2) was pointed out as the active and selective phase. The IR spectrum (Fig. 1 b) was also different from those of catalysts with a P:V ratio of band at 1240 cm

-1

~

1.0 and, in particular, a

was present that could be attributed to more condensed phos-

phates (ref. 5). Unlike the catalysts dried at 125°C, for those calcined at 380 °c, the P:V ratio slightly increased after washing; this indicated that the excess of phosphorous was not free on the surface, but irreversibly adsorbed. The washing removed a superficial

phase of vanadium (V) with a P:V ratio of about

1.0; in

fact, only after washing the d-d transition of vanadium (IV) at 775 and 640 nm was present in the diffuse reflectance (Fig. 3 d and e)spectra. The calcination at 500°C did not alter substantially the IR spectra of the catalysts with P:V >1. An increase in intensity of the e-phase lines was noted in the XRD patterns. The amounts of vanadium (V) rose from 50 to 60 % in the case of catalysts with P:V = 1.16 and from 20 to 50 % for P:V = 1.25. The washing procedure removed all the vanadium (V), but did not alter the XRD patterns. This showed that the vanadium (V) phase was amorphous. The Raman spectra (Fig. 2 d)

corresponded to a vanadyl phosphate; when the amorphous phase was removed

(Fig. 2 e) only the two bands of the vanadyl group were present. This indicated that in the amorphous phase, phosphorous was present as P0

group, 4 Bourdes and Courtine (ref. 10) reported that the XRD pattern of the S-pha-

se corresponds to (VO)2P207' but the IR spectrum is different from the one of pyrophosphate. Besides the pyrophosphate group is

active in Raman (ref. 4), un-

like those we found and we observed an endothermic reaction during the transformation of the amorphous intermediate pyrophosphate into the S-phase, while the transformation from the amorphous to the crystalline compound is

exothermic.

Therefore, we think that the S-phase is a more condensed phosphate of vanadium (IV) or a superficially modified pyrophosphate of vanadium (IV). It is also 1 interesting to note that the V = 0 stretching at 940 cm- was the same for vanadium (V) phosphate (Fig. 4 a) and vanadium (IV) phosphate (Fig; 4 b), and therefore the reactivity of the double bond is similar. In fact in the active component, as claimed in the patent literature (ref. 2), these two components were present. The V =

°

stretching in S-voPo is at higher Raman shift (ref. 9). 4 This shows that Raman spectra are very important in the study of these

catalysts, while due to the low polarizability of the constitutive atoms of phosphate (ref. 12) the bands of vanadium are more noticeable. In the case of the catalysts with P:V spectra (Fig. 1 d)

~

1.0, XRD patterns (Table 1) and IR

showed the formation of S-voPo

and chemical analysis showed 4 the complete oxidation to vanadium (V). This phase was not soluble in water. Rapid calcination in air. When the precursor dried at 125°C was directly

549 introduced into the muffle furnace at high temperature, the XRD patterns (Table 1) and IR spectra (Fig. 1 f) showed the presence of B-VoP0 but not of a-vopo 4, 4, for P:V ratios ranging between 0.9 and 1.25, and for a crystalline vanadium (IV) phosphate not reported in literature. The washing procedure reduced only partially the amounts of vanadium (IV), but did not alter the XRD patterns. At P:V ratios

the XRD patterns (Table 1) showed the presence of B-

~1.25,

phase and of other compounds not identified. The IR spectra corresponded to that of B-phase. The chemical analysis showed that 60 % of vanadium (V) could be removed

completely by washing. In this case too no substantial change in XRD pat-

terns

was noted after the treatment. Modification in the reaction conditions. When the calcination of the pre-

cursor was carried out in vacuum or in air plus 1 % butene, only the B-phase was detectable from the IR spectra and XRD patterns (Table 1) for all the P:V

ratios~

the catalysts were completely reduced to vanadium (IV).

va nad ium(IV) phases

(voh

P207 2H 20

pyrophosphate amorphous fi-phase non Identified phosphate

~

CALCINATION

vanadium(V)phases o(-VOPO" modified

GJ 0

Q

slow

non cr ystaillne8 o(-VOPO"

r:==t>

rapidO

0

ct>

ATMOSPHERE OF CALCINATION

GJ c:::::J

ft-VOPO"

0

AIR

1i::;:I:iJIA 1% BUTENE IN AIR

P:V about 1.0

o DODOr:>[±EJ P:V>l

.

"

&?;.; 0,. 01 t.'io;~:.

C

I.. ._a---'p 0 0 0 0 ~ 500·C

P: V> 1.25

•

~

Fig. 4 Block diagram of the chemical transformations which take place during calcination.

550 Even the modified a-voPo 4 and S-VOP0 could be completely reduced to S-pha4 se at 380°C under an atmosphere of 1 % butene in air (Fig. 1 e), but in the second case the reduction was very slow. When both S-phase and amophous phosphate of vanadium (V) were present, the latter partially reduced to B-phase. In the case of the catalysts prepared by rapid calcination, the treatment with air and butene did not alter very substantially the XRD patterns, but only the line intensity.

CONCLUSIONS All the results can be summarized in the block diagram of Fig. 4. The catalysts with P:V ratios between 0.9 - 1.4 showed the same hydrated vanadyl-phosphate after drying at 125°C; the excess of phosphorous with respect to P:V ratio

= 1.0 was present as mixed vanadium

(V) and (IV) phosphate on the

surface of the catalyst. During the calcination at high temperature new compounds were formed through an intermediate amorphous phase of vanadyl-pyrophosphate. The different compounds, that could be obtained varied according to : P:V ratio;

(ii) calcination temperature;

(iii) calcination atmosphere:;

(i)

(iv)

calcination rate. The presence of an excess of phosphoric acid prevented the total reoxidation of vanadium (IV) and permitted the formation of the active S-phase, as during the calcination in air. The calcination in vacuum or in the presence of butene formed for all the P:V ratios only the B-phase, while in air an additional amorphous phase of vanadium (V), similar to a-vopo

was formed. 4, For the catalysts with a P:V ratio of about 1.0, the calcination in air

caused the total oxidation of vanadium and the formation of B-vOP0

4

through an

intermediate phase of modified a-vopo

4• The treatment with air and butenes formed the B-phase in both cases, but at

a different rate. Rapid calcination did not permit the reaction between amorphous pyrophosphate and the excess of phosphorous and so formed a new compound of vanadium (IV) associated with two phases of vanadium (V). The reduction to the B-phase could not be obtained. The B-phase was probably a condensed phosphate of vanadium (IV) that in the reaction condition is in equilibrium with an amorphous phosphate of vanadium (V).

ACKNOWLEDGEMENTS The present work was carried out with the contribution of the research program "Progetto Finalizzato per la Chimica Fine e Secondaria" of the National

551 Research Council,Rome (Italy).

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

R.L. Varma and D.N. Saraf, Ind. Eng. Chern. ,Prod. Res. Dev., 18 (1979) 7-13. R.A. Mount and H. Rafe1son, u.S. Patent No. 3,330,354 (1975). G. Stefani and P. Fontana, U.S. Patent No 4,100,106 (1978). A. Hezel and S. Ross, Spectrochimica Acta, 23 (1967) 1583-89. D.E. Corbridge and E.J. Lowe, J. Chern. Soc., (1954) 493-502. A.C. Chapman and L.E. Thirlwe11, Spectrochirnica Acta, 20 (1964) 937-947. G. Martini, L. Morselli, A. Riva and F. Trifiro, Reac. Kinet. Catal. Lett., 8 (';'978) 431-435. G. Poli, I. Resta, o. Ruggeri and F. Trifiro, Appl. Catal., 1 (1981) 395404. R.N. Bhargava and R.A. Condrate, Applied Spectroscopy, 31 (1977) 230-235. E. Bordes and P. Courtine, J. Catal., 57 (1979) 236-252. B. Jordan and C. Calvo, Can. J. Chern.,51 (1973) 2621-25. M.T. Pasques-Ledent and P. Tarte, Spectrochirnica Acta, 30A (1974) 673-689.


O. Poncelet, P. Orange and P .A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

METHOD OF IMPREGNATION WITH TRANSITION METAL

553

ALKOXIDES.

VANADIUM-ALUMINA AND VANADIUM-SILICA SYSTEMS

M. GLINSKI

and J. KIJENSKI

x

Institute of Organic Chemistry and Technology, Warsaw Technical University (Politechnika), Warsaw (Poland)

ABSTRACT The method of preparation of vanadium-alumina and vanadium-silica catalytic systems is described. The basis of the proposed saturation procedure is the reaction of vanadyl triisobutoxide with OH groups on the surfaces of oxide carriers. The physicochemical properties and catalytic activity of the systems prepared have been investigated. INTRODUCTION The present work aims to describe the method of preparation of transition metal oxides systems deposited on oxide carriers. The base of the proposed method is the interaction of alkoxides molecules with the hydroxyl groups on the oxide surfaces. The results presented concern the preparation of vanadium oxide-alumina and vanadium oxide-silica systems, although the method described is useful for oxides of all transition metals, which form soluble alcoholates. In one of our previous papers (ref.l) we have presented a method for the modification of the alumina surface with alkali metal ions using alkali metal alkoxides as donors of the corresponding cations. This method gave the possibility of obtaining A1

modified by alkali ions of defined properties and known 203 alkali metal contents, which can be controlled by the precalcination temperature of the oxide and the thermal treatment of the freshly

prepared catalyst.

As in case of impregnation with alkali metal alkoxides, the application of transition metal alcoholates allows the catalytic systems of predicted metal content and properties to be designed. The impregnation procedures carried out in waterfree conditions permits the secondary effects caused by the interaction of water with the dehydrated surface and the formed catalyst to be avoided. The reaction of alkoxides with surface hydroxyls is very selective and the distribution of the deposited metal ions corresponds to the distribution of OH groups on the starting gel surface even at relatively high temperatures of calcination. The composition of the vanadium-alumina and vanadium-silica catalysts obtained by impregnation with vanadyl triisobutoxide confirms the stoichiometry of the principal reaction. unexpectedly, the change of carrier causes dramatic changes of the acidity while maintaining the same structure of surface

554 vanadyl hydroxides. EXPERlMENT~L

A1 used in the present work was obtained by hydrolysis of aluminium iso203 propoxide, previously purified by distillation under vacuum - B.p. 413 K/1.07 2 kN mThe hydrolysis procedure has been described elsewhere (ref.1). Before calcination,Al(OH)3 was dried at a temperature of 313 K for 24 hrs., at 353 K for 24 hrs. and at Si0

39~

K for 24 hrs.

was obtained by hydrolysis of ethyl orthosilicate, previously purified

2 by distillation at normal pressure - B.p. 441-2 K. 500 g of freshly distilled

ester and 1000g of bidistilled water were stirred and heated at a temperature of 318 K for several hours. The water layer was separated and dried at 333 K during 24 hrs. and then at 393 K for 24 hrs. Before impregnation, both alumina and silica were calcined in nitrogen at temperatures of 573, 773 and 873 K for 5 hrs. Grains of diameters within 0.5 1.02 mm have been used for the impregnation. Vanadium ions were introduced onto the surface in the form of vanadyl isobutoxide (prepared according to ref. 2) and distilled twice under vacuum - B.p. 2) 414-5 K/l.07 kN mfrom water-free n-hexane. After impregnation, the catalysts were calcined at a temperature of 573 K in a stream of dry air for 3 hrs. Some of these catalysts were reduced ina stream of dry hydrogen for 2 hrs. at temperatures

of 573 and 723 K.

The number of OH groups on the A1

and Si0 surfaces, as well as on vana20 3 2 dium doped catalysts, was established using the sodium naphthenide titration method (ref.3). The total amount of vanadium ions introduced was determined v5 + ions, by iodine titration of vanadium

gravimetrically and the number of

extracted with HCl from the catalysts surface. The ESR spectra of the catalysts were obtained at room temperature using a Jeol JMX spectrometer. I.r. spectroscopic investigations of the catalysts' acidity have been carried out using a Specord 75 apparatus. For the i.r. measurements, the catalysts samples were 2). 3 2 10 kN m- into thin wafers (10 mg cmA

pressed under a pressure of 2.02

wafer was placed into the vacuum cell and treated in the same way as a normal catalyst. pyridine was adsorbed at room temperature at a pressure of 1.33 kN

m~

After 10 min. exposure the cell was evacuated. The spectra were recorded after evacuation at temperatures of 298, 393, 443, and 573 K. The catalytic activity of the systems under study has been investigated in n-heptanol transformations in the absence and presence of dry air. The

reactio~s

were carried out in a typical fixed-bed flow reactor at a temperature of 573 K, HSLV being 2 (g of n-heptanol/g of catalyst . 1 hr), the flow rate of air being 1 3 5 dm hrRESULTS The amount of vanadium ions introduced onto alumina and silica and the

555 amounts of OH groups on the carriers and on the vanadium-doped catalysts are given in Tables 1 and 2. TABLE Amounts of vanadium ions deposited on A1

203

and Si0

2

and numbers of OH groups

on the precalcined oxides.

Oxide

Temperature of calcination (K)

A1

20 3

Si0

2

Concentration of surface OH groups -1 (mmole.g )

Amount of V5+ ions

Total amount of vanadium ions -1 (mval.g )

(mval.g

573 773 873

0.46 0.44 0.34

0.41 0.46 0.36

0.40 0.41 0.34

573 773 873

0.46 0.31 0.28

0.50 0.33 0.30

0.48 0.34 0.30

In the reaction of vanadyl isobutoxide with the OH groups on A1 surfaces,isobutanol is produced

/

and Si0

2

o-i-C H I 4 9

-Al-O-V=O

V=o

+

)

according to the reaction

-,

"

-Al-OH

203

-1

/

(1 )

+

I

O-i-C

4H g

During the calcination of the impregnated oxides in a stream of air the formation of butenes and 1-butanal was detected. O-i-C H \ I 4 9 -Al-O-V=O

I

I

O-i-C

OH

" I -Al-O-V=O /

(2)

J

OH

4H g

and plausibly OH

?-i-C Hg

4

/

°"

/

4Hg O-i-C H I

-Al-O-V=O

/

" I -Al-O-V=O

Al-O-V=O I O-i-C

4 9

J

°\ °I - Al-O-V=O /

(3)

I OH

J

O-i-C

4Hg 1-butanal being probably the product of oxidation of the isobutanol residue. According to Table 1 the amounts of vanadium ions deposited on A1 Si0

and 203 surfaces are stoichiometric to the number of hydroxyls determined by the

2 naphthenide titration method . The results of the determinations of the OH group

concentrations on vanadium doped catalysts give support to the supposition that

556 TABLE 2 Concentration of OH groups on surfaces of vanadyl alkoxide doped alumina and silica precalcined at 773 K.

Catalyst

Temperature of reduction with H 2

Concentration of the surface OH groups (mmole g-1)

(K)

Alumina vanadyl triisobuto~de

Silica vanadyl triisobutoxide

a

a 573 723

0.85 0.80 0.70

573 723

0.63 0.60 0.62

catalysts calcined in dry air at 573 K.

on surfaces of silica and alumina calcined at higher temperatures, the reaction (2) occurs dominantly - the supported catalysts exhibit concentration of hydroxyls two times higher than the starting oxides and than the number of deposited vanadium ions. Vanadium is present on the catalyst surfaces in the form of

v5 +

and

v4 +

cations. The ESR spectra of the systems investigated exhibit the presence of a characteristic signal' (ref.4) derived from 51 V(IV) ions (Fig. 1). TABLE 3 Changes in the content of 51 V(IV) ions on the surface of vanadium doped alumina

Form of catalyst

after after after after

impregnation calcination in air at 573 K reduction at 573 K reduction at 723 K

4 Number of v + i o n s (a.u.)

5.4 5.6 8.4 29.8

4 The amount of v + ions present on the surface of the freshly

prepared cata-

lyst increases after calcination and remarkably during reduction with hydrogen (Table 3). The catalytic activities of the systems under study are compared in Tables 4 and 5. Pure alumina and silica are practically inactive in n-heptanol transformations and oxidation: the oxidative conversion in the absence or presence of

557 air does not exceed 1 mol %. In the absence of air vanadium-doped Al exhibits 203 activity only in the dehydration of n-heptanol: under the same conditions the silica-vanadium system possesses a remarkable activity in both the dehydrogenation and the dehydration processes. The oxidation activity is much higher for the silica-containing catalysts: the selectivity of the aldehyde formation is also higher for this system. It should be underlined that in the case of the alumina-vanadium system, di n-heptyl

ether~

a favoured dehydration product as

opposed-to pure carriers and silica-vanadium systems which produce selectively n-heptene. For the most active silica-vanadium and alumina-vanadium catalysts, as well as for pure silica and alumina, acidity investigations using pyridine adsorption have been done. The results of pyridine adsorption on the catalysts studied are presented in Fig. 2. The adsorption of pyridine on A1 calcined at 773 K results in the 20 3 appearance of a series of new bands corresponding to the interaction of basic molecules with surface acidic centres (ref.5). The bands at 1440, 1448, 1573, 1 1615 cm- should be ascribed to the adsorption on Lewis type sites and the band 1 at 1487 cm- to the adsorption on Br¢nsted sites. All these bands remain after pyridine desorption even at 573 K.

-400G-05

Fig. 1 ESR spectrum of 51 V(IVl ions on the surface of vanadium triisobutoxide doped alumina; gl

=

1.938, g,

=

1.997 .

558 2000 I

4800

,4600 I

41100 I

Fig. 2 l.r. spectra of pyridine adsorbed on vanadium-silica catalyst; 1- sample of catalyst before adsorption, 2- after evacuation at 298 K, 3- after evacuation at 373 K, 4- after evacuation at 423 K.

TABLE 4 The products of n-heptanol reactions carried out over vanadium doped alumina and silica in the absence of air.

Carrier

Temperature of carrier calcination (K)

573 773 873 573 773 873 573 773 873 573 773 873 573 773 873 573 773 873

Temperature of catalyst reduction (K)

573 573 573 723 723 723

573 573 573 723 723 723

Reaction products (moles from 1 mole of substrate related to 1 mmole of vanadium) di-n-heptyl n-heptene ether n-heptanal 0.10 0.16 0.34 0.03 0.08 0.37 0.20 0.21 0.32 0.08 0.04 0.02 0.03 0.02 0.02 0.03 0.04 0.03

0.18 0.17 0.11 0.12 0.22 0.08 0.15 0.15 0.17 0.01 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01

o o o o o o o o o 0.05 0.06 0.05 0.03 0.05 0.05 0.03 0.03 0.05

559 TABLE 5 The products of n-heptanol reactions with air over vanadium doped alumina and silica.

Carrier

Temperature of carrier calcination

Temperature of catalyst reduction

(K)

(K)

573 773 873 573 773 873 573 773 873 573 773 873 573 773 873 573 773 873

Reactions products (moles from 1 mole of substrate related to 1 mmole of vanadium) di-n-heptyl n-heptene ether n-heptanal

573 573 573 723 723 723

0.17 0.31 0.35 0.12 0.18 0.23 0.10 0.12 0.12

traces 0.02 0.03 0.02 0 0.01 0 0 0

573 573 573 723 723 723

0.24 0.26 0.25 0.28 0.27 0.30 0.29 0.26 0.27

0.01 0.02 0.02 0.01 0.02 0.02 0.01 0.03 0.03

upon adsorption of pyridine on

Si~t

0.09 0.05 0.05 0.08 0.04 0.03 0.03 0.04 0.08 0.09 0.14 0.16 0.08 0.16 0.19 0.08 0.14 0.17

calcined at 773 K intense i.r. absorp-

tion bands appear at 1443 and 1590 em

,corresponding to the interaction with 1, Lewis type acidic centres, and at 1485 cmdue to pyridine in interaction with Br¢nsted centres: these bands disappear practically after evacuation at 373 K. On adsorption of pyridine on vanadium-doped alumina (precalcination at 773 K) and on the same catalyst reduced with hydrogen at 573 K, only very weak bands at 1 1 1485 cm- and 1543 cm(both connected with Br¢nsted sites) appear in the i.r. spectrum, which disappear completely after evacuation at 373 K. On the contrary, the i.r. spectrum of pyridine adsorbed on the silica-vanadium system (precalci-1 ned at 773 K) indicated the presence of intense bands at 1445, 1450, 1487 em 1 (interaction with Lewis centres) and at 1540 cm(adsorption on Br¢nsted sites), from which only the last one disappears after outgassing at 423 K. Taking into consideration the above results, it should be emphasized that the catalysts studied possess quite different acidic properties: silica and vanadyl alkoxide doped alumina are very weak acids, the first being rather of a Lewis character and the second of Br¢nsted, although alumina and silica-vanadium catalysts have strongly acidic Lewis-type properties. CONCLUSION i

The interaction of vanadyl triisobutoxide with surface hydroxyls on alumina

560 and silica leads to the formation of -O-VO(OH)2 groups in amounts corresponding to the number of hydroxyls on the starting carrier. ii

The negligible amount of 51 V(IV) ions present on the surface directly after

impregnation increases after calcination of the catalysts and after reduction with H ( 5-fold increase). 2 iii Vanadyl triisobutoxide supported Si0

and A1 differ considerably in sur2 20 3 face acidity: catalysts obtained by reacting strongly Lewis acidic alumina with

vanadium alkoxide possess very weak acidic properties: on the other hand the catalytic system formed by reacting weakly acidic Si0

2

with vanadium alkoxide

shows high acidity. iiii Differentiation in acidic properties is reflected in the catalytic activity of the catalysts investigated - the more acidic silica

vanadium system

being more active and unexpectedly more selective in n-heptanol oxidation than the alumina

containing catalyst.

Di-n-heptyl-ether seems to be the product of n-heptanol dehydration via a nonacidic pathway. REFERENCES 1. 2. 3. 4. 5.

R. Hombek, J. Kijenski and S. Malinowski, Preparation of Heterogeneous Catalysts, ed. B. Delman, G. Poncelet, p. 595, Elsevier, Amsterdam, 1978. H. Funk, W. Weiss and M. Zeising, Z. Anorg. allgem. Chern., 296 (1958) 36. J. Kijenski, R. Hombek and S. Malinowski, J. Catalysis, 50 (1977) 186. H. Takahashi, M. Shiotani, H. Kobayashi and J. Sohma, J. Catalysis, 14 (1969) 134. E.P. Parry, J. Catalysis, 2 (1963) 371.

561 DISCUSSION A. MIYN10TO : -Is it possible to prepare catalysts with higher V20S contents by repeating reactions (1)-(3) ? -Is the catalyst stable in the presence of, say, H20 at 500°C? -What is the percentage of V4 + in the total V ions supported ? -It may be interesting to investigate various catalytic reactions in addition to the reactions of toluene and n-heptanol. J. KIJENSKI

: -The introduction of the first portion of vanadium alkoxide onto the carrier surface is the starting point of the work presented. The repetition of the procedure adopted is possible, although it should be underlined that the next portion of vanadium ions fixed with their first layer should strongly depend on the acidity of the OH groups in the vanadyl hydroxides previously formed. -The catalysts examined keep their catalytic activity as well as their yellow coloration and paramagnetic properties even at 823 K and in the presence of water. -The amount of V4 + ions supported onto the carrier surface is in the range of 0.8% with respect to the total amount of vanadium. -The reactions reported have been chosen as catalytic tests; further oxidation reactions are under study.


G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publisbers B.V., Amsterdam - Printed in The Netherlands

563

THE SIGNIFICANCE OF THE MULLITE PHASE IN A SILVER CATALYST FOR THE OXIDATION OF ETHYLENE INTO ETHYLENE OXIDE LIN BING-XIONG, ZHANG WAN-JING, YAN QING-XIN, PAN ZUO-HUA, GUI LIN-LIN and TANG YOU-CHI Laboratory for structure of Matter, Institute of Physical Chemistry, Peking University, Beijing (P.R.C.)

ABSTRACT An a -alumina support with suitable pore structure and good heat transfer properties has been prepared. Silver dispersed in uniform grains of suitable size is important for the selectivity of the silver catalyst. IVe tried various preparative procedures to disperse silver on the support, and found out that the master key to success was the addition of a muL'l i, te phase to the

a -alumina

support. On recognizing the significant role played by the mullite phase, we prepared the support for the silver catalyst from corundum and alumina-silica gel. Satisfactory results have been obtained.

INTRODUCTION Silver has a unique activity in this industrially important reaction, and is normally used on an a-alumina support. The complete oxidation of ethylene to carbon dioxide and water produces more than ten times as much heat as the desired reaction. The highly exothermic reaction leading to complete combustion has a higher activation energy than that leading to ethylene oxide, and the selectivity therefore falls abruptly with increasing temperature. An increase in temperature leads to a fall in selectivity and lower selectivity causes the temperature to rise further. Thus, the reaction has a pronounced tendency to be involved in a vicious circle. Very careful attention to temperature control is required in the design of the reactor. On the other hand,we have been very attentive toward the intrinsic selectivity of the silver catalyst. This paper describes the preparation of a suitable support for the silver catalyst.

GUIDELINES FOR A GOOD

a -ALUMINA SUPPORT

Because heat transfer as well as selectivity are of vital importance for such a silver catalyst, we had to choose in the first place a supporting framework with good heat transfer properties, small specific surface and relatively coarse pores. Secondly, high selectivity requires silver dispersed in uniform grains of suitable size. For framework material we chose corundum powder. How-

564 ever, we observed that

a-alumina itself is not good at dispersing and carrying

silver. Investigations have enabled us to establish that mullite crystallites can provide the right surface for dispersing and carrying silver.

MULLITE

SU~ACE

AND DISPERSION OF SILVER

Observations Silver is poorly deposited on the naked a-alumina surface (Fig. 1a). Silver is obtained in the usual way, i.e., by reducing silver lactate soaked into an a

-alumina support. However, silver can be dispersed into fine grains on the

mullite surface

(Fig.

1b). The mullite phase consists of prismatic crystallites

(Fig. 1c). Mullite prisms of about 1

~m

in width carryon them silver grains of

comparable size. Figure 1d shows silver deposit on an a-alumina surface only partially covered by mullite prisms, and the uncovered part is seen to be barren. Our findings strongly suggest that the mullite surface may have some peculiarity which facilitates the reduction of the silver salt.

Figure 1 (a) Deposit of silver on naked a-alumina; Figure l(b) Dispersion of silver on mullite prisms of about 1 ym in width; Figure l(c) Mullite crystallites; Figure l(d) Silver deposit on an a-alumina surface only partially covered by mullite prisms

565 The structure of the mullite phase The structure of the mullite phase has been determined

(Ref. 1). It is

closely related to the structure of sillimanite and andalusite. Both of them have the composition AI

The mullite phase is richer in aluminium, and 4Si Z0 10. contains less oxygen. Its composition ranges approximately from AI4.5Sil.509.75 to AI4.SSil.Z09.6' The structure of sillimanite may

well be depicted by the

structure formula [AI~

. ,+10 [ ] -5 ] -') [ AIZSlZOZJ AI0 4

-5 chains formed by AI0 octahedrons through sharing edges run 6 4] parallel to the c-axis at each corner and the center of the unit cell. They are

where the l AI0

bound together by tetrahedral silicon and aluminium atoms with additional oxygen . +10 in the formula. Tetrahedrons are paired ZSl Z0 2] by sharing vertices. The mullite phase has a similar structure except that the atoms, as represented by [ AI

arrangement of tetrahedrons is involved in disorder. For a mullite phase of the composition AI4.SSil.Z09.6' the stucture formula is as follows: A10

-5

4

]

(AIO.ZSil.Z)AI0.S01.J +10 [ AI0 4] -5

The tetrahedral atoms which bind the

[AI0

two sets. Each of them is allocated four

-5 chains together are grouped into 4] equivalent positions. The first set is

very similar to the one occupied by the tetrahedral atoms in sillimanite. Were it not for the higher content of aluminium, all the tetrahedral atoms in the mullite phase should have occupied the four positions in the first set, though in a random distribution. In fact, a part of the tetrahedral aluminium atoms have now moved to the neighbouring positions of the' second set and each of such aluminium atoms requires that an oxygen atom moves to a new position which surrounds not only this aluminium atom but also two tetrahedral aluminium atoms occupying the first

set. Two seventh of the tetrahedral aluminium atoms occupy

randomly in this manner the positions of the second set. The structural feature described above for the mullite phase is conducive to the better distribution of charges on oxygen atoms and to the stabilization

oft~

structure as a whole. However, the mullite structure is a highly disordered one. We may reasonably imagine that structural interruptions, disturbances and distortions should occur on the surface as well as in the bulk of the mullite crystals. DISCUSSION Two observations emerge from Fig.l. First of all, concerning the dispersion of silver, the surface of mullite is preferable to that of a-alumina. And the preference is rather overwhelming. The second fact is that the mullite prisms

566 of about 1

in width carryon them silver

~m

grains of comparable size. We may

thus conclude that mullite crystals can provide a more active or satisfactory surfac~

than

a-alumina for the reduction of silver salt into silver and that on

mullite prisms, the size of silver grains can be controlled by the width of the former. In the light of the structure of the mullite phase, we may attribute the peculiar actiVity of mullite surface to the structural disturbances and distortions due to the disorder in the crystal. PREPARATION OF

TH~

MULLITE PHASE FOR THE CATALYST

Choice of starting materials As raw materials we have used kaolin and alumina-silica gel. Kaolin, A1 (OH)4' has a layer structure. By heating up to 980°C, 2(Si 205) kaolin is converted into an oxide (AIO.7SiO.3)203.3 and silica. This oxide has a defect spinel structure similar to that of

Y-AI

and is transformed into 203 fine mullite prisms when heated to 1240 DC. This'primary mullite'is the desired one. However, excess silica becomes silica glass at high temperature and, at 1580 DC, silica glass reacts with corundum in the catalyst support to form the 'secondary mullite', which causes primary prisms to grow into undesirable coarse crystallites. In order to get rid of the excess of silica in the starting material, we chose an alumina-silica gel with the A1

ratio of about 7 to 3. Differen20 3/si02 tial thermal and thermogravimetric diagrams indicate that the sample loses adsor-

bed water at 250°C and is further dehydrated at 490 °C to yield the oxide (AIO.7SiO.3)203.3 mentioned above. At 1270 DC, this oxide is completely converted into mullite crystallites. They are desirable prisms as shown in Figure 2a. For comparison, Figure 2b shows mullite crystallites nurtured by the secondary mullite, as prepared from kaolin and

a-alumina at 1580 DC.

Figure 2(a) Primary mullite from alumina-silica gel which approximates the chemical composition of the mullite; Figure 2(b) Mullite crystallites nurtured by the secondary mullite as obtained from Kaolin and

a-Alumina at 1580 DC.

567 Size control of mullite crystallites First of all, we have to realize that we are preparing a mullite phase for the catalyst. And in the preparation of the catalyst support, alumina-silica gel has to be calcined in the presence of a-alumina or corundum. In this case the basic factor which controlli the size of the mullite crystallites is obviously the A1

ratio of the alumina-silica gel. Excessive silica leads to the forma20 3/Si02 tion of s~condary mullite at high temperature and spoils the preparation. Besi-

des, we

have found two other factors which influences

the crystallites size.

Whether the calcination takes place in a closed atmosphere or under a gas flow also makes remarkable difference. Q'1antitative experiments reveal that calcination under a gas flow always gives better results. Closed atmosphere favours the formation of silica glass and gives less primary mullite.And the more so, the more the gel contains alumina. In order to enhance the strength of the catalyst support we add a small amount (about 1 %) of MgO as flux. This additive also favours the formation of silica and reduces the yield of the primary mullite. Figure 3 summarizes the influence of various factors on the formation of silica glass versus the A1

ratio of the alumina-silica gel. With this 20 3/Si02 diagram as a guide, we used a gel with A1 ratio between 61/39 and 64/36. 20 3/Si02

'< ,",. (]) I-~

O

---t I

H,

tI1

,",. ~

'0"',

J'l

64

61 39

P-

I

20

36

toI I I

()'q ~

~

tI1 CD

10

closed atmosphere

u 1-.

1•

under gas flow

0

ro

ci~

0

1.0

2.0 '.5 ratio A1 2O/Si0 2 of Gel

2.5

Figure 3. The influence of MgO, closed atmosphere and gas flow on the yield of silica glass versus the A1203/Si02 ratio of the gel.

568 Results Good catalyst supports are prepared by mixing the following constituents: Corundum Alumina-silica gel !lgO

Activated carbon Dextrin and then calcining the mixture at 1580 °C in a tunnel kiln. The support contains about 25 % of

mulli~e.

Scanning electron micrographs of the support and the

silver catalyst, respectively, are shown in Fig. 4 a and b. The catalyst contains about 15 % of silver. The silver grains have a rather uniform diameter of about 0.4

~m.

Figure 4 (a) SEM of the catalyst support;

(b) SEM of the silver catalyst.

The silver catalyst prepared in our Laboratory has been tested in industrial reactors as well as in reactors of laboratory scale, and the performance is very satisfactory. REFERENCE 1. R. Sadanaga, M. Tokonami and Y. Takeuchi, Acta Cryst., 15

(1962) 65-68

569 DISCUSSION M. FARINHA PORTELA: How did you measure the contents of the mullite phase of the

supports ? GUI LIN-LIN : The contents of the mullite phase were determined by X-ray quantitative analysis. A. KORTBEEK: Can you lndicate the activity and selectivity of your catalyst? GUI LIN-LIN: Usually we can get selectivity of 76 % and conversion of 16 %.



571

PREPARATION OF ACTIVE CARBON SUPPORTED OXIDATION CATALYSTS J.L. FIGUEIREDO, M.C.A. FERRAZ and J.J.M. ORFAO Faculdade de Engenharia, 4099 Porto Codex, Portugal

ABSTRACT A cobalt oxide catalyst supported on activated carbon was prepared by impregnation of sawdust with cobalt nitrate, followed by carbonization and partial gasi fication. Key variables in the preparation procedure were identified and their effects upon the porous structure and activity of the resulting catalyst were ascertained.

INTRODUCTION Active carbons are finding increasing application as catalyst supports in the treatment of gaseous or liquid effluents, where advantage is taken of the enhanced retention of organic solutes in the pore system of the carbon. In this way, adsorption is combined with catalytic oxidation in order to achieve the complete destruction of the pollutant (ref. 1). In the present communication, we report the preparation of a cobalt oxide catalyst supported on active carbon to be used in the oxidative destruction of organic compounds in air. Very high activity at low temperatures is required in order to promote nearly complete conversion of the organic compound without significant loss of carbon by gasification. Therefore, a good distribution of the active phase must be combined with a suitable pore structure in the support. This was obtained by impregnating a carbon precursor rather than the carbon itself. The method of preparation involves impregnation of a suitable carbon precursor (e.g. sawdust) with a metaJ salt (e.g. cobalt nitrate), followed by carbonization in inert atmosphere and activation by controlled gasification with a reac tive gas (e.g. C02)' The final product is a porous carbon containing cobalt oxide which shows high activity for the deep oxidation of organic compounds such as hydrocarbons, alcohols and acids. The role of the impregnant during the carbonization and activation steps was monitored by thermogravimetry. The kinetics of these steps were determined and correlated with the porous structure and activity of the resulting catalysts.

572

EXPERIMENTAL The impregnated active carbon catalyst (SA/Co) was prepared from pinewood sawdust. After acid washing (15 hours with a 10% H2S04 solution) and drying (15 hours at 1100C) it was impregnated at room temperature and under vacuum with 0,1

M

cobalt nitrate solution (20 cm3 solution/g dry sawdust). After drying, the

impregnated sawdust was carbonized in a tubular furnace under nitrogen flow at 100C/min up to 850 0C and held there for 1 hour. Activation was carried out by controlled gasification with carbon dioxide at 825 0C for 15 minutes. For comparison, an active step

carb~n

was prepared in the same way but without the impregnation

(SA). The activation time was

extended in order to obtain the same degree

of burnoff. Nitrogen adsorption isotherms were obtained by conventional methods and used for textural analysis. Thermogravimetric studies of the carbonization and activation steps were carried out by means of a C.I. Electronics micro force balance with a suitable flow attachment, electric furnace and a Stanton Redcroft linear temperature programmer. Catalyst activity towards the complete oxidation of organic compounds was determined in a chromatographic pulse reactor.

RESULTS Thermogravimetric studies In order to investigate the effect of the impregnant, the preparation procedure was duplicated in the thermobalance. Thus, Figure 1 shows thermograms of sawdust and impregnated sawdust under inert atmosphere, the weight loss being referred to the dry materials.

o

10K/min

20 ~'" 40 ~

.c:

~"" 60

80 ~

~

~

~O

Temperature (KI

Fig. 1. Thermograms of sawdust (curve 1) and impregnated sawdust (curve 2) under nitrogen flow (5 cm3/s).

573

It is apparent that the pyrolysis kinetics (between 520 and 650 K) is not affected by the presence of the impregnant. However, further

volatilization

occurs with SA/Co (curve 2), as compared with SA (curve 1). On the other hand, the gasification kinetics is quite different (Figure 2): the impregnated carbon is gasified at much higher rate initially, although this tends to level off at about twice the rate for the non-impregnated carbon.

0.6..------------------,

oil oil

0.4

o

oL==:i===~==~=:J. 10

30

40

Fig. 2. Gasification of carbonized sawdust (curve 1) and carbonized impregnated sawdust (curve 2) at 1099 K and 1 atm CO 2 , Carbonization temperature = 1123 K.

A detailed kinetic study of the gasification step was carried out with SA in order to establish the maximum temperature for reaction in the absence of diffusion limitations. In fact, activation can only result from an even removal of carbon atoms throughout the structure. If the reaction is diffusion limited, there is no development of the porous structure, and gasification occurs only at the external surface, leading to the shrinking of the carbon particles. Thus, Figure 3 shows the rates of gasification of carbonized sawdust in the form of an Arrhenius plot. The transition from the chemical regime (activation energy kJ/mole) to the diffusional regime (activation energy

=

238 kJ/mole)

is

=

414

well

defined, showing that the activation temperature should not exceed 1153 K for the particle size considered. A different situation occurs with the impregnated carbons, as the rates of gasification change with burnoff. Thus, the initial rates of gasification of SA/Co are two orders of magnitude higher than those observed with SA, while the final rates are only about twice the latter. Gasification of SA/Co was studied in the range of temperatures from 1062 to 1128 K and activation energies of 264 kJ/mole (for the final rates) and 58 kJ/mole (for the initial rates) were derived.

574

Fig. 3. Rates of gasification of carbonized sawdust (sawdust average particle 0.9 mm; carbonization temperature = 1273 K).

si~e

Textural studies The texture of the resulting activated carbons is quite different, as shown by the adsorption isotherms in Figure 4 and by the textural parameters collected in Table 1. Thus, while SA is essentially microporous, SA/Co exhibits considerable mesoporosity. Note also the presence of low pressure hysteresis in both carbons, which has been discussed elsewhere (ref. 2). The surface area of the impregnated carbon was found to be 210 m2/g before activation.

~

ii: ....

SA

V!

"'e ~ 'a

1/200 "Ii "Cl

..'"

e

.3 ~1dl

o' - - - - " " - - - - ' - - - - - ' - - - " " - - - - - - rtol Al!lative pmsII'e PI ~

Fig. 4. Nitrogen adsorption isotherms at 77 K on SA and SA/Co.

575 Table 1 - Textural parameters Surface areas (m2/g)

Metal load Carbon

(%)

2.31

Vme s o Vmicro Vmacro

Smeso

SBET

SA SA/Co

Pore volumes (cm3/g)

1084

17

0.03

0.44

0.51

287

114

0.34

0.14

1.87

Activity studies The impregnated carbon SA/Co was tested for the deep oxidation of several organic compounds in air. The model of Langer et a1. (ref. 3) was used to derive kinetic parameters from the data. The reaction was found to be of 1st order in the organic compound, and the activation energies were determined. Table 2 summarizes the results obtained. Table 2 - Kinetic parameters for the oxidation of organic compounds cata1ysed by SA/Co Compound

E(kJ/mo1e)

tnk o

X250

Benzene

147

29.2

0.98

Propene

92

18.9

0.98

Butanol

65

13.0

0.98

Toluene

84

14.4

0.63

Butanoic ac.

69

13.7

0.98

E, k O = parameters of the Arrhenius equation, k=kO exp(-E/RT) X250

conversion obtained at 2500C in the chromatographic pulse reactor (column: length 284mm, i.d. 4.8mm; catalyst: 89Omg; air flow rate: 5 4.1x10- mo1e/s)

DISCUSSION The results presented in the previous section are consistent with the following role of the inorganic material during the preparation of the catalyst: -In the carbonization stage, the impregnant acts merely as a spacer between the wood the

fibres, preventing their shrinkage and originating an open structure in carbon. This allows further

volatilization

to occur and, moreover, it fa-

cilitates the access of the reactant in the next stage of the preparation. -In the activation stage, the inorganic material acts as a catalyst for carbon

576 gasification, but becomes quickly deactivated as reaction proceeds. We believe that the catalyst may be originally present in the metallic state, but is then converted to the oxide CoO, which is the form actually identified in the final product by X-ray diffraction: Co

+

CO 2

=

CoO + CO. Similar results were reported

by several authors (ref. 4-6). Thus, the initial rates in the activation of SA/Co reflect the catalytic gasification of carbon, while the final rates are closer to those obtained for SA. The activation energies determined support this 238 kJ/moie for SA whe

vie~,

(264 kJ/mole for SA/Co and

:ification is diffusion limited). Gasification of SA/CO

is still faster, probauLy as a result of higher gas diffusivities associated with the mesoporous structure. Marsh et al. (ref. 6) studied the gasification of pure and doped polyfurfuryl alcohol carbons, and reported activation energies of 210 kJ/mole (Co doped) and 370 kJ/mole (pure) in the kinetic regime. It seems, therefore, that the catalytic gasification of SA/Co is also diffusion limited at the temperatures considered in the present work. Nevertheless, most of the carbon is removed while the

CR-

talyst is still active, i.e., gasification occurs at the interfaces carbon/catalyst. The degree of activation achieved is therefore quite satisfactory and the catalysts obtained exhibit an adequate texture and perform reasonably as oxidation catalysts for the destruction of small amounts of organic compounds in air.

ACKNOWLEDGEMENTS This work was supported by Instituto Nacional de Investiga~ao (INIC) and by Junta Nacional de

Investiga~ao

Cientifica

Cientifica e Tecno1ogica (JNICT,

research contract n9 45.78.05).

REFERENCES 1 M. Beltran, Chern. Eng. Progr., 70(1974)57. 2 J.L. Figueiredo and t!.C.A. Ferraz, in "Adsorption at the gas-solid and liquid-solid interfaces", ed. J. Rouquerol, K.S.W. Sing, Elsevier, in press. 3 S.H. Langer, J.Y. Yurchak and J.E. Patton, Ind. Eng. Chem., 61(1969)11. 4 S. Kasaoka, Y. Sakata, H. Yamashita and T. Nishino, Int. Chem. Eng., 21(1981) 419. 5 E.T. Turkdogan and J.V. Vinters, Carbon, 10(1972)97. 6 H. Marsh and R.R. Adair, Carbon, 13(1975)327.

577 DISCUSSION S.P.S. ANDREW There have been movie photographs of the catalytic action of metal particles on the gasification of carbon. These show that the metal particles move around "burning" a path in the graphite. If this is the phenomenon why does the gasification catalysis by the cobalt erase in your experiments? Could it be either that the cobalt particles coalesce and lose mobility or that they collide with an inorganic impurity in the carbon which they cannot burn their way through ?

J.L. FIGUEIREDO:

The acid washing treatment completely eliminates the ash content of the starting material, so there are no inorganic impurities at all in the char. We do not think that the loss of catalytic activity seen in Fig.2 (curve 2) may result from the sintering of cobalt particles, as we observed a much faster deactivation at lower temperatures (e.g. 775°C). We believe that the effect is due to the inability of cobalt to remain in the metallic state in the presence of carbon dioxide. At the end of the process, we indeed detected cobalt oxide.


579

G. Poneelet, P. Grange and P .A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.Y., Amsterdam -Printed in The Netherlands

INFLUENCE OF PREPARATION VARIABLES ON THE ACTIVITY AND ON THE MECHANICAL PROPERTIES OF AN INDUSTRIAL CATALYST FOR THE PROPYLENE OXIDATION TO ACRYLIC ACID R. COVINI (I), C. D'ANGELI (2)and G. PETRINI (3) (1) Montedipe S.p.A. - Research Center of Bollate (Italy) (2) Montefibre S.p.A. - Via Pola 14, Milano (Italy) (3) Ausind S.p.A.- Attivita Catalizzatori - Via Fauser 4, Novara (italy)

ABSTRACT Preparation of a (Ni-Mo-Te)O catalyst for the production of acrylic acid from propylene was investigated on a statistical basis: It was found that catalytic performances and mechanical properties can be independently optimized, by a proper selection of experimental conditions.

INTRODUCTION In a research for the one step oxidation of propylene to acrylic acid, a preliminary work (ref. I) had resulted in the choice of a (Ni-Mo-Te)O catalyst which, on a microreactor scale, afforded sufficiently high selectivities, under conditions providing almost complete conversion. The commercial application of the process implies the use of a tubular reactor with high enough heat transfer efficiency and requires that the catalyst is shaped in proper way, so that the pressure drop through the tubes be acceptable and that the mass transfer phenomena through the pellets be kept under control. Altogether, SxS mm hollow cylinders, with a 2 mm hole, appeared to be convenient: the aim of this work was to find preparation conditions affording proper morphology and sufficiently good mechanical properties to such pellets.

EXPERIMENTAL Catalyst preparation I To a solution of (NH ) Mo 0 (Climax) containing 4.7 moles.l4 7 MoO ,the s«achiometric a~ou~t of a Ni(NO ) .6H (C. Erba, RPE) 20 solation was added, under thorough mixingj ~he operation was carried out at 6S GC, while pH was kept constant at 6.S through addition of aqueous ammonia. After filtration, the precipitate was washed with cold distilled water, dried overnight at 110 GC and calcined 8 hours at the selected temperature in a furnace controlled by a linear temperature time programmer, to obtain a powder with a slight excess of M00

3

580

(chemical analysis: MO/Ni=I.1 atoms) The powder was thoroughly dry mixed with an amount of TezMoO 7 (ref. 2) corresponding to 5% TeO by weight, with the selec'ted amount of lubricant (stearic acia) and with 3% binder (Hydroxy propylcellulose: Klucel 250L, Degussa) By addition of distilled water, a paste was obtained which, after drying overnight at 100°C, and milling through a I mm grid, was tabletted in a rotatory type machine, equipped with forced feed and two stages compression, and fitted with tungsten carbide dies. The table~ng pressure was controlled by checking the weight of samples of 30 tablets: 10 s~ch samples were controlled for each preparation condition, and tablets were discarded when the deviation exceeded ± 0.1 gr. Activation was performed in air containing about 20% steam: a tubular isothermal reactor was used, heated at a rate of about 1°C/min and kept 8 hours at the desired temperature. All other conditions being maintained constant, the following were chosen as independent variables: - Calcination temperature of (Ni-Mo)O , from now on: Tc - Lubricant concentration (Lc) - TablettifB pressure, expressed as weight of 30 pellets (Pt) - Activation temperature of the tablets (Ta) The experimental ranges were: 610°C ~ Tc ). 690°C 5% ~ Lc ~ 9% 7. 7gr ~ Pt ~ 8. 3gr 480°C ~ Ta ~ 520°C The heat treatment had to be split into two steps (cal cination of the powder activation of the tablets) as: on the one hand it must be sufficiently severe, as a whole, to bring the surface area to the desired values; on the other, the temperature of the second step is limited by the occurrence of a phase transition in the system, because of which a heating at 580°C followed by cooling below 200°C results in a complete collapse of the tablet; and even more it is limited by the dispersion of surface area values, which is obtained at any temperatures above 530°C. Catalyst characterization Conventional methods were used for the determination of morphological properties: N adsorption for surface area (from now on: As) and Hg porosimetr~ for pore volume (Vp) and pore size distribution (Rp=average pore radius). Axial crush strength (CSa) and radial crush strength (CSr) were determined by an apparatus assembled in our laboratory and equipped with a strain gage cell and a recorder. Catalytic tests were performed in a tubular reactor 3 cm wide and 80 cm long, immersed in a melted salts bath; the feed was propylene 4%, air 56% and water 40% by volume. Propylene conversion (Cv) and selectivity to each product are given by: Cv = propylene fed - propylene recovered • 100 propylene fed

581

S

C atoms converted to each product • 100 C atoms converted

In Tab. 2 and Fig. 5, Saa, Sa and Scox stand for selectivity to acrylic acid, acrolein and carbon oxides, respectively; balance to 100% is accounted for by small amounts of by-products, mainly acetic acid.

RESULTS To reduce the number of experiments and according to reported experiences (ref. 3l,a composite design was used, made up by 8 tests selected out of a 2 factorial with a proper confusion technique; by 8 additional tests for the evaluation of the quadratic effects; and by the central point of the program, repeated 4 times, for an independent estimation of the ex?~rimental error. A drawback of this program is that, in the presence of a second degree interaction it is not possible to point out the couple of variables, which are involved; this problem was partially solved, however, by performing some extra tests on pellets a~vated at somewhat different temperatures. In the absence of indications pointing out equations with theoretical significance, an empirical quadratic polynomial has been chosen. The coefficien~were evaluated by a stepwise regression (ref. 4), which is a modification of the least squares method, and only those were retained, having a probability higher than 95% to be different from zero. Catalytic performances and surface areas were found to show a quite similar dependence on preparation variables, so that it is possible (and it was deemed more convenient) to express the first as a function of the latter. All resulting correlations are reported in Tab. 1 - 2, together with the related multiple correlation coefficient (Cc). For the same correlations, the ~isher's F test (ref. 5) shows that the regression standard error does not differ significantly from the reproducibility standard error.

DISCUSSION Morphological properties depend only on calcination and activation temperatures. Among them, specific surface area depends much more on the second (Fig. 1), as activation is made on a mixture containing Te molybdate which, with the small M00 excess, originates a 3 low melting (526°C) highly mobile eutectic composition (ref. 6). A comparison between Vp and Rp equations (Tab. 1) shows that in the absence of tellurium compound (calcination), the pore size increases at constant pore volume, as in a plain sintering process; while in the presence of tellurium (activation), the increase of pore size, beside being more pronounced, is also accompanied by a decrease of pore volume, pointing out an occlusion of smaller pores

582

TABLE 1 Dependence of morphological and mechanical properties on preparation variables. Variables normalization: Tc - 650 40

Ta - 500 20

Lc - 7

2

As

8.122 - 1.939x1 - 3.599x2 - 2.869x2 + 1.361x

Vp

0.1270 - 0.6278x2

Rp

380 + 181x

1

+ 218x

0.0148x 2

+114x~

2

- 17. 125 x

2

3

- 12.312x

2.37 - 0.50 x

1

+ 2.55 x

2

219X~

+

=

Pt - 8

--=--=0.3

(Cc=0.992)

2

+

(Cc=O.977) 2

16.420X + 30.125x2 4

+ 17.037x1x3

1x2

1x2

4

(Cc=0.899)

2

CSa = 58.875 - 13.410x1 + 49.580x

CSr

x

2

2 + 1.32x 2

(Cc=O.976) (Cc=0.953)

TABLE 2 Dependence of catalytic performances on specific surface areas. Variable normalization: As - 6.625 3.481 A) Constant salt bath temperature 315°C Cv = 41.050 + 12.880x

(Cc=0.963)

5

Saa = 11.63S + S.834xS

(Cc=0.95S)

Sa = 75.026 - 8.885xS +1.S20x Scox = 9.105 - 2.095x5

2

(Cc=0.9S6)

5

(Cc=0.831)

B) Constant conversion 80% Saa = 39.397 - 1.810x Sa = 46.520 Scox = 11.20S

2

S

- 0.913xS

(Cc=0.782)

583

C~a.(k~)

A~ (m2/gy)

-\SOI.-------r------.---..-----/-~

-i4,----,----,----,-------,

iOOI------+---+--/+~~--j

SOI------b~~~C---+_----l

21----+---+----!---'04&0

500

Ta. (O()

°460

520

Fig. 1. Specific surface area vs. activation temperature, at different Tc.

CSa. Clq~)

-\SO r - - - - , - - - - r - - - - r - - - - - - ,

500

Ta. {Oe}

520

Fig. 2. Axial crush strength vs. activation temperature at different Tc (Lc~7%; Pt~8g).

cs-

(~9)

7 .-----,-------r--~--~

-\00 1----+----+---+-----1

51-----I----+---~

31-----I---~~'#-+-----I

50 ~".e::-+_-_+--+-='.....,_--j

"''=--''''''''~--+---+-----I

Lc. (%)

9

Fig. 3. Axial crush strength vs. lubricant concentration at different Ta (Tc~650oC; Pt~8g).

04&0

500

Ta.(oC)

520

Fig. 4. Radial crush strength vs. activation temperature, at different Tc.

584

G/o 90-.-------------,

50

c>

1

I

TO GAS PLANT

:

I

~

I

COMPRESSOR~---I·--~ WASH WATER ~-----·I--""'--------'

TO FRACTIONATION

I I I

RECYCLE OIL (FRACTIONATOR BOTTOMS)

Fig. 4.

Typical Unicracking Process Configuration.

case, the first catalyst converts the organic nitrogen and sulfur compounds to ammonia and hydrogen sulfide. These molecules, although they can act as catalyst poisons for the cracking catalyst, are much less inhibiting than the organic heteromolecules. The use of two reactors allows the choice of recycling unconverted feed and hydrogen to either of the two reactors. Recyc 1i ng to the

592

second reactor containing the cracking catalyst is most common. This process scheme allows the conversion of organic heterocompound~ optimally in the first reactor into less objectionable compounds such as ammonia and hydrogen sulfide and an independent control of the levels of unconverted heterocompounds over catalysts specially tailored for the purpose. Such catalysts are typically the well-known hydrotreating catalysts comprising sulfided molybdenum or tungsten and cobalt or nickel supported on alumina. Since in hydrocracking, organonitrogen compounds are the most objectionable, nickel-molybdenum on alumina catalysts are the usual catalysts of choice. Usually only a nominal conversion of the hydrocarbon feedstock occurs (i.e., change in boiling point range), although there may be substantial hydrogenation of unsaturated hydrocarbons. Most of the hydrocracking occurs in the second reactor. The feedstock is cracked to lower its boiling point to an extent of between 40 and 60 volume percent per pass. After fractionation of the product, the unconverted oil is passed back to the second reactor inlet for further processing. The catalyst in the second reactor is usually optimized for the hydrocracking operation and is the major subject of this discussion. Only in a few specialized cases, such as when the two catalysts are the same, or when the organonitrogen compounds are very difficult to remove, is recycle to the first reactor employed. Whichever configuration is used, the important operating factor in a single-stage operation is that the hydrocracking takes place in the presence of unconverted organo-nitrogen and sulfur compounds or in the presence of ammonia or hydrogen sulfide and thus the catalyst hydrogenation components are metal sulfides. The most flexible and versatile hydrocracking process is the so-called twostage process. Basically in this process, the hydrotreating, and in some processes, part of the cracking, are carried out as in the single-stage process. Next, however, the product is fractionated and the unconverted oil is passed to an additional reactor containing hydrocracking catalyst. The recycle gases are stripped, or washed free, of most (usually all) of the contained ammonia and, depending on the process, most or part of the contained hydrogen sulfide. Thus in this process, the hydrocracking catalyst in the second stage is operating in the absence of ammonia and in a sulfur-free or sulfur-containing atmosphere. Although considerably more expensive to construct and operate, the two-stage system results in all or part of the cracking being conducted at much lower temperatures because of the absence of ammonia. For instance, first-stage cracking temperatures are usually in the range of 650-800°F whereas secondstage temperatures are usually in the range of 500-700DF.

593

A two-stage system is generally used when the nitrogen content of the feedstock is such as to require very high operating temperatures at conventional space velocities or when reactor volumes be~ome uneconomically large. A two-stage system may have two recycle gas systems and, depending on the design, one or two separator systems. As well as controlling the ammonia atmosphere, the level of hydrogen sulfide can be controlled over a relatively large range so that it is possible to hydrocrack over metal catalysts or metal sulfide catalysts. The metal sulfide catalysts are most common and only in cases where a very hydrogenated product is desired are sulfur-free systems used.

FEEDSTOCKS AND DESIRED PRODUCTS In principle, suitable feedstocks can be converted into a range of products from liquid petroleum gas to catalytic cracker feedstocks. Variations in the process configuration, catalyst and process conditions permit highly selective conversion to: gasoline kerosine fuel oil middle distillate fuels (turbine & diesel)

lubricating oils liquified petroleum gas catalytic cracker feedstocks petrochemical feedstocks

Changes in optimum production of the desired product can often be achieved by small changes in operational conditions although for some product changes, major revamping is necessary. Another important feature of hydrocracking is that the products contain low concentrations of photochemically active hydrocarbons, such as olefins, and very low, if any, concentrations of sulfur and nitrogen compounds. Changes in operating conditions of modern catalysts such as distillation column cutpoints, small reactor temperature changes, and crack per pass have been shown to produce substantial product changes as in Table 1. It is seen that the product objective for processing a Kuwait virgin heavy gas oil can be varied from a high yield of gasoline over to a high yield of heating oil simply by a change in fractionation conditions and a ten degree change in reactor temperature.

594

TABLE Hydrocracking of Heavy Virgin Gas Oil Feedstock properties Gravity, °APl Sulfur, wt% Nitrogen, ppm Boiling range, of

22.3 2.9 820 600-1000 Maximum product objective Turbine Diesel Heating Gasoline Fuel Fuel Oil

Yields on feed, vol% Cl-Lp scf/bb 1 Butane Light gasoline Heavy Gasoline Jet fuel Diesel fuel Heating fuel Chemical H? consumption, scf/bol When more drastic changes are required in the product slate, usually a change to specifically designed catalysts with the appropriate hydrogenation and cracking components is necessary. An example of the influence of catalyst and process is given in Table 2 (ref.6). TABLE 2 Catalyst and Process Variations in Unicracking Heavy Gas Oil for Gasoline and Turbine Fuel Feedstock Properties: Gravity, °APl Distillation, D 1160, of lBP 10 50 90 EP Sulfur, wt% Nitrogen, ppm

20.3 520 641 728 820 890 1.33 2770

Product Yields and Properties: Yields: Cl-C3, scf/bbl light gasoline C3-C6 C7-plus gasoline Turbine fuel Total C -plus H2 consumetion, scf/bbl Turbine Fuel Properties: Aromatics. % vol Smoke point, mm

One-Stage Cat A

One-Stage Cat B

Two-Stage Cat C

146 32.5 40.6 45.0 121.2 1750

50 18.2 34.1 61. 1 121.7 1950

110 16.5 34.4 61.3 120.8 2110

34 13.6

19 20.1

2.0 29.7

595

This table shows three situations for hydrocracking the same feed resulting in significant differences in products. The data show that Catalyst A, developed for primarily gasoline production can produce substantial amounts of turbine fuel. However, at the same operating conditions, Catalyst B, which was designed to produce larger yields of turbine fuel, produces about 35% more turbine fuel. The use of another Catalyst C with two-stage processing conditions r~sults in the same production of turbine fuel but with much reduced aromatic content and a considerable change in the hydrogen consumption in the process. Furthermore, alteration of fractionation conditions and minor changes in catalyst temperature would permit production of 100% gasoline from the feeds tock. CATALYST DESIGN AND PREPARATION The hydrocracking catalyst is a carefully formulated combination of hydrogenation components and cracking components. The catalyst can initially be considered in terms of these two basic groups of components relatively independently. Later their joint properties will be considered. The Cracking Component The cracking component provides generally two basic functions: 1) the acidic function and 2) the high surface area porous support which allows ready diffusion of reactive molecules and provides a high surface area onto which the hydrogenation metals can be dispersed. Typical support materials are amorphous metal oxides having surface areas greater than about 150 m2g-1. Pore size distributions in hydrocracking supports, in contrast to hydrotreating supports, for example, have not been shown to be of great significance except in some special applications. Additional requirements are that the supports should be stable under thermal and hydrothermal conditions for several years. The second criterion of acidity largely controls the activity of the catalyst, at least for cracking. It is generally accepted that cracking reactions are proton or Bronsted acid catalyzed. Hence, it is desirable for the support to be a proton acid or to be capable of modification into a proton acid. Acidity can have two major distinguishing properties, strength and quantity. For hydrocracking, the greater the number of sites, the more active the catalyst will be; all other properties being equal. The ro1e of streng th is more complex and is i nter- re 1ated with the type of products desired. These phenomena will be discussed later. In initial designs of hydrocracking catalysts, cracking components used were similar to those already being used in catalytic cracking, namely: alumina, silica-alumina, silica-magnesia and the like. Some supports appear to be specially developed for hydrocracking applications such as

596

silica-zirconia-titania (ref.?) and alumina-titania (ref. B). These supports are generally prepared by coprecipitation of appropriate salts followed by washing and drying. More recently, just as in catalytic cracking, molecular sieve zeol ite supports have become important and probably dominate most areas of hydrocracking. Four zeolites appear to have most commonly been used up to now, V, ZSM-5 types, erionite and mordenite, with the V-zeolite being by far the most common. The lSM-5, erionite and mordenite appear to have only been used in special applications which req~ire shape-selective reactions controlled by pore geometry. There is also a possibility that X-zeolite was a component of some early catalysts. Currently, hydrocracking is the second largest catalytic use of zeo1ites. The princlpal requirement for a commercial catalyst is that the zeolite exhibits a high cracking activity (i.e., acidity). Since catalysts are used for long periods, the activity and stability of the structure to thermal and hydrothermal conditions as well as ammonia and hydrogen sulfide at operating conditions is vital. The ability to reactivate the catalyst after use is also desi rab1e. Examination of the patent literature and various publications show that it was soon realized that the development of the necessary properties required modifications of the zeolite and this has occupied much work and skill. The as synthesized sodium zeolite has essentially no catalytic activity. However, it was known that by ion exchange with ammonium ions, followed by thermal decomposition, acidic hydroxyl groups could be generated in the zeo1ite. Na+

NH

+

4

Exchange to about 2 weight percent sodium from about 11 weight percent was readily achieved. It was found that such materials, although catulytically active, were usually unstable and would lose crystallinity and surface area and hence activity relatively rapidly. However, partial re-exchange of the ammonium zeolite with multivalent cations such as magnesium (ref.g) or rare earths (ref.10) resulted in a stable zeolite which was still highly acidic. Such zeolite supports maintain high surface areas (over 500 m2g1) and high zeolite crystallinity.

597

Subsequently, it was found that by exchanging the residual sodium to a very low level the zeolite acidity and catalytic activity could be increased. For example, in Figure 5, the catalytic activity for ortho-xylene conversion is

50

2

Fig. 5.

4 6 8 PERCENT SODIUM

10

Conversion of a-xylene as a function of sodium content.

shown as a function of the residual sodium content (ref.ll). Similar observations have become available for paraffin isomerization over a platinumhydrogen-Y catalyst (ref.12) as revealed by Table 3. TABLE 3 Influence of Sodium Content on Isomerization of n-Pentane over Pd on Hydrogen-Y (ref.12) Crystallinity %

Na 2%0 Wt

Temperature (OC) for 30% Conversion

90 80 80

2.02 0.27 0.02

305 300 250

A secondary benefit of removing sodium ions at the synthesis stage is that any residual sodium, during use of the catalyst, will migrate from its initial position into sites probably in the supercages in which the sodium ions inhibit the catalytic activity. As will be shown later, these residual ions can be removed by reactivation techniques. The removal of residual sodium ions to a very low level is possible by using a large number (10-20) of exchanges. However, such a method would be impractical on a commercial scale. It has been found that an intermediate calcination can produce a rearrangement of the ions in the structure with ions such as

598

hydrogen, magnesium and rare earths migrating into the zeolite structure and displacing sodium ions. These sodium ions can readily be removed by further ion exchange with ammonium or other cations. Thus a sodium V-zeolite can be partially exchanged with rare earth ions and then calcined at elevated temperature. The displaced sodium ions can then be removed readily by further exchange. A second type of zeolite whi ch has been frequently reported in the patent literature for hydrocrading is the stabilized V-zeolite. These zeolites are characterized by being metal cation free and thermally stable to temperatures higher than that of the parent V. A typical preparative procedure (ref.13) involves exchanging a sodium V-zeolite with an ammonium salt until it contains less than about 3 weight percent sodium. The zeolite is then calcined at about 5400e for three hours and subsequently re-exchanged with an ammonium salt. When the sodium content is sufficiently low (below about 0.2 wt%) the zeolite is washed, filtered and stabilized by calcination at 815°e for three hours. This product is thermally stable up to 10000e. The zeolite produced has a high surface area and crystallinity. It is characterized by a decrease in the unit cell constant from about 24.65 to 24.4A in the finished product. A typical catalyst base can be produced from such a zeolite by extrusion with about 20 weight percent alumina. Another method of preparing a stabilized zeolite is to heat an ammonium Vzeolite (containing about 2 wt% sodium) to between 550 and 8000e in flowing steam for up to four hours. The product can be readily ammonium ion exchanged to produce a stable low sodium zeolite suitable for a hydrocracking catalyst base (ref.14). The characteristics of a support for hydrocracking can be summarized in terms of the following analysis: alkali metal surface area stability, thermal

- minimum >150 m2g-1 hi gh

aci dity, amount acidity, strength crystall inity (zeal t te )

high variable high

Recently, selective hydrocracking of normal or slightly branched hydrocarbons has become of interest. There are at least three processes designed using this phenomenon: 1) Selectoforming, or selectively cracking linear paraffins from reformer feedstocks. These molecules are low octane molecules and thus, their removal results in increased product octane.

599

The catalyst is tailored around the small pore diameter of zeolites such as erionite and lSM-5 which do not readily admit desirable highly branched molecules but selectively crack normal and slightly branched paraffins (ref.15). 2)

Lube oil and middle distillate dewaxing processes also operate on the same principle of removing normal and slightly branched paraffins by preferential cracking. Catalysts-based on mordenite and lSM-5 have been successfully used for dewaxing (ref.16,17).

The Hydrogenation Component Many hydrogenation components have been evaluated for hydrocracking. These have generally been those well known for hydrocarbon hydrogenation and constitute the noble metals, particularly platinum and palladium and the non-noble metals of Group VIb and VIII, especially nickel, cobalt, molybdenum and tungsten. Noble metals are reported to be used in amounts less than 1 weight percent whereas the non-noble metals are used in levels similar to those in hydrotreating catalysts (i.e., 2-8 wt% nickel and cobalt and 12-30 wt% molybdenum and tungsten as oxides). Noble Metal Components Noble metals may be introduced into the catalyst support by several methods. Impregnation by pore saturation, adsorption from solution or the gas phase, comulling and ion exchange are some of the more common methods. All of these methods except ion exchange are applicable to the amorphous supports such as silica-alumina. A typical method involves the measurement of the pore volume of the support, dissolving the desired amount of the noble metal salt, e.g., palladium chloride, in the measured pore volume of water contacting the solution and support for fifteen to sixty minutes, filtering off any excess liquid, drying the impregnated support and calcining in air at around 900°F. Palladium and platinum can be impregnated from acidic, near neutral or basic, e.g., ammoniacal, solutions. Noble metals can be incorporated into molecular sieve zeolites by the above methods. Since most zeolites have an exchange capacity far greater than that necessary to accommodate the amount of noble metal, exchange is one of the preferred methods. It is necessary that the metal be present in the exchange solution as a positive cation which usually requires formation of a complex cation which is easily decomposable. In general, up to about 2 weight percent of metal can readily be exchanged into the zeolite quantitatively. Platinum and palladium are readily introduced via the tetra-amine salts in neutral or

600

ammoniacal solution. Pyridine complexes can also be used. The introduction of the noble metal is usually done after all other modifications and cation exchanges to the zeolite powder have been carried out. The major reason for this is to avoid waste of expensive metal. In a typical preparation, 250 grams of an ultrastable zeolite were stirred in 500 ml of water containing 25 ml of concentrated ammonium hydroxide solution. To the zeolite slurry was added dropwise over a period of several hours a solution of 2.88 gm o~ palladium chloride dissolved in 90 ml of water plus 20 ml of concentrated ammonium hydroxide. After standing overnight, the zeolite was filtered, washed free of chloride and dried. For activity evaluation, the zeolite would then be pelleted with about 15-25 weight percent of a binder, such as alumina. The catalyst would contain about 0.5 weight percent palladium. It is also possible to incorporate the noble metal after pelleting. With powders the metal ions will be uniformly distributed, whereas there is a strong possiblity with pellets that the metal will be located preferentially near the pellet edge. Recently, however, it has been shown that high activity catalysts can be made by impregnating pellets with a Pd(NH 3)4(N03)2 solution using the pore saturation method (ref.18). The catalyst made by this method had similar activity to that of a more conventionally made catalyst. It has been shown that catalysts made by incorporation of the noble metal as an anion are less effective. It has been found desirable to decompose the tetramine complexes in flowing air rather than by direct reduction of the cation to the metal in hydrogen. Direct reduction appears to favor agglomeration of the metal (ref.19) whereas calcination in air above 800°F will decompose the tetramine complex into the oxide and remove most of the physically absorbed ammonia and water. Non-Noble Metals Many non-noble metals have been investigated for hydrogenation components both singularly and in combinations. The metals used have mirrored the pattern used in hydrotreating catalysts. In general, the most frequently used components are selected from the non-noble metals of Group VIII and the metals of Group VIb; nickel, cobalt, molybdenum and tungsten being the most frequently chosen. Nickel and cobalt are usually used at the 2-8 weight percent and molybdenum and tungsten at the 12-30 weight percent levels. Because of the many possible combinations, numerous methods are available for incorporating the non-noble metals. If single component nickel or cobalt is used, the methods discussed under noble metals such as impregnation, poresaturation and ion exchange are applicable.

601

Since most catalysts usually contain a Group VIb and VIII component, the most frequent methods used are: 1) comull ing 2) impregnation 3) exchange and impregnation Comulling is a common method of incorporating the hydrogenation components. In a typical preparation of say a nickel molybdate catalyst, the appropriate amounts of: l~

a nickel compound a molybdenum compound the catalyst base a binder and possibly an amorphous diluent such as alumina are mulled together as solids plus a suitable amount of water until an extrudable paste is formed. The paste is then extruded and chopped into suitable lengths followed by drying and calcination in air at about 900°F. Typical nickel compounds are nitrates, carbonates, oxides and the like or their combinations. Molybdenum compounds can be ammonium heptamolybdate, molybdenum oxide, ammonium dimolybdate, etc. A suitable binder is an acid peptized alumina. Thus a typical catalyst may have the composition of:

3% NiO Mo0 3 Zeolite Alumina Binder

15% 20% 42% 20%

For pH control, if desired, it is possible to use mixtures of salts or to add small amounts of acid or ammonia. This is probably the simplest and most economical method of preparation although scientifically, the least satisfactory. A typical example of a comulling process is given in ref. 20. A modification to the above comulling method is to mull together the catalyst base and binder and add to it a solution of the hydrogenation components in water or other media. In this' process, it is often necessary to add a solubilizing agent to the metal components. Thus nickel nitrate and ammonium heptamolybdate are insufficiently soluble in water to meet the levels needed on the catalyst. However, addition of ammonia or phosphoric acid can markedly increase the solubility and form very stable solutions for impregnation purposes (ref.21)

602

2) The impregnation route basically is the addition of the metal compounds in solution to the already pelleted catalyst base. Two possible routes are available. Firstly, the catalyst pellets are contacted with a solution of the metals equivalent in volume to the pore volume of the support, followed by drying and calcination. Secondly, the catalyst pellets are contacted with an excess solution of the metals, then separated, dried and calcined. Both of these two methods can be carried out in one or two stages, that is, all the metals added at the same time or each metal added as a separate solution followed by drying and then reimpregnation. Thus a typical catalyst could be prepared by pelleting about 20% zeolite, 60% alumina and 20% binder, drying and calcining at 900°F. The pellets would then be impregnated by pore saturation with a solution containing 6 weight percent nickel as NiO, 18 weight percent molybdenum as Mo0 3 and 3 weight percent phosphorus. This solution has a pH of about 1.1 before contacting the pellets. After drying, the pellets are calcined at 900-1000°F. If nickel or cobalt and tungsten are the desired components, the solubility is such that no additional stabilizer is necessary although naturally other components could be added, for example, for pH control. The third potential method of exchange followed by impregnation simply implies that one hydrogenation component could be introduced at an early stage by ion exchange, e.g., into a zeolite during the early modification stages and the second metal at a later stage. Whereas by comulling, if carried out properly, the metal components will be uniformly distributed throughout the catalyst pellet, by impregnation, possibilities exist for non uniform distributions with metal concentrations being greater near the exterior of the pellets resulting in a rind formation. It is also possible for the metals to diffuse through the pellets at different rates which could result in uniform distribution for some components and rind effects for other components. Thus it is necessary to control the metal addition steps very carefully and to verify the disposition of the metal by some physical or chemical technique. X-ray diffraction, electron microscopy and chemisorption techniques can supply data on the state of dispersion and particle size of the supported metals while scanning electron microscopy can supply information on the uniformity of distribution. A selection of techniques for preparing nonnoble metal catalysts can be found in the patent literature. In most cases, catalysts are prepared in such a manner as to maximize the interaction and proximity of hydrogenation sites and cracking sites. Although not certain, it is possible that a close proximity of the basic components may aid in enhancing the rate of the overall reactions and in reducing the rate of carbon deposition at the acidic cracking sites. However, it has been found in

603

certain cases that excellent catalysts can be prepared in which the hydrogenation components and the cracking components are kept isolated, at least in the preparation (ref.22). Catalyst Shape Two basic types of catalyst have been used in hydrocracking: pellets (or tablets) and extrudates. The former are made by mechanically compacting the catalyst base with about 20% of a binder such as alumina in the presence of about 270 carbon as a lubricant. Pellets are usually very regular in shape and are usually in the form of 1/8" cylinders. More frequently used nowadays are extrudates. In the simplest case, the catalyst base, with or without a binder and with or without hydrogenation components, is forced under pressure through a circular orifice. The bead formed in 1/8, 1/16 or 1/32" diameter is broken into short lengths «1/2"), dried and calcined at about 900-1200°F. Amorphous catalyst bases can often be extruded without added binder, but zeolites require dilution with about 20 or more weight percent binder such as peptized 0 alumina. For many years, catalysts were universally cylindrical in shape. Recently, it has been shown that improved activity can be obtained by increasing the surface/volume ratio of the extrudate by using shaped extrudates (ref.23). Tri1obe(R), tetra10be and in general po1ylobe catalyst bases have been prepared and used commercially (ref.24,25). Although Trilobe(R) and tetralobe catalysts have been used in hydrotreating and reforming, they appear not to have yet gained great util ity in hydrocracking. In general, 1/16" and 1/8" cy1 inder extrudates are the most common shapes for hydrocracking. CATALYST ACTIVATION The next critical operation in determining the performance of a hydrocracking catalyst is the technique of activation. Noble metal catalysts are activated by reduction, usually with hydrogen. The key to success in this case is remembering that the small amount of noble metal (about 0.5 wt%) is very labile and by malhandling is easily agglomerated or poisoned. The reduction of the metal oxide can be carried out at pressures from atmospheric up to about 50 atmospheres in flowing hydrogen. In order to avoid metal agglomeration from water liberated during the reduction and physically adsorbed on the zeolite or catalyst support, the temperature should be increased slowly to around 700°F and then maintained constant until reduction is completed. The catalyst temperature is lowered in preparation for feed introduction. The final conditionsof reduction can markedly effect the activity of the catalyst. In Figure 6, the influence of temperature and time in the final stages of the reduction are illustrated for second-stage activity.

604 +60.----------------------::1

eel

la) Z HRS 700°F Ibl 6 HRS 650°F lei 6 HRS 850°F Id)6 HRS 700°F Ie) 6 HRS BOOoF I~ Z HRS BOOoF BASE

L-

-±:50

~:_;;_---~---~

100

150

200

CATALYST AGE, HR.

Fig. 6.

Influence of reduction conditions on hydrocracking activity.

Variations from 2 to 6 hours and 650 to 850°F in calcination temperature can make over 20°F difference in activity for second-stage conditions which is equal to doubling of the activity. At first-stage conditions, no changes outside experimental error are observed. Although the exact nature of the physico-chemical phenomena occurring is not known, since the activity differences occur in the second-stage process which takes place in the absence of ammonia, it would appear that some changes involving the state or location of the palladium is involved which influences the hydrogenation activity of the catalyst. Non-noble metal catalysts are usually activated by sulfiding rather than by direct reduction. Sulfiding media are generally introduced into the catalyst system at low temperatures and elevated pressures. Typical sulfiding systems are very dilute hydrogen sulfide in hydrogen, carbon disulfide in hydrogen, butyl mercaptan in kerosene and the like. The temperature is slowly raised until sulfiding starts, as indicated by an exothermic wave in the catalyst bed and by water evolution. The temperature is held until hydrogen sulfide is evolved and then it is slowly raised to about 700°F, maintaining hydrogen sulfide evolution. When sulfiding is completed, the catalyst is cooled and readied for feed introduction. Relatively minor empirical changes in conditions of activation can result in optimization of activity for different catalysts. Nickel-only containing catalysts can be activated by either direct reduction or sulfidation depending on the type of service.

605

FACTORS INFLUENCING THE BALANCE OF CATALYTIC ACTIVITY The previous sections have discussed some of the factors involved in selecting hydrogenation and cracking components. It now remains to show how process conditions and catalyst formulation can affect operations. The ability of molecular sieve catalysts to operate in the presence of a substantial concentration of ammonia is a marked contrast to that of silicaalumina based catalysts which are strongly poisoned by ammonia. This difference is illustra~ed in Figure 7. The silica-alumina catalysts require severe

....I

o ~

z

o

iii a: w >

50

Z

o

o

1·15% HYIO.4% PI) IN SIOZ Z-SiOZ-AI Z03 (0.4% PI)

00 1000 2000 NITROGEN WPPM (FROM QUINOLINE)

Fig. 7.

Influence of ammonia on hydrocracking of heptane.

hydrotreating of the feedstock and essentially complete removal of nitrogen compounds from the feed to the hydrocracking reactor. The advent of the ammonia tolerant molecular sieve catalysts led to the birth of the two catalyst singlestage operation with the resulting economics and improved gasoline product slate. It is believed that the greater ability of molecular sieves to tolerate ammonia is due to their greater acidity. Presence or absence of ammonia should alter the role of the cracking component and it should be possible to change the catalyst from one which is balanced to one which is hydrogenation limited to one which is cracking limited. Such effects can readily be demonstrated using a hydrofined gas oil as a feedstock. Table 4 shows that in the absence of ammonia, two catalysts with different zeolite levels, are of approximately the same activity.

606

TABLE 4 Effects of Ammonia on Hydrocracking Activity Ammonia Present 40% Conversion* Cracking Component Level Effects High Zeolite Low Zeolite Hydrogenation Component Level Effects High hydrogenation level Low hydrogenation level *~

Base Base + 18°F

Base Base + 2

Ammonia Absent 60% Conversion* Base Base _2°F

Base Base + 20°F

temperatures for given conversion.

However, in the presence of 2000 ppm of ammonia, the higher zeolite content catalyst is 18°F more active, which kinetically translates into double the activity. This example shows that catalysts which have the same hydrogenation activity, can be activity limited by the cracking component in the presence of ammonia. In the absence of ammonia, the activity is independent of the cracking component suggesting hydrogenation limited reactivity. This is confirmed by the second example in Table 4 in which catalysts with the same molecular sieve contents but different hydrogenation component contents. Although the catalysts are of similar activity in the presence of ammonia, in the absence of ammonia the high hydrogenation level catalyst is twice as active. This shows that the high hydrogenation component catalyst is cracking limited whereas the low level hydrogenation component is hydrogenation limited. Clearly, optimum catalyst formulation may depend strongly on the process design, as well as on the feedstock and process objectives. The influence of palladium level on catalyst activity is demonstrated in Figure 8. This figure illustrates the change in activity for a catalyst as the noble metal component is varied by a factor of almost four in an ammoniafree cracking environment. In th€ absence of ammonia, virtually no change of activity was observed.

607

+80 0 +60

1!-

..r a:

0

:::l

l-

ea:

+40

lU

A-

~ lU

I-

0

+20

BASE~---..'.--------..I;.-_.....,-',,-----_-..'.

o

0.5 1.0 1.5 2.0 PALLADIUM CONCENTRATION, WT. %

Fig. 8.

Influence of palladium level on hydrocracking activity.

In the presence of ammonia, the concentration of the cracking base not only influences the activity of the catalyst but can also have a marked effect on the selectivity. Figure 9 illustrates the change in efficiency for diesel fuel

~------------,80

70

1!- +40

~

..r a:

,:

IC

Z lU ij

U

:::l

a:

u::

lU

A-

60

~ +20

I&.

lU

I-

50

BASE

o

5

10

15

20

% ZEOLITE

Fig. 9.

Variation of activity

(~)

and efficiency (a) with zeolite content.

608

production and activity of a series of non-noble metal catalysts as the percent of zeolite in the support is changed from zero to 20 weight percent. With the constant hydrogenation activity, as level of support is increased and hence the level of cracking activity is increased, the overall catalyst activity is increased by about 25°F, roughly equivalent to doubling of the activity while the efficiency for turbine fuel production drops from about 80 to 54 volume percent. The selectivity of the hydrocracking catalyst can be changed considerably by varying the type ?f cracking components. A broad classification of catalyst characteristics has been given by Scott et al. (ref. 26) and is illustrated in Table 5. TABLE 5 Hydrocracking Catalyst Types

Desired Reaction Hydrocracking Conversion A. Naphthas to LPG (ref.27,28) Gas oils to gasoline (ref. 29,30) B. Gas oils to jet and middle distillate (ref. 31,32,33)

Catalyst Characteristics Hydrogenation Surface Area Acidity Activity Strong

Moderate

Moderate Strong

Porosity

High

Low to Moderate

High

Moderate to High

Moderate

High

Gas oils to high V.I. lubricating oils (ref.34,35) Solvent deasphalted oils and residua to lighter products (ref.36,37) Hydroconversion of Nonhydrocarbon Constitutents Sulfur and nitrogen in gas oils (ref.38,39)

Weak

Strong

Basically, for maximum extent of cracking, e.g., to make gasoline or lighter products, high cracking activity is required and can be furnished by highly acidic materials such as molecular sieve zeolites and some amorphous oxides. On the contrary, to produce high yields of middle distillate, lower acidity but high hydrogenation activity is required so that secondary cracking is minimized and aromatics are saturated. Hence, a high surface area alumina or silica-alumina with large pores· and loaded with a high level of metals constitutes a preferred middle distillate catalyst. A typical example of the effect of catalyst base is given in Table 6. This table compares

609

two catalysts containing the same hydrogenation components and the same amount of two different molecular sieves. It is seen that Catalyst A, despite being more active, also produces a substantially larger amount of turbine fuel. By changing the catalyst support to an amorphous alumina or silica-alumina, a higher turbine fuel efficiency would be expected. TABLE 6 Hydrocracking of a Gas Oil for Turbine Fuel Catalyst A Temperature for 65% Conversion to 675°F-product Turbine Fuel Efficiency, %

Catalyst B

Base -7

Base

71

55

Within the series of potential hydrogenation components, the ranking Ni-W > Ni-Mo > Co-Mo > Co-W has been found (ref.5). Platinum, when not sulfided, is the most active hydrogenation component. From a study of toluene hydrocracking (ref.5), the atomic ratio Group VIII Metal ~ Group VIII Metal + Group VIb Metal ~0.25 has been found to be an optimum.

50.0',--------=-=-----, oNi

.«

oNi - Mo .Co - Mo oCo- W

z

37.5 SUPPORT ALUMINA

o

iii II:

W

> Z

o

U

25

50 % ATOMIC

CO Co + Mo

Fig. 10.

Co Co + W:

NI

Ni""+iiO:

Influence of hydrogenation component.

NI HI + W

610

Data on which this correlation is based are plotted in Figure 10. It is seen that for the four different metal combinations supported on alumina. the maximum conversion occurs when the atomic ratio is about 0.25. Progressing a step further. the relative isomerization to hydrocracking of heptane is illustrated in Figure 11 for a series of hydrogenation components on silica-alumina in the

HYDRO ISOMERIZATION

100 I- ClI-Mo

Z- Ni-Mo 3 - NiW 4 - 0.15% PI WITHOUT S 5 - 0.3,0.5% PI WITHOUT S

20

40

60

80

HYDROCRACKING

Fig. 11.

Effect of hydrogenation component on hydrocracking activity.

presence of ammonia and hydrogen sulfide. In contrast. Figure 12 illustrates the influence of changing the silica-alumina ratio. As the silica-alumina ratio increases. with resulting increase in acidity. the conversion and cracking activity increases (ref.5). HYDROISOMERIZATION

100....---------------, Ni-Mo/SiOZ-AI Z03 1 70% 30% Z 50 50 3 30 70 +R-S-S-R + n BUTYL-AMINE

20

40

60

HYDROCRACKING C3 + C4

Fig. 12.

Effect of silica-alumina content on hydrocracking.

611

For most of the hydrocracklng processes and catalysts described above, the most desirable supports have been amorphous metal oxide mixtures or large pore molecular sieve zeolites, the smallest pore size used being that of V-zeolite. However, it waS indicated initially that for certain types of selective hydrocracking, small pore molecular sieves are highly desired since they are able to sieve the molecules and limit the accessibility of hydrocarbon molecules into the pore structure and hence to the vicinity of the active sites. Selective hydrocracking has been applied in several ways to remove normal or slightly branched paraffins. Advantages of small pore zeo1ites for the 1owering of the pour point of di s t i l 1ate fuels have been utilized by British Petroleum, using a noble metal impregnated mordenite and by Mobil Corporation using a ZSM-5 zeolite. Both processes operate by removing long-chain normal paraffins. Figure 13 illustrates the change

AFTER PROCESSING

BEFORE PROCESSING

PROGRAMMED TEMPERATURE, DC

Fig. 13.

Chromatograms of a gas oil before and after dewaxing.

from feed to product for the ZSM-5 processing showing the dramatic removal of normal paraffins (ref. 40). The change in pour point accompani ed by the slight change in boiling point distribution is shown in Table 7 (ref.40).

612

TABLE 7 Properties of a Gas Oil Before and After Hydrodewaxing Fraction

Virgin Heavy Gas Oil

MMDW Processed Heavy Gas Oi 1

TBP cut, of Yield on Crude, Vol%

650-750 7.5

Properties: Gravity, °API Pour Point, of Cloud Point, of Sulfur, wt% Diesel Index

27.8 60 66 2.3 46

25.4 -10 +22 2.5 38

658 685 732

653 682 731

ASTM 10 50 90

Dist., of vol% vol% vol%

650-750 6.3

Finally, it is of interest to compare a narrow pore shape-selective catalyst with a large pore catalyst for the hydrocracking of ~-paraffins. Jacobs et al. (ref.41) have shown that platinum supported on HISM-5 was more active than platinum supported on ultrastab1e V-zeolite for the hydrocracking of n-decane. CATALYST REACTIVATION In commercial use over several years, hydrocracking catalysts slowly lose activity. The desired conversion of feedstock to product is generally kept constant by gradually increasing the catalyst bed temperature until limiting factors such as reactor metallurgy or product distribution dictate reactivation or replacement of the catalyst. The cycle length can typically vary from about 1 year to 5 years. The activity of most hydrocracking catalysts can be restored to close to that of fresh activity by oxidative combustion of the carbon deposited on the catalyst. This can be carried out in-situ in the reactor or ex-situ in suitable equipment (ref.42). Reactivation by carbon burning has been reviewed recently (ref.43). Other causes of deactivation can be: 1) Decay of the hydrogenation function. 2) Inhibition of the cracking function. For non-noble metal hydrocracking catalysts currently used, carbon removal is usually sufficient to substantially restore the activity of the catalyst.

613

Noble metal hydrocracking catalysts are much more sensitive than nonnoble metal catalysts. After regeneration to remove carbon, although they genera lly exhi bit good recovery of fi rst- stage activity, they often exhi bit a considerable loss of second-stage activity. This activity loss is related to loss of hydrogenation activity due to metal agglomeration or redistribution. X-ray diffraction, electron microscopy and hydrogen chemisorption studies confirm the metal agglomeration. The metal agglomeration is believed to be due to the prolonged exposure of the catalyst to elevated temperatures in the presence of water vapor, ammonia and hydrogen sulfides. As discussed previously, the noble metal hydrogenation component in molecular sieve catalysts has been incorporated by ion exchange rather than by impregnation. This results in a uniform, extremely fine dispersion of the metal throughout the catalyst and hence has greater potential for agglomeration than poorly dispersed metals prepared by impregnation or other methods. Techniques have been developed which permit the redispersion of the noble metals to that approaching fresh catalyst. The earliest technique used (ref.44) was the treatment of the molecular sieve catalyst after hydration with ammonia. Basically, the catalyst is treated with a moist air stream until saturated with water and then with a gaseous ammonia stream until saturated with am~onia. It is believed that the noble metal dissolves in the strong ammonia solution in the molecular sieve pores, reforms the tetramine complex, and then is redistributed at the ion exchange sites in the molecular sieve. The rejuvenation is finished by removing the excess of ammonia with a nitrogen purge followed by calcination at a suitable temperature. Typical catalyst comparisons of fresh and rejuvenated catalyst are shown in Table 8 (ref.45). It is seen that the activity of the catalyst is improved dramatically. Later the process was modified such that the molecular sieve catalyst could be treated with aqueous ammonia rather than gaseous ammonia (ref.46). TABLE 8 Properties of Regenerated and Rejuvenated Catalyst After Regeneration Surface Area, m2g-1 Crystallinity, % (relative to fresh catalyst) Relative Activity, OF Second Stage (compared to fresh catalyst)

After Rejuvenation

100

535 100

+61

-9

550

614 It was later realized that the acidic zeolite portion of the catalyst could

have lost a substantial part of its contribution to the catalyst activity. Early molecular sieve catalysts comprised zeolites which had been partially exchanged with ammonium ions to remove sodium down to an easily achieved level, e.g., about 1.5-2.0 percent. Thus the sodium ions which are difficult to remove by straight ion exchange were left in the zeolite. It is feasible that either the initial preparati'on of the catalyst or the hydrothennal exposure the catalyst receives in use causes migration of the sodium ions from inaccessible portions of the structure to the vicinity of the acidic center resulting in a loss of cracking act ivfty , Treatment of the deactivated catalyst with an aqueous ammonium salt solution removed substantial amounts of the residual sodium from the catalyst. On evaluation, the activity was found to have been improved at first-stage conditions but little effect was found at second stage suggesting that the sodium had been removed from the zeolite with a resulting boost in acidity and cracking activity. A desirable extension of these two processes was to sequentially treat the catalyst with aqueous ammonia and then with an aqueous solution of an ammonium salt (ref.47). It was found that the catalyst could be rejuvenated with an ammoniacal ammonium salt solution (ref.48). Furthennore, it is possible to rejuvenate the catalyst before carbon burn off. Typical properties of two commercially used catalysts before and after rejuvenation are given in Table 9. TABLE 9 After Regeneration Catalyst Surface Area, m2g-1 Pellet Crush Strength, lb Attrition Loss, Wt% Relative Activity of, First Stage, compared to fresh catalyst

After Rejuvenation

A

B

A

B

513

420

494

433

22 1.7

16

18

15

1.0

+35

+45

-7

0.9 -2

The data shows that the activity of the catalyst can be substantially improved by chemical treatment, based on a knowledge of the catalyst composition and potential deactivation mechanisms. Catalysts reactivated by the above methods have been used in typical commercial operations with outstanding success. Performance at least equal to, and in several cases, superior to fresh catalyst has been obtained.

615

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616

40 41 42 43 44 45 46 47 48

H. Heinemann, Cat. Rev. - Sci. Eng. 23 (1981) 315. P. A. Jacobs, J. B. Uytterloeven, M. Steyns, G. Froment and J. Weitkamp, Proc. 5th Intn. Conf. Zeolites (1980) 607. Oil Week, Jan. 25, 1982, 12. P. K. Maher, A. J. Garrett, Oil Gas J. 75 (15) (1977) 51. R. C. Hansford, U.S. Patent 3,899,441 (1975). A. D. Reichle, L. A. Pine, J. W. Ward, R. C. Hansford, Oil Gas J. 72 (30) (1974) 137. J. W. Ward, U.S. Patent 4,107,031 (1978). D. E. Clark and J. W. Ward, U.S. Patent 3,692,692 (1972). J. W. Ward, U.S. Patent 3,849,293 (1974).

617 DISCUSSION J. GUEGUEN How many rejuvenations is it possible to apply before a regeneration? How does it affect the acidity of the catalyst? J. WARD: Reactivation of some types of hydrocracking catalysts comprises i) regeneration and then ii) rejuvenation. Rejuvenation is always accompanied by regeneration (coke burn off), allthough regeneration is not always accompanied by rejuvenation. Rejuvenation remOves alkali and other metal ions from the catalyst. Since these ions are poisoning metal sites, rejuvenation increases the catalyst proton acidity. P.A. JACOBS: In Fig. 6 you clearly showed the effect of changing the reduction temperature on the hydrocracking behaviour. In the scientific literature, a similar effect is known for noble metal zeolites when the oxygen or air treatment temperature prior to reduction is changed. Does a similar effect exist for real hydrocracking catalysts containing either noble metals or group VI B elements ? J. WARD: The conditions of heat treatment prior to reduction are very important for noble metal containing zeolites for real catalysts as for noble metal zeolites. Conditions have to be chosen carefully in order to avoid agglomeration and maldistribution of the noble metal before reduction. Catalysts containing group VI B elements appear to be less sensitive. It is routine practice to treat all the catalysts discussed in dry air pi or to reduction. The data in Fig. 6 attempt to show the influence of time and temperature of exposure to hydrogen, even though the catalyst must be seentially totally reduced prior to these changes. R.J. BERTOLACJNI: During regeneration-rejuvenation with ammonia, do you see any changes in the zeolite or zeolite type? J. WARD: No significant change in the zeolite is seen during regeneration: there is a slight reduction in the unit cell constant due to the high temperature exposure during coke burn off. During rejuvenation, the only significant change is a lowering of the sodium content due to the ammonium ion exchange. D. CHADWICK: You illustrated the stability of the activity of zeolites operating under a partial pressure of ammonia. Do you see any changes in the product spectrum as a function of ammonia partial pressure ? J. WARD: Depending upon the ammonia partial pressure and operating condition changes, there mayor may not be a change in the product spectrum. The product spectrum depends upon a complex interaction of catalyst, process conditions and whether conversion is allowed to remain constant or change on ammonia addition. M.M. BHASIN What is the difference in the ammonium ion treatment rejuvenation of the used catalyst and that of the fresh catalyst ? J. WARD: Essentially none. The ammonium ion exchange in the fresh catalyst is used to remove the zeolite sodium content from that of sodium Y to that of an ammonium Y containing about 1.5-2.5 wt % Na. This residual sodium is difficult to exchange. During use, the hydrothermal conditions result in migration of the sodium ions to easily exchangeable positions. It is this sodium which is removed during rejuvenation. C. MARCILLY: 1.Usually zeolite-based catalysts are used for producing a maximum of gasoline. Recently Union Oil has claimed the use of a zeolite-based catalyst which allows to maximize either gasoil or jet fuel. Could you make any comment on this ? 2. Is ZSM-5 used in industrial hydrocracking catalysts?

618 J. WARD: 1. You are correct in that zeolite-based hydrocracking catalysts are used for producing maximum gasoline. A new zeolite-based hydrocracking catalyst for the production of jet fuel or diesel was announced (see Hydrocarbon Processing, 1981). We believe that this is a significant breakthrough. 2. To my knowledge, at least two industrial hydrocracking processes use ZSM-5 type zeolite containing catalysts. These are lubeoil dewaxing and middle distillate dewaxing. J.A. MARTENS: Does the mechanism for hydrocracking of n-dodecane account for all the hydrocracking catalysts mentioned in your paper? In shape selective zeolites (i.e. ZSM-5), the mechanism may proceed via dimethyldecanes. J. WARD: We have not examined n-dodecane hydrocracking over the several types of catalyst discussed. The mechanism cited is that of weitkamp and coworkers (Ref. 2,3) and applies to wide pore zeolite such as Y. Recently Jacobs and coworkers (Ref. 41) examined platinum supported on Y and ZSM-5 zeolites. ZHAO JIUSHENG The surface acidity may influence a lot the cracking ability and coking. When you design the catalyst, how to select the acidity and acid amount and how to control them , by ion exchange or by additives? J. WARD The surface acidity is very closely related to the cracking activity (see Figs. 5 and 9, and Table 3). Although the tendency to coke increases with acidity, the coking rate is minimized by the hydrogenetaion activity. The concentration of active sites can be controlled by any means of controlling the popUlation of acidic hydroxyl groups. This the zeolite content, extent, of ion exchange, type of cation exchanged into the zeolite (e.g. monovalent vs. divalent), thermal treatment to change the hydroxyl group content, etc.. Few criteria are available for controlling acid strength. However, it is generally believed that amorphous oxides such as silica-alumina or silica magnesia are weaker acids than zeolites.

619


INFLUENCE OF SODIUM CHLORIDE ON THE CAT ALYTIC PROPERTIES OF TELLURIUM-LOADED Y-ZEOLITES B.E. LANGNER+ and J.H. KAGON Institut fUr Technlsche Chemie, Universitat Essen (FB 8) Universrtatsstr s J , 43 Essen (FRG)

ABSTRACT The catalytic properties of tellurium-loaded Na- Y zeolites which have been modified prior to the tellurium loading by a thermal treatment in the presence of 0-15 wt.-% sodium chloride at 670 0C have been studied by physical methods and by dehydrogenation reactions.

The results show

that both the crystallinity and

the free pore volume of the zeolite decrease with increasing amounts of added sodium chloride. The retainment of tellurium in the zeolite after a hydrogen treatment at 540

0C

exhibits a maximum at a sodium chloride content of about 4-8 wt-%. In the same range of salt addition the catalysts exhibit maximum activity for the dehydrogenation reactions of ethylbenzene to styrene, of isobutane to isobutene , and n-hexane or cyclohexane to benzene.

INTRODUCTION Tellurium-loaded zeolites have been proved to be excellent catalysts for aromatization reactions (ref. 1-4 ,5,6) leading to a yield of more than 80% in the conversion of n-hexane to benzene at 530 0C on a Na-X/Te-catalyst. Furthermore, tellurium-loaded Na- Y zeolites catalyse the dehydrocyclodimerization of

butenes

to aromatics at selectivities of more than 60% at conversions below 10% (ref.?). The nature of the active sites of these catalysts is discussed in different ways. Whereas Olson et al ,

(ref. 3) conclude from X-ray powder diffraction patterns

that telluride ions are responsible for the catalytic activity,

Hightower et al ,

(ref. 5 ,6) concluded from surface measurements that tellurium atoms in a special zeolitic environment provide the high dehydrogenation activity. Although Hightower et al , (ref. 6)

did not

find any acidic activity for the

catalysts, we could detect cracking products in the dehydrocyclodimerization of

"re

whom correspondence should be sent.

620 butene which point to the existence of residual acidic sites on a Na- Y/Te catalyst. These acidic sites could have been formed during the hydrogen treatment of the physical mixture of tellurium and Na- Y at 530 0C according to the equation:

in which telluride ions are assumed to be responsible for the catalytic activity. Furthermore, the commercial Na- Y zeolites usually contain impurities of calcium ions which are known to possess acidic activity. In any case acidic activity is not required if one looks for a pure dehydrogenation catalyst. To destroy the residual acidic activity of the catalysts we used a method first proposed by R abo (ref. 8,9). Rabo has demonstrated that double bond isomerisation of I-butene on a commercial Na- Y zeolite could be suppressed by mixing solid sodium chloride to the zeolite followed by a thermal treatment at about 500 0C. By this procedure sodium chloride migrates into the zeolite structure in the solid state and destroys acidic sites of the zeolite by ion exchange:

where Z-O-H represents the zeolitic hydroxyxl groups and ZO- stands for the zeolitic framework. From these results we expected that tellurlum-Ioaded zeolites which have been modified by a thermal treatment in the presence of sodium chloride could lead to pure dehydrogenation catalysts as residual acidic activities have been destroyed by the salt addition.

EXPERIMENTAL For each experiment and each analysis 1.00 g catalyst was prepared by a strictly kept procedure. The catalyst bases were dried commercial Na- Y zeolites in the powder form (Linde SK 40, ratio Si0 which have been mixed 2/A1203=4.8) with 0-15 wt.-% solid sodium chloride in a ball-mill for 10 min. (too long ballmilling partially destroys the zeolite structure). The mixture was held in a muffle furnace at 670 0C for 17 hours. After the thermal treatment about 25 wt. -% tellurium powder was added and mixed in a mortar. The mixture was placed into the reactor which consisted of a quartz tube (d= 15 mrn) with an internal quartz frit to uptake the catalyst. The reactor was mounted in a tube furnace (Heraeus Type B/ A 1. 7/2.5) the temperature of which could be regulated at ±3K. Prior to the reaction, the Na-Y/NaC1!Te mixture was heated in a stream of dry hydrogen (2.5 Iiter/hr ,') at 530 0C for 17 hours. By this treatment a fraction of the tellurium

621

is evaporated and condenses at cooler parts of the apparatus from where the tellurium needles are removed before the reaction starts. The partial pressures of the hydrocarbons (ethylbenzene, n-hexane, cyclohexane) were adjusted by the saturation of a stream of dry nitrogen (2.5 liters/hr.) at lOoC. The dehydrogenation of isobutane to isobutene was carried out at atmospheric pressure without any dilution. Analyses of the reaction products were made gaschromatographically on a 6mx 1/4" column filled with 17% Sebaconitril on Chromosorb for the separation of the at 250C and a 3 mx1/4" column filled with 10% Benton-34 5-hydrocarbons on Chromosorb for the separation of the monoaraomatics at 900C. To follow the

C l-C

deactivation of the catalysts product samples were taken every 20 min.

The d-

spacings of the powder diffraction patterns (apparatus:Siemens spectrometer Type M34) were calculated according to the tables of Strunz (reLIO). Adsorption isotherms were determined with nitrogen at -196 0C in a conventional BET-apparatus , The adsorption capacities of the catalysts are expresssed in x/m = gN 2,adsorbed/ g catalyst at a partial pressure of 150 torr.

The analyses of the ultimate tellurium content of the catalysts after the hydrogen treatment were made (0 by weighing out the tellurium which has been crystallized on the walls of the reactor and (li) by a potentiometric titration procedure after extracting the tellurium from the catalyst with nitric acid.

RESULTS AND DISCUSSION Influence of sodium chloride on the physical properties.

During the thermal treat-

ment of the Na- Y/NaCI mixture the physical properties of the system drastically change. Although the calcination temperature is below the melting point of

= 800 0C), sodium chloride loses its crystal structure and rnis grates into the zeolite. This is confirmed by X-ray powder diffraction patterns

sodium chloride (T

shown in Fig.I. Whereas in the physical mixture of sodium chloride and Na-Y both the strong line of the sodium chloride crystals and the zeolite lines can be observed, after 89 hours at 590 0C the sodium chloride peak has disappeared. This shows in accordance with the results of Rabo (reL8) that sodium chloride is able to migrate into the zeolite structure below its melting point. (The migration process of the salt can be accelerated by elevated temperatures; therefore we have carried out most of the experiments at a calcination temperature of 6700C for 17 hours). Also the adsorption capacities of the zeolite for nitrogen at -196 0C and 150 Torr change during the thermal treatment in the presence of sodium chloride. Fig.2 demonstrates that the addition of sodium chloride results in a decrease of the adsorption capacities of the zeolites leading to a pore volume for a Na-Y zeolite + 8%NaCI catalyst which is less than one third of a catalyst without the

622

Oh

89 h - 550°C

-,

"No CI

No- Y

c

10

Cl> ~

::l

0', "

,,

to

I

~,

,'\,'i1 .... 0

T

5

85

'l\

0~

80 ~

\\

..c 0 ~

90

~,

=

570°C

W/F = 0.018 9 h Immol 760 Torr p

o~

~.

,0 ~

ti Cl> 75

jj

70

o+---..-.----,--,--.----,--,----,,....-..--,---,-..,--_+_ 12 10 o 4 6 8 2 NoCI [wt

-%J

Fig.6. Dehydrogenation of isobutane as a function of NaCI content

627 The yield of benzene can be doubled if cyclohexane is used instead of n-hexane, as ring closure is a slow step in the reaction sequence from paraffins to aromatics. Nevertheless, even for the aromatization of cyclohexane a strong influence of sodium chloride can be stated (Fig.8). On the other hand, methylcyclopentane is no appropriate feedstock for the formation of benzene with this catalyst as only cracking products and coke formation could be observed leading to a rapid deactivation of

the catalyst. This can be explained by the lack of acidic sites

which are necessary for ring expansion reactions from a five-membered ring to a six-membered ring.

50

100

/~-g

0

o

40 ~ I

~

30

0

/

/'

x

90 x x

C

T = 530°C W/F= 0.22 9 h/mmol 60 Torr p =

(II

c

0

x

QJ

N

-0

20

QJ

co

-,

~

0

80

x

1;>

ti QJ

70

10

Qi

V1

0 0

2

3

4

NoCI [%

5

6

7

8

1

Fig.8. Aromatization of cyclohexane as a function of NaCl content

CONCLUSIONS A novel catalyst system which exhibits excellent dehydrogenation activity and selectivity can be made by modifying tellurium-loaded Na- Y zeolites with about 4-8% sodium chloride. Although the crystallinity and the pore volume of the zeolites decrease during the thermal treatment with sodium chloride at 670o C , both the catalytic activity and the amount of retained tellurium in the catalyst are increased by the addition of sodium chloride. The results indicate that the salt not only destroys residual acidic activity of

the zeolite but also stabilizes the

active form of tellurium. Nevertheless, the nature of the active tellurium site remains obscure.

628

ACKNOWLEDGEMENTS The authors want to thank Mrs.A.SchrOder for valuable technical assistance.

REFERENCES

1 W.H.Lang,R.J.Mikovsky,A.J.Silvestri, J.Catal. 20 (1971) 293 2 R.J.Mikovsky,A.J.Silvestri,E.Dempsey,D.H.Olson, J.Catal. 22 (1971) 371 3 D.H.Olson,R.J.Mikovsky, G.F.Shipman, E.Dempsey, J.Catal. 24 (1972) 161 4 A.J.Silvestri,R.L.Smith, J.Catal. 29 (1973) 316 5 G.L.Priee,Z.R.lzmagilov,J.W.Hightower J.Catal. 73 (1982) 361 6 G.L.Priee,Z:R.Ismagilov,J.W.Hightower in Proe. 7th Int.Congress Catal., Elsevier Tokyo p.708 (1980) 7 K.D.Hungenberg,J.H.Kagon,B.E.Langner, J.Catal. 68 (1981) 200 8 J.A.Rabo ACS Monograph 171 (1976) 335 9 J.A.Rabo,M.L.Poutsma,G.W.Skeels, Proe.lnt.Congr.Catal. 5th, North Holland Publ.Co. 98 (1977) 1353 10 H.Strunz "Mineralogisehe Tabellen" Akad.Verlagsgesellsehaft, Leipzig 1966

629 DISCUSSION D.D. SURESH Have you studied the effect of stronger bases such as Rb+ and Cs+ to further improve the retainment of Te and increase the activity ? B.E. LANGNER: We did not. But there may be an even greater effect on the activity of tellurium-loaded zeolites according to the greater effect of potassium salts in comparison with sodium and lithium salts. But as I mentioned before,it is very difficult to define equal calcination conditions for the zeolite/saltmixtures, as the salts have different melting points. LIN LIWU: 1) You used N2 as carrier gas to maintain lower partial pressure of reactants. If nitrogen is circulated, the H2 partial pressure of the reacted gas should be increased. Did you observe the effect of H2 partial pressure on the reaction performance for this kind of catalysts ? 2) Is it possible to maintain the concentration of tellurium and sodium chloride in the catalyst composite without any loss during the high temperature operation? B.E. LANGNER : 1) We have carried out all the experiments in afixed-bed reactor without recycling the carrier gas. But it may be probable that there is a kinetic effect of hydrogen according to a Hougen-Watson equation for dehydrogenation reactions. Furthermore, in some experiments the conversions were close to the thermodynamic equilibrium, and there should be an effect of hydrogen too. 2) If you have hydrogen or hydrocarbons in the carrier gas, then there is only minor loss of tellurium. But in the absence of hydrogen or hydrocarbons, tellurium is evaporated. A loss of sodium chloride during the reaction or during the hydrogen treatment was not examined. R. CAHEN 1) Have tou examined the stability of this very interesting catalyst? 2) Can the catalyst be regenerated ? B.E. LANGNER: l)There is deactivation of the catalyst by coke formation in all reactions, but the rate of the deactivation is strongly dependent on the kind of feed and on the reaction conditions (partial pressure, residence time, temperature). Thus the catalysts lose 50% of their initial activity in the DHCD of butene within a few hours, but in the dehydrogenation of ethylbenzene, within about two weeks. 2) We only made preliminary experiments on the regeneration of coked catalysts. But it could be shown that about 70% of the initial activity is restored, if the coke is burnt off in air followed by a new hydrogen treatment of the catalyst. T. BEIN You observe volcano curves for the catalytic activity is several reactions on NaCl/Te-Na-Y catalysts with a maximum at a NaCl content of around 6%. Could an explanantion for this effect be the competition between the following influences: on the one hand, the tellurium retention increases strongly with NaCl content up to 5% and on the other hand, crystallinity and adsorption capacity (Fig. 4) of the samples decrease with increasing NaCl content (Figs. 2,3). Could the catalytic activity, due to these effects, be a function of available tellurium sites ? B.E. ~NGNER : I agree with your hypothesis that there are two effects for the catalytic activity: salt content and crystallinity of the zeolite or at least of a part of the zeolite· structure. But we do not know which are the active sites. And we even do not know in which form the tellurium exists in the zeolite structure. Is there elemental tellurium, are there telluride ions or are there even poly-telluride ions? Therefore, we can only speculate about the role of the salt in tellurium-loaded zeolites.


631


CONTROL OF THE PORE STRUCTURE OF POROUS ALUMINA

T. ONO, Y. OHGUCHI and 0, TOGARI Chiyoda Chemical Engineering and Construction Co., Ltd. Yokohama, Japan

ABSTRACT A new and simple method to prepare porous alumina is described.

This

alumina has a large pore volume ranging from about 0.5 to 1.5 ml/g and a narrow pore distribution even at a large pore diameter (10 - 100 nm).

In the

present method, both aluminum salts of acid and alkali are admixed alternately in the gelation process resulting in a swinging pH value.

Experiments were

carried out to determine the possible effects of the principal influencing factors, such as the frequency of pH swing, the pH value and reaction time On the pore structure of alumina.

INTRODUCTION Alumina is used as a carrier in various fields, owing to its superior pore structure.

The pore structure of the alumina has a close bearing on the

catalyst activity, selectivity and life.

It is very important to produce an

alumina having a pore structure suited for the intended reaction. alumina can be obtained by the dehydration of pseudo-boehmite.

Porous

The pore

structure of the resulting alumina is greatly influenced by the size, shape and further by the aggregation and disposition of pseudo-boehmite particles. The preparation of an alumina carrier includes controlling its pore structure during gelation, drying and calcining in order to provide a carrier suited for the intended reaction.

A lot of methods have been proposed in this connection.

For example, raw materials selection [1] and gelation conditions [2] are applied to controlling the size of pseudo-boehmite.

Aging of pseudo-boehmite

[3] is applied to controlling size distribution and growing.

Other methods

include replacing water among particles in a gel with an organic solvent having a lower surface tension, such as alcohol

[4],

to control the pattern in which

they are aggregated, or utilizing the interstices formed by calcining a pseudoboehmite gel in which a water-soluble organic polymer has been mixed [5].

632 The calcining temperature is also controlled to provide a desired pore structure [6].

But these methods are not necessarily effective in preparation

an alumina having a large pore volume and a large pore diameter, and a narrow pore distribution.

The advantage of this new method is that the alumina having

a desired pore diameter and a narrow pore distribution can be prepared easily.

EXPERIMENTS Apparatus Most important for carrying out the present method is the pseudo-boehmite preparation apparatus shown in Fig. 1.

A gelation vessel has a capacity of 100

liters, and is provided with a steam jacket, a stirrer and a condenser.

A

vessel is provided over the gelation vessel for introducing prescribed quantities of sodium aluminate and aluminum nitrate. The temperature of the gelation vessel was controlled by a TRC unit.

The pH

of the pseudo-boehmite slurry was measured while it was being circulated.

A

vacuum filter was used to clean and filter the gel, and a hydraulic piston extruder to

form

it.

A hot air circulation type drier was used to dry the

product, and a muffle furnace to calcine it.

water steam gelation

Fig. 1.

~~~~~to

filter

Apparatus for alternating pH swing gelation.

Raw materials An aqueous solution of sodium aluminate (20wt% as A1 Na/Al=1.6) and 20 3, ) were used as raw materials for the alumina.

aluminum nitrate (5.4wt% as A1 0, 2

633

Experimental procedures and conditions First, 40 to 60 liters of water were placed in the gelation vessel and usually heated to a gelation temperature of 100°C.

The quantity of both sodium

aluminate and aluminum nitrate was determined in the desired pH swing range from the titration curve.

The first pH swing operation started with the

addition of an aluminum nitrate in the gelation vessel and ended with the addition of a sodium aluminate.

The second and subsequent substantial pH swing

operations consisted of alternately adding an aluminum nitrate and sodium aluminate after a prescribed reaction time between each addition. In the present method, if these operations are repeated as required, the gelation procedure is completed.

The pH swing operation referred to include

a set of operations for rendering the slurry acidic and then alkaline.

The

pseudo-boehmite prepared as described above was washed until it contained not more than 0.02% by weight of Na

20

and a cake containing 20 3, was formed by filtration. This cake was

relative to A1

about 20 to 30% by weight of A1 20 3 then formed into extrudates having a diameter of Imm¢.

The extrudates were

dried at 120 DC, and calcined at 500 DC for three hours, whereby alumina was produced.

Analyses The pore volume, pore distribution and modal pore diameter of alumina were determined by a 2,000 kg/cm 2 mercury porosimeter (Model 70, Carlo Erba). The specific surface area of alumina was measured by BET method using a surface area measurement instrument (SA-2000, Shibata Scientific Instrument Co.).

The

Scherrer's equation according to the line broadening method by X-ray diffractometer (Geiger-Flex KG-X, Rigaku Electric Co.) was used to determine the crystal size of pseudo-boehmite on the (020) plane, and the crystal size of alumina on the y-alumina (440) plane.

The size and shape of pseudo-boehmite

particles were observed by an electron microscope (H-600, Hitachi Ltd.).

RESULTS Fig. 2 shows the pore distributions of aluminas prepared by the present method, and Table 1 shows their physical properties and the preparation conditions.

Fig. 2 and Table 1 are believed to indicate clearly the features

of this method.

Aluroinas A to I were prepared under substantially the same

conditions except for the frequency of pH swinging, and showed an increase in pore volume from 0.54 to 1.49 ml/g with an increase in modal pore diameter. It is clear that the present method makes it possible to control the pore diameter in a wide range. In the following description, the pore size of alumina is expressed in modal

634 pore diameter, since it is believed to represent most clearly the pore

distributions as shown in Fig. 2.

B

20

c 15

o

Ci

E F

e

OJ

o 10

G H

Pural SB > Pural NF > Pural NG > Dispural. This behaviour appears to be caused by the shape, composition and size distribution of alumina aggregates, as mentioned before.

651

CONCLUS IONS Although a quantitative relation cannot yet be established between structure properties and extrusion behaviour of aluminas. There are strong indications that paste formation and extrusion are controlled principally by the size of the primary particles and their aggregations. Illustrating the two extre~sare gibbsite, obtained by the Bayer process and consisting of aggregates of 1 u m crystallite" size yielding paste of barely extrudable consistency and boehmites of the Condea type, composed of aggregates of about 10 nm primary particle size. The above supposition is strongly supported by the fact that Alu C of Degussa, Hanau, GFR, being a pyrolytic alumina of about 10 nm particle size containing 0.5 % (w/w) of chloride can be pelletized easily only when water is employed as pelletizing liquid. The excess of water of the Condea boehmites and their microcrystalline size additionally favour the paste formation process. Activated boehmites and gibbsites behave differently in paste formation which may be attributed to their low water content. It was demonstrated that unfavourable extrusion properties can be overcome by adding boehmite, alumina gels and sols as binder and poreforming additives. REFERENCES 1 G.Berrebi and Ph. Bernusset in B. Delmon, P.A. Jacobs and G. Poncelet (Eds.), Preparation of catalysts, Elsevier, Amsterdam, 1976, PP 13-38 2 D.L. Trimm (Ed.), Design of industrial catalysts, Chemical Engineering f'1onographs, Vol. II, Elsevier, Amsterdam, 1980, pp 3-36 3 DPS 23 49 773 (18.04.1974) 4 DAS 25 11 967 (09.10.1975) 5 DOS 26 39 285 (02.03.1978) 6 K.K. Unger, St. Doeller and K.F. Krebs in S.J. Gregg et al. (EdsJ, Character. Porous Solids, Proc. Symp., 1978, pp 291-299 7 R. Tettenhorst and D.A. Hofmann, Clays Clay Miner., 28(5) 1980, pp 373-380 8 B.E. Yoldas, Ceramic Bulletin, 54(3), 1975, pp 289-290



653

INFLUENCE OF ALUMINIUM HYDROXIDE PEPTIZATION ON PHYSICAL PROPERTIES OF ALUMINA EXTRUDATES K. JIRATOVA, L. JANACEK+ and P. SCHNEIDER Institute of Chemical Process Fundamentals, Czechoslovak Academy of Sciences, 165 02 Prague 6, Czechoslovakia

ABSTRACT Physical properties of alumina extrudates prepared from differently peptized aluminium hydroxide were studied. The effect of different peptizing acids on the physical properties of extrudates can be generalized by using the Hammett acidity function, Ho' of the peptization solution. In the range of 0 < Ho < 1, the physical properties of extrudates change most significantly. The mean radii of transport pores were determined by combining the permeation and countercurrent diffusion measurements. Correlation was proved between the mean radius of transport pores, calculated from diffusion measurements, and the volume of macropores.

INTRODUCTION Heterogeneous catalysts used in chemical industry must possessa well-defined geometric shape and convenient physical and chemical properties. As the most important are considered sufficient surface area, mechanical strength, and low resistance to internal diffusion which depends on porous structure. Recently, catalysts are manufactured very often by extrusion. This procedure consists in mixing dry aluminium hydroxide with a small amount of water and peptizing agent. By kneading, a paste is formed which is extruded through nozzles and the extrudates are dried and calcined. Extrusion permits production of catalysts and supports with smaller dimensions and at lower expenses than pelleting. For extrusion, it is necessary to prepare the mass of pasty consistence which is sufficiently plastic. The plasticity is reached by different ways (ref. 1); in case of aluminium hydroxide, the +Chemical Works, Research Institute of Hydrocarbon Utilization, 436 70 litvinov, Czechoslovakia.

654

'chemical dispersing (peptization) by inorganic or organic acids or hydroxides (refs. 2-S) is used most frequently. In the course of drying and calcining, the extrudates shrink, which leads to the formation of macropores; however, simultaneously also the mesopores in the primary aluminium hydroxide particles can be influenced. Consequently, by changing the amount and type of peptizing agent, the porous structure and therefore also the mechanical strength o~ extrudates can be influenced. This work continues our recent study (ref. 6), and its aim is to find how the amount and type of peptizing agent and the size of aluminium hydroxide particles used influence the physical properties of extrudates, namely their porous structure. EXPERIMENTAL Preparation of alumina extrudates Commercial aluminium hydroxide Pural (Condea Chemie, FRG) of boehmitic structure was used. The characteristic dimension of particles d SO was 18, 32, 47, and 19S ~m (d SO is that particle diameter for which SO % wt. of particles are smaller than d SO)' The aluminium hydroxide paste was prepared at laboratory temperature by kneading for 1 hour 2S0 g dry aluminium hydroxide with a chosen amount of peptizing agent and such an amount of water that it should be possible to extrude the resulting paste by a piston extruder at pressure 4 MPa through 2 mm nozzles. The amount of water added varied between 80 + 20 cm 3/100 g aluminium hydroxide. The kneading procedure, i.e. the kneading machine (Becken, FRG), the rate of mixing, and order of adding the components was always the same. The extrudates were dried at l20 0C and calcined for 4 hours at 600 0C. Evaluation of physical and mechanical properties of extrudates The BET surface area was determined from the low-temperature nitrogen sorption by measuring several points of adsorption isotherm in the range of relative pressures 0.05 to 0.3 at liquid nitrogen temperature using modified method of De Baun and Fink (ref. 7). The volume and distribution of mesopores (pores with radii 2 - 15.8 nm) were determined from the adsorption isotherm of benzene. The volume and distribution of macropores (pores with radii lS.8 - 6 500 nm) were determined by mercury porosimetry (porosimeter Carlo Erba, model 6S A). The mechanical strength was determined by gradual loading the pellet with an edge in normal direction to the cylindrical surface of extrudates and is given by the force which breaks a pel-

655

let of unit diameter. It is expressed in N.m- l. Determination of transport pores The mean radii of transport pores were determined by combining the permeation. and countercurrent diffusion measurements (refs. 8-10). Fifty pieces of extrudates were vertically mounted in a horizontal silicon rubber plate 4 mm thick in which holes with diameter about 0.2 mm smaller than measured particles were cut out. In th}s way, the needed tightness between measured particles and the silicone plate was ensured. RESULTS AND DISCUSSION Peptization of aluminium hydroxide particles of various size by acetic acid The samples of aluminium hydroxide with particle sizes d 50 = 17; 32; 47; and 195 ~m were used for measurements. The dependence of mechanical strength of extrudates on the amount of acetic acid added is plotted in Fig. 1.

........,

8

'E E

b

CL

4

6

(CH 3COOH) [% wt.]

Fig. 1. Dependence of the extrudate mechanical strength P on the amount of acetic acid (% wt.) for different sizes of aluminium hydroxide d 50: ~ - 17 ~m; ~ - 32 ~m; • - 47 ~m; 0 - 195 ~m The mechanical strength reached on using the same amount of acetic acid increased with decreasing aluminium hydroxide particle size. In the investigated region, a linear dependence holds approximately between the particle size and the amount of acetic acid required to reach a chosen mechanical strength (Fig. 2).

656

s:o a

u

'"

I: U

[,um]

Fig. 2. Dependence of peptizing agrnt amount required to reach mechanical strength 1; 2; 4; 6 N mm- on the aluminium hydroxide particle-size d SO Because of the strong dependence of mechanical strength of extrudates on the size of primary particles of aluminium hydroxide constant particle size was used in the following measurements. Comparison of different peptizing agents The effect of various acids on the aluminium hydroxide peptization was evaluated from the properties of calcined extrudates. The aluminium hydroxide used had the particle size d SO = 32 wm and the peptizing agents were sulphuric, nitric, hydrochloric, trichloroacetic, phosphoric, oxalic, lactic, and formic acids. The dissociation constants of the acids used varied from -6.1 to +3.8 (ref. 11). The amount of the peptizing agent used during kneading varied from O.OS to S wt. % of dry aluminium hydroxide. It is evident from the left-hand part of Fig. 3 that with increasing amount of acids in the paste, the mechanical strength increases and the macropore volume decreases. The mesopore volume and surface area change only slightly, except at high concentrations of the peptizing agent. The results obtained are in agreement with following concept of formation of porous structure of solids. The irregularly shaped primary particles of aluminium hydroxide contain certain amount of mesopores. By compacting the primary particles, the macropores are formed between them. Aluminium hydroxide peptization makes possible a closer arrangement of primary particles so that the free vo-

657

lume among particles in the paste decreases. Therefore after drying and calcination the macropore volume in extrudates decreases. On using a larger amount of peptizing agent, deeper layers of primary particles react and thus the mesopore volume decreases.

15r----,------,------,;:Jrr-:rr--,--,---,-ff-,--, p N/mm 10·

S m2.jg

@

@

Vme

V cm3/g

o

4

m [Ofo wt']

Fig. 3. Dependence of mechanical strength P, surface area S, and pore volume V , V on the amount of peptizing agent in paste m and on the va~~e o~aH : (J - HC1, () - HF, 'l) - HCOOH, • - HNO , @ (COOH)2' e - H3P0 4, 0 0 _ H2S04, ~ - CC1 300H, ~ - CH 3CH(OH)COeH It was found experimentally that different acids have different effects especially on the macropore volume. This is probably connected with their ability to react with aluminium hydroxide which depends,ingeneral,on the co nc e n t r a t i on of H+ ions. Since concentrated solutions of acids (up to 3 mol 1-1) are used for peptization, it is not possible to express the acidity of peptization solutions by a quantity appropriate for diluted electrolytes. Therefore, we have characterized the concentration of hydrogen ions in the peptization solutions with concentrations higher than 0.1 mol 1-1 by the Hammett acidity function Ho (ref. 12). H for hydrochloric, o sulphuric, and nitric acids were taken from Paul and Long (ref. 13), for phosphoric, hydrofluoric, and trichloroacetic acids from Rochester (ref. 14) and for formic acid from Milyaeva (ref. 15). Ho for mildly concentrated solutions of oxalic and formic acids were calculated from the relation

658

Ho - 1/2 pK a - 1/2 log c a proposed by Randles and Tedder (ref. 16). On the right-hand side of Fig. 3, the physical properties of extrudates in dependence on Ho of peptizing agent are plotted. When peptizing by strong inorganic acids and trichloroacetic acid, the physical properties of extrudates in the region Ho > 1 do not change and approach the values of aluminium hydroxide kneaded with water (H o = 7). For 0 < < H < 1, significant changes in physical properties of extrudates o take place. With ~ecreasing Ho' the macropores vanish and the mechanical strength increases, which proves that both these properties are mutually connected. The mesopore volume changes only slightly. The effect of Ho on the specific surface area depends on the anion of peptizing acid. Sulphuric and phosphoric acids, which form non-volatile compounds with aluminium hydroxide, increased the surface area unlike the acids whose anions are almost completely removed by calcining. Since the dependences for the surface area and the mesopore volume are not similar, we assume that by the effect of sulphuric and phosphoric acids, a change in the micropore volume (i.e. pores with radii < 1.7 nm) takes place. With a further increase in the acidity strength of peptizing agent below Ho < 0, the macropores dissappear, and the mesopore volume started to decrease more significantly. However, the mechanical strength did not increase any more, as it might be expected, and, on the contrary, after exceeding Ho < -0.25, it strongly decreased. On extruding such a peptized paste, a high internal stress arises after calcining which manifests itself by longitudial splitting of extrudates after their loading by an edge, and consequently, in a striking decrease of the mechanical strength. The aluminium hydroxide peptization by hydrofluoric acid and weak organi c aci ds takes pl ace in a somewhat di fferent way than wi th strong inorganic acids. The decrease of macropore volume and the corresponding increase of mechanical strength appears at higher values of Ho' The region of significant changes in physical properties of extrudates, localized to Ho equal to 0 - 1 in case of peptizing by strong acids, extends to H equal to 0 - 3 for weak orgao nic acids. Thus, it follows that besides the H+ ion concentration, also the nature of the acid anion and of the undissociated acid plays a role in peptizing aluminium hy d r ox i de Oreoof the possible explanations is based on the physical adsorption of anions and undissociated molecules on the aluminium hydroxide particles. It is well-known i

659

that adsorption of polar particles and ions on solids proceeds more easily and to a greater extent than adsorption of nonpolar molecules. Anions of strong inorganic acids and their undissociated molecules are more polar than organic anions, and apparently are adsorbed in larger amounts than organic anions and undissociated acids. This accounts for a less dense packing of primary particles in the dispersion system with strong inorganic acids so that the macropore volume is larger than with weak organic acids. This is supported also·by similar peptizing effect of strong trichloroacetic acid and strong inorganic acids. Effect of peptization on porous structure We have shown that peptization influences considerably the macropore volume and the mechanical strength. It was therefore interesting to investigate how the distribution of pore sizes was changed by peptization. As an example, the pore size distributions of extrudates peptized by various amounts of sulphuric acid (0 < Ho < < 2.2) are illustrated in Fig. 4.

nm

Fig. 4. Pore size distributions of extrudates peptized by various amounts of sulphuric acid m (wt. %) It is evident that the size.of the mostfrequent mesopores (radius -5 nm) is nearly independent of the acid amount. Certain changes are

660

apparent in the macropore region, however, it is not possible to quantify them accurately. Tischer in his work (ref. 17) made similar observations. For a deeper characterization of porous structure of differently peptized aluminas, we have determined the m~an radii of pores (refs. 8-10) through which the transport of gases takes place (transport pores). The resulting parameters r~, ~ characterize the transport of ~ases through the porous medium. On the left-hand part of Fig. 5 it is shown how these parameters vary owing to the acidity of peptization solution Ho' 3000

.!:..!f

v

2000 1000 0

v 0.05 0 200 rip

100 0

-1

Fig. 5. Dependence of parameters r~, ~, and r~/~ ~n H of peptization solution and on the macropore volume V a (cm g-lo) for various peptizing agents: ~ - HF, ~ - HCOOH, • - HN~3' ~ - H20, 0 - H2S04, o - CC1 3COOH, 0 - (CH3COO)2Zn It can be seen that parameter r~ decreases on deeper peptizing, the decrease being steeper for peptization by weak acids. For non-peptized aluminium hydroxide r~ = 270 nm. The lowest measured value of r~ of peptized aluminium hydroxide was 3.7 nm. Analogously to the dependence of macropore volume on Ho' higher values of r~ were observed for extrudates peptized by trichloroacetic acid. The geometric transport parameter of extrudates, ~, decreased

661

due to peptization from original value 0.095 to 0.002 and exhibited a similar S-shaped dependence on Ho as the parameter r~. By combining parameters r~ and ~, the mean radius of transport pores, (r~/~), was evaluated. Peptization decreased its value from 2 900 nm to 300 nm. We estimate that the radius of transport pores of this type of alumina can range from 600 to 1 200 nm for the usual regions of peptization. It follows from the similarity of dependences r~ vs. Ho and Vma vs. Ho khat the macropore volume is decisive for the size of transport pores. This assumption is confirmed by the right-hand side of Fig. 5: parameters r~ and (r~/~) show an exponential increase with the macropore volume Vma' whereas the geometric factor, ~, increases with Vma linearly. CONCLUSION It has been shown, using aluminium hydroxide Pural, that the size of primary aluminium hydroxide particles influences substantially the physical properties of extrudates prepared from it. Physical properties of differently peptized extrudates are dependent on the Hammett acidity function of peptized solution, Ho' The de~~n dence of the mechanical strength of the extrudates 'In the macropore vol ume has been proved. Although the conventional pore si ze di stri bution does not show a significant dependence on peptization, the transport parameters r~, ~, and (r~/~) allow to prove that a decrease of mean radii of transport pores takes place with decreasing Ho of peptization solution. A simple dependence of parameters r~, W, and (r~/~) on the macropore volume was found. It is necessary to state that the dependences obtained hold for the type of aluminium hydroxide used. Due to large variability of properties of aluminium hydroxides, it is possible to expect different values of physical properties o~ extrudates originating from aluminium hydroxides prepared by different methods. ACKNOHLEDGEtlENT The authors thank Mrs. J. Aunicka and Mrs. V. experimental assistance.

Ne~kodna

for their

SYMBOLS concentration of peptization solution (mol 1-1) mean diameter of primary particles of aluminium hydroxide (Il m)

662

Hammett acidity function dissociation constant of acid quantity of peptizing agent related to dry aluminium hydroxide (wt. %) mechanical strength of extrudates (N mm- l) P radius of most frequent pores (nm) r transport parameters for Knudsen and bulk diffusion, resp. r Co at the moment of passing of menisci through the pore "throat" (r and 6 L are respectively the radius of curt vature and surface tension of the liquid phase; e is the wetting angle). The volume of gel, Vg, is fl-1+VL ' where VL is the volume of the liquid phase (cm 3/g SiO ). The decrease in V is par2 L alleled by volume contraction which decreases for aggregated (aged) hydrogels, and also when the rate of drying, f) and 1) increase or ~ decreases. Step of drying I I determines the pore size distribution and lasts until the residual moisture content Vn is attained. At this moment the liquid phase remains solely at contact points and on the surface of globules (V approx. corresponds to the lower n point of the hysteresis loop of adsorption isotherm on xerogel). As the evaporation front moves in the grain volume, individual

zones-domains filled with liquid are formed (Fig. 3, c,e). The

671

capillary forces are directed to the surface of these domains, and therefore, at this step the local contraction in some zones is possible at minimum changes in the total pore volume Vr. • Special regimes of drying (rapid drying of gel at step I and slow drying at step II), in principle, allow preparation of silicas having the bidisperse structure (ref. 6). Step of drying III affects predominantly the surface area of the xerogel.The- destruction of the shells has been accomplished, the contact points between globules are formed, and the transfer MC processes are intensified. The maximum decrease of the surface area is observed during drying of alkaline hydrogels, i.e. when the volume filling of part of the cavities may be accompanied by the appearance of "molecular-sieve" effects (MC-UD?F mechanism) (ref. 7). Thus, the transfer of LlIlS determines rhe formation of the surface and corosf.ty of 'the xerogel : at the steps that follow sol formation the u~c transfer leads to the decrease of S, while at the steps that follow the formation of hydrogel - to the growth of V~ • The final formation of the structure occurs at the step of drying. The experimental data reported in refs. 1-9 can be satisfactorily described in terms of such mechanisms. Regime II is accomplished in the situation where the steps of a) hydrolysis of an alkaline metal silicate, b) sol formation, and c) rapid coagulation occur simultaneously (for example, interaction of silicates with the easily hydrolyzable salts or in the presence of earlier formed gel (refs. 1,2,8). The details of the formation of such silicas were described in (ref. 8) for the continuous method of silica precipitation at constant pH, temperature and the rate at which the reagents are brought together with a simultaneous removal of the formed coagel from the reaction zone. Then the precipitates were filtered, washed, plasticized and extruded. The structure of thus prepared xerogels is determined essentially by the precipitation conditions employed, being almost independent of the drying regime. One can outline two limit regimes of precipitation that allow drastically different structures to be formed: regime IIa - high value of pH, temperature, rate of stirring n, and low rate of bringing initial reagents together q; regime lIb - low value of pH, temperature, n and high q. Hydrogels prepared by operation in regime IIa consist of aggregates made from close-packed globules different in

672

size, which are pr ac t.Lce Ll.y not destroyed by mechanical treaiment in the plastifying apparatus. After drying the xerogels have the structure of nonuniform porosity and appreciable volume of ultramicropores accessible to water molecules and inaccessible to larger molecules of argon, nitrogen, etc. In typical examples the size of aggregates is 10 2_10 3 run, SAr", 20-40 m2/g, SI-/2 0 - 300 400 m2/g, the volume of ultramicropores is ~ 0.05 cm3 / g. Hydrogels prepared by operation in regime lIb, consist of loose aggregates which are readily destructible by mechanical treatment. Xerogels possess no molecular-sieve properties, their characteristic: S '" 300-700 m2/g "and V~ ~ 2-0.5 cm 3 / g. The peculiarities of silica formation in regime II are caused by that LMS, sol, gel are continuously formed and co-exist in the reaction zone. The Ll-1S groups formed can be consumed for 1) the formation of sol globules, 2) growth of available globules, and 3) cementing together of aggregates of contacting globules. It can be ShOVill that gradients ~~t , that determine the motive forces of the corresponding transfer processes of Si0 change in the order "A": 2, !J~3> A(IA 2 > 1If'11. Moreover, sol globules can be consumed to form new aggregates route ("B") or growth of already aV9.ilable aggregates route ("C"). The formation of new aggregates demands some oversaturation. The conditions of regime lIn favor the intense mass-exchange, absence of oversaturation, i.e. process occurrence via routes "A" and "B". In regime lIb the mass-exchange processes are restricted to individual zones and include consecutive steps "a-c" and processes proceeding via route "B" in each zone. For these reasons, conditions of regime IIa favor the formation of large and stable aggregates in which the space between globules is filled with loosely packed groups of LIVIS. SINTERING OF SILICA In general one may outline four types of structural variations: 1) decrease in S with increasing the mean pore size d at c.=const; 2) simultaneous decrease in Sand C at small variations of d, 3) simultaneous decrease in S, G , and d, and 4) decrease in Sand E. with increasing d. The type I variations are typical for sintering in hydrothermal conditions (ref. 10), low-temperature sintering of silicas in the presence of alkaline metal impurities (ref. 10) or water vapor (ref. 11). These variations are attributed to the LMS

673

transfer along the globule surface (or through the volume of intermiscellaneous medium in hydrothermal conditions) via the MC mechanism. Sintering occurs without coming together of globular centers ( C-const). UmJ transfer from loosely to densily packed sites is accompanied by "filling" of these latter, with the mean pore size being increased (ref. 6). The high-temperature sintering is usually followed by variations of type 2 and then of type 3. At intermediate steps the type 4 variations are also possible. These transformationp ~y be described in terms of the LM8 transfer mechanism at the expense of volume diffusion or viscous flow of 8i0 2 phase accompanied by coming together of globular centers (ref. 6). The type 2 variations are typical for silicas thorougly purified from impurities (ref. 10) and can be associated with the coalescence of globules in the domains having increased dispersion or density of packing, and also high residual content of impuritieo. Finally, the sites are formed that possess viscous-fluid properties. At the same time the domains with rigid structure are still retained. Under the action of capillary forces, the "rigid" domains become more densely packed (contraction of the type shown in Fig. 3e), while Sand c- proportionally decrease. with no essential changes in pore distribution in "rigid" domains. \fIith the further temperature rise the fraction of coalescent sites increases. In these conditions it is possible that the viscous-fluid phase would move into the porous space of "rigid" domains. If the melt fills the domains solely (completely or partially), variations of type 4 are possible leading to the increase of mean pore dimensions. Once the melt fills both the "domains" and the porous space between them, S, e, and d start to decrea se simultaneously (type 3 variations). On further increasing the temperature, the whole grain gradually transforms in the viscous-fluid state. Hence, the type 3 variations are always typical for the final step of sint e r Lng , CONCLUSION The above mechanisms of the formation and variation of the structure of silicas at all steps of synthesis may be considered to be a basis for the analysis of genesis of more complex systems. For example, the role of structural-geometrical factors determining Vo, Cmin and VL at the step of drying remains the same when

674

passing to the mUlti-component and crystallizable systems (ref.12). Variations in S with the thermal treatment can be complicated by phase transformations. but still the general scheme of sintering processes remains valid. REFERBNCBS 1 R.K. Iler, The Chemistry of Silica, Wiley Interscience Publ. Jolm Wiley Sons, New York, Chichester, Brisbane, Toronto, 1979. 2 Adsorption and Adsorbents, Vol. 1, Naukova Dumka , Kiev, 1972. 3 C. Okkerse, in B.G. Linsen (ED.), Physical and Chemical Aspects of Adsorbents and Catalysis, Aca d, .Pr-esa , London, 1970. 4 I.E. Neimark and R.Yu. She infa in , Silikagel, Ego Poluchenie, Svoistva i Primenenie, Naukova Dumka Kiev, '1973. 5 D.V. Tarasova, V.B. Fenelonov, V.A. Dzis'kO, V.Yu. Gavrilov and A.U. Khutoryanskaya, Kolloidnyi Zhurnal, 37(1977)207-212. 6 V.B. Fenelonov, D.V. Tarasova and V.Yu. Gavrilov, Kinetika i Illitaliz, 18(1977)480-487; 19(1978)222-227; Izvestiya SO AN SSSR, Ser. Khdrn, Nauk , 4(1978)116-129. 7 V.Yu. Gavrilov, A.P. Karnaukhov and V.B. Fenelonov, Kinetika i Kataliz, 19(1978)1549-1556. 8 V.B. Fenelonov, L.G. Simonova, V.Yu. Gavrilov and V.A. Dzis'ko, Kinetika i Kataliz, 23(1982)444-450. 9 V.M. Chertov, D.B. Dzhambaeva and I.E. Neimark, Kolloidnyi Zhurnal. 27(1965)279; Ukrainskii Khim. Zhurnal, 31(1965)1149,

1253-1258 10 N.B. Akshinskaya, A.V. Kiselev and Yu.S. Nikitin, Zhurnal Fiz. Khim., 36(1962)2277; 37(1963)921; 38(1964)488. 11 VI.G. Shalaffer, C.R. Adams and J.N. Wilson, J. Phys. Chem., 69(1965)1530. 12 V.A. Dzis'ko, A.P. Karnaukhov and D.V. Tarasova, in: Phisikokhemitscheskie Osnovy Sinteza Okisnykh Katalizatorov, Nauka, Novosibirsk, 1978.

675

G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catolysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

STUDY OF THE PREPARATION OF IRON CATALYSTS FOR LIQUEFACTION OF COAL BY HYDROGENATION UNDER PRESSURE

M. ANDRES (I) , H. CHARCOSSET(I) , P. CHICHE(Z) , G. rJEGA-MARIADASSOu(3) , J.P. JOLY (4)

and S. PREGERMAIN(Z)

(I) Institut de Recherches sur la Catalyse, Villeurbanne (France) (2)

CERCHAR, Verneuil en Halatte (France)

(3) Laboratoire de Cinetique Chimique, Paris (France) (4)

ESCIL, Lyon (France)

ABSTRACT The catalytic properties of various iron compounds were investigated in the high pressure hydroliquefaction of coal. In situ sulfidation of iron was carried out by addition of CS

to the coal/catalyst/tetralin charge. Iron acetylacetonaZ te soluble in tetralin, FeS0 deposited on the coal by aqueous impregnation, red 4 mud, as well as various iron supported catalysts were not found to decrease the percent residue of distillation very significantly. Precipitated iron oxide, particle size 1_3 wm was found more active. The best results were obtained with highly dispersed, particle size ~

0,05

m, iron oxide prepared by a flame me-

thod.

INTRODUCTION It has long been recognized that iron compounds, in particular pyrrhotite Fe1_xS' have a catalytic influence on hydroliquefaction of coal (refs.

I). In

this reaction, coal is in suspension in a recycle oil and is treated at a temperature of about 450°C under a hydrogen pressure of roughly 200 atm. But very rare are the studies concerning the relationships between ; - the mode of preparation-the physicochemical properties - and the catalytic activity of the catalysts (refs. Z). The present work deals. with a preselection of iron based catalysts which are sulfided in situ during liquefaction by the addition of CS foil suspension.

Z

to the catalyst/coal

676 IU:SULTS Coal The coal used was a highly volatile bituminous coal (Freyming,France) , which was pulverized to less than 80jUm prior to use. The analytical and petrographic data are listed in Table 1. TABLE 1. Proximate, petrographic and ultimate analyses of Freyming coal

~~~~~~~~~_~~~~l~~~· (wt % air dried basis)

(O_80~m)

~~~~~~~~_~~~~l~~~ (wt % MAF* basis)

Moisture

1.9

C

Ash

7.4

H

5.4

S**

0.5

Volatile matter Fixed carbon

35 55.7

Vitrinite

76

Exinite :

9

Inertinite

10

Mineral matter : 4

83.3

N

1.1

0

9.7

MAP

moisture and ash-free

Pyritic sulfur

0.3

%

Catalytic hydroliquefaction of coal Batch experiments were performed using a 830 ml rocking autoclave. In a typical experiment the autoclave was charged with tetralin (200 g), coal (100 g) , CS

(1 g) and the iron oxide catalyst. CS was used to sulfide the catalyst du2 2 ring the experiment. The autoclave was filled with cold hydrogen to an initial

pressure of 150 atm at room temperature, heated to the reaction temperature (450°C) at a heating rate of 200°C/h, held at 450°C for 3 hours and allowed to cool to room temperature. The gaseous products were then removed for analysis and the autoclave content fractionated, following the conventional scheme described in refs. 3-4. A preliminary vacuum distillation allows the removal of the volatile products in order that the bulk of the liquid product may be filtered without uncontrolled loss of volatile substances. Filtration of the stripped solution was carried out under vacuum. Recovery of solvent and distillate products arising from coal was performed by a further vacuum distillation of the filtrate. The amount of vacuum distillation residue was further determined (hard pitch with a softening

poin~

150°C),

The wet filter cake was treated with pyridine in order to extract all of the soluble material. The extraction residue was assumed to represent the part of the coal which is insoluble in tetralin after reaction (mineral matter and insoluble

677 organic matter) . The activity of any added catalyst is evaluated mainly by the value of the percent residue of distillation, which should be decreased as much as possible compared to the blank experiment. Note that the percent residue of distillation does note include the inertinite plus mineral matter content of the coal. Iron catalysts precursors, soluble in tetra lin Iron acetylacetonate was studied as a model compound (10 g were included in the

char~e,

all of the other conditions being as stated above). The percent re-

sidue of distillation (24.5 %, Table 2) was somewhat less than in the blank experiment (26.0 %). The relatively low catalytic activity may be related to the low dispersion of the iron sulfide arising from the in situ sulfidation of iron acetylacetonate. It was verified in fact that iron acetylacetonate became sulfided under the above conditions of coal hydroliquefaction. X-ray Diffraction Analysis (XRD) showed Fe as the sulfidation product with a mean crystallite size of O. 95S about 70.110 nm. TABLE 2 Hydroliquefaction of coal in the presence of iron acetylacetonate.

Catalyst wt % H2 % residue of % ~xcess consumed distillation l~quid

% insoluble %C02 +co %C 1-C 4 %H 20

Total yield

No added catalyst

4.15

26.0

46.5

16.1

1. 35

8.5

5.7

104.15

Fe (acac)3

4.5

24.5

44.0

17.3

1.8

9.1

7.8

104.5

Impregnation of coal by an aqueous solution of

Fes04'~2~(10

wt % of sulfate

Icoal). The hydroliquefaction results showed a poor reproducibility of the impregnation technique and a relatively low catalytic effect. Iron sulfide (Fe

was identified by XRD in the solid residue of the coal O. 9 5S) hydroliquefaction. Its crystallite size was too large to be evaluated from the XRD lines broadening. Here again the poor dispersion of the iron sulfide accounts for its low catalytic' activity. Natural iron containing solid precursors (i.e. red mud, possibly modified) 5 grams of red mud 4.0 % Ti0

(Composition: 42.1 % Fe203 ; 16.7 % A1

11.7 % Si0 2 20 3 ; 5.9 % CaO ; weight loss under heating up to 1000 0C :

7.5 % Na 2 20 10.3 %) were added to the charge, all other conditions being as stated above.

The results (Table 3)

showed no evidence of a significant catalytic activity (%

residue of distillation: 26.6).

678 TABLE 3 Coal hydroliquefaction in the presence of red mud, or of supported iron catalysts

Catalyst

%H %C %H % Residue of % Excess % insolu- %c0 2 2+co 1-C4 2O consumed distillation liquid ble

Total yield

No added catalyst K"d mud

4.15 4.40

26.0 26.6

46.5 46.3

16.1 17.8

1.35 1. 45

8.5 5.7 6.15 6 .10

104.15 104.4

Fe

4.1

26.6

44.8

17.9

1.4

7.1

6.3

104.1

4.1

29.0

42.2

17.6

1.5

7.2

6.6

104.1

s i o:r 4.5

26.5

43.4

18.2

1.5

7.4

7.5

104.5

4.1

29.4

42.1

17.9

1.4

7.1

6.2

104.1

= 6.5

2/g). m

Al Fe

20/ 20 3 203

Si0

1

2

Fe 20 3 /

Al Fe

20 3 20/c

This likely arises from the low dispersion of the red mud (S

BET

It was further observed that i) The catalytic actiVity of red mud or other disposable solids like fly ash is highly variable from one sample to another one. ii) We tried to increase the dispersion of iron in the red mud by acid leaching (HN0 10 %, at 60°C) followed by ammonia reprecipitation. This was unsuccesful 3 since only the alumina and not the iron oxide in the red mud was dissolved and reprecipitated under a much higher surface specific area form of A1

20 3.

Iron supported catalysts The following supports were used : 2/g SCS 59 (granulates S 100 m ; Vp = 610 mm 3/g ; ¢p 1\120 nm) 203 2/g - Si0 MAS 100 (powder; S 112 m ; Vp = 550 mm 3/g ; ¢p ~ 15 nm) 2 (75 %) HTH (powder; S 680 m2/g) - Si0 2-A1 203 - carbon black VULCAN 6 (granulates S = 113 m2/g) - Al

6 wt % Fe were deposited on each of these supports by impregnation with an aqueous solution of Fe(N0

9H followed by 1N ammonia precipitation. Water 3)3' 20, washing and air drying at 110°C were finally performed. These catalysts were tested as mentioned above (5 g of catalyst and 19 of CS

2 in the charge). The results, reported in Table 3 show no example of any decrease of the percent residue of distillation, which is on the other hand significantly increased in the presence of Fe

or of Fe 20 3/Si02 203/c. The inactiVity of those iron supported catalysts may be at least partly rela-

ted to the low % of iron versus coal (0.6 %). Low accessibility of the sulfided iron to the tetralin, and to its dehydrogenation product naphtalene, may further

679 be involved. Precipitated iron oxide The starting iron salt was Fe(N0

9H

3)3'

20

diluted in water. Two precipitating

agents were employed i) the buffer NH (pH 9.2) solution 40H-NH4CI ii) diluted NH alone (5 %) 40H and also two modes of drying of the precipitates subsequent to their water washing i) air oven drying at 110°C ii) spray-drying (Minispray dryer Buchi) Typical results are reported in Table 4. TABLE 4 Coal hydroliquefaction in the presence of precipitated iron oxide

Precipitated by

No added catalyst

Drying

NH 4OH-NH4CI Air-oven

%H consumed 2 %residue of distillation

4.15

NH 40H Air-oven 4.55

4.55

NH 40H Spray-dried 4.8

26.0

38.1

26.2

20.9

% excess liquid

46.5

34.8

46.8

52.0

% insoluble

16.1

17.45

% CO + CO 2 %C - C 4 1 %H 2O Total yield

15.3

16.75

1. 35

1.4

1.1

1. 25

8.5

6.0

6.85

7.0

5.7

7.2

8.3

6.9

104.15

104.55

104.55

104.8

The air-oven dried Fe precipitated by NH + NH gave rise to a percent 40H 20 3, 4CI, residue of distillation value (38.1 %) which is much more than the blank experiment value (26.0 %). This was proved to be related to the retention of CI

ions

by the precipitated iron oxide prepared inside a single precipitation operation, and the percent residue of distillation of the coal hydroliquefaction product also increased accordingly to (ref. 5). Iron oxide precipitated by water diluted ammonia differed very much in catalytic activity according to its mode of drying (Table 4). In fact the oven-dried solid gave results highly comparable to those of the blank experiment. On the other hand the spray-dried solid showed an important decrease in the percent residue of distillation (20.9 instead of 26.0 %). This is due to the smaller particle size of the spray-dried iron oxide ( ~ 5

rm)

(1-3~m)

compared to the oven dried one

as shown in more detail elsewhere (ref. 6).

680 ]ron oxide prepared by a flame method As a small particle size of the oxidized precursor of the iron sulfide was found to be important for a good catalytic activity (comparison of spray-dried and of oven-dried iron oxide precipitates) it was thought of interest to test a very finely dispersed (particle size

~

0.05~m)

iron oxide. Such a sample was

prepared by combustion of FeCl

vapor in a H flame (ref. 7). The results 2-02 3 obtained with only 0.5 wt % Fe in the charge (the quantity of CS was 2 203/coal maintained equal to 19) are reported in table 5. For comparison, the results obtained with V 0 and M00 also prepared by the flame method, granulometry and 2 5 3, wt %/coal being nearly the same as for Fe203, are also mentioned. TABLE 5 Coal hydroliquefaction in the presence of added (0.5 wt%/coal) Fe

Catalyst

203,

V Mo0 3 205, Total yield

%H con- %residue of %excess % inso- %C0 2 2-CO sumed distillation liquid luble

NO

4.15

Fe

20 3 V 20 3 M00 3

26.0

46.5

16.1

1. 35

8.5

5.7

104.15

4.95

20.75

54.1

16.4

0.7

6.4

6.6

104.95

4.90

28.4

45.0

17.7

0.8

6.65

6.35

104.9

4.75

24.4

50.2

16.4

0.8

6.6

6.35

104.75

The two main points relevant to this Table are i) the relatively high efficiency of that iron oxide preparation, even if used at a low wt %/coal ii) the higher activity of the Fe203 precursor, compared to the V and M00 3 20 5 ones.

DISCUSSION Certainly no definite conclusion may be drawn from the present work since the efficiency of any catalyst in the coal hydro liquefaction may be highly dependent on the coal investigated. All of our experiments were performed over a single coal and obviously,it is not

possible to extrapolate to other coals. Our pur-

pose is to find some improved disposable iron catalysts, by looking at the correlations between the (in) activity of these catalysts and their physicochemical properties. 1. Oil soluble iron organometallics are attractive catalyst precursors. They should be able to penetrate the internal texture of the coal and to give rise to a highly dispersed sulfide, located in the vicinity of the free radicals arising from the thermolysis of coal. The most probable mechanism of the hydroliquefaction is actually (ref. 8)

considered to be :

681 thermolysis ______________________

Coal

~.

Free radicals (RO)

hydroaromatics in

RO

-I- - - - - - - - - . ,~.

~

RH + Aromatics

the solvent RR (repolymerisation, to be prevented)

The role of the sulfided iron catalyst should be to improve the rehydrogenation of the aromatics (model compound : naphtalene) to H donor hydroaromatics (model compound : tetralinel. Nevertheless, at least the present Fe organometallic

(acetylacetonate) in pre-

sence of the actual coal, gives rise to coarse instead of highly dispersed iron sulfide. 2. Iron sulfate is a most interesting precursor since it is available very cheaply in very large amounts. But preparing highly dispersed iron sulfide "supported" by the coal, was not achieved via the very simple present procedure (simple impregnation of the coal by an aqueous solution of iron sulfate) . 3. Red muds and other disposable iron containing solids have the main drawback of being highly variable in catalytic activity, according to their actual compositions and physicochemical properties in general. 4. Unsuccessful results were presently obtained with various iron supported catalysts. The support has a double role : i) presumably to increase the dispersion of iron ii) to decrease the percentage of iron,

con~ared

to unsupported iron catalysts.

This second negative effect appears as dominating, in the present study. S. Active synthetic iron catalysts precursors may be prepared by

precipitatio~

provided i) the presence of chloride ions should be avoided in the precipitating medium ii) the drying of these precipitates should prevent agglomeration processes, hence the sUitability of spray-drying. 6. A still more appropriate iron oxide, as a precursor for the iron sulfide catalytic active phase, arises from the combustion of FeC1 flame. Xhis method results in small (particle size

~

vapours in a H 2-02 3 O.OSJ'ml non porous iron

oxide particles. It should be outlined that similarly prepared V 0 M00 do not 3 2 S' appear as good hydroliquefaction catalyst precursors, as opposed to Fe 3• 20 ACKNOWLEDGEMENTS The GECH (Groupe d'Etude de la Conversion du Charbon par Hydrogenation) is acknowledged for helpful discussions and financial support'.

682 REFERENCES

2

3

4

5 6 7 8

For review articles see a) W. KAWA, R.W. HITESHUE, R.B. ANDERSON and H. GREENFIELD, U.S. Bur. of Mines, Rept. Invest n° 5690 (1960), 16 pp. b) P.A. MONTANO and B. GRANOFF Fuel 59 (1980) 214-216. c) M. ANDRES and H. CHARCOSSET J. Chim. Phys. 76 (10) (1979) 887-901. a) D.K. MUKHERJEE, J.K. SAMA, P.B. CHOUDHURY and A. LAHIRI Proceeding~ of the Symposium on Chemicals and Oil from Coal (1969, (Pub. 1972», p. 116. b) R.P. ANDERSON 15th Intersociety Energy Conversion Engineering Conference, August 18-22, Seattle, Washington, 1980, p. 1557. c) J.A. GUIN, A.T. TARRER, J.W. PRATHER, D.R. JOHNSON and J.M. LEE Ind. Eng. Chern. Process Des. Dev. 17 (1978) 118-126. C.H. WRIGHT, R.E. PERRUSSELL and G.R. PASTOR (PAMCO), Development of a process for producing an ashless, low-sulfur fuel from coal. Autoclave experiments. R. and D. Report nO 53 Interim Report n° 6 prepared for Office of Coal Research (february 1975). C.H. WRIGHT, G.R. PASTOR and R.E. PERRUSSEL (PAMCO), Development of a process for producing an ashless, low-sulfur fuel from coal. Continuous reactor experiments using anthracene oil solvent. Rand D Report nO 53. Interim Report nO 7 prepared for Energy Research and Development Administration (september 1975). FE-496-T4. M. ANDRES, Thesis to be published M. ANDRES, H. CHARCOSSET, P. CHICHE, L. DAVIGNON, G. DJEGA-MARIADASSOU, J.P. JOLY and S. PREGERMAIN, Submitted to publication, Fuel. P.G. VERGNON and H. BATIS LANDOULSI, Ind. & Eng. Chern., Prod. Res. Dev., 19 (1980) 147-151. R.C. NEAVEL Fuel 55 (1976) 237-242.

683 DISCUSSION L. GUCZI: You gave many characteristics about goof iron catalysts prepared for coal liquefaction. I wonder if one could hear anything about the mechanism, in particular, about the hydrogen ~ctivation on FeS ? H. CHARCOSSET: In order to simulate the rehydrogenation of the recycle oil during the liquefaction of coal, we used the model reaction of the inverse reaction of the naphtalene hydrogenation, that is the tetra I in dehydrogenation. We have found (H. Zimmer, M. Andres, H. Charcosset, G. Djega-Mariadassou, to be published) that the specific catalytic activity is highly dependent on the S/Fe ratio. The activity of Feo is suppressed by a very small degree of sulphidation and is restored when entering into the pyrrhotite Fel-x compositions range. K.S.W. SING: It is interesting to see that the flame hydrolysed iron oxide showed the best catalytic activity of all the samples so far studied. It is possible that the high activity of this material is due in part to its surface structure, i.e. not only to its particle size distribution. H. CHARCOSSET : This hypothesis is not unlikely, provided one considers that the surface structure of the oxide influences the surface properties of the resulting sulphide. The active phase should be in fact Fel-xS.


685

G. Poncelet, P. Grange and P .A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands

IMPREGNATION OF Y-ALUMINA WITH COPPER CHLORIDE. EQUILIBRIUM BEHAVIOUR, IMPREGNATION PROFILES AND IMMOBILIZATION KINETICS R.J. OTT + and A. BAlKER Swiss Federal Institute of Technology (ETH), Department of Industrial and Engineering

Ch~mistry,

CH-8092 Zurich, Switzerland

ABSTRACT Different aspects of the impregnation of y-alumina pellets with aqueous solution of copper (II) chloride are considered. These are the equilibrium behaviour and the effect of time of impregnation, previous state of the alumina and concentration of the impregnation solution on the resulting internal distribution of copper. The measured copper profiles indicate a shell progressive immobilization. Quantitative studies of the ion concentration changes in the impregnation solution show that the main immobilization process is equivalent adsorption of CuC1 2, whereas Cu 2+ immobilization due to ion exchange becomes only important with larger impregnation times.

INTRODUCTION Supported catalysts are frequently prepared by impregnation of porous supports with a solution containing the active component. In its simplest form this method of catalyst preparation involves three steps: the contacting of the porous support with the impregnation solution, drying and calcination. Several aspects of the processes taking place during these steps of preparation have been studied in the past and were recently reviewed (ref. 1). The present work is centered on the preparation of alumina-supported copper processes (ref. 2). Liquid-phase chloride as used for gas phase oxychlorin~tion impregnation is a widely used technique for preparing such catalysts. The following aspects of the impregnation of the alumina support with an aqueous solution of copper (II) choride are considered: the equilibrium behaviour and the effect of time of impregnation, previous state of the alumina and concentration of the t Present address:Schweiz. Unfallversicherungsanstalt, CH-6002

Luzern.

8M

impregnation solution on the internal distribution of the active component within the porous support. In addition, some characteristic features of the adsorption kinetics are presented which should help to define impregnation conditions for tailoring internal profiles. EXPERIMENTAL Support Commercial y-alumina pellets (Girdler-SUdchemie,T-126) were employed as support material. The physical properties of the cylindrical alumina pellets of 4.3 x4.3 mm size were: surface area (BET) 197 mZ/g, solid density 3.43 g/cm3, apparent density 1.37 g/cm3, specific pore volume measured by mercury intrusion 0.44 cm 3/g. Impregnating compound Copper chloride (CuCli ZHZO) from Fluka AG., A.R. Grade was used in an aqueous solution. Analysis Solution concentrations of CUC1 Z were measured by visible absorption spectrophotometry, using an SP 700 A UNICAM recording spectrophotometer. The CUC1 Z immobilized on the support was extracted with 60% nitric acid at 600C and then measured spectrophotometrically. For the spectrophotometric measurements the total extract was diluted with water to give a nitric acid content of 8%. The amount of CUC1 Z measured in the extract was checked by the CUC1 uptake calculated from Z spectrophotometric measurements of the CUC1Z-solution concentration before and after impregnation. Z The internal Cu + distribution along the catalyst pellets was measured by means of an X-ray fluorescence microprobe analyzer (Novelco AMR-3) using the Cu Ka line. The samples were imbedded in plastic and cut to reveal their cross section. The analysis was carried out along the diameter of the polished cross section, in points approximately 0.05 mm apart. The copper concentration of each point was calculated relating the number of impulses measured with a calibration line obtained from uniform samples of known copper content. The chloride ion concentration in the impregnation solution was determined employing the titration method described by Volhard (ref. 3). 3+ concentration was determined by complexometric zinc back titration using The A1 ethylene-diaminetetraacetic acid (EDTA) as complexing agent and dithizon as indicater (ref. 4) after removing the Cu Z+ ions by precipitation with NaZS. o

The determination of the H+/OH- concentrations were performed with a digital pH-meter using a single rod assembly electrode.

687

Impregnation procedure Impregnations were carried out with constant bath concentration as well as with finite bath concentration of solute. For impregnation with constant bath concentration, typically, five grams of the alumina support, either dried or previously wetted, were immersed in 1 liter of aqueous CuC1 2-so1ution of a given concentration. The impregnation bath was thermostated at 250C and vigorously stirred during the sorption experiments. After the desired time has been elapsed the solution was drained off and the pellets quickly washed with distilled water and then dried at 1100C for 24 hours. The alumina support was pretreated in two different ways previous to the impregnation experiments. For the experiments with a dry support, the alumina was dried for 12 hours at 4000C before use, whereas in the case of a wet support, the alumina was preconditioned for 12 hours in distilled water at room temperature. RESULTS AND DISCUSSION Equilibrium behaviour In order to determine the equilibrium behaviour of the impregnation system, the amount of CUC1 2 taken up by the alumina was measured after different equilibration times. The uptake curves plotted in Fig. 1 indicate that the equilibration is very slow and requires more than 17 days. Equilibration times higher than

3.0

1.0

o

0.5

1.0

1.5

c, [Mol/I] Fig. 1

Dependence of CuC1 2-uptake (C a) on bath concentration (C ) for different equilibration times e (wet support, equilibration times: 05, L:.1l, 017, .23 days)

688

23 days did not lead to significantly different uptake values, indicating that the uptake values measured after 23 days are close to equilibrium. The adsorption equilibrium constant K and the total concentration of adsorption sites S on the alumina were calculated applying the linearized form of the Langmuir isotherm (Eq. 1) to the measured data. KC

(1 )

a + SK

The equilibrium parameters found for the 23 days isotherm were: K

3.4 + 0.7 l/Mol,

and S = 5.0 ~ 1.2 Mol/kg. Impregnation profiles Effect of time of impregnation The effect of time of impregnation on the internal concentration profiles of copper is shown in Fig. 2 for the wet (2.a) and dry support (2.b), respectively.

2.0

a)

~=

b)

20 min

t

~=30

1:3= 40

t=40 3 t 4=60

t 4=60

'* .... s

1=15min

t 2= 30

" " " t4

~

U

....:J

U

t4

0

1

0.5

RfR o

t1

o1

t2 t 3

0.5

0

RfR o

2 Fig. 2 Effect of time of impregnation on internal Cu + concentration profiles. a) wet support, b) dry support (constant bath concentration C = 0.5 M) e The concentration profiles determined for the wet and dry support differ, in particular, in the penetration depth. With the dry support the impregnation front moves markedly faster into the center of the pellet and more solute is adsorbed. This behaviour is due to the different way of intake of the impregnation solution into the pores: for a wet support the transport of impregnation solution into the pores takes place by diffusion only, whereas for a dry support penetration (capillary

689

flow) is dominant. In both cases, a progressive shell immobilization (ref. 5) is observed with a relatively small concentration gradient between the exterior of the pellet and the impregnation front. Figure 3 presents a quantitative comparison of the progress of the impregnation front for the wet and dry support, respectively.

1 • WET SUPPORT

o DRY SUPPORT

o ~

0.5

a:

o o

1

2

TIME (hours]

Fig. 3 Progress of the impregnation front. (constant bath concentration Ce = 0.5 M) Effect of solution concentration In order to study the concentration effects, several impregnations with different solution concentrations were performed with a short impregnation time of 15 minutes. Some of the internal profiles obtained with these impregnations are presented in Fig. 4. For both the wet and dry support, the solution concentration mainly influences the penetration depth of the impregnation front, whereas the shape of the profiles does not depend markedly on concentration. With all impregnations a sharp impregnation front is observed ,indicating a shell progressive behaviour. The internal concentration increases as the solution concentration increases; however, no great effect is observed on the internal distribution of the immobilized copper ions. For all impregnation conditions studied,more solute is immobilized using a dry support and the impregnation front penetrates deeper into the pellet.

690

Effect of subsequent precipitation step To study the effect of a subsequent precipitation step on the internal concentration profiles, the impregnated alumina pellets were immersed in a 0.2 MNaOHsolution for 12 hours after the impregnation and then dried at 1100C for 24 hours. Some profiles obtained with this impregnation method are presented in Fig. 5. 2.0~-'--"--'--"--'--"--'--"--'--

4.a)

5.a)

4.b)

5.b)

c

?P: ... ~

1.0

~c

....::I

U

C 1 C2

0

1

C3

C4

0.5

R/R o Fig. 4 Effect of concentration of impregnation solution. a) wet support, b) dry support constant bath concentrations C = 0.1, C = 0.25, C = 0.5 2 l 3 C4= 0.75 and C = 1.0 MCUC1 2 5 impregnation time = 15 minutes

o1

0.5

R/R o Fig. 5 Effect of concentration for impregnation with subsequent precipitation step. a) wet support, b) dry support conditions see Fig. 4 precipitation: 12 hours in 0.2 MNaOH

o

691

Ce - 0.15 [Mol/I)

0

8

0.5

0.25

0.75

a)

~

::l 0

.£ 6

w ~

i= 4 z

0

fi

a: 2 w

I-

Z

W

e,

0 0.9

o

0.25

0.5

0.75

1.0

c, [Mol/I] Fig. 6 Penetration times and shell progressive uptake of CUC1 Z on the alumina support a) Penetration time after which the impregnation front reached the center of the alumina pellet as a function of the impregnation bath concentration. b) Dependence of CuC1Z-uptake on the impregnation bath concentration, measured when penetration front reached the center of the pellet (shell progressive uptake). ------ pore volume contribution (wet support, constant bath concentrations)

692

For both the wet and dry support, the precipitation step leads to considerably steeper profiles. The precipitation step causes a concentration depletion in the liquid filling the pore volume, and this may lead to a reverse concentration gradient which forces the solute to migrate from the inner pore volume to the external shell, where it precipitates. This effect of the precipitation step has also been observed with the impregnation of alumina pellets with Ni(N0

3)2

solution (ref. 6).

Immobilization kinetics The progress of the impregnation front towards the center of the pellet was studied in a series- of experiments with different impregnation times and impregnation solution concentrations. The penetration depth of the impregnation front in the pellets were determined from the internal concentration profiles measured with the microprobe analyzer and from optical micrographs taken from the pellet cross section. The penetration depth obtained with both methods were in good accordance. The penetration time the impregnation front requires to reach the center of the pellet is dependent on the solution concentration as shown in Fig. 6.a) The corresponding total uptake of CUC1 after the impregnation front reached the 2 center of the pellet is plotted in Fig. 6.b). The uptake curve obeys a linear relationship between the impregnation bath concentration,C and the amount of CUC1 2 e, taken up by the support. The dashed line corresponds to the uptake which is due to solute intrapped in the pore volume. From the intercept of the uptake line we estimate a total of 0.32 mol sites per kg support which are occupied during the strong shell progressive adsorption process. Thus, two types of immobilization sites with different affinity for Cu 2+ adsorption may be distinguished based on kinetic grounds. One type is characterized by a fast, the other type by a slow, adsorption. The former leads to a shell progressive adsorption behaviour. More information about the different immobilization processes emerges from a quantitative study of the concentration changes of the ions present in the impregnation solution. The measured decrease of the Cu 2+ and Cl concentrations during impregnation are presented in Fig. 7 along with the change of the pH-value of the impregnation solution. The results plotted in Fig. 7 indicate an increase in nonequivalent adsorption of the Cu 2+ and Cl- ions with time of impregnation. The concentration of the Cu 2+ ions in the bulk solution decreases faster than the equivalent concentration of the Cl ions. After 12 hours an excess of 5.3% Cl ions was measured which increased to 11.3% after 10 days. The nonequivalent adsorption is due to ion exchange between A1 3+ from the support and Cu 2+ from the solution as is substantiated by the results plotted in Fig. 8. The amount of positive charges due to A1 3+ fits very well with the excess negative charge of Cl expected for electroneutrality reasons.

693

TIME [hours!

o

1.0

2

1.0 • Cc/-(t) 4

CCU2+

4.0

(t)

0.9

~~

J:

o

Q.

........ ~

3.75

~

o 0.8 ------L-

o

---!.....J 3.5

..J.-

4

8

12

TI MEl hours] 2+ Change of Cu , Cl and impregnation (wet support, weight of pregnation solution 50 0.25 MCuC1 2)

Fi g. 7

+ H concentrations during alumina 5g, volume of imml , initial concentration

4

";""'

3

:J

tT

QI

N

~

2

.... e

)(

~

~

o

1

• C(t)=IZcr' Ccr(t)-IZCU2+ICcU2+(t) A C(t)=IZA1,+1 CA1,+(t)

0 0

6

12

18

24

TIME [hours! Fig. 8

Kinetics of the apparent excess concentration of Cl ion~ relative to Cu 2+ due to ion exchange between A13+ and Cu : (wet support, weight of alumina 100g, volume of impregnation solution 100 ml, initial concentration 0.25 MCuC1 2)

694

The increase of the pH-value in the initial period of impregnation (Fig. 7) is to be ascribed to the adsorption of H+ on the alumina, whereas the subsequent decrease is caused by the commencing hydrolysis of the A1 3+ ions which form easily stable hydroxocomplexes under the conditions given (ref. 7). For short impregnation times, the ion exchange appears to be of minor importance; however, it becomes very significant when longer impregnation times are used, as indicated by the results shown in Figures 7 and 8, respectively. Thus, an attempt to describe the kinetics of the impregnation process for short impregnation times can proceed from the premise of only one important immobilization process, namely, an equivalent adsorption of the CUC1 2 on the alumina, whereas with longer impregnation times both immobilization processes have to be taken into account. CONCLUSIONS The impregnation of y-alumina with an aqueous solution of CUC1 2 shows a shell progressive immobilization. The previous state of the alumina support (wet or dry) mainly influences the penetration rate of the immobilization front, whereas the shape of the internal Cu 2+ concentration profiles remains about the same. Subsequent precipitation after the impregnation with 0.2 MNaOH-solution leads to a larger amount of immobilized Cu 2+and the internal profiles become steeper. Two different immobilization processes can be distinguished: equivalent adsorption of CUC1 2 and ion exchange between A1 3+ and Cu 2+. The ion exchange is comparatively slow and becomes only significant with larger impregnation times. ACKNOWLEDGEMENTS The experiments presented in this paper were carried out by one of us (R.J.O.) while spending a postdoctoral year at the Consejo Superior de Investigaciones Cientificas, Madrid. Partial financial support of the fellowship by "Stiftung fUr Stipendien auf dem Gebiet der Chemie", Basel, is kindly acknowledged. Thanks are also due to Dr. C. Martinez Perez for valuable help with the microprobe analysis. REFERENCES 1 A.V. Neimark, L.I. Kheifez and V.B. Fenelov, Ind. Eng. Chem. Prod. Res. Dev., 20 (1981) 439. 2 J. Blanco, J. Fayos, J.F. Garcia de la Banda and J. Soria, J. Catal., 31 (1973) 2~, 3 W. Fresenius and G. Jander, Handbuch der analytischen Chemie, Vol .11, Springer, Berlin, 1967, p. 98. 4 E. Wanninen and A. Ringboom, Anal. Chim. Acta, 12 (1955) 308. 5 P.B. Weisz and R.D. Goodwin, J. Catal., 2 (1963) 397. 6 J. Cervello, E. Hermana, J. F. Jimenez and F. Melo, in B. Delmon, P.A. Jacobs and G. Poncelet (Eds.), Preparation of Catalysts, Elsevier, Amsterdam, 1976, p. 251. 7 C.F. Baes and R.E. Mesmer, The Hydrolysis of Cations, Wiley, London, 1976, p. 112.

695 DISCUSSION L. RIEKERT From your observations one has to conclude that in a given preparation of Cu 2 + on y-A1203 obtained by impregnation, there must be at least two species of Cu 2 + which will also differ with respect to radial distribution. After calcination we may then have two different active components. The catalytic properties of such pellets do not follow from the distribution of Cu alone, since the variation of the distribution of copper between different types of solid compounds with radial position may be important. A. BAlKER: I completely agree with your valuable comment. As you pointed out, under certain conditions, we will have at least two different types of immobilized copper species with a different radial distribution. Evidence for this emerges from" TPR-studies we have carried out with alumina samples which have been impregnated employing different impregnation times. For pellets impregnated with short impregnation times (( 15 minutes) we obtained predominantly copper species which are reduced at about 370°C, whereas with larger impregnation times (up to 24 h) copper is preferentially immobilized in a form which is reducible at about 200°C. Hence, it is to be expected that the catalytic properties of pellets which show more than one maximum in the TPR-profiles do not follow from the copper distribution alone. S.P.S. ANDREW: Many times in this symposium during impregnation it has been pointed out that the support, particularly when it is high area and mark of imperfect and therefore reactive solid, reacts and to some extents dissolves in the impregnating solution especially when using aqueous and somewhat acidic solutions. The corrodability of the support material during impregnation is thus an important variable. A. BAlKER: I think the "corrodability" of the support material becomes an important variable in particular when long impregnation times are employed. with short impregnation times the "corrodability" may be neglected due to the fact that the corrosion process is usually considerably slower than the immobilization by adsorption. NG CHING FAI: Did you carry out XRD measurements on your resulting catalysts ? If so, did you detect bulk phase CUCI2? If so, could you suggest how y-A1203 stabilize the otherwise highly hygroscopic CuCl2 ? A. BAlKER : Preliminary XRD measurements carried out on the impregnated alumina gave evidence for the existence of crystalline CuCI2.2H20, however, only for samples with a higher copper concentration (> 2 wt %). This result seems to be in agreement with earlier findings by J. Blanco et al., J. Catal. 1l,257 (1973). In the present state of the investigation we are not able to make a suggestion in which way the y-alumina stabilizes thehighly hygroscopic CuCI2.2H20. M.V. TWIGG: Over what period is it necessary to treat the alumina support with water to observe the effects you have described ? Are the effects due to more than just surface hydration ? A. BAlKER The duration of the pretreatment of the alumina support with water is not critical, because the effects on the copper distribution observed are to be ascribed to the different way of intake of the impregnation solution. with the wet support (immersed to water) the pores are filled with water and the transport of the impregnation solution into the pores takes place by diffusion only, whereas with the dry support the pores are empty and the diffusion flux is superimposed with a capillary flow. The degree of surface hydration has, however, an effect on the type of copper immobilization as some preliminary TPR studies carried out in our laboratory indicate.

696 S. KALIAGUINE: I am interested by the technique of X-ray microprobe. Would you care for example to tell us about spatial resolution, possibility of measuring the chloride concentration,etc ... ? A. BAlKER: The X-ray microprobe or better termed electron probe microanalyzer (EPMA) has a very limited spatial resolution and is therefore only useful for purposes where examination on the micron scale is pertinent, as e.g. is mostly the case with the determination of concentration profiles in a catalyst precursor. Quantitative microanalysis of volumes 1-10 ~m3 is regarded as routine, provided that the specimen is homogeneous over the emitting volume. The spatial resolution is depending on different factors, such as chemical composition of the specimen, acceleratiQn voltage and density of the electron beam, and characteristic X-ray line. Depending on the type of detector employed (wave length-dispersive or energydispersive) the concentration of elements with atomic numbers greater than five can be determined. A review which covers your questions has been written by G. R. Purdy, R.B. Anderson in R.B. Anderson and P.T. Dawson (Editors), Experimental Methods in Catalytic Research, Vol. II, Academic Press; New York, 1976, p. 95137.

G. Poneelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

697

PREPARATION OF COPPER SUPPORTED ON METAL OXIDES AND METHANOL STEAM REFORMING REACTION H.KOBAYASHI, N.TAKEZAWA, M.SHIMOKAWABE and K.TAKAHASHI Department of Chemical Process Engineering, Hokkaido University, Sapporo 060 Japan

ABSTRACT The effects of the catalyst preparation on the title reaction were studied over various copper-containing catalysts. It was concluded that highly dispersed CuO clusters formed preferentially on metal oxides at high copper loading or high calcination temperature. These precursors were readily reduced to metallic copper under reaction conditions and provided highly active and selective catalysts. The support effect on the reaction emerged when the metallic copper surface area exceeded 100 mZjg Cu. The precursors on A1 Z0 3, ZrO Z and MnO Z were highly susceptible to the reduction and fine particles of metallic copper were formed on these oxides.

INTRODUCTION Steam reforming of methanol CH 30H + HZO = 3H Z + CO Z is thermodynamically favorable and proceeds with high selectivity and high activity over copper-containing catalysts (refs. 1-4). Hydrogen atoms in methanol as well as in water are, therefore, effectively converted into gaseous hydrogen. In the present work, the title reaction is carried out over various copper-containing catalysts which are supported on a variety of metal oxides and the effects of the preparation of the catalysts upon the reaction are studied.

EXPERIMENTAL The catalysts were prepared by the ion exchange between hydroxyl protons on metal oxide surfaces (SiO Z' A1 Z0 3, TiO Z' CeO Z' MnO Z and ZrO Z) and tetrammine copper (II) cations at pH = ll-1Z. The ion exchanged metal oxides were thoroughly washed with distilled water, dried at 110°C overnight and calcined in air at a given temperature in a range from 400° to 900°C for 3 hrs. (A10-4, ZOO mZjg), TiO Z (7.8 mZjg) and MnO Z (31 mZjg) were SiO (413 mZjg), A1 Z Z03 available from Japan Chromato Co., Catalysis Society of Japan, Wako Pure Chemicals, and Diichi Carbon Co., respectively. ZrO Z was prepared by decomposition of zirconium nitrate (Wako Pure Chemicals) at 500°C in air for 3 hrs. CeO Z was formed

698

by calcination at 500°C in air for 3 hrs after the ion exchange had been carried out over cerium hydroxide (Wako Pure Chemicals). Steam reforming of methanol was carried out in a flow system at atmospheric pressure. An equimolar mixture of methanol and water was fed with a micropump and rapidly vaporized in a nitrogen stream before entering the catalyst bed. The total inflow was always kept at 96 cc STP/min and the partial pressures of methanol and water were both kept at 0.Z4 atm. The reactants and the products were determined by gas chromatography. The UV diffuse reflectance spectra were obtained by means of a Hitachi Model 330 spectrophotometer to which an integrating sphere was attached. The presence of bulk CuO'was confirmed by XRD (X-ray diffraction method, Rigaku Denki Zl14). TPR (temperature programmed reduction) experiments were carried out in a hydrogen stream (0.04 atm.) which was diluted with nitrogen at a total flow of 50 cc STP/min. The temperature was raised at a programmed rate of 10°C/min and the hydrogen consumption was determined by gas chromatography (Ohkura Model 701). The surface area of metallic copper was determined by titration with nitrous oxide according to the method proposed by Scholten and Konvalinka (ref. 6). Correction was made for the reaction with mangania when the surface area of metallic copper was determined on copper/mangania catalysts. The turnover frequency of the reaction was estimated from the rate of hydrogen production (HZ molecules/sec) and the number of metallic copper exposed on the surface.

RESULTS AND DISCUSSION Steam reforming reaction Effect of copper loading upon the reaction Table 1 lists the kinetic parameters obtained at steady states of the reaction over a variety of copper/silica catalysts. The results previously obtained over a support-free Cu or the catalysts prepared by kneading method (ref.4) are also listed for comparison. The parameter HZ/CO Z which is related to the selectivity of the reaction is estimated from the outlet partial pressures of hydrogen and carbon dioxide. It is seen that at higher copper loadings the parameter Hz/CO z as well as the activation energy is practically kept constant irrespective of the loading. However, at lower loading these kinetic parameters vary sensitively with the loading. Methyl formate was formed to a considerable extent together with hydrogen and carbon dioxide at lower loadings while at higher loadings the latter two products predominantly formed. The reaction proceeded selectively over the latter catalyst. The rate of hydrogen production (alloted for the weight of copper used) increased with the decrease in copper loading. However, when the loading was decreased to 0.5 wt.% copper, the rate decreased markedly. Table Z shows the results obtained over the catalysts which were supported on

699

I Kinetic parameters obtained over Cu/Si0 2 catalysts with various copper loadings a) Cu loading Rate of H b) H /CO b) activation energyC) Catalyst . 2 2 2 pro ductlon (-) (kcal/mole) (wt.%) (cc STP/min.~.Cu)

TABL~

3e) Cu Cu/Si0 2 l3 d) Cu/Si02 14d) Cu/Si0 21-l Cu/Si0 2 1-2 Cu/Si0 2 1-3 Cu/Si02 1-4 Cu/Si02 1-5

100 34.6 17.4 11. 67 9.97 1.89 0.94 0.52

5.3 48.3 74.7 101 85.6 279 89.9 8.7

20.3

3.3 3.7 3.7 3.7 4.2 4.8 f)

7.7

21.8 21. 7 18.7 20.5 24.1 27.1 31. 7

a) calcined at 500aC b) obtained at 2Z0 aC c) determined from H2 formation d) support-free e) kneaded catalyst f) not determined. TABLE 2 Kinetic parameters obtained over copper supported on various metal oxides Catalys t a)

Cu/A1 203 5d) 6 Cu/Mn0 2 4d) H-l Cu/Zr02 H- 3 H-l Cu/Ti02 ld) H-l Cu/Ce0 2 H-l a) calcined at

Cu 1oadi ng (wt.%) 13.4 0.5 26.7 0.61 2.99 0.52 28.4 2.4 0.52

sooac

Rate of H2 production b) H2/C020) activation energyc) (cc STP/min.g.Cu) (--) (kcal/mole) 57.4 564 21.8 162 331 545 1.9 0.78 144

b) obtained at 220°C

4.4 4.8 4.3 4.Z 3.1 5.0 3.4 6.0 4.2 c) HZ production

16.8

°

21. 16.1 24.1 22.8 30.8 18.5 41.6 27.0

d) kneaded catalyst

various metal oxides. As obtained over copper/silica catalysts, the selectivity as well as the activation energy of the reaction was strongly affected by the copper loadings. Theactivities obtained over copper/zirconia and copper/alumina with 0.5 wt.% copper were found to be about 60 times as high as those obtained on the corresponding copper/silica catalysts.

700

N

0

150

c 0

....,

'"'-

E

0 4-

N

:r: 4-

o

....,

(1J

0.5 wt.% Cu/Si0 2

0.8

+ CV)

:r:

~

0.6

~

""c

u 0 0

u

:r:

100

E

0.4

---t0..

N l

this phase is accompanied with Cu-Zn hydroxycarbonates, which are generally of the malachite kind. The X-ray pattern of the sample Cat 7, in which both the described phases can

725

be seen, is reported in Figure 1. TABLE 1

Atomic ratios of the elements and compounds identified by X-ray diffraction in the different

precursors

prep~red

Samples

Cu:Zn:Al

Identified compounds

(as atomic ratio %) Cat 1 " 2 " 3 " 4 " 5

60.0:30.0:10.0 45.0:45.0:10.0 30.0:60.0:10.0 55.3:27.7: 17.0 41.5:41.5: 17.0 27.7:55.3:17.0 60.8:15.2:24.0 50.7:25.3:24.0 38.0:38.0:24.0 25.3:50.7:24.0 34.5: 34. 5: 31 .0

" 6 7 u 8 " 9 10

n

11

(after drying at 90°C) HY; M M; HY HY; quasi-amorphous phases HY; M HY; i'1 HY; quasi-amorphous phases HY; M HY; jVj HY HY HY; Al (OH) 3

HY

hydrotalcite-like phase (Cu,Zn)sA12C03(OH)16'4H20

M

malachite-like

(Cu,Zn)2C03(OH)2

*

*

(Cu,Zn)6AI2C03(OHl1604H20

• (Cu,Zn)2 C03 (OHh

t

>-

en

e

•

GI

•

*

*

c

10

30

70

50 2e~

Fig. 1. X-ray pattern of the sample Cat 7 dried at 90°C.

726

The samples Cat 3 and S, in which the hydroxycarbonates are quasi-amorphous and may be interpreted as auricalcite- and/or hydrozincite-like phases, are exceptions. In the case ofa(3, the aluminum in excess of the ternary phase stoichiometry is precipitated as hydroxide. The X-ray pattern and crystallographic parameters of the ternary phase are similar to those reported for the mineral hydrotalcite M96A12(OH)16C03'4H20[11]. Rhombohedral hydrotalcite cgnsists of positively charged brucite-like layers [M9

alternating with disordered, negatively charged interlayers 2+ In our compound Cu 2+ and Zn 2+ ions substitute the Mg ions. More

sA1 2(OH)lS]2+

[C0 3·4H20]2-. detailed information concerning the structure and properties of this phase will be published in a later paper[12]. Study of the calcination process

The calcination process of the precursors has been investigated by differential scanning calorimetry, thermogravimetry, X-ray diffraction and E.S.R. spectroscopy. The DSC-curves of several precursors are shown in Figure 2. The various endothermic transitions, all of which were accompained by weight loss, were interpreted on the basis of X-ray and thermogravimetric analyses. The transition in the temperature range 170-190°C may be attributed to the crystallization water-loss of the hydrotalcite-like phase, with the formation of quasi-amorphous phases (hydroxycarbonates in all probability), while the peaks at 230-240°C and at 280-290 °C may be attributed to the decomposition to oxides of those phases. In the samples in which the hydrotalcite-like phase is accompanied by Cu-Zn hydroxycarbonates, there are also peaks at 340-360°C and at 380°C, attributable respectively to hydrozincite- and malachite-like compounds[13]. The calcination at temperatures increasing up to 3S0 °C showed a gradual de2 crease in E.S.R. total intensity, suggesting that the Cu + ions are progressively grouped into clusters on the supports. This effect was less marked in the case of samples containing only the ternary phase (for example Cat 9 of Fig. 3). It is therefore possible, given the hydrotalcite-like phase as a starting point, to obtain the related oxides at temperatures lower than in the case of 2+ binary hydroxycarbonates and with a greater dispersion of the Cu ions inside the diamagnetic matrix.

727

100

500

300 Temperature (OCI

Fig. 2. Differential scanning calorimetric curves of some precursorSl a) catalyst 2; b) catalyst 5; c) catalyst 7; d) catalyst 9. I

I

10

\

• Catalyst 9

l-

."

"

7

~

~

·~5

I-

....c: Q)

o

o

-

\ ~~

-

\....

'.-J

I

200

-

I

-

:A"

I

400

Temperature (OCI

Fig. 3. E.S.R. total intensity as a function of the calcination temperature.

728

The X-ray patterns of the sample Cat 9 are shown in Figure 4: the precursor dried at 90°C is a well crystallized hydrotalcite-like phase (Fig. 4-A), whereas the compound calcined at 360°C shows a quasi-amorphous pattern (Fig. 4-B), from which the extremely low dimensions of the crystallites of the obtained oxides are clearly apparent.

A

-

I

I

I

10

30

50

I

70

2tJo -

> "00

B

c

Q)

I

of: 15

I

I

25

35

I

55

45

2tJo Cu Cu

~ I

I

15

25

~

C

I

35

45

2tJo -

I 55

Fig. 4. X-ray patterns of Cat 9: A) after drying at 90°C; B) after calcination for 24 hr at 360°C; C) after reduction in H flow (Cu-Karadiation; 2-N2 ). = 0.15418 nm) . In the case of some calcined samples showing a relatively greater sharpness of the peaks, X-ray identification of the involved phases was possible, namely CuO, ZnO and ZnA1

204. ternary precursors.

The smallest oxide crystal sizes were obtained from pure

The reduction of the oxides in HZ-N

showed that binary phases are more easiZ ly reducible than the pure ternary phase. This difficulty in reduction might be ascri bed to greater Cu-Zn i nteracti on[4]. However, the dimensions of the copper crystallites obtained from the pure hydrotalcite-like phase (Fig. 4-C) were lower (nearly 3.0 nm) than those obtained by binary precursors (5.0 - 7.0 nm)[9].

729 I

100

~

I

•

Cu -1 Zn -

•

l-

~

~

... -c

50

-

•

Q)

0

I-

...

~

•

I

0 0

10

E

-

c::

t

-

5

Q)

N

C/)

•

•

•

t

:::3 C/)

-

•

E Q)

I

I

20

-

~ .....

en

...::>-

o

I

0

•

40

AI Content (atomic%) Fig. 5. Surface area and CuD crystal size of the calcined precursors as a function of the aluminum content (the arrow shows the samples obtained from precursors in which only the hydrotalcite-like phase was identified). Figure 5 shows the surface area and CuD crystal size values, determined by profile-fitting methods, in the calcined samples in relation to the aluminum content (for the ratio Cu/Zn = 1). It may be noted that the CuD crystal sizes decrease with the increase in the aluminum content, together with the passage from samples obtained from biphase precursors and others obtained from precursors in which the ternary phase was pure or accompanied by Al(DH)3. The net increase in the surface area of the Cat 11 sample may be ascribed to the microporosity of the alumina obtained by calcination of Al(DH)3 (Fig. 6). The Cat 9 sample obtained by calcination of the pure hydrotalcite-like phase presents CuO crystallite dimension, surface area and cumulative pore volume values lower than those of the samples obtained from biphase precursors. Likewise to Figure 5, Figure 7 shows the surface area and CuO crystal size values in the case of calcined samples in relation to the ratio Cu/Zn (for an aluminum content of 24%). It may be noted that the CuO crystal sizes decrease with the increase in the zinc content. The high surface area of sample Cat 10 may also be interpreted in terms of different porosity: this sample in fact presents a pore radius distribution characterized by a large number of small

730

and large pores, therefore with a cumulative pore volume value similar to that 3

of other samples (for example Cat 10 = 0.536 and Cat 7 2/g respectively). a greater surface area (113 and 57 m

0.528 cm /g), but with

,Ol

ME

Cu/Z n= 1:1

c Q;"OA

&

E ::;,

0

"0

..

...

•

>

Q)

Catalyst 2 5 " 9 "

11

&0.2

...

.~

ell

"S

E

Bo 1.0

10 average pore radius (nm)

2

10

Fig. 6. Cumulative pore volume distribution of some calcined precursors with different aluminum content. I

• 100 _

t

I

I

I

-

AI=24%

10

OJ ~

§.

E

-

. c, perature was correlated with the 0 0.1 0.2 heat of adsorption by microcaloPy(ad) by IR (mmol/g) rimetry, and a linear relation Fig. 4. Comparison of acid strength was obtained between the heat of distribution measured by IR and microcalorimetric methods (ref. 3,9~ adsorption and the reciprocal of The evacuation temperature. amount of adsorbed pyridine with the heat of adsorption larger than various level was calculated from the infrared spectra by the aid of the above-mentioned linear relation, and was compared with those of adsorbed pyridine and ammonia measured by microcalorimetry. As shown in Fig. 4, quantitative agreement was obtained between microcalorimetry and IR method.

/a

s

O}{{

781

3. Metal Surface Area of Reference Supported Metal Catalysts. (H. Matsumoto, ref. 2,4; and N. Nojiri, ref. 11). The project of entitled subject was organized to give a chance for comparing the metal surface area measured by different researchers with different methods. Nine supported precious metal catalysts, shown in Table 2, were provided for the project and two symposia were held. As one of the objectives was the examination of the methods which had been employed, any of the experimental conditions were not fixed. The res~lts were summarized in Table 3. Participants in the project are listed in the appendix. It should be noted that the researchers in industry employed the dynamic (pulse) method of CO chemisorption, although the variety of methods were listed in Table 3. It was interesting that the results were rated high by industry researchers but low by university researchers. Most of the former felt that the results agreed pretty well with each other,except sample No.1, considering the difference in the experimental conditions and method, Table 3 Metal surface area of reference supported metal catalysts a Method

NO.1

Catalyst No.2 No.3 No.4 No.5

XRD N.D. N.D. XRD N.D. N.D. TEM 1.0 1.0 small TEM &5'1,10 7.0 Chemisorption methods CO pulse 49 78 CO pulse CO pulse 17 66 CO pulse 53 92 CO pulse 92 116 CO pulse 25 87 CO pulse 64 108 CO static 53 83 CO static 140 133 02 pulse 58 57 02 pul se 25 36 02 stati c 31 33 Hr0 2 pulse 76 124 46 02- H2 {02 44 titratn H2 130 144 H2 static 72 77 H2 TPR 56 60 CS2 7.6 34 poisoning

No.6

No.7

No.8 No.9

Reduction condition

22 22 N.D. N. D. N.D. 19 19 20 N.D. 2.2 6.8 1.0 0.9 1.8 1.7 1 & 2'1,3 & h6 2.5'1,4.5 - 0.6'1,1.5 6'1,9 1arge 53

26

53 66 78 62 67 56 84 44 28 28 94 36 108 67 45 23

28 29 21 34 22 11 6.3 7.6 25 13 38 16 19 8.9

44 39 45 47 56 55 59 66 95

38 1.7 0.4 31 43 0 8.5 0.7 55 0.6 48 5.0 51 0 58 101 84 33

15 21 47 12 61 44 39 43

16 13 20 4.7 55 1.8 9.3 1.8 52 5.6 7.5 35 2.2 32 27 2.8

60 79 63 73 138 95 110 32 172

198 147 50 158 160 33 228 112 281

42

99 94 271 86 146 90

21 95 96 94 66

-

473'U673K, lh 473'U673K 313K, 10min 453'U623K, 30mi n 723K, 10min 723K, 10min 723K, 10min 673K, 2.5h 573K, 1h 723K, 45min 823K, 30min 573K, 1h 823K, 30min 823K 823K 573K, 1h 773'U823K 423'U573K

aXRD and TEM, average diameter (nm) from XRD patterns and from TEM microphotograph. Chemisorption methods, cm 3-adsorbed gas/g-meta1.

782

and they seemed to gain confidence in their methods. On the other hand, some of the latter held the view that the re-examination under standard conditions is necessary. These led to the project of standardization of rapid measurement of metal surface area by the pulse method of CO chemisorption for industrial purpose and a joint research on the dispersion of these catalysts. In the latter, good agreement was obtained among the chemisorption data by static and TPR methods and TEM data, when reversibly adsorbed species are taken into consideration (ref 9). Further, tbe remarkable change in HZ chemisorption by the treatment with HZ and Oz was observed, and the effect of the supports, especially the effect of sulphur content, on the change was examined by K. Kunimori et al. (Univ. of Tsukuba and Mitsubishi Petrochem., ref. 3, 10). 4. Standardization of Rapid Measurement of Metal Surface Area by Pulse Method of CO Chemisorption. (N. Nojiri, ref. 3, 11). In this project, the standard pretreatment condition was fixed as follows: (1) increasing temperature from room temperature to 673 K in an inert gas flow, (2) increasing temperature from 673 K to 723 K in HZ flow, (3) holding temperature at 7Z3 K for 0.5 h in HZ flow, (4) holding temperature at 7Z3 K for 0.5 h and then cooling to room temperature in an inert gas flow, and (5) measurement of CO chemisorption by the pulse method. But the other experimental conditions were not fixed. The results are shown in Table 4. Participants in the project are listed in It also was the appendix. Again a large scatter was observed on sanp l e No 1. reported that the data on sample No 1 was remarkably affected by the pretreatment condition, and a view that milder pretreatment may be favorable, especially for sample No 1 , was presented. On the other hand, the results on samples No 5 and 6 were reproducible. The results were devided into two groups: the first group is A and B, and the second, C, D, E, F and G. The Table 4 agreement in each group is excellent. In A, the CO Chemisorption a by pulse method after b effluents were collected in a Porapak trap at standard pretreatment dry-ice temperature and then analysed. In B, Catalyst the effluents were analysed with a 70cm column No.5 No.6 No.1 of activated carbon at 330 K. In the others, A 54 40 38 effluents flowed immediately to a thermal con44 38 B 51 60 43 C 38 ductivity detector. This difference would 46 56 D 38 55 44 cause the difference between two groups. The E 43 56 53 F 40 average experimental conditions were as follows. 44 53 G 49 The purification of carrier gas is not necestext. sary. r1easurement was done around room tempera-

783

ture. The ratio of CO pulse size to catalyst weight vias about 1/1 (mm 3/mg). The interval of CO pulse was 2~3 min. The long interval may allow the desorption of reversibly adsorbed CO, and, therefore, it leads to the evaluation of only irreversibly adsorbed CO. The interval of 2~3 min may be sufficient to measure both reversibly and irreversibly adsorbed CO. The above mentioned joint research (ref. 9) concluded that the reversibly adsorbed species also should be taken into consideration to evaluate the degree of dispersion (percentage exposed). This project is stilt in progress and will be a subject of the fourth annual symposium scheduled~in October 1982. 5. Miscellaneous. In addition to the subjects mentioned above, several reports and comments on the reference catalysts have been presented in the symposia. Only the titles are shown below. (1) CH 4-D 2 exchange on reference alumina catalysts (H. Hattori, M. Uchiyama and K. Tanabe, Hokkaido Univ., ref. 1). (2) Surface reaction rate of dehydration of alcohols by pulse surface reaction rate analysis (PSRA) and an emmisionless infrared diffuse reflectance spectrometer (EDR) (T. Hattori, K. Shirai and V. Murakami, Nagoya Univ., ref. 1 and 12). (3) Turnover frequency of dehydration of sec-butylalcohol as a function of strength of Lewis acid sites (K. Nakacho, J. Take and V. Voneda, Univ. of Tokyo, comment in 3rd symposium). (4) Optimum reaction condition for the formation of ethyl ether in the dehydration of ethanol by a full automatic computer-operated reaction system in laboratory (V. Tsuchida and H. Niiyama, Tokyo Inst. Tech., comment in 3rd symposium). (5) Adsorption of Ni, Cu and Pt ions on reference aluminas (H. Niiyama, T. Ogiwara, N. Suyama and E. Echigoya, Tokyo Inst. Tech., ref. 1 and 2nd symposium). (6) IR spectra of dissociatively adsorbed hydrogen on reference supported metal catalysts (V. Soma, Univ. of Tokyo, 2nd symposium). (7) Hydrogenation of 1,3-butadiene on reference supported metal catalysts suspended in acid solution (H. Kita and K. Shimazu, Hokkaido Univ., 2nd symposium and ref. 3). (8) Methanation of CO over Ni-La203 supported on reference aluminas and over reference supported metal catalysts (T. Inui, T. Miyake and Y. Takegami, Kyoto Univ., ref. 1 and 3). (9) Support effect in the hydrogenation of cyclopentadiene on supported nickel catalysts (A. Sannomiya, M. Yano and Y. Harano, Osaka City Univ., ref. 3 and 13).

784

(10) Temperature programmed reduction of nickel catalysts supported on reference a1uminas and thier catalytic behavior in the methanation of CO and CO 2 (Y. Nakagawa and S. Ogasawara, Yokohama Nat1. Univ., comment in 3rd symposi um). REFERENCES 1 Data on Reference Catalysts, Shokubai, 22(1980)115, H. Matsumoto, Shokubai, 22(1980)107. 2 Preprints of Symposium on Metal Surface Area II, Nagoya, June 19, 1981. 3 Preprints of 3rd Imnual Symposium on Reference Catalysts, Kyoto, Oct. 11,1981. 4 H. Matsumoto, Shokubai, 22(1980)410. 5 D.H. Everett, G.D. Parfitt, K.S.W. Sing and R. Wilson, J. Appl. Chem. Biotechno1., 24(1974)199. 6 M. Takahashi, Y. Iwasawa and S. Ogasawara, J. Cata1., 45(1976)15. 7 J. Take, T. Ueda and Y. Yoneda, Bull. Chem. Soc. Japan, 51(1978)1581. 8 J. Take, H. Matsumoto, S. Okada, H. Yamaguchi, K. Tsutsumi, H. Takahashi and Y. Yoneda, Shokubai, 23(1981 )344. Further details will be published. 9 K. Kunimori, T. Uchijima, M. Yamada, H. Matsumoto, T. Hattori and Y. ~1urakami. Appl. Catal., submitted. 10 K. Kunimori, T. Okouchi and T. Uchijima, Chem. Lett., (1980)1513. K. Kunimori, Y. Ikeda, T. Okouchi, N. Nojiri, M. Soma and T. Uchijima, Shokubai, 23(1981) 365. 11 N. Nojiri, Shokubai, 23(1981 )488. 12 T. Hattori, K. Shirai, M. Niwa and Y. Murakami, React. Kinet. Catal. Lett., 15(1980)193. 13 A. Sannomiya, T. Tashiro, M. Yano and Y. Harano, Shokubai, 24(1982)112. APPENDIX Institutions participating in the project of BET surface area of aluminas Tanabe's lab., Hokkaido Univ.; Okazaki's lab., Ibaragi Univ.; Central Res. Lab., Idemitsu Kosan Co.; Mr. Takagi, Tokyo Inst. Tech.; Ogasawara's lab., Yokohama Natl. Univ.; Kinuura lab., JGC Corporation; Murakami's lab., Nagoya Univ.; Inui's l ab. , Kyoto Univ.; Yoshida's lab., Kyoto Univ.; Tsuchiya's lab., Yamaguchi Univ.; and Kagawa's lab., Nagasaki Univ .. Participants in the project of metal surface area S. Yoshida (Kyoto Univ.), T. Murata (JGC Corp.), A. Furuta and M. Yamada (JGC Corp.), H. Arai and Y. Kibe (Kyushu Univ.), Y. Akai (Idemitsu Kosan Co.), T. Hattori and Y. Murakami (Nagoya Univ.), E. Kikuchi (Waseda Univ.), K. Aika and O. Kato (Tokyo Inst. Tech.), K. Kunimori, S. Matsui, Y. Ikeda, E. Yamaguchi and T. Uchijima (Univ. of Tsukuba), K. Iida and T. Imai U~itsubishi Heavy Ind. Co.), and those participating in the following project. Participants in the project of standardization of rapid measurement of metal surface area by CO-pulse method S. Nishiyama and H. Niiyama (Tokyo Inst. Tech.), T. Mori (Gov. Ind. Res. Inst., Nagoya), E. Yasui and F. Haga (Nippon Oil Co.), T. Nakata and T. Sakurai (Nippon Engelhard), N. Nojiri and K. Kurashige (Mitsubishi Petrochem. Co.), T. Inui, T. Miyake and Y. Takegami (Kyoto Univ.), and T. Suzuki (Asia Oil Co.).

785 DISCUSSION ZHAO JIUSHENG Could you tell something about the measurement of the acid amount of Y-AI203: 1) Did you use the titration method, and if so, why? 2) What carrier gas have you used for the TPD study. Is there any difference when you use another carrier gas? 3) In the infrared measurements, have you distinguished the Lewis acid sites from the Bronsted ones? Y. MURAKAMI: 1) I do not think that the titration method is suitable for measuring the acidity of alumina catalysts, because the indicator turns into acidic color only after the alumina is treated in vacuum at high temperature and the amount of n-butylamine is too sensitive to the pretreatment. Further, the change of color of the indicator can hardly be controlled. 2) In the TPD method, nitrogen was used as a carrier gas with an FID detector. Although no attempt has been made to examine the effect of the carrier gas, I believe that any inert gas will give the same results. 3) Of course,Lewis and Bronsted acid sites were distinguished. On alumina catalysts, only coordinatively adsorbed pyridine on Lewis acid sites was observed on the IR spectra. Therefore, Fig. 4 in the text shows the relationship for the Lewis acid sites. The relationship for the Bronsted acid sites has also been examined by using H-ZSM 5, on which only pyridinium ion (Bronsted acid sites) was observed. A linear relationship has been obtained between the amount of adsorbed ammonia by microcalorimetry and the amount of adsorbed pyridine (pyridinium ion) by IR method. (Ref. 8 in the text) . J.J.F. SCHOLTEN The CO-pulse method may also be used in hydrogen as a carrier gas (McKee's method). The advantage is that: a. strongly chemisorbed hydrogen, still present from the reduction pretreatment, has not to be desorbed at temperatures as high as 500°C. Many metals sinter very strongly at this temperature. b. Pulsing is performed in the reduction hydrogen stream, and for many noble metals,the reduction temperature may be chosen very low (100°C,for instance). Sintering will not disturb the results. The hypothesis on which the method is based is that CO kicks off the hydrogen from the surface. This asks for further fundamental studies for various metals. G.C. BOND I can confirm that the method of pulsing CO in a H2 stream works perfectly well for the routine determination of Pd dispersion. The method was developed by myself and my colleagues in the Johnson Matthey laboratories in the mid-1960's. The problem of using CO in an inert carrier gas is not only that of removing adsorbed H remaining from the reduction, but also that of using an extremely pure (especially, 02-free) gas stream: if this is not done, consistent results cannot be expected. Y. MURAKAMI: (To Prof. Scholten and Prof. Bond). The project of metal surface area possesses two subjects: the research for true metal surface area and the standardization of rapid determination of metal surface area for pratical purpose. One might prefer to start the second subject after the first one, but industrial researchers are not so patient. They must determine the metal surface area of catalysts which are presently used and are going to be used. Actually, they determine it by their own methods. As the initial step of the second subject, the standardization of the most popular method has been undertaken. The next step is the examination of the standardized method for the final goal. For such purpose, it is necessary to solve the problems which have been raised, to compare the method with others, and to examine the applicability of the method to various catalysts. At present, these are in progress, and the first subject (basic research) will be of a great help in this step. The effect of the purity of the carrier gas is one of the ~roblems raised. It has been reported that the purity of the carrier gas has o"ly a little effect, as shown in Table 1, although a few contradictory results have also been privately communicated. The effect seems to depend on the metal and support. This

786 will be a topic of the next meeting of the committee. The CO-pulse method in hydrogen as a carrier gas has also been examined. The results, shown in Table 1, are in good agreement with those in He as carrier gas. Kicked off hydrogen also was measured by using nitrogen as a carrier gas, but the amount did not coincide with that of adsorbed CO, as shown in Table 1. This result may be examined further in relation to the comparison with other methods. It has been pointed out in the committee that the reduction temperature appears too high. Some contradictory results have been reported on the effect of the reduction temperature. This also will be a topic of the next meeting, and lower reduction temperatures will be employed for a new series of catalysts. A new series of catalysts listed in Table 2 has been prepared and distributed for the comparison with other methods and for the examination of the applicability of the method. The committee is ready to supply them to the foreign countries, although the total amount prepared is limited. TABLE 1.

Effect of the carrier gas on CO chemisorption CO chemisorbed and H2 kicked off

Carrier gas

He (purified

Catalyst

a)

He (unpurified) a) H2 (purified b) N2 (purified

CO (ads) CO(ads) CO (ads) H2 (des)

c c c d

No 1

No 5

No 6

48

44

38

52

44

38

48

44

37

35

31

29

a, by silica gel trap at 77K; b, by oxytrap; c, in cm d, kicked off H2 in cm 3 H2/9-P t. TABLE 2. Number 10 11

12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

3

CO/g-Pt;

List of the new series of catalysts Mark JRC-A4-0.5Pt JRC-A4-0.5Pt JRC-A4-0.5Pt JRC-A4-5.0Pt JRC-A4-5.0Pt JRC-A4-0.5Rh JRC-S3-0.5Rh JRC-A4-0.5Ru JRC-S3-0.5Ru JRC-A4-0.5Pd JRC-S3-0.5Pd JRC-A4-30Ni JRC-A4-50Ni JRC-S3-30Ni JRC-S3-50Ni JRC-A4-5.0Ni JRC-S3-5.0Ni

(1.0) a (0.5)a (0.1) a (1.0) a (0.2)a (2.0)b

(2.0)b

Metal

Metal content

Pt Pt Pt Pt Pt Rh Rh Ru Ru Pd Pd Ni Ni Ni Ni Ni Ni

0.5 wt % 0.5 0.5 5.0 5.0 0.5 0.5 0.5 0.5 0.5 0.5 30 50 30 50 5.0 5.0

a, expected dispersion; b, second batch.

Support A1203 A1203 A1203 A1203 A1203 A1203 Si02 A1203 Si02 A1203 Si02 A1203 A1203 Si02 Si02 A1203 Si02

787


THE SCljIUPAC/NPL STANDARD NICKEL-SILICA CATALYST

R. BURCH and A.R. FIAMBARD, the University of Reading, England .M.A. DAY, Imperial Chemical Industries Ltd. R.L. MJSS, Warren Spring Laboratory, Department of Industry N.D. PARKYNS

and

A. WILLIAMS, British Gas Corporation

J .M. WINI'ERBClI'I'a1 and A. IVHITE, The University of Birmingham

ABSTRACT

The intention of this work is to devise clearly defined pretreatment and reduction procedures for =nverting the pre=sor to a standard nickel-silica catalyst which will exhibit a nickel surface area and activity for benzene hydrogenation falling within specific limits when agreed test methods are adopted.

This paper describes the methods which have been used to investigate

this catalyst, and reports sane of the results obtained.

It is hoped that

improvements in procedure can be suggested and then introduced into future experiments.

INTIDDlJCrICN The task of producing a standard catalyst is a formidable one and it is important at the outset to define the objectives.

For example, these might

include the use of the standard catalyst (L) to confirm the validity of catalyst test equiprent (ii) as a reference material for establishing catalyst characterization techniques (iii) for the accumulation of a more =herent lxxiy of catalytic data and (iv) for canparison with new catalysts under investigation. For such purposes where a stable supported metal catalyst is involved, then the standard catalyst might 1I\ell be supplied in its final reduced state, thereby retaining cont.ro.l of more of the experimental procedure within a single organisation.

Often, however, catalysts with a range of properties, e.g., metal

crystallite size, are required and then it seems more convenient to supply the catalyst pre=sor and to carefully specify conditions for generating the required range of reduced catalysts.

Again it needs to be recognised that the

catalyst pre=sor may change during storage. The Working Party on Catalyst Reference materials set up by the Society of Chemical Industry (SCI), following a proposal by the IUPAC carmission on Colloid

788 and Surface ChEmistry, has available a standard nickel-silica catalyst precursor

vkiich is distributed by the National Physical Laboratory (NPL).

The material

was obtained as a single batch (100 kg), prepared by in'pregnating the silica support with a solution of nickel nitrate, follO'l.led by drying at 393 K.

Tests

for hanogeneity on different particle size fractions indicated that the +150 pm material should be ranoved., the re:nainder of the batch was then subdivided into 80 g representative sarrples.

Initially

'We

have concentrated on the changes which occur during the pre-

treabnent and reduction of the catalyst precursor as shosn by the extent of hydrogen adsorption and activity for benzene hydrogenation.

We hope to

establish a very clearly defined procedure vkiich will allow other workers, starting fran the

I

as-received' material to produce a reduced catalyst having a

nickel surface area and (benzene) hydrogenation activity falling within specj.f.ied limits.

Additionally other nickel-silica catalysts with a range of propertaes

may be generated fran the precursor using information accumulated by the participants. EXPERIMENTAL ME:I'HODS

Table 1 surrrnarises the properties of the catalyst precursor. TABLE 1

Properties of the catalyst precursor

toss on ignition (1 h at 1273 K) Ni content 004 content N~P content s~I~ca

21.6%wb 11.1%db 0.2%db 0.07%db balance

apparent bulk density cempacted bulk density surface area

-3

0.55 g em_

0.64 9 em 197 m2 g-l

3

l'iisorption measurenents Three methods 'Were used to detennine the amount of hydrogen adsorbed by the nickel, narrely static (gravimetric or volumetric), pulse, and temperatureprogranmed adsorption or desorption.

The latter twJ methods offer the advantage

of speed and simplicity, but, can only give information on the total amount of hydrogen adsorbed under dynamic conditions.

For pulse experiments, the choice of

temperature and the cemposition of the carrier gas is important, for TPD experiments, the choice of heating rate, initial temperature, final temperature, carrier gas cemposition, are all important pararreters.

Static volumetric

adsorption measurements, although more time consuming, offer greater control over the experimental conditions, and allow an isothenn to be determined.

The shape

789 of the isotherm can be a guide to the cleanliness of the metal surface. the static method it is necessary to chose a prefe=ed procedure.

Even for

The temperature

must be specified, the range of hydrogen pressures to be used must be specified, the time required for equilibrium must be established, and t.be method of determining the rronolayer coverage must be decided upon.

The experimental

conditions used in the recarrnended procedure :liar static hydrogen adsorption measurements are as follows.

The adsorption tffilJJerature, 273 K, was chosen

partly because of its convenience, and partly because at higher temperatures the pressures required to attain rronolayer coverage with supported metals increases beyond the preferred range.

The preferred pressure range chosen (0-50 torr) is

convenient and easily measured with manareters, pressure transducers, etc.

The

attainment of equilibrium was considered camplete after 1 h. Having determined the adsorption isotherm, it is necessary to extract a value for the hydrogen uptake corresponding to rronolayer coverage.

'I'No methods

are canmonly used, namely, extrapolation of the linear portion of the isotherm (at higher pressures) to zero pressure, or fran a Langmuir plot (using the equation for dissociative adsorption).

In the context of a standard catalyst

t.he best; way of presenting the adsorption data is the quantity of hydrogen

adsorbed (as moles or molecules) per unit weight of catalyst or per unit weight of nickel.

However/to allow canparisons with published data it is helpful to

recarmend a factor for calculating the metal surface area.

Hence we have

assumed that each surface Ni atan adsorbs 1 H atan, and that each H atan 2. occupies a constant area of 0.065 nm Activity for the hydrogenation of benzene The choice of benzene as a reactant for the determination of the hydrogenation activity of the reference catalyst (based on a survey of catalyst users and researchers) conveniently allowed a temperature above ambient to be used (323 K). Only a very limited number of experiments have :teen performed by one research group and it is premature to offer a recx:mrended procedure.

However, it is

relevant to indicate the reproducibility which has been obtained for the procedure used.

The conditions chosen on the basis of preliminary experiments

to give an activity (defined as % conversion of :tenzene to cyclohexane) of le, and experience has shown that reproducible results can be obtained.

Similarly,

there is little doubt that t.he other experimental variables could be optimised, and the purity of the benzene should be specified for future work.

790 It is also necessary to decide whether initial activities should be rreasured or, as v.e have done,

I

steady state I activities determined.

The

products were sampled every 10 minutes for 1.5 h , and analysed using a Perkin Elmer F33 gas chromatograph fitted with a FID, a silicone fluid column, and coupled to an Infotronics CRS 308 electronic integrator.

The activity reported

was determined by extrapolating the linear portion of the activity/time curve to zero time. RESULTS AND DISCUSSION Part A. (a)

Evolution of a preferred procedure Storage of 'as received' material.

Changes in the structure of the

material are most probably associated with the water content.

Dry sarrples

should be more stable, so it is recarmended that the 'as received' material be dried at 393 K for 16 h and then stored in a sealed container. (b)

Pretreatment of starting material - re-drying, cooling and weighing.

The outline procedure is given in Table 2. TABLE 2

Proposed 'standard' procedure for pretreatment of the reference catalyst 1.

2. 3. 4.

Dry 0.5 g

I as received I catalyst for 16 h at 393 K. Place catalyst in a desiccator and cool for 20 minutes. Transfer about 0.350 g to v.eighing bottle and v.eigh accurately. Transfer the accurately v.eighed sarrple to a glass reactor.

Sarrples (0.5 g) of the starting material are removed frem the sealed container, placed in an open crucible and redried for 16 h at 393 K. important for t:Y.o reasons.

This redrying step is

First, it gives a reproducible starting material,

the v.eight of which can be related accurately to the nickel content.

Second,

it establishes a reproducible water content, which is important because the degree of v.etness of catalyst samples can seriously affect their characteristics during, and after reduction.

After drying, the crucible is placed in a small

desiccator for 20 minutes to cool.

About 0.350 g of the material is transferred

to a ground glass stoppered v.eighing bottle, and v.eighed accurately.

The

accurately v.eighed sarrple is transferred to the reactor (a U-tube fitted with a 1 an diameter No. 2 grade Pyrex sinter) which is then connected to the gas handling system. (c)

Thermal treatment.

The starting material can be decanposed in three

fundamentally different ways, namely, calcination in air, decanposition in nitrogen, or reduction in hydrogen.

The characteristics of the final material

will depend on the choice of decanposition procedure.

Table 3 surrroarises sane

of the experimental parameters which can affect the properties of the final material.

791 TABLE 3

Experimental parameters which can affect the catalyst 1. 2. 3. 4. 5. 6. 7. 8. Gas

gas flow rate gas purity gas cauposition (air, nitrogen, hydrogen) sample size catalyst bed depth heating rate final temperature time at temperature flow rates and gas purity (especially concentrations of oxygen-containing

gases) can affect the properties of the final material because the higher the flow rate, the lower the residual water vapour pressure in the catalyst bed. The sample size and l::ed depth can similarly affect the water vapour pressure a deep bed, for example, during dehydration and/or reduction will produce water vapour at the entrance to the bed, and this will be carried down with the gas stream through the remainder of the bed.

Heating rates again detennine the

instantaneous water vapour pressure above the samples.

Heating rate also

affects the decauposition and reduction directly because of changes in the number of nuclei on the surface of the particles.

Water vapour levels and

heating rates canbine to detennine the decauposition and reduction characteristics of the material.

The final temperature and the time at this

temperature canbine to detennine the reactivity of the oxide (if air or nitrogen are used for the decanposition), and the surface area of the metal (if hydrogen is used during decanposition). The reactivity of the oxide can l::e affected in many ways.

If the decanpos-

ition temperature is too low, residual nitrate will be present, if the temperature is too high the oxide may anneal, sinter, or even canbine with the support.

Each of these possibilities decreases by varying amounts the

reducibility of the oxide, and affects the metal surface area which can be obtained.

If hydrogen is used during decauposition the maximum temperature and

time will affect the metal particle size and size distribution.

lJ:M

temper'atures favour high dispersions, but may not give cx:rnplete reduction. tE!llperatures give total reduction, but at the cost of sintering.

High

The optimum

metal surface area needs to be defined because this will determine the choice of decanposition procedure.

High surface areas, although valuable in principle,

may not be ideal in practice because of their inherently lower stability and problems of reproducibility. (d)

Decarposition versus direct reduction.

When this work cannenced there

was no clear consensus in the literature as to whether decarposition should be perfo:nred prior to, or simultaneous with, reduction.

One of our first

792

objectives was to investigate these alternatives. quite clear.

The results (see later) were

Dece:rnposition prior to reduction invariably gave lower surface

areas (by a factor of about three).

How=ver, direct reduction gave high surface

area materials whose surface area decreased rapidly with tine and tanperature. Although high surface areas are advantageous in sane respects, it was decided that stability and reproducibility were more important at this stage.

The choice

was made to perfonn the dece:rnposition and reduction reactions separate1y. (e)

Decanposition and reduction procedures.

No advantage appeared to be

gained by deccmpositioh in nitrogen rather than in air, (indeed preliminary experinents indicated that the surface area was significantly 10¥Jer if nitrogen was used), so it was decided to perfonn decompositions in air.

The recarraended

procedure for the deccmposition and reduction is sunmarized in Table 4. TABLE 4

Recarmended procedure for deccmposition/reduction of catalyst 1. 2. 3. 4. 5.

Flush reactor with dry air. Raise tanperature at 7 K/minute to 623 K. Calcine at 623 K for 2 h. Flush with nitrogen for 5 minutes at 623 K. Reduce in hydrogen for 2 h at 623 K.

I 3 The sample in the reactor is flushed with dry air (flow rate 80 an gcatminute-I) for 5 minutes, and then heated in flowing air at 7 K minute-1 to 623 K. The ternr:;erature is held at 623 K for 2 h, after which the air is turned off, the reactor flushed with nitrogen (80 em3 gcat- 1 minute-I) for 5 minutes, with the semple still at 623 K.

The nitrogen supply is turned off, and the samp.Ie reduced in flowing hydrogen (80 em3 gcat-1 minute-1) for 2 h at 623 K. (f)

O1tgassing and cooling procedure for chemisorption experiments.

To prepare the reduced catalyst for the chemisorption experiment the sample was

evacuated at the final reduction tEmperature for 0.5 h, cooled to roan tanperature, and evacuated for a further 16 h. Part B.

Preliminary results

(a) TGA.

Figure 1 shows a representative thennogravimetric analysis

profile for the starting material.

The results underline the necessity of

predrying the stored sempl.es to obtain a reproducible starting state, and show that constant weight is obtained quickly.

(Our choice of 16 h for drying is

longer than required, rot is convenient.)

On raising the tanperature to 623 K

the samp'le dece:rnposes and rapidly attains a constant weight.

Addition of

hydrogen at 623 K leads to rapid reduction and constant weight is produced within about 30 minutes.

The weight change is consistent with almost canplete

reduction of Ni(II) to Ni(O).

793

d

~

I

:E

en

·w ~

OJ

Ci.

E

o

If)

2

4

6

Time I h Fig. 1. TGA profile for the catalyst precursor. Letters refer to different experimental regiIres. a, heating in air at 10 K/ minute to 393 K; b, air, 393 K; c, heating in air at 10 K/minute to 623 K; d, air, 623 K; e, nitrogen, 623 K; f, hydrogen, 623 K.

(b) Temperature-prograrrmed reduction (TPR). obtained by various research groups.

Figure 2 shows TPR profiles

This figure is included partly to indicate

the extent to which the reducibility is affected by the reduction conditions, and partly to underline the need to establish acceptable standard procedures. * As can be seen, the conditions typicailly used by various groups range in gas

canposition fran 5 to 25 % hydrogen, and in heating rates fran 5 to 30 K/minute. Although for each set of exper:iInental parameters this technique gives very reproducible data, reliable canparisons of data fran different laboratories is virtually impossible. (c) Metal surface area rreasurements. conditions.

(i) influence of deCOItlj:X?sition

Table 5 canpares the surface area determined on samples decanposed

in air, nitrogen or hydrogen and emphasises the great variation in surface area

which can be produced. each procedure is good.

It should be emphasised that the reproducibility for As noted earlier the highest surface area was not

*It should be recognised that many of the data presented in this paper were obtained while the preferred experimental procedures were being developed. For this reason, the experimental parameters used sanet:iInes differ fran those rec:x:mtended .

794

Q e 0 ....... 0E

,,

~

Ul

e 0

....

....

U

e

d-

~

OJ

(J)

0

L..

"0 ~

:r:

750 Fig. 2. TPR profiles for the uncalcined catalyst precursor. Ex:perinEntal pararooters: a, 5 % hydrogen, 5 K/rninute: b, 10 % hydrogen, 30 minute: c, 25 % hydrogen, 7 K;1ninute: d , 25 % hydrogen, 27 Kzmimrt.e,

KI

TABLE 5

SUrface areas of samples decanposed in different atmospheres. Atmosphere

SUrface area

air nitrogen hydrogen

25 13 118

1m2

necessarily the most desirable.

gNi

-1

Figure 3 shows how the surface area can change

rapidly during the early stages of reduction.

Clearly, it would be difficult to

stop this initial reduction at a reproducible point. (ii) adsorption isotherms. ,Figure 4 shows the hydrogen adsorption isotherms

obtained in different laboratories for samples prepared in canparable ways (Le. precalcined and reduced at 623-723 K).

We note again the disparity in the

methods adopted in this preliminary VJOrk, e.g. choice of temperature for adsorption, range of pressures, sample size (fran 0.2 to 3.0 g).

795

-

100

a

OJ

L...

a

OJY

-

a z0)

U

L.

:J

N

50

E

If)

2

[.

Time I h Fig. 3. Change in surface area with time of reduction . • , uncalcined precursor : 0 , precursor calcined for 0.5 h at 673 K.

-

\J OJI

.c .Z

L...

0

0)

Vl

\J

a~

C

6

-

0 c ..-- [.

OJ

0)

0

L...

\J

X Vl OJ

-0

~ :r: E

A

2

0-

0

0

=0-

S

150

75

PI mbar Fig. 4. Hydrogen adsorption isothenns measured in different laboratories. A. laboratory A (293 K): 8, laboratory B (307 K): C, laboratory C (273 K). The variation in the shapes of theisothenns is notable.

For non-linear

isothenns such as these there are real problems in accurately detennining the rronol.ayer coverage fran an extrapolation to zero pressure.

A more accurat.e value

for the monolayer coverage can be obtained fran a Langmuir plot, using the equation for dissociative adsorption. (iii) repeatability.

This work is still at an early stage and much effort has

been concentrated on identifying generally acceptable standard procedures. Therefore, only a few data are presently available to indicate the probable reproducibility of the chemisorption experiments. relevant data.

Table 6 simnar.Lses sane

The repeatability for catalysts prepared under identical

796 TABLE 6

Repeatability of hydrogen chEmisorption data Laboratory A sample

a

l

4

f

Sample 15

25 26 26 22 19 19

2 3 5 6

SUrface area m2 gNi- 1

Laboratory C

Laboratory B

2 d 1

Surface area m2 gNi- 1

Sample

33.1 32.1 23.3 32.6 37.2 41. 9 50.3 50.3 50.3

1c 2 Ie 2

Surface area

m2 gNi- 1 44.7 47.2 124.5 112.8

asamples calcined at 673 K and reduced at 673 K, bSamples reduced at 673 K in static hydrogen, cSamples calcined at 573 K and reduced at 573 K, dsamples reduced at temperatures rising fran 303 to 643 K, and hydrogen adsorption determined by pulse method using the same sample, ~calcined

samples reduced at

723 K, fsamples calcined at 673 K and reduced at 773 K. conditions is encouraging.

(Using a procedure similar to that recanmended 1 2 earlier, a surface area of 38 m gNi- has been obtained.) The importance of agreeing on a suitable method of extracting the monolayer coverage fran the adsorption isotherm is illustrated by the data in Table 7. TABLE 7 ~nolayer

coverages fran Langmuir and zero pressure calculations.

Experiment

V (Lanqmzf.r) m

V (zero pressure) m

V (Lang.) /V (zero) m m

1 2 3 4 5 6

56.9 51.5 51.4 153.6 140.1 38.4

44.7 47.2 39.0 124.5 112.8 32.9

1.272 1.091 1.318 1.234 1.242 1.167

These data show that the ratio of the monolayer coverage fran the Lanqrnri.r plot to that fran the zero pressure extrapolation is not constant.

Indeed, the

scatter is of the same order of magnitude as the total error in the experiment. (d) Activity for the hydrogenation of benzene. been performed on the hydrogenation of benzene.

Only a very few experiments have After calcining and reducing the

samples using a procedure similar to that outlined above (see Table 4) the activity was determined.

{The main difference in procedure was that the final

797

15

-

'::!?

0

c

0 VI

10

L..

OJ

>

C 0

0

5 25 50 75 Time I minutes

Fig.

5. Change in benzene hydrogenation activity with time.

temperature for reduction was 673 K rather than 623 K.)

Figure 5 shows how the

activity decreases with time, requiring the extrapolation to zero time previously mentioned.

The recorded activities in the three determinations shown were 15. 0 ,

15.6, and 14.7 prrol/g cat.;1ninute.

Allowing for the fact that these results were

obtained on separate samples which had undergone drying, calcination, and reduction stages, the reproducibility can be considered satisfactory. CCNCLUSIONS This work has demonstrated that in addition to providing sources of catalyst reference materials, there is a clear need to establish standard techniques of pretreatment and measurement.

In

the present work, where there has been

consistency of technique, the repeatibility of the experiments has been rrost encouraging.

It is reasonable to expect that a reproducibility within

could be obtained in both nickel surface area and benzene hydrogenation activity measurements. ACKNCWLEI:X;EMENT

The Society of Chemical Industry Working Party on Catalyst Reference Materials wishes to thank Akzo Chemie Nederland bv for provision of the catalyst precursor.

±5

%

798 DISCUSSION J.W.E. COENEN Did you verify that the hydtogen adsorption isotherms really represent equilibrium states. Our experience is that adsorption and desorption "isotherms" never coincide. Deviations are worse when reduction prior measurement is poorer. A.R. FLAMBARD: The problem of deciding whether or not an isotherm truly represents an equilibrium state is a difficult one. For the standard catalyst we did not measure any desorption isotherms. I can only report the observations that we have made in our laboratories. For each point o~ the isotherm the hydrogen uptake followed an often reported scheme; a rapid adsorption followed by a much slower (activated) adsorption. In the time allowed for the measurement of each point, this second adsorption process did not in fact come to equilibrium. However, we have alsi determined that after much longer eqUilibrium times no significant change in either isotherm shape or amount of hydrogen adsorbed were observed. We therefore believe that even though the isotherms do not represent true kinetic equilibria they do in fact come quite close to it. We have also observed that with poorly reduced Ni/Si02 catalysts the contribution of the slow uptake to the total hydrogen adsorption increases quite dramatically and the measured isotherms appear more "rounded". As you have said, deviations from eqUilibrium become worse. The question must therfore be asked as to if this slower adsorption is not in fact due to some contamination effect and if it should be considered when evaluating free metal surface areas. A. FRENNET: I feel some difficulties in the use determine the monolayer coverage as this isotherm of the heat of adsorption with coverage, which is heat of adsorption of hydrogen on nickel is known with coverage.

of the Langmuir isotherm to model is based on a constancy generally not the case. The to vary in a important way

A.R. FLAMBARD: As noted in Prof. Bond's boof "Catalysis by Metals", there is no definition of surface coverage which is either generally or satisfactorily applicable. At the last Minisymposium on catalyst normalization, Prof. Scholten recommended the use of extrapolation to zero pressures in order to evaluate the monolayer coverage. Faces with an isotherm such as C in Fig. 4 and from our experience with poorly reduced catalysts, as I outlined in reply to Prof. Coenen's question, we decided that this method was perhaps not the best. We have tried to fit our data to the Freundlich and Tempkin (Slygin-Frumkin) models of adsorption as well as the Langmuir and found that the best approximations to linear plots was obtained with the latter. Of course, the heat of adsorption of hydrogen on nickel does not remain constant with coverage. However, in respect to the standard catalyst and catalyst normalization we are at present more interested in determining a standard procedure of activation and measurement that will give repeatable results rather than investigating suitable adsorption models. It is for this reason that the question of tree equilibrium raised by Prof. Coenen has not been thoroughly investigated with this catalyst. Under the conditions specified and with the model used, a repeatable figure for the free metal surface area will be obtained, which was after all our first objective. B. SMEDLER: A main topic of catalytic hydrogenation on Ni/silica catalysts is that of enhancing the selectivity in consecutive reactions. Therefore, wouldn't it be recommendable to complete your activity test (benzene hydrogenation) with a few other hydrogenations (for exampl~ olefin and aldehyde hydrogenation) in order to get some information of the sleectivity properties of the catalyst? A.R. FLAMBARD: Benzene hydrogenation was chosen as the test reaction for the standard catalyst after a survey amongst catalyst users and researchers. We have also considered the selectivity properties of this catalyst and have determined

799 its activity and selectivity for both n-hexane hydrogenolysis and carbon monoxide hydrogenation (Fischer-Tropsch) reactions. As with benzene hydrogenation, the results are encouraging with good reproducibility for a specified activation procedure. We have not considered using alkene or aldehyde hydrogenation but I thank you for your comments.



RESEARCH GROUP ON CATALYSIS, COUNCIL OF EUROPE.

801

STANDARD CATALYST PROJECTS

J.W.E. COENEN l and P.B. WELLS 2 lCatholic University, Nijmegen, The Netherlands 2Univers-ity of Hull, United Kingdom

ABSTRACT In 1975 a research group on catalysis was started in which 22 research groups from 9 European countries participate. Aim of the group is joint discussions and collaborative work in the field of metal catalysis. Two projects were started, each centred on a standard catalyst. EURONI-l is a nickel/silica catalyst which in the unreduced state contains 25% Ni and has a total surface area of 270±20 m2 g-l. Its structure in the active reduced state depends strongly on the reduction conditions appl ied. Reduced at 6500C the nickel crystallite size ranges from 1 to 10 nm, average about 4.5 nm. At lower reduction temperature, where reduction remains incomplete smaller crystallites are formed. The catalyst was subjected to a number of physical examination methods from which a reasonably consistent picture of its structure was obtained. EUROPT-l is a platinum/silica catalyst of total surface area 185±10 m2 g-l containing 6.3 wt% Pt. The platinum crystallite size distribution extends from 1 to 3.8 nm, the maximum being just below 2.0 nm. 80% of the crystallites have a diameter less than 2.2 nm. The chemisorption of hydrogen, of oxygen and of carbon monoxide has been studied in detail and the extents of adsorption at saturation defined. Concordance of results was better for the platinum catalyst than for the nickel catalyst, which is undoubtedly due mainly to the difficulty of reducing the later catalyst in a reproducible manner. INTRODUCTION At the initiative of E. Derouane of the University of Namur, Belgium, a Research Group on Catalysis was started early in 1975 under the aegis of the Committee on Science and Technology of the Council of Europe. In this group 22 catalysis research groups located at universities and research institutes in 9 European countries are represented. The group decided to start collaborative work on two projects, each centred on a standard catalyst, a quantity of which was distributed among the participating laboratories. As standard catalysts were selected a 25% nickel on silica catalyst and a 6% platinum on silica catalyst. Aim of the projects was twofold. In the first instance it was hoped to obtain insight into the possibility of obtaining concordant results in different laboratories with various methods of catalyst characterisation. Secondly it was expected that the combined measuring results would yield for the two catalysts a

802

more complete characterisation than would ever be possible in any single laboratory, so that the catalysts would be highly valuable bases for subsequent catalytic studies. In the following the results of the studies on the two catalysts are presented in compact form. The authors of this paper earlier prepared more comprehensive reports on the nickel and the platinum catalyst respectively. In the acknowledgement at the end of this paper the participating teams are identified by their leader or one member. THE EURO NICKEL CATALYST, EURONI-l Catalyst Preparation For a loading of about 25% nickel a precipitation method is more suitable than impregnation. A wide choice of precipitation methods has been described but most of them have the risk of local excess of ingredients causing precipitation of part of the nickel not attached to the support. The method of homogeneous precipitation, developed by Geus, provides an elegant method to avoid this difficulty. In this process all ingredients for the catalyst are present in the reaction vessel at the start of the precipitation: nickel nitrate and urea, together in a suitable volume of water and the silica support, aerosil 180, suspended in the aqueous solution. The essence of the method is that the precipitating hydroxyl ions are generated very gradually throughout the solution by hydrolysis of the urea upon heating of the reaction mixture to 900C. Under these conditions the support nucleates precipitation of nickel compounds so that all nickel precipitate is associated with the support. Precipitation was done at Nijmegen in a 75 litre glass reactor with stirrer and heating. To prepare the required amount two batches had to be made. Per batch 3 kg Ni(N03)2,6aq (p.a.), 1.4 kg Aerosil 180 silica ex Degussa, 1.8 kg urea (p.a.) and 50 litres demineralised water were used. The charge was kept at 900C for 20 hrs and then filtered hot. The precipitate was twice resl urried in hot water and filtered. The combined precipitates were then reslurried and spray dri ed. Catalyst Reduction To obtain the catalyst in active form the catalyst must be reduced with hydrogen at elevated temperature. It is known from the literature that this reduction for silica supported catalysts is a complex and difficult process. Water formed in the reduction acts as an inhibitor for the reduction. Hydrothermal conditions can prevail by the combination of high temperature. and significant water vapour pressure unless measures are taken for its fast elimination and this can induce formation of more difficultly reducible nickel-silicate structures. The combined result of these effects makes the result of the reduction highly sensitive to the detailed conditions temperature, purity and flow

803

rate of hydrogen, geometry of the catalyst vessel and conditions of hydrogen flow, through or over the catalyst bed. Also the quantity of catalyst reduced in one batch has significant influence on the result. Reproducibility in one laboratory already proves difficult, between different laboratories seems almost impossible. The cause of not very good concordance of results obtained in different locations will be mainly located in the reduction. Two laboratories exposed the unreduced catalyst to a hydrogen atmosphere with temperature linearly increasing with time. In one case heating at SoC min-l up to 635 0C was applied and the weight loss followed of a precalcined sample. A weight loss of about 1% occurs gradually up to 4300C, followed by an abrupt loss of 6% at this temperature. A further equally abrupt weight loss of about 2% occurs at about 5300C. Since nickel oxide formed by calcination of basic carbonate or hydroxide is easily reduced at or below 300°C, at most 10% of the nickel is present in the unreduced catalyst in these easily reducible forms, the remainder presumably as more difficult reducible nickel silicates. In the second case the temperature rise was 10°C min-l and reduction progress was followed by hydrogen consumption. Again about 10% of the nickel is reduced at low temperature, the remainder between 400 and 700 0 . Comparison with curves obtained with two model silicates indicates that this nickel is likely to be present as badly ordered nickel antigorite. Isothermal reduction with hydrogen flowing through the catalyst bed, performed in several laboratories, showed that complete reduction of nickel in the catalyst is only attained with reduction times of 24 h or more at 630°C or above. Reduction at lower temperatures, e.g. at or near 450°C produces significantly lower degrees of reduction and significant deviations between laboratories. Values as low as 50% and as high as 90% were obtained. Chemical Characterisation The nickel content of the unreduced catalyst was determined by titration after acid extraction, atomic absorption spectroscopy, X-ray fluorescence and proton induced X-ray emission. Values obtained ranged from 23% to 25.5%. Sources for variation were the hygrqscopic character of the catalyst and difficulties met in the complete extraction of nickel with acid. These causes make the higher values to be more likely to be correct. A silicon content of 25.5% was obtained. The unreduced catalyst further contains 1.0% nitrogen, 0.45% CO 2 as memory of the urea precipitation. Trace impurities were Fe, Ca, Cl and S all in the ppm range. Physical Characterisation Two laboratories obtained X-ray diffraction patterns of X-ray diffractio~. the catalyst in the unreduced state and after calcination in one case at 450°C, in the other at 5000C. Conclusions were closely concordant. The pattern of the unreduced catalyst gave no evidence for the presence of nickelhydroxide and showed most resemblance to that of badly crystallised nickelantigorite. The cal-

804

cination treatment produced only negligible change in the pattern and hardly if any evidence was found for presence of nickel oxide in the calcined sample. This confirms the earlier conclusion that at most only a small amount of easily reduceable nickel compounds can be present. One laboratory also obtained patterns of the catalyst after reduction at 45cPC and passivation and after reoxidation of the latter sample in air at 4500C. In the reoxidised sample there was clear evidence of finely divided nickel oxide, with a crystallite size of 2.4 nm. Assuming that each oxide crystal was formed from one nickel crystal in-the reoxidation the average crystallite size in the reduced sample would be 2.2 nm. However the pattern of the reduced/passivated sample showed strong evidence for the presence of nickel oxide and only weak indication for nickel metal. Clearly the passivation treatment must have oxidised most of the nickel metal produced in the reduction treatment. It is known that the type of passivation treatment applied in general oxidises about 2-3 atom layers deep and with the fine state of subdivision this would involve most of the nickel as was indeed found. The crystallite size was confirmed in a third laboratory where from the X-ray pattern of the catalyst reduced at 4500C for 70 h a crystallite size of 2.5 nm was derived. The conclusion from this investigation is that most of the nickel in the unreduced catalyst is present in a structure which is thermally stable at 4500C, e.g. sheets of nickel antigorite, which have no ordered three-dimensional stacking. The nickel atoms in this structure can however be mobilised at 4500C when they are reduced to the zerovalent state. They then cluster to crystallites of 2.2 to 2.5 nm. Electron spectroscopy. (XPS). Three laboratories did ESCA studies on several forms of the catalyst. Electron binding energies differed marginally between the laboratories and they yielded no enlightening information. The intensities were more informative. Two laboratories observed that the Ni(2p)3/2/Si(2p) ratio decreased marginally on calcination of the unreduced catalyst. This confirms the picture already obtained from the X-ray data: there is very little structural change, but some dehydration/dehydroxylation. Therefore the deeper-lying silicon in the antigorite structure is less attenuated. Two laboratories observed that upon reduction the intensity ratio dropped significantly. This again is in line with X-ray findings: a significant proportion of the nickel is now shielded by nickel - in the crystallites - whereas the silicon signal for that part of the surface from which nickel has migrated is less shielded. One laboratory also took the ESCA-pattern of the reoxidised catalyst and observed hardly any change, as compared to the passivated reduced sample, again in line with X-ray findings. Electron microscopy. Two laboratories did scanning electron microscopy at moderate magnification on the unreduced catalyst wi th- identical results: a spherical particle shape typical for spray drying. The size distribution is wide, from

805

4 to 70 ~m. The same labs did transmission electronmicroscopy on the unreduced catalyst at magnifications up to 450,000 again with closely concordant results. The typical microspheroidal structure of the Aerosil has virtually disappeared and is replaced by a loose network of thin randomly oriented wrinkled sheets. In view of ESeA and X-ray results we may identify these as antigorite sheets. We may expect these to retain their structure on heat treatment at 4500e and they are found to do so. On reduction the sheetlike structure persists, but minute speckles, th~ nickel crystallites can now be seen, which persist on reoxidation. Four laboratories did electron microscopy to obtain the crystallite size distribution in the reduced catalyst of the nickel crystals. Three laboratories us630-6500e for periods ranging from 15 to 45 h. ed similar reduction treatmen~at The results are closely concordant: the surface average crystallite size is found to be 4.6±0.3 nm and thehalfwidth of the distribution curve is about 0.4 nm. Longer reduction times increase the average crystallite size only marginally. Magnetic measurements. One laboratory did magnetic measurements on the catalyst after its reduction at 6300e for 26 h at magnetic fields up to 21 kOe. From the measurements a surface average crystallite size of 5.0 nm is derived in good agreement with the electronmicroscopic results. Another laboratory provided evidence with respect to the monolayer definition in hydrogen adsorption on the reduced catalyst. On increasing hydrogen pressure from e.g. 1 Torr to 760 Torr not only additional hydrogen adsorption is observed but also additional decrease of the magnetization. This observation indicates that this additional adsorption takes place on nickel metal and should therefore be counted in the monolayer coverage. Adsorption studies Nitrogen adsorption. On the unreduced catalyst total surface areas by the BET-method from nitrogen adsorption isotherms were measured in six laboratories. The concordance in the results is rather disappointing. Also several laboratories report that the reproducibility of the measurement is poorer than usually obtained on samples of similar surface area. The deviations bet~en laboratories are largely due to different degassing treatments, for which temperatures ranging from 200e to 1200e were used. Total surface areas ranged from about 220 to 270 m2 g-l and it appears likely that the true value is close to the latter, which is also the value obtained in one laboratory in sevenfold ±l m2 g-l. It is highly likely that poor degassing is the cause of lower values. We recall from the scanning electron microscopical data that we are dealing with relatively large particles, so that the real attainment of a suitable low pressure may well take considerable time, especially in apparatus with narrow tubing. Two laboratories did measurements on the catalyst after various reduction treatments. Reduction at 4500e reduced the total surface area to 252 m2 g-l, at

806

5000 e reduction to an area of 234 m2 g-l was obtained whereas at 700°C the surface area had shrunk to 215 m2 g-l All these values were obtained in the same laboratory. In another laboratory surface areas of 212 and 192 m2 g-1 were obtained after reductions at 650°C during 15 and 45 h respectively, which is in reasonable agreement with the figures quoted above. Hydrogen adsorption. This measurement, used to quantify the available nickel surface area in the reduced catalyst, is obviously of crucial importance in this cooperative project. The problem is that on the one hand the individual reductions done in the differe~t laboratories may well produce significantly different results, as explained earlier, but also that the methods for measuring hydrogen adsorption and their interpretation differs for different locations. For reduction in three laboratories at 430-4500C during times from 4 to 24 h hydrogen adsorption at 200C yielded values of 53, 53, 19, 46, 46 and 44 ml STP (gNi)-l. The agreement is thus quite reasonable with one exception. Ignoring this low value we find an average monolayer capacity of 48.4 ml STP (g Ni)-l. For reductions in four laboratories at 630-6500C during times from 3 to 26 h hydrogen adsorption at 20°C yielded values of 37, 32, 28, 26, 25, and 28 ml STP (g Ni)-l. Again agreement is not unreasonable, we find an average capacity of 29.3 ml STP (g Ni f l . From the average va1ues degrees of di spers i on of 25 and 15% can be calculated. Summary and conclusions From X-ray, EM, ESCA and TPR data we can conclude that in the unreduced catalyst most of the nickel is present in disordered sheets of a basic nickel silicate with possibly less than 10% of a more easily reducible nickel compound, hydroxide or basic carbonate. The total surface area of the unreduced catalyst is about 270 m2 g-l. Ignition at 450°C hardly changes the structure, apart from some dehydration. Reduction at progressively higher temperatures reduces the total area gradually to 215 m2 g-l at 700°C, referred to as unit weight of the original material. In the reduction in hydrogen th.e nickel atoms in the antigorite sheets are mobilised and cluster to small crystallites. With a hemispherical model for the crystallites used by one of the authors (JWEC) an average crystallite size of 2.1 nm can be calculated for the catalyst reduced near 4500C, which agrees excellently with the value of 2.2 and 2.5 nm obtained by X-rays. Similarly from the hydrogen adsorption value of the catalyst reduced near 650°C an average crystallite size of 4.3 nm can be calculated, again in good agreement with the electronmicroscopic observations. THE EURO PLATINUM CATALYST, EUROPT-l Preparation EUROPT-l was prepared by Johnson Matthey Chemicals of the UK. 6 kg silica

807

(Sorbsil grade AQ U30, Crossfield Chemicals) was impregnated with Pt(NH 3)4++ at pH 8.9, filtered, washed, dried, and reduced in hydrogen at 400oC. The material was granular (size range 62 to ~750 microns). Physical Characterisation Random samples contained 6.3 wt%Pt by spectrophotometry (ptSn4C11+), 6.2% by atomic absorption, and 6.2% by proton induced X-ray emission (values ±0.3%). The agreed value is 6.3%. The largest granules exhibited a somewhat lower platinum content ~s expected. Trace elements present included: Al 500 ppm, Ca 500, Na 400, Ti 400, Mg 200, K 150, Fe 90, Cl He/RT)

The underlying

yP co'

(7) +

mechanism is discussed elsewhere (ref. 5). The

parameters of eq.

(7) which have been obtained independently in the

two laboratories and which show reasonable agreement are listed in Table 4. If only a small range of hydrogen partial pressures, which TABLE 4 Parameters of eq.

k

(8);

(BO: Bochum (ref. 5)

O

mole/g h BO

4.8 10 9

ER

9

5.2 10

L'>H

K e

E 0

kJ/mole

bar

-0.5

ER: Erlangen (ref. 6)).

O

C

kJ/mole

K

L'>HH

H

bar

-0.5

kJ/mole

103

4 5.8 10-

-42.1

0.016

-16.0

101

10- 4

-40.0

0.016*

-16.0*

6.6

*fixed value for optimization of the other 4 parameters exists under industrial conditions, is considered a simpler kinetic equation may be used (ref. 6) k

to characterize the rate of reaction

~l{P;;;

(8)

1 + K Peo

For a comparison of catalysts G and R the knowledge of the kinetic parameters of G are required. Since for catalyst G only limited kinetic data, not disguised by diffusion, are presently available the following procedure was used: assuming that the adsorption constants K and K are equal for both catalysts the rate C H constant k for catalyst G is estimated by eq. (7) from the descending parts of the two curves in Figure 3 not affected by pore diffusion. Surprisingly, the intrinsic rate constants are very similar as shown in Table 5. This result means that the difference TABLE 5 Intrinsic rate constants k/mole g-l h- 1 of catalysts Rand G T /K/ 483 503

R

G

9.2 10- 2 27.5 10- 2

829

r

CH

10- 3

14

4

mole/ g h

12 10 8 6 o~

o

4

----===-=1....-.-...-:..-.

2

_

()-

0

0.10

0.05

0.15

0.20

0.25

Pco bar Fig. 3. Dependence of methanation rate on partial pressure of CO for catalyst G (particle size: 1 to 1.4 mm); effective rate: -0----0--, rate without limitation by pore diffusion: ----in activity of catalyst pellets Rand G are mainly due to their effective pore diffusion coefficients D which can be derived from e the effective Knudsen- and effective binary diffusion coefficients: De

K, i

= K

0

V

8 RT i

..f-D

11 m

T

( 9)

1,2

The required porosity data are listed in Table 6. TABLE 6 Porosity data of catalyst pellets Rand G (ref. 7) catalyst pellet G

R

K

o

0.05 10- 8 0.26 10- 8

0.014 0.086

As a first approximation the ratio of the activities of the two catalyst pellets expressed by the effective reaction rates r e at equal temperature and concentrations which corresponds to the effective rate constants k

e

can be described by eq.

(10):

(10)

830 and L being the characteristic dimensions of the two pellets. G R Inserting the relevant data into the right hand side of eq. (10)

L

results into a value of 0.4 which is very close to the ratio of the experimentally determined effective reaction rates, i.e. within less than 10 % deviation. Summarizing, it can be concluded that a more fundamental measure of activity, when pore diffusion prevails, comprises the intrinsic rate constant and the effective pore diffusion coefficient. CONCLUSION ANB OUTLOOK The present results have principally shown that catalysts should be characterized with respect to their actual performance by kinetic parameters for the chemical and transport processes. On the basis of these results further work will deal with the standardization of the experimental procedures and conditions as required for catalyst testing in the most general way possible. REFERENCES 1 B. Muller and M. Baerns, Chern. Ing. Techn. 52(1980) 826-830. 2 B. Muller, Dissertation, Ruhr-Universitat Bochum, Bochum 1981. 3 Le Duy Duc, unpublished results. 4 D. Kreuzer,

Diploma-Thesis, Ruhr-Universitat Bochum, Bochum

1981 . 5 J. Klose, Dissertation, Ruhr-Universitat Bochum, Bochum 1982. 6 Zhang Ji-Yan, unpublished results. 7 Tran Vin Loc, unpublished results.

831 DISCUSSION S.P.S. ANDREW My question is entirely of a philosophical nature concerned with the knowledge and the intellectual ability of the people required to effect a satisfactory scale-up of design from data from microreactors and gradientless reactors. The old method of using pilot plants for scale-up required less intellectual ability and less chemical and physico-chemical knowledge than scaleup from gradientless reactors and the use of Thile moduli etc.. Take for instance ammonia synthesis catalysts: it is now well known that small particles of catalyst have a much higher specific surface than do large particles due to reduction phenomena and therefore have a higher intrinsic activity. If the person engaged in a scale-up operation involving microreactor measurements, on ammonia synthesis, is unaware of this phenomenon than he will err greatly. Can we be sure that the practical user of these techniques will have the necessary enchanced intellectual capacity to be able to use them correctly ? M. BAERNS Reactor modeling based on kinetic data obtained in laboratory reactors and on transport parameters, if needed, facilitates (1) to study the effect of process variables on reactor performance by simulation and (2) to design a chemical reactor. Both possibilities may support the industrial chemist and the chemical engineer in his scientific and technical work. Having a suitable eductaion in the field of chelical reaction engineering there should be no intellectual obstacles to apply and work with reactor models. J.W GEUS: You are studying considerably exothermic reactions and consequently the problem arises whether you can determine correctly the temperature at the catalytic active site. Is it required to do measurements in a gradientless reactor at low conversions per pass or is it possible to work at higher conversions per pass and derive the temperature at the catalytic sites and possibly the distribution by calculation ? M. BAERNS The gradientless reactor requires by definition low conversions per pass through the layer of the catalyst pellets; the overall conversion of the feed to the reactor may, however, vary between very low and very high values. The temperature gradient from the gas phase to the catalytic surface depends on the heat transfer coefficient "pellet surface/gas" which can be influenced by the linear gas velocity through the catalytic bed, i.e. recycle rate. This temperature gradient and hence, the temperature of the catalytic surface, can be calculated; intraparticle temperature gradient rarely occurs. R.A. VAN NORDSTRAND: In answer to the Chairman's general question (see preceeding paper), I propose that we consider objectives. The ASTM in USA is a group of catalyst scientists trying to expedite commerce in catalysts, providing detailed and practical tests. The European group represented here seem to be academic catalyst scientists whose objectives are to promote the science of catalysis. Perhaps it is expecting too much to bring closely together groups with objectives so different. M. BAERNS: To my opinion the objectives of both groups as defined by Dr. van Nordstrand are not detrimental to each other but supplementary. The European Groups are (a) developing basic methods and respective reference materials for testing physical and chemical properties of the solid material and (b) evaluating the bases for, the experimental determination of the catalytic activity and selectivity which can be extrapolated to process conditions. Both these objectives will not only contribute to a better understanding of various scientific aspects of catalyst testing but they will eventually also improve the experimental methods and techniques applied in the characterization of industrial catalysts.


833

LIST OF PARTICIPANTS

ADM1S C., Dr.

ALTHAUS H., Dr.

ANDERSSON A.S., Dr.

ANDRES M.

ANDREW S.P.S., Dr.

A.."

Preparation of Catalysts III: Scientific Bases for the Preparation of Heterogeneous Catalysts (Studies in Surface Science and Catalysis)

Preparation of Catalysts III: Scientific Bases for the Preparation of Heterogeneous Catalysts (Studies in Surface Science and Catalysis)

Preparation of Catalysts IV: Scientific Bases for the Preparation of Heterogeneous Catalysts (Studies in Surface Science and Catalysis)

Scientific Bases for the Preparation of Heterogeneous Catalysts (Studies in Surface Science and Catalysis)

Preparation of Catalysts VI: Scientific Bases for the Preparation of Catalysts (Studies in Surface Science and Catalysis)

Scientific Bases for the Preparation of Heterogeneous Catalysts

Scientific Bases for the Preparation of Heterogeneous Catalysts

Preparation of Catalysts VII (Studies in Surface Science and Catalysis)

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Preparation of Catalysts III: Scientific Bases for the Preparation of Heterogeneous Catalysts (Studies in Surface Science and Catalysis)