Catalysis and Adsorption by Zeolites

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Studies in Surface Science and Catalysis 65 CATALYSIS AND ADSORPTION BY ZEOLITES

This Page Intentionally Left Blank

Studies in Surface Science and Catalysis Advisory Editors: 6. Delmon and J.T. Yates Vol. .65

CATALYSIS AND ADSORPTION BY ZEOLITES Proceedings of ZEOCAT 90, Leipzig, August 20-23, 1990

Editors

G. Ohlmann Zenrralinsrirur fur Physikalische Chemie, Rudower Chaussee 5, Berlin Adlershof, 0- 1 7 99 FRG

H. Pfeifer Universirar Leipzig, Sekrion Physik, Linnhstrasse 5, Leipzig, 0-70 10 FRG and

I?.Fricke Zenrralinstirur fur Physikalische Chemie, Rudo wer Chaussee 5, Berlin Adlershof, 0- 1 199 FRG

ELSEVIER

Amsterdam - Oxford - New York - Tokyo

1991

ELSEVIER SCIENCE PUBLISHERS B.V. Sara Burgerhartstraat 25 P.O. Box 2 1 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 0-444-89088-2

0Elsevier Science Publishers B.V., 199 1 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 regulationsfor 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 rnethods, products, instructions or ideas contained in the material herein. This book is printed on acid-free paper. Printed in The Netherlands

V CONTENTS Preface.........................................................XI International Scientific Committee, National Organizing Committee, Sponsors

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

XI11

INVITED LECTURES Catalysis on ZSM-5 zeolites modified by phosphorus G. Ohlmann, H.-G. Jerschkewitz, G. Lischke, R. Eckelt, B. Parlitz, E. Schreier, B. Zibrowius, E.Lijffler...........l New directions in zeolite catalysis J. Weitkamp

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

21

Zeolites as catalysts for alkane oxidations R.F. Parton, D.R.C. Huybrechts, Ph. Buskens, P . A . Jacobs..47 Sorption and separation of binary mixtures of CH4, N, and COz in zeolites L.V.C. Rees...............................................61 Use of ZSM zeolites in the liquid phase separation of alcohols R. Schollner, W.-D. Einicke.......................... .....75 Basic principles and recent results of 'H magic-angle-spinning and pulsed field gradient nuclear magnetic resonance studies on zeolites H. Pfeifer, D. Freude, J. Karger

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

89

Spectral study of lewis acidity of zeolites and of its role in catalysis V.B. Kazansky........ ~ 1 7

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

Comparative measurements on acidity of zeolites H.G. Karge.

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

133

Acidity and basicity in zeolites D. Barthomeuf.............................................l57 Matrix vs zeolite contributions to the acidity of fluid cracking catalysts R. v. Ballmoos, C.-M.T. Hayward

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

171

VI ZEOSORB HS-30 - a template-free synthesized pentasil-type zeolite K.-H. Bergk, W. Schwieger, H. Furtig, U. Hadicke..

....... 185

Selective conversion of syngas to hydrocarbons by zeolites H. Tominaga, K. Fujimoto, T. Tatsumi........... ..........203 '"Xe NMR of adsorbed xenon for the determination of void spaces Q. Chen, M.A. Springuel-Huet, J. Fraissard... 219

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

Diffusion of hydrocarbons in A and X zeolites and silicalite D.M. Ruthven, M. Eic, Z. Xu..............................233

- Present - Future ...........................................

Atlas of zeolite structure types: Past W.M. Meier....

247

Zeolites as membranes: The role of the gas-crystal interface R.M. Barrer..............................................257

The roles of metal and organic cations in zeolite synthesis D.E.W. Vaughan..

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

275

Temperature dependence of nucleation of zeolites in alkaline aluminosilicate gels in hydrothermal crystallization conditions S.P. Zhdanov, N.N. Feoktistova, L.M. Vtjurina 287

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

SUBMITTED PAPERS Catalvsis Synthesis of piperazine and triethylenediamine using ZSM-5-type zeolite catalysts J. Weitkamp, S . Ernst, H.-J. Buysch, D. Lindner..........297 Diffusion effects on the kinetics of toluene methylation and xylene isomerization on HZSM-5 zeolites F. Bauer, J. Dermietzel, W. Jockisch......

............... 305

Iron-containing ZSM-5 type zeolites used in the coupled methanol-hydrocarbon cracking (CMHC) A. Martin, S. Nowak, B. Lucke, W. Wieker, B. Fahlke......315 Dispersion dependent selectivities of syngas conversion on faujasite encapsulated Pt, Pd or Ir N.I. Jaeger, G. Schulz-Ekloff, A . Svensson...............327

VII

Contribution of 13C N M R spectroscopy to the analysis of surface compounds formed in the transformation of acetone on zeolites V. BosaEek, L. Kubelkova, J. Novakova....................337 Isopropylation of benzene over large pore zeolites A.R. Pradhan, B.S. Rao, V.P. Shiralkar

...................347

EXAFS study of local structure of Pt-Cr clusters in pentasils in relation with their reactivity in lower alkanes aromatization E.S. Shpiro, R.W. Joyner, G.J. Tuleuova, A.V. Preobrazhensky, O.P. Tkachenko, T.V. Vasina, O.V. Bragin, Kh.M. Minachev 357

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

Splitting of methane into the elements over nickel containing ZSM-5 catalysts J. Hoffmann, R. Bauermeister, B. Hunger, K. Hantel, G. Ullmann, K.-H. Steinberg, H. Siege1

...................367

Sulfur tolerant Ni-Mo-Y-zeolite catalysts for water-gas shift reaction M. Zianiecki, W. Zmierczak...........................

.....377

The influence of cations on the alkylation of toluene with ethylene over modified ZSM-5 zeolites J. Eejka, B. Wichterlova, G.L. Raurell...................

387

Multinuclear MAS NMR studies on coked zeolites H-ZSM-5 H. Ernst, D. Freude, M. Hunger, H. Pfeifer...............397 Coke oxidation in HZSM-5 zeolites. Intermediates, final products and reformation of OH groups and void volumes J. Novakova, L. Kubelkova................................405 Influence of the conditions of dealumination and the partial extraction of non-framework aluminium on the properties of ZSM-5 catalysts G . Ohlmann, H.-G. Jerschkewitz, G . Lischke, R. T. Gross, B. Parlitz, I. Schulz, K. Wehner, D.

effect of catalytic Eckelt, Timm... ...415

Spectroscopic and catalytic investigations of hydrothermally dealuminated ZSM-5 E. Loffler, L.M. Kustov, V.L. Zholobenko, Ch. Peuker, U. Lohse, V.B. Kazansky, G. Ohlmann......................425

VIII

Sorption and diffusion Computer modelling of p-xylene sorption in ZSM-S/silicalite-l K.-P. Schroder

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

435

Microdynamics of guest molecules in zeolites studied by quasi-elastic neutron scattering and NMR pulsed field gradient technique H. Jobic, M. Bee, J. Caro, M. Bulow, J. Karger, 445 H. Pfeifer

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

Permeability studies on a silicalite single crystal membrane model E.R. Geus, A.E. Jansen, J.C. Jansen, J. Schoonman, H. van Bekkum.......

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

457

The adsorptive and the catalytic diffusion of 2,3-dimethylbutane in large crystals of (aluminated) silicalite ......... 467 P. Voogd, H. van B e k k u m . . . . . . . . . . . . . . . . . . . . . . . . . Transport phenomena and reactions in 13X type zeolites M. Biilow, W. Hilgert, G. Emig

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

479

Svnthesis and structure The effect of various physical and chemical parameters on the synthesis of ZSM-5 for propene oligomerization C.T. O'Connor, S. Schwartz, M. Kojima

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

491

On controlled growth of SAPO-5 molecular sieve crystals of different sizes and shapes G. Finger, J. Kornatowski, J. Richter-Mendau, K. Jancke, M. Bulow, M. Rozwadowski......................501 Approximate assignment of vibrational frequencies of the NaX framework E. Geidel, H. Bohlig, Ch. Peuker, W. Pilz................511 The topological structure representation of zeolites B. Muller................................................521 Studies of secondary synthesis on modified pentasil zeolites W. Reschetilowski, W.-D. Einicke, B. Meier, E. Brunner, H. Ernst...........................

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

529

IX Incorporation of silicon into the framework of SAPO-5 studied by N M R and IR spectroscopy B. Zibrowius, E. Loffler, G. Finger, E. Sonntag, M. Hunger, J. Kornatowski

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

537

On the synthesis and structure of A1P04-14 B. Zibrowius, U. Lohse, K. Szulzewsky, H. FichtnerSchmittler, W. Pritzkow, J. Richter-Mendau.......

........549

Y-zeolite treated with Sicl, vapour. Structure and properties G.M. Telbiz, A.I. Prilipko, I.V. Mishin..................563 Characterization and nucleation of Na,TPA-ZSM-5 zeolite with different aluminium content G. Golemme, A. Nastro, J.B. Nagy, B. Subotic, F. Crea, R. Aiello................................................573 New molecular sieve - vanadium silicalite ms-5 J. Kornatowski, M. Sychev, V. Goncharuk, W.H. Baur.......581 Multinuclear NMR study of the crystallization of SAPO-37 N. Dumont, T. Ito, J.B. Nagy, Z. Gabelica, E.G. Derouane......................

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

591

Metastability of zeolites in tetraethylammonium media F. di Renzo, A. Albizane, M.-A. Nicolle, F. Fajula, F. Figueras, Th. des Courieres...........................603 Synthesis of ferrierites with high gallium content M.A. Camblor, J.A. Martens, P.J. Grobet, P . A . Jacobs.....613 Synthesis of artificial zeolite-like mountainite M.K. Babayev, D.M. Ganbarov, D.B. Tagiyev

................ 623

The distribution of iron in ZSM-5 type iron containing zeolites and ferrisilicates: acidic and catalytic properties G. Vorbeck, M. Richter, R. Fricke, B. Parlitz, E. Schreier, K. Szulzewsky, B. Zibrowius.................631 Zeolite ZSM-57: Synthesis, characterization and shape selective properties S . Ernst, J. Weitkamp 645

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

X

Aciditv New data on the structure and properties of acidic sites in HZSM-5 zeolites: IR-spectroscopic studies and non-empirical quantum chemical calculations I.N. Senchenya, V.Yu. Borovkov...

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

653

FTIR in-situ investigation of zeolite activation R. Salzer, B. Ehrhardt, J. Dressler, K.-H. Steinberg, P. Klaeboe.......

663

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

IR spectra of CO adsorbed at low temperature (77 K) on titaniumsilicalite, H.-ZSM5 and silicalite A. Zecchina, G. Spoto, S . Bordiga, M. Padovan, G. Leofanti, G. P e t r i n i . . . . . . . . . . . . . . . . . . . . . . . . . . ........671 The properties of boralites of various boron contents J. Datka, A. Cichocki, Z. Piwowarska.................

....681

An electrostatic model to predict the infra red characteristics of zeolite hydroxyl groups after adsorption of aromatics P.J. O'Malley 6ag

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

The influence of the acidity of zeolites on the formation of unsaturated carbenium ions I. Kiricsi, H. Forster, G . Tasi, P. Fejes

................ 697

Author Index................. Subject Index.............

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

707 711

Studies in Surface Science and Catalysis (Other volumes in the series) 715

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

XI

PREFACE There is a permanently growing interest, worldwide, in zeolite science and technology. Especially in Europe it has become an established tradition to hold specialized zeolite meetings which occur sandwiched in between the large International Zeolites Conferences. The ZEOCAT 90 Conference held in Leipzig, is the latest in a series of successful European zeolite conferences which have been held previously in Wurzburg, Nieuwpoort, Siofok, Prague and Bremen. The range of synthesized zeolites and zeolite-related molecular sieve materials is constantly expanding. The variety of structures comprises high- and low-silica zeolites, micro-porous metallosilicates, alumino-phosphates and crystalline silica polymorphs. These newly developed, modified or improved zeolites and other molecular sieves have opened up new horizons by their application as shape-selective catalysts and adsorbents, as cation exchangers or as high-tech materials. Current topics of interest are the newly developed catalytic applications of zeolites in the synthesis of fine chemicals, the use of zeolites as a new material for sensors or solid electrolytes and the setting up of sophisticated zeolite-based separation processes including molecular sieve membranes. In order to master the promising potential offered by the new zeolites, the links between the various fields of molecular sieve research are growing closer. This is especially true for merging synthesis and characterization by such powerful methods as solid state NMR and Xray or neutron diffraction, as well as for adsorption, diffusion and catalytic studies. A new trend is the growing interest in the use of computational and theoretical approaches to explain synthesis, sorption, diffusion and catalytic data. ZEOCAT 90 was comprised of the many different and diverse aspects of the rapidly expanding field of zeolite science and technology. The invited plenary lectures of this conference summarize our current knowledge and address topical areas of zeolite research. New interesting findings produced by fundamental research have led to new industrial applications and these developments have, in their turn, inspired new directions for fundamental research. The deliberate control of crystallization processes has led to the production of many zeolites which do not occur naturally. Chemically

XI1

and physically controlled treatments have been used to tailor shapeselective zeolite catalysts and adsorbents for industrial use. Newly developed physical and chemical spectroscopic and diffraction methods have given us new insights into the exciting world of molecular interactions of solid surfaces. The editors are convinced that ZEOCAT 9 0 contributed, like its European predecessors, to the successful interdisciplinary exploration of zeolites and to finding new pathways for their application in science and technology.

G. dhlmann, H. Pfeifer, R. Fricke (Editors)

XI11 ZEOCAT 90 - Leipzig International Scientific Committee R.M. Barrer, London D. Barthomeuf, Paris H.K. Beyer , Budapest H. Bremer, Berlin P. Fejes, Szeged J.P. Fraissard, Paris J. Haber, Krakow P.A. Jacobs, Heverlee H.G. Karge, Berlin V.B. Kazansky, Moscow

W.M. Meier, Zurich W.J. Mortier, Heverlee C. Naccache, Villeurbanne L.V.C. Rees, London W. Schirmer, Berlin G. Schulz-Ekloff, Bremen J. Weitkamp, Stuttgart B. Wichterlova, Prague S.P. Zhdanov, Leningrad

National Orsanizina Committee Chairmen: G. Ohlmann, H. Pfeifer Secretary: H. Miessner, B. Staudte H. Bremer, Berlin M. Bulow, Berlin D. Freude, Leipzig R. Fricke, Berlin J. Karger, Leipzig H. Lieske, Berlin SI)onsors AKZO Chemicals B.V. BASF AG BAYER AG Chemie AG Wolfen-Bitterfeld Degussa AG Deutsche Shell AG EXXON Chemical Holland-BCT Hoechst AG Intern. Zeolite Ass. (IZA) Leuna-Werke AG PCK AG Schwedt PQ Corp.

W. Schirmer, Berlin R. Schollner, Leipzig H. Stach, Berlin K.-H. Steinberg, Leipzig J. Volter, Berlin


G. Ohlmann et al. (Editors), Catalysis and Adsorption by Zeolites 0 1991 Elsevier Science Publishers B.V.. Amsterdam

1

G OHLMANN, H.-b.JtHSCHKEWlIL, b.LlSCHKt, H.ECKtLT, b.PARLIIL, t S C H R k l E R . B / I K R O W i I I S . 6 InFFLER

CentraJ I n s t i t u t e o t P h v s i c a l Chemistry o f t h e Academy o f Sciences R:idower u m i s s e p 5 . 11'JY M R L IN-ADLtRSHOF AbS1 R A L ! M o d i r v i n s (J teinplcite-tree s y n t h e s i z e d and h y d r o t h e r m a l l y dealuminuted LSIY-5 t v o e z e o l i t e w i t h o r t h o p h o s D h o r i c a c i d v i e l d s c a t a l y s t s t o r t h e h i g h - t e m p e r a t u r e MIO-reaction which a r e s u p e r i o r t o t h e n i e i ' e l y aealuniiiiot t'cl z e o l i t e s i n r e s p e c t o f Cz-Cd-olef i n s e l e c t i v i . t i e c nncl d e a c t i v a t i o n t i m e s . F o r m a t i o n o f methane, a r o m a t i c s ond coke i s : ; t i - o n y I v suppressed. I h e mechanism o f t h e P-modificnr t o n 3 5 char;ic,teriz;ld by on n t l e n s t partially r e v e r s i b l e c h e m i c a l i n t e i - i l i . t i u i 1 o i p h l i s E t j o i ' l c a c i d w i t h t h e sti-onq a c i d s i t e s and by n c t ~ i i : y e i.11 ; . o n c e n t r u t i n n and n a t u r e o t weak a c i d s i t e s which a l s o contrtbute f o c a t a l y s i s . A f t e r P-modification, dealumination of t h e z e o l i t e tramework by steoiaing i s i n h i b i t e d .

';inre t h e work n t Kciedinq and R u t t e r ( 1 ) who f i r s t hnvp shown t h e p o s i t i v e e t f r c t o f t h e PhosDhOrus m o d i t i c a t i o n o f ZSM-5 c a t a l v r t s on t h e i r r r r : a l v t i c p r o p e r t i e s i n t h e MIO-process and soniewhat l a t e r on t i l e P o c o - s e l e c t i v e a l k y i a t i o n o t t o l u e n e w i t h methono1 ( L ) , numeruus s t u d i e s have been aevoted t o t h e problem o t t h e niec;huiiisin o t ttiis i a o d i r i c a t i o n ( 3 - 11). KUt?dillg uiicl B u t t e 1 pruposed i n t h e i r Paper bonding o t PhosPhorLls t o the z e o l i t e framework t h r o u g h oxygen what f o r m a l l y i s equival e n t t o t h e reoiacpinent o f t h e h r i d g e d h y d r o x y l groups hv a HZP04QI'OUD bv means o t I P D mensuremmts o f NHs t h e y tound a s i g n i f i r a n t l v i n r r e a s f t l number o f a c i d s i t e s . which a r e weaker i n s t r e n u t h compared t i l t h those i n t h e unmodi t i e d z e o l i t e . Vedrine e t a 1 ( 4 ) s t z t e d t h a t Phosphorus n e u t r a l i z e s a c i d s l t e s

p r i m a r i l v u t t n e e n t r a n t - e a t t h e LSM-5 channels whereas t h e s t r o n aest n c i a s i t e s o r e n o t m o d i i i e d . Consequentlv t o r e x p l a n a t i o n o t t h e observed i n c r e n s e n f C2-C4-.nlefin selectivities. o slight aocl; : i r . a t i n n i n chnnnel r;ize und i n c r e a s e d t u r t u o s i t v due t o phos~ h o i ~ i i : ;comooirnd i s p r e f e r e d , compared t o changes i n a c i d s t r e n g t h . U e r e w i n s k i und Haber (6) found by NHZ-lPD t h a t m o d i f i c a t i o n w i t h r I. inie t n y 1piio 5 uii i t e ond d iilmmon i umhyd r agenp hos p ha t e e 1i m i n a t es t h e Hrrjn:;ted acirl s i . t e s and a l s o s i ~ n I f I c a n t decreases l~ the content u r W C ~ C I K C ~ Iricic'l . S I tes B~isecl on P~IISC~I~~I~Y experiments w i t h q u i n o l i n e and t r i m e t h y l p h o s p l i i t e Nunoil et o i ( L , j r;ont:liirierl, t h a t t h e most i m p o r t a n t e f f e c t o f u n o s ~ h ~ ~I:;r ~ ~r h; t : ooir,oning n t t h e s t r n n g ocirf site:, o n t h e e x t e r r i o l :;urtnr.f r i t t h e i ' . r , v s t a l s . K:IIl. Maiiit~c~arid Yii;hinio ( i i J ) s t u d i e d t h e shupe s e l e c t i v e a l k y l u t i o n n t ethvlhenzene W I tll e t h a n o l or1 P - m o d i f i e d ZSM-5 c a t a l v s t s and claim t h n t t h e S L I P P ~ ~ S S I O o~ i Para-di.ethvlbenzene L s o m e r i z a t i o n i s achieved by I'eduLing t h e amount o t s t r o n s a c i d s i t e s r a t h e r t n a n tiy r e ~ ~ u c i nthe a e t t e c t i v e pnre diameter o f LSM-5. S h i u e r u I k ( i 1 et r i l . ( 1 2 ) orepnred a c a t a l y s t f o r methanol conver(;oIir;i?,t i n u ot H'-LSM-5 oel l e t i z e d and m o d i t i e d w i t h sion onrl HPU4&- A c c o r d i n g t o t h e i r c o n c l u s i o n t h e b i n d e r reduces t h e a c i d s i t e s an t h e e x t e r n a l s i i r t u c e CJt t h e serves 0s a source o f g r a d u a l s u p p l y o f HPUqA-, which ocirl s i t e s arid r e t a r d s d e a l u m i n a t i o n . Most comprehent h e mechanism o f P - m o d i f i c a t i o n has been problein o f the wnrks n t l e r c h e r e t n l ( 7 . 8 . 1 4 , 1 5 ) . I n t h e i r w r I :FIw,i; 1, ( 8 ) tl;ev hnd develooed a concept o f c o n t r o l l e d decrease . * ;;i- -i,.; .?,i:-~r,ath hv c n n v e r s i n n o f s t r o n g a c i d s i t e s i n t o weakely i i ( i d oiler,. u i t l l o i i i alteration n t t h e OVel-all acid-base P r o p e r t i e s . 111 t t i , L: i;r:est p u b l i c a t i o n s they come t o t h e c o n c l u s i o n , t h u t t i e u ( i n e : i ~ o i t i l e zeolite w i t h H3PU4 and t r i i n e t h u l p h a s p h i t e decrea~ e ! tire , CcjiiLenti'ution O T s t r o t i s a c i u s i t e s e s s e i i t i u i l y by dealuniir x i T i n i i , I n t h e case n t t r i m e t h y l p h o s p h i n e as t h e m o d i f y i n g agent thev rniiiid ( 1 5 ) ('I hl.ocking o f s t r o n q a c i d s i t e s up t o u r e a c t i o n teinc;t?rotiir~C i t 0 O O n C . Ahc,ve t h i s t e m p e r a t u r e t h e phosphine i s de?,rjrtji?<j. i r ? q c i i e r o I 1 . i ~t h e o r i g i n a l a c i d i c p r o p e r t j . e s o f H I - L S M - 5 . Lumniar-iziriy t h i s o v e r v i e w i t may be s t a t e d , t h a t decreuse o t ;troriy i l i i i l i t y atiJ i ' t ' a u c t i o n o f p o r e d i a m e t e r a r e i n o s t l y d i s c u s s e d i n ui'tJei tc, e x p l a i n t h e e t i e c t o t phosphorus m o d i f i c a t i o n . the I e i c l i ~ v t ' 1 l l l I J l J I ~ t ~ i l oL r~ these r o c t o r s is a i t r e r e n t l y e v a l u a t e d and rfww-ids G I I t h e s t t i d i e d r e a c t i o n . C o n t r a d i c t o r y a r e t h e views o f v n r i o i i s nilthrbrs on t h e wov. i.n which s t r o n g a c i d s i t e s a r e decrenL i .

3

sed, by i n t e r a c t i o n w i t h t h e b r i d g e d h y d r o x y l groups, o r by u a r t i a l deaiumination. i n t h i s paper we d e s c r i b e t h e combined e f f e c t o f h y d r o t h e r m a l d e a l u m i n a t i o n and phosphorus m o d i f i c a t i o n on t h e s e l e c t i v i t i e s and d e a c t i v a t i o n rimes i n t h e high-temperature MTO-reaction. F o r a l l experiments t h e ZSM-5 type. template-free synthesized z e o l i t e HS-30 o f Chemie AG B i t t e r f e l d , has been used. TPD o f ammonia, I R and :*1P--.27AI-MAS-NMR soectroscopy were employed t o g e t deeper i n s i g h t i n t o t h e mechanism o f t h e z e o l i t e m o d i f i c a t i o n by o r t h o ptiosplioric a c i a . l h e work was p a r t o f a c o o p e r a t i o n w i t h t h e Researcn and Uevelopment D i v i s i o n o f Leuna-Werke-AG.

f r e P a r o r i o n v t soimles P e n t a s i l t y p e LSM-5 z e o l i t e (HS-30, Si/Al=19, Chemie AG B i t t e r t e l d ) i s c o n v e r t e d I n t o t h e H*-form by a f o u r t i m e s r e p e a t e d t r e a t m e n t w i t h a 0 . 2 N so1.ution o f HN03 i n a s i m u l e b a t c h Proced i i r e . For d e u l u s i n a t i o n . samples o f t h e H+-form a r e exposed t o a stream o t a i r and steam (pwater.=90 kPa) a t 400, 410 and 700°C. Framework aluminiirm ( A h ) i s determined u s i n g t h e ammonium exchonge method. For p a r t i a l e x t r a c t i o n o f non-framework aluminium t A 1 1 . i ~ ) torined d u r i n g deUlUminatiOn, samples a r e t r e a t e d w i t h 2 N HN03 a t 105°C f o r t w o h o u r s . E x t r a c t e d aluminium was d e t e r m i n e d o n a l y t i c a l l v i n the s o l u t i o n o f t h e e x t r a c t i n g agent. C a t a l y s t p a r t i c l e s were oreoared by m i x i n q t h e HS-30 samples w i t h Aerosil-ZGO and water and subsequent e x t r u d i n g , t h e ready made e x t r u d a t e s (d = 1 t o 2 mm) c o n t a i n i n g 6 5 % by w e i g h t o t z e o l i t e . P-modir i c a t i o n was c a r r i e d o u t by i m p r e g n a t i o n w i t h aqueous p h o s p h o r i c a c i d . i h e c o n t e n t o f phosphorus f o r c a t a l y s t s based upon e x t r a c t e d HS-30 was h e l a somewhat lower t a k i n g i n t o c o n s i d e r a t i o n t h e i r di.minished c o n t e n t s o f t o t a l . alilminium. F i n a l l y c a t a l y s t samples were c o n d i t i o n e d b y steaming a t 700°C f o r 1 h o u r .

Ammonium exchange z e o l i t e s a r e exchanged by NH4*-ions by l h e H I - i o n s o t LSM-5 treatment w i t h an aqueous s o l u t i o n o f an ammonium s a l t . F o r t h i s Purpose t h e sample i s p l a c e d on a 64 s l a s t r i t (approx. 4s) and exposed t o CI tiow o f 50u m l o t an aqueous 0,2 N NH4(CHaCOO) s o l u t i o n t o r b Iluurs a t /c) t o 80°C. A t t e r c a r e f u l washing, t h e c o n t e n t

4

o f ammonium i s d e t e r m i n e d by t h e K j e l d a h l method. D e t e r m i n a t i o n o f ahasphorus i n o w ous s o l u t i o n s Ihe suiiif d e v i c e a s i n t h e ammonium exchanse i s used t o a n a l y z e phosphorus removed from P-modified samples by e l u t i o n w i t h h o t w a t e r . Q u a n t i t a t i v e d e t e r m i n a t i o n o f t h e e x t r a c t e d Phosphorus i s based on t h e p h o t o m e t r i c molvbdo-vanada-phosphoric a c i d method. Temper a t w r o g r a m m e d des o r p t i o n o f am0n i a (NH 3-TPD) A t t e r - c l e a n i n g z e o l i t e samples by h e a t i n g t o 500°C i n an He 9ns stream ( v = 3 m l / s ) f o r one hoirr, ammonio i s odsarbed a t 120°C from a He gas stream c o n t a i n i n g 3 Vo1.-I o f N H 3 . A f t e r f l t r s h i n g by p u r e He a t 120°C f o r two ho:irs, d e s o r p t i o n o f ammonia up t o 500°C 1 s s t a r t e d crlow f a t e o f h e l i u m : 1 ml/s; heat r a t e l P C / m i n ) . I h e c o n c e n t r a t i o n o t NH3 i n t h e e x i t gas i s d e t e r m i n e d u s i n g a thernioc o n d u c r i v i t y c e l l . I h e c u r v e o f t h e desorptogram is r e c o r d e d DY a c o n v e n t i o n o l d e v i c e . A d d i t i o n a l l y ammonia i s absorbed from t h e e x i t acts stream SY on 0 . 1 N HaS04 s o l u t i o n . I t s t o t a l amount i s determined by back t j t r a t i o n o f excess s u l f u r i c o c i c l .

T R Soectroscooy LSM-5 z r o l i t e samples a r e compacted t o t h i n s e l f s u p p o r t i n g and p l a c e d i n a q u a r t z 1 R c e l l . The waTe1-s tiouohlu 7 mg/cmz) worers o r e c u r c i n e a a t 400°C i n vacuo. A f t e r c o o l i n g t o 200°C, p y r i o i n e vapour i s uailiitted i n t o t h e system f o r 30 m i n u t e s . A f t e r w u r ~ t s , t l i e c e l l 1 s degusbecl and evacuated t o e l i m i n u t e p h v s i s o r b e d P v r i d l nf Transmission s p e c t r a a r e r e c o r d e d i n t h e range from 4000 t o 3300 cm-' b o t h h e t o r e and a f t e r p y r i d i n e n d s o r p t i n n . u s i n g o SDecord M 85 spectrometer o f C a r l Z e i s s Jena w i t h a 4 c m - l r e s o l u t ion. Diffuse r e t l e c t a n c e s p e c t r a a r e r e c o r d e d w i t h t h e t - T - 1 H t e c h n i q u e t h e r e s o l u t i o n o f which i s 4 cm-I. The u s i n g u s p e c i a l aevice, z e o l i t e Powdei i s p r e t r e a t e d f o r t o u r h o u r s a t 450°C and a t 10- ' Po

L 7 A l- and P-MAS-NMK--measurement s I h e NMK s p e c t r a were o b t a i n e d on a BRUCKER MSL4OO m u l t i n u c l e a r spectronieter o p e r a t i n g a t u f i e l d o t 9 , 4 r w i t h a s t a n d a r d BRUCKER d o u b l e - b e a r i n g MAS p r o b e , I h e z i r k o n i u o x i d e r o t o r s were spun near

5

5 kHz w i r h a r y n i t r o g e n as d r i v i n g gas. About 200 my o f sample m a t e r i a l a r e f i l l e d i n t h e i n n e r r o t o r volume o f about 0 . 3 5 cm:’. T y p i c a l l y , 1500 t o 1800 f r e e i n d u c t i o n decays ( F I D ’ s ) were accumulated p e r sample f o r b o t h n u c l e i . The p u l s e w i d t h s were 3 . 9 11s (n/U-pulse) w i t h r e p e t i t i o n times between 5 t o 30 s and 0.6 11s (n/l2-puise) w i t h a r e p e t i t i o n time o f 1 s f o r a t 362 MH7 and f o r 2 7 A l a t 104 MHz, r e s p e c t i v e l y . The c h e m i c a l s h i f t s a r e g i v e n on t h e d s c a l e w i t h a maximum u n c e r t a i n t y o f fl Ppm. I n o r d e r t o o b t a i n r e l i a b l e q u a n t i t a t i v e r e s u l t s by t h e L’7Al-MAS;NMR measurements, t h e samples were h y a r a t e d i n an e x s i c c a t o r i n an atmosphere s a t u r a t e d wi.th water f o r more t h a n 20 h o u r s . Ihct ahs o l u t e i n t e n s j t v mode (AI-mode o f t h e DISMSL-software package) was used t o r e x p e r i m e n t a l d a t a a n a l y s i s . La t a l v s i s C a t a l y t i c experiments were c a r r i e d o u t i n a t u b u l a r f i x e d bed r e a c t o r (6 = 12 mm) u s i n g a m i x t u r e o f methanol/Nz ( m o l e c u l a r r a t i o = 1 : l ) as r e a c t i o n feea ( c o n d i t i o n s o f r e a c t i o n see t a b l e 1 ) . Products were analyzed DY an o n - l i n e o p e r a t e d guscnromotogrophic device C a l c u l a t i o n o f t h e c o n c e n t r n t i n n from a n a l v t i c a l d a t a i s based on i n d i v i d u a l c a l i b r a t i o n o f t h e components

lABLE I : Parameters o f r e a c t i o n ~ _ _ t enipe r a t u r e :

c o n c e n t r a t i o n o t methanol: c a t a l y s t volume: c a t a l y s t mass ( a v e r a g e ) : teed t-are ( i n i x r u r e ) : charge GHSV ( m i x t u r e ) . LHSV(methano1):

~

560°C 50 Val-% 12 m l 5.b 9 4 . 0 m l / s (Sir)

1200 m l / m l c a t 1 m 1/ml a t

I vs t I

I.

*h *h

I h e t i m e on stream when methanol f i r s t appears i n the e x i t gas o f t h e r e a c t o r i s clenGted as t 1 m e o t d e a c t i v a t i o n C a t a l y s t s a r e named a c c o r d i n g t o t h e g e n e r a l formula D X P v . x d e n o t i n g the b i / A l ~ r a t i o and Y t h e c o n t e n t o t pnosphorus us weight p e r c e n t .

6

RESUL LS .MRDILSCUSSIQN R e c e n t l y we have shown ( 2 0 ) , t h a t d e a l u r n i n a t i o n o f z e o l i t e HS-30, used as c a t a l y s t i n t h e M'10-reaction a l t e r s t h e s e l e c t i v i t i e s o f l i g h t o l e f i n s and o f a r o m a t i c s i n a s i m i l a r way as was found by Chans e t a 1 . ( 1 9 ) f o r ZSM-5 c a t a l y s t s s y n t h e s i z e d w i t h v a r i o u s S ~ / A I F r a t i o s . f o r t h e r e a c t i o n c o n d i t i o n s chosen i n t h i s worK t n e e t t e c t o t w r y i n g c a t a l y s t S i / A l F r o t i o on t h e s e l e c t i v i t i e s o t cZ-c4- o i e T i n s , L 6 - b - U r o l n a t i c s and methone as w e l l a s on t h e d e a c t i v a t i o n t i m e i s demonstrated i n F i g . 1 . S e l e c t i v i t i e s o f l i g h t o l e t i n s i n c r e a s e UD t o almost 70% wh,ile t h e f o r m a t i o n o f a r o m a t i c s ond methane i s d i s t i n c t l v decreased UP t o c a t a l ~ s t S i / A l F o r 2 6 8 . lhese changes a r e accompanied by ii inarked ~ r o l o i i ~ a t i o n o i t h e d e a c t i v a t i o n t i m e . As d e s c r i b e d i n a s e p a r a t e paper ( 7 0 ) a remarkable i n c r e a s e o f t h e d e a c t i v a t i o n t i m e may be n t t a i ilea by e x t r a c t i o n o t t h e non-tramework aluminium u t t e r s u i t i c i e n t IY l o n g d e a l u m i n a t i o n a t mediuin t e m p e r a t u r e s . F u r t h e r dealuminat i n n , however, does n o t r e s u l t i n a b e t t e r c a t a l v s t . O l e f i n s e l e c t i v i t i e s t e n d t o decrease a g a i n , t h e some h o l d s t r i i e o f t h e dent:-t i v a t i . o n t i m e . W h i l e t h e s e l e c t i v i t i e s a t a r o m a t i c s a r e n o t sn

I0

6C

50

m \

40 Y

r(

c

Y r(

10 m 9 #

! 20 L 10

k

,so '61

'9 6

268

102

1

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0

1.25

-time o r dcactiv.

F i g . l . E t t e c t o f d e a l u m i n a t i o n on p r o d u c t s e 1 e c : i v r t i e s and O e a c t i v a t i o n t i m e compured w i t h u d d i t i o n a l phosphorus inod1 t i c a t i o n

7

mucn affected, a significant growtn o f methane - in particular on the catalyst with S i / A l F 480 - is observed. lhus maxima o f olefin selectivity and a t deactivation time, attainable by dealumination. seem to exist. Concerning the deactivation time this behaviour is to be expected since the concentration of strong acid sites hecomes very sma 1. lhe behaviour of the oiefins, however, is less plausible. lhe Turtner decrease of strong acid sites should result in a further loss ot catal.ytic activity and consequently favour the accumulat on of the lower olefins as the primory products in the reaction sequence. Under the severe reaction conditions the light olefins - ethene in particular - are partly formed a l s o by crackina of higher olefins. A strong suppression of these cracking reactions by the decrease of strong acid sites as compared with tne diminished rate of the overall reaction may therefore contribute to the changes in olefin selectivity. Indeed. the portion of ethene in the c 2 - c 4 olefin fraction decreases from 30% for D 2 6 8 to around 18% for 0 4 8 0 . B e s i w s , one has to taKe into consideration, thflt Si/AlF ratios above 270 can be obtained with reasonable times o f hydrothermal treatment only at temperatures o f 700 - 750 " C . Nevertheless no loss of cristallinity could be detected by X-ray diffraction tor the sainples treated at this severe conditions. However, they undergo a phose transformation from orthorhombic to monoclinic. As wns reported ( 2 1 ) this phase transition is indeed aependent on sl/ulF ond temperature. It i s not clear, however, wtietner this i s ot importance for the observed reversed olefin selectivities or not. For further improving the selectivity of light olefins. suppressing the tormation o t aromatics and increasing the deactivation time, the modirication with phosphorus, f i r s t used by Sutter and Kaeding ( 1 ) with non-dealuminated LSM-5, was chosen. Ihe results achieved arter systematical study are examplitied by the catalyst b 2 6 8 f 1 . 2 5 in k i g . 1 . i n order to rina tile conditions Tor preparing catalysts with optimum activity, maximum light oletin selectivity and deactivation time. the influence o f qrowing amounts o f phosphorus at different degrees of dealumination on the product distributlon of methanol coilversion had to be studied. As shown in Fig.2 UP t o 2% phosphorus the selectivities O T L 2 - i 4 oieTins grow on a level which i s the higher the greater the Si/AiF ratio, except for very hign S i I A 1 ( 1 ) 3 0 0 ) . Aocive 2% i.' the octivities 01 the catalysts drop, dimetnvlether appears right from the beginning o f the reaction and the selectivities o f the CZ-C4 olefins ore drastically decreased.

8

ring

only

hydrocarbons

as reoc-

sz

t i o n p r o d u c t s . Quite t h e o p p o s i t e b e h o v i o u r show t h e s e l e c t i v i t i e s o f t h e a r o m a t i c s . They f a l l w i t h g r o w i n g c o n t e n t o t phosphorus, but agoin the catalysts with d i T r e r e n t S i / A l F r a t i o s have d i f ferent levels o f s e l e c t i v i t i e s , t h e lowest t o r the c o t o l ~ s t s w i t h the highest degree o f dea l u m i n a t i o n . Here t h e samples w i t h S i / A l i - = 3 3 0 e x h i b i t no exc:ept i o n a l behoviour. [he suwr'es-

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k i g . 5 , t t i e c t o t Phosphorus s e l e c t i v i t y O T aroinotics l o r various S i / A l F r a t i o s

0.5

1.0

1,5

2.0

2.5

3.0

S P

k l Q . 4 . tlrei1 O T P1105PtlOfU5 on s e l e c t i v i t y 0 1 inetnune for varioiis S i / A l t . rfltins

9

p u r t i c u l n r i n the r e g i o n between 1 . 5 and 2 . 0 % P . i t i s g e n e r a l l y acceated now. t h a t t h e higher o l e t i n s a r e formed mainl v hv n l k v l n t i o n o f t h e l o w e r ones w i t h m e t h a n o l o r d i n i e t h ~ l e t h e r and t o wme degree a l s o hv t h e i r o l i g o m c r i z a t i on. tthene i s the primary p r o d u c t o f t i r s t C-Cbond toi-tnation, b u t un-

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10

ably also for crackinn. while catalyst ability for the alkvlation o f oiefins by methanol is evidently less affected. lhis conclusion i s supported by an experimerit with cofeeding of Propene to the methanol stream durjng the MTO reaction. A higly dealuminated cotalyst sample with very small concentration of strong acid Bransted s i t e s , additionallv modified with 0.5% phosphorus was chosen for this purpose. The most striking effect of cofeeded propene. as f o l l o w s troin table 2, i s the distinct increase of methanol conver-

WHSVIh- I ) Methanol WHLV(h-') Prnoene Methanol conversion ( X I

1,55

1,55

0.00

0,46 98,7

90.3

Products

+8.4

(% V o l . )

Methane

1,65

0.81

-0,84

E.f ha tie

0,05 0.31 0.35

0,05 0,49

0.14

0,zo

0,oo +0,18 +o. 19 + O . 06

0,81

0,96

+0,15

Prooane I -Butane N- Hu tflne La-Aliphatics senzene I oluene Xylenes Cs-Aromatics

0.54

0,oo

0,04

0,lO

0,16

+0,04 +0,06

0,3Y

0,38 0,06

-0,Ol -0,015

0,075

sion, which most likely may be explained as the result of its consumption in the alkylation of added propene. Indeed the concentrations of all Ca+-products. except for Cs+-aromatics. are

11

increusecl. I n e t a c t , t n a t t h i s i s v a l i d a l s o f o r t h e c o n c e n t r a t i o n o f ethene i s i n d i c a t i v e o f t h e a t l e a s t p a r t l y m a i n t a i n e d c r a c k i n g a c t i v i t y o t the catalyst. To g e t deeper i n s i g h t i n t o t h e mechanism o f m o d i f i c a t i o n by phosphorus t h e i n t l u e n c e of growing amounts o f phosphorus on t h e con'*O c e n t r a t i o n o f weaK and 0-3 strong a c i d s i t e s was s t u d i e d by NHS-TPD. I h e 7mC.6 IPD t r a c e s o f ammonia de- ,+ s o r p t i o n show characteOB: r i s t i c a l l y o f ZSM-5 ( 2 2 ) two maxima, one f o r weak 'D! z acid sites of ditferent n a t u r e a t about 240°C and ' 0 '1#5 ' 0 '1.5 Po '1.0 '0.5 '2.5 '0.5 p ~ , 2 ~ '0.5 '2.5 a h j g h temperature peak -ti2*-&"-3,07the o t 420°C r e f l e c t i n strong Bronsted acid F i g . 6 . E f t e c t o t P on weak and s t r o n g sites (bridged hydro(hatched p a r t ) a c i d i t y f o r x v l groups zSi.-p--Al=). various Si/AlF of the z e o l i t e il i h e i r c o n c e n t r a t i o n c o u l d De determined q u a n t i t a t i v e l y f o r SUtTlc i e n t l v , separated peaks by t h e i r peak a r e a s . Otherwise, t n e amount o t s t r o n g a c i d si.tcts was determined s e p n r a t e l y from s p e c i a l exaeriments a f t e r NH3 a d s o r p t i o n a t 2500C. The r e s u l t s a r e demons t r a t e d i n F i g . 6 . The f i r s t column o f each group ( P O ) r e p r e s e n t s the data f o r the unmoditied c a t a l y s t s . Obviously both type o f s i t e s decrease w i t n d e a l u m i n a t i o n , a t l e a s t . above S j / A 1 ~ = 2 8 . A f t e r m o d i f i c a t i o n w i t h g r o w i n g amounts o f phosphorus, t h e concent r a t i o n o f s t r o n g Bronstea a c i d s i t e s i s d r a s t i c a l l y d i m i n i s h e d by a f a c t o r o f 10 t o 5 depending on S i / A l F r a t i o t h e most pronounced e f f e c t b e i n g ochi.eved w i t h 0 . 5 % phosphorus. a l r e a d y . F u r t h e r i n crease o f P-content r e s c i l t s i n p r a c t i c o l l v no change o f t h e i r conc e n t r a t i o n , which, i f any. l i e s w i t h i n t h e margins o f e x p e r i m e n t a l e r r o r ( * 0 . 0 0 5 mmol/g N H 3 ) . T h i s i s i n p a r t i c u l a r v a l i d f o r t h e c a t a l y s t s based on 068 and 0107. I-he c o n c e n t r a t i o n o f weak a c i d s i t e s i s reduced d i f f e r e n t l y , dePendinQ on t h e P-content and on the s l / U l F r a t i o 0s w e l l . t a c h o r t h e samples oased on D2e and D ~ H seem t o have a maximum c o n c e n t r a t i o n a t 1 . 5 % P w h i l e f o r t h e cat h e l o w e s t c o n c e n t r a t i o n o f weak a c i d s i t e s tfllvsts DlOiP0.s-2.5 i s o b t a i n e d w i t h 0 . 5 % P . I n t h i s case f u r t h e r i n c r e a s e o f P-conA

8

0"

12

t e n t i s accompanied bv a c o n t i n u o u s i n c r e n s e o f t h e c o n c e n t r a t i o n o f weak a c i d s i t e s . r e a c h i n g w i t h 2 . 5 % P an even h j q h e r v a l u e t h a n w i t h o u t phosphorus. l h i s complex dependency is a c l e a r i n d i c a t i o n o t a change i n t h e n a t u r e o f weak a c i d s i t e s w i t h P - m o d i f i c a t i o n . some o f them h e i n s removed, o t h e r s newlv c r e a t e d . I h e f a c t t h a t P-content above 1 - 1 . 5 % t o r a l l S ~ / A I Fr a t i o s p r a c t i c a l l y t h e same c o n c e n t r u c i o n o f s t r o n g a c i d s i t e s is observeo l e a d s t o t h e c o n c l u s i o n , t h a t t h e d i s t i n c t d i f i e r e n c e s i n Product s e l e c t i v i t i e s . o b t a i n e d i n t h j s ranqe o f P - c o n t e n t s . s h o u l d he due, a t l e a s t p a r t l y . t n t h e d i f f e r e n c e s i n n n t l r r e and c a n c e n t r a t i o n o f t h e w e a k a c i d s i t e s . rhus. weak a c i d s ' i t e s n t t h i s high-temperature c o n d i t i o n s a r e l j k e l v t o p a r t i c i p o t e i n c a t a l v sis. O f course, an a d d i t i o n a l shape s e l e c t i v i t y e f f e c t caused bv t n e growing amount o t P-compounds d e p o s i t e d i n s i d e t h e channels cannot iie r u i e u o u t . l h e q u e s t i o n i s now. how t h e decrease o f s t r o n g nnd weok a c i d s i t e s caused by phosphorus m o d i f i c a t i o n can he e x o l a i n e d As n l ready mentioned. t h i s problem i s s t i l l d i s D u t e d . h u t t h e n i i t h n r s OT most r e c e n t p u b l i c a t i o n s i n t e r o r e t t h i s e t f e c t e s s e n t i n l l v hv d e a l u m i n a t i o n o t t h e z e o l i t e . I n case or a c n e m i c a l i n t e r a c t i o n o f p h o s p h o r i c n c i d w i t h t h e b r i d g e d h v d r o x v l 4roiIPs as t h e a l t e r n a t i v e t o d e a l u m i n a t i o n , one would expect a c e r t n i n degree o r t n e columns r e p r e s e n t a g a i n t h e sulil of reversibility. i n Fig./. wenk and strong acid s i t e s o f a non-dealuminnt e d z e o l i t e sample. ThF! m o d i f i c a t i o n o f t h i s onrl sample w i t h 2 . 5 % P treatment for 1 hour i n a He- stream n t 500 O L causes a d i s t i n c t decrease o t s t r o n q nc;.tl sites. A T t e r e x t r a c r i o n o i phosp n o r i c a c i d w i t t i water u t 80 C'C t o r b h o u r s , nowever, t n e former concent r a t i o n o f strong uciu Fig.7.Changes o f a c i a s i t e s c o n c e n t r a sites i s fully restorea. t i o n of a P-modified s a m p l e ( D ~ ~ P ~ . o ConseqitPnt1.v ) the rlisa f t e r various treatments (hatched onpearence o f s t r o n a a c i d p a r t o t columns: c o n c e n t r a t i o n o f s i t e s has t o be unders t o o d as t h e r e s u l t o f strong acid s i t e s ) .

13

reversible

chemicnl i n t e r a c t i o n o f p h o s p h o r i c a c i d w i t h t h e b r i g -

df*d h y d r o x v l q r o i t p s and cannot be due t o d e a l u m i n a t i o n o f t h e zeo-

. I i , t e . I f t h e P - m o d i f i e d sample p r i o r t o e x t r a c t i o n i s steamed f o r 4 hours o t 40u "i r e s p . t o r 0 , 5 hours a t 700 "C a f u r t h e r decrease o r s t r o n g and weak a c i d s i t e s occurs (see F i g . 7 ) . The same e x t r a c t i o n pi-i.rr.edure, employed t o t h i s samples, a g a i n r e s t o r e s t h e aci.d s i t e s b u t the o r i g i n o l c o n c e n t r a t i o n o f t h e s t r o n g a c i d s i t e s i s nnt n t t n i n e r l l r r e v e r s i b l e changes. o b v i o u s l . . ~d e a l u m i n a t i o n , have token olnce. I t tins t o b e noted, however, t h a t a f t e r steaminq i n c o n t r a s t t o t h e i n e r e l y thermal o r e t r e a t m e n t i n He a t 500 " C . phosp h o r i c u c i a c o u l d be e x t r a c t e d o n l y by 89% and 60% r e s p e c t i v e l y . l h e c ~ u e s t i o n o r i s e s us t o whether the Presence o t t h e p h o s p h o r i c nLi.d 1.n the z e o l i i e channels i a v o u r s t h e process o f d e a l u m i n a t i o n o r n o t . i h e ciiogront i n t-19.8. shows t h e c o n c e n t r a t i o n o t weak and s t r o n g a c i d s i t e s o f t w o HS-30 samples one un1.1 m o d i f i e d and t h e other 1.0 0.9 modified w i t h 2,5% P o f 0.8 t e r steaming a t eqiinl. c 0.7 condi t i o n s and subsealtent 'b 0.6 e x t r n c t i o n o f ohosohorus .+ 0.5 from t h e m o d i f i e d sample. gE 0.4 The concentrations o f v 0.3 $0.2 strong acid s i t e s d i f f e r z 0.1 by a f a c t o r o f t w o i n t a vour o f t h e P-inoditied i i ~ . 8 . A c i u i 1cnuiiges ~ O T unmoditied sample. Consequently t h e U i i i l i'-InOC)iTieCI Hb-50 ( Z , 5 X P ) o t t e r modification o f strong steominu und e x t r a c t j o n o t P . a c i d B r o n s t e d s i t e s by phosphoric a c i d excerts a p r o t e c t i n n e t t e c t on t h e A1F-atoms r a t h e r t h e n f a v o u r i n s t h e j r t r o m t h e Traniework. I h o t e x t r a c t i o n o f phosphorus l e a d s exvu1:ion t o i u t u l y s t s w i t h p r u p e r t i e s approaching a g a i n t h o s e o f u n m o d i f i e d i ; u t a l y s t s i s cleinonstratecl i n F i g . 9 . I h i s h o l d s t r u e a l t h o u g h , due t u 11io1-2sever-e steuiiiiriy o t t h i s sample, o n l y about 16% o i pnosphor u s c o u l d be removed. A complete r e s t o r a t i o n o f t h e o r i g i n a l p r a a e r t i e r , o t IJnmOditied somples t h e r e f o r e c o u l d n o t be expected. Neverthelexr, t h e t o c t o t r e o p p r o a c h i n g t h e former p r o p e r t i e s i n d i c o t e ! : t h a t t h e v e r v molecules o t p h o s p h o r i c a c i d which hove i n t e r n c t e d w i t h t h e s t r o n a o c i d si.tes can e a s i l y be removed by e x t r a c ~ 1 0 1 1 . i h e gi.eiitt:r p o r t o f phosphoric a c i d m o l e c u l e s must e x i s t i n s i d e t h e channels i n a c l i t f e r e n t s t a t e , o b v i o u s l y c h e m i c a i l y

-

14

70

30 20 10

k1g.Y. Product s e l P c r i v i t i e s a f t e r P - e x t r a c t i o n compared w i t h [inm o d i t i e d nnd m o d i t i e d samDles bonded w i t h t h e non-framework a l i i m i n i u m k u r t n e r s u p p o r t and a d d i t i o n a l i n f o r m a t i o n on t h e mechanism o f Pm o d i t i c . a t l o n wete o b t a i n e d by means o f 1R- ond MAS-NMR-spectroscoPV I h e T i r s f two s p e c t r a i n F i g 10 a r r a n g e d one upon a n o t h e r , i l l u n t r n t e t h e e f f e c t o t P - m o d i f i c a t i o n ( 2 5% P, w i t h o u t steamiiiq)

3610 I

k i s . i U . I K - l r a n s l n i s s i o n s p e c t r a o f HS-30 samples o t t e r d i f f e r e n t o r e t r e a t m e n t s . Kegion o t O H - s t r e t c h i n g f r e q u e n c i e s .

15

nn t h e i n t e n s i t v ,if t h e s t r o n a hand o t 3610cm-i which is g e n e r a t e d bv t h e b r i d g e d OH-arnilos i n t h e 7 e o l i . t e . 1-he d r a s t i c decrease o f t h i s hnnd i s riuoarent. The next two s p e c t r a demonstrate how t h e i n t e n s i t i e s and t h e n a t u r e o f OH-groups a r e changed by steaming and SIJbSeClUent e x t r a c t i o n o f phosphorus. besides t h e d i s t i n c t l y aecreased band a t 3610 cm-I, t h e appearance n f n new hand nt 3 6 6 5 c m - I , which hos been assigned t o A1-OH groups o f non-framewnrk aluminium (23). i s r e g i s t e r e d . ConsequentI v . steomina f o r 5 hours a t 400°C causes a o a r t i a l d e a l u m i n a t i o n . Ihe l a s t two s p e c t r a show. what happens w i t h t h e u n m o d i f i e d samole a f t e r t h e sume steaming p r e t r e a t m e n t compared w i t h t h e P - m o d i f i e d one ( t h e t o p s p e c t r u m ) . l h e decrease o f t h e s t r o n g B r o n s t e d s i t e s is s z g n i r i c a n t i y g r e ( l l e r , d e a l u m i n a t i o n proceeded t o a l a r g e r degree. I h u s . t h e p r o t e c t i n g e f f e c t o f phosphorus i s c o n f i r m e d a l s o bV t h e IR-dfltfl. lhe i n t e r a c t i o n a t ohosohoric a c i d w i t h non-framework aluminium i s shown i n F 1 9 . 1 1 . M o d i r y i n g a deaJuminated sample w i t h P p r a c t i c n i l v r e s u l t s i n the disauuearance o f t h e band a t 3 6 6 5 cm-I , which reappears, however. a r t e r severe steaming a t 7OO0C, a p p a r e n t l y as u consequence o t f u r t h e r d e a l u m i n a t i o n and d e f i c i e n c y o f phosphor u s t o r c o i w l e r e oonding t h e newly c r e a t e d non-framework alum].-

3740

P at 120°C 37iC

/ :::5 -b-------L-

3900

I k i 8

at 700°C +steamed lh

Il.trrect

OT

P on 1 R -

hnnu i n t e n s i t i e s a t 5 b 6 3 and 5610 cm-'

3800

3700

3600

--

3500 ?/Em

3600

-'

Fig.lZ.IR-dittuse reflectance spectra o f dealuminated HS-30 b e f o r e ( f u l l l i n e ) and a f t e r P - m o d i f i c a t i o n

16

niiim. l h e i n t e n s e i n t e r a c t i o n w i t h t h e s t r o n g a c i d s i t e s and t h e OH-QroUPS o r non-TrameworK a l u m i n i i l m can be concluded a l s o from the d i f t u s e r e t t e c t a n c e IR-spectra presented i n F i g . 1 2 . A dealuh l z , r u l l l i n e spectrum) shows t h e t y p i c a l bands minuted sample Tor o i l o b s e r v e a b l e OH-groups as a s s i g n e d b e f o r e . A f t e r m o d i f i c o t i o n w i t h 1 . 5 % P t h e c h a r a c t e r i s t i c bands o f t h e s t r o n g a c i d s i t?-, (361.0 cm-l) rind t h e A ~ N F - O H groitos ( 3 3 6 5 cm-') a r e s i q n i f i c n n t i v reduced. l n t e r e s t i n s l v t h i s spectrum r e v e a l s a s h o u l d e r o f rJ twitid a t 3670 cm-' - a frequency r e g i o n , where P-OH-groups show absorption. i h i s muy be c o n s i d e r e d as an i n d i c a t i o n o f t h e i r p r e sence. Wh;le i t i s c l e a r trom t h e s e d a t a , t h a t p h o s p h o r i c a c i d r e a c t s w i t h t h e b r i d a e u h v d r o x v l groups and t h e non-tramework a l u m i n i u n i CIS w e l l . t h e i n t o r m a t i o n about t h e n n t u r e o f t h e p r o d u c t o f t h i s i n t e r a c t i o i i remains r a t h e r D o o r . l h e r e f o r e t h e P - m o d i f i e d sarnolea o d d i t i o n o l l v :;tiidie!d hv :I1P- and "'Al-MAS-NMR spectroscopv. I h e :"P-MAS-NMR spectrum o f a d e a l u m i n a t e d P-modified sample (spectram A i.n k i q . 1 3 ) e x h i b i t s a t l e a s t t h r e e d i f f e r e n t resonance i i n e s . one o t them ( a t =1 ppm) corresponds t o t h e o r i g i n a l o r t h o ohosphorlr: o c i d . t h e second a t - 1 5 ppm r e p r e s e n t s c h e m i c a l s h i f t s u s u a l l y o b t a i n e d t o r s h o r t c h a i n polyphasphates (241, and t h e r e inoinina l i . n e n t nboitt -;."-I opm corresoonds t o t h e range c h a r a c t e r i ~ ~ -, :;tic o f ~;IiiminiUr;i ahos- 3 r p - NMR phiites ( 2 5 ) . Residues u l A o r t h o u n o s p h o r i c o c i u and D95p1,6 Y.:traotion (80.C) dried a t 120% snort c h a i n Doiyuhosphate s ~ e ~ i e fart? , removea by e x t r u c t i o r i w i t t i wuter spzctt'uin ils L ii n s k Shown i g . 1 5 by 111

at l h 100% eteame:

A-A ax'raction

(80.C)

which t t i e col-I-esPunuinY - ," s i g t i a l s a r e absent . u n i y t n e i i n e cnurocteristic ot alumiilluni0 0 -0 -10 0 3 -0 -rG pnosphates, t h e pr'ouuct 71 0: i n t e r a c r i o n o r phosk i q 13.="P-MAS-NMR s p e c t r a o t P - m o d i t l e d . u h o r i c a c i d w i t h nondealurninated samples o t t e r v a r l o t i r f I ninework oluminium pretrmtments r emoins ITt h e sample i s steamed t o r 1 hour a t 100 " C (spectrum b) t h e s i ~ n a i s a t 1 cliiu - 1 5 ppin d i s a p p e u r and new l i n e s a t -3Y and -4b pplli appear U b v l o u s l y t h e p h o s p h o r i c anu POlyPhOSPhOt~lC a c i d

17 species undergo f u r t h e r condensation r e a c t i o n s , s i n c e t h e new lines may be i n t e r p r e t e d as caused by h i g h l y condensed p o l y ~ h o s h a t e species ( 2 6 ) . A chemical s h i f t a t -46 ppm is known f o r t h e branc h i n g groups i n P 4 h O ( 2 4 ) . These s p e c i e s can be removed by h o t water e x t r a c t i o n as is r e f l e c t e d by spectrum D, i n which t h e c o r r e s p o n d i n g l i n e s almost c o m p l e t e l y v a n i s h . Unchanged remains t n e s i g n a l a t -30 ppni c h a r a c t e r i s t i c or aluminium phosphate. Inus, d i r e c t evidence i s g i v e n f o r t h e i n t e r a c t i o n p r o d u c t o f pnosphoric a c i d w i t h non-framework aluminium. U n f o r t u n a t e l y . from these d a t a t h e e x i s t e n c e of an i n t e r a c t i o n o r o d u c t o t p h o s p h o r i c a c i d w i t h b r i d g e d hvdroxv.1 groups cannot be concluded. However. t n e "'Al-MAS-NMR p r o v i d e s o d d i t i o n a l i n d i r e c t evidence t o r t h e e x i s t e n c e O T such a p r o d u c t . The spectrum on t o p of F i g . 1 4 shows t h e c h a r a c t e r i s t i c l i n e o f A ~ Fa t 5 5 ppm and a s m a l i amount o f o c t a h e d r o l l y c o o r d i n a t e d A I N F a t -1 ppm. P - m o d i T i c a t i o n o f t h i s sample and steaming T o r 3 hours s i g n i f i c a n t l y l o w e r s t h e i n t e n s i t y o f t h e A l l . s i g n a l . The s i g n a l a t about -10 ppm i s c h a r a c t e r i s t i c o t o c t a h e d r a l l y c o o r d i n a t e d aluminium ( 2 7 ) I n aluminophosphates, which r e s u l t from t h e r e a c t l o n o f orthoDhosPhoric a c i d w i t h A ~ N F . I h e secona new s i g n a l a t 40 PPm stems from t e t r a h e d r a l l y c o o r d i n a t e d A ~ N F a l s o p r e s e n t as alulninophosphutes. Other t e t r a h e d r a l l y c o o r d i n a t e d A ~ N F species can c o n t r i b u t e o n l y t o a s m a l l e x t e n t . dy means o f n u t a t i o n NMR spectroscopy (28, t h e quadrupoie i n t e r action o t the nuclei g i v i n g r i s e t o t h i s l i n e was shown t n be s m a l l . l h e soectrum a t t h e bottom o f F i g . 1 4 of demonstrates. t h a t e x t r a c t i o n phosphoric a c i d i n c r e a s e s c o n s i d e r ably the i n t e n s i t v of the Ah-signal a t 5 5 w i n . A s r e u i u i n i n a t i o n a t these I . . . . ,. . _I.. . . . I . . c o n d i t i o n s is u n l i k e l y t o occur, D22P1 , 6 (>h/7W°C) t n i s pnenonienoii i s i n u i c a t i v e TOI- o very close i n t e r a c t i o n o r r w i t h , . . . . , . . . . I . . . . , AIP, causing a diStOrtiOn O T i t s Syininetry, Which i s e i i m i n a r e c l DY t h e D2*P,,(, eluted removal of P . so, t n e i n t e n s i t y p t t h e A1F-signal i s a t l e a s t p a r t l y reduced by t h i s i n t e r a c t i o n . $00 50 0 -50 ppo F i n a l l y a few words concerni.ng t h e n a t u r e o f weak a c i d s j . t e s . I t has F i g 14.z7A1-NMH s p e c t r a o f been P o i n t e d o u t a l r e a d y . t h a t P- DzzHS-30 b e f o r e ana n f t e r P m o d i f i c o t i o n changes n o t o n l y t h e i r m o d i t i c a t i o n and - e x t r a c t i o n

A

-

18

c o n c e n t r a t i o n b u t a l s o t h e i r n a t u r e . Some i n c l i c a t i o n as t o t h i s p o i n t was o b t a i n e d trom t h e temperature-programmed d e c o m p o s i t i o n o f t h e NH-~*-exchanaed, P - m o d i f i e d samples.

c h

,. ?

d

I

2M

F i n . l 5 . T e m p e r a t u r e - ~ r o g r a m m e d d e c o m p o s i t i o n o f an NH4+-exchanged u n m o d i f i e d ( l e f t ) and P - m o d i f i e d somple. From F i a . 1 5 i t appears, t n a t i n d i s t i n c t i o n t o t h e u n m o d i f i e d samp l e , t h e P-modified sample b e s i d e s t h e h i g h - t e m p e r a t u r e PeaK exnib i t s a second one w i t h a maximum a t about 280 " C . l h i s peok may be assigned t o a c i d s i t e s w i t h medium s t r e n g t h . [ h e i r p r o p e r t y t o he ion-exchangeable f a v o u r s t n e i r i n t e r p r e t a t i o n us f i r o n s t e d a c i d s i t e s . Whether t h e i r e x i s t e n c e i s due t o t h e i n t e r o c t i o n p r o d u c t of phosphorlc acid w i t h b r i d g e d h v d r o x y l groups o r w i t h non-framework oluminium i s d i f f i c u l t t o d e c i d e w i t h o u t f u r t h e r experimental data.

D e a l u r i n a t i o n oT LSPI-5 t y p e HS-30 z e o l i t e bv h y d r o t h e r m a l t r e a t ment decreases t h e s t r o n g a c i d S i t e s and - c o n s e q u e n t l y i s ossoc i a t e d w i t h a d i m i n i s h i n g o t c o t u l y t i c a c t i v i t y . C o n s e c u t i v e orod u c t s o f t h e o l e f i n s a r e suppressed and t h e Cz-C4-olefin s e l e c t i v i t y i s r a i s e d . I t s v a l u e seems t o he l i m i t e d . however. t o about b 5 X . M o d i t i c a t i o n of P r e v i o u s l y d e a l u m i n a t e d samoles w i t h o r t h o phosphoric a c i d i n c r e a s e s t h e s e l e c t i v i t y o t cZ-c4 o l e t i n s UP to 80%, s u p p r e s s i n g t o r m a t i o n o f o r o m o t i c s and coking. D e a c t i v a t i o n times a r e s i g n i t i c a n t l v Prolonged (UP t o 1 0 0 h ) . A l l c a t a i y t i c

19

parameters depend bn t h e o r i g i n a l degree O T d e a l u m i n a t i o n and UP t o a c e r t a i n l e v e l at S i / A l = 250-300 t h e y a r e improved. M o d i f i c a t i o n w i t h o r t h o p h o s p h o r i c a c i d causes a f u r t h e r decrease o f s t r o n g a c i d s i t e s by i t s r e v e r s i b l e i n t e r a c t i o n w i t h t h e s e s i t e s . This i n t e r a c t i o n seems t o p r o t e c t AlF-atoms from b e i n g expulsed trom t h e framework under h y d r o t h e r m a l c o n d i t i o n s . The observed t u r t h e r d e a l u m i n a t i o n o f P - m o d i f i e d samples a f t e r severe hvdrotherinal t r e a t m e n t a p p a r e n t l y i n v o l v e s P r e f e r e n t i a l l y unprotected Ah-atoms. C o n c o m i t a n t l y w i t h t h e decrease o f s t r o n g a c i d s i t e s t h e c o n c e n t r a t i o n o f weak a c i d s i t e s i s changed and t h e s i t e s a r e a l t e r e d i n t h e i r n a t u r e . A new. ion-exchanseoble a c i d s i t e w i t h medium a c i d i c s t r e n g t h has been d e t e c t e d . These chnnaes a r e p r e d o m i n a n t l y due t o an i n t e r a c t i o n o f t h e p h o s p h o r i c a c i d w i t h t h e non-framework aluminium, which r e s u l t s i n a k i n d o f a l u minium phosphate. Ihe v e r y slnail diTTerences o t t h e c o n c e n c r a t i o n o i s t r o II g a c i d s i t e s above 1%P a l o n e h a r d l y e x p i o i n t n e Uependence 0 1 sel e c t i v i t y on t h e o r i g i n a l degree o t d e a l u m i n a t i o n . The more oronounced d i f f e r e n c e s i n t h e c o n c e n t r a t i o n o f w e a k a c i d sites favour t h e i d e a o f t h e i r p a r t i c i p a t i o n i n c a t a l y s i s a t these cnnd i t i o n s . An a d d i t i o n a l shape s e l e c t i v i t y e t r e c t , can n o t be e x c l u ded and seems t o be even l i k e l y f o r coke t o r m a t i o n .

I'he a u t h o r s express t h e i r g r a t i t u d e t o Leuna-Werke-AG t o r fund i n g P a r t s o f t h i s w o r k . They a r e i n p a r t i c u l a r i n d e p t e d t o Or-. K.Wehner and D r . D 3 i m m f o r t h e i r Permanent i n t e r e s t i n th1.s work and s t i m u l a t i n g d i s c u s s i o n s . The a u t h o r s thank M r s I n a Schulz t o r t h e d e t e r m i n a t i o n o f coke on spent c a t a l v s t s .

REF LRENCtS 1 W . W Kaedinq Und S 8 . B u t t e r . J . C o t a 1 . b l . 155(1Y80) W.W.Kaeding, C.Lhu; L B.Youna. B . W e l n s t e l n , anu s . A . d u t t e r , J

2

5 4 5

Cata1.67, 159(1981) L.6.Young, S . U . B u t t e r . and W.W.Kaeuing, J . ~ u t a l . / b . 41811Y8L) J.C.Vedrine, A Auroux, P . D e j a i f r e , V Oucarme, H.Hoser. s Lnou. J . C a t a l . / 3 . 14/(1982) J Nunan. J C r o n i n . and J.Cunningham, J . C a t a 1 . 8 7 . I7(1984)

20

6 M . D e r e w i n s k i . J . Haber, J . P t a s z y n s k i , V . P . S h i r a l k a r , and S . U z w i g a j . S t u d . S u r t . S c i . C a t a l . 1 8 , 209(1984) 1 J . A . L e r c h e r , ti. Rumplmayr, H . N o l l e r , A c t a Phys .Chem. 3 1 , 7 1 (1985) 8 J . A . L e r c t i e r , 6 . Rumplmavr, UPPI. C a t u l . 2 S , 215(1986) 9 A.Hahman, G.LemaY, A.Adnot, S . K a l i a s u i n e , J . C a t a 1 . 1 1 2 . 4 5 3 ( 1 9 8 8 ) 10 J.H.Kim, S.Namba. l-.Yasnima, Bull.Chem.Soc.Japan 61, l O S l ( 1 9 8 8 ) 11 F . L o n v l , J.Enge1hardt and D . K a l l o ; P . A . J a c o b s nnd R . A . v a n S a n t e n ( t d i t o r s ) , Z e o l i t e s : F-acts, F i g u r e s . F u t u r e 1988 t l s e v i e r Pub. B . V . Amsterdam. 1 3 5 / 1%S . 1 k a i , and M . Okamo t o , H .N i s h i o k a , r . Mivainoto, K . Mat s u z a k i , K . S u z u k i , Y.Kivazumi, T.Sano, S.Shin, A ~ p l . C a t a l . 4 9 , 143(1989) 1 5 H . V i n e k , G . Rumplmavr, ana J . A . L e r c h e r , J . C a t a l , 1 1 5 , 2 Y 1( 198Y) 14 A , J e n t y s , ci. Huiiwimavr, and J . A . L e r c h e r , A p p l . C a t o l . 5 5 , 2 9 9 ( 1 9 8 9 ) 1 5 G.Ruinplmavr. ~ . A . ~ e r c n e rL,e a l i t e s , l O . 283(1990) 36 G . S e o . R.Rvoo, J . C a t a 1 . 1 2 4 , 224(1990) 1/ M . Ko j ima , F . 1.et ebvre, flnd Y . Ben Taa r it , J . Chem . Soc . Fnrad . 1 r a i l s . 86(4), /5/(1YYO) 18 G.ohlmann, H . - G . J e r s c h k e w i t z ,

G . L i s c h k e . B . P a r l i t z , R. t c k e l t , M . R i c h t e r , L . ~ . L n e n i i e28, 5 , l b l ( 1 9 8 8 ) 1 9 c.D.Chans, C . T.--W.Chu, and R . F . Socha. .I . i o t a l . 8 6 . % 8 9 (1984) 20 li.ohlinann, H.-G.Jerschkewitz, G . i i s c h k e , R . k c k e l t , r . G r o s s , b . P a r i i t z , I . S c h u l z , K.Wehner, and D . l i m m , t h i s e d i t i o n O T Stud.Surf.Sci.Cata1. 2 1 . I . R .Anderson. K . Fnqer, T.Mole, and R . A . R n j a d h v a k s h i , . I . C a t n l . S 8

118 (1919) 2 5 t.LotTler, Ch.Peuker. H . - G . J e r s c h k e w i t z . C a t a l . lodclv 3 . 415(1988) 24 A . -R . Grimmer and U . HaUDeIlrel S s e r , i hein. P h v s . Ler t . Y 9 . 48 / ( 19 8 S ) i 3 U . f ” l u i i e r , t .Junn, b . ~ a O w and i ~ U.Hauoenreisser, C n e m . r h y s . L t ? t t . lU5, 55L(1984) Zb 1 .M.Duiican and D.C.Doualass. Chern.Phys.8/. 5 ? 9 ( 1 9 8 4 ) Z/ D . M u l l e r , 1 .Grilnze. t . H o l l a s and G.Ladwig. 7 . a n o r q . f l l l ~ . C . h e m . 500. 80(1985) 28 6 . L i t ) r o w i u s nnd W.Storek, u n p u b l i s h e d r e s u l t s .

21

G . Ohlmann et al. (Editors), Catalysis and Adsorption by Zeolites 0 1991 Elsevier Science Publishers B.V., Amsterdam

New Directions in Zeolite Catalysis Jens Weitkamp Institute of Chemical Technology I, University D-7000 Stuttgart 80 (Federal Republic of Germany)

of Stuttgart, Pfaffenwaldring 55,

Summary It is likely that the catalytic performance of a much broader variety of zeolites and related microporous materials will be routinely scrutinized in the nineties. In shape selective catalysis, reactions of lar er molecules will play an increasing role. Even in petroleum refining and the manufacture o f commodity petrochemicals, there is room for novel a plications of zeolite catalysts. Two examples, viz. dewaxing by isomerization and isobutane butene alkylation are discussed in some detail. The use of zeolite catalysts for the synthesis o organic intermediates and fine chemicals continues to be a thrust area. It is desirable that synthetic organic chemists routinely employ molecular sieve catalysts. Stereoselective catalysis in zeolites is still an underdeveloped field. The same holds for base catalysis in zeolites, especially shape selective base catalysis, but new approaches can be perceived in this area. Coupling of the exothermic MTO process and the endothermic hydrocarbon cracking on ZSM-5 type catalysts allows an almost thermoneutral production of lower olefins. The role of zeolites as hosts for a variety of guests is recognized by an ever increasing number of researchers. Ship-in-the-bottle catalysts or shape selective photochemistry are just a few examples for host/guest chemistry in zeolites,

P

Introduction and scope About thirty years after its first industrial application, catalysis on zeolites continues to be a most dynamic and promising field of research. The advantages of microporous crystalline materials over more conventional solid catalysts are nowadays recognized by an ever increasing number of scientists. Among these advantages are (i) the strictly regular pore size and pore architecture; (ii) pore widths of molecular dimensions which enable shape or size selective catalysis; (iii) acid surface properties with the possibility to tune the nature (Br6nsted versus Lewis acidity), density, strength (or strength distribution) and location (e. g., inside the pores or at the external surface) of the acid sites within certain ranges; (iv) the high thermal stability; (v) the possibility to regenerate deactivated catalysts, e. g., by burning off carbonaceous deposits; (VI)the capability of zeolites to act as hosts for a variety of guests with catalytically attractive properties, such as transition metal ions, small metal clusters or highly selective transition metal complexes or chelates. Indeed, through host/guest chemistry, zeolite science has many features in common with the modern supramolecular chemistry. Although nowadays, an impressive number of industrial processes are operated with zeolite catalysts, the riunzber of strLictiiru//y different zeolites accepted as commercial catalysts has remained surpri.ying/y.mu//:To our knowledge, the zeolites utilized so far as industrial catalysts are restricted to faujasite, mordenite, ZSM-5 and, probably, erionite. The domain of zeolite Y is petroleum refining where it has been used on a very large scale in fluid catalytic cracking (FCC) [l-41and hydrocracking [5-9].-Mordenite is applied in

22

the isomerization of light alkanes for octane enhancement of gasoline [5,8-101. Zeolite ZSM-S is the prototype of shape selective catalysts. Its realm is the manufacture of commodity petrochemicals, such as ethylbenzene or para-xylene [ 11-13]. Moreover, ZSM-5 based catalysts are applied i n the production of gasoline from methanol [13-151, in the M-Forming process [16,17] for octane improvement of reformer gasoline and in dewaxing of middle distillates and lubricating oils [12,13]. For the latter process, mordenite based catalysts have ;iIso been employed [ lS,l9]. Finally, ZSM-S is used as an additive in some F C C catalysts for octane enhancement of the gasoline produced [20,21]. The titanium containing analogue of ZSM-5, named TS-1 [22,23], is employed for the hydroxylation of phenol to hydroquinone/catechol mixtures. Erionite was the catalyst in the old Selectoforming process [24,2S] and is reported to be used again, in an improved form, i n the German Democratic Republic for octane enhancement of reformate [26]. Until several years ago, most fundamental studies on zeolite catalysis, too, were restricted to ;I very small number of structural types, again mostly faujasites, mordenite and ZSM-S. Lately, this situation has changed significantly: The catalytic behavior of an ever increasing number of alumosilicates and related microporous materials is being scrutinized, which stems, in part, from the remarkable progress in hydrothermal synthesis and structure determination. Likewise, the recent progress in post-synthesis modification techniques has a strong impact on catalysis. It is another most important trend that zeolite catalysis spreads into almost all branches of chemistry with a remarkable boom in the manufacture of organic intermediates and fine chemicals. The present review paper is intended to describe some selected developments in zeolite catalysis which, in the author’s opinion, deserve particular attention in the nineties. Hroader variety of microporous materials More and more groups which have been traditionally active in zeolite catalysis are adopting the techniques of hydrothermal synthesis with the benefit that they now dispose of a broad assortment of zeolitic materials, in addition to the few ones which are commercially available. In our opinion the alumosilicate zeolites listed in ‘Table 1 will receive particular attention in catalysis over the coming decade. Even today, materials like Beta [27-311, Omega [32-35], L [35-371, offretite [3 1,351, EU-1 [38-401, ZSM-12 (35,411, ZSM-22 [38,42-441, E M - 2 3 [38,42,43] and ZSM-48 [35,38,45] are known to possess attractive catalytic properties for many applications. It is likely that, in the nineties, several other alumosilicates with eight-, ten- o r twelve-membered ring or even larger pores have to be added to this list. One example could be ZSM-18 with its unique pore system consisting of pseudo-unidimensional, puckered 12-membered ring channels with side pockets capped by 7-membered rings [46]. The zeolites enumerated in Table 1 cover a n appreciable range of pore widths from ca. 0.5 to 0.8 nm which is the relevant range for most shape selective conversions. As a relative nie:isure for the effective pore width of microporous materials, indices derived from certain cat:ilytic test reactions have been found to be very useful. The oldest and best known example is Mobil’s Constraint Index [47]. Other useful indices for this purpose are the Refined or

23

TABLE 1 Aluniosilicate zeolites with particular potential in catalytic research. (The dotted line separates zeolites with 10- and 12membered*ring pores. CI denotes the Constraint Index [47], the* data for CI were taken from [48]; CI is the Refined Constraint Index [49,50], the CI data were taken from [51]; SI stands for the Spaciousness Index [52,53], the SI data were taken from [53]). ZEOLITE

IZA CODE

RELATED STRUCTURES

Theta-1 ZSM-23 Ferrierite ZSMJ ZSM-48 ZSM-11

TON M7T FER MF1

ZSM-12 EU-1 Offre tite Mordenite Omega L Beta ZSM-20 Y

CI

CI*

MEL

ZSM-22, ISI-I, KZ-2, Nu-10 EU-13, ISI-4, KZ-1 ZSM-35, ISI-6, NU-23,FU-9 Silicalite 1, Nu-4, Nu-5 EU-2, EU-11, ZBM-30 Silicalite 2

_____.

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

MTW EUO OFF MOR MAZ LTL

CZH-5, Nu-13, Theta-3 ZSM-50, TPZ-3

FAU

Mazzite, ZSM-4 NU-2 Intergrowth of BSS1) and FAU Faujasite, X

7.3 9.1 4.5 6- 8.3 3.5 5 - 8.7 2.3 2.1 3.7 0.4 0.5 0.6- 2.0 0.5 0.4- 0.5

14.4 10.8 10.3 6.8 5.2 2.7 2.2 1.8 1.8 1.2 1.0 1.4 1.3

SI

w l

-1 w 1 w l Fa1 u l

____ 3 5 5.5 7.5

16.5 13- 19 20 21

1)Code for Breck’s Structure Six Modified Constraint Index which was defined by Jacobs et al. [49,50] and the Spaciousness Index which was introduced by Weitkamp et al. [52,53]. Wherever available, these three indices are given for the zeolites listed in Table 1. From top to bottom, i. e., with increasing effective pore width, both the Constraint Index and the Refined Constraint Index decrease whereas the Spaciousness Index increases. A number of elements other than silicon and aluminum can occur on tetrahedral sites in the crystal lattice of microporous solids [54]. This way, a huge number of materials result, part of which possess structures known from the alumosilicates. An example is titanium silicalite, TS-I, with the crystal structure of ZSMd and very interesting catalytic properties in selective oxidation reactions [22,23,55-571. Other members from this familiy of materials possess completely new structures. This is particularly true for certain alumophosphates (AIPOds), silicoalumophosphates (SAPOs) and the microporous solids derived from them by

24

incorporation of additional elements, such as titanium, manganese, cobalt or iron into the lattice [58,59]. The potential of alumophosphate based molecular sieves in catalysis has rcccntly been reviewed by Rabo et al. [60]. I

I

( c a . 0.8nm I

lea. 1.2 nm 1

Fig. 1. Structures of AIP04-11, AIP04-5 and VPI-5. Within the frimily of AIP04s, very large and super-large pore molecular sieves have recently been identified. Davis et al. recognized that V P I J is a material with unidimensional channels formed from 18-membered rings with an approximate diameter of 1.2 nm [61-631. The structure of VPI-5 is shown in Fig. 1 along with the related structures of A1P04-5, a material with unidimensional 12-membered ring pores, and AIP04-l 1, a material with unidimensional 10-membered ring pores. It is seen that the cross section of the pores in AIP04-I 1 is strongly elliptic which renders the material particularly interesting for shape selective catalysis. The discovery of the existence of super-large pore molecular sieves might be a landmark in zeolite catalysis. Such materials are of great interest for the catalytic conversion of feedstocks containing large molecules as, e. g., petroleum residues. AIPO,s, however, possess very little o r n o acidity. As soon as silicoalumophosphates or zeolitic :ilumosilicates with super-large pores can be synthesized this will have a tremendous impact o n catalysis. Another material, named MCM-9 [64,65a], does contain Si-VPI-5 but it is apparently not a pure phase. A third molecular sieve material with very large pores, i. e., channels formed by more than 12 tetrahedra, is AIPO4-8 [65a] the structure of which has been determined very recently [6Sb]. A1P04-8 possesses unidimensional channels formed from 14-membered rings with the approximate dimensions 0.79 x 0.87 nm [65b]. Shape selective catalysis: Towards larger molecules Shape selectivity effects can occur if the pore width of the catalyst is in the same order as the dimensions of reactant molecules, transition states, intermediates and/or product molecules. The principles of shape selective catalysis in zeolites have been discussed in much detail in prior review articles, e. g. [66, 671. In the past, almost all shape selectivity effects were

25

related to the conversion of aliphatic compounds or inononuclear aromatics [68] in tenmembered ring zeolites, typically HZSM-5. We perceive more and more attempts to make use of shape selective catalysis for the production of larger molecules, e.g., valuable derivatives of biphenyl or naphthalene. 4,4’-Diisopropylbiphenylwhich is an intermediate for liquid crystals could be produced from propene and biphenyl in high yields on dealuminated H-mordenite [69]. Other 12-membered ring zeolites like HY, H L or H-offretite which were tested for comparison gave very poor results. So did the original mordenite sample with an Si/AI ratio of 5. Upon massive dealumination up to an Si/AI ratio of 1300 mesopores were created. It was one of the authors’ conclusions that the mesopores are important for an efficient mass transport of the bulky molecules and that dealumination, i. e., dilution of the active sites, slows down both coke formation and undesired alkylations, e. g. in para-meta positions. 2-Methylnaphthalene which is an intermediate in the synthesis of vitamin K can be formed by isomerization of 1-methylnaphthalene on acid catalysts. Usually, however, the isomerization is accompanied by undesired side reactions, especially by transalkylation and coking, and this was found to be particularly true for zeolite H Y [70]. In HZSM-5, on the other hand, the isomerization turned out to be severely hampered by diffusional limitations. As shown in Fig. 2, certain zeolites with an intermediate pore width, characterized by Spaciousness Indices from 3 to ca. 16, gave excellent results: Initially, the equilibrium

6\”

100

>:

I

I

HZSM-12

(SI = 3)

75-

Si/AI = 35

- \m: O S

.

100

-

I

I

H-EU-1

(SI = 5) Si/AI = 20

75 - ; 05

1

-

-I

w

P

-

X

-

25-

100

I

I

I

I

H-Mordenite

1001

(SI = 7) Si/Al = 6.7-

O

,,-x,

y,*, A y,, n

- 1

-

251

I

I

I

H-Beta

(SI = 16)

--

I

STREAM, h Fig. 2. Isomerization of I-methylnaphthalene in acid zeolites with intermediate pore widths at 300 “C after [70] (SI: Spaciousness Index; Tr: Transalkylation products).

26

conversion of ca. 70 % was achieved. The bimolecular transalkylation into naphthalene and dimethylnaphthalenes which requires a very bulky transition state and/or intermediate, was almost completely suppressed. With HZSM-12, H-EU-1 and H-Beta the deactivation was moderate. Only with H-mordenite, there was a rapid deactivation due to coke formation which has probably to do with the relatively high aluminum content rather than with the unidimensional pore system; otherwise one would expect the same rapid deactivation for HZSM-12 which has unidimensional pores as well. 2,6-Dimethylnaphthalene is a particularly valuable compound. It can easily be oxidized into naphthalene-2,6-dicarboxylicacid which is a starting material for high quality polyesters, polyamides and liquid crystals. The problem in its synthesis is that it is usually formed together with 9 other isomers which are difficult to separate. In principle, zeolite catalysis offers the possibility of enhanced selectivities for the 2,6-isomer, since its molecular dimensions are smaller than those of most other isomers. Indeed, Matsuda et al. [71,72] found that the disproportionation of 2-methylnaphthalene in H Z S M J leads to a dimethylnaphthalene fraction which contains the 2,6-isomer (and the 2,7-isomer which has the same molecular dimensions) in excess of thermodynamic equilibrium. The yields, however, were very small, presumably due to diffusional limitations. By dealumination of the external surface with (NH&SiF6 it was shown that the shape selective disproportionation occurs inside the ZSM-5 pores [73]. Another way to 2,6-dimethylnaphthalene is the shape selective alkylation of naphthalene or 2-methylnaphthalene with methanol. Fraenkel et al. [74] did observe such enhanced selectivities for 2,6- and 2,7-dimethylnaphthalene on HZSM-5, but not on H-mordenite or HY. These authors proposed [75,76] that the shape selective conversion of naphthalene derivatives occurs uf the cxternul surjiuce of zeolite ZSM-5, in so-called "half-cavities". This concept which was generalized by Derouane et al. [77,78] has, however, been questioned severely [70,79]. It is more likely that the shape selective alkylation of naphthalene or 2-methylnaphthalene on HZSM-5 occurs inside the pores and is slowed down by diffusional limitations. So far, most shape selectivity effects were observed in zeolite Z S M J . It is clear from the origin of these effects [66,67] that with bulkier reactant and/or product molecules, shape selectivity can occur in zeolites with 12-membered ring pores as well. An example is the suppression of transalkylation of methylnaphthalenes in HZSM-12, H-EU-I, H-mordenite or H-Beta, as shown in Fig. 2. Even in the very open and spacious pore system of zeolite Y shape selectivity effects were observed, e. g. during the competitive hydrogenation of cyclohexene and cyclododecene in Rh/NaY [go], hydrocracking of hydropyrenes in NiW/ultrastable Y [81] or catalytic cracking of cis-decalin/trans-decalinmixtures in ultrastable Y [82]. Petroleum refining and commodity petrochemicals The application of zeolite catalysts in petroleum refining and the manufacture of commodity petrochemicals is sometimes considered as a mature technology. Even in these fields, however, there is room for improvements and new developments. Large efforts are

being undertaken by the industry to develop FCC catalysts which yield gasoline with enhanced octane numbers, especially motor octane numbers. Once the petroleum prices rise again all problems associated with catalytic cracking of residues will receive renewed attention, especially the resistance of the zeolite in FCC catalysts towards vanadium and nickel. Possibly, new zeolite based hydrocracking catalysts will be developed. For example, Idemitsu Kosan Co. claims a novel iron on Y-type zeolite catalyst for residue hydrocracking [83]. For the isomerization of light alkanes, zeolite catalysts with higher acid strength and hence higher activity are desirable, so that lower process temperatures can be applied, where the equilibrium is more favorable for the high-octane isomers. Making aromatics from liquefied petroleum gas (LPG) by BP/UOP's Cyclar [85] or Mobil's M2-Forming process [86] is ready for commercial application and the same is probably true for the production of distillates from light olefins by the MOGD process [87]. Acid pentads could have advantages [88] over the conventional organic cation exchange resins which are used as industrial catalysts for the production of methyl tert.-butyl ether. Another process which offers a large potential for zeolite catalysts is the manufacture of cumene from benzene and propene. Dewaxing by isomerization The cold flow properties, e. g. the pour point and cloud point, of petroleum fractions are largely determined by their content of normal paraffins. One method for the removal of these disturbing waxes is shape selective hydrocracking on ZSMd based catalysts [13]. In this process, the undesired paraffins in the boiling range of the middle distillate or lubricating oil feedstock are converted into gasoline and C, + C4 (LPG) hydrocarbons. Another approach is to transform the waxy paraffins into branched isomers which have considerably better cold flow properties. This way, the yield loss necessarily associated with dewaxing by selective hydrocracking could be avoided. The isomerization of long chain n-alkanes can indeed be achieved on bifunctional zeolite catalysts with a strong hydrogenation component, e. g., Pt/CaY or Pd/LaY in the presence of hydrogen [89,90]. Under these conditions, skeletal isomerization takes place at the acid sites via carbenium ions. These carbenium ions, however, can also undergo 0-scission, then the overall reaction is hydrocracking. Since skeletal rearrangement and 0-scission are consecutive reactions, the selectivity for long chain iso-paraffins is high at low conversions, but as the conversion is increased, hydrocracking becomes more and more severe. In other words, the yield of the desired iso-paraffins in dependence of the conversion of long chain n-paraffins passes through a maximum. Even with the best catalysts based on zeolite Y, this maximum yield of long chain iso-alkanes amounted to ca. 60 % (891. Significantly higher yields of long chain iso-alkanes can be attained on certain other zeolites, especially on bifunctional forms of Beta [91,92], ZSM-22 [43] and ZSM-23 [43]. Two examples are shown in Fig. 3: The maximum yields of iso-decanes from n-CloHzz or isotetradecanes from n-CI4H30 on Pd/H-Beta are above 70 %. In the future, the fundamentals of dewaxing by isomerization need further clarification. Presumably, the mechanistic reasons for the unusually high yields of iso-alkanes are different

28

for zeolite Beta on the one hand and medium pore zeolites such as ZSM-22 or ZSM-23 on the other hand. It has been argued [43] that in the latter zeolites, B-scission is slowed down because the highly branched precursors for the most favorable type of 13-scission (so-called type A O-scission) cannot form due to spatial constraints. No explanation for the interesting behavior of zeolite Beta has been proposed so far. Apparently, not all samples of zeolite Beta do give the high yields of long chain iso-alkanes [27]. It is particularly desirable to find out the key property of zeolite Beta which governs its catalytic performance in isomerization of n-alkanes. Furthermore, isomerization experiments with mixtures of homologous n-alkanes on bifunctional Beta catalysts are needed in the future. 100

-

n Decane

n-Tetradecane WIF = 6LOg-hlmol

WIF = 570g.hImol

$ 60

0 Xn-De

0 Xn-Te

>

0, tl! >

Ylso

A Ycr.

60

Ili

0

10 X

z

0 v) OL

W

20

2

0 V

210

220

230

210

250

260

270

280

190

200

210

220

230

210

250

260

270

TEMPERATURE , ' C

Fig. 3. Isomerization and hydrocracking of long chain n-alkanes on 0.27 wt.-% Pd/H-Beta after [92]. (The hydrogen partial pressure was 2 MPa). Isobutane/butene alkylation This refinery process converts the C4 by-product from catalytic crackers into high quality gasoline. Today, alkylation is carried out either with concentrated sulfuric acid or anhydrous hydrogen fluoride as catalysts. There is a very large incentive for replacing these processes by a new technology which is environmentally more acceptable and safer. In the sixties and seventies, the potential of acid faujasite catalysts in isobutane/alkene alkylation has been explored by several groups [93-1011. Typical results are depicted in Fig. 4 and Fig. 5: 011fliefiesh zeolite, the olefin is totally converted (Fig. 4), and a perfect alkylate is formed (Fig. 5 ) which consists fully of iso-alkanes. After a certain time on stream, however, the olefin conversion drops and the product quality deteriorates drastically: More and more C8-alkenes form and eventually, the zeolite no longer alkylates isobutane; rather, the butene

29

-

100

+a 0

0 I-

z

80

60

u)

U

W

5

40

0 0

TIME

ON

STREAM, rnin

Fig. 4. Conversion of isobutane with 1-butene (11:l) at 80 "C in the liquid phase on CeY-98 zeolite after [loo].

bp

I W

I

I

I

I

Fig. 5. Reaction of isobutane with I-butene on CeY-98 at 80 "C. Composition of the Cg product fraction after [loo].

oligomerizes. This dramatic loss of catalyst performance is due to the build-up of low temperature coke from the olefin. Concomitantly, the zeolite looses its hydrogen tranfer activity. The yields of alkylate which could be achieved on faujasite catalysts were much too low for an industrial application. If one arbitrarily considers the product as an alkylate as long as it contains 90 mol-% or more alkanes in the C,-fraction, then one can define an integrated yield

30

of alkylate (total mass of alkylate formed/mass of catalyst) for a given catalyst and set of reaction conditions. Typically, these yields were in the order of SO to SO0 mg/g [loo]. Zeolite HZSM-S was reported to be inactive in isobutane/butene alkylation a t temperatures up to 100 "C and this was attributed to the narrow pores [102]. In the meantime, a much broader variety of zeolitic materials and techniques for their manipulation are known. Given the high incentive for replacing the H$04 and HF catalysts we foresee a renewed interest in the use of zeolite catalysts for isobutane/butene alkylation. Organic intermediates and fine chemicals Although so far, the industrial application of zeolite catalysts has been essentially restricted to hydrocarbon chemistry, their potential in the synthesis of organic intermediates, especially of oxygen and nitrogen containing compounds, was recognized by several research groups as early as in the sixties. This early work was reviewed by Venuto and Landis [103]. With the advent of the modern molecular sieve materials, novel modification techniques and a better understanding of shape selective catalysis, a renewed interest arose in the utilization of zeolite catalysts for organic syntheses from about 1980 onward. O n e result was the introduction of at least one new commercial process, viz. the hydroxylation of phenol to hydroquinone/catechol mixtures on titanium silicalite by EN1 [22,23]. A few others, such as the conversion of methanol with ammonia with enhanced yields of methylamine and dimethylamine at the expense of the less valuable trimethylarnine on narrow pore zeolites like Rho or ZK-5 [104-1091 are considered to be ready for commercialization. In recent years, the potential of zeolite catalysts was explored in numerous organic reactions, both in the gas and in the liquid phase. Essential parts of this modern work have been discussed and evaluated in a number of excellent review articles [110-1141. The application of zeolite catalysts in the manufacture of organic intermediates and fine chemicals will continue to be a thrust area in the nineties. In the following paragraphs a few selected reactions will be addressed which, from the author's viewpoint, are examples for the specific behavior and versatility of molecular sieve catalysts. Gallezot et al. [ 1151 investigated the selective hydrogenation of cinnamaldehyde in the liquid phase at 60 "C under a hydrogen pressure of 40 bar. The first hydrogenation step can lead to either 3-phenylpropanal or cinnamyl alcohol (Fig. 6), depending on whether hydrogen adds to the carbon-carbon double bond or the carbonyl group. Ultimately, 3-phenylpropanol is formed. Two samples of Pt/Y zeolite were prepared by ion exchange of NaY with Pt(NH3)4?+ and subsequent decomposition of the ammine complex under different, but well defined conditions. The Pt loadings were 11 and 14 wt.-%, respectively. Transmission electron microscopy revealed that, in the first sample, the noble metal was homogeneously distributed inside the zeolite with a particle diameter of 1 to 2 nm. In the second sample, the platinum was still inside the zeolite pores. However, it formed much larger agglomerates with an average diameter of 5 nm, and these metal particles were preferentially located in a shell near the external surface.

31

a

cn = CH - C H ~ O H

clnnamsldehyde (CAL)

clnnemyl alcohol (COU

Catalyst

11 wt.-% PtlY (1-2 nm. insidel 14 wt.-% PtlY (5 nm, inside]

97 %

"Liquid phase, T = 6OoC. pnz = 40 bar After P. Gallerot el at.. Catal. Letters

2 169 11990)

Fig. 6. Selective hydrogenation of cinnamaldehyde to cinnamyl alcohol on platinum clusters encapsulated in zeolite Y after [ 1151.

The results summarized in Fig. 6 show that hydrogenation on platinum in Zeolite Y gave much better selectivities for cinnamyl alcohol than on a non-zeolitic reference catalyst like platinum on charcoal. This was interpreted by a special shape selectivity effect: Inside the faujasite pores, the cinnamaldehyde molecule can adsorb on the platinum clusters, sitting in the supercages, only in an end-on mode, i. e., via its carbonyl group, through the twelvemembered ring window. By contrast, the adsorption and activation of the carbon-carbon double bond in the center of the molecule is considered to be strongly hindered, but only as long as the supercage is completely filled with platinum; a small cluster of six or less metal atoms would leave enough space for cinnamaldehyde molecules to enter the supercage and adsorb laterally, i. e., via the carbon-carbon double bond, so that hydrogenation leads to 3-phenylpropanal. This is how the authors explain that on Pt/Y with the 1 to 2 nrn clusters the undesired hydrogenation of the carbon-carbon double bond occurs to a certain extent. Adamantane has been found to form readily by rearrangement of tetrahydrodicyclopentadiene (tricycIo[5.2.1.O2b]decane, see Fig. 7) which, in turn, is easily accessible from the Diels-Alder dimer of cyclopentadiene. Suitable catalysts for this interesting reaction are acid or bifunctional forms of zeolite Y, such as rare earth Y (REY), Pt-Re/REY or H Y [116,117]. For example, at temperatures around 250 "C, adamantane selectivities between 40 and 50 % could be achieved at conversions above 50 %. Likewise, mixtures of 1- and 2-methyladamantane were obtained on the same catalysts from tricyclo[6.2.1.0.2>7]undecane [ 1171. One of the conclusions from these studies was that the faujasite structure, due to the existence of almost spherical cages, is in general a promising catalyst for the formation of spherical molecules, and adamantane or its homologues might be just examples for spherical molecules which can be advantageously synthesized inside the cages of zeolites.

L

a) K. Honna et al.. ldemllsu Koaan Co., since 1985 b) G.C. Leu and W.F. Maler. LanOmulr 1987

endoTrlcyclo [5.2.1.0"]

exo-

Adamantane

decsne

Fig. 7. Formation of adamantane from tetrahydrodicyclopentadiene in zeolite Y after [116,117]. A more recent article [ 1181 reports on the formation of adamantane and its homologues from a simple alkene, viz. 1-hexene, at 240 "C in SAPO-34, a silicoalumophosphate with the chabazite structure. However, unlike in the above-mentioned studies with zeolite Y, the adamantanes did not appear in the gaseous effluent from the reactor; rather, they were built up and entrapped inside the chabazite cages (see Fig. 8), presumably again via tetrahydrodicyclopentadienes as intermediates. The adamantanes are, of course, too bulky to escape through the eigth-membered ring windows of the chabazite cage. The only way to detect them is the dissolution of the used molecular sieve, e. g., in hydrogen fluoride, followed by extraction with an appropriate solvent, e. g., CH,Cl,. It has been again concluded from these results [118] that the microporous catalyst with its cages of molecular dimensions can be considered as an ideal host for transition state guests which lead to spherical, polynuclear products. At higher reaction temperatures around 400 "C, naphthalene and its methyl derivatives were preferentially formed inside the cages (cf. Fig. 8) of SAPO-34, probably in related chemical pathways via dicyclopentadienes. It is too early to judge upon the general validity of these concepts. If zeolite cages act indeed as hosts which favor the formation of spherical products, then one could imagine a completely new direction of zeolite catalysis: A zeolite with cages of appropriate size and geometry could act as u templute which directs a chemical reaction towards the desired product, which may or may not have a spherical shape. If this product is of sufficiently high value (and/or the molecular sieve used as a reaction template sufficiently cheap) then it could even be economical to dissolve the zeolite so that the product becomes accessible through extraction. In the precise sense, the zeolite would then no longer play the role of a catalyst (with a high number of closed catalytic cycles or turnovers) but simply act as a selectivity directing reaction vessel with uppropriute moleculur dinzensions.

33

124.

After J.R. Anderson el. al.. J. Catal. 259 (1990) Flow reactor, I-hexene in N, dissolution of used catalyst in HF. extraction with CH,CI,, analysis by G C l M S

1-Hexene

-

b'2400c

1

Adamantane Methyladamantane Dimethyladamantane Ethyladamantane

=4OO0C

Naphthalene Methylnaphthalenes Dimethylnaphthalenes

Fig. 8. Formation of polycyclic hydrocarbons entrapped inside the chabazite-like cages of SAPO-34 from 1-hexene after [118]. An example for such a reaction which one could envisage in a very speculative manner is the synthesis of compounds with the dodecahedrane (C,,H,,, see Fig. 9) skeleton. Such compounds which strongly resemble a sphere became accessible in the eighties by multi-step syntheses, usually via precursors with the pagodane structure [ 119-1241. To our knowledge, no systematic and professional attempt has been undertaken so far, to utilize molecular sieves for

L H . Paquette et al., alnce ce. 1981 H. Prinzbach e l al., since ca. 1985

Fig. 9. A speculative example for the use of molecular sieves as reaction templates: Pagodanes to dodecahedranes. dodecahedrane synthesis. The reasons are obvious: Modern synthetic organic chemistry and professional zeolite chemistry are dealt with in different laboratories and separated from each other. It is a managerial challenge to bring inorganic host/guest chemistry and synthetic organic chemistry together.

34

Stereoselective catalysis Enantioselective catalysis in zeolites is often looked upon as one of the ultimate goals. It requires chiral centers next to the catalytically active sites. In principle, different methods can be used to induce chiral properties. According to Dessau [125], chiral hydrogenating catalysts result if a n acid zeolite loaded with a hydrogenation metal, e. g., Pt/HZSM-5, is neutralized with an optically active amine, such as S-(-)- a-methylbenzylamine (Fig. 10). O n such a catalyst, the hydrogenation of

2-Phenylbutene

y

5

2-Phenylbutane

Pt/(CH 3 - C x - N H , ) Z S M - 5 I H C,H,-

C - CH, I1

b-

C,H,-

0 +H2

Acetophenone

H

kCH, OH

1-Phenylethanol

Afler R.M. Dessau. US Patent 4 554 262, Mobil Oil Corp.. 1985

Fig. 10. Enantioselective hydrogenation of prochiral compounds after [ 1251. prochiral compounds, e. g., 2-phenylbutene or acetophenone, is claimed to give 2-phenylbutdne or 1-phenylethanol, respectively, in a n optically active form, i. e. with an excess of one cnantiomer. It is astonishing that, since the appearance of Dessau’s patent [ 1251, his approach has not heen dealt with more comprehensively in the scientific literature. Zeolite Beta, as it can be synthesized today, is an intergrown hybrid of two closely related structures [120,127]. One of these end members, the so-called polymorph A, forms an enantioniorphic pair while polymorph B is achiral. Since the structure of zeolite Beta has been determined, large efforts are being undertaken to develop synthesis procedures for its pure polymorph A. If, at the end, these endeavors will be successful and methods for the separation of the enantiomorphic pair will be available, then the old dream of enantioselective catalysis i n zeolites could become reality. Base catalysis Base catalysis in zeolites is still an underdeveloped field of science and technology, especially in comparison with acid catalysis. In the literature, essentially three approaches to create basic sites in zeolites can be discerned. Interestingly, these attempts were almost completely restricted to faujasites. It was recognized early [ 1281 that faujasites exchanged with large alkali cations, especially CsX and RbX, catalyze the side chain alkylation of toluene with methanol or formaldehyde, rather than ring alkylation to xylenes which occurs on acid zeolites. Later, side chain alkylation to styrene/ethylbenzene mixtures on faujasites exchanged with alkali cations has

35

been studied in detail by several groups [129-1351. It has been generally concluded that this is a base catalyzed reaction via carbanions, although the nature of the basic sites under reaction conditions remained a matter of debate [136,137]. Clusters from sodium metal can be generated in the pores of faujasites using various techniques. According to Jacobs et al. [138-1411 the preferred method from the viewpoint of catalysis is the impregnation of the dehydrated zeolite with NaN, followed by the thermal decomposition of sodium azide under carefully controlled conditions. In faujasites modified by this procedure, three types of sodium particles can be distinguished by ESR spectroscopy: Na43+ clusters located inside the sodalite cages, neutral Na, clusters inside the pore system and neutral Nay particles at the external surface. Basic framework oxygen anions next to intracrystalline Na, clusters seem to be the active sites [141] for base catalyzed reactions such as the isomerization of cis-2-butene [ 139,1401, the side chain alkylation in toluene, ethylbenzene or cumene with ethene [ 1411 or regioselective ring opening in unsymmetrical epoxides [142]. While basic zeolites prepared via the azide method appear to be very active catalysts, it remains to be seen how stable they are against potential poisons, such as water formed in many reactions or sulfur compounds present in traces in many feedstocks. The third method, introduced by Hathaway and Davis [143,144], consists of impregnating a faujasite, e. g. CsNaY, with cesium acetate followed by decomposition of the acetate at 450 "C in air or helium. It appears that the active site in such catalysts is cesium oxide rather than metallic cesium, cesium carbonate or cesium hydroxide [ 1451. These modified faujasites were tested in various base catalyzed reactions, such as the dehydrogenation of isopropanol to acetone [ 144-1461 and the alkylation of methane, ethane, acetone or toluene with methanol [146]. It will be interesting to observe whether this new method is accepted by other groups. If

Si 0, 0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

A l PO1 Al 0

Zeolite IAcid 1

0

0

H'B

/ V S /o\; i / \ / \

o \ i 0

Sip0 ?

0

0

0

0

0

0

0

0

0

He

H@

/VSi /o\si /O\$/O I \ / \ / \ / \

0

0

0

0

0

0

0

0

OH 0

OH8

OH8 0

(Base 1

i ' o

i ' o

o/\o

d'o

o/\o

Fig. 11. Neutral, acid and hypothetical basic molecular sieves.

/ \

0

0

36

so, this could foster base catalysis in zeolites including, perhaps, shape selective base catalysis with its huge potential in the manufacture of organic intermediates and fine chemicals. A hypothetical approach towards a new generation of molecular sieve Briinsted bases might be seen in the preparation of microporous silicophosphates, either by direct synthesis or post-synthesis modification. As shown schematically in Fig. 11, and for obvious reasons, a molecular sieve S i 0 2 , such as silicalite 1 or 2, has no lattice charge. The same is true for the overuff lattice in AIP04s due to the strictly alternating arrangement of @lo4)and (PO4)+ tetrahedra. Alumosilicate zeolites are cation exchangers and, in their H + forms, strong Briinsted acids. By mere analogy and in a formal manner a porous silicophosphate would be expected to be an anion exchanger and, in its OH- form, a Briinsted base. In a triangular compositional diagram (Fig. 12), the alumosilicate zeolites and the hypothetical silicophosphates are just limiting cases which are represented by positions on the Si02-A102' and SO2-PO2+ connecting lines, respectively. There exist a whole field of cation exchangers, embracing both the SAPOs and alumosilicate zeolites, and a whole field of

A

Electroneutrality

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

AI02-/ Field of cation exchangers

0.5

-

:. Al-

: rich

Field of anion /e xc ha ng e rs ?

SiO, high : silica : ) .

ZEOLITES

Fig. 12. Schematic representation of the possible compositions of molecular sieve materials in the Si02/A102/P02 system, adapted from Flanigen et al. [%I.

37

hypothetical anion exchangers, and these two fields are separated from each other by the electroneutrality line which connects the AIPO4s and SiO2. It has, in fact, been reported [147] that the overall framework charge in a zeolite can be changed from negative to positive if a synthetic zeolite is treated with a melt of ammonium dihydrogen phosphate at 230 "C. For the resulting materials with anion exchanger properties the name phosphated aluminosilicates or PASOs [147] was coined. It remains to be seen whether the new procedure is reliably applicable to other zeolites and by other groups. If so, the door to a new class of microporous Bronsted bases would be open. It is sometimes difficult to distinguish unambiguously between acid and base catalysis on molecular sieve materials. Recently, Dessau [ 148,1491 proposed the conversion of HZSM-5 (SIIAI = 35) T = 225OC

I1

I1

0

Acetonylacetone

CH

I

X=25%

2,5-Dlmethylluran

CHS-C-CH,-CH,-C-CH, 0

HC

-.,A-

H,C

-C /C%

H,C,

/CH

I

NaZSM-5 (SI/Al = 13 000) (Na/AI = 300) T = 300%

II

X=51% S=95%

$

0 3-Methylcyclopentenone

After R.M. Dessau, Zeolites 10, 205 (1990)

Fig. 13. Conversion of acetonylacetone as a valuable test reaction for distinguishing between acid and base catalysis in zeolites after [149]. 2,5-hexanedione (acetonylacetone) as a test reaction which, from the products formed, allows a safe discrimination (Fig. 13). On acid zeolites, e. g. HZSM-5, 2,s-dimethylfuran is formed almost exclusively at medium conversions; base catalysis, on the other hand leads to 3-methylcyclopentenone, again in a very high selectivity a t conversions above 50 %. A broader application of this test reaction could be very helpful in future research on base catalysis in zeolites. Engineering aspects: Coupling of exothermic and endothermic reactions Lower alkenes, i. e., mainly ethene and propene along with some butenes, are currently produced on an industrial scale by steam cracking of petroleum or natural gas derived hydrocarbon fractions, such as light naphtha. Steam cracking is a non-catalytic pyrolysis at high temperatures around 850 "C in the presence of steam. The process is highly endothermic. An alternative route to lower alkenes is the methanol-to-olefin (MTO) process [13]. Ultimately, this process relies on coal or natural gas as raw materials from which methanol

38

Alter S. Nowak el .I.. since 1983

Fig. 14. Production of lower alkenes by coupled methanol/hydrocarbon cracking (CMHC) after [150-152]. X is the conversion and Y denotes product yields. can readily be manufactured via gasification to CO and H, followed by methanol synthesis. MTO is a catalytic process: Methanol is dehydrated on Z S M J type catalysts under such conditions that the yield of C,- to C,-alkenes is maximized. This is usually the case at temperatures in the order of 400 "C. MTO which is not yet a commercial process, is strongly exothermic. Nowak et al. [150-1521 advanced the idea to combine the exothermic MTO reaction and the endothermic hydrocarbon cracking with such a feed ratio that the overall process is nearly thermoneutral (Fig. 14). This is favorable from an engineering point of view since the problems associated with heat removal in MTO and heat generation in steam cracking are avoided. One problem in the coupled rnethunol/liyrlrocurbon cracking (CMHC) is to find an appropriate reaction temperature which is not excessively high for MTO, yet sufficiently high for hydrocarbon cracking. Since the latter reaction which in the non-catalytic steam cracking process proceeds via radicals, is now catalyzed by an acid zeolite (presumably with a mechanism via carbocations), much lower temperatures in the range of 600 to 700 "C seem to suffice. Typical conversions and product yields achieved in CMHC with methanol and n-butane are given in Fig. 14. For the most part, zeolite HZSM-5 was used as catalyst, recent studies [152] revealed, however, that molecular sieves with a lower acid strength, such as iron containing ZSM-5, might offer advantages in CMHC, especially a lower coking and deactivation rate. Zeolites: Hosts for a variety of guests; ship-in-the-bottle catalysts It is evident that in the channels or cages of zeolite molecular sieves, a broad variety of guests with catalytically attractive properties can be accommodated or encapsulated. Indeed, host/guest chemistry is playing an ever increasing role in the preparation of sophisticated

39

zeolite catalysts. Zeolite host/guest chemistry which is, at this time still at its infancy, can be expected to become a thrust area in the nineties. In a few review articles [113,153,154], the potential of zeolite host/guest chemistry for catalysis has been outlined and the different approaches were described. In complete analogy with a ship-in-the-bottle, transition metal complexes which are too bulky to escape through the twelve-membered ring window of faujasite, can be synthesized in its supercages from sufficiently small building blocks which do have access to the cages. The best known examples are phthalocyanine complexes of cobalt, nickel, copper and iron [ 155-1591. For their preparation, the transition metal ion is first exchanged into the zeolite whereupon the complex is synthesized by reaction with 1,2-dicyanobenzene at temperatures around 300 "C. Usually, part of the phthalocyanine complexes are formed at the external surface, but this part can be selectively removed by Soxhlet extraction. Iron phthalocyanine in NaY has also been synthesized from [HFe3(CO),,]- which was first oxidized and then reduced under controlled conditions whereupon 12-dicyanobenzene was admitted [ 1601. Metal phthalocyanines immobilized in zeolite Y have been described to exhibit interesting catalytic properties, e. g., in oxidation reactions with iodosobenzene [ 158,1591: Reactant shape selectivity occurred with cyclohexane/cyclododecane mixtures, n-octane was oxidized in a regioselective manner and stereoselective oxidation of methylcyclohexane and norbornane took place. Recently, an electron donor/acceptor complex (Na+)4 (FePc)@/NaY was prepared from iron phthalocyanine (FePc) encapsulated in NaY by reaction with sodium naphthalene, Na+ (CloH8)-, and this complex gave unusually high ratios of trans-2-butene/ cis-2-butene in the selective hydrogenation of butadiene [ 1601. Another guest which has been synthesized in zeolite Y as a host is cobalt salen [161]. About one Co2+ per two supercages was exchanged into NaY followed by sublimation of the free ligand salen [1,6-bis(2-hydroxyphenyl)-2,5-diaza-l,5-hexadiene]into the zeolite voids. Cobalt salen, a tetradentate chelate has dimensions greater than the window diameter of zeolite Y, hence it is a true ship-in-the bottle. The complex and its pyridine adduct were found to form adducts with dioxygen and can be considered as hemoglobin mimic [161]. Metal clusters entrapped in the intracrystalline voids are of utmost importance in zeolite catalysis. Since the pioneering work of Gallezot et al. [162,163] it is known that the size and location of such clusters depend, to a large extent, on the preparation conditions. The metal can be introduced by ion exchange with its cation or a cationic complex, especially an ammine complex, followed by a controlled thermal treatment including reduction. Metals which are difficult to introduce by ion exchange in aqueous suspension, such as molybdenum, vanadium and others, can often be easily incorporated by solid state ion exchange which has recently found much interest [ 164-1661. An alternative method for preparing highly dispersed metal clusters in molecular sieves is the adsorption and controlled decomposition of volatile metal compounds, mostly carbonyls [167]. In all these methods, metal atoms or small metal oligomers are probably formed as intermediates which migrate rapidly through the zeolite pores and coalesce to form larger agglomerates. Once these particles are larger than the windows of the cages the metal agglomerates find themselves entrapped, e. g., in a supercage

40

of faujasite. In other words, it is the ship-in-the-bottle principle which enables the preparation of highly dispersed metals in zeolites. Similarly, the encapsulation af metal carbonyls in zeolites has found considerable interest because many carbonyls can act as selective catalysts. An example is the ship-in-the-bottle formation of Pd,,(CO), clusters in NaY [168]. While, in the eighties, the potential role of zeolites as hosts for a variety of guests has clearly been recognized, host/guest chemistry has been essentially confined to faujasites. It is likely that, in the nineties, host/guest chemistry will be systematically extended to additional microporous materials, such as AlPO,-8, VPI-5 or the hexagonal counterpart of faujasite, Breck's Structure Six,which can now be synthesized in a pure form using a crown-ether based supramolecule, viz. 18-crown-6,as template [ 1691. Photochemical transformations of guest molecules in zeolites as hosts It is an attractive idea to combine the host/guest principle with photochemistry. Indeed, one can expect that the selectivities of certain photochemical reactions can be altered if they are carried out under the spatial constraints inside the pores of an appropriate zeolite. Examples for shape selective photochemical reactions have already been described in the literature, and one example is summarized in Fig. 15.

it;

Norrish Type II Reaction: C6H5

C6H5

H

R

H

H L

~~~~~

~

E/C In: R=

C,H,

R= C,,H2,

~~~~

Benzene

NaX

1.9 2.7

Alter V. Aemamurlhv el 01..

CBX

NaZSM-5

NaZSM-11

1.5

1.9

>>lo0

>>I00

0.62

2.7

6.8

>> 100

>> 100

0.48

Na-Beta

J. Cham. SOC, Chsm. Commun. 1213 (1989)

Fig. 15. Shape selective photolysis of alkanophenone guests in zeolites as hosts after [170]. The Norrish type I1 reaction, i. e., the photochemical decomposition of alkanophenones was studied in non-acidic forms of zeolites X, ZSM-5, ZSM-11 and Beta [170]. The intermediate 1,4-biradical can stabilize into two directions, viz. by cyclization (C) to a cyclobutanol or elimination (E) to acetophenone and an olefin. In a liquid solvent like benzene, E/C amounts to 1.9 for octanophenone and 2.7 for octadecanophenone, i. e., the elimination is slightly preferred. Essentially the same E/C ratios are observed in zeolite X with its spacious pore system. In the pentads, by contrast, the cyclization was almost completely suppressed, and

41

this was attributed to the restriction of the rotational motion of the central u bond in the 1,4-biradical. In other words and in the language of the zeolite community, this is a photochemical example for restricted transition state shape selectivity [66,67]. Interestingly, zeolite Beta with its intermediate pore width clearly favors cyclization, especially for octadecanophenone with its long alkyl chain, which is not yet understood. Similar photochemical studies with molecules adsorbed in zeolites were carried out by Turro et al. [171,172] and Zimmermann and Zuraw [173]. It has been found, for example, that the photochlorination of n-alkanes in pentad zeolites proceeds with a high selectivity for monochlorination at terminal methyl groups [172]. Conclusion: Zeolites and host/guest chemistry Shape selective photochemistry, in a rigorous sense, is beyond the scope of a review on zeolite catalysis. Nevertheless, the topic was briefly addressed because it clearly demonstrates that through host/guest chemistry, zeolites begin to spread into various new branches of science. Indeed, we foresee an ever increasing importance of the host/guest principle in zeolite chemistry and catalysis. Given the role of zeolites as inorganic hosts, the zeolite community is adviced to watch the current progress in tailor-made organic host molecules, i. e., suprari~olecitlrrr chetnistty [ 174-1801. Enzyme mimics in zeolites, ship-in-the-bottle catalysts and shape selective photochemistry in zeolites are just a few examples which show that zeolite chemistry and supramolecular chemistry are growing together. Acknowledgements The author expresses his gratitude to the following funding institutions which have sponsored his research on catalysis in zeolites: Deutsche Forschungsgemeinschaft, Bundesministerium fur Forschung und Technologie, Fonds der Chemischen Industrie and Max Buchner-Forschungsstiftung. References

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P.E. Hathaway and M.E. Davis, J. Catal. 119 (1989) 497-507. A. Dyer, S.A. Malik, A. Araya and T.J. McConville, in: P.A. Williams and J. Hudson (Eds.), Recent Developments in Ion Exchange; Elsevier Applied Science, London, New York, 1987, pp. 257-263. R.M. Dessau, in: J.C. Jansen, L. Moscou and M.F.M. Post (Eds.), Zeolites for the Nineties; Recent Research Reports, 8th Intern. Zeolite Conference, Amsterdam, July 10-14, 1989, p .457-458. R.M. Dessau, Zeohes 10 (1990) 205-206. S. Nowak, H. Giinschel, A. Martin, K. Anders and B. Lucke, in: M.J. Phillips and M. Ternan (Eds.), Catalysis: Theory to Practice; Proc. 9th Intern. Congress on Catalysis, Vol. 4, The Chemical Institute of Canada, Ottawa, 1988, pp. 1735-1742. A. Martin, S. Nowak, B. Liicke and H. Giinschel, Appl. Catal. 50 (1989) 149-155. A. Martin, S. Nowak, B. Liicke, W. Wieker and B. Fahlke, Appl. Catal. 57 (1990) 203-214. N. Herron, Chemtech (1989) 542-548. G.A. Ozin and C. Gil, Chem. Rev. 89 (1989) 1749-1764. G. Meyer, D. Wiihrle, M. Mohl and G. Schulz-Ekloff, Zeolites 4 (1984), 30-34. E.S. Shpiro, G.V. Antoshin, O.P. Tkachenko, S.V. Gudkov, B.V. Romanovskyand Kh. M. Minachev, in: P.A. Jacobs, N.T. Jiiger, P. Jiru, V.R. Kazansky and G. Schulz-Ekloff (Eds.), Structure and Reactivity of Modified Zeolites: Studies in Surface Science and Catalysis, Vol. 18, Elsevier, Amsterdam, Oxford, New York, Tokyo, 1984, pp. 31-39. B.V. Romanovsky, in: Proc. Intern. Symposium on Zeolite Catalysis, Sibfok, Hungary, May 13-16, 1985, pp. 215-222. N. Herron, G.D. Stucky and C.A. Tolman, J. Chem. SOC.,Chem. Commun. (1986) 1521-1522. C.A. Tolman and N. Herron, Preprints, Div. Petr. Chem., Am. Chem. SOC.32 (1987) 798-805. T. Kimura, A. Fukuoka and M. Ichikawa, Catal. Letters 4 (1990) 279-286. N. Herron, Inorg. Chem. 25 (1986) 4714-4717. P. Gallezot, Catal. Rev.-Sci. Eng. 20 (1979) 121-154. P. Gallezot, in: B. Imelik et al. (Eds.), Catalysis by Zeolites; Studies in Surface Science and Catalysis, Vol. 5, Elsevier, Amsterdam, Oxford, New York, 1980, pp. 227-234. P.E. Dai and J.H. Lunsford, J. Catal. 64 (1980) 173-183. A.V. Kucherov and A.A. Slinkin, Zeolites 7 (1987) 38-42. H.G. Karge, H.K. Beyer and G. Borbdy, Catalysis Today 3 (1988) 41-52. R.F. Howe, J. Ming, W. She-Tin and Z. Jian-Him, Catal. Today 6 (1989) 113-122. L.L. Sheu, H. Kniizinger and W.M.H. Sachtler, Catal. Letters 2 (1989) 129-138. F. Delprato, L. Delmotte, J.L. Guth and L. Huve, Zeolites 10 (1990) 546-552. V. Ramamurthy, D.R. Corbin and D.F. Eaton, J. Chern. SOC.,Chem. Commun. (1989) 1213-1214. S.C. Stinson, Chern. Eng. News 66 (No. 26, June 27, 1988) 27-30. N.J. Turro, J.R. Fehlner, D.P. Hessler, K.M. Welsh, W. Ruderman, D. Firnberg and A.M. Braun, J. Org. Chern. 53 (1988) 3731-3735. H.E. Zimmerrnann and M.J. Zuraw, J. Am. Chem. Soc. 111 (1989) 7974-7989. I. Tabushi and Y. Kuroda, Adv. Catal. 32 (1983) 417-466. F. Viigtle and E. Weber (Eds.), Host Guest Complex Chemistry, Macrocycles, 421 pp., Springer-Verlag, Berlin, Heidelber , New York, Tokyo, 1985. J. Franke and F. Vogtle, Angew. Chem. 97 t1985) 224-225 F. Ebmeyer and F. Vogtle, Angew. Chem. 101 (1989) 95-96. E. Weber and F. Vogtle, Chemie in unserer Zeit 23 (1989) 210-212. F.H. Kohnke, J.P. Mathias and J.F. Stoddart, Angew. Chem. Adv. Mater. 101 (1989) 1129-1134. M.E. Tanner, C.B. Knobler and D.C. Cram, J. Am. Chem. Soc. 112 (1990) 1659-1660.

47


ZEOLITES AS CATALYSTS FOR ALKANE OXIDATIONS R.F.

P art on, D.R.C.

Huybrechts, Ph. Buskens and P.A. Jacobs

K.U. Leuven, Dept. B i o t e c h n i s c h e Wetenschappen, Oppervlaktechemie, K a r d i n a a l M e r c i e r l a a n 92, 8-3030 Belgium

Laborat orium voor Heverlee (Leuven),

SUMMARY The o x i d a t i o n o f u n a c t i v a t e d alkanes was s t u d i e d on z e o l i t e encaged p h t h a l l o c y a n i n e s w i t h t e r t i a r y b u t y l hydroperoxide and on t i t a n i u m s i l i c a l i t e w i t h aqueous hydrogen p e r o x i d e as o x i d a n t . On b o t h t y p e s o f c a t a l y s t s secondary and/or t e r t i a r y a l c o h o l s and ketones a r e formed. The o x y f u n c t i o n a l i z a t i o n occurs i n a r e g i o s e l e c t i v e way on t h e o u t e r carbon atoms o f t h e hydrocarbon c h a i n , t h u s r e f l e c t i n g t h e shape s e l e c t i v e properties o f the catalysts. INTRODUCTION The ma jo r

industrial

applications

of

zeolites

in

refinery

and

pet roc hemic a l o p e r a t i o n s a r e r e l a t e d t o r e a c t i o n s where t h e z e o l i t e i s used as

an

acid

isomerization,

or

bifunctional

alkylation,

catalyst,

etc.

e.g.

cracking,

[l]. Th eref ore,

hydrocracking,

up t o now most o f t h e

fundamental work i n t h e f i e l d o f z e o l i t e s has been c o n c e n t r a t e d on t h e i r a c i d o r metal s t a b i l i z i n g p r o p e r t i e s . On t h e c o n t r a r y ,

relatively l i t t l e

e f f o r t s have been done t o e x p l o i t t h e u n i q ue f e a t u r e s o f z e o l i t e c a t a l y s t s , such as shape s e l e c t i v i t y , i n o t h e r t y p e s o f r e a c t i o n s [ 3-51. One o f t h e l e a s t studied z e o l i t e catalyzed reactions i s t h e o x y f u n c t i o n a l i z a t i o n o f alkanes i n t h e l i q u i d phase. N e v e r t h e l e s s , t h e g r e a t abundance o f alkanes makes them t o one o f t h e g r e a t e s t r e s o u rces f o r t h e energy and f o r t h e chemical

industry.

The extreme

low r e a c t i v i t y o f

alkanes makes

their

a c t i v a t i o n and f u n c t i o n a l i z a t i o n a c h a l l e n g e f o r many chemical s c i e n t i s t s . D u r i n g t h e l a s t 10 y e a r s many e f f o r t s have been done i n t h i s area u s i n g homogeneous c a t a l y s t s [ Z ] .

Therefore, m a j or c o n t r i b u t i o n s o f r e s e a r c h e r s i n

the f i e l d o f z e o l i t e catalyzed reactions f o r t h e f u n c t i o n a l i z a t i o n

of

alkanes cannot h o l d o f f . I n t h i s work a l k a nes a r e o x i d i z e d by a p e r o x i d i c oxygen

source

(ROOH)

to

the

c o r r e s p o n ding

maintenance o f t h e carbon c h a i n s t r u c t u r e ,

alcohols

and

ketones

with

according t o t h e f o l l o w i n g

genera l r e a c t i o n scheme: CnH(2n+2)

+

ROOH

-

ROH

+

H20

+

CnH(2,,+qOH

( + CnHZnO)

48

The behav i o r o f two new t y p e s o f c a t a l y s t w i l l be discussed, namely p h t h a l l o c y a n i n e c o n t a i n i n g l a r g e - p o r e z e o l i t e s and t i t a n i u m s i l i c a l i t e 1 (TS-1). The f o r m e r one uses t - b u t y l h y d r o p e r o x i d e as o x i d a n t , aqueous hydrogen p e r o x i d e i s t h e oxygen s o urce f o r t h e l a t t e r one.

whereas

ALKANE OXIDATIONS ON ZEOLITE ENCAGED PHTHALLOCYANINES

Introduction M e t a l l o - p o r p h y r i n e s and p h t h a l l o c y a n i n e s a r e a c t i v e c a t a l y s t s f o r t h e s e l e c t i v e e p o x i d a t i o n o f alkenes and h y d r o x y l a t i o n o f alkanes and many o t h e r o x i d a t i o n r e a c t i o n s [6-81. They r e p r e s e n t models f o r t h e a c t i v e s i t e o f cytochrome P-450.

However,

t h e y l a c k a p r o p e r mimic o f t h e p r o t e i n

function.

Metallo-phthallocyanines encaged i n z e o l i t e s Y have been proposed as complete enzyme mimics [9-131.

Z e o l i t e s can r e p l a c e t h e p r o t e i n p o r t i o n o f

n a t u r a l enzymes and m o d i f y t h e r e a c t i v i t y i n t h e same way as enzymes do by imposing s t e r i c c o n s t r a i n t s on t h e environment o f t h e a c t i v e complex. I n t h e p r e s e n t work, t h e c o n s t r u c t i o n o f a mimic o f cytochrome P-450 is

at t emp t e d

supercages

by

in

situ

o f zeolite Y

synthesis

of

iron-phthallocyanines

and i n t h e channels

o f VPI-5.

in

the

I t s catalytic

a c t i v i t y and s e l e c t i v i t y i s t e s t e d i n t h e o x y f u n c t i o n a l i z a t i o n o f n-a1 kanes w i t h t e r t i a r y b u t y l hydroperoxide. ExDerimental Materials A sample o f NaY w i t h a Si/A1 r a t i o o f 2.46 was purchased f r o m Ventron. VPI-5 was s y n t h e s i z e d a c c o r d i n g t o a r e c e n t l i t e r a t u r e procedure [14]. Dicyanobenzene

(DCB)

hy dro pero x ide (t.BHP) hexane,

n-heptane,

(+98%), f e r r o c e n e

(FeCp2)

(+98%),

1,2-

t e r t i a i r y butyl

(70% i n w a t e r ) , acetone, dichloromet hane (CH2C12), n n-octane,

n-nonane

and

n-decane

(all

+99%)

were

purchased f rom A l d r i c h . H e - p h t h a l l o c y a n i n e (H2Pc) (+98%) was purchased f rom A l d r i c h and F e - p h t h a l l o c y a n i n e (FePc) (+98%) f r o m Strem Chemicals. F erric enium - Y i s p r e p a r e d by adding 5 g o f a i r - d r y NaY t o 50 m l o f a s o l u t i o n o f f e r r o c e n e i n acetone c o n t a i n i n g 84 mg (=1 complex p e r u n i t c e l l ) o f FeCp2, f o l l o w e d b y a i r d r y i n g a t 343 K. The l a t t e r s o l i d i s mixed w i t h 5 g o f DCB and h e a t e d under He atmosphere a t 423 K f o r 4 hours. T h i s khaki -coloured

sol i d

is

succesively

p y r i d i n e and a g a i n w i t h acetone,

soxhlet

extracted

with

acetone,

u n t i l a c o l o u r l e s s e x t r a c t i s obt ained.

The f i n a l c a t a l y s t i s a i r d r i e d a t 343 K. FePcVPI-5 i s s y n t h e s i z e d a t 523 i n s t e a d o f 423 K a c c o r d i n g t o t h e same procedure.

49

Characteri sat i o n I.R.

c h a r a c t e r i s a t i o n o f t h e samples i s c a r r i e d o u t u s i n g t h e K6r

t ec hnique.

spectroscopy

U.V.

was

used

for

the

semi-quantitative

d e t e r m i n a t i o n o f t h e amount o f i n t r a c r y s t a l l i n e p h t h a l l o c y a n i n e s ,

after

d i s s o l u t i o n o f t h e z e o l i t e i n c o n c e n t r a t e d s u l f u r i c a c i d (0.1 g o f c a t a l y s t i n 10 m l o f c on c e n t r a t e d H2S04 f o r 4 h ) . The i r o n c o n t e n t o f t h e samples i s determined by chemical a n a l y s i s . X - r a y powder d i f f r a c t i o n i s used t o ensure good c r y s t a l l i n i t y o f t h e z e o l i t e a f t e r t h e s y n t h e s i s and p u r i f i c a t i o n procedures. R eac t io n Drocedure O x y f u n c t i o n a l i z a t i o n r e a c t i o n s o f n - a lkanes a r e c a r r i e d o u t a t room temperature and atmospheric p r e s s u r e w i t h t.BHP as oxidans and acetone as s o l v e n t . Product a n a l y s i s was done w i t h GC on a 50 m CP S i l - 8 6 c a p i l l a r y column f rom Chrompack. R e s u l t s and d i s c u s s i o n C h a r a c t e r i s a t i o n o f encaqed FePc I n spite o f d i f f e r e n t conditions,

such as t h e r e s p e c t i v e s y n t h e s i s

c o n d i t i o n s mentioned above, a r e a p p l i e d f o r FePc and FeCp2 impregnat ion on NaY and VPI-5

is

the application

o f the

st andard s o x h l e t

extraction

procedure s t i l l s u f f i c i e n t t o remove a l l i r o n f rom t h e s o l i d s . When i n s i t u synthesis

of

FePc

is

made

in

both

molecular

sieve

structures,

the

e x t r a c t i o n procedure removes o n l y p a r t o f t h e i r o n . Thus a l l r e s i d u a l i r o n p r e s e n t i s ' a s s o c i a t e d w i t h encaged FePc.

Indeed,

after extraction the

c h a r a c t e r i s t i c l i n e s o f Cp2 (814 c m - l ) and DC6 (965 c m - l ) a r e absent as shown by I . R . d a t a [ 1 5 ] . Chemical a n a l y s i s combined w i t h U.V.

spectroscopy o f samples d i s s o l v e d

i n c onc ent rat e d s u l f u r i c a c i d a l l o w t o d et ermine t h e amount o f FePc and H2Pc pre s ent

i n t h e molecular sieves.

The NaY samples

used f o r

the

c a t a l y t i c experiments have about 1 FePc f o r e v e r y 77 supercages. I n case o f VPI-5 samples t h e amount o f i n t r a c r y s t a l l i n e FePc corresponds t o 0.021 f o r each u n i t c e l l

.

M o l e c u l a r g r a p h i c s a n a l y s i s o f FePc l o c a t e d i n t h e supercages o f NaY z e o l i t e s shows a s a d d l e - t y p e d e f o r m a t i o n o f t h e p l a n a r FePc molecule which det e rmines t h e f r e e a p e r t u r e s t o t h e a c t i v e s i t e [15]. I t c o n t r a s t s w i t h FePc encaged i n VPI-5 where no d e f o r m a t i o n o f t h e c h e l a t e occurs. C a t a l y t i c a c t i v i t v w i t h FePcY Zeolite-enclosed

FePc

are

suitable

catalysts

for

the

selective

o x i d a t i o n o f p a r a f f i n e s t o ketones and a l c o h o l s a t ambient t emperat ure and pre s s ure w i t h t.BHP as oxygen atom donor. O x i d a t i o n r e a c t i o n s a r e performed

50

b o t h i n a i r and i n n i t r o g e n atmosphere w i t h o u t s i g n i f i c a n t c o n v e r s i o n and p r o d u c t d i s t r i b u t i o n .

Fe3+-Y,

ferricenium-Y,

as w e l l as H2Pc impregnated on NaY a r e i n a c t i v e ,

changes i n

f errocenium-Y

i n d i c a t i n g t h a t t h e low

amounts o f FePc p r e s e n t i n t h e m o l e c u l a r s i e v e s a r e t h e a c t i v e s i t e s . F i g . 1 shows t h e c o n v e r s i o n o f a s e r i e s o f n - a l k a n e s w i t h d i f f e r e n t c h a i n l e n g t h on FePcY and FePcVPI-5 c a t a l y s t s a t a c o n s t a n t r a t e o f t.BHP addition.

I n FePcY

initial

reaction stoichiometry.

conversions

approach

limit

the

imposed

by

Upon f u r t h e r a d d i t i o n o f oxidans t h e c o n v e r s i o n

curve g r a d u a l l y deviates from t h e t h e o r e t i c a l l i n e ,

pointing t o catalyst

d e a c t i v a t i o n . Turnover numbers (N) p e r FePc molecule amount t o 6,000

and

exceed t h o s e o f homogeneous FePc (N r a n g i n g f rom 20 t o 30) [15] as w e l l as t h o s e r e p o r t e d i n t h e l i t e r a t u r e on FePcY c a t a l y s t s w i t h iodosobenzene as ox idans (N=5.6) dichloromethane,

[9, 121. Because o f t h e l o w a c t i v t y o f FePc, d i s s o l v e d i n it

is

not

shown

in

Fig.

1.

Initial

activities

are

presumably h i g h on homogeneous FePc c a t a l y s t s b u t t h e complexes a r e r a p i d l y d e s t r o y e d as shown by b l e a c h i n g o f t h e s o l u t i o n . FePcY t u r n s y e l l o w - b r o w n a f t e r r e l a t i v e h i g h t u r n o v e r numbers i n c o n t r a s t w i t h

FePcVPI-5 which

remains a c t i v e even a f t e r t u r n o v e r numbers over 2,000. The l a t t e r c a t a l y s t does n o t undergo any c o l o u r change a t a l l and can be reused. O bviously, t h e stability

and t h e r e f o r e

the

overall

conversion

is

improved when

the

complexes a r e encaged i n t h e m o l e c u l a r s i e v e s . It i s c l e a r t h a t t h e z e o l i t e p r o t e c t s t h e p h t h a l l o c y a n i n e s t r u c t u r e a g a i n s t o x i d a t i o n compared t o t h e homogeneous case, j u s t as t h e p r o t e i n m a n t l e does i n t h e enzyme cytochrome P-450.

However,

even a f t e r

correction f o r differences

i n experimental

c o n d i t i o n s , FePcVPI-5 remains l e s s a c t i v e t han FePcY. I n t h e f o r m e r system, f r e e t.BHP

accumulates g r a d u a l l y w h i l e w i t h FePcY a l l

t.BHP

added i s

consumed a t once. The l i n e a r i n c r e a s e i n c o n v e r s i o n observed w i t h FePcVPI5, suggests t h e absence o f c a t a l y s t d e a c t i v a t i o n , a t l e a s t up t o t u r n o v e r numbers o f 2,000. A t t h i s st a g e o f t h e work, t w o p o s s i b l e e x p l a n a t i o n s can be advanced t o r a t i o n a l i z e t h e h i g h e r a c t i v i t y o f FePcY and t h e s u p e r i o r s t a b i l i t y o f FePcVPI-5. As a r e s u l t o f t h e t u b u l a r n a t u r e o f t h e pores o f VPI-5 and ev ent u al d i f f u s i o n l i m i t a t i o n s o f t h e r e a c t i o n , o n l y t h e FePc complexes a t the external r i m o f the crystals are active i n i t i a l l y but are gradually consumed d u r i n g r e a c t i o n . Consequently, FePc complexes 1 ocat ed more towards t h e c e n t r e o f t h e c r y s t a l s become a c t i v e . Secondly, that

the

s ad d l e - c o n f o r m a t i o n

of

environment o f t h e a c t i v e i r o n ,

FePc

in

NaY

i t can be s p e c u l a t e d

changes

the

electronic

thus increasing not o n l y i t s c a t a l y t i c

a c t i v i t y b u t a l s o i t s v u l n e r a b i l i t y towards s e l f - o x i d a t i o n . The y i e l d o f a l c o h o l s and ketones on p e r o x i d e b a s i s on FePcY and FePcVPI-5,

r e s p e c t i v e l y t h e FePc c a t a l y s t s

i s 75% and l e s s t han lo%,

51

r e s p e c t i v e l y . The m a j o r s i d e r e a c t i o n i s t h e decomposition o f t h e o r g a n i c peroxide t o

molecul a r oxygen and t. b u t a n o l .

F i g . 1 a l s o shows a f a s t e r d e a c t i v a t i o n r e a c t i o n f o r an alkane when t h e c h a i n l e n g t h i n c r e a s e s f r o m 6 t o 10 carbon atoms. The reason f o r t h i s behav iour i s n o t c l e a r . 15 .~

5,

h

FePcY

3

v

10

0

octane FePcY

u

decane FePcY

C 0

. I

rA h

w

b

C

0

u

~

5

1 0 0 % ketone formation

octane FePcVPI- 5

0 0.0

0.5

1 .o

1.5

Ratio of t.BHP to alkane (mol/mol) F i g . 1. O x i d a t i o n c o n v e r s i o n o f n - a l k a nes w i t h t . B H P t o ketones and a l c o h o l s a t 298 K and 0 . 1 MPa. Wi t h FePcY 0 . 5 g o f c a t a l y s t (lFePc/77 supercages), 200 mmol o f n-alkane, 100 m l o f acetone and a t.BHP i n j e c t i o n r a t e o f 21.9 mmo1.h-' i s used. W i t h FePcVPI-5 0.1 g o f c a t a l y s t (0.021 p e r u n i t c e l l ) , 50 mmol o f n-C8, 50 m l o f acetone and a t.BHP i n j e c t i o n r a t e o f 8.76 mmo1.h-l i s used. The s t r a i g h t l i n e r e p r e s e n t s t h e maximum p o s s i b l e c onv ers io n when o n l y ketones a r e formed and t.BHP c o n v e r s i o n i s complete. Fig.

2A shows f o r n - o c t a n e o x i d a t i o n on FePcY,

FePcVPI-5 and FePc

c a t a l y s t s t h e d i s t r i b u t i o n o f ketones and a l c o h o l s . A l l t hese c a t a l y s t s e x h i b i t a h i g h s e l e c t i v i t y towards t h e f o r m a t i o n o f ketones. Encaged FePc complexes a r e more s e l e c t i v e f o r ketones t han t h e homogenous ones. S i m i l a r behav ior i s a l s o e s t a b l i s h e d by Herron e t a1 [ 9 ] and t h e y a t t r i b u t e i t t o d i f f u s i o n e f f e c t s i n t h e m o l e c u l a r s i e v e s.

The ketone t o a l c o h o l r a t i o

which ranges f r o m about 4 t o 9, i s t y p i c a l f o r f r e e r a d i c a l c h a i n o x i d a t i o n r e a c t i o n s and s i m i l a r t o what i s r e p o r t e d by Fontecave and Mansuy [ 16] f o r t h e system MnTPP(C1 )/02/ascorbate. They a t t r i b u t e t h e f o r m a t i o n o f ketones t o t h e involvement o f f r e e r a d i c a l s . A l s o t h e oxygen f o r m a t i o n i n d i c a t e s that

r e a c t i o n s o f r a d i c a l a r n a t u r e a r e i n v o l v e d i n t h e mechanism.

It

suggests t h a t t h e a c t i v e s i t e i s n o t an "oxenoi'd" species as i n enzymes [17] and t h e i r mimics [6-81, b u t t h a t t h e r e a c t i o n r a t h e r occurs v i a a f r e e r a d i c a l c h a i n mechanism. T h i s c h a i n mechanism seems t o be d i s t u r b e d by t h e z e o l i t e framework where t h e r e a c t i o n y i e l d on a p e r o x i d e b a s i s exceeds t h a t

52

o f f r e e FePc by a f a c t o r of

10. However,

Haber e t a l .

[ 18]

using the

oxidation of cyclohexane with hydrogen peroxide over c h l orotetratolylporphyrinatochromium( 111), o b t a i n e d under comparable c o n d i t i o n s k et o ne t o a l c o h o l r a t i o s between 1.4 and 4, and claimed t h a t t h e r e a c t i o n s were n o t r a d i c a l a r i n n a t u r e . Another e x p l a n a t i o n i s t h a t d i f f e r e n t k i n d o f r e a c t i o n s a r e i n v o l v e d i n t h e f o r m a t i o n o f p r o d u c t s . Indeed,

i t i s shown

[19] t h a t t h e i r o n - c a t a l y z e d t.BHP o x i d a t i o n o f cyclohexane proceeds w i t h different

parallel

r e a c t i o n pathways.

formation o f t.butoxy

However,

the total

s u b s t i t u e n t s on t h e a l k y l chain’,

lack o f

the

i n our results,

c o n t r a s t s w i t h a f r e e r a d i c a l mechanism.

A ketone

C2lC3

alcohol

100,

C2IC4

I

FePcY

FcPcVPI- 5

Catalyst

FePc

FcPcY

PePcVPI- 5

PCPC

Catalyst

2. S e l e c t i v i t y f o r k e t o n e and a l c o h o l formation (A) and r e g i o s e l e c t i v i t y (B) ;n t h e o x i d a t i o n o f n-oct ane a t l o w c o n v e r s i o n s o v e r FePcY, FePcVPI-5 and FePc c a t a l y s t s . C o n d i t i o n s a r e t h o s e o f F i g . 1. On homogeneous FePc no r e g i o s e l e c t i v i t y i s observed as t h e C2/C3 and C2/(C4tC5)

r a t i o s a r e a l l around 1. F i g . 28 i n d i c a t e s t h a t r e g i o s e l e c t i v i t y

e x i s t s f o r b o t h m o l e c u l a r s i e v e s , p o s s i b l y due t o t h e encaged n a t u r e o f t h e complex. However, l o w e r v a l u e s o f t h e C2/C3 and C2/C4 r a t i o s a r e o b t a i n e d i n VPI-5 compared t o z e o l i t e Y, p o i n t i n g t o t h e e x i s t e n c e o f shape s e l e c t i v i t y . The m o l e c u l a r g r a p h i c s a n a l y s i s enables q u a n t i f i c a t i o n o f t h e f r e e pore a p e r t u r e s , which f o r b o t h m o l e c u l a r s i e v e s a r e about 0.6 nm [ 15] . I t shows t h a t t h e d i f f e r e n c e i n s e l e c t i v i t y can h a r d l y be caused by

d i f f e r e n c e s i n t h e z e o l i t i c environment. The enhanced c o n s t r a i n t observed f o r FePcY s hou l d t h e n be r e l a t e d t o t h e s a d d l e - t y p e d e f o r m a t i o n o f t h e complex. R e g i o s e l e c t i v i t y i n t h e a l c o h o l f r a c t i o n i s i n s i g n i f i c a n t .

53 TABLE 1 R e g i o s e l e c t i v i t y i n t h e ketone f r a c t i o n f o r t h e o x i d a t i o n o f C6 t o C10 n a1 kanes on FePcY c a t a l y s t a expressed as s t a n d a r i z e d molar r a t i o s b o f C2/C3 and C2/ (C4tC5)

.

Substrate

Regi osel e c t i v i t y C Ketone f r a c t i o n C2/C3 C2/(C4tC5)

n - hexane n - heptane n -o c t a ne n-nonane n-decane

0.92 1.15 1.29 1.38 1.56

a; b; c;

1.51 1.95 2.80 2.59

R eac t io n c o n d i t i o n s : i n a m i c r o r e a c t o r o f 3 m l w i t h 2.4 mmol o f t.BHP, 6 mmol o f p a r a f f i n , 0.1 g o f FePcY and 1.5 m l o f acetone and a t 298 K, 0.1 MPa, 5 % c o n v e r s i o n . C o r r e c t i o n s a r e made so as t o have an equal number o f C2 and (C4+C5) p o s i t i o n s i n t h e n-alkane chain, i r r e s p e c t i v e o f i t s chain length. R a t i o o f o x y g e n a t i o n a t C p o s i t i o n 2 over oxygenat ion a t i n n e r C positions. T a ble 1 shows t h a t f o r n-hexane as s u b s t r a t e no r e g i o s e l e c t i v i t y i s

observed.

However,

f o r l o n g e r c h a i n s t h e z e o l i t e enclosed FePc e x e r t s a

shape s e l e c t i v e e f f e c t o f i n c r e a s i n g i n t e n s i t y on t h e i n s e r t i o n o f oxygen. Carbon atoms a t p o s i t i o n 0 - 1 a r e p r e f e r e n t i a l l y o x i d i z e d o v e r carbon atoms a t p o s i t i o n w-2, which i n t u r n a r e o x i d i z e d f a s t e r t h a n t h o s e a t p o s i t i o n 4 and 5. Thus t h e r e g i o s e l e c t i v i t y i n c r e a s es w i t h c h a i n l e n g t h , o b v i o u s l y , because l o n g e r a l k y l c h a i n s a r e more l i a b l e t o s t e r i c c o n s t r a i n t s e x e r t e d by t h e z e o l i t e framework a t t h e l e v e l o f t h e a c t i v e s i t e . Conclusions I r o n - p h t h a l l o c y a n i n e s a r e encaged i n m o l e c u l a r s i e v e s by an i n s i t u s y n t h e s i s f rom ferrocenium-Y and f e r r i c e n i u m-VPI -5.

The a c t i v i t y o f i r o n -

p h t h a l l o c y a n i n e s i n z e o l i t e Y i s h i g h e r t h a n i n t h e VPI-5 c a t a l y s t which i n t u r n exceeds t h o s e o f t h e f r e e complexes as shown by t u r n o v e r numbers o f respectively,

6,000,

2,000 and 25. However,

iron-phthallocyanine-VPI-5 i s

more s t a b l e t h a n t h e i r o n - p h t h a l l o c y a n i n e - Y c a t a l y s t , which t u r n s i n a c t i v e a f t e r t u r n o v e r numbers h i g h e r t h a n 6,000, and t h e f r e e complex s u f f e r s f rom v e r y f a s t o x i d a t i v e d e s t r u c t i o n . Shape s e l e c t i v i t y i n t h e r e g i o s e l e c t i v e o x i d a t i o n o f alkanes i s h i g h e r on FePcY t h a n on FePcVPI-5, which i n t u r n i s higher

t h an

explained framework

by

on

the

non-selective

a combined e f f e c t

of

phthallocyanine.

the

molecular

homogeneous

of

sieve

steric and

catalyst.

constraint the

This

can

imposed by

deformation

of

be the the

The shape s e l e c t i v e o x i d a t i o n o f t h e secondary carbon

atoms o f normal alkanes i n c r e a s e s w i t h t h e c h a i n l e n g t h .

54

ALKANE OXIDATIONS ON TITANIUM SILICALITE Introduction T i t a n i u m s i l i c a l i t e - 1 o r TS-1 i s a t i t a n i u m c o n t a i n i n g d e r i v a t i v e o f s i l i c a l i t e - 1 , w h i c h i n a s e l e c t i v e way c a t a l y z e s t h e o x i d a t i o n o f o r g a n i c s u b s t r a t e s w i t h d i l u t e d hydrogen p e r o x i d e as o x i d a n t [20-231. The r e a c t i o n s

i n t h e p a t e n t 1 i t e r a t u r e i n c l u d e t h e (mono)epoxidat ion o f ( d i ) o l e f i n s [24-251, t h e c o n v e r s i o n o f o l e f i n s t o g l y c o l monomethyl e t h e r s

described

i n t h e presence o f methanol [26],

t h e h y d r o x y l a t i o n o f aromat ics [27], t h e

o x i d a t i o n o f p r i m a r y o r secondary a l c o h o l s t o aldehydes o r ket ones [28], t h e ammoxidation o f ketones i n t h e presence o f NH3 [29], o f v in y lbenz ene s t o 8 - p h e n y l aldehydes [30].

and t h e c o n v e r s i o n

Very r e c e n t l y , t h e o x i d a t i o n

o f u n a c t i v a t e d a l k a n e s on TS-1 was d e s c r i b e d i n d e p e n d e n t l y b y Tatsumi e t a l . [31] and by us [32]. ExDerimental TS-1 was p r e p a r e d by t h e h y d r o t h e r m a l procedure d e s c r i b e d i n example 2 o f US 4,410,501.

I t s i n f r a r e d spectrum i s c h a r a c t e r i z e d by an I R band a t

960 cm-l, as r e p o r t e d f o r T S - 1 [20-211, and no e x t r a z e o l i t i c c r y s t a l l i n e o r amorphous phases c o u l d be d e t e c t e d by XRD o r SEM. The o x i d a t i o n r e a c t i o n s were conducted a t 373K under v i g o r o u s s t i r r i n g i n a s t a i n l e s s s t e e l b a t c h r e a c t o r p r e v i o u s l y f l u s h e d w i t h N2. A f t e r c o m p l e t i o n o f t h e r e a c t i o n , t h e two l i q u i d phases (H202 and alkane) were homogenized w i t h an excess o f acetone. The o x i d a t i o n p r o d u c t s were analyzed by GC on a 50 m CP S i l - 8 8 c a p i l l a r y column f rom Chrompack, u s i n g t o l u e n e as internal

s t a nd a r d .

W i t h i n t h e accuracy o f t h e GC a n a l y s i s ,

yields

of

a l c o h o l s and ketones add up t o 100% on a carbon b a s i s . H202 conversions were det e rmined by c e r i m e t r i c t i t r a t i o n [33]. R e s u l t s and d i s c u s s i o n F i g . 3 shows t h e o x i d a t i o n p r o d u c t s o b t a i n e d i n t h e o x i d a t i o n o f n hexane w i t h 35% aqueous H202 as a f u n c t i o n o f r e a c t i o n t i m e . The o x i d a t i o n p r o d u c t s a r e d e r i v e d f r o m n-hexane by o x i d a t i o n a t t h e secondary carbon p o s i t i o n s , w h i l e o x i d a t i o n o f p r i m a r y C - H bonds i s below d e t e c t i o n 1 i m i t s . Wi t h i n c r e a s i n g n-hexane conversion, t h e s e l e c t i v i t y f o r ketones i n c r e a s e s a t t h e expense o f t h a t f o r a l c o h o l s , i n d i c a t i n g t h a t t h e o x i d a t i o n oc c u r s i n two c o n s e c u t i v e s t e p s,

i.e.

hexane i s o x i d i z e d t o a

m i x t u r e o f 2- and 3-hexanols, w h i c h i s t h e n f u r t h e r o x i d i z e d t o 2- and 3 hexanones. The absence o f i s o m e r i s a t i o n o f t h e p r o d u c t s was c o n f i r m e d by t h e f o r m a t i o n o f o n l y 2-hexanone o r 3-hexanone when r e s p e c t i v e l y 2-hexanol o r 3-hexanol were used as s u b s t r a t e s . Theref ore, t h e o v e r a l l r e a c t i o n scheme f o r n-hexane o x i d a t i o n on TS-1 can be r e p r e s e n t e d as f o l l o w s :

55

H202 2-hexanol

H202

n-hexane

,

1

2-hexanone

1

3-hexanone

H202

3-hexanol

I r

I

3-hexanone

/ /

lZ@#2-hexanone

/

EB 2-hexanol total

n 0

1

0.5

2

3

Reaction time (h) F i g . 3. n-Hexane o x i d a t i o n by H202 on TS-1 as a f u n c t i o n o f r e a c t i o n t i m e . R eac t io n c o n d i t i o n s : 500 mg o f TS-1, 115 mmoles o f n-hexane, 240 mmoles o f H202 (35% i n H20), 45 m l o f acetone; 373K, 700 RPM s t i r r i n g r a t e . T a ble 2 shows t h e H202 y i e l d and s e l e c t i v i t i e s i n t h e o x i d a t i o n o f d i f f e r e n t n-alkanes and hexane isomers by 35% aqueous H202 [31]. TABLE 2 Alkane o x i d a t i o n s by H202 on TS - l a Substrate

H202 y i e l d b

(%I n-pentane n - hexane n -o c t a ne n-decane 2-methylpentane 3-methyl pentane 2,2-dimethylbutane a; b; c;

68 70 65 56 59 58 50

Regiosel e c t i v i t y C Total products Ketone f r a c t i o n C2/C3 C2/(C4tC5) C2/C3 C2/(C4tC5) 2.12 1.13 1 1.12 5.67 0.78

1.17 0.60

2.45 1.78 2.77 3.28 2.70

3.59 2.56

>loo

OD

R eac t io n c o n d i t i o n s : 400 mg o f TS-1, 310 mmoles o f alkane, 210 mmoles o f H202 (35% i n H20), 60 m l o f acetone, 3 hours a t 373K and 1000 RPM. H202 y i e l d = f r a c t i o n o f H202 used f o r alkane o x i d a t i o n on t o t a l H202 c onv ers io n ; H202 c o n v e r s i o n s a r e h i g h e r t han 90% i n a l l r e a c t i o n s . R a t i o o f o x y g e n a t i o n a t C p o s i t i o n 2 over oxygenat ion a t i n n e r C positions.

56

F o r t h e d i f f e r e n t n - a l k a n e s as w e l l as o f t h e f o u r hexane isomers t h e e f f i c i e n c y o f H202 use f o r o x i d a t i o n i s h i g h on TS-1, b u t decreases as t h e dimensions

of

the

substrate

p r o d u c t s i s almost

increase.

statistically

The t o t a l

amount

of

oxygenated

d i s t r i b u t e d over t h e d i f f e r e n t carbon

p o s i t i o n s , w i t h o n l y a s l i g h t s e l e c t i v i t y f o r o x i d a t i o n a t t h e w-1 p o s i t i o n o f t h e carbon c h a i n . W i t h i n t h e ketone f r a c t i o n however, a v e r y pronounced s e l e c t i v i t y f o r 2-ketones i s observed. T h i s means t h a t t h e f i r s t o x i d a t i o n step,

i.e.

the formation o f

alcohols,

i s only

l i t t l e regioselective,

whereas i n t h e second o x i d a t i o n s t e p , t h e 2 - i s o m e r s o f t h e a l c o h o l m i x t u r e a r e s e l e c t i v e l y f u r t h e r o x i d i z e d t o 2 - k e t o n e s . T h i s s e l e c t i v i t y i s imposed by t h e shape s e l e c t i v e p r o p e r t i e s o f TS-1. Furthermore, i t i s seen t h a t f o r branched

a1 kanes,

secondary ones.

tertiary

C-H

bonds

are

selectively

oxidized

over

The a l k a n e o x i d a t i o n on TS-1 t h u s f o l l o w s t h e ' n o r m a l '

r e a c t i v i t y o r d e r : t e r t i a r y C-H > secondary C-H >>> p r i m a r y C-H. Furthermore

the

H202

yield

decrease

with

increasing

substrate

dimensions. T h i s o f f e r s s t r o n g e v i d e n c e f o r an i n t r a c r y s t a l l i n e r e a c t i o n . Indeed, because o f t h e g e o m e t r i c a l c o n s t r a i n t s imposed by t h e p o r e geometry of

TS-1

on

the

catalyzed

reactions,

the

oxidation

rate

of

organic

s u b s t r a t e s decreases as t h e i r dimensions i n c r e a s e , and t h e r e f o r e t h e main side reaction,

i.e.

t h e r m a l and c a t a l y t i c d e c o m p o s i t i o n o f H202 t o 02,

becomes more c o m p e t i t i v e . F i g . 4 shows t h e dependance o f t h e TS-1 c a t a l y z e d n-hexane o x i d a t i o n on t h e amount o f s o l v e n t i n t h e r e a c t i o n m i x t u r e . 100

3-hexanone

be

80

@%%l 2-hexanone

W

.-

3-hexanol

60

v1

2-bexanol

h

2

40

C

0

0

total

20

0 0

10

20

30

45

60

90

Acetone (ml) F i g . 4 . n-Hexane o x i d a t i o n by H202 on TS-1 as a f u n c t i o n o f acetone c o n c e n t r a t i o n . R e a c t i o n c o n d i t i o n s : 500 mg o f TS-1, 115 mmoles o f n-hexane, 240 mmoles o f H202 (35% i n H20), I h r a t 373K and 700 RPM. F i g . 4 shows t h a t t h e hexane c o n v e r s i o n e x h i b i t s an optimum a g a i n s t t h e acetone c o n c e n t r a t i o n .

I n absence o r a t v e r y l o w c o n c e n t r a t i o n s o f

57

acetone, t h e s o l u b i l i t y o f t h e o r g a n i c s u b s t r a t e i n t h e aqueous H202 phase, and t h e r e f o r e t h e d r i v i n g f o r c e f o r d i f f u s i o n o f t h e s u b s t r a t e t o t h i s phase i s v e r y l o w . As a r e s u l t t h e r e a c t i o n r a t e may become d i f f u s i o n l i m i t e d . When t h e amount o f acetone i s increased, b o t h t h e hexane s o l u b i l i t y and t h u s t h e d r i v i n g f o r c e f o r d i f f u s i o n increase, which ' r e s u l t s i n an in c re as e d r e a c t i o n r a t e . When t h e acetone c o n c e n t r a t i o n i s s t i l l f u r t h e r inc re as e d however, b o t h H 2 0 2 and n-hexane become more d i l u t e d i n the reaction solution,

t h u s c a u s i n g a decrease i n r e a c t i o n r a t e .

The

compromise between h i g h hexane s o l u b i l i t i e s and d i f f u s i o n r a t e s and l o w d i l u t i o n s e x p l a i n s t h e optimum o f t h e acetone c o n c e n t r a t i o n . The t r a n s i t i o n of a diffusionally

t o a chemically

controlled

system w i t h

increasing

s o l v e n t c o n c e n t r a t i o n , i s c o n f i r m e d by t h e r e s u l t s shown i n F i g . 5, f o r nhexane o x i d a t i o n as a f u n c t i o n o f c a t a l y s t c o n c e n t r a t i o n b o t h i n t h e presence and i n t h e absence o f acetone. hexane

c onv ersi o n

increases

in

a

I n t h e presence o f acetone t h e

proportional

way

with

the

catalyst

c o n c e n t r a t i o n up t o conversions o f about 60%. I n t h e absence o f a p o l a r solvent

deviations

c onv ers io ns ,

of

this

indicating

linearity

that

the

are

reaction

already is

no

observed longer

at

low

chemically

c o n t r o l 1ed.

m

100

80

0

h

ae

0

d

W

60

0

C

0,'

0

U

.I

vl

L,

a2 P

.'

40

0

2-bexanol

O

El

$

0

20 0

++1 0

. 2 5 . 5 0 .75 1.00

TS-1

0

. 2 5 . 5 0 .75 1.00

(g)

F i g . 5. n-Hexane o x i d a t i o n by H20 as a f u n c t i o n o f TS-1 c o n c e n t r a t i o n . R eac t io n c o n d i t i o n s : 115 mmoles n-hexane, 2 4 0 mmoles o f H 2 0 2 (35% i n H 2 0 ) , 1 h r a t 373K and 700 RPM, a; 4 5 m l acetone added, b; no s o l v e n t added.

03

As i s seen i n F i g . 4 , t h e acetone c o n c e n t r a t i o n a l s o s t r o n g l y a f f e c t s

the

k et o ne/ a lc o h o l

ratio

of

the

oxidation

product s.

The

decreasing

s e l e c t i v i t y f o r ketones w i t h i n c r e a s i n g acetone c o n c e n t r a t i o n i s p r o b a b l y p a r t l y connected w i t h t h e v a r i a t i o n o f t h e conversion, b u t i s a l s o caused by t h e v a r i a t i o n o f hexane c o n c e n t r a t i o n i n t h e aqueous H202 phase. Indeed,

58

a t l o w acetone c o n c e n t r a t i o n s , t h e hexane s o l u b i l i t y and t h u s t h e s u b s t r a t e concentration i s low,

and t h e r e f o r e a hexanol m o l e c u l e w h i c h has been

formed on t h e c a t a l y s t w i l l be e a s i l y c o n v e r t e d t o t h e k e t o n e . A t h i g h e r acetone c o n e n t r a t i o n s and h i g h e r hexane s o l u b i l i t i e s however, c o m p e t i t i o n between hexane and hexanol f o r o x i d a t i o n w i l l become s i g n i f i c a n t , and o n l y a f r a c t i o n o f t h e hexanol m i x t u r e ,

will

m a i n l y 2-hexanols,

be f u r t h e r

oxidized. A l t h o u g h no e x t e n s i v e m e c h a n i s t i c s t u d y o f t h e a l k a n e o x i d a t i o n on titanium

silicalite

has

it

been made,

is

believed

that

the

reaction

proceeds v i a t h e a c t i v a t i o n o f H202 on t h e t i t a n i u m s i t e s w i t h f o r m a t i o n o f T i - p e r o x o s p e c i e s , which a r e t h e a c t u a l oxygen donors. such

species

subsequent

was

shown

vacuum

by

drying

adsorption [32,34].

disappearance o f t h e 960 cm-’ (Ti=O) group [26],

of

This

I R band,

H202

on

The f o r m a t i o n o f Ti-silicalite

treatment

results

in

and the

ascribed t o t h e surface t i t a n y l

and t h e f o r m a t i o n o f a 425 nm a b s o r p t i o n band i n t h e

v i s i b l e spectrum. The l a t t e r a b s o r p t i o n i s c h a r a c t e r i s t i c f o r T i complexes

w i t h peroxo l i g a n d s [31].

H e a t i n g o f t h e H202 t r e a t e d t i t a n i u m s i l i c a l i t e

l e a d s t o t h e disappearance o f t h e 425 nm and t h e reappearance o f t h e 960 c m - l a b s o r p t i o n bands i n t h e V I S and I R s p e c t r a , due t o d e c o m p o s i t i o n o f the

Ti

peroxo

complexes

with

reformation

of

the

titanyl

group.

The

f o r m a t i o n o f hydroxyhydroperoxo o r peroxo complexes by r e a c t i o n o f hydrogen p e r o x i d e w i t h t i t a n y l groups on TS-1 i s r e p r e s e n t e d i n scheme 1.

H202

0 II

OH

OOH

\ /

Ti

Ti

-

0-0 \

.

114 23 24 25 26 27 28 29 30 31 32

J. Rippmeester, J. Am. Chem.. SOC. 104 (1982) 289. J. Fraissard, T. Ito, Zeolites 8 (1988) 350. J. Demarquay, J. Fraissard, J. Chem. Phys. Lett. 136 (1987) 314. T. Ito, Fraissard, J. Chem. SOC., Faraday Trans. 1, 83 (1987) 451. J. Fraissard, T. Ito, L.C. de Menorval, in: Proceedings of the 8th Internat. Congress on Catalysis Berlin, 1984, p. 111.

.

T. Ito, L.C. de Menorval, E. Guerrier, J. Fraissard, Chem. Phys. Lett. 111 (1984) 3271. E.W. Scharpf, R.W. Grecely, B.C. Gates, C. Dybowski, J. Phys. Chem. 9 0 (1987) 9. R. Shoemaker, T. Apple, J. Phys. Chem. 9 1 (1987) 4024 B.F. Chmelka, R. Ryoo, S.-B. Liu, L.C. de Menorval, J. Am. Chem. SOC. 110 (1988) 4465. H. Winkler, D. Michel, Adv. Coll. Interf. Sci. 23 (1985) 149.

33 34

J. Karger, H. Pfeifer, J. Fraissard, Z. Phys. Chemie, Leipzig 195 (1987) 268. J. Karger, H. Pfeifer, and W. Heink, Adv. Magn. Res. 12 (1988) 1.

35 36 37

W. Heink, J. Karger, H. Pfeifer, and F. Stallmach, J. Am. Chem. SOC. 112 (1990) 2175. D.M. Ruthven, Canad. J. Chem. 52 (1974) 3523. J. Caro, J. Karger, G. Finger, H. Pfeifer, and R. Schollner,

Z. Phys. Chem., Leipzig, 257 (1976) 903. 38

H. Pfeifer, J. Karger, A. Germanus, W. Schirmer M. Bulow, and J. Caro, Ads. Science Techn. 2 (1985) 229.

39

Landoldt-Bornstein IIZahlenwerte und Funktionenll, Band I, Teil 1, p. 325, 370, Springer, Berlin, Gottingen, Heidelberg, 1950. J. Karger, H. Pfeifer, F. Stallmach, and H. Spindler, Zeolites 10 (1990) 288. R.M. Barrer IIZeolites and Clay Minerals as Sorbents and Molecular Sievest1,Academic Press, London, 1978 J. Karger, H. Pfeifer, Zeolites 7 (1987) 90. M. Kocirik, P. Struve, K. Fiedler, M. Billow, J. Chem. SOC. Faraday Trans. 1, 89 (1988) 3001. D.M. Ruthven, R.I. Derrah, J. Chem. SOC., Faraday Trans. I

40 41 42 43 44

7 1 (1974) 2031. 45 46 47

M.M. Dubinin, A.K. Gorlow, A.M. Voloshchuk, in: Proc. 5th Internat. onf. Zeolites, Naples, 1980, Heyden, London, 1980, p. 554. M. Bulow, U. Hartel, U. Muller, K.K. Unger, Ber. Bunsenges. Phys. Chem. 94 (1990). B. Zibrowius, J. Caro, J. Karger, Z. Phys. Chem., Leipzig, 269 (1988) 1001.

48 49

P. Demontis, E.S.Fois, G.B. Suffritti, S . Quartieri, J. Phys. Chem. 94 (1990) 4329. M. Morgan, T. Cosgrove, R. Richardson, Coll. Surf. 36 (1989) 209.

50 51 52 53 54

W. Heink, J. Karger, H. Pfeifer, Z. Phys. Chem., Leipzig, (in press). C. Forste, W. Heink, J. Karger, H. Pfeifer, N.N. Feoktistova, S.P. Zhdanov, Zeolites 9 (1989) 299. C. Forste, J. Karger, H. Pfeifer, Proc. 8th Int. Zeolite Conf., Amsterdam, Elsevier, 1989, p. 907. C. Forste, J. Karger, H. Pfeifer, L. Riekert, M. Bulow, A . Zikanova, J. Chem. S O C . Faraday Trans. 1, 86 (1990) 881. D.M. Ruthven, IIPrinciples of Adsorption and of adsorption Processes" Wiley, New York, 1984

-

115 55

H. Pfeifer, W. Meiler, D. Deininger, Ann. Rep. NMR Spectr.

56 57

D.M. Ruthven, Am. Chem. SOC. Symp. Ser. 40 (1977) 320. J. Karaer. J. Caro, J. Chem. SOC., Faraday Trans. 1, 73

58 59

J. Karger, D.M. Ruthven, Zeolites 9 (1989) 267. E.Cohen de Lara, R. Kahn, F. Mezei, J. Chem. S O C . , Faraday Trans. 1, 79 (1983) 1911. Z . Xu, personal communication. A. Germanus, J. Karger, H. Pfeifer, Zeolites 4 (1984) 188. C.J. Wright, C. Riekel, Mol.Phys.36 (1978) 695. J.Karger, D.M.Ruthven, J.Chem.Soc., Faraday Trans. 1,

15 (1983) 292. (1977)-1363.

60 61 62 63

77 (1981) 1485. 64 65 66 67 68 69 70

71 72 73

H. Jobic, M. Bee, J. Karger, H. Pfeifer, J. Caro, J. Chem. S O C . , Chem. Commun. (1990) 341. M. Bulow, W. Mieth, P. Struve, P.Lorenz, J. Chem. SOC. Faraday Trans. 1, 79 (1983) 2457. M. Eic, M. Goddard, D.M.Ruthven, Zeolites 8 (1988) 327. J. Caro, M.Bulow, W.Schirmer, J.Karger, W.Heink, H.Pfeifer, S.P.Zhdanov, J. Chem. SOC. Faraday Trans. 1, 85 (1989) 4201 H. Jobic, M. BBe, J. Caro, J. Karger, M. Billow, J. Chem. SOC. Faraday Trans. 1, 85 (1989) 4201. D.T. Hayhurst, A.R. Paravar, Zeolites 8 (1988) 27. S.D. Pichett, A.K. Nowak, J.M. Thomas, B.K. Peterson, J.F.P. Swift, A.K. Cheetham, C.J.J. den Ouden, B.Smit, M.F.M. Post, J. Phys. Chem. 94 (1990) 1233. R.L. June, A.T. Bell, D.N. Theodorou, J. Phys. Chem., in press. J.P. Chauvel, N.S. True, Chem. Phys. 9 5 (1985) 435. R.W. Taft, I.A. Koppel, R.D. Topsom, F. Anvia, J. Am. Chem. SOC. 112 (1990) 2047.


G . Ohlrnann et al. (Editors),Catalysis and Adsorption by Zeolites 0 1991 Elsevier Science PublishersB.V., Amsterdam

117

SPECTRAL STUDY OF LEWIS ACIDITY OF ZEOLITES AND OF ITS ROLE IN CATALYSIS

V .B .KAZANSKY N.D.Zelinsky Institute of Organic Chemistry, Moscow, USSR SUMMARY

The Lewis acidity of cationic forms of zeolites and of those modified by mild steaming or by Zn and Ga ions was studied using molecular hydrogen and light paraffins as molecular probes. The extent of their perturbation was controlled by IR diffuse reflectance spectroscopy. It was concluded, that Lewis sites should be considered not as low coordinated cations, but rather as acid-base pairs in which the neighboring basic oxygen is equally important. The role of extra lattice aluminum or modifying Zn and Ga ions in cracking of paraffins or their aromatization is discussed. INTRODUCTION The catalytic properties of zeolites are mainly connected with

their aci-

dity. Therefore the central problem of catalysis on zeolites is the nature of their acid active sites. The recent information on the Broensted acidity of zeolites is rather complete, since it could be easily followed by such direct spectroscopic methods as IR or high resolution solid state NMR. On the contrary the knowledge of the nature and the properties of Lewis acid sites is quite not as full. This is because they could be investigated only by indirect way by means of molecular probes with spectral control of perturbation of adsorbed molecules. For this purpose most often ammonia or pyridine adsorption is used. These

molecules have however very little in common with hydrocarbons, whose catalytic transformations on zeolites are of

the greatest

interest. In other

words, if adsorbed ammonia is protonated by a Broensted acid site or is strongly bonded by a Lewis site, this does not yet mean, that the adsorbed hydrocarbons would behave in the similar way. Another, more principal difference between strong bases and hydrocarbons consists in the chemical nature of their interaction with Lewis acid sites. In the case of hydrocarbons or hydrogen the elementary step, which is most often discussed, is the heterolytic dissociation: 6- 6 H--H

-Me--0- + H

2

--.c

-Me--O-

+

H- H+ ----L

-Me--O-

(1)

Unlike this ammonia or pyridine adsorption results in coordinative binding:

118

-Me--0- + NH3

c_)

(2)

Y3 -Me--0-

Thus, the strong bases are the tests only for Lewis sites, whereas the molecular hydrogen is rather the test for the acid-base ion pairs. Therefore the conclusions on the nature, the strength and the number of Lewis sites obtained from adsorption of strong bases could not be

so

simply transferred

to

adsorption of hydrocarbons. Of course the best molecular probes for Lewis acidity themselves. Therefore in the previous studies of aluminum

are

the reactants

oxide and H-zeo-

lites we used the adsorption of light paraffins or the low temperature adsorption of molecular hydrogen (refs. 1.2). Below the application of

the similar

approach for the investigation of Lewis acidity of zeolites modified by various methods is reported. CATIONIC FORMS OF ZEOLITES

Zodlurn-Errns The univalent cations or multivalent cations introduced by ion exchange represent the simplest examples of Lewis acid sites in zeolites. Therefore already in the very first papers on their catalytic application the probable role of such active sites was postulated and discussed (ref.3). According to these works the cations could activate adsorbed molecules due to their electrostatic polarization. The strength of electric field created by cations and of the degree of such polarization have been however never estimated experimentally. Some ideas on this point could be drawn out from our

re-

sults on low temperature molecular hydrogen adsorption (ref.2,4). IR spectra of molecular hydrogen adsorbed at 77

K and the pressure of 2-5

Torr on sodium forms of different zeolites are presented in Fig.1. They that low temperature adsorption of hydrogen

show,

results in the appearance of se-

veral bands in the region of H-H bond stretching vibrations. This indicates the existence of various types of adsorption sites, which seem

to be

sodium

ions in different positions of the zeolite framework. They are able to polarize adsorbed hydrogen molecules in various extent, resulting in the appearance of several absorption bands. When the spectra of adsorbed hydrogen contain more than one line, then the decrease of hydrogen pressure or evacuation at 77 K result

in the following

change of their relative Intensities: the high frequency bands at w

=

4120-1 4150 cm-l disappear , whereas those at lower frequencies of w = 4080-4110 cm

remain unchanged. This could be explained by the different strength of hydro-

119 gen binding by cations located in various positions of the zeolite framework.

It correlates with the values of low frequency shifts of the H-H stretching vibrations.

Fig.1 Diffuse reflectance IR spectra of molecular hydrogen adsorbed on sodium forms of zeolites at 77 K at various pressures. P=5 Torr:(b)-NaM,(c)-NaZSM-5, (e)-NaA, (f)-NaX, (g)-NaY. P=100 Torr: (a)-NaM, (d)-NaA, (h)-NaY.

At low pressure the spectra of pentasils and mordenites are the simplest (Fig.1. spectra (a)-(c) 1. For mordenite they consist of a single band at 4108 -1 cm , which is shifted by 55 cm-l towards lower frequencies as compared with the stretching vibrations of free hydrogen in gas phase at 4163 cm-l. This is consistent with conclusion of ref. 5, that in mordenite Na cations are preferentially located at one main type of sites at the centers of eight membered rings. At higher pressure of 100 Torr (spectrum (a) 1 an additional shoulder -1 of lower intensity appears near 4120 cm . It may be connected with sodium cations distributed among other available sites of localization. For NaZSM-5 zeolite there is no published data on Na cations distribution. Our results show, that there is only one type of sodium in its framework (Fig.1, spectrum (c) 1. The corresponding stretching frequency shift of hydrogen is very close to that of hydrogen adsorbed by mordenite. Therefore it

is

highly probable, that in NaZSM-5 the sodium ions are also localized at

the

similar positions. According to X-ray analysis, in NaA 8 of the overall number of

12 sodium

ions are located near the centers of the six membered rings, which form the sodalite cages. The 3 of remaining 4 Na cations are situated near the centers of 8-rings and one is statistically located over 12 fold equipoints (ref.6).

120

This is also consistent with the spectra (d) and (el of Fig.1, which show, that the majority of Na ions bind hydrogen more strongly and exhibit the

lar-

ger low frequency shifts of H-H stretching vibrations, while the rest of sodium ions could hold and perturb molecular hydrogen only at higher pressures. The spectra of hydrogen adsorbed on sodium forms of faujasites are more complicated (Fig.1, spectra (f)-(h) 1. According to our results (ref.4) they could be ascribed to H molecules perturbed by sodium ions located in SI; SI, 2 and in SIIpositions. Thus, low temperature molecular hydrogen adsorption allows to follow both the different sites of sodium ions localization in

the

zeolite frameworks and the distribution of ions among these sites. The important feature of IR spectra of hydrogen adsorbed by sodium forms of zeolites is the dependence of the line shifts on the type of

zeolite frame-

works. In this respect hydrogen behaves quite differently from adsorbed carbon monoxide (ref.7) which for the same cations exhibits very

close shifts for

different frameworks. Thus, the perturbation of adsorbed hydrogen depends not only on the nature of cations, but also on the type of

the zeolite crystal

lattice. This is in consistence with the by-point adsorption of molecular hydrogen, which was earlier supported by quantum chemical calculations for aluminum oxide (ref.8). If this conclusion is also correct for zeolites, then for their sodium forms the shifts should be dependent on the basic properties of oxygen. This is really observed experimentally. Indeed, the least basic oxygen is that

one

in high silica zeolites, which contain the lowest amount of aluminum in their framework. This results in the smallest low frequency shifts of H2

stretching

vibrations. On the contrary the largest shifts were observed for NaA zeolite, which contains the most basic oxygen. However, even in this case the value of the low frequency shift is only 90 cm-l i.e. is about twice less, than for hydrogen adsorbed by acid-base pairs of aluminum oxide sodium ions are rather weak perturbing sites of adsorbed

that

(ref.1). Thus,

hydrogen. This is

consistent with the fact, that contrary to aluminum oxide we never observed dissociative hydrogen adsorption on sodium forms of zeolites.

Bllralent-catlonlr-~orms_oy_qeoliteq The IR spectra of hydrogen, adsorbed at 77 K on alkaline earth forms of mordenite are presented in Fig.2. They contain two bands for Mg

and Ca

exchanged samples, but only single bands for the samples exchanged with Sr and Ba ions. Such difference could be easily explained by various hydrolysis extent of these cations.

121

J

Q

i C

d Fig.2. IR diffuse reflectance spectra of molecular hydrogen adsorbed at 77 K on alkaline earth forms of mordenite dehydrated at 720 K. (a)-MgNaM, (blCaNaM,(c)-SrNaM,(d)-BaNaM. The degree of cationic exchange is about 60 %. Hydrogen pressure is 30 Torr. Indeed,the second dissociation constants of (MgOHl' and

(CaOH)'

rather low (3'10-3 and 4'10-2 respectively). On the contrary for (BaOH)' cations they are much higher

(

ions are (SrOH)'

and

about 0.15-0.21. Therefore after dehy-

dration at elevated temperature magnesium and calcium ions exist in the zeolite framework in two forms: as bivalent Me2'

cations and as hydrolyzed MeOH'

species. On the contrary Sr and Ba cations are completely dehydroxylated resulting in doubly charged ions. Such an interpretation is supported by IR spectra of these samples in OH stretching region. For dehydrated Mg and Ca forms they contain both the bands from structural OH groups and those from basic hydroxyls of MeOH' On the contrary removal of water from S r ' 2

fragments.

+2

and Ba containing samples results

in complete dehydration and only the bands of silanol groups are observed. The comparison of results on hydrogen adsorption on sodium and earth forms of mordenite shows, that for Na'

and MeOH'cations

alkaline

the shifts are

very close. This also supports the above interpretation based on hydrolysis of M g ' 2

and Ca'2

cations, which behave in hydrolyzed form similar to univalent

ions. More surprising are the small shifts exhibited by nonhydrolyzed bivalent Sr"

and Ba"

cations, which are only slightly larger, than for sodium. In our

opinion, this is consistent with the above idea of bypoint adsorption of

122 hydrogen. If it is really of the bypoint nature, then the charge and

the polarizing

ability of lattice oxygen is also important. In other words the charges of cations and their dimensions are not the only factors affecting polarization of adsorbed hydrogen. In addition, for bivalent cationic forms the higher positive charge results in the partial electron density transfer from basic

oxygen

to cations. This decreases both positive charges of cations and those of

oxy-

gen in ion pairs and results in smaller shifts of stretching vibrations of adfrom

sorbed molecular hydrogen in comparison with which one could expect

the

formal electric charges. DEHYDROXYLATED HYDROGEN FORMS OF ZEOLITES Historically, one of the first mechanisms of dehydroxylation of H-faujasites was suggested by Uytterhoeven, Crystner and Hall (ref. 91. They postulated the formation of trigonally coordinated A1 and Si atoms, when

two hydroxyl

groups are removed from the zeolite framework:

-

H 0 \

2

/ \

Si /

\ /

\

/

A1 \

+

Si

d

/

\ /

/

A1 \

\

0 / \

Si

+ /

/

A1

\ /

(31

\

Later this mechanism was found to be incorrect, since according to the numerous solid state high resolution NMR data

the dehydroxylation of

hydrogen

forms of faujasites results instead of formation of lattice Lewis sites in dealumination (ref.101. Nevertheless our experimental results on low temperature molecular hydrogen adsorptioii clearly show, that f o r zeolites dehydroxylation still could occur according

high silica containing to

scheme

(31 (refs.

2,111.

IR spectra of H2 adsorbed at 77 K on HZSM-5 zeolite preheated in vacuo at different temperatures are presented in Fig.3. At moderately high

temperatu-

res, when dehydroxylation has not yet started (770K) , two bands with

maxima

at 4125 and 4105 cm-l are observed (fig.3 (a) 1. With the increase of the pretreatment temperature and of the extent of surface dehydroxylation their intensity decreases. On this ground we assign these bands to hydrogen interacting with surface hydroxyl groups. Since the position of the high frequency band is the same as when hydrogen is adsorbed on silica, this band could be attributed to the molecules, perturbed by silanol groups. Then the band with a lower frequency of 4105 cm-1 should be ascribed to hydrogen molecules interacting with the acidic bridged

hydroxyls with

stretching

123 vibrational frequency of 3610 cm-1

.

Fig. 3. IR spectra of hydrogen, adsorbed at 77 K on HZSM-5 zeolites dehydroxylated in vacuo at various temperatures: (a) - 770 K. (b) - 970 K. (c) 1220 K. (d) - HY zeolite preheated at 770 K under "deep bed" conditions. If the pretreatment temperature of HZSM-5 zeolite is increased up to 910 K

its partial dehydroxylation takes place. This results in the decrease of

the

bands intensity of the bridged hydroxyl groups. Simultaneously two new bands from adsorbed hydrogen with maxima at 4010 and 4035 cm-l and values of low frequency shifts of 125 and 150 cm-l

the very

are appearing

large

(Fig.3b).

They should be attributed to adsorbed hydrogen interacting with two different kinds of Lewis acidic sites, which are formed after removal of hydroxyl groups in accordance with scheme ( 3 ) . There are also the following additional arguments in favor of ment of these bands to hydrogen molecules adsorbed on

the assign-

trigonally coordinated

Si and A1 atoms of the zeolite crystal lattice. Firstly, if the dehydroxylation of H-high silica zeolites is carried out under still more

severe condi-

tions and the dealumination of their crystal lattice really takes place, one more low frequency band cm-'appears

of adsorbed hydrogen with

the maximum at

4060

[Fig.3c). Simultaneously the intensity of the band of adsorbed hy-

drogen with the maximum at 4010 cm-l decreases. This is easy

to explain,

if

this band is assigned to molecular hydrogen interacting with trigonaly coordinated aluminum atoms, the number of which decreases upon dealumination. Then the band at 4035 cm-l should be attributed to trigonal silicon atoms.

124 Such an interpretation is also confirmed by a similar, but much more distinct picture, which is observed after a high temperature pretreatment of HY zeolite under more severe "deep bed" conditions, when dealumination really

is

the main process. For instance. according to (ref.12) the degree of framework dealumination of Y zeolite preheated at 770 K in a "deep bed" is as high as 85 -1 %. As seen from the Fig 3(d) in this case the band at 4060 cm ,which was previously attributed t o hydrogen interacting with extra lattice aluminum, really predominates and the band at 4010 cm-l from trigonal lattice aluminum atoms is practically absent. The following simple suggestion may help in understanding the difference in the dehydroxylation of hydrogen forms of

faujasites and of high silica

zeolites: The removal of aluminum from the crystal lattaicedoes not only disturb the balance of the electric charges in their structure, but also changes the valence angles and the hybridization of the orbitals in low coordinated Si and A1 atoms being formed. Each of these sites possesses therefore a certain excess energy and is a source of some strains in the zeolite structure. When stoichiometric dehydroxylation of faujasites occurs, the number of defects is very high and is comparable with the total A1 and Si

such

content. For ratio of 2

instance, when full dehydroxylation of a zeolite with a Si to A1

takes place, half of the A1 atoms and a quarter of Si atoms should be

conver-

ted from the tetrahedral coordination into trigonal coordination. Then the total amount of low coordinated aluminum and silicon atoms will be about

30 %.

Such a perturbation of the zeolite crystal lattice is apparently too strong. Therefore the stabilization of the crystal structure takes place resulting in its dealumination and partial decomposition. Unlike this, the percentage of lattice Lewis acidic sites which are formed when high silica H-zeolites are dehydroxylated, does not exceed

several per

cent. In other words, Lewis sites appear in their crystal lattice as a relatively small number of structural defects. Therefore high silica zeolites remain stable without dealumination or decomposition up to much higher temperatures.

ON THE PROBABLE ROLE OF LEWIS ACID SITES IN CRACKING OF PARAFFINS At the moment there is no generally accepted opinion about

the role of

extra lattice aluminum in catalytic transformations of hydrocarbons on zeolites. On the other hand there is also no doubt, that their mild steaming or dealumination results in considerable increase of catalytic activity. Probably the best evidence of this effect was presented by Lago, Haag et. al. (ref. 13). They showed, that the mild steaming of HZSM-5 at 773 K resulted

125 in

a considerable, about one order of magnitude, increase of the activity in

cracking of n-hexane. The more severe treatment decreased the activity, which after prolonged steaming was even lower than that of the starting material. The authors explained this effect by the appearance after mild tion of some new superacidic Broensted active sites. However

dealumina-

they failed

in

their direct experimental observation. Therefore this explanation is only of a speculative character. The effect of the mild steaming was reproduced by us in ref.14, where we also investigated the Broensted and Lewis acidity of H E M - 5 zeolite at different stages of its dealumination. It was actually found, that

the steaming

created several new types of Broensted acid sites, connected with extra lattice aluminum ions. No one of them exhibited however any strong acid properties as evidenced by OH stretching shifts created by adsorption of benzene. Therefore the enhancement of catalytic activity after mild steaming is hardly connected with an appearance of any new superacidic Broensted active sites. On the other hand, the low temperature hydrogen adsorption indicated that the mild dealumination created the extra lattice Lewis sites. Their number was increasing with the time of water vapor treatment. This result allowed to explain the enhancement of the catalytic activity in the following more

traditi-

onal and simple way: According to ref.15 the cracking of paraffins proceeds via chain mechanism, where both Lewis and Broensted acid active sites are involved: R H + L -

+

:C=C:

+ Hsurf + Rads

+

H2

+ +

,‘c=c:

--c

(41

Rads

+ 1

:C=C:

+

R

‘ads

+

The Lewis sites play here the role of the dehydrogenating active centers, resulting in formation of olefins. The latter are then transformed by ted acid sites into adsorbed carbenium ions, which are

involved

Broensin chain

propagation via /3 scission and hydrogen transfer reactions. Since the steaming creates extra lattice Lewis active sites, this favors the dehydrogenation of paraffins and the generation of adsorbed carbenium ions at the initial stages of the treatment. At the same time the dealumination also

destroys the Broensted acid sites. These two factors do influence the cata-

lytic activity in opposite directions. Therefore after mild steaming the reac-

126 tion rate is increased. It reaches then some maximal value after moderate time of steaming and goes down for the higher extent of

the dealumination, where

the steam treatment results in a considerable decrease of Broensted acidity. This explains the extreme character of the activity dependence upon the time of high temperature water vapor treatment. To check this explanation the special experiments were carried out in which the cracking of n-hexane doped by small amounts of

1-hexene was

studied on

steam treated and on untreated HZSM-5 samples. As one should expect, the catalytic activity of the nonsteamed zeolite and of that one

treated

in optimal

hydrothermal conditions changed after 1-hexane addition in such a way

that

they reached almost the same values. In other words the activity of the steamed zeolite became two times lower, whereas for the untreated sample 2-3 times higher.

Dehydrogenating activity of Lewis sites was also confirmed by

the

increased H2 yield (by 1.5-2 times) in the cracking of n-hexane on mildly dealuminated zeolites as compared with initial H-form. Thus, the role of Lewis sites in catalytic cracking was independently proved by purely chemical exper iments . Recently some other examples of modification effects in developing the strong acidity of hydrogen forms of zeolites were reported (refs. 16-17). For instance. Lunsford et.al. (ref.16) found, that dealuminated faujasites and HZSM-20 zeolites prepared such, that they had a minimum amount of extra framework aluminum were relatively inactive in cracking of hexane, which was used as a test of strong acidity. Upon exchanging La+3 ions into the zeolites, the cracking activity first increased with respect to lanthanum loading and then went through a maximum. The authors explained these results by strong Broensted acidity. It was

suggested, that

the model of

lanthanum ions formed

[La(OH)2Lal+4 or La(OH)+2 species in 6 cages, which were responsible for

the

withdrawal of electrons from the framework hydroxyl groups, thus making protons more acidic. In our belief this explanation sounds rather illogical, since hydrolyzed lanthanum species definitely are of the basic character. Therefore they hardly would increase the acidity of structural hydroxyl groups, but rather neutralize them. It looks, that the more probable explanation is that one similar to the already discussed above for extralattice aluminum

ions.

According to it the role of dehydrogenating active sites is played by the lanthanum-oxygen acid-base pairs, which are the source of olefins, resulting

in

the chain carbenium ion mechanism of cracking. THE ACID-BASE PAIRS IN HIGH SILICA ZEOLITES MODIFIED BY Zn AND Ga IONS. Another example, where Lewis acid-base pairs play the role of dehydrogena-

127 ting active sites, represent the high silica zeolites modified by oxides. They are known as active catalysts for aromatization of

Zn and Ga light paraf-

fins (ref.18) and therefore attract the growing attention of a number of

in-

vestigators. There are rather strong indications that these catalysts are of bifunctional nature combining the dehydrogenation of light paraffins on modifying oxides with the consequent aromatization of olefins on super acid Broensted sites of high silica zeolite. Therefore in our work (ref.19) the dehydrogenating

cen-

ters of these catalysts were studied in frames of the similar approach of

the

low temperature molecular hydrogen adsorption. In addition the distribution of modifying oxides was also investigated by means of XPS and by ESR of adsorbed bulky stable free radicals, which could not penetrate inside the zeolites micropores. The most interesting results were obtained for the samples modified by Zn+2 ions. It was found, that both the ion exchange and the impregnation with zinc nitrate decreased the intensity of the band of bridged acidic hydroxyl groups with the stretching frequency of 3610 cm-l. In addition a shoulder from basic ZnOH'

groups at 3660 cm-' was observed.

On the other hand, the modification by Zn also resulted

in appearance of

several different Lewis sites. This is evident from the Fig.4, where

the

IR

spectra of molecular hydrogen adsorbed at 77 K on the sample modified

by

Zn

are represented.

z,'L +

HI

Fig.4 IR diffuse reflectance spectra of molecular hydrogen adsorbed at 77 K on HZSM-5 zeolite modified by 3 weight % of ZnO and preheated in vacuo at 770 K. (a)-hydrogen adsorption on nonmodified sample.(bl-adsorption at 77 K without any additional treatment.(c)- adsorption at 77 K after keeping the sample in hydrogen at 300 K for 10 minutes.

128 The important point is, that the largest shift of adsorbed hydrogen considerably exceeded that, which was earlier observed for hydrogen adsorbed on the surface of the bulk ZnO (ref.20). This means, that the adsorption sites on the surface of zinc oxide and in high silica zeolites modified by Zn+2 ions are of the different nature. This difference could be explained by the different nature of oxygen, which surrounds Zn+2ions in both of these cases.

In the modified

zeolites this

could be the oxygen of the zeolite framework, which certainly is different from that of ZnO. Such interpretation is confirmed by the following experimental results:

It was found, that the sites, which most strongly perturb adsorbed hydrogen at low temperature do dissociatively adsorb it at room temperature. This is evident from the spectrum (c) of the Fig.4, which shows, that after keeping of modified ZnZSM-5 sample at room temperature in hydrogen and cooling it back to

77 K the low frequency band of adsorbed molecular hydrogen was disappearing. Instead the growth of the band from the bridged acidic hydroxyl

groups with

the stretching frequency of 3610 cm-l was observed. This could be explained by the heterolytic dissociative adsorption of hydrogen, which blocks the Lewis acid-base pairs and makes them unaccessible for the adsorption of molecular hydrogen:

-Zn--0-

t

H2

-

+ H

H

-Zn--0-

(5)

Since the stretching frequency of the resulting OH groups is the same as in the hydrogen form of high silica zeolite, the corresponding oxygen atoms of the acid-base pairs are those of the zeolite framework. The acid-base pairs in zeolites modified by Zn+2 do also perturb light paraffins. This is evident from Fig.5, where the IR diffuse reflectance spectra of methane and ethane adsorbed at room temperature on ZnZSM-5 as well as on HZSM-5 zeolite dealuminated by mild hydrothermal treatment are presented.

addition to the lines from the physically adsorbed molecules marked by

In

aste-

riks, the broad bands at 2750-2860 cm-' strongly shifted towards low frequencies are observed. They resemble the corresponding IR bands from light paraffins specifically adsorbed on Lewis acid-base pairs of aluminum oxide (ref.1). The shifts observed for zinc-containing acid-base pairs are somewhat larger as compared with extralattice aluminum ions in the steamed zeolite. After evacuation these bands are removed more difficult than those from the physically adsorbed molecules. Like i n the case of hydrogen at high temperatu-

129 re paraffins are able to be adsorbed dissociatively. This is evident from disif it is adsorbed at 77 K

on

the samples preheated in hydrocarbons at elevated temperature. This effect

is

appearance of the bands of molecular hydrogen

completely reversible, since after evacuation at high temperature the low temperature adsorption of hydrogen again resulted in appearance of the corresponding IR bands. successful. For

the

sample prepared by impregnation we did not observe by IR spectroscopy the

in-

The spectral study of GaZSM-5 zeolites was Just not fluence of Ga ions neither on the intensity of

so

the bridged acidic hydroxyl

groups nor on the formation of any new Lewis acid sites. This result was

ra-

ther surprising, since the samples modified by gallium were also active in aromatization of light paraffins.

Fig.5 IR diffuse reflectance spectra of methane (a,b) and ethane (c) adsorbed at 300K and P=10 Torr on ZnO/HZSM-5 zeolite (a,c) and on the mildly dealuminated HZSM-5 zeolite (b). The explanation of this discrepancy was found, when we studied the chemical composition of modified samples by XPS .It was found, that the external surface of the zeolite micrograins was markedly enriched in Ga, whereas Zn

ions

were distributed homogeneously inside the zeolite micropores. Therefore the lack of IR observation of Lewis acid sites for gallium doped samples should be explained by their low concentration inside the channels.

130 We observed these low coordinated gallium ions on the outer surface of zeolite micrograins by more sensitive ESR technique. As a molecular probe the adsorption of 2.2.6.6 - tetramethylpiperidine 1-oxyle stable free radical was used, which due to its large dimensions could not penetrate inside the zeolite micropores. The ESR spectra of

this free

radical

exhibited

splitting from low coordinated Ga ions, which proved their

the hyperfine

existence on

the

external surface of micrograins of modified zeolite (ref. 1 3 ) . CONCLUSIONS The increase of activity in transformations of paraffins after ming or modification of zeolites by various oxides is usually

mild

stea-

connected with

formation of some new super acidic Broensted active sites (ref.13,16,21). The present paper supports an alternative explanation,according to which the main effect of modification consists in creation of Lewis acid

sites.

In

the steam treated samples their role is played by extra lattice aluminum. is shown, that such active centers could dehydrogenate paraffins and

way assist the easier formation of adsorbed carbenium ions. This

in

It this

results

in

higher reaction rate of cracking. In the case of catalytic transformations of light paraffins

to aromatics

the role of dehydrogenating Lewis sites is played by modifying Zn and Ga ions. They differ in their location, since the former are homogeneously inside the zeolite micro grains, whereas the latter are mainly

distributed localized on

their outer surface.

It was also shown, that the Lewis active sites should be considered as acid-base pairs, where both low coordinated cations and the neighboring basic oxygen of the zeolite crystal lattice are equally important. The best

molecu-

lar probes for their study are molecular hydrogen and paraffins, i e. the substrates, which are involved in catalytic reactions with participation of

such

active sites. REFERENCES V.B.Kazansky, V.Yu. Borovkov, A.V.Zaitsev, IR spectroscopic study of hydrogen and light paraffins interaction with Lewis acidic sites on 7 and -r)-aluminas,Proceedings of 9 th International Congress on Catalysis, Calgary 1988, Ed. by M.J.Phillips and M.Teman, Pub. by Chem. Inst. of Canada 3 (19881. pp.1426-33. J.A.Rabo, P.E.Pickert, D.N.Starnires, J.E.Boyle, Molecular sieve catalysis in hydrocarbon reactions, Proc.2nd Int. Congr. Catalysis, Paris 1960. V.B.Kazansky, V.Yu.Borovkov, L.M.Kustov, IR diffuse reflectance study of oxide catalysts. Use of the molecular hydrogen adsorption as a test for surface active sites, Proc of 8 th 1nt.Congr. on Catalysis, Berlin 1984, Dechema Verlag Chemie, 3 (19841,pp. 3-13.

131

4 L.M. Kustov, V.B. Kazansky, IR - spectroscopic study of an interaction of cations in zeolites with simple probe-molecules. I. Adsorption of molecular hydrogen on alkaline forms of zeolites as a test for localization sites, J . Chem. SOC., Faraday Transactions (1991) (in press). 5 W.J. Mortier, Compilation of extraframework sites in zeolites, Butterworth, Guildford, (1982). 6 R.Y. Yanagida, A.A. Amoro, K. Seff, A redetermination of the crystal structure of dehydrated zeolite 4A, J. Phys. Chem., 77(3) (1973), p p . 805-9. 7 C.L. Angell, P.C. Shaffer, Infrared spectroscopic investigation of zeolites and adsorbed molecules. 11. Adsorbed carbon monoxide, J. Phys. Chem., 70(5) (1966), pp. 1413-17. 8 I . N . Senchenya, V.B. Kazansky, Nonempirical quantum chemical study of molecular hydrogen adsorption and heterolytic dissociation on aluminum oxide, Kinetika i Kataliz, 29(5) (1988), pp. 1331-37. 9 L.B. Uytterhoeven, L.G. Crystner, W.K. Hall, Studies of the hydrogen held by solids, J . Phys. Chem., 69(6) (1965), p p . 2117-2126. 10 J.M. Thomas, J . Klinowski, The study of alumina silicates and related catalysts by high resolution solid state NMR spectroscopy, Adv. Catal. 33 (1985), Ed. D.D. Eley, H. Pines, P.W. Weisz, p p . 200-361. 11 V.B. Kazansky, On the nature of Lewis acidic sites in high silica zeolites and the mechanism of their dehydroxylation, Catalysis Today, 3 (1988), pp. 367-72. 12 D. Freude, T. Froelich, G. Pfeifer, G . Scheler, NMR studies of aluminium in zeolites, Zeolites, 3 (1983), pp. 171-177. 13 M. Lago, W.O. Haag, R.G. Mikovsky, D.H. Olson, S.D. Hellring, K.D. Schmitt and J.T. Kerr, The nature of the catalytic sites in HZSM-5 - activity enhancement, Proc. 7th International Zeolite Conference, Tokyo, Kodansha, 1986, p p . 677-84. 14 V.L. Zholobenko, E. Loeffler, L.M. Kustov, U. Lohse, Ch. Peuker, V . B . Kazansky, G. dhlmann, Zeolites, (1990), in press. 15 P.A. Jacobs, Carboniogenic activity of zeolites, Elsevier, Amsterdam, 1977. 16 R. Carvajal, Po-jen Chu, J.H. Lunsford, The role of polyvalent cations in developing strong acidity: a study of lanthanum-exchangedzeolites, J . Catal., in press. 17 V.L. Zholobenko, L.M. Kustov, V.Yu. Borovkov, V.B. Kazansky, The role of Lewis acidic sites in initial stages of the n-hexane cracking catalyzed by high silica containing zeolites, Proc. 6th Int. Symp. on Heterog. Catal., 1987, Sofia, V o l . 2 , p p . 240-45. 18 T. Mole, J.R. Anderson, G . Greer, The reaction of propane over ZSM-5-H and ZSM-5-Znzeolite catalysts, Appl. Catal., 17(1) (1985), p p . 141-154. 19 V.B. Kazansky, L.M. Kustov, A . Y u . Khodakov, On the nature of active sites for dehydrogenation of saturated hydrocarbons in HZSM-5 zeolites modified by zinc and gallium oxides, Proc. 8th Int. Zeolite Conf., Amsterdam, 1989, Zeolites: facts, figures, future, Ed. P.A. Jacobs, R.A. Van Santen, Elsevier, 1989, pp. 1173-83. 20 R.J. Kokes, Hydrogenation and related reactions over metal oxides, Proc. 5th Int. Congr. Catal., Miami Beach, 1972, Catalysis, Ed. J.W. Hightower, North Holland Pub. Co., 1973. 21 R.A. Beyerline, G.B. McVicker, L.M. Jacullo, I.J. Zilmiak, Influence of framework and nonframework aluminum on the acidity of high-silica protonexchanged FAU-framework zeolite, J. Phys. Chem., 92(7) (1988), p p . 1667-70.



133

COMPAKA'I'I V E MEASU KKMEN'FS ON ACIDITY OF ZEO1,ITES

Hellmut G. Karge Fritz-Haber-Institut der Max-Planck-Gesellschaft Berlin, 1000 Berlin 33, FRG

A €3STKACT

Methods of identification of various acidic sites in zeolites (Brensted sites, different types of Lewis sites, cations) are briefly reviewed. Similarly, techniques for determination of the density of acid sites are discussed and illustrated with selected examples. In particular, a good agreement between the results obtained by IR (using pyridine as a probe molecule) and ESR (employing NO) in determining Lewis site densities of, for instance, progressively dehydroxylated H-Y is demonstrated. Emphasis is laid on the discussion of techniques, such as titration, in aprotic solvents, IR with and without probe molecules, 'H MAS NMR, temperature-programmed desorption and microcalorimetric measurements of heats of adsorption of basic probe molecules, which are suitable for characterising the strength of acidic sites. Quantitative evaluation of TPD spectra using a kinetics model as well a s the combination of different methods is described in more detail, and results obtained by various techniques on selected examples of acidic zeolites are compared.

Most of the important reactions carried out over zeolite catalysts are acid-catalysed even though there is a n ever increasing interest in zeolite catalysis on basic sites [ l - 2I. The term "acidity of zeolites", however, is by no means unequivocal. "Acidity" could designate (i) the nature of acidic sites (e.g. Brcdnsted sites 131, various forms of Lewis centres such a s "true" Lewis sites [4-5], cations), (ii) the number or density of the respective sites or, finally (iii) their strength. Although there exists a n overwhelming evidence that a t least for the large majority of acid-catalysed reactions the acidic Brplnsted centres of zeolites (acidic hydroxyls) are the relevant active sites [6-10!, there is still a debate on the role of the Lewis centres [ll-191 in zeolite catalysis. In any event, there i s no severe difficulty to discriminate between the various types of acidic sites. A survey of the most relevant methods of distinguishing the various types of acidic sites is presented in the first section. In fact, this can be rather brief since a number of excellent earlier and also more recent review articles are available which cover this subject [8, 20-231. Similarly, the determination of the relative or absolute density or number of acidic sites will not be dealt with in great detail. Most of the techniques suitable for identifying the nature of the acidic sites can be developed in such a way that they provide quantitative information. Thus, in the second section some of the important methods for quantitative evaluation of acidic sites in zeolites will be presented and for a few instances the results ofdifferent techniques will be compared.

134

A more difficult problem concerning the acidity of zeolites is the evaluation of the acidity strength. Indeed, more recently some theoretical concepts have been developed to rationalise the acidity strength of Bransted acid sites in zeolites [24-321. They relate acidity strength to long range collective phenomena and/or more localised effects of structure and chemical composition. Hence, theoretical ideas such a s "activity" or "efficiency" (regarding the zeolite as a crystalline liquid, see Refs. [241, [27]), Sanderson electronegativity (see Refs. [25],[261, [281, [301), topological densities (see Ref. [31]), quantum mechanical effects of bond lengths and angles (see Ref. [291) and deprotonation energies (see Refs. [29], [32]) are involved. Detailed discussions of these concepts were presented by Dwyer [221 and Rabo and Gajda [231. There is, however, an urgent demand to have a t hand reliable techniques which enable us to characterise and measure the acidity strength of zeolite materials, not only to verify or falsify theoretical concepts, even though this is certainly a n important point. Progress in experimental techniques will extend the data base for further improvement of theoretical approaches to the phenomenon of acidity strength. A practical need is felt to determine the acidity strength experimentally in a reliable manner, since in many instances i t has become quite obvious that not only the number but also the strength of acidic sites is particularly decisive for the behaviour of zeolitic materials in sorption and catalysis (see, e.g. Refs. [331, [341). Quite a number of methods have been suggested, developed and employed to characterise the strength of acidic sites in zeolites. In the third part of this contribution a n attempt is made to evaluate some of the more important ones and to illustrate them by pertinent examples taken from the literature or own measurements. Special emphasis is laid on the comparison of results obtained for one and the same acidic zeolite sample by the use of different techniques for the determination of density and strength of acidic sites.

1. THE NATURE OF ACIDIC SITES IN ZEOLITES AND TECHNIQUES FOR THEIR DISCRIMINATION Table 1 presents a list of techniques proposed for discrimination between the various acidic sites encountered in zeolites. Pertinent references are also given; however, here and in the following text they are by no means exhaustive, due to the extreme broadness of the field. Possible acidic sites are the acidic OH groups, in particular the so-called bridging H ones in the = Si - 0 - A1 = configurations, i.e. potential Bransted acid centres (proton donors). Furthermore, coordinatively unsaturated sites occur (electron pair acceptors), e.g. "true" Lewis sites [4-5, 351 such a s A10+ or charged A1,0,"+ clusters or cations, Me"+, or acceptor sites on tiny oxide particles inside or outside the pore structure.

135

TABLE 1

NATURE OF ACIDIC SITES IN ZEOLITES Methods of Identification

Examples

probes

P O . 131

AI-OH; P-OH

'H MAS NMR, 15N NMR

[39-421

?AI (framework)

IR

AIO +,[AI,0y](3X*21

27AlMAS, 15N NMR

Al-oxides (extra-framework)

ESR

Bransted Bridging OH;

0

Lewis

+

Ref.

IR,IR

Other oxides Cations, Men

+

I

+

probes

+

probes

[201 [39.411 [65-671

IR + probes

Regarding the methods to distinguish between the various types, i t has been claimed several times that Hammet indicators are non-specific whereas aryl indicators should indicate only Bransted but not Lewis sites [36].This, however, was disproven [37-38,76-771. It was shown by Karge [37]via U V M S transmission spectroscopy that, for instance, triphenyl methanol, @,C(OH), reacts with a surface being free from Bransted sites as well, giving the typical spectrum of @,C+ carbenium ions. The most powerful and widely employed techniques for discrimination of acidic sites are the IR and NMR spectroscopy used with or without probe molecules [a,20,39431.Acidic OH groups may be detected by the position of the corresponding OH stretching vibration in the range of ca. 3520 to 3650 cm-l [8,9, 20, 441 or by their typical chemical shift in 'H MAS NMR [41,421. Adsorption of probe molecules on acidic OH'S gives rise to characteristic features in the spectrum which also reveal the presence of Bransted sites. Thus, after interaction with pyridine, typical bands stemming from ring deformation modes occur a t 1632 cm-' ( w 8a) or ca. 1540 cm-l ( v 19b). Reaction with ammonia results in the appearance of the v,(NH4+) band around 1445 cm-l. Similarly, adsorption of 15N-pyridineon acid Bransted sites causes a chemical shift of S = 171-174 ppm vs. solid NII,NO, [39-401. 27Al MAS NMR allows for the determination of the number of A1 atoms tetrahedrally coordinated in the framework, Al'. Since tetrahedrally coordinated A1 corresponds (in zeolites not modified via dealumination) to the presence of Bransted acid sites, this provides another means to indicate those centres [431. True Lewis sites (i.e. Al-containing species on extra-framework sites, AINF, [4-5, 35, 451)as well as cations can be identified via IR spectroscopy with the help of suitable probe molecules such as NH,, pyridine, acetonitrile or CO. Ammonia molecules coordinatively bound to true Lewis sites and I or cations give rise to a broad band around

136

1620 cm-l. The u(19b) band of pyridine attached to Lewis sites is usually more intense

and sharp bands occur around 1450 cm-I for true Lewis sites and in the range from 1440

to 1551 cm-l for cations depending on their Coulomb field [20, 46-47]. Similarly, the band position of adsorbed acetonitrile and CO is affected by the nature (e/r) of the respective cation [ 37,48-491. In the case of IR spectroscopy usually only one type of Al-containing extra-framework species is indicated by the probe, i.e. with pyridine just one band appears a t 1452 cm-l. Only in a few instances a doublet (1456,1451 cm-l) was observed [45,50]. In contrast, 27A1MAS NMR frequently reveals the presence of different types of extra-framework Al. In fact, the interpretation seems to be difficult; however, besides octahedrally coordinated extra-framework A1 also penta- and even tetrahedrally coordinated species are being discussed [41,43,51-531.

2. DENSITY (NUMBER) OF ACID SITES IN ZEOLITES Tables 2 and 3 present lists of methods which were applied to determine the den-

sity and / or number of Bransted and Lewis sites in zeolites, respectively.

'ABLE 2

DENSITY (NUMBER) OF BRQNSTED ACIDIC SITES Method

Remarks

Ref. -

Titration

Restricted applicability

[69,74-771

IR Spectroscopy

Absorbance of bands of acidic OHs andlor probes, e.g., PyH , NH4 , H2 (extinction coefficients required for absolute numbers)

[20,13-14.91

Area of respective signal

[41-42)

2 7 ~MAS 1 NMR

Area of respective signal

I431

29SiMASNMR

Combined with chemical analysis

[911

Ion Exchange

Exchange capacity

[601

Test Reactions


(34.62-641

Adsorption and Desorption of Probe Molecules

Restricted applicability (no other adsorption sites involved; or in combination with, e.g., IR)

[83-84.891 [94-1041

+

'HMASNMR

+

Those methods which are reliable in discriminating the various types of sites (e.g. IR spectroscopy, '13 NMR, 29Si and 27Al NMR spectroscopy) can be in most cases employed for quantitative measurements as well.

137

In this context, i t should be mentioned that the NMR techniques provide the absolute numbers of sites, n (sites), under investigation in a very straight-forward manner, uiz. by the area of the respective signals. In contrast, the bands of IR absorbance, which are typical of certain acidic sites, must be evaluated with the help of the appropriate extinction coefficients according to

n (sites)=

1;'vdv I

where E, d, v1 and w2 are the extinction coefficient, thickness of the sample, beginning and end of the particular band, respectively. The extinction coefficient must be determined in an independent experiment (see, e.g. Refs. [54-561 ) and may depend on the coverage of sites. However, in many cases i t is sufficient to use relative densities to obtain the relevant information about the relationship between numbers of sites and, e.g. catalytic or adsorption properties [9-10,201. Figure 1 illustrates the application of IR spectroscopy and 'H NMR spectroscopy on the determination of the number of Bronsted sites, respectively [3,57-581.

3670 3580 cm-' W

bridgin acidic 08s

v] = 49.2 0Hh.c.

= 48.8 0Hh.c.

U

z

a c

-

t-

5 wl

z

a

'I

z

7 4,s 2

I-

! E I

I(1)

IR-SPECTROSCOPY

J.E. Uytterhoeven. 1. G. Chrirtner and W. K. Hall 1. Phyr.Chem.69(1965)2117 (2) J.E. Uytterhoeven. P. Jacobs, K. Makay and R. Schoonheydt 1. Phyr. Chem. 72 (1968) 1768

I ~ H M A S N M R SPECTROSCOPY] H.Pfeifer, D. Freude and M. Hunger Zeolites 5 (1985) 274

Fig. 1 Determination of bridging acid OH groups in HY (90) by IR and 'H hIAS NMR. Most of the other techniques indicated in Tables 2 and 3 suffer from certain restrictions. With respect to the titration method, this will be elucidated in the third section. Deter-

138

mination of the ion exchange capacity, e.g. with NH4+, gives usually a good estimate of the upper limit for the total number of acidic Brernsted sites, however, often even very weak sites are involved, without any relevance to catalysis. Adsorption and desorption of probe molecules requires, inter alia, t h a t only one type of sites exists in the zeolite, otherwise the assignment of the amounts of adsorbed or desorbed probe molecules might be erroneous. In such cases, where more than one type of site is involved, the combination with IR and/or NMR measurements is advisable [59-611. Finally, the evaluation of sites via the rate of suitable test reactions is well established for a series of homologous zeolite catalysts [34,62-641. Comparison of quite different types of zeolites could be misleading. This field, i.e. application of test reactions for determination of density of active sites is not yet satisfactorily explored. As already pointed out (see also Table 3), it is possible to identify and quantitatively determine acidic Lewis sites by IR spectrosopy with the aid of suitable probe molecules such as pyridine, ammonia, carbon monoxide and hydrogen. The direct measurement of Al-containing extra-framework species (true Lewis sites) can be achieved via 27Al MAS NMR,provided the Al-content is not too low [43,53]. TABLE 3

DENSITY (NUMBER) OF LEWIS ACIDIC SITES

Ref.

Method

Remarks

0

IR Spectroscopy

Absorbance of bands of probes, e.g., Py L, NH3 -,L, CO L, H2 -,L (extinction coefficients required for absolute numbers)

[20,13-14,91

0

27AIMAS NMR Spectroscopy

Requirements:A1 content not too low, all Al observable (area of respective signal)

1431

0

29Si MAS NMR

Combined with chemical analysis

0

ESR Spectroscopy NO adsorption

[91l [65-681

-.

-

However, ESR spectroscopy using NO as a probe, provides also a valuable tool not only for identification but also for quantitative evaluation of Lewis sites in zeolites. This was first shown by Lunsford et al. [65-671, see also Ref. [591. Figure 2 shows a n ESR spectrum obtained after adsorption of NO on a n H-ZSM-5 sample [68]. It is almost identical with the ESR spectrum of NO adsorbed on alumina [66-671. Double integration provides the spin density as a measure of the density of Lewis sites. In a series of experiments, these densities were determined as a function of pretreatment temperature. As expected, the density of Lewis sites increases with pretreatment temperature as a result of increasing dehydroxylation 1681. This is indicated in Figure 3 by the corresponding increase of the spin density of NO adsorbed. In a parallel

139

set of experiments the Lewis site density was determined through IR measurements of the intensity of the 1452 cm-’ band after pyridine adsorption. Figure 3 demonstrates that the results of both techniques are in very good agreement.

r

I : : )

p(N0) = 50Pa

lo’* Adsorbent : H - ZSM - 5 (2)

3.

Part

Tad (Py) Tdel (Py) Tad(NO)

z

Fig. 2 ESR spectra of NO adsorbed on y-Al,O, and H-ZSM-5 (2) or CAZ 49

CAZ 49

I

1

= 10-5pa = 475 K (2h)

= 475 K (1 h) = 298K ( l h )

- 0.6

a W

n m

- 0.3

0

I

600

I 1 I I 700 800 900 1000 A C T I V A T I 0 N T E M P E R A T U R E [K]

10.0 1100

Fig. 3 Density of Lewis sites of H-ZSM-5 (2) or CAZ 49, measured through NO (ESR) and pyridine (IR) adsorption

140

Table 4 displays comparative data of measurements of site densities obtained via various methods. The relative densities (%) for Bransted sites were obtained via MAS NMR (see AIF ) and IR (see A[OH] and A[PyH+]), those for Lewis sites via MAS NMR (see AINF1, IR (see A[Py -B L] and ESR (see spin densities D[NO]). With the exception of the PyH+ data the agreement is satisfactory. The determination of Brflnsted sites via the density of pyridinium ions, (A[PyH+]),seems to be disturbed by the presence of a relatively high amount of extra-framework aluminium (AINF)which may give rise to (weak) Al-OH groups reactive towards pyridine or, another possibility, may affect the extinction coefficients of the PyH+ species. The presence of Al-OH in H-ZSM-5 (21, i.e. CAZ 49, was indicated by an IR band at 3680 cm-'; this band was missing in the case of H-ZSM-5 (1).This would explain the high value of A(PyH+) of CAZ 49 compared to CAZ 36.

TABLE 4

COMPARISON OF (RELATIVE) SITE DENSITIES MEASURED VIA IR, NMRAND ESR

1 [%I

Brcansted Sites

[%I

[%I

I

I [%I

I

Lewissites

["/.I

[%I

H-MD-1

100

100

100

100

100

_-

H-MD-2

65

63

84

95

84

__

100

100

100

100

100

H-ZSM-5(1)

I

I

I

100

3. CHARACTERISATION OF THE STRENGTH OF ACIDIC SITES IN ZEOLI'I'ES Titration of acidic sites Tables 5 and 6 list a number of techniques frequently used for determination of the strengths of Bransted and Lewis sites. The titration technique was developed to characterise the acidity strength of solid inorganic materials such as oxides and salts (see Tanabe, 1691 references therein) and is still widely used in this area of research [70,71]. It was also employed in studying the acidity of the classic silica-alumina cracking catalysts [37,72]. The particular procedure is described in detail in the literature (see, for instance Refs. [34,36,69,72-731). In zeolite chemistry, pioneering and careful work was carried out by Hirschler L361, Beaumont et al. 1741, Kladnig 1751, Kittelmann [761 and Unger e t al. [771. In fact, Beaumont et al. [74] obtained very interesting results when applying the titration technique on faujasite-type X and Y zeolites. These authors found a distinct

141

correlation between the degree of exchange of N a + against NH4+ (or H') and the efficiency of thus created acidic hydroxyls. This enabled them to define a n efficiency coefficient in analogy to the activity coefficient of electrolytes. In the case of H,Na-X (Si/Al =1.23 or Al / [A1 Si]=0.45) this coefficient was u,=0.16 whereas for Y-type zeolite, H,Na-Y (Si/Al = 2.43 or A1 / [A1 Si] = 0.29),a, = 0.6 was obtained. This was the first evidence for a (collective)effect of the Si/Al ratio on the acidity of zeolites (compare also Ref. [241).

+

+

TABLE 5

STRENGTH OF BRQNSTED ACIDIC SITES Method

Remarks

Ref.

0

Titration


[ 36,34,74-771

0

TPD

Probe molecules, e.g., NH3! pyridine (eventually combined with spectroscopy)

(83-84,89-901

0

Microcalorimetry

Adsorbates, e.g., NH3, pyridine, weaker bases (eventually combined with spectroscopy)

[94-104)

0

IR Spectroscopy

Shift of OH; with and without probe molecules, e.g. benzene, CO

[28,1081 [lo91

UV-VIS Spectroscopy

aromatics as probe molecules

[1101

0

'HMASNMR

Chemical shift of the 'H signal

[41-42.1111

0

Test Reactions

Various reactions requiring different acidic strength

[1141

TABLE 6

STRENGTH OF LEWIS ACIDIC SITES Method

Remarks

0

TPD

Probe molecules, e.g., NHJ, pyridine or weaker bases (eventually combined with spectroscopy)

0

Microcalorimetry Adsorbates, e.g., NH3, pyridine or weaker bases (eventually combined with spectroscopy) IR Spectroscopy

0

Adsorption of bands of, e.g. pyridine or CO coordinated t o cations

ESR Spectroscopy Desorption of NO

Ref. (83-84,89-901

[94-1041

[20,14,91

[a1

142

Even though the method was subject to frequent criticism [78-791, the agreement between the total number of acidic sites (H, 2 -3.0) and the same number evaluated, for instance, from the stoichiometry seems to provide some confidence in its reliability. However, i t turned out that the procedure was obviously not applicable to other zeolite structures such as mordenites [75-77,80411. The reason is not, as has been often claimed, that the bulky indicator molecules cannot penetrate into the channels or pores of structures other than faujasite [23,82]; mordenite for instance, is a large-port zeolite with twelve-membered rings as pore openings very similar to faujasite. Moreover, it i s questionable whether slight colour changes of indicator molecules inside the pore structure would be detectable by the eyes of the experimentalist. However, the main reason for the failure of the titration method in the case of many zeolites seems to result from a very strong interaction of the solvent molecules with the acidic sites. I

I

b : 1 rnin. after injection of excess butylamine (16 m mo1.g-1) c : 7 h after injection

3680 Y

U

z

U II-

-

I z VI

4 L*

+

I

,000

I

3500 W A V E N U M B E R [cm']

3001

Fig. 4 Interaction of solvent (nheptane) with H F (3650 cm-I ) OH groups of H,NaY and in-situ titration with butylamine

In the pore system of mordenite the replacement of the solvent molecule by the titrator base (e.g., butyl m i n e ) is thus severely impeded. This is demonstrated by Figures 4 and 5 which contrast the different behaviour of H,Na-Y and H,Na-MOR in a titration experiment followed in-situ by IR. Thus, one has to state that, regardless of other difficulties, the method fails when applied to acidic zeolites such as mordenite, erionite etc. This is in agreement with the above-mentioned findings [75-77,80-811.

143

Va

v-

a

afterdehydrationat 725 Kand 1O'Pa

b after admiwon at solvent. CCI,

c

after 1st mje(tion of n butylamine ( 0 25 m mol q 11, 8 h

d

-a

after dehydration at 1 2 5 K and 10 5 Pa

b after admiwon of ~ o l v e n l CCI. .

b after admission of solvent CCI.

c

c

after 2nd mjeamn of n-butylamme

atterlnd mjert~onof n-butylamlnc

1025mmol (I-) t 2 h

I

I

3800

WAV

2800

E N U M B E R [cm-'1

Fig. 5 Interaction of solvent (CCI,) with acidic OH groups of H,Na-MOR and in-situ titration with butylamine It seems to work, however, with faujasite-type zeolites. In fact, in the case of a homologous series of Y-type zeolites with systematically varied acidity strength a spectrum of the acidity strength distribution was obtained as is shown in Figure 6.

-

Y'

0

€ E

-

very strong H, c-8 2 strong -82 disproportionation of ethylbenzene > dehydration of cyclohexanol. In a more recent study, Guisnet and co-workers [1141 used the following set of test reactions: isomerisation and cracking of: ( l ) , n-hexane ( a t 673 K); (2), 2-methyl pentane ( a t 673 K); (3), 2,4 dimethyl pentane ( a t 623 K); (4), 1,2,4 trimethyl pentane (at 625 K); isomerisation and disproportionation of: (51,o-xylene (at 623 K);(61, 1,2,4-trimethyl benzene ( a t 625 K) and (7), 3,3-dimethyl 1-butene ( a t 473 K).The reaction rates were extrapolated to 625 K. Their results suggested t h a t the required strength of the acidic sites, catalysing these reactions, decreases in the above sequence from (1)to (7). I t appears that this approach could be further developed and may provide another possibility of scaling the acidity strength of sites quantitatively through the activation energies of a set of suitable model reactions.

CONCLUSIONS While well-established techniques are available for identification of acidic centres in zeolites and quantitative determination of their density, improved methods are still required for characterisation and reliable measurement of acid strength and strengths distribution. For this purpose, temperature-programmed desorption (TPD) of suitable probe molecules seems to be appropriate provided that possible experimental sources of error (limitations in mass and heat transfer, effects of geometrical arrangements, etc.) are avoided. In this case, evaluation of the TPD data on the basis of a valid kinetics model may provide quantitative results such a s number of types of sites involved, fractional population of these sites, activation energies of desorption (Ed,,), and distribution of Edes. However, in order to identify the nature of the sites from which the probe molecules desorb complementary experiments (such a s IR or 'H MAS NMR measurements) are necessary. TPD of NO, monitored by ESR, seems to offer a valuable tool for characterisation of Lewis sites. Microcalorimetric measurements of the heat of adsorption released by sorption of probe molecules onto acidic sites is a very attractive way to quantitatively determine their strengths. However, analogous to the case of TPD, complementary measurements are required to assign the calorific data to the types of active centres involved. Correlating of the rates of a suitable set of test reactions to the strength of the catalysing acidic sites is an attractive alternative. This method deserves further development since it could provide a relative simple and fast technique which does not require sophisticated equipment.

153

ACKNOWLEDGEMENT Contributions from colleagues, in particular from Dr. Vera Dondur, Prof. Dr. Richard Fiedorow, Dr. Linda Jozefowicz, Dr. Jurgen Ladebeck, Dr. Jurgen Schweckendiek a n d Dip].-Phys. Frank Witzel are gratefully acknowledged. The author is indebted to Dipl. Phys. Uwe Hartel, Mrs. Erika Popovic and Mr. Walter Wachsmann for excellent experimental assistance. REFERENCES 1

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53

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65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81

82 83 84 a5 86 87 a8

89

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90 V. Dondur and H.G. Karge, Surface Sci., 189/190 (1987)873-879. 91 H.G. Karge, M. kaniecki, M. Zidek, G. Onyestyak, A. Kiss, P. Kleinschmit and M. Siray, in P.A. Jacobs and R.A. van Santen (Editors) "Zeolites: Facts, Figures, Future", Proc. 8th Int. Zeolite Conf., Amsterdam, The Netherlands, July 10-14, 1989; Elsevier, Amsterdam, 1989; Stud. Surface Sci. Catalysis, 46 (1989) pp. 13271337. 92 H.G. Karge and J. Schweckendiek, in D. Shopov e t al. (Editors), Proc. 5th Int. Symp. on Heterogeneous Catalysis, Varna, Bulgaria, Oct. 3-6, 1983; Publishing House of the Bulgarian Academy of Sciences, Sofia, 1983, pp. 429-439. 93 J . Take, €1. Yoshioka and M. Misono, in M.J. Philips and M. Ternan (Editors), "Catalysis: Theory to Practice", Proc. 9th Int. Congress on Catalysis, Calgary, Canada, J u n e 26-July 1, 1988, The Chemical Institute of Canada, Ottawa, 1988 Vol. 1, pp. 372-379. 94 A. Auroux, P. Wierzchowski and P.C. Gravelle, Thermochimica Acta 32 (1979) 165-170. 95 A. Auroux, P.C. Gravelle and J.C. Vedrine, in L.V.C. Rees (Editor),Proc. 5th Int. Zeolite Conf., Naples, Italy, J u n e 2-6,1980; Heyden, London, 1980, pp. 433-439. 96 A. Auroux, in P.J. Grobet, W.J. Mortier, E.F. Vansant and G. Schulz-Eckloff (Editors), "Innovation in Zeolite Materials Science", Proc. Int. Symp., Nieuwoort, Belgium, Sept. 13-17, 1987, Elsevier, Amsterdam, 1988; Studies in Surf. gci. and Catal sis37 (1988) pp. 385-391. 97 A. Auroux, Y.8. J i n and J.C. Vedrine, Applied Catalysis 36 (1988)323-330. 9 8 A.L. Klyachko, T.R. Brueva, I.V. Mishin, G.I. Kapustin and A.M. Rubinstein, in P. Fejes (Editor), Proc. Symp. on Zeolites, Szeged, Hungary, Sept. 11-14, 1978; Acta Universitatis Szegediensis, Acta Physica et Chemica 24 (1978) pp. 183-188. 99 A.L. Klyachko, G.I. Kapustin, T.R. Brueva and A.M. Rubinstein, Zeolites 7 (1987) 119-122. 100 I . Bankus, J . Valyon, (3.1. Kapustin, D. Kallo, A.L. Klyachko and T.R. Hrueva, Zeolites8(1988) 189-195. 101 G.I. Kapustin, T.R. Brueva, A.L. Klyachko, S. Beran and B. Wichterlova, Applied Catalvis 42 (1988)239-246. 102 11. l'hamm, J. Phys. Chem. 91 (1987) 8-11. 103 H. Thamm. H . 4 . Jerschkewitz and H. Stach. Zeolites 8 (1988) 151-153. 104 S.S. Khvoshchev, V.E. Skazyvaev and E.A. Vasileva, in L.V.C. Rees (Editor), Proc. 5th Int. Zeolite Conf., Naples, Italy, J u n e 2-6, 1980; Heyden, London, 1980, pp. 476-482. 105 I,. Jozefowicz, H.G. Karge and F. Asmussen, to be published in J. Phys. Chem. 106 M.F.M. Post, T. Huizinga, C.A. Emeis, J.M. Nanne and W.H.J. Stork, in H.G. Kargc and J. Weitkamp (Editors) "Zeolites a s Adsorbents, Catalysts and Detergent Builders - Innovation and Application", Proc. Int. Symposium on Zeolites, Wurzburg, FRG, Sept. 4-6, 1988, Elsevier, Amsterdam, 1989; Stud. Surface Sci. Catalysis, 37 (1989) pp. 363-375. 107 P.A. Jacobs and J.B. IJytterhoeven, J. Chem. SOC.,Faraday Trans. I, 69 (1973) 359-371. 108 PA. Jacobs, J.A. Martens, J. Weitkamp and H.K. Beyer, Faraday Discuss. 72 (1981) 353-369. 109 L. Kuhelkova, S. Beran and J.A. Lercher, Zeolites9 (1989)539-543. 110 C. Naccache, C. Fang Ren and G.Coudurier, in P.A. Jacobs and R.A. van Santen (Editors) "Zeolites: Facts, Figures, Future", Proc. 8 t h Int. Zeolite Conf., Amsterdam, The Netherlands, July 10-14, 1989; Elsevier, Amsterdam, 1989; Stud. Surface Sci. Catalysis, 46 (1989) pp. 661-668. 111 H. Pfeifer, 1). Freude and M. Hunger, Zeolites 5 (19859 275-286. 112 J. Weitkamp, in L.V.C. Rees (Editor), Proc. 5th Int. Zeolite Conf., Naples, Italy, J u n e 2-6,1980; Heyden, London, 1980, pp. 858-865. 113 H.G. Karge, H. Kosters and Y. Wada, in D. Olson and A. Bisio (Editors), Proc. 6th Int. Zeolite Conf., Reno, Nevada, USA, July 10-15, 1983; Butterworths, London, 1984, pp. 308-315. 114 G. Bourdillon, C. Gueguen and M. Guisnet, Appl. Catalysis61 (1990) 123-139.

G. Ohlmann et al. (Editors),Catalysis and Adsorption by Zeolites 0 1991 Elsevier Science Publishers B.V., Amsterdam

157

ACDITY and BASICITY in ZEOLITES

D. BARTHOMEUF

Laboratoire de RCactivitC de Surface et Structure, URA 1106, C.N.R.S., UniversitC P. et M. Curie, Paris, France

ABSTRAm The paper presents correlations between the acidity and the basicity of zeolites. Basic sites are of the Broensted or Lewis type. The last ones which occur the more frequently consist of the oxygen atoms of the framework. The charge on the oxygen, which gives information on the basic strength, can be calculated and the basicity can be experimentally measured using for instance pyrrole as a probe molecule. Several parameters are identified as affecting the basicity. They are the electronegativity of the framework atoms, the bonds angles and bond lengths, the structure ionicity, the crystallographic sitting of the oxygen, the location of Al (pairs, Al gradients). The properties of basic zeolites in adsorption and catalysis, their resistance to poisons, are presented in relation with their basicity.

INTRODUCTION For many years the attention on the main chemical properties of zeolites has been focussed on their acidity which gives rise to a large number of applications in acidic catalysis. Little has been published on the zeolites basic properties and on their applications in catalysis and adsorption (1-5). It is well admitted that Broensted acids and bases are conjugated and that weak acids have strong conjugate bases and reciprocally strong acids have weak conjugated bases (6). The existence of protons in zeolites has to be associated with that of basic sites. The paper will consider the factors which characterize the number and strength of both sites. It will not consider the case of cationic framework which are not known presently, but it will focus on the family of (Si-AI) zeolites giving reactions typical of basic centers. One of the first reaction tested in that sense in the side chain alkylation of toluene with methanol (1,2) which is favoured on basic zeolites over the ring alkylation catalyzed by acids. The catalysts are X type faujasites i.e. materials with a high A l content and exchanged with Rb and Cs in the alkaline cation series. Dehydrogenation of isopropanol is also favoured on the same type of catalysts (7).

158

NATURE OF ACID AND BASIC SITES The study of protonic and non protonic acidity of zeolites has been the subject of very important research. The Lewis acidity arises from Al or other cationic species, alkaline, alkaline earth or other cations. In a similar way the basic sites may be considered of the Broensted or Lewis type. The first one consists of basic OH groups and the second of the framework oxygen atoms. The charge

on these oxygen determines their strength. Some major differences have to be noted specifically for zeolites in the characteristics of the acidity and of the basicity. Considering first protons, their theoretical number is equal to the number of AlOb tetrahedra whose charge has to be neutralized, i.e. to the A1 content. It is also accepted that their strength increases as the A1 content decreases. The protons are mobile, jumping from oxygen to oxygen and may approach the reactant to form a cxbocation. The number of Lewis acid sites (extraframework Al or compensating cation) depends on the zeolite pretreatment for the Al or on the degree of cation exchange for the other cations. For basic zeolites, most of the samples used do not contain much Broensted basicity, but mainly Lewis basicity. The number of potential basic sites is equal to that of oxygen atoms in the framework. The difference from one zeolite to the other is determined only by the charge on the oxygen, i.e. by the basic strength and by the degree of heterogeneity of the charge distribution, some oxygen being possibly more basic than others due to different bond angles and bond lengths. In addition and by contrast with the protons these framework oxygen are not mobile and the reactant has to adjust itself to the structure geometry which may induce structural selectivities in adsorption and catalysis.

STRENGTH OF BASICITY AND ACIDITY A large number of studies, experimental (8) or theoretical (9) have been devoted to the characterization of the acid sites, number, strength and location. The calculation based on the principle of the equalization of the Sanderson electronegativity of the atoms constituting a compound (10) gives an approach of the average charge on the proton (1 1) or on the oxygen (1 2,13). The intermediate electronegativity of a substance. Sint. gives the mean electronegativity of

the atoms of the compound, which is the same for all of them. It suggests to rank the zeolites according to their Sinl value. The results depend in fact only o n the chemical fomiula and not on structural parameters. It will be shown funher that some advantage may be taken from this. Acidity The known increase in protonic acid strength with decrease in A1 content is exemplified in figure

1, curve a, giving the charge on the proton calculated from Sin1 (10,ll). The acid strength can also be decreased by exchange of a part of the protons by metallic cations, the stronger sites being exchanged first. It is also well accepted that these results obtained by considering Sinl values give only trends. Each zeolite structure and topology will superimpose other specific parameters.

159

Basicity The paper is focussed mainly on Lewis basicity since the oxygen atoms exist in all the zeolites frameworks. The Broensted basicity is less often present and in addition some OH may be amphoteric which makes the study less clear (14). The Lewis base strength can be calculated or determined experimentally. i) Theoretical approach Several methods can be used. Ab initio quantum chemical calculations have been applied to small clusters (15.16). The use of the Sanderson electronegativity equalization principle (10) gives an overage oxygen charge value. Curve b in figure 1 reports as a function of the A1 content the average charge on oxygen for a protonic zeolite. The oxygen charge decreases as the zeolite contains less Al, i.e. as the charge on the proton rises. This is in line with the concept of conjugated acid-base pairs (12). One may at that point wonder how the material keeps its electrical neutrality with a constant number of theoretical basic framework oxygen. It has to be kept in mind that zeolites are oxyacids. Their formula can be written as T On (OH), by analogy with other oxyacids such as CI On(OH), (12,17). In zeolites m expresses the Al atomic fraction i n the framework and consequently the average number of protons per TO2 tetrahedron (n + m = 2). The fomiula for a theoretical fully protonic X type zeolite (AI/AI + Si = 0.45) is T01.55 (OH)o.45. For a ZSM-5 material with AI/AI + Si = 0.04 (Si/AI = 24) it is T01.96 (0H)o.w. It is known in oxyacids that the acid strength of the proton increases with n (18), i.e. with the number of oxygen free of protons. This is in agreement in zeolites with the higher acid strengths at the lower Al contents, i.e. lower m. The framework negative charge which compensates the positive charge of all the protons is shared between a larger number of oxygen free of protons at low content, i.e., at high n. The charge bom by each individual oxygen then decreases and the electrical neutrality is maintained. The calculation of the average oxygen charges of zeolites fully saturated with alkaline cations using the same Sanderson approach is reported in figure 1 (curves c to g) as a function of the Al content. The change in oxygen charge as a function of cations electronegativity is given in figure 2 for different Al contents which correspond to usual zeolites compositions (X, Y,L, mordenite, beta and ZSM-5). Both figures 1 and 2 show that the higher basicity strengths are obtained at high Al content and low cation electronegativity. The lowest oxygen charge is obtained for the H forms. Besides the chemical composition other parameters may affect the charges and their distribution in the framework. It has been shown for a long time that the charge on the oxygen is larger as the TOT or OTO angles are narrower and as the TO bonds are longer (19). Then the charge on oxygen depends on the structure and in a given structure on the oxygen crystallographic site considered (4 types in faujasite, 10 in mordenite, 26 in ZSM-5). The introduction in a framework of elements other than Si and Al will change not only the electronegativity but also the bond angles and bond lengths and very likely the charge on the oxygen. For instance the replacement of Si by Ge in an X zeolite increases the measured oxygen basicity (20) while the simultaneous rise in Sint value would suggest a lowering of basicity. One may expect that the presence of other elements like B, P or of all the elements known to exist in S A W S , MeAPSO's and EIAPSO's materials also generates very

160

-&1

62 n

E

aJ 0.4 m

0.4

J

7

m ru

X %

0 0

r 0

3

0.3

0.3

aJ

%

n

m L

0 1

r m

u

0.2

0.2

ln lD 7J

0.1 0

0.1

0.2

0.3

Al/Al + S i

Fig, 1. Calculated charge on oxygen and on proton as a function of the A1 content. (a) (b) Protonic zeolite and zeolites exchanged with (c) Li, (d) Na, (e) K, (f) Rb, (9) Cs specific basicities. As to the structure effect, using the Electronegativity Equalization Method @EM) (21) which differs from the Sanderson approach (10) in that it takes into account structural parameters, it has been shown that the ionicity of the bonds in a zeolite depends on its crystallographic structure. Usual zeolites are ranked in the order of ionicity FAU < LTL < MFI < MOR (21) which is different from their Sin, order ( FAU < LTL < MOR < MFI (figures 1 and 2)). This means that the ionic charge on oxygen and proton in mordenite will be increased compared to the ones in a less ionic structure, even of similar composition. In addition and particularly for zeolites with a low A1 content the inhomogeneity in the A1 distribution (possible paired sites, gradient of A1 concentration ...) will also make the true charge of some specific oxygen atoms different from the average calculated one. One may expect that the experimental measurement of basicity will reflect these influences of the structure and of Al location. ii) Experimental measurement of basicity Several methods have been proposed to measure the basicity of solid materials (22,23). Some of them have been applied to the field of zeolites. The use of Co;? as adsorbate is not well suited since infrared studies showed that physical and chemical adsorption occur simultaneously (24) and that various and not well defined carbonated species may be generated (25). The shift in the NH infrared wavenumber of pyrrole adsorbed on basic zeolites may be used to evaluate the basic strength (20). It showed that NaX are more basic than NaY (20) which is in line with figures 1,2. Applied to various zeolite structures exchanged with alkaline cations, it is seen (Table I) that some samples do not show any basicity like ZSM-5 for instance, even in the Cs form (12). For the faujasite structure pyrrole adsorption detected basicity for all the X samples from LiX to CsX. In theyseries LiY was not basic and NaY close to the limit detected. These results suggest to draw in

161 Rb Na Cs K Li

-

-61

3.0

c

g 0.4

- - - - - -3.5 -------

3

0 C

0 a,

+ .-C v)

0.3

1 m U

,4.5

9 4 0.2

I

0

1

1

2

8

E l ectronegativ i ty

Fig. 2. Calculated Sint and charge on oxygen as a function of cation electronegativity for usual A1 contents of zeolites (a) X, (b) Y, (c) L, (d) MOR, (e) Beta, (0 ZSM-5. TABLE I Shift of NH vibration of pyrrole adsorbed on zeolites and calculated average charge on oxygen Zeolite csx NaX KY NaY KL Na-MOR Na-Beta CS ZSM-5 Na ZSM-5

240 180 70 30-40 30 30 30 0 0

- 0.461 - 0.413

- 0.383 - 0.352

- 0.356 - 0.278 - 0.240 - 0.236

- 0.225

(a) Shift of NH from the liquid (from 12) (b) Charge on oxygen calculated from Sanderson electronegativity (10-12) figures 1 and 2 the dashed lines which define the range of oxygen charges above which zeolites are basic and below which the charge on the oxygen is not high enough to generate a significant basicity. Such a range will be confirmed later in catalytic studies. The limit is around 3.5 for Sint and -0.36 for the oxygen charge. It follows that none of the hydrogen form has strongly basic oxygen. This is in general agreement with acid catalysis results. In the alkaline cation materials not only ZSM-5 but also beta and mordenite should not show any basicity. Nevertheless pyrrole adsorption studies could evidence an interaction of pyrrole with the basic sites of these two zeolites (Table I). The oxygen charge in Table I is calculated for the actual composition of the materials

162

studied. It takes into account the fact that the cation exchange is not always 100 % for the major cation. For X, Y and L zeolites the shifts of NH wavenumber upon pyrrole adsorption are in line with the expected charge on oxygen while the NH shift obtained for mordenite and beta demonstrate the existence of basic sites. These two zeolites are then examples of a strong influence of the structure and/or of the Al distribution on the charge on oxygen which is not taken into account in the Sanderson electronegativity calculation. In mordenite, cations are in location of low symmetry and a rather large range of TOT angles exists (26). This structure induces a higher ionicity of bonds than other structures do (21). In addition Al pairs have been proposed to exist in niordenites (27). The existence of small bond angles, of a high bond ionicity and of Al pairs may explain the presence of basic sites. The structure of beta zeolite has been determined (28) but little is published for this zeolite on bond angles, bond lengths, bond ionicity and A l location. The behaviour of beta zeolite in catalysis (29) suggests that it is less strongly acidic than its chemical composition would suggest. Both this too weak acidity and simultaneously the present too high basicity show a very strong influence on the framework charges of parameters related to the structure and very likely on A1 location at this very low Al content (Al/Al+Si in the range of 0.07 to 0.09). In the case of faujasite another probe has been also used to evaluate the basicity. Benzene just fits the twelve ring window where it can interact weakly through the -CH with the basic framework oxygen (30-33). I n the alkaline cation series the infrared shift of yCH and the amount adsorbed in the 12-R window increases from the Li to the Cs form reaching a significant shift for the more basic Cs materials (31). The LiY sample adsorbs benzene only very weakly on the oxygen showing a weak zeolite basicity. These results are in agreement with the pyrrole adsorption results and with the calculated results of figures 1 and 2 showing the limit basicity range. In beta zeolite a significant interaction of benzene with the 12 R window is seen (34) which is surprising if one takes into account only its Al content (i.e. Sinl) but which is in line with the pyrrole results. iii) Correlations aciditv-basicitv The opposite change in the charge of proton and oxygen seen in figure 1 should also exist for the acidic strength of metal cation versus the oxygen basicity when no protons are present. It was shown by the interaction of pyridine with the alkaline cations acting as Lewis sites that the Lewis acidity decreases from Li to Cs in the alkaline cation series in faujasites (12,35). It was also shown that the acid strength for a same cation is higher in Y than X (12) which is parallel to the change in H+ charge with the Al content. One may conclude that generally speaking the acid strength of a metal cation should be high at low Al content, i.e. in highly siliceous zeolites or dealuminated materials, while the framework basicity should be low. This agrees with the existence of conjugated acid-base pairs. At this point the advantage of considering Sin1 for predicting trends in acid-base propenies of zeolites has to be considered. The cases of mordenite, beta and GeX clearly show that the predictions based on chemical composition alone are not confirmed by experimental results. A major conclusion is that a strong influence of other parameters, (structure, topology, bond ionicity, Al distribution) becomes very important for those materials. This opens a field of research for

163

Characterizing what in zeolites properties (acido-basicity, ion exchange, influence on supported metals ...) is relevant mainly to chemical composition or to other parameters. The characterization of these parameters will lead to a better understanding and then to prediction of the properties. Another consequence is the guide line given by these results for obtaining highly basic zeolites. Following the trend of increasing basicity at high Al content, i n the faujasite family the highest possible value is obtained in X. In mordenite and beta for which AVAI+Si ratios are 0.17 and 0.07 respectively one may expect quite higher basicity at high Al levels by a combined effect of Sin*, structure, topology and Al location. Once a given structure is ranked by comparison with faujasite taken as a reference, it would be possible, for this structure to prepare a large series of samples with predetermined Lewis acidity and basicity by choosing the right Al content and the right cation. The family of samples could then be used to direct the properties in adsorption and catalysis. USE OF BASIC ZEOLITES IN ADSORPTION In adsorption, non protonic zeolites are commonly used i n order to prevent any catalytic transformation of the adsorbates on acid sites. The Na or Ca forms of A, X, Y, mordenite ... have been used extensively in fundamental studies or in applications (36,37). Besides separations of mixtures based on differences in the kinetics of diffusion of the adsorbates in the zeolite pores a number of separations rely on subtle differences between the adsorbates when they are in the adsorbed state in the zeolite cavities and channels. Two examples will be given. The first one concerns the separation of the four C8 aromatics. They have very close boiling points and selective adsorption on zeolite is well suited in that it avoids very difficult distillation or crystallisation separations. The preference of the zeolite for a given isomer strongly depends on the solid. In the faujasite series KY prefers p-xylene (38-39). NaY m-xylene (40) and RbX and CsX ethylbenzene (41). Ethylbenzene is rejected from CaY or CaX (42). A correlation was observed between the Y zeolite acidity and the p-xylene selectivity (38). In the X and Y zeolites it was shown that the order of preference for the isomers may be related to the adsorbent intermediate electronegativity (5) i.e. to its basic character. The relative x basicity (43) of the aromatics increases from ethylbenzene to p-xylene and to m-xylene (Table 11). The order of increased K basicity parallels that of increased electronegativity of the zeolite selective for each isomer (Table 11). The Cg aromatics interact only with the cations through their 7c electrons and due to steric hindrance not with the framework oxygen as benzene does. The results of Table I1 suggest that the interactions aromatic-cation are governed by a balance between the aromatic basicity and the Lewis acid character of the cation in the zeolite, itself related to the zeolite electronegativity. For beta zeolite, Figure 3 reports selectivity values in the separation of ethylbenzene from p-xylene in a mixture of the four Cg aromatics (45). The selectivity factor a12expresses the ratio of the concentrations of compound 1 in the zeolite and i n the liquid phase to the corresponding concentrations of compound 2. Values of a12 higher than one indicate the preferred adsorption of compound 1 in the zeolite. Figure 3 shows that any form of beta zeolite tested (protonic or saturated with alkaline cations) is selective for ethylbenzene over p-xylene (and over the o - and m - forms

164

also) (45) as the basic faujasites NaX to CsX in Table 11. TABLE I1 K basicity of c8 aromatics and Sin1 values for zeolites selective for each isomer

Zeolite

Selectivity for EB, PX or MX@)in a mixture of the four C8 aromatics

Sint

Cs5oNa3d( Rb53Na33X RbyjMggNa14X KX NaX K%Na2Y NaY

3.03 3.08 3.18 3.15 3.25 3.38 3.54

-

-

-

EB

PX

Mx

(1.5)@)

(1.6)

(2.0)

Ref

41c 41, 41, 41c 41, 4 h 4ob

X X

X X

X X X

(a) EB : Ethylbenzene, PX : paraxylene, Mx : metaxylene (b) K basicity (from 43 )

Rb Cs K

H

Na

2 X

a m

--. W

0 1

0 9

4.0

4.1

'int

Fig. 3. Separation coefficient between ethylbenzene and p-xylene as a fonction of the intermediate electronegativity of beta zeolite exchanged with (o)Cs, (0) Rb, (n) K, @) Na, (A) H. The values of Sint for beta lie in a range - 4 to 4.2 - which is usually obtained in acidic zeolites (Figure I). As previously for pyrrole (Table I) or benzene adsorption, beta zeolite looks like much more basic that one would expect from the Sin1 calculation. This confirms the strong influence of the structure or/and Al location on the charges born by the atoms in this zeolite. This is also in line with the moderate acidity of this material (29). The second example of separation on basic zeolites is given by the changes in the separation

165

coefficient a between methyl-2 and methyl-1 naphtalenes (46). The authors observed a decrease in the C I ~ M N I I M N coefficient as Sin, increases upon alkaline and alkaline earth ion exchange. The curves are distinct for X and Y zeolites.

USE OF BASIC ZEOLITES IN CATALYSIS Basic catalvsis For a long time reactions involving zeolites containing metallic cations but no protons have been proposed (47). Many of them may be relevant to basic catalysis, like reactions involving H2S. For the transformation of furan to thiophene (47) it has been shown that H2S dissociation occurs to generate mainly HS- and H+ species which are adsorbed in NaX on the Na+ cation and framework oxygen respectively (48). The activity of X zeolites exchanged with the different alkaline cations in the reactions of CH3OH

CH3SH and C2HsOH

a C2HsSH is the

highest for the zeolites with lowest Sint, i.e. for the most basic (49). Reactions like isopropanol dehydration, alkylation with methanol of toluene or of other aromatics and reactions involving heteroatoms (N,O ...) have been reviewed elsewhere ( 3 3 . It has to be pointed out that for the toluene alkylation with methanol the alkylation of the ring or of the chain is obtained on acidic or basic zeolites respectively. A correlation was shown between the extent of alkylation in each case and the electrostatic potential of the cation, the lower the electrostactic potential, the higher the side chain alkylation (50). A similar relationship was latter obtained considering the zeolite intermediate electronegativity and the yield of products. Interestingly the limit of acidic or basic zeolites defined by the selectivity of this reaction is at a value of Sinl close to 3.5 which is in agreement with the limit range already reported in figures 1 and 2. Recently it was shown that the condensation of benzaldehyde with derivatives of malonic esters has an increased activity for the fmjasites giving the highest NH shift in adsorbed pyrrole i.e. having the strongest basicity from Table I. Similarly GeX zeolite is very active in condensation reactions (53) which is in line with the strong basicity of this zeolite (20). Supported metals A few works are related to the study of metal supported basic zeolites. It is well known in the field of acidic zeolites that the electronic properties of the metal are modified through an electron transfer from the zeolite acid sites to the platinum particles (54). One may expect for basic zeolites an effect in the opposite direction. The influence of such a metal-support interaction explains that the extent of N i reduction is increased for the more basic NaX compared to NaY and that the presence of Ce3+ cations , able to give Ce4+ ions and one electron favors this reduction in CeNaX (55). The catalytic properties of the supported metallic particles also depend on the acidic or basic

character of the zeolites. The hydrogenation of CO to olefins and paraffin over RuY zeolites increases for the formation of unsatured hydrocarbons as the zeolite basicity rises from Ru supported on Nay, to KY and CsNaY (56). In the case of PtL zeolites in the alkaline cation form the activity for the dehydrocyclization of n-hexane to benzene increases from Li to Cs form. This was related to an increased electron

166

transfer from the zeolite to the Pt particles (57). Such an explanation appears to be also valid for the alkaline earth forms (58). Figure 4 reports the change in benzene yield in n-hexane aromatization as a function of oxygen charge and Sin1 evaluated from (57,59). The more active catalysts (Rb and Cs

3.65 1

3.60

3.55

'int

'.

I

I

A

-0.34

oxygen c h a r g e

Fig. 4. Benzene yield in n-hexane dehydrocyclization at 733 K (ref 59) as a function of Sinl and oxygen charge of PtL zeolite exchanged with (0)H, ( 0 ) Li, (A) Na, 0) K, g)Rb, (A) Cs. forms) for the production of benzene have the lowest Sinlvalues. This reaction is monofunctionnal, the active sites being the Pt atoms (59). The results confirm that a high zeolite basicity increases the aromatization activity of the Pt sites. The change in the electronic state of Pt was also shown from a decrease in the amount of CO adsorbed on Pt and in the benzene hydrogenation activity in L materials (58). In a series of Pt faujasites of the X or Y type exchanged with H, Na or Cs cations, the wavenumber of CO adsorbed on Pt particles decreases simultaneously with the benzene hydrogenation activity from the acidic to basic zeolites in the order PtHY > PtHNaY > PtNaY > PtNaX > PtCsX (44a,60,61). Simultaneously the apparent activation energy increases from 48 to 120 KJ.mole-1 which precludes any explanation based on diffusion due to the steric hindrance of the big Cs ions. The results show that the intrinsic electronic properties of the platinum are gradually changed following the decrease in acidity and increase in basicity of the zeolite. Figure 5 gives a scheme summarizing the continuous changes in properties from the strongly acidic to the highly basic zeolites. Choosing the right Al content and the right cation is a way to adjust for a given zeolite the properties to the expected behaviour of the solid in adsorption or in catalysis. Resistance to sulfur The experimental results presented here indicate that the zeolite basicity may direct the properties of supported metal . Changes i n the metal particles properties may be induced not only by the

167

zeolite but also by the species adsorbed on the metal. For instance sulfur atoms formed by decomposition of sulfur compounds are electronegative and favor an electron transfer from the Pt particles to the sulfur (62). In the case of Pt supported on acidic zeolites, the Pt-S bond is loose since less electrons are available in the metal due to the Pt-acid interaction (62). One may expect that

4.0 I

0.30 1

4

4

1

3.5

3.0

1

I

I

-6

acidity

basic i ty

increase

increase

(oxygen)

strength

increase

decrease

-

decrease

increase

e N-hexane a r o m a t i za t ion

increase

decrease

*

increase

4

4

'int

0.40

benzene hydrogenation

resistance t o S

Fig. 5. Change in properties of zeolites with their electronegativity. on the opposite, due to the electron transfer from the basic zeolites to the Pt particles, more electrons are available in the metal to form a strong bond Pt-S. Sulfur should be a stronger poison for m a s i c zeolites than for Pdacidic zeolites. This is in fact observed, the PtKL or PtBaL zeolites active in the n-hexane dehydrocyclization are very sensitive to sulfur (63). CONCLUSION The basicity of zeolites, like acidity, varies i n a large range from weak to strong. Several parameters which direct this basicity have teen identified. They give trends and guide lines for the increase in basicity. The easiest of these parameters to act on are the A1 content and the electronegativity of the cation. For instance in the case of zeolites like mordenite or beta which show some basicity despite their low Al level, an increase in the A1 content should give a stronger basicity. In the search of new selectivities in adsorption and catalysis, the basic zeolites offer a large range of opportunities. REFERENCES 1

2

Y.N. Sidorenko, P.N. Galich, V.S. Gutyrya, V.G. Il'In, I.E. Neimark, Dokl. Akad. Nauk SSSR, 173 (1967) 132. T. Yashima, K. Sato, T. Hayasaka, N. Hara, J. Catal., 26 (1972) 303.

168

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 28

29 30 31

32 33 34 35 36 37 38 39 40 41

Y. Ono, in "Catalysis by zeolites" (B. Imelik et al. eds), Stud. Surf. Sci. Catal., Elsevier, Amsterdam, 5 (1980) 19. M.L. Unland, G.E. Barker, in "Catalysis of Organic Reactions" (W.R. Moser ed.), Chemical Industries Series 5, Dekker, New York, 1981, p. 51. D. Barthomeuf, G. Coudurier, J. Vedrine, Mat. Chem. and Phys., 18 (1988) 553. W.B. Jensen, "The Lewis Acis-base concepts. An overview", Wiley, 1980, p. 51. T. Yashima. H. Suzuki, N. Hara, J. Catal., 33 (1974) 486. P.A. Jacobs, "Carboniogenic Activity of Zeolites", Elsevier, Amsterdam, 1977 ; D. Barthomeuf, Mat. Chem. Phys., 17 (1987) 49 ; K.V. Topchieva, Huo Shi Thuong, Vestn. Moskov. Univ. Khim., 15 (1974) 239. A. Goursot, F. Fajula, C. D a d , J. Weber, J. Phys. Chem, 92 (1988) 4456 ; I.N. Senchenya, V.B. Kazansky, S . Beran, J. Phys. Chem., 90 (1986) 4857 ; S. Beran, J. Mol. Catal., 26 (1984) 31 ; A. Goursot, F. Fajula, F. Figueras, C. D a d , J. Weber, Helv. Chim. Acta, 73 (1990) 112 ; S. Beran, J. Phys. Chem., 94 (1990) 335. R.T. Sanderson, "Chemical Bands and Bond Energy" Academic Press, 1976. W.J. Mortier, J. Catal., 55 (1978) 138. D. Barthomeuf, J. Phys. Chem., 48 (1984) 42. A. de Mallmann, D. Barthomeuf, in "Innovation in Zeolite Materials Science", (P.J. Grobet et al., eds), Stud. Surf. Sci. Catal., Elsevier, Amsterdam, 37 (1988) 365. W. Przystajko, R. Fiedorov, I.G. Dalla Lana, Zeolites, 7 (1987) 477 ; A. Bielanski, J. Datka, J. Catal., 32 (1974) 183. E.G. Derouane, J.G. Fripiat, J. Phys. Chem., 91 (1987) 145. P.J. OMalley, J. Dwyer, Chem. Phys. Lett., 143 (1988) 97. D. Barthomeuf, J. Phys. Chem., 83 (1979) 249. R.P. Bell, "The proton in chemistry", Chapman and Hall, London, (1973). p. 92. G.V. Gibbs, E.P. Meagher, J.V. Smith, J.J. Pluth, ACS Symp. Ser., 40 (1977) 19. P.O. Scokart, P.G. Rouxhet, Bull. Soc. Belg., 90 (1981) 983. K.A. Van Genechten, W.J. Mortier, Zeolites, 8 (1988) 273. K. Tanabe, "Solid Acids and Bases", Kodanska, Tokyo, Academic Press, New York, (1970). K. Tanabe, M. Misono, Y. Ono, H. Hattori, New Solid Acids and Bases, Kodanska, Elsevier, Stud. Surf. Sci. Catal., 51 (1989). Y. Delaval, R. Seloudoux, E. Cohen De Lara, J. Chem. Soc. Faraday Trans. I, 82 (1986) 365. C. Mirodatos, P. Pichat, D. Barthomeuf, J. Phys. Chem., 80 (1976) 1335. J.L. Schlenker, J.J. Pluth, J.V. Smith, Mat. Res. Bull., 14 (1979) 751. P. Bodart, J.B. Nagy, G. Debras, Z. Gabelica, P.A. Jacobs, J. Phys. Chem., 90 (1986) 5183. M.M.J. Treacy, J.M. Newsam, Nature (London), 332 (1988) 249 ; J.M. Newsam, M.M.J. Treacy, W.T. Koetsier, C.B. De Gruyter, Proceed. R. SOC., London, Ser. A, 420 (1988) 375 ; J.B. Higgins, R.B. La Pierre, J.L. Schlenker, A.C. Rohrman, J.D. Wood, G.T. Kerr, W.J. Rohrbaugh, Zeolites, 8 (1988) 446 . J.A. Martens, J. Perez-Pariente, P.A. Jacobs, Proceed. Int. Symp. Zeol. Catalysis, Siofok, Acta Phys. Chem. Szegediensis, 1985, p. 487. A. de Mallmann, D. Barthomeuf, Proceed. 7th Int. Zeolite Conf., Tokyo (Y. Murakami, A. Iijima, J.W. Ward , eds), Kodanska, Elsevier, 1986 p. 609. A. de Mallmann, D. Barthomeuf, Zeolites, 8 (1988) 292. H. Lechert, K.P. Wittern, Ber. Bunsenges Phys. Chem., 82 (1978) 1054. A.N. Fitch, H. Jobic, A.J. Renouprez, J. Chem. SOC., Chem. Comm., (1985) 284 and J. Phys. Chem., 90 (1986) 1311. S. Dzwigaj, A. de Mallmann, D. Barthomeuf, J. Chem. SOC.Faraday Trans., 86 (1990) 431. J.W. Ward, J. Phys. Chem., 72 (1968) 421 1. R.M. Barrer, "Zeolites and Clay Minerals as Sorbents and Molecular Sieves", Academic Press, London, 1976. D.M. Ruthven, "Principles of Adsorption and Adsorption Processes", John Wiley and Sons, New York, 1984. Maomi Seko, Oil and Gas J., July 2 (1979) 81. L.O. Stine, D.B. Broughton, (1969), US 3.636.121. to UOP ; R. Baerden Jr, R.J. De Foe, 1972, U.S. 3.686.343, to Esso. a) R.W. Neuzil, (1982), U.S. 4.326.092, to UOP b) D. Barthomeuf, (1986), U.S. 4.593.150 to Exxon. a) R.W. Neuzil, D.H. Rosback, (1976), U.S. 3.943.182 to UOP

169

42 43 44 45 46 47 48 49

50 51 52 53 54 55

56 57 58 59

60 61 62

63

b) P.R. Geissler, (1979), U.S. 4.175.099 to Exxon c) D. Barthomeuf, (1986), U.S. 4.593.149 and 4.613.725 to Exxon A.J. De Rosset, (1975), U.S. 3.917.734 to UOP. G.A. Olah, Acc. Chem. Res., 4 (1971) 240. a) A. de Mallmann, Thesis, Paris (1989) b) A. de Mallmann, D. Barthomeuf, Industrial and Engineering Chemistry Research, (1990), in press. D. Barthomeuf, (1986), U.S. 4.584.424 to Exxon D. Barthomeuf, L.G. Daniel, (1988), U.S. 4.751.346 to Exxon. V. Solinas, R. Monaci, E. Rombi, M. Morbidelli, in Zeolites as Catalysts, Sorbents and Detergent Builders, (H.G. Karge, J. Weitkamp, eds.), Stud. Sci. Catal., Elsevier, Amsterdam, 46 (1989) 595. P.A. Venuto, P.S. Landis, Adv. Catal., 18 (1968) 259. H.C. Karge, J. Rasko, 1. Colloid, Interf. Sci., 64 (1978) 522 ; H.G. Karge, M. Ziolek, M. Laniecki, Zeolites, 7 (1987) 197. M. Ziolek, D. Szuba, R. Leksowski, in "Innovation in Zeolite Materials Science", (P.J. Grobet et al., eds), Stud. Surf. Sci. Catal., Elsevier, Amsterdam, 37 (1988) 427. H. Itoh, T. Hattori, K. Suzuki, Y. Murakami, J. Catal., 70 (1983) 27. N. Giordano, L. Pino, S. Cavallero, P. Vitarelli, B.S. Rao, Zeolites, 7 (1987) 131. A. Coma, V. Fornes, R.M. Martin, H. Garcia, J. Pnmo, Appl. Catal., 59 (1990) 237. A. Corma, R.M. Martin-Aranda, F. Sanchez, J. Catal., submitted. P. Gallezot, Catal. Rev. Sci. Eng., 20 (1979) 121. M. Briend-Faure, J. Jeanjean, M. Kermarec, D. Delafosse, J. Chem. SOC.Faraday Trans I, 74 (1978) 1538 ; S. Djemel, M.F. Guilleux, J. Jeanjean, J.F. Tempere, D. Delafosse, J. Chem. Soc., Faraday Trans I, 78 (1982) 835. I.R. Leith, J. Chem. SOC. Cheni. Commun., (1983) 93. C. Besoukhanova, J. Guidot, D. Barthomeuf, M. Breysse, J.R. Bernard, J. Chem. SOC., Faraday Trans I, 7 1981) 1595. G. Larsen, G.L. Haller, Catal. Lett., 3 (1989) 103. J.R. Bernard, Proceed. 5th hit. Zeol. Conf. Zeolites, (L.V.C. Rees ed.), Heyden, London, 1980, p. 86. A. de Mallmann, D. Barthomeuf, in "Zeolites as Catalysts, Sorbents and Detergent Builders" (H.G. Karge, J. Weitkamp, eds), Stud. Surf. Sci. Catal., Elsevier, Amsterdam, 46 (1989) 429. A. de Mallmann, D. Barthomeuf, J. Chim. Phys., 87 (1990) 535. P. Gallezot, J. Datka, J. Massardier, M. Primet, B. Imelik, Proceed. 6th Int. Cong. Catal., (G.C. Bond, P.B. Wells, F.C. Tompkins, eds), The Chemical Society, London, 2 (1977) 696. T.R. Hughes, W.C. Buss, P.W. Tamrn, R.L. Jacobson, Proceed. 7th Int. Zeol. Conf., (Y. Murakami, A. Iijima, J.W. Ward, eds), Kodanska, Elsevier, Tokyo, (1986), p. 725.


G . ohlmann c s t a!. (Editors 1, Cakdysis and Adsorption by Zeolites (01991 Elsevier Science Publishers R.V., Amsterdam

171

MATRIX vs ZEOLITE CONTRIBUTIONS TO THE ACIDITY OF FLUID CRACKING CATALYSTS

ROLAND VON BALLMOOS AND CHI-MI T. HAYWARD Engelhard Corporation, Menlo Park, CN28, Edison, NJ 08818, USA

SUMMARY

The matrix of fluid catalytic cracking (FCC) catalysts serves multiple functions: it gives the microspheres their physical strength and the attrition resistance necessary to withstand the stresses of rapid circulation in the FCC unit and protects the soft zeolitic particles from abrasion. The matrix also serves as a cracking component for gas oil molecules which are too large to enter the zeolitic pores. This second function is extremely important for achieving the high conversions and high gasoline selectivities of today's FCC catalysts. The proper choice of matrix chemistry and its balance with the zeolite component are the keys to reduced gas and coke makes at high catalyst activity while maximizing the liquid product yield. We investigated the influence of low, moderate, and strongly acidic matrices and their synergisitc contributions to the catalyts' activity. "Staged bed" MAT experiments demonstrated that the overall conversion and gasoline yield were drastically increased when gas oil molecules could interact with the matrix material first, before contacting the zeolite. Secondary cracking on the matrix of products initally generated inside the zeolite was negligible. The strength of the acid sites in the matrix influences the coke make. A matrix component with moderate acidity was found to result in the most beneficial selectivities. INTRODUCTION Modern fluid catalytic cracking (FCC) catalysts represent a highly sophisticated class of materials. In principle, they consist of approximately 15 to 35 wt-% of an active zeolite component in an amorphous or clay matrix with a minor amount of an oxide binder. Yet, on a microscopic scale the composition of the 60 to 9 0 micron size microspheres is considerably more complex: many different forms of zeolite Y, a

172

faujasite type material, with great variations in silicon-toaluminum ratios and rare earth levels are used depending on the selectivity demands of the refiners. Several recent reviews on FCC catalysts have been published dealing with either overall catalyst properties and performances [1,2,3] or investigating zeolite modifications and their effects on selectivity [4]. They describe in detail the effects of varying the silicon-to-aluminum ratio of zeolite Y and highlight differences resulting from ultrastabilization (dealurnination) by thermal, chemical, or thermo-chemical treatments. The beneficial influence of the zeolite's rare earth oxide (REO) content on stability and gasoline yields have been known since the early days of FCC cracking with zeolites. Ultrastabilized Y zeolites, however, generate higher octane gasolines at a yield debit. In order to achieve an optimal compromise, many of today's FCC zeolite components represent ultrastabilized rare earth exchanged faujasites. The equally significant variations in catalyst matrices and binder materials are usually given less attention in scientific investigations of cracking catalysts. Nevertheless, several reviews have been published recently [5,6]. This presentation focuses on the importance of the matrix contributions to FCC cracking, the matrix properties, and their influence on FCC catalyst selectivities. Modern FCC catalyst matrices can be separated into four classes: aluminas, silicas, silica-aluminas, and clays. The chemical and catalytic functions of these matrices may vary quite significantly, as demonstrated below, but they all share some common functions as FCC matrices. The matrix provides the suitable particle size and shape for fluidized bed operation, nominally 60-90 micron spheres. FCC zeolite particles are less than 2 microns in size and very soft. The matrix materials render the microspheres attrition resistant and reduce the loss of active zeolite components. Because the FCC unit is heat balanced without much external heat input (only for feed preheat to approximately 250 OC) the endothermic cracking reaction requires the microspheres to serve as a heat transfer medium to vaporize the feed and to crack the hydrocarbons. The heat is provided by burning coke off the catalyst in the regenerator. It is then transferred by the hot ( - 730 OC) microspheres to the riser section of the unit. The matrix furthermore serves as an adsorbent and scavenger for feed contaminants such as nickel, vanadium, sodium, iron, sulfur, and nitrogen. An additional function of the matrix is to provide easy access for feed molecules to the active sites of the zeolite. A major function of the matrix is the cracking of feed molecules which are too large to enter the zeolite's pores. The kinetic diameter of molecules with boiling points above approximately 450 OC (atmospheric equivalent) is larger than the

7.5 A pore of faujasite [7]. Depending on feed cut points a signifcant fraction of the molecules may fall into this category of compounds which have to be cracked on the matrix first. We have investigated this process in some detail by separating the functions of the matrix and the zeolite. Some aspects of this work have been published in a recent paper [a]. EXPERIMENTAL The experimental catalysts used in this study were commercial FCC microspheres with a clay based matrix which were combined with surface area-stabilized silicas doped with metal oxides to generate model matrix compounds with different intrinsic acidities. We used an Engelhard rare earth exchanged US-Y catalyst as the base case. The weakly acidic matrix material was prepared by using a commercially availbale silica support modified by a metal oxide. The moderatly acidic model matrix was based on the same,silica modified with a group IIA metal oxide. The strongly acidic matrix was generated by leaching aluminosilicate clay microspheres. In some experiments, aluminum oxide was deposited on the above silica to simulate a strongly acidic matrix with variable site density. The zeolitic FCC particles were steam deactivated separately to simulate typical FCC equilibrium catalysts for 4 hours in 100% steam at 788 OC. The model matrix components were either steamed at 732 OC for 8 hours in one atmosphere steam or calcined at 580 OC to provide the target acid strength. The catalysts' activity and selectivity patterns were then determined in a modified MAT unit (modified ASTM D-3907 test). Figure 1 shows a schematic process flow diagramm of the MAT test modified by Engelhard [9]. Figure 2 shows the catalyst bed arrangement for the two different types of experiments. In the "Catalyst on Top1@configuration, 3 g FCC catalyst were loaded into the reactor tube on top of 3 g appropriately sized matrix component. Because the reactor was of the down-flow type, the zeolite-containing FCC catalyst was contacted by the feed first. In the "Matrix on Top" configuration, the loading order was inverted and the matrix component was added on top of the FCC catalyst'bed. 1.2 g of a Mid Continent gas oil were fed in 48 s (catalyst-to-oil ratio of 5 ) at 490 OC. The liquid product syncrude was analyzed by GC simulated distillation (ASTM D-2887), the gas fraction was analyzed by GC, and the coke on catalyst was determined by a LECO coke analyzer. Activity is defined as weight percent of gas oil converted to products boiling below 216 OC and coke, or as a dimensionless activity based on second order kinetics, where Activity = Conversion/(lOO - Conversion).

w

ENGELHARDS MAT PROCESS FLOW SCHEME

CHROYATOORAPHK: 'ROGEN

DELIVERV ZONE

IL W l V E R V TUBE

CATALYST BED

GLASS REACTOR MEAT

ZONE FURNACE

I

t

CALIBRATION GAS INLET

GAS COLLECTION RESERVOIR

0 % COOUNT

Figure 1: Schematic flow diagram f o r MAT unit modified by Engelhard based on ASTM D3907 test.

P -1

Q

P S

0 a,

U a, a,

LL

I

a,

c,

.0 '

a, N

.-X L

c,

I'

a,

c,

0

.-a, N

176

Figure 2: Catalyst bed arrangement for "Staged Bed" MAT experiments used to demonstrate the importance of matrix cracking before zeolite cracking in FCC.

Synergistic Interaction Between Matrix and Zeolite Increases Cracking Activity

9 ri c r.

ID

.. W

MAT A c t i v i t y

2.5

-

I

2I

1.5

~~

10.5 -

0Matrix

Zeolite

Calculated

Measured

Catalyst In MAT .Test

J 9

--1

?

Activity = Conv./(lOO-Conv.) Conversion to Prod. < 216 C

-i

177

RESULTS AND DISCUSSION We are reporting three sets of experiments which demonstrate the importance and beneficial effects of an optimal balance between the zeolitic cracking component and the amorphous matrix material: A first experiment highlights the synergistic effects of matrix and zeolite cracking. A second set of data shows that the matrix component is most effective in cracking large molecules before they are further converted to gasoline inside the zeolite. A third experiment distinguishes between matrices of different overall activity, expressed as a combination of acid strength and acid site density. Zeolite

-

Matrix Synergism:

A rare earth exchanged zeolite catalyst with minimal matrix activity showed an experimental MAT conversion of 5 2 wt% (activity = 1.1). A moderate activity amorphous material, when tested by itself in the absence of a zeolitic cracking component, yielded 30 wt% MAT conversion (activity = 0 . 4 ) .

A third catalyst was prepared from a blend of the first two materials. It had the same zeolite activity as the first, combined with the identical amorphous matrix activity of the second. If the effects of the matrix and the zeolite were additive, the activity of Catalyst 3 would be calculated to be 1.1 + 0 . 4 = 1.5. In fact, when the MAT activity was determined experimentally it was 2.2, with a conversion of 69 wt%. These results are represented graphically in Figure 3 .

. The substantially greater than expected activity of Catalyst 3 indicates that significant interactions between the components occurs. The synergism can be explained by a mechanism of initial matrix cracking of large feedstock molecules to smaller fragments which are subsequently cracked into the gasoline and gas range inside the zeolite. Approximately 20 % of the MAT gas oil molecules are too large to enter the zeolite's pores. They have to be cracked on an easily accessible matrix first. In fact, the MAT data of Catalyst 2 showed 30% conversion, primarily of heavier molecules which are cracked more easily over a moderately acidic matrix. The matrix cracking does not produce converted products in the gasoline range, but rather it generates "intermediate feed molecules". The conversion of these harder-to-crack molecules requires the enhanced activity of the zeolite. A recently published study demonstrated the effectiveness of matrix-type "bottoms cracking additives1'to conventional FCC catalysts in increasing their activity [lo]. The results of our experiments confirmed that separate particle matrix components were in fact active in pre-cracking gas oil molecules. The hypothesis of "matrix first" cracking was further investigated by separating the matrix and zeolite components

178

TABLES TABLE I

-

Staged bed MAT tests

Moderate Matrix/REY

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

Description Conversion, wt%

Activity,conv./(lOO-conv.)

Matrix first 69.0 2.2

Yields,wt % Bottoms ,>602OF (352OC) Gasoline,Cg -421°F (216OC) Dry Gas rH2-C2 Coke Selectivities, (yield/activity) Dry Gas Coke

10.2 54.4 1.0 3.1 0.45 1.40

Zeolite first 51.0 1.0 24.5 41.9 0.6 2.2 0.60 2.20

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

TABLE I1

-

Staged bed MAT tests

Description Conversion I wt%

Activity,conv/(lOO-conv.)

Moderate Matrix/=-USY

Matrix first 75.05 3.01

Yields, wt % Bottoms, > 602OF (352OC) Gasoline,Cg -421°F (216OC) Dry Gas ,H2 -C2 Coke Selectivities, (yield/activity) Dry Gas Coke

Zeolite first 65.75 1.92

6.84 55.54 1.50 4.06

12.66 51.01 1.20 3.18

0.50 1.35

0.62 1.66

-_______________________________________------------

179

physically and by reversing the order in which the gas oil molecules contacted the active sites: Importance of Matrix Cracking Before Zeolitic Cracking: We designed "staged bed" MAT experiments to verify the concept that conversion to desirable products depends on the feed contacting the matrix first. For the matrix-on-top experiments the FCC catalyst was carefully poured into the bottom of the MAT reactor tube. The matrix component was poured on top of this bed carefully to avoid mixing of the two components. The feed was now forced to percollate through the Ilmatrix bed" first. In the zeolite-on-top experiments the order of the components was reversed. The spent catalyst beds were thoroughly mixed before the coke analysis to obtain a reliable average carbon residue number. We repeated the experiment three times: first using a lower activity grade, high rare earth catalyst combined with a moderately active matrix, secondly using a more active "octanebarrel" moderate rare earth catalyst with a moderatly active matrix, and thirdly a combination of the octane barrel catalyst with a more strongly acidic but not very active matrix (lower surface area). The activity and selectivity results are summarized in Tables I through 111, respectively. In all three cases the effect of matrix cracking of the fresh feed was clearly present. Very little conversion of zeolite products occurred when the matrix was downstream of the FCC catalyst. The MAT activity of the samples with the "matrix-ontop" was 1.5 to over 2 times higher, depending on the initial activity of the FCC component. Similar effects had been observed before by other investigators [ll]. Figure 4 shows the drastic conversion increase for the 9natrix-on-topIl sample to valuable products, accompanied by a reduction in bottoms compounds (boiling above 317 OC). The absolute conversion differences between the less active REY and the more active octane-barrel RE-USY catalyst introduce a bias to the selectivity numbers: the REY yields high gasoline (at lower octane), whereas in the other two comparisons the gasoline yield is relatively smaller, but at higher octane. The difference in conversion is largely due to the increased production of wet gas (c3 and c4 olefins and paraffins) in the experiments with the octane-barrel catalyst. Matrix Type Optimizes Cracking Selectivity: In a final set of experiments, we studied how the product selectivity and octane yield can be optimized by selecting a matrix with a balanced combination of acid strength and site density. These experiments were carried out in the traditional MAT configuration with fully blended components. REY was used as the active FCC catalyst. The matrix types varied in composition

Matrix Sets up Large Molecules for Selective Cracking in Zeolite Staged Bed MAT, wt-%

80 70 60 50 40

30 20 10

0

Conv.

Activ.

Gasol. Bottms Coke Dry Gas

Zeolite/Weak Matrix

0 Weak

Matrix/Zeolite

181

and, hence, intrinsic acid strength, and surface area. We generated MAT data at 70 wt% conversion, shown in Table IV, which are directly comparable. Significantly different selectivities resulted depending on the type of matrix used. Catalyst A, which had the lowest matrix activity, produced the highest yield of coke and gas and the lowest yield of the most valuable products, gasoline and LCO. The reason for this adverse selectivity was overcracking promoted by the few very strong acid sites on the matrix. Catalyst B, with its moderatly active matrix, had the lowest coke and gas yields and higher yields of the desirable products gasoline and LCO. Catalyst c , which had the highest matrix activity, produced yields that generally fell between those of Catalyst A and Catalyst B. These results suggest that an optimum level of matrix activity exists for each active zeolite component. If the activity of the matrix is too low the desired level of heavy molecule conversion cannot be achieved and the overall catalyst activity is negatively affected. If the matrix activity is too high, pr'imarily because the matrix acid sites are too strong, the catalyst selectivities are severely shifted to undesirable products, such as coke and gas.

Catalyst

A

Matrix Activity

LOW

Acidity Surface Area

Strong Very Low

B

C

Moderate

Moderate

Weak Moderate

Moderate High

MAT Yields @ 70 wt-% Conversion Dry Gas wt%

1.1

1.2

9.7

11.1

C5+ Gasoline wt%

1.3 11.8 53.0

55.9

54.2

Valuable Products Gasoline+LCO wt%

71.2

75.6

74.7

3.9

3.3

2.1

1.5

3.5 1.3

c3 -I-c4 wt%

Coke wt% ic4/~4=ratio

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

182

CONCLUSIONS Our experiments using REY and RE-USY catalysts with a minimal matrix activity in combination with model matrix compounds of increasing intrinsic acid strength have clearly demonstrated the following principles:

-

A strong synergism exists between matrix and zeolite cracking under FCC conditions. The resulting activity is higher than the sum of the individual contributions.

-

It is imperative that the larger feedstock molecules contact the matrix component first t o convert them to smaller fragments which can be cracked inside the zeolite to gasoline products.

-

Matrix activity is a combination of acid site strength and density. A moderately active matrix results in a properly balanced FCC catalyst with optimal gasoline and coke selectivities. The FCC catalyst matrix plays a key role in determining overall activity, selectivity, and octane trends. The FCC catalyst manufacturer can control the matrix activity and acidity, allowing him to tailormake catalysts to meet specific refining needs.

ACKNOWLEDGMENTS We acknowledge the expert technical assistance of Mr. Egberto Villanueva in preparing the Samples for this study. We are indebted to Engelhard's Petroleum Catalyst hraluation group for the many experiments, in particular those of the staged bed configurations. Engelhard's Research Services group provided the physical and chemical characterization of the samples. We also thank S.H. Brown, M. Deeba, G.S. Koermer, J.B. McLean, E.L. Moorehead, and D. Stockwell for many stimulating discussions. REFERENCES

,

31(3) , 215-354 (1989).

1.

J. Scherzer, Catal.Rev.-Sci.Eng.

2.

P.B. Venuto and E.T. Habib, IIFluid Catalytic Cracking with Zeolite Catalystsl8, Marcel Dekker, New York (1979).

3.

B.W. Wojciechowski and A . ?orma, IICatalytic Cracking Catalysts, Chemistry and Kinetics", Marcel Dekker, New York (1986).

4.

Reference [l] above, and references therein.

5.

A.G.

Oblad, Oil and Gas Journal, March 27, 84 (1972).

183 6.

C.J. Groenenboom, "Zeolites as Catalysts, Sorbents, and Detergent Builders@~,Stud.Surf.Sci.Catal. 46, Elsevier, Amsterdam, 99 (1989).

7.

A. Humphries and J.R. Wilcox, lIZeolite/Matrix Synergism in FCC Catalysis8@,paper presented at the 1988 NPRA Annual Meeting (March 1988).

8.

C.T. Hayward and W.S. Winkler, Hydrocarbon Processing, February, 55 (1990).

9.

Engelhard Corporation, @IMicro Activity Testing of FCC Catalystsll, The Catalyst Report, Engelhard Corporation, Edison, NJ, USA (1988).

10.

M.M. Mitchell, H.F. Moore, and T.L. Goolsby, llImprovementof FCC Catalyst Performance with Bottoms Cracking Additives', paper presented at the Spring AIChE Meeting, Orland0,FL (1990).

11.

R.E.. Ritter and G.W. Young, paper AM-84-57 presented at the March NPRA Meeting (1984).


G . Ohlmann et al. (Editors), Catalysis and Adsorption by Zeolites 1991 Elsevier Science Publishers B.V., Amsterdam ZEOSORB HS 30

-

185

A TEMPLATE-FREE SYNTHESIZED PENTASIL-TYPE

ZEOLITE K.-H.

'

BERGK1, W.

SCHWIEGER',

H.

FURTIG'

and U. HADICKE2

M a r t i n - L u t h e r - U n i v e r s i t y H a l l e - W i t t e n b e rg, Department o f C h e m i s t r y , S c h l o R b e r g 2, H a l l e 4020, Chemie-AG B i t t e r f e l d - W o l f e n ,

B i t t e r f e l d 4400,

BRD

BRD

SUvlMARY The s u b j e c t o f t h i s p a p e r i s t h e s y n t h e s i s o f p e n t a s i l - t y p e z e o l i t e s w i t h o u t t e m p l a t e s . We d e s c r i b e i n v e s t i g a t i o n s o f the various synthesis variables s u c h as t h e c o m p o s i t i o n o f the r e a c t i o n mixture, a p p l i c a t i o n s i n a t e c h n i c a l process and, based on t h e z e o l i t e "Zeosorb HS 30", i n v e s t i g a t i o n s o f t h e a f t e r - t r e a t m e n t f o r a s p e c i f i c adjustment o f t h e Al-content i n the l a t t i c e o f the zeolite which are important f o r t h e a p p l i c a b i l i t y o f the catalyst.

-

-

INTRODUCTION S i n c e 1966 t h e Chemie-AG B i t t e r f e l d - W o l f e n has been t h e o n l y p r o d u c e r o f m o l e c u l a r s i e v e s i n t h e GDR.

The development

s t a r t e d w i t h t h e s y n t h e s i s o f z e o l i t e 4A i n 1960. t y p e s o f m o l e c u l a r sieve, t h e z e o l i t e s 3A and 5A,

The r a n g e o f

w h i c h a r e produced f u r t h e r i n c l u d e s

t h e f a u j a s i t e s 13X,

1OX and Y,

morde-

n i t e and e r i o n i t e t y p e s and c u r r e n t l y a l s o t h e p e n t a s i l t y p e Zeosorb HS 30 ( T a b l e 1). The z e o l i t e s a r e used i n v a r i o u s a d s o r p t i v e and ca t al,,t

i c processes.

The range o f p e n t a s i l t y p e b y m o l e c u l a r s i e v e s was w i d e n e d b y u s i n g a s p e c i a l method.

F o r a l o n g time the synthesis of

t h e s e t y p e s was c o n n e c t e d w i t h t h e a p p l i c a t i o n o f s u b s t a n c e s deternining the structure, as d e s c r i b e d i n r e f s .

t h e s o - c a l l e d t e m p l a t e compounds,

1-3.

TABLE 1 Chemie-AG B i t t e r f e l d - W o l f e n as a m o l e c u l a r s i e v e p r o d u c e r TYP e A

Fauj a s i t e

E r i o n it e

~~~~

~~

Zeosorb 3A, 4A, 5A

Zeosorb 13X, lox, Y

delivery form:

as powder ( a i r ,

~

Mo r d e n i t e

Pent a s i l

Zeosorb VlOO

Zeosorb HS30

~

Erionite dried,

activated),

as g r a n u l e

186 The f i r s t work on t h e a p p l i c a t i o n o f o r g a n i c s u b s t a n c e s i n t h e p r o c e s s of z e o l i t e f o r m a t i o n was c a r r i e d o u t b y BARRER and DENNY ( r e f .

4) and KERR ( r e f .

5).

T a b l e 2 shows a l i s t o f

v a r i o u s o r g a n i c compounds, w i t h w h i c h p e n t e s i l z e o l i t e s c o u l d be pr oduc ed i n t h e y e a r s a f t e r 1966. We have grouped t h e s u b s t a n c e s i n t o categories,

such as o r g a n i c c a t i o n s ,

n e u t r a l molecules,

o r g a n i c a n i o n s and

and h b v e l i s t e d t h e names o f t h e a u t h o r s and

the year o f publication,

when t h e s e f a c t s w ere known.

s t a g e s a r e t h e w o rk s o f ARGAUER and LANDOLT ( r e f . e t el.

(ref.

KERR ( r e f .

2),

Important

l), RUBIN

5) and LOWE and AROYA ( r e f .

From t h i s v a r i e t y and g r o u p s o f s u b s tances,

6).

the r o l e o f template

compounds i n t h e s y n t h e s i s p r o c e s s c a n be deduced t o b e u n c l e a r and d i v e r s e , The a p p l i c a t i o n o f o r g a n i c compounds i n a process, w h i c h i s otherwise e n t i r e l y inorganic, process i t s e l f ,

ceuses some p r o b l e m s f o r t h e

a s w e l l a s f o r t h e economy and t h e q u a l i t y o f

t h e products.

-

-

Such pr oble m s a r e ,

f o r instance:

the cost o f t h e template; t h e p o l l u t i o n o f w e s te w a t e r ; s a f e t y problems i n t h e t e c h n i c a l process; t h e p o l l u t i o n o f t h e o r g a n i c component b y w aste gas d u r i n g thermal decomposition; t h e c ar bon d e p o s i t s i n t h e z e o l i t e caused b y a p o s s i b l e i n c o m p l e t e c o mb u s ti o n o f t h e o r g a n i c component. These d i s a d v a n t a g e s demand t e m p l a t e - f r e e s y n t h e s i s

fore,

.

There-

b e f o r e commencing r e s e a r c h and development w ork on t h e

synthesis of pentesil zeolites, Bitterfeld-Wolfen

i t was n e c e s s a r y f o r Chemie-AG

( a s a p r o d u c e r o f m o l e c u l a r s i e v e s ) and t h e

Research I n s t i t u t e a t H a l l e t o c o n s i d e r t h e b a s i c raw m a t e r i a l s and t h e t e c h n i c a l f a c i l i t i e s w h i c h were a v a i l a b l e , i n o r d e r t o make i n d u s t r i a l p r o d u c t i o n f e a s i b l e .

187 TABLE 2 The d i f f e r e n t t y p e s o f o r g n n i c compounds,

w h i c h c a n be used

f o r t h e syntheses o f P e n t e s i l z e o l i t e s Type

Example

Author

R eference/ year o f publication

Organic cations

R~N+, R ~ P +

A rg a u e r and L a n d o l t Dwyer and J e n k i n s Kerr

1, 1972 7, 1976 5, 1969

polymeric cationic conpounds

................................ Organic molecules

Amine, diamine alcohols, dioxane, 2- amino-py r i d i n e

Rubin e t e l . Bremer e t e l . R o l l me n n P o s t and Nanne Klotz Whit t am

1975 1981 3, 1978 9, 1979 10, 1983 11, 1982 2, 8,

................................ Organic anions

Alkylbenzols u l f onate, polymeric anionic compounds

Hugiwara e t a l .

12,

1981

Roscher e t a l . Lowe and Aro ya

13, 6,

1984 1983

SYNTHESIS OF PENTASIL TYPE ZEOLITE The f i r s t p o s i t i v e r e s u l t s ( T a b l e 3) were o b t a i n e d i n l a b o r a t o r y t e s t s which looked s p e c i f i c a l l y a t t h e aspect of a c o n t i n u o u s s i m p l i f i c a t i o n o f t h e t e m p l a t e compounds.

The

c l a s s i c a l t e m p l a t e compound TEA-3 ( t e t r a e t h y l a m m o n i u m i o d i d e ) , i t s f o r m a l p r e c u r s o r s TEA ( t r i e t h y l a m i n e ) and

alkyliodide

,

s i m p l e amines,

a t e c h n i c a l m i x t u r e o f amines and a l c o h o l and

NH3 were used.

By t h i s method,

s p r o d u c t c o n t a i n i n g 60 t o 80 %

p e n t e s i l c o u l d a l r e a d y be p ro d u c e d w i t h o u t u s i n g t e m p l a t e s . A f t e r optimizing, considerably.

t h i s c o n t e n t l e v e l c o u l d be i n c r e a s e d

188 TABLE 3

S y n t h e s i s o f P ,n ta s i l 175

OC

zeolites with different additions at

and r e s t e d r e a c t i o n m i x t u r e s

Number

CrystelliProduct Composition of t h e reaction mixture z a t i o n time composition Na 2 ~ ~l / 203/ s i O 2 / H20/Z t / h/ Ma.-%

Addition (2)

TPA-J

5,7

1

100 1580 9,6

142

60 ZSM-5 40 Q u a r t z

TEA + n-Butyl-bromid

;?

1

Technical B u t y l am i n mixture

8,3

1

93,5

3877 36,7

168

80 ZSM-5 20 Q u a r t z

N-P r o p y l amin

8,3

1

93,5

3877 36,7

96

70 ZSM-5 30 Q u a r t z

+

8.3

1

93,5

3877 36,7

48

NH3

28

750 16

30 amorphous

240

70 ZSM-5

20 sio2-x

C2H5-OH

80 ZSM-5

NH3

25,9

From t h e s e t e s t s ,

1

88,4

1820

55

24

8 0 ZSM-5 10 amorphous

t h e f i r s t p a t e n t a b l e r e s u l t s were o b t a i n e d ,

and t h e s e a r e l i s t e d i n T a b l e 4. C a t i o n i c and/or

a n i o n i c p o l y m e r compounds a s w e l l as a t e c h n i c a l

m i x t u r e o f amines a r e u s e d as t e m p l a t e s ( r e f s . template-free

13-14).

The f i r s t

synthesis w i t h o r without the eddition o f n u c l e i

o f c r y s t e l l i z a t i o n are described i n refs.

15-17.

F i g u r e 1 shows a f i e l d o f s y n t h e s i s f o r t h e t e m p l a t e - f r e e type. The phases w h i c h c a n be seen a r e shown b y p l o t t i n g t h e r e a c t i o n t i m e a g a i n s t v a r i a t i o n s o f t h e SiOdA1203 molar r e t i o i n t h e

reaction mixture.

I n a d d i t i o n t o t h e t a r g e t phase,

pentasil

w i t h a M F I - s t r u c t u r e , m o r d e n i t e , q u a r t z and l a y e r s i l i c a t e s can a l s o be seen. Each t i m e t h e S i O d A 1 2 0 3 m o l a r r a t i o was i n c r e a s e d a d i f f e r e n t sequence o f phases c o u l d be seen. A t a S i02/ A1 2 0 3 m o l a r r a t i o o f 20

-

amorphous phase i n t o m o r d e n i t e o c c u r s , o f 25

-

25 c o n v e r s i o n f r o m t h e a t a medium m o l a r r e t i o

55 c o n v e r s i o n f r o m t h e amorphous phase i n t o q u a r t z v i a

P e n t a s i l oc c u rs , e t an even h i g h e r r a t i o c o n v e r s i o n f r o m t h e amorphous phase i n t o q u a r t z v i a l a y e r - s i l i c e t e occurs.

189

TABLE 4

E a r l i e r p a t e n t s f o r P e n t a s i l s y n t h e s i s (1982) of Chemie-AG B i t t e r f eld- W o l f en Number o f t h e p a t e n t

Content

DD-WP 207 184

c e t i o n i c and a n i o n i c p o 1ym e r ic comp o u n d s

DD-WP 207 186

template-f ree synthesis

DD-WP 207 185

organic-free, ammonia

DD-WP 205 674

t e m p l a t e - f ree, w i t h . t h e a d d i t i o n o f a c t i v a t e d seed m a t e r i e l s

DD-VIP 206 551

template-f ree w i t h vigorous s t i r r i n g o r the addition o f grinding bodies

DD-WP 206 979

t e c h n i c a l amin m i x t u r e as t e m p l a t e

I

'

BS YO

7004

b u t b y use o f

4.-""

300

,

~

r r n d r n o r ~

k-i,f---/Yfj

mordenih

I

I

I

/

24

48

I

I

I

72

96

720

Y

I

tU

14.0 h

HS 20

Fig.

1. A t e m p l a t e - f r e e

zeolites.

synthesis f i e l d o f Pentasil type Pentosil type zeolites; SS : s h e e t s i l i c a t e s .

MFI:

190

ss

MF I

MOR

c---------r 5 um

/

c---------r 2 um

/

F i g . 2. A r a s t e r e l e c t r o n microscope image o f t h e phases observed i n t h e t e m p l a t e - f r e e s y n t h e s i s f i e l d . MFI: P e n t a s i l t y p e z e o l i t e s f SS : s h e e t s i l i c a t e e ; MOR: M o r d e n i t e s .

191 T h i s d e s c r i p t i o n shows t h a t when t h e SiO2/Al2O3 m o l a r r s t i o

i s i n c r e a s e d , c r y s t a l l i z a t i o n g e n e r a l l y s l o w s down. We have named t h e r ang e s o f s y n t h e s i s , w h i c h were most i m p o r t a n t i n o r d e r t o achieve t e c h n i c a l r e e l i s a t i o n .

On t h e b a s i s o f t h e s e

r e s u l t s t h e p r o d u c t i o n o f t h e p r o d u c t was s t e r t e d .

The f i r s t

co m m er c ial p r o d u c t t o be d e v e l o p e d was Zeosorb HS 30. further,

product8 with

I

SiOdA1203 m o l a l r a t i o o f 20 (HS 20) end

40 (HS 40) can b e m a n u f a c t u r e d and o f f e r ' e d .

F i g u r e 2 shows a p h o to g ra p h , t a k e n t h r o u s h an e l e c t r o n m i c r o scope,

o f t h e phases t o be seen i n t h e f i e l d o f s y n t h e s i s .

Each phase has i t s c h a r a c t e r i s t i c morphology,

so t h a t even

s l i g h t i m p u r i t i e s can be seen,

although these i m p u r i t i e s cannot

y e t be shown b y means o f X-ray

techniques.

THE TECHNICAL PROCESS OF SYNTHESIS Based on t h i s s t u d y o f t h e l i m i t a t i o n s o f t h e f i e l d o f s y n t h e s i s end t h e k i n e t i c s o f t h e c r y s t a l l i z a t i o n process, an e n t i r e l y i n o r g a n i c , t e c h n i c a l p r o c e s s was developed. I n F i g u r e 3 a scheme o f t h e c o u r s e o f t h e r e a c t i o n i s gi ven. Based on a sodium a l u m i n a t e s o l u t i o n as a s o u r c e o f A1203, a NaOH s o l u t i o n and water,

an a l k a l i n e p r e p a r a t i o n i s p r o d u c e d

i n a f i r s t m i x i n g p ro c e s s .

S i l i c a s o l used as a s o u r c e of S i 0 2 ,

i s added i n a subsequent m i x i n g p ro c ess. A f t e r homogenizing, t h e r e a c t i o n m i x t u r e i s t r a n s f e r r e d t o an a u t o c l a v e end c r y s t a l l i z e d under hydrothermal conditions. has ended and h a s been c o o l e d down,

After the reaction

t h e p r o d u c t is s e p a r a t e d

fro m t h e m o t h e r l i q u o r b y f i l t r a t i o n .

I t i s t h e n washed u n t i l

t h e o u t - f l o w i n g w a t e r has a pH v a l u e o f a p p r o x i m a t e l y 9 and d r i e d a t e t e m p e r a t u r e o f 110

OC.

i s a d r i e d powder o f sodium form.

The p r o d u c t

-

Zeosorb HS 3 0

-

T h i s i s i t e c o m m e r c i a l form.

I f r e q u i r e d , HS 3 0 - p r o d u c t s c a n a l s o be s u p p l i e d i on-exchanged ( l o a d e d w i t h d i f f e r e n t c a t i o n s ) o r g r a n u l a t e d (bound w i t h c l a y ) .

192

lstep

1

MlXING 2 step

I

DRYING (770°C)

I

MIXING

L

0

I o wder

F i g . 3. M a n u f a c t u r i n g p r o c e s s o f t e m p l a t e - f r e e type zeolite.

Pentasil

PROPERTIES OF THE PRODUCTS When c om pa ri n g t h e p r o p e r t i e s o f t e m p l a t e - c o n t a i n i n g and template-f ree synthesized products,

according t o t h e i r S i O d

A1203

ratio,

i t

A1203

ratio,

mo rp h o l o g y and a c i d i t y cnn be v a r i e d s y s t e m a t i -

can be f o u n d t h a t t h e i r c r y s t a l l i n i t y , S i O d

c a l l y i n t h e t e m p l a t e - f r e e v a r i s n t as w e l l , o n l y p o s s i b l e w i t h i n a s m a l l range.

although t h i s i s

I n t h i s respect,

a r e s i m i l a r and d i f f e r e n t t e n d e n c i e s .

there

These a r e g i v e n i n

T a b l e 5. For both synthesized variants, products,

the C r y s t a l l i n i t y o f the

t h e volume o f t h e m i c r o p o r e s , t h e c o n t e n t o f

alum inium and t h e B r o e n s t e d a c i d i t y a r e i n t h e same p r o p o r t i o n , depending on t h e c o n t e n t o f t h e a v a i l a b l e A l . t o this, cells,

In o p p o s i t i o n

t h e r e a r e d i f f e r e n t t e n d e n c i e s f o r t h e volume o f u n i t

t h e r a t i o o f s t r o n g l y a c i d i c t o weakly a c i d i c c e n t r e s

and f o r t h e volume o f mesopares.

193

TABLE 5 Comparison o f t h e p r o p e r t i e s o f t e m p l a t e - c o n t a i n i n g and t e m p l a t e f r e e P e n t a s i l p r o d u c t s ( g e n e r a l t e n d e n c i e s depending on t h e Si0,./A1203

ratio

S i m i l a r tendencies

D i f f e r e n t tendencies

1. C r y s t a l l i n i t y

1.

dep e n d e n c y o f t h e v o l u m e o f t h e u n i t c e l l upon t h e SiOgAl2O3 r a t i o

2.

r a t i o o f weak a n d s t r o n g a c i d i c centres

3.

volume o f mesopores

2.

volume o f m i c r o p o r e s

3.

r e l a t i o n between t h e Si02/A1 0 r e t i o s o f the reaztlon mixtures a nd t h e p r o d u c t s

4.

dependence o f t h e m o r p h o l o g y upon t h e Si02/A1203 r a t i o

5.

d i r e c t r e l a t i o n between t h e A1 c o n t e n t and t h e Broensted-acidity

F o r example,

t a b l e 6 shows a c o m p a r i s o n o f c h a r a c t e r i s t i c d a t a

o f s e l e c t e d samples f r o m b o t h s e r i e s ,

-

letter S

-

o r without template

Si02/A1203 r a t i o ,

the acidity,

a d s o r b e d q u a n t i t y o f NH3, of t h e micropores, is,

e i t h e r with template

- l e t t e r A.

which i s given here as t h e

decreases i n b o t h s e r i e s .

d e t e r m i n e d f rom n-Hexane

w i t h an e q u a l volume,

With an i n c r e a s i n g Th e v o l u m e

adsorption isotherms,

independent o f t h e SiOdA1203 m o l a r

r a t i o i n both series. T h e vo lume o f t h e mesopores o f t h e s a m p l e s s y n t h e s i z e d w i t h

TPA-J i s t w o o r t h r e e t i m e s l a r g e r i n c o m p a r i s o n t o t h e template-f ree v a r i a n t . By c o m p a r i n g t h e m o r p h o l o g y o f t h e p r o d u c t s o f b o t h t h e series (Figure 4), SiO/A1203

i t i s p o s s i b l e t o see t h a t w i t h an i n c r e a s i n g

m o l a r r a t i o t h e r e i s a l w a y s a change f r o m l o o s e

a g g r e g a t e s t o l a r g e r m o n o c r y s t a l s w i t h a smooth su-face. A l t h o u g h t h e r e i s no d e t a i l l e d i d e n t i t y b e c a u s e t h e f o r m a t i o n o f a s p e c i a l m o r p h o l o g y depends o n t o o many f a c t o r s , t e n d e n c y i s t h e same.

the

194

TABLE 6 Comparison o f p r o d u c t p r o p e r t i e s a c c o r d i n g t o t h e i r s y n t h e s i s w i t h ( S ) and w i t h o u t ( A ) t e m p l a t e s (H-forms)

Ternalate

s 5 5 5 5

Product

TPA-J TPA-3 TPA-J TPA-J TPA-J

1 2 3 4 5

no no no

A 1 A 2

A 3

NH,-TPD

Volume o f p o r e s

38

0,83

56 124 204 3 64

0,bB

0,46 0,20 0,13

1,53 1,21

22 32 58

SiOdA1203 r a t i o

0,BO

0,13 0,16 0,16 0,16 0,16

0,06 0,06 0,05 0,04 0,03

0,17 0,16 0,15

0,02 0,Ol 0,Ol

SPECIAL VARIANTS OF MODIFICATIONS The p o s s i b l e v e r i a t i o n s d e s c r i b e d so f e r o n l y r e s u l t f r o m t h e d i m e n s i o n s o f t h e shown f i e l d o f s y n t h e s i s .

However,

for

many a p p l i c a t i o n p u r p o s e s t h i s r o n c e o f p o s s i b l e v a r i a t i o n s i s not sufficient, retio.

e s p e c i a l l y w i t h regard t o t h e SiOdA1203 molar

Therefore,

i t W F S t h e a i m o f o u r f u r t h e r work t o a c h i e v e

a b e t t e r adjustment o f

t h e p r o p e r t i e s t o s p e c i a l demands b y

o p t i m i z i n g t h e s y n t h e s i s and m o d i f y i n g t h e p r o d u c t . T h i s wes a c h i e v e d i n d i f f e r e n t

WAYS:

1. By u s i n g s y n t h e s i s v a r i a n t s f o r w i d e n i n g t h e r a n g e o f t h e S i 0 2/ A1 203-m o d u 1e

.

2.

V a r i n g t h e p r o p e r t i e s b y isomorphous s u b s t i t u t i o n o f t h e n e t w o rk-f o r m i n g e l e m e n t s .

3.

I n c r e a s i n g t h e module b y subsequent d e a l u m i n i z a t i o n . By u s i n g s p e c i a l v a r i a t i o n s o f t h e s y n t h e s i s c o n d i t i o n s

(concentration o f a l k a l i metals,

c r y s t a l l i z e t i o n time)

,

i t

i s

p o s s i b l e t o extend t h e renge o f t h e module o f t h e s y n t h e s i z e d p r o d u c t s t o 20

-

60 f r o m a s t a r t i n g m o l a r r a t i o o f 20

K i t h an i n c r e a s i n g m o l a r r a t i o ,

- 70.

the rate of crystallization

-

With a s t c r t i n g m o l a r r a t i o o f 80 100 t h e r e p r o d u c i b i l i t y is p o o r , a n d t h e r e w i l l b e a d e c r e a s e i n t h e c r y s t a l l i n i t y decreases.

of t h e products.

@ O D

N

w m L O

I

y.

N

N

e,m m 4 0 E

W

r

D t I r o a m

a I-

0)

W C

L O

..

196

me,

Q)c

mrl E l rl 0 a,@ P N

ma,

0.4 0 0

Or: Le, v c

€ 0

4%

196

On t h e o t h e r hand we can see,

t h a t t h e SiO$Al2O3

molar r a t i o

o f t h e p r o d u c t is a l w a y s a p p r o x i m a t e l y 10 u n i t s l o w e r compared w i t h those s t a r t i n g compositions f o r template-free A s t o n i s h i n g l y enough,

syntheses.

an e x t e n s i o n o f t h e p o s s i b l e S i 02/A 1203

m o l a r r a t i o t o 80 was p o s s i b l e ,

when p r o d u c t s c o n t a i n i n g f i v e -

membered r i n g s as a s t r u c t u r a l e l e me nt w ere u s e d a s n u c l e i . We us ed o f f r e t i t e o r e r i o n i t e a s n u c l e i .

Obviously,

t h e r e i s an

i n t e r m e d i a t e d e c o m p o s i t i o n i n t o b a s i c el ements, w h i c h i s f a v o u r e s p e c i a l l y t h o s e w i t h MFIi s p o s s i b l e t o produce MFI-products w i t h a S i O d A1203 m o l a r r a t i o o f 70 80 f r o m r e a c t i o n m i x t u r e s w i t h a Si02/A1203 m o l a r r a t i o o f 100 and w i t h o f f r e t i t e o r e r i o n i t e as n u c l e i , w i t h o u t any o t h e r c r y s t a l l i n e byproducts. One more p o s s i b l e m o d i f i c a t i o n d u r i n g d i r e c t s y n t h e s i s i s t h e i som or phous s u b s t i t u t i o n o f Al- o r S i -atoms i n t e t r a h e d r a l p o s i t i o n s b y v a r i o u s hetero-atoms. T h e r e a r e many p o s - s i b i l i t l e s f o r such a s u b s t i t u t i o n d u r i n g s y n t h e s e s w i t h t e m p l a t e s . Even t h e c om plet e s u b s t i t u t i o n o f a l u m i n i u m i n t h e l a t t i c e b y e.g. Fe- o r B-atoms i s p o s s i b l e ( r e f s . 18-21). S o - c a l l e d f e r r i t e s o r b o r a l i t e s a r e formed. T h i s has n o t y e t been d e s c r i b e d f o r t e m p l a t e - f r e e S y n t h e s i z e d v a r i a n t s . We s t u d i e d t h e i somorphous s u b s t i t u t i o n o f B and Fe o n t h e b a s i s o f o u r s t a n d a r d t e m p l a t e f r e e s y n t h e s i z e d runs. able t o the synthesis of pentasils, structure.

It

-

The r e s u l t s of t h e s y n t h e s e s w i t h b o r o n a r e shown i n F i g u r e 5. The r e s u l t a n t r e l a t i v e c r y s t a l l i n i t y (compared t o a s t e n d a r d ) o f synthesized products obtained a f t e r a c r y s t o l l i z a t i o n time ( T K ) o f 24 o r 4 8 h o u rs ,

and a t a t e m p e r n t u r e o f 175

OC

(TK),

depending on t h e S i 0 2 / A 1 2 0 3 m o l a r r a t i o i n t h e r e a c t i o n m i x t u r e , a r e shown.

On t h e b a s i s o f a s t a n d e r d Si02/A1203 i n i t i a l r a t i o

o f 40, alum ini u m was c o n s t a n t l y r e p l a c e d t o a c e r t a i n p e r c e n t a g e by B ( u p p e r l i n e o f F i g u r e 4), w h i a l e t h e r a t i o o f S i O d A 1 2 0 3 + +B203 remained c o n s t a n t a t 40. Up t o a s u b s t i t u t i o n o f a p p r o x i m a t e l y 50% A l , w e l l - c r y s t a l l i z e d p r o d u c t s a r e o b t a i n e d . W i t h

l a r g e r boron content, suppressed.

the c r y s t a l l i z a t i o n of p e n t s a i l i s

T h i s can be e x p r e s s e d b y t h e g r e a t d r e c r e a s e i n

crystallinity.

A p a r t fro m M F I - t y p e z e o l i t e ,

amorphous phases

and l a y e r - s i l i c a t e s a r e s t i l l found, and t h e r e a r e a l r e a d y t r a c e s o f more h i g h l y - c o n d e n s e d S I O -phases, such as c r i s t o b a l i t e . 2

197

%&\ '

55 Cr

- sheet silicates - cristoba/i te am - amorphous I

I

I

700 750 21 product 5102/Ab 0 ' -ratio

50

F i g . 5. S y n t h e s i s o f P e n t a s i l t y p e z e o l i t e s w i t h b o r o n s t a r t i n g Si02/A1203 + B 2 0 3 - r a t i o = 40 TK = 175 OC Vie can draw t h e c o n c l u s i o n , t h a t alum inium i n t e m p l a t e - f r e e

a complete s u b s t i t u t i o n o f

synthesized products i s impossible,

and t h u s t h a t a l u mi n i u m p l a y s an i m p o r t e n t r o l e i n t h e f o r m a t i o n o f t h e MFI-st r u c t u r e .

The e x p e r i m e n t s gave s i m i l a r r e s u l t s w i t h r e g a r d t o t h e s u b s t i t u t i o n o f i r o n . W i t h a Fe:Al

m o l a r r a t i o o f 0,6

there i s

o n l y a s l i g h t d e c re a s e f r o m 100% t o about 95%. B e g i n n i n g w i t h a Fe : A l- m olar

ratio,

crystallinity, r a t i o o f 1:1

o f 0,7, t h e r e i s a s l i g h t decrease i n

w h i c h becomes q u i t e e v i d e n t a t 59% w i t h a m o l a r

( t h i s means a 50% s u b s t i t u t i o n o f t h e A1 b y Fe).

From t h e s e r e s u l t s , s u b s t i t u t i o n w i t h b o r o n i s c o n f i r m e d i n t h i s case a s w e l l . However, t h e s e p a r t i a l s u b s t i t u t i o n s can a l r e a d y e f f e c t c e r t a i n changes i n t h e p r o p e r t i e s o f t h e p e n t s s i l p r o d u c t s . I n p a r t i c u l a r , t h e y have an i n f l u e n c e on t h e morphol o g y , t h e a c i d i t y , t h e l a t t i c e c o n s t a n t s and, t h e r e f o r e , o n t h e i r a p p l i c a b i l i t y a s c a t a l y s t s o r components o f c s t s l y s t s . Further p o s s i b i l i t i e s f o r modification are the so-called p o s t - s y n t h e t i c processes. The p u r p o s e o f such a t r e a t m e n t is t o

TABLE 7

c

co

Q,

S t r u c t u r a l and a c i d i c p r o p e r t i e s o f HS 30 (H-forms)

Zeolite

Type o f t r e a tment

Treatment temperature

Extraction process

a f t e r t h e r m a l and h y d r o t h e r m a l t r e a t m e n t

SiOd

Total

Number o f

Lattice

A1 0 soluhe

K atoms

H-HS H-HS H-HS H-HS H-HS

30 30 30 30 30

H-HS 30 H-HS

30

-

thermal thermal thermal hydrothe rmel hydrot hermel hydrothermal

-

6 n HC1 EOTA, i n

2)

1)

-

NH OH, i n HC1 6 HC1 6 n HC1

29.6 32.4 31.4 32.5 313.5

64.0 58.8 60.6 58.6 49.9

66.3 12.0 12.0 12.0 30.7

54.1 11.5 12.7 11.6 32.3

5.2 3.4 5.4 14.1

773

6 n HC1

55.7

34.9

15.1

14.2

29.1

973

6 n HC1

71.8

27.3

2.4

-

36.7

1073 1073 1073 623

8

tic1

" L a t t i c e A l " 8 T o t a l o f t h e number o f Broensted and L e w i s centres.

*)

"Amorphous Al s o l u b l e " :

D i f f e r e n c e between t h e t o t a l number o f A 1 atoms o f t h e i n i t i a l z e o l i t e and t h e r e s p e c t i v e sample.

199 remove A 1 f r om t h e l a t t i c e i n o r d e r t o a c h i e v e t h e r e q u i r e d a c i d i t y . We t e s t e d Zeosorb HS 30 as a b a s i c m a t e r i a l i n t h e r m a l and h y d r o t h e r m a l t r e a t m e n t processes. These p r o c e s s e s can b e f o l l o w e d b y e x t r a c t i o n s t e p s f o r t h e d i s p o s a l o f t h e removed alum inium ( t h e e x t r a - l a t t i c e a l u m i n i u m ). The e f f e c t s t h a t

can b e a c h i e v e d a r e shown i n f a b l e 7.

Th er m al t r e a t m e n t was c a r r i e d o u t a t t e m p e r a t u r e 8 f r o m 500 t o 1000

OC

i n a f l a t - b e d f o r a p e r i o d o f 0,5 t o 24 h.

some examples a r e g i v e n . p e r i o d o f treatment,

OC

I n Table 8

0 y c h o o s i n g t h e t e m p e r a t u r e and/or

t h e c o n t e n t o f A1 i n t h e l a t t i c e can b e

varied systematically. TABLE 8 P e n t a s i l t y p e z e o l i t e Zeosorb HS 30 f i e l d s o f a p p l i c a t i o n i n t h e GDR

-- cc aa tt aa ll yy ss tt -- cc aa tt aa ll yy ss tt However,

for for for for i t

i n t h e samples,

a h y d r o - c r a c k - f o r m i n g p r o c e s s (Leuna) a dewaxing p r o c e s s (Schwedt, BBhlen) h y d r o r a f f i n a t i o n (Schwedt) h y d r o t r e a t i n g (Schwedt)

i s i m p o s s i b l e t o re d u ce t h e t o t a l c o n t e n t o f A1 even u n d e r d r a s t i c e x t r a c t i o n c o n d i t i o n s .

The

s p e c i e s o f A 1 o b t a i n e d b y t h e r m a l t r e a t m e n t remai n i n t h e sample as e x t r a - l a t t i c e aluminium. H y d r o t h e r m a l t r e a t m e n t makes a s p e c i f i c a d j u s t m e n t o f t h e content o f A1 i n the l a t t i c e possible,

end,

apart from t h 3 t ,

t h e p c r t i o l e x t r a c t i o n o f t h e removed a l u m i n i u m ( u p t o 60%) from t h e l a t t i c e system o f t h e z e o l i t e . T h i s i s i m p o r t a n t f o r t h e i r a p p l i c a b i l i t y as c a t a l y s t o r c a t s l y s t c a r r i e r s .

Comparing t h e

e f f i c i e n c y o f b o t h processes o f after-treatment,

we f i n d t h a t :

- with

-

b o t h p r o c e s s e s p r o d u c t s cen b e manufactured,

that other-

w i s e can o n l y be s y n t h e s i z e d w i t h t h e h e l p o f t e m p l a t e s , d u r i n g hydrothermal treatment the tempersture o f adjustment f o r a c e r t a i n c o n t e n t o f A 1 i n t h e l a t t i c e can be 150 O C t o 200 OC l o w e r, i n comparison w i t h t h a t ,

thermal treatment.

l e s s equipment i s needed f o r t h e

A? P L I CAT I ONS F o r s p p l i c a t i o n as c e t o l y s t c a r r i e r s ,

n e c e s s a r y i n most cases.

F o r r e a c t i o n s u s i n g an a c i d i c c a t e l y s t

t h e H-form i s s u i t a b l e i n most coses.

+ !JH4 - i o n

i o n exchange w i l l be

I t can be a c h i e v e d b y

e x c h a n g e o r by t h e d i r e c t a c t i o n o f a c i d s ,

as Zeosorb

HS 30 i s s t a b l e t o a c i d s . F o r s p e c i a l p r o c e s s e s a m o d i f i c a t i o n w i t h v a r i o u s m e t a l c a t i o n s w i l l be necessary. o f HS 30 w i t h Zn2+-

A modification

o r K + - i o n s p r o v e d t o be s u i t a b l e f o r o b t a i n -

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

The c a t a l y t i c e x p e r i -

ments were c a r r i e d o u t a t t h e C e n t r a l I n s t i t u t e of O r g a n i c C h e m i s t r y ( Z e n t r a l i n s t i t u t f u r O r g e n i s c h e Chemie) under t h e supervision o f Prof.

at Leipzig

Anders.

O t h e r a p p l i c c t i o n s o f Z e o s o r b I-IS 30 i n i n d u s t r i z l ~ r c c t ? . " E i

in

t i l e ?'X :,r? choi!ii

i n T a b l e 8.

201

REFERENCES L a n d o l t , US-Potent 3 702 886 (1972) ;\rc!cvcr c n d G.R. 1 x.K. R o s i n s k i and Ch.J. P l a n k , DE-Patent 2 M.V. Rubin, E.J. 2 4 4 2 240 ( 1 9 7 5 ) Rollmann, US-Petent 4 1 0 8 881 (1978) 3 L.D. 4 R.M. B a r r e r and P.J. Denny, 3. Chem. SOC. 971 (1961) 5 G.T. K e r r , US-Potent 3 459 6 7 6 (1969) 6 B.M. Lowe and R. Aroya, EP 77 624 ( 1 9 8 3 ) 7 Y.N. Dwyer e n d E.E. J e n k i n s , US-Patent 3 9 4 1 8 7 1 (1976) 8 H. Bremer, V!. R e s c h e t i l o w s k i , D.Q. Sou, K.P. Wendlandt, P.N. Nau und F. Vogt, A c i d i t a t u n d k a t e l y t i s c h e E i g e n s c h a f t e n des Z e o l i t h s ZSM-5, 2. Chem. 2 1 (1981) 77-78 9 M.F.M. P o s t and J.M. Nanne, DE-Patent 2 913 5 5 2 (1979) 10 M.R. K l o t z , US-Petent 4 377 5 0 2 ( 1 9 8 3 ) 11 T.V. W h i t t a m , EP 59 359 (1982) 1 2 H. Hugiwara, Y. I is a function of 6a and the distance travelled by the xenon atom between two successive collisions. In fact, 6, = 6a when the xenon leaves the surface. 6 , t h e n decreases during the journey between sites 1 and 2, whence the need to determine a mean value,

*>. In order to obtain by this technique precise data on the void space of a zeolite of unknown structure and on the dimensions of structural defects,

222

-

Fraissard et a1 (refs. 5,6) have calculated the mean free path, 1 , of xenon

-

imposed by the structure and determined the dependence of 6s on 1 : a 6 s = 6a a+ 1 where a is a constant. The case of the infinite cylinder and the sphere can be rigorously solved by calculation:

-

-

infinite cylinder : 1 = Dc - Dxe sphere : 1 = 1/2(Ds-Dxe) Dc, Ds and Dxe are the diameters of the cylinder, the sphere and the xenon atom, respectively.

F: Ferrierite Z-5, 48, 12: Z S M J , 48 and 12 A-5, 11, 17: A1m4-5, 1 1 and 17 S-34, 37: SAPO-34 and 37

0

b 1

2

Fig. 3. Variation of

4

6

e A i

-

against the mean free path 1 .

With a more complicated structure we must feed the computer the real structure of the zeolite considered. Then we circulate a xenon atom within this space, assuming that the velocity vector is random and all directions are

equally probable. The mean free path,

-

223

1 , is obtained by averaging the

distances between successive collisions. The result thus obtained for various model zeolites is shown in Fig. 3-1. This curve has enabled us to determine the characteristics of the void volumes of unknown structures or structural defects. But certain points have to be considered more closely. For example, is 26OC a high enough temperature for the rapid exchange model to be used? Up to what point is the effect of the A1 concentration negligible? And, more generally, what is the effect of the chemical composition of the solids, in particular of AlPO4, SAPO, etc? It is obvious that if one increases 6 a or the probability of state A one must in theory increase the experimental 6 value. But, as we shall see later, the magnitude of this variation depends also on , which depends not only on 6 a but also on 1 , compared to 6a. Therefore, it is necessary to

-

study the influence of the different parameters aforementioned. INFLUEiNCT OFTEMPERATURE The chemical shift variations with xenon concentration 6 = f(n) at different temperatures are illustrated in Figs. 4 and 5 for NaY and ZSM-5 samples, respectively, pretreated at 4OOOC and 10-4 torr. Figure 4 can be divided into two regions, I and 11, corresponding to 6 the xenon concentration range of E a at Ocn,cl and 1SOO torr signals for PXe>80 torr

Conclusion intergrowth intergrowth intergrowth intergrowth

at at at at

short distance long distance long distance long distance

CONCLUSION These complementary studies indicate more exactly what experimental conditions should be used for the 129Xe-NMR technique, depending on the nature of t h e sample and the property studied. This technique is increasingly able to provide original results difficult to obtain by other methods.

1

2 3 4 5

T. Ito and J. Fraissard, NMR study of the interaction between xenon and zeolites A, X and Y, in: L.V.C. Rees (Ed.), Proc. 5th Int. Conf. on Zeolites, Naples, Italy, June 2-6, 1980, Heyden, London, 1980, pp. 510-515. J. Fraissard and T. Ito, 129Xe n.m.r. study of adsorbed xenon: A new method for studying zeolites and metal-zeolites, Zeolites, 8 (1988) 35036 1 , (and references therein). T. Ito and J. Fraissard, 129x8 nuclear magnetic resonance study of xenon adsorbed on zeolite NaY exchanged with alkali-metal and alkaline-earth cations, J. Chem. SOC.,Faraday Trans. 1, 83 (1987) 451-462. A. Gedeon, J.L. Bonardet, T. Ito and J. Fraissard, Application of 129Xe NMR to the study of Ni2+Y zeolites, J. Phys. Chem.. 93 (1989) 2563-2569. J. Demarquay and J. Fraissard, 129Xe NMR of xenon adsorbed on zeolites. Relationship between the chemical shift and the void space, Chem. Phys. Letters, 136 (1987) 314-318.

232

M.A. Springuel-Huet, J. Demarquay, T. Ito and J. Fraissard, 129Xe NMR of xenon adsorbed on zeolites: determination of the dimensions of the void space from the chemical shift 8(129Xe). Studies in surface science and catalysis, 37 (1988) 183-190. 7 T.T.P. Cheung, C.M. Fu and S. Wharry, 129Xe NMR of xenon adsorbed in Y zeolites at 144K. J. Phys. Chem. 92 (1988) 5170-5180. 8 Q. Chen and J. Fraissard, in preparation. 9 G. Debras, A. Gourgue, J.B. Nagy and G. De Clippeleir, Physico-chemical characterization of pentasil type materials. 111. High power solid state 27AI. 23Na and 29Si n.m.r. of precursors and calcined samples, Zeolites, 6 (1986) 161-168. 1 0 A. Auroux, P.C. Gravelle and J.C. Vedrine, Microcalorimetric study of the acidity of H-ZSM-5 zeolite, in: L.V.C. Rees (Ed.), Proc. 5th Int. Conf. on Zeolites, Naples, Italy, June 2-6, 1980, Heyden, London, 1980, pp. 433439. 1 1 C. Naccache and Y. Ben Taarit, Recent developments in catalysis by zeolites, in: L.V.C. Rees (Ed.), Proc. 5th Int. Conf. on Zeolites, Naples, Italy, June 2-6, 1980, Heyden, London, 1980, pp. 592-606. 6

233

G . Ohlmann et al. (Editors), Catalysis and Adsorption by Zeolites 0 1991 Elsevier Science PublishersB.V., Amsterdam

DIFFUSION OF HYDROCARBONS IN A AND X ZEOLITES AND SILICALITE DOUGLAS M. RUTHVEN, MLADEN EIC AND ZHIGE XU Department of Chemical Engineering, University of New Brunswick, Fredericton, NB Canada ABSTRACT Diffusion of a range of different hydrocarbons in A and X zeolites and silicalite has been studied by gravimetric, chromatographic and zero length column (ZLC) methods. The ZLC diffusivities for CHq and C2H6 in 5A and 13X at low temperatures are only slightly smaller than the corresponding NMR self diffusivities, extrapolated to zero concentration. Data for the Cg aromatics in silicalite show satisfactory conformity between gravimetric and ZLC measurements but the diffusivity values are higher than most previously reported values, with only about one order of magnitude difference between the isomers. The diffusional activation energies for p-xylene, ethylbenzene and o-xylene are almost identical. First results of a detailed study of diffusion of large aromatic molecules (dimethyl naphthalenes and tri-isopropyl benzene) in NaX zeolite are also reported. INTRODUCTION Because of the practical importance of zeolites as catalysts and selective adsorbents, coupled with their academic interest as examples of geometrically regular micropore systems, the study of zeolitic diffusion has attracted much attention from both experimentalists ad theoreticians.

A wide variety of

experimental techniques have been applied, including both 'microscopic' and 'macroscopic' methods.

The microscopic methods (principally NMR and neutron

scattering) yield direct information concerning the molecular mobility, and hence the self-diffusivity, whereas the macroscopic measurements generally yield the transport diffusivity (the quotient of the flux and the negative concentration gradient) although, by the use of isotopically labelled species the macroscopic methods can be adapted to measure self-diffusion. Since self-diffusion and diffusive transport are physically different phenomena, their coefficients are in general different. However, simple physical reasoning, as well as most detailed quantitative theories, suggest that the self-diffusivity and the transport diffusivity should converge in the low concentration limit.

This has been

confirmed experimentally for several systems although there are also some well documented discrepancies.( 1 * 2 ) A simple classification of the commonly used macroscopic experimental methods, according to the quantity measured and the nature of the measurement, is given in Table 1. The methods of category A3 account for the large majority of the reported data.

They have the advantage of experimental simplicity but

they are subject to the intrusion of extracrystalline resistances to heat and mass transfer,

so

straightforward.

the analysis and interpretation of the rate data is not always

234

Our own recent experimental studies, by chromatographic and gravimetric methods, have been directed to the study of three classes of zeolite system for which there is either little available data (large hydrocarbons in zeolite X ) or for which there are large discrepancies in the reported diffusivity values (light alkanes in A and X zeolites, aromatics in silicalite).

A summary of these

results is presented here. Table 1 Experimental Methods for Measurinn Transport Diffusion in Microporous Solids A.

A1

Transient Measurements

Following the evolution of the concentration profile in response to a change in external concentration. - Optical techniques - X-ray techniques - NMR spin mapping ESR spin mapping

-

A2

Flux measurements - Time lag measurements for diffusion through a membrane - Transient Wicke-Kallenbach

A3

Uptake rate measurements. (Following the variation of the mean sorbate concentration averaged over a particle). - Batch methods (gravimetric, volumetric, piezometric, frequency response) - Flow methods (chromatography, ZLC etc.)

B. Steady State Measurements B1

Steady state flux through a membrane Permeation - Wicke-Kallenbach

B2

Effectiveness factor in catalytic reaction.

-

EXPERIMENTAL METHODS Gravimetric Gravimetric uptake rate measurements were performed in a Cahn vacuum microbalance systems, using small zeolite samples (10-12 mg) with incremental pressure steps (to avoid the problems associated with the analysis of integral uptake rate data). To maximize heat transfer and minimize external mass transfer resistances, the zeolite sample was spread as thinly as possible over the balance pan. Where possible, measurements were repeated with different crystal sizes and different adsorbent sample configurations in order to confirm the dominance of intracrystalline resistance. In the experiments reported here the volume of the system was sufficiently large relative to the quantity of sorbent that the sorbate pressure, following the initial step, remained essentially constant. The simple expressions for the uptake curve for an infinite isothermal system were

235

therefore used to interpret the uptake curves(3): m

t = I - -6 -

m

I n2 n=l

Spherical Particles: (radius R)

2 2 2 -1 e -n II Dt/R n2

m m -t-- 1 - -8 I mm n2 n=O (2n+1)

-exp[-(Zn+l)

Parallel-sided Slab: (half-thickness 1 )

(1)

2 2 2 n Dt/4?. 1

(2)

ZLC In the zero length column (ZLC) meth~d(~-~) a small sample of zeolite crystals (- 2 mg) held between two sinter discs, is equilibrated with sorbate and then purged with helium (or argon) at a high flow rate. The hydrocarbon content of the effluent stream is monitored continuously using a chromatography detector (thermal conductivity or flame ionization).

The conditions of the experiment are

adjusted to ensure that the desorption rate is controlled by diffusion out of the crystals, rather than by the purge rate.

This may be achieved by using a

sufficiently high purge flow. The absence of significant external resistance may be confirmed by replicate measurements with He and Ar as the purge gases and the dominance of intracrystalline diffusion may be confirmed by varying the crystal size. The mathematical model used to interpret the ZLC desorption curves assumes a linear isothermal system with perfect mixing through the thin bed and adsorption equilibrium maintained at the crystal surface. For such a system the desorption curve (for a set of uniform spherical particles) is given by:

- pn2Dt/R2

m

- -- 2 L 0

I n=l [ p

(3) n

t~(L-1)1

where pn are the roots of the equation:

pn cot pn t L - 1 EVR L = 3(1-~)KDz

and

For large values of L, pn

+

-3

= 1

= 0

2 Purge Flowrate . . . Crystal Volume KD

(4) (5)

nv and Eq. 3 reduces, in the long time region to:

The diffusional time constant is readily found from the slope of a plot of log (c/co) vs t. If external film resistance is significant the analysis remains the same except that the expression for L (Eq. 5 ) is replaced by:

236

It is evident that external film resistance may modify the intercept of the semilog plot of c/co vs t but it has no effect on the limiting slope. For a parallel sided slab of half-thickness Q, the expressions corresponding to Eqs. 3-6 are: m

-= 0

where

2L

-a 2Dt/Q2 n

n=l z e [an2+L(L+1)1

are the roots of the equation: 2

a t a n a = L = EVQ ( 1-E)KZD n n For large L, an

(9)

(2ntl)n/2 and Eq. 8 reduces, in the long time region, to: 2 2 -r- 2 -n Dt/4Q +

co

(Ltl) e

(10)

DIFFUSION OF AROMATIC HYDROCARBONS IN SILICALITE Representative uptake curves showing the difference in kinetic behaviour between o-xylene, p-xylene and ethylbenzene in large crystals of silicalite (40x50~105pm) under comparable conditions, are shown in figure 1.

The curves

all conform well to the simple isothermal diffusion model for a parallel sided slab (Eq. Z), rather than for a sphere (Eq. 11, since the intercepts approximate 8/n2

(rather than 6/n2).

This suggests that

diffusion probably

occurs

predominantly through the straight channels of the silicalite pore system (which run between the large parallel faces of the crystals).

This conclusion has been

confirmed by measurements with different crystal sizes which show the time constant varies with the square of the crystal thickness, rather than with the mean equivalent spherical radius. The diffusivities 'for ethylbenzene are almost independent of concentration in the low concentration region but the diffusivities for p-xylene show a stronger trend (increasing with concentration) presumably reflecting the more favourable form of the equilibrium isotherm. Only limited uptake measurements were performed with o-xylene since, as a result of the low diffusivity, these experiments are tedious and time consuming. Diffusion of the same three sorbates (as well as benzene for which the data have been previously reported was also studied by the ZLC method. Representative response curves are shown in figure 2.

The coincidence between the curves

measured, under similar conditions, with He and Ar vs purge gases confirms the absence of any significant extracrystalline resistance to mass transfer. In view of the evidence from the uptake measurements the parallel sided slab model (Eq. 8) was used to derive the time constants which are summarized in Table 2.

1.0

8

E

'- 0.1 E

-

8

1

0.01

0

Figure 1

\ PX,

p=0;0.12

To;r

p=0-0.103 Torr

,

,

,

50

60

~/12.~~10-3

10

20

30 LO Time (rnins)

Gravimetric uptake curves at 125OC ( %

70 =

20 vm).

1.9 0-

,cc.-c.

I

I

I

I

2.3

2.5

2.7

2.9

3.1

1 0 ~ 1 ~

0

I.L

1

2.1

Figure 3

Temperature dependence of limiting diffusivities for benzene (x), p-xylene ( 0 ) . ethylbenzene ( ) and 0-xylene ( A ) . Filled symbols, gravimetric,open symbols ZLC.

ta w -a

238

Table 2 Summary of ZLC Diffusivity Data for Aromatic Hydrocarbons in Silicalite Crystals Sorbate

Temp. (Deg. C)

Benzene

50 75 100 100 150

p - Xylene

Ethylbenzene

o-Xylene

Pu e Flow (cm'gSTP/min)

4 4 4 2 4

L

DX$O9-,) (cm .s

227 105 91 46 87

1.0 1.8 3.1 2.7 11.0 2.8 2.9 4.1 11.9 11.8 43

80 80 100 150 150 (Ar) 200

60 100 60 60 60 60

11 23 37 55.6 40.0 51.3

70 70 100 150 150 (Ar) 200

60 100 60 60 60

19.5 25 28.5 66.7 55.5 91.0

0.8 0.92 2.0 7.3 5.6 29

30 30 15 15

323 95 476 571

0.36 1.24 2.9 7.84

100 150 200 250

60

E W/mol)

27.0

30

30

33

Purge was He except where indicated as Ar. In ref.(7) the ZLC data for benzene were interpreted according to the spherical particle model. These values have been recalculated based on the parallel sided slab model for consistency with the xylene data. Figure 3 shows a comparison between the ZLC data and the gravimetric values (extrapolated to zero concentration). satisfactory.

It is evident that the agreement is

There is little difference in diffusivity between benzene and

p-xylene and the diffusional activation energies are all similar (- 30 kJ/mol). In contrast with the results of earlier studies, the present data show no evidence of any significant deviation from the simple diffusion model and the differences in diffusivity between the C8 isomers are much smaller than has been previously rep~rted.(~*~)The similarity in the activation energies implies that the difference in diffusivity cannot be attributed to differences in repulsive interaction energies resulting from diameters.

the

difference in

critical molecular

239 DIFFUSION OF METHANE AND ETHANE IN ZEOLITE A AND X The ZLC method provides one of the more useful techniques for studying rapid diffusion processes since, by careful attention to detail it is possible to reduce the dead volume of the experimental system to a fraction of a millilitue and hence to reduce the response time essentially to that of the detector.

By

the addition of a refrigeration system the operating temperature range has been extended to -1OO"C, thus making it possible to measure diffusion of the light alkanes (CH4 and C2H6) in 5A and 13X zeolites. The results of such measurements are summarized in figures 4 and 5 and Table 3 where comparative NMR selfdiffusivity data(1°-12), extrapolated to zero concentration, are also shown. For both 5A and 13X the measurements performed with two different crystal sizes show good agreement, thus providing convincing evidence of intracrystalline diffusion control. In contrast to earlier ZLC data for the higher paraffins in NaX(6)

which

show large discrepancies with the NMR self-diffusivities, the agreement of the present data for CHq and C2Hg in both 13X and 5A can be considered satisfactory. A discrepancy of this magnitude is easily accounted for by the natural tendency of an NMR self-diffusion measurement to err on the high side (as a result of relaxation effects) and for a sorption rate measurement to err on the low side because of the emphasis on the tail of the desorption curve. The activation energies also show qualitative agreement and the surprising observation that the activation energy for C2H6 in 13X is higher than in 5A is confirmed by both NMR and ZLC data. However, the c2%-13X

system at the higher temperatures is close

to the limit of the ZLC technique (half time less than 1 sec.) so these data and consequently the activation energy for this system are subject to a large margin of error. Table 3 Comparison of NMR and ZLC Diffusivity Data for CH4 and C2Hg in 5A and 13X Zeolites ZLC Data Do at 200K system CH4-5A C2H6-5A C2H6-13x

( cm2.s-1)

10-6 31c10-~ 10-5

(kJ/mole)

(cm2. s-1)

E

(kJ/mole)

3.7

4~10-~

6.3

5.9

1.4~10-~

6.3

9.6

2.5 ~ 1 0 - ~

(14.6)

ZLC data are at zero concentration; NMR data extrapolated to zero from data at higher concentrations.

240

:.

I .

; * i

1 .I;

5A

( 55 pm)

0 13X

( 1 0 0 pm)

A 13x

( 50 pm)

0

'.4- '..*

1 I ~

1

ClCO

.01

,001

0

10

t

Figure 4

(I)

ZLC desorption curves for C2H6 in 5A and 13X crystals at 183K.

4

Figure 5

30

20

5

6

4 1 0 ~ 1 (~ K1)

5

6

Temperature dependence of limiting diffusivities (ZLC) and NMR self-diffusivities for CH4 and C 2 h j in 5A and 13X crystals.

241

TRANSITION STATE THEORY Because of its simple cubic structure 5A zeolite provides an ideal model system for the application of transition state theory. This was recognized many years ago(13*14)

but the available diffusivity data used

in the earlier

comparisons were derived from measurements with small commercial 5A crystals. More recent evidence suggests that a substantial fraction of the windows in these samples were probably blocked thus invalidating the quantitative comparisons between theory and experiment. It is therefore appropriate to reconsider such a comparison on the basis of the more reliable diffusivity data now available. According to transition state theory, in the low concentration limit where self- and transport diffusivities become identical, the diffusivity is given by:

*

where uz and uz represent the potential energies of a molecule within the cage (equilibrium state) and within the window (transition state). To avoid the difficulties associated with the evaluation of the partition function for a molecule within the cage (f,)

one may make use of the equilibrium relationship:

-AHo/kT fZ (ug-uz)/kT =K = K e kT'Fe B

(12)

where ug is the potential energy for the vapour phase (zero), thus obtaining: .b

or comparing with the Arrhenius form (D = , D e-E/RT):

*

D = -6 2 m

where Vo

hKo

. -fZ f' g

.

'

*

- = u - u - A H o

R

vO

E -

k

k

is simply the difference in potential between the transition and

equilibrium states.

fi,

the partition function per unit volume of the gaseous

species may be expressed as the product of the internal, rotational and translational contributions:

fi

so,

(15) = fCrans * frot * fint if it is assumed that the molecule in the transition state retains the same

internal and rotational freedom as in the equilibrium state and there is no significant additional freedom of motion in the plane of the window:

* fZ =

frot

*

fint

* . -fZ = 7 1 ;

.

f,

g

trans

D, =

ti2

(16)

;ran,

Table 4

N

Absolute Rate Theory: Coqmrism of Predicted a d IIc.8ured Oiffusivities i n

System

Crystals

6

K x10

(molecule/ cage.Torr)

mmmtaic

tqs'

(a.s

)

2 -1

(an .s

Method

1.06

14

4.4

1.2

3.7

24.2

grav.

3.2 rn ( L i d )

1.26

17.7

1.2

0.12

10

33.8

grw.

Xe-5A

20-50 pn

1.29

22.5

1.0

2.8

0.36

(1.0)

WR PFG

cn -4A

36 Im, 7.3 In

1.5

18

12

1

12

24.2

grw.

CH4-5A

20-50

c

1.9

10.8

10

45

0.22

3.8

mR

(15) Derrah

PFG

CF4-5A

3.6 p (Lindc)

0.38

24.7

3.7

2.5

1.5

3.8

grav.

CF -5A

27.5 p, 55 p

1.49

21-7

0.93

19

0.05

27.6

grav.

0 -4A

3.2 p (Lindc)

1.23

13.4

5.2

6.6

0.8

18.8

grsv.

N -4A

36 P 7.3 m

1.o

17.6

7.0

5.0

1.6

24.2

grw.

55 P, 27.5

0.35

21 .o

41

42

1.o

10.0

UC

36 m, 7.3 m

0.019

47.5

211

9.0

23

grw.

1.39

24.7

5.0

4.8

34.3

grav.

Y'ucel

5.9

WR PFG

(11) Caro

6.7

ZLC

xu

grav.

Yucel

2 W -5A 2 u) -4A

2

c n

-u n m.

2 6

7.3 P

23.4 1.oc

17 1.7 10 20-50 ~n 20.0 0.4 2 6 7.3 17 20.8 2.3 atamic C H -5A 27.5. 55 P 0.4 2 6 3.0 1.o 3.1 nC H -5A 27.5 pin, 55 42.6 0.86 4 10 3.0 3.0 42.6 1.o nC tl -5A 34 p, 7.3 P 0.86 4 10 *The nargin of u u e n a i n t y in the experirrntal a c t i v a t i m energies i s probably about ZkJ/nole. This corrcopmS t o a factor of 2-3 i n the valucs dcrived for 0., Ibpblished experimental data obtained at U.N.B. Experimental 0, values are frm taperatwe depenance of 0 or D o woo' ply-

Ref.

)

3.2 p (Linde)

2

linear triatanic

2 -1

Kr-4A

4

diataic

(kJ/nole)

Em.*

6 ODX1O

oDmlo

Ar-4A

4

'Spherical

Theorg

Ano

b

4A and 5A Zeolites

C H -5A

19.2 18.8

(17)

4.

ZLC

Eic

(19)

(6)

where fkrans = (21nnlcT/h~)~/~e is the translational partition function (per unit volume) for the free gas phase. It is in principle possible to estimate the potential energies of molecules in the cage and windows from theoretical potential calculations. Such estimates are, however, very sensitive to the precise values assumed for the molecular radii since, particularly for the molecule in the window, the attractive and repulsive forces are of similar magnitude.

In comparing experimental data with

the theory it is therefore more satisfactory to consider only the pre-exponential factors. Such a comparison is shown, for several simple sorbates, in Table 4 . In view of the obvious approximations in the theory and the uncertainty in the experimental values of D,,

the agreement between theory and experiment is

remarkable. The ratio (D,)theory/(Dm)expt is, for most sorbates, close to unity. Values less than unity can be rationalized by restricted rotation of the transition state and values greater than unity can be explained by

some

contribution from degrees of freedom in the plane of the window.

Evidently, for most sorbates, these two effects must either be small or they must compensate. The close agreement for polyatomic molecules such as n-butane seems at first sight surprising since one might anticipate significant restriction of rotation. However, the butane molecule is probably not in the linear conformation and even though it probably does not have complete rotational freedom in the window, the additional degrees of freedom (torsional oscillations about the centre of mass) may very well compensate fully for restriction of the three dimensional rotation. The most significant discrepancy is for C02 for which the predicted value of D, is substantial greater than the measured value.

Since the free rotation of a

long linear molecule such as CO2 is bound to be severely reduced in the transition state this deviation is clearly understandable. In the original application of transition state theory to diffusion in 5A zeolite, attempts were made to estimate the effect of restricted rotation and the contribution from degrees of freedom in the plane of the zeolite window. The broader perspective which is now possible as a result of the improved data base suggests that such detailed analysis was probably not justified. The simplified treatment leading to Eq. 16 appears to be quite adequate for most small molecules while Eq. 14 may provide a useful basis for more refined calculations. DIFFUSION OF LARGE AROMATIC MOLECULES IN ZEOLITE X Results of an experimental study of diffusion of large molecules in 13X zeolite are summarized in figures 6 and 7.

The data of tri-isopropyl benzene at

200°C show exactly the same pattern of behaviour as has been commonly observed for smaller molecules in 5A and for the c8 aromatics in 13X.

The strong

244

-

1,3,5 Tri Isopropyl Benzene NaX

N

Y

10

0

30

20

60

50

40

Concent rot ion (mg/g) Concentration dependence of diffusivity (D) and corrected diffusivity (Do) for tri-IPB in 13X crystals at 473K.

Figure 6

10-12

1.5

'

1

1.6

'

1

1.7

'

Gravirnetric Data,

I

1.8

I

' ' ' 1.9

2.0

' ' '

2.1

2.2

1 0 ~ 1 (K-') ~ Figure 7

Temperature dependence of corrected (limiting) diffusivities for dimethyl naphthalene isomers in 13X zeolite.

245 concentration dependence appears to be due entirely to non-linearity of the equilibrium isotherm and corrected diffusivities, calculated according to:

D = Do (dllnp/dllnc) are essentially constant.

(17)

The data obtained by gravimetric and chromatographic methods (for the dimethyl naphthalenes) are quite consistent. Despite the differences in critical molecular diameter (Table 5 ) there is evidently little difference in either diffusivity or activation energy between the DMN isomers. These data thus support the conclusion drawn previously from the similarity in the diffusivities (in NaX) between the xylene isomers, that the diffusional activation energy is determined by the kinetic diameter of the freely rotating molecule rather than by the critical diameter (the diameter of the smallest circumscribing cylinder). Table 5 Critical Molecular Diameters of Some Aromatics Molecules Diameter A 6.9 Benzene 6.9 Toluene 6.9 p-Xylene 7.3 o-Xylene 7.3 Naphthalene 7.3 2-6,2-7 DMN 7.9 1-2,l-5,1-6 DMN 8.6 1-3 DMN 9.1 1-4 DMN 8.4 Mesitylene 9.3 Tri-isopropylbenzene Notation c/co ratio of outlet concentration to initial concentration in ZLC experiment diffusivity (intracrystalline) (cm2.s-1)

D

or corrected diffusivity (Eq. 17) (cm2.s-l) (cm2. s-1)

Do Dm

limiting value of D (as c + molecule diffusivity

D

self diffusivity (intracrystalline)

E

activation energy

fz,f2

0 )

(cm2.s-1) (J/mole.deg)

partition functions for adsorbed molecule in equilibrium and transition states

frot,fint rotational and internal contributions to partition function ( cm-3 partition function per unit volume for free gas fi (erg. sec) h Planck's constant (erg/deg.) k Boltzmann constant pre-exponential factor

K, KO Henry's Law constant II

half-thickness of crystal

L

defined by Eqs. 5 , 7 or 9

m

mass of a molecule

(moleucles/cage.dyne/cm2) (cm)

246

mt/m, fractional approach to equilibrium p sorbate partial pressure R

equivalent radius of crystal

T

absolute temperature

uz-ug difference in potential between transition state and free gas

Vo

defined by Eq. 14

z

thickness of adsorbent bed (ZLC)

Sh

Sherwood Number

%,Bn

roots of Eqs. 9 and 4

E

voidage of adsorbent bed (ZLC)

6

lattice parameter heat of adsorption at zero coverage

-AHo

Acknowledgement:

(cm) (erg/molecule)

We are grateful to Dr. David Hayhurst for providing us with

samples of large silicalite crystals. Financial support was provided, in part, by the Imperial Oil Company of Canada. References 1. 2.

J. Karger and D.M. Ruthven, Zeolites 2, 267 (1989). J. Karger and D.M. Ruthven, Diffusion in Zeolites, John Wiley, New York

3.

J. Crank, Mathematics of Diffusion, Oxford University Press, London (1956). M. Eic and D.M. Ruthven, Zeolites 8, 40 (1988). M. Eic, M. Goddard and D.M. Ruthven, Zeolites 8, 327 (1988). M. Eic and D.M. Ruthven, Zeolites g, 472 (1988). M. Eic and D.M. Ruthven, 8th Internat. Zeolite. Conf., Amsterdam, July 1989, Proceedings, p. 897, P.A. Jacobs and R.A. van Santen eds., Elsevier, Amsterdam (1989). P. Wu, A. Debebe and Y.H. Ma, Zeolites 2, 118 (1983). K. Beschmann. G.T. Kokotailo and L. Riekert, Chem. Eng. Process 22, 223

(1991). 4.

5. 6. 7. 8. 9. 10. 11.

12. 13.

14. 15. 16. 17. 18. 19.

(1987). J. Caro, J. Karger, H. Pfeifer and R. Schollner, Z. Phys. Chem. Leipzig 256, 698 (1975). J. Caro, J. Karger, G. Finger, H. Pfeifer and R. Schollner, Z. Phys. Chem. Leipzig 257, 903 (1976). J. Karger, H. Pfeifer, M. Rauscher and A. Walter, J. Chem. SOC. Faraday Trans I, 76, 717 (1980). D.M. Ruthven and R.I. Derrah, J. Chem. SOC. Faraday Trans I. 8,2332 (1972). J. Karger, H. Pfeifer and R. Haberlandt, J. Chem. SOC. Faraday Trans I, 76, 1569 (1980). D.M. Ruthven and R.I. Derrah, J. Chem. SOC. Faraday Trans I, 2. 2031 (1975). J. Karger, H. Pfeifer, F. Stallmach and H. Spindler, Zeolites - in press. H. Yucel and D.M. Ruthven, J. Chem. SOC. Faraday Trans I, 76, 60 (1980). H. Yucel and D.M. Ruthven, J. Chem. SOC. Faraday Trans I, 76, 71 (1980). H. Yucel and D.M. Ruthven, J. Colloid Interface Sci. 2,186 (1980).

241

G. Ohlmann et al. (Editors), Catalysis and Adsorption by Zeolites 0 1991 Elsevier Science Publishers B.V., Amsterdam

ATLAS OF ZEOLITE STRUCTURE TYPES: PAST- PRESENT- FUTURE

W.M. MElER Institute of Crystallography, ETH Zurich, 8092 Zurich, Switzerland

ABSTRACT The 'Atlas of Zeolite Structure Types' is a periodically updated data compilation of observed zeolite-type frameworks. Apart from being useful as a work of reference it reflects the progress in zeolite structural chemistry. This paper describes some uses of Atlas data including framework densities, pore diameter values, loop configurations and fault planes. In addition, some recurrent misconceptions are discussed.

INTRODUCTION For over two decades the 'Atlas of Zeolite Structure Types' (refs. 1,2) and its forerunner (ref. 3) have reflected the enormous progress made in the elucidation of the framework structures of zeolites and zeolite-like materials. Fig. 1 shows that the current growth rate of over 6 new structure types per year is quite unprecedented. What looks like an exponential growth is due to two developments: (a) the recent advances in the methods of crystal structure analysis, in particular powder diffraction methods (refs. 4,5), and (b) the greatly expanded range of compositions of zeolite-type molecular sieves explored in more recent years (refs. 6-8).

I

"I 53

Fig. 1.

61

Development of the number of known zeolite structure types over the years.

248

The available structural information on zeolite-type materials is very substantial indeed. It raises the need for the Atlas and the other data compilations published on behalf of the 1ZA Structure Commission (refs. 9,lO). The principal aims of the Atlas project have been: (a) to provide a reference work (both for crystallographers and non-crystallographers) based on data which has been subjected to critical evaluation (normally published), (b) to promote some desirable uniformity in recording such data, (c) to fill in gaps in available data, and (d) to facilitate studies and analyses of data. The following account describes some examples of uses of the Atlas which were not dealt with previously (ref. 11).

SILICATE AND PHOSPHATE MICROPOROUS MATERIALS From a chemical point of view zeolite-type silicates and phosphates apparently constitute two distinctive categories of microporous materials. This appears to be particularly evident in synthesis. Table I (which is based on the isotypes listed in the Atlas) shows, however, that there are three, rather than two distinct groups of framework types. Apart from those associated with silicates and phosphates there is a sizeable group of structure types which have been found to occur both in silicates and phosphates. Comparative in-depth investigations of isotypes in this latter group in particular offers new insight into synthesis/structure relationships. An example is the structure refinement of SSZ-24, the silica analogue of AIP04-5 (ref. 12). Table I also shows that while the present structure-orientedactivity is greatest in the field of phosphates, the silicates still make up the majority of zeolite structure types. The structure types which have been approved by the IZA Structure Commission (SC) since the last Atlas appeared in 1988 have been marked by a plus (+) in the Table. In addition to these, seven structure types (listed in Table I) are under consideration for clearance by the SC at present. A current listing of the zeolite structure type codes can be found in the Appendix.

249

TABLE 1

Microporous zeolite-type materials excluding interrupted frameworks

Silicates

Both silicates and phosphates more abundant as a silicate

ABW AFG BIK BOG + BRE CAS + DAC DDR DOH EAB ED1 EPI EUO FER GME GOO

MON + HEU MOR KFI JBW + MTN LAU M U LIO MTW NAT LOS LOV NON LTL OFF LTN PAU MAZ PHI ME1 + SGT MEL STI MEP THO MER TON MFI YUG MFS + (a) (b)

(a) Beta (b) ZSM-20 (c) AIP04-8 (d) MAPO-39

ANA CAN CHA ERI FAU

Phosphates

first established as a phosphate AFI AST BPH +

GIS LEV LTA RHO SOD

AEL AFS A N + AFY APC APD AllATV + Aww + VFI + (c) (d) );: (g1

(e) AIP04-18 (f) AlP04-31 (9) AIP04-41

+

not included in ref. 1

POROSITY AND FRAMEWORK DENSITY

The pore volume of zeolite-type frameworks can be expressed in terms of the framework density (FD), which is defined as the number of tetrahedral atoms per nm3 and is listed in the Atlas for each structure type. The distribution of FD-values as a function of the smallest ring(s) in the network has been shown to be of considerable interest (refs. 13,14). Fig. 2 is an updated version of the earlier diagram prepared in

250

1988. The gap defining the boundary between microporous and dense networks has gained further support by the new data, and the postulated range for zeolite-type materials has been confirmed. A particularly noteworthy addition is the data point relating to ZSM-18 (MEI), the second framework containing 3-rings, which was reported only recently (ref. 15). Earlier, only lovdarite, a beryllosilicate, was known to contain 3-rings. The new structure type ME1 is very significant for it demonstrates that silica can also form tetrahedral nets containing 3-rings (without Be being needed).

T-atoms / nm3

size of smallest rings

Fig. 2.

Distribution of framework densities (FD) as a function of the smallest ring(s) in the network. Updated version of diagram in ref. 13. Data points refer to dense (+), previous (*) and new (0)zeolite-type structures, and low density tetrahedral networks (O).

251

This bodes well for the synthesis of low-density materials based on 3-ring structures in the silica field. The FD is obviously related to the pore volume but does not reflect the size of the pore openings. Customary values are frequently given or assumed for the free diameters of channels with 8-, 10- and 12-ring apertures. It should be clear from Fig. 3 (which is also based on Atlas data) that the crystallographic free diameters for the various types of apertures fall into a wide range. There is even a considerable overlap of the respective ranges for 8-, 10- and 12-ring openings. Hence, sorption properties have to be interpreted with caution when predicting the type of ring openings in structurally unknown materials.

...._ I...... li

18-ring aperture

........ i........ l i

14-ring

#

12-ring

1O-ring 9-ring

[

I 15

J

........... ........... 1 2 :

8-ring

Fig. 3.

Range of crystallographicfree diameters for different ring openings. The arrows indicate the customary values assigned to 8-, 10- and 12-ring apertures. The number of pertinent structure types is given in each bar.

252

The customary values for 8-, 10- and 12-rings (applying to LTA-, MFI- and FAU-type materials) are marked by arrows in Fig. 3. This Figure also includes relevant data for VPI-5 with 18-ring channels (refs. 16-18) and for AIP04-8 with 14-ring channels (ref. 19). These are the most open framework structures to date, and both have unidimensional channel systems.

SOME MISCONCEPTIONS Among the most widespread misconception is the belief that large unit cell constants are indicative of large apertures. Fig. 4 should make it clear that there is no such correlation, no matter whether minimum or maximum unit cell constants are considered.

LTN-

LTN-

-

PAU-

-

AFS-

FAU-

Fig. 4.

Minimum (a) and maximum (b) unit cell constants compared to largest ring openings.

-

253

A more serious illusion is the belief that close resemblance between powder diffraction patterns is sufficient to assign a structure type. This may hold in some special cases. In general, however, the assignment or the establishing of a structure type based on the similarity of XRD powder patterns is full of pitfalls. This has become evident in a number of instances. For example, PHI and GIS were confused for well over a decade, and later EAB and ERI were repeatedly mixed up. In one instance, even the Structure Commission assigned a code to a structure type which turned out to be in error (ref. 20). The code ATF of the erroneous structure type of AIP04-25 had to be discredited, and according to the rules of the SC is not to be used again. The correct structure has been assigned the code ATV. DLS is certainly an indispensable tool for simulating and evaluating possible framework structures but a reasonable agreement between an experimental XRD pattern and one based on DLS atom coordinates does not give enough evidence for the correctness of a framework topology. Even in cases in which this qualitative approach led to basically correct framework topologies subsequent refinements have invariably revealed significant differences. A good example of this kind is ZSM-12 (refs. 21,22). In the originally proposed structure the channel dimensions were 5.5 x 5.9 A but structure refinement later on showed these to be 5.6 x 7.7 A.

SIGNIFICANCE OF LOOP CONFIGURATIONS AND FAULT PLANES The utility and significance of loop configurations (LC) has been demonstrated before (ref. 11). In a study on the evaluation of hypothetical zeolite frameworks, Brunner (ref. 23) determined the relative abundance of various possible LC. One of the more frequently occurring LC consists of a pair of edge-sharing 4-Yktgs. This is shown in Fig. 5 together with two pertinent conformations. While the 'cis' conformation (i) had been observed in 22 known zeolite frameworks, the 'trans' conformation (ii) had not been recorded but was noted to occur in the proposed framework for VPI-5. Brunner's findings cast some doubt on the correctness of the VPI-5 framework structure at an early stage. Meanwhile, it has been found by extensive structure refinements and NMR studies that the VPI-5 framework is not purely tetrahedral (ref. 18). Instead, it is 4-connected (containing some octahedral At) and it even appears that a strictly tetrahedral network of this type is unlikely to be obtainable.

254

Fig. 5.

A common loop configuration and associated conformation (from Brunner, ref. 23).

For a considerable proportion of the frameworks covered in the Atlas, fault planes have been listed. This allows predictions at a glance as to the likelihood of stacking faults, twinning, and disorder. These possible defects are of considerable importance in the characertistics of these materials and their catalytic applications.

OUTLOOK The number of established zeolite-type framework configurations will soon approach one hundred. Compilations, which are extensive enough and which contain carefully examined data, can be expected to be of increasing interest to both theoreticians and practitioners in the field. The need for computer-based data files can be clearly foreseen. The available printed versions of the compilations issued by the SC are already based on computer files to a large extent . In the future it is to be expected that the huge amount of structural information, once prepared for efficient use in the form of well-documented data bases combined with good search routines, can be used as a source for new insights in zeolite chemistty. The first step in this direction has already been made (ref. 24). One important goal is the ability to predict which of the enormous number of hypothetical networks can be synthesized and in what system(s).

ACKNOWLEDGEMENTS The author thanks Dr. Lynne B. McCusker and other members of his group for helpful discussions. Continued support by the Swiss National Science Foundation (grant 20-25256.88 at present) is also gratefully acknowledged.

255

Appendix List of zeolite structure type codes approved by the IZA Structure Commission (on behalf of IUPAC) up to June 1990 Structure types not included in the 1988 Atlas are marked by a plus (+)

ABW AEL AFG AFI AFS AFT AFY ANA APC APD AST ATT ATV AWW BIK BOG BPH BRE CAN CAS CHA -CHI DAC DDR DOH EAB ED1 EPI ERI EUO FAU FER GIS GME GOO HEU JBW

+

+ + +

+ +

+

Li-A(BW) AIP04-11 Afghanite AIP04-5 MAPSO-46 AIP04-52 COAPO-50 Analcime AIPO4-C AIPO4-D AIP04-16 AIPO4-12-TAMU AIP04-25 (revised structure) AIP04-22 Bikitaite Boggsite Beryllophosphate-H Brewsterite Cancrinite Cs-Aluminosilicate (Araki) Chabazite Chiavennite Dachiardite Deca-Dodecasil 3R Dodecasil 1H TMA-E(AB) Edingtonite Epistilbite Erionite EU-1 Faujasite Ferrierite Gismondine Gmelinite Goosecreekite Heulandite Na-J(BW)

KFI LAU LEV LIO LOS LOV LTA LTL LTN MAZ ME1 MEL MEP MER MFI MFS MON MOR MTN M-n

+

+ +

MMI NAT NON OFF - PAR PAU PHI RHO - ROG SGT SOD STI THO TON VFI -WEN YUG

+

ZK-5 Laumo ntite Levyne Liottite Losod Lovdarite Linde Type A Linde Type L Linde Type N Mazzite ZSM-18 ZSM-11 Melanophlogite Merlinoite ZSM-5 ZSM-57 Montesommaite Mordenite ZSM-39 ZSM-23 ZSM-12 Natrolite Nonasil Offretite Partheite Paulingite Phillipsite Rho Roggianite Sigma-2 Sodalite Stilbite Thomsonite Theta-1 VPI-5 Wenkite Yugawaralite

256

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

W.M. Meier and D.H. Olson, Atlas of Zeolite Structure Types (IZA Structure Commission, 1978). W.M. Meier and D.H. Olson, Atlas of Zeolite Structure Types, Second extended edition, Buttetworths, 1988. W.M. Meier and D.H. Olson, Adv. Chem. Series, 101 (1970) 155-168. Ch. Baerlocher, Zeolites, 6 (1986) 325-333. L.B. McCusker, Acta Cryst. (in preparation). S.T. Wilson, B.M. Lok, C.A. Messina, T.R. Cannan and E.M. Flanigen, J. Am. Chem. SOC.,104 (1982) 1146-1147. 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) 6092-6093. E.M. Flanigen, B.M. Lok, R.L. Patton and S.T. Wilson, Proc. 7th Int. Zeolite Conf., Tokyo (Elsevier, 1986) 103-112. W.J. Mortier, Compilation of Extra-Framework Sites in Zeolites. Butterworths (1982), pp. 67. R. von Ballmoos and J.B. Higgins, Collection of Simulated XRD Powder Patterns for Zeolites. Zeolite Special Issue 10 (1990) 313s-514s. W.M. Meier, Studies in Surface Science and Catalysis, 49 (1989) 691-699. R. Bialek, W.M. Meier, M. Davis and M.J. Annen, Zeolites (submitted for publication). G.O. Brunner and W.M. Meier, Nature, 337 (1989) 146-147. M.E. Davis, Nature, 337 (1989) 117. S.L. Lawton and W.J. Rohrbaugh, Science, 247 (1990) 1319-1322. C.E. Crowder, J.M. Garces and M.E. Davis, Advances in X-ray Analysis, 32 (1988) 507-514. J.W. Richardson, Jr., J.V. Smith and J.J. Pluth, J. Phys. Chem., 93 (1989) 8212-8219. L.B. McCusker, Ch. Baerlocher, E. Jahn and M. Buelow, Zeolites (submitted for publication). R.M. Dessau, J.L. Schlenker and J.B. Higgins, Zeolites, 10 (1990) 522-524. J.W. Richardson, Jr., J.V. Smith and J.J. Pluth, J. Phys. Chem., 94 (1990) in press. R.B. LaPierre, A.C. Rohrman, J.L. Schlenker, J.D. Wood, M.K. Rubin and W.J. Rohrbaugh, Zeolites, 5 (1985) 346-348. C.A. Fyfe, H. Gies, G.T. Kokotailo, B. Marlerand D.E. Cox, J. Phys. Chem. 94 (1990) 3718-3721. G.O. Brunner, Zeolites, 10 (1990) 612-614. J.M. Newsam and M.M. Treacy, Zeofile, in preparation.


251

ZEOLITES AS MEMBRANES : THE ROLE OF THE GAS-CRYSTAL INTERFACE

Richard M. Barrer Chemistry Department, Imperial College, London, SW7 2AZ, United Kingdom

ABSTRACT Steady flow through a porous cystal membrane is associated with concentration distributions across the gas/crystal interfaces which cannot represent true equilibrium. Steady state distributions have been modelled from mass balance equations for a membrane where a fraction of the flow enters the crystal directly from the gas phase and the remainder enters via an externally adsorbed layer. The same fractions are assumed for the nett flow through the exit surface. Special cases of the general equations investigated numerically or otherwise include : the extent of departures from equilibrium across interfaces; the ideal flow when these departures tend to zero; the ratios of ideal to measured flows; and ratios of true intracrystalline diffusivities to apparent diffusivities obtained from steady flows by assuming equilibrium across interfaces. In fast sorptions and desorptions non-equilibrium distributions across the gas/solid interface may also result in differences between true intracrystalline diffusivities and those obtained by incorrectly assuming equilibrium. INTRODUCTION Zeolite membranes provide in principle a means of continuous scparation based on differences in the shapes and sizes of molecules. If they contain catalytic centres they can also be envisaged as membrane reactors. In steady flow, independently of any model, more molecules of diffusant enter the crystal than leave at the ingoing surface. T h a c imbalances require that concentrations just within the crystal cannot have their equilibrium values (ref. 1) at either surface, so that it is of interest to examine their scale. The imbalances have recently been modelled quantitatively for gases obeying Langmuir's isotherm when all molecules enter the crystal via an externally adsorbed layer (ref. 2). In this paper the model will be extended to include, for very wide windows, possible entry directly from the gas phase simultaneously with entry from the externally adsorbed layer. Also considered is the effect of imbalance betwecn forward and reverse flows across the gas/crystal interface in transient state sorption and desorption kinetics. Aspects of surface resistances have already formed the bases of other papers (refs. 3-5). The treatment for a single crystal applies also to membrana consisting of a monolayer of porous crystals on a macroporous support, in which gaps between porous crystals in the monolayer are filled by a non-permeable medium which adheres permanently to the sides of the crystals, but leaves their top and bottom surfaces free of the adhesive. In another kind of membrane the porous crystals comprise a discontinuous phase embedded in a continuum. Such membranes will not be considered here.

258

(a) CRYSTAL

GAS

el x=

%

0

DISTANCE

-

GAS

%-I

9n x=l

(b) GAS

CRYSTAL

GAS

AE

x=o

DISTANCE

-

x =I

Fig. I . (a) The energydistanre curve for entry to the rrystal from an externally adsorlml layer. ( b ) The energv4istance curve alien entry to tlic crystal takes plare directly froin t l w gas phase. C C denote constant Concentrations in the gas phase at x = 0 and x = Ircspertively. go’ gl Oo , @,denote fractions of external sites of type A occupied at x = 0 and x = 1 respectively. O1. On denote fractions of internal sites occupied a t x = 0 and x = I respcctively.

.

E, E, denote activation energies for entry from external sites and for intracrystalline diffusioii rcsprtively. AEs , AE are energies of dcsorption from sites of type A and from intrarrystallinr sites respcctivrly.

259

MODEL Sorption is assumed to obey Langmuir's isotherm for which diffusivity is independent of concentration (ref. 6). A single crystal membrane, bounded by planes x = 0 and x = 1 separates a constant gas phase concentration C at x = 1, with C > C Inside the crystal activatcd

d

go

gr

diffusion occurs involving energy harriers of height El. Molecules of diffusant may enter or leave the crystals via an externally adsorbed layer, for which the cncrgy of desorption is A E ~ . Fig. l a shows the energy - distancc configuration, the energy levels and the nomenclaturc involvcd. If the windows are widc cnough some moleculcs may pass directly from the gas phase to intracrystalline sites, for which the energy - distance curves and nomenclature are shown in Fig. lb. Of the nett flow J through unit area of surface the fraction F2 J occurs as in Fig. l a and a fraction F1 J as in Fig. lb, wlicre (F1

+ F2) = 1.

A t and around channel mouths at x = 0 there are sites of type A from which the molecules comprising F2 J enter the crystal and possibly sites of type I3 from which they do not. There is however constant exchange of molecules between the two kinds of site and between each kind ol site and the gas phase. In steady flow there is nett transport f2 J from the gas to the B-typc sites and hcncc from B- to A-type sitcs. There is also nctt transport of fl J from the gas phase to sites of type A and therefore (fl

+ f2) J = F2 J from these sitcs to the interior of the crystal.

In addition there is nett transport F1 J from the gas phase directly into the crystal. These mechanisms are shown schematically in Fig. 2. Exit at x =I Sit,esesp

Entrv at x = 0 Gas Phase

F1

7

/Sites A 0 (44) ratlicr than solutions bascd upon the assumption O1 = Olcq and OI1 = 0 for all t > 0 (15) ncq (ii) The sccontl example is that of sorption and tlcsorption kinctics, in which flow decays from an initial niaximum va.luc to zero at eqiiilil~riiim. At time t = 0 for sorption the concentration at x = 0 a.nd x = 1 is instantaneously raised from C to C and kcpt gl go constant at that value. Tlic system tliercafter relaxes from its prcvious sorption equilibrium to t,hc new equilibrium. For a cryst,al with a I-dimensional channel system parallel with the xdircctioii J

( t ) = 0 = - J ( t ) = / = [@cq - 0 ( t ) ] R = -Csat D((l@/dx)x = 0

For tlcsorption, when at t = 0, C

go

(46)

is instantancously lowered to C the corresponding

.d

expression is -J (1,)

= 0 = J (t,)

= 1= (0 ( t ) - 0

C(l

] 0 = - C sa.t D ( t l B / d ~ ) ~ =

(47)

I n tlirsc expressions a positive sign is given to J (t) whcre flow is in the direction of x incrca.sing. The symmct,ry of tlic situation at each intcrfacc means that R1 = RI1 = R in

equations 42 and 43. Otherwise comments made for transicnt, flow in tlic membrane apply equally to solption~llesorptionkinetics. Especially if J ( t ) is initially large the time dependence of 0 ( t ) a n d its difference from 0 may be such that solut,ions of Fick's ccl second equation ba.scd on 0 = 0 for all t > 0 at x = 0 and x = 1 (equation 45) arc no cq

212

longer suitable for evaluating tliffusivities. In many past attempts to reconcile tliffusivitics obta.inetl from sorption kinetics with the large diffusivity values mcasurahlc by pulsed field gradient N M R , the sorption kinetics method has involved extremely small half-lives for the sorption and initially large flows .](I,). Initial tirnc tlependcncc of 0 (I,) is tlien masimiscd as arc initial differences between 0 and 0 (t). Time dependence of 0 just within the intcrfa.ccs i n such circiimstanccs m y cq therefore have to he atldctl to the othcr known limitations to obtaining meaningfill tliffusivities from very fast sorpt,ion rates. For the pnrpose of finding diffusivitics the sorpt,ion kinetics method should best he confined to slow or very slow uptakes wlicn nearly all the limitations encountered with very fast uptakes are avoided. t,lie relation between 0 (t,) just wit.liin tlic crystal and tlic While, for consta.nt C go ' t.iine is not known, it coiild for fast upthtkes he a st,rong fnnction of time. One empirical function of tinic for 0 ( t ) just, within s = 0 or x = 1 is @ ( t ) = O (l-cxp(-/3t))fort>O

(48)

crl

when the cryst,al is init,ially free of sorhate antl at t = 0 is exposcd t,o a const,ant external concentra.tion of the sorhate. For p = IX and all t > 0 equation 4s givcs 0 ( t ) = 0 eq while, as t -> 0 and for finite p, it givcs 0 ( t ) -> 0. Solutions of the diffusion equation have hccn given by Crank( lo) for tlic bounclary condition of equation 4s for diffusion i n I-, 2- or klirnensions. For a splicrc of ratliris x his plots of relative upt,akes as a fiinct,ion of tlinicnsionless time, Dt/a2 , become increasingly signioid as /3 tlecreases and t,lie half-lives, tlI2, of the sorption process increase strongly wilh decreasing values of Dtl/'_/a 2 show:

3

/3 for constant D/a- , as thc following approximate

pa"/D = o;, 5.0 1.0 0.5 0.1 3 Dtl/,/a- = 19 SI 1!J0 5S0 2S70 In the above circumstances apparcnt diffusivit,ies ohtainc~lfrom half-lives by neglecting tlic role of 0 can, for finite 8, he very significa,nt,ly less than the true value. The larger the 2 value of pa / D the more nearly is Dt 1/2 /a2 given 11y t ~ i csolution of the diffusion equation for 0 = 0 at the surface of entry for all t > 0. cq

CONCLIJDING REMARKS It has becn the purpose of this paper antl its pictlecessor (ref. 2 ) to model the transpoit piopcrtics of single crystal membranes taking into acconnt the processes occurring at and acioss the mcmbiane surfaces as well as tliose witliin the crystal. In the present paper the iiiotlel has hwn extentled to include the possibility of simultaneous entry to the crystal both via an externally adsorbed lajer antl tliicctly fioin the gas phi1sr. Tlic role of tlic mass balance equations across tlic surface has also 1w.w extentled to sorption~esorption kinetics.

273

I n t.hc previous paper (ref. 2) thc important role of partial blockagcs located along l-dimcnsional ( 1 - D ) channcl systems was niodcllcd and examined numerically. The papcrs show how the modcrating effect of the mass balance equations at and across mcmbranc surfaces will increase with decreasing thickness and falling tempcraturc. Some numerical estimates indicate that undcr conditions when the intcrface cffccts havc littlc influcncc upon flow at room tcrnpcrature these effects may dominatc flow at low tcmpcraturcs, such as OOK. \Vliere flow is reduccd by partial blockages distributed along 1 - D clianncls thc reduct.ion is indcpcndent of membrane thickness and can be vcry large (ref. 2).

REFERENCES R.M. Barrer, in "The structure and Propcrtics of porous Matcrials". Eds., D.H. Everett and F.S. Stone, (Butterworth). 195S, p.170 2 R.M Barrcr, J.Chcm. SOC.Faraday Trans., 1990, SG, 1123. 3 M. Kocirik, P. Struvc, K. Fiedlcr and hi. Bulow, -1. Chcm. Soc. Faratlay Trans. I , lOSS, 81, 1

3001. 4 5 G

7 S

9 10

M. Kocirik, A . Zikanova, P. Struvc and hl. Bulow, i n "Zeolitcs: Facts, Fiqurcs, Future". Eds. P . A . Jacobs and R.A. van Santcn (Elscvicr), 19S9, p.925. J . Kargcr, II. Pfcifcr and W. Ileink, i n Proc., Gth Int. Zeolite Conferencc, Eds. D. Olson and A. Bisio (Butterworth) 19S4, p.lS4. I1.M. Barrcr, Langmuir, 19S7,3, 309 D.L. Wernick and E.J. Osterhuher, i n Proc. 6th Int. Zcolite Confcrcncc, Etls. D. Olson and A. Bisco (Butterworth) 1983, p.123. A. Paravar and D.T Hayhurst, in Proc. Gtli Int. Zcolitc Confcrcncc, Eds. D. Olson and A Bisio (Buttcrworth) 1984, p.217. D.T Hayhurst and A.R Paravar, Zcolitcs, ISSS,g, 27. J.Crank, "Thc Mathematics of Diffusion" 2nd Edition, (Clarcntlon Prcss), 1975, pp.53, 75 and 92.


G. Ohlmann et al. (Editors), Catalysis and Adsorption by Zeolites 1991 Elsevier Science Publishers B.V., Amsterdam

275

THE ROLES OF METAL AND ORGANIC CATIONS IN ZEOLITE SYNTHESIS

D.E.W. VAUGHAN Exxon Research and Engineering Company, Rt. 22 East, Annandale, New Jersey, U.S.A., 08801.

SUMMARY The position is taken that the crystallization products of a gel are the key to understanding the important gel intermediate structures and therefore the mode of zeolite genesis. Reviewing the products of reactions in sodium and potassium aluminosilicate gels leads to the conclusion that extended sheet (Na) and columnar ( K ) sub-structures are the important structure building units. The smaller base metal cation controls the nature of the sub-structure and the larger cation the ways in which such units interconnect to form specific zeolites.

INTRODUCTION Much recent interest in modes of zeolite genesis has focused on the role of small molecular clusters as major participatory species in the direction and development of zeolite structure types, relying mainly on developments i n such techniques as NMR (1,2,3) and Raman spectroscopy (4) combined with increasingly powerful computer hardware and software capabilities which facilitate infinite peak deconvolution and search-match abilities. S u c h methods reveal much new information about the complexities of the silicate and gel systems of interest, but they give little guidance to those researchers interested in creating, and preferably tailoring, new zeolite structures. Indeed, it is probable that species observed in solution during zeolite crystallization are primarily "spectator species" in a state of dynamic near equilibrium in a reactive system in which the "participatory species" are constantly scavenged by the growing zeolite(s) or their structure additive sub-units. An alternative approach, examined i n part by Liebau for silicates i n general ( 5 ) and by Barrer for zeolites i n particular (6), is to look at larger

216

entities, such as sheet or chain structures, and to examine how these interconnect in various ways to form t h e many different structures which characterize the crystallization systems of interest. The products and their inter-relationships are the starting point rather than the gel systems. T o this author it is this latter approach which not only explains much of the available experimental data, but offers some opportunity for designed syntheses within the limitations of the specific system of interest. This paper will evaluate numerous minor cation substitutions on the products of separate series of syntheses in sodium aluminosilicate and potassium aluminosilicate systems. The focus will be mainly on the low silica: alumina ratio gels and products where the reactivity, and therefore the multiplicity and diversity of zeolites, is high. At lower temperatures less stable products survive longer and can therefore be analyzed and recognized. The more rapid the approach to thermodynamic equilibrium (represented by higher density zeolite mineral equivalents) the lower the probability of detecting novel and interesting products. CRYSTALLIZATION PRODUCT OVERVIEW T h e approximate composition range discussion is:

of

primary

interest

in

this

2-4 (Na,K)20: xM20: A1203: 6-15SiO2: 100-200H20 where M is either a Group 1A or simple alkylammonium cation and x is less than 1.O. T h e initial ("metastable") main crystallization products at temperatures between 90 and 180°C using sodium or potassium silicates or colloidal silica are:

Na

K

FAU LTL FA U - b ss in t ergr o w t h s OFF GME ERI MAZ ECR-I MOR beta The phases stable at long reaction times are CIS, PHI and MER, with the first dominating the sodium system and mixtures of the lattcr two the potassium system. Mixtures are common in most of the experiments in this composition range, but pure products can be made in high yield by suitable "fine tuning"

277

of the gel compositions and choice of raw materials. ANA (pollucite) dominates the crystallization of any gel composition containing caesium and, with the exception of CSZ-1 and -3, frequently inhibits nucleation over most of this composition range. In understanding the products of these systems recent high resolution structure imaging results have been invaluable (7,8,9,10). (Throughout the IZA structure codes will be used to indicate structure type (1 l ) , with lower case indicating proposed structures, or "inter g r o w n " p h a s e s where the i n d i c a t e d s t r u c t u r e p r e d o m i n a t e s .) (Oualification: This discussion is based on broad generalizations made on the basis of an overview of many experiments. Specific phases and phase combinations are influenced by numerous experimental factors, particularly raw materials and nucleation effects. Specific references should be consulted for these details.)

GME GIS

MAZ LTL OFF

MER PHI

MAZ

Figure 1. The building units characteristic of the many structures under discussion can be readily derived from the Si12 unit in the center of the figure by rearranging a few S i - 0 linkages. Rapid re-equilibration of the system maintains the supply of those removed by the growing zeolites.

278

In terms of derived secondary building units (sbu) or similarly sized clusters all the structures may be readily derived from the Si12 unit shown in Figure 1 by a small number of S i - 0 bond changes. The rapid equilibration of such a system would ensure a constant supply of nutrient sbu to the growing zeolite crystals, but these would always be at a low concentration and therefore not expected to be observed in the solution or gel. LAYER STRUCTURES DOMINATE THE SODIUM ALUMINOSILICATE SYSTEM Various Si/AI ratio faujasites are the dominant products in this system for almost the whole range of M species evaluated, as detailed in Table 1 . In the cases of K and Rb substitution only morphology effects are evident, with the tendency to form platelet crystals of FAU. Substitution of larger cations such as Li (hydrated) and C s not only accentuates the development of platelet forms (ZSM-2 and 3; CSZ-I and 3), but introduces faults and defects associated with the bss connectivity of the "faujasite" sheets. (The so called Breck structure-6, first proposed as a theoretical structure by Moore and Smith (1 3), formed by connecting "faujasite" sheets together to give the hexagonal structure shown in Figure 2 (16)) These effects become more pronounced with increasing Si/AI ratios, but phase stability pushes the system more towards MOR at gel Si/AI over about 6, with that zeolite dominating as the primary product when the Si/AI exceeds about 8. The addition of alkylammonium cations yields higher ratio zeolite Y (13), but gmelinite like impurities (gme) co-crystallize more readily than in the organic free gels in the absence of agitation. (Growth of these gme products is usually greatly accelerated in stirred crystallizations, probably as a result of "collision breeding" of nuclei of this "phase". As the crystallizing solutions are homogenized before each sample is removed i n a single batch, sequentially sampled to obtain a timed crystallization profile, the sampling mode may i n fact promote the gme component. One notes that t h e basic building u n i t of GME is the same as that for CIS, MER and PHI, as shown i n Figure 1.) The notable exceptions are the development of MAZ in TMA systems and ECR-1 i n DMDE(0H) containing gels at higher temperatures, indicating that the "mazzite" sheet is more stable than the "faujasite sheet. FAU usually precedes MAZ in these crystallizations at MC=O), 153.0 ppm ( >* CP =) and probably by a weak signal hidden in the shoulder of the signal at 124 ppm ( = C " < ) . The signal of this a-carbon would be weak, as it is created from a carbon type which has not been enriched with the 13C isotope.

342

a

~

. 300

Fig.2.

.

.I

.

. . . . , .

200

...

1

.

100

.

.

.

1

.

...

ppm

13C CPMAS NMR spectra at 50.32 MHz on (a) 2.5 mmol/g of acetone-2-13C(lO%) days,after 2 months, NS = 560 (b) sample (a) after heating at 180°C (c) sample (b) after heating at 280°C ( normalized spectra as in Fig.1.).

1

.

.

.

.

1

.

,

0

HZSM-5 at 25'C

....after

for lh, f o r 0.5h, NS = 2 5 6 0

2

343

Another interesting signal in the N M R range of carbonyl compounds is the broad signal with a maximum at 177 ppm which could indicate the formation of a carboxylic compound, e.g. acetic acid, or which could correspond to Some form of a cyclic ketone (isophorone?) The signals at 137 and 129 ppm are characteristic for simple aromatic compounds: their presence has also been demonstrated in the desorption products. Spectrum (c) in Fig.2. obtained for the same sample after further heating to 28OoC for 0.5 h corresponds to the formation of alkylaromatics. In this spectrum, the signals at 138.5 and 129.3 ppm predominate and are very close to those exhibited by trimethylbenzene and similar alkylaromatics (ref.17). The high-field-shifted signal of -CH3 is also evidence for this fact. Both the signal at 180 ppm and the methyl signal at 20 ppm most probably correspond to the acetic acid which was found by mass spectrometry in the desorption products. Similarly, C7 - Cll aromatics were also found in the desorption products. Isobutene was not found in the adsorbed state while it appeared in the desorbable gaseous products.

.

Analysis of the coke deposits after reaction of acetone Similar problems to those ocurring in the analysis of coal and other fossil fuels (refs.18-20) are encountered in the analysis of coke deposits on catalysts. The distinction between aromatic and olefinic hydrocarbons represents an especially difficult problem as they exhibit NMR signals in an overlapping region at 110 - 140 ppm. In this respect, the combination of the chemical treatment of coke deposits with 13C NMR analysis seems to be useful (ref.21); this combination permits us to distinguish between olefinic and aromatic bonds on the basis oftheir different reactivities. The spectra depicted in Figs.3 and 4 illustrate the difference between coked HZSM-5 and HY zeolites. The broad and poorly resolved band at 130 ppm is usually attributed either to aromatic compounds or to polyenic chains. In addition, with both types of zeolite signals in the range of paraffinic carbons (-CH3,-C2H5) bonded to the aromatic rings are observed as follows from the high-field-shifted value with a maximum at 19.8 ppm. HZSM-5 zeolite differs from HY in a clearly separated broad signal at 154 ppm, which has also been observed by other authors on coked zeolites (refs.19-21). However, the interpretation of this signal is not yet clear: some authors assigned it to carbocations

344

20 0

100

0 PPm

Fig.3. 13C CPMAS NMR spectra of coke deposits after acetone reaction on Y-type zeolites at 50.32 MHz : (a) 28 w.% of coke on H,NaY 70 zeolite, (b) 16.0 w.% of coke on HY dealuminated zeolite, NS=8000, (c) 13.9 w.% of coke after pyrolysis of the sample (b) at 600°C for 2h, NS=8000. (normalized spectra as in Fig.1.)

K130*8

19,8

Fig.4. 13C CPMAS NMR spectra of coke deposits after acetone reaction on HZSM-5 zeolite at 50.32 MHz: (a) 1.0 w.% of coke after acetone-1,2,3-13C(lO%) reaction, NS=240, (b)sample as ad (a) after partial oxidation at 200°C , ( c ) sample (b) after pyrolysis at 600°C for 2h, NS=7000.

345

(ref.l9), others to olefines (ref.20). In order to solve this problem, we have carried out a number of experiments on the chemical treatment of coke deposits. Partial oxidation (see spectra in Fig.3,4) at temperatures higher than 5OO0C did not lead to any change in the spectrum. Therefore, the creation of carbocations does not seem probable. The easy addition of bromine to olefinic bonds led us to perform bromination experiments with bromine vapours at 25 and 100°C. As the effective radius of bromine permits its adsorption in the microporous structure and no significant effect of bromination in the NMR spectra was observed, we concluded that the mentioned signal cannot be associated with olefins. The pyrolysis of the deposits in vacuum at 6OO0C yielded only evidence that the dealkylation took place (the signal at 19.8 ppm disappeared) but did not lead to any change inthe signalat 154 ppm. We assume (on the basis of its resistance to chemical treatments) that this signal could be associated with carbons belonging to an aromatic ring bonded to oxygen, as is true of phenols, cresols or ethers (ref.l7).We suggest that phenoxy groups are formed on the surface of HZSM-5 zeolites, analogous to the surface methoxy groups (ref.22-26). For the wide pore Y zeolite, the condensation of the aromatic rings is preferred, as is indicated by the relatively strong shoulder at 140 ppm, whereas the signal at 154 ppm is almost indistiguishable from that of HZSM-5.

.

CONCLUSIONS The 1 3 C CPMAS NMR represents a significant contribution to the solution of problems connected with less mobile chemisorbed species which can participate as intermediates in the transformation of acetone. In combination with chemical treatment of catalysts with deposits the NMR spectroscopy can help considerably in elucidating problems in the formation and analysis of coke residues on zeolites. REFERENCES 1 2

3

C.D. Chang and A.J. Silvestri, J.Cata1. C.D. Chang, W.H. Lang and W.K. Bell, Catalysis in Organic Reactions, M. 1981,p.73. Y. Servotte, J. Jacobs and P.A. Jacobs, on Zeolite Catalysis, Siofok 1985, Acta Szegediensis, Szeged, 1985, p. 609.

47 1977) 249. in: W.R. Moser (Ed.), Dekker, New York, in: Proc. Int. Symp. Physica et Chimica

346 4

J. Novakova, L. Kubelkova and Z. Dolejsek, J. Molec. Catal.,

5

L. Kubelkova, J. Cejka, J. Novakova, V. Bosacek, I. Jirka and P. Jiru, in: P.A. Jacobs and R.A. van Santen (Eds.), Zeolites, Facts, Figures, Future, Elsevier, Amsterdam, 1989, p. 1203. V. Bosacek and L. Kubelkova, Zeolites 10 (1990) 64. L.M. Baigrie et al., J. Am. Chem. SOC., 107 (1985) 3640. H. Pfeifer, W. Meiler and D. Deininger, Annual Rep. NMR Spectroscopy, 15 (1983) 291. G. Engelhardt and D. Michel, High Resolution Solid State NMR of Zeolites and Related Systems, J.Wiley & Sons, 1987, Chap.7. D. Freude, in: Advances in Interface Sci., Elsevier, 23 (1985)

39 (1987) 195.

10

21 11

M.W. Anderson and J. Klinowski, J. Am. Chem. SOC., 112 (1990)

12 13

22

L. Kubelkova, J. Cejka and J. Novakova, Zeolites, in press. T. Bernstein, D. Michel and H. Pfeifer, J. Coll. Interfaces Sci., 84 (1981) 310. G.A. Olah and A.M. White, J. Am. Chem. SOC., 90 (1968) 1884. G.A. Olah and D.J. Donovan, J. Org. Chem., 43 (1978) 860. Z. Dolejsek, J. Novakova, V. Bosacek and L. Kubelkova, Zeolites, in press. H.O. Kalinowski, S. Berger and S. Braun, 13C NMR Spektroskopie, G. Thieme Verlag, 1984. C.E. Snape, D.E. Axelson, R.E. Botto, J.J. Delpuech, P. Tekely, B.G. Gerstein, M. Pruski, G.E. Maciel and M.A. Wilson, Fuel, 68 (1989) 547. J.P. Lange, A. Gutsze, J. Allgeier and H.G. Karge, Appl. Catal., 45 (1988) 345. . E.A. Lombardo, J.M. Dereppe, G. Marcelin and W.K. Hall, J. Catal., 114 (1988) 167. L. Carlton, R.G. Coppertwhite, G.J. Hutchings and F.C. Raynhardt, J. Chem. SOC. Chem. Commun., (1986) 1008. V. Bosacek and Z. Tvaruzkova, Coll. Czech. Chem. Commun., 36

23 24

P. Salvador and J.J. Fripiat, J. Phys. Chem., 79 (1975) 1842. P. Salvador and W. Kladnig, J.C.S. Faraday Trans.1, 73 (1977)

10.

14 15 16 17 18 19 20 21

(1971) 551. 1153. 25

G. Senkyr and H. Noller, J.C.S.

Faraday Trans.I.,71

(1975)

997. 26

L. Kubelkova, J. Novakova and (1990) 441.

K.

Nedomova, J.

Catal.,

124

G . Ohlmann et al. (Editors),Catalysis and Adsorption by Zeolites 0 1991 Elsevier Science PublishersB.V., Amsterdam

341

ISOPROPYLATION OF BENZENE OVER LARGE PORE ZEOLITES

A.R. PRADHAN, B.S. RAO and V.P. SHIRALKAR Catalysis Group, National (India)

Chemical Laboratory, Pune 411 008

ABSTRACT Isopropylation reaction of benzene is carried out over large pore zeolites with characteristic structural differences (namely La-H-Y, H-mordenite and H-ZSM-12). The activity and deactivation pattern are correlated with structural and acidic properties. The deactivation of La-H-Y is due to blocking of active sites while that o f H-mordenite is due to blocking of channels. The stable activity and selective nature o f H-ZSM-12 for cumene can be attributed to siliceous nature, lower acidity and presence of non-interpenetrating channels. INTRODUCTION Isopropylation o f benzene using solid phosphoric acid (SPA) catalyst and Fridel-Crafts catalysts (refs. 1-3) for the production of cumene is an industrially important reaction. The drawbacks suffered by these processes (environmental and corrosion) can be overcome by using solid acid catalysts like zeolites. Major side products formed in this reaction are isomeric diisopropylbenzenes (DIPB) and at higher temperatures n-propylbenzene (nPB). Even though the medium pore zeolite ZSM-5 is reported (ref. 4) a s a potential catalyst for this reaction, better stability and selectivity were observed over large pore zeolites (ref. 5). In view of this, isopropylation of benzene was carried out over large pore zeolites with characteristic structural differences, like (1) La-H-Y with cubic crystal symmetry and three directional channel system having pore opening of 7.4 k , (2) mordenite with orthorhombic structure and unidirect(12 MR) and 2.9 X ional dual pore system with 6.7 X 7.0 5.7 (8 MR) connected via side pockets of 2.9 i, ( 3 ) ZSM12 with monoclinic symmetry and linear non-interpenetrating channels o f 5.7 X 6.1 "A. The stability, selectivity and deactivation pattern were correlated with structural, acidic

348

and sorption properties of these are reported in this communication.

zeolites and

the

results

EXPERIMENTAL Materials Benzene (

X pure) and propylene (having 4

99.98

% propane)

were used for catalytic studies. Catalysts La-H-Y (SK-500) and H-mordenite from M/s

Carbide, USA

Union

and

(Zeolon 100) were procured Norton, USA, respectively.

ZSM-12 was prepared in this laboratory following the procedure reported (ref. 6 ) earlier in the literature. ation of zeolite beta was avoided.

Co-crystallis-

Characterisation the

Crystalline framework

techniques

phase purity and the state of aluminium in of these samples were characterised by the

XRD,

like

of the samples was ammonia. Adsorption

P/Po

=

0.5

using

sensitivity of

4'

IR

and

MASNMR

spectroscopy.

Acidity

measured by the irreversibly adsorbed studies were carried out at 25°C and

a

McBain

balance

with

silica

spring

of

50.0 cm gm-'.

Catalytic reactions The catalyst was pressed and crushed into 10-20 mesh binder free self supported pellets. Prior to catalytic runs, the catalyst was activated hrs.

Catalytic

bed,

down

Benzene while

was

runs were

flow, fed

propylene

in a flow of dry air at 45OOC for 8

silica

by was

a

carried

out

reactor

at

syringe pump

metered

through

in an integral, fixed atmospheric

(Sage a

pressure.

Instruments, USA)

mass-flow

controller

(Matheson, USA). The products were analysed by gas-chromatography (Shimadzu, Model 15A) using Apiezone L column for liquids and Poropak Q column for gaseous samples. RESULTS AND DISCUSSION The structural features, silicon to aluminium ratio, acidity values in terms of irreversibly retained ammonia and equilibrium sorption capacity for benzene are compared in Table 1.

TABLE 1 S t r u c t u r a l and physico-chemical

Catalyst

La-H-Y

Channel structure

H-ZSM-12

i

7.4

Unit c e l l crystal symmetry Si/Al

Cubic

total

5.7 x 7,l 1 2 . 9 X 5.7 ‘A (8 MR)

Monoclinic

Orthorhombic

60.5

0.535

9.8

13.2

20.3

is

acidity

6.4

0.067

1.338

Equil. sorption capacity for benzene (wt Z)

Unidirectional 8-membered i n t e r connecting

5 . 7 X 6.1

2.3

Acidity m mole o f NH3/gm

H-mordenite

Unidirectional l i n e a r non-inte r p en e t r at i n g

Three directional with interconnecting channels

P o r e opening (12-membered ring)

The

p r o p e r t i e s of c a t a l y s t s .

to

found

be

in

accordance

with

the

aluminium c o n t e n t o f t h e z e o l i t e , w h i l e t h e s o r p t i o n of b e n z e n e does

not

the

follow

same

trend

due

to

structural

the

differences. The

catalytic

propylene

over

performance the

all

in

three

alkylation

zeolites

has

of

benzene

been

with

compared

in

T a b l e 2. c a t a l y s t s showed

A l l

But

the

and

higher

and

impurities

of

in

than

200

catalytic

aliphatics,

fractions

after

even

(Fig.

hrs

fast

reaction

Due

were

studies

cumene

to

C8

toluene,

are is

conversions.

aromatics

more very

deactivation

in high

was

La-H-Y i n

the

noticed

w h i l e s t e a d y a c t i v i t y was o b s e r v e d

the

1).

propylene

(H.B.F)

A

catalyst.

and H-mordenite,

H-ZSM-12

X)

( )99

Selectivity

H-ZSM-12

i n La-H-Y

like

boiling

H-mordenite.

case

high

to

its

performed

was

performed

for

steady activity, over

H-ZSM-12

more

further

catalyst.

Influence of temperature In on At

Table

the

3

product

temperatures

not completed.

the

results

on

distribution below

200°C,

the over

the

influence H-ZSM-12

conversion

With t h e i n c r e a s e of

of

temperature

are p r e s e n t e d . of p r o p y l e n e i s

temperature,

a continuous

350 decrease i n the

DIPB formation

is noticed,

while

appreciable

q u a n t i t i e s o f nPB a r e o b s e r v e d a b o v e 230°C.

TABLE 2 I s o p r o p y l a t i o n o f benzene o v e r z e o l i t e c a t a l y s t s R e a c t i o n t e m p e r a t u r e = 230OC; P r e s s u r e = Atmospheric;-TOS = 3 h r s ; Benzene t o p r o p y l e n e molar r a t i o = 6 . 5 ; WHSV = 2 . 5 h r

Catalyst

La-H-Y

H-ZSM-12

Product d i s t r i b u t i o n (wt % ) Aliphatics Benzene T o l u e n e t C8 a r o m a t i c s Cumene nPB C9-C11 a r o m a t i c s DIPB H.B.F

0.74 77.10 0.97 18.52 0.40 0.29 1.75 0.18

0.06 77.80 0.04 20.50 0.02 0.04 1.55 0.01

0.30 78.30 0.27 18.10 0.74 0.12 1.78 0.26

C3 = c o n v e r s i o n Cumene s e l e c t i v i t y S e l e c t i v i t y (cumene t DIPB)

99.4 81.1 88.5

99.9 92.3 99.3

99.8 82.9 91.6

1./F

H-mordenite

t,

w h 19

a

H.ZSM-I2

* H MORDENITE rLa H Y

2

4

6 TIME

ON STREAM (hra)

F i g . 1 . C a t a l y t i c p e r f o r m a n c e o f t h e wide p o r e z e o l i t e s i n t h e i s o p r o p y l a t i o n of b e n z e n e w i t h t i m e o n s t r e a m (TOS). R e a c t i o n temp. = 230OC; WHSV = 2 . 5 h r - 1 ; Benzene t o p r o p y l e n e m o l a r r a t i o = 6 . 5 .

351 The increase in selectivity to cumene with temperature Above is a result of transalkylation of DIPB with benzene. 230°C, the selectivity towards cumene decreases on account of the formation of nPB. Also unwanted products like aliphatics, C9-C11 aromatics and higher boiling fractions increase with the increase in temperature as a result of cracking of higher alkylbenzenes (ref. 7). TABLE 3 Influence of temperature on product distribution Catalyst = H-ZSM-12; Benzene to propylene molar ratio WHSV = 2.5 hr-1

Temperature ("C)

170

190

210

Product distribution (wt X ) Aliphatics 0.03 0.03 Benzene 84.42 82.38 Tol. t C8 arom. 0.26 0.07 Cumene 11.47 13.39 nPB 0.01 C9-C11 arom. 0.06 DIPB 4.01 3.86 H.B.F -

0.05 80.65 0.11 16.68 0.03 0.04 2.41 0.02

0.06 80.00

99.8 86.2 97.1

99.8 91.2 98.0

C3 = conversion 85.3 Select. to cumene 73.6 Select (cumene 99.4 + DIPB)

.

95.3 76.0 97.8

230

0.18 18.24 0.11 0.06 1.35 -

=

6.8;

250

270

0.12 79.71 0.22 18.46 0.31 0.10 1.05 0.03

0.14 78.87 0.23 18.74 0.63 0.17 1.03 0.09

99.6 90.9

99.4

96.2

88.7 93.4

Influence o f mole ratio With increase in benzene to propylene molar ratio, selectivity to cumene increases, even though the total selectivity to cumene and DIPB remained almost constant during present investigation (Fig. 2). This is due to high propylene concentration at lower mole ratios, resulting in the successive alkylations of cumene. Relative amounts of unwanted byproducts are less at higher mole ratios.

352

> 100t >

F

_-

95-

0

w

d

xCUMENE*UPB CUMENE

90.

v)

s

85-

5

IS

10

20

MOLE RATIO

Fig. 2. Influence of reactant mole ratio on the isopropylation of benzene over H-ZSM-12 catalyst. Reaction temp. = 2 3 0 ° C ; WHSV = 2 . 5 hr-l Influence of weight hourly space velocity (WHSV) Low space velocities are found to be favourable selective to

formation

cumene

and

of

(Fig.

cumene

3).

Total

for

selectivity

DIPB again remains constant over the entire

range of WHSV.

100

t

t 1 c

95 90

; 0

. '

CUMENE *DIP6

.

.

:\

CUMENE

85.

fn

s

80

1

2

3

4

5

6

7

SPACE VELOCITY ( W H S V I h i l

F i g . 3.

Influence o f weight hourly space velocity (WHSV) in the isopropylation of benzene over H-ZSM-12 catalyst. Reaction temp. = 2 3 0 ° C ; Benzene to propylene molar ratio = 6 . 5 .

353

Thus

the

optimised condition for propylation of benzene over H-ZSM-12 catalyst are at temperature 230°C, WHSV, 2.5 hr-l, and reactant mole ratio, 6-8. As already mentioned, the ageing studies indicated faster deactivation o f La-H-Y and H-mordenite catalysts. Whereas H-ZSM-12 catalyst did not deactivate even after 200 hrs of time on stream (TOS), therefore faster deacpivation was carried out by accelarated ageing. All the three coked samples were subjected to thermal and sorption studies to find out the Fig. 4 presents the cumene probable cause of deactivation. sorption kinetics on fresh and coked samples.

r

I CATALYST

Lo H Y

o FRSH SAhlPLE DEACTIVATED SAMPLE

1

0

V , 6

n

a w

2

w

5u

I= -=+/ t

l4

10

2

I

f

r

5t

= I

10

30

20

TIME

(

'

120

min 1

Fig. 4 . Kinetics o f cumene sorpkion on fresh ( 0 ) and coked zeolite catalysts at 25 C (P/P, = 0.5).

( 0 )

354

and

The equilibrium sorption capacity for cumene over fresh deactivated samples and the % coke formed for the

cummulative

feed

passed

over

each

catalyst

is

presented

in

Table 4 .

TABLE 4 Physico-chemical studies on fresh and coked catalyst samples

Catalyst

La-H-Y

H-mordenite

H-ZSM-12

Equil. sorption of cumene over fresh sample (wt X)

17.6

4.5

12.55

Equil. sorption of cumene over deactivated sample (wt X)

11.16

1.4

6.36

X sorption capacity retained

63

31

51

7.48

5.80

8

10

250

0.37

0.15

0.003

14.78

Amount of coke formed (wt X) Time required for deactivation,hrs ( 10 of initial activity)

X

Amount of coke formed per 100 gm of catalyst per gm of feed (gm)

and

Although the rate of sorption of cumene is same for fresh coked La-H-Y the equilibrium sorption capacities are

different. to

high

The deactivation of this catalyst may be attributed

acid

irreversibly structure

site

density

adsorbed

wherein

the

(ref.

ammonia)

8)

in

reactants

(as the

revealed three

(especially

by

total

dimentional

propylene)

and

product molecules are strongly adherent to the active sites. Also due to dehydrocyclisation reaction,bulkier coke precursors are (ref.

readily

9).

(14.78 %

formed

in

the

large

intra-zeolite

cavities

This results in higher amount of coke formation calculated b y thermogravimetric methods) in the

catalyst. In spite o f this, the coked sample shows more than half the void volume still available ( 6 3 % o f

case of La-H-Y

355

initial sorption capacity) for sorption of reactant molecules without any further activity. Thus the deactivation to blocking of the active sites.

is due

The phenomenon of deactivation in H-mordenite is relatively slower on account of lower acid site density compared to La-H-Y (ref. 10) (Si/A1 = 6 . 4 and 0.535 m moles of irreversibly adsorbed

ammonia

per

gm

of

catalyst).

ivation in the unidirectional

However,

the

deact-

pore system of mordenite leads

to a drastic decrease in sorption capacity in the coked sample. In

fact,

capacity 30

X)

the in

sorption

the

suggest

kinetics

coked mordenite only

surface

and

equilibrium

(retained

adsorption

sorption

sorption

capacity

indicating

blocking

of the channels. The

least

coking

tendency

is

observed

sample (0.003 gm of coke formed per gm

of

feed

density

passed)

(Si/A1

again

on

60.5 and

=

in

the

H-ZSM-12

100 gm of catalyst

account

0.067 m

adsorbed ammonia per gm o f catalyst).

of

lowest

acid

per site

of irreversibly In addition, the linear moles

non-interpenetrating unidirectional channel system with virtual absence of larger intra-zeolitic cages (like those in zeolite

Y)

and

highly

siliceous nature

of bulkier coke precursors. by

ZSM-12

activity

zeolite for

not

favour the

formation

The sorption capacities exhibited

support

prolonged

do

these

period

of

findings. time

Thus

(more

the

than

stable

200 hrs)

can be explained for H-ZSM-12.

CONCLUSIONS H-ZSM-12

is

a

superior

and

selective

catalyst

in

the

alkylation of benzene with propylene. The catalytic activity and stability are dependent on the acidic and structural properties. Coking

of

La-H-Y

is

due

to

acid

site

blocking,

in

H-mordenite, it is due to channel blocking while deactivation of H-ZSM-12 required accelarated ageing. ACKNOWLEDGEMENT We

sincerely

thank

Dr.

P. Ratnasamy,

for

his

constant

encouragement throughout this investigation. We also thank Dr. V.G. Gunjikar and Mr. S . P . Mirajkar, for helping in

356

the

thermal and s o r p t i o n s t u d i e s .

T h e w o r k was p a r t l y f u n d e d

by t h e UNDP.

REFERENCES

2 3

4 5

6 7 8

9 10

Y.C. Y e n , S t a n f o r d Res. I n s t . E c o n . R e p . , 22 A ( 1 9 7 2 ) and 22 B (1973). I b i d , 49 (1969) E.S. M o r t i k o v , S.R. Mirzabekova, A.G. Pogorelov, N.F. Konov, R . F . M e r h a n o v a , A.Z. D o r o g o c h i n s k i i a n d Kh. M . M i n a c h e v , N a f t e k h i m i y a , 16 ( 1 9 8 8 ) 7 0 1 . W . W . K e a d i n g a n d R.F. H o l l a n d , J . C a t a l . , 109 ( 1 9 8 8 ) 212. B.S. Rao, I. B a l a k r i s h n a n , V.R. Chumbhale, A . R . Pradhan and P. Ratnasamy, P a p e r p r e s e n t e d a t F r i s t Tokyo C o n f e r e n c e o n A d v a n c e d C a t a l y t i c S c i e n c e a n d T e c h n o l o g y (TOCAT l ) , J u l y 1-5, 1990. E . J . R o s i n s k i a n d M . K . R u b i n , U.S. P a t e n t 3 , 8 3 2 , 4 4 9 ( 1 9 7 4 ) . D . A . B e s t a n d B.W. W o j c i c h o w s k i , J . C a t a l . , 4 7 ( 1 9 7 7 ) 11. H.G. Karge and E.P. Boldingh, Catalysis Today, 3 ( 1 9 8 8 ) 53. P.B. V e n u t o a n d P . S . L a n d i s , Adv. C a t a l . , 18 ( 1 9 6 8 ) 2 5 9 . P.E. E b e r l y J r . a n d C.N. K i m b e r l i n J r . , I n d . Eng. Chem. P r o d . Res. D e v . , 9 ( 1 9 7 0 ) 335.


357

EXAFS STUDY OF LOCAL STRUCTURE OF Pt-Cr CLUSTERS IN PENTASILS IN RELATION WITH THEIR REACTIVITY IN LOWER ALKANES AROMATIZATION

E.S. S H P I R O ~ , R.W. ~ JOYNER~, 2 G . J . T U L ~ O V A ' . A.V. PFEOBR~IENSKY'. O.P. TKACHENKO , T.V. VASINA , O.V. BRAGIN and Kh.M. MINACHEV

'N.D.Zelinsky Institute of Organic Chemistry, USSR Academy of Sciences, Moscow, YSSR The Leverhulme Centre for Innovative Catalysis, The University of Liverpool, Liverpool, England SUMMARY Pt-Cr/H-ZSM-S samples were characterized by EXAFS and XPS and their catalytic activity in ethane and methane aromatization was examined. EXAFS shows clear evidence of Pt-Cr alloy particles formation in samples reduced at 823 K. Most of these particles is located within zeolite structure and coordinated to zeolitic oxygen. The changes of catalytic activity in C1-C3 alkane aromatization and hydrogenolysis caused by chromium are related to alloying effect.

INTRODUCTION It is well known that VIII Group metal additives play an important role in catalysis of lower alkanes aromatization which proceeds on modified high-silica zeolites (refs. 1-4). Among other metals platinum behavior is established rather in details and such characteristics as dispersion and electronic state was shown to be key factors for high catalytic performance in ethane and propane aromatization (refs. 4-6). Recent EXAFS studies (ref. 7 ) proved earlier suggestion that platinum clusters are located within zeolite structure in the proximate vicinity t o Bronsted acidic centers and they are most probable candidates for active sites in alkane dehydrogenation step (refs. 5-71. Platinum might play even greater role in methane aromatization. This reaction was discovered (ref. 8 ) to proceed over modified pentasils at 1023 K to yield up to 4-14% of aromatics. In connection with aromatization reactions two features of platinum loaded Hpentasils became important: (a) stabilization of very dispersed Pt particles at temperatures as high as 823-1023 K; (b) reducing Pt hydrogenolysis activity and as consequence coke formation and catalyst deactivation. To maintain these characteristics of platinum in reforming catalysis promotion by second metal (Re, Sn, In, Ga, Cr) is widely used, chromium is definitely suppressed platinum activity in hydrogenolysis (refs. 9,101. Our previous studies (ref. 6 ) also suggested the strong influence of chromium on platinum behavior in ethane hydrogenoly-

358 sis but mechanism of this effect remained unclear.

This paper presents a study of the influence of chromium on Pt-HZSM-5 catalyst where we have able to establish a detailed connection between structure and catalytic function via EXAFS and XPS characterization of Pt-Cr/H-ZSM-5 and their reactivity in aromatization of lower alkanes inclusive methane. To elucidate possible alloying effects the activity of Pt-Cr/H-ZSM-5 and Pt/H-ZSM-5 in ethane and propane hydrogenolysis was also compared. EXPERIMENTAL Catalyst containing 0.5 wt % Pt and different contents of chromium (0.13, 0.75 and 1.25 wt % I were prepared by simultaneous impregnation of H-ZSM-5 (Si/A1 a mixture of H PtC16 and (CrO 1 solutions, 0.5% Cr/H-ZSM-5 was ob2 3 x tained by impregnation with (CrO 1 and 0.75% Pt/H-ZSM-5 was prepared by ionic 3 x exchange of [Pt(NH3l41Cl2 with NH -EM-5 (ref. 4). Samples were calcined in an 4 air at 623 K for 2 h and subsequently at 790 K for 3 h. = 16.5) with

EXAFS measurements were performed at the Daresbury synchrotron radiation SQUrce and reduction of the catalyst specimen was carried out in-situ at 1 bar in flowing hydrogen at 623 and 823 K. Data were collected for the Pt L I11 edge and analyzed by standard methods (ref. 11). XPS analysis of the Pt-Cr/H-ZSM-5 has been performed after in-situ reduction in 1 bar of flowing hydrogen at 823-873 K in a specially developed cell attached directly to Kratos XSAM 800 spectrometer and using Mg K radiation. The analysis of Pt-Cr/H-ZSM-5 samples after methane a aromatization reaction was performed with Kratos ES 200B spectrometer according procedure described in (refs. 4 , 6 ) . Pt 4f + A 1 2p and Cr 2p peaks were analyzed by using peak synthesis programme with PDP 11/03L computer. The details of peak synthesis treatment for these particular spectra are described in (ref. 12). Ethane and methane aromatization were tested in microcatalytic pulse

reactor

in He flow at 823 K and 1023 K respectively. Propane and ethane hydrogenolysis were performed in flow reactor at hydrocarbon/H2 ratio equal to 1/10 rential mode (hydrocarbon conversion does not exceed 5-7%).

in diffe-

RESULTS AND DISCUSSION Characterization of the reduced catalysts Studies of Pt/H-ZSM-5 catalysts by XPS, EXAFS and electron microscopy revealed that most of platinum in pre-calcined samples can be stabilized as very dispersed particles within zeolite structure (refs. 4,6,7). The lower limit of ave0

rage particle size, 7-8 A is close to the channel size EXAFS indicating formati-

on of one Pt-Pt shell clusters consisted of 13

5 atoms (ref. 7). Upon increase

of calcination temperature from 723 K to 793 K average particle size increased 0 + 75 up to 13 A and number of atoms in cluster up to 120 - 40. Nevertheless these

359

particles remain to be located inside zeolite matrix (ref. 7). The particular 0 features of 7-13 A particles are 1-4% contraction of nearest Pt-Pt distance with respect to the bulk and bonding between metal and oxygen of zeolitic framework. XPS positive shifts of Pt 4f level observed for such particles have been

inter-

preted in terms of charge transfer from metal to zeolite (refs. 4-6). Fig.1 and Table 1 shown that main structure characteristics of Pt particles remained eventually the same after promotion of 0.5% Pt/H-ZSM-5 with chromium

a

0.03

k)/k

kX(

0.08 0.04 0.00

0.04 0.08 0.12

1 b

I Fig. 1. Pt L I 1 1 EXAFS for the Pt-Cr/HZSM-5 catalyst reduced at 623 K (a) and 823 K (b), solid line experiment; dashed line calculated using the parameters given in Table 1.

360 (0.75%) and reduction at 623 K. The average coordination number

for

the first

0

Pt-Pt shell is equal to 5 and Pt-Pt distance is of 0.08 A shorter than for Ptfoil. Pt-0 distance with C.N. 1.4 is also contributed

to EXAFS spectrum. The

comparison of these data with those obtained for 0.5% Pt/H-ZSM-5 calcined at the same temperature (793 K) (refs. 6,7)indicated some stabilization effect

caused

by chromium. TABLE 1 Best fit Pt L I 1 1 edge EXAFS data for 0.5% Pt-0.75% Cr/H-ZSM-5 ~~

Tred,K

Neighbor

Interatomic 0

distance, A

Coordination

Debye-Waller

number

factor, X2

623

platinum oxygen

2.70:O. 01 1.94-0.03

5.0;1.0 1.4-0.5

0.012:O. 003 0.017-0.005

823

platinum oxygen chromium

2.7110. 01 1.97~0.03 2.65-0.02

4.511.0

0.015:0.003 0.016;O. 005 0.015-0.003

1.8~0.5

1.7-0.5

The increase of reduction temperature up to 823 K did not result in noticeable sintering Pt particles but clear evidence of Pt-Cr distance was

found from

analysis of EXAFS spectra (Fig. lb). Including a Pt-Cr interatomic distance into the analysis decreased the fitting index by 51% and this is significant at excellent statistical level of < 0.5%. As was shown in (ref. 13) a

the

significant

level of 5% is considered sufficient if a new shell of neighbors is to be accepted. Preliminary analysis of EXAFS data for second series of Pt-Cr/H-ZSM-5 samples (0.5%Pt-0.75% Cr and 0.5% Pt-1.25% Cr) which differs from above sample only by Si/A1 ratio demonstrate again Pt-Cr coordination (ref. 1 4 ) . The existence of platinum-chromium bond in the catalysts is thus well established and the observ0

ed distance is in the range expected for metallic bonding (2.65-2.70 A ) . It

was

demonstrated that alloying of chromium with platinum decreases the lattice parameter, the intermetallic of Pt3Cr being often observed (ref. 10). The coordination numbers for 0.5% Pt-0.75% Cr/H-ZSM-5 suggests a Pt/Cr ratio of 3/1 for the first series of samples and up to 1011 for second series (ref. 14). This implies Pt3Cr and more diluted in Cr alloys are formed even in catalysts contained an excess of chromium. The additional information on chemical state of chromium and its distribution over zeolite was deduced from XPS data

(Table 2).

In fresh sample chromium

exists as a mixture of Cr(VII and Cr(II1). the rather strong enrichment of

ex-

ternal surface with Cr was observed. Both calcination and reduction lead to de-

361

TABLE 2 XPS analysis of 0.5% Pt-0.75% Cr/H-ZSM-5 Treatment

Pt ( 0 )

Pt(I1)

fresh

B.E..eV %

air,793 K B.E. ,eV

x

72.3 44.3

Cr(0)

Cr(II1)

Cr(V1)

0.44a)

74.0 100

-

577.5 74.8

579.5 25.2

73.9

-

577.0 78.8

580.8 21.2

0.033

55.7

H2,823 K B.E. ,eV %

72.3 100

-

573.4 12.3

-

577.2 87.7

02,298K

B.E.,eV

x

%

0.032

0.037 577.2 100

72.8 100

H2,873 K B.E. ,eV

Cr/Si

0.029 72.2

-

100

577.4

573.0 16.2

83.8

a) Cr/Sibulk = 0.009 crease of Cr/Si ratio which can reflect migration of part of Cr

ions into the

channels. At 623 K in hydrogen only Cr(II1) was found but increase of

reduction

temperature up to 823 K gives rise new doublet in Cr 2p envelope which is characteristic of Cr(0) (refs. 6.12). The degree of Cr(0) reduction reached to 10-15% at 823-873 K. Catalytic activity of Pt-Cr/H-ZSM-5 (1) Hydrogenolysis reactions. Adding 0.75% Cr to Pt/H-ZSM-5 suppressed

its

activity in both ethane and propane hydrogenolysis (Table 3). The difference between two hydrocarbons is that an decrease of reaction rate with C2 was observed only at relatively low temperatures (623-648K), the activity also strongly depends on chromium content. As temperature increased up to 673 K

the difference

in activity of Pt- and Pt-Cr-samples becomes negligible and at higher temperatures the activity of bimetallic sample even exceeds the activity of purePt/H-ZSM-

5. This is formally explained by higher activation energy observed for Pt-Crsamples. In contrast, with C3 the drop of reaction rate by an order of magnitude was observed on whole temperature range (573-648K) with Pt-Cr sample and difference of activation energies for Pt-, and

the

Pt-Cr-samples is smaller than

with ethane. On other hand, the main decrease of reaction rate was found for 0.5% Pt-0.75% Cr and further increase of Cr effects C hydrogenolysis. 3

content to

1.25% only slightly

362

One of the explanation of the observed difference is that zeolite could contribute to hydrogenolysis (dehydrogenation) or cracking of alkanes at higher temperatures. This explanation is rather speculative because nor H-ZSM-5 neither 0.75% Cr/H-ZSM-5 shown no noticeable activity in C2H6 or C3H8 hydrogenolysis in the temperature range studied. But we could not ruled out possible dualfunctional behavior of Pt-Cr/H-ZSM-S. Despite of complications connected with possible zeolite-catalyzed reactions it is obvious that chromium suppressed intrinstic hydrogenolysis activity of platinum. This is particularly confirmed by the fact that 0.5% Pt-0.75% Cr/H-ZSM-5 reduced at 623 K, when no metallic Cr

is

formed, has the same activity as Pt/H-ZSM-5. TABLE 3 Hydrogenolysis activities of Pt- and Pr-Cr/H-ZSM-S Sample

Ethane (Tred = 823 K) 623K

648K

673K

698K

723K

Reaction rate, U x 103, pmole/g 0.SLPt O.YLPt-0.7SLCr 0.5%Pt-1.25%Cr

1.2 0.36

3.0 1.8

0.056

0.45

7.2 9.8 7.3

20

748K s

280 2100

39

40

-

36

120

Ea' kJ/mole

134 232 260

Propane (Tred = 823 K) 573K

598K

623K

Reaction rate, W x 104, pmole/g 0.5%Pt 0.5%Pt-0.75%Cr 0.5%Pt-1.25%Cr

4.6 0.35 0.22

8.6 0.69 0.82

29 2.9 2.5

698K s

1.2 2.0

76 119 143

( i i ) Ethane and methane aromatization. The 0.5% Pt-0.75% Cr/H-ZSM-5 exhibit

higher aromatization activity with C H than pure Pt/H-ZSM-5. About 25% or BTX 26 was found on former catalyst at 823 K while 0.5% Pt/H-ZSM-5 gives only 15% of aromatics at equal conditions. XPS studies of Pt state after different number of C H pulses shown some increase of B.E. Pt 4f7/2 similar to observed earlier 26 with Pt/H-ZSM-5 (refs. 4.5). This means that platinum in small Pt-Cr particles also possessed electron deficiency which is favorable for aromatization reactions (refs. 4-6). In addition the increase of aromatics yield can be related to Pt-Cr interaction in alloy particles which can modify Pt electronic state. Fig.2 shows the dependence of yield of aromatics produced from methane at

363

1023 K over different compositions of Pt-, Cr-, and Pt-Cr/H-pentasils. H-

pentasil is inactive but Cr, Ga, Zn/H-ZSM-5 give a few percent of aromatics. Including platinum substantially increased aromatics yield from 4 to 9%. But most dramatic increase of aromatization activity was found when Cr was added t o 0 . 5 % Pt/H-ZSM-5. Starting from 0.13% Cr aromatization activity jumped up to 14%. The other important function of chromium is prolongation of catalyst life. While Pt/H-ZSM-5 activity sharply and irreversibly declined after 10-15 pulses of methane (ref. 8 ) (not shown on the figure) activity of Pt-Cr samples retained on the maximum level especially for samples with higher Cr content (1.25% Cr). chromium improved the resistance of the catalyst to deactivation.

So,

1 2

T = 1023 K g = 500

mg cat.

VHe = 1.2 1 h-'

a

rl 6

2

0

,

0

Number of cH4 pulees Fig.2. The dependence of aromatics (B+T) yield on CH4 pulse number: 1 - 0.5% Pt-0.75% Cr; 2 - 0.5% Pt-0.75% Cr (was studied by P S I ; 3 - 0 . 5 % Pt-1.3% Cr; 4 - 0 . 5 % Pt-0.13%Cr; 5 - 0.5% Pt; 6 - 0.75% Cr. 5

10

15

Post-reaction studies The enhancement of methane aromatization activity with Pt-Cr sample seems to be indicative of platinum-chromium interaction rather than simple additive effect. This interaction may appear in the course of the reaction which proceeds at high temperature and i n purely reducing atmosphere. To verify this assumption

XPS spectra were monitored after treatment of 0 . 5 % Pt-0.75% Cr/H-ZSM-5 with He and with 1, 3, 10 and 17 pulses of methane at 1023 K (Table 4). As was expected (refs. 4-7) platinum was part,?lly reduced even during air calcination and

its

reduction was progressing in He at 1023 K. After interaction with methane extent of Pt reduction increased but about 1 5 2 0 % of Pt gives spectrum with higher B.E. than for pure metal. By analogy with Pt/H-ZSM-5 treated with hydrogen or ethane this spectrum can be assign to Pt"

clusters. The lack of significant change of

364

Pt/Si ratio during reaction could suggest no severe sintering of metallic particles. Although most of chromium is present as Cr203 the formation of a few percent of metallic chromium and its alloying with platinum might be possible. These results imply that structure of Pt-Cr catalysts after methane reaction resembles the structure of reduced catalyst or sample exposed to ethane at 823 K. In latter case EXAFS suggest the formation of diluted Cr-Pt alloy

particles (ref. 14). TABLE 4 XPS analysis of 0 . 5 % Pt-0.75% Cr/H-ZSM-5 after methane aromatization Treatment

Pt(I1)

Pt(0)

Cr (YI

Cr(II1)

air, 793 K B.E. .eV

73.9

72.3 44

579.6 21

577.4 79

0.12 56

%

He, 1023 K B.E. ,eV

73.8 % 18 CH4,1023 K,lth pulse

71.9 82

B.E. ,eV

71.5

72. 14

%

-

0.23

576.9 100 0.23

-

577* 5 100

86

CH4,1023 K, 3 pulses B.E.,eV

0.56

73.44 23

%

71.4

577.0

77

100

CH4,1023 K, 10 pulses B.E. ,eV

73.3a) 26

%

0.27 71.5

576.Sb)

74

100

CH4,1023 K, 17 pulses 73.3a)

B.E.,eV

22

%

a) P t ' +

;

Pt/Cr

0.33 71.6

-

577.5

78

100

b) a few Cr(Ol was found from peak synthesis

CONCLUSION Let us discuss in conclusion some points concerning local structure of Pt-Cr zeolites and its possible effect on catalysis. Both EXAFS and XPS

data clearly

demonstrated that Cr(0) is formed during H2 treatment at 823-873 K part of which is alloyed with platinum. Other properties of Pt particles such as dispersion,

location, crystallographic structure are rather similar to those in Pt/H-ZSM-5 (ref. 7). Thus we can neglect size effect and discuss the data in terms of Cr promotion or poisoning. Chromium in H-ZSM-5 is present in several forms

:

(a)

365

Pt-Cr alloy; (b) separate Cr(0); (c) Cr(II1) ions in channels; (d) chromia on external surface. Higher XPS Cr(O)/Pt(O) ratios with respect to found from EXAFS suggest that a part of chromium exist in separate phase, although there is no indications from TEM o r XANES that big Cr(0) particles are formed. In addition about 90% of total chromium content is not reduced and it can be located or in the channels either on external surface. The latter is more probable when we take into account higher Cr/Si ratios on the surface. Size and composition of the alloy particles may be deduced from EXAFS results. The sum of Pt-Pt and Pt-Cr nearest neighbor coordination number is 6.2, very close to that found in 13-atom clusters which is 6 . 5 .

For

other samples

even less chromium is included into Pt-Cr particles (ref. 14) although chromium content in bulk alloys can reach to 71% (ref. 15). We can therefore assume that in diluted alloys chromium is homogeneously distributed over the particle. The limit of chromium concentration in alloy can be explained by the fact that only

a part of chromium i s within zeolite structure. Also, Pt particles which size is limited by channel cross-section can dissolve only a limited number of chromium atoms. Besides, the surface of platinum particles can be decorated by chromia. Thus, the difference in catalytic behavior of Pt/H-ZSM-5 and Pt-Cr/H-ZSM-5 is likely to be due to platinum-chromium interaction in small platinum-chromium particles and not so well-defined interaction between Pt particles and Cr20g. In agreement with data (ref. 10) we ascribe the suppression of hydrogenolysis activity to alloying of Pt-Cr. The ensemble model can be proposed as simplest one to explain, for instance, stronger poisoning of propane hydrogenolysis than ethane one for sample 0.5% Pt-0.75% Cr/H-ZSM-5. But stronger drop of ethane hydrogenolysis activity on 0.5-1.25% Cr requires more complex model. It should taken into account platinum-chromium electronic interaction, strong hydrocarbon adsorption on Cr sites and so on. Pt and Cr lie on opposite sides of their respective volcano curves for alkane hydrogenolysis (ref. 16). Hydrocarbons are insufficiently strongly adsorbed on Pt while adsorption is too strong on chromium for optimal activity. This can inhibit hydrogenolysis which proceeds at relatively low temperatures and on pure metallic surface (low conversions) but at

the aromatiza-

tion conditions (high temperatures, coking) the situation may change in such way that stronger hydrocarbon adsorption on Pt-Cr would play a positive role. This is especially important for methane which activated very hardly. Taken into account mechanism of methane oxidative coupling and thermodynamic hindrances of

direct methane aromatization (refs. 8,171 one can expect that first stage of methane activation must be oxidative one and zeolite or oxide oxygen can participate in initializing reaction. But even in pulse mode the methane aromatization does not look as simple stoichiometric reaction and it definitely involves catalytic steps. For this particular system the presence of Cr203as source of mobi-

366

le oxygen is not enough to provide high activity. In contrast, the system PtCr/Cr 0 /H-ZSM-5 gives rise to maximum effect. The Cr reduction by reaction mlx23 ture was recently found for 0.5% Pt-1.25% Cr/H-ZSM-5. Similar activity of three Pt-Cr samples in methane aromatlzation 1s in accordance with EXAFS data lndlcatlng formation Pt-Cr alloys of close composition in samples with very different total Cr/Pt ratios (ref. 14). REFERENCES O.V. Bragln, T.V. Vasina, Ya.1. Isakov, N.V. Pallshklna, A.V.Preobrazhensky, B.K. Nefedov and Kh.M. Mlnachev, Stud. Surf. Scl. Catal., 18 (1984) 31-36. C.W.R. Engelen, J.P. Wolthulzen, J.H.C. van Hoof, H.W. Zundbergen. Proc. 7th Int. Zeolite Conf., 1986, pp.709-716. T. Inul, J.Maklno, F. Maganos, A. Mlymoto. Ind. Eng. Chem. Res. and Develop., 26 (1987)647-652. O.V. Bragln, E.S. Shplro. A.V. Preobrazhensky. S.A. Isaev. T.V. Vasina, B.B. Dysenbina, G.V. Antoshin and Kh.M. Mlnachev, Appl.Catal., 27 (1986) 219- 231. Kh.M. Mlnachev and E.S. Shpiro, React. Klnet. Catal. Lett., 35 (1987) 195206. E.S. Shplro, G.J. Tuleuova, V.A. Zalkovskii, O.P. Tkachenko. T.V. Vasina,O.V. Bragin and Kh.M. Minachev, in H.G. Karge and J. Weitkamp (Eds.), Zeolites as Catalysts, Sorbents and Detergent Builders, Elsevier, Amsterdam, 1989, pp. 143-152. E.S. Shplro, R.W. Joyner, Kh.M. Minachev and P.D.A. Pudney, J. Catal., to be published O.V. Bragin, T.V. Vaslna, A.V. Preobrazhensky and Kh.M. Minachev, Izv. AN SSSR, Ser. Khlm., 1989, pp. 750-751. K. Anders, R . Feldhaus, H.-G. Vieweg, S. Engels, H. Lausch et al., Chem. Tech.(Lelpzlg) 37 (1985) 65. 10 S . Engels, H. Lausch, B. Peplinski, M. Wilde, W. Morke, P. Kraak, Appl. Catal., 55 (1989)93-107. 11 S.J. Gurman, N. Blnstead, I . Ross, J. Phys. C (Solid State Phys.), 17 (1984) 143. 12 W. Grunert, E.S. Shpiro, R Feldhaus, K. Anders, G.V. Antoshln and Kh.M. Minachev, J. Catal.,100 (1986) 138-148. 13 R.W. Joyner, K.J. Martln and P. Meehan, J. Phys. C (Solid State Phys.), 20 (1987)4005. 14 E.S. Shpiro, R.U. Joyner , unpublished results 15 M. Hansen, Constitution of Binary Alloys, 2nd edn. (Mc Graw-Hl11, New York,1958). 16 J.H. Sinfelt, Catal. Revs. Scl. Eng., 3 (1970) 175-205. 17 Kh.M. Mlnachev, N.Ya. Usachev, V.N. Udut and Yu.S. Khodakov, Russian Chemical Reviews, 57 (1988)385-404.

G . Ohlmann eta!. (Editors), Catalysis and Adsorption by Zeolites 0 1991 Elsevier Science Publishers B.V., Amsterdam

367

SPLITTING OF METHANE INTO THE ELEMENTS OVER NICKEL CONTAINING ZSM-5 CATALYSTS

J. HOFFMANN. R. BAUERMEISTER. 8. HUNGER, K. HANTEL, G. ULLMANN. K.-H. STEINBERG and H. SIEGEL' Department of Chemistry, Karl Marx University, Leipzig. DDR-7010 (GDR) 'Chemieanlagenbau Grimma, Grimma, DDR-7240(GDR) SUMMARY Methane splitting was studied over nickel containing Na-ZSM-5 catalysts and at a Ni/AI,O, catalyst for comparison The reaction took place between 673 and 1073 K The highest activity was found between 923 and 1023 K The ZSM-5 catalyst showed the best long time behaviour Good working conditions were only obtained in fluid bed realized by a vibration reactor. In the temperature region between 673 and 973 K a rate equation containing a first order term with respect to methane and a reversible second order one concerrung hydrogen best fulfilled the experimental results INTRODUCTION The formation of carbon filaments (or whiskers) as a result of the decomposition of hydrocarbons on metal surfaces or, respectively. catalysts containing Fe. Ni or Co particles has been known for a long time 11-31, Since this time the practical interest has been mainly directed on the question how to avoid the whisker growth because it is an important reason for the formation of carbon deposits on reactor walls and for mechanical break-down of catalyst particles. A good review about the production of filamentous carbon is given by BAKER [ 4 :I who emphasizes that the scientific interest should be drrected not only on the prevention of carbon-filament formation but also on the possibilities to produce such materials for useful aims, thus for electrodes with high surface area and electrical conductivity, for new generations of

3-D composites 141,for carbon fibres [ 5 ] and possibly as materials for adsorption processes. As could be shown by electron microscopy the whisker consists of a tubular filament with a co-axial channel [ 6 ] and a metal particle at the growing end of the filament. The whisker structure is graphitic [7,81 with the basal planes parallel to the whisker axis. The core of the whisker may be void or filled with carbon material of lower density.

It could be shown that after an induction period which is necessary for saturation of the nickel particle with carbon the single whisker grows after a rate equation of zero order 191. Moreover the rate of growth depends on the reciprocal root of the dlameter of the particle. The growth can be inhibited when the leadmg face is encapsulated b y a layer of amorphous carbon. Whereas the whisker growth is obviously determined by the diffusion of carbon through the metal particle leading to a concentration gradient in the metal there are different interpreta-

368

tions about the driving force of the carbon segregation. In the case of the decomposition of alkenes and alkines it is assumed that the end of the particle connected with the whisker is cooler than the leading face because of the exothermic splitting of the unsaturated hydrocarbons and therefore the solubility of carbon in the metal is lower. But this explanation cannot be valid in case of the splitting of methane or other alkanes because of their endothermicity. Therefore in recent work other explanations are favourized. KOCK et al. [ 10 1 suggest that an instable bulk carbide is formed as the intermediate phase whereas ALSTRUP [ 1 1 1 assumes the formation of a surface carbide and the splitting of methane only on (100) and (110) nickel facets and segregation on ( 1 I 1) facets.

Whereas in literature extensive studies are published about whisker formation on metal surfaces and on supported metal catalysts almost no results can be found regarding pssibilities to lead the process of alkane splitting continuously and to use it for the production of larger amounts of whisker carbon. With respect to this the observation made in this work that the process can be effectively brought about for a period of some hours using a fluidbed reactor is of significance.

EXPERIMENTAL Preparation of catalysts Pellets of catalyst material consisting of 70 mass per cent of Na-ZSM-5 zeolite and of 30 mass per cent of y-A1203 were impregnated with such amounts of solutions of nickel nitrate that the nickel content of the dry material was 20, 10 and, respectively, 5 mass per cent. After heating in dry air for about 3 hours at 773 K the pellets were broken up and classified to a fraction of 0.2 _..0.4 mm.

Catalvtic measureme& For catalytic measurements a vibration reactor was used. It consists of a quartz tube with an inner diameter of 1 cm which is elastically suspended from springs. By means of a magnetic oscillation apparatus the reactor can vibrate with an amplitude of about 0.5 cm and a frequency between I and 50 Hz. usually at I5 Hz. The flow rate is controlled by a Brooks flow controller and measured at the reactor outlet. The gaseous reaction products are analyzed by GC using a 2 m PORAPAK T column. Catalytic experiments were carried out using 0.1 ... I g of catalyst. The catalyst was heated in a stream of dry He up to 773 K and kept at this temperature for 3 hours. then cooled down to 473 K, newly heated in a stream of hydrogen up to 673 K and reduced at this temperature for 3 ... 4 hours. After having switched the gas stream to pure methane the vibration of the reactor was started and the sample was heated with a rate of 10 K per min up to the reaction temperature.

369

Tpr emeriments In order to find out the optimum reduction conditions samples of the catalysts were reduced in a tpr apparatus consisting of a quartz reactor, a furnace controlled electronically and a heat conductivity cell. The reducing gas mixture was argon containing 5 per cent of hydrogen. The heating rate could be chosen between I and 20 K p e r min and was usually 10 K per min.

Evaluation of the experiments

It was assumed that because of the mechanical vibration of the reactor and of a relatively high flow rate a fluid bed reactor was approached which can be kinetically described by the model of a perfectly mixed reactor. For kinetic evaluation it must be considered that the

-

CH, C 2 H, +

reaction is connected with a change of the mole number. In order to count the degree of conversion X first of all the molar fractions

xcH4and xH2were determined by GC.

Then the conversion degree could be expressed as

From this the momentary consumption of methane was calculated after

"CH4

=

VCH4

(2)

'

(vCH; inlet flow rate of methane). The whole amount of carbon formed until the reaction had been stopped could be calculated by numerical integration: t

a

SVCHddt

(3)

For the kinetics of methane splitting a rate equation of the following form was assumed:

370

In case of the perfectly mixed flow reactor the rate

results from

I

ngH4= molar flow at the reactor inlet nCH4=molar flow at the reactor outlet V

=

volume of the reactor

The partial pressures of methane and hydrogen can be written in the following manner considering the change of the mole number during reaction ( p

When substituting k, by k , / K ngn4(X3

+

2X2

P +

- total pressure):

it results from equation ( 4 ) :

XI

=

klpV - klpV(l

+

4p/Kp).X2

(7)

From equ (7) k , and Kp can easily be calculated by regression and from the rate and equilibrium constants the activation energy EA1 and the heat of reaction ARH can be determined using the equations of Arrhenius and van t Hoff RESULTS AND DISCUSSIGN Studies of the methane splitting reaction were made mainly over Na-ZSM-5 zeolites containing 20 mass p e r cent of nickel (0.2 NiNa-ZSM-5) because these catalysts provide the best results with respect to activity and resistance against deactivation. For comparison same experiments with NiNaX. NiNaY. Ni/A1,0,

and NiNa-ZSM-5 catalysts of lower nickel content

were carried out. As could be observed in all cases carbon formed by decomposition of methane under

fluid-bed conditions balls together to almost spherical aggregates above the catalyst bed but still containing microscopic particles of the catalyst.

In order to study the conditions of reduction of the Ni" containing catalyst precursors more in detail tpr experiments were carried out. Fig. I shows a typical reductogramme of a ZSM-5 catalyst containing 20 per cent of nickel. The heating rate was 10 K per min. The reduction seems to occur as a relatively

371

uniform process as can be seen from the only less structured peak'with the maximum at about 690 K. Taking into account that the peak-maximum temperature decreases when the heating

rate is diminished it can be concluded that a reduction temperature of about '100 K should be

Fig. 1. Tpr spectrum of a 0.2 NiNa-ZSM-5 zeolite (heating rate 10 K per min)

H

heating period

tim on stream [min]

Fig. 2. Catalytic activity in dependence on time of a 0.2 NiNa- ZSM-5 zeolite

372

sufficient. Indeed, a reduction degree of about 80 per cent could be calculated from the peak area related to the whole amount of nickel. The measured peak-maximum temperature is in good agreement with results of other authors who studied the reduction of NiO species [ 121. Considering the activity of the various catalysts it can be stated that over nickel faujasites only a very low amount of carbon was formed until the reaction broke down by rapid deactivation (at 1 g of 0.45 NiNaX only 0.13 g of carbon were formed. the same amount of

0.5 NiNaY led to 0.2 g of carbon). Na-ZSM-5 catalysts containing 5 or. respectively. 10 mass per cent of nickel showed lower activity and more rapid deactivation compared to catalysts with 2 0 per cent of nickel. Comparison of the activity and long-time behaviour of convenient Ni/AI2O3 hydrogenation catalysts containing about 50 per cent of nickel with that of 0.2 NiNa-ZSM-5 catalysts was of special interest: Whereas the catalytic activity of the N i l A1,0,

catalyst is comparable

to that of the 0.2 NiNa-ZSM-5 its long-time behaviour is much worse. Thus in an experiment with 0.2 NiNa-ZSM-5 after 7 hours of time on stream the fourfold amount by weight of carbon related to the weight of catalyst was formed but the degree of conversion diminished only from 0.6 to 0.4 whereas the Ni/AI,O,

catalyst produced only

twice of its own weight until it deactivated dramatically. In fig. 3 the momentary methane consumption at constant inlet flow rate of methane is shown in dependence on temperature for the 0.2 NiNa-ZSM-5 catalyst. Catalytic activity can be observed between 673 and 1073 K with a maximum in the 1000 . . . 1030 K region.

Fig. 3. Catalytic activity of a 0.2 NiNa-ZSM-5 catalyst depending on temperature

373

Similar behaviour was found by other workers [6 1 which used Ni / MgU catalysts. The loss of activity at temperatures higher than loo0 K should not be due to deactivation by carbon

deposition but by adsorption effects in a rate determining surface reaction [ 13 1. The results obtained in this work for the Ni-ZSM-5 and Nil A1,0,

catalysts are contrary to this explanation

because almost no catalytic activity was observed at temperatures beolw 1000 K when the catalyst had worked before at temperatures over 1100 K (in the temperature region where the reaction rate is decreasing) indicating a poisoning of the active nickel centres by deposition

of amorphous carbon. In fig. 2 the conversion is shown in dependence on time on stream. As can be seen the conversion degree decreases in the course of 7 hours only from 0.6 to 0.4. When carrying out the same experiment without vibration of the reactor the activity dramatically decreases after short reaction times because of sticking together of the catalyst pellets. Kinetic studies were carried out in a temperature region between 673 and 923 K. In contrast to other authors 1141 an inhibition by hydrogen was not observed, thus a rate equation ex-

pressing the conditions for the equilibrium reaction CH,

-

C

+

2H2 could be used (equ. (4)).

In case of the 0.2 NiNa-ZSM-5 catalyst the activation energy of the splitting was (1 13

2.2) kJ

per mole, the reaction heat ( 1 24 : I . 2 ) kJ per mole. For the Ni/A1203 catalyst a n activation energy of ( 1 10 : 1.4) k] per mole and a heat of reaction of ( I 25.4

1.2) kJ per mole could

be calculated. These values are in good agreement with results of other authors

r 151.

In figs. 4 and 5 Arrhenius and van? Hoff plots are shown for the 0.2 NiNa-ZSM-5 catalyst. In Tab. 1 the equilibrium constants calculated from the experiments or, respectively. from thermodynamic data are listed together with the respective equilibrium degrees of conversion.

TABLE I Equilibrium constants and equilibrium degrees of conversion in dependence on temperature

T[Kl theor.

Kplbarl 0.2 NiNa-ZSM-5

xNi/AI,O,

theor.

0.2 NiNa-ZSM-5 ~

873 923 973 1023 1073

2.1 3.92 7.32 12.35 20.7

I .34 3.44 7.89 16.70 33.0

0.921 2.35 5.43 11.6 2 3.0

0.59 0.70 0.8 I 0.8 7 0.9 2

0.50 0.6 8 0.8 2 0.90 0.94

Ni/A1203 0.4 3 0.6 1 0.76 0.86 0.9 2

The results show that the equilibrium conversion can be higher than the value calculated from the theoretical thermodynamic data, especially in the upper temperature region. This can be explained by the fact that the gas phase does not contact with the pure graphitic carbon surface but with a carbidic one at the exposed surface of the nickel partic!e (e.g. Ni3C ).

374

B

6

Fig. 4. Arrhenius plot for a 0.2 NiNa-ZSM-5 catalyst

1

.....

. . -..

...

. _..._ _ . _..-.

'\

C b?

r;

-1

+

-2

'4, \ '

-3

-4

\. ,Q\,\

'\

1

1.6

375 CONCLUSIONS The splitting of methane can be well realized on nickel containing Na-ZSM-5 zeolites. When using a vibration reactor which allows fluid bed condtions a quick deactivation of the system by sticking together of the catalyst pellets can be avoided. The good activity and long-time behaviour of the NiNa-ZSM-5 catalysts could be explained: by their high thermal stability by their properties with respect to fluidized-bed conditions (e. g. low density) by a favourable distribution of the nickel-particle size with respect to whisker formation (in our case between 5 and 30 nm as could be proved by electron microscopy) and possibly by the avoidance of formation of carbon forms other than whisker-like Kinetics of the reaction follows a rate equation which contains the methane concentration in a first order term and the hydrogen concentration in a second order one expressing the reversible step. The activity of carbon does not appear in the rate equation and no inhibition by hydrogen was observed. At the temperature which represents the region of highest activity (about 1023 K ) a degree of conversion of nearly 0.9 can be realized (higher than the equilibrium degree calculated from thermodynamic data). REFERENCES

I P. A. Tesner and I.S. Rafalkes. Dokl. Akad. Nauk SSSR, 87(1952)821 2 R.T.K. Baker and P.S. Harris. J. Phys. E., 5(1970)793 3 J.R. Rostrup-Nielsen. J. Catal.. 2 7 ( 19 7 2 134 3 4 R.T.K. Baker. Carbon. 27(1989)315 5 G.G. Tibbetts. Appl. Phys. Lett.. 42(1983)666 6 J.R. Rostrup-Nielsen, Catalytic Steam Reforming, in: J.R. Anderson and M. Boudart (Ed.). Catalysis - Science and Technology. Vol. 5, Springer Berlin, Heidelberg. New York. Tokyo, 1984. pp. 7 4 - 7 5 7 T. Baird. J.R. Fryer and 8. Grant, Carbon, I2(1974)591 8 S.D. Robertson, Carbon, 8(1970)365 9 R.T.K. Baker, P.S. Harris, R.B. Thomas and R.J. Waite. 1. Catal., 96(1985)468 10 A.J.H.M. Kock. P.K. de Bokx. E. Boellaard. W. Klop and J.W. Geus, J. Catal.. 96(1985)468 I 1 I. Alstrup. J. Catal.. 109(1988)241 I 2 N.W. Hurst. S.J. Gentry, A. Jones and B.D. McNicol. Catal. Rev. - Sci. Eng. 24(1982)2 3 3 I 3 J.R. Rostrup-Nielsen and D.L. Trimm. J. Catal.. 48 (1 977) 155 1 4 C.A. Bernardo. I. Alstrup and J.R. Rostrup-Nielsen. J. Catal.. 96 (1 9 8 5) 5 1 7 15 B.K. de Bokx. A.J.H.M. Bock, E. Boellaard. W. Klop and J.W. Geus. I. Catal.. 96(1985)454



SULFUR TOLERANT SHIFT REACTION

Ni-Mo-Y-ZEOLITE

CATALYSTS

377

FOR

WATER-GAS

M-LANIECKI and W.ZMIERCZAK Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan (Poland) SUMMARY Molybdenum sulfide and nickel-molybdenum sulfide catalysts were prepared from Mo(CO), encaged in Y and stabilized HY zeolites. These catalysts were characterized by XRD, IR, ESR spectroscopy and NO sorption capacity. Mo(CO& based catalysts are highly reactive in water-gas shift reaction and this is related to the high dispersion of molybdenum sulfide. Ni and Mo containing zeolites indicate the existence of synergetic effect. The synergy is confirmed by the presence of a band at 2082 cm during adsorption of CO. Catalysts prepared from dealuminated supports show low activity, mainly due to the limited access of molybdenum carbonyl into the zeolite cavities. ESR spectroscopy indicates the formation of sulfur chain biradicals after interaction of HIS with molybdenum containing supports. INTRODUCTION It is known that both high-temperature and low-temperature water-gas shift catalysts, based on iron oxides and copper-zinc oxides respectively, are highly sensitive towards sulfur contamination of the feed. In order to overcome this problem a new class of catalysts, similar to the hydrodesulfurization catalysts, based on alumina-supported Co-Mo sulfides has been prepared and recently introduced into the market. Literature concerning the application of zeolites either as catalysts or as the supports in water-gas shift (WGS) reaction is scarce (ref.1-4). The Ru-X (ref.1) and Ru-Y (ref.2) zeolites have been reported to be active in WGS reaction, but their activity and especially stability were relatively low. Iwamoto et al. (ref.3) studying transition metal ion-exchanged X and Y zeolites established that only C U * ~and Ni” containing zeolites showed good performance in WGS and this was related to the low electronegativity of the exchanged transition metal. Lee and co-workers (ref.4) studying the kinetics of WGS reaction over Ni-Mo-Y-zeolite catalysts in the sulfided state established high reactivity in this reaction and excellent resistance against H,S and NH,.

378

Studies performed in the previous years by different groups (ref.5-7) and concerning molybdenum loaded zeolites indicate that saturation with molybdenum hexacarbonyl leads towards the formation of highly and homogenously dispersed molybdenum. This can be subsequently transformed into highly dispersed supported molybdenum sulfide species (ref.8,9). In contrast, molybdenum containing zeolites prepared from ammonium heptamolybdate (AHM) form bulky, weakly dispersed Mo sulfides. Our recent studies (ref.9) showed that Mo(CO), loaded Y-zeolites in the sulfided state indicate high activity in WGS reaction. In the present study the catalytic activity in WGS reaction over sulfided Ni-Mo-Y and Ni-Mo-USY zeolites was measured and the characterization of these catalysts has been given. EXPERIMENTAL Mat er i a1s NaY from KATALISTIKS (Si/A1=2.56) was applied as a starting material for preparation of nickel and amonium exchanged zeolites. The ammonium containing samples were obtained by a single or multiple exchange with 0.25M aqueous solution of NH,C1 at 345 K. Three series of stabilized Y-zeolites, denoted hereafter as USY-1, USY-2 and USY-3, were prepared from NH4NaY forms, differing in the degree of exchange, by hydrothermal treatment at 875 K for 4.5 hours under 100% steam. Nickel was introduced into the zeolites before molybdenum by ion-exchange applying nickel nitrate solution, both for NaY and USY series. Molybdenum was loaded into the zeolites by saturation with molybdenum hexacarbonyl vapours. Zeolitic supports after activation at elevated temperatures in hydrogen were exposed to the Mo(CO), vapours passing the support in a hydrogen stream for 10-15 hours at room temperature. Samples completely saturated with Mo(CO), were subsequently partially decarbonylated at 425 K for one hour in a stream of hydrogen and next slowly exposed to the air at room temperature with the use of leaking valve. Each sample, before any experiment, was presulfided for two hours at 675 K with a 10 vol.& mixture of hydrogen sulfide in hydrogen, flowing with the rate of 2 dm3 h-'. Chemicals a_nd Gases Doubly sublimed Mo(CO), from Aldrich was used for saturations. Hydrogen and helium were freed from impurities by passing through

379

the MnO/SiO, oxygen-water scavenger (ref.10). CO and NO were from Fluka and H,S from Intermar (The Netherlands) and were used without further purification. Characteri zatLog Zeolitic supports as well as Ni and Mo containing samples were characterized by XRD, NO sorption capacity. FTIR, ESR and MAS NMR spectroscopy. The experimental details concerning NO adsorption and ESR measurements are given elsewhere (ref.9). For infrared measurements self-supporting wafers with the -1 were mounted in the infrared cell "thickness" of 3-5 mg cm operating under dynamic conditions. Samples after activation were exposed for 30 minutes to the Mo(CO), vapours according to the procedure already described in the previous section (Materials). IR spectra were recorded applying FTIR Bruker IFS 113v spectrometer. 29 27 Si and A1 MAS NMR measurements of USY and Ni-USY were performed for non-sulfided samples applying Bruker MSL-300 spectrometer. Catalytic Activity Measurements Grained samples (0.5-lmm) of zeolites weighing 0.25 or 0.59 were pleaced in a continous-flow reactor operating at atmospheric pressure. Samples after presulfidation were purged 30 minutes with He at 6 7 5 K and next the temperature was lowered to 6 2 5 K and the WGS reaction started. Gas mixture introduced into the saturator was composed from equimolar amounts of CO and H, and 2 vol.% of H,S. Reaction was usually performed for 20 hours and the hydrogen sulfide concentration was mantained constant during the reaction time. H20 : CO ratio was changed in different experiments and varied between 0 . 7 to 3. RESULTS AND DISCUSSION It was found previously (ref.9) that molybdenum sulfide supported on sodium or hydrogen form of Y-zeolite indicate relatively high activity in WGS reaction but with the tendency to the slow deactivation. It was expected that certain loss of the zeolite crystallinity can be responsible for the decrease in activity. The XRD measurements indicated, however, negligible changes in the support crystallinity during or after the reaction.In the present study three series of stabilized Y-zeolites have been apllied as the supports in order to minimalize the changes in the catalysts crystallinity during the reaction time as well as to study the influence of the support dealumination on the behaviour in WGS reaction.

380

TABLE 1 Characterization of Ni-, Mo- and Ni-Mo-Y zeolite catalysts Cata 1yst

Si/A1 29 Si NMR AAS

Composition (wt.%)

Ni NaY NaN i Y HNaY HNaN i Y USY-1 USY-I-Ni USY-2 USY-2-Ni USY-3 USY-3-Ni

2.56 2.63 2.60 2.78 2.62 2.48 2.76 2.48 2.56 2.55

10.3 10.0 0 10.3 1.1 8.0 0 5.5 1.2 5.0 0 4.8 1.0 4.6 0 4.5 4.0 0.8 0

2.8

3.28 3.78 4.19 4.47 5.02 5.30

Mo

Surface * area 2

- 1

[m 9

900 830 930

840 860

-

830

-

800 780

I

NO adsorption ** capacity [mmol g- ‘ 1 0.184 0.134 0.166 0.121 0.053 0.136 0.056 0.145 0.097 0.154

* “BET analogous surface area” of the support activated at 675 K * * after activation at 675 K , MO(CO)~ deposition, decarbonylation at 425 K and sulfidation at 675 K Table 1 shows certain properties of the catalysts prepared by standart procedure described in the previous section. A l l supports were activated 2 hours at 675 K before Mo(CO), admission. “BET analogous surface area” was practically the same for sulfided or non-sulfided molybdenum-free supports. After deposition of tiiolybdcnum onto non-stabilized supports, followed by sulfidation, t h e surface area was usually reduced by about 30-40%. This is indicative of the location of molybdenum sulfide-like species inside the zeolite porous system (ref.9). In contrast to the non-stabilized zeolites, steam calcined samples indicate a lower surface area, due to the dealumination process. After deposition of molybdenum and subsequent sulfidation, ultrastable samples indicated only minimal decrease of surface area. High concentration of the non-framework aluminum atoms at the zeolite surface (ref.ll), or the formation of boehmite-like structures inside the supercages (ref.12) can cause a partial blockage of the zeolite porous system towards nitrogen molecules and can protect Mo(CO), molecules from entering zeolite porous system. The lowered ability to adsorb Mo(CO), at the inital preparation for USY based catalysts seems to confirm via steam calcination this suggestion. However, dealumination causes changes in the acidity of the support and these changes also influence the ability to adsorb Mo(CO), during catalyst stage of

381

s-'

,

30Q WAVENUMBER

Fig.l.Infrared spectra of USY supports activated at 675 K. HY-obtained from NH,,Y (78% exchanged) calcined 2 hours at 675 K.

34 CM-1

Fig.2.Infrared spectra of nickel exchanged NaY zeolites (NaNiY) a)after activation at 675 K b)after activation at 475 K c)after activation at 475 K , saturation with M O ( C O ) ~ ,and decarbonylation at 425 K.

preparation (ref.7). It is known that basically one aluminum atom in the zeolite framework of HY zeolite can give rise to one acid group. Hence, a decrease in the intensities of OH stretching bands at 3550 and 3650 cm-l with progresive dealumination was expected. Y-zeolites. Fig.1. shows OH bands for the series of dealuminated In our case, ultrastabilization practically eliminated OH groups from the small cages (absence of the band at 3550cm-') and reduced significantly the amount of acidic OH groups in supercages (3650cm-I). The appearance of the OH stretching bands at 3690 and 3610 cm-', non reactive towards NH, and pyridine (ref.l3), as well as silanol groups (at 3745 cm-') is indicative of strong 29 dealumination (compare also SiNMR results). The shoulder bands at

382

Fig.3. Infrared spectra of molybdenum loaded NaNiY zeolite a)after activation at 675 K b)after activation at 675 K, saturation with M~(CO)~.followed by decarbonylation at 425 K c)as b) with subsequent interaction with HZS at room temperature.

b

1

A

3675 and 3590 cm-' appearing for USY-3 can be attributed to the hydroxyls connected with extraframework aluminum species (ref. 14). Nickel-containing ultrastabia lized samples showed additional BOO 170 WAVENUMBER CM-' decrease of OH bands intensity, similarily to the findings presented in ref.14. The characteristic OH bands formed upon nickel exchange with NaY are shown in Fig.2. Here, after activation at 675 K the typical acidic OH groups (3645 and 3554 cm-') are present. Activation at 475 K leads towards the appearance of new weak band at 3610cm-'. This is usually ascribed to the characteristic dissociation of water molecules over the zeolites containing divalent cations (ref.13) with simultaneous formation of Me+(OH) species. Adsorption of Mo(CO), at room temperature followed by thermal decomposition at 425 K yielded the subcarbonyl species with general formula cd Mo(CO), (ref.8,15). Irrespective of the support used, after partial decarbonylation, more or less complex infrared spectra in the region of carbonyl groups vibrations were observed (example presented in Fig.3.). Usually USY and HY supports give rise to the more complex infrared spectra in the reg on of carbonyl vibrations than for NaY or NaNiY samples. The decr ase in intensity of OH bands for USY and NaNiY after partial decarbonylation at 425 K is indicative of the oxidation of molybdenum by zeolite protons (ref.5). Partially decarbonylated samples after intereaction with H,S at room temperature show a significant reduction in the

383

carbonyl bands quantity (2031 sh, 1984, 1946 sh, 1845 cm-') and absorbance value (see Fig.3.). Exposition of partially decarbonylated samples towards H,S at 425 K resulted in a complete disappearance of carbonyl groups and further decrease in the OH bands intensity, suggesting the formation of sulfided molybdenum species. Interaction with H,S at 675 K additionally lowered the intensity of OH groups vibrations. These observations show that after decarbonylation, interaction with H,S at room temperature leads towards Mo(CO),-SH like species (ref.8), whereas at higher temperatures,MoSx structures are formed (1< x . At small 0 values, using the relationship between continuous and jump diffusion, D = /67,the HWHM becomes equal to h ( Q=) DO2. Clt high (1 values, the half-width approaches t h e mean j u m p rate

r. The broadening values obtained for ethane and benzene in NaX were fitted with the jump diffusion model of Singwi and Sjolander /21/. When the molecule spends more time in i t s oscillary state than diffusing, the shape of the quasi-elastic peak i s still a Lorentzian, but the HWHM i s given by

This model w a s used for ethane and benzene in NaX because the broadening curves converged more slowly towards their asymptotic values at high Q than for methane in NaZSM-5. b) NMH pulsed field gradient technique In pulsed field gradient NMR, spin echo attenuation W d u e to diffusion in an isotropic system such a s zeolite X with a selfdiffusion coefficient D follows the relation /3/

448 with the

respectively,

the i n t e n s i t y ,

which a r e i n the given case t h e protons o f

the hydrocarbon molecules.

-

deno-

t h e w i d t h and t h e separation o f

Y stands f o r t h e gyromagnetic r a t i o o f

f i e l d g r a d i e n t pulses.

the n u c l e i under study,

n

6: and fl

the parameters o f t h e s p i n echo experiment g,

ting,

The measurements a r e based on t h e n / Z

-

echo pulse sequence.

For a n i s o t r o p i c systems l i k e d i f f u s i o n i n Z S M - J / s i l

calite

eq

.

( 4 ) has t o be replaced /3,22/ by

+ -b

Pr(g,S,A) = exp I-Y with

-b + D

vector,

2

2-*--+ 6 g D g

(5

+

and g denoting t h e d i f f u s i o n tensor and t h e f i e l d respectively,

I t has been found, no

( A - 6/3)3

significant

i n s t e a d o f the s c a l a r q u a n t i t i e s however,

gradient

D and 9.

t h a t f o r methane i n ZSM-5

d i f f e r e n c e between t h e values

p r i n c i p a l elements o f the d i f f u s i o n tensor

of

/22/,

r i m e n t a l data are c o r r e c t l y described by eq.

the

there i s

individual

so t h a t the expe-

( 4 ) w i t h a mean s e l f -

+ D Z Z ) denoting one t h i r d c o e f f i c i e n t D = 1/3 (Dxx + D YY o f t h e t r a c e o f t h e d i f f u s i o n tensor /3, 22/. diffusion

RESULTS a ) Methane d i f f u s i o n i n ZSM-5 The

results

elastic

obtained

i n the

QENS f o r t h e

broadening

peak versus Q2 a t 200 K a r e shown i n F i g .

of

the

1 for different

loadings. Fig. 1 Elastic peak broadening versus GI2 for CH i n ZSM-5 a t 2OOK for the loadings 2, ( I4, )and 8 CH4 per U.C.

(v)

(A)

I t f o l l o w 5 from Fig.1

QENS

t h a t i n the

experiments the broadening

of

the e l a s t i c peak as a f u n c t i o n o f Q deviates that

from

a straight

line

distribution

l e n g t h s has t o be interpretation

o f the

considered.

occur

ments vidual

This

lengths

w i t h i n t h e channel

a s w e l l as across channel

a

jump

i m p l i e s t h a t molecu-

l a r jumps w i t h v a r y i n g jump may

so

a jump d i f f u s i o n model w i t h

Gaussian

2

the

intersections.

segindiThe

s e l f - d i f f u s i o n c o e f f i c i e n t s obtained a r e presented i n Table 1.

O

oI2

0:4

'

6.6

a

449

These data yield an energy of activation for t h e self-diffusion o f the order of 4...5 kJ mol-l. The mean jump lengths are found to be larger at small loading and high temperatures /12,14/. For methane in NaZSM-5, the mean jump lengths calculated for small and high loadings at 200 K amount to 1.04 and 0.87 nm, respectively. For the NMR PFGT experiments, Fig. 2 represents the spin echo attenuation due to molecular self-diffusion of methane in the NaZSM-5 at 250 K at different sorbate concentrations. In complete agreement with the behaviour predicted by eq.(4), the plots of 1nPc y2 92 ( A - 1/3 6 ) provide straight lines, whose slopes VS. should coincide with the intracrystalline diffusivities. Cornparing the slopes of the spin echo attenuations and using eq. (41, the mean diffusion coefficients D = 1/3 ( D x x + Dyy + D Z z ) are found t o decrease slightly with increasing sorbate concentration from 6.3 x 10-5 cmzs-l ( 4 CH4/u.c.) to 4.5 x 10-5cm2~-1 ( 1 2 CH4/u.c.). The diffusivities thus obtained are shown in Table 1. From the Arrhenius plots of the NMR self-diffusion coefficients of methane 1251, the energy o f activation for the intracrystalline self-diffusion of being in good methane in NaZSM-5 amounts to (4.7 f 0.7) kJ mole-' o f methane in NaZSM-5

agreement with the QENS data of Ed. In addition to the values of D and Ed, a further agreement between P E N S and NMR PFGT i s found by comparing the individual molecular j u m p lengths. In neutron scattering experiments /12,14/ a s well a s in a previous NMH study /25/ dealing with the microdynamics of hydrocarbons in ZSM-5 type zeolites, it has been shown that the mean jump lengths of light paraffins decrease from 1...1.2 nm at low sorbate concentration to a nm for high loadings. value of 0.7...0.8 Fig.2 Spin echo attenuations d u e to intracrystalline selfdiffusion of CH4 in NaZSM-5 at 250 K for the loadings ( 0 ) 4 ; (.)8and ( 0 ) 12 molecules per u.c., respectively. For calibration, the echo attenuations for neat water ( ) and liquid ) , whose selfammonia diffusion coefficients taken

100

m

80 60

c C

-=!a 2 .-. *

I

*O

(A

10 1

-2

3 4 5 6 7 8 9 $ g 2 6 * ( A - + & I / re\.units

10

v

from literature amount t o 2.3 x loF5 cm2s-' /23/ and 1.5 x cm2s-' /24/, respectively, are included.

450 TABLE 1

Intracrystalline self-diffusion coefficient of methane in NaZSM-5 measured by QENS and NMR PFGT

loading/ CH4iu. c

.

D

T

= 200 K

~

~

99.5

were

performed

in

a

fixed-bed

continuous f l o w glass

w i t h 6% 2,3-DMB (>99 %), 6% 3-methylpentane (>99.5%) o r 2%

%) i n d r y

helium a t 101 kPa. The products o f t h e cracking

reactions were gaschromatographical l y analysed. Apparent

first

order

r a t e constants,

kaPP,

were determined according t o

Sill

and Si12 revealed t h a t these

conventional equations (3). RESULTS AND DISCUSSION Characteri sation Scanning crystals show

a narrow

fraction pm i n

the

e l e c t r o n microscopy are

invariably

(SEM)

twinned.

of

No gel phase i s detected. The batches each

p a r t i c l e s i z e d i s t r i b u t i o n . Sample Si12 does c o n t a i n a c e r t a i n

o f agglomerates. SEM o f Si13 shows s p h e r u l i t i c c r y s t a l l i n e m a t e r i a l 5 size. The ZSM-5 sample consists o f smaller s p h e r u l i t e s (0.1-1 pm). From

silicon-to-aluminium

ratios

i n Table

1 it

i s clear that true Al-free

470

silicalite

is

n o t r e a l i s e d i n the a l k a l i n e - f r e e synthesis g e l , mainly because of the i m p u r i t y of the s i l i c a source (Ludox AS40/Dupont, 100 ppm A l ) . A1 uminat i o n Alumination

of

and Si12 has r e s u l t e d i n a s u b s t a n t i a l increase o f the

Sill

aluminium content (Table 2) whereas t h e adsorptive p r o p e r t i e s f o r n-hexane have remained

unchanged.

Results

of

the

elemental

analysis

reveal

TABLE 2 Characterisation o f aluminated s i l i c a l i t e samples SilAll

SilA12

Si/A1 (before NHi-exchange)*

31

46

Si/A1 ( a f t e r NH;

70

85

9.98

10.09

Sample

-exchange)*

n-Hexane ads. (wt%)

t h a t ammonium removes between 20 and 30 % o f t h e deposited aluminium from the sample. This l o s s o f aluminium can be explained w i t h t h e r e a c t i o n scheme proposed by Chang e t a l . ( l l ) , e.g. an i n s e r t i o n o f aluminium i n t o t h e z e o l i t e framework w i t h a concomitant formation o f counter c a t i o n i c aluminium. Figure 1 the s i l i c o n and aluminium d i s t r i b u t i o n across an a- ( b - ) section

In of

a polished c r y s t a l o f sample SilA12 are shown. The A l - l e v e l does n o t vary

significantly

over

the

major

part

of

the

polished crystal

surface.

Nevertheless, some Al-zoning seems t o e x i s t over a very small distance, v i z . 2 3 pm as i n d i c a t e d by t h e i n t e r v a l o f t h e step advancement. Adsorot ive d i f f u s i on An analysis o f adsorption serious samples.

deviations Thus

in

isotherms o f 2,3-DMB revealed t h a t t h e r e are no

adsorptive p r o p e r t i e s o f t h e (dealuminated) s i l i c a l i t e

a comparison

of

data

on

diffusion

obtained w i t h d i f f e r e n t

adsorbent samples i s j u s t i f i e d as the adsorption c h a r a c t e r i s t i c s are comparable f o r each sample. In

Figures

coverage

2 and 3

uptake curves are shown f o r 2,3-DMB d i f f u s i o n a t low

(maximum 3.5 molec. 2,3-DMB/u.c.)

i n a volumetric system a t 373 K and

471

--: 5 0 0 0 . 4000 L

b

SI

A

.r

%

c

Al

3000.

1.2

1.6

1.4

K"

10'/T,

b-dlractlon (mlcron) Flg. 1:

FIg. 4: Tamprraturr rffoct on k- for 2,3-dlmothylbutano crocklng on sarnplrs SIIAll. A and SllA12, and on H-ZSM-5, 0

SI- an Al-dlstrlbutlon across an alurnlnotrd crystal of SIIAIZ.

.

r;

1.0

I 1.0'

Y

0.8 0.6

0.4

0.2

0.0 1 0

Y 0.8

I

100

0.6

0.4

0.2 *

J

300

200

400

0.0

4 0

1000

3000

2000

t, Flg. 2: Uptako of 2.3-dlmrthylbutana In Slldl, 0 and SlldZ, at 373 K. Sorb. covrragr= 0.5 molrc./u.c.

+

FIg. 3: As Flguro 2. T=473 K Sorb. covorago=0.15 maloc./u.c.

472

473 K , respectively. It is clear that the theoretical curve follows the data reasonably well up to high relative uptake. TABLE 3 Apparent and intrinsic diffusivity of 2,3-DMB in dealuminated silicalite samples Silld and Sil2d at 373 K and 473 K

Sil Id 373 473

4.i*io-16 1 .5*10-14

Sil2d 1.0*10-15 1 .4*10-14

Si 1 Id 2.8*10-16 1 .7*10-14

Si 12d 5.5*10-16 1 .6*10-14

In Table 3 results are given o f the curve fitting procedures. Apparent together with intrinsic diffusivities are given, the latter being obtained by calculating Darken’s correction factor from the respective isotherms (10). Furthermore, for each silicalite sample a specific diffusion length is given, taken t o be half the breadth o f a crystal. From the diffusivities obtained at 373 K and at 473 K an activation energy o f diffusion is calculated o f about 52 kJ/mol e . Both the diffusivities and the activation energy compare well with data obtained (using other techniques) on diffusion o f other hydrocarbons in silicalite/ZSM-5 at other temperatures. The diffusivity o f 2,3-DMB is, after extrapolation t o corresponding temperatures, substantially lower than that of 3-methylpentane in ZSM-5, viz. by a factor o f 100 (10). Contrarily, compared to 2,2-dimethylbutane (Deff= 6.5*10-19 m2/s) the migration rate o f 2,3-DMB is enhanced by a factor o f about 500 (2). ’Catalytic’ diffusion First order rate constants have been determined for the 2,3-DMB cracking reaction over the directly synthesised H-ZSM-5 sample and over the aluminated silicalite samples SilAll and SilA12. The rate constants relate t o steady state situations. Figure 4 shows an Arrhenius plot revealing that the apparent activation energy of reaction is highly dependent on the size o f the zeolite

473

c r y s t a l s . I f t h e c o n c e n t r a t i o n s o f a c t i v e s i t e s i n t h e d i f f e r e n t samples do n o t diverge

t o a l a r g e e x t e n t t h i s phenomenon i s i n d i c a t i v e o f a r e a c t i o n which i s

l i m i t e d by zeolite

the

diffusional

r a t e o f r e a c t a n t and/or p r o d u c t molecules i n t h e

channels.

As 2,3-DMB i s a f a r more b u l k i e r molecule t h a n i t s c r a c k i n g pro duc t s i t i s presumed t h a t t h e r e a c t i o n i s l i m i t e d by r e a c t a n t d i f f u s i o n . The t h e ZSM-5 sample has t h e h i g h e s t a c t i v a t i o n energy o f w e l l over

r e a c t i o n ov er

100 kJ/mole. T h i s i s i n d i c a t i v e o f a c o n v e r sion n o t l i m i t e d by i n t r a c r y s t a l l i n e

2,3-DMB d i f f u s i o n . The

use

requires determined

o f t h e Thiele theory i n determining t h e d i f f u s i v i t y o f t h e reactant knowledge o f t h e i n t r i n s i c a c t i v i t i e s o f t h e z e o l i t e samples. We have intrinsic

activities

by

employing

a reference reaction, viz. n-

butane c r a c k i n g . T h i s r e a c t i o n i s expected t o occur w i t h o u t any i n t e r f e r e n c e o f diffusion

i n t h e ZSM-5 c r y s t a l s because t h e k i n e t i c d i a m e t e r o f n-but ane ( 0 . 4 9

nm) i s l o w e r t h a n t h a t o f 2,3-DMB (0.56 nm). F i r s t o r d e r r a t e c o n s t a n t s f o r t h e n-butane c r a c k i n g r e a c t i o n over t h e H-ZSM5 sample and ov e r t h e a l u m i n a t e d s i l i c a l i t e s a r e again present ed i n t h e f orm o f an A r r h e n i u s be to

p l o t i n F i g u r e 5. From t h e r e s p e c t i v e a c t i v a t i o n e n e r g i e s i t can

concluded t h a t a l l c o n v e r s i o n s o c c u r w i t h an e f f e c t i v e n e s s f a c t o r (0) equal unity

and

that

the

cracking

rate

is

therefore

not

retarded

by

i n t r a c r y s t a l l i n e n-butane d i f f u s i o n . I f t h e c a t a l y s t p a r t i c l e i s c o n s i d e r e d t o be o f p l a t e geometry, permeable f o r reactant

molecules

i n one

direction

only,

the

effectiveness

factor

is

t h e o r e t i c a l l y g i v e n by:

4 i s t h e T h i e l e modulus:

R i s equal t o h a l f t h e b r e a d t h o f a twinned c r y s t a l . The a d s o r p t i o n c o n s t a n t Kc is incorporated i n o r d e r t o r e l a t e t h e d i f f u s i v i t y t o t h e adsorbat e concentration

inside

the

zeolite

c r y s t a l s . kintr

can be c a l c u l a t e d f rom t h e

r e s p e c t i v e apparent f i r s t o r d e r r a t e c o n s t a n t s u s i n g t h e n-but ane r e a c t i o n da t a . I t i s assumed t h a t t h e c r a c k i n g o f 2,3-DMB proceeds u n r e t a r d e d over t h e H-ZSM-5 sample.

474

of

I n Figure 6 T h i e l e p l o t s are given. I t can be seen t h a t a p e r f e c t f i t r e s u l t s evaluated data w i t h theory (eq. 1) which i s a marked improvement compared

with

results

obtained

improved homogeneity

e a r l i e r using d i r e c t l y synthesised ZSM-5 (3). Thus the of

the

distribution

of

active

sites

throughout the

aluminated c r y s t a l s now j u s t i f i e s t h e use o f t h e simple T h i e l e theory. Thiele

i n an e f f e c t i v e d i f f u s i v i t y times a dimensionless Deff*Kc. I n order t o c a l c u l a t e t h e t r u e Deff the adsorption constant, based on the 2,3-DMB concentration i n a z e o l i t e c r y s t a l , has t o be determined. Two types o f adsorption experiments have been performed. A t temperatures below 523 K the volumetric method was used. Above 523 K the adsorption constant was determined gaschromatographically from n e t t o r e t e n t i o n times using conventional r e l a t i o n s h i p s . I n both experiments sample Si13 was used. I t was assumed t h a t adsorption c h a r a c t e r i s t i c s o f s i l i c a l i t e do n o t d i f f e r s i g n i f i c a n t l y from t h a t o f the aluminated samples used i n c a t a l y t i c experiments. The r e s u l t s showed t h a t a good c o r r e l a t i o n e x i s t s between the experimental data from both methods, and t h e Van’t H o f f equation: adsorption

analysis

resulted

constant,

e.g.

Kc=Ko*exp( -AH/RT) with

the

pre-exponential

enthalpy, (-AH),

factor,

(3) KO,

equal

t o 1 .8*10-4 and the adsorption

equal t o 57 kJ/mol.

TABLE 4 Reaction l i m i t i n g d i f f u s i v i t i e s o f 2,3-DMB i n aluminated s i l i c a l i t e .

673 7 23 748 773

2.8*10 7. 9.o*10-l2 1 .3*10-11

4.8 2.4

1.7 1.3

5. 8 * d 3 3.0*10-12 5.3*10-12 1.0*10 -

Table 4 shows the r e s u l t s o f estimations o f 2,3-DMB d i f f u s i v i t i e s a t d i f f e r e n t temperatures. I n Fig. 7 an Arrhenius p l o t i s given i n which both adsorptive and ’ c a t a l y t i c ’ 2,3-DMB d i f f u s i v i t i e s are p l o t t e d i n dealuminated and aluminated s i l i c a 1 i t e , r e s p e c t i v e l y . The c o r r e l a t i o n a f t e r e x t r a p o l a t i o n o f

475

1=723 K

-4

k. s-'

m2/s

D,~'K, = 7.3.

1.00

0.10 P

1=773 K

0.0 1 1.2

1.6

1.4 lO'/T,

DeffK, = 1 . 2 6 lo-'' m 2 / s

K-'

Fig. 5: A s Fig. 4 for n-butane

cracking. 0

Fig. 6: Effectiveness f a c t o r as a function of Thiele modulus. 2.3-dimethylbutane cracklng at 7 2 3 K and a t 773 K.

Def f

..

10-14

10-15

1

I -

1.00

1.45

1.90

2.35

2.80

10'/T,

K-'

1.2

1.4

1.6

10-'/T,

K-'

Fig. 7: Adsorptive ( m ) ond 'catalytic'( 0 ) dlffuslvltles of 2,3-dlmethylbulane In (olumlnoted) slllcallte.

Fig. 8: As Fig. 4 f o r 3-methylpentane

cracklng.

476

a d s o r p t i v e d i f f u s i v i t i e s t o l o w e r r e c i p r o c a l t emperat ures i s w i t h i n an o r d e r o f magnitude. However, t h e a c t i v a t i o n e n e r g i e s o f d i f f u s i o n , ED, d e v i a t e , v i z . 121 kJ/mole f o r t h e ' c a t a l y t i c ' and 52 kJ/mole f o r t h e a d s o r p t i v e d i f f u s i v i t y .

I n o r d e r t o f i n d o u t whether t h i s d e v i a t i o n o r i g i n a t e s f r o m t h e p r o p e r t i e s o f t h e r e a c t a n t molecule, 2,3-DMB, o r t h e c a t a l y s t , we have repeat ed t h e c r a c k i n g experiments u s i n g 3-methylpentane. U s i n g d i r e c t l y s y n t h e s i s e d l a r g e ZSM-5 crystals

we

have determined d i f f u s i v i t i e s a t 723 K and a t 811 K which, a f t e r

extrapolation,

corresponded

satisfactorily with

1 ow-temperature

adsorptive

d i f f u s i v i t i e s (3). An

Arrhenius

cracking,

the

plot

of

t h e r e s u l t s ( F i g . 8) shows t h a t , c o n t r a r y t o 2,3-DMB

3-methylpentane

conversion

is

diffusion

l i m i t e d o v e r sample

same method as i n t h e 2,3-DMB case t h e ' c a t a l y t i c ' d i f f u s i v i t y o f 3-methylpentane i n a l u m i n a t ed s i l i c a l i t e has been determined. A d s o r p t i o n d a t a r e p o r t e d e a r l i e r ( 3 ) have been used t o c a l c u l a t e t h e t r u e Deff S ilA 12

only.

from Deff*Kc.

Using

the

I n Table 5 t h e r e s u l t s o f t h e e s t i m a t i o n s a r e compared w i t h d a t a

TABLE 5 Reac t io n l i m i t i n g d i f f u s i v i t i e s o f 3-methylpentane i n aluminat ed s i l i c a l i t e and i n ZSM-5. Data on ZSM-5 and a d s o r p t i o n c o n s t a n t s a r e t a k e n f r o m r e f . 3.

A1 uminated

723 773

1.0*10- lo 1.4*10-1°

4.2 2.3

2 . 4*10-11

Silica1 i t e

81 1

2.3*10-1°

1. 5

1. 5*10-1°

723

5.6*10- lo

4.2

1. 3*10-1°

811

4.6*10- lo

1.5

3. 1 * 1 0 - l o

ZSM-5

on

3-methylpentane

that

correspondence

diffusion exists

6. 1*10-l1

i n d i r e c t l y s y n t h e s i s e d ZSM-5 ( 3 ) . I t i s c l e a r within

an

order

of

magnitude.

However,

the

r e s p e c t i v e a c t i v a t i o n e n e r g i e s o f d i f f u s i o n a r e h i g h l y d e v i a t e , v i z . 48 kJ/mole versus 100 kJ/mole. I t can

diffusion

be

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

between d i r e c t l y s y n t h e s i s e d ZSM-5 and aluminat ed s i l i c a l i t e a r e due

471

t o t h e c a t a l y s t . I n f a c t , a discrepancy e x i s t s because t h e a c t i v a t i o n energy o f the

cracking r e a c t i o n over

enough that

to of

result the

aluminated s i l i c a l i t e does n o t decrease sharply

i n an a c t i v a t i o n energy o f d i f f u s i o n which i s comparable t o

adsorptive

diffusivity.

This

is

an important d i f f e r e n c e w i t h

r e s u l t s obtained by Voogd e t a l . ( 3 ) on ZSM-5 samples where a c t i v a t i o n energies of

r e a c t i o n on

the

largest

crystals

amounted

t o about h a l f t h e i n t r i n s i c

a c t i v a t i o n energy. This d i f f e r e n c e i n c a t a l y s t behaviour between ZSM-5 and aluminated s i l i c a l i t e i s as y e t unclear. Some

insight

features

of

into this

problem might be gained by i n v e s t i g a t i n g t h e basic

countersorption

behaviour

i n submicroporous systems. Studies on

countersorption o f l i q u i d hydrocarbons i n f a u j a s i t e type z e o l i t e s and i n mordenite have been reported (12-15). The r a t e o f countersorption was observed to

be considerably

phase.

Although

lower than the sorption r a t e o f these sorbates i n the gas

experimental

difficulties

can

be

rather

tedious,

is

it

recommended t o study co- and countersorption behaviour o f gaseous adsorbates i n p e n t a s i l z e o l i t e systems. Results from experiments l i k e t h i s w i l l undoubtedly shed more l i g h t on p e c u l i a r i t i e s o f ’ c a t a l y t i c ’ d i f f u s i o n . CONCLUSIONS I n t h i s study 2,3-DMB has been chosen as probe hydrocarbon species i n d i f f u s i o n experiments w i t h (aluminated) s i l i c a l i t e c r y s t a l s . The uptake o f t h i s compound

in

adsorption

experiments

i n t r a c r y s t a l line diffusion,

not

at

373

influenced

K and a t 473 K was c o n t r o l l e d by by

external

heat

and/or

mass

t r a n s f e r resistances. Cracking experiments using

intracrystalline

aluminated s i l i c a l i t e .

diffusion o f

deposited

randomly

cracking

activity

synthesised

were performed w i t h t h e probe hydrocarbon a t 673-773

of

large crystals

through of

a

2,3-DMB.

large

aluminated

It was

silicalite silicalite

K

Reactions were l i m i t e d by found

that

crystal. equal

to

aluminium

Also that

was

an n-butane of

directly

the

( o f comparable s i l i c o n - t o - a l u m i n i u m r a t i o ) was found. As a c t i v e s i t e s were d i s t r i b u t e d homogeneously throughout t h e z e o l i t e c r y s t a l

the

rates o f

crystal.

H-ZSM-5

diffusion

Therefore

an

and o f r e a c t i o n were equal a t any l o c a t i o n w i t h i n a important

condition

for

using Thiele’s model i n i t s

simple form was f u l f i l l e d . ’ C a t a l y t i c ’ d i f f u s i v i t i e s were estimated between 673 K and 713

K.

478

I t was found t h a t a l a r g e discrepancy e x i s t s between t h e a c t i v a t i o n energy o f

o f 2,3-DMB determined by adsorption k i n e t i c s and by performing cracking experiments. Experiments w i t h 3-methylpentane l e a d t o t h e conclusion t h a t t h i s discrepancy i s caused by t h e a p p l i e d c a t a l y s t r a t h e r than t h e probe

diffusion

molecule.

ACKNOWLEDGEMENTS The authors w u l d l i k e t o thank M r . E.J.A. van Dam, M r . J.F. van Lent and D r . W.G. S l o o f o f t h e Laboratory o f M e t a l l u r g y f o r Electron-Probe Microanalysis, SEM and X-ray d f f r a c t i o n analyses as w e l l as f o r discussions. M r . J. Padmos i s thanked f o r the AAS analyses. REFERENCES 1. W.O. Haag,

R.M.

Lago and

P.B.

Weisz,

Faraday Discuss. Chem. SOC., 72

(1981) 317. Post, J. van Amstel and H.W. Kouwenhoven, i n "Proceedings, 6 t h I n t e r n a t i o n a l Z e o l i t e Conference" (D.H. Olson and A. B i s i o , Eds.), p. 517. Butterworths, Guildford, 1983. P. Voogd and H. van Bekkum, Appl. Catal. 59 (1990) 311. D.B. Shah, D.T. Hayhurst, G. Evanina and C.J. Guo, AIChE J. 34 (1988) 1713. M. Ghamami and L.B. Sand, Z e o l i t e s , 3 (1983) 155. R.W. Grow and E.M. Flanigen, U.S. Patent 4,061,724, (1977). C.A. Fyfe, J.H. O'Brien and H. S t r o b l , Nature 326 (1987) 281. R.J. Argauer and G.R. Landolt, U.S. Patent 3,702,886, (1972). K. Yamagishi, S. Namba and T. Yashima, J. Catal., 121 (1990) 47. P. Voogd and H. van Bekkum, i n "Proceedings o f an I n t e r n a t i o n a l Symposium on Z e o l i t e s as Catalysts, Sorbents and Detergent B u i l d e r s " , (H.G. Karge and J. Weitkamp, Eds.), p. 519, E l s e v i e r , Amsterdam, 1989. C.D. Chang, C.T.-W. Chu, J.N. Miale, R.F. Bridget- and R.B. C a l v e r t , J. Am. Chem. SOC., 106 (1984) 8143. C.N. S a t t e r f i e l d , J.R. Katzer and W.R. Vieth, Ind. Eng. Chem., Fundam., 10

2. M.F.M: 3. 4. 5. 6. 7. 8. 9. 10.

11. 12.

(1971) 478. 13. R.M. Moore and J.R. Katzer, AIChE J., 18 (1972) 816. 14. C.N. S a t t e r f i e l d and J.R. Katzer, Adv. Chem. Ser., 102 (1971) 193. 15. C.N. S a t t e r f i e l d and C.S. Cheng, AIChE J., 18 (1972) 724.


479

T R A N S P O R T PHENOMENA AND R E A C T I O N S I N 13X T Y P E ZEOLITES

M.

BULOWl, W .

HILGERT2 a n d G .

EMIG3

' C e n t r a l I n s t i t u t e o f P h y s i c a l C h e m i s t r y , Academy o f S c i e n c e s o f t h e G.D.R., B e r l i n - A d l e r s h o f , DOR-1199 (German Dem. R e p . ) 2 1 n s t i t u t e o f T e c h n i c a l C h e m i s t r y I,U n i v e r s i t y o f E r l a n g e n - N u r e m b e r g , E g e r l a n d s t r . 3, 0 - 8 5 2 0 E r l a n g e n ( F e d . Rep. o f Germany) 3 1 n s t i t u t e o f Chemical Technology, U n i v e r s i t y o f K a r l s r u h e , K a i s e r s t r a a e 1 2 , 0 - 7 5 0 0 K a r l s r u h e 1 ( F e d . Rep. o f Germany)

SUMMARY I n f o r m a t i o n a b o u t Inass t r a n s f e r p r o c e s s e s d u r i n g h e t e r o g e n e o u s c a t a l y t i c r e a c t i o n s on m i c r o p o r o u s s o l i d s h a s become o f s p e c i a l i m p o r t a n c e . To i n v e s t i g a t e b o t h s o r p t i o n a n d k i n e t i c s o f t r a n s p o r t and r e a c t i o n o f e d u c t s and p r o d u c t s , e s p e c i a l l y o f t h e d i s p r o p o r t i o n a t i o n o f e t h y l b e n z e n e o n 13X z e o l i t e s , a h i g h p r e s s u r e a p p a r a t u s with a B e r t y r e c y c l e r e a c t o r i s described. D i f f u s i o n c o e f f i c i e n t s o f a r o m a t i c h y d r o c a r b o n s i n 13X t y p e z e o l i t e s a r e m e a s u r e d as dependences o n t e m p e r a t u r e and c o n c e n t r a t i o n i n t h e n o n - r e a c t i o n s o r p t i o n r e g i o n b y means o f b o t h t h e h i g h - p r e s s u r e c h r o m a t o g r a p h i c t e c h n i q u e and t h e p i e z o m e t r i c method. I n p a r t i c u l a r , a comparative study o f the transport parameters f o r the p - d i e t h y l b e n z e n e / l 3 X z e o l i t e s y s t e m i s c a r r i e d o u t . The d a t a o b t a i n e d a r e i n g o o d a g r e e m e n t , e s p e c i a l l y i n t h e h i g h t e m p e r a t u r e r e g i o n . They complement each o t h e r w i t h r e s p e c t t o t e m p e r a t u r e and c o n c e n t r a tion. INTRODUCTION I n t h i s work i t i s a t t e m p t e d t o d e t e r m i n e t h e c o e f f i c i e n t s o f

d i f f u s i o n a n d s o r p t i o n o f v a r i o u s a r o m a t i c h y d r o c a r b o n s o n 13X t y p e z e o l i t e s b y means o f t h e h i g h p r e s s u r e c h r o m a t o g r a p h i c ( G C ) and t h e s o r p t i o n u p t a k e (SU)

methods i n o r d e r t o a p p l y t h e r e s u l t s

t o t h e m o d e l l i n g and i n t e r p r e t a t i o n o f c o n t i n u o u s d i s p r o p o r t i o n a t i o n o f e t h y l b e n z e n e on a p p r o p r i a t e m o l e c u l a r s i e v e ( M S ) lysts. (373

I n t h e GC (SU)

... 563

s t u d y t h e t e m p e r a t u r e r a n g e i s 373

cata-

...6 8 3

K

K ) a n d t h e p r e s s u r e r a n g e s f r o m v a l u e s less t h a n 1 T o r r

i n vacuo (SU)

t o a b o u t 140 b a r ( G C ) .

The t r a n s a l k y l a t i o n r e a c t i o n

w i l l b e p e r f o r m e d w i t h i n t h e same t e m p e r a t u r e r a n g e , b u t p r e s s u -

res t o b e u s e d w i l l b e h i g h e r t h a n 400 b a r i n o r d e r t o e x c e e d t h e Besides the s o r p t i o n study, p r e l i m i n a r y explo-

c r i t i c a l point.

r a t i o n s a r e needed t o f i n d a most a c t i v e and s t a b l e c a t a l y s t f o r the disproportionation reaction.

480

EXPERIMENTAL The e x p e r i m e n t a l t e c h n i q u e s t o m e a s u r e m a s s f l u x e s o f s o r b i n g s p e c i e s i n m i c r o p o r o u s c a t a l y s t s , e . g . MS c r y s t a l s , a r e c o n f r o n t e d with t h e f o l l o w i n g problems:

-

n o n - i s o t h e r m i c i t y of t h e s o r p t i o n p r o c e s s ;

- v e r y s m a l l p a r t i c l e (MS c r y s t a l ) s i z e s ; - i n t e r p l a y between i n t r a c r y s t a l l i n e and i n t e r c r y s t a l l i n e t r a n s -

p o r t f o r p e l l e t s o r p a c k i n g s o f MS c r y s t a l s . B e s i d e s t h e c l a s s i c a l SU m e t h o d , t h e G C e x p e r i m e n t s h a v e e v o l v e d a s a major source f o r r e l i a b l e diffusionhon-equi1ibrium)data f o r MS s y s t e m s . I n a d d i t i o n , t h e N M R t e c h n i q u e s h a v e e x t e n s i v e l y b e e n u t i l i z e d t o determine s e l f - d i f f u s i o n c o e f f i c i e n t s (equilibrium d a t a ) . U n f o r t u n a t e l y , one f r e q u e n t l y e n c o u n t e r s l a r g e d i s c r e p a n cies between v a r i o u s t y p e s of e x p e r i m e n t a l r a t e p a r a m e t e r s , e s p e c i a l l y f o r h i g h l y m o b i l e systems, e . g . f o r h y d r o c a r b o n s i n 13X t y p e z e o l i t e s . O f t e n t h i s f l a w i s b e i n g a t t r i b u t e d t o u n s a t i s f a c t o r y d e s i g n of experiments or improper measurement c o n d i t i o n s , c f . t h e reviews ( r e f s . 1-3). Therefore, i n t h i s investigat i o n t h e i n f o r m a t i o n o n m o l e c u l a r t r a n s p o r t i s o b t a i n e d by t w o i n d e p e n d e n t e x p e r i m e n t a l m e t h o d s , i . e . t h e GC a n d t h e SU techniques. Chromatographic method I f o n e uses u n p e l l e t i z e d MS c r y s t a l s a s s o r b e n t , t h e d i s t r i b u t i o n o f c o n c e n t r a t i o n of a s o r b i n g m o l e c u l a r s p e c i e s b e t w e e n t h e g a s e o u s a n d s o r p t i o n ( p a r t i c l e ) p h a s e s c a n b e d e s c r i b e d by t h e following equations: gaseous phase:

with

p a r t i c l e phase:

481

The required measurements of transport rate without any chemical reaction were performed by the experimental setup shown in Fig. 1 schematically. The GC column is filled with zeolite Fig. 1. Flow diagram for GC measurements at high pressures.

bl

chrornslopraphlc column

' U nmpllllsr

- --

I

hsnlsd llnes

recorder

crystals (without binder) diluted by inert material to reduce the retention times in actual GC experiments. This arrangement ensures a more uniform gas flow than in the case of the Zero Length Column method (ref. 4). The carrier gas velocity was always high enough to guarantee a quasi-isothermic sorption-diffusion process and to exclude limiting external transport resistance at the particle-gas interface. In addition, the influence of axial dispersion on chromatograms is eliminated by high gas velocities. The pressure drop along the column axis is accounted for by the transport model of the column (Blake-Kozeny equation). A very similar model for GC experiments with zeolite crystals was proposed by Chiang et al. (ref. 5). It is most essential for further use o f the experimental data that the concentration dependence of the coefficients of diffusion and sorption is determined. For this purpose, the carrier gas nitrogen was loaded with the corresponding aromatic in a saturator and a peak is injected on top of the elevated base-line. All measurements were performed at different temperatures. Therefore, it i s possible to calculate both the activation energy and sorption heat for the systems under study. Fig. 2 shows the dependences of diffusion coefficients on temperature for various

482 a r o m a t i c h y d r o c a r b o n s on NaX t y p e z e o l i t e ( c r y s t a l s i z e 1...2

pm).

The d i f f u s i o n c o e f f i c i e n t s a r e t a k e n f r o m t h e r e g i o n w h e r e t h e y F i g . 2. A r r h e n i u s p l o t s f o r v a r i o u s systems aromatic hydrocarbon/ NaX t y p e z e o l i t e (values 0 obtained by t h e GC t e c h n i q u e ; Do... c o r r e c t e d by t h e Darken eqn. benzene ( E / k J mol-1; 0 5 ) ; A t o lu8ne ( 6 9 ) ; O e t h y l b e n z e n e (72); a m - x y l e n e (81).

1.5

1.6 1.7 1 / T [lo3 K - ' I

1.8

remain unchanged w i t h c h a n g i n g c o n c e n t r a t i o n a f t e r a p p l y i n g t h e D a r k e n e q u a t i o n t o t h e p r i m a r y d a t a . An e x a m p l e o f t h e c o n c e n t r a t i o n dependence o f d i f f u s i o n c o e f f i c i e n t s i s g i v e n i n F i g . 3 f o r t h e p-diethylbenzene/NaX

z e o l i t e system.

U

0)

ul N \

-5

Y

10-8 A

c

.-aJ .-U

Y .c -

A

g U C

.-0 ul 3

Y-

'c

u ,040

1

2

3

I

concentration [ m o l e c u l e s / cage]

F i g . 3 . C o n c e n t r a t i o n dependence o f t h e d i f f u s i o n c o e f f i c i e n t o f p - d i e t h y l b e n z e n e o n NaX z e o l i t e a t 6 9 3 K ( o b t a i n e d by t h e GC t e c h n i q u e ) .

483

A second apparatus was built in order to

- decide if the results obtained by the GC technique are transferable to the disproportionation of ethylbenzene (ref. 6 ) ; - derive a kinetic model f o r prediction o f selectivities and conversions on various MS catalysts at different temperatures, pressures and residence times. The corresponding experimental setup is shown in Fig. 4. I n the recycle reactor, pressures up to 400 bar and temperatures up to 770 K can be maintained, so that measurements at the critical

--

I-7-

,

-

I

t

Fig. 4. Flow diagram of the experimental setup f o r kinetic measurements in the supercritical region, point and in the supercritical region are feasible. The experimental principle prevents pulsations in both the gaseous and the liquid phases. First investigations indicate a lower coking tendency at supercritical conditions (refs. 7 - 9 ) . Fig. 5 shows an Arrhenius plot of diffusion data (corrected by the Darken equation) f o r the p-diethylbenzene/NaX zeolite system investigated by both

484

t h e G C ( c r y s t a l s i z e 1...2 s i z e 8 0 pm) m e t h o d s . e a c h o t h e r on t h e

pm) a n d t h e p i e z o m e t r i c S U ( c r y s t a l

The r e s u l t s f r o m b o t h t e c h n i q u e s c o m p l e m e n t

temoerature scale. F i g . 5 . Compar i s o n o f GC ( c i r c l e s ) and SU ( s q u a r e s ) d i f f u s i o n data f o r p-diethylbenzene on NaX t y p e z e o l i tes.

S o r p t i o n method I n t h e SU e x p e r i m e n t s a c o n s t a n t v o l u m e - v a r i a b l e p r e s s u r e system w i t h v a l v e - e f f e c t was e m p l o y e d .

c o r r e c t i o n s as d e s c r i b e d i n ( r e f s .

10,ll)

The v a l v e - e f f e c t c o r r e c t i o n s r e l a t e t o d e a d - t i m e

moments o f t h e a p p a r a t u s a m o u n t i n g t o s 0.2 s .

To a v o i d t h e i n -

f l u e n c e o f i n t e r c r y s t a l l i n e d i f f u s i o n on t h e u p t a k e r a t e and t o r e a l i z e quasi-isothermic conditions,

SU was m e a s u r e d u s i n g ,

r e s p e c t i v e l y , a monolayer o f s i n g l e c r y s t a l s and f a v o u r a b l e e x t e r n a l temperature conditions ( i . e . ,

To
p-xylene > p-diethylbenzene. ACKNOWLEOGEMENTS W.H. and G . E . thank the Oeutsche Forschungsgemeinschaft for financial support. M.B. thanks the Akademie der Wissenschaften der D D R for the possibility to work on diffusion phenomena in molecular sieve systems. NOTATIONS diffusion coefficient coefficient of axial dispersion ?3x constant of sorption equilibrium K column length L mass flux density across the NRc zeolite crystal interface (cf.eqn.2) external r a d i u s of zeolite crystals RC C gaseous phase concentration 9 total sorbate concentration in CT gaseous phase = Kc , sorbate concentration in C intrgcrystalline void volume o f zeolite r radial coordinate of zeolite crystal time t D

*

cm m m o l cm

-2 s -1

cm mmol cm-3 mmol cm-3 mmol cm-3 cm S

490 V

l i n e a r gas v e l o c i t y

cm s

X

molar f r a c t i o n

'

i n t e r p a r t i c l e v o i d volume of column p a c k i n g (volume f r a c t i o n )

-

7

= r/Rc,

5

dimensionless a x i a l coordinate

of

dimensionless radius zeolite crystal

-1

-

-

REFERENCES 1 2 3

4 5 6 7

8 9

10

11 12 13 14 15

J . K a r g e r a n d D.M. R u t h v e n , Z e o l i t e s , 9 ( 1 9 8 9 ) 2 6 7 - 2 8 1 . H.W. H a y n e s , J r . , C a t a l . Rev. - S c i . E n g . , 30 ( 1 9 8 8 ) 5 6 3 - 6 2 7 . M. B u l o w , J . K a r g e r , M . K o z i r f k a n d A . M . V o l o g z u k , Z . Chem., L e i p z i g , 2 1 (1981) 175-182. D.M. Ruthven and M. E i c , Z e o l i t e s , 8 (1988) 40-45. A . S . C h i a n g , A . G . D i x o n a n d Y . M . Ma, Chem. Eng. S c i . , 3 9 (1984) 1451-1459. W.J. Thomas a n d U . U l l a h , Chem. Eng. Res. D e v . , 6 6 ( 1 9 8 8 ) 138146. N . A . C o l l i n s , P . G . O e b e n e d e t t i a n d 5. S u n d a r e s a n , A I C h E J . , 3 4 (1988) 1211-1214. G . Manos, Ph. 0 . T h e s i s " U n t e r s u c h u n g e n d e r V e r k o k u n g v o n Z e o l i t h e n und i h r e r R e a k t i v i e r u n g m i t u b e r k r i t i s c h e n F l u i d e n " U n i v e r s i t a t E r l a n g e n - Nurnberg, 1989. H . S c h a f e r , Ph. 0 . T h e s i s " U n t e r s u c h u n g e n z u r K i n e t i k d e r Desa k t i v i e r u n g und R e a k t i v i e r u n g p o r o s e r K o n t a k t e m i t u n t e r - bzw. u b e r k r i t i s c h e n R e a k t i o n s g e m i s c h e n " U n i v e r s i t a t E r l a n g e n - Nurnb e r g , 1990. P . S t r u v e , M . KoEi?? 3 pm and t h e alcohols c r y s t a l sizes > 4 pm. Fig. 1 shows the r e l a t i v e e f f e c t o f aluminium content on propene conversion t o l i q u i d products. An unusually h i g h r e a c t i o n temperature o f 300°C was used t o ensure t h a t 2-487-1 would show s u f f i c i e n t a c t i v i t y . When samples were prepared w i t h S i / A l r a t i o o f 48 and 64 r e s p e c t i v e l y and c r y s t a l s i z e o f -1.1 pm i n both cases, the aluminium content was more s i g n i f i c a n t than c r y s t a l s i z e f o r propene oligomerization a c t i v i t y . Fig. 2 shows t h e c a t a l y s t u t i l i z a t i o n values obtained throughout t h i s study f o r d i f f e r e n t c r y s t a l sizes i r r e s p e c t i v e o f the method o f synthesis used o r aluminium content. The c r i t i c a l importance o f the c r y s t a l size i s c l e a r l y evident f r o m t h i s f i g u r e . The best c a t a l y s t u t i l i z a t i o n values (CUV

=

g.liq/g.cat)

were obtained

using samples w i t h c r y s t a l sizes < lpm and Si/A1 = 40.

C r y s t a l s o f 2.4 pm

showed very poor oligomerization a c t i v i t y (CUV = 110 g/g).

The CUV values f o r

the 1,6 HD and TPA based samples ( m x h a n i c a l l y s t i r r e d ; Si/A1 = 40) were 409 and

494

C

100

0

n V

e r

80

0

I 0

n

80

0

I P

40

r

0

P

n e

x

0'

0

Fig. 1 MPa, T

=

2

14

16

18

E f f e c t o f A1 content on propene o l i g o m e r i z a t i o n a c t i v i t y ( P 3OO0C, WHSV = 12 h-')

0 0.5 1

Fig. 2

6 8 10 12 Time on stream (hrs)

4

1.5

2

2.5 3 3.5 4 Crystal size (urn)

4.5

5 5.5 6

E f f e c t o f c r y s t a l s i z e on c a t a l y s t u t i l i z a t i o n value.

=

5

495 385 g/g

and t h e i r magnetically

s t i r r e d equivalents were 286 and 258 g/g

r e s p e c t i v e l y . Their c r y s t a l sizes are shown i n Table 2.

A t a constant Si/A1 r a t i o o f about 40 i t was found t h a t an optimum CUV = 520 g/g was obtained f o r a synthesis t i m e o f 3 days. For a Si/A1 r a t i o o f about 20 t h e optimum CUV (380 g/g) was obtained f o r a synthesis time o f 1 day. The former optimum corresponded t o t h e highest concentration o f strong a c i d s i t e s as determined by TPD, v i z . 0.40 mmol/g, whereas the l a t t e r optimum corresponded t o the smallest c r y s t a l size. I n a l l these runs the r e a c t i o n temperature was increased g r a d u a l l y from

220

- 290°C

over t h e e n t i r e run o f about 40h and the

r e p r o d u c i b i l i t y was found t o be very good. The f o l l o w i n g general propene oligomerization a c t i v i t y sequence was observed when d i f f e r e n t templates were used: TPA, TBA, C, diamines > C, amines > Cp,3,4 amines

and diamines

> alcohols.

The f i r s t mentioned group

all

had

comparable CUVs o f -300 g/g. When samples o f ZSM-5 (TPA-Z-41-3) were extruded w i t h s i l i c a o r alumina i t was found t h a t t h e r e was no change i n the CUV o f t h e c a t a l y s t which was found t o be 400 and 375 g/g r e s p e c t i v e l y

as opposed t o 385 g/g f o r t h e powder.

When

s i l i c a - a l u m i n a was used as a binder the CUV increased from 409 t o 497 g/g.

Fig.

3 shows c l e a r l y t h a t the a c t i v i t y and l i f e t i m e o f t h e c a t a l y s t

s i g n i f i c a n t l y when p e l l e t s were used,

decreased

the CUV dropping from 380 g/g

for

extrudates o f TPA-Z-38-3 t o 60 g/g f o r 10 ton p e l l e t s o f t h e same sample. Fig. 4 shows the l i q u i d product analysis obtained from t h e o l i g o m e r i z a t i o n o f propene using powder and extrudates.

The values obtained f o r t h e powder are

representative o f those f o r almost a l l runs i n t h i s work. High r e a c t i o n temperatures l e d t o a s h i f t t o l i g h t e r products l a r g e l y because o f t h e increased e x t e n t o f cracking a t these temperatures.

This f i g u r e shows a d i s t i n c t t r e n d t o

heavier products i n the case o f the extruded c a t a l y s t independent o f t h e binder used.

Gautier's c o r r e l a t i o n ( r e f . 24) was used t o c a l c u l a t e cetane numbers from

the 'H-NMR spectra o f hydrogenated l i q u i d samples. The values obtained were i n the range 39-52 f o r both powder and extrudates. The best c a t a l y s t t e s t e d was alumina-bound extrudates o f (TPA)-Z-38-3 (mechanically s t i r r e d ) which had a CUV o f 1100 g/g and a product cetane number o f 44.

A g r a v i m e t r i c analysis o f the carbonaceous deposit on t h e c a t a l y s t a t the end o f a run showed t h a t i n the case o f extrudates bound w i t h alumina t h e t o t a l mass o f these deposits, v i z . " g r a p h i t i c " material p l u s h i g h b o i l i n g p o i n t hydrocarbons, was 15% o f the t o t a l mass o f deactivated c a t a l y s t . This was about

496 Conversion of orooene (a)

2 Ton9

5 Tons

0

5

10

20

15

25

30

35

40

45

Time on stream (hr)

Fig. 3

Effect of extrusion and pelletizing on propene oligomerization activity (P = 5 MPa, T = 250"C, WHSV = 12 h- )

40

Mass%

30

20

10

0

Dlmer

Tetramer

Trimer

Pentamer

Hexamer,

Oligomer Group Powder

@ Silica

extrudates

Fig. 4 Liquid Product Analysis

0Alumina Extrudetes

497

half that found in the case of powdered, extrudates.

silica or silica-alumina bound

DISCUSSI ON The number average crystal sizes of 2-18-1, 2-19-3 and 2-26-6 were larger than what was expected from the trend observed by Van der Gaag et a1 .(ref. 25) and Jacobs and Martens (ref. 13). For the 6-day catalyst this may be due to amorphous material being deposited on the zeolite surface. The increase in crystal size (2-19-3 and 2-26-6 larger than 2-18-1) has been observed by other workers (ref. 26). Mechanical stirring is expected to cause more attrition than a magnetic stirrer bar in a synthesis mixture and hence as observed in the present study should cause a decrease in crystal size. The narrow size distributions obtained with mechanical stirring could be a result of more effective agitation throughout the autoclave and a more even distribution of stress on the crystals. This is distinctly advantageous with respect to the propene oligomerization activity. When different templates were used it was found that 1 BA, 1,2 ED, 1,2 PD as well as the alcohol-based samples produced larger crystal sizes than TPA-ZSM-5. This may happen if the formation of the template-silicate complex is difficult resulting in the formation of only a few nuclei which then grow to a larger size. The activity for propene 01 igomerization increased with decreasing Si/A1 ratio. TPD studies indicated a high concentration of Bronsted sites (refs. 2326). Moreover little dehydroxylation appeared to have taken place. The relationship between aluminium content and activity is thus probably indicative of a relationship between activity and Bronsted acidity for propene ol igomerization. Calcination temperatures less than 500°C however do not favour optimum catalytic performance (ref. 22,23). The synthesis used in this study was similar to type ‘A’ of Gabelica et al. (ref. 27), who reported an aluminium rich outer layer for this synthesis procedure. Aluminium zoning of this nature was reported by Von Ballmoos and Meier (ref. 28). Suzuki et al. (ref. 29) also confirmed this finding and observed a decrease in activity for methanol conversion as synthesis time increased once maximum crystal1 inity was attained. They attributed the decrease in activity to increased coking on the catalyst surface. The results of the present study support these observati ons. This work has shown that a decrease in crystal size enhances lifetime and activity for propene ol igomerization as has been reported for methanol

498

conversion (ref. 3 0 ) , xylene isomerisation (ref. 31) and propene oligomer sation using N i - and Zn-impregnated ZSM-5 (ref. 3 2 ) . The improved performance of small crystals may be related to coking This has been found by Behrsing et al. (ref. 3 3 ) who compared ZSM-5 crystals of differing crystal sizes using methanol conversion. They observed that ZSM-5 with a crystal size < 1 pm had the longest lifetime for methanol conversion, even though it contained more coke than the other catalysts. The long lifetime of small ZSM-5 crystals may also be attributed to higher pore volumes, higher external surface areas and more external acid sites when compared to large crystals. Higher pore volumes will increase resistance to coking and more external acid sites ensure that a large fraction of the total acid sites are easily available for reaction. In larger crystals, with a lower ratio of external to internal surface area, many acid sites may not be accessible for reaction due to excessively long diffusional pathways. Moreover heavier hydrocarbons produced inside a large crystal may not be able to diffuse out of the crystal and will therefore block the passage of other molecules further into the crystal. This would decrease the amount of acid sites available for reaction and cause quicker deactivation than in small crystals. The low crystallinities and primarily the large crystal sizes of the alcohol-based ZSM-5 samples were probably responsible for their low activities and short lifetimes. Surprisingly extrusion did not decrease the activity for propene ol igomerization although the high conversions involved may have masked any difference in activity between powdered and extruded catalysts. Attempts to verify this observation at low conversions were difficult due to temperature runaway problems. The observed decrease in lifetime and activity with increasing pelletisation pressure is probably due to increased diffusional resistance in the intercrystalline pores and blockage of the ZSM-5 micropores by binder particles. The activity and lifetime of catalysts synthesized using different amine templates increased in the order: 1 , 2 ED = 1,2 PD < 1 BA < 1 HA (crystal size = 3 . 4 prn, 4 . 7 pm, 2 . 9 pm and 1 . 0 pm respectively). At larger crystal sizes this trend became indistinct. No reference has been made to the importance which different morphologies may have on the catalytic properties and this has been considered el sewhere (ref. 2 3 ) . Temperature control for extruded catalysts was much easier than for powdered catalysts due to the dilution effect of the binder and the void spaces between individual extrudates in the catalyst bed. Although it may have been expected that the lighter products would thus be produced by powdered ZSM-5 due to cracking at hotspot temperatures, it was observed that

499

p e l l e t s y i e l d e d t h e l i g h t e s t l i q u i d products. lo we r c o nv ers io n s .

T h i s was

p o s s i b l y due t o t h e

S i l i c a - a l u m i n a e x t r u d a t e s produced more coke t h a n t h e o t h e r e x t r u d a t e s i n v e s t i g a t e d and t h i s may be due t o t h e a c t i v i t y o f t h e b i n d e r f o r propene 01 i g o m e r i s a t i o n . T h i s i n c r e a s e d i d not, however, adversely a f f e c t t h e 1 i f e t i m e o f these extrudates. 1/2

The l o w coke c o n t e n t o f t h e alumina-bound e x t r u d a t e s (-

o f t h e coke c o n t e n t o f t h e s i l i c a e x t r u d a t e s )

did not result

i n any

d i f f e r e n c e i n l i f e t i m e o r a c t i v i t y when compared t o t h e s i l i c a e x t r u d a t e s . The o b s e r v a t i o n t h a t 1 i g h t l y coked e x t r u d a t e s d i d n o t c o n t a i n g r a p h i t i c coke, w h i l e t h e h e a v i l y coked e x t r u d a t e s d i d , seemed t o i m p l y t h a t d e a c t i v a t i o n would o n l y occ ur once h i g h q u a n t i t i e s o f g r a p h i t i c coke were deposit ed. CONCLUSION

T h i s s t udy has shown t h a t a t Si/A1 r a t i o s o f about 40, t h e c r y s t a l s i z e o f ZSM-5 has a c r i t i c a l e f f e c t on propene o l i g o m e r i z a t i o n a c t i v i t y . S m a l l e r s i z e and narrow er c r y s t a l s i z e d i s t r i b u t i o n s a r e o b t a i n e d when u s i n g m e c h a n i c a l l y s t i r r e d a ut o c lav e s .

When t h e powder i s e x t r uded t h e r e i s no adverse e f f e c t on

o l i g o m e r i z a t i o n a c t i v i t y and when alumina i s used as a b i n d e r b e t t e r q u a l i t y distillate

fuel

i s produced.

P e l l e t i z i n g t h e powder reduces a c t i v i t y

in

p r o p o r t i o n t o t h e p r e s s u r e s used. Other t e m plat es such as 1,6 hexanediamine and 1,2 cyclohexanediamine produce ZSM-5 o f s i m i l a r a c t i v i t y t o TPA-based ZSM-5 b u t o t h e r amines and a l l a l c o h o l s t e s t e d produced poor o l i g o m e r i z a t i o n c a t a l y s t s . ACKNOWLEDGEMENT

The a ut h or s w i s h t o acknowledge f i n a n c i a l support f r o m t h e U n i v e r s i t y o f Cape Town, N a t i o n a l Energy Council and Sasol

.

REFERENCES

1 2 3 4 5 6 7 8 9 10 11 12 13

S.A. Tabak, F.J. Krambeck and W.E. Garwood, J.AIChE, 32(9) (1986) 1529. R.J. Ouann. L.A. Green, S.A. Tabak and F.J. Krambeck, I n d . Eng. Chem. Res., '27 (1'988) 567. . M.L. O c c e l l i , J.T. Hsu and L.G. Galya, J. Mol. Cat al. , 32 (1985) 378. K.G. W i l s h i e r , P. Smart, R. Western, T. Mole and T. Behrsing, Appl. C at a l. . 31 119871 345. S. B e s s e l l and 0: Seddon, J. Catal., 105 (1987) 272. S.P. Zhdanov, N.N. Fe o k t i s t o v a , N . I . Kozlova, N.R. Bursian, S.B. Kogan, V.K. Daragan and N.V. Aleksandrova, Acta Phys. Chem., 31 (1985) 131. V.S. Nayak and V.R. Choudary, Appl. C a t al. , 4 (1982) 339. F.J. van d e r Gaag, J.C. Jansen and H. van Bekkum, Appl. C a t a l . , 17 (1985) 264. M. Sugimoto, H. Katsuno, K. Takatsu and N. Kawata, Z e o l i t e s , 7 (1987) 503. P. Ratnasamy, G.P. Babu, A.J. Chandwadkar and S.B. K u l a r n i , Z e o l i t e s , 6 (1986) 98. M.L. O c c e l l i , J.T. Hsu and L.G. Galya, J . Molec. Cat al. , 32 (1985) 379. U. Hammon, M. K o t t e r and L. R i e k e r t , Appl. Cat al. , 37 (1988) 155. P.A. Jacobs and J.A. Martens, S t u d i e s i n Surf ace Science and C a t a l y s i s , 33 (1987) 72.

500

15

W. Holderich, H. Eichorn, R. Lehnert, L. Marosi, W. Mross, R. Reinke, W. Ruppel and H. Schlimper, Proc. 6 t h I n t . Z e o l i t e Conf., Reno, USA, 1983, Butterworths (1984) 545-555. K. Suzuki, Y. Kiyozumi, K. Matsuzaki and S. Shin, Appl. Catal., 42 (1988)

16 17 18 19 20

N . Y . Chen, S.J. Lucki and W.E. Garwood, US Patent 3700585 (1972). S.A. Tabak, US Patent 4254295 (1981). P. Chu, US Patent 3709979 (1973) A. Araya and B.M. Lowe, Zeolites, 6 (1986) 111-118. F.J. van der Gaag, J.C. Jansen and H. van Bekkum, Appl. Catal., 17 (1985)

14

35.

261-271.

25 26 27 28

R.J. Argauer and G.R. Landolt, US Patent 3702886 (1972). S.Schwarz, M.Kojima and C.T.O’Connor, Appl. Catal.,56 (1989) 263-280, S. Schwarz, PhD Thesis, U n i v e r s i t y o f Cape Town, 1990. S . Gautier, Diplome-Ingenieur Thesis, Conservatoire National des A r t s e t Metiers, Paris (1988). N-Y. Topsoe, K. Pedersen and E.G. Derouane, J. Catal., 70, (1981), 46. Z . Gabelica, E.G. Derouane and N . Blom, Catal. Minerals, 12, (1984), 219. R. von Ballmoos and W.M. Meier, Nature (London), 289, (1981). 782. K. Suzuki, Y. Kiyozumi, K. Matsuzaki and S. Shin, Appl. Catal., 42,

29

M. Sugimoto, H. Katsuno, K. Takatsu and N. Kawata, Z e o l i t e s , 7, (1987),

21 22 23 24

(1988), 35. 503. 30

P. Ratnasamy, G.P.

Babu, A.J. Chandwadkar and S.B. K u l k a r n i , Z e o l i t e s , 6,

(19861. 98. 31 32

S.J. M i l l e r , US Patent 4423269 (1983). T. Behrsing, H. Jaeger and J.V. Sanders, Appl. Catal.,

54, (1989), 289.

G. Ohlmann et al. (Editors),Catalysis and Adsorption by Zeolites 01991 Elsevier Science PublishersB.V., Amsterdam

501

ON CONTROLLED GROWTH OF SAPO-5 MOLECULAR SIEVE CRYSTALS OF DIFFERENT SIZES AND SHAPES

G. FINGER, .J. KORNATOWSKI~,J. RICHTER-MENDAU, K. JANCKE, M. RIJLOW, and M. ROZWADOWSKIl Central Institute of Physical Chemistry, Academy of Sciences of the G.D.R., Berlin (German Democratic Republic) ‘Institute of Chemistry, Nicolaus Copernicus University, Toruh (Poland)

SUMMARY Systematic studies of the SAPO-5 system have been performed. Roth the crystal size and shape as well as the phase purity can be controlled by the reacting gel composition and also by the duration and/or temperature of crystallization. Evidence is given for the interdependence between the shape of crystals and their sorption properties. INTRODUCTION Porous silicoaluminophosphates (ref. 1) are promising materials for numerous scientific and potential practical applications. However, thoroughly refined applications require materials of strictly defined physical featiires, e.g.

crystal size and shape. Unfortunately, the control of these parameters in

the case of zeolitic materials did not represent a field of major interest until now, except for the tailoring of type A zeolites for washing powder production (refs. 2 , 3 cited in ref. 4) and for the growing of MFI type crystals (refs. 5-10). Obviously, the growth of tailored crystals requires the knowledge of both the kinetics of the process and the conditions enabling the formation of larger crystals. The latter problem has been solved for both the AlPO4-5 (refs. 11-16) and the SAPO-5 molecular sieves ( r e f s . 17-19). Recently, the kinetics of SAPO-5 crystallization has been investigated in detail (ref. 2 0 ) but not with respect t o the size of the crystals. We aim at a better understanding of the SAPO-5 crystallization process and at getting control of size and shape of the crystals. The paper presents a number of results and conclusions regarding these goals. EXPERIMENTAL SAPO-5 molecular sieve crystals were synthesized hydrothennally in teflonlined steel autoclaves in air-heated w e n s at 4 5 3 to 473 K from gels of the formal molar composition

a A1203 with a

=

1, b

*

b P2O5 =

-

c TEA

1 t o 2.4, c

=

d Si02

e H20

1.5 t o 12, d

=

*

x H2SO4

0.05 t o 1 , e

(1) =

300 t o 2000, x ? 0.

F i r s t , g e l s were formed by mising a s t a b i l i z e d aluminiumoxidehydrate s o l ( p u r c h a s e d from CTA SAURESCHUTZ, B e r l i n ) w i t h s i l i c a s o l (LUDOX AS 40 from DUPONT/Germany). Next, a s o l u t j o n of 85% p h o s p h o r i c a c i d and t r i e t h y l a m i n e / T E A / (Merck) i n t w i c e d i s t i l l e d w a t e r was added under v i g o r o u s s t i r r i n g . Varioiis amounts o f H3PO4, TEA, and w a t e r were admixed a c c o r d i n g t o t h e g e l rompr~siticln s t u d i e d . A t t h e same t i m e , s u l f u r i c a c i d was added, i f n e c e s s a r y , t o a d j u s t t h e pH v a l u e t o 3 , 3 + 0 , 2 . A s e r i e s of r u n s was made a t d

=

0 t o grow AlP04-5 c r y s -

t a l s a s r e f e r e n c e samples. These g e l s were p u t i n t o t h e a u t o c l a v e s and exposed t o t h e c r y s t a l l i z a t i o n t e m p e r a t u r e . A f t e r a p p r o p r i a t e time p e r i o d s , t h e v e s s e l s were c o o l e d down, t h e samples were washed, s e p a r a t e d from b y p r o d u c t s , i f n e c e s s a r y , by d e c a n t a t i o n ( o b v i o u s l y , a number of smaller SAPO-5 c r y s t a l s were l o s t a t t h e same t j m e ) , and d r i e d a t 3 7 3 K. Furthermore, s e l e c t e d samples were c a l c i n e d under s t a t i c a i r atmosphere a t about 1050 K f o r 24 h r s . The p r o d u c t s were c h a r a c t e r i z e d by XRD ( G u i n i e r t e c h n i q u e ) , MAS NMR, I R s p e c t r o s c o p y , a d s o r p t i o n , and SEM. The c o n t e n t of c r y s t a l s i n t h e samples was e s t i m a t e d and t h e c r y s t a l dimensions were measured by l i g h t microscopy accor-di n s t o t h e methods d e s c r i b e d i n d e t a i l e l s e w h e r e ( r e f . 2 1 ) .

RESUILTS The procedure e l a b o r a t e d ( r e f s . 17-19) i s a r e l i a b l e r c c j p e € o r t h e synt h e s i s of l a r g e hexagonal SAPO-5 p r i s m s , though sometimes i n a broad p a r t i c l e s i z e d i s t - r i b u t i o n . I n comparison w i t h t h e similar p r o c e d u r e t o grow A1P04-5 m o l e c u l a r s i e v e s ( r e f s . 13-16), t h e SAPO-5 s y s t e m is more complex. However, t h e c r y s t a l s o b t a i n e d i n a l l e x p e r i m e n t s e x h i b i t X-ray p a t t e r n s c o r r e s p o n d j n g t o t h e A F I topology and MAS NMR measurements prove s i l i c o n i n c o r p o r a t i o n i n t o t h e framework. The c r y s t a l l i z a t i o n was s t u d i e d w i t h i n t h e range of t e m p e r a t u r e from 423 t o 4 7 3 K and p e r i o d s up t o 10 days. Whereas a t t h e lower l i m i t of t.emperature f o r e i g n phases a r e formed, t h e c r y s t a l l i z a t i o n of SAPO-5 is a c c e l e r a t e d w i t h r i s i n g t e m p e r a t u r e . T h i s l e a d s t o y i e l d s of SAPO-5 up t o 100 p e r c e n t depending on t h e g e l composit.ion. A t 463 K , t h e l a r g e s t c r y s t a l s were obt.ained. Theref o r e , we r e p o r t o n l y about t h e e x p e r i m e n t s a t t h i s t e m p e r a t u r e , The g e l c o m p o s i t i o n r e p r e s e n t s t h e most i m p o r t a n t f a c t o r f o r t h e c o n t r o l of b o t h c r y s t a l s i z e and s h a p e . To t h e d e g r e e t o which t h e r o l e of s j l i c o n can be n e g l e c t e d , i . e .

a t s u f f i c i e n t l y low s i l i c a c o n c e n t r a t i o n i n t h e gel, t h e

e f f e c t s of c o n t e n t s of H 2 0 , TEA, and P2O5 are s i m i l a r t o t h o s e which govern t h e n u c l e a t i o n and growth

of AlPO-5 c r y s t a l s ( r e f . 1 6 ) . The i n f l u e n c e s of b o t h t h e

c o n t e n t of H 2 0 ( F i g . 1 ) and t h e c o n t e n t of TEA o r TEA and P2O5 t o g e t h e r ( F i g . 2 )

503

have been chosen as representative illustrations of the system, The maximum length of the hexagonal prism grown from gels of various compositions are depicted in a graph (Fig. 3 ) . Dilution of the reacting gel results always in the growth of larger crystals (Fig. 1). However, this is accompanied by a rising content of a byproduct and a considerable prolongation of the crystallization time (more than one week for the most diluted gels). Occasionally, a new nucleation of hexagonal prisms occurs in the bulk volume of the gel and on the surface of crystals already formed. A water content 750 < e

c 1000

seems t o be an advant.ageous compromise.

Then, the largest crystals can grow up to a length of 580 pm (Fig. 4D1 and the yield of the SAPO-5 phase (regardless of the size distribution) can amount up to 80 per cent.

Fig. 1. SAPO-5 crystals (after decantation) synthesized from gels with rising water content (a:b:c:d = 1:1:1.55:0.2): ( A ) e = 300, (B) e = 450, ( C ) e= 600, (D) e = 750.

504

Fig. 2. SAPO-5 crystals (after decantation) synthesized from gels with constant water content (e = 300 H20, d = 0 . 2 Si02): ( A ) b = 1 , c = 1.55; ( B ) b = I , c = 3.1,; ( C ) b = 1, c = 6 . 2 ; (D) b = 2, c = 3.1.

6 00I

0

1 1 2

E 400-

-

2 E -.

-

1.55

0.2

3.1 6.2

0.2

3.1

0.2

/

+,

0.2 0

200I

I

I

400

I

I

600

I

I

800 H20ImoLe

Fig. 3. Maximum length of AFI hexagonal prisms depending on the gel composition.

505

F i g . 4 . Various c r y s t a l morphology i n SAPO-5 s y n t h e s i s Gel composition

a

b

c

d

e

vicinal faces

1 1

1 2

3.1 3.1

0.2

( B ) growth aggregate

0.2

450 300

1 1 1

1 1 1

1.55 1.55 1.55

0.3 1.0 0.4

750 300 300

(A) (C)

byproduc t

(D) SAPO-5 c r y s t a l s (E) d i s t o r t e d c r y s t a l s ( F ) secondary n u c l e a t i o n

For AlPO-5, recrystallization of AFI type crystals into tridymite and/or crystobalite type phases have been reported (ref. 2 2 ) . We could not observe this even in runs of one week duration (optimum period for the largest crystals growth). Small contents of a tridymite phase can be observed with SAPO-5 grown from gels with an overabundance of phosphorus, i.e. b:a

=

2.4 (ref. 2 3 ) . Simi-

lar results have been already reported (ref. 20). At a fixed water content in the gel, the crystal size can be influenced by the content of template or template and phosphoric acid toget.her (Fig, 2). Their excess can result in the formation of smaller crystals with a higher yield and in a decreasing crystallization period. Distortions of the cr-ystals or rose-like looking vicinal faces start to occur (Fig. 4A). This behaviour underlines the role of the template in the process of the molecular sieve formation and its significance for the nucleation stage in particular. An excess of both P2O5 and TEA leads to the strongest effect on the morphology of the product and, at low water content., even polycrystalline growth aggregates are formed (Fig. 48). On the other hand, the samples do not contain any byproduct and the crystallization is completed within 24 hrs. Depending on the gel composition, small spherulitic particles can be formed together with the SAPO-5 crystals (Fig. 4C). According to an

MAS

NMR

investigation (ref. 24), this byproduct represents an aluminophosphate which we failed to identify until now. Its occurrence is correlated with HzO/TEA 2 100 (molar ratio), i.e. just with the conditions to synthesize the large SAPO-5 crystals. However, using the differences in s i z e , both phases can be separated, e.g. by decantation. Occasionally, a small number of rhombohedric crystals have been observed in the samples, too (Fig. 4C). Concerning the silicon incorporation into the AFI framework, it is generally accepted that only small amounts can substitute phosphorus atoms esclusively (ref. 2 5 ) . As it was stated for the system investigated, even the smallest content of silica in the gel (i.e. d = 0.05) seems to hitlder the crystals to grow so large as in the AlP04-5 system. With increasing silicon content up to about d = 0.25 the system remains almost insensitive with regard to the crystal size and shape. Surprisingly, the largest crystals ( u p to about 580 pm long) can be synthesized at d = 0.3 (Fig. 4D), whereas at higher d values, only crystals as illustrated in Figs. 4E and 4F are formed. A similar appearance (Fig. 4F) was observed in the FAPO-5 system and attributed to secondary 11uc1eation (ref. 26). Unquestionably, this behaviour of the system should be attrihuted to the presence of heteroatoms. This is underlined by the absence of similar findings in the systems forming large AlP04-5 crystals (ref. 1 6 ) . Up to the same composition limit ( d

= 0.31, silicon replaces almost esclu-

sively phosphorus atoms in the framework of the crystals in this study. This

507 i s proved by 29S1 MAS NMR ( r e f . 24) and I R measurements a r e c o n s i s t e n t w i t h t h i s s t a t e m e n t a s w e l l ( r e f . 2 4 ) . I n r e f . 20, a similar l e v e l of s i l i c a c o n t e n t in t h e g e l ( d = 0 . 4 ) h a s been s u g g e s t e d

as a c r i t i c a l v a l u e € o r t h e s i l i c o n i n -

c o r p o r a t i o n i n t o t h e A F I framework and f o r t h e optimum c r y s t a l l i z a t i o n of g e l s c o n t a i n i n g n-tripropylamine.

1

I

I

-2

I

1

.4

I

.6

I

I

.8 PIP,

F i g . 5 . Adsorption i s o t h e r m s of n-hexane on SAPO-5 m o l e c u l a r s i e v e c r y s t a l s of d i f f e r e n t morphology ( T = 298 K): 1-hexagonal p r i s m s ; 2-growth a g g r e g a t e s ; 3 - d i s t o r t e d c r y s t a l s .

The a d s o r p t i o n behaviour of t h e v a r i o u s SAPO-5 m o l e c u l a r s i e v e s a m p l e s d i f f e r s depending on t h e c r y s t a l morphology ( F i g . 5 ) . A t h i g h r e l a t i v e a d s o r b a t e p r e s s u r e ( p / p s ) , o n l y t h e well-formed hexagonal prisms e x h i b i t t h e i d e n t i c a l a d s o r p t i o n and d e s o r p t i o n i s o t h e r m s , whereas b o t h t h e d i s t o r t e d c r y s t a l s and t h e growth a g g r e g a t e s show a n a d s o r p t i o n h y s t e r e s i s o c c u r r i n g most p r o b a b l y i n t h e secondary p o r e systems. I € t h e l a t t e r p o r e system is assumed as s e r v i n g a s a channel f o r t r a n s p o r t , t h e a d s o r p t i o n k i n e t i c s might b e a c c e l e r a t e d , b u t i n t h e s e p a r a t i o n p r o c e s s e s , t h e a d s o r p t i o n h y s t e r e s i s would be a draw back due t o r e d u c t i o n of t h e a d s o r b e n t - a d s o r b a t e i n t e r a c t i o n . The d i f f e r e n c e s i n t h e amounts adsorbed i n comparison t o t h e f i n d i n g s i n r e f . 27 can be a t t r i b u t e d t o t h e n o t optimized c a l c i n a t i o n c o n d i t i o n s i n t h e present case.

CONCLUSIONS The s t u d i e s performed have shown t h e c o n d i t i o n s € o r t a i l o r i n g t h e c r y s t a l s of t h e SAPO-5 molecular s i e v e w i t h c r y s t a l s i z e s r a n g i n g from 25 t o 580 pm i n

508 l e n g t h . P a r t i c u l a r a t t e n t i o n h a s been p a i d t o t h e c o n d i t i o n s i n f l u e n c i n g c r y s t a l morphology. The f o r m a t i o n and growth of t h e c r y s t a l s a r e i n f l u e n c e d by t h e c o n t e n t of a l l components i n t h e g e l . D i l u t i o n of t h e s y s t e m enhances t h e growth a l o n g t h e c-axis

l e a d i n g t o t h e f o r m a t i o n of l a r g e hexagonal p r i s m s . On t h e o t h e r h a n d ,

an e x c e s s of amine o r amine and p h o s p h o r i c a c i d t o g e t h e r r e s t r i c t s t h e c r y s t a l growth v i a an a c c e l e r a t i o n of t h e n u c l e a t i o n r a t e . Then, t h e d i s t o i . t e d c r y s t a l s

o r even growth a g g r e g a t e s a r e formed. The s i l i c o n c o n t e n t r e p r e s e n t s a n a d d i t i o n a l v a r i a b l e i n f l u e n c i n g t h e c r y s t a l shape. The e u h e d r a l hexagonal p r i s m s c a n c r y s t r a l l i z e o n l y from g e l s c o n t a i n i n g up t n about 0 . 3 s i l i c o n ( m o l a r r a t i o ) which is incorporat.ed i n t o t h e s t r u c t u r e a l m o s t o n l y on phosphorus-T-sites. The f o r m a t i o n of t h e p u r e SAPO-5 phase is c o r r e l a t e d w i t h t h e c o n d i t i o n H?O/TEA < I 0 0 ( m o l a r r a t i o ) . T h e r e f o r e , l a r g e hexagonal SAPO-5 prisms a r e always accompanied by s m a l l p a r t i c l e s of an u n i d e n t i f i e d aluminophosphate which can be s e p a r a t e d , i n p r i n c i p l e , because of t h e l a r g e d i f f e r e n c e s i n t h e s i z e of t.he particles. The shape of t h e c r y s t a l s i n f l u e n c e s c o n s i d e r a b l y t h e i r s o r p t i o n properties.

ACKNOWLEDGEMENTS The authoi

i

a r e i n d e b t e d t o Dr. Janchen f o r a d s o r p t i o n measurements and t o

D I . M . Hndnn f o r h e l p f u l d i s c u s s i o n s . Thr work was p a r t i a l l y s u p p o r t e d by Leuna WilrkF' AG. by t h e fund of t h e P r e s i d e n t of t h e Academy of S c i e n c e s of tht, GDR,

and by t h e P o l i s h M i n i s t r y of N a t i o n a l Education w i t h i n t h e P r o j e c t CPBP 01.06.

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Bulow, P 280 471 ( 1 9 8 9 ) . (1990). i n press. and M. Bulow, s u b m i t t e d f o r

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C. F i n g e r , J. Kornatowski, J. Richter-Mendau, publication. G. F i n g e r , J . Kornatowski, E. Jahn, M. Bulow, M. Rozwadowski, and B. Zibrowius, DD WP 330 567/7 ( 1 9 8 9 ) . J. Kornatowski, M . Rozwadowski, G. F i n g e r , E. J a h n , M. Bulow, and H. Zibrowius, P 280 470 ( 1 9 8 9 ) . G . F i n g e r and J. Kornatowski, Z e o l i t e s , lo (19901, 615. H . Weyda and H. L e c h e r t , Z e o l i t e s , lo ( 1 9 9 0 ) , 251. H . Beyer and H. Riesenberg, Messen und Zahlen m i t dem Mikroskop, i n : Handbuch d e i Mikroskopie, 3. Auflage, VEB V e r l a g Die Techriik, B e r l i n ( 1 9 8 8 ) . pp. 401-408. S.T. Wilson, B.M. Lok, C . A . Messina, and E.M. F l a n i g e n , Proc. 6 t h I n t . Z e o l l t e Conf., Reno, 1983, D. Olson, A. B i s i o ( E d s . ) , B u t t e r w o r t h , G u i l d f o r d , 1984, p p . 97-109. G . F i n g e r , E. J a h n , D. Zetgan, B. Zibrowius, K . S z u l z e w s k i , J. R i c h t e r Mendau, and M. Bulow, B u l l . S O C . Chim. Belg., % ( 1 9 8 9 ) . 291. B. Zibrowius, E . L o f f l e r , G. F i n g e r , E. Sonntag, M. Hunger, and J . Kornatowski, t h i s volume. N.B. Milestone and N.J. Tapp, Stud. S u r f . S c i . Catal., 36 ( 1 9 8 8 ) , 553. w. P a w , S . Q i u , Q. Kan, Z . W u , S . Peng, G . Fan, and D. T i a n , S t u d . S u r f . S c i . C a t a l . , 49 ( 1 9 8 9 1 , 281. V . R . Choudhary, D.B. Akolekar, A . P . Singh, and S . D . S a n s a r e , J . C a t a l . , 1 1 1 (1988). 23.

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G. Ohlmann et al. (Editors),Catalysis and Adsorption by Zeolites 1991 Elsevier Science Publishers B.V., Amsterdam

511

APPROXIMATE ASSIGNMENT OF VIBRATIONAL FREQUENCIES OF THE NaX FRAl.iEI.VORK E. GEIOEL',

H.

9uHLIG2,

CH.

PEUKER'

and W.

PILZ'

' C e n t r a l I n s t i t u t e o f P h y s i c a l C h e m i s t r y , Academy o f S c i e n c e s , Rudower Chaussee 5, B e r l i n - 1 1 9 9 (GDR) 2Department o f C h e m i s t r y , K a r l - M a r x - U n i v e r s i t y T a l s t r a R e 35, L e i p z i g - 7 0 1 0 (GDR)

Leipzig,

SUMMARY O n t h e b a s i s o f s y s t e m a t i c a l l y d e v e l o p e d s u b u n i t c l u s t e r mode l s o f NaX z e o l i t e framework n o r m a l c o o r d i n a t e a n a l y s e s h a v e been c a r r i e d o u t by means o f WILSON'S GF m a t r i x method i n t e n d i n g t o u n d e r s t a n d t h e c o r r e s p o n d i n g e x p e r i m e n t a l v i b r a t i o n a l spect r a . An a p p r o x i m a t e a s s i g n m e n t o f v i b r a t i o n a l f r e q u e n c i e s o f t h e NaX z e o l i t e framework i s g i v e n o n t h e b a s i s o f c a l c u l a t e d e i g e n v a l u e s , p o t e n t i a l e n e r g y d i s t r i b u t i o n s and e i g e n v e c t o r s . The r e s u l t s a r e d i s c u s s e d i n c o n n e c t i o n w i t h t h e FLANIGEN c o n c e p t i o n o f i n t e r n a l and e x t e r n a l v i b r a t i o n s .

INTRODUCTION Z e o l i t e s have been w i d e l y used f o r i o n exchange, s e l e c t i v e a d s o r p t i o n and c a t a l y s i s .

Related t o t h a t r e s u l t s o f a great

number o f e m p i r i c a l s t u d i e s o f i n f r a r e d and Raman s p e c t r a have been p u b l i s h e d m a i n l y c o n c e r n i n g w i t h OH s t r e t c h i n g v i b r a t i o n s and more r e c e n t l y c o n s i d e r i n g t h e v i b r a t i o n s o f t h e z e o l i t e framework on t h e b a s i s o f FLANIGEN c o n c e p t i o n ( r e f .

1). I n t h e

most c a s e s t h e r e a r e d i f f i c u l t i e s w i t h r e s p e c t t o t h e i n t e r p r e t a t i o n o f v i b r a t i o n a l s p e c t r a because o f t h e c o m p l i c a t e d c r y s t a l s t r u c t u r e on one s i d e and t h e s t r o n g l y o v e r l a p p e d bands i n t h e z e o l i t e s p e c t r a on t h e o t h e r s i d e .

Beside t h a t i t i s i n s u f f i -

c i e n t l y known t h e i n f l u e n c e o f c a t i o n s , s t r u c t u r a l OH g r o u p s , a d s o r b e d w a t e r and o t h e r m o l e c u l e s on t h e v i b r a t i o n a l spectrum. F u r t h e r d i f f i c u l t i e s a r i s e w i t h i n a more t h e o r e t i c a l t r e a t ment o f v i b r a t i o n a l b e h a v i o u r o f z e o l i t e s c o n n e c t i n g w i t h t h e l a r g e u n i t c e l l s and t h e i n s u f f i c i e n t l y known f o r c e r e l a t i o n s between t h e l a t t i c e atoms and i o n s . The n o r m a l c o o r d i n a t e a n a l y s i s a s a r o u t i n e method c a n be u s e d t o i n t e r p r e t v i b r a t i o n a l spectra, but i s r e s t r i c t e d t o the s o l u t i o n o f d i r e c t eigenvalue p r o b l e m s because o f t h e l a c k o f e x p e r i m e n t a l d a t a f i t t i n g t h e model f o r c e c o n s t a n t s . So f a r some n o r m a l c o o r d i n a t e a n a l y s e s

512

have been c a r r i e d o u t on t h e b a s i s of s u b u n i t cluster models ( r e f s . 2 , 3 ) a s s u m i n g t h e d e c o u p l i n g p o s t u l a t e o f 6LACICI”IELL ( r e f . 3 ) and s i m p l i f i c a t i o n s w i t h i n t h e p s e u d o - l a t t i c e method ( r e f s . 4 - 6 ) , r e s p e c t i v e l y . R e c e n t l y t h e m e t h o d s of m o l e c u l a r m e c h a n i c s h a v e b e e n u s e d , t o o ( r e f s , 6-8). I n t h i s s t u d y , n o r m a l mode c a l c u l a t i o n s were d o n e f o r some s y s t e m a t i c a l l y s e l e c t e d s u b u n i t c l u s t e r models on t h e b a s i s of a simplified force f i e l d for understanding the vibrational spectra o f NaX z e o l i t e framework. SUBUNIT CLUSTER MODELS AND CALCULATIONS T h e s u b u n i t c l u s t e r m o d e l s o f NaX z e o l i t e f r a m e w o r k h a v e b e e n s e l e c t e d f o r t h e normal c o o r d i n a t e t r e a t m e n t s u s i n g a t e t r a h e d r a l s u r r o u n d i n g o f T ( T = S i , A l ) atoms a n d a s s u m i n g t h e v a l i d i t y o f LOEWENSTEIN r u l e ( r e f . 9 ) . G e o m e t r i c a l p a r a m e t e r s h a v e b e e n t a k e n from OLSON‘S X-ray d i f f r a c t i o n s t u d y o f h y d r a t e d N a X zeol i t e ( r e f . 10). The s e l e c t e d c l u s t e r m o d e l s t o g e t h e r w i t h t h e i r p o i n t g r o u p s y m m e t r i e s , t h e d i s t r i b u t i o n o f norrnal v i b r a t i o n s t o t h e i r r e d u c i b l e r e p r e s e n t a t i o n s a n d t h e number o f i n f r a r e d a n d Raman a c t i v e m o d e l v i b r a t i o n s a r e shown i n T a b l e 1. T h e r e s u l t of a n u c l e a r s i t e group a n a l y s i s f o r t h e complete u n i t c e l l o f NaX z e o l i t e b a s e d o n Fd3 s p a c e g r o u p symmetry a n d s u b t r a c t i n g t h e m o t i o n s o f Na’ c a t i o n s i s g i v e n i n t h e l a s t r o w . MARONI ( r e f . 1 1 ) a n d GODBER a n d OZIN ( r e f . 1 2 ) o b t a i n e d an o t h e r r e s u l t o n t h e b a s i s o f Fd3m s p a c e g r o u p s y m m e t r y . W i t h t h e e x c e p t i o n o f S i 0 4 ( A 1 0 3 ) 4 a n d A 1 0 4 ( S i 0 3 ) 4 A l g t h e number o f i n f r a r e d a n d Raman a c t i v e model v i b r a t i o n s i s a l w a y s smaller t h a n t h e c o r r e s p o n d i n g number o f o p t i c a l a c t i v e v i b r a t i o n s o f t h e u n i t c e l l . T h e n o r m a l mode c a l c u l a t i o n s were d o n e by m c a n s o f t h e GF m a t r i x m e t h o d o f WILSON ( r e f . 1 3 ) o n t h e b a s i s o f t h e JONES p r o g r a m ( r e f . 1 4 ) u s i n g i n t e r n a l c o o r d i n a t e s i n t h e u s u a l way w i t h e x c e p t i o n o f t o r s i o n a l o n e s w h i c h were c h o i c e d t o t a k e i n t o c o n s i d e r a t i o n o n l y o n e d i h e d r a l a n g l e b e n d i n g f o r o n e n o n - t e r m i n a l bond. A c c o r d i n g t o BLACKWELL ( r e f . 3 ) we h a v e u s e d t h e s o - c a l l e d bond l e n g t h s c a l e d f o r c e f i e l d o n t h e b a s i s o f t h e BADGER r u l e (BLSF-BR)

.

513

TABLE 1 D i s t r i b u t i o n o f n o r m a l v i b r a t i o n s o n t h e symmetry s p e c i e s o f d i f f e r e n t z e o l i t e c l u s t e r models C l u s t e r models; / p o i n t o r space group/

D i s t r i b u t i o n o f normal vibrations

S i 0 4 , A104

u(1R)

u(RA)

9

9

O3 S i -0 ( i) -A103

21

21

4R(hP),

42

42

44

44

40

40

21

21

4R(S)-

r=22a+22e

6 R ( h P ) ; /C3/ 06R;

r=20au t 2oa

/S6/

r=21a

Si04(A1)4 Si04 (A103)4 A104 P i 0 3 14 ( A 1 19 A124Si24096 ; /Fd3, (Th 4 ) /

9

+ 20eu+20e

9

r=57a

57

57

r=84a r=18A +18Au+18E +18Eu+ 9 9 54 Fa+53F,

84 53

84 90

( u ( 1 R ) number o f i n f r a r e d and u(RA) number o f Raman a c t i v e m o d e l v i b r a t i o n s ; i = 1 - 4 ; 4R(hP) 4 - r i n g i n t h e h e x a g o n a l p r i s m ; 4R(S) 4 - r i n g i n t h e s o d a l i t e u n i t : 6R(hP) 6 - r i n g i n t h e h e x a g o n a l p r i s m ; D6R d o u b l e - 6 - r i n g ) RESULTS AND DISCUSSION The r e s u l t s o f o u r c a l c u l a t i o n s f o r some s e l e c t e d s u b u n i t c l u s t e r models a r e r e p r e s e n t e d i n F i g .

1 and compared w i t h t h e

e x p e r i m e n t a l i n f r a r e d s p e c t r u m o f NaX z e o l i t e framework t a k e n f r o m MIECZNIKOWSKI and HANUZA ( r e f .

1 5 ) . Whereas t h e o b s e r v e d

spectrum i s g i v e n i n t r a n s m i s s i o n u n i t s t h e c a l c u l a t e d frequenc i e s a r e p l o t t e d a g a i n s t t h e number o f f r e q u e n c i e s p e r f r e q u e n c y interval i n arbitrary units.

I t c a n be seen t h a t an i n c r e a s e o f

s u b u n i t c l u s t e r s i z e l e a d s t o i n c r e a s i n g d e n s i t y o f bands p e r f r e q u e n c y i n t e r v a l . The A10 and SiO s t r e t c h i n g v i b r a t i o n s a b s o r b i n c l e a r l y d i f f e r e n t f r e q u e n c y r e g i o n s whereas OAlO and O S i O b e n d i n g ones appear i n t h e same r e g i o n what i s shown i n r e p r e s e n t a t i o n (b).

T h e r e f o r e t h e l a t t e r v i b r a t i o n s a r e d e n o t e d by OTO

bendings. T h i s p r i n c i p a l d i f f e r e n c e remains v a l i d a l s o i n t h e o t h e r s u b u n i t c l u s t e r s m o d i f i e d , however,

by c o m p l i c a t e d coup-

lings.

I f t e r m i n a l bonds e x i s t i n t h e c l u s t e r models as i n t h e c a s e s (b)-(e)

t h e c a l c u l a t e d h i g h f r e q u e n c y A10 s t r e t c h i n g v i b r a t i o n s

f a l l i n t h e e x p e r i m e n t a l f r e q u e n c y gap (780-850 cm").

Lacking

t e r m i n a l bonds ( f ) t h e above m e n t i o n e d v i b r a t i o n s a r e s h i f t e d t o

514 I

I

600

1200

1000

800

400

-

600 400 krn-11

515

l o w e r f r e q u e n c i e s by s t r o n g c o u p l i n g s .

T h i s r e s u l t i s supported

by FLANIGEN's i n f r a r e d s t u d i e s o f sodium a l u m o s i l i c a t e s i n d i f ferent crystallization states (ref.

16). Increasing the crystal-

l i z a t i o n degree and t h u s d e c r e a s i n g t h e number o f t e r m i n a l bonds i n t h e m i x t u r e l e a d s t o d i s a p p e a r a n c e o f an a b s o r p t i o n band a t 850 cm"

c h a r a c t e r i z i n g t e r m i n a l A 1 0 bonds.

The c a l c u l a t e d f r e q u e n c y r e g i o n s o f s t r e t c h i n g and OTO bending vibrations,

t o g e t h e r w i t h an a p p r o x i m a t e c l a s s i f i c a t i o n o f

t h e model v i b r a t i o n s a r e c o l l e c t e d i n t h e T a b l e s 2 and 3. The TOT b e n d i n g s and t o r s i o n a l v i b r a t i o n s a p p e a r i n g i n t h e f a r i n f r a r e d r e g i o n a r e n o t s u b j e c t o f t h i s s t u d y . V i b r a t i o n a l modes o f t h e s u b u n i t c l u s t e r models d e t e r m i n e d by t h e p o t e n t i a l e n e r g y d i s t r i b u t i o n and t h e e i g e n v e c t o r s show a c o m p l i c a t e d p i c t u r e . I t c a n be d i s t i n g u i s h e d between 3 t y p e s o f s t r e t c h i n g v i b r a -

tions: terminal,

bridging ( w i t h regard t o linkages connecting 2

t e t r a h e d r o n s ) and t e t r a h e d r a l v i b r e t i o n s ,

a l l o f t h e s e c a n be

f u r t h e r d i v i d e d i n s y m m e t r i c and a s y m m e t r i c ones.

I n addition t o

t h i s t h e p o t e n t i a l e n e r g y can be d i s t r i b u t e d i n a d i f f e r e n t way. T h i s statement i s v a l i d i n p r i n c i p a l f o r t h e bending v i b r a t i o n s , too, The b r i d g i n g and t h e t e t r a h e d r a l t y p e v i b r a t i o n s o f t h e subu n i t c l u s t e r models c a n be u s e d t o i n t e r p r e t t h e v i b r a t i o n a l s p e c t r a o f an a c t u a l z e o l i t e .

The a p p r o x i m a t e a s s i g n m e n t t h u s

o b t a i n e d i s shown i n T a b l e 4. So t h e h i g h f r e q u e n c y framework a b s o r p t i o n s can be a s s i g n e d t o a s y m m e t r i c b r i d g i n g s t r e t c h i n g vibrations.

The m a i n p a r t o f p o t e n t i a l e n e r g y o f t h e s e i s l o -

c a t e d i n t h e S i O bonds. Then f o l l o w a s y m m e t r i c b r i d g i n g s t r e t c h i n g v i b r a t i o n s w i t h dominant A 1 0 c h a r a c t e r ( S i O c h a r a c t e r less t h a n 10%). Symmetric b r i d g i n g and t e t r a h e d r a l v i b r a t i o n s a p p e a r a t l o w e r f r e q u e n c i e s . The d o u b l e r i n g and p o r e o p e n i n g modes m e n t i o n e d i n T a b l e 4 have been d e s c r i b e d i n d e t a i l e l s e w h e r e (ref.

Fig.

1 7 ) . The a b s o r p t i o n s between 510 cm"

1. Observed

and 408 cm"

c a n be

IR s p e c t r u m and c a l c u l a t e d v i b r a t i o n a l f r e q u e n -

c i e s o f NaX z e o l i t e framework ( T = t r a n s m i s s i o n , nf=number o f f r e quencies per frequency i n t e r v a l ; NaX z e o l i t e (Si/A1=1.12) (ref.

15),

a c c o r d i n g t o MIECZNIKOWSKI and HANUZA

( b ) S i 0 4 ( f u l l l i n e ) and A104 ( d o t t e d l i n e ) c l u s t e r ,

( c ) 03Si-0(1)-A103 prism,

( a ) e x p e r i m e n t a l IR s p e c t r u m o f

cluster,

( e ) double-6-ring

(d) 6 - r i n g c l u s t e r i n t h e hexagonal

cluster,

( f ) A104(Si0,)4(A1)9

cluster).

TABLE 2 C a l c u l a t e d f r e q u e n c y r e g i o n s and a p p r o x i m a t e a s s i g n m e n t s f o r t h e s t r e t c h i n g v i b r a t i o n s o f s e l e c t e d z e o l i t e c l u s t e r models Cluster

Si04 / A104

C a l c u l a t e d f r e q u e n c y r e g i o n and a p p r o x i m a t e a s s i g n m e n t

1011-978

828-805

(SiO)

Ztt,as(AIO)

V

t,as

03S i - C ( i ) - A 1 0 3

1086-1041 Y

b, a s

T,

6R(hP)

1C16-982

s

b, S

991

( S i o ,A10)

1095-1065

b,s

v,

564-561

(SiO.Al0)

1039-1027

Si C )

Y

T,s

5,705-690 ( s i o Ale) 8

% 744-682

vb, ( SiO ,A 10 )

1098-1045,936-901 vb, as ( Si0 , A 10 )

814-803

687-682 ( Si0 )

800-791 uT, as (

609-604 ( A10 )

“;-,

VT,

534-523 V

756 vb, as (A1O)

347,233

?,as

789-721,694-595 yb, a s (

785

5,a s (A1o)

vb,s(AIO,SiO)

(A10)

902-894

b.as ( S i 0 , A l O )

889,768

827 %,as (A1O)

866-864

669

(Al0,SiO)

Y ( A 10, t,s

820-803

4,

yt ,as (A10)

Y,as(SiO)

732 D6 R

(A10)

(A10,SiO)

1082-1052

t.S

t.s

239-231 Y

b, a s ( S i O . A l 0 ) ‘V

574 2r

845-811

5 ,a s ( S i O )

(Si0,AlO)

571-559 v (A10)

720 Vt,s(SiO)

T,s

(A10,SiO)

812,779 a (

*b,

569,365,260 vb, ( A 1 0 , S i O ) 584-539 VT,s(A1O)

(wave numbers i n cm-’; d i s p l a c e m e n t s o f t e r m i n a l , o f b r i d g i n g and o f t h e bonds o f a c o m p l e t e t e t r a h e d r o n a r e d e n o t e d by t , b and T, r e s p e c t i v e l y ; s and a s d e n o t e s y m m e t r i c a l ( t h e same s i g n ) and a s y m m e t r i c a l ( o p p o s i t e s i g n ) di s pl ac ement s , r e s p e c t i v e l y ; m c h a r a c t e r i z e s m ixed v i b r a t i o n a l modes; S i O a n d A10 d e n o t e t h e m a i n p a r t o f p o t e n t i a l e n e r g y w i t h i n t h e p o t e n t i a l energy d i s t r i b u t i o n )

TABLE 3

C a l c u l a t e d f r e q u e n c y r e g i o n s and a p p r o x i m a t e a s s i g n m e n t s f o r t h e OTO-bending v i b r a t i o n s o f s e l e c t e d z e o l i t e c l u s t e r models C a l c u l a t e d f requenc y r e g i o n and approx i mat e assignment

Si04

/

A104

t t393-381 , a s (Oslo 1

if

I

I

03Si-0 ( i)-A103

544-443 &tb.

304-376 Jt * a s ( O A l O 1 410-404 m '

363-356 Jt

381-376 dtt (OT0

362-357 ( O M 01

dt *

* (Oslo 1 276- 27 1 'tb, a s

6R(hP) 06 R

.

590-549

dbb (OT0) 219,136-114

524-503

dtb,

481-397

353-351.214-162

am

Jm

(OT0)

325-222 atb, a s

abb. a s

I ( t t * t b and bb c h a r a c t e r i z e a n g l e d e f o r m a t i o n s .

i f t h e a n g l e c o n s i s t s o f two t e r m i n a l ( t t ) .

o f a t e r m i n a l a n d a b r i d g i n g ( t b ) a n d of two b r i d g i n g bonds ( b b ) ; s and a s d e n o t e symmetr i c a l and a s y m m e t r i c a l d e f o r m a t i o n s , v i b r a t i o n a l nodes)

respectively;

OTO=OA10,

O S i O ; m c h a r a c t e r i z e s mixed

518

a s s i g n e d t o OTO b e n d i n g s .

I t s h o u l d be n o t e d t h a t so f a r an ex-

p l a n a t i o n o f t h e e x p e r i m e n t a l s p e c t r a c a n be o b t a i n e d o n l y by c o n s i d e r i n g t h e v i b r a t i o n a l b e h a v i o u r o f some d i f f e r e n t s u b u n i t c l u s t e r models,

TABLE 4

Approximate assignment o f observed v i b r a t i o n a l f r e q u e n c i e s o f NaX z e o l i t e ( S i / A l = l . l 2 ) cm-'

obs. IF! b,

Assignment a ) RA

1078

sh

975

ve

752

m

1082

m

985

m

800

w b,as

(A10)

700

m

ub, ( S i 0 , A l O )

612

vw

562

m

vl-s(AIO) d o u b l e r i n g mode

461

S

408

W

510

m

p o r e o p e n i n g mode

m

( s h s h o u l d e r , w weak, strong) ')notation

'bb,

$( OTO ) 375

364

vs

v w v e r y weak,

m medium,

s strong,

vs very

see t a b l e s 2 and 3

b ) t a k e n f r o m MIECZNIKOWSKI and HANUZA ( r e f .

15)

measurements

As a r e s u l t t h e FLANIGEN c o n c e p t i o n o f e x t e r n a l and i n t e r n a l t e t r a h e d r a l v i b r a t i o n s h a s t o be m o d i f i e d because o f t h e s t r o n g c o u p l i n g between t h e z e o l i t e framework v i b r a t i o n s .

Each bond has

b r i d g i n g and i n t e r n a l t e t r a h e d r a l c h a r a c t e r s i m u l t a n e o u s l y . m i l a r c c n c l u s i o n s have been drawn by WALTHER ( r e f s .

18,19)

Sifrom

c o r r e s p o n d i n g c a l c u l a t i o n s f o r ZSM-5 z e o l i t e s u b u n i t c l u s t e r models. The c o u p l i n g between t h e t e t r a h e d r o n s d o m i n a t e s t h e c o u p l i n g w i t h i n t h e t e t r a h e d r o n s . T h e r e f o r e we p r e f e r t o d i s -

519 t i n g u i s h between l o c a l i z e d and d e l o c a l i z e d v i b r a t i o n s ( s e e ref.

20) t h a n between i n t e r n a l and e x t e r n a l ones.

CONCLUDING REMARKS Z e o l i t e s r e p r e s e n t an e x t r a o r d i n a r y c h a l l e n g e f o r t h e v i b r a t i o n a l s p e c t r o s c o p y because o f t h e i r l a r g e u n i t c e l l s .

They a r e

c o n n e c t e d w i t h a l a r g e number o f v i b r a t i o n a l modes l y i n g c l o s e l y t o g e t h e r i n a s m a l l frequency region.

Vibrational calculations

o f z e o l i t e s u b u n i t c l u s t e r models c a n be a c c o u n t e d f o r an i n t e r mediate step i n t r e a t i n g t h e complete s p e c t r o s c o p i c a l l y s i g n i f i cant u n i t c e l l .

I f t h e c l u s t e r p r o b l e m w i l l be s o l v e d ,

q u e s t i o n f o r a p p r o p r i a t e f o r c e f i e l d s r e m a i n s open.

the

In a d d i t i o n

t o t h i s methods f o r t h e e s t i m a t i o n o f v i b r a t i o n a l i n t e n s i t i e s o f z e o l i t e s seem t o be n e c e s s a r y e x p l a i n i n g c o m p l e t e l y t h e e x p e r i m e n t a l s p e c t r a . W i t h r e s p e c t t o b o t h t h e problems mentioned above quantum c h e m i c a l methods have t o c o n t r i b u t e c o n s i d e r a b l y . REFERENCES E.M. F l a n i g e n , H. K h a t a m i and H.A. Szymanski, Adv. Chem. Ser. 101 (1971) 201 2 Y.S. Kong, M.S. 3hon and K.T. No, B u l l . Kor. Chem. SOC. 6 (1985) 57 C.S. B l a c k w e l l , 3. Phys. Chem. 83 (1979) 3251, 3257 3 4 K.T. No and M.S. Jhon, J. Phys. Chem. 8 7 (1983) 226 5 K.T. No, O.H. Bae and M.S. ahon, J. Phys. Chem. 90 (1986) 1772 K.T. No, B.H. Seo, 3.M. Park and M.S. Jhon, J. Phys. Chem. 6 9 2 (1988) 6783 7 K.T. No, Y.Y. Huh and M.S. Jhon, 3. Phys. Chem. 93 (1989) 6413 K.T. No, B.H. Seo and M.S. Jhon, Theor. Chim. A c t a 75 (1Y89) 8 307 K.T. No, H. Chon, T. Ree and M.S. Jhon, 3 . Phys. Chem, 85 9 (1981) 2065 10 D.H. Olson, 3. Phys. Chem. 74 (1970) 2758 11 V.A. M a r o n i , A p p l . S p e c t r o s c . 4 2 (1988) 487 12 J. Godber and G.A. Ozin, J. Phys. Chem. 9 2 (1988) 2841 jr., J.C. D e c i u s and P.C. Cross, M o l e c u l a r 13 E.B.Wilson, V i b r a t i o n s , M c G r a w - H i l l Book Company, Inc., New York, 1955 301188, Computer Programs f o r I n f r a r e d S p e c t r o p h o t o m e t r y 14 R.N. -Normal C o o r d i n a t e A n a l y s i s N.R.C.C. B u l l e t i n No. 15, Canada, 1976 15 A. M i e c z n i k o w s k i and J. Hanuza, Z e o l i t e s 5 (1985) 188 16 E.M. F l a n i g e n , S t r u c t u r a l A n a l y s i s by I n f r a r e d S p e c t r o s c o p y , ACS Monograph No. 171, Z e o l i t e C h e m i s t r y and C a t a l y s i s , 1976, chap. 2, pp. 80-117 17 E. G e i d e l , H. B o h l i g , Ch. Peuker and W. P i l z , Z. CIiem. 28 (1985) 155 18 P. W a l t h e r , 2. Chem. 26 (1986) 189, 222 19 P. W a l t h e r , Z. phys. Chem. L e i p z i g , 269 (1988) 809 20 R.A. v a n S a n t e n and D.L. Vogel, L a t t i c e Dynamics o f Z e o l i t e s , Advances i n S o l i d S t a t e C h e m i s t r y , V o l . 1, JAI P r e s s Inc., 1989, pp. 151-224 1

-



THE TOPOLOGICAL STRUCTURE REPRESENTATION

521

OF ZEOLITES

B. MULLER I n s t i t u t f u r P h y s i k a l i s c h e Chemie, Friedrich-Schiller-Universitat Jena. LessingstraRe 10, 0-6900 Jena, BRD SUMMARY I t i s shown t h a t z e o l i t e s can be c h a r a c t e r i z e d

and c l a s s i f i e d by means o f t o p o l o g i c a l s t r u c t u r e a n a l y s i s . The t o p o l o g y used i s a non-metrical geometry. The advantage o f u s i n g t h e s e t o p o l o g i c a l s t r u c t u r e d e s c r i p t i o n i s t h a t t h e general s t r u c t u r a l p r o p e r t i e s r e p r e s e n t i n g t h e frame f o r t h e p h y s i c s and c h e m i s t r y o f m a t e r i a l s can be d e c l a r e d w i t h s i m p l e numbers o f t h e t o p o l o g i c a l parameters Nk1. The l o c a l r e c i p r o c a l curves o f t h e c e l l u l a r c o n f i g u r a t i o n b31/l11, expressed i n t h e t o p o l o g i c a l parameters, a r e measures f o r both t h e compactness and t h e c a t a l y t i c a l p r o p e r t i e s o f z e o l i t e s . The general s t r u c t u r e f u n c t i o n N 2 1 = N21(NS2,NO',N12) o f t h e independent t o p o l o g i c a l parameters N k 1 i s deduced f r o m t h e t h e o r y of c e l l u l a r configurations. The d i s t r i b u t i o n f u n c t i o n s g21i and 9 3 2 1 o f t h e f a c e s and polyhedra, r e s p e c t i v e l y , which may b u i l d up t h e v a r i e t y o f z e o l i t e c o n f i g u r a t i o n s , a r e given. INTRODUCTION

The

simplest,

e 1ement s

of

topo 1ogy

obtains

but

spatial

a l s o t h e most e s s e n t i a l systems a r e o f

the

a

connective s t r u c t u r e

i n v a r i a n t a t continuos transformations

s non-metrical.

topo 1ogy ture

ana y s i s

consists

relations

topological

relations

l),

(ref.

(refs. uniquely

Atomic

mapped

i n the determination

condensed

into

which

are the

The problem o f s p a t i a l t o p o l o g i c a l s t r u c -

between those c o n s t r a i n e d t h e

2,3).

The

therefore,

of

both

t o p o l o g i c a l s t r u c t u r e elements and i n t h e corresponding relations

between

nature.

structure

intrinsic connective

configurations

systems such as z e o l i t e s

space-filling

cellular

may

be

configurations

(ref .4). One

k-celloid

structure make

it

element

of

t h e dimension k correponds

o f t h e concrete

zeolite.

inquely

These

p o s s i b l e f o r t h e t o p o l o g i c a l parameters o f

configurations

space t o u n i q u e l y r e p r e s e n t ,

parameters

Nk1

the

as w e l l as

and t h e i r d i s t r i b u t i o n

constrain

n o t o n l y t h e s p a t i a l c o n f i g u r a t i o n and

also

s p a t i a l e x i s t e n c e and s t a b i l i t y i n

the

nected system represented b y t h e z e o l i t e s ( r e f . 6 ) .

cellular classify, 5).

functions arrangement

every

one

homomorphisms

t h e t o p o l o g i c a l s t r u c t u r e o f atomic condensed system ( r e f . topological

to

The gkli but

cellular-con-

522

TOPOLOGICAL STRUCTURE CONFIGURATIONS The can

topological

be

s t r u c t u r e elements o f

zeolite

configurations

r e a l i z e d by k - c e l l o i d s w i t h d i f f e r e n t dimensions

0,1,2.3)

on a 4-dimensional c o n f i g u r a t i o n space.

celloids

represent one k-dimensional

(k-l)-celloids (k>O).

Each o f

f i g u r e bounded a t

=

(k

k

the

k-

least

k+l

These f i g u r e s can be r e a l i z e d as k-dimensio-

n a l polyhedra e x h i b i t i n g a t l e a s t k + l v e r t i c e s ( r e f . 5 ) . 0-celloids zeolites,

both

celloid, with

the

1.2.

Si-

...,N o ) :

I n our case

and t h e A l - t e t r a h e d r a

of

do

(dangling-bonds)

crystalline

represent

which i s a l s o r e f e r r e d t o as the v e r t i c e s .

single

atoms)

=

(p

mOp

o r two chemical bonds

n o t represent O - c e l l o i d s because o f

the

0-

Atoms o r i o n s (as

the

the

0-

general

pro-

p e r t i e s o f connected c e l l u l a r systems. 1-celloids neighbouring

=

(q

mlq

Si-

1,2,.

. . .N1 1:

The "bonds" e x i s t i n g

between

o r A l - t e t r a h e d r a by means o f t h e i n c l u s i o n

0-ion

represent t h e 7 - c e l l o i d s o f t h e z e o l i t e c o n f i g u r a t i o n s . 2 - c e l l o i d s nPr ( r

=

1,2,

...,Nz):

The 7 - c e l l o i d s which a r e

c a l l e d the edges o f the c e l l u l a r c o n f i g u r a t i o n may be b u i l t or rings.

cycles

Only t h e r i n g s which seperate two neighbouring polyhedra

are 2 - c e l l o i d s , tions

also

of

i.e.,

the 2-celloids depict c o r r e la t io n d i s t r i b u -

the third-order.

This i s a condition o f

the

homotopic

invariance o f c e l l u l a r c o n f i g u r a t i o n s ( r e f . 6 ) . 3-celloids either

dimensional formed

are

(s = 1 , 2 , , . . , N a ) :

nP.

The polyhedra

belonging

the same o r d i f f e r e n t classes cover compactly over c o n f i g u r a t i o n space. by t h e 2 - c e l l o i d s and

d i s t r i b u t i o n s o f the fourth-order, figurations

The 3 - c e l l o i d s represent

(the

spatial

to

the

3-

polyhedra) correlation

The polyhedra o f c e l l u l a r

do n o t agreed w i t h the conventional co-ordinate

conpoly-

hedra o f s t r u c t u r e chemistry ( r e f , 7 ) . CONNECTIVE STRUCTURE RELATIONS

The

simplest

O,l, . . . ,d: of

the

then, result

structure

p = 1,2,

relations

...,N k ) o f

i n the

only

of

the

then,

t h e border o f t h e 1 - c e l l o i d the

means mkp

mrq,

if

k-celloid

limboide connective s t r u c t u r e r e l a t i o n s can

mapped t o t h e connective r e l a t i o n m a t r i x ve s t r u c t u r e r e l a t i o n a m a t r i x element

Mk1

mklpO

g i v i n g each

=

k

(&p:

The k - c e l l o i d

i n t e r s e c t i o n between both i s

Thouse second-order

M

k - c e l l o i d s can be produced by

i n t e r s e c t i o n between a l l elements. and

set

is the &p.

be

connecti-

w i t h t h e values

523 1

i f mkp n

0

o t h e r wise

ml0

= rnkp

= I

mklpq

he mean v a l u e

which i s a l s o c a l l e d t h e t o p o l o g i c a l

Nk1

parame-

t e r c h a r a c t e r i z e s t h e number o f l i m b o i d e connective r e l a t i o n s o f typ cal

k - c e l l o i d i n regard t o i t s 7 - c e l l o i d s .

the

topological

the

typical

condensed

parameters

The m a t r i x

represents t h e g l o b a l as

Nk1

topological structure invariants o f

system.

The

a

d i s t r i b u t i o n f u n c t i o n gkli

a

Nk1

of

well

as

given

atomic

obtains

types

n k l i of t h e limboide connections between t h e k- and t h e

7-celloids

(ref. 7 ) . TOPOLOGICAL STRUCTURE CONSTRAINTS Three-dimensional three and

c e l l u l a r s t r u c t u r e s must be c h a r a c t e r i z e d

o f t h e f o u r independent t o p o l o g i c a l parameters (ref.

N32

by

No1,N12,N21,

The general g l o b a l s t r u c t u r e f u n c t i o n o f such a

7).

c o n f i g u r a t i o n i s given by equation ( 2 ) :

The

topological structure function ( 2 ) constrains not only

spatial

configuration

stence ted

and arrangement b u t a l s o t h e

and s t a b i l i t y i n every c e l l u l a r connected system

by

the zeolites.

theory

that

the

I t i s d e r i v e d from

t o p o l o g i c a l parameters

the

exi-

represen-

topological

the

o r d e r s o f t h e f a c t o r groups as w e l l as t o t h e o r b i t l e n g t h s o f

the

permutation group i n t h e c o n f i g u r a t i o n space (eqn.

N3

and

No a r e t h e d e n s i t y numbers o f t h e

respectively,

which

3-

related

group to

Nk1

are

the

spatial

(3) and r e f . 7 ) .

and

a r e i n v a r i a n t i n regard t o b o t h

0-celloids, homeomorphic

and permutative t r a n s f o r m a t i o n s . The

distribution

limboide celloids.

connections The

function of

the

g k l i gives the types k-celloids i n

respect

trizes

Mk1

between t h e d i s t r i b u t i o n f u n c t i o n 9211 and

Especially,

on

they y i e l d the following the

the

the

d i f f e r e n t r e l a t i o n s e x i s t between t h e

tions

7).

of

to

d i s t r i b u t i o n f u n c t i o n s g k l i a r e dependent

o t h e r as a d d i t i o n a l , (ref.

nkli

1each

submarela-

topological

524

Parameters N Z 1 and N 3 2 (eqns. ( 4 ) - ( 7 ) ,

and

7):

i=l

i=l

This

ref.

i s t h e p h y s i c a l and chemical reason why

different

atomic

bonding p r o p e r t i e s g i v e d i f f e r e n t s o r t s o f c o n f i g u r a t i o n s

evolutions.

and

The i n t e r a c t i o n o f l o c a l energy m i n i m a z a t i o n and topo-

logy produces complex t h r e e - d i m e n s i o n a l

of

c e l l u l a r configurations

zeol it e s . TOPOLOGICAL CURVATURE MEASURE The s p e c i f i c t o p o l o g i c a l c u r v a t u r e measures b ’ * k / v 1 1 1 both

the s t a b i l i t y ,

the zeolites ( r e f . trical

property

surface.

determine

t h e compactness and a d s o r p t i o n p r o p e r t i e s 7).

that

of

These measures have t h e i n t e r e s t i n g geomethey a r e t h e u n i t d i s t a n c e

of

a

parallel

A p a r a l l e l s u r f a c e i s formed by t r a n s f o r m i n g each p o i n t a

c o n s t a n t d i s t a n c e a l o n g t h e s u r f a c e normal a t t h e p o i n t . A z e o l i t e c r y s t a l c o n f i g u r a t i o n can be d e s c r i b e d as

consisting

o f a number o f d i f f e r e n t t y p e s o f p o l y h e d r a whose r e c i p r o c a l

unit,

c u r v a t u r e , i s o n l y g i v e n by t h e t o p o l o g i c a l parameters ( r e f . 7).

COMPUTING METHODS OF THE TOPOLOGICAL PARAMETERS

A

case those

complete e v a l u a t i o n o f a l l t o p o l o g i c a l parameters i s i n possible, in

if

t h e s e t o f atomic c o o r d i n a t e s i s know

c r y s t a l l i n e z e o l i t e structures.

The

algorithm

this

as

are

of

our

computational program NWS i s as f o l l o w s ( r e f . 8 ) : 1.

Generate t h e s e t o f atomic c o o r d i n a t e s i n t o a f i n i t e

2.

Determinate

structu-

r e model o f c r y s t a l l i n e z e o l i t e . the

c o n n e c t i v e m a t r i x Mo1 by means

of

metrical

525 distance

r e l a t i o n s between t h e g i v e n model

coordinate

poly-

hedra. 3.

Search

t h e c o n f i g u r a t i o n r i n g s and d i s c r i m i n e

faces ( r e f . Evalute

4.

between

their

9).

both the topological d i s t r i b u t i o n functions gk’i

and

t h e t o p o l o g i c a l parameters N k l . 5.

Correct

the

t o p o l o g i c a l p a r a m e t e r s Nkl w i t h

model s i z e e f f e c t ( r e f .

regard

to

the

o f most o f

the

10).

TOPOLOGICAL STRUCTURES OF ZEOLITES The t o p o l o g i c a l p a r a m e t e r s N 0 1 1 N 1 2 , N 2 1 ,

and

N32

z e o l i t e s a r e shown i n t a b l e 1 . TABLE 1

The t o p o l o g i c a l p a r a m e t e r s o f t h e z e o l i t e s

: Zeol it e

1

NZ1

NO1

N32

b 3 * 1 / ~ 1 * 1 N3/N0

1Afghanite

1 4 ; 4 1Bikitait.e 1 4 1Chabazite 1 4 1Deca dodecasil 1 4 1Dodecasil 1H 1 4 1 TMA-E( AB) 1 4 IEdlngtonIte 1 4 1 Ericnite 1 4 ; Faujasite 1 4 1Ferrierite 1 4 1Gismondine 1 4 1Gmenilite 1 4 1Heulandite 1 4 1 Levyne 1 4 1Liottite 1 4 1 Losod 1 4 1 L inde-A 1 4 1 L inde-N ; 4 1 L inde-W 1 4 1Mordenite 1 4 1Natrolite 1 4 1Phillipsite 1 4 1 RHO 1 4 1Sodalite 1 4 1Thomsonite 1 4 1 ZK-5 : 4 !Quartz 1 4 1Cristobalit.e 1 4 1 T r 1dymit e 1 4 !Random Voronoi 1 4 Li-A

Moreover,

3 3.5 4 3 3 3 3 4 3 3 3 3.5 3.4 5 3 3 3 3 3 3.28 3.72 4 3 3 3 3.5 3 9 6 5 3

5.08 5.6 6 5.2 5.14 5.14 5.2 6 5.2 5.25 5.4 5.6 5.66 6.67 5.14 5.2 5.25 5.22 5.19 5.41 5.63 5.82 5.33 5.33 5.14 5.6 5.2 6 6 6 5.23

10 8 16 14 14 15 8 15 16 20 10 12 6 14 16 15 15.33 14.8 11.33 8.25 7.33 18 18 14 10 15 3 4 5 15.5

3.0 3.0 3.5 3.0 3.0 3.25 3.0 3.25 3.5 4.5 3.0 3.5 3.0 3.0 3.5 3.25 3.3 3.2 3.0 2.68 2.67 4.0 4.0 3.0 3.0 3.25 1.75 2.0 2.25 3.39

.182 .25 .333 .143 .166 ,166 .154 .333 .154 .143 .111 .25 .2 .5 .166 .143 .154 .150 .156 .214 .32 .375 .125 .125 .166 .33 .154 2 1 .667 .148

132

P/1000 -~

2.89 4.75 6.73 2.09 2.92 3.07 2.37 5.53 2.40 1.82 1.96 3.85 2.92 8.5 2.52 2.17 2.43 1.93 2.37 3.42 5.50 6.67 1.95 1.78 2.86 6.56 2.26 53.0 23.2 15.11 ?

!

121

1 -1

~

2 1 1 2 3 2 3 2 3 3 2 1 3 2 2 3 2 3 5 3 4 2 1 2 1 2 4 1 1 1 ?

2 3 3 3 3 3 3 2 3 3 4 2 4 6 3 2 2 3 3 3 4 2 2 3 2 3 3 1 1 1

?

1 1 1 1 1 1 : 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 : 1 1 : 1 :

1

t h i s t a b l e 1 a l s o shows t h e t o p o l o g i c a l i n t r i n s i c r a t i o

between t h e numbers o f p o l y h e d r a end v e r t i c e s N 3 / N 0 ,

the

recipro-

526

cal topological u n i t curvature b3,l/v1,1,

t h e polyhedron

P / I O O o g i v e s t h e number o f polyhedron per 1000

of b o t h t h e p o l y h e d r a t y p e s In

order

132

and t h e

AS,

and f a c e s t y p e s

density, number

respectively.

121,

t o compare w i t h t h e r e l a t e d parameters

of

zeolites,

t h e t o p o l o g i c a l q u a n t i t i e s o f some S i O z m o d i f i c a t i o n s as w e l l as

a

random Voronoi c o n f i g u r a t i o n a r e a l s o shown i n t a b l e 1. Topological. c o n s t r a _ i n t s of_t h e ZeOl.ite c o n f i w r a t i o n s The

value

one c o n f i g u r a t i o n o b t a i n e d i n t a b l e 1.

with

four

This

respectively,

zeolite

the

for

vertex A10412-

o f the z e o l i t e configurations.

The t o p o l o g i c a l mean v a l u e parameters N O 1 , realize

four

typical and

i n c i d e n t edges r e p r e s e n t s t h e S i O 4 1 z -

tetrahedra, each

i s equal

o f t h e t o p o l o g i c a l parameter N O 1

each

c o n f i g u r a t i o n (and a l s o o f

N 1 2 , N Z 1 , and N 3 2 o f

each

the topological structure equation

SiOz-modification)

(eqn.

2),

z e o l i t e c o n f i g u r a t i o n s a r e t o p o l o c a l l y determined.

Each

i.e.,

the

topologi-

c a l parameter NR1 depends on each o t h e r . TABLE 2

P o s s i b l e face d i s t r i b u t i o n s F Z 1 1 o f a t y p i c a l p o l y h e d r o n w i t h N j 2 10 f o r t h e t o p o l o g i c a l parameters NO1 4, N1z = 3, and N21 = 4.8, r e s p e c t i v e l y _ _ 3 4 5 6 7 8 9 10 1 ;F21i\n21i 1

=

\

I

I

1

I -

1 1 ; 1 1 1 1

1 2 3 4 5 6 7

1

0

2

8

: 1

0

9

1 1 I

1 0 1 0 1 2

3 1 4 2

1

8

1

0

! f

9 10 11 12 13 14 15 16 17

5 3 1 6 4 2 0 3 1

6 7 4 5 6 2 3 4 0 1 2 3 0 1

0

0

1

: 1 1 1 1

:

I

I

I I

1

1

1

1 1 : 1

:

I

1 2 0 1 2 3 2 3 4

0

0 0

0 0 0 0

1 1 2 2 2

0 0

0

0 0

3 3

0

3 0

0

0

0

0 0 0

0

0

1

0

0

1 :

0 0 0

0 0 0

0

0

1 1 1 1 1 : 1

: I I

o f the topological constraints f o r c e l l u l a r

tion

spaces w i t h

only

be

affect

0

0 0

I

Because

0

0

0 0 0

0

1

0 0

0

0 0

1

1 1 : 1

0

0

1

0 0

0 0

0

0 0 0 0

0

0 0

0 0

0

4 4 4 4 5 5 6

0 0 0

0 0

0

0

0

NO1

=

i n t h e range 5 the

zeolite

f a c e s 9211 must e x i s t .

4,

B>A>D>E.

z (t) 1.0

0.5

0 0

5

10

15

20

25

t/h Fig. 3 . Crystallization curvcs for the ZSM-5 samples.

The hystograms o f the granulornetric distributions in the final products of each run can be constructed together with the monotonic curvcs approximating them. These monotonic

curves and the

curves of the maximum average diametcrs of crystals are used t o derive

thc

be1 1

(sce r c f . 16)

The

shaped

curves

representing

nucleation

kinetics

(Fig. 4 ) .

linear growth

rates of crystals, and

also the times at

which nucleation is maximum in each run, are reported in Table 2.

Thc detailed quantitative analysis will be included

in a future

publication (see ref. 1 3 ) .

TABLE 2 Linear growth rate of crystals and maximum nucleation time

A (AL/2At)/(pm/h) tmax.""cl. /h

1.35 3.6

B

2.25 3.15

C 0.89 1.9

D 0.89

5.3

E 0.63 8.8

578

40

IC

0

ik 30

20

10

I 5

0

15

10

t/h F i g . 4 . Nucleation

rate

as

a

function

of

time

for

the

ZSM-5

samplcs.

DISCUSSION Previous studies (see ref. 8) showed that the conversion rate and the yield of silicalitc-1 arc favored by the amount of TPA ions up t,o TPA/Si m o l a r ratio equal to 0.08. Hence, an excess of ‘ W A beyond that, v a l u e i s not influent, any more on these parameters. The amount of organic ion in our samples (TPA/Si=0.088) is t h e r e f o r e optimal and does not. c n n R t , i t r i t . e PL 1 i m i t . i n r f f n * o * n r

579 cithcr for the yield or for the crystallization rate. The crystallization C>B>A>D>E.

The

rates

in the five runs are

in the order

relevant data t h a t can be found

in

literature

(see refs. 13,17-19) for similar studies also confirm that there is a n optimum amount of aluminium to speed up the crystallization

kinetics of ZSM-5. The linear growth rates of crystals (Table 2 ) are in the ordcr B>A>C,D>E.

The granulometric distributions, and

hencc the kinetic nucleation curves, are narrow (Fig. 4 ) and show

A1,0, content.

a tendency to broaden in the systems with higher

The results shown in Figs. 3 and 4 demonstrate that the faster crystallization rate found for ZSM-5

in run C is not a consequen-

ce of a higher crystallite growth rate, but is rather the effect of a faster nucleation.

I t is clear from Fig.

4

that nucleation of ZSM-5 in o u r systems

starts at time zero, o r even before. It must be pointed out that maximum crystalline dimensions are an averaEe of the diameters of the largest crystals found in each sample. S o , e.g., in the sample of run B at 20.5 h it is possible to find one crystal as long as 44 pm

(lcngth to width 1) whereas the average diameter of the

15 largest crystals is 10 pm less.

This fact is a strong evidence

for the autocatalytic theory (see ref. 3) according to which nuclcation starts immediately in the gel already at room temperature. The analyses of the solid phases during the syntheses show that both the gcl and the crystals of zeolite enrich in A1 with time. This means that the crystallisation of zeolite ZSM-5

proceeds via

the formation of an A1 poor core, and an aluminous outer shell. An exception to this behaviour seems to be run E .

The number of TPA'

ions per unit cell of ZSM-5

is nearly cons-

t a n t in all of the phascs of the crystallyzation process, ranging

from 3 . 1 to 3.8,that is close to the theoretical value of 4 . Elemental analyses always show a lower content of organic ion with may with

respect to the thermogravimetric measurements. This effect be due to the annealing of lattice defects

R =

11,

Na',

TPA')

during

thermogravimetric

which produccs water molecules. Nmr studies on

*'Si

(groups Si-OR, measurements, in ZSM-5

(see

ref. 20) showed that calcination at 440'C for 24 hours eliminated most of the latticc defects.

CONCLUSIONS It is shown that the nucleation starts when the crystallization

580 temperature

is reached

or

evcn

earlier, the gel still being at

temperature. The crystal1 ization rates as expressed by crystal1 inity pcr unit time are essentially linked to the nucleation rates. The analyses carried out on the solid phases show that the crystallization of Na,TPA-ZSM-5 in the five series examined proceeds by the formation of aluminium poor cores. The concentration of this element is higher on the external parts of t h e crystallitcs. room

ACKNOWLEDGEMENTS We gratefully acknowledge D r . Nicola La Rosa for kindly supplying calculation facilities. This work h a s been supported by CNR (Italian National Rcsearch Council), Progetto Finalizzato Chimica Fine e Sccondaria. REFERENCES 1 R.J. Argauer and G.R. Landolt, U . S . Pat. 3,702,886. 2 J.C. Jansen, C.W.R. Engelen and 11. van Rekkum, ACS Symp. Ser., 398 (1989) 257.-68 3 4

5 6 7 8

B. Subotib, ACS Symp. Ser., 398 (1989) 110-21. R . A . Van Santen, J. Keijsper, G . Ooms and A.G.T.G. Kortbcek, Stud. Surf. Sci. Catal., 26 (1986) 169-75. D.C. Hayhusrt, R. Aiello, J. B.Nagy, F. Crea, G . Giordano, A . Nastro and J .C. Lee, ACS Symp. Ser., 368 (1988) 277-91. G . Boxhoorn, 0. Sudmeijer and P . 1 I . G . van Kasteren, J. Chem. SOC.,Chem. Commun., (1983) 1416-8. J. B.Nagy, P. Bodart, E.G. Derouane and Z. Gabclica, Stud. Surf. Sci. Catal., 26 (1986) 231-8. F . Crea, A . Nastro, J. B.Nagy and R . Aiello, Zeolites, 8

(1988) 262-8. R . Aiello, F. Crea, A. Nastro and C. Pellegrino, Zeolites, 7 (1987) 549-53. 10 A. Araya and B.M. Lowe, Zeolites, 6 (1986) 111-8. 11 B.M. Lowc, Stud. Surf. Sci. Catal., 37 (1987) 1-12. 12 G . Bellussi, G . Perego, A. Carati, U . Cornaro and V. Fattore, Stud. Surf. Sci. Catal., 37 (1987) 37-44. 13 G. Golemmc, A. Nastro, J. B.Nagy, B. Subotik, F. Crca, and R .

9

Aiello, in preparation. 14 P.T. Cardew and R . J . Davey, Proc. R . SOC. London A, 398 (1985) 415-28. 15 Z. Gabelica, J. B.Nagy, P. Bodart, N. Dewaele and A. Nastro, Zeolites, 7 (1987) 67-72. 16 S . P . Zhdanov and N.N. Samulcvich, in: L.V.C. Rees (Ed.), Proc. 5th Intl. Conf. on Zeolites, lleyden, London, 1980, p p . 75-84. 17 V.N. Romannikov, V.M. Mastikhin, S . HoEevar and B . Driaj, Zeolites, 3 (1983) 311-20. 1 8 S . B . Kulkarn i , V.P. Shi ralkar, A.N. Kotasthane, R.B. Borade and P . Ratnasamy, Zeolites, 2 (1982) 313-8. 19 K.-J. Chao, T.C. Tasi and M.-S. Chen, J. Chem. SOC., Faraday Trans. I , 77 (1981) 547-55. 20 P. Bodart, J . B.Na y , 2. Gabalica and E;.G. Derouane, J. Chim. Phys., 83 (11-12) fi986) 777-90.


NEW MOLECULAR SIEVE

J . Kornatowskil,

581

- VANADIUM SILICALITE KVS - 5

M. Sychev2. V. GoncharukZ, W.H.

Baur3

1Institute of Chemistry, N. Copernicus University, Gagarina 7, 87-100 Toruh, Poland 2Institute of Colloid and Water Chemistry, Ukrainian Academy of Sciences, Vernadsky Ave. 42, 252680 Kiev, USSR 31nstitut fur Kristallographie und Mineralogie der J.W. Goethe-Universitat, Senckenberganlage 30. D-6000 Frankfurt am Main 1, PRG

SUMMARY Vanadium silicalite KVS-5 has been grown under conditions of hydrothermal synthesis. Its structure corresponds to the MFI type. The product can be synthesized with a yield of 100% in form of slightly coloured large crystals up to 300 m. The incorporation of V5+ ions on T-sites in the zeolitic framework has been investigated by a variety of methods: XRD, MAS M,SSIMS, ESR, TGIDTA. IR. and XRF.

INTRODUCTION The catalytic properties of vanadium compounds and especially of its oxides have been investigated and used since the 1960's. These studies of vanadium catalysts were dealing mainly with the changes of oxidation state of vanadium during the catalytic process as well as with the role of support. Commonly. silica or alumina [l-31 have been used as supports for the vanadium compounds. No catalysts have been prepared with an exactly defined state of this metal except for the synthesis of heteropolyacids [4]. An introduction of vanadium into the ZSM-5 zeolite crystals has been already reported: Kucherov et al. 15-81 and later Sass et al. [9] introduced V4+ ions into extra-framework cationic positions by a solid state reaction. Inui e l al.

[lo] introduced vanadium compounds instead of the A1 component already into the reaction gel for synthesis of the zeolitic catalyst. The possibility of isomorphous substitution of a large number of elements with vanadium in three different oxidation states 111, IV, and V has been published by Xu Ruren et al.[ll]. Nevertheless, they have given neither synthesis conditions nor any proof or method for checking the actual incorporation of those elements into the crystal structure except that they report lattice constants. The differences in these lattice constants compared to ZSM-5 seem to be much too high considering the amounts of heteroatoms which could be possibly incorporated into the framework.

582

The aim of this work was to prepare a crystalline porous zeolite with isomorphously substituted Vt5 ions in the framework as a precursor of a catalyst for oxidation reactions and to check the product with a variety of methods which could show real substitution. EXPERIMENTAL The zeolite was synthesized in teflon-lined autoclaves by reacting silica s o l ,

alkali metal vanadate, sodium bicarbonate, tetrapropylammonium hydroxide (TPAOB) and water at 160-190.C

and autogeneous pressure for 5-9 days followiiig

the procedures given in refs. [12.13]. Reference samples were prepared by impregnation of silicalite with VOSO4 and by synthesis of VA1-ZSM-5 using analogous methods to KVS-5. The impregnation procedure was the following: siljcalite

was stirred for 24 hs at RT and pH

=

3 in an 0,1 M water solution of VOSO4 ( 2 0

ml/lg of zeolite), then washed with water, dried at 80'C calcined at 500.C 500'C

for 3 hs and finally

for 1 h in air. Calcination of KVS-5 samples was made at

for 48 hs in a slow air stream and 24 hs i n an 02 stream.

The products were characterized by XRD. SEM. MAS NMR, SSIMS, ESR, TG/DTA, and IR techniques. Apparatus used and condi.tionsof measurements: XRD: DRON 3 with PC, CuKa radiation, 0,5'/min SEM: NOVOSCAN 30. samples covered with Au

MAS NMR: BRUKER CXP 200 equipped with 4.8 T magnet ESR: TSN-254 and SE/X 2544 Radiopan

TG/DTA: MOM Q-1500, heating rate 1O0C/min, sample 400 mg IR: UR 20 SSIMS: LAS-3000 RIBER. range 0-200 amu. time 30 min. RESULTS AND DISCUSSION The experiments were successful in obtaining vanadium silicalite with a 100% yield and in form of slightly violet large crystals up to 300 vm in length. Fig. 1 shows the material of euhedral habitus of MFI topology from a sample with -200

long crystals. Crystal sizes distribution was usually within the

range of 210% and their aspect ratio length to width was about 2 - 2 . 5 . shows the crystals grown together with Al3'

F.ig.

2

ions. They were obtained only up to

-90 pm in length and with distinctly wider size distribution. The XRD pattern

was typical for an MFI topology for both cases. The KVS-5 crystals had lattice constants 20.087, 19.943, and 13.420 I for the a, b, and c axes, respectively. They were changing from one sample to another but it was impossible to get a relation between the lattice constants and content of vanadium as the differences were usually smaller than experimental error. A decrease of lattice

583

la

lb

Fig. 1.

Scanning electron micrographs of molecular sieve KVS-5.

584

2a

2b Fig. 2. Scanning electron micrographs of vanadoaluminosilicalite molecular sieve VAL-ZSM-5.

585

constants larger than the experimental error occurred after calcination which suggests a relaxation of the framework after removal of the template molecules. Thermogravimetry analysis (Fig. 3 ) shows that the calcination process proceeds with three thermal effects: endo- and exo-thermal effects at about 350

-

360'C overlapping one another and coinciding with a weight loss and the third

T PC

DTG 800 -.----L

-'\~

\

-

600

LOO

200

Fig. 3 .

Thermogravimetry analysis curves for KVS-5.

strong exothermal effect at -520.C not followed by a weight loss. The endo-effect. is connected with the template decomposition. Then the decomposition products leave the sample (weight loss) causing a relaxation of the framework (exo-effect and decrease of lattice constants). The exo-effect at -520.C can be due to combustion of the organic decomposition products outside the sample. The last effect is not observed when the calcination proceeds in a stream of gas (the decomposition products are taken away from the system). The possibility of a two step mechanism of the template removal was already reported [14]. All these effects are probably well resolved due to both the large dimensjons of the crystals and the high amount of sample hindering diffusion processes. The static SINS data (Fig. 4) show that KVS-5 samples are free from A1 and F e impurities (confirmed in ESR spectra) and the vanadium content (lines 51 and

67 for V(+) and VO(+), respectively, and no 181 line for V*O5(+)

[15]) can

586 72 LL

I

4. Static secondary ions mass spectroscopy data for vanadium s i l i c a l i t e Fig.

KVS-5.

Fig. 5.

Infrared spectra of vanadium s i l i c a l i t e KVS-5 before ( a ) and a f t e r (b) calcination.

l

1500

l

.

l

l

l

1000

.

l

l

l

l

500

l

cm-'

587

reach about 1%. This is i n agreement with t h e X-ray fluorescence a n a l y s i s res u l t s . For unknown reasons V atom cannot be d e t e c t e d by e l e c t r o n probe microa n a l y s i s of t h e samples. The I R s p e c t r a of KVS-5 (Fig. 5 ) show t h e weak band at -950 cm-l.

I t w a s ob-

served f o r t h e f i r s t time f o r T i s i l i c a l i t e [16] and its i n t e n s i t y was l a t e r r e l a t e d t o t h e T i content [17]. The band of o u r KVS-5 material i n c r e a s e s its i n t e n s i t y a f t e r c a l c i n a t i o n (Fig. 5b) which might suggest a n improvement of t h e ordering of t h e vanadium atoms i n t h e c r y s t a l .

3PPH

I

H

__c

200 G

I-----+

DPPt

F i g . 6. ESR s p e c t r a of KVS-5 samples with complete ( a ) and incomplete ( b ) inc o r p o r a t i o n of V i n t o t h e t e c t o s i l i c a t e framework. S p e c t r a l parameters: g~ = 1,961; ggg = 1.939; geff = 1.954; b = 8.78 mT; A,, = 19.25 mT; t h e broad l i n e i n spectrum ( b ) geff = 1,965, AH e 50 mT.

The ESR spectrum of t h e as-prepared KVS-5 sample (Fig. 6 ) shows t h a t vanadium occurs as V4+. The w e l l resolved hyperfine s t r u c t u r e shows a very high

d i s p e r s i o n of t h e V4+ ions. As a pure s i l i c a l i t e s t r u c t u r e would have no pref e r r e d p o s i t i o n s , a reason f o r such d i s p e r s i o n could be i n c o r p o r a t i o n of t h e

588 V atoms i n t o t h e framework. The p a r a m e t e r s of t h e s p e c t r u m are similar t o t h o s e

f o r V s p e c i e s i n n e a r l y s q u a r e p l a n a r environment [ 6 ] though t h e d i f f e r e n c e s

are large enough t o allow a l s o o t h e r d i s t o r t i o n s of t h e f o u r f o l d c o o r d i n a t i o n [ 9 ] . The v e r y broad l i n e i n F i g . 6b serves as a background f o r t h e h y p e r f i n e

s t r u c t u r e and c a n be a s s i g n e d t o c l u s t e r e d extra-framework V4+, as i t is i n t h e

s i l i c a l i t e impregnated w i t h VOSO4 ( F i g . 7 a ) .

Fig. 7. ESR s p e c t r a of a ) sample p r e p a r e d by impregnat ion of sit i c a l i t e w i t h VOSO4, b ) VAL-ZSM-5 a f t e r calcination.

I

I

3,lO

I

I

I

330

35 0

I

37 0 H [kGl

F i g . 8. 51V s o l i d s t a t e MAS NMR spectrum of vanadium s i l i c a l i t e KVS-5 a f t e r c a l c i n a t i o n (6 = 0 f o r VOC13. f r e q u e n c y 52,6 MHz, s p e c t r a l w i d t h 720 kHz. r o t a t i o n 3 , l kHz, D 1 = 0 , 5 vsec, D 0 = 0 , 3 s e c , p u l s a n g l e 22,5', 70.000 t r a n s i e n t s )

F i g . 9. 51V s o l i d s t a t e MAS NMR spectrum of s i l i c a l i t e impregnated w i t h VOSO4 ( c o n d i t i o n s s e e Fig. 8 , 100,000 transients).

589

After calcination, the vanadium ions represented by the hyperfine ESR spectrum are completely oxidized to V5+ and the ESR signal vanishes. The broad lines in Figs. 6b and 7a remain unchanged after calcination showing that the clustered V4+ is not oxidized. In the ESR spectrum of calcined VAl-ZSM-5. the hyperfine structure is not vanishing completely (Fig. 7b) which might suggest a hindering role of A1 in the oxidation of V4+. The 51V MAS NMR spectrum of calcined KVS-5 shows the band at -557 ppm (Fig. 8 ) corresponding to vanadium complexes with a tetrahedral oxygen environment [ I , 181 which might indicate again the incorporation of vanadium into the framework. Fig. 9 presents the spectrum for silicalite impregnated with VOSO4 (measured immediately after calcination). As V4+ should not give any NMR spectrum, the extremely broad band (Av

0

110 kHz) at about -620 ppm can be

identified as the band due to a small amount of V5+ ions broadened because of the influence of V4+ present in excess. A similar band at about -530 ppm with a small shoulder at --I000 ppm is observed for as-prepared KVS-5 except for a narrower width -17,5 kHz probably due to a much lower content of vanadium.

As it appears from 27Al MAS NMR spectrum of as-prepared VAL-ZSM-5 (Fig.10). 5

-

20% of the Al3+ ions are in octahedral coordination ( s e e the band at -0

pprn). This band vanishes completely after calcination and only the band for

tetrahedral A 1 at +50 ppm remains, thus proving the tendency of A1 for entering tetrahedral coordination during heat treatment [ 1 9 ] .

Fig. 10. 27Al MAS NMR spectrum of VA1-ZSM-5 before calcination ( 6 = 0 for A ~ ( H * O ) ~ ~in + aqueous solution, frequency 52.2 Mlz, spectral width 42 kHz, rotation 3 , 1 kHz, D 1 = 10 vsec, D 0 = 0,3 sec, 5.000 transients).

80

60

LO

20

0 ' -20

-LO

6

pprn

590

CONCLUSIONS The results show the possibility of preparation of vanadium silicalite KVS-5 containing approximately 1% vanadium. The vanadium atoms in the as-prepared material exist mainly as V4+ ions and after calcination they are oxidized to V5+. In both cases, they are tetrahedrally coordinated and they occur in a highly dispersed state. Depending on synthesis conditions, a part of the vanadium can occur as clustered extra-framework V4'

species which are difficult to

oxidize. After calcination, the vanadium atoms in KVS-5 appear

to

be in a more

uniform state than before. The work was partially supported by the Polish Ministry of National Education within the Project CPBP 01.06. REFERENCES 1 V.M. Mastikhin, K.l. Zamaraev. Z. Phys. Chem. Neue Folge, 152 (1987). 317. 2 B. Taouk, M. Guelton, J . Grimbolt. J.P. Bonnelle, J . Phys. Chem., 92 (1988). 6700. 3 K.J. Zhen, M.M. Khan, C.H. Mak, K.B. Lewis, G.A. Somorjai, J . Catal., 96 (1985). 501. 4 M.A. Fox et al., J . Am. Chem. SOC. 109 (19831, 6347. 5 A.V. Kucherov. A.A. Slinkin, Zeolites, (1986). 175. 6 A.V. Kucherov, A.A. Slinkin, Zeolites, I_ (1987), 38. 7 A.V. Kucherov. A.A. Slinkin, Zeolites, L (1987). 43. 8 A.V. Kucherov. A.A. Slinkin, Zeolites, 7 (1987). 583. 9 C . E . Sass, Xinhua Chen, L. Kevan, J . Chem. SOC., 86 (1990). 189. 10 T. Inui, A. Miyamoto. H. Matsuda, H. Nagata, Y. Makino, K. Fukuda. F. Okazumi, Proc. 7th Int. Zeolite Corif.. Tokyo, 1986, Y. Murakami et al. (Eds.). Kodansha, Tokyo, 1986. 859 and refs. 12-14 therein. 1 1 X u Ruren. Pang Wenqin, Proc. Tnt. Symp. Zeolites, Portoroz, 1985, B. Drzaj et al. (Eds.), Elsevier, Amsterdam, 1985, Stud. Surf. Sci. Catal.. 24 (1985). 27. 1 2 J . Kornatowski, M. Rozwadowski, Pol. Pat. Appl., P 281513, (1989). 13 J . Kornatowski. Zeolites, 8_ (1988), 77. 14 G . Debras, A. Gourgue, J.B. Nagy, G. De Clippeleir, Zeolites. 2 (1985), 377. 15 C. A. Altomare, G.S. Koermer, E. Martins, P.F. Schubert, S.L. Suib, W.S. Willis, Appl. Catal., 45 (1988). 291. 16 G . Perego, S. Bellussi, C. Corno, M. Taramasso, F. Buonorno, A. Esposito, Proc. 7th Int. Zeolite Conf., Tokyo, 1986, Y. Murakami et al. (Eds.), Kodansha, Tokyo, 1986, 859. 17 B. Kraushaar. J.H.C. van HooIf, Catal. Lett., L(1988). 81. 18 L.R. Le Coustumer. B. Taouk, M. Le Meur, E. Payen, M. Guelton. J. Grimblot, J. Phys. Chem.. 92 (1988), 1230. 19 J. Kornatowski. M. Rozwadowski, W. Schmitz, A. Cichowlas, Proc. 8th Int. Zeolite Conf., Amsterdam, 1989. Recent Research Reports Val., J.C. Jansen et al. (Eds.). L.. Moscou, Akzo Chemicals, Amsterdam, 1989, 79.

G. Ohlmann et 01. (Editors),Catalysis and Adsorption by Zeolites 01991 Elsevier Science Publishers B.V., Amsterdam

591

MULTINUCLEAR NMR STUDY OF THE CRYSTALLIZATION OF SAPO-37

N. Dumontl, T. lto2, J. B.Nagy1, Z. Gabelical and E.G. Derouanel Facultks Universitaires N.-D. de la Paix, Laboratoire de Catalyse, 61, rue de Bruxelles, B-5000-Namur (Belgium) Tamai Sangyo CO., LTD, Zenibako 3-chome, 524-11, Otaru 047-02 (Japan)

ABSTRACT A series of intermediate phases, isolated during the synthesis of SAPO-37, have been characterized by XRD, SEM, NMR of adsorbed Xe, and solid state 27Al- and 31P-NMR, in order to evidence the successive steps occurring during the crystallization process. SAPO-37 stems from a direct gel restructuration: large cavities form in the amorphous phase during the aging period at ambient temperature. Upon heating at 200°C, aluminum and phosphorus respectively incorporate in configurations Al(4P) and P(4AI) in the framework, at the expense of the amorphous phase, giving rise to the formation of well-defined SAPO-37 crystals, showing an octahedral morphology, and growing with time. The crystallinity reaches a maximum after 32h. For excessive synthesis times (149h), the appearing of structural defects observed by NMR illustrates the partial degradation of the SAPO-37 framework, whereas a side-phase, SAPO-40, possibly involving interconnected channels limited by 12T puckered rings, co-crystallizes from the mother liquid phase. INTRODUCTION The synthesis of SAPO-37, a silicoaluminophosphate involving a Faujasite topology [ref. 11 was first reported by Lok et al. [ref. 21. Because of its open pore structure, this material possesses potential interesting and attractive applications in catalysis or adsorption, this explaining the increasing attention that is now being devoted to the understanding of its synthesis and to its better characterization. We have recently examined the influence of various synthesis variables (aging time and temperature, agitation and crystallization time) on the nature and final properties of the various SAPO-type open structured phases obtained by two different methods currently yielding SAPO-37 [ref. 31. Our study allowed us to define the optimal conditions under which pure SAPO-37 could be obtained in high yield by using both the conventional "aqueous" synthesis [ref. 21 or

592

the new biphasic (water-hexanol) route [ref. 41. The short range structural configuration and coordination of Si, Al and P and their distribution in the SAPO-37 framework, as well as the mechanisms governing the potential framework substitution of Al or P by Si in the Faujasite structure, have been thoroughly investigated by high resolution solid state NMR (29Si, 27AI, and 31P) [refs. 3, 5-71. The same techniques [ref. 81, complemented by 129Xe-NMR [ref. 91 were used to follow the structural changes occurring when water and organic template molecules progressively escape from the SAPO-37 intracrystalline volume upon various thermal treatments. Finally, we have recently taken advantage of the multiple potentialities of the 129Xe-NMR method, linked to the great sensitivity of the xenon nucleus to its close environment [refs. 10, 111 to characterize a series of intermediate phases isolated during the hydrothermal synthesis of SAPO37 [ref. 121. In the present work, we confirm and complete these preliminary 129Xe-NMR data by investigating more in depth the successive stages of the crystallization (gel restructuration, crystal growth, formation of a side-phase) and by characterizing all the solid intermediates using complementary 27Al- and 31 P-NMR, in combination with XRD and SEM techniques.

EXPERIMENTAL The optimized procedure followed to synthesize the intermediate and final SAPO-37 phases was described in details in our previous papers [refs. 3, 121. Several aliquots of a hydrogel of molar composition 1.0 A1203 : 0.9 P2O5 : 0.4 Si02 : 0.86 (TPA)20 : 0.023 (TMA)20 : 50 H20, previously aged at 20°C for 48h, were heated at 200°C under stirring conditions for various periods of time. For the present study, we have selected the more representative samples among the intermediates isolated during the crystallization course. These specimens were obtained after 0, 5, 10, 32 and 149h of heating, and are referred to as Pn, where n denotes the crystallization time (Tablel). The as-synthesized materials were checked for nature and purity by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). The percentage of SAPO-37 in each phase was estimated by comparing the total surface area of the characteristic XRD peaks measured for this sample to the area of the same peaks recorded for the most crystalline SAPO-37 of the series, arbitrarily considered 100% crystalline (P32) (Tablel). The xenon adsorption isotherms were measured at 34% using a classical volumetric apparatus, following operating conditions described

593

TABLE 1 Nature, relative xenon adsorption capacities and relative crystallinities of the intermediate phases isolated during the crystallization course of ;APO-3; Sample a Pn

e f

9 h

129Xe-NMR (mean diameter)

Relative Xe adsorption capacities b

% of crystallinity C XRD 129Xe-NMR d

p10

p25

SAPO-37

p32

SAPO-37

octahedra (15.1 pm)

SAPO-37

100

100

100

SAPO-37

octahedra

SAPO-37

135 g

h

55

p149

d

SBA

amorphous amorphous SAPO-37

PO p5

C

Nature

m

+

amorphous cavities amorphous cavities octahedra SAPO-37 (7.7 w ) f octahedra (14.8 l m )

+

+

81

0 0 78

e e 83

f

95

f

18 18

SAPO-40 platelets SAPO-40 n denotes the crystallization time (h). Relative xenon adsorption capacities calculated from the isotherms with respect to the most crystalline SAPO-37 phase (P32) arbitrarily considered to have a 100% adsorption capacity. Weight percentage of SAPO-37 in each intermediate phase determined with respect to the most crystalline SAPO-37 (P32) considered to be 100% crystalline. Determined from the ratio between the slope of the straight line &=f(nXe/g) for each crystalline intermediate phase and the slope obtained for P32 [ref. 121 The crystallinities of Po and P5 could not be determined by 129Xe-NMR because these two phases do not contain well-defined SAPO-37 crystals. not investigated. Xe isotherms only allow to calculate the global Xe adsorption capacity of the mixture (SAPO-37 + SAPO-40). It is not possible to estimate the XRD crystallinity of SAPO-37 in the mixture Pi49 , as most of the XRD peaks of SAPO-37 and SAPO-40 overlap.

elsewhere [ref. 121. The 129Xe-NMR spectra of xenon adsorbed at different pressures on all the intermediate phases were recorded at t h e same temperature, with a Bruker CXP-200 spectrometer operating at 55.3 MHz. The 27Al- (105.3 MHz) and 31P- (162.0 MHz) high resolution solid state MAS N M R measurements were performed o n a Bruker MSL-400 spectrometer. The pulse length and recycle time were lps and 200ms for

594

27Al and 3ps ( d 2 ) and 30s for 31P, respectively. Typical measuring conditions for the 31P- CP-MAS NMR experiments were a 1H pulse length of 2.5ps, a contact time of l m s and a recycle time of 5s.

RESULTS AND DISCUSSION i ) XRD, SEM and 129Xe-NMR The structural and morphological characteristics of the SAPO-37 intermediate phases, determined by X-ray diffraction and scanning electron microscopy, are summarized in Table 1. These data are compared to those obtained previously by 129Xe-NMR [ref. 121. The relative xenon adsorption capacities of the calcined samples are compared to their relative crystallinities estimated by XRD and IzgXe-NMR, respectively. For Po and P5, we observe a broad 129Xe-NMR line, distinguishable from the one belonging to the gaseous xenon, and characterized by a quasi constant chemical shift value ( about 45 ppm), whatever the xenon pressure. This signal arises from the interactions of the xenon atoms with the walls of large cavities of about 25 A. We therefore conclude that, at these early stages of the crystallization, the gel already contains a preliminary void structure, responsible of the non-negligible adsorption capacity of these materials (about 20% of the maximum observed in the case of 100°/~ crystalline SAPO-37, P32). The degree of connection between these cages is small, but increases as the crystallization proceeds. These cavities do not show any regular ordering, which explains why no crystalline phase is detected in PO and P5, by XRD and SEM. P l o is found to contain already about 80% crystalline SAPO-37. The crystals show the typical octahedral morphology characterizing Faujasitetype materials (Fig. 1) [ref. 31. The crystal size increases with the crystallization time (Table 1). X-ray diffractograms indicate that P32 is a 100% crystalline SAPO-37, whereas, for longer synthesis times, a crystalline side-phase, SAPO-40, consisting of platelet-like crystallites, is found admixed with SAPO-37 (P149) (Fig. 1). In contrast to SAPO-37 that stems from a direct gel restructuration, SAPO-40 nucleates in the liquid-phase , probably because of the marked change in its composition observed as soon as 100% crystalline SAPO-37 is formed [ref. 121. The relative xenon adsorption capacities of P10 and P32 are in very good agreement with their relative crystallinities measured by XRD or calculated from the slopes of the straight lines 6 1 2 9 ~=f(nXe/g) ~ [ref. 121. We took advantage of this latter method to determine the weight percentage of SAPO-37 admixed with SAPO-40 in P14g (Table 1) [ref. 121, which is rather difficult to estimate by XRD, as most of their

595

Fig. 1. SEM micrographs of the various intermediate phases. (a) Po, (b) P5, (c) Pio,

(4 P25, (el P32, (f) PIN. diffractogram peaks overlap [ref. 21. Distinct 129Xe-NMR signals characterize these two SAP0 materials. The chemical shift of the high field resonance extrapolates to 53 ppm at zero xenon concentration for samples Plo, P32 and PI49 and confirms the presence of SAPO-37. For the low field signal, the corresponding parameter 1 3 ~is larger (95 ppm), which suggests that SAPO-40 has a pore structure narrower than SAPO-37, the cavities of which are connected by 12 T windows. n-hexane adsorption measurements confirm this assumption. Indeed, the porous volume of SAPO-40 was found to be about 60% of the void volume defined by the supercages of SAPO-37. We recently proposed a tridimensional structure for SAPO-40, consisting of interconnected straight channels limited by puckered 12 T rings [ref. 121. These structural properties, in line with the typical tetragonal crystal morphology, justify the behavior of this material upon adsorption. Indeed, whereas SAPO-40 shows a n-hexane adsorption capacity quasi identical to that of ZSM-5, xenon atoms diffuse

596

much more rapidly in the S A P 0 than through the ZSM-5 structure. Moreover, marked confinement effects, due to the puckered channels, give rise to a particularly high xenon sorption capacity explaining why, in spite of its narrower pore system, SAPO-40 adsorbs more xenon than SAPO-37 (Table 1) [ref. 121. For too long heating times, the SAPO-37 framework starts collapsing, as indicated by the broadening of the high field signal for the phase P14g.

and 3 1 P - N M R The progressive structuration of the gel into crystalline SAPO-37 is illustrated by the 27AI-NMR spectra presented in Fig. 2. As the octahedral aluminum resonates at higher fields (see below), we tentatively attribute the single line observed for samples Po and P5 at about +6 ppm (line P) to penta-coordinated aluminum in the amorphous phase. No signal is detected at lower fields, which confirms the absence of any crystalline material at these stages of the crystallization. The pure and well-crystalline SAPO-37 phases P l o , P25 and P32 exhibit an additional 27AI-NMR line at about +37 ppm (line T) due to tetrahedral aluminum in the configuration Al(4P) [refs. 5-61. From P i 0 to P32, the intensity of line P decreases, and that of line T increases, as the crystallization proceeds, i.8. as the amount of SAPO-37 increases. This reflects the progressive incorporation of aluminum into the tetrahedral positions of the SAPO-37 framework, at the expense of the amorphous phase. However, line P does not disappear completely, even for P32 which does not contain any amorphous phase detectable by XRD or SEM. We therefore conclude that an additional contribution to signal P, probably due to some (otherwise tetrahedral) aluminum atoms distorted by secondary coordinations with occluded molecules, adds on the contribution of the penta-coordinated aluminum belonging to the amorphous phase. Indeed, as shown in Fig. 3, this resonance is sensitive to the effects of dehydration and calcination. Upon heating to various temperatures (1 20, 230°C), the intensity ratio I(lineT)/l(line p) for sample P25 progressively increases. Moreover, line P tends to disappear when the sample is calcined under dry air up to 550°C. Saldarriaga et al. [ref. 51 and Blackwell et al. [ref. 61 have also suggested that such interactions with intracrystalline water molecules, OH- groups or organic templates could modify the geometry around some framework aluminum atoms and give rise to this additional signal appearing at a chemical shift value that characterizes aluminum in a coordination larger than 4. However, we observe that this "penta"coordination is not restored upon rehydration, which supports the

ii)

27AI-

597

T

100

0

-100

PPM

Fig. 3. 27AI-NMR spectra of phase p25.

100

0 PPM

-100

Fig. 2. 27AI-NMR spectra of the assynthesized intermediate phases. (a) Po, (b) P5, (c) Pie, ( 4 P25, (el p32, (f) p149.

(a) as-synthesized, (b) heated in a N p flow to 230°C, (c) heated in a N2 flow to 550°C then calcined in air at 550°C, ( d ) sample c left overnight in moist atmosphere,(e) sample c dispersed in water for 15 min then dried at 100°C, (f) sample e recalcined to 550°C.

598

hypothesis of a distortion of the Al tetrahedra by organic compounds, such as TMA+ or TPA+. Indeed, when calcined SAPO-37 is exposed overnight to atmospheric moisture or dispersed in water for 15 minutes and then dried at 100°C, another signal appears at about -11 ppm. It is attributed to octahedral aluminum stemming from a reversible coordination of some framework aluminum atoms with water molecules (Fig. 3). The hydration state of SAPO-37 also affects the chemical shift of line T: the higher the degree of hydration, the larger the chemical shift (calcined P25: +29 ppm, as-synthesized P25: +37 ppm, hydrated P25: +42 ppm). According to an NMR study reported by Muller et al. on a series of AIP04 phases [ref. 131, the 27AI chemical shift depends linearly on the AI-0-P bond angle. In the case of SAPO-37, such angular changes are probably induced by the presence of water molecules. The instability of SAPO-37, in its H-form, in the presence of water was well-recognized [ref. 81. We have also previously evidenced by 129XeNMR the formation of larger cavities and a partial pore blockage in SAPO37 leading to a dramatic decrease of the adsorption capacity [ref. 91. This loss of crystallinity, due to the partial structure collapse, is confirmed by XRD: the samples hydrated by contact with atmospheric moisture or by dispersion in water, become totally amorphous to X-rays. By contrast, the SEM micrographs reveal that the octahedral crystals still keep intact their outer shell, which suggests that moisture induces a true perimorphic transformation. However, as indicated by the persistency of line T, the tetrahedral geometry of some framework aluminum atoms is retained. The two main signals characterizing the crystalline SAPO-37 intermediate phases are also found in P14g (mixture of SAPO-37 and SAPO-40) (Fig. 2) but the ratio of their respective intensities is completely reversed: line P appears only as a shoulder. This inversion could be interpreted in terms of a decrease of the relative number of penta-coordinated aluminum atoms in the SAPO-37 framework but, if we balance the absolute intensity of this resonance by the percentage of SAPO-37 in the mixture, we find that this decrease is only apparent. The main cause of this ratio inversion is the large increase of the intensity of signal T. It is explained by the marked incorporation of aluminum into the tetrahedral positions of the SAPO-40 network, from the mother liquid phase. The 27AI-NMR spectrum of sample PI49 shows a broad shoulder at around -15 ppm, a ppm range that usually characterizes octahedrally coordinated aluminum. This shoulder noteworthly decreases when sample PI49 is calcined at 550°C, and nearly completely disappears after another

599

T

100

-100

0 PPM

Fig. 4. 27AI-NMR spectra of phase p i 49.

(a) as-synthesized, (b) heated in a N p flow to 550°C then calcined in air at 550°C, (c) sample b left overnight in moist atmosphere then recalcined to 550°C.

I . . . . I . . . . I . . . . I

0 I

-25

'

I

-35

I

PPM Fig. 6. Simulation of the 31P-NMR spectrum of as-synthesized phase p25.

-50

..

PPM

Fig. 5. 31P-NMR spectra of the assynthesized intermediate phases. (a) Po, (b) P5, ( 4 Pie, (4 P25, ( 4 p32, ( f ) p149.

600

rehydration-calcination cycle (Fig. 4). We therefore assign this line to structural defects (terminal AI(0H)” groups, ...) created during a long hydrothermal crystallization (149h) at a rather high temperature (200°C). Such defects easily recombine upon successive calcinations at 550°C. These findings go in line with the information obtained from 129Xe-NMR investigations of phase Pi49 in which SAPO-37 was found to have undergone a partial structural degradation. Finally, a weak line located at +19 ppm (P’) can be tentatively assigned to aluminum atoms in the SAPO-40 framework involving a secondary coordination with occluded organic molecules. As for SAPO-37, this line logically disappears upon calcination of PI49 at 550°C, a temperature at which all the organic molecules are released. Decoupled 31P-NMR spectra of non crystalline samples Po and P5 show a single but broad line located at about -20 ppm (Fig. 5). It obviously characterizes phosphorus atoms belonging to the amorphous phase. Its important width (about 2900 Hz) accounts for different types of phosphorus atoms located in various environments. They possibly could be terminal P(OH), groups. For sample P10, in which about 20% of amorphous phase is still present, this line still remains visible next to the resonance due to framework tetra-coordinated phosphorus (Fig. 5 ) . It becomes hardly detectable (and only by using cross polarization) for P25 and completely disappears in the case of the 100% crystalline P32. This progressive decrease in intensity confirms the attribution of this resonance to the presence of amorphous phase and quite well illustrates the progressive disappearing of this latter, as the crystallization proceeds. The main resonance at -31 ppm, due to SAPO-37 framework P(4AI) configurations [refs. 5-61 shows an asymmetrical character. An accurate decomposition of the spectrum (Fig. 6) suggests the presence of an additional small line at about -33 ppm. Its presence is not a simulation artifact, as evidenced by a series of calcination or ethanol washing experiments. Indeed, after such treatments, this shoulder completely disappears and the main line becomes symmetrical and narrow. We therefore assign the small shoulder at -33 ppm to slight deformations of P-O-AI angles produced by weak interactions of the framework atoms with the neighbouring organic molecules. The main line is broader by 100 Hz for PI49 with respect to that present in P32 (pure SAPO-37). Obviously such a broadening can be explained by the superposition of two very similar resonances of P(4AI) configurations belonging to both SAPO-37 and SAPO40 ordered structures. Blackwell et al. [ref. 61 have indeed observed small differences in position of the 3 1 P-NMR lines characterizing identical

601

P(4AI) configurations in various SAP0 frameworks. Finally, 31P-NMR confirms a partial collapse of the SAPO-37 lattice upon long heating times (149 h). Indeed, the presence of P-OH defects that appear upon partial degradation of the framework can be evidenced by a weak NMR line at -20 ppm, markedly enhanced under cross polarization. Internal P-OH terminal groups are indeed characterized by a small resonance in the -20 ppm range for a series of AIP04-n and SAPO-n partially degradated structures [ref. 141. CONCLUSION 129Xe-, 27Al- and 31 P-NMR techniques, combined with XRD and SEM, have been proved a useful tool to investigate the successive steps occurring during the crystallization process of SAPO-37. 129Xe-NM R was of particular interest to detect the presence, at the early stages of the crystallization, of preliminary void structures in the gel, which are not observable by XRD or SEM. It allowed to follow to progressive increase of the crystallinity of SAPO-37 and to evidence the partial degradation of its framework, as well as the formation of the side-phase SAPO-40, upon too long hydrothermal crystallization. The progressive incorporation of phosphorus and aluminum into tetrahedral positions in the SAPO-37 network, at the expense of the phosphorus and penta-coordinated aluminum belonging to the amorphous phase, was well illustrated by the 31P- and 27AI-NMR spectra of the successive intermediate phases. 27AI- and 31 P NMR also allowed to evidence the formation of structural P(OH), and AI(0H)n defects in SAPO-37, and the appearing of SAPO-40 after a long period of heating at 200°C, as well as the interactions of some framework Al and P atoms with occluded organic molecules. REFERENCES 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) 6092-6093. B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R. Cannan and E.M. Flanigen, U.S. Patent 4, 440, 871 (1984). L. Maistriau, N. Dumont, J. B.Nagy, 2. Gabelica and E.G. Derouane, Zeolites, 10 (1990) 243-250. E.G. Derouane and R. von Ballmoos, Eur. Patent 185, 525 (1986). L.S. Saldarriaga, C. Saldarriaga and M.E. Davis, J. Am. Cham. SOC, 109 (1987) 2686-2691. C.S. Blackwell and R.L. Patton, J. Phys. Chem., 92 (1988) 3965-3970. J.A. Martens, C. Janssens, P.J. Grobet, H.K. Beyer and P.A. Jacobs, Stud. Surf. Sci. Catal., 49A (1989) 215-225. M. Goepper, F. Guth, L. Delmotte, J.L. Guth and H. Kessler, Stud. Surf.

602

Sci. Catal., 498 (1989) 857-866. N. Dumont, T. Ito and E.G. Derouane, Appl. Catal., 54 (1989) Ll-L6. 9 1 0 J. Fraissard and T. Ito, Zeolites, 8 (1988) 350-361, and references therein. 1 1 T. Ito, J. Fraissard, J. B.Nagy, N. Dewaele, Z. Gabelica, A. Nastro and E.G. Derouane, Stud. Surf. Sci. Catal., 49A (1989) 579-588. 1 2 T. Ito, N. Dumont, J. B.Nagy, Z. Gabelica and E.G. Derouane, in: Proc. Intern. Symp. on Chemistry of Microporous Crystals, Tokyo, Japan, July 27-29, 1990, Kodansha/Elsevier, in press. 1 3 D. Muller, E. Jahn and G. Ladwig, Chem. Phys. Lett., 109 (1984) 332336. 1 4 L. Maistriau, Z. Gabelica, E.G. Derouane, E.T.C. Vogt and J. van Oene, Zeolites, in press.

G. Ohlmann et al. (Editors), Catalysis and Adsorption by Zeolites

603

0 1991 Elsevier Science Publishers B.V., Amsterdam

METASTABILITY OF ZEOLITES I N TETRAETHYLAMMONIUM MEDIA

F r a n c e s c o D I R E N Z O I , A b d e r r a h m a n ALBIZANE1, Ma rie -A gn8s NICOLLE1, F r a n q o i s FAJULAI, F r a n c o i s FIGUERASl a n d T h i e r r y DES COURIERES2 1 L a b o r a t o i r e d e Chimie Organique Physique e t CinCtique Chimique A p p l i q u e e s , URA 418 d u CNRS, E c o l e N a t i o n a l e S u p e r i e u r e d e C h i m i e , 8 r u e d e 1 ’ E c o l e N o r m a l e , 34053 M o n t p e l l i e r C e d e x 1 ( F r a n c e ) * C e n t r e d e Recherche ELF-France,

Solaize

SUMMARY T h e i n f l u e n c e of o r g a n i c c o n c e n t r a t i o n , S i / A 1 r a t i o , s t i r r i n g and n a t u r e o f t h e s i l i c a s o u r c e on z e o l i t e s y n t h e s i s i n t h e p r e s e n c e o f t e t r a e t h y l a m m o n i u m h a v e b e e n s t u d i e d . Examples o f m e t a s t a b i l i t y o f z e o l i t e b e t a a n d ZSM-5 t o w a r d s ZSM-12 p r o p o s e t h e l a s t a s t h e more s t a b l e z e o l i t i c p h a s e i n t e t r a e t h y l a m m o n i u m m e d i a f e a t u r i n g h i g h e s t Si/A1 r a t i o . E v i d e n c e s of h e t e r o g e n e o u s n u c l e a t i o n o f z e o l i t e s o v e r amo r p h o u s o r c r y s t a l l i n e p h a s e s a r e discussed. 1NT RODUCT I ON

Many s t u d i e s h a v e d e a l t w i t h z e o l i t e s y n t h e s i s i n t h e p r e s e n c e o f t e t r a e t h y l a m m o n i u m , s i n c e t h e f i r s t r e p o r t of t h e s y n t h e s i s o f z e o l i t e b e t a ( r e f . 1 ) . Recent p a p e r s have been e s p e c i a l l y d e v o t e d t o t h e s y n t h e s i s o f z e o l i t e b e t a ( r e f s . 2 , 3 ) and t o t h e c o m p e t i t i v e f o r m a t i o n o f z e o l i t e b e t a a nd ZSM-PO

( r e f s . 4 , 5). T h e

open l i t e r a t u r e p r e p a r a t i o n s o f t h e s e z e o l i t e s a r e a l w a y s b a s e d on

te t r a a l k y l o r t h o s i l i c a t e s , w h i l e p a t e n t s y n t h e s e s p r o p o s e i n o r g a n i c s o u r c e s o f s i l i c a b o t h i n t h e case o f z e o l i t e b e t a ( r e f s .

1, 6 )

and i n t h e case of ZSM-12 ( r e f . 7 ) . The c r y s t a l l i z a t i o n s e l e c t i v i t y i n t e t r a e t h y l a m m o n i u m m e d i a when c o m m e r c i a l i n o r g a n i c s o u r c e s of s i l i c a a r e u s e d appears h en c e a s a p r o m i s i n g f i e l d of investigation. T h e n a t u r e of

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

c r y s t a l l i z a t i o n i n a m u l t i p l e way. Beyond t h e d i f f e r e n t d e g r e e o f polymerization of t h e s i l i c i c species i n s o l u t i o n , t h e s u r f a c e p r o p e r t i e s o f t h e a m o r p h o u s s o l i d p r e s e n t i n t h e s y n t h e s i s medium are deeply a f f e c t e d . A s a consequence, heterogeneous n u c l e a t i o n of

d i f f e r e n t p h a s e s can o c c u r and modify t h e s e l e c t i v i t y of t h e p r e p a r a t i o n . E v i d e n c e s of p r e f e r e n t i a l n u c l e a t i o n of z e o l i t e s o v e r

604

non-crystalline

s o l i d s i n o t h e r s y n t h e s i s media have been r e c e n t l y

p r o p o s e d ( r e f s . 8 - 1 1 ) . M o r e o v e r , t h e o r i e n t e d n u c l e a t i o n of a z e o l i t e framework o v e r a d i f f e r e n t z e o l i t i c s t r u c t u r e is a well known phenomenon ( r e f s . 1 2 - 1 4 ) .

I t seems t h e n e s p e c i a l l y

i n t e r e s t i n g t o pay a t t e n t i o n t o t h e i n t e r f a c e phenomena r e l a t e d t o t h e h e t e r o g e n e i t y o f t h e s y n t h e s i s medium. METHODS C r y s t a l l i z a t i o n e x p e r i m e n t s h a v e b e e n c a r r i e d o u t u s i n g two t y p e s o f e x p e r i m e n t a l d e v i c e s : 120 a1 s t a i n l e s s s t e e l a u t o c l a v e s f o r e x p e r i m e n t s i n s t a t i c c o n d i t i o n s and a 0.5 l i t r e s s t a i n l e s s s t e e l r e a c t o r f o r experiments under s t i r r i n g . T h i s l a s t autoclave wad e q u i p p e d w i t h a n a n c h o r - s h a p e d

s t i r r e r and a sampling o u t l e t

a l l o w i n g specimens t o be withdrawn w i t h o u t a l t e r i n g t h e s y n t h e s i s c o n d i t i o n s . The s y s t e m w a s d e s i g n e d s o a s t o a v o i d a c c u m u l a t i o n of

material i n s i d e s a m p l i n g p i p e and v a l v e s i n t h e c o u r s e of t h e r e a c t i o n ( r e f . 1 5 ) . The r e a c t o r was h e a t e d b y a n e l e c t r i c a l f u r n a c e r e g u l a t e d t h r o u g h a t h e r m o c o u p l e immersed i n t h e r e a c t i o n m e d i u m . T h e h e a t i n g r a t e was 3 "C/min

a n d t h e s t i r r i n g r a t e 150

rpm. S a m p l e s o f 1 U m l w e r e p e r i o d i c a l l y w i t h d r a w n f r o m t h e a u t o c l a v e . T h e s o l i d f r a c t i o n was r e c o v e r e d b y f i l t r a t i o n . The ::lcar

s o l u t i o n , c o n t a i n i n g t h e d i l u t e d m o t h e r l i q u o r , was

p r e s e r v e d f o r e l e m e n t a l a n a l y s i s . The s o l i d w a s washed w i t h d e i o n i z e d w a t e r up t o p H

Y a n d d r i e d a t 70

"C

i n a i r . The

temperature of a l l syntheses, both s t i r r e d and i n s t a t i c c o n d i t i o n s , was 1 5 0 ° C . T h e r e a g e n t s w e r e s i l i c a s o l ( C e c a s o l 3 0 , S i O z 2 5 % , Na 0 . 2 % , A 1 6 0 ppm, pH 8 . 8 , g r a i n s i z e 1 2 - 1 8 n m ) , p r e c i p i t a t e d s i l i c a ( Z e o s i l

175MP f r o m Rhane P o u l e n c , Na U . Y % , A 1 0.4%, Hz0 6 . 5 % , 2-2U

grain s i z e

u , p o r e volume 0 . 0 8 m l / g ) . s o d i u m a l u m i n a t e ( C a r l o E r b a R L E ) ,

t e t r a e t h y l a m m o n i u m (TEA!

h y d r o x i d e s o l u t i o n ( A l d r i c h ) , sodium

h y d r o x i d e ( P r o l a b o KP N o r m a p u r ) , d e i o n i z e d w a t e r . T h e r e a g e n t s were m i x e d u n d e r s t i r r i n g i n t h e o r d e r : a l k a l i n e s o l u t i o n , o r g a n i c

a g e n t , a l u m i n a t e , s i l i c a . T h e m i x t u r e was s t i r r e d f o r 4 h r s a t roam t empe r a t u r e b e f o r e t he b e g i n n i n g o f t h e s y n t lies i s . X - r a y p o w d e r d i f f r a c t i o n ( C t i R T h e t a 60 d i f f r a c t o m e t e r , C u K a r a d i a t i o n ) was u s e d t o i d e n t i f y t h e p h a s e s p r e s e n t i n t h e s o l i d f r a c t i . o n . C r y s t a l s i z e and h a b i t were d e t e r m i n e d b y s c a n n i n g e l e c t r o n m i c r o s c o p y ( C a m b r i d g e S l 0 0 i n s t r u m e n t , r e s o l v i n g power 7 nm).

605 RESULTS A N D DISCUSSION

C.om0p e t i Lian...be-ttwaen. an al.c.ine_,_. mx.deni..f_e.and..ze~l,i.t a .bet a S y n t h e s i s b a t c h e s o f s t o i c h i o m e t r y 8.10[xNa-(l-x)TEA]~Alz03~ l Y . O S i 0 2 . 6 3 0 H z O (OH-/SiOz 0.72), w h e r e x s p a n n e d f r o m 0 t o 1, h a v e b e e n h e a t e d a t 150°C f o r 72 h o u r s i n s t a t i c c o n d i t i o n s . P r e c i p i t a t e d s i l i c a was u s e d a s s i l i c a s o u r c e . T h e p h a s e s f o r m e d , t h e e l e m e n t a l composition o f t h e p r o d u c t and t h e y i e l d s of s i l i c o n and aluminum t h r o u g h o u t t h e p r e p a r a t i o n a r e r e p o r t e d i n t a b l e 1.

As t e t r a e t h y l a m m o n i u m c o n c e n t r a t i o n i n c r e a s e s t h e main p h a s e formed p a s s e s from a n a l c i m e t o z e o l i t e b e t a t h r o u g h m o r d e n i t e . The r o l e of t h e tetraethylammonium c a t i o n i n t h e formation of s i l i c a r i c h l o w - d e n s i t y c r y s t a l l i n e p h a s e s is e v i d e n t . From t a b l e 1 i t

emerges t h a t t h e aluminum i n c o r p o r a t i o n p r e s e n t s a minimum f o r i n t e r m e d i a t e v a l u e s o f t h e t e t r a e t h y l a m m o n i u m c o n c e n t r a t i o n . The phenomenon seems t o b e c o n s i s t e n t w i t h t h e f o r m a t i o n of m o r d e n i t e more s i l i c i c t h a n t h e c o r r e s p o n d i n g z e o l i t e b e t a . T h e m o r p h o l o g y o f some p r o d u c t s a r e d e p i c t e d i n f i g u r e 1. A t a TEA/SiOz r a t i o o f 0 . 5 0 ( f i g u r e l a ) r o u g h i r r e g u l a r g r a i n s o f z e o l i t e b e t a u p t o 1 . 5 cc l a r g e a r e f o r m e d , t o g e t h e r w i t h t a b u l a r c r y s t a l s of mordenite 12

CI

l o n g . Some o f t h e g r a i n s o f z e o l i t e

b e t a a p p e a r t o h a v e grown a r o u n d t h e e d g e s o f t h e m o r d e n i t e c r y s t a l s . Rare s p h e r e s o f a n a l c i m e ( n o t d e p i c t e d ) p r e s e n t a d i a m e t e r up t o 7

u . A t a TEA/SiOz r a t i o o f 0 . 1 6 ( f i g u r e

co-crystallization

lb)

of m o r d e n i t e and a n a l c i m e can be o b s e r v e d .

A n a l c i m e c r y s t a l s a r e l a r g e r ( u p t o 16

u)

a n d p r e s e n t t h e {211}

t r a p s z o h e d r o n h a b i t . When n o t e t r a e t h y l a m m o n i u m was p r e s e n t i n t h e s y n t h e s i s m i x t u r e ( f i g u r e l c ) t h e p r o d u c t i s composed by 2 5

u

l a r g e a n a l c i m e t r a p e z o h e d r a . The c r y s t a l e d g e s a r e e t c h e d by

c o r r o s i o n l i n e s , s u g g e s t i n g t h a t t h e s y s t e m h a s grown u n d e r s a t u r a t e d f o r a n a l c i m e . Analcime is s p r e a d of 2

CI

large

TABLE 1 Phase composition, e l e m e n t a l composition and material b a l a n c e a s a f u n c t i o n a f t h e tetraethvlaamonium c o n t e n t af-the s y n t h e s i s b a t c h .

T E AL

cristallinity %

mole f r a c t i o n - Al

beta

MUR

74

80

62 -

28

10

0.087

0.5U

52

48

-

-

0.185 0 252

u

0.16 0.00

Yield

(ele.ment/Si+Al>-

S i02

ANA

1UO

0.061

N

Na.

Si

A I-

0.083 0.069 0.022

0.027

0.94 0 61 0.35

0.85

0.000

0.160 0.179 0.305

0 25

0.37

U 75 0.78

606

F i g . 1. S y n t h e s e s a t d i . f f e r e n t t e t r a e t h y l a m m o n i u m c o n c e n t r a t i o n . F r o m t o p t o b o t t o m : TEA/Si02 0 . 5 0 ( a ) , 0 . 1 6 ( b ) a n d 0 . 0 0 ( c ) .

607

-\ 0.038

Q, 0.020

-a \

0.002

100

t (h)

200

F i g . 2 . N u c l e a t i o n l a g s o f z e o l i t e b e t a ( 0 ) a n d ZSH-12 ( 0 ) a s f u n c t i o n s o f t h e m o l e f r a c t i o n of a l u m i n u m .

f l a k e s o f g i s m o n d i n e , d e t e c t a b l e i n t r a c e amount f r o m t h e p o w d e r X-ray d i f f r a c t i o n s p e c t r a .

F ~ o u .~ e ~ l i tb.afa.t~-..Z.~n--_l.Z e Mixtures of s t o i c h i o m e t r y 0.08Na~0.30TEA-(x/2)Al~O~~(l-x)SiO~~

16Hz0 (OH-/SiOz

0.35) w i t h x r a n g i n g f r o m 0.038 t o 0 . 0 0 2 h a v e b e e n

h e a t e d a t 150°C i n a s t i r r e d r e a c t o r . S i l i c a s o l w a s t h e s o u r c e of s i l i c a . T h e i n d u c t i o n lags p r e c e d i n g t h e d e t e c t i o n of t h e c r y s t a l l i n e phases are r e p o r t e d i n f i g u r e 2. I n moderately s i l i c i c s y s t e m s ( S i / A l 2 5 - 5 0 ) z e o l i t e b e t a f o r m s b e f o r e ZSM-1%. T h e n u c l e a t i o n l a g o f ZSH-12 d e c r e a s e s when S i / A 1 i n c r e a s e s a n d a t h i g h e r S i / A 1 r a t i o ( 5 0 0 ) ZSM-12

i s t h e f i r s t and u n i q u e

c r y s t a l l i n e p h a s e f o r m e d . Aluminum seems n e e d e d t o f o r m t h e l a d d e r s o f 4-membered r i n g s t y p i c a l o f z e o l i t e b e t a ( r e f s . 16, 17), a l r e a d y p r o p o s e d a s a l u m i n u m l o c a t i o n ( r e f . 18). On t h e o t h e r hand, t h e p r e s e n c e of s i l i c o a l u m i n a t e s p e c i e s a p p e a r s t o i n h i b i t tlie f o r m a t i o n of t h e c r i s t o b a l i t e - l i k e w a l l s of i n t e r c o n n e c t e d 6-membered r i n g s t y p i c a l o f ZSM-12

( r e f . 19).

O n t h e o t h e r hand a l u m i n u m a l s o m o d i f i e s t h e p h y s i c s o f t h e

s y s t e m . I f t h e amount o f s o l i d f r a c t i o n r e c o v e r e d f r o m i n t e r m e d i a t e s a m p l i n g is c o n s i d e r e d , m a i n d i f f e r e n c e s a r e f o u n d a s a f u n c t i o n of composition.

I n t h e more a l u m i n i c s y s t e m a n

amorphous s o l i d is p r e s e n t t h r o u g h o u t t h e h y d r o t h e r m a l t r e a t m e n t . I n t h e c a s e o f t h e more s i l i c i c s t o i c h i o m e t r y t h e s y s t e m i n i t i a l l y c o n s i s t s o f a c l e a r s o l u t i o n . G e l l i n g , and r e c o v e r y of a n y s o l i d f r a c t i o n from s a m p l i n g , b e g i n o n l y a f t e r one d a y a t t h e s y n t h e s i s t e m p e r a t u r e . Anyway a n a m o r p h o u s s o l i d i s a l w a y s p r e s e n t when zeolite nucleates.

608

C ~ m p e tf;iQo...he.tween i z.e.Q.l.i.t.e..hzta...an.d._ZS.M.-...5 T h e n a t u r e o f t h e s o u r c e o f s i l i c a may i n f l u e n c e t h e p h a s e s e l e c t i v i t y o f t h e s y n t h e s i s . Two g e l s o f s t o i c h i o m e t r y

1.2Na20.9.2TEA~0~A120~~78SiOz~128O (O HH zO -/Si02

0.24) h a v e b e e n

p r e p a r e d f r o m p r e c i p i t a t e d s i l i c a o r s i l i c a s o l . Under hydrothermal treatment i n a non-stirred

a u t o c l a v e t h e experiment

with p r e c i p i t a t e d silica has given o r i g i n t o z e o l i t e b e t a . In t h e e x p e r i m e n t w i t h s i l i c a s o l u n d e r t h e same h y d r o t h e r m a l c o n d i t i o n s

ZSH-5 h a s b e e n f o r m e d . T h i s s e l e c t i v i t y e f f e c t h a s n o t b e e n o b s e r v e d when t h e s y n t h e s i s h a s b e e n c a r r i e d o u t u n d e r s t i r r i n g . Two g e l s o f stoichiometry 2.2Na~0-8.8TEA~0~A1~0~~51Si0~~ (OH-/Si02 1040Hz0 0.36) have been p r e p a r e d from p r e c i p i t a t e d s i l i c a or s i l i c a s o l . Under h y d r o t h e r m a l t r e a t m e n t b o t h g e l s g a v e o r i g i n t o z e o l i t e b e t a . The c r y s t a l l i z a t i o n from s i l i c a s o l f e a t u r e d a l o n g e r i n d u c t i o n t i m e (22 h o u r s v e r s u s 15) a n d b i g g e r c r y s t a l s o f z e o l i t e ( 1 . 5 v e r s u s 0.8 p ) .

T h e n u c l e a t i o n o f z e o l i t e b e t a is h e n c e e a s i e r when t h e s y n t h e s i s g e l is p r e p a r e d f r o m p r e c i p i t a t e d s i l i c a , b o t h i n s t i r r e d a n d s t a t i c c o n d i t i o n s . On t h e c o n t r a r y , s t i r r i n g seems t o h i n d e r t h e f o r m a t i o n o f ZSH-5.

T h i s e f f e c t a p p e a r s i n agreement

w i t h t h e l o c a t i o n o f ZSH-5 e m b r y o s a t t h e e x t e r n a l s u r f a c e o f t h e a m o r p h o u s g r a i n s ( r e f . 8), w h e r e s t i r r i n g may a f f e c t t h e e p i t a x i a l cond i t i o n s .

F r ~ mZ.SMk5 t Q ZSM-12 I n t h e p r e v i o u s l y c i t e d e x p e r i m e n t o f s t o i c h i o m e t r y 1.2Na2O.

Y . Z T E A 2 0 ~ A 1 ~ 0 s ~ 7 8 S i 0 2 ~ 1 2 8f0eHa~t O u r i n g s i l i c a s o l , ZSH-5 p r o v e d u n s t a b l e on l o n g e r s y n t h e s i s t i m e . The p h a s e c o m p o s i t i o n o f t h e s y s t e m is r e p r e s e n t e d i n t a b l e 2 a s a f u n c t i o n o f t h e s y n t h e s i s

t i m e . A f t e r t e n d a y s i n h y d r o t h e r m a l c o n d i t i o n s ZSH-12 n u c l e a t e s

a n d ZSM-5 b e g i n s t o d i s s o l v e . A f t e r t w e n t y d a y s ZSM-12 is t h e main c r y s t a l l i n e p h a s e p r e s e n t . The e v o l u t i o n o f t h e s o l i d c o m p o s i t i o n i s a l s o r e p o r t e d i n t a b l e 2 . The o r g a n i c c o n t e n t f e a t u r e s a

minimum i n c o r r e s p o n d e n c e w i t h t h e h i g h e r y i e l d o f ZSH-5. The e l e c t r o n m i c r o g r a p h s o f t h e s o l i d a t d i f f e r e n t s y n t h e s i s t i m e s

a r e r e p o r t e d i n f i g u r e 3 . F i g u r e 3a r e p r e s e n t s t h e z e o l i t e f o r m e d a f t e r 10 d a y s i n h y d r o t h e r m a l c o n d i t i o n s . T h e X-ray d i f f r a c t i o n p a t t e r n o f t h i s s a m p l e i n d i c a t e s t h e p r e s e n c e o f t r a c e s of ZSM-12 besides well-crystallized

ZSM-5. The s a m p l e c o n s i s t s o f 35

c h a r a c t e r i s t i c e u h e d r a l c r y s t a l s o f ZSM-5.

p

long

609

Fig. 3. Synthesis of ZSM-5 and ZSM-12. Crystallization time, from top to bottom: ( a ) 10, (b) 15, ( c ) 2 0 d a y s .

610

TABLE 2 P h a s e c o m p o s i t i o n , e l e m e n t a l c o m p o s i t i o n and material b a l a n c e d u r i n g the time (days) ._

3 10 15 20

4-~xma!i.ii~n.nf _zSK:3xd-_zSHzL2, Y-isI d

crisfa.lLi~fy--% m o l e f r a c t i o n L e l e m e n t / S i + A l L

ZSMz5 __ - ZSK=l2..

-Al-..-.-.--N--_ 0.026 0.022 0.024 0.025

amorphous 70 3 64 14 39 40

---N& 0.053 0.041 0.045 0.053

0.027 0.007 0.007 0.006

_Si-Al0.57 0.83 0.94 0.91

0.59 0.73 0.89 0.89

E x t e n d e d t w i n n i n g i m p l i e s t h a t t h e l a t e r a l f a c e s of t h e p r i s m s a r e e s s e n t i a l l y ( 1 0 0 ) f a c e s . T h e c r y s t a l s p r e s e n t a l s o some l e s s common f e a t u r e s . T h e i r l a t e r a l f a c e s a r e s p r e a d o f r e c t a n g u l a r e t c h i n g f i g u r e s , c l e a r l y showing t h a t t h e s y n t h e s i s system h a s grown u n d e r s a t u r a t e d f o r ZSM-5.

This e f f e c t should be accounted

f o r by t h e p r e s e n c e o f a l e s s s o l u b l e c r y s t a l l i n e p h a s e . I n d e e d a d e n s e p o p u l a t i o n o f s u b m i c r o n p r i s m a t i c c r y s t a l s are embedded i n t h e ( 1 0 0 ) f a c e s of ZSM-5.

T h e smaller p r i s m s a p p e a r t o h a v e g r o w n

up f r o m t h e s u r f a c e p r e c i s e l y o r i e n t e d w i t h r e g a r d s t o t h e ZSM-5 l a t t i c e . T h e i r main a x i s l a y s i n t h e (010) p l a n e a n d f o r m s a 53” angle w i t h the

direction

o f ZSM-5.

Also t h e n o r m a l t o o n e

f a c e o f t h e smaller p r i s m s l a y s i n t h e ( 0 1 0 ) p l a n e o f ZSH-5. I t c a n b e o b s e r v e d t h a t t h e o r i e n t a t i o n of t h e smaller c r y s t a l s

c o u l d c o r r e s p o n d t o 234-12 c r y s t a l s g r o w i n g i n t h e d i r e c t i o n f r o m a ( 2 0 1 ) f a c e l a y i n g o n t h e ZSM-5 ( 1 0 0 ) f a c e . T h i s o r i e n t a t i o n a c c e p t e d , t h e ZSM-12

< 0 1 0 > d i r e c t i o n s h o u l d c o r r e s p o n d t o t h e ZSM-

5 < 0 1 U > d i r e c t i o n . An e p i t a x i a l r e l a t i o n c a n b e f o u n d b e t w e e n t h e b parameters:

5 . 0 and 1 9 . 9 A , r e s p e c t i v e l y . O r i e n t e d n u c l e a t i o n of

a z e o l i t e over a n o t h e r z e o l i t e f e a t u r i n g e p i t a x y i n only one d i r e c t i o n h a s a l r e a d y b e e n o b s e r v e d , f o r i n s t a n c e i n t h e case o f o f f r e t i t e a n d m a z z i t e ( r e f . 14). F i g u r e 3b d e p i c t s t h e s o l i d f o r m e d a f t e r 1 5 d a y s o f s y n t h e s i s . The s u r f a c e o f t h e ZSM-5 c r y s t a l s is d e e p l y e t c h e d a n d a h o p p e r p i t i s open i n t h e ( 0 1 0 ) f a c e s . The s u r f a c e o f t h e l e s s - e r o d e d c r y s t a l s is s p r e a d o f s l i g h t l y f l a t t e n e d s p h e r e s 3

LI

large, with a

r o u g h s u r f a c e . X-ray d i f f r a c t i o n a l l o w s t o i d e n t i f y t h e s p h e r i c a l c r y s t a l s a s ZSM-12.

Some c h a r a c t e r i s t i c p i t s i n t h e ZSM-5 s u r f a c e

seem t o c o r r e s p o n d t o t h e o r i g i n a l l o c a t i o n o f d e t a c h e d s p h e r o i d a l c r y s t a l s . The i n war d g r o w t h o f a c r y s t a l a f t e r i t s n u c l e a t i o n on

611 a n h e t e r o g e n e o u s s u r f a c e is a we1 -known p h e n o m e n o n . I t h a s b e e n carefully described, for instance

i n t h e case o f ZSM-5 n u c l e a t i o n

o v e r amorphous s i l i c a ( r e f . 8 ) . A c h a n g e o f m o r p h o l o g y o f ZSM- 2 f r o m e u h e d r a l p r i s m s t o h i g h -

i n d e x s u r f a c e s p h e r e s i s s u c h a n u n u s u a l phenomenon t h a t i t d e s e r v e s a comment. I t c o u l d b e e x p l a i n e d o n l y b y a s h a r p r i s e o f s u p e r s a t u r a t i o n , as i f a p r e v i o u s l y confined source of s o l u b l e s p e c i e s had b e e n made a c c e s s i b l e t o t h e s o l u t i o n . T h e h o p p e r p i t s i n t h e (010) f a c e s o f t h e ZSM-5 c r y s t a l s p r o p o s e t h e c o r e o f t h e ZSM-5 c r y s t a l s a s a p o s s i b l e s o u r c e o f more s o l u b l e s p e c i e s . U n d e r t h i s h y p o t h e s i s , once t h e o u t e r l a y e r of (100) t w i n n i n g d i s s o l v e d , t h e r a p i d d i s s o l u t i o n o f t h e ( 0 1 0 ) f a c e s would i n c r e a s e t h e r a t e o f g r o w t h o f ZSM-12 b e y o n d t h e t h r e s h o l d o f d e v e l o p m e n t o f h i g h index f a c e s from t h e e d g e s of t h e f l a t f a c e s . The f i g u r e 3 c , r e p r e s e n t i n g t h e s o l i d a f t e r 20 d a y s o f s y n t h e s i s , c l e a r l y shows t h e growth of t h e f l a t t e n e d s p h e r e s of ZSM-12,

t h e s h r i n k i n g o f t h e ZSM-5 c r y s t a l s a n d t h e d e e p e n i n g of

t h e hopper p i t s i n t h e (010) f a c e s of t h e l a t t e r .

CONCLUSIONS C h e m i c a l and p h y s i c a l f a c t o r s a f f e c t t h e s e l e c t i v i t y o f formation of t h e c r y s t a l l i n e p h a s e s t y p i c a l of t h e tetraethylammonium m e d i a . The i s s u e of b o r d e r l i n e s i t u a t i o n s i n w h i c h s e v e r a l p h a s e s c a n b e f o r m e d may b e d i r e c t e d b y t h e n a t u r e of s o l i d s u r f a c e s p r e s e n t i n t h e s y n t h e s i s s y s t e m .

ACKNOWLEDGMENTS Many t h a n k s a r e d u e t o t h e S e r v i c e C e n t r a l d ' A n a l y s e o f CNRS i n S o l a i z e f o r e l e m e n t a l a n a l y s i s and t o George Nabias f o r e l e c t r o n microscopy e x p e r i m e n t s . REFERENCES 1

2

E . L . W a d l i n g e r , G . T . Kerr a n d a n d E . J . R o s i n s k i , C a t a l y t i c c o m p o s i t i o n o f a c r y s t a l l i n e z e o l i t e , US P a t . 3 , 3 0 8 , 0 6 9 ( 1 9 6 7 ) . J . P e r e z - P a r i e n t e , J . A . M a r t e n s and P.A. J a c o b s , C r y s t a l l i z a t i o n m e c h a n i s m o f z e o l i t e b e t a f r o m ( T E A ) z O , NazO and K2O c o n t a i n i n g a l u m i n o s i l i c a t e g e l s , A p p l i e d C a t a l . , 31 ( 1987) 35-64.

3

J . P e r e z - P a r i e n t e , J . A . M a r t e n s and P . A . J a c o b s , F a c t o r s a f f e c t i n g t h e s y n t h e s i s e f f i c i e n c y o f z e o l i t e BETA f r o m a l u m i n o s i l i c a t e g e l s c o n t a i n i n g a l k a l i and t e t r a e t h y l a m m o n i u m i o n s , Z e o l i t e s , 8 (1988) 46-53.

612

U e w a e l e , L . M a i s t r i a u , J . B . Nagy a n d E . G . Derouane, D i r e c t i n g p a r a m e t e r s i n t h e s y n t h e s i s of z e o l i t e s ZSM-20 and b e t a , ACS Symp. S e r i e s , 3 9 8 ( 1 9 8 9 ) 5 1 8 - 5 4 3 . D.E.W. V a u g h a n , M . M . J . T r e a c y , J . M . Newsam, K.G. S t r o h m a i e r a n d W.J. M o r t i e r , S y n t h e s i s a n d c h a r a c t e r i z a t i o n o f z e o l i t e SZM-20, ibidem 544-559. Y . F . C h u , P r o c e s s f o r ZSM-11 p r o d u c t i o n , US P a t . 4 , 8 4 7 , 0 5 5 (1989). E . J . K o s i n s k i and M.K. R u b i n , C r y s t a l l i n e z e o l i t e ZSM-12, US P a t . 3,832,449 (1974). J . C . J a n s e n , C . W . R . E n g e l e n a n d H . v a n Bekkum, C r y s t a l g r o w t h r e g u l a t i o n a n d m o r p h o l o g y o f z e o l i t e s i n g l e c r y s t a l s o f t h e MFI t y p e , ACS Symp. S e r . , 398 ( 1 9 8 9 ) 2 5 7 - 2 7 3 . J . B.Nagy, Ph. B o d a r t , H . C o l l e t t e , C . F e r n a n d e z , Z . G a b e l i c a , A . N a s t r o and R . A i e l l o , C h a r a c t e r i z a t i o n of c r y s t a l l i n e a n d a m o r p h o u s p h a s e s d u r i n g t h e s y n t h e s i s o f (TPA,M)-ZSM-5 Z e o l i t e s ( M = L i , Na, K ) . J . Chem. S O C . F a r a d a y T r a n s . 1 , 85 ( 1 9 8 9 ) 2749-2765. F . D i R e n z o , F . Remoue, P . M a s s i a n i , F . F a j u l a , F . F i g u e r a s a n d T . Des C o u r i e r e s , C r y s t a l l i z a t i o n k i n e t i c s o f z e o l i t e TON, Zeolites, submitted. C . S . Cundy, p e r s o n a l c o m m u n i c a t i o n . I.Y. Chan a n d S . I . Z o n e s , A n a l y t i c a l e l e c t r o n m i c r o s c o p y (AEM) o f C u b i c P z e o l i t e t o Nu-3 z e o l i t e t r a n s f o r m a t i o n , Z e o l i t e s , 5 ( 1 9 8 9 ) 3-11. E . d e Vos B u r c h a r t , J . C . J a n s e n a n d H . v a n Bekkum, O r d e r e d o v e r g r o w t h o f z e o l i t e X o n t o c r y s t a l s o f z e o l i t e A, Z e o l i t e s , 9 ( 1 9 8 9 ) 432-435. F . F a j u l a , F . F i g u e r a s , C . Gueguen a n d R . D u t a r t r e , B i n a r y z e o l i t i c s y s t e m s , t h e i r s y n t h e s i s a n d t h e i r u t i l i z a t i o n , US P a t . 4,847,224 (1989). F . F a j u l a , S . N i c o l a s , F . D i R e n z o , C . G u e g u e n and F . F i g u e r a s , K i n e t i c s and m e c h a n i s m o f c r y s t a l g r o w t h o f z e o l i t e o m e g a , ACS Symp. S e r . , 398 ( 1 9 8 9 ) 4 9 3 - 5 0 5 . R . B . L a P i e r r e , A . C . Kohrman J r . , J . L . S c h l e n k e r , J . D . Wood, M . K . Kubin and W . J . R o h r b a u g h , T h e f r a m e w o r k t o p o l o g y o f ZSM-12: A h i g h - s i l i c a z e o l i t e , Z e o l i t e s , 5 ( 1 9 8 5 ) 3 4 6 - 3 4 8 . J . M . Newsam, M.M.J. T r e a c y , W . T . K o e t s i e r a n d C . H . d e G r u y t e r , S t r u c t u r a l c h a r a c t e r i z a t i o n of z e o l i t e b e t a , Proc. ti. S O C . ( L o n d o n ) A, 42il ( 1 9 8 8 ) 375-4135. N.A. B r i s c o e , J . L . C a s c i , J . A . D a n i e l s , D.W. J o h n s o n , M.D. S h a n n o n a n d A. S t e w a r t , Some a s p e c t s o f t h e s y n t h e s i s , c h a r a c t e r i z a t i o n a n d p r o p e r t i e s o f z e o l i t e Nu-2, S t u d . S u r f a c e S c i . C a t a l y s i s 45A, E l s e v i e r , A m s t e r d a m 1 9 8 9 , 1 5 1 . J . B . H i g g i n s , K.B. L a P i e r r e , J . L . S c h l e n k e r , A . C . Rohrman J r . , J . D . Wood, G . T . Kerr a n d W . J . R o h r b a u g h , T h e f r a m e w o r k topology of z e o l i t e b e t a , Z e o l i t e s , (1988) 446. Z . Gabelica, N .

10

11 12 13 14

15 16

17 18

19

G. Ohlmann et al. (Editors),Catalysis and Adsorption by Zeolites 0 1991 Elsevier Science Publishers B.V., Amsterdam

613

SYNTHESIS OF FERRIERITES WITH HIGH GALLIUM CONTENT M.A. Camblor*, J.A. Martens, P.J. Grobet and P.A. Jacobs Laboratorium voor Oppervlaktechemie, Kathol ieke Universitei t Leuven, Kardinaal Mercierlaan 92, 8-3030 Leuven, Belgium

* on leave from Instituto de Catalisis y Petroleoquimica, C.S.I.S, Madrid, Spain.

SUMMARY Ferrierite samples with Ga/GatAl compositions ranging from 0 t o 90% are synthesized hydrothermally in the presence o f piperidine. The incorporation of aluminium and gallium in the zeolite is monitored with 27Al, 29Si and 71Ga MAS - NMR . INTRODUCTION The isomorphic substitution of A1 and Si by other elements in the framework of zeolites is of considerable interest. For example, gallium zeolites possess surface chemical properties which are different from those o f aluminosil icate zeolites and have special catalytic properties, e.g. in LPG aromatization (refs. 1 and 2). A number of (Si, Ga)-analogs o f (Si,Al)-zeolites have been made by direct synthesis (see e.g. the references cited in ref. 3). In this paper we report on the synthesis of aluminosilicate ferrierite samples with extensive isomorphous substitution by Ga. We developed a new synthesis method according to which alumino-galloferrierites can be obtained with Si/(GatAl) ratio’s ranging from 7.1 t o 3.4. The gallium content of alumino-galloferrierites can be considerably higher than the aluminium content of aluminosilicate ferrierites, for which the lower limit of Si/A1 is ca. 6 (ref. 3). EXPERIMENTAL Synthesis mixtures with the following molar compositions are used: 23.3 Na20 : T2O3 : 46.5 Si02 : 18.6 CSHIONH : 976 H20 where T stands for Ga and Al. The Ga/(GatAl) fractions used were 0.0, 0.5, 0.75 and 0.9. The gels were prepared as follows. In a first solution NaOH (Merck) was added t o waterglass (Merck) with stirring and then piperidine (Aldrich) was added. The second solution, consisting o f A12(S04)3.18H20

614

(U.C.6)

and Ga(N03).xH20 (Janssen) solved i n water, was added t o t h e f i r s t

solution

with

agitation.

The

synthesis

mixtures

were

transferred

into

s t a i n l e s s steel autoclaves o f 120 m l ; a g i t a t i o n was performed by r o t a t i n g t h e autoclaves a t 50 rpm. crystallization

C r y s t a l l i n e phases were obtained a f t e r ca.

22 h o f

a t 473 K.

The phase p u r i t y o f t h e f e r r i e r i t e products was v e r i f i e d using XRD patterns, recorded w i t h a Siemens instrument, which was equipped w i t h a Mc Brawn p o s i t i o n s e n s i t i v e detector. The chemical compositions were determined by I C P . The MAS NMR spectra were obtained w i t h a Bruker 400 MSL spectrometer.

The

experimental conditions used were as f o l l o w s :

I

Parameter NMR frequency (MHz) Pulse angle ( * ) Pulse l e n g t h ( p s ) R e p e t i t i o n time ( s ) Spinning r a t e (kHz) Number o f scans Reference f o r chemical s h i f t The

29Si

spectra

were

I

29Si

79.5 45 4 3 3 18,000 TMS

deconvoluted

1

27Al 104.2 20 0.6 0.1 5 3000 AlC13

according

71Ga 122.0 20 0.6 0.1 15

150,000 Ga(N03) 2 to

a

procedure

described

previously (ref. 4). RESULTS AND DISCIUSSION The XRD p a t t e r n s o f t h e c r y s t a l l i z a t i o n products are shown i n Fig.1. The XRD l i n e s which are c h a r a c t e r i s t i c f o r t h e f e r r i e r i t e f a m i l y o f m a t e r i a l s are present. Traces o f quartz may be present i n samples S3 and S4. The T-atom composition o f synthesis g e l s and c r y s t a l l i z a t i o n products are given i n Table 1. I t can be seen t h a t the Ga/GatAl

r a t i o o f the f e r r i e r i t e product r e f l e c t s

t h a t o f the synthesis mixture. Scanning e l e c t r o n micrographs o f t h e samples are shown i n Fig.2. Sample S 1 c o n s i s t s o f elongated c r y s t a l s w i t h a l e n g t h o f up t o 20 have t h e shape o f hexagonal

prisms.

pm.

Some c r y s t a l s

I n sample S2 p l a t e l e t and r o d - t y p e

morphologies are observed. The rods reach a l e n g t h o f up t o 40 Fm. Samples S3 and S4 e x h i b i t the p l a t e l e t - t y p e morphology. I t seems t h a t t h e morphology changes from r o d - l i k e t o p l a t e l e t - l i k e when t h e g a l l i u m content i s increased.

615

s1 5

10

15

20

25

30

35

40

45

50

55

50

55

50

55

50

55

2e

5

10

15

20

25

30

35

40

45

2 e

5

10

15

20

25

30

35

40

45

2e

5

10

15

20

25

30

35

40

45

2e Fig. 1. The XRD patterns o f samples S1-S4.

616

Fig. 2 . The scanning electron micrographs o f the samples Sl-S4.

617

TABLE 1 T-atom c o mp os it i o n o f s y n t h e s i s m i x t u r e and f e r r i e r i t e p r o d u c t s Sample

S y nt h es i s q e l Ga/(GatAl )

C r y s t a l 1 i z a t i o n Droduct Ga/(GatAl ) S i / ( G a t A l ) Si/(GatAl ) ICP

ICP

Ga/U.C.

Al/U.C.

MAS NMR

s1

0

0

7.6

7.1

0

4.4

s2

0.5

0.4

6.4

5.9

2.1

3.2

s3

0.75

0.7

4.1

4.4

4.7

2.0

s4

0.9

0.9

4.1

3.4

7.4

0.8

D i r e c t evidence f o r t h e A1 and Ga i n c o r p o r a t i o n i n t h e f e r r i e r i t e s t r u c t u r e i s f o und i n t h e 27Al and 71Ga MAS-NMR s p e c t r a ,

shown i n F igs. 3

and 4,

r e s p e c t i v e l y . The 2 7 A l resonance a t 54 ppm ( F ig. 3) and t h e 71Ga MAS-NMR s i g n a l a t 152 ppm (F ig. 4 ) a r e c h a r a c t e r i s t i c o f a t e t r a h e d r a l environment. The "Si

MAS-NMR

s p e c t r a o f t h e samples a r e shown i n Fig.5.

chemical s h i f t v a l u e s o f t h e "Si spectrum o f S4,

l i n e s summarized i n T able 2.

and t h e The "Si

which c o n t a i n s t h e l a r g e s t amount o f g a l l i u m (T able

l),

c l e a r l y d i s p l a y s f i v e l i n e s c o r r e s p o n d i n g t o t h e f i v e p o s s i b l e Si(n(A1,Ga)) s t r u c t u r a l s urro u n d i n g s , w i t h 0 < n < 4. I n t h e 29Si s p e c t r a o f t h e S2 and S3 samples,

which c o n t a i n i m p o r t a n t amounts o f b o t h aluminium and g a l l i u m ,

no

d i r e c t d i s c r i m i n a t i o n between Si(nA1) and S i (nGa) s i g n a l s c o u l d be made, s i n c e no

extra

splittings

in

the

Si(n(A1,Ga))

lines

are

observed

(Fig.5).

Nev ert heles s , t h e n = 0 l i n e s have a tendency t o s h i f t t o l o w e r ppm values when h i g h e r Ga l o a d i n g s a r e achieved ( Ta b l e 2); t h e Sil(Ga,Al) g a l l i u m content

and SiZ(Ga,Al)

t h e chemical s h i f t v a l u e s o f

l i n e s , f o r example, move u p f i e l d w i t h i n c r e a s i n g

.

T h i s i n f l u e n c e o f t h e presence o f Ga on t h e 2 9 S i chemical s h i f t was a l s o observed i n o t h e r g a l l i u m s i l i c a t e s ( r e f s . 5 and 6 ) . Probably,

a homogeneous d i s t r i b u t i o n o f A1 and Ga i n t h e f e r r i e r i t e framework can be assumed. The Si/GatAl

r a t i o s o f t h e samples were c a l c u l a t e d f r o m t h e i n t e n s i t i e s o f

t h e i n d i v i d u a l S i (n(A1 ,Ga)) 1 i n e s o f t h e d e convolut ed 29Si s p e c t r a (T able 1). The r e s u l t s a r e i n agreement w i t h t h e r e s u l t s f rom ICP.

618

Fig. 3. The 2 7 A l MAS NMR spectra o f samples Sl-S4. The *’s indicate the position o f the spinning side bands.

B

52 51

Fig. 4 . The 71Ga MAS NMR spectra o f samples Sl-S4.

619

SiO(AI,Ga)

I Sil(A1,Ga)

I

Fig. 5. The "Si

MAS NMR spectra o f samples Sl-S4.

620

TABLE 2 2 9 S i chemical s h i f t s of S i (n(A1 ,Ga)) 1 i n e s i n f e r r i e r i t e . 29Si chemical s h i f t s ( m m l

Sample

Si4(A1 ,Ga) Si3(A1 ,Ga) Si2(A1 ,Ga) S i l ( A 1 ,Ga) SiO(A1 ,Ga)

s1

/

/

-99.1

-105.8

-112.4

s2

/

/

-97.4

-105.1

-112.3

s3

/

-90.6

-96.5

-105.2

-112.9

s4

-85.0

-90.5

-96.5

-104.6

-112.6

I f one assumes t h e complete i n c o r p o r a t i o n o f b o t h Ga and A1 i n t h e z e o l i t e as suggested by t h e absence o f s i g n a l s due t o o c t a h e d r a l c o o r d i n a t i o n i n t h e 71Ga and 27Al s p e c t r a ( F i g s . 3 and 4 ) , one can c a l c u l a t e t h e amount o f Ga and A1 atoms p e r u n i t c e l l f r o m a c o m b i n a t i o n o f t h e Ga/(GatAl) and S i / ( G a t A l ) r a t i o s , measured b y I C P and MAS NMR, r e s p e c t i v e l y and c o n s i d e r i n g a u n i t c e l l o f 36 T-atoms ( T a b l e 1). S4 c o n t a i n s 7.4 g a l l i u m atoms p e r u n i t c e l l . T h i s i s a h i g h e r g a l l i u m c o n t e n t compared t o t h e 4.7 g a l l i u m atoms p e r u n i t c e l l i n an a l u m i n i u m - f r e e g a l l o f e r r i e r i t e , r e p o r t e d by Sulikowski e t a l . ( r e f . 3). A l u m i n o s i l i c a t e f e r r i e r i t e s c o n t a i n t y p i c a l l y 6 o r l e s s aluminium atoms p e r u n i t c e l l ( r e f . 7 ) . It seems t h a t i n t h e presence o f g a l l i u m , t h e u n i t c e l l can c o n t a i n up t o 8 t r i v a l e n t atoms (T able 1 ) . The y i e l d o f z e o l i t e , c a l c u l a t e d as t h e w e i g h t o f t h e p r o d u c t over t h e w e i g h t o f framework,

Si02,

Ga2O3 and A1203 i n t h e s y n t h e s i s

mixture,

i s p l o t t e d against

the

Ga/GatAl f r a c t i o n i n Fi g . 6 . I t shows t h a t t h e replacement o f aluminium w i t h g a l l i u m reduces t h e p r o d u c t y i e l d c o n s i d e r a b l y . When no aluminium was p r e s e n t i n t h e s y n t h e s i s m i x t u r e (Ga/(GatAl) = l . O ) , t h e c r y s t a l l i z a t i o n o f f e r r i e r i t e f a i l e d . T h i s r e s u l t suggests t h a t t r a c e s o f A1 a r e necessary, p r o b a b l y a t t h e n u c l e a t i o n s t a ge o f t h e s y n t h e s i s , and t h a t A1 can be r e p l a c e d by Ga i n t h e c r y s t a l gro w t h process. The A1 c o n t e n t p e r u n i t c e l l i s p r o p o r t i o n a l t o t h e a b s o l u t e i n t e n s i t y o f t h e 2 7 A l resonance, as shown i n F i g u r e 7 . The 71Ga a b s o l u t e l i n e i n t e n s i t i e s do n o t show p r o p o r t i o n a l i t y

w i t h t h e Ga c o n t e n t o f t h e u n i t c e l l

Q u a n t i f i c a t i o n o f t h e g a l l i u m c o n t e n t w i t h 71Ga

(Fig.7).

MAS-NMR i s n o t p o s s i b l e

d e s p i t e t h e use o f a resonance magnetic f i e l d o f 9.4 T, a s t r o n g R F - f i e l d o f 10 mT, a s h o r t RF-pulse o f 0.6

ps

and a h i g h s p i n n i n g f requency o f 15 kHz. The

71Ga MAS NMR l i n e s a r e s t i l l b r o a d ( F i g . 4 ) and, p r o b a b l y , s t r o n g l y a f f e c t e d by

621

\

a

M

0

'2 3 0 d 0 20

.-

c1

6

N

Irl m

10

cd c1

m h

6

o 0.25

0.00

1 .oo

0.75

0.50

Ga/(Ga+Al) in initial mixture Fig. 6. The yield o f alumino-galloferrierite zeolite as function of the Ga/(GatAl) fraction in the synthesis mixtures o f samples S1-S4.

120 100 9 80

cd 60

v

n

40 20 0

0

1

2

3

4

5

6

7

8

Al or Ga / (u.c.) Fig. 7. The 27Al and 71Ga NMR line intensities versus their content per unit cell in the samples Sl-S4.

622

s p i n n i n g s i d e bands due t o t h e i n c o m p l e t e r e d u c t i o n o f t h e second o r d e r qu adru polar e f f e c t s . CONCLUSIONS The f e r r i e r i t e

structure

seems

to

be p a r t i c u l a r l y

susceptible t o

Ga

i n c o r p o r a t i o n . The l o w e s t Si/Ga r a t i o o f alumino g a l l o f e r r i e r i t e s o b t a i n e d i n t h i s work i s ca. 4, whereas t h e S i / A l r a t i o s f ound w i t h a l u m i n o s i l i c a t e f e r r i e r i t e s a r e equal t o o r l a r g e r t h a n 6.

Traces o f aluminium seem t o be

necessary i n o r d e r t o c r y s t a l l i z e f e r r i e r i t e s w i t h h i g h g a l l i u m c o n t e n t s . The g a l l i u m c o n t e n t o f alumino g a l l o f e r r e i r i t e s c o u l d n o t be q u a n t i f i e d w i t h 'lGa MAS-NMR,

even when u s i n g a s p i n n i n g f r e q u e n cy o f 15 kHz.

ACKNOWLEDGMENTS The a u t h o r s acknowledge t h e Fl e m i s h NFWO and t h e B e l g i a n M i n i s t e r y f o r Science P o l i c y f o r a r e s e a r c h g r a n t . J.A.M. and P.J.G acknowledge t h e F lemish NFWO f o r a re s e a r c h p o s i t i o n as Research A s s o c i a t e and S e n i o r Research A s s oc iat e , r e s p e c t i v e l y . REFERENCES

1 J.R. Mowry, R.F. Anderson and J.A. Johnson, O i l Gas J., 83 (1985) 128. 2

H. Kitagawa, H. Sendoda and Y. Ono, J. C a t a l . ,

3

B. S u l i k o w s k i and J. K l i n o w s k i , J. Chem. SOC., Chem. Commun., 1289 (1989).

4

P.J.

5

D.E.

6

S.

7

SOC. Jpn., 58 (1985) 52. P.A. Jacobs and J.A. Martens, Stud. S u r f . S c i . C a t a l . 33 (1987) 217.

Grobet, H.

Geerts, M. T i e l e n , J.A.

101 (1986) 12.

Martens and P.A.

Jacobs,

Stud.

S u r f . S c i . C a t a l . , 46 (1989) 721. Vaughan,

Dwyer (Eds.),

M.T.

M e l c h i o r and A.J.

Jacobson,

i n : G.D.

St uck and F.G.

I n t r a z e o l i t e Chemistry, ACS Symp. Ser. 218 (1983) 231.

Hayashi, K. Suzuki, S. Shin, K. Hayamizu and 0. Yamamoto, B u l l . Chem.


S Y N T H E 5 I S O F ARTIFICIAL %EC>LIYE-LIKF M.Ii.EPBA?iX\',

623

N1O~If:'lAIPJITE

L!.n4.C-AP.'l~f~h.(-:V, C.B.TP.GIYEV

Irlstitute of Inorganic a n d Fhysica.1 Ckemistry Gf t h e A zerbaijan S S R Academy of S c i e n c e s , 370143 Baku., Narimanov p r o s p e c t 29, USSF

suR.I MAR 1T h e p e p e r clenlsi wit11 kiyclrotl-:errv. e.1 t l cLricfi.>r mation of artificial g k s s e s into zeolite-.like silica.t€?s,systr?r~leticallS. studied.Syr.thetic mowntainite,teing a n a n a l o g u e of prirrary n r i r e r a l , w a s p r o d u c e d for t l i e first tir!ic.Ffft:(.t:- (.I vc:riijt .c I . ~c t c r s , i . e . t e r ~perclture,coricentration,g r a s s grading,nature o f s c l ver,t, solid plie.se/liquid p h a s e ratic, on p h a s e forniaticiri w a s c o x s i d e r e d . Zeolite-.like n a t u r e of sodiuri hydros.ilica.te w e s p r o v e d by radiography., t h e r m o g r a ~ i m e t r i c u l , chenlical &nd electrorir;,icro..copic methc.-ds of irivesti

-

g,i+tiCIr..

I i i T K ODUCTIOP I Due to uriique s t r u c t u r a l p r o p e r t i e s a n d wide r a n g e of c h a - a c t e r i u t i c s ze o lites a r e u s e d iri cateil)rtic, a d s o r p t i o r

p r o c e s s e s a n d in vm-ious bran-

c h e s of ratiorial econoniy. To meet gron-irig dec;ands gi cal

of nioderri t e c h r o l o -

p r o c e s s e s it i s n e c e s s a r y tG s t u d y a n d select n o v e l nELtLral Emcl

syrithetic alunmiriosilicate

s,yster,is which c a n b e u s e d for

z eo lites witk? predeterniinec!

production

of

crysteillirte s t r u c t u r e , compositior. e n d proper-

ties. In t h e F r o c e s s of z e ol i t e s s y n t h e s i s

tt?s with

natural a n d

s y n t h e t i c alur.?iriosilica-

c r) stallirie arid a ni orphous s t r u c t u r e ( t h e latter being p r e s e n t e d

by gels arid g$rsses)

were used

2.5

an iritial s t c c k . High reactivity of

g l a s s e s b ein g ar. a m orphous material arid r i e c e s c i w of reproductiorl o f rlatural p r o c e s s . e s with

a c t i ve

Farticipatiori

of n i i r . e r d s formation in labora-

tory, w ad e r e s e e . r c k e s to s t u d y thorotighly ra.w n.aterial.However,ccmplexi-

t-,

a n d uristability

of chemical c o n i p o s i t i o ~ ,i t s

clu.sioris, admixtLires iri

urivaria.Dilit4.,cryste.llic in-

natureil g l a s s e s turr-ed r e s e a r c h e s attention

to

sy n th etic glasses. A nuntber of w o r k s

has b e e n d e v o t e d

from v a r i o u s syr~tl-ieticg l a s s e s . O n e of si l icate, e.g. r h ode si t e -

hydrosi l i c a t e of

h a v e p r c d L . c e c i two syr:thetic forms-K-

to crystellizatiori of zeolites

t h e s e i s s y n t h e s i s of zeolite-like alkali a n d alkali-earth a n d Na-rhodesits.It

metetle.WE

must Ire r.otec!

that z.ccordir.g to its, s t r u c t u r e mountainite a s well a s r h o d e s i t e could b e r e g a r d e d as n:irierel of c.elhq:elite grc'up.

624 TABLE

1.

Ccmparison of minerals

N a m e of mirt e r a1

charecteristics

~

-

Chemical composition

-_____

Lattice p r r a m e t e r s , A b

a

-------

--

Space group

C

- - - - - ---

I

( H 2 0 ) 2] 4 H 2 0

23,8

6,54

7,05

Pnin2

Delheyelite K,Na F a 2 ( E i A1)801g] F, C1 2

6,56

24,6

7,lO

Pmn2

6,65

23,84

7,07

Pmri2

3 3,51

13,51

13,51 Pniri2

BaH2 F a 2 S i e O l 9 ( H 2 0 ) 8H20 14,OE

13,08

23,52 CniCnc

23,91

13,09 B m a t

&odesite

KNa[ Ca2Si8019

Hydrcdelhayelite

KF

4

C2 ( S I N ) 7039 (H 2 0 ) 44H20 2

K 2 N a 2 k a 2 S i g ) 03 9 ( O H ) 2] 4 H 2 0

Mountainte W..ecdonaldite

Moriteregia.nite K N a 2 [ Y S i80 1 9

]

;I

10K20

0 r . e c a n s e e analogy- between

14,OI

peranletere a n d chenlice.1 p r o p e r t i e s

-

of r e L - e r a l minerals and mountairiite. T h i s a n d o g y i s d u e to t h e s t r u c t u ral l i k e n e s s of t h e s a i d mir.ernls. Mirierals of t h i s eigkt-

nierrber c a v i t i e s

* 4A0 ,

g r o u p h a v e wide

filled with l a r g e ce8tior.s a n d w&er molecu-

les: K + cations a n d water rr.olecules in delhayelite, hydrodelha.yelite,m~un-

tairiite a.rid

r h o d e s i t e ; R a 2 + a n d water n i o l e c d e s in macdor:aldite. I t

found t h a t b a s i c delhayelite dimensiocal by C a -

/

s t r u t u r o l L.r,its of

ref.21

and

rhodesite

/

r e f . l / , a n d also h y d r o

rrountairiite a r e C.ct-octnhedron

r a d i c a l ( two-

was

l e v e l 1cttice) .Colurr.ns a l o n g tk.e r x i s fornlod

octe.hedr0r.s a r e c o n n e c t e d by

wollastonitr c h a i n whick i s arrar.-

g e d aroL1r.d t h e columns s o that two silica-

o x y g e n t e t r a h e d r o n s in t h e

s a i d chair1 a r e corijugated witti r i k of Cz-

o c t a . h e l r c n s andtk.e third

c o n n e c t e d with tkle adjacer?t columns. Wollastonite c h a i n s additional

diorthogbGps fornmir g two-

. Togetk.er

positiort [sic;olg]

-

c o h m r - e.nd two-

le\ el

a r e tied

is

by

lattice cef t h e chemical com-

t h e y form tt-ireedinier,sic,r.e~lstrL:cture

with

N.

zeolite-like the

cavities. Cations of Ir.etals a n d water n.olecules a r e p l ~ c e c ?in

c h a n n e l s . Water rrolecules

frame tetre.hedrons. Water

a r e conrNected Icy t-,ydroSerr h0r.d witti

p&rtially t & k e s p e r t in Ca-

atoms coordinaticr.

(fis. 3.). ME: T H 0 D S Res,u.lts of

s y s t e m a t i c kvestigatiori of

tic gle,ss with chemical

crystallization p r o c e s s of syrlthe-

composition, m a s s %: S i 0 2

71,6

; A1203

3,2 ;

625

Fig.1. Stt cictcire of R h o d e f i t e .

Fe203

0,3 ;

CaO

MgO

6,4 ;

3,6 ; N a 2 0

pres er . t p a p e r . Wk.en treetiris g l a s s mourtainitc v * a s

pt.ciduced for

tl:esis were: g l a s s NaOE ; g l a s s

/

14,4

a r e s i v e n in t h e

powder by NaOW solution s ynthetic

t h e first time. OptirrJuni conditions of syri-

fraction 1 , O O nies,h. ccncentratior,-

solutior. ratior. = 1:Fs

;

1 n solutior. of 0

t e mperature cf s y r , t h e s i s 170 C ;

crysta.llizatior time is, 2 0 dr?ys. E x p e r i a e r t s w e r e c a r r i e d out a t 60dr o p of f

0

2 C. In l ow-

25OCC with t h e terxperature 0

~ to p 90 C ) ql;artz

t e m pe ra t ure experinierlto (

g k s s h b e s witk. i nne r volume of 3 0 c m 3 w e r e u s e d . At higker tempereh i r e s "Mori" a u t o c l a v e s rr,ade of 45 M h - F T s t a i n l e s s steel of differert of 3 ('7.5, 1 9 . 6 , 13.0, 9 G cm ) w e r e used.Hydrotk~erma1crystalliza-

volumes

tior, ar.d

r ecr y s t a l l i z a t i on experimerlts

w e r e c a r r i e d out in a u t o c l a v e s ,

without building t h e t e m pe ra t ure gra di e nt (&?'=O)

and

without

s tirring

the r eactio n mixture. After finishing t he

experinNer,t

n a c e a n d water quer.ched. P r o d u c t s

e u t o c l a b e was t a k e n out of t h e furw e r e filtered, w a s h e d u p to pEI=e,

dried a.t 80 c C , horrogenized a n d further StLdied. Qualitative a n d quantitative ar.alysis of s a m p l e s w a s c a r r i e d out by powder r~ etl.od orr DEC)b.j-3,0 Irrier a n d o u ter s t a n d a r d s

diffractor. e t e r

( C.LLK d

-

radia.tion, Ni-filter).

K a C I , S i 0 2 ( q u a r t s , ) as well

tes w e r e applied. Diffractrometry w a s kV, c u r r e n t 1 4 mA, velocities 4 ar.d

c a r r i e d out

a s p u r e zeoli-

a t a n o d e b-oltage 3 2

2 grad/rr.ir. Survey w a s d o n e in t L J e

626 r a c g e of reflection a n g l e s 8 = 2 + o w a s of 0.1 .

-

70

0

. Angles m e a s u r e m e n t

accuracy

-

Thermogravimetrical a n a l y s i s of t h e s a m p l e s was c a r r i e d out in dynamic mode with t h e u s e of “(3-

Derivatograph 1500-D” of

MOM. T e m p e r a t u r e r a n g e w a s within 20

0

-

Eiurigariari firm

1000°C.

0

S u r v e y conditi0ns:heating r a t e i s 10 /min ; pa.per t r a v e l s p e e d is miri ; sensitivity of DTA, D T G a n d TG i s 500

rniV;

2,s

nim/

ceramic cubicles were

u s e d ; standa.rd i s A1203. Chemical s i l i c a t e a n a l y s i s of th.e nitial material a n d of transformation

c a r r i e d out with t h e u s e of

WBS

- 18 s r e c t r o m e t e r ( W e s t G e r m a n y ) . M e e s u r e m e n t mode: P d - ar.ode ; v o l t a g e -- -- ~-p o s u r e period i s 1 0 0 s ; sensitivity’ limit w e r e p r e p a r e d a s ftrllbws: t h e

prrJducts

niultichannel X-ray

Of

CPW

i s 2 5 kV; c u r r e n t i s 7 0 niA; ex-2 i s 1 0 . S a m p l e s for ariG1~~sis

s u b s t a n c e beir.5 a n a l y s e d w a s fLsed with

f1v.x Li B 0 ( r a t i o 3. : 1 0 ) a t 1 2 5 0 2 4 7 up to 300 m e s h a n d p r e s s u r i z e d at

0

C. Ohttiiried g l e i s s

2 0 ton / c m

2

WES

powdered

with time l a g of 1. minu-

te. t h e u s e of e l e c -

Electronmiroscopic investigation w a s c a r r i e d ol;t with tron

s c a r i n i r g m i c r o s c o p e B S - 3 0 1 ‘TESLA. T h i s vetkiod a l l c ~ w sto irives-

t i s a t e s p z c e a n d time relatioriship of microtlrysteils. It that t h e s e relationships a r e more complicated

hits

been shown

thar. t h o s e obteiir e d by ro-

entyenonl etry.

RE; SUL’I’S Diffredonietry

d a t a for synthetic rrokntainite

d a t a for i t s n a t u r a l a n a l o g u e /ref.

(Table 2 )

.?I.

TARLE: 2. Ciffrectometr)- d a t a f a r s y n t h e t i c moLnteir.ite ar.d i t s thermally t r e s t e d ( a t 370

C

) fvrr.;.

C

Mourlttrinite --d

exp

A0

-J

I

13,18 6,58

6,OO 5, 59 4,72 4,18 3,76 3,62 3,41 3,22 3,04 2,92

90 90 16 14 62 44 16 11 25 13 21 100

dexp

P.

0

J

9

11, 24 6,38 5, 7 8 4, 3 6 4, 06 3,9 2 3, 06 2, e9 2, 7 7

46

6 52 8

1c 5 21 26 16

627 2, 2, 2, 1, 1,

Mounta.inite

84

36

70

18 23 32 22

34 96 74

i s crystallized iri mor1ocliriic

p a r a m e t e r s of w.it cell: a= 13,53 A VolLme of the unit

cell i s 2 4 2 8 A

3

0

,

systec. with

t h e fcillovuing

b = 13,O.S A@, c = 13,53 A

.

0

.

To determine relative s t a t i l i t y of newly fcjrnied p h a s e s orid tc: estotr-

lish their

ste.bili@ limits influence of v a r i o u s p m a n l e t e r s of syntkiesis ori

p r o d u c t s crystetllizi’.tiori in this s y s t e m w a s studied. Influence cf temperature. To c r e a t e fo\-orc?..ble conditions for t h e transformiition of t h e ir.itial raw material into

rriouritainite a n d a t t h e sane time

to s u p p r e s s q u a r t z formation optinla1 temperature region w a s s e a r c k e d . I n spite of coniplexity of tt:e

prot,len: c a u s e d by pi.opensib of c . q s t a l l i z a t i c n

of both niir e r a l s to high-teniper&tLire conditions, coritrolliriy hydrothernial crystallizatior; we singled out wide t e m p e r a t ~ i r eregiorr of

nicmntairiite c r y s t a l s 150

-

195

0

C

. Syr:tk.esis

of riiaxirnurri yield

of nlol;r.t&inite from

higher temperatures g a v e r.0 r e s d t . TemperatLre r i s e r e s u l t e d in c h a n g e of reaction direction, witti

fornation of pec tolite. Higher teniperatu-es

bL.t low cor.centrations of solutions a r e more quarts

rather

peratures

prefers.ble fcr

ec‘ucing

than pectolite. Attempts to s y n t h e s i z e mouritiiinite s t teni-

lower than 150

0

C were unsuccessful.

Influence of concentration. A s a therrnd solution sodium bydroxide w a s

-- - __ - - - -__

- - --

-

c k o s e n . It w a s found t k t c h i w g e

CJf

ccirocc~ntrfiticn had

greatly inf1uer.c-

ed pl-lase formatior.. I n c r e s s e o f tkie solution pH prcrniotes solubility gle.ss.es, h r e a k i n g down their silicaI~itiori as.r.ccintiorl of p11i:c-ez. Q z

+

o x y g e r lattice. In 0,3 N

Gf

NaOH

SO-

F c i s crystiilliaed arid in t h e c o u r s e

of r i s i n g solu.tior, cor!cer.tra.tior. from 0 , 3 N to l,ON abovesaicl essocieticin i s su.bstituted by n,ountainite ( T a b l e 3 ) . I n c r e a s e in coficer.trettion of

NaGHcclutior, u p to 1 K l e a d s to destruction of rhodesite. F u r t h e r r i s i n g of cor.certratiori of N a O E solution fron

lr’! to 2N a n d more w l f a v o u r a t l y in-

f l u e n c e s the p r o c e s s of nlourrta.inite cry-stalliz.eticr. Mear:wkiIf? syt>thc?tic mounteinite i s b e i n s reple.ced by- pectolite. 1nfluer.ce of s o l v e n t

riatL

re. In our e x p e r i n er.ts direction of t h e crystalli-

- - - -- -- -- - - - - - - - -

za,tior p r o c e s s w a s determiried f:y ttie s.ol\.ent nature. At t h e i d e c t i c a l concentratior.s of h’a-

lization and

c a t i o n s but with ce.rlr*onate solL.tion acting, c r y s t a l -

of mountainite d e c e l e r a t e s ar.d

quertz i s o b s e r v e d , t h e latter

forniaticln of k,oth n.,c)uritairiite

o n e predomina.tiny. At low cor,centra-

628 q u a r t s i s formed, a t ccncentraticir: of 1 N asso-

tiori of P\’a2C03 solution ciation of cluartz urider

a n d r h o d e c i t e apFeara.. Mour.ta.inite i s not crq’stdlized

the action of soliunl c a r b o n a t e solution.

I r f l u e n c e of solid p h a s e /liquid p h a s e ratio. Glass/solu.tior; ratio g o v e r n s pot only cor.ipositior of mother liquor, but e k o conipcjsitic,r: a.nd structL r e of t h e products. In t h e c o u r s e cf interaction of alkali s o l u t i o n s with s i l i c a rich g l a s s ratio glass/ solution g r e a t h , i n f l u e n c e s t h e p r o c e s s of p r o d c c ts

( T a b l e 3 ). Obtained exprriniental dwta confirm obser-

crysterllizatior

vation of the p r o c e s s of silicalution

/

r i c h z e o l i t e s formation. Thus., a t

l o w so-

gJnss ratio boundery k y e r i s s a t i a t e d witL S i 0 2 a n d alkali catinrls

n e c e s s a r y for silica-rick

z e o l i t e s formatior.. At high solution / g l a s s ratio,

i. e. a t soluticri excess, s i o ,

L.

-

diffuses from t h e t o u n d a r y l a y e r of s o h .

tion to more f r e e s p c c e , a n d pec.tolite c r rkJodesite r a t h e r tk.an mountainite i s formed. TABLE 3. Ccnditions a n d crq.stallizetiori p r o d u c t s of silica-

r i c h g l a s s i r NaOH ar.d

N a 2 C 0 3 so1utior.s

---- ---

_ _ - --_

--

___ ___----------c___---_I-

i m

Ccr.centration of NaOH,N

Concentration Solid F h h s e of N a 2 C 0 ,hT liquid p h a s e ratio

--1

0,3

2 3. 4. 5. 6. 7. 8. 9. 10. 11. 3 2. 13. 14. 1 5 16. 17. 18. 19

0,3 0,3

Note:

hln Rd Qz Pc

--

-

-

1 1

1:5 1:5

mountainitc rk.odecite quartz pectolite

C

---

O,-? 0: 5

085 0,5 0,5 1 1 1 3 1 1 1 2 - 6

o

--

I _ ~

25 1:10 1:15 1:5 1:1 5 1:15 1:5 1:5 1:3 1:5 1:5 1:1 5 1:5 1:10 1:5 1:5 1:5

03

TeniFerature, F r o d L c t s

170 170 200 170 170 PO0 200 170 170 150

1eo 200

200

170 170 170 170 170 200

-

-

-

~

Mn Rd

45: Rd

+ +

Rd Mn

Rd Rd

92 Mn amorphous a n 1 CI r p k 10LI s hlrr + P c Pc Pc Pc Pc

Qz Qz -t Rd amorphous on.orplious,

629 It could b e s e e n from T a b l e 3 that solid p h a s e respor.ding

/

to 1:10 l e a d s to d i s a p p e a r a n c e of

of pectolite. At 1 N

liquid p h a s e ratio corn ountainite a n d i-..olation

concentration of NaOH solution cryste.llization of so-

diun. r h o d e s i t e i s o b s e r v e d . A s to lower r a t i o n s solid pklase/ liquid p h a s e

-

e.g. 1:3

-

s u c h proportions a r e unefficient for p h e s e formation in tk-.is

system. Thermogravimetrical investigations s h o w that dehydration of mountainite

,

i s continuous

whick c o r r e s p o n d s to

hydratior c h a r a c t e r i s t i c of zeolites. DI'A c u r v e of eristic t h r e e endo- a n d tvro exo-effects.

t h e 3--5t

synthetic

type of d e

nlom-lta.inite den;onst 0

Ecclotkrermal p e a k s a t 1 7 0

-

-

and.

320° C w e r e indticed Icy dehydration of water which got lost r e v e r s i b l y 0

at 340 C. Water losses constitute 3.2,A %. Week exotk.ermal band a t 3 7 0 ° ~ a s c r i k e d to destruction. Diffraction d a t a of t h e n e w p r o d u c t

could b e

0

a p p e a r i n g a t 370 C a r e given iri T a b l e 2. Smee.rir.g of endo-therrral 0

a t 7 2 0 OC could b e ascrik,ed to fLsior, exo-

effect a t 8 3 5 C-

peak

to produc-

tion of pseudowollastonite. Chemical conipositiori of synthetic mour.ta.ir.ite i s ( m a s s 43; CaO

-

1 4 , l l ; hIg0

-

2,61; K 2 0

-

/ref. 31. Electron microccoFic investigction h a s synthetic mountainite c o u l c

b e not

-

11,38; H 2 0- 12,8. niounteinite a r e fibrous formaticrs

3,79; N a 2 0

It i s known that c r y s t a l s of nature.1

%): Si02- 55,

only

shown that crystetls of

fibrous, but also needle-like

ar.d in the form of thin pellets. When studying ior.tior, with N H

4.

+

'

e x c h a n g e ability of synthetic mountainite in rela2+ K a n d C.a c a t i o n s singnifica.ct c h a n g e s in i n t e n s i t i e s of

reflections d = 13,18; 6,58; 3,22 A

0

were

observed.

R e v e r s i b l e chare.cter of dehydration, ion-exchange

ability, p r e s e n c e

of specific cavities, c h a r a c t e r r i s t i c for z e o l i t e s , in tkje s t r u c t u r e cor.firr.

that rr.otir.tainite b e l o c g s to s i l i c e t e s

having

z e o l i t e s piroperti es.

R E F E R E N CE S

1. H.P... H e s s e " Verfeinerung d e r s t r u c u r d e s R h o d e s i t s H K 2 C a 4 S i 1 6 O Z 8 . 1 0 H 2 0 . " Zeifur Kristallograpl-y 1 9 8 7 , V 178, N 1-2, p.99-98. 2. Rd. D. P o r f n a n , M. I. Chiragov, Noviye darmiye c mineralack S S S R , 1 9 7 9 , NO. 28, p. 172-175

.

3. J. P. C-ard, H. F. W. Taq-lor, "An irivcstigation of tvro r e w minerals: r h o d e s i t e a n d mountainite" Mireral. Was., 1957, V. 31, p. 611-623 4. R. H. Mariner, R C S u r d a n o , "Alkaliri a n d forrt1e.tic.r c.f zeolites in s a l i n e alkaline lakes". S c i e n c e , 1 9 7 0 , V . 70, p. 977-582



THE D I S T K I B U ' r ~ O N OF IRON I N Z S M - 5 TYPE

AND FERRI SI L I CATES:

IRON

63 1

CONTAINING

ZEOLITES

ACIDIC AND CATALYTIC PROPERTIES

G . VOREECK, M. RICHTER, R. FRICKE, B. PARLITZ, E. SCHREIER, K . SZULZEWSKY and E . ZIBROWIUS lnstitute of Physical Chemistry, Academy o f Sciences, 1199 Berlin, SUMMAKY

The state of iron in microporous ferrisilicates with MFI structure as well as in template free synthesized FeZSM-5 zeolites has been examined by XRD, IR, ESR, Si MAS NMR, ammonia TPD, and catalysis. The percentage of iron in the ferrisilicate framework could reliably be determined by two independent methods, ammonia TPD and Si MAS N M R . Catalytically, the Fe related acid sites are weaker than those of the parent ZSM-5 but. nevertheless, they isomerize m-xylene and convert ethylbenzene. Nonframework iron dehydrogenates ethylbenzene to styrene, where the selectivity correlates with the amount of nonframework iron. INTRODUCTION

One possibility to modify the acidic and catalytic properties of zeolites and zeolite-like metallosilicates (1) consists in varying their chemical composition at the stage of synthesis. Advantageously, this a1 lows the tailored design of active molecular sieve components via substitution of A1 or Si by several elements (e. 8 . E, Ga, Cr, V, Co, and others) ( 1 - 15). Iron containing zeolites or pure ferrisilicates with MFI structure are of special interest as catalysts for the conversion of several aromatic compounds, above a l l C 8 nlkylaromatics. Numerous investigations have been performed in order t o confirm the incorporation of iron into the molecular sieve framework and to reveal the nature of iron in both framework and nonframework positions (1,16 - 2 6 ) . Incontestably, the location and the state of iron are essentially for their acidic, catalytic and shape-selective properties. But the knowledge about the exact distribution of iron between the lattice and channels is still incomplete. Therefore, this contribution emphasizes the quantitative determination of iron throughout the matrix of aluminum free ZSM-5 type ferrisilicates. The consequences for the aciditiy and catalytic activity of the samples are discussed and the results are related to reference samples such as iron

632

containing

ZSM-5

zeolites,

where

is

aluminum

only

partially

r e p l a c e d by i r o n , a n d a p u r e ZSM-5 z e o l i t e . EXPERI MENTAL

Samale w&am.Lu?n . . . . . with ( i ) Perrlslllcates ( F e S i 1 ) . T h e A 1 f r e e f e r r i s i l i c a t e s Z S M - 5 s t r u c t u r e were s y n t h e s i z e d b y m X i n g a n a c j.d s o l u t i o n of i r o n s u l f a t e w i t h h a l f t h e v o l u m e of a s o d i u m s i l i c a t e s o l u t i o n a t room t e m p e r a t u r e . An a q u e o u s s o l u t on o f tetrapropylammonium bromi de and t h e r e m a i n d e r o f t h e d i l u t e d so d iu m s i l i c a t e s o l u t i o n were a d d e d t o t h e r e s u l t i n g g e l ( a f t e r a n a g i n g p e r i o d o f 1 h ) . T h e mixture w 3 s s t i r r e d f o r 1 h, then placed i n a t e f l o n coated

-

s t a i n l e s s s t e e l a u t o c l a v e an d k e p t u n d e r a u t o g e n o u s p r e s s u r e a t 4 4 3

K f o r 48 h . The r e s u l t i n g w h i t e o r p a l e y e l l o w s o l i d was washed a n d d r i e d a t 383 K . T h e t e m p l a t e was removed b y

filtered, calcination

a t 773 K f o r 7 h i n a i r f o l l o w e d b y a t h r e e f o l d e x c h a n g e o f Na'. by NH4+ i . o n s a t 353 K for 2 h . AAS was u s e d f o r a n a l y s i s ( T a b l e l a ) . ( i i j

E.e

. . contalnlna Z S M - 5 0 .

Ferric

a l u m i n a t e , s i l i c a s o l a n d s o d i u m h y d r o x i d e were materials f o r t h e s y n t h e s i s . S i l i c a s o l solution containing the aluminate, t h e

nitrate,

added

was

sodium

as

used to

sodium starting

a

hydroxide

diluted a nd

the

f e r r i c n i t r a t e . T h e m o l a r r a t i o s o f t h e c o m p o n e n t s were: 2-8 N a g O

*

*

(Al2O3+FezO3j * 3 0 - 9 0 S i Oz 6 0 0 - 2 , 5 0 0 HzO. The r a t i o Fe 203 : A 1 2 0 3 was v a r i e d b e t w e e n 0 . 1 a n d 0 . 8 . Th e c r y s t a l l i z a t i o n o f t h e z e o l i t e s

was p e r f o r m e d h y d r o t h e r m a l l y u n d e r a u t o g e n e o u s p r e s s u r e w i t h i n 8

-

24 h a t 4 5 3 - 4 8 3 K . T h e a d d i t i o n of s e e d c r y s t a l s ( a b o u t 3 w t . % of

was f o u n d t o b e n e c e s s a r y f o r p r o m o t i n g the c r y s t a l l i z a t i o n p r o c e s s . Ho wev er , n o o r g a n i c t e m p l a t e was n e e d e d . ferrisilicate)

The r e m o v a l o f Na+

cations

in

the

as-synthesized

products

was

a c c o m p l i s h e d b y i o n - e x c h a n g e ( T a b l e l b ) . T h e f i n a l ammonium f o r m o f t h e z e o l i t e s c o n t a i n e d less t h a n 0 . 1 w t . % N a p O . T h e d e s i g n a t i o n t h e samples is t o

be

read

as

HFeSil

for

the

H

form

of

of the

f e r r i s i l i c a t e s r e s p . HFeZSM-5 f o r t h e i r o n c o n t a i n i n g z e o l i t e s . The a n a l y t i c a l (Si0z/Fez03)z

r a t i o s are g i v e n i n p a r e n t h e s e s .

( i i i ) Reference samples.A c o m m e r c i a l

ZSM - 5 p r o d u c t

(HS 3 0 , a silicalite i m p r e g n a t e d w i t h a F e( N0 3 ) 3 s o l u t i o n t o r e a c h a n i r o n c o n t e n t o f 1

C h e m i e - AG

Bitterfeld)

i n t h e H form a s

well

w t . % a f t e r c a l c i n a t i o n ( 7 7 5 K , 2 h ) were u s e d a s lb).

as

reference

(Table

633

TABLE 1 List of samples and characterization data a) ferrisilicates a

NH3

Overall HPeSil HFeSil HFeSil HFeSil HFeSil HFeSil HFeSil HFeSil HFeSil

Htpd

(58)

91

0.44

0.25

657

130

45

(74)

97

0.45

0.25

653

129

58

(76)

97

0.47

0.26

648

126

61

(78)

84

0.45

0.31

655

104

75

(94)

98

0.40

0.25

653

132

72

(155)

91

0.29

64 5

172

89

(180)

95

0.19 -

-

-

-

(259)

86

0.14

0,12

274

93

(508)

68

0.10

0.06

520

97

b ) iron containing zeolites

Sample

T~(KP

desorbed

(

sio2) Fe203 E

HFeZSM-5 ( 1 5 2 ) HFeZSM-5 ( 1 1 8 ) HFeZSM-5 (100) HFeZSM-5 ( 0 )

(

sio2 Me203

fe203 Z A1203

1

cXRDe Z

NH3 desorbedb d Overall Htp 0.57

705

90

40

0.31 0.52

87

1.04

0.54

695

41

0.70

80

0.99

0.53

680

40

0.00

100

1.30

0.84

720

36

1.07

Tm(K)'

a: Crystallinity parameter, estimated from n-hexane adsorption, b: Desorbed ammonia in inmol/g, c: Temperature of the peak maximum (high temperature peak), d : High temperature peak, e: Crystallinity parameter derived from XRD, Subscripts X. and f: Analytical r e s p . framework values, Me: sum o f Fe and Al, %Fef: percentage of framework iron. SamDle characterization The XRD data were obtained with a HZG 4 diffractoineter by using qi-filtered Cu K, radiation. IR spectra were recorded with a Specord M 85 (Zeiss Jena) using the KBr pellet technique. ESH measurements were performed with a ZWG ERS - 220 X - band

634

s p e c t r o m e t e r a t 7 7 an d

2 9 5 K. T h e 2 9 S i HAS N M K s p e c t r a were a B r u k e r MSL 400 s p e c t r o m e t e r a t a r e s o n a n c e f r e q u e n c y o f 7 9 MHz u s i n g s i n g e - p u l s e e x c i t a t i o n w i t h high-power p r o t o n d e c o u p l i n g . NH3-TPD m e a s u r e m e n t s were c a r r i e d o u t a t n o r m a l 3 p r e s s u r e i n a f l o w r e a c t o r w i t h He a s c a r r i e r g a s ( f l o w r a t e 1 cm s - l j c o n t a i n i n g 3 Vo1.X o f NH3. T h e h e a t i n g r a t e a m o u n t e d t o 12 K measured

min-l.

with

Sample w e i g h t s of

200

mg

were

used

(mesh

Pa, T

=

size

25-60).

Details are g i v e n e l s e w h e r e ( 2 9 ) . B d s o r D t i q n and catalvsis A f t e r a c t i v a t i o n in VUCUO ( p < =

570 K , t > =

5

t h e f e r r i s i l i c a t e s were c h a r a c t e r i z e d b y i s o t h e r m a l a d s o r p t i o n d e s o r p t i o n of n-hexane (T = 293 K , saturation pressure) using

a

HFeZSM-5 > HFeSil. The iron-impregnated silicalite does not possess isomerization activity, but converts ethylbenzene at T > 675 K, yet to a minor extent (5 X at 775 K). Within the ferrisilicates series the sample HFeSil(58) with the highest Fe content shows t,he highest activity. It is, however, low in comparison to the parent ZSM-5 (especially for the ethylbenzene conversion). The lower activity may primarily be attributed to the lower concentration of framework acid sites (theoretically 3 . 2 Fe 3+ per unit cell in case of the ferrisilicates compared to approximately 4.6 A13+ per unit cell in case of ZSM-5).

-s

60

- 50 5 40

._

Ln

& 30

5 20 10

600

700

T(KI

800

600

700

800

T(K)

Fig. 7. Catalytic properties of the catalysts. (a) conversion of ethylbenzene (b) m-xylene isomerization ( A , HZSM-5 (Si02/A1203 = 4 U ) , ( 0 ) HFeZSM-5(152), ( 0 ) HFeZSM-5(100), ( ) HFeSil(58), (x) HFeSil(SU8), ( V ) iron impregnated silicalite (1 wt.X Fe). The dashed line indicates equilibrium conditions.

As regards the m-xylene isomerizatinn sample HFeSil(58) compares better to the HFeZSM-5(100) resp. to the parent HZSM-5 zeolite. However, it has t o be taken into account that the m-xylene isomerization is a reversible reaction with an equilibrium conversion of about 50 X . That is, higher conversions are only possible due to side reactions ( e .6. disproportionation,

64 1

I

10

20

40

30

m- Xylene Conversion

Fig.

Shape

8.

selectivity

isomerization of HFeZSM-5(100),

(ratio

m-xylene.

(YO1 of

p-/o-xylene)

HZSM-5

(m)

(A) H F e S i 1 ( 5 0 8 ) ,

50

(Si02/A 1203

=

for

the

401,

(0)

( X ) H F e S i 1 ( 2 5 9 ) , (+) H F e S i l ( S 8 ) .

The d a s h e d l i n e i n d i c a t e s e q u i l i b r i u m c o n d i t i o n s a t 5 7 3 K. d e a l k y l a t i o n ) . Moreover,

the

isomerization

of

comparison

to

the

ZSM-5

as

is

reflected

on

m-xylene

f e r r i s i l i c a t e s b e n e f i t s f r o m t h e low mass t r a n s f e r

the

resistances

by

the

in

low

shape

with

shape

s e l e c t i v i t y (see b e l o w ) .

. . shaee s e l e c t i v i t s During selective

the

p r e d i c t e d by m-xylene

isomerization

properties

maintain

thermodynamics.

of

m-xylene p/o-isomer

Th e

isomer

catalysts ratios ratio

c o n v e r s i o n ( 3 0 ) . I t is e v i d e n t ( F i g . 8 )

higher depends

that

sample sh o w s t h e s t r o n g e s t s h a p e s e l e c t i v i t y f o l l o w e d b y

than on

the

the

HZSM-5

HFeZSM-5.

selective e f f e c t s . decreasing iron c o n t e n t s . This c o n t r a d i c t s c u r r e n t i d e a s t h a t f e r r i s i l i c a t e s should r e v e a l h i g h e r s h a p e s e l e c t i v i t y d u e t o t h e nonframework i r o n which i n e v i t a b l y is p r e s e n t a f t e r s y n t h e s i s a nd w h i c h r e n d e r s mure d i f f i c u l t t h e mass t r a n s p o r t i n s i d e t h e p o r e s y s t e m . I n t h e p r e s e n t c a s e a l l i n v e s t i g a t e d s a m p l e s h a v e ZSM-5 s t r u c t u r e w h e r e t h e p o r e d i a m e t e r s h o u l d b e t h e same. Th e e n l a r g e m e n t o f t h e u n i t c e l l dimensions as observed f o r t h e f e r r i s i l i c a t e s should b e n o t great e n o u g h a s t o e x p l a i n t h e d i f f e r e n t s h a p e s e l e c t i v i t i e s . The c o k e d e p o s i t i o n c a n n o t b e t h e r e a s o n s i n c e more c o k e i s f o r m e d on t h e f e r r i s i l i c a t e s a s a l r e a d y i n f e r r e d f r o m v i s u a l i n s p e c t i o n . REM p h o t o g r a p h s s h o w , h o w e v e r , t h a t t h e ZSM-5 h a s t h e l a r g e s t c r y s t a l s i z e f o l l o w e d by t h e F e a n a l o g u e s . Large c r y s t a l s d o n o t o n l y The p u r e f e r r i s i l i c a t e s e x e r t o n l y weak

shape

The d a t a seem t o i n d i c a t e a s l i g h t i n c r e a s e

with

642

prolong

t h e d i f f u s i o n pathways

but

they

also

a lower enhance t h e

exhibit

p e r c e n t a g e of o u t e r s u r f a c e area. Both f a c t o r s s h o u l d

s i z e of t h e f e r r i s i l i c a t e c r y s t a l s Sample HFeSi1(508), f o r example, d e p e n d on t h e F e c o n t e n t (35). r e s e m b l e s i n i t s s h a p e t h e p a r e n t ZSM-5 but, nevertheless, the shape s e l e c t i v i t y i s s t i l l low. T h i s p o i n t s t o a f u r t h e r e f f e c t superimposing t h e i n f l u e n c e of t h e c r y s t a l s i z e . Presumably, the s h a p e s e l c t i v i . t y . Form

and

f r a m e w o r k i r o n a t l o w c o n c e n t r a t i o n i s o v e r w h e l m i n g l y b e l o c a t e d on t h e o u t e r s u r f a c e what a l l o w s t h e

approximative

establishment

of

equilibrium regarding the isomers.

DehvdruaenationQ€ethvlben,ene 7 indeDendenceQnucontentnf

nonframeworkksDecies Since t h e iron impregnated s i l i c a l i t e has

no

Fe

in

p o s i t i o n s t h e a c t i v i t y observed f o r t h e c o n v e r s i o n of originates

from

nonframework

Fe

species.

dehydrogenate ethylbenzene t o s t y r e n e ( F i g .

framework

ethylbenzene

These The

9).

b e t w e e n t h e s t y r e n e s e l e c t i v i t y an d t h e n o n f r a m e w o r k

species

relationship iron

content

is n o n l i n e a r . O b v i o u s l y , n o t o n l y t h e c o n c e n t r a t i o n o f n o n f r a m e w o r k Fe b u t

also

its

dispersity

determines

the

f o r m a t i o n . A similar r e l a t i o n s h i p between -:bvlbenzene

an d t . h e o v e r a l l

iron

extent

styrene

content

is

of

styrene

formation

reported

for

P i g . 9 . S t y r e n e s e l e c t i v i t y enhancement w i t h i n c r e a s i n g c o n t e n t

nonframework i r o n i n t h e temperatures.

(0)

673 K ,

ferrisilicate ( 0 )

773 K.

series

for

two

from the

of

reaction

643

HFeZSM-5 s e r i e s

But

(29).

contrary

to

ferrisilicates

the

s t y r e n e s e l e c t i v i t y was f o u n d t o b e c o n s i d e r a b l y

lower.

course, occurs due t o t h e A 1 r e l a t e d a c i d sites p r e v a i l i n g HFeZSM-5 s a m p l e s w h i c h ,

above

all,

lead

a

to

the

This, in

of the

dealkylation

of

ethylbenzene. B u t as a whole t h i s c a t a l y t i c e f f e c t

of

opens t h e p o s s s i b i l i t y t o prove

existence

the

nonframework of

species

Fe

species,

such

a d d i t i o n a l l y t o o t h e r c h a r a c t e r i z a t i o n t e c h n i q u e s , b y m e a ns o f

the

c a t a l y t i c dehydrogenation of ethylbenzene. CONCLUSIONS

Microporous f e r r i s i l i c a t e s , synthesized with

Si02/Fe203

o f 58 - 5 0 8 , i n c o r p o r a t e i r o n t o a p e r c e n t a g e > 80 %

ratios

at

only

low

i r o n c o n c e n t r a t i o n s an d w i t h i n c r e a s i n g a m o u n t s o f q u a r t z ( u p t o 30 X).

At

higher

iron

i n c r e a s e s u p t o 50 % .

content

part

the

of

species

nonframework

I r o n i n framework p o s i t i o n s e n l a r g e s t h e u n i t

c e l l v o l u m e and s h i f t s

the

T-0-T

vibration

frequency

to

lower

v a l u e s . Si0H:Pe B r e n s t e d s i t e s are weaker t h a n Si0H:Al

sites.

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

related

t h e c o n c e n t r a t i o n of

Brensted

sites.

directly

N onfra m e w ork

iron

dehydrogenate ethylbenzene t o s t y r e n e t h e formation t h u s be t a k e n a s Template-free

evidence

synthesized

for

the

FeZSM-5

presence zeolites

of

of

to

species

which

these

show

The

can

species.

intermediate

p r o p e r t i e s c o m p ar ed t o t h e p a r e n t ZSM-5 a nd t h e f e r r i s i l i c a t e s . T h e A 1 r e l a t e d a c i d s i t e s of

FeZSM-5

zeolites

decide

the

catalytic

p r o p e r t i e s predominantly. ACKNOW LEDGEMENT

The a u t h o r s g r a t e f u l l y ack n o wl ed g e t h e

supply

of

FeZSM-5

and

s i l i c a l i t e samples b y Dr. U . H P d i c k e ( C h e m i e AG B i t t e r f e l d ) a n d

by

Dr. B . F a h l k e ( I n s t . I n o r g . C h e m . ) , resp., a nd s u p p o r t

M.

H u n g e r ( U n i v e r s i t y of L e i p z i g , 'H

(REM),

MAS N H R ) , Mr.

J.

by

Dr.

Richter-Mendau

D r s . H . K o s s l i c k ( S y n t h e s i s ) and J . Janchen ( A d s o r p t i o n ) .

REFERENCES

1

2 3 4

5 6

R . S z o s t a k , Molecular s i e v e s : P r i n c i p l e s of S y n t h e s i s and I d e n t i f i c a t i o n (Van N o s t r a n d R e i n h o l d New Y o r k , 1989), 205. B . U n g e r , T h e s i s , TH " C a r l S c h o r l e m m e r " M e r s e b u r g , 1 9 8 8 . S . R . E l y , M . H . K l o t z . Belg. P a t . 8 8 0 8 5 8 ( 1 9 8 0 ) . M . Taramasso, G. Perego, B . N o t a r i , P r o c . 5 t h I n t e r n . Conf. Z e o l . , London 1980, 4 0 . M. T i e l e n , M . G e e l e n , P . A . J a c o b s , P r o c . I n t e r n . Symp. Z e o l . C a t a l . , S i o f o k 1 9 8 5 . 1. C . T . W . Ch u , C . D . C h a n g , J . P h y s . Chem. 89 ( 1 9 8 5 ) 1 5 6 9 .

644

7 8 Y

I0 11 12 13

14 15 lb:

17 18

19

20 21 22 23 24 25 26 27 28 29 30 31

32 33

34 35

S . tlayashi, K . Suzuki, 5. Shin, K .

H a y a m i z u . 0 . Yamamoto, B u l l .

Chem. S O C . J p n . 5 8 ( 1 9 8 5 ) 5 2 . G . P . H a n d r e c k , T . D . S m i t h , J . Chem. S O C . F a r a d a y T r a n s . 1 , 85 (1989) 3215. T . J . G r i c u s K o f k e , R . J . G o r t e , G. T . K o k o t a i l o , A p p l . C a t a l . 54 ( 1 9 8 9 ) 1 7 7 . L . M a r o s i , J . S t a b e n o w , M . S c h w a r z m a n n , DE 2 8 3 1 6 3 0 ( 1 9 8 0 ) . S. J . M i l l e r , DE 3 0 3 1 1 0 2 ( 1 9 8 1 ) . R . B . B o r a d e , A . B. H a l g a r i , T . S . R . P r a s a d a R a o , P r o c . 7 t h Nat. Symp. " A d v a n c e s i n C a t a l y s i s , S c i e n c e and Technology" ( J o h n Wiley, New York, 1985) 385. A . B . H a l g a r i , R . B . E o r a d e , T . S . R . P r a s a d a Ra o, P r o c . 2nd Indo-Soviet. Conf. Catal., I n d i a 1986, 94. L . M a r o s i , J . S t a b e n o w , M . S ch war z m a nn, DE 2 8 3 1 6 3 1 ( 1 9 8 0 ) . A . J . D a b r o w s k i , K. M o s t o w i c z , 9 . C z e r w i n s k a , M . S o l i k D a b r o w s k a , B u l l . P o l i s h Acad . S c i . , Chem. 36 ( 1 9 8 8 ) 2 4 3 . P . R a t n a s a m y , R. B . B o r a d e , 5 . S i v a s a n k e r , V . P . S h i r a l k a r , S. G . H e g d e , P r o c . I n t e r n . Symp. Z e o l . C a t a l . , S i o f o k 1 9 8 5 , 137. H . B. E o r a d e , Z e o l i t e s 7 ( 1 9 8 7 ) 3 9 8 . G. C a l i s , P. F r e n k e n , E . d e B o e r , A . S w o l f s , M . A. H e f n i , Zeolites 7 (1987) 319. T . I n u i , H . N a g a t a , 0 . Yamase, H . M a t s u d a , T . K u r o d a , M . Y o s h i k o w a , T . T a k e g u c h i , A . Mi y amoto, A p p l . C a t a l . 2 4 ( 1 9 8 6 ) 257. W . J . B a l l , J . Dwy er , A . A . G a r f o r t h , W . J . Smith, Proc. 7th I n t e r n . C o n f . Z e o l . , To k y o 1 8 8 6 , 1 3 7 . K . S z o s t a k , V . N a i r , T . L . T h o mas , J . Chem. S O C . F a r a d a y T r a n s . 1, 8 3 ( 1 9 8 7 ) 4 8 7 . A . Meagher, V. N a i r , R . S z o s t a k , Z e o l i t e s 8 ( 1 9 8 8 ) 3. L . M . Kustov, V . B . Kazansky, P . Ratnasamy, Z e o l i t e s 7 (1987) 79. G. P . H a n d r e c k , T . D . S m i t h , J . Chem. S O C . F a r a d a y T r a n s 1. 85 (1989) 3195. G . V . Kharlamov, V . N. Romanikov. V. J . Kuznetzov, V . F. A n u f r i e n k o , K i n . i R a t . 30 ( 1 9 8 9 ) 1 1 8 2 . D . H . Lin, G . Coudurier, J . C . Vedrine, Proc. 8 t h I n t e r n . Conf. Z e o l . , Amsterdam 1 9 8 9 , 1 4 3 1 . A . N . K o t a s t h a n e , V . P . S h i r a l k a r , S . G . H e gde , 5 . B . K u l k a r n i , Z e o l i t e s 6 ( 1 9 8 6 ) 253. L . M. K u s t o v , V . B . Kazansky, Proc. 3rd Indo-Soviet C o n f . C a t a l . , Baku 1 9 8 8 , 1 2 8 , 2 8 . M . R i c h t e r , R. F r i c k e , B . P a r l i t z , U . H a d i c k e , G . O hlm a nn, Z . Phys. Chemie, ( L e i p z i g ) , i n press. M . R i c h t e r , W . F i e b i g , H.-G. J e r s c h k e a i t z , G . L i s c h k e , G . ohlmann, Z e o l i t e s , 9 (1989) 238. G . E n g e l h a r d t , D . Michel, High R e s o l u t i o n Solid-state NMR of S i l i c a t e s an d Z e o l i t e s , ( W i l e y , C h i c h e s t e r . 1 9 8 7 ) . G . E n g e l h a r d t , B . F a h l k e , M . M a e g i , E . L i p p m a a . Z . P h y s . Ch m. ( L e i p z i g ) , 266 ( 1 9 8 5 ) 239. D. F r e u d e , M . H u n g e r , H . P f e i f e r , Z . P h y s . Chem. (NF) 1 5 2 ( 1 9 8 7 ) 171. A. R. Halgeri, T. S. R. P r a s a d a Ra o, P r o c . I n t e r n . Symp. Acid-Base C a t a l . , Sapporo 1988, 319 J . Richter-Mendau, M. R i c h t e r , G. V o r b e c k e t a l . , t o be published.

645


ZEOLITE ZSM-57: SYNTHESIS, CHARACTERIZATION AND SHAPE SELECTIVE PROPERTIES S.ERNST and J. WEITKAMP Institute of Chemical Technology I, University of Stuttgart, Pfaffenwaldring 55, D-7000 Stuttgart 80 (Federal Republic of Germany)

SUMMARY Zeolite ZSM-57 can be synthesized from sodium-containing alumosilicate els using hexaethyl-C5-diquatas organic template and silica sol as silica source. ZSM-57 can e readily distinguished from the structurally closely related zeolites with the FER framework topology by its X-ray powder attern and mid-infrared spectrum. From thermogravimetric analysis of as-synthesized ZSM-!7 it follows that a plying an air calcination at 550 "C for several hours is sufficient to remove the organic temp ate from the pores of the zeolite. The results of two selected test reactions for probing the effective pore width of zeolites, viz. ethylbenzene dis ro ortionation and n-decane isomerization su est that the pore width is lar er for ZSb-f7 as compared to ZSM-35 (ferrierite) and 8 M - 5 . This is in agreement wit8; their crystallographic structures and offers new opportunities in shape selective catalysis and molecular sieve separation.

%

P

INTRODUCTION ZSM-57 is a medium pore, high silica zeolite with a structure very similar to ferrierite (refs. 1,2). Both zeolites possess channel systems consisting of intersecting linear eightmembered and ten-membered ring pores. However, the ZSM-57 framework contains fourmembered rings which are not present in ferrierite (refs. 1,2). Interestingly, the structure of ZSM-57 appeared in a systematic derivation of hypothetical structures based on the ferrierite net (ref. 3). Although the apertures of the eight-ring channels in ZSM-57 and ferrierite are of a comparable size (0.33 x 0.48 nm versus 0.35 x 0.48 nm), the ten-ring channels are significantly larger in the former zeolite (0.51 x 0.58 nm versus 0.42 x 0.54 nm) (refs. 1,2), and they are even slightly larger than the sinusoidal (0.51 x 0.54 nm) and the linear (0.54 x 0.56 nm) channels in zeolite Z S M J (ref. 4). Hence, from their crystallographic structures differences in the shape selective properties of these zeolites can be expected. The hydrothermal synthesis of zeolite ZSM-57 was first described by Valyocsik et al. (ref. 5 ) from sodium containing alumosilicate gels using N,N,N,N,N,N - hexaethylpentamethylenediammoniumdibromide (hexaethyl-C5-diquat) as organic template. Here we report on our attempts to synthesize zeolite ZSM-57, and on its characterization by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), thermogravimetry (TGA), midinfrared- (IR-) spectroscopy and selected catalytic test reactions. For comparison, data obtained with a material of the FER structure type, viz. ZSM-35, are also included.

-

646

EXPERIMENTAL The template required for the synthesis of zeolite ZSM-57 was prepared by refluxing 1J-dibromopentane and a surplus of triethylamine in ethanol as solvent for several hours (ref. 5). After cooling the mixture with an ice bath, a white solid precipitated which was recovered by filtration, washed with diethylether und then used for the synthesis experiments without further purification. As silica sources, either sodium waterglass (Merck; 28.5 wt.-% Si02, 8.8 wt.-% Na20, ca. 62.7 wt.-% H20) or colloidal silica sol (Ludox HS-40, DuPont; 40 wt.-% Si02 in water) were used. All synthesis experiments were conducted at 160 "C and the crystallization time was restricted to 6 days. After this time the autoclaves were quenched in cold water and the solid products recovered by filtration, washed with distilled water and dried at 120 "C. Characterization of the as-synthesized materials occurred by X-ray powder diffraction (Cub-radiation; 40 kV, 30 mA) and IR-spectroscopy in the region of the lattice vibrations using the KBr pellet technique. The product of an optimized synthesis was further subjected to scanning electron microscopy and thermogravimetric analysis. HZSM-57 was obtained by calcining the as-synthesized material for 16 hours at 540 "C in air, followed by an ion exchange with a 1 n aqueous solution of NH&l and a second air calcination for 16 hours at 350 "C. The bifunctional form (HZSM-57 loaded with 0.27 wt.-% of palladium) was prepared via an optimized procedure which has been described previously (ref. 6). For a characterization of the shape selective properties of ZSM-57, two established test reactions, viz. the disproportionation of ethylbenzene (ref. 7) and the bifunctional conversion of n-decane (ref. 8) were selected. Both reactions were performed in a flow-type apparatus with fixed bed reactor under atmospheric pressure. The partial pressures of the feed hydrocarbons ethylbenzene (E-Bz) or n-decane (n-De) amounted to pHC= 1.3 kPa. Nitrogen or hydrogen was used as the carrier gas with ethylbenzene or n-decane as feed, respectively. Product analysis was achieved by automatic sampling and on-line analysis via temperature programmed capillary glc. For a comparison of zeolite ZSM-57 with materials of the ferrierite structure, zeolite ZSM-35 was synthesized according to a published method using 1,Zdiaminoethane as template (ref. 9). RESULTS AND DISCUSSION In the first synthesis experiments, sodium waterglass was used as the silica source. The synthesis gel was prepared by subsequently adding to the sodium waterglass, aqueous solutions of the alumina source (usually Al(NO3)3 * 9H20, in some cases Al2(SO& * 18 H20) and the template, and concentrated (98 wt.-%) sulfuric acid. The gel composition was systematically varied within the following molar ratios (calculated as described in ref. 10): Si02/Al,03 = 30 to 120, H20/Si02 = 20 to 60, Na+/Si02 = 0.6 to 0.8, OH-/Si02 = 0.1 to 0.3 and R/Si02 = 0.05 to 0.2. No ZSM-57 at all crystallized from these gel compositions under our synthesis conditions. Instead, in some cases a not yet identified crystalline component, which reversibly adsorbs/desorbs ca. 7 wt.-% of water, was obtained. Even if seeds of uncalcined ZSM-35 were added to the gel, the system could not be forced to produce

647

a ferrierite material or the structurally related zeolite ZSM-57. However, if sodium waterglass is replaced by colloidal silica sol as the silica source, pure ZSM-57 was obtained after 6 days at 160 "Cfrom a gel with the composition: Si02/A1203 = 60, H20/Si02 = 40, Na+/Si02 = 0.6,OH-/Si02 = 0.1 and R/Si02= 0.1. Note that in this case no sulfuric acid was required to adjust the pH, instead a concentrated aqueous solution of NaOH was added. This example demonstrates that, like in numerous other zeolite syntheses, the nature (reactivity) of the silica source plays an important role in directing the structure of the crystallizing material. The X-ray powder pattern of as-synthesized zeolite ZSM-57 is shown in Figure 1 (upper part). The agreement of line positions and relative intensities with literature data (ref. 5) is very good. However, ZSM-57 and ZSM-35 can be readily distinguished on the basis of their X-ray powder patterns: The most significant differences are the presence of an additional reflexion of zeolite ZSM-57 around 7.5 degrees 2 8 and the absence of a peak of medium I

I

I

I

I

I

I

I

ZSM-57

>

k v)

5

10

15

20

25

30

35

40

45

I

I

I

I

I

I

I

I

10

15

20

30

35

40

45

50

Z

w IZ

-

5

25

50

ANGLE 28 , d e g Fig. 1. X-ray powder patterns of as-synthesized zeolites ZSM-57 and ZSM-35 (Cubradiation; 40 kV, 30 mA).

648

intensity at ca. 14 degrees 2 8 . Indeed, it was stated in the original patent (ref. 5) that due to the latter difference, ZSM-57 can be "readily distinguished from ferrierite-type zeolites, such as ZSM-35". Mid-infrared spectra of ZSM-57 and ZSM-35 are depicted in Figure 2. The pattern for ZSM-35 essentially agrees with those published in the literature (refs. 11,12). The pattern for ZSM-57 is reported here for the first time. Both zeolites have two characteristic absorption bands around 1230 cm-1. These two bands can be assigned to the vibrations of five-membered oxygen rings (refs. 12,13), which is in agreement with the crystallographic structures of both materials. Slight shifts in the frequency of each IR-band may be explained by the unique framework structure of every zeolite (ref. 14). One significant difference between both zeolites appears in the region around 600 cm-1, which is known to be sensitive to structural changes (ref. 15). Whereas ZSM-35 shows a single band in this range, ZSM-57 possesses a doublet. This additional band may be tentatively assigned to the presence of four-membered rings in the structure of the latter zeolite (ref. 1,2) which are absent in the ferrierite framework. Hence, in addition to X-ray powder diffraction, IR-spectroscopy also offers a means to distinguish between both zeolites. 60

55 50 45 40 35 30

25 20 15 10 5 1400

1200

1000

800

600

1200

1000

800

600

40 0

60 55 50 45 40 35 30

25 1400

WAVE NUMBER , c m

-1

Fig. 2. Mid-infrared spectra of ZSM-57 and ZSM-35.

400

649

Fig. 3. shows a scanning electron micrograph of a typical preparation of zeolite ZSM-57. The sample consists of small platelet-like crystallites with a diameter of ca. 0.5 to 1 p m and a thickness of ca. 0.1 to 0.2pm. Frequently, these platelets are intergrown.

Fig. 3. Scanning electron micrograph of zeolite ZSM-57. The white scale bar corresponds to 10pm. 100

M

95

I--

I

c,

LLJ

B

90

85

80

'

0

70

140 210 280 350 420 490 5 6 0 630 7 0 0 770

Temperature,

O C

Fig. 4. Weight-loss of as-synthesized ZSM-57 upon heating from room temperature to 770 "C (rate: 5 K/min) in an air flow ($air = 50 cm3/min). Full line: Sample weight in dependence of temperature, dotted line: first derivate of that curve.

650

As-synthesized ZSM-57 was further characterized by thermogravimetric analysis in order to obtain guidelines for the removal of the organic template occluded in the pores during synthesis. For this purpose, the sample was heated in a flow of dry air (Vab = 50 cm3/min) from room temperature to ca. 770 "C with a heating rate of 5 K/min. The weight-loss curve and its first derivative are plotted in Fig. 4. The main weight-loss occurs between temperatures of 350 "C and 550 "C and amounts to ca. 11 wt.-%. It can be attributed to the desorption of water and the combustion of the organic template occluded inside the pores. There is a further weight-loss corresponding to ca. 3 wt.-% at temperatures above 550 "C which can be tentatively assigned to dehydroxylation of Bronsted-acid sites at these high temperatures. However, combustion of an additional, more strongly bound template species may also be envisaged. As a whole, it follows from the TGA-experiments that an air calcination at ca. 550 "C for several hours should be sufficient to remove the organic material from the pores. The catalytic and shape selective properties of HZSM-57 were characterized by the disproportionation of ethylbenzene to benzene and diethylbenzenes. This test reaction was initially proposed by Karge et al. (refs. 16,17) to characterize the Bronsted-acidity (number and/or strength of acid sites) in zeolites. Later the method was shown to be also useful for probing the shape selective properties of zeolites (ref. 7). From the time on stream behaviour of the catalyst four criteria can be derived which enable to characterize the effective pore width of the investigated material: (i) the presence or absence of an induction period, (ii) the ratio of yields of diethylbenzenes and benzene, (iii) the selectivities for ortho-, meta- and para-diethylbenzene and, (iv) the rate of catalyst deactivation. The results with HZSM-57 are plotted in Figure 5. There is no induction period, viz. conversion of ethylbenzene ( X E - ~ ~ ) decreases continuously with time on stream, and the ratio of yields of diethylbenzenes (YDE-BJ and benzene (YBz)amounts to ca. 0.77. Both features are typical for medium pore (ten-membered ring) zeolites (ref. 7). By contrast, the selectivities for the three diethylbenzene isomers resemble more closely to what could be expected with a large pore zeolite. However, there may be a marked contribution of the non-shape selective outer surface of the small zeolite crystallites of the sample used in this case. Again typical for medium pore zeolites is the increase of the selectivity for the slim p-diethylbenzene at the expense of the bulkier m- and o-diethylbenzenes with time on stream and hence coke content. This effect has been referred to as "coke selectivation" in the patent literature (ref. 18). As a whole HZSM-57 behaves in the disproportionation of ethylbenzene like a medium pore zeolite. This is in agreement with its proposed framework structure. For a more accurate ranking of ZSM-57 among other members of the group of ten-membered ring zeolites, the Modified Constraint Index (CI*) of 0.27 Pd/HZSM-57 was determined. It is defined as the ratio of selectivities for the formation of 2-methylnonane and 5-methylnonane in the isomerization of n-decane at an isomer yield of 5 Yo (ref. 8). A Modified Constraint Index of 4.3 was determined for ZSM-57. This is a typical value for a medium pore zeolite. As compared to the corresponding CI*-values for ZSM-5 and ZSM-35 (6.8 and 8.1, respectively,

651

M

L

i n

10

-1

W -

>.

75

12

-

T W/FE-,z

= 250 "C = 200 gh/mol= 0.2 g

8

t

IY

50

0 X

6

Z

0 -

cn of W >

25

4

2

0

2

4

6

8

1

0

0

2

4

6

8

1

0

TIME ON STREAM, h Fig. 5. Disproportionation of ethylbenzene in HZSM-57; P E - B= ~ 1.3 kPa, PN2 = 100 @a. ref. 19), this indicates a larger effective pore width of ZSM-57 compared to ZSM-5 or ZSM-35, which is in good agreement with the structures of these zeolites. CONCLUSIONS Provided that the right silica source is used, ZSM-57 can be readily synthesized with the hexaethyl-C5-diquat cation as organic template. It can be readily distinguished from the structurally very closely related zeolites with the FER structure-type, e. g. ZSM-35, on the basis of their X-ray powder patterns and mid-infrared spectra. Thermogravimetric analysis revealed that, like with many ofther high-silica zeolites, the template occluded inside the pores during synthesis can be readily burned off by air calcination for several hours at 550 "C. According to the results of selected catalytic test reactions the effective pore width of zeolite ZSM-57 is larger than that of ZSM-35 and ZSM-5. Hence, this new high-silica zeolite offers the opportunity to close the gap in the effective pore widths between ZSMJ and the twelvemembered ring zeolite with the most restricted pore system, ZSM-12. This probably offers new possibilities for shape selective catalysis in zeolites and molecular sieve separations. ACKNOWLEDGEMENTS The authors gratefully acknowledge financial support by Deutsche Forschungsgemeinschaft, Fonds der Chemischen Industrie and Max Buchner-Forschungsstiftung.

652

REFERENCES 1 J.L. Schlenker, J.B. Higgins and E.W. Valyocsik, in: J.C. Jansen, L. Moscou and M.F.M. Post (Eds.), Zeolites for the Nineties; Recent Research Reports, 8th Intern. Zeolite Conference, Amsterdam, July 10-14, 1989, pp. 287-288. 2 J.L. Schlenker, J.B. Hi ins and E.W. Valyocsik, Zeolites 10 (1990) 293-296. 3 R. Gramlich-Meier, Z%istallographie 177 (1986) 237-245 4 D.H. Olson, G.T. Kokotailo, S.L. Lawton and W.M. Meier,J. Phys. Chem. 85 (1981) 2238-2243. 5 E.W. Val ocsik and N.M. Page, Europ. Patent Appl. 174 121, Mar. 12, 1986, assigned to

Mobil OirCom. 6 J. Weitkamp, %’.Gerhardt and P.A. Jacobs, in: Proc. Intern. Symp. on Zeolite Catalysis, Si6fok, Hungary, May 13-16, 1985, pp. 261-270. 7 J. Weitkamp, S. Ernst, P.A. Jacobs and H.G. Karge, Erdol, Kohle-Erdgas-Petrochem. 39 (1986) 13-18. 8 J.A. Martens, M. Tielen, P.A. Jacobs and J. Weitkamp; Zeolites 4 (1984) 98-107. 9 P.A. Jacobs and J.A. Martens, Synthesis of Hi h-Silica Aluminosilicate Zeolites, Elsevier Science Publishers, Amsterdam, Oxford, New$ork, Tokyo, 1987, p. 8. 10 L.D. Rollmann and E.W. Valvocsik. Inoreanic Svnthesis 22 f 1982) 61-68. 11 Reference 9 12 K. Suzuki, $ . k ~ ~ ~ m m S. iShin, , K. Fujisawa, H. Watanabe, K. Saito and K. Noguchi, Zeolites 6 (19861 290-298. 13 G. Coudurier, C. Nacchache and J.C. Vedrine, J. Chem. SOC.,Chem. Commun. (1982) 1413-1415. 14 J.C. Jansen, F.J. van der Gaag and H. van Bekkum, Zeolites 4 (1984) 369-372. 15 E.M. Flanigen, in: J.A. Rabo (Ed.), Zeolite Chemistry and Catalysis; ACS Monograph 171, American Chemical Society, Washington, D.C., 1976, pp. 80-1 17. 16 H.G. Karge, J. Ladebeck, Z. Sarbak and K. Hatada, Zeolites 2 (1982) 94-102. 17 H.G. Karge, K. Hatada, Y. Zhang and R. Fiedorow, Zeolites 3 (1983) 13-21. 18 R.M. Dessau, US Patent 4 444 986, Apr. 24, 1984, assigned to Mobil Oil Corp. 19 P.A. Jacobs and J.A. Martens, in: Y. Murakami, A. Iijima and J.W. Ward (Eds.), New Developments in Zeolite Science and Technology; Proc. 7th Intern. Zeolite Conference, Kodansha/ Elsevier, Tokyo/Amsterdam, 1986, pp. 23-32.


653

NEW DATA ON THE STRUCTURE AND PROPERTIES OF ACIDIC SITES IN HZSM-5 ZEOLITES: IR-SPECTROSCOPIC STUDIES AND NONEMPIRICAL QUANTUM CHEMICAL CALCULATIONS

1.N.SENCHENYA and V.Yu.BOROVKOV N.D.Zelinsky Institute of Organic Chemistry, Academy of Sciences of the USSR, Leninsky prospect,47. Moscow B-334 117913 (USSR)

SUMMARY The quantum chemical analysis of the influence of geometry on properties of zeolite acidic OH-groups and on the strength of the A1-0 bond in SiOAl bridges is given. It is shown that bridge hydroxyls possessing high values of SiOAl angle (-180O) are able to form a strained hydrogen bond with the neighboring lattice oxygen atom attached to aluminum resulting in the appearance of HZSM-5 zeolites of the broad absorption band at 3250-3300 cm-l in IR spectra The increase of SiOAl angle in bridged hydroxyl and alkoxide leads to continuous weakening of A1-0 bond. Therefore in the presence of basic molecules the A1-0 bond in the straitened SiOAl bridges of alkoxides could be broken with the formation of trigonal A1 cation coordinated with the base being a strong Lewis acid. Such unusual manifestation of Lewis acidity of alkoxylated HZSM-5 zeolite is proved spectroscopically. The new hypothetical mechanism of the methanol conversion with the participation of such Lewis acidic sites is proposed.

.

INTRODUCTION Decationated ZSM-5 zeolites are active in the production of high quality gasoline from non-oil row materials (methanol, light olefins, paraffins etc). From the scientific point of view they are convenient model objects for the study of the influence of different structural factors on properties of their acidic sites. Indeed the structure of the lattice and pores for these catalyst are well established by X-ray analysis (ref.1). The high Si/Al ratio in HZSM-5 zeolites provides rather homogeneous chemical surrounding of their active sites (ref.2). Moreover among other zeolites they have a higher thermal stability and practically do not undergo dealumination during thermovacuum treatment. The present report deals with the theoretical analysis of two novel phenomena detected recently by ourselves using IR spectroscopy: 1. The existence in HZSM-5 zeolites of the considerable amount of acidic OH-

groups hydrogen bonded with the basic oxygen atom of the lattice (ref.3). 2. An unusual manifestation of Lewis acidity in the presence of adsorbed

sic molecules for zeolites in which a fraction of OH-groups

ba-

is substituted by

alkoxide fragments (ref.4). In addition, on the basis of considered theoretical and experimental data the new mechanisms of acid catalyzed reactions over HZSM-5 zeolites are discussed.

654

METHODS IR spectra of powdered samples were measured in diffuse reflected light as described in ref.5. Before study HZSM-5 zeolites (Si/Al = 17 and 351 were pretreated in vacuum for 2-3 h at 50OoC. The partial substitution of groups of zeolites for alkoxide fragments was performed by small quantities of propene on the zeolites at 300'K.

acidic

OH-

chemisorption of

MAS 13C NMR

spectra were

measured using CXP-300 "Brucker" spectrometer. Quantum chemical calculations were carried out using nonempirical SCF method with a "Gaussian-80''program (ref.6). Five different basis sets ranged from mi** nimal STO-3G to 6-31G were used (ref.71. Zeolite lattice was simulated by clu-

(I),

sters of following composition: H3A10HSiH3

(OH)3A10HSi(OH)3

and

(11)

OH(H)A1(OH)20Si(OH)ZOHAl~OH)3 (1111, (OH)3SiOSi(OH)20HA1(OH)3 (IY). Calculations for cluster 11-IY were made only with STO-3G basis set. All

calculations were

carried out with complete geometry optimization using gradient technique. RESULTS AND DISCUSSION

The literature data suggest that acidic OH-groups of HZSM-5 zeolites are nonhomogeneous (ref.8-9). Thus the substitution of only 20 % of acidic hydroxyls for methoxide fragments by zeolite treatment with methanol vapor is accompanied by the complete suppression of zeolite activity

in ethylene oligomerisation

(ref.8). At the same time the remained 80 % of OH-groups are "free" as they are still able to adsorbe benzene molecules with the larger kinetic diameter than that of ethylene. The detailed analysis of the band shape in IR spectra of

aci-

dic OH-groups perturbed by hydrogen bonding with adsorbed cyclohexane and benzene molecules showed the existence at least of 5 types of hydroxyls with different acidic strength in HZSM-5 zeolites. It was also found (ref. 9 ) that poisoning no more than 10

%

of the sites (counted as lattice aluminum ions) was

suf-

ficient to eliminate the activity of zeolites. Quantum chemical interpretation of the broad IR band at 3250-3300 cm-' The typical spectrum of OH-groups measured at

room

temperature in diffuse reflected

light is shown in Fig.1. It consists of two narrow lines at 3740 and

3610 cm-l

corresponding to terminal silanols and isolated acidic hydroxyls respectively. Additionally the broad absorption band with the maximum at

3250-3300 cm-l

present. This band is always detected in the spectra of HZSM-5 zeolites but

is the

position of its maximum could be changed a little for the samples of different origin. The broad absorption band displays an unusual behavior. Under cooling of

the

sample its maximum is shifted to higher wave numbers as in complexes with hydrogen bond. Adsorption of weakly basic molecules (C -C paraffins, CC1 , cyclohe-

655

xane, C02, etc) on zeolite leads to vanishing of both 3610 and 3250-3300 cm-l bands and to the development of a single band at 3480-3530 cm-' characteristic of acidic OH-groups forming H-complexes with these adsorbed molecules. The inteis nsity of the broad 3250-3300 cm-l band in the IR spectrum measured at 500'C greatly reduced but can be completely restored after sample cooled to room temperature. On the basis of these observations this band was assigned in ref.3 to two different states of the same bridged hydroxyls such as isolated OH-groups and those forming the strained hydrogen bond with the nearest basic oxygen atom attached to aluminum.

, 3610 Fig.1. IR diffuse reflectance spectrum of HZSM-5 zeolite (Si/A1=35) pretreated in vacuum at 70 K for 4 h.

I

2400

I

2800

1

3200

I

3600

v, cm-1

We tried to elucidate theoretically what type of hydroxyls is able to form such hydrogen bond. It is known that zeolites of higher protic acidity have a range of T-0-T bond angles higher than those of lower protic acidity. For instance, in ZSM-5 these values are changed from 133 to 177' (ref.1). and those in mordenites are varied in the range of 143-180' (ref.10). At the same time in Y zeolites the T-0-T angle variation is much smaller, from 137 to 143'.

It is reasonable to propose that the increase of SiOAl angle in the bridged hydroxyl will promote the formation of strained hydrogen bonding of OH-group with the nearest basic lattice oxygen atom due to the decrease of the distance between H and 0 atoms. To verify this assumption we performed corresponding calculations for clusters simulating the lattice fragments with different SiOAl angle values in the bridged hydroxyl. The obtained results demonstrate that the straighten SiOAl angle leads to the significant elongation of the T-0 and 0-H bonds as well as to the increase of the OH-group acidity (ref.11).

Fig.2 shown

the geometry of cluster I 1 with the SiOAl bond angle close to 180'.

There is al-

so the noticeable flattening of the A103 fragment. As a result the distance between the proton of the bridged OH-group and the basic oxygen atom in such clus-

656

ter becomes equal to 1.56

1 which is characteristic of a strong hydrogen bond.

The incline of the OH-group towards basic site equals to 12'. Thus the broad 3250-3300 cm-' band in I R spectra of HZSM-5 zeolites indeed could be assigned to the bridged hydroxyls with the

large SiOAl angle which to

forms rather strong and strained hydrogen bond with the oxygen atom attached

aluminurn.

Fig.2

Completely optimized geometry of cluster (LSiOA1 = 178O).

I1

The main difference in the catalytic action of HZSM-5 and other zeolites is usually explained by the higher protic acidity of the pentasil. Taking into account that the acidic strength of the bridged hydroxyls should rise with

the

increase of SiOAl angle we can assume that the hydroxyls characterized by

the

broad 3250-3300 cm-l I R band would have the highest

acidity.

worthwhile to pay special attention to the analysis of broad

It is therefore I R bands of OH-

groups of high silica zeolites. Lewis acidity of HZSM-5 zeolites The nature of Lewis acidic sites (L-sites) and their role in catalysis by

zeolites is widely discussed in the

literature

(ref.12). Now the existence of several types of Lewis sites well established. Among them there are the extralattice aluminum containing species and the lattice trigonal aluminum or silicon cations appeared as a result of zeolite dehydroxylation at high pretreatment temperatures. In our opinion these sites, however, could be considered as the active sites in the catalysis by

zeolites with

caution. The reasons for this are as follows: 1. HZSM-5 zeolites hardly undergo dealumination during their activation under conventional conditions and therefore the concentration of extralattice aluminum is usually small. Moreover these sites are believed to participate in the coke

formation which is not characteristic of HZSM-5 zeolites.

2. The formation of lattice L-sites are known to proceed at temperatures above 5OO0C (ref.12). So their appearance under usual

activation conditions also

has a low probability. Moreover during catalytic reactions these L-sites could be easily poisoned by moisture traces of the feedstock.

657

In this connection we would discuss the idea about two different ways of the interaction between basic molecules and the acidic OH-groups in zeolites proposed by Uytterhoeven et a1 in 1965 (ref.13). The first one is the usual reaction of the base protonation: HB+ H

The second route is associated with the break of the Al”’0 bond in the Si-0-A1 bridge accompanied by the formation of complex between basic molecule and the trigonal A1 cation being the strong Lewis acid:

H

H

To estimate the probabilities of these two reaction paths we performed the quantum chemical calculations of the A1-0 bond energies in model clusters I-IY. These values were estimated as the energy of the reaction H

which is equal to the difference between total energy of the initial cluster and / the sum of total energies of 3i-OH and A ~ Kseparated fragments having equilibrium geometries. The energies of complete A1-0 bond break for cluster I calculated with different basis sets are listed in Table 1. TABLE 1 Basis set dependence of A1-0 bond energy (AE, kJ/mole) for cluster I 9

Basis set

STO-3G

3-21G

6-31G

MF’2/6-31G

AE

176.5

156.5

79.5

91.0 9

According to the estimation at the MF’2/6-31G level the energy of Si-0 and 0H bonds breaking in the molecule H3SiOH which respectively correspond to enthalpies of reactions H3SiOH - H3SiO’ + H’ and H3SiOH - H3Si‘ + HO’ is approximately the same and equal to 500-550 kJ/mole. Therefore the strength of the A1-0 bond in the SiOAl bridge is much smaller than that of the Si-0 and 0-H bonds.

It is clear that minimal cluster model I is only a very primitive model of the zeolite lattice. Therefore we also calculated the BE values of the A1-0 bond in more extended clusters 11-IY. The transition from cluster I to I 1 is accompanied by the decrease of the A1-0 bond energy from 176.5 to

136.5 kJ/mole.

The

658

substitution of one H atom in cluster I1 for

real silicon-oxygen tetrahedron

does not lead to significant changes in cluster geometry whereas similar substitution of the same H atom for aluminum-oxygen tetrahedron is accompanied by noticeable shortening of the A1-0 bond. This evidences the increase of the A1-0 bond strength with the decrease of the Si/Al ratio in the zeolite lattice. The direct calculation indicates that the A1-0 bond strength in cluster I 1 1

is the

same as in cluster 11. The existence of aluminum-oxygen tetrahedron in the neighborhood of bridged hydroxyl (cluster I Y ) increases the A1-0

bond

strength

by the factor of 1 . 5 (197 kJ/mole). Quantum chemical analysis (clusters I and 1 1 ) of the influence of zeolite lattice geometry and substitution of the acidic proton for sodium cation or methoxy fragment on the A1-0 bond strength led us to the following conclusions: 1.

The change of the SiOAl angle from its equilibrium value to -180’ leads to

a significant decrease of the A1-0 bond strength and to the flattening of

Al-

atom surrounding. 2. The substitution of the bridged proton for Na’ is accompanied by

shorte-

ning of the Si-0 and A1-0 bonds along with sharp strengthening of the A1-0 bond as compared to that in the cluster containing bridged hydroxyl. 3. The energies of the A1-0 bond in the

methoxylated cluster I

calculated

with STO-3C and 3-21G basis sets are noticeable lower than those in the cluster containing OH-group, by 20 and 50 kJ/mole, respectively. On the contrary the decrease of the A1-0 bond energy in the methoxylated cluster I 1 is much smaller

(*

5 kJ/mole) due to formation of the intermolecular hydrogen bond between the hyd-

rogen atom of the methoxy group and the oxygen atom attached to aluminum. The weakness of the Al-0 bond strength in high-silica zeolites could be manifested in the course of chemical reactions as well as during interaction of

the

acidic hydroxyls with adsorbed basic molecules. Let us demonstrate this using the interaction of the bridged OH-group with the ammonia molecule as an example. The nonempirical STO-3G calculations indicate that the interaction of ammonia with the SiOAl bridge, which leads to the breaking of the A1-0 bond

(scheme 2 )

proposed by Uytterhoeven is allowed energetically. Nevertheless the ammonia protonation (scheme 1) which in practice usually takes place is energetically more favorable. There is, however, an opportunity to switch off the protonation channel by substitution of the acidic proton for an alkoxy fragment. As was mentioned above such a substitution also results in weakening of the Al-0 bond. Therefore the interaction of the NH3 molecule with the alkoxy group by the route

R

R 0

>Si’ \AIL \

0 + B

-+ >Si’

is even more probable.

B’.’Alf

(4)

659

IR spectroscopic studies of the basic molecule adsorption (NH3. CD3CN) on HZSM-5 zeolites support this assumption (ref.4). Fig. 3 displays the IR spectra of NH3 and CD CN molecules adsorbed both on 3 fresh HZSM-5 zeolite and on the sample with propene preliminary chemisorbed at 300 K. There are only two bands at 1460 and 2295 cm-l in IR spectra of the initial H-form which correspond respectively to bending 6N-H and stretching v

CeN vibrations of ammonium ions and CD3CN molecules coordinated with the acidic OHgroups. Additional bands at 1610 and 2370 cm-' appear in the zeolites containing chemisorbed C3H6. They are characteristic of ammonia (aNH = 1610 cm-'1 and deuteroacetonitrile ( uC=N = 2370 cm-'1 molecules adsorbed on L-sites.

I

2400

I

2300

I

I

u.cm

-1

1700

I

1600

I

1500

I ,

-1

v,cm

Fig.3. IR spectra of NH3 (A)and CD3CN (B) adsorbed on fresh HZSM-5 zeolite (1) and on the sample with propene preliminary chemisorbed at 300 K (21. Line ( - -1 corresponds to the background. The process of such an L-sites formation is a reversible one. They disappear after removal of chemisorbed hydrocarbons by evacuation of the sample at higher temperatures but are restored again after repeated chemisorption of the olefin.

It is worthwhile to emphasize that in this case no additional coordinatively unsaturated L-sites was found from IR spectra of adsorbed weak basic molecules (H2 and CO). The conclusion was made that the formation of L-sites in the presence of strongly basic molecules in zeolites Containing chemisorbed olefins proceeds according scheme (4).where the oligomer hydrocarbon chains play a role of the alkoxide R. The formation of alkoxy structures is evidenced by the appearance of the intense line at 70 ppm corresponding t o resonance of C-atom in the fragment in MAS 13C NMR spectrum of chemisorbed 13C-enriched ethylene (Fig.4 ) . The energy calculated for reaction (4) (B = NH3) indicates that this process

'0-bH

/

I

is energetically favorable: AE = 33.5 kJ/mole for cluster I (3-21C1 and AE = 13

660

kJ/mole for cluster I1 (STO-3G). For the acetonitrile molecule the energetics of such reaction is less advantageous. It, however, became much more favorable f o r bridged alkoxy groups with the larger SiOAl bond angle. Indeed, the increase of this angle by 50'

from its equilibrium meaning leads to essential enhancement of

system total energy and, as consequence, to sharp lowering the energy of A1-0 bond breaking in the presence of strong bases. At the same time for zeolites with the low Si/A1 ratio the energy of the A1-0 bond is higher, than the energy of the interaction between the basic molecule and the L-site. This explains why L-sites are not formed in methoxylated zeolites of such type with strong basic molecules adsorbed.

29

Fig.4.

MAS 13c NMR spectrum of chemisorbed I3C enriched ethylene on HZSM-5 at 370 K.

75

50

25

0

The theoretical results considered above demonstrated that the formation of L-sites in the presence of strong bases occurs only in the moieties of the

zeo-

lite framework having large values of angles in SiOAl bridges. As only protons localized on such SiOAl bridges are able to form a strained hydrogen bond with neighbor lattice oxygen atom, the substitution of OH-groups characterized by the broad IR band at 3250-3300 cm-l f o r alkoxides results in the manifestation of the unusual L-sites in the presence of adsorbed basic molecules.

It is reasonable to suppose that such L-sites could also appear during chemical reactions with the participation of basic molecules, for instance, methanol. In this case they can play a role of active sites for methanol conversion. L-sites and chemical reactions. Let us analyze this assumption in more detail. Indeed, the first step of the CH OH molecule interaction with acidic OH-groups 3

is the formation of bridge methoxy fragments. .H

The next methanol molecule is able to break A1-0

bond

and

consequently

to

661

transfer the bridge methoxide into the terminal one, as well as to dissociate on the pair consisting of the formed trigonal A1 cation and the basic oxygen atom attached to aluminum. This process could proceed via two energetically favorable ways with the formation of either terminal A1-0 group and bridged methoxide or A1-OCH3 fragment and bridged hydroxyl. The second structure is more preferable in the presence of the methanol excess due to the formation o f the additional strong hydrogen bond between the acidic OH-group and CH OH molecule. 3

In the first case the production of methane and formaldehyde could be accomplished from the formed structure by the following synchronous mechanism (scheme 6 ) . It includes the attachment of the basic OH-group to bridge methoxide accompanied by hydride ion abstraction. The interaction of hydride ion with the nelghboring terminal methoxide results in the break of the 0-C bond and formation of the methane molecule. Simultaneously the restoration of initial SiOAl bridge and the formation of an unstable bridged hydroxymethoxide group take place.

Abstraction of the proton from the latter complex being accompanied by

the 0-C

bond breaking produces the formaldehyde molecule and regenerates the acidic OH-group.

initial

When second type structure is formed via CH30H dissociation on the A1-0 pair the next adsorbed methanol molecule is activated by the acidic OH-group. Its .further conversion could be presented by the scheme:

H I

H3C'

662

The proton transfer from bridged hydroxyl to the adsorbed CH30H molecule accompanied by the nucleophylic attack of terminal methoxide by

is

the protonated

methanol molecule. As a result the abstraction of hydrogen atom from terminal methoxlde to neighboring oxygen and the formation of

the acidic OH-group and

terminal ethoxide occur. In this case the regeneration of

the

initial SiOAl

bridge unit and acidic hydroxyl proceeds throughout the desorption of methanol and ethylene molecules. The schemes similar to (6) and (7) could be also suggest for dimethylether conversion. The realization of scheme (7) would be accomplished only

in the excess of

methanol. In the lack of the CH OH molecule the structure containing the A1-OCH 3 3 group and acidic OH-group would be isomerized into a more energetically preferable one, which leads to methane and formaldehyde formation. It is worthwhile to mention that the schemes presented here allow to explain the difference observed in the products of methanol conversion at low (methane and

formaldehy-

de) and high (hydrocarbons) pressures of alcohol (ref.161. REFERENCES

K.J.Chao, J.C.Lin, Y.Wang, G.H.Lee, Single crystal structure refinement of TPA ZSM-5 zeolites, Zeolites 6(1) (1986)pp. 35-38. V.B.Kazansky, Theory of Broensted acidity of crystalline and amorphous aluminosllicates: quantum chemical cluster model and IR spectra Kinet. Catal. 23(6) (1982) pp.1334-1348. V.L.Zholobenko, L.M.Kustov, V.Yu.Borovkov, V.B.Kazansky. A new type of acidic hydroxyl groups in ZSM-5 zeolite and in mordenite according to diffuse reflectance i.r. spectroscopy, Zeolites 8 ( 5 ) (1988)175-178. A.S.Medin, V.Yu.Borovkov. V.B.Kazansky, A.G.Pelmenshchikov,G.M.Zhidomirov, On the unusual mechanism of Lewis acidity manifestation in HZSM-5 zeolites, Zeolites 1990 (in press) V.B.Kazansky, Diffuse reflectance IR spectroscopy and its new potentialities in studying chemisorbed species and the structure of surface oxide catalysts, Izv. AN SSSR 1 (1984) pp.40-51. J.S.Binkley, R.A.Whiteside,R.Krishnan, R.Seeger, D.J.DeFrees. H.B.Schlege1, S.Topio1, L.R.Kahn, J.A.Pople, QCPE, 13 (1981) 507. W.J.Hehre, L.Radom. P.v.R.Schleyer, J.A.Pople, Ab initio molecular orbital theory, Willey Intersci., New York etc., 1986. A.S.Medin, Ph.Diss., Moscow, 1990. E.A.Lombardo. G.A.Si11.W.K.Hal1. The assav of acid site on zeolites as measured by ammonia poisoning, J. Catal., 119(2) 1989 pp.426-40. 10 J.L.Schlenker, J.J.Pluth, J.V.Smith,Position of cations and molecules in zeolites with the mordenite framework. IX. Dehydrated H-mordenite via acid exchange, Mat. Res. Bull., 14(7) (1979)pp.849-56. 11 I.N.Senchenya, V.B.Kazansky, S.Beran, Quantum chemical study of the effect of the structural characteristics of zeolites on the properties of their bridging OH groups, J. Phys. Chem., 90(20) (19861 4857-4859. 12 V.B.Kazansky, On the nature of Lewis acidic sites in high silica zeolites and the mechanism of their dehydroxylation, Catalysis Today,3 (1988) 367-72. 13 L.B.Uytterhoeven, L.G.Crystner,W.K.Hal1, Studies of the hydrogen held by solids, J. Phys. Chem., 69(6) (1965) pp.2117-2126. 14 L.Kubelkowa, J.Novakova, P.Jiru, Reaction of small amounts of methanol on HEM-5, HY and modified Y zeolites, In: Structure and Reactivity of Modified Zeolites, Stud. Surf. Sci. Catal. 18 (1984) pp.217-224.


663

FTIR IN-SITU INVESTIGATION OF ZEOLITE ACTIVATION

R. SALZER', B. EHRHARDT', J. DRESSLER', K.-H. STEINBERG' and P. KLAEBOE~ 'Department of Chemistry, University of Leipzig, Talstr. 35, Leipzig 7010, GDR *Department of Chemistry, University of Oslo, N-0315 Oslo, Norway

ABSTRACT Activation of powdered, undiluted zeolites was studied in-situ by FTIR diffuse reflectance (DRIFT) spectroscopy. From the DRIFT spectra we constructed contour plots, which display the process quasi continuously. At temperatures below 2OO0C, the diffusion of water and ammonia towards the cation coordination sphere is seen. The maximum loss of a particular NH species was observed at 500°C. The concentration of OH species increases most strongly at 430"C, after the on-set of an almost uniform decomposition of all NH species. The activation of 0.93 NH,' erionite was completed at around 600°C. INTRODUCTION Diffuse

Reflectance Infrared Fourier-Transform (DRIFT-) spectroscopy has a high potential for in-situ studies of heterogeneous catalytic systems. Even very slow processes can be monitored over long periods (ref. 1). Most samples may be investigated without further pretreatment in a wide range of temperatures and pressures (ref. 2). Lateral resolution in the millimeter range can be achieved. We used DRIFT spectroscopy to investigate the activation process of zeolites i n - s i t u under atmospheric pressure between room temperature and 640°C. NH,' exchanged erionites were selected as test compounds. The DRIFT results were compared to the results of a thorough study of erionites by conventional infrared transmission technique (ref. 3). Here we describe the experimental technique and present the first results.

664

SAMPLING SYSTEM

DRIFT spectra are given in Kubelka-Munk units f (R) (ref. 4 ) , which are directly related to the absorbance coefficient k of the sample and its scattering coefficient s via ( 1 ) f(R) = (1 - R/R')' / 2R/R' = k / S . R is the single-beam reflectance spectrum of the sample, R' the single-beam reflectance spectrum of a non-absorbing standard. The Kubelka-Munk equation (1) was derived for the ideal case of purely diffuse reflectance. For real samples, diffuse reflectance is always accompanied by specular reflectance, which upsets the ordinate scale and deteriorates the detection limits. The only way to prevent contributions of specular reflectance from reaching the detector is to select a DRIFT attachment of a particular optical design, the so-called off-axis geometry (ref. 5), where the incoming beam and the normal to the sample surface do not share a common plane (or axis) with the reflected beam (Fig. 1). A heatable sample stage for DRIFT spectroscopy, which is commercially available (HVC-DRP chamber of Harrick Scientific Corp., Ossining/NY), was tested over a wide temperature range. The vacuum chamber body, which encloses the sample stage, was omitted because the investigation was carried out at ambient pressure. Any change in the sample position during the measurement series, including realignment, was avoided.

reflected beam incoming beam sample to power supply heater thermal insulator adjustment screw base

Fig. 1. Scheme investigations.

of

the

sample

heater

for

the

DRIFT

665

S i n g l e beam s p e c t r a of t h e t e s t sample ( c o r r e s p o n d i n g t o R i n e q n . 1) a r e summarized i n F i g . 2 . The a r e a below a p a r t i c u l a r c u r v e r e f e r s t o t h e t o t a l amount of modulated r a d i a t i o n r e a c h i n g t h e d e t e c t o r . Upon h e a t i n g t h e HVC-DFW sample s t a g e , t h e s i n g l e beam s p e c t r a become i n c r e a s i n g l y f l a t ( F i g . 2 a )

. The

observed r e d u c t i o n

i n e n e r g y t h r o u g h p u t i s more s e v e r e a t s h o r t e r w a v e l e n g t h s (compare the

lowering

of

t h e c u r v e maxima

4 0 0 0 a n d 2200 cm-I).

at

This

s u g g e s t s a g e o m e t r i c a l e f f e c t dominating t h e energy d r o p d u r i n g h e a t i n g , p r o b a b l y b e due t o t h e r e l a t i v e l y l o n g h e a t i n g zone of t h e sample s t a g e , c h a n g i n g t h e sample p o s i t i o n r e l a t i v e t o t h e

HVC-DRP

reflectance mirrors. 1

1

i 0,5

0

/p

25 *C

I

a)

i

~

0.

, 6000

0 2000

4000

+-

cm-'

0

6000

4000

2000

+-

cm-'

F i g . 2 . S i n g l e beam s p e c t r a of 0 . 9 3 NH,' e r i o n i t e a t d i f f e r e n t t e m p e r a t u r e s . a ) Sample s t a g e of HVC-DRP chamber. b ) Oven a s i n F i g . 1. The o b s e r v e d e n e r g y l o s s d u r i n g h e a t i n g r e d u c e s t h e S / N r a t i o ,

causes

background

ordinate

non

fluctuations

linearities.

in

These

the

spectra

effects

and

can

introduces be

reduced

c o n s i d e r a b l y , when t h e r e f e r e n c e s p e c t r u m i s measured a t t h e same t e m p e r a t u r e a s t h e sample spectrum. The b e s t

results

according t o Fig.

1,

c o u l d be

observed

using a

sample

stage

whose t h e r m a l e x p a n s i o n was minimized by

i n t r o d u c i n g t h e h e a t e r d i r e c t l y i n t o t h e sample b u l k ( F i g . 2 b ) .

666

EXPERIMENTAL

A Bruker FTIR spectrometer IFS 88 was used for all tests of the heatable sample stage of the HVC-DRP reaction chamber. The sample stage was inserted into a diffuse reflectance attachment D R A 3 (Harrick Scientific Corp. , Ossining/NY, USA) . All the other FTIR spectra were scanned on an IRF 180 (ZWG, Akademie der Wissenschaften der DDR, Berlin), equipped with a the off-axis diffuse reflectance attachment of the same manufacturer. 128 interferograms were co-added before Fourier transformation. The thermal insulator of the heatable sample stage was made of glass ceramics (Fig. 1). A micro thermocouple was used for temperature measurement. The alignment of the thermocouple to the uppermost layer of the powder is important for exact measurement. Samples were heated by 2 K min-', and during the spectral measurements the temperature was kept constant. The synthesis and ion exchange of the erionites used in this study have already been described (ref. 3). The samples were investigated as received. Finely ground KBr was used as reference. No features, e.g. peaks due to atmospheric C02, have been omitted from the spectra. RESULTS AND DISCUSSION The OH and NH stretching bands are selected in order to monitor the activation of undiluted 0.93 NH,' erionite in the open

atmosphere (Fig. 3). All the band locations and intensity ratios correspond completely to the transmission spectra (ref. 3). The graduate loss of physisorbed species is indicated by the overall reduction in the size of the band complex. The almost complete desorption of NH species up to 550°C is seen by the disappearance of the corresponding NH stretching bands below 3 3 0 0 cm-'. Simultaneously, the band appearing above 3600 cm-! indicates the formation of OH groups. A detailed investigation of the whole activation process (Fig. 3) is difficult because of the complicated shape of the bands. For this purpose we applied a complex evaluation procedure to the spectra. It is important to note, that no individual assumptions are necessary in course of the following computations.

667 A s a first step,

Fig. 3. DRIFT spectra of 0.93 NH,' erionite during activation at the open atmosphere.

all the constant (background) features apparent during heating are removed by s u b t r a c t i n g consecutive spectra. It depends upon the available software, w h e t h e r t h e temperature intervals have to be constant or not. A high S/N ratio and a constant ordinate accuracy in the single beam s p e c t r a a r e prerequisites. The ordinate accuracy of DRIFT spectra is mainly determined by the scattering coefficient s in eqn. 1. It varies remarkably for different sample preparations, even for samples from the same batch. One must only consecutive series of

evaluate spectra, which belong to one measurements. The D R I F T difference spectra in Fig. 4 enhance all the changes taking place during activation, which have been discussed regarding Fig. 3. The removal of all the constant features did not simplify the shapes significantly. In the following, we shall regard the complete set of curves in Fig. 4 as a landscape. The height of the hills corresponds to the increase of a particular XH species per temperature step, the

668

Fig. 4 . DRIFT d i f f e r e n c e 0 . 9 3 NH4’ e r i o n i t e

spectra

for

the

activation

of

d e e p n e s s of t h e v a l l e y s t o t h e d e c r e a s e i n c o n c e n t r a t i o n . A t e v e r y v i e w i n g a n g l e mountains i n t h e f o r e g r o u n d h i d e f e a t u r e s b e h i n d , whereas a map, t h e s o - c a l l e d c o n t o u r d i a g r a m , p r o v i d e s immediate a c c e s s t o a l l t h e d e t a i l s of t h e measured s e r i e s . Contour d i a g r a m s f o r t h e DRIFT d i f f e r e n c e s p e c t r a i n F i g . 4 a r e g i v e n i n F i g s . 5 and 6 . To e n s u r e c l e a r n e s s of t h e monochrome d i s p l a y , t h e r e g i o n s of n e g a t i v e r e f l e c t a n c e c h a n g e s ( d e c r e a s i n g c o n c e n t r a t i o n s ) d u r i n g h e a t i n g a r e h i g h l i g h t e d i n F i g . 5, r e g i o n s of p o s i t i v e r e f l e c t a n c e c h a n g e s ( c o r r e s p o n d i n g t o i n c r e a s i n g concentrations) i n Fig. 6 . A t t e m p e r a t u r e s below 200°C t h e I R r e f l e c t a n c e d e c r e a s e s i n

t h e r e g i o n 3700

-

2800 cm-’ due t o d e s o r p t i o n o f w a t e r and ammonia

( F i g . 5 ) . The r e f l e c t a n c e l o s s e s s t a r t most

pronounced

around

3450 cm-’ (OH s p e c i e s ) and 3 1 0 0 cm-’ ( N H s p e c i e s ) . Above 150°C t h r e e bands f o r d e s o r b i n g NH c a n be s e e n ( 3 3 0 0 , below 3100, below

2900 cm-I). Up t o 200°C a s i m u l t a n e o u s i n c r e a s e i n r e f l e c t a n c e i s o b s e r v e d between 2800 and 2200 cm-’. The i n c r e a s e d r e f l e c t a n c e i s e a s i l y s e e n

669

640 O C

530

420

310

200 130

Fig. 5. Contour plot of DRIFT difference spectra in Fig. 4. The bright area indicates decreasing concentrations.

640 O C

530

420

310

200

130 Cm-'

Fig. 6. Contour plot of DRIFT difference spectra in Fig. 4. The bright area indicates increasing concentrations.

670

in Fig. 6, whereas it could only be guessed in Figs. 3 and 4. The increased reflectance amounts to ca. 10% compared to the decrease mentioned above. It points to diffusion towards the cation coordination sphere. The assignment of the bands to water and ammonia is under study. At 26OoC, a minimum of reflectance changes across the investigated spectral region is indicated in Figs. 5 and 6 . Above this temperature the decomposition of the NH,+ cations starts. Three bands are observed (ca. 3 2 5 0 , 3 0 3 0 and 2 7 8 0 cm-') . The decomposition is first indicated for the band having the highest frequency. This band a l s o shows the maximum loss of NH,' ions at 500°C. The on-set of an almost uniform decomposition of all the NH,' species is indicated by the spread of the contour line at 400°C (Fig. 5). The formation of new OH groups is first observed for species absorbing above 3 6 0 0 cm-' (Fig. 6 ) . This band has been assigned to strong Brernsted acid sites (ref. 3 ) . An OH species, absorbing at 3 5 6 0 cm-' and located in the six-membered ring units (ref. 3 ) , is formed at somewhat higher temperatures. The stretching frequencies of both OH species are red shifted with increasing temperature. The concentration of OH groups increases most strongly around 430"C, after the beginning of the uniform decomposition of all NH,' species. At ambient pressure the activation of 0 . 9 3 NH,' erionite is completed at around 650°C. ACKNOWLEDGEMENT

This research was supported by The Norwegian Research Council for Science and the Humanities (NAVF). REFERENCES 1 R. Salzer, K.-H. Ste nberg, P. Klaeboe, B. Schrader, Zeolites submitted. 2 S. A. Johnson, R.-M Rimkus, T. C. Diebold and V. A. Maroni Appl. Spectr. 42 1 9 8 8 ) 1 3 6 9 3 A. Kogelbauer, J. A . Lercher, K.-H. Steinberg, F. Roessner, A. Soellner and R V. Dmitriev: Zeolites 9 ( 1 9 8 9 ) 2 2 4 . 4 P. Kubelka and F. Munk: 2. Techn. Phys. 1 2 ( 1 9 3 1 ) 5 9 3 . 5 D. M. Hembree and H. R. Smyrl: Appl. Spectr. 4 3 ( 1 9 8 9 ) 2 6 7


671

IR SPECTRA OF CO ADSORBED AT LOW TEMPERATURE (77 K) ON TITANIUMSILICALITE, €I-ZBMS AND SILICALITE

A. ZECCHINA~,G. SPOTO~,s. BORDIGA~,M. PA DO VAN^, G. LEO FAN TI^ and G. PETRINI~ 'Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Via P. Giuria 7, 10125 Torino (Italy)

'Montedipe, Unit& di Ricerca, Via San Pietro (MI) (Italy)

50,

28100 Bollate

ABSTRACT

The IR band at 960 cm'l observed on TS is due to a local stretching mode of a [Ti04] unit in the silicalite framework. Framework Ti does not show Lewis acidity as probed by CO adsorption at 77 K. TiOH and SiOH have undistinguishable IR properties. At 77 K they form with CO very weak 1:l OH...CO adducts. Small amount of extralattice Ti in form of Ti02 can be probed by CO adsorption at 77 K. INTRODUCTION

The substitution of Ti and A1 for Si in zeolites of the pentasil family (Silicalite) leads to Ti-Silicalite and ZSMS which are two important catalysts for oxidation with HZOZ (refs. 1-3) and acid catalyzed reactions (ref. 4 ) . The characterization of Titanium-Silicalite (TS) by means of physical methods has been recently published (ref. 5). However, a few problems are still open concerning: i) the presence of extralattice Ti (as Ti02 microparticles); ii) the interpretation of the vibrational spectrum of the solid; iii) the crystallinity of TS with respect to pure S and ZSMS; iv) the presence of titanols in the channel8 and cavities. The CO molecule is an efficient probe of Lewis and Broensted acidity and can be used to explore the acidity of OH groups in the channels (TiOH and SiOH in TS, (Si,Al)OH, SiOH and AlOH in H-ZSM5). On dehydroxylated samples coordinatively unsaturated Ti4+ and A13+ ions in lattice and extralattice positions can be revealed as well, because they can form Lewis adducts with CO. In this contribution we report on the IR spectra of CO adsorbed at 77 K on TS, s and ZSMS. The vibrational spectra of s,

TS and Na-ZSM5 are also compared and discussed. EXPERIMENTAL

Silicalite and Titanium-Silicalite have been synthesized in Montedipe laboratories following the method described in ref. 2; Na-ZSM5 and H-ZSM5 (external surface area 60 m’9-l) have been provided by the same laboratory. The ER spectra have been obtained on a Bruker IFS 113V FTIR spectrometer using a specially designed silica cell permanently attached to a vacuum manifold and allowing in situ outgassing procedures at temperatures in the 273-1073 K interval and gas dosing at 7 7 K. The samples were either in form of thin pellets or of films deposited on KBr or Si plates. RESULTS AND DISCUSSION IR modes associated with framework Ti

The spectra recorded at 7 7 K of S, TS and ZSM5 (sodium form; Si/A1=35) films outgassed at 573 K are compared in Fig.1. The major difference between S and ZSM5 on one side and TS on the other side is represented by the presence in the spectrum of TS of a (finger print) peak at 960 cm-l. In order to give a correct assignment of this band the following considerations have to be made: a) it cannot be assigned to an optical mode of TiOZ microparticles entrapped into the channels or at their intersection because neither rutile nor anatase show strong IR bands at this frequency; b) despite the similarity with the IR manifestations of homogeneous titanyl containing analogs , the 960 cm-’ peak cannot be attributed to the stretching mode of internal (Ti=O)’+ because: i) it is not perturbed by filling the pores and channels with CO (results not reported), which, being a weak Lewis base, is expected to interact with the positive centres and to perturb them; ii) it does not have the expected counterpart in the UV-Vis diffuse reflectance spectrum (absorption in the 25000-35000 cm’l range) (ref. 6); iii) it cannot be removed by reductive treatments in H2 and CO even at very high temperature or under plasma discharge conditions (Ha). On the basis of the previous **negativet* evidences, the hypothesis has been made that the 960 cm-l band is due to a local mode of the Titanium-doped pentasilic structure (essentially a stretching mode of a [Si04] unit perturbed by an adjacent Ti) (ref. 5 ) .

673

F'LI

0.5

A

/ I

1

I0

YRVENUHBER C H - I

Fig.1. IR absorbance spectra (recorded at 77 K) of: a) Silicalite, b) Na-ZSM5 (Si/A1=35) and c) Ti-Silicalite. In the inset: W-Vis reflectance spectra of a) Silicalite and c) Ti-Silicalite.

A full assignment of the IR spectra of s, TS and ZSM5 requires a detailed unit cell vibrational analysis and is outside the scope of this contribution. However, an explanation of the presence of the extra-absorption in the TS spectrum can be equally given on the basis of the following qualitative considerations. The vibrational representation of the stretching modes of an llisolatedll tetrahedral [ S i O , ] unit is:

rstret.= T2 *I (TZ: IR active; A1: IR inactive). Packing of the [Sio,] (by sharing the corners) to form the zeolitic structure has two consequences: i) the local *'site1@symmetry of each [SiO,] primary +

674

building unit is lowered to C2 (ref. 7) so that the triply degenerated T2 mode is splitted into three, one weak (A) and two strong (B), components and ii) due to the high number of [SiO,] units forming the unit cell (where 9 6 tetrahedral units are present) broad bands are expected because of the further splitting of each component into a maximum of 96 sub-components. On these basis, the spectrum of S in the stretching region can be interpreted as follows: i) the two clear absorption at 1235 (mw) and 1200-1050 (vs) cm-l, are essentially associated with the A and B modes of the primary unit broadened by unit cell splitting effects; ii) the complex (vw) band centered at =770 cm-l derives from the A1 (IR inactive) mode (the fine structure observed at 77 K being associated with the unit cell sub-components). The bands at lower frequency belong to skeletal modes having bending character and will not be considered here for sake of brevity. We only mention that the peak at ~ 5 5 0cm-l is absent on amorphous silica and is considered as a gfcrystallinityfg (structure sensitive) band (ref. 5). As far as TS is concerned, some 1-2% of the [SiO,] building blocks of pure silicalite framework are substituted for Igheavierff [TiO,] units. The substitution does not dramatically change the overall IR spectrum other than for the appearence of localized fgimpuritygg modes associated with the [TiO,] dopant units. In the stretching region four additional local modes are, in principle, expected, three (A+2B) deriving from the T2 degenerate vibrations of the free [TiO,] unit and one (A) from the A1. These modes should be shifted to lower frequencies with respect to those of Silicalite, because of the mass effect. Actually only the band deriving from the strongest absorption at 1120 cm-l (presumably of B simmetry) has enough intensity and is enough shifted to show up clearly in the gap between the 1250-1050 and 820-750 cm-l absorptions. The previous analysis predicts that, due to the small difference in the mass of A1 and Sir the impurity modes associated with [AlO,] should be not osservable at all. This explains the great similarity between the Silicalite and Na-ZSM5 spectra. The assignment of the 9 6 0 cm" peak to local modes of [TiO,] units is in agreement with: i) the characteristic reflectance spectrum of TS (inset of Fig.1) where an high intensity band at 48000 cm-', with charge transfer character, is indicative of the presence of [TiO,] units (ref. 5 and references therein) ; ii)

675

the IR spectrum of Si02/Ti02 glasses where a similar, although broader, band is observed (refs. 8,9). Of course these considerations, based only on the mass effect, are an heavy approximation. In fact, due to the higher ionicity of the Ti-0 bond, the Ti-0 and the Si-0 stretching constants are not the same. This assignment is slightly different with respect to that given in ref. 5, where a larger ionicity of the Ti-0 bond was assumed. It can be easily demonstrated that they transform the one into the other when the ionicity of the Ti-0 bond is gradually changed from pure covalent to ionic. Ir spectra of CO adsorbed on Silicalite, Ti-Silicalite and HZSM5: assianment and comarison The spectra of CO adsorbed at 77 K on S and TS samples outgassed at 573 K under vacuo are reported in Fig.2a (3800-3000 cm-l region) , and Fig.2b (2250-2050 cm-l) and Fig.3a and 3b respectively. Similar spectra have been obtained for samples outgassed at lower and higher temperature. They are not described in detail for sake of brevity. The following can be commented. Silicalite The IR spectrum in the OH stretching region (Fig.2a) is characterized by two main absorptions at 3750-3700 cm-' (composit with narrow components at 3750 and 3730 cm-l and a broader one at These two bands 3710 cm-l) and at 3460 c m ' l (0))112=130 cm") essentially correspond to free (isolated and terminal, external and internal) and hydrogen bonded silanols respectively. Upon CO dosage the bands due to free silanols (both isolated and terminal) are eroded, while two new peaks are formed at 3640 and 3585 .''nrc The presence of a clear isosbestic point indicates that the free OH (isolated and terminal) are transformed into hydrogen bonded species (because of formation of 1:l OH...CO adducts) as illustrated in the following scheme:

-

OH.. .CO

I

.

OH...OH...OH...CO

(isolated)

Si Upon CO adsorption the peak at ic changes, in agreement with small but clear shift at lower reinforcement of the hydrogen

I

l

si

Si

l

(terminal)

si

3460 cm" does not undergo dramatthe given assignment. However, a frequency is indicative of a small bond between adjacent OH groups.

676

t

a

a 2

Fig.2. IR spectra of CO adsorbed at 77 K (increasing doses) on Silicalite outgassed at 573 K. a) OH stretching region: the evolution of'the bands is indicated by the arrows; Aindicate the isosbestic points. b) CO region. The spectrum of CO adsorbed on H-ZSM5 (at 77 K and maximum coverage) outgassed at the same temperature is reported for comparison.

This can be easily explained on the basis of the previous scheme: in fact the formation of the 1:l adducts involving terminal OH groups and CO induces the polarization in the terminal 0-H bond and this in turn reinforces the hydrogen bonding between the vicinal groups of the chain. Similar effects have been observed also in homogeneous conditions (ref. 10). The previous assignment is confirmed by the following other experiments: i) by outgassing Silicalite at 973 K the peaks at 3460 and 3710 cm-' simultaneously disappear; ii) the adsorption of CO on samples containing only isolated OH both on silica and silicalite gives the 3750 cm-' peak only. In the part b of the figure the parallel spectra in the CO stretching region is illustrated. Two main absorptions are clearly observed at 2159 and 2137 cm-'. The first peak, with frequency slightly higher than that of the CO gas (a7=16cm-l) , is due to

677

the CO stretch of the OH...CO (1:l) adducts (both isolated and terminal). The second peak (with frequency nearly coincident with that of (CO)liq) is due to CO molecules physically adsorbed (or in a liquid-like state) in the channels and pores. In the same figure (2b) the spectrum of CO on H-ZSM5 (Si02/A1203=28) outgassed at the same temperature is reported for sake of comparison. We can immediately notice that the two type of spectra are very similar but for the peak at 2170 cm-' present only on H-ZSM5: this peak, characterized by a blue shift of 27 cm" with respect to CO gas, is associated with Broensted acid-CO complexes. This comparison shows that CO is a good probe of the strong Broensted acidity of H-ZSM5 and that SiOH groups are much less acidic in agreement with ref. 11. Titanium-Silicalite An identical experiment carried out on TS outgassed at 573 K gives the spectra reported in Fig.3a and 3b (OH and CO stretching region respectively). The great similarity with the spectra of Fig.2 is immediately evident. This observation has two main consequences: i) titanols (either on framework or extra-framework Ti) cannot be easily distinguished from silanols by IR spectroscopy (especially when the Titanium content is small) and ii) titanols (either framework or extraframework) have Broensted acidity not substantially different from silanols. In conclusion, the presence of Ti does not substantially modify the OH distribution and their acidity. In these conditions we can ask wether TiOH groups are really present (specially if we consider that if Ti is totally framework and if the Ti-0-Si bridges are not at least partially hydrolyzed TiOH should be absent). For the time being we cannot answer this question. However, we shall come back again on this problem later on, on the basis of further data. As known from indipendent experiments on the Ti02/C0 system (ref. 12) , the adsorption of CO on microparticles of Ti02 outgassed at temperature t 873 K, gives surface Ti4+cus-C0 (cus: coordinatively unsaturated) species characterized by a well defined high intensity peak at 2179 cm-'. The presence, or the absence, of this peak on properly outgassed TS samples can be so used as a text of the presence or the absence of extraframework Ti in form of Ti02 microparticles. The results of this experiments on a TS sample outgassed in vacuo at 873 K are reported in Fig.4a and 4b. In the same figure the spectrum of CO (CO-stretch-

678

1

t TO'

a

Ii1

n

b

0.5

-z

r 0 3

Y

u

z U

m w 0 VI

U m

3888

3688

3400

YRVCNUHBtR CM-I

3288

380022

8 YRVt"ljHBtR

CH-I

Fig. 3. IR spectra of CO adsorbed at 77 K (increasing doses) on Ti-Sil icalite outgassed at 573 K. a) OH stretching region. The arrows and A have the same meaning as in Fig.2. b) CO region.

ing region) adsorbed at 77 K on TiOZ (anatase) outgassed at the same temperature is reported for sake of comparison. From these figures we can conclude that: i) the bands at 3450 and 3700 cm" disappear simultaneously upon outgassing, in agreement with the given assignment ; ii) isolated silanols (either external or internal) are the main species present on the sample and give the expected OH...CO 1:l complexes characterized by peaks at 3650 cm" (OH) and 2155 cm'l (CO); iii) very low intensity band at ~ 2 1 8 0cm-l reveals that Ti4+cus-C0 adducts are indeed present on the sample outgassed at high temperature. However, comparison with the spectrum of CO adsorbed on Ti02 clearly indicates that the amount of extralattice Ti under form of Ti02 microparticles is negligible. The last observation confirm that the major part of Ti is in framework position and does not have relevant Lewis acidity. We can now go back to the question concerning the presence or

679

a

I!

h '

I

3600

3400 YRVENUHBER

3200

CM-I

31

12250

2200 2150 2100 YRVENUMBER C H - I

2

0

Fig.4. IR spectra of CO adsorbed at 77 K (increasing doses) on Ti-Silicalite outgassed at 873 K. a) OH stretching region. The arrows and A h a v e the same meaning as in the previous figures. b) CO region. The spectrum of CO adsorbed at 77 K (maximum coverage) on pure Ti02 outgassed at the same temperature is reported for comparison.

absence of TiOH groups. First of all we can now say with confidence that TioH or Ti02 microparticles do not play any relevant role in the spectrum of TS (OH stretching region), and secondly that TioH (if present) must comes only from partial hydrolysis of polar Si-0-Ti bridges, as already hypothesized in ref. 5. In order to prove or reject the hypothesis that Si-0-Ti bridges exposed in the channels and cavities are preferential sites for H20 adsorption, the following comparative experiment has been designed. Silicalite and Ti-Silicalite samples were outgassed at 973 K in high vacuum for 4 hrs to eliminate all the hydrogen bonded hydroxyl groups. After this treatment the IR spectra in the OH stretching region were substantially undistinguishable (with only one peak left at 3720 cm-' similar to that shown in Fig.4a). H20 vapuor was then dosed (10 torr) on the sample and

680

left to stand for 15 minuts. After this dehydration step the unreacted H20 was pumped off at 573 K and the hydration state of the surface monitored by recording the IR spectrum in the OH stretching region. The result was as follows: after the thermal treatment Silicalite shows hydrofobic character, as H20 dosage does not restore the original hydroxyl concentration; on the contrary, Ti-Silicalite is slightly more hydrophylic and can be partially rehydrated. We think that this behaviour could be a consequence of the presence of Si-0-Ti bridges which, being more polar, can easily undergo H20 attack giving silanols and titanols in vicinal position. CONCLUSIONS

The IR band observed in the spectrum of TS at 960 cm-', associated with framework Ti, is due to a local impurity stretching mode of the [Ti04] unit in the Silicalite lattice. The small fraction of extralattice Ti in form of Tio2 microparticles does not contribute to the IR spectrum of TS. TiOH and SiOH cannot be distinguished by IR spectroscopy and have very similar acidity (as tested by CO adsorption at 77 K). Moreover, comparison with the spectra of OH...CO adducts on H-ZSM5 shows that their acidity is much lower. Lewis acidity associated with Ti4+ ions is also pratically absent even on samples outgassed at 973 K. REFERENCE8

W. Holderich, M. Messe and F. Naumann, Angew. Chem. Int. Ed. Engl., 27 (1988) 26. C. Neri, A. Esposito, B. Anfossi and F. Buonomo, Eur. Pat. 100 119.

C. Neri, M. Taramasso and F. Buonomo, U.K. Pat. 102 665. J.A. Rabo, Catal. Rev. Sci. Technol., 24 (1982) 202. M.R. Boccuti, K.M. Rao, A. Zecchina, G. Leofanti and G. Petrinil Spectroscopic characterization of Silicalite and Titanium-Silicalite, in: C. Morterra, A. Zecchina and G. Costa (Eds.), Structure and Reactivity of Surfaces, Elsevier, Amsterdam, 1989, pp. 133-144. P. Comba and A . Merbach, Inorg. Chem., 26 (1987) 1325. A. Miecznikowski and J. Hamuza, Zeolites, 7 (1987) 249. M.F. Best and R.A. Condrate, J. Mat. Sci. Letters, 4 (1985) 994. B.G. Varshal, V.N. Denisov, B.N. Maurin, G . A . Paulova, V.B. Podobedov and K.E. Stebin, Opt. Spectrosc. (USSR), 47 (1979) 344. 10 G.C. Pimentel and A.L. McClellan, The hydrogen bond, W.H. Freeman and Co., San Francisco and London, 1960. 11 V.B. Kazanskii, Kinet. Catal., 28 (1987) 482. 12 G. Spoto, C. Morterra, L. Marchese, L. Orio and A. Zecchina, Vacuum, in press.


681

THE PROPERTIES OF BORALITES OF V A R I O U S BORON CONTENTS

J . DATKA. A.

CICHCCKI and Z. PIWOWARSKA

F a c u l t y of C h e m i s t r y . J a g i e l l o n i a n U n i v e r s i t y . Karasia 3, 30-060 Krakdw C Poland>

SUMMARY The p r o p e r t i e s of Bog u n i t s and a c i d p r o p e r t i e s of

boralites

w i t h v a r i o u s boron c o n t e n t s w e r e s t u d i e d b y I R s p e c t r o s c o p y . The band of a n t i s y m m e t r i c s t r e t c h i n g v i b r a t i o n s of 9-0 i s s p l i t i n t o t w o m a x i m a . I t s e e m s t o b e d u e t o t h e removal of d e g e n e r a t i o n of t h i s vibration. The i n t e n s i t y of t h e B03 doublet increases l i n e a r l y w i t h t h e b o r o n c o n t e n t b o t h i n N a - and H - b o r a l i t e s . The e x t i n c t i o n c o e f f i c i e n t of 9-0 v i b r a t i o n i s d i s t i n c t l y l h i g h e r i n t h e . . c a s e of H-boralites. The c o n c e n t r a t i o n of 3720 c m hydroxyls CBronsted a c i d s i t e s 3 i n c r e a s e s l i n e a r l y w i t h b o r o n c o n t e n t i n H - b o r a l i t e s and i s close t o t h e t h e o r e t i c a l v a l u e s . S m a l l amounts of L e w i s a c i d sites a r e p r e s e n t i n H-boralites. their acid s t r e n g t h b e i n g h i g h e r t h a n i n HZSM-5. I NTRODUCTI ON Isomorphically studied

substituted

extensively

because

zeolites of

their

have

recently

interesting

been

chenucal

p r o p e r t i e s and p o t e n t i a l i n d u s t r i a l a p p l i c a t i o n [ f o r a r e v i e w see ref. the

C1>1. small

zeolites.

Boron s u b s t i t u t e d z e o l i t e s . s i z e of draw

t r i -coordinated

boron

the

atom

greatest

show

boralites,

properties

attention.

The

i n d e h y d r a t e d b o r a l i tes , t h e B03

t h e spectrum Cref.

2.31. The p r o p e r t i e s of B03

which owing t o

not

observed

boron

atom

in

is

band a p p e a r s i n

units i n boralites

of v a r i o u s b o r o n c o n t e n t s w e r e s t u d i e d b y I R s p e c t r o s c o p y a n d t h e

r e s u l t s a r e described i n t h e present paper. Our p r e v i o u s I R s t u d y C r e f . 3,4> h a s shown t h a t f o u r k i n d s of -1 OH g r o u p s exist i n H - b o r a l i t e s . Only 3720 cm h y d r o x y l s were f o u n d t o b e B r z n s t e d a c i d sites. lower

than

in

zeolites

Cref.

Their

2-51.

B r z n s t e d a c i d s i t e s i n H-boralites

of

acid

The

strength

concentration

of

much the

v a r i o u s boron c o n t e n t s w a s

s t u d i e d by I R s p e c t r o s c o p y and t h e r e s u l t s a r e a l s o t h i s paper.

is

presented i n

682 EXPERIMENTAL Five

samples

of

MFI

were

boralites

synthesized

in

the

N a 0-TPABr-B 0 -SiO -H 0 s y s t e m as d e s c r i b e d i n d e t a i l i n r e f . 7 . 2 2 3 2 2 C o n c e n t r a t e d s i l i c a sol, s o l i d o r t h o b o r i c a c i d H3B03. sodium hydroxide,

and

distilled

tetrepropylammonium

water

bromide

Individual

synthesis

C=Si02/B203

in

the

were

differed

reaction

used

as

TPBABr only

in

mixture).

substrates

as

a

template

the

which

ratio

molar

varied

was

r a n g e 5-100. C r y s t a l l i z a t i o n w a s c a r r i e d o u t i n steel lined

with

at

PTFE.

438

K.

for

days,

7

without

p r e c i p i t a t e w a s washed w i t h d i s t i l l e d w a t e r o r d e r t o decompose o r g a n i c species

mml.

p r e p a r e d are d e n o t e d as Na-boralites. the

NH4-form

at

solution

by

triple

XRD

studies

have

without

autoclaves

stirring.

The In

for

The

that

in

ammonium

compositions analysis)

all

crystalline

Boralites

4 h.

thus

They w e r e n e x t t r a n s f o r m e d

from chemical

shown

RM

the

a n d d r i e d i n air.

exchange

temperature.

room

NH - b o r a l i t e s ( o b t a i n e d 4 T a b l e 1.

crystalline

ion

n

in

boralites were c a l c i n a t e d i n

a i r a t 773 K i n a t h i n l a y e r C l - 2

into

and

substance.

the

chloride and

Na-

of

are presented

were

samples

admixtures.

in

highly

They

had

the

boron

loss

from

or thor hombi c MFI s t r u c t u r 8 . The N a / N H 4 boralites. paper

This

Cref.

exchange

resulted

problem is d i s c u s s e d

The c a l c i n a t i o n

81.

boron

further H-BOR-5.

ion

loss

H-BOR-4.

Cref.

H-BOR-3.

8).

0.69. 0.85 a n d 1 . 0 5 B/u. c. r e s p . for

The

our

previous

w e r e f o u n d t o be 0.88, 0.59,

upon c a l c i n a t i o n i n a i r a t 773 K .

71.

For I R s t u d i e s NH - b o r a l i t e s C4-7

i n

h C s a m p l e numbers a r e t h e s a m e as i n o u r p r e v i o u s p a p e r ,

4

ref.

i n detail

NH - b o r a l i t e s r e s u l t s i n a 4 b o r o n c o n t e n t s i n H-BOR-8.

of

H-BOR-2

some

in

m g cm-').

4 The w a f e r s

vacuum at. 773 K , during

this

for

activation

1 h.

was

were p r e s s e d

w e r e activated It

was

the

u n d e r c o n d i t i o n s described i n r e f .

assumed

same

as

into thin

wafers

in situ i n I R c e l l that

during

the the

boron

in

loss

calcination

8.

T h e s p e c t r a w e r e r e c o r d e d u s i n g a SPECORD 75 I R s p e c t r o m e t e r CCarl

Zeiss J e n a l

working on l i n e w i t h

a K S R 4100 minicomputer.

Pyr i d i ne C PCCh-G1 i w i c e l u s e d w a s o f a n a l y t i c a l g r a d e .

683 TABLE 1 Compositions of N a -

and NH -boralites 4

*

Sample

composition ~~

Na-BOR-8

6’

30H0.10CCB02’0. 3OcAlo2’0. 1OcSi02’Q5.

NH4 -BOR-8

03CNH4’0.37ccB02’0.2QcA102’0. 1lcsi02’S5. 6’

Na-BOR-5 NH 4 -BOR-5

2’

80CCB02’0. 8OcSi02’Q5.

04CNH4’0.69[cB02’0. 63CA102’0.10csi02395.2’

N a -BOR -4

N a lOO[CBo2’l. .

NH4-BOR-4

0’

0OcSi02’Q5.

Na0..02CNH4’0. 84CCB02’0. 78cA10220.0ScSi02’Q5. 0’

N a -BOR -3

N a l30H0. . 10CcB0231. 4OCSio2’Q4.

NH4-BOR-3

6’

01CNH4’0.98[cB02’0. 99csi022Q4. 63

Na-BOR-2

N a l60H1. , 0OCCB02’2.

4’

6OcSi02’Q3.

OlCNH4’1. 36[cB02’1. 37csi022Q3.4’

NH4-BOR-2

*

~~

s a m p l e numbers a r e t h e same a s i n p r e v i o u s p a p e r C r e f .

73

RESULTS AND DISCUSSION BO -3

vi b r a t i ons I R spectra

cm

-1

B-0

of

dehydrated

b o r a l i t e s show a band about 1400

, c h a r a c t e r i s t i c of v3 a n t i s y m m e t r i c s t r e t c h i n g v i b r a t i o n s of T h i s band i s p r e s e n t b o t h i n t h e s p e c t r a of N a -

i n B03 u n i t s .

and H - b o r a l i t e s Cref. 53

that

Cref.

boron

The MAS NMR s t u d i e s h a v e also p r o v e d

2.32.

is

tri-coordinated

in

dehydrated

boralites.

Because of i t s s m a l l s i z e , boron i s n o t s i t u a t e d i n t h e c e n t r e of TO,

t e t r a h e d r o n b u t on ‘ o n e of

its f a c e s .

Our

previous s t u d y has

shown t h a t t h e B03 band i s s p l i t i n t o t w o maxima: 1380 a n d 1 4 0 5 -1

cm

.

The o r i g i n of t h i s s p l i t w i l l be now d i s c u s s e d . interpretations existence

of

will

two

be

kinds

considered. of

boron

One

sites

of in

c h a r a c t e r i z e d by d i f f e r e n t B-0 f o r c e c o n s t a n t . d i f f e r e n t f o r c e s of

them

Two p o s s i b l e assumes

boralite

the

lattice

I t c o u l d be d u e t o

i n t e r a c t i o n between t h e b o r o n a n d t h e f o u r t h

684 o x y g e n i n 0 B---.O u n i t s . A n o t h e r i n t e r p r e t a t i o n a s s u m e s t h e removal 3 o f d e g e n e r a t i o n o f t h e v3 €3-0 v i b r a t i o n i n B03 u n i t s . This c o u l d be d u e t o t h e f a c t t h a t B03 u n i t s a r e n o t i s o l a t e d b u t e a c h o x y g e n

i s bonded that

the

t o a s i l i c o n a t o m of v3

vibration

band

I t s h o u l d be n o t e d

t h e lattice.

in

the

spectra

of

many

crystalline

i n o r g a n i c b o r a t e s i s a l s o s p l i t C r e f . 91. In

to

order

get

information

more

s p l i t t i n g , t h e s p e c t r a of

on

a n d H-boralites

Na-

c o n t e n t s C a c t i v a t e d a t 773 KI w e r e r e c o r d e d . if in

the

lattice.

boralite

one

of

them

p r e f e r e n t i a l l y a t l o w boron c o n t e n t s .

of

the

w i t h v a r i o u s boron

I t w a s supposed t h a t

t w o k i n d s o f b o r o n sites d i f f e r i n g i n B-0 the

origin

force c o n s t a t e x i s t be

should

I n t h i s case,

occupied

the intensity

r a t i o of

b o t h m a x i m a 1380 a n d 1 4 0 5 c m - l

content.

On t h e o t h e r h a n d i f t h e s p l i t t i n g t h e v 3 b a n d w a s d u e t o

should change w i t h boron

t h e d e g e n e r a t i o n removal,

t h e i n t e n s i t y r a t i o is expected n o t

depend

The s p e c t r a of H-boral i tes c o n t a i n i n g :

on boron c o n t e n t . 0.59,

0.28,

1 . 0 5 B/u. c .

0 . 6 9 and

i n t e n s i t y r a t i o of

It

suggests

that

reason f o r t h e s p l i t t i n g of units.

Of

course t h e

i n

Fig.

The

1.

bands p r a c t i c a l l y does n o t

T h e s a m e w a s a l s o o b s e r v e d i n t h e case of

depend on b o r o n c o n t e n t .

Na-boralites.

are presented

1380 a n d 1 4 0 5 c m - l

to

the

degeneration

t h e v3 b a n d o f

possibility

d i f f e r i n g i n B-0 f o r c e c o n s t a n t

that but

is

the

€3-0 s t r e t c h i n g i n

removal

B03

t w o kinds

without

of

boron

sites

preference i n their

o c c u p a t i o n c a n n o t b e t o t a l 1y e x c l u d e d . Fig.

2

shows

the

plots

of

intensities

d o u b l e t as a f u n c t i o n of boron c o n t e n t i n N a t h e case o f

H-boralites.

t h e loss of

of

1380,

1405 cm-l

a n d H-boralites.

In

boron d u r i n g t h e a c t i v a t i o n

o f NH -form w a s t a k e n i n t o a c c o u n t . B o t h p l o t s a r e l i n e a r . I n t h e 4 case of H - b o r a l i t e s t h e s l o p e i s much h i g h e r t h a n i n N a - b o r a l i t e s .

M o s t probably t h i s is due t o t h e higher e x t i n c t i o n c o e f f i c i e n t of t h e v3 B - 0 s t r e t c h i n g Cand h e n c e t o h i g h e r 8-0 bond p o l a r i z a t i o n > . We

can

however

Na-boralites

exclude a possibility

not

are n o t

i n B03

units

and i t

that

some

B atoms i n

could r e s u l t

in a low

i n t e n s i t y o f t h e B 0 3 band.

Our p r e v i o u s s t u d y C r e f . boron

content

results

in

the

101 h a s shown t h a t t h e i n c r e a s e i n contraction

of

the

unit

cell

of

b o r a l i t e s ( b o r o n i s s m a l l e r t h a n s i l i c o n > . T h e force c o n s t a n t s o f Si-0 s t r e t c h i n g i n

content.

boralites

w e r e f o u n d t o be i n d e p e n d e n t

of

B

T h i s l a s t e f f e c t i s d i s c u s s e d i n t e r m s of t h e c o l l e c t i v e

685

Fig. 1. The B-0 band in t h e spectra of *H-boralites of various B contents: a - 0.28, b - 0.59, c - 0.69, d - 1 . 0 B1u.c.

686

7 c

I

5 -

er, \

E \

In 0

.

2

3 -

-

0

(0

$ a

1 2

1

3 D I U.C.

F i g . 2. - The i n t e n s i t i e s of B - 0 bands i n t h e s p e c t r a of: N a - b o r a l i t e s Col and H - b o r a l i t e s Col as a f u n c t i o n of B c o n t e n t .

-P

/

0 .-

theoret.

U

Cl 0

a

c

v)

C :0

115;

-,,

LewisL

1.o (B+Al-Na)/u.C. F i g . 3. - The c o n c e n t r a t i o n s of Brgnsted a c i d sites COD, t h e o r e t i c a l values of c o n c e n t r a t i o n of Brgnsted sites Co> and t h e i n t e n s i t y of PyL band i n H - B o r a l i t e s a s a f u n c t i o n of boron c o n t e n t Co3

model

zeolites

of

Cthe

of

electronegativities

and

Si

are

B

practically the same>. Aci d proper ti es of H-bor a1 i tes N a - b o r a l i t e c o n t a i n s o n l y 3740 c m to

analogous

those

in

zeolites.

d e c o m p o s i t i o n of NH - f o r m CB-OH>.

OH g r o u p s

3460 cm-'

The

CtermFnal Si-OH> ion

NafiH4

Cref.

3.4):

CSi-OH.--03.

3720 cm-'

Only 3720

cm

groups,

exchange

r e s u l t s i n t h e a p p e a r a n c e of

4

k i n d s of

-1

and

t h r e e new

CSi-OH---B>. 3680 cm-' -1 hydroxyls w e r e found

t o be B r o n s t e d a c i d sites. T h e i r a c i d s t r e n g t h

i s much l o w e r t h a n

i n z e o l i t e s C r e f . 2-5) e v e n though t h e e l e c t r o n e g a t i v i t y of

boron

t h a n t h a t of aluminum C 2 . 9 3 a n d 2,22 r e s p . > . I t c a n be

is higher

e x p l a i n e d by w e a k i n t e r a c t i o n between t h e OH g r o u p a n d s m a l l b o r o n a t o m i n t h e Si-OH-.--B u n i t s .

The c o n c e n t r a t i o n of B r o n s t e d a c i d sites C3720 cm-' in

H-boralites

various

of

pyridine sorption.

boron

contents

P y r i d i n e molecules

+

react

determined

with

BrGnsted

sites f o r m i n g p y r i d i n i u m i o n s CPyH 3 f o r which 1 5 4 5 cm-' is

characteristic.

H-boralites band.

portion

Small

of

Si -OH-.-B>

was

pyridine

was

by acid

I R band sorbed

in

a t 430 K up t o t h e c o n s t a n t i n t e n s i t y of t h e 1 5 4 5 cm-'

The c o n c e n t r a t i o n of B r z n s t e d a c i d sites w a s c a l c u l a t e d f r o m t h e 1 5 4 5 cm-'

t h e m a x i m a l i n t e n s i t y of coefficient

determined

c o n c e n t r a t i o n of i n Fig.

in

a

previous

band a n d i t s e x t i n c t i o n study

Cref.

B r z n s t e d a c i d sites i n H-boralites

3 a s a f u n c t i o n of

The

is p r e s e n t e d

boron c o n t e n t C t h e loss of

t a k e n i n t o a c c o u n t > . The t h e o r e t i c a l v a l u e s :

11>.

boron is

t h e c o n c e n t r a t i o n s of

B + A1 - N a ( o u r b o r a l i t e s c o n t a i n e d small amounts of Al> are also p r e s e n t e d i n t h e same f i g u r e . The c o n c e n t r a t i o n of BrGnsted sites in

H-boralite

increases

linearly

with

the

e x p e r i m e n t a l v a l u e s are close t o t h e o r e t i c a l

boron

content.

The

ones. Small d e f i c i t

of B r z n s t e d sites may be d u e t o s o m e d e h y d r o x y l a t i o n p a r t i c u l a r l y -1 band of p y r i d i n e bonded t o Lewis a c i d sites s i n c e a w e a k 1460 c m

CPyL3 i s p r e s e n t i n t h e s p e c t r u m of p y r i d i n e s o r b e d i n b o r a l i t e s . The i n t e n s i t y of

t h e 1 4 6 0 cm-'

PyL band ( F i g .

2) i s l o w a n d

p r a c t i c a l l y t h e same i n a l l t h e b o r a l i t e s s t u d i e d i n d i c a t i n g t h a t t h e d e g r e e of content.

d e h y d r o x y l a t i o n i s small a n d i n d e p e n d e n t

The f r e q u e n c y of

h i g h e r t h a n i n t h e case of

PyL band

of

i n b o r a l i t e s C1460 cm-'>

234-5 z e o l i t e s C1450

ern-',

ref.

boron is

11).

T h i s means t h a t t h e e l e c t r o a c c e p t o r p r o p e r t i e s of Lewis a c i d sites

688 i n H - b o r a l i t e s are s t r o n g e r t h a n i n H-ZSM-5.

Cref 3. 12.131. I t i s

p r o b a b l y d u e t o s m a l l s i z e and h i g h e l e c t r o n e g a t i v i t y of b o r o n .

The

authors

thank

Professor

Jerzy

Haber

of

Institute

C a t a l y s i s a n d S u r f a c e C h e m i s t r y of t h e P o l i s h Academy of

of

Sciences

i n Krakdw f o r a g r a n t t h a t made t h e p r e s e n t w o r k p o s s i b l e .

REFERENCES 1 2 3

4

5

6 7 8 9

10 I1 12 13

M. T i e l e n . M. G a l l e n a n d P. A. J a c o b s , P r o c . I n t . Symp. Z e o l i t e C a t a l y s i s , , S i o f o k , Hungary, May 1985. p. 1. G. C o u d u r i e r , J . C. V e d r i n e . P u r e Apl. Chem. , 58. C1986>, 1389. J. D a t k a and Z . Piwowarska. J . Chem. Soc. , F a r a d a y T r a n s . I , 85, C1989>, 47. J . D a t k a and Z. Piwowarska. J . Chem. Soc., F a r a d a y T r a n s . I . 85, C19893, 837. K. F.M.G. J . S c h o l l e . A.P.M. Kentgens. W . S . Veeman, P. F r e n k e n a n d G. P. M. van d e r Velden. J . Phys. Chem. , 88. C19843, 5. P. Ratnasamy, S.G. Hegde a n d A. J . Chandwalkar, J . C a t a l . , 102, 1986. 467. A. C i c h o c k i , M. M i c h a l i k , M. Buf a n d J . P a r a s i e w i c z Kaczmarska, Zeolites. 10. (19893. i n p r i n t . A. C i c h o c k i . M. M i c h a l i k . M. BUS. W. Lasocha a n d Z. Saw&owicz, Zeolites. 10. Cl989>, i n p r i n t . C.W. Weir and R . A . S c h r o e d e r . J o u r n a l of R e s e a r c h of t h e N a t i o n a l Bureau of S t a n d a r d s A. 6 8 A C 5 > , 465. 1964. A. Cichocki. J. Datka, M. M i c h a l i k . A. O l e c h , a n d Z . Piwowarska. J . Chem. Soc., F a r a d a y T r a n s a c t i o n s 1 . 86C4). c19903. 753. J . Datka and E. TuPnik, J . C a t a l . , 102. C19863. 43. J . W . Ward, J. C a t a l . , 10. C19681, 34. J . W. Ward, J . C o l l . I n t e r f a c e Ski. , 28, ClQ683. 269.


689

AN ELECTROSTATIC MODEL TO PREDICT THE INFRA RED CHARACTERISTICS OF ZEOLITE

HYDROXYL GROUPS AFTER ADSORPTION OF AROMATICS P a t r i c k J . O'Malley, Department o f Chemistry, UMIST, Manchester M60 l Q D U.K SUMMARY

The p e r t u r b a t i o n o f t h e h i g h frequency (h.f.1, h y d r o x y l s t r e t c h i n g i n f r a r e d band o f a HY z e o l i t e i s i n v e s t i g a t e d a f t e r a d s o r p t i o n o f benzene, t o l u e n e , o-xylene, p-xylene, m-xylene and cumene. A s h i f t o f t h e band t o a lower frequency accompanied by an i n c r e a s e i n i n t e n s i t y i s noted. The f r e q u e n c y s h i f t v a r i e s i n t h e o r d e r , o-xylene > m-xylene = U t i l i s i n g a t h e o r e t i c a l model f o r t o l u e n e = cumene > benzene = p-xylene. t h e i n t e r a c t i o n which t a k e s account o f o n l y t h e d i e l e c t r i c p r o p e r t i e s o f t h e h y d r o x y l group environment i t i s demonstrated t h a t t h e v i b r a t i o n a l p r o p e r t i e s p r e d i c t e d by such a model i . e . f r e q u e n c y s h i f t and i n t e n s i t y change a r e i n c l o s e agreement w i t h e x p e r i m e n t a l l y determined q u a n t i t i e s . INTRODUCTION

The i n t e r a c t i o n o f aromatic molecules w i t h t h e h y d r o x y l groups o f z e o l i t e s r e s u l t s i n a decreased frequency and an i n c r e a s e d i n t e n s i t y f o r t h e h y d r o x y l i n f r a r e d band.

The e x t e n t o f p e r t u r b a t i o n o f t h e h y d r o x y l

band i s o f t e n r e l a t e d t o t h e a c i d s t r e n g t h (1,21. I n t h i s s t u d y a s e r i e s o f aromatic molecules i . e . o-xylene,

p-xylene,

benzene, t o l u e n e ,

m-xylene and cumene a r e absorbed on a 100% exchanged

HY sample and t h e i r e f f e c t on t h e h i g h f r e q u e n c y ( h . f . )

monitored. h.f.

h y d r o x y l group i s

Both t h e decrease i n frequency and i n c r e a s e d i n t e n s i t y o f t h e

band a r e m o n i t o r e d and an e l e c t r o s t a t i c model, due o r i g i n a l l y t o

Coggeshall ( 3 1 , i s u t i l i s e d t o p r e d i c t t h e e x p e r i m e n t a l l y observed i n f r a red properties.

EX PER I MENTAL D e t a i l s o f t h e sample p r e p a r a t i o n and i n f r a r e d s t u d i e s a r e as described previously (4).

100% exchange o f Na by NH4 (NH4Y-100) was

achieved by t r e a t i n g t h e sample w i t h c o n c e n t r a t e d NH&l c o n d i t i o n s f o r s e v e r a l weeks. HY-100.

under r e f l u x

Subsequent c a l c i n a t i o n g i v e s r i s e t o

For benzene, t o l u e n e and o-xylene s a t u r a t i o n o f t h e a d s o r p t i o n

s i t e s was achieved r a p i d l y , however f o r p-xylene, m-xylene and cumene t h e o r g a n i c vapour had t o be l e f t i n c o n t a c t w i t h t h e z e o l i t e d i s c f o r 1 hour f o r reasonable coverages t o be a t t a i n e d .

690

:I

\

::

z

s

p

v1

B bp

\J

3600

3400

3200

~/c,'~

F i g 1 . E f f e c t o f a d s o r D t i o n on t h e hydroxyl s t r e t c h i n g ' region of HY-100. ( a ) HY-100, ( b ) HY-1001 benzene, ( c ) HY-lOO/toluene, ( d ) HY-lOO/o-xylene.

3600

3400 3200

p/c In-i F i g . 2. E f f e c t of a d s o r p t i o n on t h e hydroxyl s t r e t c h i n g x g i o n o f HY-100. ( a ) HY-100, ( b ) rlY-lOO/ p-xylene, ( c ) HY-lOO/m-xylene, ( d ) HY-lOO/cumene.

691 RESULTS AND DISCUSSION The e f f e c t o f a d s o r p t i o n o f benzene, t o l u e n e and o-xylene on t h e h y d r o x y l s t r e t c h i n g bands o f HY-100 i s i l l u s t r a t e d i n f i g . 1 w h i l e f i g . 2 shows t h e e f f e c t o f a d s o r p t i o n o f p-xylene,

m-xylene and cumene.

In all

cases, a d s o r p t i o n leads t o a s h i f t t o lower f r e q u e n c i e s o f t h e h . f . band and a l s o causes a c o n s i d e r a b l e broadening and i n c r e a s e i n i n t e n s i t y of t h i s band.

I n a l l cases, a d s o r p t i o n does n o t l e a d t o p e r t u r b a t i o n o f t h e

1 . f . band which i s s i m i l a r t o t h a t found by p r e v i o u s workers f o r a d s o r p t i o n o f r a r e gases and alkane molecules (5).

T h i s can be e x p l a i n e d

by t h e i n a c c e s s i b l e p o s i t i o n s o f t h e h y d r o x y l groups g i v i n g r i s e t o t h i s band.

The magnitude o f t h e s h i f t o f t h e h . f . band t o lower f r e q u e n c i e s

decreases i n t h e o r d e r o-xylene > m-xylene = t o l u e n e = cumene > benzene = p-xylene (see Table 1 ) . The i n c r e a s e i n i n t e n s i t y and s h i f t t o l o w e r f r e q u e n c i e s on a d s o r p t i o n can be a t t r i b u t e d as r e s u l t i n g f r o m e l e c t r o s t a t i c i n t e r a c t i o n s o f t h e OH d i p o l e w i t h t h e adsorbed molecules.

T h i s r e s u l t s i n a displacement o f t h e

e l e c t r i c a l charge i n t h e s e molecules, and as a r e s u l t , t o an i n c r e a s e i n t h e d i p o l e moment as w e l l as a decrease i n t h e f o r c e c o n s t a n t o f t h e OH bond.

Coggeshall (3) has g i v e n a t h e o r e t i c a l s o l u t i o n t o t h e problem of

an o s c i l l a t i n g d i p o l e p e r t u r b e d by an e l e c t r o s t a t i c f i e l d .

This theory

was subsequently extended by White e t a l . (5) and used q u i t e s u c c e s s f u l l y i n p r e d i c t i n g t h e s h i f t s and changes i n i n t e n s i t y undergone b y t h e h i g h frequency OH s t r e t c h i n g band when t h e OH o s c i l l a t o r i n t h e supercage was surrounded by p o l a r i s a b l e adsorbed molecules.

I n order t o deal w i t h t h e

problem o f an OH group p e r t u r b e d by an e l e c t r o s t a t i c f i e l d Coggeshall has adopted a s i m p l i f i e d f o r c e f i e l d s i t u a t i o n and assumed t h a t t h e h y d r o x y l group can be c o n s i d e r e d as a d i a t o m i c group possessing a d i p o l e moment immersed i n a c o n s t a n t e l e c t r o s t a t i c f i e l d .

T h i s w i l l r e s u l t i n an

i n t e r a c t i o n energy between t h e d i p o l e and t h e e l e c t r i c f i e l d which w i l l depend on t h e d i p o l e moment, t h e angle o f o r i e n t a t i o n and t h e e l e c t r i c f i e l d strength.

Since a l l t h e v i b r a t i o n a l energy terms w i l l be s h i f t e d b y

t h e same amount, t h e r e w i l l be no change i n t h e t r a n s i t i o n e n e r g i e s between t h e v i b r a t i o n a l s t a t e s as a r e s u l t o f t h i s i n t e r a c t i o n energy term.

There w i l l , however, be a change i n t h e p o t e n t i a l energy o f t h e

d i a t o m i c group due t o p o l a r i s a t i o n o f t h e OH bond by t h e e l e c t r i c f i e l d . T h i s w i l l be g i v e n by -qE z where q i s t h e charge unbalance f o r t h e group, P i s t h e e l e c t r i c f i e l d component p a r a l l e l t o t h e d i r e c t i o n o f t h e P valence bond and z i s t h e i n t e r n u c l e a r displacement f r o m e q u i l i b r i u m .

E

F o l l o w i n g r e f . (3) t h i s leads t o an e x p r e s s i o n f o r t h e p o l a r i s a t i o n f o r c e

Q,

N W

TABLE 1 Observed and t h e o r e t i c a l p e r t u r b a t i o n s o f h . f . z e o l i t e hydroxyi band

Adsorb a t e

Benzene To1 uene 0- Xy 1e ne m-Xy 1e ne p-Xylene Cumene

t

'obs.

2.274 2.379 2.568 2.374 2.270 2.380

310 350 380 350 300 340

Ap2 pl

Elql

5.33 6.88 8.82

3.729 4.070 4.330 -

-

-

-

2

6 -

42

P1

41

1.093 1 .lo6 1.128 -

-

2.31 2.57 2.95 -

1.61 1.58 1.47 -

5.33 6.64 8.73 -

-

-

-

-

E

M't

P E

2.37 2.54 2.91 2.53 2.35 2.54

1.49 1.51 1.55 1.51 1.48 1.51

(

",

5.61 6.45 8.47 -

-

''talc.

300 330 410 330 295 330

693 qE as qE P = [I - $(v/uo) - v / u o ~ + l / 1 6 ( u / v o ~ ) D

P

(1)

w D ) i

where

a

=

I f t h e wave number o f t h e f r e e OH v i b r a t i o n i s 3750 cm-’ d i s s o c i a t i o n energy ( D ) o f t h e OH bond i s 461.077 kJmol-’,

and t h e then t h e

Equation 1 P a l s o enables us t o c a l c u l a t e t h e v a r i a t i o n i n t h e p o l a r i s a t i o n f o r c e (Eq)

p o l a r i s a t i o n f o r c e (qE

)

can be deduced f r o m t h e r a t i o v/uo.

on t h e hydrogen atom o f t h e OH o s c i l l a t o r b r o u g h t about by t h e p h y s i c a l a d s o r p t i o n of gases and vapours and i t can be expressed as

where E2 and E l a r e t h e f i e l d s a t t h e d i p o l e , i n t h e presence and t h e absence of adsorbed molecules r e s p e c t i v e l y and u2 and v1 a r e t h e c o r r e s p o n d i n g wave numbers. F o l l o w i n g t h e methods adopted by White e t a l . (5) a model f o r t h e i n t e r a c t i o n o f t h e OH group w i t h adsorbed a r o m a t i c s can be proposed which t r e a t s t h e h y d r o x y l group as a p o l a r i s a b l e d i p o l e which i s p l a c e d a t t h e c e n t e r o f an e l l i p s o i d a l c a v i t y .

T h i s i n t u r n i s immersed i n a c o n t i n o u s

d i e l e c t r i c of d i e l e c t r i c c o n s t a n t

which i s a c t e d on by a homogeneous

electrostatic f i e l d E parallel t o the a axis of the e l l i p s o i d .

Because of

t h e p a r t i c u l a r geometry o f t h e OH group, t h e e l l i p s o i d adopted was c o n s i d e r a b l y f l a t t e n e d i n t h e d i r e c t i o n o f t h e f i e l d and had dimensions o f i n t h e d i r e c t i o n o f t h e f i e l d and a l e n g t h of 2.486

0.994

perpendicular t o t h e f i e l d .

a

The former v a l u e i s a p p r o x i m a t e l y t h e l e n g t h

o f t h e O H bond w h i l e t h e l a t t e r i s r o u g h l y t h e d i a m e t e r o f t h e oxygen atom i n silicates.

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

a t t h e d i p o l e i n t h e presence o f t h e d i e l e c t r i c a r e subsequently g i v e n ( 5 ) by MS/M =

Eh/E

=

(3)

(1.397+

y3+ CH3-C-CH-CH-CH3

-_____

“I

t , CHZ-CH-CH-CH2-CH2-CH3 - - -. - - - -

CH2- C- CH- CH2- CH3 __---

-_--

t h e r e should be an e q u i l i b r i u m m i x t u r e o f v a r i o u s monoenyl carbenium i o n s commonness o f which c o n s i s t s i n a c 6 backbone a b s o r b i n g a t d i f f e r e n t

the

frequen-

c i e s . The s p e c t r o s c o p i c measurements were performed on z e o l i t e s w i t h d i f f e r e n t B/L r a t i o s . I t i s accepted t h a t t h e B r o n s t e d a c i d s i t e s p l a y t h e dominant r o l e i n oligomerization o f olefins, reaction amount

b u t t h e p a r t i c i p a t i o n o f Lewis c e n t e r s i n t h i s

cannot be n e g l e c t e d ( r e f s . of

olefins

Bronsted a c i d s i t e s ,

and

reaction

consequently

the

21,22).

It f o l l o w s t h a t the higher

t h e more pronounced t h e a l k e n y l carbenium

absorb a t h i g h e r wavelengths.

ions

oligomerization formed

during

This i s the experimental

the of this

observation

w i t h b o t h propene and a l l e n e on b o t h t y p e s o f z e o l i t e s (see F i g s . 3 , 4 ) . In range

the

s p e c t r a o f adsorbed a l l e n e t h e p r e v a i l i n g band was

359-380

nm

characteristic f o r dienyl

f o l 1 owing t r a n s f o r m a t i on :

carbenium

found

ions.

in

the

Assuming

the

t

CH2-C-CH-CH-CH2

-_------

t h e g e n e r a t i o n o f i n c r e a s i n g l y u n s a t u r a t e d carbenium i o n s s h o u l d be The

i d e a was t h a t i f t h e number o f B r o n s t e d a c i d s i t e s o f t h e z e o l i t e

was decraesed by d e h y d r o x y l a t i o n , ion

exchange

carbenium creasing Fig,

(ref.

ions

4c),

the

i n s t e a d o f l o w e r i n g t h e degree o f e x i s t e n c e o f more and

s h o u l d be observed by U V - V I S

dehydroxylation the strength o f

increases, in

expected.

more

spectroscopy,

Bronsted acid s i t e s

simple since

sample ammonium monoenyl

with

probably

inalso

t h e r e b y p r o m o t i n g t h e s t a b i l i t y o f a g i v e n carbenium i o n . As shown

4

our

e x p e c t a t i o n came t r u e :

t h e lower the

B/L

ratio

the

more

703

pronounced

290 nm on b o t h z e o l i t e s .

band was observed a t ca.

explanation

detailed

above

f o r the s h i f t s o f U V - V I S

bands

Accepting to

the

the

higher

wavelengths, t h e i n t e r p r e t a t i o n o f t h e r e v e r s e s h i f t o c c u r r i n g upon e v a c u a t i o n o f t h e sample a t i n c r e a s i n g temperatures i s t h e f o l l o w i n g : higher

the

remaining

formed

o l i g o m e r s undergo c r a c k i n g

and/or

A t about 400 K

desorb,

thereby

carbenium i o n s ( i n c l u d i n g t h e a l k e n y l t y p e s ) a r e o f s h o r t e r

or the

carbon

c h a i n type, thus a b s o r b i n g a t h i g h e r f r e q u e n c i e s . The

maximum a b s o r p t i o n o f t h e a l l y 1 c a t i o n b e i n g

carbenium

i o n was

predicted

to

be

a t 293

the

nm as

a

simplest result

monoenyl

of

LCAO-MO

18). The LVSE e n e r g i e s c a l c u l a t e d by CNDO/S and MNDO-HE methods a r e l i s t e d i n Table 2. calculations

(ref.

Table 2. Lowest v e r t i c a l s i n g l e t e x c i t a t i o n e n e r g i e s and s t a n d a r d heats o f f o r m a t i o n o b t a i n e d by s e m i e m p i r i c a l quantum chemical methods Carbocation

LVSE energies/nm CNOO/S//MNDO MNDO-HE//MNDO

Standard h e a t o f f o r m a t i o n bv MNDO k c a l /&l rel. abs.

abs

re1 .

abs.

rel.

220

0

281

0

221.4

is o p r o p y l a1 l y l

237

17

289

8

200.0

-21.4

n-propyla1 l y l

237

17

289

8

195.8

-25.6

l-ethyl-2methyl-ally1

253

33

314

33

195.2

-26.2

l-ethyl-3methyl-ally1

234

14

300

19

187.0

-34.4

241 286

21 0

313 299

32 0

185.4 219.8

-36.0 0

pentadienyl

290

4

306

7

207.3

-12.5

2-methyl pentadienyl

297

11

311

12

214.7

-5.1

305

17

328

29

188.7

-31.1

325

37

330

31

182.8

-37.0

a1 l y l

0

1,1,3-tri methyl -a1 l y l pentadienyl

1 -methyl -

1,1,3,5tetramethylpentadienyl

1,1,5,5-

tetramethyl pentadienyl

It

can be seen t h a t t h e more s u b s t i t u t e d monoenyl and d i e n y l

should ions

absorb a t h i g h e r wavelengts.

Futhermore,

under study a l s o i n c r e a s e d w i t h i n c r e a s i n g

results

carbeniurn

the s t a b i l i t y o f methyl

a r e i n good accordance w i t h t h e e x p e r i m e n t a l

carbenium

substitution.

observations

ions These

discussed

704

above.

According t o t h e r e s u l t s o f quantum chemical c a l c u l a t i o n s , t h e a s s i g n -

ment o f a g i v e n U V - V I S band measured t o a w e l l - d e f i n e d a l k e n y l i o n seems t o be rather d i f f i c u l t .

By t h e e x p e r i m e n t a l l y observed band o n l y a group o f a l k e n y l

i o n s can be c h a r a c t e r i z e d . The

generation

of

unsaturated

carbenium i o n s

was

proven

also

by

IR

spectroscopy. The band g e n e r a l l y observed a t 1505-1510 c d was regarded as t h e t y p i c a l band o f these types o f carbenium i o n s . CONCLUSIONS The lower

the

B/L a c i d i t y r a t i o o f t h e z e o l i t e ,

the higher w i l l

be

the

frequency o f t h e a l k e n y l carbenium i o n s formed f r o m propene and a l l e n e on b o t h types

of

zeolites.

T h i s c h a r a c t e r i s t i c can be a t t r i b u t e d

to

the

enhanced

f o r m a t i o n o f o l i g o m e r s on t h e z e o l i t e s o f h i g h e r B r o n s t e d a c i d i t y . The h i g h e r t h e B/L r a t i o o f t h e sample,

the l e s s intenses the formation o f

monoenyl carbenium i o n s a b s o r b i n g a t 280-310 nm. by

assuming

an

increase o f the strength o f the

T h i s f a c t m i g h t be e x p l a i n e d Bronsted

decreasing c o n c e n t r a t i o n s (as a r e s u l t o f dehydroxyl a t i o n ) ,

acid

sites

with

thereby promoting

t h e s t a b i l i t y o f t h e monoenyl carbenium i o n s formed f r o m a l l e n e . ACKNOWLEDGEMENT One o f t h e a u t h o r s ( I . K.) thanks t h e Alexander von Humboldt F o u n d a t i o n f o r a r e s e a r c h f e l l o w s h i p . The f i n a n c i a l s u p p o r t o f t h e Hungarian Academy o f Sciences (No. OTKA 217/88) and t h e Deutsche Forschungsgemeinschaft i s g r a t e f u l l y acknowledged REFERENCES 1 H F o r s t e r , R . Seelemann, J. C . S . Faraday Trans. I , 74 (1978) 1435 2 H F o r s t e r , J. Seebode, Z e o l i t e s , 3 (1983) 63 I . K i r i c s i , G. T a s i , P . F e j e s , F. Berger, J . Mol. C a t a l . , 51 1989) 341 3 I . K i r i c s i , G. T a s i , I. Hannus, P . F e j e s , H. F o r s t e r , J. Mol. C a t a l . , i n press 4 ( a ) M. Zardkoohi, J. F . Haw, J. H. Lunsford, J. Am. Chem. SOC , 109 (1987) 5278 ( b ) A . S. Medin, V . Yu. Borovkov, V . B. K a z a n s k i i , K i n e t i k a i K a t a l iz. 30 (1989) 177 ( c ) H . F o r s t e r , 3 . Seebode, P . F e j e s , I . K i r i c s i , J . C . S. Faraday Trans. I , 83 (1987) 1109 5 E. A . Lombardo, R . P i e r a n t o z z i , W. K. H a l l , J . C a t a l ., 110 (1988) 171 6 J . P . van den Berg, J. P . Wolthuizen, A. D. H. Clauqe, G. R . Hays, R . Huis, J . H. C . van H o o f f , J. C a t a l . 80 (1983) 130 7 P. Fejes, H. F o r s t e r , I . K i r i c s i , J. Seebode, i n " S t r u c t u r e and R e a c t i v i t y of M o d i f i e d Z e o l i t e s " , D. A. Jacobs,N. I . Jaeger, P. J i r u , V . B . K a z a n s k i i G. S c h u l z - E k l o f f ( e d s . ) , E l s e v i e r , Amsterdam, 1984, p. 9 1 8 M. L . Poutzma, i n " Z e o l i t e Chemistry and C a t a l y s i s " , Am. Chem. SOC. Washington, Ed. J . A . Rabo (ACS Monograph 171) pp. 437 9 H. F o r s t e r , S. Franke,J. Seebode, J. C . S. Faraday Trans. I , 79 (1983) 373 10 I . K i r i c s i , G. T a s i , H. F o r s t e r , P . F e j e s , A c t a Phys. e t Chem. Szeged, 33 (1987) 69 11 G. Tasi, I . K i r i c s i , P . F e j e s , H. F o r s t e r , S. Lovas, Magy. Kem. F o l y o i r a t , 95 (1989) 520

705

12 13 14 15 16 17 18 19 20 21 22

M. J. S . Dewar, J. A. Hashmall, C . G. V e n i e r , J. Am. Chem. SOC., 90 (1968) 1953 H. H. J a f f e , R . L. E l l i s , i n " S e m i e m p i r i c a l Methods o f E l e c t r o n i c S t r u c t u r e C a l c u l a t i o n s " , G. A. Segal ( e d . ) , Plenum P r e s s , New York, 1977 I . K i r i c s i , H. F o r s t e r , J. C . S. F a r a d a y T r a n s . I , 84 (1988) 491 P. Kos. I . K i r i c s i . K. Varqa. - . P. Fe.ies. - . A c t a Phvs. e t Chem. Szeged, 33 (1987) 109 I . K i r i c s i , G. T a s i , H. F o r s t e r , P. F e j e s , J. M o l . S t r u c t . 218 (1990) 369 I . K i r i c s i , H. F o r s t e r , G. T a s i , P. F e j e s , J. C a t a l . , 115 1989) 597 T. Sorensen, i n "Carbonium I o n s " , e d s . G. A. O l a h and R. P S c h l e y e r ( W i l e y - I n t e r s c i e n c e ) New York, 1970, V o l . 2, p . 807 G. A. Olah, C . U. P i t t m a n n , M. C . R. Symons, i b i d . V o l . 1, p. 153 N. C . Deno, i b i d , V o l . 2, p . 783 L . Kubelkova, J . Novakova, B. W i c h t e r l o v a , P. J i r u , C o l l e c Czech. Chem. Commun., 45 (1980) 2290 J. Datka, J. C . S. F a r a d a y T r a n s . I , 77 (1981) 1309, 2633

.


70 7

AUTHOR INDEX

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

573 Aiello. R Albizane. A 603 Babayev. M.K 623 Barrer. R.M 257 Barthomeuf. D 157 Bauer. F 305 Bauermeister. R 367 Baur. W.H 581 Bee, M 445 Bergk. K.-H 185 Bordiga. S 671 Bohlig. H 511 Borovkov. V.Yu 653 BosaEek. V 337 Bragin. O.V. 357 Brunner. E 529 Biilow. M 445.479. 501 Buskens. Ph 47 Buysch. H.-J 297 Camblor. M.A 613 Caro. J 445 Eejka. J 387 Chen. Q 219 Cichocki. A 681 Crea. F 573 Datka. J 681 Dermietzel. J 305 Derouane. E.G 591 Des Courieres. Th 603 Di Renzo, F 603 Dressler. J 663 Dumont. N 591 Eckelt. R 1.415 Ehrhardt. B 663 Eic. M 233 Einicke. W.-D .............. 75. 529 Emig. G ................... 479 Ernst. H 397. 529 Ernst. S 297. 645 Fahlke. B 315 Fajula. F 603 Fejes. P 697 Feoktistova. N.N 287 Fichtner-Schmittler. H 549 Figueras. F 603 Finger. G 501. 537 Forster. H 697 Fraissard. J .............. 219 Freude. D 89. 397 Fricke. R 631 Fujimoto. K 203 Fiirtig. H 185 Gabelica. 2 ...............591 Ganbarov. D.M 623 Geidel, E 511 Geus. E . R 457 Golemme. G 573 Goncharuk. V 581

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

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

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

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

708

............... 613

Grobet. P.J Gross. T Hadicke. U Hantel, K Hayward. C.-M.T Hilgert. W Hoffmann, J Hunger. B Hunger, M Huybrechts. D.R.C Ito, T Jacobs. P.A Jaeger. N.1 Jancke, K Jansen, A.E Jansen. J.C Jerschkewitz.H G Jobic, H Jockisch. W Joyner. R.W Karge. H.G Karger. J Kazansky, V.B Kiricsi. I Klaeboe. P Kojima. M Kornatowski. J Kubelkova. L Kustov, L.M aaniecki. M Leofanti. G Lindner. D Lischke. G Loffler. E Lohse. U Lucke. B Martens. J.A Martin. A Meier, B Meier, W.M Minachev. Kh.M Mishin, 1.V Muller, B Nagy. J.B Nastro. A Nicolle, M.-A Novakova. J Nowak, 5 O'connor. C.T ohlmann. G O'Malley, P.J Padovan. M Parlitz, B Parton. R.F Petrini. G Peuker, Ch Pfeifer, H Pilz, W Piwowarska. 2 Pradhan. A.R

.................. 415 ................185 .................367 ...........171 ................ 479

............... 367 ................. 367 ................. 397,537 ..........47 .................... 591 ................ 47. 613 ............... 327 ................. 501 ...............457 ............... 457 .- ........... 1. 415 .................. 445 ............... 305 ...............357 ................133 .................. 89. 445 .............117.425 ................ 697 ................ 663

.................491 ............501.537, 581 ..............337. 405 ...............425 ............... 377 ...............671 ................297 ..................1. 415 .................. 1.425. 537 .................. 425. 549 ..................315 ..............613 .................315 ..................529 ................247 ............357 ............... 563 ................. 521 .................573. 591 ................. 573 ............. 603 ...............337. 405 .................. 315 .............491 .................. 1.415. 425 .............689

................ 671 ..................1,415.631 ................ 47 ................671 ................425. 511 .................89,397. 445 ...................511 .............681

.............. 347

709

Preobrazhensky. A.V Prilipko. A.1 Pritzkow, W Rao. B.S Raurell. G.L Rees. L.V.C Reschetilowski. W Richter, M Richter-Mendau. J Rozwadowski, M Ruthven. D.M Salzer. R Schollner. R Schoonman, J Schreier. E Schroder. K.-P Schulz. I Schulz-Ekloff. G Schwartz. S Schwieger. W Senchenya. 1.N Shiralkar. V . P Shpiro, E.S Siegel. H Sonntag. E Spoto, G Springuel-Huet, M.A Steinberg, K.-H Suboti6. B Svensson. A Sychev. M Szulzewsky. K Tagiyev. D.B Tasi, G Tatsumi. T Telbiz. G.M Timm, D Tkachenko. 0.P Tominaga. H Tuleuova. G.J Ullmann. G Van Bekkum, H Vasina. T.V Vaughan, D.E.W Von Ballmoos. R Voogd. P Vorbeck. G Vtjurina. L.M Wehner. K Weitkamp. J Wichterlova. B Wieker. W xu. z Zecchina. A Zhdanov. S.P Zholobenko. V.L Zibrowius, B Zmierczak. W

.......357

.............563 ............... 549 ..................347

.............. 387

................61 .........529 ................631 .........501. 549

............501 .............. 233 .................663 ...............75 ..............457 ................. 1. 631 ............ 435 .................415 ..........327 ............... 491 .............. 185 ............653 ............347 ............... 357 ................. 367 ................ 537 .................. 671 .......219 ...........367. 663 ................ 573 ............... 327 .................581 ............. 549. 631 .............. 623 ...................697 ................203 ...............563 ...................415 ............357 ............... 203 .............357 ................367 ............. 457,467 ...............357 ............275 ........... 171 .................. 467 ................ 631 .............287

................. 415 ................21.297. 645 ............387 .................315 ..................... 233 ...............671 .............. 287 ...........425

................ 1,537,549, 631 ..............377


711

SUBJECT INDEX

............... 337 .........405

Acetone adsorption Acetone. conversion from Acid-base pair Acidity Acid site characterization Activation Active sites Adsorption Adsorption of CO Alcohol-water mixtures Al in zeolites Alkali metal exchanged ZSM-5 Alkene oxidation Alkylation of toluene Allene Alloying particles Al-0 bond strength

...................1171.89.563.631.681. 697 ............................ .......133 ....................... 663 ..................... 425 ....................... 689 .................133.157.435.479. 671 ............75 ................... 387 ..... 297 .................. 47 ............387 ........................... 697 ...............357 ...............653 549 AlPO-14 .......................... Alumination...................... 467 Alumino-galloferrierite..........613 Approximate assignment...........511 Aromatic hydrocarbons............233 ........................ 203. 689 ............................ 247 .................... 21 ......................... 1571563 ...............203 ........................ 681 .................... 681 ...................697 .........337 ..................327 .................367 ........................ 157 ........315 ...............397 ..........645 ...........521 ................. 5731645 .................. 415 ........ 337 .................. 405 ........ 501 ......................... 117. 467 .............171 ................... 2871603 .................... 529 ..................2871591 .........573 ..................... ..247 491

Aromatics Atlas Base catalysis Basicity Bimetallic cluster B03 units Boralites MFI Carbenium ions Carbocations on zeolites Carbon deposits Carbon filaments Catalysis Catalyst characterization Catalytic activity Catalytic test reaction Cellular configuration Characterization Coke deposition Coke deposits on zeolites Coke in H-ZSM-5 Controlled crystal growth Cracking Cracking selectivity Crystal growth Crystallinity Crystallization Crystallization kinetics Crystallographic pore diameters Crystal size Curvature of configuration DABCO (triethylenediamine) Deactivation Dealumination Density of acid sites para-Diethylbenzene Diffuse of reflectance

....... 521 ....... 297 ..................... 347. 397 ...................... 1.185.415. 425 ............133 ..............479

...........663

712

........................ 2331305145714671479 ...................... 257 ....................... 327 ................ 157 .............653 ......................... 663 ............ 357 ..........6 3 1 EXAFS ............................ 357 Extraction of aluminium..........415 Extrudates....................... 491 Faujasite........................ 27Sl 327 FCC catalysts.................... 171 Ferrisilicates...................6 3 1 Fe-ZSM-5......................... 631 Framework density distribution...247 Heteroatom incorporation.........5 8 1 Heterogeneous nucleation.........603 n-Hexane cracking................425 High pressure chromatography.....479 H-mordenite...................... 347 Host/guest chemistry ..............2 1 Hybrid catalyst..................203 Hydrocarbon pyrolysis ............315 Hydrothermal crystallization.....623 Hydroxyls......................... ag1563168i H-ZSM-5 zeolite.................. 3 3 7 14 2 5 H-ZSM-12 ....................... ..347 Influence of geometry ............653 In-situ study .................... 663 Interface........................ 257 Intergrowth of structure.........219

Diffusion Diffusivity Dispersion Electronegativity Electronic structure Erionite Ethane hydrogenolysis Ethylbenzene conversion

....1 1 7 ..................4 2 5 1 5 3 7 1 6 6 3 1 6 8 1 .................... 3 0 5 ....5 8 1 .........537 .................... 203 ........347 ......................... 4 4 ......................... 2a 7 1 3 0 5 ......347 ............5 8 1 ........................... ...................35 40 7l I 5 a i ..............347 .................... i1 7 1 6 5 3 .......................... 275 ........... 7 5 MAS NMR .......................... 397 MAS N M R ( 2 7 Al) .................. 529 Matrix activity..................1 7 1 Matrix vs zeolite................1 7 1 Mechanism .......................... il i a 5 Membranes ........................ 2 5 7 1457 Methane aromatization............357 Methane splitting................367 Methanol ......................... 563 Methanol conversion ..............3 1Sl653 Methanol conversion to olefins...4 1 5 IR diffuse reflectance spectr IR spectroscopy Isomerization Isomorphous substituted ZSM-5 Isomorphous substitution Iso-paraffins Isopropylation of benzene Isotherms Kinetics Kinetics of cumene sorption KVS-5 molecular sieve La-H-Y Large crystals Large pore zeolites Lewis acidity Linde L Liquid phase adsorption

713

................1 ...................... 305 .................... 445 ........................ 257,689 ............ 435 ....................... 1. 1171327 ..................501 ...................... 623 MO-Y ............................. 377 Multinuclear N M R .................591 Nay ............................... 4 7 1 61 Ni-Mo-Y .......................... 377 Nitrogen bases ...................297 Ni-ZSM-5 catalyst................367 N M R in alumino-galloferrierite...613 NMR spectroscopy ............337 Methanol-to-olefins Methylation Microdynamics Modelling Modelling by computer Modification Molecular sieve Mountainite

... ....... ....................... (13c)

NMR spectroscopy. multinuclear 537. 549 Normal coordinate analysis 511 Nucleation 287. 591 Nucleation kinetics 573 Oligomerization 491 Organic intermediates 297 Para selectivity 387 Path of oxidation 405 Pelletization 75 Pentasil-type 185 Pentasil zeolites 75 Permeation 457 Phase transitions in AlPOs 549 Piperazine 297 Porosity 247 Propene 697 Properties 623 Proton-MAS-NMR 89 Pt-Cr pentasils 357 Pulsed field gradient N M R 89. 445 Quasi-elastic neutron scattering.445 Realumination 529 Regeneration 315 Restoration of active sites 405 Restoration of void volumes 405

.............. .................. ............ ................. ................ ..................... .................... ................. ....................... ....... ....................... ......................... .......................... ....................... .................... .................. ......... .................... ..................... ...... ...... SAPO-5 ........................... 5011537 SAPO-37 .......................... 591 SAPO-40 ........................ ..591 Secondary synthesis..............529 a gI445 Self-diffusion .................... Separation........................ 61 Shape selective catalysis .........21 Shape selective photochemistry ....21 Shape-selectivity ................387 Ship-in-the-bottle catalysts......21 ....................... ...................... ............ ................... .......................... ..................... ...........

Silicalite 233.457. 4 6 7 1 6 7 1 Silicalite-1 61.435 Silicon incorporation 537 Single crystal 457 Sorption 611257 Spectroscopy 697 Strength of acid sites 133 Structure defect .................219 Structure effect 157

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

714

.................. 247 ..........511 ...................... 377 ................327 ........................ 1851275157312 .................... 203 ........................ .................... 501 185 ................. 549 ........................ 491 ............... 603 ................ 75 ................ 367 .................... 467 ..................... 471671 .......... 521 ........... 521 . .. ...................521 415 .......... 233 ................... 377 ........ 581 ..........511 ................ 367 ....................... 219 ............................. 47 .................. 377 ........................ 219 .............................. 549 ...613 ...................... 435 ........631 ..................... 479 ........................ 233 ................. 603 171 ........................ .................. .................. 275 ...........397 623 ...................... 511 ......................... 20714451689 ................ ................275 645 ........................ 233 ........................ 563 ................... 645 ............................ 3 0 ~ 1 3 ~ ~ 1 3 a 7 1 4 3 5 1 ~ ~ ~ l ~ ~ ~ 1 ZSM-12 ........................... 603 ZSM-35 ........................... 645 ZSM-57 ........................... 645

Structure types Sub-unit cluster models Sulfidation Syngas conversion Synthesis Synthesis gas Tailoring Template-free Template removal Templates Tetraethylammonium Thermal desorption Thermal reduction Thiele theory Ti-silicalite Topological constraints Topological parameters Topological structure functions TPD of ammonia Transition state theory USY as support Vanadium silicalite KVS-5 Vibrational frequencies Vibration reactor Void space VPI-5 Water-gas shift Xenon-= XRD XRD in alumino-galloferrierite para-Xylene meta-Xylene isomerization Zeolite 13 x Zeolite A Zeolite activity Zeolite 8 Zeolite genesis Zeolite H-ZSM-5 Zeolite-like silicates Zeolite NaX Zeolites Zeolite sub-units Zeolite synthesis Zeolite X Zeolite Y Zeolite ZSM-57 ZSM-~

1

715

STUDIES IN SURFACE SCIENCE AND CATALYSIS Advisory Editors: 6. Delmon, Universite Catholique de Louvain, Louvain-la-Neuve,Belgium J.T. Yates, University of Pittsburgh, Pittsburgh, PA, U S A .

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Preparation of Catalysts 1. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the First International Symposium, Brussels, October 1417,1975 edited by B. Delrnon, P.A. Jacobs and 0. Poncelet The Control of the Reactivity of Solids. A Critical Survey of the Factors that Influence the Reactivity of Solids, with Special Emphasis on the Control of the Chemical Processes in Relation to Practical Applications by V.V. Boldyrev, M. Bulens and B. Delrnon Preparation of Catalysts It. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Second International Symposium, Louvain-la-Neuve, September 4-7,1978 edited by B. Delrnon, P. Grange, P. Jacobs and G. Poncelet Growth and Properties of Metal Clusters. Applications to Catalysis and the Photographic Process. Proceedings of the 32nd International Meeting of the Societe de Chimie Physique, Villeurbanne, September 24-28, 1979 edited by J. Bourdon Catalysis by Zeolites. Proceedings of an InternationalSymposium, Ecully (Lyon), September 9- 1 1,1980 edited by B. Irnelik, C. Naccache, Y. Ben Taarit, J.C. Vedrine, G. Coudurierand H. Praliaud Catalyst Deactivation. Proceedings of an InternationalSymposium, Antwerp, October 13-15,1980 edited by B. Delmon and G.F. Frornent New Horizons in Catalysis. Proceedings of the 7th InternationalCongress on Catalysis, Tokyo, June 30-July 4, 1980. Parts A and B edited by T. Seiyama and K. Tanabe Catalysis by Supported Complexes by Yu.1. Yerrnakov, B.N. Kuznetsov and V.A. Zakharov Physics of Solid Surfaces. Proceedings of a Symposium, Bechyfie, September 29-October 3, 1980 edited by M. UzniEka Adsorption at the Gas-Solid and Liquid-Solid Interface. Proceedings of an International Symposium, Aix-en-Provence, September 2 1-23, 198 1 edited by J. Rouquerol and K.S.W. Sing Metal-Support and Metal-Additive Effects in Catalysis. Proceedings of an International Symposium, Ecully (Lyon), September 14-16, 1982 edited by B. Irnelik, C. Naccache, G. Coudurier, H. Praliaud, P. Meriaudeau, P. Gallezot, G.A. Martin and J.C. Vedrine Metal Microstructures in Zeolites. Preparation - Properties - Applications. Proceedings of a Workshop, Brernen, September 22-24, 1982 edited by P.A. Jacobs, N.I. Jaeger, P. Jird and G. Schulz-Ekloff Adsorption on Metal Surfaces. An Integrated Approach edited by J. Benard Vibrations at Surfaces. Proceedings of the Third InternationalConference, Asilomar, CA. September 1-4, 1982 edited by C.R. Brundle and H. Morawitz

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Heterogeneous Catalytic Reactions Involving Molecular Oxygen by G. I.Golodets Preparation of Catalysts Ill. Scientific Bases for the Preparationof Heterogeneous Catalysts. Proceedings of the Third International Symposium, Louvain-la-Neuve, September 6-9, 1982 edited by G. Poncelet, P. Grange and P.A. Jacobs Spillover of Adsorbed Species. Proceedings of an International Symposium, LyonVilleurbanne, September 12-1 6, 1983 edited by G.M. Pajonk, S.J. Teichner and J.E. Germain Structure and Reactivity of Modified Zeolites. Proceedings of an International Conference, Prague, July 9-1 3, 1984 edited by P.A. Jacobs, N.I. Jaeger, P. Jirir, V.B. Kazansky and G. Schulz-Ekloff Catalysis on the Energy Scene. Proceedings of the 9th Canadian Symposium on Catalysis, Quebec, P.Q., September 30-October 3, 1984 edited by S.Kaliaguine and A. Mahay Catalysis by Acids and Bases. Proceedingsof an International Symposium, Villeurbanne (Lyon), September 25-27, 1984 edited by B. Imelik, C. Naccache, G. Coudurier, Y. Ben Taarit and J.C. Vedrine Adsorption and Catalysis on Oxide Surfaces. Proceedings of a Symposium, Uxbridge, June 28-29,1984 edited by M. Che and G.C. Bond Unsteady Processes in Catalytic Reactors by Yu.Sh. Matros Physics of Solid Surfaces 1984 edited by J. Koukal Zeolites: Synthesis, Structure, Technology and Application. Proceedings of an InternationalSymposium, Portoroi-Portorose, September 3-8, 1984 edited by B. Driaj, S.HoEevar and S.Pejovnik Catalytic Polymerization of Olefins. Proceedings of the International Symposium on Future Aspects of Olefin Polymerization, Tokyo, July 4-6, 1985 edited by T. Keii and K. Soga Vibrations at Surfaces 1985. Proceedings of the Fourth International Conference, Bowness-on-Windermere, September 15-1 9, 1985 edited by D.A. King, N.V. Richardson and S.Holloway Catalytic Hydrogenation edited by L. Cervenl New Developments in Zeolite Science and Technology. Proceedings of the 7th InternationalZeolite Conference, Tokyo, August 17-22, 1986 edited by Y. Murakami, A. lijima and J.W. Ward Metal Clusters in Catalysis edited by B.C. Gates, L. Guczi and H. Knozinger Catalysis and Automotive Pollution Control. Proceedings of the First InternationalSymposium, Brussels, September 8-1 1, 1986 edited by A. Crucq and A. Frennet Preparation of Catalysts IV. Scientific Bases for the Preparationof Heterogeneous Catalysts. Proceedings of the Fourth International Symposium, Louvain-la-Neuve, September 1-4, 1986 edited by B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet Thin Metal Films and Gas Chemisorption edited by P. Wissmann Synthesis of High-silica Aluminosilicate Zeolites by P.A. Jacobs and J.A. Martens Catalyst Deactivation 1987. Proceedings of the 4th International Symposium, Antwerp, September 29-October 1, 1987 edited by B. Delmon and G.F. Froment

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Volume 5 1 Volume 52

Volume 53

Keynotes in Energy-Related Catalysis edited by S. Kaliaguine Methane Conversion. Proceedings of a Symposium on the Production of Fuels and Chemicals from Natural Gas, Auckland, April 27-30, 1987 edited by D.M. Bibby, C.D. Chang, R.F. Howe and S. Yurchak Innovation in Zeolite Materials Science. Proceedings of an International Symposium, Nieuwpoort, September 13-1 7,1987 edited by P.J. Grobet, W.J. Mortier, E.F. Vansant and G. Schulz-Ekloff Catalysis 1987. Proceedings of the 10th North American Meeting of the Catalysis Society, San Diego, CA, May 17-22, 1987 edited by J.W. Ward Characterization of Porous Solids. Proceedings of the IUPAC Symposium (COPS I),Bad Soden a. Ts., April 26-29, 1987 edited by K.K. Unger, J. Rouquerol, K.S.W. Sing and H. Kral Physics of Solid Surfaces 1987. Proceedings of the Fourth Symposium on Surface Physics, Bechyne Castle, September 7-1 1, 1987 edited by J. Koukal Heterogeneous Catalysis and Fine Chemicals. Proceedings of an International Symposium, Poitiers, March 15-1 7, 1988 edited by M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, C. Montassier and G. Perot Laboratory Studies of Heterogeneous Catalytic Processes by E.G. Christoffel, revised and edited by 2. Pa61 Catalytic Processes under Unsteady-State Conditions by Yu. Sh. Matros Successful Design of Catalysts. Future Requirements and Development. Proceedings of the Worldwide Catalysis Seminars, July, 1988, on the Occasion of the 30th Anniversary of the Catalysis Society of Japan edited by T. lnui Transition Metal Oxides. Surface Chemistry and Catalysis by H.H. Kung Zeolites as Catalysts, Sorbents and Detergent Builders. Applications and Innovations. Proceedings of an International Symposium, Wurzburg, September 48,1988 edited by H.G. Karge and J. Weitkamp Photochemistry on Solid Surfaces edited by M. Anpo and T. Matsuura Structure and Reactivity of Surfaces. Proceedings of a European Conference, Trieste, September 13-1 6, 1988 edited by C. Morterra, A. Zecchina and G. Costa Zeolites: Facts, Figures, Future. Proceedings of the 8th International Zeolite Conference, Amsterdam, July 10-14, 1989. Parts A and B edited by P.A. Jacobs and R.A. van Santen Hydrotreating Catalysts. Preparation, Characterizationand Performance. Proceedings of the Annual International AlChE Meeting, Washington, DC, November 27-December 2, 1988 edited by M.L. Occelli and R.G. Anthony New Solid Acids and Bases. Their Catalytic Properties by K. Tanabe. M. Misono, Y. Ono and H. Hattori Recent Advances in Zeolite Science. Proceedings of the 1989 Meeting of the British Zeolite Association, Cambridge, April 17-1 9, 1989 edited by J. Klinowski and P.J. Barrie Catalyst in Petroleum Refining 1989. Proceedings of the First International Conference on Catalysts in Petroleum Refining, Kuwait, March 5-8, 1989 edited by D.L. Trimm, S. Akashah, M. Absi-Halabi and A. Bishara

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Future Opportunities in Catalytic and Separation Technology edited by M. Misono, Y. Moro-oka and S. Kimura Volume 55 New Developments in Selective Oxidation. Proceedings of an International Symposium, Rimini, Italy, September 18-22, 1989 edited by G. Centi and F. Trifiro Volume 56 Olefin Polymerization Catalysts. Proceedings of the International Symposium on Recent Developments in Olefin PolymerizationCatalysts, Tokyo, October 23-25, 1989 edited by T. Keii and K. Soga Volume 57A Spectroscopic Analysis of Heterogeneous Catalysts. Part A: Methods of Surface Analysis edited by J.L.G. Fierro Volume 578 Spectroscopic Analysis of Heterogeneous Catalysts. Part 6: Chemisorption of Probe Molecules edited by J.L.G. Fierro Volume 58 Introduction t o Zeolite Science and Practice edited by H. van Bekkum, E.M. Flanigen and J.C. Jansen Volume 59 Heterogeneous Catalysis and Fine Chemicals II. Proceedings of the 2nd International Symposium, Poitiers, October 2-6, 1990 edited by M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, G. Perot, R. Maurel and C. Montassier Volume 6 0 Chemistry of Microporous Crystals. Proceedings of the International Symposium on Chemistry of Microporous Crystals, Tokyo, June 26-29, 1990 edited by T. Inui, S. Namba and T. Tatsumi Volume 6 1 Natural Gas Conversion. Proccfedings of the Symposium on Natural Gas Conversion, Oslo, August 12-1 7, 1990 edited by A. Holmen, K.-J. Jens and S. Kolboe Volume 62 Characterization of Porous Solids II. Proceedings of the IUPAC Symposium (COPS II),Alicante, May 6-9, 1990 edited by F. Rodriguez-Reinoso, J. Rouquerol, K.S.W. Sing and K.K. Unger Volume 63 Preparation of Catalysts V. Proceedings of the Fifth International Symposium on thescientific Bases for the Preparation of HeterogeneousCatalysts, Louvain-laNeuve, September 3-6, 1990 edited by 0. Poncelet, P.A. Jacobs, P. Grange and 6. Delmon Volume 6 4 New Trends in CO Activation edited by L. Guczi Volume 65 Catalysis and Adsorption by Zeolites. Proceedings of ZEOCAT 90, Leipzig, August 20-23, 1990 edited by G. Ohlmann, H. Pfeifer and R. Fricke Volume 66 Dioxygen Activation and Homogeneous Catalytic Oxidation. Proceedingsof the fourth International Symposium on Dioxygen Activation and Homogeneous Catalytic Oxidation, Balatonfured, September 10- 14, 1990 edited by L.I. Simandi Volume 67 Structure-Activity and Selectivity Relationships in Heterogeneous Catalysis. Proceedings of the ACS Symposium on Structure-Activity Relationships in Heterogeneous Catalysis, Boston, MA, April 22-27, 1990 edited by R.K. Grasselli and A.W. Sleight Volume 68 Catalyst Deactivation 1991. Proceedings of the fifth International Symposium, Evanston, IL, June 24-26, 1991 edited by C.H. Bartholomew and J.B. Butt

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