Zeolite Chemistry and Catalysis: Proceedings of an International Symposium, Prague, Czechoslovakia, September 8-13, 1991 (Studies in Surface Science and Catalysis)

Studies in Surface Science and Catalysis 69

ZEOLITE CHEMISTRY AND CATALYSIS

This Page Intentionally Left Blank

Studies in Surface Science and Catalysis Advisory Editors: B. Delmon and J.T. Yates Vol. 69

ZEOLITE CHEMISTRY AND CATALYSIS Proceedings of an International Symposium, Prague, Czechoslovakia, September 8-1 3 , 1 9 9 1

Editors P.A. Jacobs Laboratorium voor Oppervlaktechemie, Katholieke Universiteit Leuven, Kardinaal Mercierlaan 92, 8-3030 Leuven (Heverlee), Belgium

N.I. Jaeger Universitat Bremen, Forschungsgruppe Ange wandte Katalyse, Postfach 330440, D-2800 Bremen, Germany and

L. Kubelkova and B. Wichterlova J. He yrovskp Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, DolejSkova 3, 182 23 Prague 8, Czechoslovakia

ELSEVIER

Amsterdam - Oxford - N e w York -Tokyo

19 9 1

ELSEVIER SCIENCE PUBLISHERS B.V. Sara Burgerhartstraat25 P.O. Box 21 1, 1000 AE Amsterdam, The Netherlands Distributors for the United States and Canada:

ELSEVIER SCIENCE PUBLISHING COMPANY INC. 655, Avenue of the Americas New York, NY 10010, U.S.A.

ISBN 0-444-88245-6

0 Elsevier Science Publishers B.V.. 1991 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science Publishers B.V./ Academic Publishing Division, P.O. Box 330, 1000 AH Amsterdam, The Netherlands. Special regulations for readers in the USA -This publication has been registered with the Copyright Clearance Center Inc. (CCC), Salem, Massachusetts. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the USA. All other copyright questions, including photocopying outside of the USA, should be referred to the publisher. No responsibility is assumed by the Publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein.

Although all advertising material is expected to conform to ethical (medical) standards, inclusion in this publication does not constitute a guarantee or endorsement of the quality or value of such product or of the claims made of it by its manufacturer. This book is printed on acid-free paper. Printed in The Netherlands

CONTENTS Preface Acknowledgements Organizing and Scientific Committee Financial Support Hydrocarbon Transformations over Analogues and Derivatives of Zeolite Y (plenary lecture) Dwyer J . , Dewing J . , Karim K., Holmes S., Ojo A.F., Garforth A.A., Rawlence D.J.

XI XI I XI11 XIV 1

Isomorphous Substitution in Zeolitic Frameworks: Procedures and Characterization (plenary lecture) Vedrine J. C.

25

Introduction of Cations into Zeolites by Solid-state Reaction (plenary lecture) Karge H . G . , Beyer H.K.

43

Zeolite-hosted Metals and Semiconductors as Advanced Materials (plenary lecture) Schulz-Ekloff G.

65

Isomorphous Substitution in Zeolites: a Route for the Preparation of Novel Catalysts (plenary lecture) Bellussi G . , Fattore V.

79

Zeolite Synthesis with Metal Chelate Complexes Balkus Jr. K.J . , Kowalak S., Ly K.T., Hargis D.C.

93

Synthesis of Ferrous Cyanide Complexes inside Zeolite Y Bresinska I., Drago R.S.

101

Genesis of Gallosilicates with ZSM-5 Structure. Insertion of Ga and Zeolitic Properties at Various Steps of Crystallization Kosslick H., Richter M., Tuan V.A., Parlitz B , Szulzewsky K., Fricke R.

109

Studies on the Phosphorus Substituted Zeolites Prepared by Secondary Synthesis Reschetilowski W . , Einicke W.-D., Meier B., Brunner E.. Ernst H

119

Synthesis of Zeolite Beta in Boron-Aluminium Media Derewinski M., Di Renzo F. ,Espiau P., Fajula F . , Nicolle M. -A.

127

VI On the Possibility of Generation of Brrensted Acidity by Silicon Incorporation in Very Large Pore Alp04 Molecular Sieves Martens J . A . , Balakrishnan I . , Grobet P.J . , Jacobs P.A.

135

Crystallization of Porous Alurninophosphates and Metal Substitutions Lechert H . , Weyda H., Hess M . , Kleinworth R., Penchev V . , Minchev Ch.

145

Factors Affecting the Crystallization of Zeolite ZSM-48 Giordano G., Dewaele N., Gabelica Z., Nagy J.B.,Nastro A , , Aiello R . , Derouane E.G.

157

Synthesis and Characterization of Cr-modified Silicalite-1 Cornaro U . , J i r g P., Tvarfiikova Z.,Habersberger K.

165

Synthesis, Characterization and Catalytic Activity of V-ZSM-5 Zeolites Fejes P., Marsi I . , Kirisci I . , Halasz J., Hannus I . , Rockenbauer A , , Tasi Gy., Korecz L . , Schoebel Gy.

173

A Study of Acid Sites in Substituted AlPO-5 Gorte R.J . , Kokotailo G.T . , Biaglow A. I., Parrillo D., Pereira C.

181

Structure and Photocorrosion of NaX Hosted Q-Size Metal Sulfide Particles Wark M . , Schulz-Ekloff C . , Jaeger N . I . , Zukal A .

189

Faujasite-Hosted Methylene Blue: Synthesis, Optical Spectra and Spectral Hole Burning Hoppe R., Schulz-Ekloff G., Woehrle D., Ehrl M., Brauchle C.

199

Preparation and Characterization of Zinc-ZSM-5 Catalyst Liang J . , Tang W., Ying M.-L., Zhao S.-Q.,Xu B.-Q., Li H.-Y.

207

The Formation of Well Defined Surface Carbonyls of

215

Ru and Ir with Highly Dealurninated Zeolite Y as

Matrix Burkhardt I . , Gutschick D., Landmesser H., Miessner H.

VII A Comparative Study of State and Reactivity of Copper Ions Embedded in Various Molecular Sieve Materials Wendlandt K.-P., Vogt F., Moerke W., Achkar I.

223

Acidity, Redox Behaviour and Stability of CoAPO Molecular Sieves of Structure Types 5, 1 1 , 34 and 16 Kraushaar-Czarnetzki B. , Hoogervorst W.G.M. , Andrea R.R., Erneis C.A., Stork W.H.J.

231

State of Iron and Catalytic Properties of AkaliMetal-Exchanged Ferrisilicate Zeolite Molecular Sieves Kan Q., Wu Z.,Xu R., Wei Q., Peng S., Xiong G., Sheng S., Huang J.

241

Framework and Extraframework Ti in TitaniumSilicalite: Investigation by Means of Physical Methods Zecchina A , , Spoto G . , Bordiga S . , Ferrero A , , Petrini G . , Leofanti G . , Padovan M.

25 I

Studies on the State of Copper and the Formation of Its Oxidic and Metallic Phases in Zeolite CuNaY Piffer R., Hagelstein M., Cunis S., Rabe P., Foerster H . , Niemann W.

259

ESCA Study of Incorporation of Copper into Y Zeolite Jirka I., Wichterlova B., Mary5ka M.

269

Preparation of Ga-Doped Zeolite Catalysts via Hydrogen Induced Solid-state Interaction between Ga 0 and HZSM-5 Zeolite

277

2 3

Kanazirev V . , Price G.L., Dooley K.M. Comparison of Kydrosulfurization Zeolite Catalysts Prepared in Different Ways Onyestyak Gy., Ka116 D . ,Papp J.,Jr.

287

Effect of the Introduction of Ni(I1) on the Catalytic Properties of SAPO-5 Molecular Sieves Mavrodinova v., Neinska Ya., Minchev Ch., Lechert H., Minkov V., Valtchev V . , Penchev V.

295

Study of Broensted and Lewis Acid Sites in Phosphates, Silicates and Silica Gels with Molecular Sieve Properties Kustov L.M., Zubkov S.A , , Kazansky V. B., Bondar L.A

303

VIII Influence of Framework Phosphorus on the Acidic Properties of Faujasite Type Zeolite Briend M., Lamy A., Dzwigaj S., Barthomeuf D.

313

Zn-Doped HZSM5 Catalysts for Propane Aromatization Guisnet M., Gnep N.S.,Vasques H . , RamBa Ribeiro F.

32 1

Sulfided Ni-Mo-Y Zeolites as Catalysts for Hydrogenation and Hydrodesulfurization Reactions Laniecki M., Zmierczak W.

331

Reduction of SO on Molybdenum Loaded Y Zeolite Soria J. , Gonzafez-Elipe A. R. , Conesa J.C.

339

Contribution of Metal Cations to the Para-Selectivity of Small Crystals of H-ZSM-5 Zeolite in Toluene Alkylation with Ethylene Cejka J . , Wichterlova B., Krtil J . , Ki-ivanek M., Fricke R.

347

NO Decomposition on Cu-Incorporated A-Zeolites under the Reaction Condition of Excess Oxygen with a Small Amount of Hydrocarbons Inui T., Kojo S., Shibata M., Yoshida T., Iwamoto S.

355

A

Comparison of the Catalytic Properties of SAPO-37 and HY Zeolite in the Cracking of n-Heptane and 2,2,4-Trimethylpentane Lopes J.M., Lemos F., RamBa Ribeiro F., Derouane E.G.

365

Cracking of Light Alkanes over MeAPO-5 Molecular Sieves Meusinger J . , Vinek H. , Dworeckow G . , Goepper M. , Lercher J.A.

373

Promoting Effect of Pt Supported on Galliumsilicate in n-C4HI0 Aromatization Dmitriev R.V., Shevchenko D.P., Shpiro E . S . , Dergachev A. A. , Tkachenko 0.P. , Minachev Kh.M

381

Conversion of Ally1 Alcohol to Oxygenated Products over Zeolite Catalysts Hutchings G.J . , Lee D.F.,Williams C.D.

389

Cation Exchange Influence on the Activity of Zeolites in Reactions between Alcohols and Hydrogen Sulphide Ziolek M . , Hildebrand-Leksowska K.

397

Possible Intermediates during C3H8 aromatization over Ga-HZSM-5 Catalyst Meriaudeau P., Naccache C.

405

IX Dehydrocyclodimerization of Short Chain Alkanes on Ga/ZSM-5 and Ga/beta Zeolites Corma A., Goberna C., Lopez Nieto J.M., Paredes N., Perez M.

409

Bifunctional Cobalt-ZSM-5 Catalyst for the Synthesis of Hydrocarbons from the Products of Biomass Gasification Krylova A , , Lapidus A., Rathousky J . , Zukal A , , JanCalkova M.

417

Shape Selective Reforming: Possible Reaction Pathways on Platinum-Containing Erionite/Alumina Catalysts Kalies H., Roessner F., Karge H . G . ,Steinberg K. -H.

425

Framework Ordering2+n Aluminophosphate Molecular Sieves Studied by A1 Double Rotation NMR Chmelka B.F., Wu Y . , Jelinek R . , Davis M.E., Pines A.

435

A Computer Analysis of ESR Powder Spectra of Silver and Sodium Clusters in Molecular Sieves Uytterhoeven M. G., Schoonheydt R.A.

443

Magic-Angle-Spinning Nuclear Magnetic Resonance and Infrared Studies on Modified Zeolites Brunner E., Freude D., Hunger M., Pfeifer H., Staudte B

453

129Xe NMR Study of Intra- and Inter-Crystallite Diffusion of Cations in Faujasite Zeo 1i tes Fraissard J . , Gedeon A , , Chen Q . , Ito T.

461

Intracrystalline Diffusion of Benzene in Ga-Silicate Zikanova A,, Struve P., Buelow M., Wallau M., KoCiiik M., Micke A , , Tissler A , , Unger K.K.

469

Intracrystalline Diffusivities of HZSM-5 Zeolites Hashimoto K., Masuda T., Murakami N.

477

New Porous Materials from Layered Compounds (plenary lecture) Clearfield A . , Kuchenmeister M . , Wang J . , Wade K.

485

Author Index

499

Subject Index

505

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

511

This Page Intentionally Left Blank

XI

Preface

The International Symposium entitled "ZEOLITE CHEMISTRY AND CATALYSIS", held in Prague from September 8 to 1 3 , 1991 and elected by the International Zeolite Association as a local IZA Symposium, is one of a series of European Symposia which has been organized during the past decade. As the field of zeolite science is continually growing, each of the previous locally organized European Zeolite Symposia (Szeged, Villeurbanne, Bremen, Prague, Siofok, Nieuwpoort, Wurzburg, Leipzig) has focussed on a particular area in zeolite science and technology. The present Symposium emphasizes the effect of modifying components on the structure and reactivity o f molecular sieves. The plenary lectures and contributed papers concentrate

on the problem of isomorphous substitution in a zeolitic framework; on the occlusion and the structure of metal, metal oxide, and metal sulfide clusters and complexes in the intracrystalline void volume of molecular sieves and zeolites as well as in the interlaminar space of layered compounds. Attention has been paid to synthesis, structural characterization and the mobility of charged encapsulates in such phases or their mixtures. Moreover, not only has the impact of such modifications on catalysis been examined, but also the use of such materials as active components in photo or chemical sensors. New developments are to be expected from the recent growth of knowledge in traditional areas of zeolite applications and from the recent progress which has been made with such systems in material science. The use of zeolitic materials as hosts for specific chemical entities and their application in SUPRAMOLECULAR chemistry and catalysis looks particularly promising.

XI1 We expect that the Prague 1991 Symposium and its Proceedings will become a milestone in this evolution and will stimulate not only the use of molecular

sieves in new research areas but also in applications involving new and sophisticated experimental and theoretical methods. The Peer review system to which all the contributed papers were subjected, guaranteed the high quality of the zeolite science papers in the present volume.

Prague, June 1991

Peter A . Jacobs Nils I. Jaeger Ludmila Kubelkova BlankaWichterlova

Acknowledgements The Organizing Committee of the International Symposium on "Zeolite Chemistry and Catalysis" held in Prague from September 8 to 13, 1991 highly appreciated the efforts of all the participants who contributed to the Scientific Program of the Symposium and presented their results in plenary lectures, contributed papers and in recent research reports. We thank all the organizations and companies which sponsored this Symposium, thereby enabling, especially, our younger colleagues to participate. The work of the Scientific Committee in accomplishing the difficult task of selecting the contributed papers deserves a special mention. The Editors would also like to thank the authors for their careful preparation of the camera ready manuscripts and the reviewers who conscientiously evaluatedthese papers in short time.

XIII Organizing Committee

R. Zahradnik (chairman) J. V. BosaEek J . Cejka J. K. Habersberger J. I. Jirka L. P. JirPl J.

Kapieka

Z . Tvarfiikova

Koubek

B, Wichterlovh

Krtil

N. iilkova

Kubelkova Novakova

Scientific Committee

H. van Bekkum (Delft University, The Netherlands) H.K. Beyer (Academy of Sciences, Budapest, Hungary) A.C. Corma (Institute of Chemical Technology, Valencia, Spain)

D. Barthomeuf (University P.& M. Curie, Paris, France) J. Dwyer (UMIST, Manchester, Great Britain) G.J.

Hutchings (Liverpool University, Great Britain)

P.A . Jacobs (Catholic University, Leuven, Belgium) N.I. Jaeger (Bremen University, Germany)

H G. Karge (Fritz-Haber-Institute,Berlin, Germany) A. Kiss (Degussa AG, Hanau, Germany) J.A . Lercher (Technical University, Vienna, Austria)

W. Mortier (Exxon Chemical Holland B.V.,Rotterdam, The Netherlands) G. Schulz-Ekloff (Bremen University, Germany) A . A. Slinkin (Academy of Sciences, Moscow, USSR) D. E.W .

Vaughan (Exxon Research and Engineering Co. , Annandale, USA)

J . C . Vedrine (Research Institute for Catalysis, Vil eurbanne, France

J. Vblter (Central Institute of Physical Chemistry, Berlin, Germany)

XIV Financial support

BP International Ltd., Sunbury on Thames, Great Britain Chemical Works, Litvinov, CSFR Czechoslovak Academy of Sciences, Prague, CSFR Degussa, AG, Frankfurt, FRG

Dow Benelux, Terneuzen, The Netherlands Eniricerche S.p. A . Milano, Italy Exxon Chemical Holland B. V., Rotterdam, The Netherlands Grace GmbH, Worms, FRG International Zeolite Association (IZA) Sudchemie AG, Munich, FRG

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 1991 Elsevier Science Publishers B.V., Amsterdam

J D q e r , J Dew-, D J Rawlence (a)

1

K Karhn, S Holmes, A F eo, A A Garforth and

UMIST Chemistry Saclrville Street, MANcHEsTER M60 lQD, UK

(a)Crosfield cfieshire, UK

Catalysts,

4

Liverpool Road,

WARRINGION WA5

lAB,

zeolites of type Y are prepared by either primary or secondary synthesis. S t r u a e s include zeolite Y in both the cubic and hexagonal forms, SAFC-37 and faujasitic frameworks Containing Ga or Zn. These materials are characterised using solid state NMR, X-ray powder diffraction, infrared spectroscopy, surface analysis and sorption. Catalysts are then evaluated for the conversion of n-hexane, cyclohexane andgas-oil. Resultsare interpretedin terms of the effectiveness of catalytic sites in alkane activation and in the effect of both density and distribution of active sites.

lTmamaTm The extensive use of zeolite Y as an acid catalyst for hydrccarbn conversion has had a mjor impad on the petroleum refining industry. Zeolite Y has the faujasite f m w o r k structure (1) consisting of SOddLite u n i t s ( p cages) linked through rings of six tetrahedra, via OF-, to generate layers of scdalite units linked by double-six r T s (hexagonal prisms). The layers are also linked by hexagonal prisms in an ABc seqgence to generate a tetrahdral array of scdalite u n i t s having cubic symmetry and the same space group (Fd3m) as diamond. In fact the structure of zeolite Y is readily derived from that of diamond if carbon atcnns are replaced by scdalite units and C-C bonds by double-six rings. This arrangement of sodalite units generates wide pores of 12 linked tetrahedra, with free diameter 7 - 8A, which provide entrances into larger supercages of 11 - 12 A diameter. The supercages and linked tetrahedrally viathe 12 rings to form an openthree dimensional pore system (Fig 1). analogue of zeolite Y, sconetimes called hexagonal Y, has also been synthesised recently (2). In this structm layers of linked scdalite units are linked in the sequence ABAB, by mtatirg every second layer, to produce hexagonal syrmnetry (Fig 1). These framework structures represent end members with other intermediate structures for example ZSM-20 ( 2 ) represented by varying stacking sequences. In the hexagonal framework there are five, 12-ring openings in each supercage, two of which (in the 001 direction) are planar, the other three being elliptical (Fig 1).

An

2

the last decade or so heteratom have been intrcduced into the faujasitic framework, and a phosphate-based material (SARI - 37) has also been synthesised with the faujasitic framework (3).

rxlring

?Lpically, zeolite Y is synthesis&, in aqueous alkaline media, with a framework ccanposition Si/Al I 3 and in order t o enhance thermal/hydrothennal stability and t o modify catdlytic function, postsynthesis modifications are employed. Typical post-synthesis procedures for siliceous zeolite Y involve hydrothermal treatment ( 4 ) or dealmination by chemical methcds. chemical dealunination can u t i l i s e ccanplexing agents such as EDTA (5) which remove a l m i n i m w i t h very limited replacement of silicon in the vacated site, or they u t i l i s e secondary synthesis prooeCtures i n which a second source of s i l i c o n is available for "healingll framaJork vacancies prcduced by extraction of aluminium. Secondary synthesis may involve gas/solid reactions for example reaction of zeolites w i t h Sic14 (6) or solid/ solution reactions for exanple reaction of zeolites and aqueous (MIq)2SiF6 ( 7 ) . Both of these approaches canbe usedto h q m r a t e heteroatms into zeolite frameworks (8) ( 9 ) . The present lecture describes recent work by the authors, on siliceous forms of zeolite Y (cubic and hexagonal), SAFO-37, and gallated Y zeolites. The materials used are well-characterised and are evaluated

catalytically using the mnversion of n-hexane, cyclohexene and gas-oil t o camment on (i) site activity (ii) site density (iii) site distribution and (iv) the role of gallium i n FCC catalysts.

cl~emically s t a b i l i s e d y zeolites (CSY) w e r e prepared by reacting a (Si/Al = 2.5) a t 70 C and buffered a t pH = 6.5 w i t h an aqueous solution of (MIq)2Ss6 ( 9 ) . Heteroatm Y zeolites w e r e prepared i n a similar way but using fluorides of gallium and zinc in place of silicon hexafluoride (12). A semi-batch reactor was used and in a l l cases products w e r e washed free of fluoride.

slurry of W Y

zeolite Y w a s also synthesiseddirectly i n b o t h c u b i c and hexagonal forms (2) using the templated aqueous fluoride system and the intergrowth zm-20 (10) w a s dlso synthesis&. SAFO-37 was

siliceous

synthesised using published procedures (3).

olaraderisation procedures The XRD data w e r e obtained using an XIlS 2000 SCINTAG diffractcm&er (Cu Kcr radiation) over a range of 2 - 60" 2 Theta. The unit cell dimensions were determined using silicon as internal standard fo1lmh-g AS'IM methcds. Nitrcgen sorption i s o t h m and surface areas w e r e

3

~ l g u r e1 ,

770‘

J 10

I

‘243

20 30 0 10 NUflBER OF FRAflEWDRK ALUMINIUM SUBSTITUTED BY m

20

Substitution of m (m: Si,Ga.Zn) intu the faujasitic framework a Changes in frequency of symmetric stretch IIR) b Changes in unit cell parameters (XRDI

Figure 2 .

A 158 316 L13 631

‘

FLUENCE pA mrn Icrn’l 7 9 0 941 11001260

$161

Figure 3.

Hexagonal faqarih framework

Cubic faujarite framevork

i8

2

.

Depth profiling of faujasitic zeolites B [Ga/AIIY2 prepared by fluorogallate ACUB-Y prepared using crown ether template NaY ex crusfield catalysts

A 0CS-Y prepared by reaction of NH,-Y and INHI, SiF,

30

4

deterrmned ' using a Micromeretics ASAP 2400 porosimeter at Crosfield Catalysts. ’IheIRspectroscopic studywasperformed usinga Mattson Cysnus 100 FTZR spectrcweter. Framework-region absorptions were recorded using JBr discs while the hydroxyl spectra were obtained using self-supporting zeolite wafers activated at 400 C/10’5 torr/4 hours. 29Si, 27Al and 7 k a Solid S t a t e NMR spedra were recorded at 59.6, 78.2 and 91.4 MHZ respectively, using a Varian VXR 300 niltinuclear spectrameter at theuniversity of Ilxham. Detailed procectures are described elsewhere (11). The surface ccanpositional depth profiles were obtained using a VG SINS spectmmter. An Ar+ ion beam of 10 W energy and 20 @ current w a s used to etch the surface. catalysis

Catalytic evaluation was made using an intennittent micro-flow reactor (11)(13)(14) a continuous micro-flaw reactor and a conventional MAT unit. catdlysts enployed w e r e either as 100% zeolite or as 25% zeolite bound with silica and formed into a typical FCC matrix. For the latter tests, catalyst were elutriated, preheated at 590 C for 3 hours and fluid bed steam de-activated at 760 C for 5 hours in 100 Cprior to catalyst evaluation.

The substitution of Si for framework Al is well reported (4) (6) (7) arid it is also reported (9) that Fe, Ti, Cr and Sn may be substituted. m e unit cell size changes progressively (12) with substitution of si or ~a and probably Zn for Al (Fig 2). Solid state N M R confirm the substitution of Ga into the framaJork (Fig 4) and IH - 29Si cross polarisation indicates that few silanol defectsare prcdud since relative intersities of 29Si signals are not significantly affected by cross polarisation. ?his is confirmed by FTIR analysis of the hydroxyl region (14). Prcduc3.s are highly crystalline (XRD) with good sorption capacity (Table 1). kpth profiling using sm shows that camposition gradients are not extmae at the levels of substitution considered but as substitution of either Si or Ga inrreases there is enri-t of the substituting a m in layers closer to the surface (Fig 3 ) suggesting some diffusion limitation (shrinkixq core model) at higher levels of substitution.

5

’T

F w p e 4.

'9S~-'H (crosspolansation)

Sold state MASNMR spectra for H-lGa/AII-YI Zeolite Figure 5 29S1 MASNMR spectra of (a) CU0.V (SllA1=38

( b ) ZSM-20 ( S I A1;3

Silicon

NMR

So

s1

Parameter S25SL

Si/Al

25 70 5 0 0

2 6 53 21 0 0

12 50 33.5 0

Figure 6 .

3

Fauysite structure Centres of symmetry D6R links

6 ) and (c) CSY (SlIAl

4)

I

6 TABLE 1 Conparison o f Physical Properties of Faujasite-type Z e o l i t e s

Zeolite

Unit C e l l

Sanptes

S i z e (A)

Crystallinity (X )

Si/Al (Bulk)

Si/AI

(NMR)

%/A1

(XRD)*

Surface Area (cm'g-') BET

Langrmir

Micropore Volm

(c2g-l) NH4Y CUB-Y WB-Y ZSM-20 CSY 1 CSY 2 CSY 3 CSY 4 [Gal A l Y 1

[Gal ALY2 [Gal A l Y 3 [Gal ZSM-20 [GalCSY 4 [Zn/All Y

*

24.69 24.62 24.57 24.57 24.40 24.45 24.58 24.50 24.74 24.77 24.79 24.72 24.53 24.73

100 110 115 110 89 92 102 95 109 109 105 110 92 98

2.60 3.20 3.50 3.70 7.35

2.50 3.10 3.80 3.60 7.0

2.33 2.88 3.40 3.40 6.9

6.9 4.37 4.30

5.3 4.70 4.40

5.4 4.40 4.87

2.94 3.58 4.91 4.55 3.1

2.66 2.74 4.36

-

-

860 859 869 894 584 817 820 750 607 792 779 829 776

894 915 945 930 670 848 860 767 820 793 867 790

Gallium or Zinc Oxide (ut%)

0.32 0.33 0.34 0.31 0.28

0.31 0.30

0.32 0.31 0.29 0.27 0.31 0.32

2.01 3.62 5.95 7.95 3.3 2.3

Calculated from Breck-Flanigen equation

a) Activity.

Ccmersicn of n+xxme

Fig (7) shaws a plot of conversion against contact t b (W/F) for the reaction of n-hexane over CSY2 (Si/Al = 5.3) at 400 C. A clear induction period isapparent. R e s u l t s m y b e d e m i b e d b y t w o rate constants, kl at lawer conversion and k2 a t higher conversion. In order to minimise effects dueto reactor geometry, whichmight lead t o spurious results at higher flaw r a t s and lower conversion, results are cbtained using various experimental approaches. Conversion is varied by chanqiq flaw rate of n-haare aver fixed amounts of catalyst and by varying the amount of catalyst in a bed of fixed volune, a t constant f l m rate of feed. In all cases the experimental trials are randcrmised and same trials consist of single-point determinations on fresh c a t a l y s t to rninimise any affects due t o catalyst deactivation. The concordance of results suggests that the pattern evident in Fig (7) does not result frcnn geometric factors nor f r m deactivation. The probable explanation for these and similar results (11) is that the

Figure7.

Rate of n-hexane cracking Over X [Ga/AIIYZ and 0 CSYZ at 400°C.

15

05

25

35

45

55 CMTACT TlME/rec

Figure 8 .

Rate of n-hexane cracking over CSYl zeolite (framework S i / A I d )

( ) 1 ’ -

2425

Flgure 9

24 35

I

ZL 15

i

2455

24 65 UNIT CELL SIZE

2475

1/11

Isomerisation/hydrogen transfer as a function o f Zeolite unit Cell size (1) NHLY OCSY x ZSMZO x CSY-S OSAPO-37(38) REUSY Kheng J Catai ,19891

8

slower i n i t i a l rate largely reflects the activation of hydrmwbon molecules w i t h generation of sorbed active species, presumably carhnium ions or t h e i r precursors w h i c h are then involved i n propagation of the reaction. Tkis view is supported by the effect of adding mall amounts of olefins to the feed, which can enhance the i n i t i a l rate of reaction (14), o r by the presence of h e t e r o a t m w h i c h can pmvide active sites for n-hexane activation. The effect of gallium, w h i c h is discussed subsequently, is also seen in Fig (7). Fig (8) shows the effect of teqxmture on C S Y l (Si/Al= 6.9) in the range 350 C t o 450 C (temperatures less than 350 C can result i n problems associated with sorption (15)). E s t i m a t e s of activation ensuggest that the i n i t i a l rate is associated with a higher activation (Ea - 102 k J mol-I) than the subsequent rate (E - 85 k J mo1-l) hplying that a t higher temperatures the rate of generation of c a r b a t i o n Separate precursors is enhanced relative t o the subsequent reactions. pulse studies (14) using results a t higher conversion (15 - 30%), corresponding to the second stage of reaction gave an activation energy (E,) of (E 85 k J nr0l-l) in agreement w i t h results for k2 from Fig (8). A recent paper (15) reports activation energies a t hiqher conversion, of 88 and 85 k J m1-l for the reaction of n-hexane over zeolite Y (LZY -82; 41 Al/vC) in nitrogen or helium respectively.

-

b)

Selectivity in n-Hexam Coenrersim

The activation of hydmcarbns over zeolites is widely held t o result f m direct protmnation a t C-C or a t C-H bonds (16) (17) as proposed for reaction in superacid media (18) (19). mesent results (14) are exemplified by Fig (11) and Table 2. Frrrm the l i m i t i n g slopes of plots of weight selectivity against conversion (20) the p r d u c t s a t zero conversion may be estimated (Schm 1).

75

c1 + c5 0.05 (0.04)

n

c2 + c2= + c4 0.02 (0.23)

+ c4=

c3 + c3= 0.7 (0.67)

- hexane

H2 4- c6= 0.05 (0.05)

and C, signify olefins and alkanes respectively ard the n W r s refer t o weight selectivities determined from limiting slopes of selectivity/conversion plots. The figures in brackets are weight selectivities calculated by extrapolation of results in Fig (11) t o zero conversion.

Cn=

9 Table 2

Z e o l i t e CsY2 ( S i / A L = 5.3) * Reaction Tenp 400°C Feed: n-Hexane (mole o f Product per 100 moles o f n-hexane converted)

CT U s e c

0.021 8.2 0.21

U/F (ghlmol)

Conversion (%) Hydrogen Methane Ethane Ethene Propane Propene iButane n-Butane tran-2-butene 1-butene iso-butene c is- butene Butene Pentane Pentene i- hexane Hexene Heptane Heptene Aromatics P/O C/H (Total) iC4/nC4 c3/c3= c2/c2= CMR

0.00 11.1 18.2 10.5 59.9 82.0 4.53 5.13 3.68 3.68 1.47 8.86 0.00 2.22 0.00 4.36 0.00 0.0

(w)

0.049 11.19 0.491

0.085 16.34 0.588

0.16 26.07 0.698

0.18 28.92 1.290

0.00 6.377 13.9 7.36

10.00 6.30 13.4 7.38 54 80.5 5 .8 5.6 2.84

7.5 4.5 9.5 6.7 67.2 80.0 6.8

4.90 2.58 10.3 0.16 2.7

9.87 6.05 12.95 7.61 59.63 84.20 6.14 6.16 2.19 0.66 2.85 1.76 7.47 0.96 1.92

8.4

4.7

48.7 78.03 5.0 5.3 3.43 2.49 1.25 7.2 0.0 2.7 0.0 3.2 0.0

0.83 0.43

0.83 0.43

0.837 0.623 1.i3 8.81

0.944 0.62 1.89 5.29

0.78 0.44 1.032 0.66 1 .a1 4.67

0.87 0.43 0.99 0.70 1.70 4.33

0.32 31.472 5.45

1.66 142.40 18.98

2.36 0.99 5.6 5.90 1.86 5.11 3.41 0.17

1.44 0.79 1.68 5.68 69.9 31.5 23.0 7.55 1.24 0.69 2.01 0.89 4.83 10.60 1.35 8.8 1.32 0.26

0.41

2.10

0.97 0.43 1.10 0.83 1.41 3.04

1.41 0.42 1.93 1.24 0.67 1.03

2.74

6.1 1.91 1.19 2.39 1.43 6.95 2.20 1.98 3.58

2.68 1.75 3.86 5.74 72.9 58.63 10.9 5.7 1.40 0.79

0.42 3.07 2.21 0.295 0.35

iC4 Total Mass (%)

99.9

99.9

98.4

99.8

99.8

100

59.9

Total Mass

8389

8392

8266

8386

8390

8405

8395

*

C a l c u l a t e d f r o m NMR

10

Extrapolation produces scam= uncertainty but provides an approximation to the initial prcduct distribution. ’Ihe initial product distribution (Table 3 ) derived from results in Table 2 can be rationalised in t e r m s of a possible reaction network (Fig 10) w h i c h suggests that the overall reaction involves a 90% contribution f m momlecular protolytic attack at C-C andC-H bonds, the reminder involving bklecular secondary processes particularly bblecular hydmgen transfer. Table 3

I n i t i a l ProrLrt Selectivity of n-llexane Cracking

(Moles/lM Moles of

A

-

-

-

_

13

-

11

50

50

-

C

7

-

Total

7

13

20

11

11 10 - 1 4 50 85

EXPT

14

13

20

11

50

B E&D

ovw CSYZ ( S i / A l = 5.3) at

W ' C

% Converted)

-

20

-

-

_

85

-

-

-

10 10

10 10

2

9

11

2

2

-

5

5 5

300 78 180 42 600

1400

64lO

1412

This mechanl’stic network, which is based on accounting and is not proven, Suggests that, at the low conversions observed at 4OO0C, moncanolecular processes d&te the reaction of n-hexane over Y zeolites enriched in silica. As activegaseous andsurface species becaw available at higher conversion, the reaction proceeds more extensively by bhlecular processes producing the induction period seen in Fig (7). This view is supported by results for the CMR values (21) shown in Fig (12) which hcrease very markedly as conversion approaches zero. Estimations of activation energies indicate that, as temperature is increased, mnmlecularprccesses tend todmhate the overall conversion. A recent detailed study of n-hexane cracking at 500 C over zeolite H-Y ( M e SK40), based on extrapolation using theonstream theory (22), reports that primary products are consistent with direct pmtonation at C-C bonds. Frcduct distributions (22) differ frmn t h e reported (Fig 11) here in that no hydrqen, methane, or hexane are observed as primary praducts at 500 C but isohexane is detected ( 2 2 ) . Apart f m differences in activation treatments the discrepancy between results at 500 C (22) and thcse reported here for 400 C may reflect differences in the types of catalyst used since the present results refer to the u s e of siliceous Y zeolites which are much stronger acids than typical Y zeolites. Figure 10 suggests the following order of protolytic crackh-q in n-hexane at 400 C (Table 4 ) , over the cSY2 zeolite.

at bonds

11

17 0

=PRODUCT

L-REACTlON M--NoOF

No MOLES

Figure10,

Schematic of n-hexane converslon over Y type zeolite (Si/A1=53 CSY2 ,Temp 400OC)

t

! % CONVERSION

Figure 11

% CONVERSION

Conversion of n-hexane over CSYZ zeolite (framework Si/AL=53) at 400°C- intermittent flow reactor

12

Table 4

mitial

scissicgl in n-kxam cxackiq over S - Y ( S i w = 5.3) at

400°C.

Bond

wlative Scission

The preference for attack at C-C rather than C-H (other than tertiary Cis reprtd for paraffin reactions in superacid solutions ( 2 3 ) and Mind0/3 calculations ( 2 4 ) support this. Olah ( 2 3 ) gives the order of protolytic attack in superacids at lm temperature as:

H)

(tertc-H)

> C - C >

(secc-H)

>> (prirraryC-H)

Although there is scnne cxnnplication arising frcnn a -1 contribution frum secondary reactions at 4 0 0 ° C it is noted that on H-ZSM-5, a stronger acid than conventional Y zeolite, h y m e n is observed ( 1 6 ) and is attributed to protolytic attack at C-H. Moreover, recent studies on conversion of both n o m l and iso-butane over H-ZSM-5 shm cleavage of C - H bonds at low conversion ( 1 3 ) .

The preference for attack on the centre C - C bond (Table 4 ) is expect& i n linear paraffin cracking ( 2 5 ) . The absence of ism-hexane in our study and its presence in conversion over H-Y ( 2 2 ) is more difficult to explain. In the present work, amounts of ism-hexane at low conversion were never more than impurity levels in the feed. If these are taken into account in previcus studies ( 2 2 ) differences may relate to temperature and acid strength. Iso-butxe/n-butane ratios are close to equilibrium values as mnversion decreases to zero and iso-butene is never more than 50%of the total C4 olefins (the equilibrium value at 400 C) suggesting that the butenes are close to equilibrium. There is uncertainty because 1-butene is not detected but, s h at equilibrium only around 10% of the butenes is present as 1-butene at 400"C, this is probably due to difficulty in detection. These C4= results suggest that is0 C4= and i s o C 4 are not extensively produoed by cracking of oligcnners and this view is supported by the absence of c7+ material in the product stream. since the isamerisation of the n-C4 carbem’um ion is energetically unfavourable it that consideration should he given to the possible isomerisation of the c6 HIS+ carbonium ion to give isoC6 H15'. This ion may then crack directly, or lose hydrcgen to given an iso-cg H13+ carbenium ion which, in turn, may crack by an unfavourable (D-cracking)p-Scission (%heme 2 ) .

13

& c c *

Similarly in scheme (2a) primary carknium ions are included. These are energetically unfavourable species in m i u m ion p-scission but the suggestion from these and previous results (13) is that the generation of primary w i u m ions by scission of carbonium ions may be less unfavourable. At higher conversion increased a m u n t s of iC4 are observed which, presumbly, are readily produced by oligamerisation to give %+ (n > 7) carbenium ions followed by isamerisation and cracking (Scheme 2b)

.

overall the present results are consistent with an induction period largely involving primary (mnmlecular) activation processes which generate surface precursors for carbenium ion processes. stabilisation of these carbnium ion precursors results in an increased rate of nhexane conversion with increasing contribution from bhlecular processes at higher converson, lmer temperatures and longer contact t i m e s . The balance of involvement of monamolecular versus bimolecular processes depends upon temperature, as reflected in activation energies, and upon strength and proximity of acid sites. The strength of acid sites strongly influences the equilibrium concentrations of surface carbenium ions which, for stronger acids, have longer surface lifetimes and can undergo secondary bimolecular processes providm that neighbow% sorption sites are available (13, 26). Consequently prcduct distributions can usually be explained by ptulathq different proportional contributions of primary and secondary reactions.

14

me

qui1ibriumbetwee.n olefinsand s0rbedcark.m’urn ions is a key feature of the overall reaction.

.mis reaction is endothesmic ard psh& to the right when temperature is raised. This shortens the lifetime of the carbenium ion and its surface as does reaction a t short contact time and l m concentration,

conversion, limiting its participation in oligcmrisation and hydrosen transfer - hence the increasing contribution from the primary m&am'SmS as temperature hmeases and conversion decreases. The effect of acid strength is ccanplicated by the fact that strength is UflldLly increased by reducing the framework aluminium content and therefore the proximity of sites ami the surface polarity ( 2 7 ) . Stronger acids stabilise the durn ions enhancing t h e i r lifetime, but the reduced proximity of sites can make bimolecular hydrosen transfer less favourable, a s discussed sukquently, and changes i n polarity can affect relative sorption of olefins versus paraffins (27). In the range of ccmrpositions studied here, the strorqer acids (more siliceous zeolites) shm an increase in the r a t e s o f both primary andsecondary processes as compared t o the parent Y zeolite. In scheme 2 we depict the activation of n-hexane involving direct protoMtion t o the cxaimnium ion. Hmever, this is not t o inply that these species are stabilised on zeolitic surfaces tplt rather t o provide passible recham'stic pathways. In fact, a t this tim, the nature of sorb& activat&hydmca&onspecies i s n o t k n m n andforexanple, a role for radicals o r radical ions cannot be excluded (28) nor can the involvement of Lewis acid sites w h i c h continue to a t t r a c t interest

-

(29)

DlENsITy

OF ACITW SITJB IN l?AIKR&mC

F o l l a w i ~ the ~ ~ generation of initial reactive surface species from hydmcarbn feedstocks the prcduct distrilxltion is largely governed by the balance of moncanolecular processes such a s isomerisation and Beta scission and b h l e c u l a r processes (for example oligoanerisation and b h l e c u l a r hydrogen transfer). A measure of the relative contribution of b b l d a r versus mncanolecular carbeniutn-ion like processes can be p m i d e d from the distribution of prcducts from cycloheene conversion (30). The basis for distindion may be seen from Scheme ( 3 ) .

15

c r a c k i n g t olkylation

Y - transfer

,

/+CHE

MI: PA

Readion of cyclohexene over acidic zeolites: CHE, cyclahexene; MCPA methylcyclocpentane; aIA cycl0heXi;me; MCPE, methylcyclopentene.

lm conversion over H-Y zeolits a t 250"C, the major prcducts are aIA, MBE and MCPA. consequently, by considering initial selectivities t o these products it :is possible t o define the relative rates of ismerisation (monmlecliLar:~and bimolecular hydrosen transfer

A t very

as:

Isamerisation Hydrqa-l Transfer

._ .-

I (MBE) + I (MBA) I (CNA) + I (MCPA)

Where 1 refers t o the i n i t i a l selectivities taken as the slope of the appropriate yield/conversion (x) 1 x u v r 3 a t the origin (x -> 0)

.

This function is platted against unit cell s i z e in Figure 9, w h i c h is related to the rnnober of framework a.luminiums and hence to site density (31). Previous results (30) dem~nstratet h i s effect for more siliceous ZSM-5 and zeolite Y materials and it is reported that mre than one site per large cage is r e q u i r d f o r b h o l e c u l a r hydrosentransfer in H-Y (32). T h e present results suggest I - h t the effect of site density is also evident i n the range of ompcx;itions studied here. Unit cell parameter is used for sirrp?le correlation and is adequate for zeolite Y and the CSY materials but is not s t r i c t l y adequate for ZSM-20 which is an i n of hexagonal an3 cubic Y nor for SARI-37 w h i c h although faujasitic i n strudure is not dir13ztly related by cell s i z e to the almincsilicate faujasites. Nevertheless the effect of site density aplpears to be detectable also i n SAPQ-37. Table (5) shows that a slight hcrease in silica content, w h i h can increase the density of acid sites, results in an increase i n the iscanerisation/hydmgen transfer function.

16

Table 5 Catalytic Cunmsicn of

2MIm

W Y -37 A sAK+37 B

%

si 6.6 3.6

w & m c e m ewer = s~1+37 ard Zeolite Y a t MCPE 0.49 0.34 0.30

MCPA

0.236 0.245 0.254

CHA

0.29 0.41 0.50

250°C

ISOMERISATION/ H-TRANS??ER

1.40 0.90 0.70

couzse, the relative sorption p a n n ~ t e r sfor alkanes and olefins can influence the balance of cracking o r isclmerisation versus b h l e c u l a r hydrogen transfer, particularly a t very high Si/Al (27), and t h i s effect is under investigation for these lower Si/M materials. Of

DEXREWTIPI OF ACITVE SITES IN F'AUIXITIC !ZEDIZl!ES

Previous Camments concern the activity and average density of active sites. It is of interest to know whether variation in the distribution of aluminium sites in faujasites having the same framework ccarposition (Si/AL) canbe achieved. In thepresent workthis isattempted by producing cubic faujasitic zeolites by synthesis i n the presence of fluoride anions (2) for amparison with prcducts of similar ccanpositon made by secondary synthesis using m4siF6 ( S Y zeolites). Details are given in Table (1). Solid state NMR can hxtkectly provide infornation on aluminium distribution and the relative intensities of the si(nAl)configurations (Fig 5) fllggest interesthq differences in the distribution of "T1' atcans in synthetic products asccanpared to those produced by secondary synthesis. The relative populations of S i ( W ) are higher and those of Si(0Al) are lmer for zeolites prepared by primary synthesis than for zeolites produced by secordary synthesis (11). Wt the enhance3 si (W) peak contains no s y f i c a n t contribution froan silanol grovps (sim) is clear frcnn the 1~ 9si cross-polarisation spectmm w h i c h shms negligible tof thesi(lA1) peak suggesting a very low concentration of silanols. ?his is confirmed by FTlR spectra of the hydroxyl region (14).

mereas

it is possible t o calculate the 2 9 ~ NMR i parameters for a faujasite structure tihere the aluminium orderirq is kncwn, it is not generally possible to define aluminium ordering f m experimental NMR spectra. Aluninium onkrhq i n zeolites m y be based on selection of unique structures (33) (34) or on mre detailed statistical models (35) (36) ( 3 7 ) . Althcugh it is simplistic to presume that 29Si spectra can be simulated by a single &asen aluminim ordering scheme it is

17

IL

6

20 '

Aromatar 2

3

4

CMIVERSION 19 1 . Figure12.

5

6

7

CARBDI WtiBER

Cracking mechanism ratm ICMRI for n-hexane conversion +ZSH-5 ZSM-11 3cCSY2 0 H-Y X CUB-Y 0 ZSM-20 A CUB-Y

F w r e 14.

Pmduct selectivhes far n-hexane conversion l1%1 aver 0 lGa/AIlY2. I NHLY at LW0C

-1s

5 a

. 1

z a 20.5

-

0

0

1

2

3

L

5

6

7

CaWERSION 19 1 .

O

O

l

L

t

CONVERSlON 1%1

0

c m s m DL1 Figura 15.

Ratio of prad~ctsfor n-hexane tonversmn aver NH'Y, x 1Ga/AUYZ, o tGa/AIIY3, at 400 C

Figure 13.

02

04

06 08 CONTACI TIME

10

li'l

n-hexane cracking wer CSY ISiIAI-LLI CUB-Y WAl-361 and ZSM-20 (Si/A1=35)

18

instructive to use specific distributions for ccenparative purposes (33). the synt2eticmateridls used i n t h i s study are close in ccmposition (Si/Al = 3.8 and 3.1) to values for centm-qmmetric stmctures (34) w i t h 40 and 48 alurniniums per unit cell (Si/Al = 3.8 and 3.0 respectively). Ihe faujasitic structures represented by two p cages (Fig 6) give calculated 29Si NMR parameters w h i c h are reasoMbly close to expervalues (11). The NMR results, therefore, suggest strongly thatthe alurniniu150-% i s d i f f e r e n t i n the synthetic materials asccanparedto thcsemadeby secondarysynthesisand i t i s then of considerable interest to knmwhether this is reflected in catalytic properties. 'Ihere is a f a i r l y widely held view that isolated aluminiumS provide the most active framework sites and these appear t o be mre numerous (for a given Si/Al) in the products made by primary synthesis since si(lA1) u n i t s a r e increased and Si(0Al) units are

TWO of

deQ-eased.

Fig (13) shms results for hexane cracking a t l m conversion over CUB-Y and CSY zeolites of similar ccanposition. It does appear that the synthetic pruc3uct.s are mre active and it is tapting to ascribe this to diffin alrmciniUm ordering. Hawever, a t this t h e this conclusion should betaken a s t e n t a t i v e s i n c e otherfactorSsuch as traces of s c d i u m ions can markedly affect rates and, although extensive ion exchangeprocedures areused, theanalysis f o r t r a c e sodim is subject to error and further work on t h i s aspect is i n progress.

'MEEEFECl!OFGALUCJM

T h e foreyohq discussion is confined to active sites generated in the Y strucbm by. aluminium boded via oxygen to silica. mere is wnsiderable interest i n themle of hetematom in zeolites and particularly i n the m l e o f g a l l i u m which isused i n the Qclar technology (38) i n association w i t h H-ZSM-5. In this present paper we discxlss briefly the effect of g a l l i u m incorpOration into zeolite Y, to

pruduce H-[&/All-Y oil. a)

zeolites, on the conversion of

n-hexane and gas

w€kxar~Mn304>MnS04. In contrast to the case of NiCl,, however, even with MnC1, no complete elimination of the acidic OH'S was achieved. Moreover, a fraction of the manganese compound admixed always remained unreacted. In the case of MnC12/H-ZSM-5the solid-state ion exchange was studied as a function of reaction time and temperature. Figure 13 clearly demonstrates the effect of temperature. Raising the reaction temperature from 570 K to 770 K resulted in a significant increase in the degree of exchange. Most of the manganese ions were introduced during the first stage of reaction (within 1 h), and then the further reaction proceeded very slowly.

-

1

Y

I

I

0-

P / O - O

50-

670K

3

0 W e* 0 P 25-

-

\;I.-- I I

770 K

&?

cn

1

o-

W

I 3

z

0 U

-

0Heat treatment in vacuum

I

I

0

I

I

I

5 10 15 R E A C T I O N T I M E [hl

I

20

I

Fig. 13. Solid-state ion exchange with manganese chloride. Number of bridgin OH groups consume via solid-state exchange in a mixture of MnCl, and H-Z8M-5 (SiiA1=13.5, M$+iOH=0.33) as a function of reaction time a t 570, 670 and 770 K (after Ref. [141,with permission). Modification of H-ZSM-5 zeolites through solid-state reaction with ZnO was described by Yang et al. [32]. On the basis of XPS results they reported that, upon heattreatment of a ZnO/II-ZSM-5 mixture, Zn ions migrated from the outer surface into the channels of the zeolite. This finding was supported by TPDA, IR (decrease of acidic Br#nsted sites upon solid-state reaction between ZnO and H-ZSM-5) and temperatureprogrammed reduction (TPR).The latter showed increased uptake and reducibility after thermal treatment of ZnO/H-ZSM-5 compared to ZnO. Zeolites Zn,H-ZSM-5 exhibited, after reduction in H,, pronounced selectivity in propane aromatization. More recently, Karge et al. [361 have shown that also noble metals can be easily introduced into zeolites via solid-state reaction. Various zeolites, such a s NH4-Y, US-Y,

63

H-MOR and H-ZSM-5, and noble metal compounds (PdCl,, Pd(NO,),, PdO, PtCl,, orPtC1,) were used. It was demonstrated with the help of several techniques (IR, TPDA, TPE etc.) that the noble metal cations upon solid-state reaction occupy cation sites inside the zeolite structure. After reduction in H, the thus-obtained materials possessed hydrogenation properties. Provided a suitable balance between the acid function (residual acidic OH groups) and the hydrogenation function (noble metal aggregates) was established, these catalysts were efficient in hydroisomerisation of, for instance, ethylbenzene. Role of Water i n Solid-state Ion Exchange In most cases solid-state ion exchange in zeolites was conducted in the presence of ambient moisture or residual water vapour. However, it was shown that this type of exchange also occurs whenever traces of water are carefully excluded [ 121. Moreover, solid-state ion exchange into zeolites was also achieved with compounds insoluble in water, e.g. with AgCl o r Hg,Cl,. This suggests that the presence of residual water is not necessarily a prerequisite for the solid-state ion exchange in zeolites to occur, even though small amounts of water such as the crystal water might facilitate the low-temperature solid-state reaction (vide supra). However, more subtle details of solid-state ion exchange in zeolites as, for instance, the particular mechanism of ion migration remain a mystery, and their clarification needs further experimental work. Acknowledgment Financial support by the Bundesminister fur Forschung und Technologie (BMFT, Project No. 03C 257 A7) is gratefully acknowledged.

K K F E KENCES 1 J.A. Rabo, M.L. Poutsma and G.W. Skeels, in J.W. Hightower (Editor), Proc. 5th Int. Congress on Catalysis, Miami Beach, Flo., USA, August 20-26, 1972, NorthHolland Publishing Co., New York, 1973, pp. 1353-1361. 2 J.A. Rabo and P.H. Kasai, Progress in Solid State Chemistry 9 (1975) 1-19. 3 J.A. Rabo, "Salt Occlusion in Zeolite Crystals", in J.A. Rabo (Editor), "Zeolite Chemistry and Catalysis", ACS Monograph 171, Am. Chem. SOC.,Washington, D.C., USA, 1976, pp. 332-349. 4 A. Clearfield, C.H. Saldarriaga and R.C. Buckley, in J.B. Uytterhoeven (Editor), Proc. 3rd Int. Conference on Molecular Sieves; Recent Progress Reports, Zurich, Switzerland, Sept. 3-7, 1973; University of Leuwen Press, 1973, Leuwen, Belgium, Paper No. 130, pp. 241-245. 5

6 7 8

9 10

C.A. Fyfe, G.T. Kokotailo, J.D. Graham, C. Browning, G.C. Gobbi, M. Hyland, G.J. Kennedy and C.T. DeSchutter, J. Am. Chem. SOC.108 (1986) 522-523. J.H. Lunsford, Adv. Catal. 22 (1972) 265-344. H.G. Karge, S. Trevizan de Suarez and I.G. Dalla Lana, J. Phys. Chem. 88 (1984) 1782-1784. J.B. Uytterhoeven, L.G. Christner and W.K. Hall, J. Phys. Chem. 69 (1965) 21172126. M.L. Hair, "Infrared Spectroscopy on Surface Chemistry", Marcel Dekker Inc., New York, 1967. H.G, Karge, Z. Phys. Chem. [NF] 122 (1980) 103-116.

64

11 H.K. Beyer, H.G. Karge and G. Borbely, Zeolites 8 (1988) 79-82. 12 H.G. Karge, V. Mavrodinova, 2. Zheng and H.K. Beyer, in D. Barthomeuf, E.G. Derouane and W. Holderich (Editors), "Guidelines for Mastering the Properties of Molecular Sieves", NATO AS1 Series, Series B, Physics Vol. 221, Plenum Press, New York, 1990, pp. 157-168. 13 G. Borbely, H.K. Beyer, L. Radics, P. SBndor and H.G. Karge, Zeolites 9 (1989) 428-431. 14 S. Beran, B. Wichterlovh and H.G. Karge, J. Chem. SOC.Faraday Trans. I 86 (1990)3033-3037. 15 R. Schollner, P. Nobel, H. Herden and G. Korner, in P. Fejes (Editor), Proc. Symp. on Zeolites, Szeged, Hungary, Sept. 11-14,1978, Acta Universitatis Szegediensis, Acta Physica e t Chemica, Nova Series 24 (1978) 293-298. 16 H.K. Beyer and I. Belenykaja, in B. Imelik, C. Naccache, Y. Ben Taarit, J.C. Vedrine, G. Coudurier and H. Praliaud (Editors), Proc. Int. Symp."Catalysis by Zeolites", Ecull (Lyon), France, Sept. 9-11, 1980; Elsevier, Amsterdam, 1980; Stud. Surf. Sci. 5 (1980)203-210. 17 H.K. Beyer, I.M. Belenykaja, F. Hange, M.Tielen, P.J. Grobet and P.A. Jacobs, J. Chem. SOC. Faraday Trans. I81 (1985) 2889-2901. 18 B. Sulikowski, G. Borbely, H.K. Beyer, H.G. Karge and I.W. Mishin, J. Phys. Chem. 93 (1989)3240-3243. 19 H.G. Karge, H.K. Beyer and G. Borbely, Catalysis Today 3 (1988)41-52. 20 A.E. Hirschler, J. Catal. 2 (1963) 428-439. 21 C.J. Plank, in W.M. Sachtler, G.C.A. Schuit and P. Zwietering (Editors), Proc. 3rd Congress on Catalysis, Amsterdam, The Netherlands, July 20-25, 1964, NorthHolland Publ. Comp., Amsterdam, 1965, pp. 727-728. 22 D. Keir, E.F.T. Lee and L.V.C. Rees, Zeolites 8 (1988) 228-231. 23 S. HoEevar and B. Dr&aj,in L.V.C. Rees (Editor), Proc. 5th Int. Conf. Zeolites, Na les, Italy, June 2-6,1980,Heyden, London, 1980, pp. 301-310. 24 H.8. Karge, G. Borbely, H.K. Beyer and G. Onyestyhk, in M.J. Philips and M. Ternan (Editors), Proc. 9th Int. Congress on Catalysis,Calgary, Ottawa, Canada, June26-July 1,1988, Chemical Institute of Canada, Ottawa, 1988, pp. 396-403. 25 H.G. Karge and H.K. Beyer, in DGMK-Berichte-Tagungsbericht 9101, DGMKFachbereichstagung "Cl-Chemie - Angewandte Heterogene Katalyse - C4Chemie", Leipzig, FRG, Febr. 20-22, 1991, ISBN No. 3-928164-07-4, ISSN No. 0938-068 X, pp. 191-206. (English Version to be published in Erdol & Kohle, Erdgas, Petrochemie). 26 A.V. Kucherov and A.A. Slinkin, Zeolites 6 (1986) 175-180. 27 A.V. Kucherov and A.A. Slinkin, Zeolites 7 (1987) 38- 42. 28 A.V. Kucherov and A.A. Slinkin, Zeolites 8 (1988) 110-116. 29 A.V. Kucherov and A.A. Slinkin, Zeolites 7 (1987)43-46. 30 A.V. Kucherov, A.A. Slinkin, D.A. Kondrat'ev, T.N. Bondarenko, A.M. Rubinstein and Kh.M. Minachev, Zeolites 5 (1985)320 - 324. 31 A.V. Kucherov and A.A. Slinkin, Zeolites 7 (1987) 583-584. 32 Y. Yang, X. Guo, M. Deng, L. Wang and Z. Fu, in H.G. Karge and J. Weitkamp (Editors), Proc. Int. Symp. "Zeolites as Catalysts, Sorbents and Detergent Builders-Applications and Innovations",Wiirzburg, FRG,Sept. 4-8,1988;Elsevier, Amsterdam, 1989; Studies Surface Sci. Catalysis 46 (1989) 849-858. Faraday 33 A.V. Kucherov, A.A. Slinkin, H.K. Beyer and G. Borbely, J. Chem. SOC. Trans. I, 85 (1989) 2737-2747. 34 B. Wichterlovh, S. Beran, S. BednaFovh, K. NedomovB, L. Dudikovh and P. J iru in P.J. Grobet, W.J. Mortier, E.F. Vansant and G. Schulz-Ekloff (Editors), Proc. Int. Symp. "Innovation in Zeolite Materials Science", Nieuwpoort, Belgium, Sept. 13-17,1987, Elsevier, Amsterdam; Studies Surf. Sci. Catalysis 37 (1988) 199-206. 35 8. Wichterlovh, S. Beran, L. Kubelkovh, J, NovhkovB, A. SmiegkovA and R. Sebik, in H.G. Karge and J. Weitkamp (Editors), Proc. Int. Symp. "Zeolites as Catalysts, Sorbents and Detergent Builders - Applications and Innovations", Wurzburg, FRG, Sept. 4-8, 1988, Elsevier, Amsterdam, 1989; Studies Surf. Sci. Catalysis 46 (1989)347-353. H.G. Kame. Y. Zhanp and H.K. Bever. Dublication in DreDaration.

catalysis

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 0 1991Elsevier Science Publishers B.V., Amsterdam

65

Zeolite-hosted metals and semiconductors as advanced materials G. Schulz-Ekloff

I n s t i t u t f u r Angewandte und Physi kalische Chemie, Universitat Bremen, 0-2800 Bremen 33, FRG

Abstract Preparations and characterizations o f zeolite-hosted materials, 11 k e metals, semiconductors o r dyes are described. Photophysical and photochemical properties are reviewed. Potential applications, e.g. i n optical switching, microwave absorption, optical data storage, microsensor devices o r dispersion electrolysis, are summarized.

1.

INTRODUCTION

The regularly s t r u c t u r e d pores and cages o f molecular sieves represent host systems which can accomodate guest particles o r molecules i n a colloidal dispersion and , thus, o f f e r t h e properties of a solid solvent. The fixation of colloids i n zeolitic ceramic materials o f f e r s advantages f o r practical applications due t o t h e h i g h chemical, mechanical and thermal stability of a molecular sieve s u p p o r t and t h e isotropic access o f t h e colloid f o r guest molecules. The preparation of s t r u c t u r e d cluster dispersions exhibiting tailored particle sizes i n t h e nanometer range is o f practical importance f o r t h e development o f advanced quantum-confined electronic, optical o r opto-electronic devices, e.g. f o r lasers, switches, t r a n s i s t o r s o r information storage, since t h e low dimension o f quantum-dot (QD) systems w i l l make electron excitation o r t r a n s f e r processes faster and more energy selective and will exhibit novel photophysical properties. The development o f zeolite-based host-guest systems as heterogeneous or immobilized homogeneous catalysts is a permanent a t t r a c t i v e challenge, whereas t h e design o f advanced molecule separation materials o r t h e creation of molecule sieving sensor devices seem t o be r a t h e r a t t h e beginning. Other new fields are photochemistry, photocatalysis and applications r e q u i r i n g composite systems on a nanometer scale. Many of t h e f u t u r e potential applications o f zeolite-based advanced materials have been mentioned i n recent review articles. I n t h e following, some problems w i l l be pointed o u t i n connection with t h e application of zeolite hosts f o r t h e preparation of well-defined colloidal dispersions o f guests, which have t o be overcome f o r a breakthrough in t h e development of new materials f o r advanced usages.

66 I

-

NONINlIRCONNECTIN(I CHANNELS

a.

c. d.

P

P 7

.FtRRlElllTF

;::lK

It. INTERCONNECTING CHANNEL5

\ l l . INTERCONNECTING C H A N N t L S AND C A G E S

Figure 1. I l l u s t r a t i o n of zeolite void f i l l i n g guest particles f o r various s t r u c t u r a l t y p e s o f channel networks (from E.G. Derouane i n "Intercalation Chemistry" (M.S. Whittingham and A.J. Jacobson, Eds.) Academic Press, New York 1982, p. 101)

2.

ZEOLITE-HOSTED

TRANSITION M E T A L S

2.1 Application of metal-loaded zeolites in catalysis Zeolite catalysts containing reduced metals are broadly applied i n t h e upgrading of l i g h t gasoline fractions and hydrocracking of heavy Beyond t h e established usages in petrochemical feedstocks [l-41. processes a variety o f prospective catalytic properties o f transition metal containing zeolites are investigated intensively, Ii k e oligomerization, hydrogenation, dehydrogenation, selective oxidation [5,6], carbonylation [7], hydroformylation [8], low temperature water gas s h i f t [9] o r syngas conversion [lo]. The aromatization o f lower alkanes, which appears to be of great importance f o r economic and ecological reasons, seems t o be close t o an industrial realization [ll-131. The break-even point is already reached f o r t h e application o f titanium silicalite i n t h e selective oxidation The o f hydrocarbons using hydrogen peroxide as oxidant [14-151. applications o f transition metal containing zeolites f o r t h e catalyzed synthesis o f chemical intermediates and f i n e chemicals have been reviewed recently [16,17].

2.2 Preparation of zeolite accomodated metal dispersions Incorporation o f metals i n a zeolite void system o f f e r s advantages as well as shortcomings as compared t o dense ceramic carriers, l i k e silica o r alumina. The open crystalline polyanion framework enables controlled reproducible stoichiometric metal loadings v i a ion exchange from aqueous solutions. Solid-state ion exchange can be applied i n some cases [18,191. The reduction by hydrogen produces, however, protons f o r charge compensation, which might f a v o r undesired side-reactions, like coke

61

a

b

Figure 2. Transmission electron micrographs o f iridium crystallites hosted i n NaX (a) and platinum crystallites hosted in ZSM-5 (b). The phase contrast imaging o f t h e zeolite lattices represents an internal scale.

deposition o r removal of framework atoms, o r affect t h e selectivity o f conversions. Alternative loadings can be obtained v i a carbonyls o r nitrosylcarbonyls [lOc, 20-241 as well as b y organometallics l i k e rr-allyls, metallocenes or phosphines [25-271 from vapor phase o r solution, f a v o r i n g t h e incorporation of zerovalent metals. The generation o f zeolite-hosted metal c r y s t a l s o r large clusters, respectively, r e q u i r e s subsequent steps of calcination and reduction. The reducibility o f t h e metal and t h e resulting dispersion depend sensitively on t h e parameters f o r these treatments, e.g. zeolite t y p e and acidity, manner and extent of loading, calcination medium, reducing agent, temperature programs, additionally exchanged unreducible cations o r presence of noble metals [28-331. Dispite t h e large number o f parameters influencing t h e preparation of the zeolite-supported metals, the preparation o f monodisperse metal phases having narrow particle size histograms and being located exclusively i n t h e zeolite void is achieved repeatedly.

2.3 Characterization For any judgement on possible correlations between t h e state o f a zeolite-hosted metal phase and i t s physical o r chemical properties in classical o r novel applications detailed informations about location, dispersion, geometric and electronic s t r u c t u r e under working condition are needed. I n fact, there is a scarcity o f knowledge about s t r u c t u r e p r o p e r t y correlations f o r most of t h e described systems. I n many cases a

68

Figure 3. I l l u s t r a t i o n o f a supra-supercage c r y s t a l hosted i n a mesopore formed by zeolite framework fragmentation d u r i n g guest partical growth.

lack of information exists f o r t h e location of t h e metal phase, i.e. inside t h e channels and cages o r at t h e external surface o f t h e zeolite crystals. Presumably, an exclusive internal accomodation o f metal crystallites can be proved by photoelectron spectroscopy only [34-361. With few exceptions no histograms o f metal dispersions based on electron micrographs are given. Usually, rough estimates are made from X-ray diffraction and/or probe molecule chemisorption, o r averaged values are drawn from magnetization measurements [37]. Charged clusters are internally located i n lattice positions and can exhibit distinct sizes, e.g. (Ag)6+ [38-411. Location and coordination o f metal atoms o r small aggregates i n lattice positions o r well-defined sites can be identified b y XRD [421 o r EXAFS [43, 441. Electronic s t r u c t u r e s might be studied favorably by means of t h e reactivity o f probe molecules [45, 461 o r by photoelectron spectroscopy [471. Origin and catalytic effect of a partial positive charge on metal clusters are not yet f u l l y understood [48, 491. New silicon o r aluminum containing phases o r alloys, which might result from s t r o n g metal-support interactions i n analogy t o observations on dense s u p p o r t s [50-521, have not yet been found with metal loaded zeolites. A prospective way f o r t h e design of tailored cluster sizes might be t h e anchoring o f reducible metal at non-reducible transition metal ions [53, 541. Resulting bimetal clusters can exhibit interesting novel catalytic properties [13, 551. However, f o r t h e interpretation o f bimetal catalysis with zeolite supports, effects o f distribution, acidity o r stability have t o be taken into account [56]. The metal dispersion effect on a c t i v i t y and

69

F i g u r e 4. Model o f a platinum c r y s t a l l i t e i n a faujasite supercage exhibiting a host-guest orientation-relationship due t o s t r u c t u r a l accomodation. selectivity i s well-established [57,58]. The influence o f t h e dimensions o f t h e zeolite channels and cages on t h e metal dispersion, as depicted i n Fig. 1, is not clear. There is no doubt, t h a t metal crystallites can grow beyond these dimensions under zeolite framework fragmentation and reorganization (Fig. 2) [59-601. The zeolite framework removal around t h e growing metal c r y s t a l s results i n t h e generation o f halos (Fig. 3) indentified as mesopores from t h e evaluation o f hysteresis loops i n adsorption isotherms [61]. The zeolite fragments can be reorganized b y i n-situ recrystal Iization forming secondary micropores as identified f r o m an appropriate adsorption isotherm analysis [62]. The zeolite framework fragmentation decreases with increasing Si/AI ratio, resulting in a corresponding limitation of metal crystal growth [58a]. The embedding o f metal c r y s t a l guests i n a zeolite host can result i n a host-guest orientation relationship, i.e. a parallel orientation of crystal axes of host and guests [59]. This effect is related t o a s t r u c t u r a l accomodation, i.e. an optimum f i t t i n g o f metal crystallites into t h e supercage space (Fig. 4). This orientation relationship w i l l be maintained d u r i n g f u r t h e r growth, since a particle is no longer f r e e to rotate once it has reached t h e confinement o f t h e supercage [ 6 3 ] .

2.4 Potential novel applications For s i l v e r sodalites a variety of physical and chemical properties, e.g. photochromy, barochromy o r thermochromy, are summarized which could f i n d possible utilization i n h i g h resolution imaging o r high density data storage [64]. The effects are observed following t h e reduction o f silver

70

ZEOLITE-SUPPORTED

ULTRAMICROELECTRODES

Figure 5. Schematic representation of a dispersed-particle electrode as contained between feeder electrodes with an exploded view o f faujasitesupported platinum (from ref. 69).

ions and/or t h e formation of silver clusters, i.e. are related t o nonreversible processes. Corresponding effects are expected f o r other zeolite-hosted redox systems which can be influenced b y d i f f e r e n t k i n d s o f energy impact under change o f optical o r dielectric properties. I n general, t h e fabrication o f conductor o r semiconductor nanostructures in zeolite channels o r cages could be of importance f o r nonlinear optics o r ultimate microelectronic v e r y large scale integration. Nanoparticulate metals o r semiconductors o f colloidal dispersion i n ceramic supports exhi b i t three-dimensional confinements o f charge carriers, resulting i n changes o f t h e s t r u c t u r e o f electronic levels and alterations o f t h e complex dielectric functions [65,66]. A prospective usage in optical signal processing is based on t h e possible complete absorption saturation f o r interband transitions i n t h e small discrete level system of a QD. The particle size dependent change in absorption and/or r e f r a c t i v e index could be v e r y large as compared t o b u l k materials. Furthermore, t h e refractive index could be changed by feedback effects on local fields surrounding a QD. This effect could be used f o r optical storage devices. The size-induced metal-insulator transition f o r QDs from metal atoms could be applied f o r t h e engineering of resistors o r microwave absorbing materials 167,681. Up t o now, size-induced metal-insulator transitions have not yet been studied at zeolite-hosted metal crystallites. I n t e r e s t i n g app I ications o f metal-loaded zeolites as intracrystal Ii ne electrodes are proposed. The use of feeder electrodes and dispersions of metalated zeolites results i n electrode functions without a direct electrical contact (Fig. 5). I t was demonstrated t h a t dispersion electrolysis can be achieved on platinum-loaded NaY zeolites [69].

71

3. ZEOLITE-HOSTED SEMICONDUCTORS 3.1 Preparation

I n analogy t o t h e experiences with metal dispersions inside a zeolite matrix t h e preparation of corresponding semiconductor colloids with quantum-sized particles i n t h e range 1-10 nm should be possible. The generation of zeolite-hosted dispersions has been described repeatedly f o r CdS [70-751, PbS [70,74] and ZnS [71, 751. The sulfidation o f t r a n s i t i o n metal ion-exchanged zeolites can be achieved b y treatment o f t h e dehydrated samples with H S [70, 72-75] o r by adding Na2S t o an aqueous s l u r r y [71]. However, t h e sulfide formation remains incomplete since t h e metal ion w i l l competitively interact with H S and t h e zeolite lattice [76]. Furthermore, t h e tendency f o r sulfide formation decreases with decreasing sulfide cluster size due t o t h e corresponding increase of t h e solubility constants o f quantum-sized sulfide [77,78]. The charge zeolite compensating protons formed during su I f idat ion faci Iitate framework fragmentation around t h e growing particle in analogy t o metalloaded zeolites [59-611, thus, resulting i n mesopores accomodating suprasupercage sulfide aggregates [76]. Precipitation of metal hydroxides inside t h e zeolite c r y s t a l o r a t i t s external surface d u r i n g ion exchange has t o be considered. The extent o f t h i s side-reaction depends on t h e solubility p r o d u c t o f t h e metal hydroxides, t h e alkalinity of t h e zeolite and t h e acidity o f t h e salt solution [79]. The tailored preparation and characterization o f transition metal oxide clusters i n zeolite channels and cages has not been investigated, up t o now. A possible way could be t h e conversion o f transition metal clusters, e.g. prepared v i a zeolite-accomodated carbonyls o r vapor-deposited volatile metal compounds, t o encaged oxide cluster under appropriate conditions. The incorporation of semiconductor oxide clusters, e.g. Ti02 o r SnO2, v i a ion exchange s u f f e r s from s t r o n g h y d r o l y s i s and polymeric cation formation i n t h e exchange s l u r r y . The incorporation o f transition metals b y isomorphous substitution d u r i n g molecular sieve synthesis and subsequent aggregation i n extra-framework positions v i a removal from Tsites would be an alternative approach.

3.2 Characterization EXAFS has been used f o r t h e analysis o f location or s t r u c t u r e o f zeolite-hosted CdS and CdSe clusters [73]. The persuasive power of t h i s method suffers, however, from t h e lack o f well-defined reference s t r u c t u r e s i n model compounds needed f o r exact data analysis procedures, i.e. f o r precise values o f t h e bond length and t h e coordination number [SO]. Always convincing are optical spectra g i v i n g t h e blue-shifted absorption edge o f a quantum-sized particle as expected theoretically from an increasing band gap with decreasing particle size [81, 821. Blueshifted absorption edges were identified f o r zeolite-hosted CdS [70, 71, 741, PbS [70, 741 and ZnS [75]. A less pronounced blue-shift is obtained upon several measures, l i k e increasing loading o f t h e zeolite host with t h e semiconductor guest, calcination procedures o r sulfidation of non73a, 741 and is interpreted by cluster dehydrated samples [71, aggregation. The particle sizes deduced from t h e absorption edge positions exceed the supercage dimensions by far. However, photocorrosion and photoreaction measurements point t o an internal location of t h e aggregates [71, 741. Different aggregation mechanisms have

12

a

b

F i g u r e 6. Models (a) o f a (CdS)4 cluster i n a sodalite cage and ( b ) of occupied adjacent sodalite cages (from ref. 73a).

been proposed f o r these zeolite-accomodated sulfide agglomerates, i.e. ( i ) interconnection of clusters in adjacent supercages [71], (ii) interconnection o f filled sodalite cages (Fig. 6) [73a] o r ( i i i ) growth o f sulfide particles to supra-supercage size under zeolite lattice fragmentation and mesopore formation [74,75]. The l a t t e r mechanism is supported by adsorption isotherm analysis. The application of transmission electron microscopy gives a s t r i c t correlation between cluster sizes gleaned from t h e optical absorption edge position and those drawn from t h e electron micrographs [74]. The increase o f particle size is accompanied b y characteristic colour changes (Table 1). The observed increase of t h e rate of photocorrosion with decreasing sulfide particle size [75, 761 is i n agreement with theoretical expectations. Small semiconductor particles are characterized b y a high extent of lattice defects affecting bond lengths [83] and solubility [77,78] and leading t o surface states energetically located within t h e band gap [84]. The surface states t r a p electrons and, therefore, suppress t h e recombination of lightinduced electron-hole pairs. The holes remain available f o r photocorrosion, i.e. t h e oxidation of sulfide ions belonging t o t h e lattice of t h e semiconductors. I t i s an open question, whether photocorrosion can be used t o eliminate certain fractions i n cluster size distributions i n o r d e r t o prepare single-size particles.

3.3 Potential Applications The use o f o f QD semiconductors f o r optical switching are suggested repeatedly [65, 661. The suggestions are based on the experimental finding, t h a t t h e excitation absorption is bleached d u r i n g t h e presence of a trapped electron-hole pair and recovers as t h e exciton pair decays [go, 911. The effect is r e f e r r e d t o a s t r o n g reduction of t h e excitation oscillator s t r e n g t h in t h e presence o f a surface-trapped electron-hole pair [91]. More sophisticated applications, l i k e cellular automation computers, are i n early stages o f development [85]. Contacting o f QDs requires t h e development o f quantum wires. F i r s t attempts are reported f o r t h e preparation o f zeolite-hosted selenium chains [86, 871. The inclusion of conducting polymer chains in zeolite channels is under investigation [88, 891.

73 Table 1 Colour changes o f size-tuned (nm) zeolite-supported PbS clusters 0.5

1 greenish l i g h t yellow beige

2 dark beige

4

yellow earth

6 8 red r u s t y brown

10 dark r u s t y brown

> 10 black brown

The use of zeolite-hosted semiconductor oxides as chemical sensors towards oxidizing o r reducing gases might be attractive. Since t h e alteration of t h e conductivity depends on changes of t h e oxide s h o r t e r diffusion distances i n smaller clusters stoichiometry [93,94], should r e s u l t i n shorter response times of t h e sensors. Fast response is a prerequisite f o r t h e application of sensors based on changes of t h e bulk composition, e.9. in air/fuel ratio control devices. Furthermore, t h e application of t h e molecule sieving effect o f zeolites f o r t h e development o f molecule selective sensors i s studied. One route of preparation aims t o coat t h e sensor material b y a glassy t h i n film hosting zeolite c r y s t a l s as molecule sieving gates [95, 961. The shortcoming o f t h i s technique is t h e limited thermal and mechanical stability of t h e glassy t h i n film. It seems t o be more prospective t o use zeolite single c r y s t a l s hosting t h e sensor materials. The changes i n t h e stoichiometry o f t h e sensor oxides by interaction with gas molecules can be detected b y methods which do not r e q u i r e electric contacts, e.g. optical absorption, dielectric p e r m i t t i v i t y o r phonon absorption. A t present only a few studies on t h e application o f zeolite-hosted semiconductors i n photochemistry [71, 741 exist. The potentials of nanocomposite systems i n a zeolite host, comprising photosemiconductor, catalyst, photosensitizer, sacrificial donor and acceptor in optimum spatial arrangement mediated by t h e void s t r u c t u r e o f t h e host, give reasons for f u r t h e r intensive investigations. The applicability o f zeolite-based molecular multicomponent systems f o r light-induced charge separation o r photochemical H2 evolution has been demonstrated impressively [97].

4.

MISCELLANEOUS ZEOLITE-HOSTED

SYSTEMS

4.1 Optical processing systems Optical processing systems are proposed based on relative intensity changes of second harmonic generation i n sorbate complexes of p n itroan i I i ne and 2-meth y I-p-n itroan i Ii ne i n molecu lar sieve hosts [98]. Optical data storage would be one o f a variety o f potential electro-optical applications. I n addition, molecular optical effects b y t h i r d harmonic generation are suggested f o r optical data storage based on t h e possible formation o f bistabilities f o r t h e local fields [65,991. 4.2 Zeolite-hosted dyes

Zeolite-hosted dyes can be prepared by ion-exchange o f cationic dye molecules o r b y incorporation d u r i n g hydrothermal crystallization [loo]. Optical absorption bands o f molecular sieve-accomodated dyes can be

broadened, presumably, due t o t h e variety o f d i f f e r e n t possible interactions based on t h e d i s t r i b u t i o n of t h e aluminum atoms in t h e zeolite framework. This effect favours optical data storage by spectral methylene blue shows a hole b u r n i n g [ l o l l . Faujasite-encapsulated decoupling o f t h e guest electron excitation and t h e host phonon movement, which is a prerequisite f o r high temperature hole b u r n i n g [loo]. A f u r t h e r advantage of t h e zeolite host i s its relatively h i g h thermal, chemical and mechanical stability as compared t o polymer hosts. Reversible optical data storage based on reversible changes o f molecular structures, .e.g. cis-trans-isomerization o f thioindigo [102], is possible.

5.

ACKNOWLEDGEMENT

I am grateful t o Prof. D r . N.I. Jaeger f o r f r u i t f u l cooperation and critical reading o f t h e manuscript, t o D r . R. Lamber f o r electron micrographs t o A. Kleine f o r molecular modelling, t o M. Wark f o r PbS particle size t u n i n g and t o Academic Press and American Chemical Society f o r permission o f p i c t u r e reproduction. Financial s u p p o r t by t h e Bundesmi n i s t e r f u r Forschu n g u nd Technolog ie (BMFT-423-4003-03C 2583, BMFT-NT 20 606) and t h e Max Buchner-Forschungsstiftung (MBFStKennziffer 1557) are gratefully acknowledged.

6

REFERENCES

1

A.P. Bolton, i n "Zeolite Chemistry and Catalysis" (J.A. Rabo, Ed.), ACS Monograph 171, American Chemical Society, Washington 1976, p. 552. E. Gallei, Chem.-1ng.-Techn. 52 (1980) 99; and references therein. H. Heinemann, i n "Catalysis: Science and Technology" (J.R. Anderson and M. Boudart, Eds.), Springer, Berlin 1981, vol. 1 , p. 1. K. Becker, H. John, K.-H. Steinberg, M. Weber and K.-H. Nestier, in "Catalysis on Zeolites" (D. Kallo and Kh. M. Minachev, Eds.), Akademia Kiado, Budapest 1988, p. 515. C. Naccache and Y. Ben Taarit, in "Zeolites: Science and Technology" (F.R. Ribeiro, A.E. Rodrigues, L.D. Rollmann and C. Naccache, Eds.) Martinus Nijhoff, The Hague 1984, p. 373. Kh. M. Minachev, V.V. Kharlamov and V.I. Garanin, in "Catalysis on Zeolites" (D. Kallo and Kh. Minachev, Eds.), Akademia Kiado, Budapest 1988, p. 489. B.K. Nefedov, A.A. Slinkin, A.V. Kucherov, N.S. Sergeeva and Ya. T. Eidus, I z v . Akad. Nauk (SSSR), Ser. Khim. (1974) 2119. B.K. Nefedov, N.S. Sergeeva and L.L. Krasnova, Izv. Akad. Nauk (SSSR), Ser. Khim. (1977) 614. (a) J.J. Verdonck, P.A. Jacobs and J.B. Uytterhoeven, J. Chem. SOC., Chem. Commun. (1979) 181. ( b ) M. Niwa, T. I z u k a and J.H. Lunsford, J. Chem. SOC., Chem. Commun. (1979) 684.

2 3 4

5 6 7 8

9

75 10

11

12

13 14

15

16

17 18 19

20

21 22 23 24

25 26 27 28

29 30 31

(a) H.H. Nijs, P.A. Jacobs and J.B. Uytterhoeven, J. Chem. SOC., Chem. Commun. (1979) 1095. ( b ) D. Fraenkel and B.C. Gates, J. Am. Chem. SOC. 102 (1980) 2478. (c) T. Bein, G. Schmiester and P.A. Jacobs, J. Phys. Chern. 90 (1986) 4851. O.V. Bragin, A.V. Preobrazhensky and A.L. Liberman, I z v . Akad. Nauk (SSSR), Ser. Khim. (1974) 2751. O.V. Bragin, E.S. Shpiro, A.V. Preobrazhensky, S.A. Isaev, T.V. Vasina, B.B. Dysenbina, G.V. Antoshin and Kh. M. Minachev, Appl. Catal. 27 (1986) 219. O.V. Bragin, T.V. Vasina, A.V. Preobrazhensky and Kh.M. Minachev, I z v . Akad. Nauk (SSSR), Ser. Khim. (1989) 750. B. Notari, i n "Innovation i n Zeolite Materials Science" (P.J. Grobet, W.J. Mortier, E.F. Vansant and G. Schulz-Ekloff, Eds.), Elsevier, Amsterdam 1988, Stud. Surf. Sci. Catal., vol. 37, p. 413. (a) U. Rornanov, A. Esposito, F. Masperi, C. Neri and M. Clerici, i n "New Developments i n Selective Oxidation" (G. Centi and F. Trifiro, Eds.), Elsevier, Amsterdam 1990, Stud. S u r f . Sci. Catal., vol. 55, p. 33. ( b ) P. Roffia, G. Leofanti, A. Cesana, M. Mantegazza, M. Padovan, G. Petrini, S. Tonti and P. Gervasutti, ibid., p. 43. ( c ) C. F e r r i n i and H.W. Kouvenhoven, ibid., p. 53. W.F. Holderich, i n "Zeolites: Facts, Figures, F u t u r e " (P.A. Jacobs and R.A. van Santen, Eds.), Elsevier, Amsterdam 1989, Stud. Surf. Sci. Catal., vol. 49A, p. 69. W.F. Holderich, Angew. Chem. 100 (1988) 232. H.G. Karge, H.K. Beyer and G. Borbely, Catalysis Today 3 (1988) 41. B. Wichterlova, S. Beran, S. Bechnarova, K. Nedomova, L. Dudikova and P. Jiru, i n "Innovation i n Zeolite Materials Science" (P.J. Grobet, W.J. Mortier, E.F. Vansant and G. Schulz-Ekloff, Eds.), Elsevier, Amsterdam 1988, Stud. Surf. Sci. Catal., vol. 37, p. 199. D. Ballivet-Tkatchenko, G. Coudurier and N.D. Chau, i n "Metal Microstructures i n Zeolites" (P.A. Jacobs, N.I. Jaeger, P. J i r u and G. Schulz-Ekloff, Eds.) Elsevier, Amsterdam 1982, Stud. Surf. Sci. Catal., vol. 12, p. 123. R.K. Unger and M.C. Baird, J. Chem. SOC., Chem. Commun (1986) 643. D.K. Lee and S.K. Ihm , J. Catal. 106 (1987) 386. W.R. Hastings, C.J. Cameron, M.J. Thomas and M.C. Baird, I n o r g . Chem. 27 (1988) 3024. L.F. Rao, A. Fukuoka, N. Kosugi, H. Kuroda and M. Ichikawa, J. Phys. Chem. 94 (1990) 5317. D.R. Corbin, W.C. Seidel, L. Abrams, N. Herron, G.D. Stucky and C.A. Tolman, I n o r g . Chem. 24 (1985) 1800. T.N. Huang and J. Schwartz, J. Am. Chem. SOC. 104 (1982) 5244. A. Mahay, G. Lemay, A. Adnot, I.M. Szoghy and S. Kaliaguine, J. Catal. 103 (1987) 480. J.A. Rabo, V. Schomaker and P.E. Pickert, in "Proc. 3 r d I n t . Congr. Catal." (W.M.H. Sachtler, Ed.), North-Holland, Amsterdam 1965, vol. 2, p. 1264. R.A. Dalla Betta and M. Boudart, i n "Proc. 5th I n t . Congr. Catal." (J.W. Hightower, Ed.), North-Holland, Amsterdam 1972, vol. 2, p. 1329. P. Gallezot, Catal. Rev.-Sci. Eng. 20 (1979) 121. M. Briend-Faure, J. Jeanjean, M. Kermarec and D. Delafosse, J. Chem. SOC., Faraday Trans 1 , 74 (1978) 1538.

76 32 33

34

35 36 37 38 39 40 41 42 43 44 45

46 47 48 49 50 51 52 53

54 55 56

57 58

59

60

J.T. Richardson, J. Catal. 21 (1971) 122. P.A. Jacobs, i n "Metal Microstructures i n Zeolites" (P.A. Jacobs, N.I. Jaeger, P. J i r u and G. Schulz-Ekloff), Elsevier, Amsterdam 1982, Stud. Surf. Sci. Catal., vol. 12, p. 71. Kh. M. Minachev, G.V. Antoshin, E.S. Shpiro and Yu. A. Yusifov, i n "Proc. 6th Int. Congr. Catal." (G.C. Bond, Ed.), Chemical Society, London 1977, vol. 2, p. 621. J.H. Lunsford and D.S. Treybig, J. Catal. 68 (1981) 192. G. Schulz-Ekloff, D. Wright and M. Grunze, Zeolites 2 (1982) 70. W. Romanovski, Z. anorg. allg. Chem. 351 (1967) 180. D. Herrnerschmidt and R. Haul, Ber. Bunsenges. Phys. Chem. 84 (1980) 902. L.R. Gellens, W.J. Mortier and J.B. Uytterhoeven, Zeolites 1 (1981) 11. G.A. Ozin, M.D. Baker and J. Godber, J. Phys. Chem. 88 (1984) 4902. J.R. Morton and K.F. Preston, Zeolites 7 (1987) 2. G. Bergeret, P. Gallezot and B. Imelik, J. Phys. Chem. 85 (1981) 411. K. Moller and T. Bein, J. Phys. Chem. 93 (1989) 6116. Z. Zhang, H. Chen, L.-L. Sheu and W.M.H. Sachtler, J. Catal. 127 (1991) 127. P. Gallezot, J. Datka, J. Massardier, M. Primet and B. Irnelik, in "Proc. 6th I n t . Congr. Catal." (G.C. Bond, Ed.), Chemical Society, London 1977, vol. 2, p. 696. L.L. Sheu, H. Knozinger, W.M.H. Sachtler, J. Am. Chem. SOC. 111 (1989) 8125. E.S. Shpiro, R.W. Joyner, Kh.M. Minachev and P.D.A. Pudney, J. Catal. 127 (1991) K.-P. Wendlandt, H. Bremer, F. Vogt, W.P. Reschetilowski, W. Morke, H. Hobert, M. Weber and K. Becker, Appl. Catal. 31 (1987) 65. G. Larsen and G.L. Haller, Catal. Lett. 3 (1989) 103. B.C. Beard and P.N. Ross, J. Phys. Chem. 90 (1986) 6811. T. Sheng, X. Guoxing and W. Hongli, J. Catal. 111 (1988) 136. R. Lamber, N.I. Jaeger and G. Schulz-Ekloff, J. Catal. 123 (1990) 285. N.I. Jaeger, P. Ryder and G. Schulz-Ekloff, i n " S t r u c t u r e and Reactivity of Modified Zeolites" (P.A. Jacobs, N.I. Jaeger, P. Jiru, V.B. Kazansky and G. Schulz-Ekloff, Eds.), Elsevier, Amsterdam 1984, Stud. Surf. Sci. Catal., vol. 18, p. 299. M.S. Tzou, H.J. Jiang and W.M.H. Sachtler, Appl. Catal. 20 (1986) 231. L.F. Rao, A. Fukuoka and M. Ichikawa, J. Chem. SOC., Chem. Commun. (1989) 458. M. Dufaux, M. Lokolo, P. Meriandeau, C. Naccache and Y. Ben Taarit, i n "New Developments in Zeolite Science and Technology" (Y. Murakami, A. I i j i m a and J.W. Ward, Eds.), Elsevier, Amsterdam 1986, Stud. Surf. Sci. Catal., vol. 28, p. 929. N.I. Jaeger, G. Schulz-Eklaff and A. Svensson, cf. ref. 56, p. 923. (a) N.I. Jaeger, G. Schulz-Ekloff and A. Svensson, i n "Catalysis and Adsorption by Zeolites" (G. dhlmann and H. Pfeifer, Eds.), Elsevier, Amsterdam 1991, Stud. Surf. Sci. Catal., vol. 65, p. 327. (b) N.I. Jaeger, A. Jourdan and G. Schulz-Ekloff, J. Chem. SOC., Faraday Trans.1, 1991 in press. (a) A. Kleine, P.L. Ryder, N. Jaeger and G. Schulz-Ekloff, J. Chem. SOC., Faraday Trans. 1, 82 (1986) 205. ( b ) D. Exner, N. Jaeger, A. Kleine and G. Schulz-Ekloff, J. Chem. SOC., Faraday Trans. 1, 84 (1988) 4097. G. Schulz-Ekloff and N. Jaeger, Catalysis Today 3 (1988) 459.

61 62 63 64 65 66 67 68 69 70 71 72

73

74 75

76 77 78 79

80

81 82 83 84

85 86

N.I. Jaeger, J. Rathousky, G. Schulz-Ekloff, A. Svensson and A. Zukal, cf. ref. 16, vol. 498, p. 1005. J. Rathousky, A. Zukal, N.I. Jaeger and G. Schulz-Ekloff, in preparation. A. Kleine a n d P.L. Ryder, Catalysis Today 3 (1988) 103. G.A. Ozin, A. Kuperman and A. Stein, Angew. Chern. 101 (1989) 373. S. Schrnitt-Rink, D.A.B. Muller a n d D.S. Chemla, Phys. Rev. B 35 (1987) 8113. G.D. S t u c k y and J.E. Mac Dougall, Science 247 (1990) 669. P. M a r q u a r d t , L. Borngen, G. Nimtz, H. Gleiter, R. Sonnberger and J. Zhu, Phys. L e t t e r 114 A (1986) 39. G. Nirntz, P. M a r q u a r d t and H. Gleiter, J. C r y s t a l Growth 86 (1988) 66. D.R. Rolison, Chern. Rev. 90 (1990) 867. Y. Wang and N. Herron, J. Phys. Chem. 91 (1987) 257. M.A. Fox a n d T.L. Pettit, Langrnuir 5 (1989) 1056. (a) R.D. Strahrnel, T. Nakamura and J.K. Thomas, J. Chern. SOC., Faraday Trans. 1, 84 (1988) 530; ( b ) X. L u i and J.K. Thomas, Langmuir 5 (1989) 58. (a) N. Herron, Y. Wang, M.M. Eddy, G.D. S t u c k y , D.E. Cox, K. Moller and T. Bein, J. Am. Chem. SOC. 1 1 1 (1989) 530; ( b ) K. Moller, M.M. Eddy, G.D. S t u c k y , N. H e r r o n and T. Bein, J. Am. Chem. SOC. 111 (1989) 2564. M. Wark, G. Schulz-Ekloff and N.I. Jaeger, Catalysis Today 8 (1991) 467. M. Wark, G. Schulz-Ekloff, N.I. Jaeger and A. Zukal, in "Zeolite Chemistry and Catalysis" (P.A. Jacobs, N.I. Jaeger, L. Kubel k o v a and B. Wichterlova, Eds.), Elsevier, Amsterdam 1991; Stud. S u r f . SCi. Catal., t h i s book, vol. , p. M. Wark, G. Schulz-Ekloff and N.I. Jaeger, Mater. Res. SOC. Symp. Proc.; Materials Research Society, P i t t s b u r g h 1991, in press. M.I. Vucemilovic, N. Vukelic and T. Rajh, J. o f Photochem. and Photobiol. 42 A (1988) 157. S. Gallardo, M. Guiterrez, A. Henglein and E. Janata, Ber. Bunsenges. Phys. Chem. 93 (1989) 1980. W. Lutz, H. Fichtner-Schmittler, M. Bulow, E. S c h i e r k o r n , N. Van Phat, E. Sonntag, I. Kosche, S. Arnin and N. Dyer, J. Chem. Soc., Faraday Trans. 1, 86 (1990) 1899. D.E. Sayers and B.A. Bunker, i n "X-ray Absorption" (D. Koningsberger a n d R. Prins, Eds.), Wiley, New York 1988; Chemical Analysis, vol. 92, p. 211. H.M. Schmidt and H. Weller, Chem. Phys. Lett. 129 (1986) 615. L.E. Brus, J. Phys. Chem. 90 (1986) 2555. Y. Wang, A. Suna, M. Mahler and R. Kasovski, J. Chern. Phys. 87 (1987) 7315. (a) A.N. Chestnoy, T.D. Harris, R. Hull a n d L.E. Brus, J. Phys. Chern. 90 (1 986) 3393. (b)Y. Wang and N. Herron, J. Phys. Chem. 92 (1988) 4988. E. Corcoram, Spektrum d e r Wissenschaft, Januar 1991, p. 76. (a) V.N. Bogomolov, S.V. Kholodkevich, S.G. Rornanov, L.S. Agroskin, Solid State Cornrnun. 47 (1983) 181; ( b ) 0. Terasaki, K. Yamazaki, J.M. Thomas, T. Ohsuna, D. Watanabe, J.V. Sanders and J.C. B a r r y , Nature 330 (1987) 58.

78 J.B. Parise, J.E. Mac Dougall, N. Herron, R. Farlee, A.W. Sleight, Y. Wang, T. Bein, K. Moller and L.M. Moroney, I n o r g . Chem. 27 (1988) 221. 88 P. Enzel and T. Bein, J. Phys. Chem. 93 (1989) 6270. 89 J.V. Caspar, V. Ramamurthy and D.R. Corbin, J. Am. Chem. SOC. 113 (1991) 600. 90 (a) A. Henglein, A. Kumar, E. Janata and H. Weller, Chem. Phys. Lett. 132 (1986) 133. ( b ) M. Haase, H. Weller and A. Henglein, J. Phys. Chem. 92 (1988) 4706. E. Hilinsky, P. Lucas and Y. Wang, J. Chem. Phys. 89 (1988) 3435. 91 92 Y. Wang, A. Suna, J. McHugh, E. Hilinsky, P. Lucas and R.D. Johnson, J. Chem. Phys. 92 (1990) 6927. 93 D. Baresel, W. Gellert, W. Sarholz and P. Scharner, Sensors and Actuators 6 (1984) 35. 94 D. Kohl, Sensors and Actuators 18 (1989) 71. 95 T. Bein, K. Brown and C.J. Brinker, cf. ref. 16, vol. 49 B, p. 887. 96 T. Bein, K. Brown, G.C. F r y e and C.J. Brinker, J. Am. Chem. SOC. 111 (1989) 7640. 97 (a) Z. Li, C. Lai and T.E. Mallouk, I n o r g . Chem. 28 (1989) 178. ( b ) J.S. Krueger, J.E. Mayer and T.E. Mallouk, J. Am. Chem. SOC. 110 (1988) 8232. (c) L. Persaud, A.J. Bard, A. Campion, M.A. Fox, T.E. Mallouk, E.S. Webber and J.M. White, J. Am. Chem. SOC. 109 (1987) 7309. 98 S.D. Cox, T.E. Gier, G.D. Stucky and J. Bierlein, J. Am. Chem. SOC. 110 (1988) 2986. 99 Y . Wang and N. Herron, J. Phys. Chem. 95 (1991) 525. 100 R. Hoppe, G. Schulz-Ekloff, D. Wohrle, M. Ehrl and C. Brauchle, cf. ref. 75 ( t h i s book). . 101 W.E. Moerner (Ed.), Persistent Spectral Hole Burning: Science and Applications, Topics i n C u r r e n t Physics, Springer, New York 1988, p. 251. 102 R. Hoppe, G. Schulz-Ekloff and D. Wohrle, submitted. 87

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 01991 Elsevier Science Publishers B.V., Amsterdam

I9

ISOMORPHOUS SUBSTITUTION IN ZEOLITES: A ROUTE FOR THE PREPARATION OF NOVEL CATALYSTS G. Bellussi and V. Fattore ENIRICERCHE S.p.A., Via F.Maritano 26, 20097 San Donato, Milan, Italy Abstract In the recent years a growing interest in the preparation, characterization and utilization of Titanium-silicalite has been observed. New interesting applications of TS-1, such as the low temperature oxidation of paraffins to alcohols and ketones, were reported. In this paper we will review the state of the knowledge concerned with the preparation of titanium-containing zeolites and the characterization of lattice Ti-sites. We will try to depict the emerging routes for research activities in this field. 1. Introduction With the growth of the interest in the utilization of zeolites as catalysts, more attention was devoted to the introduction of various elements as substitute for lattice silicon or aluminum. Prof. Barrer, some years ago, classified four types of isomorphous replacement in zeolites [l]: 1- One guest molecule by another (i.e.: substitution of sodium chloride by sodium sulphate transforms Sodalite into Nosean). 2- One cation by another (i.e.: the more common method is the treatment of a zeolite with an aqueous solution of the salt containing the different cation; it is the base of water sweetening). 3 - One element by one of its isotopes (i.e. : mainly hydrogen, oxygen and silicon). 4 - One element in tetrahedral position by another (i.e.: substitution of silicon or aluminum with sterically compatible elements). Type one substitution has low relevance from the point of view of zeolite applications. Type two substitution is important because through it is possible to obtain active catalysts and to eliminate certain cations contaminating water or solutions. Type three substitution could be useful to investigate the zeolite synthesis and for Characterization purposes. Studies on type four substitution were initiated many years ago. Goldsmith [ 2 ] referred in 1952 his success in the synthesis of

80

Thomsonite samples in which Si was replaced by Ge in the zeolite framework. Again in the fifties appeared the remarkable work of Barrer et al. [ 31 : Thomsonite, zeolite A, Faujasite and Harmotome wereobtained havinggalliumand/or germaniuminthe lattice. These works were followed by a worldwide scientific effort to identify zeolites in which isomorphous substitution could give rise to: - new structures, - new chemical compositions, - new properties and, consequently, new applications. This paper will deal with type four substitution taking in consideration one of the most interesting case: the replacement of Ti for Si in Silicalite-1. The synthesis of titano-silicates considered to exhibit zeolitic properties was described the first time in the patent literature by D.A. Young in 1967 [4]. The information given in the Young’s patents were not sufficient to draw clear conclusions about the crystalline structure of that compounds. More than ten years later, again in the patent literature, Taramasso et al. describedthe synthesis of a titanium containing silicalite [5]. In the following years, several other patent applications claimed the possibility to prepare Ti-containing zeolites, but clear evidences for the presence of titanium in framework positions were not reported until 1986 when Perego et al. described in a paper the synthesis and the characterization of the Titanium-silicalite-1 (TS-1) [6]. The presence of titanium in the silicalite structure gave to the TS-1 original properties in oxidation reactions with hydrogen peroxide [6-131. The peculiar TS-1 catalytic activity, never observed before with other Ti-containing materials, open new and very interesting perspectives for industrial applications of shape selective catalysis. In the recent years, several papers dealing with the synthesis, the characterization and the catalytic activity of the TS-1 have been published, nevertheless many questions about its preparation, activation and characterization of active sites remain still open. 2. Physical-chemical characterization of TS-1

Only for few elements the possibility to substitute Si or A1 in zeolitic structure has beendemonstrated. In factonlycationswith specific steric requisites can fit in the tetrahedral positions of the zeolite lattice. K.G. Ione et al. [14], by applying the Pauling criterion, concluded that only the cations for which the r a t i o g = rc/ro, where rc andr,arethe radiiofthe consideredcationan the oxygen respectively, is in the range 0.414>p>0.225 should be stable in a tetrahedral surrounding. Then we have to expect that for cations for which p is out of that range, the substitution should be impossible or it could take place only on a limited scale. In the latter case it appears very difficult to demonstrate whether an element is really sitting in the lattice and not only supported

81

in an extraframework position. In the case of titanium,p is equal to 0.515; this can explain the difficulties encountered to recognize its presence in the TS-1 lattice. Many different physical-chemical methods where used to carry out such kind of characterization. X-ray diffraction analysis was the more effective technique. For samples havinga Ti molar fractionx =Ti/(Si +Ti) lower than 0.025 a clear increase of the unit cell volume was observed with the increasing of x [ 6 ] . The monoclinic lattice symmetry, characteristic of Silicalite-1, is preserved up to x = 0.01; for higher values of x, orthorombic symmetry was observed. The unit cell volume variation was correlated to the higher length of the Ti0 bond with respect to the Si-0 bond. The cell volume was related to the framework composition through the equation:

is the cell where V(,i)is the cell volume of the silicalite, V volume of TS-1, x is the Ti molar fraction, dTi andxAsi represent the Ti-0 and Si-0 bond length respectively. By applying the above equation to the TS-1, a value of 1.79 A was obtained forthe Ti-0 bond length. This valuewas in agreement with Ti-0 bond distances measured for BaZTiOq in which Ti displays a tetrahedral coordination. Since this equation does not consider T-0-T angle variations, the presence of framework T atoms in a distorted tetrahedral symmetry can affect the T-0 bond distance values derived from it. As a difference with the silicalite, TS-1 shows a characteristic IR band at 960 cm-l. Adsorption of water shifts upwards the 960 cm-l peak to 970-975 cm-lj151. By increasing the Ti molar fraction in TS-1 from 0 to 0.025 a linear increase of the above mentioned IR band intensity with x was observed [15]. This means that the 960 cm-l IR band is in someway related to the presence of lattice titanium. SEM-EDX analysis of TS-1 showed an homogeneous distribution of Ti through the TS-1 crystals [ 6 ] . Evidence in favour of the structure homogeneity of TS-lwas achieved a l s o by FABMS analysis [161. More recently, A. Tuel et al. demonstrated the possibility to reduce the tetravalent titanium in TS-1 giving rise to the formation of Ti3+ species [ 171. The signals on EPR spectra of the reduced TS-1 sample were attributed to Ti3+ in tetrahedral coordination; from that the authors concluded about the framework sitting of the precursor tetravalent Ti. All the above mentioned experimental results and the peculiar TS-1 catalytic activity, lead to conclude about the presence of titanium atoms in framework position but as we will see in one of the next paragraphs, several different hypothesis can be thought to describe the environment of such titanium atoms.

a2

3 . Direct synthesis of TS-1

The synthesis of TS-1 is particularly difficult, probably because of the higher tendency displayed by Ti4+ to assume the octahedral coordination with respect to the tetrahedral in compounds with the oxygen. In the patent of Taramasso et al. [5] two methods for the hydrothermal synthesis of TS-1 are described. In the first method the reaction mixture is prepared by hydrolysis of tetraethylsilicate and tetraethyltitanate, while in the second it is prepared from colloidal silica and tetrapropylammonium peroxotitanate. One of the critic point of these synthesis is the presence of alkali cations, even in trace amounts, in the reaction mixture. Several authors [18-211 proved that the presence of sodium or potassium can prevent the insertion of titaniuminto the silicate framework. These cations are contained as impurity in most of the commercial solution of tetrapropylammonium hydroxide. The addition of increasing amount of NaOH to a solution of high purity tetrapropylammonium hydroxide, was showed to produce an increase of the amount of Ti detected in the solid recovered at the end of the synthesis and a d e ~ r e a s e o f t h e 9 6 0 c m ~ ~ I R b a n d[21]. At the highest level of sodium, the presence of anatase beside the silicalite crystals, was revealedby X - r a y d i f f r a c t i o n a n a l y s i s . The presence of sodium in the reaction mixture seems to favour the formation of insoluble titanium-silicate species which reduce the amount of titanium available for the formation of the TS-1 crystals. In few cases, the synthesis of a titanium containing silicalite prepared in the presence of sodium was reported in the literature [22-241. Starting f r o m a g e l h a v i n g t h e f o l l o w i n g m o l a r c o m p o s i t i o n : Si02, 0.004-0.04 TiO2, 0.14 Na20, 0.1 TPA-Br, 23.5 H20, Kornatowsky et al. [22] reported the formation of very large silicalite crystals (160-180 pm) containing 1 Ti atom every 60 Si atoms. The influence of the molar SiOZ/Ti02 ratio, in the starting reaction mixture, on the Ti content of TS-1 is reported in Fig. 1. For these experiments we used a synthesis procedure similar to thatdescribedinthe exarnplelof [5]. The syntheseswereperformed in the absence of alkali cations, by using tetraethyltitanate and tetraethylsilicate as sources of Ti02 and Si02 respectively. The TS-1 results always with a higher SiOz/TiO2 ratio compared to the starting solution. At the higher concentration of titanium in the reaction mixture, the formation of bulky Ti02 was observed. Crystallization temperature has a strong influence on the TS-1 composition. Fig. 2 shows how variable is the Si02/Ti02 ratio in the TS-1 with the variation of the crystallization temperature. It is relevant that from 100 " C to 200 " C no anatase is observed; its formation starts at higher temperatures. The maximum amount of framework Ti obtained in these experiments was correspondent to 2 Ti atoms per elementary cell.

83

Si02/Ti0 10 20 50

13

(reagent mixtures)

100

300

110

42 59

325

Si02/Ti02 (products) Figure 1. Influence of the reaction mixture composition on Ti contents in TS-1.

SiO /Ti0 20

2

23 30

(reagent mixture)

2

40

60

97

Si0,/Ti02 (products) Figure 2. Influence of the crystallization temperature on Ti contents in TS-1. An interesting method for the synthesis of TS-1 was recently reported by Padovan et a1 [ 251. A sample of dried microspheroidal silica was impregnated up to incipient wetness with an aqueous solution obtained by hydrolizing Ti isopropoxide in aqueous tetrapropylammonium hydroxide. The sample was then sealed in a glass tube and kept at 448 K for several hours. A well crystallized titanium containing silicalite was obtained after 10 hours. At longer crystallization times, the formation of octahedralTi02was revealed by UV-Vis. spectroscopy. J.M. Popa et al. described the preparation of a titanium containing silicalite from a reaction mixture having a very low pH (6.5 - 7.5) andcontaining fluoride anions [26]. The crystalline product obtained by this method had a monoclinic symmetry. No evidence was given to support the presence of titanium in lattice position. Several efforts has been devoted to the synthesis of silicalite

.

84

crystals containingtitanium and atrivalent element. The interest in this kind of material is not only related to the possibility of having catalysts active in both oxidizing and acid catalyzed reactions, but also to investigate forthe presence of synergysms between the two sites. Several patents describe the synthesis of silicalites containing Ti and A1 [27-291, Ti and Fe [30], Ti and Ga [31], Ti and B [32]. In many of them, only applications in acid catalyzed reactions were reported. We demonstrated in a recent paper the possibility to insert Ti and A1 or Ga or Fe in lattice position in the Silicalite-1 [21]. The amounts of framework titanium and trivalent element are limited to a restricted range of compositions. In fact, when the amount of framework trivalent element is increased above a certain value ( Si02/M203 - 150), the amount of lattice titanium begins to decrease. We observed a different catalytic behaviour between TS-1 and Al(Ga,Fe)-TS-1 in the butene epoxidation with H20e2. While TS-1 is very selective toward the formation of the epoxide, the other catalysts, because of the presence of acid sites, are more selective toward the formation of glycols. The rate of H 2 0 2 conversion was lower when the selectivity to glycols was higher. This was explained by considering that the slow diffusion rate through the silicalite channels of glycols and polyglycols formed on the acid sites hinders the diffusion of H 2 0 2 and of the olefins. The difference in the rate of H 2 0 2 conversion accounts for the presence of lattice Ti and Al(Ga,Fe) in the same crystals. The direct synthesis of titanium-containing zeolites with a framework topology different from the MFI was also investigated. The preparation of a TS-2 (titanium-silicalitewith MEL framework topology) has been reported [33-341. The TS-2 can be prepared in the same way as the TS-1 but substitutingthe tetrapropylammonium cation with the tetrabutylammonium during the synthesis. As already observed in the case of the corresponding aluminosilicates [35-361 and boro-silicates [37], the use of mixed alkylammonium cations allow the formation of titanium-silicalites witha structure intermediatebetween that of MFI andMEL structure type 1341 * The synthesis of titanium containing Mordenite, Sodalite and ZSM-12 is also reported in the patent literature [28]; the informations given are not sufficient to drawn conclusions about the situation of titanium. Apart from the MFI and MEL structure type, there are not yet clear evidences about the possibility to insert titanium atoms in other zeolite structures by direct synthesis. Recently D.M. Chapman et al. described a method for the synthesis of crystalline microporous titanium-silicates. These materials are different from the zeolites since their lattice is constitued by tetrahedral SiO, and octahedral Ti02 units [ 3 8 ] . Xuznicki reported in a patent the preparation of a crystalline microporous titanium-silicate with pore size of approximately 8 A [39]. It is not clear if this new material is really constitued of Si02 and Ti02 tetrahedra or if the TiO, units are in octahedralcoordination as in the case mentioned above.

85

4. Secondary synthesis of TS-1 A procedure able to modify the lattice composition of a preformed zeolite leaving the framework topology relatively unchanged is indicated with the terms "indirect synthesis" or "secondary synthesis". The method consists in contacting the zeolite crystals with a suitable compound of the element to be inserted in the framework. This procedure has been mainly used to substitute silicon for aluminum atoms in Y zeolite [40]. Several examples describing the indirect synthesis of titanium containing zeolites are reported in the literature (Table 1). The treatment of the zeolite crystals with titanium compounds are made either through the contact with a liquid phase [41], or witha gas phase [42-441.The titanium salts used forthe secondary synthesis are gaseous TiC14 or an aqueous solution of (NH,),TiF,. Skeels and Flanigen [41] reported the preparation of several titanium containing zeolites: Faujasite, Phillipsite, Omega, L, ZSM-5. They put in contact the zeolite crystals with a solution of (NH4)2TiF6 at 75 - 95 " C . The reaction with the ammonium f l u o r o t i t a n a t e p r o d u c e s t h e r e m o v a l o f acertainamountof aluminum and the deposition of a relevant quantity of titanium on the solid phase. In spite of the high amount of titanium detected on the treated samples no new IR band, attributable to the presence of titanium, was observed in the region between 900 and 1000 cm-l. Table 1. Examples of secondary synthesis of Ti-containing Zeolites Precursor

SiOz/ A1 7 0 -

Pretreatment

Fauj asite Omega L Phi11ip ZSM-5 ZSM-5 ZSM-5 ZSM-5 Faujasite Beta Bor-C Bor-C

5.08 6.62 5.80 3.77 30.82 46.49 50.00 25.00 (el (el 73(f) 73(f)

NH4+ NH4+ NH & NH4+ NH 4+

.

form form form form form

--

dealum. -_

H+ H+

form form

--

--

Ti salt

Si02/ SiO,/ A1 0, TiOz (2) (c)

Ref,

7.76 7.55 8.04 8.82 7.36 12.74 4.88 6.00 58.34 13.22 62.20 90.00 2000 s 80 25.00 36.00 (el (el (el (el 86(f) 40.00 99(f) 27.00

41 41 41 41 41 41 42 43 43 43 44 44

(a) zeolite composition (molar ratio) before the treatment with the titanium salt. (b) zeolite composition (molar ratio) after the treatment with the titanium salt. (c) (NH4),TiF6 in aqueous solution. (d) TiC1, in the gas phase. (e) not reported. (f) Si02/B203 molar ratio.

86

T h e p r e p a r a t i o n o f t i t a n i u m c o n t a i n i n g ZSM-5 bycontacting a ZSM5 sample with TiC1, in the gas phase at 200 - 500 ' C was described byKraushaarandVanHooff I 4 2 1 a n d l a t e r b y F e r r i n i a n d K o u v e n h o v e n [43]. In the first case the secondary synthesis is performed on a sample of dealuminated ZSM-5 and the substitution presumably takes place on the sites made vacant from the removal of aluminum. In the second case the synthesis is performed on a ZSM-5 not dealuminated and the treatment with TiC14 does not reduce the aluminum content; in this case, if titanium replace some lattice aluminum, extraframework aluminum oxide must be formed in the zeolite crystals. In both the above mentioned works samples treated with TiC1, have an IR absorbance band at 960 cm-’. Ferrini observed the appearance of such a band also on Ti-containing Beta and Y samples prepared by secondary synthesis, We tried the secondary synthesis of a titanium containing silicalite by having as a precursor a sample of MFI structure-type boro-silicate (Bor-C) [44]. The treatment with TiC1, produced a decrease of the boron content and the deposition of titanium on the Bor-C crystals. In the samples s o prepared an IR absorption band at 960 cm-I was detected. Analyzing samples after different times on stream of gaseous TiC14 by XPS spectroscopy, we observed at the beginning the formation of a Ti(2p) signal at the same binding energy as in the case of a TS-1 prepared by direct synthesis. Proceeding the treatment with TiCl,, a second Ti(2p) signal at a different b.e. appeared. The b.e. of this signal was equal to that observed for Ti4+ species in bulk titanium oxide compounds. Fromthese experimentwe c o n c l u d e d t h a t a t t h e b e g i n n i n g of the treatment a part of boron atoms, probably near to the crystals surface, could be substitutedbytitanium, but very soon, because of the steric hindrance of the TiCl,, the reaction of this compound with surface -OH groups gave rise to the formation of extraframework titanium oxide. 5 . The Titanium environment in TS-1 lattice

If a cation M4+ is a substituent for Si,’ in a zeolite framework, it is expected to be bonded to four silicon atoms through bridging oxygens in tetrahedral coordination. In the case of TS-1, doubts arose about the real environment of Ti4+, mainly because of the possible different attributions for the characteristic TS-1 IR peak at 960 cm-l. In the same region of the IR spectrum are located the absorption lines for the following groups: (SiO), Si-OH ; (SiO), Si-OTi ;

(SiO), Ti=O

[15].

An extensive spectroscop’ic characterization of TS-1 was carried out by Boccuti et al. [15]. The authors concluded that the 960 cm-’ IR peak is due to the Si-0 stretching of the polarised Si-Od---Ti (IV) bond. This hypothesis was supported on the ground of the following: a) the electronic transitions of the titanyl group (expected at

87

25000-35000 cm-l) are not present in the U.V.-Vis. reflectance spectrum of TS-1; b) the peak at 960 cm-I does not show any tendency to exchange with 1802 even at high temperature (700 C); c) the peak at 960 cm-I is totally insensitive to the reduction in molecular H2 at 700’C. A strong transition band at 48000 cm-’ observed in the UV-Vis spectrum of TS-1 was attributed to a transition having charge transfer character involving the Ti (IV) sites schematized in the following:

0

\

\

/ Ti / \

/

Si / \ 0 0

0

0

\

\

/

/

/

Si / \

Si / \

H H

H H

H H

00 \ /

00 \ /

00 / \

Ti / \ 0 0

0 0

/

Si / 0 0

\

\

Si / \ 0 0

/

Si / I

Ti / \ 0 0

Other authors, on the ground of results obtained in olefins epoxidation [18] and paraffins oxidation [12] suggested the following situation for the titanium site:

H \

H

fTil

/O

Si /

\

/ 0 0

0 \

/

Si \

/ 0 0

\

In this case the TS-1 IR peak at 960 cm-I should be attributed to the (SiO),Si-OH or to the Ti=O stretching. A recent study on interactions at low temperature (20-60 "C) of H2170, H2180, and D,O with TS-1 adds new information related to this argument [45].The interaction of H2017 with large Silicalite-1 crystals ( s l mm) does not produce variation on 170-MAS-NMR spectra while the interaction of H2017 with small Silicalite-1 crystals ( 0.1 nun) produces the appearance of peaks due to the exchange of -OH group with H2170 on Si-OH defective sites. The same was observed for large crystals and small crystals TS1 with the exception that for both these samples a new signal appeared at 370 ppm chemical shift. The latter was probably due to the interaction of H2170 with titanium sites. The IR band at 960 cm-l was shifted to lower frequencies after exchange with H 2 1 7 0 or with H2180 as expected, but no variation was observed upon exchange with D20. From this, the attribution of the 960 cm-’ IR band to the group (SiO), Si-OH must be excluded. Moreover, the above mentioned results indicate that even at low temperature there is an exchange of water in proximity of the titanium sites. The exchange can be represented by the two schemes:

88

SiO

OSi \

I

/ SiO

\

Ti

+ H20*

Sio O*H \ / / Ti + HO-Si/

OSi

\

SiO

\

-

Sio o*Si \ / Ti + H20 /

-

OSi

SiO

\

OSi

O*H \

Ti=O

+

/

\

H20*

\ Ti=O* + H20 /

/

Ti / \ OH

B y c o n s i d e r i n g a l l t h e p o s s i b i l i t i e s , the following overall scheme

can be drawn to describe all the possible interactions of the Tisites with water: SiO

SiO

OSi

\

HOSi

/

HOSi

\

HOSi Ti-OH + I \ SiO OSi

H20 / /

SiO

OH HOSi

The possible existence of these equilibria even at room temperature, can make very difficult to state about the absence of one or another of the above depicted intermediates. 6 . TS-1 interaction with hydrogen peroxide

By wetting the TS-1 microcystalline powder with a solution of hydrogen peroxide, its color turns from white to bright yellow. The treatment causes also the appearance of a band in the visible spectrum at 425 nm [12]. These experimental results are in agreement with the formation of titanium peroxoderivatives. The interaction of TS-1 with H202 causes the disappearance of the 960 cm-I IR band; by heating the sample this band reappears. This indicatesthat thedecompositionofthe peroxo-derivative restores the original site [12]. According to literature data [46] two different peroxo forms can be thought:

89

HO-Si SiO

SiO

0

1

\

\Ti/ \ SiO 0 HO-Si /

and

OSi /

Ti / ’0-OH SiO HO-Si

In a recent paper we reported the results of our investigation on the peroxo-derivatives of TS-1 (hereafter referred.as TS-1-0,) [45]. TS-1-02 shows an acidity that is much higher than that of TS-1 or Silicalite-1 or Silicalite-1/H202: the first one in fact hydrolizes much more faster the trans-2-3-epoxybutane than the latter three. The acidity of TS-1-0, may come from one of these two situations:

H\ /"i sio O o

sio

0'

/si

o

Si0

SiO

0

SiO

$ + H +

Ti

)l C2H50H > H20. This behaviouris consistentwith the presence of cyclic complexes like: R *H SiO-Ti / \ 0-O~H sio

H and

SiO-Ti / \ SiO 0-O’H

Although, according to literature data [ 4 6 1 ,groups IV-VI t r a n s i t i o n m e t a l p e r o x o c o m p o u n d s have the structure (I), theabove mentioned experimental results strongly suggest the presence of the hydroperoxo form ( 1 1 ) in the TS-1 peroxo derivative. Another question still open concerns whether the oxidation of organic substrates goes through an homolytic or an heterolytic

90

pathway . The recent discovery of the TS-1 capability to oxidize paraffins and the TS-lactivityin aromatic hydroxilation could indicate the involvement of radical species in the oxidation mechanism. This has been shown to be most likely in the oxidation of paraffins [ 12131. On the other hand, in the epoxidation of olefins higher than C3, no isomerization products has been observed: cis-epoxide is formed only from cis-olefins [7]. The stereoselectivity in this reaction is more consistent with the heterolytic pathway. 7 . Conclusions

The substitution of Ti4+ for Si4+ in zeolitic structure is a certainly new and interesting field of study for researchers involved in zeolite synthesis. Although strong research effort is still required for a full u n d e r s t a n d i n g o f t h e T i s i t u a t i o n inTS-landofthe transformation it undergoes upon interaction with different molecules, it is already possible to state that Ti in TS-1 produces a high performance, high flexible and high stable catalytic site. This three properties allowed to carry out several oxydation reactions with unexpected activity and selectivity. The possibility to prepare Titanium-silicalites containing also lattice trivalent elements beside titanium open new opportunity for the application of these materials in heterogeneous catalysis. There is still anotherwide space for research activities related to titanium containing zeolites: the synthesis of large pores Tizeolites, the activationof TS-lwith oxidants different fromHZOZ, the comprehension of the mechanism of reactions catalyzed by TS1. These are just some of the more actractive perspectives for future research in this field. 8 . References

R.M. Barrer, inD. OlsonandA. Bisio, Proc. of 6th Int. Zeolites Conf., Reno July 10-15 1983, Butterworth Ltd UK, (1984) 870. 2 J.R. Goldsmith, Min. Mag., 29 (1952) 952. 3 R.M. Barrer, J.W. Baynham, F.W. Bultitude, W.M. Meyer, J. Chem. SOC., (1959) 195. 4 D.A. YOUng, US Patent No 3 329 481 (1967). 5 M. Taramasso, G. Perego, B. Notari, US Patent No 4 410 501 (1983) ; M. Taramasso, G. Manara, V. Fattore, B. Notari, US Patent No 4 666 692 (1987). G. Perego, G. Bellussi, C. Corno, M. Taramasso, F. Buonomo, Stud. in Surf. Sci. and Catal., 28 (1986) 129. U. Romano, A. Esposito, F. Maspero, C. Neri, M.G. Clerici , Stud. in Surf. Sci. and Catal., 55 (1990) 3 3 . P. Roffia, G. Leofanti, A. Cesana, M. Mantegazza, M. Padovan, G. Petrini, S. Tonti, P. Gervasutti, Stud. in Surf. Sci. an Catal., 55 (1990) 43. 9 M.G. Cleric:, G. Bellussi, U. Romano, J. of Catal., in press. 1

91

10 A. Thangaraj, R. Kumar, P. Ratnasamy, Applied Catal., 57 (1990) Ll-L3. 11 T. Tatsumi, M. Nakamura, S. Negishi, H. Tominaga, J.Chem. SOC., Chem. Comm., (1990) 476. 12 D.R.C. Huybrechts, L. De Bruycker, P. Jacobs, Nature, 345 (1990) 240. 13 M.G. Clerici, Applied Catalysis, 68 (1991) 249. 14 K.G. Ione, L.A. Vostrikova, M. Mastikhin, J. Mol. Cat. 31 (1985) 355. 15 M.R. Boccuti, K.M. Rao, A. Zecchina, G. Leofanti, G. Petrini, Stud. in Surf. Sci. and Catal., 48 (1989) 133. 16 A.G. Ashton, J. Dwyer, I.S. Elliott, F.R. Fitch, G. Qin, M. Greenwood,J. Speakman, in D. Olson and A. Bisio, Proc. of 6th Int. Zeolites Conf., Reno July 10-15 1983, Butterworth Ltd UK, (1984) 704. 17 A. Tuel, J. Diab, P. Gelin, M. Dufaux, J.F. Dutel, Y. Bee Taarit, J. of Mol. Catal., 63 (1990) 95. 18 B.Notari, Stud. in Surf. Sci. and Catal., 37 (1987) 413. 19 J. El Hage A1 Asswad, J.B. Nagy, Z. Gabelica, E.G. Derouane, inJ.C. JansenandL.MoscouEd.s,Proc. 8thInt. ZeolitesConf., Amsterdam July 10-14 1989, Recent Research Reports, (1989) 475. 20 B. Notari, Proc. of Int. Symp. onChem. of Microporous Crystals, Tokyo June 26-29 1990, Elsevier, in press. 21 G. Bellussi, A . Carati, M.G. Clerici, A . Esposito, Proc. of 5th Symp. on Scient. Bases for the Preparation of Het.Catalysts, Louvain-la-Neuve September 3-6 1990, Preprints (1990) 201. 22 J. Kornatowski, M. Malinowski, J.C. Jansen and L. Moscou Ed.s, Proc. 8th Int. ZeolitesConf., AmsterdamJulylO-14 1989, Recent Research Reports, (1989) 49. 23 X. Ruren, P. Wenquin, Stud. in Surf. Sci. and Catal., 24 (1985) 27. 24 R.Y. Saleh, Eur. Pat. Appl. No 132 550 (1985). 25 M. Padovan, F. Genoni, G. Leofanti, G. Petrini, G. Trezza, A. Zecchina, Proc. of 5th Symp. on Scient. Bases for the Preparation of Het. Catalysts, Louvain-la-Neuve September 36 1990, Preprints (1990) 221. 26 J.M. Pope, J.L. Guth, H. Kessler, Eur. Pat. Appl. No 292 363 (1988). 27 H. Baltes, H. Litterer, E.I. Leupold, Eur. Pat. Appl. No 77 522 (1982). 28 B.M.T. Lok, K.M. Bonita, E.M. Flanigen, US Patent No 4 707 345 (1984). 29 G. Bellussi, A. Giusti, A . Esposito, F. Buonomo,Eur. Pat.App1. No 226 257 (1988). 30 G. Bellussi, M.G. Clerici, A. Giusti, F. Buonomo, Eur. Pat. Appl. No 226 258 (1988). 31 G. Bellussi, M.G. Clerici, A. Carati, A. Esposito, Eur. Pat. Appl. No 266 285 (1988). 32 K.L.S.L. Kee, Eur. Pat. Appl. No 104 107 (1983). 33 J.S. Reddy,R.Kumar,P. Ratnasamy, AppliedCatalysis, 58 (19901 Ll-L4. 34 G. Bellussi, A. Carati, M. G. Clerici, A. Esposito, R. Millini,

92

F. Buonomo, Bel. Pat. No 1 001 038 (1989). 35 G.R. Millward, S . Ramdas, J.M. Thomas, M.T. Barlow, J. Chem. SOC. Faraday Trans. 11, 79 (1983) 1075. 36 G.A. Jablonski, L.B. Sand, J.A. Gard, Zeolites, 6 (1986) 396. 37 G. Perego, G. Bellussi, A. Carati, R. Millini, V. Fattore, in M.L. Occelli and H.E. Robson, Zeolite Synthesis, ACS Symposium Series 398, (1989) 360. 38 D.M. Chapman, A.L. Roe, Zeolites, 10 (1990) 730. 39 S.M. Kuzniki, US Patent No 4 853 202 (1989). 40 G.S. Skeels, D.W. Breck, in D. Olson and A. Bisio, Proc. of 6th Int. Zeolites Conf., Reno July 10-15 1983, Butterworth Ltd UK, (19841 87. 41 G.W. Skeels, R. Ramos, D.W. Breck, WO Patent No 85/04854 (1985). 42 B. Kraushaar, J.H.C. VanHooff, Catal. Letters, 1 (1988) 81; ibid. 2 (1990) 43. 43 C. Ferrini, H.W. Kouvenhoven, Stud. in Surf. Sci. and Catal., 55 (1990) 53. 44 A,Carati, S . Contarini, R. Millini, G. Bellussi, ACS Symposium on Synthesis and Properties of New Catalysts, Boston 26 Nov.1 Dec. 1990, Mat. Res. SOC. Ext. Abstract (EA-24), (1990) 47. 45 G. Bellussi, A. Carati, M.G. Clerici, G. Maddinelli, R.Millini, submitted to J. of Catalysis. 46 J.A. Connor, E .A.V. Ebsworth, in H. J. Emeleus and A.G. Sharpe Ed.s "Advances in Inorganic Chemistry and Radiochemistry", Academic Press, New York, Vol. 6 (1964) 279. 47 M.G.Clerici, G.Bellussi, Eur.Pat. Appl. No 315248 (1989).

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 0 1991 Elsevier Science Publishers B.V., Amsterdam

93

ZEOLITE SYNTHESIS WITH METAL CHELATE COMPLEXES K. J. Balkus, Jr.,

S.

Kowalak, K. T. Ly, and D. C. Hargis

Department of Chemistry, University of Texas at Dallas, Richardson, TX 75083-0688, United States

Abstract

X type zeolites have been synthesized in the presence of metallophthalocyanines (MPc, where M = Fe, C o , Ni, Cu) resulting in partial inclusion of these complexes. Synthesis variables such as metal loading, order of mixing, and aging will be discussed.

1. INTRODUCTION

The synthesis of zeolites and molecular sieves often requires a template or directing agent. Generally, these templates are cationic or nuetral organic molecules. The manner in which such templates affect zeolite crystallization is not straightforward. The organic additives may function as a structure-directing agent, gel modifier, buffer, and void filler [l]. Some zeolites such as synthetic faujasite type X can be prepared in the absence of organic additives. In this case the alkali metal ions may function as directing agents. If zeolites and molecular sieves crystallize around cationic and nuetral organic molecules as well as metal ions, then it is reasonable to expect that cationic and nuetral metal complexes might also act as directing agents. Surprisingly, the previous reports of zeolite synthesis in the presence of metal complexes appears limited to a few patents [2-41. We recently reported the preparation of X and A type zeolites [ 5 , 6 ] as well as A1P04-5 [7] in the presence of cationic and nuetral metal chelate complexes. Although, zeolites X and A are easiliy prepared in the absence of a template there is evidence that the metal complexes modify the crystallization. A feature of these syntheses is the partial encapsulation of the metal complexes. There are many examples of organic additives that become occluded in the zeolite during crystallization such as tetrapropylammonium ion in ZSM-5 [ 8 ] .

94

Nuetral templates may not be bound to the zeolite surface but simply trapped. This is the basis of our z e o l i t e s y n t h e s i s method for the preparation of ship-in-a-bottle metal complexes, ie crystallization of the zeolite around a metal chelate complex. Zeolite encapsulated metal complexes have many applications, ranging from shape selective catalysis [9] to magnetic resonance imaging contrast agents [lo]. X type zeolites are well suited for the preparation of encapsulated complexes by virtue of the large supercage (12A in diameter) and the restricted openings (7.4A) to the supercage. Several strategies have been explored for the entrapment of metal chelate complexes in synthetic faujsite type zeolites including the f l e x i b l e 1 i g a n d and t e m p 1 a t e s y n t h e s i s approach. The first method involves reacting a metal exchanged zeolite with a flexible chelate that can diffuse into the zeolite whereupon complexation becomes too large to exit. The tetradentate Schiff base N , N ' (salicyla1dehyde)ethylenediimine or SALEN has been used to prepare metal complexes in X and Y type zeolites [ 11-14]. In the t e m p l a t e s y n t h e s i s method the ligand precursors diffuse into the zeolite where they form the chelate and complex around a metal ion template. For example intrazeolite metallophthalocyanines (figure 1)have been prepared by the

Metallophthalocyanine (MPc)

condensation of four dicyanobenzene molecules inside a metal ion exchanged zeolite [15-241. The resulting complex is much too large to escape through the zeolite pores. In comparison to the z e o l i t e s y n t h e s i s approach there are many disadvantages associated with the preparation of intrazeolite complexes by the f l e x i b l e l i g a n d and t e m p l a t e synthesis methods. The complexes are difficult to characterize, especially if the ligand has multiple coordination modes available and some of the target metal ions may remain uncomplexed which will complicate any reactivity studies. Additionally, there are limitations to the types of metal complexes that might be encapsulated in a zeolite. The only criteria for incorporating metal complexes

95

during zeolite crystallization is that the complex must be stable at high pH and moderate temperatures. In addition to the variety of complexes that might be encapsulated during crystallization there are now many zeolites and molecular sieves that might be modified with metal complexes. For example we reported the crystallization of A type zeolites around copper (11) phthalocyanines [5]. The 4.1A apertures to the large cavity in A type zeolites are too small for dicyanobenzene to enter which precludes the template synthesis method for preparing the intrazeolite MPc. The potential exists for encapsulating metal chelates in a whole host of different zeolite and molecular sieve structures. In this paper we report the results for crystallization of X type zeolites in the presence of MPc complexes where M = Fe, Co, Nil and Cu (MnPc decomposed under synthesis conditions). The crystallization is affected by both the amount of complex added and the order of mixing. Additionally, the amount of complex encapsulated depends on the metal ion. 2.

EXPERIMENTAL

Silica gel and aluminum isopropoxide were purchased from Aldrich. Metallophthalocyanines were obtained from Strem Chemical and were used without further purification. Freshly prepared silicate and aluminate solutions were combined in the ratio 1 A 1 2 0 3 : 3.2Si02: 4Na20: 155H20 to produce an X type zeolite. The metal complexes were added in various amounts to either the silicate solution, aluminate solution or the initial gel mixtures combined in a ratio close to the X type recipe. The mixtures were crystallized in polypropylene bottles at 90 C. The resulting zeolites were washed with copious amounts of distilled water then Soxhlet extracted with pyridine and dried at 1OOC. Surface adsorbed metal complexes were removed by vacuum sublimation at 450-5OOC. FT-IR spectra were recorded as KBr pellets on a Nicolet 5DX spectrophotometer. X-ray powder patterns were obtained with a Scintag XDS 2000 diffractometer. Elemental analyses were performed by Galbraith Laboratories, Knoxville. 3. RESULTS

and DISCUSSION

The results for the synthesis of an X type zeolite in the presence of FePc, CoPc, NiPc, and CuPc are shown in table 1. In all cases here the metal complex or metal complex solution was added to the aluminosilicate gel immediately after mixing the silicate and aluminate solutions. The mixture was magnetically stirred for 15 minutes before heating. The resulting crystals were washed with water extracted with pyridine and sublimed. The surface MPc complexes can not be removed by solvent extraction. However, vacuum sublimation appears to be completely effective for removing non-intrazeolite complexes. The product zeolites were various shades of blue but became a very pale blue-green after

96

sublimation. The characteristic electronic spectra for phthalocyanines have been used previously to characterize the entrapped complexes. However, in our samples the level of encapsulation varies from -5-25% or between 1 MPc in 20 unit cells to 1 in 4 . At these l o w loadings interpretable spectra were not obtained. Except for MnPc the intact complexes were recovered from extractions and sublimations which provides additional evidence for stability during synthesis and purification. A highly crystalline X type zeolite can easily be prepared in the absence of organic additives in less than 4 hours. Slight variations in this recipe such as the amount of water does not have a dramatic effect on the synthesis. The addition of small amounts of metallophthalocyanine (-2% by weight relative to the silica gel) nearly doubles the crystallization time. Generally, phthalocyanines are Table 1 Results for X crystallization with MPc added to initial gel Si02:Na20:H20:MPca

Hrs

3.2: 4: 155 3.1: 3.7: 141:O. 007 3.0:3.6:138:0.007 3.1:3.7:141:0.005 3.0:3.5:138:0.007 3.1:3.7:141:0.006 3.0:3.6:139:0.007 3.1:3.7:141:0.005 3.0:3.5:138:0.009

10 10 10 10 10 10 10 8

4

T (C) Zeoliteb %M 90 90 90 90 90 90 90 90 90

-0.034 24 h no further modification of the shape selectivity is observed although the transformation of gallium into framework positions continues. The amount of: nonframework species is too low to cause a visible effect. 4 . CONCLUSION

During the course of crystallization, but especially in the subsequent recrystallization process the physico-chemical properties of the samples vary to a considerable extent with respect of the nature of the gallium, the crystal size and shape, and the perfection of the lattice, although the overall composition ot the Ga-ZSM 5 samples as well as their structure remains essentially unchanged. The manufacturing of catalysts with desired properties on the basis of Ga-ZSM 5 has to take into account these circumstances. 5. REFERENCES 1. C. T. W. Chu, G. H. Kuehl, R. M. Lago, C . 2. 3. 4.

5.

6. 7. 8.

D. Chang, J.

Catal., 93 (1989) 451. G . Choudurier, A . Auroux, J. C . Vedrine, R. D. Farlee, L . Abrams, R. D. Shannon, J. Catal., 108 (1987) 1. R . Szostak, V. Nair, T. L. Thomas, J. Chem. SOC., Faraday Trans. I, 83 (1987) 487. 0 . K. Simmons, R. Szostak, P. K. Agrawal, T . L. Thomas, J. Catal. 106 (1987) 787. H . K. Beyer, G. Borbely, in New Developments in Zeolite Science and Technology, Y. Murakami et al. (Eds.), Kodansha, Tokyo 1986, 867. T. Kanai, N. Kawata, Appl. Catal. 55 (1989) 115. M. Richter, W. Yiebig, H.-G. Jerschkewitz, G. Lischke, G. Ohlmann, Zeolites 9 (1989) 238. L. M. Parker, D. M. Bibby, J. E. Patterson, Zeolites 4 (1984) 168.

This Page Intentionally Left Blank

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 0 1991 Elsevier Science Publishers B.V., Amsterdam

119

STUDIEG ON THE PHOSPHORUS SUBSTITUTED ZEOLITES PREPARED BY SECONDARY SYNTHESIS W. Reschetilowski, W.-D. Einicke, B. Meler. E. Brunner and

H.

EfnSt Karl MarX University Leipzig, Department of Chemistry and Physics, Linndstr. 3-5, 7010-~eipzig, Federal Republic of Germany

Abstract A study was made on the phosphorus substitution on pretrea-

ted ZSM-5 zeolites, The phosphorus lnsertion is demonstrated by means of MAS NMR, X-ray diffraction and I R investigat ions.

1. INTRODUCTION

The possibility of isomorphous substitution of aluminium and/or silicon by other chemical elements in pentasil zeolites and the consequences concerning the properties of these zeolites have been recently lnvestigated (refs. 1, 2 ) . The formation of such products is possible by means of hydrothermal synthesis and also by the post-modification of the parent synthesis products with aqueous solutions of different elements to insert them into the zeolltic lattice (refs. 3-51. own experiments concernlng the realumination of dealuminated pentasils have shown that also the secondary synthesis of lsomorphously substituted pentasils by the treatment of templat-free synthesized zeolites wlth aqueous solutions of different compounds at higher temperatures seems to be useful (ref. 5). This work deals with systematic studies on the phosphorus modified ZSM-5 zeolites prepared by secondary synthesis to demonstrate relations between the structural, surface-chemical and adsorptional properties of these materials.

120

2. EXPERIMENTAL

The insertion of phosphorus was investigated for ZSM-5 zeolite (Si/A1=15) from template-free synthesis. which were modified by thermical, combined thermlcal-mechanical and ultrasonic pretreatment. For this reason the parent zeolite was calcinated for 16 hrs at 6OO0C and taken into a vibration box for 10 hrs. Furthermore a sample treated with ultrasound was also used. For the experiments of phosphorus insertion a certain amount of phosphoric acid (87 wt-X) was diluted by water and contacted with all zeolites mentioned above. In each case the phosphorus content in the suspension was calculated as six phosphorus atoms per zeolitic unit cell at solld/liquid ratio of 0.1. The mixtures were treated hydrothermally at 70 OC in an autoclavic vessel. The products were filtered, washed very carefully and dried on air. BY means of the X-ray diffraction pattern the products were identified as ZSM-5 zeolites with a crystallinity of about 93 % (Leuna standard sample = 100%). The Si/Al ratios were determined by 27Al MAS NMR investigations on a home-made puls spectrometer at 70.3 MHz in comparison to a standard reference sample HZSM-5 (Si/A1=15). 3 1 MAS~ and 3 1 CP ~ MAS NMR experiments were carried out at 121.4 M H Z with proton decoupling on a Bruker spectrometer MSL 300 with phosphoric acid (60 wt-%) as reference for the chemical shifts. For the determination of the phosphorus content the free induction decay extrapolated to time zero were compared with those of the standard N H 4 H 2 P 0 4 . For the 31P CP MAS investigations a radio frequency field corresponding to a n / 2 puls of 6 /-ISwas used. The reference samples were AlPO molecular sieve and NH4H2POq. For further characerization pellets prepared by the KBrtechnique (zeolite:KBr=lr400, pressure =15 MPa) were lnvestigated in the lattic vibration region on a SPECORD M 80 (Carl Zeiss Jena).

121

The adsorption properties were determined by the measurements of nitrogen isotherms at 77 K on a SATORIUS balance. The designation of the zeolites prepared by secondary synthesis. the phosphorus content per unit cell (P/u.c.) the Si/A1 and P/(P+Al) ratios, the volume of the unit cell determined from X-ray data (v/u.c.) ,the 1550/1450 ratio from I R spectra and the nitrogen adsorption capacity at 0.1 MPa are shown in table 1. I

TABLE 1 Sample characterization

.

v/u c. 1550/1450 preparation Sl/A1 P/U.C. P/(P+Al) 7 N r v conditions nm h h h 3 h 4 h 5 h ’6 7++12 h 1 2

3 6 12 16 24 12

3 70 70 70 3 C

at 70 at 70 oC

at at at

C

at 70 at 70

+ultrasonic

OC

OC

and

19 19 20 19 20 17 20

0.025 0.65 1.25 1.46 4.70 0.30 1.92

0.052 0.119 0.215 0.233 0.569 0.053 0.296

5.3783 5.3809 5.3820 5.3859 5.3937 5.3778 5.3892

0.713 0.710 0.714 0.724 0.741 0.708 0.729

mg,g

133.80 131.90 129.68 128.80 121.05 137.30 128.93

++combined thermical-mechanical pretreatment

3. RESULTS AND DIsCussI:ON

The 3 1 M~A S NMR spectra of some P-contalning ZSM-5 zeolites prepared after a thermical pretreatment for 16 hrs at 600 C by the secondary synthesis are given in fig 1 a-c. The time of zeolite treatment with phosphoric acid was varied from 3 to 24 hrs. A s shown by the spectra, a narrow line of the chemical shift in the region from -19 to -25 ppm appears which should be connected with phosphorus atoms In tetrahedrical positions in the zeolitic lattice. Furthermore i t is demonstrated a significant correlation between the time of secondary synthesis and the number of inserted phosphorus atoms. The increase of the secondary synthesis time from 3 to 24 hrs leads to an Increase of the inserted P-atoms per unit cell from 0.025 to 4.7 in comparisan to 6 atoms offered in the synthesis mixture (see

122

0

-1 8.9

1

- 25.3

,

50

50

0

-29.6

-50 -100 d/pprn

* I .

0

-50

-100

- C/ppm

Flgures 1 and 2

3 1 MAS ~ NMR spectra of the samples 4Cb). 5 (a) and samples 6(b) 7(a)

2~c) I

table 1). With a higher number of inserted phosphorus atoms in the lattice the chemical shift of the P-signal tends t o higher values. For the sample with 4.7 P-atoms the chemlcal shift with about -25 ppm agrees well with the results of Blackwell and Patton (ref. 6) for SAP0 zeolites. The position of the Psignal in the 3 1 MAS ~ N M R spectra at -25 ppm is not caused by the number of pho8phorus atom8 in tetrahedrical coordination as shown by fig. 2 a-b for the spectra of samples prepared by secondary synthesis after the pretreatment of the Parent Zeolite carried out a s combined thermical and mechanical and by means of ultrasound. The degree of the phosphorus insertion into the zeolitic lattice depends on the tlme of the seconary Synthesis. The number of the inserted P-atoms of both zeoli-

123

tes do not achieve the values obtalned after 24 hrs with 4.7 P/u.c.. Certainly i t can be seen that the mechanical pretreatment in combinatlon with the calclnation has a positive influence on the phosphorus insertlon. This behavlour could not be detected in the case of the ultrasonlc pretreatment. The value of the chemical shift of both samples of -29 and -27 ppm is higher than for the thermicelly pretreated zeolites and agrees well with those of A l P O molecular sieves (ref. 7 ) . Further information concerning the state of phosphorus in the lattice are avallable from selected X-ray Patterns. The different ionic radii of A13+ (0.051 nmII Si4+ (0.041 nm) and P5+ (0.035 nm) should lead to a change of the lattice parameters with an increased number of inserted phosphorus atoms after the secondary synthesis. W i t h the assumption that the phosphorus atoms were inserted into lattice positions. the lattice constants should decrease with increasing phosphorus content. In opposition to this assumption the higher the tetrahedrlcally coordinated P-atoms in the lattice the higher the volume of the unit cell as ehown in table 1. The increase of the lattice constants a5 a result of a realumination can be excluded, because the Si/Al ratios of the prepared zeolites are nearly the same. From the experimental results i t becomes clear that for the products of the secondary synthesis the isomorphous substltution of aluminium and/or silicon by phosphorus can play only a subordlnate Part. I t seemsl that the increase of the volume of the unit cell should be due to a phosphorus insertion into "activated" regions of the zeolitic surface. For the constructlon of the post-synthesized phosphorus containing ZSM-5 zeolltes we want to submit the followlng structure mentioned by Lercher (ref. 8 ) in another connection.

Ho\ /OH H \

/

O

/

A1

+

H3P04

-

\ /

A1

61

/ \

/

\

+

H20

124

on the basis of our recent investigations concerning the realumination of dealuminated ZSM-5 zeolites in alkaline medium, the provable phosphorus insertion is coupled wlth the presence of lattice defects in the activated zeolites. The concentration of lattice defects and their distribution depends on the aluminium distribution n the Parent zeolite and the kind of pretreatment. The structural change of the zeol tes during the process of the secondary synthesis can be determined by means of I R spectra of the zeolites. Particulary the intensity of the absorptlon band at 550 cm is sensitive for lattice defects and lattice reconstructions. This behaviour la shown in table 1 for the post-synthesized Phosphorus containing ZSM-5 zeolites by the ratio of the intensities of the bands at 550 and 450 cm- . I t is demonstrated that wlth increasing Phosphorus content the ratio 1550/1450 is di8tinCtlY increasing which is due to the reconstruction of the lattlce defects during the phoephorus Insertion. A s shown by the nitrogen adsorption in table 1 this process leads to a significant decrease of the adsorption capacity of the samles with the highest phosphorus content, because the geminal hydroxyl groups on the phosphorus species can block a part of the free volume of the zeolitic channels. Further information on the chemical state of phosphorus in the zeolites are available from the 31P CP MAS NMR investigations (ref. 9). By means of this method phosphorus in the neighbourhood of protons can be detected which allows a differentia of phosphorus in lattice position and in the state suggested above. In the CP experiments the intensity of the P-signal can be increased at the most on the factor I c p / I F I D = YH/Yp

x NH

(NH

+

Np)

For the given strucural model this value is 1.67. Therefore the lntensities of the free induction and the cross polarlzatlon should not differ considerably. In the case of phosphorus atoms In lattlce positions the values tend to zero cau-

125

sed by the large distance of the phosphorus atoms to the protons and should give no contribution to the NMR signal. The spectra for the FID MAS and CP MAS lnvestlgations for sample 2 and 4 are shown in the flge. 3 a-b and 4 a-b.

I I A _ . . .

20

0 -20 -40

Figures 3 and 4

-60 dlppm

-

20

0

3 1 CP ~ (a) and F I D (b) MAS les 2 (left) and 4 (right)

-20 -LO -60 dippm

-

NMR

spectra of samp-

For these experiments the zeolites were transfered into their Proton forms by cation exchange with 0.1 N NH4NQ3 and calcination at 55OoC. While the CP intensity for sample 2 is about six-fold higher as the FID intensity, sample 4 shows a n

126

opposite behaviour, beCaUBe the FID intensity is about fiveteen-fold higher than the CP intensity. From the results i t can be drawn, that in sample 2 the phosphorus is inserted into regions of the ’activated" surface, while the P-atoms in sample 4 are positioned in the zeolitic lattice. This conclusion can be supported by the corresponding chemlcal shifts. In the MAS spectra for the free induction of sample 4 a sig- nal at -30.2 ppm appears, which is connected to phosphorus atoms in the lattice (ref. 7). In the case of sample 2 this signal is absent in both MAS experiments. The signal at -19.5 ppm should be correspond to the phosphorus on the *activated" zeolitic surface. The signal at 0.8 ppm should be assumed as phosphoric acid which was built during the tranfer of the zeolites into the proton form. From a comparison of the FID spectra i t can be seen that phosphorus was migrated during the proton exchange Into lattice positions. 4. REFERENCES

1 2 3 4

5

9

M. Tielen, M. Geelen and P.A. Jacobs, Proc. Int. Symp. Zeolite Catal., Bzeged 1985, Acta Phys. Chem. Szeged. , 1985, P. 1 K.G. Ione and L.A. Vostrikova, usp. Khim. 88 (1987) 393 G.W. Skeels and E.M. Flanigen, ACS Symp. Ser. (M.L. Occelli and H.E. Robson, Eds.) 388 (1989) 420 8 . Sulikowski and J. KlinOwski, ACS Symp. Ser. (M.L. occelli and H.E. Robson, Eds.) 398 (1989) 393 w. Reschetilowski, W.-D. Einlcke, B. Meier, E. Brunner and H. Ernst, Zeocat 90 8 ’Catalysis and AdSOrptiOn by Zeolltes’, Leipzig, 1990 C . S . Blackwell and R.L. Patton, J . Phys. Chem. 92 (1988) 3965 D. MUller, E. Jahn, 8. Fahlke, G . Ladwis and U . Haubenreisser , Zeolites 5 (1985) 53 J.A. Lercher, G. Rumplmayr and H. NOller, Proc. Int. S m p . zeolite Catal.. Szeged 1985, Acta Phys. Chem. Szeged., 1985, p. 71 H. Mehring, ’Principles of High Resolution NMR in Solids". Springer-Verlag Berlin, Heidelberg, New York. 1983

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 01991Elsevier Science Publishers B.V., Amsterdam

127

Synthesis of zeolite beta i n boron-alurmnium media Miroslaw DEREWINSKIa, Francesco DI RENZOblC, Pierre ESPIAUb, Francois FAJULAb and Marie-Agnes NICOLLEb alnstitute of Catalysis and Surface Chemistry, polish Academy of Science, NieZdpOminaJeK 1 , 3 0 2 3 9 Krakow, Poland bLaboratoire de Chimie Organique Physique et Cinetique Chimique Appliquees, URA 418 du CNKS, ~ c o l eNationale Superieure de Chimie, 8 rue de 1 Ecole Normale, 34053 Montpellier, France Cto whom all correspondence should be sent

Abstract E m o n and a l m m u m compete in the crystallization of zeolite beta. A l m n i w n incorporation is faster than boron incorporation Non-linear effects of the B/A1 ratio on the (B+Al)/Si ratio are observed m e particle size is affected by the composition of the synthesis gel, the bigger crystals being f o m d fromUUghly boric gels.

1.

INTRODUCTION

Zeolite beta was m n g the first zeolites m c h underwent successful replacement of boron for alurmmum ( 1 ) . The main ground for inserting boron in zeolitic frameworKs is the modulation of the strength of the acid sites ( 2 - 5 ) , but structural boron proved to be less stable tnan alwnium in the activation trea-nts, especially in hydrothermal conations (6, 7). W s drawback may be turned into advantage when a networlc quite unstable under dealurmnating conations is concerned, as in the case of zeolite beta (8). ?he mlder conations required for deboration are likely to affect to a lesser extent the lattice stability. B-beta could then represent a suitable precursor of the activated form of the zeolite ( 9 ) . Moreover the afferent Kinetics of incorporation of boron and alurmmum are llKely to influence other properties of the solid, like the size andhabit of the crystals and the defect patterns (10-12).

128

The reagents usedhave been precipitated silica (Zeosil 175MP from RhOne Poulenc, Na 0.9%, A1 0 . 4 L , H20 6.5 %, gram size 2-20 p, pore volunr? 0.08 rnlig), tetraethylammmum (TEA) hydroxide solution (Aldrich), sodlum alumnate (Car10 E2-U W ) , so&un tetraborate decahydrate, sodmm hydroxide (Frolabo RP N o r m a p ) , deionized water. The reagents were m x e d under stirring in the following order: alKaline solution, organic agent, alumnate anWor borate, silica The mxture was stirred for 4 hrs at room temperature before the beginning of the synthesis. The crystallization experimnts have been carried out at 150°C in 120 ml stainless steel autoclaves, without stirring. For all gels the (Al+B)/(Si+Al+B)mlar ratio WaS 0. 07, TEA/(Si+Al+B) 0.35, CH-/Si02 0. 35, H20/Si02 17. ?he B/(B+Al) ratio was 0.00, 0.36, 0.48, 0.64, 0.77and 0. 88 for experlmnts from 1 to 6, respectively. Tne solid fraction has been recovered by filtration, washed with deionized water up to FH 9 and dried at 70 O C in air. The proctucts have M e n characterized by powder X-ray dlffraction, scanrtlng electron mcroscopy (Canibridge S1W instrument), atormc absorption spectroscopy and nitrogen adsorption. Characterization By llBand 27Al MAS-NMR spectroscopy and thermal gravlrretry have been already reported ( 7 ) . 3. RESULTS AND DISCUSSION

Zeolite beta was obtained as a pure D s e from all experiments. A conplete crystallization, as indlcated by XF3, w a s attamed after 24 hours for experlments from 1 to 4, whereas 72 hours were needed for experlrrents 5 and 6. On longer crystallization time other phases appeared beside zeolite beta. Quartz was formed after 6 days in the concZltions of experiment 1, zeolite 2'3%-12 after 3 days in the condltions of experiment 4, and zeolites ZSM-5 and ZSM-12 after 6 days in the condltions of the experiments 5 and 6. The longer time needed to obtam full crystallization in the experIm2nts at hgher boron content suggests that alurmnlum features an efficiency of incorporation h@er than boron A more detaled evidence comes from the yields of crystallization (ratio between the amount of element recovered in the crystals and the amount intromced in the synthesis pel) of the tetrahedron-fomng elements, as reported in table 1. m ' e yields of alummum and silicon are nearly constant for all t h e experients, m e yield of alumnlum being slightly hlgher. The yield of incorporation of boron, instead, is very low when the synthesis gel is almnium-rich (exp. 2-4). Boron and almnium are incorporated at t h e s a m extent only when the available boron largely exceeds alurmniun (exp. 5 , 6). W s behaviour can be accounted for by a corrpetition between the kinetics of incorporation of borosilicate and alumnosilicate units in the zeolite. No rate constants can be established without a Knowledge of the partition coefficients of the elen-ents between the liq-ud and solid phases of the synthesis gel. Anyway, the lugher crystallization efficiency of the

129

Table 1 G e l m l a r comsitlon, zeolite crystallization yielcts and sodrm content in the products B Yield

B/ (B+A1)1 gel

A1 yield

2

0. 00 0. 36

0. 92 0. 88

0. 06

0. 73

0.003

3

0. 48

0. 92

0. 08

0. 82

0. 004

4

0. 64

0. 88

0. 32

0. 78

0.005

5

0. 77 0. 88

0. 90

1.00 0. 86

0. 84

0. 007

0. 78

0. 005

experiment # 1

6

yield

-

0. 95

Na/ (Si+Al+B)

Sl

0. 83

0. 002

almnosilicate units is evident. The changes in the yield ratios between the tetrahemon-fomng elements correspond to a non-linear evolution of the zeolite composition, as shown in figure 1, where the B/(B+Al) and (B+Al)/(B+Al+Si) ratios in the products are reported as functions of the B/(B+Al) ratio in the syntnesis gel. When boron replaces a part of the alurmnium in the gel, a trivalent-poor, mre silicic zeolite is oDtained (exp. 2-4).At bgher boron concentration, the m l e fraction of trivalent elements climbs again to the values of t h e silicoalurmnate zeolite ( e m . 5, 6). It can be observed that the experiments from 1 to 3, in m c h very few bOrosi1icate units are incorporated in the zeolite, present a fairly linear correlation between the alurmnium content in the zeolite and the alurmnium/silica ratio in the synthesis gel. The change of the boron content of the gel from notlung to an m u n t equal to the amount of alurmniumhardly affects the Si/Al ratio. Hence the polymerization degree of the silica in the silicoalurmnate growth units should depend only on the Si/Al ratio in the gel, independently of the boron concentration A sirmlar behaviour in the case of the borosilicate units can easily explain why the trivalent content of the

0.06

'

Figure 1.

Composition of the zeolite as a +unction of the composition of the gel.

(7

0.2

II 0

L

0.2

0

'

m

0

I

0.6

B/B+Al ( G e l )

I

1

130

zeolite increases again when the boron concentration in the gel increases (exp. 5, 6). The mole fraction of s o d ~ u min the crystals is also reported in table 1. The slight increase of the sodmnn content with the insertion of boron could suggest that the more boric solids present a lower cristallinity or a mgher defect concentration. The lattice paramters of zeolite beta as a function of the mole fraction of almnium in the solid are reported in figure 2. A fairly linear shrinl(ing of the unit cell is observed when the alurmmum content decreases. The Si/A1 ratio can account for the @x?nomnon, independently from the m u n t of boron incorporated in the solid As a consequence, the shrinliing of the m t cell alone could not be considered as an evidence of the insertion of boron in the framworK, at least when alummum is competing for incorporation llB MAS-NMR evidences appear m c h m r e suitable (7, 12-15). From the figure 2 it can also be inferred that the &latation module of paramter c at increasing alumllvum content is sligTkly Wgher than the &latation module of parameter a. The crystal size cbstributions for all expermnts are reported in figure 3. The average crystal size as a function of the composition of the synthesis gel is reported in figure 4. WE crystals f o m d in boron-rich m&a (exp. 4-6) are sigmficantly larger than the silicoalurmnate crystals (exp. I ) , in good WeeIEnt w i m results concerning other Kinds of Zeolites (10, 16). However, the correlation is f a r from monotone, as testified by the data reported in figure 4. The crystal size presents a deep m n l m when small m u n t s of boron substitute for alurmmum (exp. 2). AS a consequence, no mrect correlation between nucleation rate and alurmmum or boron content can hold over the whole crystallization field. Very liKely, the size of the crystals is strongly affected by the myslcal properties Of the synthesis gel. The slrmltaneous presence of borate and almnate ions m y indeed influence in a non-strwgth omardway the agglomeration of the mrphous silica (17, 18).

12.4

Figure 2. Lattice parameters as functions of the mole fraction of a l m m m

I

0.02

I

I

1

I

0.1 0 .0 6 Al/tet. ( C r y s t . )

131

20 10

20 10

30

20 10

1

2

I

I

3 I

L

I

Ilt

30 b2

20

G: 4

u

.d ci

10

L4

a

14 d.3

30 20 10

30

20 10

1

2 Size (

Figure 3. Particle size experiments from 1 to 6 .

3 I.I )

a s t r i b u t i o n of zeolite beta From top to bottom:

132 2

Figure 4. Average crystal size as a function of the gel composition.

1

m

0.2

0.6

1

B/B+Al ( G e l )

The non-linear influence of boron on the crystal size confirms that the lughest caution is needed when the interpretation of any crystallization experiment carried out in Pyrex vessels is attempted (15, 1 6 ) . In figure 5 the mcrograpx of s o m samples of zeolite beta are reported. The more alurmruc samples (exp. 1 , 2) feature flattened smeroids with irregular surfaces (figure 5a). When some boron substitutes for alununium (em, 3, 4) almnd-llKe crystals with a four-fold axis are obtaned (figure 5b). The mre boron-rich crystals (exp. 5, 6) feature a s m l a r habit, but some flat faces are present at the outer run of the square almonds (figure 5c). The angle that the flat faces f o m with the axis of the crystal corresponds to the orientation of the face (101) of zeolite beta Ihe slupe of the crystals probably does not depend arectly on the composition of the solid, but it is a function of the crystallization rate. A slower crystallization brings to m r e developed flat, low-index faces. As an example, a solid obtained at lower allcaliruty is depicted in figure 5d 4. CONCLUSlQNS

Partial substitution of boron for alurmnium in the synthesis is a mtable tool to control not only the composition, but also the crystal size of the zeolite beta ?he nucleation flow of the zeolite is a nonlinear function of the composition of the parent gel. The composition of the crystals, Instea& depends on the relative rate of sticking of independent borosilicate and alurmnosilicate species.

133

Figure 5. Micrograghs of zeolite beta. Top left (a): experimnt 1 (no boron present). Top right (b): experiment 4 (B/(BtAl) 0.38). Bottom left ( c ) : experiment 6 (B/(BtAl)0,87). Bottom right (d): solid crystallized at oH-/s102 0.20.

134 5. 1

M. Taramasso, G. Perego and B. Notari, in L.V. Rees (Ed ) , E m c . 5 t h Int. Zeolite Conf., Napoli, June 2-6, 1980, Heyden, London, 1980, pp. 40-48.

2

W. Holderich H, Eichhorn, R. Lehnert, L. Marosi, W. Mross, R. Reinke, W. Ruppel and H. Schlinpr, in D. Olson and A. Bisio (Eds. ) , Proc. &th Int. Zeolite Conf., Reno, July 10-15, 1983, Buttemorths, Guilclford, 1984, pp. 545-555.

3

4

EG. Derouane, L. Baltusis, R.M. Dessau and K. D. Schrmtt, St. S’urface Sci. Catal., 20 (1985) 135-146. A. Auroux, G. Coudurier, R. Shannon and J.C. Vedrine, Cal. Anal. T h e m , 16 (1985) 68-75.

G. Coudurier and J.C. Vedrine, Pure Awl. Chem , 58 (1986) 1389-1396. Sayed, J. Qlem Soc., Faraday l'rans. I, 83 (1987) 1751-1759. M. Derewinslu, P. Massiani and F. FaJUla, in J.C. Jansen, L. Moscou and M. F,M. Post (Eds.) I Recent Research Reports 8th Int. Zeolite Conf. , Amsterdam, July 10-14, 1989, pp. 103-104. N.A. Bsiscoe, J.L. Casci, J . k Daniels, D.W. Johnson, M.D. Shannon and 8 A. Stewart, St. QSrf. Sci. Catal., 49 (1989) 151. C.D. Chang and P. B. Weisz, US Pat. 4,701, 313 (1987) to Mobil Oil Co. 9 10 P. modart, J. B. N a y , 2. Gabelica and EG. Derouane, Applied Catal., 24 5 6 7

M.B.

(1986) 315-318. 11

J.C. Jansen, C.W.R. Fngelen and H.

van BewcUm

ACS

Sm.

Ser., 398

(1989) 257-273. 12

T. R. Gaffney, R. Pierantozzi and M. R. Seger, ACS Symp. Ser., 398 (1989) 374-392.

K.F.M.G.J. Scholle and W,S. V e e m , Zeolites, 5 (1965) 118-122. H. Kessler, J.M. Chezeau, J.L. G u m H. S t r u b and G. Coudurier, 14 Zeolites, 7 (1987) 360-366. 15 2. Gabelica, J. E l Nagy, P. Bodart and G. Debra, chem Lett., 1984, 13

1059-1062. 16

A. Cichoclu, Zeolites, 5 (1985) 26-30.

17

R.K. Iler, m e Chemstry of Silica, Wiley, New York, 1979, pp. 13, 190, 381.

18

6.

C. J. Brinker and G.W. Diego, 1990, p. 225.

Scherer, Sol-Gel Science, Acadermc Press, San

ACKN-

Many thanks are due to the staff of the Service Central d’Analyse du CNRS in Solaize for elemental analysis and to Roger Dutartre for electron rmcroscopy.

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 0 1991Elsevier Science Publishers B.V., Amsterdam

135

On the possibility of generation of Bransted acidity by silicon incorporation in very large pore AlP04 molecular sieves J.A. Martens, I. Balakrishnana, P.J. Grobet and P.A. Jacobs Centrum voor Oppervlaktechemie en Katalyse, KU Leuven Kardinaal Mercierlaan 92, B-3001 Heverlee, Belgium a, on leave from National Chemical Laboratory, Pune 411008, India Abstract Si-VPI-5 is synthesized according to a novel method using aluminium isopropoxide as source of aluminium and using a recipe from literature. The samples are compared with SAPO-5 and SAPO-11 using 27Al and 29Si MAS NMR, thermoanalysis and the decane catalytic test reaction. The Si for P isomorphic substitution mechanism, which generates Bronsted acidity in SAPO-5 and SAPO-11 is not active in Si-VPI-5. An explanation for the fundamental difference between SAPO-n materials and Si-VPI-5 is offered.

1. INTRODUCTION

In some of the AlPO4-n molecular sieves discovered in 1982 [l], it is possible to substitute part of the P and Al framework elements with Si [2]. In the resulting SAPO-n materials, isolated Si atoms occupy P sites, while patches of Si atoms replace localIy P as well as Al atoms [3,4]. The degree of Si substitution and the substitution mechanism depend on the topology of the framework and on the synthesis method [4,5]. While the synthesis method does not seem to be critical for the incorporation of traces of Si in SAPOJ and SAPO-11, extensive Si incorporation in these structures can be achieved only by using very specific synthesis recipes [4,6]. Silicon-rich crystals of SAPO-5 and SAPO-11, e.g., can be prepared by using aluminium isopropoxide as a source of aluminium and specific templates, viz. dipropylamine for SAPO-11 and cyclohexylamine for SAPO-5 [4,6]. There are indications in literature that it is possible to incorporate silicon during the crystallisation of very large pore AlP04 molecular sieves of the type VPI-5 [7] and MCM-9 [S]. We report now on attempts of Si incorporation in VPI-5 using synthesis methods which have proven to be succesful with SAPO-5 and SAPO-11. The generation of acidity following such incorporation was probed by catalytic testing of SAPO-8, the thermal transformation product of Si-VPI-5. Such material seems to have 14-membered ring pore openings [9].

136

2. EXPERIMENTAL 2.1. Techniques The conversion of decane was performed in a fixed bed, tubular microreactor. The H2/decane molar ratio in the feed was 100. The pressure in the reactor was 0.35 MPa and the space time of decane 0.5 kg s mmol-1. Powder X-ray diffraction patterns were recorded on a Siemens instrument, equipped with a McBraun position sensitive detector. TG-DTA patterns were recorded on a Setaram TG-DTA92 thermobalance. 27Al, 29% and 31P MAS NMR was performed on a Bruker 400 MSL instrument, using the following operational parameters:

Parameter MAS frequency (MHz) Pulse length (ms) Pulse angle (O ) Repetition time (s) Spinning rate (Mz) Number of scans Chemical shift reference

29~i

27Al

31P

79.5 4.0

104.2 0.6 15 0.1 5-15 3,000 AlCl3

161.9 3.0

45

5 3 10,000 TMS

45

60 15 8 H3P04

2.2. Synthesis of Si-WI-5 Si-VPI-5(1) was obtained as follows. To a mixture of 32.1 g of aluminium isopropoxide (Janssen Chimica) in 34 g of water, 17.7 g of phosphoric acid (85%) (Janssen Chimica) diluted with 12 g of water was added under stirring. Then, 4.6 g of colloidal silica (Ludox AS-40, DuPont) was added, and finally 10 g of dibutylamine (DBA)(Janssen Chimica). This final mixture was stirred to obtain a homogeneous gel with composition

DBA.0.4SiO~.Al203.P205.4OH20. The gel was transferred to a stain ess steel autoclave with a capacity of 150 ml and heated statically at a temperature of 423 K for 8 h. Si-VPI-5(2) was prepared according to a synthesis procedure reported by Davis et al. [7] using pseudobeuhmite (Vista), orthophosphoric acid (85%, from Janssen Chimica), colloidal silica (AS-40 from DuPont)) and dipropylamine (DPA) (Janssen Chimica) as reagents. The gel composition was: DPA.SiOp?l203.P205.40H20. The method involves two ageing steps and hydrothermal treatment at a temperature of 415 K [7]. After the hydrothermal treatments, the autoclaves were cooled to room temperature, the contents centrifugated, and the solids washed and dried in air at a temperature of 313 K. The XRD pattern of SI-WId(1) and Si-VPI-5(2) are shown in Fig.1. That of SiW I d ( 1 ) is in agreement with the one of VPI-5 reported in literature [lo]. The Si-WI5(2) sample contains crystalline impurities of the AlPO4-H3 or MCM-1 type. SAPOJ and SAPO-11 were the SAPO-11/1 and S A P 0 - 5 / 2 samples used in previous work [4]. The Si/(Si+Al+P) fraction in the SAPOJ and SAPO-11 samples is 0.15 and 0.04, respectively.

137

Si-WI-5( 1) Si-VPI-5(2)

15

5

25

45

35

5

15

25

35

45

28

26)

Figure 1. XRD pattern of Si-VPIJ(1) and Si-VPI-5(2). 2.3.Preparation and activation of catalysts The Si-VPI-5 samples were loaded with platinum by impregnation of 1 g of sample with 8.5 mg of Pt(NH3)4Cla dissolved in a minimum quantity of water. The powders were shaped into pellets hamng a diameter of 0.3-0.5 mm by compressing, crushing and sieving. A 200 mg sample of the pellets was charged into a reactor tube having an internal diameter of 1cm. Pt/SAPO-S(1-0) was obtained by calcination of Pt/Si-VPI-5( 1) in flowing oxygen. The temperature was increased from 291 K to 773 K at a rate of 6 K per minute. After 1 h of calcination at 773 K, the catalyst was purged with nitrogen and cooled to 573 K. A flow of hydrogen was conducted over the catalyst for another hour at 573 K to reduce the platinum ions. Pt/SAPO-8( 1-V) and Pt/SAPO-S(Z-V) were prepared by evacuation of Pt/Si-VPI5(1) and Pt/Si-VPI-5(2) under vacuum (15 mPa) at a temperature of 291 K during 12 h. Subsequently, the temperature was increased with a rate of 1 K per minute to 653 K. After 12 h of calcination, the sample was cooled to 291 K. The samples were subjected to an oxygen/hydrogen activation as described for Pt/SAPO-8(1-0). The phase transition of Si-VPI-5 into SAPO-8 was verified with XRD and 3IP MAS NMR on the rehydrated used catalysts. Due to the presence of platinum, XRD lines at low angles were scattered. Fig.2 shows the XRD pattern of Pt/SAF'O-8 (1-0) and Pt/SAPO-8 (2-V). I

I

2.’

20

Pt/SAPO-8( 1-0)

8

.

25

'

'

1

'

30

'

35

'

I "

40

.

45

1

L "

20

'

'

25

'

" '

'

30

29

Figure 2. XRD pattern of Pt/SAF'O-8( 1-0)and Pt/SAPO-8(2-V).

'

35

.

*

I

'

'

40

28

' 45

138

These XRD patterns are in agreement with that for the AlPO4-8 topology [l].The 31P MAS NMR spectrum of Pt/SAPO-8 (1-0) shown in Fig3 is also representative of the AlPO4-8 topology [ 111.

-20

PPH

-40

Pt/SAPO-5 and Pt/SAPO-11 were prepared by impregnation of calcined samples with Pt(NH3)4C12, followed by oxygen/hydrogen activation.

RESULTS AND DISCUSSION The three-dimensional structures comprised in the AlP04-n phases of Wilson et al. [l] have been classified by Bennett et al. as (i) aluminophosphate molecular sieves, (ii) semi-dense phases and (iii) hydrates [ 121. The latter two categories were distinguished from the first one by the presence of Al atoms in five and six coordination with oxygen atoms, four of which belonging to the framework and the additional ones to water molecules or hydroxyl groups [12]. In the early work [3,5,12-141, aluminium in tetrahedral coordination was thought to be essential in order to obtain molecular sieving properties. Meanwhile, it has been shown that in several of these molecular sieves, the coordination number of part of the aluminium can change from IV to VI after the removal of the template and adsorption of water [15,16]. Changes in the aluminium coordination can be monitored conveniently with highfield 27Al fast spinning MAS NMR and DOR NMR [17,18]. In the 27Al MAS NMR spectra of SAPO-5, SAPO-11, Si-VPI-5(1) and Si-VPI-5(2) shown in Fig.4, signals of Alw appear at chemical shifts ranging from 37 to 41 ppm. The 27Al NMR signals at ca. 10 ppm and in the range from -10 to -20 ppm represent Alv and Alm respectively [17]. As-synthesized SAPO-5 and SAPO-11 contain small amounts of Alv and Am.In the Si-VPI-5 samples, ca. one third of the aluminium atoms have the Alw coordination. A similar Alm content is found in VPI-5 [17,18]. The Si-VPI-5(2) sample contains traces of Alv. After calcination and hydration, the Alw signal in SAPO-5 has increased significantly and the Al distribution in calcined hydrated SAPO-5 is AlIV (58%), Alv (9%) and Alm (33%). Meinhold and Tapp reported that in calcined AlPO4-5 up to 40% of the AlIv can be converted reversibly into Alm u on adso tion of water 1151. At the magnetic field of 4.7 T used in that work [15], and Al% signals were not resolved. In calcined hydrated SAPO-5, a comparable share of the Al atoms is found now to be coordinated to one or two water molecules (Fig.4). AlPOg-11 and SAPO-11 undergo a reversible phase transition upon calcination and hydration [19], which is responsible for further structural inequivalency among the

Alt

139

AlIv crystallographic sites, and splitting of the AlIv signal (Fig.4). The contribution of A l v in calcined hydrated SAPO-11, estimated by integration of the 27Al signals of Fig.4, amounts to ca. 22%. Alv is not observed in calcined hydrated SAF'O-11 (Fig.4).

SAPO-11 AS.

11111111111!IIII 50

PPM

0

11111111111111(1 50

SAPO-11 C.H.

Si-VPI-5(2) A.S

11111111))111111 50

0

PPM

0

IIIIIIIIIII(IIII

50

0

SAPO-5 A.S.

1111111111111111 50

PPM

0

SAPOJ C.H.

11111111111111)1 50

0

PPM PPM PPM Figure 4. 27Al MAS NMR spectra of Si-VPI-5(1), Si-VPI-5(2), SAPO-5 and SAPO-11; A.S. stands for as-synthesized, C.H. for calcined hydrated. The TG-DTA results on Si-VPIJ( 1) are shown in F i g 5 Endothermic weight losses due to water desorption are observed at temperatures of 353 K and 400 K. The absence of weight losses associated with exothermic reactions indicates that the micropores do not contain organics. Similar observations were made previously with W I - 5 [7]. Assynthesized VPI-5 is silent in I3C MAS NMR [18] and Duncan et al. have shown that organics are not essential in the synthesis [20]. The absence of organics in the pores of Si-VPI-5 after synthesis explains its similarity in 27Al MAS NMR with calcined hydrated SAPO-5 and SAPO-11 samples (Fig.4).

140

EX0 ENDO

\1.

-

-10

-

-20

I-I

PR,

atmosphere, using a flow rate of 50 ml per minute and a heating rate of 10 K per minute.

29Si MAS NMR spectra are shown in Fig.6. The most intense 2% resonances in SAPO-5 are at chemical shifts of -93 ppm and -111 ppm. These signals represent Si(4Al) and Si(4Si) environments, respectively [4]. The -1 11 ppm signal representing the Si(4Si) environment predominates in the SAPO-11sample (Figd).

n Si-VPI-5 (1)

--

~~

-100

PPM

A

-150

-100

PPM

Si-WI-5(2)

-100

PPM

- 150

1'1

-150

SAPO-11

1 1 1 1 1 1 , 1 1 1 1 1 -100

PPM

-150

141

The 29Si resonance of the Si-VPI-5 samples exhibits a maximum at ca.-111 ppm, indicative of the presence of Si(4Si) environments. These 29Si MAS NMR spectra do not allow to decide on the presence of other Si environments. The conversion of decane over the different catalysts at increasing reaction temperatures is shown in Fig.7. Based on the conversion curves of Fig.7, the activity of the catalyst decreases in the order: Pt/SAPO-5, Pt/SAPO-11 > > Pt/SAPO-8(1-V) > Pt/SAPO-8(1-0) > Pt/SAPO-8(2-V) , Pt/AlP04-5. This activity sequence reflects the Bransted acidity of the samples. The Si, Al, P composition is generally not homogeneous throughout the individual SAPO-n crystals [4]. SAPO-n crystals contain aluminosilicate domains (SA), where the silicon is concentrated, and silicoaluminophosphate (SAPO) domains. The Bransted acid sites of the SAPO-5 crystals of this work are located in the SAPO domains [21]. The SA domains do not contain aluminium and are catalytically inactive [21]. The Si(4M) environment generates the Bransted acidity in SAPO-5. It represents 4% of the Si+Al+P atoms in this particular sample [21]. In the SAPO-11 crystals used in this work, the Brmnsted acid sites are located in the SA crystal domains and at the interface of SA and SAPO domains [21]. Si(nAl) environments responsible for the Brcinsted acidity of SAPO-11 are not resolved from the 29Si resonance envelop (Fig.6). Their amount was estimated at ca. 1% of the Si+Al+P atoms [21]. From the high reaction temperatures, necessary to render the SAPO-8 samples active in decane conversion, it can be concluded that the number of Bransted acid sites in these samples is substantially lower than in SAPO-5 and SAPO-11. The catalytic activity of SAP0-8(2-V) is comparable to that of AlPOq-5. It can be concluded that the silicon in this sample doesnot give rise to Brmnsted acidity. Pt/SAPO-8(2) is more active and calcination in vacuum results in a higher activity compared to activation in oxygen (Fig.7). The refined constraint index, CIO, is the 2-methylnonane/5-methylnonaneproduct ratio at 5% isomerisation conversion of decane [22]. Large pore zeolites have CIO values between 1.0 and 2.2. The CIO criterion does not allow to distinguish 12-membered ring zeolites from structures with larger ring sizes. For 10-membered ring zeolites, CIO is larger than 2.7. The 10-membered ring properties of SAPO-11 are reflected in its CIO value of 3.5 (Fig.8). SAPO-5 with a CIO value of 1.5 fits into the family of 12-membered ring structures (Fig.8). SAPO-8(1-0) and SAP0-8(2-V) have CIO values of 2.1 and 2.2, respectively, classifying them among the open structures. SAPO-8(l-V), which is the most active SAPO-8 sample (Fig.7) has a CIO value of 3.5. This 10-membered ring characteristic could arise from a trace of SAPO-11 impurity in this particular catalyst batch, or else from a trace of SAPO-11 by-product in the Si-VPI-5 into SAPO-8 transformation. From the very low catalytic activity of the SAPO-8 samples, it can be concluded that Si for P substitution does not occur in SAPO-8, and probably neither in its precursor, Si-VPIJ. A plausible reason is that in the absence of organic amines in the micropores and in the absence of alkali cations, no species are available to play the role of charge compensating agent. Si for P substitution in AlP04 molecular sieves requires filling of the micropores with template [4,6].

142

The 2% MAS Nh4R spectra suggest the occurrence of substitution of Si atoms for P and Al atom pairs, a mechanism which doesnot create net negative framework charges. However, if present, the silica patches cannot be large as it is expected that micropores in siliceous domains of the crystals are filled with organics. h

8

100

Figure 7. Conversion curves of decane on Pt/SAPO-n catalysts.

U

-d 0

m LI dl

*d

6 r

't

v

-

60

SAPO- 1 I

40

-.-

20

--- SAPO-8 (I-V)

SAPO-5

0 450

550

650

750

"+

SAPO-8 (2-V)

---

SAPO-8 ( 1 - 0 )

-a-

ALPO-5

Reaction temperature (K)

CI" Figure 8. CIO values of the Pt/SAPO-n catalysts.

4 10-MR

3 .

J.12-MR

2 1 0 -

SAPO-8 SAPO-8 SAPO-8 SAPO-11 SAPO-5 (1-0)

(1-V)

(2-V)

CONCLUSIONS Si for P substitution during the synthesis of aluminophosphates generates net negative framework charges and potential Br~nstedacidity. In SAPO-5and SAPO-11 materials, these framework charges are compensated by organic cations filling the micropores. Si for P substitution in VPI-5 is suppressed since during synthesis the micropores of as-synthesized Si-VPI-5 do not contain organic molecules nor alkali cations. The catalytic activity of Si-WI-5 is much lower than that of SAPO-5 and SAPO11 and is probably due to traces of impurities. The 2% MAS NMR spectra suggest the occurrence of Si for Al+P pairwise substitution. The absence of organics in Si-VPI-5 precludes the presence of siliceous crystal domains. In as-synthesized Si-VPI-5, part of the Al atoms have a coordination number of VI.Al coordinationnumbers larger than IV are found in SAPO-5 and SAPO-11 only after evacuation of the organic template from the micropores and hydration.

143

ACKNOWLEDGMENTS

J.A. Martens and P.J. Grobet acknowledge the Flemish National Fund for Scientific Research for research positions as Research Associate and Senior Research Associate, respectively. This work has been sponsored by the Belgian Government in the frame of "Geconcerteerde Onderzoeksakties". REFERENCES 1. S.T. Wilson, B.M. Lok and E.M. Flanigen, US Patent No. 4 310 440 (1982). 2. B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R. Cannan, E.M. Flanigen, US Patent No. 4 440 871 (1984). 3. S.T. Wilson, R.M. Lok, C.A. Messina, T.R. Cannan and E.M. Flanigen, J. h e r . Chem. SOC.104 (1982) 1146. 4. M. Mertens, J.A. Martens, P.J. Grobet and P . k Jacobs, in Guidelines for Mastering the Properties of Molecular Sieves, D. Barthomeuf et al. (eds.), Plenum Press, New York, 1990, p. 1. 5. E.M. Flanigen, R.L. Patton, S.T. Wilson, S.T. Wilson, Stud. Surf. Sci. Catal. 37 (1988) 13. 6. J.A. Martens, M. Mertens, P.J. Grobet and P.A. Jacobs, Stud. Surf. Sci. Catal. 37 (1988) 97. 7. M.E. Davis, C. Montes, P.E. Hatthaway and J.M. Garces, Stud. Surf. Sci. Catal. 49A (1989) 199. 8. E.G. Derouane, L. Maistriau, Z. Gabelica, A. Tuel, B. Nagy and R. von Ballmoos, Appl. Catal. 51 (1989) L13. 9. R.M. Dessau, J.L. Schlenker and J.B. Higgins, Zeolites 1990, 10,522. 10.M.E. Davis, C. Saldarriaga, C. Montes, J. Garces and C. Crowder, Zeolites 8 (1988) 362. ll.J.A. Martens, H. Geerts, P.J. Grobet and P.A. Jacobs, to be published. 12.J.M. Bennett, W.J. Dytrych, J.J. Pluth, J.W. Richardson, Jr. and J.V. Smith, Zeolites 6, 1986,349. 13.R.M. Lok, C.A. Messina and E.M. Flanigen, Proceed. 6th Int. Zeolite Conf., Ed. D. Olson and A. Bisio, Buttenvorths, Guildford, 1984, 97. 14.E.M. Flanigen, B.M. Lok, R.L. Patton, S.T. Wilson, Proceed. 7th Int. Zeolite Conf., Y. Murakami A Lijima and J.W. Ward, eds., Kodansha, Elsevier, 1986,103. 15.R.H. Meinhold and N.J. Tapp, J. Chem. SOC.Chem. Commun. (1990) 219. 16.M. Goepper, F. Guth, L. Delmotte, J.L. Guth and H. Kessler, in Zeolites: Facts, Figures, Future, ed. P.A. Jacobs and R.A. van Santen, Stud. Surf. Sci. Catal. 1989, 49B, 857. 17.Y. Wu, B.F. Chmelka, A. Pines, M.E. Davis, P.J. Grobet and P.A. Jacobs, Nature 346 (1990) 550. 18.P.J. Grobet, J.A. Martens, I. Balakrishnan, M. Mertens and P.A. Jacobs, Applied Catal. 56 (1989) L21. 19.R. Khouzami, G. Coudurier, F. Lefebvre, J.C. Vedrine and B.F. Mentzen, Zeolites 10 (1990) 183. 20.B. Duncan, R. Szostak, K. Sorby and J.G. Ulan, Catal. Lett. 7 (1990) 367. 21.J.A. Martens, P.J. Grobet and P.A. Jacobs, J. Catal. 126 (1990) 299. 22.P.A. Jacobs and J.A. Martens, Pure Appl. Chem., 58(10) (1986) 1329.

This Page Intentionally Left Blank

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 01991Elsevier Science Publishers B.V., Amsterdam

145

CRYSTALLIZATION OF POROUS ALUMINOPHOSPHATES AND METAL SUBSTITUTIONS

H. LECHERT*, H. WEYDA**,M . HESS*, R. KLEINWORTH*, v. PENCHEV***

***

AND CH. MINCHEV

*

**

of

Institute

of

Physical Chemistry of t h e University

Hamburg,

Bundesstrasse 45, 2000 Hamburg 1 3 , Germany SUD-CHEMIE AG, Katalyse-Labor

,

Waldheimer S t r

. 13,

8206 Bruckmiihl/Heu f e l d , Germany

***I n s t i t u t e

of Organic Chemistry, Bulgarian Academy of Sciences

S o f i a , Bulgaria ABSTRACT phenomena of a series of t h e molecular s i e v e s MeAP04-5

Crystallization and

MeASPO-5

have

been s t u d i e d under t h e

influence

of

Me-

different

components. A s M e components Be, Mg. Zn, N i and Fe have been used. These components

been o f f e r e d i n some excess t o study t h e e x t e n t of t h e

have

incorporation

which

was determined by SEM-EDAX a t t h e product c r y s t a l s .

For Be and Mg almost no i n f l u e n c e on t h e k i n e t i c s can

be

observed

compared

s t r u c t u r e with about 0.12 M@ shows

a

phosphate,

reduced The

with a pure

of

crystallization

batch.

f o r one A1203 P2O5 u n i t .

c r y s t a l l i n i t y and a

ZnO

ALP04-5

Mg

enters

A batch with

cocrystallization

of

content of t h e c r y s t a l s shows about t h e

an

the Zn

unknown

same

molar

r a t i o as t h e MgO. Extended experiments with N i O show t h a t t h e N i i s not incorporated i n t o ALP04-5 and SAPO-5 c r y s t a l s . I n t h e presence of N i O pure ALP04-5 and SAPO-

5 c r y s t a l l i z e which undergo r e c r y s t a l l i z a t i o n t o a c r i s t o b a l i t e

like

phase. Fe i s incorporated i n t o t h e c r y s t a l s f o r both valency states t a k i n g

about

Generally

the

presence

of an excess

of

the

Me-ions favours some

r e c r y s t a l l i z a t i o n t o denser phosphate phases o r t o c r i s t o b a l i t e o r

mite

.

up

0,l Fez03 f o r one A1203 P2O5 u n i t i n t h e s t r u c t u r e . tridy-

146 INTRODUCTION Porous aluminophosphates and silicoaluminophosphatehave been

first

described by E.M. Flanigan et al. (ref 1-5). Later on this family has been expanded introducing the so-called metalloaluminophosphates, containing an additional metal component (ref. 6-13). Excellent reviews about these substances have been given by WILSON and FLANIGEN (refs. 14,15). In a preceding paper we have thouroughly studied the kinetics of the crystallization of ALP04-5 and SAPO-5 in dependence on all relevant parameters (ref. 17). For

a variation

of the ratio Al/P

a distinct maximum

crystallinity can be observed always near a value of Al/P = 1.0.

of the Starting

with a general batch composition of for

A1203 * P2O5 * s Si02 * r TP * 50 H20 the incorporation of Si the following typical values can be observed

f o r the formation of 100% crystalline samples.

< s < 0.5 , r > 0.5 , TP = 0.4 and a temperature of 473 K

SAPO-5: 0 For

batches with s =

Pr3N the templates P q N ,

Et3N and Et2NH cause nearly the same induction time of about two hours. The crystallization time is about 2 hours €or the triamines and about 4 hours for the Et2NH. Pr4NOH shows an induction period of 4 hours and a very short crystallization time of less than 2 hours. In the following, f o r a further study of the incorporation of ions into the ALP04-5 structure kinetic experiments shall be reported. At first Be and Mg have been studied. According to the literature both give welldescribed MeAPO-5 structures, which have been characterized by solid state NMR (refs. 15.16). This holds also for Zn (see also ref. l 9 ) . As a contrast, a series of experiments with Ni has been carried out. Ni occurres in its ionic compounds almost only in sixfold coordination. In the literature no Ni-containing ALP04-5 o r SAPO-5 has been reported. Kinetic studies with Fe have been done with both valencies from which

the

respective ALP04-5 analogues are wellknown. Generally, the experiments were carried out with some excess of the Me compounds in the batch, to study the maximum content of the incorporation beside the influence on the kinetics. As far as possible the incorporation of the ions into the crystal shall be studied by SEM-EDAX experiments.

147

EXPERIMENTAL METHODS Svnthesis The

aluminium s o u r c e was always a pseudoboehmite phase obtained

CONDEA-Chemie.

Fumed

silica,

85%-orthophosphoric a c i d and

the

from sodium

hydroxide were products from Merck, as w e l l as t h e template tripropylamine

(Pr3N). For a l l r e a c t i o n mixtures d i s t i l l e d water was used. The

was

crystallization

steel

stainless

temperatures

autoclaves.

under

carried out i n

teflon

bottles

The mixtures were heated

autogeneous

pressure.

For t h e

to

placed

the

in

reaction

preparation

of

the

b a t c h e s two b a s i c r e a c t i o n mixtures were used: an aluminophosphate gel and another

gel,

reaction

t h e s i l i c a and

containing

the

template.

The

mixture t o p r e p a r e a SAPO- (AlPO4-) s t r u c t u r e had t h e

'standard' following

molar composition:

*

A1203

TP denotes t h e template,

P2O5

*

s Si02

*

r TP

f o r example Pr3N;

*

50 H20

A s r e a c t i o n temperature 473 K

was choosen throughout t h e experiments. A t s u i t a b l e t i m e i n t e r v a l s samples were taken,

filtered,

washed with d i s t i l l e d water t o near n e u t r a l i t y and

d r i e d o v e r n i g h t a t 460K.

For t h e experiments with t h e metals u s u a l l y batch compositions A1203

*

P205

* 0.4

Me0

*

r TP

*

50 H20

were used. Characterization For t h e a n a l y s i s of t h e products X-ray powder d i f f r a c t i o n p a t t e r n s were taken u s i n g a d i f f r a c t o m e t e r ISO-DEBYEFLEX 1000 with copper K The

X-ray

fluorescence

spectrometer.

measurements were c a r r i e d

out

with

radiation.

a

Philips

The method used t o o b t a i n t h e % c r y s t a l l i n i t i e s i s based on

a

of

the

The SEM-EDAX experiments have been c a r r i e d o u t a t a EDAX terminal

PV-

the

peak

areas

i n t h e i n t e r v a l 20=5"-40" a f t e r

subtraction

background. 9900 i n connection with a P h i l i p s scanning microscope SEM 515/D806.

RESULTS The g e n e r a l composition o f t h e batches used for t h e

experiments

was

given by A1203 This

*

P2O5

*

0.4 Me0

*

2 Pr3N

*

50 H20

means t h a t i n case of a f u l l i n c o r p o r a t i o n of t h e metal about

each

148

100

-

-

80

-

U

w 1 ) .

60 -

c

b

.A

40 -

m c,

cn

20 -

u

-

0

Fig.

I

1

A

I

I

1. Kinetics of the crystallization of a batch

* P205 * 0.4 Be0 * 2 Pr3N * 50 H20 u denotes the presence of an unknown phosphate phase A1203

I

100 w

80

\ ) .

2 60 c

.A

e

nJ

40

c,

L

20

u

0

10

0 Fig.

20

30

time /

40 hours

50

2. Kinetics of the crystallization of a batch A1203

*

P2O5

*

0.4 MgO

*

2 Pr3N

* 50 H20

u denotes the presence of an unknown phosphate phase

149

fifth

A 1 o r P should be replaced by a metal atom.

amounts

have

been o f f e r e d t o study t h e e x t e n t of

The

relatively

incorporation

large

to

and

obtain d i s t i n c t e f f e c t s f o r the kinetics.

1 and 2 show t h e r e s u l t s of t h e experiments with B e and Mg. Both

Fig.

elements are added t o t h e batch as oxides. There is no d i f f e r e n c e of t h e course of t h e c r y s t a l l i z a t i o n with t h e pure ALP04-5 and t h e SAPO-5 with s = 0.4.

compared

The i n d u c t i o n time

n e a r two hours and t h e c r y s t a l l i z a t i o n i s completed i n another two

is

hours.

The Be-containing sample c o u l d n ' t be analyzed by SEM-EDAX. The Mg c o n t e n t was a t about 0.1 MgO f o r one A1203

*

which

P2O5 u n i t

i s i n accordance t o t h e c o n t e n t s r e p o r t e d i n t h e l i t e r a t u r e ( r e f s . 15,16). After

about

one

day by X-ray some p e r c e n t of an unknown

observed i n t h e X-ray diagrams,

phase

can

be

t h e c o n c e n t r a t i o n of which remains almost

c o n s t a n t f o r a long t i m e . This phase cannot be i d e n t i f i e d i n t h e SEM. In

t h e r e s p e c t i v e experiments with ZnO t h e mentioned

phase

appears the

a l r e a d y a f t e r s i x hours i n a l a r g e r q u a n t i t y and t h e c r y s t a l l i n i t y of ZnAPO-5 phase does n o t exceed 60 %. The Zn-content i s n e a r 0.1 ZnO f o r A1203 In

*

a

P2O5 u n i t . comparison,

some Ni-containing samples

have

been

studied.

Ni-

c o n t a i n i n g materials with ALP04-5 s t r u c t u r e have never been r e p o r t e d .

In Fig.

are

4a

series of experiments with d i f f e r e n t N i - and S i

summarized.

contents

It can be seen t h a t a f t e r 6 hours a batch with 0.2

and 0.2 N i O h a s a c r y s t a l l i n i t y of more than 90 % compared with batch with 0.4 Si02. is s t a b l e f o r t h e

a

Si02 SAPO-5

It can be seen t h a t t h e sample c o n t a i n i n g only

Si02

48 hours o b s e r v a t i o n t i m e . The Ni-containing phases show The Si02 i n c r e a s e s t h e l i f e t i m e

a recrystallization t o cristobalite.

of

t h e m e t a s t a b l e SAPO-5 phase. Because

of t h e p o s s i b l e i n t e r e s t of t h e s e substances

experiments

for

catalysis,

f o r t h e f u r t h e r c h a r a c t e r i z a t i o n of t h e s t a t e of t h e N i

have

been c a r r i e d o u t . P r i m a r i l y EDAX experiments have been done. t h e g e n e r a l p r o p e r t i e s of t h e Ni-ions, the

nickel

is never i n t h e c r y s t a l s ,

b e s i d e t h e c r y s t a l s . UV-VIS-spectra coordination. the

Ni

Thus,

b u t always i n an

the

amorphous

phase

show, t h a t t h e Ni2+-ion has o c t a h e d r a l

a c r y s t a l l i z a t i o n o f pure ALP04-5 occurs

completely o u t of t h e c r y s t a l s .

accelerates

from

A s i t may be expected

t h e s e experiments show c l e a r l y t h a t

change t o c r i s t o b a l i t e .

The presence

of

NiO

Using Ni-acetate

leaving obviously for

the

150

100 2 s

' 80 =c,

c

.A

60

.A

A

ro

A

40

c,

cn

L

20

0

0

10

0 Fig.

20

30

40

time / hours

50

3. Kinetics of the crystallization of a batch A1203

*

P2O5

* 0.4

ZnO

*

2 Pr3N

*

50 H20

u denotes the presence of an unknown phosphate phase

100 be

'80 ) .

5 60 c .A

m

40

c, Ln

p 20 0

0

151 crystallization The

time.

experiments t h e c r y s t a l l i n i t y remains f o r a longer

crystallinity

decreases,

but

no

cristobalite

can

be

observed.

a t t h e BET-data f o r a sample without S i and 0.4 N i a f t e r 6 hours

Looking

crystallization

time

48

m2/g.

decreases t o

261 m2/g can be found.

A f t e r 52 hours

this

value

For t h e pure SAPO-5 with s = 0.4 310-330 m2/g

are

observed. For t h e Si-containing samples t h e decrease of t h e pore volume i s much slower.

a

The preceding experiments with t h e Ni-containing batches show t h a t characterization whole

5 and 6 show experiments with an Fe-containing

Figs. be

by X-ray accompanied only by a chemical a n a l y s i s of

the

sample may be sometimes misleading. batches.

It can

seen t h a t t h e c r y s t a l l i z a t i o n occurres f a s t e r than i n t h e presence

of

N i . S i m i l a r t o t h e N i c o n t a i n i n g batch a r e c r y s t a l l i s a t i o n t o c r i s t o b a l i t e

is

observed ( F i g . 5 ) .

EDAX experiments show t h a t i n c o n t r a s t t o

t h e Fe i s p r e f e r a b l y i n s i d e t h e c r y s t a l s r e g a r d l e s s

product

has been two- o r t h r e e v a l e n t .

Fe-source

This agrees with

the

Ni-

whether results

the which

have been obtained from MoBbauer experiments ( r e f . 18). The

comparatively

c r y s t a l l i n i t y i n t h e case of

low

Fez03

is

probably

the

SEM-EDAX

caused by t h e low s o l u b i l i t y of t h i s substance. The c r y s t a l composition was throughout t h e experiments n e a r A1203 which

means

that

*

*

1.1 P205

within

0.12 Fez03

t h e l i m i t s of t h e

accuracy

of

determination t h e aluminium about 10% of t h e A 1 have been replaced by Fe. In

the

X-ray diagram and i n t h e colour of t h e c r y s t a l s and

v a l u e s no d i f f e r e n c e s could be observed using Fe2+ o r Fe3'

in

the

BET

compounds a s an

Fe source. From preliminary experiments could be seen t h a t s i m i l a r t o t h e with

Ni

longer

t h e samples with Si02 c r y s t a l l i z e slower and a r e time.

To

study t h e amount of t h e i n c o r p o r a t i o n of

samples

stable the

for

Fe,

a the

following batches have been used. A1203 with n = 0.1:

*

P2O5

*

n Fe2O3

0.2; 0.3; 0.5;

*

0.2 SiO2

. As

*

2 Pr3N

*

50 H20

Fe-source Fe3+-acetate was used. The

c r y s t a l l i z a t i o n temperature was again

473 K.

By X-ray and by SEM-EDAX t h e following compositions could be obtained.

It could be shown t h a t a f t e r about 10 hours almost 100% c r y s t a l l i n i t y

can be obtained which i s s t a b l e f o r a long time. Most of t h e c r y s t a l s have t h e shape of hexagonal prisms which a r e surrounded by some amourphous

152

100 w

80

1 ) .

2 60 c

.A

40

m

c,

cn

=-. 20 L

u

0 10

0 Fig.

20

30 40 t i m e / hours

50

5. Kinetics of the crystallization of a batch

*

A1203

P2O5

*

0.4 FeC12’

2 Pr3N

* 50

H20

and the recrystallization to cristobalite (open circles)

100 w

80

\

>

Z, 60 c

-I+

d

40

m

c,

cn

=- 20

L

0

0 0 Fig.

10

20

30 40 t i m e / hours

50

6. Kinetics of the crystallization of batches A1203 * P2O5 * 0.4 Fe R * 2 Pr3N * 50 H20 with different Fe sources Fe R R

(CH3COOH)2 ( rn ) ; R = C12 ( 0 ) , FeR = 1/2 Fe203 ( A

153 material

and some s q u a r e p l a t e l e t s with t h e a molar r a t i o Fe/P

of

about

0.8 and about A1/P of about 0.2. I n t h e s e c r y s t a l s only very few S i can be found

From

. 1 can be s e e n t h a t t h e Si02-content is reduced t o

Table

compared with t h e b a t c h composition.

formulae t h e v a l u e s i n l a s t column can be found.

n

=

0.3

these

data

about

are i n good agreement

1/2 these

C a l c u l a t i n g t h e n e t charge o f

Apart from t h e v a l u e f o r

with

those

found

in

the

l i t e r a t u r e ( see e . g . r e f . 1 4 ) . The balance of t h i s charge may be p o s s i b l y given by some a c i d sites o r t h e r e s p e c t i v e c a t i o n of t h e template. Composition of t h e FeASPO samples with d i f f e r e n t Fe

Table 1.

contents

i n t h e batch. Fez03 c o n t e n t

Composition o f hexagonal prisms

Net charge of a

of t h e batch

FeASPO c r y s t a l s i n t h e product

(Fe,Al,P,Si)O2unit

A1203

n = 0.1: n = 0.2:

A1203

n = 0.3:

A1203

n = 0.5:

A1203

* * * *

1 . 2 P2O5

*

0.12 Fe203

1.1 P2O5

*

0.09 Fez03

1.1 P2O5

* *

0.16 Fez03

1.1 P2O5

0.21 Fez03

* * * *

0.10 Si02

-0.01

0.16 Si02

-0.03

0.12 Si02

’0.18

0.08 Si02

-0.09

CONCLUSIONS If

an

incorporation

of t h e a metal

into

an

ALPO-5

structure

is

observed, t h e i n d u c t i o n period and a l s o t h e r a t e of c r y s t a l l i z a t i o n i s n o t i n f l u e n c e d a p p r e c i a b l y compared with t h e d a t a observed f o r ALP04-5. The amount of i n c o r p o r a t i o n of t h e Mg, appreciable Fe3'

this

variation

Zn, Fe2+ and Fe3'

and lies g e n e r a l l y between

amount i s n o t changed a p p r e c i a b l y ,

shows no

5 and 1 0 %. For t h e

i f additionally

Si

is

offered i n t h e batch. Pure s u b s t a n c e s can always obtained remaining i n t h e batch below

these

values. N i i s not incorporated i n t o the c r y s t a l s .

I n t h i s c a s e only ALP04-5 o r

SAPO-5 c r y s t a l l i z e s . The presence of c a t i o n s and t h e compound i n which they are t h e b a t c h have, SAPO-5 phase.

added

to

g e n e r a l l y , i n f l u e n c e s on t h e s t a b i l i t y of t h e ALPO4-5 and

154 phosphates of t h e incorporated metals a r e u s u a l l y

The

water.

The c r y s t a l l i z a t i o n o f t h e s e phosphates o r of an

insoluble

in

aluminophosphate

with h i g h e r d e n s i t y may t a k e p l a c e i n d i f f e r e n t ways. It can be amorphous, crystallize

in

d i f f e r e n t s t r u c t u r e s on o r b e s i d e t h e ALPO4-5

and may cause a r e c r y s t a l l i z a t i o n c r i s t o b a l i t e - o r

structure

or

SAPO-5

tridymite-.

l i k e aluminophosphate. Which of t h e s e cases i s p r e s e n t i n a p e c u l i a r system must

be

studied

separately. ACKNOWLEDGEMENTS

W e thank t h e "Deutsche Forschungsgemeinschaft" f o r t h e generous support of

our work. REFERENCES

1 E.M. Flanigan, B.M.

Lok, R. Lyle P a t t o n and S.T. Wilson, Pure and Appl. Chem.,

2

B.M.

3

B.M. Lok, C.A.

Lok, C.A.

E.M. Flanigan, U S P a t . E.M.

4 S.T. 5 B.M.

4 440 871 (1984)

Messina, R.L.

106 (1984) 6092

Wilson, B.M. Lok, E.M. Flanigan, EP 0 043 Lok, C.A.

Messina, R.L. P a t t o n , R.T. EP 0 103

562 (1984)

Gajek, T.R. Cannan and

117 (1986)

Lok, R . Lyle P a t t o n and S.T. Wilson,

E.M. Flanigan, B.M.

EP 0

7 8 9

P a t t o n , R.T. Gajek, T.R. Cannan and

Flanigan. J . A m e r . SOC.,

E.M. Flanigan,

6

58 (1986) 1351

Messina, R . L . P a t t o n , R.T. Gajek. T.R. Cannan and

158 976 (1985)

B.M. Lok. L.D. Vail and E.M. Flanigan, EP 0 B.M.

Lok. L.D. Vail and E.M.

B.M.

Lok, B.K. Marcus and E.M. Flanigan, EP

Flanigan, EP 0

158 348 (1985) 161 491 (1985) 0 161 490 (1985)

10 B.M. Lok, B.K. Markus, C . A . Messina, R . L . P a t t o n , S.T. Wilson and E.M. Flanigan. EP 0

11

158 349 (1985) 158 350 (1985) S.T. Wilson, EP 0 158 977 ( 19851

B.M. Lok, B.K. Marcus and E.M. Flanigan, EP 0

12 B.M.

Lok, B.K.

Marcus, C.A.

Messina and

13 B.M.

Lok, B.K.

Marcus, L.D.

V a i l , E.M.

S.T. Wilson, EP 0

159 624 (1985)

Flanigan, R.L. P a t t o n and

155

14 R.

Khouzami, G . Coudurier, B.F. Mentzen and J . C . Stud. S u r f . S c i . ,

15 E.M.

Vedrine,

37 (1988) 355

Flanigen. B.M. Lok, R.L. P a t t o n and S.T. Wilson i n

"New Developments i n Z e o l i t e Science and Technology" Y . Murakami, A. Iima. J.W. Ward ( E d s . ) , Kodansha, E l s e v i e r

Amsterdam, Oxford, New York. Tokyo (1986) , p . lo3 16 S.T. Wilson and E.M. Flanigen i n " Z e o l i t e S y n t h e s i s " . M.L. O c c e l l i and H.E. Robson (Eds.) ACS Symposium S e r i e s 398 (1989) 329

17 H. Weyda and H. Lechert, Z e o l i t e s 10 (1990) 251 18 Hong-Xin L i , J . A . Martens, P.A. Jacobs, S. Schubert, F. Schmidt, H.M.

Ziethen and A.X.

Trautwein, i n "Innovations i n Z e o l i t e Materials

Science" P . J . Grobet e t a l . (Eds.) S t u d i e s i n S u r f . Science and Catal.

37 (1988) 75

19 G.C. Bond. M.R.Gelsthorpe, K.S.W. Sing and C . R . Theocharis. J . Chem. SOC. Chem. Commun., 1056 (1985)

This Page Intentionally Left Blank

P.A. Jacobs et al. (Editors),Zeolite Chemistry and Catalysis 0 1991 Elsevier Science Publishers B.V., Amsterdam

157

FACTORS AFFECTING THE CRYSTALLIZATION OF ZEOLITE ZSM-48 G. Giordanoa, N. Dewaeleb, 2. Gabelicab, J. B.Nagyb, A. Nastroa, R. helloa and E.G. Derouaneb b Laboratory of Catalysis, FacultCs Universitaires N.D. de la Paix, rue de Bruxelles, 61, B-5000 NAMUR (Belgium) a Dipartimento di Chimica, Universita della Calabria, Arcavacata di Rende,

1-87030 RENDE (CS), (Italy)

Abstract Zeolite ZSM-48 was synthesized in the presence of either hexamethonium ions or octylamine admixed with tetramethylammonium ions. It was observed that at high temperature and in presence of ammonium ions, the hexamethonium ions are decomposed into hexyltrimethylammonium and trimethylammonium ions, when ZSM-48 is formed, while no decomposition occurs when EU-1 crystallizes from the same but slightly Al-richer reaction mixture. The relative amount of defect groups can be rationalized if one supposes that the hydroxyl nests resulting from missing T atoms must be created in the framework as to better accomodate the trimethylammonium terminal groups of the hexamethonium ions located along the linear ZSM-48 channels. 1. INTRODUCTION

In the last few years, the preparation of a series of structurally similar zeolites involving one-dimensional channels, namely ZSM- 12, ZSM-22, ZSM-23, ZSM-48, KZ-2, EU-1 and NU-10, h a s been reported in the literature[ 1- 111. The high-silica zeolite ZSM-48 can be synthesized from aqueous silica hydrogels, with or without alkali cations or aluminium, in presence of a variety of N-containing organic molecules (mono or diaminoalcanes) [4, 12- 171 b u t also from mixtures containing quaternary imidazole compounds [51 in presence of ethyleneglycol, glycerol or butanol as solvents [181 or even in non-aqueous systems 1161. The structurally related zeolite EU- 1 was claimed to crystallize for SiOz/A1203 ratios of 120 or lower in systems containing hexamethonium cations [2,4,12,13], but recently, the synthesis of high-silica EU-1 was realized when benzyl dimethylamine and benzylchloride were used as templates [lo].

158

On the other hand, it was recognized that the presence of ammonium ions in hydrogels leading to high-silica zeolites, such as ZSM-5 plays an inhibiting role on the nucleation process [ 191. Finally one should bear in mind that, because of the high alkalinity of the synthesis mixtures, a rather high amount of defect groups (= SiOX, where X = H, alkali or organic cation) are formed in the final zeolite crystals. Van Santen et al. 1201 proposed a hypothesis to explain the presence of these groups in the silicalite-1 and also supposed the existence of "hydroxyl nests" created by the missing tetrahedral sites in the MFI framework. The relative amount of the various kinds of defect groups could be easily determined by using 29Si-NMR [21,22]. A series of preliminary investigations [13] revealed that the presence of ammonium ions in the starting hydrogel induced a partial decomposition of hexamethonium ions into the corresponding hexyltrimethylammonium and trimethylammonium fragments that were found incorporated in the final ZSM-48 framework and that this decomposition noteworthyly influenced the final amount of defect groups in the zeolite lattice. In order to investigate the actual role played by ammonium ions on the synthesis of ZSM-48 and on its final "defected" structure, we have investigated more systematically the influence of a series of synthesis variables (nature of the organic guests, presence of alkali hydroxides, and of Si- and Al- bearing ingredients in various concentration) on the behaviour of ammonium ions during synthesis. 2. EXPERIMENTAL

2.1. synthesis

Two series of hydrogels having the following molar composition have been investigated: Gels of type 1: xNa2O-yHMBra -z(NH4)20-wAl203-6OSi02 -3000H20 where HMBr2 stands for hexamethonium bromide and 05 x 510, 01 y 110. 01 z 110, 05 w 51.5. The syntheses were run in static conditions under autogeneous pressure at 200 f 2°C in 60 ml Teflon-lined Morey-type autoclaves for variable periods of time. Gels of type 2: 1 5 N a 2 0 - 15.6~MABr-80.40cNH2-0.48Al20310.8H2S04-6OSi02-3258H20

where TMABr stands for tetramethylammonium bromide and OcNH2 for n-octylamine. The syntheses were performed under autogeneous pressure in stirring conditions at 160 5 2OC in the same autoclaves utilized for the type 1 synthesis. The detailed synthesis procedures for type 1 and 2 systems were reported elsewhere [4,13].

159

2.2. Characterization

The identification of the solid phases and the determination of their crystallinities were carried out by X-ray powder diffraction. The alkali and Al contents were determinated by PIGE [23], while the amount of organic and water molecules was evaluated by thermal analysis. The amount of defect groups in the zeolite framework was calculated from solid state MAS 29Si-NMR spectra [21]. Finally, the identification of the decomposition products of hexamethonium ions were performed by combining TG-DTA analysis and MAS 13C-NMR [ 141.

3. RESULTS AND DISCUSSION A detailed study of the systems of type 1 allows to define adequately the influence of organic and inorganic cations, on the synthesis course. The nature of the crystalline products so-obtained as a function of the relative a m o u n t s of sodium hydroxide, ammonium ions and hexamethonium ions, is reported in Table 1.

Table 1 Nature and crystallinity of the products obtained from the system: xNa~O-yHMBr~-z(NH~)~O-O.5Al~0~-60Si0~-3000H~O at 200°C. ~

~~

Sample

~

~

mole in hydrogel X

1

2 3 4 5 6 (a) 7 8

0 0.5

5 5 5 5 5 5

Y

5 5 5 0.5 0 0 5 5

Synthesis Nature [crystallinity] time (days) of the products Z

10 5

5 5 5 5 0,5 0

5and9 5and9 2.7 7 5 16 2.7 2.7

amorphous amorphous ZSM-48 [63%] ZSM-5 [25%]+a-quartz ZSM-5 [20%l+a-quartz ZSM-5 [50?'0] ZSM-48 [8O%l ZSM-48 [81%]

(a)Syntheses carried out at 170 "C. In these systems zeolite ZSM-48 does not form if HM++ions are absent (samples 5 and 6) or if their relative concentration is low (sample 4). Under such conditions the reaction leads to the formation of ZSM-5, probably because the formation of 5-1 SBU is favoured in a high-silica hydrogel containing both Al and Na+ [13,24,25]. The presence of a-quartz admixed with ZSM-5 is caused problably by the high synthesis temperature. Indeed if the synthesis temperature decreases no dense phases are detected (sample 6). In absence or with a low NaOH content even in the presence of NH4+ ions, the starting hydrogel does not crystallize, showing that the presence of alkali cations is indispensable for

160

the nucleation to proceed (samples 1 and 2). Low NH4+ concentrations neither influence the crystallinity. nor the crystallization rate of ZSM-48 (samples 7 and 8). For a better understanding of the role played by the NH4+ ions on the crystallization rate of ZSM-48 the ammonium contents was vaned from 0 to 5 moles in the hydrogel of type 1 (Table 1 samples 3. 7 and 8). It can be seen that the crystallinity of ZSM-48 decreases with increasing NH4+ content. Similar behaviour was observed for the nucleation and growth of ZSM-5 [19]. Dodwell et al. 1121 also observed a decrease of the crystallization rates of ZSM-48 and EU- 1 with increasing ammonium content. A decrease of the crystallization rates due to the presence of NH4+ ions was also observed during the formation of silicalite-2 in presence of F- ions [261. The presence of NH4+ ions, in type 1 hydrogels, favours the partial decomposition of HM++ ions in hexyltrimethylammonium (HTMAm+)and trimethylammonium ions (TMAm+).The results of this investigation are reported in Table 2. Table 2 Nature and crystallinity of the products obtained from the system: 5 N a ~ O - 5 H M B r ~ - x ( N H ~ ) ~ O - y A l ~ O ~ - 6 O S i Oas ~ -a3 O function O O H ~ Oof synthesis conditions. Sanple

mole

x 9 8

0 0

y 0 05 0

Synth.

synth

time, (d-1

temp.

("a

2 200 2.7 200 10(d 5 25 180 ll(d 5 025 33 180 11 5 025 2.7 200 3 5 05 2.7 200 12 5 1 6 200 13(b) 5 15 7 200 (a): under stirred conditions. (b):using 10 Na2O and 10 HMBr2.

Nature[crystalluutyl oftheprodmts ZSM-48 [97%] ZSM-48 [819'01 ZSM-48 [95%] ZSM-48 [88?!] ZSM-48 [8!3?!] ZSM-48 [63%] ZSM-48 [26%] EU-1 [78%]

Decomposit. of HM ions NO NO NO NO YES YES YES NO

In a previous work, we have shown that the decomposition products are also incorporated in the ZSM-48 framework 1141. The degradation of HM++ ions was shown to occur by nucleophilic substitution of the trimethylamine groups by NH3 [ 141. The decomposition is essentially influenced by the synthesis temperature and not by the actual Al content in the hydrogel (sample 11). On the other hand the channel dimension of zeolite ZSM-48 also seems to play an important role in the decomposition of the HM++ ions. Indeed, in the case of sample 13 which turns to be zeolite EU-1, the HM++ ions were found intact. We believe that the terminal trimethylammonium groups of each HM++ entity can be easily

161

accomodated in the side pockets of EU-1 channels so that their decomposition, probably otherwise not very favourable energetically, does not need to occur. Table 3 gives the chemical composition of the various of ZSM-48 samples. The amount of incorporated water per unit cell is similar in all samples, its low value confirming the hydrophobicity of the high-silica zeolite. Considering the low value of Na/u.c.. it can be supposed that the alkali cations do not play a major role in the crystallization of ZSM-48 [12,13,15,27]. In fact, when diamines (15,271 or even HM++ ions [12] are used, ZSM-48 crystallizes easily in absence of alkali cations. The alkali cations probably play a role on the nucleation rate, on the morphology and on the crystal size by neutrlizing the Si-0- defect groups along with the (Si-0-Al)-negative charge, when available. In all syntheses carried out in presence of HM++ ions (samples 8 and 9) the organic content per unit cell is close to one (Table 3). Most likely the organic cations stabilize the framework by a pore filling action and neutralize the (Si-0-All-negative charges and a part of the Si-0- defect groups. On the other hand the role of HM++ ions as counterions and as pore fillers is confirmed by the observation that in the presence of NH4+ ions, although the HM++ ions are decomposed, ZSM-48 is obtained. The presence of OcNH2 also leads to the formation of zeolite ZSM-48 (sample 14). In this case, it is supposed that the TMA+ ions, added to the hydrogel, along their possible pore filling action, essentially act as counter cations together with the Na+ ions, to the framework negative charges 14,151. The HM++ions insure a good filling of the intracrystalline pore volyme of zeolite ZSM-48, because the length of hexamethonium, is. 14.05 A, is close to the channel length of one unit cell of ZSM-48 (16.8 A). MorFover, considering that the diameter of one hydrated sodium ion is 4.6 A and that these ions are located in the channels one obtains a good pore filling. Sample 3 differs from the other samples (pore filling 75%), since in this case the contribution of the NH4+ ions present in the pore volume has not been taken into account. Higher values of pore filling are obtained for the ZSM-48 synthesized in presence of OcNH2 and TMA+ ions, since the dimension of the TMA+ ions insure an even better filling of the zeolitic channels. In the syntheses carried out in the presence of HM++ions only quite a number of defect groups are created, most likely due to the more bulky terminal trimethylammonium groups that have a larger dimepion (6.9 A) than the average ZSM-48 channel diameter (about 5.3 x 5.6 A) [131. To explain how the HM++ ions are responsable of the creation of framework defect groups, it must be supposed that the presence of each terminal trimethylammonium bulky groups generates a missing tetrahedral site in the zeolitic framework. This consequently corresponds to 4 defect groups (hydroxyl nests) per trimethylammonium group. For sample 8 that contains about one HM++ion per unit cell, 8 hydroxyl nests are created in the framework by the terminal ends of the template, the

Table 3 Crystallinity and chemical composition of various ZSM-48 zeolites. mole in hydrcgel

Sample

9 8(a) 3(a) 14

%Cryst.

Na

R

NH4+ Al

5 5 5 15

5 5 5 96

0 0

0 97% 0.5 75%

5

0.5 0.4

0

73% 85%

H20 (2)

Na (1)

Al (1)

HM* (394)

1.80 2.25 2.05

n.d 0.24 0.26 0.20

n.d 0.72 0.94 0.95

0.98 1.00

2.05

0.23

-

Composltionperunitcel HTMAm' TMAm' OcNH2 TMA' (3) (3) (2) (2)

0.54

0.26

-

0.20

1.90

Pore filling SiOR

Yo

(5) 11.1 10.1 5.1 2.0

82% 84%

68% 90%

R= HMBr2 for samples 3, 8 and 9, and OcNH2 + TMABr for sample 14. System of sample 3 . 8 and 9 : x Na2O y HMBr2 z (NH4)2O w A1203 60 Si02 3000 H2O System of sample 14: 15 Na2O 15.6 TMAI3r 80.4 OcNH2 0.48 A1203 10.8 H2SO4 60 SiO2 3258 H20 syntheses carried out at 160 f 2 OC under stirring conditions [ 151. (1): Evaluated by PIGE 1231 (2):Evaluated by TG-DTA 1141 (3):Evaluated by TG-DTA and 13C-NMR [14] (4):For sample 8 and 9 evaluated by TG-DTA and ammonia titration (131 (5):Evaluated by 29Si-NMR [21] (6):Percentage of filling as calculated by considering the total channel length of one ZSM-48 unit cell equal t.0 16.8 A and the length of each organic molecule respectively: HM++ = 14.05 A; HTMAm+ = 11.91 A: TMA+=TMAm+= 6.60 A and OcNH2 = 12.90 A [131 (a):The difference in the crystallinity percentage of samples 3 and 8, compared to the same samples in Tables 1 and 2, is due to the different synthesis times.

163

remaining 2.1 defects, as measured by 29Si-NMFt, are probably those that are usually statistically created throughout the framework at high synthesis pH values. Oppositely, in syntheses ran in the presence of NH4+ ions, a lower number of defects is detected (e.g. sample 3). Indeed, in that case, part of the HM++ ions is decomposed by NH4+, HTMAm+ yielding the equivalent a m o u n t of hexyltrimethylammonium a n d trimethylammonium "fragments". Because of their small steric dimension, these latter probably do not induce additional defects. The remaining "bulky" organics, namely 0.23 HM++ and 0.54 HTMAm+ present in the unit cell, respectively generate 0.23 x 2 x 4 = 1.84 and 0.54 x 4 = 2.16, thus a total of about 4, hydroxyl nests per unit cell. As for the preceding case, the remaining 1.1 defects stem from the synthesis condition and are statistically distributed along the framework. Finally, ZSM-48 crystallized from systems involving OcNH2 and TMA+ ions, do not show a large number of defects (Table 3, sample 14). Indeed, these molecules are not bulky enough to generate missing T sites, their actual dimension corresponding well to that of the average channel diameter, therefore ensuring their good fitting. On the other hand this synthesis requires a long synthesis time to obtain the crystalline ZSM-48 zeolite [151. The resulting ZSM-48 therefore contains a low amount of defects, this explaining well the relatively long crystallization time necessary to obtain a highly crystalline material [15], as also observed in the case of ZSM-5 [221. 4. CONCLUSION

The presence of NH4+ ions in the hydrogel involving HM++ ions reduces the cristallization rate of ZSM-48 zeolite in agreement with a previously publication [12]. On the other hand, ammonium ions also favour the partial decomposition of hexamethonium ions at high temperature. The decomposition occurs if Z S M - 4 8 is formed, while no decomposition is observed if the reaction mixture leads to EU- 1. It seems therefore that the zeolite plays a catalytic role in the decomposition of hexamethonium ions. I t is observed that a much larger relative amount of defect groups is formed when hexamethonium ions are incorporated in the ZSM-48 zeolitic structure. The results can be rationalized, if it is supposed that each terminal trimethylammonium groups leads to one missing tetrahedral site in the zeolitic structure, therefore inducing the formation of 4 hydroxyl nests. ACKNOWLEDGEMENTS

The authors are indebted to Mr. G . Daelen for his skillful help in obtaining the N M R spectra. They also thank Dr. P. Ratnasamy and Dr. R. Kermar for fruitful discussions.

164

5. REFERENCES 1 2 3 4

5 6 7 8 9 10 11

12 13 14 15 16 17 18 19 20 21 22

23

24 25 26 27

A. Araya and B.M. Lowe, Zeolites, 4 (1984) 280. J.L. Casci, Stud. Surf. Sci. Catal., 28 (1986) 215. B. Marler. Zeolites, 7 (1987) 393 N. Dewaele, 2. Gabelica, P. Bodart, J. B.Nagy, G. Giordano and E.G. Derouane, Stud. Surf. Sci. Catal., 37 (1988) 65. S.I. Zones, Zeolites. 9 (1989) 458. S . Emst, P.A. Jacobs, J.A. Martens and J. Weitkamp, Zeolites 7 (1987) 458. C. Pellegrino, R. Aiello and 2. Gabelica, in M.L. Ocelli and H.E. Robson (Eds.). Zeolite Synthesis (ACS Symp. Series 3981, Am. Chem. SOC.. Washington DC, 1989, p 161. R.A. Lefebre, H. Kouwenhoven and H. van Bekkum, Zeolites 8 (1988) 60. S. Emst, J. Weitkamp, J.A. Martens and P. Jacobs, Appl. Catal.. 48 (1989) 137. G.N. Rao, P.N. Joshi. A.N. Kotasthane and P. Ratnasamy, Zeolites, 9 (1989) 483. C.A. Fyfe, G.T. Kokotailo. H. Strobl, C.S. Pasztor, G. Barlow and S. Bradley. Zeolites 9 (1989) 531. G.W. Dodwell. R.P. Denkewicz and L.B. Sand, Zeolites, 5 (1985) 153. G. Giordano, J. B.Na@. E.G. Derouane, N. Dewaele and 2. Gabelica, in M.L. Ocelli and H.E. Robson (Eds.), Zeolite Synthesis (ACS Symp. Series 398), Am. Chem. SOC.. Washington DC, 1989, p 587. G. Giordano, N. Dewaele, 2. Gabelica, J. B.Nagy and E.G. Derouane, Appl. Catal., in press. G. Giordano, Z . Gabelica, N. Dewaele. J. B.Nagy and E.G. Derouane, Proc. Int. Symp. Chemistry of Microporous Crystals, Tokyo, J u n e 26-29, 1990. in press, and references cited therein. X. Wenyang, L. Jianquan and L. Guanghuan, Zeolites 10 (1990) 753. J. Quingzhu and P. Wenqin, Huaxue Xuebao, 48 (1990) 761, (C. Abstr.) 113, 2146986, 1990. H. Qisheng, F. Shduhua and X. Ruren. J. Chem. SOC. Chem. Commun. (1988) 1486. 2. Gabelica, N. Blom and E.G. Derouane, Appl. Catal., 5 (1983) 227. R.A. van Santen, J. Keijsper, G. Ooms and A.G.T.G. Kortbeek, Stud. Surf.Sci. Catal., 28 (1986) 169. J. B.Nagy, P. Bodart, H. Colette, J. El-Hage Al-Asswad. 2. Gabelica, R. Aiello, A. Nastro and C. Pellegrino. Zeolites, 8 (1988) 209. J.M. Chezeau, L. Delmotte, J.L. Guth and Z . Gabelica, Zeolites in press. G. Debras, E.G. Derouane, J.P. Gilson, 2. Gabelica and G. Demortier. Zeolites, 3 (1983) 37. A. Nastro, C. Colella and R. Aiello. Stud. Surf. Sci. Catal., 24 (1985) 39. G. Bellussi, G. Perego, A. Carati, U. Cornaro and V. Fattore, Stud. Surf.Sci. Catal., 37 (1988) 37. R. Mostowicz , personal comunication to one of the authors. A. Araya and B.M. Lowe, J o u m . Catal., 85 (1984) 135.

PA. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 01991Elsevier Science Publishers B.V., Amsterdam

165

SYNTHESIS AND CHARACTERIZATION OF Cr-MODIFIED SILICALITE-1

U.Cornaroa, P.Jirub, Z.Tvaruzkovab

and K.Habersbergerb

a Snamprogetti, Via Maritano 26, S.Donato Mi., Italy The Heyrovsky Institute of Physical Chemistry, Dolejskova 3, Prague 8 , Czechoslovakia Abstract

A series of Cr-modified silicalites-1 was prepared and characterized. Acidic and dehydrogenating properties were observed. Electron-acceptor Cr(II1) species were detected by IR adsorption experiments. 1.INTRODUCTION

Few atoms are recognized as compatible with a Silicon based MFI framework topology: All Gal Fe, B, Ti, Ge among others, with different degrees of stability [1,2,3]. Cr2O3 [ 4 ] and Cr-based materials are well known catalysts in alcohol [5], alkane [ 6 ] dehydrogenation and olefins polymerization [7]. Cr-zeolites have already been recognized as interesting materials [ a ] . Cr(II1) is very likely to be unstable in framework tetrahedral coordination [ 9 ] , although isomorphous substitution has also been claimed [lo]. Aims of this work are: - To synthetize a series of Cr-modified Si-MFI zeolites, with attention paid to obtain homogeneous, XRD-single phase materials. - To characterize the nature of chromium sites using model chemical reaction and physico-chemical methods. 2.EXPERIHENTAL 2.1

Syntheses and preliminary characterization

Zeolites syntheses were carried out by mixing: Si(OC2H5), A1 free (Huls); N(Pr0p)qOH (aq.so1. 14% wt), alkaline ion free: Cr(N03)3 (Carlo Erba), ethanol and water. After hydrolysis of alkoxide the reactions mixtures were aged overnight and hydrothermally crystallized in static 250 ml autoclaves at 150 " C for 7 days. The products filtered and washed were dried at 120 "C. Thermal treatment in air at 550 "C burns out the organic ternplating agent. Samples in C form were obtained in this way. Calcined samples were ionically exchanged with NH4+ (1 mol/l, 10 cc/gr zeolite). By thermal treatment in air at 550 " C

166

samples in H form were obtained. Chromium content was determined by inductively coupled plasma analysis, silicon content gravimetrically. Structure characterization was carried out via MID-FTIR (KBr pellets, 0.3 % ) . Phase purity was also confirmed via XRD measurements. Morphology of crystals was observed via optical microscopy. 2.2 Physico-chemical characterization

The adsorption of NO and CD3CN were studied by FT-IR (Nicolet Mx-1E). Heatable, high vacuum IR cuvette with samples in the form of self supporting pellets(zl0 mg/cm2) were used. Before measurements, all samples were evacuated at 350 C overnight. In all the experiments the equilibrium amount of either NO (2 Torr) or CD3CN ( 1 Torr) was adsorbed at 25 C for 15 minutes and spectrum recorded. The normalized absorbance of the respective bands was measured after desorption at room temperature and 100 "C; its variation was selected to characterize the stability (strength) of the bond between the adsorbate and the sample. 2.3 Catalytic characterization

Ter-butanol dehydration and n-hexane cracking were studied on samples in H form, isobutane dehydrogenation was studied on dried samples activated in situ. Reactions were carried out on st.stee1 or pyrex integral, fixed bed, plug flow reactors at atmospheric pressure. Catalyst (1-2 cc) was crushed to 2 0 - 4 0 mesh size. On-line chromatographic analyses were carried out. Experimental conditions are outlined in Table 1. Kinetic constants were evaluated by applying eq.(l). Table 1 Catalytic tests i n flow reactors: Experimental corditions Acidic a c t i v i t y Activation : Conditions :

1. Ter-Butanol dehydratim

2. n-Hexane cracking

500 C'CI, Nitrogen flow T= 140-180 [ T I ; PtButOH" 0.5 CAtml;

500 pC1, Nitrogen flow T= 450-575 [ T I ; p n - ~ a =0.5 [Atml; PtOt= 1 Wtml; T= 2.3 + 3 [ g r * s e c * ~ c - ~ l

pH20=0.05 [ntd Ptot= 1 [ntml; T= 0.3+3 [gr*sec*cc-lI

Dehvdrogenatins a c t i v i t y 3. lsobutane dehydrogenation : P form, 3 0 4 0 msh Catalyst Activation : 550 [ ' C I Air flow, 650 [ " C I CH4 flow. Conditions : l= 580 ["Cl, pi^^= 1 tatml T= 9 Isecl

k7 = l n I l / ( l - X ) l 7

(1) apparent contact tim

tgr*sec*cc-l~

k kinetic constant [gr-l*sec'l*ccl X

reactant conversion

Ethylene oligomerization was investigated with a McBain balance in static arrangement at the temperature of 80 "C and pressure of 40 Torr. The mass increase of the sample was determined as a function of time, and from the data obtained the initial rate ra of oligomerization was calculated. The experimental details were published previously [ll].

167

3.RESULTS AND DISCUSSION 3.1 synthesis and preliminary characterization A series of Cr,Si-MFI zeolites was prepared from reaction mixtures with composition as reported in Table 2. Attention had to be paid in order to avoid polymeric Cr-oxide species formation [12]. Syntheses were carried out in the absence of alkaline metals and at a pH as low as possible. A pH varying from 9.2 in the absence of Cr (scpl0) and 8.2 ( Si/Cr =loo, scp4) was observed. Preparations at the lower pH values were scarcely reproducible and amorphous phases (scp42) were occasionally obtained. The structural characterization of synthetized materials was carried out via Mid-FTIR and XRD techniques. MFI structures were observed as evidenced by integrated absorbance ratio ( 5 5 0 / 8 0 0 cm-1) (Table 2 ) . No IR bands related to the presence of Cr were evident, nor were extraphases detected in XRD diffraction patterns. A monoclinic elementary cell was observed for samples in H form. Large hexagonal prismatic crystals were observed. (Table 2 ) . A comparison of Cr content for samples in C (calcined)and H form evidences Cr leaching in the ionic exchange step, suggesting a weak zeolite-Cr interaction (Table 2).

.

Table 2 Zeolites synthesis and preliminary characterization Preparations

Chemical analysis Si/Cr

sample Si/Cr TPAISi HzO/Si

pH

form C

form H

Characterization Cryst. Yield

[cl (548/797)

IAR

[dl form form

XRD

Cryst a 1s [el Morphology l u

C H [a1 [bl _ - - - - - _ _ _ _ - _ _ _ _ - - _ _ - ~ - - - - - - - - - - - - -- -.- _ _ _ _ _ _ _ _---_ _ --_ - _ _ _ _ _- _--scpl0 scpl3 scp7

00

0.05

500 300

scp4 scp42

100 100

0.05 0.05 0.05 0.05

40 40 40 40 40

9.2 8.8 8.3 8.1 8.2

00

00

2803 556 85 110

6348 928 248 146

77 75 81

1.5 1.6 1.5

m c m c

40 n.m.

1.1 0.1

or mc amorphous

or mc mmc

60 20 40 20

25 7 15 5

[a]: Reaction mixture molar ratios; [bl: (Zeolite ueight form C) / (Si02+ Cr2~),eact.mixt. *loo; Ccl: Integrated Absorbance R a t i o 1R band 550 crn-'/ 800 an-'; [dl: Symxtry of elementary c e l l . Orthorhorrbic (or), Monoclinic (mc); [el: Crystals dimensions [XI. Length (1). Width (u)

3.2

Physico-Chemical characterizations

(i) NO adsorption The IR spectrum of NO adsorbed on Cr,Si-MFI sample (scp4H) at 25 " C exhibits a weak band at 1757 cm-l (Fig.1). In the case of pure silicalite (where no NO adsorption takes place) , such band was not observed. From this it follows that in the case of Cr,Si-MFI sample this band The corresponds to a weakly bonded NO com lex with Cr(II1) normalized absorbance of the 1757 cm-y band ( 0 . 2 8 ) decreases after the desorption at 25 C by about 50%, after further desorption at 100 C by about 70%. A similar band, only shifted

.

168

towards higher wavenumbers (1780 cm-l), thus corresponding to a higher bond strength, was found after the adsorption of NO on Cr exchanged Si.Al-MFI zeolite (HCrZSM5), prepared and characterized according to [13]. We suggest in this case the formation of a similar surface complex Cr(III)--.NO [13], only more weakly bonded. This result seems to indicate that: - The predominant part of Cr(II1) present in the Cr,Si-MFI is accessible to the interaction with molecules in the gaseous phase in the same way as in the Cr exch,Si.Al-MFI sample, with Cr in cationic positions. - No Cr (or only a very small part of it) is inserted in the zeolite skeleton. Besides that, the presence of a minor part of Cr in a higher valency state (V,VI) (which does not form adsorption complexes with NO) cannot be excluded. Fig.

A

r-

I

Lu U

Fig. 2

1

ul I-

z

a

m

$In m

a

I

1700

1900

-1 crn

...

SO0

*

2000.

2500

zoo0

1

cm' Fig.1. I R spectra o f NO adsorption on Cr-SiLicalite: (1)adsorption of 2 (2) desorption o f NO a t 25'C. Fig.2.

15 min. (3) Oesorption of NO a t lOO'C,

IR spectra o f CD3CN adsorption on Cr,Si-MFI (scplrH),(A); and

TOW

of NO at 25-C.15 min.

1 hour. S i - M F I (scplOH),(B).

(1) Adsorption of CD3CN a t 25°C. 15 min, g l l o r r . (2) Oesorption o f CD3CN a t 25 " C , 15 min. (3) desorption o f CD3CN a t 100 "C, 15 min.

(ii) CD?CN adsorDtion The IR spectra of CD3CN adsorbed on a Cr,Si-MFI (scp4H) as well as those after its desorption at 25 and 100 "C, respectively, are presented in Fig.2. The corresponding spectra obtained with pure silicalite (scpl0) are also reported for comparison. In the spectrum three bands were found in the wavenumber range characteristic for adsorption complexes of CD3CN with proton-donor and electron-acceptor sites [14,15,16]. The band at 2300 cm-1 corresponds to the interaction of CD3CN with electron acceptor centers exhibiting a good thermal stability: the value of the normalized absorbance remains the same after the desorption at 25 C. The shift of the corresponding band towards lower wavenumbers (by

169

21 cm-1) indicates the minor strength of the electron acceptor centers when compared with those on HZSM5 [15]. The lower value of the normalized absorbance (0.15) of 2300 cm-1 band indicates also their lower concentration in comparison with the HZSM5 zeolite (0.75). On pure silicalite after the adsorption of CD3CN no bands in the 2300 cm-1 region were found. These centers are therefore connected with the presence of Cr in Cr-silicalite. In the case of the Cr-silicalite the other bands in the range 2260-2240 cm-1 correspond to the non- specific sorption of CD3CN and the VC-D vibration. Both bands are thermally unstable in the course of the desorption at both 25 and 100 C. No bands corresponding to the proton donor center were found in the spectrum. The conclusion drawn from NO-adsorption experiments may be completed by the statement that the active sites represented by the part of cr(II1) accessible to the interaction with NO and CD3CN have electron-acceptor properties. 3.3 catalytic Characterization

[i) Ter-Butanol dehydration Dehydration reaction was studied in order to evidence the occurrence of weak acidic sites. Expected first order kinetics was experimentally observed when plotting In (1/1-x) versus apparent contact time. Silicalite and Cr,Si-MFI samples were studied. Arrhenius plots Fig 3(a) allowed a determination of activation energy, summarized with kinetic constants ratios,(kcatalyst/ksc 10,at 180 "C) in Table 3. When comparing Cr,Si-MFI and silicayite catalysts? the same activation energy and an activity correlation with Cr content are observed. The higher loaded scp4 sample exhibits a decline in observed kinetic constant due to a decreased specific activity or number of active sites. No differences are observed in activation energy values thus reinforcing the latter hypothesis. Acidic activity is related to Cr sites whose strength cannot be differentiated from weak sites on silicalite by means of the easy dehydration reaction. As Cr loading increases,sites pairing is likely to occur, this resulting in a lower overall kinetic constant.

(ii) n-Hexane crackinq cracking reaction was studied to evidence the occurence of sites capable of a more acid strength demanding reaction. A first order kinetics widely used in the literature [17] allowed us to obtain kinetic constants and the activation energy. Results are summarized in Arrhenius plots Fig.3(b) and in Table 3 . Activation energy is reduced in the presence of Cr sites, their involvement in catalytic reaction is also evident when comparing correlation with (kcat/ksil)550 and Cr content. A Cr sites pairing effectl as previously discussed,is also evident in this case Products distribution analysis evidences two main effects (Table 3): - Aromatics selectivity increases with Cr content, thus suggesting the presence of dehydrogenating species related to Cr sites.

.

170

- Enhanced

p-xylene isomers selectivity compared to the thermodynamic one is observed, suggesting that the reaction takes place mainly in the pore system.

3

-2

2

1 Y

0

3-1 -2

-3

:zq\ \ ,

-4

u

fi -6

0

Fig.3.

SWIO

0

A

-8

Arrhmius p l o t s . (a) Ter-butanol dehydration. (b) n-Hexane cracking.

Table 3 Catalytic characterization: Acidic Properties. t-butanol dehydration Catalyst pmlCr/gr

Nature

Ea [a1 [Kcal/moll (Rk)18oaC

Ea tKcal/moll

n-hexane cracking [a1 Arm. (Rk)18o'C

Yield

p/o Xylme i s a x r a t i o [bl

--__-_-_______-_-_______________________------~--------------------------------------------------~-----scplOH scpl3H scp 7H scp 4H [a]: tbl:

0 3 18 67

Si-MFI Cr,Si-MFI

Cr,Si-MFI Cr,Si-MFI

23 28 26 27

1 4.4 16.4 11.1

45 26 23 20

1 1.2 6.8

7.2

.3 1.1 2.4 10.7

__

2.6 6.7 10.1

(Rk)TeC = ( k catalyst/ k scplo), k i n e t i c constant evaluated a t the tenperature T; (p/o Xylene)5500~, thenodynamic e q u i l i b r i u n r a t i o =0.91

(iiil Iso-Butane Dehvdroqenation Dehydrogenation activity was further characterized by comparing isobutane reaction on silicalite (scpl0): Cr,Si-MFI (scp7,scp4): cr/siO2 (scp42) and on a traditional Cr/A1203 (ca) catalyst, prepared according to [18]. Results are outlined in Table 4. Cr modification of silicalite results in an increase of both isobutane conversion and isobutene selectivity, thus evidencing Cr involvement in dehydrogenation reaction. Effects of crystalline versus amorphous matrix in determining acidic versus dehydrogenating properties appear when comparing Cr,Si-MFI and cr-sioz samples. Higher yield ratios, both for cracking and skeletal isomerization products on Cr,Si-MFI than in Cr/SiO2 or Cr/A1203 are observed. Appearance of isomerization products suggests the presence of acidic sites, also enhancing cracking reaction. Higher conversion and isobutene yields per pmol of Cr

171

(Table 4 ,X/pmol; Y/pmol) are observed on Cr,Si-MFI samples compared with the conventional catalyst. Table 4 lsotutane dehydrogenation.

scp42P ca

150 2589

Cr/Si02 Cr,K/AL2$

17.7 48.9

83.4 91.3

0.11 0.10

0.09

0.06

0.16

0.01

0.01

0.09

[a1 : lsobutane conversion / w o l of Cr; [bl: lsotutene Yield / p o l of Cr; [cl: (Linear C4 y i e l d ) / lsobutene yield; [dl : (Cl+C2+C3) yield/ Isotutene y i e l d

[iv) Ethvlene oliqomerization The values of mass increase of samples during the oligomerization as a function of time are presented in Fig.4 in the case of CrfSi-MFI(scp4H), pure silicalite, (scpl0) and the Cr exchanged Si,A1-MFI (HCrZSM5) The values of ro for the individual curves are also given. For Cr,Si-MFI the value ro=1.0*10-2 [min-l] is significantly higher than for pure silicalite where practically no C2Hj oligomerization takes place (r* 4. 4’.4") can be explained by dehydration and ligand exchange (water vs. zeolite framework). A s described above, propositions for copper coordination (column 5) were derived from g,, and A g , , /@gP Additional information on the distortion of coordination

226

Table 1: EPR and FMR results for Cu2+ containing zeolites and oxides pretreated at 5OOOC sample C uN aY

4II +0.005

g, tO.O1

A l l- 1 0 - 4 [cm-11

2.362 2.317 2.380 2.322 2.383 2.368 2.359 2.338 2.328 2.306 2.466 2.340 2.331 2.313 2.372 2.320 2.340

2.06 2.06 2.07 2.07 2.07 2.07 2.07 2.07 2.06 2.06 2.11 2.08 2.06 2.06 2.07 2.06 2.06

133.2 179.6 136.4 177.1 167.9 167.9 120.2 172.4 165.5 173.4 111.2 143.3 168.4 151.0 143.5 165.4 146.3

2.362 g Cu/SiOz/ A1203> Cu/A120, ) , whereas in the zeolites it is only in CuZSM-5 that a tetrahedral distortion can be observed. The covalence parameters a * are given in the last column of table 2 . If there is a given kind of polyheder in all samples (and this is true for the pyramidal complex with 4 (=Si-0-) groups as ligands), from a2 values the covalent character of the copper-zeolite interaction can be evaluated f a 2 = 1 for ionic bond, a 2 = 0 . 5 for covalent bond [ll]).

221

Thus the following sequence of a 2 is obtained indicating an increase of covalent bond character with increasing silica content from CuNaA t o CuNaZSH-5: CuNaA= 800 K) in H2 and CO: however the intensity and reproducibility of the ESR signal is always low. Na vapors are more efficient: but the still high T required to run the experiment (>= 520 K) suggests some caution in the interpretation of the results. Moreover a spurious signal associated with excess sodium in form of (Na), clusters trapped in the channels, is always observed. Reduction in H2 at 300 K under X-ray irradiation (but other radiation sources can be utilized as well) gives well reproducible results. However as they are not substantially

258

different from those obtained with a very diluted solution of Na in liquid ammonia, we shall describe only the latter results (which, by the way, correspond to the mildest reduction proceThe results for the two samples (TS1 and TS2) are shown dure) in fig.4. In presence of NH3, the samples (TS1 and TS2) give the same, essentially isotropic, spectrum centered at g=1.920 corresponding to Ti3+ in octahedral and or tetrahedral coordination with low contribution of tetragonal field. Elimination of weakly adsorbed NH3, induces a main modification of the spectrum with appearance of a new gzz component at g = 1.968 which is definitely stronger on TS2, i.e. the sample with extraframework Ti. The modification is totally reversible. The explanation is as follow: framework Ti3+ gives an isotropic signal which is not substantially affected by the removal of the NH3 ligands present in the channels. On the contrary extraframework Ti3+ changes its coordination state passing from a fully coordinated (octahedral) situation in presence of NH3, to a lower coordination state characterized by a larger g factor This effect has been alanisotropy, after NH3 elimination ready documented for similar Ti02-SiO, systems (ref.12). In conclusion ESR spectroscopy confirms that framework Ti is much more abundant on TS1, while some reduced Ti3+ species deriving from an extralattice precursor is definitely more abundant on TS2.

.

.

5.

BIBLIOGRAPHY

1) W. Holderich, M. Messe and F. Naumann, Angew. Chem. Int. Ed Eng. 27 (1988) 26. 2) C . Neri, A . Esposito, B. Anfossi and F. Buonomo, Eur Pat. 100, 119. 3) C. Neri, M. Taramasso and F. Buonomo, U. K. 102, 665.

4) M. Boccuti, K. M. Rao, A. Zecchina, G. Leofanti and G. Petrini, Structure and Reactivity of Surfaces, Elsevier, Amsterdam, (1989) 133. 5) A . Zecchina, G. Spoto, S. Bordiga, M. Padovan, G. Leofanti and G. Petrini, Zeocat 90, Leipzing August 90 Proccedings Elsevier, Amsterdam, in press. 6) C. K. Jorgensen, Prog. Inorg. Chem., 12 pp. 101 S. J. Lippard ed. Intersci. Pub., John Wiley N. Y. 1970. 7) I. R. Beattie and T.R. Gilson, Proc. Roy. SOC. A307 (1968) 407.

T. Ohsaka, F. Izumi and Y. Fujiki, J. Raman Spectr. 1 (1978) 321. 9) N. T. Mc Devitt and W. L. Baun, Spectrachimica Acta, 20 (1964) 799. 10) C. U. Ingemar Odenbrand, S . Lars T. Andersson, Lars A. H. 8)

Andersonn, Jan G. M. Brandin and Guido Busca, J. of Catal. 125 (1990) 541. 11) F. Boccuzzi, A. Chiorino, G . Ghiotti, C. Morterra, A. Zecchina, J. Phys. Chem., 82 (1978) 1298. 12) V. A. Shvets and V. B. Kasanskii, Kinet. Katal., Vol 12, N.4, (1971) 935.

P.A. Jacobs et al. (Editors),Zeolite Chemistry and Catalysis 0 1991 Elsevier Science Publishers B.V., Amsterdam

259

Metallic

S t u d i e s on t h e S t a t e o f Copper anq t h e F o r m a t i o n o f i t s O x i d i c and Phases i n Z e o l i t e CuNaY R . P i f f e r ' , M. H a g e l s t e i n 2 , S . C u n i s 2 , P . Rabe',

I n s t i t u t e o f P h y s i c a l Chemistry, U n i v e r s i t y 0-2000 Hamburg 13, FRG 'Fachhochschule O s t f r i e s l a n d ,

H. F o r s t e r ' and W . Niemann3

o f Hamburg, B u n d e s s t r . 45,

C o n s t a n t i a p l a t z 4, D-2970 Emden,

FRG

3 H a l d o r Topsoe R e s e a r c h L a b o r a t o r i e s , DK-2800 Lyngby, Denmark*

Abstract The o x i d a t i o n s t a t e o f Cu i o n s i n z e o l - i t e Y depends on t h e p r e t r e a t m e n t p r o c e d u r e . A f t e r a c t i v a t i o n f o r more t h a n 8 h o u r s a t 675 K phases s i m i l a r t o CuO o c c u r . A d m i t t i n g h y d r o g e n a t 575 K, a r e d u c t i o n t o C u ( 1 ) b u t n o t t o Cu(0) m e t a l l i c c l u s t e r s i s observed. P r o b a b l y a r e a c t i o n w i t h extraframework oxygen t a k e s p l a c e . However, a r e d u c t i o n t o r a t h e r s m a l l m e t a l l i c c o p p e r c l u s t e r s i s o b s e r v e d a f t e r c o - a d d i t i o n o f w a t e r v a p o u r and h y d r o g e n . T h i s demonstrates t h e i m p o r t a n t r o l e o f s t r o n g l i g a n d s l i k e w a t e r i n t h e r e d u c t i o n mechanism, e n a b l i n g a c o n t r o l l e d f o r m a t i o n o f c o p p e r c l u s t e r s .

INTRODUCTION Copper-exchanged z e o l i t e s w i t h f a u j a s i t e s t r u c t u r e have f r e q u e n t l y been b y means o f d i f f e r e n t t e c h n i q u e s , t o e n l i g h t e n t n e i r the object o f studies, r e d o x as w e l l as t h e i r c a t a l y t i c p r o p e r t i e s [I-51. E s p e c i a l l y t h e o x i d a t i o n s t a t e and t h e c r y s t a l l o g r a p h i c e n v i r o n m e n t o f t h e c o p p e r i o n s a r e o f g r e a t i n t e r e s t . The i n t e n t i o n o f t h i s work was t o s t u d y t h e i n f l u e n c e o f t h e p r e t r e a t m e n t c o n d i t i o n s . We a p p l i e d X-Ray A b s o r p t i o n Near Edge S t r u c t u r e (XANES) and E x t e n d e d X-Ray A b s o r p t i o n F i n e S t r u c t u r e (EXAFS) as a s i t e s p e c i f i c p r o b e . The o x i d a t i o n s t a t e c a n be d e r i v e d f r o m t h e t h r e s h o l d e n e r g y . The c o - o r d i n a t i o n s p h e r e o f t h e c o p p e r i o n s i s g i v e n b y t h e r a d i a l d i s t r i b u t i o n f u n c t i o n which i s d i r e c t l y d e r i v e d from t h e F o u r i e r transform o f t h e EXAFS o s c i l l a t i o n s . The r e d u c t i o n b y h y d r o g e n and t h e i n t e r a c t i o n w i t h w a t e r a t d i f f e r e n t t e m p e r a t u r e s w i l l be e x p l o r e d . The f o r m a t i o n o f a c i d i c h y d r o x y l g r o u p s and s o r p t i o n c o m p l e x e s o f w a t e r has been s t u d i e d b y I R spectroscopy.

*

P r e s e n t address: P h i l i p s Research L a b o r a t o r i e s , V o g t - K o l l n - S t r . D-2000 Hamburg 54, FRG

30,

260

EXPERIMENTAL CuNaY z e o l i t e s were prepared by i o n exchange o f z e o l i t e NaY w i t h an aqueous 0.03 M s o l u t i o n o f Cu(NO,), ( Merck, p r o a n a l y s i ) a t 300 K. A f t e r washing and d r y i n g a t ambient temperature, t h e composition determined by AAS was Cu,, ,Na,, ,Y. The samples were pressed i n t o s e l f - s u p p o r t i n g wafers o f t h i c k n e s s f o r I R and X-Ray measurements, r e s p e c t i v e l y . about 8 o r 80 mg They were dehydrated i n vacuo up t o 12 h a t temperatures between 625 and 675 K . Water, p y r i d i n e (Merck, p r o a n a l y s i ) and hydrogen (Messer Griesheim, 99.999%) were i n t r o d u c e d w i t h o u t f u r t h e r p u r i f i c a t i o n . The I R s p e c t r a were recorded a t ambient temperature ( i f n o t e x p r e s s l y s t a t e d o t h e r w i s e ) on a F o u r i e r t r a n s f o r m spectrometer DIGILAB FTS 20E w i t h r e s o l u t i o n s between 1 and 4 cm-’. The EXAFS and XANES d a t a were o b t a i n e d a t t h e Cu K edge on t h e E4 and X beam l i n e s a t the s y n c h r o t r o n r a d i a t i o n source o f HASYLAB/DESY i n Hamburg, a t a r i n g energy o f 5 . 3 GeV w i t h a r e s o l u t i o n o f 2-4 eV by means o f S i - 1 1 1 and Si-400 (DEXAFS) monochromators and e l e c t r o n c u r r e n t s o f 20-40 mA. F o r t i m e - r e s o l v e d s t u d i e s e n e r g y - d i s p e r s i v e XAS a l l o w s t o o b t a i n a s i n g l e XAS spectrum w i t h i n about t h r e e seconds a t t h e D i s p e r s i v e EXAFS (DEXAFS) spectrometer [ 6 ] . F o r t h e examination o f t h e o x i d a t i o n s t a t e o f copper, t h e XAS s p e c t r a o f Cu,O ( R i e d e l de Haen) as w e l l as o f CuO (Merck, p r o a n a l y s i ) and Cu metal f o i l (Goodfellow) were examined under i d e n t i c a l c o n d i t i o n s . A l l s p e c t r a were recorded i n s i t u i n t h e same a l l metal sample c e l l equipped w i t h KBr windows f o r t h e I R range and w i t h b e r y l 1ium windows f o r t h e XAS measurements.

RESULTS AND D I S C U S S I O N Hydrated, A c t i v a t e d and Rehydrated Samples The r a d i a l d i s t r i b u t i o n f u n c t i o n o f t h e hydrated sample e x t r a c t e d by EXAFS proves t h e e x i s t e n c e o f a s i n g l e c o - o r d i n a t i o n s h e l l w i t h n e a r l y 4 oxygen neighbours a t a d i s t a n c e o f about 200 pm. T h i s s h e l l must be a t t r i b u t e d t o a copper-aquo complex f l o a t i n g i n s i d e t h e supercages w i t h o u t any bonding t o z e o l i t e oxygen, o t h e r w i s e f u r t h e r c o - o r d i n a t i o n spheres should be observable. T h i s i s supported by comparable measurements o f hydrated Cu(NO,), showing n e a r l y t h e same EXAFS and t h e r e f o r e the same Fourier transform. A f t e r d e h y d r a t i o n a t temperatures between 625 and 675 K i n h i g h vacuo f o r about 12 h, a l l copper i o n s should l o o s e t h e i r s u r r o u n d i n g water molecules. Therefore, t h e i o n s s h o u l d occupy d i s t i n c t c r y s t a l l o g r a p h i c s i t e s w i t h i n t h e z e o l i t e , c o - o r d i n a t e d by t h e z e o l i t e oxygen [ 1 , 7 ] . Indeed, t h e simultaneous decrease o f the f i r s t c o - o r d i n a t i o n sphere and the b u i l t - u p o f h i g h e r s h e l l s d u r i n g d e h y d r a t i o n i n d i c a t e s a c a t i o n movement t o these s i t e s ( F i g . 1, l o w e r and i n t e r m e d i a t e spectrum). The corresponding EXAFS i s n e a r l y i d e n t i c a l w i t h t h e EXAFS o f CuO c l u s t e r s which are s m a l l e r t h a n 800 pm i n diameter [ 8 ] . The Cu-0 d i s t a n c e t u r n s o u t t o be n e a r l y 196 pm, approaching t h a t o f CuO w i t h a peaks s l i g h t l y decreased c o - o r d i n a t i o n number. The l a c k o f any CuO o r Cu,O i n t h e X-ray d i f f r a c t i o n p a t t e r n s i n d i c a t e s t h e absence o f any m a c r o c r y s t a l l i n e copper o x i d e phases. A l a r g e r amount o f Cu(1) o x i d e a s w e l l as Cu(0) may be excluded from t h e comparison w i t h t h e r e s p e c t i v e EXAFS o s c i l l a t i o n s and i s supported by XANES measurements. The i o n s seem t o be b r i d g e d by extraframework oxygen under f o r m a t i o n o f

261 I

0.03

I

1

I

,

I

(

N Y

c

a -i.-l---l

0

2

1

3

4

atomic distance

5

6

(A)

F i g . 1. F o u r i e r t r a n s f o r m o f s i g n a l X ( k ) : - h y d r a t e d sample ( a ) - a c t i v a t e d sample ( b ) - r e h y d r a t e d sample ( c ) window: B e s s e l , w e i g h t i n g : k 2 , k - r a n g e : 2 . 5 - 7 . 5

A- 1

300

0 0 c

F i g . 2 . IR s p e c t r a o f adsorbed pyridine i n the - a c t i v a t e d CuNaY sample ( a ) - H2-reduced CuNaY ( b ) - H2/H20-reduced CuNaY ( c )

c

QJ U

ru c

n h ul 0

n

10

0 1400

1500

1600

wavenumber ( l / c m )

1700

262

Cu-0-Cu complexes. From EPR r e s u l t s CHAO e t a l . have suggested them t o be a p o s s i b l e c o n f i g u r a t i o n among o t h e r s [ 91. During the d e h y d r a t i o n process some o f t h e w a t e r molecules d i s s o c i a t e under f o r m a t i o n o f an i n c r e a s i n g amount o f Bronsted a c i d OH groups i n the supercages and t h e s o d a l i t e u n i t s [ 8 ] . However, t h e amount o f these a c i d s i t e s i s small, as has been proved by p y r i d i n e a d s o r p t i o n . From I R s p e c t r a bands a t 1390, 1488, 1542 and 1632 cR’ should be expected [ l o ] . F i g u r e 2 ( l o w e r spectrum) proves t h e almost t o t a l absence o f these bands. On t h e o t h e r hand the s t r o n g bands a t 1455, 1490 and 1605 c m i n d i c a t e t h e presence o f Lewis a c i d s i t e s e . g . Cu c a t i o n s [ l o ] . The r e s p o n s i b l e band a t 1455 cm-’ s p l i t s i n t o a d o u b l e t i n d i c a t i n g two d i f f e r e n t Lewis a c i d s i t e s which probably a r e due t o t h e f r e e C u ( I 1 ) c a t i o n s and t h e t r u e Lewis a c i d s i t e s o r r e s i d u a l Na’ i o n s . Treatment o f t h e Dehydrated Sample w i t h Hydrogen Admission o f hydrogen a t 575 K t o a dehydrated sample causes a s t r o n g i n c r e a s e o f b o t h a c i d i c OH groups due t o t h e r e d u c t i o n o f C u ( I 1 ) i o n s t o Cu(1) accompanied by t h e f o r m a t i o n o f p r o t o n s ( F i g . 3 ) . Simultaneously t h e amount o f Lewis a c i d s i t e s decreases, observable b y t h e corresponding I R bands o f adsorbed p y r i d i n e ( F i g . 2, i n t e r m e d i a t e spectrum). The lowfrequency band o f t h e 1455 cm-’ d o u b l e t i s reduced t o a shoulder, i n d i c a t i n g a loss o f accessible cations, probably the Cu(I1) ions. The XANES and EXAFS prove t h e immediate i n t e r a c t i o n between hydrogen and the copper i o n s a t 575 K ( F i g . 4 ) . The a b s o r p t i o n edge s i g n i f i c a n t l y s h i f t s towards lower photon e n e r g i e s and a change i n t h e EXAFS i n d i c a t e s a d i f f e r e n t l o c a l environment. Both, XANES and EXAFS o f t h e sample i n t h e o r copper f i n a l s t a t e d i d n o t show any resemblance t o those of CuO, Cu,O metal ( F i g . 5 ) . A comparison o f t h e XANES w i t h s e v e r a l c r y s t a l l o c r a p h i c a l l y well-known compounds c o n t a i n i n g copper [ 111 and t h e observed chemical s h i f t i n d i c a t e t h e f o r m a t i o n o f Cu(1) i o n s . Rather s i m i l a r phases appear when butadiene was used as t h e r e a c t i v e gas a t 575 K [ 8 ] . Reduction o f a Rehydrated Sample w i t h Hydrogen Completely d i f f e r e n t r e s u l t s were o b t a i n e d w i t h samples a c t i v a t e d f o r o n l y one hour a t 500 K. A f t e r repeated o x i d a t i o n - r e d u c t i o n c y c l e s m e t a l l i c copper c l u s t e r s were o b t a i n e d [12]. W i t h t h e assumption t h a t under these a c t i v a t i o n c o n d i t i o n s t h e s t r o n g l y bound water c o u l d n o t be c o m p l e t e l y removed, t h i s r e s u l t i n d i c a t e s t h e i m p o r t a n t r o l e o f water f o r r e d u c t i o n . To e n l i g h t e n t h e mechanism o f t h e r e d u c t i o n a new sample was a c t i v a t e d a t 570 f o r 8 h i n h i g h vacuo. A f t e r having reached ambient temperature again 3 . 1 0 Pa water vapour was i n t r o d u c e d . Having heated t h e sample up t o 575 K under t h i s atmosphere, no dramatic change i n t h e l o c a l environment o f t h e copper i o n s c o u l d be observed ( F i g . 1, upper spectrum). The i n t r o d u c t i o n o f an excess o f hydrogen y i e l d e d copper m e t a l . Time-resolved XAS measurements a l l o w f u r t h e r c l a r i f i c a t i o n o f the r e d u c t i o n process. The v a r i a t i o n o f the XANES i n d i c a t e s a two s t e p r e a c t i o n . Immediately a f t e r a d d i t i o n o f hydrogen t h e XANES changes i n t h e same way as f o r t h e c o m p l e t e l y dehydrated sample ( F i g . 4 ) , b u t i s f o l l o w e d by a f u r t h e r change o f t h e XANES r a t h e r s i m i l a r t o t h a t o f copper metal ( F i g . 6 ) . A t f i r s t the hydrogen r e a c t s w i t h t h e b r i d g i n g oxygen under f o r m a t i o n o f w a t e r . T h i s i s supported by I R measurements which show small b u t d i s t i n c t bands a t 1640 and near 3300 cm-l, due t o adsorbed w a t e r ( F i g . 3 , lower spectrum).

5

263

F i g . 3 . IR d i f f e r e n c e spectra o f CuNaY a f t e r reduction compared t o the activated sample : - a f t e r H2 ( a ) - a f t e r H2/H20 ( b ) reduction

0

S 5

n

L 0

v)

n

5

8.965

8.985

9.005

9.025

9.045

energy (keV) F i g . 4 . Cu K-edge XANES measured d u r i n g t h e r e d u c t i o n o f C u ( I 1 ) c a t i o n s i n CuNaY. The r e a c t i o n proceeds from t h e f a t t o t h e dashed spectrum. F o r comparison see t h e XANES o f a Cu f o i l ( d o t t e d ) .

264

2

1.5 F i g . 5 . Compari t i ve Cu K-edge XANES of - CuO ( a ) - C U ~ O( b ) - Cu metal ( c ) - H2-reduced CuNaY ( d )

"1 1

m

0.5 0

c

0 8.96

9

8.98

9.02

energy (keV)

8.965

8.985

9.005

9.025

9.045

energy (keV)

Fig. 6. CLI K-edge XANES measured d u r i n g t h e r e d u c t i o n o f C u ( I 1 ) c a t i o n s i n r e h y d r a t e d CuNaY. The r e a c t i o n proceeds w i t h t h e s h i f t t o l o w e r energy, r e a c h i n g t h e f i n a l s t a t e (dashed). F o r comparison t h e Cu K-edge o f a Cu f o i l r e f e r e n c e ( d o t t e d ) .

265

0.2

0

atomic distance

(A)

F i g . 7 . F o u r i e r transform of EXAFS signal ~ ( k ) :The r e d u c t i o n o f Cu(I1) s p e c i e s t o m e t a l l i c Cu c l u s t e r s s t a r t s with the bottom graph ( r e h y d r a t e d s a m p l e ) . For compzrison s e e the FT of a C u f o i l r e f e r e n c e spectrum ( d o t t e d ) .

F i g . 8 . IR d i f f e r e n c e s p e c t r a o f the H2-exposed CuNaY v s . tne rehydrated saxpie recorded a t 575 I(: . 1mmed;ately a f t e r hydroge!] adrrlisslon ( a ) - a f t e r 30 m n ( b ) - a f t e r 60 r i i n ( c ) .

4000

3400

2800

2200

wavenumber ( 1 /cm)

1600

266

I n t h e second s t e p t h i s makes t h e C u ( I ) i o n s a c c e s s i b l e t o w a t e r m o l e c u l e s so t h a t f u r t h e r r e d u c t i o n may o c c u r . The c o r r e s p o n d i n g radial d i s t r i b u t i o n f u n c t i o n o f t h e f i n a l s t a t e shows a s t r u c t u r e identical t o t h a t o f a Cu m e t a l f o i l ( F i g . 7 ) . I n comparison t o t h e l a t t e r t h e f i r s t c o o r d i n a t i o n s h e l l d i s p l a y s a s m a l l e r h e i g h t , w h i c h can be e x p l a i n e d by a reduced number o f n e a r e s t copper n e i g h b o u r s i n agreement w i t h r e c e n t f i n d i n g s i n t h e l i t e r a t u r e [ 1 3 ] . T h i s i n t e r p r e t a t i o n i s s u p p o r t e d by t h e comparison o f t h e XANES o f t h e f i n a l s t a t e ( F i g . 6 ) w i t h t h a t o f s m a l l copper p a r t i c l e s [14,15]. Comparative I R s t u d i e s show t h e e f f e c t o f i n t r o d u c i n g hydrogen a t 575 I< t o t h e w a t e r t r e a t e d sample. I m m e d i a t e l y a f t e r exposure t h e bands o f adsorbed w a t e r a t 1640 and around 3500 cm-’, a t t r i b u t e d t o t h e bending and the s t r e t c h i n g vibrations, r e s p e c t i v e l y , disappear. Simultaneously a small b u t d i s t i n c t band a t 3650 cm?, due t o a c i d i c OH g r o u p s i n t h e supercage, i n c r e a s e s ( F i g . 8, l o w e r s p e c t r u m ) . T h i s i s caused by t h e g r o w i n g amount o f p r o t o n s c r e a t e d d u r i n g r e d u c t i o n . T h i r t y and s i x t y m i n u t e s a f t e r a d m i s s i o n o f hydrogen a s i g n i f i c a n t i n c r e a s e o f t h e absorbance i s observed i n d i c a t i n g t h e p r o g r e s s i v e c r e a t i o n o f copper c l u s t e r s ( F i g . 8, i n t e r m e d i a t e and upper spectrum). Preceding the r e d u c t i o n t o m e t a l l i c c l u s t e r s , a vanishing o f the amount o f adsorbed w a t e r i s observed. P y r i d i n e a d s o r p t i o n shows a d r a m a t i c i n c r e a s e o f B r o n s t e d a c i d c e n t e r s p a r a l l e l e d by t h e decrease o f Lewis a c i d c e n t e r s ( s e e u p p e r spectrum o f F i g . 2 ) . T h i s i s i n agreement w i t h t h e o b s e r v a t i o n o f t h e g r o w i n g amount o f reduced Cu p a r t i c l e s accompanied b y a l o s s o f Lewis a c i d s i t e s . From o u r e x p e r i m e n t s a c o r r e l a t i o n between c l u s t e r s i z e and t h e w a t e r and hydrogen p a r t i a l p r e s s u r e must be assumed. CONCLUSIONS The r e d u c t i o n of t h e copper i o n s i n a c t i v a t e d CuNaY z e o l i t e depends on t h e p a r t i a l p r e s s u r e o f b o t h hydrogen and w a t e r . A r e d u c t i o n by hydrogen t o copper m e t a l o n l y o c c u r s i f t h e p r e s s u r e o f w a t e r exceeds a c e r t a i n l i m i t . O t h e r w i s e t h e r e d u c t i o n w i l l be i n c o m p l e t e and s t a b l e Cu(1) phases show up. The i n c r e a s e d m o b i l i t y o f copper i o n s due t o t h e a v a i l a b i l i t y o f s t r o n g l i g a n d s seems t o be i m p o r t a n t . Only t h e s e i o n s a r e a b l e t o be reduced t o copper atoms by m o l e c u l a r hydrogen f o l l o w e d by an a g g l o m e r a t i o n t o c l u s t e r s . ACKNOWLEDGEMENT A p a r t o f t h i s p r o j e c t i s s u p p o r t e d by t h e BMFT under c o n t r a c t No. DAI.

05419

REFERENCES 1 P . G a l l e z o t , Y . Ben T a a r i t and B . I m e l i k , J . C a t a l . , 26 (1972) 295. P.A. Jacobs, W . de Wilde, R . A . Schoonheydt, J.B. U y t t e r h o e v e n and H . Beyer, JCS F a r a d . Trans. I, 72 (1976) 1221. 3 I.E. Maxwell, J . J . de Boer and R.S. Downing, J . C a t a l . , 61 (1980) 493. 4 R . A . Schoonheydt, J . Phys. Chem. S o l i d s , 50 (1989) 523. 5 I . E . Maxwell and J . J . de Boer, J. Phys. Chem., 79 (1975) 1874.

2

267 6

7 8 9

10 11 12 13 14 15

M. Hagelstein, S . Cunis, R . Frahrn, W. Niemann and P . Rabe, Conf. P r o c . , V o l . 25, 2nd European C o n f . P r o g r . X-Ray S y n c h r o t r o n R a d i a t i o n Res., A . B a l e r n a , E. B e r n i e r i and S. M o b i l i o , Eds., SIF, Bologna, (1990) 407. W.J. M o r t i e r and R . A . Schoonheydt, P r o g r . S o l i d S t a t e Chern., 16 (1985) 105. R . P i f f e r , H . F o r s t e r and W. Niemann, C a t a l . Today, i n p r e s s . C.C. Chao and J.H. L u n s f o r d , J. Chem. Phys., 57 (1972) 2890. H. Karge, Z . Phys. Chem., 76 (1971) 133. L.S. Kau, D.J. Spira-Solomon, J.E. Penner-Hahn, K.O. Hodgson and E.I. Solomon, 3 . Am. Chem. SOC., 109 ( 1 9 8 7 ) 6433. M. H a g e l s t e i n , S . Cunis, R . Frahm, W . Niemann, R . P i f f e r and P. Rabe, XAFS V I , 1990, P r o c . Conf., i n p r e s s . S . Tanabe and H. Matsumoto, B u l l . Chem. S O C . Jpn., 63 (1990) 192. Shenoy, E.E. Alp, W. S c h u l z e and J. Urban, Phys. P.A. Montano, G.K. Rev. L e t t . , 56 (1986) 2076. G.N. Greaves, P . J . Durham, G. Diakun and P . Q u i n n , Nature, 294 (1981) 139.

This Page Intentionally Left Blank

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Cafalysis 0 1991 Elsevier Science PublishersB.V., Amsterdam

269

ESCA STUDY OF INCORPORATION OF COPPER INTO Y ZEOLITE Ivan Jirkae Blanka Wichterlovaa and Martin Maryskab

aJ. Heyrovsky Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, 182 23 Prague 8 , Czechoslovakia ’Institute of Chemical Technology, Department of Silicates, 166 28 Prague 6, Czechos1ovakia Abstract Both low and high temperature mode of contact interaction between Cu2O and NH4-Y zeolite in a mechanical mixture has been observed by means of XF'S and XAES spectroscopies. Moreover, hydration of this mixture significantly increases extent of this interaction.

1. INTRODUCTION

It has been shown that solid-state (or contact) interaction can occur among various metal compounds and zeolites in mechanical mixtures, resulting in deaggregation of the metal compound phase and migration of metal ions into the zeolite channels [l-61. Generally, both the low temperature and high temperature modes of solid-state interaction can take place 131. The detail mechanism of this interaction is not known being affected by the type of a zeolite and a metal compound in the mixture /4-61. It has been found previously that a solid-state ion exchange occurs in the mixture of Cu oxides and NHs-Y or H-ZSM-5 zeolite after heating above 670 K [61. This information belongs to the changes in the bulk of zeolite crystals. I t seems probable that some changes in the Cu20/NH4-Y (CuzO/H-ZSM-5) interface may occur at much lower temperature. The electron spectroscopy for chemical analysis (ESCA) and scanning electron microscopy (SEM) have been used to investigate the changes in the Cu20 - zeolite interface resulting from heating and hydration of the mixture. 2. EXPERIMENTAL

The mixture was prepared by mechanical grinding of Cu20 (Merck) and NH4-Y zeolite in an agate mortar for 60 minutes. The chemical composition of NH4-Y was (wt.%): SiO2 = 67.72, A1203 = 22.21, Na2O = 1.41 and (NH4)20 = 8.65. The concentration of CuzO in the mixture was 112 mg/g of zeolite, corresponding to a Cu/OH (bridging) molar ratio equal to 0.5. In some cases the mixture was exposed to water vapour (p(H20)- 440 torr) in a static air atmosphere at 358 K for 0.5, 11.0 and 20.5 hours. The spectra of the mixtures were measured without any heat treatment and after vacuum heat treatment at 420, 620 and 770 K for 1 hour. The photoelectron and Auger lines were measured on an ESCA 3 Mk I1

270

spectrometer at ambient temperature and at a base pressure typically lower than lo-* torr. The AlKa ( E = 1486.7 eV) and MgKa (E = 1253.4 eV) lines were used to excite the photoelectrons. The Cls line (Eb = 284.4 eV1 was employed to calibrate the energies of spectra. The error of Eb (Ek) estimation was typically 0 . 3 eV. An analytical information from the electron spectra may be obtained from their intensities, binding energies of the photoelectrons and kinetic energies of the Auger electrons of a given atom. As investigated mixtures were substantially heterogenous (see bellow), the Cu concentrations estimated by ESCA were only semiquantitative. The simplest equation was used: Cu/Si = I(CuZp)~(SiZp)/I(SiZp)u(Cu2p)

(1)

where Cu/Si is a copper-silicon atomic ratio, I(Cu2p) and I(Si2p) are intensities of Cu 2p3/2 and Si 2p photoelectron lines, respectively, and cr(Cu2p) and u(Si2p) are photoionization cross-sections of Cu2pw2 and Si2p levels, respectively [71. Scanning electron microscopy (SEMI was done on a JEOL JEM 1008. Accelerated voltage was 40 kV. The surface charge of the sample was compensated by evaporated layer of Pd/Au alloy. 3. RESULTS AND DISCUSSION 3.1. ESCA of Cu ions

The estimation of the location of Cu ions in the mixture is based on a fingerprint method, i.e. on comparison of copper core level binding energies Eb and kinetic energies Ek of Auger electrons of copper with standard values of Eb and Ek of Cu compounds. This method may be complicated by charging effects resulting from the emission of electrons from insulating materials like zeolites. The results then should be carefully checked, whether they are in accordance with recent interpretations of Eb and Ek values. It is known that Eb of Cu 2p3/2 line of Cul’compounds do not substantially differ each other while their kinetic energies Ek of an Auger Cu CVV transition depend on the type of Cu compound. Both the core level E b and Auger Ek values of Cu2+are dependent on the type of a compound. These effects can be explained by screening theory proposed in literature IS]. Two channels are available for the screening - local and non-local one. Two lines are then observable in the core level photoelectron spectrum of divalent copper. Lower energy main line, screened by 3d9 electrons of copper and by another electron from a ligand localized during screening in the Cu 3d orbital and a higher energy satellite screened by Cu 3d9 electrons only. According to the interpretation of van der Laan et al. [Sl the main line is for the case of divalent copper sensitive on its chemical surroundings due to screening mechanism, while the energy of a satellite is almost independent on the chemical surroundings. F o r monovalent copper only 10 one screening channel is available (3d configuration which exclude any charge transfer from the ligands) and so this line should be, according to the above interpretation, insensitive to the chemical surroundings of the Cul+ ion. The use of the Eb values of Cu 2p3/2 line of copper enable to distinguish changes in the coordination of Cu2+ ion in the investigated mixtures. Moreover, knowing the origin of the satellite in the Cu 2p3/2

271

TABLE 1 The Cu 2p3/2 binding energies Eb (eV) and Cu L3M4,5M4,5 Auger kinetic energies Ek (eV) of Cu compounds and Cu ions in zeolites. Compound

Ref.

Eb

Ek

cu20 Cu20 (dispersed) cuc1 cul+-y

932.2 932.7 932.6 932.4

916. a 915.9 915.0 913.2

this work

CUO CUCl2 Cu (OH12 Cu"-Y

933.5 934.4 935.1 936.2

917.9 915.5 913.1

this work

9 10 11

8 10 11

TABLE 2 The Cu/Si. 10’ratio of CuzO/NH4-Y ( A ) unhydrated, hydrated for (B) 0.5 h., (C) 11 h., (D) 2 0 . 5 h., and heated at temperature T (K) in situ.

A

B

(4.5)

(7.5)

-

3.7 3.8 3.3

-

4.6 4.1 3. 6

C 12.2 10.7 10.6 7.1

D

T

(28.7) 23.8 19. a

293 293 420 620 770

-

2.3

number in brackets - Cu/Si ratio after 5 . 5 minutes of measurement (see the text) TABLE 3 Binding energies Eb(eV) of the Cu 2p3/2 lines of copper in hydrated Cu20/NH4-Y mixture and their atomic ratios of Cul /Si and Cu2+/Si estimated from eq. ( 1 ) af-ter 5 . 5 minutes of measurement- see the text). Eb(CU1+) 931.4 932.2 931.8

Eb(CU")

Cul+/Si.10'

934.5 934.9 935.0 935.1

2.4 2.0 3. a

-

Cu2+/Si.lo2 Hydration (h) 2.1 5.5 9.4 28.7

0.5 11.0 20.5

272

spectrum (no satellite is observable in this spectrum for CulC), we can estimate the oxidation state of Cu in the mixture. Similar screening effects influence two hole Auger final states. It has been shown that kinetic energy Ek of L3M4,5M4 5 transition of both Cu2+ (3d9 and 3d8 Auger final states) and Cu1+(3d8’ Auger final state) depends on chemical surroundings of the copper ion. Our previous data on the ion exchanged Cu-Y and Cu-ZSM-5 zeolites are in accordance with the above theory. The E b of Cu 2p3/2 line of Cul+ ion in zeolites do not substantially differ from the values which belong to other cuprous compounds. On the contrary, the Ek values of the Auger Cu CVV transition of Cul+ ions in zeolites are much lower in comparison with any other Auger Cu CVV kinetic energy (see Table 1). In accordance with the screening theory the Eb of Cu 2$1*3/2line of Cu2+ in Y zeolite is much higher than that of the other Cu compounds. It follows that the Eh of Cu 2p3/2 line of Cu2+ species and the E k of Cu L3M4,5M4,5 (Cu CVV) line of both Cul+ and Cu2’ species may be used to distinguish qualitatively the coordination and, therefore, location of Cu in the mixture. 3.2. CU O/NH -Y 2 4

The SEM reveals that the unhydrated mixture was composed from the grains with a diameter of about 1 pm and from the agglomerates with a diameter of about 3 - 6 pm. The hydration of the mixture at 358 K resulted in a disappearance of these agglomerates. The structureless spots with a diameter of 10 pm appeared in the mixture hydrated for 20.5 hours. As no changes induced by hydration were observed for pure Cu20, the disappearance of agglomerates and the presence of structureless spots in the heavily hydrated mixtures may be explained by the deaggregation of CuaO caused by the zeolite induced hydrolysis. This was also indicated by the dependence of the Cu/Si intensity ratios on the time of hydration estimated by ESCA (Table 2 ) . A pronounced increase of this ratio with the time of hydration confirms deaggregation of copper oxide phase Further details on the copper oxide-zeolite interaction were gained from the binding energy values and shapes of the Cu 2p3/2 photoelectron spectra (Figure 1). Two lines abbreviated as line I and I1 (at -932 and -935 eV, respectively) with a satellite at a higher binding energy were resolved by a fitting procedure for unhydrated mixture and for that hydrated for 0.5 and 11.0 hours. The high energy Cu 2p3/2 line and a satellite disappeared during the measurement (after -240 minutes 1. The accumulation time of the Cu 2p3/2 spectra was thus minimized (5.5 minutes). The only one Cu 2.~312 line with a satellite was observed for the mixture hydrated for 20.5 hours at a binding energy Eh = 935.1 eV (not shown in Figure 1). Line I belongs to cuprous species and its Eh was slightly lowered in comparison with standard Eb values. However, this deviation (except of unhydrated mixture) was about what is expected from experimental error. A lowering of Eb of line I which belongs to unhydrated mixture was most probably caused by wrong calibration (see discussion of this problem in [ 1 2 1 ) . Alternative explanation of this effect as a consequence of charge donation from the zeolite to Cul+ species is in disagreement with the results of discussion presented below. The high energy Cu 2p3/a line with a satellite corresponds to the cupric ions bonded most probably in Cu(0H)a and no Cu2+ ions were observed by ESCA to be exchanged into the zeolite by hydration. This follows from a comparison of the binding energy Eb value

-

273 (Table 3 ) with that of the standard compounds (see Table 1). The longer time of hydration, the higher was observed concentration of Cu(0H)z. This effect seemed to be quantitative for heavily hydrated mixture, as no 1+ substantial concentration of Cu was found. Migration of some copper species into the zeolite channels occurred under vacuum during spectra measurement. This follows from a decrease in the Cu/Si ratio with time of measurement (see Table 2 and Figure 2 ) . A further decrease in the Cu/Si ratio was caused by heating of the samples at 770 K. Again, this effect was most pronounced for the heavily hydrated mixture (Table 2 ) . It follows from the above discussion that the Cu/Si ratio estimated by XPS was influenced by two effects - deaggregation of the copper oxide phase, which increases the Cu/Si value, and subsequent migration of copper species into the volume of the zeolite crystals, decreasing, on the contrary, the Cu/Si ratio. The more hydrated the copper oxide phase (corresponding to a higher initial concentration of Cu(0H)z in the mixture), the greater extent of incorporation of copper species inside the zeolite was found. The conclusion on the deaggregation and diffusion of (at least part of) the copper species into the zeolite channels is supported by discussion of the shape and energy of Auger CVV spectrum of copper. The numerical values of Ek presented here are only rough estimations being discussed here only qualitatively. Their exact values can be obtained, in principle, by curve fitting of Auger spectra. However, this procedure cannot be unfortunately used because of a lack of information on the line shapes of the fitted components, which may be very different

A

91 9

A

934

B

Eb( e v ) 949

FIGURE 1 ( A ) Cu 2p lines of copper in CuzO/NH4-Y mixture (a) unhydrated; hydrated for (b) 0.5 h, (c) 11.0 h ( B ) Typical fit of the Cu 2p line

274

B A

lop

0

0

0

0 0

0 0

0

IL . 0

I

100

., % I 200

0

0

100

200

t (min ) FIGURE 2 ( A ) Dependence of Cul+/Si (open points) and Cu2+/Si (full points) of the and hydrated ( 0 1 for 11.0 h on the time t (min) sample unhydrated ( of measurement. ( B ) Dependence of Cu/Si ratios (Cu/Si = Cu’+/Si + Cu2+/Si) of the unhydrated mixture ( c) and the sample hydrated for 11.0 h ( 0 ) and 20.5 h ( A on the time t (min) o f measurement.

a)

Figure 3 depicts the Cu CVV Auger lines of copper in the unhydrated mixture and in the samples hydrated for 11.0 and 20.5 hours. For the unhydrated mixture and f o r that hydrated for 0.5 (not shown in Figure 2) and 11.0 hours the Cu CVV spectra were composed of two lines at a kinetic 917 eV (close to the value observed for Cu20, 917.4eV) and at energy Ek Ek 913 eV (close to the value of Cul’ion exchanged in the Y zeolite, 913.2 eV). The interpretation of an additional line (at lower kinetic energy) found in this spectrum, also observed in a pure zeolite, is not yet clear. The intensity of the Auger line at 913 eV increases with the time of hydration and decreases with the heating of the mixture (Figure 2). Only one broadened Cu CVV line was observed for heavily hydrated sample 917 eV. Heating of this sample at 770 K caused a substantial line at Ek shape change and shift to 913.5 eV. These effects can be explained in terms of migration of at least part of the Cu species into the zeolite channels, even at ambient temperature (see discussion of the Cu/Si ratio above). More extensive incorporation of Cu species into the zeolite channels due to heating of the sample increases with the time of hydration, In the sample hydrated for 20.5 hours followed by heating at 770 K, all the copper species can be assumed to be incorporated into the zeolite channels (no Cu CVV line of Cul+in Cu20 was observed - see Figure 3).

-

-

-

275

I

I

I

I

1

1

I

1

915.8

912.9

Ek(eV)

916.8 912.8

Ek(eV)

I

FIGURE 3

Auger Cu CVV spectra of copper in Cu20/NII4-Y mixture ( 1 ) unhydrated; ( 2 ) and (3) unhydrated followed by heating at 620 and 770 K, resp.; (4) hydrated for 11.0 h; (51 hydrated for 11.0 h followed by heating at 770 K; ( 6 ) hydrated for 20.5 h; (7) and ( 8 ) hydrated for 20.5 h followed by heating at 420 and 770 K, resp. 4. CONCLUSIONS

The core level and Auger shifts of copper ion exchanged in zeolites compared to those in various Cu compounds may be explained by the screening theory proposed in literature. The core level Eb of Cul’ions depends only weakly on their chemical surroundings. The kinetic energy of the Auger Cu CVV line of the Cul’ion exchanged in zeolite may be used as a fingerprint^ value due to nonlocal screening of the Auger final state of the Cul+ ion. In the case of Cu2+both the core level E b and Auger E k values may be used due to the nonlocal screening of final states. The first step of contact interaction of CuzO with NH4-Y zeolite in their mechanical mixture is deaggregation of Cu20 and its oxidation to Cu(0H)z. A part of Cu species is incorporated into the zeolite channels during measurement of photoelectron spectra likely as a consequence of sample heating during measurement and/or by photodissociation of the Cu(0H)a thin layer. Heating of the mixture already at 420 K causes further migration of Cu species into the zeolit,e channels. This migration was previously indicated for the zeolite bulk after heating of the mixture

216

above 620 K. Oxidation of CuzO and a low as well as high temperature migration of Cu species into the zeolite are substantially increased by pre-exposure of the mixture to water vapour. This effect is explainable by an increased fraction of Cu bonded in Cu(0H)a due to hydration. As a lattice energy of cupric hydroxide is lower than that of cuprous (cupric) oxide, dissociation of a former compound is energetically more favourable and expectably an easier incorporation of Cu into the zeolite channels takes place.

REFERENCES 1. 2. 3. 4.

5.

D.W. Breck in Zeolite Molecular Sieves - Structure, Chemistry and Use, Wiley, New York, 1974, pp 588 - 592. A . V . Kucherov and A.A. Slinkin, Zeolites 7 (1987) 38. H.K. Beyer, H.G. Karge and G. Borbely, Zeolites 8 (1988) 79. B. Wichterlova, S. Beran, L. Kubelkova, J. Novakova, A. Smieskova and R. Sebik, Stud. Surf. Sci. Catal. 46 (1989) 347. S. Beran, B. Wichterlova and H.G. Karge, J. Chem. SOC. Faraday Trans. 86 (1990) 3033.

6. B. Wichterlova and H.G. Karge, submitted for publication. 7. J.H. Scofield, J. Electron Spectroscopy, 8 (1976) 129. 8. G. van der Laan, C. Westra, C. Haas and G.A. Sawatzky, Phys. Rev.B 23 (1981) 4369.

I. Jirka, Thesis, J. Heyrovsky Institute of Physical Chemistry and Electrochemistry, Prague 1989. 10. Practical Surface Analysis by Auger and X-Ray Photoelectron Spectroscopy (D. Briggs and M.P. Seah eds. 1, John Wiley, New York, 1983. 11. I. Jirka and V. Bosacek, Zeolites 11 (19911, 77. 12. T.L. Barr and M.A. Lischka, J. Am. Chem. SOC.,108 (1986) 3178. 9.

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 0 1991 Elsevier Science PublishersB.V., Amsterdam

277

PREPARATION OF Ga-DOPED ZEOLITE CATALYSTS VIA HYDROGEN INDUCED SOLID STATE INTERACTION BETWEEN Ga203 AND HZSM-5 ZEOLI’TE b

V.Kanazireva,G. L.Priceb and K. M. Dooley

armtitUte of Organic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria bDepartment of Chemical Engineering, Louisiana State University, Baton Rouge, Louisiana 70803, USA

Abstract Ga-doped catalysts were prepared by mechanical mixing of 6 Ga203 and HZSM-5 zeolite. Several physical methods (XRD, XPS, IR, TPR, TEM/EDAX) unanbigously evidence that in the presence of Hz Ga203 reduction process occurs leading to gradual depletion of the Ga203 crystalline phase and simultaneous transfer of the gallium species into the zeolite The formation of GaaO is considered as the first step of the reduction process The zeolite acidity seems to be the most important factor in facilitating this process via trapping in cationic zeolite positions of gallium species in a lower (probably Ga oxidation state. The role of the hydrogen reduction in creating of a new active state of Ga-containing zeolites is confirmed by the dramatic enhancement of propane and n-pentane aromatization activity as well as by the strong increase in the ethylbenzene conversion.

1. INTRODUCTION

Ga containing zeolites have received extensive current interest since their utilization in the Cyclar process for- light paraffin aromatization [ll. Ion exchanged [21 or impregnated gallium [31 as well as gallosilicates [41 and even mechanical mixtures of GaZ03 and HZSM-5 zeolites [51 have been shown to exhibit catalytic activity in this new reaction. Examination of several Ga zeolite catalysts preparations has disclosed that all of the common synthesis techniques can conceiveably generate an intimate mixture of a gallium oxide or hydroxide with the zeolite 161. Moreover, recent a bifunctional reports [5,7,81 in the literature have focused on (Ga~Os)/acidiczeolite) mechanism for the catalytic conversion process. We have found, however, that mechanical mixtures of Ga20UHZSM-5 undergo chemical transformation upon treatment with hydrogen or via treatment with propane feedstock [9,1O] and have suggested that the bifunctional theory needs to be re-evaluated to include the possibility of a reduced galliurn oxidation state [111. This paper reports some additional evidence, which confirms that under suitable temperature conditions, the hydrogen induces gallium transfer from the GaZ03 crystalline phase into the HZSM-5 zeolite.

The increased catalytic activity of the resulting Ga-modified zeolite catalyst is corroborated in the conversion of both n-pentane and C8 aromatic hydrocarbons.

2.

EXPERIMENTAL

Catalysts were prepared by mixing powdered Ga203 (4N5-grade, Ingal International Co) and HZSM-5 zeolite (Union Carbide,Linde Div.) in a stainless steel ballmill .The manufacturer reported composition of the HZSM-5 zeolite is 3.73% A1203, 94,95%Si02 and 0.03% Na2O by weight. We refer to these catalysts as Ga/HZSM-5 catalysts with a numerical prefix indicating the gallium loading by weight. The catalysts were pelletized and crushed to 40-60 mesh prior to use in reduction or catalytic experiments. A Scintag PAD-V X-ray diffractometer equipped with a C u K a radiation source operated at 1.6 KW and Kevex Peltier-cooled solid state silicon detector was used to characterize the catalysts as described in ref. 10. The content of the gallium oxide crystalline phase in "fresh","reduced" or "used" ( i n catalytic experiments) catalyst samples was determined by using a M-80 Carl-Zeiss spectrometer and the KBr-pellet technique. XPS measurements were carried out in the analysis chamber of an ESCALAB MK I 1 (VG Scientific) electron spectrometer. The spectra were excited with A1Ka radiation (KV=1486eV). The powder samples were pressed into stainless steel sample holders and then introduced into the preparation chamber and pumped down to 10-8-10-’ombar. After transferring the samples in the analysis chamber of the XPS spectrometer, several photoelectron and Auger lines were recorded and Ar ion bombardment was used for depth profiling. The procedure applied is described in detail in [12]. The IR spectra were recorded on a DIGILAB FTS 20E Fourier transform spectrometer. The sample was pressed into a self-supporting wafer and dehydrated in vacuo at 783 K. 300 Torr of highly pure hydrogen was used as a reducting agent. After introduction of the hydrogen the temperature was raised again to 783 K and kept at this temperature for 1.5 h including a short evacuation and renewed admission of 300 Torr H2. A transmission electron microscope (PHILIPS EM 420) equipped with an X-ray spectral analyzer (EDAX) was used to obtain the selected area electron diffraction pattern and for microanalysis of the catalyst. Speciments for microanalysis were prepared by dispersing the powders in ethanol, placing a drop of this suspension on a thin carbon support net and allowing the solvent evaporate. The temperature programmed reduction (TPR) experiments were performed in a SETARAM TGDTA 92 microbalance. After evacuation of the sample to lo-’ mbar, the sample was equilibrated at room temperature with a 65 ml/min pure argon flow.The temperature was then raised with rate of 10 K/min up to 823 K and kept at this temperature for 1 h. Then the sample was cooled down to 360 K and 50 ml/min of the argon flow was replaced by the equal amount of hydrogen (80 kPa H2 in the total flow). Finally the sample was purged in hydrogen at 360 K for 15 min and TPR was performed at a scan rate 10 K/min up to 1073 K. The catalytic experiments for propane, n-pentane and C8 aromatics conversion were carried out in fixed bed type reactors operated in an inert gas (He or N2) stream at atmospheric pressure. The catalyst amount varied from 0.1 to 0 . 8 g depending on the selected experimental conditions and the

279

particular reaction investigated. The analysis of the feed and reactor products was performed with HP-5880 and HP-5890 gas chromatographs equipped with high performance fused silica capillary columns (Supelco SPB-1 and HP-PONA). The detailed procedure for the catalysts testing is described in

[lo].

3. RESULTS AND DISCUSSION

3.1 Gallium state in hydrogen treated GaXI3 catalysts

The relatively simple technique of temperature programmed reduction (TPR) was employed to elucidate whether the mixed Ga203/HZSM-5 catalysts undergo a chemical transformation in the presence of hydrogen as a reducing gas. The TG and DTA curves in Fig. 1 show a process of weight l o s s taking place in hydrogen flow whereas there are almost no changes in the sample weight when the same experiment is conducted in an inert gas atmosphere. In both cases, DTG and TG curves are calculated by the computer facility dividing the data for the pure HZSM-5 sample gathered in a separate experiment from those for the 5Ga/HZSM-5 catalysts. This approach significantly increases the accuracy of the weight loss estimation. The DTG band at 853 K clearly indicates a fast process of hydrogen reduction of the Ga203 leading to a weight loss of approx. 0.9%. This process is followed by a slower one, which accounts for approximately one third of the total weight loss. The above results confirm our previous observation [ l o ] that the gallium oxide reduction with H2 can be greatly facilitated in the presence of an intimate admixed acidic ZSM-5 zeolite. Due to the higher H2 partial pressure used in the present investigation, the DTG reduction band is shifted to a lower temperature than those reported in ref 10. Moreover, the applied procedure of monitoring difference TPR spectra allows verification that the weight loss effects observed are not due to processes such as dehydration and dehydroxylation of the zeolite component of the mixed catalyst.

l..'---:I:lGE I., Nlu

'

bn

U/minl

-0.00

_---

0.3

0 00%

+ t -

.

580

,\

-0.1

\

-0.10 -0.m

ARGON

-

\

-0.m

\

'\O

7

1-

t-O'O

-0.90

HVOROGEN

\

...

-1.7

.sax

80

-0.m

*

-TuE 3w

490

roo

roo

700

Figure 1. TG and DTG curves of 5Ga/HZSM-5 in Hz and

N2

El

-0.m

280

We have shown previously, that the two HZSM-5 zeolite XRD bands at about 24.2’ and 29.2’ 26 and the two lines of the 6 Ga203 at about 39.6’ and 35.2’ do not interfere, therefore, these bands were used to measure the 13 Ga203 crystalline phase content of the mixed catalyst. The XRD examination of the 5Ga/HZSM-5 sample before and after reduction with hydrogen at S30 K shows that, after the reduction of the sample in the microbalance, only traces of gallium oxide crystalline phase are present. The process of Ga203 IR depletion can be more conveniently recorded by using a simple KBr-pellets technique as it is ilustrated in Fig. 2. From this figure i t can be seen that one of the most intense IR bands of Ga203 appears in the 600-800 cm-’ region of the IR spectrum of HZSM-5 zeolite. Examination of several Ga20UHZSM-5 mechanical mixtures confirmed the intensity ratio of the peaks at 698 and 456 cm-’ as a suitable measure of the gallium oxide content even in the case of catalysts containing less than lwt% Ga203. In application of the approach described above for investigation of the 5Ga/HZSM-5 sample after the TPR experiment (Fig.1) as well as for characterization of a number of other "reduced" and "used" gallium catalysts, we did not observe any detectable amount of remaining Ga203 phase in the 2GaIHZSM-5 and 5Ga/HZSM-5 samples pretreated with hydrogen at a temperature equal to or higher than 763 K. In contrast, the degree of reduction of the 10Ga/HZSM-5 sample treated under the same experimental conditions does not exceed 50%. Therefore, we assume that only a limited amount of gallium can be effectively reduced with H2 in the presence of an acidic zeolite component. Finally, we should note that both XRD and I R methods used for gallium oxide determination produce well-correlated

456

548

A

Ga(3d) :,i: oj2s) :,: I I

.I.

5 Figure 2. I R spectra of the mixed catalyst and its components

B.E.,eV

35

Figure 3. XPS spectra of 5GaIFZSM-5 a-before and b-after Ha reduction

281 results. Due to the small sample size used in the microbalance and catalytic experiments, however, the described IR approach was found to be more suitable and even solely applicable in the determination of the GaaO3, crystalline phase in these samples. The transmission electron microscopy TEM coupled with EDAX microanalysis along with X-ray photoelectron spectroscopy (XPS) helped to establish the processes occurring in the mixed Gaz03/HZSM-5 catalysts after their treatment with hydrogen. Transmission electron micrographs of the SGa/HZSM-5 sample before the hydrogen reduction clearly indicate two kinds of partic1es:O.l-0.3 pm zeolite prisms and spherical particles ranging from 0.05 to aprox. 0.3 pn in size and containing gallium oxide. After its reduction in the microbalance, several micrographs of the same sample show that the gallium oxide particles have almost disappeared. Selected data from EDAX microanalysis of a number of zeolite particles listed in Table 1 provides convincing evidence that, during the TPR experiment shown in Fig.1, a process of gallium species transfer into the zeolite has occured. It is difficult to decide on the basis of the data in Table 1 whether the variations in the composition reflect a non-random gallium distribution among the zeolite particles. Nonetheless, there is no doubt that the major part of the gallium oxide crystalline phase undergoes a degradation process and the resulting gallium species are distributed into the zeolite microcrystallites. Neither gallium foreign phases on the zeolite crystallites nor gallium enrichment of their surface are observable. Table 1 EDAX microanalysis of selected zeolite particles ~

Composition wt%

A 1203 Si02 Ga203

~

Partical number (5Ga/HZSM-5 sample) before 1 4.238 95.762 0.000

reduction 2 4.010 95.990 0.000

after 3 4.571 90.944 4.485

reduction 4 4.838 90.042 3. 120

5 4.542 90.944 3.598

Additional important evidence on the process of gallium transfer into the zeolite was obtained by XPS. As one can see from Fig. 3 the reduction of the 5GaIHZSM-5 sample with hydrogen causes an increase in the Ga (3d) intensity by factor of about ten. This phenomenon can be well understood assuming that the hydrogen reduction leads to spreading of the Ga over the zeolite, which greatly increases the effective surface area of the Ga-containing material. On the other hand, the removal of 15-20 monolayers of the sample by Ar’ bombardment does not change more than 1-2% of the Ga (3d) intensity as measured from the total peak area of Ga (3d)+ + 0 ( 2 s ) lines, which clearly shows that Ga is transferred into the bulk of the zeolite. Finally, the Ga(3d) peak of the reduced sample shifts to a binding energy that is 0 . 7 eV lower compared to the non-reduced mechanical mixture, which can be interpreted as a lowering of the gallium formal oxidation state. However, the determination by XPS of the normal Ga oxidation state of the mixed catalysts is rather complicated for several reasons and a separate publication [121 is devoted to this subject.

282

In a previous paper [lo1 we assumed that the zeolite acidity is the driving force, that allows a substantial lowering of the reduction temperature of the admixed gallium oxide phase. Recently we proved the involvement of the acidic zeolite OH groups in the process of gallium transfer into the zeolite [131. As it can be seen from Fig 4, the narrow band at 3610 cm-* due to the acidic OH groups of the HZSM-5 zeolite greatly decreases in intensity after reduction of the 5Ga/HZSM-5 sample with hydrogen. A rough estimation shows that approximately 50% of the initially present OH groups are lost despite the mild conditions (static apparatus and relatively low temperature) of the reduction experiment. In contrast, numerous investigations e.g.[14, 151 failed t o confirm any significant changes in the OH group content of the HZSM-5 zeolite, when the gallium was introduced by the common wet techniques of ion exchange or impregnation. This effect can be readily explained by taking into account the large size of the hydrated Ga3+ ion as well as the constraints due to electrostatic disbalance when three isolated ne ative charges of the zeolite framework have to be compensated by one Ga3’ ion. Furthermore, the IR data reported in the literature are not related to Ga/ZSM-5 zeolites, that have been subjected to H2 treatment at elevated temperatures. We suggest that the hydrogen reduction plays a key role in facilitating the introduction of gallium species in the bulk of the zeolite even if the gallium source is present in the form of a separate crystalline phase. It seems that the reduction to a lower oxidation state of polyvalent cations such as Ga3+ helps to avoid the above mentioned constraints and assure 15 the introduction and random 0 0 distribution of these cations in W the HZSM-5 zeolite. The gallium U B suboxide appears to be the first ' product of the reduction process, 10 a 0 which is readily trapped by the VI m acidic sites and further a 5distributed in the cationic A positions of the zeolite framework, probably as a Ga’ cation. ’3;OO’ ' * . 3600 ' . ' . * 3700 ' WAVENUMBERS [IICM] Figure 4.,IR spectra of 5Ga/HZSM-5 sample A-before and B-after hydrogen reduction

-

F

5

3.2 Catalytic properties of Ga203/HZSM-5 catalysts

Catalytic experiments reveal the hydrogen reduction of Ga203/HZSM-5 mixed catalysts as a process which strongly affects the catalytic properties of these catalysts. As has been reported elsewhere [10,111, the aromatization of propane is greately enhanced after reduction of the catalysts which either hydrogen o r propane reactant. The data listed in Table 2 show that there is a gradual increase of both total conversion and aromatic selectivity of the 5GaIHZSM-5 catalyst with the time on stream at the same time a process of Ga203 reduction by the propane reactant or hydrogen envolved during the reaction takes place. The IR examination of

283

the "used" catalyst after the catalytic run mentioned above showed almost complete disappearance of the gallium oxide phase. No development of catalytic activity,however,was observed when pure HZSM-5 and Gaa03+HZSM-5 separated by a 0.5 cm long quartz wool bed were used as catalysts. From Table 2 it can also be seen that, after 1500 minutes on stream with propane the aromatic selectivity is enhanced by a factor of about 100 and there is a threefold increase in the total conversion compared with the HZSM-5 sample containing no Ga. While the methane yield does not change significantly, the content of saturated hydrocarbons increases steadily, reflected in the olefin/paraffin ratios 2. Characteristic changes also occur in the shown in Table isobutane/n-butane ratio. Table 2 Propane conversion on HZSM-5 and 5Ga/HZSM-5 catalysts Sample Time on stream, min

HZSM-5 90

5Ga/HZSM-5

40

90

180

400

1500a

Total conversion, wt%

7.49

11.26

13.99

16.53

18.28

20.20

Yield of products, wt% Methane Ethy1ene Ethane Propylene Butenes Bytanes C5 aliphatics

2.24 3.72 0.30 1.02 0.12 0.04 0.02

2.70 4.69 0.48 2.06 0.41 0.14 0.03

2.93 4.82 0.59 2.70 0.52 0.27 0.04

2.89 4.77 0.75 3.40 0.57 0.36 0.04

2.89 4.50 1.00 3.38 0.60 0.44 0.04

2.70 4.29 1. 29 3.48 0.60 0.49 0.05

Total aliphatics Total aromatics

7.44 0.03

10.51 0.75

11.87 2.12

12.78 3.75

12.85 5.43

12.88 7.32

Aromatic selectivity, %

0.4

6.7

15.1

22.7

29.7

36.2

12.4 3.0 0.61

9.8 2.9 0.63

Selected product ratios Ethylene/Ethane ButenedButanes Isobutaneh-Butane ~

8.2 1.9 0.70

6.4 1.6 0.76

4.5 1.4 0.71

3.3 1.2 0.64

~~

Conditions: Temperature 803 K; WHSV 1 h-l, Propane partial pressure 14 kPa Total pressure 123 kPa ( H e as a diluent gas) a Catalytic testing at a higher temperature (833 and 858 K) was performed between 400 and 1500 rnin of the experiment. The effect of Ha reduction of the 2Ga/ZSM-5 sample is demonstrated in Tables 3 and 4 for n-pentane and c8 aromatics conversion respectively. After a short catalytic testing in Nz the same catalyst sample was purged with pure nitrogen at 773 K, then reduced with pure Ha at this temperature for 1.5 h and finally subjected again to the catalytic testing in nitrogen atmosphere.

284 Table 3 Pentane conversion at 693 K, WHSV 1.8

Table 4 Conversion of C8 aromatics

Catalyst

Cata 1yst

HZSM-5 2GaZSM-5 A

Conversion wt% Selectivity % : Aromatics Aliphatics Distribution of aliphatics %

c1 c2

c3 c4

24.1

36.5

5.4 6.8 95.6 93.2

13.6 86.4

1.9 1.9 27.8 27.6 49.3 48.9 21.0 21.6

2.7 26.7 45.6 25.0

1.59 0.55 0.91 0.53

1.60 0.53 0.93

0.55

2Ga/HZSM-5

B

24.6

Ethylene/ethane Propylene/propane Butenes/Butanes Isobutane/n-butane

HZSM-5

1. 11 0.40 0.35 1.57

Product Composition wt% Benzene 1.29 Toluene 1.77 Ethylbenzene 15.87 Xy 1enes 78.20 3.01 Cg aromatics Ethylbenzene 19.3 Conversion %

A

B

1.66 2.51 1.95 2.54 16.06 13.8s 77.70 77.00 2.60 4.09 18.3

29.4

A-before and B-after H2 reduction WHSV 1.5 h-I, Nz/feed=3, T=553 K Feed composition wt%: Toluene 1.41 Ethylbenzene 19.66,Xylenes 78.SO

A-before and B-after H2 reduction As shown in Tab. 3 an increase in the aromatization activity is observed after the hydrogen reduction. The hydrogen reduction also strongly influences both isobutaneln-butane and olefidparaffin product ratios whereas the production of methane and aromatics distribution do not change as much.It seems that the Ha reduction introduces into the Ga/HZSM-5 catalysts an increased ability to produce more saturated and aromatic hydrocarbons converting a paraffinic feedstock.This feature of the reduced catalyst is likely related to the catalyst capability to greatly facilitate hydrogen transfer reactions as compared to both HZSM-5 and nonreduced Ga/HZSM-5 zeolite.This assumption is in agreement with the results in Tab. 4 for the conversion of c8 aromatics. Indeed, the hydrogen reduction of the 2Ga/HZSM-5 sample leads to an enhanced ethylbenzene conversion and to an increased rate of the disproportionation and transalkylation reactions Since such reactions require a bimolecular transition state, i t is reasonable to assume that the new active catalyst state contributes to acceleration of the hydrogen transfer processes. On the basis of the catalytic results it is rather difficult to determine the reason for such dramatic changes in the catalytic properties of the Ga/HZSM-5 catalysts after hydrogen treatment. Along with the above mentioned changes in the gallium state,the zeolite acidity is also altered considerably by the process of gallium transfer. The replacement of a part of the acidic OH groups by new Lewis sites containing cationic gallium may be one of the most important factors contributing to the specific catalytic action of the Ga203IHZSM-5 catalysts.

285 4. CONCLUSIONS

The increased reducibility of the gallium oxide in the presence of both hydrogen and acidic ZSM-5 zeolite causes a process of gradual depletion of the gallium oxide and simultaneous transfer of cationic gallium species into the zeolite.The solid-state reaction proceeding through galljum suboxide as an intermediate represents a new route to high quality catalysts,which requires no wet operation for the catalyst preparation.The theories pointing to a bifunctional gallium oxide/acidic zeolite mechanism of the light paraffin aromatization need to be re-evaluated to include the possibility of a reduced gallium oxidation state, as well as the impact of gallium on the zeolite acidity.

5. ACKNOWLEDGEMENTS

The authors gratefully acknowledge the financial support of the Bulgarian Academy of Sciences and the Exxon Foundation.The authors a r e indebted to Drs. V. Mavrodinova, C. Tyiliev, V. Valtshev, Mrs. L. Kosova and M. Stojanova for their helpful1 assistance and stimulating discussions. V. K. thanks Prof. H. Forster (Hamburg University, Germany) for providing the FTIR facilities.

6. REFERENCES

1 J.R.Mowry, R.F.Anderson and J.A. Johnson,Oil Gas J., (1985) 128 2 H.Kitagama, Y.Sendova and Y.Ono, J.Catal., 101 (1986) 12

J.Y.Doyemet, A.M.Seco, F.Ramoa Ribeiro and M.Guisnet, Appl.Catal., 43 (1986) 155 J.M.Thomas and Xiu-Cheng Liu, J.Phys.Chem., 90 (1986) 4843 N.S.Gnep, J.Y.Doyemet and M.Guisnet, J , M o l . Catal., 45 (1988) 281 G.L.Price, K.M.Dooley and V. Kanazirev, submitted for publication T. Inui, Y.Makino, F.Okazumi, S.Nagano and A.Miyamoto, Ind.Eng.Chem.Res.,

3 N. S. Gnep,

4 5 6 7

26 (1987) 647

8 P.Meriadeau and C. Naccache, J.Mol.Catal., 59 (1990) L31 9 V. Kanazirev, G.L.Price and K.M. Dooley, J.Chem.S O C . ,Chem.Commun., 9 (1990) 712

G.L.Price and V.Kanazirev, J.Catal., 126 (1990) 267-278 G.L.Price and V.Kanazirev, in press V. Kanazirev, G.L.Price and G.Tyuliev, submitted for publication V.Kanazirev, R.Piffer and H.Forster, submitted f o r publication 14 V. B. Kazansky, L.M.Kustov and A. Yu.Khodakov, Stud.Surf.Sci.Catal. 10 11 12 13

49 (1989) 1173

15 V. I. Yakerson, T.V. Vasina, L. I.Lafer, V.P.Syntyk, G.L.Dikh and 0.V. Bragin in Abstracts of ZEOCAT 90, Leipzig, p.55

This Page Intentionally Left Blank

P.A. Jacobs et al. (Editors),Zeolite Chemistry and Catalysis 0 1991 Elsevier Science Publishers B.V., Amsterdam

281

COMPARISON OF HYDROSULFURIZATION ZEOLITE CATALYSTS PREPARED IN DIFFERENT WAYS

Gy. Onyestyak, D. Ka116 and J. Papp, Jr. Central Research Institute for Chemistry, Hungarian Academy of Sciences, 1525 - Budapest, P. 0. Box 17, Hungary

Abstract

Transition metal ions can be introduced into zeolites by solid state ion-exchange for preparing catalysts to be used in hydrosulfurization. Chlorides of Co, Mn, Zn, Cd, Ca were contacted with NH4-Y; CdClz with NH4-MOR and NH4-X; and different compounds of Cd with NHs-Y. Ion-exchange was followed by i. r. spectroscopy: decrease of OH band intensity and bands generated by dissociative HzS adsorption were detected. The rearrangement of adsorbed bases after HzS adsorption observed in i. r. spectra indicated more extensive dissociative adsorption of H2S on Cd-faujasites prepared in solid state than in liquid phase ion-exchange. Dehydrosulfurization tests also revealed this recognition.

1. INTRODUCTION

Hydrosulfurization of olefins, i e , the addition of Has to C=C double bond resulting in the formation of thiols and thioethers can selectively be catalysed by transition metal-forms of different zeolites ( 1 ) . I t has been found that on the same catalysts thiols and olefins are converted to thioethers, furthermore, thioethers with HzS transform into thiols (2). Catalysts were prepared with conventional ion-exchange carried out in an aqueous solution of the corresponding metal salt (1). For transition metal ions this method involves some limitations. At low pH values, required to avoid hydrolysis of the salt, the zeolite lattice may be damaged, moreover, deep ion-exchange for transition metals can hardly be achieved. Solid state ion-exchange seems to be promising, as it proved to be suitable for the introduction of alkali, alkaline earth and earth metal ions as well ( 3 - 6 ) . According to these investigations NH4- and H-forms readily react with chlorides and the desired cationic form is produced. Incorporation of Mn and Fe ions into the cationic sites of H-ZSM-5 has been observed when solid state reactions proceed between the zeolite and the corresponding salts or oxides at elevated temperatures ( 7 ) . In this way exhaustive ion-exchange can be achieved. The resulting cationic forms are mostly of higher activity than those prepared with usual ion-exchange, as it has been found for

288

partly hydrated La-Y in ethylbenzene disproportionation (51 and for Fe- and Mn-ZSM-5 in methanol transformation to aromatics and toluene disproportionation (7) owing to the deeper ion-exchange. We wish to examine (i) whether solid state ion-exchange can be effectively carried out with transition metal compounds, and (ii) how these samples behave in comparison with samples prepared in usual aqueous ion-exchange.

2.

EXPERIMENTAL

Ma ter ia 1s Na-Y and Na-MOR were produced in Wolfen/Germany, Na-A by Bayer AG/Germany and Na-X by BDH Chemicals Ltd/Great-Britain. 100 g of Na-zeolite in 2 1 1 N aqueous solution of NH4C1 was refluxed for 5x7 hours, then washed with distilled water. The degrees of ion-exchange f o r NH4 were NH4-X 85%

NH4-Y 96%

NH4-MOR 90%

NH4-A 98%

Ion-exchange with 2 1 of 0.2N solution of metal chlorides was performed for 5x7 hours under refluxing. The degrees of io~-exchangewere: CdX 94%

CdY 87%

CdMOR 79%

CdA 99%

ZnY 94%

MnY 77%

Solid state ion-exchange was carried out as follows: generally stoichiometric amounts of metal compounds were mixed with 2 g of NH4-zeolite in an agate mortar and stored in a glass flask. HzS was a Linde product of 99.6% purity, ethanethiol (EtSH) and diethyl sulfide (EtzS) (Fluka), ammonia (Matheson) and pyridine (reanal/Hungary) were of GC purity. Methods I. r. spectra of samples pressed into wafers were recorded usually after heat treatment at 5OO0C at lo-' Pa f o r 1 hour; adsorption of water: 20 Pa for 10 min. at 35OoC, evacuation: 10 min. 35OOC; adsorption of HzS: 6.6 kPa for 10 min. at 2 5 O C , evacuation: 10 min. 25OC. X. r. d. patterns of the samples investigated did not show any loss in crystallinity. Decomposition of EtSH and EtzS was tested by ethylene formation i n a pulse reactor. 100 mg catalyst was inserted and pretreated at 500°C in Nz-flow. At 377 C first 4 pulses of HzS ( 2 5 0 p1 each), than 10-10 pulses of 1 1.11 EtSH and EtzS were injected in Nz flow.

289 3. RESULTS AND DISCUSSION

Indication of ion-exchange Solid state ion-exchange can be favourably performed when the process results in the formation of volatile compounds. NH4-zeolites are preferred since the ammonium salts formed are removed easily at elevated temperatures (3). During the heating of thoroughly mixed powders of a metal salt and NH4-zeolite, ammonium salt (or its dissociation product) is released in the extent of the ion-exchange. (We found, e. g . , that NH4Y violently reacts with NazS even at room temperature with the evolution of NH3 and H2S and Na-Y is formed stoichiometrically.) The sites not exchanged are deammoniated at higher temperatures and acidic OH groups are formed. Formation of these groups indicate the incompletness of introduction of the desired cations. The i. r. spectra of well mixed NH4-Y and CdClz wafers treated (i) at Pa for 1 hour at each increasing temperatures between 200 and 500’C temperatures), (ii) at 400’C at 1,0-2 Pa for 1-12 hours, (iii) with excess Pa for 1 hour) show that the of Cd salt (pretreatment at 300 C, intensity of the LF band (OH vibration band at 3540 cm-*I decreases and thereafter the intensity of the H F band (OH vibration band at 3640 cm-’1 diminishes. After disappearance of the LF band, the HF band also disapears, pointing to perfect ion-exchange. Since in the strong electrostatic field of multivalent cations water dissociates heterolytically, resulting in the formation of acidic OH groups (81, corresponding absorption bands appear after adsorption of water on Cd-Y prepared with solid state ion-exchange (Fig. la HF band of the heat treated sample reflects incomplete exchange f o r Cd , while its intensity increase and the apearance of the LF band after the adsorption of water indicate Cd ions in cationic sites. For comparison the behaviour of solid state ion-exchanged La-Y is shown in Fig. lb. It has been proved (1) that the adsorption of H2S introducing hydrosulfurization proceeds similarly to the adsorption of water (figs. lc and d): both HF and LF bands are generated on Cd-Y and La-Y. However, H 2 S adsorption on the catalytically most active Cd-Y (1) is larger than on La-Y showing low activity in hydrosulfurization; the situation is reversed compared to water adsorption. H2S adsorption on Ca- or Na-Y does not generate detectable OH bands. An intense OH band is recorded at 3580 cm-I for NHs-MOR after deammoniation at 500’C (Fig. 2, curve 1). When CdClz is admixed, the same treatment results in a weak OH band (curve 2 ) owing to ion-exchange for Cd+2. H2S adsorption increases the intensity of a diffuse OH-band (curve 3j, and simultaneously a band appears at 2520 cm-’, which is assigned to SH stretching vibration ( 9 1 . Appearance of the SH band indicates the dissociative chemisorption of H2S on the introduced cations. The SH groups formed in H2S dissociation could not be fully removed even at 2OO0C (curve

+&

4).

Factors affecting solid state ion-exchange In the first run of experiments chlorides of bivalent metals were used and solid state ion-exchange capability of dufferent metals was thus compared (Fig. 3). Increasing order of ion-exchanges based on the intensity decrease of the

290

NHLY Lac13

3630

la1

3520

Ib1

NH4Y+LaCI3

3640 3530

10

3500

1

10

3500

3000

WAVENUMBER ICM-’)

Fig. 1. I. r. spectra of NH4-Y with admixed CdCla and Lac13 after standard pretreatment for 3 hours (upper curves), after water and H2S adsorption (lower curves).

LOO0

3Si0, 3500

I

I

3000

2500

00

WAVENUMBER (CM-’)

Fig. 2. I. r. spectra of NH4-MOR; 1: after standard pretreatment; 2: with admixed CdCla, standard pretreatment for 3 hours; 3: followed by adsorption of H2S; 4: pumped off at 2OO0C for 30 rnin.

HF band of deammoniated NHI-Y is CoiMniCd-Zn 573 K i t i s probably overshadowed by a strong signal F. In the case of sample 111-295 signal A i s formed i n the T, = 423-473 K range, reaching an i n t e n s i t y plateau in the T, = 473-523 K range, f i g u r e 4b, and showihg a marked decrease above t h a t temperature. Signal C i s observed, more o r l e s s isolated i n the 1, = 423-523 K range and overlapping with signal F i n the T, = 573-623 K range and w i t h signal E f o r T, 2 623 K. Signal 0 i s observed f o r T, L 573 K, f i g u r e 4c, reaches a maximum f o r T, = 673 K, f i g u r e 4d, and i t i s s t i l l observed f o r T, = 723 K (the maximum T, used i n t h i s study) For sample 111-373 signal A i s f i r s t observed a t T, = 423 K, reaches a maximum a t T, = 473 K and decreases sharply f o r T, = 523 K, showing a marked broadening; above T, = 523 K the signal i s no longer modified. Signal C i s observed without interference in the T = 423-473 K range and T, = 773 K, but i s overlapped by signal F in t h e T, = s23-623 range and by signal E i n the T = 623-723 K range. Signal D i s observed a t T, = 523 K, reaches a maximum af: T, = 623 K and disappears a t T, = 773 K. We examined a l s o a sample 111-473 K , the main differences observed in r e l a t i o n with 111-373 are t h a t now signal E i s not observed, and t h a t signal D, which appears w i t h lower i n t e n s i t y in the T, = 523-623 K range, is no longer observed f o r T, 2 673 K.

.

Table 1 Parameters and assignment o f the ESR s i g n a l s Signal A A’ 6

C D E F

g-Val ues

Assignment

91 = 1.934, 911 = 1.884 g, = 1.944, g,, = unresolved g = 2.002 (AH = 46) gl, = 2.009, 91 = 2.002 g, = 2.043, g, = 2.030, g3 = 1.999 g1 = 2.014, g = 2.010, g3 = 2.002 g = 2.004 (A$= 86)

MO~+

MO~+

Trapped electrons

so -

Sutfur r a d i c a l s (S,-) 0-

sb3-

343

r5 \

x2.

m

a-

Figure 3. ESR spectra o f sample 11: (a) outgassed at 573; (b) outgassed at 295 K, and SO, adsorbed at (b) 473 K, (c) 523 K and (d) 573 K.

Figure 4. ESR spectra o f sample 111: (a) outgassed at 573 K; (b) outgassed at 295 K and SO, adsorbed at (b) 523 K, (c) 573 K and (d) 623 K.

344

All t h r e e samples were examined a l s o f o r higher temperatures T, of preoutgassing. Similar behaviour was obtained i n a l l cases: A and F s i g n a l s were generated f o r T, 2 423 K 4. ASSIGNMENT OF

THE SIGNALS

The l a r g e l i n e width of signal A i n d i c a t e s t h a t i t i s not due t o r a d i c a l s b u t t o t r a n s i t i o n metal ions; here i t must be due t o Mo5+ ions. Signals with s i m i l a r g values have been observed f o r Mo5+ ions in MoNaY z e o l i t e s [6] and assigned t o such s p e c i e s belonging t o Mo,O, clusters produced during the preparation treatments ( i s o l a t e d Mo5+ g i v e s sharper peaks, s e e r e f . 6 ) . Modifications in t h e environments of t h e Mo5+ ions can produce changes i n t h e g-values as in t h e case of signal A’. Signal B i s often observed a f t e r thermal treatments under vacuum in many c a t a l y s t s ; i t is assigned generally t o e l e c t r o n s trapped i n oxygen vacancies or t o carbon impurities. Considering t h a t t h e samples have been previously calcined a t 823 K the f i r s t assignment seems more l i k e l y . As t o t h e other symmetric signal F , although i t i s located c l o s e t o t h e p o s i t i o n o f signal B and i s only a l i t t l e broader than i t , i t s c o n s i s t e n t s h i f t t o higher g-value i n d i c a t e s t h a t i t must have a d i f f e r e n t o r i g i n . I t s parameters a r e s i m i l a r t o those of s i g n a l s assigned by d i f f e r e n t authors t o SO,- s p e c i e s [8], we will adhere t o t h i s assignment. Species s i m i l a r t o our signal C have been observed a f t e r adsorption of SO, on vacuum t r e a t e d Y z e o l i t e s [9] and many other compounds [ l o ] ; following these works, i t can be assigned t o SO,- r a d i c a l s . As t o signal E i t s g values a r e not much d i f f e r e n t from those obtained f o r 0,- bound t o Mo6’ ions [5]. We would not d i s c a r d , however, t h e p o s s i b i l i t y t h a t i t corresponds t o an 0-, fragment bonded t o a chain of s u l f u r atoms as suggested in [ l l ] . In any case, t h e unpaired e l e c t r o n d e n s i t y would have t o be concentrated on the 0 atoms, in view of t h e small deviation of i t s g values from g,. F i n a l l y , signal D i s q u i t e s i m i l a r t o t h a t found upon adsorption of HS, on MoO,/A1,0, samples [12], wh5;e i t could be c l e a r l y ascribed, on t h e b a s i s of i s o t o p i c s u b s t i t u t i o n with S and 95M0, t o symmetric S-, s p e c i e s bonded t o Mo ions. I t i s t o be noted t h a t s u l f u r chain r a d i c a l s present r e l a t i v e l y s i m i l a r g parameters [13]; without discarding such a p o s s i b i l i t y , we consider more l i k e l y the assignation t o S,-, e s p e c i a l l y s i n c e t h e d e v i a t i o n (9,-g,) < 0 presented by signal D i s shown by S-, b u t n o t by S., 5. DISCUSSION

The f i r s t observation t o be made from t h e presented s p e c t r a i s t h a t Mo5+ ions a r e thermally generated more e a s i l y by i n t e r a c t i o n with SO, than under vacuum. SO, can obviously a c t as reductant; t h e overall basic process f o r t h i s i n our case (where Mo5’ i s generated) would be 2 Mo6+ t 2 OH- t SO,

-

2 Mo”

t HO ,

t SO,

(1)

o r s i m i l a r . SO, can, however, a c t a l s o as e l e c t r o n acceptor ( i . e . o x i d a n t ) , which makes p o s s i b l e t h e generation of SO,- by r e a c t i o n o f SO, w i t h an e l e c t r o n donor as Mo” i t s e l f

345

Mo5+ t SO, + Mo6+ t

(2)

SO,-

It is to be noted that a combination of (1) and ( 2 ) would amount to a disproportionation of SO, (with formal redox state t 4 for sulfur) into product with S in formal redox state higher (6t for SO,) and lower (3t for SO,-). The implication of Mo in this process is clear, since the parent zeolite does not produce comparable quantities of SO,- under these mild conditions. The formation of SO,- can be explained either as an intermediate step in (1): Mo6+ + 2 OH-

t SO,

-

-

Mo5+ t HO,

t SO,-

(3)

or as a result from an electron transfer similar to reaction ( 2 ) : Mo5+ t SO,

Mo6+ t SO,-

(4)

favoured by the oxidizing character of the SO, molecule. In any case, this radical appears in substantial amounts specially when the sample has been previously outgassed at temperatures of 373 K and higher; this suggests that its stabilization i s achieved by insertion into coordination vacancies produced on Mo ions by outgassing. More remarkable is the observation of S-, radicals; this corresponds to a level of reduction of SO, deeper than SO,-; i .e. elemental sulfur and beyond. The overall process for this could be described as: 4 (Mo5+ - OH-) t SO,

-

4 (Mo6+ = 0) + 2 HO,

t 1/2 S,(5)

although instead of this a direct auto-reduction of SO, can be also formulated: 3 SO, t 2 OH-

-

2 H SO,-

t

1/2 S,

(6)

Any of these reactions would be then followed by stabilization o f the sulfur radical on Mo itself: MO~+

-

H

t

s, +Mo6+

. ..

S,

(7)

Such a particular stability of monoanion radicals in a cation-exchanged zeolite has been also observed in the case of 02- formed on Ce-loaded Y zeolite [ 1 4 ] . In that case, this was ascribed to the presence o f a Ce4+ (OH-) (0,J complex, with the Ce-(OH) group stabilized by the particular coordination constraints imposed to the cation by the rigid zeolite framework. In our case, it may well be that the final product of (7) is particularly stable if an (OH-) group is also bonded to the Mo6+ ion, resulting in a Mo6+ (OH-) ( S J species. Then, this would explain that this species is particularly stable in proton-exchanged zeolites since in the absence o f H+, [Mo6+ = OI4+ species, will dominate, and their smaller positive charge rather than [Mo6+ - (OH)-]'+, will be less stabilizing for the coordinated Sz- anions. Also, the excess of protons will be eliminated (desorbed as H,O) upon outgassing at increasing temperatures and this agrees with the fact that the S-, species disappears at lower temperatures in pre-outgassed samples. Since the acidity of the samples is lowered by the exchange with Co [ 1 5 ] , the lower thermal stability of the Sz- species in the Co-containing samples can be explained with the same arguments. On the other hand, the reason why S,- is formed at lower temperature in the less acidic samples is less clear; it must be related to the details

346

of t h e p a r t i c u l a r elementary s t e p s which sum up t o r e a c t i o n s (3-4). A s i m i l a r consideration can be made about t h e 0,--type s p e c i e s ; they a r i s e probably from t h e process in which SO, l o s e s oxygen t o give f i n a l l y S,-type species. A p o s s i b l e mechanism might be:

2 SO,-

SO,oos0,-

SO,-

-

H'

[0, S-S0,]2-

- -

t (0,s-S0,H)-

so,

t (S-S0,H)-

+

SO,-

(0, S-SO, H)-

t (OS-SOZH)'

+

(8)

(0-OS0,)- t (S-S0,H)-

(9)

t 0-,

SO,

t S,- t

OH-

but many o t h e r r e a c t i o n schemes can be devised, where b a s i c a l l y SO, o r SO,a b s t r a c t oxygen from a species where t h e S-S bond has been already formed; in a l l c a s e s , c a t a l y s i s of some s t e p s by Mo ions would not be excluded. Since some of t h e a b s t r a c t i o n r e a c t i o n s could r e s u l t in formation of 0-0 bonds (as in (9) above), t h e generation of 0,--type intermediates could be explained. In t h e end, a f t e r heating under SO, a t high temperatures, only those r a d i c a l s will remain which a r e able t o become s t a b i l i z e d on Mo ( o r perhaps Co) c a t i o n s ; seemingly, t h i s a p p l i e s t o S,- only in proton-rich samples, otherwise only SO,o r SO,- withstand the treatment. Thus, t h e d i f f e r e n t behaviour observed w i t h i n t h i s s e t of samples seem t o depend mostly on t h e amount of H,O and/or protons i n them. No s p e c i f i c e f f e c t of Co i s revealed beyond those which can be explained in terms of t h e reduction in a c i d i t y induced by the exchange w i t h co In conclusion, SO, a c t s on Mo-loaded z e o l i t e s b o t h a s reductant (giving Mo5+ and SO, o r SO,-) and as e l e c t r o n acceptor, giving SO,- and/or S,- r a d i c a l s , which become s t a b i l i z e d on Mo ions i n t h e presence of Bronsted a c i d i t y . 02-type r a d i c a l s a l s o appear in t h e process, being probably a s i g n a t u r e of t h e mechanisms which a b s t r a c t 0 atoms from SO, t o g i v e f i n a l l y S,- o r s i m i l a r .

.

6. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

12. 13. 14. 15.

A.R. Gonzalez-Elipe and J . S o r i a , Z . Phys. Chem. N.F. 132 (1982) 67. A.R. Gonzalez-Elipe and J. S o r i a , J . Catal. 51 (1983) 235. A.R. Gonzalez-Elipe and J. S o r i a , J.C.S. Faraday 182 (1986) 739. A.R. Gonzilez-Elipe and J. S o r i a , J . Catal. 103 (1987) 506. M. Che and A.J. Tench, Adv. Catal. 32 (1983) 1. J.L.G. F i e r r o , J.C. Conesa and A. L6pez Agudo, J . Catal. 108 (1987) 334. R.F. Howe and H. Minming, Proc. gth Intern. Congr. Catal. 4 (1988) 1585. Y. Ono, H. Takagiwa and S. Fukuzumi, J.C.S. Faraday 1 7 5 (1975) 1613. Y. Ono, H. Tokunaga and T. Keii, J. Phys. Chem. 79 (1975) 752. R.A. Schoonheydt and J.H. Lunsford, J . Phys. Chem. 76 (1972) 323. M. S t e i j n s , P. Koopman, B. Nieuwenhuijse and P. Mars, J . C a t a l . 42 (1976) 96. A.K. Kolosov, U . A . Shvets, M.D. Chuvylkin and V.B. Kazansky, J. Catal. 47 (1977) 190. R. Steudel, J. Albertsen and K. Zink, Ber. Buns. Phys. Chem. 93 (1989) 502. J.C. Vedrine, G . Wicker and S. Krzyzanowski, Chem. Phys. Let. 45 (1977) 543. R. Cid, F. Orellana and A. Ldpez Agudo, App. Catal. 32 (1987) 327.

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 0 1991 Elsevier Science Publishers B.V., Amsterdam

341

CONTRIBUTION OF METAL CATIONS TO THE PARA-SELECTIVITY OF SMALL CRYSTALS OF H-ZSM-5 ZEOLITE IN TOLUENE ALKYLATION WITH ETHYLENE J. Cejka, B. Wichterlova, J. Krtil, M. Krivanek and R. Fricke

1

The J. Heyrovsky Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, Dolejskova 3 , C S - 1 8 2 2 3 Prague 8 , CZECHOSLOVAKIA ’Central Institute of Physical Chemistry, Rudower Chaussee 5 , D-01199 Berlin, FEDERAL REPUBLIC OF GERMANY Abstract

The alkylation of toluene with ethylene yielding a mixture of ethyltoluenes, coke formation and its reoxidation were investigated on small crystals of H-ZSM-5 zeolites containing Fe, Mn and A1 cations. The cations were located either in the zeolite channel intersections or attached to the zeolite surface, where strong acid OH groups were annihilated by silylation. It appears that the "surfacettmetal cations contribute to the lower p-ethyltoluene selectivity of the zeolite and, therefore, enhance isomerization of ethyltoluene mixture. In contrast, the cations located in the channel intersections increase the zeolite para-selectivity as a result of steric hindrances for transport of bulkier isomers and likely due to para-selectivity of the initial alkylation step. Even though the metal cations as electron acceptor sites slightly enhance the deactivation by by coking, their redox properties contribute significantly to coke removal in the regeneration process. 1. INTRODUCTION

For application of zeolites as catalysts in industrial processes, high activity and easy removal of coke deposits are required. To meet these requirements, small crystals of zeolites (0.5-1.0 um) should be advantageously used. On the other hand, the para-shape selectivity of zeolites in alkylaromatic transformations is connected especially with large crystals of the ZSM-5 zeolite structure, modified by silicon, boron, magnesium and phosphorus [1-5]. However, no definite conclusion has been drawn on the contribution of various species to restricted transport of the bulkier isomers through the zeolite crystals, selectivity of the initial

348

alkylation step and subsequent isomerization of para to meta and ortho isomers in the zeolite channels and/or on the outer zeolite surface [l-81. Further, it has been found that depending on the number of strong acid bridging OH groups and reaction conditions, even small crystals of ZSM-5 zeolites can exhibit an over equilibrium concentration of para-isomer in toluene alkylation with ethylene [ 9 ] . Moreover, the presence of Na and K cations and A 1 0 species in the zeolite channels X Y contribute to the increased zeolite para-selectivity likely through the initial alkylation step [lo]. This paper deals with the activity and para-selectivity of small crystals of H-ZSM-5 zeolites modified by various metal cations located at different sites. The effect of Fe, Mn (which can be expected to enhance coke removal) and A 1 cations, modelling an electron acceptor site without redox properties, located mostly in the zeolite channels or on the zeolite outer surface, has been investigated. Attention has been paid to the alkylation of toluene with ethylene, including coke formation and its removal by oxidation. The surface deposition of silicon is discussed to explain some effects of the metal cation location in the zeolite on its para-selectivity. 2. EXPERIMENTAL AND ZEOLITE CHARACTERIZATION

H-ZSM-5 zeolites (prepared from Na forms by ion exchange with 0.5 M HN03) with Si/Al ratio from 22.5 to 600 and crystal size in the range of 0.5 to 1.5 pm were supplied by the Research Institute for Oil and Hydrocarbon Gases,Czechoslovakia The ion exchange of Fe3+ , Mn2+ and A13+ into H-ZSM-5 (abbrev. MeH-ZSM-5) was carried out at 330 K using 0.5M FeC13, A1(N03)3, and MnC12 under conditions (pH 3-4) avoiding precipitation of hydroxo-oxidic species. The solid-state interaction of Mn304 oxide and H-ZSM-5 zeolite, carried out in a nitrogen stream at 770 K f o r 6 hours yielded Mn304H-ZSM-5 zeolite containing surface Mn ions in the Mn304 phase and some Mn 2+ in the cationic sites. The location of Mn 2+ in the latter sites was reflected in a decrease in the number of strong acid zeolite OH groups compared with the parent H-ZSM-5 zeolite and

349 2+

in the ESR signal of isolated Mn ions; for details see ref. [11].The surface silylated zeolites were prepared by suspending the H-ZSM-5 or FeH-ZSM-5 zeolite in n-hexane into which a calculated amount of tetraethyl orthosilicate was added to obtain addition of 1.5 wt. % of Si in the final product (abbrev. SiHZSM-5 or SiFeH-ZSM-5). n-Hexane was evaporated and the zeolites were dried and calcined in an oxygen stream at 770 K for 5 hours. To add some Fe cations to the surface of silylated zeolites, the SiH-ZSM-5 was introduced into a FeC13 solution, filtered and dried (abbrev. FeSiH-ZSM-5). The characteristics of the parent and modified zeolites are given in the Table. The alkylation of toluene with ethylene was carried out in a vapour phase continuous flow microreactor at atmospheric pressure. Nitrogen as a carrier gas was saturated with toluene to 18.5 vol. % I the toluene to ethylene molar ratio was 3 . 8 . The reaction products were analyzed by an vlon-linevvgas chromatograph (Hewlett-Packard 5890) with MS and FID detection.

3. RESULTS AND DISCUSSION

It has already been reported that the conversion of toluene in toluene alkylation with ethylene on pure H-ZSM-5 zeolites with different Si/Al ratios is proportional and the para-selectivity is inversely proportional to the number of zeolite strong acid bridging OH groups [9,10]. MeH-ZSM-5 zeolites, containing Fe, Mn and A 1 mainly in the cationic sites in the inner channel intersections and with a lower number of OH groupsI exhibit correspondingly lower conversion in comparison with the parent H-ZSM-5 zeolite (Fig. 1 and the Table). It indicates that no significant contribution of metal cations to the zeolite activity in these zeolites was observed. However, the para-selectivity of MnH- and FeH-ZSM-5 was slightly higher than corresponded to the conversion vs. para-selectivity relationship for the pure H forms of ZSM-5 zeolites (see Fig. 1). The investigation of the reaction in time-on-stream (T-0-S) at relatively high space velocities (WHSV = 20 h-l) revealed that zeolites containing metal cations at the cationic sites were

350 Table Characteristics of zeolites and their conversion and para-ethyltoluene selectivity in the toluene alkylation witlh ethylene after 15 minutes of time-on-stream (T 620K,WHSV 10 hT/E = 3.8). Zeolitea OH groupsb metal (mmol/g) cation

f

conversion

(%I

(wt.% )

p-ETf coke amount’ selectivity (mg/g) (wt.%)

H-ZSM-5

0.72

-

25.4

33.4

1.34

FeH-ZSM-5

0.63

0.242

24.1

48.4

3.34

A1H-ZSM-5

0.67

0.2oe

23.5

36.1

2.37

MnH-ZSM-5

0.63

0.293

24.7

39.6

3.47

25.2

32.0

18.8

83.1

Mn304H-ZSM-5 0.64

10.80

S iH-ZSM-5

0.58

FeSiH-ZSM-5

0.58

19.9

58.2

SiFeH-ZSM-5

0.51

20.6

88.4

H-ZSM-5A

0.30

16.8

55.3

H-ZSM-5B

0.02

6.7

85.0

%i/Al

d

1.50e

= 22.5 (H-ZSM-5 and related zeolites), 45 (H-ZSM-5A), 600 (H-ZSM-5B) bestimated from temperature programmed desorption of ammonia (see.ref.9) Cmg coke per g of a zeolite after the alkylation of toluene with ethylene for T-0-S of 200 minutes dcalculated value from the chemical analysis emeans the amount of A1 or S i added to the H-ZSM-5 zeolite ’for simplicity toluene conversion and p-ET selectivity are presented complete aromatic product composition was (wt.%): B 0.2, T 68.5, EB 0.47, pX 0.26! mX 0.25, OX 0.07, PET 9.46, mET 17.34, oET 0.7, diEB 0.6, Clo 0.9

351

- 75 - 50 I-

- 25

W

n

i 0.5

0.25

0.75

OH groups

20

10

(mmo~/~)

Conversion

30

(%)

Fig. 1 Alkylation of toluene with ethylene at WHSV 10 h-l, 6 2 0 K, after 15 minutes in T-0-S on ZSM-5 zeolites. A ) Dependence of toluene conversion and p-ET selectivity on the number of strong acid OH groups, B) Dependence of p-ET selectivity on toluene conversion H-(O,.), MnH-(O,.), FeH-(A,4), AlH-(V,v), Mn304H- (0,4), SiH- (@,el, FeSiH- ( 0 , O ) , SiFeH-(0,O). A

30 -

B

-

w

IT\

3

33 c

8

-75 u

- _ h

3

u

s20-

Y

.-C0In

/ a - Q

L aJ

>

g

0

10-

r

a I

-50

--

a W

2

U

-YaJ

0

-25

U

v,

t; CL I

I

1

I

I

I

I

I

I

I

Time-on-stream (min)

Fig. 2 Alkylation of toluene with ethylene at WHSV 10 h-l, 6 2 0 K, in dependence on T-0-S on Z S M - 5 zeolites. A l Toluene con) , FeH-( 8 ) , version,-B) p-ET selectivity. H-( e ) , FeSiH-( SiFeH-( @ ) . SiH-( ),

a

6

352

deactivated to a larger extent in comparison with the H-ZSM-5 zeolite. The following toluene conversion decrease within T-0-S of 200 minutes has been observed for H-ZSM-5 (79.8-77.6), FeH-ZSM-5 (79.5-72.5) , MnH-ZSM-5 (74.2-64.1) and A1H-ZSM-5 (78.6-74.1). This was in agreement with a higher amount of Ifcokedeposits" formed with MeH-ZSM-5 zeolites compared to that formed with H-ZSM-5 in the course of reaction under the same conditions and with similar conversion values (the Table). However, despite of the higher amount of coke deposits on MeH-ZSM-5 zeolites, no significant decrease in the conversion was observed at WHSV = 10 h-l (Fig. 2). A substantially higher para-selectivity was observed for silylated SiH-ZSM-5 zeolite (Fig. 1). However, a lower conversion of toluene was found than would correspond to the number of strong acid OH groups (estimated by ammonia desorption) present in the silylated SiH-ZSM-5 zeolite (Fig. 1). Because of the large molecule of tetraethyl orthosilicate used for silylation, only the "surface" OH groups (or those in the mouth of the zeolite channels) were captured by silicon. A s the number of strong acid OH groups (ca 15 % lower than the value for the parent H-ZSM-5 zeolite) is higher than would correspond to the toluene conversion value, it can be assumed that some of the bridging OH groups accessible to ammonia are not accessible to reactants. Therefore, even though a very low amount of Si was added, the silylation most likely caused plugging of some zeolite pore openings and/or considerable decrease in the free diameter of the zeolite channel mouths. Similar results were obtained for the SiFeH-ZSM-5 zeolite, where the surface strong acid sites were poisoned by subsequent silylation of the Fe ion-exchanged zeolite. Similarly, the para-selectivity of SiFeH-ZSM-5 was substantially increased compared with the FeH-ZSM-5 zeolite. The combined effect of higher para-selectivity in the initial alkylation step (due to the presence of Fe ions) and suppression of isomerization reaction on the outer surface on the resulting ethyltoluene para-selectivity is likely (see Figs. 1, 2 and the Table). On the other hand, when some Fe cations were attached to the zeolite surface covered by Si

353

(assuming that the Fe ion-exchange did not occur to a larger extent inside the zeolite) the para-selectivity of this sample was considerably decreased in comparison with SiH-ZSM-5. Similarly, the Mn304H-zeolite containing Mn cations in the channels at the cationic sites as well as on the outer surface exhibits significantly lower p-ethyltoluene selectivity than MnH-ZSM-5 having Mn2+ only at the cationic sites. It follows from the above results that the presence of metal cations mostly in the zeolite intersections (MeH-ZSM-5) increases the zeolite para-selectivity, while their contribution to the zeolite alkylation activity is not significant. On the other hand, the metal cation presence in the "surface" sites (FeSiH-ZSM-5, Mn304H-ZSM-5) substantially enhances the zeolite isomerization activity. A s the para-selectivity of the MeH-ZSM-5 zeolites is higher than that of the pure H-ZSM-5, the metal cation location in the channel intersections should cause steric hindrances owing to the diameter of the metal cations (Fe3+ 0.64 8 , Mn2+ 0.80 8 and A13+ 0.50 2); this effect (which is also necessarily affected by the number of cations present in the zeolite) was not found with the relatively small A 1 cation. Then the contribution of the metal cation to the transport limitation of the bulkier isomers and/or to the selectivity of the initial alkylation step should exceed the contribution of the cations to the isomerization activity. The presence of isolated metal cations in the zeolite channels also plays an important role in the oxidation of coke formed during the alkylation of toluene with ethylene. The facility of coke burning was characterized by the initial temperature at which the coke started to be removed as CO and C 0 2 and the temperature for the CO and C 0 2 concentration maxima. It appears that Mn and especially F e cations enhance significantly coke burning. The initial temperature for coke oxidation and the concentration maxima for CO and C 0 2 evolution were 470, 590 and 780 K, resp., for FeH-ZSM-5 and 5 2 0 , 765 and 8 3 0 K, resp., for pure H-ZSM-5. On the other hand, A1 cations apparently slightly retard coke oxidation (CO is evolved at higher temperatures) likely owing to steric hindrances.

354 4. CONCLUSION

It can be summarized that the presence of metal cations in zeolites, exhibiting redox properties, increases both the zeolite para-selectivity and the burning off coke deposits, however, their electron acceptor properties enhance slightly coke formation during the alkylation reaction. Depending on the location of the metal cations in the zeolite structure, they may strongly affect the zeolite para-selectivity and contribute much more to ethyltoluene isomerization, than to the alkylation reaction. When metal cations are lodged on the zeolite surface they substantially enhance the ethyltoluene isomerization reaction. On the other hand, their location in the zeolite channel intersections, causes greater steric hindrances because of their larger diameter in comparison with protons, resulting in a higher para-selectivity of the zeolite. Finally, it can be stated that even small crystals of H-ZSM-5 zeolites, when properly modified both by metal cations at cationic sites with redox properties and by subsequent silylation, can give a catalyst exhibiting the paraselectivity exceeding 9 5 % at a high conversion level and, moreover, enabling coke removal at relatively low temperatures. 5. REFERENCES

1.

W.W. Kaeding, C. Chu, L.B. Young and S.A. Butler, J.Catal.67,

2. 3.

W.W. Kaeding, L.B. Young and C. Chu, J.Catal. 89 ( 1 9 8 4 ) 2 6 7 . W.W. Kaeding, C. Chu, L.B. Young and S.A.Butler, J.Cata1. 69

4.

L.B.Young, S.A.Butler, W.W.Kaeding, J.Cata1.X ( 1 9 8 2 ) 4 1 8 . W.W. Kaeding, G.C. Barile and M.M. Wu, Catal.Rev.Sci.Eng. 26

(1981)

(1981)

5.

(1984)

,

159.

392.

597.

J. Wei, J.Cata1. 76 ( 1 9 8 2 ) 4 3 3 . I. Wang, C. Ay, B.Lee M. Chen, Appl. Catal. 54 ( 1 9 8 9 ) 2 5 7 . P. Ratnasamy and S.K. Pokhriyal, Appl. Catal. 55 ( 1 9 8 9 ) 2 6 5 . J. Cejka, B.Wichterlova, S.Bednarova, Appl. Catal., in press. J. Cejka, B. Wichterlova and G.L. Raurell, Stud. Surf. Sci. Catal., in press. 11. S. Beran, B. Wichterlova and H.G. Karge, J.Chem.Soc., Faraday Trans. I, 86 ( 1 9 9 0 ) 3 0 3 3 .

6. 7. 8. 9. 10.

355

P.A. Jacobs et al. (Editors),Zeolite Chemistry and Catalysis 0 1991 Elsevier Science Publishers B.V., Amsterdam

NO DECOMPOSITION ON CU-INCORPORATED A-ZEOLITES

UNDER THE REACTION

CONDITION OF EXCESS OXYGEN WITH A SMALL AMOUNT OF HYDROCARBONS

Tomoyuki I N U I , IWAMOTO

S h i n i c h i KOJO,

Masashi SHIBATA, Takashi YOSHIDA,

Department o f Hydrocarbon Chemistry, U n i v e r s i t y , Sakyo-ku, Kyoto 606 (Japan)

and S h i n j i

F a c u l t y o f Engineering,

Kyoto

SUMMARY Copper c o n t a i n i n g A - t y p e z e o l i t e s w h i c h c o n t a i n e d copper w i t h c o n s i d e r a b l y h i g h c o n c e n t r a t i o n s were s y n t h e s i z e d t h r o u g h c r y s t a l 1 i z a t i o n . I t was c o n f i r m e d t h a t Cu' i o n s i n t h e c r y s t a l s c o u l d be s t a b l y m a i n t a i n e d compared w i t h t h o s e i n t h e c o p p e r - l o a d e d s a m p l e s p r e p a r e d b y an i o n NO d e c o m p o s i t i o n a c t i v i t y o n t h e CU-A c a t a l y s t e x c h a n g e d method. c o r r e s p o n d e d t o t h e c a p a c i t y o f r e d o x response. Even u n d e r an e x c e s s oxygen c o n d i t i o n t h e NO d e c o m p o s i t i o n progressed smoothly a t around 300 350°C b y t h e a d d i t i o n o f a v e r y s m a l l e x p l a i n these unusual n o n - l i n e a r r e a c t i o n phenomena, M i c r o s c o p i c S e q u e n t i a l R e a c t i o n mechanism was p r o p o s e d and t h e n e c e s s a r y c o n d i t i o n s t o r e a l i z e t h i s mechanism were discussed. INTRODUCTION D i r e c t d e c o m p o s i t i o n o f NO w i t h o u t u s a g e o f a n y r e d u c t a n t has been an o u t s t a n d i n g t a s k o f c a t a l y s t s t u d y f o r NO r e m o v a l f r o m e x h a u s t gases, e s p e c i a l l y w h i c h comes f r o m d i e s e l engines. oxygen;

Some k i n d s o f reduced m e t a l -

can decompose NO t o n i t r o g e n and

o r p a r t l y reduced m e t a l - o x i d e - c a t a l y s t s

however, t h e oxygen formed i s i m m e d i a t e l y adsorbed on t h e s u r f a c e

of t h e c a t a l y s t and d e a c t i v a t e s i t (ref. t h e r e a c t a n t gas,

7).

When oxygen i s c o n t a i n e d i n

t h e c a t a l y s t i s e a s i l y o x i d i z e d and deactivated.

The c a t a l y t i c r e d u c t i o n o f NO w i t h NH3 i s w i d e l y adopted f o r NO removal i n t h e s t a t i o n a r y generators.

I n t h i s r e a c t i o n , c o e x i s t e n c e o f oxygen

w i t h a p r o p e r c o n c e n t r a t i o n r a t h e r enhances t h e NO r e d u c t i o n :

however,

a

l a r g e excess o f oxygen s t i l l d e a c t i v a t e s t h e c a t a l y s t and t h e NH3 i s a p t t o b u r n b e f o r e t h e r e a c t i o n w i t h NO.

Therefore,

e v e n t h e NH3 r e d u c t i o n

m e t h o d c a n n o t b e a d o p t e d f o r t h e r e m o v a l o f NO i n t h e e x h a u s t gas f r o m d i e s e l e n g i n e s , i n w h i c h a l a r g e e x c e s s oxygen, as h i g h as 13%, r e m a i n s

356 unconsumed.

i t was reported (ref.

Recently,

H-ZSM-5

2) t h a t an excessively Cu-ion-exchanged

e x h i b i t e d an NO decomposition a c t i v i t y under 02-absent c o n d i t i o n

w i t h o u t s i g n i f i c a n t deactivation: concentration 02 i n t h e feed gas,

however,

w i t h coexistence o f even a low

t h e a c t i v i t y could n o t be exerted.

I n t h i s study, i n o r d e r t o overcome t h e s e d i f f i c u l t problems, a n o v e l r e a c t i o n mechanism,

M i c r o s c o p i c S e q u e n t i a l R e a c t i o n mechanism (MSR The MSR mechanism was b u i I t f r o m t h e v i e w

mechanism), was considered.

p o i n t t h a t t h e e s s e n t i a l p r o p e r t y o f t h e r e a c t i o n on t h e s o l i d c a t a l y s t must be non-1 i n e a r phenomena i n v o l v i n g m i c r o s c o p i c a l l y sequential r e a c t i o n processes,

which are d i f f e r e n t from the conventional Langmuir-Hinshelwood

r e a c t i o n mechanism based on l i n e a r phenomena. mechanism and achieve t h e NO decomposition,

To r e a l i z e t h e MSR

novel metal c o n t a i n i n g z e o l i t e

c a t a l y s t s were prepared and t h e r e a c t i o n c o n d i t i o n s were investigated. EXPERIMENTAL Catalyst Four study.

k i n d s o f c o p p e r c o n t a i n i n g z e o l i t i c c a t a l y s t s were used i n t h i s The a b b r e v i a t i o n and b r i e f explanation o f t h e c a t a l y s t s are l i s t e d

i n T a b l e 1. Characterization Behaviors o f redox treatments a t 250°C and temperature programmed r e d u c t i o n (TPR) f o r t h e p r e - o x i d i z e d samples were measured by a TG-DTA Shi madzu Thermal Analyzer DT-30.

For t h e measurements o f redox responses,

a 20 mg p o r t i o n o f t h e sample was p l a c e d i n a sample pan, and 5% 02 o r 5%

H2 d i l u t e d w i t h N p was allowed t o f l o w w i t h a feed r a t e o f 40 ml/min.

The

amounts o f O2 o r H2 supplied was s u f f i c i e n t l y excess f o r t h e o x i d a t i o n o r The TPR was measured f o r t h e same amount of

r e d u c t i o n o f t h e samples.

samples w i t h a constant h e a t i n g r a t e o f 10"C/min. Reaction The c a t a l y s t i n powder form was t a b l e t e d w i t h a t a b l e t machine. crashed and s i e v e d t o 1 5

-

24 mesh t o p r o v i d e t o t h e r e a c t i o n .

It was

A 0.5 g

(ca. 0.7 m l ) p o r t i o n o f t h e c a t a l y s t was packed i n a quartz t u b u l a r r e a c t o r o f 8 mm i n n e r diameter.

The c a t a l y s t - b e d

l e n g t h was 1.4 cm.

The

357 TABLE 1 Copper c o n t a i n i n g z e o l i t i c c a t a l y s t s used. Catalyst

Abridged notation

Description

No. Cat. 1

Cu/H-ZSM-5

H-ZSM-5 having S i / A 1 a t o m i c r a t i o 40 prepared b y t h e r a p i d c r y s t a l l i z a t i o n method (ref. 3) was by u s i n g 0.5 m o l a q u e o u s ion-exchanged s o l u t i o n o f Cu n i t r a t e a t room temperature. The Cu l o a d i n g was 1.00 w t % w h i c h c o r r e s p o n d e d t o 80% ion-exc han ged.

Cat, 2

Cu-silicate

MFI-type C u - s i l i c a t e prepared by t h e r a p i d c r y s t a l 1 i z a t i o n method. The Cu c o n t e n t was 0.57 w t % .

Cat. 3

Cu/NaA

The c o m p o s i t i o n o f m i x e d g e l was S i / A 1 r a t i o = 1, N a / A 1 r a t i o 5. I t was h y d r o t h e r m a l l y c r y s t a l l i z e d a t 85°C f o r 6 h, c a l c i n e d a t 430°C f o r 1.5 h. I t was i o n - e x c h a n g e d b y Cu n i t r a t e aqueous s o l u t i o n a t r o o m t e m p e r a t u r e . The Cu l o a d i n g was 18.2 w t % w h i c h c o r r e s p o n d e d t o 84% ion-exchanged.

Cat. 4

Cu-NaA

Added Cu n i t r a t e t o t h e p r e p a r a t i o n p r o c e d u r e f o r NaA d e s c r i b e d on Cat. 3. The Cu c o n t e n t was 8.6 w t % .

c a t a l y s t was u s e d f o r t h e r e a c t i o n a f t e r d r y i n g i n an He f l o w a t 400°C. The r e a c t i o n gas was i n t r o d u c e d a t t e m p e r a t u r e range f r o m 200 t o 500°C w i t h an SV r a n g e f r o m 500 t o 2500 h-’.

The r e a c t i o n gases and p r o d u c t s w e r e

analyzed by u s i n g a gas chromatograph equipped w i t h an i n t e g r a t o r . RESULTS AND DISCUSSION Comparison

of c a t a l y t i c

p r o p e r t y between Cu-ion-exchanged-ZSM-5

and Cu-

i n c o r p o r a t e d MFI-type s i l i c a t e Cu/H-ZSM-5

(Cat.

as t h a t o f H-ZSM-5, oxides.

and no i n d i c a t i o n f o r t h e e x i s t e n c e o f i s o l a t e d copper

NO conversion was measured on b o t h c a t a l y s t s under t h e 02-absent

condition. h-l

1) and C u - s i l i c a t e (Cat. 2) gave t h e XRD p a t t e r n s same

4% NO d i l u t e d w i t h N2 was f e d t o t h e r e a c t o r w i t h a SV 2000

a t 500°C and C h a t t e m p e r a t u r e was m a i n t a i n e d f o r 8 h.

c o n v e r s i o n s a t t h e s t e a d y s t a t e on Cats. respectively.

The

NO

1 and 2 w e r e 42% and 12%.

The i n t e g r a t e d amount o f NO c o n v e r t e d t i l l 8 h on s t r e a m

358 1 and 2 were 85 t i m e s and 54 t i m e s o f t h e Cu q u a n t i t i e s ,

f o r Cats.

r e s p e c t i v e l y , i n d i c a t i n g t h a t t h e s e c a t a l y s t s had t h e NO d e c o m p o s i t i o n acttvity.

Under an 0 2 - p r e s e n t c o n d i t i o n , b o t h c a t a l y s t s a c c e p t e d t h e

o x i d a t i o n o f copper,

and no NO c o n v e r s i o n a c t i v i t y was e x h i b i t e d any

more. I n order t o c o n f i r m t h e d i f f e r e n c e i n s t a b i l i t y o f both catalysts,

the

CO o x i d a t i o n t e s t according t o t h e Forced-Oscillating r e a c t i o n method (ref.

4) was adopted.

By t h i s method,

t h e redox processes o f c a t a l y s t surface

d u r i n g t h e r e a c t i o n can be r e a l i z e d forcedly, therefore,

through examining

t h e redox response repeatedly, t h e s t a b i l i t y o f c a t a l y s t s can be evaluated. As a r e s u l t , t h e r e d o x c y c l e s f o r Cat. 2 were shown v e r y r e p r o d u c i b l y , i n d i c a t i n g t h a t t h e c o p p e r p a r t can be s t a b l e t h r o u g h t h e r e d o x c y c l e s . On t h e o t h e r hand, as f o r Cat. 1 a w i d t h o f t h e h y s t e r e s i s became n a r r o w w i t h an i n c r e a s e o f number o f t h e r e d o x cycle.

This indicates t h a t the

s i n t e r i n g o f t h e i r copper p a r t would progress.

Further,

the temperature

dependence o f CO conversion on Cat. 2 was much sharp compared w i t h t h a t on Cat. 1, b o t h i n t e m p e r a t u r e r i s i n g and l o w e r i n g .

T h i s corresponds t o a

h i g h d i s p e r s i o n o f Cu and a very narrow Cu-particle d i s t r i b u t i o n range i n Cat. 2.

This was supported by t h e d i f f e r e n c e o f e f f e c t i v e pore-diffusion

c o e f f i c i e n t between Cats.

1 and 2 , i.e.,

t h e l a t t e r was t w i c e o f t h e

former. C h a r a c t e r i s t i c s o f Cu-containing z e o l i t e s S i n c e i t was supposed t h a t a z e o l i t e , w h i c h c o n t a i n s l a r g e r amount o f Cu,

would have a h i g h e r p o t e n t i a l f o r NO decomposition,

f o r v a r i o u s k i n d s o f z e o l i t e s was i n v e s t i g a t e d .

Cu i n c o r p o r a t i o n

As a r e s u l t , 8.6 w t X Cu

c o u l d be u n i f o r m l y i n c o r p o r a t e d i n t o t h e c r y s t a l s o f z e o l i t e s A(Cat. 4). The NO decomposition a c t i v i t y o f Cat. 4 a t 350 C was the same as Cat. 2 a t 500°C.

The a c t i v i t y p e r Cu i n v o l v e d i n Cat. 4 was l o w e r than t h a t o f Cat.

2; however, t h e a c t i v i t y p e r c a t a l y s t volume o f Cat. 4 was 5.5 t i m e s t h a t o f Cat. 2. Since the s t a b i l i t y o f i n t e r m e d i a t e o x i d a t i o n - s t a t e o f Cu i s one o f t h e key p o i n t s t o r e a l i z e t h e NO decomposition as shown above, TPR response f o r The Cu-ion-exchanged NaA (Cat. 3) showed 270 C h a v i n g a s h o u l d e r a t 240°C. On t h e o t h e r hand, t h e C u - c o n t a i n i n g NaA (Cat. 4) showed t w o d i s t i n c t peaks appeared a t 160 various c a t a l y s t s were compared. one peak a t around

359 and 250°C.

The h i g h e r t e m p e r a t u r e one was near t o t h e s i n g l e peak o f Cat.

3; h o w e v e r , t h e l o w e r one was f a r f r o m t h e s h o u l d e r o f Cat. 3.

The t w o

p e a k s o f Cat. 4 c o r r e s p o n d e d t o t h e c h a n g e f r o m CuO t o Cu20 and f r o m Cu20 t o Cu, r e s p e c t i v e l y .

I n case o f t h e CuO supported by ion-exchange method

(Cat. 3 ) i t was somewhat d i f f i c u l t t o r e d u c e compared w i t h Cat.4, r e d u c t i o n s h i f t e d t o h i g h e r temperature:

however,

and t h e

once r e d u c t i o n began i t

progressed s u c c e s s i v e l y f r o m CuO t o Cu w i t h o u t showing a s t a b l e Cu20 state. These p r o p e r t i e s r e f l e c t t h a t i n t h e c a s e o f t h e i o n - e x c h a n g e m e t h o d Cu f o r m s c o n s i d e r a b l y l a r g e r c l u s t e r s o r t h a t t h e Cu p a r t i c l e s b l o c k t h e p a r t o f p o r e c h a n n e l s , and r e t a r d t h e d i f f u s i o n o f h y d r o g e n and f o r m e d w a t e r , r e s u l t i n g t h e d e l a y o f hydrogen r e d u c t i o n . Effect

of

hydrocarbon a d d i t i o n on NO c o n v e r s i o n under

an

excess

oxygen

condition The s e n i o r a u t h o r e t a l . ( r e f . 5 ) r e p o r t e d p r e v i o u s l y t h a t t h e o r d e r o f r a t e c o n s t a n t f o r hydrogen r e d u c t i o n o f t h e p r e o x i d i zed s u p p o r t e d c o p p e r o x i d e w i t h v a r i o u s k i n d s o f r e d u c t a n t s were: CO > H2 > CH4 > C3H8, and t h i s o r d e r was t h e same as t h e o r d e r o f t h e r a t e o f O2 a d s o r p t i o n t o t h e reduced s u r f a c e s w i t h these reductants.

T h i s means t h a t t h e s t a t e s o f t h e reduced

s u r f a c e s a r e changeable w i t h t h e k i n d s o f r e d u c t a n t s s u g g e s t i n g t h a t t h e i m p o r t a n c e o f t h e m i c r o s c o p i c change o f t h e s u r f a c e state. Furthermore,

-

we have a l r e a d y s t u d i e d t h e c a t a l y t i c combustion r a t e o f C1

C14 s t r a i g h t c h a i n s a t u r a t e d h y d r o c a r b o n s on a s u p p o r t e d Pt-Ce02

catalyst.

I t was f o u n d t h a t t h e c o m b u s t i o n r a t e s o f c a r b o n number C7

C10 h y d r o c a r b o n s w e r e maximum among them,

-

and t h a t t h o s e o f above C8

hydrocarbons g r a d u a l l y decreased w i t h an i n c r e a s e o f carbon number o w i n g t o t h e i n c o m p l e t e combustion ( r e f . 6).

I t was a l s o found t h a t t h e h y s t e r e s i s

i n t h e f o r c e d o s c i l l a t i n g r e a c t i o n t e s t l a r g e l y d i f f e r e n t f r o m each other. I t i s c o n s i d e r e d t o b e n e c e s s a r y t h a t t h e a m o u n t o f h y d r o c a r b o n s added

s h o u l d d i s t r i b u t e i n a c a t a l y s t bed b e f o r e t h e c o m b u s t i o n a s w i d e l y as possible t o play the role effectively. hydrogen and carbon monoxide,

The o t h e r r e d u c t a n t s s u c h as

w h i c h combust t o o e a s i l y ,

give l i t t l e effect

t o t h e o b j e c t i v e r e a c t i o n because t h e s e r e d u c t a n t s a r e consumed j u s t a t t h e e n t r a n c e o f t h e c a t a l y s t bed. According t o t h i s c o n s i d e r a t i o n , t h e NO decomposition i n t h e presence o f e x c e s s 02 o n Cu-NaA(Cat.

4) was s t u d i e d w i t h an a d d i t i o n o f l o w

360 c o n c e n t r a t i o n s t r a i g h t c h a i n s a t u r a t e d C2

-

n-C,

hydrocarbons.

The

t e m p e r a t u r e dependence o f NO c o n v e r s i o n r o u g h l y c o r r e s p o n d e d t o t h e temperature dependence o f the combustion r a t e o f hydrocarbon added. From these r e s u l t s , chosen,

i t was expected t h a t when a proper hydrocarbon was

t h e NO decomposition c o n d i t i o n and t h e r e d u c t i o n c o n d i t i o n o f an

o x i d i z e d c a t a l y s t surface could be adjusted, $6

and then n-C8,

n-ClO,

and n-

saturated hydrocarbons were selected as t h e hydrocarbons t o be added

here.

The amount o f added hydrocarbons was s e t a t about 0.6 molar r a t i o

o f complete combustion stoichiometry.

As shown i n Fig. 1, t h e o r d e r o f

m a g n i t u d e o f t h e NO c o n v e r s i o n was n-cs < n-$O 350°C each NO conversion a t t a i n e d maximum. (n-C16)

addition,

< n-CI6.

and around 300

-

Especially, i n case o f cetane

t h e c o m p l e t e NO c o n v e r s i o n was a c h i e v e d a t t h a t

temperature range, and moreover, even a t temperature range above 350"C, t h e degree o f decrease i n t h e NO conversion was v e r y l i t t l e compared w i t h o t h e r cases.

As shown i n Fig. 2 t h e conversions o f hydrocarbons t o C02 and

H20

d u r i n g t h e NO conversion were detected above ca. 200°C and these increased e x p o n e n t i a l l y up t o ca. 300°C. s u d d e n l y s l o w e d down.

and above t h a t t e m p e r a t u r e t h e i n c r e a s e

I t i s n o t e w o r t h y t h a t t h e o r d e r o f NO c o n v e r s i o n

was i n v e r s e o f t h e o r d e r o f h y d r o c a r b o n c o n v e r s i o n f o r t h e k i n d o f As shown i n Fig. 3, when NO was n o t i n v o l v e d i n t h e

hydrocarbons added.

r e a c t i o n gas t h e c o n v e r s i o n o f each h y d r o c a r b o n i n c r e a s e d e x p o n e n t i a l l y w i t h an increase o f t h e r e a c t i o n temperature and reached a t 100% conversion u n t i l 300

-

350°C.

and above t h a t t e m p e r a t u r e t h e t o t a l c o n v e r s i o n was

maintained, t h a t was d i f f e r e n t from t h e case o f coexistence o f NO. As can be u n d e r s t o o d f r o m t h e c o m p a r i s o n between Figs. 1 and 2, t h e increase o f NO conversion up t o 300°C was markedly l a r g e r than t h e increase o f hydrocarbon conversion,

a1though t h e temperature range f o r increase o f

each c o n v e r s i o n c o i n c i d e d .

T h i s suggested t h a t t h e NO c o n v e r s i o n

progresses e f f e c t i v e l y w i t h coexistence o f a l e s s amount o f hydrocarbon on a considerably small

number o f a c t i v e s i t e s .

The d e c r e a s e i n NO

conversion a t higher temperature must be a t t r i b u t e d t o t h a t t h e o x i d a t i o n of the c a t a l y s t surface progresses predominantly and the a c t i v e s i t e s f o r

NO conversion diminish. I n case o f t h e c e t a n e a d d i t i o n , products such as aldehydes, 2-ketones,

s m a l l amounts o f p a r t i a l o x i d a t i o n a-a1 kylfuranes, etc.,

were detected.

I t i s c o n s i d e r e d t h a t t h e s e p r o d u c t s s t r o n g l y adsorbed on t h e c a t a l y s t

361

I

t

I

300

I 400

I

I

500

Temperature ("C) F i g . 1. E f f e c t o f k i n d o f h y d r o c a r b o n s added o n NO c o n v e r s i o n u n d e r an e x c e s s o x y g e n c o n d i t i o n . Cat. : Cu-NaA(Cat.4). NO 9600ppm, 02 11.0%, SV, 0 : n-CgH18 6500ppm. a: n-C10H22 4100ppm. m : n-C16H34 2600pprn. 2500 h-',

I

300 Temperature

400

I

I

500

("C)

F i g . 2. E f f e c t of t e m p e r a t u r e on h y d r o c a r b o n c o m b u s t i o n d u r i n g t h e NO c o n v e r s i o n shown i n Fig. 1.

362

s u r f a c e and r e t a r d e d a d e e p o x i d a t i o n o f t h e c a t a l y s t s u r f a c e , consequently,

t h e decrease o f t h e NO c o n v e r s i o n would be moderate even a t

t h e h i g h e r t e m p e r a t u r e range. S i n c e i t was s u g g e s t e d as m e n t i o n e d above t h a t a v e r y s m a l l amount o f h y d r o c a r b o n s was s t i l l e f f e c t i v e f o r t h e NO c o n v e r s i o n , t h e n we t r i e d t o reduce t h e c o n c e n t r a t i o n o f cetane added, 4.

and t h e r e s u l t s a r e shown i n Fig.

The m o l a r r a t i o s o f c e t a n e added t o t h e c o m p l e t e c o m b u s t i o n

s t o i c h i o m e t r y were, 0.56, 2600,

700,

and 190ppm,

0.15, and 0.04 respectively.

f o r t h e concentration o f cetane The d e g r e e o f d e c r e a s e i n NO

c o n v e r s i o n was v e r y 1 i t t l e , c o n s i d e r i n g t h e d e c r e a s e o f t h e c o m b u s t i o n s t o i c h i o m e t r y , and even 190ppm cetane a d d i t i o n , s t i l l 50% NO c o n v e r s i o n was realized. C o n s i d e r a t i o n on t h e r e a c t i o n mechanism The r e a c t i o n mechanism i n w h i c h NO c a n b e decomposed e v e n u n d e r t h e c o e x i s t e n c e o f excess oxygen w i t h t h e c a t a l y t i c combustion o f a v e r y s m a l l amount o f h y d r o c a r b o n o f a c o n s i d e r a b l y l a r g e c a r b o n number,

can be

c o n s i d e r e d as f o l l o w s ;

By c o n s i d e r i n g t h e successive o c c u r r e n c e o f t h e c o n s e c u t i v e r e a c t i o n s (1-1). (1-2) a n d (2-1). (2-2). and p a r a l l e l r e a c t i o n s (1-3) and (3-1). t h e whole e x p e r i m e n t a l r e s u l t s can be reasonably understood.

363

Temperature ( " C ) Fig. 3. C a t a l y t i c combustion of n-cg, n-Cl0, and n-C16 hydrocarbons. Cat. : Cu-NaA(Cat.4), S V 2500 h-', O 2 12.0%, 0 : n-CgH18 4600ppm. 0: n-C10H22 4800ppm, M: n-C16H34 3400ppm.

w v

0

I

200

I

I

300

I

I 400

-I e n o e r a t u r e ("C)

I

I 500

Fig. 4. E f f e c t of cetane c o n c e n t r a t i o n on NO conversion. Cat. : Cu-NaA(Cat.4). NO 9600ppm, O2 11.0%, SV 2500 h-'. Cetane; 0 : 2600ppm. 0 : 700ppm, A: 190pprn.

364 I n o t h e r words,

a p r o p e r hydrocarbon,

w h i c h adsorbs on t h e oxygen-

adsorbed c a t a l y s t s u r f a c e , combusts w i t h consuming t h e oxygen on t h e surface explosively.

Successively,

t h e combustion products.

C02 and HZO.

d e s o r b and t h e a c t i v e s i t e s f o r NO d e c o m p o s i t i o n a r e recovered. recovered a c t i v e s i t e s would be o x i d i z e d by t h e oxygen:

however,

The

NO can be

adsorbed and decomposed on t h e a c t i v e s i t e s a t a p r o p e r t e m p e r a t u r e . T h i s r e a c t i o n mechanism i s based on t h e u n d e r s t a n d i n g o f t h e non-1 i n e a r phenomena l i k e t h e o s c i l l a t i n g r e a c t i o n on t h e s o l i d c a t a l y s t surface (ref. 7).

Therefore,

we propose t o c a l l t h i s mechanism Microscopic Sequential

R e a c t i o n mechanism smoothly,

(MSR mechanism).

F o r t h i s mechanism t o o p e r a t e

t h e f o l l o w i n g c o n d i t i o n s are necessary.

(i)m e t a l oxides,

w h i c h a r e c o n s i d e r a b l y easy t o be o x i d i z e d and

reduced, are supported w i t h a h i g h l y d i s p e r s i o n b u t s t a b l y on a microporous crystal. (ii) the r e a c t i o n r a t e s o f

NO

decomposition and t h e combustion r a t e o f

hydrocarbons added are comparable a t around 300

- 400°C.

I n order t o s a t i s f y these c o n d i t i o n s i n t h e l i g h t o f t h e new mechanism i t i s expected t h a t many o t h e r new c a t a l y s t s w i l l be able t o be developed.

REFERENCES H. Niiyama, K. Sasamoto. S. Yoshida, and E. Echigoya, J. Chem. Eng. Jpn.. 14(4) (1981) 301-306. 2 M. Iwamoto, H. Yahiro, T. Yoshioka, and N. Mizuno, Chem L e t t . 1990(11), 1967-1 970. 3 T. I n u i , Mechanism o f Rapid Z e o l i t e C r y s t a l l i z a t i o n s and i t s A p p l i c a t i o n t o C a t a l y s t S y n t h e s i s , in: M. L. O c c e l l i and H. E. Robinson (Ed.), Z e o l i t e Synthesis :ACS Symp. Series, Vol. 398, 1989, pp. 479-492. 4 T. I n u i , H. Wakita, and H. Fukuzawa, A n a l y s i s on C h a r a c t e r i s t i c s o f Supported Pd, P t , and Rh i n Methane Combustion by the Forced O s c i l l a t i n g R e a c t i o n Method, i n : Y. Morooka (Ed.), MRS I n t e r n a t i o n a l M e e t i n g on Advanced Materials, Vol. 2, 1989, pp. 271-176. 5 T. I n u i , T. Ueda, and M. Suehiro, J. Jpn. Chem. SOC., 1977(7), 934-940. 6 T. I n u i , Y. Adachi, T. Kuroda, M. Hanya, and A. Miyamoto. Chem. Express, 1( 4 ) ( 1986) 255-258. 7 T. I n u i and T. Iwana, A n a l y t i c a l S t u d y o f an O s c i l l a t i n g R e a c t i o n on Copper C a t a l y s t s and i t s S i m u l a t i o n , i n : S. K a l i a g u i n e and A. Mahay (Ed.), Studies i n Surface Science and C a t a l y s i s 19, Elsevier, Amsterdam, 1984, pp. 205-212. 1

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 0 1991Elsevier Science Publishers B.V., Amsterdam

A

comparison o f t h e

365

and HY z e o l i t e i n

c a t a l y t i c p r o p e r t i e s o f SAPO-37

t h e c r a c k i n g o f n-heptane and 2,2,4-trimethylpentane J.M.

Lopesa, F. Lemosa, F. Ramaa R i b e i r o a and E.G.

Derouaneb

a Grupo de Estudos de Cata”l i s e Heterogknea, I n s t i t u t o S u p e r i o r Te‘cnico,

Av. Rovisco P a i s , ‘1096 L i s b o a Codex, P o r t u g a l Faculte‘s U n i v e r s i t a i r e s N.D.

de l a P a i x , L a b o r a t o r y o f C a t a l y s i s ,

Rue de B r u x e l l e s , 61, B-5000-Namur,

Belgium

Abstract The c a t a l y t i c a c t i v i t y o f t h e p r o t o n i c forms o f SAPO-37 and HY z e o l i t e were

compared

in

the

cracking o f

n-heptane

and

2,2,4-trimethylpentane.

HY z e o l i t e p r e s e n t s a h i g h e r i n i t i a l a c t i v i t y w h i c h i s i n agreement w i t h its

higher

o f ammonia. r e l a t i ve

acidity

characterized

by

temperature

programmed

desorption

T h i s i s c o n f i r m e d by t h e f a c t t h a t SAPO-37 e x h i b i t s a h i g h e r

cracking

activity

(2,2,4-trimethylpentane/n- heptane f

than

HY

zeo I it e . Cracking product d i s t r i b u t i o n s a r e very simi l a r f o r both c a t a l y s t s : C3 and C4 hydrocarbons

i n quasi

e q u i m o l a r amounts c o n s t i t u t e more t h a n

90% o f t h e c r a c k i n g p r o d u c t s and t h e i s o / n C q r a t i o always p r e s e n t s h i g h values.

1. INTRODUCTION S i licoaluminophosphates

crystal line

microporous

and phosphorus 11-31.

(SAPO’s)

molecular

framework

the

species,

of

SiIv

sieves,

a

containing

novel

class

silicon,

of

aluminurn

SAPO-37 i s i s o s t r u c t u r a l t o f a u j a s i t e , and has l a r g e

p o r e s and an a n i o n i c presence

constitute

12-41. The n e g a t i v e charge a r i s e s froin partially

substituting

Pv

in

a

neutral

366 aluminophosphate

framework

structure.

Thus,

i t i s possible t o

generate

Bronsted a c i d s i t e s upon c a l c i n a t i o n o f SAPO-37 i n t h e f o l lowing manner: t h e o r g a n i c c a t i o n s used a s t e m p l a t e agents d u r i n g t h e s y n t h e s i s ,

and

which remain i n t h e s t r u c t u r e compensating some o f t h e framework charges, a r e decomposed and generate

a c e r t a i n number o f p r o t o n i c s i t e s

a subsequent ion-exchange w i t h

+ NH4

cations

followed

by

14,5l;

calcination

can

lead t o an o c c u p a t i o n o f c a t i o n i c s i t e s a l m o s t e x c l u s i v e l y by protons. The

acidity

of

this

material

spectroscopy e x a m i n a t i o n o f

the

has

hydroxyl

been

confirmed

region

12,41

by

infrared

and a l s o t e s t e d

b y the n-butane c r a c k i n g r e a c t i o n 11,2/, showing an a c t i v i t y i n t e r m e d i a t e between t h a t o f aluminophosphates and z e o l i t e s . we w i l l

I n t h e p r e s e n t work,

compare 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 p r o t o n i c forms o f SAPO-37 and Y z e o l i t e f o r t h e c r a c k i n g o f n-heptane and o f 2,2,4-trimethylpentane.

C a r r y i n g o u t these two r e a c t i o n s w i I I g i v e

us

SAPO-37’s

a

better

knowledge

characterization

wi I I

of

also

be

made

acid

by

strength.

ammonia

Acid

temperature

strength programed

d e s o r p t i o n (TPD). The

data

presented here

for

Y

zeolite,

and which

i s given f o r

comparison purposes, concerns NaY (LZY-52 f r o m Union Carbide) and a p r o t o n i c form HY.

2. EXPERIMENTAL 2.1.

P r e p a r a t i o n and c h a r a c t e r i z a t i o n of t h e c a t a l y s t s SAPO-37 was synthesized by t h e s i n g l e phase method f r o m a g e l h a v i n g

the

f o I Iowing mo I a r composition:

(TMA)20:50H20

A 12O3:0.9P205

:O. 4Si 02:0.86( TPA)20:0.0250

based on example 43 o f t h e p a t e n t o f Lok e t a I.

is d e s c r i b e d e I sewhere I7 I

.

The m o l a r c o m p o s i t i o n o f SAPO-37

i s about 50% A l ,

I61 , and

sample used i n t h e p r e s e n t s t u d y

40% P and 10% S i as T atoms. The ammonium f o r m o f SAPO-

37 was o b t a i n e d by t h r e e i o n i c exchanges w i t h a 1 M s o l u t i o n o f amnonium n i t r a t e a t room temperature. i

HY was prepared b y NH4

ion

exchange

of

a

NaY

sample

( f r o m Union

Carbide) by t r e a t i n g t h e z e o l i t e w i t h a 2 M s o l u t i o n o f ammonium n i t r a t e 3 times a t 20°C

and 5 times a t 100°C

( l o g z e o l i t e p e r 40 cm3 s o l u t i o n ) ;

367

the degree of ion exchange was 92%. After exchange i t was washed, then dried a t 120OC f o r 8 h , and calcined a t 5OOOC under a low flow of dry a i r t o obtain t h e protonic form. The Si/P81 r a t i o was about 2.6. Samples were characterized by X-ray d i f f r a c t i o n and shown t o present a good level of c r y s t a l l i n i t y . Acidity was characterized by ammonia TPD; the c a t a l y s t s were submitted t o a pretreatement a t 45OoC f o r 12 h under a flow of dry helium (60 m ' l / m i n ) . NH3 adsorption was performed a t 90°C, a f t e r which the temperature was raised from 90°C t o 5OO0C a t a r a t e of 1(IoC/min.

Reaction Conversions of n-heptane and 2,2,4-trimethylpentane were c a r r i e d out in a flow r e a c t o r a t 350oC, a t a tota.1 pressure of 1 bar w i t h a nitrogen-to-hydrocarbon r a t i o equa.1 t o 9 and a WHSY (weight of a.lkane per hour per u n i t weight of zeo.lite) equii'l t o 6.9. Previously, the cata.lysts were pretreated in s i t u a t 450% f o r 12 ti under a flow of dry nitrogen. Since t h e SAPO-37 s t r u c t u r e i s degraded by moisture 15,81, the protonic form was generated from the ammonium one by t h i s i n situ 2.2.

pretreatement.

After

the

cata.lytic t e s t s the c r y s t a l l i n i t y was checked

by Xray d i f f r a c t i o n , and t h e r e were no g1oba.l .losses detected. The

reaction

products

were

separated

and

identified

by

Gas

Chromatography ( G C ) on a 50 rn PLOT c a p i H a r y column coated w i t h alumina deactivated by KC.1. The coke content of the c a t a l y s t s a f t e r 5 h reaction was determined by therrnogravimetric combusti on.

3. RESULTS AND DISCUSSION In Figure 1 we present ammonia TPD data both f o r the protonic form of SAPO-37 and f o r HY. As can be readi ly seen, t h e r e iire s i g n i f i c a n t d i f f e r e n c e s in t h e high temperature region. The protonic form of SAPO-37 c l e a r l y has a lower acid strength than z e o l i t e HY a s evidenced by the absence of NH3 desorption above 40OoC.

368 I n agreement w i t h these r e s u l t s ,

SAPO-37 was found t o be much l e s s

a c t i v e than HY z e o l i t e f o r t h e c r a c k i n g o f n-hepane (Table 1). T h i s r e a c t i o n requests t h e f o r SAPO-37

presence o f s t r o n g a c i d s i t e s .

2,2,4-trimethylpentaneY

of

weaker a c i d t =

Nevertheless,

i s s i g n i f i c a n t l y h i g h e r than t h a t f o r Nay. sites,

a

SAPO-37

reaction that

can

be performed w i t h much

p r e s e n t s a reasonable i n i t i a l a c t i v i t y ( f o i -

5 min), a l t h o u g h q u i t e lower than t h a t observed f o r

90

300

the a c t i v i t y

For t h e c r a c k i n g

T("C)

F i g u r e 1. Thermoprogrammed d e s o r p t i o n

HY.

500

of

ammonia

on

(-)

SAPO-37

and

HY z e o l i t e (---).

Table 1 1 n i t i a . l a c t i v i t i e s o f HSAPO-37,

C 7 ) and 2,2,4-trimethylpentane

SAPO- 37

n-C7 Zy2,4-tmC5

cracking

.

(mol h - l . 9 - l )

(t=5min)

I n i tia.1 a c t i v i t y

~____

HY and NaY a t 350oC i n t h e n-heptane (n-

(2,2,4-tmC5)

HY

NaY

2.3~10-4

8.4~10-3

5.4~10-~

1.4~10-2

3.6~10-~

4. 5x10q4

_~

369 If we compute t h e r e l a t i v e i n i t i a l a c t i v i t i e s ( 2 , 2 , 4 - t r i m e t h y l p e n t a n e

/ n-heptane

cracking va.Iue

with

cracking)

SAPO-37

(60.9)

for

than

both

with

catalysts, (4.3),

HY

we

in

obtain

a

agreement

higher

with

the

e x i s t e n c e o f s t r o n g e r a c i d s i t e s on HY z f d i t e . 0vera.I.I 37

HY:

and

cracking product d i s t r i b u t i o n s are at

the

beginning o f

the

very simi.lar f o r

n-heptane

SAPO-

r e a c t i o n C3+C4 p r o d u c t s

c o n s t i t u t e r e s p e c t i v e l y 95% and 91% o f t h e p r o d u c t s . F o r t h e 2 , 2 , 4 - t r i m e t h y l pentane

reaction,

catalysts.

C4 c o n s t i t u t e

Iso/nCq

c r a c k i n g and

distribution,

cracking

2,2,4-trimethylpentane).

for

significant

A

the

products

r a t i o s p r e s e n t always h i g h v a l u e s (5-6

36-58

t h e c l a s s i c a . 1 carbenium i o n 19,101.

90% o f

6-scission

difference

Thus,

for

both

f o r n-heptane

it i s dear

that

c r a c k i n g mechanism i s d o m i n a t i n g

is

the

symmetry

of

the

product

measured by t h e C4/C3 r a t i o , which i s a b o u t 1.3 f o r HY and

v e r y c l o s e t o u n i t y f o r SAPO-37. ( i n c l u d i n g coke f o r m a t i o n )

T h i s i n d i c a t e s t h a t secondary r e a c t i o n s

do n o t a f f e c t t h e p r i m a r y d i s t r i b u t i o n i s s u e d

b y t h e 6 - s c i s s i o n mechanism.

1,

From T a b l e

with

those

o b t a i n e d w i t h NaY z e o . l i t e . The ' l a t t e r has a n e g . l i g i b . l e a c i d i t y and,

thus,

its

n-heptane

reactions. the

can

cracking

The

cracking

we

higher products

a.lso compdre

activity

activity

of

distribution

SAPO-37

activities

corresponds

practically

SAPO-37

n-heptane

which

for

i s observed,

to

thermal

cracking,

confirms

and

to

the

conc'l u s i o n t h a t t h e SAPO-37 a c t i v i t y corresponds t o c a t a l y t i c c r a c k i n g . and HY d e a c t i v a t e

Both SAPO-37 coke d e p o s i t i o n .

However,

the

coke

very

r a p i d l y as

c o n t e n t o f SAPO-37

a

consequence

of

obtained w i t h

n-

heptane c r a c k i n g i s u n e x p e c t e d l y h i g h ( a p r o x i m a t e l y 15% whi l e f o r HY i t was 13%), d e s p i t e t h e .lower c r a c k i n g a c t i v i t y of SAPO-37. between

cracking

and

coking a c t i v i t i e s

was

a.lso

This discrepancy

observed e a r - l i e r w i t h

p a r t i a l l y exchanged RENaY z e o l i t e 1111. I n fact, sites

we t h i n k t h a t f u r t h e r

i n v o l v e d i n SAPO-37

performance w i t h a Y

i n f o r m a t i o n about the kind o f a c i d

can be o b t i i i n e d i f we

compare

zeo.lite having a s i m i l a r a c t i v i t y .

i t s cata'lytic The

comparison

o f SAPO-37 w i t h a fu1.I HY f o r m i s r a t h e r u n f a i r s i n c e t h e f o r m e r has o i l y mi.Id a c i d s i t e s .

The same '1eve.I o f c a t X I y t i c a c t i v i t y has,

however,

been

a c h i e v e d w i t h PrNaY z e o . l i t e s w i t h a r e l a t i v e l y .low .level o f P r 3 + c a t i o n s Tab'le 2 shows a comparison of

introduced

1101.

n-heptane

cracking

for

SAPO-37

( PrU. 16Na0.52)A'I 02( s i 0212.36.

and

a

Pr3+

t h e main parameters f o r exchanged

NaY

zeo.lite

370

Table 2 Comparison o f HSAPO-37 and PrNaY a t 350% i n n-heptane cracking. Values taken a t 5 min TOS: Cracking a c t i v i t y , C4 t o C3 r a t i o (C4/C3), propane t o propylene r a t i o (C3-/=) and coke c o n t e n t a f t e r 5 hours TOS.

. tl

Acti (mo I vih- g - l )

SAPO-37 PrNaYa

0.23 0.36

c4/c3

c3-/=

I .o

0.4 0.4

1.3

Coke (wt.%)

15 15

aValues taken f r o m r e f . 10. A c t i v i t y computed from d e a c t i v a t i o n parameters. As can be seen f r o m these r e s u . l t s t h e r e a r e g r e a t s i m i l a r i t i e s between these

two

catalysts.

which

shoild

have

comparable amounts o f

protonic

s i t e s . SAPO-37 has about 0.1 H+/T atom, whi.le t h i s PrNaY should have between

0.05 and 0.1 H+/T atom depending on t h e f o r m o f t h e P r 3 + c a t i o n s . A

significant difference,

d i s t r i b u t i o n f o r SAPO-37 f r o m a simple

however,

i s t h e C4/C3 r a t i o :

i s v i r t u a . l . l y symmetrical,

6 - s c i s s i o n mechanism,

the product

as one would expected

w h i l e Y z e o l i t e s u s u a l l y g i v e an

asymmetrical d i s t r i b u t i o n w i t h a C4/C3 r a t i o g r e a t e r t h a n one.

T h i s means

t h a t s i d e r e a c t i o n s o c c u r a t much 'lesser e x t e n t i n SAPO-37 than i n PrNaY. S t r u c t u r e t y p e and pore dimensions a r e s i m i l a r f o r SAPO-37

and Y

zeo.lite. Thus, t h e observed d i f f e r e n c e i n a c i d i t y must on.ly be a consequence of

different

framework

charge

and composition.

For z e o . l i t e s t r u c t u r e s ,

i t i s g e n e r a l l y accepted t h a t t h e a c i d s t r e n g t h o f a s i t e i n c r e a s e s as

t h e number o f d o s e A l neighbors decreases

1151:

t h i s corresponds t o an

i n c r e a s e i n p r o t o n charge w i t h A l c o n t e n t r e d u c t i o n . such an e f f e c t ,

a l l Si(nA.1) c o n f i g u r a t i o n s (n=O-4) Si(4A.l)

Taking i n t o account

t h e above s i t u a t i o n i s favoured on Y z e o l i t e which has

i n c o n t r a s t t o SAPO-37 which o n l y has

s i t e s a s can be seen b y 2 7 A l ,

and 29Si-NMR

141. However,

the

comparison w i t h PrNaY shows t h a t f o r a comparable number o f a c i d s i t e s , b o t h materia.ls behave i n much t h e same way. The c a t a . l y t i c p r o p e r t i e s o f severa.1 SAP0 m a t e r i a l s have been i n s p e c t e d by s e v e r a l o t h e r a u t h o r s ,

11 I ,

xylenes

methylation

isomerization 1131.

f o r t r a n s f o r m a t i o n s such as n-butane c r a c k i n g 1121,

Most o f them,

propy-lene o l i g o m e r i z a t i o n and and s p e c i f i c a l l y SAPO-37,

a c i d c h a r a c t e r , s i m i . l a r t o t h e one r e v e a l e d i n t h i s study.

toluene

presented m i l d

371

A c i d i c c n a r a c t e r i s t i c s o f these matelria I s are, however, much dependent on

composition,

catalyst

as

shown

containing

in

recent

species

Pt

was

work

1141.

compared

A

to

bifunctional for

Pt-HY

SAPO-37

the

decane

conversion. The r e s u l t s g e n e r a l l y showed a c a t a l y t i c a c t i v i t y f o r HY much higher

than

that

SAPO-37.

of

However,

with

materials.,

has

an

enriched

Si

SAPO-37,

a c t i v i t i e s became compa r a b I e.

4. CONCLUSION SAPO-37,

as

other

SAPO

t o some forms o f Y z e o l i t e s .

Nevertheless,

acidic

p r o p e r t i e s simi l a r

t h e usual s y n t h e s i s does n o t

produce a m a t e r i a l h a v i n g t h e s t r o n g e,cid s i t e s r e q u i r e d f o r demanding r e a c t i o n s , namely n-heptane c r a c k i n g . These s i t e s a r e found i n HY, SAPO-37's i s o s t r u c t u r a l analogue.

5 . ACKNOWLEDGEMENTS T h i s work was p a r t i a ' l ' l y supported by Junta Nacional de I n v e s t i g a c a o C i e n t i f i c a e Tecnol6gica under r e s e a r c h c o n t r a c t no. 856.86.160.

The SAPO-

37 m a t e r i a l was prepared by Mrs. N. Dumont and L. M a i s t r i a u a t t h e Facu.lte's Uni v e r s i t a i r e s de Namur.

6. REFERENCES 1 B.M.

Lok,

C.A.

Messina,

R.L.

Flanigen, J. Am. Chern. SOC.,

2 t.M. E.F.

F l a n i g e n , R.L.

P a t t o n and S.T. Schultz-Ekloff

Vansant and G.

Science,

Studies

Patton,

R.T.

Gajek,

T.R.

Cannan and E.M.

106 (1984) 6092-6093. Wilson, i n P.J. (Eds),

Grobet, W.J.

Mortier,

Innovation i n Zeolite Materials

in Surface Science and C a t a l y s i s No.

37,

Elsevier,

i n Y.

Murakami,

Amsterdam, 1988, pp. 13-27. 3 L.M. A.

Flanigen, Lijima,

Technology, Amsterdam,

J.W.

B.M.

Lok,

Ward

R.L.

(Eds),

P a t t o n and S.T. New Developments

Wilson,

i n Zeolite

S t u d i e s i n Surface Science and C a t a l y s i s No. 1986, pp. 103-112.

Science and 28,

Elsevier,

312

4

L.S.

Saldarriaga,

S a l d a r r i a g a and M.k.

C.

Davis,

J.

Chem.

Am.

SOC.,

109 (1987) 2686-2691. 5 N. Dumont, T. I t o and E.G.

Derouane, Appl. Catal.,

6

R.L.

Lok,

B.M.

Messina,

C.A.

Patton,

Flanigen, U.S.

P a t e n t 4 440 871 (1984).

7

L.

N.

8

M. Briend, A. Shikholeslarni, M.J.

Maistriau,

Dumont,

J.B.

R.T.

Nagy,

Z.

54 (1989) Ll-L6.

Gajek,

T.R.

Cannan,

Gabelica and E.G.

E.M.

Derouane,

Zeo lit e s , 10 (1990) 243-250. J . Chem.

9

SOC. D a l t o n Trans.,

(1989) 1361-1362.

B.W.

Wojciechowski

Inc.,

New York, 1986, ch. 5, pp. 127-194.

10 F.

Lemos,

and A.

P e l t r e , D. Delafosse and D. Barthomeuf,

Corma.

Lopes and F.

J.M.

C a t a l y t i c Cracking,

Ramda R i b e i r o ,

Marcel Dekker,

J. Mol. Catal.,

53 (1989)

265- 273. 11 F. Lemos, Ph. D. Thesis, Univ. Tec. Lisboa, 1989.

12 D.R.

Pyke,

Whitney and H.

P.

Houghton,

Appl.

Catal.,

18 (1985) 173-

190. 13 R.J. J.W.

Pel l e t ,

Long and J.A.

G.N.

Ward (Eds),

Rabo,

i n Y.

Murakami,

L i j i m a and

A.

New Developments i n Z e o l i t e s Science and Technology,

S t u d i e s i n Surface

Science and C a t a l y s i s No.

28,

Elsevier,

Amsterdam,

1986, pp. 843-849. 14 J.A.

Martens,

i n P.A.

C.

Janssens,

Jacobs, R.A.

S t u d i e s i n Surface

P.J.

Grobet,

H.K.

Beyer and P.A.

Jacobs,

van Santen (Eds), Z e o l i t e s : f a c t s , Figures, f u t u r e , Science and C a t a l y s i s No.

49,

E l s e v i e r , Amsterdam,

1989, pp. 215-225. 15 U.

Barthorneuf,

i n f.

C.

Naccache (Eds),

E,

80,

Ramda R i b e i r o , A.E. Zeolites:

Science and Technology,

Martinus N i j h o f f Publishers,

pp. 317-345.

Rodrigues,

The Hague,

L.D.

Rollmann and

NATO AS1 SERIES

Boston,

London, 1984,

P.A. Jacobs e t al. (Editors), Zeolite Chemistry and Catalysis 0 1991Elsevier Science Publishers B.V., Amsterdam

373

Cracking of light alkanes over MeAPO-5 molecular sieves J. Meusinger', H. Vinelt, G. Dworeckow', M. Goepperb and J.A. Lercher' Institut f i r Physikalische Chemie und Christian Doppler Laboratorium fir Heterogene Katalyse, Technische Universitat Wien, Getreidemarlct 9, A-1060 Vienna, Austria

a

mole Nationale Supeneure de Chimie de Mulhouse, 3, Rue Alfred Werner, 68093 Mulhouse, France Abstract The catalytic activity and selectivity of SAPOS, MgAPO5, CoAPO5 and ZnAPOS for cracking and dehydrogenation of n-butane was investigated. At 773 K the turnover frequencies of cracking according to the monomolecular pathway are constant for all SAP05 samples (1.21 +/-0.17*10" molec./p+].s). The turnover frequencies for MgAPOS and CoAPOS were considerably higher (2.96and 3.2*105 molec./~+].s).ZnAPOS did not show appreciably cracking activity along the monomolecular pathway. The higher turnover frequencies of MgAPO5 and CoAPOS are not due to a higher strength of the acid sites but should rather be caused by lateral interactions of n-butane close to the accessible metal cation.

INTRODUCTION The changes of the acid - base properties of metal substituted aluminophosphate based molecular sieves (MeAPO) as function of the chemical composition and the crystal structure are proposed to be complicated and to be substantially different compared to zeolites (1,2). Three mechanisms for the incorporation of Si or other metals (Me) into ALP05 frameworks have been proposed: Substitution of A1 by Si (Me), substitution of phosphorus by Si (Me) and the simultaneous (formal) substitution of A1 and P with Si or other tetravalent cations. The consequences of these substitution depend upon the ionic charge of the two partners in the substitution. If Si is substituting phosphorus strong Bronsted acid sites should be produced, if it is substituting aluminum strong basic sites should be produced and if two silicon are substituting aluminum and phosphorus the neutral charge of the framework should not change. Jacobs et al. (3) demonstrated that large domaines of silica lattices can be incorporated in this way. High concentrations of either one of the components (Si, P or Al) may lead to extraneous material partially blocking the molecular sieve pores (23. Because the concentration of tetrahedrally coordinated metal cations of a kind is frequently not constant throughout the crystal, it might be difficult to rationalize the catalytic activity or the acid - base properties as function of the overall lattice charge or the overall composition of the material (3). This complicated situation is also reflected in widely varying values for the n-butane cracking rate constants for even a material of one given kind, e.g.

374

for SAP05 (1,6). Note that no apparent correlation between the chemical composition, the intensity of OH bands and other indications were used to assess the acid strength and the concentration of acid sites for this molecularsieves (7). In contrast, Halik et al. (5) showed that the specific rates for cracking light n-alkanes (number of cracked molecules per second and strong acid site) is approximately the same for SAP05 samples synthesized with one template. Recently Jacobs et al. (3) reported identical rates of hydrocracking of n-decane on four Pt-loaded SAPO5-samples. In order to probe these differences further, we investigated a series of SAP05 molecular sieves synthesized with different templates and samples of the same structure but containing: Mg (MgAPOS), Co (CoAPO5) and Zn (ZnAPOS). The conversion of n-butane was used as test reaction and t.p.d. of pyridine to assess the strength and the concentration of acid sites.

EXPERIMENTAL Temperature-programmed desorption (t.p.d.) T.p.d. was carried out in vacuum (p = lo6 mbar) using a temperature increment of 10 Wmin. The sample was calcined in siru at 873 K for 1 hour, cooled to 293 K and contacted with 5 mbar pyridine for 30 min. Subsequently, the system was evacuated at 433 K, 493 K and 553 K, in order to determineweakly, moderately and strongly chemisorbed pyridine (4). Cracking of n-butane The conversion of n-butane was studied in continuous flow mode. 50 to 110 mg molecularsieve powder were mixed with different amounts of washed and calcined quartz (MERCK) to achieve the same sample volume for all experiments. For activation, the temperature was increased with an increment of 10 Wmin up to 873 K in He flow (10 mllmin). After 30 min. at 873 K He was replaced with air for further 30 min. to remove any carbonaceuous residues. The rates of reaction of 2 mol% n-butane in He were measured between 733 and 833 K in intervalls of 20 K. The conversion was kept below 5 mol % . The absence of thermal cracking of n-butane was confirmed with blank experiments. Material The synthesis procedures followed the description given in ref. (8). The removal of various templates from the molecular sieves was achieved by calcination in air at 873 K for 1 h. After calcination, the only crystalline phase detected in all samples by XRD was SAP05 (MeAPO5). The size of the crystals was determinded by scanning electron microscopy. The chemical composition was determined by electron microprobe. analysis. The chemical composition, the templates used for synthesis and the crystal size are listed in Table 1.

RESULTS Temperature programmed desorption (t.p.d.) of pyridine The variations of the rates of desorption of pyridine from SAPO5-1 and SAPO5-3 during t.p.d. can be seen in Figs. la and lb. For all samples, but ZnAP05, three maxima in the rate of desorption were observed and attributed to desorption from weak, moderate and strong Br6nsted acid sites. We have no indication that Lewis acid sites did contribute significantly to the strongest of these sites. The concentration of the acid sites and the upper limit of the concentrations of strong Brijnsted acid sites expected from the bulk chemical composition are compiled in Table 2. It should be noted that the values found experimental-

375

ly were considerably lower than those estimated from the overall composition and that the low values indicate isolated strong Bronsted acid sites.

Table 1 Chemical composition, template used and crystal size of MeAPO-5 samples Sample

Template

composition

Crystal size

SixA$P,Oz

Si (Me)

A1

P

(pm)

SAPOS-1

TEA

0.12

0.50

0.38

4

SAP05-2

DEOLA

0.10

0.50

0.40

7

SAPO5-3

DEOLA

0.33

0.42

0.25

3

SAP05-4

TEOLA

0.32

0.41

0.27

2

MgAPO5

TEA

0.02

0.48

0.50

9

CoAPO5

TEA

0.04

0.46

0.50

5

ZnAPO5

TEA

0.06

0.44

0.50

TEA DEOLA TEOLA

15

tetraethylamine diethanolamine triethanolamine

Table 2 Concentration of acid sites determined by t.p.d. of pyridine per 100 Si,A$P,O, Sample

acid

sites

T

09'

strong

moderate

weak

expected

SAPOS-1

2.42

0.48

0.56

12

733

SAP05-2

1.OO

0.25

0.06

10

703

SAP05-3

0.93

0.38

0.53

17

703

SAP05-4

1.30

0.18

0.29

14

703

MgAPOS

0.59

n.d.

n.d.

2

643

CoAPOS

0.69

n.d.

n.d.

2

723

ZnAPOS

0.00

n.d.

n.d.

6

1

n.d.

__-

Temperature of the maximum of the rate of desorption of pyridine from strong acid sites not determinded

376

Cracking of n-butane The product distribution, the rates of total conversion of n- butane, the rates for monomolecular cracking and for dehydrogenation as well as the turnover frequency (TOF) for cracking are compiled in Table 3. For SAP05-3 the selectivity as a function of the reaction temperature is shown in Fig. 2, respectively. For the SAP05 samples the TOF for cracking was approximately constant, irrespectively of the concentration of the Briinsted acid sites. Note that the rates based on the sample mass varied considerably which corresponds to the variations reported in the literature (1,6). The apparent energies of activation varied between 138 and 150 kT.mo1'. With the exception of SAPOS- 1 the rates of dehydrogenation were significantly lower than those of cracking indicating a minor importance of this reaction pathway at 773 K. The catalytic activity of the MeAPOS samples varied strongly as a function of the metal cation. The samples with Mg and Co exhibited rates of cracking similar to those observed with the SAP05 samples while ZnAPOS was virtually inactive for cracking. The rates for dehydrogenation varied between 0.3*10-'and l.2*lO" rnol.g-'.s-'. ZnAPO5 showed primarily activity for dehydrogenation under our experimental conditions.

Table 3 Selectivity (mol %), rates (mol.g-'.s-') and turnover frequencies (molecules . Irr+]"s-') for reactions of n-butane at 773 K SAPO5-1

SAPO5-2

SApO5-3

SAP054 MgAPOS

COAPOS

W

0

5

methane

19.0

22.8

19.0

19.1

23.2

17.7

0.0

ethane

11.5

13.7

10.2

11.5

10.7

5.5

0.0

ethene

24.6

34.6

36.1

36.9

25.9

35.4

17.5

0.0

0.0

propane

0.0

0.0

1.7

0.0

0.0

propene

24.6

26.8

30.7

29.9

26.3

39.0

47.1

butene

16.6

2.1

2.2

2.5

13.8

2.4

35.4

i-butane

3.9

0.0

0.0

0.0

0.0

0.0

0.0

rate (total)' *lo9

8.03

2.73

3.20

3.92

4.9

8.52

1.40

rate (cracking)' *lo9

5.49

1.94

2.03

2.25

2.89

3.47

0.00

rate (dehydr.)' *lo9

3.19

0.11

0.09

0.18

1.17

0.35

1.01

TOF (~racking)~ *los

1.38

1.16

1.30

1.04

2.96

3.2

0.00

' '

,

rate (cracking, total) = 114 (rc + 2rc + 3rc ,) rate (cracking, monomolecular) = r (methane) + r (ethane) rate (dehydr.) = r @utene) rate (cracking, monomolecular) normalized for the concentration of strong Brcinsted acid sites

317

DXSCUSSION The strength and the concentration of acid sites T.p.d. of pyridine indicates that the acid sites of the samples investigated could be classified into weak, moderate and strong on the basis of the maximum of the rate of desorption. Each of these desorption maxima corresponds to OH groups as sites for the adsorption of pyridine. From separate i.r. measurements we have no indication that large a Concentration of pyridine are desorbing from metal cations (Lewis acid sites). In the light of the large difference between the concentration of sites measured and that expected as the maximum value (Table 2), we conclude that relatively large domaines of pure silica structure should exist in our samples. This is confirmed by the relatively low intensity of the SiOH and POH bands in comparison with the intensity of the bands of SiOHAl groups and is in agreement to the literature (3,9). If we accept that an upper limit of Si incorporation in the SAP05 phase exists (3) the silica rich crystalline phase (or amphorphous phase) must contain alumina and thus should exhibit Bronsted acidity. It is interesting to note that neither the t.p.d. of pyridine nor the cracking of n-butane indicates a significant contribution of these sites to the acidity and the catalytic activity, respectively.

Fig. 1 Rates of desorption of pyridine during t.p.d. from SAPOS-1 (a) and SAP05-3 (b) Although it should only be used with great caution (lo), the similar temperature of the maximum attributed to desorption of pyridine from strong Bronsted acid sites indicates a similar strength of sites for all samples (see table 2). Note that this agrees very well with our previous conclusions (4,5). The subtly higher temperature of this maximum found with SAPOS-1 in comparison with the other SAP05 samples is concluded to be caused by the higher concentration of acid sites in the former sample. With HZSMS, the variation in the concentration of the acid sites without the change of the heat of adsorption was demonstrated to give a similar effect (11). While CoAPOS showed a maximum in the rate of desorption at a temperature of maximum (723 K) close to the values found for SAPOS, MgAPO5 exhibited the maximum at considerably lower temperatures (643 K) indicating somewhat weaker acid strength. It is not clear at present whether the absence of strong Bronsted acid

378

sites is an intrinsic property of ZnAP05 or if our sample did not contain any Zn2+in the zeolite lattice. It should be emphasized, however, that the sample did not show any peak due to the possible desorption of pyridine from accessible Znz+ cations which was found with ZnO at approximately 783 K. Based on this indirect evidence we suggest that amorphous impurities containing Zn” are not important in the sample studied.

Cracking of n-butane Two possibilities exist to crack n-butane via an ionic intermediate: (i) via the formation of a carbonium ion or (ii) via the formation of a carbenium ion. According to Haag et al. (12) cracking via the carboniumion is a monomolecular reaction. The proton is added to a saturated hydrocarbon and the carbon - carbon bond adjacent to the carbon atom bearing the positive charge is broken. In this case for each molecule butane cracked one molecule of methane and propene or of ethane and ethene is formed. In addition, to the cleavage of the carbon - carbon bond, the cleavage of two C-H bonds (dehydrogenation) might be possible. In the case of the route via the carbenium ion a hydride ion is abstracted from the saturated molecule, either by the surface or by an adsorbed carbenium ion (hydride transfer). The carbenium ions usually cleave the C-C bond next nearest to the carbon atom bearing the positive charge (B-rule). This is not to likely for n-butane, because the mechanism requires either a primary carbonium ion or a methyl carbenium ion in the reaction pathway. Furthermore, it was pointed out that a low partial pressure of the hydrocarbon, low concentrations of acid sites, high temperatures and narrow zeolite pores favor the monomolecular pathway. Except for the pore size all other parameters are adjusted to favor primarily cracking via the carbonium ion route.

Selectivity [mol%]

40 35 30

*

25

% Methane % Ethane

.x % Ethene

20

0 % Propane 15

.X % Propene

10

\

f % Butene

0 5

730

740

750

760

770 780 790 800

810 820

Temperature [Kl

Fig. 2 Product selectivity for reactions of n- butane over SAP053

379

The selectivity of the conversion of n-butane over MeAPOS materials differs from the product distributions found with e.g. HZSMS. As it can be seen in Fig. 2, at reaction temperatures around 730 K the reaction products are dominated by unsaturated hydrocarbons in excess to the cracking products formed. Only at reaction temperatures as high as 800 K, the product distribution is that expected for the monomolecular pathway. At 730 K more than 50 % of ethene and propene formed are concluded to be produced via the monomolecular pathway of cracking. At present, it is impossible for us, however, to asses unequivocally the reaction pathway by which these unsaturated hydrocarbons were formed. We would only like to point out that in general the apparent energies of activation for cracking (138 - 150 ldlmol) were higher than for dehydrogenation (45 - 90 kl/mol) indicating a higher energy barrier for the cleavage of the carbon - carbon bond than for the carbon - hydrogen bonds. Furthermore, the apparent energies of activation for formation of methane and ethane were similar, that of ethane being slightly higher. Because this suggests similar rate determining steps for the formation of both products and because ethane can only be formed from butane via monomolecular (protolytic) cracking we conclude that the monomolecular pathway dominates. The rates of cracking of n-butane showed a direct proportionality to the concentration of strong Bronsted acid sites of the SAP05 samples. Thus, the catalytic activity per strong Bronsted acid site and hence the acid strength of these sites are identical for all of the SAP05 molecular sieves investigated. This is in good agreement with the conclusions drawn by Halik et al. (5) and by Jacobs et al. (3), but we can now extend this for several other templates and site concentrations. Thus, the preparation with different templates leads to samples of the same or very similar intensive acid - base properties. All correlations of this and other studies (13,14,15) indicate that only Br6nsted acid sites contribute to the catalytic activity. Because it was proposed earlier that an upper limit of approximately 6 mol% of Si incorporation in SAP05 exists, the question arises whether we probe only the acidity of the SAP05 phase or also and indifferentiable that of a (crystalline or amorphous) silica - alumina phase. Neither the i.r. spectra nor the t.p.d. of bases gives indication of acid sites of appreciable strength in the silica - alumina phase. Thus, also the catalytic activity is supposed to be low. This is certainly in part due to the presence of phosphorus acid during the preparation which has shown to decrease the acid strength of high silica zeolites remarkably (16,17,18). The MeAPOS samples that exhibited strong Br6nsted acid sites had a higher turnover frequency than that of any of the SAP05 samples. We conclude that this is not due to a higher acid strength of the hydroxyl group, Neither the calculation of the partial charge at the proton according to Sanderson (19) nor the position of the t.p.d. maxima suggest acid strengths higher than those observed with SAP05 samples. Therefore we are inclined to speculate that the presence of larger metal cations tends to modify the environment around the Bronsted acid site, i.e. primarily the charge at the oxygens. This should increase the strength of interaction of the hydrocarbon with the zeolite by lateral interactions (20) which migth compensate the lower density of strong acid (in comparison to the SAP05 samples) and have a positiv influence upon the reaction rate by increasing the transition state entropy. In order not to overemphasize this effect, it should be noted that these turnover frequencies (2.96 and 3.2*10-’ molec./[H+]/s)are still significantly lower than that found with HZSM5 3*104 molec./[H+]/s. In contrast to MgAPO5 and CoAPO5, ZnAPOS was not active for monomolecular

380

cracking of n-butane. It should be emphasized that we did not observe strong Bronsted acid sites with this sample. Thus, the absence of cracking confirms quantitatively the direct correlation of cracking with the presence of the strong Bronsted acid sites. It also shows that the presence of (more accessible) metal cations alone does not suffice for cracking of hydrocarbons and that the proton of the strong Bronsted acid site is indispensable. As the rate of dehydrogenation (the second highest of all samples investigated) was not affected by the lack of Bronsted acid sites, we would like to speculate that dehydrogenation uses at least in part different catalytically active sites than cracking.

ACKNOWLEDGEMENT The supply of SAP05 samples by Dr. L.Puppe, Bayer AG and the financial support of the Christian Doppler Society are gratefully acknowledged.

REFERENCES 1

2 3 4

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

E.M. Flanigen, B.M. Lok, R.L. Patton, R.T. Gajek and S.T. Wilson, Stud. Surf. Sci. Catal., 28 (1986) 103. E.M. Flanigen, R.L. Patton and St-T. Wilson, Stud. Surf. Sci. Catal., 37 (1987) 13. J.A. Martens, P.J. Grobet and P.A. Jacobs, J. Catal., 126 (1990) 299. C. Halik and J.A. Lercher, J. Chem. Soc., Faraday Trans. 1, 84 (1988) 4457. C. Halik, S.N. Chaudhuri and J.A. Lercher, J. Chem. Soc., Faraday Trans. 1, 85 (1989) 3879. B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R. Cannan and E.M. Flanigen, J. Am. Chem. Soc., 106 (1984) 6092. X. Quinhua, Y. Aizhen, B. Shulin and X. Kaijun, Stud. Surf. Sci. Catal., 28 (1986) 835. B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R. Cannan and E.M. Flanigen, US Patent No. 4 440 871 (1984). N.J. Tapp, N.B. Milestone and D.M. Bibby, Stud. Surf. Sci. Caral., 37 (1987) 393. R.J. Gorte, J. Catal. 75 (1982) 164. G.I. Kapustin, T.R. Brueva, A.L. Klyachko, S. Beran and B. Wichterlova, Appl. Cat. 42 (1988) 239. W.O. Haag and R.M. Dessau, Proc. 8th Int. Congr. CataI. (Verlag Chemie Weinheim, 1984), 2 (1984) 305. R.B. Borade, S.G. Hegde, S.B. Kulkarni andP. Ratnasamy, Appl. Cat., 13 (1984) 27. D.H. Olsen, W.O. Haag and R.M. Lago, J. Catal., 61 (1980) 390. J.G. Post and J.H.C. van Hooff, Zeolites, 4 (1984) 9. H. Vinek, G. Rumplmayr and J.A. Lercher, J. Catal., 115 (1989) 291. J.A. Lercher and G. Rumplmayr, Appl. Cat., 25 (1986) 215. A. Jentys, G. Rumplmayr and J.A. Lercher, Appl. Cat., 53 (1989) 299. R.T. Sanderson, Chemical bonds and bond energy, Academic press, New York, 1976. A. Jentys, G. Mirth, J. Schwank and J.A. Lercher, Stud. Surf. Sci. Cat., 49 (1989) 847.

P.A. Jacobs et al. (Editors),Zeolite Chemistry and Catalysis 0 1991 Elsevier SciencePublishersB.V., Amsterdam

381

PROMOTING EFFECT OF Pi. SUPPORTED ON GALLIUMSILICATE IN n-C4HI0 AROMATIZATION

R. V. Dmitriev, D. P. Schevchenko, E. S. Shpiro, A. A. Dergachev, 0. P. Tkachenko and Kh. M. Minachev

N. D. Zelinsky Institute of Organic Chemistry of USSR Academy of Sciences, MOSCOW B-334, USSR Abstract

Catalytic properties of both Ga-silicate and Pt/Ga-silicate in n-CaHio aromatization have been investigated in a wide range of reaction conditions (at temperatures of 573-773 K, space velocities of 300-10000 h-l, total pressures of 0.1-0.5MPa, partial pressures of butane and hydrogen of 5-100 kPa and 5-400 kPa, respectively. The introduction of platinum into the Ga-silicate was shown to result in dramatic effects on butane aromatization: a decrease of the reaction temperature, an increase in the reaction rate and a drastic change in the product distribution. Platinum was found to accelerate both the initial paraffin and alicyclic intermediate dehydrogenation and suppressed the cracking process. 1. INTRODUCTION

The A1 isomorphous substitution by Ga in zeolites with ZSM-5 structure permits the preparation of crystalline galliurnsilicates with Ga in the framework tetrahedral positions [ l l . Such material exhibits rather higli catalytic activity and selectivity for lower alkane aromatization [ 2 , 3 1 The activity of galliumsilicate was related to the Broensted acidity produced by the Ga framework ions as well as to Lewis acidity which results from Ga extraframework species [41. Recently [5,61, a strong platinurn promoting effect on the aromatization activity of Ga-silicate has been found. The main platinum functions were proposed to be the enhancement of the initial paraffin dehydrogenation activity, decrease of coking and facilitating of the regeneration process due to the effects of both hydrogen and oxygen spillover [71. To elucidate the Pt promoting effect in more detail we have investigated various catalytic features of n-butane aromatization with Ga- and Pt/Ga-silicates. XPS and TEM characterization of the catalysts studied was also performed. 2. EXPERIMENTAL

The starting sample was Ga-silicate with ZSM-5 structure, SiO2/Ga2Os=GO, pretreated in a Nz stream at 823 K. According to MASNMR 59Ga most of the Ga is in tetrahedral coordination in the zeolite framework [41. The 0.5%

Pt/Ga-silicate was prepared by impregnation of the starting material with [Pt(NH3)41C12 solution. The catalysts were treated in the air at 723 K and then in hydrogen at 773 K, the heating rate being 1 K/min. The n-butane aromatization was performed, using an automatic flow unit, in a quartz microreactor (the charge was 0.5g). The reaction conditions were varied in a wide range: temperatures of 573-773 K, space velocities of 300-10000 h-l, total pressure of 0.1-0.5 MPa, butane partial pressure in He stream of 0.5-100 kPa. Some runs were made in hydrogen atmosphere, the hydrogen partial pressure was varied from 5 to 400 kPa. To avoid the flow gradient caused by butane condensation the feed was injected as a liquid at 1.5 MPa into a mixer heated to 673 K. The use of separate flow lines f o r catalyst activation and for the feedstock make it possible to measure the conversion and product distribution immediately after feedstock injection. The probes for analysis were taken automatically every 3-5 min. Normally, stable activity was reached after 5-10 min and remained practically constant over 4 hours. The Ga-silicates and Pt/Ga-silicates were investigated by XPS according to the procedure described in [&I. Pt dispersion in the reduced samples was determined by TEM [91. 3. RESULT

AND DISCUSSION

1. Catalyst Characterization

The Table listed XPS data obtained for the initial samples and after their calcination and catalytic reaction followed by regeneration. I t should be noted that, in contrast to Ga impregnated ZSM-5 [ S ] , n o substantial surface enrichment with Ga was observed after redox treatment. Thus if even the Ga is released from the framework during calcination and/or catalytic reaction [41, it is located mainly inside the zeolite channels. Pt has no significant effect on the Ga distribution. Platinum reduction in the Pt/Ga-silicate occurred during calcination which was a l s o observed with Pt/HZSM-5 [91. The amount of Pto remained the same after the reaction and regeneration but it seems to be higher in the prereduced sample, which w a s confirmed by TEM data. They indicate three types of Pt particles in the Ga-silicate: large crystals with sizes exceeding 100 A (minor fraction). hemispherical particles of 20-50 A in diameter (major fraction) located on the surface and very dispersed clusters less than 10 A i n size located inside the zeolite channels. 2. Catalytic Properties 2.1.

Temperature dependence

Fig.1 shows the butane conversion and selectivity to aromatics in dependence of the reaction temperature for both Ga- and Pt/Ga-silicates. Pt introduction substantially increased Ga-silicate activity. This is manifested as a tctal conversion increase and dramatic enhancement of the aromatics selectivity. These effects are much more pronounced at lower reaction temperatures. When Pt is included the minimal temperature of aromatics formation drops by 50-70 K with respect to Ga-silicate. separate hydrocarbon selectivities The temperature dependences of demonstrated additional differences between the PtKa-silicate and the Ga-silicate (Fig.21, which are again more distinct in the low temperature

383 region. Under these conditions cracking and isomerization are the main reactions with Ga-silicate while Pt/Ga-silicate yields more olefins. T h e Table

XPS spectra parameters of Ga- and Pt/Ga-silicates

B.E. , eV

Samples

Atomic ratios

SL___-

si_ _ _ _

?t

-

40

40

-

21.5

-

-

40

-

1118.7

21.6

-

53

48

-

103.7

1118.5

21.1

72.9

50

50

-

103.7

1118.4

21.2

71.7a;73.0

59

-

0.003

103.8

1118.5

21.5

71.5a;73.0

53

59

0.001

(treatment )

Si 2p

Ga 2p

Ga 3d

Ga-silicate (initial) Ga-silicate (talc. Ga-silicate (reac.+regen.) Pt/Ga-silicate (initial) Pt/Ga-silicate (calc.) Pt/Ga-silicate (reac.+regen.1

103.7

1118.6

21.4

103.6

1118.5

103.6

Pt 4f

Ga(2p) Ga(3dl Si

"1 Pto fraction is equal to 45%

Conversion,

Selectivity to aromatics,

"I 80

573

673

773

T, K

Fig.1 Temperature dependence of n-C.rHio conversion ( 0 ) and selectivity to aromatics ( 0 ) for Pt/Ga-silicate (solid line) and Ga-silicate ( b r o k e n line). WHSV = 2000 h-’

maximum olefin selectivity on Pt/Ga-silicate was observed at the lowest temperature studied (573 K) while, on the Ga-silicate, the maximum is shifted to 673 K. The aromatic products on the Pt/Ga-silicate are enriched

384

in xylene (50%) especially at low temperature whereas on the Ga-silicate, the relative xylene content does not exceed 25% and remains constant in the whole temperature range. 2.2. Contact t i me dependence Fig.3 presents the dependece of the total conversion and aromatics yield on the relative contact time for several temperatures. Conversions on the Ga-silicate comparable with the Pt/Ga-silicate can be obtained at contact

Selectivity,

Selectivity, ( a )

wt.

(b)

wt. %

40

:

o z20

20

1

0 573

673

773

Temperature. K Fig.2 Temperature dependence of different hydrocarbon selectivities for Pt/Ga-silicate (a) and Ga-silicate (b) : 0 - aromaticsi 0 - C4H8, a - C3H6, x - C4Hio, A - ZCi-C3 WHSV = 2000 h-

times that are an order of magnitude longer than on the Pt/Ga-silicate. Fig.4 shows the dependence of the different hydrocarbons yield on contact time for two catalysts at 673 K. C3-C4 paraffins yield increased linearly with contact time and aromatics content rose also gradually. C3-C4 olefins yield rapidly reached steady values with Ga-silicate while a distinct maximum in their concentration was observed with the Pt/Ga-silicate. The analysis of these trends confirmed suggestions made in the literature [ l o ] that aromatization process involves several parallel and consecutive stages of both initial paraffin and intermediates transformations, which can depicted as the following scheme: 1

n-C4Hio

n-CaH8 Ca-Cs-olefins Ci-Cs-paraffins -c

I

13

385

To elucidate the Pt promoting effect in the Pt/Ga-silicate we have comparethe reaction product distribution at similar conversions f o r Ga-silicate and Pt/Ga-silicate. Comparable conversions have been obtained at the same temperatures by varying contact time (see the diagram on Fig.5). Again at lower temperature we can see prominent differences between the two catalysts. The following main conclusions have been drawn from the analysis of these features: (i) higher ( 2 times) butene fraction over Pt/Ga-silicate than on Ga-si licate; (ii) lower fraction of C I - C ~ cracking products over Pt/Ga-silicate; Conversion, ( a ) %

Aromatic yield, (b)

wt.%

II

I 40

-. -

_- --

---I'

20

0

4

8

12

4

16

8

12

16

Contact time (relative unit) Fig.3 Contact time dependence of n-CaHio conversion f a ) and yield of aromatics ( b ) for Pt/Ga-silicate (solid line) and Ga-silicate (broken line) at several temperatures: 623 K - A , 673 K - 0 , 773 K - 0

(a)

Yield, wt.%

Yield, wt.%

I

I

I

I

(b)

1

./ n

"C 4

8

-

t

12

16

l k E 3 .

U ( , P

0

4

1

I

8

12

16

Contact time (relati\re unit) Fig.4 Contact time dependece of different hydrocarbons yield Pt/Ga-silicate ( a ) and Ga-silicate ( b ) at 673 K: 0 - aromatics, 0 - C4H8, a - C3H6, X - i-C4H10, A - Cl-c3.

for

386 (iii) similar total Cs-yields but higher C3Hc/C3H8 ratio for Pt/Ga-silicate at lower temperatures. The CS/C3 ratio on this catalyst is equal to 1 : l at all temperatures while on Ga-silicate it varied from 1:5 (623 K) to 1: 1.5 (773 K); (iv) higher yields of Cs-hydrocarbons over Pt/Ga-silicate. At 623 K the Cs-yield reached 14-1677 while f o r Ga-silicate it did not exceed 3-4%; (v) the aromatic products on Pt/Ga-silicate are enriched in Cs-aromatics;

2.3.

Hydrogen Effect

The product distribution determined at equal conversions in dependence of the hydrogen partial pressure demonstrates significant difference between the two catalysts (Fig.6). When partial pressure of hydrogen varied from 0 to 0.4 MPa, the C3/C5 ratio on Pt/Ga-silicate changes more than 30 times while this ratio on Ga-silicate decreases by only 2-2.5 times. The butene fraction also decreased more strongly over the Pt/Ga-silicate. Aroma: i c e C

623K (1)

Conv.=32.1%

(2) Conv.=27.3% 673K Conv.=56.9% (2) Conv.=50.0% Fig.5 Product distribution (C-wt%) for Pt/Ga-silicate ( 1 ) and Ga-silicate (2) at equal conversion of n-CeHio Nevertheless, despite the strong hydrogenation platinum activity, the butene fraction remained higher over the Pt/Ga-silicate. The above data indicate that at a low hydrogen partial pressure and rather high butane yield aromatization is completely suppressed. This evidenced that, first of all, hydrogen influenced intermediate chemical transformations rather than butene formation. The stronger platinum hydrogenation activity found in these experiments is likely to facilitate coke precursor hydrogenation in real reaction mixtures, where the hydrogen partial pressure is rather low. Based on the data obtained and the described trends, we could consider a plausible mechanism of platinum promoting action in Pt/Ga-silicate. Inui [71 ascribed to platinum the role of strong dehydrogenation agent for starting paraffins which, in turn, increases the intermediate concentration. The higher butene yield obtained with Pt/Ga-silicate confirmed this suggestion. But our data clearly shown that platinum is

387

likely to accelerate another important step in aromatization - alicyclic hydrocarbon dehydrogenation. This follows from the aromatics distribution and propane/propene ratio for two catalysts. Since the main dirner product in butane aromatization should be CsHi6 the enrichment of aromatics with Cs-hydrocarbons indicates greater Pt activity in the alicyclic hydrocarbon dehydrogenation:

On galliumsilicate, particularly at lower temperatures, aromatics a r e formed via hydrogen transfer:

(y+3c=c-c - 3c-c-c

+

@f

This explains why, at equal conversions and propane concentrations, the propene fraction on Ga-silicate is much lower than on Pt/Ga-silicate. This route of arene formation is catalyzed by acidic centers and it competes with cracking. At higher temperatures, gallium became more active in alicyclic hydrocarbon dehydrogenation [7,81 and selectivities for the both catalysts became similar. 679K

pH L

Pt/Ga-silicate

Conv . 36 - 7%

Ga-silicate

0

.o

Conv . 34.2%

-4

26.6%

C

I

iC H 4

0

Arom.

'

10

50

100

0

50

Fig.6 Products distribution [wt%) in dependence of pressure at equal conversions.

100 partial hydrogen

The difference in Cs-hydrocarbon concentrations observed for Pt/Ga-silicate and Ga-silicate can be related to the Pt reactivity in hydrogen spillover. Since Cs-hydrocarbons are likely to be Cs-dimer cracking products produced over acidic sites, they can also be involved in subsequent oligomerization reactions over acidic sites, too. Cs-hydrocarbon reactions are more probable than C3-hydrocarbon reactions, because Cs-hydrocarbons are more volatile. However, Cs-unsaturated intermediates are rapidly hydrogenated over Pt/Ga-silicate by hydrogen which was activated on platinum and spills over the acidic sites. Consequently, they did not participate in further conversion and their concentration remained constant. The platinum hydrogenation activity was confirmed by the data on the hydrogen effect on the activity for Pt/Ga-silicate and Ga-silicate.

388 4. CONCLUSIONS

Platinum introduction into Ga-silicate resulted in an increase of b o t h the activity and selectivity in lower paraffin aromatization. This effect is very strong in the temperature range where galliumsilicate has no appreciable aromatization activity. Platinum promoted the dehydrogenation of both the initial paraffin and alicyclic intermediates. Platinum provides lower olefins and catalyzes direct a higher concentration of dehydrogenation of aromatic precursors. Pt and Ga synergic action cannot be ruled out at least for the highly dispersed Pt fraction located inside the channels. These effects can be of great importance to provide higher aromatization activity in the medium temperature region a s well as to improve the catalyst stability due to Pt efficiency in hydrogen spillover and backspillover processes. 5. REFERENCES

C. T. - W. Chu and C. D. Chang, J.Phys.Chem.,89 (1985) 1569 D. K. Simmons, R. Szostak, P. K. Agrawal and T. L. Thomas, J.Cata1, 106 (1987) 287 Kh. M. Minachev, V. B. Kazansky, A, A. Dergachev, L. M. Kustov, 3 T. N. Bondarenko and A. Yu., Khodakov, Bull.Acad.Sci.USSR, 1 (1990)311 4 A. Yu. Khodakov, L. M. Kustov, T. N. Bondarenko, A. A. Dergachev, V. B. Kazansky, Kh. M. Minachev, G. Borbely and H. K. Beyer, Zeolites, 10 (1990) 603 T. Inui, 0. Yamase, K. Fukuda, A. Itoh, J. Tarumoto, N. Morinaga, 5 T. Hagiwara and Y. Takegami, Proc. 8th Intern. Cong. Catal., Berlin, 1984, Vol. 1 1 1 , p.569 T. Inui, Y. Makino, F. Nagano and A. Miyamoto, Ind. Eng. Chem. Res., 6 26 (1987) 647 7 T. Inui, Y. Ishihara, K. Kamachi and H. Matsuda, Stud. Surf. Sci. Catal. 49 (1989) 1183 0. P. Tkachenko, E. S. Shpiro, T. V. Vasina, A. V. Preobrazhensky, 8 0. V. Bragin and Kh. M. Minachev, Bull. Acad. Sci. USSR, (1991) in print E. S. Shpiro, G. J. Tuleuova, V. 1. Zaikovskii, 0. P. Tkachenko, 9 T. V. Vasina, 0. V. Bragin and Kh. M. Minachev, Zeolites as Catalysts, Sorbents and Detergent Builders, 1989, Amsterdam, p. 143 10 N. S. Gnep, J. Y. Dovement, A. M. Seco, F. Ramoa Ribeiro and M. Guisnet, Stud. Surf. Sci. Catal., 43 (1988) 155 11 Kh. M. Minachev, V. B. Kazansky, A. A . Dergachev, L. M. Kustov and T. N. Bondarenko, Bull. Acad. Sci. USSR, 303 (1989) 412

1 2

389

P.A. Jacobs et al. (Editors),Zeolite Chemistry and Catalysis 01991 Elsevier Science Publishers B.V., Amsterdam

Conversion of a l l y l alcohol t o oxygenated products over zeolite catalysts Graham J . Hutchings, Darren F . Lee and Craig D. W i l l i a m s Leverhulme Centre f o r Innovative C a t a l y s i s , Department U n i v e r s i t y of Liverpool, PO Box 147, Liverpool L69 3BX

of

Chemistry,

Abstract The r e a c t i o n of a l l y 1 a l c o h o l over z e o l i t e s H-ZSM-5, Na-ZSM-5 and H-Y i s d e s c r i b e d and discussed. Over t h e a c i d i c forms of t h e z e o l i t e s s i g n i f i c a n t are s e l e c t i v i t i e s of C, oxygenates ( p a r t i c u l a r l y CH,CHCHO and CH,CH,CHO) observed. I n p a r t i c u l a r , under s o m e r e a c t i o n c o n d i t i o n s i n v e s t i g a t e d acetone c a n become a s i g n i f i c a n t product. The mechanism of t h e r e a c t i o n i s i n v e s t i g a t e d using m o d e l compounds as r e a g e n t s and it i s proposed t h a t p r o t o n a t i o n of t h e carbon-carbon double bond i s t h e i n i t i a l r e a c t i o n s t e p l e a d i n g t o t h e formation of C, oxygenated p r o d u c t s .

1. INTRODUCTION The conversion of a l c o h o l s t o hydrocarbons u s i n g z e o l i t e c a t a l y s t s forms t h e b a s i s of a number of commercial o r near commercial production p r o c e s s e s , eg. t h e methanol t o g a s o l i n e process ( r e f . l), t h e methanol t o o l e f i n s p r o c e s s and a s s o c i a t e d production of g a s o l i n e and d i s t i l l a t e ( r e f . 2 ) , as w e l l as a number of o t h e r v a r i a n t s ( r e f . 3 ) . Since t h e r e a c t i o n w a s i n i t i a l l y described i n s o m e d e t a i l by Chang and S i l v e s t r i ( r e f . 4 ) it has a t t r a c t e d s i g n i f i c a n t r e s e a r c h e f f o r t from both i n d u s t r i a l and academic l a b o r a t o r i e s ( r e f . 1 ) . The mechanism of formation of t h e i n i t i a l products, methane and ethene remains a c o n t r o v e r s i a l t o p i c , although t h e r e i s a g e n e r a l consensus t h a t t h e i n i t i a l s t e p i n t h e r e a c t i o n involves t h e formation of a s u r f a c e methoxyl i n t e r m e d i a t e v i a a methylation process (ref. 1,5). More r e c e n t l y , t h i s o b s e r v a t i o n h a s been used t o develop p r o c e s s e s f o r t h e s y n t h e s i s of l i n e a r m i n e s using methanol as a co-reagent However, t w o major with ammonia over s m a l l p o r e z e o l i t e s ( r e f . 6 ) . problems p e r s i s t f o r t h e r e a c t i o n of a l c o h o l s over z e o l i t e c a t a l y s t s . F i r s t , t h e products are almost e x c l u s i v e l y hydrocarbons with e t h e r s being t h e o n l y s i g n i f i c a n t oxygen c o n t a i n i n g product; second, s e l e c t i v i t y t o a s p e c i f i c product i s t y p i c a l l y very low, p a r t i c u l a r l y f o r high carbon number p r o d u c t s . The e x c l u s i v e loss of oxygen i s a consequence of t h e previously c i t e d methylation mechanism, s i n c e t h e a l c o h o l OH group i s i n i t i a l l y protonated by t h e Bronsted a c i d s i t e of t h e z e o l i t e and as a consequence Considerable advantage would be water i s e l i m i n a t e d from t h e molecule. achieved if t h e oxygen could be r e t a i n e d i n t h e product, p a r t i c u l a r l y i f high s e l e c t i v i t y t o oxygenated p r o d u c t s could b e obtained. T o d a t e , t h i s a s p e c t of a l c o h o l conversion has received l i t t l e o r no a t t e n t i o n . We have now addressed t h i s area and i n t h i s paper w e r e p o r t our preliminary f i n d i n g s f o r t h e conversion of a l l y l a l c o h o l over z e o l i t e c a t a l y s t s , which demonstrate t h a t oxygenated products can be formed i n high s e l e c t i v i t y .

390 2. EXPERIMENTATA Z e o l i t e H-ZSM-5 ( S i O , / A l , O , = 35) was prepared according t o t h e method of Howden ( r e f . 7 ) . The ZSM-5 prepared by t h i s method was converted i n t o t h e hydrogen form (H-ZSM-5) by i o n exchange. ZSM-5 ( l o g ) was s t i r r e d i n aqueous ammonium n i t r a t e ( 1 0 0 m 1 , 0.1M) under r e f l u x f o r 4h. The s o l i d was recovered by f i l t r a t i o n and t h e procedure was repeated twice. The s o l i d was then c a l c i n e d a t 65OOC f o r 3h. The sodium form of t h e z e o l i t e (Na-ZSM-5) was prepared using a s i m i l a r ion exchange method using aqueous sodium n i t r a t e . Z e o l i t e Y w a s purchased from Union Carbide i n t h e a c i d form (Zeolite HZY-82). Both z e o l i t e samples were calcined a t 660°C f o r 3h p r i o r t o use a s c a t a l y s t s . The c a t a l y t i c r e a c t i o n s were c a r r i e d o u t using a microreactor i n which a l l y l alcohol was vaporised i n a stream of dry nitrogen a t a c o n t r o l l e d flow r a t e t o achieve t h e required WHSV of 0 . 5 h - l . The a l l y l alcohol vapour was then r e a c t e d over t h e z e o l i t e c a t a l y s t (0.59) The products i n a t h e r m o s t a t i c a l l y c o n t r o l l e d microreactor ( i . d . = lorn). were analysed by gas chromatography. I n a d d i t i o n , products were c o l l e c t e d i n a low temperature t r a p and analysed using g . c . m a s s spectroscopy (VG7070E with DEC PDP 11-24 d a t a system). Blank thermal r e a c t i o n s i n t h e absence of c a t a l y s t were found t o be n e g l i g i b l e and s a t i s f a c t o r y mass balance was obtained f o r a l l d a t a presented.

3. RESULTS AND DISCUSSION 3.1 Conversion of a l l y l alcohol over H-Z91-5 The r e s u l t s € o r t h e conversion of a l l y l alcohol over H-ZSM-5 a t 25OOC I n i t i a l l y t h e products comprise mainly of a r e shown i n Figure 1. hydrocarbons a s would be expected from t h e conversion of an alcohol over an a c i d i c z e o l i t e c a t a l y s t . However, as t h e conversion decreases with time on CH,CHCHO and stream, t h e s e l e c t i v i t y t o C, oxygenated products (CH,COCH,, CH,CH,CHO) becomes s i g n i f i c a n t . Product i d e n t i f i c a t i o n was confirmed by 80% with lesser amounts of g.c.m.s. and s e l e c t i v i t i e s t o a c r o l e i n of propanal and acetone could be achieved. A t 100°C t h e s e l e c t i v i t y t o t h e s e oxygenated products was higher a t t h e expense of conversion (Figure 2 ) . Although t h e r e a c t i o n conditions have yet t o b e optimised, it i s c l e a r t h a t s i g n i f i c a n t s e l e c t i v i t i e s t o oxygenates can be obtained from t h i s r e a c t i o n .

3.2 Conversion of a l l y l alcohol over H-Y The r e s u l t s f o r t h e conversion of a l l y l alcohol over z e o l i t e H-Y a t A t 25OoC t h e products 250°C and 350OC a r e shown i n Figures 3 and 4 . comprise mainly hydrocarbons i n i t i a l l y , b u t as t h e conversion decreases due t o c a t a l y s t d e a c t i v a t i o n , t h e s e l e c t i v i t y t o C, oxygenates increases s t e a d i l y . However, t h e maximum s e l e c t i v i t y achieved with z e o l i t e H-Y f o r C, oxygenates i s lower than t h a t f o r z e o l i t e H-ZSM-5 under comparable conditions. A t a higher r e a c t a n t f e e d r a t e and higher temperature (Figure 4) t h e s e l e c t i v i t y t o C, oxygenates can be s i g n i f i c a n t l y enhanced, i n d i c a t i n g t h a t t h e r e e x i s t s considerable scope t o optimise t h i s r e a c t i o n f o r t h i s z e o l i t e . I t i s c l e a r from t h e d a t a t h a t t h e c a t a l y s t l i f e t i m e f o r both ZSM-5 and z e o l i t e H-Y a r e s h o r t f o r t h e conversion of a l l y l alcohol. This s h o r t l i f e t i m e w a s due t o t h e formation of coke during t h e r e a c t i o n 4% carbon a f t e r r e a c t i o n f o r 3h. and t h e c a t a l y s t s t y p i c a l l y contained The rapid formation of coke was probably due t o a l d o l condensations occurring f o r t h e C, oxygenated products of t h e r e a c t i o n .

391 100

80

60

40

20

0 0

w

lw

too

Time on Stream (minutes)

F i g u r e 1. Conversion of a l l y 1 a l c o h o l over H-ZSM-5 a t 25OoC, WHSV = 0 . 5 h - l . ethene, .f propene, 13 b u t e n e s , i( pentenes, 0 C, oxygenates (CH,COCH,, CH,CHCHO, CH,CH,CHO), I conversion of a l l y l a l c o h o l .

+

100

0

7

0

20

40

60

80

HWI

120

NO

11)O

Time on Stream (minutes)

F i g u r e 2. Conversion of a l l y l a l c o h o l over €3-ZSM-5 a t 100°C, WHSV = 0.5h”; Key as i n F i g u r e 1, except ethene, 2-propanol

x

3 . 3 Conversion of allyl alcohol over Na-ZSM-5

and effect of added water

The r e s u l t s f o r t h e conversion of a l l y 1 a l c o h o l over Na-ZSM-5 are shown 5. It is c l e a r t h a t t h e production of C, oxygenates i s s i g n i f i c a n t l y lower f o r Na-ZSM-5 when compared w i t h H-ZSM-5 a t comparable conversion and r e a c t i o n c o n d i t i o n s . This i n d i c a t e s t h a t Bronsted a c i d s i t e s a r e important f o r t h i s r e a c t i o n . I n t e r e s t i n g l y , a d d i t i o n of 3% water t o t h e a l l y l a l c o h o l r e a g e n t i n c r e a s e s the s e l e c t i v i t y for C, oxygenates o v e r Na-ZSM-5 ( F i g u r e 6 ’ ) , whereas a similar e f f e c t is n o t observed with H-ZSM-5 under comparable c o n d i t i o n s ( F i g u r e 7 ) . However, for H-ZSM-5 t h e a d d i t i o n of water a l s o d e c r e a s e s c a t a l y s t d e a c t i v a t i o n , which is c o n s i s t e n t from coke formation being t h e r e s u l t of a l c o h o l condensation r e a c t i o n of t h e C, oxygenated p r o d u c t s .

i n Figure

392

i

40

n

10

2

4

n v

0

-

100

60

160

200

TOL (mind

Figure 3. Conversion of allyl alcohol over zeolite H-Y at 25OoC, WHSV = 0.5h-l, I# ethene, -4- propene, butenes, C l dimethyl ether, X C, oxygenates (CH,COCH,, CH,CHCHO, CH,CH,MO), 2-propano1, .$ unconverted allyl alcohol; TOL = Time on Line

*

70

60 50

x

40

30 20 10

-

n 0

60

100

160

200

TOL (mins)

Figure 4 . Conversion of allyl alcohol over zeolite H-Y at 350QC, WHSV 1.6h-’. propene, t methanol, % C, oxygenates, X 2-propanol, 13 unconverted ally alcohol; TOL = Time on Line

=

3 . 4 Reaction of model reactants

The conversion of 1-propanol over H-ZSM-5 or H-Y was not found to yield any C, oxygenated products for a range of reaction conditions and the products are mainly propene and butenes. This confirms that the introduction of the carbon-carbon double bond into the reactant molecule significantly affects the reactivity. Conversion of 2-propanol over H-ZSM-5 was found to give significant selectivity to acetone at low f l o w rates and this indicates that this could be a possible reaction intermediate. In addition, reaction of propene oxide over H-ZSM-5, under comparable conditions to those utilised for allyl alcohol, produced significant selectivities of both acetone and allyl alcohol.

393 rv

60

60 40

x

30 20 10 0

0

60

100

160

200

TOL (mind

Figure 5. Conversion of allyl alcohol over Na-ZSM-5 at 25OCC, WHSV = 0.5h-l; ethene, + propene, .# dimethyl ether, 131 butenes, X C, oxygenates, 0 unconverted allyl alcohol; TOL = Time on Line

80 70

I

0

60

100

160

200

TOL (mins)

Figure 6. Conversion of allyl alcohol/3% water over Na-ZSM-5 at 25OoC, WHSV = 0.Sh-l; B methane, t ethene, X propene, X C, oxygenates, 0 2-propanol, C'l unconverted allyl alcohol; TOL = Time on Line 3.5 CoIwents on the reaction mechanism

Ally1 alcohol possesses two functional groups that could be protonated by the Bronsted acid sites of the zeolite. Protonation of the OH group would lead to loss of water via an elimination mechanism (Figure 8) resulting in the formation of hydrocarbons as exclusive products. This is demonstrated both by the reaction of 1-propanol and by the initial reaction of allyl alcohol of H-ZSM-5. However, protonation of the carbon-carbon double bond leads to oxygen retention via the formation of a carbenium ion intermediate, which could yield acetone via a 1,2 oxygen shift. A reaction mechanism consistent with the observed reaction of allyl alcohol, 1-propanol, 2-propanvl and propene oxide over zeolite catalysts is given in Figure 8 .

394

40 U

30

20 10 0

0

60

100

160

200

TOL (mind

Figure I . Conversion of allyl alcohol/3% water over H-ZSM-5 at 25OoC, WHSV = 0.5h-l; il ethene, T propene, % butenes, 17 C, oxygenates, $2-propanol, 'x unconverted allyl alcohol; TOL = Time on Line

Figure 8. Proposed reaction mechanism for the conversion of a l l y l alcohol over zeolite catalysts. It is possible that the intermediate formed from initial loss of water via elimination could also be important in the formation of C, oxygenates. Reaction of this intermediate with water could be expected to lead to the formation of C, oxygenates with oxygen at either the primary or secondary carbons. However, this possibility can be discounted, since the addition of water as a co-reagent significantly decreased the selectivity to C, oxygenates when H-ZSM-5 was used as catalyst. The observation that the reaction of allyl alcohol/3% water over Na-ZSN-5 produces significant selectivities of the C, oxygenates requires further comment. In the absence of co-fed water, Na-ZSM-5 is not particularly selective to C, oxygenates. The interaction of Na’ and H,O within zeolite pore systems has been well studied (ref. 8) and it is possible that polarization of the solvation shell of Na’ within the zeolite pore may be sufficient to induce the required acidity f o r this reaction.

395 The r e s u l t s of t h i s p r e l i m i n a r y study have shown t h a t t h e i n t r o d u c t i o n of a carbon-carbon double bond i n t o an a l c o h o l r e a g e n t can l e a d t o t h e formation of oxygenate p r o d u c t s i n high s e l e c t i v i t y and t h i s may be of s i g n i f i c a n c e f o r t h e u s e of z e o l i t e s f o r t h e s y n t h e s i s of f i n e chemicals.

W e thank t h e I n t e r f a c e s and C a t a l y s i s I n i t i a t i v e , SERC, f o r f i n a n c i a l support and Alan M i l l s f o r o b t a i n i n g t h e g . c . m.s. r e s u l t s .

5 . REPERENCES

7 8

C.D. Chang, Stud. S u r f . S c i . C a t a l . , 36 (1988) 127. S.A. Tab& and S . Yurchak, Catal. Today, 6 (1990) 307. L.V. McDougall, C a t a l . Today, i n p r e s s . C.D. Chang and A . J . S i l v e s t r i , J. C a t a l . , 4 9 (1977) 247. G . J . Hutchings and R. Hunter, C a t a l . Today, 6 (1990) 279. R.G. Copperthwaite, G . J . Hutchings and T. Themistocleous, ' C a t a l y s t s f o r t h e p r o d u c t i o n of methyl m i n e s ' , S . African P a t e n t A p p l i a t i o n 1990. M.G. Howden, CSIR Report C.Eng 4 1 3 ( C S I R , P r e t o r i a , South A f r i c a , 1982). J . W . Ward, J . C a t a l . , 17 (1970) 355; 22 (1971) 237.

This Page Intentionally Left Blank

P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis 01991 Elsevier Science Publishers B.V., Amsterdam

397

C A T I O N E X C H A N G E I N F L U E N C E O N T H E A C T I V I T Y OF ZEOLITES

IN REACTIONS B E T W E E N ALCOHOLS A N D H Y D R O G E N SULPHIDE

M . Z I 6 t E K a n d K.HILDEBRAN0-LEKSOWSKA F a c u l t y of C h e m i s t r y , A . 6 0 - 7 8 0 Poznar5, P o l a n d

Mickiewicz U n i v e r s i t y ,

SUMMARY The i n f l u e n c e o f t h e a l k a l i c a t i o n e x c h a n g e i n f a u j a s i t e t y p e z e o l i t e s on t h e z e o l i t e a c i d i t y and e l e c t r o n e g a t i v i t y i s p r e s e n t e d . C o r r e l a t i o n s b e t w e e n t h e c h a n g e s of t h e s e p a r a m e t e r s and t h e a c t i v i t y and s e l e c t i v i t y of z e o l i t e s i n t h e h y d r o s u l p h u r i z a t i o n of a l c o h o l s a r e d i s c u s s e d . I t was s t a t e d t h a t f o r t h e s e p r o c e s s e s i n w h i c h t h e d i s s o c i a t i v e l y a d s o r b e d H2S t a k e s p a r t , t h e i n c r e a s e of t h e z e o l i t e e l e c t r o n e g a t i v i t y c a u s e s t h e decrease i n the activity.

INTRODUCTION Most of t h e c a t a l y t i c r e a c t i o n s w h e r e o n e o f t h e r e a c t a n t s i s H2S e x h i b i t s i m i l a r f e a t u r e s , s u c h a s ( r e f . 1 ) : - h i g h e r a c t i v i t y of X-type

z e o l i t e s t h a n Y-type

- h i g h e r a c t i v i t y of a l k a l i metal c a t i o n exchanged z e o l i t e s than acidic z e o l i t e s - t h e i n c r e a s e of t h e a c t i v i t y w i t h t h e i n c r e a s e of t h e

alkali cation radius. T h e s e f e a t u r e s were o b s e r v e d for s u c h r e a c t i o n s a s : h y d r o g e n s u l p h i d e o x i d a t i o n , r i n g t r a n s f o r m a t i o n of r - b u t y r o l a c t o n e into (-thiobutyrolactone

and r e d u c t i o n o f n i t r o compounds w i t h

hydrogen s u l p h i d e i n t o amines. T h e aim of o u r s t u d y was t o p r e s e n t t h e c h a n g e s i n p r o p e r -

t i e s of z e o l i t e s a f t e r a l k a l i c a t i o n e x c h a n g e a n d t h e i r i n f l u e n c e on t h e h y d r o s u l p h u r i z a t i o n of a l c o h o l s .

398 EXPERIMENTAL Catalysts Z e o l i t e s NaX L i n d e ( L o t No. 2 1 2 4 9 8 / 5 8 2 ) NaY Leuna w i t h S i / A 1 = 2 . 5 6

with S i / A l = 1 . 1 3 and

were used a s p a r e n t m a t e r i a l s .

M o d i f i e d f o r m s were p r e p a r e d by an i o n - e x c h a n g e w i t h 0 . 1 M s o l u t i o n s o f t h e r e s p e c t i v e a l k a l i m e t a l c h l o r i d e s . The f o l l o w i n g c a t a l y s t s were o b t a i n e d (degree o f exchange i n b r a c k e t s ) : Li,NaX K,NaY

( 2 8 % ) ; K,NaX

( 6 0 % ) ; Cs,NaY

( 6 1 % ) ; Cs,NaX

(20%); L i , N a Y ( 4 3 % ) ;

(46%).

A l l z e o l i t e s w e r e s t u d i e d b y I R ( i n t h e 300-1500

c m - l re-

g i o n ) and X-ray methods.

o f p y r i d i n e was

The t e m p e r a t u r e - p r o g r a m m e d d e s o r p t i o n ( T P D ) used f o r c h a r a c t e r i z i n g t h e Y-type z e o l i t e s .

A w a f e r o f 1 0 mg

o f z e o l i t e was a c t i v a t e d u n d e r vacuum ( w 4 P a ) a t 673K f o r 2h. A f t e r c o o l i n g t o 473K, t h e s a m p l e was e x p o s e d t o p y r i d i n e and lh-outgased.

P y r i d i n e d e s o r p t i o n was m i n i t o r e d w i t h a B a l z e r s

QMG 3 1 1 mass s p e c t r o m e t e r .

R E A C T I O N CONDITIONS The c o n t i n u o u s f l o w t e c h n i q u e was u s e d t o m e a s u r e t h e c a t a l y t i c a c t i v i t y of z e o l i t e s i n t h e r e a c t i o n between methanol o r e t h a n o l and h y d r o g e n s u l p h i d e ( s e e r e f . 2 ) . a c t i v a t e d i n h e l i u m a t 674K f o r 4 h o u r s .

The c a t a l y s t s w e r e The r e a c t i o n was

c a r r i e d o u t a t 523K by u s i n g a r e a g e n t m i x t u r e c o n t a i n i n g Merck r e s e a r c h g r a d e H2S ( 5 % v o l . ) ,

a l c o h o l (2,5% v o l . )

and

h e l i u m as a c a r r i e r gas and on l i n e G C a n a l y s i s . RESULTS C h a r a c t e r i z a t i o n of z e o l i t e s The s t r u c t u r e o f X - t y p e z e o l i t e s i s m o r e s e n s i t i v e t o t h e c a t i o n exchange and r e a c t i o n c o n d i t i o n s t h a n t h a t o f Y - z e o l i t e s (ref.3,Y).

Therefore,

were Y-type z e o l i t e s ,

t h e m a j o r c a t a l y s t s i n our s t u d y

d e s p i t e t h e f a c t t h a t X z e o l i t e s show

h i g h e r a c t i v i t y i n t h e r e a c t i o n s w i t h H2S c o n t r i b u t i o n . X-ray

IR a n d

s t u d i e s of a l k a l i c a t i o n exchanged Y z e o l i t e s i n d i c a t e d

t h a t no s t r u c t u r a l changes o c c u r e d a f t e r t h e m o d i f i c a t i o n o f

399

w

Fig. 1. MS/TPD spectra of pyridine preadsorbed on alkali cation exchanged Y-type zeolites; temperature of activation: 6 7 3 K .

313

17 3

57 3

673

TEMPERATURE C K 1

Y-zeolite. The acidity of zeolites can be estimated on the basis of the strength of pyridine chemisorption. Fig.1 shows the results of TPD of pyridine from Me IY zeolites. They confirmed the well known fact that pyridine i s adsorbed only on cations. The highest strength of cation acid sites i s observed for Li,NaY. With the increase of the alkali cation radius, the acidic strength of zeolites decreases, except for the Cs,NaY zeolite. The cesium form shows the maximum at a little bit higher desorption temperature than the maximum of pyridine desorption from K,NaY. Generally, alkali cation exchanged faujasite type zeolites are considered to be basic catalysts. However, it i s important to stress their acidity to explain selectivity changes in the reaction between alcohols and hydrogen sulphide. One of the parameters which are changed a s a result of the cation exchange in zeolites is their electronegativity. Fig.2 presents the electronegativity of Me I X and Me I Y zeolites in comparison with the electronegativity of the hydrogen sulphide molecule, which i s one of the reagents in the described reaction. The electronegativity of zeolites was calculated using the equation presented by Mortier (ref.5). The electronegativity of Cs,NaX i s higher than K,NaX because of a lower

400

3.1

3.2

3.3

3.4

3.6

3.5

3.1 I

I I

3.8

electronegativity K Na Li

I

I

I ]I

Y

I

K

Cs Li

cs

1

I I t

Na

H2S Fig. 2. The electronegativity of used faujasi.te type zeolites and H 2 S molecule.

degree of exchange of cesium ions than of potassium. All zeolites have lower electronegativity than hydrogen sulphide rnolecule. The differences between the zeolite electronegativity and H2S electronegativity i s higher for X type zeolites than for the Y-type. Activity and selectivity of zeolites The activity and selectivity of Me I Y zeolites in the reaction between methanol and H2S are showed in Fig.3. T h e increase of the zeolite electronegativity causes the

- 100

100

- 90 200°C ), B N - , ~should increase and the minimum should

be more pronounced if the La3+ cations stayed in the supercages. However, the chemical shifts are seen to decrease, the minimum weakens and shifts to smaller N =0 values, while the 8La = f [ N ] variation becomes again a straight line for (Ttz6OO0C). This evolution shows that in the range 200