Preparation of Catalysts V

Studies in Surface Science and Catalysis 63 PREPARATION OF CATALYSTS V Scientific Bases for the Preparation of Heterogeneous Catalysts

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Studies in Surface Science and Catalysis Advisory Editors: B. Delmon and J.T. Yates Vol. 63

PREPARATION OF CATALYSTS V Scientific Bases for the Preparation of Heterogeneous Catalysts Proceedingsof the Fifth International Symposium, Louvain-la-Neuve, September 3-6,1990

Editors G. Poncelet Catalyse et Chimie des Materiaux Divises, Groupe de Physico-Chimie Minerale et de Catalyse, Universite Catholique de Louvain, Louvain-la-Neuve, Belgium

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

and P. Grange and B. Delmon Catalyse et Chimie des Materiaux Divises, Groupe de Physico-Chimie Minerale et de Catalyse, Universite Catholique de Louvain, Louvain-la-Neuve, Belgium

ELSEVIER Amsterdam - Oxford - New York - Tokyo

1991

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V

CONTENTS Organizing Committee Foreword Acknowledgements Financial Support

X XI XIII XTV

Studies of unit operations in catalyst preparation Illustration of process scale-up in heterogeneous catalyst preparation I. Biay, G. Dessalces, C. Hypolite, F. Kolenda, J.P. Reymond

1

Deposition precipitation onto pre-shaped carrier bodies. Possibilities and limitations K.P. de Jong

19

Influence of the preparation procedure on the physical properties, surface acidity and dispersion of MoP/A1203 catalysts R. Prada Silvy, Y. Romero, J. Guaregua, R. Galiasso

37

Synthesis of non-stoichiometric spinel-type phases : a key tool for the preparation of tailored catalysts with specific activity M. Piemontese, F. Tnfir6, A. Vaccari, E. Foresti, M. Gazzano

49

Effect of preparation variables on catalytic behaviour of copper/zirconia catalysts for the synthesis of methanol from carbon dioxide R.A. Koeppel, A. Baiker, Ch. Schild, A. Wokaun

59

Preparation of Tia-Al203 by impregnation with TiC4-CC4 Liu Yingjun, Zhang Qinpei, Zhu Yongfa, Gui Linlin, Tang Youqi

69

Interactions of the impregnating solution with the support during the preparation of Rh/Ti@ catalysts R.J. Fenoglio, W. Alvarez, G.M. Nuiiez, D.E. Resasco

77

Impregnation of controlled-porosity silica : Cu/Si@, Co/SiO2 and Cu-Co/SiO2. Investigation of the parameters affecting selectivity in CO hydrogenation M.A. Martin Luengo-Yates, Y. Wang, P.A. Sermon

87

Selective hydrogenation of cyclododecatriene isomers to cyclododecene catalyzed by Cu-Al2O3 V. Di Castro, M. Gargano, N. Ravasio, M. Rossi

95

Preparation and characterization of highly selective Fe-Cu/Si& catalysts for partial hydrogenation of alkynes Y. Nitta, Y. Hiramatsu, Y. Okamoto, T. Imanaka

103

Some remarks on the preparation of Fe-WCa-Cr catalyst for styrene production Z. Dziewiecki, E. Ozdoba

113

Hydrogenation of 2-ethyl hexen-2-al on Ni/SiOz catalysts. Role of preparation parameters A.F. da Silva Jr, V.M.M. Salim, M. Schmal, R. Frety

123

Preparation and properties of a Wsilica and its comparison with Europt-1 S.D. Jackson, M.B.T. Keegan, G.D. McLellan, P.A. Meheux, R.B. Moyes, G. Webb, P.B. Wells, R. Whyman, J. Willis

135

VI

Factors analysis for mechanical strength in pelleting process of Fe-based high temperature shift catalyst Yongdan Li, Jiusheng Zhao, Liu Chang

145

Studies on pore size control of alumina : preparation of alumina catalyst exmudates with large unimodal pore structure by low temperature hydrothermal treatment M. Absi-Halabi, A. Stanislaus, H. Al-Zaid

155

Production of nickel-on-alumina catalysts from preshaped support bodies L.M. Knijff, P.H. Bolt, R. van Yperen, A.J. van Dillen, J.W. Geus

165

Development of a methodology for investigating the adsorption of species containing catalytically active ions on the surface of industrial carriers N. Spanos, Ch. Kordulis, A. Lycourghiotis

175

Scaling down of the calcination process for industrial catalyst manufacturing G. Groen, J. Ferment, M.J. Groeneveld, J. Decleer, A. Delva

185

Hydrothermal sintering of the active phase in alumina supported fixed bed nickel catalysts during reduction E.K. Poels, J.G. Dekker, W.A. van Leeuwen

205

Catalyst preparation via the sol-gel route The influence of silica support on polymerisation catalyst performance C.E. Marsden

215

Preparation and catalytic effects of Ce0,-MOy-A12@ (M = Ba, La, Zr and Pr) by an improved sol gel method for automotive catalysts K. Masuda, M. Kawai, K. Kuno, N. Kachi, F. Mizukami

229

Influence of preparation parameters on pore structure of silica gels prepared from tetraethoxy orthosilicate B. Handy, K.L. Walther, A. Wokaun, A. Baiker

239

Preparation of catalysts from layered structures and pillaring of clays Aspects of the synthesis of aryl sulfonic acid h4ELS@catalysts D.L. King, M.D. Cooper, W.A. Sanderson, Ch.M. Schramm, J.D. Fellmann

247

Preparation of basic silicates and their use as supports or catalysts G.A. Martin, M.C. Durupty, C. Mirodatos, N. Mouaddib, V. Pemchon

269

Soils as unusual catalysts S.A. Moya, A. Flores, M. Escudey

279

Thermal stability, acidity and cracking properties of pillared rectorite catalysts M.L. Occelli

287

Preparation and properties of large-pore RE/Al-pillared montmorillonite. A comparison of RE cations J. Sterte

30 1

Preparation of pillared montmorillonite with enriched pillars E. Kikuchi, H. Seki, T. Matsuda

311

VII

Intercalation of La203 and La2e-NiO oxidic species into montmorillonite layered structure A.K. Ladavos, P.J. Pomonis

319

Mixed Al-Fe pillared laponites :preparation, characterization and catalytic properties in syngas conversion F. Bergaya, N. Hassoun, L. Gatineau, J. Barrault

329

Zirconium pillared clays. Influence of basic polymerization of the precursor on their structure and stability E.M. Farfan-Torres, 0. Dedeycker, P. Grange

337

Control of the acidity of montmorillonites pillared by Al-hydroxy cationic species D. Tichit, Z. Mountassir, F. Figueras, A. Auroux

345

Preparation and modification of zeolite-based catalysts The chemistry of dealumination of faujasite zeolites with silicon tetrachloride J.A. Martens, P.J. Grobet, P.A. Jacobs

355

Factors affecting the formation of extra-framework species and mesopores during dealumination of zeolite Y D. Goyvaerts, J.A. Martens, P.J. Grobet, P.A. Jacobs

38 1

Treatment of galloalumino-silicate (ZSM-5 type zeolite) with KOH solution. Dispersion of aggregated zeolites into small particles J. Kanai, N. Kawata

397

Design and preparation of vanadium resistant FCC catalysts D.J. Rawlence, K. Gosling, L.H. Staal, A.P. Chapple

407

Double substitution in silicalite by direct synthesis : a new route to crystalline porous bifunctional catalysts G. Bellussi, A. Carati, M.G. Clerici, A. Esposito

42 1

Study on titanium silicalite synthesis M. Padovan, F. Genoni, G. Leofanti, G. Pemni, G. Trezza, A. Zecchina

43 1

Carbon supported catalysts Activated carbon from bituminous coal J.A. Pajares, J.J. Pis, A.B. Fuertes, J.B. Parra, M. Mahamud, A.J. PLrez

439

Carbon-supported palladium catalysts. Some aspects of preparation in connection with the adsorption properties of the supports A.S. Lisitsyn, P.A. Simonov, A.A. Ketterling, V.A. Likholobov

449

Preparation of palladium-copper catalysts of designed surface structure Zs. BodnAr, T. Mall&, S. Szab6, J. Petr6

459

Optimization and characterization of Pt-Fe alloys supported on charcoal P. Fouilloux, D. Goupil, B. Blanc, D. Richard

469

Supported metallic catalysts achieved through graphite intercalation compounds F. Beguin, A. Messaoudi, A. Chafik, J. Barrault, R. Erre

479

VIII

Prepantion of graphite-iron-potassiumcatalysts for ammonia synthesis K. Kalucki, A.W. Morawski

487

Preparation of oxidation catalysts Synthesis of V-P-0 catalysts for oxidation of Q hydrocarbons V.A. Zazhigalov, G.A. Komashko, A.I. Pyatnitskaya, V.M. Belousov, J. Stoch, J. Haber

497

Preparation of well dispersed vanadia catalysts by ultra-high intensity grinding at ambient temperature Z. Sobalik, O.B. Lapina, V.M. Mastikhin

507

Dispersion and physico-chemical characterization of iron oxide on various supports Weijie Ji, Shikong Shen, Shuben Li, Hongli Wang

517

The use of chelating agents for the preparation of iron oxide catalysts for the selective oxidation of hydrogen sulfide P.J. van den Brink, A. Scholten, A. van Wageningen, M.D.A. Lamers, A.J. van Dillen, J.W. Geus

527

Preparation of oxidation catalysts with a controlled architecture Y.L. Xiong, L.T. Weng, B. Zhou, B. Yasse, E. Sham, L. Daza, F. Gil-Llambias, P. Ruiz, B. Delrnon

537

Structure and selectivity changes in vanadia-titania-deNOxcatalysts M. Kotter, H.-G. Lintz, T. Turek

547

Binary oxide catalysts synthesized by sequential precipitation C.S. Brooks

557

Zr@ as a support :oxidation of CO on CrO-

T. Yamaguchi, M. Tan-no, K. Tanabe

Methane oxidative coupling by definite compounds (e.g. perovskite, cubic or monoclinic structure, ...) obtained by low temperature processes J.L. Rehspringer, P. Poix, A. Kaddouri, A. Kiennemann

567

575

Novel and unusual preparation methods Preparation of strong alumina supports for fluidized bed catalysts M.N. Shepeleva, R.A. Shkrabina, Z.R. Ismagilov, V.B. Fenelonov

583

Synthesis and regeneration of Raney catalysts by mechanochemical methods A.B. Fasman, S.D. Mikhailenko, O.T. Kalinina, E.Yu. Ivanov, G.V. Golubkova

591

Controlled preparation of Raney Ni catalysts from Ni2Al3 base alloys. Structure and properties S. Hamar-Thibault, J. Gros, J.C. Joud, J. Masson, J.P. Damon, J.M. Bonnier

601

Novel type of hydromating catalysts prepared through precipitation from homogeneous solution (PFHS) method K. Somasekhara Rao, V.V.D.N. Prasad, K.V.R. Chary, P. Kanta Rao

611

IX

Preparation of manganese oxide catalysts using novel NHqMnO4 and manganese hydroxide precursors. Comparison of unsupported and alumina supported catalysts A.K.H. Nohman, D. Duprez, C. Kappenstein, S.A.A. Mansour, M.I. Zaki

617

Influence of surface OH groups and traces of water vapor during the preparation of Ti@-Si@ samples A. Muiioz-Paez, G. Munuera

627

Catalysts and preparation of new titanates R.G. Anthony, R.G. Dosch

637

New methods of synthesis of highly dispersed silver catalysts N.E. Bogdanchikova, V.V. Tretyakov

647

Preparation of high-surface-area V-Si-P oxide catalysts M. Ai

653

Preparation of fine particles of ruthenium-alumina composite by mist reduction method H. Imai, J. Sekiguchi

66 1

Designed catalysts for hydrodechlorination, reduction and reductive amination reactions J.L. Margitfalvi, S. Gobolos, E. Tilas, M. Hegediis

669

Preparation of high surface area hydrogen-molybdenum bronze catalysts C. Hoang-Van, 0. Zegaoui, B. Pommier, P. Pichat

679

New preparation of supported metals. Hydrogenation of nitriles M. Blanchard, J. Barrault, A. Derouault

687

Preparation of highly dispersed gold on titanium and magnesium oxide S. Tsubota, M. Haruta, T. Kobayashi, A. Ueda, Y. Nakahara

695

Preparation of monodisperse colloidal Pt-Re@ particles using microemulsions A. Claerbout, J. B.Nagy

705

New organometallic active sites obtained by controlled surface reaction of organometallic complexes with supported metal particles B. Didillon, A. El Mansour, J.P. Candy, J.M. Basset, F. Le Peltier, J.P. Bournonville

717

Conversion coatings on stainless steel as multipurpose catalysts L. Aries, A. Komla, J.P. Traverse

729

Author Index

74 1

Studies in Surface Science and Catalysis (other volumes in the series)

745

X ORGANIZING COMMITTEE President Prof. B. DELMON, Universitt Catholique de Louvah Executive Chairmen Dr P. GRANGE, Universitt Catholique de Louvain Prof. P.A. JACOBS, Katholieke Universiteit Leuven Dr G. PONCELET, Universitt Catholique de Louvain Dr P. RUIZ, Universitt Catholique de Louvain

SCIENTIFIC COMMITTEE

Dr D. ARNTZ, Degussa AG, Germany Dr J.L. CIHONSKI, Catalytica, U.S.A. Dr Ph. COURTY, Institut FranGais du Pttrole, France Prof. B. DELMON, Universitt Catholique de Louvain, Belgium Prof. E.G. DEROUANE, Facultts Universitaires N.-D. de la Paix, Belgium Dr T. EDMONDS, BP Research Centre, U.K. Dr J.W. GEUS, Rijksuniversiteit Utrecht, The Netherlands Dr P. GRANGE, Universitt Catholique de Louvain, Belgium Dr J. GROOTJANS, Labofina, Belgium Mr C. HAMON, Zeocat, France Dr H. HINNEKENS, Labofina, Belgium Prof. P.A. JACOBS, Katholieke Universiteit Leuven, Belgium Dr W.T. KOETSIER, Unilever Research Laboratonum, The Netherlands Dr 0. KRAUSE, Neste Oy, Finland Dr L. LEROT, Solvay & Cie, Belgium Dr G. MATHYS, Exxon Chemical International Inc., Belgium Dr T. MEURIS, Belgian Shell, Belgium Dr G. PONCELET, Universitt Catholique de Louvain, Belgium Dr L. PUPPE, Bayer AG, Germany Dr P. RUIZ, Universitk Catholique de Louvain, Belgium Dr P. SCHWARZ, Enichem SPA, Italy Dr M. TOKARZ, Eka Nobel AB, Sweden Dr D. VANDE POEL, Catalysts and Chemicals Europe, Belgium Mr A. VAN GIJSEL, UCB SA, Belgium Dr R. van HARDEVELD, DSM Research, The Netherlands Mr A. VASTEELS, Kemira SA, Belgium Dr D.E. WEBSTER, Johnson Matthey, U.K.

XI

FOREWORD The organizers are pleased to present the Proceedings of the Fifth International Symposium on the "Scientific Bases for the Preparation of Heterogeneous Catalysts". These Proceedings correspond to the fourth organized in Louvain-la-Neuve, the first having taken place in Brussels. Throughout the five symposia, held successively in 1975, 1978, 1982, 1986 and 1990, the organizers have not departed from their initial objectives, namely to bring together experts from both Industry and Universities in order to discuss the scientific problems involved in the preparation of heterogeneous catalysts, and to encourage, as much as possible, the presentation of research work on catalysts which are of real, industrial significance. Indeed, even if industrial researchers have easy access to the work carried out in university laboratories or research centers, the reverse is not always true. But this feedback is nonetheless indispensable, as the university staff is not always sufficiently aware of the needs of industry and of the problems encountered in the preparation of real catalysts, which correspond ultimately to the most challenging issues. This is one of the reasons why at least 50% of the members of the scientific committees have always come from industrial research and development organizations (at this symposium, 20 out of the 27 members came from industry). This major goal of linking Industry and Universities was partly fulfilled at the Fifth Symposium : indeed, out of the 338 participants, 182 belonged to industry. Although only 25 abstracts were submitted by industrial laboratories, the quality of the corresponding work was outstanding : 17 were selected by the Scientific committee. Another established highlight of these symposia is the reservation of a substantial part of the program to new developments in catalyst preparation, new preparation methods and new catalytic systems. Indeed, the fact that chemical reactions which were hardly conceivable a few years ago have now become possible through the development of appropriate catalytic systems proves that catalysis, like all industrial and academic activities, is in a constant state of progress. Because of the very large number of submitted abstracts (234), the unanimous wish expressed by the Scientific Committee to avoid parallel sessions, and the desire to accept the largest possible number of contributions which could be accomodated in a reasonable sized volume of the Proceedings, it was decided to organize a poster session, to suppress the half-day session devoted in previous symposia to normalization methods, and not to print the discussions. This decision allowed us to accept 70 papers, half presented orally, the other half as posters. In these Proceedings, the papers (including three extended communications) are grouped under the following headings : . Studies of unit operations in catalyst preparation (19) . Catalyst preparation via the sol-gel route ( 3 ) . Preparation of catalysts from layered structures and pillaring of clays (10) . Preparation and modification of zeolite-based catalysts (6) . Carbon supported catalysts (6) . Preparation of oxidation catalysts (9) . Novel and unusual preparation methods (17)

XI1

Finally we would like to express special thanks to 26 industrial companies for their financial support, and especially to Catalysts and Chemicals Europe who generously provided the reception on the occasion of their 25th anniversary. The financial contribution of these companies permitted us to rearrange the budget. In this way, their support allowed several participants from countries with economical difficulties to benefit from financial aid so that they could attend the Symposium and present their communication.

Prof. B. DELMON Dr P. GRANGE Prof. P.A. JACOBS Dr G. PONCELET

XI11 ACKNOWLEDGEMENTS

The Organizing Committee thanks Professor P. Macq, Rector of the Universite Catholique de Louvain, who allowed the Fifth International Symposium to be held in Louvain-la-Neuve. We also gratefully acknowledge the University Authorities for providing us with facilities, and in particular Dr L. Van Simaeys, Head of the Library of Sciences, who provided us with the lecture room where the Poster session was organized. The organizers also thank Professor V. Hanssens for his welcome address to the participants. At this Symposium, even more than in the previous ones, the members of the Scientific Committee were faced with a very difficult task in selecting the communications. They are all most sincerely thanked for the outstanding job which they accomplished. The Organizing Committee gratefully thanks the authors of the 240 submitted abstracts, those who contributed an oral or a poster presentation, as well as those whose contribution could not be selected, mainly because of the limitations of time and space. The Organizers are pleased to thank the authors of the stimulating extended communications, and in particular Dr J.P. Reymond, Dr G. Groen, Dr D.L. King and Dr K.P. de Jong for their excellent oral presentations. Sixteen people deserve special thanks for their performance as session chairmen during the symposium : Dr D. Arntz, Dr J. Cihonski, Prof. E. Derouane, Dr E.B.M. Doesburg, Prof. J. Geus, Dr C. Hamon, Mr K. Johansen, Dr G . Mathys, Prof. J. B.Nagy, Prof. J.T. Richardson, Dr D.S. Thakur, Dr D. Van de Poel, Dr D.E. Webster and Dr F. Wunde. The hostesses of the REUL (Relations Extkrieures de 1'Universitk de Louvain), and particularly Mrs F. Volon-Bex, are congratulated on their perfect achievement. We also want to extend our gratitude to Mr M. Van Windekens, of the "Service du Logement", for his dedication to the symposium, We also owe our particular thanks to the secretaries, F. Somers, M. Saenen and especially P. Theys who had the hidden part of the organization of the symposium in their charge, from its inception to its end. Finally, the Organizers want to mention in their acknowledgements all the people from the "Unit6 de Catalyse et Chimie des MatCriaux DivisCs" and the "Centrum voor Oppervlaktechemie, K.U. Leuven", who contributed to the success of the symposium, in particular : F. Bautista, N. Blangenois, R. Castillo, S. Colque, L. Daza, S. Giraldo, E. Lament, R. Maggi, H. Matralis, R. Molina, S. Moreno, E. and N. Paez, C. Papadopoulou, G. Pelgrims, E. Ponthieu, L. Portela, M. Remy, P. Ruiz, M. Ruwet, R. Sosa, M. Tielen, A. and M. Vieira Coelho, and L.T. Weng.

XIV

FINANCIAL SUPPORT

The following companies agreed to provide financial support to the Fifth Symposium. The Organizers are grateful to them for their generosity.

AKZQ Catalysts AUSIMONT CATALEZATORI BRITISH PETROLEUM International Ltd. CATALYSTS AND CHEMICALS EUROPE DEGUSSA AG DOW BENELUX B.V. DSM Research EKA NOBEL AB EXXON CHEMICALS INTERNATIONAL HALDOR TOPSOE A/S JOHNSON MATTHEY CHEMICALS Kontaktgruppe Forschungsfragen (CIBA-GEIGY AG, HOFFMANN-LA-ROCHE AG, LONZA AG, SANDOZ AG) LABOFLNA S.A. METALLURGIE HOBOKEN-OVERPELT MONSANTO EUROPE NORSK HYDRO PROCATALYSE REILLY CHEMICALS REPSOL PETROLEO TEXACO TOLSA UNION CHIMIQUE BELGE (UCB)

The Netherlands IdY U.K. Belgium Germany The Netherlands The Netherlands Sweden Belgium Denmark U.K. Switzerland

Belgium Belgium Belgium Norway France Belgium Spain U.S.A. Spain Belgium

The Organizing Committee would like to especially thank CATALYSTS and CHEMICALS EUROPE for the reception which they generously offered on the occasion of their 25th anniversary. We are also grateful to ENGELHARD-DE MEERN (The Netherlands) for supplying the conference folders.

G . Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors),Preparation of Catalysts V 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

1

ILLUSTRATION OF PROCESS SCALE-UP IN HETEROGENEOUS CATALYST PREPARATION I. BIAY, G . DESSALCES, C. HYPOLITE, F. KOLENDA* and J.P. REYMOND "Labomtoire GCnie de la Fabrication des Catalyseurs Hitiroghes" UM36 CNRS-IFP ; IFP-CEDI - BP 3 - 69390 VERNAISON (France) *Institut Franqais du Pitrole, IFP-CEDI - BP 3 - 69390 VERNAISON (France) SUMMARY The command of heterogeneous catalyst preparation and process scale-up from laboratory to industrial plant requires research at a pilot scale. This paper presents the methodology followed to study catalyst preparation. The case of amorphous silica-alumina dried solids, involving a sol-gel route is here developped. Influence of operating variables on catalyst properties (i.e texture) at each stage of the process and scale effect are analyzed. Pilot plant runs point out operating problems, equipment effects on product specifications and simulate key steps in manufacturing process.

INTRODUCTION The world market for heterogeneous catalysts is expected to reach 4billion dollar global business (1). Despite a slow growth and small profit expectation, catalysts suppliers andusers arevery active in new catalyst developments. This can be explained by the strategic role a catalyst plays in production sales (2). New chemical products are requiring more elaborate catalysts. Heterogeneous catalysts can be compared to high performance materials which greatly enhance production process competitiveness andeconomics. A new catalyst can boostprocessefficiency andreduce production costsormakeitobsolete (3) In spite of numerous scientific studies (4-6), the industrial preparation of heterogeneous catalysts is regarded asempirical and still remains an "art" proceeding through a "know-how" jalously kept by the catalyst manufacturers. As aresult, it is crucial to master catalyst preparation and scale-up from the laboratory to the manufacturingplant (7). Each step of theprocess ofpreparation must be characterized not only with physical and chemical techniques but also by using chemical engineering science (8). Physical (heat, mass and momentum transfers) and chemical phenomena (reactions, kinetics) of each unit operation are pointed out and studied using chemical engineering concepts (9). These determinations are performed at the laboratory and then reproduced on a pilot plant. The size of each element of thispilotplantmust bechoseninorder to takeintoaccount limitingeffectssuchaswalleffect orsizeeffect(l0). Pilotplantpreparation using industrial equipmentis anecessaryintermediate steprequiredforasucessfull scale-up of the process. It offers the following assets.

2

I. Preparation of prototvpe catalvsts which will be as similar as possible to the industrial catalyst in terms of activity, selectivity, life time performance, shape and evaluation of the production cost. 2. Simulation of the kev sm in the manufacturingprocess and quantificationof the influenceof each step on the catalyst characteristics (porosity, surface area, chemical composition, bulk properties).

3. Establishment of extrapolationrules using chemical engineering concepts.

4.Definition of product suecificatioqchart for each manufacturingstep in order to assure catalyst performance and reproductibility.

5. Avoidanceof process scale-uu pitfalls, such as mixing problems,raw material purity and availability, fluid rheology which can make fluid transport impossible,solid handling...which cannot be clearly shown during the earIy phases of the catalyst development in the laboratory (7). Advantages and valuable information brought by pilot scale catalyst preparation are illustrated in this paper. Itrepresentspartofthe workcarried out by ourresearchp u p which associatesthe"Institut FranCais du P6trole" and the "Centre National de la Recherche Scientifique".

I - PROCESS SCHEME

In the field of heterogeneouscatalysts, silica-alumina are widely used as acid supportsor catalyst matrices. Their preparation involves a large number of unit operations, usualy present in catalyst manufacturing processes, such as precipitation or gelation, filtration, drying, ion exchange...These unit operations are gathered on figure 1. Figure 1- Example of unit operations in catalyst elaboration

.PRECIPITATION .GELATION

.HYDROTHERMAL TREATMENT .SOLID-LIQUID SEPARATION

.WASHING IMPREGNATION .ION EXCHANGE .KNEADING, COATING .CRYSTALLIZATION .DRYING .CALCINING GRINDING, SOLID CLASSIFICATION .SHAPING

.

.

3

Intensiveuse of fluid bed type reactors led us to study the preparationof solidsused in suchcatalytic reactors. In this paper we examine the preparationof spray-dried silica-aluminamatrices at the laboratory (several hundred of grams) and on a pilot plant (several tens of kilograms). The figure 2 shows the chosen process scheme. The first step of this industrial process is the silica hydrogel formation obtained by successive additions of reactants (batch operation) using a sol-gel technique. A dilute sodium silicate solution is partly neutralized by adding a sulfuric acid solution to form colloldal particles (sol state) which link together giving a tridimensional network (hydrogel state). Addition of aluminiumsulphatefollowedby pH adjustmentof the suspensionwith ammoniainduces alumina precipitation and its incorporationwith silica. A catalytically active phase or other solids such as clays can be added to the hydrogel. This hydrogel is then filtered, washed and repulped in Figure 2 - Catalyst elaboration process

SODIUM SILICATE

ACID

7

CLAYS

-----

SILICA HYDROGEL I

SILICA - ALUMINA HYDROGEL

I

FILTRATION ,WASHING SPRAY - DRYLNG IONIC EXCHANGES FINAL DRYING

I

5

water to make it pumpable. The suspension is spray-driedin order to produce spherical particles which are washed and dried. At the laboratory such a preparation takes place in a five liter s t i d glass vessel. This vessel is equipped with sensors (pH, temperature, stirrer torque) which allow the control of the gelation. Filtrations and washings are performed on a vacuum filter. The filter cake is repulped in water and the suspension is dried in a laboratory spray-dryer (3 kg/hr water evaporation rate). This dryer produces 20-30 micrometers diameter particles with 15-25 % moisture content. A scheme of the pilot plant unit dedicated to the preparation of the catalysts is shown on figure 3. The four first steps of the process (gelation, filtration, drying and washing) arerepresented. The final stage of the preparation which could involve drying alone or drying and calcination depending on the application, is not covered here. All the equipments are of industrial type. The size of each piece of equipment (agitators, filters, dryers, pumps ...) has been set by the need to extrapolate the manufacturing process to industrial scale. Gelation is performed in a stainless-steel reactor (1000 liters) equipped with sensors allowing us to follow the hydrogel formation (pH, temperature, stirrers rotary speed, power consumption). The hydrogel is filteredon avacuumdrumfilter. The filter cake (= 15 % solidcontent) is repulped with water and the suspension is dried in a pilot spray-dryer (100 k g h r water evaporation rate at 400°C). The spherical particles of 65 pm average diameter contain 15-25 % of residual water. Centrifugal atomization is carried out using a vane atomizer wheel. The impureties contained in the spray dried particles (sulphates) are removed by successive filtrations and washings of aqueous suspensions of these particles. These steps are simultaneously operated on a vacuum belt filter. The last step of the process consists in drying the washed particles at 350OC.

II - EXPERIMENTAL METHODS 1. Analvtical methods Sampling procedures including pretreatments have been defined in order to obtain representative samples of solids, liquids and suspensions by means of grinder, spinning rifler, sampler probe.,.. 1.1. Chemical analvsis and moisture content Inductively coupled plasma (I.C.P.) and atomic absorption quantitative chemical analysis of Na, Si, Al, S and Nare achieved by a C.N.R.S. laboratory, the "Service CentraldAnalyses (S.C.A.)". Mass balances of all catalyts preparation steps are based on these chemical analysis.

6

1.2. Textural and momholoeical determinations Pore texture of a catalyst (i.e. pore size distribution P.S.D., total pore volume and surface of the pores) governs catalytic performance such as activity and selectivity, through diffusion of reactants and products in the pore system of the solid, density, mechanical strength and thermal stability (5, 1 1, 12). 1.2.1. Thermooorometry The hydrogel formation and the spray-dryingsteps are the key operationsof the process in terms of influenceon the catalyst properties (texture and morphology). It is important to measure the influence of operatingvariables for each step on the pore textureof the catalyst. The difficulty arises from the necessity of measuring the porosityof an hydrogel (95 % water and 5 % solid) and of axemgel (drysolid particles) with the same technique. To our knowledge, thermoporometry is the only method which can apply (13,14). It is acalorimetrictechniquebased on the measurementof the temperatureof solidification of aliquidconfined or divided into a porous texture. Pore diameters ranging from 2 to 150nm (mesopores) can be measured.

1.2.2. Other textural methods The texture of solids is evaluated using well-known techniques : - structural density : helium picnometry - total pore volume of xerogels : high pressure mercury porosimeter - surface area and pore diameter distribution : nitrogen adsoption (B.E.T. and B.J.H. methods)

colloidal silica particle surface area as well as xerogel silica particle can be measured by the Sears' analytical method (15). -

Typical silica-alumina we producehavea specificarea of 250-600 m2g-l, a skeletaldensityof 2,l2,4g. cm~3,aporevolumeof0,5-0,9cm3-g~1 (poresofdiameterlessthan7,5pasdeterminedbymercury porosimetry). 1.2.3. Morphologv and size of particles Optical and scanningelectronicmiscrocopiesgive information on the morphology and the size of solid particles. The photographies of figure 4 show that silica (photo a) and silica-alumina (photo b) particles we obtain are quite spherical. Silica-aluminaparticles seem to have arougher surface than silica particles. Size distribution of powdery raw materials,suspensionsand xerogels are determined by laser diffraction (size range : 1,2-560 pn). Size measurements of powders are performed on either aqueous suspension and dry aerosols (interactions between water and dried particles can result in breakage of particles).

7

Figure 4

a) spray-dried silica

b) Spray-dried silica-alumina

Diameter of droplets generated by the cenb-ifugalatomizing device have been measured with this technique and results are discussed further in the text. 1.2.4. Other characterisations The knowledge of therheological behaviour of hydrogels is very important for pipe, pump and mixer sizing and design. It allows also a theoretical approach of internal structure of gels and interaction between gel particles. Hydrogel flow curves are established by means of a rotational rheometer. During the gelation viscosity evolution of the fluid is followed in the reactor by means of a torque sensor set up on the agitator shaft (laboratory reactor). Kinetic study of the filtration and establishment of mass balances during this unit operation allow the determination of the specific resistance, the compressibility and the filtrability coefficients of the filtration cakes (16). The optimisation of pilot filter operation can be. deduced from these determinations. Theresistance to attrition is an important characteristic of catalysts usedin fluid beds (5). This mechanical property is evaluated with an air jet Gwyn-type-apparatus (17).

8

I11 - RESULTS AND DISCUSSIONS The process involves two key steps which need to be mastered:

- the gelation step which gives to the catalyst its textural and catalytic characteristics - the spray-drying step which gives to the catalyst its morphology (shape and size of the particles).

The results presented is this paper concern these two steps. Catalytic activity of the solids will not be described here. 1. Textural studv

The influenceof operating variableson the silicaand silica-aluminatextures isquantified by means of thermoporometry. During the gelation step these variables are :

. pH of the sol-gel transition

. temperature of hydrogel formation . mixing . batch or continuous preparation . percentage of alumina . concentration of reactants . aging time of hydrogels. We also study the influence of drying on the textural characteristics of silica and silica-alumina. Figure 5 shows the differences observed between the textureof silica and silica-alumina hydrogels. Silica presents a very narrow pore size distribution (2-6 nm) with a median pore radius around 3 nm and pore volume of 0,5 cm3 g-l. The addition of alumina broadens the P.S.D. which ranges for silicaalumina from 2 to 20 nm and pore volume (1- 1,4 cm3 g-'). The figure 6 illustrates the effect of spraydrying on the texture of silica and silica-alumina ( 25 % of alumina). Drying results in an important skrinkage and noticeable reduction of pore volume and median pore diameter for the two-types of hydrogels. The effect of drying is more drastic on the silica-aluminagel. Drying tends to minimize the influenceof each operating variables of the precipitation step. Nevertheless xerogels are similar to initial hydrogels in terms of texture.

9

-

Figure 5 Comparison of pore size distribution (cumulativecurves) of silica and silica-alumina hydrogels

" .

5

0

10

I

20

15

R (nm.)

Figure 6 - Effect of drying on pore size distribution of hydrogels

- .- -

Si02 +A1203 hydrogel (I) Si02 hydrogel (2) 5 0 2 xerogel(3)

-I--

1400 1

1200 1000 -

> 600 -

0

5 R (nm.)

10

15

3

10

The figure 7 shows the scale effect on the texture of hydrogels prepared at laboratory and on pilot plant. No differences can be observed on silica-aluminasamples (curves 3 and 4). On the contrary P.S.D. of pilot plant silica sample is broader than P.S.D. of laboratory silica sample (curves 1and 2). Differences in mixing efficiency of each reactor are probably the cause.

Figure 7 - Comparison between laboratory and pilot plant preparation

------ --

SiO2 laboratory ( I ) Si02 pilot plant ( 2 ) SiO2lAUO3 laboratory (3)

0

5

R (nm.)

15

10

20

TABLE 1 Effect of subhates on Dorous texture of silica gel filtration

gel washing

Hydrogel

X

X

Xerogels

X X

X

particle washing

X X

pore surface m2.g-1

radius nm

659

476

3,4

236 233 137

282 237 117

22 22 2,1

pore volume mm3.g-1

11

TABLE 2 Effect of subhates on vorous texture of silica-alumina gel filtration

Hydrogel Xerogel

gel washing

X

X

X

X

X

particle washing

X X

porous volume -34-1

porous surface m2.g-1

radius nm

1561

601

4.4

500

373

2s 23 2,1

267

345

239 308

Thermoporometrycan indicate optimunway and timing to realize an operationalong the manufacturing process. An example is provided by theelimination of sulphatesproduced during silicaand alumina precipitations. Table 1 and 2 show that silica and silica-aluminatextures are depending on the presence of sulphatesduring the drying. In tables 1and 2, the crosses indicate the operations (column) realizedon samples(line). In each table comparison between the hydrogel andcorrespondingxerogels shows the influence of drying. Comparison between xerogels shows the influence of the presence of sulphatesduring the drying. It has been checked that xerogelparticles washingdoes not affect theirporous texture. The texture of silica xerogels is detrimentallymodified by the presence of sulphates (18). Sulphatescristallizationanddeposition duringdryingdecreasethe pore diameter and must be avoided. Sulphatesmust be washed out before the drying of silica hydrogels in order to preserve the hydrogel texture. The same observations can be made for silica-alumina (see table 2).

2. Influence of rheolotkal behavior of eel on meparation Simulation of catalyst preparation in the laboratory and then at a pilot plant are complementary. Pitfalls along the way can lead to unsucessfullscale-up. These problemscan only be studiedat the pilot plant where all the equipment used is of an industrial conception. One of the major problems encounteredwith hydrogelsconcerns their ability to be transfered from one vessel to another. These problems are not observedin the laboratory and can result in huge operating difficultiesin industrialplants if not studied carefully. Study of the hydrogel rheology is very important for the design of agitators, pump selection and pipe sizing.

The figure 8 shows hydrogel rheograms for silica and silica-alumina prepared at the pilot plant. Hydrogels are non-newtonian plastic fluids of Bingham type and can be described by the followingrelation:

12

-

Figure 8 Rheograms of silica and silica-alumina hydrogels

-

SIUCA GEL

80 I60 240 SHEAR RATE(S-1)

320

4

zc (yield stressvalue; Pa), the minimum stress to develop in order to establish flow, is acrucial characteristic for scaling.

+, is theplastic (orBingharn) viscosity

(Pa.s) and y is the shear rate(s'*).

A slight hysteresis can be observed, showing a slight thixotropy. As a result from these measurements the minimum pressure drop,

AP

(TImi*

by unit lenght in a pipe of diameter D, needed to establish the flow can be calculated from AP

Fluids after gelation have low stress values due to a low solid content (=5 %). After filtration and repulping, zc is noticeably modified and strongly related to the solid content in the hydrogel. The Bingharn viscosity remains small (few mPa.s). The effect of the solid content in silica-alumina hydrogels is illustrated on figure 9. The value of zc increases from 2 Pa to 20 Pa if the solid content increases from 7 3 to 15%. After a certain level, an increaseof 0,l percent in the solid content can cause a drastic change of zc which must be controlled during the repulping of the filtration cakes (figure 10). Pressure drop is affected in the same way and can reach high values, as shown on figure 10, depending

13

-

Figure 9 Influence of water content of a silica-alumina hydrogei on its rheologkal properties

--------

25

84.89 % of wafer 8S.69 % of water 86.23 90ofwater 87.98 90ofwater

I

20

0

I

I

30 60 90 SHEAR RATE(S-I) Figure 10

- Repulping test

REPULPING TEST [3 RHEOLOGY MEASUREMENTS

50.0

10.0 Q

5.0

crw 4 4

I.0

9 0.5 rn 0.I

120

I50

14

on pipe diameter and pipe lay out. A bad control of the solid content can result in an inability to pump the fluid at the suction of the pump or no flow at all at the discharge side if pressure drops are excessive. Pilot plant can also avoid pitfalls in equipment selection. The two rheograms on figure 11 show how a screw type pump can affect the rheological properties of silica-alumina hydrogel. The shear stress imposed by the rotor-stator couple of the pump results in an increase of the yield value of the hydrogel. In the pump, the size of the particles which compose the hydrogel is decreased by a milling effect and the forces of cohesion are increased. This can stop the flow if the pump characteristics have been underestimated. This results also in modification of the gel structure and ability to be spray dried.

-

Figure 11 Pump effect on the rheological properties of a silica-alumina hydrogel

-

SUCTION SIDE

3. Studv of the smav-drving The hydrogel suspension is atomized using centrifugal force into a hot air stream. The drying step is rather fast (few seconds to 30 seconds). Residual moisture of the spherical particles is still high (around 20 % wt). It is necessary to master the shape, the size andresistance to attrition of spherical particles produced by the dryer. The properties of the solid are determined by the action of two phenomena: - droplet formation, which depends on gel composition, viscosity, surface tension, density and the pulverisation technique.

15

- drying phase which is the result of interactions between acontinuous phase (hot air stream) and adiscontinuous, highly dispersed phase (droplets of hydrogel). The efficiency of the drying will depend on the droplet size, hydrogel properties (texture, composition), hot air characteristics (temperature, flow rate) and air hydrodynamics in the drying chamber. It is important to understand and quantify these underlying phenomena in order to master spray drying process and its extrapolation. The centrifugal atomizing technique produces spherical particles with a size distribution between 20 to 150 microns as demonstrated on figure 4. Research is being conducted tocorrelate physico-chemical properties of the feed to droplet size distribution at the exit of the atomizing device. These experiments are conducted in a mock-up. Atomizing wheel dimension and rotational speed can be adjusted in order to have a peripheral speed of ejection varying between 50 and 100m.s-l, common valuesencountered in industrial practice. Droplet sizing is realized usinglight scattering technique. The laser beam goes through perpendicular to thedroplet umbrella created by the turbine. The figure 12 shows droplet size distribution for water and hydrogel suspension. The atomizing speed is 80 m.s-l and the feed flowrate 3 kg.hr-l (curves 1 and 2). Curve 3 represents the particle size distribution of a xerogel, which is produced from the hydrogel of curve 1dried in the laboratory spray dryer using the atomizing device described here.

-

Figure 12 Diameter distribution (cumulative curves) of droplets geuerated by atomizing vaned wheel

100 90 80

70 60 50

€3 20 10

0 5

10 50 100 DIAMETER (micron)

500

16

Droplet size distribution of atomized water and hydrogel are quite similar. These two-fluids (a newtonian one and anon-newtonianone) have same surfacetension and, from ourresults, we think that their apparentviscosities arevery similarat the high yiel stressexisting at the atomizingwheel periphery.

Comparison of curves 1 and 3 shows that drying does not affect droplet size distribution and that phenomena such as coalescenceof dropletduringdrying are not very importantfor this kind of product. Correlationscan be establishedbetween droplets and dried particle size distributions, for spray dryers of different sizes.

CONCLUSION The formulation of an industrialcatalyst dependson the choice of catalytic reaction and reactor design (19). Once the formulation has been determined, the first step of the catalyst manufacture is the choice of the type of preparation. Laboratoryexperimentsallow to specifyinfluencesof operatingvariables on the catalystcharacteristicsanddefine processoperation specifications. Pilot plantpreparation is the necessary and complementarystep, devoted to check faisability and reproductibilityof the chosen process using industrial types of equipmentsas well as pointing out scale effects and operating problems. This paper is an attempt to illustrate this methodology through the elaborationof silica-alumina microspheres usable in fluid bed reactors. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14

15

i6

S. Wilkinson and D. Hunter, Chem. Week, 144, (1989), 24-40. L.L. Hegedus (Ed.), "Catalyst Design: Progress and Perspectives", J. Wiley and Sons, NewYork, (1987), p 1-10. M.M. Van Kessel, R.H. Van Dongen and G.M.A. Chevalier, O.G.J., Feb. 16, (1987), p 55. B. Delmon and Coll. (Eds), "Preparationof Catalysts: ScientificBases for the Preparationof Heterogeneous Catalysts"; vol. I, 11, 111and IV, Elsevier, Amsterdam, 1975, 1979, 1983, 1987. J.F. Le Page and Coll. (Eds); "Catalysede Contact : Conception et Mise en Oeuvre des Catalyseurs Industriels", Editions Technip, Paris, 1978. D.L. T r i m , "Design of Industrial Catalysts", Chem. Eng. Monographs 11, Elsevier, Amsterdam, 1980. E.F. Sanders and E.J. Schlossmacher,in B.E. Leach (Ed.),"Applied Industrial Catalysis", Academic Press, London, 1,(1983),pp. 31-40. P. Trambouze, J.P. Reymond,D. Vanhove and F. Kolenda, Information Chimie n0294, (1988), pp. 275-282. J.N. Fulton, Chem. Eng., July 7, (1986), pp. 59-63. P. Trambouze, Chem. Eng. Progr., 86, (1990) pp. 23-31. K.D. Ashley and W.B. Innes, Ind. Eng. Chem., 44, (1952), pp. 2857-2863. 0. Levenspiel (Ed.), "The chemical Reactor Omnibook", OSU Book Stores Inc., Corvallis, Oregon, (1984), ch.23. M. Brun, A. Lallemand, J.F. Quinson and Ch. Eyraud, ThermochimicaActa, 21, (1977),pp. 5988. J.F. Quinson, M. Brun, in K.K. Unger and Coll. (Eds), "Characterization of Porous solids", Elsevier, Amsterdam, (1988), p 307-315. G.W. Sears Jr., Anal. Chem., 28, (1956) pp. 1981-1983 P. Rivet, "Guide de la skparation liquide-solide", SociCtC Franqaise dz Filtration, Idexpo, Ed. Cachan, (1981

17

17 J.E. Gwyn, A.I.Ch.E., 15, (1969) 35-39. 18 M.E. Winyall, in B.E. Leach (Ed.), "AppliedIndustrial Catalysis", Academic Press, London, 3, (1984), ch. 3, pp. 43-62. 19 R. Montarnal and J.F. Le Page, in B. Claude1 (Ed.), "LaCatalyse au Laboratoire et dans 1'Industrie", Masson Paris, (1967), pp. 231-287.

This Page Intentionally Left Blank

G.Poncelet,P.A.Jacobs,P.Grange and B. Delmon (Editors),Preparation of Catalysts V 0 1991 Elsevier Science PublishersB.V., Amsterdam - Printed in The Netherlands

19

DEPOSITION PRECIPITATION ONTO PRE-SHAPED CARRIER BODIES. POSSIBILITIES AND LIMITATIONS

K.P. de Jong Koninklijke/Shell-Laboratorium, Amsterdam

1003 AA

Amsterdam, The Netherlands.

(Shell Research B.V.) P.O. Box 3003,

ABSTRACT Several features of deposition precipitation onto pre-shaped carrier bodies are dealt with. Firstly, the kinetics and mass transfer effects of catalyst synthesis via deposition precipitation onto pre-shaped and powder carriers have been studied under pseudo-stationary conditions. The precipitation of manganese hydroxide onto silica by urea hydrolysis has been used as a model reaction. The overall disappearance of manganese ions from the aqueous solution could be described as a first-order process. The rate-determining step for Mn deposition is related as expected - to the urea hydrolysis. From the distribution of Mn over the silica granules after precipitation the rate constant for the surface deposition process has been determined. The latter process has a much higher rate constant than the rate-determining hydrolysis reactions. The surface reaction appears to determine the ultimate distribution by a combined process of reaction and diffusion. The consequences of this study for the viability of deposition precipitation onto pre-shaped carriers for practical application are addressed. Secondly, transient phenomena taking place initially during deposition precipitation onto large carrier particles have been considered. More specifically, the occurrence of transient pH gradients over carrier bodies directly after liquid imbibition has been utilized to control the local rate of precipitation in carrier bodies. This could be effected by applying a precipitation reaction whose rate depends on the pn. It has been found that thereby nonuniform activity distributions can be realized in a controlled manner. The method seems to be especially useful to produce egg-yolk type catalysts by the application of redox reactions for ureciuitation. Several catalvst formulations _. have been prepared via this novel synthesis method, viz. Mo/Si02, Cu/A1203 and Ag/A1203.

INTRODUCTION Catalyst synthesis by means of deposition precipitation comprises application of the active component onto an existing support via a chemical reaction. This reaction gives rise to the formation of an insoluble compound which involves the active element. The insoluble compound can be formed by an increase of the pH of the solution (hydroxide precipitation), a valence change of the element in question and the like. Under certain conditions, such as a suitable interaction between the compound and the carrier, the preparation

20 method has a number of unique features, e.g. a uniform distribution of the active component over the carrier surface even at high loadings. The deposition precipitation method has been known for quite some time; its history is summarized in Table 1. It has been extensively studied by Geus and co-workers ~51. TABLE 1 History of deposition precipitation. Year

Assignee/author

~~~

~~

Remarks

Reference

~

1943

IG Farben

Precipitation of (hydr)oxides, sulfides, selenides

1

1967

Stamicarbon

Precipitation with urea from homogeneous solution; powder carriers

2

1970

Unilever

Precipitation with urea from concentrated solutions; prevention of evaporation

3

1970

Stamicarbon

Precipitation with reduction reaction from homogeneous solutions; powder carriers

4

1983

Geus et al.

Basic studies on deposition precipitation with powder carriers

5

1988

Shell

Controlled non-uniform activity distributions 6 with large, pre-shaped carriers

The work of Geus et al. [5] on deposition precipitation has been directed towards the use of support materials consisting of small particles (powders, particle size smaller than say 1 pm). For practical applications, deposition of the active component onto powders implies that shaping of the small particles into larger bodies has to be done afterwards. The shaping procedure might invoke a number of possible disadvantages such as occlusion of the active component and a low mechanical strength. Therefore, the scope of deposition precipitation for the production of uniform metal distributions would be enhanced considerably if it were applicable with larger (pre-shaped) carrier particles. There are indications in the patent literature [ 3 ] that this is indeed possible. Recent publications [7,8],on the other hand, suggest that with medium-size silica particles (50-100 p m ) precipitation of nickel hydroxide via urea hydrolysis gives rise to significantly enhanced, but uncontrolled concentrations of the active component at the outer surface of the silica particles. In this paper we report on some work undertaken to assess the scope cf deposition precipitation onto large, pre-shaped carrier particles (about 1 mm).

21 Specifically two aspects are addressed: (i) the kinetics and mass transfer effects under pseudo stationary conditions, and (ii) the utilization of a unique feature of precipitation onto large carrier particles, viz. transient concentration gradients occurring directly after liquid imbibition. The kinetics and mass transfer effects are studied with the often considered method of precipitation of metal hydroxydes via a controlled increase of the pH of the solution by the hydrolysis of urea [5,9]. The deposition of manganese hydroxide onto silica was chosen as a model reaction because of its relative simplicity (e.g. no formation of silicates, no formation of basic carbonates). The kinetics o f the relevant reactions have been established and the mass transfer effects during precipitation assessed. Based on the quantitative data a model for the precipitation reaction is formulated. Implications for precipitation onto large carrier particles are addressed. In the course of our work on deposition precipitation onto pre-shaped carrier particles we have found that under certain circumstances the technique can also be used for realizing controlled non-uniform activity distributions over carrier bodies [ 6 ] . After careful consideration it turned out that the basic phenomenon underlying this finding is connected with transient pH gradients occurring in carrier particles directly after liquid imbibition. In case the rate of the precipitation reaction in question depends on the pH of the aqueous phase, the metal (oxide) distribution after precipitation will reflect the previous pH gradient. In this paper the principles of this new application of precipitation reactions in catalyst synthesis are elaborated. Furthermore, several examples of catalysts with their respective metal distributions are described. EXPERIMENTAL Precipitation of manganese onto silica (pseudo-stationarv) Precipitation of manganese hydroxide in the presence of silica was brought about by the hydrolysis of urea according to the overall reaction Mn2+

+

CO(NH2)2

+

3 H20

__ > Mn(OH)2

+

C02

+ 2 NH4'

To this end Mn(N03)2.6H20 was dissolved in 250 ml water; the solution was acidified with 1 ml of nitric acid ( 6 5

%)

and transferred to a double-walled,

thermostatted vessel equipped with baffles and a stirrer. Five gram of silica was added to this solution in experiment U30 and U31, while experiment U33 was carried out without silica and experiment U34 with 10 g of silica. For further details on the concentrations of reactants the reader is referred to Table 2. After heating the mixture to 90

OC

the urea was added at time zero (cf.

Figs. 1-5). At that temperature the reaction was continued for several hours while nitrogen was bubbled through (to prevent oxidation of the di-valent

22

manganese precipitate) and the pH monitored continuously. At certain intervals of time a small sample (about 2 ml) of the liquid was withdrawn, filtered, weighed and acidified. The total manganese content of these samples was determined using Atomic Absorption Spectrometry. From these data the concentration

of manganese in the solution was calculated. TABLE 2 Experimental conditions and kinetics of the precipitation of manganese. kl is the first-order rate constant of the urea decompodition. kt is the first-order rate constant of the manganese precipitation. Exp.

Carrier

Temp. Mn2+ conc. (OC) (mmol .I-')

U30 granulesa) U31 powderb) u33 ~ 3 4 granulesa)

90.5 90.5 88.5 88.5

Urea conc. (mmol.1-1)

9.1 9.1 36 36

kt *lo5 (s-1)

46 46 182 182

3.3 3.3

1.1 2.7

n.d. = not determined a) Grace Davison silica gel: 0.6-1.4 mm, pore volume 1.2 ml.g-', area (BET) 310 rn2.g-'. b) Degussa Aerosil: surface area (BET) 2 8 1 m2.g-'.

kl*

lo5

(S-5

n.d 3.8 2.7 2.7

surface

Precipitation of molybdenum onto silica spheres (transient phenomenal As has been reported before [4], molybdenum can be deposited onto silica powder via the redox reaction: 4M004~-+ 3N2Hq

+

4H20 ->

4Mo(OH)3

+

3N2

+

8OH-

A molybdate solution was prepared by dissolving the appropriate amount of (NH4)6M07024.4H20 and adding concentrated ammonia till pH=8.7. A hydrazine solution of similar pH was obtained by neutralizing N2H5QH with acetic acid. Further experimental details can be found in Table 3. The procedure of contacting the carrier with the solution is further described below. Experiment RK15 was done by first contacting the silica spheres with the Mo solution (0 OC), followed by addition of the cooled hydrazine solution. Subsequently, the mixture was rotated in a round-bottomed flask under nitrogen and heated slowly. In experiments RK29, FX33 and RK37 (Table 3) a mixed Mo/N2H4 solution was applied. This solution (0-10 OC) was circulated for several hours through a small vessel, which was maintained at higher temperatures and contained 25 g of silica spheres. Under these conditions M o was deposited exclusively in the inner part of the spheres except for catalyst RK33 where some precipitation in the liquid occurred. Catalyst RK77 (Table 3) was also prepared using a mixed solution. The reaction was observed to start at the

23

centre of the spheres and continued for 15 min at 0 OC. Subsequently, the reaction was 'quenched' by diluting the reaction mixture with water at 0 OC. TABLE 3 Synthesis conditions of Mo/Si02 catalysts. Cat.

Si02

Soln.

[Mo]

[N2H4]/[Mo]

(g)a)

(ml)

(g/l)

(mol/mol)

pH

Method

T;y?fzk;;e

(%W)

79

2.8

8.7 rotating 1 h at OC flask 0 --* 60 OC in 20 h

15.0

2.0

8.5

1300 15.4

2.0

8.7 recirculation

RK15

25

95

RK29

25

500

RK33

25

Mob)

recirculation

16

19 h at 50 OC

6.7

40 min. at 1 OC

20.6

1

6 h at 7 OC RK37

25

1300 15.4

2.0

9.0

recirculation

30 min at 2 OC

1.5

1

22 h at 30 OC 6.7

RK77 ~~~~

~

~

40

72

2.0

8.5 quench

15 min. at 0

OC

0.66

~~

a) Si02 carrier used: 1.5 mm Shell silica spheres, pore volume 0 . 9 8 ml.g-l, surface area (BET) 263 m2.g-'. b) Determined by X-ray fluorescence.

Precipitation of copDer and silver onto alumina (transient Dhenomena) Copper and silver were deposited onto alumina extrudates applying the reactions and conditions summarized in Table 4. The mixed solution of the metal salt and reducing agent was poured onto the extrudates at ambient conditions. It was observed visually that the precipitation started at, and was restricted to, the central part of the extrudates. After some time the reaction was 'quenched' by dilution with water. Catalyst characterization After drying in air the metal distribution was established using electron microprobe analysis. The scanning electron microscope used was a Jeol-35. A step size between 20 and 30 pm was applied. The metal distribution was determined by carrying out a scan along the shortest line through the centre of the catalyst body.

24

TABLE 4 Synthesis conditions of Cu/y- A1203 and Ag/y- A1203 (egg - yolk) Catal. Formulation RK129

A1203 S o h . Reactants

Cu/A1203

(g)a)

(ml)

10

64

2.7 ml N2H50H solution (80 %w)

pH

Conditions

Reaction

Metal loading')

2.4

2 h at 4Cu2++ N2Hq+2H20 1.8 %w 21 OC + 2Cu20J+N2 Cu + 8H+

1.5 ml acetic acid (99.9 %w)

11.4 g C~(N03)2.3H20 After a spontaneous reaction at 21 OC for 1.5 h and filtration the resulting clear solution was used for precipation onto alumina. RK130

Ag/A1203

41

10

5.0 g AgNO3 0.87 g formalin (37 %w '3320) nitric acid (65 %w)

0.94 3 min. 2Ag+ + CH20 + at H20+2AgJ 21 OC + CHOOH + 2H+

0.38 %w Ag

a) A1203 carrier used: AC300, 1.5 mm cylindrical extrudates, pore volume 0.68 m2.g-'. h) Determined by X-ray fluorescence. RESULTS AND DISCUSSION Kinetics and mass transfer effects under pseudo-stationarv conditions pH measurements. Figures 1 and 2 show the pH records as obtained during the four Mn precipitation experiments. I n all experiments a rapid initial increase o f the pH after addition of the urea at zero time is observed. This increase

coincides with the neutralization of the nitric acid by the ammonium hydroxide generated via the hydrolysis of urea according to the equation CO(NH2)2

+

kl 3 H20

~

> Cog + 2 NH4+ + 2 OH-

(1)

25

The rate of urea decomposition has been calculated from the increase of the pH in the range pH 2-4. The rates obtained directly from the linearized pH curves have been interpreted using the first-order kinetics for the hydrolysis reaction which are clearly indicated in the literature

[lo].

From the litera-

ture mentioned it is furthermore apparent that the rate of hydrolysis of urea is independent of pH. The first-order rate constants kl calculated and summarized in Table 2 , therefore, are valid throughout the experiments. Note that the value of kl is not affected by the presence or the absence of the carrier. The higher value of kl obtained from experiment U31 is not caused by the nature of the silica carrier but by the fact that the temperature is slightly higher in experiment U31 than in experiments U33 and U34. The qualitative features of similar pH curves have already been described by Hermans et al. [9]. In the absence of a carrier (U33 in Fig. 2), after the initial rapid increase of the pH an overshoot develops after 1.5 to 2.0 hours. This overshoot is characteristic of the precipitation process being activated due to the required formation of nuclei of the solid manganese hydroxide phase. Following the nucleation phase (pH=6.6) the pH drops, which coincides with the growth of the nuclei formed. This process of growth takes place as a pseudostationary process at virtually constant pH (pH=6.3). In the presence of a carrier no overshoot o f the pH is observed and the pH level of the pseudo-stationary precipitation phase (pH=5.8) is lowered (Fig. 2 ) . In accordance with previous studies [ 9 ] these observations are ascribed to the smooth, non-activated nucleation at the carrier and to the stabilization of the MII(OH)~ phase by interaction with silica, respectively. The pH curves obtained with silica powder on the one hand and silica granules on the other (Fig. 1) do not display any relevant differences. The apparent difference of the plateau of the records (pH=6.1 with powder and pH=6.2 with granules) is within the experimental error of the pH measurements. PH

PH

1

10

1 2

1

1 4

1

1

6

1

1

8

1

1

10

TIME, h

TIME, h

Fig. 1. Records of pH as a function of time for experiments U30 (granules) and U31 (powder).

Fig. 2. Records of pH as a function of time for experiments U33 (no carrier) and U34 (granules),

26

Removal of manganese from solution. The total concentration o f manganese species dissolved in the aqueous phase as a function of time has been established in the experiments (results in Figs. 3 and 4 ) . From Fig. 3 it is apparent that the rate of removal of Mn from the solution is very much alike for the silica powder and the silica granules. The presence or absence of silica, however, has a large impact on the kinetics of the removal of Mn (Fig. 4 ) . In the absence of silica the precipitation process is slowed down considerably. Furthermore, the shape of the plots is different. The nonlinear shape of the plots (Figs. 3 and 4 ) in the presence of silica suggests a reaction order higher than zero, whereas the linearity of the graph in the absence of silica (experiment U 3 3 , Fig. 4 ) suggests a zero-order process.

In Fig. 5 we have gathered the experimental data on the total Mn concentration in the form of a first-order plot. The experiments carried out in the presence of silica ( U 3 0 , U 3 1 and U 3 4 ) give rise to a linear relationship in Fig. 5 , which supports the idea that the overall precipitation process in the presence of silica can be described by first-order kinetics. For experiment U 3 3 (no carrier), too, a linear plot is obtained, but we hesitate to conclude

from this that the precipitation kinetics are first-order in this case. This hesitation arises from (i) the low levels of manganese 'conversion' attained in experiment U 3 3 , and (ii) the linear relationship obtained in Fig. 4 for this experiment, which suggests zero-order kinetics. From the slopes of the fitted straight lines (Fig. 5) the first-order rate constants (kt) for the overall precipitation reaction of Mn from solution in the experiments were obtained and tabulated (Table 2 ) .

0 50

-

040

-

0 0 2 03 0 L

TIME, h

TIME. h

Fig. 3 . Manganese concentration in the Fig. 4 . Manganese concentration in the liquid as a function of time for liquid as a function of time for experiments U 3 0 (granules) and U 3 1 experiments U 3 3 (no carrier) and U 3 4 (granules). (powder).

Clearly, kt is much larger in the presence than in the absence of silica. From the kt values for experiments U30 and U31, it turns out that the size of the silica particles does not affect the overall kinetics of the precipitation process. The difference with respect to kt between experiments U30 and U 3 4 is caused by the higher rate of urea hydrolysis in experiment U30, which itself is brought about by the slightly higher temperature in that experiment. The four-fold difference in initial Mn concentration (experiment U30 versus U 3 4 ) giving rise to comparable values for kt - especially when the temperature difference is taken into account - further supports the overall precipitation being adequately described as a first-order process. Since the rate of Mn deposition is increased in the presence of silica and is not affected by the silica particle size, it is tempting to conclude that the precipitation process is not influenced by mass transfer effects. In the next section it will be demonstrated, however, that these effects still play an important role. From the rate constants and concentration data gathered in Table 2 it can be easily seen that the rate of urea hydrolysis (mol/s) exceeds the rate of manganese removal (mol/s) by a factor of 5 or more. This implies that the reaction between manganese ions and urea is not a stoichiometric one and it is proposed that part of the ammonium hydroxide formed (cf. reaction (1)) will

0.8

-

Ln (C&) 7

-+ -

0.6

U30 U31

- 8 - u33

--4-u34

-

0.4

-

02-

0

-

I

I

2

I

I

4

I

I

6

I

I

8

I

I

10 TIME, h

Fig. 5 . First-order plot of the rate of removal of manganese from solution. Go manganese concentration at zero time C actual manganese concentration

28

leave the solution as ammonia while the remaining part brings about the hydrolysis reactions of the manganese species. The reactions can be written as follows:

NH~+

+

OH-

-> NH3 t

Mn2+

+

OH-

->

+

H20

(2)

M~(oH)+

(3)

Hereafter we will assume that as a first approach reactions (2) and ( 3 ) can be considered to be in equilibrium. Although the rate of hydrolysis of urea is larger than that of the removal of Mn, reaction (1) is the rate-determining step for the overall precipitation process. The ’selectivity’of the utilization of the generated ammonium hydroxide is determined by ‘equilibria‘ (2) and ( 3 ) . It is expected that higher temperatures and/or a higher pH will favour (2) over ( 3 ) . This indeed explains that for experiment U33 the rate constant kt is lower than in the other cases, seeing that, as is shown in Fig. 2, the pH at which precipitation takes place is much higher. Deuosition reaction of manganese. The final reaction which leads to deposition of the partly hydrolysed Mn species onto the silica carrier is tentatively described as

Mn(OH)+

+

SiOH

+

OH-

k4 >

SiO-Mn(0H)

+

H20

(4)

The kinetics of reaction (4) cannot be assessed from the overall precipitation reaction since - as outlined above

-

the latter is dominated by the urea hydro-

lysis in combination with the ‘selectivity’determining reactions (2) and ( 3 ) . The kinetics of the consecutive reaction ( 4 ) could be found, however, from the resulting distribution of manganese over the silica granules. An example of such a distribution as obtained from electron microprobe analysis is shown in Fig. 6. The qualitative shape of the distribution indicates that reaction (4) is influenced by both kinetic and diffusion effects. In order to further extract quantitative information from the manganese distributions we have assumed that the kinetics of reaction ( 4 ) can be approached by a first-order dependence in manganese and a zero-order dependence for the other components. With these assumptions one can make use of the Thiele concept of diffusionlimited reactions [ll].

lob

29

CONCENTRATION, C ( z / L l / C ( - 1 )

I.

0.8

1

0.6

O2 -

-

.

EXPERIMENT -MODEL

0

F 5. 6 . Distribution of manganese over a representative silica granule from

experiment U 3 4 . The drawn lines have been calculated for different values for the Thiele modulus ( @ ) . We start with the quantitative determination of k4 by defining the Thiele

modulus of reaction (4) as (5)

0 = V/S *qkq/De with

V

=

S

=

(average) surface area of the carrier particles

De

=

effective diffusivity of manganese species in the

(average) volume of the carrier particles

porous silica particles Furthermore we use the known relationship [ll] between the Thiele modulus and the concentration of manganese at the outer surface ( G o ) and that at the centre of the particles (Ci) written as Ci/Co

=

1 / (cash 0)

(6)

From an average value of Ci/C0=0.32

0 . 0 2 as determined for seven granules

obtained in experiment U 3 4 a value of +1.8 granules as cubes with a length of From an estimate of De (lo-'

follows. By approaching the silica

m we conclude that V/S=1.67*10-4

m.

m2/s) according to relationships given by Perry

[12] for the bulk diffusivity and by Satterfield [ 1 3 ] for the tortuosity of the

carrier in combination with equation (5) it follows that k4=O.12 l/s. Comparison of this value with those of the rate constants reported in Table 1 shows that the consecutive precipitation reaction ( 4 ) is much more rapid than the rate-determining urea hydrolysis.

30

Development of a semi-quantitative model. In this section we will briefly discuss a model which summarizes the above kinetic and mass-transfer effects. We distinguish three steps. viz. (i) the hydrolysis of urea, which leads to hydrolysis and precipitation of manganese, (ii) transport of (hydrolysed) manganese species from the bulk of the solution to the outer surface of the carrier particles, and (iii) reaction parallel with mass transfer inside the porous particles. These steps lead to the following expressions for the rate of removal of Mn from solution (Rt):

*

Rt

s

Rt

kl

*

=

S

kl

*

ak

(7)

C1

*

(‘B

-

‘B,i,l)

selectivity defined as the rate of Mn hydrolysis over urea hydrolysis, mol(Mn)/mol(urea).

kl kl

=

rate constant for urea hydrolysis (s-l)

- mass transfer coefficient, liquid/solid - concentration of urea, mo1/m3

c1

(m/s)

CB = concentration of Mn(OH)+ (-B) in liquid phase, mol/m3. CB,i,l= concentration of B at liquid side of interface between liquid and carrier particle, mol/m3. ak d

=

external carrier surface area, m2/m3.

=

V/S, m.

- tanh(0)/0

11

(effectiveness factor).

CB,i,s= concentration of B at solid/liquid interface at solid side, mo1/m3. and realizing that ‘B,i,s

*

=

‘B,i,l

with e

=

porosity of the carrier, m3/m3

the interfacial concentrations for B can be eliminated and equation (11) is obtained. S*Cl Rt

4-

CB

=

l/kl + l/kp*ak + l/kq*ak*d*q*r With respect to the model summarized by equation (11) it is noted that a crude

31

approximation lies in the selectivity factor

s

(equation (7)) which describes

the complex interplay between the urea hydrolysis, ammonia evaporation and manganese hydrolysis in a simplified manner. However, in the absence of more detailed data a more sophisticated description of the process cannot be validated. The advantage of equation (11) is that it presents insight into the three main resistances for manganese deposition, viz. urea hydrolysis (l/kI), transport to the outer surface of the catalyst particles (l/kl*ak), and diffusion and reaction inside the particles (l/k&*ak*d*q*e).

The first and the third

resistance follow from this work and the second one can be estimated from the literature [14]. To a first approximation, for experiment U34 the three resistances as mentioned rate as follows: 90:1:8. In other words, the largest resistance is related to the urea hydrolysis, whereas the transport to the external surface causes a negligible resistance only. From equation (11) it can be easily shown that depending on the specific value of the selectivity parameter

(s)

the overall process of Mn removal from

the solution can or cannot be described by a first-order process. In view of the importance of the surface deposition reaction for the distribution of the active components over the carrier particles, k4 has been determined for a number of catalytic components. The results shown in Table 5 indicate that the values for k4 can vary considerably; the distributions of the active component will vary correspondingly. TABLE 5 Rate constants for the surface precipitation reaction for several active components onto silica carriers. The precipitation reaction involves urea hydrolysis. Active metal Mn Rh Ni

Rate constant, k4 (s-l) 1.2 * 10-1 3 . 6 * lo-* 9.0 * 10-3

Reference This work 15 16

Effects of transient concentration eradients A novel method to apply deposition precipitation which makes use of

transient concentration gradients will be illustrated by the synthesis of eggyolk type molybdenum-on-silica catalysts. Two methods have been elaborated to start and restrict the deposition of Mo at the centre of the silica spheres (cf. Table 3 ) . Note that in all experiments the volume of the solution exceeds the carrier pore volume considerably (wet 'impregnation'). The first method uses relatively dilute solutions and mild reaction conditions (pH, temperature) such that a reaction starts at the centre of the spheres and terminates spon-

32

taneously (typically after 1 h). This method was applied for experiments RK29 and RK37. The line scans for catalyst RK37 (Fig. 7 ) reveal - in accord with visual inspection of the dried catalyst

-

the Mo to be concentrated in the

centre of the spheres (a so-called egg-yolk catalyst). Note the small variation of the metal distribution between the separate spheres (Fig. 7). The second method to control the local rate of deposition was investigated in experiment RK77 (Table 3 ) , where the reaction was allowed to continue for 1 5 min at 0 OC and then quenched by diluting the reaction mixture with water, also at 0 OC. From the line scan (Fig. 8) it is apparent that again an egg-yolk catalyst was obtained, although less pronounced than with RK37 (Fig. 7 ) . In future work it might be worthwhile to consider quenching the reaction by lowering the temperature of the reaction mixture.

Fig. 7. Electron microprobe analysis of the molybdenum distribution over silica spheres of catalyst RK37 (1.5 %w Mo).

Fig. 8 . Electron microprobe analysis of the molybdenum distribution over a representative silica sphere of catalyst RK77 ( 0 . 6 6 %w Mo).

In Fig. 9 line scans of Mo/Si02 catalysts have been collected for different Mo loadings. Clearly, with increasing load the metal distribution shifts from convex to flat and, finally, to concave.

In searching an explanation for the controlled precipitation reaction at the centre of the spheres, it is important to note that the reaction between Mo0h2and N2Hh is strongly accelerated at reduced pH. This effect has been observed experimentally by us and is related to the liberation of hydroxyl ions during the reaction (cf. Experimental sectionj. Furthermore, we have observed that a more uniform Mo distribution was obtained when the reactants had contacted the carrier for an extended period under conditions which precluded any reaction. Based on the above observations, the controlled precipitation is ascribed to the existence of temporary concentration gradients inside the spheres. After imbibition with part of the solution containing the reactants, the 'acidic'

33

A ) 1 5 o/ow

Mo (RK 3 7 )

B)6.7°/owMo(RK

C ) 16 '/OW

MO (RK 15)

D) 2 1 o/ow

29)

Mo (RK 33)

Fig. 9 . Microprobe analysis of the molybdenum distribution over silica spheres of Mo/Si02 catalysts with various metal loadings. silica support will give rise to a decrease o f the pH of the penetrated solution in comparison to the pH o f the 'excess' solution outside the spheres. This will lead to concave pH profiles within the spheres, which will exist typically for 1 h". This concave pH profile results in a convex Mo distribution as the rate of precipitation increases with decreasing pH. We now propose to generalize the above described phenomena as follows:

- The combination of (i) a precipitation reaction proceeding via redox chemistry and liberating OH-(or H+) ions with (ii) the use of an acidic (or a basic) carrier may give rise to accelerated precipitation of the active component at the centre of the carrier body (egg-yolk distribution).

- The combination of (i) a precipitation reaction proceeding via redox chemistry and liberating OH-(or H+) ions with (ii) the use of a basic (or an acidic) reacting carrier may give rise to accelerated precipitation of the active component at the edge of the carrier body (egg-shell distribution). Of course, this method may be even further generalized to other types of precipitation reactions and to concentration gradients other than that of H+. In this paper, however, we will restrict ourselves to the combination of redox reactions and pH gradients.

*

Typical times to efface concentration gradients by diffusion under these conditions range from 0.5 - 1.0 h related to the Fourier process in question [17]. Adsorption phenomena may even greatly prolonge these periods (18,191.

34

We note in passing that the question whether a carrier will react basic or acidic will depend both on the nature of the carrier and on the pH of the aqueous solution. Roughly speaking, one can say that if the pH o f the solution is abovefielow the iso-electric point (IEP)'

o f the oxide in question, the

carrier will react acidic/basic. Two precipitation reactions have been designed to check the generalization concerning the egg-yolk catalysts, viz. the liquid-phase reduction of Cu2+ and of Ag+ with N2H4 and CH20, respectively (for details see Table 4 ) . Both reactions generate H+ ions and it is predicted that a basic carrier, such as y-Al2O3 at sufficiently low pH, will lead to an egg-yolk distribution. Indeed, the line scans shown in Figs. 10 and 11 prove the metals to be concentrated in the inner part of the extrudates.

Fig. 1 0 . Electron microprobe analysis of the copper distribution over an alumina extrudate of catalyst RK 129 (1.8 % m/m Cu).

Fig. 11. Electron microprobe analysis of the silver distribution over an alumina extrudate of catalyst RK 130 ( 0 . 3 8 % m/m A g ) .

The second generalization related to the egg-shell catalysts has been confirmed by repeating the copper precipitation under identical conditions with (acidic) silica spheres, Visual inspection of the precipitation process revealed the copper oxide to be concentrated at the outer surface of the spheres. Practical application of this method to produce egg-shell catalysts, however, involves the problem of precipitation in the liquid separate from the carrier. Therefore, the method seems to be especially suited for preparing egg-yolk type catalysts. GENERAL DISCUSSION AND CONCLUSIONS Kinetics and mass transfer effects (pseudo-stationarv conditions) Several conclusions for deposition precipitation onto pre-shaped carrier particles utilizing urea hydrolysis emerge from this study. Firstly, as

#

The IEP of 7 -A1203 is between 7 and 9 [ 2 0 ] while the IEP of Si02 has been reported to range from pH 0.5 to 3 . 7 [21].

35

expected, the rate-determining step is connected with the urea hydrolysis. Secondly, the ultimate distribution of metal over the carrier body is dictated by the rapid consecutive precipitation reaction (rate constant kb) at the carrier surface. An important implication is that the rate of the urea hydrolysis (and thus of the overall precipitation process) does not affect the metal distribution. Consequently, the rate may be much enhanced without affecting the distribution. Furthermore, it might be inferred that higher metal concentrations will not change the picture since we have indications that the processes are first-order in the metal concentration. In our laboratory [16] we have obtained evidence for the precipitation of nickel onto silica spheres that this is indeed the case. With respect to the viability of using deposition precipitation onto pre-shaped carriers for industrial applications the feasibility of the method is apparent from the work presented in this paper. Pore mouth plugging by deposition of the metal component will occur only with extremely rapid surface reactions. However, in general concentration gradients will result from applying this synthesis method with larger carrier particles. Whether these concentration gradients are acceptable or not depends on the catalyst and its specific application. Effects of transient concentration eradients A unique feature of deposition precipitation onto large carrier bodies has been found to be related to the occurrence of concentration gradients direct after imbibition with the reactant mixture. In general, when the iso-electric point of the carrier deviates from the pH of the solution, a pH gradient inside the carrier body will exist for some time (typically 1 h). By applying a precipitation reaction whose rates depend on the pH the

local rate

of precipitation

in the carrier body may be controlled. It has been shown that deposition precipitation via redox chemistry can be used for the production of egg-yolk type catalysts. The synthesis o f Mo/Si02, Cu/A1203 and Ag/A1203 catalysts displaying an egg-yolk type distribution has been described to illustrate this novel preparation route. ACKNOWLEDGEMENTS The skilful experimental assistance of Mr. E.J.G.M. Romers and Mr. R. van Kempen is gratefully acknowledged. Furthermore, the valuable review of this paper by Prof. Dr. J.A.R. van Veen is appreciated.

36 REFERENCES

1 German Patent No. 7 4 0 , 6 3 4 to IG Farben ( 1 9 4 3 ) . 2 Netherlands Patent Application 6 7 , 0 5 2 5 9 to Stamicarbon ( 1 9 6 7 ) . 3 US Patent 3 , 6 6 8 , 1 4 8 to Lever Brothers Company ( 1 9 7 0 ) . 4 Netherlands Patent Application 6 8 , 1 6 7 7 7 to Stamicarbon ( 1 9 7 0 ) . 5 J.W. Geus, in (B. Delmon et al., Eds.) Preparation of Catalysts 111, Elsevier, Amsterdam, 1 9 8 3 , p. 1. 6 European Patent Specification 2 5 8 , 9 4 2 ( 1 9 8 8 ) to S.1.R.M.-B.V. 7 M. Montes, Ch. Penneman de Bosscheyde, B.K. Hodnett, F. Delannay, P. Grange and B. Delmon, Appl. Catal. 12 ( 1 9 8 4 ) 3 0 9 . 8 B. Delmon, Solid State Ionics 16 ( 1 9 8 5 ) 2 4 3 . 9 L.A.M. Hermans and J.W. Geus, in (B. Delmon et al., Eds.), Preparation of Catalysts 11, Elsevier, Amsterdam, 1 9 7 9 , p. 1 1 3 . 10 R.C. Warner, J . Biological Chemistry USA 1 4 2 ( 1 9 4 2 ) 7 0 6 . 11 K.R. Westerterp, W.P.M. van Swaaij and A.A.C.M. Beenackers, Chemical Reactor Design and Operation, John Wiley, Chichester, 1 9 8 4 , p. 3 8 3 . 1 2 J .H. Perry, Chemical Engineers’ Handbook, 4th Edition, McGraw-Hill, 1 9 6 3 , pp. 1 4 - 2 3 . 1 3 C.N. Satterfield, Heterogeneous Catalysis in Practice, McGraw-Hill, 1 9 8 0 , p. 3 3 6 . 14 F. Kneule, Chemie-1ng.-Techn. 2 8 ( 1 9 5 6 ) 2 2 1 . 1 5 K.P. de Jong, J.H.E. Glezer, H.P.C.E. Kuijpers, A . Knoester and C.A. Emeis, J . Catal., accepted for publication. 16 K.P. de Jong, unpublished results. 17 J. Crank, The Mathematics of Diffusion, Clarendon Press, Oxford, 1 9 7 5 , p. 8 9 . 18 P.B. Weisz, Trans. Faraday S O C . 6 3 ( 1 9 6 7 ) 1801. 1 9 M. Komiyama, Catal. Rev.-Sci. Eng. 27 ( 1 9 8 5 ) 3 4 1 . 2 0 J . P . Brunelle, Pure C Appl. Chem. 5 0 ( 1 9 7 8 ) 1211. 2 1 R.K. Iler, The Chemistry of Silica, John Wiley, New York, 1 9 7 9 , p. 188.

G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands

37

INFLUENCE O F THE PREPARATION PROCEDURE ON THE PHYSICAL PROPERTIES, SURFACE A C I D I T Y AND D I S P E R S I O N OF MoP/A1,0,

CATALYSTS

R. PRADA S I L V Y , Y. ROMERO, J . GUAREGUA, R. GALIASSO

I N T E V E P S.A, Seccion de C a t a l i s i s ' A p l icada, Apdo. 76343, Caracas 1070-A, VENEZUELA.

SUMMARY The i n f l u e n c e o f t h e p r e p a r a t i o n procedure on t h e p h y s i c a l p r o p e r t i e s , s u r f a c e a c i d i t y , d i s p e r s i o n and d i s t r i b u t i o n o f t h e supported phases i n MoP/A1,0, c a t a l y s t s i s here investigated. The r e s u l t s i n d i c a t e t h a t t h e b e h a v i o u r o f t h e molybdenum-phosphorus c a t a l y s t s depend s t r o n g l y on t h e i m p r e g n a t i o n sequence used ( c o i m p r e g n a t i o n o r d o u b l e i m p r e g n a t i o n ) . Coimpregn a t i o n procedure (Mo+P) a l l o w s t h e o b t e n t i o n o f c a t a l y s t s w i t h l a r g e r C o n c e n t r a t i o n o f a c i d s i t e s and s u r f a c e d i s p e r s i o n ! when compared t o consecuti v e i m p r e g n a t i o n procedures (P->Mo, Mo->P). T h i s sample a l s o .showed h i g h a c t i v i t y f o r t h e HDS, HDN and MHC r e a c t i o n s . The d i f f e r e n c e s i n b e h a v i o u r observed f o r t h e d i f f e r e n t samples c o u l d be m a i n l y a t t r i b u t e d t o t h e e x i s t e n c e o f d i f f e r e n t molybdenum d i s t r i b u t i o n phases, p r o b a b l y Mo-P . h e t e r o p o l compounds a r e formed f o r t h e coimpregnated sample, . w h i l e t h e f o r m a t i o n o f b u l t MOO, species c o u l d be f a v o r i z e d f o r t h e sample preimpregnated wJth phosphorus. I t was a l s o observed t h a t phosphorus produces t h e f o l l o w i n g e f f e c t s on alumina: i ) i t improves t h e . s u r f a c e a c i d i t y , ii) i t i n c r e a s e t h e mechanical s t r e n g t h o f t h e e x t r u d a t e s , iii) i t decreases t h e s u r f a c e a r e a o f t h e s u p p o r t . We e x p l a i n o u r r e s u l t s on t h e b a s i s o f t h e mechanism proposed f o r t h e a d s o r p t i o n o f molybdate. phosphate and phosphomolybdate compounds on alumina. INTRODUCTION Molybdenum supported alumina c a t a l y s t s promoted by m e t a l s o f t h e group

V I I I ( N i , Co) a r e w i d e l y used f o r h y d r o p r o c e s s i n g heavy o i l s o r c o a l d e r i v e d fuels.

E f f o r t s have been made t o improve c a t a l y t i c a c t i v i t y by f i n d i n g appro-

p r i a t e a d d i t i v e s (such as P,

B,

S i , T i , F , Ga, e t c . ) . , Phosphorus can be c o n s i -

dered one o f t h e most e f f e c t i v e a d d i t i v e s o f t h e molybdenum supported alumina c a t a l y s t s . I n f a c t , i t appears as a component i n a number o f f o r m u l a t i o n s o f commercial h y d r o t r e a t i n g c a t a l y s t s . The p a t e n t l i t e r a t u r e c l a i m s t h a t i t s use p r o v i d e s b e t t e r c a t a l y t i c performance, i n c r e a s i n g h y d r o d e s u l f u r i z a t i o n (HDS), h y d r o d e n i t r o g e n a t i o n (HDN), hydrodemetall i z a t i o n (HDM) , and i m p r o v i n g t h e m i l d hydrocracking

(MHC)

activity

(ref.

1-41.

phosphorus produces s e v e r a l e f f e c t s , such as:

I t has been demonstrated t h a t

i) i t p r o v i d e s a more s t a b l e

i m p r e g n a t i n g s o l u t i o n and t h u s b e t t e r d i s p e r s i o n o f t h e metal ( r e f . 1 , 4 ) , ii) i t i n h i b i t s t h e f o r m a t i o n o f Ni(Co)Al,O,-like

on

t h e support species and

enhances t h e f o r m a t i o n o f N i z t ' i o n s w h i c h a r e p r i m a r l y r e s p o n s i b l e f o r t h e f o r m a t i o n o f c a t a 1 . y t i c a l l y a c t i v e n i c k e l s u l f i d e o r NiMoS-like phase ( r e f . 3 ) ,

iii) i t a l t e r s t h e a c i d s t r e n g t h d i s t r i b u t i o n on alumina, t h e c o n c e n t r a t i o n o f medium a c i d s i t e s i n c r e a s e s p r o g r e s s i v e l y w i t h i n c r e a s i n g phosphorus l e v e l

38

5) and i v ) i t improves t h e thermal s t a b i l i t y o f gamma alumina w i t h t o s i n t e r i n g and and phase t r a n s i t i o n t o alpha alumina ( r e f . 5). The b e n e f i c a l e f f e c t s o f phosphorus has s t i m u l a t e d research on i t s i n f l u e n c e on molybdena based c a t a l y s t s . However, most o f t h e above mentioned (ref.

respect

1 i t e r a t u r e e s s e n t i a l l y focused on t h e i n f l u e n c e o f phosphorus on t h e c a t a l y t i c p r o p e r t i e s o f t h e m o d i f i e d system and i t s e f f e c t on t h e d i s p e r s i o n on the a c t i v e phase deposited on t h e alumina surface has n o t y e t been i n v e s t i g a t e d . Also,

very few works deal w i t h t h e i n f l u e n c e o f t h e sequence o f phosphorus c a t a l y s t s on t h e

i n c o r p o r a t i o n d u r i n g t h e p r e p a r a t i o n step o f the MoP/A1 ,O,

surface a c i d i t y , d i s p e r s i o n and d i s t r i b u t i o n o f t h e supported phases.

As the

phosphate i o n s s t r o n g l y i n t e r a c t w i t h alumina, competing w i t h molybdate ions, a f a c t o r o f p o s s i b l e importance i s t h e p r e p a r a t i o n procedure. Three

different

procedures

can

be

followed .to

prepare a MoP/A1 ,03

catalyst.

(1) P A1,0,

-->

MO

(2) MO + P ( 3 ) Mo --> P

->

MOP/ A1,0,

-->

This c o m u n i c a t i o n i s p a r t o f a research program aimed a t a systematic i n v e s t i g a t i o n o f t h e p r e p a r a t i o n procedure o f MoP/A1,0, catalysts.

Essentially,

we study

mild-hydrocracking

the e f f e c t o f phosphorus i n c o r p o r a t i o n

sequence on t h e s t a t e o f d i s p e r s i o n o f t h e a c t i v e phase, surface a c i d i t y and physical p r o p e r t i e s . the

following

For t h i s purpose, t h e samples were c h a r a c t e r i z e d using

physico-chemical

techniques:

BET

strength, X-ray p h o t o e l e c t r o n spectroscopy (XPS).

(SEM),

surface

area,

mechanical

scanning e l e c t r o n microscopy

surface a c i d i t y determined by p y r i d i n e adsorption.

EXPERIMENTAL Catalyst preparation Three MoP/A1,0, c a t a l y s t s having constant molybdenum and phosphorus contents (P->Mo

(10 w t % MOO,

and 4.5

w t % P,O,)

were prepared by e i t h e r consecutive

o r Mo-->P) o r simultaneous (Mo+P) impregnation o f alumina extrudates

(1/20 i n c h l e n g t h ) w i t h aqueous s o l u t i o n s o f ammonium heptamolybdate and/or orthophosphoric acid. The alumina has a s p e c i f i c surface area o f 269 m 2 / g and a pore volume of 0.69 cc/g. The impregnating s o l u t i o n s were adjusted t o pH =

1.5-2.0 by adding n i t r i c a c i d b e f o r e c o n t a c t i n g w i t h t h e support. The samples were d r i e d a t 373 K f o r 2 h and then c a l c i n e d i n two steps a t 623 K f o r 2 h and a t 7 7 3 K f o r 3 h.

39

Characterization Surface areas o f c a t a l y s t s were determined by t h e BET method from N, analyzer. An Erweka apparatus was used t o measure t h e s i z e c r u s h i n g s t r e n g t h o f c a t a l y s t extrudates.

The method determines p a r t i c l e c r u s h i n g s t r e n g t h by

measuring t h e f o r c e i n kilograms ( k ) r e q u i r e d t o crush an e x t r u d a t e o f measurA l a r g e number o f extrudates (about f o r t y o f each sample) were t e s t -

ed s i z e .

ed and t h e average value was e s t a b l i s h e d . X-ray

p h o t o e l e c t r o n spectra

(XPS)

o f c a t a l y s t s were recorded u s i n g a

Leybold Heraeus LHS-11 apparatus equipped w i t h a computer system, which a1 lowed

the determination o f

Alka l i n e ( E = 1486 eV).

peak areas.

The e x c i t a t i o n r a d i a t i o n was

the

A l l t h e samples were grounded and then pressed i n t o

Cis.

Alpp, A l Z s . M o ~ ~M , o ~ ~P2p , and P 2 s e n e r g y l e v e l s were recorded. The CIS energy l e v e l (284.5 eV) was taken as a reference. Atomic surface c o n c e n t r a t i o n o f t h e sample h o l d e r s b e f o r e t h e a n a l y s i s .

Signals corresponding t o

supported elements was evaluated form t h e peak i n t e g r a t e d areas and t h e sensit i v i t y f a c t o r s provided by t h e equipment manufacturer. The elemental p r o f i l e d i s t r i b u t i o n o f b o t h molybdenum and phosphorus, across t h e t r a n s v e r s a l s e c t i o n o f t h e alumina extrudates, was obtained u s i n g the

scanning

e l e c t r o n microscopy

technique

(SEM).

An

ISI-60 apparatus

equipped w i t h an energy d i s p e r s i v e X-ray analyzer (Kevex 5-7000) was used f o r these measurements.

C a t a l y s t extrudates were mounted on an epoxy s l i d e and

then p o l i s h e d b e f o r e scanned under t h e e l e c t r o n beam. The a c i d i t y measurements were c a r r i e d o u t i n a Cahn 1000 e l e c t r o b a l a n c e u s i n g p y r i d i n e as

probe molecule adsorbed on t h e c a t a l y s t

surface.

The

i r r e v e r s i b l y adsorbed p y r i d i n e amounts were determined a t 273 K, 473 K and 573 K.

Results a r e expressed as m o l o f p y r i d i n e i r r e v e r s i b l y adsorbed p e r

surface area o f c a t a l y s t . RESULTS Surface areas and mechanical s t r e n g t h Specific

surface areas corresponding t o MoP/A1 209 c a t a l y s t s prepared

f o l l o w i n g d i f f e r e n t procedures a r e given i n Table 1. t h e same t a b l e t h e values obtained f o r A1,0, Mo/A1,0,

samples.

I t i s also reported i n

support and f o r P/A1,0,

and

40

TABLE 1 S p e c i f i c s u r f a c e areas and mechanical s t r e n g t h corresponding t o d i f f e r e n t p r e p a r a t i o n procedures Sample

Surface Area (m'/g)

A1 Z 0 3 P / A 1 ,03 Mo/A1 ,O,

Crushing S t r e n g t h ( k g / p e s t l e )

269 241 248 214 222 234

P->Mo Mo->P MotP

For b o t h Mo/A1,0,

5.2 6.0 5.3 6.1 5.5 6.1

and P/A1,0,

samples,

t h e surface area decreased i n

8% and 11%, r e s p e c t i v e l y , a f t e r impregnation w i t h ammonium -heptamolybdate o r phosphoric acid, whereas f o r MoP/A1 ,03 c a t a l y s t s t h e l o s s

approximatively,

i n surface areas was more pronounced.

One can observe t h a t t h e s u r f a c e area

value o b t a i n e d f o r t h e sample prepared by coimpregnation i s s l i g h t l y higher than t h a t observed f o r those samples prepared by consecutive impregnation.

A l l MoP/A1,0,

c a t a l y s t s presented a pore volume i n t h e 0.52 - 0.55 c c / g range.

Concerning t h e mechanical s t r e n g t h measurements,

i t i s observed i n Table

1 t h a t phosphorus s l i g h t l y improves t h e mechanical s t r e n g t h o f alumina e x t r u dates, w h i l e molybdenum seems t o have no i n f l u e n c e on these p r o p e r t i e s .

The

p r e p a r a t i o n procedures (P->Mo) and (Mo+P) produce c a t a l y s t s w i t h simi 1 a r c r u s h i n g s t r e n g t h values. X-ray p h o t o e l e c t r o n spectroscopy (XPS) The XPS r e s u l t s obtained f o r P/A1 ,O,,

Mo/A1 ,O,

and MoP/A1 ,O,

catalysts,

a r e presented i n Table 2. The percentage o f surface d i s p e r s i o n o f both Mo and P elements ( I e / I A l x 100) i s r e p o r t e d as a f u n c t i o n o f c a t a l y s t p r e p a r a t i o n

procedure. Mo/A1,0,

When comparing t h e (Mo/A1) o r ( P / A l )

o r P/A1,0,

i n t e n s i t y r a t i o obtained f o r

s w p l e s w i t h t h a t o f MoP/A1,0,

c a t a l y s t s , we can observe

s t r i k i n g d i f f e r e n c e s i n t h e d i s p e r s i o n s t a t e o f t h e supported species.

Molyb-

denum d i s p e r s i o n o f samples v a r i e s as f o l l o w s :

while,

i n t h e case o f t h e phosphorus d i s p e r s i o n , t h e observed sequence i s as

follows: Mo+P

>

P/A1,0,

>

Mo->P

>

P->Mo

41

MO

MO

-070

0

070

Radial Position (mm) Fig. 1. Electron Microprobe p r o f i l e of Mo and P corresponding t o MoP/A1,0, c a t a l y s t s prepared following d i f f e r e n t procedures.

A t 373 K (weak acid s i t e s ) , t h e pyridine adsorbed amounts per surface area v a r i e s as follows: Mo+P > P->Mo > Mo->P = A1,0,

Whereas, f o r desorption temperatures of 473 K and 573 K (medium and strong acid s i t e s , r e s p e c t i v e l y ) , the observed sequence i s :

42

TABLE 2 XPS r e s u l t s c o r r e s p o n d i n g t o t h e c a t a l y s t s prepared u s i n g d i f f e r e n t

procedures. Sample

IMojp/ I A 1

2P

I P 2 p / I A 1 2P

__

Mo/A1 ,O,

4.5

2.7 __

P->Mo

4.1

2.3

Mo->P

4.4

2.5

Mo+P

4.8

3.3

P / A 1 *03

Scanning e l e c t r o n microscopy (SEM) F i g u r e 1 r e p r e s e n t s t h e p r o f i l e d i s t r i b u t i o n , across t h e t r a n s v e r s a l sect i o n o f t h e alumina e x t r u d a t e s , b o t h f o r phosphorus and molybdenum elements, obtained

through

procedures

(Mo+P)

SEM

and

technique. (Mo->P),

For

catalysts

b o t h Mo and

prepared

P elements

following are

the

distributed

homogeneously i n t h e s u p p o r t , whereas sample prepared a c c o r d i n g t o procedure (P->Mo),

c l e a r l y shows some h e t e r o g e n e i t i e s .

Strength o f a c i d i t y S u r f a c e a c i d i t y r e s u l t s , o b t a i n e d t h r o u g h p y r i d i n e a d s o r p t i o n , correspond i n g t o A1,0,

s u p p o r t and d i f f e r e n t MoP/A1,0,

catalysts,

Table 3 as a f u n c t i o n o f t h e d e s o r p t i o n temperature.

are presented i n

Differences i n pyridine

i r r e v e r s i b l y adsorbed amounts p e r s u r f a c e a r e a u n i t can be observed i n t h e 373 -573 K temperature range, depending on t h e p r e p a r a t i o n method used. TABLE 3 I r r e v e r s i b l y a c i d i t y o f MoP/A1,0,

prepared c a t a l y s t s ( m o l p y r i d i n e l m ' )

Sample 373

TEMPERATURE ( K ) 473

573

1.41

0.67

0.11

Mo->P

1.78 1.44

0.83 0.76

0.42 0.32

Mo+P

2.06

0.85

0.45

3'2

P->Mo

x lo3

43

DISCUSSION The above r e s u l t s show how a c i d i t y and s u r f a c e d i s p e r s i o n o f supported phases found i n MoP/A1,0,

c a t a l y s t s can be s t r o n g l y i n f l u e n c e d by t h e prepara-

Tables 2 and 3 i n d i c a t e t h a t c o i m p r e g n a t i o n o f Mo and P

t i o n procedure.

a l l o w s t h e o b t e n t i o n o f a c a t a l y s t w i t h l a r g e r c o n c e n t r a t i o n o f a c i d s i t e s and surface dispersion,

(P->Mo,

Mo->P).

t h e mechanical

i n comparison w i t h c o n s e c u t i v e i m p r e g n a t i o n procedures

Phosphorus seems t o improve t h e a c i d i t y s t r e n g t h as w e l l as p r o p e r t i e s o f alumina e x t r u d a t e s .

However,

this additive

s t r o n g l y a f f e c t s t h e s p e c i f i c surface area o f t h e support. L e t u s d i v i d e t h e d i s c u s s i o n o f o u r r e s u l t s i n two p a r t s :

we s h a l l f i r s t

d i s c u s s t h e r e s u l t s o b t a i n e d on t h e sample prepared by c o i m p r e g n a t i o n (MotP) and subsequently, examine t h e b e h a v i o u r o f those c a t a l y s t s p r e p a r e d by consec u t i v e impregnations (P->Mo and Mo->P). P r e p a r a t i o n Procedure (Mo+P) Table 2 shows t h a t c o i m p r e g n a t i o n method improves t h e s u r f a c e d i s p e r s i o n

o f b o t h molybdenum and phosphorus.

T h i s agrees w i t h r e c e n t r e s u l t s o b t a i n e d

by Atanasova e t a l . ( r e f . 6), who s t u d y i n g a s e r i e s o f NiMoP/Al,O, XPS

and c a t a l y t i c

measurements,

stated

that

c a t a l y s t by

c o i m p r e g n a t i o n method and a

s u i t a b l e alumina c a r r i e r can l e a d t o a b e t t e r d i s p e r s i o n o f a c t i v e components and consequently, t o an i n c r e a s e i n

HDS a c t i v i t y .

We s h a l l a t t e m p t t o e x p l a i n now o u r r e s u l t s supported on fundamental s t u d i e s d e a l i n g w i t h a d s o r p t i o n o f Mo and/or P

on

alumina.

L e t us r e f e r t o t h e

more i m p o r t a n t r e s u l t s o f t h e s e s t u d i e s . L i t e r a t u r e shows t h a t phosphorus i n c r e a s e s t h e s o l u b i l i t y and s t a b i l i t y o f molybdenum s o l u t i o n s

(ref.

1,4). A d s o r p t i o n s t u d i e s proposed t h a t when

alumina i s coimpregnated w i t h s o l u t i o n s c o n t a i n i n g molybdate and phosphate, t h e r e i s a c o m p e t i t i o n between b o t h i o n s f o r t h e same a d s o r p t i o n s i t e s ( b a s i c hydroxyl

groups

of

alumina),

thus,

a d s o r p t i o n o f rnolybdates (3,6-9).

the

adsorbed

phosphate

inhibits

the

However, most o f t h e s e s t u d i e s deal w i t h

molybdate and phosphate a d s o r p t i o n s e p a r a t e l y w i t h o u t c o n s i d e r i n g f o r m a t i o n o f phosphomolybdate compounds i n t h e i m p r e g n a t i n g s o l u t i o n .

the

R e c e n t l y , Cheng and L u t h r a ( r e f . 8 ) , u s i n g t h e NMR t e c h n i q u e , s t u d i e d t h e a d s o r p t i o n o f v a r i o u s phosphomolybdate compounds on alumina spheres.

Authors

observed t h a t when phosphoric a c i d i s added t o a s o l u t i o n c o n t a i n i n g amonium hep tam0 1 y b d a t e , p e t amol ybdodi phosphate compounds ( P ,Mo s o l u t i o n s c o n t a i n i n g P/Mo m o l a r r a t i o h i g h e r t h a n 0.4, remained i n f o r m o f phosphates.

1 a r e formed.

For

amounts o f phosphorus

T h i s suggests t h e e x i s t e n c e o f a chemical

e q u i l i b r i u m between b o t h phosphate and molybdate i o n s i n s o l u t i o n .

44

8 H * + 5 MOO:'

+ 2 HP0:-

=

P,Mo,O,,

6-

+

(2)

5H20

A c c o r d i n g t o t h e above e q u a t i o n , decomposition o f phosphomolybdate i n t o s i m p l e molybdate and phosphate c o u l d be f a v o u r e d by a r i s e o f s o l u t i o n pH, which would s h i f t t h e chemical e q u i l i b r i u m t o l e f t . ,

Indeed, t h i s behaviour

was observed d u r i n g phosphomolybdate a d s o r p t i o n on alumina.

The i n c r e a s e i n

pH o f t h e i m p r e g n a t i n g s o l u t i o n was a t t r i b u t e d t o w a t e r f o r m a t i o n d u r i n g i o n exchange r e a c t i o n . L u t h r a and Cheng ( r e f . 10) observed t h a t e q u i l i b r i u m b e t w e e n heptamolybdate Mo,O:i rise Mo,O,,

and molybdate MOO:-

i o n s i s a l s o a f f e c t e d by a

i n pH. 6-

+

4 H,O

I n short,

=

2-

7 MOO,

+

(3)

8 H

r e s e a r c h o f Cheng and L u t h r a c l e a r l y i l l u s t r a t e s t h a t t h e h i g h

s o l u b i l i t y and s t a b i l i t y observed when p h o s p h o r i c a c i d i s added t o molybdenum i s m a i n l y due t o t h e f o r m a t i o n o f 'phosphomolybdate compounds.

These compounds

a r e v e r y s e n s i t i v e t o changes i n t h e i m p r e g n a t i n g s o l u t i o n pH. The f a c t t h a t t h e r e a r e d i f f e r e n c e s i n a c i d s i t e s d i s t r i b u t i o n and s u r f a c e d i s p e r s i o n when u s i n g d i f f e r e n t procedures t o p r e p a r e a MoP/A1 ,O,

catalyst,

suggests t h a t n a t u r e and c o n c e n t r a t i o n o f t h e o x i d i c supported phases p r e s e n t i n these c a t a l y s t s are d i f f e r e n t . Our h y p o t h e s i s i s c o n s i s t e n t w i t h t h e r e s u l t s o b t a i n e d by o t h e r researche r s ( r e f . 3, 6, 9, 111, who combining v a r i o u s c h a r a c t e r i z a t i o n techniques, s t u d i e d t h e s t r u c t u r a l changes t h a t o c c u r r e d when phosphorus i s used as an a d d i t i v e o f Mo/A1 ,O,

catalysts.

Atanasova and Halachev ( r e f . l l ) , s t u d y i n g

t h r o u g h I R spectroscopy t h e phases p r e s e n t i n NiMoP/Al ,O, by coimpregnation, observed bands c o r r e s p o n d i n g t o AlPO, and Ni-Mo-P

heteropoly

compounds.

IR-bands

c a t a l y s t s , prepared and t o a mixed A1-Mo

c o r r e s p o n d i n g t o b u l k MOO,,

Al,(MoO,), and NiMoO, were n o t observed i n t h o s e samples. Authors observed t h a t h i g h phosphorus c o n t e n t l e a d s t o an i n c r e a s e i n degree o f molybdenum p o l y m e r i z a t i o n and t o changes i n t h e r a t i o between t h e d i f f e r e n t t y p e s o f h e t e r o p o l y compounds, Ni-Mo-P/Al-Mo loadings.

Lopez Corder0 e t a l .

r a t i o i n c r e a s e s w i t h i n c r e a s i n g phosphorus ( r e f . 9) s t u d i e d by TPR and DRS t h e s u r f a c e

d i s t r i b u t i o n o f molybdenum on two s e r i e s o f MoP/A1 ,O,

c a t a l y s t s which were

prepared u s i n g simul taneous (P+Mo) o r double i m p r e g n a t i o n (P->Mo) methods. I n c o n t r a s t w i t h t h e r e s u l t s o b t a i n e d by Atanasova and Halachev ( r e f . 111, t h e a u t h o r s observed t h e presence o f b u l k MOO,

and a l s o small c l u s t e r s o f p o l y -

molybdate m u l t i l a y e r s f o r b o t h c a t a l y s t s e r i e s . l a t t e r specieswas

The c o n c e n t r a t i o n o f t h e

more i m p o r t a n t f o r t h e preimpregnated samples.

45

C o n s i d e r i n g o u r r e s u l t s t o g e t h e r w i t h t h o s e found i n l i t e r a t u r e ,

two

p o s s i b l e e x p l a n a t i o n s can be proposed: The f i r s t e x p l a n a t i o n i s based on t h e mechanism proposed f o r molybdate o r phosphate a d s o r p t i o n on alumina ( r e f . 121, because phosphomolybdate compounds were observed t o decompose i n t o these two species d u r i n g a d s o r p t i o n . These s t u d i e s suggest t h a t phosphate as w e l l as molybdate i n t e r a c t f i r s t w i t h b a s i c h y d r o x y l groups o f alumina, g e n e r a t i n g a w a t e r molecule. Competition between phosphorus and molybdenum t a k e s p l a c e . Phosphate i s adsorbed on alumina more r a p i d l y t h a n molybdate i s adsorbed.However, t h e r a t e o f a d s o r p t i o n o f b o t h compounds depends on s e v e r a l f a c t o r s , such as: i ) n a t u r e and c o n c e n t r a t i o n o f t h e i o n s i n t h e i m p r e g n a t i n g s o l u t i o n , ii) pH o f s o l u t i o n , iii) t y p e o f alumina and i v ) a d s o r p t i o n temperature. For each exchanged h y d r o x y l group by phosphoric a c i d molecule, two new a c i d s i t e s a r e c r e a t e d . T h i s would e x p l a i n t h e i n c r e a s e s i n a c i d s i t e s c o n c e n t r a t i o n observed a f t e r 12) suggested phosphorus i n c o r p o r a t i o n i n t o alumina. Morales e t a l . ( r e f . t h a t when a l l b a s i c h y d r o x y l groups a r e t i t r a t e d , t h e a c i d h y d r o x y l groups b e g i n t o be t i t r a t e d and t h e n a monolayer o f phosphate i s formed by f u r t h e r a d d i t i o n o f p h o s p h o r i c a c i d . I n t e r a c t i o n s between n e i g h b o r i n g adsorbed phosphates c o u l d occurs l e a d i n g t o t h e f o r m a t i o n o f p o l y m e r i c phosphate c h a i n s . I n t h i s p a r t i c u l a r case, t h e a u t h o r s mentioned t h a t t h e number o f a c i d s i t e s remains almost c o n s t a n t because t h e s u b s t i t u t i o n o f two a c i d h y d r o x y l groups o f alumina would y i e l d two a c i d s i t e s a s s o c i a t e d t o phosphorus. I n t h e c o m p e t i t i v e system, we can propose t h a t b o t h molybdate and phosp h a t e i o n s c o u l d be adsorbed i n n e i g h b o r i n g s i t e s . T h i s would i m p l y t h a t adsorbed molybdates impede t h e p o l y m e r i z a t i o n o f adsorbed phosphate and phosphates would produce t h e same e f f e c t on molybdates. The l a t t e r may e x p l a i n t h e h i g h e s t s u r f a c e d i s p e r s i o n o f b o t h P and Mo observed f o r t h e coimpregnated sample. Since p o l y m e r i z a t i o n o f phosphorus as we1 1 as molybdenum was f a v o u r e d by h i g h c o n t e n t s o f t h e s e elements, we c o u l d suggest t h a t i n t e r a c t i o n s between n e i g h b o r i n g molybdate and phosphate adsorbed species may occur c a u s i n g t h e f o r m a t i o n o f Mo-P h e t e r o p o l y compounds a f t e r c a l c i n a t i o n . One may s p e c u l a t e t h a t a c i d i t y produced f o r t h e l a t t e r compounds should be d i f f e r e n t f r o m t h a t produced by phosphates on alumina. The second e x p l a n a t i o n would be t o c o n s i d e r t h a t phosphomolybdate remains stable during impregnation. In t h i s case, t h e b e h a v i o u r o f these compounds towards a d s o r p t i o n would be d i f f e r e n t f r o m t h a t o f p h o s p h a t e a n d molybdate. One may s p e c u l a t e t h a t Mo-P h e t e r o p o l y compounds may be formed f r o m t h e adsorbed phosphomolybdate a f t e r c a l c i n a t i o n . T h e r e f o r e , t h e h i g h e s t d i s p e r s i o n observed f o r t h e coimpregnated sample may be a t t r i b u t e d t o t h e f o r m a t i o n o f Mo-P h e t e r o p o l y compounds, which were observed on coimpregnated samples i n a r e c e n t study ( r e f . 11). L e t us now complement o u r d i s c u s s i o n showing some i m p o r t a n t e f f e c t s

46

observed through scanning e l e c t r o n microscopy technique i n t h e a n a l y s i s o f t h e samples.

Figure 1 shows t h a t b o t h phosphorus and molybdenum a r e homogeneously

d i s t r i b u t e d i n t h e coimpregnated sample. and L u t h r a ( r e f .

8)

during adsorption o f

However, r e s u l t s obtained by Cheng

showed an i n t e r e s t i n g chromatographic e f f e c t appearing phosphomolybdate on alumina spheres,

p r e f e r e n t i a l l y l o c a t e d a t t h e edge, w h i l e molybdenum c e n t e r o f t h e spheres.

phosphorus was

was concentrated a t t h e

The l a t t e r o b s e r v a t i o n makes e v i d e n t t h e decomposition

o f phosphomolybdate d u r i n g a d s o r p t i o n and t h e c o m p e t i t i o n o f b o t h phosphate and molybdate

ions

for

the

same adsorption

sites.

I n our o p i n i o n ,

we

a t t r i b u t e t h e d i f f e r e n t behaviour observed i n b o t h s t u d i e s t o d i f f e r e n c e s i n p r e p a r a t i o n c o n d i t i o n s o f t h e samples, as w e l l as t h e type o f alumina employed. Preparation Procedures (P->Mo) and (Mo->P) As shown i n Tables 1 and 3, phosphorus seems t o improve t h e mechanical

p r o p e r t i e s as w e l l as t h e a c i d i t y s t r e n g t h o f alumina extrudates,

However,

t h i s a d d i t i v e s t r o n g l y a f f e c t s t h e s p e c i f i c surface area o f support. Our r e s u l t s a r e i n agreement w i t h those obtained by several i n v e s t i g a t o r s ( r e f . 7,9,12).

Lopez Cordero e t a l . ( r e f . 9)

suggested t h a t l o s s i n surface

area o f alumina a f t e r phosphorus i n c o r p o r a t i o n i s probably due t o a c o r r o s i v e e f f e c t o f surface caused by phosphoric a c i d molecules o r t o a pore blockage by phosphate species. t h e 30-60 explanation

A'

We have observed t h a t pores having an average diameter i n

range were t h e most a f f e c t e d by phosphorus d e p o s i t i o n .

An

o f these r e s u l t s c o u l d be t h e f a c t t h a t these pores c o u l d be

s e l e c t i v e l y plugged by polymeric phosphate adsorbed species,

However, we can

n o t d i s c a r d a p o s s i b l e c o r r o s i v e e f f e c t produced by t h e phosphoric a c i d molecules on t h e alumina surface.

An e l e c t r o n microscopy study o f t h e P/A1,0,

sample c o u l d reveal p o s s i b l e morphological changes due t o phosphorus. Table 2 i n d i c a t e s t h a t f o r t h e sample prepared f o l l o w i n g procedure ) was a f f e c t e d b y phosphorus ( P - > M o ) . t h e Mo d i s p e r s i o n ( I M o / I A 1 3P 2P i n c o r p a t i o n . The I P 2 p / I A 1 2 p i n t e n s i t y r a t i o a l s o decreased w i t h respect t o t h e value obtained f o r P/A1,0,

sample,

a f t e r molybdenum d e p o s i t i o n .

These

r e s u l t s can be explained by t h e f a c t t h a t preimpregnation o f alumina w i t h phosphorus reduces t h e number o f s i t e s a v a i l a b l e f o r molybdate adsorption. Therefore, changes i n d i s p e r s i o n and d i s t r i b u t i o n o f molybdenum species should be expected. I n t h i s case, one c o u l d suggest t h a t phosphorus promotes t h e formdtion o f b u l k MOO, species, which i s i n agreement w i t h t h e r e s u l t s o b t a i n ed by Lopez Cordero e t a l . deposited on t h e AlPO,

( r e f . 9).

monolayer,

A p a r t o f t h e l a t t e r species may be

which would e x p l a i n t h e decreasing i n

47

phosphorus

intensity

e l e c t r o n microscopy

r a t i o observed f o r study

confirms

the

this

sample.

Additionally,

heterogeneous d i s t r i b u t i o n

molybdenum species when alumina i s preimpregnated w i t h phosphorus.

the

o f the

A similar

s i t u a t i o n m i g h t be expected when t h e c a r r i e r i s preimpregnated w i t h molybdenum.

I n this

particular

case,

mechanical

strength

as w e l l

as a c i d i t y

p r o p e r t i e s s h o u l d n o t be improved by phosphorus. A c t i v i t y o f t h e (MotP) and (P->Mo) samples I n t h i s work we have e v a l u a t e d t h e c a t a l y t i c p r o p e r t i e s o f t h e molybdenum-phosphorus

catalysts.

For

this

purpose,

we

have

prepared

two

Ni-Mo-P/Al,O,

samples f o l l o w i n g t h e i m p r e g n a t i o n sequences (Mo+P->Ni) and

(P->Mo->Ni).

These samples p r e s e n t t h e same chemical

MOO,,

7 . 5 w t % P,O,

and 5.0 w t % N i O ) .

c o m p o s i t i o n (15 w t %

The c a t a l y t i c r e a c t i o n was c a r r i e d o u t

i n a h i g h p r e s s u r e f i x e d bed r e a c t o r u s i n g a vacuum g a s o i l under t y p i c a l mild-hydrocracking conditions

(T = 653K, P

= 5 MPa, LHSV = 0.65 l / h , H,/Hc

=

600). We p r e s e n t i n Table 4 t h e a c t i v i t y r e s u l t s o f t h e NiMoP/Al,O, Both samples

show comparable a c t i v i t y

i n MHC.

However,

r e a c t i o n s a r e h i g h e r f o r t h e coimpregnated sample.

catalysts.

t h e HDS and HDN

The same b e h a v i o u r was

observed by o t h e r r e s e a r c h e r s ( r e f . 13 and 1 4 ) . TABLE 4 Activity o f the

NiMoP/Al,O,

samples

% CONVERSION

HDS

HDN

MHC

(Mo+P-> N i l

82

59

13

(P-> Mo-> N i )

77

50

11

SAMPLE

I n o r d e r t o analyze t h e p o s s i b l e changes i n t h e molybdenum d i s t r i b u t i o n phases induced by phosphorus, we have c a r r i e d o u t I R measurements MoP/A1 ,O, samples b e f o r e n i c k e l i m p r e g n a t i o n . published

elsewhere

(ref.

molybdenum d i s t r i b u t i o n ,

15),

depending

confirm

the

on

both

The r e s u l t s , which w i l l be fact

modify

the

on t h e p r e p a r a t i o n procedure f o l l o w e d .

phosphorus

No

evidences about t h e presence o f phosphomolybdate compoundswere observed f o r t h e coimpregnated sample.

48

To summarize. we may conclude t h a t The d i f f e r e n c e s i n a c t i v i t y observed f o r t h e (MotP-,

N i l and (P->Mo->

N i ) samples c o u l d be associated t o changes

induced by phosphorus on t h e s u r f a c e molybdenum d i s t r i b u t i o n phases. CONCLUSIONS In

the

frame

of

the

present

work

we

conclude

that

coimpregnation

p r o c e d u r e i s t h e more a p p r o p r i a t e method o f p r e p a r i n g MoP/Al *03c a t a l y s t s . This procedure a l l o w s t h e o b t e n t i o n o f samples w i t h h i g h e r d i s p e r s i o n , surface a c i d i t y and c a t a l y t i c p r o p e r t i e s . Changes observed i n molybdenum d i s t r i b u t i o n phases caused by phosphorus depend on t h e impregnation sequence employed.

Coimpregnation procedure would

l e a d t o t h e Mo-P heteropoly compound formation,

w h i l e preimpregnation w i t h

phosphorus c o u l d induce t h e b u l k MOO, formation. I t was a l s o observed t h a t phosphorus produces t h e f o l l o w i n g e f f e c t s on

i ) i t improves t h e surface a c i d i t y , ii) i t increases t h e mechanical

alumina:

s t r e n g t h o f extrudates, iii) i t decreases t h e surface area o f support.

REFERENCES

L. Hilfman, U.S.

A. Morales, M.M. K. G i s h t i , A.

Patent 3.617.528

(1971).

Ramirez de Agudelo, Appl. Catal. 23 (1986) 23. i a n n i b e l l o , S. Marengo, G. M o r e l l i , Appl. Catal. 1 2 (1984)

381. G.A.

Mickelson, U.S.

Patent 3.749.663

S. S t a n i s l a u s , A. Asi-Habi,

(1973).

C. Dolana, Appl. Catal.,

39 (1988) 239.

P. Atanasova, T. Halachev, J. U c h y t i l , M. Kraus, Appl. Catal., 38 (1988), 235.

D. Chadwick, D.W.

Atchison, R.

C a t a l y s t s 111, (G. Poncelet, Amsterdan, p 323, 1983. W.C.

R.

B a d i l l a , L. Josefsson, P.

Grange,

P.

" I n preparation o f

Jacobs

eds).

Elsevier,

Cheng, N.P. Luthra, J. Catal., 109, (1988), 163. Lopez Cordero. N. Esquivel, J . Lazaro, J.L.G. F i e r r o , A. Lopez Agudo.,

Appl

. Catal . 48

(1989), 341.

Luthra, W.C., Cheng, J . Catal., 107, (19871, 154. 11 P. Atanasova, T. Halachev, App. Catal ., 48, (19891, 295.

10 N.P.

12 A. Morales, M.M. 261. 13 J.L.G.

Ramirez de Agudelo, F. Hernandez, Appl. Catal.,

41 (1988),

F i e r r o , A. LBpez Agudo, N. Esquivel, R. L6pez Cordero, Appl. Catal.

48 (1989), 353. 14 P. Atanasova, T. Halachev, J. U c h y t i l , M. Kraus, Appl. C a t a l . 38 (1988) 235. 15 R . Prada S i l v y , Y . Romero, M. GonzBlez, t o be published.

G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors),Preparation of Catalysts V 01991Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

49

SYNTHESIS OF NON-STOICHIOMETRIC SPINEL-TYPE PHASES: A KEY TOOL FOR THE PREPARATION OF TAILORED CATALYSTS WITH SPECIFIC ACTIVITY Michele Piemontese1,Ferruccio Trifiro'l, Angelo Vaccaril, Elisabetta Foresti and Massimo Gazzano2 'Dipartimento di Chimica Industriale e dei Materiali, Viale del Risorgimento 4,40136 BOLOGNA

pa"). Dipartimento di Chimica "G. Ciamician" ,

and CSFM (CNR) , Via Selmi 2 , 40126 BOLOGNA

(Italy). SUMMARY The preparation, stability and catalytic activity of non-stoichiometric spinel-type phases used in the synthesis of methanol were investigated as a function of the composition, heating temperature and atmosphere. It was shown that these phases formed mainly via amorphous chromates, especially for copper-rich catalysts. High activities in the synthesis of methanol were observed for zinc-rich samples (with a maximum for a catalyst in which 20% of the zinc ions were substituted by copper ions) and associated with the presence of a non-stoichiomemc spinel-type phase, stable also in the reaction conditions. On the other hand, the low activity of copper-rich catalysts was attributed to the instability of the spinel-type phase where much of the copper segregates into well crystallized metallic copper, with a further poisoning effect by zinc and cobalt. INTRODUCTION The majority of catalytic devices used in the modem chemical industry (i.e., both usual heterogeneous catalysts and materials based on applications of the catalytic properties) are based on mixed oxides (1- 3). The synthesis of specific tailor-made mixed oxides able to perform complex functions is one of the most current topics in solid state chemistry (4). Non-stoichiometric Zn/Cr and Cu/Cr mixed oxides are one of the principal examples of these unusual solids. They have applications as both solid state gas sensors ( 5 ) and catalysts for hydrogenation reactions (of CO to methanol and/or methanol-higher alcohol mixtures, and of many organic molecules) (6-12). These systems have been widely investigated over the last few years, and results obtained show that their peculiar catalytic properties may be associated with the presence of non-stoichiometric phases (with a M2+/M3+ratio higher than 0.5, M= metal), in which some of the zinc or copper ions are present in octahedral positions, i.e., with an unusual coordination. However, until now very few data have been reported regarding the changes in structure and reactivity as a function of the composition in ternary systems (for instance Cu/Zn/Cr). The aim of the research reported here was to study the changes in structure, stability and reactivity as a function of catalyst composition. In particular, attention was focused on the role of the Cu/Cu+Zn ratio and, for copper-rich catalysts, on the differences related to the partial substitution of copper by zinc or cobalt. In all samples the chromium content was 50% (as atomic ratio) in order to favour the formation of monophasic systems (13-16). EXPERIMENTAL The precursors with different atomic ratios were obtained by coprecipitation at pH= 8.0 k 0.1

50

Table 1. ComPosition of the catalyst investigated (as atom %) Sample

(Table 1). A solution of the nitrate salts of the elements was added to a continually stirred solution containing a slight excess of NaHC03 at 333K. Subsequent filtration

Cu:Zn:Co:Cr 0.0:50.0:0.0:50.0 was performed, followed by washing until the sodium 1 0 ~ 0 ~ 4 0 ~ 0 ~ o ~ o ~ 5content 0 ~ 0 was lower than 0.05% (as Na20). The precipitates Cat B were dried at 363K and heated at different temperatures Cat C 25.025.0:0.050.0 and in different atmospheres. Catalyst compositions were Cat 40’0:10.0:0*050’0 confirmed by atomic absorption using a Perkin Elmer 40.0:0.0:1~.0:50.0 mod 360 spectrophotometer. Cat E 50.0:0.0:0.0:50.0 The XRD analysis was carried out with a Philips Cat F PW1050/81 diffractometer controlled by a PW1710 unit, using Nickel-filtered CuKa radiation, h= 0.15418 nm (40kV, 40mA). The data were processed on a Olivetti M240 computer. The lattice constants were determined from diffractometric data by least squares refinements. The crystal sizes were determined by the Scherrer equation, using Warren’s correction for instrumental line broadening. Possible contributions to the line broadening due to disorder effects and/or lattice strains were not taken into account. The quantitative analysis of oxide phases in the catalysts was carried out using the method suggested by Klug and Alexander (17). The cation distribution between tetrahedral and octahedral sites in the cubic spinel-type phase was evaluated as an extension of the Bertaut method (18,19) on the basis of the I400/I440 ratio. A C.Erba Sorptomatic 1826 apparatus with N2 adsorption was used to measure the surface area. IR spectra were recorded using the KBr disk technique and a Perkin Elmer 1700 Fourier-transform spectrometer. U.v.-visible diffuse reflectance @R) spectra were recorded using a Uvikon 860 spectrophotometer, equipped with an integrator sphere. The amounts of CuO and chromates were determined after extraction with NH4OH:NH4NO3 (1:l v/w) (20) at 61511x11 and 446nm, respectively, using a Uvikon 860 spectrophotometer; in the last case, the solutions were previously buffered at pH= 5.0 f 0.1 with concentrated CH3COOH. The catalytic tests were carried out in a copper-lined piston flow reactor, operating at 6.0MPa and 500-600K, using a GHSV= 16,000h-’ and a H2:CO:C02= 65:32:3 (v/v). Before the catalytic tests, the catalysts were activated in-situ by hydrogen diluted in nitrogen; the hydrogen concentration and temperature were progressively increased during this pretreatment. Outlet gases were monitored on-line by gas-chromatography, while the liquid products were condensed under pressure in a cold trap at 253K during the time-on-stream (6h), then weighed and ‘analyzed off- line by gas-chromatography . Cat A

RESULTS AND DISCUSSION Nature of the precipitates The precipitates dried at 363K show XRD powder patterns typical of quasi-amorphous phases, identified as hydroxycarbonates on the basis of the IR spectra (21). Further information may be obtained on the basis of the values of empirical parameters A and B35, calculated from the DR spectra (22). All precipitates (Fig. 1) show similar values of the A parameter (related to the Cr-0 distance), while the B35 parameter (inversely proportional to the C3’- Cr3+ interaction) shows a minimum for the Cu/Cu+Zn= 0.5 ratio. Therefore, the same type of structure may be hypothesized for all precipitates, with an increase in surface crystallinity for Cat C .

51

I

I

I

18000

Nature of the samples heated at 653K

Figures 2a and b report the XRD powder patterns of the precipitates heated at 653K in air and in a reducing atmosphere (H2:N2= 10:90 v/v), respectively. Calcined samples (Fig. 2a) show the presence only of spinel-type phases, whose XRD patterns become more and more broad as the copper content increases. IR spectra confirm the presence, for all calcined samples, of spinel phases, and also show he presence of 0 0.5 1 .o dichromate-type phases (25), the amounts of Cu/Cu+M(IIl which increase with increasing copper content. In Fig.l.Empirica1 parameters calculated from the previous papers it was shown that nonDR spectra of precipitates dried at 363K: stoichiometric Zn/Cr spinel-type phases formed zinc (m.0); cobalt (0,0). by decomposition of amorphous chromates and that some amounts of residual Cr6' ions are present in these phases (8,15). Taking into account that copper and zinc may form mixed spinel-type phases (with cubic symmetry for high zinc contents) (20,24), we may hypothesize the formation up to a ratio Cu/Cu+Zn= 0.5 of cubic non-stoichiometric spinel-type phases, containing both elements and characterized by an I excess of bivalent ions. On the other hand, on the basis of the XRD spectra of Figure 2a, we cannot speculate about the number and/or nature of the phases present in the copper-rich catalysts. After the samples had been heated in an H m 2 atmosphere, the XRD powder patterns (Fig. 2b) again showed the presence only of spinel-type phases for Cu/Cu+Zn 20.5, while for the copper-rich samples the main phases present were Cu (Cat D and E) or CuO (Cat F). The lack of reoxidation for the metallic copper in Cat D and E, cannot be justified on the basis of differences of crystal size, but most probably can be attributed to the formation of copper-rich alloys at the surface of the particles. The presence of small amounts of zinc or cobalt does not modify the XRD powder pattern of the copper particles, but may strongly influence their physicochemical or catalytic properties (25-27). For all catalysts, the IR spectra show the presence, together with small amounts of residual carbonates, of the typical bands of spinels (even if not well resolved), wavenumbers cn except the Cu/Cr= 1.0 sample (Cat F) (Fig. 3) for which only a broad peak at 554 cm-' is present i n the low frequency Fig* 3. IR spectra Of Cat heated at 653K for 24h in air (a), N2 (b) region, attributable to the overlapping of CuO and Cr203 and H m 2 (c). absorptions (13,28).

k

\

D

El

Cat E

,

60

50

221P

40

30

60

50

2W"

40

30

60

.

,

50

.

221P

I

40

,

,

30

I

I

60

.

1

50

.

2+/"

I

40

.

I

30

. XRD powder patterns of the different catalysts heated at different temperatures and in different conditions. (a) 653K in air; (b) 653K in an H2/N2 (10:90 v/v) mixture; (c) 753K in air and (d) 853K in air. (&Tetragonal phase (ASTM 34-424); (A)ZnO (ASTM 5-664); (m) Cu (ASTM 4-836); (e)CUO(ASTM 5-661); (0)c0304 (ASTM 9-418); without symbol: cubic phase (ASTM 22-1 107 and/or 26-509) .

Fig. 2

53

Table 2 Amounts of CuO (w/w %) (a) and of Cr(h2- (w/w %) (b ) extracted for the catalysts heated in different conditions and after catalytic tests of the synthesis of methanol.

T, K 653K/air

Cat A

Cat B

Cat C

Cat D

Cat E

Cat F

a

b

a

a

a

a

a

-

6.0

b

6.0 13.5

b

13.5 26.5

b

28.5 27.5

b

20.0 13.5

b

29.5 28.0

2.5

n.d. n.d.

11.0 3.5

n.d. n.d.

n.d. n.d.

653IVI32-N~ -

0.5

10.0 0.5

24.5 1.0

40.0 2.0

41.0 2.0

48.0 1.0

853K/air

3.0

2.0

653K/N2

5.5

n.d. n.d. -

2.5

3.0

1.0

5.0

1.5

6.0

3.0

2.5

Afterreact.

-

15.0 1.5

38.5 1.0

n.d. n.d.

cuomax*

-

10.1

25.4

25.5

25.7

25.6

cuomax**

-

10.1

25.4

40.8

41.2

51.1

7.5

1.0

1.0

1.0

47.5 1.5

* calculated on the basis of a phase composition ZnCnO4 (or c O c n04) + CuCnO4 +CuO. **calculated on the basis of a phase composition CuO + ZnO (or COO) + Cr203. Further information on the effect of the heating conditions may be obtained from the values of surface area (Fig. 4) and the amounts of CuO and chromates extracted with NH40WNH4N03 solutions (Table 2). From Figure 4, it is possible to observe that the heating atmosphere has a small effect on the surface of the samples. In both cases, samples with large surface areas may be obtained, especially in the range 0s Cu/Cu+Zn 10.5. Furthermore, it is possible to observe that cobalt is a better physical promoter than zinc in both conditions. Table 2 shows that in the samples heated in a mixture of H m 2 practically all the Cu2+ ions present may be extracted. Therefore we may hypothesize that in these conditions copper gives rise to a separate phase, while the formation of spinel phases is due essentially only to zinc or cobalt, in agreement with the IR data for Cat F (Fig. 3). zoo I I I I However, it should be pointed out that copper 0 H a N 2 M Zinc AlrM 28°C containing phases were detected only for the 6 ~~rM=Coball j HUNZM=CobalI copper-rich catalysts, while for Cu/Cu+Zn ratios 150,,a 5 0.5 they escape XRD detection. Table 2 confirmes the presence in the calcined samples of increasing amounts of chromates, with a maximum for Cat F in which ca 43% of the total chromium is present as Cr6' ions. Up to a Cu/Cu+Zn ratios 0.5, the amount of CuO extracted is lower than both the theoretical value 50 and the chromate content, and does not depend directly on the latter. This is particulary true for Cat B, taking into account also the values of the 0 0.2 04 06 08 1 samples heated at 653K in N2. Therefore, two Cu / Cu + M(II) (atomic ratio) Cu2+ containing fractions are present in these spinel-type phases, which show different Fig.4. Surface area of the samples heated at 6S3K for 24h in air and H7h-7 .. - - . solubilities in the m 40H /N H 4N 03 solution, but

1

~

~~

3

~

Table 3 Crystallographic data for the samples calcined at 753 and 853 K -~~ -

~-~~_ _ ~

~

Sample T , K

Phase

a, nm

..- -

.

c,nm

c.s.,nm

cuoa

-

~

C U O ~ ZnOa

ZnOb

-~

~~

Xr

XO

xc ~~

CatA CatB CatC CatD CatE

753

cubic + ZnO

0.8356(1)

7.5

853

cubic + ZnO

0.8336(5)

13.0

753

cubic

0.8342(2)

853

cubic + CuO

0.8334(3)

753

cubic + CuO

0.8344(5)

4

25.8

0.246

0.137

0.41 1 0.448

14

25.5

0.246

0.082

7.5

-

10.1

-

15.5

0.242

0.158

0.400

12.0

2

10.1

-

15.5

0.266

0.124

0.406

6.5

3

25.4

-

0.249

0.137

0.409

853

cubic+ CuO

0.8325(1)

69.0

18

25.4

-

0.253

0.049

0.465

753

cubic + CuO

0.8323(5)

7.0

15

25.5

-

0.243

0.077

0.453

853

cubic + CuO

0.834(3)

10.0

25

25.5

-

0.242

0.005

0.502

cubic + CuO

0.833(3)

753 853

tetragonal + CuO 0.5969(4)

0.798(2)

7.O

12

25.7

-

0.23 1

0.107

0.441

28.0

27

25.7

-

n.d.

n.d.

n.d.

7.5

13

25.6

-

n.d.

n.d.

n.d.

22

25.6

-

n.d.

n.d.

n.d.

i Co,04

CatF

753

cubic > tetragonal 0.832(4)

853

tetragonal + CuO 0.6025(6)

0.780(3)

16.0

CuCr02

0.2977(6)

1.704(5)

15.0

n.d.

n.d.

n.d.

CuCrOp

0.2975(2)

1.7105(9)

22.0

n.d.

n.d.

n.d.

+ CUO CatF

753/N, 853/N, -

C.S.= crystal size of the spinel-type phase; (a) = amount detected; (b) = amount calculated on the basis of a phase composition ZnCr20, (or CoCr204)

+ CuCr,O, + CuO. xT,xc, xo = on the basis of structural formula (M2txT)

'e'rohedro'

(MZtxo,C?',c)

oclahedral

0

55

are both not detectable by XRD and IR analysis. On the other hand, the amount of CuO extracted in the copper-rich samples is mainly related to the presence of Cr6' ions (compare Cat D heated in air or N2). However, it must be noted that for all samples the amount of CuO overlaps that of chromates ( if both are expressed on an atom basis), indicating that also in these samples a consistent fraction of CuO ( more than 33% of the total CuO extracted) may be present as excess of Cu2+ ions inside a spinel-type phase. Unfortunately, no further support for this hypothesis may be obtained from XRD powder patterns. In recent papers (13,29) Bonnelle et al. have reported the formation, by heating at 643K in N2, of cubic non-stoichiometric spinel-type phases for copper chromite catalysts, with Cu/Cr ratios from 0.8 to 1.5. However it must be pointed out that these authors claimed as necessary for the stability of the spinel-type phases, the formation of consistent amounts of Cr6' ions (36% or more of the total chromium), during the controlled decomposition of the hydroxynitrate precursors. In our case, the Cu/Cr= 1.0 sample gives rise upon heating in N2, mainly to the formation of CuCro;! (Fig. 3), as c o n f i i e d also by XRD patterns, the amount of which increases with increasing temperature. This difference may be explained taking into account the different natures of the precursors; however, it must be pointed out that the amount of Cr6' ions present in Cat F calcined at 653K as well as its IR spectrum are similar to those reported in the literature (13,29). Therefore, for copper-rich compositions the formation of non-stoichiometric phases may be related mainly to a controlled oxidation of the precursor obtained by coprecipitation, while, on the contrary, this does not seem to be a key factor for the zinc-rich catalysts, in agreement with that previously reported for Zn/Cr catalysts (14). Furthermore, the data reported in this section, suggest that non-stoichiometric phases do not form by heating the precipitates in a reducing atmosphere. Thermal stabilitv of the samples.

In order to test the thermal stability of the catalysts, the precipitates were calcined at 753 and 853K for 24h. The XRD powder patterns of these samples are shown in Fig. 2c and d, and the main Surface area (rn2/g) XRD data are summarized in Table 3. A cubic spinel-type phase is the main component present in ,Ki= Zlnc 853K M= Zmc all samples calcined at 753K, also for the copperrich catalysts, suggesting that in these samples it forms via the amorphous phases discussed in the I5O previous section. Therefore a general mechanism of formation of the spinel-type phases by decomposition of chromate phases may be hypothesized. When the temperature is increased further to 853K3,the cubic phase is again detected, with the exception of Cat E and F, in which teuagonal CuCnO4 is present. With increasing calcination temperature, increased segregation of oxide phases takes place. 0 0.2 0.4 06 0.8 1 However, it must be noted in regard mainly to the Cu I Cu + M(II) (atomic ratio) CuO, that ZnO is observed only for the binary Cat A. Fig. 5 . Surface area of the samples calcined Furthermore, CuO segregation is less marked for at 753K and 853K. Cu/Cu+Zn I 0.5 ratios, while in the copper-rich

t

1

56

catalysts the values detected at 853K approach the calculated ones. The high stability towards calcination temperature of Cat B is worth noting, in which only 20%of the zinc ions are substituted by copper ions. For this catalyst ZnO is never observed, whereas 20% CuO is detected only after calcination at the highest temperature. On the other hand, the partial substitution of copper ions with zinc ions stabilizes the cubic spinel-type phase, whereas for cobalt ions this effect is less marked probably because of their tendency to segregate as c0304. In the literature it is reported that the cubic CuCr204 phase is stable only at high temperatures (30). Furthermore, it is noteworthy that no formation of CuCrO:! was observed when Cat D was heated in N2 up to 853K. The increase in stability for the ternary catalysts also is reflected by the surface areas of the samples calcined at 753K (Fig. 5). After calcination at 853K all samples show a dramatic decrease in surface area and appreciable differences are no longer detected. The collapse of the catalyst structure is also responsible for the low values of CuO extracted (Table 2), taking into account that a sample of CuO (E.Merck, Germany) calcined at 853K for 24h was fully soluble in the N H 4 0 W N 0 3 solution. As already mentioned the 14oO/I440 ratio may be assumed to be a measure of the distribution ratio between the occupancy of the tetrahedral and octahedral sites in the cubic cell of the spinel-type phase. According to Miller data of the octahedral site preference energies (31), it is assumed that the Cr3+ ions are all located in octahedral sites, whereas the Zn2+ and Cu2+ ions are present in both tetrahedral and octahedral sites, according to results previously reported for Zn/Cr catalysts (14-16). From Table 3, it is possible to observe that the tendency of the M2+ ions to be retained in the octahedral positions decreases with increasing calcination temperature and copper content, indicating that the CuO side phase detected may be mainly attributable to the segregation of Cu2+ ions present in octahedral positions of the spinel- type phase. The behaviour of Cat B is noteworthy; in this catalyst more M2+ ions tend to be retained in octahedral positions than is the case for ZdCr catalysts (16). Catalytic activitv in methanol svnthesis Methanol productivity (g/h kg Cat.) The catalytic activity in methanol synthesis of the catalysts investigated is reported in Figure 6 as a function of the Cu/Cu+Zn ratio (Cat E, containing cobalt, was practically inactive in the temperature range investigated). The progressive substitution of zinc ions with copper ions gives rise to considerable differences in the catalytic activity, as a function of the copper content. However, two general behaviours are found: 1) Up to a Cu/Cu+Zn ratio5 0.5, the presence of copper considerably increases the activity in methanol synthesis, with a very high selectivity in methanol (the main by-product being water). 2) For the highest ratio, a dramatic ' deactivation is observed, accompanied also by a 0 0.25 0.5 0.75 Cu / Cu + Zn (atomic ratio) change in selectivity, especially for Cat D, for which Fig- 6. Catalytic activity as a function of the the formation of hydrocarbons (mainly methane) is Cu/Cu+Zn ratio. also detected. On the other hand, Cat F shows the

51

formation at T2 550K of dimethylether, with a corresponding decrease in the values of productivity of methanol. However, the value of productivity in methanol for Cat F at 540K is in very good agreement with that reported by Apai er al. (32). It must be pointed out that the main increase in catalytic activity takes place for Cat B, in which only 20% of zinc ions is substituted by copper ions. The methanol productivity of this catalyst is similar to the best values reported in the literature , if based on kg of catalyst (33- 34), but clearly better if calculated on the basis of kg of copper, thus indicating the formation of very active copper-containing centers. XRD powder patterns of the catalysts after reaction show the presence until a Cu/Cu+Zn ratios 0.5 only of cubic spinel-type phases, while for copper-rich samples the main phases identified are metallic copper (Cat E) and Cu20 (Cat F). Furthermore, from Table 2, it is possible to observe that for these last catalysts practically all the theoretical CuO is extracted by NH40WNH4N03, while for Cat B and C values very similar to those of the calcined samples are obtained. In a previous paper, it was shown that there is a rough correlation between the catalytic activity in methanol synthesis and the whole chemisorption activity towards CO (35). Therefore, the low catalytic activity of copper-rich catalysts may be attributed to the segregation of much of the copper as well crystallized metallic particles (36). The further decrease in activity for Cat D and, especially, Cat E is in agreement with the hypothesis of the formation of alloys at the surface of the copper particles (26,27), taking into account the poisoning effect of small amount of cobalt (37). On the other hand, the high activity of Cat C and, especially, Cat B may be correlated to the presence of a non-stoichiometric spinel-type phase, in which some copper and zinc can be found in octahedral positions or to an interaction between highly dispersed metallic copper formed in reducing conditions and the spinel-type phase. However, it must be pointed out that this copper fraction is so dispersed and stable that it is not detected even after the catalytic tests. The role of a not detectable fraction of copper has already been reported in the literature (20, 36). Furthermore, the activity of Cat B higher than that of Cat C (notwithstanding its slightly lower CO chemisorption capacity) suggests that an important role is played by the oxide matrix, very probably in the hydrogen activation step (38,39). CONCLUSION Non-stoichiometric spinel-type phases may be obtained mainly via amorphous chromates and their stability and reactivity are strongly influenced by the composition. Very stable spinel-type phases active in the synthesis of methanol may be obtained at low copper contents, while copper-rich catalysts show a considerable tendency for segregation of metallic copper with a considerable decrease in catalytic activity . However, these last catalysts, especially those containing cobalt or zinc, may be very active in the hydrogenation of 0x0-aldehydes (12). Furtehr work will be directed towards obtaining a better understanding of the nature of the active sites, taking into consideration that the reaction conditions adopted are quite different from those reported by other authors (10,ll). REFERENCES

1 J.J. Burton and R.L. Garten (Ed.s), Advanced Materials in Catalysis, Academic, N.Y., 1977. 2 O.T. Sorensen (Ed.), Non-Stoichiometric Oxides, Academic, N.Y., 1981. 3 H. Yanagida, Angew. Chem. (Engl. Ed.) 27 (1988) 1389-1392.

58

4 G. Centi, F. Trifiro’ and A. Vaccari, Chim. Ind. (Milan), 71 (1989) 57-62. 5 A. Jones, P. Mosely and B. Tofield, Chem. Brit. 8 (1987) 749-766. 6 E. Errani, F. Trifiro’, A. Vaccari, M. Richter and G. Del Piero, Catal. Lett. 3 (1989) 65-72. 7 P. Courty, D. Durand, E. Freund and A. Sugier, J. Mol. Catal. 17 (1982) 241-254. 8 A. Riva, F. Trifiro’, A. Vaccari, G. Busca, L. Mintchev, D. Sanfilippo and W. Manzatti, J. Chem. SOC.,Faraday Trans. 1 83 (1987) 2213-2225. 9 G. Fornasari, S. Gusi, F. Trifiro’ and A. Vaccari, I&EC Res. 26 (1987) 1500-1505. 10 R. Bechara, G. Wrobel, M. Daage and J.P. Bonnelle, Appl. Catal. 16 (1985) 15-27 11 L. Jalowiecki, G. Wrobel, M. Daage and J.P. Bonnelle, J. Catal. 107 (1987) 375-392. 12 G. Braca, A.M. Raspolli Galletti, F. Trifiro’ and A. Vaccari, Italian Pat. n. 21831A (1989). 13 G. Wrobel, J. Arsene, M. Lenglet, A. d’ Huysser and J.P. Bonnelle, Materials Chem. 6 (1981) 19-34. 14 G. Del Piero, F. Trifiro’ and A. Vaccari, J. Chem. Soc., Chem. Commun. (1984) 656-658. 15 G. Del Piero, M. Di Conca, F. Trifiro’ and A. Vaccari, in P. Barret and L.C. Dufour (Ed.s), Reactivity of Solids, Elsevier, Amsterdam, 1985, pp. 1029-1034. 16 M. Bertoldi, B. Fubini, E. Giamello, G. Busca, F. Trifiro’ and A. Vaccari, J. Chem. Soc., Faraday Trans. 184 (1988) 1405- 1421. 17 H.P. Klug and L.E. Alexander, X-Ray Diffraction Procedures, Wiley, N.Y., 1974, ch. 7. 18 E.F. Bertaut, C.R. Acad. Sciences 230 (1950) 213-215. 19 L. Weil, E.F. Bertaut and L. Bochirol, J. Phys. Radium 11 (1950) 208-212. 20 J. Escard, I. Mantin and R. Sibut-Pinote, Bull. Soc. Chim. France (1970) 3403-3408. 21 K. Nakamoto, Infrared and Ranian Spectra of Inorganic and Coordination Compounds, Wiley, N.Y., 1978. 22 L. Nondek, D. Mihajlova, A. Andreev, A. Palazov, M. Kraus and D. Shopov, J. Catal. 40 (1975) 46-51. 23 J.A. Campbell, Spectrochim. Acta 21 (1965) 1333-1343. 24 C. Delorme, C.R. Acad. Sciences 241 (1955) 1588-1589. 25 L. Aitchison and W.R. Barclay, Engineering non-ferrous metals and alloys, Frowde and Hodder & Stoughton, London, 1923, ch. VII. 26 T. Van Henvijnen and W.A. De Jong, J. Catal. 34 (1974) 209-214. 27 R. Cao, W.X. Pan and G.L. Griffin, Langmuir 4 (1988) 1108-1 112. 28 R.A. Nyquist and R.O. Kagel, Infrared Spectra of Inorganic Compounds, Academic, N.Y., 1971. 29 A. d’ Huysser, G. Wrobel and J.P. Bonnelle, Nouv. J. Chem. 6 (1982) 437-442. 30 V.M. Ust’yantsev and V.P. Mar’evich, Izv. Akad. Nauk. SSSR 9 (1973) 336-337. 31 A. Miller, J. Appl. Phys. Suppl. 30 (1959) 245-251. 32 J.R. Monnier, M.J. Hanrahan and G. Apai, J. Catal. 92 (1985) 119-126. 33 K. Klier, in D.D. Eley, H. Pines and P.B. Weisz (Eds), Advances in Catalysis, Academic, N.Y., 1982,31, pp. 243-312. 34 E.B.M. Doesburg, R.H. Hoppener, B. de Koning, X. Xiaoding and J.J.F. Scholten, in B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet (Eds), Preparation of Catalysts IV, Elsevier, Amsterdam, 1987, pp. 767-780. 35 M. Piemontese, F. Trifiro’, A. Vaccari, B. Fubini, E. Giamello and I. Rumori, XI1 Simp. Iberoamericano de Catalisis, Rio de Janeiro, Brasil, July 27-August 3, 1990. 36 S. Gusi, F. Trifiro’, A. Vaccari and G. Del Piero, J. Catal. 94 (1985) 120-127. 37 E. Errani, G. Fornasari, T.M.G. La Torretta, F. Trifiro’ and A. Vaccari, in F. Cossio, G. del Angel, 0. Bermudez and R. Gomez (Ed.s), Act. XI Simp. Iberoamericano de Catalisis, IMP, Mexico D.F., 1988,111, pp 1239-1247. 38 G. Busca and A. Vaccari, J. Catal. 108 (1987) 491-494. 39 G. Busca, M.E. Pattuelli, F. Trifiro’ and A. Vaccari, in C. Morterra, A. Zecchina and G. Costa (Eds), Structure and Reactivity of Surfaces, Elsevier, Amsterdam, 1989,239-248.

G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

59

EFFECT O F PREPARATION VARIABLES ON CATALYTIC BEHAVIOUR OF COPPER/ZIRCONIA CATALYSTS FOR THE SYNTHESIS O F METHANOL FROM CARBON DIOXIDE R.A. KOEPPEL', A. BAIKER', Ch. SCHILD' and A. WOKAUN' 'Department of Industrial and Engineering Chemistry, Swiss Federal Institute of Technology, ETH - Zentrum, CH - 8092 Zurich (Switzerland) 2Physical Chemistry 11, University of Bayreuth, D-8580 Bayreuth (FRG)

SUMMARY A series of copper-zirconia catalysts have been prepared by methods of sequential precipitation, coprecipitation and deposition precipitation. The influence of various pretreatments and of the copper/zirconla ratio on the structural and chemical properties of these samples are examined. High activity and selectivity of the catalysts is shown to be correlated to the presence of amorphous zirconia which is stabilized by copper ions. The results indicate that the structural and chemical properties of the support and particularly the interface copper/zirconia are most decisive in governing the catalytic properties of these methanol synthesis catalysts. INTRODUCTION The synthesis of methanol from syngas (CO/CO,/H,) using Cu/ZnO/Al,O, catalysts is a well established industrial process (ref. 1,2). More recently zirconia-supported copper catalysts were found to be active and selective for this reaction (refs. 3-9). Although methanol synthesis catalysts have been studied intensively for several years there is still much controversy about the nature of the active components and the reaction steps that take place on them. Many aspects of the reaction mechanism are still not fully understood and are the subject of an active debate. Several investigations (refs. 10,ll) have shown that over typical commercial catalysts practically all of the methanol is formed from CO, under industrial conditions and that support effects are minimal for these catalysts (ref. 12). Other workers reported a marked support effect for the synthesis of methanol over copper catalysts prepared by different methods (refs. 3,7,8,13) showing that the activity of supported copper catalysts depends strongly on both the choice of the support and the nature of the feedstock. The results suggest that more than one mechanism may lead to methanol. As the choice of the preparation method and also of the further thermal and chemical treatments control the behaviour of a catalytic system to a large extent, considerable differences are found in catalysts of the same nominal composition but prepared in different ways (ref. 6). The morphology of a catalytic system as well as the appearing crystallographic phases are determined by the method of preparation.

60

While the conventional methanol synthesis reaction from syngas has been studied intensively, little attention has been paid so far to the synthesis from CO, and H,. In the present work a series of Cu/ZrO, catalysts were prepared and tested for methanol synthesis from carbon dioxide and hydrogen. Special emphasis was devoted to the influence of the preparation variables on the structural, chemical and catalytic properties of the catalysts. EXPERIMENTAL Catalyst Preparation A first type of catalyst precursors was prepared by sequential precipitation at constant pH and temperature (Samples S). An aqueous solution of ZrO(CH,C00),-2H20 or ZrO(NO,), .2H20 (0.6 M) and an aqueous solution of sodium hydroxide/sodium formate (2 M each) were poured into two separate dropping funnels. The reagents were added dropwise with vigorous stirring into a Pyrex flask containing 250 ml deionised water at 363-368 K. The addition was adjusted to keep the pH constant at about 7. The precipitation was complete after 5 min and the precipitate was further aged for 15 min at the same temperature. An aqueous Cu(N0,),.3H20 or Cu(CH,COO),.H,O solution (1.5 M) was then added simultaneously with the alkaline solution under the same conditions as described above. Finally the precipitate was aged for further 30 min in the mother-liquor at 368 K and then filtered using a G-4 glass filter. The residue was washed four times by redispersing it in 200 ml deionised water. After washing of the precipitate with 200 ml methanol the voluminous gel was dried I t 333 K in a vacuum drier (the vacuum was kept at 1.25.1U Pa by a small stream of air passing through the drier) for 15 h to yield a rigid solid. This material was crushed to a grain size of 50 - 150 pm using an agate pestle and mortar. Sample A was prepared in the same way except that zirconia was substituted by alumina. Pure zirconia was prepared analogously by precipitation of zirconyl nitrate. Sample C was made by coprecipitation instead of sequential precipitation. Sample H was prepared by the method of deposition precipitation using urea. A suitable amount of amorphous zirconia was suspended in deionised water. After the addition of copper(I1)nitrat and urea the temperature was brought to 363 K under constant stirring. The reaction was accompanied by a rise of the pH to a final value of 8. The final product was treated in the same way as the sequentially precipitated catalysts. Nitrate- and acetate-precursors of copper and zirconium, respectively, were used for preparations to avoid the presence of chloride species in the final catalysts. All chemicals used were of analytical grade. The dried precursors were studied in both the freshly prepared state and after calcination in air at appropriate temperatures. Catalyst characterization The catalysts were characterized by means of nitrogen adsorption, nitrous oxide titration, X-ray diffraction (XRD), thermal analysis (TG/DSC), temperature-programmed reduction

61

(TPR) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Surface areas (SBm)were calculated using a value of 0.162 nm2 for the cross-sectional area of the nitrogen molecule (ref. 14). Pore size distributions were determined following the BJHmethod (ref. 15) using the equation of Halsey (ref. 16). Nitrous oxide pulse experiments were carried out using the procedure to that reported by Evans et al (ref. 19). Samples were first reduced in a flux of 75 ml min-’ 5 % H,/Ar by heating at 5 K m i d from 373 K to 523 K. Subsequently the samples were hold at this temperature for 30 min and then exposed to a flux of 75 ml m i d pure hydrogen for 1 h at the same temperature. The hydrogen was purged with 50 ml m i d He at 523 K for 5 min. After cooling to 363 K under He, pulses of nitrous oxide (0.5 cm3) were injected. Copper metal surface areas were calculated assuming 1.46.1e9 copper atoms per mz (ref. 15). Back-titration of the oxidized copper surface was realized either by pulses of CO (0.5 cm3) at 423 K or by TPRmeasurements starting at 263K. X-ray diffraction patterns were measured using a powder diffractometer (Philips PW 1700) and Cum-radiation. Mean crystallite sizes were estimated from the peak width at half maximum of the (111) reflection of Cu or CuO, respectively, using the Scherrer equation. The measured peak width was corrected for instrumental broadening using the function proposed by Warren (ref. 17). Thermogravimetric- (TG) and differential scanning calorimetric- (DSC) studies were carried out using a Mettler TA 2000C thermoanalyzer. Measurements were performed under air with a heating rate of 10 K min.’. The apparatus used for the TPR studies was described in an earlier report (ref. 18). TPR profiles were measured under the following conditions: heating rate 10 K m i d , flow rate 75 ml m i d 5 % H,/Ar. The conditions of the IR-measurements have been reported in detail elsewhere (ref. 20). Catalytic tests The apparatus used for the catalytic tests consisted of a continuous tubular fixed-bed reactor (8 mm id.) which was operated at 1.7 MPa. Details are reported in (ref. 9). The premixed gas contained 25 mol% CO, and 75 mol% H, and was fed from a high-pressure cylinder. The reaction flow rate was typically 90 ml m i d (STP). All experiments were carried out in the temperature range 433 - 533 K using 1.0 g of catalyst (50 - 150 pm sieve fraction). The prereduction of the catalysts prior to the kinetic tests was performed according to the following procedure: heating to 473 K at a heating rate of 15 K m i d in 1.25 vol% HJN, at a pressure of l@Pa. The H,-concentration was then increased stepwise (30 min per step) in the sequence 2.5/5/10/20/50 to 100%. After replacement of H, with reactant gas the temperature was brought to 533K under a pressure of 1.7 MPa. RESULTS Influence of chemical comDosition Samples with more than 20 at% copper resulted in black precipitates which is indicative

62

for the formation of cupric oxide under hydrothermal conditions. DSC curves of the decomposition behaviour of the dried precipitates are shown in Fig. 1. The endotherm events between 323 and 423 K are due to the volatilization of physisorbed and crystalline water, respectively. The corresponding TG curves showed the same characteristics as found for a zirconia gel (ref. 21). Dehydration occurs primarily between 300 and 700 K with a final weight loss of about 10% of the initial sample weight. Pure zirconia (Fig. la) exhibits an exothermic peak at about 710 K, characteristic for zirconia prepared by wet-chemical routes. The socalled glow-exotherm is commonly associated with the transition of an X-ray amorphous zirconia phase into a crystalline modification of zirconia. The presence of copper shifts the exothermal crystallization peak to 823 K for sample Sl (10 at% Cu)and to 893 K for samples S3-S7 (30-70 at% Cu, Fig. 1). The exothermic peaks at 473 K and 550 K (Fig. lc) are associated with the presence of acetate in the precursor, as emerges from a comparison with the DSC curve of the pure nitrate precursor (Fig. If). Calcination at 623 K i n air for 3 h results in an acetate free catalyst (Fig. Id). The exothermic signals appearing at 480 and 613 K after exposing this catalyst to methanol synthesis

v

I

273 373 473 573 673 773 873 973 Temperamre (K) Fig. 1. DSC curves of samples a) ZrO, , 6) SI, c) SS, d ) SS-6234 e) S5-623K after

reaction, f) S5N.

3

373

473

573

Temprmfrrrc

673

773

873

(IS)

Fig. 2. Temper~tiire-progr~irt~r, icd rdiictiori profiles or samples o) S2,1)) S.7, C ) SS, (1) S5-623 , e) S5-923/(, j) SSN.

R

63

conditions are accompanied by a small weight gain due to reoxidation of reduced copper species (Fig. le). TPR-profiles are shown in Fig. 2 for samples with various copper contents. Note that all copper of calcined samples is quantitatively reduced to cu" below 523 K in 5% H,/Ar. Increasing the calcination temperature results in a shift of the T,,-values of the reduction profiles to lower temperatures (Fig. 2d,e). The small peak at about 740 K occurring with all samples, except the one calcined at 923 K, was accompanied by a measurabIe exothermicity and is attributed to the crystallization of amorphous zirconia. Pure uncalcined zirconia support yielded also a small peak at 718 K, followed by two broad peaks at 793 and 913 K, while a calcined sample (723 K) did not show the crystallization-peak at 718 K. Uncalcined samples resulted in TPR profiles which depended on the nature of the precursor salts used for preparation. While pure nitrate precursors showed a single reduction peak at 500 K (Fig. 2f), samples prepared in the presence of acetate-ions resulted in TPR profiles consisting of three peaks at about 500,567 and 623 K (Fig. 2b,c). We conclude that the reduction peaks at 500 and 567 K are attributable to the reduction of CuO, whilst the peaks at 623 K are due to the reductive decomposition of acetate-modified zirconia. Calcination at 623 K led to the disappearence of the reduction peak at 623 K and to a shift of the peaks at 500 and 567 K to lower temperatures (Fig. 2d). Nitrogen adsorption/desorption isotherms were measured after drying/calcination as well as after methanol synthesis reaction. All the isotherms were of type IV (BDDT classification), indicating the presence of well developed mesoporous systems (ref. 14). The shape of the hysteresis loop changed from type H2 (IUPAC classification, ref. 14) for both samples dried at 393 K and calcined at 623 K, to type H1 for samples dried at 923 K. Note that all samples were degassed at 523 K. Mesopore size distributions calculated from the adsorption and desorption branch of the isotherms showed the presence of pore size maxima in the range 3 - 5 nm for all samples except for those calcined at 923 K which exhibited a shift of the maxima to 8 - 9 nm. The mesopore volume decreased simultaneously from about 0.20 cm'/g to 0.12 cm3/g. Calculated t-plots (ref. 22) indicated the presence of microporosity for uncalcined samples before and after methanol synthesis reaction. Surface areas for all samples following methanol synthesis are listed in Table 1. Influence of calcination temnerature The X-ray patterns of the uncalcined zirconia support (Fig. 3a) shows only two broad bands in the range of 20" to 40" and 40" to 75" for 26, indicating the presence of zirconia with very low degree of crystallinity. Calcination of the sample at 723 K for 3 h resulted in metastable, probably tetragonal ZrO, and some stable monoclinic ZrO,. Note that a distinction between tetragonal and cubic zirconia is not possible based on XRD data alone. However, based on thermodynamical arguments tetragonal ZrO, is more likely. Higher calcination temperatures lead to an increase of the fraction of the monoclinic phase (Fig. 3b shows a sample calcined at 770 K for 3 h in air) until an almost pure monoclinic phase is obtained after calcination at 970 K.

64

TABLEI Textural properties of the catalysts Catalyst Precursor@’ Composition Cu/Zr (at%)

T,, (K)

SI

10/90 20/80 30/70 40/60

623 623

70/30 50/50

ni/ni ni/ac ni/ni

623 623 923

18/82

ac/ni

50/50

623 823

s2 s3 s4 s5 s7 S5-623 S5-923 S5N c5 H H-823 A fb)

ac/ac

ni/ni ac/ac

50/50

SBET

(mYguT)

169 181 183 174 174

57 138 44 141 161

80 177

dcu (nm)

SC,

@’/SCAT) 1.70

2.25 3.80 4.80 7.10 8.60 6.90 2.50 5.70 6.30 3.00 3.20 7.85

from N,O from XRD 21.0 33.0 30.5 37.5 29.7 37.5 30.6 84.5 37.1 33.5 25.0 23.3 41.7

30.0 31.0 16.5

25.0 40.0 39.0

(a) ac = acetate, ni = nitrate. (b) zirconia was substituted by alumina Sample H, calcined at 823 K for 3 h, yielded XRD-patterns characteristic of crystalline CuO and tetragonal ZrO,. No indication for the presence of the monoclinic phase of zirconia could be found. The XRD-patterns of the dried sample S5 are shown in Fig. 3c and are indicative of copper oxide and amorphous zirconia. The CuO particles had a mean size of about 15 nm as estimated from the line broadening of the CuO (111) reflection. Calcination at 970 K for 3 h resulted in well crystallized tetragonal zirconia and in CuO-particles of about 29 nm mean diameter (Fig. 3d). No reflections of the monoclinic phase were found with this sample. The XRD patterns of a sample after use for methanol synthesis are shown in Fig. 3e. The occurence of reflections due to crystalline Cu,O beside of the reflections of crystalline copper particles can be explained by the exposure of the sample to air during its transfer from the reactor to the XRD-measurement. Zirconia existed as amorphous phase. Mean copper particle sizes calculated from the line broadening of the Cu (111) reflections and copper surface areas measured by N,O-titration are listed in Table 1. By using a half-sphere model the average copper particle sizes were calculated from the copper surface areas. Back titration using CO-pulses at 423 K yielded values for copper surface areas identical to those measured by N,O-pulses. No CO, could be detected in the effluent gas stream indicating that none of the adsorbed carbon monoxide was removed as carbon dioxide at 423 K in the case of zirconia containing catalysts. This contrasts the behaviour of alumina or silica supported copper catalysts where all CO evolved as CO,. Back titration using TPR resulted in reduction peaks between 333 and 373 K for all samples. A characteristic feature of all catalysts was the appearence of a broad, intense desorption peak in the temperature range 513 to 573 K.

65

Catalvtic behaviour Preliminary experiments with respect to possible influences caused by interparticle and intraparticle mass transfer limitations confirmed that such limitations could be ruled out under the conditions used in this study. The results of the CO, hydrogenation experiments over the different catalysts are summarized in Table 2 and reflect the steady-state behaviour of the catalysts at 493 K after 15 h on stream. Carbon containing products were only methanol and carbon monoxide for all catalysts. Acetate precursors yielded some ethanol in the initial stage of reaction at 533 K, probably due to the hydrogenolysis of acetate species in the acetatemodified oxides. No ethanol could be detected after calcination of these samples at 623 K. The catalytic behaviour of some catalysts was compared on the basis of uncalcined samples. Note that the decomposition of the precursor under reducing conditions resulted in improved catalytic properties compared to the oxidative decomposition (Table 2). The activities of the catalysts were also compared on the basis of turnover frequencies (TOF) which were calculated either as molecules of methanol formed or as molecules CO, reacted per copper surface atom and per second.

stnt.

h

Ih

I.

~~

<-

I

0.05

I

3500 2e (degree.7) Fig. 3. X-my diffraction patterns (Cu,) of surnples n ) ZrO, , b) Zr02-770K, c) SS, rl) S5-970K rind e) S5 rrfer reriction.

3000

2500

2000

IS00

ZOO

wavenritiiber (cm ') Fig. 4. Diffuse reflectance FTIR spectra of sntnple S7, reaction of CO, nn H2 (1.3) nt 47.1 K ond 0.25 MPn.

66

Prevalent surface species Diffuse reflectance F I I R spectroscopy was used to investigate the species prevailing on the catalyst surface under CO, hydrogenation conditions at 2.5 bar. Before the FTIR measurements, the catalysts were heated in a hydrogen flow to the desired reaction temperature. The measurements were started by switching from the hydrogen flow to a defined flow of CO, and H,. Subsequently the surface reactions were studied by recording time-resolved FI'IR spectra as shown in Fig. 4. The spectra indicate that surface formate is readily formed after the reactant feed has been switched on. After 10 minutes peaks grow at 1150, 1050 and around 2900 cm-' reflecting the formation of surface-bound formaldehyde, methylate and methanol species. The peaks due to methanol are enhanced if the flow is stopped and the surface is exposed to static conditions. Methanol formation is paralled by the observation of gas phase CO originating from the reverse water gas shift reaction. It is noteworthy to mention that in all FTIR measurements performed with either CO,/H, or CO/H, no conclusive correlation between the disappearance of the prevalent formate species and the formation of methanol could be observed. On the other hand, the appearance of gas phase methanol was always associated with the observation of some CO, as well as formaldehyde and methylate surface species (ref. 20). DISCUSSION An importand aspect of the work concerns the nature of the zirconia phase produced and its influence on the catalytic behaviour. The occurence of ethanol species in the initial stage qf rraction under methanol synthesis conditions as well as the DSC and TPR data indicate the formation of anion modified oxides from acetate precursors as proposed earlier by Yurieva (ref. 23) for other oxides. We conclude from measurements with sequentially precipitated catalysts (ex zirconyl acetate/copper nitrate) and from the occurence of cupric oxide in the preparation step that oxygen ions in the zirconia are partially substituted by acetate ions. It is interesting to note that calcination of copper containing catalysts results in the formation of metastable tetragonal zirconia, while calcination of pure zirconia leads to the formation of stable monoclinic ZrO,. Although, an oxidative thermal treatment (DSC under air) shows a marked influence on the crystallization temperature of the amorphous zirconia, under reducing conditions (TF'R) the same crystallization temperature is observed for both zirconia as well as copper containing samples. These results confirm that probably copperions are responsible for the stabilizing effect onto amorphous zirconia and that the transformation of these ions into metallic copper eliminates this effect. The activity measurements (TOF values) summarized in Table 2 indicate that the intrinsic activity of the catalysts decreases with increasing copper content. An increase in CO, conversion is accompanied by a concomitant decrease in methanol selectivity. Calcination of samples S5 and H at 923 and 823 K, respectively, resulted in the transformation of the initially amorphous zirconia to crystalline tetragonal zirconia. The most striking effect of the crystallization process was the collaps of the activity and selectivity of the catalysts which was accompanied by a simultaneous decrease in the BET-surface area. It is interesting to note that

67

TABLE 2 Catalytic properties of the catalysts Catalyst Precursor"

Sl s2 s3 s4 s5 s7 S5-623 S5-923 S5N C5 H ff-823 A (b)

ac/ac

Composition Cu/Zr (at%) 10/90

20/80

30/70

T,, (K) 623 623

40/60

ni/ni ac/ac

50/50 70/30 50/50

ni'ni ni/ac ni/ni

18/82

ac/ni

50/50

623 623 923 623 823

Conversion (%)

3.8 4.1 5.9 5.9 6.6 8.1 5.2 3.6 5.9 6.0 4.6 2.8 5.3

Selectivity TOF,,,, to methanol (d) (%)

73 73 70 70 66 61 67 51 66 67 69 49 37

Z24.103 6.18 5.14 3.89 2.89 2.52 2.33 3.35 3.22 2.87 4.81 1.90 1.13

TOF,,, (s-') 9.17-10' Z 48 6.37 5.04 3.81 3.86 3.09 5.91 4.25 3.91 6.29 3.59 2.77

(a) ac = acetate, ni = nitrate. (b) zirconia was substituted by alumina. in the case of sample H the copper surface area remains constant, however the intrinsic activity decreases. As sample A (Cu/Al,O,) had about the same copper surface area as the zirconia based catalysts, we may conclude that the copper surface area alone cannot explain the catalytic behaviour of the catalysts. High activity and selectivity are related to the presence of amorphous zirconia. Owen et a1 (ref. 24) proposed that oxygen anion vacancies characteristic of the fluorit structure of zirconia could be important in the methanol synthesis reaction. The presence of anions like acetate, formate or nitrate in the precursors could result in a distortion of the oxide structure of zirconia and their removal may generate vacancies in the anion lattice which are known to stabilize the cubic structure of zirconia. The crystallization of the amorphous zirconia is likely to result in a drastic decrease of the copper/zirconia interfacial area which certainIy contributes to the loss of activity observed upon crystallization. Our investigations provide further support for the crucial role of the interfacial area in copper/zircania catalysts. Further work focusing on the structural and chemical properties of this interphase and its role in methanol synthesis is presently undertaken . As to the surface species observed by in situ FTIR measurements during methanol synthesis, the most striking result is that the surface concentration of the prevalent formate species seems not to be directly influenced by methanol formation. This behaviour will be discussed in detail elsewhere (ref. 20).

CONCLUSIONS The present studies confirm that highly active and selective Cu/ZrO, catalysts can be

68

prepared by precipitation. The catalytic behaviour of these catalysts depends strongly on the structural and chemical properties of the zirconia support. High activity and selectivity are related to the presence of amorphous zirconia which is stabilized by copper ions. Crystallization of the amorphous zirconia by calcination in air at appropriate temperatures results in the formation of metastable tetragonal zirconia and leads to a drastic decrease of the catalytic activity and selectivity. The results indicate that the structural and chemical properties of the support and particularly of its interface with the copper-species play an important role in methanol synthesis from CO,/H,. On all catalysts surface formate was found as an abundant surface species. However, the appearance of methanol is not correlated with the disappearance of formate, but with a decrease in surface formaldehyde and methylate signals. ACKNOWLEDGEMENTS This work has been supported by grants of the "Swiss National Science Foundation" (2.102086), the "Bundesamt f i r Bildung und Wissenschaft" and the "Deutsche Forschungsgemeinschaft" (SFB 213). REFERENCES 1 J.C.J. Bart and R P A . Sneeden, Catal. Today, 2 (1987) 1-124. 2 G.C. Chinchen, P.J. Denny, J.R Jennings, M.S. Spencer and KC. Waugh, Appl. Catal., 36 (1988) 1-65. 3 B. Denfie and R P A . Sneeden, Appl. Catal., 28 (1986) 235-239. 4 Y.Amenomja, AppL Catal., 30 (1987) 57-68. 5 B. Pommier and S.J. Teichner, in M.J. Philips and M. Ternan (Eds.), Proc. 9th Int. Congr. Catal., Calgary, 1988, The Chemical Institute of Canada, Ottawa, 1988, Vo1.2, pp. 610-617. 6 Y.Amenomiya, I. T. Emesh, K W Oliver and G. Pleizier, in M.J. Philips and M. Ternan (Eds.), Proc. 9th Int. Congr. Catal., C a l g q , 1988, The Chemical Institute of Canada, Ottawa, 1988, Vo1.2, pp. 634-641. 7 G.J.J. Bartley and R Burch, Appl. Catal., 43 (1988) 141-153. 8 B. Denise, 0. Cheriji, M.M. Bettahar and R.P.A. Sneeden, Appl. Catal., 48 (1989) 365372. 9 D. Gasser and A. Baiker, Appl. Catal., 48 (1989) 279-294. 10 A.Ya Rozovskii, YaB. Kagan, G.I. Lin, E.K Slivinskii, S.M. Loktev, L.G. Liberov and A.N. Bashkirov, Kinet. Catal., 17 (1976) 1132-1138 (engl). I1 G.C. Chinchen, P.J. Denny, D.G. Parker, G.D. Short, D.A. whan, M.S. Spencer and X C . Waugh, Am. Chem. SOC.Div. Fuel Chem., 29 (1984) 178-188. 12 G.C. Chinchen, KC. Waugh and D.A. Whan, Appl. Catal., 25 (1986) 101-107. 13 L R Jennings and M.S. Spencer, in C. Morterra, A. Zecchina and G. Costa (Eds.), Structure and Reactivity of Surfaces, Elsevier, Amsterdam, 1989, pp. 515-524. 14 KS.W Sing, D.H. Everett, RA.W Haul, L. Moscou, RA.Pierotti, J. Rouquh-ol and T. Siemieniewska, Pure & Appl. Chem., 57 (1985) 603-619. 15 E.P. Barrett, L.S. Joyner and P.P. Halenda, J. Am. Chem. SOC.,73 (1951) 373-380. 16 G. Halsq, J. Chem. Phys., 16 (1948) 931-93% 17 B.E. Wmen, J. AppL Phys., 12 (1941) B75. 18 D. Monti and A. Baiker, J. Catal., 83 (1983) 323-335. 19 J.K Evans, M.S. Wainwright,A.J. Bridgewater and D.J. Young, Appl. Catal., 7 (1983) 7583. 20 Ch. Schild, A. Wokaun and A. Baiker, submitted for publication. 21 P.D.L. Mercera, J.G. Van Ommen, E.B.M. Doesburg, A.J. Butggraaf and J.RH. Ross, Appl. Catal., 57 (1990) 127-148. 22 B. K Lippens and J.H. de Boer, J. CataL, 4 (1965) 319-323. 23 T.M. Yurieva, React. Kinet. Catal. Lett., 29 (1985) 49-54. 24 G. Owen, C.M. H a w k s and D. Lloyd, AppL Catal., 58 (1990) 69-81.

G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation ofCata2ysts V 01991 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands

PREPARATION OF Ti0,-Al,O,

69

BY IMPREGNATION WITH TiC1,-CCl,

Liu Yingjun, Zhang Qinpei, Zhu Yongfa, Gui Linlin and Tang Youqi Institute of Physical Chemistry, Department of Chemistry Peking University, Beijing 100871 (China)

ABSTRACT Catalyst carrier Ti0,-AI,O, was prepared through impregnation y-Al,O, with non-aqueous solution of TiCI, (e.g. carbon tetrachloride or acetone as solvent), followed by calcination at 550'c for 24 h. Series of Ti0,-AI,O, samples with various TiO, loadings have been characterized by XRD, XPS, TEM and HEED techniques. The maximum dispersion capacities for TiO, on y-Al,O, measured by XRD and XPS are 0.12g TiO,/ g y-AI,O, and 0.1 lg TiO, / g y-Al2O3, respectively. It was also verified by TEM and HEED techniques. This value illustrates that TiO, disperses on the surface of y-Al,O, as a submonolayer and the observed monolayer coverage for TiO, on y-Al,O, is 58% as compared with a close-packed monolayer model.

INTRODUCTION With its special properties, TiO, attracts more attention recently (refs. 1-2). Especially, to be used for hydrodesulfurization (HDS) or hydrodenitrogenation (HDN) in the petroleum refining process (refs. 3-4), the character of the catalyst with TiO, carrier is superior to that with y-Al,O, carrier. However, TiO, is seldom used as a catalyst carrier in commercial process. The reason is that as compared with the widely used industrial catalyst carrier y-AI,O, it has two disadvantages, one is that its specific surface area is rather small, usually of 10 mz / g and can only reach some tens mz / g prepared in special ways, the other is its strength rather poor with its mechanical strength five times less than that of y-Al,O,. In order to improve the properties of TiO, as a carrier many authors have tried several preparing methods for modifying y-Al,O, with TiO,, such as coprecipitation (ref.5), impregnation with aqueous solution (ref.6), mixed gelatinization (ref.7) and kneading process (ref.8). This research prepared monolayer dispersion type Ti0,-AI,O, carrier with y-Al,O, as a matrix impregnated with non-aqueous solution of TiCI, (e.g. carbon tetrachloride or acetone as solvent) and characterized it with XRD, XPS, TEM and HEED techniques.

70

EXPERIMENTAL Sample preparation y-Al,O, supplied by Changling Oil Refinery of Chinese Petro-Chemical Corporation, was ground down into small particles. The 40-80 mesh of y-Al,O, particles was calcinated in a muffle furnace at 650C for 4h. After calcination the specific surface area of y-Al,O, is 205m2/ g. The samples with various titanium oxide content of Ti0,-AI,O, carrier were prepared by the following procedures: impregnation of y-Al,O, with TiCI,-CCI, (or TiCI,-CH,COCH,) solution; giving off the solvent; neutralization with 1:1 ammonia liquor; washing with dilute ammonia solution until no CI- to be detected, then calcination at 550'c for 24 h. X-ray diffraction The XRD analysis, involving qualitative and quantitative analysis, have been carried out on a home made BD-86 diffractometer, with Cu Ku radiation, Ni filter and a scintillation couter. The residual crystalline TiO, after being dispersed on y-Al,O, can be measured by the matrix-flushing method (refs. 9-10) for quantitative X-ray diffraction analysis and calculated from the equation

ZI/ I ,

=kx, /x,

(1)

where I, is the X-ray intensity of the peak (101) for TiO,; I, is the X-ray intensity of the peak (200) for the flushing agent, KCl; x l and x , are the weight fractions of TiO, to the flushing agent, KCI, respectively; the constant k is the reference intensities of TiO, to KCl. In this case, the value of k is 0.99, calculated from the Equation(1). Specific surface area Specific surface areas of y-Al,O, and Ti0,-AI,O, desorption gas chromatography method.

were determined with thermal

XPS measurements All XPS spectra were acquired by using a VG ESCA LAB 5 Spectrometer equipped with a 4025 Data System. An aluminum anode (A1 Ku 1486.6eV) operated a t 12 kV and 40 mA was used. During recording spectrum, pressure inside the sample chamber was ca. 5 x 10-*m bar. Intensities of Ti2p and A12p photoelectron lines were measured by the peak area with 4025 Data System. Sample was well spread on one side of double-sided adhesive tape and fixed on the sample stub. The extent of carbon contamination on samples controlled to be nearly the same by monitoring the CIS peak area, and then the influence of carbon contamination can be neglected.

71

u

5 -m

0.40

4 0 A

N

m

i

Crd Z L

9

o'\

0.30

4 -

d

;2 2

N

m

0.20

a

0.10

0.00 0.00

0.10

Ti0

0.40

0.30

0.20 2

0.50

loading ( g / g Y - A 1 2 0 3 )

Fig.1 Dispersion threshold of TiOz on y-Al20, by XRD quantitative extrapolationmethod (prepared with TiC1,- CCI, solution)

N 3

d

Y \

3

0.20-

.3

c

l i

0.00

0.20

0.40

0.60

T i O Z loading ( g / g f - A 1 2 0 3 )

Fig.2 Relationship between ITizp/ IAnP and TiO, loading of Ti0,-AI,O, (prepared with TiC1,- CCI, solution)

72

TEM and HEED measurements Transmission Electron Microscopy (TEM) image and High Energy Electron Diffraction (HEED) pattern were carried out with JEM-200CX Electron Microscope operated at 200 kV.

RESULTS AND DISCUSSION XRD quantitative measuremental results The intensities of TiO, (101) peak of series of Ti0,-AI,O, samples were measured by XRD quantitatively. With residual amount of TiO, crystallite in samples after calcination as ordinate and with loading of TiO, as abscissa, plot has been got in Fig.1. The plot is a broken line consisting of two straight lines and shows a threshold value. Before the threshold TiO, disperses on the y-Al,O, surface as a monolayer and no TiO, peaks were observed. After the threshold, the residual amount of TiO, is in crystalline form on y-AI,O, surface. There is a linear relationship between the residual amount of TiO, crystallite and the added loading of TiO, on y-Al,O,. As impregnation y-Al,O, with TiCI,-CCI, solution, from Fig. I , the maximum dispersion has been obtained as 0.12g TiO, / g y-Al,O,. In the case of preparing through impregnation y-Al,O, with TiCI,-CH,COCH, solution, the maximum dispersion is 0.1 Ig TiO, / g y-Al,O,. The specific surface of y-Al,O, used is 205m2/ g. In terms of surface area, the dispersion threshold for TiO, on y-Al,O, is 5.6 x 104g TiO, / m2. We can estimate the utmost monolayer capacity by employing the simple close packed monolayer model. Assuming that 02-ions from TiO, form a close-packed layer on the surface of y-Al,O, and the Ti4+ ions occupy the interstices formed by 02-ions. Taking 1.40A as the radius of 02-ion, the surface area occupied by a TiOz "molecule" on the surface of y-A1203 can be calculated as 2 . 0 4 ~ 10-'9m2, in other words, forming each m2 close-packed monolayer of TiO, requires 9.7 x 104g TiO,, i.e. the monolayer capacity of TiO, on y-Al,O, should be 9.7 x 104g TiO,/ m2. Comparing the observed dispersion threshold for TiO, on y-Al,O,as mentioned above, thus the observed monolayer coverage for TiO, on y-Al,O, is 58%. This value illustrates that TiO, disperses on the surface of y-AI,O, as a part covered monolayer or a submonolayer, rather than as a full covered monolayer. XPS measuremental results The intensities ratio ITi2, / I,,,, of XPS for series of TiO, / y-Al,O, have been determined (refs. 11-12). The relationship between ITi2,/ I,,,, and TiO, loading is shown in Fig.2. The plot consists of two straight lines with different slope, and the amount of TiO, at intersection point happened to correspond with the value of monolayer dispersion threshold by XRD as mentioned above. When the TiO, loading is less than the threshold, the monolayer coverage increases with the increase of TiO, loading. Thus the intensity I,,, increases in proportion to the TiO, loading directly and the intensity I,,,, from matrix

73

Fig.3 TEM image of Ti02-A1203 Samples (prepared with TiCI,- CCI, solution) a. y-Al,03 b. Ti02 (Anatase) c. 0.061g Ti02/ g y-Al,O, d. 0.44g TiO, / g y-Al,O,

14

Fig.4 HEED patterns of Ti0,-AI,O, Samples (prepared with TiC1,- CC1,solution) a. y-A120, b. TiO, (Anatase) c. 0.06 1g TiO, / g y-Al,O, d. 0.44g Ti02/ g 9-A120,

75

y-Al,O,

remains unchanged essentially. As a result, intensity ratios ITiZp / IAIZP increase

linearly with the increase of Ti02 1oading.When the TiO, loading is higher than the threshold, the residual crystalline TiO, appears and resides on the top layer of the sample. / I,,,, increases with the increase of TiO, loading. Since XPS is Also the intensity ratio ITiZp a surface sensitive technique, the photoelecton contribution to the peak of Ti2p depends on the inelastic mean free path. Therefore the intensity contributed by crystalline TiO, is much less than that by monolayer TiO, on the surface of y-Al,O, at the same amount. Although two lines from two sources in plot are present in linear way, the slopes of them are quite different. In Fig.2 the loading amount of TiO, at intersecting point of two straight lines corresponds to the threshold of monolayer dispersion, 0.1 l g TiO, / g y-Al,O,. The threshold a o cords with the XRD quantitative measurement by extrapolation. The result shows that both methods, XPS intensity ratio method and XRD quantitative extrapolation method, are complementary to each other in determining the dispersion and studing the surface state of TiO, / y-Al,O,. TEM and HEED measuremental results The results of XRD quantitative measurement and XPS measurement were also verified by TEM and HEED techniques. Fig.3 and Fig.4 only show the results for samples prepared with TiC14-CCI, solution omitting similar results with TiCI,-CH,COCH, solution. As the loading amount of TiO, on y-Al,O, is less than the dispersion threshold, the TEM image (see Fig.3~)and the HEED pattern (see Fig.&) show just like that of matrix y-A1,0, (see Fig.3a and Fig.4a). As the loading amount of TiO, on y-Al,O, is more than the dipsersion threshold, the TEM image (see Fig.3d) and the HEED pattern (see Fig.4d) show like that of y-Al,O, and additional crystalline TiO,.

CONCLUSIONS The results mentioned above have illustrated that as the samples of Ti0,-Al,O,carrier were prepared with impregnation of TiCI, nonaqueous solution (carbon tetrachloride or acetone as solvent), the maximum dispersion capacities for TiO, on y-Al,O, is 5.6 x 104g TiO,/ mz with coverage of 58%, and TiO, with molecule^^ disperses on y-Al,O, surface in form of a submonolayer, rather than as a full covered monolayer. It is different from the preparation with impregnation of Ti(SO,), aqueous solution. In this case strong acidity of Ti(SOJ, solution (PH 1-2) would cause y-Al,O, partially solubilizing during impregnation and then depositing with Ti4+ on y-Al,O, to form Ti0,-AI,O, mixed gelatination. Thus, the method described in this paper may be employed as a method for monolyer modification of y-Al,O, with TiO,.

76

ACKNOWLEDGMENT The authors are grateful to the National Laboratory for Structural Chemistry of Unstable and Stable Species (Beijing, China) for financial support.

REFERENCES 1 S.J. Tauster, S.C. FungandR.L. Garten, J.A.C.S., 1(1978), 170-175 2 R.T.K. Baker, E.B. Prestridge and R.L. Garten, J.Catal., 56(1979), 390-406 3 M. Takeuchi, S. Matsuda, H. Okada, H. Kawagoshi, F. Nakajima, Ger Offen 2838231 (1979) 4 H.B. Jones, and R. Smith, US Pat 4206038 (1980) 5 E. Rodenas, T. Yamaguchi, H. Hattori and K. Tanabe, J. Catal., 69(1981), 434-444 6 G.B. McVicker, and J.J. Ziemiak, J. Catal., 95(1985), 473-481 7 Zhu Yongfa, Gui Linlin and Tang Youqi, CUIHUA XUEBAO (Journal of Catalysis. China), 10(1989), 118-122 8 Liu Yingjun, Luo Shengcheng, Gui Linlin, Annual Report of National Laboratory for Structural Chemistry of Unstable and Stable Species (Beijing, China), 1987, pp.61 9 F.H. Chung, J.Appl.Crst., 7(1974), 526-531 10 Liu Yingjun, Xie Youchang, Ming Jing, Liu Jun and Tang Youqi, CUIHUA XUEBAO (Journal of Catalysis, China), 3(1982), 262-267 11 S.C. Fung, J.Catal., 58(1979), 454-469 12 Gui Linlin, Liu Yingjun Guo Qinlin, Huang Huizhong and Tang Youqi, Scientia Sinica (Series B), Chinese Ed., No.6 (1985), 509-517; English Ed., 28(1985), 1233-1242

77

G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 01991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

INTERACTIONS OF THE IMPREGNATING SOLUTION WITH THE SUPPORT DURING THE PREPARATION OF Rh/Ti02 CATALYSTS

R.J. FENOGLIO, W . ALVAREZ, G . M . N U N E Z , INTEHA ( U n i v e r s i d a d N a c i o n a l d e Mar

J u a n B. J u s t o 4302, ( 7 6 0 0 ) Har d e l P l a t a , A r g e n t i n a

D . E . RESASCO d e l Plata-CONICET)

SUHHARY W e have a n a l y z e d t h e d i f f e r e n t phenomena o c c u r r i n g d u r i n g t h e impregnation of t i t a n i a w i t h aqueous s o l u t i o n s of rhodium chloride. Our r e s u l t s d e n o n s t r a t e t h a t t h e d e p o s i t i o n of t h e Rh p r e c u r s o r s on t i t a n i a , a t low pH v a l u e s ( a b o u t 3), i n v o l v e s t h r e e d i f f e r e n t p r o c e s s e s : a ) a d s o r p t i o n o f a n i o n i c species, b ) l i g a n d e x c h a n g e r e a c t i o n and c ) a c i d a t t a c k o f t h e s u p p o r t .

INTRODUCTION The i n t e r a c t i o n s

taking

place

between

the

metal

precursor

s p e c i e s and t h e s u p p o r t d u r i n g i m p r e g n a t i o n p r o c e s s e s , may p l a y decisive role in catalysts.

determining

the

resulting

properties

of

a the

I n t h e case of t h e i m p r e g n a t i o n of o x i d e s u p p o r t s w i t h

a q u e o u s s o l u t i o n s c o n t a i n i n g t h e metal p r e c u r s o r s , w e c a n i d e n t i f y

we c a n mention t h e a d s o r p t i o n o f m e t a l p r e c u r s o r species on t h e o x i d e

s e v e r a l p o s s i b l e t y p e s o f i n t e r a c t i o n s ( r e f . 1 and 2).

First,

s u p p o r t ( r e f . 3), which h a s b e e n a w i d e l y i n v e s t i g a t e d phenomenon. W e c a n a l s o c o n s i d e r t h e p o s s i b i l i t y t h a t t h e a d s o r b e d s p e c i e s may undergo a s u b s e q u e n t s u r f a c e r e a c t i o n w i t h t h e s u p p o r t ( r e f . 4 ) . F i n a l l y , when t h e i m p r e g n a t i n g s o l u t i o n is a c i d i c , t h e r e e x i s t s t h e p o s s i b i l i t y t h a t t h e s u r f a c e of t h e s u p p o r t be a t t a c k e d by t h e This acid impregnating s o l u t i o n c a u s i n g a partial d i s s o l u t i o n . a t t a c k w a s f i r s t p r o p o s e d by S a n t a c e s a r i a e t a l . ( r e f . 5 ) f o r t h e i m p r e g n a t i o n of H2PtC16 on A 1 2 0 3 . More r e c e n t l y , i t h a s a l s o been

I t h a s been s u g g e s t e d t h a t t h e acid a t t a c k m a y n o t only involve t h e partial d i s s o l u t i o n found t o o c c u r on o t h e r systems ( r e f . 6 ) . of t h e s u p p o r t , b u t a l s o

a

T h i s , i n turn,may r e s u l t

in

subsequent

p r e c u r s o r s b y s u p p o r t e d species.

a

re-deposition

partial

These

coverage

ideas

may

a s s o c i a t e d w i t h t h e w e l l known SMSI e f f e c t ( r e f .

of be 8),

7). metal immediately (ref.

the

which

also

78

i n v o l v e s a p a r t i a l c o v e r a g e o f t h e m e t a l by t h e s u p p o r t .

However,

w h i l e t h e SMSI e f f e c t r e q u i r e s a h i g h t e m p e r a t u r e r e d u c t i o n s t e p t o become e v i d e n t , an a c i d a t t a c k t o t h e s u p p o r t f o l l o w e d by r e - d e p o s i t i o n m a y have s i m i l a r e f f e c t s a f t e r low t e m p e r a t u r e reduction. In t h i s contribution we w i l l

consider

these

particular

case

of

p r e p a r e d by i m p r e g n a t i o n of T i 0 2

with

rhodium

interactions for the

of

three

types

Rh/TiOp

of

catalysts

chloride

species

from a q u e o u s s o l u t i o n s . EXPERIMENTAL To s t u d y t h e phenomena a s s o c i a t e d w i t h t h e i m p r e g n a t i o n p r o c e s s i n v o l v e d i n t h e p r e p a r a t i o n o f Rh/Ti02 catalysts, w e have a n a l y z e d t h e c h a n g e s o c c u r r i n g i n t h e i m p r e g n a t i n g s o l u t i o n and t h e s u p p o r t when 60 cc o f a 3.0 x lo-’ l4 a q u e o u s s o l u t i o n of RhC13.3 H20 (from A l f a P r o d u c t s ) w a s p l a c e d i n c o n t a c t w i t h one gram o f TiOz(Degussa P25. BET 55 m 2 / g ) used a s r e c e i v e d . The rhodium c h l o r i d e complexes p r e s e n t i n s o l u t i o n , b e f o r e and a f t e r b e i n g c o n t a c t e d w i t h T i 0 2 , w e r e c h a r a c t e r i z e d by UV-visible spectroscopy. The e v o l u t i o n of p H a f t e r p l a c i n g t h e s u p p o r t i n c o n t a c t w i t h i m p r e g n a t i n g s o l u t i o n w a s c o n t i n u o u s l y m o n i t o r e d by a d i g i t a l pH-meter.

The amount of Rh d e p o s i t e d on

remaining

in

solution

during

the

the

catalyst

different

p r e p a r a t i o n were d e t e r m i n e d by a t o m i c a b s o r p t i o n . reduced

catalysts

were

further

characterized

stages

The by

and

that

of

the

fresh

and

transmision

e l e c t r o n m i c r o s c o p y (TEH) and X-ray d i f r a c t i o n ( X R D ) . RESULTS AND DISCUSSION The Rh (111) c h l o r i d e s y s t e m i n a c i d i c s o l u t i o n s can e x h i b i t a +3-nwhich m a y v a r y series of complexes o f t h e type RhC1,(HzO)6-n from RhC16 3- t o Rh(H20)6 3+ . T h e s e species c a n b e e a s i l y c h a r a c t e r i z e d by UV-visible a b s o r p t i o n s p e c t r o s c o p y . The s p e c t r a o f c h l o r o - c o m p l e x e s o f R h ( I I 1 ) e x h i b i t two m a x i m a i n t h e 300-600 nm r e g i o n a s c r i b e d t o d-d t r a n s i t i o n s and a very i n t e n s e c h a r g e t r a n s f e r band i n t h e s h o r t e r w a v e l e n g t h r e g i o n ( r e f . 9 ) . The p o s i t i o n o f t h e b a n d s d e p e n d s on t h e number o f c h l o r i d e and water l i g a n d s . When t h e number of w a t e r l i g a n d s i n c r e a s e s , t h e l i g a n d f i e l d t h e o r y p r e d i c t s t h a t t h e d-d t r a n s i t i o n m a x i m a must s h i f t t o s h o r t e r wavelengths.

I n f a c t , it is e x p e r i m e n t a l l y o b s e r v e d

that

79

t h e y s h i f t from 412-518 nn f o r t h e h e x a c h l o r o complex ( n = 6 ) t o 305-393 nm f o r t h e hexa-aquo complex (n=O). T h e r e f o r e , t h e n a t u r e of t h e species p r e s e n t i n s o l u t i o n can b e , a t least q u a l i t a t i v e l y , e s t i m a t e d from t h e s p e c t r a . F i g u r e 1 shows a b s o r p t i o n spectra of t h e aqueous s o l u t i o n of RhC13 used i n o u r i m p r e g n a t i o n p r o c e s s . The p o s i t i o n of t h e m a x i m a f o r t h e f r e s h i m p r e g n a t i n g s o l u t i o n i n d i c a t e t h a t t h e impregnating s o l u t i o n h a s a of a n i o n s w i t h 5 and 6 c h l o r i d e l i g a n d s .

high

concentration

In order t o s a t i s f y

the

m a s s and c h a r g e b a l a n c e s , t h e s e complexes must b e compensated by a

comparable number of aquo complexes, a l t h o u g h t h e low p H v a l u e of t h e s o l u t i o n i n d i c a t e s t h a t a f r a c t i o n o f t h e s e aquo complexes may have undergone h y d r o l y s i s f o r m i n g

Rh(OH)m(HgO)6-n n- complexes.

I

300

400

500

nm

600

F i g u r e 1. A b s o r p t i o n spectra of rhodium chloride supernatant solutions after different t i m e s of c o n t a c t w i t h t i t a n i a .

80

The s p e c t r a of t h e s u p e r n a t a n t s o l u t i o n s r e s u l t i n g a f t e r t h e

5

min and 24 h c o n t a c t w i t h t h e T i 0 2 are a l s o shown i n F i g . A 1. c l e a r d e c r e a s e i n t h e c o n c e n t r a t i o n o f a n i o n i c s p e c i e s compared t o t h e i n i t i a l s o l u t i o n is o b s e r v e d

in

the

supernatant

solutions,

i n d i c a t i n g t h a t a large f r a c t i o n o f t h e rhodium s p e c i e s have

been

a d s o r b e d on t h e T i 0 2 s u p p o r t .

been

This adsorption

process

has

q u a n t i f i e d by a t o m i c a b s o r p t i o n . As shown i n T a b l e 1, a f t e r 5 min c o n t a c t w i t h t h e s u p p o r t , a b o u t h a l f o f t h e Rh i n i t i a l l y p r e s e n t i n s o l u t i o n h a s been removed, s u g g e s t i n g t h a t most o f t h e a n i o n i c complexes have been a d s o r b e d on t h e TiOi(T h i s w a s indeed t h e e x p e c t e d b e h a v i o r o f t h e s y s t e m . When T i 0 2 is immersed i n an aqueous medium a t a p H v a l u e below its i s o - e l e c t r i c p o i n t , it becomes p o s i t i v e l y c h a r g e d and a b l e t o a d s o r b n e g a t i v e l y c h a r g e d species ( r e f . 3). However, as a l s o shown i n T a b l e 1, a f t e r l o n g e r o f Rh r e t a i n e d by the support I t w a s o b s e r v e d t h a t a f t e r 24 h i n c o n t a c t w i t h t h e a c i d i c s o l u t i o n a l m o s t 40 X of t h e Rh i n i t i a l l y d e p o s i t e d on t h e TiOg w a s l o s t from t h e s u r f a c e . contact

times,

the

amount

unexpectedly d e c r e a s e d .

TABLE 1

Rh c o n c e n t r a t i o n i n s o l u t i o n and on t h e contact t i m e s with TiOZ. SAMPLE

fresh solution 5 min c o n t a c t 24 h c o n t a c t

Rh mmol/l in the solution

support

after

different

w t % Rh i n t h e catalyst

3.0

-

1.3 1.9

1.1 0.7

To d e t e r m i n e t h e n a t u r e of t h e species d e p o s i t e d on t h e s u p p o r t

w e have u s e d d i f f u s e r e f l e c t a n c e s p e c t r o s c o p y . A s i l l u s t r a t e d i n F i g . 2 , t h e i n t e n s i t y of t h e band c o r r e s p o n d i n g t o t h e sample c o n t a c t e d f o r 24 h w i t h t h e i m p r e g n a t i n g s o l u t i o n is s i g n i f i c a n t l y lower t h a n t h a t of t h e 5 min c o n t a c t . This d i f f e r e n c e confirms t h e unexpected r e s u l t s o b t a i n e d by a t o m i c a b s o r p t i o n a n a l y s i s of t h e s u p e r n a t a n t s o l u t i o n f o r t h e 24 h c o n t a c t r e p o r t e d i n T a b l e 1.

81

3 80

430

480

530

580

nm

F i g u r e 2. D i f f u s e r e f l e c t a n c e s p e c t r a of t i t a n i a samples a f t e r d i f f e r e n t c o n t a c t t i m e s w i t h t h e rhodium c h l o r i d e s o l u t i o n

The a n a l y s i s of t h e band i n t e n s i t i e s d o e s n o t add much r e s u l t s d i s c u s s e d above.

But,

an

important

drawn from t h e a n a l y s i s of t h e p o s i t i o n of

o b s e r v e d t h a t when t h e c o n t a c t t i m e i n c r e a s e s

A s d e s c r i b e d above, l i g a n d s by i n d i c a t e a s u b s t i t u t i o n o f weaker

Accordingly, w e could propose t h a t , a f t e r t h e

the

can

be

maxima.

It

is

the

wavelengths t a k e s p l a c e .

to

conclusion

a

shift

this

to

lower

shift

would

stronger initial

ligands. adsorption

p r o c e s s , t h e a d s o r b e d s p e c i e s undergo a l i g a n d exchange r e a c t i o n

82

on t h e s u r f a c e by which some c h l o r o l i g a n d s ( w e a k e r ) are exchanged by s u r f a c e OH g r o u p s ( s t r o n g e r ) .

This

two-step

process

can

d e s c r i b e d i n t h e f o l l o w i n g scheme: a ) anion adsorption: Ti-OH

+

H+

+

HLx -

______

b) ligand exchange reaction: T -OHZ+ HLx + Ti-OH ______

Ti-OH2+ HLx -

Ti-OHZ+ HLX-* -0 -T i

+ HL

where L is t h e l i g a n d and H t h e c e n t r a l metal atom.

3.20

PH

3.10 0

I

I

100

2 00

time (sec)

F i g u r e 3. E v o l u t i o n o f t h e p H o f t h e 3 x lo-% rhodium c h l o r i d e s o l u t i o n a f t e r t h e a d d i t i o n of T i 0 2 .

300

be

83

T h i s scheme would p r e d i c t

that

the

pH

s o l u t i o n should v a r y d u r i n g t h e d e p o s i t i o n

of

the

process.

impregnating He

e x p e c t t h a t , i n i t i a l l y , as t h e a n i o n a d s o r p t i o n p r o c e e d s ,

should the

pH

s h o u l d i n c r e a s e b e c a u s e H+ are consumed from t h e s o l u t i o n . But, after a while, t h e l i g a n d exchange r e a c t i o n o f t h e a d s o r b e d species s h o u l d b e g i n and, c o n s e q u e n t l y , t h e p H s h o u l d d e c r e a s e as H+ are evolved from t h e s u p p o r t . H e have shown t h a t t h i s is indeed t h e case. U e have a n a l y z e d t h e e v o l u t i o n o f t h e p H o f t h e rhodium c h l o r i d e s o l u t i o n a f t e r t h e a d d i t i o n o f T i O Z . A s shown i n F i g . 3, t h e p H i n i t i a l l y i n c r e a s e d r a p i d l y from an i n i t i a l v a l u e of 3.14, r e a c h i n g a maximum a f t e r a f e w s e c o n d s , and t h e n d e c r e a s e d . By c o n t r a s t , when H C 1 s o l u t i o n s o f t h e same i n i t i a l p H were p u t i n c o n t a c t w i t h T i O Z i n s t e a d o f t h e rhodium s o l u t i o n s , no maximum w a s d e t e c t e d b u t a c o n t i n u o u s i n c r e a s e u n t i l a c o n s t a n t v a l u e w a s r e a c h e d . L i k e w i s e , when SiOz w a s used i n s t e a d o f T i O Z a l m o s t no change i n p H w a s d e t e c t e d . These observations d e m o n s t r a t e t h a t t h e a p p e a r a n c e of m a x i m a i n t h e e v o l u t i o n o f t h e p H is r e l a t e d t o p a r t i c u l a r i n t e r a c t i o n s between t h e Rh complexes and t h e T i O z s u p p o r t and n o t t o an a r t e f a c t o f t h e e x p e r i m e n t a l procedure. To e x p l a i n t h e pronounced d e c r e a s e i n t h e amount o f Rh r e t a i n e d by t h e s u p p o r t as t h e t i m e of c o n t a c t w i t h t h e s o l u t i o n i n c r e a s e s , w e have c o n s i d e r e d t h e p o s s i b i l i t y o f a n a c i d a t t a c k t o t h e support surface. W e s p e c u l a t e t h a t i f t h e s u r f a c e of t h e support is a t t a c k e d by t h e a c i d i c s o l u t i o n , a l o s s o f a d s o r b e d species can

t a k e place d u r i n g t h a t p r o c e s s . H e have a n a l y z e d t h e s u r f a c e t h e T i O Z by TEH t o i n v e s t i g a t e whether an a t t a c k is e v i d e n t .

of

As

i l l u s t r a t e d i n F i g . 4 , a T i O z sample which had been c o n t a c t e d w i t h t h e i m p r e g n a t i n g rhodium c h l o r i d e s o l u t i o n f o r 24 h e x h i b i t e d s u r f a c e f e a t u r e s t h a t d i d n o t appear e i t h e r on t h e f r e s h T i 0 2 n o r on t h e one which had o n l y been i n c o n t a c t w i t h t h e s o l u t i o n f o r 5 min. The most marked f e a t u r e o f t h e samples t h a t showed e v i d e n c e s of a t t a c k w a s t h e p r e s e n c e of o v e r l a y e r s . These o v e r l a y e r s m a y b e t h e r e s u l t of a re-deposition.

partial

dissolution

of

the

Ti02

followed

by

84

F i g u r e 4. Transmission e l e c t r o n micrograph of t h e t i t a n i a s u p p o r t c o n t a c t e d w i t h t h e rhodium c h l o r i d e s o l u t i o n f o r 24 h and r e d u c e d a t 323K W e have a l s o a n a l y z e d by XRD t h e r u t i l e / a n a t a s e

ratio

in

the

T i 0 2 s u p p o r t , which is a b o u t 1/4 i n t h e o r i g i n a l

samples.

r a t i o s were o b t a i n e d from t h e peak

corresponding

intensities

These to

t h e s p a c i n g s d = 3 . 2 5 , r u t i l e (100) and d=3.52, a n a t a s e (101). shown i n T a b l e 2 t h i s r a t i o is

significantly

increased

sample which h a s been i n c o n t a c t w i t h t h e s o l u t i o n f o r 24 h . a n a t a s e - r u t i l e t r a n s f o r m a t i o n would agree w i t h t h e

As

for

the This

dissolution

/

r e - d e p o s i t i o n h y p o t h e s i s , s i n c e r u t i l e is t h e most s t a b l e form o f T i 0 2 and i t s f o r m a t i o n c o u l d b e e f f e c t e d d u r i n g t h e r e - d e p o s i t i o n process.

85

TABLE 2 Anatase-to-rutile

t r a n s f o r m a t i o n e f f e c t e d by t h e

acid

attack

as

d e t e r m i n e d from XRD p e a k i n t e n s i t y , d = 3 . 2 5 r u t i l e (1001, 6 ~ 3 . 5 2 a n a t a s e (101).

a partial c o v e r i n g of t h e metal p r e c u r s o r s . W e have looked f o r a n o t h e r e v i d e n c e o f t h i s phenomenon by m e a s u r i n g hydrogen c h e m i s o r p t i o n c a p a c i t i e s o f a Rh/Si02 c a t a l y s t , u s e d a s r e f e r e n c e , a f t e r b e i n g i n c o n t a c t w i t h T i 0 2 i n b o t h , n e u t r a l and a c i d i c l i q u i d media. If a p a r t i a l c o v e r a g e of t h e Rh s u r f a c e by T i 0 2 species o c c u r r e d , w e s h o u l d e x p e c t a d e c r e a s e i n t h e hydrogen c h e m i s o r p t i o n capacity. A s shown i n T a b l e 3, t h e r e w a s n o much change i n t h e c h e m i s o r p t i o n v a l u e s when t h e r e f e r e n c e c a t a l y s t w a s e i t h e r c o n t a c t e d w i t h T i 0 2 i n a non a c i d i c medium o r t r e a t e d i n an a c i d i c medium w i t h o u t T i 0 2 - However, t h e r e w a s a clear d e c r e a s e when it w a s c o n t a c t e d w i t h a s l u r r y c o n t a i n i n g T i 0 2 and a HC1 s o l u t i o n ( p H = 3 . 1 4 ) . A s mentioned a b o v e , t h e r e - d e p o s i t i o n may r e s u l t i n

_ ,

TABLE 3 Hydrogen c h e m i s o r p t i o n d a t a o b t a i n e d on t h e same Rh/SiOz contacted w i t h T i O z under d i f f e r e n t c o n d i t i o n s

catalyst

..................................................................

Sample

H/Rh

86

CONCLUSIONS W e conclude t h a t when T i 0 2 is p l a c e d i n c o n t a c t w i t h rhodium c h l o r i d e s o l u t i o n s o f low p H v a l u e s , s e v e r a l phenomena, l e a d i n g t o t h e d e p o s i t i o n of m e t a l p r e c u r s o r s on t h e T i 0 may o c c u r : 2 i ) anion adsorption.

A s the

oxide

particles

become

positively

charged i n t h e low p H s o l u t i o n , t h e a d s o r p t i o n of a n i o n i c

species

is r e a d i l y e f f e c t e d .

A f t e r t h e a d s o r p t i o n of t h e a n i o n i c by means of an exchange of c h l o r o l i g a n d s by s u r f a c e OH g r o u p s . i i ) ligand exchange reaction.

complexes, f u r t h e r a n c h o r i n g t o t h e s u p p o r t t a k e s place

iii) acid attack.

The a c i d i c s o l u t i o n a t t a c k s t h e s u r f a c e of

a partial support causing r e - d e p o s i t i o n of o x i d e species.

ACKNOWLEDGEMENTS Financial support acknowledged.

from

dissolution

CONICET

of

followed

Argentina

is

by

the the

gratefully

REFERENCES K . Foger, i n Anderson J . R., and Boudart M.(Eds.), C a t a l y s i s : S c i e n c e and Technology, S p r i n g e r , B e r l i n , H a i d e l b e r g , N.York, 1984, Ch. 4, p . 227. M . Che and C.O. B e n n e t t , Adv. C a t a l . , 36 (1989) 55 J. B r u n e l l e , i n Delnon e t a l . ( E d . ) , P r e p a r a t i o n of H e t e r o g e n - C a t a l y s t s , Vol. 11, E l s e v i e r . Amsterdam, 1979. J.C. Summers and S . A . Ausen, J. C a t a l . , 52 (1978) 455. E. S a n t a c e s a r i a , S . C a r r a , and I. Adami, I n d . Eng. Chem. Prod. R e s . & Dev., 16 (1977) 41. W.J. van den Boogert, G. van d e r L e e , H. Luo, V . Ponec, A p p l . C a t a l . (1987) Y.J. L i n , R . J . F e n o g l i o , D.E. Resasco, G.L. H a l l e r . i n Delmon e t a l . ( E d . ) , P r e p a r a t i o n of Heterogeneous C a t a l y s t s , Vol. I V , E l s e v i e r , Amsterdam, 1987, p.125. G . L . H a l l e r and D . E . Resasco, Adv. C a t a l . , 36 (1989) 173. C . K - J o r g e n s e n , Acta Chim. Scand., 10 (1956) 500.

G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

IMPREGNATION OF CONTROLLED-POROSITY SILICA:

87

Cu/Si02, Co/SiO2 and Cu-Co/Si02

I N V E S T I G A T I O N OF THE PARAMETERS AFFECTING SELECTIVITY I N CO HYDROGENATION

M.A.

MARTIN LUENGO (YATES), Y. WANG and P.A.

SERMON*

S o l i d s and Surfaces Research Group, Department o f Chemistry, Brunel U n i v e r s i t y , Uxbridge, UB8 3PH, UK

SUMMARY S i l i c a spheres o f c o n t r o l l e d p o r e s i z e (83% o f pores between 6-10nm i n r a d i u s ) were impregnated w i t h aqueous n i t r a t e s o l u t i o n s t o g i v e 8%Cu/SiO , 8%Co/Si02 and 8%Cu-4%Co/SiO . Temperature-programmed r e d u c t i o n , microproge a n a l y s i s , and propane h y d r o t e n o l y s i s a l l i n d i c a t e d t h a t t h e Cu and Co i n t h e I n t h e c a t a l y s i s o f CO bimetallic catalyst i n i n t i m a t e contact. h y d r o g e n a t i o n r a t e o f CO c o n v e r s i o n i n c r e a s e d i n t h e sequence: Cu/SiO < Cu-Co/Si02,< Co/SiO w h i l e s e l e c t i v i t y t o methine i n c r e a s e d i n t h e sa&e sequence (and s e l e c t i v i t y t o methanol decreased). These s e l e c t i v i t y t r e n d s a r e t o be expected s i n c e t h e a b i l i t y o f Co t o d i s s o c i a t e CO and produce methane i s w e l l known. However, none o f these c a t a l y s t s produced s i g n i f i c a n t amounts o f e t h a n o l . T h e r e f o r e t h e s t u d y o f these i d e a l i s e d c a t a l y s t s suggests t h a t Cu-Co i n t e r a c t i o n s a r e n o t s u f f i c i e n t t o induce s e l e c t i v i t y t o e t h a n o l i n CO h y d r o g e n a t i o n a t 523K and 2MPa b u t must a l s o r e q u i r e an i n t e r a c t i o n w i t h a ZnO o r A1203 component i n t h e f u l l catalyst.

were

INTRODUCTION

A t t h e l a s t conference c o n s i d e r i n g s c i e n t i f i c bases f o r t h e p r e p a r a t i o n o f heterogeneous c a t a l y s t s some o f t h e p r e s e n t a u t h o r s d e s c r i b e d ( r e f . 1) a s t u d y o f t h e parameters a f f e c t i n g t h e d i s p e r s i o n and l o c a t i o n o f P t d u r i n g i m p r e g n a t i o n o f a s i l i c a o f c o n t r o l l e d pore s i z e w i t h solutions o f hexachloroplatinic acid. Using t h i s approach i t was p o s s i b l e t o d i f f e r e n t i a t e p o r e d i a m e t e r and s o l u t i o n t e m p e r a t u r e - v i s c o s i t y e f f e c t s i n t h e p r e p a r a t i o n o f a model mono-metallic catalyst.

T h i s approach has now been extended t o c o n s i d e r t h e importance

o f Cu-Co i n t e r a c t i o n s i n s i l i c a - s u p p o r t e d CO h y d r o g e n a t i o n c a t a l y s t s i n d e f i n i n g s e l e c t i v i t y t o ethanol. Cu-Co heterogeneous c a t a l y s t s a r e o f some c u r r e n t ' i n t e r e s t i n t h e c o n v e r s i o n

o f CO/H2 and CO/C02/H2 m i x t u r e s w i t h good s e l e c t i v i t y t o h i g h e r a l c o h o l s such as ethanol ( r e f s . 2,3).

I t seems u n c e r t a i n whether t h e Co i s e n t i r e l y reduced i n

such c a t a l y s t s ( r e f . 3 ) , b u t i t s a d d i t i o n t o t h e Cu-only c a t a l y s t i n c r e a s e s alkane f o r m a t i o n .

The p r e c i s e i n t e r a c t i o n of Cu and Co i n such c a t a l y s t s i s

c e r t a i n l y u n c l e a r and hence t h e p r e s e n t work was undertaken.

However, i t i s

88

appreciated that the present catalysts are f a r simpler than alkali-treated Cu-Co/ZnO/A1203

samples w h i c h a t 563K, 6MPa and CO/H2=0.5 gave 21-24%CO c o n v e r s i o n

w i t h 30% s e l e c t i v i t y t o e t h a n o l ( r e f . 3 ) . A g a i n s t t h i s i n t r i g u i n g b a c k g r o u n d t h e p r e s e n t r e s u l t s a r e now r e p o r t e d . EXPERIMENTAL Materials

A s i l i c a ( S h e l l I n t e r n a t i o n a l Chemical Company L t d . ) was chosen because o f i t s c o n t r o l l e d p o r o s i t y , h o m o g e n e i t y a n d p u r i t y ; i t s p r o p e r t i e s have been d e s c r i b e d p r e v i o u s l y ( r e f . 1 ) b u t i t s main c h a r a c t e r i s t i c s a r e g i v e n i n Table 1 below. TABLE 1 P r o p e r t i e s o f s i l i c a u s e d 2.5

p a r t i c l e s i z e (mm)

sN2 ( m 2 / g )

21 1

% p o r e s i n t h e r a d i u s r a n g e 6-10nm

83

I t s t o t a l s u r f a c e a r e a was e s t i m a t e d f r o m a n a l y s i s o f N assuming i t s m o l e c u l a r c r o s s - s e c t i o n a l a r e a was 0.162nm

a d s o r p t i o n d a t a a t 77K

5.

The a d s o r p t i o n

i s o t h e r m was o f t y p e I V ( r e f . 4 ) w i t h a h y s t e r e s i s l o o p t y p i c a l o f t h a t f o r a mesoporous m a t e r i a l ( i . e .

t h a t designated type H1) ( r e f . 4).

Samples o f t h i s p r e - d r i e d s u p p o r t ( 2 3 9 ) were 75cm3 aqueous s o l u t i o n o f Cu o r Co o r Cu-Co n i t r a t e s o f a s u f f i c i e n t s t r e n g t h t o g i v e t h e c a t a l y s t c o m p o s i t i o n s below: 8%Cu/silica 8%Cu-4%Co/silica 8%Co/silica These were t h e n a g i t a t e d f o r lOmin, e v a p o r a t e d t o d r y n e s s , d r i e d f o r 16h i n a i r a t 403K, and c a l c i n e d f o r 3h a t 523K i n a i r . Methods T o t a l s u r f a c e a r e a s were e s t i m a t e d f r o m N2 a d s o r p t i o n a t 77K i n a S o r p t y a p p a r a t u s ( C a r l o E r b a ) a f t e r o u t g a s s i n g a t 523K f o r 2h.

The r e d u c i b i l i t y o f t h e

samples was a s s e s s e d i n a c o n v e n t i o n a l f l o w temperature-programmed r e d u c t i o n system ( r e f .

5 ) u s i n g 5%H2/Ar f l o w i n g a t 50cm3/min o v e r 50-100mg c a t a l y s t samples.

The r a t e and s e l e c t i v i t y o f CO h y d r o g e n a t i o n was f o l l o w e d o v e r samples ( 0 . 1 9 ) o f c a t a l y s t s ( w h i c h had been p r e t r e a t e d t o 573K i n 6%H2/N2 and t h e n c o o l e d t o 523K) i n CO/H2

( = 2 ) f l o w i n g a t 20cm3/min a t 2MPa and 523K; o v e r a p e r i o d o f 3-4h

p r o d u c t s were a n a l y s e d b y gas c h r o m a t o g r a p h y u s i n g gas c h r o m a t o g r a p h s f i t t e d w i t h F I D d e t e c t o r s and Porapak Q a n d T columns.

The a c t i v i t y o f t h e c a t a l y s t s i n t h e

h y d r o g e n o l y s i s o f p r o p a n e a t 523K was a l s o d e t e r m i n e d c h r o m a t o g r a p h i c a l l y .

The

89

c a t a l y t i c c o n d i t i o n s u s e d were a s f o l l o w s : 0.29 samples p r e - r e d u c e d t o 573K i n H2 were s u b j e c t e d t o a r e a c t a n t s t r e a m (140cm3/min a t 1 0 l k P a ) c o n s i s t i n g o f p r o p a n e : N2:H2=10:30:100 mol r a t i o .

Reduced c a t a l y s t s were s e c t i o n e d and c r o s s - s e c t i o n s

a n a l y s e d i n a s c a n n i n g e l e c t r o n m i c r o s c o p e b y EDAX-microprobe methods f o r Cu and Co c o n c e n t r a t i o n p r o f i l e s . CHARACTERISATION The r e s u l t s o f BET a n a l y s i s o f N2 a d s o r p t i o n a t 7 7 K showed t h a t t h e t o t a l

2

s u r f a c e a r e a o f t h e s u p p o r t (211m / g ) d e c r e a s e d on i m p r e g n a t i o n w i t h t h e m e t a l Thus t h e C u O / s i l i c a and C u 0 - C o O x / s i l i c a samples h a d t o t a l s u r f a c e a r e a s o f o n l y 187 and 160m2 / g n i t r a t e s and c a l c i n a t i o n t o t h e s u p p o r t e d m e t a l o x i d e s .

r e s p e c t i v e l y and t h i s i s p r o b a b l y due t o p a r t i a l b l o c k i n g o f t h e s u p p o r t p o r e s by t h e metal oxide p a r t i c l e s . Cu0-CoOx/silica

r e d u c e d i n temperature-programmed r e d u c t i o n w i t h a maximum

r a t e a t 533K ( s e e F i g . 1) w h i c h was s i g n i f i c a n t l y above t h a t f o r C u O / s i l i c a o r CuO and s i g n i f i c a n t l y b e l o w t h a t f o r C o O x / s i l i c a o r COO.

T h i s suggested t h a t

t h e Cu and Co phases were i n t i m a t e l y m i x e d o n t h i s s i l i c a s u p p o r t u n d e r t h e present conditions.

I n each c a s e t h e h y d r o g e n c o n s u m p t i o n s were 34-71% above

t h e e x p e c t e d v a l u e i f CuO and COO were b e i n g r e d u c e d t o t h e z e r o - v a l e n t m e t a l s , b u t h y d r o g e n s p i l l o v e r may b e r e s p o n s i b l e f o r t h i s .

The i n t i m a t e m i x i n g o f

CuO and Coox may n o t b e s u r p r i s i n g i n t h e l i g h t o f t h e f a c t t h a t COO c a n a c c e p t up t o 25mol% CuO w i t h o u t s t r u c t u r a l change ( r e f . 6 ) .

The i n t i m a c y o f Cu and Co

phases was c o n f i r m e d b y m i c r o p r o b e a n a l y s i s o f t h e m e t a l c o n c e n t r a t i o n s i n p a r t i c l e c r o s s - s e c t i o n s ( s e e F i g . 2).

These show some p r e f e r e n c e f o r b o t h m e t a l s

a t t h e o u t e r edge o f t h e s u p p o r t p a r t i c l e s ( a s e x p e c t e d f r o m i m p r e g n a t i o n ( r e f . 1 ) b u t m o s t i m p o r t a n t l y t h a t t h e t w o m e t a l s a r e l o c a t e d i n t h e same p a r t s o f t h e s u p p o r t p a r t i c l e s ; t h i s t h e n i s c o n s i s t e n t w i t h temperature-programmed r e d u c t i o n .

F i g u r e 1 TPR o f 8%Cu-4%Co/Si02

90

Figure 2.

C o n c e n t r a t i o n p r o f i l e s o f Cu ( ) and Co ( 0 ) i n c r o s s - s e c t i o n s o f c a l c i n e d S i O ( a ) and 8%Cu-4%Co/Si02 ( b ) determined b y k c r o p r o b e a n a l y s i s o f s e c t i o n e d c a t a l y s t p a r t i c l e s whose edge i s marked by arrows .

The i d e n t i c a l depth o f p e n e t r a t i o n o f t h e two m e t a l s i n ( b ) suggest these w i l l be i n good c o n t a c t on s i l i c a impregnated i n t h e manner d e s c r i b e d .

91

Alkane h y d r o g e n o l y s i s i s a s u i t a b l e probe f o r t h e p r e s e n t model c a t a l y s t s i n t h a t z e r o - v a l e n t Cuo i s expected t o have low a c t i v i t y i n t h e r e a c t i o n , w h i l e on t h e o t h e r hand z e r o - v a l e n t Coo s h o u l d have h i g h a c t i v i t y i n t h e r e a c t i o n (ref. 7).

I f a b i m e t a l l i c phase e x i s t s a t t h e s u r f a c e o f t h e p a r t i c l e s i n t h e

C u - C o / s i l i c a sample t h e n one would e x p e c t some i n t e r m e d i a t e a c t i v i t y , u n l e s s t h e Cu was p r e s e n t t o t h e e x t e n t t h a t Co ensembles were fragmented t o such a degree t h a t s t r u c t u r e - s e n s i t i v e h y d r o g e n o l y s i s c o u l d n o t o c c u r a t a s i g n i f i c a n t Thus t h e h y d r o g e n o l y s i s r e a c t i o n may i n d i c a t e t h e e x t e n t t o which Cu and

rate.

Table 2 g i v e s t h e s t e a d y - s t a t e

Co phases a r e i n c o n t a c t i n t h i s c a t a l y s t .

a c t i v i t i e s and s e l e c t i v i t i e s o f t h e p r e s e n t c a t a l y s t s i n propane h y d r o g e n o l y s i s T h i s shows t h a t s i l i c a - s u p p o r t e d Cuo ( l i k e S i 0 2 a l o n e ) has no a c t i v i t y

a t 523K.

i n the reaction.

As expected f o r t h e z e r o - v a l e n t s t a t e s i l i c a - s u p p o r t e d Co

does show s u b s t a n t i a l a c t i v i t y ; f u r t h e r m o r e i t shows h i g h s e l e c t i v i t y t o t o t a l h y d r o g e n o l y s i s t o methane. I f one l o o k s a t t h e d a t a f o r t h e b i m e t a l l i c c a t a l y s t , t h e n i t i s c l e a r t h a t a c t i v i t y i s v e r y l o w i n t h i s r e a c t i o n , which suggests t h a t t h e Cu and Co a r e i n i n t i m a t e c o n t a c t and t h a t t h e s u r f a c e s o f t h e b i m e t a l l i c p a r t i c l e s must be p r e f e r e n t i a l l y Cu ( w i t h few l a r g e Co ensembles). T h i s i s c o n s i s t e n t w i t h X-ray p h o t o e l e c t r o n spectroscopy evidence ( r e f . 8 ) on single crystals.

However, even w i t h t h e b i m e t a l l i c t h e r e i s some s e l e c t i v i t y t o

t o t a l h y d r o g e n o l y s i s t o methane. Thus temperature-programmed r e d u c t i o n , EDAX-microprobe a n a l y s i s and a l k a n e h y d r o g e n o l y s i s a l l i n d i c a t e t h a t i n t h e impregnated b i m e t a l l i c c a t a l y s t t h e Cu and Co phases a r e i n i n t i m a t e c o n t a c t ;

t h i s i s i n t r i g u i n g given t h e simple

p r e p a r a t i v e method used. TABLE 2

A c t i v i t i e s and s e l e c t i v i t i e s i n propane h y d r o g e n o l y s i s a t 523K

*

catalyst

% conv propane

'CH4

8%Cu/SiO 2

0

-

8%Cu-4%Co/Si02

0.53

2.51

0.25

0.66

8%Co/SiO2

9.25

2.85

0.07

11.50

* TABLE 3

'C2H6

rate 0

r a t e o f propane c o n v e r s i o n i n mmol propane/g c a t / h

A c t i v i t i e s and s e l e c t i v i t i e s i n CO h y d r o g e n a t i o n a t 523K

c a t a 1y s t

*

%CH4

%C2H6

%CH30H

%C2H50H

45.5

7.8

33.8

5.3

8%Cu-4%Co/Si02 26.2

51.3

10.8

24.0

0.0

8%Co/Si02

43.4

75.8

5.5

8.8

1.1

(SiO2)

(0.96)

8%Cu/Si02

*

rate

6.18

(40.8)

r a t e i n pmol/g cat/min

(37.7)

92

RESULTS OF CATALYSIS OF CO HYDROGENATION The r e s u l t s f o r t h e a c t i v i t i e s and s e l e c t i v i t i e s o f t h e c a t a l y s t s prepared here a r e shown i n Table 3; h e r e p a r t i c u l a r a t t e n t i o n s h o u l d be g i v e n t o t h e e f f e c t o f h a v i n g t h e z e r o - v a l e n t Cu and Co i n c o n t a c t i n t h e b i m e t a l l i c sample. These d a t a a g a i n r e l a t e t o s t e a d y - s t a t e c o n d i t i o n s a f t e r 3-4h r e a c t i o n t i m e a t 523K and 2MPa. F i r s t , t h e o v e r a l l a c t i v i t i e s o f t h e c a t a l y s t s i n terms o f t h e r a t e o f CO c o n v e r s i o n i s a l m o s t z e r o f o r S i 0 2 a l o n e and t h e n i n c r e a s e s as Co i s added t o t h e s i l i c a - s u p p o r t e d Cu and i s h i g h e s t f o r t h e C o / s i l i c a .

Therefore f o r the

b i m e t a l l i c c a t a l y s t i t would seem t h a t t h e r e r e a l l y a r e p a r t i c l e s c o n t a i n i n g b o t h m e t a l s , r a t h e r t h a n some Co p a r t i c l e s and some Cu p a r t i c l e s .

Again, t h i s

i s i n t r g u i n g f o r such a s i m p l e c a t a l y s t p r e p a r a t i v e method. Second, as t h e Co c o n t e n t i n c r e a s e s t h e s e l e c t i v i t y t o alkanes i n c r e a s e s and t h i s i s e s p e c i a l l y so f o r methane. T h i r d , as t h e Co c o n t e n t i n c r e a s e s so t h e s e l e c t i v i t y t o CH30H decreases. These t r e n d s towards methane and away from methanol a r e t o be expected i n terms o f t h e g r e a t e r a b i l i t y o f Co t o d i s s o c i a t e CO and t o f a c i l i t a t e t h e methanation reaction (ref. 9). Fourth, t h e r e i s almost no evidence f o r e t h a n o l p r o d u c t i o n f o r any o f these c a t a l y s t s ; t h i s must be i m p o r t a n t i n terms o f mechanisms and s i t e s i n t h e Cu-Co sample. D I S C U S S I O N AND CONCLUSIONS

Temperature-programmed r e d u c t i o n , EDAX-microprobe a n a l y s i s and a l k a n e h y d r o g e n o l y s i s a l l suggest t h a t i n t h e s e model impregnated heterogeneous c a t a l y s t s Cu and Co, when b o t h p r e s e n t i n t h e s i l i c a pores, e x i s t as l a r g e l y bimetallic particles.

I t may however be t h a t t h e i r c o m p o s i t i o n v a r i e s w i t h

r a d i a l p o s i t i o n i n t h e s u p p o r t p a r t i c l e s ; t h i s i s t h e s u b j e c t o f f u r t h e r study. I t i s an i m p o r t a n t f i n d i n g h e r e t h a t such b i m e t a l l i c p a r t i c l e s (which a r e

p r o b a b l y q u i t e w e l l reduced under c o n d i t i o n s o f p r e t r e a t m e n t h e r e ) a r e a c t i v e i n f a c i l i t a t i n g CO h y d r o g e n a t i o n t o e t h a n o l .

not

Alkane h y d r o g e n o l y s i h i n t s

a t Cu b e i n g p r e f e r e n t i a l l y a t t h e b i m e t a l l i c s u r f a c e s , b u t t h i s may n o t be t h e case under c o n d i t i o n s o f CO hydrogenation.

U s i n g Cu-Co/ZnO/A1203

( i n the

presence o f a l k a l i ) o r w i t h o t h e r s u p p o r t i n g o x i d e s ( r e f . 10) may produce Cuo-Coxt

p a i r s b y s t a b i l i s i n g Co i n p o s i t i v e o x i d a t i o n s t a t e s v i a i n t e r -

c a l a t i o n o f Co c a t i o n s o r f o r m a t i o n o f a s u r f a c e s p i n e l .

These would

c e r t a i n l y show d i f f e r e n t p r o p e r t i e s from t h o s e seen here and m i g h t w e l l be a c t i v e i n t h e s e l e c t i v e c o n v e r s i o n o f CO/H2 t o e t h a n o l . For t h e moment, t h e p r e s e n t s t u d y has shown t h a t Cuo-Coo on ' i n e r t '

93

silica are not in themselves and in isolation from support o r promoter effects sites of effective production of ethanol from CO/H2. This has been shown unequivocally using the model impregnation approach used here. Further work in which ZnO, A1203 and other oxides are added is in hand. It i s hoped that the study o f such model and well-defined precursors and catalysts will allow the critical sites and interactions in such catalysts to be defined. ACKNOWLEDGEMENTS The authors gratefully acknowledge SERC and Royal Society support of MAML(Y) and YW respectively during the course of this work. REFERENCES 1 M.A.Martin-Luengo, P.A.Sermon and K.S.W.Sing 'Preparation of Catalysts IV' ed. B.Delmon, P.Grange, P.A.Jacobs and G.Poncelet (1987) Elsevier p.29 2 J.G.Nunan, C.E.Bogdan, K.Klier, K.J.Smith, C.W.Young and R.G.Herman J.Cata1. 116 (1989) 195 3 P.Courty, D.Durand, E.Freund and A.Sugier J.Molec.Cata1. 17 (1982) 241; H.F.Hardman and R.Beach US patent (1978) 905,703 4 K.S.W.Sing, D.H.Everett, R.A.W.Hau1, L.Moscou, R.A.Pierotti, J.Rouquero1 and T.Siernieniewska Pure Appl .Chem. 57 (1985) 603 5 G.C.Bond and S.Namijo J.Cata1. 118 (1989) 507 6 C.Delorine Bull.Soc.Chim.Fr. Miner.Cryst. 8 1 (1958) 19 7 S.A.Goddard, M.D.Amiridis, J.E.Rekoske, N.C.Martinez and J.A.Dumesic J.Cata1. 117 (1989) 155; Z.Paa1, P.Tetenyi and M.Dobrovolszky React. Kin.Catal.Lett. 37 (1988) 163 8 D. C hadwi c k (pri va te communi cati on) 9 M.A.M.Luengo (Yates) and P.A.Sermon (unpublished results) 1 0 M.Mouaddib and V.Perrichon Proc. 9th. Intern. Cong. Catal. 2 (1988) 521; A.J.Marchi, J.di Comino and C.R.Apestiguia Proc. 9th. Intern. Cong. Catal. 2 (1988) 529; M.Mouaddib, V.Perrichon and M.Primet J.Chem.Soc.Faraday Trans. I 85 (1989) 3413

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G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors),Preparation of Catalysts V 0 1991Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

95

SELECTIW HYDROGENATION OF CYCLODODECATRIENE ISOMERS TO CYCLODODECENE CATALYSED BY Cu-A1203. V. Di Castro', M. Garganoz,N. Ravasio2and M. Rossi3 Dipartimento di Chimica - Universiti3 " La Sapienza" - P.le A. Moro, 5 - 00185 Roma - Italy C.N.R. Cenm di Studio sulle Metodologie Innovative di Sintesi Organiche - Via Amendola, 173 - 70126 Bari - Italy Centro C.N.R. and Dipartimento di Chimica Inorganica e Metallorganica - Universiti di Milano Via Venezian, 21 - 20133 Milano - Italy

SUMMARY The behaviour of supported copper catalysts has been investigated in the hydrogenation of cyclododecamene to cyclododecane. The activity of these catalysts, which is influenced by the nature of oxide support and Cu(0) surface dispersion, is lower than that obtainable with conventional heterogeneous noble metal systems, whereas the selectivity is higher. In particular, Cu/A1203, prepared by chemisorption of CU(NH~)~*on alumina and preactivated in H2 atmosphere, allows high yield of cyclododecene (94%). INTRODUCTION The selective hydrogenation of mixtures of cyclododecamene (CDT) isomers, easily available through cyclotrimerization of butadiene, represents a convenient route to cis- and trans-cyclododecene (CDE). These latter compounds are of great practical importance because can be employed to prepare compounds such as 1,12-dodecandioic acid, 1,12-diaminododecane, 12-amminododecanoic acid lactam, which are monomers for polyamide manufacture, and cyclododecanecarboxilicacid, a pesticide, and its esters used as plasticizers. Highly selective catalysts are requested because the by-products, cyclododecane (CDA) and cyclododecadienes (CDD), cannot be easily separated fiom cyclododecene. Many efforts have been recently done in order to achieve this goal and different homogeneous and heterogeneouscatalysts have been tested. Best results have been obtained under homogeneous conditions by using transition metal complexes. In particular the use of Co or Ru complexes, at temperature ranging between 120 and 160°C. allows CDE yields greater than 98% (ref. 1,2). According to patent literature, heterogeneous catalysts for CDE production are based on

96

transition metals, particularly Ru, Pd, Ni, Co, dispersed on different materials such as functionalised polymers, A1203, SO2, zeolites, activated carbon, ecc. . The selectivity achieved by these catalysts is generally lower than that reported for homogeneous ones and usually CDE yields are less than 90%; only in few cases higher yields are reported, as in the case of a bimetallic Pd-Co system which produces CDE up to 95%(ref. 3). The use of copper based catalysts has been scarcely explored in this hydrogenation reaction, CDE yields never exceeding 83%(ref. 4). The high selectivity obtained during our studies on the reduction of polyenes (ref. 5-7) and alkynes (ref. 8) by using supported copper catalysts prompted us to investigate the selective hydrogenation of CDT to CDE. Preliminary results have been already claimed (ref. 9). In this paper the full investigation is reported. EXPERIMENTAL SECTION Materials

-

Cyclododecatriene (supplied by Fluka) was distilled, purified by filtration

through activated alumina and stored under nitrogen atmosphere at OOC. According to GLC analysis, the distilled product was a mixture of isomers with the following composition : trans-trans-trans-CDT = 58.7% , cis-trans-trans-CDT = 34.7% , cis-cis-trans-CDT = 2.7%, unidentified = 3.9% (probably cis-cis-cis-CDT). All oxides and salts were reagent grade products and were used without further purification. Catalysts preparation

-

Chemisorption method: all catalysts were prepared following a

procedure similar to that used by Koritala for the CuO/Si02 (ref. 10). Ammonia was added to an aqueous solution of C U ( N O ~ ) ~ . ~until H ~ Ocomplete dissolution of the initially precipitated Cu(OH), occurred. The oxide support (M&)

was added to the solution; the slurry, under

continuous stirring, was then slowly diluted with H20. The solid was separated by filtration, washed with H20, dried over night at 1lOOC and lastly calcined at 35OOC for 3 hrs. Coprecipitation method: all catalysts were obtained by addition of NaHC03 to a solution containing C U ( N O ~ ) ~ . ~ and H~O the metal nitrate corresponding to the oxide support. The solid was washed with H20, in order to eliminate Na+, dried over night at llO°C, then calcined at 35OOC for 3 hrs. Catalysts reduction - The reduction was carried out in glass reactors provided with a side arm connected to a sampler. All the catalysts were heated for 1 hr under vacuum to 27OOC before

reductive treatment. Reduction with H2 at 100 kPa was carried out either at constant temperature (270°C) or increasing the temperature, from 100 to 20O0C, at 2OC/min. Cu(0) specific area determination - The Cu(0) surface area was determined by using the

97

N20 chemisorption method (ref. 11). The measurements were carried out on samples of reduced catalysts by using a Carlo Erba Sorptomatic 1800 automatic gas burette

. N20

(75 Wa) was

introduced in the sampler and allowed to react for 0.5 hrs at 37OC. Then the gas in the sampler was analysed for N2 content. The Cu(0) specific area was computed by using a surface coverage factor ( moles of oxygen atoms per moles of surface Cu(0) atoms) 8 = 0.35 and a mean surface area for a copper site of 7.41 A2(ref. 11). Samples for ESCA determinations were stored under inert atmosphere in sealed vials and analysed with a VG ESCA 3 spectrometer employing an A1 K, source (hv = 1486.6eV) (ref. 12). Hydrogenation reactions - These were performed in glass reactors or, when H2 pressure was greater than 100 Wa, in stainless AISI 316 autoclave (60ml) . The rate of H2 uptake was measured by using a Brooks electronic mass flow meter connected by an AD interface to an IBM XT personal computer. Analysis

-

The products composition was determined by GLC with a 50 m x 0.2 mm

Carbowax 20M capillary column using a Hewlett Packard 5880A gas chromatograph equipped with a flame ionization detector. N2 from N20 chemisorption was analysed by GC with a 3 m x 2 mm stainless steel column packed with Carbosieve S using a Carlo Erba Fractovap gas chromatographprovided with a thermal conductivity detector. RESULTS AND DISCUSSION The hydrogenation of CDT catalysed by copper dispersed on different oxides, in the initial step of the reaction, follows in all cases a first order kinetic respect to CDT concentration. However, the activities of the catalysts are quite different. As shown in table 1, in fact, initial turnover number ranging from 0 to137 h-' were obtained. In all experiment the CDE produced was a mixture of trans- (65%)and cis-isomers (35%); the cislrruns ratio was independent from the catalyst, indicating that the same hydrogenation mechanism was operating for all the active systems.

The wide range of catalytic activities showed by different catalysts can be related both to a different dispersion of the active copper species on the surface of the catalytic material and to the different cooperative effect of oxide support. As reported in experimental section, all catalytic systems were prepared by reduction in

hydrogen atmosphere of CuO/Mfly

precursors; ESCA data (ref.5,6,12) showed that this

pretreatment causes the almost quantitative reduction of copper to Cu(0). This species , as suggested in our previous work, plays an essential role in promoting the hydrogenation reaction,

98

particularly with cyclic polyenes (ref.5-7). TABLE 1 Hydrogenation of Cyclododecamene catalysed by Cu/M,Oy systemsa Catal?

Pr.C

%Cu Red.temp. (OC)

Tvd (h-9

nHdnCDT

Reaction products (%moles) CDE CDD CDA

Cu/Alz03 Cu/A1203 Cu/Si02 Cu/Cr203g C Can0

I C I

7.0 9.5 7.0 37 8.4

130 9 67 16 122 77 137

2.00 1.37 1.87 2.03

94.0 45.1 83.0 90.7 93.0 93.6 91.0

3.0 42.0 12.7 5.9 3.5 3.2 4.5

3.0 1.7 2.3 3.4 3.5 3.2 4.5

40 86

1.94 2.00

90.0 90.8

7.8 4.6

2.2 4.6

Cu/MnOz Cum203

cu/Mgo

Othersh

C C I I I I I

8.7 8.7 6.3 7.4

270 100-270 100-200 270 100-200 270

<5 <5

a PH~= 100 KPa ; T = 18OOC ; CDT/Cu molar ratio = 100. All system were derived by corresponding CuO/Mfly by reduction in H2 atmosphere. Preparation method, C = coprecipitation, I = chemisorption. Turnover number at the beginning of the reaction in moles of Hz consumed per mole of copper per hr. Moles of Hz consumed per mole of CDT whcll the reaction was stopped. 2 % ca. of by-products are f m e d during the reaction. g Derived by commercial copper chromite catalyst by reduction. prepared by reduction of CuO supported on TiO,, BeO, SnOz, ZrOz,CeOz, TazOs, ZnO, CdO, Crz03,Nb0.

determined by means of NzO The values of specific Cu(0) surface area (SCU(O)), chemisorption, are reported in table 2 for different catalysts. A comparison between SCU(O)and tumover number indicates that the nature of oxide support has a predominant influence on the activity of the catalytic system. In fact, Cu/A1203 shows a higher tumover number respect to Cu/SiOz, Cu/ZnO, Cu/MgO in spite of a lower Scu(0). In particular, the best results were obtained using Cu/A1203. This catalyst, which was prepared reducing in hydrogen atmosphere a material obtained by chemisorption of Cu(NH3)4*

on alumina, showed best selectivity (CDE yield = 94% when 2 moles of Hz were consumed per mole of CDT) and high activity (Tv = 130 h-l). Also interesting for their activity and selectivity are Cu/MnOz/ and C a n 0

which

exhibited activity comparable with that of Cu/A1203 and selectivity only slightly lower. Cu/MgO, Cu/Y203and Cu/Cr203, although less active, allowed also a CDE yield 2 90%. By using Cu/Si02 we observed a gradual loss of activity which didn't allow

the reaction

to go to completion. This behaviour was probably due to the acidity of support ; the catalyst

99

surface being slowly poisoned by reaction by-product. The systems Cu/riOz, Cu/BeO, Cu/Sn02, CuiZrOz, Cu/CeOz, Cu/Ta205, C a n O , Cu/CdO, Cu/Cr,03 and Cu/NbO, prepared by the same method of Cu/AI,O,, were all almost inactive. The dependence of the activity from the nature of support indicates a strong interaction between copper and oxide support; this latter playing a role during the molecular Hz activation. This is in agreement with the literature concerning the interaction of H, with copper chromite and copper aluminate (ref. 13,14). However, for catalysts with the same support, SCU(O) strongly influences the catalytic activity: conditions that increase the Cu(0) dispersion also increase catalyst performance. Thus the higher turnover numbers observed with CuOEnO pre-reduced at low temperature (100-200 "C), respect to that of the same catalyst pre-reduced at high temperature (270 "C), is related to the change of SCU(O) . The growth of the copper particles on the surface and the consequent diminution of Cu(0) dispersion by increasing the pre-reduction temperature occur also for other supports (examples for Cu/A1203and Cu/SiO, are reported in tab. 2). TABLE 2 Cu(0) specific surface area of CUFlxOy systemsa

pr.b

Catal. Cu/A1203 Cu/A1203 Cu/SiOz Cu/SiO, CuEnO

I I I C I I C C I I I I

%Cu 7.0 9.5 7.0 8.4 7.8 7.4 7.9

Red.temp. ("0 100-200 270 350

270

100-200 270 100-200 100-270 270 100-200 100-200

Sup.Cu(0)C Disper? (m2/gcu) 91 64 45 7 180 210 74 135 11

76

17 <3

0.12 0.09 0.06 0.01 0.26 0.30 0.10 0.19 0.010 0.10 0.02

a All systems were derived by corresponding CuO/Mfly by reduction in H2 atmosphere. key as tab. 1. Determined by N 2 0 adsorption. Dispersion of Cu(0) species in moles of surface Cu(0)per moles of total Cu. CU/TiO2,Cu/Ce02, zr02

The dramatic loss of activity exhibited by Cu/Mn02 when pre-reduced at 270°C deserves a word of comment. As reported in a previous work (ref. 15), this behaviour is related to the

100

diffusion of the copper in the bulk of oxide support which produces a very low dispersion of Cu(0). The method employed for supporting copper also influences Cu(0) dispersion and the consequent catalyst activity. Thus, in the case of C a n 0 and Cu/Cr203, the coprecipitation method produces active catalysts whereas the chemisorption method produces almost inactive materials. On the contrary, Cu/A1203is best prepared with the chemisorption method (tab.1). The relation between preparation method and activity can be explained, once again, in term of Cu(0) dispersion (tab. 2). In our conditions, in fact, all systems with a Cu(0) dispersion below 0.02 were almost inactive regardless of the oxide support and preparation method. The relation between total copper concentration and activity was studied for the Cu/A1203 catalyst. In table 3 are reported the turnover number and SCU(O)value for a series of catalysts with copper content ranging from 3.1 to 14.5%. TABLE 3 Turnover numbersaand Cu(0) specific area for Cu/A1203bsystems with a different Cu content %

cu

3.1 4.7 6.5

8.4

9.8 14.5

SCu(0)

SCU(0)d

TvSe

Tv

141 105 95 85 65 47

139 88 93

540 670 858 900 720

108 101 110 100 65 42

53 49

595

= 300 kPa; T = 1 W C . CDT/Cu molar ratio = 100. Re-reduction temp. = 100 - 2WC. Derived by N 2 0 chemisorption. Derived by ESCA method. Initial specific turnover in moles of H2 consumed per mole of surface Cu(0) (derived by N20 chemisorption) per hr.

In order to exclude the presence of chemisorbed hydrogen (ref 14), which produces overestimated values when the Cu(0) specific area is determined by N20 chemisorption, we have also determined these values from ESCA data. In particular we have derived specific surface area of Cu(0) from Cu

2p signal intensity ratio (ref. 12). The SCU(O)

values derived by means the two different techniques show to be in agreement within a 15% (tab. 3, col. 2 and 3) and indicate that reduced catalyst doesn't contain significative amounts of

chemisorbed

hydrogen.

is observed as copper loading From the data in table 3, a progressive decrease of SCU(O) increases. Since the temperature used for pre-reduction should avoid the diffusion of copper in the bulk (ref. 16), this trend indicate an increment of average size of Cu(0) crystallite (from 2 to 7

101

nm (ref. 11)). The growth of Cu(0) particles scarcely influences the turnover number referred to the total copper (Tv) for catalysts with a copper loading less than 8.4%. This behaviour is apparently in contrast with the great variation of SCU(O), but can be explained by considering that the specific surface turnover (TvS) does not show a constant value but a maximum at c.a. 8% (tab 3 col. 4). The lowering of TvS at the extremities of the concentration range can be due to either higher effect of poisoning (low copper loading) or a less favourable surface ratio between copper and alumina (high copper loading). Finally, with the aim to optimize the performance of the Cu/A1203 systems, we have studied the influence of the operative conditions on the reaction selectivity. TABLE 4 Hydrogenation of Cyclododecatrienecatalysed by Cu/A1203 a Catal.

TPC)

C~/A120,

180 200 140 180 160 C U O / A ~ ~ O ~ ~ 180

P(kPa) CDE

Tv(h-') CDD

Reaction Products (mole%)c CDE

100

130 133 7 207 90 <5

94 92

4

4

86 91

7 4.5

7 4.5

250 300 100

3

3

a CDT/Cu molar ratio = 100. Re-reduction temp. = 27OOC; Cu% = 7.0: when 2 moles of Hz were consumed per mole of CDT. Untreated catalyst.

As shown in table 4 the catalyst was satisfactorily active at temperature higher than 14OOC, while the selectivity decreases as temperature and pressure increase. Yields up to 94% of CDE were obtained at 180°C and 100 H a . The reductive pretreatment was also essential for the catalyst activity, the untreated material being almost inert with no evidence of in situ activation. This behaviour confirms that Cu(0) is an active species in the hydrogenation of CDT. From the data above exposed we can conclude that copper carefully dispersed on alumina and other M f l ~oxides represents a effective and cheap system for selective CDE production.

102

REFERENCES

1 A. Misono and I. Ogata, Bull. Chem. Soc.Jap., 40 (1967)2718 2 D.R. Fahey, J. Org. Chem., 38(1) (1973)80 3 K.Kihara, K. Katsuragawa and K. Yoshimitsu, Jap. Pat. 7479,990- C.A. 83:27714m 4 N.V. Unilever Neth. Pat. 6,608,993- C.A. 675393511 5 C. Fragale, M. Gargano, M. Rossi, J. Am. Oil Chem. Soc.,59 (1982)465 6 C. Fragale, M. Gargano, N. Ravasio, M. Rossi, I. Santo, Inorg. Chim. Acta, 82 (1984)157 7 C. Fragale, M.Gargano, N. Ravasio and M. Rossi, Gazz. Chim. Ital., I17 (1987)43 8 M.Gargano, N.Ravasio. M. Rossi, I. Santo, La Chimica e l'Indusma, 694 (1987)1. 9 C.Fragale, M. Gargano, M. Rossi, Italian Patent 24437 A/82 10 S.Koritala, J. Am. Oil Chem. Soc.,49 (1972)83 11 T.J. Osinga, B.G. Linsen, W.P. van Beek, J. Catal., 7 (1976)277 12 V.Di Castro, C. Furlani, M. Gargano, N.Ravasio, M. Rossi., I. Electr.Spectr. (1990)in press 13 R. Bechara, G. Wrobel, M. Daage, J.P. Bonnelle. Appl. Catal., 16 (1985)15 14 L. Jalowiecki, G. Wrobel, M. Daage, J.P. Bonnelle, J. Catal., 107 (1987)375 15 V. Di Castro, C. Furlani, M. Gargano, M.Rossi, Appl. Surf. Science 28 (1987)270 16 B.R. Strohmeier, D.E. Leyden, R.S. Field, D.M. Hercules, J. Catal.,94 (1985)514.

G . Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catulysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed i n T h e Netherlands

103

PREPARATION AND CHARACTERIZATION OF HIGHLY SELECTIVE Fe-Cu/Si02 CATALYSTS FOR PARTIAL HYDROGENATION OF ALKYNES

Y u r i k o NITTA, Yoshifumi HIRAMATSU, Y a s u a k i OKAMOTO and Toshinobu IMANAKA Department of Chemical E n g i n e e r i n g , F a c u l t y of E n g i n e e r i n g S c i e n c e , Osaka U n i v e r s i t y , Toyonaka, Osaka 560, J a p a n SUMMARY A h i g h l y s e l e c t i v e Fe-Cu/Si02 c a t a l y s t w i t h a n enhanced a c t i v i t y f o r p a r t i a l h y d r o g e n a t i o n of p h e n y l a c e t y l e n e w a s p r e p a r e d by c o p r e c i p i t a t i o n of i r o n and c o p p e r s u l f a t e s o v e r u l t r a f i n e s i l i c a g e l w i t h a n e x c e s s a m o u n t of s o d i u m The e f f e c t s o f t h e p r e c i p i t a t i o n v a r i a b l e s and carbonate a t a l o w temperature. t h e d o p i n g o f c o p p e r and s u l f a t e i o n s o n t h e r e d u c t i o n b e h a v i o r , s u r f a c e s t a t e s , and c a t a l y t i c p r o p e r t i e s of t h e p r e c u r s o r s a n d / o r r e d u c e d c a t a l y s t s were s t u d i e d by u s i n g TGA, XRD, and XPS. The p r e s e n c e of c o p p e r lowers t h e d e c o m p o s i t i o n and r e d u c t i o n t e m p e r a t u r e s of a-FeOOH t o a-Fe20g a n d Fe2+, t h u s d e c r e a s i n g t h e amount o f water produced a t h i g h e r t e m p e r a t u r e s and p r e v e n t i n g t h e s i n t e r i n g of i r o n m e t a l . The l o w p r e c i p i t a t i o n t e m p e r a t u r e and t h e u s e of metal s u l f a t e s are b o t h e f f e c t i v e t o p r o v i d e h i g h l y - d i s p e r s e d metal s p e c i e s i n t h e p r e c u r s o r , which r e s u l t i n a n i n c r e a s e i n t h e i r o n d i s p e r s i o n of t h e reduced c a t a l y s t . INTRODUCTION I r o n catalysts show s p e c i f i c a c t i v i t y and s e l e c t i v i t y f o r t h e h y d r o g e n a t i o n of c a r b o n monoxide.

For p a r t i a l hydrogenation of alkynes, analogously prepared

i r o n catalysts e x h i b i t h i g h s e l e c t i v i t y (refs. 1-5), b u t e x t r e m e l y l o w a c t i v i t y , which h i n d e r s p r a c t i c a l u s a g e of

iron-based

catalysts.

Development o f

iron

c a t a l y s t s with h i g h a c t i v i t y for a l k y n e hydrogenation is r e q u i r e d f o r i n d u s t r i a l applications.

Recently, w e have r e p o r t e d t h a t ,

c a t a l y s t s , t h e u s e o f f e r r i c s u l f a t e , non-porous

i n t h e p r e p a r a t i o n of F e / S i 0 2 u l t r a f i n e s i l i c a g e l , and a n

e x c e s s amount of Na2C03 r e s u l t s i n a n a c t i v e and s e l e c t i v e c a t a l y s t f o r t h e h y d r o g e n a t i o n o f p h e n y l a c e t y l e n e t o s t y r e n e u n d e r m i l d r e a c t i o n c o n d i t i o n s , and t h a t t h e s e l e c t i v i t y f o r s t y r e n e i n c r e a s e s w i t h i n c r e a s i n g d i s p e r s i o n of i r o n metal ( r e f . 6 ) .

In

o r d e r t o i n c r e a s e t h e d i s p e r s i o n of i r o n ,

and

hence

to

i n c r e a s e t h e s e l e c t i v i t y as w e l l as t h e a c t i v i t y o f t h e c a t a l y s t , t h e e f f e c t of c o p r e c i p i t a t i o n of

d i f f e r e n t metals w a s examined.

The a d d i t i o n of

proper

amount of c o p p e r w a s f o u n d t o b r i n g a b o u t r e m a r k a b l e i n c r e a s e s i n t h e a c t i v i t y and s e l e c t i v i t y of

the

r e s u l t i n g c a t a l y s t s ( r e f . 7).

Moreover,

simultaneous

a d d i t i o n o f c o p p e r and s u l f a t e i o n s and a l o w p r e c i p i t a t i o n t e m p e r a t u r e a p p e a r e d e s s e n t i a l t o t h e p r e p a r a t i o n of a h i g h l y a c t i v e and s e l e c t i v e c a t a l y s t . work,

t h e e f f e c t s o f t h e p r e p a r a t i o n v a r i a b l e s and t h e d o p i n g o f

In this

c o p p e r and

s u l f a t e i o n s o n t h e p r o p e r t i e s o f F e / S i 0 2 (1:l) c a t a l y s t s were s t u d i e d i n d e t a i l by u s i n g TGA, XRD, a n d XPS.

104 EXPERIMENTAL Catalyst preparation

C a t a l y s t p r e c u r s o r s were p r e p a r e d by a p r e c i p i t a t i o n method a s f o l l o w s : s o l u t i o n o f a n e x c e s s amount of Na2C03 (1.5-1.7

m o l a r e q u i v a l e n t ) i n 2 5 m l of

d i s t i l l e d water was a d d e d t o a n a q u e o u s s u s p e n s i o n (150 m l ) c o n t a i n i n g non-porous

ultrafine silica gel

iron(lI1) sulfate,

and

(Nippon

Aerosil,

copper(l1) s u l f a t e

usually at 7540°C

Aerosil-300,

3 2 0 m'g-l),

Th e p r e c i p i t a t i o n t e m p e r a t u r e

The p r e c i p i t a t e w a s a g e d t h r e e times

an d f i l t e r e d , f o l l o w e d by w ash i n g

water a n d d r y i n g a t 110°C f o r 20 h .

2 g of

( t o t a l amount o f metal = 36 mmol)

o v e r a p e r i o d of 1-2 min u n d e r v i g o r o u s s t i r r i n g .

w a s c ha nge d i n t h e r a n g e of 8 t o 8 0 ° C .

A

f o r 15 min with hot

O t h e r d e t a i l s were i d e n t i c a l t o t h o s e

described f o r the preparation of silica-supported b a s i c n i c k e l carbonate ( r e f s . 8,9).

A f e w p r e c u r s o r s were p r e p a r e d by u s i n g i r o n ( I I 1 ) and c o p p e r ( I 1 ) n i t r a t e s

f o r c ompa ris on.

Fe-Cu/Si02

c a t a l y s t s (metal l o a d i n g = ca. 50 w t % ) were p r e p a r e d

by h e a t i n g d r i e d p r e c u r s o r s (1 g ) i n f l o w i n g h y d r o g en ( 8 1 h - l ) t e m p e r a t u r e ramp of 10°C min-' otherwise noted.

an d h o l d i n g a t

up t o 500°C a t a

temperature

for 1 h u n l e s s

C a t a l y s t s a r e d e n o t e d by t h e a t o m i c c o m p o s i t i o n a n d t h e

p r e p a r a t i o n ( p r e c i p i t a t i o n and aging) Fe-Cu(7:3)/Si02

this

temperatures

s u c h as

c a t a l y s t p r e c i p i t a t e d a t a low t e m p e r a t u r e

for

the

( u s u a l l y , 20°C)

F7C3-LH

and

s u b s e q u e n t l y a ge d a t a h i g h t e m p e r a t u r e ( u s u a l l y , 75-80%). Hydrogenation The h y d r o g e n a t i o n o f p h e n y l a c e t y l e n e (0.6 m l ) w a s c a r r i e d o u t i n e t h a n o l ( 2 7 m l ) with a freshly-reduced

c a t a l y s t a t 60°C u n d e r a h y d r o g en p r e s s u r e of 1 MPa

by u s i n g a g l a s s a u t o c l a v e e q u i p p e d w i t h a m a g n e t i c s t i r r i n g s y s t e m . i n i t i a l h y d r o g e n a t i o n rates (Ro)

Th e

and s t y r e n e s e l e c t i v i t i e s , d e f i n e d as t h e mo l ar

p e r c e n t a g e of s t y r e n e i n a l l t h e p r o d u c t s , a t 50% c o n v e r s i o n (S50) were o b t a i n e d from t h e GLC d a t a o f t h e p r o d u c t s a t d i f f e r e n t r e a c t i o n times.

Analysis Powder X-ray

d i f f r a c t i o n (XRD)

p a t t e r n s of t h e c a t a l y s t s s e p a r a t e d from t h e

r e a c t i o n m i x t u r e w e r e m easu r ed u s i n g a Shimadzu VD-1 radiation.

T h e m e an c r y s t a l l i t e s i z e (D,)

diffractometer with C u b

of i r o n metal i n a c a t a l y s t w a s

c a l c u l a t e d f r o m t h e half-maximum b r e a d t h o f t h e (110) p eak o f a-Fe a f t e r correct i o n f o r i n s t r u m e n t a l broadening ( r e f .

10).

The c r y s t a l l i t e s i z e d i s t r i b u t i o n s

(CSD) of i r o n i n some c a t a l y s t s were o b t a i n e d a c c o r d i n g t o t h e F o u r i e r t r a n s f o r m method ( S t o k e s m e thod ) f o r X-ray l i n e p r o f i l e a n a l y s i s ( r e f . 11). T h e r m o g r a v i m e t r i c a n a l y s e s (TGA) an d d i f f e r e n t i a l t h e r m a l a n a l y s e s (DTA) of c a t a l y s t p r e c u r s o r s were c a r r i e d o u t w i t h a Shimadzu DT-30 h e a t i n g i n a stream of h y d r o g e n t o 800°C a t a otherwise noted.

rate

t h e r m a l a n a l y z e r by

of 1 0 ° C m i n - l

unless

The d e g r e e o f r e d u c t i o n o f a c a t a l y s t w a s e s t i m a t e d from t h e

105 weight l o s s i n t h e range above

1 0 0 ° C on t h e b a s i s of t h e t h e o r e t i c a l w e i g h t

l o s s f o r 100% r e d u c t i o n of t h e p r e c u r s o r . The X-ray p h o t o e l e c t r o n s p e c t r a (XPS) of some p r e c u r s o r s and f r e s h l y - r e d u c e d c a t a l y s t s were measured o n a H i t a c h i 507 p h o t o e l e c t r o n s p e c t r o m e t e r u s i n g a n A 1 The sample w a s mounted on a d o u b l e - s i d e d

anode.

adhesive tape.

Binding

e n e r g i e s (BE) were r e f e r e n c e d t o t h e S i ( 2 p ) band a t 103.1 eV due t o t h e c a t a l y s t s u p p o r t . The peak areas were u s e d f o r a q u a n t i t a t i v e s u r f a c e a n a l y s i s of

the

D e t a i l e d procedures have been described elsewhere ( r e f . 12).

sample. RESULTS

Reduction behavior

of copper-promoted

precursors

The d i f f e r e n t i a l t h e r m o g r a v i m e t r i c (DTG) p r o f i l e s i n F i g . 1 i n d i c a t e t h a t t h e r e d u c t i o n of Fe-Cu/Si02

precursors v i r t u a l l y

p r o c e e d s i n two s t e p s , v i z . ,

a

s h a r p p e a k a t a b o u t 200°C and a b r o a d p e a k a t a r o u n d 500OC. On t h e b a s i s of t h e s e p a r a t e DTG a n d DTA e x p e r i m e n t s i n H2

a n d N2

f o r Cu/Si02

and Fe/Si02

p r e c u r s o r s , i t was c o n c l u d e d t h a t t h e s h a r p DTG peak a t a r o u n d 200°C c o r r e s p o n d s t o a r a p i d w e i g h t l o s s due t o t h e r e d u c t i o n of Cu2+ t o Cuo s u p e r p o s e d by a b r o a d peak d u e t o t h e d e c o m p o s i t i o n of a-FeOOH t o a-Fe203,

and t h a t

t h e b r o a d peak a t

a r o u n d 500°C c o r r e s p o n d s t o t h e r e d u c t i o n of i r o n o x i d e s o r s i l i c a t e t o metallic i r o n . T h i s i s c o n s i s t e n t

-

with t h e results i n literature

1 4 , 15).

(refs.

The p e r c e n t a g e

"C ( A , )

and above 3OOOC ( A , )

c u 90% Cu 70%

w e i g h t l o s s e s i n t h e r a n g e of 100-300 l i s t e d i n T a b l e 1, t o g e t h e r

c u 100%

are

with t h e

h y d r o g e n a t i o n r e s u l t s on t h e r e d u c e d catalysts.

It i s noteworthy t h a t t h e

s h a r p DTG peak due t o Cu2+ r e d u c t i o n s h i f t e d t o a lower t e m p e r a t u r e f o r

c u 20%

t h e most a c t i v e and s e l e c t i v e c o n t a i n i n g ca. 30% Cu.

catalyst

addition,

"C (A;)

In

t h e w e i g h t l o s s a b o v e 300

close t o a t h e o r e t i c a l value,

f o r t h e c o m p l e t e r e d u c t i o n of Fez+ t o whereas t h e weight l o s s e s f o r t h e catalysts were

larger

c u 0%

9.0%,

Feo (FeO - C u / S i 0 2 + F e - C u / S i 0 2 ) , other

c u 5%

f o r t h e F7C3-HH c a t a l y s t w a s

than

those e x p e c t e d , i n d i c a t i n g t h a t p a r t

of Fe3+ s p e c i e s s t i l l e x i s t s i n t h e s e

I

100

I

200

I

300

400

500

I

600

T e m p e r a t u r e / "C F i g . 1. DTG-in-H2 p r o f i l e s of Fe-Cu/Si02 p r e c u r s o r s w i t h d i f f e r e n t content of copper.

106 TABLE 1 E f f e c t s o f c o p p e r c o n t e n t on t h e r e d u c t i o n b e h a v i o r of Fe-Cu/Si02

precursorsa

and on t h e c a t a l y t i c p r o p e r t i e s a f t e r r e d u c t i o n a t 500°C f o r 1 h. Catalyst

F10-HH F9.5CO. 5-HH F9C1-HH F8C2-HH F7C3-HH F6C4-HH F5C5-HH F4C6-HH F3C7-HH FlC9-HH c10-HHg

Content

Tcu

o f CU/%

"C

0 5 10 20

30 40 50 60 70 90 100

227 200 185 183 183 184 189 190 212 190-220

Weight l o s s c

A,/%

A2/% A2'/%

10.1 13.1 14.2 15.7 15.8 16.9 17.2 17.8 18.6 22.8 22.9

16.8 10.9 10.3 8.9 7.8 7.3 5.8 5.6 5.4 4.6

1.9

18.6 12.5 12.0 10.6 9.2 8.8 6.9 6.9 6.6 6.0 2.5

D.R.d

Dce

%

nm

95.2

19 20 17 15 12 14 15 16 17

93.5

93.4 93.1 98.2 88.2 83.1 90.6 91.9 91.7 93.1

'50

r0 mmol-min-lg-l 1.9~10-~ 3.3x10-' 4.9x10-' 7.3x10-' 1. 9x1OF1 1. 0x10-1 7.9x10-' 8.2~10-~ 4 . 5 ~ 1 0 ~ 1.2x10-3 7.3~10-~

z

96.5

97.3 97.6 98.9 99.5 99.1 98.8 93.3f ~89.5f 94.6f 98.4f

~~~~~~

a T e m p e r a t u r e s o f p r e c i p i t a t i o n and a g i n g are b o t h 75°C. b Peak t e m p e r a t u r e f o r r e d u c t i o n o f C u * + s p e c i e s . c P e r c e n t a g e w e i g h t l o s s e s d u r i n g 100-3OO0C( AI) and a b o v e 3OO0C(A2), and t h e p e r c e n t a g e l o s s b a s e d on t h e w e i g h t a t 3OO0C(A;). d Degree o f r e d u c t i o n measured by TGA i n H2. e Mean c r y s t a l l i t e s i z e of w F e . f S e l e c t i v i t y a t 10%c o n v e r s i o n . g Reduced at 300°C f o r 1 h i n s t e a d of 500'C. TABLE 2 E f f e c t s of p r e c i p i t a t i o n v a r i a b l e s on t h e p r o p e r t i e s of Fe-Cu(7:3)/Si02

and

F e / S i 0 2 p r e c u r s o r s and t h e r e d u c e d c a t a l y s t s . Catalyst

Starting

Temperature/OC

salt

Ppt.

Aging

F7C3-HH F7C3-MH F7C3-LH F7C3-LHa F7C3-LHb F7C3-L H F7C3-LL. F7C3-HH(N) F7C3-LH( N)

Sulfate Sulfate Sulfate Sulfate Sulfate Sulfate Sulfate Nitrate Nitrate

81 55 20 20 20 8

81

20 80 20

80 80 80 80 80 20 80 80

F10-HH F10-LH F 10-L ' L FlO-HH(N)

Sulfate Sulfate Sulfate Nitrate

75 20 9 75

75 83 9 75

S B E ~ Tcu m2g-l

"C

D.R.

Dc

%

nm

r0 mmol.min-lg-l

'50 %

17 16 14 11 13 18 13 20 22

1.3~10-1 1.4~10-1 3.6~10-1 5.3~10-1 3.6~10-1 3.OxlO-' 6.3~10-~ 2.4x10-' 8.6x10-'

99.3

485 46 2 437 484

98.2 97.9 98.7 100 100 170 97.1 178 94.5 183 <83 182 95.8

428 833 508 408

95.2 <77 <79 <75

19 40 32 45

1.9~10-~ 5 . 4 ~ 1 0 ~ 5.6~10-~ 2 . 6 ~ 1 0 ~

96.5 87.0' ~ 92.6' 78.1' ~

463 4 31 535

183

183 172

99.0 99.4 99.3 99.5 99.4 98.4 97.2 99.3

a R e d u c t i o n w a s c a r r i e d o u t by h e a t i n g a t a r a t e of 2.5"C/min t o 400°C and h e l d a t 400°C f o r 5 h. b R e d u c t i o n w a s c a r r i e d o u t a f t e r h e a t e d a t 300°C f o r I h i n a n N2 stream. c S e l e c t i v i t y a t 10% c o n v e r s i o n .

107 c a t a l y s t s a t 300°C.

These reduction

p r o c e s s e s were c o n f i r m e d by XPS a s s t a t e d below. lower and

The a b o v e r e s u l t s s u g g e s t t h a t t h e the

r e d u c t i o n t e m p e r a t u r e s of

Cu2+

a-FeOOH a r e , t h e m o r e a c t i v e t h e

r e d u c e d c a t a l y s t becomes.

~E f f e c t s of In

precipitation variables

t h e course of

preparation conditions,

the

s t u d i e s on

i t was found

that

t h e p r e c i p i t a t i o n t e m p e r a t u r e had a g r e a t e f f e c t on t h e c a t a l y t i c p r o p e r t i e s of r e s u l t i n g catalyst.

the

The p r o p e r t i e s o f Fe-

Cu/Si02 and F e / S i 0 2 p r e c u r s o r s p r e p a r e d a t different precipitation and/or aging t e m p e r a t u r e s are l i s t e d together

i n Table

2,

p r e p a r e d f r o m metal n i t r a t e s .

p r e c u r s o r s p r e c i p i t a t e d a t a r o u n d 2OoC had l a r g e r BET s u r f a c e a r e a s w i t h f l o s s y t e x t u r e and e x h i b i t e d h i g h e r h y d r o g e n a t i o n activities,

when r e d u c e d ,

p r e c i p i t a t e d a t 55-80°C.

catalysts,

I

.",

was

i

1 F7C3-LH

I n t h e case of t h e c o p p e r - f r e e however,

/ "C

F i g . 2. DTG p r o f i l e s of F7C3-LH and -HH p r e c u r s o r s : ( A ) a s prep a r e d , ( B ) a f t e r h e a t e d a t 300°C f o r 3 . h i n N2, and ( C ) measured i n N2.

than those

With t h e a g i n g

p r o c e s s , t h e h i g h t e m p e r a t u r e (75-80°C) preferable.

Temperature

Irrespective

of t h e s t a r t i n g s a l t , t h e Fe-Cu/Si02

500

300

100

with t h e data f o r t h e c a t a l y s t s

the low precipitation

t e m p e r a t u r e had n e g a t i v e e f f e c t s on t h e r e d u c i b i l i t y of t h e p r e c u r s o r s and on t h e catalytic properties. As c a n b e s e e n f r o m F i g .

2,

t h e DTG

peak a t a r o u n d 200°C f o r t h e most a c t i v e c a t a l y s t , F7C3-LH, lower

temperature

Dc / nm

shifted further to a than

that

f o r F7C3-HH.

The DTG p r o f i l e s measured a f t e r h e a t i n g a t

F i g . 3. CSDs of a-Fe i n F7C3-LH and F7C3-LH(N) c a t a l y s t s .

3OOOC for 3 h i n a n N2 stream, which have no s u p e r p o s i t i o n of

t h e d e c o m p o s i t i o n p r o f i l e s of

a-FeOOH,

show more c l e a r l y

t h a t t h e r e d u c t i o n of CuO i n F7C3-LH p r e c u r s o r o c c u r s a t a t e m p e r a t u r e l o w e r t h a n t h a t i n F7C3-HH

precursor.

The d i f f e r e n c e

30-50°C

i n t h e decomposition

t e m p e r a t u r e of a-FeCQH t o a-Fe203 w a s l e s s t h a n 10°C between F7C3-HH and F7C3-LH a s r e v e a l e d i n t h e p r o f i l e s measured

i n an N2

stream.

Therefore,

the

108 r e d u c i b i l i t y of Cu2+ i n t h e p r e c u r s o r seems t o h a v e a s t r i k i n g e f f e c t on t h e c a t a l y t i c p r o p e r t i e s of t h e r e d u c e d c a t a l y s t . is also s i g n i f i c a n t .

The F7C3-LH

The e f f e c t of t h e s t a r t i n g s a l t

catalyst prepared from s u l f a t e s e x h i b i t e d

a l m o s t f i v e times h i g h e r a c t i v i t y t h a n t h e c a t a l y s t

F7C3-LH(N), a l t h o u g h t h e y had similar v a l u e s o f Dc. i n F i g . 3.

c a t a l y s t s are compared

p r e p a r e d from n i t r a t e s ,

The CSDs of a-Fe

i n these

E v i d e n t l y , the p r e s e n c e of v e r y small

c r y s t a l l i t e s of i r o n e x p l a i n s t h e h i g h a c t i v i t y of t h e F7C3-LH i n c r e a s e d r e d u c t i o n t e m p e r a t u r e of Cu2'

catalyst.

The

f o r F7C3-LH(N) r e n d e r s f u r t h e r e v i d e n c e

t o t h e i m p o r t a n c e of t h e r e d u c i b i l i t y of Cu2+ i n t h e p r e c u r s o r .

XPS s t u d i e s _-_-

of v a r i o u s p r e c u r s o r s and r e d u c e d c a t a l y s t s

The XPS p a r a m e t e r s f o r v a r i o u s p r e c u r s o r s and the c a t a l y s t s r e d u c e d a t 300°C

f o r 0.5 h o r a t 500°C f o r 1 h are summarized i n T a b l e 3.

The s p e c t r a of t h e

Fe2p l e v e l f o r t h e F7C3-LH p r e c u r s o r and t h e r e d u c e d s a m p l e s are shown i n F i g . 4 .

TABLE 3 XPS b i n d i n g e n e r g i e s (BE), k i n e t i c e n e r g i e s (KE), and i n t e n s i t y r a t i o s f o r Fe-Cu/Si02 Catalyst

p r e c u r s o r s and t h e r e d u c e d c a t a l y s t s . Reduction Temp/"C Time/h

F10-HH F9C1-HH F8C2-HH F7C3-HH F7C3-LH F7C3-LH(N) F6C4-HH F5C5-HH F3C7-HH FlC9-HH c10-HH

710.9 711.5 712.1 711.4 711.6 712.0 711.7 711.5 712.7 713.5

F7C3-HH F7 C 3-LH F10-HH F10-LH

300 300 300 300

F10-HH F8C2-HH F7C3-HH F7C3-LH F7C3-LH(N) F5C5-HH F3C7-HH

500 500 500 500 500 500 500 300

C10-HH

BE/eV KE/eV BE/eV Fe2p3/, C ~ 2 p ~ / , ( S a t e ) ~ Cu(LMM) S2p

0.5 0.5 0.5 0.5

710.7 710.0 711.4 710.7 710.0 710.4 710.4 710.5 710.6 710.9 710.0

706.7 706.9 706.9 707.0 707.1 706.5 707.0

934.1 935.5 935.1 934.4 934.0 935.0 934.5 935.5 935.2 935.7

(942.4) (943.2) (943.1) (942.2) (941.9) (943.1) (942.5) (943.2) (942.7) (943.6)

932.2 931.9

c c c c c

169.5 168.9 169.9

915.5 919.3

c c

c c

c C

932.1 931.7 931.9 931.9 931.9 931.7 932.0

a I n t e n s i t y r a t i o : FeZp, Cu2p3/2, S2p, and S i 2 p . b S a t e l l i t e peak.

c Not d e t e c t e d .

C

916.0 916.0 915.5 917.0 915.5 916.1 916.6 916.0 915.9 915.6

918.9 919.0 919.1 919.5 919.0 919.3 919.1

C

c c

c c

c

161.6 169.0

ea Si

2.31 4.74 5.09 5.93 8.86 8.20 4.79 4.16 3.47 2.36 5.72 5.67 5.82 5.21 0.91 1.61 2.00 2.84 2.94 1.61 2.87

Si

0.48 1.01 1.62 2.34 1.72 2.29 3.39 4.37 4.86 12.69

2 Si

0.12 0.16 0.25

1.75 0.54

0.30 0.43 0.79 0.58 0.82 1.23 2.60

0.04 0.10

109 The b i n d i n g e n e r g i e s Fe(2p3/2)

(BE)

of

the

l e v e l f o r t h e unreduced

precursors with t h e copper content less t h a n 50% were v e r y c l o s e t o t h a t

f o r a-FeOOH,

w h i l e t h e BEs f o r t h e

p r e c u r s o r s c o n t a i n i n g 70 and 90% Cu were c l o s e t o t h a t f o r F e 2 ( S 0 4 ) 3 . The BE of t h e Cu(2p3/2) l e v e l and t h e k i n e t i c e n e r g y (KE) o f t h e Cu(LMM) Auger l e v e l f o r a l l p r e c u r s o r s are c o n s i s t e n t w i t h t h o s e f o r CuO ( r e f .

12).

With t h e c a t a l y s t s r e d u c e d a t

5OO0C f o r 1 h , t h e BE and KE v a l u e s i n d i c a t e t h a t i r o n e x i s t s as Fe2'

and

Feo r e g a r d l e s s o f t h e c o m p o s i t i o n ,

\\

w h i l e c o p p e r e x i s t s o n l y as Cu metal.

As

for

the

catalysts partially

r e d u c e d a t 300°C f o r 0.5 h , t h e BEs

I

750

740

i r o n e x i s t a s Fe2+ i n F7C3-LH and as

+

Fe3+ i n F7C3-HH

catalysts.

T h i s i s i n good a g r e e m e n t w i t h t h e result

of

TGA

experiments

d e s c r i b e d above.

I

720

710

700

BE/eV

of t h e Fe( 2p3/2) l e v e l i n d i c a t e t h a t Fe2'

I

730

F i g . 4. Fe2p XP s p e c t r a f o r F7C3-LH. ( A ) P r e c u r s o r , ( B ) r e d u c e d a t 300°C f o r 0.5 h , and ( C ) r e d u c e d a t 500°C f o r 1 h .

as 10

T h e Cu(LMM) KE

I

v a l u e s f o r t h e s e two c a t a l y s t s show t h a t t h e o x i d a t i o n state of c o p p e r a l s o d i f f e r s f r o m e a c h o t h e r : Cuo f o r F7C3-LH

and

catalysts. detected

Cut

f o r F7C3-HH

T h e S ( 2 p ) s p e c t r a were only

in

the

catalysts

a

N .r(

m

sa

\

N

3

V

a

c o n t a i n i n g more t h a n 60% Cu, as S042-

N .d

in the

\

p r e c u r s o r s and

as

metal

s u l f i d e s i n t h e reduced catalysts.

m

a

N

a

L

Therefore, t h e extremely l o w a c t i v i t y of t h e catalyst w i t h such a h i g h c o n t e n t of c o p p e r (see T a b l e 1) c a n be, a t least p a r t l y , a t t r i b u t e d t o t h e p o i s o n i n g e f f e c t of t h e r e m a i n i n g sulfur. In Fig.

5, t h e Fe2p/Si2p

and

Cu2p3/2/Si2p i n t e n s i t y r a t i o s f o r t h e

C u / ( F e t C u ) i n Bulk F i g . 5. F e 2 p / S i 2 p (0) and Cu2p3/,/Si2p (A) XPS i n t e n s i t y r a t i o s f o r Fe-Cu/Si02 p r e c u r s o r s p r e c i p i t a t e d a t 75OC a s a f u n c t i o n o f Cu c o n t e n t i n b u l k . ( 0 ) F e 2 p / S i 2 p a f t e r r e d u c e d a t 5OO0C f o r 1 h.

110 p r e c u r s o r s p r e c i p i t a t e d a t 75OC are p l o t t e d a g a i n s t t h e b u l k c o m p o s i t i o n .

The

F e 2 p / S i 2 p r a t i o i n c r e a s e d s i g n i f i c a n t l y w i t h i n c r e a s i n g c o n t e n t of c o p p e r i n t h e b u l k up t o 30%, which i m p l i e s t h e i n c r e a s e d d i s p e r s i o n o f i r o n i n t h e p r e s e n c e of copper.

I n t h e p r e c u r s o r o f t h e most a c t i v e F7C3-LH

catalyst, further

i n c r e a s e s i n t h e XPS r a t i o s were o b s e r v e d as c i t e d i n T a b l e 3. DISCUSSION H y d r o g e n a t i o n r e s u l t s p r e s e n t e d i n T a b l e s 1 and 2 show t h a t t h e c a t a l y s t w i t h higher a c t i v i t y e x h i b i t s higher s e l e c t i v i t y . previous r e p o r t s (refs.

6, 7 ) .

T h i s is i n conformity with our

The a c t i v i t y of t h e Fe-Cu(7:3)/Si02

p r e c i p i t a t e d a t a r o u n d 2OoC by u s i n g m e t a l s u l f a t e s , F7C3-LH,

catalyst

was three orders

h i g h e r t h a n t h a t of a n o r d i n a r y Fe/Si02 c a t a l y s t prepared from ferric n i t r a t e , FlO-HH(N).

T h e h i g h s e l e c t i v i t y of t h e F7C3-LH

c a t a l y s t w a s k e p t almost

c o n s t a n t d u r i n g t h e r e a c t i o n and w a s 99% e v e n a t 98% c o n v e r s i o n . Although t h e r e d u c t i o n e x t e n t of i r o n i s u s u a l l y i n c r e a s e d by t h e p r e s e n c e of copper ( r e f s .

16, 1 7 ) , t h e r e i s no s i g n i f i c a n t

difference

in

t h e percentage

r e d u c t i o n s of t h e catalysts under t h e p r e s e n t r e d u c t i o n c o n d i t i o n s , suggesting t h a t t h e d i f f e r e n c e i n a c t i v i t y is n o t due s o l e l y t o t h e d i f f e r e n c e i n t h e reduction degree of i r o n . t h e H2-reduction

It h a s b e e n r e p o r t e d t h a t t h e water produced d u r i n g

s t r o n g l y a f f e c t s t h e d i s p e r s i o n o f i r o n metal,

sintering especially at

high temperatures ( r e f s .

r e d u c t i o n c o n d i t i o n s on t h e c a t a l y t i c consistent with

these observations.

causing

The e f f e c t s of

p r o p e r t i e s , shown i n T a b l e 2 , a r e

When t h e F7C3-LH

reduced a t a lower temperature for a longer employed h e r e (50O0C, 1 h ) ,

18, 19).

precursor

was

slowly

time t h a n t h e u s u a l c o n d i t i o n s

a d e c r e a s e i n Dc and much improved a c t i v i t y w e r e

obtained, probably due t o e f f i c i e n t

e l i m i n a t i o n o f water ( r e f . 7 ) .

Our

DTG

e x p e r i m e n t s showed t h a t t h e r e d u c t i o n o f CuO and a-Fe00H i n t h e p r e c u r s o r of t h e a c t i v e catalysts c o n t a i n i n g

ca.

those i n the other catalysts.

30% Cu p r o c e e d s a t l o w e r t e m p e r a t u r e s t h a n

I n l i n e w i t h t h i s , Wielers e t a l . h a v e v e r y

r e c e n t l y r e p o r t e d t h a t c o p p e r f a c i l i t a t e s t h e r e d u c t i o n of Fe3+ t o Fe2+ as w e l l a s t h e r e d u c t i o n t o Feo ( r e f . 2 0 ) .

The i n c r e a s e i n t h e r e d u c i b i l i t y of Fe3+ i n

t h e p r e s e n c e o f c o p p e r i s c l e a r l y e v i d e n c e d by t h e XPS r e s u l t s i n T a b l e 3. A c c o r d i n g l y , one o f t h e p r o m o t i o n a l e f f e c t s of c o p p e r c a n b e e x p l a i n e d i n terms of t h e i n c r e a s e d r e d u c i b i l i t y of Fe3+:

t h e amount of water produced a t h i g h

t e m p e r a t u r e s becomes smaller i n t h e p r e s e n c e of

copper,

thus preventing t h e

s i n t e r i n g o f i r o n metal. A s f o r t h e effect of p r e c i p i t a t i o n t e m p e r a t u r e ,

it h a s been r e p o r t e d t h a t

amorphous o r smaller p a r t i c l e s of a-FeOOH are s t a b l e i n c o l l o i d a l s o l u t i o n s a t t e m p e r a t u r e s below 5 5 " C ,

w h i l e l a r g e (50-6Om) p a r t i c l e s of h e m a t i t e , cx-Fe203,

are formed a b o v e 85 OC ( r e f .

21).

It seems l i k e l y t h a t smaller p a r t i c l e s of

o x i d i z e d i r o n s p e c i e s i n t h e p r e c u r s o r are r e d u c e d more e a s i l y ,

resulting i n

111 smaller metal p a r t i c l e s .

The h i g h

BET s u r f a c e a r e a s a n d t h e

-15.0

flossy

nature of t h e precursors p r e c i p i t a t e d

a t t h e low t e m p e r a t u r e are also favorable f o r of

water

process. at

efficient

elimination

during the reduction Moreover, t h e p r e c i p i t a t i o n

lower

temperatures

cause

will

smaller e x t e n t of m e t a l - s u p p o r t

i n t e r a c t i o n and l e a d t o i n c r e a s e d r e d u c i b i l i t y o f Cu2+ a n d F e 3 + ,

as

1

e v i d e n c e d by t h e TGA and XPS r e s u l t s .

Fe2p / S i 2 p

The e x o t h e r m i c r e d u c t i o n of Cu2+ a t a lower t e m p e r a t u r e g r e a t l y f a v o r s t h e

F i g . 6. Dependence of t h e hydrogenat i o n a c t i v i t y on t h e F e 2 p / S i 2 p XPS i n t e n s i t y r a t i o s f o r Fe-Cu(7:3)/Si02 p r e c u r s o r (0)and t h e r e d u c e d c a t a l y s t (0) p r e p a r e d f r o m metal s u l f a t e s . b,& P r e p a r e d from metal n i t r a t e s .

r e d u c t i o n of Fe3+ i n a n e a r l y s t a g e

of t h e a c t i v a t i o n .

Accordingly,

it

is e v i d e n t t h a t t h e p r e c i p i t a t i o n a t the

low t e m p e r a t u r e

catalysts with

provides

the

high dispersion of

i r o n and hence h i g h a c t i v i t y and s e l e c t i v i t y .

As s h o w n i n F i g . 6 , t h e XPS

r e s u l t s support t h e i d e a t h a t t h e increase i n

a c t i v i t y is a t t r i b u t e d t o a n

improved d i s p e r s i o n o f i r o n . C o n c e r n i n g t h e e f f e c t s of catalyst,

prepared

t h e s t a r t i n g s a l t , t h e F7C3-LH(N)

from metal

nitrates,

had

relatively

p r e c u r s o r and

l a r g e XPS i n t e n s i t y

r a t i o s f o r s u r f a c e i r o n , a l t h o u g h t h e c a t a l y s t e x h i b i t e d much l o w e r a c t i v i t y than t h a t prepared from s u l f a t e s .

The CSD i n F i g . 3 showed t h e l a c k of

i r o n c r y s t a l l i t e s i n t h e F7C3-LH(N) l a r g e i r o n p a r t i c l e s i n F7C3-LH(N)

catalyst.

small

These f i n d i n g s suggest

that

e x i s t i n t h e o u t e r s u r f a c e , whereas copper

p a r t i c l e s e x i s t i n t h e i n n e r s u r f a c e i n t e r a c t i n g w i t h t h e s u p p o r t , a s is e v i d e n c e d by t h e i n c r e a s e d r e d u c t i o n t e m p e r a t u r e of Cu2+ f o r F7C3-LH(N) 2).

Thus,

provide

(Table

t h e presence of s u l f a t e i o n s i n p r e c i p i t a t i o n appears c r u c i a l

well-mixed

to

s m a l l p a r t i c l e s of i r o n and copper i n t h e p r e c u r s o r .

F u r t h e r s t u d i e s a r e n e e d e d t o make c l e a r t h e r o l e of s u l f a t e i o n s i n t h e p r e c i p i t a t i o n process. CONCLUSIONS I n t h e p r e p a r a t i o n o f h i g h l y a c t i v e a n d s e l e c t i v e Fe-Cu/Si02 c a t a l y s t f o r t h e p a r t i a l h y d r o g e n a t i o n o f p h e n y l a c e t y l e n e , c o p r e c i p i t a t i o n of 70% F e and 30% Cu, t h e u s e o f metal s u l f a t e s , a n d a r e l a t i v e l y l o w p r e c i p i t a t i o n t e m p e r a t u r e ( a r o u n d 2OoC) were f o u n d t o b e v e r y e f f e c t i v e . catalyst was

three orders higher

The a c t i v i t y of t h i s improved

than t h e o r d i n a l Fe/Si02

c a t a l y s t and

the

112 s e l e c t i v i t y r e a c h e d 99% a t 98% c o n v e r s i o n . The p r e s e n c e of c o p p e r lowers t h e d e c o m p o s i t i o n and r e d u c t i o n t e m p e r a t u r e s of a-FeOOH

toa-Fe203 and Fez+, t h u s d e c r e a s i n g t h e amount of water produced a t

h i g h e r t e m p e r a t u r e s and p r e v e n t i n g t h e s i n t e r i n g of i r o n metal.

The p r e c i p i -

t a t i o n a t a low t e m p e r a t u r e p r o v i d e s t h e p r e c u r s o r s w i t h h i g h d i s p e r s i o n o f b o t h a-FeOOH and Cu2+ s p e c i e s , which r e s u l t i n t h e c a t a l y s t s w i t h h i g h l y - d i s p e r s e d i r o n metal.

The u s e of metal s u l f a t e s a p p e a r s c r u c i a l t o p r o v i d e well-mixed

small p a r t i c l e s of i r o n a n d c o p p e r i n t h e p r e c u r s o r .

REFERENCES P a u l and G . H i l l y s , B u l l . SOC. Chim. F r . [ 5 ] , 6 (1939) 218. F. Thompson and S. B. W y a t t , J. Am. Chem. SOC., 62 (1940) 2555. Reppe, Ann., 596 (1955) 38. T a i r a , B u l l . Chem. SOC. J p n . , 35 (1962) 840. R. S. Mann a n d K. C. Khulbe, Can. J . Chem., 45 (1967) 2755. Y . Nitta, S. M a t s u g i and T. Imanaka, Chem. E x p r e s s , 4 (1989) 547. Y. N i t t a , S. M a t s u g i and T. Imanaka, Catal. L e t t . , i n p r e s s . Y . Nitta, F. S e k i n e , T. Imanaka and S. T e r a n i s h i , J . C a t a l . , 74 (1982) 382. Y. N i t t a , T. Imanaka and S. T e r a n i s h i , J. Catal., 96 (1985) 429. B. E. Warren, J. Appl. P h y s . , 12 (1941) B75. A. R . S t o k e s , P r o c . Phys. SOC. London, 61 (1948) 382. Y. Okamoto, K . F u k i n o , T. Imanaka and S. T e r a n i s h i , J. Phys. Chem., 87 (1983) 3740 and 3747. M. Rameswaran a n d C. H. Bartholomew, J. Catal., 117 (1989) 218. J . W. Geus, i n : G. P o n c e l e t , P. Grange and P . A. J a c o b s ( E d s . ) , P r e p a r a t i o n of C a t a l y s t s 111, E l s e v i e r , Amsterdam, 1983, pp.1-33. A. F. H . Wielers, A . J. H . M. Kock, C. E. C. A. Hop, J. W. Geus and A. M. van d e r K r a a n , J. C a t a l . , 117 (1989) 1. R . N. P e a s e , H. S. T a y l o r , J. Am. Chem. SOC., 43 (1921) 2179. Y. Murata a n d S. Kasaoka, Kogyo Kagaku Z a s s h i , 62 (1959) 1195. Y. M u r a t a and S. Kasaoka, Kogyo Kagaku Z a s s h i , 61 (1958) 1440. E. R u c k e n s t e i n a n d X. D. Hu, J . C a t a l . , 100 (1986) 1. A. F. H . Wielers, C. E. C. A . Hop, J. van Beijnum, A . M. v a n d e r Kraan, and J. W. Geus, J . Catal., 121 (1990) 364. J. W. G e u s , Appl. C a t a l . , 25 (1986) 313.

1 R. A. W. S.

2 3 4 5 6 7 8 9 10

11 12

13 14 15 16 17 18 19 20 21

G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 01991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

SOME REMARKS ON THE PREPARATION OF

Fe-K/Ca-Cr

113

CATALYST FOR STYRENE PRODUCTION

Z.DZIEWII$KI and E. OZDOBA Institute of Chemistry, Pedagogical University, 25-020 Kielce (Poland) SUMMARY Several title catalyst specimens with an identical Fe, K, Ca, and Cr content had been prepared from the different FeOOH or Fe203 phases, and then tested at the same conditions in dehydrogenation of ethylbenzene to styrene. An influence has been discussed of the kind and the contamination degree of FeOOH or Fe 0 on the catalyst properties. The most selective but moderately active catal&$ was obtained from P-FeOOH slightly contaminated with C1 which contained traces of cc-FeOOH phase. The less selective but more active catalyst was obtained from pure K-FeOOH phase. INTRODUCTION Actually, iron oxide based catalysts are the most widespread contacts utilized in industrial dehydrogenation of ethylbenzene (EB) to styrene, and many propositions how to enhance their efficiency are still reported in the literature (refs. 1-3). A majority of the propositions has concerned, however, promoters variation rather than proper selection of the main components of the initial mixture from which the catalyst is prepared. The latter problem seems to be of importance because of a consolidating opinion that just Fe atoms are included, apart from 0 and K atoms, into such proposed active phases of the catalyst as KFe02 (ref. 4 ) , K2Fe220j4 (ref. 5), o r solid solution of potassium in y-Fe203 (ref. 6 ) . It is conceivable that at least some amount of those phases can be produced at the stage of heat treatment of the mixture from which the catalyst is obtained. The easiness of such a primary formation of the active phase will be conditioned by the kind, structure, and texture of the initial iron compounds used. It is also conceivable that the active centers are still created during the EB dehydrogenation. A feasibility of their effective formation ought to depend on the kind of iron oxide matrix adjacent to the K atoms on the catalyst surface. It is evident that a type of the matrix formed will be, in turn, conditioned by the kind of initial iron compound used. The purpose of the present work was, therefore, to elucidate which one of the numerous FeOOH or Fe203 phases could be used as the most appropriate material for the production of such a title catalyst which could exhibit high selectivity and/or satisfactory activity in the EB dehydrogenation to styrene. It seemed a l s o interesting to check to what extent a possible contamination of the initial iron oxide hydroxides could affect activity or selectivity of the catalyst prepared from the contaminated initial iron compound. Sulphates or chlorides were

114 chosen as t h e contaminants, because j u s t these substances could be predominantly incorporated i n t o FeOOH or Fe203 d u r i n g t h e i r large-scale solutions

previously

prepared

by

iron

scrap

production from the

dissolution

HC1.

i n H2S04 or

EXPERIMENTAL I n i t i a l i r o n compound p r e p a r a t i o n AnalaR reagents were used throughout a l l t h e procedures a p p l i e d . To c o n t r o l surface area and p a r t i c l e dimensions o f t h e specimen obtained, the appropriate precipitation,

ageing, and c a l c i n a t i o n c o n d i t i o n s had been chosen. Some speci-

50:-

or

C1-

mens were purposely contaminated with

i o n s . The degree o f t h e

contamination was c o n t o l l e d by a v a r i a t i o n o f t h e ageing and washing c o n d i t i o n s . The f o l l o w i n g procedures were a p p l i e d t o o b t a i n t h e FeOOH and Fe203 specimens: A solution o f

(i)a-FeOOH.

Fe(N03)3.9 H20

p e r a t u r e with t h e equivalent amount o f

3.5 M

(1 M)

was mixed a t room tern-

ammonia water, and the superna-

t a n t l i q u i d was removed a f t e r some time from above t h e t r a n s i e n t hydrated i r o n which was mixed i n t u r n w i t h approximately the same volume o f oxide s l u r r y (I), 3.5 M

KOH s o l u t i o n and heated a t p r e c i p i t a t e (11)

a-FeOOH

b u f f e r (pH.8.0) o f a-FeOOH

thoroughly,

60

2 hr.

under s t i r r i n g f o r

OC

0.01 M

was washed with

105

and d r i e d a t

s o l u t i o n o f NH4N03+NH3H20

i n air.

OC

50:-

t h a t were p a r t i a l l y contaminated with

So obtained

The other specimens i o n s were prepared ac-

c o r d i n g t o (refs. 7-8). (ii)j3-FeOOH.

0.25 mole o f

been d i s s o l v e d i n

1 dm3

of

Fe(NO3I3.9 H20

H20 ,and then

s o l u t i o n . The s o l u t i o n was b o i l e d a t about to

24 hr.

0 . 1 mole o f

and

OC

for

1 hr. and then subjected 95 'C

supernatant l i q u i d decantation, t h e obtained p r e c i p i t a t e (111)

C1-

had

1 mole o f urea was added t o t h i s

95

ageing a t the temperatures decreasing from

t h e above mentioned b u f f e r u n t i l no

NH4C1

i o n was detected with

to

20

OC.

After

was washed w i t h AgN03

solution

i n t h e f i l t r a t e . More i n t e n s e washing was r e q u i r e d , however, i f t h e intended C 1 content i n d r y

p-FeOOH

t e was d r i e d a t

60

t h a t contained more

ought n o t t o exceed

OC

under vacuum f o r

C1

were prepared from

with e i t h e r ammonium acetate ( r e f . 9)

1wt. %

24 hr.

. The obtained p r e c i p i t a The specimens o f

FeC13

P-FeOOH

s o l u t i o n by p r e c i p i t a t i o n

or pure water ( r e f . 1 0 ) used as h y d r o l y -

z i n g agents. (iii)y-FeOOH.

The procedure r e p o r t e d i n (ref. 11) was a p p l i e d t o o b t a i n

t h i s i r o n oxide hydroxide by means of t h e o x i d a t i o n w i t h bubbled a i r o f t h e suspension o f i r o n ( I 1 ) hydroxide, which had been obtained p r e v i o u s l y by the p r e c i p i t a t i o n w i t h ammonia water a t oxygen-free atmosphere from (iv)

Ge2c3.

cination o f the o f the lized

(11)

Very f i n e c r y s t a l l i n e

(I) precipitate at

precipitate at

cc-Fe203

400-550

350 0 C

FeS04

solution.

Fe203 powder was obtained by c a l -

OC

for

for

l hr. i n a i r . The c a l c i n a t i o n

2-6

w i t h a lower s p e c i f i c surface area.

hr.

yielded well crystal-

115

At first, the obtained (11) precipitate had been dehydrated at 320 OC for 1 hr. to hematite, which was subsequently reduced at 360 OC with hydrogen f o r 1 hr. to magnetite. So obtained magnetite powder was in turn reoxidized at 300 OC in air enriched with oxygen until pure maghemite phase of Fe203 was produced.

(v)

y-Fe,03.

Catalyst preparation To avoid the possible masking effects of some commonly used activators, such as Mo, V, W , and particularly Ce oxides, the relatively simple composition was chosen for the catalyst specimens investigated. In addition, each of the catalyst specimen was prepared strictly in the same manner. Before the catalyst preparation Fe content (and K content, if needed) had been determined in FeOOH o r Fe203 used actually for the catalyst preparation, and such an amount of this iron compound which contained 220 g of Fe was weighted. That amount of FeOOH or Fe203 was mixed with 28 g KOH ( o r l e s s , if FeOOH or Fe203 used had already contained some K content), 25 g CaO, and 14 g K2Cr207 . Some amount of was added to those components to obtain, by a H20 thorough kneading, a tough paste from which 4 mm diameter extrudates were obtained. The extrudates were, after drying at 105 OC and cutting into ca 6 mm length cylinders, calcined in two steps, at 350 O C and 550 OC respectively, for 4 hr. Both the cooling and the storage of so obtained specimens were accomplished at H20 and C02 free atmosphere. Some part of the cylinders were crushed and sieved to obtain the catalyst fraction with 0.5 mm mean size grains. Initial iron compounds and catalyst characterization Conventional analytical methods were applied to determine C1 ( r e f . 12) or S (ref. 13) content in the specimens obtained. BET method was used to determine specific surface areas of the specimens. X-ray powder diffraction patterns were recorded on a DRON 3 diffractometer with CuK, , FeKa , or MoKx radiation. The results were compared wlth the literature data (ref. 14) to determine phase composition of the specimens investigated. IR spectroscopy for some amorphous specimens was performed on BECKMANN IR Spectrometer. The obtained spectra were compared with those reported in the 11terature (ref. 15). Catalyst testing in ethylbenzene dehydrogenation to styrene The measurements were carried out in a conventional fixed-bed flow reactor, which had been equipped with a feed-stream overheater and with a cooler for the products. The values of catalyst bed temperature, feed-stream composition and rate, and total gas pressure were controlled by means of additional attachments.

116

Before the main test, an influence of the possible diffuse retardation on the reaction rate had been established. A microreactor housing ca 10 cm3 of the catalyst was used for a relatively quick detection if any difference appear in the catalyst activity when the catalyst grains of different dimensions had been separately investigated in the reaction. Since no substantial difference had been found for such beds of the same catalyst in the microreactor experiments, the 4 mm diameter cylinders of the catalyst were then used in the main test, which was carried out as follows: A 200 cm3 portion of the catalyst was placed in the reactor, the bed being first heated to 420 OC with a nitrogen stream and then up to 530 OC with an overheated steam passed through the bed. The catalyst temperature was then raised to 590 OC and the feed-stream was gradually enriched with ethylobenzene until its flow rate reached 7 5 cm3 EB/hr . Then, the reaction was studied under the following conditions: temperature, 590 OC, total pressure, ca 100 kPa, H20/EB molar ratio, 10, NTP space velocity, 10,000 hr-', overall reaction time 30 hr. Ouring each run, samples of the feed and product were taken at intervals and analyzed. An ELWRO N-504 gas chromatograph with a column packed with 5 % SP 1200/1.75 % Bentonite 34 on 100/120 Supelcoport was used to the feed and product analysis. Gaseous products were additionally analyzed, if needed, by means of the conventional chemical method. The results obtained at stationary state of the reaction, usually after 30 hr. , were taken for calculations. Styrene (STY) selectivity was calculated as lOO.(STY/(EB inlet - EB outlet)), styrene yield as 100.(STY/EB inlet), ethylbenzene conversion was calculated as 100.((EB inlet - EB outlet)/EB inlet). The two latter magnitudes were arbitrary assumed as a measure of the catalyst activity. RESULTS AN0 DISCUSSION

Initial iron compounds purity Of the compounds that were to be pure in intention, only cc-FeOOH, oc-Fe203, and y-Fe20j proved to be single-phase materials not contaminated with C 1 or S atoms. All the attempts to obtain non-contaminated single-phase P-FeOOH failed. This fact seems to be consistent with the supposition of many researchers that the existence of C1 ( o r theoretically F too) atoms in P-FeOOH lattice is required necessarily for the stability of this compound to be secured. There is no uniformity of views, however, to what extent the chlorine content could be lowered without a disturbance of p-FeOOH phase homogenity. Theoretically, the chlorine content could be calculated from various proposed "real" formulae, e.g. Fe403(OH)5C1 (ref. 16) or Fe8(0,0H)16C1 <2 (ref. 17), but no firm argument is so far available in favour of the definite proposed formula. Hence, we decided to apply the appropriate ageing and intense washing of (111) precipitate until the traces of other FeOOH phases would start to appear. ~

117 No references were found on the necessity of the presence of SO:ions in 7-FeOOH lattice as a prerequisite of this phase stability. Nevertheless, no y-FeOOH specimen could be prepared without 5 contamination from FeS04 solution, and an attempt to obtain chemically pure y-FeOOH from the other available iron salt solutions failed. Apparently, too strong inclusion or adsorption of the 50:- ions on y-FeOOH precipitate caused that even an intense washing of the precipitate was ineffective. Hence, we were forced to recognize the least contaminated specimens of the p-FeOOH or y-FeOOH phases as sufficiently "pure" for the aims of this work.

Relationships between the catalyst and the initial iron compound properties (i) Mutual phase composition and surface area relations All the pure FeOOH phases calcined without a presence of other substances at 550 OC were always finally transformed into cc-Fe203 after 4 hr. calcination. In spite of this, some amount of Y-Fe203 was formed apart from cx-Fe203 in some catalysts when the mentioned FeOOH phases were calcined together with the other components of the initial mixture. Table 1 illustrates which Fe203 phases appeared in the catalyst obtained from different FeOOH phases o r their mixtures. The contribution of respective Fe20j phases in the catalyst specimens was not quantitatively determined, but it might be approximately estimated.

TABLE 1 Effect of kind and contamination of FeOOH on selected properties of the catalyst FeOOH used for the catalyst preparation

Catalyst characterization

Type of the phase used

Mean surgace area, (m /g)

OL

,(amorph)

cc ,(fine cr)

cc ,(needles)

cc cc

Contamina- Mean sursace ting ele- area, (m /g) ment, (%) 240

92 48

22 19 12

45 38

11

,(plates) ,(amorph) ,(needles) y ,(fine cr) 7 ,(fine cr)

52 116 46 88 76

10 18 10 19 16

7 9 % ~+ 2 l % p 6 2 % ~+ 38%p 4 8 % ~ +. 52%p

49 49 50

13 15 11

13 p p

10

Type of the Fe 0 phase foZnd

Relatively high content of maghemite in the catalyst prepared from Y-FeOOH was confirmed in two ways. First, the line with 2.52 d spacing value, was the most intense line in XRD patterns of the catalysts prepared from T-FeDOH, similarly as it was seen in XRD pattern of phase pure y-Fe203. Secondly, the extrudates of these catalysts were easily attracted by a magnet, although a presence of no other paramagnetic materials could be detected in the extrudates. A l o wer content of maghemite must have been present in the catalysts prepared from more contaminated with C1 specimens of f3-FeOOH phase (cf. Table 1). Only two weak lines, with 1.47 and 2.95 d spacing values, could be attributed to maghemite phase in XRD patterns of those catalysts. Even fine grains of those catalysts were not attracted by a magnet. It is conceivable, that two-step mechanism of type: (j3 or y )-FeOOH -y-FegD3 oc-Fep03,with the second step inhibited e.g. by contaminants,is valid here, but more detailed study is required to confirm or deny this supposition. It is worthy noticing that the catalysts in which only cc-Fe203 was formed, had been produced from a-FeOOH or pure P-FeOOH. Calcination of the initial mixture caused expected reduction in the specific surface areas. The reduction degrees seem not to be, however, in any relation with the kind or contamination degree of the FeOOH phase used. Nevertheless, the catalysts witii higher specific surface areas could be obtained from amorphous o r fine-crystalline FeOOH specimens. (ii) Catalyst properties vs. kind and contamination dearee of FeOOH phase. Fig. 1 illustrates the values of EB conversion at 590 OC over the catalysts prepared from different pure phases of FeOOH or Fe203. As it is seen from the Figure, the catalysts prepared from iron oxide hydroxides exhibited more

-

/-. 60 o \ -

v

c .. 0 -I

ln

Ll

40

-

20

-

(u 1

C 0 0

m W

oc

Fig. 1. ( IZZZZZJ

P

;Y

cc

Y

EB conversion,(%), over the catalysts prepared either from FeOOH o r Fe20j ( EXZZ3 Kind of the phase is given under X axis

>.

119

differentiated activity than those prepared from iron oxides introduced to the initial mixture from which the catalyst was then prepared. The considerable differences observed in the catalyst specimen activity suggest that active centers of the catalyst were apparently formed from the introduced iron compound rather than from iron oxides formed ultimately in each catalyst specimen due to calcination. If the centers had been prepared from oxides formed during the calcination process, the observed differences would have been slighter, not greater than the difference in activity of the catalysts prepared directly from E-Fe 203 and y-Fep03 phases. As it can be seen from the Figure, the catalyst prepared from cc-Fe203 displays somewhat higher activity than that prepared from y-Fe203. This observation correlates with Subrt et al. opinion (ref. 18) that active phase KFe02 forms more easily from a-Fe203 than from y-Fe203. Changes in the catalysts selectivity are shown in Fig. 2. It is clearly seen from the Figure that the catalyst prepared from pure (in earlier mentioned sense) P-FeOOH phase exhibits much higher selectivity than the remainilcg catalysts investigated. On the other hand, the catalyst prepared from Y-FeOOH displays the lowest selectivity among the catalysts investigated. So considerable differencies in selectivity suggest that the kind of initial iron com-

t

A

o\-

.

v

+ x

94

i cc

P

oc

Y

Fig. 2. Selectivity, ( % I , of the catalysts prepared either from FeOOH (-1 or from FeZOj ( ) phases. Kind of the phase used is given under X axis pound used for t h e catalyst production can determine remarkably the catalyst selectivity. It is conceivable that a number of different phase features, such as e.g. FeOOH structure ability to be rearranged into somewhat differently distorted iron(II1) oxide matrices formed in the catalyst itself, or the ability of different FeOOH phases to be transformed into the iron oxides with a somewhat different surface acidity o r to induce different pore structures in the catalyst, can affect the selectivity of the catalyst.

120

Apart from the kind of the initial phase used, its contamination can exert an influence on the catalyst selectivity and activity. Fig. 3 shows the changes in selectivity o r activity values, if the same FeOOH phases, but with the different 32

42

(a)

o\*

u

(b)

A

v

-I

36

87 o\-

v

30

40

.a .,I

x

..

+J ..-I

86

95

85

94

>

.A

4 0

*x

28

38

2a, ffl

v)

26

I

1

I

I

2

3

S (a) o r C 1

36 (0)

84

I

1

I

I

2

3

33

content, (%) , in initial FeOOH

Fig. 3. Relatioship between the: (a) styrene yield, (b) selectivity of the catalysts prepared from: p-FeOOH contaminated with C1 ( -) or y-FeOOH contaminated with S (-+-m--), and the degree of contamination

contamination degree were used for a preparation of the catalysts investigated. Although both chloride and sulphate ion contaminants undoubtedly affect the catalysts activity and selectivity, scale of their influence on these features can be different and depends mainly on kind and concentration of the contaminant The 50:concentrations h i g h e r than 4 % but lesser than 10 present in T-FeOOH, exert rather a weak influence on the selectivity, and almost none on the acivity of the catalyst prepared from y-FeOOH (cf. Fig. 3). From a practical point of view there is, therefore, no need to decrease too high 50;content in the precipitated 7-FeOOH below 7 % . Theoretically, however, more thorough purification of 7-FeOOH can either substantially enhance activity of g-FeOOH based catalyst or can have no substantial effect on the activity. Since non-contaminated 7-FeOOH was so far not obtained, there was no opportunity to check which one of those two suppositions was true. For the catalysts based on contaminated P-FeOOH more substantial decrease in selectivity has been observed only when j3-FeOOH used was contaminated with more than 2 % C1 .If that oxide hydroxide was less contaminated, the lower decrease in activity, and particularly in selectivity, was observed with the C1 content increase. If the contamination degree did not exceed 0.7 % the selectivity decrease could be hardly observed. High selectivity of the catalyst based on the "pure" (i.e. least contaminated) P-FeOOH suggest that C1 incorporated into (3-FeOOH lattice cannot affect strongly the catalyst selectivity.

+

121

(iii) Properties of the catalysts prepared from mixed FeOOH phases High selectivity of the catalyst prepared from phase pure (in earlier mentioned sense) p-FeOOH, and satisfactory activity of the catalyst prepared from phase pure cc-FeOOH, prompted us to study the properties of the catalysts prepared from a mixture of the mentioned phases. Some results of such a catalysts testing are shown in Fig. 4 . Apart from experimental curves shown in the Fi-

n .\v

60

U al

4 .r(

x

50

al

c

al

FI

x

+ Lo

40 I

I

I

I,

t

Fig. 4. Activity (a) and selectivity (b) of the catalysts prepared from the mixture of cx-FeOOH and P-FeOOH pure phases. Full lines illustrate experimentally found relationship, broken lines show how these relationships would have run if additivity rule had been preserved for the features of the catalysts gure with full lines, theoretical runs, calculated under assumption that a rule of features additivity is valid for the catalysts prepared from phase mixtures, were also plotted in Fig. 4 with broken lines, It is evident from the Figure, that experimental values for selectivity are higher, if the P-FeOOH content is higher than 23 %, than the calculated values for the selectivity. It is worthy noticing that even high contribution of a-FeOOH , up t o 60 % , in the initial mixture of both considered phases practically does not decrease the high selectivity of the catalyst based on the "pure" P-FeOOH. Unfortunately, experimental values of activity of the catalysts based on mixed FeOOH phases are always lower, regerdless of p-FeOOH phase contribution, than calculated values for predicted activity. Only small changes in activity are observed (cf. Fig. 4 a> when the P-FeOOH content in the initial mixture was increased. Rather considerable decrease in activity of the catalysts based on the mixed FeOOH phases is, of course, unfavourable for an attempt to enhance low activity of the catalyst based on "pure" P-FeOOH by addition of a-FeOOH phase to the initial mixture, o r to enhance selectivity of the catalyst based on s-FeOOH by addition of the inducing selectivity P-FeOOH phase.

122

CONCLUSIONS It has been stated in this work that the kind and contamination degree of the initial iron compound used for the Fe-K/Ca-Cr catalyst preparation, which can be used in ethylbenzene dehydrogenation to styrene, exerted rather substantial influence on the catalyst activity and selectivity in the mentioned reaction. Hence, a proper selection of the kind of initial iron compound, as well as the selection of appropriate conditions for these compounds preparation can substantially help us to enhance the catalyst activity and selectivity. The most effective catalyst was prepared from pure crystalline x-FeOOH, that had been previously obtained from pure iron nitrate solution by the described in this work precipitation method. An application of the solutions obtained by iron-scrap dissolution in H2S04 or HC1 is not reccornended for FeOOH or Fe203 preparation. To obtain the high selective catalyst , the P-FeOOH phase was used as the initial iron compound. Its contamination with C1 was so lowered as it was possible by 24 hr. ageing and thorough washing of p -FeOOH precipitate. REFERENCES 1 J.L. Smith, 8.S. Masters and D.J. Smith, U.S. Patent, 4,467,046 K. Sarumaru, T. Iwakura, A. Watanabe and M. Mori, Japan Kokai, 85193,934 J. Kryska, J. Spevatek and M. Novotny, CS Patent, 230,464 (1986) 4 T. Hirano, Appl. Catal., 26 (1986) 81-90 5 M. Muhler, R . SchlOgl and G. Ertl, Surf. Interface Anal., 12 (1988) 233-238 6 J. Koppe, I. Raphtel and P. Kraak, Chem. Techn. (Leipzig), 40 (1988) 81-83 7 G. Buxbaum and F. Hund, Ger. Offen. 2,556,406 (1977) 8 A. Krause and A. Borkowska, Roczniki Chem., 29 (1955) 999-1006 9 S . Hirai, ,A. Matsumoto and K. Wakai, Japan Kokai, 78-88,698 (1978) 10 J.M. Gonzalez-Calbet, M.A. Alario-Franco and M. Gayoso-Andrade, Inorg. Nucl. Chem., 43 (1981) 257-264 11 A. Solcovd, J. Subrt, F. Hanousek, P. Holba, V. Zapletal and J. Lipka, Silikaty, 24 (1980) 133-141 12 Polish Industrial Standards, Tests PN-75/C-04617 (1975) and PN-80/C-04617.04 (1980) f o r chlorine and its compounds determination 13 Polish Industrial Standards, Test PN-74/C-04566 f o r sulphur and its compounds determination 14 ASTM 4-0755, 8-98, 13-157, 17-536, 24-72 15 T.V. Kalinskaya, L.8. Lobanova, I.V. Pologih, Zhurn. Prikl. Xhim., 40 (1982) 2463-2467 16 W. Feitknecht, R. Giovanoli, W. Michaelis and M. Mueller, Helv. Chim. Acta, 56 (1973) 2847-2856 17 P. Keller, Neues Jahrb. Miner. Abh., 113 (1970) 29-49 18 J. Subrt and J. Vins, Thermochim. Acta, 93 (1985) 489-492

123

G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

HYDROGENATION OF 2 ETHYL HEXEN-2-AL

ON N i / S i O ,

CATALYSTS.

ROLE

OF

PREPARATION PARAMETERS

A.F.

DA S I L V A JUNIOR1,

'PEQ/COPPE/UFRJ, *IRC/CNRS,

V.M.M.SALIM1,

M.SCHMAL1

TELEFAX 5 5 2 1 - 2 9 0 6 6 8 6

TELEFAX

-

and R.

FRETYZ

R i o de J a n e i r o ( B r a z i l )

78894769 - V i l l e u r b a n n e

(France)

SUMMARY T h e i n f l u e n c e o f a m a c r o p o r o u s FC C e l i t e carrier and t h e p r e p a r a t i o n parameters were i n v e s t i g a t e d i n o r d e r t o o p t i m i z e presupport by p a r a t i o n and v e r i f y t h e i n t e r a c t i o n o f N i w i t h t h e a c c o m p l i s h i n g t h e m o d i f i e d p r o p e r t i e s b y TPR, q u i m i s s o r p t i o n and t h e i r e f f e c t s on t h e h y d r o g e n a t i o n o f 2 - e t h y l - h e x e n - 2 - a l . Precipl t a t i o n w i t h NaHCO,, F!H4HC0, and PIH,OH, w i t h o p t i m i z a t i o n o f a g i n g time, a 1 l o w us t o p r e p a r e c a t a l y s t s w i t h h i g h e r m e t a l l i c s u r f a c e and b e t t e r a c t i v i t y t h a n t h e impregnated and commercial N i C r , used a s r e f e r ence. The m o s t a c t i v e c a t a l y s t r e d u c e s t h e f o r m a t i o n o f heavier p r o d u c t s due b a s i c n a t u r e o f p r e c i p i t a t e d a g e n t . INTRODUCTION S i l i c a supported n i c k e l catalysts, important i n practical catalysis, t i o n s t h e y can promote.

Therefore,

z a t i o n o f such c a t a l y s t s ,

w i t h c o n t r o l l e d properties are

due t o t h e g r e a t number o f

reac

t h e p r e p a r a t i o n and characterj.

e v e n i f w e l l d o c u m e n t e d ( I - 6 ) , remains

an

a r e a where c o n s t a n t e f f o r t s a r e done. F o r example,

i n recent years,

H e r m a n s a n d Geus s h o w e d t h e i n t e r e s t

o f homogeneous p r e c i p i t a t i o n ( 6 - 7 ) ,

Montes e t a l .

( 8 - 9 ) analyzed t h e

r e l a t i o n s h i p between t h e p r e p a r a t i o n method and t h e i n t e n s i t y nickel-silica

interaction,

the m e t a l l i c area.

as w e l l

More r e c e n t l y ,

6s

the extent o f reduction

Mile

e t a1 (10) used

reveal various types o f m e t a l l i c n i c k e l

in

c a t a l y s t s a n d Wilson e t a1 ( 1 1 ) d e t a i l e d t h e

TPR

impregnated textural

of

and to

Ni/SiO,

variations

brought about by p r e c i p i t a t i o n w i t h urea o f N i c a t a l y s t s w i t h h i g h o r low N i loadings. With the aim t o hydrogenate edible o i l , loaded Ni/SiO,

catalysts,

we h a v e p r e p a r e d h i g h l y

by a d e p o s i t i o n - p r e c i p i t a t i o n

s h o w e d t h a t t h e u s e o f FC C e l i t e ,

a s i l i c a support,

method and

t u r n e d t h e ca;a-

l y s t s more e f f i c i e n t t h a n t h e ones p r e p a r e d w i t h o t h e r C e l i t e mate rials,

e i t h e r w i t h lower o r higher surface area

with high N i loading,

(12).

i t i s d i f f i c u l t to correlate the

However, variations

124

o f metal p r o p e r t i e s w i t h p o s s i b l e i n t e r a c t i o n s between t h e

nickel

and t h e s i l i c a . The p r e s e n t w o r k s t a r t e d

to verify the influence of

preparg

t i o n p a r a m e t e r s a n d t h e e f f e c t on FC C e l i t e a s s u p p o r t on l o w

loaded

N i samples.

phase

The c a t a l y s t s w e r e a l s o t e s t e d i n t h e

hydrogenation o f 2 e t h y l 2 hexenal

This r e a c t i o n h a s b e e n p r e v i o u s l y s t u d i e d , and l i q u i d phase ( 1 5 - 1 7 )

b o t h i n t h e g a s (13-14)

as t h e main products,

( H O L ) h a v e some i n d u s t r i a l

and 2 e t h y l hexanol

2 e t h y l hexanal (HAL) interest.

used d u r i n g t h e p r o d u c t i o n o f d i o c t y l p h t a l a t e , t u r i n g o f PVC softeners,

liquid

(EPA).

HOL

f o r the

is

manufac-

whereas t h e o c t a n o i c a c i d can be o b t a i n e d

b y o x i d a t i o n o f HAL. EXPERIMENTAL Preparation o f catalysts For the preparation,we support,

u s e d t h e FC C e l i t e ( M a n v i l l e ) ,

w i t h 1 w t % A1,0,.

SiO,

a

This i s a diatomaceous earth,

with

t o t a l s u r f a c e a r e a o f 4 5 mYg a n d a p o r e v o l u m e o f 3 . 7 c m 3 / g . d e d u c e d b y N,

a As

t h e p u r e FC C e l i t e w i t h o n l y 0 , 1

a d s o r p t i o n a t 77 K,

cm3/g o f mesopores volume and a n e g l i g e a b l e m i c r o p o r e s volume,

is

m a i n l y a macroporous support. Two g r o u p s o f c a t a l y s t s w e r e p r e p a r e d .

I n t h e f i r s t one,

two

s e r i e s o f c a t a l y s t s w e r e o b t a i n e d b y deposition-precipitation(Ni-D1 Ni-D2),

u s i n g Ni(NO,),.6

p r e c i p i t a t i n g agent. s o l u t i o n o f Ni(NO,),

a s s t a r t i n g N i c o m p o u n d a n d NaHCO,

H,O

( 1 7 0 cm3, 0,4

s t i r r i n g (1100 rpm).

?I) a n d s u b m i t t e d

A q u e o u s s o l u t i o n o f NaHCO,

The p r e c i p i t a t i o n o f t h e N i - D 1

to

constant

( 1 3 6 cm3, 0.8

c m 3 / m i n ) a t room temperature

PI)

(298 K).

s e r i e s was s t o p p e d f o r a N a / N i r a t i o

and t h e p r e c i p i t a t i o n o f Ni-D,

Na/Ni r a t i o o f 1.5.

as

T h e FC C e l i t e ( 3 6 g ) was s u s p e n d e d i n aqueous

was a d d e d t o t h e s u s p e n s i o n ( 1 ,7 equal t o 2,

and

series stopped f o r

a

T h e s l u r r y was t h e n m a i n t a i n e d u n d e r s t i r r i n g

i n t h e r e a c t i o n medium f o r thereafter "aging time".

different periods Then,

of

time,

called

t h e s u s p e n s i o n was f i l t e r e d ,

w a s h e d a n d d r i e d f o r 2 3 h a t 3 8 3 K,

before being submitted t o

a

c a l c i n a t i o n under a i r ,

a t 7 2 3 K,6

i n c l u d e d f o u r samples,

N i I was p r e p a r e d b y s i m p l e w e t p o i n t i m p r e g

nation;

NiDHeNH,,

NiDHoNH,

The s e c o n d g r o u p o f c a t a l y s t s

a n d NiDHoNa w e r e p r e p a r e d a s

for deposition-precipitation NH4HC0, a n d NaPCO,,

h.

water

procedure,

described

using respectively

NH,nH,

a s p r e c i p i t a t i n g compounds andar, a g i n g t i m e

3 h (optimized i n the f i r s t group o f catalysts).

of

DHe a n d DHo i n d j

125

cate t h a t the deposition p r e c i p i t a t i o n procedure i s considered h e t e r o g e n e o u s a n d homogeneous, A Ni-Cr/SiO,

catalyst,

as

respectively.

u s e d i n d u s t r i a l l y f o r EPA h y d r o g e n a t i o n ,

was u s e d a s a r e f e r e n c e c a t a l y s t . Characterization R e d u c i b i l i t y and d i s p e r s i o n t h r o u g h TPR i n s t r u m e n t s . i n (12).

catalysts

were e s t i m a t e d

The e q u i p m e n t i s s i m i l a r

of

the

t o that described

+

T h e r e d u c t i o n was c o n d u c t e d w i t h a n A r g o n

t u r e and h e a t i n g r a t e o f 8 K / m i n u p t o 8 2 3 K . was t r a p p e d

1 , 5 % H,

water

i n t o a 3 A z e o l i t e s located before the thermal

t i v i t y cell.

mix-

The e n v o l v e d

The d i s p e r s i o n o f t h e r e d u c e d N i

conduc-

was e s t i m a t e d ,

at

t h e e n d o f t h e TPR,

b y q u i c k l y c o o l i n g down o r

t h e reduced sample,

i n t h e argon-hydrogen mixture.

These h i g h tem-

p e r a t u r e v a r i a t i o n s generate e i t h e r an a d s o r p t i o n ,

or a desorption

peak t h a t , i n H,

after correction f o r flow

volume.

h e a t i n g up ( 1 1 O K / m i n )

r a t e v a r i a t i o n s , were

For the dispersion estimation,

converted

we a s s u m e d t h a t

K a n d a t r o o m t e m p e r a t u r e ( 2 9 8 K ) , hydrogen coverage was

823

t i v e l y c l o s e t o 0 and t o 1. s u r f a c e atom,

The r a t i o H / N i * ,

was t a k e n e q u a l t o 1 .

with Ni*

at

respecbeing

a

Values issued from adsorption

and d e s o r p t i o n peaks were i n good agreement. C a t a l y t i c measurements T h e h y d r o g e n a t i o n o f E P A was p e r f o r m e d i n a s e m i - b a t c h a t a t m o s p h e r i c p r e s s u r e . Sample w e i g h t c o r r e s p o n d i n g was r e d u c e d i n " s i t u " , o u t g a s s e d w i t h N, tor.

a t 7 7 3 K f o r 1 7 h w i t h a n H,

A t t h e end o f t h e r e d u c t i o n ,

12 l / h .

N,

t o 0,09

t h e reduced

reactor, g of

flow close catalyst

Ni

to was

f l o w a n d 150 m l EPA w e r e i n t r o d u c e d i n t o the r e a c

b u b b l i n g was m a i n t a i n e d d u r i n g e s t a b l i s h m e n t o f t h e r e a c t i o n

t e m p e r a t u r e . Then,

a H,

while

f l o w o f 2 0 5 l / h was a d d e d

starting

L i q u i d samples flow. n e e d e v a l v e and anawere c o l l e c t e d p e r i o d i c a l y t h r o u g h a s e a l e d t h e m e c h a n i c a l s t i r r i n g a n d s t o p p i n g t h e N,

l y z e d c h r o m a t o g r a p h i c a l l y w i t h a C a r b o w a x c o umn. RESULTS Catalysts preparation T h e f i n a l pH v a l u e s , agent,

a

t h e end o f t h e a d d i t on o f t h e p r e c i p i t a t i n g 7.0,

were r e s p e c t i v e l y 5.0,

7.0

and 8.0,

for

impregnation

a n d d e p o s i t i o n - p r e c i p i t a t i o n w i t h N a H C 0 3 , N H 4 C 0 3 a n d NH40H. T h e pH evolution,

d u r i n g p r e c i p i t a t i o n w i t h NaHCO,,

t i t r a t i o n c u r v e f o r Ni(NO,), same c u r v e f o r p u r e Ni(NO,),.

+

showed

FC C e l i t e was a l w a y s

that beneath

the the

126

Catalytic Characterization T a b l e s 1 and 2 s u m m a r i z e some c h a r a c t e r i z a t i o n p a r a m e t e r s , different catalysts, of

a n d F i g u r e s 1,

for

2 and 3 gave r e d u c t i o n profiles

typical catalysts.

TABLE 1 I n f l u e n c e o f N i c o n t e n t a n d a g i n g t i m e on r e d u c i b i l i t y a n d m e t a l l i c area f o r N i / C e l i t e catalysts. Catalyst

Ni/Na

Ni-D1-0 Ni-01-2 Ni-D1-3

0.50 0.50 0.50

Ni-D2-0 Ni-D2-3 Ni-D2-6 Ni-D2-22

0.66 0.66 0.66 0.66

A

:B

a(%)

0 2 3

8.5 10.0 10.0

0.20 0.16 0.15

55 70 84

36 42 47

0 3 6 22

5.8 6.7 7.3 8.5

0.14 0.17 0.17 0.15

59 64 60 68

23 36 31 33

Ni-4-3

i\, Ni-D2-,

e

r"

%Na

-_---.-

22

n

%Ni

A g i n g Time ( h )

~

,-.. i

U.

i.

-

o f aging time

S N ~( m z / g N i )

at

room

temperature i s given f o r Ni-D1 and N i - D Z catalysts, samples p r e c i p i t a t e d NaHCO,.

In

Figs. 1

and

by

para1l e l ,

2

show

127

The p r o p e r t i e s o f i m p r e g n a t e d a n d c o m m e r c i a l o f t h e d e p o s i t e d - p r e c i p i t a t e d ones, p r e s e n t e d i n T a b l e 2.

for

c a t a l y s t s and t h a t

aging times

3 h,

of

The c o r r e s p o n d i n g r e d u c t i o n p r o f i l e s are

is given

i n F i g u r e 3. TABLE 2 I n f l u e n c e o f t h e p r e p a r a t i o n m e t h o d on t h e r e d u c t i b i l i t y a n d text! r a l p r o p e r t i e s o f t h e second s e t o f c a t a l y s t s . C a t a l y s t s BET ( m 2 / g )

NiI NiDHeNH, NiDHoNH, N i DHoNa N i - C r Com.(a)

33 30 29 32 11

Vp ( c m 3 / g )

1.6 2.6 2.6 2.7 0.7

%Ni

%Na

a(%)

10.4 8.4 8.0 9.3 9.3

-

91 96 95 91 83

-

-

0.18

-

SNi(mz/gNi)

13 38 36 53 14

a% C r = 1.7%

- - - - - - - - NiDHeNH4 .............. .. ......... NiCr Gm -. -- -- -. - NiO -.-1(-1-

Fig.3

Temperature I K I

NiDHoNo NiWNH4 NiI

- Influence of Preparation Methods on the TPR Profiles.

a

d) 530 180 190 130 480

128

Catalytic Activity Preliminary experiments,

using the catalyst with

the

m e t a l l i c area, were performed i n o r d e r t o minimize t h e v a r y i n g t h e gas f l o w r a t e and t h e c a t a l y s t w e i g h t . gNi/lEpA

a n d a H,

higher diffusion, u s i n g 0,6

Then,

f l o w r a t e o f 2 0 5 l / h we c o n s t r u c t e d r e a g e n t - p r o d u c t s d i s t r i b u t i o n curves, w i t h time,

for

different

the

tempera

t u r e s ( 3 9 3 , 403 and 4 1 3 K ) . The v a r i a t i o n of area

the

metallic

being l a r g e r

for set

the of

alysts,

second cat-

the

cata-

l y t i c measurements were

1imited

to

this latter

set.

F i g . 4 shows a t y p i c a l curve

-

Fig. 4 - Typical distribution of reagent products for catalyst NiDHoNa at 413 K .

-

t i o n o f E P A and t h e ucts

using

413 K. From

the

F i g u r e 5,

80

t h a t presents

70

consumption o f EPA

8 60

U

A NiDHoNH4 4 NiDHeNH4

50

the

for differents cat

A Ni I

alysts,

0 NiDHoNH4

we

lated the

L

z

f e r s t o t h e consump

c a t a l y s t N i DHoNa a t

90

=

re

formation o f prod-

€PA Consumption T 413 K

100

repre

s e n t a t i o n which

40

calcuinitial

reaction rate the

&? 30

and

corresponding

20

TON a t 4 1 3 K . These

10

v a l u e s a r e reported

0

i n T a b l e 3 together 0

1

2

3

4

Reaction Time ( h )

5

6

7

Fig. 5 - Comparison of EPA Consumption for the second set of catalyst at 413 K.

w i t h a p p a r e n t act! v a t i o n energy.

129

TABLE 3 I n f l u e n c e o f t h e p r e p a r a t i o n m e t h o d on t h e a c t i v i t y . I n i t i a l r a t e * 102 sN i (mol/gNi*min) (m'/gNired)

Catalyst

TON(s-')

EAt(KJ/mol)

~~

N i DHoNa NiDH,NH, N i DHoNH, NiI N i C r Com.

10.1 6.8 6.2 1.2 1.0

53 38 36 13 14

1.2 1.2 1.1 0.62 0.44

69

-

52 59

The m a i n p r o d u c t s o f t h e h y d r o g e n a t i o n o f EPA a r e t h e d i a t e HAL a n d t h e f i n a l p r o d u c t HOL.

r e a d y i d e n t i f i e d b y GC-mass s p e c t r o m e t r i c a EPA ( 2 b u t h y l b u t e n - 2 - a l ,

interme-

I n a d d i t i o r ? t o i t we h a v e

c a l l e d 1SOM.EPA)

al-

p o s i t i o n isomer

of

a n d some h e a v i e r

com-

pounds l i k e 2 e t h y l h e x y l h e m i a c e t a l o f 2 e t h y l h e x a n a l and

con-

s e q u e n t l y b i s - ( 2 e t h y l h e x y l ) a c e t a l o f 2 e t h y l h e x a n a l (both c a l l e d

SOM. ACT). Table 4 presents the actual s e l e c t i v i t i e s i n standard conditions f o r a constant conversion

of 2 5 % ( 4 1 3 K ) .

pounds were d e t e c t e d o n l y f o r h i g h e r c o n v e r s i o n s .

reaction

H e a v i e r com-

I n the l a s t

o f T a b l e 4 we h a v e p r e s e n t e d t h e s e l e c t i v i t i e s o f t h e s e for

t o t a l c o n v e r s i o n o f EPA,

column

compounds

f o r t h e most a c t i v e c a t a l y s t s .

TABLE 4 I n f l u e n c e o f t h e p r e p a r a t i o n m e t h o d i n s e l e c t i v i t y a t 413 K. Selectivity

S~~~

SHOL

Conversion Catalv s t NiI NiDH,NH, N i DHoNH NiDHoNa N i C r Corn.

'ISOM.EPA

'SOM.ACT

1.10 0.55 0.75 0.43 1.30

'HOL

100%

25% 85.6 88.7 89.0 87.5 90.0

'HAL

0.41 0.42 0.18

13.5 10.8 10.5 12.0 8.8

-

91.5 88.2 90.0

-

8.1 11.4 9.8 -

DISCUSSION D u r i n g t h e i r s t u d y on d e p o s i t i o n - p r e c i p i t a t i o n d e c o m p o s i t i o n o r c o n t r o l l e d NaOH a d d i t i o n ,

analyze t h e i n t e r a c t i o n between N i i o n s and h i g h silicas.

o f Ni,

usingurea

H e r m a n s a n s Geus surface

(6-7) area

B a s e d o n v a r i o u s m e a s u r e m e n t s and i n p a r t i c u l a r f o l l o w i n g

t h e e v o l u t i o n o f pH d u r i n g t h e p r e c i p i t a t i o n ,

they concluded

that

t h e i r e x p e r i m e n t a l c o n d i t i o n s a l l o w e d an homogeneous d e p o s i t i o n o f

130 N i precursor onto the s i l i c a support

.

I n t h e p r e s e n t case,

with a

m a c r o p o r o u s m i d d l e s u r f a c e a r e a s i l i c a a s t h e s u p p o r t a n d NaHC0,as the precipitant, ented i n (6). (II),

t h e pH v a r i a t i o n s ,

Therefore,

we t h i n k ,

a r e s i m i l a r t o t h e ones

pres

i n agreement w i t h Wilson

e t a1

t h a t o u r N i / C e l i t e c a t a l y s t s a l s o have a r a t h e r

homogeneous

d i s t r i b u t i o n o f N i onto the c a r r i e r . By i n c r e a s i n g t h e p r e c i p i t a t i o n t i m e

N i c a t a l y s t s v i a urea decomposition, t h a t the N i content increases,

during the preparation of

Richardson e t al.(18)

showed

l e a d i n g b o t h t o an increased

mean

N i p a r t i c l e s i z e and a decrease o f t h e t e x t u r a l parameters.

The

v a r i a t i o n s o f t h e t e x t u r a l p r o p e r t i e s between l o w and h i g h

loaded

N i / s i l i c a c a t a l y s t s were f u r t h e r c o n f i r m e d by W i l s o n e t a1

(11).

T e x t u r a l a n d m e t a l l i c a r e a m o d i f i c a t i o n s r e s u l t i n g f r o m an increase i n the

aging time,

Ni/Celite catalysts,

after precipitation, w i t h 50 w t % N i ( 1 9 ) .

were a l s o observed

methodology n o t o n l y changes t h e p r o p e r t i e s of m e t a l can a l s o a l t e r t h e i r a c c e s s i b i l i t y ,

on

Therefore, the p r e p a r a t i o n

by a l t e r a t i o n

particles,

of

the

but

support

texture. I n t h e p r e s e n t work,

using a constant p r e c i p i t a t i o n time,

we

re

h a v e o b s e r v e d t h a t t h e TPR p r o f i l e s a n d t h e r e f o r e t h e e x t e n t o f duction,

as w e l l as t h e m e t a l l i c a r e a a r e s l i g h t l y changed whenthe

i l i c o n t e n t o r t h e a g i n g t i m e i s changed. a g r e e m e n t w i t h known r e s u l t s ( l o ) , major r e d u c t i o n peaks, t o 773-823 N i species,

K.

way,

in

t h e TPR p r o f i l e s p r e s e n t

I n a general

two

one a r o u n d 573-643 K and t h e o t h e r

T h e f i r s t o n e seems c h a r a c t e r i s t i c o f n o n - i n t e r a c t i n g

whereas t h e second one can be c o n s i d e r e d as a

p r i n t o f the Ni-SiO,

t

i s observed a t

interaction.

Sometimes,

finger-

a l i m i t e d H,

uptake

510 K and a t t r i b u t e d t o t h e e l i m i n a t i o n o f

s t o i c h i o m e t r i c oxygen.

non

T h e n o n i n t e r a c t i n g N i seems p r e s e n t

l a r g e r q u a n t i t y when t h e p e r c e n t N i i n c r e a s e s ( F i g . the

around

aging time increases (Fig.

2).

1)

and

The c o n t r i b u t i o n o f t h i s

i n t e r a c t i n g N i t o t h e m e t a l l i c area i s however l e s s c l e a r l y l i s h e d , as i n T a b l e 1, the aging time,

non estab

the m e t a l l i c area increases s l i g h t l y

up t o 3 h,

i n when

with

b u t tends t o l e v e l o f f o r even decrease

f o r higher aging times. When c o m p a r i n g i m p r e g n a t e d w i t h p r e c i p i t a t e d c a t a l y s t s and Table 2 ) ,

i s r i c h i n non i n t e r a c t i n g N i , m e t a l l i c area. situation,

(Fig.

3

i t can be seen c l e a r l y t h a t t h e impregnated c a t a l y s t

situation leading t o a rather

The p r e c i p i t a t e d c a t a l y s t s p r e s e n t

a s m a j o r i t y o f N i i s i n t e r a c t i n g w i t h FC

an

low

opposite

Celite

and

131

generates a l a r g e r m e t a l l i c area. l y s t i s more complex: area

The c a s e o f t h e c o m m e r c i a l c a t a

the reduction i s inhibited but the metallic

lessened b e c a u s e some C r i s p r o b a b l y c o v e r i n g a f r a c t i o n o f t h e l i m i t i n g t h e r e f o r e H,

metallic particles,

adsorption.

d i f f er

The

ences o f p r o p e r t i e s between t h e v a r i o u s p r e c i p i t a t e d c a t a l y s t s a r e v e r y l i m i t e d , compared t o t h e d i f f e r e n c e s n o t e d catalysts,

as expected.

Therefore,

for

agent i s o f secundary importance i n our preparation. seems d u e t o v a r i o u s p a r a m e t e r s , precipitation,

impregnated

the nature of the precipitating This situation

among w h i c h t h e l o w temperature

the high s t i r r i n g during the preparation

and

of the

r a t h e r l o w s u r f a c e a r e a o f t h e s u p p o r t c a n b e r e t a i n e d . The b e t t e r m e t a l l i c N i a r e a o b t a i n e d f o r N i DHoNa c a n b e r e l a t e d b o t h t o l o w e r q u a n t i f y o f e a s i l y r e d u c e d N i a n d t o a m o r e homogeneous a c t i o n between N i and t h e s i l i c a ( t h e r i g h t hand s i d e o f t h e temperature Ni-SiO,

r e d u c t i o n peak i s n o t pronounced).

This

high

smaller

i n t e r a c t i o n i s t e n t a t i v e l y a t t r i b u t e d t o t h e presence

r e s i d u a l Na,

the inter

a s n o c l e a r t e x t u r a l v a r i a t i o n was o b s e r v e d .

of

Finally,

i t i s i n t e r e s t i n g t o n o t e t h a t t h e t e x t u r a l p a r a m e t e r s d r o p i s more i n t e n s e f o r t h e impregnated sample t h a n f o r t h e p r e c i p i t a t e d ones. Catalytic activity The h y d r o g e n a t i o n o f EPA c a n b e r e p r e s e n t e d b y

the

following

scheme: 2 e t h y l hexen-2-a1

J

+H, ------+

2 e t h y l hexanal

Isomerization

>-

+H*

2 e t h y l hexanol

Condensation

2 b u t h y l buten-2-a1

2 ethyl h e x i l hemiacetal o f 2 e t h y l hexanal Condensation

bis(2 ethyl hexil) acetal of 2 ethyl hexanal With the present catalysts, never observed. sumption o f EPA,

t h e u n s a t u r a t e d a c o h o l , HEOL, The c a t a l y t i c a c t i v i t y e x p r e s s e d t h r o u g h t h e and g i v e n i n T a b l e 3,

v a r i e s by a f a c t o r

of

was con

10,

t h e t h r e e p r e c i p i t a t e d c a t a l y s t s b e i n g i n one g r up a n d t h e i m p r e g n a t e d and commercial exception of

ones b e i n g i n a n o t h e r group.

t h e commercial

catalyst,

However,

with t h e

f o r which t h e m e t a l l i c

area

m e a s u r e m e n t i s p r o b a b l y n o t o p t i m i z e d , t h e TON ( s - ' ) for all cat a l y s t s i s r a t h e r s i m i l a r , as a r e t h e apparent a c t i v a t i o n energies. T h i s means t h a t ,

i n a f i r s t analysis,

t h e EPA h y d r o g e n a t i o n , i n

the

132

.

l i q u i d phase,

can be c o n s i d e r e d i n i t i a l l y as i n s e n s i t i v e

to

the

structure o f the catalysts. Concerning the s e l e c t i v i t i e s , products, studies.

HAL a n d I S 0 E P A a p p e a r a s i n i t i a l

e v e n i f I S 0 E P A was n e v e r m e n t i o n e d ,

u p t o now,

i n basic

W i t h t h e e x c e p t i o n o f t h e c o m m e r c i a l c a t a l y s t , modified

by

C r a n d l e a d i n g b o t h t o a s l i g h t l y b e t t e r s e l e c t i v i t y i n HAL a n d

l o w e r s e l e c t i v i t y i n I S 0 EPA,

a

and t h e impregnated c a t a l y s t showing

a l a r g e r p r o d u c t i o n o f HOL, a l l o u r c a t a l y s t s p r e s e n t r a t h e r c o m p a rable selectivities. i n i t i a l l y produced,

However,

l o o k i n g a t t h e l o w q u a n t i t i e s o f HOL

i t appears t h a t t h e lower

f o r t h e more a c t i v e c a t a l y s t s .

s e l e c t i v i t y i s obtained

As HOL i s t h e e n d p r o d u c t

of

the

h y d r o g e n a t i o n , o n l y t h e s u p p r e s s i o n o f HAL r e a d s o r p t i o n w o u l d a b l e t o s u p p r e s s t h e f o r m a t i o n o f HOL.

25%

A t

s t r o n g e r EPA a n d I S 0 E P A a d s o r p t i o n s a r e p r o b a b l y l i m i t i n g r a l l y t h e HAL a d s o r p t i o n .

But,

a t higher conversion,

c e n t r a t i o n s o f EPA a n d I S 0 EPA a r e v e r y l o w , have a good N i a c c e s s i b i l i t y ( i . e

be

conversion,

the

natu-

when t h e

coy!

i t seems n e c e s s a r y t o

t h e h e l p o f t h e macroporous struc

t u r e ) a n d a n e f f i c i e n t a g i t a t i o n t o l i m i t t h e HAL r e a d s o r p t i o n . fact, for

even a t 100% c o n v e r s i o n ,

t h e b e t t e r s e l e c t i v i t y i n HAL

In are

the c a t a l y s t s having the l a r g e r surface area. Increasing the conversion,

t h e HAL s e l e c t i v i t y i n c r e a s e s s l i g h t l y ,

p a s s e s t h r o u g h a maximum a n d t h e n d r o p s f o r c o n v e r s i o n s b e t w e e n 9 0 a n d 9 5 % . I n p a r a l l e l , some h e a v y p r o d u c t s , been r e c o g n i z e d ,

a r e f o r m e d . If

among w h i c h a c e t a l s h a v e

the formation o f these by-products

i s c a t a l y t i c a l o r n o t i s n o t known,

but it i s interesting to

see

t h a t p r e s e n c e o f a l a r g e r N i a r e a ( o r t h e p r e s e n c e o f Na i n t h e a l y s t ) seems t o l i m i t t h e i r f o r m a t i o n .

F u r t h e r work i s needed

cat to

t r y t o c o r r e l a t e c a t a l y s t s t r u c t u r e and f o r m u l a t i o n w i t h t h e form?

t i o n o f t h e s e h e a v y compounds. CONCLUSIONS T h e p r e p a r a t i o n o f N i c a t a l y s t s o n t h e FC C e l i t e , porous c a r r i e r , some a g i n g ,

by p r e c i p i t a t i o n a t room t e m p e r a t u r e ,

a l l o w s t h e o b t e n t i o n o f c a t a l y s t easy t o

presenting a satisfactory metallic dispersion.

a SiO,

macro

followed reduco

by and

The n a t u r e o f

the

p r e c i p i t a t i n g agent i s n o t i m p o r t a n t and t h e macroporous s t r u c t u r e o f the support i s maintained,

i n a large part.

phase hydrogenation o f 2 e t h y l 2 hexenal, s e l e c t i v e and 3 t o 4 t i m e s more a c t i v e reference catalyst.

Used i n t h e

these c a t a l y s t s are

than

a

commercial

The f o r m a t i o n o f h e a v y b y - p r o d u c t s seems

m a i n t a i n e d a t an a c c e p t a b l e l e v e l .

liquid as Ni-Cr

also

133 REFERENCES 1 2 3 4 5 6. 7 8 9 10

11

12

13 14 15 16

18

19

G.C.A. S c h u i t , L.L. van R e i j e n - Advances i n C a t a l y s i s - V o l . 10, Academic P r e s s London/New Y o r k , 1358, pp. 242-267. J.W.E. Coenen, B.G. L i n s e n - " P h y s i c a l a n d C h e m i c a l A s p e c t s o f Adsorbents a n d C a t a l y s t s , B . G . L i n s e n ( E d . ) , A c a d e m i c Press, London/New Y o r k , 1 9 7 0 , p p . 4 7 2 - 5 2 5 . J.W.E. Coenen (G. P o n c e l e t , P . G r a n g e a n d P . A . J a c o b s , Eds.), P r o c . o f t h e 3 r d I n t e r n a t i o n a l C o n g r e s s on The S c i e n t i f i c B a s e s of C a t a l y s t P r e p a r a t i o n , E l s e v i e r , A m s t e r d a n , 1 9 7 9 , p p . 89-111. J.W.E. Coenen - I n d . Eng. Chem. F u n d . , 25 ( 1 9 8 6 ) 4 3 - 5 2 . P . T u r l i e r , H. P r a l i a u d , P . M o r a l , G . A . Y a r t i n , J.A. Dalmon A p p l . C a t . 18 ( 1 9 8 5 ) 3 8 9 . Hermans, J . v a n d e r Meijden, A . J . v a n D i l l e n , J.W. Geus, L.A.M. P r o c . S i x t h I n t . C o n g r e s s on C a t a l y s i s L o n d o n , 1 9 7 6 , pp. 677685. on L.A.M. H e r m a n s , J.W. G e u s , P r o c . o f t h e 3 r d I n t . C o n g r e s s t h e S c i e n t i f i c B a s s o f C a t a l y s t P r e p a r a t i o n , E l s e v i e r , Amsterdan, 1979, pp. 113-130. M . M o n t e s , Ch. Penneman d e B o s s C h e y d e , B.K. Hodnett, F. Delannay, P . G r a n g e , B. D e l m o n , - A p p l . C a t . , 1 2 ( 1 9 8 4 ) p p . 3 0 9 - 3 3 0 . M. M o n t e s , J.B. S o u p a r t , M . S a e d e l e e r , B . K . H o d n e t t , B . Delrnon J . Chem. S . C . , Faraday T r a n s . 8 0 ( 1 ) (1980), pp. 3209-3220. B. M i l e , D. S t i r l i n g , M . A . Z a m m i t t , A . L o v e l l , M. Webb - J . C a t a l . , 114 ( 1 9 8 8 ) pp. 217-229. M.F. W i l s o n , 0. A n t i n l u o m a , J.R. B r o w n - S y m p o s i u n on t h e P r e p a r a t i o n and C h a r a c t e r i z a t i o n o f C a t a l y s t s P r e s e n t e d before the D i v i s i o n o f P e t r o l e u m C h e m i s t r y , I n c . , - A m . Chem. S O C . - L o s A n g e l s M e e t i n g - 1 9 8 8 , pp. 669-675. V.M.M. S a l i m , M . S c h m a l , R . F r e t y , M. R o d r i g u e s , M . C . S i l v e i r a 50 S e m i n a r i o de C a t a l i s e - B r a s i l ( 1 9 8 9 ) , pp. 93-101. C. P l i k l a s s o n , G . S m e d l e r , I n d . Eng. Chem. Res. 26 ( 1 9 8 8 ) 4 0 3 410. J . S m e d l e r - I n d . Eng. Chem. Res, 2 7 ( 1 9 8 8 ) , 2 0 2 3 - 2 0 3 0 . E.F. Souza A g u i a r , M . Schmal - V I I S i m p 6 s i o I b e r o - A m e r i c a n 0 d e C a t a l i s e - A r g e n t i n a , 1980. J . L . A l m e i d a , L . D i e g u e z , M. S c h m a l , I X S i m p 6 s i o I b e r o A m e r i c a no de C a t a l i s e , P o r t u g a l , 1984. e L.R.R.Araujo, L.C. D i e g u e z , R. F r e t y , M . S c h m a l , X I Simp6sio I b r o American0 de C a t a l i s e s , Mexico, 1988, pp. 1195. J . T . R i c h a r d s o n , R.J. Dubus, J.G. Crump, P . Desai, U. Osterwalder, I n t e r n a t i o n a l Congress on t h e T.S. C a l e - P r o c e d . o f t h 3 r d . Amsterdan, S c i e n t i f i c Bases o f C a t a l y s t P r e p a r a t i o n , E l s e v i e r , 1979, pp. 131-142. V.M.M.Salim, M.A. Duarte, M.C. S i l v e i r a , R. F r e t y M.Schma1 X I 1 S i m p o s i o I b e r o A m e r i c a n 0 de C a t a l i s e - B r a s i l - 1 9 9 0 . (in press.

This Page Intentionally Left Blank

G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V

135

0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

PREPARATION AND PROPERTIES OF A PT/SILICA AND ITS COMPARISON WITH EUROPT-1 S.D. JACKSON3, M.B.T. KEEGAN', G.D. McLELLANl, P.A. MEHEUX', WEBB1, P.B. WELLS', R. WHYMAN4 AND J. WILLIS3

R.B. MOYES',

G.

'Department of Chemistry, University of Glasgow, Glasgow, 612 8QQ, Scotland (UK 'School

of Chemistry, University of Hull, Hull, HU6 7RX (UK)

31CI Chemicals and Polymers Ltd., The Catalysis Centre, Research and Technology Dept., PO Box 1, Billingham, Cleveland, TS23 1LB (UK) 41CI Chemicals and Polymers Ltd., Research and Technology Dept., PO Box 8, The Heath, Runcorn, Cheshire, WA7 4QD (UK) SUMMARY A 0.76% Pt/silica has been prepared by conventional impregnation from aqueous solution using hexachloroplatinum( IV) ions as the Pt source. Its adsorption characteristics and reactivity are compared with those of the standard reference catalyst EUROPT-1 (a 6.3 wt% silica-supported Pt) which was prepared by ion exchange using platinum( 1I)tetrammine ions as the Pt source. Platinum dispersion by HRTEM and particle morphology by EXAFS are reported. The catalysts both showed high metal dispersion and comparable behaviour in oxygen chemisorption and butadiene hydrogenation, whereas they differed with respect to mean Pt particle size and showed different behaviours in carbon monoxide chemisorption, cyclopropane hydrogenolysis, and enantioselective methyl pyruvate hydrogenation. Clearly the choice of comparators is of importance in catalyst evaluation. INTRODUCTION The relationship between catalyst preparation and performance is complex and curious. Catalyst characterisation provides the bridge in that preparation can be directed towards the attainment of certain structural and chemical characteristics, and attempts can be made to relate performance to those same characteristics. The authors are collaborating in an extensive joint study in which the preparation, characterisation and function of a range of supported platinum catalysts is being evaluated. In part, this study will compare platinum catalysts prepared using conventional supports (e.g. silica, alumina) with those using less conventional supports (e.g. molybdena) or those prepared by less conventional methods (e.g. metal vapour deposition). The restricted object of the present paper is to compare the conventional Pt/silica prepared within this programme with the standard reference silica-supported Pt codenamed EUROPT-1 for which full preparation and characterisation details have been published (refs. 1-5).

136

NOMENCLATURE Throughout this paper the silica-supported p atinum prepared and characterised in this investigation is referred to as Pt/silica, whereas the si 1 ica-supported platinum reference catalyst is referred to as EUROPT-1. EXPERIMENTAL METHODS Catalyst preparation is described in the next Section. Conventional means were used to obtain electron microscopic images of the platinum particles in each catalyst at a magnification of ca. 450,000~. EXAFS spectra of samples at the Pt L3-edge were obtained at the Synchrotron Radiation Source at the SERC Daresbury Laboratory. Powdered samples were reduced at 523 K and examined under pure hydrogen at ambient temperature in specially constructed glass cells fitted with mylar windows. Chemisorption studies were performed in a dynamic mode using a pulse-flow microadsorption apparatus. Samples of precursor (0.3-0.5 g) were reduced in After s i t u in flowing 5% hydrogen in nitrogen by heating to 573 K at 7 K min-'. reduction the flow was changed to helium, the temperature held at 573 K until hydrogen elution ceased and then lowered to ambient. Adsorbate was injected into the helium stream ahead of the adsorbent as pulses of known size (typically 0.05 cm3 at 0.101 MPa). The amount adsorbed was determined from the difference between the peak obtained and a calibration peak. The detection limit for adsorption was 2 x 10l6 molecules. Chemisorption of [14C] carbon monoxide was investigated at ambient temperature in a static system using pressures up to 533 Pa (ref. 6 ) . FTIR spectra of adsorbed-C0 were obtained in transmission at a resolution of 2 cm-' using a Nicolet 5DXC spectrometer with TGS detector. Hydrocarbon reactions were carried out in a static reactor (200 ml) attached to a conventional grease-free high vacuum apparatus. Samples of precursor were placed in the reactor, evacuated, and reduced in pure hydrogen at 523 K for 0.5 or 1.0 h. Reactants (cyclopropane, butadiene) were admitted to the catalysts in the order (i) hydrocarbon, (ii) hydrogen; pressure fall was measured by use of a pressure transducer, and analysis was by glc. Enantioselective ester hydrogenation was conducted in the liquid phase in a stirred glass reactor (Fischer Porter). 10 ml methyl pyruvate, 20 ml ethanol, and d i h y d r o c i n c h o n i d i n e - t r e a t e d catalyst (0.1 g EUROPT-1 or 1.0 g Pt/silica) were placed in the reactor, hydrogen was admitted and maintained constant at 10 bar pressure, and the contents stirred vigorously. After the required hydrogen uptake the product was filtered, distilled, and analysed by glc and polarimetry according to procedures published elsewhere (ref. 7) to determine the optical

137

yield in methyl lactate formation. THE CATALYSTS: PREPARATION, ANALYSIS, MICROSCOPIC AND SPECTROSCOPIC PROPERTIES A stock of the precursor to Pt/silica was prepared as follows. The chosen support was M5 Cab-0-Sil silica (surface area 203 m2 g-l; zero pore volume; impurities, C1=540 f 50 ppm (by neutron activation) Na = Cu = 20 ppm (by ICP-MS)). 9,76 g chloroplatinic acid (Johnson Matthey, platinum assay 41%) was dissolved in deionised water (0.60 1) in a five litre flask. Silica (M5 Cab-OSil) was added and mixed until the suspension began to gel, at which point further deionised water ( c a . 0.50 1) was added to promote mobility. This process was repeated until 398 g of silica had been introduced; at that point the total volume of water added had become 2.5 1. The flask was then attached to a Buchi rotary evaporator and water slowly removed by maintaining the contents at 353 K under a partial pressure of dry nitrogen. After 48 h a pale yellow free-flowing powder was obtained. Its uv-visible spectrum showed bands at 456, 374 and 264 nm consistent with the presence of chloroplatinate ions (ref. 8) and a further band at 206 nm which., is assigned to an 0-ligand to Pt charge transfer (ref. 9) and which indicates the presence of hydroxochloro- or oxochloro-platinum anions, e.g. [PtC150H]2-. The Pt-content of the precursor was 0.73 wt %. The Pt(4f7/2) binding energy measured on two occasions was 72.4 and 72.7 eV. Temperature programmed reduction (TPR) of the precursor was examined over the range 180 to 773 K. With a heating rate of 10 K min-' th'e major reduction feature was an asymmetric peak having a sharp maximum in the range 400 to 410 K. Reduced Pt/silica provided values for the Pt(4f712) binding energy of 71.2 and 71.4 eV, the Pt content was 0.76 wt% and the C1 content 610 f 60 ppm. Thus, the chlorine content o f the reduced P t l s i l i c a was no greater than that o f the original support material, within the f 10% experimental error inherent in neutron activation analysis. EUROPT-1 was prepared in a 6 kg. batch by Johnson Matthey Chemicals plc (ref. 2). Sorbsil AQ U30 silica was treated with Pt(NH3)4C12 and Pt(NH3)4(0H)2 at pH 8.9, filtered, washed until free of C1-, dried at 378 K and reduced at 673 K (ref. 2). The catalyst became oxidised by air before issue (refs. 3,lO). The total surface area of EUROPT-1 is 185 m2 g-', the Pt-content is 6.3 wt% and the impurities (in ppm) are: A1 = Ca = 500; Ti = Na = 400; Mg, 200; K , 150; Fe, 90; C1, < 50; Cr < 10. The Pt(4f7/2) binding energy in the re-reduced catalyst was measured in three laboratories at 71.3, 71.4, and 71.5 eV. The catalyst may be re-reduced in pure hydrogen without sintering at temperatures up to 673 K . TPR (this work) showed that re-reduction commenced at 248 K and was complete at 425 K , the maximum in the reduction profile occurring at 340 K; this concurs

138

with a previous report (ref. 11). The platinum particle size distribution (PSD) in Pt/silica, measured by HRTEM was: ,< 1.0 nm, 60%; 1.0 - 1.6 nm, 18%; 1.6 - 2.2 nm, 14%; 2.2 - 2.8 nm 6%; 2.8 4.0 nm, 2%. It is probable that some of the smallest particles present escaped detection, and hence this distribution is to be regarded only as a guide to the very high dispersion (approaching 100%) of the platinum active phase in this catalyst. The platinum PSD in EUROPT-1 reduced below 673 K contains maximum in the distribution at 1.8 nm, 75% of the platinum particles are c 2nm in diameter, and the dispersion i s 60% (ref. 3 ) . The morphology of the platinum particles in Pt/silica and in EUROPT-1 has been studied by EXAFS spectroscopy. Catalysts were reduced in pure H2 at 573 K for 80 min (Ptlsilica) or 60 min (EUROPT-1) and spectra were taken with the catalysts in a hydrogen atmosphere. The experimental and computed spectra for Pt/silica are shown in Fig. 1, that for EUROPT-1 and a Pt foil (for reference)

-8

0

200

400

eV

600

800

Fig. 1. Pt L3-edge EXAFS spectrum of 0.76 wt% Pt/silica. Full curve experimental (unsmoothed) spectrum: dashed curve = theoretical spectrum.

=

were of comparable quality. Structural information from these spectra is shown in Table 1. The d-, JZd, /3d, and 2d spacings are observed in each spectrum together with the coordination numbers (CN) and Debye Waller factors (DWF) appropriate for each shell. The values of the coordination numbers provide

139

direct information concerning the likely morphology of the average platinum particle present in each catalyst. Fig. 2 shows three 14-atom clusters o f Pt atoms of which b and c possess coordination numbers in reasonable agreement with experiment (Table 2). Similar calculations show that an average particle in TABLE 1 Structural parameters* obtained by EXAFS spectroscopy Platinum foil

Pt/silica

EUROPT-1

Pt-Pt/nm

CN

DWF

Pt-Pt/nm

CN

DWF

Pt-Pt/nm

CN

DWF

0.277 0.392 0.481 0.547

12.0 6.1 21.9 14.0

0.010 0.014 0.016 0.012

0.276 0.391 0.477 0.543

4.4 1.6 1.3 2.1

0.012 0.011 0.009 0.010

0.276 0.391 0.477 0.543

5.5 2.1 3.8 3.9

0.013 0.018 0.017 0.015

* CN

=

Coordination number; DWF

=

Debye Waller factor

TABLE 2 Structural parameters for proposed model particles a, b, c, and d o f Fig. 2 Pt/silica Coord. Nos

EUROPT-1 Coord. Nos

P t-P t/nm

Expt

a

b

C

Expt

d

0.276 0.391 0.477 0.543

4.4 1.6 1.3 2.1

4.1 0.0 2.7 2.3

4.4 0.9 2.1 1.7

5.1 1.1 1.0 1.1

5.5 2.1 3.8 3.9

5.5 0.7 3.7 3.4

a

C

Fig. 2. Model configurations for Pt particles in Pt/silica (a.6.c) and in EUROPT-1 (d). EUROPT-1 i s larger and is reasonably described by model d of Fig. 2. This model will be discussed in greater detail elsewhere. HRTEM and EXAFS each demonstrate that the average platinum particles in Pt/silica are very small ( c a . 0.1 nm) and i n EUROPT-1 are larger ( c a . 0.2 nm). In addition the Pt particles i n each

140

catalyst appear to consist of a raft of (111)-structure, with some atoms present in a partial second layer. ADSORPTION PROPERTIES Isotherms for [14C]carbon monoxide adsorption at 298 K over the range 0 to 526 Pa were measured using reduced Pt/silica (0.268 g) and EUROPT-1 (0.150 9). The adsorption capacities of the two catalysts were similar, and each isotherm showed a primary and secondary region (ref. 12) the transition occurring at about 26 Pa. Evacuation for 0.5 h at 298 K caused the desorption of 47% (Pt/silica) or 5% (EUROPT-1) of the adsorbed-C0. Subsequent equilibration with '658 Pa [12C]carbon monoxide resulted in 90% (Pt/silica) or 58% (EUROPT-1) removal of the remaining [14C]carbon monoxide. The FTIR spectrum of CO adsorbed on Pt/silica contained one strong band at 2085 cm-l having a slight shoulder on the high frequency side, whereas that for EUROPT-1 contained three bands: 2075(s), 1849(w) and 1720(vw) cm-l attributed to linear, bridged and capped species (ref. 5). Clearly, from the exchange measurements, more CO is reversibly adsorbed on Pt/silica than on EUROPT-1, and this is consistent with the higher frequency observed for CO adsorbed in the linear form on Pt/silica i n comparison with that on EUROPT-1 which implies a stronger Pt-C bond in the latter system. Similarly, CO adsorption on Pt/silica measured by the pulse technique over the range 256 to 294 K is an activated process (to be published), whereas no such claim has yet been made for CO adsorption on EUROPT-1. In oxygen adsorption on Pt/silica at 273 K measured by the pulse technique, 5.73 x 10l8 molecules (9. cat.)-' were adsorbed at saturation which corresponds to an O:Pttotal ratio of 0.5:l.O. The corresponding stoichiometry on EUROPT-1 was 0.65:l.O (ref. 5). However, after allowance for differences in dispersion, O:PtSurf i s 0.5:l.O for Pt/silica and 0.9:l.O for EUROPT-1. Furthermore, there is evidence for bulk oxidation of EUROPT-1 by oxygen (ref. 5 ) . Thus, although comparisons must be made with caution, there is a consensus in these results that adsorption of CO and perhaps that of O2 i s stronger on EUROPT-1 than on Pt/silica. CATALYTIC PROPERTIES 1,3-Butadiene hydrogenation The kinetics and mechanism of this reaction are well understood (ref. 13); in particular a greater than expected extent of 1:4-addition is indicative of the presence of electronegative contaminants ( S , C1) at the active sites (ref. 14). Hydrogenations were conducted at 290 K over Pt/silica and EUROPT-1, each reduced at 523 K and, for reference purposes, over an evaporated Pt film at 326 K (initial pressures: C4H6, 6.6 kPa; H2, 19.7 kPa; conversion, 20%). Butene and

141

butane yields were about 67% and 33% respectively and the butene compositions were: 1-butene 76 78 75

Pt film Pt/silica EUROPT-1

t-2-butene 18 14 18

c-2-butene 6 8 7

Pt/silica and EUROPT-1 surfaces behave similarly to that of the clean evaporated film confirming the absence of C1 in the neighbourhood of the Pt sites in the supported catalysts and showing that there is no particle size effect on selectivity in this reaction. Cyclopropane hydrogenolysis The failure of butadiene hydrogenation to distinguish between Pt/silica and EUROPT-1 may be related to the very strong adsorption of this hydrocarbon on Pt. We therefore examined the conversion of cyclopropane to propane, as this cyclic hydrocarbon is among the most weakly adsorbed of those that undergo hydrogen addition. Bond has reported that orders for this reaction vary with reactant pressures such that rate passes through a maximum with increasing pressure of either reactant; moreover this behaviour conforms to expectation based on Langmuir-Hinshelwood theory (ref. 16). Pt/silica and EUROPT-1, each reduced in pure hydrogen at 523 K for 1 h were used as catalysts for this reaction at 313 K ; the variation of rate with hydrogen pressure is shown in Fig. 3. The expected maxima were observed but the behaviour o f Pt/silica conforms to expectation based on Langmuir-Hinshelwood kinetics (eq. 1) whereas that of EUROPT-1 does not. (In eqn. 1 the symbols have their usual significance, r

=

kOC

3 1

OH

=

kecOl

=

kbcPc(bH1/nPH”n)2/(1

+

bcPc + bA/nPH1/n)3

(1 1

c = cyclopropane, n is defined by the process: 2H(ads)H2(g) + n(vacant sites)). For Pt/silica the experimental points are well modelled by an equation having n = 1 (firm curve, Fig. 3a, k = 10.2, bH = 0.70, b, = 0.0.33) and n = 0.5. However, we have failed to model the behaviour of EUROPT-1 adequately. The dashed curve in Fig. 3b i s a poor fit (n = 1) and we note that only the sharp maximum is well modelled by the dotted curve (n = 2, for which there is no ready interpretation). Thus the surfaces of the small Pt particles in Pt/silica behave as an energetically homogeneous surface on the Langmuir model whereas those of the larger particles in EUROPT-1 do not. It is not clear whether failure in the latter case i s due to the larger average size of the particles or to their wider size distribution.

142

P4I T o r r

0

100

2 00

300

Pi;l Tor r

Fig. 3. Dependence o f initial rate, R/Torr min- 1 , on initial hydrogen pressure, Pi/Torr, in the hydrogenolysis of cyclopropane to propane catalysed Po = 125 Torr. [ I Torr = at 313 K by Pt/silica (a) and by EUROPT-1 (b). 133.3 Pa]. In (a) the full curve represents a vakation of rate given by eqn. 1 (see text), and the dotted and dashed curves show the corresponding theoretical variations o f surface coverages. In (b) the full curve describes the experimental variation of rate, and the dotted and dashed curves represent predicted behaviour according to eqn.1 (see text).

Asymmetric Hydrogenation

Each catalyst has been modified by cinchonidine(1) and used for the high MeCOCODEae, to MeCH(0H)COOMe according (refs. 7, 17). The reaction provides optical yields (%R %S) at 290 K and

-

the deposition on its surface of pressure hydrogenation of methyl pyruvate, to our variation of Orito's method an excess of R-(+)-lactate. Typical 25 to 50% conversion were 87% over

143

( I ) Cinchonidine. R = vinyl

Dihydrocinchonidine. R = ethyl

EUROPT-1 and 54% over Pt/silica. We have proposed that ordered adsorption of several L-shaped alkaloid molecules occurs on each platinum particle of EUROPT-1 leaving exposed shaped ensembles of platinum atoms most of which accommodate methyl pyruvate in the conformation which, on hydrogenation, gives R-(+)-methyl pyruvate (ref. 7). Pt/silica contains much smaller particles (Fig. 2) most of which may be unable to accommodate more than one alkaloid molecule; hence the steric situation is less well defined and the optical yield much lower. Indeed, it might be argued that the typical 1 nm Pt particle in Pt/silica does not contribute to the optical selectivity but catalyses the formation of racemic lactate, and that a smaller proportion of larger Pt particles produces the optical yield observed. CONCLUSIONS 1. A Pt/silica has been prepared from a C1-contain ng source in such a way that the Pt particles show no evidence of contamination by C1. 2. The platinum particles in this Pt/silica are structurally similar to, but smaller than, those in the standard reference catalyst EUROPT-1. 3 . Substances which are weakly or only moderately strongly adsorbed (cyclopropane, CO) exhibit different characteristics when adsorbed on the Pt surfaces of these two catalysts, whereas substances that are strongly adsorbed (02, butadiene) show similar or identical behaviour. 4 . Where the adsorption of a molecular template on the Pt surface is required in order to induce enantioselectivity, the optical yield i s diminished as size of the Pt particles approaches that of the template.

144

ACKNOWLEDGEMENTS We thank SERC and ICI C & P Ltd for financial support in the context of a Cooperative Award. SERC is also thanked for SRS beam time and for the award of studentships to MBTK and PAM. REFERENCES 1 2 3 4 5 6

G.C. Bond and P.B. Wells, Applied Catal., 18 (1985) 221. G.C. Bond and P.B. Wells, Applied Catal., 18 (1985) 225. J.W. Geus and P.B. Wells, Applied Catal., 18 (1985) 231. A. Frennet and P.B. Wells, Applied Catal., 18 (1985) 243. P.B. Wells, Applied Catal., 18 (1985) 259. S. Kinnaird, G. Webb and G.C. Chinchen, J. Chem. SOC. Faraday I, 8 3 (1987)

7 8 9 10 11 12. 13

I.M. Sutherland, A. Ibbotson, R.B. Moyes and P.B. Wells, J. Catal., accepted for publication. C.K. Jorgensen, Acta Chem. Scand., 10 (1956) 518. D.L. Swihart and W.R. Mason, Inorg. Chem., 9 (1970) 1749. R.W. Joyner, J. Chem. SOC. Faraday Trans. I , 76 (1980) 357. G.C. Bond and M.R. Gelsthorpe, Applied Catal., 35 (1987) 169. J.U. Reid, S.J. Thomson and G. Webb, J. Catal. 29 (1973) 421. J.J. Phillipson, P.B. Wells and G.R. Wilson, J. Chem. SOC. (A), (1969)

14

M. George, R.B. Moyes, D. Ramanarao and P.B. Wells, J. Catal., 52 (1978)

15

A.G. Burden, J. Grant, J. Martos, R.B. Moyes and P.B. Wells, Discuss. Faraday SOC., 72 (1981) 95. G.C. Bond and J. Turkevich, Trans. Faraday SOC., 50 (1954) 1335; G.C. Bond and J. Newham, Trans. Faraday SOC., 56 (1960) 1501. Y. Orito, S. Imai, and S. Niwa, Nipp. Kag. Kaishi, 8 (1979) 1118.

16 17

3399.

1351.

486.

G. Poncelet,P.A. Jacobs,P.Grange and B. Delmon (Editors),Preparation of Catalysts V 01991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

145

FACTORS ANALYSIS FOR MECHANICAL STRENGTH IN PELLETING PROCESS OF Fe-BASED HIGB TEMPERATURE SHIFT CATALYST

YONGDAN LI, JIUSHENG ZHAO and LIU CHANG Department of (China)

Chemical

Engineering,

Tianjin

University,

Tianjin

300072,

ABSTRACT The pelleting process and the factors influencing mechanical strength for Fe-based catalyst have been investigated. The horizontal crushing strength data is used in the text, their mean value and their corresponding Weibull modulus are taken as parameters for analysis. It is found that the processing precision of the pelleting machine, the pretreatment of the powder and the pelleting conditions all have strong effect on the final strength of pellets. The experimental result shows that a potentiality of increasing in overall properties exists, which means a better quality catalyst could be expected. INTRODUCTION Formation of pellets is one of the important processes in catalyst preparation, which

significantly affects the mechanical strength and the

texture, on which

the reliability of

catalyst performance in industrial

reactor and the effectiveness of pellet depend, even though it usually does not change the intrinsic catalytic activity apparently. The influence of pelleting conditions on the properties of catalyst has been studied by several authors

(ref.l,2).

pelleting pressure.

Their publications stressed mainly on

the effect of

But actually in the process of pelleting, there are

many factors which will influence the properties of the resulting pellets. In most cases these factors are interdependent on one another. Yet in general idea, they can be distinguished into three different categories, that is the processing precision of pelleting machine, the property and the treatment of

the

powder

material

pelletization. This

paper

and

effectiveness influencing both properties of

additives,

will

report

and some

the

processing

important

pressure

factors and

the mechanical strength and the

resulting pellets, and

finally

it will

also

of

their

overall

tell us

the

potentiality of getting better catalyst as a whole. SAMPLES AND EXPERIMENTAL Samples The powder material of mixed oxides was obtained from industry made

146 X

Fig.1

I

I

I

I

20

30

40

50

I

60

70

Degrees (28) The XRD diagram of the pre-pelleting mterial

by coprecipitation process. It contains about 10wt% of Cr2O3, 90wt% of Fez03 and a small amount of volatile combines. The XRD pattern (Fig.1) shows r-Fe203 phase is the most significant one. Several brands of commercial Fe-based catalyst pellets with a length of 4-7mm and a diameter of 9mm are taken for comparison. Pelletinq The pelleting was performed by using a hydraulic press with a set of specially designed die and punch, from which air can be released easily. Experiment shows that the contactsurface of both the die and the punch must be extremely smooth, or else during ejection there would be a big shearing stress "u" (refer to Fig.2) developed at the edge plane of the pellet, which in turn will induce big flaws within the pellet; at some cases there was even no complete pellet made. The precision of filling weight is also very important for strength, because under a definite operating condition the stress distribution in pelleting within the pellet is sensitive to the filling weight. The set of die and punch was carefully machined so that a variety of the strength data with a close distribution could be obtained.

The purpose

of which is to guarantee the repeatability of data for analyzing different factors involved. The process is illustrated as in Fig.2.

(a). Transfer

the material into the cylindrical cavity of the die, and rest it on the upper side of the lower punch. upper punch.

(b). Compress the powder by lowering the

(c). Eject the pellet by the upper movement of lower punch.

The pellets obtained in our laboratory under a normal processing condition with the industrial material possess a mean horizontal crushing strength (HCS)

those

of 45.8 and a Weibull modulus of 10.1, which are much higher than coming from the industry.

All the pellets in this paper made in

our laboratory have the same size, with a length of 6mm and a diameter of 9mm.

147

Fig.2

Pelleting process

Measurement The pellet strength is characterized by the HCS value, which has been shown in accordance with tensile fracture stress and proved to follow Weibull distribution (ref.3) both theoretically and practically, with the probability of failure under certain radial compression force given by

where p is the maximum stress, or in this paper, the maximum compression force, m is the Weibull modulus,

is a size factor, F(p) is the probability

of failure under p. The HCS data of solid catalyst scatters in a certain mode characterized by

the different value

of

Weibull

parameters m

and

p,

indicating the

discrepancy of preparation technology. We can see from equation (l), that higher m and lower

means less scattering of the data. In this paper, both

the mean value of HCS and the Weibull parameters of the catalyst strength were used in the analysis. The XRD pattern was taken by Rigaku 2038 X-ray diffractometer, Cu KO(, and the texture data was measured by AUTO PORE 9220 I[ porosimeter. RESULTS AND DISCUSSION Statistical result of commercial catalyst pellets Statistical results of

some properties of several brand

commercial

catalyst pellets show that the scattering of length of different samples also follows Weibull distribution reflecting the precision of the pelleting machine and bears some relationship with the scattering of HCS data, as indicated in Fig.3 and Table 1. Sample 1 and 2 are two different brands of commercial catalyst, but were pelleted by same model of pelleting machine. Sample 3 and 4 are another two brands but were pelleted by another same model of machine. We can see the Weibull modulus of strength and length given by same model machine are very close. In our

other paper, the relationship between the scattering

148 of

strength and density was

Scattering of

studied

strength, length

and

(ref.4) and density

shown the same trend.

in most

cases display

the

performance of the pelletinq machine.

h cl Q

-

0.6

6

Q

I

0 10 Fig.3

20 30 40 0 10 20 30 40 50 3.04.05.06.03.04.05.06.07.0 l(m) HCS(kg/pellet) Strength and length distribution of commercial catalysts a & b: strength distribution of sample 3 & 1 in Table 1, c & d: length distribution of sample 3 & 1.

Table 1.

Weibull parameters of length and strength

B1

mS

BS

ml

3.59

4.75 x 10-6

11.8

2

3.41

4.16 x

3

6.23

1.96

4

6.44

1.98 x

sample 1

Note:

ms

&

ps - Weibull

m1 s -

'

"the

S

1.40 x

27.8

11.5

3.25 x

43.2

2.10

10-30

24.9

46.9

2.20

10-32

31.3

10-9

34.2

parameters of strength:

- Weibull parameters of length:

most probable strength (HCS).

Factors Influencing the Mechanical Strength of Pellet For

simplicity and

convenience, two

sets

of

two

level

fractional

factorial experiments were carried out. First Set Factorial Experiment

FOUr factors, pelleting pressure,

powder size distribution, water content, and graphite amount are taken into account. Table 2 gives the factors and levels of the experiment. The results and the experimental matrix as well as the comparative effectiveness are shown in table 3. From these two tables, we can see that all the factors are sensitive. The powder

size distribution and water

content are most

effective.

The

149 pressure has a negative effect, it means as the pressure increases, the strength decreases. Notice that the change of density with strength is not in a simple manner, which shows it may be possible to get good mechanical strength with comparatively low desity after appropriate treatment.

Table 2.

Factors and levels of first set factorial experiment factors

Levels

A(kbars) pelleting pressure

B(mesh ~ 0 1 % ) powder size distribution

C(Wt%) graphite content

D(Wt%) water content

0

3

60-100, 10 < 100, 60 20-60, 30

0.5

14.5

1

5

60-100, 10 < 100, 30 20-60, 60

1

10.1

0.5

4

60-100, 10 < 100. 45 20-60, 45

0.75

11.76

Table 3.

Results and effectiveness of factors

experimental matrix

results

No.

A

-

HCS (mean) (kg/pellet)

D

B

C

0 0 1 1

0 1 0 1

0 0

0

1

1

0

0 1 0 5

0 1

0

1 2 3 4 5

0 0 0

6

1

7

1

8 9

1 0 5

1 1 0 5 A

B

C

D

Eff. for HCS Eff. for Density

-3.22 0.257

-12.3

2.07 0.013

-12.3 -0.31

0 1

0

1 0

0 5

Second Set Factorial Experiment

In this

-

2.46 2.13 2.14 2.47 2.43 2.73 2.72 2.40 2.48

58.1 47.8 43.6 48.8 31.9 57.9 42.5 29.8 45.5

1

Density (mean) (g/cm’)

AD

-11.6 0.017

BD 9.2 0

CD 0 -0.032

set, another 4 factors,

150 pelleting pressure, HNO3 concentration, r-A100H doping, and grinding time are examined. The factors and levels are shown in Table 4. Levels of E denotes same volume but different concentration of HNO3 was added into the material. F refers to doping with r-Al00II (below 300 mesh), and G stands for the time

of hand grinding of the material before pelleting.

Table 4.

Factors and levels of second set experiment

__

factors Levels

A(kbars) pelleting pressure 3 5 4

0

1 0.5

F(Wt%) r-A100H cont

E(N) HNO3 conc

0 2 1

0 1

0.5

Table 5.

0

20 10

Results and effectiveness of factors

experimental matrix No.

results

A

E

F

G

0

0

0 1 1

0 1 0 1

0

0 0 0

1 1 1 1 0.5

Eff. for HCS Eff. for Density

HCS (mean)

(kg/pellet) 54.6 69.3 54.2 41.6 103.2 71.7 42.1

1 1 0 1

0 0

0

1 1 0.5

0

0 0

1 0.5

1

79.0

0.5

63.8

A

E

F

8.73 0.697

G(min) grinding time

1

-20.6 0.306

8.29 -0.041

G 20.1 0.039

Density (mean) (g/cm’) 2.22 2.22 2.19 2.09 2.56 2.50 2.44 2.44 2.52

AG 20.4 0

The results and effectiveness are given in Table 5 .

FG

-12.6 0

The factors in

the second set have more striking effect on the mechanical strength, while the effect on density is still less sensitive. It is shown that by grinding and doping of r-AlOOH, the strength i s greatly enhanced, but impregnation

151 with HNO3 gives bad result. Effect of grinding shows the same effect as particle size distribution, that means smaller particle size is beneficial for strength. The highest HCS in Table 5 is 103.2, which is approximately 5 times as high as that of ordinary commercial catalyst, as its density is only 2.56, still in the range of commercial ones. The experiment shows that a great potentiallity in increasing mechanical strength is existing. Effect of Pelleting pressure Pressure on Density

Normally as pressure increases, the mean strength

of pellets increases, and the density increases too. Yet in our experiment, pellets made directly from industrial

-

50

4

c)

. i . i

40

2

\

2 -

:: X

powder

> ,’

60

pressure upper

HCS

on

limit

density of

20

pre-treatment

and

both

between

pelleting

in a

density

as

There appears strength certain

pressure.

and

range

Such

fact

incompressibility

of

mother crystals in pellets. As

for

the

gas

suggests

10

the

diffusion

limiting

water

shift reaction (ref.51, a relatively

0 1 2 3 4 5 Pelleting Pressure (kbars) Fi9.4

any

illustrated in Fi9.4.

/o-----‘

30

without

shows some limitations of pelleting

Dependence of strength and density on pelleting pressure

high

strength

and

low

density

to

a

comparatively enhance

the

effectiveness factor of the catalyst are preferred.

Table 6. Effect of pelleting pressure on the pore structure of oxidized state ~~

P

HCS

2

40.4

3 5

V

R

71.6

0.233

13

2.22

45.8

67.0

0.184

11

2.30

58.4

63.5

0.159

10

2.64

S

D

c

m 9.56 10.1 7.51

2.99 x 10-16 1.21

10-17

3.46

10-14

In the table, P(kbars): pelleting pressure, HCS (kg/pellet): mean horizontal crushing strength, S (m2/9): specific surface area, V (ml/g): porosity, R (nm): most probable pore diameter, D (g/cm’): density, m and @ : the Weibull parameters. Pressure on Pore Structure and Activity

Table 6 and 7 display the

effect of pelleting pressure on the mechanical strength and the pore structure

152 of oxidized and reduced state catalysts respectively. The reduction was performed under an optimum condition for strength developed by a set of optimization experiments Table 7.

HCS

P

S

(ref.6).

Effect of pelleting pressure on the properties of reduced state of the catalyst V

R

2

35.5

70.6

0.287

16.3

2.10

3

63.0

65.0

0.244

14.4

2.28

5

58.5

60.1

0.189

12.6

2.46

B

m

D

2.93 12.8 4.60

A

1.99 x

57.3

6.60 x

51.2

4.89

10-9

46.3

A: the apparent activity of CO conversion in a microreactor under normal high temperature shift reaction condition (ref.6). From the above two tables, we noticed that all of the parameters changed in the same trend as pelleting pressure changed, except Weibull parameters m and

p.

They give optimal values at P=3 kbars, under which the most reliable

catalyst in mechanical strength can be expected as shown in Fig.5.

Both

the HCS and the Weibull modulus of the sample pelleted at P=3 kbars is much superior to that of commerial catalysts (refer to Table 1). Meanwhile, its density and activity are quite acceptable. As a matter of fact the reliability or the probability of failure may be the most important in industrial point of view.

1.0 0.8 0.6

0.4

1.0

f

0.8

0.6

/a a'

0.4 0.2

0.2 0 10 20 3 0 40 50 60 70

40 50 60 70 8 0

20 30 40 50 60 70 80 90 100 HCS (kg/pellet)

Fig.5

HCS distribution of reduced state at different pelleting pressures a: 2 kbars, b: 3kbars. c: 5 kbars.

REFERENCES 1

J. Uchytil, M. Kraus and P. Schneider, Influence of Pelleting Conditions

on Catalyst Pore Structure and Effectiveness, Appl. Catal., 28 (1986) 13-14.

153 I. Brasoveanu, S.I. Blejoiu, A. Ssabo, P. Rotaru and I.V. Nicolescu, Structural Strains Appearing in the High Temperature Shift Conversion Fe-Cr Catalyst, Revue Roumaine de Chimie, 25(8) (1980) 1159-1169. 3 Yongdan Li, Liu Chang and Zhou Li, Measurement and Reliability Analysis of Mechanical Strength of Cylindrical Metallic Oxide Catalyst, Journal of Tianjin University, 1989 ( 3 ) 9-17. 4 Yongdan Li et al., Statistical Analysis for Mechanical Strength of Cylindrical Fe-based Catalyst. Journal of Fuel Chemistry and Technology, in press. 5 Hans Bohlbro, An Investigation on the Kinetics of the Conversion of Carbon Monoxide with Water Vapour over Iron Oxide Based Catalysts, Second Edition, Gjellerup, Copenhagen, 1969. 6 Yongdan Li et al., Factors Analysis on Mechanical Strength in Heating and Reduction of High Temperature Shift Catalyst by Dn Saturation Optimum Experimental, C1 Chemistry and Chemical Industry, in press. 2

This Page Intentionally Left Blank

G . Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

STUDIES ON

155

PORE SIZE CONTROL OF ALWINA: PREPARATBN OF ALWINA CATALYST EXTRUDATES

WITH LARGE UNMODAL PORE STRUCTURE BY LOW TEMPERATURE HYDROTHEWW TREATMENT

M. Absl-Halabi, A. Stanislaus and H. Al-Zaid

Petroleum Technology Department, Petroleum, Petrochemicals and Materials Division, Kuwait Institute for Scientlflc Research, P. 0. Box 24885, 13109 Safat, Kuwait

ABSTRACT

In the present work, the application of low temperature hydrothermal treatment method for preparing y-alumina supports with large monomodal pore size distribution has been investigated. Gamma alumina in the form extrudates was subjected t o hydrothermal treatment in an autoclave for various durations in the temperature range 150-3OO0C. The effect of ammonia vapor during hydrothermal treatment was also studied. The treated catalysts were characterized for surface area, pore size distribution and mechanical strength. The samples were also examined by x-ray diffraction and transmission electron microscope. The results revealed that pores can be widened selectively with greater than 70% pore volume In the mesopore range 100-250 A' and with improved crushing strength by low temperature hydrothermal treatment (e.g. 150°C). Further shift of pore size t o any desired larger range (250-500 Ao or 500-1500A') is possible by increase of temperature and treatment time. Ammonia was found to enhance the pore enlargement. The undesirable effects on loss of surface area and mechanical strength were signlficantly low and alumina phase transition was negligible in this process.

INTRODUCTION

Alumina has been widely used as a catalyst support in many catalytic processes of industrial importance.

This is largely because it is reasonably stable, contains acidic and

basic sites and can provide, through its different phases a wide range o f surface areas and porosities which are suitable for many catalytic applications. For some applications like hydroprocessing of petroleum residues and coal derived liquids it is desirable to have catalysts with wide pores since the large complex molecules present in heavy residues must have access t o the active surface sites within the catalyst pellets through an appropriate pore network (1.2).

A greater emphasis has been placed in recent years on the development of

wide pore catalysts with small external dimensions t o overcome the problem o f diffusion limitations (3,4,5.6).

Catalysts with bimodal pore size distribution having mesopores and macro-

pores in different proportions have been recommended t o prevent rapid deactivation In residue hydroprocessing. Large pore unimodal catalysts have also been found t o be more effective in residue hydroprocessing.

156 Pore size control of supported catalysts Is effected mainly by controlling the pore size of the support.

Several methods have been described in the literature for controlling

the pore size of alumina supports (7,8.9,10).

Thermal sintering has been chosen as a meth-

od for obtaining catalyst supports with monomodal pore size distribution In the mesopore or macropore (11,12,13) range.

The use o f some inorganic salt additives and different atmos-

pheres in enhancing the rate of sintering and lowering the calcination temperature to produce large pore alumina catalyst supports has also been explored (12.14.15).

Thus, for

example, by using Moo3 doped alumina, Tischer noticed that the sintering temperature could be lowered from 1000 t o 700°C t o produce a similar pore size distribution.

Sintering In

presence of steam has also been found t o result in enlargement o f pores (7,11,13,16). However, in these studies temperatures as high as 700% have been found necessary to promote sintering and t o produce aluminas with desired large pore size. The large pore aiuminas prepared by the high temperature sintering process usually have reduced surface area and low mechanical strength. phase is converted t o other types active as the y-aiuminas.

(6. 8

&

Further, part o f the y-alumina

a) of aluminas that are catalytically not

as

These undesirable effects on surface area, mechanical strength

and alumina phase transformation may probably be minimized if selective sintering of the narrow pores could be achieved at lower temperatures. A major obJective of the present work was t o investigate the influence o f low temp-

erature hydrothermal treatment on pore size enlargement of y-alumina support.

The material

(y-alumina) In the form o f extrudates was subjected t o hydrothermal treatment in an autoclave in the presence of water vapor for various duration In the temperature range 150-300°C. gated.

The influence o f ammonia vapor during hydrothermal treatment was also investi-

The treated catalyst samples were characterized for surface area, pore volume,

pore size distribution and mechanlcai strength.

The samples were also examined by x-ray

diffraction and transmission electron microscope t o assess possible changes in the alumina phase and the extent of sintering.

The results o f the studies reveal that low temperature

hydrothermal treatment can be used for the preparation of alumina support extrudates with large unimodal pore structure by selective enlargement of pores.

EXPERIMENTAL

Alumina extrudates were prepared from Condea Chemie Pural SB boehmite gel by kneading and extrusion.

Alumina paste suitable for extrusion was prepared by peptizing and

kneading the alumina powder with the peptizing solution.

In a typical experiment, 250 g of

alumina powder and appropriate quantity o f peptizing agent were used for each batch.

The

peptizing agent(l.5X HN03) was added at a constant flow rate over 20 min duration with continuous mixing using a kneader (Linden Model D5277, Germany). The paste was extruded through 1.5 mm nozzles, then dlred at 110°C in an oven for 24 h. 370'C

The dried extrudates were calcined under programmed temperature condltions (at for 2 h, 450°C for 1 h and 550'C

for 2 h).

A slngle screw type extruder model No.

157 250 (Netzsch, Germany) was used in making the extrudates.

For hydrothermal treatment

studies y-alumina extrudates prepared from Condea Pural SB alumina by the above procedure were used as starting material.

A weighed portion o f the sample (about 10 g) was

heated In an autoclave at temperatures ranging between 150 and 300'C. reagents, namely, water and ammonium hydroxide were used in the study.

Two types of

The treatment time

and the ratio o f water t o alumina were also studied. A mercury porosimeter (Micromeritics model 9305) was used t o determine pore size A Quantasorb adsorption unit (Quantachrome Corporation, USA) was used for

distribution.

BET surface area measurements. Pharma test model PTB 300 equipment was used to meas-

ure the side crushing strength of the alumina extrudates.

X-ray diffraction patterns were

obtained using a Phillips PW 1410 x-ray spectrometer operated at 30 kV and 20 mA with C u Ka radiation.

Transmission electron micrographs were made with JEM-1200 EX microscope.

RESULTS AND DISCUSSION

Gamma alumina in the form of 1.5 mm extrudates was subjected t o hydrothermal treatment in an autoclave In the presence of water vapor, for different time periods in the temperature range 150-300°C.

The effect of water t o alumina ratio and the presence of

ammonia vapor during hydrothermal treatment in the same temperature range on the modification of pore size was also Investigated. The results are presented and discussed below.

H f e c t o f treatment t i m e .

Influence of the duration of heating on pore size

distribution was investigated at a constant temperature

.

The results obtained at a con-

stant temperature of 150°C for water t o alumina ratio 1:l (w/w) are presented in Table 1. Table 1.

Effect of Hydrothermal Treatment Time on Pore Size Distribution (Reagent: water; Temp. 150°C)

60-1 00 100-250 250-500 500-1 500 1500-4000 4000-1 0000 10000-100000 > 100000 Tota I

0.405 0.025

0.006 0.004 0.001 0.002 0.000 0.005 0.448

0.408 0.03 0.008 0.003 0.002 0.001 0.000 0.007 0.459

0.230 0.287 0.01 0.006 0.004 0.001 0.000 0.012 0.55

0.124 0.377 0.008 0.005 0.006 0.000 0.000 0.007 0.527

0.430 0.022 0.010 0.004 0.0 0.0 0.0 0.004

0.47

It can be noticed that the amount of pores in the 60-100 A' diameter range progressively

decreases

y-7 L;,increasing

time o f heating with a corresponding increase in the amount of

158 100-250 A' pores.

Thus, the amount o f 60-100 A' pores is reduced from 90.4 t o 36.9%

with an increase in the amount of 100-250 A' pores from 5.6 t o 59.7% when the heating time is increased from l h t o 4 h. Further increase o f heating duration t o 8 h resulted in a further increase in the amount of 100-250 A" pores (from 59.7 t o 71.5%) with a corresponding decrease in the amount o f the 60-100 A" pores.

Thermal treatment in the pres-

ence o f water at a temperature o f 150°C, thus, increases Selectively the amount of 100-250 A' pores. A similar effect o f heating t i e on selectively increasing the amount of pores o f a particular diameter was also noticed for samples heated at higher temperatures. However, the pore size range that is widened or enlarged depends t o a large extent on the temperature o f heating as shown below.

E f f e c t of temperature.

The temperature o f hydrothermal treatment was varied

between 150 and 300°C t o study its effect on pore size modification. Table 2 presents pore volume distribution data for samples heated at 150, 200 and 300°C for a fixed time of

8 h.

It Is seen that temperature has a remarkable effect in widening the pores.

Thus, a

sample heated at 150°C for 8 h contains 23.5% and 71.5% o f the pore volume in pores of diameter 60-100 Ao and 100-250 A",

respectively.

In this sample, only about 5% of the

total pore volume is contributed by pores larger than 250 Ao. Table 2.

Effect o f Temperature on Pore Size Distribution of Alumina During Hydrothermal Treatment in Presence of Water for 8 h Pore Volume

60-1 00 100-250 250-500 500-1 500 1500-4000 4000-1 0000 10000-1 00000 > 100000 Tota i

0.124 0.377 0.008 0.005 0.006

0.000 0.000 0.007 0.527

23.5 71.5 1.5 1.2 0.0

.o

0.002 0.076 0.247 0.188 0.007 0.005

0.0

0.000

1.3 100

0.013 0.538

1

0.4 14.1 45.9 34.9 1.3 1 .o 0.0 2.4 100

0.009 0.018 0.131 0.34 0.006 0.004 0.001 0.025 0.534

I .7 3.4 24.5 63.7 1.I

0.7 0.2 4.5 100

On increasing the temperature t o 2OO0C, the pore size distribution pattern is altered. The amount of pores in the 250-500

Ao is increased from 1.5 t o 45.9%. Substantial

increase (1.5 to 34.9%)is also noticed in the 500-1500 Ao dia. pores. Further enlargement of the pores with maximum pore volume (about 63.7%) in the 500-1500 noticed with increase o f temperature t o 300°C.

A'

dia. range is

159 The results show that pores can be widened and pore size distribution in alumina

sup-

port can be shifted from the narrow pore size range (e.g. 60-100 Ao dia.) t o the larger range (e.g. 100-250 A', 250-500 A' or 500-1500 A' dia.) by increasing the temperature of hydrothermal treatment.

E f f e c t of the Alumina: Water Ratio.

The effect o f the amount o f water

used in hydrothermal treatment studies in modifying the pore size distribution was investigated by varying the amount of water between 10 and 40 mi for a given weight (10 g) of alumina at a fixed temperature and duration. The results for the experiments conducted at a constant temperature of 150°C for a fixed duration of 1 h are shown in Table 3. The data indicate that the amount of water or in other words, the ratio between the alumina and water, used in hydrothermal treatment has no significant influence on the pore size distribution.

Similar observations were also made

for the experiments conducted at higher temperatures.

Table 3.

Influence of the Amount o f Water Used for Hydrothermal Treatment on Pore Size Distribution of Alumina at 150°C for 1 h Pore Volume

10 m i H20

-----______Pore Dia ( A " )

60-1 00 100-250 250-500 500-1 500 1500-4000 4000-1 0000 10000-1 00000 > 100000 Tota I

m i g-l

m i g-l

%

0.405

40 m i H20

___---_____

90.4

0.413

5.6

0.026

0.006 0.004

1.3 0.9

0.001

0.2

0.002 0.000 0.005 0.448

0.4 0.0 1.1 100

0.008 0.007 0.003 0.000

0.025

0.002

0.005 0.464

%

89.0 5.6 1.7 1.5 0.6 0.0 0.4 1.1 100

The results of the studies presented above clearly show that low temperature hydrothermal treatment can lead t o widening of pores.

The extent of pore enlargement is

dependent on the treatment temperature and duration, but not on the amount of water. The exact type of chemical interaction or mechanism that leads t o pore enlargement is not clearly understood. Transmission electron microscope examination of the hydrothermaliy treated samples showed a progressive increase in the alumina crystaiiite size with increasing treatment time (Fig. 1).

X-ray diffraction analysis showed progressive narrowing of y-alumina peaks indi-

cating increase of particle size.

No peaks corresponding t o other phases o f alumina were

noticed. Since porosity originates from the volume of the space between the packed alumina particles, Increase in the partlcie size may be expected to result In pore enlargement.

160

Fig. 1

. TEN 01 sluminas trailed with water a1 2 0 O O C lor 111 I hr .Ibl 2 hr and icl 8 hr

Although the exact nature of chemical interactions that has resulted in particle size growth during thermal treatment in presence of water vapor at relatively low temperatures is not clear, It would be useful t o consider the following: Sintering of alumina generally requires material transport in the solid state. bulk diffusion (17.18).

This may proceed via surface diffuslon or

The surface diffusion is very responsive t o the presence o f impuri-

ties such as adsorbed gas (19.20) and ions (14,15,21).

During the process o f heating of

y-alumina in presence of water vapor, hydroxylation and dehydroxylation of alumina are possible.

This may enhance the mobility of the oxide and hydroxyl Ions on the alumina surface

leading t o acceleration of their surface diffusion, and thus may promote the rate of particle size growth.

E f f e c t o f ammonia. The influence of ammonia on the modification of pore size distribution o f alumina during hydrothermal treatment was studied in the temperature range 150-300°C. Fig. 2 shows the effect of treatment time at a fixed temperature (150°C) on the pore size distribution of alumina.

It is seen that the pore diameter is increased progressively

when the heating duration is increased, as in the case o f hydrothermal treatment with water A comparison o f the pore size distribution data (Table 4) of hydrothermally treated

alone.

alumina in presence and absence o f ammonia indicates that ammonia has a promoting effect on pore enlargement. Ammonia is a basic gas and it may strongly enhance the rate of hydroxyiation of y-Ai203

by cleavage of the AI-0-AI

bond. Such enhanced hydroxylation during the hydroth-

ermal treatment may increase mobility of OH ions on alumina surface and enhance the rate of recrystallization and particle agglomeration and consequently lead t o pore enlargement.

161

PORE DIAMETER (A) Fig. 2. The effect of hydrothermal treatment time at 1 5OoC in presence of NH7 on the pore size distribution of alumina extrudates. Table 4.

Comparison of Pore Size Distribution Data o f Alumina Hydrothermally Treated in Presence and Absence of Ammonia at 300T Pore Volume

60-1 00 100-250 250-500

500-1 500 1500-4000 4000-1 0000 > 10000 Total

m l / g (%) 0.00 (0.0)

0.08 0.27 0.17 0.01 0.01 0.01 0.55

(14.5) (49.0) (31 .O) (1.8) (1.8) (1.8)

ml/g (%) 0.0 (0.0)

0.02 0.10 0.41 0.01 0.01 0.01 0.56

E f f e c t o f hydrothermal

(3.6) (17.8) (73.2) (1.8)

(1.8) (1.8)

ml/g

0.01 0.02 0.13 0.34 0.01 0.01 0.03 0.55

(%I

ml/g (%)

(1.8) (3.6) (23.6) (61.8) (1.8) (1.8) (5.5)

0.01 0.02 0.05 0.47 0.01 0.01 0.02 0.59

(1.7) (3.4) (8.5) (79.7) (1.7) (1.7) (3.4)

treatment on surface area and mechanlcal

strength. The surface area of hydrorthermally treated catalyst samples are plotted in Fig.

3 as a function of treatment time. increasing treatment time.

All samples show a decrease h surface area with

However, the drop in surface area Is significantly high for sam-

ples treated at higher temperatures (>150°C)for longer duration.

This is not surprising in

view of the presence of the large amount of macropores in these samples. The crushing strength of the samples show an interesting behavior (Fig. 4). For the alumina hydrothermally treated at 1 50aC, the crushing strength Increases progresslveiy with

z2

162

I -

300 .250 -E 200 M

20 r C

N

Q

w

I

4

a?

Q

w 1500

2

5 vl

E l . . , .

100-

zo

0

2

4

6

8

TIME (h)

Fig. 3. Effect of hydrothermal treatment time and temperature on surface area of alumina extrudates.

0

2

4

6

8

TIME (h) Fig. 4. Influence of hydrothermal treatment time and temperature on side crushing strength of alumina extrudates.

increasing duration of heating, although there is a considerable increase (about 65%) in the volume o f 100-250 Ao diameter pores. However, at higher temperatures, a reverse trend is noticed.

A similar effect was also noticed for samples treated in the presence of ammonia.

During hydrothermal treatment at moderate temperatures (e.g. about 150°C). rehydration of the y-alumina is possible.

This may lead to the creation of chemical functions with hydroxyl

groups which on further calcination may increase the cohesion and consequently increase the mechanical strength.

In the case of of hydrothermal treatment at higher temperatures

(e.g. 3OO0C), hydroxylation-dehydroxylation cycles leads t o larger particles that are probably loosely packed. A weak cohesion between the alumina particles can result in weak mechanical resistance. Currently, the effects o f other reagents are being investigated and further experiments t o obtain a better understanding of the mechanism through which pore widening takes place are being undertaken.

ACKNOWLEDGEMENT

The authors thank Dr. S. Mansour for the TEM work, and MS. K. assistance in catalyst characterization. for Scientific Research, Kuwait.

Al-Dolama for her

This is KlSR Publication No. 3401, Kuwait institute

163 REFERENCES 1.

R. J. Quan, R. A. Ware, C. W. Hung and J. Wei.

Advances in Chemical Engineering,

14(1988) 95. 2.

H. Toulhoat and J.

C. Plumall.

In "Catalysts in Petroleum Refining 1989". D. L. Trhm,

S. Akashah, M. Absi-Halabi and A. Bishara (editors), Elsevier, Amsterdam, 1990.

p.

463. 3.

C. T. Adams, A. A. Del Pagglo, H. Schaper, W. H. J. Stork and W. K.

Schiflett, Hydro-

carbon Processing, September 1989, p. 57. 4.

J. Wei.

In "Catalyst Design Progress and Perspectives", L. L. Hegedus (editor), John

Wiiey and Sons, New York, 1988, p. 245.

5.

R. L. Howell, C. W. Hung, K. R. Gibson and H. C. Chen. Oil and Gas J., 83(1985) 121.

6.

K. Onuma.

In "Preparation of Catalysts IV", B. Delmon. P. Grange, P. A. Jacobs and G.

Poncelet (editors). Elsevier, Amsterdam, 1987. p. 543. 7.

V. J. Lostaglio and J. D. Carruthers, Chem. Eng. Progr. March 1986, p. 46.

8.

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9.

T. Ono, Y. Ohguchi and 0. Togari.

In "Preparation of Catalysts HI, G. Poncelet, P.

Grange and P. A. Jacobs (editors), Elsevier, Amsterdam. 1983. p. 631. 10.

B.

C. Lippens and J. J. Steggerda.

In "Physical and Chemical Aspects o f Adsorbents

and Catalysts, Linsen (editors), Academic Press, London, 1970, p. 171. Chem. Eng. Tech. 56(1984) 455.

11.

U. Hammon and M. Kotter.

12.

R. E. Tischer, J. Catal. 72(1981) 255.

13.

D. J. Young, P. Udaja and D. L. Trimm.

In "Catalyst Deactivation", B. Delmon and G. F.

Froment (editors), Elsevier, Amsterdam, 1980, p. 331. 14.

W. H. Gitzen, Alumina as a Ceramic materials, The American Ceramic Society, Columbus,

Ohio, 1970. 15.

P. Burtin, J. P. Brunella, M. Pijolat and M. Sousteli, Appi. Catai. 34(1987) 225.

16.

D. Aldcroft, G. C. Eye, J. G. Robinson and K. S. W. Sing, J. Appl.

17.

J. Haber.

Chem. 18(1968) 301.

In "Studies in Physical and Theoretical Chemistry", P. Lacombe (editor),

Elsevier Amsterdam, 1984, p. 123. 18.

A. Kapoor, R. T. Yang and C. Wong, Catal. Rev. Sci. Eng., 31U989) 129.

19.

A. E. B. Presiand. G. L. Price and D. L. Trimm. Progr. In Surf. Scl.. X1972) 63.

20.

N. A. Gjostein, in Surface and Interfaces, T. T. Burke, N. L. Reed and V. Weiss (editors) Syracuse Univ. Press (1 967).

21.

H. Schaper, E. B. M. Doesburg and L. L. Van Reijen, Appl. Catal. 7(1983) 211.

This Page Intentionally Left Blank

G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors),Preparation of Catalysts V 01991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

165

Production of Nickel-on-Alumina Catalysts from Preshaped Support Bodies L.M. Knijff, P.H. Bolt, R. van Yperen, A.J. van Dillen, and J.W. Geus State University of Utrecht, Department of Inorganic Chemistry, Croesestraat 77A, 3522 AD Utrecht, The Netherlands. SUMMARY To apply nickel uniformly into preshaped support bodies of a - and y-alumina, two procedures based on incipient wetness impregnation were investigated. The first one involved fixation of nickel ions by the alumina surface. Deposition-precipitation by hydrolysis of urea or nitrite within the pores of the support attached the impregnated nickel almost completely to the y-alumina; impregnation with nickel nitrate solution affixed a smaller fraction of the nickel. The a-alumina took up appreciably less nickel. According to the second procedure, a stable, high dispersion of nickel on a-alumina was obtained by precipitation of nickel magnesium oxalate within the pores. Decomposition leads to small nickel particles attached to magnesium oxide.

Introduction The most straightforward procedure to apply a catalytically active component into preshaped support bodies is incipient wetness impregnation with a solution of a precursor of the active component, followed by drying and thermal treatment. To distribute the active component uniformly over the entire support surface, the pore volume has to be filled completely by the impregnating solution. Occlusion of air and poor wetting of the support have to be avoided. Evacuation of the support bodies prior to impregnation improves the imbibition of the support considerably, especially when atmospheric pressure is readmitted after addition of the solution to the evacuated support.

To obtain a uniform distribution of the active component, two additional obstacles must be envisaged. Rapid extensive adsorption of the dissolved precursor to the support surface will cause deposition to proceed mainly at the pore mouths. Lack of interaction with the support, on the other hand, causes the distribution of the active component to be affected by migration of the solution, which may occur during the drying stage that follows impregnation. Thus the precursor is deposited mainly where the solvent evaporates. An important objective of our study was therefore to avoid the critical character of drying by immobilizing the precursor before the drying stage. Migration of the liquid phase does not disturb the distribution of the precursor when the latter has been fixed to the support. This can be achieved by (deposition-) precipitation. Geus et al. most extensively described this method for suspensions of powdered support materials [ 11. The additional requirement imposed by preshaped support bodies is that the precursor as well as the precipitant have to be distributed uniformly throughout the pore volume before the onset of precipitation. To this end we impregnated with solutions containing nitrite ions or urea, intended for in situ generation of hydroxyl ions. The reactions of urea and nitrite are, respectively: H2N(CO)NH;!

+

3 H 2 0 ->

2NH4+

+

C02(g)

+ 2OH-

3NOz- + H20 -> 2 N 0 (g) + N03- + 2OH' to beprevented: 2 N 0 + 0, -> 2 N 0 , ; 3 NO, + H20

-->

2HN0,

+ NO

166

To investigatethe feasibility of the procedure outlined above, this papex will deal with application of nickel within a - and y-alumina support bodies. With a-alumina an optimum activity per unit catalyst mass is not secured merely by a uniform distribution of the active component: the low reactivity and smoothness of the a-alumina surface causes a deposited component to be liable to sintering. The advantage of wide-porous a-alumina, providing good transport facilities, can be combined with a high catalytically active surface area by application within the alumina bodies of small clusters of a second support stabilizing a high dispersion of the active component. To this end nickel magnesium oxalate will be applied into a-alumina extrudates. Reportedly [2,3], mixed oxalates are excellent precursors for nickel catalysts. The mixed oxalate decomposes in inert atmosphere at about 350 O C to v e q fine particles consisting of metallic nickel and magnesium oxide. [4,5].However, the powder cannot readily be shaped to tablets without the (pyrophoric) nickel being oxidized. The applicability in fixed bed reactors can be improved appreciably when the mixed oxalate is incorporated into porous bodies of a support, such as a-alumina, and is decomposed in situ in the reactor. Since the oxalate precursor is insoluble, a special impregnation procedure is required. In separate experiments without a-alumina, the length of the induction period preceding precipitation of the mixed oxalate will be assessed. If the induction period is sufficiently long, the support bodies can be impregnated with a solution obtained by rapidly mixing solutions of oxalate ions, and of nickel and magnesium nitrate (co-impregnation). According to our first objective, the nickel is immobilized as the precipitating mixed oxalate before the drying stage. Alternatively, the support can be impregnated successively with the nickel-magnesiumsolution and the oxalate solution or vice versa, with an intermediate drying stage (two-step impregnation).

Experimental: Materials: Engelhard De Meern B.V. (The Netherlands) provided y-alumina supports of various shapes (specific surface areas ranging from 200 to 240 m2/g, pore volumes from 0.38 to 1.10 ml/g) produced from the same pseudo-boehmite, and a-alumina extrudates (9.1 m2/g, 0.55 ml/g). Gases. either high purity grade (quality 5.0) or purified over Linde molecular sieve 4A and reduced copper BTS-catalyst (BASF) to remove water and oxygen, respectively, were obtained from Hoekloos. -nation orocedure: Typically 1 to 5 gram of a support was impregnated to incipient wetness. In all cases, therefore, the volume of the impregnating solution was equal to the total pore volume of the support bodies. Prior to impregnation the support bodies were evacuated to a few mm Hg for at least 15 minutes to avoid occlusion of air. After the vessel had been closed, the impregnation solution was added from a syringe through a rubber septum. To allow distribution of the fluid throughout the support bodies, the vacuum was maintained for at least 10 minutes. Finally the vessel was opened to the atmosphere to apply additional force on the penetrated liquid. Impremation with urea or nitrite solutions: A slight excess of urea was added to a 0.88 M nickel nitrate solution. The solution of nickel nitrite (0.837 M) was prepared by combining solutions of barium nitrite and, in slight excess, nickel

167

sulphate and removing the barium sulphate by filtration. Alternatively, a combined solution of nickel nitrate and potassium nitrite was used. In both cases the nickel-to-nitrite ratio was determined by titration to be 0.60, theoretically sufficient to precipitate 56 % of the nickel as Ni(OH)2. After impregnation the vessel was shortly evacuated to avoid reaction of NO with 02,closed, and kept at 90 O C for either 3 or 20 hours in order to bring about hydrolysis of the urea or the nitrite. Evaporation of water was prevented. After the heating period the amount of nickel that had remained in solution was determined by crushing some of the wet impregnates, extracting nickel with distilled water, and determining the amount of nickel by atomic absorption spectrometry or complexometric titration. The remainder of the impregnates was dried in air at 120 OC. To assess the extent of fixation by the alumina without hydroxyl ions being generated, support bodies were impregnated with 0.938 M nickel nitrate solution only, and kept at 90 OC for 20 hours while evaporation of water was prevented. The nickel that was not bound by the alumina was determined as described above. Preparation of unsupported oxalates and application within a-alumina: Nickel magnesium oxalates with Ni-Mg ratios of m,3.55, 1.04,0.37, and 0 were prepared. The precipitates were centrifugated and washed with distilled water. In the application of mixed oxalates into a-alumina bodies, a Ni-Mg ratio of 6 was chosen for the nitrate solutions in order to attain a suitable nickel weight loading. Procedures, concentrations, and sample codes are to be found in Table IT. In two-step impregnations hot solutions were used in order to obtain a maximum concentration of oxalate ions. The impregnates were kept at 70 OC for about 1 hour before being dried to achieve complete reaction to nickel magnesium oxalate. Drying was performed at 100 OC in air. Besides the samples that were used in TPH (see below) the oxalates were decomposed by heating in a nitrogen flow with 5 OC/min to 400 O C , which temperature was held for 2 hours. Characterization: All samples were examined with scanning and transmission electron microscopy (SEM and E M ) . For comparison support bodies impregnated with 0.938 M nickel nitrate solution, and dried subsequently in air at 90 or 120 O C or in vacuum at 20 OC were studied. The distribution of nickel throughout the support bodies was studied with a Cambridge Stereoscan 150 S scanning electron microscope equipped with detectors for secondary and backscattered electrons and with a Link AN 10000 X-ray analysis system with energy dispersive detector. Impregnated support bodies were split, and mounted on an aluminium stub with carbon glue. A carbon layer was vapor-deposited onto the samples to provide a conducting surface. TEM samples were made by ultrasonic treatment of ground impregnates suspended in alcohol, and spreading a droplet of the suspension on a holey carbon film. The samples were examined in a Philips EM 420 "EM using an accelerating voltage of 100 kV. The reducibility of the deposited nickel species was investigated with temperature-programmed reduction (TPR) performed in 10 % H2/Ar flow with a heating rate of 5 OClmin. Except the oxalatebased catalysts, the impregnates had previously been calcined in air for two hours at 450 OC. A cold trap containing dry ice retained the water produced. The following characterization techniques were only used with the oxalate-based catalysts. Oxalate decomposition was studied with temperature-programmed heating in helium (TPH) using the

168

same apparatus as with TPR. The temperature was raised with 5 OC/min to 500 OC. Mean nickel particle sizes were determined with hydrogen chemisorption, performed in a conventional glass apparatus. The samples, typically containing 0.05 g nickel, were decomposed either in vacuum or in a flow of nitrogen. Evacuation was performed at 22 OC to a final pressure of about 5. Pa. Small doses of hydrogen were admitted to the sample at time intervals of 20 minutes unless complete uptake was attained earlier. An adsorption isotherm was measured up to a pressure of about 10 kPa. Next the samples were treated in hydrogen at 500 OC for 2 hours, the hydrogen was removed at 300 OC, evacuation was continued at 22 OC, and another chemisorption measurement was performed. The hydrogen up- take attributed to a monolayer was obtained by extrapolating the isotherm to zero hydrogen pressure. Calculation of the nickel metal surface area was based on a Ni : Had ratio of 1, and a mean surface area per nickel atom of 6.5. m2. Vibrating sample magnetomehy (VSM), a technique that was described in detail by Van Stiphout [6],was performed with the unsupported mixed oxalates, which were decomposed in a 10% H2/N2 flow at 400 OC. The magnetization of the sample was measured at 77 K as a function of the applied field strength (maximum 12 kOe). The size distribution of the nickel particles was obtained by fitting the thus measured magnetization curve with theoretical curves calculated for discrete particle sizes. Oxidation of methane, performed in an automated flow apparatus, was used as a test reaction for the stability of the nickel particles with respect to surface structure and sintering. 1.0 g of the sample A-Amox-2 (see Table II) was decomposed in a N2-flow and pretreated in a 10 % H2/He flow for one hour at 400 OC. The reaction feed, 1 mole % CH4 and 4 mole % 0, in He, passed through the catalyst bed at a space velocity of 4000 h-l. Methane conversion was measured with a Perkin Elmer 8500 gas chromatograph. As oxygen was present in excess, the nickel particles were completely oxidized. A measurement comprised a successive increase and decrease of the temperature between 350 and 750 OC in steps of 10 OC. Before each measurement the sample was reduced at 400 or at 850 OC.

Results and discussion Imuremation with urea or nitrite solutions: In Table I the extent to which nickel ions were attached to the alumina support is represented for various impregnations with and without urea or nitrite. Impregnation with nickel nitrate alone already leads to fiation of a considerable fraction of the nickel, especially at 90 OC; at 22 OC, the attachment is lower. Per unit surface area a-alumina takes up more nickel than y-alumina. The hydroxyl ions provided by hydrolizing urea or nitrite cause the fixation of nickel by y-alumina to be about complete, and by a-alumina to be much higher than with nickel nitrate alone. The distribution of nickel in the y-alumina supports was completely uniform, irrespective of the preparation technique and the drylng procedure. SEM showed a homogeneous concentration throughout the support bodies (figure 1). The degree of fiiation at the onset of drying appears to be of no importance. Probably, the pore structure of this y-alumina support prevents migration of the solution over macroscopic distances whatever the drying rate. Furthermore, it can be concluded that the produced carbon dioxide or nitric oxide can be discharged without expelling the liquid from the pore system, which would result in deposition of the active component on the outer edge of the support body.

169 TABLE I

Fixation of nickel ions in alumina supports impregnated to incipient wetness concentrations (mole/l)

support

heating time (h)

temperature fixation (OC)

(2)

?'-A1203

17

90

66

0.938

Y-A1203

20

22

35

0.938

a-Al2O3

20

90

27

90

95

65

0.938

0.88;

0.95

Y-A1203

20

0.88;

0.95

a-A1203

20

90

Y-A1203

20

90

97

"f-Al2O3

3

90

87

cc-Al2O3

19

90

72

0.837* 0.837;

1.674"

0.837"

The N02- concentration was 1.4 mole/l

On a small scale, as was revealed by E M , dried as well as calcined impregnates could not be distinguished from the fresh, unloaded alumina. Only upon reduction nickel particles developed (figure 2). Complete reduction asks for heating in hydrogen to about 800 OC or prolonged treatment at a lower temperature, as follows from TPR. Equal particle sizes were found with different reduction procedures. The size of the nickel particles ranged for some preparations from 3 to 9 nm, and for other from 5 to 18 nm. The nickel particles were evenly distributed over all clusters of alumina needles, demonstrating uniformity on a small scale as well. The interaction of dissolved nickel with the alumina surface can be assessed more in detail by TPR (figure 3). Since all samples had been calcined at 450 OC,differences must be due to different conditions during impregnation and drying. From the profiles obtained from the samples impregnated with nickel nitrate solution it appears that interaction leads to nickel species that are difficult to reduce: the sample that was dried rapidly at 22 OC, thus minimalizing interaction, exhibits reduction at temperatures much lower than the sample kept for 17 hours at 90 OC. Compared to the latter, also the samples prepared with nitrite are more readily reduced, demonstrating that generation of hydroxyl ions diminishes the extent of interaction with the alumina. The effect is more pronounced when the production of hydroxyl ions proceeds rapidly, as with KNO,-Ni(NO&. De Bokx demonstrated that the interaction between y-alumina and dissolved nickel involves for-

figure 1: Typical backscattered electron image of a cross-sectionof a nickelly-aluminacatalyst body; brightness indicates a high nickel concentration in the alumina matrix; a line profile of nickel is obtained by passing the electron beam along the projected straight line and recording the emitted Ni K u radiation.

170

mation of Feitknecht compounds ( Ni,A1y(OH-)2x-3y-z(N03-)z ) [7].The mixed compound of nickel and aluminium is converted to a species upon calcination that requires a high reduction temperature, probably nickel aluminate. With y-alumina impregnated with nickel nitrate solution and kept at 90 O C , the anchoring reaction proceeds to a considerable extent, which is, however, limited by the decrease in pH brought about by the continuing hydrolysis of nickel ions. Generation of hydroxyl ions by hydrolysis of urea or nitrite compensates for the protons released, and allows fixation to proceed to completeness. On the other hand, rapid production of hydroxyl ions leads to precipitation of Ni(OH)2 less strongly interacting with the alumina surface, resulting in an easily reducible nickel species. This effect is already apparent from the better reducibility exhibited by the sample prepared with nickel nitrite, but is obviously present in the KN02-Ni(N03), impregnate, which displayed a high rate of decomposition of nitrite. The improved reducibility is an important advantage of the method of in situ generation of hydroxyl ions when large batches of catalyst are to be reduced. The incomplete fixation of nickel to a-alumina may be due to the limited rise in pH attainable with urea (caused by the NH4+- NH3-equilibrium [8]) and the insufficient amount of nitrite present. Alternatively, instead of the concentration of hydroxyl ions, the surface area of the alumina may be the restricting factor. Possibly a fraction of the nickel hydroxide particles is not attached to the alumina surface and can be removed with distilled water. Pretreatment of the alumina or application of larger amounts of urea or nitrite may be useful to achieve complete fixation.

50nm

,-,

figure 2: TEM image of a typical nickelly-alumina catalyst after reduction

figure 3: TPR profiles of y-alumina impregnates:

_--

heated at 90 OC; Ni-nitrate .+..*..+. Ni-nitrite dried at 100 OC Ni-nitratelK-nitrite Ni-nitrate, dried subsequently at 20 O C H2 -consumptionper mmole Ni (arbitrary units)

____

Unsumorted and a m n’ckel mayesium oxalates: Results of preparations of nickel magnesium oxalates without alumina and within a-alumina extrudates are represented in Table II. To apply nickel magnesium oxalate into support bodies by coimpregnation the induction period for nucleation has to be sufficient to distribute the solution throughout the support bodies. We established the induction period in the absence of alumina under various conditions. It decreases with increasing concentration, temperature, and acidity of the solution produced by rapidly mixing a solution of magnesium and nickel nitrate with a solution supplying oxalate ions. Even when an induction pencd of half a minute is acceptable and a high Ni-Mg ratio is used, a

171

nickel weight loading of only 1.5 % can be achieved in a support having a pore volume of 0.5 ml/g. Therefore we will focuss on two-step impregnation, which allows application of solutions of a concentration correspondingto the solubility,resulting in a much higher nickel weight loading. From the data in Table II for unsupported oxalates, it is seen that with oxalic acid a considerable fraction of the magnesium is not taken up in the precipitate. Magnesium oxalate is soluble in acid solution. Ni-Mg ratios of unity and 0.25 in the solution result in ratios of 3.55 and 0.37, respectively, in the precipitate. To maintain the Ni-Mg ratio of the solution in the oxalates precipitating within the extrudates, ammonium oxalate is to be preferred over oxalic acid, in spite the higher solubility of the acid. When the a-alumina extrudates were firstly impregnated with a solution of ammonium oxalate and subsequently with the nickel magnesium nitrate solution ( A-Amox-2 ), a fairly homogeneous distribution was observed with X-ray analysis in SEM (the distribution of magnesium could not be esablished, as the energy of Mg Ka photons is too close to that of the abundant A1 Ka photons). The reverse impregnation order ( A-Ox-2 and A-Amox-1 ) led to an egg-white distribution: a relatively high amount of nickel was present in a narrow band inside the extrudates as a result of a combination of depletion and diffusion processes. Decomposition of the oxalates sets free carbon monoxide and dioxide. It turned out that the flow of gas evolved may displace the fine nickel-on-magnesiaclusters within the pores of the a-alumina. Although the uniform distribution of A-Amox-2 was not affected significantly, the aforementioned egg-white distribution became more diffuse. Decomposition of the unsupported mixed oxalates leads to severe shrinkage and a change from pale green to black. In TEM, decomposition of separate platelets of mixed oxalate by the electron beam can be observed. Platelike structures, consisting of small nickel metal and magnesium oxide particles, remain (figure 4). Mostly, the clusters of closely packed particles are irregularly shaped (figure 5a) and separate nickel particles are only discernible in a dark field image (figure 5b). It is noted that in the dark field image only a small fraction of the nickel particles shows up. In fact nickel constitutes about 38 volume percent of the specimen shown. Obviously, a relatively small amount of magnesium oxide is effective in preventing the nickel particles from sintering. Mixed oxalates that are contained in a-alumina extrudates display the same structure of packed particles as is exhibited by the unsupported samples. TABLE I1 Unsupported and a-alumina supported nickel magnesium oxalates I

Code

Ni/Mg 3.55 Ni/Mg 1.04 Ni/Mg 0.37 A-Ox-1 A-OX-2 A-Amox-1 A-Amox-2

impregnation method

1.0

co-impregnation lSt Ni-Mg,Znd Ox lSt Ni-Mg,2nd Amox lSt Amox,Znd Ni-Mg

I

I

Ni:Mg ratio

1.0 0.25 6.0 6.0 6.0 6.0

,

3.55 1.04 0.37 n.m. n.m. n.m. n.m.

concentrat ion1 particle size (nm) (mole/1) H ~ 0.66* 0.25* 0.62* 0.46* 2.3 2.33 2.33

22 8.2 2.7 16.4 16.6 16.5

n.m.

* concentration after combination with Ni-Mg-nitrate solution Ox= oxalic acid; Amox= ammonium oxalate; A= a-alumina; n.m.= not measured chs= chemisorption

13.2 7.9 7.6 n.m. n.m. n.m. n.m.

M

172

figure 4: TEM micrographs of nickel magnesium oxalate platelet before (a) and afler (b) decomposition

100 nm

a

b

figure 5: TEM micrographs of nickel magnesium oxalate NUMg 1.04: bright field (a) and dark field (b)image

In figure 6 TPH profiles of unsupported oxalates with Ni-Mg ratios of 0, 0.37, 3.55, and m are shown. Pure nickel oxalate decomposed from 280 to 350 O C . The sample with a small amount of magnesium exhibited decomposition within about the same temperature range. At higher magnesium contents a shoulder at higher temperatures developed. This may be attributed to a separate phase of a higher magnesium content, but not to pure magnesium oxalate, since the latter decomposed at a higher temperature, viz., above 400 OC. TPR indicated that nickel was not completely converted to the metallic state; X-ray powder diffraction provided evidence for nickel carbide and, possibly, for nickel oxide, besides for the expected nickel metal and magnesium oxide. Nickel carbide may have originated from disproportionation of carbon monoxide to carbon dioxide and carbon at the nickel metal surface. Since the main diffraction maxima of the four components coincide, neither their relative amounts nor their particle sizes, to be obtained from line broadening, can be properly estimated. Nickel metal mean particle sizes obtained from hydrogen chemisorption and vibrating sample magnetometry are given in table 11. The values mentioned for the VSM measurements have been deduced from the particle size distributions obtained. Samples that had only been decomposed consumed an extra amount of hydrogen with increasing hydrogen pressure, probably reflecting reaction of nickel carbide (and possibly oxide). Extrapolation to zero pressure, however, led to the same cal-

173

onidizeo 2 h (50

TPH

'.

/'?

200

300

400

500

figure 6: TPH profiles of unsupported oxalates; the detector signal is normalized with respect to the amount of oxalate ions

"C

T( 3C)

? '' I I 7 3 .

o

200

400

660

800

lono

figure 7: TPR profiles of A-Amox-2; Hp consumption per mmle Ni (arbitrairy units)

culated nickel surface area as was found in the second measurement, after reduction in hydrogen. The results from the three unsupported samples demonstrate that the nickel particle size strongly depends on the Ni-Mg ratio in the original oxalate. The VSM results only qualitatively display this relation; only the value obtained for Ni/Mg 1.04 agrees well with the chemisorption measurements. Ni/Mg 0.37 and 3.55 definitely contain nickel particles exceeding the upper and lower limit, respectively, of the size range to which the VSM theory applies (2 to 15 nm) [6]. The nickel particle size measured in a-alumina impregnates was about 16.5 nm, irrespective of the impregnation procedure. Considering the high Ni-Mg ratio applied (6, and probably higher in case oxalic acid had been used as precipitant), this diameter is small compared to the unsupported oxalates (see table 11).Nevertheless, much smaller nickel particles are expected to result with a mixed oxalate of lower Ni-Mg ratio. Increasing the amount of magnesium in the impregnating solution, implying a lower attainable nickel weight loading, will therefore be more effective in obtaining a higher nickel surface area than increasing the nickel content by means of multiple impregnation steps. In figure 8 conversion plots are displayed for the oxidation of methane over A-Amox-2. It is noted that upon exposing the (completely reduced) catalyst to the reaction feed, which contains excess oxygen, the nickel is almost immediately converted to nickel oxide, the active phase in the oxidation of methane. The hysteresis between the curves at increasing and decreasing temperature indicates severe 120

figure 8: Conversion of methane by A-Amox-2 increasing temperature : decreasing temperature: - - - - - run 1 : fresh sample, reduced at 400 OC, 2 h run 2: deactivated sample, reduced at 400 OC, 2 h run 3:deactivated sample, reduced at 850 O C , 1 h

'

conversion

80 60

-

40

:

300

400

500

600

700

8OC

174

deactivation.Comparison of TPR profiles from samples of the same catalyst that were heated in air to 450 or 850 OC (figure 7)indicates that a nickel species difficult to reduce is formed at elevated tem-

perature. Figure 8 shows that reduction at 400 OC for two hours partly restores the activity. Reduction at 850 OC (one hour) brings the activity back to the original level, which implies, moreover, that the nickel particles did not suffer from sintering. Formation of mixed magnesium nickel oxide nicely explains the observations. Nickel and magnesium ions have about equal radii and may react readily to a mixed oxide of the same crystal structure. Diffusion of magnesium into the nickel oxide may lead to an inactive nickel oxide surface, probably by diminishing the amount of excess surface oxygen, which is the reactive species in the oxidation of methane. Removal of the magnesium ions from the nickel oxide can only be achieved by reducing the nickel to the metallic state. Nickel metal nucleation will be increasingly difficult for lower Ni-Mg ratios in the oxide. The reducibility of the nickel species depends on the overall Ni-Mg ratio and the degree to which interdiffusion has proceeded. The reduction procedure at 450 ‘W is therefore sufficient to reduce the nickel oxide particles containing a low concentration of magnesium ions, while nickel that has diffused into the magnesium oxide requires a more severe reduction treatment. Conclusions: Fixation of nickel ions to the inner surface of y- and a-alumina support bodies, impregnated to incipient wetness, takes place to a considerableextent, especially at elevated temperature. In y-alumina fixation proceeds to completeness upon generating hydroxyl ions by in situ hydrolysis of urea or nitrite. The gases released are discharged smoothly, without expelling liquid from the pore system. This procedure leads to an improved reducibility compared to catalysts prepared by ordinary impregthe surface area appears to restrict the amount of nickel nation and successive drying.With a - a l u m i ~ that can be anchored. In order to realize sufficient interaction with the support surface in wide porous catalyst bodies, s m a l l particles of nickel-on-magnesium oxide can be synthesized within a-alumina by decomposition of nickel magnesium oxalate. A uniform distribution of mixed oxalate in the extrudates was obtained

by two successive impregnation steps, viz., with a hot concentrated solution of ammonium oxalate, and with a solution of nickel and magnesium nitrate. Application of a low nickel-magnesium ratio strongly enhances the dispersion of the nickel particles developing upon decomposition. Thus a lower nickel weight loading may exhibit a higher specific surface area. REFERENCES 1 J.W. Geus, in: Preparation of Catalysts 111, Scientific Bases for the Preparation of Heterogeneous Catalysts, B. Delmon, P. Grange, P. Jacobs, and G. Poncelet, Eds., Elsevier Amsterdam, 1982 2 W. Langenbeck, H. Dreyer, D. Nehring, and J. Welker, Z.anorg.allg.Chem. 281 (1955)90-98 3 V. DaneS and P.Jir6, Coll.Czechoslov.Chem.Comm.21 (1956)765-767 4 M. Ralek and V. DaneH, Coll.Czechoslov.Chem.Comm.24 (1959)1908-1913 5 V. Ponec and V. DaneS, Coll.Czechoslov.Chem.Comm.25 (1960)820-828 6 P.C.M. van Stiphout, Ph.D. thesis, University of Utrecht, Utrecht, The Netherlands, 1987 7 P.K. de Bokx, Ph.0. thesis, University of Utrecht, Utrecht, The Netherlands, 1985 8 L.A.M. Hermans and J.W. Geus, in: Preparation of Catalysts II, Scientific Bases for the Preparation of Heterogeneous Catalysts, B. Delmon, P. Grange, P. Jacobs, and G. Poncelet, Eds., Elsevier Amsterdam, 1979

G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

175

DEVELOPMENT OF A METHODOLOGY FOR INVESTIGATING THE ADSORPTION OF SPECIES CONTAINING CATALYTICALLY ACTIVE IONS ON THE SURFACE OF INDUSTRIAL CARRIERS

N. SPANOS, CH. KORDULIS and A. LYCOURGHIOTIS* Department o f Chemistry-Research I n s t i t u t e o f Chemical Engineering and High Temperature Processes, P.O.Box 1239, U n i v e r s i t y Campus GR-26110, Patras, Greece. ABSTRACT A methodology f o r e l u c i d a t i n g t h e mechanism o f a d s o r p t i o n on i n d u s t r i a l o x i d i c s u p p o r t s o f species c o n t a i n i n g c a t a l y t i c a l l y a c t i v e i o n s has been developed. This i n v o l v e s t h e c o r r e l a t i o n between t h e surface c o n c e n t r a t i o n corresponding t o monolayer and t h e s u r f a c e groups o f o x i d i c supports, t h e combined use o f p o t e n t i o m e t r i c t i t r a t i o n s and m i c r o e l e c t r o p h o r e s i s which a l lows t h e d e t e r m i n a t i o n of t h e surface and e l e c t r o k i n e t i c charge d e n s i t i e s , r e s p e c t i v e l y as w e l l as t h e mathematical a n a l y s i s o f t h e isotherms obtained. The methodology a p p l i e d t o t h e a d s o r p t i o n o f molybdates and t u n g s t a t e s on y-alumina, t o t h e adsorption o f molybdates on t i t a n i a and t o t h e adsorption o f Co2+ and N i 2 + i o n s on y-alumina l e d t o t h e f o l l o w i n g c o n c l u s i o n s : (i) Responsible f o r t h e c r e a t i o n o f a d s o r p t i o n s i t e s f o r n e g a t i v e ( p o s i t i v e ) species are, mainly, t h e protonated (deprotonated) s u r f a c e hydroxyls o f t h e o x i d i c supports. ( i i ) These species are adsorbed on e n e r g e t i c a l l y e q u i v a l e n t s i t e s o f t h e I n n e r Helmholtz Plane o f t h e double l a y e r around t h e y-Al,O, p a r t i c l e s suspended i n t h e aqueous medium. (iii)L a t e r a l i n t e r a c t i o n s a r e operational between t h e adsorbed species, t h e magnitude o f which depends on t h e nature o f t h e support and t h e species t o be adsorbed.

INTRODUCTION Although a r e l a t i v e l y l a r g e number o f supported c a t a l y s t s are prepared by a d s o r p t i o n o f a species c o n t a i n i n g t h e a c t i v e i o n on t h e s u r f a c e o f an o x i d i c s u p p o r t , e.g.

y-Al,O,,

SiO,,

TiO,,

studies dealing with catalysts

prepared by e q u i l i b r i u m adsorption f o l l o w e d by f i l t r a t i o n are r a t h e r scarce i n t h e l i t e r a t u r e [ e . g . l - 8 1 . T h i s i s p r o b a b l y t h e main reason f o r which a c l e a r methodology a l l o w i n g t h e e l u c i d a t i o n o f t h e mechanism o f a d s o r p t i o n from aqueous suspensions has n o t y e t been e s t a b l i s h e d . T h i s method should t a k e i n t o account t h e u s u a l l y i g n o r e d e x i s t e n c e o f an e l e c t r i c a l double l a y e r around t h e suspended support p a r t i c l e . Moreover, t h i s method should enable us t o i n v e s t i g a t e t h e f o l l o w i n g p o i n t s : (i) Are t h e groups responsible f o r t h e c r e a t i o n o f s o r p t i v e s i t e s , the n e u t r a l * A l l correspondence t o t h i s Author.

surface or t h e

176

charged hydroxyls? (ii)The p a r t o f t h e double l a y e r where t h e species are located, i . e . t h e surface o f t h e c a r r i e r , t h e I n n e r Helmholtz Plane ( I H P ) o r t h e d i f f u s e p a r t o f t h e double l a y e r .

i.e.

(iii)The n a t u r e o f adsorption,

whether i t i s l o c a l i z e d o r nonlocalized.

( i v ) The existence o f l a t e r a l i n -

t e r a c t i o n s between t h e adsorbed species. The e s t a b l i s h m e n t o f such a methodology i s t h e purpose o f t h i s comm u n i c a t i o n . To i l l u s t r a t e t h e proposed methodology we use s e v e r a l r e s u l t s taken from our r e c e n t adsorption s t u d i e s o f molybdates [9,10] and tungstates [ll] on y-alumina, o f molybdates on t i t a n i a [12] and o f Co2+ and N i 2 + on y-alumina [13]. EXPERIMENTAL The experimental method used t o determine t h e c o n c e n t r a t i o n o f t h e adsorbed species,

r(mo1 .m-'),

a t a given equilibrium concentration o f the

species i n t h e suspension, Cq(mol .dm?), has been described elsewhere [9,10]. P o t e n t i o m e t r i c t i t r a t i o n s have been used t o determine t h e s u r f a c e charge d e n s i t y , oo(pC.cm-2), i n t h e absence and presence o f t h e species t o be adsorbed h y d r o x y l s,

and t h e c o n c e n t r a t i o n SOH,':

of

the

surface groups

[SOH:

neutral

p r o t o n a t e d h y d r o x y l s, SO-: deprotonated h y d r o x y l s ]

d e t a i l s have been g i v e n elsewhere [14-17].Microelectrophoretic

.

Full

mobility

measurements were used t o determine t h e e l e c t r o k i n e t i c charge d e n s i t y , o,(pC.cm-'),

i n t h e absence and presence o f t h e species t o be adsorbed. F u l l

d e t a i l s are g i v e n elsewhere [18]. DESCRIPTION OF THE METHODOLOGY On t h e n a t u r e o f t h e a d s o m t i o n s i t e s . I t i s w e l l known t h a t t h e s u r f a c e o f t h e p a r t i c l e s o f a simple oxide

l i k e y-Al,O, and SiO, i s g e n e r a l l y charged i n e l e c t r o l y t e s o l u t i o n s . The w e l l e s t a b l i s h e d surface i o n i z a t i o n model [I91 describes q u i t e w e l l t h e charging mechanism. This process may be represented as:

SOH,'

K, SOH

K,

SOH

+

Hs+

SO- + Hs+

Hs+, H i : denote hydrogen ions on t h e surface o f t h e support and i n t h e b u l k s o l u t i o n , respectively.

177

I n t h e case o f TiO,,

which i s a m i x t u r e o f r u t i l e and anatase, t h e above

e q u i l i b r i a should be w r i t t e n f o r each component[20].

I t i s obvious t h a t i n o r d e r t o i n v e s t i g a t e which o f t h e s u r f a c e groups i s mainly responsible f o r t h e c r e a t i o n o f adsorption s i t e s ,

i t i s necessary

t o c o r r e l a t e t h e surface c o n c e n t r a t i o n o f t h e adsorbed species corresponding t o t h e p l a t e a u o f t h e isotherm,

rm

(see f i g u r e 8) w i t h t h e c o n c e n t r a t i o n o f

t h e d i f f e r e n t t y p e s o f groups. The l a t t e r may be r e g u l a t e d by doping t h e c a r r i e r o r by changing e i t h e r t h e pH o r t h e temperature o f t h e suspension [14-171.

WxOyz-,

I f t h e species t o be adsorbed i s n e g a t i v e l y charged, l i k e Mo,Ot-

or

i t seems reasonable t o assume t h a t t h e n e u t r a l o r t h e p o s i t i v e groups

are r e s p o n s i b l e f o r t h e c r e a t i o n o f adsorption s i t e s . T h i s may be t e s t e d by plotting

rm

t h e c o n c e n t r a t i o n o f SOH,'

o r SOH. A t y p i c a l example i s il-

l u s t r a t e d i n f i g u r e l. S i m i l a r t r e n d s were observed f o r t h e a d s o r p t i o n o f

[ll] as w e l l as f o r t h e adsorption o f t h e MoxOyz- ions on

WxOyz- i o n s on y-Al,O, TiO,,

though i n t h e l a t t e r case, uo, was used i n s t e a d o f t h e c o n c e n t r a t i o n o f

t h e p o s i t i v e groups [ 1 2 ] .

The above shows t h a t t h e p o s i t i v e groups are

r e s p o n s i b l e f o r t h e c r e a t i o n o f adsorption s i t e s f o r n e g a t i v e ions. The c o r responding p l o t s o f

rm

RlOH

t h e n e g a t i v e l y charged and t h e n e u t r a l surface

/ sites.nmF2

6.0

6.5

7.0

.05

0.55

2.05

7.5

8.0

1.55

2.

5

A~oH; / sites.nm-2 F i g . 1. S a t u r a t i o n surface Mo(V1) c o n c e n t r a t i o n obtained a t v a r i o u s temperat u r e s (ref.10) as a f u n c t i o n o f the c o n c e n t r a t i o n o f t h e protonated (curve a) and n e u t r a l (curve b) surface hydroxyls r e g u l a t e d by v a r y i n g t h e temperat u r e of t h e impregnating suspension o f t h e y-Al,O, (ref.16).

178 AlOH

-2

/ sites.nm

n

2

4

0

2

4

6

6

8

8

-2 A ~ O - / sites-nm

Fig. 2. Saturation surface Ni2' concentration obtained for the system Ni2'/yAl,O,-F at room temperature (ref.13) as a function of the concentration o f the negative (curve a) and neutral (curve b) surface hydroxyls regulated by varying the F- content (ref.15)

I

SOLID

SOLTIT ION

Shear

I Fiq. 3 . Structure of the solid-solution interface according to the "triple layer model". up, ud and uek refer to the total charge from the surface o f the support t o the IHP, to the OHP, and t o the shear plane,respectively,

179

groups f o r t h e Ni2' ions, shown i n f i g u r e 2, show t h a t t h e n e g a t i v e groups are mainly responsible f o r the c r e a t i o n o f adsorption s i t e s f o r p o s i t i v e ions [13]. S i m i l a r r e s u l t s were obtained f o r t h e Co2+ ions. P a r t o f t h e double l a v e r where t h e adsorbed soecies are l o c a t e d - Q u a l i t a t i v e aDDroach. Adopting t h e " t r i p l e l a y e r model" f o r t h e double l a y e r ( f i g . 3 ) t h r e e p o s s i b i l i t i e s do e x i s t : t h e adsorbates may be l o c a t e d on t h e surface, on t h e

I H P o r i n t h e d i f f u s e p a r t o f t h e double l a y e r . I n t h e f i r s t case adsorption o f negative ions i s expected t o cause a decrease i n t h e surface charge den-

s i t y , whereas i n t h e t h i r d case t h i s type o f adsorption

would r e q u i r e p o s i -

t i v e e l e c t r o k i n e t i c charge d e n s i t y . F i g u r e s 4 and 5, and s i m i l a r ones observed f o r t h e WxOyZ-/y-A1203 and MoxOyZ-/TiO, systems, show t h a t l o c a t i o n o f t h e adsorbates on t h e surface o r i n t h e d i f f u s e p a r t o f t h e double l a y e r i s precluded. Therefore, t h e o n l y p o s s i b i l i t y i s t h e a d s o r p t i o n a t t h e IHP. I n f a c t , i n t h a t case t h e negative i o n s are expected t o promote t h e appearence o f a d d i t i o n a l SOH,'

groups on t h e surface by forming i o n p a i r s [14,

15, 171

and t h e r e f o r e t o i n c r e a s e t h e p o s i t i v e s u r f a c e charge d e n s i t y ( F i g .

4).

Moreover, t h e presence o f negative ions a t t h e I H P i s i n agreement w i t h t h e negative e l e c t r o k i n e t i c charge d e n s i t y i n t h e pH range s t u d i e d ( F i g . 5). Based on r e s u l t s i l l u s t r a t e d i n f i g u r e s 6 and 7 and f o l l o w i n g t h e above reasoning we may conclude t h a t t h e p o s i t i v e i o n s are a l s o l o c a t e d a t t h e I H P

600

N I

300

E

Y

4

\

0"

-300

-600 3

4

5

6

7

8

9

PH

Fig. 4. Surface charge d e n s i t y o f y-A1203 as a f u n c t i o n o f pH o f t h e suspens i o n a t 25OC. ( a ) i n t h e presence o f MoxOyZ- i o n s (ammonium heptamolybdate s o l u t i o n , C0=l*10-3 mol Mo(VI)/dm3, i o n i c strength, I = O . 1 mol/dm3 NH,N03), (b) ions (0.1 mol/dm3 NH,NO, s o l u t i o n ) . i n t h e absence o f MoxO:'

180

0.5 \

x

0"

0.0

- 0.5 - 1.0

Fig. 5. Electrokinetic charge density of y-A1 0 as a function of pH of the suspension at 25OC. (a) in the presence of Moxb:- ions (ammonium heptamolybdate solution, C0=l*10-3 mol Mo(VI)/dm3, ionic strength, I=O.Ol mol/dm3 NH,NO,), (b) in the absence of MoxOt- ions (0.01 mol/dm3 NH,NO, solution). of the double layer. Useful information concerning the mechanism of adsorption may also be drawn from the form of the isotherms obtained. For the systems already mentioned at various temperatures [lo] as well as for the systems

N

I

6

3.5

4.5

5.5

6.5

7.5

PH

Fig. 6. Surface charge density of y-Al,O, as a function of pH of the suspensolution, sion at 25OC. (a) in the presence of Co2+ ions ( Co(N0,),.6H20 C0=l*10-3 mol Co2+/dm3, ionic strength, I=O.1 mol/dm3 NH,NO, ), (b) in the absence of Co2+ ions (0.1 mol/dm3 NH,NO, solution) .

181

N I

5

U

5

0.0

\

\

2

0"

'0.5

-1.0

I

4

5

6

7

8

9

10

1

PH

Fig. 7 . Electrokinetic charge density of y-A1 0 as a function of pH of the solution, suspension at 25°C. (a) in the presence of Co2' ions ( Co(N03),.6H,0 C0=l*10-4 mol Co2+/dm3, ionic strength, I=O.Ol mol/dm3 NH,NO,), (b) in the absence of Co2+ ions (0.01 mol/dm3 NH,NO, solution). Moody-Al,O,-Na [lo], Co2+/y-Al,0,-F [13] and Ni2+/y-A1203-F [13] at room temperature, the isotherms may be classified as S and I type suggesting localized, Langmuir type, adsorption at the IHP with strong and weak lateral interactions, respectively [21,22]. Typical examples of the $ type isotherms obtained are illustrated in figure 8. Part of the double laver where the adsorbed sDecies are located-Ouantitative Amroach. The next step is to analyse the isotherms obtained on the basis o f the following assumptions: (i) More than one kind of ions (e.g. MOO:-, Mo,O,"-), are specifically adsorbed at the IHP, as it has been inferred above. (ii) The adsorbed ions are located on energetically equivalent sites as suggested from the Langmuirian shape of the isotherms. (iii) One specifically adsorbed ion, i , replaces one water molecule from the IHP [9]. Assuming no lateral interactions between the adsorbed species we may derive [9] the "Stern-Langmuir" equation

where rm represents the saturation surface concentration of the adsorbed species (maximum in the and S type isotherms). The constant K is given by eqn (2).

182

N

'E

10

-

4

5

\

-

L

0.0 1

n nn L."V

0. 3

0.02

c,,

/ mol.dm

-3

F i g . 8. Surface c o n c e n t r a t i o n o f M o ( V 1 ) as a f u n c t i o n o f t h e e q u i l i b r i u m

Mo(VI) c o n c e n t r a t i o n a t v a r i o u s temperatures o f t h e impregnating suspension o f t h e y-Al,O,. pH=5, 1 4 . 1 M NHN , O., 13: 20°C, 0 : 3OoC, A : 45OC. K

=

Ii[(ai/55.5)exp(-AGaads,i/RT)],

where ai and

AGOads,,

(2)

represent a c o e f f i c e n t (independent from t h e Cq

b u t de-

pH and t h e n a t u r e o f t h e species i) and t h e

pendent on t h e temperature,

standard f r e e energy o f adsorption f o r t h e i o n i, r e s p e c t i v e l y . Assuming l a t e r a l

i n t e r a c t i o n s between t h e adsorbed species we may

d e r i v e [ 9 ] t h e "Stern-Langmui r-Fowl e r " equation

where

E

i s t h e energy o f t h e l a t e r a l i n t e r a c t i o n s ,

i s given by eqn (4):

I

K

=

Ii[(ai/55.5)exp(-ZiFUlg

(4)

/RT-AGocsJRT)],

where Z i and Wg r e p r e s e n t t h e charge o f t h e ith k i n d o f t h e i o n s t o be adsorbed and t h e p o t e n t i a l

at

IHP,

respectively.

Equations

(1) and ( 3 )

describe a l s o t h e adsorption o f one k i n d o f species b u t i n t h i s case simpler expressions f o r t h e values o f K and

are a v a i l a b l e [ 9 ] .

It was found t h a t i n a l l cases studied, eqn(3) described b e t t e r t h e experimental r e s u l t s as compared t o eqn(1). This i n d i c a t e s t h a t l a t e r a l i n t e r a c t i o n s e x i s t between t h e adsorbed species. However, t h e magnitude o f these i n t e r a c t i o n s depends on t h e k i n d o f t h e support and t h e ions adsorbed (Table 1). With regard t o t h e support, i t may be observed t h a t t h e "support-

183

adsorbed species interactions", estimated by the value of t, are stronger in the case of TiO,. This justifies the relatively weaker lateral interactions observed with this carrier in the case o f adsorption of the molybdates. Concerning the adsorbate phase, it may be suggested that the lateral interactions between the Co2+ or Ni2' ions are negligible, in comparison with those observed for MoxO:- and WxO:in which an S type of isotherm was obtained. TABLE 1 Values of the lateral interactions energy (E) and the adsorption constant (K) determined for different catalytic systems at temperature 25T. No

Catalytic System

PH

E / KJ.mol-'

4.6

2.2 10.5 6.8

-K / mol-'.dm3

~

1 2 3 4 5

MoxOyz-/ TiO, MoXOt-/ y-Al,O, WxOt-/ y-Al,O, CO'+ / y-Al,O, Ni2' / y-Al,O,

5.0 5.1 4.5 4.5

. 4

+

0.0 0.0

2392 116 2610 242 385

REFERENCES 1. L.Wang and W.K.Hal1, J.Catal., 77(1982)232. 2. S.Kasztelan, J.Grimblot, J.P.Bonnelle, E.Payen, H.Toulhoat and Y.Jacquin, Applied Catalysis, 7(1983) 91. 3. C.V.Caceres, L.G.Fierro, A.L.Agudo, M.N.Blanko and H.J.Thomas, J.Catal., 95(1985)501. 4. J.A.R. Van Veen, H.De Wit, C.A. Emein and P.A.J.M. Hendriks, J.Catal., 107(1987)579. 5. P.A.J.M.Hendriks and J.A.R. Van Veen, Polyhedron, 5(1985)75. 6. D.S.Kim, Y.Kurusu, I.E.Wachs, F.D. Hardcastle and K.Segawa, J.Catal., 120(1989)325. 7. K.Y.S. Ng and E.Gulari, J.Catal., 92(1985) 340. 8. J.P.Brunelle, Pure Appl.Chem., 50(1978)1211. 9. N.Spanos, L.Vordonis, Ch.Kordulis and A. Lycourghiotis, J.Catal., in press. lO.N.Spanos, L.Vordonis, Ch.Kordulis, P.Koutsoukos and A.Lycourgiotis, J.Cata1, in press. 11.L.Karakonstadi s, Ch .Kordul is and A. Lycourghiot i s, in preparation. lE.N.Spanos, Ch.Kordulis and A.Lycourghiotis, in preparation. lS.N.Spanos, L.Vordonis, Ch.Kordu1 is, P.Koutsoukos and A. Lycourghiotis, in preparation. 14.L.Vordonis, P.G.Koutsoukos and A.Lycourghiotis, J.Catal., 98(1986)296. 15. L. Vordoni s , P.G.Koutsoukos and A. Lycourghiot i s , J. Catal ., 101 ( 1986) 186. 16.K.Akratopoulou, L.Vordonis and A.Lycourghiotis, J.Chem.Soc. Faraday Trans I . 82(1986)3697.

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17.K.Akratopoulou, L.Vordonis and A.Lycourghiotis, J.Catal., 109(1988)41. 18.P.H.Wiersema, A.L.Loeb and J.Th.G.Overbeek, J. Colloid Interface Sci., 22(1966)78. 19.C.P.Huang and W.J.Stumm, J.Colloid Interf.Sci., 43(1973)409. 20.K.Ch.Akratopoulou, Ch.Kordulis and A.Lycourghiotis, submitted. El.J.Lyklema, in "Adsorption from solution at the solid/liquid Interface" G.P.Parfitt and C.H.Rochester eds, Acad.Press, London, 1983 ch.5. EE.C.H.Giles, D.Smith and A.Huitson, J.Colloid Interface Sci., 47(1974)755.

G . Poncelet,P.A.Jacobs,P.Grange and B. Delmon (Editors),Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands

185

SCALING DOWN OF THE CALCINATION PROCESS FOR INDUSTRIAL CATALYST MANUFACTURING G. GROEN' , J . FERMENT',

M.J . GROENEVELD' , J . DECLEER2 and A. DELVA2

lKONINKLIJKE/SHELL-LABORATORIUM, AMSTERDAM (Shell Research B.V. ) Badhuisweg 3 , 1031 CM Amsterdam, The Netherlands 2SHELL GHENT - CATALYST PRODUCT TECHNOLOGY, Passagierstraat 100, 9000 Gent, Belgium

SUMMARY A computer model for the calculation of the temperature/time profile and the composition of the gas atmosphere in rotary kilns is described. The model is applied for scaling down the continuous commercial calcination of catalyst materials to batch calcination in laboratory rotary kilns as used in catalyst development work. The results of the model are compared with measurements carried out in the rotary kiln of the catalyst plant at Ghent, which produces y-alumina extrudates used as carrier for heterogeneous catalysts. The plug flow transport model assumed for the solids is confirmed by residence time distribution measurements. The measured temperature profiles are in agreement with the calculated profiles after adjustment of the kinetic rate constants. INTRODUCTION Industrial catalyst manufacturing involves several process steps such as preparation and mixing of solutions or suspensions, crystallization, filtration, washing, drying, mixing and kneading o f powders, shaping, drying, calcination and impregnation. Catalyst recipes to be developed in the laboratory must be properly translatable to an industrial processing scheme, taking into account the equipment and instrumentation of an existing catalyst plant. Hence the translation problem is merely that o f scaling down rather than that of scaling up: in view of the possible commercialization of a catalyst recipe each process step in the laboratory needs to be carried out in a way representative of the commercial production. Tests on small-scale process steps are representative if the process conditions and the scale applied are such that the target product properties are similar to those to be obtained in the commercial process step. Representative equipment and methods should already be used in the generally small-scale experiments in the initial phase of the laboratory recipe development, but often a larger scale is necessary. Muffle furnaces might be used in the laboratory, but it should be realized that these furnaces are usually not representative of commercial calcination in rotary kilns. Computer models simulating the performance of commercial equipment are therefore applied to an increasing extent to assist in scaling. This paper will discuss a computer model for the calcination of catalyst materials in rotary kilns. The pur-

186

pose of such a model is, among other things, to calculate the temperature/time profile and gas composition in a commercial rotary kiln in order to be able to apply the same conditions (preferably batchwise) in a laboratory rotary kiln at the smallest conceivable scale (e.g. 50 ml catalyst). The model can also be used for translation from one type or size of commercial rotary kiln to another and for optimization of existing calcination practices. Also in the manufacturing of special catalysts (e.g. based on zeolites), calcination might be the critical factor for the performance of the final catalyst. Representative calcination experiments will therefore remain essential for successful commercialization. The scaling down of the calcination process for industrial catalyst manufacturing requires knowledge of both the processing characteristics of the commercial rotary kiln and, for each different catalyst material, the physical and chemical processes taking place during the calcination. In this paper the elements of the model will be described in more detail and the problems of its validation discussed. It should be realized that the model is still in the development phase. Therefore, the most important heat and mass transfer phenomena occurring in a rotary kiln must be described properly first. A description of the development of important catalytic properties such as surface chemistry, crystallinity, pore structure and metal dispersion is still beyond the scope of the present model. DESCRIPTION OF ROTARY KILNS Calcination can be carried out in various types of process equipment such as fixed beds, moving beds, fluid beds, tunnel kilns, moving belts and rotary kilns. Because of their versatility rotary kilns are widely used in the catalyst manufacturing industry both for calcination of zeolite powders and for calcination of shaped carriers and catalysts. Other types of equipment (e.g. moving belts) are used less frequently. Only the use of rotary kilns will be discussed. Various types of rotary kilns are found in commercial catalyst plants:

(1) Directly gas-fired rotary kilns (Fig. 1); (2) Indirectly gas-fired rotary kilns e.g. for the production of powders or in cases where combustion flue gases are harmful to the catalyst material ; ( 3 ) Electrically heated rotary kilns for small-scale production or for use

in the laboratory; Rotary kilns are often equipped with internals such as longitudinal strips on the wall to prevent slipping of the solids bed over the tube wall and with solids flow restrictions such as dams and slotted diaphragms (Fig. 2). Flights for raining down of particles through the gas phase, as used in rotary driers

187 are not commonly used for calcination of catalyst materials. Commercial-scale rotary kilns are as a rule continuously operated. Rotary kilns in the laboratory might be operated batchwise or continuously and are usually indirectly heated.

EXTRUDATES F R O M S I E V E S

-

SOLIDS FEED L I N E

-

PROPANE PRIMARY AIR SECONDARY AIR

#

EXTRUDATES TO HOPPER

Fig. 1. Rotary kiln o f the alumina catalyst plant at Ghent.

SOLIDS FLOW

TYPE

I

Fig. 2. Slotted diaphragms used in rotary kilns

TYPE

II

188 DESCRIPTION OF COMPUTER MODEL The following elements are essential in any computer model of rotary kiln calcination:

(1) A description of the transport of solids and gas through the rotary kiln;

(2) A description of the heat-transfer processes taking place between solids, gas and walls, including heat losses from the outside walls of the kiln to the surroundings; ( 3 ) A description of the physical and chemical conversions with their heat

effects, including mass transfer between the solids and bulk gas phases. The first two items are common elements for rotary kiln models and are discussed in many publications. A brief summary will be given and a few areas where uncertainties still persist will be indicated. The third item varies with the material to be calcined. A few general outlines will be given here. More details will be presented in subsequent sections. Gas and solids transuort Gas and solids are both assumed to flow in plug flow from one end of the kiln to the other, either cocurrently or countercurrently. Hence, neither of the two phases is assumed to mix in axial direction. For the gas phase this is justified because the gas flow in commercial roc:ry kilns is usually turbulent and the length-to-diameter ratio of rotary kilns is usually large (i.e. larger than 10). For these reasons the bulk gas phase is assumed to be completely mixed in a cross section of the kiln (except for thin film layers adjacent to the tube wall and the solids bed surface). The transport mechanism of the solids depends on the surface roughness o f the inside wall, the inside diameter of the kiln, the solids properties and the operating conditions of the kiln. Henein et al. [l] distinguish slipping, slumping, rolling, cascading, cataracting and centrifuging beds. The latter two are not relevant for catalyst calcination. Existence regions have been experimentally determined as a function of fill percentage (bed depth) and the Froude number (rotation speed) and have been founded with theoretical arguments [l]. The present model assumes a rolling bed, which has been confirmed for the rotary kiln of the catalyst plant at Ghent. The rolling bed is characterized by two regions (Fig. 3 ) : a thin layer of sliding or rolling particles at the bed surface, and the bulk part of the bed, in which the particles are stagnant with respect to the rotating tube wall. Differences in axial velocity between particles travelling only in the middle of the bed (i.e. small particles) and particles travelling only on the periphery of the bed (i.e. large particles) are averaged out for particles following random paths in the solids bed (see right-hand side of Fig. 3 ) . Plug flow is therefore justified for

189

non-segregating beds of particles and this has also been experimentally confirmed, as shown in a subsequent section.

SOL FE

I t

CROSS SECTION OF T H E KILN

CROSS SECTION A-A (VIEW OF BED SURFACE) INSIDE THE RED -SLIDING/ROLLING OVER THE BED SURFACE

Fig. 3 . Particle trajectories in a rolling bed. In a cross section of the kiln, the rolling type of solids bed is assumed to be well mixed for heating up calculations and for kinetic calculations of relatively slow chemical or physical conversions in spite of the absence of relative motion in the stagnant part of the bed. This is justified because the Fourier time of a solids bed in commercial kilns is in the order of hours, while the cyclus time of a particle is in the order of seconds. Hence, bed heating occurs mainly by the continuous replacement of particles in the bed rather than by conduction. For a rolling bed, Saeman [2] has derived the following formula for the rate of volumetric solids transport:

dh dz is the slope of the bed surface with respect to the kiln axis, which makes

where qh

=

Eq. (1) an ordinary differential equation with as boundary condition:

h = hL at z = L

(3)

where hL is slightly larger than zero for rotary kilns without a flow constriction at the solids discharge end of the kiln or slightly larger than the dam height for kilns with a dam at the solids discharge end. Eq. (1) can easily be solved numerically taking into account possible bed volume changes as a consequence of chemical or physical conversions. For kilns with dams at several intermediate locations, the integration can be carried out section by section in

190

upstream direction. For kilns with slotted diaphragms the solution procedure involves the evaluation of the solids flow characteristics of the slotted diaphragms as a function of the operating conditions: qSD

VSDN ( a + f(Ah/h,,Fr,B-d))

=

(4)

where the first term in braces a represents the "pumping" action of the slotted diaphragm and the second term the "levelling out" action as a function of the bed depth difference over the diaphragm, the Froude number and the dynamic angle of repose of the material corrected for the slope of the kiln. The solids flow characteristics have to be determined experimentally by testing of a (preferably full-scale) model of the slotted diaphragm or have to be derived from the overall solids hold-up characteristics of the kiln under various conditions. For rotary kilns without solids flow constrictions and with a large length-to-bed depth ratio (shallow beds) Eq. (1) can be considerably simplified. Saeman [ 2 ] has derived for that case the following equation:

He validated his model with experimental data of Sullivan et al.[3]. Description of heat transfer The heat transfer in the kiln is described in terms of enthalpy flows of the solids and gas phases and in terms of heat losses to the environment:

a'_d _ dz - 'loss 'st

=

'loss

'gs =

+

' g u

"us

- 'us

-

' s b

where f

=

+1 for flow in the positive z direction and

E

=

-1 for flow in the negative z direction,

T,

=

Ts for gas species produced in the solids bed and

T,

=

T

g

for gas species consumed by the s o l i d s bed.

g and 0, are the enthalpy flows of the gas and the solids, respectively and is the heat lost to the surroundings of the kiln. The heat fluxes on the right-

P,