Zeolites, science and technology

Zeolites: Science and Technology NATO AS1 Series Advanced Science Institutes Series A Series presenting the results o...

Author: Fernando Ramôa Ribeiro

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Zeolites: Science and Technology

NATO AS1 Series Advanced Science Institutes Series A Series presenting the results of activities sponsored by the NATO Science Committee, which aims at the dissemination of advanced scientific and technological knowledge, with a view to strengthening links between scientific communities. The Series is published by an international board of publishers in conjunction with the NATO Scientific Affairs Division A

Life Sciences Physics

Plenum Publishing Corporation London and New York

C

Mathematical and Physical Sciences

D. Reidel Publishing Company Dordrecht and Boston

D

Behavioural and Social Sciences Applied Sciences

Martinus Nijhoff Publishers The HagueiBostonlLancaster

Computer and Systems Sciences Ecological Sciences

Springer-Verlag BerliniHeidelbergiNew York

B

E F

G

Series E: Applied Sciences - No. 80

Zeolites: Science and Technology edited by

F. Ram6a Ribeiro lnstituto Superior Tecnlco Technical University of Lisbon, Portugal

Alirio E. Rodrigues Department of Chemical Engineering University of Porto, Portugal

L. Deane Rollmann Mob11Research and Development Corporation Central Research D~vision Princeton, NJ, USA

Claude Naccache lnstitut de Recherches sur la Catalyse Villeurbanne. France

1984 Martinus Nijhoff Publishers The Hague 1 Boston 1 Lancaster Published in coo~eratlonwlth NATO Scientific Affairs Division

Proceedings of the NATO Advanced Study Institute on Zeolites: Science and Technology, Alcabideche, Portugal, May 1-1 2, 1983

Library of Congress Cataloging in Publication Data NATO Advanced Study Institute on Zeolites--Science and Technology (1983 : Alcabideche, ~ortugal) Zeolites--science and technology. (NATO advanced science institutes series. Series E , Applied sciences ; no. 80) "~roceedin~s of the NATO Advances Study Institute on Zeolites: Science and Technology, Alcabideche, Portugal, May 1-12, 1983"--~.p. verso. "Published in cooperation with NATO Scientific Affairs Division." 1. Zeolites--Congresses. I. Ribeiro, F. Rarnoa. 11. Title. 111. Series. ~ ~ 2 .4~ 55 ~ 3 1983 8 660.2'8423 83-25486

ISBN 90-247-2935-1 (this volume) ISBN 90-247-2689-1 (series)

Distributors for the United States and Canada: Kluwer Boston, Inc., 190 Old Derby Street, Hingham, MA 02043, USA Distributors for all other countries: Kluwer Academic Publishers Group, Distribution Center, P.O. Box 322, 3300 AH Dordrecht, The Netherlands

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, mechanical, photocopying, recording, or otherwise, without the prior written permission of the publishers, Martinus Nijhoff Pubiishers, P.O. Box 566, 2501 CN The Hague, The Netherlands Copyright O 1984 by Martinus Nijhoff Publishers, The Hague Printed in The Netherlands

PREFACE Z e o l i t e s have been t h e f o c u s o f i n t e n s i v e a c t i v i t y and growth i n a p p l i c a t i o n s o v e r t h e p a s t 25 y e a r s i n i o n exchange, i n a d s o r p t i o n and i n c a t a l y t i c p r o c e s s t e c h n o l o g y . Beginning w i t h t h e synt h e t i c z e o l i t e s A , X and Y , c o n t i n u i n g i n t o t h e emerging ZSM s e r i e s , and i n c l u d i n g s e l e c t e d n a t u r a l z e o l i t e s , a p p l i c a t i o n s s p a n t h e r a n g e From l a r g e - s c a l e p u r i f i c a t i o n and s e p a r a t i o n t o such major p e t r o l e u m and p e t r o c h e m i c a l p r o c e s s e s a s c a t a l y t i c c r a c k i n g anFi a r o m a t i c s alkylation. The f u t u r e promises s e v e r a l new a r e a s o f s i g n i c i a n t u s e a s o u r energy r e s o u r c e b a s e i s expanded. A s a r e s u l t , a NATO Advanced Study I n s t i t u t e on Z e o l i t e s was h e l d i n A l c a b i d e c h e , P o r t u g a l , May 1-12, 1983. I t s p u r p o s e was t o summarize t h e s t a t e - o f - t h e - a r t i n z e o l i t e s c i e n c e and t e c h n o l o g y , w i t h p a r t i c u l a r emphasis on r e c e n t developments. T h i s summary i s i n t e n d e d t o complement p r e s e n t a t i o n s of t h e l a t e s t r e s e a r c h r e s u l t s a t t h e 1983 I n t e r n a t i o n a l Z e o l i t e s A s s o c i a t i o n meeting i n Reno, Nevada - USA. Both t h e fundamentals c o n c e p t s and i n d u s t r i a l a p p l i c a t i o n s a r e addressed i n t h e l e c t u r e s of t h e I n s t i t u t e . I n d i v i d u a l c h a p t e r s cover 3 i s t o r i c a l development, s t r u c t u r e , c r y s t a l l o g r a p h y and s y n t h e s i s techniques. B a s i c p r i n c i p l e s of a d s o r p t i o n , d i f f u s i o n , i o n exchange and a c i d i t y a r e reviewed. A s e c t i o n on c a t a l y s i s a d d r e s s e s shape s e l e c t i v i t y , t r a n s i t i o n m e t a l s , b i f u n c t i o n a l c a t a l y s i s and "methanolto-gasoline"

.

.a

I n c l u d e d i n t h e $ k c t i o n on i n d u s t r i a l a p p l i c a t i o n s a r e c h a p t e r s on r e a c t o r and adsorbek d e s i g n , c a t a l y t i c c r a c k i n g , x y l e n e and n- p a r a f f i n s i s o r n e r i z a t i 8 n , a s w e l l a s i o n exchange and a d s o r p t i o n . \! We would l i k e t o Wank t h e members o f t h e Advisory Board f o r t h e s u p p o r t given by them t o t h e o r g a n i z a t i o n of t h i s m e e t i n g ; and a l l l e c t u r e r s a r e t o be thanked f o r t h e i r c l e a r p r e s e n t a t i o n s and w r i t t e n c o n t r i b u t i o n s . We p a r t i c u l a r l y wish t o acknowledge t h e e f f o r t s o f t h e l o c a l committee.

This NATO Advanced Study I n s t i t u t e was made p o s s i b l e t h r o u g h +-he NATO-AS1 Programme. We a r e much i n d e b t e d t o t h i s o r g a n i z a t i o n . May 1983 Alcabideche

F. Ramba R i b e i r o ~ l i r i oE . Rodrigues L . Deane Rollmann Claude Naccache

D i r e c t o r : P r o f . F. ~ a & a R i b e i r o Co-Directors:

D r . C . Naccache, P r o f . A. R o d r i g u e s , D r . D. Rollmann

Advisory Board P r o f . L. A l v e s , I n s t i t u t o S u p e r i o r Tgcnico, P o r t u g a l P r o f . B. Delrnon, U n i v e r s i t e C a t h o l i q u e de Louvain, Belgium

D r . G . M a r t i n o , I n s t i t u t F r a n ~ a i sdu p e t r o l e , F r a n c e P r o f . R. Maurel, ~ n i v e r s i t ed e P o i t i e r s , F r a n c e P r o f . M. F. P o r t e l a , I n s t i t u t o S u p e r i o r Tecnico , P o r t u g a l

Lectures P r o f . R.M. B a r r e r , I m p e r i a l C o l l e g e , London, UK

3r. D . Barthomeuf, Exxon, Linden, USA P r o f . E . Derouane, M o b i l , P r i n c e t o n , USA 3r. Z . G a b e l i c a , ~ n i v e r s i t ed e Namur, Belgium

P r o f . M . G u i s n e t , ~ n i v e r s i t ede P o i t i e r s , F r a n c e P r o f . C . Kenney, U n i v e r s i t y of Cambridge, UK 3 r . P. J a c o b , E l f - A q u i t a i n e ,

France

3 r . G . K o k o t a i l o , U n i v e r s i t y of D r e x e l , USA P r o f . H. de L a s a , U n i v e r s i t y of Western, O n t a r i o , Canada "of.

H. L e c h e r t , U n i v e r s i t y o f Hamburg, FRG

3 r . C . Naccache, I n s t i t u t Recherches C a t a l y s e , F r a n c e 3 r . J . Rabo, Union C a r b i d e , Tarrytown, USA I r o f . F. ~ a m c aR i b e i r o , I n s t i t u t o S u p e r i o r T e c n i c o , P o r t u g a l I r o f . A. R o d r i g u e s , U n i v e r s i t y of P o r t o , P o r t u g a l 2 r . D . Rollmann, Mobil, P r i n c e t o n , USA 3 r . J . Sherman, Union C a r b i d e , Tarrytown, USA

Local Committee

Eng. M. J . P i r e s , I n s t i t u t o S u p e r i o r Tgcnico, P o r t u g a l Eng. J . M. L o u r e i r o , U n i v e r s i t y of P o r t o , P o r t u g a l Eng. F. F r e i r e , I n s t i t u t o S u p e r i o r ~ e c n i c o ,P o r t u g a l Eng. F. Lemos, I n s t i t u t o S u p e r i o r ~ e c n i c o ,P o r t u g a l Eng. M. L. P a l h a , I n s t i t u t o S u p e r i o r Tecnico, P o r t u g a l

TABLE OF CONTENTS Preface

Part I.

History, structure and synthesis E. M. Flanigen Molecular sieve zeolite technology: the first twenty five years

3

R. M. Barrer Zeolite structure G. T. Kokotailo Zeolite crystallography L. D. Rollmann Synthesis of zeolites, an overview

F. Roozeboom, H. Robson, S. Chan Study on the mechanism of crystallization of zeolites A, X and Y Part 11.

127

Physical characterization and sorption fundamentals

H. Lechert The physical characterization of zeolites

151

Z. Gabelica, J. Nagy, Ph. Bodart, G. Debras,

E. Derouane, P. Jacobs Structural characterization of zeolites by high resolution magic-angle-spinning solid state 2 9 ~ i - ~ Mspectroscopy R

193

Ph. Bodart, Z. Gabelica, J. B. Nagy, G. Debras Multinuclear solide-state NMR study of mordenite crystallization

211

R . M. B a r r e r S o r p t i o n by z e o l i t e s :

- E q u i l i b r i a and e n e r g e t i c s I1 - K i n e t i c s and d i f f u s i v i t i e s I

P a r t 111.

Catalysis

J . Rabo Unifying p r i n c i p l e s i n z e o l i t e c h e m i s t r y and catalysis D

291

. Barthomeuf

Acidic C a t a l y s i s w i t h z e o l i t e s E . G. Derouane Molecular s h a p e - s e l e c t i v e c a t a l y s i s by z e o l i t e s

347

C . Naccache, Y . Ben T a a r i t T r a n s i t i o n metal exchanged z e o l i t e s : p h y s i c a l and catalytic properties M. G u i s n e t , G. P e r o t Zeolite bifunctional catalysis

Part IV.

Industrial applications

A . Rodrigues, C . C o s t a , R . F e r r e i r a , J . L o u r e i r o ,

S . Azevedo Design a s p e c t s of c a t a l y t i c r e a c t o r s and adsorbers

H. Lasa E n g i n e e r i n g a s p e c t s of c a t a l y t i c c r a c k i n g

373

E. Derouane, Z. Gabelica Conversion of methanol to gasoline over zeolite catalysts I

Reaction mechanisms

I1

Industrial processes

F. R. Ribeiro Use of platinum HY zeolite and platinum H mordenite in the hydroisomerization of n-hexane

M. Guisnet, N. S. Gnep Zeolites as catalysts in xylene isomerization processes

J. D. Sherman Ion exchange separations with molecular sieve zeolites

P. Jacob Selective adsorption processes: N-Iself C. N. Kenney, N. F. Kirkby Pressure swing adsorption

List o f P a r t i c i p a n t s

545

PART I HISTORY, STRUCTURE AND SYNTHESIS

MOLECULAR SIEVE ZEOLITE TECHNOLOGY: FIVE YEARS*

E d i t h M.

THE F I R S T TWENTY-

Flanigen

Union C a r b i d e C o r p o r a t i o n Tarrytown Technical Center T a r r y t o w n , New Y o r k 1 0 5 9 1 , USA

ABSTRACT I n twenty-five years molecular s i e v e z e o l i t e s have s u b s t a n t i a l l y impacted adsorption and c a t a l y t i c process technology throughout the chemical process industries; p r o v i d e d t i m e l y s o l u t i o n s t o e n e r g y and e n v i r o n m e n t a l p r o b l e m s ; and grown t o o v e r a hundred m i l l i o n d o l l a r industry worldwide. The e v o l u t i o n i n z e o l i t e m a t e r i a l s w i t h improved o r n o v e l p r o p e r t i e s h a s s t r o n g l y i n f l u e n c e d t h e e x p a n s i o n o f t h e i r a p p l i c a t i o n s , a n d p r o v i d e d new f l e x i b i l i t y i n t h e d e s i g n of p r o d u c t s and p r o c e s s e s . INTRODUCTION The y e a r 1979 marked t h e t w e n t y - f i f t h a n n i v e r s a r y of t h e commercial b i r t h of m o l e c u l a r s i e v e z e o l i t e s a s a new c l a s s o f i n d u s t r i a l m a t e r i a l s . They w e r e i n t r o d u c e d i n l a t e 1 9 5 4 a s a d s o r b e n t s f o r i n d u s t r i a l s e p a r a t i o n s and purifications. S i n c e t h a t time t h e f a s c i n a t i o n w i t h and t h e e l e g a n c e o f , t h i s u n i q u e c l a s s of m a t e r i a l s h a s g e n e r a t e d a masss of s c i e n t i f i c l i t e r a t u r e d e s c r i b i n g t h e i r s y n t h e s i s , p r o p e r t i e s , s t r u c t u r e and a p p l i c a t i o n s , w h i c h p r o b a b l y now n u m b e r s w e l l o v e r 1 5 , 0 0 0 s c i e n t i f i c c o n t r i b u t i o n s and over 10,000 issued p a t e n t s . The m o l e c u l a r s i e v e i n d u s t r y h a s been p r o j e c t e d t o h a v e grown i n t o a n e s t i m a t e d q u a r t e r of a b i l l i o n d o l l a r market {I) s e r v i n g a l l o f t h e m a j o r s e g m e n t s o f t h e c h e m i c a l process i n d u s t r i e s including major applications i n t h e p e t r o l e u m r e f i n i n g and p e t r o c h e m i c a l i n d u s t r i e s , and h a s g e n e r a t e d a myriad of o t h e r a d s o r p t i o n , c a t a l y t i c ,

*

This a r t i c l e i s used a s r e f e r e n c e m a t e r i a l f o r J. A. l e c t u r e on " H i s t o r i c a l A s p e c t s of Z e o l i t e s " .

Rabo's

and most r e c e n t l y

i o n exchange a p p l i c a t i o n s .

Milton reviewed t h e beginnings and development of molecular s i e v e z e o l i t e s i n 1967 (21. He t r a c e d t h e e a r l y d i s c o v e r i e s a n d s y n t h e s i s o f t h e new z e o l i t e s A , X , and Y, which l e d t o t h e i r commercial a p p l i c a t i o n s It w i l l be the a s s e l e c t i v e adsorbents and c a t a l y s t s . p u r p o s e of t h i s p a p e r t o r e v i e w t h e e v o l u t i o n o f molecular sieve materials, t h e i r synthesis, properties and a p p l i c a t i o n s , over t h e span of 1954 t o 1979, w i t h emphasis on t h e major m i l e s t o n e s and t r e n d s i n t h e s e areas. There w i l l be no attempt here t o r e p e a t t h e i n d e p t h , c o v e r a g e o f z e o l i t e m o l e c u l a r s i e v e s g i v e n by B r e c k {3} o r B a r r e r 1 4 3 , o r r e c e n t u p - t o - d a t e r e v i e w a r t i c l e s on t h e a p p l i c a t i o n s of m o l e c u l a r s i e v e z e o l i t e s a s a d s o r b e n t s { 6 } , c a t a l y s t s { 7 , 8 ) , and i o n exchangeers ( 9 1 , a n d on n a t u r a l z e o l i t e s and t h e i r a p p l i c a t i o n s {10,11,1,5}. Success of molecular s i e v e z e o l i t e s h a s been due p r i m a r i l y t o t h e d i s c o v e r y o f new m a t e r i a l s w h o s e p r o p e r t i e s h a v e b e e n e n g i n e e r e d i n t o i m p r o v e m e n t s i n known p r o c e s s e s a n d i n t o t h e d e v e l o p m e n t o f new o n e s . This d i s c u s s i o n w i l l t h e r e f o r e emphasize t h e m a t e r i a l s and p r o p e r t i e s a s p e c t s of molecular s i e v e z e o l i t e s a s t h e y e v o l v e d and developed i n t o v a r i o u s a p p l i c a t i o n a r e a s . THE EVOLUTION I N MATERIALS T h e t h e m e i n r e s e a r c h on m o l e c u l a r s i e v e z e o l i t e m a c e r i a l s o v e r t h e twenty-five y e a r period h a s been a q u e s t f o r new s t r u c t u r e s a n d c o m p o s i t i o n s . Because zeol i t e s a r e unique a s c r y s t a l l i n e porous m a t e r i a l s , with s t r u c t u r e as well a s composition controlling properties, t h e r e a r o s e t h e s t r o n g conviction t h a t novel and u s e f u l p r o p e r t i e s w o u l d r e s u l t f r o m t h e d i s c o v e r y o f n e w comp o s i t i o n s and s t r u c t u r e s . L e t u s now t r a c e t h e w e b o f change i n t h o s e d i s c o v e r i e s over twenty-five years. " L o w - S i l i c a " Z e o l i t e s o r Aluminum-Rich Z e o l i t e s . The d i s c o v e r y of z e o l i t e s A and X by M i l t o n {12} a t t h e Union C a r b i d e C o p o r a t i o n L a b o r a t o r j e s r e p r e s e n t e d a f o r t u n a t e optimum i n c o m p o s i t i o n , p o r e volume, a n d c h a n n e l s t r u c t u r e , g u a r a n t e e i n g t h e s e two z e o l i t e s t h e i r l a s t i n g commercial prominence o u t of more t h a n 1 5 0 s y n t h e t i c s p e c i e s known a n d d i s c o v e r e d o v e r t h e l a s t twenty-five years. Both z e o l i t e s a r e n e a r l y " s a t u r a t e d " i n aluminum i n t h e framework c o m p o s i t i o n w i t h a m o l a r r a t i o o f S i I A 1 n e a r o n e , t h e maximum a l u m i n u m c o n t e n t p o s s i b l e i n t e t r a h e d r a l a l u m i n o s i l i c a t e frameworks i f

one a c c e p t s Loewenstein's r u l e . A s a consequence they c o n t a i n t h e maximum n u m b e r o f c a t i o n e x c h a n g e s i t e s b a l a n c i n g t h e framework aluminum, and t h u s t h e h i g h e s t c a t i o n c o n t e n t s and exchange c a p a c i t i e s . T h e s e comp o s i t i o n a l c h a r a c t e r i s t i c s combined g i v e them t h e most h i g h l y h e t e r o g e n e o u s s u r f a c e k n o w n among p o r o u s m a t e r i a l s , due t o exposed c a t i o n i c charges n e s t e d i n a n a l u m i n o s i l i c a t e framework which r e s u l t s i n h i g h f i e l d g r a d i e n t s . Their s u r f a c e i s h i g h l y s e l e c t i v e f o r w a t e r , p o l a r and p o l a r i z a b l e m o l e c u l e s w h i c h s e r v e s a s t h e b a s i s f o r many o f t h e i r a p p l i c a t i o n s p a r t i c u l a r l y i n d r y i n g and p u r i f i c a t i o n . T h e i r p o r e v o l u m e s o f n e a r l y 0 . 5 cm 3 / c m 3 a r e t h e h i g h e s t known f o r z e o l i t e s a n d g i v e t h e m a d i s t i n c t e c o n o m i c a d v a n t a g e i n b u l k s e p a r a t i o n and p u r i f i c a t i o n s where h i g h c a p a c i t y i s e s s e n t i a l t o and economic d e s i g n . Their 3 - d i m e n s i o n a l c h a n n e l s t r u c t u r e s a l l o w t h e maximum i n diffusion characteristics. By a j u d i c i o u s s e l e c t i o n o f c a t i o n c o m p o s i t i o n a c h i e v e d by f a c i l e i o n exchange r e a c t i o n s , n e a r l y t h e e n t i r e s p e c t r u m o f known p o r e s i z e s i n z e o l i t e s can be obtained. The p o r e s i z e s a c h i e v a b l e by c a t i o n e x c h a n g e of t y p e s A and X s p a n t h e e n t i r e r a n g e from t h e s m a l l e s t p o r e - s i z e d z e o l i t e known, C s - A a t 0.2nm i n s i z e ( 1 3 ) t h r o u g h t h e 0.3nm p o t a s s i u m A , t h e 0.4nm sodium A , t h e 0.5nm c a l c i u m A , t o t h e l a r g e s t known w h i c h i s a b o u t 0 . 8 n m i n s o d i u m X. T h i s l a r g e p o r e s i z e o f z e o l i t e X was a key t o i t s introduction a s a catalytic cracking catalyst. "Intermediate Silica" Zeolites. The n e x t e v o l u t i o n i n z e o l i t e m a t e r i a l s was t h e i m p e t u s t o s y n t h e s i z e more s i l i c e o u s z e o l i t e s , p r i m a r i l y t o improve s t a b i l i t y I t was r e c o g n i z e d c h a r a c t e r i s t i c s , b o t h t h e r m a l and a c i d . i n t h e e a r l y 1 9 5 0 ' s by s c i e n t i s t s a t Union C a r b i d e L a b o r a t o r i e s t h a t t h e t e t r a h e d r a l framework aluminum p r o v i d e d a s i t e o f i n s t a b i l i t y f o r a t t a c k by a c i d and water vapor o r steam. A l s o , t h e s i l i c e o u s m i n e r a l zeol i t e m o r d e n i t e w a s known w i t h a S i / A l m o l a r r a t i o o f 5 and p o s s e s s i n g s u p e r i o r s t a b i l i t y c h a r a c t e r i s t i c s . Breck provided t h e f i r s t success i n t h i s quest with t h e d i s c o v e r y of t h e t h i r d c o m m e r c i a l l y i m p o r t a n t m o l e c u l a r s i e v e z e o l i t e t y p e Y ( 1 4 1 , w i t h a n S i / ~ 1r a t i o o f f r o m 1 . 5 t o 3 . 0 , and a framework topology l i k e t h a t of zeol i t e X and t h e r a r e z e o l i t e m i n e r a l f a u j a s i t e . Not o n l y was t h e d e s i r e d improvement i n s t a b i l i t y o v e r t h e more a l u m i n o u s X a c h i e v e d , b u t a l s o t h e d i f f e r e n c e s i n comp o s i t i o n and s t r u c t u r e had a s t r i k i n g , u n p r e d i c t e d e f f e c t on p r o p e r t i e s t h a t h a s l e d t o t h e preeminence of z e o l i t e Y b a s e d c a t a l y s t s i n n e a r l y a l l of t h e i m p o r t a n t c a t a l y t i c applications involving hydrocarbon conversion,

( i . e . , c r a c k i n g , h y d r o c r a c k i n g and i s o m e r i z a t i o n ) i t s i n i t i a l commercial i n t r o d u c t i o n i n 1959.

since

The n e x t commercially i m p o r t a n t s y n t h e t i c z e o l i t e i n t r o d u c e d i n t h e e a r l y 1 9 6 0 ' s was a l a r g e p o r e m o r d e n i t e made b y t h e method o f Sand { 1 5 } a n d m a r k e t e d a s " Z e o l o n " by t h e N o r t o n Co.{16}, which c o n t i n u e d t h e p r o g r e s s i o n toward h i g h e r S i / A l r a t i o , i n t h i s c a s e a v a l u e n e a r 5. Again, t h e r m a l , h y d r o t h e r m a l , and a c i d s t a b i l i t y improvement was e v i d e n t . T h i s improved s t a b i l i t y coupled w i t h i t s s p e c i f i c s t r u c t u r a l and compositional c h a r a c t e r i s t i c s found i t a small but a s i g n i f i c a n t commercial market a s both an adsorbent and hydrocarbon conversion c a t a l y s t . Type L z e o l i t e , d i s c o v e r e d i n t h e e a r l y 5 0 ' s by B r e c k and Acara ( 1 7 ) w i t h a n S i / A l r a t i o of 3.0 and a u n i q u e framework topology, has only r e c e n t l y r e c i e v e d a t t e n t i o n a s a commercial c a t a l y s t i n s e l e c t i v e h y d r o c a r b o n conv e r s i o n r e a c t i o n s {18}. Other z e o l i t e s with "intermediate" Si/Al compositions o f f r o m 2 t o 5 a n d t h e i r own u n i q u e f r a m e w o r k t o p o l o g i e s which have achieved commercial s t a t u s a r e t h e z e o l i t e m i n e r a l s m o r d e n i t e , e r i o n i t e , c h a b a z i t e , and c l i n o p t i l o l i t e , a n d t h e s y n t h e t i c z e o l i t e omega [ 1 9 ] w i t h a t y p i c a l S i / A l of 3 t o 4. T h e i r p r o p e r t i e s e x h i b i t a common c h a r a c t e r i s t i c i n t e r m s of improved s t a b i l i t y o v e r t h e "low" s i l i c a zeolites. However, u n i q u e p r o p e r t i e s a s a d s o r b e n t s , c a t a l y s t s and i o n exchange m a t e r i a l s a r e a l s o e x h i b i t e d which r e f l e c t t h e i r unique s t r u c t u r a l f e a t u r e s . The s u r f a c e of t h e s e i n t e r m e d i a t e s i l i c a z e o l i t e s i s s t i l l h e t e r o g e n e o u s and e x h i b i t s h i g h s e l e c t i v i t y f o r w a t e r and other polar molecules. "High S i l i c a " Z e o l i t e s . The most r e c e n t s t a g e s i n t h e q u e s t f o r more s i l i c e o u s m o l e c u l a r s i e v e c o m p o s i t i o n s was a c h i e v e d i n t h e l a t e 1 9 6 0 ' s and t h e e a r l y 1 9 7 0 ' s w i t h t h e s y n t e h s i s a t t h e Mobil Research and Development Laboratories of t h e "high s i l i c a z e o l i t e s " , compositions e x e m p l i f i e d f i r s t by z e o l i t e b e t a d i s c o v e r e d by W a d l i n g e r , K e r r a n d R o s i n s k i [ 2 0 } , a n d l a t e r ZSM-5 d i s c o v e r e d b y S u b s e q u e n t l y , ZSM-11 { 2 2 } , Argauer and Landolt {21). ZSM-21 { 2 4 } , a n d ZSM-34 { 2 5 } w e r e d e s c r i b e d . T h e s e compositions are molecular sieve zeolites with Si/Al r a t i o s from 1 0 t o 1 0 0 o r h i g h e r , and w i t h u n e x p e c t e d , s t r i k i n g l y different surface characteristics. In contrast to the lllown a n d " i n t e r m e d i a t e " s i l i c a z e o l i t e s , r e p r e s e n t i n g heterogeneous hydrophilic surfaces within a porous c r y s t a l , t h e s u r f a c e of t h e h i g h s i l i c a z e o l i t e s approaches a more h o m o g e n e o u s c h a r a c t e r i s t i c w i t h a n organophilic-hydrophobic selectivity. They more s t r o n g l y a d s o r b t h e l e s s p o l a r

o r g a n i c m o l e c u l e s and o n l y weakly i n t e r a c t w i t h w a t e r and other strongly polar molecules. In addition to this n o v e l s u r f a c e s e l e c t i v i t y , t h e h i g h s i l i c a z e o l i t e comp o s i t i o n s s t i l l c o n t a i n a s m a l l c o n c e n t r a t i o n of aluminum i n t h e framework and t h e accompanying s t o i c h i o m e t r i c c a t i o n exchange s i t e s . Thus, t h e i r c a t i o n exchange prop e r t i e s a l l o w t h e i n t r o d u c t i o n o f a c i d i c OH g r o u p s v i a t h e w e l l known z e o l i t e i o n e x c h a n g e r e a c t i o n s , e s s e n t i a l t o t h e development of acid hydrocarbon c a t a l y s i s properties. S i l i c a Molecular Sieves. The u l t i m a t e i n s i l i c e o u s m o l e c u l a r s i e v e c o m p o s i t i o n s , a n d a much d i s c u s s e d a s p i r a t i o n of e a r l y workers i n z e o l i t e s y n t h e s i s i n t h e 1950%, was a l s o a c h i e v e d i n t h e 1 9 7 0 ' s w i t h t h e s y n t h e s i s of t h e f i r s t pure s i l i c a molecular s i e v e , s i l i c a l i t e (261, c o n t a i n i n g e s s e n t i a l l y no aluminum o r c a t i o n s i t e s . In t h e complete absence of s t r o n g f i e l d g r a d i e n t s due t o framework aluminum and e x c h a n g e a b l e m e t a l c a t i o n s which serve a s hydrophilic s i t e s , s i l i c a l i t e exhibits a high d e g r e e of o r g a n o p h i l i c - h y d r o p h o b i c c h a r a c t e r , c a p a b l e of s e p a r a t i n g o r g a n i c m o l e c u l e s o u t of w a t e r - b e a r i n g s t r e a m s (261. S i l i c a l i t e does however c o n t a i n e x t r a n e o u s o r def e c t hyroxyl groups which c o n t r i b u t e a s m a l l concentrat i o n of h y d r o p h i l i c s i t e s c a p a b l e of i n t e r a c t i n g w i t h A r e l a t e d new c o m p o s i t i o n , water and p o l a r molecules. f l u o r i d e - s i l i c a l i t e (271, completely f r e e of hydroxyl groups, e x h i b i t s t h e u l t i m a t e i n near p e r f e c t hydrop h o b i c i t y , a d s o r b i n g l e s s t h a n 1 w t . % w a t e r a t 20 t o r r the and 25"C, and e v e n e x h i b i t s b u l k h y d r o p h o b i c i t y : c r y s t a l s (d = 1 . 7 g/cm3) a c t u a l l y f l o a t on w a t e r . S i l i c a l i t e r e p o r t e d l y {26a} h a s t h e same framework t o p o l o g y Other s i l i c a molecular sieve a s z e o l i t e ZSM-5 { 2 8 } . compositions have been reported, including s i l i c a l i t e - 2 {29}, and T E A - s i l i c a t e {30). Chemically Modified Z e o l i t e s . An a l t e r n a t e m e t h o d o r producing h i g h l y s i l i c e o u s z e o l i t e c o m p o s i t i o n s had t s b e g i n n i n g s i n t h e mid 1 9 6 0 ' s when t h e r m o c h e m i c a l m o d i f i c a t i o n r e a c t i o n s t h a t l e a d t o framework dealuminat i o n were f i r s t r e p o r t e d ( 3 1 ~ 3 2 ) . These r e a c t i o n s i n c l u d e those described as "stabilization," or "ultrastabilizat i o n " i n v o l v i n g h i g h t e m p e r a t u r e s t e a m i n g o f ammonium e x c h a n g e d o r a c i d f o r m s o f t h e z e o l i t e (311, a n d f r a m e work aluminum e x t r a c t i o n w i t h m i n e r a l a c i d s o r c h e l a t e s . R e p e t i t i v e treatments i n e f f e c t produce z e o l i t e s with framework S i / A l c o m p o s i t i o n s and s t a b i l i t y c h a r a c t e r i s t i c s comparable t o those observed i n t h e synthesized high s i l i c a zeolite compositions. Such h i g h l y s i l i c e o u s v a r i a n t s h a v e b e e n d e s c r i b e d by S c h e r z e r f o r z e o l i t e Y

{ 3 3 } , by Chen { 3 4 } a n d o t h e r s f o r m o r d e n i t e , a n d b y Eberly e t a l . (351 and P a t t o n e t a l . 1361 f o r e r i o n i t e . The s t a b i l i z e d Y z e o l i t e of McDaniel and Maher {31} and t h e h i g h l y s i l i c e o u s m o r d e n i t e p r o d u c t s o f Chen 1 3 4 1 , were reported t o be hydrophobic. Although t h e ultrastabilized and o t h e r d e a l u m i n a t e d forms o f z e o l i t e s emerged a t t h e same t i m e a s t h e s y n t h e s i z e d h i g h s i l i c a z e o l i t e b e t a , focus i n t h e former case i n t h e l a t e 60's was on t h e i r improved s t a b i l i t y c h a r a c t e r i s t i c s and c a t a l y t i c a p p l i c a t i o n s , r a t h e r than on t h e i r s u r f a c e selectivity. O t h e r h i g h l y s i l i c e o u s a n a l o g s o r "pseudomorphs" of c l i n o p t i l o l i t e and mordenite prepared d u r i n g t h e same p e r i o d by a c i d e x t r a c t i o n {37} h a v e S i / A l composit i o n s l i k e t h e h i g h s i l i c a z e o l i t e s and s i l i c a m o l e c u l a r sieves. However, t h e i r c r y s t a l l i n i t y , s t a b i l i t y , and hydrophobicity a r e s u b s t a n t i a l l y less than t h e thermochemically d e r i v e d u l t r a s t a b i l i z e d and dealuminated c o m p o s i t i o n s , presumably due t o t h e p r e s e n c e of h i g h c o n c e n t r a t i o n s of hydroxyl d e f e c t s {37b}, and t h e a b s e n c e of s u b s t a n t i a l s i l i c o n r e i n s e r t i o n i n t o framework t e t r a h e d r a l s i t e s . Natural Zeolites. I n c o n t r a s t t o t h e development of t h e s y n t h e t i c z e o l i t e s which required t h e i r discovery and s u c c e s s f u l s y n t h e s i s i n t h e l a b o r a t o r y , t h e evolut i o n o f t h e n a t u r a l o r m i n e r a l z e o l i t e s d e p e n d e d on t h e i r a v a i l a b i l i t y i n mineable deposits. The d i s c o v e r y i n 1957 of m i n e a b l e d e p o s i t s of r e l a t i v e l y h i g h p u r i t y zeolite minerals i n volcanic tuffs i n the western U n i t e d S t a t e s and i n a number of o t h e r c o u n t r i e s r e p r e s e n t s t h e b e g i n n i n g of t h e c o m m e r c i a l n a t u r a l zeoP r i o r t o t h a t t i m e t h e r e was no r e c o g n i z e d l i t e e r a (101. indication that zeolite minerals with properties useful a s molecular sieve materials occurred i n large deposits. C o m m e r c i a l i z a t i o n of t h e n a t u r a l z e o l i t e s c h a b a z i t e , e r i o n i t e , and mordenite a s molecular s i e v e z e o l i t e s commenced i n 1 9 6 2 w i t h t h e i r i n t r o d u c t i o n a s new a d s o r bent m a t e r i a l s with improved s t a b i l i t y c h a r a c t e r i s t i c s i n various acid natural gas drying applications. Their improved s t a b i l i t y o v e r t h e t h e n p r e v a l e n t s y n t h e t i c z e o l i t e a d s o r b e n t s , t y p e s A and X , a g a i n r e f l e c t s S i / A l r a t i o of 3-5. The their higher intermediate a p p l i c a t i o n s of c l i n o p t i o l i t e i n r a d i o a c t i v e w a s t e r e c o v e r y a n d i n w a s t e w a t e r t r e a t m e n t d u r i n g t h e same p e r i o d o f t h e 6 0 ' s was b a s e d n o t o n l y on s u p e r i o r s t a b i l i t y c h a r a c t e r i s t i c s b u t a l s o a h i g h c a t i o n exchange s e l e c t i v i t y f o r cesium and s t r o n t i u m , o r f o r t h e ammonium i o n .

A summary o f t h e e v o l u t i o n o f m o l e c u l a r s i e v e m a t e r i a l s a s d e v e l o p e d above i s g i v e n i n T a b l e I , w i t h emphasis on t h e framework S i / A l v a r i a t i o n .

TABLE 1 T H E EVOLUTION OF MOLECULAR SIEVE MATERIALS

"Low" S i / A I Z e o l i t e s A, X

(I to 1.5):

" I n t e r m e d i a t e " S i / A l Z e o l i t e s (-2 t o 5 ) : A) Natural Zeolites: erionite, chabazite, clinoptilolite, mordeni t e B) Synthetic Zeolites: Y , L , l a r g e p o r e m o r d e n i t e , omega " H i g h " ~ i / A lZ e o l i t e s ( - 1 0 t o 1 0 0 ) : A) By t h e r m o c h e m i c a l f r a m e w o r k m o d i f i c a t i o n : h i g h l y s i l i c e o u s v a r i a n t s of Y , mordenite, erionite B) By d i r e c t - s y n t h e s i s : ZSM-5 S i l i c a Molecular

Sieves:

Silicalite

During t h i s p e r i o d of t w e n t y - f i v e y e a r s of r e s e a r c h and d e v e l o p m e n t o v e r 1 5 0 s p e c i e s of s y n t h e t i c z e o l i t e s h a v e b e e n s y n t h e s i z e d a n d some 7 m i n e r a l z e o l i t e s h a v e Yet been found i n s u b s t a n t i a l q u a n t i t y and p u r i t y { 3 8 } . commercially only twelve b a s i c types a r e u t i l i z e d . The major Table 2 l i s t s t h e s e twelve types from Ref. 5. l a r g e volume commercial m o l e c u l a r s i e v e z e o l i t e s used i n a d s o r p t i o n and c a t a l y s i s remain a f t e r twenty-five y e a r s , t h e z e o l i t e s A, X and Y .

TABLE -

2

ZEOLITE TYPES I N COMMERCIAL APPLICATIONS { 5 1 Zeolite Minerals

Synthetic Zeolites

Mordeni t e Chabazite Erionite Clinoptilolite

L Omega IIZeolon", Mordenite ZSM-5 F W

N a , K , Ca f o r m s N a , C a , Ba f o r m s N a , C a , NHq, r a r e e a r t h forms K , NH4 forms Na, H f o r m s H , Na f o r m s Various forms K form K form

TRANSITION I N PROPERTIES The t r a n s i t i o n i n p r o p e r t i e s of m o l e c u l a r s i e v e The emphasis m a t e r i a l s i s summarized i n T a b l e 3. chosen i n on t h e framework S i / A l i n c r e a s i n g from aluminum s a t u r a t e d " S i / A l = 1 " t o i n f i n i t y , a s r e p r e s e n t e d by t h e aluminum-free, p u r e s i l i c a m o l e c u l a r s i e v e , silicalite. The p r o p e r t y t r a n s i t i o n s shown a r e somewhat g e n e r a l i z e d and s h o u l d b e c o n s i d e r e d o n l y a s t r e n d s .

TABLE 3 THE TRANSITION I N PROPERTIES Transition in: Si/Al,

from 1 t o

Stability, -1300°C

. Natural Zeolites. T h e r e h a v e b e e n a n u m b e r o f comp r e h e n s i v e and e x c e l l e n t r e v i e w s of t h e u s e s o f n a t u r a l A summary zeolites i n the l a s t f i v e years {1,10,11). The a d a p t e d f r o m t h e s e r e f e r e n c e s i s shown i n T a b l e 7 . major use of n a t u r a l z e o l i t e s i s i n bulk mineral applii n Europe i n t h e b u i l d i n g and c o n s t r u c t i o n cations {I): i n d u s t r y , w h e r e p r o x i m i t y t o b u i l d i n g l o c a t i o n makes them c o s t e f f e c t i v e ; a n d i n t h e E a r E a s t a s f i l l e r i n t h e paper i n d u s t r y , l a r g e l y because of t h e u n a v a i l a b i l i t y As discussed previously, of a l t e r n a t e m i n e r a l r e s o u r c e s . a modest market f o r z e o l i t e m i n e r a l s has developed a s a nolecular sieve adsorbent i n acid gas drying i n the n a t u r a l g a s i n d u s t r y , i n NHh r e m o v a l i n w a t e r t r e a t m e n t

s y s t e m s b y i o n e x c h a n g e , and i n the p r o d u c t i o n of oxyg e n and n i t r o g e n v i a a d s o r p t i v e a i r s e p a r a t i o n , e s p e c ia lly i n J a p a n (591. I n g e n e r a l , h o w e v e r , their p e n e t r a t i o n into m o l e c u l a r s i e v e a p p l i c a t i o n s h a s b e e n q u i t e limited. TABLE 7 S U M M A R Y OF USES O F N A T U R A L Z E O L I T E S {1,10,5) Bulk Applications: Filler in Paper Pozzolanic Cements and Concrete Dimension Stone Lightweight Aggregate Fertilizers and Soil Conditioners Dietary Supplement in Animal Nutrition

Molecular Sieve Applications: Separation of Oxygen and Nitrogen from Air Acid-resistant Adsorbents in Drying and Purification Ion Exchangers in Pollution Abatement Processes

E V O L U T I O N IN COMMERCE -T h e synthetic zeolite markets progressed from a r e l a t i v e l y s m a l l a d s o r b e n t m a r k e t of t h e o r d e r of a m i l l i o n d o l l a r s i n the l a t e 50's, through a rapid growth w h i c h w a s influenced by the u s e of z e o l i t e s X and Y i n c a t a l y t i c cracking to a published e s t i m a t e (1) of $ 4 0 M M i n 1 9 7 0 , and a projected $ 2 5 0 M M i n 1979. T h e projected m a r k e t in t h e l a r g e v o l u m e , bulk c o m m o d i t y d e t e r g e n t a r e a f o r z e o l i t e b u i l d e r s i n reported to b e $ 2 5 M M , o r a p p r o x i m m a t e l y l O O M M lbs. of z e o l i t e A i n 1 9 8 0 (791, and a n o p t i m i s t i c p r o j e c t i o n f o r growth t o a 4 0 0 M M lb. m a r k e t i n 1 9 8 2 {78a}. T h e i n t r o d u c t i o n of z e o l i t e s i n t o c r a c k i n g catalysts caused a c o m m e r c i a l as w e l l as a t e c h n i c a l r e v o l u t i o n a i n c a t a l y t i c cracking. I t is r e p o r t e d that z e o l i t e c reking c a t a l y s t s h a v e saved r e f i n e r s over $ 2 5 0 M M per y e a r , and increased g a s o l i n e c a p a c i t y s u b s t a n t i a l l y (80). T o d a y , type Y z e o l i t e h a s 100% of t h e U.S. market, and 75-80% of that o u t s i d e of the United S t a t e s {681. T h e t o t a l w o r l d w i d e c o n s u m p t i o n of z e o l i t e s i n catalytic c r a c k i n g i n 1 9 7 8 is estimated a t b e t w e e n 7 0 - 9 0 M M p o u n d s per y e a r (681, r e p r e s e n t i n g t h e s i n g l e l a r g e s t u s e o f s y n t h e t i c m o l e c u l a r s i e v e z e o l i t e s to date. It is lik e l y t h a t b u l k u s e of s i n t h e t i c z e o l i t e s i n d e t e r g e n t s may s u r p a s s that v o l u m e if p r o j e c t i o n s a r e r e a l i z e d .

I n a r e c e n t e s t i m a t e of t h e n a t u r a l z e o l i t e m a r k e t , Leonard { I } s u g g e s t s worldwide s a l e s beginning i n 1965 o f 24MM l b s . a t a v a l u e o f $ ~ M M 1, 6 0 MM l b s . a t $8MM i n 1 9 7 0 , a n d a 1 9 7 9 p r o j e c t i o n o f 560MM l b s . a t a v a l u e o f $35MM. G r e a t e r t h a n 90% of t h e m a r k e t i s i n b u l k m i n e r a l a p p l i c a t i o n s , and o n l y a b o u t 2% of t h a t i n North America. The m a j o r m a r k e t s a r e i n E u r o p e , R u s s i a , and t h e F a r E a s t , especially Japan The s y n t h e t i c m o l e c u l a r s i e v e z e o l i t e s a s i n d u s t r i a l xaterials are appropriately classified as specialty chemicals, o r preferably a s engineered products. The n o l e c u l a r s i e v e i n d u s t r y i s technology and engineering intensive. The m a j o r i t y of m o l e c u l a r s i e v e z e o l i t e a p p l i c a t i o n s a r e engineered i n a l l r e s p e c t s , from t h e s y n t h e s i s of t h e z e o l i t e m a t e r i a l , t h e m o d i f i c a t i o n of t h e i r p r o p e r t i e s , t h e s e l e c t i o n of a t a i l o r m a d e z e o l i t e ?roduct, t o a process engineering design and execution t h a t i n t e g r a t e s and o p t i m i z e s t h e m a t e r i a l w i t h t h e ?recess. The s u c c e s s f u l g r o w t h of m o l e c u l a r s i e v e zeol i t e s a s a n i n d u s t r i a l c h e m i c a l depended most s t r o n g l y on t h e i r d e v e l o p m e n t a s a n e n g i n e e r e d p r o d u c t . As developed previously, the e f f e c t of outside forces i n t h e changing world surrounding molecular s i e v e s ?ad a profound impact on t h e i r commercial growth and 2evelopment. T h i s i s e s p e c i a l l y true i n t h e c a s e of t h e ? r o b l e m s and c r i s e s i n e n e r g y and environment which svolved i n t h e 6 0 ' s and e r u p t e d i n t h e 7 0 7 s , and which s f f e r e d o p p o r t u n i t i e s f o r t h e i r uniquely s u i t e d pro? e r t i e s i n s e p a r a t i o n s and c a t a l y s i s . This is seen i n t h e n a t u r a l gas p u r i f i c a t i o n a r e a where l a r g e growth 5 a s r e c e n t l y o c c u r r e d i n t h e t r e a t m e n t o f LNG i n g i a n t h s e load f a c i l i t i e s i n t h e Middle East {6}, i n increased FCC p r o d u c t i o n of g a s o l i n e and o c t a n e improvement f o r -nleaded g a s o l i n e , i n t h e f l e d g i n g u s e of z e o l i t e i o n exchangers i n waste water treatment, i n insulated ~ i n d o wa d s o r b e n t s , i n s u b s t i t u t i o n o f z e o l i t e s f o r ;hosphate i n d e t e r g e n t s , and p o s s i b l y i n t h e f u t u r e , =ethanol o r biomass t o g a s o l i a e . The n o r m a l / i s o - p a r a f f i n ~ r o c e s s e su s i n g m o l e c u l a r s i e v e z e o l i t e s w e r e i n f l u e n c e d 5- t h e n e e d f o r b i o d e g r a d a b l e d e t e r g e n t s . The x y l e n e s e p a r a t i o n p r o c e s s e s w e r e i n f l u e n c e d by t h e i n d u s t r i a l :.eed f o r r a w m a t e r i a l f o r p o l y e s t e r f i b e r p r o d u c t i o n .

THE PAST AS INDICATOR OF THE FUTURE: YEARS --

THE NEXT TWENTY-FIVE

The i n i t a l d i s c o v e r i e s of t h e s y n t h e t i c m o l e c u l a r s i e v e z e o l i t e s A, X and Y i n t h e l a t e 4 0 ' s and e a r l y 5 0 ' s spawned a n immense w o r l d w i d e s c i e n c e a n d t e c h n o l o g y , u t i l i z i n g t h e r e s o u r c e s o f many m a j o r i n d u s t r i a l R a n d D organizations throughout t h e world. Zeolites are " r e s e a r c h e d 3 ' and used commercially i n e v e r y major c o u n t r y i n t h e world. (Recently China displayed molecular s i e v e s a s one of i t s i n d u s t r i a l p r o d u c t s a t t h e Shanghai I n d u s t r i z l Exhibit 1811.) W o r l d w i d e t h e r e a r e o v e r a d o z e n manufacturers of zeolites o r zeolite-containing products. A s a n i n d u s t r i a l m a t e r i a l , t h e i r m a r k e t h a s grown t o hundreds of m i l l i o n s of d o l l a r s . Since t h e mid-19601s, t h e l a t e r developing n a t u r a l z e o l i t e s have reached the s t a t u s of a n i m p o r t a n t i n d u s t r i a l m i n e r a l r e s o u r c e 82 w i t h more t h a n 300,000 t o n s mined worldwide, and used principally i n bulk applications. The P a s t . What was n e c e s s a r y f o r t h e s u c c e s s o f t h e molecular sieve z e o l i t e industry' The d e v e l o p m e n t of any new c o m m e r c i a l p r o d u c t a n d p r o c e s s i s u s u a l l y t h e r e s u l t o f c o m p l e x i n t e r a c t i o n s among many c o n t r i b u t i n g f a c t o r s . I w i l l a t t e m p t h e r e t o h i g h l i g h t some k e y f a c t o r s t h a t f a c i l i t a t e d the zeolite "explosion". The commercial u s e o f z e o l i t e s d e p e n d s on: 1) useful p r o p e r t i e s c o n t r o l l e d by t h e i r s t r u c t u r a l c h e m i s t r y ; The p i o n e e r i n g work 2 ) a v a i l a b i l i t y ; a n d 3 ) c o s t {5}. o f B a r r e r i n t h e 4 0 ' s i n o u t l i n i n g t h e l a r g e number o f molecular s i e v e s e p a r a t i o n s p o s s i b l e gave t h e impetus t o t h e i n d u s t r i a l r e s e a r c h e r s , M i l t o n a n d a s s o c i a t e s , who made i n i t i a l key d i s c o v e r i e s i n novel s y n t h e t i c z e o l i t e c o m p o s i t i o n s and a p r a c t i c a l method f o r t h e i r m a n u f a c t u r e . The s c i e n t i f i c and e n g i n e e r i n g r e s o u r c e s were t h e n committed by a l a r g e , major c h e m i c a l c o r p o r a t i o n , Union C a r b i d e C o r p o r a t i o n , w i t h a v a i l a b l e t e c h n i c a l a n d cornmercial resources. A s a r e s u l t t h e y became a v a i l a b l e t o t h e i n d u s t r i a l and s c i e n t i f i c communities i n l a t e 1954. The m a j o r e a r l y s y n t h e s i s e f f o r t s i n t h e l a t e 4 0 ' s and e a r l y 50's could n o t have been successful without t h e development of r a p i d , e f f e c t i v e 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 t o e v a l u a t e t h e s y n t h e s i z e d p r o d u c t s {2}. Such t e c h n i q u e s w e r e d e v e l o p e d t o d e t e r m i n e t h e i r s t r u c t u r e , c h e m i c a l compostion, p u r i t y , and a d s o r p t i o n proA s a r e s u l t t i m e e f f i c i e n t a n a l y s i s of a v e r y perties.

l a r g e number o f s y n t h e s i s e x p e r i m e n t s was p o s s i b l e w h i c h f a c i l i t a t e d t h e d i s c o v e r y o f t w e n t y - s o m e new z e o l i t e s p e c i e s and d e l i n e a t e d t h e i r optimum s y n t h e s i s s y s t e m . The g e n e r a l s y n t h e s i s method d e v e l o p e d by M i l t o n provided a simple, cost e f f e c t i v e manufacturing process, involving r e a d i l y a v a i l a b l e , cheap raw m a t e r i a l s such a s hydrated alumina, s o l u b l e a l k a l i s i l i c a t e s , c a u s t i c , and w a t e r , and p r o c e s s c o n d i t i o n s of r e l a t i v e l y low t e m p e r a t u r e and p r e s s u r e a n d s h o r t c r y s t a l l i z a t i o n t i m e s . The u n i t o p e r a t i o n s of b a t c h c r y s t a l l i z a t i o n , f i l t r a t i o n and drying, were w e l l e s t a b l i s h e d i n t h e p r a c t i c e s manufacturing art. Thus, molecular s i e v e z e o l i t e s could be manufactured t o c o m p e t e o n a c o s t p e r f o r m a n c e b a s i s w i t h o t h e r known commercial adsorbent, c a t a l y s t , and i o n exchange m a t e r i a l s . The development of formed o r bonded z e o l i t e p r o d u c t s n e c e s s a r y f o r c o m m e r c i a l a p p l i c a t i o n i n s u p p o r t e d o r movi n g bed s y s t e m s , r e q u i r e d e x t e n s i v e development of f o r m i n g technology. T h e a s - s y n t h e s i z e d 1-5ym z e o l i t e c r y s t a l s a r e formed i n t o b e a d s , p e l l e t s , o r mesh, t y p i c a l l y by u s e of a c l a y b i n d e r . Over t h e p e r i o d o f t w e n t y - f i v e y e a r s , t h e development of an unsung and l i t t l e d i s c u s s e d forming technology has resulted i n the a b i l i t y t o control p a r t i c l e p r o p e r t i e s s u c h a s a s t r e n g t h and a t t r i t i o n r e s i s t a n c e and mass and h e a t t r a n s f e r c h a r a c t e r i s t i c s , and t o optimize t h e formed p r o d u c t s ' p r o p e r t i e s and performance.

A s t h e new m o l e c u l a r s i e v e z e o l i t e s b e c a m e k n o w n and a v a i l a b l e and s u b s e q u e n t l y u s e d , e x t e n s i v e r e s e a r c h e f f o r t i n i n d u s t r i a l and academic c i r c l e s provided a w e a l t h of s c i e n t i f i c i n f o r m a t i o n on t h e p h y s i c a l , chemical, aned s t r u c t u r a l c h a r a c t e r i s t i c s of t h i s unique c l a s s of m a t e r i a l s . The r e s u l t i n g i n - d e p t h u n d e r s t a n d i n g o f p r o p e r t i e s a l l o w e d t h e s e l e c t i o n of z e o l i t e p r o d u c t f o r a s p e c i f i c a p p l i c a t i o n , t h e i d e n t i f i c a t i o n of a p p l i c a t i o n s f o r t h e p r o d u c t , and t h e d e s i g n and e n g i n e e r i n g of t h e a p p l i c a t i o n p r o c e s s . Their application a s adsorbents required a major development i n a d s o r p t i o n p r o c e s s design and e n g i n e e r i n g zechnology. The u n i t o p e r a t i o n of a d s o r p t i o n h a s undergone a m a j o r development i n t h e l a s t t w e n t y y e a r s m a i n l y a s a r e s u l t of t h e i n t r o d u c t i o n o f m o l e c u l a r s i e v e s a s 2ommercial a d s o r b e n t s {6,2}. I t i s now a m a t u r e e n g i n e e r Lng p r a c t i c e t h a t h a s b r o u g h t a d s o r p t i o n t o t h e f o r e f r o n t a s a major t o o l of t h e chemical process industry.

I n d i s p e n s a b l e t o t h e c o m m e r c i a l s u c c e s s of m o l e c u l a r s i e v e z e o l i t e s h a s been t h e d e d i c a t i o n and c o n t r i b u t i o n s o f t h e s c i e n t i s t s a n d e n g i n e e r s who p r o v i d e d t h e k e y t o t h e i r discovery and development, and subsequently unfolded t h e i r e l e g a n t s t r u c t u r a l and chemical a r c h i t e c t u r e and novel properties. The z e a l w i t h which t h e s e m o l e c u l a r sieve "apostles" preached t h e i r gospel t o t h e s c i e n t i f i c w o r l d , t o i n d u s t r i a l t e c h n o l o g y management, and t o t h e hard-to-sell chemical p r o c e s s i n d u s t r i e s , was e s s e n t i a l t o t h e i r success. Hundreds of t h e s e a p o s t l e s and champions became committed, i n a d d i t i o n t o t h e o r i g i n a l p i o n e e r s . The g r o w t h of m o l e c u l a r s i e v e s c i e n c e and t e c h n o l o g y h a s e v o l v e d a community of o u t s t a n d i n g s c i e n t i s t s and e n g i n e e r s . whose c o n t r i b u t i o n s i n c l u d e c r e a t i v e and p r a c t i c a l s c i e n t i f i c work, and t h e s u c c e s s f u l t r a n s l a t i o n of R and D r e s u l t s t o commercial manufacture, s a l e and u t i l i t y . It i s e s t i m a t e d a t t h e o n s e t of t h e i r s e c o n d t w e n t y - f i v e y e a r s , t h a t over f i v e thousand s c i e n t i s t s and engineers d e v o t e a s u b s t a n t i a l p o r t i o n of t h e i r t e c h n i c a l e f f o r t t o molecular sieve zeolites. The F u t u r e . The f u t u r e t r e n d s i n m a t e r i a l s w i l l no d o u b t s e e t h e d e v e l o p m e n t o f new c o m m e r c i a l z e o l i t e s s e l e c t e d from newly d i s c o v e r e d c o m p o s i t i o n s and s t r u c t u r e s , chemical m o d i f i c a t i o n s of p r e s e n t commercial products t o g e n e r a t e new a n d u s e f u l p r o p e r t i e s , a n d a r e e v a l u a t i o n o f t h e h o s t o f k n o w n z e o l i t e s w h i c h n e v e r a c h i e v e d comI t seems l i k e l y t h a t w i t h t h e i n c r e a s mercial success. i n g number o f l a b o r a t o r i e s d e v o t i n g r e s o u r c e s t o t h e s e a r c h f o r new s t r u c t u r e s a n d c o m p o s i t i o n s , new c l a s s e s o f molecular sieve materials w i l l be discovered. The m o d i f i c a t i o n c h e m i s t r y of z e o l i t e s p r a c t i c e d t o d a t e , s u c h a s s t e a m i n g and c h e m i c a l e x t r a c t i o n , l e a v e s a v a s t a r e a of c h e m i c a l and s t r u c t u r a l m o d i f i c a t i o n of s o l i d s a s y e t unexplored with zeolites. A d d i t i o n a l types of n a t u r a l z e o l i t e s w i l l probably n o t achieve commercial prominence s i n c e t h e l a r g e g e o l o g i c a l e x p l o r a t i o n e f f o r t s f o r zeol i t e d e p o s i t s throughout t h e world during t h e l a s t ten t o f i f t e e n y e a r s have probably i d e n t i f i e d a l l of t h e z e o l i t e m i n e r a l s p e c i e s of commercial s i g n i f i c a n c e . The commercialization of " s t o r e d " o r "shelved" z e o l i t e s h a s l a r g e l y b e e n h a m p e r e d by t h e i r l a c k o f g e n e r a l a v a i l a b i l i t y and t h e i r apparent i n a b i l i t y t o compete performance-wise w i t h t h e c u r r e n t commercial products. With t h e worldwide expansion of s c i e n t i f i c z e o l i t e c e n t e r s w i t h t h e c a p a b i l i t y of s y n t h e s i z i n g non-commercial z e o l i t e s and d e t e r m i n i n g t h e i r p r o p e r t i e s and p o t e n t i a l a p p l i c a t i o n s , i t i s l i k e l y t h a t s e v e r a l

"old"

z e o l i t e s w i l l achieve commercial s t a t u s .

It i s l i k e l y t h a t t h e r e w i l l b e more development and change i n z e o l i t e manufacturing p r o c e s s e s during t h e next decade than during t h e l a s t twenty years, due t o t h e c o s t i n c e n t i v e s of t h e b u l k chemical and consumer m a r k e t s , and t h e a v a i l a b i l i t y of t h e n a t u r a l z e o l i t e s .

B r e c k h a s r e c e n t l y r e v i e w e d (5) a l a r g e n u m b e r o f new p o t e n t i a l a p p l i c a t i o n s a r e a s f o r z e o l i t e s . His c o m p i l a t i o n o f p r o p o s e d a p p l i c a t i o n s b a s e d on t h e r e p o r t e d l i t e r a t u r e i s reproduced i n Table 8.

TABLE 8 SOME PROPOSED APPLICATIONS O F ZEOLITES Adsorption New a d s o r b e n t s f o r s i e v i n g Hydrophobic a d s o r b e n t s Gas s t o r a g e s y s t e m s Carriers of chemicals Nuclear

Industry Applications

Environmental Weather m o d i f i c a t i o n S o l a r energy

(51

Agricultural F e r t i l i z e r s and s o i l s Animal c u l t u r e Consumer A p p l i c a t i o n s Beverage carbonation Laundry d e t e r g e n t s Flame e x t i n g u i s h e r s E l e c t r i c a l conductors Ceramics New c a t a l y s t s

The m a j o r t r e n d s i n f u t u r e commercial a p p l i c a t i o n s u i l l p r o b a b l y comprise s u b s t a n t i a l growth i n most of t h e 3 r e s e n t l y e x i s t i n g s e p a r a t i o n s and c a t a l y s i s a r e a s , and t h e d e v e l o p m e n t o f new a p p l i c a t i o n s . The emergence of z e o l i t e i o n exchange a p p l i c a r i o n s could p a r a l l e l t h a t of z e o l i t e adsorption technology over the l a s t twenty-five ::ears. The m a t u r i n g o f t h e d e v e l o p i n g i o n e x c h a n g e 3 r o c e s s d e s i g n and e n g i n e e r i n g t e c h n o l o g y , w i t h t h e z a p a b i l i t y of advanced s y s t e m s d e s i g n and e n g i n e e r i n g c o n c e p t s , should s t i m u l a t e t h e growth and a c c e p t a n c e of z e o l i t e i o n exchange s e p a r a t i o n s i n t h e chemical process F n d u s t r y a l o n g s i d e t h o s e b a s e d on a d s o r p t i o n a n d c a t a l y s i s .

The growth of b u l k c h e m i c a l and consumer a p p l i c a tions for synthetic a s well a s natural zeolites appears to be certain. I n a d d i t i o n t o t h o s e now p r e v a l e n t f o r natural zeolites, Table 8 includes t h e i r use i n agric u l t u r e , b e v e r a g e c a r b o n a t i o n , and raw m a t e r i a l s f o r ceramics

.

N a t u r a l z e o l i t e s s h o u l d c o n t i n u e t o grow a s a n important i n d u s t r i a l mineral resource used p r i n c i p a l l y i n bulk application areas. The " e n g i n e e r i n g 1 ' of t h e mined z e o l i t e s , by b e n e f i c i a t i o n t o u p g r a d e p u r i t y and c h e m i c a l m o d i f i c a t i o n t o t a i l o r p r o p e r t i e s , w i l l no doubt emerge a s t h e l e v e l of t e c h n i c a l l y i n t e n s i v e e f f o r t on n a t u r a l They s h o u l d t h e n e n j o y a l a r g e r z e o l i t e s expands {I}. s h a r e of t h e " e n g i n e e r e d 1 ' m o l e c u l a r s i e v e t y p e a p p l i c a tions. The e x t e n t of s u c h g r o w t h w i l l n o t l i k e l y b e r e l a t e d t o t h e i r lower c o s t but r a t h e r improved p r o p e r t y and performance c h a r a c t e r i s t i c s . The e x p a n s i o n w i l l c o n t i n u e t o b e a r e l a t i v e l y minor p o r t i o n of t h e t o t a l m o l e c u l a r s i e v e a p p l i c a t i o n s m a r k e t , l a r g e l y b e c a u s e of t h e i n c r e a s i n g c a p a b i l i t y i n t h e m a n u f a c t u r e and a v a i l a b i l i t y of a l a r g e number of s y n t h e t i c z e o l i t e s , and t h e a b i l i t y t o c o n t r o l p u r i t y and p r o p e r t i e s d u r i n g manufacture. The t r e n d s i n m o l e c u l a r s i e v e p r o c e s s d e s i g n s h o u l d s e e more compound, m u l t i s t e p p r o c e s s s y s t e m s u t i l i z i n g m u l t i p l e o r composite molecular s i e v e m a t e r i a l s and combined u n i t o p e r a t i o n s s u c h a s i n t e g r a t e d a d s o r p t i o n and c a t a l y t i c systems. The e n e r g y s a v i n g s p o s s i b l e i n adsorption s e p a r a t i o n s has received emphasis only It is l i k e l y t h a t t h e energy e f f i c i e n c y r e c e n t l y (831. i n molecular s i e v e a d s o r p t i o n and c a t a l y t i c processes ' w i l l b e more f u l l y e x p l o i t e d i n t h e p r o c e s s d e s i g n . Over t h e l o n g r a n g e , m o l e c u l a r s i e v e z e o l i t e t e c h n o l o g y s h o u l d c o n t i n u e t o b e s t r o n g l y i n f l u e n c e d by t h e new e m p h a s i s on c l e a r e n v i r o n m e n t , a n d e n e r g y a n d r e n e w able resource technology. Cost e f f e c t i v e and n o v e l s e p a r a t i o n and recovery processes w i l l b e r e q u i r e d t o meet p o l l u t i o n s t a n d a r d s and m a t e r i a l and energy r e Developsource limitations i n thenext several decades. ment of a d e q u a t e e n e r g y r e s o u r c e s , e s p e c i a l l y t h e p r e sently considered alternate synthetic f u e l technologies b a s e d on s y n t h e s i s g a s , o i l s h a l e , c o a l a n d g a s o h o l among o t h e r s , a l l i n v o l v e t e c h n i c a l l y d i f f i c u l t and complex s e p a r a t i o n s and c a t a l y t i c problems.

Molecular s e i v e z e o l i t e s a r e w e l l positioned hist o r i c a l l y and o f f e r a most a p p r o p r i a t e t e c h n o l o g y j e c a u s e of t h e i r u n i q u e p r o p e r t i e s which g i v e them a n e a r - i n f i n i t e f l e x i b i l i t y t o t a i l o r p r o d u c t and p r o c e s s . The r e c e n t e x t e n s i o n of t h e i r s h a p e and s u r f a c e s e l e c t i v i t y c h a r a c t e r i s t i c s w i t h t h e a d v e n t of h i g h s i l i c a z e o l i t e s a n d s i l i c a m o l e c u l a r s i e v e s o f f e r s new o p p o r t u n i t i e s i n d e s i g n p a r a m e t e r s , a s e x e m p l i f i e d by t h e m e t h a n o l t o g a s o l i n e p r o c e s s w i t h ZSM-5. The a v a i l a b i l i t y of hydrophobic molecular s i e v e adsorbents opens u p a new a p p l i c a t i o n a r e a i n r e m o v i n g a n d r e c o v e r i n g organic molecules from aqueous systems. Combined h y d r o s h o b i c and h y d r o p h i l i c a d s o r b e n t s y s t e m s would a l l o w t h e c o n c e n t r a t i o n o r removal of a n o r g a n i c m o l e c u l e from an aqueous s o l u t i o n , and e f f i c i e n t d r y i n g of t h e r e c o v e r e d organic. S i m i l a r s e p a r a t i c n s c h e m e s a r e now u n d e r i n v e s t i g a t i o n i n t h e production of gasohol from g r a i n (84). The z e o l i t e f u t u r e l o o k s b r i g h t .

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18,

206,

ZEOLITE STRUCTURES

R.M.

Barrer

Chemistry Department Imperi a1 College of Science and Technology London SW7 2AY England ABSTRACT An a c c o u n t h a s been given of s i l i c a t e a n i o n s from t o p o l o g i c a l and c o n f i g u r a t i o n a l a s p e c t s , w i t h s p e c i a l r e f e r e n c e t o z e o l i t e s . D i f f e r e n t ways of c o n s t r u c t i n g z e o l i t e a n i o n s a r e d e s c r i b e d which l e a d t o known frameworks and t o a l a r g e number o f n o v e l ones. B r i e f c o n s i d e r a t i o n i s a l s o given t o r e s u l t a n t channel systems and t o A l , S i o r d e r and d i s o r d e r .

1.

INTRODUCTION:

SILICATE ANIONS

The s t r u c t u r a l s i d e of z e o l i t e chemistry i s concerned w i t h t h e i r three-dimensional g i a n t anions and t h e a s s o c i a t e d i n t r a z e o l i t e c a t i o n s and w a t e r m o l e c u l e s . Much t h e most a c c u r a t e s t r u c t u r a l i n f o r m a t i o n r e l a t e s t o t h e a n i o n i c frameworks which Lave g e o m e t r i c a l e l e g a n c e , p r o v i d e numerous honeycomb s t r u c t u r e s and determine i m p o r t a n t z e o l i t e p r o p e r t i e s . However z e o l i t e anions a r e a p a r t only of a remarkable a r r a y of s i l i c a t e anions Ln which many s u b - u n i t s found i n z e o l i t e s a r e a l s o p r e s e n t . I t i s t h e r e f o r e of c o n s i d e r a b l e i n t e r e s t t o g i v e examples of t h e s e znions i n a b u i l d up t o t h e main t o p i c of z e o l i t e a n i o n s . Z e o l i t e s a r e u s u a l l y s y n t h e s i s e d from g e l s o r from o t h e r s i l i c a t e s . 3ecause n u c l e a t i o n can be i n f l u e n c e d by t h e s t a r t i n g m a t e r i a l s sxamples of t h e d i v e r s e s i l i c a t e anions which could be chosen a s s o u r c e s of s i l i c a a c q u i r e an a d d i t i o n a l s i g n i f i c a n c e . Two a s p e c t s of t h e s t r u c t u r e of s i l i c a t e a n i o n s a r e t h e i r z3pology and t h e i r c o n f i g u r a t i o n . By topology we w i l l mean t h e

Table 1:

S i l i c a t e s grouped a c c o r d i n g t o anion t y p e s

Type

Sub-types

F i n i t e anions ("Island" anions)

Ortho- and p y r o - s i l i c a t e s . Short unbranched c h a i n s . S i n g l e r i n g a n i o n s . Branched s i n g l e r i n g s . Double r i n g p r i s m s . S t r u c t u r e s w i t h two o r more i s l a n d a n i o n s .

I n f i n i t e c h a i n anions

Unbranched s i n g l e c h a i n s . Openbranched s i n g l e c h a i n s . Loopbranched s i n g l e c h a i n s ( l i n k e d r i n g s ) . M u l t i p l e c h a i n s (two, t h r e e , four o r f i v e cross-linked Hybrid c h a i n s . Tubular chains) chains.

.

I n f i n i t e sheet anions

Unbranched s i n g l e s h e e t s . Branched s i n g l e s h e e t s . Double s h e e t s . Hybrid t r i p l e s h e e t s .

I n f i n i t e three-dimensional anions

Non-porous and porous t e c t o silicates.

p a t t e r n of t h e Si-0-T bonds (T = A 1 o r S i ) and by c o n f i g u r a t i o n we w i l l mean t h e s p a t i a l d i s p o s i t i o n of t h e bond p a t t e r n . For example, S i 0 4 t e t r a h e d r a can j o i n w i t h o t h e r Si04 t e t r a h e d r a t o g i v e unbranched l i n e a r c h a i n s . These can, a s we s h a l l s e e , be v a r i o u s l y puckered. These a n i o n s a l l have t h e same topology i n terms of t h e bond p a t t e r n , b u t t h e y have d i f f e r e n t c o n f i g u r a t i o n s . Silicates can be grouped a c c o r d i n g t o t h e i r t o p o l o g i e s as i n Table 1. I n t h e f o l l o w i n g s e c t i o n s examples of t h e v a r i o u s c a t e g o r i e s of Table 1 w i l l be g i v e n , l e a d i n g up t o and w i t h emphasis on z e o l i t e s .

2.

FINITE ANIONS Some i s l a n d anions a r e a s f o l l o w s (1) :-

Single tetrahedron

~i02-

Orthosilicates

Two l i n k e d t e t r a h e d r a

si20?-

Disilicates

Chain o f - t h r e e t e t r a h e d r a

S~~O:;

Trisilicates

Ring of t h r e e t e t r a h e d r a ( 3 - r i n g )

si30;-

Benitoite

4-ring

si40?;

Taramellite

6-ring

si60:i-

Beryl

8-ring

sig0i;-

Muirite

9-ring

si90i?-

Eudialite

12-ring

Traski t e

Double 3 - r i n g

( t r i a n g u l a r prism)

si60?;

(Ni (En) 3) 3CSi6015126H20

Double 4 - r i n g

(cubic u n i t )

si80g;

Ekanite

Double 6-ring

(hexagonal prism)

s i l2 0 i i -

Milarite

si50:g-

Zunyite

si60i;-

Eakerite

Branched c h a i n of three tetrahedra

Branched 4-ring 4

Branched 6 - r i n g

S i l 8028- T i e n s h a n i t e

b

The l i n e diagrams g l v e t h e t o p o l o g i e s of t h e l a s t t h r e e a n i o n s . The d o t s r e p r e s e n t t h e Si atoms and t h e l i n e s show t h e l i n k i n g . The oxygen atoms a r e n o t shown: they a r e between t h e p a i r s of Si atoms t h e y l i n k . The non-linking oxygens r e q u i r e d t o complete t h e t e t r a h e d r a l groups a r e , f o r c l a r i t y , o m i t t e d . A d d i t i o n a l l y t o t h e above examples a c r y s t a l l i n e tetra-alkylammonium s i l i c a t e has been r e p o r t e d which c o n t a i n s t h e anion S ~ ~ ~ bOe l i~e v ~e d - t o, be a p e n t a g o n a l p r i s m ( 2 ) .

3.

INFINITE CHAIN ANIONS

Unbranched s i n g l e c h a i n s a r e found i n a v a r i e t y of metas i l i c a t e s . They can have t h e c o n f i g u r a t i o n s i l l u s t r a t e d i n F i g . 1 ( 3 ) , t h e c o n f i g u r a t i o n s b e i n g c h a r a c t e r i s e d by t h e p e r i o d i c i t i e s , i . e . t h e number of l i n k e d t e t r a h e d r a which must be counted a l o n g t h e c h a i n b e f o r e t h e arrangement r e p e a t s i t s e l f . The ~ e r i o d i c i t i e sof F i g . 1 a t o k and s i l i c a t e s t o which they r e f e r a r e a s follows: L e t t e r i n Fig. 1

Mineral Pyroxenes Ba2[Si2o61 Wollastonite Krauskop f i t e

Periodicity

Haradite Rhodoni t e Stokesite Pyroxyferroit e F e r r o s i l i t e I11 Alamosi t e Even-period c h a i n s t e n d t o become l e s s s t r e t c h e d w i t h h i g h e r mean e l e c t r o n e g a t i v i t y and mean v a l e n c e of t h e c h a r g e - b a l a n c i n g c a t i o n s w h i l e f o r o d d - p e r i o d c h a i n s t h e e x t e n t of c h a i n p u c k e r i n g i s s t r o n g l y c o r r e l a t e d w i t h mean e l e c t r o n e g a t i v i t y of t h e c a t i o n s b u t less so with cation radius (4). I n s t a n c e s of open- and loop-branched c h a i n s a r e shown i n F i g . 2 a t o g ( 4 ) . The examples and t h e i r p e r i o d i c i t i e s a r e : Letter i n Fig. 2

Miner a 1

Periodicity

Aeni gmati t e Astrophyllite Deeri t e V l asovit e Lemoyni t e Pellylite Nordi t e ( a ) and ( b ) a r e open-branched s i n g l e c h a i n s ; ( c ) t o ( g ) a r e loopb r a n c h e d s i n g l e c h a i n s . The l a t t e r produce r i n g s l i n k e d t o o t h e r r i n g s v i a Si-0-Si bonds. Some d o u b l e c h a i n s w i t h d i f f e r e n t p e r i o d i c i t i e s a r e shown i n F i g . 3 a t o i (5) f o r t h e m i n e r a l s named below: Letter i n Fig. 3

Mineral

Periodicity

Sillimanite 1 Amphibole 2 S y n t h e t i c Lit, 1 ( ~ i ~ e ~ ) 0 ~ 0 ] 2 Xonotli t e 3 Devi t r i t e 3 S y n t h e t i c Na2Be2HCSi601510H 3 Narsarsukite 4 Inesit e 5 Tuhualite 6 ~ x a m p l e s ( a ) ( c ) ( f ) and ( i ) a r e a l l l a d d e r a n i o n s composed o f 4 - r i n g s l i n k e d by s h a r e d e d g e s , w i t h t h e same t o p o l o g i e s b u t d i f f e r e n t c o n f i g u r a t i o n s . A s y n t h e t i c a l u m i n a t e , Na7iA13081 (6) h a s t h e same t o p o l o g y a s ( e ) ( d e v i t r i t e ) b u t w i t h A 1 r e p l a c i n g S i .

F i g . 1.

Fig. 2.

C o n f i g u r a t i o n s of some s i n g l e c h a i n unbranched anions ( 3 ) . F o r i d e n t i f i c a t i o n of s p e c i e s t o which t h e y r e f e r see text.

Some branched and loop-branched s i n g l e c h a i n a n i o n s ( 4 ) . S p e c i e s t o which t h e y r e f e r a r e i d e n t i f i e d i n t h e t e x t .

F i g . 3.

Some d o u b l e c h a i n a n i o n s w i t h d i f f e r e n t p e r i o d i c i t i e s f o r species identified i n the t e x t (9).

Fig. 4.

S e v e r a l k i n d s of u n b r a n c h e d s h e e t a n i o n f o r s p e c i e s identified i n the text (9).

As f i n a l examples of anions based on i n f i n i t e c h a i n s one may i n c l u d e m u l t i p l e , t u b e and h y b r i d c h a i n anions ( 1 , 7 ) : M u l t i p l e c h a i n anions Three c r o s s - l i n k e d

c h a i n s (8)

Example

Periodicity

Synthetic

2

Four c r o s s - l i n k e d c h a i n s , g i v i n g s t r i p of hexagons, p a t t e r n a s above, t h r e e hexagons wide (8)

S y n t h e t i c Ba5CSi80211

Five c r o s s - l i n k e d c h a i n s , g i v i n g s t r i p of hexagons, p a t t e r n a s above, f o u r hexagons wide (8)

S y n t h e t i c Ba6CSi100261

2

L i t i d i o n i t e NaKCuCSi4Ol01

3

Tube a n i o n s (7)

Hybrid c h a i n anion

Tinaksi t e ~ ~ ~ a C a ~ T i C S i ~ ~ 0 ~ ~3 1 0 H Yodels of complex c h a i n a n i o n s , such a s t u b u l a r a n i o n s , when opened o u t and f l a t t e n e d y i e l d open branched s t r i p a n i o n s ( 7 ) . The n a r s a r s u k i t e a n i o n i n t h i s way opens t o c h a i n of branched 6 - r i n g s ; t h e l i t i d i o n i t e a n i o n g i v e s a s t r i p h y b r i d a n i o n of 4- and 8 - r i n g s ; and t h e m i s e r i t e a n i o n g i v e s a s t r i p h y b r i d anion Various h y p o t h e t i c a l t u b u l a r a n i o n s and of 4-, 6- and 8 - r i n g s . t h e s t r i p a n i o n s t h e y can y i e l d when opened and f l a t t e n e d have a l s o been c o n s i d e r e d ( 7 ) . The s t r i p s can be extended t o make i n f i n i t e s h e e t s composed of v a r i o u s combinations of r i n g s . Some of t h e s e combinations a r e known t o occur i n s h e e t s i l i c a t e s , f o r example 4-, 6- and 12-rings i n manganpyrosmalite, (Mn, Fe) 8 CSi60151iOH, C1) 10. There seems no r e a s o n why complex c h a i n a n i o n s of t h e k i n d s e n v i s a g e d may n o t b e found o r s y n t h e s i s e d i n t h e r i c h f i e l d of s i l i c a t e c h e m i s t r y , and used a s s t a r t i n g materials for zeolite synthesis.

4.

INFINITE SHEET ANIONS

The n e x t s t a g e of p o l y m e r i s a t i o n i n s i l i c a t e a n i o n s y i e l d s i n f i n i t e s h e e t s . Unbranched s h e e t s of s e v e r a l k i n d s a r e shown i n F i g . 4a t o e ( 9 ) . These a r e i d e n t i f i e d below: ( a ) Hexagon b a s e d l a y e r s of t e t r a h e d r a Periodicity found i n mica, where a l l a p i c e s p o i n t i n one d i r e c t i o n .

=

2

(b) Hexagon based l a y e r s i n s e p i o l i t e ; bands w i t h a p i c e s p o i n t i n g up a l t e r n a t e w i t h bands h a v i n g a p i c e s p o i n t i n g down. ( c ) The s i n g l e l a y e r i n d a l y i t e , K2Zr[Si6ol51, (4-, 6- and 8-rings i n l a y e r ) . (d) The s i n g l e l a y e r i n a p o p h y l l i t e , KCa4[Si8020?F.8H20 (4- and 8rings i n layer). ( e ) The manganpyrosmalite, k 6[ S i 1 6 0 3 0 1 ( O H ) ~ C s~h~e e, t , c o n t a i n i n g 4-, 6and 1 2 - r i n g s . Openbranched s i n g l e l a y e r s a r e a l s o found, two such t y p e s b e i n g t h o s e i d e n t i f i a b l e i n p r e h n i t e , Ca2 (Al, Fe) C ( S i 3 , A1)0101 (OH) 2 w i t h p e r i o d i c i t y 2 , and i n z e o p h y l l i t e , Gal 3CSi5OI4l2F8(OH?f i H 2 0 , w i t h p e r i o d i c i t y 4 and a s h e e t of 1 2 - r i n g s . A loop-branched s i n g l e l a y e r of p e r i o d i c i t y 4 o c c u r s i n s y n t h e t i c NaPr[Si6ol41. Double l a y e r s h e e t a n i o n s a r e i l l u s t r a t e d i n F i g . 5 a , b and c , (10) i n which (a) (b) (c)

i s t h e unbranched double l a y e r of h e x a c e l s i a n , Ba[Si2Al2O81 of p e r i o d i c i t y 2; i s t h e loop-branched double l a y e r of d e l h a y e l i t e , Ca4 (Na3, Ca)K7CSi14A120381C12F4 of p e r i o d i c i t y 3 ; and i s t h e loop-branched double l a y e r of c a r l e t o n i t e , K2Na8Ca8[si160361( 0 3 ) 8 (OH,F) 22H20, which h a s a p e r i o d i c i t y of 6 .

Two s u b - l a y e r s of each k i n d a r e l i n k e d v i a t h e oxygen atoms marked w i t h d o t s and l y i n g on m i r r o r o r pseudo-mirror p l a n e s . The bottom p a r t s of t h e diagram show t h e double l a y e r s edge on

5.

SILICATES CONTAINING TWO ANIONS

I n some s i l i c a t e s t h e r e may be more than one k i n d of a n i o n . Examples of s i l i c a t e s of t h i s type i n c l u d e t h e f o l l o w i n g ( 1 ) : Silicate

Anions P r e s e n t

~ o i s i t e ,e p i d o t e , ganomalite, vesuvian, s e r e n d i b i t e , rus tumite

Csi0,14-

[si20716-

Kilchoanite, ardennite

[sio414-

C

S

Meliphanite

[sio414-

C

S

Traskite

C S ~ ~ O ~C I ~S - ~

~

~

O

Miserit e

[si20716-

~

~

O

Baveni t e

C S ~ ~ O [ ~s i 6 ~ 0 I1 6~~ *- -

Eudi a l i t e

~ s i ~ 0 ~[sig 1 ~( O - ,OH)~~I~~*-

Ches t e r i t e

CSi401

Reyerite

~

High p r e s s u r e "garnet1'

[sio4i4-

synthetic S i 5 ( ~ 0 4 ) 6 0

C S ~ O , I ~ - csio618-

High p r e s s u r e K2Si409

[si30916-

~

C

S

~

~

~

O

~

~

~

O

~

~

~

i

~ ~ 0 ~ ~ i ~ ~l

~~

-~

C S ~ O ~ I ~ -

[sio618-

SiO,+ t e t r a h e d r a shar6ng: S i d i s t a n c e (A) Si

corners 3.24

edges 1.87

faces 1.08

SiOb o c t a h e d r a s h a r i n g : Si Si distance

corners 3.56

edges 2.52

faces 2.06

. ..

(2)

~

~

-

~

~

-

~

~

~

~

~

~

~

~

~

0

~

[si601618-

I t i s of i n t e r e s t t h a t under h i g h p r e s s u r e c o n d i t i o n s S i octahedr a l l y co-ordinated w i t h 0 a p p e a r s . I n s t i s h o v i t e , a high pressure form of c r y s t a l l i n e s i l i c a , o c t a h e d r a S i 0 6 appear t o be l i n k e d by edge s h a r i n g a s much a s by c o r n e r s h a r i n g . No example of f a c e s h a r i n g of S i 0 6 o c t a h e d r a i s known. Also, t e t r a h e d r a S i 0 4 ( o r A104) l i n k only by c o r n e r s h a r i n g . The d i s t a n c e s between c e n t r a l S i atoms a r e :

...

~

Edge and f a c e s h a r i n g of S i 0 4 t e t r a h e d r a and f a c e s h a r i n g of o c t a h e d r a draw si4+ c e n t r a l i o n s t o o c l o s e f o r s t a b i l i t y r e l a t i v e t o c o r n e r s h a r i n g f o r S i 0 4 p a i r s and edge and c o m e r s h a r i n g f o r Si06 p a i r s .

1

Fig. 5.

Examples o f unbranched d o u b l e l a y e r a n i o n s i d e n t i f i e d i n the t e x t (10).

PRESSURE, '.01

Fig. 6 .

psi

S o r p t i o n of He i n a- and i n B 1 - t r i d y m i t e

(12)

Among t h e i n f i n i t e s i n g l e c h a i n anions ( F i g . 1) t h e p e r i o d i c i t y a p p e a r s t o be determined o r i n f l u e n c e d by t h e a s s o c i a t e d c a t i o n s . Even-period c h a i n s a r e l e s s s t r e t c h e d t h e h i g h e r t h e mean e l e c t r o n e g a t i v i t y and mean v a l e n c e of t h e c a t i o n s . For oddp e r i o d c h a i n s mean e l e c t r o n e g a t i v i t y more t h a n mean r a d i u s of t h e cations influences the chain puckering (11).

6.

TECTOSILICATES

Three-dimensional frameworks ( c r y s t a l l i n e s i l i c a s , f e l s p a t h o i d s , f e l s p a r s and z e o l i t e s ) have O/(Al + S i ) = 2 , i n d i c a t i n g t h e p r e s e n c e only of t e t r a h e d r a TO4 (T = A 1 o r S i ) where each t e t r a h e d r o n s h a r e s i t s f o u r oxygens w i t h f o u r o t h e r t e t r a h e d r a . I n c l a y m i n e r a l s on t h e o t h e r hand o c t a h e d r a l l a y e r s of A106 o r MgO6 a r e p r e s e n t a t t a c h e d e i t h e r t o one t e t r a h e d r a l l a y e r of 6-rings ( k a n d i t e s ) o r t o two such l a y e r s one one each s i d e of t h e l a y e r of o c t a h e d r a ( s m e c t i t e s , v e r m i c u l i t e s and m i c a s ) . The r e s u l t a n t m u l t i p l e s h e e t s may be uncharged a s w i t h t h e k a n d i t e s o r they may be a n i o n i c a s w i t h most of t h e t h r e e - l a y e r s h e e t s t r u c t u r e s . I n t h e l a t t e r case t h e c h a r g e - n e u t r a l i s i n g c a t i o n s a r e l o c a t e d between s u c c e s s i v e s h e e t s . I n t e c t o s i l i c a t e s t h e r e i s one e q u i v a l e n t of c a t i o n s p e r g. atom of t h e A 1 r e p l a c i n g S i , and t h e i o n s a r e d i s t r i b u t e d i n t h r e e dimensions i n t h e i n t e r s t i c e s of t h e network. The t e c t o s i l i c a t e s may be sub-divided i n t o t h o s e which a r e n o t and t h o s e which a r e porous (Table 1 ) . The f e l s p a r s , t h e d e n s e r c r y s t a l l i n e s i l i c a s such a s q u a r t z and v a r i o u s f e l s p a t h o i d s ( e g s . n e p h e l i n e , k a l i o p h i l i t e , k a l s i l i t e and e u c r y p t i t e ) a r e nonporous; i n them t h e i n t e r s t i c e s a r e n o t l a r g e enough t o c o n t a i n even t h e s m a l l e s t g u e s t m o l e c u l e s . Tridymite and c r i s t o b a l i t e can however take up c o n s i d e r a b l e amounts of He and Ne, a s shown f o r t r i d y m i t e i n F i g . 6 (12) and s o can j u s t be c o n s i d e r e d as porous tectosilicates. Other porous c r y s t a l l i n e s i l i c a s a r e s i l i c a l i t e s 1 and 2(4) dodecasil-3C (15) and melanophlogite ( s e e 5 . 7 ) . Some f e l s p a t h o i d s such a s s o d a l i t e and c a n c r i n i t e ( ~ i g .7 ) a l s o have frameworks w i t h c a v i t i e s o r channels l a r g e enough t o c o n t a i n v a r i o u s s a l t s and/or z e o l i t i c w a t e r . The most i m p o r t a n t porous t e c t o s i l i c a t e s a r e found among t h e l a r g e and growing number of z e o l i t e s , each type h a v i n g i t s i n d i v i d u a l c o n f i g u r a t i o n of windows, c a v i t i e s and c h a n n e l s of m o l e c u l a r dimensions. The pore volumes a c c e s s i b l e t o w a t e r , and o f t e n t o numerous o t h e r g u e s t m o l e c u l e s , range from $0.18 cm3 p e r cm3 of c r y s t a l f o r t h e l e a s t porous (analcime) t o %0.50 cm3 p e r cm3 of c r y s t a l f o r t h e most porous (such a s f a u j a s i t e , c h a b a z i t e and z e o l i t e s A , H-RHO, ZSM-2 and ZSM-3. The open s t r u c t u r e s allow ready m i g r a t i o n of w a t e r molecules and of i o n s i n c a t i o n exchange. The exchange c a p a c i t y of some z e o l i t e s i s given f o r t h e i d e a l i s e d compositions i n Table 2. This c a p a c i t y can of

Table 2:

Exchange c a p a c i t i e s i n meq/lOOg of some z e o l i t e s

Zeolite

I d e a l i s e d composition

Natrolite

Na2 LA1 S t 3 O1 12H20

Analcime

NaCA1Si2O61H20

Levyni t e

CaCA12Si401216H20

Chabazi t e

( ( i ) C a , ~ aC ) A l ~ i ~ 0 ~ 1 3 ~ ~ 0

Gmelini t e

( ( 1 ) C a , ~ a [)A l S i 2 0 6 1 3 ~ 2 0

Edingtoni t e

BaCA12Si301014H20

Fauj a s i t e

(Ca,Na2) CAl2Si5O1t+16.6H20

Harmo tome

(K2,Ba) CAl2Si5Olt+I5H2O

Heulandi t e

Ca[A12Si601615H20

Stilbite

(Na,

Mordeni t e

( ( $ ) C a , ~ a~) A l S i ~ O ~3H20 ~l3.

Exchange c a p a c i t y

( i ) Ca) CAlSi3O8I3H2O

c o u r s e v a r y w i t h t h e S i / A l r a t i o which can u s u a l l y be a l t e r e d according t o s y n t h e s i s conditions. Z e o l i t e s l i k e ZSM-5 o r -11 w i t h S i / A 1 r a t i o s v a r y i n g between a 2 5 and 1000 c a r r y v e r y much l e s s n e g a t i v e charge p e r lOOg and a r e t h u s l e s s p o l a r t h a n t h e most aluminous z e o l i t e s of Table 2 .

7.

ZEOLITE GROUPS

Attempts have been made t o group t o g e t h e r z e o l i t e s which have s t r u c t u r a l elements i n common. For numerous s y n t h e t i c z e o l i t e s t h e s t r u c t u r e s a r e s t i l l unknown, b u t examples of grouping a r e g i v e n i n Table 3. This t a b l e c o n t a i n s i n s t a n c e s of framework anions which a r e v a r i a n t s of a s i n g l e topology, i . e . i s o t y p e s ( e g s . a l l members of t h e analcime group; h e u l a n d i t e and c l i n o p t i l o l i t e i n t h e h e u l a n d i t e group; s t i l b i t e , s t e l l e r i t e and b a r r e r i t e , a l s o i n t h e h e u l a n d i t e group; p h i l l i p s i t e and harmotome i n t h e p h i l l i p s i t e group; gismondine and Na-P a l s o i n t h e p h i l l i p s i t e group; and n a t r o l i t e s c o l e c i t e and m e s o l i t e i n t h e n a t r o l i t e g r o u p ) . V a r i a n t s o f a given topology a r i s e a s a r e s u l t of d i f f e r i n g chemical compos&tions. For examples analcime h a s a c u b i c u n i t c e l l of edge + 13.72 A and u n i t c e l l c o n t e n t Na16CA116Si32096116H20. The ~ a and

T a b l e 3:

1.

C l a s s i f i c a t i o n of some z e o l i t e s and p o r o u s t e c t o s i l i c a t e s

A n a l c i n e Group Analcim Wairaki t e Leucite (felspathoid) Rb-analcime ( " ) Polluci t e Viseite (aluminosilicophosphate) Kehoei t e ( a l u m i n o p h o s p h a t e )

2.

F a u j a s i t e ( z e o l i t e s X and Y) Z e o l i t e ZSM-2 Z e o l i t e ZSM-3 Paulingi t e Zeolite A Z e o l i t e RHO Zeolite 5 5

5.

7.

H e u l a n d i t e Group Heulandi t e Clinoptilolit e Brews t e r i t e

M o r d e n i t e Group Mordeni t e Ferrierite Dachiardit e Epis t i l b i t e Biki t a i t e

8.

N a t r o l i t e Group Natrolite Scolecite Mesoli t e Thomsonite Gonnardi t e Edingtonit e Metanatrolite

Clathrasil-3C

F a u j a s i t e Group

Laumonti t e Group Laumon t i t e ~ u g a w a r a ltie

C l a t h r a t e Group Melanophlogite Z e o l i t e ZSM-39,

4.

6.

C h a b a z i t e Group Chabazite Gmelinite Erioni t e Offretite Levyni t e M a z z i t e ( z e o l i t e a) Zeolite L Sodalite hydrate Cancrinite hydrate

3.

S t i l b it e Stellerite Barrerite

9.

P e n t a s i l Group Z e o l i t e ZSM-5, S i l i c a l i t e I Z e o l i t e ZSM-11, ~ i l i c a l i t e I 1

1 0 . P h i l l i p s i t e Group Phillipsite Harmo tome G i smondine Z e o l i t e Na-P Amici t e Garroni t e Mer l i n o i t e Z e o l i t e Li-ABW -

Fig. 7.

Frameworks of ( i ) s o d a l i t e ( 1 . h . s . ) and ( i t ) c a n c r i n i t e ( r . . I n (i) 14-hedral cages a r e i d e n t i f i a b l e . ( i i ) shows ( a ) 11-hedra t y p i c a l of c a n c r i n i t e and (b) a s e c t i o n normal t o t h e c - d i r e c t i o n , i n d i c a t i n g a wide c h a n n e l c i r c u m s c r i b e d by 1 2 - r i n g s . A1 o r S i a r e c e n t r e d a t e a c h c o r n e r and 0 atoms a r e c e n t r e d n e a r b u t n o t on t h e mid-point of e a c h edge o f F i g . 7 and a n a l o g o u s s u b s e q u e n t framework r e p r e s e n t a t i o n s . The s c a l e s of t h e two d r a w i n g s a r e n o t t h e same.

. .

Radius

Fig

8. W a t e r c o n t e n t s of d i f f e r e n t c a t i o n i c forms of p h i l l i p s i t e plotted against cation r a d i i (16).

Exchange of Na+ by K+ g i v e s t h e H20 occupy d i f f e r e n t s u b - l a t t i c e s . l e u c i t e wihh a t e t r a g o n a l u n i t c e l l h a v i n g a = 1 2 . 9 8 and c = 13.68 A . The c r y s t a l s a r e anhydrous and t h e IZi i o n s a r e i n t h e s u b - l a t t i c e occupied by H20 i n analcime. The topology of t h e analcime anion i s however unchanged. Another way i n which v a r i a n t s of a g i v e n topology may a r i s e i s through changes i n S i / A l r a t i o s i n t h e framework. This changes t h e c a t i o n d e n s i t y , which can a l s o be a l t e r e d by exchanges such a s 2 ~ a ' 2 c a 2 + . These two f a c t o r s o p e r a t e f o r t h e i s o t y p e s h e u l a n d i t e Ca4EAlgSi28872124H20, (momclinic, a = 17.72 b = 17.90 c = 7.43 A , 6 = 116O25') and c l i n o p ~ i l o l i t e ,Na66A16Si30072124H20, (monoclinic, a = 17.64 A , b = 17.90 A , c = 7.40 A , f3 = 1 1 6 0 2 2 ' ) . When one c a t i o n i s r e p l a c e d by a n o t h e r of d i f f e r e n t s i z e o r charge t h e d i s t r i b u t i o n s of t h e i o n s between s u b - l a t t i c e s may change a s w e l l a s t h e w a t e r c o n t e n t under ambient c o n d i t i o n s . Such changes can i n t u r n cause minor o r s i g n i f i c a n t framework d i s t o r t i o n s w i t h o u t a l t e r i n g t h e topology. An example of t h e v a r i a t i o n of w a t e r c o n t e n t w i t h exchange c a t i o n i s shown i n F i g . 8 f o r p h i l l i p s i t e ( 1 6 ) . F o r i o n s of t h e same charge w a t e r c o n t e n t s d e c r e a s e as i o n r a d i u s i n c r e a s e s . A d i f f e r e n t curve i s o b t a i n e d f o r a s e r i e s of d i v a l e n t i o n s than f o r a s e r i e s of u n i v a l e n t ones.

2

i,

2,

Guest s p e c i e s o t h e r than w a t e r can a l s o r e s u l t i n v a r i a n t s of a g i v e n topology. S o d a l i t e ( N a g [A16Si602412NaCl; c u b i c w i t h a = 8.87 2 ) and nosean (Na6[A16Si60241Na2S04;c u b i c w i t h a = 9 . 0 8 ) b o t h have t h e s o d a l i t e framework, s l i g h t l y d i l a t e d i n t h e c a s e of nosean. I n g e n e r a l , t e c t o s i l i c a t e frameworks although s t r o n g and r i g i d can undergo s m a l l a d j u s t m e n t s of c r y s t a l l o g r a p h i c s i g n i f i c a n c e The m a j o r i t y of t h e frameworks i n most z e o l i t e groups a r e n o t v a r i a n t s of a given topology b u t p o s s e s s d i f f e r e n t t o p o l o g i e s , w i t h s t r u c t u r a l s i m i l a r i t i e s which j u s t i f y t h e i r b e i n g grouped t o g e t h e r . Such c l a s s i f i c a t i o n s a r e n o t w i t h o u t a l t e r n a t i v e s because on a b a s i s of t h e p o s s e s s i o n of s t r u c t u r a l s u b - u n i t s i n common c e r t a i n z e o l i t e s could e q u a l l y w e l l appear i n more t h a n one group. Thus i n s o d a l i t e p l a c e d i n t h e c h a b a z i t e group 14-hedral c a v i t i e s e x i s t which a r e a l s o found i n some members of t h e f a u j a s i t e group However s o d a l i t e and c a n c r i n i t e a r e ( f a u j a s i t e and z e o l i t e A) s t r u c t u r a l l y r e l a t e d and c a n c r i n i t e i s w e l l p l a c e d i n t h e c h a b a z i t e group and s o i s s o d a l i t e . The c l a s s i f i c a t i o n of z e o l i t e s i n t o groups i s t h u s somewhat a r b i t r a r y .

.

Z e o l i t e s can a l s o be c o n s i d e r e d i n terms of t h e d e n s i t i e s of Certain bonds Si-0-T ( T = A 1 o r S i ) i n t h e x , y and z d i r e c t i o n s . network a n i o n s have bond d e n s i t i e s comparable i n a l l d i r e c t i o n s ( e . g . groups 1, 2, 3, 4 and 9 of Table 3) ; o t h e r s have bond densi t i e s g r e a t e r i n two d i r e c t i o n s t h a n i n t h e t h i r d (group 5 ) ; and o t h e r s have t h e s e d e n s i t i e s g r e a t e r i n one d i r e c t i o n t h a n i n t h e Bond remaining two ( " f i b r o u s " z e o l i t e s of t h e n a t r o l i t e group) d e n s i t y d i f f e r e n c e s can i n f l u e n c e thermal s t a b i l i t y and r i g i d i t y

.

.

Fig. 9 .

Fig.

10.

Dodecahedra ( b ) , t e t r a d e c a h e d r a (a) and h e x a d e c a h e d r a ( c ) found i n m e l a n o p h l o g i t e ( ( a ) + ( b ) ) and i n dodecasil-3C o r z e o l i t e ZSM-39 ( ( b ) + ( c ) ) ( 1 8 ) .

S t a c k i n g of p e n t a g o n a l dodecahedra i n (a) m e l a n o p h l o g i t e and ( b ) d o d e c a s i l - 3 C o r z e o l i t e ZSM-39 ( 1 8 ) .

and t h e e x t e n t of deformation when undergoing m o d i f i c a t i o n by i o n exchange o r i n changes of t h e Si/A1 r a t i o s and i n d e h y d r a t i o n and rehydration. I n t h e f i b r o u s z e o l i t e s t h e dense-bond c h a i n s a r e c r o s s - l i n k e d t o l i k e c h a i n s by s i n g l e Si-0-T bonds and t h i s appears On t h e o t h e r hand i n t o make t h e frameworks r e a d i l y deformable. z e o l i t e A 14-hedral s o d a l i t e cages a r e each l i n k e d through t h e s i x A-ring f a c e s t o s i x o t h e r s o d a l i t e cages by f o u r Si-0-T bonds c r e a t i n g a c u b i c l i n k a g e u n i t . Cross-connecting s o d a l i t e cage subm i t s by m u l t i p l e Si-0-T bonds produces a more r i g i d l e s s e a s i l y deformable framework t h a n when, a s i n t h e n a t r o l i t e group, c r o s s l i n k i n g i n v o l v e s s i n g l e Si-0-T bonds. S i m i l a r l y i n f a u j a s i t e each s o d a l i t e cage i s l i n k e d t o f o u r o t h e r such cages through f o u r of i t s e i g h t 6 - r i n g f a c e s by s i x Si-0-T bonds, c r e a t i n g a hexagonal ? r i s m l i n k i n g u n i t . R i g i d i t y and thermal s t a b i l i t y i s found f o r such z e o l i t e s a s A and f a u j a s i t e (X and Y) d e s p i t e t h e s m a l l e r S e n s i t y , g r e a t e r i n t r a c r y s t a l l i n e p o r o s i t y , and s m a l l e r number of ( A 1 + S i ) atoms (and hence of Si-0-T bonds) p e r u n i t volume. I n z e o l i t e A, f a u j a s i t e , n a t r o l i t e and thomsoni t e t h e s e numbers a r e as f o l l o w s : Zeolite (A1 + S i ) , p e r 1000

natroli te 14.5

thomsoni t e 14.4

zeolite A 12.9

faujasit e 12.7

I n c l u d e d i n T a b l e 3 a r e examples of porous c r y s t a l l i n e s i l i c a s . I h e s e a r e m e l a n o p h l o g i t e , r a r e i n Nature and only r e c e n t l y synr h e s i s e d ( 1 7 ) , dodecasil-3C (15) and s i l i c a l i t e s I (14) and I1 (15) !lelanophlogite h a s t h e s t r u c t u r e of c l a t h r a t e h y d r a t e t y p e I , i n ;;hi ch t h e p e n t a g o n a l dodecahedra and t e t r a d e c a h e d r a w i t h twelve 3 - r i n g and two 6 - r i n g f a c e s of F i g . 9 (18) b and a r e s p e c t i v e l y (1) 2re s t a c k e d t o a s t o f i l l a l l s p a c e . There areotwo dodecahedra and s i x t e t r a d e c a h e d r a p e r c u b i c u n i t c e l l of 13.4 A edge ( F i g . 10a) 8 ) I n c l a t h r a s i l - 3 C as i n i t s i s o t y p e ZSM-39 (19) p e n t a g o n a l iodecahedra a r e s t a c k e d w i t h hexadecahedra having twelve 5-ring and ?our 6 - r i n g f a c e s t o f i l l a l l space ( F i g . l o b ) . There a r e 6 dodecaIf nedra and 8 hexadecahedra p e r c u b i c u n i t c e l l of edge 19.4 t h e r e was one g u e s t molecule, G, p e r void t h e l i m i t i n g composition 2f m e l a n ~ ~ h l o g i twould e be 23Si02.2G and t h a t of c l a t h r a s i l - 3 C would 3e 13Si02.2G. I f only t h e l a r g e r v o i d s c o n t a i n e d a g u e s t molecule the l i m i t i n g compositions would be 23Si02. 3G and l7SiO2 .G r e s p e c t i v e l y . Because of t h e i r analogy w i t h c l a t h r a t e h y d r a t e s they have been termed t h e c l a t h r a t e group i n Table 3. S i n c e a range of c l a t h r a t e h y d r a t e s i s known s y n t h e s i s may r e v e a l more ?orous s i l i c a s o r z e o l i t e s i n t h i s group s t r u c t u r a l l y l i k e t h e i r : l a t h r a t e h y d r a t e c o u n t e r p a r t s . A l l a r e r i c h i n 5 - r i n g s and indeed t5e c l a t h r a t e , mordenite, h e u l a n d i t e and p e n t a s i l z e o l i t e groups zre a l l related i n t h i s respect.

.

a.

The p e n t a s i l z e o l i t e s ZSM-5 and ZSM-11 and t h e i r r e s p e c t i v e 2nd members s i l i c a l i t e s I a n d I I d i f f e r from m e l a n o p h l o g i t e and

F i g . 11.

Fig.

12.

Frameworks of ZSM-5

(1.h.s.)

and ZSM-11

(r.h.s.).

Formal r e p r e s e n t a t i o n o f c h a n n e l p a t t e r n s i n ( a ) ZSM-11, (c) ZSFI-5 and (b) an i n t ~ l m e d i a t e s t r u c t u r e (20)

.

c l a t h r a s i 1-3C i n h a v i n g r e l a t i v e l y open c o n t i n u o u s channe 1s r a t h e r t h a n s e m i - i s o l a t e d v o i d s p e r m e a t i n g t h e i r s t r u c t u r e s . A view of t h e i r frameworks i s shown i n F i g . l l a and b , and a f o r m a l r e p r e s e n t a t i o n of t h e t h r e e - d i m e n s i o n a l c h a n n e l p a t t e r n s i n F i g . 1 2 c and a t o g e t h e r w i t h t h i s p a t t e r n f o r an i n t e r m e d i a t e s t r u c t u r e , o f which many a r e p o s s i b l e ( F i g . These c h a n n e l s h a v e minimum f r e e d i m e n s i o n s from 5 . 1 12b) t o 5.6 (20B , by c o n t r a s t w i t h t h e l a r g e s t windows i n m e l a n o p h l o g i t e o r d o d e c a s i l - 3 C o f %2.6 2 a t m o s t .

.

T a b l e 3 , w i t h t h e e x c e p t i o n o f kehoeite, r e f e r s o n l y t o s i l i c a and a l u m i n o s i l i c a t e s . A1P04 can c r y s t a l l i s e i n f o m s i s o s t r u c t u r a l w i t h q u a r t z , t r i d y m i t e and c r i s t o b a l i t e and i n a d d i t i o n , u s i n g o r g a n i c b a s e s as t e m p l a t e s , a number o f o t h e r A1P04 s p e c i e s have b e e n made which i n c l u d e a s t r u c t u r a l a n a l o g u e of s o d a l i t e and of e r i o n i t e / o f f r e t i t e ( 2 1 ) . Analogues o f z e o l i t e s a r e known i n which Ga r e p l a c e s Al and Ge r e p l a c e s S i ( 2 2 ) . T a b l e 3 i s n o t i n t e n d e d t o i n c l u d e such m a t e r i a l s .

8.

CONSTRUCTING 3-D FRAMEWORKS

I n what f o l l o w s s e v e r a l ways of c o n s t r u c t i n g z e o l i t e frameworks w i l l b e c o n s i d e r e d . These a r e most i n f o r m a t i v e i n t h a t t h e y show n o t o n l y how known s t r u c t u r e s emerge b u t a l s o many n o v e l ones which p o i n t t h e way f o r f u r t h e r s y n t h e s i s . T h i s a p p r o a c h i s Dreferred t o describing individual crystallogranhic structures. Among t h e ways of v i s u a l i s i n g and c o n s t r u c t i n g z e o l i t e a n i o n s a r e the following: 1. S t a c k i n g of p o l y h e d r a 2. Use of t h e sigma t r a n s f o r m a t i o n 3. Use o f o p e r a t o r s 4 . C r o s s - l i n k i n g c h a i n s of v a r i e d c o m p l e x i t y 5 . C r o s s - l i n k i n g v a r i o u s k i n d s of l a y e r 8 . 1 . Frameworks a s A s s e m b l i e s of P o l y h e d r a l Voids T h i s method of b u i l d i n g p o r o u s frameworks was i l l u s t r a t e d i n Polyhedral voids are v e r y common i n z e o l i t e s t r u c t u r e s , a s i l l u s t r a t e d i n T a b l e 4 ( 2 3 ) , and many frameworks can be b u i l t by s t a c k i n g p o l y h e d r a of one o r n o r e d i f f e r e n t s h a p e s and s i z e s s o as t o occupy a l l s p a c e a v a i l a b l e o r t o c r e a t e open c h a n n e l s s u r r o u n d e d by l i n k e d p o l y h e d r a . The s i m p l e s t example i n v o l v e s t h e s o d a l i t e c a g e ( F i g . 7 ) , i . e . t h e l i - h e d r o n of t y p e I . T h i s i s one of F e d e r o v ' s s p a c e - f i l l i n g ? o l y h e d r a w h i c h , s t a c k e d by f a c e s h a r i n g w i l l f i l l a l l s p a c e w i t h l r s f e l l o w s , i n t h e same o r i e n t a t i o n . The r e s u l t i s t h e s o d a l i t e s r r u c t u r e . The forms o f some of t h e p o l y h e d r a l i s t e d i n T a b l e 4 - r e shown i n F i g . 1 3 (24)

5 . 7 . f o r m e l a n o p h l o g i t e and d o d e c a s i l - 3 C .

.

TABLE 4 Some polyhedra in zeolited23)

Polyhedron

Faces

Vertices"

Approximate free dimension

6-hedron (cube) 8-hedron (hexagonal prism)

2.3 in plane of 6-rings

10-hedron (octagonal prism) 1I-hedron

4.5 in plane of 8-rings 4 . 7 along c axis 3 . 5 normal to c

14-hedron Type I (truncated octahedron) 14-hedron Type 11

6 . 6 for inscribed sphere

17-hedron Type I 17-hedron Type I1 18-hedron (oblate spheroidal form)

26-hedron Type I (truncated cubo-octahedron) 26-hedron Type I1

"If

11

Examples

(-4

6 . 0 along c 7 . 4 normal to c 9 . 0 along c 7 to 7 . 3 normal to C 7.7 along c 6 . 4 normal to c 1 0 . 8 ~ 6 . (6.6 6 is measured between centre planes of opposite 8-rings) I I along c 6 . 5 nornlal to c

zeolite A faujasite, zeolite ZK-5, chabazite, erionite, offretite, levynite paulingite, zeolite

RHO cancrinite, zeolite L, erionite, offretite, zeolite losod sodalite, faujasite, zeolite A gmelinite, offretite, mauite (zeolite Q) levynite zeolite losod paulingite, zeolite ZK-5

chabazite

15 along c 6 . 3 normal to c

erionite

11.4 for inscribed sphere

paulingite, zeolite ZK-5, zeolite A, zeolite RHO faujasite (zeolites X and Y)

11.8 for inscribed sphere

denotes the number of faces, 2 n - 4 gives the n~~rnber of vertices.

Next i n s i m p l i c i t y t o s o d a l i t e a r e frameworks made by s t a c k i n g two k i n d s of polyhedron o n l y , a s e x e m p l i f i e d by m e l a n o p h l o g i t e and dodecasil-3C i n 5 . 7 . Then one may c o n s i d e r s t r u c t u r e s composed of a s s e m b l i e s of t h r e e k i n d s of polyhedron such a s z e o l i t e A ( c u b i c u n i t s , s o d a l i t e cages and 26-hedra of t y p e I) and f a u j a s i t e (hexagonal p r i s m s , s o d a l i t e cages and 26-hedra of t y p e 11). Next come s t r u c t u r e s made by a s s e m b l i n g p o l y h e d r a , of t e n i n columns, which do n o t f i l l a l l s p a c e b u t which l e a v e c o n t i n u o u s c h a n n e l s i n t h e framework. Examples of t h e s e ways of b u i l d i n g z e o l i t e s a r e g i v e n i n T a b l e 5 ( 2 5 ) . Wide p a r a l l e l c h a n n e l s c i r c u m s c r i b e d by 12-rings a r i s e i n u n f a u l t e d , i . e . c r y s t a l l o g r a p h i c a l l y i d e a l , c a n c r i n i t e h y d r a t e , g m e l i n i t e , o f f r e ti t e and mazzi t e Where such c h a n n e l s do n o t o c c u r windows of 8 - r i n g s ( c h a b a z i t e , e r i o n i t e , z e o l i t e A, z e o l i t e ZK-5), of o c t a g o n a l p r i s m s ( z e o l i t e RHO) o r of 12-rings ( f a u j a s i t e ) allow molecules t o migrate, i n t h e s e i n s t a n c e s i n a 1 1 t h r e e dimensions t h r o u g h o u t t h e frameworks

.

.

8 . 2 . The Sigma T r a n s f o r m a t i o n

'

The sigma t r a n s f o r m a t i o n i s a p u r e l y c o n c e p t u a l d e v i c e f o r i n t e r - r e l a t i n g and b u i l d i n g known and hypothe t i c a l z e o l i t e frameworks ( 2 6 ) . A t e t r a h e d r a l l y connected s t r u c t u r e i s expanded by i m a g i n a r y f i s s i o n of T atoms (T = A 1 o r S i ) l y i n g on s p e c i f i e d p l a n e s r u n n i n g through t h e s t r u c t u r e , and c r e a t i n g new oxygen b r i d g e s c o n n e c t i n g p a i r s r e s u l t i n g from t h e f i s s i o n . As a n example F i g . 14, i n t h e t o p h a l f , shows how a s i n g l e t e t r a h e d r o n becomes a p a i r , a 4 - r i n g , a 6 - r i n g and an 8 - r i n g ; and how t h e s e t h r e e r i n g s become r e s p e c t i v e l y cube, hexagonal p r i s m and o c t a g o n a l p r i s m . The bottom h a l f of t h e f i g u r e shows t h e s t a g e s of t r a n s f o r m a t i o n of a s o d a l i t e cage i n t o t h e 26-hedron of t y p e I found i n z e o l i t e A , The b a s i c r e q u i r e m e n t i s t h a t e v e r y T atom l y i n g i n t h e t r a n s f o r m a t i o n p l a n e must have two of i t s l i n k a g e s l y i n g i n t h e p l a n e and t h e o t h e r two emerging from o p p o s i t e s i d e s of t h e p l a n e . An i n v e r s e o f t h e sigma t r a n s f o r m a t i o n may o c c u r w i t h a p l a n e c o n t a i n i n g no T atoms i f e v e r y oxygen b r i d g e c u t by a p l a n e i s a common edge of two 4 - r i n g s . T h i s d e v i c e may n o t i o n a l l y r e d u c e double r i n g s ( p r i s m s ) t o s i n g l e r i n g s o r l a d d e r s t o s i n g l e chains. The t r a n s f o r m a t i o n of s o d a l i t e i n t o z e o l i t e A ( c . f . t h e b o t t o m h a l f of F i g . 14) t a k e s p l a c e v i a two h y p o t h e t i c a l i n t e r m e d i a t e z e o l i t e s . S t a r t i n g w i t h s o d a l i t e numerous o t h e r t r a n s f o r m a t i o n s were e f f e c t e d , t o z e o l i t e RHO w i t h two i n t e r m e d i a t e s t r u c t u r e s , f a u j a s i t e w i t h t h r e e i n t e r m e d i a t e s and c h a b a z i t e w i t h two i n t e r m e d i a t e s . T r a n s f o r m a t i o n s of c a n c r i n i t e y i e l d e d o f f r e t i t e and g m e l i n i t e ; t r i d y m i t e gave p a r a c e l s i a n and p h i l l i p s i t e ; and c r i s t o b a l i t e gave z e o l i t e ABW and gismondine a s w e l l a s two unknown s t r u c t u r e s (26).

Sodal~teunit

FIG.14 Examples of the "sigma transformation". The top half of the figure illustrates the transformation of a single tetrahedron to yield 4-, 6- and 8-rings and 4-4, 6-6- and 8-8- prisms. The bottom half shows stages in transforming a sodalite cage (14-hedron of Type I) to the 26-hedral cage of zeolite ~(16).

-.

.-ig.

15.

Two c o n f i g u r a t i o n s of l a d d e r s of 4 - r i n g s three-fold operator axis (27)

.

relative to a

Fig. 16.

I n ( a ) two connected NFNF l a d d e r s do n o t f a c e e a c h o t h e r d i r e c t l y and g e n e r a t e t h e p r o j e c t i o n ( b ) . I n ( c ) t h e s e connected l a d d e r s f a c e each o t h e r d i r e c t l y and g e n e r a t e t h e p r o j e c t i o n (d) ( 2 7 ) .

8.3.

Use of O p e r a t o r s

I n 5 . 3 . t h e e x i s t e n c e of l a d d e r - l i k e c h a i n s was n o t e d . They c o n s i s t e d of 4-rings each 4 - r i n g l i n k e d t o two o t h e r s by s h a r i n g o p p o s i t e e d g e s . Such l a d d e r s v a r i o u s l y buckled s e r v e t o i l l u s t r a t e t h e u s e of o p e r a t o r s i n producing z e o l i t e frameworks ( 2 7 ) . F i g . 15 shows two c o n f i g u r a t i o n s of 4-ring l a d d e r s r e l a t i v e t o a t h r e e - f o l d o p e r a t o r a x i s . The t e r m i n a l unshared oxygen atoms i n t h e s e l a d d e r s a r e d e s i g n a t e d N and F ("near" and " f a r " a c c o r d i n g t o t h e i r d i s t a n c e from t h e a x i s . Thus on t h e l e f t t h e c h a i n i s i n an c o n f i g u r a t i o n and on t h e r i g h t an NNFF configuration. NFNF

.. .

.. .

The t h r e e - f o l d o p e r a t o r of F i g . 15 s e r v e s t o l i n k t h e l a d d e r s through t h e unshared t e r m i n a l oxygens n e a r t h e a x i s t o produce columns of p o l y h e d r a c h a r a c t e r i s t i c of some z e o l i t e s of t h e c h a b a z i t e group ( s e e Table 5 ) . There a r e t h e n t h r e e ways of e x t e n d i n g t h e column i n t h e p l a n e normal t o t h e o p e r a t o r a x i s :

1. 2.

The c h a i n c o n s i d e r e d i s s h a r e d by two o p e r a t o r s . The c h a i n i s l i n k e d t o a second c h a i n b e l o n g i n g t o a n o t h e r o p e r a t o r i n such a way t h a t ( a ) t h e two connected c h a i n s do n o t f a c e each o t h e r d i r e c t l y ( F i g . 1 6 a ) , and (b) t h e two connected c h a i n s f a c e each o t h e r ( ~ i g .16c)

.. .

The NFNF c h a i n s of F i g . 15 t h e n i n F i g . 16a and c l e a d r e s p e c t i v e l y t o t h e p r o j e c t i o n s normal t o t h e channel a x i s shown i n F i g . 16b and d. These have channels c i r c u m s c r i b e d by puckered 12- and 24-rings. For v a r i o u s c h a i n s h a v i n g p e r i o d i c i t i e s n a l o n g t h e i r lengths (the c d i r e c t i o n ) the structures obtained f o r n < 8 a r e g i v e n i n Table 6 . A l l s t r u c t u r e s i n column 4 have t h e p r o j e c t i o n normal t o t h e c - d i r e c t i o n shown i n F i g . 16b, and a l l s t r u c t u r e s i n column 5 have t h e p r o j e c t i o n s e e n i n F i g . 16d. The l a r g e f r e e d i a m e t e r s o f z e o l i t e s of column 5 and t h e o b s e r v a t i o n t h a t t h e y a r e n o t blocked by s t a c k i n g f a u l t s would make such z e o l i t e s of s p e c i a l i n t e r e s t . The t a b l e i n c l u d e s 33 unknown z e o l i t e frameworks. Other members of t h e chabazi t e group such a s s o d a l i t e , e r i o n i t e , c h a b a z i t e and l e v y n i t e a s w e l l as a d d i t i o n a l unknown s t r u c t u r e s can be g e n e r a t e d by a p p l y i n g 6 3 and 7 axes a s o p e r a t o r s ( 2 7 ) .

8.4.

C r o s s - l i n k i n g of Chains

I n 5 . 8.3 o p e r a t s r s s e r v e d t o c r o s s - l i n k l a d d e r - l i k e c h a i n s and t o form z e o l i t e s . Smith and R i n a l d i (28) c o n s i d e r e d o t h e r a s p e c t s of t h e c r o s s - l i n k i n g o f l a d d e r s based on 4 - r i n g s . Each v e r t e x of a g i v e n r i n g may p o i n t up (U) o r down ( D ) , g i v i n g r i s e t o the f o u r t y p e s of r i n g shown i n F i g . 17 ( 2 8 ) . The l a s t of t h e s e w i t h a n o t h e r of l i k e k i n d can form only t h e c u b i c u n i t shoxm i n t h e

TABLE 5 Exanlples of combinations of polyhedra in zeolites(25) Zeolite

Polyhedra and other features

Cancrinite hydrate

1I-hedra (cancrinite cages). Wide channels 11' to c

Losod

1 1-hedra in columns A 17-hedra of Type I1 in columns B Hexagonal prisms 20-hedra

Chabazite

Gmelinite

Hexagonal prisms. 14-hedra of Type II Wide channels 11' to C

Erionite

Hexagonal prisms I I-hedra 23-hedra

Mazzite

Unknown Faujasite

Hexagonal prisms I I-hedra 14-hedra of Type 11 Wide channels 11' to c 14-hedra of Type I1 Wide channels 11' to c. Narrow channels 11' to c Id-hedra of Type 11. Wide channels 11' to C Hexagonal prisms 14-hedra of Type 1 26-hedra of Type 11

Proportions

Mode of combination 11-hedra in columns 11' t o c. Six linked coiumns surround and create each wide channel. Alternate columns are displaced by c/2. A column of A is surrounded by six of B, 11' to c. B-columns are alternately displaced by c/2. Prisms and 20-hedra alternate in columns 11' t o c. A given column is surrounded by six like columns, three displaced by c/3 and three by 2~13. Prisms and 14-hedra alternate in columns 11' to c. Six linked columns surround and create wide channels. Alternate columns displaced by c/2. Prisms and I I-hedra alternate in columns A , 23-hedra form columns B, both A and B 11' to c. Each column A is surrounded by six of B, where columns B a r e alternately displaced bq cl2. Prisms and I I-hedra alternate in columns A ; 14-hedra form columns B; both A and B are 11' t o c. Three A and three B surround and create each wide channel. Six linked columns of 14-hedra surround and create wide and narrow channels 11' to c. Alternate columns displaced by c?. Six linked columns of 14-hedra surround and create wide channels. All at same height. A given 14-hedron is linked by four prisms arranged tetrahedrally on four of its eight hexagonal faces, to its four nearest 14-hedron neighbours. Arrangement of 14hedra is as are the atoms in diamond. This creates 26-hedral voids, also arranged like atoms in diamond.

TABLE 5-rontin~ted Zeolite Zeolite A

Polyhedra and other features

Proportions

Cubic units 14-hedra of Type 1 26-hedra of Type I

Zeolite RHO Octagonal prisms 26-hedra of Type I Zeolite ZK-5 Hexagonal prisms I 8-hedra 26-hedra of Type I

Mode of combination

A given '14-hedron is linked by cubic units through its six 4-ring faces to a 4-ring face of each of six other 14-hedra. This arrangement creates 26-hedra. A given 26-hedron is linked by octagonal prisms through each of its six 8-ring faces to a n 8-ring face of one of six other 26-hedra. A given 26-hedron is linked by hexagonal prisms through its eight 6-ring faces to a 6-ring face of each of eight other 26-hedra. This creates the 18-hedral voids.

TABLE 7 Schemes for interconnecting chains of Fig. 18( 3 I). Number

Direct

Unitary

Inverse

Rotational symmetry of scheme

T a b l e 6:

A c t u a l and h y p o t h e t i c a l s t r u c t u r e s b a s e d on l a d d e r s of 4 - r i n g s (27)

.

Periodicity n

2 3a 3b 4 5a 5b 6 7a 7b 7c 7d 8a 8b 8c

Type of Chain

Three-fold o p e r a t o r Chain s h a r e d Chain n o t s h a r e d not facing facing

c a n c r i n i Pe

NF NNF 1 FFN NNFF FNNFN NNFNFF NFNFNFN FNFNFNF

offretite gmelini t e unknown

1 1

'ITFFNFF FFNNFNN FNFFNFN NFNFFNFF FNFNNFNN

]

unknown unknown unknown unknown unknown

unknown zeolite L unknown unknown unknown unknown unknown unknown unknown unknown unknown unknown unknown unknown

unknown unknown unknown* unknown* unkn ownj: unknown unknown* unknown unknown unknown* unknown,? unknown* unknown* unknown

0

a(hex) i n A c(hex) i n Free diameter ofo Main c h a n n e l i n A Ring c i r c u m s c r i b i n g main c h a n n e l Stacking f a u l t s

2

1 2 . 8 t o 13.7 n x 2.5 a

6.5

12-ring Block main Channe 1

12-ring Do n o t b l o c k

24-ring Do n o t b l o c k

*FF l e a d s t o h e a v i l y d i s t o r t e d &-membered r i n g s . f i g u r e . From t h e o t h e r t h r e e k i n d s o f 4 - r i n g t h r e e t y p e s of c h a i n can be made, a s shown i n F i g . 1 7 . The UUDD c h a i n r e s e m b l e s t h e NNFF c h a i n of T a b l e 6 and F i g . 15. From i t 17 o t h e r frameworks were made by v a r i o u s l y c r o s s - l i n k i n g t h e c h a i n s , i n which r e p e a t i n t h e p l a n e normal t o t h e c h a i n d i r e c t i o n were l e s s These i n c l u d e d f e l s p a r , p a r a c e l s i a n , p h i l l i p s i t e and t h a n 15 dis a s y n t h e t i c m a t e r i a l termgd phase A (29) &BaCAlSi2O61C1, OE< group 14/mmm; a = 14.194 A and c = 9.934 A ) . O t h e r s were n o v e l s t r u c t u r e s . Chains can be l i n k e d i n two ways:

tancea. 1.

F l e x i b l e mode. T e t r a h e d r a connected t o one a n o t h e r a l o n g t h e c h a i n a r e b o t h c o n n e c t e d t o t h e same a d j a c e n t c h a i n .

These i n c l u d e p a r a c e l s i a n , p h i l l i p s i t e , gismondine and p h a s e A. I n f l e x i b l e mode. T e t r a h e d r a connected t o one a n o t h e r along the chain a r e linked t o d i f f e r e n t adjacent chains. An example o f t h i s i s f e l s p a r .

2.

I n t h e f l e x i b l e mode 4 - r i n g s can r o t a t e c o - o p e r a t i v e l y b u t t h i s i s n o t p o s s i b l e i n t h e i n f l e x i b l e mode. The second k i n d of c h a i n (UDUD) a p p e a r s u n - c r o s s - l i n k e d a s ~ h ea n i o n i n n a r s a r s u k i t e , Na4(Ti0?2CSi80201, ( s e e 5. 3 and F i g . 3 g ) . I h e s e c h a i n s c o u l d be c r o s s - l i n k e d i n f o u r ways, one of which i s found i n t h e n o n - z e o l i t e b a n a l s i t e , Ba,Na2CA14Si4016I . The t h i r d k i n d of c h a i n (UUUD) could a l s o be c r o s s - l i n k e d t o y i e l d frameworks af n o v e l k i n d s (28)

.

The c h a i n s i n F i g . 17 a r e examples o n l y of t h o s e which may l e a d t h r o u g h c r o s s - l i n k i n g t o t e c t o s i l i c a t e s . O t h e r examples of s i n g l e and d o u b l e c h a i n a n i o n s were r e f e r r e d t o i n 5. 3. A d d i t i o n a l c h a i n s from which z e o l i t e a n i o n s may be c o n s t r u c t e d o c c u r i n t h e x a t r o l i t e group and m o r d e n i t e group. I n t h e n a t r o l i t e c h a i n UDUD i - r i n g s a r e l i n k e d t o g e t h e r by s i n g l e t e t r a h e d r a a s shown i n F i g . 1 8 (30) f o r n a t r o l i t e and t h o m s o n i t e . These c h a i n s d i f f e r i n S i / A l r a t i o , which i s 312 f o r n a t r o l i t e and 1 f o r t h o m s o n i t e . The A1,Si i i s t r i b u t i o n s on t e t r a h e d r a l s i t e s a r e o r d e r e d and t h e p e r i o d i c i t y The h e i g h t s of f r e e v e r t i c e s of t e t r a in t h e c - d i r e c t i o n i s 6 . 6 h e d r a a r e m u l t i p l e s of c / 8 . A l b e r t and G o t t a r d i (31) c o n s i d e r e d Yays of c r o s s - l i n k i n g a c h a i n t o f o u r o t h e r c h a i n s t h r o u g h i t s f r e e -:ertices. Each c h a i n can be coded by t h e h e i g h t , n ( n c / 8 ) of t h e z e n t r e of i t s s i n g l e t e t r a h e d r o n . The l i n k a g e of a g i v e n c h a i n zo i t s n e i g h b o u r s i s t h r o u g h t h e p a i r of U and t h e p a i r of D t e t r a xedra. For t h e l a t t e r t h e p o s s i b i l i t i e s a r e :

2.

1.

The c r o s s - l i n k s a r e b o t h t o D t e t r a h e d r a of two o t h e r c h a i n s ( t h e s e c h a i n s b e i n g a t h e i g h t n) . One c r o s s - l i n k i s t o D and t h e o t h e r t o U ( t h e two a d j a c e n t c h a i n s b e i n g a t h e i g h t s n and ( n - 2 ) ) . Both c r o s s - l i n k s a r e t o U ( t h e a d j a c e n t c h a i n s b e i n g a t h e i g h t s (n-2)) .

2.

3.

Three s i m i l a r ~ o s s i b i l i t i e sa r i s e f o r t h e p a i r of U t e t r a h e d r a , namely b o t h c r o s s - l i n k s t o U t e t r a h e d r a ; one t o U and one zo D ; and b o t h t o D . I n U-D bonds t h e h e i g h t of t h e a d j a c e n t :hain i s n + 2. T a b l e 7 (31) g i v e s t h e schemes f o r c r o s s - l i n k i n g . I n i t t h e symbols =, + and + have t h e f o l l o w i n g meanings: n n n

= +-

+

n f o r a c r o s s - l i n k w i t h a c h a i n a t t h e same h e i g h t n + 2 f o r a cross-link with a chain a t height n + 2 n - 2 f o r a cross-link with a chain a t h e i g h t n - 2

Fig. 17.

Four t y p e s of 4 - r i n g w i t h a p i c e s of t e t r a h e d r a p o i n t i n g up (U) o r down (D). Below, t h r e e k i n d s of c h a i n a r e shown b a s e d on UUDD, UDUD and UDUUD r i n g s , t o g e t h e r w i t h t h e c u b i c u n i t made from two 4 - r i n g s of t h e f o u r t h kind (28).

F i g . 18.

The c h a i n s p r e s e n t i n z e o l i t e s of t h e n a t r o l i t e group ( a ) r e f e r s t o n a t r o l i t e , (b) t o t h o m s o n i t e . (30). Shaded t e t r a h e d r a denote A104, unshaded d e n o t e S i 0 4 . There i s o r d e r i n g i n t h e A1,Si d i s t r i b u t i o n s , and t h e p e r i o d i c i t i e s i n view of t h e o r d e r i n g a r e 3 f o r ( a ) and 6 for (b).

Fig.

19.

Fig. 20.

The framework o f e d i n g t o n i t e viewed a l o n g C1101 ( 3 2 ) .

F o u r k i n d s o f h e x a g o n a l s i n g l e s h e e t d i f f e r e n t i a t e d by p a t t e r n s i n which a p i c e s o f t h e l i n k e d t e t r a h e d r a p o i n t up W ) o r down (35).

(v)

I f c o r r e s p o n d i n g i n v e r s e and d i r e c t schemes a r e c o n s i d e r e d a s a s i n g l e scheme t h e n i n e d i f f e r e n t p o s s i b i l i t i e s i n T a b l e 7 a r e reduced t o s i x . I n f i b r o u s z e o l i t e s two k i n d s o f S i / A l o r d e r i n g have b e e n o b s e r v e d , a s a l r e a d y n o t e d f o r n a t r o l i t e and thomsonite i n F i g . 18. I f p o s s i b i l i t i e s of Si/A1 o r d e r i n g a r e combined w i t h t h e s i x schemes t h e n t h e 15 p o s s i b i l i t i e s of Table 8 (31) a r e found. Ten of t h e s e r e f e r t o unknown s t r u c t u r e s . As an example, F i g . 19 (32) shows a p o r t i o n of t h e e d i n g t o n i t e framework, viewed a l o n g E1101. The c h a i n c h a r a c t e r i s t i c of some z e o l i t e s o f t h e m o r d e n i t e group i s b a s e d on 5 - r i n g s (33) . The c h a i n s r u n i n t h e c d i r e c t i o n E i g h t s t r u c t u r e s which c a n b e made by with a p e r i o d i c i t y 7.52 c r o s s - l i n k i n g t h e c h a i n s a r e g i v e n i n Table 9 ( 3 4 ) . Only two of t h e s e r e f e r t o known m i n e r a l s .

g.

8.5. Frameworks made by c r o s s - l i n k i n g

Sheets

Many k i n d s of s h e e t can be made from polygons s h a r i n g edges w i t h l i k e o r u n l i k e p o l y g o n s , a number of which have b e e n i d e n t i f i e d i n s i l i c a t e s (5.4). These s h e e t s were composed o n l y of 6 - r i n g s (mica) ; of 4- and 8 - r i n g s (apophyl li t e ) ; of 4-, 6- and 8 - r i n g s ( d a l y l i t e ) ; and of 4-, 6- and 1 2 - r i n g s (manganpyrosmalite). Each t y p e of s h e e t can b e f u r t h e r s u b - d i v i d e d , a c c o r d i n g t o t h e Four p a t t e r n s o f un-linked v e r t i c e s p o i n t i n g up (U) and down ( D ) . such p a t t e r n s a r e shown f o r hexagonal s h e e t s i n F i g . 20 ( 3 5 ) . There a r e a c c o r d i n g l y many ways of c r o s s - l i n k i n g s h e e t s , which may i n a d d i t i o n be b u c k l e d i n a v a r i e t y of ways. Puckered s h e e t s of type ( i ) a r e f o u n d , p a r a l l e l t o C O O 1 1 i n b i k i t a i t e , and such s h e e t s a l s o o c c u r i n t h e n o n - z e o l i t e s n e p h e l i n e and c a r n e g i e i t e . I n

T a b l e 8:

Space groups o f t h e n a t r o l i t e group of z e o l i t e s (31)

S t r u c t u r a l Type

2 3

4 5 6

D i s o r d e r e d (A1,Si) distribution

Pman Gonnardi t e ( ?) 162 d Te t r a g o n a l natrolite Pmma Imma Pmna

Natrolite type o r d e r

Thomsonite t y p e order

PZ1Z12 Edingtoni t e ~ 2/ b

~ 4 c2

Fdd2 Natrolite P2/a B2/b PZ1/a

P cnn Thomsonite Not p o s s i b l e

Pcca Not p o s s i b l e Not ~ o s s i b l e

r...-,--

0

,

, # ; , ,

I

:......: .

:

/

,

I

.

.

;

.,".. . ... . . . ...

.........:, . .. ..

.

z

. . , 3 ,, ,

.

.

r 1 i

1

I

J

: o : .

> :...... . ':. '

;

... .

:

'., ... : .:........... .

:

: n :

.

a

......b,

.

:......:

:

.

.

.,, ........:,.' : : 0 : ........ .. .. . .

............ ..,

..

F l c . 2 1 The puckered conformations of hexagonal sheets found in some zeolites are shown edge on (full lines). The interconnections between sheets are shown as dashed lines. The zeolites concerned are: (a) rnordenite; (b) dachlsiiite; (c) epistilbite; (d) ferrierite; (e) bikitaite; and (f) zeolite Li-~Bw(36).1.

0

Heulandite

Stilbite

Brewsterite

F i g . 22.

L a y e r s w i t h a common s t r u c t u r a l u n i t f o u n d i n h e u l a n d i t e , s t i l b i t e and b r e w s t e r i t e ( 3 8 ) .

I 0

Fig. 23.

I bsiny

The laumontite l a y e r showing s u b - u n i t s composed of f o u r 6 - r i n g s and two 4-rings

(39).

b i k i t a i t e t h e s h e e t s a r e l i n k e d t o one a n o t h e r through s i n g l e t e t r a h e d r o n c h a i n s p a r a l l e l t o C0101, whereas i n n e p h e l i n e and c a r n e g i e i t e t h e s h e e t s a r e l i n k e d d i r e c t l y t o one a n o t h e r . S h e e t s of type ( i i ) a r e d i r e c t l y connected t o one a n o t h e r i n Those of type ( i i i ) a r e found i n d a c h i a r d i t e , z e o l i t e ABW. e p i s t i l b i t e and f e r r i e r i t e . They a r e l i n k e d by s i n g l e 4 - r i n g s f o r t h e f i r s t two z e o l i t e s and by s i n g l e 6 - r i n g s f o r f e r r i e r i t e . Type ( i v ) s h e e t s connected through s i n g l e 4-rings form t h e mordenite s t r u c t u r e s . F i g . 21 (36) shows t h e puckered s h e e t s edge on , a s t h e b o l d l i n e s , w h i l e t h e c o n n e c t i o n s between s h e e t s a r e i n d i c a t e d as t h e dashed l i n e s , f o r t h e v a r i o u s z e o l i t e s r e f e r r e d t o i n t h i s and t h e p r e c e d i n g p a r a g r a p h , I n t h e c h a b a z i t e group s h e e t s c o n t a i n i n g e i t h e r 6 - r i n g s o r hexagonal prisms (6-6--rings) can be i d e n t i f i e d . These a r e l i n k e d t o s i m i l a r s h e e t s above and below i n t h e v a r i o u s s t a c k i n g sequences of Table 10 ( 3 4 ) , where l a y e r s c o n t a i n i n g s i n g l e 6 - r i n g s a r e denoted by s m a l l l e t t e r s and t h o s e c o n t a i n i n g 6-6-rings by c a p i t a l l e t t e r s . Novel z e o l i t e s i n v o l v i n g sequences of A , B and C l a y e r s analogous t o those of a , b and c l a y e r s i n l o s o d , l i o t t i t e , a f g h a n i t e and f r a n z i n i t e a r e p o s s i b l e , a s w e l l as sequences of double and s i n g l e 6 - r i n g l a y e r s i n a d d i t i o n t o t h o s e g i v i n g o f f r e t i t e , e r i o n i t e and l e v y n i t e . Two of t h e s e p o s s i b i l i t i e s w i t h sequences ABCB and ABCACB have beenoexamined ( 3 7 ) . have hexagonal u n i t c e l l s w i t h a % 13.7 A and c % 20 and 30 respecti v e l y . Each c o n t a i n s e l o n g a t e d 26-hedra of a t h i r d k i n d , w i t h n i n e 8 - r i n g , t o 6 - r i n g and f i f t e e n 4-ring f a c e s b The f r e e l e n g t h approaches 20 and t h e f r e e d i a m e t e r about 6 . 5 A. In addition the ABCB s t r u c t u r e c o n t a i n s g m e l i n i t e type 14-hedra and t h e ABCACB s t r u c t u r e c o n t a i n s b o t h g m e l i n i t e and 20-hedral c h a b a z i t e type cages.

TheH

B

H e u l a n d i t e , s t i l b i t e and brews t e r i t e can be r e p r e s e n t e d i n terms of o t h e r l a y e r s each c o n t a i n i n g t h e same u n i t composed of f o u r 5 - r i n g s and two 4 - r i n g s , a s shown i n F i g . 22 ( 3 8 ) . The bond d e n s i t y i n t h e l a y e r s i s g r e a t e r t h a n t h a t of t h e l i n k s between l a y e r s , r e s u l t i n g i n t h e i r p l a t y c h a r a c t e r . Laumontite c o n t a i n s s h e e t s of t h e type shown i n F i g . 23 ( 3 9 ) i n which s t r u c t u r a l u n i t s composed of f o u r 6 - r i n g s and two 4-rings o c c u r . Z e o l i t e s can be c o n s t r u c t e d from l a y e r s o f , o r c o n t a i n i n g , s o d a l i t e cages. I n s o d a l i t e i t s e l f t h e l a y e r c o n s i s t s of t h e s e cages each l i n k e d t o f o u r o t h e r cages by s h a r i n g 4-ring f a c e s , and t o a cage i n an upper l a y e r and one i n a lower l a y e r a l s o by s h a r i n g i t s remaining two 4-ring f a c e s . I n z e o l i t e A a s o d a l i t e cage i n a l a y e r i s l i n k e d t o f o u r o t h e r cages by double & - r i n g s ( c u b i c u n i t s ) and t o a s o d a l i t e cage i n t h e l a y e r above and one i n t h e l a y e r below, a l s o by c u b i c u n i t s . From a t o p o l o g i c a l v i e w p o i n t

Table 9 :

Some S t r u c t u r e s Obtainable by Cros s - l i n k i n g Mordeni te Chains (34)

Number

Space group

1 2 3

4 5 6 7 8

~2/m B2/m Pmnm Amam Cmcm Imm Bb cm Bmm

a(%

b

(2)

c

6)

Y

107. g o 123O

.

Example

Dachiardite Modified d a c h i a r d i t e Unknown Unknown Mordeni t e Modified mordeni t e Unknown Unknown

Table 10:

S t r u c t u r e s formed by l i n k i n g l a y e r s c o n t a i n i n g s i n g l e 6 - r i n g s ( a , b , c) and l a y e r s c o n t a i n i n g 6-6-rings (A,B, C) ( 3 4 ) .

No. of l a y e r s i n repeat unit

Structural t Y Pe

ab abacac ab cab cbacb Ab Ab Ac aB aC AbCaBc AB AB C

Space Group

a

(2)

c

(2)

N ame

Canc Sodalit e Los od Liottite Afghani t e Franzinite Offretite Erionit e

-

Levyni t e Gme l i n i t e Chabazi t e

s o d a l i t e c a g e s may r e p r e s e n t t h e p o s i t i o n s occupied by s p h e r e s i n c u b i c close-packing ( 4 0 ) . An i n d e f i n i t e number of s t r u c t u r e s can be made from hexagonal l a y e r s of close-packed s p h e r e s i n v a r i o u s corresponds w i t h c u b i c symsequences. The sequence ABCABC metry a s i n f a u j a s i t e , i n which t h e 14-hedra a r e j o i n e d by hexagonal p r i s m u n i t s t o f o u r o t h e r 14-hedra, t h e ABC l a y e r corsequence b e i n g a l o n g C1111. The l a y e r sequence ABAB responds w i t h a hexagonal s t r u c t u r e , t h e s o d a l i t e 14-hedra b e i n g , l i n k e d , a s i n f a u j a s i t e , by hexagonal p r i s m u n i t s t o f o u r o t h e r 14-hedra. The r e s u l t a n t v e r y open s t r u c t u r e h a s cages and c h a n n e l s comparable w i t h t h o s e i n f a u j a s i t e . This sequence i s shown f o r m a l l y a s F i g . 24a. F i g . 24b i s t h a t i n f a u j a s i t e and F i g . 24c i s t h e sequence ABABCA of one of many p o s s i b l e novel s t r u c t u r e s . Z e o l i t e ZSM-3 might, i t was t h o u g h t , be r e l a t e d t o f a u j a s i t e as one of t h e s e s t a c k i n g arrangements of l a y e r s of s o d a l i t e cages ( 4 0 ) . I t had a hexagonal u g i t c e l l w i t h a = 17.5 A and a maximum p o s s i b l e v a l u e of c of 129 A., S i n c e t h e o d i s t a n c e between a d j a c e n t s o d a l i t e cage l a y e r s i s 1 4 . 3 A, c = 129 A r e p r e s e n t s a 9 - l a y e r sequence.

. ..

...

Foore and Smith (41) gave f u r t h e r c o n s i d e r a t i o n t o s t r u c t u r e s based on s o d a l i t e cages a n d / o r on 26-hedra of t y p e I ( a s found i n z e o l i t e A). I n a short-hand d e s c r i p t i o n of p o s s i b i l i t i e s they used t h e symbols H

=

H'

hexagonal f a c e ( 6 - r i n g )

= hexagonal prism (6-6-ring)

S = s q u a r e f a c e (4-ring) S ' = c u b i c u n i t (4-4-ring) 0 = octagonal face (8-ring) 0 ' = o c t a g o n a l prism (8-8-ring)

The type of c o n t a c t between p o l y h e d r a was i n d i c a t e d by t h e a p p r o p r i a t e one of t h e s e l e t t e r s and t h e l e t t e r s f o l l o w i n g ( i n p a r e n t h e s e s ) denoted t h e t y p e s of p o l y o n a l f a c e s opposing each o t h e r a c r o s s t h e c o n t a c t . The r e s u l t s a r e summarised i n Table 11 i n which frameworks of f o u r s o f a r unknown z e o l i t e s a p p e a r . High r e s o l u t i o n e l e c t r o n microscopy (HREM) has r e v e a l e d a tendency t o r e c u r r e n t twinning i n s y n t h e t i c f a u j a s i t e ( z e o l i t e Y) ( 4 2 ) . T h i s can g e n e r a t e a new s t r u c t u r e w i t h i n t h e z e o l i t e w i t h e l l i p t i c a l a p e r t u r e s along Cl101 and an e l o n g a t e d "hypercagel' t h e l e n g t h of which depends on t h e e x t e n t of twinning. I f A denotes a b u i l d i n g u n i t r e p e a t a l o n g C l l l l ( a 1 4 . 3 A r e p e a t d i s t a n c e ) and V d e n o t e s a twin J a m e l l a , t h e n . . .AAA.. . i s t h e normal l a y e r V/A/V/A.. i s a t u n n e l s t r u c t u r e i n which a twin sequence and . l a m e l l a bounded by a p a i r of { I l l ) twin p l a n e s , denoted by s l a s h e s , a l t e r n a t e s e v e r y 14.2 with the regular f a u j a s i t e lamellae. When t h e r e a r e n twin p l a n e s t h e t u n n e l i n t h e s t r u c t u r e i s %14.3 ( n + l ) + 6.95 i n l e n g t h . The sequence . .AA/V/AA/V..

..

.

2

.

.

..

.-I

cnd

rim

CO ri

d d

- m

0 111

Ill A

-

. .

m

ri

rl d

4m

i n d i c a t e s a twin l a m e l l a between f l a n k i n g p a i r s of r e g u l a r r e p e a t long u n i t s and g i v e s a cage sequence i n which each cage i s 4 9 . 6 i n f r e e d i a m e t e r . This p a r t i c u l a r and v a r i e s between 1 3 and 7.4 sequence h a s been observed ( 4 2 ) . R e c u r r e n t twinning a t t h e u n i t c e l l l e v e l could a l s o c o n v e r t o t h e r known z e o l i t e s t r u c t u r e s i n t o n o v e l ones.

1

a

9.

INTRAZEOLITE CHANNELS

Each d i f f e r e n t z e o l i t e topology has a d i f f e r e n t system of channels and c a v i t i e s . N e v e r t h e l e s s a broad c l a s s i f i c a t i o n i s p o s s i b l e , from t h e viewpoint of m i g r a t i o n of g u e s t m o l e c u l e s , i n t o three divisions :

1. I n t r a c r y s t a l l i n e channels a r e p a r a l l e l and a r e n o t i n t e r connected (1-D d i f f u s i o n ) . 2 . Channels a r e i n t e r - c o n n e c t e d t h e t h i r d (2-D d i f f u s i o n ) . 3. Channels a r e i n t e r - c o n n e c t e d diffusion)

.

i n two dimensions b u t n o t i n i n t h r e e dimensions (3-D

The geometry of channel and c a v i t y systems i s d e f i n e d as p r e c i s e l y a s a r e t h e p o s i t i o n s of framework atoms. I n a d d i t i o n t o v a r i e d s p a t i a l arrangements of t h e open pathways, a s i n t h e above t h r e e c a t e g o r i e s , i n d i f f e r e n t t o p o l o g i e s c r o s s - s e c t i o n a l a r e a s and shapes a r e s p e c i f i c t o each topology and c o n f i g u r a t i o n . C a t i o n s may a l s o be l o c a t e d i n t h e d i f f u s i o n pathways and i f t h e y a r e p r e s e n t a t s t r a t e g i c ~ o i n t s ,such a s t h e windows o r n a r r o w e s t p o i n t s a l o n g a pathway t h e y can a c t a s p a r t i a l o r complete b a r r i e r s t o t h e movement of g u e s t molecules. The l o c a t i o n and numbers of t h e c a t i o n s can be modified by exchanges such a s 2 ~ a +? ca2+ and t h e b l o c k i n g e f f e c t s of c a t i o n s can be d r a m a t i c a l l y changed by such means. Some examples of z e o l i t e s w i t h 1-D, systems a r e : 1-D.

2-D.

3-D.

2-D and 3-D channel

C a n c r i n i t e h y d r a t e ( 1 2 , 6 . 2 ) ; l a u m o n t i t e ( 1 0 , 4 . 0 ~ 5- 6 ) ; m a z z i t e , z e o l i t e Q (12, - 7.4) ; mordenite (12, - 6.7x7.0) ; z e o l i t e L (12, 7.1). D a c h i a r d i t e T 1 0 , 3.7x6.7 and 9 , 3.6x4.8) ; f e r r i e r i t e (10, 4.3x5.5 a n d 8, 3.4x4.8) ;-levynite (8 , 3.3x5.3) ; s z l b i t e ( 1 0 , 4.126.2 and 8, 2.7x5.7) . Chabazite 78, - 3.6x3.7) ; e r T o n i t e ( 8 , 3.6x5.3.) ; f a u j a s i t e (12, 7 . 4 ) ; o f f r e t i t e ( 1 2 , 6.4 and 8, 3 . 2 x 5 . 2 ) ; z e o l i t e A (8,4.1) ; z e o l i t e RHO (8-8, - 3.9x5.i) z e o l i t e ZK-5 (8'. - 3.9).

The u n d e r l i n e d f i g u r e s i n t h e b r a c k e t s i n d i c a t e t h e numbers n of l i n k e d t e t r a h e d r a forming t h e n a r r o w e s t a p e r t u r e s a l o n g t h e d i f f u s i o n pathways and t h e o t h e r f i g u r e s a r e t h e f r e e dimensions of t h e s e a p e r t u r e s . They show, f o r example f o r % r i n g s , t h a t t h e r i n g s may have v a r i o u s c o n f i g u r a t i o n s a c c o r d i n g t o t h e framework i n which i t a p p e a r s .

10.

A l , S i ORDERING

According t o Lowenstein's r u l e (43) A1-0-A1 bonds do n o t o c c u r i n t e c t o s i l i c a t e s f o r which ~ i / A 1> 1. Accordingly where S i / A l = 1 S i and A1 must a l t e r n a t e on a l l t e t r a h e d r a l s i t e s . Recent magic a n g l e s p i n n i n g n u c l e a r magnetic resonance (MASNMR) measurements appeared t o c h a l l e n g e t h e v a l i d i t y of t h i s r u l e f o r z e o l i t e A w i t h S i / A l = 1 b u t s p e c t r a o b t a i n e d on z e o l i t e s A w i t h v a l u e s of ~ i / A 1> 1 l e d t o a r e - a p p r a i s a l which confirmed t h e r u l e ( 4 4 , 4 5 ) . It i s now almost c e r t a i n t h a t t h e r e a r e no known e x c e p t i o n s t o Lowenstein's r u l e f o r t e c t o s i l i c a t e s w i t h ~ i / A 1> 1, and hence t h a t t h e r e i s A l , S i o r d e r i n g whenever ~ i / A l= 1. There can a l s o be ~ 1 u n i t y , an A l , S i o r d e r i n g i n t e c t o s i l i c a t e s i n which ~ i / exceeds example r e f e r r e d t o i n 5 . 8 . 4 and F i g . 1 8 b e i n g n a t r o l i t e where Si/A1 = 312. F a u j a s i t e s h a v i n g wide ranges i n ~ i / A 1r a t i o s gave MASNMR s p e c t r a which have been i n t e r p r e t e d i n terms of o r d e r i n g of A 1 and S i a t s p e c i f i c r a t i o s corresponding w i t h i n t e g r a l numbers of A 1 and S i atoms p e r s o d a l i t e cages ( 4 6 , 4 7 ) . There i s c o n s i d e r a b l e i f n o t y e t t o t a l agreement on t h e o r d e r i n g schemes. The s p e c t r a themselves s e r v e t o g i v e t h e r e l a t i v e numbers of S i atoms l i n k e d ( t h r o u g h 0) w i t h 4 , 3, 2, 1 and 0 A 1 atoms, denoted r e s p e c t i v e l y a s S i ( 4 A l ) , Si(3A1), Si(2A1), S i ( l A 1 ) and S i (OA1) . Table 12 g i v e s a s a f u n c t i o n of t h e S i / A l r a t i o t h e r e l a t i v e p e r c e n t a g e p o p u l a t i o n s of t h e s e u n i t s f o r any t e c t o s i l i c a t e s t r u c t u r e , a c c o r d i n g t o a binomial d i s t r i b u t i o n and when Lowenstein's r u l e i s v a l i d . Where A 1 and S i a r e d i s t r i b u t e d on t e t r a h e d r a l s i t e s i n v o l v i n g more t h a n one s u b - l a t t i c e t h e r e may, a s i n t h e f e l s p a r s (48) be o r d e r e d and d i s o r d e r e d forms, s u b j e c t t o t h e o v e r - r i d i n g i n f l u e n c e of t h e Lowenstein r u l e . I n o r d e r e d a l k a l i m e t a l f e l s p a r s a t e q u i l i b r i u m a t low t e m p e r a t u r e s t h e A 1 atoms a r e found on one subl a t t i c e . High temperature e q u i l i b r i u m forms have A1 atoms d i s t r i b u t e d on more t h a n one s u b - l a t t i c e . O r d e r - d i s o r d e r changes of t h i s k i n d a r e s l u g g i s h and may b e s t be approachable by s y n t h e s i s under h i g h and low t e m p e r a t u r e c o n d i t i o n s provided r e a c t i o n i s a l s o slow. Rapid hydrothermal s y n t h e s e s of a l k a l i m e t a l f e l s p a r have y i e l d e d d i s o r d e r e d forms, presumably m e t a s t a b l e ( 4 9 ) . T h i s may be an example of Ostwald's law of s u c c e s s i v e t r a n s f o r m a t i o n s a c c o r d i n g t o which l e s s s t a b l e phases appear b e f o r e t h e more s t a b l e ones.

Table 12:

11.

R e l a t i v e p e r c e n t a g e p o p u l a t i o n s of S i (nAl) s t r u c t u r a l u n i t s , c a l c u l a t e d from t h e binomial formula f o r d i f f e r e n t t e c t o s i l i c a t e compositions ( 4 6 ) . Lowenstein's r u l e i s obeyed.

CATIONS AND WATER MOLECULES

Accurate and q u a n t i t a t i v e l o c a t i o n of c a t i o n s and w a t e r molecules h a s been more d i f f i c u l t t h a n d e f i n i n g t h e p o s i t i o n s of oxygen and T atoms i n t h e anions of z e o l i t e s . Reasons f o r t h i s include: (i)

The t o t a l number of c a t i o n s i s r a t h e r s m a l l compared w i t h t h e number of 0 and T atoms i n t h e a n i o n s . ( i i ) These c a t i o n s a r e u s u a l l y d i s t r i b u t e d o v e r a number of s u b - l a t t i c e s , and o f t e n t h e r e i s only p a r t i a l occupancy of s i t e s on a g i v e n s u b - l a t t i c e . Thus t h e number nn a g i v e n s u b - l a t t i c e may be low. ( i i i ) I n a number of i n v e s t i g a t i o n s powder X-ray photography has of n e c e s s i t y been used, g i v i n g more l i m i t e d information. (iv) I n t r a c r y s t a l l i n e water i s not necessarily a l l t i g h t l y h e l d on s p e c i f i c s i t e s .

A c o m p i l a t i o n of p u b l i s h e d work on c a t i o n and w a t e r l o c a t i o n s i s a v a i l a b l e , which however cannot a s s e s s accuracy i n t h e l o c a t i o n s claimed ( 5 0 ) . Such an assessment w i l l n o t be a t t e m p t e d h e r e , a l t h o u g h t h e r o l e of c a t i o n s a s m o d i f i e r s of molecular s i e v e behaviour i s of major s i g n i f i c a n c e .

F i g . 24.

S t a c k i n g of l a y e r s o f s o d a l i t e c a g e s i n f a u j a s i t e and r e l a t e d s t r u c t u r e s ( 4 0 ) . The c a g e s a r e r e p r e s e n t e d as the l i n e junctions. The view i s p e r p e n d i c u l a r t o t h e c-axis, 110 p r o j e c t i o n . ( a ) Hexagonal AB s e q u e n c e ; ( b ) c u b i c ABC s e q u e n c e ; ( c ) ABABC s e q u e n c e .

Qdb 5-1

Fig. 25.

4-4-1

Secondagy b u i l d i n g u n i t s (SBU) i n z e o l i t e b u i l d i n g . T h e i r c h a r a c t e r i s t i c i s t h a t a s e l e c t e d z e o l i t e framework can be b u i l t e n t i r e l y f r o m an a p p r o p r i a t e l y chosen one o f t h e s e SBU ( 5 1 ) .

12.

CONDLUDING REMARK

T h i s a c c o u n t of z e o l i t e s t r u c t u r e s has c o n c e n t r a t e d upon t h e a n i o n s which a r e t h e most a c c u r a t e l y d e f i n e d p a r t s and p r o v i d e t h e key t o z e o l i t e d i v e r s i t y , When t e c t o s i l i c a t e frameworks a r e cons t r u c t e d by a s e r i e s of d i f f e r e n t p r o c e e d u r e s one o b t a i n s i n a d d i t i o n t o t h e many known s t r u c t u r e s a remarkable number of o t h e r frameworks, porous on t h e s c a l e of m o l e c u l a r d i m e n s i o n s , which r e p r e s e n t s o f a r unknown z e o l i t e s and p o i n t t h e way t o f u r t h e r chemical d i s c o v e r y . Complementary t o t h e methods of 5 5 . 8 . 1 t o 8 . 5 , one may c o n s i d e r t h e s m a l l e s t number of secondary b u i l d i n g u n i t s (SBU) from which a l l s t r u c t u r e s can be made, assuming t h a t t h e e n t i r e framework i s made up of one t y p e of SBU o n l y . Nine a r e r e p o r t e d , a s shown i n F i g . 25 ( 5 1 , 5 2 ) . I t c a n n o t however b e assumed t h a t t h e s e a r e chemical u n i t s a c t u a l l y added t o a z e o l i t e c r y s t a l d u r i n g i t s growth. A l s o , of t h e many h y p o t h e t i c a l t o p o l o g i e s r e f e r r e d t o i n 5 . 8 , some may be i m p o s s i b l e t o make because o f c o n f o r m a t i o n a l r e s t r i c t i o n s ( 5 2 ) , f o r example i n T-0-T bond a n g l e s .

ACKNOWLEDGEMENTS I w i s h t o thank P r o f e s s o r J . M . Thomas f o r Table 12 and e s p e c i a l l y P r o f e s s o r F . Liebau f o r p e r m i s s i o n t o u s e m a t e r i a l from h i s work on s i l i c a t e c l a s s i f i c a t i o n , and t h e Academic P r e s s f o r p e r m i s s i o n t o use c o n s i d e r a b l e m a t e r i a l from my two r e c e n t books ( r e f s . 22 and 2 3 ) .

REFERENCES

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2. 3.

4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

14. 15. 16. 17. 18. 19. 20.

21. 22. 23. 24. 25. 26.

27.

c f . L i e b a u , F . , Reviews i n M i n e r a l o g y Vol. 5 . O r t h o s i l i c a t e s , ( E d i t o r P.H. R i b b e , M i n e r a l o g i c a l S o c i e t y of America, 1980) Chap. 1. Hoebbel, D . , W. Wieker, P . F r a n c k e and A. O t t o , Z . Anorg. A l l g . Chemie 418, (1975) 35. ~ i e b a u , Xand I . P a l l a s , Z. K r i s t . 1 5 5 , (1981) 139. Ref. 1, p . 9 . R e f . 1, p . 1 0 . B a r k e r , M . G . , P.G. Gadd and M. J. B e g l e y , Chem. Comrn. (1981) 379. H e f t e r , J . and M.E. Kenny, I n o r g . Chem. 21, (1982) 2810. H e s s e , K.-F. and F. L i e b a u , Z . K r i s t . 15% (1980) 3 . R e f . 1, p . 11. R e f . 1, p . 12. L e i b a u , F . , S t r u c t u r e and Bonding i n C r y s t a l s (Academic P r e s s I n c . , 1981) Chap. 2 3 . B a r r e r , R.M. and D.E.W. Vaughan, T r a n s . F a r a d a y S o c . , 63, (1967) 2275 . F l a n i g e n , E.M., J . M . B e n n e t t , R.W. G r o s e , J . P . Cohen, R.L. (1978) P a t t o n , R.M. K i r c h e n e r and J . V . S m i t h , N a t u r e , 512. Bibby, D . M . , N.B. M i l e s t o n e and L.P. A l d r i d g e , N a t u r e , 280, (1979) 664. Edition G i e s , H . , F. L i e b a u and H. Gerke, Angew. Chem. I n t . ( E n g l i s h ) 21, (1982) 205. B a r r e r , R.M. and B.M. Munday, J . Chem. Soc. A, (1971) 2904. G i e s , H . , H . Gerke and F. L i e b a u , Neues J a h r b . M i n e r a l o g . Mh., (1982) 1 1 9 . B a r r e r , R.M. N o n - s t o i c h i o m e t r i c Compounds ( E d i t o r , L. Mandelc o r n , Acadmic P r e s s I n c . , 1963) pp. 314-5. S c h l e n k e r , J . L . , F.G. Dwyer, E.E. J e n k i n s , W . J . Rohrbaugh, G.T. K o k o t a i l o and W.M. M e i e r , N a t u r e , 294, (1981) 340. K o k o t a i l o , G.T. and W.M. M e i e r , p r o p e r t = and A p p l i c a t i o n s o f Z e o l i t e s ( E d i t o r , R.P. Townsend, Chem. Soc. S p e c i a l P u b l i c a t i o n No, 33, 1979) p . 1 3 3 . Wilson, S . T . , B.M. Lok, C.A. M e s s i n a , T.R. Cannan and E.M. F l a n i g e n , J . Amer. Chem. S o c . , 1 0 4 , (1982) 1 1 4 6 . B a r r e r , R.M., Hydrothermal chem=ry o f Z e o l i t e s (Academic P r e s s I n c . , 1982) p . 282. B a r r e r , R.M., Z e o l i t e s and Clay M i n e r a l s a s S o r b e n t s and M o l e c u l a r S i e v e s , (Academic P r e s s I n c . , 1978) p . 36. R e f . 23, p . 38. R e f . 23, p . 41. Shoemaker, D.P., H.E. Robson and L. B r o u s s a r d , P r o c . of 3 r d I n t C o n f e r e n c e on M o l e c u l a r S i e v e s ( E d i t o r J. B. U y t t e r h o e v e n , Z u r i c h , S e p t . 3 r d - 7 t h , 1973) p . 138. (1969) 352. B a r r e r , R.M. and H . V i l l i g e r , Z . C r i s t . ,

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28. Smith, J . V . and F . R i n a l d i , M i n e r a l o g . Mag., (1962) 202. 29. S o l o v ' e v a , L.P., S.V. B o r i s o v and V.V. Bakakin, Sov. Phys.C r y s t . 16, (1972) 1035. 30. Ref. 1,g.60. 31. A l b e r t i , A . , and G. G o t t a r d i , Neues J a h r b . M i n e r a l o g , Mh. (1975) 396. 32. Ref. 23, p . 6 3 . 33. Meier, W . M . , Z. K r i s t . , 115, (1961) 439. 31 34. M e r l i n o , S . , Soc. I t a l . s e r a l o g . P e t r o l o g . R e n d i c o r t i , (1975) 513. 35. Ref. 2 3 , p . 48. 36. R e f . 23, p. 49 37. K o k o t a i l o , G . T . and S.L. Lawton, N a t u r e , (1964) 621. 38. Ref. 2 3 , p . 5 0 . 39. Ref. 23, p . 5 1 . 40. K o k o t a i l o , G . T . and J . C i r i c , M o l e c u l a r S i e v e Z e o l i t e s - I , Advances i n Chemistry S e r i e s No. 101 ( E d i t o r R . F . Gould, Amer. Chem. S o c . , 1971) p. 109. 4 1 . Moore, P.B. and J . V . Smith, Mineralog. Mag. 33, (1964) 1008. 4 2 . Thomas, J . M . , M. Audier and J . K l i n o w s k i , ~ h % . Comm., (1981) 1221. (1954) 92. 43. Lowenstein, W . , Amer. M i n e r a l o g . , 44. Thomas, J . M . , C. A. F y f e , S. Ramdas, J . ~ l i n o w s k iand G . C. Gobbi, J . Phys. Chem., ( 1 9 8 2 ) , 3061. 45. elc chi or, M . T . , D.E.W. Vaughan, R . H . Jarman and A. J . Jacobson, (1982) 455. Nature, 46. K l i n o w s k i , J . , S. Ramdas, J . M . Thomas, C.A. F y f e and J . S . Harman, J . Chem. Soc. F a r a d a y , ( 1 9 8 2 ) , 1025. $ 7 . M e l c h i o r , M.T., D.E.W. Vaughan and A. J. J a c o b s o n , J. Amer. Chem. S o c . , 104, (1982) 4859. 48. Senderov, E . K Phys. Chem. M i n e r a l . 6 , ( 1 9 8 0 ) , 251. 59. B a r r e r , R.M. and E . A . D . White, J. hem. S o c . , (1952) 1561. 50. M o r t i e r , W. J. Compilation of Extra-framework S i t e s i n Z e o l i t e s (Butterworth S c i e n t i f i c L t d . , 1982). 51. Meier, W.M. and D.H. Olson, A t l a s of Z e o l i t e S t r u c t u r e Types ( S t r u c t u r e Commission of t h e I n t e r n a t i o n a l Z e o l i t e A s s o c i a t i o n , 1978) p. 9 . 52. Gramlich-Meier, R., and W.M. Meier, J . S o l i d S t a t e Chem., 44 (1982) 41.

203,

3,

86,

298,

78,

ZEOLITE CRYSTALLOGRAPHY

G. T. Kokotailo Drexel University Physics Department Philadelphia, Pennsylvania

INTRODUCTION The importance of zeolites in science and technology is well established and the indications are that it will increase. The number of new zeolites synthesized and structures determined is increasing. The lack of large crystals suitable for single crystal structure determination has hindered the progress of structure determination of the new zeolites synthesized. The knowledge of zeolite structures is limited; however the topology or the general features of the framework structure of a fairly large number of zeolites are known. Good structure information regarding metal framework atom distribution, the position of cations, water and organic molecules is available for only a few zeolites. The properties of a zeolite are dependent on the topology of its framework, the size of the free channels, the location, charge and size of the cations within the framework, the presence of faults and occluded naterial, and the ordering of T atoms (framework metal atoms). Therefore, structural information is important in understanding the absorptive and catalytic properties of zeolites. There have been a number of reviews of the structure, chemistry and use of zeolites (1-5). This paper is directed to the review of the structures of zeolites ranging from the topology of the frameworks, cation location,T-atom distribution, faults and imperfections to model building. CLASSIFICATION OF KNOWN ZEOLITE STRUCTURES The framework structure of zeolites consists of linked tetrahedra (metal atoms are tetrahedrally coordinated to four oxygen atoms)

Figure 1 Secondary Building Units (2)

Figure 2 Frameworks of (a) analcite (b) laumontite (5)

Figure 3 Chains i n ( a ) n a t r o l i t e , (b) B r e w s t e r i t e ( c ) ZSM-5

Figure 4 O f f r e t i t e and E r i o n i t e Framework a) o f f r e t i t e (b) c - p r o j e c t i o n of of f r e t i t e ( c ) E r i o n i t e (d) c - p r o j e c t i o n of e r i o n i t e

which form three dimensional four connected nets. The corner sharing of tetrahedra requires that there are twice as many framework oxygens as T-atoms (metal atoms). Cations are required for T-atom charge balance. The first classification of zeolites on the basis of common structural units such as parallel 6-rings was made by Smith (6). Meier (2) classified zeolites into seven groups based on secondary building units or polyhedral building blocks which on linking form the framework structures. These units (Figure I ) (single 4-ring, single 6ring, single 8-ring, double 4-ring, double 6-ring, and 4+1, 5 -F1 and 4+4+l combinations) are sufficient to describe a zeolite framework although 3 and 9 rings should be added. This classification into seven groups should be extended to nine with the addition of the melanophlogite group based on the aluminosilicate analogs of the gas hydrates and the lovdarite group based on 3,5 and 9-rings. The nine groups of zeolites identified on the basis of their framework structure are given in Table I with idealized cell contents and crystallographic data and channel systems. The isotypes of the species listed in Table I are listed in Table 11. ANALCITE GROUP The frameworks of the two members of this group, analcite and laumontite, can be derived by interconnecting 4 and 6-rings as shown in Figure 2. NATROLITE GROUP The chains (Figure 3a) characteristic of this group consist of linkei four 4-ring units. There are three different ways to link these chains resulting in the natrolite (lo), edingtonite (12) and thomsonite (10) frameworks (Figure 4). All the structures have a two dimensional 8-ring channel system. CHABAZITE GROUP The chabazite group frameworks consist of parallel 6-rings. The offretite-erionite a and c projections are shown in Figures 4a-4d. The stacking sequence involves single 6-rings A, B or C, double 6-rings AA, BB or CC or a combination of both. The stacking sequence of the members of the group are: Cancrinite Gmelinite Chabazite Offretite Erionite

AB MBB MBBCC AAB AABAAC

(30) (16) (14) (18) (17)

Levynite Afghanite Losod Liottite TMA-E(AB)

AABCCABBC ABABACAC ABAC ABABAC ABBACC

(19) (25) (27) (28) (29)

TABLE I. Classification and Crystallographic Data for Zeolites Typical Unlt Cell Contents Analcite Group Analcite

Framework Density

Crystal Data

Cubic Ia3d a=13.7A Monoclinic Am or A2 a=7.6, b=14.8, c=13.1A, y=112"

Laumontite

17.7

Natrolite -Group Natrolite Thomsonite Edinqtonite

Orthorhombic Fdd2 a=18.3, b=18.6, c=13.2A Orthorhombic Pnn2 a=l3.1, b=13.1, c=13.2A

17.8

Orthorhombic P2,2,2, a=9.6, b=9.7, c=6.5A

16.6

Chabazite Group Trigonal R3m a=13.2, c=15.1

Chabazite

Hexagonal Ph?/mmc a=13.8, c=lO.OA Hexagonal P63/mmc a=13.3, c=lS.lA Hexagonal ~ 6 m 2 a=l3.3, c=7.6P

Erionite Of fretite

Triqonal R3m a=13.3, c=23.OA Hexagonal P63/mmc

Linde L

Hexagonal P 6 / m a=18.4, c=7.5A Hexagonal P63/mmc

Af ghanlte

a=l2.8, c=lO.iA Losod

Hexagonal ~ 6 2 a=12.9, c=10.5A

Liottite

(CaNa2K2),(AlO,)

,,(Si02)

- (CaNa2K 2 ) n (SO4,C03,Cl)8.2H20 TMA-E

(AB)

Cancrinite

(Me N) 2Na7 (A10 ) (Si02)27.26H20 4 2 9

Hexagonal ~ 6 m 2 a=12.8, c=5.1A Hexagonal P63Immc a=13.3, c=15.2A Hexagonal Pb3 a=12.8, c=5. IA

~

17.7

Channel System

Reference

03 03

Typical Unit Cell Contents

Framework Density

Crystal Data

Channel system

Reference

Phillipsite Group Phillipsite

Monoclinic P21 /m

Gismondite

15.4 Monoclinic P21/a a=9.8, b=10.0 c=10.6A, y=90° Monoclinic PC 18.3 a=6.7,b=14.0,c=10DA,B=112°

Yugawaralite

15.8

Orthorhombic Pna2 a=10.3, b=8.2, c=5.OA

Li A(BW)

19.0

Heulandite Group Heulandite

Monoclinic C m 17.0 a=17.7,b=17.9,~=7.4A,B=~16~

Brewsterite

Monoclinic P2l/m

'

17.5

a=6.8,b=17.5,~=7.7A,B=95~

16.9 Monoclinic F 2/m a=13.6,b=18.2,c=17.8A,B=9l0

Stilbite Mordenite Group

Orthorhombic Cmcm a=18.l,b=20.5,~=7.5A

17.2

Ferrierite

Orthorhombic Imnun a=i9.2,b=14.l,c=7.5A

17.7

Dachiardite

Monoclinic C2/m 17.3 a=18.7,b=7.5,~=10.3A,B=108~

Bikitaite

Monoclinic P2 1 20.2 a=7.6,b=8.6,~=5.OA,y=114~

Epistilbite

Monoclinic C2/m 18.0 a=8.9,b=17.7,~=10.2,B=124~ Orthorhombic Pnma 17.9

Mordenite

ZSM-5

Na, (AlO,), (SiO,) 40.24H20

a=20.l,b=19.9,~=13.4A Tetragonal 15m2 a=20.1, c=13.4A

17.7

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TABLE 11. Zeolite Species Isotypes Zeolite

Isotype Species

Analcite

CaD (61), Kehoite (62), Leucite,NaB (63), Pollucite, Viseite (64), Wairakite (65)

Laumontite

Leonhardite (66)

Natrolite

Scolecite, Laubanite

Thomsonite

Gonnardite

Edingtonite

K-F (67)

Chabazite

Herschelite, Linde D (68), Linde K (69)

Gmelinit e Offretite-Erionite

s (70) Linde T (68), O (71)

Levynite

ZK-20 (72)

Eazzite

Omega (73), ZSM-4 (74)

Linde L

BaG (75), P-L (76)

Cancrinite

Cancrinite Hydrate (77)

Phillipsite

Harmatome, Wellsite (78), ZK-19 (79)

Gismondite

Linde B (80), Garronite (81), PC (82), Pt (82), P (70)

Yugawaralite

Sr-Q (75)

LiA (BW)

CaAlSiOq (83), RbAlSiO4 (83)

Feulandite

Clinoptilolite (84)

Stilbite

Barrerite (85), Desmine

Mordenite

Na-D (63), Ptilolite, Zeolon

Ferrierite

Sr-D (75)

3achiardite

Svetlozarite (86)

Faujasite

Linde X (87), Linde Y (88)

Linde A

Alpha (89), ZK-4 (go), ZK-21 (91,92), ZK-22 (91,92)

: 2

5

BaP (93), BaQ (93), P(C1)

(931, Q(Br) (93)

Xerlinoite

K-M (67), Linde W (80)

Sodalite

Basic sodalite (63), Danalite, Hydroxysodalit: (94), Nosean, Tetracalcium trialuminate (95), Tugputite (96), Ultramarine, Zh (97)

Figure 5 Framework of m a z z i t e (a) c - p r o j e c t i o n ( b ) l i n k i n g of g m e l i n i t e columns

Figure 6 Three Types of O-ring Chains UUDD, UDUD, and UDUU (117)

Figure 7 Secondary U n i t s i n (a) h e u l a n d i t e group ( b ) mordenite group ( 2 )

Figure 8 Framework P r o j e c t i o n s Along Main Channels of (a) mordenite (b) d a c h i a r d i t e (c) f e r r i e r i t e (d) e p i s t i l b i t e ( e ) b i k i t a i t e

The framework of mazzite (21) consists of columns of gmelinite cages linked through 8-rings with alternate columns staggered by c/2 (Figure 5). If cancrinite cages are connected through double 6rings to form columns and these columns are linked through 6-rings to form 12-ring channels parallel to the C-axis, as in mazzite, the framework of Linde L (24) is formed. PHILLIPSITE GROUP The framework of members of this group are based on 4-rings with variations of U (up) and D (down) linkages. Three of the four such variations can be linked to form chains (Figure 6). Phillipsite (32,33) and gismondite (34,35) consist of cross-linked UUDD chains. Li-A(BV) (37) and yugawaralite (36) consist of linked single &rings. EEULANDITE GROUP

A building block containing four 5-rings and two 4-rings is common to the framework of all the members of this group. If linked throug?. a common edge, chains are formed (Figure 3b) which when linked together yield brewsterite (40). Linking these blocks through common vertices yield chains which are constituents of heulandite (38,39) and stilbite (41,42). This group of structures contain some 5-rings. MORDENITE GROUP The secondary building block consisting of four 5-rings is common to all members of this group except bikitaite. In mordenite and dachiardite they are linked to form complex chains which in turn are linked in different ways (2). Epistilbite and ferrierite are lamellar structures and they also contain the building block in Figure 7b. The lamellae are normal to a in ferrierite and to b in epistilbite. The projections along the main channels are shown in Figures 8a-8e (2). The projection of bikitaite along b is essentially the same as the projection of epistilbite along a and dachiardite along c. ZSM-5 and ZSM-11 are better described by the chain in Figure 3c. The configurational unit in this chain contains eight 5-rings. If these chains are linked so that alternate pairs are related by a reflection, a layer is formed which is the basic layer in the ZSM-5 and ZS11-11 structures. If this layer is linked so that alternate layers are related by a reflection, S, the ZSM-11 framework is formed (57). If the layers are linked such that alternate layers are related by an inversion I, the ZSM-5 framework results (49). If the stacking sequence is varied, a family of structures results with ZSM-5 and ZSM-11 as the end members. With the lattice parameter doubled there are only two possible stacking sequences, SSII and SISI. The framework for SSII is shown in Figure 9. This variation in stacking sequence has been confirmed by Thomas and Millward (98) with lattice imaging using ultra high resolution electron microscopy.

Figure 10 Arrangement of A Cages in (b) Rho ( c ) Paulingite

(a) ZK-5

Figure 1 1 Polyhedral Units in ZSM-39 and Melanophlogite Structures (a) 12-hedron (b) 14-hedron (c) 15-hedron (d) 16-hedron

Figure 12 Melanophlogit e Framework

FAUJASITE GROUP There are three types of cages in this group, sodalite (30), A and ZK-5. Linde A (55) is formed by linking sodalite cages through double 4-rings. The large cages in Linde A will be referred to as the A cage. If the A cages are linked through double 6-rings, the framework of ZK-5 is formed (58). If they are linked through double 8-rings, the rho (51a) structure results (Figure 10). The cage formed by the octagonal faces of the A cages in ZK-5 is analagous to the gmelinite cage and will be referred to as the ZK-5 cage. Columns of ZK-5 cages linked through double 8-rings when connected together form the merlinoite (53a)framework. Feldspar and phillipsite are closely related to merlinoite formed by sliding layers such that the double 8-rings are broken up. Figure 10a shows part of the framework of paulingite (52a), the sequence of cages along the cubic axis A, ZK-5, ZR-5, A each connected to the other through double 8-rings. No one cage can be considered as the building block, the combination of cages is required. The faujasite framework (52) consists of sodalite cages linked through double 6-rings. If layers of these connected sodalite cages are linked in an ABC sequence the framework is that of faujasite with Fd3m symmetry, the same as diamond. If the stacking sequence is changed to AB, the result is a hexagonal structure. There are now five 12-ring openings to the large cavity compared to four in faujasite. The c parameter and the number of stacking sequences is dependent on the number of layers in the identity period. ZSM-3 (59) was found to be hexagonal with a=17.5 and c=129 A indicating a nine layer stacking sequence. Sodalite cages linked through common 6-rings form the sodalite framework (30). Falth and Anderrson (54a) found that Linde N, a cubic structure with a=36.9AY was an intergrowth of ZK-5 and sodalite. YELANOPHLOGITE GROUP The relationship between hydrogen bonds linking oxygens in gas hydrates (clathrates) and oxygens linking T-atoms in zeolites is well known. Hexagonal ice I is isostructural with 6-tridymite (58a). Appleman (57a) found that the rare mineral melanopnlogite is isostructural with the 12A cubic gas hydrate. The melanophlogite framework consists of interwoven layers of 12 and 14-hedra (Figures 11 and 12). It is a dense structure with only 5 and 6-ring openings. ZSK39 (59a), a high silica synthetic zeolite, was found to be isostructural with the 17A cubic gas hydrate. The ZSM-39 framework consists of 12 and 16-hedra as shown in Figure 11 , with layers of face sharing 12-hedra arranged as in Figure 13. These layers are stacked

F i g u r e 13

ZSM-39 Framework

F i g u r e 14 L o v d a r i t e Framework ( a ) a - a x i s p r o j e c t i o n (b) c - a x i s p r o j e c t i o n

in ABC sequence. The openings in the framework are limited to 5 and 6-rings. Zeolite analogs of water clathrate structures which typically consist of cages comprising of 5-rings constitute a new family of zeolites. LOVDARITE GROUP Lovdarite (60), a unique beryllium zeolite, has a framework consisting of 4 and 8-rings linked via corner sharing 3-rings. It has a two dimensional intersecting channel system bounded by 9-rings (Figure 14). The 9-rings are unique as no other known zeolite has a 9ring channel system. The 129' equilibrium angle of the Be-0-Si linkage indicates that the three-membered rings are not strained. T-ATOM DISTRIBUTION It is desirable to have some knowledge regarding T-atom distribution in zeolites. For several zeolites there is unequivocal evidence of Si,Al ordering (natrolite and gismondite). X-ray and other evidence are consistent with random ordering of T-atoms. The x-ray evidence for the occupancy of tetrahedral sites by Si or Al is based on the Si,Al-0 interatomic distances which differ by about .13A (100). This requires accurate atomic coordinates. The available evidence is also in accord with Lowenstein's rule forbidding A1-0-A1 bonds. Olson (101) found that hydrated Linde X crystals had Fd3 rather than Fd3m symmetry. The cell content prohibits complete ordering (88 Al, 104 Si). It was determined (102) that A1,Si alternate in the 4-rings in mordenite. CATION LOCATION Precise information as to cation positions in zeolites is still rather limited as faults,thermal and positional disorder, partial occupancy act as hindrances. The cation sites and their population in dehydrated mordenite are given in Table 111.

TABLE 111. Site I or I'

_

H

_

.6Na

N_a

Cation Population in Mordenite _

K-

-

Rb -

-

Cs

3.1

II

1.7 3.3

3.6

3.8

111 IV

Ca

Ba 0.3Ca 1.9Ba

0.6

0.3Ba

2.6

3.0

3.1

1.9

0.5

1.1Ba

SG

Cmcm

Pbcn

Pbcn

Pbcn

P2lcn?

Cmcm

Pbcn

Ref.

(103)

(104)

(105)

(106)

(107)

(108)

(109)

Site I lies at the end of the side pocket and is too small for large cations. Site I1 is in the side pocket at the center of the 8-ring. Site IV is in the center of another 8-ring at the junction of the side pocket with the main channel. This site is occupied by all types of cations. Site VI is coordinated to oxygens in wallsof the main channel. Lowering of space groups is partially due to displacement of cations. The number and position of cations is a function of temperature and degree of hydration. The cation site on the threefold axis, outside the 6-rings in ZK-5 is fully occupied in the hydrated state, while at 150°C it is empty (58). In calcium exchanged offretite and erionite the calcium displaces the R from the center of the cancrinite cages on dehydration (110,111). In Ce exchanged faujasite the ~e* ions occupy sites in the center of the 12-ring. On dehydration the cerium ions move into the center of the sodalite cage where each metal ion is coordinated to three framework oxygen ions with Ce-0=2.52, f. O ~ Aand up to three water oxygens with Ce-0=2.44 2.08 (112). This sodalite cage complex is highly stable. STACKING FAULTS The occurrence of stacking faults in a number of zeolites is quite prevalent. Stacking faults can be detected by the presence of broad odd R lines in the diffraction pattern of offretite, by the presence of contrast lines in transmission electron micrographs of erionite (113),by lattice images of ZSM-5 using high resolution electron microscopy (98). STRUCTURE DETERMINATION With increasing knowledge of zeolite frameworks there is a considerable understanding of zeolite chemical principles but many zeolites have been synthesized as small or larger but poorerquality crystals. Increased x-ray intensity sources should help in determining the structure of some of these crystals but if it is not possible to use single crystal methods then we have to turn to other methods. Information regarding lattice parameters, symmetry can be obtained from diffraction studies, estimates of channel dimensions from IR and diffusion studies, and ring ellipsisity from diffusion rates. Physical models using tetrahedral stars to represent T-atoms are connected via tubing to depict zeolite frameworks. These models are built to scale for rapid and accurate estimation of unit cell parameters and atomic coordinates and are useful for determining the symmetry of a structure. The basic building units can be readily identified in these open frameworks.

Trial models may be built using all the available information and satisfying all the known conditions. X-ray patterns can be simulated and compared to experimental ones. SIMULATION OF PATTERNS Interatomic distances and bond angles for zeolites of known composition can be predicted within fairly narrow limits. If the lattice paraaeters are known, the atomic coordinates of individual atoms can be adjusted so that the interatomic distances correspond as closely as possible with predicted distances. The atomic positional parameters can be computed from the prescribed interatomic distances, D y , by a least squares procedure which minimizesthe residual function

where wj is the weight ascribed to the interatomic distance of type j. This DLS (Distance Least Squares) method of refining the positional parameters was described by Meier and Villiger (114). This refinement gives idealized framework models using prescribed interatomic distances and unit cell constants for a given space group. The weight wj of each error equation is based on bonding considerations or observed bond length variations (115). In zeolites the Si-O bond length relation to the Si-0-Si bond angle is given by the function,

Convergence of the least squares refinement is usually rapid for chemically reasonable structures. A final "R-factor" is provided which can be used to estimate the "goodness" or chemical reasonableness of the structure. If the positional parameters of some of the atoms in a structure are determined by single crystal methods, the missing atoms may be located by model building as in the case of Linde N (54) and the structure further refined. Simulated structures can be altered in a manner not possible with actual structures. The size of the atoms can be changed by changing the interatomic distances. We can impose restrictions on the lattice parameters. We can put cations in prescribed locations. The DLS refinement will give us the changes in geometry. This should give us valuable insights into chemical principles. EFFECT OF STACKING FAULTS ON DIFFRACTION PATTERN Stacking faults can be determined by contrast lines in trarismt~sioa electron microscopy (113) and by lattice imaging (98) bat his is a tedious process and is applicable to individual crystals and not the bulk. In order to determine the effect of stackfriz faults t h c

Figure 15 Simulated Plots of Ferrierite

II

Ferrierite-Framework C a q o n s

A c t or=1

Mg C o m p l e x in Ferrieritd

,

v

,

.

AL

I

Li

" .-,--.. .-.

Actor=l

*I

Ferrierite

-

with Mg Comp ex R e m o v e d

!i

%I ,I

two structures are superimposed and an occupancy of 1 is assigned to all atoms which coincide and an occupancy of % to split atoms. The application of this method to offretite-erionite (116) resulted in the determination of the concentration of stacking faults and whether they were random or ordered. CONTRIBUTION OF CATIONS AND WATER 70 POWDER PATTERN The structure of ferrierite was determined by Vaughn (44). He found M~(H~o)~* cation complexes in the cavities with 8-rings. Difference in powder pattern intensities for samples from various locations were found to be due to the Mg complexes in the 8-ring cages. Smith plots of the contribution of the cation complexes (116) are shown in Figure 15. By obtaining plots of structures with partial occupancy better agreement of powder data can be obtained. ACKNOWLEDGEMENTS

I am grateful to W. M. Meier, J. L. Schlenker, S. Sawruk, S. L. Lawton and A. C. Rohrman for valuable discussion, advice and encouragement. REFERENCES 1.

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SYNTHESIS OF ZEOLITES, AN OVERVIEW

L. Deane Kollrnann Mobil Research

& Development Corporation Central Research Division

PO Box 1025 Princeton, NJ 08540

Zeolites have been an important component in petroleum process technology for mare than twenty years -- and there is considerable indication that kheir role will continue and will expand as utilization of our energy xesource base broadens worldwide. Despite the large and growing number of distinctive structures and compositions, only a handful1 of zeolites, nearly all synthetic, have achieved commercial significance. This paper is directed at the preparation of zeolites of this special group. The first section develops general principles and techniques in the synthesis of these zeolites, particularly the concepts of ternplating and compositional control. The second presents examples, to illustrate practical application of these concepts. Composition in zeolites can be controlled during or after synthesis. In the third section, an example of the lakter is discussed, the dealuminization of mordenite.

The synthesis of zeolites is an art better chronicled in the patent literature perhaps than in classical scientific publications, Nevertheless, several books and review articles can be cited which will provide useful background information and supplementary detail (1-10). The discussion below summarizes and updates, where appropriate, those reviews. In the interest of clarity, discussion is restricted to three-dimensional,

crystalline networks whose framework composition is mol percent Si02, the remainder being Al2O3.

at

least

50

PATTERNS

Most commonly, zeolite crystallization is a nucleation-cantrolled process occurring from molecularly inhomogeneous, alkaline, aqueous gels, at temperatures between about 80 and 300°C, The particular framework structure which crystallizes can be strongly dependent on the cations present in the gel. m Reaction mixture composition in a crystallization experiment is best defined by the set of mol ratios given in Table 1 (10).

Table I. Reaction Mixture Composition Mol Ratio sio2/A1203

Primary Influence Framework Composition Hate, crystallization mechanism Silicate molec. w t , OH- conc Structure, cation distribution Framework aluminum content

A large number of silica and alumina source materials can be used in formulating a gel, and the product obtained is often dependent on the sources selected and on their treatment prior to formulation. For example, Na2Si03, silica gel, silica sol, NaA102, aluminum sulfate, aluminum turnings or alumina itself are all routinely considered. Clays, either directly or after varying heat treatment, can be used as well (11). In describing a reaction mixture composition in terms of the above mol ratios, NaZSi03 i s treated as a mixture of Sio2 and NaOH; NaA102, as A1203 and NaOH; aluminum sulfate, as A1203 and H2S0+, each with the appropriate amount of accompanying H20. Hydroxide in the O H - ~ S ~ O ratio ~ is calculated by subtracting equivalents of acid added from those of hydroxide. It is not hydroxide i o n concentration in the resultant mixture. Organics such as m i n e s , for example, are never included i n calculating OH-/Si02 ratios. In careful work, and as OH-/Si02 ratios approach zero, it is important to recognize that A1203 is incorporated into a zeolite framework as ~ 1 0 ~ -i.e., , each rnol acts as two

equivalents of acid and consumes two equivalents of hydroxide: A l 2 G 3 f 20H2A102 + +

To a first approximation, the

mol ratios in Table 1 can be divided according to primary functi.on. S i 0 2 / A 1 2 0 3 , for example, places a constraint on the framework composition of the zeolite produced, and it may define the structural competitors in a nucleation-controlled experiment. With the exception of the aluminum-rich NaA, zeolites normally incorporate all of the aluminum present in a reaction mixture into their framework structure, leaving varying amounts of silica ( silicate ) in solution according to hydroxide ion concentration, reaction conditions, etc . a.nd OH-/Si02 strongly influence the "molecular" or "polymeric" species present in a reaction mixture composition and the rate at which those species interconvert, by hydrolysi,s, to form the ordered, three-dimensional network of the zeolite hydrolysis rates and product. Through their control of polysilicate (or polyaluminosilicate) distribution, they can significantly influence the "winner" among competing, possibly metastable structures in a crystallization experiment. EI20/SiO2

Cations present in a reaction mixture are often the dominant factox determining which zeolite structure is obtained, as will be discussed below. In addition, their incorporation and presence in a product can be important considerations in subsequent use and handling. Anticipating that discussion, literature data (2) for the synthesis of four different zeolites, Y, Omega, L and TMA-O, are given in Table 3 1 . The primary variable, it is asserted, was the cation. (Each preparation was static, I U U ° C , w i t h sio2/A1203 = 16-20, HZO/Si02 = 14-25, o~-/.Sio,= 0.7-0.9. )

Table 11.

Zeolite Y

omega L TMA-0

Product Dependence on Cation

N ~ + s ~ u ~

TMA+/S~O~

K+/SIO~

0.8 0.6 0.0 0.0

As a result of small changes in the cation content of the reaction rn~eure, completely different zeolite products were obtained .

Product integrity is best "authentic" sample.

r

judged

by

comparison with

an

Zeolite identification is made largely on the basis of X-ray diffraction, and powder patterns are the common measure of purity in a crystallization product. If a pattern shows no evidence for crystalli.ne or amorphous contaminants, purity is estimated by comparing intensities of reflections (at d-spacings smaller than about 6 A) with those of an authentic sample o f the same composition and crystal size. Reflections at higher d-spacing are not used in estimating crystallinity since their intensities are dependent on variables such as moisture content. Except for such large scale commercial products as N a A and Nay, "authentic" samples are normally obtained by repeated and varied crystallization experiments, Once the X-ray diffraction pattern of a preparation is defined, elemental analysis becomes useful, to corroborate assertions of purity, to detail cation content and to critically explore the "ternplating" in a particular crystallization question of sequence. In contrast to more conventional chemical synthesis, elemental analysis is generally not a satisfactory or sufficient criterion for purity in zeolites. Almost all zeolite structures have been prepared in a range of framework compositions, as will be shown below. X-ray diffraction, supported by elemental analysis, can often directly probe composition within a particular zeolite framework. Sorptive and cation exchange properties are a third measure of among well-characterized zeolites. For product integrity example, high purity samples of N a A and N a Y will sorb about 25 g H20 per 100 g of dry zeolite ( 2 5 " C , 1-4 torr). HZSM-5 and N a Y samples should respectively sorb about 11 and 19 percent of their dry weight in n-hexane ( 2 5 O C , 10-20 torr). an as-synthesized, high p u x i t y zeolite sample will conventionally contain at least one cation for every aluminum in the framework. Microscopy is the fourth essential ingredient in product characterization. crystal size and crystal morphology are both important, controllable aspects of a synthesis experiment. Moreover, recent electron microbe analysis of aluminum distribution within individual zeolite crystals offers significant insight into actual crystallization processes, as will be discussed below (12).

a

Composition i s a variable for any given framework structure.

When composition is cited in recent patents on zeolites, a range is almost always presented. Although the limits o f , this range may not be accurately known, it is now widely recognized that no elemental composition is unique to any s p e c i f i c framework structure. Table I11 presents examples which span the Si:Al range from L : 1 to infinity.

Table 111. Variable Composition in Zeolites Framework

~ame

sio2/Al2O3

Linde A N-A

2 2.5 to 6 2.5 to 4

ZK-4

Sodalite Sodalite

Faujasite Faujasite ZSM-5

Sodalite TMA Sodalite

2 10 2 to 3 3 to 6

Linde X Linde Y ZSM-5

References

5

to infinity

16

mere is abundant evidence in the literature to show that these ranges indeed reflect differing framework compositions ( 2, 17-22j

.

That composition can be controlled by cation type was originally discovered for the A framework (13,141. It has been emphatically demonstrated with sodalite ( 15 ), where two "model" compositions are obtained, depending on which cations are present during crystallization. When sodium ions are used, the sodalite obtained has a composition Na3Al3Si3OI2; when tetramethylammonium ions are used in place of sodium, (CH3)4NA1Si5012 is produced. Bath cornpositions have the same framework structure, a three-dimensional network built entirely of truncated octahedra. Each octahedron in TMA sodalite contains (and can accomodate) only one tetramethylammonium ion, thereby restricting ( by the requirement of charge balance) the number of negatively charged AL04 tetrahedra. On average, three sodium ions are present in each octahedron in Na sodalite. Framework composition can often be varied by simply changing the SiU2/'A.L203 ratio of the reaction mixture. ZSM-5 and the synthetic faujasites are examples cited in Table 111. With ZSM-5, framework compositions ranging to over 20,000 can be easily prepared by this technique. Very high Si02/A1203 ratios require special effort to

e x c l u d e a d v e n t i t i o u s aluminum. N u c l e a t i o n and c r y s t a l growth may o c c u r w i t h d i f f e r i n g f a c i l i t y as t h e Si02/A12U3 r a t i o o f a r e a c t i o n m i x t u r e i s changed. crystals may grow f o r example i n a Si02/A1203 range where n u c l e a t i o n would b e d i f f i c u l t - and v i c e v e r s a . S e e d i n g , low-temperature a g i n g and the u s e o f r e a c t i v e g e l s o r d i s p e r s i o n s are all p r a c t i c a l t e c h n i q u e s f o r c o n t r o l l i n g z e o l i t e p u r i t y and/or composition. me synthesis of z e o l l t e Y illustrates the low-temperature aging approach. Four r e a c t i o n m i x t u r e s w e r e p r e p a r e d uslng s l l l c a s o l , NaA102 and NaOH, two a t S102/A1203 = 10 and two a t S102/fi1203 = 30. K u l r a t i o s i n both sets were 08-/S1O2 = 0 . 6 - 0 . 7 , Na / S 1 O Z = 0.8 a n d H 2 0 / S ~ 0 2 = 16, One from e a c h p a l r w a s p l a c e d ~ m m e d l a t e l y l n t o a steam chest at 90 - 95'C; the second was aged f o r 2 4 h o u r s a t room t e m p e r a t u r e b e f o r e h e a t l n g . The r e s u l t s are p r e s e n t e d In T a b l e IV. Except where n o t e d , the Y s a m p l e s w e r e a l l 95 - 1001 c r y s t a l l m e .

Table I V .

Initial Si02/A1203

10

S y n t h e s i s of Z e o l i t e Y

Aging ( hours )

24

crystn (days )

5

Product Zeolite

Y

(trace P )

Product Si02/A12CJ3

5.3

I t i s g e n e r a l l y a c c e p t e d that c r y s t a l l i z a t i o n o f Y w i l l c o n t i n u e u n t i l t h e aluminum i n a r e a c t i o n mixture i s e x h a u s t e d . S u p p o r t i n g that a s s e r t i o n was the fact t h a t , in all t h e above cases, y i e l d exceeded 90% based on a l u m i n a . As a r e s u l t , there w e r e large d i f f e r e n c e s i n r e s i d u a l silica (soluble s i l i c a t e ) i n the two cases. At Si02/A1203 = 1 0 , 50 - 60% o f t h e s i l i c a w a s incorporated into t h e zeolite product; at Si02/A1203 = 30, only a b o u t 20%.

Low-temperature a g i n g i s commonly used s i l i c a sols, and i t s p r i m a r y f u n c t i o n "equilibration of t h e h e t e r o g e n e o u s gel It is l i k e l y to be i m p o r t a n t i n t h e w e l l , however ( 23-26 ), which i n t r o d u c e s

to obtain p u r e Y f r o m i s probably p r e - d i g e s t i o n , w i t h t h e solution" ( 2 ) . i n i t i a l n u c l e a t i o n step as t h e n e x t assertion.

Composition is often variable even within crystals.

individual zeolite

NaX seeds have been frequently used to initiate crystallization of zeolite Y (23), and they of course introduce compositional

heterogeneity into the crystals which result. Sil~ceous external shells have been grown onto aluminum-containing ZSM-5 crystals (27). Evidence is i.ncreasing however that non-uniform composition may be a common, intrinsic characteristic of synthetic zeolite crystals. Such a result should not be particularly surprising since both solution and gel composition are continuously changing during the course of a crystallization (i.e., as zeolite product is formed, effectively removing constituents from the reaction mixture ) . In a large-crystal NaX preparation, for example, there is microprobe evidence that SiO2/Al2O3 ratio increases with increasing distance from the crystal core (28 ). X-ray photoelectron spectroscopy ( X P S ) data show aluminum depletion in the surface of NaA, X, Y, and synthetic mordenite czysta1.s t 29 ) . Changes in unit cell dimension during crystallization have been cited as evidence for S i O Z j A 1 2 0 3 gradients in NaY ( 2 4 1 , a conclusion however which relies heavily on X-ray diffraction analysis of partially crystalline materials.

Large-crystal ZSM-5 preparations have provided the most striking examgle of intrinsi.~ cornpositional heterogeneity to date. Aluminum content increased from core to outer crystal rim, the respective concentrations sometimes differing by a factor of 10 or more (12,30). One note of caution is warranted in generalizing from these observations however. Special techniques have been required to obtain zeolite crystals sufficiently large to permit electron microprobe analysis. These techniques may influence is aluminum distribution in the crystals produced. It nevertheless probable that gradients e x i s t in more conventionally prepared, smallex crystals. Modern analytical techniques will soon deflne site occupancy witbxn zeollte crystals on an atomic level and may contribute to reaction mlxture definition as w e l l . Until recently there has been no dixect probe for aluminum or silicon siting in zeolites, and assertions have been made regarding only average (distributed) siting, 1.argelyon the basis of X-ray diffraction data, sorption, ion exchange or catalytic properties. Nuclear magnetic resonance (NMK) promises to provide the desired direct probe.

In very strong magnetic fields, high resolution magic angle spinning N M N spectra have been obtained far both 2 9 ~ i( I = 1/2, 4 . 7 % abundance ) and 2 7 ~ 1 (I = 5 / 2 , 1008) in a variety of zeolites (31-38). In structures based on 4-membered rings, 2 9 ~ ichemical shift differences clearly distinguish SiU4 tetrahedra according to the number of ~ 1 neighbors 0 ~ (0,1, 2, 3...) (34,37). In more complex structures, like ZSM-5, numerous crystallographically distint sites exist for Si atoms in the lattice, and numerous different Si04 types have been detected ( 36 ) .

a direct probe for aluminum within zeolite crystals and has confirmed that Al is tetrahedrally coordinated In the within a multitude of framework structuxes ( 3 5 ) . ultrastabilization and dealuminization of synthetic faujasites, both tetrahedral and octahedral (non-framework) aluminums have been detected (37). Only the tetrahedral type was present in the original Nay. In "silicalite" 2 7 ~ 1showed all the aluminum to be tetrahedrally coordinated, with at least two distinct environments, and the authors concluded that, structurally, silicalite and zsM-5 are essentially indistinguishable (36). NMK further provides

Organic (and inorganic) catsons can framework structures.

"template" particular

a While the ability of organics to alter the course of crystallization process is becoming increasingly apparent, the specific function of those organics is sensitively dependent on the details of a given experiment. In general, additionof organics to a reaction mixture can effect changes of four types: (a) A different zeolite structure is obtained; (b) Crystallization rate is strongly enhanced (or inhibitedl ); (c) The same framework is obtained but with a significantly new chemical composition; and (d) "Microscopic texture", e.g., crystal size, habit, etc. (39), is altered. It is not uncommon i n exploratory crystallization experiments for organics to be superfluous, i,e,, to simply provide cation balance for varying hydroxide. Only the first and the second of the above (and the second only when the rate is enhanced) represent "templating effects" and then only if it can be shown that the change is not due to the virtually inevitable system perturbations which accompany a new reaction mixture component. A striking example of the first type was found in crystallization experiments with cationic polyelectrolytes ( 40 ) , and it demonstrates the detailed analysis which must be performed before "templating" can be suggested. Several relatively low molecular weight polymers of the following type were prepared by reaction of 1,4-diazaBicyclo[2.2.2Joctane (Dabco) with the compounds Br(CX2 ),Br:

where n = 3 , 4 , 5 , 6 a n d 10 a n d x = 10-60. W i t h 1,4-dibromobutane for example, t h e p o l y m e r was d e s i g n a t e d "Dab-4 B r " . When a d d e d t o a r e a c t i o n m i x t u r e t h a t p r o d u c e d zeolite Y ( a n d / o r P ) a t 85-90°C, t h e s e polymers p r o d u c e d dramatic c h a n g e s as shown i n Table v ( s1O2/Al2o3

= 30, B 2 0 / S i D 2

= 20, 0 H / S i 0 2

=

1.2,

Na/Si02

=

3.2,

3-13 days).

Table V .

Polymer

N+/s~o*

o

None Dab-4

Polymer E f f e c t s i n C r y s t a l l i z a t i o n

Br

Dab-4 Br Dab-4 Br Dab-4 Br

0.01 0.14 0.23 0.43

Product Z e o l i t e ( s ) Y + P Gmelinite ( faulted ) G l n e l i n i t e ( faulted ) Pure gmelinite Amorphous

T h e r e s u l t s show clearly t h a t a very small amount of organnc can completely alter t h e course of these nucleation-controlled

reactions. Moreover, with these polyelectrolytes, a large excess, which cannot be accommodated w i t h i n the zeolite product, actually inhibits all crystallizatian. " t e r n p l a t i n g " the gmelinite structure, the r e s u l t s s h o u l d be s e n s i t i v e t o c h a n g e s i n p o l y e l e c t r o l y t e m o l e c u l a r s t r u c t u r e . T a b l e VI shows t h a t t h i s i s indeed the case. A series o f p o l y e l e c t r o I y t e s , t o g e t h e r with t h e monomeric analogs (prepared by t h e r e a c t , i o n of D a b c o w i t h propyl o r b u t y l b r o m i d e or i o d i d e ) , was s u b s t i t u t e d f o r Dab-4 B r . O n l y Dab-4, -5 and -6 p o l y e l e c t r o l y t e s were e f f e c t i v e i n p r o d u c i n g gmelinite. If t h e Dab-4 Br i n T a b l e V i s i n d e e d

Table K t .

Polymer Dependence in Crystallization

Organic

Product Zeolite( s )

Dab-3 Elr

P Pure gmelinite Gmelinite ( faulted ) Gmelinite ( faulted ) Y + P

Dab-4 Br Dab-5 Br Dab+ Br D a b - 1 0 Br

In theory, gmelinite has a large, 12-ring pore system which should readily admit molecules such as cyclohexane. In fact, both natural and synthetic gmelinites behave like small-pore zeolites, a behavior attributable to chabazite stacking faults. chabazite fault planes, often observable by x-ray diffraction, effectively block or restrict access to the large gmelinite channels (41). The "pure gmelinites" described in Tables V and VI are believed to be f a u l t - f r e e , They sorb 7 . 3 9 cyclohexane. For comparison, a natural (faulted) sample soxbed only 1.0%. It is proposed that the plyelectrolyte is present in the pore as the gmelinite framework forms, the polymeric nature of the organic preventing formation of stacking faults across that pore. If the polymer is an integral part of the product structure, located in t.he pores, certain size and charge balance requirements must be fulfilled. The unit cell in gmelinite is traversed by a single 12-ring channel (7-8 A in diameter) for a distance of 10.0 A.

The D a b c o unit is cylindrical with a dlameter of about 6.1 A and can thus fit comfortably within the gmelinite pore. In length, the repeating units of Dab-3, -4, -5, -6 and -10 measure 7.5, 8.7, 9.9, 11.0 and 14.5 A . Comparing these measurements with the results in Table VX, all polymers which effected gmelinite synthesis had repeating units 9-11 A in length, matching the unit cell dimension of the pore. Charge balance is the second constraint. A repeating unit of the Dab-4 p o l y m e r contains two equivalents of cation and extends about 8.7 A. The unit cell in gmelinite contains a single 12-ring channel 10 A in length and could therefore hold little more than two quaternary cations, Seven different gmelinite preparations with Dab-4 Br averaged 2.3 N/unit cell. Furthermore, elemental analysis suggested that the polymer was intact. The C/N atomic ratio in the seven samples averaged 5 . 4 , compared with 5,l in t,he

srlginal polymer. Once encapsulated within a zeollte framework, quaternary ammonium cations are protected from hydrolysis.

It is well known that quaternary ammonium cations decompose readily in hot. caustic. As the number of examples o f their successful use in zeolite synthesis increases however, it is ~nstructive to examlne / ratios in the synthesis products. Table VI I shows remarkably little dl fference between the C/N expected if the organic remained intact and that found in four as-synthesized zeolites:

Table V I I .

Average C/N Ratios in Zeolite Products c/N Atomic Ratio

Zeolite

Organic

Uffretite

TMA

TMA Sodalit-e

TMA

Dab-4 Gmelinite

Dab-4

!rMA

Expected

5.1

Found

( ref. )

5.4 (40)

That these organics can indeed be intact within a product zeollte has now been directly demonstrated by 13c N M H on TPA ZSM-5 ( 4 4 ) .

EXRMPLES

IN SYNTHESIS

Patent literature is the primary information source in zeolite synthesis, but a set of instructional preparations has been assembled which require no specialized equipment and which can serve as an introduction to experimentation in the area. Details of those preparations will be published shortly (43), but an abbreviated version is presented here for easy reference. Three examples will be given, describing synthesis routes to zeolites A , Y and ZSM-5. They thus include framework compositions known to occur with S i 0 2 / A 1 2 0 3 ratios from L:l to essentially infinity; they demonstrate such techniques as low-temperature nucleation, templating and variable reactant sources. [A fourth example, TMA Uffretite, is given in the above reference but need not be included here.)

r

Zeolite A, a preparation for freshman chemistry.

bench-scale Crystals of N a A can be made in 3-4 hours, a preparation requiring only a stirred, heated beaker. A boiling solution of sodium aluminate and NaOB is added'to one of sodium meta-silicate, and the resultant mixture is heated with stirring at about 90°C until the suspension will settle quickly when the stirring is stopped. The suspension is then filtered (hot), washed repeatedly with water and dried at about 110°C to yield an 80-90% yield of N a 2 0 ' A 1 2 0 3 ' 2 S i 0 2 ' - 4 H 2 0 . Product purity is determined by comparing the X-ray diffraction pattern of the solid with that of an authentic sample of NaA. fie product, after dehydration at 350-40U°C, should sorb about 25% of its weight in water. In the

crystallization, reactants per mole of S i 0 2 are one 20 ( NaOH) and 550 ( H2(3), the water being divided between ( A 1 2 0 3 ), (Commercial aluminate and silicate solutions in the ratio 3:2. sodium aluminate analyzes about 40% A 1 2 0 3 , 33% NaZO and 27% water. )

Zeolite Y, low-temperature aging.

solution of sodium aluminate and NaOH is added, with vigorous stirring, to 30% silica sol (a colloidal silica suspension). The resultant mixture is aged at room temperature for 2-3 days and then crystallized in a steam chest (about 9 5 ' ~no~ stirring) for 1-2 weeks. Solid is withdrawn, filtered, and analyzed by X-ray diffraction every 2-3 days until Nay purity (diffraction pattern intensity) reaches a limiting value. After hot filtration, washing and drying, a 5U-6O% yield (based on SiOZ) is obtained of NaZO'A1203'5.3Si02'5~ with an approximate composition Without the aging, the S i 0 2 / A l Z 0 3 ratio of the product will not exceed 5 and P will be a common contaminant. As with NaA, the purity of the NaY preparation is determined by comparing its X-ray diff ractivn pattern with that of a authentic sample. The sample should sorb 25% of its weight in water, after dehydration. A

Moles of zeackants per mo1.e of silica should be 0.1 (ALZ03), 0 . 8 ( N a O H ) and 16 (water), with the water evenly divided between the silica suspension and the sadium aluminate solution.

TPA ZSM-5, probable templating.

A solution of sodium aluminate and NaOH is added simultaneously with one of TPA Br and H2SO4 to 16% silica sol, and the mixture is

lrnmediately mixed, to form a gel. Placed in a steam chest a-t: about 95°C and sampled periodically, the mixture will produce an 80-90s yield of ZSM-5 (based on Si02) in 10-14 days. Its molar cmposition will be approximately as follows : l.B(TPA)20'1.2Na20'1.3A12033100Si02'7H20. Again, the X-ray diffraction pattern should be compared with that of a known sample. A purified, dehydrated sample of ZSM-5 will sorb about 11% n-hexane, Solutions should be prepared such that moles of reactant per mole of silica are 0.012 ( ~ 1 ~ 00.54 ~ ) (N ~aOH), 0.1 (TPA Br), 0 . 2 (H2SU4) and 45 ( B Z u ) . sodium-stabilized 30% silica sol is the starting material in above preparation. Additional water is divided among the various solutions in the ratio, one (aluminate): two ( W A Br): one (silica sol ) . Higher temperatures, for example 140-180'~~will reduce crystallization time. Furthermore, with' appropriate adjustment for acid and base, no aluminum need be added to the reaction mixture. In that case, only the aluminum present as a contaminant in the various other reactants will be found in the product ZSM-5.

is one of a number of framework structures which can be crystallized with essentially no alumina. Several other structures can apparently exist in or near that compositional (45,461, but must first be synthesized in an range aluminum-containing form. Although direct synthesis routes will likely be discovered, this section reviews experimental techniques for dealminizing these structures, with particular emphasis on the very siliceous compositions. ZSM-5

8 Acld extraction dealuminization.

alone

often

achieves

only

partial

The zeolites Y and mordenite are the most commonly targets for dealuminization, and a substantial literature exists on aluminum removal from both. In the case of Y, it is now generally accepted that controlled, direct addition of acid (such as ethylenediamine tetracetic acid (H4EDTA) can remove at least 5 0 % of the aluminum, to a SiO2/Al2O3 ratio of about 12 (47,481, without significant loss in crystallinity. With mordenite, direct acid leaching can remove up to about 80% of the aluminum (SiCJ2fA1203 = "60) without structure collapse ( 49)

.

Beyond this point, i .e., to a c h i e v e SiO2/'AI2O3 yati os 2309 confbined t h e r m a l a n d c h e m i c a l (acid) t r e a t m e n t s are r e q u i r e d , Samples of s y n t h e t i c f a u j a s i t e w i t h S i 0 2 / A 1 2 0 3 = 100-200 have been r e p o r t e d l y p r e p a r e d by a l t e r n a t e acid l e a c h i n g and s t e a m i n g of u l t r a s t a b l e Y' s ( 45 j . M o r d e n i t e s i n t h i s c o m p o s i t i o n r a n g e a r e p r e p a r e d by t h e r m a l a n d / o r h y d r o t h e r m a l t r e a t m e n t a t t e m p e r a t u r e s dbove 5 0 0 ° C , f o l l o w e d by v a r y i n g a c i d e x t r a c t i o n (46,49 ). Examples are g i v e n i n T a b l e V I I I

.

Table V I I Z . I n i t i a l Sample

D e a l u m i n i z a t i o n of Y a n d of Mordenite Treatment

Product

Reference

( Si02/A1203 1

( Si02/A1203 )

Y (5.3) Y (5.1)

H4EUTA,

U S Y ~( 5 . 2 )

2N HCI,

Hord (15) Moxd ( 1 5 )

6 N H C 1 , 1 h, reflux

slow addition Na2H2EDTA + slow HCl 1

h,

9 0 0 ~

Fau jasite ( 10 ) Faujasite (12)

48

Faujasite

45

47

(10s)

Mord ( 1 5 )

Mordenite ( 20 ) Mordenrte (59) Mordenlte ( 60)

49

6Nac1, 16 h, reflux 6N BCI, 24 h, reflux

Mord (15)

6 5 0 * ~ ,3h; 0.5NHU1,

Mordenite

49

( 73 )

49

49

16 h, refllux

a

-

"Ultrastable Y " , prepared by treatment under self-steaming conditions at 7 6 U - B 1 5 * ~ (45)

techniques for dealuminization, which promise to extend the range i n the above s t r u c t u r e s even further, have very recently appeared (50-52). When Nay was treated with SiC14, for example, a h i g h l y c r y s t a l l i n e , e s s e n t i a l l y aluminum-free faujasite r e p o r t e d l y r e s u l t e d ( 52 ) New

Si02/AL2U3

.

a

In many structures, aluminum removal is site-dependent.

Not all aluminum tetrahedra within a given, siliceous ( S i 0 2 / A 1 2 0 3 > 5 ) framework are e q u i v a l e n t . I n m o r d e n i t e , f o r example, f o u r d i f f e r e n t c r y s t a l l o g r a p h i c s i t e s f o r aluminum p o t e n t i a l l y e x i s t ( 49 ) Aluminum t e t r a h e d r a of d i f f e r i n g acidity are r e c o g n i z e d w i t h i n t h e s y n t h e t i c faujasite framework ( 53-55 ) I t is t h e r e f o r e very r e a s o n a b l e t o e x p e c t that aluminum removal, i . e . , ease of h y d r o l y s i s , w i l l be site d e p e n d e n t .

.

.

The s t r o n g e s t e v i d e n c e t o date f o r s u c h a n a s s e r t ~ o ni s the marked n o n - l i n e a r i t y of kh@ l a t t x c e p a r a m e t e r c o n t r a c t i o n s as aluminum

( framework charge) IS removed from mordenite (49). A very plausible correlation can be developed between the d i f f e r i n g a , b and c projections of t h e four p o t e n t i a l Alo4 sites and the respective non-linearities in t h e t h r e e lattice p a r m e t e r d e p e n d e n c i e s on aluminum content i n this orthorhombic unit cell. HOLY? powerful evidence c a n be e x p e c t e d as 2 Y ~ iand 2 7 ~ 1NMR techniques d e v e l o p . I n a d d i t i o n t o p r o v i d i n g a d i r e c t probe i n dealuminization, those t e c h n i q u e s should clarify a more basic and unanswered question, namely, what is the r e l a t i o n s h i p b e t w e e n an as-synthesized and a dealurninized s t r u c t u r e w i t h t h e same o v e r a l l cornposit ion?

T h l s b r i e f summary oE current thlnklng I n zeolite synthesis

draws e x t e s ~ s l v e l y on the numerous new zeolites a n d new preparation technxques d~scovered In Mobll's Prxnceton and Paulsboro Laboratories over the past 20-25 years. Thanks are due the many authors whose names appear i n the references and whose names wlll be found on p a t e n t s d e s c r ~ b l n g t h e s e dlscoverles f o r t h e ~ r valuable lnput, advlce and suggestions as my own experiments progressed.

1.

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2.

D . W.

Breck, "Zeolite Molecular Sieves," John Wiley and

Sons, N e w York, 1974, 3.

L. B . Sand, Pure A p p l , c%em.,

4.

E . M . F l a n i g e n , Pure Appl.

5.

M. Mengel, Chem.-Tech.,

6.

G. T . Kerr, Catal. R e v . - S c i .

7.

H . Robson, C'hemTech, 8,

8,

E, M, F l a n i g e n , Adv.

9.

S. P. Zhdanov and N . N . Samulevich, P r o c , Int. Uonf. Zeolites, 5th, ed. L. V . C . R e e s , p . 7 5 {lYSU).

52, -

hem.,

2105 ( 1 9 8 0 ) .

52,

2.191 ( 1 9 8 0 1 .

1 1 3 5 ( 1981).

10, -

Eng., 23, 281 ( 1 9 8 1 ) .

176 ( 1 9 7 8 ) .

em.

Ser., 1 2 1 , 119 (1473).

173,

10.

L, D. Hollmann, Adv. Chem. Ser.,

11,

W, L, Baden, J r . , and P . J. Dzierzanowski, U S P a t e n t s 3,663,165 and 3,657,154 ( 1 9 7 2 ) , for example.

12.

R. von ~ ( 1 9 8 1 ).

13.

H. M. B a r r e r and P. J . Denny, J. Chem. Soc.,

14.

G. T. K e r r , I n o r g . Chem.,

15.

C. B a e r l o c h e r and W. 1 8 5 3 1 ~( 1959 )

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H. J . Argauer and G. H. L a n d o l t , U S P a t e n t 3,702,886

387 ( 1 9 7 9 ) .

a l - 0 and ~ ~ W. M. Meier, Nature, 289,

.

-5,

782

971 ( 1 9 6 1 ) .

1537 ( 1 9 6 6 ) .

M. Meier, Helv. Chirn. Acta,

52, -

(1972), 17.

U. M . Bibby, L. P. A l d r i d g e and N . B. M i l e s t o n e , J . C a t a l y s i s , 72, 373 ( 1 9 8 1 ) .

18.

D. H . Olson, W. 0. Baag and H. M . Lago, J . C a t a l y s i s , 390 ( 1 9 8 0 ) .

61, -

19.

E. L. Wu, S . L. Lawton, U. H . Olson, A . and G . T. K o k o t a i l o , J . Phys. Chem.,

C , Hohrman,

2 , 2777

20.

Jr.,

(1979).

G. T. K o k o t a i l o , S . L. Lawton, D. 8. Olson and W. M. Meier, Nature, 272, 437 ( 1 9 7 8 ) .

-

21.

D. W.

22.

H. O l s o n , M. M e i e r ,

S . L. Lawton and J. Phys. Chem., 85, 2 2 3 8 ( 1 9 8 1 ) . G. T. K o k o t a i l o ,

E. M. Flanigen, J. M. B e n n e t t , R. W. Grose, J. P. Cohen, R. L. P a t t o n , R. M. K i r c h n e r a n d J . V. Smith, N a t u r e ,

271, -

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STUDY O N THE MECHANISM OF CRYSTALLIZATION O f ZEOLITES A , X AND Y

F r e d Roozeboorn a t * ) , H a r r y E . Robson a ) and S h i r l e y S. Chan b,

a ) Exxon R e s e a r c h and Development L a b o r a t o r i e s , P.O. Box 2226, Baton Rouge, La. 70821, U.S.A. and b ) Exxon R e s e a r c h and E n g i n e e r i n g Company, P . O . Box 4 5 , L i n d e n , h . J . 07036, U.S.A. ABSTRACT Samples o f A , X , and Y s y n t h e s i s g e l s a t 98 O C w e r e withdrawn a f t e r c r y s t a l l i z a t i o n f o r v a r i o u s t i m e s , t h e n c e n t r i f u g e d w h i l e h o t . The s o l i d and l i q u i d p h a s e s w e r e examined by L a s e r Raman s p e c t r o s c o p y {LRS), X-ray d i f f r a c t i o n ( X R D ) and c h e m i c a l a n a l y s i s ( A l , S i , Na). LRS and XRD on s o l i d s a m p l e s d e t e c t z e o l i t e s t r u c t u r e s a t a b o u t t h e same time, i . e . , when c r y s t a l l i t e s a r e a b o u t 500 fi i n s i z e . LRS on t h e l i q u i d s g i v e s a d d i t i o n a l i n f o r m a t i o n . For i n s t a n c e , t h e Al(OH)4- i n t h e l i q u i d s ( b a n d a t 621 cm-1) d i s a p p e a r e d b e f o r e any z e o l i t e structure was d e t e c t e d . Some s o l u t e s p e c i e s a p p e a r e d i n t h e c o u r s e o f X and Y c r y s t a l l i z a t i o n , h a v i n g b r o a d , weak Raman bands ( a r o u n d 448, 6 0 0 , 777, and 936 c m - I ) , d u e t o f r e e , u n r e a c t e d monomeric and d i m e r i c s i l i c a t e i o n s . The above a n d c h e m i c a l a n a l y s i s r e s u l t s i n d i c a t e t h e d i s s o l u t i o n o f

some s i l i c o n c o n t a i n i n g (monomeric and p o l y m e r i c ) i o n s from t h e amorphous a l u m i n o s i l i c a t e g e l s . The h y d r o x y l a t e d i o n s c o n d e n s e t o form a l a r g e v a r i e t y o f complex a g g r e g a t e s . w i t h Al(OH)4' T h e s e c o m p l e x e s may b e s o l u b l e p o l y m e r i c a l u m i n o s i l i c a t e s r e l a t e d t o z e o l i t i c f r a g m e n t s and t h u s form t h e n u c l e i o f c r y s t a l growth. A mechanism f o r t h e f o r m a t i o n o f 4 and 6 r i n g v s . 5 r i n g s y s t e m s is proposed.

* Present address: E s s o Chemie B.V.,

P.O.

Box 7225, 3000 HE R o t t e r d a m , The N e t h e r l a n d s .

INTRODUCTION

D u r i n g t h e p a s t two d e c a d e s a number o f p a p e r s h a s b e e n p u b l i s h e d on t h e k i n e t i c s and mechanism o f z e o l i t e f o r m a t i o n . I n most c a s e s t h e s y n t h e s i s o f z e o l i t e A and f a u j a s i t e z e o l i t e s was s t u d i e d (1-17) s i n c e t h e s e a r e t h e e a s i e s t a n d f a s t e s t t o s y n t h e s i z e . R e c e n t l y a l s o h i g h l y s i l i c e o u s z e o l i t e s , s u c h as t h e ZSM-5 ( 1 0 , 1 7 ) a n d m o r d e n i t e s y s t e m s ( 1 1) h a v e r e c e i v e d a t t e n t i o n . From t h e b e g i n n i n g r e s u l t s h a v e b e e n c o n t r o v e r s i a l . Zhdanov a r g u e d i n a d e t a i l e d r e v i e w ( 1 ) i n f a v o r of a s o l u t i o n t r a n s p o r t mechanism, f i r s t p u t f o r w a r d by B a r r e r e t a l . ( 2 ) and l a t e r by many o t h e r i n v e s t i g a t o r s (3-11). B a r r e r et a l . proposed t h e n u c l e a t i o n t o b e t h e r e s u l t o f t h e p o l y m e r i z a t i o n o f a l u m i n a t e , s i l i c a t e and p o s s i b l y more complex i o n s i n t h e l i q u i d p h a s e , t h e i o n s b e i n g c o n t i n u o u s l y s u p p l i e d by t h e d i s s o l u t i o n o f t h e s o l i d g e l m a t e r i a l . McNicol e t a l . s t u d y i n g A and f a u j a s i t e t y p e z e o l i t e s ( 1 2 , 1 3 ) p r o p o s e d a s o l i d p h a s e t r a n s f o r m a t i o n mechanism, i n v o l v i n g z e o l i t e c r y s t a l l i z a t i o n i n t h e s o l i d g e l p h a s e v i a c o n d e n s a t i o n between h y d r o x y l a t e d Si-A1 t e t r a h e d r a . T h i s mechanism was a l s o f a v o r e d by P o l a k e t a 1 ( 1 4 , 1 5 ) who s t u d i e d t h e mechanism of f o r m a t i o n o f X and Y z e o l i t e s . F l a n i g e n ( 1 6 ) p r o p o s e d a s i m i l a r mechanism, involving a reordering o f t h e hydrogel t o an ordered c r y s t a l l i n e state v i a s u r f a c e d i f f u s i o n i n t h e absence o f l i q u i d phase t r a n s p o r t . Derouane e t a l . (17, 1 8 1 , s t u d y i n g t h e s y n t h e s i s o f z e o l i t e s w i t h ZSM-5 t o p o l o g y , c o n c l u d e d t h a t b o t h t h e l i q u i d p h a s e i o n t r a n s p o r t a t i o n mechanism and s o l i d h y d r o g e l p h a s e t r a n s f o r m a t i o n mechanism are i m p o r t a n t , d e p e n d i n g on t h e s i l i c a s o u r c e and t h e g e l f o r m u l a t i o n u s e d . I n t h e f o r m e r mechanism o n l y a few n u c l e i a r e formed, y i e l d i n g l a r g e c r y s t a l l i t e s , w h e r e a s t h e l a t t e r i n v o l v e s numerous n u c l e i y i e l d i n g p o l y c r y s t a l l i n e a g g r e g a t e s . T h e s e w o r k e r s a l s o p e r f o r m e d i n f r a r e d s p e c t r o s c o p i c tests s h o w i n g t h e e x i s t e n c e o f ZSM-5 which was n o t X-ray d e t e c t a b l e i n i n t e r m e d i a t e p h a s e s ( 1 7 , 1 9 ; . Most i n v e s t i g a t o r s a t t h e p r e s e n t t i m e t e n d t o a g r e e w i t h B a r r e r ' s c o n c e p t (21, e s p e c i a l l y s i n c e some r e c e n t Raman s p e c t r o s c o p i c (20, 21) and 2 9 ~ NMR i s p e c t r o s c o p i c (22-25) d a t a s u g g e s t t h e e x i s t e n c e o f s o l u t e a l u m i n o s i l i c a t e s p e c i e s . S o f a r , Raman i n v e s t i g a t i o n s h a v e b e e n c o n f l i c t i n g . McNicol e t a l . ( 1 2 , 13) o b s e r v e d no s p e c t r a l c h a n g e s i n t h e l i q u i d p h a s e d u r i n g c r y s t a l l i z a t i o n and t h u s p r o p o s e d a s o l i d p h a s e t r a n s f o r m a t i o n . However, similar e x p e r i m e n t s by A n g e l 1 and F l a n k ( 9 ) showed s p e c t r a l c h a n g e s i n t h e l i q u i d , g i v i n g e v i d e n c e f o r a s o l u t i o n t r a n s p o r t mechanism. Guth e t a l . ( 2 0 ) r e p o r t e d i n d i r e c t e v i d e n c e f o r t h e o c c u r r e n c e o f aluminos i l i c a t e c o m p l e x e s i n s o l u t i o n by c o m p a r i n g t h e s p e c t r a o f s i l i c a t e and a l u m i n a t e i o n s s e p a r a t e l y i n NaOH s o l u t i o n w i t h t h e s p e c t r u m o f s i l i c a t e and a l u m i n a t e i o n s p r e s e n t t o g e t h e r i n s o l u t i o n .

The o b j e c t i v e o f t h e p r e s e n t work was t o s e e k e v i d e n c e f o r t h e existence of (soluble o r solid) z e o l i t i c precursor species i n the e a r l i e r s t a g e s o f z e o l i t e f o r m a t i o n and t o add t o o u r unders t a n d i n g o f t h e mechanism o f z e o l i t e f o r m a t i o n . T h u s , we a n a l y z e d b o t h s o l i d and l i q u i d components o f z e o l i t e A , X a n d Y s y n t h e s i s g e l s by l a s e r Raman S p e c t r o s c o p y ( L R S ) f o r i d e n t i f i c a t i o n o f m o l e c u l a r s p e c i e s and c h e m i c a l a n a l y s i s f o r A l , S i and Na c o n t e n t . S o l i d s w e r e a l s o t e s t e d by X-ray d i f f r a c t i o n (XRD) f o r c r y s t a l l i n i t y . The main r e a s o n f o r u s i n g Raman s p e c t r o s c o p y is i t s u n i q u e a b i l i t y t o examine s o l i d s a m p l e s a s well as s o l u t i o n s a m p l e s , and its a b i l i t y t o m e a s u r e t h e i r l o w e r f r e q u e n c y modes as opposed t o i n f r a r e d s p e c t r o s c o p y . U n l i k e t h e o t h e r Raman i n v e s t i g a t o r s ( 9 , 1 2 , 2 0 ) we had a Raman s p e c t r o m e t e r e q u i p p e d w i t h a n o p t i c a l m u l t i c h a n n e l a n a l y z e r f o r s i g n a l a v e r a g i n g , and t h u s h a d t h e p o s s i b i l i t y o f r e s o l v i n g i n t e r m e d i a t e s p e c i e s which o f t e n exist a t r e l a t i v e l y low c o n c e n t r a t i o n s s p e c i e s . EXPERIMENTAL Z e o l i t e s y n t h e s i s . T h r e e s i m p l e z e o l i t e s y n t h e s i s c a s e s were s e l e c t e d i.e. t y p e A , and t y p e X a n d Y , b e c a u s e o f t h e i r f a s t f o r m a t i o n a t r e l a t i v e l y IQW t e m p e r a t u r e s . The g e l s s t u d i e d w e r e formed by m i x i n g a q u e o u s s o l u t i o n s o f sodium a l u m i n a t e , sodium h y d r o x i d e and c o l l o i d a l s i l i c a s o l ( t u d o x HS-40). Z e o l i t e A c r y s t a l l i z a t i o n was from a g e l w i t h c o m p o s i t i o n No a g i n g was c a r r i e d o u t s i n c e 2.1 Na20.A1203.2Si02.80H20. i t h a s b e e n r e p o r t e d t o h a v e no e f f e c t ( 9 ) and no s e e d was added. F o r z e o l i t e s X and Y t h e g e l c o m p o s i t i o n s were 3.5 NazO.Al203. 5Si02.80H20 and 3 . 3 Na20.A1203.9Si02.140H20 r e s p e c t i v e l y . The z e o l i t e s were s y n t h e s i z e d u s i n g p u b l i s h e d methods of s e e d i n g ( 2 6 , 27), t h e seed s l u r r y composition being 13.3 Na20.Al2O3.12.5SiO2.267H20. The amount o f s e e d was 5 mol 7; By s e e d i n g t h e c r y s t a l l i z a t i o n t i m e was r e d u c e d of t h e t o t a l A l . t o 8 h o u r s f o r z e o l i t e X and 1 2 h o u r s f o r z e o l i t e Y. Raman S p e c t r o s c o p y . P o l y e t h y l e n e b o t t l e s o f t h e master s y n t h e s i s q e l s were w i t h d r a w n from a c o n s t a n t t e m p e r a t u r e oven ( 9 8 O ~ ) a f t e r Garious times of s t a t i c c r y s t a l l i z a t i o n , c e n t r i f u g e d while still h o t and f i l t e r e d t o s e p a r a t e t h e l i q u i d phase and s o l i d p h a s e . The s o l i d s a m p l e s w e r e f u r t h e r washed w i t h w a t e r , d r i e d a t llO°C and c a l c i n e d a t 500°C f o r 2 h r i n a i r t h e n c o o l e d t o room t e m p e r a t u r e . An a r g o n i o n l a s e r was t u n e d t o t h e 514.5 nrn l i n e f o r e x c i t a t i o n . L i q u i d s a m p l e s a t room t e m p e r a t u r e were p l a c e d i n 10mm x 10mm q u a r t z c u v e t t e s . The Raman s i g n a l s were c o l l e c t e d u s i n g 90° s c a t t e r i n g geometry w i t h a F1.2 l e n s . The l a s e r power a t t h e The s o l i d s a m p l e s were s a m p l e l o c a t i o n was s e t a t 70-90 mW. p e l l e t i z e d t o 13 mm d i a m e t e r w a f e r s f o r m o u n t i n g i n a s a m p l e h o l d e r .

Fig. 1

Raman s p e c t r a o f s o l i d phase in zeolite A synthesis

0 hr.

300

Fig. 2

400

500

600 700 FREQUENCY SHIFT (cm-I)

800

900

1000

Raman spectra o f l i q u i d phase i n z e o l i t e A synthesis

The c o l l e c t i o n o p t i c s i n t h i s c a s e was a b a c k s c a t t e r i n g geometry a n d t h e l a s e r power was l o w e r e d t o 10-30 mW. The Raman s p e c t r a were a n a l y z e d by a t r i p l e t monochromator, model DL203 w i t h F4 o p t i c s made by I n s t r u m e n t SA, Metuchen, NJ, and an o p t i c a l m u l t i c h a n n e l a n a l y z e r s y s t e m , model OMA2 e q u i p p e d with a n

i n t e n s i f i e d p h o t o d i o d e a r r a y d e t e c t o r , made by P r i n c e t o n A p p l i e d R e s e a r c h , P r i n c e t o n , NJ. T h i s s y s t e m made i t p o s s i b l e t o c o l l e c t a c o m p l e t e s p e c t r u m o v e r a r a n g e o f t h o u s a n d cm-I s i m u l t a n e o u s l y i n t h e m a t t e r o f s e c o n d s o r m i n u t e s . Thus e a c h s a m p l e c o u l d b e examined i m m e d i a t e l y a f t e r i t s p r e p a r a t i o n p r o c e d u r e w i t h o u t any c h a n c e o f a g i n g which m i g h t a f f e c t its s t r u c t u r a l i n t e g r i t y . The t o t a l a c c u m u l a t i o n time n e e d e d f o r e a c h s p e c t r u m r e p o r t e d h e r e was i n t h e r a n g e 20-100 s e c . The q i g i t a l d i s p l a y o f t h e s p e c t r u m was c a l i b r a t e d t o g i v e 1 . 7 cm- / c h p n n e l w h e r e a s t h e o v e r a l l s p e c t r a l r e s o l u t i o n was a b o u t 8 cm which was a d e q u a t e f o r v i b r a t i o n a l band w i d t h s o v e r 2 0 cm-I. X-Ray d i f f r a c t i o n . S o l i d p h a s e s a m p l e s were s c a n n e d w i t h t h e a i d o f a P h i l i p s X-ray d i f f r a c t o m e t e r , u s i n g CuK-alpha r a d i a t i o n . C h e m i c a l a n a l y s i s . C h e m i c a l c o m p o s i t i o n s (Al, S i and Na c o n t e n t ) o f b o t h s o l i d and l i q u i d p h a s e s were d e t e r m i n e d by p l a s m a s p e c t r o s 111 I n d u c t i v e l y Coupled Plasma/ copy u s i n g a ~ a r r e l l - ~ s h Atomic E m i s s i o n S p e c t r o m e t e r which p e r f o r m s a s i m u l t a n e o u s m u l t i e l e m e n t measurement. S o l i d s a m p l e s w e r e f u s e d i n a s a l t m i x t u r e by a Claisse F l u x e r , which a u t o m a t i c a l l y p o u r s t h e m o l t e n f l u x i n t o a d i l u t e a c i d s o l u t i o n f o r f i n a l d i s s o l u t i o n . The i n i t i a l f u s i o n m i x t u r e c o m p r i s e s 0.1 g . of s a m p l e p l u s 1 . 5 g. o f a Li2COj/ T h i s melt was d i s s o l v e d i n 1 0 0 m l 5% HNO3, Li2B407 m i x t u r e . t h e n d i l u t e d t o 250 m l a n d f i n a l l y i n j e c t e d i n t o t h e a r g o n plasma. L i q u i d p h a s e s were i n j e c t e d d i r e c t l y i n t o t h e a r g o n plasma.

tomc corn^

RESULTS AND DISCUSSION Raman S p e c t r o s c o p y . F i g u r e 1 g i v e s t h e r e s u l t i n g s p e c t r a f o r t h e s o l i d p h a s e s i n z e o l i t e A s y n t h e s i s a f t e r 0 , 1 , 2 , 3 , and 4 h o u r s o f c r y s t a l l i z a t i o n . The s p e c t r a o f t h e i n i t i a l g e l ( 0 h o u r s ) and a f t e r 1 h o u r h a v e no i n t e r p r e t a b l e p e a k s . The p e a k s i n d i c a t e d i n t h e o t h e r s p e c t r a are a l l d u e t o z e o l i t e A f o r m a t i o n and a g r e e r e a s o n a b l y well w i t h t h o ~ e ~ r e p o r t ebyd A n g e l 1 ( 2 8 ) . He reported o n e s t r o n g band ~t 490 cm- and f o u r weak b a n d s a t 700, 4 1 0 , 3 4 0 , and 280 cmThe c o r r e s p o n d i n g band f r e q u e n c i e s o$ o u r measurement c e n t r e a r o u n d 4 9 2 , 7 1 4 , 4 0 5 , 347 and 281 cmW e d i d n o t o b s e r v e s i g n i f i c a n t s h i f t s o f t h e s e band p o s i t i o n s , which m i g h t s u g g e s t t h e g r o w t h from a s o l i d p r e c u r s o r .

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F i g u r e 2 shows t h e s p e c t r a o f t h e c o r r e s p o n d i n g l i q u i d p h a s e s . I t i s s e e n t h a t i n t h e e a r l y s t a g e s , i . e . , d u r i n g i n d u c t i o n and e v e n a f t e r 2 h o u r s when r y s t a l l i z a t i o n h a s set i n , a s t r o n g p e a k was -7 whlch . . o b s e r v e d a t 621 cm 1s a s s i g n e d t o t h e A104 s y m m e t r i c s t r e t c h i n g mode o f t h e monomeric Al(OH)4- a l u m i n a t e a n i o n ( 2 9 ) . O t h e r a u t h o r s f o nd t h i s peak a t f r e q u e n c i e s r a n g i n g from 618 crn-I ( 1 2 ) t o 6 2 5 cm ( 2 9 ) . No e v i d e n c e f o r s o l u t e a n i o n i c a l u m i n o s i l i c a t e p r e c u r s o r s p e c i e s was n o t e d i n t h e l i q u i d p h a s e d u r i n g t h e c o u r s e of t h e c r y s t a l l i z a t i o n . I t is n o t l i k e l y t h a t Raman i n a c t i v e p r e c u r s o r s p e c i e s a r e formed s u g g e s t i n g t h e a b s e n -c e o f well d e f i n e d p r e c u r s o r s p e c i e s i n s o l u t i o n o t h e r t h a n A1(OHI4 T h i s means t h a t numerous i l l - d e f i n e d p r e c u r s o r s p e c i e s may be f o r m e d , e a c h h a v i n g i n s u f f i c i e n t c o n c e n t r a t i o n f o r a Raman p e a k t o be r e s a l v e d . The o b s e r v a t i o n s a l s o i n d i c a t e t h a t n o s i g n i f i c a n t n e t d i s s o l u t i o n of t h e s o l i d g e l phase occurs throughout t h e s y n t h e s i s o f z e o l i t e A: o n l y a l u m i n a t e is b e i n g consumed from t h e l i q u i d p h a s e ( d i s a p p e a r i n g Rarnan p e a k ) on i n c o r p o r a t i o n i n t h e s o l i d p h a s e and no s i l i c a t e p e a k s a p p e a r .

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F i g u r e s 3 and 4 show t h e c o r r e s p o n d i n g d a t a f o r z e o l i t e X a n d F i g u r e s 5 and 6 f o r z e o l i t e Y . The p e a k s i n d i c a t d i n F i g u r e 3 a l l -7 a r i s e from s o l i d z e o l i t e X ( 5 1 7 , 3 7 6 , and 291 cm ) a n d t h o s e i n F i g u r e 5 from z e o l i t e Y ( 5 1 1 , 3 6 9 , and 2 9 8 c m - I ) . Literature r e p o r t s t h e p e a k s t o be - ~i 505, 3 7 5 , and 282 crn-' ( p l u s v e r y weak a t 1 1 0 7 5 and 990 c~ ) f o r z e o l i t e X a n d a t 5 0 3 , 3 5 0 , and 3 0 0 cm- ( a n d 1 1 1 0 cm- ) f o r z e o l i t e Y ( 2 8 ) . T h u s , t h e s e measurements a g r e e reasonably w e l l with t h e l i t e r a t u r e d a t a t o w i t h i n e x p e r i m e n t a l limits.

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The p e a k s o r b a n d s i~ F i g u r e s 4 a n d 6 c l e a r l y show t h a t t h e Al(OH)4 i o n ( p e a k a t 621 cm- ) i s p r e s e n t i n t h e i n i t i a l g e l o f b o t h A s i n t h e c a s e of z e o l i t e A , t h i s z e o l i t e X and z e o l i t e Y . p e a k d i s a p p e a r s a f t e r t h e f i r s t 2-3 h o u r s o f c r y s t a l l i z a t i o n . B u t u n l i k e z e o l i t e A , some s o l u b l e s p e c i e s a r e o b s e r v e d i n t h e course o f the crystallization. The i d e n t i f i c a i o n of t h e s e s p e c i e s , h a v i n g b a n d s a r o u n d 448, 600, 7 7 7 , and 936 cm- f o r z e o l i t e X a n d 600 and 777 cm-I f o r z e o l i t e Y , is d i f f i c u l t b e c a u s e o f t h e b r o a d n e s s and weakness of t h e bands. I n g e n e r a l , a b r o a d Raman band p o i n t s t o a v a r i e t y o f s t r u c t u r e s r a t h e r t h a n t o a v e r y well d e f i n e d s t r u c t u r e of one d i s t i n c t i o n ( l i k e A ~ ( O H ) ~ - ) . T h i s v a r i e t y o f s t r u c t u r e s is common f o r s i l i c a t e i o n s , which a r e known t o b e p r e s e n t i n d i f f e r e n t d e g r e e s o f h y d r o x y l a t i o n a n d p o l y m e r i z a t i o n d e p e n d i n g on c o n c e n t r a t i o n , pH, t e m p e r a t u r e and p r e s s u r e ( 3 0 ) .

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Chemical A n a l y s i s T a b l e s I and I1 summarize t h e a n a l y s i s r e s u l t s , along with t h e other r e s u l t s . Z e o l i t e A . I n a c c o r d a n c e w i t h what was o b s e r v e d w i t h Raman spectroscopy, t h e alumina concentration decreased a f t e r 3 hours o f s y n t h e s i s t i m e . From t h e r e s u l t s on t h e s o l i d p h a s e , we c a n c o n c l u d e t h a t a l u m i n a t e d i s a p p e a r e d from s o l u t i o n and is i n c o r p o r a t e d i n t h e s o l i d phase. A s t o t h e s i l i c a t e c o n c e n t r a t i o n o f t h e l i q u i d p h a s e , t h i s c o n c e n t r a t i o n d e c r e a s e d less d r a s t i c a l l y t o c o m p a r a b l y low l e v e l s . Z e o l i t e s X and Y . From T a b l e s I and I1 we c a n s e e t h a t i n c a s e o f b o t h z e o l i t e X and z e o l i t e Y i n t h e f i r s t 2-3 h o u r s , i . e . , b e f o r e any c r y s t a l l i t e s w e r e d e t e c t e d , a l u m i n a t e was d i s a p p e a r i n g from s o l u t i o n a s o b s e r v e d w i t h Raman s p e c t r o s c o p y and i n c o r p o r a t e d i n t h e amorphous a l u m i n o s i l i c a t e p h a s e . F o r b o t h z e o l i t e s w e o b s e r v e d i n t h e same p e r i o d a n i n c r e a s e i n s i l i c a t e c o n c e n t r a t i o n i n t h e l i q d i d and a d e c r e a s e i n s i l i c o n c o n c e n t r a t i o n o f t h e s o l i d phase. A f t e r t h i s 2-7 h o u r p e r i o d , t h e s i l i c o n c o n t e n t d e c r e a s e d o r r e m a i n e d t h e same f o r t h e l i q u i d p h a s e s and r o u g h l y r e m a i n e d t h e same f o r t h e s o l i d s . From t h e s e d a t a i t is c l e a r t h a t i n t h e f i r s t 2-3 h o u r s o f t h e c r y s t a l l i z a t i o n o r n u c l e a t i o n some s i l i c o n cont a i n i n g i o n s (monomeric o r p o l y m e r i c ) w e r e d i s s o l v e d from t h e amorphous ( a 1 u m i n o ) s i l i c a t-e g e l . T h e s e h y d r o x y l a t e d i o n s a p p a r e n t l y c o n d e n s e d w i t h t h e A1(OH)4 i o n s p r e s e n t t o form a l a r g e v a r i e t y o f a g g r e g a t e s , which may be t h e n u c l e i f o r c r y s t a l growth. Thus, t h e whole mechanism o f z e o l i t e A , X and Y f o r m a t i o n is a s o l u t i o n t r a n s p o r t mechanism. X-Ray D i f f r a c t i o n The X-Ray d i f f r a c t i o n p a t t e r n s f o r t h e v a r i o u s s o l i d p h a s e s o f z e o l i t e s A , X , and Y a r e summarized i n T a b l e 11. I n comparing XRD and Raman r e s u l t s , we c a n c o n c l u d e t h a t Raman s p e c t r o s c o p y d e t e c t s z e o l i t e f o r m a t i o n i n t h e s o l i d p h a s e s a t a b o u t t h e same t h e c h a r a c t e r i s t i c XRD p e a k s a p p e a r a t t h e same s t a g e a s XRD: p o i n t i n t h e c r y s t a l l i z a t i o n as t h e Raman p e a k s , i . e . , when c r y s t a l l i t e s a r e a b o u t 500 8 i n s i z e a s o b s e r v e d w i t h S c a n n i n g E l e c t r o n Microscopy (SEM). A s i n t h e Rarnan s p e c t r a , no o t h e r XRD p e a k s a p p e a r e d t h a n t h o s e o f t h e z e o l i t e we i n t e n d e d t o s y n t h e s i z e Thus, no i n t e r m e d i a t e p h a s e s were d e t e c t e d by t h e two t e c h n i q u e s . Background F l u o r e s c e n c e Raman s p e c t r o s c o p y o f t h e s o l i d s a m p l e s d o e s n o t g i v e a d d i t i o n a l i n f o r m a t i o n ; by t h e t i m e t h a t z e o l i t e c r y s t a l s a r e l a r g e enough t o b e Raman d e t e c t a b l e , t h e s e c r y s t a l s g i v e s t r o n g XRD p a t t e r n s ( > 500 8 ) .

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L i t e r a t u r e d a t a have r e c e n t l y been r e p o r t e d f o r v a r i o u s s i l i c a t e i o n s ( 1 9 ) : 1 0 0 5 ( s h o u d e r ) , 9 3 0 ( i n t e n s e ) , 830 ( s h o u l d e r ) , 778 ( i n t e n s e ) a n d 448 cm- (medium) f o r t h e monomeric s p e c i e s , 2-1 (medium) f o r d i m e r i c s i l i c a t e , and 6 3 2 , S i O p ( ~ ~ ) 2, 6 0 5 cm 5 5 5 , 5 4 2 , and 5 2 0 cm ( a l l weak) f o r p o l y m e r i c , e s p e c i a l l y c y c l i c , t r i m e r i c a n i o n s . Thus o n e may ? s c r i b e t h e band p o s i t i o n s a t 9 3 6 , 7 7 7 , 600, a n d 448 cm- t o monomeric a n d dirneric s i l i c a t e anions. A s discussed l a t e r i n t h i s paper (Tables I a n d 111, t h e c h e m i c a l a n a l y s i s r e s u l t s show t h a t a f t e r some 2-3 hours o f s y n t h e s i s t h e s i l i c o n concentration i n t h e l i q u i d phase of X a n d Y r e m a i n s t h e same o r d e c r e a s e s , w h e r e a s t h e Rarnan b a n d s become more i n t e n s e . T h u s , i t is c l e a r t h a t t h e b a n d s a r e d u e t o t h e d e p o l y r n e r i z a t i o n o f p o l y m e r i c s i l i c a t e s p e c i e s i n t o monomeric and d i m e r i c s i l i c a t e i o n s .

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The Si02/A1203 r a t i o i n b o t h t h e i n i t i a l g e l f o r m u l a t i o n Thus a n d t h e f i n a l p r o d u c t o f z e o l i t e A s y n t h e s i s was 2.0. a l l s i l i c a t e s p e c i e s p r e s e n t were consumed by c o n d e n s a t i o n w i t h aluminate species. In t h e c a s e of t h e f a u j a s i t e s t h e s y n t h e s i s g e l s were r i c h i n s i l i c a : t h e Si02/A1203 r a t i o s i n t h e i n i t i a l g e l a n d f i n a l p r o d u c t were 5.0 a n d 3.0 f o r z e o l i t e X a n d 9.0 a n d 4.8 f o r z e o l i t e Y . Thus, i n c o n t r a s t t o z e o l i t e A , t h e a p p e a r a n c e o f f r e e s i l i c a t e i o n s was o b s e r v e d d u r i n g t h e c r y s t a l lization reaction. A n g e l 1 a n d F l a n k ( 9 ) o v s e r v e d some s i l i c a t e s p e c i e s w i t h b a n d s a r o u n d 450 a n d 800 cm- ( p r o b a b l y m o n o s i l i c a t e ) i n t h e s e p a r a t e d l i q u i d phase of t h e i r i n i t i a l z e o l i t e A s y n t h e s i s g e l , T h i s is p r e s u m a b l y d u e t o t h e s o d i u m s i l i c a t e , w h i c h t h e y u s e d a s a s i l i c a s o u r c e . We u s e d c o l l o i d a l s i l i c a s o l w h i c h d o e s n o t g i v e r i s e t o any s i g n a l i n t h e g e l . A p p a r e n t l y t h e c o n d e n s a t i o n o f ~ l ( 0 I - l ) ~a -n d t h e d e p o l y m e r i z e d , h y d r o x y l a t e d s i l i c a t e i o n s r e q u i r e s t h e same 2-3 h o u r s f o r a l l t h r e e z e o l i t e s . A f t e r t h i s i n d u c t i o n period t h e aluminate d i s a p p e a r s and, i n c a s e o f X and Y , f r e e s i l i c a t e i o n s a p p e a r i n s o l u t i o n . Schwochow a n d H e i n z e (31) a r g u e d t h a t t h e z e o l i t e s p e c i e s c r y s t a l l i z e d from s e p a r a t e l i q u i d p h a s e s d e p e n d s on t h e c o m p o s i t i o n o f t h e l i q u i d p h a s e a n d , a t c o n s t a n t c o m p o s i t i o n , a l s o on t h e s i z e o f t h e p o l y m e r i c s i l i c a t e i o n s . They c o n c l u d e d t h a t p o l y m e r i c i o n s promote t h e f o r m a t i o n o f a p h i l l i p s i t e t y p e phase o v e r a f a u j a s i t e t y p e a n d t h a t a t t h e time o f n u c l e a t i o n t h e d i s s o l v e d s i l i c a must b e p r e d o m i n a n t l y i n a monomeric s t a t e t o c r y s t a l l i z e a s f a u j a s i t e type structures.

A s t o t h e mechanism o f z e o l i t e , f o r m a t i o n , t h e Raman s p e c t r a s u p p o r t t h e s o l u t i o n t r a n s p o r t mechanism ( s e e l a t e r ) .

P a r t o f t h i s " l a t e " Raman d e t e c t a b i l i t y may b e d u e t o t h e l a r g e f l u o r e s c e n t background t h a t a l l s o l i d z e o l i t e s s a m p l e s g i v e rise t o . T h i s background h a s a l s o been o b s e r v e d by o t h e r a u t h o r s (28, 3 2 , 3 3 ) on t h e e x a m i n a t i o n o f m e t a l o x i d e m a t e r i a l s . They a l s o r e p o r t e d , a s we found i n o u r p r e s e n t work, t h a t p r o l o n g e d e x p o s u r e t o t h e l a s e r beam d e c r e a s e d t h e f l u o r e s c e n c e , i n g e n e r a l . However, e v e n t h i s r e d u c e d t h e f l u o r e s c e n c e h a r d l y s u f f i c i e n t l y f o r t h e o b s e r v a t i o n o f Raman s p e c t r a i n s e v e r a l s a m p l e s , e . g . , AO-A2,XO-X4, a n d YO-Y6. It h a s been s u g g e s t e d t h a t t h e c a u s e o f t h e e x c e s s i v e b a c k g r o u n d c o u l d be the f l u o r e s c e n c e d u e t o t r a n s i t i o n m e t a l i m p u r i t i e s , e s p e c i a l l y ~ e ~ cr3+ + , and Mn 2+ (13). Angel1 ( 2 8 ) o b t a i n e d a h i g h p u r i t y z e o l i t e Y which c o n t a i n e d less t h a n 1 7 ppm F e and showed much less f l u o r e s c e n c e . Q t h e r h i g h p u r i t y m a t e r i a l s ( z e o l i t e A and X ) g a v e r i s e t o a r e l a t i v e l y s t r o n g Raman s p e c t r u m . Thus, i t may v e r y well b e t h a t i n o u r c a s e some r e l e v a n t i n f o r m a t i o n was o b s c u r e d by f l u o r e s c e n c e . E g e r t o n e t a 1 ( 3 4 , 35) r e p o r t e d t h a t i n t h e c a s e o f s i l i c a a n d s i l i c a - a l u m i n a m a t e r i a l s t h e background f l u o r e s c e n c e c a n b e minimized by h e a t i n g t h e s a m p l e s i n oxygen a t h i g h e r t e m p e r a t u r e s (500°C). I t i s r e p o r t e d f o r z e o l i t e s , however, t h a t a c t i v a t i o n o f z e o l i t e s under s i m i l a r c o n d i t i o n s always r e s u l t s i n h i g h e r backgrounds ( 2 8 ) .

ON THE RAMAN DETECTABILITY AND IDENTIFICATION OF COMPLEX ALUMINOSILICATE SPECIES IN SOLUTION R e c e n t l y , Guth e t a 1 . ( 2 0 , 21) p u b l i s h e d some p a p e r s on a Raman i n v e s t i g a t i o n o f s t r o n g l y b a s i c (NaOH) s o l u t i o n s o f d i l u t e d sodium-l a l u m i n a t e and s i l i c a t e . They r e c o r d e d s p e c t r a i n t h e 580-680 cm r a n g e onlyLl I n a l u m i n a t e s o l u t i o n s , a n i n t e n s e p o l a r i z e d peak a t 623-625 c m was o b s e r v e d and t h e y a t t r i b u t e d t h i s peak t o t h e free A ~ ( O H ) ~ i- o n . Only t h i s band had s t r o n g enough Raman c r o s s s e c t i o n t o a l l o w e x p l o i t a t i o n w h e r e a s t h o s e o f t h e s i l i c a t e and a l u m i n a t e o r complex a n i o n s w e r e much t y o weak. I n t h e p r e s e n c e o f s i l i c a t e , t h e a l u m i n a t e band a t 6 2 3 cm- ( S i / A l = l ) d e c r e a s e d t o v a r y i n g e x t e n t s as we f o u n d , t h u s d e n o t i n g i n d i r e c t l y t h e f o r m a t i o n o f a l u m i n o s i l i c a t e s p e c i e s , as i n o u r c a s e w i t h z e o l i t e A. F o r s i l i c a t e / a l u m i n a t e m i x t u r e s w i t h Si/A1=6, Guth e t a l . ( 1 9 ) o b s e r v e d , b e s i d e s t h e d i s a p p e a r g n- c e o f t h e Al(OH)4- peak a t 623 cm-' a n d a d e c r e a s e o f t h e Si02(OH)2 peaks, the appearance of sm~ll p e a k s o r s h o u l d e r s . T h e i r most " i n t e n s e " peak was a t 577 cmYet, i n a l l t h e s e c a s e s t h e s p e c t r a a r e t o o p o o r t o c o n d u c t a s t r u c t u r a l s t u d y o f t h e s e complexes ( 2 0 ) . Again, t h i s might i n d i c a t e t h a t a l a r g e v a r i e t y a f p o l y m e r i c a l u m i n o s i l i c a t e complexes is formed, none o f them h a v i n g enough c o n c e n t r a t i o n o r e x i s t i n g f o r l o n g enough time t o be r e a s o n a b l y d e t e c t a b l e .

.

ON THE MECHANISM OF R I N G FORMATION

Meier ( 3 6 , 37) c l a s s i f i e d z e o l i t e s t r u c t u r e s a c c o r d i n g t o t h e s e c o n d a r y b u i l d i n g u n i t s (SBU). T h e s e u n i t s a r e b a s i c a l l y s i n g l e or d o u b l e 4 , 6 o r 8 r i n g s y s t e m s and more complex 4-1, 4-4-1 and 5-1 u n i t s . F o r s i m p l i f i c a t i o n we w i l l s u b d i v i d e them i n t o even-number membered r i n g s y s t e m s [ 4 , 6 , 81 and 5 r i n g s y s t e m s . The l a t t e r g r o u p may b e e x t e n d e d t o odd-number membered r i n g s y s t e m s 15 and 9 1 , i f we i n c l u d e t h e 9 r i n g systems, r e c e n t l y r e p o r t e d f o r l o v d a r i t e by M e r l i n o ( 3 8 ) . F o r s o l u t i o n s w i t h S i / A l > 5 , Guth e t a l . ( 2 0 ) s u g g e s t e d t h a t a r e l a t i v e f 3 - s t a b l e s p e c i e s e x i s t s as an i n t e r m e d i a t e , i.e. t h e A1(OSi03)4 s p e c i e s . Derouane e t a l . ( 1 7 ) a l s o p o s t u l a t e d t h i s s p e c i e s t o b e t h e s p e c i e s t r a n s p ~ r t e dt h r o u g h t h e l i q u i d p h a s e o f silicon-rich gels. We assume t h i s s p e c i e s w i l l b e formed a l r e a d y a t S i / A 1 = 4 i n s o l u t i o n . A t Si/Al > 4 t h e s o l u t e Al-species a r e completely s a t u r a t e d w i t h A1-0-51 b o n d s formed by t h e c o n d e n s a t i o n r e a c t i o n :

We p o s t u l a t e t h e r e m a i n i n g s i l i c a t e a n i o n s i n t h e s i l i c o n - r i c h m i x t u r e ( S i / A 1 > 4 1 , which h a v e n o t been u s e d f o r t h e c o n d e n s a t i o n l a , t o c o n d e n s e t o p o l y s i j f c- a t e s t r u c t u r e s which c a n e v e n t u a l l y r e a c t w i t h t h e A1(OSi03)4 s p e c i e s t o 5 r i n g systems:

-O,Si \

8

\

0, ,OSi / \ 0-

/O

OR, OR, \ I HOSi I

I

HOSi I OR,

OR,

OR, OR, R 1 t o R4 r e p r e s e n t g r o u p s r a n g i n g from h y d r o g e n t o more c o m p l i c a t e d S i - 0 - S i and 51-0-A1 n e t w o r k s .

A t S i / A l < 4 t h e s o l u t e A ~ ( o H ) ~s p- e c i e s a r e n o t c o m p l e t e l y s a t u r a t e d w i t h A1-0-Si b o n d s a n d t h e c o n d e n s a t i o n r e a c t i o n may f o r examplf be: 10 -

3SiO2(OH),*- + 3 0 H - + AI(OH),-

-

O,Si \

0 \

I

0'

+ 6 H,O (Ha)

Al

/ /

0

OH

O,Si S i n c e t h e m i x t u r e i s now r e l a t i v e l y r i c h i n A 1 we may h a v e o t h e r ( s o l i d o r s o l u t e ) polymeric aluminosilicate s p e c i e s r e a c t i n g with t h e s o l u t e u n s a t u r a t e d a l u r n i n o s i l i c a t e s p e c i e s from r e a c t i o n I I a , y i e l d i n g e v e n number r i n g s y s t e m s . F o r t h e 4 r i n g s y s t e m a n example may be:

0=03Si \

0-

HO

OR, \ 1 AIL 0R,

\ k

S i O -

/

+

.o'

/

-0-Si 4\ OR,

OR,

-

0-

0 Al

L'

'0

\

0R3 A h OR, 4'

+ 20H-

F o r t h e 6 and 8 r i n g s y s t e m s o n e c a n d e s i g n s i m i l a r r e a c t i o n s . The S i / A 1 r a t i o s i n t h e l i q u i d p h a s e o f t h e i n i t i a l g e l ( s e e T a b l e I ) w e r e m e a s u r e d t o b e 0 . 4 4 , 1.44 a n d 1 . 0 5 f o r t h e A , X a n d Y s y n t h e s i s g e l s r e s p e c t i v e l y , t h u s a l l < 4 and a l l o w i n g t y p e I I a condensations. R 1 , R2, R3, a n d R4 i n d i c a t e d i n r e a c t i o n I l b c a n r a n g e f r o m h y d r o g e n t o more c o m p l i c a t e d Si-O-Si a n d Si-O-A1 n e t w o r k s . T h i s means t h a t a l a r g e v a r i e t y o f c o m p l e x i o n s i s p r e s e n t i n t h e s o l u t i o n , e a c h s p e c i e s h a v i n g t o o low c o n c e n t r a t i o n d u e t o i t s g r o w t h t o l a r g e r s p e c i e s ( e . g . D-4-Rings f o r z e o l i t e A a n d D-6-Rings f o r z e o l i t e X/Y and p o s s i b l y l a r g e r s p e c i e s ) .

In our opinion t h i s variety of species i n solution prevents the o b s e r v a t i o n o f s p e c i f i c Rarnan p e a k s d u r i n g n u c l e a t i o n . E v e n t u a l l y t h e g r o w i n g s o l u t e s p e c i e s become v i a b l e c r y s t a l l i z a t i o n c e n t e r s , p r e c i p i t a t i n g f r o m s o l u t i o n . The p r e c i p i t a t e c o n s i s t s o f c r y s t a l s l a r g e enough t o b e Rarnan and X-ray d e t e c t a b l e , t h e s i g n a l s s h o w i n g up s i m u l t a n e o u s l y i n t h e c o u r s e o f c r y s t a l l i z a t i o n . ACKNOWLEDGEMENT T h a n k s a r e d u e t o P r o f . P.J. G e l l i n g s ( T w e n t e U n i v e r s i t y of T e c h n o l o g y , The N e t h e r l a n d s ) a n d P r o f . D . P . Shoemaker ( O r e g o n S t a t e U n i v e r s i t y , C o r v a l l i s , O r e g o n ) f o r t h e i r comments on t h e m a n u s c r i p t a n d t o E x x o n R e s e a r c h & E n g i n e e r i n g Co. f o r p e r m i s s i o n t o p u b l i s h t h i s p a p e r . Mrs. K. G e r t h i s a c k n o w l e d g e d f o r t y p i n g t h e manuscript.

REFERENCES 1. 2.

3. 4. 5. 6. 7. 8. 9. 10.

11. 12. 13. 14 15

16 17 18 19 20. 21.

22.

23, 24. 25. 26. 27. 28. 29.

101

Zhdanov, S.P., Advan. Chem. Ser. (1971) 20 B a r r e r , R.M., J.W. Baynham, F.W. B u l t i t u d e and W.M. Meier, J. Chem. Soc. 195 (1959) K e r r , G.T., J . p h y s . Chem. 70 (1966) 1047 K e r r , G.T., J. phys. Chern. 72 (1968) 1385 C u l f a z , A * , and L.B. Sand, Advan. Chem. Ser. 121 (1973) 140 Cournoyer, R.A., W.L, K r a n i c k and L.B. Sand, J . phys. Chem. 75 (1975) 1578 K a c i r e k , H., and H. Lechert, J. phys. Chem. 79 (1975) 1589 Freund, E.F., J. C r y s t a l Growth 34 (1976) 11A n g e l l , C.L., and W.H. F l a n k , i n - ~ o l e c u l a r Sieves 11" (Katzer. J.R., ed.), ACS Symposium s e r i e s 40 (1977) 194 T.C. Tasi, M.S. ~hen, 1 / 2 have an electric quadrupole moment which experiences a torque in e l e c t r i c field gradients caused by the crystal fields and bond electrons within a solid structure. Whereas from single crystals very detailed informations can be obtained about the field gradient tensor and the field distribution around the nucleus, these informations are for powders and especi-

Fig. 11. 2 9 ~ ispectra of faujasites with different s$/A~ ratios. The measured s p e c t r a are simulated by constructing structures from different amounts of the cubooctahedra shown in Fig. 10 ( 3 9 ) .

ally for complicated structures like zeolites rather poor. The e l e c t r i c field gradient tensor can be described by its value in the direction of the highest field inhomogeneity, called

L3

Fig. 12. Comparisons of two Na spectra of a dehydrated NaX sample with S F / A ~= 1.03 at different amplification and span with a computer simulated spectrum, using the data of model calculations of the electric field gradients at the different sites of the sodium ions.

"field gradient" and by the deviation from rotational symmetry perpendicular to this direction, which is described by a dimensionless parameter between 0 and 1 called "asymmetry parameter". A very careful single crystal experiment of natrolite has been published by PETCH and PENNINGTON (42). The authors showed that the A1 as well as the Na ions in the natrolite structure are in chemical equivalent positions and obtained very exact values for the field gradient tensors.

The absolute values for the field gradient at the sites of the A1 show that the AIOq tetrahedron is only weakly distorted. The field gradient at the Na sites is rather large compared with other sodium containing crystals. In a series of papers published by the author ( 4 3 - 4 6 ) the Na resonance and the resonances of other nuclei of the alkali group in faujasite and A zeolites have been measured. The results have been compared with model calculations of the electric field 23

gradient tensor. The aim of these investigations was to find a model suitable to describe the electric field distribution within the zeolite cavities, which is interesting for the explanation of the phenomena connected with the catalytic activity of these substances. In Fig. 12 two spectra of an X zeolite are compared with a 'calculated spectrum. It can be seen that in the calculated and in the measured spectra two peaks appear at lower fields the position of which agrees well with the position calculated for the S2 and the S ; sites. The strong intensity in the center of the line cannot be assigned exactly, but it must belong to the extra framework ions for which several different sites are reported (see e.g. the atlas of MORTIER ( 2 0 ) ) . The spectra can be explained by the assumption that these sites have varying field gradients and high asymmetry parameters, which seems to be plausible. For the calculation of the field gradients a point multipole model has been used. In this model at first the point charge contribution to the fields and field gradients at the sites of all ion's of a cubooctahedron was evaluated. With the obtained values, the strengths of the induced dipoles and quadrupoles were calculated for different polarizabilities of the oxygen ions and the contributions of these multipoles added to the point charge contribution. A similar model has been used by DEMPSEY (47) who has carried out the calculation self-consistent without the induced quadrupoles

.

The calculations have been done for different positions of the mentioned extra framework cations in the dehydrated zeolite and for different models with adsorbed molecules. In the fully hydrated zeolite NaX a single slightly asymmetric line can be observed which is appreciably narrower than the central line belonging to the transitions m = -1/2 +-) m = + 1 / 2 in the dehydrated zeolite. By measurements of pulsed experiments on Nay samples BASLER (48) has found that this resonance must be attribuked to S2 sites

with an adsorbed water molecule which changes its site rather quickly. The field gradients which can be obtained from this experiment for the S2 sites are in good agreement with the calculated values for the described configuration of a S2 Na ion with an attached water molecule. 27

BOSACEK et al. (49) have studied the A1 resonance in decationated samples of Y zeolite in comparison to the respective Nay, and samples with different degrees of cation exchange. The spectrum obtained in dehydrated Nay can be explained by the presence of sites with a distribution of field gradients and

and asymmetry parameters.

+

d-

When t h e Na i o n s a r e exchanged a g a i n s t NH4 i o n s and t h e samples a r e t r e a t e d a t 4 0 0 ° C OH groups a r e formed n e a r t h e A 1 i o n s and a l a r g e f i e l d g r a d i e n t i s c r e a t e d causing a v e r y broad and weak l i n e which cannot be observed any more. The formation of u l t r a s t a b i l i z e d z e o l i t e i n which a c e r t a i n amount of A 1 i o n s i s i n e x t r a l a t t i c e p o s i t i o n s i s s t u d i e d by complexing t h e s e i o n s by acetylacetone. The complex provides a r a t h e r symmetric surrounding of the A 1 nucleus and shows a narrow l i n e from which t h e number of these i o n s can be e a s i l y e v a l u a t e d . 4.6 Some S p e c i a l Questions Solved by Proton NMR Spectroscopy

I n t h i s s e c t i o n some problems of s p e c i a l s t r u c t u r a l i n v e s t i gations by p r o t o n resonance measurements s h a l l be d i s c u s s e d . The problems of s o r p t i o n and m o b i l i t y i n v e s t i g a t i o n s by p r o t o n NMR spectroscopy a r e beyond t h e scope of t h i s a r t i c l e . I n a paper of BASLER and MAIWALD ( 5 0 ) t h e OH groups i n t h e c a v i t i e s of A z e o l i t e has been s t u d i e d on a v a r i e t y of samples of d i f f e r e n t o r i g i n . These samples have been c a r e f u l l y checked f o r s t r u c t u r a l i n t e g r i t y by X-ray and s o r p t i o n measurements. A s h a s been observed a l s o f o r samples of f a u j a s i t e z e o l i t e s , t h e water molecules i n t h e supercages and t h e cubooctahedra can be q u a n t i t a t i v e l y s e p a r a t e d by NMR p u l s e experiments. The r e s p e c t i v e decay functions a f t e r a 90'-pulse a r e shown f o r a number of z e o l i t e s i n f i g . 13. The s h o r t decay belongs t o t h e water molecules i n s i d e t h e cubooctahedra and t h e long one t o t h e molecules i n t h e supercages. By a c a r e f u l a n a l y s i s of t h e temperature behaviour of both decay f u n c t i o n s it could be shown t h a t i n t h e s h o r t decay a t h i r d decay f u n c t i o n should be hidden w i t h almost t h e same decay t i m e , which could be a s s i g n e d t o A l ( 0 ~ ) o r s i m i l a r e n t i t i e s w i t h i n t h e 3 cubooctahedra. The amount of t h e s e e n t i t i e s i s d i s t i n c t l y dependent on t h e c o n d i t i o n s of t h e growth o f t h e samples. I t could be demonstrated t h a t o u t of 31 samples grown under d i f f e r e n t condit i o n s o r i n d i f f e r e n t l a b o r a t o r i e s no sample was without t h e s e aluminate i n c l u s i o n s .

S i m i l a r i n v e s t i g a t i o n s have been c a r r i e d o u t w i t h t h e z e o l i t e ( 5 1 ) . This s t r u c t u r e i s exc e l l e n t l y s u i t e d for s t u d i e s of h y d r o l y s i s p r o c e s s e s w i t h i n t h e s e large c a v i t i e s .

ZK 5 , c o n t a i n i n g o n l y l a r g e c a v i t i e s

D i r e c t l y a f t e r t h e removal of t h e o r g a n i c c a t i o n about two OH groups could be observed i n one c a v i t y , c a u s i n g an exchange of

protons w i t h t h e water molecules. This exchange e x p r e s s e s i t s e l f i n a minimum o f t h e tempera-

--

Nay

Fig. 13.

4W ppm Fe

k

ih---

Na Y 90 pp m F e

-

I

NaX

~agnetizationdecays after a 90°-pulse of a NMR pulse experiment on protons in different zeolites. The short decay belongs to water molecules inside the cubooctahedra, the long decay to the molecules inside the large cavities.

ture function of the transverse relaxation time as it is shown in fig, 14a. Treating the sample with 0.1 n NaOH the signal of the OH groups falls below the limit of detection and only one kind of protons can be observed in the magnetization decays as well as in the temperature functions of the relaxation times (fig. 14b). The absolute values of the relaxation time in the short component can be calculated assuming an interaction of the protons with the aluminium of the lattice taking the usual distances of an A ~ O Hbond. Finally, the problem of the mobility of the protons of the acid OH groups in the cavities of zeolites shall be discussed in short. This problem has been studied with NMR methods by MESTDAG et al. ( 5 2 ) , by FREUDE et al. ( 5 3 ) and by FREUDE and P F E I F E R ( 5 4 ) . FREUDE and PFEIFER have shown from the temperature dependence of the transverse relaxation times in decationized Y and.mordenite that the correlation times of the motion of the protons are dependent on the temperature of the pretreatment. A strong increase of the mobility with residual ammonia could be observed. A systematic study showed that the pyridinium ions have an increased mobility, too. The number of the mobile protons could be obtained by measuring the proton NMR relaxation after the sorption of deuterated pyridine. The absolute number of pyridinium ions could be also determined by 13c resonance measurements, where characteristic shifts can be observed in the spectra of the protonated and the nonprotonated species. The ratio of the number of the acid sites and the correlation

-- (H,Na)ZK-5 TI t o t a l :- (a) 0000 00 0 0

C

NaZK-5

o 0 0 O

rn

00%'

-

T2 H 2O

w w 7

TI t$o 0

8 0

%a

i (b) .2.0,.0

0

r

--

T2 H20 @a,

w

a

-.

L

I

o

-,

I

L

m

. -

e

E-

A T2OH

--

,r

Fig.

I

a

a

*

A

1

14. Temperature functions of t h e l o n g i t u d i n a l r e l a x a t i o n times T1 and t h e temperature functions of t h e nuclear r e l a x a t i o n times T I and T2.

a.

T2 of water (black c i r c l e s ) , T2 of OH groups (triangles) T of a l l protons (open c i r c l e s ) of (H,Na)ZK 5 with 2 4 8 rng of water/g;

b.

T2 (black c i r c l e s ) and TI (open circles) of water i n NaZK 5 with 317 m g of water/g.

time of t h e mobility of i t s protons which can be regarded a s t h e r a t e of c r e a t i o n of f r e e o r protons a t t a c h e d t o a sorbed molecule i s a good measure f o r t h e a c i d i t y of t h e r e s p e c t i v e OH group.

4.7 The Application of Moessbauer Spectroscopy in Metal Containing Zeolites In zeolite research Moessbauer spectroscopy is applied mostly for the investigations of iron in its different oxidation states and local environments. The oxidation state may be seen from the shift of the observed lines and the environment expresses itself in a characteristic structure of the spectrum because the 5 7 ~ enucleus usually applied for these investigations has an electric quadrupole moment which may interact with gradients of the electric field of its surroundings as it has been discussed in connection with the NMR experiments with quadrupolar nuclei. Furthermore, magnetic ordering in a sample can be detected because the 5 7 ~ enucleus has the spin 3 / 2 . Investigations of this kind have received considerable attention because of their importance in adsorption and catalysis. The different oxidation states of the iron and especially the conditions of the existence of the zerovalent iron inside the cavities has become important for the preparation of catalysts for hydrogenation reactions. The characteristic argumentation shall be shown from results obtained by SCHMIDT ( 5 5 ) . Fig. 15 shows a Moessbauer spectrum of a Y zeolite which has been exchanged under very careful pH conditions under nitrogen with ferrous sulphate to prevent the formation of ferric hydroxide. The samples were partly reduced, by exposing them to Na vapour at 673 K. The central doublet is due to iron clusters, which can be shown to be superparamagnetic by magnetic measurements. This superparamagnetism expresses itself in the weak six peak component in the spectrum. The results obtained from these investigations have been confirmed by electron microscopic experiments. Numerous investigations by Moessbauer spectroscopy have been reported by REES et al. (56, 57) and by DELGASS et al. ( 5 8 ) .

4.8 Further Application of Spectroscopic Methods In this section some further spectroscopic methods shall be mentioned in short which have been applied to solve special problems in zeolite research which are not typical characterization problems.

A special field of research in zeolite chemistry is the investigation of transition metals and transition metal complexes. The transition metal complexes have been proposed since a long time as reactive intermediates in heterogeneous catalysis. Inspite

.

-8.0

-4.8

I. 6

-1.6

4.8

8.0

velocity [ mm/sec] 0

2+

Fig. 15. Moessbauer spectrum of a Fe - Y / F ~ -Y z e o l i t e (a) at 298 K and its computer fit (b).

Fig. 16. ESR powder absorption spectrum for an axially symmetric g-tensor (a) and its derivative (b). of the great importance comparatively few work has been done in this field. Some of the most important papers have been published by LUNSFORD (59). The theoretical background has been summarized by KASAI and BISHOP (60). The shape of an electron spin resonance ( E S R ) spectrum is mainly determined by the g-tensor and the hyperfine coupling tensor. The g-tensor measures the deviation of the g-value from the value ge of the free electron caused by a spinorbit coupling which is dependent on the direction in the crystal. Because zeolites are usually not available in single crystals which are large enough to measure this tensor only the socalled powder patterns are usually observed. Fig. 16 shows a powder pattern of an axially symmetric g-tensor, and its derivative which is measured in the ESR technique. The hyperfine splitting is caused by an interaction with the nuclear spins in the neighbourhood of the electronic spin. In the usual practice the parameters of the g-tensor and the hyperfine splitting are taken approximately

Fig.

17. ESR The 1 4 due

f

spectrum o f NO adsorbed on Cu -Y z e o l i t e . s p l i t t i n g i s due t o t h e h y p e r f i n e c o u p l i n g w i t h t h e s~p i n . The d i s t u r b a n c e i n t h e c e n t r a l p a -r t i s p o s s i b l y t o t h e p r e s e n c e of a s m a l l amount of c u 2 + i o n s .

from t h e measured spectrum and t h e n a computer f i t i s made t o f i n d the e x a c t v a l u e s , and t o d e c i d e whether a d d i t i o n a l i n t e r a c t i o n s have t o be t a k e n i n t o a c c o u n t . A t y p i c a l spectrum i s shown i n f i g . 1 7 w i t h NO sorbed on Cu(1)Y. The spectrum i s s i m i l a r t o t h a t shown by KASAI and B I S H O P ( 6 0 ) . The s p l i t t i n g i s due t o t h e i n t e r a c t i o n w i t h t h e 1 4 n~u c l e u s .

G e n e r a l l y from t h e symmetry o f t h e g - t e n s o r c o n c l u s i o n s on t h e symmetry o f t h e c r y s t a l f i e l d a t t h e s i t e o f t h e c a t i o n o r o f the s o r p t i o n complex can be drawn, which can be r e f i n e d by t h e u s e of t h e d a t a from t h e h y p e r f i n e i n t e r a c t i o n . The t r a n s i t i o n m e t a l c a t i o n s and a l s o t h e s o r p t i o n complexes have been s t u d i e d a l s o by o p t i c a l s p e c t r o s c o p y i n t h e r e f l e c t a n c e technique g i v i n g a l s o i n f o r m a t i o n on t h e c r y s t a l l i n e o r t h e l i g a n d f i e l d a t t h e s i t e of t h e i o n . The b a s i c problems of t h i s f i e l d have been summarized by K L I E R and KELLERMANN ( 6 1 ) . A r e c e n t comp a r i s o n o f a l l methods on t h e rhodium complexes i n z e a l i t e s h a s

been made by LUNSFORD ( 6 2 ) . Some papers on t h e valence s t a t e of t r a n s i t i o n m e t a l s i n zeol i t e s can be found by t h e X P S method ( 6 2 ) .

5. THE CHARACTERIZATION BY ADSORPTION OF SPECIFIC PROBE MOLECULES Sorption p r o c e s s e s w i l l be t r e a t e d i n a s e p a r a t e l e c t u r e of t h i s c o u r s e . Therefore, i n t h i s c h a p t e r o n l y some s o r p t i o n e x p e r i ments s h a l l be mentioned, which may be used a s a means of charact e r i z a t i o n of a s p e c i f i c z e o l i t e . A f i r s t information on t h e pore volume can be o b t a i n e d from t h e s o r p t i o n c a p a c i t y f o r water. The amount of l a r g e r p o r e s may be determined by t h e s o r p t i o n c a p a c i t y f o r l a r g e r molecules which cann o t e n t e r e , g . t h e cubooctahedra of f a u j a s i t e . A molecule o f t e n used f o r t h e s e purposes i s cyclohexane.

Sorption c a p a c i t i e s a r e u s u a l l y determined by balance t e c h n i ques o r f o r gaseous components volumetric. Another p o s s i b i l i t y i s t h e g a s chromatographic method. The accuracy o f such a determinat i o n i s mostly b e t t e r than one p e r c e n t . Thus, t h e s o r p t i o n capacit i e s a r e o f t e n used t o c h a r a c t e r i z e t h e p u r i t y e . g . of a s y n t h e t i c z e o l i t e from i n c l u s i o n s of s i l i c a o r o t h e r d i s t u r b a n c e s of t h e structure. I n e a r l i e r times t h e z e o l i t e s have been c l a s s i f i e d according t o t h e i r a b i l i t y t o adsorb o r t o exclude molecules of a p a r t i c u l a r s i z e . D e t a i l s a r e summarized i n t h e book of BRECK ( 1 0 ) . The s o r p t i o n isotherms c h a r a c t e r i z i n g t h e s o r p t i o n behaviour f o r any s p e c i f i c s u b s t a n c e can be measured i n more d e t a i l by t h e same methods a s t h e s o r p t i o n c a p a c i t i e s . The balance technique can be regarded a s having t h e h i g h e s t a c c u r a c i e s , where e s p e c i a l l y a l s o d i f f u s i o n d a t a can be obtained o b s e r v i n g t h e e s t a b l i s h m e n t of t h e equilibrium. Very conveniently t h e isotherms can be measured by g a s chromatographic s t e p o r p u l s e methods, where t h e c o n c e n t r a t i o n of t h e s o r b a t e i n a g a s stream over t h e z e o l i t e i s changed i n a s t e p o r a p u l s e of t h e substance i s i n j e c t e d i n t o t h e stream of a c a r r i e r g a s . From t h e response f u n c t i o n t h e isotherm can be obtained. For a p u l s e t h e isotherm may be c a l c u l a t e d e.g. by t h e r e l a t i o n s given by HUBER and GERRITSE (63)

.

The g a s chromatographic method i s p r e f e r a b l e i f isotherms o v e r a wide range o f e q u i l i b r i u m p r e s s u r e s a r e needed. E s p e c i a l l y f o r l i q u i d s low p a r t i a l p r e s s u r e s a r e d i f f i c u l t t o m a i n t a i n exactl y o v e r a long time which i s necessary i f t h e isotherms a r e meas-

ured by the balance. From the isotherms at different temperatures the heats and entropies of sorption may be evaluated giving information on the strength of the interaction of the molecules with the cavity system and among themselves. Special effects have to be expected, if reactions inside the cavities take place, as this is the case e.g. at sorption processes of bases in zeolite cavities with acidic groups. An example will be discussed in connection with the thennoanalytical methods.

6. CHARACTERIZATION OF ZEOLITES BY THERMAL ANALYSIS

Thermoanalytical methods are among the most important tools of the characterization of zeolites. Generally, thermal analysis describes a group of methods whereby the dependence of the parameters of any physical property of a substance on temperature is measured. The two techniques measuring the change of heat and the change of weight are the methods used preferably for the characterization of zeolite properties. These methods are called differential thermal analysis (DTA) and thermogravimetric analysis (TGA) . In both methods the sample and possibly a reference sample are heated or cooled at a controlled rate. In the DTA technique the difference in temperature between a substance and a reference material against either time or temperature is recorded. If any heat releasing or heat consuming process takes place in the sample, the temperature of the sample increases or remains behind the temperature of the reference. If the process is finished the temperatures of both specimen become equal again. The peak, obtained in the recorded curve can be evaluated to get the kinetics and the amound of the heat transfer. In the thermogravimetric analysis the weight of the sample is recorded in dependence on the temperature. In modern devices both principles of measurement are often realized in one apparatus. The DTA method has a sensitivity of about lo-* Joule. With the TGA method weight changes of about 10-8 g can be detected. A summary of the most important effects observed in zeolites has been recently published by DIMITROV et.al. (64).

temperature [ K ] Fig. 18, DTA curves in the region of the dehydration for A zeolite with different cations.

In the typical behaviour of a zeolite being heated and subjected to differential thermal analysis three typical regions can be distinguished. The first region begins slightly above room temperature, has its maximum mostly near 500 K and is finished at about 750 K. This region expresses itself as an endotherm in the DTA curves and is caused by the evolution of water and possibly other volatile substances in the zeolite cavities. Between about 900 and 1500 K often two exotherms can be observed which are associated with the collapse of the zeolite lattice and sometimes at much higher tempqratures recrystallization to a new phase.

In the first region often a stepwise evolution of water can be observed. Fig. 18 shows DTA curves for A zeolites containing different cations.

A LiA

900

3000 1100 1200 1300

temperature [ K ] Fig. 19. High temperature effects in the DTA curves of different A zeolites.

A detailed thennoanalytical study has been carried out by DYER and WILSON ( 6 5 ) on NaA, who measured beside the DTA curves thermogravimetric data which were compared with X-ray experiments looking at the sites of the cations and water molecules. According to these studies the first endothermic effect at about 395 K is connected with an evolution of 10 water molecules per unit cell which are bonded very loosely. A second peak at 438 K is connected with a loss of 8 water molecules, which have been sorbed at the sodium ions in the S1 positions. Of the remaining 10 molecules six are released at temperatures between 445 and 625 K. The four molecules inside the sodalite cavities leave the structure at higher temperatures.

Changing the cations characteristic changes in the DTA curves are observed which depend on the degree of the exchange and may often be explained by the typical complexes with these ions. The high temperature effects have been extensively studied by BERGER et al. (66). Some typical exothermic effects of A zeolites with different cations are shown in Fig. 19.

The process of the dehydration of faujasite zeolites is more complicated. This is especially also due to the fact that at higher temperature dehydroxylation occurs. Special attention has been devoted to the deammoniation and the formation of the hydrogen form. In dependence on the composition of the sample for the end of the deamrnoniation of various zeolites temperatures between 570 and 770 K are reported, so that the dea-oniation and the dehydroxylation often cannot be resolved. The process can only be studied exactly by analyzing the released substances simultaneously to the DTA measurement. Furthermore, in the high temperature effects influences of ultrastabilization can be detected. A summarizing article on this topic has been published by McDANIEL and MAHER ( 6 7 ) . For the two methods a large number of variations have been reported giving answer to questions different properties of zeolites and zeolite catalysts. Finally, a special method shall be mentioned with which centers of different acid strength can be characterized. The acid form of the zeolite in question is exposed to ammonia at temperatures near 500 K, where no more physical adsorption occurs and the sorbed ammonia is then desorbed by programmed heating at higher temperatures near 850 K detecting the amount in a TGA experiment. The temperature at which an evolution of ammonia is observed, is then a good measure of the acidity of the respective site. Although thermal analysis gives valuable information on a series of properties of zeolites and zeolite catalysts, it has not successfully provided a reproducible and standard method for measuring thermal properties because too many factors of the particular instrument and the conditions of the experiment influence the measured parameters. ~lthoughit is often quite useful for direct comparisons, it is difficult to compare results reported by different authors.

References

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4 2 . P e t c h , H.E. and K.S. P e n n i n g t o n , J o u r n a l o f Chemical P h y s i c s 36 (1962) 1216 43. L e c h e r t , H., Ber. d . Bunsenges. p h y s i k . Chemie 77 ( 1 9 7 3 ) 697 44. L e c h e r t , H. and H. Benneke, B e r . d . Bunsenges. p h y s i k . Chemie 78 (1974) 347 45. L e c h e r t , H . , C a t a l y s i s Reviews S c i . - E n g . 14 ( 1 ) (1976) 1 46. L e c h e r t , H. and H . Henneke, 2 3 ~ a - ~ e s o n a n cien Z e o l i t e s of t h e A-Type and i t s I n t e r p r e t a t i o n by Computer S i m u l a t i o n of t h e Measured S p e c t r a , i n M o l e c u l a r S i e v e s -11 ACS Symposium, S e r i e s 4 0 , J.R. K a t z e r , e d . ( A m e r . Chem. Soc. Washington D . C . 1977) p . 53 47. Dempsey, E., J o u r n a l o f P h y s i c a l C h e m i s t r y 73 ( 1 9 6 8 ) 3660 48. B a s l e r , W . D . , Z e i t s c h r . f . N a t u r f o r s c h u n g 35a ( 1 9 8 0 ) 645 49. Bosgcek, V . , D . F r e u d e , T . F r o h l i c h , H . P f e i f e r and H. Schrnied e l , J o u r n a l o f C o l l o i d a n d I n t e r f a c e S c i e n c e 85 (1982) 502 50. B a s l e r , W.D. and W . Maiwald, J o u r n a l o f P h y s i c a l C h e m i s t r y 83 (1979) 2148 J o u r n a l o f P h y s i c a l C h e m i s t r v 81 (1977) 2102 51. B a s l e r , W . D . , 52. Mestdagh, H . H . , W.E.Stone a n d J . J . ~ r i p i a t , ~ o u r n a lo f p h y s i c a l C h e m i s t r y 76 ( 1 9 7 2 ) 1226 53. F r e u d e , D., W. O e h m e , H. S c h m i e d e l and B. S t a u d t e , J o u r n a l of C a t a l y s i s 49 ( 1 9 7 7 ) 123 54. F r e u d e , D . a n d H . P f e i f e r , NMR S t u d i e s C o n c e r n i n g ~ r d n s t e d t A c i d i t y of Z e o l i t e s , i n P r o c e e d i n g s o f t h e 5 t h I n t e r n a t i o n a l C o n f e r e n c e on Z e o l i t e s , L . V . C . R e e s , e d . (Heyden, London 1980) p. 732 55. S c h m i d t , F . , W. G u n s s e r and J . Adolph, F o r m a t i o n o f I r o n C l u s t e r s i n Z e o l i t e s w i t h D i f f e r e n t S u p e r c a g e S i z e s , i n Molec u l a r S i e v e s 11, ASC Symposium S e r i e s 4 0 , J.R. K a t z e r , e d . (Arner. Cherp. Sac. Washington D . C . 1977) p. 56. R e e s , L.V.C., Moessbauer S p e c t r o s c o p i c S t u d i e s o f F e r r o u s I o n Exchange i n Z e o l i t e A , i n S t u d i e s i n S u r f a c e S c i e n c e and C a t a l y s i s , V o l . 1 2 , M e t a l M i c r o s t r u c t u r e s i n Z e o l i t e s , P.A. J a c o b s , N . I . J a e g e r , P. J i r u a n d G . S c h u l z - E k l o f f , e d s . ( E l s e v i e r Publ. Comp., Amsterdam, O x f o r d , New York 1982) p. 33 57. D i c k s o n , B.L. and L.V.C. R e e s , J o u r n . Chem. Soc. F a r a d a y T r a n s a c t i o n s I 70 (1974) 2 0 3 8 58. D e l g a s s , W.N., R.L. G a r t e n and M . B o u d a r t , J o u r n a l o f Chemical P h y s i c s 50 ( 1 9 6 9 ) 4603 59. L u n s f o r d , J . H . , T r a n s i t i o n M e t a l Complexes i n Z e o l i t e s , i n Mol e c u l a r S i e v e s 11, ACS Symposium S e r i e s 4 0 , J . R . K a t z e r , e d . ( A m e r . Chem. S a c . , Washington D . C . 1977) p. 473 60. K a s a i , P.H. and R.J. B i s h o p , E l e c t r o n S p i n Resonance S t u d i e s o f Z e o l i t e s , i n J , A . Rabo, 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 , ACS Monograph 171 (Amer. Chem. S o c . , Washington D . C . 1976) p. 350 61. K e l l e r m a n n , R . a n d K . K l i e r , I n t r a z e o l i t i c T r a n s i t i o n - m e t a l I o n Complexes, i n S p e c i a l i s t P e r i o d i c a l R e p o r t s , S u r f a c e and D e f e c t P r o p e r t i e s o f S o l i d s - V o l . 4 (The Chemical S o c . , B u r l i n g t o n House, London, W1V OBN 1975) p . 1

62. Lunsford, J.H., The Chemistry of Ruthenium in Zeolites, in Studies in Surface Science and Catalysis, Vol. 12, Metal Microstructures in Zeolites, P.A. Jacobs, N.I. Jaeger, P. Jiru and G. Schulz-Ekloff, eds. (Elsevier Publ. Comp., Amsterdam, Oxford, New York 1982) p. 1 63. Huber, J.F.K. and G. Gerritse, Journal of Chromatography 58 (1971) 137 64. Dimitrov, Ch., Z. Popova, S. Mladenov, K.-H. Steinberg and H. Siegel, Zeitschrift fur Chemie 21 (1981) 387 65. Dyer, A. and M.J. Wilson, Thermochimica Acta 5 (1973) 91 66. Berger, A.S., T.I. Samsonova and L.K. Jakovlev, Izvest. Akad. Nauk USSR, Ser. Chim. (1971) 2129 67. McDaniel, C.V. and P.K. Maher, Zeolite Stability and Ultrastable Zeolites, in J.A. Rabo, Zeolite Chemistry and Catalysis, ACS Monograph 171 (Arner. Chem. Soc. Washington D.C. 1976) p. 285

STRUCTURAL CHARACTERIZATION OF ZEOLITES BY HIGH RESOLUTION MAGIC-ANGLE-SPINNING SOLID STATE 2 9 ~ i SPECTROSCOPY - ~ ~ ~

a a a Z e l i m i r G a b e l i c a , J a n o s 3.Nagy , P h i l i p p e B o d a r t Guy Debras a , E r i c G . Derouane a , C and P e t e r A . J a c o b 's b a - F a c u l t & U n i v e r s i t a i r e s de Namur L a b o r a t o i r e de C a t a l y s e , Rue de B r u x e l l e s , 61 B-5000 Namur, Belgium b-Centrum v o o r O p p e r v l a k t e s c h e i k u n d e e n C o l l o i d a l e Scheikunde, K a t h o l i e k e U n i v e r s i t e i t Leuven, D e C r o y l a a n , 4 2 , B-3030 Leuven ( H e v e r l e e ) Belgium c-Present address : Mobil T e c h n i c a l C e n t e r , C e n t r a l Research D i v i s i o n , P.O. Box 1025, P r i n c e t o n , N J 08540, U.S.A. ABSTRACT The s t r u c t u r e o f v a r i o u s n a t u r a l and s y n t h e t i c z e o l i t e s was i n v e s t i g a t e d by h i g h r e s o l u t i o n magic-angle-spinning (HRMAS) o s c~ o p y~ . The NMR a n a l y s i s i s b a s e d on s o l i d s t a t e 2 9 ~ sip e-c t r~ the g r e a t s e n s i t i v i t y of t h e 2 9 ~ i - c h e m i c a l s h i f t s e i t h e r on t h e chemical environment ( i . e . t h e number o f aluminium atoms i n t h e second c o o r d i n a t i o n s h e l l of s i l i c o n ) , on t h e c r y s t a l symmetry ( i . e . t h e number of c r y s t a l l o g r a p h i c a l l y d i f f e r e n t s i t e s ) o r on local s t r a i n s i n the c r y s t a l . Silicon-aluminium o r d e r i n g s c o u l d b e d e s c r i b e d i n z e o l i t e s such a s f a u j a s i t e s , m o r d e n i t e , f e r r i e r i t e , ZSM-5, ZSM-8, ZSM-11, ZSM-39 and ZSM-48 by examining s y s t e m a t i c a l l y e i t h e r p r o g r e s s i v e ly d e a l u m i n a t e d samples o r s y n t h e t i c z e o l i t e s w i t h d i f f e r e n t S i / A l r a t i o s . The S i / A l r a t i o s were d e t e r m i n e d from t h e r e l a t i v e l i n e i n t e n s i t i e s . It i s concluded t h a t aluminium atoms occupy p r e f e r e n t i a l l y s p e c i f i c s i t e s i n ZSM-5, ZSM-11, m o r d e n i t e , f e r r i e ZSM-8 and ZSM-48 z e o l i rite,ZSM-35 and p r o b a b l y a l s o i n ZSM-39. t e s a r e shown t o c o n t a i n combined 4- and 5- membered r i n g s forming layer sequences s i m i l a r t o those c h a r a c t e r i z i n g p e n t a s i l s t r u c t u r e s . M R a l s o used t o follow t h e d i f f e r e n t F i n a l l y , HRMAS ~ ~ S ~ - N was silicon-aluminium arrangements t h a t occur d u r i n g p r o g r e s s i v e t r a n s f o r m a t i o n s of amorphous g e l s i n t o c r y s t a l l i n e ZSM-5 p h a s e s .

1 . INTRODUCTION The h i g h r e s o l u t i o n magic-angle-spinning (HRMAS) s o l i d s t a t e NMR h a s become a p o w e r f u l t o o l f o r d e t a i l e d s t r u c t u r a l i n v e s t i g a t i o n s of s o l i d z e o l i t i c m a t e r i a l s . S h o r t - r a n g e s i l i c o n - a l u m i n i u m framework o r d e r i n g s w i t h i n c r y s t a l l o g r a p h i c a l l y n o n - e q u i v a l e n t s i t e s of v a r i o u s n a t u r a l and s y n t h e t i c A l - r i c h z e o l i t e s have been r e s o l v e d (1-9). The p o s i t i o n of t h e 2 9 ~ i r-e s~o n a~n c~e e s s e n t i a l l y depends on t h e number of t e t r a h e d r a l A 1 atoms i n t h e second c o o r d i n a t i o n s p h e r e o f S i ( 1 0 ) . An i n t e r v a l of a b o u t 5 ppm u s u a l l y separ a t e s two n e i g h b o u r i n g NMR r e s o n a n c e s b e l o n g i n g t o each Si(nA1) c o n f i g u r a t i o n . The a s s i g n m e n t of t h e l i n e s t o t h e v a r i o u s Si-A1 o r d e r i n g s i s u s u a l l y a c h i e v e d by f o l l o w i n g t h e i r i n t e n s i t y v a r i a t i o n s f o r d i f f e r e n t dealuminated m a t e r i a l s . Such a p r o c e d u r e was s u c c e s s f u l l y a p p l i e d i n t h e c a s e o f f a u j a s i t e (11-13), m o r d e n i t e ( 9 ) and p e n t a s i l m a t e r i a l s ( 1 4 ) . The p e r c e n t a g e of e a c h Si(nA1) u n i t i n t h e s t r u c t u r e c a n b e e v a l u a t e d by measuring t h e corresponding l i n e i n t e n s i t y

.

Quantitative determination of S i / A l r a t i o s i n the z e o l i t e framework c a n a l s o b e d e r i v e d from NMR d a t a ( 4 , 6 ) . Recent s t u d i e s have shown t h a t t h e c h e m i c a l s h i f t s which b e l~ o n g~ ing t o d e f i n e d Si(nA1) c h a r a c t e r i z e t h e 2 9 ~ lii n-e s ~ conf i g u r a t i o n s were a l s o i n f l u e n c e d by t h e a c t u a l geometry of t h e T-0-T l i n k a g e s (T = S i o r A l ) o r by l o n g e r range s t r u c t u r a l f e a t u r e s such as t h e (T-O)p r i n g s i z e s ( 1 5 , 1 6 ) . T h i s was t u r n e d t o account a d v a n t a g e o u s l y f o r t h e e f f e c t of t h e a c t u a l r i n g s t r u c t u r e and s t r a i n s t h a t o c c u r i n h i g h s i l i c e o u s z e o l i t e s (ZSM-5, ZSM-11, silicalite, .) ( 1 4 , 1 7 , 1 8 ) , where Si(OA1) c o n f i g u r a t i o n s a r e predominant.

..

I n a d d i t i o n , 2 9 ~ il i n-e s~b e~ l o n~ g i n g t o s i l a n o l groups (-Si-0-H) i n d e a l u m i n a t e d f a u j a s i t e s ( 1 2 ) and i n ZSM-5 m a t e r i a l s (1 9) w e r e unambiguously i d e n t i f i e d , u s i n g t h e c r o s s - p o l a r i z a t i o n technique. The p u r p o s e of t h e p r e s e n t p a p e r i s f i r s t t o show how t h e i n f o r m a t i o n s o b t a i n e d from t h e HRMAS s o l i d s t a t e 2 9 ~ cia n- be~ ~ ~ used i n r e s o l v i n g framework d i s t r i b u t i o n of Si-A1 atoms i n v a r i o u s z e o l i t i c s t r u c t u r e s . Secondly, t h i s t e c h n i q u e i s a l s o u s e d t o f o l l o w t h e p r o g r e s s i v e s t r u c t u r a l changes of t h e Si-A1 o r d e r i n g s d u r i n g t h e t r a n s f o r m a t i o n of amorphous g e l s i n t o c r y s t a l l i n e ZSM-5 phases.

2 2.1

EXPERIMENTAL Materials

Na-X and Na-Y z e o l i t e s were s y n t h e s i z e d by D r D . Barthomeuf (Lyon, F r a n c e ) . Mordenite, a Na-zeolon s a m p l e , was s u p p l i e d by t h e Norton Company. ZSM-5 and ZSM-11 were s y n t h e s i z e d a c c o r d i n g t o p u b l i s h e d methods ( 2 0 - 2 3 ) , u s i n g d i f f e r e n t S i / A l r a t i o s . The h i g h l y pure ZSM-5 z e o l i t e , p a r e n t m a t e r i a l f o r v a r i o u s c h e m i c a l modificat i o n s , was o b t a i n e d u s i n g d i l u t e d c o n d i t i o n s , a s d e s c r i b e d e a r l i e r ( 2 4 ) . P r o g r e s s i v e c r y s t a l l i z a t i o n of ZSM-5 z e o l i t e s was conducted f o l l o w i n g two d i f f e r e n t p r o c e d u r e s a s r e p o r t e d p r e v i o u s ly ( 2 1 , 2 5 , 2 6 ) . From e a c h p r o c e d u r e , i n t e r m e d i a t e p h a s e s w i t h i n c r e a s i n g d e g r e e s of c r y s t a l l i n i t y were i s o l a t e d and a n a l y z e d . ZSM-5 was o b t a i n e d from b o t h p a t e n t ( 2 7 ) and own-modified ( 2 2 ) recepies ZSM-39 was s y n t h e s i z e d a s d e s c r i b e d by Dwyer e t a1. ( 2 8 ) . The o t h e r z e o l i t e s : f e r r i e r i t e , ZSM-35 and ZSM-48 were o b t a i n e d by own s y n t h e s e s , a c c o r d i n g t o p r o c e d u r e s p u b l i s h e d i n t h e c u r r e n t or p a t e n t l i t e r a t u r e ( 2 9 ) .

.

The chemical c o m p o s i t i o n of each sample was d e t e r m i n e d by Proton Induced y-Ray Emission ( P I G E ) ( 3 5 ) , by Energy D i s p e r s i v e X-Ray A n a l y s i s (EDX) (23) o r by Atomic A b s o r p t i o n S p e c t r o s c o p y Water and o r g a n i c c o n t e n t s o f t h e p e n t a s i l m a t e r i a l s were (AAS) The s t r u c t u r a l i d e n measured by t h e r m a l methods (21-23,31,32). t i t y of a l l t h e z e o l i t e s was a s c e r t a i n e d by t h e c l a s s i c a l powder X-ray d i f f r a c t i o n method.

.

2.2

2 9 ~ measurements i - ~ ~ ~

The HRMAS *'S~-NMR s p e c t r a were o b t a i n e d a t room t e m p e r a t u r e on a Bruker CXP-200 s p e c t r o m e t e r o p e r a t i n g i n t h e F o u r i e r Transform mode, u s i n g a "one c y c l e " t y p e measurement. An r . f . - f i e l d o f 49.3 O e was used f o r t h e ~ / 2p u l s e s of 2 9 ~(uo i = 39.7 MHz) Magic-angle-spinning was a t a rate of 3.1 kHz. u s i n g D e l r i n c o n i c a l r o t o r s . Time i n t e r v a l s between p u l s e sequences were 3.0 s. 2,UUu t o 20,600 f r e e i n d u c t i o n decays w e r e accumulated p e r sample. Chemical s h i f t s ( 6 1 , i n ppm, were measured w i t h r e s y e t t o t e t r a n i e f h y l s i l a n e (TMS), used a s e x t e r n a l r e f e r e n c e .

.

3 3.1

RESULTS AND DISCUSSION Zeolites with crystallographically equivalent tetrahedral sites : faujasite

The s t r u c t u r e of f a u j a s i t e i s b u i l t up of c o r n e r s h a r i n g TO4 t e t r a h e d r a ( T = S i o r A l ) . 24 t e t r a h e d r a are j o i n e d t o form a cuboctahedron o r s o d a l i t e cage. Cages a r e s t r o k e d t e t r a h e d r a l l y

t o form a c u b i c diamond l a t t i c e . A l l t h e 1 ': s i t e s are crystallog r a p h i c a l l y (but n o t c h e m i c a l l y ) e q u i v a l e n t ( 3 3 ) . Five types of g r o u p i n g s can b e distinguished w i t h S i l i n k e d through oxygen t o 0 , 1 , 2 , 3 o r 4 A 1 atoms. I n t h e c a s e of f a u j a s i t e , t h e r e i s no chemical i s h i f t s f o r t h e above groupings and a overlap i n the 2 9 ~ s t r u c t u r a l model could b e developped t h a t s u c c e s s f u l l y e x p l a i n s t h e S i , A l framework d i s t r i b u t i o n , i n r e l a t i o n w i t h t h e NMR d a t a ,over a wide range of c o m p o s i t i o n s . Provided t h e Lowenstein r u l e i s obeyed i n t h e z e o l i t e , i . e . no A1-0-A1 l i n k a g e s a r e p r e s e n t , t h e Si/A1 r a t i o of t h e f a u j a s i t e l a t t i c e can be e s t i m a t e d from t h e r e l a t i v e s i g n a l i n t e n s i t i e s of 29si (4) :

It can be used a s an e x c e l l e n t q u a n t i t a t i v e method of measuring z e o l i t e composition i n d e p e n d e n t l y of t h e e l e m e n t a l chemical anal y s e s . A non-coincidence between NMR Si/A1 r a t i o s and t h o s e obt a i n e d by chemical methods i n d i c a t e s t h a t non-framework A 1 o r S i a r e p r e s e n t (34)

.

D e t a i l e d s t r u c t u r a l models and S i , A l o r d e r i n g s i n f a u j a s i t e have been developped and d i s c u s s e d i n many r e c e n t p a p e r s (1-8, 10-13,16,35-37). F i g u r e 1 shows examples of HRMAS s p e c t r a of t h r e e f a u j a s i t e s , a N a X z e o l i t e w i t h S i / A 1 = 1.25 and two Nay zeol i t e s w i t h Si/A1 = 1.82 and 2.4. The i n t e n s i t i e s of t h e NMR l i n e s c h a r a c t e r i z i n g t h e f i v e Si(nA1) ( n = 0 , 1 , 2 , 3 , 4 ) c o n f i g u r a t i o n s a r e computed i n Table 1 Good agreement i s o b t a i n e d between t h e ( S i / A l ) r a t i o s a s c a l c u l a t e d u s i n g e q u a t i o n [ l ] and t h o s e measured by t h e EDX t e c h n i q u e .

.

TABLE 1 2 9 ~ i c-h a~r a ~ c t e~r i s t i c s of f a u j a s i t e s Si'A1

(EDX)

R e l a t i v e p o p u l a t i o n s of v a r i o u s Si(nA1) con£i g u r a t i o n s , i n % (6 i n pprn from TMS)

Si/A1

(NMR)

Si(4A1) Si(3A1) Si(2A1) S i ( l A 1 ) Si(OA1) (-84.6) (-89.1) (-94.5) (-99.5) (-103.4)

Na-X

1.25

53.8

26.5

12.2

5 .O

2.5

1 .23

Na-Y

1.82

15.9

31.4

32.7

15.5

4 ,5

1.7

Na-Y

2.4

2.9

15.0

39.4

37.3

5.4

2.3

b(oom) I TMS

Figure 1 .

3.2

HRMAS 2 9 ~ si p e-c t~ r a of ~ t~ hree faujasite zeolites.

Z e o l i t e s with c r y s t a l l o g r a p h i c a l l y non-equivalent t e t r a h e d r a l sites

I n z e o l i t e s whose s t r u c t u r e s e x h i b i t d i f f e r e n t framework topol o g i e s , some o v e r l a p i n t h e ~ ~ S ~ - N lM i nRe s c h a r a c t e r i z i n g v a r i o u s Si(nA1) c o n f i g u r a t i o n s c a n o c c u r b e c a u s e t h e c h e m i c a l s h i f t s of a n NMR r e s o n a n c e b e l o n g i n g t o a g i v e n c o n f i g u r a t i o n does a l s o depend on t h e a c t u a l symmetry of t h a t Si(nA1) s i t e a n d / o r on t h e s i z e of t h e (T-0)p r i n g t o which t h a t c o n f i g u r a t i o n b e l o n g s . T y p i c a l examples are e n c o u n t e r e d i n nlordenite ( 9 , 3 7 ) , p e n t a s i l z e o l i t e s ( 1 5 , 1 6 , 1 9 ) and ZSM-39 ( 3 8 ) z e o l i t e . I n s u c h c a s e s , t h e unambiguous a t t r i b u t i o n of a n NMR l i n e w i t h i n the o v e r l a p r e g i o n becomes h i g h l y u n c e r t a i n . T h e r e f o r e , v a r i o u s e x p e r i m e n t a l p r o c e d u r e s a r e developped t o d e r i v e t h e a c t u a l S i , A1 o r d e r i n g s i n z e o l i t i c frameworks from 2 9 ~ i - ~ M R s p e c t r a : p r o g r e s s i v e d e a l u r n i n a t i o n of t h e sample o r s y n t h e s i s of t h e z e o l i t e w i t h v a r i o u s ~ i / A lc o n t e n t s .

3 . 2 . 1 . Mordenite g r p u p . A common f e a t u r e o f t h e z e o l i t e frame...................... works b e l o n g i n g t o t h e m o r d e n i t e group i s t h e e x i s t e n c e of 6 - r i n g s h e e t s . I n mordenite, t h e s e a r e l i n k e d t o each o t h e r through s i n g l e

4-membered rings.

r i n g s and i n f e r r i e r i t e

,

t h r o u g h s i n g l e 6-membered

S i n g l e c r y s t a l X-ray r e f i n e m e n t of m o r d e n i t e (39) h a s suggest e d t h a t t h e A 1 atoms must b e l o c a t e d on t h e 4-membered r i n g s . Using t h a t p r o p o s a l , the t h e o r e t i c a l d i s t r i b u t i o n o f t h e A 1 atoms w i t h i n t h e m o r d e n i t e s t r u c t u r e can b e c a l c u l a t e d f o r v a r i o u s Si/AI r a t i o s . The s o o b t a i n e d t o t a l number of Si(nA1) c o n f i g u r a t i o n s d i s t r i b u t e d w i t h i n one u n i t c e l l c a n b e computed and t h e correspond i n g t h e o r e t i c a l . NMR l i n e i n t e n s i t i e s compared t o t h e e x p e r i m e n t a l data. The H W S 2 9 ~ si p e-c t~ ra ~ of Na-mordenite ~ ( S i / A l = 5 . 5 ) shows t h r e e l i n e s a t -95, -105 and -110 ppm ( F i g . 2 ) , which a r e a t t r i b u t e d r e s p e c t i v e l y t o t e t r a h e d r a l a r r a n g e m e n t s of Si(2A1) , ~ i ( l A 1 ) and Si(OA1). The p a r a l l e l d e c r e a s e o f t h e f i r s t two l i n e s and t h e i n c r e a s e of t h a t a t -1 10 ppm ,with p r o g r e s s i v e d e a l u r n i n a t i o n of the p a r e n t sample ( F i g . 2 ) , c o r r e s p o n d s t o t h e t h e o r e t i c a l p r e d i c t i o n and c o n f i r m s t h e a s s i g n m e n t . Q u a n t i t a t i v e d a t a a r e g i v e n i n T a b l e 2. A more complete NMR s t u d y of m o r d e n i t e and i t s dealumin a t e d forms i s p r e s e n t e d e l s e w h e r e ( 9 ) .

TABLE 2 HRMAS 2 9 ~ cih a-r a ~ cte~ r i z a~ t i o n of v a r i o u s m o r d e n i t e samples

Sample

Si/Al (AAS)

Na-Mor

5.5

Relative l i n e i n t e n s i t i e s (2) (a) C o n f i g u r a t i o n : Si(2A1) 6/ppm : -95 13.2

Si(lA1) -105

Si(OA1) -1 1 0

Si/Al (NMR)

44.7

42.1

5.4

H-MorlCb) 20.5

0

29.8

70.2

21.6

H - M o ~ ~)( ' 3 1 . 2

0

22.4

77.6

30.7

( a ) Normalized t o 100 f o r Na-Mor ( b ) Na-Mor, l e a c h e d w i t h 4 M HNO3 a t 90°c f o r 24h ( c ) Na-Mor, l e a c h e d w i t h 14M HNO3 a t 90°c f o r 24h.

The HRMAS 2 9 ~ NMR i s p e c t r u m o f f e r r i e r i t e and of i t s S i - r i c h e r a n a l o g u e H-ZSM-35 shows t h r e e l i n e s l o c a t e d a t -101, -108 and -1 13 ppm. They a r e a t t r i b u t e d t o t h e ~ i ( 2 A 1 ) , Si(lA1) and S i ( 0 ~ 1 ) configurations respectively The f e r r i e r i t e u n i t c e l l c o n t a i n s two t y p e s o f t e t r a h e d r a l l y coord i n a t e d T atoms. The r e l a t i v e i n t e n s i t i e s o f t h e t h r e e l i n e s , a s e x p e r i m e n t a l l y measured on b o t h f e r r i e r i t e and H-ZSM-35, c o r r e s pond w e l l t o t h o s e e v a l u a t e d t h e o r e t i c a l l y f o r s i m i l a r ~ i / A 1rat i o s . F o r t h e l a t t e r , i t was a s s u m d t h a t A l atoms were s p e c i f i c a l l y l o c a t e d i n 6-membered r i n g s l i n k i n g one d i m e n s i o n a l s h e e t s

.

i n t h e s t r u c t u r e ( s i t e s T I ) ( 4 0 ) , and t h a t a 6-membered r i n g w i t h one s i n g l e Alatom i s more s t a b l e than one c o n t a i n i n g two A 1 a t o m s . A s i m i l a r t h e o r e t i c a l c o m p u t a t i o n , a s s u m i n g t h e A 1 atoms prefer e n t i a l l y l o c a t e d i n 6-membered r i n g s o f t h e s h e e t s , a t a maximum d i s t a n c e from each o t h e r ( s i t e s T 2 ) , d o e s n o t f i t t h e e x p e r i m e n t a l d a t a (Table 3 ) .

-90

-130

- 1 10

b (PP~) Figure 2 .

HRMAS 2 9 ~ of i m o~ r d e~ n i t e~ s : e f f e c t of d e a l u m i n a t i o n .

TABLE 3 H W S 2 9 ~ i of - ~f e r~t i ~e r i t e and H-ZSM-35 z e o l i t e : comparison of t h e o r e t i c a l and e x p e r i m e n t a l l i n e i n t e n s i t i e s i

Sample

S i /AL

R e l a t i v e l i n e i n t e n s i t i e s (%) configuration : Si(2Al) 6/ppm : -101

f i c t i t i o u s (Alon T2) f i c t i t i o l u s (Alon T i ) Ferrierite

S i(lA1) -1 0 8

Si(OA1: -1 13

8.2 8.2 8.2

0 5.9 8

48.6 36.8 39

51.4 57.3 53

f i c t i t i o u s (A1 on T2) 1 0 . 4 f i c t i t i o u s ( A l o n T i ) 10.4 H-ZSM-35 10.4

0 3.7

39 31.7 34

61 64.6

6

60

r a o~ f H-ZSM-5 ~ and 3.2.2 P e n t a g--i 1 ze_~liLes. The HRMAS 2 9 ~ sip e-c t ~ -----------H-ZSM-11 z e o l i t e s h a v i n g d i f f e r e n t S i / A 1 r a t i o s (from 30 t o 1000) have been s t u d i e d i n d e t a i l ( 1 4 , 1 6 ) . A t low r e s o l u t i o n , e s s e n t i a l l y t h r e e r e s o n a n c e s were o b s e r v e d a t -105, -113 and -115 pprn. The d e c r e a s i n g i n t e n s i t y of t h e -1 15 pprn l i n e w i t h t h e i n c r e a s i n g SiIA1 r a t i o i s d i r e c t l y l i n k e d t o t h e p a r a l l e l i n t e n s i t y i n c r e a s e of t h e -1 13 pprn l i n e , w h i l e t h e i n t e n s i t y of t h e -1 15 pwm l i n e remains c o n s t a n t i n b o t h H-ZS11-5 and H-ZSM-I I ( F i g . 3)

.

Figure 3 .

V a r i a t i o n of t h e 2 9 ~ i - ~ lm i n e i n t e n s i t i e s , as a f u n c t i o n of S i / A l r a t i o s inH-ZSM-5 ( w h i t e s p o t s ) and i n H-ZSM-11 (black spots) z e o l i t e s

.

These v a r i a t i o n s a r e e a s i l y e x p l a i n e d i f t h e -105 and -113 ppm l i n e s are a t t r i b u t e d t o S i (1Al) and Si(OA1) c o n f i g u r a t i o n s r e s p ec t i v e l y . The SiIA1 r a t i o s computed u s i n g e q u a t i o n [I] , I t o t a l / 10.25 I (-105) (every A 1 i s s u r r o u n d e d by 4 Si), match e x a c t l y t h e e x p e r i m e n t a l v a l u e s . I n a d d i t i o n , t h e i n t e n s i t y r a t i o I (-115) ZSM-11/I (-1151, ZSM-5, e q u a l t o 2, i s d i r e c t l y p r o p o r t i o n a l t o t h e amount of 4-membered r i n g s p r e s e n t i s one u n i t c e l l of e a c h z e o l i t e : f o u r i n ZSM-5 and e i g h t i n ZSM-11. The -115 pprn l i n e can t h e r e f o r e b e a t t r i b u t e d t o Si(OA1) c o n f i g u r a t i o n s l o c a t e d i n 4-membered r i n g s . T h i s also i m p l i e s t h a t A 1 atoms a r e e x c l u s i v e l y l o c a t e d i n t h e 5-membered r i n g s of b o t h z e o l i t e s and t h u s n o t s t a t i s t i c a l l y d i s t r i b u t e d throughout t h e p e n t a s i l l a t t i c e . In t h e 2 9 ~ sip e-c t r~ um~ of ~ h i g h l y s i l i c e o u s H-ZSM-5 (Si/Al=lOOO), up t o 8 l i n e components c a n b e d i s t i n g u i s h e d ( F i g . 4 ) .

Figure 4 .

High r e s o l u t i o n 2 9 ~ i spectrum - ~ ~ ~ of H-ZSM-5 ( S i / A l = 1000)

.

T h i s m u l t i p l i c i t y a r i s e s from c r y s t a l l o g r a p h i c a l l y non equiUsing t h e i n t e n s i t i e s of t h e w e l l valent S i ( O M ) arrangements resolved s i g n a l s l o c a t e d a t -109.2 ppm and a t -116.3 ppm a s a base u n i t of one, t h e t o t a l i n t e n s i t y i s found t o be approximatel y 2 4 , i n agreement w i t h previous f i n d i n g s ( 1 6 ) . The number of Si atoms i n t h e 24 non-equivalent s i t e s i n t h e r e p e a t - u n i t of t h e ZSM-5 s t r u c t u r e i s given i n p a r e n t h e s e s i n F i g . 4. The unambiguous assignment of a l l t h e s e l i n e s s t i l l needs f u r t h e r i n v e s t i g a t i o n s . N e v e r t h e l e s s , t h e f o l l o w i n g i n f e r e n c e s can be made : ( i ) t h r e e m a g n e t i c a l l y d i f f e r e n t S i atoms i n t h e 4-membered r i n g s c o n t r i b u t e t o t h e t h r e e l i n e components observed i n t h e --,. , . --T h l s r e v e a l s t n a t s t r a l n must e x .l s c. -1 I5 ppm r e g l o n ( 1 4 ) i n t h e s e r i n g s , i n b o t h ZSM-5 and ZSM-I I ; ( i i ) t h e s u b s t i t u t i o n o f S i by A 1 only t a k e s p l a c e i n t h e 5-memb e r e d r i n g s , where t h e energy i s pnobably lower t h a n i n t h e s t r a i n e d 4-membered r i n g s .

.

.

.

The chemical s h i f t s and r e l a t i v e l i n e i n t e n s i t i e s i n 2 9 ~ i NMR s p e c t r a of samples which by XRD were found t o b e ZSM-8 and ZSM-48,appear t o be v e r y s i m i l a r t o t h o s e of ZSM-5 and ZSM-ll ( F i g . 5 ) . T h i s s u g g e s t s t h a t b o t h z e o l i t e s could a l s o belong t o

t h e p e n t a s i l f a m i l y . I n p a r t i c u l a r , t h e r e s o n a n c e l i n e a t -115 ppm shows t h a t ZSM-8 a n d ZSM-48 frameworks a l s o c o n t a i n 4-membered r i n g s . F o r ZSM-8, t h e r e l a t i v e i n t e n s i t y o f t h i s l i n e f a l l s b e t w e e n t h o s e m e a s u r e d f o r ZSM-5 a n d ZSM-11. T h i s s u g g e s t s t h a t ZSM-8 c o u l d be a mixture o f ZSM-5 and ZSM-11 o r a n i n t e r g r o w t h of t h e s e two s t u c t u r e s a s s u g g e s t e d by K o k o t a i l o ( 4 1 ) . -

1-

r

I

I

1

pZ.1

1

I

Si/AI = 5 0

Si/AI= 5 0

1

I

Si/AI = 4 8 0

- 80

I

I

-100

- 120

F i g u r e 5. HRMAS "S~-NMR

-80

1

I

-100

-120

s p e c t r a of four p e n t a s i l z e o l i t e s .

3 . 2 . 3 . ZSM-39 z e o l i t e . T h i s S i - r i c h z e o l i t e b e l o n g s t o t h e " C l a t h ...................... r a t e g r o u p t t . I t s framework c o n s i s t s o f a n a r r a n g-e m e n t of TO4 12-and l b - h e d r a i n w h i c h t h r e e t y p e s of T a t o m s (TI , T z a n d ~ 3 w) e r e i d e n t i f i e d ( 4 2 ) . T h e i r n a t u r e and t h e i r d i s t r i b u t i o n i n t h e u n i t c e l l a r e d e t a i l e d i n T a b l e 4 . The HRMAS ~ ~ S ~ - N M s pRe c t r u m of ZSM-39 shows t h r e e w e l l r e s o l v e d r e s o n a n c e s a t 6 = -109,-115 a n d -120 ppm ( ~ i g . 6 ) ~ A l t h o u g h t h e number of r e l e v a n t S i ( n A 1 ) c o n f i u r a t i o n s i s r e s t r i c t e d t o S i ( J A 1 ) a n d Si(OA1) d u e t o t h e h i g h S i A 1 r a t i o s , t h e i n t e n s i t y r a t i o of t h e t h r e e NMR l i n e s c a n n o t b e e x p l a i n e d w i t h o u t computing a l l t h e p o s s i b l e A 1 p o t e n t i a l s i t i n g s i n t h e l a t t i c e . This has been envisaged i n d e t a i l elsewhere (38).

7

TABLE 4

R e p a r t i t i o n o f T atoms i n t h e u n i t c e l l o f ZSM-39

T atom

Ring s t r u c t u r e

T1

s h a r e d between

T2

in

0

0

Number/u. c .

Number of n e i g h b o u r i n g T atoms of t y p e

T1

*2

T3

8

0

4

0

32

1

0

3

96

0

1

3

s h a r e d be tween

*3

Figure 6 .

0 and

0

HRMAS 2 9 ~ spectrum i - ~ ~o f ~ ZSM-39 z e o l i t e .

T a b l e 5 compares t h e e x p e r i m e n t a l and t h e v a r i o u s t h e o r e t i c a l l i n e i n t e n s i t i e s o b t a i n e d assuming e i t h e r a s p e c i f i c l o c a t i o n of t h e A 1 atoms on Tl,T2 ar T 3 s i t e s o r t h e i r s t a t i s t i c a l d i s t r i b u t i o n between a l l t h e T s i t e s i n t h e l a t t i c e .

'

TABLE 5 Cbmparison b e t w e e n t h e r e l a t i v e t h e o r e t i c a l i n t e n s i t i e s c a l c u l a t e d f r o m v a r i o u s A 1 s i t i n g s i n t h e l a t t i c e and t h e e x p e r i m e n t a l ones m e a s u r e d f o r H-ZSM-39 ( S i / A l = 54.3)

6 (ppm)

R e l a t i v e t h e o r e t i c a l i n t e n s i t i e s (%) assuming A 1 s i t e d i n TI

-104 - 109 -1 15 - 120

0 11.5 16.6 71.9

in

T2

I .8 4.2

27.6 66.4

i n T3 0 7.8 27.7 64.5

statistically

0.5 7.1 27 .O 65.4

Exper. v a l u e s (NMR) O 9

29 62

L

The b e s t a g r e e m e n t between e x p e r i m e n t a l a n d t h e o r e t i c a l v a l u e s i s o b t a i n e d when A 1 c a t i o n s a r e s p e c i f i c a l l y l o c a t e d o n s i t e s T3 i . e . i n t h e 6-membered r i n g s . However, b e c a u s e o f t h e h i g h proport i o n of t h e T s i t e s , a s t a t i s t i c a l d i s t r i b u t i o n a l s o g i v e s a c l o s e 3 agreement. 2 9 ~ i - N k B s p e c t r a of ZSM-39 s a m p l e s w i t h a l o w e r ~ i / r~a tli o a r e needed t o differentiate u n e q u i v o c a l l y b e t w e e n t h e two d i s t r i b u t i o n s .

3.3 C r y s t a l l i z a t i o n o f ZSM-5 from amorphous g e l s I n f o r m a t i o n s on s t r u c t u r a l e v o l u t i o n o f v a r i o u s c o n s t i t u e n t s p e c i e s formed w i t h i n g e l m i x t u r e s o r s o l u t i o n s , p r e c u r s o r s t o crys t a l l i z a t i o n o f m o r d e n i t e (43) o r ZSM-5 ( 2 5 , 2 6 1 , h a v e b e e n o b t a i n e d r e c e n t l y u s i n g HRMAS m u l t i n u c l e a r ( I ~ c2, 7 ~ 12, 9 ~ i )NMR. The h y d r o t h e r m a l s y n t h e s i s of ZSM-5 c a n i n v o l v e a t l e a s t two d i f f e r e n t mechanisms whose v a r i o u s a s p e c t s h a v e b e e n d e v e l o p p e d in d e t a i l e l s e w h e r e ( 2 1 , 2 3 ) . P r o c e d u r e A y i e l d s l a r g e s i n g l e c r y s t a l s o f ZSM-5 which grow s l o w l y i n a n A l - r i c h g e l t h r o u g h a l i q u i d p h a s e i o n t r a n s p o r t a t i o n mechan2sm. P r o c e d u r e B g i v e s r a p i d l y small ZSM-5 p o l y c r y s t a l l i n e a g g r e g a t e s w h i c h a p p e a r v e r y e a r l y w i t h i n the h y d r o g e l , w h e r e t h e y r e m a i n s t a b i l i z e d a s v e r y s m a l l s i z e d "X-ray amorphous" z e o l i t e s ( 2 1 , 2 5 , 4 4 ) . S e v e r a l i n t e r m e d i a t e p h a s e s formed d u r i n g t h e c r y s t a l l i z a t i o n of ZSM-5 c o n d u c t e d u s i n g b o t h p r o c e d u r e s A a n d B were isolated and i n v k s t i g a t e d by HRMAS 29~i-NMR. The s p e c t r a of g e l s e x h i b i t v e r y b r o a d r e s o n a n c e s . T h e i r maxima a r e l o c a t e d b e t w e e n -100 and - 1 1 1 ppm, d e p e n d i n g o n t h e i r a c t u a l S i / A l r a t i o . The l i n e s become n a r r o w e r and a r e s h i f t e d t o w a r d s h i g h e r f i e l d s , a s ZSM-5 i s p r o g r e s s i v e l y formed w i t h i n t h e g e l s . T h i s e v o l u t i o n i s shown i n F i g . 7 f o r 3 p h a s e s f r o m s y n t h e s i s A .

~helinssappearing below - 1 1 1 ppm must essentially belong to Si atoms which have only S i as neighbours, while those located above - 100 ppm should reflect various Si(nA1) configurations, where n > 0. In that case, Si atoms are surrounded either by Al, randomly arranged in amorphous phase, or by silanol groups which should appear in that region in amorphous or crystalline ZSM-5 (1 9). The variation of the corresponding relative intensities, respectively noted by I (6 < - I 1 1) and I ( 6 > -loo), as well as that of the total 2 9 ~ i - ~ ~ ~ intensity, as a function of synthesis time, leads to interesting conclusions.

~ ~ ~ of some intermediate phases Figure 7. HRMAS 2 9 ~ i -spectra obtained using procedure A.

3.3.1. ZSM-5 synthesized according---to procedure ----------------------------------------A . I(&-loo) respectively increase and decrease as the crvstallization proceeds, confirming that the number of Si(nA1) (n > 0) configurations decreases in a parallel way with the decreasing global A1 concentration in the solid phases ( ~ i / ~progressively l increases from 1.8 (0 % crystallinity) to 13.2 (100 X crystallinity)], while more Si(OA1) configurations appear ordered in a crystalline zeolite phase. (Fig; 8,A) d

Figure 8. Variation of the relative 2 9 ~ line i - intensities ~ ~ of the peaks located below - 1 1 1 ppm (6 < - 1 1 1) and above -100 ppm (6 > -100) and of the % crystallinity (XRD) for various A and B-type intermediate phases, as a function of crystallization time.

The progressive Al-depletion of the solid phases is also characterized by the sygmoidal increase of the total intensity, which follows the XRD crystallinity (Fig. 9A).

Synthesis time (h)

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Figure 9 Variation of the total 2 9 ~ i intensities - ~ and of the % of crystallinity of A and B-type intermediate phases, as a function of synthesis time.

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3.3.2, ZSM-5 synthesized _ _ _ _ - _ _ _ _ _ _according _ _ _ _ _ _ _ ____ to procedure __________B. Oppositely to -100) is the synthesis A, in the beginning of the process B, I (6 low, while I (6 < -1 11) is a l r e a d y high (Fig. 8 B ) . Both intensities show little variations as the crystallization proceeds. This only reflects the low amount of Si(nA1) configurations within the Si-rich

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( g e l + z e o l i t e ) p h a s e s i n which S i / A l 35 and remains c o n s t a n t during the synthesis course. By c o n t r a s t , I ( t o t a 1 ) remains c o n s t a n t o n l y d u r i n g t h e t i m e i n t e r v a l i n which t h e X-ray amporphous ZSM-5 c r y s t a l l i t e s a r e d e t e c t e d ( ~ i 9~ ~ .) . I t shows, however, a s h a r p i n c r e a s e when t h e ZSM-5 cryst a l l i t e s b e g i n t o grow. Because ~ i / remains ~ l constant during the c r y s t a l l i z a t i o n , t h e o r d e r i n g of S i atoms w i t h i n a ZSM-5 l a t t i c e i s t h e o n l y wa t o e x p l a i n t h a t i n c r e a s e . T h i s l a t t e r i s p r o b a b l y due t o s h o r t e r $9 Si-NMR r e l a x a t i o n t i m e s f o r S i l o c a t e d w i t h i n a n o r d e r e d l a t t i c e t h a n f o r S i surrounded by a random environment i n t h e g e l . T h i s phenomenon i s p r e s e n t l y b e i n g i n v e s t i g a t e d by determining t h e T I r e l a x a t i o n t i m e s i n t h e g e l and c r y s t a l l i n e p h a s e s . his inc r e a s e however a p p e a r s t o b e a r e m a r k a b l e q u a l i t a t i v e i l l u s t r a t i o n of t h e o r d e r i n g p r o c e s s which o c c u r s d u r i n g t h e growth of z e o l i t e crystals.

4. CONCLUSIONS HRMAS 2 9 ~ porved i - ~ t o b e a powerful t e c h n i q u e t o determine t h e d i s t r i b u t i o n of t h e A 1 atoms i n v a r i o u s z e o l i t i c frameworks. The chemical s h i f t s on t h e 2 9 ~ lii n-e s ~e s s e n t i a l l y depend on t h e chemical environment of t h e Si atoms. Moreover, f o r a given S i ( n A l ) c o n f i g u r a t i o n , t h e y a r e a l s o s e n s i t i v e t o t h e a c t u a l symm e t r y o f t h e Si-0-T l i n k a g e s a s w e l l a s t o l o c a l framework s t r a i n s . I n such c a s e s , t h e s i l i c o n - a l u m i n i u m o r d e r i n g can be r e s o l v e d by f o l l o w i n g t h e r e l a t i v e i n t e n s i t i e s of e a c h l i - n e component c h a r a c t e r i a i n g a g i v e n c o n f i g u r a t i o n , a s a f u n c t i o n of t h e S i / A l r a t i o i n t h e z e o l i t e . I t i s concluded t h a t most of t h e i n v e s t i g a t e d m a t e r i a l s ( z e o l i t e s b e l o n g i n g t o t h e m o r d e n i t e o r ~ e n t a s i lf a m i l y and ZSM-39) a r e c h a r a c t e r i z e d by referential d i s t r i b u t i o n of A 1 atoms on specific sites. F o r z e o l i t e s whose s t r u c t u r e and Si-A1 o r d e r i n g s a r e known, the measure of t h e i n t e n s i t y of each l i n e component l e a d s t o a s t r a i g h t forward d e t e r m i n a t i o n of t h e Si/A1 r a t i o s . However, f o r i n t e r m e d i a t e a l u m i n o - s i l i c a t e phases obtained during t h e s y n t h e s i s , t h e l i n e int e n s i t i e s were found t o b e p r o p o r t i o n a l t o t h e d e g r e e of o r d e r i n g of t h e p h a s e s . T h i s p r o p e r t y was t h e r e f o r e e x p l o i t e d t o f o l l o w q u a l i t a t i v e l y t h e p r o g r e s s i v e c r y s t a l l i z a t i o n and growth of (ordered) z e o l i t i c frameworks from amorphous ( r a n d o m ) a l u r n i n o - s i l i c a t e g e l s .

5. ACKNOWLEDGEMENTS The a u t h o r s wish t o t h a n k M r . G. Daelen f o r h i s s k i l f u l h e l p i n t a k i n g t h e NMR s p e c t r a . P.A. J a c o b s acknowledges a r e s e a r c h p o s i t i o n a s I 1 S e n i o r R e s e a r c h A s s o c i a t e ' ' from NFWO-FNRS ( ~ e l g i u r n ) and P. Bodart t h a n k s IRSIA (Belgium) f o r f i n a n c i a l s u p p o r t .

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1. 2. 3.

4. 5. 6. 7.

8.

9. 10.

11. 12.

13.

14. 15. 16. 17.

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MULTINUCLEAR SOLID-STATE NMR STUDY OF PIORDENITE CRYSTALLIZATION

P h i l i p p e B o d a r t , Z e l i m i r G a b e l i c a , J g n o s B.Nagy and Guy Debras

a

F a c u l t g s U n i v e r s i t a i r e s d e Namur L a b o r a t o i r e de C a t a l y s e , Rue de B r u x e l l e s , 61 B-5000 Narnur, Belgium a- P r e s e n t a d d r e s s : L a b o f i n a S . A . , Zoning I n d u s t r i e l , B-6520 F e l u y , Belgium.

ABSTRACT S o l i d i n t e r m e d i a t e p h a s e s o b t a i n e d from amorphous a l u m i n o s i l i cate g e l s d u r i n g m o r d e n i t e c r y s t a l l i z a t i o n h a v e b e e n c h a r a c t e r i z e d by m u l t i n u c l e a r s o l i d - s t a t e NMR s p e c t r o s c o p y . The p r o g r e s s i v e r e o r g a n i z a t i o n of t h e amorphous g e l i n t o t h e c r y s t a l l i n e o r d e r e d z e o l i t i c phase h a s b e e n d e t e c t e d by h i g h r e s o l u t i o n magic-an l e - s p i n n i n g (HNIAS) s o l i d s t a t e 2 9 ~ i -Broad ~ ~band ~ . s o l i d s t a t e 2 9A 1 and 2 3 ~ a were - ~ ~ used ~ t o f o l l o w t h e i n c o r p o r a t i o n o f A l - and Na-atoms i n t o t h e m o r d e n i t e l a t t i c e and c h a n n e l s . An e v i d e n c e of aluminium g r a d i e n t i n t h e c r y s t a l l i t e s i s o b t a i n e d from t h e comparison b e t ween s u r f a c e and b u l k a n a l y s i s methods.

1

INTRODUCTION

During t h e l a s t t h i r t y y e a r s , t h e s y n t h e s i s o f z e o l i t e s h a s been e x t e n s i v e l y d e v e l o p e d t o p r o v i d e new o r improved m a t e r i a l s f o r c a t a l y s i s a n d / o r m o l e c u l a r s i e v i n g (1,2) However, t h e s y n t h e s i s p r o c e s s e s , i. e n u c l e a t i o n and c r y s t a l growth from amorphous aqueous aluminosilicate g e l s a r e s t i l l poorly understood (3) R e c e n t l y , mult i n u c l e a r s o l i d s t a t e NMR h a s p r o v e n t o b e a p o w e r f u l t o o l i n t h e T h i s method was a l s o i n v e s t i g a t i o n o f z e o l i t e s t r u c t u r e s (4-14) s u c c e s s f u l l y used t o c h a r a c t e r i z e i n t e r m e d i a t e s o l i d p h a s e s o b t a i ned d u r i n g c r y s t a l l i z a t i o n of ZSM-5 m a t e r i a l s (13-15).

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S i n c e t h e f i r s t s y n t h e s i s o f t h e s i l i c a - r i c h z e o l i t e mordenite by B a r r e r ( I b ) , c o n s i d e r a b l e l i t e r a t u r e d a t a have b e e n p u b l i s h e d on i t s p r e p a r a t i o n (17-23), t h e mechanism of t h i s c r y s t a l l i z a t i o n proc e s s remains however q u a s i unknown. The aim of t h i s p a p e r i s t o c h a r a c t e r i z e by s o l i d s t a t e m u l t i n u c l e a r NMR t h e s t r u c t u r a l r e a r rangements t h a t o c c u r d u r i n g t h e p r o g r e s s i v e t r a n s f o r m a t i o n of amorphous g e l s i n t o c r y s t a l l i n e m o r d e n i t e .

2 2.1

EXPERIMENTAL Mordenite s y n t h e s i s

The s y n t h e s i s of m o r d e n i t e was b a s e d on p u b l i s h e d methods of p r e p a r a t i o n ( 1 9 , 2 2 ) . An aqueous a l u m i n o s i l i c a t e g e l h a v i n g t h e m o l a r c o m p o s i t i o n 2 . 4 Na20 A1203 1 1 1 S i 0 2 220 H20 was p r e p a r e d from N a - s i l i c a t e (Merk, a r t . 5 6 2 1 ) , s i l i c a g e l (Davison g r a d e 950) and Na-aluminate ( R i e d e l de Haen, a r t . 1 3 4 0 4 ) . I t was d i v i d e d i n t o s e v e r a l p o r t i o n s a n d s e a l e d i n i d e n t i c a l 20 m l p y r e x t u b e s . The l a t t e r were h e a t e d a t 1 6 5 ' ~u n d e r a u t o g e n e o u s p r e s s u r e f o r g i v e n p e r i o d s of t i m e and p r o g r e s s i v e l y removed from t h e oven, i n o r d e r t o i s o l a t e m a t e r i a l s w i t h i n c r e a s i n g d e g r e e s of c r y s t a l l i n i t y . A f t e r cool i n g , e a c h sample ( g e l + z e o l i t e ) was f i l t e r e d , washed w i t h c o l d w a t e r and d r i e d a t 1 2 0 ' ~ o v e r n i g h t , b e f o r e c h a r a c t e r i z a t i o n . The p e r c e n t a g e of m o r d e n i t e i n t h e a s - s y n t h e s i z e d p h a s e s was e v a l u a t e d by t h e c o n v e n t i o n a l X-ray d i f f r a c t i o n t e c h n i q u e (XRD) ( P h i l i p s PW 1349/30 d i f f r a c t o m e t e r , Cu Ka r a d i a t i o n ) , u s i n g r e f l e c t i o n s occu100% r i n g a t 28 a n g l e s o f 2 2 . 3 , 2 5 . 6 , 2 6 . 3 and 27.9 d e g r e e s . c r y s t a l l i n i t y was a s s i g n e d t o t h e most c r y s t a l l i n e p h a s e o f t h e s e r i e s , which proved t o be 115% c r y s t a l l i n e w i t h r e s p e c t t o a comm e r c i a l H-Zeolon from t h e Norton Company, g e n e r a l l y u s e d a s s t a n dard reference

.

.

.

.

2 . 2 NMR s p B c t r a S o l i d s t a t e NMR s p e c t r a were o b t a i n e d a t room t e m p e r a t u r e , u s i n g a B r u k e r CXP-200 s p e c t r o m e t e r o p e r a t i n g i n t h e F o u r i e r t r a n s form mode. An r . f . f i e l d of 4 9 . 3 Oe was u s e d f o r t h e ~ r / 2p u l s e s of 2 9 ~ (3i 9 . 7 MHz). The D e l r i n c o n i c a l r o t o r was s p u n a t a r a t e of 3 . 1 kHz. Time i n t e r v a l s between p u l s e s e q u e n c e s were 3 . 0 s and o v e r 15000 f r e e i n d u c t i o n decays were accumulated p e r sample. Chemical s h i f t s ( 6 i n ppm) were measured from t e t r a m e t h y l s i l a n e (TMS). 2 7 ~h 1 i g h power NMR s p e c t r a were r e c o r d e d a t 5 2 . 1 MHz. Chemical s h i f t s were measured w i t h r e s p e c t t o AI ( ~ ~63+, 0 ) used a s e x t e r n a l r e f e r e n c e . Time i n t e r v a l s of 0 . 1 s were u s e d between p u l s e s e q u e n c e s and 5000 f r e e i n d u c t i o n d e c a y s were a c c u m u l a t e d p e r sample. 2 3 ~ a - , l i s~p~e c t r a were r e c o r d e d a t 5 2 . 9 MHz, i n s t a t i c c o n d i t i o n s . F!aiting t i m e s between p u l s e s e q u e n c e s were 0 . 2 s and 2000 f r e e i n d u c t i o n d e c a y s were a c c u m u l a t e d p e r s a m p l e .

3 3.1

RESULTS AND DISCUSSION XRD c r y s t a l l i n i t v of t h e i n t e r m e d i a t e p h a s e s

Figure 1 mediate s o l i d sigmoFd c u r v e crystallizing

shows t h e . v a r i a t i o n o f c r y s t a l l i n i t y f o r some i n t e r phases a s a f u n c t i o n of s y n t h e s i s time. A c l a s s i c a l i s obtained a s u s u a l l y observed f o r v a r i o u s z e o l i t e s from non-seeded s y s t e m s (21-23).

Time (h)

Figure 1 .

3.2

V a r i a t i o n of r e l a t i v e c r y s t a l l i n i t y of t h e i n t e r m e d i a t e s o l i d p h a s e s a s a f u n c t i o n of s y n t h e s i s t i m e .

S o l i d s t a t e HmAS " S ~ - N M R

study

u m ~o f ~100% c r y s t a l l i n e The s o l i d s t a t e HRMAS 2 9 ~ sip e-c t r~ m o r d e n i t e i s r e p o r t e d i n f i g . 2 . The t h r e e r e s o n a n c e l i n e s t h a t a p p e a r a t 6 = -1 1 0 , -105 and -96 ppm, c o r r e s p o n d t o Si-atoms hav i n g r e s p e c t i v e l y 0 , 1 and 2 Al-atoms i n t h e i r s e c o n d c o o r d i n a t i o n s h e l l s ( 1 1).

Figure 2.

S o l i d s t a t e HRMAS

29

Si-NMR s p e c t r u m of m o r d e n i t e .

F i g u r e 3 shows HRMAS 2 9 ~ sip e-c t~ r a of ~ i~ n t e r m e d i a t e phases o b t a i n e d a t v a r i o u s c r y s t a l l i z a t i o n t i m e s . The f i r s t s p e c t r u m ( A ) o o n s i s t s 6f a b r o a d r e s o n a n c e l i n e c e n t e r e d a t 6 = -95 ppm, char a c t e r i z i n g A l - r i c h amorphous g e l p h a s e s . With i n c r e a s i n g c r y s t a l l i z a t i o n times ,the amorphous p h a s e becomes r i c h e r i n s i l i c o n and a s a r e s u l t , i t s s t i l l b r o a d r e s o n a n c e l i n e s h i f t s t o h i g h e r f i e l d s (Table I ) ( f i g . 3 , A,B,C).

Figure 3 .

E v o l u t i o n of t h e s o l i d s t a t e HRMAS 2 9 ~ NMR i spectrum of i n t e r m e d i a t e p h a s e s formed d u r i n g m o r d e n i t e c r y s tallization.

Time (h)

F i g- u r e 4.

V a r i a t i o n of 2 9 ~ il i n-e ~ i n t e~n s ~ i t i e s a t 6 > -94 ppm (amorphous p h a s e ) and 6 < - 108 ppm ( m o r d e n i t e ~i(0~ 1 ) r e s o n a n c e l i n e ) a s a f u n c t i o n of s y n t h e s i s time (NMR line intensities relative t o the t o t a l intensity).

The l i n e c o r r e s p o n d i n g t o t h e amorphous phase d e c r e a s e s r a p i d l y and d i s a p p e a r s a t t h e end of t h e c r y s t a l l i z a t i o n s t e p . O p p o s i t e l y , t h e m o r d e n i t e Si(OA1) l i n e i n t e n s i t y i n c r e a s e s w i t h i n c r e a s i n g c r y s t a l l i n i t y . These r e s u l t s i n d i c a t e t h a t 2 9 ~ lii n-e ~ ~ ~ i n t e n s i t i e s can b e v a l i d l y used t o c h a r a c t e r i z e t h e c r i s t a l l i n i t y of t h e i n t e r m e d i a t e p h a s e s formed d u r i n g t h e s y n t h e s i s c o u r s e of mordenite. S i m i l a r r e s u l t s have been o b t a i n e d i n t h e s t u d y of ZSM-5 s y n t h e s i s (15). Broad band s o l i d s t a t e 2 7 ~ 1s t-u d~ v ~ ~ 27 F i g u r e 5 shows A1-NMR s p e c t r a of a s y n t h e t i c Na-mordenite and o f t h e same sample t r e a t e d w i t h aqueous H C 1 0 . 2 N . The Nam o r d e n i t e i s c h a r a c t e r i z e d by a r e s o n a n c e l i n e a t 6 2 53 ppm, c o r r e s p o n d i n g t o Al-atoms s i t e d i n t e t r a h e d r a l p o s i t i o n 9n t h e framework. Another r e s o n a n c e l i n e a p p e a r s n e a r 6 = 0 ppm a f t e r t h e HC1 l e a c h i n g . It i s due t o e x t r a - l a t t i c e o c t a h e d r a l l y coord i n a t e d Al-atoms, which have b e e n e x t r a c t e d o u t of t h e a l u m i n o s i l i c a t e framework and d e p o s i t e d i n t h e z e o l i t e c h a n n e l s o r on i t s e x t e r n a l s u r f ace ( 1 1) 3.3

.

TABLE 1 Variations of ~ i / A lr a t i o s and 2 9 ~ chemical i s h i f t s ( 6 ) , of some amorphous i n t e r m e d i a t e phases formed d u r i n g t h e f i r s t s t e p of mordenite c r y s t a l l i z a t i o n . C r y s t a l l i z a t i o n time

Si/A1 (a)

O P ~ ()b )

0.5

3.2

-95.4

8

4.8

-97.8

16

5.2

-102.3

(h)

( a ) determined by energy d i s p e r s i v e X-ray a n a l y s i s (EDX) (24) ( b ) from TMS The l i n e i n t e n s i t y measured above -94 ppm ( 6 > -94 ppm) c o r r e s ponds e s s e n t i a l l y t o t h e amorphous s o l i d , w h i l e t h e one a t 6 - 9 4 npm 6 < - 1 0 8 p p m crystallinity

0.5

0

43.6

6.9

8

0

14.1

8.5

16

11

6.9

25.8

24

37

10.1

38.2

48

99

1.9

41.6

68

1 00

1 .8

39.8

120

100

0.7

47.0

100

0.7

47.9

280

-

(a) r e l a t i v e t o t h e t o t a l 2 9 ~ i - l~i nl e~ i n t e n s i t y

I

I

- H

F i g u r e 5.

- MOR

S o l i d s t a t e 2 7 ~ 1s p-e c~t r a~ of~ m o r d e n i t e : a . Na-mordenite ( ~ i / A l= 6.5) b . t h e same, l e a c h e d w i t h HC1 0 . 2 N ( S i / A l = 6.5)

T a b l e 3 sums up t h e 2 7 ~ chemical 1 s h i f t s ( 1 5 ) and l i n e w i d t h s f o r d i f f e r e n t s o l i d phases t h a t appear d u r i n g mordenite c r y s t a l h e m i c a l s h i f t s s u g g e s t s t h a t Al-atoms l i z a t i o n . The v a l u e of 2 7 ~c 1 i n t h e i n t e r m e d i a t e p h a s e s b a s i c a l l y occupy t e t r a h e d r a l p o s i t i o n s d u r i n g a l l t h e c r y s t a l l i z a t i o n p r o c e s s . Furthermore, t h e correson ding l i n e w i d t h s d e c r e a s e a s t h e c r y s t a l l i z a t i o n p r o c e e d s and remain s t a b i l i z e d a t about 1 . 9 kHz a s soon a s 100% c r y s t a l l i n e m o r d e n i t e i s o b t a i n e d (Table 3 and F i g u r e 6 ) .

TABLE 3 2 7 ~ 1 characterization - ~ ~ ~ of the intermediate solid phases formed during mordenite crystallization.

-

100 h

aP

Y

I

-

0-6

I

XRD crystallinity

7

-

-4 -2

AH

100

200

300

Time (h)

figure 6 .

Variation of the *'A~-NMR linewidths (AH) and of the XRD crystallinity of the intermediate phases as a function of synthesis time.

T h i s d e c r e a s e can b e r e l a t e d t o t h e p r o g r e s s i v e o r d e r i n g o f t h e g e l p h a s e d u r i n g t h e growth p r o c e s s . I n d e e d , an 2 7 ~ 1l i n-e w ~i d~ th ~ can b e c o n s i d e r e d as d i r e c t l y r e l a t e d t o t h e homogeneity of A l - s i t e s d i s t r i b u t i o n w i t h i n t h e s o l i d p h a s e . I n amorphous p h a s e s , t h e d i s t r i b u t i o n i s random,while i n o r d e r e d c r y s t a l l i n e p h a s e s a l l t e t r a hedral s i t e s a r e regularly\! arranged. TABLE 4

E v o l u t i o n of 2 7 ~ 1l i -n e~i n~ t e n~s i t i e k w i t h XRD c r y s t a l l i n i t y and t h e A l - c o n c e n t r a t i o n XRDcrystallinity

(W>

CAI x 1 04 (a) (mol g-1)

1 ~ (1a . u . )

e

I

0

33.0

18.5

0

23.0

13.6

11

23,4

14.0

37

22 .O

13.4

99

19.4

12.8

100

18.9

12.2

100

17.6

12.3

10 0

14.4

10.8

( a ) d e t e r m i n e d by EDX (24)

Figure 7.

C o r r e l a t i o n between 2 7 ~ 1l i-n e~i n~t e n ~s i t i e s and A1 c o n c e n t r a t i o n , measured i n some i n t e r m e d i a t e p h a s e s obtained during mordenite c r y s t a l l i z a t i o n . = s m a l l s i z e d p a r t i c l e s e n t i r e l y probed by EDX = l a r g e r p a r t i c l e s n o t e n t i r e l y probed by EDX

---

The s t r a i g h t l i n e ( c a l i b r a t i o n c u r v e ) , shown i n f i g u r e 7 , was e s t a b l i s h e d from a n a l y s e s o f p h a s e s c o n s i s t i n g of s m a l l s i z e d ( l e s s than 2 pm; p a r t i c l e s which a r e e n t i r e l y probed by t h e EDX t e c h n i q u e (Table 4 ) ( 2 4 ) . The more c r y s t a l l i n e and S i - r i c h e r p h a s e s a r e composed of p a r t i c l e s a v e r a g i n g 30 u m i n d i a m e t e r , f o r which t h e EDX ocated i n the outer s h e l l . technique d e t e c t s o n l y A l - and Si-atoms As t h e amount of aluminium d e t e c t e d by 2JA1-NMR (a b u l k a n a l y s i s method) i s d i f f e r e n t from t h a t d e t e r m i n e d by a " s u r f a c e " a n a l y s i s (EDX) ( f i g u r e 7 , d o t t e d l i n e ) , i t i s concluded t h a t an Al-concent r a t i o n g r a d i e n t must e x i s t i n t h e 100 % c r y s t a l l i n e m o r d e n i t e p a r t i c l e s : t h e i r i n n e r c o r e i s r i c h e r i n aluminium t h a n t h e i r o u t e r rim. S i / A l r a t i o s of a 100 % c r y s t a l l i n e m o r d e n i t e , a s measured by d i f f e r e n t a n a l y t i c a l methods, a;e compared i n t a b l e 5 . T h e i r regular d e c r e a s e w i t h t h e d e p t h of t h e a n a l y s i s c o n f i r m s t h e e x i s tence of an A l - g r a d i e n t . TABLE 5

V a r i a t i o n of t h e S i / A l r a t i o of m o r d e n i t e ( c r y s t a l l i z a t i o n t i m e 68 h o u r s , 100 % XRD c r y s t a l l i n i t y ) a s a f u n c t i o n of t h e d e p t h of a n a l y s i s . ' ~ n a l ~ t i c amethod l

Depth of a n a l y s i s

~ i / A l

0

XPS (ESCA) ( a )

EDX

PIGE ( b )

~ ~ A I - N M R( c )

30 A

8.0

2 Pm

6.5

10 pm

5.2

bulk analysis

5.7

(a) X-ray p h o t o e l e c t r o n s u r f a c e a n a l y s i s (25) (b) P r o t o n induced y-ray e m i s s i o n (26) (c) Determined u s i n g t h e c a l i b r a t i o n c u r v e of f i g . 7.

3.4

Broad-band

23Na-~M~ study

The 2 3 ~ a s- p e~c t~r a ~of i n t e r m e d i a t e p h a s e s o b t a i n e d d u r i n g mordenite c r y s t a l l i z a t i o n mainly c o n s i s t of a b r o a d r e s o n a n c e l i n e c e n t e r e d a t 6 = -17 ppm ( v s aqueous NaClO 4 ) ( f i g u r e 8 ) .

-

6 (ppm) 1 aq. NaCIO,

F i g u r e 8.

2 3 ~ a s-p e ~ c t r~a ~ of some i n t e r m e d i a t e p h a s e s o b t a i n e d during mordenite c r y s t a l l i z a t i o n .

During t h e s y n t h e s i s p r o c e s s , t h e 2 3 N a - ~l i~n ~ e w i d t h (AH) r a p i d l y d e c r e a s e s and remains n e a r l y c o n s t a n t as t h e c r y s t a l l i z a t i o n i s complete ( f i g . 8 and 9 ) .

11 10

XRD crystallinity

A

N

I Y

Y

I

a

-

*

AH '

-

--

5

Yo

0

100

200

300

Time (h)

Figure 9 .

V a r i a t i o n of 2 3 ~ a l-i n~e w~i d t~h s and XRD c r y s t a l l i n i t y a s a f u n c t i o n of s y n t h e s i s t i m e .

T h i s i l l u s t r a t e s t h e p r o g r e s s i v e i n c o r p o r a t i o n of sodium i o n s into the z e o l i t e l a t t i c e . I n the intermediate phases, t h e d i s t r i Moreover, t h e e l e c t r i c bution of t h e chemical s h i f t s i s b r o a d . f i e l d g r a d i e n t pn t h e 2 3 ~ a - n u c l e u s must a l s o b e d i f f e r e n t i n an amorphous i n t e r m e d i a t e phase w i t h r e s p e c t t o t h e 100 % c r y s t a l l i f i e ~ h a s e , h e n c ea b z o a d N M R l i n e i s o b s e r v e d . O p p o s i t e l y , i n t h e o r d e r e d c r y s t a l l i n e p h a s e s , t h e chemical s h i f t d i s t r i b u t i o n i n n a r r o w e r and t h e l i n e w i d t h of t h e c o r r e s p o n d i n g NMR r e s o n a n c e d e c r e a s e s . When t h e 100 % c r y s t a l l i n e m o r d e n i t e i s l e f t i n t h e a u t o c l a v e f o r a long t i m e , a new r e s o n a n c e l i n e a p p e a r s a t 6 = - 2 ppm. It can be t e n t a t i v e l y a t t r i b u t e d t o sodium i o n s i n c o r p o r a t e d i n t o a p a r a s i t e s p e c i e s (such a s analcime) ( f i g . 8 ) . F i g u r e 10 and T a b l e 6 compare t h e r a t i o of t h e n o r m a l i z e d i n t e n s i t i e s of 27~1l i n e s w i t h t h e Al/Na a t o m i c r a t i o measured by EDX. A r e l a t i v e l y good c o r r e l a t i o n i s o b s e r v e d . T h i s iques demonstrates t h a t t h e combination of 2 7 ~ 1a- n d 2 3 ~ a -t e~c h~n ~ can be v a l u a b l y used b o t h t o show t h e i n c o r p o r a t i o n of A l - and Naatoms i n t h e z e o l i t e and t o e s t i m a t e t h e ~ 1 / r~a tai o i n t h e s a m p l e , provided a c a l i b r a t i o n c u r v e .

TABLE 6 V a r i a t i o n of t h e NMR i n t e n s i f y r a t i o

N a-NMR

'

with t h e

atomic Na

r a t i o measured by EDX i n t h e i n t e r m e d i a t e p h a s e s . LA1

A1 Na (EDX)

3G

0.83 0. 91 0.98 1.04 1.13 1.14 1.13 1.13

0.58 0.48 0.60 0.59 0.74 0.68 0.77 0.66

F i g u r e 10.

+

27 E v o l u t i o n o f the . A l - and 2 3 ~ a i-n t~e n~s i t~y r a t i o w i t h Al/Na a t o m i c r a t i o as d e t e r m i n e d by EDX

4 CONCLUSIONS M u l t i n u c l e a r s o l i d s t a t e NMR r e v e a l s t o b e a p o w e r f u l t o o l f o r t h e c h a r a c t e r i z a t i o n of i n t e r m e d i a t e s o l i d p h a s e s o c c u r i n g a can ~ be ~ during c r y s t a l l i z a t i o n of zeolites.HRMAS 2 9 ~ sip e-c t r~ v a l i d l y r e l a t e d t o t h e o r d e r i n g of t h e i n i t i a l amorphous s o l i d . The p r o g r e s s i v e i n c o r p o r a t i o n of Al-atoms i n an o r d e r e d z e o l i t i c l a t t i c e i s c h a r a c t e r i z e d by "AI-NMR. This technique i s a l s o able t o d i s c r i m i n a t e between t e t r a - and o c t a - c o o r d i n a t e d Al-atwms It can be used t o d e t e r m i n e t h e Al-content of a s o l i d p h a s e , p r o v i d e d a c a l i b r a t i o n c u r v e . From t h e comparison of t h e Si/A1 r a t i o s determined by d i f f e r e n t s u r f a c e and b u l k a n a l y s i s methods, e v i d e n c e can be o b t a i n e d f o r t h e aluminium g r a d i e n t s i n t h e s m a l l c r y s t a l l i t e s . I n t h e p a r t i c u l a r c a s e of m o r d e n i t e , t h e o u t e r s h e l l of t h e 100 % c r y s t a l l i n e p a r t i c l e s c o n t a i n s l e s s aluminium t h a n t h e i r i n n e r core. The e v o l u t i o n of ~ ~ N ~ - N Ml i Rn e s shows t h e i n c o r p o r a t i o n of sodium i o n s i n t o t h e z e o l i t e l a t t i c e . T h i s t e c h n i q u e can be used t o g e t h e r w i t h 2 7 ~ 1 t-o ~d e~t e r~m i n e t h e Al/Na r a t i o o f t h e s t u d i e d sample.

.

The a u t h o r s wish t o thank M r . G. Daelen f o r h i s a p p r e c i a t e d help i n t a k i n g t h e NMR s p e c t r a and M r . F. V a l l e t t e f o r h i s t e c h n i c a l a s s i s t a n c e . P. Bodart t h a n k s IRSIA (Belgium) 6 o r f i n a n c i a l support.

6

REFERENCES

1 . D.W. B r e c k " Z e o l i t e M o l e c u l a r S i e v e s " , J . W i l e y & S o n s , New York ( 1 9 7 4 ) . 2 . R.M. B a r r e r "Hydrothermal Chernis t r y o f Z e o l i t e s " , Academic P r e s s ,London ( 1 982) 3 . L.B. Sand i n " P r o c e e d i n g s o f t h e 5 t h I n t e r n a t i o n a l C o n f e r e n c e on Z e o l i t e s " , (L.V. R e e s , e d . ) , Heyden, London, p . 1 ( 1 9 8 0 ) . 4. G. E n g e l h a r d t , U . L o h s e , E. Lippmaa, M. Tarmak a n d M. Mggi, Z. Anorg. A l l g . Chem., 482, 49 ( 1 9 8 1 ) . 5 . J . K l i n o w s k i , S . Ramdas, J . M . Thomas, C.A. F y f e a n d J . S . Hartrnan, J . Chem. Soc. , F a r a d a y T r a n s 11, 78, 1025 ( 1982) 6. J.M. Thomas, C.A. F y f e , J . K l i n o w s k i and GZ. Gobbi, J . P h y s . Chem., 8 6 , 3061 ( 1 9 8 2 ) . G.C. , Gobbi, J . S . Hartman, R.E. L e n k i n s k i a n d J . H . 7. C.A. F ~ Z O ' B r i e n , J . Magn. Reson., 47, 168 ( 1 9 8 2 ) . J.M. Thomas a n d S. Ramdas, 8 . C.A. F y f e , G.C. Gobbi, J . = i n o w s k i , 296, 530 (1982). Nature, 9 . J. B.Nagy, J.-P. G i l s o n and E.G. D e r o u a n e , J . Chem. Soc.,Chem. Cornrnun., 1981, 1129. 10. J. B.Nagy, Z . G a b e l i c a , E .G. Derouane and P.A. J a c o b s , Chem. L e t t . , 1982, 2003. 1 1 , G. D e b r a s , J . B.Nagy, Z . ~ a b e l i c a ,P. B o d a r t and P.A. J a c o b s , Chem. L e t t . , 1983, 199. 12. J . B.Nagy, Z . G a b e l i c a , G. D e b r a s , P. B o d a r t , E.G. Derouane a n d P.A. J a c o b s , J . Mol. C a t a l . , i n p r e s s . 13. Z . G a b e l i c a , J . B.Nagy, P. B o d a r t , G. D e b r a s , E . G . Derouane a n d P.A. J a c o b s , i n " Z e o l i t e s S c i e n c e and T e c h n o l o g y v , L i s b o n , 1983 ( t h i s m e e t i n g ) 14. J. B.Nagy, Z . GabelicaandE.G.Derouane,Zeolites, 3 , 4 3 ( 1 9 8 3 ) . Proc. 6th 1 5 , Z . G a b e l i c a , J . B.Nagy, G. D e b r a s and E.G. ~erouan;, I n t . Conf. Z e o l i t e s , Reno, 1983 ( s u b m i t t e d ) . 16. R.M. B a r r e r , J . Chem. S o c . , 1948, 2518. 17. L.L. Ames and L.B. Sand, Amer. M i n e r . , 4 3 , 476 ( 1 9 5 8 ) . 18. L. B. Sand i n " ~ o l e c u l a rs i e v e s " , ~ o c i e t y o fChemical I n d u s t r y , London, p . 7 1 ( 1 968) 19. L.B. Sand, U.S. P a t . 3 , 4 3 6 , 1 7 4 ( 1 9 6 9 ) . 5 7 , 1146 ( 1 9 7 2 ) . 20. O . J . Whitternore J r . , h e r . M i n e r . , 1 2 1 , 140 ( 1 9 7 3 ) . 21. A. C u l f a z and L.B. Sand, Adv. Chem. S e r . , 22. P.K. B a j p a i , M.S. Rao a n d K.V.G.K. G o k h a l e , I n d . Eng. Chem. 223 ( 1 9 7 8 ) . P r o d . R e s . Dev., 23. S. Ueda, H . M u r a t a , M. Koizumi and H . N i s h i m u r a , Amer. M i n e r . , 6 5 , , 1012 ( 1 9 8 0 ) . 5, 227 24. G a b e l i c a , N . Blom and E.G. D e r o u a n e , Appl. C a t a l . ( 1983) 25. E.G. D e r o u a n e , J.-P. G i l s o n Z . G a b e l i c a C . Ilousty-Desbuquoit and J. V e r b i s t , J . C a t a l . , 5 1 , 447 ( 1 9 6 1 j . 26. G. D e b r a s , E.G. D e r o u a n e , J.-P. G i l s o n , Z . ~ a b e l i c aand G. D e m o r t i e r , Z e o l i t e s , 2, 37 ( 1 9 8 3 ) .

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17,

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SORPTION BY ZEOLITES PART I. E Q U I L I B R I A AND ENERGETICS

R.M.

Barrer

Chemistry Department I m p e r i a l C o l l e g e of S c i e n c e and Technology London SbJ7 2AY

ABSTMCT S t e r i c f a c t o r s g o v e r n i n g p e n e t r a t i o n of t h e h o s t z e o l i t e by g u e s t s p e c i e s have b e e n c o n s i d e r e d . A f t e r e q u i l i b r i u m i s e s t a b l i s h e d q u a n t i f i c a t i o n of t h e r e s u l t s can b e made i n s e v e r a l ways. S e l e c t i v i t y of s o r ~ t i o nr e q u i r e s i n t e r n r e t a t i o n of h e a t s and e n t r o n i e s of u p t a k e i n terms r e s p e c t i v e l y o f u n i v e r s a l ( o r nons p e c i f i c ) and of s p e c i f i c components of h e a t , and i n t e r n s of t h e ohysical s t a t e o f t h e s o r b e d s p e c i e s . These a s ~ e c t shave a l s o b e e n considered. I s o t h e r m m o d e l l i n p h a s b e e n d i s c u s s e d i n terms o f t h e v i r i a l i s o t h e r m e q u a t i o n , and i n terms OF a s i t e model i n which a s i t e i s i d e n t i f i e d w i t h a c a v i t y and s o i s a b l e t o h o l d a s m a l l c l u s t e r of m o l e c u l e s .

Z e o l i t e s and Dorous c r y s t a l l i n e s i l i c a s v r o v i d e s t a b l e , h i p h capacity, m i c r o p o r e s o r b e n t s w i t h d i v e r s e m o l e c u l e s i e v i n g p r o p e r t i e s . Each framework t o ~ o l o g yp r o v i d e s i t s own u n i q u e s y s t e m of c h a n n e l s and c a v i t i e s . For example, i n z e o l i t e s i n which wide s t r a i p h t channels a r e found t h e d e t a i l of t h e s e c h a n n e l s i s a u i t e d i f f e r e n t , rYlAZ and MER i n F i g . 1 ( 1 ) . One as i l l u s t r a t e d f o r z e o l i t e s LTL, expects and f i n d s d i f f e r e n c e s i n s o r o t i o n b e h a v i o u r f o r e a c h topology. The b e h a v i o u r i s f u r t h e r m o d i f i e d f o r z e o l i t e s by t h e number, l o c a t i o n and s i z e of t h e i n t r a c r y s t a l l i n e c a t i o n s which n e u t r a l i s e t h e n e g a t i v e c h a r g e on t h e framework. Thus, s i n c e c a t i o n s are exchangeable, m o d i f i e d s o r b e n t s c a n b e made from a g i v e n frarnework t o ~ o l o g yby c a t i o n exchange. The u p t a k e of g u e s t m o l e c u l e s b y z e o l i t e h o s t c r y s t a l s h a s e q u i l i b r i u m and e n e r g e t i c p r o p e r t i e s ,

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.

Some p o l y h e d r a l v o i d s found i n z e o l i t e s . The c h a b a z i t e 20-hedron, capped w i t h hexagonal (i) prisms. The g m e l i n i t e 14-hedron of t y p e 11. (ii) ( i i i ) The e r i o n i t e 23-hedron The l e v y n i t e 17-hedron of t y p e I . (iv) The l o s o d 17-hedron of t y p e I1 w i t h a s s o c i a t e d (v) 11-hedral c a n c r i n i t e cage.

considered i n t h i s P a r t , and k i n e t i c a s p e c t s , d i s c u s s e d i n P a r t 11. I n terms of t h e d e f i n e d s t r u c t u r e s of c h a n n e l s and c a v i t i e s z e o l i t e s provide model systems f o r q u a n t i t a t i v e s t u d i e s of s o r p t i o n .

2.

CONDITIONS FOR PENETRATION OF ZEOLITE LATTICES

Provided a z e o l i t e c r y s t a l i s open enough t o admit t h e g u e s t species under c o n s i d e r a t i o n s o r p t i o n c o m ~ l e x e sform i n which t h e amount of g u e s t s o r b e d i n a g i v e n w e i g h t of z e o l i t e depends o n l y upon t h e p r e s s u r e of t h e vapour of t h e g u e s t and t h e t e m ~ e r a t u r e . However, t h e e a s e of p e n e t r a t i o n of t h e o u t g a s s e d z e o l i t e by g u e s t molecules depends upon t h e f o l l o w i n g f a c t o r s :

1.

2. 3.

4. 5.

6.

The s i z e and shape of t h e windows c o n t r o l l i n g e n t r y t o t h e c h a n n e l s and c a v i t i e s i n t h e z e o l i t e . The s i z e and shane of t h e g u e s t m o l e c u l e s . The number, l o c a t i o n s and s i z e of t h e exchangeable c a t i o n s . The p r e s e n c e o r absence of d e f e c t s such a s s t a c k i n g f a u l t s which may narrow d i f f u s i o n pathways a t p l a n e s where such f a u l t s occur. The p r e s e n c e o r absence of d e t r i t a l m a t e r i a l l e f t i n t h e channels during synthesis o r introduced subsequently, f o r e x a m ~ l e ,by chemical means such a s s i l a n a t i o n ( 2 ) . The p r e s e n c e o r absence of o t h e r s t r o n g l y h e l d g u e s t m o l e c u l e s l i k e w a t e r , ammonia ( 3 ) and s a l t s ( 4 , 5 ) , introduced i n t e n t i o n a l l y i n metered amounts.

The s i x t h f a c t o r i s r e f e r r e d t o i n P a r t 11, Here we w i l l refer primarily t o the f i r s t four factors. 2.1.

F r e e Dimensions of Llindows

The windows o r o ~ e n i n g swhich c o n t r o l e n t r y t o t h e i n t r a c r y s t a l l i n e p o r e s and c h a n n e l s a r e r i n g s of l i n k e d (A1,Si) o4 tetrahedra c i r c u m s c r i b i n g c h a n n e l s l i k e t h o s e shown i n F i g . 1, o r allowing a c c e s s t o p o l y h e d r a l c a v i t i e s l i k e t h o s e i l l u s t r a t e d i n Fig. 2 . The i m p o r t a n t o p e n i n g s from t h e p r e s e n t v i e w p o i n t a r e those composed of 8, 1 0 o r 12 l i n k e d t e t r a h e d r a . These 8-, 10and 12-rings a r e l i n e d on t h e i r i n n e r p e r i ~ h e r i e sby oxygen atoms, and i n t h e frameworks of z e o l i t e s h a v i n g d i f f e r e n t t o p o l o g i e s t h e y may b e v a r i o u s l y e l o n g a t e d , o r puckered t o d i f f e r e n t c o n f o r m a t i o n s , such a s crown, b o a t o r c h a i r c o n f i g u r a t i o n s . T h e r e f o r e a g i v e n n-ring may have d i f f e r e n t f r e e dimensions and s o may imnose d i v e r s e molecule s i e v i n g b e h a v i o u r f o r t h e same v a l u e of n, a c c o r d i n g t o the z e o l i t e i n which i t o c c u r s . The v a r i a t i o n s i n f r e e dimensions a r e shown i n T a b l e 1 f o r t y p i c a l z e o l i t e s w i t h 8-, 1C- and 1 2 - r i n p windows. By 11 f r e e dimensions" one means t h a t t h e s p a c e t o which the dimensions r e f e r i s n o t impinged upon by t h e p e r i p h e r a l oxygens l i n i n g t h e i n s i d e of each r i n g .

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2.2.

Molecular Dimensions

The i d e a of u s i n g s i m p l e molecules of known dimensions t o c h a r a c t e r i s e m o l e c u l a r s i e v e z e o l i t e s w a s ' i n t r o d u c e d some t i m e ago (6,7) and h a s proved most u s e f u l i n g r o u p i n g z e o l i t e s i n t o d i f f e r e n t Based on P a u l i n g ' s v a l u e s c a t e g o r i e s of m o l e c u l a r s i e v e ( 6 , 7 , 8 ) . of bond l e n g t h s and van d e r FJaals r a d i i ofoatoms and m o l e c u l e s t h e dimensions of some y a r d s t i c k molecules i n A a r e a s f o l l o w s (8) :

0

For CH4, 4 . 0 A i s a v a l u e assuming i t t o be a smooth s p h e r e . The value i n b r a c k e t s t a k e s i n t o a c c o u n t t h e t e t r a h e d r a l s t r u c t u r e . For t h e o t h e r m o l e c u l e s t h e f i g u r e s I n b r a c k e t s a r e t h e d i a m e t e r s of t h e c i r c u m s c r i b i n g s p h e r e s and t h o s e n o t i n b r a c k e t s a r e t h e h e i g h t s o f t h e m o l e c u l e s s i t t i n g on a t r i a n g u l a r b a s e . One o r other of t h e s e dimensions s h o u l d b e c r i t i c a l i n d e t e r m i n i n g w h e t h e r t h e molecule w i l l e n t e r t h e z e o l i t e l a t t i c e , For s e v e r a l dumbell-shaped m o l e c u l e s t h e c r o s s - s e c t i o n a l diameter and ( i n b r a c k e t s ) t h e l e n g t h s a r e a s f o l l o w s (8) :

Idhere t h e l e n g t h e x c e e d s t h e c r o s s - s e c t i o n a l d i a m e t e r i t w i l l b e the l a t t e r which i s t h e c r i t i c a l dimension f o r p e n e t r a t i n p t h e zeolite. S i t u a t i o n s have been a b u n d a n t l y d e m o n s t r a t e d i n which one molecular s p e c i e s i s t o t a l l y e x c l u d e d w h i l e a n o t h e r i s r e a d i l y The s e n a r a t i o n can be quantsorbed by a g i v e n z e o l i t e (6,7,8) i t a t i v e i n a s i n g l e s t e p and c a n b e most i m p o r t a n t f o r l a r g e s c a l e s e p a r a t i o n s and i n shape s e l e c t i v e c a t a l y s i s . T a b l e 2 i l l u s t r a t e s s i e v e c h a r a c t e r i s t i c s f o r t h r e e z e o l i t e s , Ca-A, ZSM-5 and f a u j a s i t e ( ~ a - X )f o r which t h e windows a r e r e s p e c t i v e l y 8-, 10- and 1 2 - r i n g s . For f u l l y s t r e t c h e d n-para£ f i n s t h e c r o s s - s e c t i o n a l dimension c r i t i c a l f o r p e n e t r a t i o n i s t h a t i n t h e p l a n e o f t h e zig-zap (4.9 2); f o r benzene t h i s dimension i s t h e d i s t a n c e a c r o s s i n t h e p l a n e o f the m o l e c u l e . For non-symmetrical m o l e c u l e s t h e b e s t o r i e n t a t i o n f o r p e n e t r a t i o n can b e seen by ? r e s e n t i n g a s c a l e model of t h e g u e s t t o a s c a l e model of t h e window.

.

The c r i t i c a l dimensions f o r e n t r y a r e s e e n t o be g r e a t e r than t h e f r e e dimensions of t h e openings (Table 2 ) . This happens because the g u e s t molecules and the l a t t i c e atoms (here oxygens) a r e n o t h a r d s p h e r e s b u t a r e deformable and a l s o because included i n l a t t i c e v i b r a t i o n s t h e r e a r e b r e a t h i n g f r e q u e n c i e s f o r t h e openings a s a whole. Accordingly n - p a r a f f i n s d i f f u s e r a t h e r e a s i l y i n t o Ca-A, although s i g n i f i c a n t energy b a r r i e r s a r e involved. The energy b a r r i e r s i n c r e a s e r a p i d l y a s t h e c r i t i c a l dimensions i n c r e a s e , and those f o r t o t a l e x c l u s i o n a r e soon reached, a s i n d i c a t e d i n Table 2. Temperatures a t which t h e uptake occurs can p l a y a n imnortant p a r t i n molecule s i e v i n g . Because of t h e energy b a r r i e r s involved d i f f u s i v i t i e s , D, follow an h r r h e n i u s r e l a t i o n : D = Do ~ X ~ - E / ~ T . Thus, the lower t h e temperature t h e more s e n s i t i v e the molecules i e v i n g p r o c e s s becomes. A t low temperatures (-183OC) i t i s poss i b l e t o s e p a r a t e 0 2 and A r q u a n t i t a t 8 v e l y u s i n Na-A o r l e v y n i t e ( t h e i r c r i t i c a l dimensions being 2 . 8 A and 3 . 8 a l s o t o o b t a i n l a r g e r a t e d i f f e r e n c e s between O2 Such d i f f u s i o n a l a s p e c t s w i l l be r e f e r r e d t o i n P a r t 11.

I

2.3.

Blocking of Windows by Cations

I n Ca-A, ZSM-5 and f a u j a s i t e t h e windows a r e n o t blocked by c a t i o n s and t h e s i e v e c h a r a c t e r could t h e r e f o r e b e a s s e s s e d from t h e f r e e dimensions of t h e windows deduced from the s t r u c t u r e s . However a f t e r exchanges c a 2 + -+ 2 ~ a +o r 2 ~ +the number of c a t i o n s in z e o l i t e A i s doubled and ~ a and + K+ i o n s occupy p o s i t i o n s i n 8 - r i n g windows. Na-A no longer admits n - p a r a f f i n s a t room temperature, but s t i l l s o r b s 02, N2, A r and small p o l a r molecules. Likewise Ca-chabazi t e w i l l s o r b a-paraf f i n s b u t Na-chabazite does n o t sorb even oxygen ( 1 1 ) . Z e o l i t e RHO s o r b s only water and ammonia u n t i l converted i n t o i t s H-form when i t s o r b s permanent gases and n - p a r a f f i n s c o p i o u s l y . The windows a r e octagonal prisms of f r e e diameter 3.9 x 5 . 1 8, b u t t h e s e windows appear t o be s e l e c t i v e c a t i o n t r a p s (12). When i n z e o l i t e Na-A one exchanges 2 ~ a +by ca2+ i n s t a g e s n - p a r a f f i n s b e g i n t o be sorbed f r e e l y when about 30Z of t h e ~ a + i s replaced ( 1 3 ) . This s e n s i t i v e range occurs when enough 8-ring windows have been f r e e d of c a t i o n s t o g i v e a f r a c t i o n of c l e a r pathways through t h e three-dimensional network of channels. S i m i l a r behaviour i s observed i n o t h e r exchanges of mono- by d i v a l e n t c a t i o n s (11,14)

.

2.4.

Control of Access by S t a c k i n g F a u l t s

Stacking f a u l t s can occur i n c a n c r i n i t e and g m e l i n i t e and sequence of l a y e r s sometimes i n of f r e ti t e . I n c a n c r i n i t e an ab..

.

U

-k

Uar

rn

.d

k

X

0

c o n t a i n i n g s i n g l e hexagons ( 6 - r i n g s ) c a n be i n t e r r u p t e d by a n abc. sequence t y p i c a l of s o d a l i t e . I n g m e l i n i t e a n A3. t sequence of l a y e r s c o n t a i n i n g hexagonal p r i s m s i s i n t e r r u p t e d by a n ABCT sequence, and i n o f f r e ti t e an Ab sequence i s i n t e r sequence of e r i o n i t e . I n a l l t h e s e s i t u a t i o n s r u p t e d by AbAcwide c h a n n e l s curcumscribed by 1 2 - r i n g s a r e b l o c k e d , by windows no w i d e r t h a n t h e 6 - r i n g s of s o d a l i t e o r t h e 8 - r i n g s of c h a b a z i t e and e r i o n i t e f o r c a n c r i n i t e and f o r g m e l i n i t e and of f r e ti t e respectively, I t i s n o t d i f f i c u l t however t o make o f f r e t i t e s f r e e of such i n t e r growths and s t a c k i n g f a u l t s .

..

.

..

.

.-

I n t h e c a s e of c a n c r i n i t e i n p a r t i c u l a r s t a c k i n g f a u l t s may n o t be t h e o n l y cause of b l o c k i n g . Both c a n c r i n i t e and s o d a l i t e are n o t a b l e f o r t r a p p i n g s a l t s and c a u s t i c soda a l o n g w i t h z e o l i t i c w a t e r d u r i n g s y n t h e s i s , and t h e r e i s some a n a l y t i c a l e v i d e n c e t h a t d e t r i t a l s i l i c a t e may b e a f u r t h e r b l o c k i n g f a c t o r i n c a n c r i n i t e

(5) 3.

DISTRIBUTION PATTERNS OF GUEST MOLECULES

The p o r e s p a c e i n z e o l i t e s i s p a r c e l l e d up i n t o c a v i t i e s and/ o r c h a n n e l s of m o l e c u l a r dimensions. The r e s u l t a n t p a t t e r n s of pathways through which g u e s t molecules of t h e r i g h t s i z e and shape can move can be p l a c e d i n t h r e e c a t e g o r i e s :

1.

2.

A l l pathways a r e p a r a l l e l and n o n - i n t e r c o n n e c t e d (l-dimensi o n a l (1-D) channel systems a s i n m o r d e n i t e , m a z z i t e , l a u m o n t i t e o r z e o l i t e L. The pathways whether p a r a l l e l o r n o t a r e i n t e r c o n n e c t e d t o g i v e 2-dimensional (2-D) channel s y s tems Guest molecules may m i g r a t e between l a y e r s b u t cannot move from one l a y e r t o a p a r a l l e l layer i n the c r y s t a l (heulandite, levynite, s t i l b i t e and f e r r i e r i t e ) The pathwavs may be s o i n t e r c o n n e c t e d a s t o a l l o w guest molecules t o m i g r a t e i n 3-dimensions (3-D channel s y s tems a s i n c h a b a z i t e , e r i o n i t e , z e o l i t e A and z e o l i t e s ZSM-5, RHO - and ZK-5)

.

.

3.

.

D e t a i l e d c h a n n e l g e o m e t r i e s a r e however d i f f e r e n t f o r each framework topology, a s was s e e n i n F i g . 1 f o r t h e 1-D channels of z e o l i t e s L (LTL) - , m a z z i t e (MAZ) and m e r l i n o i t e (ME?@). The d i s t r i b u t i o n of t h e g u e s t molecules i n t h e h o s t z e o l i t e s i s determined by t h e 1-D, 2-D o r 3-D n a t u r e of t h e c h a n n e l s i n which t h e y a r e l o c a t e d . Thus i n 1-D systems t h e y a r e p r e s e n t a s p a r a l l e l filaments s u p p o r t e d by t h e c h a n n e l w a l l s . For example i n z e o l i t e L t h e r e are r e s t r i c t i o n s a l o n g each channel of 7 . 1 8 f r e e d i a m e t e r a l t e r n a t i n g w i t h w i d e r p a r t s of % 12 f r e e d i a m e t e r . Gihen t h e channel i s f u l l of s m a l l molecules l i k e w a t e r o r oxygen t h e s e form l i q u i d - l i k e beads o r c l u s t e r s i n t h e wide p a r t connected i n t o f i l a m e n t s through the

-

8

0

7 . 1 A openings t o o t h e r b e a d s . I n o f a u j a s i t e each 26-hedron of type I1 of f r e e d i a m e t e r a b o u t 12 A i s connected through 7.4 b: openings t o f o u r more such t e t r a h e d r a . L i q u i d - l i k e c l u s t e r s i n each 26-hedron a r e connected t o t h o s e i n i t s f o u r n e i g h b o u r s t o give a 3-D p a t t e r n of connected c l u s t e r s a r r a n g e d l i k e t h e bond p a t t e r n i n diamond. The s m a l l e r t h e f r e e d i a m e t e r o f t h e c o n n e c t i n g windows t h e more i s o l a t e d each m o l e c u l a r c l u s t e r becomes from i t s n e i g h b o u r s while t h e s m a l l e r t h e c a v i t y a n d / o r t h e l a r g e r t h e g u e s t molecule

Table 3:

zeolite

Chabazi t e

C l u s t e r s i z e s a t s a t u r a t i o n of c a v i t i e s i n c h a b a z i t e and faujasite (zeolite XI.

Cavities

20-hedra (6 x 8 - r i n g s 2 x 6-rings 12 x 4 - r i n g s )

Guest molecules p e r c a v i t y

12-14 H 2 0 -7.7 NH3 ~6 A r , N 2 , O2 $ 4 . 9 CH3NH2 ~ 4 . 3 CH3C1 $3.1 C P 2 C I 2 -2.0

Faujasite

26-hedra (4 x 1 2 - r i n g s 4 x 6-rings 18 x 4 - r i n g s )

l2

a32 H 2 0 (28 + 4)' 17-19 A r , N2, O2 ~ 7 . 5I 2 %7.8 C F 4 -6.5 SF6 Q5.8 C2F6 a5.6 c y c l o ~ e n t a n e ~ 5 . 4benzene ~ 4 . 6t o l u e n e ~ 4 . 5n-CgH12 % 4 . 1 cyclohexane $4.1 p e r f l u o r o c y c l o b u t a n e $4.1 C2FqC12 Q3.5 n-C7H16 -3.4 C 3 F g

-2.9 n-C4F1o

~ 2 . 8 SO-C8H1 ~ 2 . 3perfluoromethylcyclohexane

-2.1 B

perfluorodimethylcyclohexane

Four of t h e w a t e r molecules a r e t h o u g h t t o be i n t h e s o d a l i t e - t y p e 14-hedra a l s o p r e s e n t i n f a u j a s i t e .

t h e fewer a r e t h e m o l e c u l e s p e r c l u s t e r (Table 3 ) . Thus i n t h e 14-hedral c a v i t i e s o o f s o d a l i t e h y d r a t e each c a v i t y h a s a f r e e d i a m e t e r of % 6.6 A and i s connected by 6 - r i n g windows of Q 2 . 1 A f r e e d i a m e t e r t o e i g h t n e a r e s t neighbour 14-hedra. I t can,accoamodate f o u r w a t e r molecules (van d e r Waals d i a m e t e r % 2.8 A ) , a s a n e a r l y i s o l a t e d c l u s t e r due t o t h e s m a l l f r e e d i a m e t e r of t h e windows. A t h i g h p r e s s u r e and t e m p e r a t u r e i t o c a n accommodate one o n l y Ar o r K r (15) of d i a m e t e r 3 . a 3 and 3.g4 A r e s p e c t i v e l y . This corresponds with i n t e r s t i t i a l s o l u t i o n as t h e l i m i t t o the l a r g e r c l u s t e r s i l l u s t r a t e d i n Table 3 f o r t h e 20-hedra n r e s e n t i n chabazi r e and t h e 26-hedra i n f a u j a s i t e .

4.

TYPES OF ISOTHERM AND GUEST-ZEOLITE COMPLEX

Under a p p r o p r i a t e c o n d i t i o n s z e o l i t e s can s o r b non-polar and p o l a r molecules, s a l t s o r metals. I n a d d i t i o n m e t a l s may b e i n t r o duced by r e d u c i n g c a t i o n i c forms of t h e z e o l i t e o r by s o r b i n g v o l a t i l e m e t a l l i c compounds such a s c a r b o n y l s and t h e n decomposing t h e s e . S a l t s may be i n t r o d u c e d , i n c o m p e t i t i o n w i t h w a t e r , during hydrothermal s y n t h e s i s , o r from aqueous s o l u t i o n i n t o t h e a l r e a d y formed z e o l i t e , a s w e l l a s from s a l t m e l t s o r vapours. Isotherms of non-polar g u e s t molecules a r e a s a r u l e of type T i n B r u n a u e r ' s c l a s s i f i c a t i o n (16) a s shown i n F i g . 3 ( 1 7 ) . The more condensable t h e g u e s t molecule o r t h e lower t h e t e m p e r a t u r e t h e more r e c t a n g u l a r t h e i s o t h e r m s become. On t h e o t h e r hand t h e l e s s condensable t h e s o r b e d molecule o r t h e h i g h e r t h e t e m p e r a t u r e the more n e a r l y does t h e i s o t h e r m approach t h e Henry's law l i m i t (uptake proportional t o pressure). There a r e however s o r p t i o n complexes c h a r a c t e r i s e d by v e r y s t r o n g molecule-molecule i n t e r a c t i o n s between p a i r s o f g u e s t molec u l e s which d r a m a t i c a l l y change t h e i s o t h e r m c o n t o u r s t o t y p e s IV o r V i n Brunauer's c l a s s i f i c a t i o n . These w i l l b e i l l u s t r a t e d f o r a n e l e c t r o n e g a t i v e e l e m e n t , a s a l t and a m e t a l . Thus F i g . 4 (18) shows i s o t h e r m s n e a r l y of type V f o r t h e r e v e r s i b l e u ~ t a k eof phosphorus i n z e o l i t e Na-X. The s t r o n g upward i n f l e x i o n may a r i s e from t h e o n s e t of p o l y m e r i s a t i o n of s m a l l e r phosphorus s p e c i e s such as Pq w i t h i n t h e z e o l i t e . When s a l t s a r e i n c o r p o r a t e d i n t o s o d a l i t e d u r i n g hydrothermal s y n t h e s i s k e e p i n g w a t e r , c a u s t i c soda and m e t a k a o l i n i t e c o n s t a n t i n t h e r e a c t i o n m i x t u r e t h e i s o t h e r m s of s a l t u ~ t a k ea r e a g a i n of type I a s shown i n F i g . 5 ( 1 9 ) . However when s a l t s were t a k e n up from aqueous s o l u t i o n s i n t o pre-formed z e o l i t e s t h e i s o t h e r m s i n F i g . 6 (20) were of type 111, w i t h c u r v a t u r e i n t h e o p p o s i t e s e n s e t o those i n F i g . 5. T h e i r shape i s determined by a Donnan e q u i l i b r i u m whereas t h e shape i n F i g . 5 may a r i s e because t h e s a l t s a r e a c t i n g a s tem!hen p l a t e s d u r i n g a c t u a l c r y s t a l n u c l e a t i o n and growth ( 2 1 ) .

Pressure (cmHg)

Fig. 3 .

Type I isotherms of CFI, i n ~ a - f a u j a s i t e a t v a r i o u s absolute temperatures ( 1 7 ) .

Phosphorus pressure cm Hg

~ i g .4 ,

Isotherms f o r uptake of phosphorus i n Na-X (18). Temperatures a r e i n O C . 0 = a d s o r p t i o n p o i n t s ; = desorption points.

0

1

02

L

06

-

J

10

-

l

1.4

L

U

1.8

Mdarity of salt In synthes~ssolution

Fig. 5.

I s o t h e r m s f o r u p t a k e of NaC1Q4 (0) and NaC103 (a) from aqueous s o l u t i o n s d u r i n g s y n t h e s i s of s o d a l i t e ( 1 9 ) . 32g NaOH; 2g metaThe c o n d i t i o n s of f o r m a t i o n were: k a o l i n ; 200 m l d i s t i l l e d w a t e r , t o which t h e d e s i r e d amounts o f s a l t were added. R e a c t i o n i n p o l y p r o p y l e n e b o t t l e s r o t a t e d a t 80°C f o r 6 d a y s .

Mdes MCI per litre of solution

Fig. 6 .

I s o t h e r m s f o r s a l t s a t 2 5 O ~i n pre-formed z e o l i t e X (20) a = KC1 a = LiCl 0 = NaCl A = CsCl A = CaC1,

Pressure ( mm Hg

Fig. 7 .

I s o t h e r m s f o r u p t a k e s i n z e o l i t e K-L a t 245OC of A V a p o u r i s e d N H 4 C 1 ( i . e . NH3 + H C 1 i n 1:l r a t i o ) B H C 1 gas a l o n e C NH3 a l o n e ( 2 2 ) .

Pressure cm Hc

Fig. 8.

Isotherms f o r uptake of H g curve represents the l i m i t s a t u r a t i o n vapour p r e s s u r e experimental temperature. p o i n t s ; @, A , I and are

+

i n Na-X ( 2 3 ) . The u p p e r t o t h e u p t a k e s e t by t h e of l i q u i d Eg a t each 0, A , 0 and o are a d s o r p t i o n desorption points.

Pressure cm Hg

Fig. 9.

S o r p t i o n of H g i n Ag-X ( 2 3 ) 0 ( a ) Uptakes a t two t e m p e r a t u r e s ( C) 0 (b) A s o r p t i o n - d e s o r p t i o n c y c l e a t 235.2 C 0 (c) Two s u c c e s s i v e s o r p t i o n - d e s o r p t i o n c y c l e s a t 270 C

F i g . 10.

A s o r p t i o n - d e s o r p t i o n c y c l e f o r u p t a k e of p-xylene i n z e o l i t e H-ZSM-5 (sio2/A1203 = 226) a t 70°c ( 2 4 ) .

NH4Cl was sorbed from i t s vapour i n t o z e o l i t e s a t h i r d i s o t h e r m contour was found, of type V , and r a t h e r s i m i l a r t o t h a t i n Fig. 4 . This i s i l l u s t r a t e d i n F i g . 7 ( 2 2 ) . When ammonium c h l o r i d e i s vapourised i t d i s s o c i a t e s v i r t u a l l y completely i n t o a 1:l mixture of HC1 + NH3. Therefore from i t s vapour i t i s t h i s mixture which is sorbed i n t o t h e z e o l i t e . I n Fig. 7 curve C i s the r e v e r s i b l e isotherm of NH3 alone i n z e o l i t e K-L a t 245OC; curve B i s t h i s isotherm f o r pure H C 1 ; and curve A shows what happens when t h e 1:l mixture of NH3 + H C 1 ( i . e . NH4C1 vapour) i s sorbed. There i s clearly a s t r o n g i n t e r a c t i o n between NH3 and H C 1 w i t h i n t h e z e o l i t e .

When metals a r e i n i t i a l l y a t o m i c a l l y d i s p e r s e d i n z e o l i t e s , f o r example by r e d u c t i o n s such a s

and the system is s u b j e c t e d t o f u r t h e r h e a t i n g , t h e metal atoms tend t o n u c l e a t e i n t o c l u s t e r s w i t h i n t h e z e o l i t e o r o u t s i d e i t as small c r y s t a l l i t e s . This tendency can be s t u d i e d f o r i n t r a c r y s t a l l i n e n u c l e a t i o n u s i n g mercury a s t h e g u e s t s p e c i e s . When the c o n c e n t r a t i o n of mercury atoms i s low n u c l e a t i o n does n o t occur and i n t r a z e o l i t e s o r p t i o n follows Henry's law, a s shown i n F i g . 8 + pb2+ a r e d e r i v e d from for uptake i n Na- and Pb-X ( 2 3 ) . ~ a and elements h i g h e r i n t h e e l e c t r o c h e m i c a l s e r i e s than Hg. I n c o n t r a s t with t h i s behaviour when s o r p t i o n occurred i n Hg- o r Ag- z e o l i t e s the isotherms became once more of type I V i n t h e Brunauer c l a s s i f i c a t i o n , a s seen i n Fig. 9 ( 2 3 ) . The i o n s Hg2+ o r Ag+ o r i g i n a l l y in the z e o l i t e a r e d e r i v e d from elements as low a s o r lower i n t h e electrochemical s e r i e s than mercury, s o t h a t r e d u c t i o n s can occur:

Such r e d u c t i o n s , which may r e p r e s e n t t h e f i r s t s t e p i n the isotherms of Fig. 9 , appear t o t r i g g e r o f f c l u s t e r i n g processes:

Ag + xHg

-t

+ xHg Hg2 2+

AgxHg

(2) -t

~

~

x+2

2

+

Isotherms w i t h contours l i k e those of P , N H 4 C l o r Hg a r e r a r e compared with those shown f o r CF4 i n Fig. 3. It i s thus of i n t e r e s t t h a t a type I V i s o t h e r m h a s been found f o r p-xylene i n ZSM-5 i n which a c l e a r s t e p , t h i s time w i t h some h y s t e r e s i s , occurs ( 2 4 ) . The explanation o f f e r e d was n o t i n terms of s t r o n g molecule-molecule interaction, but t h a t a t a c e r t a i n i n t r a c r y s t a l l i n e concentration

t h e y pack i n a new c o n f i g u r a t i o n , more economical of s p a c e ( F i g . 10).

5.

SELECTIVITY I N MIXTURE SEPARATION AND HEATS OF SORPTION

That m i x t u r e s c a n b e s e p a r a t e d , o f t e n q u a n t i t a t i v e l y and i n a s i n g l e s t e p , by m o l e c u l e s i e v i n g h a s b e e n v e r y f u l l y e s t a b l i s h e d . T h i s i s i l l u s t r a t e d i n T a b l e 4 i n which t h e c r y s t a l l i n e z e o l i t e was powdered n a t u r a l c h a b a z i t e (7,s). Ca-chabazite i s one of a c l a s s of z e o l i t e s a b l e t o s e p a r a t e n - p a r a f f i n s from i s o - neo- and c y c l o p a r a f f i n s and a r o m a t i c s . O t h e r s i n t h i s c l a s s a r e Ca-A, e r i o n i t e , z e o l i t e ZK-5 and t h e hydrogen form o f z e o l i t e RHO. S t r o n g s e l e c t i v i t i e s a r e a l s o p o s s i b l e when b o t h components of a b i n a r y m i x t u r e c a n e n t e r t h e z e o l i t e and e q u i l i b r a t e w i t h i t . To u n d e r s t a n d t h e s e s e l e c t i v i t i e s one may c o n s i d e r t h e components of t h e p h y s i c a l bond between g u e s t m o l e c u l e s and h o s t c r y s t a l s . These i n c l u d e D i s p e r s i o n e n e r g y , $D Close-range r e p u l s i o n e n e r g y , $R P o l a r i s a t i o n e n e r g y , +p Field-dipole energy, F i e l d gradient-quadrupole energy, $ FQ Guest-guest self-energy, $ SP ~ l e c t r i cmoments i n t h e g u e s t molecule may b e permanent, o r they may b e i n d u c e d by t h e l o c a l e l e c t r o s t a t i c f i e l d of s t r e n g t h F . Thus p o l a r i s a t i o n e n e r g y can have components o f f i e l d - i n d u c e d dipole o r dipole-induced dipole. The components bD, $R and $p are termed " n o n - s p e c i f i c " because t h e y a r e always i n v o l v e d i n t h e h o s t - g u e s t bond. w i t h t h e advent of high s i l i c a z e o l i t e s , hydrogen z e o l i t e s ~d porous c r y s t a l l i n e s i l i c a s l o c a l f i e l d s F and f i e l d g r a d i e n t s F can be much reduced s o t h a t @p can become s m a l l . The components I $ ~ + , and for m o l e c u l e s w i t h permanent d i p o l e moments p o r m o l e c u l a r quadrupole moments Q are termed " s p e c i f i c " components of t h e h o s t-gues t bond b e c a u s e t h e y do n o t a r i s e w i t h non-polar m o l e c u l e s l i k e t h e r a r e g a s e s . They c a n b e v e r y l a r g e i n aluminous z e o l i t e s and c a n l e a d t o u n u s u a l l y h i g h h e a t s of s o r p t i o n and s e l e c t i v i t i e s (e.g. H 2 0 o r NH3).

$iQ

The g u e s t - g u e s t i n t e r a c t i o n s g i v i n g 4Sp may i n extreme cases be c h e m i c a l a s w e l l a s p h y s i c a l i n n a t u r e , a s f o r P and f o r (NH3 + HC1) o r i n c l u s t e r i n g of Hg atoms ( 5 . 4 ) . U s u a l l y however o n l y p h y s i c a l i n t e r a c t i o n s a r e i n v o l v e d . These a r e u n i v e r s a l l y d i s p e r s i o n and c l o s e - r a n g e r e p u l s i o n , and f o r m o l e c u l e s w i t h permanent e l e c t r i c moments t h e r e may b e terms i n 4 such a s SP

Condit~onsand Comments - .- - - - As l i q u ~ dat -20 C Rapd and quantltntlVC As above As ahove

Componcnt(r 1 Sorbed

Mixture

-

.

CHaOH t (CH.),

-

I C.H OH i CHCI:

cb

As abovc

+

As above

odj

c,k,, C,H,OH

I H,O (C,H,):O C,H,OH I CH,COC,H,

I

i

I

N,O, HzS

a,

I

I CH NH

c,h.od'

As liquid at 112 C . Separation rapid and comnktc AS ii&irl at -20 0. L a b e l l i n g may f o r example be w i t h radio-carbon, o r w i t h deuterium. The boundary c o n d i t i o n s of 5. 2.1 a r e convenient and with them t h e mathematical r e l a t i o n s a l r e a d y given a r e a p p l i c a b l e . Another e f f e c t i v e way of measuring s e l f - d i f f u s i o n i s by pulsed f i e l d g r a d i e n t NMR. For a bed of z e o l i t e powder an o v e r - a l l o r e f f e c t i v e d i f f u s i v i t y i s given by

I n t h i s e x p r e s s i o n A i s the time i n t e r v a l between p u l s e s of width <w2) i s t h e mean square displacement of a o and amplitude g. molecule over the i n t e r v a l A ; T i s t h e l i f e t i m e of guest molecules w i t h i n t h e c r y s t a l s ( a l l of r a d i u s r o ) . y i s t h e gyromagnetic r a t i o , p i s t h e f r a c t i o n of the molecules i n t h e bed which a r e i n t h e gas phase and D" i s t h e d i f f e r e n t i a l s e l f - d i f f u s i v i t y of the g u e s t i n t h i s gas p a s e . There a r e two extremes i n t h e above expression:

a

: I

1. 2.

-r >> A and so (w2) r > r

I n t h i s l i m i t-D e f f Here D e f f IpD8.

+

Dk.

These two extremes can be i l l u s t r a t e d by r e s u l t s f o r cyclohexane i n Na-X, when Rn Deff i s p l o t t e d a g a i n s t K/T ( F i g . 3 ( 6 ) ) . Below -65OC D e f f -+B* while above +lo0 D e f f + P6g, the temperature c o e f f i c i e n t b e i n g p r i m a r i l y t h a t of p . The r a t h e r f l a t intermediate region i s one where t r a n s l a t i o n a l d i f f u s i o n i s r e s t r i c t e d mainly t o t h e i n t r a c r y s t a l l i n e pore space of Na-X because the thermal energy i s i n s u f f i c i e n t f o r most molecules t o evaporate from the c r y s t a l s . Besides g i v i n g and pE* the NMR method, from p l o t s of D e f f against K / T , allows one t o f i n d b e temperature range where gas phase and i n t r a c r y s t a l l i n e d i f f u s i o n a r e both i n f l u e n c i n g D e f f , a s seen i n Fig. 3 .

E*

2.3.

Determining the Enhrrrv of A c t i v a t i o n f o r iffu us ion

I n t r a c r y s t a l l i n e d i f f u s i o n involves m i g r a t i o n of molecules from one s i t e t o a n o t h e r o f t e n with a squeeze p a s t an o b s t r u c t i o n such as a narrow window. Thus u n i t d i f f u s i o n s t e p s r e q u i r e an a c t i v a t i o n energy, E , and D can be expressed i n terms of t h e Arrhenius r e l a t i o n

I n p l o t s of M ~ / Ma, g a i n s t t a t two temperatures T1 and T2 one measures t h e times tl and t 2 f o r M ~ / Mt o~ reach a chosen value. Then, from eqn. 3 f o r example, one must have f o r c o n s t a n t pressure

Accordingly, from eqns. 11 and 1 2 i-

1

Fig.

Fig. 4.

for cyclohexane i n Na-X (10) w i t h 'L 2-molecules p e r l a r g e c a v i t y ( D ) , and comparison w i t h D* (dashed l i n e ) and w i t h corresponding v a l u e s from s o r p t i o n r a t e s using t h e method of moments ( 9 ) .

Deff

Sorption-desorption k i n e t i c s of n-hexane i n z e o l i t e ( 4 ) . Qt i s the amount sorbed o r desorbed a t time t and &D i s the f i n a l amount sorbed o r desorbed. Temperatures of runs i n OC a r e : H-RHO

Sorption

0, 33.4

x, 95.6 A , 141.4

Desorption

0 , 33.4

m,

95.6

m , 14.4

s o t h a t E i s o b t a i n e d . T h i s method i s a p p l i c a b l e f o r any d i s t r i b u t i o n o f c r y s t a l s h a p e s and s i z e s because i t i n v o l v e s o n l y eqn. 10 and e x p e r i m e n t a l c u r v e s of M / M ~a g a i n s t t , t

3.

COMPLICATING FACTORS

The e x p r e s s i o n s i n § 2.1 r e l a t i n g M ~ / M and t i m e r e f e r t o s t r i c t boundary c o n d i t i o n s , c o n s t a n t d i f f u s y v i t y , c r y s t a l s a l l of one s i z e and s h a p e , i s o t h e r m a l c o n d i t i o n s t h r o u g h o u t e a c h bed w i t h i n e a c h c r y s t a l and i n t r a c r y s t a l l i n e d i f f u s i o n a s t h e r a t e c o n t r o l l i n g s t e p . However e q n s . 5 and 9 , i n which M ~ / iEs ~ p r o p o r t i o n a l t o fi, a p p l y t o s m a l l t f o r any d i s t r i b u t i o n of c r y s t a l s i z e and s h a p e . I t i s o n l y n e c e s s a r y t h a t t i s s u c h t h a t e a c h c r y s t a l l i t e i s b e h a v i n g a s a s e m i - i n f i n i t e medium ( i . e . t h e deep i n t e r i o r s a r e f r e e o f d i f f u s a n t ) . Idhen t h e above r e q u i r e m e n t s of t h e r e l a t i o n s h i p s i n 5 2 . 1 are compared w i t h r e a l s i t u a t i o n s t h e f o l l o w i n g d i s t u r b i n g f a c t o r s may a r i s e (cf. § 2):

1. Boundary c o n d i t i o n s a s g i v e n i n 5 2 . 1 a r e n o t m a i n t a i n e d , and i n t h e c a s e of v a r i a b l e p r e s s u r e c o n s t a n t volume r u n s s o r p t i o n i s o t h e r m s do n o t obey H e n r y ' s law. 2. Where i n t r a c r y s t a l l i n e d i f f u s i o n i s f a s t t h e r a t e d e t e r m i n i n g s t e p may c e a s e t o be governed o n l y by i n t r a c r y s t a l l i n e d i f f u s i o n b u t by s u r f a c e b a r r i e r s o r by i n t e r - p a r t i c l e f l o w (5 2 . 2 . , eqn. 1 0 ) .

3. I n t r a - c r y s t a l l i n e d i f f u s i v i t i e s may b e f u n c t i o n s of t h e c o n c e n t r a t i o n of d i f f u s a n t .

4 . Because o f l i b e r a t i o n of t h e h e a t of s o r p t i o n , t h e uptake of g u e s t s d o e s n o t o c c u r u n d e r i s o t h e r m a l c o n d i t i o n s . H e a t i n g and t h e n c o o l i n g of t h e bed and of e a c h c r y s t a l l i t e s u p e r p o s e s d r i f t s in e q u i l i b r i a and r a t e s e x p e c t e d f o r i s o t h e r m a l u p t a k e s . These e f f e c t s a r e f u r t h e r d i s c u s s e d i n o r d e r , below. 3.1. Maintenance of Boundarv c o n d i t i o n s The problem of m a i n t a i n i n g r i g o r o u s boundary c o n d i t i o n s i s i l l u s t r a t e d by a r e c e n t s t u d y of n-hexane u p t a k e i n t h e hydrogen form of z e o l i t e RHO ( F i g . 4 ( 4 ) ) . The boundary c o n d i t i o n s sought f o r s o r p t i o n were t h o s e of eqn. 2, and t h e c o r r e s p o n d i n g ones f o r d e s o r p t i o n were

(i)

C = Co i n e a c h c r y s t a l a t time t = 0

( i i ) C = 0 j u s t w i t h i n e a c h c r y s t a l a t ro f o r t > 0.

The uptakes were measured g r a v i m e t r i c a l l y u s i n g a s i l i c a s p r i n g balance w i t h a bed of 0.3 g of z e o l i t e powder i n a g l a s s bucket. Fig. 4 shows t h a t (a) s o r p t i o n r a t e s measured a s Q ~ / Q , vs t were much l a r g e r than d e s o r p t i o n r a t e s ; and (b) f o r s o r p t i o n Q ~ / Q , a t given t had a n e g a t i v e temperature c o e f f i c i e n t whereas i n d e s o r p t i o n QI/&O had a p o s i t i v e temperature c o e f f i c i e n t . The h i g h e r t h e temperature t h e r e f o r e t h e more n e a r l y did Qt/Om vs t f o r s o r p t i o n and d e s o r p t i o n approach one a n o t h e r . The above behaviour was considered i n t h e following terms. Although t h e c o n s t a n t p r e s s u r e boundary c o n d i t i o n s of eqn. 2 were sought, when the v a l v e from t h e l i q u i d n-hexane r e s e r v o i r t o t h e sorption volume was opened a c o n s i d e r a b l e p e r i o d e l a p s e d b e f o r e the e q u i l i b r i u m vapour p r e s s u r e was e s t a b l i s h e d i n the s o r p t i o n volume. A t low temperature, say T I , a l l p r e s s u r e s i n t h e s o r p t i o n volume correspond w i t h p o i n t s along the f l a t top of t h e very rectangular s o r p t i o n isotherm, and t h e r e f o r e w i t h s a t u r a t i o n uptake. The boundary c o n d i t i o n s of eqn. 2 were accordingly met:

(i)

C = 0 a t t = 0 i n each c r y s t a l

(ii)

C = Cw a t t > 0 j u s t w i t h i n each c r y s t a l a t ro.

As T i n c r e a s e s the isotherm becomes l e s s and l e s s r e c t a n g u l a r s o t h a t the changing p r e s s u r e means t h a t C j u s t w i t h i n a c r y s t a l Thus Qt/qw i s varies with time and only slowly approaches C,. l e s s than i t would have been a t time t i f the second of t h e above boundary c o n d i t i o n s had been v a l i d a t once. I f t h e r a t e of uptake by t h e A second f a c t o r a l s o o p e r a t e s . c r y s t a l s i s comparable with the r a t e of supply of vapour by i n t e r c r y s t a l flow t h e r e w i l l be a p r e s s u r e g r a d i e n t i n t o the bed. A t the low temperature, TI, the r e c t a n g u l a r isotherm s t i l l e n s u r e s the approximate v a l i d i t y of the second o f the above boundary conditions. However, f o r the l e s s r e c t a n g u l a r isotherms a t h i g h e r T t h i s boundary c o n d i t i o n w i l l be l e s s and l e s s c o r r e c t and the deviation w i l l i n c r e a s e with denth i n t h e bed. This behaviour augments t h a t of the previous paragranh i n making Q ~ / \ even l e s s than i t would have been had the boundary c o n d i t i o n s of eqn. 2 been met. I f the two e f f e c t s i n f l u e n c e the temperature dependance of Q ~ / Q more ~ than does t h e d i f f u s i v i t y , with i t s p o s i t i v e temperature dependance , then the observed n e g a t i v e temperature c o e f f i c i e n t i n curves of ~ ~ / &vso t f o r s o r p t i o n w i l l r e s u l t .

We n e x t consider why d e s o r p t i o n i s s o much slower than s o r p t i o n and has a p o s i t i v e temperature c o e f f i c i e n t ( F i g . 4 ) . On reducing the p r e s s u r e of s o r b a t e o u t s i d e the bed t o t h e lowest p o s s i b l e vacuum l e v e l , w i t h i n the bed, a s the c r y s t a l s shed n-hexane, t h e r e

w i l l remain r e s i d u a l p r e s s u r e s which a r e g r e a t e s t a t depth. A t t h e l o w temperature, TI, the r e c t a n g u l a r i s o t h e r m ensures l a r g e d e p a r t u r e s from t h e second boundary c o n d i t i o n f o r d e s o r p t i o n ( C = 0 f o r t > 0 j u s t w i t h i n each c r y s t a l s u r f a c e ) . Therefore ~ ~ i1s much % l e s s a t given t than i t would be had t h i s second boundary c o n d i t i o n been met. I t w i l l t h e r e f o r e be much l e s s a l s o than Q ~ / Qf o~r s o r p t i o n a t time t and temperature TI where the boundary c o n d i t i o n s of eqn. 2 a r e met.

A s temperature i n c r e a s e s and the isotherms become l e s s and l e s s r e c t a n g u l a r the r e s i d u a l p r e s s u r e s a t depth i n the bed w i l l r e s u l t i n s m a l l e r d e p a r t u r e s from t h e c o n d i t i o n C = 0 f o r t > 0 j u s t w i t h i n each c r y s t a l s u r f a c e , and O t / k f o r given t w i l l inc r e a s e a s compared with qt/Qm a t t h e low temperature TI. When t h i s e f f e c t i s combined w i t h the p o s i t i v e temperature c o e f f i c i e n t of i n t r a c r y s t a l l i n e d i f f u s i v i t y t h e p o s i t i v e t e m ~ e r a t u r ecoeff i c i e n t i n curves of Ot/Qm vs t f o r d e s o r p t i o n (Fig. 4 ) i s t o be expected. I t can be concluded t h a t f o r r e c t a n g u l a r isotherms, provided s o r p t i o n i s s u f f i c i e n t l y slow t o minimise h e a t i n g of the bed and t o avoid r a t e c o n t r o l by i n t e r p a r t i c l e flow, s o r p t i o n k i n e t i c s can s e r v e t o give d i f f u s i v i t i e s , D, b u t d e s o r p t i o n may n o t . Also, as isotherm c u r v a t u r e becomes l e s s and l e s s , i n t h e Henry's law l i m i t curves of Q ~ / Q , f o r s o r p t i o n and d e s o r p t i o n should coincide and e i t h e r could, under c o n s t a n t p r e s s u r e o r c o n s t a n t volume v a r i a b l e p r e s s u r e c o n d i t i o n s , s e r v e t o give D.

The major importance of boundary c o n d i t i o n s i s thus c l e a r , through t h e i r i n f l u e n c e on t h e i n t e r p r e t a t i o n of t h e k i n e t i c s . Equations such a s those i n 5 2 . 1 a r e v a l i d f o r t h i s purpose only when the boundary c o n d i t i o n s f o r which they were derived a r e s t r i c t l y maintained. This can experimentally be d i f f i c u l t t o achieve.

3 . 2 . The R a t e - d e t e r m i n i n ~ S t e ~ For i n t r a - c r y s t a l l i n e d i f f u s i o n t o be rate-determining the process must be much slower than t h e r a t e of supply of g u e s t molecules t o t h e s u r f a c e s of t h e c r y s t a l s comprising t h e bed. In some z e o l i t e s , such a s &-A o r f a u j a s i t e ( z e o l i t e X and Y) , t h i s i s f a r from the c a s e f o r many g u e s t molecules. Attempts t o e v a l u a t e s e l f - d i f f u s i v i t i e s f o r i n t r a - c r y s t a l l i n e d i f f u s i o n from s o r p t i o n k i n e t i c s have o f t e n l e d t o gross d i s c r e p a n c i e s when compared w i t h those determined from pulsed f i e l d g r a d i e n t NMIZ, ( 5 2.2) as i l l u s t r a t e d by the f i g u r e s i n Table 1 ( 7 ) . The NMR procedure can give D* f o r i n t r a c r y s t a l l i n e d i f f u s i o n f o r high v a l u e s of D* and i s t h e r e f o r e i n t h e s e s i t u a t i o n s a s u i t a b l e s t a n d a r d f o r comparisons (8, 9 , 10). The t a b l e shows t h a t t h e

Temp. (OC)

Molecules p e r cavity (NMR) 2x10-5 2x104 5x10-~ 5x10-~ 4.5~10'~ 2~ 1 0 ' ~ 2 ~ 1 0 - 5(0% A ~ ) 7 x 1 0 (10% ~ ~ Ag) 2x10-~(20%Ag) 2 ~ 1 0 '(507, ~ A ~ ) 10- ( iooz ~ g )

(NMR)

D*

7

-

5x10-I O 10-10 3x10;-I l 3x10-~ 4x10-~ 10-10 5x10'~ (0% Ag) 10-7(iox A ~ ) 5x10'~ (20% Ag) 2x10'~ (50% Ag) 10-8 (loox Ag)

D* (sorption rates)

Comparison of D* (cm2 s - l ) from pulsed NMR and from sorption Kinetics ( 7 ) ,

System

Table 1:

d i s c r e p a n c i e s can be v e r y g r e a t i n d e e d ; t h e y i n d i c a t e t h a t i n sorption k i n e t i c s the. rate-controlling s t e p i s not j u s t i n t r a c r y s t a l l i n e d i f f u s i o n b u t must i n v o l v e o t h e r f a c t o r s such a s i n t e r c r y s t a l flow and h e a t e f f e c t s . The k i n e t i c d a t a have i n many c a s e s been wrongly i n t e r p r e t e d i n terms of i n t r a - c r y s t a l l i n e d i f f u s i o n alone. I f i n t r a - c r y s t a l l i n e d i f f u s i o n i s rate-controlling then the v a l u e s of t h e d i f f u s i v i t y o b t a i n e d from s o r p t i o n k i n e t i c s must ( a ) b e independent of t h e s i z e of t h e c r y s t a l l i t e s used i n t h e measurements, f o r beds of t h e same form and d e p t h ; and (b) be independent of t h e t h i c k n e s s of t h e bed f o r c r y s t a l l i t e s a l l of t h e same s i z e . These c r i t e r i a have n o t o f t e n been a p p l i e d . A s a t h i r d c r i t e r i o n , of c o u r s e , k i n e t i c s and p u l s e d NMR must g i v e t h e same v a l u e s of s e l f - d i f f u s i v i t y f o r e q u a l l o a d i n g s of t h e c r y s t a l s . Attempts which have been r e a s o n a b l y s u c c e s s f u l have been made t o r e c o n c i l e d i s c r e p a n t v a l u e s o f B* from p u l s e d f i e l d g r a d i e n t NMR and from s o r p t i o n k i n e t i c s by making t h e c r y s t a l l i t e s l a r g e r ( F i g . 5) and making t h e bed of extreme t h i n n e s s (6, l l ) , o p t i m a l l y no more t h a n t h e t h i c k n e s s of a s i n g l e c r y s t a l . Extreme c a r e i n the k i n e t i c measurements i s r e q u i r e d even s o , a s e x e m p l i f i e d by t h e h a l f - l i f e t i m e s , t l = 2 . 1 ~ 1 0 - r~ $ / ~ i,n v o l v e d f o r s y n t h e t i c z e o l i t e s i n t h e u s u a l s i z e range 0 . 1 t o 1 0 p ( i . e . r a d i u s ro from cm t o cm). t l i s given i n Table 2 (12) f o r v a r i o u s values of ro and D. The dashea l i n e i n t h e t a b l e d i v i d e s t h e h a l f - l i v e s from s o r p t i o n r a t e s i n t o t h o s e too s m a l l f o r a c c u r a t e measurements w i t h normal equipment from t h o s e which a r e l o n g enough f o r a c c u r a t e d e t e r m i n a t i o n . I n k i n e t i c measurements c r y s t a l dimensions and d i f f u s i v i t i e s should be such a s t o g i v e v a l u e s of t i below t h e dashed line.

Table 2:

H a l f - l i f e times, t l ( s ) f o r : 2

Measurements of D* by p u l s e d NMR on t h e o t h e r hand a r e normally cm2 s - I . s u i t a b l e only f o r s e l f - d i f f u s i v i t i e s above a b o u t This method would t h e r e f o r e be u n s u i t a b l e f o r s m a l l d i f f u s i v i t i e s f o r example t h o s e of n - p a r a f f i n s i n some z e o l i t e s such a s (Na,Ca)-A, c a - r i c h c h a b a z i t e o r e r i o n i t e . I n (Na,Ca)-A t r a c e r d i f f u s i o n u s i n g hydrocarbons l a b e l l e d w i t h r a d i o c a r b o n has been s u c c e s s f u l i n measurements of E* ( 1 3 ) . T a b l e 2 a l s o shows t h a t by s u i t a b l y inc r e a s i n g t h e c r y s t a l dimensions v a l u e s of d i f f u s i v i t i e s o f any magnitude, from v e r y s n a l l (< 10-l4 cm2 s - l ) t o v e r y l a r g e cm2 S-I) a r e p o s s i b l e , u s i n g s o r p t i o n k i n e t i c s . (> Under c o n d i t i o n s where i n t r a c r y s t a l l i n e d i f f u s i v i t i e s do n o t wholly o r even p a r t i a l l y govern s o r p t i o n k i n e t i c s i t may s t i l l be p o s s i b l e t o d e s c r i b e t h e system i n terms of an e f f e c t i v e d i f Conditions can be made such t h a t i n t h e i n t e r fusivity, Deff. c r y s t a l s p a c e s m o l e c u l e - c r y s t a l c o l l i s i o n s g r e a t l y exceed moleculemolecule c o l l i s i o n s , s o t h a t m o l e c u l a r s t r e a m i n g (Knudsen flow) o c c u r s i n t h e i n t e r - c r y s t a l s p a c e s . We c o n s i d e r flow through a c y l i n d r i c a l bed bounded by p l a n e s x = 0 ( t h e e n t r y f a c e ) and x = R ( t h e e x i t f a c e ) , and w i t h no flow through t h e curved s u r f a c e ( i . e . t h e bed i s i n an open c y l i n d r i c a l c o n t a i n e r ) . The t o t a l flow, J , e n t e r i n g t h e bed a c r o s s t h e p l a n e x = 0 p e r u n i t a r e a of t h i s p l a n e can be w r i t t e n a s

Deff

i s an e f f e c t i v e d i f f u s i v i t y , E i s t h e p o r o s i t y of t h e bed ( e x l u d i n g i n t r a c r y s t a l l i n e p o r o s i t y ) , C i s t h e t o t a l number of molecules p e r u n i t volume of porous medium a t x = 0 , and C; and C: a r e t h e numbers of molecules p e r u n i t volume of gas phase and of c r y s t a l s , averaged a c r o s s t h e p l a n e a t x = 0 . Each g r a d i e n t i s an average of a l l l o c a l g r a d i e n t s a c r o s s t h e p l a n e . Also we may w r i t e

where J g and J i a r e r e s p e c t i v e l y gas phase and i n t r a c r y s t a l l i n e components of J and

where t h e g r a d i e n t s a r e a g a i n averages over t h e p l a n e x = 0 , Dg i s

274 the Knudsen flow d i f f u s i v i t y and D;- the i n t r a c r y s t a l l i n e d i f f u s i v i t y . Combination of eqns. 14 t o 15b gives

Because i n the t r a n s i e n t s t a t e the g r a d i e n t s i n eqn. 1 7 a r e functions of time Deff may have s i g n i f i c a n t t i m e dependance when I n the s t e a d y s t a t e of flow, however, such time dependDg # D;. ances w i l l have disappeared, f o r the boundary c o n d i t i o n s (i.1 (ii) (iii)

C = O f o r O < x < R a t t = O C = C o a t x = O f o r t > O C = C 0 R

A p a r t i c u l a r i n t e r e s t i s t o show i n a given system whether i n t e r - o r i n t r a c r y s t a l l i n e flow i s t h e dominant component of J through x = 0. This i s p o s s i b l e t o f i n d when Henry's law governs s o r p t i o n e q u i l i b r i u m (C: = kc'), Dg i s independent of C i and i n this g range D; i s independent of Ci. Eqn. 1 7 now reduces t o

s o t h a t Deff i s constant. For uniform packing of the bed dC/dx i n t h e steady s t a t e i s t h e same a t a l l planes x = X and equals - ( c ~ - C ~ ) so / ~ t h a t , from J = D ~ ~ ~ ! c ~ - C ~D )e f/f ! ~i s, found. E and k a r e a v a i l a b l e independently. Dg i s b e s t a s s e s s d using helium as a Re v i t u a l l y non-sorbed r e f e r e n c e gas t o e v a l u a t e Dg from J~~ = J!~. I f M i s t h e molecular weight of the d i f f u s a n t and MH, t h a t of helium, then

Thus D;

i n eqn, 18 can be found and thence the r a t i o

The s t e a d y - s t a t e flow method has been a p p l i e d not t o z e o l i t e Some results compacts alone, b u t t o g r a p h i t e - z e o l i t e A compacts (14) a r e given f o r n i t r o g e n i n Table 3, i n which an extended a n a l y s i s served t o give n o t only Dg and D i b u t a l s o a s u r f a c e d i f f u s i v i t y Ds, a s s o c i a t e d l a r g e l y with the g r a p h i t e . The i n t r a - c r y s t a l l i n e d i f f u s i v i t i e s of N2 i n z e o l i t e A a t the experimental temperatures a r e reasonable i n v a l u e . The method m e r i t s f u r t h e r a t t e n t i o n for large i n t r a c r y s t a l l i n e d i f f u s i v i t i e s .

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I n t r a c r y s t a l l i n e s e l f - d i f f u s i v i t y (0) f r o m p u l s e d f i e l d g r a d i e n t NMR and apparent d i f f u s i v i t y from s o r p t i o n rates of CH4 i n c h a b a z i t e s a t OOC, a s functions of c r y s talli t e r a d i i ( 6 ) .

Fig. 6.

p l o t s of MJM, = ( Q ~ - Q/ (~Q) - - Q ~ ) a g a i n s t fi f o r n-C6Hl4 i n 30.55Z~a-exchanged (Ca,Na)-A, a t c o n s t a n t p r e s s u r e and a t 348K ( 1 5 ) . Q_ = 120.05 mg g-l. Curve 1: Curve 2:

Qo = 0 Q, - 48.08 mg g-l

Curve 3: Curve 4:

Qo = 6 3 . 3 7 m g g-l. Qo = 96.99 mg g-l,

277 3 . 3 . Concentration-Dependent D i f f u s i v i t i e s When the d i f f e r e n t i a l i n t r i n s i c d i f f u s i v i t y i n t h e z o l i t e i s a function of concentration C w i t h i n the c r y s t a l s * one may add d i f fusant i n small increments, so t h a t under constant p r e s s u r e boundary -conditions (eqn. 2) (Cm - C,) i s small and, i n t h i s i n t e r v a l i n C , D may be considered c o n s t a n t . Eqns. 3 t o 6 a r e then s t i l l v a l i d and h e a t evolution a s a complicating f a c t o r i s minimised. When isotherms a r e r e c t a n g u l a r i n shape the e q u i l i b r i u m p r e s s u r e

i s often s o small t h a t constant p r e s s u r e s o r p t i o n of increments i s impossible. Constant volume v a r i a b l e p r e s s u r e a d d i t i o n i s a l s o n o t suitable because even though (Cw-Co) i s small t h e p r e s s u r e surge may i n i t i a l l y r a i s e i n t r a c r y s t a l l i n e concentrations w e l l above Coo n e a r the surface of each c r y s t a l , and the assumption t h a t 5 i s c o n s t a n t because (Cw-Co) i s small i s no longer t e n a b l e . The method of small The c r y s t a l s increments has t h e r e f o r e been modified a s follows (15) were e q u i l i b r a t e d t o an i n i t i a l uptake, Qo, and then were brought t o s a t u r a t i o n uptake a t Q, under n e a r l y constant p r e s s u r e of s o r b a t e vapour. The value of Q was p r o g r e s s i v e l y i n c r e a s e d , while Qm was 9 % of course constant. Initially l i n e a r p l o t s of ~ ~ =1 (Qt-Qo)/ (Q -Qo) a g a i n s t Jf were s t i l l obtained (Fig. 6 ) b u t , i n t h e ex, D must be an average o r i n t e g r a l d i f f pression M ~ / M , = 6

.

(O

0

R

usivity over the i n t e r v a l (Qm-Q,)

.

Assuming t h e r e f o r e t h a t

i s the d i f f e r e n t i a l d i f f u s i v i t y corresponding t o a p a r t i c u l a r where Qt(Q, > Qt > Q ) one may e v a l u a t e i n t e g r a l d i f f u s i v i t i e s from Mt/Mm = j and from p l o t s of D a g a i n s t (&, f o r any value of =o Qo, because eqn. 22 can a l t e r n a t i v e l y be w r i t t e n as

6

This method was used t o e v a l u a t e d i f f u s i v i t i e s of n-Cb, n-C6 and n-Cg p a r a f f i n s i n (Na,Ca)-A. The e x t e n t s of exchange were r e g u l a t e d so t h a t uptake was slow, t o ensure t h a t i n t r a c r y s t - a l l i n e d i f f u s i o n was r a t e c o n t r o l l i n g and p e r t u r b a t i o n of the k i n e t i c s by h e a t *The s u b s c r i p t i of 5 3 . 2 i s omitted h e r e where only i n t r a c r y s t a l l i n e flow need be considered.

278 e v o l u t i o n was minimised. I l l u s t r a t i v e values of 6 f o r n-C4HI0 i n ( ~ a , C a ) - A a r e given i n Table 4. Isotherms of t h e p a r a f f i n s i n z e o l i t e A a r e of type 1 i n Brunauer's c l a s s i f i c a t i o n (16) and often approximate t o Langmuirts isotherm (17). For the i d e a l isotherm based on h i s p o s t u l a t e s t h e i n t r i n s i c d i f f e r e n t i a l d i f f u s i v i t y should be p r o p o r t i o n a l t o (1-0)LdRn p/dRn07 s i n c e (1-0) i s the chance t h a t a vacant s i t e occurs i n t o which a molecule on an a d j a c e n t s i t e may jump and dRnp/dgn0 i s the Darken a c t i v i t y corr e c t i o n . This c o r r e c t i o n f o r Langmuir's isotherm equation (kp = 0/(1-08,is ( 1 - 0 ) so t h a t 5 should be c o n s t a n t , and theref o r e 5 = D. The t r e n d s i n 5 i n Table 4 can be considered a s one measure of d e v i a t i o n s of t h e a c t u a l isotherms from Langmuir's i d e a l i s a t ion. Analysis of the temperature c o e f f i c i e n t s of 5 i n terms of the Arrhenius equation D = Do exp-E/RT e x t r a p o l a t e d t o Qo = 0 a r e given i n Table 5 (15). E tends t o decrease a s the e x t e n t of exchange Table 4:

-

D for n-C4H10 i n 30.55, Z e o l i t e A (15).

32,54 and 34.10% Ca-exchanged

Table 5

% Ca exchange i n (Ca,Na)-A

Diffusant

(b)

Do (m2 s - l )

in

5

= DO exp-E/RT f o r Qo = 0

of ~ a 'by ca2+ i n c r e a s e s , and Do tends t o decrease a s E d e c r e a s e s . The energy b a r r i e r s a r e c o n s i d e r a b l e f o r t h e r a t e - c o n t r o l l i n g u n i t d i f f u s i o n s t e p s because i f t h e r e i s a s i g n i f i c a n t ~ a c+o n t e n t many of these i o n s p a r t i a l l y block t h e 8-ring windows through which d i f f u s a n t must pass. Even i n absence of c a t i o n s t h e windows have free diameters of about 4 . 1 A compared w i t h a c r i t i c a l dimension of a s t r e t c h e d hydrocarbon c h a i n of about 4 . 9 9 .

3.4. Evolution of t h e Heat of Sorption The exothermal h e a t of s o r p t i o n produces both l o c a l and extended temperature changes i n t h e bed of s o r b e n t . That i s , i n each c r y s t a l l i t e t r a n s i e n t temperature g r a d i e n t s appear, t h e h e a t being produced i n i t i a l l y a t t h e s u r f a c e s , w i t h t h e temperature wave then spreading both i n t o t h e body of t h e c r y s t a l s a s w e l l as h e a t i n g the ambient gas phase. A f t e r an i n t e r v a l , t h e temperature of t h e whole bed becomes more uniform b u t i s above t h a t of t h e thermostat i n which i t may be immersed. A t s t i l l l o n g e r times t h e temperature f a l l s a s y m p t o t i c a l l y t o t h a t of t h e t h e r m o s t a t . Attempts have been made t o allow f o r the h e a t e f f e c t s i n t h e second and t h i r d s t a g e s (18) b u t any comprehensive t r e a t m e n t allowing f o r h e a t e v o l u t i o n and heat and m a t t e r flows and t h e i r i n t e r a c t i o n , t o g e t h e r w i t h timedependent d r i f t s i n e q u i l i b r i a d u r i n g h e a t i n g and c o o l i n g of beds must be very d i f f i c u l t . I t i s t h e r e f o r e p r e f e r a b l e t o aim a t conditions a s n e a r i s o t h e r m a l a s p o s s i b l e by:

1.

2.

Making t h e beds a s t h i n a s p o s s i b l e , o p t i m a l l y a l a y e r one ,crystal thick. C i r c u l a t i n g t h e vapour o r l i q u i d g u e s t through t h e bed t o remove h e a t a s q u i c k l y as p o s s i b l e .

2 80 3.

4. 4.

Avoiding very r a p i d s o r p t i o n , by having s u i t a b l y l a r g e c r y s t a l s ( s e e Table 2 ) . Adding only small increments of d i f f u s a n t a t a time.

SOME ADDITIONAL EFFECTS

D i f f u s i o n i n s i d e z e o l i t e c r y s t a l s may be i n f l u e n c e d by f a c t o r s o t h e r than those d e s c r i b e d above. It has been suggested t h a t s u r f a c e s k i n s may sometimes be p r e s e n t which form an added r e s i s t a n c e t o flow. Such s k i n s may e x i s t i n p a t c h e s o n l y , s o t h a t the a r e a through which d i f f u s a n t s e n t e r t h e z e o l i t e c r y s t a l s i s reduced o r t h e s k i n s could cover t h e whole e x t e r n a l s u r f a c e . I n t h e l a t t e r circumstance t h e c r y s t a l s could become non-sorbents i f the s k i n was g l a s s y and impermeable. For t o t a l c o n t r o l of s o r p t i o n r a t e s by s u r f a c e s k i n s (1-Mt/&) should d e c l i n e e x p o n e n t i a l l y w i t h time (19) r a t h e r than following i n i t i a l l y t h e eqn. 5 ( M ~ / M _= 6 - ) However eqn. 5 h a s been w e l l v e r i f i e d i n many

.

0

r a t e s t u d i e s which s u g g e s t s t h a t s u r f a c e s k i n s a r e n o t normally important. Hydrothermal t r e a t m e n t s which diminish s o r p t i o n r a t e s have on t h i s evidence been considered t o r e s u l t i n s k i n s (19 a and b) Pre-treatments may be such t h a t t h e c r y s t a l s s u f f e r chemical damage (e.g. g r i n d i n g , hydrothermal t r e a t m e n t s and even t h e outWith bonded p e l l e t s t h e bond may block some g a s s i n g procedures) a r e a s of t h e c r y s t a l l i t e s u r f aces. Within t h e c r y s t a l l i t e s there may be v a r i a b l e amounts of d e t r i t a l m a t e r i a l occluded d u r i n g s y n t h e s i s ; and l a t t i c e d e f e c t s such as s t a c k i n g f a u l t s may occur p e r i o d i c a l l y o r a t random t o an e x t e n t varying between preparations. Also t h e Si/A1 r a t i o s of d i f f e r e n t p r e p a r a t i o n s of t h e same zeolite may vary according t o t h e methods of p r e p a r a t i o n , and t h e r e f o r e the c a t i o n c o n c e n t r a t i o n s and l o c a t i o n s i n t h e i n t r a c r y s t a l l i n e channels and c a v i t i e s . A l l t h e s e f a c t o r s can r e s u l t i n d i f f e r e n t values of d i f f u s i v i t i e s among d i f f e r e n t samples of the same z e o l i t e . When such r e s u l t s , obtained i n d i f f e r e n t l a b o r a t o r i e s , a r e t o be compared pre-treatment and s y n t h e s i s c o n d i t i o n s should be s t a n d a r d i s e d and/ o r samples from each l a b o r a t o r y 'should b e exchanged and t h e measurements made on both samples i n each l a b o r a t o r y .

.

A purpose of § § 2 t o 4 has been t o draw a t t e n t i o n t o ways i n which t h e study of s o r p t i o n k i n e t i c s can be improved f o r determina t i o n s of i n t r a c r y s t a l l i n e d i f f u s i v i t i e s , by i n d i c a t i n g some complications and ways of avoiding o r minimising t h e s e .

5.

SELF-DIFFUSIVITY AND DARKEN'S

RELATION

Exact r e l a t i o n s have been derived between d i f f e r e n t i a l intrinsic

281 and d i f f e r e n t i a l s e l f - d i f f u s i v i t i e s (20,21,22). Here we w i l l consider only the simpler approximately v a l i d Darken r e l a t i o n which i s

The a c t i v i t y c o r r e c t i o n d~na/dRnCf o r an i d e a l gas phase i s replaceable by dknp/dRn~, which can be found from t h e s l o p e s of isotherms ( p l o t t e d a s RnC a g a i n s t Rnp). The c o r r e c t i o n s o d e t e r mined assumes t h a t everywhere t h e r e i s l o c a l e q u i l i b r i u m between gaseous and i n t r a z e o l i t e s o r b a t e . In the Henry's law range of s o r p t i o n ( C p) d~np/dknCi s unity. Outside t h i s range f o r type I isotherms dQnp/dQnCbecomes greater than u n i t y and f o r r e c t a n g u l a r isotherms may become very large a s the uptake approaches i t s s a t u r a t i o n value. Darken's relation h a s been t e s t e d f o r water d i f f u s i n g i n n a t u r a l c h a b a z i t e , Both i n t r i n s i c and s e l f - d i f f u s i v i t i e s heulandi t e and gmelini t e ( 3 ) were measured, the l a t t e r using H20 t o d i s p l a c e D20. The c r y s t a l s were l a r g e p i e c e s of t h e z e o l i t e , and an i n e r t c a r r i e r gas was charged with water vapour a t constant r e l a t i v e p r e s s u r e s and c i r culated c o n t i n u a l l y through t h e samples suspended from a s i l i c a spring balance i n an open mesh g l a s s - f i b r e bucket. Conditions were such as t o ensure v i t u a l l y complete s a t u r a t i o n of z e o l i t e by e i t h e r D 2 0 or H 2 0 .

.

The r e s u l t s i n Table 6 show t h a t Darken's r e l a t i o n i s , a s expected from the more r e f i n e d theory (20,21,22), only approximately valid. However the dominant c o r r e c t i o n i s c e r t a i n l y t h a t f o r t h e a c t i v i t y (d!Znp/dRnC)

.

Pulsed f i e l d g r a d i e n t NMR has been used t o study s e l f - d i f f u s i o n of n-C4 t o n-CI8 alkanes i n z e o l i t e X over the range -100 t o +200°c (23). This very complete study showed t h e following behaviour:

1. 2,

3. 4.

5.

For comparable degrees of f i l l i n g of the i n t r a c r y s t a l l i n e pore space D* decreased with i n c r e a s i n g carbon number. A t c o n s t a n t temperature D* decreased monotonically f o r each p a r a f f i n as the amount sorbed i n c r e a s e d . It has a l r e a d y been i n d i c a t e d t h at for an i d e a l Langmuir s o r p t i o n one -0 (1-0) where O i s the degree of would expect D* = D * f i l l i n g . For n-para f ~ i n s t h i s i s normally an overi d e a l i s a t i o n , but a decrease i n D* w i t h O i s as expected. For a given temperature and p a r a f f i n D* was w i t h i n an o r d e r of magnitude of D* f o r the corresponding l i q u i d p a r a f f i n . For a c o n s t a n t amount sorbed D* depended e x p o n e n t i a l l y on temperature = Do exp-E/RT) . The h e a t of s o r p t i o n f o r a given degree of f i l l i n g , 0 , of i n t r a c r y s t a l l i n e pore space i n c r e a s e d continuously with i n c r e a s i n g carbon number, b u t a c t i v a t i o n e n e r g i e s , E , f o r

(E*

s e l f - d i f f u s i o n approached an asymptotic l i m i t and were never more than a r a t h e r s m a l l f r a c t i o n of the h e a t of s o r p t i o n ( ~ i g .7 ( 2 3 ) ) . I t has been suggested (24) t h a t t h e behaviour of E means that a s t h e n-paraffin chains i n c r e a s e i n length each u n i t d i f f u s i o n s t e p tends i n c r e a s i n g l y t o involve segmental r o t a t i o n s of p a r t only of t h e molecule round a C-C bond. This r e s u l t s i n changes of p o s i t i o n of t h e c e n t r e of mass of t h e molecule, b u t n o t n e c e s s a r i l y i n t r a n s l a t i o n of a l l p a r t s of t h e molecule simultane o u s l y , The longer t h e chains t h e more probable t h e segmental mechanism. I f f o r longer chains t h i s mechanism involves segments of s i m i l a r s i z e , e v e n t u a l l y n e a r l y independent of chain length, then, a s observed, E would become almost constant. Segmental u n i t d i f f u s i o n s t e p s can a l s o account f o r the decrease i n with i n c r e a s i n g carbon number, because the number of u n i t s t e p s required

E*

Table 6 :

Relation between 6 and % f o r water i n s e v e r a l z e o l i t e s near s a t u r a t i o n of the z e o l i t e ( 3 ) .

Zeolite

T (OC)

10axD*

d Rnp (a) d RnC

(cm2 s - l )

Chabazite B

75 65 55 45 35

Heulandi t e

46.2 31.8 21.4

14.1 9.0

1o=x5* dan~ d RnC (cm2 s-1)

23.0 (b (b 24 "(b) 25.5 27 .O (b 2 8.0 (b

106

5

cn2 s-1

10.6,

11.7

7.60 5.4, 3.80 2.50

8.9 6.6 4.8 3.4

75 65 55 45 35

Gmelini t e

(a) Determined from t h e n e a r l y f l a t top of the isotherm. (b) Determined from isotherms measured i n sample A of chabazite.

Fig. 7 .

Heats of sorption and energies of a c t i v a t i o n f o r s e l f d i f f u s i v i t i e s , f o r n-alkanes i n Na-X z e o l i t e , as functions o f carbon number. The upper curve gives the hear of sorption, the lower curve the a c t i v a t i o n energy ( 2 3 ) . The ordinate i s i n kJ mol-I and the abscissa i s the carbon number.

Fig. 8 .

(a) Sorption r a t e s of 0 2 , N2 and Ar i n l e v y n i t e a t -184OC ( 2 5 ) . For 02, Q = 10.02, for N2 i t is 9 . 7 7 and f o r Ar, 10.01, a l l ?n cm3 a t s . t . p , g-l. (b) Sorption rates of 0 2 ,N2 and A r i n Ca-mordenite a t -78O(26)

.

Fig. 9.

Sorption rates a t 30°c in Na-Y of liquid 1,3,5trimethyl-(El) , 1 , 3 , 5 - t r i e t h ~ l - ( 0 ) and 1,3,5-triisopropyl-benzene (01 (2 7)

.

Fig. 10.

Diffusivities in cm2 s - I of HZ, 02, N p and A r i n mordenite a t -183OC, as functions of the amount of pre-sorbed NH3 ( 3 0 ) .

285 ,for migration of an e n t i r e molecule from one c a v i t y t o t h e n e x t i n zeolite X w i l l i n c r e a s e . with chain l e n g t h .

6.

MOLECULE SIEVING

According t o the f r e e dimensions of t h e meshes through which guest molecules must migrate i n t r a c r y s t a l l i n e d i f f u s i v i t i e s a r e found t o change very g r e a t l y w i t h t h e molecular dimensions of t h e guest. Windows o r meshes c o n t r o l l i n g molecule s i e v i n g a r e 6-, 8-, 10- and 12-rings of v a r i e d conformations, and-octagonal prisms. With some z e o l i t e s t h e r e i s a d d i t i o n a l o b s t r u c t i o n due t o l o c a t i o n of cations i n o r n e a r t h e windows. C r y s t a l s with r e s t r i c t e d windows d i f f e r e n t i a t e between groups of small molecules and exclude larger ones a l t o g e t h e r ; c r y s t a l s w i t h l a r g e r windows d i f f e r e n t i a t e between groups of b i g g e r molecules, and s o on, Even q u i t e small differences i n molecular dimensions can r e s u l t i n d r a m a t i c a l l y d i f f e r e n t r a t e s of s o r p t i o n . This i s i l l u s t r a t e d i n Fig. 8 f o r uptakes of 02, N2 and A r i n l e v y n i t e (25) and Ca-mordenite (26) which function a s fine-mesh s i e v e s . F i g . 9 shows a s i m i l a r behaviour i n a wide-mesh s i e v e , Na-Y, f o r 1,3,5-trimethyl-l,3,5t r i e t h y l - and 1,3,5-triisopropyl-benzene, a l l as l i q u i d s ( 2 7 )

.

The r e l a t i o n between molecular dimensions and d i f f u s i v i t i e s i s i l l u s t r a t e d f o r f i n e mesh s i e v e s K-A and K-mordenite i n Table 7 (28). Them a r e f i v e o r d e r s of magnitude between d i f f u s i v i t i e s of Ne ( 3 . 2 d i a m e t e r ) and Kr (3.9 1) i n K-A a t 20°c; and a l s o between HZ (2.4 x 3.1 2) and K r ( 3 . 9 4 A) i n K-mordenite a t -78OC. There i s a p a r a l l e l i n c r e a s e i n the energy b a r r i e r , E , involved i n each u n i t d i f f u s i o n s t e p .

1

The s t r o n g i n f l u e n c e of t h e exchange c a t i o n upon t h e uptake of A r by v a r i o u s ion-exchanged forms of mordenite a t -78OC i s shown i n changes of B/t$ (26) with c a t i o n : Ca-mordeni t e K-mordeni t e Ba-mordeni t e Na-mordeni t e Li-mordeni te NH4 (H) -mordeni t e From the sequence above d i f f e r i n g c a t i o n p o s i t i o n s as well as c a t i o n s i z e may p l a y a p a r t . Again t h e extreme range i s f i v e o r d e r s of magnitude

.

One may a l s o a l t e r the d i f f u s i v i t i e s of g u e s t molecules by presorbing metered amounts of a s t r o n g l y h e l d guest (e.g. H20 o r N H ~ ) which i s v i r t u a l l y immobile a t t h e temperature a t which l e s s p o l a r guest molecules a r e t o be sorbed. This e f f e c t can be very l a r g e as

Table 7:

Zeolite

R e l a t i o n between d i f f u s i v i t i e s and molecular dimensions (27) Molecule

Equilibrium, Dimensions (A)

-

D o r D-k(cm2 s - l )

(and T°C)

E ( k c a l mol-l)

i n Fig. 10 when W2, 02, N2 and A r were sorbed a t -183OC i n mordenite c o n t a i n i n g metered pre-sorbed amounts of NH3 (30)

.

7.

CONCLUDING REMARK

This account has emphasised some experimental problems and means of overcoming t h e s e , because much previous work on r a t e s of uptake of simple gases i n z e o l i t e A and of t h e s e and hydrocarbons i n zeoli t e s X and Y h a s , as shown by pulsed f i e l d g r a d i e n t NMR, been m i s interpreted, Some p r o p e r t i e s of i n t r a c r y s t a l l i n e d i f f u s i v i t i e s have been d e s c r i b e d , emphasising i n p a r t i c u l a r t h e i r g r e a t s e n s i t i v i t y t o molecular dimensions of d i f f u s i n g s p e c i e s . A s i n d i c a t e d i n § 1 d i f f u s i o n can o e o r f i r s t importance n o t o n l y i n s e p a r a t i o n process e s , b u t p a r t i c u l a r l y i n c a t a l y s i s where r e a c t a n t s must reach active c e n t r e s w i t h i n c r y s t a l s and r e s u l t a n t s must migrate out of the c r y s t a l s . Such counter-current flows a r e much more d i f f i c u l t t o i n t e r p r e t because of cross-coef f i c i e n t s be tween flows. Formulation of t r a n s p o r t i s now b e s t done i n terms of s o - c a l l e d i r r e v e r s i b l e thermodynamics and phenomenological s t r a i g h t and cross-coef f icients. Not much h a s been done s o f a r i n t h e a r e a s of co- and counterd i f f u s i o n of mixtures i n z e o l i t e s (31,32).

REFERENCE S

1. 2. 3.

-

T i s e l i u s , A , , Z . Phys. Chem., AL69, (1934), 425. T i s e l i u s , A , , Z . Phys. Chem., A174, (1935), 401. B a r r e r , R.M. a n d B.E.F. Fender, J . Phys. Chem. S o l i d s , 21, (1961), 12. 4, B a r r e r , R.M. and M.A. Rosemblat, Z e o l i t e s , 2 , (1982), 231. Ash, R . , R.M. B a r r e r and R . J . B . Craven, J. Chem. Soc. Faraday 5. Trans. TI, 74, (1978), 40. 6. Karger, J . , and J . Caro, J . Chem. Soc. Faraday T r a n s . I , 73, (1977), 1363. 7. Barrer, R.M., Z e o l i t e s and Clay M i n e r a l s and S o r b e n t s and Molecular S i e v e s (London, Academic P r e s s , 1 9 7 8 ) , p . 317. g 248, , (1971), 27. Karger, J . , Z . Phys. Chem., ~ e i ~ z i 8. 9. Karger, J . , S.P. Shdanov and A. W a l t e r , Z . Phys. Chem. ~ e i p z i g , 256, (1975) , 319. 10. Caro, J . , J. K a r g e r , H. P f e i f e r and R. S c h o l l n e r , 2 . Phys. Chem. Leipzig, 256, (1975), 698. I, 11. Karger, J . , and D.M. Ruthven, J. Chem. Soc. Faraday T r a n s . 77, (1981) , 1485. 12. Ref. 7 , p. 318. 13. Quig, A . , and L.V.C. Rees, Proc. 3rd I n t e r n a t . Conference on Molecular S i e v e s ( Z u r i c h , S e p t . 3-7, 1973) p. 277. 3, (1965), 14. B a r r e r , R.M. , and J . H . P e t r o p o u l o s , S u r f a c e S c i , 126 and 143. 15. B a r r e r , R.M., and D . J . C l a r k e , J . Chem. Soc. Faraday T r a n s . I , 70, (1974), 535. 16. Brunauer, S . , The Adsorption of Gases and Vapours, ( ~ e wYork, Oxford Univ. P r e s s , 1944) p. 150. 17. Ref. 7 pp. 112-114. 18. Ruthven, D . , i n P r o p e r t i e s and A p p l i c a t i o n s o f Z e o l i t e s , E d i t o r , R.P. Townsend (London, The Chemical s o c i e t y , 1 9 8 0 ) , p. 43. 31, ( 1 9 7 4 ) , 19. Karger, J . , and P. Hermann, Ann. Phys. L e i p z i g , 277. 19a. Bulow, M . , P. S t r u v e and S . P i k u s , Z e o l i t e s , 2 , (1982), 267. 19b. Karger, J . , W. Heink, H. P f e i f e r , M. ~ a u s e h e r - a n d J . Hoffmann, z e o l i t e s , 2 , (1982), 275. 20. Ash, R . , azd R.M. B a r r e r , S u r f a c e S c i . , 8 , (1967), 461. 36, (1973), 791. 21. Karger, J . , S u r f a c e S c i . , 22. Karger, J . , S u r f a c e S c i . , 5 7 , (1976), 749. 23. K a r g e r , J . , H. P f e i f e r , ~ . R a u s c h e rand A. W a l t e r , Z . Phys. Chem. , L e i p z i g , 259, (1978), 784. 24. B a r r e r , R.M., Symposium on t h e C h a r a c t e r i s a t i o n of Porous S o l i d s (Neuchatel , 9-13th J u l y , 1978). 25. B a r r e r , R.M., N a t u r e , 159, (1947), 508. 26. B a r r e r , R.M., T r a n s . Faraday Soc., 45, (1949), 358. 2 7 . S a t t e r f i e l d , C . N . , and C.S. Cheng, A m e r . I n s t . Chern. Eng., 6 8 t h N a t i o n a l Mtg., Houston, Feb. 28-Mar. 4 t h (1971) Adsorption P t . 1, Paper 1 6 f .

.

28.

29.

30.

31. 32.

R e f . 7 , p . 292. W a l k e r , P . L . Jr., L.G. Austin and S.P. N a n d i , i n C h e m i s t r y and P h y s i c s of Carbon, V o l . 2 , ( D e k k e r , 1966) pp. 257-371. B a r r e r , R.M., and L.V. C . R e e s , T r a n s . F a r a d a y Soc. , 50, (1954), 852 a n d 989. K a r g e r , J . , M. Bulow a n d W. S c h i r m e r , Z . Phys. Chem. ~ e i p z i g , 256, (19751, 144. Moore, R.M. and J . R . K a t z e r , J. Amer. Inst. Chem. Eng., 18, (1972) , 816.

PART Ill CATALYSlS

UNIFYING P R I N C I P L E S I N ZEOLITE C H E M I S T R Y A N D CATALYSIS

JULE A .

RABO

Union C a r b i d e C o r p o r a t i o n Tarrytown, New York 10591

INTRODUCTION I n r e c e n t y e a r s molecular s i e v e c a t a l y s t s have assumed a n i n c r e a s i n g l y i m p o r t a ' n t r o l e i n i n d u s t r i a l catalysis. A p p l i c a t i o n s of z e o l i t e c a t a l y s t s a r e exp a n d i n g f r o m t h e t r a d i t i o n a l p e t r o l e u m r e f i n i n g t o new and i m p r o v e d f u e l p r o c e s s i n g a p p l i c a t i o n s , a n d t o new r o l e s i n b o t h t h e p e t r o c h e m i c a l and chemical i n d u s t r i e s . Up t o t h e p r e s e n t , a l l c o m m e r c i a l a p p l i c a t i o n s o f z e o l i t e c a t a l y s t s have been c a r r i e d out with a c i d i c z e o l i t e s . Recent i n v e s t i g a t i o n s of z e o l i t e c h e m i s t r y r e v e a l e d s e v e r a l i m p o r t a n t f e a t u r e s w h i c h a p p e a r common t o b o t h T h ' i s new c h e m i c a l e v i d e n c e a l k a l i and a c i d i c z e o l i t e s . r a i s e s t h e p o s s i b i l i t y t h a t t h e underlying p'hfrsicochemical f e a t u r e s of b o t h t y p e s of z e o l i t e s p l a y a r o l e i n c a t a l y s i s . Chemistry and C a t a l y s i s With A l k a l i Z e o l i t e s Chemistry The i n t r a c r y s t a l l i n e p o r e - c a v i t y s y s t e m i n z e o l i t e s , o f t e n c a l l e d t h e z e o l i t i c s u r f a c e , i s s u r r o u n d e d by t h e z e o l i t e c r y s t a l l a t t i c e , and consequently i t i s s t r o n g l y i n f l u e n c e d by t h e z e o l i t e c r y s t a l f i e l d . This c r y s t a l f i e l d , pervading t h e i n t r a c r y s t a l l i n e pore-cavity system, r e n d e r s z e o l i t e s s o l i d e l e c t r o l y t e s . Depending on t h e i o n i c c h a r a c t e r of t h e c r y s t a l which i s m a i n l y cont r o l l e d by t h e a l u m i n a c o n t e n t , z e o l i t e s show p r o p e r t i e s A unique characterof w e a k t o v e r y s t r o n g e l e c t r o l y t e s . i s t i c of t h e s e s o l i d e l e c t r o l y t e s i s t h a t b y a d m i t t i n g

v a r i o u s a d s o r b a t e s o r r e a c t a n t s , o n e c a n c h a n g e t h e chemici composition of t h e e l e c t r o l y t e . M o r e o v e r , by e v a l u a t i n g t h e e f f e c t of t h e z e o l i t e upon a p p r o p r i a t e l y c h o s e n o c c l u d e d m o l e c u l e s , one can m o n i t o r t h e s t r e n g t h of zeol i t e electrolyte. I t s e e m s r e a s o n a b l e t o e x p e c t t h a t t h e e f f e c t of t h e z e o l i t e a s e l e c t r o l y t e upon o c c l u d e d m o l e c u l e s , and v i c e v e r s a , i s m a i n l y i n f l u e n c e d by t h e r m o d y n a m i c s , namely by t h e t e n d e n c y t o m i n i m i z e t h e f r e e e n e r g y of t h e z e o l i t e guest m a t e r i a l system. Unfortunately, the l a t t i c e e n e r g i e s o f z e o l i t e c r y s t a l s a r e n o t known. Their calc u l a t i o n w i t h any d e g r e e of r e l i a b i l i t y i s i m p o s s i b l e w i t h o u t k n o w i n g t h e c o v a l e n t a n d i o n i c c h a r a c t e r of t h e l i n k a g e s between l a t t i c e atoms. Several observations, e x p e c i a l l y t h e p r e f e r e n c e of z e o l i t e c a t i o n s f o r w a t e r o v e r f r a m e w o r k o x y g e n , i n d i c a t e t h a t t h e bond b e t w e e n framework c a t i o n s and oxygen i s s t r o n g l y c o v a l e n t . N e v e r t h e l e s s , a c e r t a i n d e g r e e o f i o n i c c h a r a c t e r must e x i s t , and a c r y s t a l f i e l d c o r r e s p o n d i n g t o t h e i o n i c c h a r a c t e r must pervade t h e whole porous c r y s t a l . Occluded molecules, especially the polar or strongly polarizable o n e s , w i l l b e p o l a r i z e d b y t h e z e o l i t e , a n d t h e y thems e l v e s w i l l e x e r t a s i m i l a r e f f e c t on t h e z e o l i t e c r y s t a l as well. T h e o v e r a l l i n t e r a c t i o n b e t w e e n z e o l i t e and a d s o r b a t e must t e n d t o m i n i m i z e t h e f r e e e n e r g y of t h e zeolite-adsorbate system. T h e r e f o r e , t h e s i m p l e s t way t o f o r e c a s t t h e r m o d y n a m i c a l l y f a v o r e d r e a c t i o n s i n zeol i t e s i s t o e v a l u a t e t h e i r e f f e c t on t h e s t a b i l i t y of the zeolite crystal.

I n s p e c t i o n of t h e i o n i c l a t t i c e ene-gy e q u a t i o n shows t h a t w i t h o r d i n a r y i o n i c c r y s t a l s t h e f r e e energy i s r e d u c e d by p l a c i n g m o r e i o n p a i r s o r i o n s o f h i g h e r According t o v a l e n c e w i t h i n t h e same c r y s t a l v o l u m e . t h i s s i m p l e r e l a t i o n s h i p , t h e o c c l u s i o n of added i o n i c m a t e r i a l i n t h e porous z e o l i t e l a t t i c e should generally l o w e r t h e l a t t i c e e n e r g y , and c o n s e q u e n t l y i n c r e a s e the O f course, thermodynamic s t a b i l i t y of t h e o v e r a l l system. t h e o c c l u s i o n o f a d d e d i o n i c m a t e r i a l may a f f e c t t h e z e o l i t e s t r u c t u r e a n d s y m m e t r y , u l t i m a t e l y a f f e c t i n g the Madelung c o n s t a n t o f t h e z e o l i t e c r y s t a l . T h i s may render t h e i n t e r a c t i o n between z e o l i t e and t h e o c c l u d e d i o n i c s p e c i e s c o m p l i c a t e d and d i f f i c u l t t o a n a l y z e . In spite of t h e s e l i m i t a t i o n s , t h e s t u d y o f i n t e r a c t i o n s between z e o l i t e s and o c c l u d e d i o n i c m a t e r i a l i s a c o n v e n i e n t t e c h n i q u e t o a s s e s s t h e r o l e of t h e z e o l i t e e l e c t r o l y t e u p o n z e o l i t e c h e m i s t r y , a n d u l t i m a t e l y on z e o l i t e catalysis.

In order to evaluate the influence of z e o l i t e s as e l e c t r o l y t e s on c a t a l y t i c phenomena, some o f t h e r e l e v a n t d a t a on s a l t o c c l u s i o n a n d r e d o x c h e m i s t r y i n z e o l i t e s w i l l be reviewed. It has been suggested t h a t the porous z e o l i t e l a t t i c e has s t r o n g a f f i n i t y f o r t h e o c c l u s i o n o f a d d e d i o n i c species (1). T h i s o c c l u d e d i o n i c m a t e r i a l may c o n s i s t of i o n i c c o m p o u n d s s u c h a s s a l t s , o r i t may c o n s i s t o f atoms o r m o l e c u l e s which a r e n o t i o n i c o u t s i d e t h e z e o l i t e , b u t become i o n i z e d u p o n o c c l u s i o n i n t o t h e z e o l i t e c r y s t a l (2). T h e r e a r e s e v e r a l e x a m p l e s of b o t h phenomena.

The o c c l u s i o n of s a l t s i n z e o l i t e s h a s been d e s c r i b e d earlier (1). The e x p e r i m e n t a l e v i d e n c e w i t h z e o l i t e s A ( 3 ) and Y c l e a r l y s h o w s t h a t i o n i c c o m p o u n d s , s u c h a s s a l t s , r e a d i l y p e n e t r a t e t h e z e o l i t e c r y s t a l , f i l l i n g up t h e available space i n large i n t r a c r y s t a l l i n e c a v i t i e s . Salts, especially s a l t s of univalent anions, can even p e n e t r a t e This is surprising the s o d a l i t e cages i n Y z e o l i t e ( 1 ) . because t h e s i z e of t h e 06-ring p o r t of t h e s o d a l i t e cage I t was i s o n l y a b o u t - 2 . 4 8 , much s m a l l e r t h a n a n a n i o n . found t h a t t h e o c c l u s i o n of s a l t s i n t o t h e s o d a l i t e c a g e s requires high temperatures i n order t o provide the high a c t i v a t i o n energy needed t o e n l a r g e t h e 06-ring p o r t , p r o b a b l y b y t h e t e m p o r a r y c l e a v a g e o f 0-A1 o r 0 - S i b o n d s . I n t e r e s t i n g l y , b o t h h a l i d e s a l t s as w e l l a s s a l t s of l a r g e c o m p l e x a n i o n s s u c h a s C103- a n d N O 3 - c a n b e o c c l u d e d i n t h e s o d a l i t e cage. From t h e p o i n t o f v i e w o f t h i s d i s c u s s i o n , t h e m o s t important a s p e c t of s a l t o c c l u s i o n i s t h e enhanced s t a b i l i t y of t h e z e o l i t e - s a l t o c c l u s i o n compound. It has been reported t h a t i n every successful s a l t occlusion experiment b o t h t h e Y z e o l i t e a n d t h e o c c l u d e d s a l t w e r e r e n d e r e d t h e r m a l l y more s t a b l e , showing d e c o m p o s i t i o n t e m p e r a t u r e s w e l l beyond t h e d e c o m p o s i t i o n t e m p e r a t u r e of t h e z e o l i t e I t was a l s o f o u n d t h a t t h e o r of t h e g u e s t s a l t i t s e l f . g u e s t s a l t o c c l u d e d i n t h e s o d a l i t e c a g e s c a n n o t b e removed f r o m t h e h o s t Y z e o l i t e c r y s t a l e v e n b y s t e a m i n g Thus t h e z e o l i t e l a t t i c e i s s t a b i l i z e d by a t 700°C ( 1 ) . t h e occluded salt and v i c e versa. The a f f i n i t y o f z e o l i t e s f o r i o n i c s p e c i e s i s b e s t shown b y t h e i o n i z a t i o n o f o c c l u d e d a t o m s a n d m o l e c u l e s . The m o s t f r e q u e n t l y o b s e r v e d i o n i z a t i o n r e a c t i o n i n z e o l i t e s i s t h e i o n i z a t i o n of w a t e r by c a t i o n h y d r o l y s i s . This is a particularly important reaction i n zeolites b e c a u s e t h i s r e a c t i o n g e n e r a t e s a c i d i c OH g r o u p s a t t a c h e d

t o framework c a t i o n s which a r e d i r e c t l y r e s p o n s i b l e f o r acidic behavior ( 4 , l T h e s e a c i d i c OH g r o u p s i n z e o l i t e s have been w e l l c h a r a c t e r i z e d by I R s p e c t r o s c o p y (5) as w e l l a s by o t h e r t e c h n i q u e s ( s e e n e x t s e c t i o n ) . Other i o n i z a t i o n e f f e c t s i n z e o l i t e l a t t i c e s have been observed by t h e o c c l u s i o n of a l k a l i m e t a l s and I t w a s r e p o r t e d t h a t a l k a l i metals certain gas molecules. r e a d i l y reduce b o t h a l k a l i X and a l k a l i Y z e o l i t e s ( 6 ) . The sodium i o n s of t h e h o s t c r y s t a l c a p t u r e t h e e l e c t r n $ from t h e occluded a l k a l i atoms forming symmetrical ~ a i o r Na;' centers in the large cavities: NaY

NaX

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It has a l s o been reported t h a t c e r t a i n t r a n s i t i o n m e t a l z e o l i t e s r e a d i l y i o n i z e a d s o r b e d NO r a d i c a l s , forming e l e c t r o n t r a n s f e r complexes w i t h z e o l i t e cations (2) :

A n o t h e r i n t e r e s t i n g i o n i z a t i o n phenomenon i s t h e i n t e r a c t i o n o f N O a n d NO2 r a d i c a l s w i t h i n z e o l i t e s . H e r e , t h e s e r a d i c a l s become i o n i z e d , p r e s u m a b l y forming a s a l t - l i k e complex i n t h e z e o l i t e ( 2 ) :

The e l e c t r o l y t i c s t r e n g t h o f X a n d Y z e o l i t e s i n t h e s e r e d o x r e a c t i o n s i s b e s t d e m o n s t r a t e d by t h e f a c t t h a t these redox r e a c t i o n s a r e a l l highly endothermic in t h e gas phase outside t h e z e o l i t e c r y s t a l . F o r example, t h e f o r m a t i o n o f t h e NO+ a n d N O 2 - i o n s f r o m N O a n d NO2 r a d i c a l s i s e n d o t h e r m i c by a b o u t 5 . 5 e V ("126 k c a l / r n o l ) , n o t c o n s i d e r i n g c o u l o m b i c i n t e r a c t i o n b e t w e e n t h e products, I n s p i t e o f t h i s , t h i s r e a c t i o n r e a d i l y o c c u r s o n Nay a t moderate temperatures. S i m i l a r l y , t h e i n t e r a c t i o n between s o d i u m m e t a l a n d Nay, t h e h i g h l y e n d o t h e r m i c i o n i z a t i o n o f s o d i u m m e t a l ( I . P . = 5 . 1 4 e V , - 1 1 8 k c a l / m o l ) , i s readily a c c o m p l i s h e d a t 3 0 0 ' ~ w i t h a ~ a $ +c e n t e r f o r m e d i n e a c h large cavity.

These examples d e m o n s t r a t e w e l l t h e h i g h a f f i n i t y of zeolites f o r ionic species. They a l s o p r o v i d e i n f o r m a t i o n on t h e l a r g e c o n t r i b u t i . o n o f z e o l i t e s t o t h e i o n i z a t i o n of o c c l u d e d s p e c i e s , d i s p l a y i n g t h e c h a r a c t e r i s t i c s o f a very strong e l e c t r o l y t e . I t s h o u l d a l s o b e n o t e d t h a t seve'ral of t h e i o n i z a tion processes described above r e a d i l y occur both w i t h Na-zeolite and w i t h a v a r i e t y of c a t i o n exchanged z e o l i t e s .

Catalysis with Alkali Zeolites The i m p o r t a n t i n d u s t r i a l a p p l i c a t i o n s o f z e o l i t e c a t a l y s t s a r e based on carbenium i o n c h e m i s t r y . This c h e m i s t r y i s i n d u c e d by s t r o n g - a c i d h y d r o x y l g r o u p s i n A t present, no a c i d - f r e e , zeolites (see next section). a l k a l i c a t i o n z e o l i t e c a t a l y s t i s used i n i n d u s t r i a l l y important processes. N e v e r t h e l e s s , i t seems w o r t h w h i l e to review here t h e i n f l u e n c e of a l k a l i z e o l i t e s a s e l e c t r o l y t e s upon h y d r o c a r b o n r e a c t i o n s b e c a u s e of t h e l a r g e chemical i n f l u e n c e t h e y e x e r t upon o c c l u d e d m o l e c u l e s (see previous section). The important e f f e c t s d i s c u s s e d above i n c l u d e s t r o n g a d s o r p t i o n a n d a v e r y s t r o n g a f f i n i t y f o r i o n i c m a t e r i a l s , even i n c l u d i n g t h e i o n i z a t i o n of occluded molecules. Such a s t r o n g d i s p l a y of c h e m i c a l activity, particularly i n the ionization reactions, s u g g e s t s t h a t t h e s e phenomena a r e p r o b a b l y a l s o r e l e v a n t i n some way t o t h e c a r b e n i u m i o n t y p e c a t a l y s i s c a r r i e d In order t o evaluate the e f f e c t o u t on a c i d i c z e o l i t e s . o f t h e z e o l i t e as e l e c t r o l y t e , w i t h o u t c o n t r i b u t i o n f r o m acidic hydroxyl groups, t h e e f f e c t of a l k a l i z e o l i t e s in hydrocarbon cracking w i l l be b r i e f l y reviewed. T h e ' T n f l u e n c e o f a l k a l i z e o l i t e s (K-Y ) f r e e o f a c i d i c h y d r o x y l s o n t h e c r a c k i n g of h y d r o c a r b o n s was i n v e s t i g a t e d A t a b o u t 5 0 0 ° c t h e K-Y with hexanes a s r e a c t a n t s {7,8}. c a t a l y z e d c r a c k i n g gave c o n v e r s i o n l e v e l s up t o 5 t i m e s h i g h e r r e l a t i v e t o t h e same r e a c t o r f i l l e d w i t h q u a r t z The p r o d u c t c o m p o s i t i o n s c h i p s a t t h e same t e m p e r a t u r e . o b t a i n e d f r o m n - h e x a n e a n d i t s i s o m e r s o v e r K-Y w e r e markedly d i f f e r e n t from t h o s e o b t a i n e d o v e r a c i d i c zeolites. Significantly, they w e r e also quite different from t h e p r o d u c t o b t a i n e d i n a r e a c t o r f i l l e d w i t h q u a r t z c h i p s a t t h e same r e a c t i o n c o n d i t i o n s ( s e e T a b l e 1 ) .

TABLE 1 P r o d u c t s f r o m P y r o l y s i s a n d K-Y c a t a l y z e d a C r a c k i n g at 500'~ (mo1/100 mol converted) n-Hexane Thermal K-Y Hydrogen Me thane Ethylene Ethane Propylene Propane l-Butene 2~Butene Isobutene Isobutane l-Pentene 2-Pentene 2-Methyl-l-butene 3-Methyl-l-butene 2-Methyl-2-butene

a

Rate enhancements:

#5-fold.

2-Methylpentane Thermal K-Y -

{7},

3,-Methylpentane Thermal K-Y -

The a n a l y s i s o f t h e r e a c t i o n m e c h a n i s m b a s e d o n p r o duct d i s t r i b u t i o n s o b t a i n e d f r o m a l l h e x a n e i s o m e r s showed t h a t o v e r K-Y t h e f r e e r a d i c a l t y p e m e c h a n i s m p r e v a i l e d , b u t w i t h s p e c i f i c changes i n t h e r a t e s of c e r t a i n r e a c t i o n steps r e l a t i v e t o t h e "hot tube." A mechanistic study r e v e a l e d t h a t t h e K-Y d o e s n o t c h a n g e t h e s e l e c t i v i t y o f alkyl r a d i c a l s e i t h e r i n t h e c h o i c e between d i f f e r e n t 0-scission o p t i o n s o r between d i f f e r e n t H atom a b s t r a c t i o n s t e p s when p o s i t i o n a l l y d i f f e r e n t c h o i c e s a r e a v a i l able. H o w e v e r , t h e K-Y h a s a v e r y l a r g e e f f e c t u p o n t h e r a t i o of t h e r a t e s of H a t o m a b s t r a c t i o n s t e p s o v e r t h e @-scission steps. Mechanistic a n a l y s i s of t h e cracked products o b t a i n e d from t h e h e x a n e i s o m e r s shows t h a t o v e r K-Y t h e r a t i o o f r a t e o f H - a b s t r a c t i o n / r a t e o f 8 - s c i s s i o n i s i n c r e a s e d f o r f r e e a l k y l r a d i c a l s by a This e ffect accounts for a l l signif a c t o r of 6 t o 9 . f i c a n t d i f f e r e n c e s i n p r o d u c t f o r m a t i o n between t h e nonc a t a l y z e d a n d t h e K-Y " c a t a l y z e d " r e a c t i o n s { 7 - 9 ) . S p e c u l a t i n g o n t h e r e a s o n why z e o l i t e s e n h a n c e t h e r a t e of H - a b s t r a c t i o n o v e r $ - s c i s s i o n , i t was c o n s i d e r e d t h a t H - a b s t r a c t i o n i s a b i m o l e c u l a r r e a c t i o n s t e p whose r a t e i s s t r o n g l y i n f l u e n c e d by t h e c o n c e n t r a t i o n of t h e reactant, whereas $-scission, being a unimolecular proc e s s , i s u n a * f f e c t e d by c h a n g e s i n r e a c t a n t c o n c e n t r a t i o n . A c c o r d i n g l y , t h e " c a t a l y t i c " e f f e c t o f K-Y i s t h e r e s u l t of i n c r e a s e d r e a c t a n t c o n c e n t r a t i o n w i t h i n t h e z e o l i t e cavities r e l a t i v e t o t h e surrounding gas phase (9,lO). The c o n c e n t r a t i o n o r a d s o r p t i o n o f h y d r o c a r b o n w i t h i n z e o l i t e c r y s t a l s i s , of c o u r s e , w e l l documented i n adsorpt i o n s t u d i e s c a r r i e d o u t a t low t e m p e r a t u r e s . For t h e temperature range used i n t h e hexane cracking s t u d y , t h e h e x a n e l o a d i n g o n K-Y a t 5 0 0 ' ~ w a s e s t i m a t e d f r o m t h e r e p o r t e d A r r h e n i u m p l o t of n-hexane l o a d i n g on NaX a t t e m p e r a t u r e s u p t o 3 0 0 ~ ~ T. h e e s t i m a t e d h e x a n e l o a d i n g a t 5 0 0 ' ~ i s s u b s t a n t i a l , and i t i s c o n s i s t e n t w i t h t h e r e a c t a n t c o n c e n t r a t i o n s u g g e s t e d by t h e m e c h a n i s t i c analysis. It i s concluded from t h e hexane c r a c k i n g s t u d y t h a t zeolites concentrate hydrocarbon r e a c t a n t s within t h e zeolite crystal, that t h i s concentration is substantial, and t h a t t h i s e f f e c t i s r e s p o n s i b l e f o r t h e s u b s t a n t i a l changes i n p r o d u c t f o r m a t i o n . Methanistically, the s p e c i f i c r e s u l t of t h i s e f f e c t i s a s u b s t a n t i a l enhancement o f t h e r a t e s o f b i m o l e c u l a r r e a c t i o n s t e p s ( 9 ) . The h i g h h y d r o c a r b o n c o n c e n t r a t i o n w i t h i n Y z e o l i t e c r y s t a l s i s t h e d i r e c t r e s u l t o f t h e h i g h a f f i n i t y of this strong electrolyte fo r ionic or polar matter.

Adsorbed h y d r o c a r b o n s become p o l a r i z e d upon a d s o r p t i o n , and they are consequently s t r o n g l y h e l d i n t h e z e o l i t e crystal. S i n c e t h e c o n c e n t r a t i o n . e f f e c t of z e o l i t e s depends mainly on t h e i o n i c c h a r a c t e r and t h e r e l a t e d e l e c t r o l y t i c s t r e n g t h o f t h e c r y s t a l , i t i s o f i n t e r e s t t o corn p a r e t h e c r a c k i n g of n-hexane o v e r monacidic moleculars i e v e s o f s t r o n g a n d weak e l e c t r o l y t i c c h a r a c t e r : F o r c o n t r a s t , t h e K-Y a n d S i l l i c a l i t e m o l e c u l a r s i e v e s were c h o s e n as c a t a l y s t c a n d i d a t e s . The K-Y r e p r e s e n t s a n aluminum r i c h , v e r y s t r o n g e l e c t r o l y t e . On t h e o t h e r h a n d , S i l i c a l i t e i s s u b s t a n t i a l l y f r e e o f b o t h a l u m i n u m and z e o l i t e c a t i o n s , a n d c o n s e q u e n t l y i t i s a v e r y weak electrolyte. F i g u r e 1 shows t h e r e a c t i o n p a t h o f r a d i c a l - t y p e c r a c k i n g f o r n-hexane and i t s c r a c k e d f r a g m e n t s . The measurement of t h e c o n c e n t r a t i o n e f f e c t r e s t s upon a comparison between Reactions a 3 and a2 because t h e relat i v e r a t e s o f t h e s e r e a c t i o n s t e p s r e v e a l t h e enhancement of t h e H - t r a n s f e r r e a c t i o n s t e p o v e r 8 - s c i s s i o n , respect i v e l y , o n t h e same c r a c k e d f r a g m e n t . E x p e r i m e n t s showi n g t h e r a t e s o f R e a c t i o n s a 2 a n d a 3 f o r K-Y a n d S i l i c a I n a d d i t i o n , t h e t a b l e inl i t e a r e shown i n T a b l e 2 . c l u d e s e x p e r i m e n t a l d a t a s h o w i n g t h e e f f e c t o f a b o u t tenf o l d helium d i l u e n t a p p l i e d t o t h e n-hexane feed during t h e hexane cracking experiments. The S i l i c a l i t e used i n t h e experiment was t r e a t e d w i t h p o t a s s i u m s a l t s o l u t i o n t o remove t r a c e s o f a c i d i c hydrogen i o n s . The d a t a show t h a t t h e a p p l i c a t i o n o f h e l i u m gas d i l u e n t r e s u l t s i n a s h a r p r e d u c t i o n o f t h e r a t e o f Ha b s t r a c t i o n s t e p (a3) r e l a t i v e t o t h e 8-scission step ( a 2 ) b o t h w i t h t h e r e a c t o r f i l l e d w i t h q u a r t z c h i p s as T h i s r e s u l t i s expected, w e l l a s w i t h K-Y a n d S i l i c a l i t e . b e c a u s e t h e a d d i t i o n of d i l u e n t t o t h e r e a c t a n t r e d u c e s t h e n-hexane c o n c e n t r a t i o n , r e s u l t i n g i n l e s s frequent i n t e r a c t i o n between n-hexane molecules r e q u i r e d f o r the H-abstraction step. T h e d i f f e r e n c e b e t w e e n t h e K-Y and S i l i c a l i t e "catalysts" is very substantial. S i l i c a l i t e , i n sharp c o n t r a s t t o K-Y, performs l i k e i n e r t quartz chips. It h a s , w i t h i n e x p e r i m e n t a l e r r o r , no e f f e c t on t h e r a t e

the H-abstraction step. T h i s m o l e c u l a r s i e v e , 2n s p i t e its h i g h s u r f a c e a r e a (-400 m 2 / g ) , h a s no i n f l u e n c e on thermal c r a c k i n g of n-hexane. Considering the strongly covalent c h a r a c t e r of t h e Si-0 l i n k a g e s forming t h e s i l i c a l i t e c r y s t a l , i t i s n o t s u r p r i s i n g t h a t t h e n-hexane c r a c k i n g e x p e r i m e n t s show t h a t S i l i c a l i t e d o e s n o t d i s p l a y the p r o p e r t i e s of z e o l i t e e l e c t r o l y t e s . From t h e a b s e n c e o f s k e l e t a l i s o m e r i z a t i o n i n t h e c r a c k i n g of n-hexane o v e r K-Y, t h e a b s e n c e of i o n i c (carbenium i o n ) r e a c t i o n s can b e i n f e r r e d . Hence t h e K-Y f r e e o f OH g r o u p s i s n o t c a p a b l e o f i o n i z i n g e i t h e r hexane o r t h e r a d i c a l s formed upon i t s f r a g m e n t a t i o n . This i s somewhat s u r p r i s i n g c o n s i d e r i n g t h e f a c i l e i o n i z a t i o n of o c c l u d e d i n o r g a n i c compounds ( s e e f o r m e r s e c t i o n ) . One h a s t o r e c o g n i z e , h o w e v e r , t h a t t h e i o n i z a t i o n o f hydrocarbons r e q u i r e s v e r y high energy. For example, i o n i z a t i o n of a tertiary-C-H bond t o form a t e r t i a r y carbenium i o n a n d h y d r i d e i o n i s e n d o t h e r m i c by 240-250 kcal/mol w h i l e t h e i o n i z a t i o n p o t e n t i a l of a t e r t i a r y r a d i c a l i s -170 kcal/mol. Even i f t h e a b s e n c e of d i r e c t i o n i z a t i o n of h y d r o carbons i s w e l l demonstrated i n t h e s e experiments, it i s reasonable t o expect t h a t strong, zeolite-type e l e c t r o l y t e s e x e r t a s t r o n g i n f l u e n c e upon carbenium i o n r e a c t i o n s . F i r s t , by t h e c o n c e n t r a t i o n o f t h e h y d r o c a r b o n r e a c t a n t s i n t h e c a t a l y s t c r y s t a l , and s e c o n d , b y a i d i n g t h e f o r mation of hydrocarbon c a t i o n s and by s t a b i l i z i n g t h e s e ionic species a f t e r formation.

C h e m i s t r y a n d C a t a l y s i s W i t h H-Acid Formation of

Zeolites

Acid S i t e s

The h y d r o x y l g r o u p s w i t h v e r y a c i d i c hydrogen have been w e l l r e c o g n i z e d a s t h e s o u r c e of a c i d i c a a t a l y t i c activity i n zeolites. P r i o r t o t h e discovery of a c i d i c z e o l i t e c a t a l y s t s , t h e m o s t n o t a b l e a c i d c a t a l y s t was t h e silica-alumina gel. Here t h e s o u r c e o f a c i d i t y i s h y d r o x y l g r o u p s g l i n k e d t o aluminum o r t o s i l i c o n - a l u m i n u m s i t e s . With s i l i c a - a l u m i n a g e l t y p e c a t a l y s t s , a l l m e t a l c a t i o n s These c a t i o n s were r e c o g n i z e d as p o i s o n s f o r a c i d s i t e s . r e p l a c e d a c i d i c h y d r o x y l h y d r o g e n s and f u l l y removed a c i d activity. With t h i s background i n a c i d c a t a l y s t chemistry, i t w a s s u r p r i s i n g t h a t z e o l i t e s N a X a n d N a y , when c a t i o n exchanged w i t h b i - o r m u l t i v a l e n t m e t a l c a t i o n s , gave rise t o a c i d i c c a t a l y t i c a c t i v i t y , w e l l exceeding t h e acid s t r e n g t h of silica-alumina g e l . I t was a l s o found

t h a t N a y , w h e n N H e~ x c h a n g e d a n d a c t i v a t e d a t h i g h t e m p e r a t u r e s t o remove ammonia, f u l l y r e t a i n e d t h e c r y s t a l f r a m e w o r k o f t h e o r i g i n a l Nay, a n d i t g a v e r i s e t o v e r y strong a c i d i c c a t a l y t i c a c t i v i t y , w e l l exceeding i n 1301. s t r e n g t h a l l o x i d e - t y p e a c i d c a t a l y s t s known b e f o r I n c o n t r a s t t o t h i s m a t e r i a l , t h e NaX f o l l o w i n g N H f4 e x c h a n g e a n d h e a t t r e a t m e n t showed c o m p l e t e l o s s of c r y s t a l s t r u c t u r e , and i t d i s p l a y e d no s i g n i f i c a n t a c i d i c catalyt i c activity. These phenomena, d t s c o v e r e d i n t h e l a t e 1950s, were subsequently thoroughly i n v e s t i g a t e d and further characterized. The c a u s e of t h e g r e a t d i f f e r e n c e s i n t h e a p p l i c a t i o n of b i - o r m u l t i - v a l e n t c a t i o n s i n X and Y-type z e o l i t e s v e r s u s s i l i c a - a l u m i n a g e l was a s c r i b e d t o t h e s t r o n g I t was tendency of z e o l i t e s toward c a t i o n h y d r o l y s i s . found t h a t t h e tendency t o c a t i o n hydrolysis increases The cause w i t h t h e Si/AI r a t i o o f t h e z e o l i t e framework. of t h i s r e a c t i o n i s t h e i n c r e a s i n g d i s t a n c e between t h e a n i o n s i t e s (A104)- a t h i g h e r S i / ' A l r a t i o s . The i n c r e a s e d d i s t a n c e betweenthe s i n g l y charged anion sites renders t h e e l e c t r o s t a t i c s h i e l d i n g of h i g h l y charged c a t i o n s ineffective. I t w a s a l s o s u g g e s t e d t h a t Y z e o l i t e i s a very s t r o n g e l e c t r o l y t e , and c o n s e q u e n t l y i t f a v o r s r e a c t i o n s r e s u l t i n g i n t h e f o r m a t i o n of added i o n s i n t h e z e o l i t e Upon c a t i o n h y d r o l y s i s i n Y c r y s t a l (hydrolysis) (93. z e o l i t e , two d i s t i n c t h y d r o x y l g r o u p s a r e formed: one of them i s a t t a c h e d t o t h e z e o l i t e c a t i o n w h i l e t h e other i s l i n k e d t o t h e f r a m e w o r k c a t i o n s { l l ) . I t i s a l s o expected t h a t cation hydrolysis i n Y z e o l i t e s occurs f i r s t I t was w i t h c a t i o n s a t low c o o r d i n a t i o n s i t e s ( S i t e 2 ) . determined t h a t , following hydrolysis, t h e hydroxylated c a t i o n s s h i f t from t h e low c o o r d i n a t i o n s i t e s t o t h e With two c a t i o n s moving i n t o t h e s o d a l i t e cage E l l , 12). s o d a l i t e c a g e , a c a t i o n - o x y g e n / h y d r o x y l c l u s t e r of g r e a t s t a b i l i t y i s f o r m e d , e n h a n c i n g t h e t h e r m a l s t a b i l i t y of The c a t a l y t i c a l l y e f f e c t i v e the Y zeolite crystal. acid s i t e s i n these bi- or multivalent cation Y zeolites a r e t h e a c i d i c h y d r o x y l s l i n k e d t o framework Si-A1 s i t e s . S e v e r a l i m p o r t a n t c h a r a c t e r i s t i c s of b i - and multiv a l e n t c a t i o n Y z e o l i t e s have been i d e n t i f i e d . F o r example, t h e a b s e n c e of s t r o n g a c i d a c t i v i t y w i t h a l k a l i n e e a r t h Y u p t o a b o u t 4 5 % c a t i o n e x c h a n g e was e x p l a i n e d o n t h e b a s i s t h a t bivalent cations corresponding t o t h i s cation e x c h a n g e l e v e l o c c u p y t h e o c t a h y d r a l s i t e s i n t h e hexagonal Extensive r i n g , and t h e y do n o t undergo c a t i o n h y d r o l y s i s . i n f r a r e d s p e c t r o s c o p i c s t u d i e s o f t h e 0-H b o n d i n b i v a l e n t c a t i o n Y reveaLed an i n f l u e n c e of t h e s i z e of c a t i o n s a n d t h e S i / A l r a t i o o f t h e z e o l i t e f r a m e w o r k on

the wave number of t h e 0-H acidic h y d r o x y l s (51.

band

(-3640 c m - I )

representing

The c h e m i c a l and s t r u c t u r a l c h a n g e s o c c u r r i n g i n Y z e o l i t e s u p o n NH4 e x c h a n g e f o l l o w e d b y t h e r m o l y s i s o f the NH4 groups have been e x t e n s i v e l y i n v e s t i g a t e d {13). It i s assumed t h a t c r y s t a l r e t e n t i o n of .the a c t i v a t e d N R 4 - Y z e o l i t e v e r s u s t h e s i m i l a r l y t r e a t e d NH4-X r e s t s on t h e l a r g e r n u m b e r o f s t a b l e S i - 0 - S i l i n k a g e s {30]. The a c i d i t y h e r e , s i m i l a r t o b i l m u l t i v a l e n t c a t i o n z e o l i t e s , i s c e n t e r e d on hydroxyls a t t a c h e d t o framework cations. Infrared spectroscopic investigations revealed several hydroxyl types. One o f t h e s e c a t e g o r i e s i s r e p r e s e n t e d by s p e c i e s i n t h e l a r g e c a v i t y , g i v i n g r i s e t.0 a c i d i c b e h a v i o r i n c a t a l y s i s w h i l e o t h e r s a r e o c c l u d e d in the s o d a l i t e cage o r i n the hexagonal ring. Both t h e r m a l and s t e a m t r e a t m e n t s r e s u l t i n c h e m i c a l A t the mildest thermal changes i n t h e NH4Y z e o l i t e . treatment t h e t e t r a h e d r a l c o o r d i n a t i o n of t h e S i and A 1 a t o m s a s s o c i a t e d w i t h a c i d s i t e s may b e r e t a i n e d I 1 4 ) . In the p r e s e n c e o f nascent moisutre o r added steam, an e x t e n s i v e h y d r o l y s i s o f framework aluminum b e g i n s {15, 17-22). Upon e x t e n s i v e h y d r o l y s i s t h e a l u m i n u m i s l i n k e d with hydroxyl groups, l e a v i n g t h e z e o l i t e framework t o U l t i m a t e l y , h y d r o x y l a t e d aluminums assume c a t i o n s i t e s . form s t a b l e o x i d e / h y d r o x y l c l u s t e r s o c c u p y i n g t h e s o d a l i t e cage. The framework s i t e s v a c a t e d by aluminum a r e o c c u p i e d by s i l i c o n w h i c h i s r e n d e r e d more m o b i l e b y t h e r e m o v a l of a d j a c e n t aluminum. U l t i m a t e l y , a r e h e a l i n g of t h e c r y s t a l s t r u c t u r e occurs w i t h t h e s i l i c o n atoms f i l l i n g up t h e t e t r a h e d r a l framework s i t e s v a c a t e d by aluminum. A r e d u c t i o n o f t h e number o f u n s t a b l e d e f e c t s i t e s ( c a t i o n vacancy) a l s o t a k e s p l a c e i n o r d e r t o r e e s t a b l i s h s h o r t A s a r e s u l t of t h e s e p r o c e s s e s , range c r y s t a l c o n t i n u i t y . It empty pockets" a r e formed throughout t h e z e o l i t e c r y s t a l . The r e c r y s t a l l i z e d p a r t o f t h e Y z e o l i t e f r a m e w o r k i s now enriched i n s i l i c a , and t h e c r y s t a l i s f u r t h e r s t a b i l i z e d by t h e aluminum o x i d e / h y d r o x y l c l u s t e r s o c c u p y i n g s o d a l i t e cages.

The s u c c e s s o f " s t e a m s t a b i l i z a t i o n " d e p e n d s o n a d e l i c a t e b a l a n c e between t h e r a t e s of aluminum h y d r o l y s i s , s i l i c o n m i g r a t i o n , and s i l i c o n r e s u b s t i t u t i o n r e a c t i o n s . I n a p p r o p r i a t e r e a c t i o n c o n d i t i o n s , c a u s i n g r a p i d aluminum hydrolysis without adequate s i l i c o n substitution,readily r e s u l t i n a c o l l a p s e of t h e c r y t a l s t r u c t u r e . Conseouently, temperature, steam p r e s s u r e , and time are a l l important variables.

C h a r a c t e r i z a t i o n o f Acid S i t e s Some o f t h e c h e m i c a l a n d s t r u c t u r a l a s p e c t s o f s t e a m s t a b i l i z a t i o n remain unresolved i n s p i t e of extensive a t t e m p t s a t c h a r a c t e r i z a t i o n u s i n g m a i n l y i n f r a r e d spectroscopy and c r y s t a l l o g r a p h y . Furthermore, characteristics o f s a m p l e s p r e s u m a b l y o f t h e same t y p e p r e p a r e d b y different i n v e s t i g a t o r s , o f t e n s h o w d i f f e r e n t b e h a v i o r b e c a u s e of d i f f e r e n c e s , n o m a t t e r how s m a l l , i n t r e a t m e n t a n d i n crystal source. The m o s t i m p o r t a n t s t r u c t u r a l a s p e c t o f steams t a b i l i z e d Y z e o l i t e i s t h e f u l l r e t e n t i o n of crystallinity i n s p i t e o f t h e e x t e n s i v e i o n i c m i g r a t i o n i n v o l v e d i n the process. Crystallographic studies, particularly the s t u d y of Si(A1)-0 bond l e n g t h s , s u g g e s t t h a t p r o b a b l y t h r e e o u t o f t h e f o u r c r y s t a l l o g r a p h i c a l l y d i s t i n c t oxygen atoms p a r t i c i p a t e i n t h e f o r m a t i o n of a c i d i c hydroxyl The s i l i c o n e n r i c h m e n t i n t h e groups {12, 23, 24). c r y s t a l framework o f s t e a m - s t a b i l i z e d Y i s i n d i c a t e d by s h r i n k i n g o f t h e c r y s t a l u n i t c e l l , c o r r e s p o n d i n g t o the A s m a l l e r l e n g t h o f S i - 0 b o n d s v e r s u s A1-0 l i n k a g e s . s i m i l a r r e l a t i o n s h i p between u n i t c e l l s i z e and increasing S i / A 1 r a t i o h a s b e e n w e l l r e c o g n i z e d b e f o r e , and i t has been used t o estimate t h e Si/A1 r a t i o of t h e Y z e o l i t e With s t e a m - s t a b i l i z e d Y , t h e determinaframework ( 2 5 ) . t i o n o f t h e f r a m e w o r k S i / A 1 r a t i o b a s e d on x - r a y d i f f r a c The u n i t c e l l s i z e i s tion pattern alone i s difficult. a f f e c t e d b o t h by s i l i c o n e n r i c h m e n t o f t h e f r a m e w o r k and by t h e s t u f f i n g o f t h e s o d a l i t e c a g e w i t h aluminum oxide/ h y d r o x i d e c l u s t e r s , and t h e s e two phenomena a r e expected t o c o n t r i b u t e t o changes i n u n i t c e l l s i z e i n opposing T h e r e f o r e , t h e d e t e r m i n a t i o n of d i r e c t i o n s (1, 2 5 1 . c a t i o n e x c h a n g e c a p a c i t y r e m a i n s a more r e l i a b l e technique t o d e t e r m i n e framework aluminum c o n t e n t . The c o m p l e x i t y of t h e c h e m i s t r y and s t r u c t u r e of s t a b i l i z e d Y i s r e f l e c t e d i n t h e i n f r a r e d s p e c t r u m of 0-H b o n d s . Here, depending on t h e d e g r e e o f framework a l u m i n u m h y d r o l y s i s , s e v e r a l i n f r a r e d 0-H b a n d s a r e d e t e c t e d , r e p r e s e n t i n g b o t h " a c c e s s i b l e " and "nonaccessible" hydroxyls. Since the extinction coefficient f o r a l l t h e s e i n f r a r e d b a n d s i s n o t known, i t i s n o t p o s s i b l e t o d e t e r m i n e t h e r e l a t i v e c o n c e n t r a t i o n of h y d r o x y l s o f d i f f e r e n t 0-H i n f r a r e d f r e q u e n c y w i t h p r e cision. N e v e r t h e l e s s , a s s i g n m e n t s o f 0-H b a n d s t o a s s u m e d s t r u c t u r a l h y d r o x y l s h a v e b e e n m a d e , a n d t h e y are used f r e q u e n t l y t o monitor s t e a m s t a b i l i z a t i o n and a c i d i t y (13).

It h a s been d i s c u s s e d above t h a t t h e hydroxyls formed i n a c t i v a t e d NH4Y r e p r e s e n t s e v e r a l d i s t i n c t s p e c i e s differing i n l i n k a g e , c o o r d i n a t i o n , and a c c e s s i b i l i t y . C o r r e s p o n d i n g l y , t h e d e t e r m i n a t i o n of z e o l i t e a c i d The strength i s d i f f i c u l t by any s i n g l e measurement. u s e of c o l o r e d d y e s o f t e n u s e d w i t h o t h e r s o l i d a c i d s i s not p o s s i b l e b e c a u s e o f t h e l a r g e s i z e o f t h e s e m o l e c u l e s . T i t r a t i o n w i t h s m a l l b a s e s s u c h a s NH3 i s u s e f u l . However, one m u s t c o n s i d e r t h a t NH3 i s s t r o n g l y a t t a c h e d t o b o t h Lewis a n d B r o n s t e d a c i d s i t e s . C o n s e q u e n t l y , NH3 s o r p t i o n measures b o t h a c i d i c h y d r o x y l s and a c c e s s i b l e c a t i o n s . One o f t h e m o s t r e l i a b l e m e a s u r e m e n t s o f a c i d c o n c e n t r a t i o n r e s t s on t h e i n s p e c t i o n of t h e i n f r a r e d s p e c t r u m of pyridine adsorbed on Y c a t a l y s t s . Here, the infrared bands r e p r e s e n t i n g d i s t i n c t p r o t o n a t e d o r p o l a r i z e d pyridine s p e c i e s g i v e q u a n t i t a t i v e account of Brons t e d a n d L e w i s a c i d i t y C5, 1 3 ) . Steric hindrance introduced onto t h e p y r i d i n e by m e t h y l - o r e t h y l - s u b s t i t u t i o n s a t a p p r o p r i a t e p o s i t i o n s g i v e s f u r t h e r i n f o x m a t i o n o n OH accessibility i n the zeolite structure.

One o f t h e s h o r t c o m i n g s o f N - b a s e t i t r a t i o n r e s u l t s from t h e h i g h e n e r g y o f i n t e r a c t i o n b e t w e e n s t r o n g b a s e s and z e o l i t e a c i d s i t e s , r e n d e r i n g t h e i n t e r a c t i o n a l m o s t i r r e v e r s i b l e a t low t e m p e r a t u r e s a n d p r e v e n t i n g e q u i l i b r a tion a m o n g . s i t e s of v a r i o u s a c i d s t r e n g t h . A g o o d "compromise" t e c h n i q u e u s e s t h e r m o m e t r i c t i t r a t i o n ( 2 6 - 2 8 ) with a m i n e s a p p l i e d i n a s o l v e n t . Here an aromatic solvenz serves t o e x p e d i t e t h e d e s o r p t i o n and t h e e q u i l i b r a t i o n of t h e amine a p p l i e d . This r e s u l t s i n aiding equilibration, f i r s t n e u t r a l i z i n g t h e s t r o n g e s t and t h e n g r a d u a l l y the weaker a c i d s i t e s . Unfortunately, even t h i s technique has s t e r i c o r d i f f u s i o n - r e l a t e d l i m i t a t i o n s , a n d r e l i a b l e data a r e obtained only w i t h c r y s t a l s w i t h three-dimens i o n a l p o r e s y s t e m s (Y z e o l i t e ) . These t i t r a t i o n s a r e much l e s s s u c c e s s f u l w i t h z e o l i t e s o f l e s s o p e n s t r u c t u r e (mordenite) (29). A s t r o n g enhancement i n c r y s t a l s t a b i l i t y and a c i d activity with increasing Si/A1 r a t i o w a s recognized i n the e a r l y d a y s of z e o l i t e c a t a l y s i s , u s i n g z e o l i t e s X a n d Y (30). I t was shown t h a t b a s e d on e l e c t r o s t a t i c c o n s i d e r a tions, the charge density at a cation s i t e increases with I t was c o n c e i v e d t h a t t h e s e increasing Si/A1 r a t i o . phenomena a r e r e l a t e d t o a r e d u c t i o n o f e l e c t r o s t a t i c i n t e r a c t i o n between framework s i t e s , and p o s s i b l y t o a d i f f e r e n c e i n o r d e r i n g of aluminum i n t h e z e o l i t e c r y s t a l (31). Later, using t i t r a t i o n s with various amines, i t was f o u n d t h a t t h e s l o p e o f t h e t i t r a t i o n c u r v e o f NaH-Y This slope zeolites i s l i n e a r with c a t i o n exchange {32}.

changes w i t h t h e S i / A 1 r a t i o , and i t r e f l e c t s t h e degree Higher o f " e f f i c i e n c y " of t h e a v e r a g e (A1O4)- s i t e { 3 3 ) . a l u m i n u m c o n t e n t s r e s u l t i n " i n t e r f e r e n c e " b e t w e e n alumina s i t e s , and t h u s i n l e s s t h a n a l i n e a r i n c r e a s e of t i t r a t a b l e strong a c i d i t y (33). Conversely, lowering the alumina c o n c e n t r a t i o n i n t h e Y z e o l i t e framework f i r s t r e s u l t s o n l y i n a d e p l e t i o n of t h e w e a k e r a c i d s i t e s w i t h o u t a f f e c t i n g t h e s t r o n g a c i d s i t e s (>90X H2S04) 1 3 4 ) . Only e x t e n s i v e framework aluminum h y d r o l y s i s , r e s u l t i n g i n l e s s t h a n 1 6 A 1 p e r u n i t c e l l , r e s u l t s i n a + d e c l i n e of s t r o n g a c i d s i t e s {13}. The d i r e c t r e l a t i o n s h i p b e t w e e n a c i d s t r e n g t h and S i / A 1 r a t i o s e e m s t o a p p l y i n some cases e v e n f o r zeol i t e of d i f f e r e n t s t r u c t u r e . I t was r e p o r t e d t h a t t h e a c i d s i t e s p r e s e n t i n H-ZSM-5 (Si/Al = 1 9 . 2 ) a r e stronger In a t h a n t h o s e p r e s e n t i n H-zeolon ( S i / A l = 5) {351. d i f f e r e n t a p p r o a c h i t was found t h a t , u s i n g a n e l e c t r o n e g a t i v i t y model f o r c a l c u l a t i n g t h e r e s i d u a l charge for hydrogen atoms i n z e o l i t e s , t h e a c i d s t r e n g t h d e c l i n e s w i t h i n c r e a s i n g aluminum c o n t e n t 1 3 6 ) . Catalytic data c o n s i s t e n t w i t h t h e s e c a l c u l a t i o n s confirm t h e chemical tendency s u g g e s t e d by t h e s e c a l c u l a t i o n s ( 3 7 1 . C a t a l y t i c P r o p e r t i e s of

Acid Z e o l i t e s

The main c a t a l y t i c c h a r a c t e r i s t i c s of a c i d zeol i t e s i n hydrocarbon r e a c t i o n s a r e t h e i r s e l e c t i v i t y a s m o l e c u l a r s i e v e s , t h e i r h i g h a c i d s t r e n g t h , and t h e i r e l e c t r o l y t i c p r o p e r t i e s which i n f l u e n c e reaction kinetics. 1.

Molecular Sieve Effect.

Shape S e l e c t i v i t y

I n many a c i d - c a t a l y z e d r e a c t i o n s t h e r e a c t i o n r a t e depends on t h e r a t e of carbenium i o n f o r m a t i o n . Normally, t e r t i a r y c a r b o n s f o r m c a r b e n i u m i o n s e a s i e r t h a n secondary carbons. Primary carbons do n o t form carbenium ions r e a d i l y under m i l d c o n d i t i o n s , and c o n s e q u e n t l y tend t o be unreactive. The d i f f e r e n c e s i n t h e e a s e o f i o n form a t i o n a r e r e a d i l y e x p l a i n e d by s i m i l a r d i f f e r e n c e s i n t h e e n e r g y r e q u i r e d f o r t h e r e m o v a l o f H- i o n f r o m C-H bonds formed w i t h t e r t i a r y , s e c o n d a r y , p r i m a r y carbon atoms. F o r t h i s r e a s o n , i s o p a r a f f i n s a r e n o r m a l l y much more r e a c t i v e t h a n n - p a r a f f i n s . The o r d e r o f t h i s welle s t a b l i s h e d r e a c t i v i t y f o r p a r a f f i n s i s , h o w e v e r , often r e v e r s e d i n c e r t a i n z e o l i t e s as a r e s u l t o f s t e r i c h i n d e r a n c e o r d i f f u s i o n l i m i t a t i o n i m p o s e d b y t h e zeolite structure. E f f e c t s o f t h i s t y p e a r e o f t e n referred t o as s h a p e s e l e c t i v i t y ( 3 8 1 .

With z e o l i t e s c o n t a i n i n g p o r e s j u s t l a r g e enough t o admit s m a l l , m o n o - s u b s t i t u t e d p a r a f f i n s , t h e r a t e o f d i f f u s i o n f o r t h e i s o p a r a f f i n s i s v e r y s u b s t a n t i a l l y less than t h e r a t e of d i f f u s i o n of n-paraf f i n s . Additionally, the d i f f u s i o n r a t e of n - p a r a f f i n s can d i f f e r by s e v e r a l orders of magnitude between s m a l l p o r e z e o l i t e s and t h e large pore z e o l i t e Y. Changes i n t h e d i f f u s i o n r a t e by several o r d e r s of magnitude can r e a d i l y reverse t h e o r d e r of r e a c t i o n s e l e c t i v i t y , r e s u l t i n g i n h i g h e r r e a c t i o n rates f o r t h e molecule of smaller c r i t i c a l dimension { 3 9 ) . In t h e c a s e of d i f f u s i o n - c o n t r o l l e d s e l e c t i v i t y , t h e s i z e of t h e c r y s t a l a l s o b e c o m e s a n i m p o r t a n t s e l e c t i v i t y parameter. Molecular s i e v e e f f e c t s i n c a t a l y s i s can be d i s t i n guished w i t h r e s p e c t t o whether t h e z e o l i t e p o r e s i z e prevents t h e p a s s a g e of the r e a c t a n t o r t h e p r o d u c t through t h e z e o l i t e p o r e s , o r w h e t h e r t h e f o r m a t i o n of a p a r ticular transition s t a t e required f o r product formation is prevented. T h e r e a r e c o n v i n c i n g e x a m p l e s f o r e a c h of T h e r e a r e many e x a m p l e s o f m o l e c u these c a t e g o r i e s { 3 9 ) . lar s i e v e effects-shape s e l e c t i v i t y - u s i n g mixtures of small and l a r g e r o l e f i n s o r a l c o h o l s , w i t h t h e s m a l l e r molecules r e a c t i n g w h i l e t h e l a r g e r ones remain i n t a c t { 4 0 ) . Molecular s i e v e e f f e c t s showing product-size s e l e c t i v i t y have been demonstrated i n t h e isomerization of n - p a r a f f i n s w i t h small-pore t y p e a c i d i c z e o l i t e s . Here, i n s p i t e of t h e p r e s e n c e of a c i d s i t e s , no i s o p a r a f f i n s are r e l e a s e d i n t h e product, and t h e product c o n t a i n s only a m i x t u r e of methane and e t h a n e i n a d d i t i o n t o t h e n-hexane r e a c t a n t . Here t h e s m a l l z e o l i t e pores prevent t h e d e s o r p t i o n of i s o p a r a f f i n s whkch a r e p r e s u m a b l y f o r m e d i n t h e l a r g e c a v i t i e s of t h e z e o l i t e c r y s t a l . An i n t e r e s t i n g e x a m p l e o f p r o d u c t - s i z e t y p e s e l e c t i v i t y i s shown i n t h e i s o m e r i z a t i b n of n-hexane For t h i s with t h e medium p o r e s i l i c a l i t a ( 4 3 2 . e x p e r i m e n t t h e s i l i c a l i t e was s y n t h e s i z e d w i t h a s m a l l aluminum i m p u r i t y ( c a l c u l a t e d as 0 . 6 w t . % a l u m i n a ) . Following t r e a t m e n t w i t h a n NH4 s a l t s o l u t i o n and t h e r m a l a c t i v a t i o n , t h e silfc a l i t e c o m p o s i t i o n s h o w e d a m o d e s t d e g r e e of a c i d i t y , a s shown i n T a b l e 3 .

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From t h e d i f f e r e n c e i n r e a c t i o n t e m p e r a t u r e r e q u i r e d t o reach s i m i l a r conversions f o r n-hexane, i t i s c l e a r that t h e a c i d i t y of t h e S i l i c a l i t e sample i s v e r y i n f e r i o r Nevertheless, a t higher r e l a t i v e t o s t e a m - s t a b i l i z e d Y. temperatures t h e S i l i c a l i t e r e a d i l y produced a mixture of methylpentanes, c l o s e t o t h e e q u i l i b r i u m composition. However, i n s h a r p c o n t r a s t t o t h e s t e a m - s t a b i l i z e d Y c a t a l y s t , t h e S i l i c a l i t e produced no s i g n i f i c a n t amounts of 2,2-dfmethylbutane. The a b s e n c e o f t h e r e l a t i v e l y large s i z e 2,2-dimethylbutane i n t h e product obtained with S i l i c a l i t e i s c o n s i s t e n t w i t h i n d e p e n d e n t a d s o r p tion experiments, showing t h a t s i l i c a l i t e does n o t adsorb t h e s i m i l a r s i z e n e o p e n t a n e m o l e c u l e . Other examples of molecular s i e v e e f f e c t s are r e In this case, lated t o t r a n s i t i o n s t a t e s e l e c t i v i t y . certain reactions a r e prevented because the transitions t a t e complex i s t o o l a r g e f o r t h e p o r e o r p o r e - c a v i t y system of a z e o l i t e . I n some of t h e s e c a s e s , n e i t h e r the r e a c t a n t s nor t h e products a r e prevented from passing t h r o u g h t h e z e o l i t e p o r e s . Examples of s u c h t r a n s i tion s t a t e s e l e c t i v i t y a r e displayed i n t h e transa l k y l a t i o n and c e r t a i n i s o m e r i z a t i o n r e a c t i o n s of a l k y l benzenes ( 3 9 , 4 1 ) . 2.

The C r a c k i n g of H y d r o c a r b o n s Zeolites

Over A c i d i c Y

T h e c r a c k i n g o f n - h e x a n e o v e r H-Y z e o l i t e s h o w s markedly d i f f e r e n t b e h a v i o r { 4 2 ) from t h a t shown by a l k a l i c a t i o n z e o l i t e s C7). Here c a t a l y t i c a c t i v i t y i s much h i g h e r , s k e l e t a l i s o m e r i z a t i o n o f f r a g m e n t s becomes d o m i n a n t , a n d t h e p r o d u c t d i s t r i b u t i o n i s generally consistent with t h a t occurring with other strong Bronsted a c i d c a t a l y s t s f o r which a carbenium ion r a t h e r t h a n f r e e r a d i c a l mechanism i s g e n e r a l l y accepted. Significantly, the product contains a relat i v e l y low c o n c e n t r a t i o n o f o l e f i n s , and i t a l s o cont a i n s s i g n i f i a a n t amounts of a r o m a t i c s .

The i s o m e r i z a t i o n o f t h e h e x a n e i s o m e r s was s t u d i e d a t low c o n v e r s i o n l e v e l s u s i n g a p a l l a d i u m - l o a d e d zeolite catalyst. H e r e , t h e primary products derived from the i n d i v i d u a l hexane isomers cannot be explained i n terms of an i n t r a m o l e c u l a r rearrangement of a carbenium It w a s observed t h a t isomerization ion intermediate. i s i n v a r i a b l y accompanied by h y d r o c r a c k i n g , b u t t h e hydrocracked porducts a r e n o t c o n s i s t e n t w i t h a simple A b i m o l e c u l a r mechanism cleavage of t h e hexane molecules. was p r o p o s e d w h i c h s a t i s f a c t o r i l y e x p l a i n s b o t h t h e

observed p r o d u c t s from b o t h isornerization and hydrocracking r e a c t i o n s as w e l l as t h e presence of heptanes i n the product { 4 9 ) . In the industrial c a t a l y t i c cracking process, the a c i d i c Y z e o l i t e - b a s e d c a t a l y s t s show c e r t a i n new f e a t u r e s n o t found w i t h t h e e a r l i e r used a c i d c a t a l y s t s ( 4 4 , 45). F i r s t , t h e a c t i v i t y o f Y z e o l i t e - t y p e c a t a l y s t s i s much higher relative to silica-alumina gel. In addition, z e o l i t e c a t a l y s t s produce s u b s t a n t i a l l y l a r g e r gasoline yields. The c o m p o s i t i o n of t h e g a s o l i n e produced over Y z e o l i t e s c o n t a i n s much l e s s o l e f i n s a n d s u b s t a n t i a l l y m o r e a r o m a t i c s r e l a t i v e t o t h e g a s o l i n e made w i t h s i l i c a alumina g e l (see Table 4 ) . M e c h a n i s t i c s u g g e s t i o n s by Weisz ( 4 6 ) r e g a r d i n g t h e s e i n t e r r e l a t e d d i f f e r e n c e s i n g a s o l i n e y i e l d a n d c o m p o s i t i o n h a v e b e e n made t o t h e e f f e c t t h a t t h e r e i s a more e f f i c i e n t hydrogen r e d i s t r i bution between hydrocarbon molecules over t h e z e o l i t e catalyst. A c c o r d i n g t o t h e g a s o l i n e s e l e c t i v i t y m o d e l of Weekman a n d N a c e ( 4 7 1 , t h e p r i m a r y c r a c k i n g r e a c t i o n p r o d u c e s g a s o l i n e a n d g a s , w h e r e a s the s e c o n d a r y c r a c k i n g r e a c t i o n produces o n l y g a s , and t h u s lower g a s o l i n e yield. Gas

oil

>-

primary Gasoline

The d i f f e r e n c e i n product d i s t r i b u t i o n between z e o l i t e and s i l i c a - a l u m i n a can b e e x p l a i n e d q u a n t i t a t i v e l y by t h e g r e a t e r o c c u r r e n c e o f t h e f o l l o w i n g o v e r a l l hydrogen redistribution reaction in the zeolite: Olefins

+

n a p h t h e n e s ~ pj a r a f f i n s

+

aromatics

I t was s u g g e s t e d 1 4 6 ) t h a t t h i s r e a c t i o n e f f e c t i v e l y r e d u c e s t h e s e c o n d a r y c r a c k i n g i n t h e c r a c k i n g o f gas o i l by c o n v e r t i n g t h e i n i t i a l l y produced o l e f i n s and n a p h t h e n e s t o more r e f r a c t o r y p a r a f f i n s and a r o m a t i c s before they crack f u r t h e r t o gas. The h i g h e f f i c i e n c y of t h e c o n v e r s i o n of o l e f i n s and naphthenes t o paraffins a n d a r o m a t i c s may b e e x p l a i n e d b y t h e s u p e r i o r h y d r o g e n r e d i s t r i b u t i o n v i a h y d r i d e i o n s h i f t between carbenium i o n s and n e u t r a l hydrocarbons over t h e z e o l i t e catalyst,

It i s apparent from t h e information presented here t h a t t h e c r a c k i n g o f h y d r o c a r b o n s o v e r K-Y a n d a c i d i c Y z e o l i t e s s h o w a n i m p o r t a n t common f e a t u r e . I n b o t h cases t h e e f f i c i e n c y of t h e hydrogen t r a n s f e r s t e p between r e a c t a n t m o l e c u l e s i s g r e a t l y e n h a n c e d by b o t h t y p e s of I n t h e K-Y c a t a l y z e d c r a c k i n g o f zeolite catalysts. n-hexane, t h i s e f f e c t r e s u l t e d i n a s u b s t a n t i a l increase i n t h e f o r m a t i o n of p r o p a n e and a s i m i l a r l y s u b s t a n t i a l d e c r e a s e i n t h e p r o d u c t i o n o f f r a g m e n t s s m a l l e r t h a n C3. I n t h e c a s e of gas o i l cracking o v e r a c i d i c Y z e o l i t e b a s e d c a t a l y s t s , a n e x t e n s i v e r e d i s t r i b u t i o n of hydrogen i s achieved t o t h e e x t e n t t h a t hydrogen i s t r a n s f e r r e d from naphthenes t o o l e f i n s forming p a r a f f i n s and aromatics. T h i s o v e r a l l r e a c t i o n , c a t a l y z e d by a c i d i c Y z e o l i t e s , f o l l o w s t h e thermodynamics p r e v a i l i n g a t t h e catalytic cracking process conditions. O n e o f t h e new c h e m i c a l f a c t o r s i n t r o d u c e d by z e o l i t e c a t a l y s t s i s t h a t t h e y a r e a b l e t o c a r r y o u t b o t h h y d r o g e n a t i o n a n d dehydrog e n a t i o n r e a c t i o n s e f f i c i e n t l y v i a m u l t i p l e h y d r i d e transfer steps. T h i s phenomenon i s q u i t e s z l m i l a r t o t h e enhancement of t h e hydrogen t r a n s f e r s t e p s found w i t h the n o n a c i d i c K-Y.

S i m i l a r d i f f e r e n c e s i n p r o d u c t f o r m a t i o n were rep o r t e d r e c e n t l y i n t h e d i r e c t s y n t h e s i s of hydrocarbons f r o m s y n g a s o v e r two m o l e c u l a r s i e v e c a t a l y s t s ( 4 8 ) . One o f t h e c a t a l y s t s c o n s i s t e d o f a m i x t u r e o f i r o n m e t a l z e o l i t e w h i l e t h e o t h e r c o n s i s t e d o f a mixture a n d H-ZSM-5 o f a s i m i l a r f r a c t i o n o f i r o n m e t a l a n d S i l i c a l i t e . By comparison w i t h a p u r e i r o n c a t a l y s t , b o t h molecular-sievebased c a t a l y s t s s u b s t a n t i a l l y reduced t h e b o i l i n g range T h e H-ZSM-5 c a t a l y s t d i s of t h e hydrocarbon product. p l a y e d b o t h h i g h e r a c t i v i t y and h i g h e r b o i l i n g - r a n g e selectivity than the silicalite-based catalyst. I n spite o f t h e s i m i l a r i n f l u e n c e o f b o t h ZSM-5 a n d ~ i l i c a l i t eon p r o d u c t b o i l i n g r a n g e , t h e two c a t a l y s t s p r o d u c e d hydroc a r b o n s of e n t i r e l y d i f f e r e n t c o m p o s i t i o n s . The i r o n H-ZSM-5 c a t a l y s t produced hydrocarbons r i c h i n aromatics and v e r y low i n o l e f i n s w h i l e t h e i r o n - s i l i c a l i t e catalyst produced m a i n l y o l e f i n s and o n l y minor amounts of a r o matics. The d e t e r m i n a t i o n as t o w h a t e x t e n t t h e a c i d strength, c o n c e n t r a t i o n , and e l e c t r o l y t i c s t r e n g t h a r e responsible f o r t h e c o n t r a s t i n product compositions is not possible without establishing the i n t r i n s i c acid strength, acid c o n c e n t r a t i o n , and e l e c t r o l y t e s t r e n g t h i n each c a t a l y s t . However, t h e s i m i l a r i t y w i t h t h e c r a c k i n g d a t a discussed above f o r t h e s t r o n g e l e c t r o l y t e Y z e o l i t e v e r s u s s i l i c a l i t e s u g g e s t s t h a t i n t h e h y d r o c a r b o n s y n t h e s i s experiments

t h e H-ZSM-5 p l a y s t h e r o l e o f a s t r o n g a c i d a s w e l l as a relatively stronger electrolyte. The e x p e r i m e n t a l e v i d e n c e s t r o n g l y s u g g e s t s t h a t the cause of t h e hydrogen t r a n s f e r enhancement w i t h both K-Y a n d w i t h a c i d i c Y z e o l i t e s i s common, b o t h r e l a t e d t o t h e h i g h c o n c e n t r a t i o n of r e a c t a n t hydrocarbons i n t h e zeolite c r y s t a l r e l a t i v e t o the surrounding gas phase. The h i g h c o n c e n t r a t i o n o f reactant m o l e c u l e s e n h a n c e s t h e r a t e of b i m o l e c u l a r r e a c t i o n s t e p s - t h e hydrogen t r a n s f e r s t e p - w i t h b o t h K-Y a s w e l l a s w i t h a c i d i c Y , o v e r t h e unimolecular cracking s t e p . The h i g h e r c o n c e n t r a t i o n of r e a c t a n t s , p e r s i s t i n g even a t h i g h r e a c t i o n temperat u r e s , i s t h e r e s u l t of s t r o n g i n t e r a c t i o n between t h e p o l a r i z a b l e hydrocarbons and t h e s t r o n g l y p o l a r i n t r a crystalline zeolite surface. The d e g r e e o f t h i s i n t e r action r e f l e c t s t h e strength of t h e z e o l i t e e l e c t r o l y t e .

CONCLUSIONS

A l a r g e body of e x p e r i m e n t a l e v i d e n c e i n z e o l i t e chemistry and z e o l i t e c a t a l y s i s s u g g e s t s t h a t t h e key distinguishing. f e a t u r e s of z e o l i t e c a t a l y s t s used i n i n d u s t r i a l a p p l i c a t i o n s are:

1.

The c a t a l y t i c s e l e c t i v i t y b a s e d on m o l e c u l a r s i e v e e f f e c t s o r on d i f f u s i o n l i m i t a t i o n s .

2.

The h i g h c o n c e n t r a t i o n of s t r o n g l y i o n i c h y d r o g e n (H+) a t o m s a t t a c h e d t o f r a m e w o r k oxygen atoms.

3.

The l a r g e enhancement of a n d the s t a b i l i z a t i o n of

4.

The h i g h c o n c e n t r a t i o n of hydrocarbon r e a c t a n t s within zeolite crystals, resulting in the enhancement of b i m o l e c u l a r r e a c t i o n s t e p s o v e r unimolecular reaction steps.

ionization reactions carbenium ions.

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1 ~

{51 161

C71 C8)

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(13

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10,

-

18,

A C I D I C CATALYSIS WITH ZEOLITES

Deni se B a r t homeuf Exxon Research and E n g i n e e r i n g Company Linden, New Jersey 07036

1.

Uses o f Z e o l i t e s a s A c i d i c C a t a l y s i s

A f t e r t h e work o f B a r r e r , z e o l i t e s have been used a s s e l e c t i v e adsorbent s. The d i scovery o f t h e i r c a t a l y t i c p r o p e r t i e s i n 1960 s t a r t e d a new e r a i n t h e f i e l d o f c a t a l y s i s . (1,2) Less t h a n t e n y e a r s l a t e r , 90% o f c a t a l y t i c A cracking u n i t s i n t h e U n i t e d S t a t e s used z e o l i t e s . continuous improvement of zeolites properties led t o a simultaneous i n n o v a t i o n and cornpetiton i n u n i t s design. A t t h e present t i m e t h e f o u r main i n d u s t r i a l a p p l i c a t i o n s o f z e o l i t e s The most i m p o r t a n t i s i n v o l ve t hei r a c i d i c p r o p e r t i e s . c a t a l y t i c c r a c k i n g which uses z e o l i t e s t h e r m a l l y s t a b i 1 i z e d by v a r i o u s treatments, The h y d r o c r a c k i ng process superimposes t h e cracking due t o t h e a c i d p r o p e r t i e s o f t h e z e o l i t e w i t h t h e hydrogenation due t o n o b l e metal. The selectoforrning adds a shape s e l e c t i v e e f f e c t o f t h e z e o l i t e cages t o t h e hydrocracking by t h e c h o i c e o f z e o l i t e s ( e r i o n i t e , o f f r e t i t e ) w h i c h s e l e c t i v e l y c o n v e r t o n l y C 5 t o Cg n - p a r a f f i n s from naphthas and reforrnates. ( 3 ) O i 1 dewaxi ng a1 so uses a shape s e l e c t i v e c a t a l y s t f o r t h e hydrocracking o f n-paraffins. Resides t h o s e i n d u s t r i a l uses, z e o l i t e s a r e a1 so very good c a t a l y s t s f o r other r e a c t i o n s i n v o l v i n g a c i d i c c a t a l y s i s such a s isomerization, hydration-dehydration, a l k y l ation, is o m e r i z a t i o n and d i s p r o p o r t i o n a t i o n o f a1k y l benzene, etc. Z e o l i t e s have been a b l e t o r e p l a c e o l d c a t a l y s t s as i n c a t a l y t i c c r a c k i n g o r generate novel a p p l i c a t i o n s a s in s e l e c t o f o r m i n g o r o i l dewaxing m a i n l y because o f t h e i r very high s e l e c t i v i t y . T h i s i s s t r o n g l y r e l a t e d t o t h e i r open s t r u c t u r e which may induce several speci f i c and d i f f e r e n t t y p e s

o f selectivity. The shape s e l e c t i v e c a t a l y s i s known i n ZSM-5 o r m o r d e n i t e i s d i s c u s s e d i n d e t a i l s elsewhere i n t h i s course. A s e l e c t i v i t y r e l a t e d t o chemical e f f e c t s o f t h e cages i s involved i n z e o l i t e s w i t h l a r g e r cavities. Zeolites give a g r e a t e r amount of p a r a f f i n s and a r o m a t i c s t h a n amorphous s i 1 i c a alumina c a t a l y s t s i n i n d u s t r i a l c r a c k i n g . T h i s i s e x p l a i n e d by an o v e r a l l hydrogen r e d i s t r i b u t i on r e a c t i o n i n t h e z e o l i t e . ( 4 ) Scheme 1 ( f r o m Ref. 4) Saturates

+

Saturates

+ 01 e f i n s

>-

Secondary P r o d u c t s

f

nap h t hene s haturates

+

Aromatics

T h i s p r o d u c t d i s t r i b u t i o n l e a d s t o h i g h gas01 i n e y i e l d s and low coke f o r m a t i o n . Usual ly, t o t a k e advantage o f those p r o p e r t i e s , z e o l i t e s a r e mixed w i t h a m a t r i x i n a p r o p o r t i o n c l o s e t o 15%. T h i s o p t i m i z e s t h e p e r c e n t c o n v e r s i o n and t h e g a s o l i n e and coke formations. I t a l so g i v e s t o t h e c a t a l y s t o t h e r i m p o r t a n t p r o p e r t i e s such a s r e s i stance t o a t t r i t i o n , l o w e r c o s t , improved heat t r a n s f e r performances.

..

2.

Main Parameters Which Determine C a t a l y t i c P r o p e r t i e s i n A c i d i c Zeol i t e s

conversion, selectivity) The c a t a l y t i c p r o p e r t i e s ( depend on z e o l it e s t r u c t u r e , chemi c a l c o m p o s i t i o n and v a r i o u s p r e t r e a t m e n t s w h i c h may change l o c a l l y t h e c o m p o s i t i o n o r atom location i n the zeolites.

2.1

A1 Content ( S i / A l

Ratio)

The A1 c o n t e n t o f a z e o l i t e may be expressed a s t h e r a t i o A l / A l + S i (molar f r a c t i o n o f A104 t e t r a h e d r a i n t h e t o t a l number S i ce the o f A104 and SiO t e t r a h e d r a ) o r a s t h e r a t i o S i / A l . atorn a c i d i t y a r i s e s r m t h e replacement o f a t e t r a v a l e n t S i " by a t r i v a l e n t A l g + atom i n t h e a l u m i n o - s i l i c a t e framework, the number o f a c i d s i t e s generated should p a r a l l e l v e r y s t r o n g l y Any change i n S i / A l r a t i o should t h e n be t h e A1 c o n t e n t . reflected i n c a t a l y t i c properties. I n f a c t an i n c r e a s e i n the S i /A1 r a t i o improves several z e o l it e p r o p e r t i e s . I t increa ses

4

the conversion o f hydrocarbons, a c i d i t y strength.

t h e thermal

s t a b i l i t y and t h e

With regards t o c a t a l y t i c a c t i v i t y , i n t h e f a u j a s i t e series t h e X z e o l i t e s (Si/A1 = 1.25, A l / A l + S i = 0.44) a r e l e s s active than Y t y p e s ( S i / A l = 2.4, A l / A l + S i = 0.29) d e s p i t e Similar t h e i r h i g h e r t h e o r e t i c a l number o f a c i d sites.(1,5-7) kinds o f i n c r e a s e w i t h t h e Si/A1 r a t i o a r e o b t a i n e d i n cumene cracking, (5,7-12) o-xylene isomerization, ( 6 ) gas oi1 cracki n g ( l 3 ) o r cyclopentane isomerization. (14) They can be explained by a h i g h e r e l e c t r o s t a t i c f i e l d i n Y than i n X z e o l i t e s ( 1 5 ) o r by an i n c r e a s e i n t h e s o - c a l l e d e f f i c i e n c y o f By c o n t r a s t reverse r e s u l t s a r e acid s i t e s from X t o Y. (16) obtained i n t h e low A1 range: the c a t a l y t i c a c t i v i t y increases, a s one c o u l d expect, w i t h t h e A1 content. Recent r e s u l t s have been p u b l i shed f o r ZSM-5 z e o l i t e i n a very l a r g e range o f composition a t a low A1 l e v e l r a n g i n g from 10 t o 10,ODO ppm. (17,18) A 1 inear re1a t i o n s h i p is observed between the i n c r e a s e i n c a t a l y t i c a c t i v i t y f o r n-hexane c r a c k i n g and the A1 content, For a1 1 t h e t y p e s o f r e a c t i o n mentioned above which a l l need s t r o n g a c i d s i t e s , one can t h e n expect a maximum i n t h e c a t a l y t i c p r o p e r t i e s f o r i n t e r m e d i a t e A1 contents. Values o f Si/A1 between 4 and 8 have been found f o r f a u j a s i t e structure (Figure 1)(7-14) o r between 8 and 25 f o r mordenites. (l9,20) F o r r e a c t i o n s which i n v o l v e o n l y weak a c i d s i t e s such a s isopropanol d e h y d r a t i o n a l i n e a r r e l a t i o n s h i p i s found i n a wide range o f composition(21,22) f o r v e r y d i f f e r e n t z e o l i t e s s t r u c t u r e s when c o n s i d e r i n g t h e t u r n - o v e r number versus t h e Sanderson e l e c t r o n e g a t i v i t y which r e f l e c t s a c i d i t y strength. A s i m i l a r r e l a t i o n s h i p is a1 so observed f o r t h e hydroconversion o f n-decane. (22) The Sander son e l e c t r o n e g a t i v i t y o f zeol it e s discussed ! a t e r i n d e t a i 1 s i s very h e l p f u l t o p r e d i c t c a t a l y t i c p r o p e r t i e s i n such r e a c t i o n s . It i s nevertheless not able t o depict c a t a l y t i c p r o p e r t i e s a t very low A1 c o n t e n t i n t h e range where l o c a t i o n o f s i t e s and m a l l changes i n t h e i r number a r e o f prime importance.

O f very g r e a t i n d u s t r i a l importance a r e t h e u l t r a stab1 e i n which t h e S i / A l r a t i o i s i n c r e a s e d by z e o l i t e s (U.S.) special t r e a t m e n t i n o r d e r t o remove A1 atoms from t h e framework. T h i s can be done e i t h e r by chemical e x t r a c t i o n w i t h c h e l a t i n g agents such a s EDTA o r acetylacetone(8,23-25) o r by procedures invol v i ng steaming o f ammoni um zeol it e s forms. (26) These z e o l i t e m o d i f i c a t i o n s l e a d t o a u n i t c e l l shrinkage and a considerable i n c r e a s e i n s t a b i l i t y ( 2 6 , 2 7 ) which i s o f m a j o r improvement f o r p r a c t i c a l use. No s i g n i f i c a n t improvements i n c a t a l y t i c a c t i v i t y o r s e l e c t i v i t y have been described. A

FIGURE 1: Change in cumene cracking as a function of Si/AI in faujasites (a) ~ a form ~ +(8-12) (b) ca2+ form (8-12) (c) H+ form (7-12)

r e 1a t i v e l y 1ow amount o f fundamental r e s u l t s has been r e p o r t e d z e o l i t e s Y a s opposed t o t h e usual n o t s t a b i l i z e d on those U.S. Y z e o l i t e s o f l e s s p r a c t i c a l value. A good thermal s t a b i l i t y of z e o l i t e s i s needed f o r t h e i r use i n r e a c t o r s and f o r t h e i r r e g e n e r a t i o n u s u a l l y by coke r e s i d u e burning. The a c i d i c forms o b t a i n e d by replacement of exchangeable c a t i o n s by p r o t o n s a r e u s u a l l y l e s s s t a b l e than t h e p a r e n t forms. An i n c r e a s e i n t h e S i / A l r a t i o improves g r e a t l y t h e thermal o r hydrothermal s t a b i l i t y . (27) Thi s has been a s c r i b e d t o t h e lower d e n s i t y o f hydroxyl groups which p a r a l l e l t h a t o f A1 content. A l o n g e r d i stance between h y d r o x y l s decreases t h e p r o b a b i l i t y o f d e h y d r o x y l a t i o n then a1 so t h a t o f d e f e c t generation. (28)

2.2

C a t i o n Content and I d e n t i t y

The a c i d c a t a l y s i s i n z e o l i t e s i s s t r o n g l y dependent on the p r o t o n content and t h e a c i d i t y strength. B o t h parameters vary w i t h remaining c a t i o n s . The c a t a l y t i c a c t i v i t y i n cracking, isomerization etc. r e a c t i o n s i n c r e a s e s a s t h e sodi urn content is decreased. Figure 2 i s t y p i c a l o f r e s u l t s obtained with Y z e o l i t e s i n r e a c t i o n s such a s curnene c r a c k i n g , ( 2 9 ) isooctane cracking(30,31) o r o-xyl ene isomerization (32) which require medium and s t r o n g a c i d s i t e s . S i m i l a r behavior i s observed f o r o t h e r zeol i t e s t r u c t u r e s such as o f f r e t i t e ( 3 3 ) b u t i t shows some exceptions. For i n s t a n c e Mg HY z e o l i t e s show a maximum i n c a t a l y t i c p r o p e r t i e s r e l a t e d t o t h e formation o f The L t y p e z e o l i t e a l s o g i v e s a very s t r o n g a c i d s i t e s . ( 3 4 ) maximum a t 50% exchange o f t h e c a t i o n s which cannot be explained by any c r y s t a l 1 i n i t y l o s s o r ul t r a s t a b i 1 i z a t i o n effect.(35) Nevertheless t h e usual e x p l a n a t i o n f o r F i g u r e 2 ' s results correlates the increase i n a c t i v i t y w i t h the higher acid s t r e n g t h o f s i t e s generated i n t h e h i g h l y p r o t o n a t e d zeolites. The exchange o f monovalent i o n s by p o l y v a l e n t c a t i o n s Those h i g h l y charged improves t h e c a t a l y t i c p r o p e r t i e s . ( l , 2 ) a hydrolysi s cations create very acidic centers by phenomenon (36)

Na PER

U.C.

FIGURE 2: Change in acid catalysis activity as a function of ~ a content + in Y zeolite

The new a c i d c e n t e r s formed i n c r e a s e c a t a l y t i c a c t i v i t y i n many r e a c t i o n s such a s cumene c r a c k i ng(7) o r o-xyl ene i somerization.(37) The r a r e e a r t h z e o l i t e s appear t o be very attractive, more t h a n c a l c i um forms f o r instance. They generate a h i g h e r a c t i v i t y , a h i g h e r thermal s t a b i l i t y and a lower s i t e deactivation. 2.3

I n f l u e n c e o f P r e t r e a t m e n t T e m ~ e r a t u r es

For most a c i d c a t a l y z e d r e a c t i o n s t h e a c t i v i t y i n c r e a ses up t o a maximum f o r p r e t r e a t m e n t temperatures compri sed between 700 and 900K. (38-41) A s e l e c t i v i t y change i s observed f o r 2propanol d e h y d r a t i o n over v a r i o u s Y z e o l i t e s a f t e r degassing a t 673K. (42) E t h e r is formed p r e f e r e n t i a l l y a t t h e maximum hydroxyl c o n c e n t r a t i o n c o n f i rrni ng t h e hypothesi s t h a t i t s f o r m a t i o n r e q u i r e s a p a i r o f hydroxyl groups.(43) For p a r a f f i n c r a c k i n g t h e maximum i n a c t i v i t y o c c u r s a f t e r p r e t r e a t m e n t a t temperatures 100 t o 300 degrees h i g h e r t h a n t h e maximum hydroxyl c o n c e n t r a t i o n . F i g u r e 3 g i v e s a s an example t h e change i n C3 f o r m a t i o n from n-hexane a t 623K(40) a d t h e t o t a l z e o l i t e hydroxyl c o n c e n t r a t i o n (3660 and 3550 cm-' bands) a s determined by i n f r a r e d spectrometry(44) a s a f u n c t i o n o f preThe e x p l a n a t i on usual l y t r e a t m e n t temperatures o f NH4Y. accepted r e l i e s on t h e f a c t t h a t only t h e s t r o n g e s t a c i d s i t e s a r e r e q u i r e d f o r c a t a l y s i s and t h e y a r e generated a t h i g h temperatures. (40) A simi 1 a r b e h a v i o r has been observed f o r The temperature f o r t h e d i f f e r e n t zeol it e s t r u c t u r e s . (38) maximum o f c a t a l y t i c p r o p e r t i e s a l s o depends on which a c i d s t r e n g t h i s needed, i.e. which c a t a l y t i c r e a c t i o n i s under study. Jacobs(41) g i v e s a c l a s s i f i c a t i o n of r e a c t i o n s i n i n c r e a s i n g o r d e r o f a c t i v a t i o n temperature which c o r r e l a t e s w i t h a decreasing number o f s i t e s and an i n c r e a s i n g s t r e n g t h o f t h e i r a c i d i t y (Table 1).

3.

C o r r e l a t i o n s Between A c i d i c and C a t a l v t i c P r o ~ e r t i e si n Zeol i t e s

The importance o f a c i d i t y f o r c a t a l y t i c p r o p e r t i e s has been p o i n t e d o u t i n t h e p r e v i o u s paragraphs. Many c o r r e l a t i o n s have been p u b l i s h e d between b o t h o f t h o s e p r o p e r t i e s ( f o r example 45,46). Most.. o f a c i d c a t a l y t i c r e a c t i o n s a r e r e l a t e d t o t h e presence o f Bronsted a c i d i t y . For i n s t a n c e i t has been shown t h a t t h e maximum i n c a t a l y t i c p r o p e r t i e s as a f u n c t i o n o f p r e t r e a t m e n t temperature c o r r e l a t e s r a t h e r we1 1 w i t h Bronsted a c i d i t y which shows a maximum i n t h e same temperature range. There i s no obvious c o r r e l a t i o n w i t h t e w i s a c i d i t y . (44) Nevert h e l e s s i t has n o t been p o s s i b l e up t o now t o f i n d a unique

PRETREATMENT TEMPERATURE (K) FIGURE 3: a: Total OH content (44) and b: n-Hexane cracking (40) as a function of pretreatment temperature

c o r r e l a t i o n between a l l t h e d i f f e r e n t t y p e s o f a c i d c a t a l y z e d r e a c t i o n s and t h e p r o t o n i c zeol i t e a c i d i t y . Moreover t h e p r o t o n i c a c i d i t y i t sel f depends on several parameters. Thi s e x p l a i n s why so much work has been devoted t o t h e study o f A b e t t e r understanding o f t h i s p r o p e r t y zeolite acidity. should be a b a s i s f o r a more v a l u a b l e p r e d i c t i o n of c a t a l y t i c properties.

4.

Zeolite Acidity

The main d i f f e r e n c e between zeol i t e s and o t h e r o x i d e s is t h e i r very open s t r u c t u r e which makes t h e l a r g e r p a r t o f t h e framework A1 0 and S i O4 t e t r a hedra acce s s i h l e t o ad sorbed molecules. t r i d i m e n s i o n a l network c r e a t e s c a v i t i e s and channel s i n which molecules may undergo c a t a l y t i c r e a c t i o n s . Then t h e surface o f a z e o l i t e i s i n f a c t i n s i d e t h e bulk o f t h e c r y s t a l and one may expect t h a t t h e surface p r o p e r t i e s , i.e. t h e c a v i t y wall p r o p e r t i e s , a r e s t r o n g l y dependent upon t h e framework c o n s t i t u t i n g atoms. Thi s dependence should be

fie

m h

m

c

o

e

c

c

m

c

m

a

e f f e c t i v e n o t o n l y on a s h o r t range b u t a l s o on a r a t h e r l o n g range a l l over t h e framework. I n t h e l a s t few y e a r s r e s e a r c h on z e o l i t e a c i d i t y has been moved from t h e c h a r a c t e r i z a t i o n o f the c o n c e n t r a t i o n and s t r e n g t h of isol a t e d s i t e s towards attempts t o present an o v e r a l l view of t h e a c i d p r o p e r t i e s . Before c o n s i d e r i n g those approaches i t i s u s e f u l t o p o i n t o u t the main parameters which c h a r a c t e r i z e a c i d i t y i n zeol i t e s and t h e methods which a r e used t o study these p r o p e r t i e s .

4.1

A c i d i t y Study Methods

The i d e a l method o f a c i d i t y measurement should g i v e i n f o r mation on several parameters: t h e number, nature, s t r e n g t h , l o c a t i o n , envi ronment and mean l i f e t i m e of a c i d s i t e s . Hence i t should be a b l e t o c h a r a c t e r i z e a c i d c e n t e r s p r e c i s e l y enough t o assign one t y p e t o t h e s e l e c t i v e t r a n s f o r m a t i o n o f a reactant molecule. I n f a c t each method g i v e s i n f o r m a t i o n b u t none f u l l y d e s c r i b e s t h e a c i d s i t e s . Spectrometric met hods have been w i d e l y used. C e r t a i n l y I R spectrometry i s t h e more usual. It has g i v e n a 1arge number o f r e s u l t s r e l a t e d t o hydroxyl c h a r a c t e r i z a t i o n and t o Bronsted P y r i d i n e and t o a l e s s e r e x t e n t NH3 and Lewis a c i d i t y . (47,48) a r e t h e bases commonly used f o r s e m i - q u a n t i t a t i ve o r q u a n t i t a Table 2 t i v e e v a l u a t i o n s o f s t r e n g t h o r c o n c e n t r a t i o n . (49,50) gives t h e assignments of p y r i d i ne a b s o r p t i o n bands. 2- 6 d i m e t h y l p y r i d n e has been suggested a s a base s p e c i f i c f o r p r o t o n i c c e n t e r s c h a r a c t e r i z a t i o n s i n c e s t e r i c hindrance o f t h e n i t r o g e n atom p.revents i t s c o o r d i n a t i o n t o A1 atoms. (51,52) Recently t h e Bronsted a c i d s t r e n g t h has been evaluated i n zeol i t e s from t h e frequency s h i f t o f hydroxyl groups upon i n t e r a c t i o n w i t h hydrogen bond a c c e p t o r molecules such a s UV spectroscopy was used t o study t h e benzene. (22,53,54) adsorption of v a r i o u s m o l e c u l e s on zeol ites. (55,56) I n the case o f p y r i d i n e on X z e o l i t e w i t h sodium c a t i o n s ( 5 6 ) t h r e e k i n d s o f adsorbed species c h a r a c t e r i z e d r e s p e c t i v e l y t h e I t was i n t e r a c t i o n s w i t h c a t i o n s , p r o t o n s and Lewis s i t e s . noted t h a t UV methods have t h e advantage t o be very s e n s i t i v e but i t i s d i f f i c u l t t o d i s t i n g u i s h t h e p o s i t i o n s o f t h e r e s p e c t i v e peak maxima. O p t i ca1 e l e c t r o n i c spectroscopy was a1 so used to di fferenti ate protonic and non-protoni c sites. ( 5 7 ) Besides t h e study o f redox p r o p e r t i e s o f z e o l i t e s , (58) ESR has been a p p l i e d t o study atomic hydrogen formed on y i r r a d i a t i o n and which i s r e l a t e d t o p r o t o n i c a c i d i t y . (59) R e l a x a t i o n and 1 inewidt h s t u d i e s i n NMR have been employed t o c h a r a c t e r i z e t h e p r o t o n m o b i l i t y and i n t e r a c t i o n s w i t h cations.(60,61) These s t u d i e s a f f o r d a f u r t h e r i n s i g h t It i s proposed t h a t t h e i n t o the nature o f protonic a c i d i t y . h i g h e r i s t h e jump frequency ( i n v e r s e mean l i f e t i m e a t l a t t i c e

.

Table 2 Assignment* I R Band; o f P y r i d i ne (Py ) Adsorbed on B r o n s t e d (H ) o r L e w i s S i t e s ( L )

V i h r a t i o n a l Mode

oxygen atoms), t h e h i g h e r i s t h e s t r e n g t h o f t h e p r o t o n . (60,61) I t i s shown t h a t an adsorbed base such as p y r i d i n e i n c r e a s e s (up t o 60 t i m e s a t 200") t h e p r o t o n m o b i l i t y p r o b a b l y because d u r i n g p y r i d i n i u m i o n f o r m a t i o n and d e c o m p o s i t i o n a h y d r o x y l p r o t o n must be f i r s t a t t a c h e d t o t h e p y r i d i n e m o l e c u l e and t h e n I t is hence suggested t h a t g i v e n back t o a n o t h e r oxygen atom. i n v e s t i g a t i o n s o f p r o t o n m o b i l i t y can l e a d t o c o n c l u s i o n s on a c i d i t y o n l y i f t h e study i s made i n t h e presence o f b a s i c molecules.(61) NMR measurements a l s o p e r m i t s t h e computation o f an elementary " p r o t o n c a p t u r e p r o b a b i l i t y " by t h e a c c e p t o r m o l e c u l e d i f f u s i ng upon t h e surface. T h i s p r o b a b i 1it y decreases f r o m 1 t o 0.06 a f t e r a l o n g o u t g a s s i n g performed between 0' and 300°C(60) ( f o r NH3). B e s i d e s t h e s p e c t r o s c o p i c met hods several approaches have been used t o c h a r a c t e r i z e a c i d i c p r o p e r t i e s . Minachev, Bremer e t a l . performed s e v e r a l works u s i n g Hz-0 exchange t o study Several therma? methods have been t h e p r o t o n mobi 1 ity. (62-64) used t o study t h e i n t e r a c t i o n s o f bases w i t h a c i d s i t e s (DTA, (65) c a l o r i m e t r y and chromatography). (66-68) I t was shown t h a t NH3 o r b u t y l a m i n e g i v e a ma11 heat o f a d s o r p t i o n on cations. The d i s t r i b u t i o n o f t h e s t r e n g t h s o f a c i d s i t e s c o u l d be o b t a i n e d from t h e h e a t o f a d s o r p t i o n o f benzene on p r o g r e s s i v e l y p y r i d i ne p o i soned sample s. ( 6 6 ) A met hod based on t h e d e t e r m i n a t i o n o f t h e amount o f oxygen used f o r t h e o x i d a t i o n o f NH3 i n t h e ammonium forms o f z e o l i t e s has been d e s c r i b e d t o I t d i s t i n g u i s h e s between measure t h e number o f a c i d c e n t e r s . B r o n s t e d and Lewis a c i d i t i e s . ( 6 9 ) A f t e r t h e e a r l y work o f H i r s c h l e r , ( 7 0 ) t i t r a t i o n w i t h b u t y l a m i n e and c o l o r e d i n d i c a t o r s The method a l l o w s an easy d e t e r m i has been used.(25,71-75) n a t i o n o f a c i d s i t e c o n c e n t r a t i o n and s t r e n g t h t o be done b u t i t does n o t g i v e i n f o r m a t i o n on t h e p r e c i s e n a t u r e o f a c i d centers. The q u e s t i o n a r i s e s a l s o a s t o t h e s i z e o f t h e

reactants may modify t h e r e s u l t s . I n fact i n Y zeolites, the number o f a c i d s i t e s determined i n t h i s way i s c l o s e t o t h a t deduced from I R experiments u s i n g p y r i d i n e . ( 4 9 ) The r e s u l t s performed w i t h v a r i o u s bases, i n d i c a t o r s and zeol it e s suggest that w i t h r e g a r d t o t h e a p e r t u r e o f t h e z e o l i t e channels, t h e size o f t h e base molecule i s more c r i t i c a l than t h a t o f t h e indicator. Assuming t h a t a t t h e e q u i l i b r i u m t h e base, small enough t o move i n t h e channel s, n e u t r a l i z e s t h e same f r a c t i o n o f a c i d s i t e s wherever t h e y a r e l o c a t e d ( i n s i d e o r o u t s i d e t h e p a r t i c l e s ) , t h e l a r g e i n d i c a t o r s molecules may d e t e c t t h e end o f n e u t r a l i z a t i o n from t h e r e a c t i o n w i t h t h e o n l y a c c e s s i b l e sites. (74,75) Among a l l t h e s e methods, i n f r a - r e d spectroscopy i s t h e most powerful since i t g i v e s a very l a r g e number of i n f o r mations on t h e a c i d s i t e s . However t h e d i f f i c u l t y t o c o r r e l a t e preci sely i n f r a - r e d r e s u l t s w i t h o t h e r p r o p e r t i e s such a s c a t a l y t i c behavior, p o i n t s o u t t h e f a c t t h a t t h e c h a r a c t e r i zation o f s t a t i c and d e f i n i t e hydroxyl groups i s n o t p r e c i s e enough t o e x p l a i n which p r o t o n s a c t i n t h e dynamic c a t a l y t i c processes.

4.2

Nature and Generation o f A c i d S i t e s

The n e g a t i v e charges i n excess due t o t h e replacement of Si04 t e t r a h e d r a by A104- t e t r a h e d r a i n t h e framework a r e n e u t r a l i z e d by p r o t o n s o r o t h e r c a t i o n s . The p r o t o n i c a c i d centers a r e generated i n v a r i o u s ways.

i. The thermal decomposition o f ammoni um exchanged zeol it e s y i e l d s t h e hydrogen form. Deammi n a t i o n i n anhydrous c o n d i t i o n s o f a1 kylammoni um, p i p e r i d i n i um o r p y r i d i n i u m Y z e o l i t e s produces a s t o i c h i o m e t r i c hydrogen z e o l i t e o n l y i n t h e case o f p r i m a r y alkylammonium ions. With o t h e r c a t i o n s a c o n s i d e r a b l e dehydroxyl a t i o n is observed p r o d u c i n g a s o - c a l l e d dehydroxylated z e o l i t e w i t h Lewi s a c i d s i t e s . (76) Thi s dehydroxyl a t i o n e f f e c t is a1 so observed d u r i n g t h e a d s o r p t i o n o f ami nes, particularly with p y r i d i ne. (77,78) One can wonder whether t h e h i g h p r o t o n m o b i l i t y i n t h e presence o f p y r i d i n e ( 6 1 ) does n o t f a c i l i t a t e t h e d e h y d r o x y l a t i o n phenomena. ii. The ~ r o n s t e da c i d i t y due t o t h e w a t e r i o n i z a t i o n on p o l y v a l e n t c a t i o n s (36) a l r e a d y described (Scheme 2 ) has been s t u d i e d by v a r i o u s methods. NMR was a p p l i e d t o t h e c a l c u l a t i o n o f t h e i n t e r p r o t o n d i stance (dH20) i n water c o o r d i n a t e l y bonded t o t h e c a t i o n s . (79)

iii. The r e d u c t i o n by hydrogen o f t r a n s i t i o n metal c a t i o n s i n z e o l i t e g was supposed t o form a hydrogen zeolite.(80,81) Such Bronsted a c i d i t y has been observed i n an I R study o f a hydrogen reduced CU'+Y zeol i t e. (82)

The r e d u c t i o n o f cu2+ c a t i o n w i t h CO (R2) o r i t s s e l f r e d u c t i o n (83) g i ve s cuprou s i o n s and Lewi s a c i d i t y . The c o n c e n t r a t i o n o f OH groups o f Y z e o l i t e s c o n t a i n i n g N i , Co o r Cu was n o t e d t o i n c r e a s e by r e d u c t i o n w i t h hydrogen a t 250-450" and t o increase w i t h the r i s e o f the r e d u c t i o n temperature. (84) A r e d u c t i o n by hydrocarbon s o f cations t o metals w i t h formation o f protonic a c i d i t y has been shown i n t h e case o f N i , Fe and Co z e o l i t e s d u r i n g t h e cumene cracking. A similar reduction i s p o s t u l a t e d w i t h C r and Cd z e o l i t e s : (85)

iv.

~ r G n s t e d a c i d s i t e s a r e a l s o generated i n b i v a l e n t cation-contai ning Y z e o l i t e s on exposure a t room temperature t o h a l i d e compounds(86) o r a t 150-400°C t o C02* (87)

The v a r i o u s and independent ways o f a c i d s i t e g e n e r a t i o n show t h a t t h e experimental c o n d i t i o n s o f z e o l i t e p r e t r e a t m e n t s o r a c i d i t y mea surement s coul d rnodi f y g r e a t l y the i n t r i n s i c acidity. F u r t h e r t h i s suggests t h a t z e o l i t e a c i d i t y may be changed by t h e presence o f (dehydroxyl a t i on by reactants, catalytic reagents r e d u c t i o n o f r e d u c i b l e c a t i o n s by hydrocarbons, r e a c t i o n w i t h a c i d i c compounds). Hence independently o f a g i ng effects, t h e r e may be l a r g e d i f f e r e n c e s i n a c i d i t y between t h e f r e s h and t h e a c t u a l c a t a l y s t . Scheme ( 5 ) d e p i c t s t h e f o r m a t i o n o f Lewis a c i d i t y from Bron sted s i t e s(88) :

H

The ~ r o n s t e d (OH) and Lewis ( A - s i t e s can be p r e s e n t simultaneously i n t h e s t r u c t u r e a t h i g h temperature. In faujasi tes, t h e d e h y d r a t i o n r e a c t i o n occurs above 873K which decreases t h e number o f p r o t o n s and i n c r e a s e s t h a t o f Lewis sites. (44) Scheme 5 has been c o n f i rmed by I R spect romet rye A t temperatures h i g h e r t h a n 673K t h e sum o f t h e number o f Bronsted sites p l u s t w i c e t h a t o f Lewi s s i t e s is almost constant. (44)

4.3

Hydroxyl Groups i n Zeol i t e s

I n t h e e a r l y works on z e o l i t e a c i d i t y t h e f o r m a t i o n o f two d i f f e r e n t hydroxyl groups has been r e p o r t e d i n f a u j a s i t e s upon ammonium i o n decomposition. (47,88,89) They a r e asigned t o OH v i b r a t i n g i n two d i f f e r e n t cages, t h e supercage and t h e soda1 i t e e For most z e o l i t e s several hydroxyl bands a r e reported corresponding t o OH groups v i b r a t i n g i n d i f f e r e n t c a v i t i e s (Table 3).(90) The hydroxyl s i n very small c a v i t i e s are not a c c e s s i b l e t o hydrocarbons o r base molecules. I f they are a c i d i c , they can n e v e r t h e l e s s i n t e r a c t w i t h those molecules, t h e p r o t o n b e i n g a t t r a c t e d i n t h e l a r g e cages. This occurs f o r t h e 3550 cm-I h y d r o x y l s i n f a u j a s i t e s which move from t h e s o d a l i t e cage t o t h e supercage upon base adsorption. The q u e s t i o n then a r i s e s t o know w h i c h hydroxyl is i n v o l v e d i n the c a t a l y t i c process. I t was shown t h a t i n t h e case o f faujasite the active s i t e s cumene c r a c k i n g a r e t h e hydroxyls v i b r a t i n g a t 3650 cmmfor The low frequency h y d r o x y l s (3550 cm-l) s t a r t t o i n t e r a c t w i t h t h e hydrocarbon o n l y a t higher temperatures (>365"C) when t h e y become s u f f i c i e n t l y activated. (91) Each o f t h e two hydroxyl groups i n f a u j a s i t e s behaves separately. Exchange o f p r o t o n s w i t h i n c r e a s i n g amounts o f cations such a s sodium decreases more r a p i d l y t h e i n t e n s i t y o f the 3540 cm-' hydroxyl band because o f t h e p r e f e r e n t i a l l o c a t i o n o f t h e f i r s t c a t i o n s i n t h e hexagonal p r i s m s and t h e s o d a l i t e cages.(32) Each o f t h e h y d r o x y l s a l s o i n c l u d e s a large range o f p r o t o n i c a c i d strengths. The weaker s i t e s a r e n e u t r a l i z e d f i r s t upon t h e exchange o f p r o t o n s by o t h e r cations. It has been r e p o r t e d t h a t approximately 30% o f t h e s i t e s a r e weak. (71) The o r i g i n o f t h e d i f f e r e n c e i n wavenumber a c c o r d i n g t o t h e hydroxyl group l o c a t i o n has been f o r a l o n g t i m e a m a t t e r o f speculation. The e x i stence o f hydrogen bonding w i t h c l o s e oxygen atoms has been p o s t u l a t e d . (28,47) Recently i t has been shown t h a t t h e h i g h wavenumber band r e l a t e s t o an unperturbed hydroxyl. (90) The s h i f t t o a l o w e r wavenumber f o r t h e hydroxyl s i n small c a v i t i e s a r i s e s from t h e i r d i s t u r b a n c e by e l e c t r o s t a t i c f i e 1 d c r e a t e d by t h e n e a r e s t oxygen s. The

Table 3 H y d r o x y l S t r e t c h i n g F r e q u e n c i e s i n Hydrogen-Zeol it e s and T h e i r Proposed Assignment (from Ref, 9 0 )

Zeol i t e FAU

OH 7:;yygncy

365ga 3584

3578

Assignment 01-H i n supercages 0 -H i n soda1 i t e cages (2-memh. r i n g , s i t e I*) 02(04)-H i n 6-nemb. r i n g s ( s i t e 11)

FAU*

RE-FAU

3560

Crystal termi nating, i n f l uenced by sample compo s i t i on I n 8-memb. r i n g s ( p o r e s )

36 3oa 3540

8-memb. 6-memb,

(3650)

Amorphous phase w i t h ST1 composition

36.20~ 3575

C r y s t a l terrni n a t i n g , i n f 1 uenced by sarnpl e composi t i on 8-memb. r i n g s

ERI

3612~ 3563

8-memb. 6-memb.

MO R

3720

SiOH o f u n i d e n t i f i e d

3650

nature Occluded i m p u r i t i e s

HEU

3620a

ring ring

r ings rings

Zeol i t e

OH F r e q ency (cm-'I

A s s i gnmen t I n pores E x t r a l a t t i c e Si-OH I n pore i n t e r s e c t i o n s E x t r a - 1 a t t i c e S i -OH I n pore i n t e r s e c t i o n s

a

MAZ

I n pores

LTL

I n pores

FER

I n p o r e s (10-memb. rings)

OFF

I n pores (?)

RHO

I n p o r e s ( ? ) (8-memb. r i ngs)

These f r e q u e n c i e s r e p r e s e n t OH groups, v i b r a t i n g i n t h e l a r g e s t p o r e s and/or cages o f t h e r e s p e c t i v e z e o l i t e s For r e f e r e n c e s see (90)

frequency s h i f t from t h e u n p e r t u r b e d h y d r o x y l s f o l l o w s a l i n e a r r e l a t i o n s h i p w i t h t h e i n v e r s e o f t h e squared d i s t a n c e o f t h e proton t o t h e n e a r e s t oxygen f o r a s e r i e s o f z e o l i t e s c o n t a i n i n g t h e p e r t u r b e d h y d r o x y l s i n 6- o r 8-membered r i n g s .

4.4

Number o f S i t e s

The p o t e n t i a l number o f a c i d s i t e s i z e o l i t e s equal s t h a t The number o f o f A1 atoms p e r any r e f e r e n c e u n i t (g, c a c i d s i t e s p r e s e n t i n a sample i s u s u a l l y l o w e r s i n c e i t depends on many p a r a m e t e r s : degree of crystallinity, dehydroxyl a t i o n , p a r t i a l n e u t r a l i z a t i o n w i t h c a t i o n s o r bases etc. The number o f a c i d s i t e s a c t i v e i n a g i v e n c a t a l y t i c r e a c t i o n can be even s m a l l e r due f o r i n s t a n c e t o t h e i n a c c e s s i b i l i t y o f s i t e s ( f o r i n s t a n c e OH groups i n small cages) o r t o t h e r e q u i r e m e n t f o r t h e r i g h t a c i d s t r e n g t h ( T a b l e 1). A quantitative evaluation of both types o f hydroxyls i n f a u j a s i t e s g i v e s a maximum v a l u e o f 16 h y d r o x y l s p e r u n i t c e l l

v i b r a t i n g i n t h e supercage a t 3650 crn-l,(50,92) i.e. one hydroxyl group on average p e r hexagonal prism. Titration with p y r i d i n e g i v e s c l o s e t o 35 OH p e r u n i t c e l l i n t h e f a u j a s i t e supercage(92) which i s c l o s e t o t h e number o f p y r i d i n i u m formed. T h i s i g h t a r i se from a t t r a c t i o n by p y r i d i n e o f some o f t h e 3550 cm-PI h y d r o x y l s i n t h e supercage.

A l a r g e number o f s t u d i e s have been performed u s i n g base They g i v e t h e t o t a l number t i t r a t i o n wi$h c o l o r e d i n d i c a t o r s . o f s i t e s Bronsted and Lewis. A d e t a i l e d study o f a f a u j a s i t e t y p e s e r i e s a s a f u n c t i o n o f c a t i o n c o n t e n t (Na, K , Ca, La) o r S i / A l r a t i o showed t h a t f o r each c a t i o n exchanged by one p r o t o n o n l y one f r a c t i o n o f an a c i d s i t e c o u l d be t i t r a t e d . This f r a c t i o n a. i s small i n h i g h l y alumineous z e o l i t e s such a s X and i t i n c r e a s e s a t lower a1 urninurn c o n t e n t ( F i g u r e 4). ( 2 5 ) Such an e f f i c i e n c y o r s e l f - i n h i b i t i o n c o e f f i c i e n t should r e f l e c t t h e h i g h d e n s i t y of a c i d s i t e s a t h i g h A1 content. It c o u l d work a s an a c t i v i t y c o e f f i c i e n t i n concentrated solutions. (16) T h i s would e x p l a i n t h e lower c a t a l y t i c a c t i v i t y o f X z e o l i t e s compared t o Y mentioned e a r l i e r .

4.5

A c i d i t y Strength

The a c i d s t r e n g t h o f s i t e s i n z e o l i t e s depends on t h e S i / A l ratio. From a l l t h e r e s u l t s p u b l i s h e d two i d e a s emerge emphasizing t h e s u p e r i m p o s i t i o n o f short range and l o n g range e f f e c t s i n d e t e r m i n i n g t h e a c i d strength.

20

40

Y60

80 X

ALUMINUM PER U.C.

FIGURE 4: Dependence of efficiency of acid site ao on the aluminum content in faujasites ( 2 5 )

4.5.1

Short Range I n t e r a c t i o n s

The q u e s t i o n o f whether t h e A1 o r d e r i n g i s t h e same f o r X and Y z e o l i t e s was r a i s e d a l o n g t i m e ago.(93) An a c i d i t y study o f these z e o l i t e s i n which A1 atoms were p r o g r e s s i v e l y removed from t h e framework by dea 1 umi n a t i o n suggested t h a t t he acid s t r e n g t h o f s i t e s was dependent upon t h e S i o r A1 atom environment, d e s p i t e t h e f a c t t h a t a l l t h e T (A1 o r S i ) positions are s t r u c t u r a l l y equivalent. (25,30,71) Seve r a 1 attempts have been made t o account f o r t h e h e t e r o g e n e i t y o f acid strengths. The general i d e a o f these approaches is t h a t the p r o t o n i c a c i d i t y s t r e n g t h a s s o c i a t e d w i t h tetrahedron i s h i g h e s t f o r t h e s m a l l e s t number o f an c l o sAe1 O ~ ; neighbors. Since, except f o r z e o l i t e s w i t h a Si/A1 r a t i o o f 1 (A o r X t y p e ) , t h e A1 d i s t r i b u t i on i s n o t p e r f e c t l y homogeneous a range o f a c i d s t r e n g t h s i s expected t o occur. Dempsey was the f i r s t t o r e l a t e q u a n t i t a t i v e l y t h e a c i d s t r e n g t h o f t h e proton t o t h e geometry o f t h e s t r u c t u r e and t h e environment o f the A1 atoms. (94) I n a more general model t h e s t r e n g t h o f t h e protons was d e r i v e d from t h e s t a t i s t i c a l d i s t r i b u t i o n o f A1 atoms i n t h e fauja s i t e s t r u c t u r e . (95) A f u r t h e r extension considers t h e next n e a r e s t n e i g h b o r s o f each A1 atom and t h e b u f f e r i n g a c t i o n o f t h e c a t i o n s t o e x p l a i n t h e known changes i n proton a c i d s t r e n g t h and t h e thermal s t a b i l i t y of h y d r o x y l groups. (96) I n t h i s model t h e parameters o f importance a r e t h e distance and number o f c l o s e A1 atoms, o f c a t i o n s and o f hydroxyl s. U n t i l r e c e n t l y no i n f o r m a t i o n was a v a i l a b l e on t h e A1 d i s t r i b u t i o n i n t h e framework. The use o f 29% NMR(97) and 2 7 A l NMR(98) has made p o s s i b l e t h e d e t e r m i n a t i o n o f t h e arrangement o f S i and A1 atoms. T h i s i s a very v a l u a b l e source of i n f o r m a t i o n f o r c a l c u l a t i o n s on t h e a c i d s t r e n g t h d i s t r i b u t i o n of protons e x i s t i n g i n a given structure. The e x i s t e n c e o f such d a t a f o r v a r i o u s l y t r e a t e d z e o l i t e s , f o r instance dealuminated Y, (99) and u l t r a s t a b l e ( 9 8 ) may h e l p i n t h e understanding o f t h e i r changes i n a c i d i c p r o p e r t i e s , A1 so t h e presence o f d i f f e r e n t a c i d s t r e n g t h s observed by Jacobs e t al. (54) i n h i g h l y s i 1 iceous zeol it e s o f simi l a r chemical composition (ZSM-5, ZSM-11 and d e a l umi nated f a u j a s i t e ) c o u l d be r e l a t e d t o environmental e f f e c t s d e t e c t a b l e by 2 9 S i NMR o r 2 7 ~ 1 NMR

.

Besides t h e e f f e c t s due t o t h e geometry o f t h e A1 d i s t r i bution, o t h e r e f f e c t s a r e w e l l known t o modify t h e a c i d s t r e n g t h o f protons. (48) The most i m p o r t a n t is t h e exchange o f The s t r o n g e s t s i t e s a r e n e u t r a l i z e d f i r s t protons by c a t i o n s . and t h e a c i d site distribution moves t o weaker a c i d i t y . Polyvalent c a t i o n s generate s t r o n g a c i d s i t e s by w a t e r

h y d r o l y s i s. These a c i d i t y changes a r e due t o l o c a l i z e d e f f e c t s (chemical r e a c t i o n s ) and a r e t h e n a l s o r e l a t e d t o s h o r t range in t e r a c t ions. 4.5.2

Overall I n t e r a c t i o n s

The i d e a o f an i n f l u e n c e o f a l l t h e A1 atoms on t h e p r o p e r t i e s of z e o l i t e s has grown a c i d i c and c a t a l y t ic progressively. A t t e m p t s have been made t o q u a n t i f y it. (14,16,21,22,25,71,90,100) I n c o n t r a s t t o t h e p r e v i o u s models which deduce t h e d i s t r i b u t i o n o f a c i d s t r e n g t h s from t h e n a t u r e o f t h e c l o s e n e i g h b o r s atoms (A1 o r S i ), models t a k i n g i n t o account a very l a r g e number o f atoms o n l y c a l c u l a t e t h e e f f e c t s o f t h e average They g i v e a mean va1 ue o f a c i d e n v i ronment. (21,22,100-102) a given d e n s i t y o f s t r e n g t h f o r a g i v e n A1 c o n t e n t , i.e. charges i n t h e s t r u c t u r e .

A very successful approach uses t h e Sanderson e l e c t r o n e g a t i v i t y equal i z a t i o n concept. (103) It has been a p p l i e d t o t h e c a l c u l a t i o n o f t h e z e o l i t e e l e c t r o n e g a t i v i t y and the charges on v a r i o u s framework atoms and on cations.(21,22,100) T h i s i n t e r m e d i a t e e l e c t r o n e g a t i v i t y is p o s t u l a t e d t o be the geometric mean o f t h e compound atoms of t h e molecule, i.e. o f a g i v e n p o r t i o n o f a z e o l i t e network. Very i n t e r e s t i n g l y i t has been shown t h a t t h e charge on t h e p r o t o n i n c r e a s e s a s t h e A1 c o n t e n t (A1 /A1 + S i ) decreases f o r f a u j a s i t e s w i t h v a r i o u s A1 c o n t e n t s o r f o r v a r i o u s zeol i t e s t r u c t u r e s (L, mordenite, Thi s f o l 1 ow s t h e experimental order c l i n o p t i lo1i t e . (21) observed f o r t h e i n c r e a s e i n s t r o n g p r o t o n i c a c i d i t y f o r the Correlations w i t h catalytic same z e o l i t e series. (16,41) p r o p e r t i e s g i v e a good r e l a t i o n s h i p o n l y w i t h r e a c t i o n s which r e a c t i o n s c a t a l y z e d by i n v o l v e a l l t h e hydroxyl groups, i.e. t h e p r o t o n i c s i t e s r e g a r d l e s s o f t h e i r s t r e n g t h (i sopropanol d e h y d r a t i o n ) o r r e a c t i o n s which woul d i n v o l v e a constant f r a c t i o n of h y d r o x y l s (n-decane hydroconversi on). (22) For in stance t h e isopropanol d e h y d r a t i o n t u r n - o v e r numbers(21,22) a r e p r o p o r t i o n a l t o t h e c a l c u l a t e d charge on t h e proton. All t h e s e s t u d i e s show t h a t t h e mean charge on t h e p r o t o n i s s h i f t e d r e g u l a r l y towards h i g h e r v a l u e s a s t h e A1 content decreases. Simultaneously t h e t o t a l number of acidic h y d r o x y l s, governed by t h e A1 content, has t o decrease. This s t r o n g l y suggests t h a t t h e e n t i r e a c i d s t r e n g t h d i s t r i b u t i o n (weak, medium, s t r o n g s i t e s ) i s s h i f t e d towards stronger values. The weaker a c i d s i t e s should become s t r o n g e r w i t h the Thi s i s i n f a c t observed. The decrease i n t h e A1 content. i n f r a r e d wavenumber o f a c i d i c hydroxyl s ( h i g h frequency band), f o r a l a r g e number o f z e o l i t e s t r u c t u r e s , has been shown t o

decrease w i t h t h e A1 c o n t e n t . (41,104) The c o r r e s p o n d i n g decrease i n t h e f o r c e c o n s t a n t c h a r a c t e r i z e s an i n c r e a s e i n acid strength. More p r e c i sely, i n f o r m a t i o n may be o b t a i n e d on the weakest a c i d s i t e s . S t a r t i n g from a z e o l i t e i n a f u l l y cationated form, Na f o r i n s t a n c e , t h e exchange o f t h e f i r s t cations c r e a t e s weak a c i d i c OH groups ( h i g h frequency band) detectable by I R spectroscopy. values o f these f i r s t hydroxyls formed decrease w i t h c o n t e n t i n d i c a t i n g an increase i n t h e i r a c i d s t r e n g t h . B o t h CNDO(101-102) and Sander son c a l c u l a t i o n s g i v e o n l y an average p r o t o n i c a c i d s t r e n g t h and l i t t l e i n f o r m a t i o n on t h e strongest s i t e s . I t has been known f o r a l o n g t i m e t h a t t h e strength o f s t r o n g p r o t o n i c s i t e s i n c r e a s e s a s t h e A1 c o n t e n t decreases. F o r i n s t a n c e , t h i s has been shown e x p e r i m e n t a l l y from t h e changes i n t h e s t r e n g t h o f base a d s o r p t i o n ( l 6 , 4 1 and references t h e r e i n ) , t h e wavenumber o f a c i d i c OH(41,104) t h e i n t e g r a t e d e x t i n c t i o n c o e f f i c i e n t o f OH groups(22) o r t h e s h i f t upon t h e i n t e r a c t i o n o f t h e h y d r o x y l s w i t h a hydrogenbond-acceptor mol e c u l e. (22,53)

kH

I n t r y i n g t o understand how a change i n c o m p o s i t i o n may increase t h e a c i d s t r e n g t h two f e a t u r e s have t o be considered. F i r s t l y z e o l i t e s are inorganic acids, secondly t h e y a r e polymeric a c i d s . These two p r o p e r t i e s modify t h e a c i d s t r e n g t h separately b u t always s i m u l t a n e o u s l y . I n an a t t e m p t t o c l a r i fy these p o i n t s , i t i s p o s s i b l e t o d i s t i n g u i s h t h e two c o n t r i b u t i o n s i n an a n a l y t i c a l approach. As oxyacids, z e o l i t e f o r m u l a s may be w r i t t e n a s TO,(OH),, ( T = S i o r A1 ) w i t h m = A1 /Al+Si and n = 2-m.(19) T h i s i s comparable t o t h e way o f w r i t i n g , f o r instance, t h e s e r i e s o f o x y c h l o r o a c i d s C10 (OH),. I t i s well known t h a t t h e i r a c i d s t r e n g t h i n c r e a s e s :ith n i n the series

E x p l a n a t i o n s based e i t h e r on e l e c t r o s t a t i c o r e l e c t r o n d e l o c a l i z a t i o n c o n s i d e r a t i o n s have been proposed. C a l c u l a t i o n s based on t h e Sanderson e l e c t r o n e g a t i v i t y e q u a l i z a t i o n p r i n c i p l e g i v e the p a r t i a l charge on t h e p r o t o n w h i c h p a r a l l e l s t h e i n c r e a s e i n n and t h e pKA o f t h e s e acids. I n z e o l i t e s n v a r i e s from 1.5 (Al/Al+Si = 0.5) t o 2 ( A l / A l + S i = 0). The i n c r e a s e i n n, i.e. increase i n a c i d s t r e n g t h , p a r a l l e l s t h e decrease i n A1 c o n t e n t which is i n l i n e w i t h t h e s t r e n g t h s o f t h e o x y c h l o r o a c i d s . The value o f n, l i m i t e d t o between 1.5 and 2, c l o s e t o t h a t o f s u l f u r i c a c i d S02(OH)2, i s i n agreement w i t h what i s known since z e o l i t e s a r e c o n s i d e r e d a s s t r o n g a s s u l f u r i c a c i d . T h i s analogy w i t h o x y c h l o r o a c i d s p o i n t s o u t t h e importance o f t h e A1 content which d e t e r m i n e s n and t h e n t h e a c i d s t r e n g t h . (16)

This contribution t o the composition c o n t r i b u t i on.

acid

strength

may

be

called

a

As t o t h e second p o i n t , i n p o l y m e r i c acids, t h e charge on t h e p r o t o n depends on t h e i n t e r a c t i o n s between t h e OH groups themselves and/or t h e p r o t o n s and t h e surroundi ng molecules. T h i s e f f e c t i s comparable i n some ways w i t h what happens i n c o n c e n t r a t e d s o l u t i o n s where e m p i r i c a l a c t i v i t y c o e f f i c i e n t s have t o be considered. I n z e o l i t e s , t h e h i g h e r t h e number o f t h e h i g h e r t h e A1 content, t h e a n i o n s i n t h e polymer, i.e. g r e a t e r t h e OH-OH and OH/framework i n t e r a c t i o n s and t h e weaker is t h e acid. The importance o f these i n t e r a c t i o n s suggests the n e c e s s i t y t o c o n s i d e r a c t i v i t y c o e f f i c i e n t s i n z e o l i t e s ; these would reduce t h e e f f i c i e n c y o f p r o t o n s i n c a t a l y s i s . (16) T h i s c o n t r i b u t i o n t o a c i d s t r e n g t h may be c a l l e d t h e c o n c e n t r a t i o n c o n t r i b u t ion. I n c r e a s i n g t h e A1 c o n t e n t i n a z e o l i t e simultaneously and i t s concentram o d i f i e s t h e n a t u r e o f t h e m o i e t y (TO,) tion. Roth e f f e c t s decrease t h e a c i d s t r e n g t h o f t h e m protons and a r e concornittent. The q u e s t i o n t h e n a r i s e s a s t o whether o r n o t t h e charge on t h e p r o t o n , c a l c u l a t e d from t h e Sanderson e l e c t r o n e g a t i v i t y model, r e p r e s e n t s t h e t r u e a c i d s t r e n g t h o f I t o b v i o u s l y t a k e s i n t o account t h e so-called the zeolite. composition c o n t r i b u t i o n b u t i t i s n o t c l e a r i f i t i n c l u d e s the concentration contribution. The c a l c u l a t i o n i n v o l v e s the composition of o n l y one i s o l a t e d molecule. For i n s t a n c e i n z e o l i t e s t h e c a l c u l a t e d p r o t o n charge i s e x a c t l y t h e same f o r one fragment o f s t r u c t u r e c o n t a i n i n g one p r o t o n o r f o r any 1a r g e r domain considered, having a l a r g e number o f i n t e r a c t i n g OH groups. I n s p i t e o f these u n c e r t a i n t i e s , t h e t h e r o e t i c a l calculat i o n s allowed much p r o g r e s s t o be made i n our knowledge of The CND0/2 c a l c u l a t i o n t y p e i s z e o l i t e overall properties. genera1 i n i t s concept since i t c o n s i d e r s t h e Sf, A1 d i s t r i b u t i o n but i t has o n l y been a p p l i e d t o c l u s t e r s s i m u l a t i n g the No r e s u l t s f a u j a s i t e s t r u c t u r e i n i t s X o r Y forms. (101,102) a r e a v a i l a b l e f o r z e o l i t e s i n which t h e Al/Al+Si r a t i o l i e s between 0 and 1/6 s i n c e t h e t o t a l number o f ( A l + S i ) atoms i n t h e c l u s t e r i s 6. Very l a r g e c l u s t e r s should be used t o r e p r e s e n t h i g h l y s i 1 i c e o u s zeol i t e s i n which t h e A1 / A l + S i r a t i o v a r i e s from 0.1 t o 0.01 o r even 0.005 ( f o r i n s t a n c e ZSM-5 w i t h S i / A l r a t i o from 10 t o 200). These l i m i t s r e s t r i c t t h e genera l i z a t i o n t o v a r i o u s s t r u c t u r e s and t o low A1 c o n t e n t s which a r e b o t h of g r e a t i n t e r e s t f o r c a t a l y t i c purposes. The Sander son e l e c t ronegat iv i t y t y p e c a l c u l a t i o n s a r e a1 so general i n t h e sense t h a t t h e y a r e based o n l y on t h e chemical A t low A1 composition, and a r e independent o f t h e s t r u c t u r e .

content t h e r e s u l t s o b t a i n e d a r e l e s s s i g n i f i c a n t since t h e c a l c u l a t i o n does n o t d i s t i ngui sh between two d i f f e r e n t A1 d i s t r i b u t i o n s which may g i v e very d i f f e r e n t a c i d strengths. The Sanderson e l e c t r o n e g a t i v i t y , which appears t o be e x t r e m e l y i n rationalizing overall acid properties h e l p f u l (21,22,90,100) and some c a t a l y t i c p r o p e r t i e s o n l y a l l o w s a comparison o f zeolites w i t h s i m i l a r acid strength d i s t r i b u t i o n . No model i s quite satisfactory t o describe the properties o f the highly s i 1iceou s zeo1 ites. Another p o i n t concerns t h e wavenumber o f acid hydroxyl s ( h i g h frequency band). The observed, a l m o s t constant, value o f FOH f o r A l / A l + S i r a t i o s l o w e r than 0.17-0.15 was proposed t o show t h e absence o f any s i g n i f i c a n t i n t e r a c t i o n s between t h e a c i d s i t e s . The a c t i v i t y c o e f f i c i e n t s I n fact, would be 1 f o r t h e p r o t o n s i n these z e o l i t e s . ( l 0 5 ) the Sander son e l e c t r o n e g a t i v i t y o f these z e o l i t e s c o r r e l a t e s well w i t h t h e 'jOH v a l u e s b u t n o t w i t h t h e c a t a l y t i c p r o p e r t i e s o r w i t h t h e a c i d s t r e n g t h measured i n a d i f f e r e n t way. (22) An attempt was made t o c o r r e l a t e t h e CoH s h i f t t o a f i e l d param e t e r ( l 0 5 ) b u t t h e r e l a t i o n s h i p i s n o t l i n e a r a t low A1 contents. I n view o f these r e c e n t r e s u l t s i t becomes more and more d o u b t f u l t h a t i n t h e range of A l / A l + S i r a t i o s lower t h a n 0.16 o v e r a l l p r o p e r t i e s can be deduced from simple model s. In the f a u j a s i t e s t r u c t u r e t h i s value corresponds t o one A1 p e r six-membered r i n g . For such " d i l u t e " composition, local pa ir s o f envi ronment effects (gradient of compo s i t ion, sites.. ) would p r e v a i 1 over t h e whole chemical composition. I n t h i s range, t h e o v e r a l l p r o p e r t i e s would s t i l l be o f s i g n i f i c a n c e o n l y f o r z e o l i t e s which would show comparable A1 d i s t r i b u t i o n in t h e framework.

.

4.5.3

Connection Between Short Range and Long Range Protonic Acid Strength

From t h e evidence p r o v i d e d by t h e s h o r t range and t h e l o n g range approach i t i s suggested t h a t two k i n d s o f p r o t o n i c strengths have t o be considered. An o v e r a l l a c i d s t r e n g t h is a c h a r a c t e r i s t i c o f t h e z e o l i t e and i s determined by an i n t r i n s i c parameter such as t h e A l / A l + S i r a t i o o r t h e Sanderson e l e c t r o negativity. An e n v i ronmental a c i d s t r e n g t h modi f i e s t h e f i r s t one by t a k i n g i n t o account t h e l o c a l n e i g h b o r i n g e f f e c t s and gives t h e d i s t r i b u t i o n o f s t r e n g t h s around t h e average o v e r a l l value. The f i r s t o r i g i n a t e s from o v e r a l l p r o p e r t i e s and t h e second from s h o r t range i n t e r a c t i o n s .

A t low A1 c o n t e n t s ( A l / A l + S i < 0.15-0.17) small changes i n A1 d i s t r i b u t i o n would g r e a t l y modi fy t h e e n v i ronmental a c i d strength. I n t h e i n t e r m e d i a t e and h i g h A1 c o n t e n t range a decrease i n A l / A l + S i r a t i o s h i f t s t h e whole a c i d s t r e n g t h

towards s t r o n g e s t a c i d i t y . The a b s o l u t e o v e r a l l a c i d s t r e n g t h o f each p r o t o n i s increased. 4.5.4

Lewi s A c i d i t y S t r e n g t h

-

I n t e r a c t i o n Between Close S i t e s

Si nce t h e importa-pce o f Lewi s a c i d i t y i n c a t a l y s i s is much l e s s than t h a t of Bronsted a c i d i t y , no d e t a i l e d s t u d i e s have been performed t o c h a r a c t e r i z e it. It i s u s u a l l y considered as a "by-product" o f p r o t o n i c a c i d i t y . The connections between t h e two t y p e s o f s i t e s make i t i n t e r e s t i n g t o look a t t h e i r changes i n s t r e n g t h upon v a r i o u s treatments. I t has been known f o r a l o n g t i m e t h a t f o r a g i v e n a c i d i c z e o l i t e t h e Lewis a c i d i t y i s always s t r o n g e r t h a n t h e Bronsted a c i d i t y (1013-150" d i fference i n t h e temperature o f complete p y r i d i n e evacuation). (48) Such a para1 l e l ism s t i 11 e x i s t s when t h e p r o t o n i c a c i d i t y s t r e n g t h i s changed upon v a r i o u s t r e a t ments. The s t r e n g t h o f Lewi s s i t e s i n c r e a s e when t h e A1 content decreases. I t v a r i e s as t h e Bronsted s i t e s s t r e n g t h . ( l 0 7 ) A s i m i l a r c o r r e l a t i o n between t h e simultaneous decrease i n B r o n s t e d and Lewi s a c i d i t y s t r e n g t h s upon dehydroxyl a t i o n i n NaHY zeol it e s has been p o i n t e d o u t very r e c e n t l y . (108) A1 1 t h e r e s u l t s p o i n t o u t a very strong interdependence between t h e s t r e n g t h o f t h e two t y p e s o f s i t e a t l e a s t f o r a consta*?t A1 /Al+Si r a t i o . L u n s f o r d proposed t h a t t h e s t r e n g t h o f Bronsted s i t e s may be i n c r e a s e d by e l e c t r o n a t t r a c t i o n by Lewis s i t e s . (109) From h i s new data, D a t k a ( l 0 8 ) suggests t h a t t h e i n v e r s e e f f e c t a1 so e x i s t s ; t h e s t r e n g t h of b o t h t y p e s o f s i t e depends on t h e l a t t i c e p o l a r i z a t i o n . I n contrast t o p r e v i o u s models, t h i s one t a k e s i n t o account t h e i n t e r a c t i o n s between s i t e s o f a very d i f f e r e n t n a t u r e f o r a g i v e n Al/Al+Si ratio. I t i m p l i e s t h a t s i t e s a r e c l o s e enough so t h a t t h e p o l a r i z a t i o n e f f e c t can occur. I t e x p l a i n s t h e presence o f very s t r o n g a c i d s i t e s i n steamed m o r d e n i t e s ( l l 0 ) which a r e supposed t o be formed a s superacids i n s o l u t i o n , (Al0)p d e p o s i t s a c t i n g a s Lewis s i t e s . I n t h i s hypothesis o f a recipr o c a l i n f l u e n c e of s i t e s , t h e Lewi s a c i d s t r e n g t h should a1 so be very high. T h i s i s i n good agreement w i t h a c a l c u l a t i o n o f Lewi s s i t e s t r e n g t h o f e x t r a framework c a t i o n i c aluminum. (11 1) As a r e s u l t o f t h e i d e a o f r e l a t e d s t r e n g t h s , i t may be expected t h a t t h e a d s o r p t i o n o f a molecule on one s i t e w i l l m o d i f y t h e e q u i l i b r i u m between t h e s t r e n g t h o f b o t h t y p e s o f site. The a c i d s t r e n g t h of t h e s i t e which i s adsorbing the molecule will i t s e l f be d i sturbed. T h i s may happen i n a c i d i t y mea surement s and a1 so d u r i n g a c a t a l y t i c reaction.

I n a s i m i l a r s i t u a t i o n i t has been suggested t h a t i n t e r actions between c l o s e o x i d i z i n g and r e d u c i n g s i t e s e x i s t i n zeolites. I t has been shown t h a t z e o l i t e s possess e l e c t r o n acceptor and e l e c t ron-donor c e n t e r s whi ch i n t e r a c t w i t h e l ectron-donors ( p e r y l ene f o r i n s t a n c e ) o r electron-acce+ptors ( t e t r a c y a n o e t h y l ene) t o form paramagneti c p o s i t i v e (Pe ) o r Strong i n t e r a c t i o n negative (TCNE-) ions, r e s p e c t i v e l y . (58) between t h e two t y p e s o f s i t e s i s shown by enhancement (up t o t e n f o l d ) o f t h e r e d u c i n g power o f z e o l i t e samples when c e r t a i n electron-donor molecules a r e adsorbed on t h e surface. Studies o f the e f f e c t o f the s i t e density or strength ( i o n i z a t i o n p o t e n t i a1 ) o f t h e e l ectron-donor mol ecul e show t h a t t h e enhancement requires neighboring s i t e s o f not too high energy. The same e x p l a n a t i o n has been used t o d e s c r i b e t h e enhancement o f t h e zeol it e e l e c t ron-donor p r o p e r t i e s upon interaction of small Pt particles with the oxidizing sites. (112) The metal p a r t i c l e s become e l e c t r o n - d e f i c i e n t . Various p r o o f s have been g i v e n o f t h e e l e c t r o p h i l i c c h a r a c t e r o f these m a l l P t p a r t i c l e s encaged i n t h e z e o l i t e channels. I n summary, b o t h a c i d i t y r e s u l t s and t h e behavior o f reducing and o x i d i z i n g s i t e s p r o v i d e good evidence of a s t r o n g interdependence between s i t e s i n c l ose p r o x i m i t y .

5.

Overal l Concepts o f Z e o l i t e P r o p e r t i e s

Several u n i f y i n g p r i n c i p l e s emerged physicochemical f e a t u r e s o f z e o l i t e s .

from

underlying

D e t a i l e d s t u d i e s on i o n i z i n g p r o p e r t i e s o f zeol it e ( l l 3 115) l e d Rabo t o p r e s e n t an o v e r a l l view o f z e o l i t e s considered as e l e c t r o l y t e s . (116) Strong i n t e r a c t i o n s between t h e p o l a r i z a b l e hydrocarbons and t h e s t r o n g l y p o l a r i n t r a c r y s t a l l i n e surface g i v e r i s e t o a h i g h c o n c e n t r a t i o n o f r e a c t a n t s p e r s i s t i n g even a t h i g h temperatures. As a consequence, t h e r a t e o f b i m o l e c u l a r r e a c t i o n steps is enhanced over t h a t o f unimolecul a r r e a c t i o n steps. An example i s t h e hydrogen t r a n s f e r step (Scheme 1) from c y c l o a l k a n e s t o o l e f i n s g i v i n g t h e h i g h aromatics + p a r a f f i n s y i e l d c h a r a c t e r i s t i c o f c r a c k i n g i n zeol ites. Hydrogen t r a n s f e r r e a c t i o n s a r e observed w i t h . b o t h K Y o r a c i d i c HY. They a r e then n o t r e l a t e d t o t h e Bronsted a c i d i t y only b u t t o a cage e f f e c t . The b e h a v i o r o f z e o l i t e s a s e l e c t r o l y t e s i s a l s o r e f l e c t e d i n a l a r g e enhancement o f i o n i z a t i o n r e a c t i o n s and i n t h e s t a b i 1i z a t i o n o f carbonium ions. S t u d i e s on t r a n s i t i o n metal complexes i n z e o l i t e s ( l l 7 , 1 1 8 ) showed t h a t zeol it e s e x h i b i t some p r o p e r t i e s o f c o n v e n t i o n a l s o l v e n t s and behave a s a s o l i d m a t r i x . Nevertheless t h e cage

geometry i s s t i l l very i m p o r t a n t f o r t h e complex f o r m a t i o n and usual s t a b i l i z a t i o n and i t d i s t i n g u i s h e s z e o l i t e s from so1 vents. C a l c u l a t i o n s , u s i n g t h e Sanderson e q u a l i z a t i o n p r i n c i p l e , on charges on atoms were made i n o r d e r t o r a t i o n a l i z e t h e p r o p e r t i e s o f z e o l i t e s . (21,100) F u r t h e r c a l c u l a t i o n s on a l a r g e v a r i e t y o f z e o l i t e s t r u c t u r e s w i t h d i f f e r e n t chemical composit i o n s gave a s t r o n g b a s i s f o r a u n i f y i n g concept.(22) Some m a j o r zeol i t e p r o p e r t i e s (wavenumber o f OH groups, a c i d i t y s t r e n g t h , t u r n over number i n isopropanol decomposition o r ndecane hydroconversi on) can be r e l a t e d t o t h e z e o l i t e Sanderson e l e c t r o n e g a t i v i t y ( 2 2 ) which can be i d e n t i f i e d t o t h e negative chemical p o t e n t i a l (119) ( F i gure 5). Thi s g i v e s an i m p o r t a n t t o o l f o r t h e p r e d i c t i o n and t h e understanding o f a c i d i c and c a t a l y t i c properties. F u r t h e r improvements o f t h e model a r e needed i n t h e range o f low A1 c o n t e n t since t h e Sanderson e l e c t r o n e g a t i v i t y does n o t t a k e i n t o account t h e great importance o f A1 d i s t r i b u t i o n s , i.e. o f l o c a l geometry. The f i n d i n g t h a t sel f - i n h i b i t i o n c o e f f i c i e n t s i n a c i d i t y changed u n i f o r m l y w i t h t h e zeol it e A l c o n t e n t (25) generated the Activity i d e a t h a t z e o l i t e s behave a s solutions.(l6,120) c o e f f i c i e n t s should e x i s t and would g r e a t l y reduce any c a t a l y t i c r a t e a t h i g h a c i d i t y c o n c e n t r a t i o n i.e. h i g h A1 content. By c o n t r a s t a t low A1 l e v e l , a s i n d i l u t e sol u t i o n s , no i n t e r a c t i o n should decrease t h e r e a c t i o n r a t e and t h e a c i d The s i t e s should behave a s i f t h e y were f u l l y i s o l a t e d . e x i stence o f such a c t i v i t y c o e f f i c i e n t s e x p l a i n s t h e maxima observed i n v a r i o u s r e a c t i o n s ( F i g u r e 1 ) a s a f u n c t i o n o f the S i / A l ratio. The l i n e a r i n c r e a s e i n n-hexane c o n v e r s i o n w i t h t h e ZSM-5 z e o l i t e A l c o n t e n t ( l 7 , 1 8 ) i s a l s o i n l i n e w i t h a c o n s t a n t value o f 1 f o r t h e a c t i v i t y c o e f f i c i e n t i n t h e low A1 l e v e l range. The c a l c u l a t e d p r o b a b i l i t y f o r having no close neighbor i n 4 - r i n g s f a u j a s i t e s t r u c t u r e , i.e. no c l o s e s i t e i n t e r a c t i o n s , f o l l o w s t h e sel f - i n h i b i t i o n c o e f f i c i e n t curves a s a f u n c t i o n o f A1 c o n t e n t a s g i v e n i n F i g u r e 4. (14) The analogy o f z e o l i t e s w i t h s o l u t i o n s may a1 so be extended t o e l e c t r o c h e m i s t r y f o r metal-loaded z e o l i t e s .

6.

Concl u s i on s

Many r e a c t i o n s i n v o l v i n g carboni urn i o n s i n t e r m e d i a t e s are c a t a l y z e d by a c i d i c z e o l i t e s . (121) W i t h respect t o a p u r e l y chemical standpoint t h e r e a c t i o n mechani sms a r e n o t fundament a l l y d i f f e r e n t w i t h z e o l i t e s o r w i t h any o t h e r a c i d i c oxides. What z e o l i t e s add a r e cage e f f e c t s , even i n the absence o f geometrical shape s e l e c t i v i t y , and o v e r a l l properties. The cage e f f e c t s , a r i s i n g from t h e h i g h l y i o n i z i n g power

o f z e o l i t e s , (116) a r e p r o b a b l y r e s p o n s i b l e f o r t h e l a r g e v a r i e t y o f s e l e c t i v i t i e s observed. I t seems r a t h e r d i f f i c u l t t o r a t i o n a l i z e and p r e d i c t those e f f e c t s i n t h e very near future. The o v e r a l l p r o p e r t i e s have been more deeply They a l l o w a b e t t e r understood i n t h e l a s t years. (16,22) understanding and improved p r e d i c t i o n s of t h e e x t e n t o f t r a n s f o r m a t i o n o f any r e a c t a n t . They a1 so a f f o r d a f u r t h e r i n s i g h t i n t o t h e n a t u r e o f a c i d i t y and i t s c o r r e l a t i o n w i t h c a t a l y t i c properties. R e s p i t e a l l t h e p r o g r e s s made, these are s t i 11 o p p o r t u n i t i e s f o r new d i s c o v e r i e s i n t h e fundamental and applied f i e l d s o f the acid c a t a l y s i s w i t h zeolites.

Refe -ence s

1. 2. 3.

4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

15. 16. 17. 18.

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-

-

R. Beaumont and D. Barthomeuf, J. Catal. 1972, 27, 45. K. V. Topchieva and H, S. Thuoang, Zh. F i z . ~ x m . 1973, 47, 2103. 1. Moscou and R. Mone, J. Catal. 1973, 30, 417. W. F. K l a d n i g, J. Phys. Chem. 1979, 76583, and 1976, 80, 262. D. Barthomeuf, J. Phys, Chem. 1979, 83, 766. P. A. Jacobs and J. B. ~ytterhoeven,~. Catal. 1972, 26, 175. P. A. Jacobs, B. K. G. Theng and J. B. Uytterhoeven, J. Catal. 1972, 26, 191. M. F. G u i l l e u y J. F. Tempere and D. Delafosse, J. Chern. Phys. 1974, 6, 963. V. G. ~ v a k a G y a , V. I. K u l i v i d z e and G. V. T s i t s i s h v i l i , 223, 273. Dokl. Akad. Naurk. SSSR 1975, J. A. Rabo, V. Schomaker and P. E. P i c k e r t , Proc. Tnterrn. Cong. Catal., 3 r d Amsterdam, 1964, 2, 1264. D. M. Breck, C. R. C a s t o r and R. F. M i l t o n , U.S. P a t e n t 3,013,990 (1961). C. Naccache and Y. Ben T a a r i t , J. C a t a l . 1971, 22, 171. P. A. Jacobs and H. K. Reyer, J. Phys. ~ h e m . 7 9 7 9 , 83, 1174. K. H. S t e i n b e r g , Kh.M. Minachev, H. Bremer, R. V. D m i t r i e v and A. N. Detyuk, Z. Chem. 1975, 15, 372. K. Tsutsumi, S. F u j i and H. Takahashi, J. Catal. 1972, 24, 8. C. L. Angel1 and M. V. Howell, J. Phys. Chem. 1970, 74, 27 37. C. M i r o d a t o s , P. P i c h a t and D. Barthomeuf, J. Phys. Chem. 1976, 80, 1335. L. G. C h r i s t n e r and W. K. H a l l , J. J. B.'Uytterhoeven, Phys. Chem. 1965, 69, 2117. J. W, Ward, J. ~ a t x . 1967, 9, 225. 2, 266. P. A. Jacobs and W. J. ort tier, Z e o l i t e s 1982, J. W. Ward, J. C a t a l . 1968, 11, 259. P. A. Jacobs, L. J. D e c l e r T L. J. Vandamme and J. B. Uytterhoeven, J.C.S. Faraday Trans. I , , 1975, 71, 1545. P. E. P i c k e r t , J. A. Rabo, E. Dempsey and ~ T ~ c h o m a k e r , Proceed. 3 r d I n t . Cong. C a t a l . Amsterdam, 1964, 714. E. J. Dempsey, J. C a t a l . 1974, 33, 497 and 1975, 39, 155. R. J. Mikowsky and J. F. ~ a r s h a l l , J. C a t a l . 1976, 44, 170. W. Wachter, B r i t i sh Z e o l i t e Ass. Meeting, Chi s l e h u r s t , 1981. G. Engel h a r d t , D. Zeigan, E. Lipmaa and M. Magi, Z. Anorg. A1 1 g. Chem. 1980, 468, 35. J. K l i n o w s k i , J. M. ~ h o m C.i A. F y f e and G. C. Gobbi, N a t u r e 1982, 296, 533.

G. Engelhardt, U. Lohse, A. Samoson, M. Magi, M. Tarmak and E. Lippmaa, Z e o l i t e s 1982, 2, 59. W. J. M o r t i e r , J. C a t a l . 1978, 5 5 , 138. S. Reran and J. Dubsky, J. ~ h y s 7 c h e m . 1979, 83, 2538. W. J. M o r t i e r and P. G e e r l i n g s , J. Phys. hex 1980, 84, 1982. R. T. Sanderson, "Chemical Bonds and Bond Energy", Academic Press, New York, 1976. O. Barthomeuf, J.C.S. Chem. Comm. 1977, 743. D. Barthomeuf i n " C a t a l y s i s by Z e o l i t e s " , Elsevier, Amsterdam, 1980, 55. N. Y. Topsoe, K. Pedersen and E. G. Derouane, J. C a t a l . 1981, 70, 41. R. Beaumont, P. P i c h a t , D. Barthomeuf and Y. Trambouze, " C a t a l y s i s", J. W. H i ghtower, ed., N o r t h Hol l a n d , Amsterdam, 1973, 1, 343. F a r a d . Trans. I, 1981, 77, 2877. J. Datka, J.C.S. J. H. Lunsford, J. Phys. Chem.. 1968, 72T4163. C. M i r o d a t o s and D. Barthomeuf, Chem. z m m . 1981, 39. S. Beran, P. J i r u and B. W i c h t e r l o v a , J. Phys. Chem. 1981, 85, 1581. M. Dufaux, C. Naccache and B. I m e l i k , J. c.-Vedrine, J.C.S. Farad. Trans. I, 1978, 74, 440. P. H. Kasai and R, J, bishop:^. Phys. Chem. 1973, 77, 2308. J. A. Rabo and P. H. Kasai, Prog. S o l i d S t a t e Chem. 1975, 9, 1. J. A. Rabo i n " Z e o l i t e Chemistry and C a t a l y s i s " , J. A. Rabo ed., ACS, Washington, DC, 1976, 332. J. A. Rabo, Catal. Rev. Sci. Eng., 1981, 23, 293. J. H. Lunsford, Catal. Rev. Sci. Eng. 1 9 7 K 12, 137. Y. Ben T a a r i t and M. Che i n " C a t a l y s i s by Z e o l i t e s " , (B. I m e l i k e t al., ed.) E l s e v i e r , Amsterdam, 1980, 5, 167. R. G. Parr, R. A. Donne1 l y , M. Levey and W. E T ~ a l k e ,J. Chem. Phys. 1978, 68, 3801. D. Barthomeuf, C . C A c a d . Sci, P a r i s , Ser. C, 1978, 181. M. L. Poutsma i n " Z e o l i t e Chemistry and C a t a l y s i s " , (J. A. Rabo ed,) ACS Monograph 1976, 171, 437.

286,

MOLECULAR SHAPE-SELECTIVE CATALYSIS BY ZEOLITES

Eric G. Derouane Mobil Research and Development Corporation Central Research Division P. 0. Box 1025 Princeton, New Jersey 08540 U.S.A.

1.

INTRODUCTION

Zeolites are three-dimensional framework aluminosilicates presenting a large intracrystalline free volume which consists of cavities and/or pores. Building-in catalytically active sites within such structures is the essence of molecular shape-selective catalysis. These active sites can pertain to the zeolite framework itself: Br$nsted acidic sites are associated to the presence of framework aluminum atoms; they can be converted into Lewis acidic sites by dehydroxylation. Catalytic centers can also be metals or their ions which can be introduced into the zeolite framework by a variety of techniques, ion-exchange being most commonly used. Weisz and Frilette (1) were first to report 23 years ago on molecular shape-selective catalysis. It is now recognized that molecular shape-selectivity can be achieved by virtue of diffusional effects, steric constraints, aqd even coulombic field interactions. Since the original article by these authors, more than 300 essential contributions have appeared in the journal and patent literatures, Critical discussions and reviews of molecular shapeselective effects in catalysis have been proposed recently by Csicsery (2), Weisz ( 3 ) , and Derouane (4,5)

.

The concept of molecular shape-selective catalysis (3) is based on the action of catalytically active sites, internal to the z e o l i t i c framework, on molecular structure(s) which can exist and/ or diffuse in the structural environment where such sites have been generated. Clearly, it implies an intimate interaction between the shape, size, and configuration of the molecules taking part in the

reaction, and the dimension, geometry, and tortuosity of the channels and cages of the zeolite used as catalyst or catalytic support. Molecular shape-selective catalysis must therefore be considered as one of the first, and an outstanding, illustration of "molecular engineering" (6). Several types of molecular shape-selective effects exist: (1) REACTANT OR CHARGE SELECTIVITY will take place if the reactants can be divided into two or more classes of molecules, of which one, at least, will not be able to enter or diffuse freely within the intracrystalline volume of the zeolite because of diffusion constraints, selective sorption, or molecular sieving effects. Molecular shape-selective cracking, hydrocracking, and selectoforming are typical processes which take advantage of this property. (2) PRODUCT SELECTIVITY occurs when similar restrictions apply to the product molecules. It plays an important role in the selective production of para-aromatic compounds over ZSM-5 zeolite based catalyst and also affects the deactivation by coking of zeolite catalysts in general. ( 3 ) RESTRICTED TRANSITION-STATE MOLECULAR SHAPE SELECTIVITY is observed when local configuration constraints, acting in the direct environment of the catalytically active sites, will prevent or decrease the occurrence probability of a given (bimolecular, for example) transition state, characteristic of an elementary catalytic step. It can act directly on the reactants as proposed in the cracking of paraffins (7) or by impeding the formation of bimolecular complexes, as claimed to justify the near-absence of transalkylation in the isornerization of the xylenes (8) and the typical hydrocarbon distribution from the methanol conversion (9) over ZSM-5 based catalysts.

The former molecular shape-selective effects are schematized in Fiqure 1. Other manifestations of molecular shape-selectivity are the concentration effect proposed by Rabo (lo), the general concept of molecular traffic control (11-13) which still needs definitive support, and the importance of molecular circulation in the internal free volume of zeolites such as erionite and offretite (14,15). Clearly, the discussion of molecular shape-selective catalysis hence requires (a) a brief description of the relevant zeolite frameworks and pore systems (5,16), (b) the delineation of the essential factors whichgovern intracrystalline diffusion (3,5,17), and (c) a classification and an illustration of typical molecular shape-selective effects and their uses. Table 1 lists some of the major industrial processes based on molecular shape-selective zeolites (3).

roduct sekctivii (Pam-dlrected orornatics reactions)

w.4

0 Restricted transition state selectivity

(Prevention of t rum-alkylation)

Figure 1. Idealized representation of typical molecular shapeselective effects in zeolite catalysis (from reference 4). TABLE 1

Industrial Molecular Shape-Selective Processes (adapted from reference 3) process

Major chemical/ Process Characteristics

Objective

SeZectofoming

Octane number increase in gasoline; LPG production

Selective n-paraffin cracking

M- Forming

Octane number increase in gasoline

Cracking depending on degree of branching; aromatics alkylation by cracked fragments

Light fuel from heavy fuel oil; reduction of lubes pour point

Cracking of high molecular weight n- and mono-methyl paraffins

Xy Zene Isomerization

High yield para-xylene production

Ethy Z Benzene

High yield ethyl benzene production

Toluene Disproportionation

Benzene and xylenes from toluene

MethanoZ-toGas0 line

Methanol conversion to high grade gasoline

1

II

High yield, long cycle life; suppression of side reactions

Synthesis of hydrocarbons restricted to gasoline range, including aromatics

2.

PORE SYSTEM CHARACTERISTICS OF INDUSTRIALLY IMPORTANT ZEOLITES

Zeolites of which the industrial importance has been widely recognized are the A, X I and Y zeolites, ZSM-5, eri~nite~offretite, and mordenite. Figure 2 summarizes the major features of their pore structure (5) and compare their critical dimensions to those of typical hydrocarbon molecules. Zeolites A, X, Y, erionite and offretite have both channels and cages while ZSM-5 and mordenite only have channels. The intersecting channels of ZSM-5 may allow a three-dimensional motion for molecules of the proper size whilst the differentiated pores of mordenite renders the latter structure essentially unidimensional with respect to the diffusion of hydrocarbon molecules. Asevidenced from Figure 2, zeolite ZSM-5 (as well as ZSM-11) has unique channel dimensions (ca. 0 . 5 5 nm) and bridges the two classical zeolite categories, i.e., the large pore mordenite and faujasite-structure (x,Y) materials which accept in their free intracrystalline space most simple organic molecules (linear and branched aliphatics and single ring aromatics) and the small pore structures which only adsorb linear aliphatics. Zeolite ZSM-5 can adsorb, by decreasing order of preference, normal paraffins, isoparaffins, other monomethylsubstituted paraffins, and single ring aromatic hydrocarbons containing up to ca. 10 C-atoms (18). Detailed descriptions of the ZSM-5 zeolite adsorptive properties have been given by Dessau (19), Olson ( 2 0 ) , Gabelica (21), and Jacobs ( 2 2 ) .

-4 -LINEAf?

PARAFFINS

-7 NAPHTHALENE

Figure 2. Pore structure of industrially important zeolites (from reference 5) .

Table 2 compares hydrocarbon sorptions by zeolites ZSM-5 and ZSM-ll ( 2 2 ) . As expected from its structural characteristics, ZSM-5 has a larqer sorption capacity. The comparison of methylnonanes adsorptions over both zeolites also indicates that the substituted carbon atom and its methyl side-chainprefersto sit at the channel intersections. Such preferential.molecular configurations affect the shape-selective behavior of these zeolites.

TABLE 2

Hydrocarbon Adsorptions over Zeolites HZSM-5 and HZSM-11 (adapted from reference 22) Sorption Temperature Sorbate

(K)

Maximum number of molecules per unit cell HZSM-5 HZSM-11

C3* C4* i-C4* C5* i-C5*

C6 *

c7*-* C8* *

C9** C10* * 2MC9 * *

3MC9* * 4MC9* * 5MC9* *

Neopentane* p-Xylene** m-Xylene** o-Xylene* *

*Derived from adsorption isotherms. **Derived from thermogravimetric data at p/po

=

0.5.

3.

DIFFUSION IN ZEOLITES

The classical theory of diffusion considers two regimes: the normal diffusion regime in which the pore size of the host material is greater than the mean free path of the diffusing molecules and the Knudsen regime for which the diffusivity decreases with the pore dimension. As emphasized by Weisz, a new diffusion regime exists in zeolites, i.e., configurational diffusion (23). It implies that molecular migration within the zeolite framework necessitates the matching of size, shape, and configuration of the diffusing species to the corresponding parameters of the zeolite. Figure 3 illustrates the various diffusion processes encountered in porous solids. Barrer (24) has reviewed the major features of diffusion in zeolites. Some of those were also discussed by Derouane (5) who insisted on the distinction to be made between classical non-equilibrium measurements and self-diffusion (at equilibrium) experiments (using NMR pulsed field-gradient techniques for example (25)). Self-diffusion becomes equivalent to counterdiffusion when all the diffusing molecules are identical. Weisz and Prater (26) demonstrated that the observed rate of catalytic reactions in zeolites are moderated by an effectiveness parameter q which is itself a function of a dimensionless variable 4' defined as:

CONFIGURATIONAL

ANGSTOMS

I

/-L

PORE SIZE

Figure 3. Diffusion mechanisms, diffusivity vs. pore size, in porous materials (from reference 23).

D being the diffusivity coefficient in the particle of equivalent radius R, C the concentration of the reactant(s), and dn/dt the observed reaction rate (27). Figure 4 illustrates the former dependence. The observed rate constant, kobs, is then related to the intrinsic rate constant, k, by kobs = q-k The interaction between diffusion and kinetics is then obvious: reactions characterized by a small ? value l (formation or transformation of antiselective species) will be selectively retarded with respect to tSose having a higher effectiveness factor (formation or transformatioc of proselective species) .

.

4. GEOMETRIC VS. ELECTROSTATIC EFFECTS Geometric effects (pore size, shape, and tortuosity) are generally claimed as the major factors affecting the molecular shape0 ~ decreases, selective behavior of zeolites. As the S i O 2 / ~ 1 ~ratio however, the influence of electrostatic fields will become more noticeable as a result of the higher framework charge and the larger concentration in counterions, offering thereby a better discrimination between polar molecules ( 6 , 2 8 ) and leading to selective sorption.

Figure 4. Effectiveness factor, ll, as a function of the dimensionless variable @ (from references 26 and 27).

Electrostatic interactions may take place between the net dipolar moment of the reactant moLecules and the electrostatic field at the pore mouths (windows), orienting the diffusing molecule in a favorable or critical position with respect to the channel (window). The role of electrostatic effects has been demonstrated most convincingly for type A molecular sieves, in particular for the isomerization of 1-butene to cis- and trans-2-butenes (28) (see Figure 5).

5.

MOLECULAR SHAPE-SELECTIVITY MODIFICATIONS

Before discussing specifically molecular shape-selective effects, it is necessary to make a formal distinction between the molecular shape-selective active sites present in the intracrystalline volume of the zeolite and those present on the external surface of the crystallites. Those will also show activity but no shape-selectivity. Typically, the "external" surface area will represent about one percent of the total zeolite surface area for a crystallite size of one micron (29).

But -I-ene

cis-But2-ene

trans-But- 2-ene

Figure 5. Electrostatic effects in molecular shape-selective catalysis. (A) Dipolar moment orientation in the butene isomers; (B) molecular orientation with respect to the pore openings (from reference 28) .

That reactions occur in the intracrystalline volume of zeolites was demonstrated in the very early stages of molecular shape-selective catalysis ( 6 , 3 0 ) . Zeolite Linde type A was found to crack selectively linear paraffins (30). A (Pt,Na)-mordenite hydrogenation catalyst was able to remove selectively ethylene from mixed propylene-ethylene feeds by converting the latter to ethane (6). Small crystallites, with a larger external surface, will have decreased molecular shape-selectivity as it can be illustrated, for example, by the increased production of unwanted durene in the methanol-to-gasoline conversion (MTG) over HZSM-5 type catalysts (see Table 3) . It is then easily conceived that the molecular shape-selective groperties of a zeolite are maximized when it is feasible to deactivate its external surface. Several such improvements have been reported recently for ZSM-5 catalysts. The active ZSM-5 phase can be bound by a preferably alumina-free material (32) or its surface coated by a metacarborane-siloxane polymer ( 3 3 ) . The most elegant way to deactivate the ZSM-5 zeolite external surface is, however, to take advantage of the fact that it can be prepared with nearly infinite SiO2/A12O3 ratio. ZSM-5 crystallites terminated by a virtually aluminum-free outer shell are more selective for the production of para-aromatic compounds: for example, the para/metaxylene ratio in the products is nearly doubled in the conversion of a mixed C6-Cg aliphatic-aromatic feed (315OC, 14 atm., hydrogen/ hydrocarbons = 3.6) (34)

.

As also demonstrated for zeolite ZSM-5, a fine tuning of the molecular shape-selective properties can also be achieved by selective coke deposition which may restrict the pore mouths in addition to deactivating the external surface or by chemical modifications with P ( 3 5 ) , Sb ( 3 6 , 3 7 ) , B ( 3 8 ) , Mg (39) containing compounds. Large cations such as CS+ and ~ a were ~ +claimed to increase the ethylene production selectivity in the methanol conversion (40) while the addition of ~ a + cations or of a group Va element maximizes the ~ ~ + - h ~ d r o c a r byield on in certain operating conditions (41). Most of these observations are understandable in terms of a modification of the catalyst diffusion characteristics, notwithstanding, however, secondary effects on its acidity.

TABLE 3 Durene Production in the MTG Conversion Over HZSM-5 Catalysts of Varying Crystallite Size (31) (Temperature = 371°C; Si02/AZ203 = 185; WHSV fh-1) = 2 ) Crystallite Size (microns) 0.02 2-5

Durene in HC Product (wt.% ) 5.9 2.6

6. REACTANT MOLECULAR SHAPE-SELECTIVITY A major application of molecular shape-selective catalysis is the removal of linear paraffins from liquid reformates to improve their octane number, or from distillates to lower their viscosity, pour point, and freezing point.

In Mobil's selectoforminu, linear paraffins are hydrocracked selectively from a mixture of paraffinic and aromatic hydrocarbons using a low potassium (Ni,H)-erionite zeolite as catalyst (42). Branched and cycloparaffins and aromatic hydrocarbons are not affected. As described by Chen and Garwood (43-45), linear chain hydrocarbons only can enter the erionite framework. Maxima are observed in the catalytic activity pattern for the cracking of C g and C10-11 hydrocarbons. They are attributed to a "cage" effect which is analogous in essence to the "window effect" described by Gorring (46) to justify the product distribution from the cracking of n-tricosane over H-erionite and the variation of the n-paraffins diffusion coefficients in zeolite T.

CAf3BCWl NUMBER OF NORMAL PARAFFIN

Figure 6. Product distribution from the cracking of n-tricosane over H-erionite (A) and diffusion coefficients of linear paraffins in zeolite T (B) (both at 340°C) (from reference 46).

In .the latter case, there is obviously a close relationship between the parameters which govern intracrystalline diffusion and the catalytic selectivity (see Figure 6 ) . Zeolite T is mainly offretite with a minor intergrowth of erionite; the erionite structure being less open than the offretite structure, erionite will limit the diffusion behavior of hydrocarbons in the intracrystalline free space of zeolite T. The erionite structure has a "window of high transmittance" for molecules ( C 3 - 5 and Cg-13) which can either orient themselves quickly with respect to the 8-membered ring window of the erionite cage or retain some orientation because they extend through the window limiting the cage. As a maximum is observed in the product distribution (C6-9) when large pore zeolites or silicaalumina are used as catalysts ( 4 7 1 , this observation demonstrates the superposition of a shape-selective pattern of diffusivities onto an intrinsically continuous reaction product distribution. As demonstrated recently by Haag et al. ( 4 8 ) , diffusion inhibition effects can be dissociated from the action of steric constraints on the transition state complex by considering zeolite crystallites with different sizes and activities. Table 4 lists pertinent data for the cracking of paraffinic and olefinic hydrocarbons over two HZSM-5 catalysts with different particle size. TABLE 4 Observed and Intrinsic Rate Constants for the Cracking of Hydrocarbons over HZSM-5 Catalysts (at 5 3 8 O C ) *

CRYSTAL SIZE, R(pm)

kobs 0.025 1.35 --

k

0 0.025 1.35 -

COMPOUND

Hexane 3-Methylpentane 2,2-Dimethylbutane Octane 2-Methylheptane Nonane 2,2-Dimethylheptane Dodecane

*See text for definition of symbols; from reference 48.

2 R k

1.35

They indicate that mass-transport limitations occur in the cracking of hexenes and of gem-dimethyl-paraffin isomers. Branching of the aliphatic chain is the essential factor which affects the relative effective diffusivities of the reactants at steady-state reaction conditions. The effects of chain length and branching on the relative cracking rates of Cg-7 paraffins have been described in detail by Chen and Garwood (49). As seen from Table 5, the following trends hold in the cracking of these paraffins:

(b) straight chain > 2-methyl > 3-methyl > dimethylor ethyl substituted. In contrast to the "window" or "cage" effect which is observed in erionite ( 4 6 , 4 7 ) , pore size has more importance for ZSM-5 cracking catalysts than the actual channel tortuosity. These unique molecular shape-selective properties of ZSM-5 catalysts constitute the essence of the Mobil distillate dewaxing (MDDW) process (50,511 in which a mixed feed of linear paraffins, isoparaffins, highly branched paraffins, and aromatics (gas-oil distillate) is selectively hydrocracked. Linear and isoparaffins react preferentially, as illustrated in Figure 7 , and the freeze and pour points of the distillate are lowered, thereby enabling one to adapt the properties of the product to climatic or utilization conditions requirements.

TABLE 5

Reactant Molecular Shape-Selective Effects in the Cracking of Paraffins over HZSM-5 (from reference 49) (Temperature = 340°C, Pressure = 35 atm., LHSV (h-1) = 1.41 Paraffin n-Heptane

2-Methylpentane

Relative Cracking Rate 1

0.38

I

50

'

m

~

l

g

.

,

,

I$ 200 260 320(hold) PROGRAMMED TEMPERATURE ("C)

Figure 7. Dewaxing of a midcontinent distillate ( 3 4 0 - 3 9 0 ° C ) on a ZSM-5 based catalyst : (A) before hydrocracking; ( B ) after processing, the n-C16-28 paraffins being selectively removed (from reference 5 0 ) . Reactant molecular shape-selectivity can also play a role in metal-catalyzed hydrogenation and oxidation (52). It has been observed, for example, that a Pt-ZSM-5 catalyst hydrogenates preferentially linear olefins while a Cu-ZSM-5 catalyst oxidizes mostly para-xylene when it is admixed with its ortho-isomer, the latter of course because of the higher diffusivity of para-xylene in ZSM-5.

7.

PRODUCT MOLECULAR SHAPE-SELECTIVITY

The striking analogy of the hydrocarbon product distributions stemming from the methanol and several triglycerides conversions on ZSM-5 illustrates product molecular shape-selectivity (see Figure 8) (3,531The methanol-to-9asoline (MTG) conversion is a large scale application of the concept of product molecular shape-selectivity. It is discussed at length elsewhere in this volume (54). The effect of pore size on the selectivity of the methanol or dimethylether conversion to olefins has been identified by Cormerais et al. (55) who compared the activities and selectivities of H-erionite, HZSM-5, and zeolite H-Y. The smaller the pore opening, the higher is the yield in (22-3 olefins. The best demonstration of product molecular shape-selectivity is found in a variety of reactions which use ZSM-5 based catalysts and aim at the selective preparation of para-aromatic compounds. Such processes are the disproportionation and alkylation (by methanol) of toluene (56-60) and the xylenes isomerization (56).

0 Porgffin

W ~ ~ A ~ O ~ Q Non Wmatics

Com oil Cs7H& 450dC, WHSV 2.4,3O/t caka 0

Methanol 45m,W HSV 0.67

C

O

~

Figure 8. Hydrocarbon product distributions from the conversions of methanol (A) and corn oil triglyceride, C57HIo406 ( B ) on HZSM-5 (from reference 3) .

Yields in para-xylene, exceeding the expected equilibrium values, can be observed because of diffusion/reaction interaction. Factors which increase the diffusion path length (larger crystals) or decrease the effective pore size of the ZSM-5 catalyst (bulky atoms such as P, bulky counterions, presence of inorganic fillers) and lower the activity of its non-selective external surface, favor the formation of the para-isomer ( 5 6 - 6 0 ) . Considering the disproportionation of toluene, Haag and Olson (61) have correlated the para-xylene selectivity of ZSM-5 catalysts of different crystal sizes and pore tortuosities (because of the presence of inorganic salts or coke plugging) with a diffusion residence time for orthoxylene obtained from separate sorption measurements (see Figure 9). Para-xylene selectivity is noticeably enhanced when diffusion/ reaction interactions increase. Young et al. (60) have recently analyzed in detail the reaction paths which lead to the formation of para-xylene in the toluene disproportionation or its alkylation by methanol and the xylenes isomerization. Figure 10 shows the

a

SORPTION TIME FOR 0-XYLENE, ~ , ~ % ( m i n )

Figure 9. Selectivity to para-xylene in the toluene disproportionation reaction vs. ortho-xylene sorption time for various ZSM-5 based catalysts. 0 = different crystal sizes; A = tortuosity increased by inorganic salts; = coked catalysts (from references 3 and 61). reaction paths,for the isomerization of the pure xylene isomers over non-modified and modified ZSM-5 catalysts as well as over nonzeolitic catalysts. Paths A, B, and C correspond to non-shape selective catalysts such as silica-alumina or phosphoric acid on Kieselguhr while paths A', B ' , and C ' are those followed with shapeselective catalysts. These are Mg and P-modified ZSM-5; as readily seen they yield a para-xylene concentration in excess of the expected thermodynamic equilibrium value. The same authors have compared the relative activities for toluene alkylation and xylene isomerization of the same catalysts. Modified shape-selective catalysts are characterized by a toluenemethanol alkylation rate which is about 2.5 to 15 times that of xylene isomerization. In contrast, non-modified HZSM-5 is ten times more active for xylene isomerization than for toluene alkylation. These observations are explained by an increased diffusion resistance in the shape-selective catalysts and by the relative diffusivities of toluene and methanol (high), para-xylene (medium), and orthoand meta-xylenes (low)

.

The selectivity of these reactions to yield para-xylene is favored additionally by restricted transition-state molecular shape selective constraints as discussed in the next section. These constraints apparently prevent the formation of the himolecular transition state which is necessary to transalkylate toluene within the zeolite and explain the high value of the rate ratio xylene isomerization/toluene disportionation, i-e., 100-1000 over nonmodified HZSM-5 ( 6 0 , 6 2 ) .

Figure 10. Reaction paths for the isomerization of the pure xylenes over shape-selective (A',B',C1) and non-shape-selective ( A , B , C ) catalysts (from reference 60).

8.

RESTRICTED TRANSITION STATE MOLECULAR SHAPE S E L E C T I V I T Y

Restricted transition state molecular shape-selectivity is observed when steric constraints in the environment of the catalytic site affect or prevent the formation of intermediate complex structures. The inability to reach a given transition state will affect both monomolecular and bimolecular reactions; it can, of course, also discriminate between mono- and bimolecular complexes which occur along the various possible reaction paths for a given These constraints will act on intrinsic kinetics reaction ( 3 , 4 ) . rather than by diffusion/reaction interaction.

As mentioned in Section 7, this type of molecular shape-selectivity explains the low xylene disproportionation activity of HZSM-5. Haag and Dwyer (62) and Gnep et al. (63) have correlated the activity of vaxious zeolites (ZSM-5, ZSM-4, mordenite, and

Type Y) for the former reaction with their effective pore size. As expected, the transalkylation is dramatically inhibited as the pores become more restrained. Restricted transition state molecular shape-selectivity is essential to account for the high ethylbenzene selectivity which characterizes the formation of ethylbenzene by the Mobil-Badger process (64,65). In contrast to other zeolites such as mordenite or faujasite which rapidly deactivate as a consequence of coking, ZSM-5 based catalysts have a stable activity for cycle lengths of several weeks and yield ethylbenzene almost stoichiometrically. Further alkylation of ethylbenzene is prevented by the complementary actions of restricted transition state and diffusion constraints. The same argumentation and advantages hold to describe and justify the high para-methylstyrene yields obtained in the direct alkylation of toluene by ethylene over ZSM-5 class catalysts ( 6 6 ) . This particular type of selectivity, as discussed elsewhere (4,54,67,68), explains probably partially the selectivity of the formation of aromatic compounds (cut-off at C10) in the methanol conversion using HZSM-5 catalysts. The bimolecular cyclo-addition of an olefin and a carbeniurn ion is only possible at the channel intersections of ZSM-5, the smaller size products (following further dehydrogenation, alkylation, and isomerization) being able to diffuse out through its channels. Constraint index measurements which consist in the evaluation of the ratio of the cracking rates of n-hexane and 3-methylpentane, are recognized as means to characterize zeolites ( 6 9 ) . It has been demonstrated recently (48,70) that shape-selective constraints on the local kinetics govern these reactions. Indeed, the relative cracking rates of these two hydrocarbons are independent of crystal size (as shown for HZSM-5 catalysts) and then, apparently, free of diffusional effects ( 7 0 ) . The cracking mechanism implies hydrogen transfer between the reacting molecule and a carbenium ion as schematized in Figure 11. It is obvious that the larger transition state required for 3-methylpentane will lead to more severe steric inhibitions and lower conversions, in particular over ZSM-5 catalysts with critical pore diameter of ca. 0.6 nm. Constraint index measurements can then be considered as indirect evaluations of the free space available in the direct environment of the catalytic sites, which give support to their use as zeolite characterization means.

9.

MOLECULAR SHAPE-SELECTIVE EFFECTS I N THE COKING AND AGING OF ZEOLITES

Coke deposition in zeolites originates mainly from olefinic (71) and aromatic (72) compounds condensation and dehydrogenation reactions. The formation of coke can be viewed as follows (5,73):

-

n hexane

c. cross -section 4 . 9 6% ~

Fiqure 11. Transition states in the cracking of the hexane isomers (from reference 4 8 ) .

olefins (possibly resulting from paraffins dehydrogenation) are first cyclo-oligomerized to naphthenes which, in turn, can be converted to aromatics by successive dehydrogenation and hydrogen transfer steps; these aromatic compounds can be further alkylated and dehydrogenated to yield fused-ring aromatic compounds. Ultimately, those are progressively dehydrogenated into coke. Rollmann and Walsh (74-77) have investigated in detail the deposition of coke over a variety of zeolite catalysts and proposed an impressive correlation between coking activity and molecular shape-selectivity (77) (measured by the ratio of the cracking rates for n-hexane and 3-methylpentane). The latter plot, which is shown in Figure 12, was obtained by reacting a mixed feed of C6 hydrocarbons over various zeolites (425OC, 15 atm, H ~ / H C = 3). Intracrystalline coking clearly depends on the pore structure of the zeolite. The alkylation of aromatic compounds which can be converted to fused-ring products at a later stage, was found to be the initial and decisive step in the coking of mordenite and zeolite H-Y. For the small pore structures such as ferrierite and erionite, the Low coking activity seems to be related to constraints acting on the formation of cyclic coke precursors (naphthenes and cycloparaffins) from aliphatic reactants. The unusually low coking activity of ZSM-5 is attributed to restricted transition-state shape-selectivity which prohibits secondary reactions of alkylaromatics in its intermediate gore-size channels (74). As discussed by Derouane (5) and Dejaifve et al. (78), zeolite aging because of the deposition of carbonaceous residues is a function of two factors, namely the probability P(t) that an active site is accessible at time t and the corresponding conditional probability S(t) that it is not poisoned at the same time (79,80).

Figure 12. Coke yield vs. molecular shape-selectivity (relative cracking rate of n-hexane vs. 3-methylpentane) in the conversion of hydrocarbons over zeolite catalysts (from reference 77).

?(t) is a function of the channel network geometry while S(t) is

related to the characteristics (activity and molecular shape-selectivity) of the active site and its environment. A comparison of the aging and rejuvenation behaviors of HZSM-5, offretite, and mordenite indicated that they were intimately depending on the actual pore system structure. It also confirmed the proposal (5) that aging was less rapid for zeolites possessing interconnected channels. The initial coking activity was found to be directly related to the availability of the acid catalytic sites (78). When cavities are present, products can be formed which have a size too large to be desorbed through the windows leading to other cages or to the channels (a reversed product molecular shape-selective effect). Such bulky molecules will act as coke precursors and deactivate rapidly the zeolite. This situation is illustrated by

the "Faujasite Trap", described by Venuto et al., which occurs in the isomerization-oligomerization of 1-hexene over rare-earth exchanged zeolite-X (81). 10. MISCELLANEOUS MOLECULAR SHAPE-SELECTIVITY EFFECTS

Acidic Y-zeolite based catalysts show an increased H-abstraction rate compared to that of 6-scission when compared to silica-alumina gels. While the latter reaction is a monomolecular process, the former one is a bimolecular event which involves a hydride shift from a neutral hydrocarbon to a carbenium ion. Rabo et al. (82) rationalized this observation by proposing that "zeolites concentrate hydrocarbon reactants to a large extent...this concentration effect enhancing the rate of bimolecular reaction steps...over unimolecular (fragmentation) stepstf. Consequently, secondary cracking reactions become less important as olefins and naphthenes are more readily converted into more refractory (paraffinic) products, and larger net gasoline yields are observed. Jacobs et al. (83) have used the bifunctionaZ conversion of n-decane over Pt-loaded zeolites to characterize the catalyst molecular shape-selective properties. They demonstrated that: the distribution of the feed isomers, in particular the yield of 2-methylnonane, could be used to discriminate between the ZSM-5 and the ZSM-11 structures. The conversion of n-decane over Pt-ZSM-5 shows a high (ca. 60%) and unusual production of the above-cited decane isomer, far above equilibrium and at the expense of the other methylnonanes. Protonated cyclopropane structures were postulated as reaction intermediates and possibly explain the observed differences between ZSM-5 and ZSM-11 based catalysts. A preliminary evaluation of these results indicate that this type of test could be used to evidence intergrowths in pentasil zeolites and to detect inhomogeneities in the distribution of aluminum through the zeolite crystallites. Recently, the concept of rnoZecuZar traffic controz was proposed (11) on the basis of sorption measurements on zeolite ZSM-5 and

generalized (12) to include transformations in which (some of) the reactants reach the active sites through diffusion pathways less readily accessible to (some of the) products, or vice versa. Principally, molecular traffic control can occur in zeolite structures presenting non-equivalent but intersecting channels and should be effective when diffusion-limited kinetics take place. Although derived from near-equilibrium sorption measurements, the concept of molecular traffic control obviously applies only to dynamic systems (13). The alkylation of para-xylene by methanol over HZSM-5 (linear and zig-zag channels) and HZSM-11 (straight channels only) has been used to test this concept ( 8 4 ) . The higher aromatic-ring alkylation activity of HZSM-11, compared to HZSM-5, can be explained

by the absence of molecular traffic control although differences in these zeolite acid strengths (83) could also play a non-negligible role. The molecular traffic control concept clearly deserves more attention. Support for its existence should eventually be gained in dynamic reaction conditions, at or near stationary state. Mirodatos and Barthomeuf (85,86) have proposed from comparative studies of the cracking of n-heptane and n-octane on offretite and mordenite that moZecuZar circuzation was an important factor affecting the catalytic activity of zeolites. If molecular circulation is impeded, for example, at high cationic content levels or by the presence of coke, molecules that enter cages or channels can undergo more severe transformations. The molecular circulation effect seems apparented to the "window effect" put forward by Gorring (46).

11.

CONCLUSIONS

The matching of size and configuration of diffusing (reacting) species to those of the zeolite channels affect the kinetics of their catalytic conversion. Conversely, the diffusion of product molecules is influenced by the same factors. A more subtle molecular shape-selectivity effect is that of restricted transition-state shape-selectivity which implies local constraints on the active site kinetics, possibly leading to discrimination between unimolecular and bimolecular transition states.

The above factors give zeolites unique catalytic properties: their high activity which results from their ability to operate at high temperature with minimal deactivation is often complemented by an unusual selectivity because of their particular structural peculiarities.

ACKNOWLEDGMENTS

The author thanks the following publishers for having released their copyrights on the following figures and tables: Elsevier Publishing Company (Figure 1); Academic Press (Figures 2, 6, 10, and 12); American Chemical Society (Figure 3); American Association for the Advancement of Science (Figure 4); Petroleum Publishing Company (Figure 7); Pergamon Press (Figures 8 and 9, and Table 1) ; The Royal Society of Chemistry (Figure 11 and Table 4); Butterworth & Co., Publishers (Table 2).

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TRANSITION METAL EXCHANGED ZEOLITES : PHYSICAL AND CATALYTIC

PROPERTIES

Claude Naccache and Younes Ben T a a r i t I n s t i t u t de Recherches s u r l a C a t a l y s e , CNRS 2, avenue A. E i n s t e i n , 69626 V i l l e u r b a n n e , CcSdex, France

INTRODUCTION C a t a l y s i s by t r a n s i t i o n m e t a l compounds i n s o l u t i o n o r s u p p o r t e d on s o l i d s i s an a c t i v e f i e l d o f r e s e a r c h o f c o n s i d e r a b l e i n t e r e s t . I t i s worthwile t o r e c a l l t h a t t r a n s i t i o n m e t a l i o n s were found act i v e and s e l e c t i v e f o r a g r e a t number o f r e a c t i o n s among them, o x i dation of e t h y l e n e t o a c e t a l d e h y d e , hydroformylation o f o l e f i n s t o aldehydes, c a r b o n y l a t i o n and homologation o f a l c o h o l s , hydrogenation and i s o m e r i s a t i o n o f o l e f i n s , o l i g o m e r i s a t i o n and c y c l o d i m e r i s a t i o n of o l e f i n s , w a t e r g a s s h i f t r e a c t i o n e t c . I n recent years a new a r e a o f r e s e a r c h developed which c o n s i s t e d t o anchor o r imrnob i l i z e t o a s o l i d s u p p o r t a s o l u b l e t r a n s i t i o n m e t a l complex t o produce a p o t e n t i a l a c t i v e and s e l e c t i v e new t y p e o f heterogeneous c a t a l y s t . I n a d d i t i o n i t i s thought t h a t r e l a t i o n s h i p between homogeneous and heterogeneous c a t a l y s i s may be found through t h e s t u d i e s o f such h e t e r o g e n e i z e d c a t a l y s t s . S e v e r a l d i s t i n c t ways f o x "heterogeneizing homogeneous c a t a l y s t s " have been proposed. One approach was t o anchor t h e s o l u b l e m e t a l complex t o an o x i d e s u r f a ce e i t h e r through s u r f a c e oxygen bond r e s u l t i n g from t h e r e a c t i o n of t h e metal l i g a n d s w i t h hydroxyl groups, o r by l i g a n d exchange with f u n c t i o n a l i z e d o x i d e s u r f a c e . An a l t e r n a t i v e means f o r convert i n g homogeneous m e t a l complexes i n t o heterogeneous c a t a l y s t s i s t o introduce t h e a c t i v e complex i n t o t h e i n t e r c r y s t a l space o f a l a y e r l a t t i c e s i l i c a t e by exchanging t h e ~ a ' c a t i o n s o f t h e l a y e r s i l i c a t e by c a t i o n i c t r a n s i t i o n m e t a l i o n s . Z e o l i t e s c o n t a i n a l s o exchangeable c a t i o n s , t h u s t h e y can be used f o r anchoring s o l u b l e t r a n s i t i o n metal complexes. The z e o l i t e s t r u c t u r e t h u s behaves a s a " s o l i d solvent" and l e a d s t o a new c l a s s o f c a t a l y s t s when t h e m a t e r i a l i s exchanged w i t h t r a n s i t i o n m e t a l i o n s . E x t e n s i v e s t u d i e s on t r a n s i t i o n metal exchanged z e o l i t e s have been performed d u r i n g t h e l a s t

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two decades. S e v e r a l e x c e l l e n t reviews have been p u b l i s h e d i n recent y e a r s (1-8). I n t h i s review we have n o t a t t e m p t e d t o i n c l u d e every m a t e r i a l s o f i n t e r e s t concerning t r a n s i t i o n m e t a l i o n s i n z e o l i t e s , b u t r a t h e r we have p u t o u r e f f o r t s t o p r o v i d e a comprehensive s u r vey on t h e c h e m i s t r y o f t r a n s i t i o n m e t a l i o n s i n z e o l i t e s and t o show how such c h e m i s t r y h a s opened a v e r y promising new a r e a i n heterogeneous c a t a l y s i s . I t i s o u r hope t h a t t h e few examples assemb l e d i n t h e s e l e c t u r e s w i l l s t i m u l a t e t h e i n t e r e s t o f c a t a l y s t scient i s t s i n t h e s e m a t e r i a l s . I t i s worthwhile t o n o t e t h a t most of the m a t e r i a l s t h a t w i l l be d i s c u s s e d a r e of t h e f a u j a s i t e - l i k e s t r u c t u r e , which among o t h e r t y p e z e o l i t e s o f f e r t h e advantages o f having a t h r e e dimensional p o r e arrangement, and r e l a t i v e l y l a r g e c a v i t i e s .

1

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Procedures f o r t h e p r e p a r a t i o n o f z e o l i t e - s u p p o r t e d t r a n s i t i o n metal ions.

S e v e r a l methods have been used t o p r e p a r e z e o l i t e s c o n t a i n i n g t r a n s i t i o n metal i o n s . The most common method i s t h e w e l l known c o n v e n t i o n a l i o n exchange t e c h n i q u e . Other methods such a s reaction w i t h c h l o r i d e s a l t s , a d s o r p t i o n from t h e vapor phase o r i n s o l u t i o n o f m e t a l c a r b o n y l compounds, r e a c t i o n o f c a t i o n s p r e s e n t i n t h e zeol i t e c a v i t i e s w i t h a n i o n i c metal complexes, impregnation w i t h a sol u t i o n o f t h e m e t a l s a l t s . The d i s p e r s i o n o f t h e c a t i o n s and t h e i r l o c a l i z a t i o n w i t h i n t h e z e o l i t e framework o r / a n d t h e e x t e r n a l surfac e depend s t r o n g l y on t h e method employed. T h i s paragraph w i l l refer t o t h e s e v a r i o u s methods.

1-1- P r e p a r a t i o n by i o n exchange t e c h n i q u e . The procedure which i s c e r t a i n l y t h e most s u i t a b l e t o i n t r o d u c e c a t i o n s i n t o t h e zeolite framework c o n s i s t s o f exchanging t h e Na+ c a t i o n s which e q u i l i b r a t e t h e n e g a t i v e charge b e a r e d by A l O 4 t e t r a h e d r a , w i t h a s o l u t i o n of t h e m e t a l s a l t , through c o n v e n t i o n a l i o n exchange t e c h n i q u e . This p r o c e d u r e h a s been s u c c e s s f u l l y a p p l i e d i n most c a s e s , e s p e c i a l l y w i t h a l k a l i , a l k a l i - e a r t h and r a r e - e a r t h c a t i o n s . I n r e c e n t years s e v e r a l s t u d i e s on t h e k i n e t i c s and t h e e q u i l i b r i u m o f t h e exchange r e a c t i o n have been p u b l i s h e d . The g e n e r a l c o n c l u s i o n s which may be g i v e n a r e t h a t t h e exchange r e a c t i o n e q u i l i b r i u m depends on t h e conc e n t r a t i o n o f t h e exchanging metal s a l t s o l u t i o n , on t h e temperature. The g e n e r a l f e a t u r e s o f t h e e q u i l i b r i u m and k i n e t i c a s p e c t s have been reviewed r e c e n t l y ( 9 ) . I t was shown t h a t many exchange react i o n s i n X and Y z e o l i t e s f a i l t o proceed t o completion and i t was s u g g e s t e d t h a t t h e l a c k o f t o t a l sodium exchange i s due t o t h e diffic u l t y o f d i s p l a c i n g t h e r e s i d u a l Na+ c a t i o n s p r e s e n t i n t h e sodalite c a g e s , i n Nay z e o l i t e a b o u t 16 ~ a a+r e l o c a l i z e d i n t h e s o d a l i t e cages. S i n c e t h e exchangeable c a t i o n s a r e g e n e r a l l y h y d r a t e d , the free d i a m e t e r o f such s o l v a t e d c a t i o n s i s g e n e r a l l y l a r g e r than t h e d i a m e t e r o f the 6-membered r i n g window o f t h e s o d a l i t e cage, about 0 . 2 2 nun, t h u s t h e ~ a i+n t h e s o d a l i t e c a g e s a r e n o t a c c e s s i b l e by t h e exchangeable h y d r a t e d c a t i o n s . I t r e s u l t s t h a t a complete exchange w u l d o c c u r i f t h e r e i s a r e d i s t r i b u t i o n o f t h e c a t i o n s

between t h e s o d a l i t e and t h e supercages. T h i s w i l l n e c e s s i t a t e a p a r t i a l dehydration of t h e s o l v a t e d exchangeable c a t i o n . The r e s u l t s on lanthanum i o n exchange a r e s i g n i f i c a t i v e of t h i s r e s p e c t . Hydra+ ted ~ a c ~ a t i o+n s were found t o r e p l a c e Na p r e s e n t i n t h e supercage on NaY z e o l i t e o n l y . However a t 1 8 0 ° C t h e exchange p r o c e e d s t o comp l e t i o n , ~ a i+n t h e s o d a l i t e cages b e i n g removed by ~ a ~ I+t .was concluded t h a t a t 180°C La-H 0 bond a s weakened t h u s allowing t h e 2 s p l i t t i n g o f H 0 l i g a n d s from t h e L a (H 0 ) complex. T h i s r e s u l t s i n a f a c i l e d i g f u s i o n o f t h e dehydrated ?,a3' c a t i o n s i n t h e s o d a l i t e cages a t 180°C where exchange with ~ a t+a k e s p l a c e (10-11). I n conclusion o f t h e s e d a t a i t i s now w e l l e s t a b l i s h e d t h a t h y d r a t e d cations, hydrated ~ a h~a s ' a diameter of 3 -96 A , cannot p e n e t r a t e t h e s o d a l i t e cages o f X and Y z e o l i t e s . Only a t high temperature t h e hydrated c a t i o n s l o s e t h e i r c o o r d i n a t e d H20 molecules which a l l o w them t o p e n e t r a t e t h e s o d a l i t e cages. However during t h i s p r o c e s s hydroxylation of t h e c a t i o n through water i o n i z a t i o n may occur r e s u l t i n g i n t h e formation of Men+ - OHx e n t i t i e s . A t high temperature OH condensation between Men+ - OHx s p e c i e s would r e s u l t i n t h e f o r mation of Me-0-Me bonds. According t o t h i s OH condensation oxide inside t h e z e o l i t e framework may be formed t h u s l e a d i n g t o c a t i o n collapse d u r i n g t h e subsequent c a l c i n a t i o n of t h e z e o l i t e . I t i s clear t h a t t h i s undesired c a t i o n c l u s b r i n g r e q u i r e s t h e e x i s t e n c e of hydroxometal c a t i o n s , which a r e produced by h y d r o l y s i s o f t h e exchangeable c a t i o n s , i n s u f f i c i e n t number.

Y+

To summarize t h e a b v e d i s c u s s i o n one may conclude t h a t although

it i s r e l a t i v e l y e a s y t o change t h e n a t u r e of t h e c a t i o n s i n zeol i t e s by i o n exchange, t h e d i f f u s i o n o f t h e bulky hydrated c a t i o n s w i l l be l i m i t e d by t h e z e o l i t e pore s i z e . Reduction i n s o l v a t e d cation s i z e would be n e c e s s a r y i n o r d e r f o r t h e i o n s t o p a s s f r e e l y through t h e oxygen membered r i n g s . I t i s a l s o known t h a t c a t i o n exchange i n t o hydrogen form z e o l i t e , i s o f t e n a d i f f i c u l t p r o c e s s , due t o t h e s t r e n g h t o f t h e bonding o f t h e p r o t o n s w i t h t h e l a t t i c e oxygen. To overcome t h e p r o t o n exchange l i m i t a t i o n i t i s recommended t o transform t h e hydrogen form i n t o ammonium form, N H c ~a t i o n s w i l l behave s i m i l a r l y t o ~ a c+a t i o n s . The s t u d y of t r a n s i t i o n metal i o n s exchanged z e o l i t e h a s r e v e a l e d t h a t the s t a t e of t h e t r a n s i t i o n metal c a t i o n s i s s t r o n g l y dependent on t h e method and on t h e experimental c o n d i t i o n s employed f o r the i o n exchange (11). I t was shown t h a t sodium form z e o l i t e s i n s o l u t i o n g i v e a b a s i c r e a c t i o n w i t h t h e subsequent i n c r e a s e o f the pH of t h e exchanging s o l u t i o n . Since t r a n s i t i o n metal i o n s i n b a s i c s o l u t i o n a r e e a s i l y hydrolyzed, t h e i o n exchange can be accompanied with h y d r o l y s i s o f t h e t r a n s i t i o n metal i o n s and t h e subsequent p r e c i p i t a t i o n o f t h e hydroxy anion t r a n s i t i o n m e t a l , Thus t h e t r a n s i t i o n metal i o n s would be adsorbed i n t h e form o f metal hydroxide e i t h e r i n s i d e t h e z e o l i t e c a v i t i e s o r on t h e e x t e r n a l s u r f a c e . Prec i p i t a t i o n of hydroxide s p e c i e s o f t r a n s i t i o n m e t a l i o n s i n , in/on t h e z e o l i t e would be avoided i s t h e pH o f t h e s o l u t i o n i s

maintained low enough such t h a t no c a t i o n h y d r o l y s i s o c c u r s . However a t low pH, s e v e r a l t y p e z e o l i t e s such a s A, X I Y a r e r e l a t i v e l y unst a b l e , t h e i r s t r u c t u r e s c o l l a p s e i n a c i d i c s o l u t i o n , p H < 4. The ext e n t t o which t h e m a t e r i a l l o s e s i t s c r y s t a l l i n i t y i n a c i d i c medium depends on t h e n a t u r e o f t h e z e o l i t e and t h e e x t e n t t o which c a t i o n h y d r d y s is o c c u r s depends on t h e p H o f t h e s o l u t i o n and on t h e n a t u r e o f t h e t r a n s i t i o n m e t a l i o n , t h e second and t h i r d s e r i e s of t r a n s i t i o n metal i o n s b e i n g more e a s i l y hydrolyzed than t h e f i r s t series. The g e n e r a l e q u a t i o n f o r i o n exchange i s

a t high p H hydrolysis occurs following

Copper i o n exchange p r o c e s s on X and Y z e o l i t e s h a s been s t u d i e d ext e n s i v e l y . The i n t e n s i t y o f t h e esr s i g n a l o f cu2+ exchanged Y-zeolit e h a s been followed a s a f u n c t i o n of t h e pH of t h e exchanging solut i o n ( 1 2 ) . The pH was v a r i e d from 3 t o 10. Ammonia s o l u t i o n was used f o r samples p r e p a r e d a t h i g h pH. A s t h e pH i n c r e a s e d t h e e s r signal i n t e n s i t y d e c r e a s e d u n t i l a minimum was reached a t a p H o f 8-9. The d e c r e a s e o f t h e esr s i g n a l i n t e n s i t y was a t t r i b u t e d t o a decrease of copper i o n d i s p e r s i o n which produced a e s r l i n e broadening through d i p o l e - d i p o l e i n t e r a c t i o n . The lower cu2+ d i s p e r s i o n a s t h e pH of t h e s o l u t i o n i n c r e a s e d i s due t o c a t i o n h y d r o l y s i s producing c u 2 + ( 0 ~ ) s p e c i e s w i t h t h e subsequent formation o f Cu-0-Cu b r i d g e s upon dehyd r a t i o n . Thus when t h e exchange was c a r r i e d o u t a t h i g h pH, upon d e h y d r a t i o n o f t h e m a t e r i a l , cu2+ e x i s t s i n t h e z e o l i t e i n t h e form o f a polymeric hydroxy anion copper s p e c i e s which e x h i b i t a r e l a t i v e l y broad e s r s i g n a l . However a t v e r y h i g h pH, h i g h e r t h a n 10, i n NH OH medium t h e e s r s i g n a l i n c r e a s e d a b r u p t l y , which i n d i c a t e d that 4 t h e formation o f Cu-0-Cu b r i d g e s was h i n d e r e d . The i n h i b i t i o n w a s t h e consequence o f t h e formation o f ammonia-copper complexes cu2+ (NH ) 3 4 which allowed c u p r i c i o n s t o remain d i s p e r s e d i n t h e z e o l i t e . Cob a l t and n i c k e l have t h e same c h a r a c t e r i s t i c f e a t u r e s a s those ex i b i t e d by copper i n z e o l i t e , I t i s p o s s i b l e f o r b o t h ~ i and ~ + Co t o be s t a b i l i z e d i n t h e z e o l i t e framework a s i s o l a t e d c a t i o n s by mixing t h e sodium form o f t h e z e o l i t e w i t h a s o l u t i o n o f t h e met a l s a l t , provided t h e p H o f t h e s o l u t i o n be k e p t low enough. By c o n t r a s t t h e exchange c o n d i t i o n s must be more c a r e f u l l y c o n t r o l l e d when p r e p a r i n g chromium, i r o n o r aluminium exchanged z e o l i t e s , part i c u l a r l y w i t h X o r Y-type z e o l i t e s . Indeed t h e s e c a t i o n s form very e a s i l y hydroxy a n i o n s which polymerize t o form c l u s t e r s . Furthermore e x t e n s i v e exchange w i t h t h e s e c a t i o n s produces o f t e n an appreciable l a t t i c e d e s t r u c t i o n . A n i m p o r t a n t l o s s o f l a t t i c e c r y s t a l l i n i t y was observed f o r X I Y z e o l i t e s e x t e n s i v e l y exchanged (13, 1 4 ) while when cr3' exchange l e v e l was low, l e s s t h a n 2 5 % of ~ a being +

4+

exchanged, no framework d e s t r u c t i o n was observed by X-ray analys i s ( 1 5 ) . S i m i l a r l y it h a s been shown t h a t iron-exchanged z e o l i t e s were u n s t a b l e toward high temperature t r e a t m e n t ( 1 6 ) . Group V I I I metal ion-exchanged z e o l i t e s have been widely used a s s t a r t i n g m a t e r i a l s t o p r e p a r e h i g h l y d i s p e r s e d supported noble metal c a t a l y s t s . While i n t h e c a s e of t h e f i r s t s e r i e s o f t r a n s i t i o n metal ions one can use v a r i o u s metal s a l t s , c h l o r i d e , n i t r a t e , s u l f a t e , oxalate, i n aqueous s o l u t i o n f o r exchange, group V I I I metal i o n s were g e n e r a l l y i n t r o d u c e d i n t h e z e o l i t e framework by i o n exchange using metal ammine complexes. Platinum exchanged f a u j a s i t e $Ype z e o l i t e was o b t a i n e d when Nay sample was t r e a t e d with a P t (NH3)4 solution ( 1 7 ) Since p t 2 + (NH3) c a t i o n i s s t a b l e and n o t s u b j e c t e d to h y d r o l y s i s o v e r a wide pH range t h e ion-exchange may be p e r formed over a wide range pH v a l u e s , s o l u t i o n s w i t h p H up t o 9 have been used. The exchange p r o c e s s i s r e p r e s e n t e d by t h e f o l l o w i n g expression P t (NH ) + ? N ~ + z+ p t 2 + ( N H ~ 4) 2 2 + 2Na when 3 4 p t 2 + ( N H 3 ) 4-Z i s h e a t e d i n oxygen t h e complex decomposed with t h e subsequent removal o f NH l i g a n d . However i n t h e temperature range 3 200-600°C no c a t i o n h y d r o l y s i s occurs. I t h a s been shown by X-ray d i f f r a c t i o n a n a l y s i s t h a t p t 2 + i o n s remain d i s p e r s e d i n t h e z e o l i t e framework ( 1 8 ) .

.

+

Palladium exchanged z e o l i t e s were prepared u s i n g t h e same procedure a s f o r p t 2 + - ~ a y , u s i n g a s o l u t i o n o f P ~ ~ + ( N H ~f )o *r t h e exchange (19). I t appeared t h a t pd2+ (NH ) c a t i o n s d i d n o t e x p e r i e n c e h y d r o l y s i s i n the pH range 6-9. Fur$hermore, a s it h a s been observed f o r p l a t i num exchanged z e o l i t e , pd2+ c a t i o n s i n Nay do n o t show a tendency t o h y d r o l y s i s . The palladium ammine complex pd2+ ( N H ~4I, following thermal t r e a t m e n t l o s e s i t NH l i g a n d s and t h u s m i g r a t e s from t h e 3 supercage towards t h e s o d a l i t e cage. The l o c a l i z a t i o n of p t 2 + and pd2' has been determined by X-ray d i f f r a c t i o n a n a l y s i s (18-20). Rhodium, i r i d i u m and ruthenium c a t i o n s have been exchanged i n z e o l i t e s u s i n g t h e i r a m i n e complexes. However i t was a l s o claimed t h a t , a t l e a s t f o r rhodium and ruthenium, aquo complexes could be used. However s i n c e t h e s e R h , I r and R u c a t i o n i c forms a r e e a s i l y hydrolyzed it i s n e c e s s a r y t o c a r r y o u t t h e exchange r e a c t i o n i n w e l l c o n t r o l l e d experimental c o n d i t i o n s . F h 3 + c a t i o n s can be e i t h e r uniformly d i s t r i b u t e d through t h e z e o l i t e framework when proper rhodium s a l t s and experimental c o n d i t i o n s a r e used o r predominantly supported on t h e e x t e r n a l s u r f a c e . RhC13, 3H 0 i n aqueous s o l u t i o n forms seve2 r a l rhodium s p e c i e s such a s RhC1 1 3-, [%(H 6 2 0 )4 C 12 +, [ R ~ ( H ~ o ) ~ c+ ~ ] e t c , t h e i r r e l a t i v e concentrations depend on t h e pH and t h e temperature of t h e s o l u t i o n . The c a t i o n i c c h l o r o aquo rhodium complexes a r e favoured a t 80-90°C, t h u s rhodium exchange w i l l be favoured when t h e r e a c t i o n i s c a r r i e d o u t a t 80-90°C. Rh-Nay sam3H20 sop l e s have been p r e p a r e d by mixing Nay z e o l i t e with a RhCl 3' l u t i o n a t 80-90°C ( 2 1 - 2 2 ) . From t h e d i f f u s e r e f l e c t a n c e spectrum of fi3+and t h e Xps measurement of atomic r a t i o Cl/Rh i t was concluded

. ..

L

1

9i o n s

a r e exchang ?$ a s [ R ~ ( H ~ o ) ~ 1 12+ and complex h a s Eeen used a l s o t o prep a r e &-Nay by i o n exchange. Ton exchange w a s c a r r i e d o u t e i t h e r a t room t e m p e r a t u r e -or a t 8 0 ° C ( 2 3 ) . Pentammine c h l o r o i r i d i u m chloride salt [1r ( N H ~5~ ) C l h a s a l s o been used t o p r e p a r e i r i d i u m ex2 changed z e o l l t e s ( 2 4 ) . Although RuC13, H 2 0 s a l t has been used i n aqueous s o l u t i o n t o p r e p a r e Ru exchanged z e o l i t e it appeared t h a t t h i s s a l t was n o t s u i t a b l e f o r exchange s i n c e r a p i d h y d r o l y s i s occur e d . I n aqueous s o l u t i o n t h e ruthenium s a l t most o f t e n used f o r exchange i s Ru(NH ) C 1 ( 2 5 ) . 3 6 3 t h a t indeed

[m ( H

0)

J

. [a( N H ~ 5) ~ 1 ]

11

1-2- Ion exchange by i n t e r a c t i o n of anhydrous m e t a l s a l t s with t h e z e o l i t e framework. Ion exchange i n l i q u i d phase o c c u r s r e a d i l y when t h e exchanging m e t a l i s i n i t s c a t i o n i c form i n t h e s o l u t i o n . I t was found t h a t i o n exchange occured by i n t e r a c t i o n between anhydrous s a l t s and z e o l i t e s . H e a t i n g a t 300°C NH C 1 w i t h sodium form + 4 + (26). Similarly z e o l i t e r e s u l t e d i n t h e replacement o f N a by NH i t was shown t h a t zirconium phosphate Zr(HPOq)Z 4n t h e d r y s t a t e ex+ changed H w i t h c a t i o n s when h e a t e d i n t h e p r e s e n c e o f metal c h l o r i d e , HC1 g a s evolved d u r i n g t h e r e a c t i o n ( 2 7 ) T r a n s i t i o n metal ions s u c h a s t i t a n i u m , chromium, molybdenum a r e d i f f i c u l t t o exchange into z e o l i t e s because t h e y a r e s t a b l e i n t h e i r c a t i o n i c form a t low pH where i n g e n e r a l z e o l i t e s decompose. E f f e c t i v e exchange i s p o s s i b l e through r e a c t i o n o f t h e H-form z e o l i t e w i t h m e t a l c h l o r i d e . The exchange procedure c o n s i s t s i n : i ) exchange of t h e sodium form with + f o l l o w i n g b y h e a t t r e a t m e n t t o produce H-form z e o l i t e

.

NH4

Na-Zeol

+

+ NH4

+

NH

4

-

Zeol.

ii) r e a c t i o n o f H-Zeol w i t h m e t a l c h l o r i d e :

n H-Zeol

+

Me C l n

-t

Me

n+

Zeol

+

n HCI

Titanium exchanged Y z e o l i t e s were p r e p a r e d by t h i s method. Recently molybdenum c o n t a i n i n g Nay z e o l i t e s were p r e p a r e d by s o l i d - s o l i d react i o n ( 2 8 ) . The NH -Y form was deaminated by h e a t i n g t h e s o l i d a t 4 3 5 0 ° C . MoC15 was ground w i t h H-Y which formed a H-Y z e o l i t e support e d MoOC14. When t h e mixture was h e a t e d a t 4 0 0 ° C exchange occured f o l l o w i n g t h e r e a c t i o n MoOC14 + 4H-0-Zeol -t O=Mo (0-Zeol) + 4HC1. 1-3- Z e o l i t e - s u p p o r t e d m e t a l complexes,

t r a n s i t i o n metal i o n s by r e a c t i o n w i t h

A - Uniform d i s t r i b u t i o n o f t h e metal i o n s on t h e z e o l i t e could be o b t a i n e d by t h e r e a c t i o n between t h e m e t a l exchanged z e o l i t e and a m e t a l - c o n t a i n i n g c o o r d i n a t i o n compound such as a metal cyanide complex ( 2 9 ) . The method c o n s i s t s t o r e a c t t h e metal exchanged zeol i t e , i r o n z e o l i t e f o r example, w i t h a s o l u b l e i r o n cyanide complex i n solution :

The i r o n f e r r o cyanide r e s u l t i n g i s i n s o l u b l e and t h u s p r e c i p i t a t e d within t h e z e o l i t e c a v i t i e s . Subsequently it r e s u l t e d i n a uniform d i s t r i b u t i o n o f t h e complex w i t h i n t h e z e o l i t e framework. I r o n potassium Y z e o l i t e was o b t a i n e d by r e a c t i n g K4 [ F ~ ( c N ) ~ w i t h Fe, NH -Y z e o l i t e , I r o n cobalt-Y was o b t a i n e d through t h e r e a c t i o n o f 4 Co, NH -Y w i t h (NH ) [F~(cN)~] 4

4 4

B - Reaction o f t h e z e o l i t e w i t h m e t a l carbonyl compounds. The a d s o r p t i o n o f m e t a l carbonyl compounds on z e o l i t e h a s been used a s an a l t e r n a t i v e r o u t e t o p r e p a r e z e r o v a l e n t t r a n s i t i o n m e t a l supported on z e o l i t e . The method c o n s i s t e d i n adsorbing z e r o v a l e n t met a l carbonyl compounds w i t h i n t h e z e o l i t e c a v i t i e s followed by a thermal decomposition t o remove t h e c a r b o n y l l i g a n d s ( 3 0 ) . T h i s procedure was f i r s t a p p l i e d w i t h Mo (CO) and Ru3 (CO) 12 ' Fe(C0I5, Fe2(C0I8 Fe3 (CO) 12 supported on Y z e o l i t e samples have been p r e p a r e d ( 3 ) . I t was shown t h a t upon t h e r m a l decomposition o f t h e c a r b o n y l , t h e f i n a l oxidation s t a t e of t h e m e t a l depended on t h e a c i d i t y o f t h e s t a r t i n g z e o l i t e on H-form Y z e o l i t e t h e metal was o x i d i z e d by t h e p r o t o n s with t h e subsequent e v o l u t i o n of hydrogen

The i n t e r a c t i o n o f i r o n carbonyl w i t h HY z e o l i t e h a s been s t u d i e d by I R and g r a v i m e t r i c methods (31, 3 2 ) . I t was shown than i n HY, i n t e r a c t e d s t r o n g l y w i t h t h e OH groups p r e s e n t i n t h e s u p e r )::::: 3 2 ) . During t h e t h e r m a l decomposition t h e i r o n i s p a r t i a l l y oxidized f o l l o w i n g t h e r e a c t i o n

?

1-4- Location and o x i d a t i o n s t a t e o f t r a n s i t i o n m e t a l i o n s i n zeolites . S i n c e t h e c a t a l y t i c behaviour o f z e o l i t e - c o n t a i n e d t r a n s i t i o n metal i o n s i s dependent o f t h e l o c a t i o n and t h e o x i d a t i o n s t a t e of the c a t i o n s , i t i s i m p o r t a n t t o e s t a b l i s h t h e d e g r e e o f d i s p e r s i o n of the t r a n s i t i o n metal i o n s , t h e i r l o c a t i o n w i t h i n t h e z e o l i t e cav i t i e s o r on t h e e x t e r n a l s u r f a c e o f t h e z e o l i t e c r y s t a l , t h e i r oxydation s t a t e . S e v e r a 1 methods have been used f o r t h e s e s t u d i e s such as X-ray d i f f r a c t i o n , W s p e c t r o s c o p y i n f r a r e d , e l e c t r o n s p i n r e s o nance, M6ssbauer s p e c t r o s c o p y , xps. A m o n g t h e s e t e c h n i q u e s X-ray p h o t o e l e c t r o n s p e c t r o s c o p y was s u c c e s s f u l l y employed t o determine both t h e o x i d a t i o n s t a t e and t h e l o c a t i o n o f c a t i o n s . Thus Minachev exe t a 1 (33,34) have shown t h a t upon r e d u c t i o n o f N i 2 + , cu2+, changed z e o l i t e t h e c a t i o n s a r e s t a b i l i z e d t o lower o x i d a t i o n s t a t e and a l s o t h a t t h e y m i g r a t e t o t h e e x t e r n a l s u r f a c e . The xps s p e c t r a a r e c h a r a c t e r i z e d by xps peaks which p o s i t i o n s a r e c h a r a c t e r i s t i c o f t h e t r a n s i t i o n m e t a l i o n and i t s o x i d a t i o n s t a t e ,

and i n t e n s i t i e s c h a r a c t e r i s t i c o f t h e c o n c e n t r a t i o n of t h e c a t i o n s on t h e s u r f a c e ; depending o f t h e element examined t h e xps w i l l rev e a l a s u r f a c e d e p t h o f a b o u t 2-5 nm. The peak i n t e n s i t y i s given by t h e r e l a t i o n :

where n i s t h e c o n c e n t r a t i o n o f t h e element, 0 t h e c r o s s s e c t i o n for p h o t o e l e c t r o n e m i s s i o n from t h e l e v e l , X i s t h e escape d e p t h , F e t K p a r a m e t e r s depending of t h e s p e c t r o m e t e r . Thus t h e r e l a t i v e s u r f a c e c o n c e n t r a t i o n o f two elements w i l l be g i v e n by t h e r e l a t i o n : n1/n2

=

I1 a2 / I2

2+ where 1 e t I2 a r e t h e peak a r e a s [ R ~ ( N H ~~ ) l ] - NaY h a s been 1 s t u d i e d by xps ( 3 5 ) . The b i n d i n g e n e r g i e s 0% Rh 3d3/2 and Rh 3d5/2 were found e q u a l t o 3 10.8 and 3 15.7 eV as e x p e c t e d f o r Rh3+. Furthermore atomic r a t i o s determined by xps and chemical a n a l y s i s a r e very c l o s e as shown i n t a b l e I :

Table I : xps r e s u l t s f o r

Chemical analysis

1

Rh(NH3)+C1

Nay

5

These r e s u l t s i n d i c a t e t h a t t h e rhodium complex i s i n t r o d u c e d without decomposition and homogeneously d i s t r i b u t e d o v e r t h e z e o l i t e framework. F u r t h e r i n v e s t i g a t i o n s o f rhodium exchanged z e o l i t e by xps have been made ( 3 6 ) . The d a t a g i v e n i n ( 3 6 ) confirmed t h a t t h e atomic r a t i o Rh : N : CL found i n rhodium pentarnmine exchange X zeolite i s a s expected f o r t h e c a t i o n [ R ~ ( N H ~ ) ~ c ~ The ] Si/Rh r a t i o determined by xps i s c l o s e t o t h a t p r e d i c t e d on t h e b a s i s o f t h e bulk composition f o r homogeneous d i s t r i b u t i o n of t h e c a t i o n i n t o t h e zeol i t e . A d d i t i o n a l xps evidence o f homogeneous d i s t r i b u t i o n o f Rh cat i o n s i n Nay z e o l i t e , when [R~(NH ) was used f o r i o n exchange w a s g i v e n i n ( 3 7 ) . The atomic &/gi r a t i o s measured from xps exp e r i m e n t s a r e i d e n t i c a l t o t h e t h e o r e t i c a l v a l u e s assuming t r u e ion exchange. I n c o n t r a s t i t was found t h a t when t h e i o n exchange was c a r r i e d o u t i n a s o l u t i o n o f Rh(N03)3, 2W 0 t h e m e t a l i o n s were pre2 dominantly l o c a l i z e d on t h e e x t e r n a l s u r f a c e o f t h e z e o l i t e . Indeed for t h e s e samples t h e xps atomic r a t i o s R.h/Si were one o r d e r of mag n i t u d e h i g h e r t h a n t h e t h e o r e t i c a l v a l u e s . Molybdenum-containing z e o l i t e s p r e p a r e d by s o l i d - s o l i d r e a c t i o n between MoOCl and HY 4 z e o l i t e were examined by xps ( 2 8 ) . The b i n d i n g e n e r g i e s f o r t h e Mo(3d3/2) and ~ o ( 3 d 5 / 2 )w e r e r e s p e c t i v e l y 235.8 and 232.7 eV

.

~11 *'

i n d i c a t i n g that molybdenum i o n s were p r e s e n t as Mo(V1) i n t h e z e o l i t e s . Mo-exchanged z e o l i t e o b t a i n e d by s o l i d - s o l i d r e a c t i o n between MoOC14 and HY e x h i b i t e d a xps Mo/Si r a t i o c l o s e t o t h e t h e o r e t i c a l value, which s u g g e s t s t h a t Mo i o n s were homogeneously d i s t ~ i b u t e di n the z e o l i t e framework, w h i l e t h e ~ o / S ir a t i o i n samples p r e p a r e d by impregnation o f Nay w i t h MoCl s o l u t i o n i s a b o u t s e v e n f o l d g r e a t e r 5 than t h e t h e o r e t i c a l v a l u e , molybdenum b e i n g mainly on t h e z e o l i t e external surface ( 2 8 ) .

-

2

R e a c t i v i t y of t r a n s i t i o n metal i o n s i n z e o l i t e s

The i n t r a c r y s t a l l i n e p o r e and c a v i t y system o f z e o l i t e s and t h e important l a t t i c e i n d u c e unusual r e a c t i v i t y t o t h e e n t r a p p e d t r a n s i t i o n metal i o n s . I n a d d i t i o n t h e r e a c t i v i t y o f t h e s e i o n s i n zeol i t e s appeared t o be v e r y s i m i l a r t o t h e i r homologous i n s o l u t i o n . This paragraph w i l l p r o v i d e some i n t e r e s t i n g r e a c t i v i t i e s o f t r a n s i t i o n metal i o n s exchanged z e o l i t e s . 2-1-

I o n i s a t i o n o f molecules i n z e o l i t e s .

The v e r y s t r o n g i o n i s a t i o n p r o p e r t i e s of z e o l i t e s a r e r e s p o n s i ble f o r t h e i o n i s a t i o n of w a t e r by m u l t i v a l e n t exchanged c a t i o n s . There a r e now l a r g e number o f e x p e r i m e n t a l e v i d e n c e s showing t h e ~ + and i o n i s a t i o n o f H 0 w i t h t h e subsequent f o r m a t i o n of ~ e (OH) 2 a c i d i c OH groups (38) When ~ e i s~ a +t r a n s i t i o n m e t a l i o n , ?henna1 decomposition o f t h e h y d r a t e d sample o f t e n l e d t o t h e r e d u c t i o n o f the c a t i o n . I t h a s been shown t h a t d e h y d r a t i o n o f F ~ ~ + - N ~z Ye o l i t e produced ~ e i o~n s +( 3 9 ) From Mijssbauer s p e c t r o s c o p y s t u d i e s and q u a n t i t a t i v e measurements o f 0 adsorbed i t h a s been assumed t h a t the f o l l o w i n g r e a c t i o n s o c c u r e a on f e r r o u s ion-exchanged z e o l i t e (40)

.

.

These o b s e r v a t i o n s were f u r t h e r extended t o s e v e r a l t r a n s i t i o n metal ions and it was concluded t h a t t h e z e o l i t e s have t h e remarkable p r o p e r t i e s t o decompose w a t e r i n t o oxygen and hydrogen f o l l o w i n g a thermochemical c y c l e ( 4 1 ) . The r e a c t i o n s producing w a t e r s p l i t t i n g are t h e f o l l o w i n g : H 0 ionisation :

2

0

2

2 Cu

2+

+

2 H20

-t

2

+ +

CU-OH

2~'

and R 0 d e s o r p t i o n 2

2 OHReduction

+

2 cu2+

H ~ O+

+

2 e-

1/2 +

o2 +

2 cu

2e

-

f

Rehydration The i o n i s a t i o n p r o p e r t y o f z e o l i t e s appeared t o be r e s p o n s i b l e f o r t h e h y d r o l y s i s o f group V I I I t r a n s i t i o n m e t a l i o n s i n z e o l i t e .

Ru3+(NH ) -exchanged z e o l i t e s have been i n v e s t i g a t e d by e s r and 6 IR ( 4 2 ) . $he white sample R U ~ + ( N H ) -N Y showed a n e s r spectrum w i t h g = 2.20 and was a t t r i b u t e d t o 6Rugf (NH3) Reflectance spectroscopy showed a b a n d a t 38.000 cm-I (43) and IR sPlowed a band a t 1360 cm(42,43) due t o NH l i g a n d s i n R U ~ + (NH?) complex. Thus t h e 3 r e s u l t s p r e s e n t e d confirmed t h a t on a f r e s h y p r e p a r e d ruthenium exchanged z e o l i t e from hexammine ruthenium s o l u t i o n , t h e complex i s i n t h e form of R U ~ + ( N H 3 ) 6 l o c a l i z e d i n t h e supercage. When t h e samp l e was outgassed t h e sample t u r n e d p r o g r e s s i v e l y red-wine along w i t h a d r a s t i c change b o t h of t h e e s r spectrum of R U ~ ' i o n s and the appearance of I R band a t 1460 cml due t o t h e formation of NH (42). 2+ I t was concluded t h a t (Ru ( N H ) OH) , and ruthenium r e d were formed 3 following thermal t r e a t m e n t a ?ow temperature (42,43,25,44)

.

+

Ru

3+

2+

+ H20 -+

(NH316

Ru(NH ) OH

3 5

2+

[RU(NH~) 50~]

+

NH

+ 4

polymerisation of

would lead t o ruthenium r e d :

Thus it appeared t h a t upon o u t g a s s i n g h y d r o l y s i s of t h e ruthenium ammine complex occured. S i m i l a r l y when NH i s adsorbed on a f r e s h l y 3 p r e p a r e d R U ~ (NH + ) -Nay t h e samp e immediatly showed an e s r spec$+ trum a t t r i b u t e d go6 and I R s p e c t r a i n d i c a t e d t h e + (45) Th2 ) 2OH) formation o f NH4 a c t t h a t o u t g a s s i n g Ru(NH adsorbing NH on t h i s sample produced t h e same e f f e c t 3 t i a l h y d r o l y s i s of t h e ruthenium hexammine complex was i n t e r p r e t e d i n t h e following manner : 3+ Ru (NH3) i s r e l a t i v e l y u n s t a b l e i n b a s i c media. However when i n t r o d u c e 3 i n t h e supercage o f t h e z e o l i t e and when t h e c a v i t y i s f u l l y h y d r a t e d , R U ~ + ( N H ) 6 i s p r o t e c t e d from t h e i o n i z i n g power of z e o l i t e by t h e water mo?ecules f i l l i n g t h e c a v i t y . Upon dehydration following o u t g a s s i n g , t h e complex i s s u b j e c t e d t o i o n i s a t i o n with t h e subsequent h y d r o l y s i s . S i m i l a r l y by adsorbing NH NH forms with 3' 3 t h e water molecules p r e s e n t i n t h e c a v i t y NH OH, which i s immediate4 l y i o n i z e d forming high c o n c e n t r a t i o g + o f OH- groups. I n t h e presence (NH3)6 occurs. of t h e s e OH- groups h y d r o l y s i s o f Ru

LRU(NH .

T h i s s t r o n g i o n i z i n g p r o p e r t y of z e o l i t e i s t h u s r e s p o n s i b l e f o r t h e f a c i l e h y d r o l y s i s o f group V I I I t r a n s i t i o n metal i o n s i n z e o l i t e . Upon dehydration a t h i g h temperature t h e hydrolyzed group V I I I t r a n s i t i o n metal i o n s form r a p i d l y metal o x i d e s which migrate on t h e e x t e r n a l surface of t h e z e o l i t e . 2-2-

Oxygen-transition metal i o n s

1t i s w e l l known t h a t tetraphenyl-porphyrin

c o b a l t (11) adsorbs molecular oxygen, t h e e l e c t r o n c o n f i g u r a t i o n i n t h i s oxygen-cobalt adduct approaches C o ( I I 1 ) one e l e c t r o n being t r a n s f e r e d from

05,

the c o b a l t o r b i t a l t o t h e oxygen molecule. Pentaamrnine Co(I1) forms also with oxygen a p-peroxodicobalt amrnine complex ( N H 3 ) 5 CO-0 2 t h e oxygen molecule b r i d g i n g two c o b a l t complexes. IdenCo (NH3),-, t i c a l r e a c t i o n s occured w i t h C o ( I 1 ) exchanged N a y z e o l i t e ( 4 6 ) . The p.-peroxodicobalt complex was formed when 0 r e a c t e d w i t h C o ( I 1 ) ( N H 3 ) 5 2 entrapped complex. I n c o n t r a s t only t h e monomeric oxygen c o b a l t spec i e s was formed when NH l i g a n d s were r e p l a c e d by propylene diarnmine 3 ligand ; t h e s t r u c t u r e 3f t h e oxygen adduct was :

Rhodium I1 porphyrin i n dimethyl-formamide adsorbs H d i s s o c i a t i v e 2 ly ( 4 7 ) following t h e r e a c t i o n :

R~(I)was very s e n s i t i v e t o oxygen and was o x i d i z e d w i t h t h e subsequent formation o f Rh(11) and H20 :

Rhodium exchanged z e o l i t e s behave s i l i l a r l y . E s r s t u d y o f oxygen adsorption i n a c t i v a t e d Rh-Nay r e v e a l e d t h e formation of a y-peroxodirhodium adduct ( 2 3 ) . I t was suggested t h a t f o l l o w i n g thermal act i v a t i o n of R h ( I I 1 )-Nay s e l f r e d u c t i o n of R h ( I I 1 ) t o Rh(I) occured Rh(1) i o n s were bound t o l a t t i c e oxygen i o n s . Upon a d d i t i o n o f O2 molecules t h e u-peroxodirhodium complex Rh (11) - 0;- - Rh(11) was formed one e l e c t r o n from each Rh(1) i o n being t r a n s f e r r e d t o t h e 0 molecule. 2 Z e o l i t e was found t o s t a b i l i z e new type o f oxygen adducts which were not r e v e a l e d i n s o l u t i o n . Example i s given by palladium exchanged z e o l i t e ( 4 7 ) . When P d ( I 1 ) - m o r d e n i t e was a c t i v a t e d i n vacuum e s r s t u d i e s showed t h e formation o f P d ( 1 ) i o n s due t o s e l f r e d u c t i o n of t h e Pd(I1) i o n s . T e a d s o r p t i o n o f O2 a t room temperature was s t u d i e d P7 by e s r , using O e n r i c h e d oxygen gas. I t was suggested t h a t O2 molecule was trapped between t h r e e P d ( 1 ) i o n s each P d ( I ) t r a n s f e r r i n g to t h e oxygen molecule one e l e c t r o n . The r e s u l t i n g oxygen s p e c i e s 3- complex, t h e e l e c t r o n conf i g u r a was d e s c r i b e d a s a [ P ~ ( I I ) ] t i o n o f t h e charged O2 molecu eing si m i l a r t o ~ 1 ion. 2

2-3-

N i t r i c oxide a d d u c t s

T r a n s i t i o n metal i o n exchanged z e o l i t e s showed high a f f i n i t y f o r n i t r i c o x i d e and it was thought t h a t t h e s e m a t e r i a l s could be advantageously used a s c a t a l y s t s i n t h e d i s s o c i a t i o n and r e d u c t i o n of n i t r i c oxide. This h a s prompted s e v e r a l s t u d i e s of t h e formation of n i t r o s y l complexes w i t h i n t h e z e o l i t e c a v i t i e s . E s r and i n f r a r e d techniques were used t o i n v e s t i g a t e t h e e l e c t r o n i c s t r u c t u r e o f t h e metal n i t r o s y l complexes. Although t h e i o n i z a t i o n p o t e n t i a l of NO i s

r e l a t i v e l y h i g h (9.3 eV) i n s e v e r a l c a s e s , due t o t h e i n t r a c r y s t a l l i n e i o n i z i n g power o f the z e o l i t e , t h e formation o f t h e n i t r o s y l complex was accompanied by a t r a n s f e r o f e l e c t r o n from NO t o t h e t r a n s i t i o n m e t a l i o n w i t h i t s subsequent r e d u c t i o n . 2+ Nickel exchanged Y z e o l i t e ( N i -Y) does n o t e x h i b i t an esr s i g n a l . When Ni2+y i s c o n t a c t e d w i t h NO a s t r o n g e s r s i g n a l i s observed. n (3dY e l e c t r o n c o n f i g u r a t i o n ) . T h i s s i g n a l was a t t r i b u t e d t o ~ i i o+ Furthermore I R d a t a i n d i c a t e d t h e p r e s e n c e o f a n i t r o s y l adduct (VNO = 1892 c m - l ) (48-49). The i n t e r e s t i n g a s p e c t o f t h e s e s t u d i e s + i s t h a t NO c o o r d i n a t e s t o ~ i + i n+z e o l i t e t o form N i NO+ complex. + + T h i s complex N i NO was c h e m i c a l l y i n e r t toward oxygen (48-49). 4N i t r i c oxide forms a l s o w i t h cr2+ i n Nay z e l i t e ( C r NO+) complex S+ which was a l s o i n e r t toward oxygen ( 4 9 ) . C r -Nay was o b t a i n e d by Hg-reduction o f cr3+-Nay. N i t r i c oxide r e a c t s w i t h f e r r o u s i o n s i n i r o n exchanged z e o l i t e t o form an i r o n n i t r o s y l complex ( 5 0 ) . Both + were present low s p i n ( s = 1 / 2 ) and h i g h s p i n ( s = 3 L 2 ) Fe(1)NO i n t h e z e o l i t e c a v i t i e s and were i d e n t i f i e d by e s r and i n f r a r e d . The h i g h s p i n i r o n n i t x o s y l complex showed an esr spectrum w i t h g = 4.07 I and g, = 2.003 and an TR band a t 1890 c m - l . The s t u d y o f t h e i n t e r a c t i o n o f NO w i t h t r a n s i t i o n m e t a l i o n exchanged z e o l i t e s h a s reinf o r c e d t h e i d e a concerning t h e h i g h i o n i z i n g p r o p e r t y o f z e o l i t e which f a c i l i t a t e s e l e c t r o n t r a n s f e r r e a c t i o n s . Furthermore metal nit r o s y l complexes e x h i b i t e d an unusuallyhigh s t a b i l i t y toward oxidat i o n by 0 2'

2-4-

R e a c t i v i t y w i t h CO

The r e a c t i v i t y o f 1 s t s e r i e s t r a n s i t i o n m e t a l i o n s exchanged z e o l i t e s h a s been c a r r i e d o u t p a r t l y t o i n v e s t i g a t e t h e l o c a t i o n o f t h e t r a n s i t i o n m e t a l i o n s i n t h e z e o l i t e s t r u c t u r e ( f o r example c a t i o n s w i t h i n t h e s o d a l i t e o r t h e hexagonal p r i s m o f Nay w i l l be hidden from CO i n t e r a c t i o n ) and p a r t l y t o probe t h e o x i d a t i o n s t a t e o f t h e t r a n s i t i o n m e t a l i o n from t h e CO I R f r e q u e n c i e s . I n general CO forms weak bondswith t h e 1 s t s e r i e s t r a n s i t i o n m e t a l i o n s and n+ t h e Me -CO complex i s e a s i l y d e s t r o y e d by o u t g a s s i n g t h e samples. The I R s t u d i e s of carbon monoxide adsorbed on t r a n s i t i o n metal ion exchanged z e o l i t e s have been reviewed i n ( 5 1 ) . More r e c e n t spectrosc o p i c s t u d i e s o f t h e i n t e r a c t i o n o f carbon monoxide w i t h group V I I I t r a n s i t i o n m e t a l ion-exchanged z e o l i t e s have shown t h a t w e l l defined m e t a l carbonyl compounds were formed w i t h i n t h e z e o l i t e c a v i t i e s ( 8 ) . I t a p p e a r s i n t e r e s t i n g t o d e s c r i b e t h e formation and i d e n t i f i c a t i o n o f t h e mononuclear and p o l y n u c l e a r m e t a l c a r b o n y l complexes which were s y n t h e s i z e d i n t h e s u p e r c a g e s of t h e f a u j a s i t e l i k e s t r u c t u r e N a Y z e o l i t e i ) z e o l i t e e n t r a p p e d mononuclear carbonyl compounds Rh(II1) -Nay r e a c t s w i t h CO a t room temperature forming Rh ( I ) dicarbony1 compound which s t r u c t u r e was i d e n t i f i e d by I R spectroscopy. I n a d d i t i o n xps measurements were o b t a i n e d t o f u r t h e r confirm t h e r e d u c t i o n o f R h ( 1 l I ) t o Rh(I) d u r i n g t h e r e a c t i o n w i t h CO. I R spect r a o f t h e carbonyl complex formed w i t h i n t h e z e o l i t e c a v i t i e s showed bands a t 2100-2020 cm-' a s e x p e c t e d f o r Rh(1) ( C O ) * species.

The g e n e r a l e q u a t i o n f o r t h e r e a c t i o n i s ( 3 5 )

Rh(II1) Rh(1)

+

+ 2CO

CO

H2° + +

Rh(I)

+

2 ~ +

+

C02

Rh(1) ( C 0 I 2

S i m i l a r l y Ir ( 111)-Nay r e a c t s w i t h CO t o form monovalent i r i d i u m c a r bony1 s p e c i e s ( 5 2 ) . Q u a n t i t a t i v e measurement o f CO uptake a t 170°C i n d i c a t e d a C O / I r r a t i o o f a b o u t 4. Furthermore I R bands a t 2086 and 2001 cm-l were observed. The d a t a were i n t e r p r e t e d i n t h e same manner a s for Rh-Nay, 1r (111) b e i n g reduced by CO i n t o I r ( I ) w i t h t h e subsequent formation o f Ir ( I )(CO) carbonyl s p e c i e s .

ii) Z e o l i t e - e n t r a p p e d m e t a l carbonyl c l u s t e r s . R e c e n t l y t h e r e h a s been i n t e r e s t i n s u p p o r t e d metal carbonyl c l u s t e r s . The i n v e s t i g a t i o n s have been d i r e c t e d towards : h e t e r o g e n i z i n g s o l u b l e c a t a l y s t s , preparing h i g h l y d i s p e r s e d m e t a l c a t a l y s t s , p r e p a r i n g s u p p o r t e d subcarbonyl metal c l u s t e r s showing unique c a t a l y t i c p r o p e r t i e s . In g e n e r a l , it was found t h a t t h e carbonyl m e t a l c l u s t e r immobilized on s o l i d s u r f a c e s such a s s i l i c a , alumina, m a i n t a i n t h e i r molecular s t r u c t u r e o n l y when t h e s u r f a c e was h i g h l y dehydroxylated o r when the s u r f a c e was f u n c t i o n a l i z e d by phosphine l i g a n d s . The Nay f a u j a site type z e o l i t e appeared an i n t e r e s t i n g m a t e r i a l for s t a b i l i z h g within t h e z e o l i t e c a v i t i e s m e t a l carbonyl c l u s t e r s such a s R h (CO) 6 16 ' Ir4(CO) S i n c e t h e c r i t i c a l dimension o f t h e c a r b o n y l c l u s t e r s a r e g e n e r a l l y l a r g e r t h a n t h e z e o l i t e cavity-windows, i t appeared necessary t o s y n t h e s i z e t h e carbonyl m e t a l c l u s t e r d i r e c t l y w i t h i n t h e cavities (6). Rhodium and i r i d i u m c a r b o n y l c l u s t e r s a r e p o t e n t i a l l y a c t i v e c a t a l y s t s for r e a c t i o n s such a s o x i d a t i o n , hydrogenation and hydroformylation of o l e f i n s . I t i s e x p e c t e d t h a t i m m o b i l i z a t i o n o f t h e c a r b o n y l met a l c l u s t e r i n c r e a s e s i t s s t a b i l i t y toward a g g r e g a t i o n . T h i s was accomplished i n two d i f f e r e n t ways : a ) a d i r e c t s y n t h e s i s of t h e metal c l u s t e r w i t h i n t h e z e o l i t e c a v i ties, b ) an i n s e r t i o n o f t h e m e t a l c l u s t e r w i t h i n t h e c a v i t i e s by sublimation. Our r e s u l t s have shown t h a t t h e f i r s t procedure i s more s u i t a b l e t o obtain h i g h l y d i s p e r s e d m e t a l c a r b o n y l c l u s t e r s * a

e n t r a p p e d w i t h i n t h e Nay z e o l i t e c a v i t i e s was o b t a i n e d Nay w a s exchanged w i t h m3+. A f t e r dehydration o f t h e W a Y sample a t 5 7 3 K , t h e z e o l i t e was t r e a t e d with a mixture of CO : H2 a t a b o u t 15 atmospheres and a t 3 0 0 K f o r few hours. The samples turned c o l o r e d and e x h i b i t e d a w e l l d e f i n e d i n f r a r e d spectrum w i t h two s t r o n g c a r b o n y l bands a t 2095 and 1765 cm-l. By comparison w i t h o t h e r rhodium c a r b o n y l compounds t h i s c l u s t e r formed and stai n f r a r e d spectrum was a s c r i b e d t o Rh6(CO) b i l i z e d w i t h i n t h e z e o l i t e c a v i t i e s . I d e n t g c a l s p e c i e s were a l s o w a s f i r s t supported s t a b i l i z e d w i t h i n t h e z e o l i t e when Rh (CO) 6 on t h e z e o l i t e s u r f a c e by s u b l i m a t i o n a 37163( followed by a d e c a r bonylation and r e c a r b o n y l a t i o n a t 373 K ( 5 4 ) . The i n f r a r e d spectrum Rh6(CO)

i n t h e hgJlowing ways (53, 8)

.

o f t h i s sample a l s o showed two s t r o n g bands a t 2095 and 1765 cm-I a t t r i b u t e d t o Rh6(CO) e n t r a p p e d i n t h e z e o l i t e . The s t a b i l i t y of 1 t h e rhodium carbonyl c f u s t e r w a s f u r t h e r i n v e s t i g a t e d . The i n f r a r e d r e s u l t s i n d i c a t e d t h a t z e o l i t e - e n t r a p p e d Rh (CO) may be decarbo6 1 n y l a t e d , w i t h o u t s i g n i f i c a n t a g g r e g a t i o n , by r e a c t i n g t h e sample a t 373 K w i t h H 2 . 0 r 02. Recarbonylation by CO a t 373 K regenerated t h e i n f r a r e d bands a t 2095 and 1765 cm-l. These s t u d i e s show t h a t t h e r e t e n t i o n o f t h e c l u s t e r i n t e g r i t y i s p o s s i b l e by u s i n g a zeol i t e support. Ir4(CO) I r 6 ( C 0 ) 1 6 have a l s o been s y n t h i z e d and s t a b i l i z e d within z e o l i t e s ( 8 ) . The exchanged I r NaY z e o l i t e was reduced by a mixture o f CO : H2 a t atmospheric p r e s s u r e and a t 443 K. The sample t u r n e d c o l o r e d and showed i n t h e c r b o n y l s t r e t c h i n g region - An i d e n t i c a l i n f r a r e d i n f r a r e d bands a t 2086, 2 0 4 0 and 1813 cm spectrum was observed when Ir (CO) was d e p o s i t e d on t h e z e o l i t e 4 1 from s o l u t i o n . These r e s u l t s show c l e a r l y t h a t t h e z e o l i t e matrix i s p a r t i c u l a r l y s u i t a b l e n o t only f o r t h e preparation of highly disp e r s e d m e t a l c a t a l y s t s b u t a l s o f o r s t a b i l i z i n g w e l l d e f i n e d metal clusters.

f.

2-5-

Reactivity with H

2

: formation o f s m a l l m e t a l p a r t i c l e s

S i n c e t h e importance o f metal d i s p e r s i o n i n t h e e f f i c i e n t use o f m e t a l c a t a l y s t s h a s been w e l l e s t a b l i s h e d e x t e n s i v e work h a s been c a r r i e d o u t t o develop methods o f p r e p a r a t i o n t h a t should produce m e t a l c a t a l y s t s f i n e l y d i s p e r s e d . The h i g h e s t d e g r e e o f metal d i s p e r s i o n was o b t a i n e d by f i x i n g metal c a t i o n s on a c a r r i e r , generally s i l i c a o r alumina, by i o n exchange t e c h n i q u e , f o l l o w i n g by reduction i n hydrogen (55-56). Because o f t h e i r p a r t i c u l a r s t r u c t u r e s and t h e i r ion-exchange p r o p e r t i e s , z e o l i t e s appeared p a r t i c u l a r l y approp r i a t e f o r s t a b i l i z i n g f i n e l y d i s p e r s e d m e t a l p a r t i c l e s . Rabo e t a 1 ( 5 7 ) were among t h e f i r s t t o r e p o r t on z e o l i t e - s u p p o x t e d platinum. From t h e i r r e s u l t s t h e y concluded t h a t p l a t i n u m was almost atomicall y d i s p e r s e d w i t h i n t h e zeolite-framework. F u r t h e x i n v e s t i g a t i o n s on z e o l i t e - s u p p o r t e d p l a t i n u m were performed by s e v e r a l a u t h o r s (58-60). G e n e r a l l y it was concluded t h a t t h e p l a t i n u m p a r t i c l e - s i z e depended s t r o n g l y on t h e p r e t r e a t m e n t c o n d i t i o n s o f t h e m a t e r i a l s b e f o r e H - r e d u c t i o n . G a l l e z o t e t a 1 ( 5 9 ) from t h e i r e l e c t r o n micros2 copy and X-ray d i f f r a c t i o n s t u d i e s conc uded t h a t t h e metal dispers i o n would depend on t h e p o s i t i o n o f P t c a t i o n s i n t h e z e o l i t e cag e s . S i n c e much o f t h e work concerned p l a t i n u m c a t a l y s t s , we have i n v e s t i g a t e d t h e behaviour of o t h e r z e o l i t e - s u p p o r t e d group V I I I m e t a l s i n o r d e r t o p r o v i d e a g e n e r a l t r e n d on t h e p r e p a r a t i o n and t h e p r o p e r t i e s o f z e o l i t e - s u p p o r t e d group V I I I m e t a l s . The p r e p a r a t i o n o f z e o l i t e - s u p p o r t e d noble m e t a l s i n v o l v e d t h e subst i t u t i o n o f ~ a ' c a t i o n s i n Nay, by ion-exchange w i t h group V I I I met a l s i n t h e i r c a i o n i c form,. For i o n e change w e have used a ueous S+ 9+, s o l u t i o n s o f ~t (m3) pd2+ ( N H ~ ) Ru (NH ) 6 , ( ~ (NH h ) ~ 1 ) ( I r ( N H ) C1I2'. A f t e r b e i n g c a r e f u l l y washe2 and dried3$e exchanged 3 5 f a u j a s l t e - t y p e z e o l i t e s were f i r s t c a l c i n e d i n oxygen i n t h e

iJ+

3+

*,

temperature range 473-773 K , followed by H2-reduced i n t h e temperature range 383-773 K. Metal d i s p e r s i o n and p a r t i c u l e s i z e measurements were o b t a i n e d by H a d s o r p t i o n and t r a n s m i s s i o n e l e c t r o n micros2 copy. X-ray d i f f r a c t i o n , e l e c t r o n s p i n resonance, X p s and i n f r a r e d were used t o i n v e s t i g a t e t h e s t a t e o f t h e metal c a t i o n s w i t h i n t h e z e o l i t e framework b e f o r e H - r e d u c t i o n , s i n c e it h a s been a l r e a d y 2 s t a t e d f o r P t t h a t t h i s parameter may s t r o n g l y i n f l u e n c e t h e metal dispersion. A s a l r e a d y observed by o t h e r s (58-60) t h e average p a r t i c l e diameter obtained from e l e c t r o n micrographs and t h a t c a l c u l a t e d from hydrogen adsorption d a t a a r e i n good agreement i n t h e c a s e of Pt-Nay samples calcined i n oxygen a t 573 K before H2-reduced. I n c o n t r a s t Pt-Nay calcined i n oxygen a t 773 K and then H2-reduced a t 773 K showed by H2-adsorption an a p p a r e n t p a r t i c l e s i z e of about 5nm while t h e e l e c tron micrographs i n d i c a t e d t h a t t h e p a r t i c l e s i z e were around 2 nm. This discrepancy was i n t e r p r e t e d i n terms o f t h e e x i s t e n c e of p l a t i num a t o m i c a l l y d i s p e r s e d i n t h e s o d a l i t e cages, which d i d n o t adsorb hydrogen. X-ray d i f f r a c t i o n a n a l y s i s ( 6 1) have i n d i c a t e d t h a t t h e increase o f the a t i v a t i o n temperature p r i o r t o r e d u c t i o n produced a migration o f P t c a t i o n s from t h e supercages t o t h e s o d a l i t e cages. Thus r e d u c t i o n o f p t 2 + c a t i o n s p r e s e n t i n t h e supercages produced platinum a g g r e g a b s of about 1 nm i n d i a m e t e r , while t h e reduction of pt2' c a t i o n s i n t h e s o d a l i t e cages would l e a d t o a t o m i c a l l y dispersed platinum.

5+

Ruthenium, rhodium o r i r i d i u m exchanged z e o l i t e s behaved d i f f e r e n t l y . Samples p r e c a l c i n e d i n oxygen below 510 K p r i o r t o H -reduction d i d 2 form h i g h l y d i s p e r s e d metal c a t a l y s t s . The p a r t i c l e s i z e s a s d e t e r mined by e l e c t r o n microscopy and by H2 a d s o r p t i o n were i n good agreement and i n t h e range 1-1.5 nm. Furthermore t h e e l e c t r o n micrographs i n d i c a t e d t h a t t h e metal p a r t i c l e s were l o c a t e d w i t h i n t h e z e o l i t e c a v i t i e s , probably i n t h e supercages. I n c o n t r a s t very l a r g e metal p a r t i c l e (10-20 nm) were formed f o r t h o s e samples p r e c a l c i n e d i n oxygen a t 7 7 3 K, E l e c t r o n micrographs showed t h a t t h e s e p a r t i c l e s were on t h e e x t e r n a l s u r f a c e of t h e z e o l i t e . These r e s u l t s c l e a r l y i n d i c a t e a d i f f e r e n t behaviour o f R u , Rh, I r exchanged Nay z e o l i t e s compared with Pt-Nay samples. To b e t t e r understand t h e e f f e c t of oxygen p r e t r e a t m e n t on t h e R u , R h , I r d i s p e r s i o n , t h e s t a t e of t h e metal p r e c u r s o r b e f o r e H - r e d u c t i o n , was i n v e s t i g a t e d . 2

It_ was shown ( 3 ) t h a t upon a c t i v a t i o n o f Ru-exchanged Y z e o l i t e t h e (NH~) 3+ complex was p r o g r e s s i v e l y transformed i n t o s p e c i e s , f o l l o w i n g h y d r o l y s i s of t h e hexammine [RU ( N H ~ ) (OH) ruthenium compyex. Hence ruthenium i n i t s c a t i o n i c form i s p r o g r e s s i vely transformed i n an a n i o n i c s p e c i e s which w i l l be no more bound t o the z e o l i t e l a t t i c e by e l e c t r o s t a t i c f i e l d . C a l c i n a t i o n a t a temper a t u r e of 773 K produced a dehydration o f t h e ruthenium hydroxyanions and t h e subsequent formation of Ru 0 Ru02 on t h e e x t e r n a l 2 3' surface o f t h e z e o l i t e . The e f f e c t of t h e oxygen t r e a t m e n t o f Ru(NH ) -Nay z e o l i t e s p r i o r H2-reduction on t h e f i n a l metal p a r t i 3 6 c l e s IS now w e l l understandable : up t o a temperature o f about 523 K ,

LRU

63

1

3+

complexes remained well d i s p e r s e d i n t h e supercages of t h e z e o l i t e . These w e l l d i s p e r s e d s p e c i e s formed upon H -reduction very s m 11 metal p a r t i c l e s . A t higher temperature t h e hy roxyanions of Ru?+ were dehydrated i n t o l a r g e Ru203 c r y s t a l l i t e s , l o c a t e d on the e x t e r n a l s u r f a c e o f t h e z e o l i t e . Obviously t h e H -reduction of these l a r g e oxide c r y s t a l l i t e s generated l a r g e meta? p a r t i c l e s . Ru

5

S i m i l a r s t u d i e s were c a r r i e d with ( I r(NH C 1 ) 2+ Nay. X-ray d i f fract i o n and i n f r a r e d s5ydies i n c i d a t e d h a t upon c a l c i n a t i o n i n c a t i o n s were p r o g r e s s i v e l y t r a n s formed i n t o oxygen ( I r(NH3) 5 C 1 ) hydroxyanions ( I r (NH3)5-x (OH) x C l ) which upon complete o x i d a t i o n a t 773 K formed l a r g e Ir02 c r y s t a l l i t e s . The g e n e r a l conclusion which may be derived from t h i s study i s the following : z e o l i t e s a r e s u i t a b l e m a t e r i a l s t o prepare and t o stabil i z e h i g h l y d i s p e r s e d metal c a t a l y s t s , provided t h a t t h e metal prec u r s o r s remain h i g h l y d i s p e r s e d i n t h e z e o l i t e c a v i t i e s before H 2 reduction. Group V I I I t r a n s i t i o n metals may be c l a s s i f i e d i n t o two groups :

i) those f o r which t h e c a t i o n i c form i s r e l a t i v e l y s t a b l e , such as platinum, palladium ; t h e s e c a t i o n s remain always h i g h l y dispersed i n t h e z e o l i t e c a v i t i e s , t h u s form h i g h l y d i s p e r s e d metal c a t a l y s t s . ii) those f o r which t h e c a t i o n i c form i s u n s t a b l e , rhodium, ruthenium, i r i d i u m , and e a s i l y transformed i n t o an hydroxyanion. A s long a s t h e hydroxyanions remain d i s p e r s e d i n t h e z e o l i t e framework, one o b t a i n s upon H -reduction h i g h l y d i s p e r s e d metal c a t a l y s t s . However 2 s i n c e a t high temperature t h e hydroxyanions a r e dehydrated i n t o t h e oxide form, one should avoid such formation of l a r g e oxide c r y s t a l l i t e i n o r d e r t o form h i g h l y d i s p e r s e d metal c a t a l y s t s . 3

-

C a t a l y t i c p r o p e r t i e s of t r a n s i t i o n metal ion-exchanged z e o l i t e s

I t i s w e l l recognized t h a t z e o l i t e s a r e widely used a s c a t a l y s t s for

a broad range of hydrocarbon transformation. Although t h e major a p p l i c a t i o n s of z e o l i t e s a s c a t a l y s t s , e s p e c i a l l y i n t h e Petroleum I n d u s t r y , a r e based on a c i d form z e o l i t e s s e v e r a l i n v e s t i g a t o r s have discovered t h a t s p e c i f i c c a t a l y t i c p r o p e r t i e s a r e shown by t r a n s i t i o n metal i o n exchanged z e o l i t e s toward r e a c t i o n s generally c a t a l y z e d by t h e p a r e n t metal i o n s ' i n s o l u t i o n . I n t h i s paragraph some r e c e n t a s p e c t s of t h e c a t a l y t i c p r o p e r t i e s of "immobilized t r a n s i t i o n metal i o n s " i n z e o l i t e s w i l l be given. I t i s worthwhile t o r e c a l l t h a t most of t h e work which w i l l be described d e a l s with f a u j a s i t e - t y p e z e o l i t e s , t h e s e m a t e r i a l s appearing t h e most suitable c a r r i e r f o r "Immobilizing" s o l u b l e c a t a l y s t s because of t h e r e l a t i vely l a r g e space of t h e i r c a v i t i e s . 3-1- Oligomerisation, a d d i t i o n , cyclodimerisation of unsatured hydrocarbons The homogeneous c a t a l y t i c formation of irkines by a d d i t i o n of

primary a l i p h a t i c amines t o a c e t y l e n e s i n t h e presence of z i n c acetate needs high p r e s s u r e ( 6 2 ) Z n ( I 1 )-exchanged Y z e o l i t e was found active f o r t h e a d d i t i o n o f methylamine t o methylacetylene a t atmospheric pressure.N-isopropylidene methylamine r e s u l t e d f o l l o w i n g t h e reaction ( 6 3 )

.

CH NH 3 2

+

CH - C r

3

CH

+

(CH ) C = N C H 3 2 3

However N i , Pd, P t and Cu(1) exchanged z e o l i t e s ( i s o e l e c t r o n i c w i t h Z n ( I 1 ) ) were found i n a c t i v e i n t h i s r e a c t i o n . The l a c k of a c t i v i t y was a t t r i b u t e d t o a r a p i d r e d u c t i o n of t h e t r a n s i t i o n metal i o n s . Thus it i s c l e a r t h a t when u s i n g t h e s e m a t e r i a l s f o r r e a c t i o n s catalyzed by i o n s i n s o l u t i o n , i t i s important t o avoid t h e r e d u c t i o n of t h e exchanged t r a n s i t i o n metal i o n s i n t o t h e i r m e t a l l i c forms. Benzene formation from t h e t r i m e r i s a t i o n o f a c e t y l e n e was c a t a l y z e d by N ~ ~ + - Nz ~e oYl i t e . The r e a c t i o n was found t o occur w i t h i n t h e l a r ge c a v i t i e s o f the z e o l i t e ( 6 4 ) . The d i m e r i s a t i o n o f e t h y l e n e over rhodium exchanged z e o l i t e h a s been i n v e s t i g a t e d 5 6 5 ) . I t was found t h a t t h e r a t e of formation of n-butenes depended on the temperature of a c t i v a t i o n of RhY, the maximum r a t e b e i n g reached when RhY was a c t i v a t e d around 400°C. Furthermore it was found t h a t t h e dimerisation a c t i v i t y was lowered by t h e a d d i t i o n o f p y r i d i n e o r CO which are known t o i n t e r a c t w i t h t h e R h c a t i o n s , while an a p p r o p r i a t e amount o f HC1 i n c r e a s e d t h e r e a c t i o n r a t e . Xps measurements i n d i c a ted t h e e x i s t e n c e o f monovalent R . ( I ) on t h e a c t i v e Rh-Y samples. I t was concluded t h a t Rh(1) i n Nay was t h e a c t i v e s i t e s f o r t h e d i merization o f e t h y l e n e . The e f f e c t of HC1 i s s i m i l a r t o t h a t encountered i n homogeneous systems f o r which it h a s been shown t h a t the a d d i t i o n o f H C 1 t o &(I) complexes a c t i v a t e s t h e c a t a l y s t ( 6 6 ) . Copper-exchanged z e o l i t e s have shown i n t e r e s t i n g c a t a l y t i c propert i e s f o r t h e c y c l o d i m e r i s a t i o n o f b u t a d i e n e t o vinylcyclohexene ( 6 7 ) . Nickel ( 0 ) complexes i n s o l u t i o n c a t a l y z e t h e c y c l o d i m e r i s a t i o n of butadiene i n t o 4-vinylcyclohexene ( 6 8 ) . Thus it appeared t h a t it i s Cu(1) which i s i s o e l e c t r o n i c with N i ( 0 ) which i s t h e a c t i v e s i t e f o r the cyclodimensation o f butadiene by Cu(I1)-Y z e o l i t e s Cu(1)-Y were f u r t h e r i n v e s t i g a t e d f o r t h e c y c l o d i m e r i s a t i o n of butadiene ( 6 9 ) . Cu(1)-Y samples were p r e p a r e d e i t h e r by d i r e c t exchange w i t h cuprous s o l u t i o n o r by r e d u c t i o n of Cu(I1)-Y by CO following t h e procedure given i n ( 7 0 ) , monovalent-copper c o n t a i n i n g z e o l i t e s e x h i b i t e d a h i g h s e l e c t i v i t y (90 % ) f o r t h e formation of 4-vinylcyclohexene . However Cu(1.)-Y samples o b t a i n e d from CO r e d u c t i o n showed a r a p i d d e a c t i v a t i o n due t o t h e formation and d e p o s i t i o n o f polymer b u t a diene on t h e z e o l i t e s u r f a c e . The a c t i v e s i t e s f o r p o l y m e r i s a t i o n were Br8nsted/Lewis c e n t r e s generated d u r i n g r e d u c t i o n of Cu(I1)-Y t o Cu(I)-Y. When t h e a c i d s i t e s were n e u t r a l i z e d by ammonia t h e cat a l y s t d e a c t i v a t i o n d u r i n g t h e c y c l o d i m e r i s a t i o n o f butadiene was lowered. Cu(1)-Y p r e p a r e d by d i r e c t exchange with cuprous s o l u t i o n was found s u b s t a n t i a l l y more s t a b l e due t o t h e absence o f a c i d s i t e s .

I n conclusion, i t was s t a t e d t h a t CO-reduced C u ( I 1 ) -Y i s due t o t h e The r o l e o f t h e z e o l i t e framework i o n s , which allowed t h e o x i d a t i v e les.

t h e d i f f e r e n c e between Cu(1)-Y and a c i d i t y g e n e r a t e d by CO-reduction. i s t o s t a b i l i z e monovalent copper coupling of two butadiene molecu-

3-2- Oxidation r e a c t i o n s . T r a n s i t i o n metal i o n s exchanges Y z e o l i t e s were found a c t i v e f o r s e l e c t i v e o x i d a t i o n o f hydrocarbons. I n t h e o x i d a t i o n of e t h y l e n e i n t o acetaldehyde (Wacker p r o c e s s ) which i n t h e l i q u i d phase i s c a t a l y z e d by a s o l u t i o n c o n t a i n i n g C u ( I 1 ) and P d ( I 1 ) i o n s was found t o occur i n t h e g a s - s o l i d r e a c t i o n u s i n g P d ( I I ) , Cu(I1)-exchanged Y z e o l i t e ( 7 1 ) . I t should be noted t h a t t h e y i e l d o f acetaldehyde decreased a t temperatures h i g h e r than 115°C a s t h e r e s u l t of r e d u c t i o n of P d ( 1 I ) and C u ( I 1 ) i n t o metal forms

.

C u ( 1 I ) -Y z e o l i t e was a l s o i n v e s t i g a t e d i n t h e vapor phase oxidation o f benzyl a l c o h o l a t a temperature range o f 300-390°C ( 7 2 ) . The act i v e s i t e s f o r t h e o x i d a t i o n of benzyl a l c o h o l i n t o benzaldehyde were Cu (11) i n t h e z e o l i t e framework. The conversion o f benzylalcohol i n c r e a s e d a b r u p t l y beyong 30 % C u ( I 1 ) i o n exchange, which indicated t h a t C u ( I 1 ) i o n s were, below 30 % exchange l e v e l i n hidden s i t e s . Beyong 30 % exchange C u ( I 1 ) i o n s were l o c a t e d w i t h i n t h e z e o l i t e supercages and t h u s a c c e s s i b l e t o t h e r e a c t a n t s . C o b a l t ( I 1 )-exchanged Nay z e o l i t e was a l s o found a c t i v e and s e l e c t i v e i n t h e oxidation o f benzyl a l c o h o l i n t o benzaldehyde ( 7 3 ) . Co-Nay was found much more s e l e c t i v e t h a n Cu-Nay. A s i n t h e c a s e o f Cu-Nay, t h e y i e l d of benzaldehyde i n c r e a s e d a b r u p t l y beyong about 2 0 & exchange. The e f f e c t o f amine a d d i t i o n on t h e benzaldehyde y i e l d demonstrated t h e sirnil a r i t y o f t h e behevior o f C o ( I 1 ) i n s o l u t i o n and w i t h i n t h e z e o l i t e framework. The a d d i t i o n o f p y r i d i n e o r p i p e r i d i n e would p u l l o u t of t h e s o d a l i t e cages Co (11) and t h u s i n c r e a s e s t h e number of access i b l e C o ( I 1 ) i o n s , which would l e a d t o an i n c r e a s e of t h e benzaldehyde y i e l d . However when e t h y l e n e diamine was adsorbed, t h e y i e l d o f benzaldehyde decreased. T h i s was a t t r i b u t e d t o t h e formation of Co (111)( e n ) 0; complex i n t h e l a r g e c a v i t i e s which appeared rel a t i v e l y s t a b f e . I n c o n t r a s t C o ( 1 I ) i n t h e presence of p y r i d i n e or ammonia forms with 0 a dimeric Cobalt-oxygen adduct such a s 2 which was considered a s t h e p r e c u r s o r for LxCo (11)-02-Co (11)L d i s s o c i a t i o n o f 0 mofecule. The formation of Co-0 i s t h u s f a c i l i 2 t a t e d i n t h e presence of p y r i d i n e r e s u l t i n g i n t h e i n c r e a s e o f the benzaldehyde y i e l d . The o x i d a t i o n o f cyclohexene over molybdenum z e o l i t e s i n t h e Liquidphase was found t o occur with r e l a t i v e l y high s e l e c t i v e l y toward e p o x i d a t i o n ( 7 4 ) a t 50 8 conversion t h e s e l e c t i v i t y f o r cyclohexane oxide was about 50 %. I t was concluded t h a t a t low Mo c o n t e n t the cyclohexene e p o x i d a t i o n w a s i n i t i a t e d by a r a d i c a l mechanism, the formation of cyclohexenyl hydroperoxide being t h e l i m i t i n g s t e p Z e o l i t e s c o n t a i n i n g both molybdenum and c o b a l t e x h i b i t e d a c t i v i t i e s and s e l e c t i v i t i e s i n cyclohexene o x i d a t i o n comparable t o homogeneous

c a t a l y s t s such a s Co ( a c a c ) /MOO ( a c a c )2 . 2 2

3-3-

Carbonylation of methanol

The c a r b o n y l a t i o n of methanol i n t o a c e t i c a c i d o r m e t h y l a c e t a t e was developed by Mmsanto i n t h e l i q u i d phase u s i n g rhodium based c a t a l y s t s , i n t h e p r e s e n c e o f an i o d i d e promotor ( 7 4 ) . Rhodium exchanged z e o l i t e s were found a c t i v e and s e l e c t i v e f o r t h e vapor phase carbonylation o f methanol ( 7 5 ) . S i m i l a r l y i r i d i u m exchanged z e o l i t e showed i n t e r e s t i n g a c t i v i t y i n t h i s r e a c t i o n . The r e a c t i o n was c a r r i e d o u t a t 150-180°C and a t atmospheric p r e s s u r e . The k i n e t i c s t u d i e s r e v e a l e d i d e n t i c a l r a t e e x p r e s s i o n b o t h i n l i q u i d phase and i n vapor phase t h a t i s : f i r s t o r d e r w i t h r e s p e c t t o CH 1 promotor and ( R h ) c o n c e n t r a t i o n and z e r o o r d e r w i t h r e s p e c t t o C$ OH and CO. 3 The v a r i o u s s t e p s of t h e r e a c t i o n were i n v e s t i g a t e d by i n f r a r e d and the r e s u l t s p a r a l l e l t h o s e o b t a i n e d w i t h t h e homogeneous rhodium c a t a l y s t s . I n t h e presence o f CO R .( 1 1 1 ) i o n s were reduced t o Rh ( I ) which c o o r d i n a t e d two CO molecules t o g i v e Rh ( I )( C O ) a c t i v e s i t e s . These s p e c i e s added CH I fhrough a n o x i d a t i v e a d d i t i o n w i t h t h e subsequent formation -Rh (111)( C 0 ) (I)] complexes. T h i s complex was r e l a t i v e l y u n s t a l e and r e a r r a n g e d r a p i d l y i n t o a rhodium a c e t y l complex . Methanol o r w a t e r r e a c CO) Rh (CO) ( 1 ted e a s i l y w i t h t h e acety?. adduct and methyl a c e t a t e o r a c e t i c a c i d were evolved. The r o l e o f t h e z e o l i t e appeared t o be a s a c a r r i e r f o r b e t t e r s t a b i l i z a t i o n and f o r b e t t e r d i s p e r s i o n o f & ( I ) k p e c i e s .

02 LCH 2 L(CH

12

These few examples a r e s u f f i c i e n t t o show t h a t a l a r g e number o f r e a c t i o n s c a t a l y z e d by s o l u b l e m e t a l complexes i n l i q u i d phase can be c a r r i e d o u t i n t h e vapor phase, by z e o l i t e s exchanged w i t h t h e

analogous metal i o n s . I n g e n e r a l t h e z e o l i t e m a t r i x a f f o r d s t h e h i g h e s t metal i o n d i s p e r s i o n i n comparison w i t h o t h e r s u p p o r t s and a f f o r d s h i g h s t a b i l i s a t i o n f o r c a t i o n s i n low o x i d a t i o n s t a t e . I n a d d i t i o n i n s e v e r a l c a s e s , on z e o l i t e s t h e r e a c t i o n can be c a r r i e d out a t much lower p r e s s u r e s t h a n t h o s e r e q u i r e d by o t h e r homogeneous c a t a l y s t s i n l i . q u i d phase. The a p p l i c a b i l i t y o f z e o l i t e s c o n t a i n i n g t r a n s i t i o n m e t a l i o n s i s f a r from b e i n g exhausted, and it i s c l e a r t h a t s e v e r a l new a p p l i c a t i o n s f o r producing chemicals w i l l be found in the near futur.

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ZEOLITE BIFUNCTIONAL CATALYSIS

M. G u i s n e t and G . P e r o t

L a b o r a t o i r e A s s o c i 6 au CNRS - C a t a l y s e Organique Universitg de P o i t i e r s , France

B i f u n c t i o n a l z e o l i t e c a t a l y s t s , g e n e r a l l y metal-loaded a c i d z e o l i t e s a r e employed i n numerous p r o c e s s e s i n p e t r o l e u m r e f i n i n g and i n p e t r o c h e m i c a l i n d u s t r i e s (1,2) : h y d r o c r a c k i n g , s e l e c t o f o r ming, dewaxing, h y d r o i s o m e r i z a t i o n of C5-C6 a l k a n e s , h y d r o i s o m e r i z a t i o n of C8 a r o m a t i c s The c a t a l y s t s used i n t h e s e p r o c e s s e s p r e s e n t two t y p e s o f s i t e s : m e t a l l i c s i t e s whose main f u n c t i o n i s t o h y d r o g e n a t e and t o d e h y d r o g e n a t e and a c i d s i t e s whose main function is t o c r a c k o r t o isomerize.

...

Because o f t h e p r o g r e s s i v e e v o l u t i o n i n t h e s u p p l y towards h e a v i e r c r u d e s and i n t h e demand towards l i g h t p r o d u c t s , hydroc r a c k i n g , t h e most i m p o r t a n t p r o c e s s i n v o l v i n g b i f u n c t i o n a l c a t a l y s i s , i s d e s t i n e d t o have a n i m p o r t a n t development. Compared t o c a t a l y t i c c r a c k i n g , i t h a s t h e a d v a n t a g e of b e i n g more f l e x i b l e indeed improvements i n p r o c e s s e s and i n c a t a l y s t s have made i t p o s s i b l e t o c o n v e r t a l a r g e v a r i e t y of f e e d s t o c k s (from n a p h t h a t o heavy g a s o i l s ) and t o o b t a i n a l a r g e v a r i e t y of h i g h q u a l i t y prod u c t s . Hydrocracking r e q u i r e s c a t a l y s t s w i t h a h i g h a c i d i t y count e r b a l a n c e d by a h i g h h y d r o g e n a t i o n a c t i v i t y , which i m p l i e s t h e u s e of b i f u n c t i o n a l z e o l i t e c a t a l y s t s . The c r a c k i n g f u n c t i o n i s p r o v i d e d by a Y z e o l i t e i n which t h e sodium i o n s h a v e been r e p l a c e d by hydrogen, by r a r e e a r t h o r by d i v a l e n t c a t i o n s and t h e h y d r o g e n a t i n g f u n c t i o n i s p r o v i d e d by n o b l e o r non-noble metals ( 1 , 3 ) . I n marked c o n t r a s t t o amorphous s i l i c a - a l u m i n a c a t a l y s t s , z e o l i t e c a t a l y s t s can o p e r a t e i n t h e p r e s e n c e o f s u b s t a n t i a l amounts of ammonia and o t h e r b a s i c n i t r o g e n compounds. T h i s g r e a t e r a b i l i t y of the z e o l i t e s t o t o l e r a t e b a s i c compounds c a n b e a t t r i b u t e d t o t h e i r g r e a t e r number o f a c i d s i t e s . Moreover t h i s g r e a t e r a c i d i t y enhances the r e s i s t a n c e of t h e h y d r o g e n a t i n g f u n c t i o n t o p o i s o n i n g by s u l f u r

compounds and hydrocracking catalysts show a high stability with regards to sulfur poisoning. Selectoforming and Mobil Dewaxing (MDDW (4)) processes take advantage of the shape-selective properties of zeolites. Selectoforming allows the selective hydrocracking of n-paraffins in a gasoline reformate. In this process, a bifunctional catalyst with a non-noble metal as hydrogenating component and a small pore zeolite (T zeolite) as a cracking component is employed. The MDDW process uses a Pd or Ni exchanged ZSM-5 zeolite (an intermediate pore-size zeolite) which cracks preferentially the n-paraffins particularly those with higher boiling temperatures. Hydroisomerization of light naphtha (C -C6 alkanes) is carried out in the Hysomer process (Shell) on a nob?e metal highly dispersed on a large pore zeolite with a high acidity. Bifunctional catalysts containing mordenite or ZSM-5 zeolite can also be used for the isomerization of C 8 aromatic cuts. Numerous parameters govern the activity and the selectivity of bifunctional zeolite catalysts. This is namely the case for the characteristics of the hydrogenating and acid functions as well as for their "balance". To show the influence of these parameters, examples will be selected from the most typical reactions of the industrial processes that is the isomerization and the cracking of n-alkanes. Naturally, the characteristics of bifunctional zeolite catalysts depend largely on the preparation procedure and particularly on the way the hydrogenating component is introduced. However the preparation of metal-loaded catalysts,, described elsewhere ( 5 , 6 ) will not be discussed here.

1. MECHANISMS OF ALKANE TRANSFORMATION ON BIFUNCTIONAL CATALYSTS 1.1

Bifunctional Catalysis

1.1.1 Generalities. In bifunctional catalysis, reactions occur in successive steps involving two different types of sites (7). As an example, the conventional bifunctional process of isomerization of n-hexane into methylpentanes found on platinum-silica alumina is shown in Fig. I. This catalyst presents two types of sites : i) platinum sites whose function is to dehydrogenate n-hexane into n-hexenes (reaction 1) and to hydrogenate methylpentenes into methylpentanes (reaction 5) ; ii) acid sites whose function is to isomerize hexenes into methylpentenes (reaction 3). Beside these chemical steps, a bifunctional process requires diffusion steps of the intermediate species. In this case, olefin intermediates diffuse from the metallic to the acid sites (step 2) and from the acid to the metallic sites (step 4).

U l C-C-C-C-C-C

C-C-C-C-C-C

-

C:c~C~C-xLC

.I.t Si02-A1 0

C:.%x=c 1

49

Cr'c-X%%%

2P 3

F

+H21T-H2 C

1

C-C-C-C-C

\

!

C,C^X%%

3.f

4

.

C

It

cLT=cz.-x Pt -

2

C-~-xI=cC

5

+H2

it+?

C

I

C-C-C-C-C

I

T I C

I

C-C-C-C-C

Fig. 1 .

C

I

C-C-C-C-C

Bifunctional process of n-hexane isomerization on platinum-silica alumina.

The existence of this bifunctional process is now well established : i) although highly unfavoured thermodynamically under the usual operating conditions,the intermediate olefins were detected by GLC or by mass spectrometry (8,lO). Moreover, the skeletal isomerization of olefins (reaction 3 in Fig. 1 .) is known to occur very readily on acid catalysLs (7,8,11) ; ii) the participation in the reaction of both acid and hydrogenating centers was clearly demonstrated by using physical mixtures

of an a c i d c a t a l y s t and of a metal d e p o s i t e d on a n i n e r t c a r r i e r (7,12-14) : t h e a c t i v i t i e s of t h e m i x t u r e s were d e f i n i t e l y g r e a t e r than t h e sum of t h e a c t i v i t i e s of t h e components ; i i i ) t h e change i n t h e i s o m e r i z a t i o n a c t i v i t i e s of bifunct i o n a l c a t a l y s t s ( d i f f e r i n g by t h e i r platinum c o n t e n t ) a s a function of t h e i r hydrogenation a c t i v i t i e s was t h e change expected from the m u l t i s t e p b i f u n c t i o n a l process (15) : - f o r low hydrogenation a c t i v i t i e s t h e l i m i t i n g s t e p i s t h e n-hexane dehydrogenation ( r e a c t i o n 1 ) o r t h e methylpentene hydrog e n a t i o n ( r e a c t i o n 5) on t h e m e t a l l i c s i t e s ; under t h e s e condit i o n s , t h e isomerization a c t i v i t y increases proportionally t o the hydrogenation a c t i v i t y (Fig. 2 . ) ; - f o r high hydrogenation a c t i v i t i e s , r e a c t i o n s 1 and 5 become very f a s t compared t o r e a c t i o n 3 which i s t h e n t h e r a t e - l i m i t i n g s t e p ; under t h e s e c o n d i t i o n s t h e i s o m e r i z a t i o n a c t i v i t y of t h e b i f u n c t i o n a l c a t a l y s t s no l o n g e r depends on t h e i r hydrogenation a c t i v i t y b u t only on t h e i r a c i d i t y . Thus, t h e i s o m e r i z a t i o n a c t i v i t y of b i f u n c t i o n a l c a t a l y s t s with a given a c i d c a r r i e r remains constant beyond a c e r t a i n v a l u e of t h e hydrogenation a c t i v i t y (Fig. 2 . ) . Moreover, t h e g r e a t e r t h e a c i d i t y of t h e c a r r i e r , t h e h i g h e r t h e maximum v a l u e of t h e i s o m e r i z a t i o n a c t i v i t y of t h e b i f u n c t i o n a l catalvst

.

Fig. 2.

n-Hexane i s o m e r i z a t i o n on p l a t i n u m - s i l i c a alumina ( P t / S A ) . I n f l u e n c e of t h e hydrogenating a c t i v i t y (H) and of t h e a c i d i t y (SAI i s more a c i d t h a n SA2) of t h e c a t a l y s t s on t h e i r i s o m e r i z a t i o n a c t i v i t i e s ( I ) (from r e f e r e n c e 12).

1.2 Mechanisms of t h e r e a c t i o n s o c c u r r i n g on b i f u n c t i o n a l c a t a lysts.

I s o m e r i z a t i o n and c r a c k i n g of a l k a n e s can t a k e p l a c e by t h e b i f u n c t i o n a l p r o c e s s b u t can a l s o b e c a t a l y z e d independently by t h e acid or by t h e m e t a l l i c s i t e s . 1 . 2 . 1 Acid - c a t a l y z e d r e a c t i o n s . The mechanism of a l k a n e isomer i z a t i o n and c r a c k i n g on a c i d s i t e s i s shown i n F i g . 3. Carbocations, involved a s i n t e r m e d i a t e s , a r e formed e i t h e r by a l k a n e a d s o r p t i o n on Br6nsted o r Lewis a c i d s i t e s :

o r by hydride t r a n s f e r from t h e a l k a n e t o a preadsorbed c a r b o c a t i o n :

1.2.2 Metal - c a t a l y z e d r e a c t i o n s . Two mechanisms have been proposed t o account f o r a l k a n e i s o m e r i z a t i o n on m e t a l s , p a r t i c u l a r l y on platinum : t h e b o n d - s h i f t mechanism a t t r i b u t e d t o t h e f o r m a t i o n of a a y - t r i a d s o r b e d i n t e r m e d i a t e s bonded t o two a d j a c e n t metal atoms (16,17) and t h e c y c l i c mechanism which i n v o l v e s t h e f o r m a t i o n of c y c l o p e n t a n i c i n t e r m e d i a t e s and t h e i r s c i s s i o n (18,19). On l a r g e platinum c r y s t a l l i t e s , i s o m e r i z a t i o n o c c u r s mainly by t h e bond-shift mechanism whereas on small platinum c r y s t a l l i t e s , t h e c y c l i c mechanism i s p r i v i l e g e d ( 2 0 ) . The a l k a n e c r a c k i n g on metal s i t e s (hydrogenolysis) i n v o l v e s t h e s c i s s i o n of h i g h l y dehydrogenated s p e c i e s adsorbed by two adjacent carbon atoms on a d j a c e n t metal s i t e s ( 2 1 ) . Contrary t o t h e cracking r e a c t i o n s by a c i d o r b i f u n c t i o n a l c a t a l y s i s , hydrogenolysis produces methane and e t h a n e .

Fig. 3 .

I s o m e r i z a t i o n and c r a c k i n g of p a r a f f i n s on a c i d c a t a l y s t s . : c a r b o c a t i o n ; x,y : numb e r of carbon atoms. P : p a r a f f i n ; 0 : o l e f i n ; C'

1.2.3 Reactions involved i n t h e b i f u n c t i o n a l p r o c e s s . This bifunct i o n a l p r o c e s s of a l k a n e i s o m e r i z a t i o n h a s a l r e a d y been developed i n t h e n-hexane example ( F i g . 1 .) The c r a c k i n g r e a c t i o n s o c c u r by s c i s s i o n o f o l e f i n i n t e r m e d i a t e s . The H o r i u t i P o l a n y i mechanism i s commonly a c c e p t e d . t o e x p l a i n o l e f i n h y d r o g e n a t i o n ( r e a c t i o n 5) o r a l k a n e d e h y d r o g e n a t i o n ( r e a c t i o n I ) . The c r a c k i n g and t h e i s o m e r i z a t i o n of i n t e r m e d i a t e o l e f i n s on a c i d s i t e s i n v o l v e carbenium i o n s formed by o l e f i n a d s o r p t i o n o n p r o t o n i c s i t e s ( F i g . 4 . ) . The r e a r r a n g e m e n t ( s t e p 2) and t h e c r a c k i n g ( s t e p s 3 , 3 ' ) of c a r b o c a t i o n s a r e t h e l i m i t i n g s t e p s of i s o m e r i z a t i o n and c r a c k i n g r e s p e c t i v e l y , s i n c e t h e c a r b o c a t i o n f o r m a t i o n and d e s o r p t i o n a r e v e r y rapid. It i s w i d e l y a d m i t t e d t h a t s k e l e t a l r e a r r a n g e m e n t s o f carbocat i o n s w i t h o u t change i n t h e c h a i n l e n g t h (termed t y p e A ( 2 2 ) ) proceed v i a a l k y l - s h i f t :

.

whereas r e a r r a n g e m e n t s w i t h change i n t h e c h a i n l e n g t h ( t y p e B) proceed v i a p r o t o n a t e d c y c l o p r o p a n e i n t e r m e d i a t e s (23,24) :

+ c-c-c-c-c-c

#%\

4

-

.

/H c-d----t-c-c

C I + c-c-c-c-c

T h i s p r o c e s s a v o i d s t h e f o r m a t i o n of t h e v e r y u n s t a b l e p r i m a r y c a r b o c a t i o n s which would b e i n v o l v e d i n a n a l k y l - s h i f t b r a n c h i n g mechanism :

P+

C-C-C-C-C-C

-P +

C-C-C-C-C

F o r t h e s a k e of c o n c i s e n e s s , t h e p r o t o n a t e d c y c l o p r o p a n e s w i l l be p i c t u r e d a s f a c e - p r o t o n a t e d c y c l o p r o p a n e s ( a ) . Such a n e n t i t y seems however t o have l i t t l e p h y s i c a l r e a l i t y compared t o a l k y l b r i d g e d (b) o r e d g e - p r o t o n a t e d c y c l o p r o p a n e s ( c ) (25) :

Fig. 4.

I s o m e r i z a t i o n and c r a c k i n g of o l e f i n s on a c i d c a t a l y s t s . 0 : o l e f i n ; C+ : c a r b o c a t i o n ; x,y : number of carbon atoms.

The 8 - s c i s s i o n of a c a r b o c a t i o n l e a d s t o an o l e f i n and a carbocation :

A primary c a r b o c a t i o n i s o b t a i n e d by @ - s c i s s i o n of a l i n e a r carbocation :

+Or\

C-C-C-C-C-C-C

+

--+C-C=C

4

C-C-C-C

Consequently, t h i s t y p e of s c i s s i o n ( s t e p 3 , F i g . 4 . i f 0, i s a l i n e a r o l e f i n ) w i l l be v e r y slow ; a l i n e a r c a r b o c a t i o n w l l l o n l y be isornerized ( t y p e B rearrangement) and t h e c r a c k i n g w i l l occur by s c i s s i o n of c a r b o c a t i o n s w i t h mono and multibranched s k e l e t o n s ( s t e p 3 ' ) , l e a d i n g t o secondary o r t e r t i a r y c a r b o c a t i o n s :

fl+

C-C-C-C-C-C I

C I C-C-C-C-C

D+

I

j

----t

+ c-c-C

+

C-C-C

1

+

C=C-C-C

+ C=C-C

This i s the r e a s o n why t h e c r a c k i n g of n-alkanes f o l l o w s t h e i r isomerizat ion.

2. ACTIVITY OF BIFUNCTIONAL ZEOLITE CATALYSTS

2.1

I n f l u e n c e of t h e hydrogenating a c t i v i t y

On pure a c i d z e o l i t e s , pentanes and hexanes isomerize and c r a c k whereas long-chain a l k a n e s undergo only c r a c k i n g . The deact i v a t i o n i s r a p i d a t low hydrogen p r e s s u r e b u t becomes very slow a t high p r e s s u r e (26). The i s o m e r i z a t i o n t o c r a c k i n g r a t e r a t i o depends n o t only on t h e z e o l i t e but a l s o on o p e r a t i n g c o n d i t i o n s (27) : t h u s , f o r n-hexane t r a n s f o r m a t i o n on H mordenite a t atmosp h e r i c p r e s s u r e , t h i s r a t i o i s p r a c t i c a l l y nu1 a t 4 0 0 " ~but i s equal t o 0.2 a t 2 5 0 " ~; i t i n c r e a s e s with c a t a l y s t d e a c t i v a t i o n by coke d e p o s i t (27,28) and with hydrogen p r e s s u r e (26). However the i n i t i a l i s o m e r i z a t i o n r a t e d e c r e a s e s when hydrogen p r e s s u r e increases (29). When a hydrogenating component i s added t o t h e z e o l i t e , i t s a c t i v i t y , i t s s t a b i l i t y and i t s i s o m e r i z a t ion s e l e c t i v i t y generally i n c r e a s e . Fig. 5. shows, f o r n-hexane i s o m e r i z a t i o n and cracking t h e changes of t h e a c t i v i t i e s of Pt-HY c a t a l y s t s (measured a f t e r an aging p e r i o d ) a s a f u n c t i o n of t h e i r m e t a l l i c s u r f a c e a r e a s (30). These c a t a l y s t s , with a platinum c o n t e n t ranging from 0 t o 17.7 wt%, were prepared by exchange of a s t a b i l i z e d Y z e o l i t e : t h e platinum c r y s t a l l i t e s were between 2 t o 5 nm depending on t h e samples. The i s o m e r i z a t i o n a c t i v i t y i n c r e a s e s very r a p i d l y a t low m e t a l l i c s u r f a c e a r e a s , then remains p r a c t i c a l l y c o n s t a n t above 0.5 m 2 g- 1 (Fig. 5 . ) . This type of curve, expected i n t h e c a s e of a bifunct i o n a l process ( s e e F i g . 2 . ) , was found f o r t h e same r e a c t i o n w i t h o t h e r c a t a l y s t s :Pt-Lay (31), Pt-H mordenite ( 3 2 ) . I t i s a l s o the c a s e f o r t h e i s o m e r i z a t i o n and t h e c r a c k i n g of long-chain n-alkanes (up t o nCI6) w i t h p h y s i c a l mixtures of mordenite and platinum depos i t e d on i n e r t alumina, provided t h a t t h e a c t i v i t i e s a f t e r aging be taken i n t o c o n s i d e r a t i o n ( 1 3) Both i s o m e r i z a t i o n and crackinga c t i v i t i e s per gram of mordenite f i r s t i n c r e a s e p r o p o r t i o n a l l y t o t h e hydrogenation a c t i v i t y and then r e a c h a p l a t e a u ( s e e a s an example F i g . 6 . f o r n-octane t r a n s f o r m a t i o n ) ; f o r a l l t h e a l k a n e s , aHI, t h e v a l u e of t h e hydrogenation a c t i v i t y required t o which i s o b t a i n t h e p l a t e a u f o r i s o m e r i z a t i o n i s g r e a t e r than a t h e value required t o obtain t h e f o r cracking 6.). I f one c o n s i d e r s t h e a c t i v i t i e s b e f o r e d e a c t i v a t i o n , t h e plateau i s reached f o r c r a c k i n g but n o t f o r i s o m e r i z a t i o n (27). The same o b s e r v a t i o n can be made f o r n-heptane t r a n s f o r m a t i o n s on mixtures w i t h Y z e o l i t e . On platinum-alumina, ZSM-5 m i x t u r e s , t h e plateau i s n o t o b t a i n e d e i t h e r f o r i s o m e r i z a t i o n nor f o r c r a c k i n g of n-heptane. This means t h a t t h e hydrogenation a c t i v i t y i s n o t suffic i e n t t o "balance" t h e high i n i t i a l a c i d i t y of t h e s e z e o l i t e s and ~ a r t i c u l a r l yt h a t of ZSM-5 z e o l i t e . The same c o n c l u s i o n was reached by J a c o b s e t a l . f o r n-decane t r a n s f o r m a t i o n on 0.5 w t W Pt-ZSM-5 ( 3 3 ) . P h y s i c a l mixtures of mordenite and NiMo s u l f i d e s deposited on

.

in

?gig.

250°C : 24 b a r PH2 p n-hexane : 6 b a r

*

Fig. 5. R a t e s of n-hexane i s o m e r i z a t i o n (I) and c r a c k i n g (C) a g a i n s t t h e metal s u r f a c e a r e a of t h e samples (Spt) (from r e f e r e n c e 30). alumina were a l s o used. I n n-decane t r a n s f o r m a t i o n a t 400°c under 30 bar hydrogen p r e s s u r e , a s y n e r g i s t i c e f f e c t was observed, which accounts f o r t h e b i f u n c t i o n a l c a t a l y s i s . However t h e poor hydrogenation a c t i v i t y of NiMo s u l f i d e s was n o t s u f f i c i e n t t o "balance" the high a c i d i t y of t h e mordenite and t h u s t h e i s o m e r i z a t i o n and cracking a c t i v i t i e s p e r gram of mordenite i n c r e a s e d w i t h t h e NiMo s u l f i d e c o n t e n t without r e a c h i n g a p l a t e a u (34) It can be noted t h a t on a l l t h e s e m i x t u r e s of a z e o l i t e and a hydrogenating compon e n t , the b i f u n c t i o n a l c h a r a c t e r o f i s o m e r i z a t i o n and c r a c k i n g i s c l e a r l y shown by t h e f a c t t h a t t h e i r a c t i v i t i e s ( c o n s i d e r e d b e f o r e o r a f t e r d e a c t i v a t i o n ) are much g r e a t e r t h a n t h e sum o f t h e a c t i v i t i e s of b o t h components.

.

F o r t h e i s o m e r i z a t i o n of C5-C6 a l k a n e s on n o b l e metal-loaded acid z e o l i t e s , v a r i o u s a u t h o r s c o n t e s t t h e e x i s t e n c e of a bifunct i o n a l p r o c e s s , They s u g g e s t t h a t t h e i n c r e a s e i n i s o m e r i z a t i o n a c t i v i t y caused by the i n t r o d u c t i o n of a hydrogenating component i n t h e mordenite would o n l y be due t o t h e i n c r e a s e i n t h e

A c t i v i t y per gram of mordenite -1 - 1 mo1e.h .g 350°C, 1 atm. H2 : hydrocarbon : 9.0

10,

.. n

-

I

U

I

I I

I I I

I I

C

I

a

I I

I I

I

a

HC

5

I

a~~

1

10

Benzene hydrogenation a c t i v i t y -1 - 1 mo1e.h .g Fig. 6.

n-Octane t r a n s f o r m a t i o n . E f f e c t of platinum-alumina c o n t e n t (numbers i n p a r e n t h e s e s ) o r hydrogenation a c t i v i t y on t h e hydroisomerizat ion ( I ) and hydrocracking (C) a c t i v i t i e s of p h y s i c a l l y mixed c a t a l y s t s (from r e f e r e n c e 13).

c a t a l y s t ' s s t a b i l i t y ; t h e o n l y r o l e of t h e hydrogenating component would be t o keep t h e s u r f a c e of the rnordenite f r e e of coke deposit. A c t u a l l y , when working under c o n d i t i o n s where t h e pure mordenite doqs n o t d e a c t i v a t e , i . e . under high hydrogen p r e s s u r e , t h e addition of t h e hydrogenating component has only a s l i g h t p o s i t i v e e f f e c t on t h e i s o m e r i z a t i o n a c t i v i t y . I n c e r t a i n c a s e s , even a decrease in t h i s a c t i v i t y h a s been observed (35). The p o s i t i v e e f f e c t i s also v e r y weak a t low hydrogen p r e s s u r e , when t h e i n i t i a l a c t i v i t i e s are taken i n t o c o n s i d e r a t i o n : Chicks e t a l . (28) found t h a t t h e i n c r e a s e i n i s o m e r i z a t i o n a c t i v i t y o c c u r s e s s e n t i a l l y a t t h e expense

of t h e c r a c k i n g a c t i v i t y ; they e x p l a i n t h e a c t i o n of t h e hydrogenating component by a m o d i f i c a t i o n i n t h e a c i d p r o p e r t i e s of t h e mordenite. For a l l t h e above a u t h o r s , i s o m e r i z a t i o n on b i f u n c t i o n a l mordenite c a t a l y s t s t h e r e f o r e occurs by t h e a c i d mechanism d e s c r i b e d in Fig. 3. However, a combination of a c i d and b i f u n c t i o n a l mechanism has a l s o been proposed (26). Again, i f the b i f u n c t i o n a l mechanism a l l o w s one t o e x p l a i n t h e cracking of long-chain a l k a n e s (with more than 6 carbon atoms), i t i s not t h e c a s e f o r t h e c r a c k i n g of l i g h t a l k a n e s (32,36). The cracking s t e p s of l i g h t carbenium i o n s involving u n s t a b l e carbocations a r e very slow. Thus t h e 6-cracking of a secondary c a r b o c a t i o n n ~ +always l e a d s t o a primary c a r b o c a t i o n : 6

+q

C-C-C-C-C-C

.-*

C-C=C

+ + C-C-C +

In the same way, t h e c r a c k i n g of t h e c a r b o c a t i o n s i C 6 w i t h a monobranched o r a bibranched s k e l e t o n i n v o l v e s , i n t h e most favorable c a s e , two secondary c a r b o c a t i o n s :

P+ >-

C-C-C-C-C

4-

C-C-C

+

C=C-C

and could be slow compared t o t h e isomer d e s o r p t i o n and t o t h e alkane s c i s s i o n on m e t a l l i c s i t e s (hydrogenolysis). It i s e f f e c t i vely what i s observed i n n-hexane c r a c k i n g on t h e s e r i e s of Pt-HY and Pt-H mordenite c a t a l y s t s ( 3 2 ) : t h e d i s t r i b u t i o n of t h e c r a c k i n g products i s t h e one expected from a simple t y p e hydrogenolysis r e a c t i o n , namely a n important formation of methane and ethane a s well a s a (C1 + C2)/(C,4 + C5) molar r a t i o of about 1 . Moreover, the g r e a t e r t h e metal a r e a t h e g r e a t e r t h e a c t i v i t y ( F i g . 5 . ) .

2.2

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

A s expected from a b i f u n c t i o n a l p r o c e s s ( a s well a s from a n acid-catalyzed r e a c t i o n ) , t h e i s o m e r i z a t i o n and c r a c k i n g r a t e s depend s t r o n g l y on t h e z e o l i t e a c i d i t y . Thus, t h e s m a l l e r t h e sodium c o n t e n t of t h e Y z e o l i t e , t h e g r e a t e r t h e a c t i v i t y f o r n-pentane i s o m e r i z a t i o n of Pd-HY c a t a l y s t s w i l l be ; t h e e f f e c t i s e s p e c i a l l y pronounced a t low sodium c o n t e n t s . Thus a d e c r e a s e from 0.27 t o 0.02 w t % Na20 enables a r e d u c t i o n of 50°c i n r e a c t i o n temp e r a t u r e f o r a 30 % n-pentane conversion (26).

The p a r t i c i p a t i o n of BrBnsted a c i d c e n t e r s i n n-decane hydrocracking i s demonstrated by t h e p o s i t i v e e f f e c t of water on t h e a c t i v i t y of a Pd-Re X c a t a l y s t . Indeed, t h i s p o s i t i v e e f f e c t can be a t t r i b u t e d t o t h e g e n e r a t i o n of p r o t o n i c s i t e s v i a r a r e e a r t h c a t i o n h y d r o l y s i s . On t h e o t h e r hand, w i t h platinum on a nons t a b i l i z e d HY z e o l i t e a n e g a t i v e e f f e c t due t o a d e c r e a s e of t h e

a c i d s t r e n g t h by h y d r a t i n g . protons i s observed (37). The e f f e c t of t h e Si02/A1203 r a t i o h a s a l s o been determined i n n-pentane i s o m e r i z a t i o n on b ~ f u n c t i o n a lmordenite c a t a l y s t s : a maximum a c t i v i t y was observed f o r a r a t i o of about 16 (26) ; t h i s optimum v a l u e would be t h e r e s u l t of two a n t a g o n i s t i c e f f e c t s of t h e dealumination : one p o s i t i v e , due t o t h e e l i m i n a t i o n of obstruct i o n s i n t h e mordenite channels, t h e o t h e r n e g a t i v e , due t o t h e d e c r e a s e i n t h e number of a c i d c e n t e r s ( 1 ) . Because t h e i s o m e r i z a t i o n a c t i v i t y depends so h i g h l y on Na removal, comparing z e o l i t e s with d i f f e r e n t s t r u c t u r e s i s d i f f i c u l t . However, t h e comparison between low sodium Pd-H mordenite and low sodium Pd-HY shows t h a t both m a t e r i a l s have about t h e same a c t i v i t y f o r n-hexane i s o m e r i z a t i o n (26)

.

2.3

I n f l u e n c e of poisons

The e f f e c t of s u l f u r poisoning was s t u d i e d on a Pd-Ca Y catal y s t by Rabo e t a l . (38) u s i n g an n-pentane feed i n which s u l f u r c o n c e n t r a t i o n s ( a s n-bu t y l mercaptan) were v a r i e d from zero t o 3000 ppm. S u l f u r c o n c e n t r a t i o n s of 6 pprn had no e f f e c t on c a t a l y s t a c t i v i t y ; t h i s o b s e r v a t i o n can be r e a d i l y explained by t h e bifunct i o n a l mechanism i f t h e hydrogenating a c t i v i t y of t h e c a t a l y s t remains high enough t o maintain a pseudo-equilibrium between p a r a f f i n s and o l e f i n s and t o keep a s l i m i t i n g s t e p t h e s k e l e t a l o l e f i n i s o m e r i z a t i o n on a c i d s i t e s . Above t h i s c o n c e n t r a t i o n , sulfur a c t e d a s a temporary poison : it decreased t h e i s o m e r i z a t i o n a c t i v i t y but t h i s a c t i v i t y was r e s t o r e d on r e v e r s i o n t o a c l e a n feed. M o d i f i c a t i o n s i n a c t i v i t y and i n s e l e c t i v i t y caused by poisoning with dimethyl d i s u l f i d e (220 ppm) and n-butylamine (800 ppm) of a Pt-HY z e o l i t e w i t h 6 w t X of platinum were a l s o those expected from t h e b i f u n c t i o n a l mechanism (30). Since s u l f u r poisons reduce t h e a c t i v e platinum a r e a , t h e poisoned c a t a l y s t w i l l a c t l i k e a c a t a l y s t ~ i t ah s m a l l e r platinum c o n t e n t . T h i s was a c t u a l l y observed : t h e poisoned c a t a l y s t had t h e i s o m e r i z a t i o n s e l e c t i v i t y and t h e a c t i v i t y of a c a t a l y s t with a 0.09 t o 0.5 w t % platinum c o n t e n t . A b a s i c poison reduces t h e number of t h e a c i d s i t e s ; s i n c e with t h e c a t a l y s t used, t h e l i m i t i n g s t e p was t h e s k e l e t a l o l e f i n i s o m e r i z a t i o n on a c i d s i t e s , poisoning by n-butylamine must provoke a decrease i n a c t i v i t y with no m o d i f i c a t i o n i n s e l e c t i v i t y . This i s e f f e c t i v e l y what was observed. The a d d i t i o n of d i m e t h y l d i s u l f i d e (1 %) t o n-decane caused a s i g n i f i c a n t decrease i n both t h e i s o m e r i z a t i o n ( I ) and cracking (C) a c t i v i t i e s of a mixture of H mordenite and platinum-alumina. As expected from t h e well known I / C decrease w i t h d e c r e a s i n g hydrog e n a t i o n a c t i v i t y , I w a s much more a f f e c t e d than C. By r e v e r s i o n t o pure n-decane, t h e c a t a l y s t recovered h a l f of i t s o r i g i n a l isomer i z a t i o n a c t i v i t y b u t o n l y 10 % of i t s o r i g i n a l c r a c k i n g a c t i v i t y and I / C was 4 . 5 times g r e a t e r than on t h e f r e s h c a t a l y s t . Since a poisoning of t h e metal should r e s u l t i n a lowering of I/C, i t would

appear t h a t t h e i r r e v e r s i b l e poisoning e f f e c t i s due t o a modification of t h e mordenite r a t h e r than t o a poisoning of t h e metal (39). C a t a l y s t d e a c t i v a t i o n and coke formation were examined on mixtures of platinum-alumina and z e o l i t e . Nearly a l l t h e coke was found t o be d e p o s i t e d on t h e z e o l i t e (14) ; t h e r e f o r e t h e d e a c t i vation can be a t t r i b u t e d t o a d e c r e a s e i n t h e a c i d i t y of t h e bifunct i o n a l c a t a l y s t . The d e c r e a s e i n c r a c k i n g a c t i v i t y a s a f u n c t i o n of time on stream was always g r e a t e r than t h e d e c r e a s e i n isomerization a c t i v i t y . To e x p l a i n t h i s phenomenon one could propose t h a t isomerization and c r a c k i n g occur on d i f f e r e n t c a t a l y t i c c e n t e r s , the cracking s i t e s being more r e a d i l y poisoned by coke than t h e isomerization s i t e s ( 14) However, t h i s phenomenon can a l s o be explained by t h e f a c t t h a t c r a c k i n g i s c o n s e c u t i v e t o i s o m e r i z a t i o n . Indeed t h e comparison of d e a c t i v a t e d c a t a l y s t s with f r e s h c a t a l y s t s shows t h a t poisoning by coke r e s u l t s i n a d e c r e a s e i n z e o l i t e concentration without any change i n t h e i s o m e r i z a t i o n t o c r a c k i n g r a t i o . The f a c t t h a t poisoning by coke does n o t cause any segregation among t h e c a t a l y t i c s i t e s i s i n f a v o r of a s i n g l e type of s i t e s for i s o m e r i z a t i o n and c r a c k i n g (40). Moreover, i t was found t h a t t h e amount of alkane transformed into coke during one experiment was p r o p o r t i o n a l t o t h e t o t a l amount of cracked a l k a n e whereas no c l e a r r e l a t i o n s h i p e x i s t e d between coking and i s o m e r i z a t i o n a c t i v i t i e s . This i n d i c a t e s t h a t coke formation probably r e s u l t s from a secondary t r a n s f o r m a t i o n of o l e f i n s produced by t h e c r a c k i n g r e a c t i o n (40,41).

.

3. SELECTIVITY OF BIFUNCTIONAL ZEOLITE CATALYSTS

3.1

n-Hexane i s o m e r i z a t i o n

3.1.1 I n f l u e n c e of hydrogenating a c t i v i t y . The s e l e c t i v i t y of n-hexane t r a n s f o r m a t i o n on pure z e o l i t e s has been d e s c r i b e d by numerous a u t h o r s . The k i n e t i c model r e p r e s e n t e d i n Fig. 7. allows u s t o account f o r t h e n-hexane i s d m e r i z a t i o n on H mordenite o r on HY z e o l i t e ( 3 2 , 3 5 ) : n-hexane (nC6) l e a d s d i r e c t l y t o a thermodynamic e q u i l i b r i u m mixture of methylpentanes (MP) and 2,3-dimethylv e r y slow "'6

7

2,2-DMB

.low\\

2-MP

F i g . 7.

.A .

K i n e t i c model of n-hexane

3-MP

i s o m e r i z a t i o n an a c i d z e o l i t e s .

b u t a n e (2,3-DMB) and t o a s m a l l q u a n t i t y o f 2 , 2 - d i m e t h y l b u t a n e (2,2-DMB). T h i s i s w e l l e x p l a i n e d by t h e mechanism i n F i g . 3 . i n which t h e l i m i t i n g s t e p i s s t e p 1 t h a t i s t h e f o r m a t i o n of second a r y nc6* c a r b o c a t i o n s . T h i s s t e p i s s l o w e r t h a n t h e c a r b o c a t i o n i s o m e r i z a t i o n ( s t e p . 2 ) and s l o w e r t h a n t h e d e s o r p t i o n of MP o r 2,3-DMB ( s t e p 4 ) which i n v o l v e s t e r t i a r y c a r b o c a t i o n s . The 2,2-DMB f o r m a t i o n i s slow b e c a u s e t h e c a r b o c a t i o n s c o r r e s p o n d i n g t o t h i s a l k a n e are e i t h e r p r i m a r y o r s e c o n d a r y . 2,3-DMB f o r m a t i o n r a t e d e c r e a s e s when t h e m e t a l l i c s u r f a c e area o f Pt-HY o r Pt-H m o r d e n i t e c a t a l y s t s i n c r e a s e s (30,32) : t h u s , t h e 2,3-DMB c o n t e n t i n t h e m i x t u r e of MP and 2,3-DMB which i s c l o s e t o i t s thermodynamic v a l u e on p u r e m o r d e n i t e , d e c r e a s e s down t o 40 % of t h i s v a l u e f o r t h e h i g h p l a t i n u m c o n t e n t m o r d e n i t e c a t a l y s t s . The d e c r e a s e i s more pronounced i n t h e c a s e of Pt-HY c a t a l y s t s : t h e 2,3-DMB c o n t e n t d r o p s t o 10 Z of i t s thermodynamic v a l u e when t h e p l a t i n u m c o n t e n t i s e q u a l t o 0 . 5 %. T h i s s e l e c t i v i t y i s between t h e s e l e c t i v i t y e x p e c t e d o f a n a c i d mechanism and t h a t of a bifunct i o n a l p r o c e s s i n which t h e l i m i t i n g s t e p i s t h e i s o m e r i z a t i o n of t h e i n t e r m e d i a t e o l e f i n s ( s t e p 3 i n Fig. 1 . ) . Indeed, w i t h t h i s b i f u n c t i o n a l p r o c e s s , t h e m e t h y l p e n t a n e s must b e t h e o n l y primary p r o d u c t s o f n-hexane i s o m e r i z a t i o n : t h e m e t h y l p e n t e n e s formed by n-hexene r e a r r a n g e m e n t s on t h e a c i d s i t e s a r e immediately hydrogen a t e d and c a n n o t i s o m e r i z e i n t o d i m e t h y l b u t e n a s . To e x p l a i n t h e d i r e c t f o r m a t i o n o f 2,3-DMB, a t l e a s t two p r o p o s a l s c a n be made (32) : - a n a c i d and a b i f u n c t i o n a l mechanism would b o t h p a r t i c i p a t e s i m u l t a n e o u s l y i n t h e i s o m e r i z a t i o n . On t h e p o r t i o n o f t h e c a t a l y s t c a r r y i n g p l a t i n u m c r y s t a l l i t e s , nC6 would i s o m e r i z e i n t o MP t h r o u g h t h e b i f u n c t i o n a l mechanism. MP would r a p i d l y i s o m e r i z e i n t o 2,3-DMB o n a c i d s i t e s d i s t a n t from p l a t i n u m c r y s t a l l i t e s . T h i s would imply t h a t t h e p a r t of c a t a l y s t w i t h p l a t i n u m and t h a t w i t h o u t p l a t i n u m work i n d e p e n d e n t l y . - a more p r o b a b l e e x p l a n a t i o n i s t h a t t h e m i g r a t i o n o f t h e i n t e r m e d i a t e o l e f i n s from one m e t a l s i t e t o a n o t h e r m e t a l s i t e i s s l o w e r t h a n t h e i r i s o m e r i z a t i o n on a c i d s i t e s . I n t h i s c a s e , n-hexenes have t h e p o s s i b i l i t y of r e a c t i n g s u c c e s s i v e l y on s e v e r a l a c i d s i t e s b e f o r e b e i n g hydrogenated and t h e r e f o r e 2,3-DMB w i l l a p p e a r as a p r i m a r y p r o d u c t o f nC6 i s o m e r i z a t i o n . The l a r g e s t direct 2,3-DMB f o r m a t i o n on Pt-H m o r d e n i t e would be d u e i) t o t h e g r e a t e r r e a c t i v i t y o f o l e f i n s on t h e s t r o n g e r a c i d s i t e s o f m o r d e n i t e i i ) a n d / o r t o t h e d i f f u s i o n a l l i m i t a t i o n s i n t h e one-dimensional p o r o u s s t r u c t u r e of m o r d e n i t e which a r e g r e a t e r t h a n i n t h e threed i m e n s i o n a l s t r u c t u r e of Y z e o l i t e ( 3 2 ) . 3.1.2 I n f l u e n c e of t h e z e o l i t e p o r o u s s t r u c t u r e . On ZSM-5 bifunct i o n a l c a t a l y s t s , t h e d i r e c t f o r m a t i o n o f 2,3-DMB i s much l e s s pronounced t h a n on Y b i f u n c t i o n a l c a t a l y s t s and t h e f o r m a t i o n of 2,2-DMB d o e s n o t o c c u r ( 2 7 ) . T h i s c a n b e e x p l a i n e d by l i m i t a t i o n s t o t h e d i f f u s i o n of t h e o l e f i n s w i t h a 2 , 3 - o r e s p e c i a l l y w i t h a 2,2-DMB s k e l e t o n . Moreover, t h e f o r m a t i o n of 2-MP i s more favoured

with ZSM-5 b i f u n c t i o n a l c a t a l y s t s than w i t h Y c a t a l y s t s . A p o s s i b l e explanation f o r t h i s i s t h a t t h e e.nvironment of t h e ZSM-5 a c i d s i t e s i s such t h a t it p r i v i l e g e s s c i s s i o n a of t h e protonated cyclopropane i n t e r m e d i a t e i n comparison t o s c i s s i o n b :

C

b

L

a

I +

C-C-C-C-C

c-6--2-c-c

c-c-c-c-c On b i f u n c t i o n a l c a t a l y s t s with a small pore s i z e z e o l i t e a s acid component ( e r i o n i t e ...), n-alkanes o n l y undergo c r a c k i n g ; the formation of branched a l k a n e s does n o t o c c u r . Indeed, o n l y l i n e a r compounds can r e a c h t h e a c i d s i t e s and be desorbed from t h e porous s t r u c t u r e . This c h a r a c t e r i s t i c i s used t o e l i m i n a t e l i n e a r alkanes (which have a low o c t a n e number) from naphthas without transforming t h e o t h e r components, e s p e c i a l l y t h e branched a l k a n e s (selec t o £orming)

.

3.1.3 I n f l u e n c e of r e a c t i o n temperature. On a 0 . 6 w t % Pt-H mordenite i t has been shown t h a t temperature i s a determining f a c t o r i n t h e i s o m e r i z a t i o n s e l e c t i v i t y (42) : - a t 250°c, 2,3-DMB c o n t e n t i n t h e mixture of MP and 2,3-DMB formed by n-hexane i s o m e r i z a t i o n i s v e r y c l o s e t o i t s thermodynamic value a s was t h e c a s e w i t h pure z e o l i t e s . - a t 400°c, 2,3-DMB formation i s v e r y slow and t h e s e l e c t i v i t y i s p r a c t i c a l l y t h a t expected from t h e conventional b i f u n c t i o n a l mechanism. I n conclusion, t h e conventional b i f u n c t i o n a l mechanism o r t h e acid mechanism o r t h e i r combination a l l o w one t o e x p l a i n t h e s e l e c t i v i t y of n-hexane i s o m e r i z a t i o n on b i f u n c t i o n a l z e o l i t e c a t a l y s t s . However, it can be noted t h a t a n o t h e r mechanism, invoking cyclohexane-type bimolecular i n t e r m e d i a t e s , was proposed by Bolton and Lanewala t o e x p l a i n t h e p a r t i c u l a r s e l e c t i v i t y of a Pt-Re NH4 Y c a t a l y s t (43).

3.2

Long-chain a l k a n e i s o m e r i z a t i o n and c r a c k i n g

3.2.1 " I d e a l b i f u n c t i o n a l c a t a l y s i s . A d e t a i l e d a n a l y s i s of t h e hydroisomerization and hydrocracking of long-chain a l k a n e s has been reported by Weitkamp (36,44-49). The r e a c t i o n s were c a r r i e d o u t on a 0.5 w t Z Pt-Ca Y c a t a l y s t a t temperatures ranging from 200 t o 300°c, w i t h a t o t a l p r e s s u r e of about 40 b a r and an H2/alkane r a t i o of about 20. I n t h e s e c o n d i t i o n s , i s o m e r i z a t i o n and cracking occur by t h e conventional b i f u n c t i o n a l process i n which t h e l i m i t i n g steps a r e t h e rearrangement o r t h e c r a c k i n g of carbenium i o n

intermediates ; according to Weitkamp's terminology the bifunctional catalysis is then "ideal" (44). Various conversion rates were obtained by varying the contact time or more generally the reaction temperature. As an example, the change of the isomerization and cracking conversion of n-tridecane versus the reaction temperature is shown in Fig. 8. With all alkanes the hydroisomerization is the only reaction observed at low conversion (up to 40 % for n-tridecane, Fig. 8.). The hydroisomerization conversion passes through a maximum which is due to the consumption of branched isomers by hydrocracking. The isomer distribution is markedly dependent on the conversion rate. The branching occurs by consecutive reactions ( 4 9 ) : at low conversion monobranched alkanes (methyl and also ethyl, propyl ...) are formed almost exclusively (at least 95 %) ; by increasing the conversion rate, the content of bibranched alkanes increases. The distribution of monobranched isomers was examined in detail ( 4 9 ) . At low conversion the 2-methyl isomers were generally formed at a lower rate than the 3-methyl isomers. The branching mechanism via

0

200

220

240

260

React ion temperature Fig. 8.

(OC)

Hydroisomerization (I) and hydrocracking (C) of n-tridecane on Pt-Ca Y zeolite (from reference 4 9 ) .

protonated cyclopropanes (type A rearrangement) can explain this observation. However, it does not explain the formation, observed even at low conversion rate, of ethyl, propyl monobranched isomers. To account for their formation, type A rearrangements of methyl branched carbocations, which are known to occur faster than type B rearrangements ( 2 4 , 4 7 ) were proposed (49). Thus the formation of ethylpentane from n-heptane should involve the following carbocation rearrangements :

...

+

CH3-CHCH2-CH CH CH CH

2SW

very fast

-

+ CH3 I CH3-CH-CH-CH2-CH2-CH3

slow

/CH3

CH CH+ 13 I CH~-CH~-CH-~H-CH~-CH~ --,CH~-CH~-CH-CH~-CH~

U

Even at high conversion rates (up to 70-90 2 ) the cracking product distribution was fully symmetrical and the sum of fragments amounted to 200 moles per 100 moles of alkane cracked, indicating pure primary cracking. Methane and ethane were not formed, which rules out hydrogenolysis on platinum. There was a significant amount of branched alkanes (essentially monobranched) in the cracking product ( 3 6 ) . The iso to normal alkane ratio was generally higher than its thermodynamic value which demonstrates that the branched alkanes are primary products of the cracking reaction and do not result from a secondary isomerization of normal alkanes (50). The cracking product distribution is actually the one expected from the cracking of monomethylbranched tertiary carbocations provided that one assumes that these cations are equally reactive and that their relative concentrations are represented by the relative concentrations of methyl isomers ( 3 6 ) . However this cracking reaction requires the highly endothermic (55-60 kcal/mole) 6-scission of a tertiary cation to form a primary cation (22).

ex. :

Such a r e a c t i o n i s most u n l i k e l y . Yet, 6 - s c i s s i o n could be concert e d with hydride s h i f t s o t h a t a secondary c a r b o c a t i o n would be produced d i r e c t l y (22) :

I n any c a s e , t h i s mechanism cannot e x p l a i n t h e high i s o t o normal r a t i o i n t h e cracked p r o d u c t s , p a r t i c u l a r l y i n butanes ( 5 0 ) , and t h e absence of c r a c k i n g up t o a 40 % i s o m e r i z a t i o n of methylnonanes ( 4 7 ) . To e x p l a i n t h i s , c r a c k i n g of c a r b o c a t i o n s w i t h a bibranched s k e l e t o n was proposed (50,51). This r o u t e i n v o l v i n g secondary and t e r t i a r y carbocations i s e n e r g e t i c a l l y favorable :

I n c o n c l u s i o n , it can be s a i d t h a t n-alkane t r a n s f o r m a t i o n by " i d e a l " b i f u n c t i o n a l c a t a l y s i s occurs v i a t h e s u c c e s s i v e r e a c t i o n s shown i n t h e rake-type scheme of F i g . 9 . The t r a n s f o r m a t i o n s of adsorbed c a r b o c a t i o n s a r e t h e l i m i t i n g s t e p s , t h e i r d e s o r p t i o n into alkanes v i a t h e o l e f i n s being very r a p i d . Consequently, monobranched isomers can be o b t a i n e d with a high s e l e c t i v i t y , p r a c t i c a l l y i n

~rh

IA

II

1 1 fast

I I I t fast

f!

br

,

I

IA

I

1fast

*!

e mb~+

n ~ + s 1ow

In

1A I I t I fast I I VI

I1

l

vi

fast

1~++110

I C + + ~ 1O'

dbc*

s 1ow "'OW

slow Y

O

W

coke

-Fig. 9. "Ideal" b i f u n c t i o n a l t r a n s f o r m a t i o n of an n-alkane. P: a l k a n e ; 0 : o l e f i n ; c+: c a r b o c a t i o n ; mb: monobranched; db: dibranched ; 1 and 11: l i g h t product from primary and secondary c r a c k i n g .

thermodynamic e q u i l i b r i u m w i t h t h e n-alkane ; w i t h a l o n g e r c o n t a c t time, t h e thermodynamic mixture of mono and bibranched a l k a n e s could be s e l e c t i v e l y formed ; with an even longer c o n t a c t time primary cracking products would be o b t a i n e d . Secondary c r a c k i n g and coking r e a c t i o n s w i l l occur only f o r extremely long c o n t a c t time. To obtain " i d e a l " b i f u n c t i o n a l c a t a l y s i s , t h e b i f u n c t i o n a l c a t a l y s t must be such t h a t a l l t h e s t e p s involved i n t h e a l k a n e d e s o r p t i o n from c a r b o c a t i o n s be v e r y r a p i d i . e . a) t h e d e s o r p t i o n of o l e f i n s from c a r b o c a t i o n s , b ) t h e d i f f u s i o n of o l e f i n s from t h e a c i d t o t h e m e t a l l i c s i t e s , C ) t h e o l e f i n hydrogenation on t h e m e t a l l i c s i t e s . 3,2.2 Deviations from t h e " i d e a l " b i f u n c t i o n a l c a t a l y s i s . S t e p a i s commonly considered t o be more r a p i d than t h e c a r b o c a t i o n rearrangement o r i t s c l e a v a g e , b u t s t e p s b and c can become t h e l i m i t i n g s t e p s of t h e a l k a n e t r a n s f o r m a t i o n . This i s t h e c a s e f o r step b when t h e d i s t a n c e between hydrogenating s i t e s , i . e . t h e d i f f u s i o n a l p a t h of i n t e r m e d i a t e o l e f i n s , i s t o o long and consequently comprises too many a c t i v e a c i d s i t e s ; t h e r e f o r e , o l e f i n s with a monobranched s k e l e t o n formed on an a c t i v e a c i d s i t e , can adsorb on o t h e r a c i d s i t e s and s u c c e s s i v e l y l e a d t o o l e f i n s w i t h a dibranched s k e l e t o n , c r a c k i n g products and coke. Under t h e s e conditions, dibranched a l k a n e isomers and c r a c k i n g p r o d u c t s w i l l b e primary products of n-alkane t r a n s f o r m a t i o n . The same behaviour w i l l be observed i f t h e o l e f i n hydrogenation i s t h e l i m i t i n g s t e p . This occurs when t h e hydrogenating a c t i v i t y i s too small i n r e l a t i o n t o the a c t i v i t y of t h e a c i d s i t e s . Then b i f u n c t i o n a l c a t a l y s i s w i l l be "ideal" only i f t h e c a t a l y s t s have many h i g h l y a c t i v e hydrogenating s i t e s w e l l d i s p e r s e d among t h e a c i d s i t e s . I n t h e l i m i t c a s e t h e d i f f u s i o n a l p a t h of t h e i n t e r m e d i a t e o l e f i n (between two hydrogenating a c t i v e s i t e s ) w i l l comprise o n l y one a c i d s i t e of such s t r e n g t h t h a t it w i l l allow o n l y one o l e f i n t r a n s f o r m a t i o n d u r i n g one sojourn.

As a l r e a d y noted, dibranched isomers a r e formed a s primary products of n-hexane i s o m e r i z a t i o n on Pt-HY and above a l l on Pt-H mordenite c a t a l y s t s . D i f f u s i o n a l l i m i t a t i o n s i n t h e o l e f i n migrat i o n have been invoked t o e x p l a i n t h e non-"ideal" behaviour of these c a t a l y s t s . I n t h e same way dibranched isomers b u t a l s o cracking products a r e found a s primary products of t h e n-heptane transformation on p h y s i c a l mixtures of platinum-alumina and Y o r ZSM-5 z e o l i t e s ( 4 0 ) . The i s o m e r i z a t i o n / c r a c k i n g r a t e r a t i o (I/C) increases and t h e percentage of bibranched alkanes d e c r e a s e s when the platinum-alumina c o n t e n t i n c r e a s e s . However, even with a platinum-alumina c o n t e n t equal t o 90 Z, t h e d i s t a n c e between hydrogenating s i t e s i s n o t small enough and t h e hydrogenating a c t i v i t y high enough t o o b t a i n a s e l e c t i v e t r a n s f o r m a t i o n of n-heptane i n t o i t s monobranched isomers. D i f f u s i o n a l l i m i t a t i o n s i n t h e narrow porous s t r u c t u r e of t h e ZSM-5 z e o l i t e account f o r t h e v e r y low value of I / C found w i t h t h e platinum-alumina, ZSM-5 mixtures ( 4 0 ) .

On 0.5 w t 2 Pt-ZSM-5 c a t a l y s t a l m o s t no h y d r o i s o m e r i z a t i o n of n-decane i s found (52) ; moreover a s e c o n d a r y c r a c k i n g i s observed a t r e l a t i v e l y low c o n v e r s i o n r a t e s (E 2 0 %). T h i s c a n b e due t o an "imbalance" of t h e a c i d and t h e h y d r o g e n a t i n g f u n c t i o n s ( t h e hydrog e n a t i n g a c t i v i t y would b e t o o s m a l l t o c o u n t e r b a l a n c e t h e h i g h a c t i v i t y of t h e v e r y s t r o n g ZSM-5 a c i d s i t e s ) a s w e l l as t o c o n f i g u r a t i o n a l l i m i t a t i o n s . Another e x p l a n a t i o n would b e t h a t t h e s t r e n g t h o f ZSM-5 a c i d s i t e s would b e s u c h t h a t s e v e r a l c o n s e c u t i v e t r a n s f o r m a t i o n s c o u l d o c c u r d u r i n g t h e same s o j o u r n of o l e f i n s on a z e o l i t e site. 3.2.3 I n f l u e n c e o f t h e z e o l i t e porous s t r u c t u r e . The i s o m e r i z a t i o n and c r a c k i n g s e l e c t i v i t i e s o f b i f u n c t i o n a l c a t a l y s t s w i t h Y o r with ZSM-5 z e o l i t e were s i g n i f i c a n t l y d i f f e r e n t ( 4 0 , 5 2 , 5 3 ) . Thus i n n-heptane i s o m e r i z a t i o n , t h e 2-methylhexane/3-methylhexane molar r a t i o i s h i g h e r w i t h ZSM-5 t h a n w i t h Y c a t a l y s t s and 2,3- and 2,4d i m e t h y l p e n t a n e s a r e formed i n d e f i n i t e l y s m a l l e r q u a n t i t i e s ; t h e 2,2- and 3 , 3 - d i m e t h y l p e n t a n e s were formed w i t h Y b u t n o t w i t h ZSM-5 c a t a l y s t s . These s e l e c ? i v i t i e s a r e v e r y similar t o t h o s e observed i n n-hexane i s o m e r i z a t i o n and t h e r e f o r e c a n b e e x p l a i n e d i n t h e same way. On a l l t h e c a t a l y s t s , c r a c k i n g g i v e s m a i n l y propane and b u t a n e i n a p p r o x i m a t e l y e q u i m o l a r amounts. C u r i o u s l y , t h e i s o / n b u t a n e m o l a r r a t i o i s h i g h e r w i t h ZSM-5 t h a n w i t h Y c a t a l y s t s whereas i t was much smaller i n n-octane and n-decane c r a c k i n g . S i n c e t h e f o r m a t i o n of b i b r a n c h e d h e p t a n e s i s v e r y u n f a v o u r e d on ZSM-5 c a t a l y s t s , t h e f o l l o w i n g c r a c k i n g r e a c t i o n i s proposed t o explain t h e high iso/n-butane r a t i o :

However t h e a u t h o r s (40) do n o t e x c l u d e a n o t h e r p o s s i b i l i t y : t h e p r e f e r e n t i a l c r a c k i n g of carbenium i o n s w i t h a 2,4- o r e s p e c i a l l y w i t h a 2 , 2 - d i m e t h y l p e n t a n e s k e l e t o n , whose d e s o r p t i o n s from t h e ZSM-5 p o r o u s s t r u c t u r e a r e i n h i b i t e d . 6 - s c i s s i o n of t e r t i a r y monobranched d e c y l c a t i o n s and even of l i n e a r s e c o n d a r y d e c y l c a t i o n s w e r e f i r s t proposed by J a c o b s e t a l . (52) t o e x p l a i n t h e p r o d u c t s of n-decane c r a c k i n g on Pt-ZSM-5. However, i n a more r e c e n t p u b l i c a t i o n (53) a n o t h e r p r o p o s a l was p r e f e r r e d : c r a c k i n g would imply two r e a c t i o n s : i ) b i b r a n c h e d carbenium i o n c r a c k i n g i i ) monobranched s e c o n d a r y carbenium i o n c r a c k i n g producing s e c o n d a r y carbenium i o n s s u c h as

/L+ --+C-C-C +

C-C-C-C-R

I

+

C=C-R

The d i s t r i b u t i o n of methylnonanes formed from n-decane i s o m e r i z a t i o n on b i f u n c t i o n a l z e o l i t e c a t a l y s t s depends v e r y much on t h e porous s t r u c t u r e o f t h e z e o l i t e (53). With Y z e o l i t e , t h e d i s t r i bution a p p r o a c h e s thermodynamic e q u i l i b r i u m a t h i g h c o n v e r s i o n whereas w i t h ZSM-5 z e o l i t e , 2-methylnonane i s much f a v o u r e d . Curiously, w i t h ZSM-11, a n o t h e r p e n t a s i l z e o l i t e , a d i f f e r e n t rnethylnonane d i s t r i b u t i o n i s o b t a i n e d . These d i f f e r e n c e s between ZSM-5 and ZSM-11 would b e mainly t h e r e s u l t s of t r a n s i t i o n - s t a t e shape s e l e c t i v i t y (53).

CONCLUSION The a c t i v i t y , t h e s t a b i l i t y and t h e s e l e c t i v i t y of b i f u n c t i o n a l z e o l i t e c a t a l y s t s are c l e a r l y governed by t h e c h a r a c t e r i s t i c s o f t h e i r a c i d and h y d r o g e n a t i n g s i t e s . A s was shown f o r a l k a n e isomer i z a t i o n and c r a c k i n g , t h e h i g h e s t a c t i v i t y would b e o b t a i n e d i f a l l t h e a c i d s i t e s were v e r y a c t i v e and working a t t h e i r maximum. For t h i s t o o c c u r , t h e a c i d s i t e s must be s u f f i c i e n t l y s u p p l i e d with o l e f i n i c i n t e r m e d i a t e s which r e q u i r e s numerous and w e l l d i s t r i buted a c t i v e h y d r o g e n a t i n g s i r e s . I n t h e " i d e a l " c a s e , t h e d i f f u s i o n a l p a t h o f o l e f i n s (between two h y d r o g e n a t i n g s i t e s ) w i l l c o n t a i n o n l y o n e a c t i v e a c i d s i t e . Here t h e c a t a l y s t w i l l a l s o b e the most s e l e c t i v e one i n t h e s e r i e s of c o n s e c u t i v e r e a c t i o n s : thus, when t r a n s f o r m i n g n - a l k a n e s , i t w i l l g i v e t h e b e s t y i e l d of monobranched i s o m e r s and t h e most s e l e c t i v e i s o m e r i z a t i o n . On s u c h a c a t a l y s t , c o k e f o r m a t i o n w i l l be v e r y slow and c o n s e q u e n t l y t h e s t a b i l i t y very g r e a t . Z e o l i t e s a r e p e r f e c t l y adapted t o t h e prep a r a t i o n of " i d e a l " b i f u n c t i o n a l c a t a l y s t s : t h e i r a c i d s i t e s a r e numerous and h i g h l y a c t i v e ; moreover, a h i g h d i s p e r s i o n and a h i g h a c t i v i t y of h y d r o g e n a t i n g s i t e s c a n b e o b t a i n e d when i n t r o d u c i n g t h e m e t a l by a n ion-exchange p r o c e s s .

ACKNOWLEDGMENTS The a u t h o r s t h a n k t h e f o l l o w i n g p u b l i s h e r s f o r h a v i n g r e l e a s e d t h e i r c o p y r i g h t s on t h e f o l l o w i n g f i g u r e s . Academic P r e s s ( F i g . 5 . ) , Heyden ( F i g . 6 . ) , American Chemical S o c i e t y ( F i g . 8. )

.

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PART IV

INDUSTRIAL APPLICATIONS

DESIGN ASPECTS OF CATALYTIC REACTORS AND ADSORBERS

A.Rodri~ues,C.Costa,R.Ferreira,J.Loureiro

and S.Azevedo

Department of Chemical Engineering University of Porto 4099 Porto Codex, Portuaal

This l e c t u r e i s intended t o provide the fundamentals f o r the desion of c a t a l y t i c r e a c t o r s and adsorbers , t h a t i s , the equi pment necessary f o r the "mise-en-oeuvre" of z e o l i t e s as c a t a l y s t s and adsorbents. Industrial applications of z e o l i t e s have been l i s t e d by several authors (Breck I 1 ,Lee I2 1 ,Hei neman 13 J ,Meuqel ) 4 l , e t c . ) ; Tab1 e s I and I1 summarize c a t a l y t i c and adsorption processes , r e s p e c t i v e l y , which use zeol i t e s . A r e a c t o r ( o r adsorber) i s an ensemble of p a r t i c l e s ( c a t a l y s t or adsorbent); i t i s then worthwhile t o look f i r s t a t the s t r u c t u r e of the p a r t i c l e and the mechanisms of mass ( h e a t ) t r a n s f e r a t the p a r t i c l e 1eve1 .

A z e o l i t e p e l l e t can be considered 181 as havinq two pore systems : mi cropore o r adsorbi na pores of zeol i t e c r y s t a l s and transport p o r e s , i . e . , i n t e r s t i c e s between contactinq c r y s t a l s . This biporous s t r u c t u r e i s sketched in Fiaure 1' and i s s i m i l a r , f o r mathematical purposes , t o o t h e r bidisperse materials ,such as macror e t i c u l a r r e s i n s ,a1 though in zeol i t e s micropores have a very uniform size in the ranqe of 3 t o 10 anqstroms.

For pel l e t s of diameter dp=0.16 cm i n packed beds with porosity 0.32 some typical values f o r desion a r e : i n t r a c r y s t a l voids i n t e r c r y s t a l voids

0.191 0.227

TABLE I - Some c a t a l y t i c processes u s i n g z e o l i t e s (3,41

- Cracking (10-40% r a r e e a r t h exchanged H-Y Z e o l i t e dispersed i n a m a t r i x o f s i l i c a alumina o r c l a y )

- Hydrocracking ( l a r g e pore Y z e o l i t e s w i t h 0.5% P t )

-

I s o m e r i z a t i o n o f Cg and C6 p a r a f f i n i c hydrocarbons ( l a r g e pore mordenite w i t h Pd)

- Shape s e l e c t i v e h y d r o c r a c k i ng

-

I s o m e r i z a t i o n o f aromatics Methanol t o g a s o l i n e (ZSM-5)

TABLE II - Some a d s o r p t i o n processes u s i n g zeol it e s 14 Separations o f

-

n - p a r a f f i n s from i s o - p a r a f f i n s (Flolex 15 ; N - I s E l f

aromatics ,e.g. ,p-xylene from a m i x t u r e o f xylene isomers and e t h y l benzene (Parex ( 5 1 )

- o l e f i n s from p a r a f f i n s

-

a i r 17)

Epuration o f

-

s y n t h e s i s gas streams c o n t a i n i n g HC1 ,SOx ,NOx

(Pura-Siv-N)

steam-cracker e f f l u e n t c o n t a i n i n g C02

- LPG (sweetening) Dehydration o f

-

c r a c k i n g gases f o r e t h y l e n e p r o d u c t i o n olefins

- hydrogen

-

16))

solvents

micro sphere pore particle

Figure 1

-

Bidisperse s t r u c t u r e of a z e o l i t e p e l l e t

i n t e r p e l l e t voids 0.32 s o l i d portion of c r y s t a l s 0.199 sol i d i or ti on of binder 0.063 pa ( p e l l e t ) 1.12 g/cm3 Pb 0.76 g/cm30 iZp (40-260 O C ) 0.22 c a l / g C c r y s t a l s i z e of the order of urn Either mass t r a n s f e r through t r a n s p o r t pores o r d i f f u s i o n i n the zeol it e c r y s t a l s can be the control 1ing r a t e process. Garg and Ruthven 191 s t a t e t h a t the r e l a t i v e importance of macropore and micropore r e s i s t a n c e s can be measured by the parameter R ,

where w i s the volume f r a c t i o n of the p e l l e t occupied by z e o l i t e crystals i s the p e l l e t porosity,Dc and D a r e the d i f f u s i v i t i e s '&P P in the zeol i t e c r y s t a l s and i n macropores , r e ~ ~ e c t i v e and l ~ ,rc~ ~ are the r a d i i of p e l l e t and c r y s t a l ,respectively and dq/dc i s the slope of the adsorption equilibrium isotherm. If R < 1 micropore diffusion i s the c o n t r o l l i n p s t e p while f o r R > 100 macropore diffusion i s the l i m i t i n g one. Macropore d i f f u s i o n has been found ( l o ! t o be the control1 ing

mechanism f o r 1 i q u i d s i n m o l e c u l a r s i e v e p e l l e t s (Davison 525) ; f o r a t r a c e r C6H12 i n a f e e d c o n t a i n i n ? C6H6,Lee and Ruthven f o u n d D = 0 . 2 9 ~ 1 0 - ~cm2/s. P D i f f u s i o n measurements i n z e o l i t e s have been made by numerous workers; Ma and Lee I11 I r e p o r t e d v a l u e s f o r d i f f u s i o n of n-butane i n X z e o l i t e p e l l e t s o f R = 0.23 cm and r c = 1 . 0 8 ~ 1 O - ~ c mequal t o D

D = 0.046 cm2/s and Dc= 5: 1x10-" cm2/s. Ruthven and Doetsch 112 1 P d e a l i n g w i t h n-C7H16,C6H12 and C6H5CH3 i n 13X z e o l i t e found d i f f u s i v i t i e s i n t h e range 1 0 - ~ - 1 0 - ~ c m ~ w / sh i l e Doe1 l e and R i e k e r t 1131 f o r t h e system n-butane/NaX z e o l i t e o b t a i n e d a d i f f u s i v i t y o f 2x10-'cm2/s a t 300 K ( l a r g e c r y s t a l z e o l i t e w i t h dc= 80 um so ~ ~ ~ 8 s ) . Kumar e t a1 ( 1 4 1 u s i n g chromatographic t e c h n i q u e s s t u d i e d d i f f u s i o n o f i-C4H10 i n 4A z e o l i t e o b t a i n i n g D = 0.021 cm2/s ( m o l e c u l e t o o

P

l a r g e t o p e n e t r a t e t h e z e o l i t e c r y s t a l ) w h i l e f o r He i n N2 c a r r i e r t h e y o b t a i n e d Dc= 1 .7x1 0-" cm2/s. F o r 4A z e o l it e p e l 1e t s (R = 0.39cm; P r c = 1.74 urn) t h e y a l s o r e p o r t e d r e c i p r o c a l v a l u e s o f t h e d i f f u s i o n t i m e c o n s t a n t s ( l / ~ ~a t ) 306 K f o r CH4,Ar and C 0 , r e s p e c t i v e l y 4 ~ 1 0 - ~ s ,10-2s-1and -l 1.7~10-~s-'. The s u b j e c t o f d i f f u s i o n i n z e o l i t e s i s a v e r y e x c i t i n g one since,as p o i n t e d o u t b y Weisz 1151 , " t h e f i e l d o f shape s e l e c t i v e c a t a l y s i s r e 1 i e s on t h e d i f f e r e n t d i f f u s i v i t i e s o f molecules through spaces o f n e a r - m o l e c u l a r dimensions". C l a s s i c d i f f u s i o n regimes are we1 1 known : - Knudsen d i f f u s i o n ,when p o r e s i z e < m o l e c u l a r mean f r e e path and t h u s d i f f u s i v i t y a ( p o r e d i a m e t e r ) - '

-

o r d i n a r y d i f f u s i o n , w h e n p o r e s i z e > m o l e c u l a r mean f r e e path and t h e n d i f f u s i v i t y = ( c o n s t r i c t i o n f a c t o r ) x ( o r d i n ary diffusion coefficient)

Now a new regime appears c a l l e d t h e " c o n f i g u r a t i o n a l regime" where t h e d i f f u s i o n i s a f f e c t e d b y t h e s i z e and c o n f i g u r a t i o n o f t h e molecules ( F i g u r e 2 ) . I n view o f t h e b i d i s p e r s e n a t u r e o f t h e s e c a t a l y s t / a d s o r b e n t s , mass c o n s e r v a t i o n e q u a t i o n s f o r a volume element o f t h e p e l l e t should be c o r r e c t l y f o r m u l a t e d and as a r e s u l t t h e concept o f c a t a l y s t e f f e c t i v e n e s s f a c t o r s h o u l d be extended. L e t us r e c a l l t h e d e f i n i t i o n o f c a t a l y s t e f f e c t i v e n e s s f a c t o r I f (cs,TS) a r e t h e concenf o r a homogeneous p a r t i c l e o f volume V P' t r a t i o n and temperature a t t h e s u r f a c e t h e e f f e c t i v e n e s s factor,^,

I

0

I

CONFIGURATIONAL

A

(pore

Firlure 2

-

SIZL)

D i f f u s i o n a i regimes ( a f t e r Weisz 115 1 )

i s d e f i n e d as t h e r a t i o between t h e observed r a t e , r o b s = and t h e i n t r i n s i c r a t e ,rint(cs

1 /I/

r(V)dV V~ VP ,TS) c a l c u l a t e d a t t h e s u r f a c e c o n d i -

t i o n s . F o r i r r e v e r s i b l e nth o r d e r r e a c t i o n t h e s t e a d y s t a t e mass balance i n t h e p a r t i c l e volume element,in i s o t h e r m a l o p e r a t i o n , i s :

where f = c / c s i s t h e reduced r e a c t a n t c o n c e n t r a t i o n i n s i d e t h e c a t a l y s t , x = z/R i s t h e reduced space c o o r d i n a t e f o r t h e c a t a l y s t , f o r c y l i n d e r , 3 f o r sphere) and

a i s t h e shape f a c t o r ( 1 f o r slab,2

4 = L!

i s t h e T h i e l e modulus ( k

- characteristic

dimension:

h a l f - t h i c k e n e s s o f t h e s l a b o r p a r t i c l e r a d i u s ; De- e f f e c t i v e d i f f u s i v i ty; k - k i n e t i c c o n s t a n t ) . The c o n c e n t r a t i o n p r o f i l e i s e a s i l y o b t a i n e d f o r f i r s t o r d e r r e a c t i o n s i f we t a k e i n t o account t h e boundary c o n d i t i o n s ,x = 1 ,f = 1 and x = 0 , d f / d x = 0;the e f f e c t i v e n e s s f a c t o r IT i s then:

slab

'1 = 7 th$

c y l in d e r

q=Tm

; diffusional

regime ($ h i g h ) q

1

=

2 ' 11 ( 4 )

3

(rn

3

sphere

1

u=T

- 1

;

w i t h 11 , I o m o d i f i e d Bessel f u n c t i o n s o f f i r s t k i n d , f i r s t orders respectively.

and z e r o

F o r z e r o o r d e r r e a c t i o n s c o n c e n t r a t i o n p r o f i 1es w i l l r e a c h , i n some cases,zero v a l u e s a t a p o i n t x* i n s i d e t h e c a t a l y s t where a l s o d f / d x = 0. S o l u t i o n o f t h e model e q u a t i o n s w i l l l e a d t o t h e e f f e c t i v e n e s s f a c t o r 11 = 1- x * ~shown i n F i g u r e 3. T h i e l e modulus v a l u e s below which t h e c a t a l y s t i s o p e r a t i n g i n t h e chemical regime a r e d 2 a w h i l e t h e a s y m p t o t i c e x p r e s s i o n o f t h e effectiveness f a c t o r f o r high $ , i s simply a /TI$. The i m p o r t a n c e o f t h e e f f e c t i v e n e s s determines t h e r e a l q u a n t i t y o f c a t a l y s t a c e r t a i n degree o f c o n v e r s i o n ;moreover c a t a l y s t can be c r u c i a l when l o o k i n g f o r

f a c t o r i s obvious s i n c e i t t o be used i n o r d e r t o reach t h e w o r k i n g regime o f t h e hiqh s e l e c t i v i t i e s .

F o r b i d i s p e r s e c a t a l y s t s Ihm e t a1 1161 proposed an o v e r a l l e f f e c t i v e n e s s f a c t o r which i s c a l c u l a t e d f r o m t h e knowledge o f m i c r o and m a c r o e f f e c t i v e n e s s f a c t o r s . T h e s e a u t h o r s c o n s i d e r e d a p e l l e t as made o f microspheres w i t h pores among them ; moreover a f r a c t i o n y o f t h e a c t i v e s i t e s i s a t t h e m i c r o s p h e r e s u r f a c e o r pore walls.

inlinite cylinder

I

Figure 3

-

I

I

Effectiveness factor reactions

.

T-I

versus $ f o r z e r o o r d e r

~

The governing steady s t a t e equations a r e then: pore space

R J ~ C : - ~ / ( V ~ D , ~ ,) fa= c a / c s P a pellet.

w i t h 4-,

, xa= R /

R

P

and Va- volume o f

mi crosphere

with $i=r c

kc,n-1 /(nlViDi)

, f.=ci/cS, 1

xi=r/rC,

Vi

-

volume

o f a microsphere and n ' - number o f microspheres i n t h e p e l l e t . I t i s obvious t h a t t h e e f f e c t i v e n e s s f a c t o r f o r a microsphere

is d i f f u s i o n a l f l u x a t the m i c r o p a r t i c l e surface 'i' i n t r i n s i c r a t e i n t h e microsphere a t t h e s u r f a c e c o n d i t i o n s

The average m i c r o e f f e c t i v e n e s s f a c t o r Ti i s d e f i n e d as t h e r a t i o between the d i f f u s i o n f l u x i n t o a l l t h e microspheres o f t h e p e l l e t and t h e i n t r i n s i c r a t e i n a l l t h e microspheres a t t h e surface concentration, c s ; then

On t h e o t h e r hand t h e o v e r a l l e f f e c t i v e n e s s f a c t o r , no,,

i s the

r a t i o between t h e d i f f u s i o n a l f l u x a t t h e p e l l e t s u r f a c e , x a = l , and t h e i n t r i n s i c r a t e i n t h e whole p e l l e t , i . e . , pore space and microcs; then spheres , supposed a t t h e s u r f a c e c o n d i t i o n s ~,

I f we i n t r o d u c e na d e f i n e d a s :

'a=

diffusional f l u x a t the p e l l e t surface [intrinsic rate i n \ (diffusional f l u x i n a l l ) t h e pores a t t h e ( + microspheres w i t h s u r concentration cs j face c o n c e n t r a t i o n cs

I

1

we can show t h a t

and f i n a l l y

Equation ( 9 ( c o n t a i n s some 1i m i t i n ? cases: a) Reaction takes p l a c e o n l y i n t h e mi crospheres ;y = 0

-

1 9a I

vov= nina b ) Reaction o n l y i n t h e pore w a l l s ; y = 1

I n t h e case o f zero-order r e a c t i o n t h e treatment becomes more complex s i n c e t h e c o n c e n t r a t i o n p r o f i1es , f o r t h e macrosphere and/or t h e microsphere can reach zero values a t c e r t a i n r a d i a l p o s i t i o n s . I n F i g u r e 4 e f f e c t i v e n e s s f a c t o r s nayqov and qi are p l o t t e d as f u n c t i o n s o f @i

f o r second o r d e r r e a c t i o n a t g i v e n values o f 4,

and y

2. FUNDAMENTALS OF CATALYTIC REACTION ENGINEERING

I n t h i s s e c t i o n we w i l l discuss some aspects o f design and o p e r a t i o n o f c a t a l y t i c r e a c t o r s F i r s t o f a l l i t should be stressed

.

Figure 4 - nov, n a , T~ versus @ i .

that the choice of the type of r e a c t o r i s the f i r s t s t e p t o be done; then preliminary estimates of concentration and temperature g r a d i e n t s , b o t h i n the film and i n s i d e the p a r t i c l e , a r e needed in order t o guide the use of a s u i t a b l e model. For homogeneous c a t a l y s t s we have ( 17 1 :

-

external concentration gradient , Ace=

C ~ b - -C ~ ~robsR

-Ca=

C ~ b

f Ab

internal concentration gradient , E i = - = 1 - Ca C ~ b

maximum internal temperature r i s e ,AT;=

-

maximum external temperature r i s e , AT:= -

(-AH)DecAb Bi,

-

'eTb

Bib

Tmax-T~= B= ( -AH 1DecAs s 'eTs Ts , r n a ~ - ~=b gb = Tb

These c a l c u l a t i o n s a r e u s e f u l p a r t i c u l a r l y when we a r e i n v o l v e d w i t h t h e d e t e r m i n a t i o n o f r e a c t i o n k i n e t i c s i n t h e l a b o r a t o r y and we t r y t o a v o i d f a l s i f i c a t i o n o f k i n e t i c s due t o d i f f u s i o n i n t r u s i o n . 2.1

-

T u b u l a r F i x e d Bed C a t a l y t i c Reactors (TFBCR)

H i s t o r i c a l l y t h i s i s one o f t h e o l d e s t arrangements f o r conduct i n g gas-sol i d o r 1 i q u i d - s o l i d chemical r e a c t i o n s on an i n d u s t r i a l scale

.

A TFBCR i s s i m p l y an assembly o f u n i f o r m l y s i z e d p a r t i c l e s ( v i z . t h e c a t a l y s t ) whtch a r e randomly a r r a n g e d and which a r e h e l d f i r m l y i n p o s i t i o n w i t h i n a tube o r pipe ( v i z . t h e r e a c t o r ) . Intimate c o n t a c t i s achieved between t h e p a r t i c l e s and t h e r e a c t a n t f l u i d as t h e 1a t t e r f l o w s i n a random manner between, around and, i n t h e case o f porous c a t a l y s t s , i n t o t h e p a r t i c l e s .

Examples o f f i x e d bed c a t a l y t i c processes employed i n b a s i c chemical i n d u s t r i e s a r e steam r e f o r m i n g , carbon monoxide o x i d a t i o n and methanation, t h e s y n t h e s i s o f ammonia, s u l p h u r i c a c i d and methanol C a t a l y t i c r e f o r m i n g , is o m e r i z a t i o n and p o l y m e r i z a t i o n processes a r e u t i l ised i n t h e p e t r o l e u m r e f i n i n g i n d u s t r y . The o x i d a t i o n o f o l e f i n s and a r o m a t i c s , t h e s y n t h e s i s o f v i n y l a c e t a t e , t h e p r o d u c t i o n o f s t y r e n e and o t h e r dehydrogenation processes a r e examples i n t h e p e t r o c h e m i c a l i n d u s t r y .

.

2.1.1 - T h e o r e t i c a l Aspects on M o d e l l i n g . The g e n e r a l problem may be p l a c e d i n p e r s p e c t i v e b y v i e w i n g t h e r e a c t o r i n terms o f l o n g and s h o r t range g r a d i e n t s a l o n g t h e s p a c i a l c o o r d i n a t e s . G r a d i e n t s of s p e c i e s c o n c e n t r a t i o n s and f l u i d - s o l i d t e m p e r a t u r e s which p r e v a i 1 t h r o u g h o u t t h e g e o m e t r i c c o n f i n e s o f t h e r e a c t o r a r e termed i n t e r p a r t i c u l a t e . Those which may p e r s i s t i n t h e f l u i d i m m e d i a t e l y s u r r o u n d i n g t h e c a t a l y s t p a r t i c l e s a r e d e f i n e d as i n t e r p h a s e g r a d i e n t s w h i l s t t h o s e w i t h i n t h e s o l i d porous c a t a l y t i c phase a r e defined as i n t r a p h a s e g r a d i e n t s . The t r a n s p o r t c h a r a c t e r i s t i c s i n s i d e t h e p a r t i c l e a r e d i s t i n c t f r o m t h o s e o u t s i d e and c o n s e q u e n t l y a r e a l i s t i c model f o r a TFBCR s h o u l d c o n s i d e r t h e s i n g l e p a r t i c l e case and t h e n c o n s t r u c t f r o m t h i s t h e o v e r a l l r e a c t o r model i n which t h e c o n s e r v a t i o n e q u a t i o n s i n c l u d e t r a n s f e r between t h e phases i n c o n t a c t .

A model r e s u l t i n g f r o m t h i s approach i s c l e a r l y a complex l e a r n i n g mode1 w i t h such c o m p u t a t i o n a l c o s t s as t o p r e c l u d e i t s use f o r o p t i m i s a t i o n and c o n t r o l purposes. Hence p r e d i c t i v e s i m p l i f i e d models a r e c o n t i n u a l l y sought b o t h a t p a r t i c l e and o v e r a l l r e a c t o r levels. The mathematical t r e a t m e n t o f t h e b e h a v i o r of porous c a t a l y s t

p e l l e t s r e s t s on r e l a t i o n s which l i n k the fluxes of the reacting species t o gradients i n composition, pressure and temperature. A variety of f l u x r e l a t i o n s of varying degrees of complexity i s available in the l i t e r a t u r e 1181. I t i s q u i t e apparent t h a t the point of i n t e r e s t l i e s i n the calculation of the global r a t e f o r a c a t a l y s t p a r t i c l e and where t h i s leads t o a r a t e of reaction per u n i t volume of r e a c t o r bed when the analysis i s extended t o the whole TFBCR. The method of expressing t h i s global r a t e i n a c a t a l y s t p e l l e t as i t s i n t r i n s i c r a t e under surface conditions multiplied by an effectiveness f a c t o r i s now well established. In the developments which follow i t i s assumed t h a t such global rates can be obtained in t h a t way. The focus of a t t e n t i o n will then be on t h e form of conservation models f o r the integral r e a c t o r , i . e . , on how they account f o r i n t e r p a r t i c l e gradients of species concentrations in t h e f l u i d , i n t e r p a r t i c l e gradients of f l u i d and s o l i d temperatures and a l s o f o r the interphase t r a n s p o r t of mass and heat. The more complex s t r u c t u r e s recognize the existence of both phases and form the group of the heterogeneous models. Resulting from the simp1 i f i c a t i o n of these l a t t e r an a1 t e r n a t i v e group of e s s e n t i a l l y predictive s t r u c t u r e s has emerged, v i z . the pseudo-homogeneous models. In these the assumptions a r e made t h a t the r e s i s t a n c e t o interphase t r a n s p o r t of mass and heat i s n e g l i g i b l e , t h a t the physical properties of the f l u i d vary only s l i g h t l y across a p a r t i c l e diameter and t h a t the f l u i d film around a c a t a l y s t part i c l e i s small - such t h a t each p a r t i c l e with i t s surrouding boundary i s regarded as a point source within a homogeneous f i e l d . The two most common forms of d e t e r m i n i s t i c description of the physical and chemical processes occurring in a TFBCR a r e the Fickian models ( a convenient general name f o r those models based on Fick's and F o u r i e r ' s laws f o r mass and thermal dispersion, r e s p e c t i v e l y ) and the c e l l models of Deans and Lapidus 1191 where i n t e r s t i c e s between packing elements a r e idealized as p e r f e c t l y s t i r r e d mixers which give r i s e t o dispersion-type behavior. The c l a s s i f i c a t i o n suggested by Froment 120 1 f o r Fickian models has been extended by Gros and Bugarel 121 1 t o include a l l models and i s presented in Table 111. One of the reasons f o r the wider use of Fickian models i s possibly the f a c t t h a t previous mass and heat t r a n s f e r experiments have been analysed almost exclusively on the basis of such models

Sr W

TABLE I 1 1 - C l a s s i f i c a t i o n o f F i c k i a n and C e l l Models f o r T u b u l a r F i x e d Bed C a t a l y t i c Reactors

-

F i c k i a n Models

Pseudo-homogeneous

C e l l Models

Heterogeneous

Pseudo-homogeneous

Characteristic

Code

Characteristic

Code

Plug f l o w

PHI

PHl +i nterphase transport Hl+intrapartic l e transport Hl+axial dispersion HIltaxial dispersion

HI

Hl + r a d i a1 dispersion

H3

HIl+radial dispersion

HI3

H2+radi a1 dispersion

H4

HI2+radial dispersion

HI4

Onedimensional PHl + a x i a l dispersion

PHl + r a d i a1 dispersion

pH2

PH3

Twodimensional PH2+radi a1 dispersion

PH4

10.

Heterogeneous

Characteristic

Code

Characteristic

Code

Cells i n series

C2

C2+i nterphase transport

CH2

Two-dimensional C4 arrays o f c e l l s

C4+interphase transport

CH4

HI1 H2 HI2

and consequently a 1arge amount o f compatible parameter values a r e available. The o t h e r reason i s concerned w i t h numerical procedure. The c e l l model was o r i g i n a l l y i n t r o d u c e d when i t appeared t h a t t h e s o l u t i o n o f l a r g e number o f a l g e b r a i c equations f o r t h e s t e a d y - s t a t e o r o r d i n a r y d i f f e r e n t i a l equations f o r t h e unsteady-state was simpl e r than t h e s o l u t i o n o f t h e p a r t i a l d i f f e r e n t i a l equations r e q u i r e d by the d i s p e r s i o n model s. Indeed, even today, t h e simpl e imp1 ementation o f methods o f s o l u t i o n o f non-1 i n e a r a l g e b r a i c equations a r e g e n e r a l l y more cumbersome. During t h e p a s t t h r e e years some works have been p u b l i s h e d which q u e s t i o n t h e v a l i d i t y o f t h e F i c k i a n approach 122,23 1 . I t i s indeed apparent t h a t a t p r e s e n t no second-order continuous model s a t i s f i e s a l l t h e requirements which t h e experimental and t h e o r e t i c a l a n a l y s i s i n packed beds suggest.

A t t h e p r e s e n t s t a t e o f t h e a r t , however, t h e F i c k i a n models are a f e a s i b l e and g e n e r a l l y s a t i s f a c t o r y approach f o r design purposes.

-

F i c k i a n Model A n a l y s i s . I n t h i s s e c t i o n a heterogeneous, a 2.1.2 h y bsr i d- a models f o r t h e s t e a d y - s t a t e o f TFBCRs w i l l be examined. The aim i s t o develop e s s e n t i a l l y p r e d i c t i v e s t r u c t u r e s f o r design purposes. I t i s obvious from t h e l i t e r a t u r e t h a t r a d i a l d i s p e r s i o n o f heat and mass i n c o o l e d - w a l l r e a c t o r s i s o f c o n s i d e r a b l e s i g n i f i c a n c e and t h e r e f o r e has t o be i n c l u d e d throughout.

C r i t e r i a which may be a p p l i e d t o determine t h e e x t e n t o f t h e i n f l u e n c e o f thermal and mass a x i a l m i x i n g (Young and F i n l a y s o n 1241, Mears 125) ) show t h a t i n general such i n f l u e n c e i s minimal f o r c o n d i t i o n s o f i n d u s t r i a l p r a c t i c e ( f l o w r e l a t i o n s and l e n g t h / p a r t i c l e diameter r a t i o ) . B u t f o r a d i a b a t i c regimes such a x i a l mixing phenomena a r e u s u a l l y n e g l e c t e d as t h e survey o f t h i r t y - t w o experimental works presented by Feyo de Azevedo 126 1 c o n f irms

.

F o l l o w i n g t h e c l a s s i f i c a t i o n o f Table 111 t h e models t o be developed a r e then o f t h e types H3 o r HI3 and PH3. The Heterogeneous Model The c o n t i n u i t y equations f o r t h e key r e a c t i n g component A and the energy equations a r e seen t o c o n s t i t u t e a s e t o f p a r a b o l i c p a r t i a l d i f f e r e n t i a l equations coup1 ed w i t h a non-1 i n e a r a1 g e b r a i c equation, v i z .

F l u i d Phase (C,T w i t h o u t s u b s c r i p t )

S o l i d Phase ( s u b s c r i p t p f o r p e l l e t v a r i a b l e )

The i n i t i a l and boundary c o n d i t i o n s a r e f o r zl=O ;

C= Co and T = T o ( r l ) 3T

f o r O O , the wave i s dispersive in nature; in the p a r t i c u l a r case of an isotherm with constant separation f a t t o r K 1 , t h e f i r s t t r a n s i t i o n i s gradual w h i l s t

< 1 and E34= h34/h31 1 1 a r e a b r u p t . R23" h 24/ h 21

Location o f the t r a n s i t i o n s :

c ) D i f f u s e boundary 1.2:

- For t h e s i d e o f zone ( 1 )

-

For t h e s i d e o f zone ( 2 )

Times a t which t r a n s i t i o n s a r e l o c a t e d ( o u t l e t o f t h e bed): a ) t r a n s i t i o n 1.2 side (1)

ul.l'=

side (2)

0.393 cm/min

t1

.2 = 1.678 cm/min

b ) t r a n s i t i o n 2.3

u

c ) t r a n s i t i o n 3.4

u

2.3 3.4

I

t 2 ,s 2 = 23.24 min

=1.688cm/min

t2.3=23.11 min

= 2.434 cmlmin

t 3 . 4 16.03 = min

C a l c u l a t i o n o f compositions (mole f r a c t i o n s

-

znd

= 99.34 m i n

-

Eq. 1361)

plateau

(h12-a11)(h22-a11)(h32-a11) X12 =

("12-

= 0.4729

all)

and then ( f r o m Eq. 1381 ) : c12 = 0.5293 moel /!?, f o r components 2 and 3:

bed = 1.2603 mole/!?, s o l u t i o n xZ2 = 0 ; cZ2 = 0 X32 = 0 ; C 3 2 = 0

T h i s i s a v e r y small p l a t e a u s i n c e we a r e near a watershed p o i n t ( h 2 4 = 3 . 0 4 2 7 ~h 1 4 = a 1 2 = 3 ) .

- srd p l a t e a u x1 = 0.2049

c13 = 0.2294 mole/!?, bed = 0.5461 m o l e l a s o l u t i o n

0 x33 = 0.2442

cz3 = 0 = 0.2532 mole/!?, bed= 0.6028 mole/!?, s o l u t i o n C33

X23 =

For t h e d i f f u s e boundary, we have f o r components 1 and 2:

when xkk = y k k = 0 and hkl = alk t h e species k appears i n t h e d i f f u s e boundary; then:

Xk

=

( u k / ~ k ) 1 / 2- 9 k

X

hk,k+l - al k Yk =

k ,k + l

( ~ ~ / u - ~Clkl) ~ / ~ Yk ,k+l l/hk,k+l - "kl

t h i s i s t h e case f o r s p e c i e s 1 i n o u r case; so we use Eq. lB1 1 :

TI= l / u l -

x1 = 0.0276

t = 82.37 m i n

T1 = 3.5

x1

0.0792

t =61.77 m i n

T1 = 2

x1 = 0.1811

t = 41.18 m i n

T =1

x1 = 0.3540

t = 27.46 m i n

Tl = .6962

x1 = 0.4729

t = 23.24 m i n

= 5

-

-1

=

F o r s p e c i e s 2 we use Eq. lA1 ( ; i n t h e same p o i n t s we o b t a i n : -

TI = 5

x2 = 0.4293

t = 82.37 m i n

3.5

0.3796

61.77

2

0.281 3

41.18

1

0.1146

27.46

0.6926

0

23.24

I n F i g u r e 9 we compare t h e s e p r e d i c t i o n s w i t h t h e experimental and c a l c u l a t e d r e s u l t s o b t a i n e d by S a n t a c e s a r i a e t a1 1471. 3.2

-

Non I s o t h e r m a l A d s o r ~ t i o n

The b a s i c t h e o r e t i c a l framework f o r t h e u n d e r s t a n d i n g o f non i s o t h e r m a l , and p a r t i c u l a r l y , a d i a b a t i c a d s o r p t i o n has been developed b y Rhee e t a1 149,50,51,521, Pan and Basmadjian 153,54,551 and Basmadjian 3 a1 156,57,58,59 1 f o r s i n g l e and mu1 ticomponent cases. A r e v i e w o f t h e a r e a has been p r e s e n t e d by Sweed 1601. P r a c t i c a l systems have been c o n s i d e r e d i n d r y i n g 161,62,63 1 and f o r t h e s e p a r a t i o n o f gas m i x t u r e s o f n-pentane f r o m iso-pentane b y m o d u l a t i o n o f f e e d temperature 164,651. Methods f o r t h e obtention o f a d s o r p t i o n e q u i l i b r i u m d a t a 166,67.( and k i n e t i c laws 168,69,70( have been implemented, s p e c i a l l y u s i n g s i n g l e p a r t i c l e technique and m i c r o b a l a n c e apparatus (71,72,73,741 ; we s h o u l d emphasize t h e work done by Ma and h i s qroup on ZSM-5 z e o l i t e and s i l i c a l i t e . M o d e l l i n g o f such processes i s o b j e c t o f a number o f papers 175,76, 771 u s i n g a staged approach and s i m p l i f i e d h y p o t h e s i s . A mechanistic

Figure 9

-

Breakthrough curves f o r case 2, Experimental p o i n t s ( 4 7 ( : ( 0 ) m-xylene; ( A ) p-xylene; ( 0 ) toluene. equilibrium C a l c u l a t e d curves: ( . . ) from 1471 ; (-) t h e o r y ( t h i s work).

.

explanation of nonisothermal s o r p t i o n has been r e c e n t l y p u b l i s h e d by H e l f f e r i c h 1781. 3.2.1 - Non i s o t h e r m a l s o r p t i o n i n a p e r f e c t l y mixed sorber. Let us consider t h e system sketched i n F i g u r e 10, where T i s t h e temperature, F t h e t o t a l mass f l u x (Fi = U p . ; i f o r a c t i v e species,

I f o r i n e r t s ) , V t h e m i x t u r e volume and

1-

4 = UA

(To - T) t h e h e a t f l u x

removed by t h e c o o l i n g medium. A f t e r i n t r o d u c i n g dimensionless q u a n t i t i e s , mass and h e a t balances t a k e t h e form 1791 :

F i g u r e 10 with

X i =

pi/"

-

, yi

The nonisothermal p e r f e c t l y mixed sorber

=

qi/Q

-

, T = T/To , e = t/tst.

The model parameters are: 5,

, mass

B

,

capacity f a c t o r

thermicity factor

5 = - 1-E Q m E PO

( - A H ) Po

B=

a

, heat

t r a n s f e r number

a = Uv A

;,

When developing t h e model equations one should note t h a t : a ) s p e c i f i c e n t h a l p y a t temperature T f o r species i hi (T) = hio

+ cpi (T - To)

b ) s p e c i f i c e n t h a l p y o f t h e adsorbent

c ) s p e c i f i c e n t h a l p y o f adsorbed species i h?(T) = hio

+

cpi

( T - To)

+

AH

where AH < 0 (-AH i s t h e h e a t o f a d s o r p t i o n i n c a l / g ) The assumptions made were: (rp i n cal K - ~ )

(f,, i i i ) vscpi i v ) cpi

i n cal s-l

1

K-I

qi nd

introducing

x = c/c 0 ,

( c o i s t h e maximum s o l u t e c o n c e n t r a t i o n i n t h e

f l u i d phase, f o r a s t e p change a t t h e i n l e t i n t h e case o f a d s o r p t i o n alone, o r f o r an impulse i n t h e case o f r e a c t i o n a l o n e ) we g e t 1841 :

with

K = 1 + K' c0 Em = ( 1 -E)~'/(EC') N r = ( 1 - ~ ) k - r ( 6c ) n-1 /E

For a s t e p change i n c o n c e n t r a t i o n we o b t a i n : i ) zero order reaction

i i ) f i r s t order reaction

For a Dirac impulse we o b t a i n : i ) zero order reaction

i i ) f i r s t order reaction

with x A ( o t ) defined by equation 1621 :

and shown in Figure 17. In Fiqure 18 we show the response of a p e r f e c t l y mixed tank t o s t e p and Dirac changes i n concentration f o r zero order reaction and Langmuir adsorption. Figure 19 shows t h e response of a p e r f e c t l y mixed adsorptive reactor f o r f i r s t order reaction. The determination of parameters can be e a s i l y made f o r f i r s t order reactions: a ) From a t r a c e r experiment we g e t E . b ) By comparing impulsional responses without and with reaction t h e corresponding we see t h a t a t a given o u t l e t concentration, cA1, .. times a r e , r e s p e c t i v e l y , el and (l+Nr)B1 from which Nr can be obtained; a l s o from the area under the curve in the presence of reaction, we o e t area = c O / ( l + ~ , ) . c ) From the impulsional response and taking i n t o account t h a t

Figure 17

- xA(ot)

versus 6 and K.

F i g u r e 18

-

Response o f a p e r f e c t l y mixed a d s o r p t i v e r e a c t o r t o : a ) s t e p chanae i n i n l e t c o n c e n t r a t i o n ; b ) D i r a c impulse. ( z e r o o r d e r r e a c t i o n , 5=2, K=5)

F i g u r e 19

-

Response of a p e r f e c t l y mixed a d s o r p t i v e r e a c t o r t o : a ) s t e p change i n i n l e t c o n c e n t r a t i o n ; b ) D i r a c impulse. ( f i r s t o r d e r r e a c t i o n , 5=2, K=10)

we g e t f o r each c A isotherm.

t h e c o r r e s p o n d i n g f ( c A ) and t h e n t h e e q u i l i b r i u m

I t i s a l s o i n t e r e s t i n g t o n o t i c e t h a t , i n t h e case o f D i r a c response, t h e ratios(Rln) between t h e areas under t h e responses i n

t h e case o f r e a c t i o n a l o n e and a d s o r p t i o n a l o n e a r e : area under t h e response f o r r e a c t i o n (order n) area un e r t e response f o r a d s o r p t i o n

zero order reaction, n = 0

f i r s t order reaction, n = 1

I

R1l = l-Tq

second o r d e r r e a c t i o n , n = 2 On t h e o t h e r hand, t h e r a t i o s (R2n) between t h e areas under t h e D i r a c responses f o r r e a c t i o n coupled w i t h l i n e a r a d s o r p t i o n and a d s o r p t i o n a1 one a r e : area under t h e D i r a c response f o r r e a c t i o n + t l i n e a r adsorption

1

area under t h e D i r a c response f o r a d s o r p t i o n

f i r s t order reaction, n = 1

second o r d e r r e a c t i o n , n = 2

I

R21 =

Giq

1+E Nr R22 = - l n ( 1 t-)1tc r

I t can be seen t h a t we o b t a i n R2n f r o m Rnl

g i v i n g Rln

we r e p l a c e Nr by

The r a t i o s

i f i n t h e expressions

Nr(l+E)l-n.

i2n f o r t h e case

o f Langmuir t y p e a d s o r p t i o n

isotherms are:

T h i s k i n d o f a n a l y s i s can be u s e f u l when c a r r y i n g o u t experiments i n m i c r o r e a c t o r s i n view o f t h e d e t e r m i n a t i o n o f k i n e t i c parameters.

NOMENCLATURE e x t e r n a l p a r t i c l e s u r f a c e area p e r u n i t r e a c t o r volume

a, A

r e a c t i o n ( a d s o r p t i o n ) component

b

r a t i o between t h e mass and t h e heat B i o t numbers

Bi

B i o t number

Bif

f l u i d / w a l l B i o t number

Bi

s o l i d / w a l l B i o t number

P

Bih

h e a t B i o t number

Bim

mass B i o t number

Bo

Bodenstein number

c

specificheat

P

ci Ci Cpi cs dp

c o n c e n t r a t i o n o f component i i n t h e f l u i d phase c o n c e n t r a t i o n o f component i c o n c e n t r a t i o n o f component i i n s i d e t h e p e l l e t reactant concentration a t the surface o f the p e l l e t p a r t i c l e diameter

D

r e a c t o r diameter, d i f f u s i v i t y

Da

Damkdhler number

Dc

d i f f u s i v i t y i n zeolite crystlas

De

effective d i f f u s i v ity

D

d i f f u s i v i t y i n macropores

P Dr e

emissivity

E

a c t i v a t i o n energy

f

reduced r e a c t a n t c o n c e n t r a t i o n i n s i d e t h e c a t a l y s t ( c / c s )

F

massflux

h

i n t e r p o l a t e d h e a t t r a n s f e r c o f f i c i e n t f o r f i l m surrounding a particle

hc

convection h e a t t r a n s f e r c o e f f i c i e n t f o r f i l m surrounding a particle

r a d i a l e f f e c t i v e mass d i f f u s i v i t y

hc,hr,hp hr

c o n v e c t i v e , r a d i a t i v e and c o n d u c t i v e c o n t r i b u t i o n s f o r e a t t r a n s f e r c o e f f i c i e n t f o r f i l m surrounding a p a r t i c l e

r a d i a t i o n c o n t r i b u t i o n f o r the f i l m heat t r a n s f e r c o e f f i c i e n t

r a d i a t i v e contribution t o the apparent wall heat t r a n s f e r coeffi c i en t apparent wall heat t r a n s f e r c o e f f i c i e n t hw hwf wall heat transfer c o e f f i c i e n t f o r the f l u i d wall heat t r a n s f e r c o e f f i c i e n t f o r the s o l i d phase hwD ( - A H ) heat of reaction (adsorption) I o , I 1 modified Bessel functions of the f i r s t kind, zero and f i r s t orders respectively k k i n e t i c constant gas phase mass t r a n s p o r t c o e f f i c i e n t r e f e r r e d t o u n i t i n t e r kc f a c i a l area kf film mass t r a n s f e r c o e f f i c i e n t k molecular thermal conductivity of the f l u i d 9 pel l e t e f f e c t i v e thermal conductivity P kr radial e f f e c t i v e thermal conductivity krad r a d i a t i v e contribution t o the s t a t i c radial thermal conductivity

brad

radial thermal conductivity of the f l u i d defined by equation 114rl radial thermal conductivity of the sol i d k r ~ K constant separation f a c t o r f o r adsorption isotherm L r e a c t o r (column) length n' number of microspheres in the pel l e t Nr reaction number Ns interphase heat t r a n s f e r group Nu f l uid/sol i d Nussel t number fp NUwff l uidlwall Nussel t number P pressure f Peha f l u i d phase axial Peclet number Peh, radial Peclet number f o r heat t r a n s f e r peLr radial f l u i d Peclet number f o r heat t r a n s f e r Pemr radial Peclet number f o r mass t r a n s f e r Pehz,Pema-axial Peclet number f o r mass t r a n s f e r Per ( m ) tlirbul e n t 1imi t Pecl e t number q adsorbed sol i d concentration krf k0r

adsorbed s o l i d concentration in equilibrium with co r dimension1 e s s radial coordinate r' radial coordinate i n t r i n s i c r a t e of disappearance of component A per u n i t r A vol ume of pel 1e t rc zeol i t e c r y s t a l radius R reactor radius Re p a r t i c l e Reynolds number c a t a l y s t pel l e t radius R~ s Laplace transform parameter Sc Schmidt number t time breakthrough time t~~ tst stoechiometric time T temperature To reference temperature T temperature inside the p e l l e t P temperature a t the surface of the p e l l e t Ts T wall temperature -wT , T reduced temperature u s u p e r f i c i a l gas velocity u 1. i n t e r s t i c i a l velocity U flowrate U overall heat t r a n s f e r c o e f f i c i e n t v r e a c t o r (adsorber) volume V V p e l l e t volume P' a Vi volume of a microsphere x reduced f l u i d phase concentration X r a d i a l l y averaged r a t i o , C/Co

Q

Greek l e t t e r s a a

6

-

B y

y E

E

n

P

shape f a c t o r heat t r a n s f e r number Prater thermici t y f a c t o r P r a t e r thermicity f a c t o r in the bulk conditions f r a c t i o n of a c t i v e s i t e s a t the microsphere surface o r a t pore wall s Arrehnius number (-AH/RTo) r e a c t o r (adsorber) voidage p e l l e t porosity effectiveness f a c t o r

e f f e c t i v e n e s s f a c t o r , defined by equation 181 effectiveness f a c t o r f o r a microsphere T I average microeffectiveness f a c t o r qOv overall effectiveness f a c t o r 8 dimensionless time A, e f f e c t i v e thermal conductivity a' ,Ps apparent density of support bulk density (density of the c a t a l y s t bed) pb

n n -

pt T

r*

Sh 5, w

R

i

t r u e density space time time constant (2" order dynamic system) heat capacity f a c t o r mass capacity f a c t o r volume f r a c t i o n of the p e l l e t occupied by z e o l i t e c r y s t a l s parameter, defined by equation 11 1 . Measures the r e l a t i v e importance between macropore and micropore mass t r a n s f e r resistances damping f a c t o r (2nd order dynamic system)

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(4) Meugel ,M., Informations Chimie 227 (1982) 99 (5) Rosset,A. ,Neuzil ,R. and D. Broughton, Industrial Applications of Preparative Chromatography, in A. Rodrigues and D.Tondeur, eds . , Percolation Processes: Theory and Appl ications (A1phen den Ri jn: Sijthoff & Noordhoff, 1981), pp. 249-281.

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ENGINEERING ASPECTS OF CATALYTIC CRACKING

H . de LASA

Chemical Engineering Department Faculty of Engi neeri ng Science The University of Western Ontario London, Ontario, Canada N6A 5B9 The t e c h n o l o a .

S t a t e of the a r t and expected progress.

The technology of f l u i d i z e d bed c a t a l y t i c cracking ( F C C ) has shown a remarkable change i n the l a s t 30 y e a r s , The conventional FCC process, i ntensi vely appl i ed durinq the 5 0 ' s , bei ng basi cal l y the combination of two dense f l u i d i z e d beds ( t h e r e a c t o r and the regenerator) and two t r a n s p o r t l i n e s , may be considered as the 6-innA: genenation of c a t a l y t i c crackers. The heat required f o r the endothermic cracking reactions was supplied by the exothermal coke combustion. From an overall view point the i n d u s t r i a l process was operated under conditions close t o the thermal equilibrium where the silica-alumina c a t a l y s t was t r a n s f e r r i n g the heat from t h e hot regions (regenerator) t o the col d regi ons ( r e a c t o r ) and v i ce-versa. ( ~ i g .1 ) .

Fig. 1

Schematic Description of the F i r s t Generation of Crude Oil C a t a l y t i c Cracki ng Process

However, t h e commercial introduction of t h e microcrystalline s i l ica-alumina c a t a l y s t s (zeol i t e s ) a t t h e beginning of t h e 6 0 ' s , indicated t h e need of handling t h e overall process of t h e c a t a l y t i c cracking i n a new system where t h e remarkable a c t i v i t y and select i v i t y of z e o l i t e s (1 ) were f u l l y used. This s i t u a t i o n generated t h e i n t e r e s t i n a new FCC concept, the necond genaaLLon of catal y t i c crackers, where t h e c a t a l y t i c reaction was conducted in a s h o r t e r period of time ( 2 ) , ( 3 ) , ( 4 ) . In f a c t , t h e cracking react i o n s were s t a r t e d i n t h e t r a n s p o r t l i n e , conveying t h e cata1,yst t o the r e a c t o r (3-4s) and they were proceeded in a shallow dense f l u i d i z e d bed (3-4s) reducing in t h i s way the undesirable overcracking phenomena ( 3 ) , (Fig. 2 ) . T h i s overcracking, r e s u l t i n g from t h e secondary reactions and normally generating excessive amounts of coke and gases, was too dominant in deep f l u i d i z e d beds c o n s t i t u t e d by t h e so a c t i v e z e o l i t e s ( 5 ) .

.

feed

Fig, 2 Schematic Description of the Second Generation of Crude Oil C a t a l y t i c Cracking Process

Consequently, t h e second generation of transported-shallow bed FCCs represent an important e f f o r t t o take f u l l advantage of zeol i t i c c a t a l y s t s minimizing a t t h e same time these undesirable cracking transformations. A typical example of t h i s process i s t h e design patented by Texaco ( 6 ) . However, during t h e l a t e 60's i t was s t r e s s e d the importance of a f u l l use of t h e s p e c i f i c a c t i v i t y of t h e new microcrystalline s i l ica-a1 umina c a t a l y s t s ( z e o l i t e s ) r e c e n t l y introduced in the market ( 6 ) , ( 7 ) . Some d i f f i c u l t i e s were nevertheless forecasted ( 2 ) f o r t h e use of these materials having a dominant microporous

structure (2-7A), (9), much smaller than the amorphous silicaalumina (30-70A) (10). In favour of zeol i tes a 100-70000 times improved activity, in relation with the amorphous silica-alumina, was claimed (1 ) . This fact coupled with an improved hydrogen transfer capability, presumably a consequence of hydrogen mobility between cracked molecules, was indicating an important potential of zeolitic materials for increasing conversions to gasoline with low coke yields (9),(11 ), (12). Besides that, the growing refinery problem resulting from the necessity of processing crudes of' different source and quality, showed that an accurate control of reaction times between the catalyst and the hydrocarbons was a must for FCC crackers. This approach was a very promising concept for the FCC units. In this way was born the Zkitrd genehatLon of crackers based on a reactor which was, in fact, a transport line unit (21, (13 J, (8),(14). This concept was developed in several patents, case of Exxon in the Flexicracking unit, Gulf in Gulf FCC unit, Kellogg in the Orthoflow and the Heavy oil processes, Standard Oil in the Ultracat technology, UOP in the riser cracker units (2), (~~),(16)y(~7)y(18)y(19)y(~O). The peculiar characteristics of FCC transported reactors (21 ) provided better control of gasoline yields (60-65%) improved feedstock conversions (75-80%) and smaller coke yields (coke/feed:3-6%). For instance, the capabil ity of zeol itic catalysts for cracking naphtenic and parafinic hydrocarbons avoiding at the same time the slow decomposition of the aromatic molecules, normally generating high coke,yields, was a significative progress for the FCC technology (41). It must be stressed at the same time that the introduction of the riser reactors in the Z k i n d genehatLon of FCC units was still combined with dense fluidized bed regenerators. However, the high thermal resistance of the zeolites allowing high regeneration temperatures, 740°C instead of 630°C without a noticeable particle deactivation or sintering (16),(22) gave fast coke combustion rates and low CRC (coke on regenerated catalyst) levels. In this way, CRC of the order of 0.05 - 0.2% were obtained (23),(24), (25),(4), (26). These low CRC levels seem to be an important condition for reactivating the zeolitic sites on the catalysts and for achieving a maximum use of zeolite surface (27). Other advantages like a better overall plant thermal balance (14), a reduced recycled oil streams (8) were also claimed for the riser cracking reactors. Finally, more recent developments in the manufacturing of the cracking catalyst have been oriented to increase the zeolite performance. For instance, it has been analysed in which way the type of zeolite affects the conversion and the selectivity (28), (29), how the resistance to the attribution or the R.O.N. yields on gasoline could be improved (30), (2), how the hydrocarbon molecule size affects the zeol i te performance (1 ), (9).

I n s p i t e o f a l l t h i s progress, d u r i n g t h e mid 70s t h e FCC u n i t s were challenged again. The energy c r i s i s r e s u l t i n g from t h e f i r s t Arab embargo showed t h a t new m o d i f i c a t i o n s o r changes were r e q u i r e d t o face t h e new s i t u a t i o n . I n f a c t , new problems needed t o be considered f o r e f f i c i e n t l y processing t h e heavy crudes (14). The bottom o f t h e b a r r e l o r heavy crudes, f r a c t i o n s t h a t c o u l d n o t have been economically cracked b e f o r e t h e 1975 c r i s i s , were more and more considered f o r t h i s purpose (31 ). T h i s s i t u a t i o n cont i n u e s t o be t h e present challenge f o r t h e r e f i n e r i e s . However, another problem appeared t o g e t h e r w i t h t h e ones a l r e a d y described. P a r t i c u l a r l y t h e growing tendency t o mix a b i g g e r r e c y c l e d o r heavy crude f r a c t i o n i n t h e c r a c k e r feed (26), increased t h e c a t a l y s t contamination w i t h n i c k e l and vanadium metals (33). These metals once deposited on t h e z e o l i t i c m a t r i x f a v o u r t h e dehydrogenation r e a c t i o n s g i v i n g more hydrogen and more coke (33), (14), (34), (35), (36), (37),(38). A t t h e same t i m e metals such as n i c k e l and vanadium reduce t h e a c t i v e s i t e s and t h e s p e c i f i c a c t i v e s u r f a c e per u n i t weight decreasing t h e o v e r a l l c a t a l y t i c a c t i v i t y and select i v i t y t o produce gas01 i n e . Some authors claimed t h a t n i c k e l i s around f o u r times as bad as vanadium a t t h e same c o n c e n t r a t i o n s (39), (36), (38). As a consequence and f o r having a standardized b a s i s o f r e f e r e n c e an " e f f e c t i v e metal c o n c e n t r a t i o n " on t h e catal y s t i s d e f i n e d . T h i s e f f e c t i v e parameter i s f o u r times t h e n i c k e l It was a l s o l e v e l p l u s t h e vanadium c o n c e n t r a t i o n (36),(32). p o i n t e d out, however, t h a t n i c k e l and vanadium a c t i v i t y would proba b l y be r e l a t e d t o o t h e r f a c t o r s 1 i k e thermal a c t i v i t y h i s t o r y (40), (38), (41 ). The r e f i n e r i e s would have then, w i t h t h e heavy crudes, t h e p o t e n t i a l problems r e s u l t i n g from a d d i t i o n a l coke f o r m a t i o n and s m a l l e r c a t a l y s t a c t i v i t y (14). I n f a c t , t y p i c a l coke y i e l d s f o r heavy crudes have g i v e n coke c o n c e n t r a t i o n s values as h i g h as 8-16%. (The b a s i s f o r these A p o s s i b l e way o f t a c k l i n g y i e l d s i s t h e f r e s h feed), (31),(14). t h e problem i s e i t h e r t h e use o f antimony p a s s i v a t o r s o f n i c k e l and vanadium as suggested by P h i l 1 i p s Petroleum (42), (39) o r t h e employment o r t h e new f a m i l i e s o f z e o l i t i c c a t a l y s t , t h e Redsicat, t h e GRZ and t h e F i l t r o l (24), (34). These c a t a l y s t s would a l l o w t h e c r a c k i n g process t o perform w i t h s i g n i f i c a n t n i c k e l and vanadium l e v e l s , p e r m i t t i n g a t t h e same t i m e adequate o p e r a t i o n o f t h e FCC u n i t s w i t h r e l a t i v e l y low coke and gas y i e l d s . It i s expected t h a t these c a t a l y s t s would p r o v i d e h i g h crude o i l conversions, w i t h eff e c t i v e (4Ni + V ) c o n c e n t r a t i o n s as h i g h as 5000 ppm, c o n d i t i o n s where a standard c r a c k i n g c a t a l y s t would have shown a n o t i c e a b l e a c t i v i t y decay (24). We may v i s u a l i z e then t h a t we a s s i s t today t o t h e development o f a s i g n i f i c a n t t e c h n i c a l e f f o r t towards t h e p r o d u c t i o n o f new FCC c a t a l y s t s r e q u i r e d f o r t h e processing of heavy crudes. A t t h e same time, t h e o p e r a t i o n o f t h e FCC u n i t s has been cont e s t e d again by t h e new environmental r e g u l a t i o n s . The problem o f

how to reduce the CO emissions, transforming CO in C02, and gaining in the same operation the CO heat of combustion has been one of the major refinery concerns during the 70s, (44),(45), (43). .4 first approach involved the use of CO burners, located in the plant after the regenerator. These burners transformed the CO gases producing at the same time steam for the cracking plant (44). However, a more advanced concept seeks a direct CO to C02 conversion in the regenerator itself. With this purpose catalytic materials named comb unto^ were developed (23), (45). Three basic types of combustors have been suggested: - a platinum group metal (Pt, Pd, Ir) deposited in small concentrations on the cracking catalyst, - a solid promotor mixed to a cracking catalyst without additives (0.1-1%) - a 1 iquid additive injected into the regenerator, (42). Unfortunately, the specific references about the chemicals promoting these effects are very incomplete. For the case of the platinum group metal it would seem that the metal is incorporated to the zeolitic matrix in such a way that would be only contacted by CO and 02 but not by the bigger hydrocarbon molecules. In this manner the CO transformation would be promoted and the adverse dehydrogenation reactions catalyzed by the same metal control 1 ed (45). These combustors, which normally 1 imi t the CO emissions to levels below 500 ppm (maximum allowed CO environmental emission concentration) (46) have shown to be a more appropriate technology than the CO external boilers (23), (45). In fact, through the advanced CO transformation method significant capital gains have been claimed as well as important reductions of CO concentrations in the dilute regenerator phase (250-500 ppm) (43), (35), (14). This low CO concentration in the entrainment phase region is possibly a very important factor for eliminating the troublesome CO oxidation in the 1 ean regenerator phase (postburning) (45). In this way, the uncontrolled temperature increase in the upper part of the regenerator, consequence of CO postburning and frequent cause of catalyst sintering and cyclone damages, is avoided. However, in spite of this progress no technical information is, at least to our knowledge, available to predict the behaviour of the catalytic combustors in front of heavy oils with high nickel and vanadium contents. This is, in our opinion, a research area where significant technological improvement must be achieved in the next few years to really make of the FCC a technological breakthrough. KINETICS OF CATALYTIC CRACKING When a gas oil is catalytically cracked, an ample diversity of chemicals, going from hydrogen and methane to coke forming polymeric materials deposited on the catalyst, are generated. The most popular kinetic models used for the process description are

basically based on a simp1 ified representation involving three characteristic groups of reactants and products: t

A1 : gas oil ; A2: gasoline(C5 - 210°C);

A3: butanes, light, gases and coke

This certainly leads to the conception of the catalytic cracking of the gas oil as follows (48),(49),(50), (51 ), (52), (53).

A1 (gas oil)

1

*

A2 (gasoline)

(1 1

(butanes, light gases and coke) It is important to point out that the catalytic process may be interfered to some extent by the thermal cracking reactions which simultaneously occur. Corrections have been suggested to the gas oil conversion to discount the noncatalytic effect (54), (55), (56). These corrections are around 2% at 490°C (51). Consequently, the scheme presented above with equation [I ] may be a usual approach to predict both the overall gas oil conversion and the selective transformation of gas oil in gasoline. Certainly the second aspect is a key factor for an efficient operation of the FCC units. These two parameters have been estimated by several kinetic models. The most frequently adopted kinetic representation (521,(571,(58), (50), (51 ), (59), considers that the overall gas oil transformation is a second order decomposition and the further gasoline cracking is a first order reaction. r1

=

kO+l(tc)Y12

(gas oil cracking)

[21

r2

=

k2@2(tc)y2

(gas01 ine cracking)

[31

This difference between the gas oil and the gasoline cracking reaction orders may be explained as follows:

- The gasoline is a mixture of hydrocarbons with boiling points

varying in a 1 imited range. In this sense it is normal to expect that the gas01 ine will closely behave like a pure hydrocarbon showing a reaction order equal to one (51 ).

- Conversely, t h e gas o i l i s a f a r more complex m i x t u r e w i t h an ample d i v e r s i t y o f c r a c k i n g r e a c t i o n r a t e s . I n t h i s r e s p e c t t h e gas o i l c r a c k i n g i s t h e summation o f a l a r g e number o f r e a c t i o n s a c t i n g i n p a r a l l e l w i t h v e r y d i f f e r e n t k i n e t i c c o n s t a n t s . The consequence i s an o v e r a l l r e a c t i o n o r d e r l a r a e r t h a n one (60). These f a c t s have been j u s t i f i e d b o t h i n t u i t i v e l y and t h e o r e t i c a l l y ( 5 7 ) , (61 ), (62). A n o t h e r k i n e t i c scheme (63), (49) based on a s i m i l a r i d e a t o e q u a t i o n 121, t h a t use i n s t e a d o f w e i g h t f r a c t i o n s t h e more approp r i a t e m o l a r f r a c t i o n s , i n t r o d u c e s a parameter named c r u d e o i l r e f r a c t o r i n e s s . T h i s parameter lumps i n a s i n g l e f a c t o r t h e c r a c k i n g f e e d s t o c k and c a t a l y s t c h a r a c t e r i s t i c s : (cracking o f crude o i l ) [ 4 ] The parameter W i s c e r t a i n l y d i f f e r e n t f r o m z e r o f o r c o n v e n t i o n a l crudes which n o r m a l l y have b o i l i n g p o i n t s v a r y i n g i n a wide t e m p e r a t u r e range (around 280-550°C). The i n t e r e s t i n g aspect o f t h i s model i s t h e c o n d i t i o n o f a W parameter o s c i l l a t i n g between 0 and 3.3 and showing some t e m p e r a t u r e dependence ( 6 4 ) , (55), ( 6 5 ) , ( 6 6 ) , (49), ( 6 7 ) , ( 6 2 ) . The l o w e r W v a l u e s corresponded t o experiments w i t h a v a r i e t y o f c r a c k i n g c a t a l y s t s where some d i f f u s i o n a l l i m i t a t i o n s were suspected. The h i g h e r W were c o n s i d e r e d f o r La'{ z e o l it i c c a t a l y s t s where d i f f u s i o n a l 1 i m i t a t i o n s c o u l d p r o b a b l y be n e g l e c t e d (28). I n any case i t would seem t h a t f o r z e o l i t e s c r a c k i n g heavy f e e d s t o c k s t h e c a t a l y s t o p e r a t i o n w i t h i m p o r t a n t d i f f u s i o n a l 1i m i t a t i o n s i s h i g h l y p o s s i b l e ( 9 ) .

A t t h e same t i m e on e q u a t i o n s ( 2 ) , ( 3 ) and ( 4 ) i t may be n o t i c e d t h a t two timedependent parameters a r e i n c l uded. Both $l ( t c ) and $ 2 ( t ) i n t r o d u c e i n t h e s e e x p r e s s i o n s t h e c a t a l y t i c a c t i v i t y decay f a k o r s . I t may a l s o be observed t h a t b o t h @ ( t ) and 4 ( t ) a r e f u n c t i o n s o f tc, t h e fieaction lime o r t h & c & n & h ~ on a.$xeh .time. T h i s t i m e i s a key v a r i a b l e when m o d e l i n g a h e t e r o geneous r e a c t i v e system l i k e t h e FCC process. The cause o f t h i s a c t i v i t y decay i s m a i n l y r e l a t e d t o t h e d e p o s i t i o n o r a d s o r p t i o n o f t h e p o l y a r o m a t i c compounds which p o l y m e r i z e on t h e c a t a l y s t s u r f a c e f o r m i n g t h e coke. ( 5 9 ) , (61 ), ( 6 8 ) , ( 2 9 ) . T h i s p o l y m e r i z a t i o n appears a c t i v a t e d by t h e o l e f i n s c a p a b i l i t y o f a b s t r a c t i n g h y b r i d e i o n s f r o m n a p h t e n i c and a r o m a t i c hydrocarbons ( 9 ) , ( 5 6 ) , ( 6 9 ) which produces a h i g h l y condensed a r o m a t i c s o l i d r e s i d u e ( 7 0 ) . I n s p i t e of t h e g e n e r a l a c c e p t a t i o n o f t h e s e f a c t s some arguments were p r e s e n t e d ( 5 6 ) , (71 ) i n d i c a t i n g t h e d i f f i c u l t i e s o f f o r m u l a t i n g a s i m p l e r e l a t i o n s h i p between c a t a l y s t a c t i v i t y and coke c o n c e n t r a tion. Moreover and because i t i s n o r m a l l y assumed t h a t t h e same t y p e o f a c t i v e s i t e s w i l l c r a c k b o t h gas o i l and g a s o l i n e molecules,

an i d e n t i c a l s e t o f @ ( t ) and @ 2 ( t G ) have been proposed ( 5 8 ) . Under t h e s e c o n d i t i o n 1 eGuations [ 2 j and [4] may be w r i t t e n as f o l l ows :

D i f f e r e n t t y p e s o f ? ( t c ) f u n c t i o n s have been proposed t o r e p r e s e n t b o t h t h e c a t a l y s t a c t i v i t y decay and t h e coke f o r m a t i o n . I n p a r t i c u l a r , two o f t h e s e mathematical forms a r e t h e most p o p u l a r ones. One o f them c o n s i d e r s t h a t t h e c a t a l y s t a c t i v i t y decay may be r e p r e s e n t e d w i t h a power law, ? ( t c ) = a t - n ( 7 2 ) , ( 6 0 ) , ( 5 1 ) , ( 7 3 ) . The o t h e r one approaches t h e @ ( t c ) f u n c t i g n u s i n g a d e c r e a s i n g e x p o n e n t i a l @ ( t c ) = b e x p ( - a t c ) , ( 5 9 ) , ( 5 2 ) . Some arguements l i k e a f i n i t e c a t a l y s t a c t i v i t y when t+O ( 5 9 ) and t h e p o s s i b l e r e l a t i o n s h i p o f coke f o r m a t i o n w i t h an i r r e v e r s i b l e a d s o r n t i o n mechanism would s u p p o r t t h e e x p o n e n t i a l decay l a w ( 7 4 ) . I t must however, be i n d i c a t e d t h a t some e f f o r t s were o r i e n t e d t o m o d i f y t h e power l a w c o r r e c t i n g e s s e n t i a l l y t h e anomaly when tc+O, ? ( t ) = a ( 1 + G t c ) - n w i t h G = ( m - l ) K and n = l / ( l - m ) (61), (75), (645, ( 6 3 ) , (65), (76), ( 7 7 ) , ( 7 8 ) . Moreover, t h i s e q u a t i o n has t h e a d d i t i o n a l advantage o f a l l o w i n g t h e d e r i v a t i o n o f an e x p o n e n t i a l l a w when m=l. I n o t h e r words t h e e x p o n e n t i a l l a w i s a p a r t i c u l a r case o f t h e m o d i f i e d power l a w ( 6 3 ) . T h i s m o d i f i e d power l a w e x p r e s s i o n i s a l s o supported by e x p e r i m e n t a l evidence which showed n and m v a l u e s changing between 0.6-30 and 1.03-2.57, r e s p e c t i v e l y (64), (66). Then t h e observed m v a l u e s , depending on t h e coup1 e c a t a l y s t f e e d s t o c k and on t h e o p e r a t i n g c o n d i t i o n s , match t h e e x p o n e n t i a l behaviour, m= 1 o n l y i n a fewcases which c e r t a i n l y showed t h e convenience o f a more q e n e r a l 4 = a ( l + G t c ) - n expression.

I t i s i m p o r t a n t t o mention, however, t h a t b o t h e x p o n e n t i a l o r m o d i f i e d power l a w s c o n t a i n a m a j o r simp1 if i c a t i o n , t h e dependence o f hydrocarbon c o n c e n t r a t i o n on @ ( t c ) i s i g n o r e d . T h i s hypothesis does n o t always seem a p p r o p r i a t e f o r f i x e d bed c a t a l y t i c c r a c k i n g l a b o r a t o r y u n i t s ( 7 9 ) , ( 2 0 ) . A m o d i f i e d e x p o n e n t i a l l a w has been proposed (81 ) as a more s u i t a b l e @ ( t c ) r e l a t i o n s h i p .

A t t h e same t i m e o t h e r e f f e c t s , h a v i n g consequences on c a t a l y s t a c t i v i t y , were d e s c r i b e d . F o r i n s t a n c e t h e a c t i o n o f n i t r o g e n a t e d bases on s i l i c a - a l u m i n a and z e o l i t i c c a t a l y s t s r e c e i v e d p a r t i c u l a r a t t e n t i o n . These compounds, coming a l o n g w i t h t h e f e e d s t o c k stream, would decrease t h e a c i d i t y o f t h e c a t a l y s t m a t r i x r e d u c i n g i n t h i s way i t s a c t i v i t y and s e l e c t i v i t y (82). N e v e r t h e l e s s , t h e appropr ~ i a t eway o f i n c l u d i n g t h i s e f f e c t i n t h e k i n e t i c model i s , a t the present time, q u i t e unclear.

We w i l l c o n s i d e r now t h e i n t e r e s t o f equations [5] and [6] f o r d e f i n i n g t h e gas o i l c o n v e r s i o n and g a s o l i n e f o r m a t i o n r a t e s . I n f a c t , once those expressions a r e d e r i v e d , i t i s p o s s i b l e t o e s t i m a t e t h e s e l e c t i v e t r a n s f o r m a t i o n o f gas o i l i n g a s o l i n e . A t t h e same t i m e because gas o i l c o n v e r s i o n t e s t s were f r e q u e n t l y performed i n 1a b o r a t o r y s c a l e f i x e d bed r e a c t o r s , some i n t e r e s t i n g o b s e r v a t i o n s were developed f o r those systems. I t has been r e a l i z e d , f o r i n s t a n c e , t h a t i n l a b o r a t o r y s c a l e u n i t s tv, t h e residence t i m e o f t h e hydrocarbon m i x t u r e , i s n o r m a l l y much s m a l l e r than t h e c a t a l y s t on stream time ( t v 3,000 nm3/hour) i s mainly due t o t h e i n c r e a s i n g isothermal e f f i c i e n c y of t h e a i r compressors. I n g e n e r a l t h e energy consumption f o r producing oxygen by t h e adsorption p r o c e s s e i t h e r with z e o l i t e molecular s i e v e s o r c o a l molecular s i e v e s i s r e l a t i v e l y high and it seems t h a t it cannot be decreased by i n c r e a s i n g t h e s i z e o f t h e a d s o r p t i o n p l a n t s and it i s apparent t h a t it i s b e t t e r s u i t e d t o small-volume u s e r s . This works t o t h e advantage of PSA s i n c e cryogenic p l a n t s do not s c a l e down w e l l below 30 tons/day. Usually though p l a n t s with a lower s p e c i f i c power consumption (cryogenic and carbon s i e v e ) w i l l be more expensive t o b u i l d . The economics of p u r i f y i n g hydrogen by PSA a r e determined by t h e c o s t of t h e equipment and t h e c o s t of t h e f e e d , s i n c e no v i r t u a l l y u t i l i t i e s a r e r e q u i r e d . The equipment c o s t i s a f u n c t i o n o f t h e throughput, type and q u a l i t y of t h e i m p u r i t i e s t o be removed and adsorbent p r o p e r t i e s . Feed c o s t s may be from zero f o r a gas being f l a r e d but a p p r e c i a b l e i f t h e f e e d has f u e l value o r an a l t e r n a t i v e use. Nontheless, t h e PSA H p r o c e s s i s a simple p l a n t 2 with no r o t a r y equipment, l e a d i n g t o low c a p i t a l expenditure and more r e l i a b l e p l a n t o p e r a t i o n . PSA has advantages i n t h a t it can handle broad ranges of i m p u r i t i e s , low H content i n t h e f e e d gas and i s capable of producing a higher2% p u r i t y H t h a n cryogenics. Bergbau-Forschung and Petrocarbon s e e a major advan?age of t h e PSA u n i t s over cryogenic systems i n t h e f a c t t h a t c a p i t a l c o s t s f a l l s t e a d i l y a s t h e y a r e s c a l e d down, because of t h i s economic advantage of s c a l e PSA purge stream p u r i f i c a t i o n u n i t s a r e of p a r t i c u l a r i n t e r e s t t o o p e r a t o r s o f p l a n t s producing a s m a l l s c a l e purge stream. A summary of t h e e x i s t i n g a p p l i c a t i o n s of PSA and cryogenic systems was included i n Wolf's paper i n 1976. This i n d i c a t e s main a p p l i c a t i o n s of PSA s t i l l l i e w i t h a i r s e p a r a t i o n and t h e p u r i f i c a t i o n of hydrogen c o n t a i n i n g streams. F u r t h e r improvements and a p p l i c a t i o n s a r e f o r e s e e a b l e i n t h e f u t u r e of PSA and w i l l be due t o t h e use of b e t t e r adsorbents and newer c y c l e s . B e t t e r c o n t r o l components could l e a d t o f u r t h e r improvements. The r e s u l t w i l l be t h a t t h e economics of PSA w i l l become more a t t r a c t i v e and more i n d u s t r i a l gases w i l l be s e p a r a b l e by PSA due t o c a p i t a l c o s t r e d u c t i o n and h i g h e r product y i e l d .

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-

NOMENCLATURE a

1

a a

oxygen bed c a p a c i t y c o e f f i c i e n t , eqn. ( 2 3 ) :

kmol/(m)(~/rn').

n i t r o g e n bed c a p a c i t y c o e f f i c i e n t , eqn. (23) : 3

c

= al - a 2 = w(KA - Kg).

coefficient defined a s a

c o n c e n t r a t i o n i n t h e g a s phase :

kmol/m3. kmol/( k g a d s o r b e n t ) (N/m2 1.

K ,K a d s o r p t i o n e q u i l i b r i u m c o n s t a n t s : A B L b e d l e n g t h : m. AL n

n e t f r o n t displacement:

kmol/(m) (N/m2).

m.

number o f c e l l s . number o f c y c l e s i n p r e s s u r e s w i n g p r o c e s s . m o l a r d e n s i t y o f a d s o r b e d p h a s e p e r u n i t volume o f b e d : kmol m-3 moles o f g a s a d s o r b e d p e r u n i t mass s o l i d :

N/m2

t o t a l pressure:

o r bar.

p a r t i a l p r e s s u r e o f component ' L ' : molar flow r a t e s :

kmol/kg a d s o r b e n t

N/m2.

kmol/s.

r a t e of adsorption :

kmol/( k g a d s o r b e n t ) ( s )

R

gas l a w constant.

t

time:

T

temperature :

v

i n t e r s t i t i a l o r front velocity:

w

weight of adsorbent p e r u n i t l e n g t h of bed:

Y Z Z

mole f r a c t i o n i n g a s p h a s e .

.

s. 0

K.

a x i a l p o s i t i o n from i n l e t :

ms

.

-1

m.

d i m e n s i o n l e s s p o s i t i o n i n bed ( Z / L ) .

kg/m.

bed f r a c t i o n a t i o n f a c t o r ( a / a 1. 2 1 r a t i o of i n t e r s t i t i a l velocity t o f r o n t velocity. e x t e r n a l p o r o s i t y o f bed. d i m e n s i o n l e s s c y c l i n g time. duration of s t e p i n cycle:

s.

c o n d i t i o n s ahead and b e f o r e shock wave. oxygen :

B nitrogen.

p e r t a i n s t o feed. high pressure in cycle. i n i t i a l condition.

TABLE 1 COMPANY

BEDS

OPERATING PRESSURE: RANGE

CYCLE FEATURES

4

Superambient

P r e s s u r i s e - Dynamic p r o d u c t r e l e a s e BPE - P r o v i d e p u r g e - c o m p l e t e blowdown - r e c e i v e p u r g e - BPE - e t c .

>2

Superambient

S i m u l t a n e o u s p r e s s u r i s a t i o n from f e e d a n d p r o d u c t e n d s , non-dynamic p r o d u c t release.

L'air Liquide

1 to 10

From Superto Subambient

1) Each b e d h a s a t h i r d p i p e c o n n e c t e d h a l f w a y up b e d 2 ) Non-dynamic p r o d u c t r e l e a s e t h r o u g h a second bed a t lower p r e s s u r e 3 ) Vacuum a p p l i e d a t mid-point o f bed 4 ) A i r feed drying s e c t i o n of s i l i c a g e l , t o product z e o l i t e .

BayerMahler

3


1 ) Used s p e c i a l 4A s i e v e s ( c a 2 + and sr2+exchanged) 2 ) Each bed h a s d r y i n g s e c t i o n 3 ) Vacuum r e g e n e r a t i o n i n c l u d i n g drying

Union Carbide

.

Nippon Steel

3


1 ) C y c l e s s i m i l a r t o Bayer-Mahler above 2 ) N a t u r a l z e o l i t e used: Calcareous Tufa.

W.R.Grace & Co.

2 t 1 tank

Superambient and Supersubambient

Tuned up E s s o P r o c e s s 1 ) Dynamic p r o d u c t r e l e a s e 2 ) Bed p r e s s u r e e q u a l i s a t i o n 3 ) Vent t o t a n k 4 ) Blowdown t o a t m o s p h e r e o r vacuum pumped.

BOC Techsep

2

Subambient

1 ) Composite b e d s : s i l i c a g e l d r y i n g , 5A z e o l i t e 2 ) Dynamic p r o d u c t r e l e a s e - f e e d s u c k e d t h r o u g h by p r o d u c t compressor 3 ) Vacuum r e g e n e r a t i o n , r e p r e s s u r i s a t i o n from p r o d u c t .

-

TABLE 1 c o n t i n u e d

COMPANY

BEDS

BOC Techsep

3

OPERATING PRESSURE RANGE

CYCLE FEATURES

Subambient

A s above, w i t h 4 ) Breakthrough from p r o d u c t r e l e a s e f e d t o r e g e n e r a t e d bed and r e l e a s e d a s second-out p r o d u c t .

Air 2 + SuperProducts 2 suband t a n k s ambient Chemicals

Produces b o t h oxygen and n i t r o g e n Offspec. 02 p r o d u c t and N2 p r o d u c t c o l l e c t e d s e p a r a t e l y , and used t o c r o s s purge beds i n a 4 - p a r t c y c l e . S e p a r a t e d r y e r s r e q u i r e d u s i n g 13x zeolite. 4 pumps used, f o r recomp r e s s i o n o f purge g a s e s .

TABLE 2

Summary o f Numerical S o l u t i o n s

AUTHOR

TYPE OF SOLUTION AND ASSUMPTIONS

Szolcsanyi

E x p l i c i t f i n i t e d i f f e r e n c e scheme. 1 ) One dimensional flow - p l u g flow no r a d i a l dispersion 2 ) No a x i a l d i f f u s i o n 3 ) Hydrodynamics dominated by v a l v e formulae a t i n l e t and o u t l e t , and Darcy's law w i t h i n t h e bed 4 ) E q u i l i b r i u m a d s o r p t i o n i s maintained 5 ) Linear isotherms 6 ) I d e a l gas b e h a v i o u r 7) Isothermal operation 8 ) Simple i n i t i a l and boundary c o n d i t i o n s .

e t a2 (1975) (46)

Sebasti a n (1978) (47

Garg & Ruthven ( 1 9 7 3 ) & (1974) (48)

Kowler & Kadlec (1972) and Turnock & Kadlec (1971) (491, ( 5 0 ) .

E x p l i c i t f i n i t e d i f f e r e n c e scheme, convergence c o n t a i n e d by monotonicity c o n d i t i o n s . Vacuum d e s o r p t i o n s t e p o n l y . A mathematical paper. 1 ) P l u g flow w i t h a x i a l d i f f u s i o n 2 ) Ergun e q u a t i o n f o r p r e s s u r e drop a c r o s s bed 3 ) I s o t h e r m a l o p e r a t i o n j i d e a l gas b e h a v i o u r 4 ) Unspecified r a t e of adsorption functions 5 ) Complex boundary c o n d i t i o n s . I m p l i c i t f i n i t e d i f f e r e n c e scheme o f t h e CrankNicholson t y p e . Constant p r e s s u r e a d s o r p t i o n process considered only. 1 ) P l u g flow w i t h a x i a l d i f f u s i o n 2 ) No s p a t i a l p r e s s u r e g r a d i e n t s 3 ) I s o t h e r m a l o p e r a t i o n / i d e a l gas b e h a v i o u r 4 ) Non-linear mass t r a n s f e r model s e p a r a t e l y c o n s i d e r i n g micro and macro p o r e d i f f u s i o n . P r o c e s s o p t i m i s a t i o n on a c e l l model. 1) I s o t h e r m a l o p e r a t i o n / i d e a l gas b e h a v i o u r 2 ) Darcy's Law p r e s s u r e drop a c r o s s bed 3 ) V i s c o s i t y o f t h e gas i s composition independent 4 ) P l u g flow 5 ) Instantaneous equilibrium 6 ) Freundlich isotherm a p p l i e s 7) R e l a t i v e v o l a t i l i t y r e l a t e s a d s o r p t i v e c a p a c i t y and i s composition i n v a r i a n t . 8 ) The e q u i l i b r i u m amount adsorbed i s independent o f composition 9 ) Each c e l l i s i d e a l l y mixed and a t c o n s t a n t pressure

LIST OF PARTICIPANTS

BELGIUM

-

Ph. B o d a r t

F a c u l t g s Univ.

d e Namur

-

B 5000 Namur

- U n i v . ~ i s g e- B 4000 ~ i g g e

J. C o s t a

J. Martens

-

K a t h o l i e k e Univ.

-

Univ. N e w B r u n s w i c k

Leuven

-

B 3030 L e u v e n

CANADA D.

Ruthven

F.

Smith

-

Univ.

-

Fredericton

P r i n c e Edward I s l a n d

-

Charlotte

DENMARK N.

Blom - H a l d o r T o p s e A/S

-

DK 2800 Lyngby

T e c h n o l o g i c a l I n s t i t u t - Denmark

J. -sonFRANCE F.

Fajula

N.

Gnep

-

J. L u c i e n

-

Univ. M o n t p e l l i e r

Univ.

-

T. L a b o u r e l

Poitiers

-

-

34075 M o n t p e l l i e r

86022 P o i t i e r s

S h e l l Recherche

-

76530 G r a n d C o u r o n n e

-

-

69360 S t . S y m p h o r i e n

E l f Recherche d ' Ozon

GERMANY D.

A r n t z - Degussa

M.

Baacke

-

-

Degussa

D 6450 Hanau 1

-

D 6450 Hanau 1

GREECE D.

Z a m b o u l i s - Univ.

Thessaloniki - Thessaloniki

ICELAND G. Einarson

-

Technological Institute

-

Reykjavik

ITALY F. Ciambelli - Univ. Napoli - 80134 Napoli P. Corbo - Univ. Napoli - 80134 Napoli I. Ferino - Univ. Cagliari - 09100 Cagliari A. La Ginestra - Univ. Roma - 00185 Roma G. Gubitosa - Donegani Research Institute - 28100 Novara R. Maggiore - Univ. Catania - 95125 Catania R. Monacci - Univ. Cagliari - 09100 Cagliari P. Porta - Univ. Roma - 00185 Roma A. Villanti - Anic/Chisec - 20097 S. Donato Milanese ISRAEL M. Steinberg

-

Univ. Hebrew - Jerusalem

NETHERLANDS C. W. Engelen - Univ. Eindhoven - 5600 MB Eindhoven E. Groenen - Koninklijke-Shell Laboratorium - 1031 CM Amsterdam H. Okkersen - Dow Chemical - Terneuzen F. Roozeboom - Esso Chemie - 3000 HE Rotterdam W. Van Erp - Koninklijke-Shell Laboratorium - 1003 Amsterdam NORWAY G. Boe - ELKEM - N 4620 Vagsbygd G. Haegh - Central Institute Industrial Research - N Oslo 3 K. Kinnari - Statoil - N 4001 Stavanger S. Kolboe - Univ. Oslo - N Oslo 3 0 . Onsager - Univ. Trondheim - N 7034 Trondheim

-

J.Raeder

-

SPAIN A. Corma

-

J. A. C. A. J. A.

Univ. Oslo

-

N Oslo 3

Instituto Catalisis Petroleoquimica - Madrid 6 Juan Aguera - ENPETROL - Cartagena Lopez Agudo - Instituto Catalisis Petroleoquimica - Madrid 6 Ballesteros Martin - Univ. Madrid - Madrid 3 Lucas Martinez - Univ. Madrid - Madrid 3 Pajares - Instituto Catalisis Petroleoguimica - Madrid 6 Villarroya Palomar - Industrias Quimicas del Ebro - Zaragoza

TURKEY E. Alper

-

A. U. Fen Fakultesi

SWITZERLAND G. Gut - ETH-Zentrum

-

-

Ankara

CH 8092 Zurich

UNITED KINGDOM S. Fegan - Univ. Edinburgh - Edinburgh EH93JJ A. Hope - Univ. College London - London WCIH OAJ D. Rawlence - Joseph Crosfield Sons - Warrington WA5 1AB M. Sanders - Univ. College London - London WCIH OAJ D. Swindells - Univ. Aberdeen - Old Aberdeen AB92UE D. Whan - Univ. Edinburgh - Edinburgh EH93JJ U.S.A. L. Sand

-

Worcester Polytechnic Institute

-

Worcester

-

-

PORTUGAL M.

-

J . Bordado

M.

-

Brotas

J. C a e i r o

C. C o s t a M.

-

J. Afonso

C.

-

Inst.

-

Quimigal

1000 L i s b o a

Barreiro

F a c u l d a d e d e ~ i g n c i a s- L i s b o a

-

Petrogal

Lisboa

Faculdade de Engenharia

-

Dias

-

S u p e r i o r TGcnico

Inst.

-

J. F i g u e i r e d o

-

Superior Tgcnico

4099 P o r t o Codex

-

1000 L i s b o a

-


4099 P o r t o

Codex

-

I n s t . S u p e r i o r TGcnico - 1000 L i s b o a

F.

Freire

C.

Henriques

J. J u s t i n 0 F . Lemos

-

-

-

Inst.

-


1000 L i s b o a

I n s t . S u p e r i o r ~ G c n i c o- 1 0 0 0 L i s b o a


Lemos - I n s t .

M.

A.

L.

S o u s a Lobo

-

S u p e r i o r ~ G c n i c o- 1 0 0 0 L i s b o a

-

U n i v . Nova d e L i s b o a

2825 .Vonte d a

Caparica J . M.

-

Loureiro


-

4099 P o r t o

Codex

-

M . A . Mendes

M.

Petrogal

-

F. M a r t i n s

-

Lisboa


-

L. P a l h a

Inst.

-

S u p e r i o r Tgcnico

1000 L i s -

boa

-

M.

J. P i r e s

M.

F. P o r t e l a

M.

F.

Ribeiro

M.

C.

Rodrigues

A. N .

Santos

-

I n s t . Superior Tgcnico

-

1000 L i s b o a

Superior Tgcnico

-

1000 L i s b o a

I n s t . S u p e r i o r TGcnico

-

1000 L i s b o a

Inst.

-

Inst.


U n i v . Nova d e L i s b o a Caparica

-

-

1000 L i s b o a

2825 - Monte d a

... ,

p:

-

:.

b

e is

I

.

1:

,"X.*l,

: a.

ei.

L

:

afid

.

.

Included in the section on industrial applications are chaptersbn reactor and adsorber design, catalytic uacking, xylme and n-paraffins ipmerization, as well as ioriexchange and adsqrhtion. Whilg'pjmariiy intended for sienti8ts and engihers concarned with catalysis, ad$orbtion and ion-exchange, or reaction.engineering, this boo& wit1 also ,b.suitable graduate dunes incatalysis.

9

.,

,"

2

. , i, .

'

i s a sumpary of the pmceediqof a NATO A&@pxd study I&&&; he& i i b ~..Pbrtug&& %lay 1083, whe,n,par$cula~emphasiswas p l a ' & ~ ~ & ~ ~ e y 6 l o p ~ k in the fietd.of zsol.ite science technology.Lndividual chgpisrs.$f the book]@& hiitorical development, stwmre. crystallqgraphy snd bynmarjis'!fbhn&6sIIII~asic principles @@wrbtion diffusion, ion.excfisr@e and M i t y are reviawed,an$i q soction on datalyrie add&s@s shape-electivity, tramition mewls, bifme-tionalCat* lysis and 'methanol-to-sasolind.

i$is

I

.

Prerented in this volu'& is an updated and integrated picture of the prarenf kboyiidi;; ledae - of the fund&entd$af zeolites a aubiect of growin@impoitme to iWuStVJafw!'A and academic cwnmunitles.

Zeolites, science and technology

Compost Science and Technology

Aerosols: Science and Technology

Science and Technology Ethics

Cotton Science and technology

Dairy Science and Technology

Wool: Science and Technology

Photocatalysis: science and technology

Science and technology almanac

Nanocomposite Science and Technology

Corrosion Science and Technology

Food Science and Technology

Nanowires Science and Technology

Dairy Science and Technology

Transport Science and Technology

Polymer Science and Technology

Science and Technology

Chocolate Science and Technology

Nanofluids: Science and Technology

Hydrocracking science and technology

Corrosion Science and Technology

Friction Science and Technology

Polymer Science and Technology

Aerosols: Science and Technology

Cotton: Science and Technology

Wool: Science and Technology

Polymer Science and Technology

Surfactant science and technology

Nanoscale science and technology

Emulsion Science and Technology

Zeolites, science and technology

Compost Science and Technology

Aerosols: Science and Technology

Science and Technology Ethics

Cotton Science and technology

Dairy Science and Technology

Wool: Science and Technology

Photocatalysis: science and technology

Science and technology almanac

Nanocomposite Science and Technology

Corrosion Science and Technology

Food Science and Technology

Nanowires Science and Technology

Dairy Science and Technology

Transport Science and Technology

Polymer Science and Technology

Science and Technology

Chocolate Science and Technology

Nanofluids: Science and Technology

Hydrocracking science and technology

Corrosion Science and Technology

Friction Science and Technology

Polymer Science and Technology

Aerosols: Science and Technology

Cotton: Science and Technology

Wool: Science and Technology

Polymer Science and Technology

Surfactant science and technology

Nanoscale science and technology

Emulsion Science and Technology

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