Acidic and Catalytic Properties of Zeolites

38 Acidic and Catalytic Properties of Zeolites DENISE BARTHOMEUF

Downloaded by PURDUE UNIV on March 8, 2016 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0040.ch038

Laboratoire de Catalyse Organique, L.A. CNRS n° 231, Ecole Supérieure de Chimie Industrielle de Lyon, 43, boulevard du 11 novembre 1918, 69621 Villeurbanne, France

ABSTRACT In the first part a review of zeolite acidity is presented. The concepts of proton and cation mobilities which become more widely considered these last years, are emphasized. The dynamics of zeolite systems appears also in the second part related to correlations between catalytic and acidic properties. A model based on the comparison of zeolites with polyacids in solution is presented. Recent reviews in the field of catalysis with zeolites have been published (1-5). They point out that the major applications of zeolites in catalytic systems are related to their acidic properties. A - ZEOLITE ACIDITY A - I A c i d i t y Models. The f i r s t r e p o r t s on t h e c a t a l y t i c p r o p e r t i e s o f z e o l i t e s X and Y ( 6 , 7 ) were soon f o l l o w e d by works w h i c h n o t e d t h e p r e s e n c e o f a c i d c e n t e r s and h y d r o x y l g r o u p s i n t h e s e c a t a l y s t s ( 6 , 8 - 1 0 ) . The a c i d i t y measurements methods a l r e a dy u s e d f o r t h e s t u d y o f t h e amorphous s i l i c a - a l u m i n a c a t a l y s t s gave p r o g r e s s i v e l y a l a r g e amount o f r e s u l t s and i n t e r e s t i n g l y p o i n t e d o u t d i f f e r e n c e s between t h e s e m a t e r i a l s and z e o l i t e s . I n c o n t r a s t w i t h s i l i c a - a l u m i n a , z e o l i t e s c o m p l e t e l y exchanged w i t h p o l y v a l e n t c a t i o n s h o l d c o n c o m i t a n t l y a l a r g e number o f p r o t o n i c s i t e s (jH . B e s i d e s , t h e i m p o r t a n c e o f t h e c a t i o n s was p o s t u l a t e d i n d e t e r m i n i n g t h e s t r e n g t h o f t h e a c i d i t y . I n 1963 H i r s c h l e r ( £ ) suggested t h a t " t h e p o l a r i z i n g a c t i o n of t h e f i e l d of the c a t i o n t e n d s t o f r e e (make a c i d i c ) a p r o t o n o f a h y d r o x y l g r o u p a t t a c h e d t o an a d j a c e n t s i l i c o n o r aluminum a t o m , o r a p r o t o n o f a w a t e r m o l e c u l e a d s o r b e d on t h e c a t i o n i t s e l f . The g r e a t e r t h e f i e l d s t r e n g t h o f t h e c a t i o n , t h e s t r o n g e r w o u l d be t h e r e s u l t a n t a c i d i t y " . A f e w y e a r s l a t e r some more e l a b o r a t e d m o d e l s were p r e s e n t e d . R i c h a r d s o n (11) c o n s i d e r e d t h a t i n c r e a s i n g t h e i o n i c p o t e n t i a l (e/r) of t h e c a t i o n r e s u l t s i n a s h i f t of e l e c t r o n charge 453 Katzer; Molecular Sieves—II ACS Symposium Series; American Chemical Society: Washington, DC, 1977.


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d i s t r i b u t i o n toward the v i c i n i t y of the c a t i o n v i a a conduction band m o d e l . T h i s weakens t h e OH bonds on t h e s u r f a c e o f t h e s u p e r c a g e making them more a c i d i c . The l a r g e number o f p o s s i b l e environments f o r the h y d r o x y l groups induce a wide a c i d s t r e n g t h d i s t r i b u t i o n . O n l y a s m a l l number o f h y d r o x y l a r e s u f f i c i e n t l y a c i d i c t o be a c t i v e i n c a t a l y s i s . T h i s f r a c t i o n depends on t h e i o n i c p o t e n t i a l o f t h e c a t i o n . The r e s u l t s o f Ward (12) o v e r MgHY z e o l i t e s a r e e x p l a i n e d on t h a t b a s i s . A s e c o n d p a r a m e t e r w h i c h may i n c r e a s e t h e a c i d s t r e n g t h i s t h e d e h y d r o x y l a t i o n ( 1 3 , 1 4 ) . L u n s f o r d (13) p r o p o s e d t h a t d e f e c t s i t e s due t o d e h y d r o x y l a t i o n a c t i n d u c t i v e l y on l o c a l h y d r o x y l g r o u p s t o f o r m s t r o n g p r o t o n i c a c i d s . R e c e n t l y J a c o b s and a l . ( 1 5 , 1 6 ) s u g g e s t an a l t e r n a t i v e e x p l a n a t i o n which imply the f o r m a t i o n of u l t r a a c t i v e Bronsted s i t e s d u r i n g h e a t t r e a t m e n t o f z e o l i t e s . Tung and M c l n i n c h (17_) have d i s c u s s e d a d i f f e r e n t t y p e o f mechanism f o r t h e m o d i f i c a t i o n o f a c i d s t r e n g t h s . Movements o f t h e c a t i o n s i n t h e f r a m e w o r k g e n e r a t e changes i n t h e s u r f a c e f i e l d s t r e n g t h and c o n s e q u e n t l y i n t h e s t r e n g t h o f p r o t o n i c s i t e s . F r e u d e and a l . (16) a p p l y t h e i d e a o f t h e s e a u t h o r s t o t h e m o b i l i t y o f p r o t o n s and c o n s i d e r a t i m e f l u c t u a t i n g B r o n s t e d a c i d i t y . The c o r r e l a t i o n between 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 and t h e s i t e e n v i r o n m e n t was c o n s i d e r e d by Dempsey (19) who d e s c r i b e d a s t r u c t u r a l model t o e x p l a i n why c l o s e t o 35 % o f t h e aluminum and sodium atoms a r e a s s o c i a t e d t o weak a c i d i t y i n Y z e o l i t e s ( 2 0 , 2 1 ) . He r e l a t e s t h e weak a c i d s i t e s t o p r o t o n s a s s o c i a t e d , t h r o u g h O^H h y d r o x y l s , w i t h t h e aluminum atoms o f t h e s q u a r e f a c e s c a r r y i n g two aluminum atoms i n t h e s o d a l i t e cages. The v a r i o u s m o d e l s p r o p o s e d b e l o n g t o one o f t h e two t y p e s , s t a t i c o r dynamic. In s p i t e of the f a c t t h a t the s t a t i c approach has been a b l e t o e x p l a i n a l a r g e number o f a c i d i t y r e s u l t s , one must c o n s i d e r t h a t t h e r e a l a c i d s i t e s ( p r o t o n i c s i t e s ) a r e mo v i n g c o n t i n u o u s l y i n t h e s t r u c t u r e , t h e l i f e t i m e o f a p r o t o n on a s i t e b e i n g between 10~2 and 10"? s e c o n d s . Hence t h e mean p o p u l a t i o n of p r o t o n s a t each s i t e r e s u l t s from a k i n d of e q u i l i b r i u m between t h e o c c u p a n c y f a c t o r s a t t h e v a r i o u s oxygen a t o m s . These c o n s i d e r a t i o n s i m p l y t h a t t h e a c i d i c p r o p e r t i e s w i l l be i n f l u e n ced b o t h by t h e c h e m i c a l e n v i r o n m e n t o f t h e f r a m e w o r k oxygen a toms and by t h e i n t r i n s i c p r o t o n m o b i l i t y . T h i s i s r a t h e r i m p o r t a n t t o c o n s i d e r when a c i d i t y m o d e l s a r e used i n r e l a t i o n w i t h c a t a l y t i c p r o p e r t i e s which of course i m p l y dynamics of the system. As t o t h e number o f a c i d c e n t e r s i n z e o l i t e s , i n t h e c a s e o f p o l y v a l e n t exchanged m a t e r i a l s , t h e r e i s , f o r a l a r g e number o f c a t i o n s , a l i n e a r r e l a t i o n s h i p between t h e p r o t o n i c a c i d i t y c o n c e n t r a t i o n and e l e c t r o s t a t i c f i e l d o f t h e c a t i o n s ( 2 2 , 2 3 ) . I n a s e r i e s o f X and Y z e o l i t e s , i t was shown t h a t o n l y a f r a c t i o n o f e a c h a c i d s i t e c o u l d be t i t r a t e d ( Z 4 ) . T h i s a l l o w e d an e f f i c i e n c y o f s i t e s t o be d e f i n e d . A - I I - A c i d i t y Measurement M e t h o d s . The i d e a l method o f a c i d i t y measurement s h o u l d g i v e i n f o r m a t i o n on s e v e r a l p a r a m e t e r s ,

Katzer; Molecular Sieves—II ACS Symposium Series; American Chemical Society: Washington, DC, 1977.


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t h e n a t u r e , number, s t r e n g t h , l o c a t i o n , e n v i r o n m e n t and mean l i f e t i m e o f a c i d s i t e s . Hence i t s h o u l d 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 enough p r e c i s e l y t o d i f f e r e n t i a t e w h i c h t y p e i s r e s p o n s i b l e f o r 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 of a r e a c t a n t m o l e c u l e . In f a c t e a c h 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 acid sites. S p e c t r o s c o p i c methods have been w i d e l y u s e d . C e r t a i n l y , i . r . s p e c t r o s c o p y i s t h e more u s u a l and i t has g i v e n a l a r g e number o f r e s u l t s r e l a t e d t o h y d r o x y l c h a r a c t e r i z a t i o n and t o B r o n s t e d and L e w i s a c i d i t y ( 2 5 , 2 6 ) . A l a r g e number o f works w i l l be d e s c r i b e d i n t h e next paragraphs. A c i d s i t e c o n c e n t r a t i o n s are g e n e r a l l y d e t e r m i n e d s e m i - q u a n t i t a t i v e l y . Q u a n t i t a t i v e e v a l u a t i o n s have been r e c e n t l y p e r f o r m e d ( 1 5 , 2 7 ) . 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 a r e t h e b a s e s commonly u s e d . R e c e n t l y 2-6 d i m e t h y l p y r i d i n e was s u g g e s t e d as 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 s i n c e s t e r i c h i n d r a n c e o f t h e n i t r o g e n atom p r e v e n t s i t s c o o r d i n a t i o n t o a l u minum atoms ( 2 8 ) . I t has been employed w i t h f a u j a s i t e ( 2 9 ) . UV s p e c t r o s c o p y was used t o s t u d y t h e a d s o r p t i o n o f v a r i o u s m o l e c u l e s on z e o l i t e s ( 3 0 , 3 1 ) . I n t h e c a s e o f p y r i d i n e on X z e o l i t e w i t h s o dium c a t i o n s (31) t h r e e k i n d s o f a d s o r b e d s p e c i e s 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 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 L e w i s s i t e s . I t was n o t e d t h a t UV method have t h e a d v a n t a g e t o be v e r y 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 of the r e s p e c t i v e peak maxima. O p t i c a l e l e c t r o n i c s p e c t r o s c o p y was a l s o used t o d i f f e r e n t i a t e p r o t o n i c and non p r o t o n i c s i t e s ( 3 2 ) . B e s i des t h e s t u d y o f r e d o x p r o p e r t i e s o f z e o l i t e s (33) ESR has been a p p l i e d t o s t u d y a t o m i c h y d r o g e n f o r m e d on γ i r r a d i a t i o n and r e l a t e s i t t o p r o t o n i c a c i d i t y ( 3 4 ) . R e l a x a t i o n and l i n e w i d t 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 c a t i o n s ( 1 8 , 3 5 - 3 8 ) . 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 i n t o t h e nature of p r o t o n i c a c i d i t y . I t i s proposed t h a t t h e h i g h e r i s t h e jump f r e q u e n c y ( 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 oxygen a t o m s ) , 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 ( 1 6 , 3 6 ) . I t i s shown t h a t an a d s o r b e d base s u c h 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 2 0 0 ° ) 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 b e c a u s e 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 g i v e n back t o a n o t h e r oxygen atom. I t i s hence s u g g e s t e d t h a t 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 s t u d y i s made i n t h e p r e s e n c e o f b a s i c m o l e c u l e s ( 1 8 ) . NMR measurements a l s o p e r m i t s t h e c o m p u t a t i o n o f an e l e m e n t a r y " 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 d e c u l e d i f f u s i n g upon t h e s u r f a c e . T h i s p r o b a b i l i t y d e c r e a s e s 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 p e r f o r m e d between 0 ° and 300°C (36) ( 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 methods s e v e r a l a p p r o a c h s 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 . M i n a c h e v , Bremer e t a l . p e r f o r m e d s e v e r a l w o r k s u s i n g H D 2 exchange t o s t u d y t h e p r o t o n m o b i l i t y ( 3 9 - 4 1 ) . The c o n c e n t r a t i o n o f e x c h a n g e a b l e OH g r o u p s ( 4 0 , 4 2 ) c o u l d a l s o be d e t e r m i n e d . S e v e r a l t h e r m a l methods have been used t o s t u d y t h e i n t e r a c t i o n s o f b a s e s w i t h a c i d s i t e s [OTA ( 4 3 ) , c a l o r i m e t r y and c h r o m a t o g r a p h y ( 4 4 - 4 6 ) ] . I t was shown t h a t _

2



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IMHg ° r b u t y l a m i n e g i v e a s m a l l h e a t o f a d s o r p t i o n on c a t i o n s . 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 f r o m 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 n e p o i sonned s a m p l e s ( 4 5 ) . A method 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 f o r m s o f z e o l i t e s has been d e s c r i b e d t o measure t h e number o f a c i d c e n t e r s . I t d i s t i n g u i s h e s between B r o n s t e d and L e w i s a c i d i t i e s (47) . 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 [9), t i t r a t i o n with butylamine and c o l o r e d i n d i c a t o r s has been used ( 2 0 , 2 3 , 2 4 , 4 8 , 4 9 ) . The method a l l o w an e a s y d e t e r m i 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 c e n t e r s . The q u e s t i o n a r i s e s a l s o as t o t h e s i z e o f t h e r e a c t a n t s may m o d i f y t h e r e s u l t s . I n f a c t i n Y z e o l i t e s , t h e num b e r o f a c i d s i t e s d e t e r m i n e d i n t h i s way i s c l o s e t o t h a t deduced f r o m i . r . e x p e r i m e n t s u s i n g p y r i d i n e ( 1 5 ) . The r e s u l t s p e r f o r m e d w i t h v a r i o u s b a s e s , i n d i c a t o r s and z e o l i t e s s u g g e s t t h a t w i t h r e gard t o the a p e r t u r e of the z e o l i t e channels, the s i z e of the b a se m o l e c u l e i s more c r i t i c a l t h a n t h a t o f t h e i n d i c a t o r . A s s u m i n g t h a t a t t h e e q u i l i b r i u m t h e b a s e , s m a l l enough t o move i n t h e c h a n n e l 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 w h e r e v e r t h e y a r e l o c a t e d ( i n s i d e ou 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 m o l e c u l e s 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 f r o m t h e r e a c t i o n w i t h the only a c c e s s i b l e s i t e s . Among a l l t h e s e methods, t h e one w h i c h has b r o u g h t t h e newest i d e a s i s t h e NMR w h i c h d e m o n s t r a t e d t h e p r o t o n m o b i l i t y even i n the absence of adsorbed m o l e c u l e . At f i r s t t h e p r o t o n m o b i l i t y a f f o r d s a new d e f i n i t i o n o f a c i d s t r e n g t h f r o m t h e jump f r e q u e n c y v a l u e s . T h i s a p p r o a c h seems v e r y i n t e r e s t i n g w i t h r e g a r d s t o t h e c h a n g e s upon h e a t i n g o r u l t r a s t a b i l i z a t i o n ( 3 6 ) . N e v e r t h e l e s s i t i s n o t known i f t h e n u m e r i c a l v a l u e depends o n l y on t h e a c i d s t r e n g t h . F o r example t h e i n c r e a s e i n jump f r e q u e n c y upon p y r i d i n e a d s o r p t i o n i s r e l a t e d t o the help of p y r i d i n e which improves the p r o t o n m i g r a t i o n ( 1 8 ) . T h i s m i g h t be compared t o t h e h i g h p r o t o n m o b i l i t y i n aqueous s o l u t i o n s . S e c o n d l y t h e p r o t o n m o b i l i t y i s a r a t h e r new f a c t w h i c h i s i n agreement w i t h t h e i d e a t h a t z e o l i t e f r a m e w o r k have some p r o p e r t i e s o f e l e c t r o l y t e s ( 5 0 ) . The q u e s t i o n a r i s e s as t o w h e t h e r i t w i l l be p o s s i b l e t o p r o c e e d t h e a n a l o g y with a c i d i c s o l u t i o n s . A t h i r d aspect of proton m o b i l i t y i s that i t may e x p l a i n some phenomena s u c h as an u n u s u a l d e h y d r o x y l a t i o n w h i c h w i l l be m e n t i o n n e d f u r t h e r . Among t h e o t h e r m e t h o d s , i n f r a r e d s p e c t r o s c o p y i s t h e most p o w e r f u l s i n c e i t g i v e s a v e r y l a r g e number o f i n f o r m a t i o n s 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 to c o r r e l a t e p r e c i s e l y i n f r a - r e d r e s u l t s with other properties s u c h as c a t a l y t i c b e h a v i o r , 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 z a t i o n o f s t a t i c and d e f i n i t e h y d r o x y l g r o u p s i s n o t p r e c i s e enough t o e x p l a i n w h i c h 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 p r o cesses. A - III Generation of A c i d S i t e s . Acid centers are generated i n v a r i o u s ways, i ) The t h e r m a l d e c o m p o s i t i o n o f ammonium e x c h a n ged z e o l i t e s y i e l d s t h e h y d r o g e n f o r m (51 ). Deammination i n a n h y -



38.

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and Catalytic

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Zeolites

457

d r o u s c o n d i t i o n s o f a1Kyiammonium, p i p e r i d i n i u m 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 c a s e o f p r i m a r y alkylammonium i o n s . W i t h 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 d e h y d r o x y l a t i o n i s observed producing a s o - c a l l e d dehydroxyl a t e d z e o l i t e w i t h L e w i s a c i d s i t e s (52_). T h i s d e h y d r o x y l a t i o n e f f e c t i s a l s o observed d u r i n g t h e a d s o r p t i o n of amines, p a r t i c u l a r l y w i t h p y r i d i n e C 5 3 , 5 4 ) . One c a n wonder w h e t h e r 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 p r e s e n c e o f p y r i d i n e (18) 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, i i ) The B r o n s t e d a 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 ( 2 , 2 5 , 5 5 ) a l r e a d y w e l l d e s c r i b e d has been s t u d i e d by v a r i o u s m e t h o d s . 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 s t a n c e (dH2u) i n w a t e r 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 ( 5_6 ). I n v a r i o u s p o l y v a l e n t Y z e o l i t e s , s e v e r a l (up t o 6) t y p e s o f OH g r o u p s were f o u n d (4Ό) d i f f e r i n g by t h e i r c o n c e n t r a t i o n , a c i d s t r e n g t h and t h e r m a l s t a b i l i t y , i i i ) The r e d u c t i o n by hydrogen o f t r a n s i t i o n m e t a l c a t i o n s i n z e o l i t e s was s u p p o s e d t o f o r m a hydrogen z e o l i t e ( 5 7 , 5 8 ) . Such B r o n s t e d a c i d i t y has been o b s e r v e d i n an i . r . s t u d y o f a hydrogen r e d u c e d C u Y z e o l i t e (59) (scheme 1) : 2 +

2 Cu + H + 2 Cu + 2 H , 2 Cu + Ho + 2 Cu° + 2 H The r e d u c t i o n o f C u ^ c a t i o n w i t h CO (59_) o r i t s s e l f - r e d u c t i o n (60) g i v e s c u p r o u s i o n s and L e w i 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 g r o u p s 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 2 5 0 - 4 5 0 ° and t o i n c r e a s e w i t h t h e r i s e o f t h e r e d u c t i o n t e m p e r a t u r e ( 6 1 ) . A r e d u c t i o n by h y d r o carbons of c a t i o n s t o metals w i t h f o r m a t i o n of p r o t o n i c a c i d i t y has been shown i n t h e c a s e 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 c r a c k i n g . A s i m i l a r r e d u c t i o n 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 (62) \ ? + Ni° + 2 Η [2] i i i i ) B r ô n s t e d a c i d s i t e s are also generated i n b i v a l e n t c a t i o n c o n t a i n i n g Y z e o l i t e s on e x p o s u r e a t room t e m p e r a t u r e t o h a l i d e compounds (63) o r a t 1 5 0 - 4 0 0 ° C t o C 0 ( 6 4 ) . I n t h e c a s e o f n - p r o p y l h a l i d e , a c i d h a l i d e i s f o r m e d . The h a l i d e i s f i x e d t o t h e c a t i o n and H i s a t t a c h e d t o an oxygen o f t h e z e o l i t e t o g i v e a h y d r o x y l g r o u p ( 6 3 ) . I n t h e second c a s e , C 0 r e a c t s r e v e r s i b l y w i t h M ( 0 H ) r e s u l t i n g f r o m w a t e r h y d r o l y s i s on Mg o r Ca c a t i o n s and g i v e s an u n i d e n t a t e c a r b o n a t e s p e c i e s w h i l e t h e h y d r o g e n atom f o r m s an a c i d i c OH g r o u p w i t h a f r a m e w o r k oxygen atom g i v i n g i . r . bands a t 3640 and 3550 cm"1 (64) (scheme 3) : 2 +

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The v a r i o u s and i n d e p e n d e n t 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 e x p e r i m e n t a l 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 measurements c o u l d m o d i f y g r e a t l y t h e i n t r i n s i c a c i d i t y . F u r t h e r t h i s s u g g e s t s 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 p r e s e n c e o f c a t a l y t i c r e a g e n t s ( d e h y d r o x y l a t i o n by r e a c t a n t s , 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 h y d r o c a r b o n s , r e a c t i o n w i t h a c i d i c compounds). Hence i n d e p e n d e n t l y o f a g i n g e f f e c t s , t h e r e may be


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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 talyst. A - I V - A c i d S i t e Number and S t r e n g t h . The number o f a c i d s i t e s has been i n v e s t i g a t e d by v a r i o u s methods ( 2 , 2 5 , 6 5 ) . The me t h o d based on changes i n a c i d i c QH g r o u p s c o n c e n t r a t i o n has been w i d e l y and s u c c e s s f u l l y used (25_). J a c o b s e t a l . (53) s t u d i e d t h e a d s o r p t i o n o f d i f f e r e n t amines on N a - h y d r o g e n X and Y z e o l i t e s They showed t h a t a d s o r p t i o n i n v o l v e s t h e r e a c t i o n w i t h a c i d i c h y d r o x y l s , t h e r e s i d u a l s o d i u m i o n s and t h e d e h y d r o x y l a t e d s i t e s . They c o n c l u d e t h a t t h e d e c r e a s e o f t h e i n t e g r a t e d a b s o r b a n c e o f amines c a n n o t be used t o d e t e r m i n e t h e a p p a r e n t i n t e g r a t e d i n t e n s i t y o f OH b a n d s . I n s l i g h t l y d i f f e r e n t c o n d i t i o n s , B i e l a n s k i e t a l . (27) were a b l e t o e v a l u a t e e x t i n c t i o n c o e f f i c i e n t o f OH bands and t h e a p p r o x i m a t e c o n c e n t r a t i o n s o f b o t h t y p e s o f OH g r o u p s . Ir r e s p e c t i v e l y t o t h e v a l u e o f S i / A l r a t i o s and p r o v i d i n g 50 % a t l e a s t o f t h e c a t i o n s have been e x c h a n g e d , t h e h i g h e s t c o n c e n t r a t i o n o f t h e 3650 c m ' OH g r o u p s i s e q u a l t o a b o u t 16 OH g r o u p s p e r u n i t c e l l , i . e . one OH g r o u p on a v e r a g e p e r h e x a g o n a l p r i s m . S i m i l a r v a l u e s were o b t a i n e d by J a c o b s e t a l . by q u a n t i t a t i v e i . r . s p e c t r o s c o p y ( 1 6 ) . The same a u t h o r s o b t a i n e d h i g h e r v a l u e s u s i n g r e a c t i o n w i t h p y r i d i n e which i s p o s t u l a t e d to t i t r a t e supercages a c i d i c OH v i b r a t i n g a t 3650 c m " . They f o u n d c l o s e t o 35 OH g r o u p s p e r u n i t c e l l f o r a h y d r o g e n Y sample 80 % exchanged and h e a t e d a t 4 0 0 ° C . The number o f h y d r o x y I s depends on t h e S i / A l f r a m e w o r k r a t i o . B r o n s t e d a c i d i t y deduced f r o m i . r . bands o f p y r i d i n i u m i o n s showed s i m i l a r v a l u e s (1_5). The t i t r a t i o n w i t h amine and c o l o r e d i n d i c a t o r s showed t h a t c l o s e t o 30 % o f t h e a c i d s i t e s a r e weak (20) i n h y d r o g e n Y z e o l i t e s . They a r e r e l a t e d t o t h e 30 % o f w e a k l y bonded aluminum atoms e a s i l y r e m o v a b l e upon d e a l u m i n a t i o n o r p a r t i c u l a r h e a t i n g ( 2 0 , 5 1 ) . The number o f a c i d s i t e s o f t h e s o - c a l l e d a l u m i n u m - d e f i c i e n t , d e e p bed o r u l t r a s t a b l e z e o l i t e s has been e v a l u a t e d . The c o m p a r i s o n w i t h h y d r o g e n Y i s n o t e a s y s i n c e s i m u l t a n e o u s l y when t h e aluminum framework c o n t e n t i s d e c r e a s e d , the t h e r m a l s t a b i l i t y i s i n c r e a s e d and g e n e r a l l y t h e sodium c o n t e n t i s l o w e r e d . J a c o b s , U y t t e r h o e v e n e t a l . (15) f o u n d t h a t t h e i r d e e p - b e d s a m p l e s a r e l e s s a c i d t h a n HY w h i l e T o p c h i e v a e t a l . (49) showed t h a t d e a l u m i n a t e d and u l t r a s t a b l e s a m p l e s t h e y s t u d i e d a r e more a c i d i c t h a n Y z e o l i t e . The r a t i o s t r o n g / w e a k a c i d i t y c o n c e n t r a t i o n i n c r e a s e s upon d e a l u m i n a t i o n ( 2 0 , 4 6 ) . Beaumont e t a l . (66) showed m o r e o v e r t h a t t h e i r d e a l u m i n a t e d s a m p l e s c o n t a i n s t r o n g e r B r o n s t e d and L e w i s s i t e s t h a n t h e p a r e n t h y d r o g e n f o r m . A h i g h e r jump f r e q u e n c y o f p r o t o n s , i . e. a h i g h e r a c i d s t r e n g t h i n d e e p - b e d Y z e o l i t e s has a l s o be n o ted (36). Other kinds of z e o l i t e s m o d i f i c a t i o n s imply a c i d i t y changes. With regard to d e c a t i o n i z a t i o n , B i e l a n s k i e t a l . (67) o b s e r v e d a r i s e i n B r o n s t e d a c i d i t y s t r e n g t h a t exchange l e v e l s h i g h e r t h a n 70 %. The m o b i l i t y o f p r o t o n s was a l s o r e p o r t e d t o be t h e h i g h e s t w i t h h i g h l y exchanged HNaY z e o l i t e s ( 3 9 ) . The d e c a t i o n i z a t i o n a l s o leads t o a r e i n f o r c e m e n t of the e l e c t r o n - a c c e p t i n g p r o p e r t i e s of 1

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Zeolites

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t h e s u r f a c e ( 6 8 ) . Changes upon s t e a m i n g were n o t e d f o r a number o f z e o l i t e s . W i t h r a r e e a r t h X and Y z e o l i t e s t h e r e i s a d e c r e a s e i n t h e number o f s t r o n g a c i d s i t e s w h i l e t h e number o f s i t e s w i t h i n t e r m e d i a t e and weak a c i d s t r e n g t h r e m a i n s unchanged ( 4 8 ) . W i t h Y z e o l i t e s of various S i / A l r a t i o s the a c i d s i t e concentration i s a l s o d e c r e a s e d ( 6 9 , 7 0 ) . The p r e s e n c e o f B r o n s t e d and L e w i s a c i d i t y i s m a i n t a i n e d i n L a z e o l i t e s even a f t e r s t e a m i n g a t 820°C (71_). D e h y d r o x y l a t i o n o f Y z e o l i t e s upon h e a t i n g i n c r e a s e s t h e e l e c t r o n a f f i n i t y of Lewis c e n t e r s [72]. Hence t h e s t r e n g t h and t h e m o b i l i t y o f p r o t o n s i n c r e a s e upon d e a l u m i n a t i o n and h i g h exchange o f c a t i o n s . M o r e o v e r t h e L e w i s a c i d i t y s t r e n g t h i s a l s o i n c r e a s e d by d e h y d r o x y l a t i o n . I f t h e se s t r e n g t h changes d u r i n g d e c a t i o n a t i o n and d e h y d r o x y l a t i o n a r e l o g i c a l , t h e i n c r e a s e due t o d e a l u m i n a t i o n and t h e d e c r e a s e r e l a t e d t o s t e a m i n g a r e more d i f f i c u l t t o be p r e d i c t e d . E s p e c i a l l y i n the case of steaming t h e r e a c t i o n s a r e complex, i n v o l v i n g chemist r y o f t h e s o l i d s t a t e and i t w o u l d be v e r y e x i t i n g t o f i n d a model describing the r e s u l t s . The a c i d i t y d e t e r m i n a t i o n o f z e o l i t e s w i t h r a t h e r s m a l l p o r e s i s o f c o u r s e c o m p l i c a t e d by t h e a c c e s s i b i l i t y o f s i t e s p a r t i c u l a r l y w i t h p y r i d i n e . Hence D e t r e k o y and a l . (73) showed t h a t o n l y 10 % o f t h e OH g r o u p s o f c l i n o p t i l o l i t e r e a c t w i t h p y r i d i n e . They c o n c l u d e t h a t o n l y 10 % o f t h e l a t t i c e OH a r e on t h e e x t e r n a l s u r f a c e . I n t h e c a s e o f L z e o l i t e s (74) t h e a c i d i t y d e c r e a s e a t exchange l e v e l s h i g h e r t h a n 50 % i s e x p l a i n e d i n t e r m s o f a p r o t o n m i g r a t i o n t o w a r d h i d d e n s i t e s i n c a g e s made f r e e o f c a t i o n s by e x change. With m o r d e n i t e , t h e r e s t r i c t i o n f o r p y r i d i n e r e a c t i o n w i t h the a c i d s i t e s i n the channels p a r a l l e l s the i o n i c radius : H < Be < Ca < Ce ( 7 5 ) . M o r d e n i t e has s t r o n g e r ( 4 4 , 7 6 , 7 7 ) and t h e r m a l l y more s t a b l e (750 p r o t o n i c s i t e s t h a n f a u j a s i t e . The a c i d i t y o f d e a l i m i n a t e d m o r d e n i t e measured by NH3 c h e m i s o r p t i o n d e c r e a s e s r o u g h l y l i n e a r l y w i t h A l c o n t e n t (7_8). A - V - H y d r o x y l Groups i n Z e o l i t e s . A s s i g n m e n t s and A c i d - B a s e Properties. The h y d r o x y l g r o u p s o f f a u j a s i t e t y p e a r e now w e l l d e s c r i b e d ( 2 5 , 2 6 ) . The new i n f o r m a t i o n w h i c h a p p e a r e d a r e m a i n l y r e l a t e d t o t h e r e f i n e m e n t s o f t h e a s s i g n m e n t s o f t h e i . r . bands and t o t h e c h a r a c t e r i z a t i o n o f t h e h y d r o x y l a c i d and b a s i c p r o p e r t i e s . J a c o b s and U y t t e r h o e v e n (79) b r o k e down i n t o s i x d i f f e r e n t components t h e s p e c t r a o f h y d r o x y I s i n X and Y m a t e r i a l s . The n a r row band a t 3650 c m " i s a s s i g n e d t o O i H h y d r o x y l g r o u p s w h i l e t h e b r o a d and a s y m é t r i e band a t 3550 c m " c o n t a i n s components due t o O2H, O3H and O4H g r o u p s . I n h y d r o l y z e d z e o l i t e s ( d e e p - b e d ) non a c i d i c OH bands a r o u n d 3680 and 3600 c m " a r e due t o s u r f a c e OH g r o u p s n e a r A l d e f e c t l o c a t i o n . The non a c i d i c OH bands i n s y n t h e t i c f a u j a s i t e exchanged w i t h p o l y v a l e n t c a t i o n s ( i . e . La) a r e e x p l a i n e d on t h e same b a s i s . P e r i (80) r e p o r t e d t h a t i n u l t r a s t a b l e f a u j a s i t e a 3700 c m ' band was e l i m i n a t e d by NH3 a d s o r p t i o n b u t bands a t 3750 c m " and 3620 c m ' , a c c e s s i b l e t o NH3, were r e l a t i v e l y u n a f f e c t e d . S c h e r z e r and B a s s (71) s t u d i e d l o w - s o d a l a n t h a n u m 1

1

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460

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Y z e o l i t e s t h e r m a l l y and h y d r o t h e r m a l l y t r e a t e d and showed t h a t s t e a m i n g p r o d u c e s d e a l u m i n a t i o n . The bands a t 3650 and 3600 c m " a r e a c i d i c ( r e a c t i o n w i t h NH3, p y r i d i n e , NaOH) whereas t h e band a t 3700 c m " i s non a c i d i c . The band a t 3540 c m " i s a c i d i c i n l o w - L a Y z e o l i t e s but i s p r a c t i c a l l y n o n - a c i d i c i n h i g h - L a z e o l i t e s . The bands a t 3700, 3650 and 3540 c m " a r e g e n e r a t e d p r i m a r l y by O4, 0 i and O3 f r a m e w o r k atoms r e s p e c t i v e l y . The 3540 c m " band i s a l s o g e n e r a t e d by OH g r o u p s a t t a c h e d t o La i o n s . The r e s u l t s p r e s e n t e d by t h e s e a u t h o r s show some d i s c r e p a n c i e s w h i c h c a n n o t be e x p l a i n e d on t h e b a s i s o f v e r y s i m p l e m o d e l s . T h i s s u g g e s t s t h a t p a r a m e t e r s s u c h as t h e t h e r m a l t r e a t m e n t s m i g h t be as i m p o r t a n t o r even more i m p o r t a n t t h a n t h e c h e m i c a l c o m p o s i t i o n i n d e t e r m i n i n g t h e h y d r o x y l s p r o p e r t i e s . M o r e o v e r i t has been now c l e a r l y d e m o n s t r a t e d t h a t p r o t o n s a r e m o b i l e . T h i s d y n a m i c p r o p e r t y may r e n d e r t o o s i m p l e t h e s t a t i c model o f h y d r o x y l s p r o p o s e d f o r z e o l i t e s . One m i g h t s u g g e s t t h a t j u m p i n g f r o m oxygen t o oxygen t h e p r o t o n s w o u l d p r e f e r some o f t h e s e atoms f o r e n e r g e t i c r e a s o n s . Hence w h a t e v e r t h e i r s o u r c e s , p r o t o n s c o u l d g i v e r i s e t o t h e same i . r . b a n d , t h e f r e q u e n c y o f w h i c h w o u l d be d e t e r m i n e d by t h e t y p e o f f r a m e w o r k oxygen i n v o l v e d . S i n c e a d e f i n i t e i . r . band i s b r o a d enough t o i n c l u d e a s m a l l r a n g e o f f r e q u e n c i e s e a c h o f t h e s e bands would c h a r a c t e r i z e a v a r i e t y of h y d r o x y l s w i t h d i f f e r e n t c h e m i c a l p r o p e r t i e s ( a c i d i c o r non a c i d i c ) d e t e r m i n e d by t h e v a r i o u s m i c r o e n v i r o n m e n t s o f t h e oxygen atom i n v o l v e d . B e s i d e s t h e c a s e o f t h e s e t h e r m a l l y t r e a t e d f a u j a s i t e , new r e s u l t s have been r e p o r t e d on o t h e r z e o l i t e s . I n t h e c a s e o f non r e d u c e d N i - h y d r o g e n Y, Ward (81 ) r e p o r t e d a band a t 3680 c m " a s s i g n e d t o NiOH and two bands a t 3635 and 3540 c m " a n a l o g o u s t o t h o s e o b s e r v e d i n hydrogen Y z e o l i t e s . Deammination o f NH4X and PdNH4X z e o l i t e s g i v e s bands a t 3660, 3625 and 3560 c m " ( 8 2 ) . The 3625 c m " band w h i c h i s n o t o f ten observed i n c r e a s e s w i t h the e x t e n t of exchange. I t i s assigned t o OH g r o u p s i n t h e s u p e r c a g e . As t o b a s i c p r o p e r t i e s , B i e l a n s k i et D a t k a (83) i n d i c a t e d a r e a c t i o n o f t h e OH v i b r a t i n g a t 3660 c m " and 3565 c m ' i n Y z e o l i t e s w i t h a c i d i c compounds s u c h as f o r m i c , a c e t i c and b e n z o i c a c i d s . T h i s p o i n t s o u t an a m p h o t e r i c p r o p e r t y o f t h e s e h y d r o x y l s . The non a c i d i c bands i n Mg and Ca hydrogen Y z e o l i t e s , v i b r a t i n g a t 3685 and 3675 c m " r e s p e c t i v e l y were shown t o r e a c t r e v e r s i b l y w i t h C02- The i n c r e a s e i n band i n t e n s i t y w h i c h p a r a l l e l s t h e c a t i o n c o n t e n t and t h e r e v e r s i b l e f o r m a t i o n o f an u n i d e n t a t e c a r b o n a t e s u g g e s t t h a t t h e s e b a s i c OH a r e l i n k e d t o t h e c a t i o n (640 (scheme 3 ) . The OH g r o u p s o f m o r d e n i t e were s t u d i e d by means o f i . r . s p e c t r o s c o p y . Two OH bands o c c u r i n NH4 m o r d e n i t e a t 3637 c m " and 3740 c m " and a r e s h i f t e d t o 3612 c m ' and 3715 c m " r e s p e c t i v e l y upon h e a t i n g (£34). Karge n o t e d i n H m o r d e n i t e an a c i d i c band a t 3610 c r r r and a non a c i d i c OH band a t 3640 c m " (85). 1

1

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A - V I - Base C a t i o n I n t e r a c t i o n s . A c i d i t y o f T r a n s i t i o n M e t a l Zeolites. The s i m p l e s t m o l e c u l e a d s o r b e d , w a t e r , i s known t o g e n e r a t e a c i d i t y t h r o u g h t h e d i s s o c i a t i o n o f w a t e r on p o l y v a l e n t



38.

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461

i o n s and t o i n d u c e c a t i o n movement ( 6 6 ) . I n 1 9 6 4 , B a u r (67) p o i n t e d o u t t h a t i n z e o l i t e s t h e w a t e r m o l e c u l e s and c a t i o n s a c t as a m o b i l e e l e c t r o l y t e s o l u t i o n . I n f a c t i t has now been shown t h a t c a t i o n s may a l s o move e a s i l y i n t h e p r e s e n c e o f m o l e c u l e s o t h e r t h a n w a t e r . I t has been f o u n d t h a t c a t i o n s i n t e r a c t w i t h p y r i d i n e (25) o r a r o m a t i c s ( 1 1 ) . L e i t h e t a l . (88_) r e p o r t a c o p p e r m i g r a t i o n a f t e r treatment of the z e o l i t e with but-1-ene. G a l l e z o t et a l . showed v e r y a c c u r a t e l y t h a t s e v e r a l m o l e c u l e s c o u l d m o d i f y t h e c a t i o n l o c a t i o n and f o r m o r g a n o m e t a l l i c c o m p l e x e s i n CuY (69) and N i Y (90) z e o l i t e s . A n i c k e l i o n m i g r a t i o n i n N i X z e o l i t e has a l s o been r e p o r t e d ( 5 4 , 9 1 ) . Due t o t h e i m p o r t a n c e o f t h e i n t e r a c t i o n s and m i g r a t i o n s , t h e se e f f e c t s have t o be c o n s i d e r e d i n d e t e r m i n i n g t h e a c i d i c p r o p e r t i e s o f z e o l i t e s w i t h t r a n s i t i o n m e t a l c a t i o n . They r e n d e r t h e e v a l u a t i o n o f t h e number o f base m o l e c u l e r e a c t i n g o n l y w i t h t h e a c i d s i t e s d i f f i c u t . The c o r r e l a t i o n s w i t h c a t a l y t i c a c t i v i t y w o u l d have a l s o t o c o n s i d e r t h a t c a t i o n l o c a t i o n may d i f f e r i n a c i d i t y and c a t a l y t i c measurements. O n l y a f e w works have been r e p o r t e d on t h e a c i d i t y m e a s u r e ments o f t r a n s i t i o n m e t a l c a t i o n z e o l i t e s ( 2 5 ) . The p r o t o n i c a c i d i t y o f a s e r i e s o f NiHY z e o l i t e s (81) d e t e r m i n e d f r o m t h e c o n c e n t r a t i o n of pyridinium ions f o l l o w s very c l o s e l y the population of t h e 3635 c m " h y d r o x y l g r o u p s . No L e w i s a c i d s i t e s were o b s e r v e d . The r e d u c t i o n o f t r a n s i t i o n m e t a l c a t i o n c r e a t e s a new a c i d i t y as e v i d e n c e d w i t h a c o p p e r Y z e o l i t e c o n t a i n i n g 6.5 % c u p r i c i o n s (12 c a t i o n s p e r u n i t c e l l ) ( 5 9 ) . S t e i n b e r g e t a l . (61) a l s o p o i n t e d o u t t h a t t h e c o n c e n t r a t i o n o f OH g r o u p s o f Y z e o l i t e s w i t h N i , Co and Cu c a t i o n s i s i n c r e a s e d upon r e d u c t i o n w i t h h y d r o g e n and upon i n c r e a s e o f r e d u c t i o n t e m p e r a t u r e . However w i t h n o b l e m e t a l s , t h e u s u a l c o n t e n t s i n c a t a l y s t s w i l l r e n d e r t h i s a c i d i t y change v e r y s m a l l . S t u d i e s o f CaHY w i t h 0.5 % o f P t , i . e * c l o s e t o 0 . 3 c a t i o n s p e r u n i t c e l l , d i d n o t show any m e a s u r a b l e a c i d i t y m o d i f i c a t i o n s due t o t h e r e d u c t i o n o f p l a t i n u m w i t h h y d r o g e n (92^). I n P t Y m a t e r i a l s w i t h a h i g h P t c o n t e n t o f 6 t o 14 %, G a l l e z o t e t a l . [93] showed a s t r o n g N H P t i n t e r a c t i o n f o r P t - a g g l o m e r a t e s 10 A i n d i a m e t e r . They s u g g e s t a c o m p e t i t i v e a d s o r p t i o n between p l a t i n u m and B r o n s t e d s i t e s f o r N H 3 . To c o n c l u d e t h i s c h a p t e r one may f i r s t p o i n t o u t a g a i n t h e i m p o r t a n c e o f p r e t r e a t m e n t s c o n d i t i o n s and t h e m o d i f i c a t i o n s i n t r o d u c e d by t h e base i n t h e i o n ( c a t i o n o r p r o t o n ) l o c a t i o n and m o b i l i t y . S e c o n d l y t h e e x i s t e n c e o f b a s i c OH g r o u p s i n p o l y v a l e n t forms o f z e o l i t e s suggest t h a t such b a s i c h y d r o x y l s might e x i s t i n o t h e r z e o l i t e s , f o r example US o r d e a l u m i n a t e d m a t e r i a l s w h i c h h a ve s e v e r a l non a c i d i c OH g r o u p s n e a r 3600 and 3700 c m " . 1

3

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Β - CORRELATIONS BETWEEN ACIDITY AND CATALYTIC ACTIVITY B - I - G e n e r a l Comments. The r e s e a r c h o f c o r r e l a t i o n s between t h e a c i d i c and c a t a l y t i c p r o p e r t i e s o f a c i d i c o x i d e s has been a m a t t e r o f i n t e r e s t f o r a v e r y l o n g t i m e . The new f i e l d o f z e o l i t e s has p r o v o k e d an i n c r e a s e i n t h e number o f works r e l a t e d t o t h e


MOLECULAR

462

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s u b j e c t and v e r y i n t e r e s t i n g r e s u l t s have been d e s c r i b e d ( 2 , 2 5 ) . T r y i n g t o r e f i n e the d e s c r i p t i o n of a c t i v e s i t e s i n terms of a c i d i t y , i t soon a p p e a r e d t h a t t h e q u i t e w e l l d e f i n e d z e o l i t e s t r u c t u r e d i d n o t a l l o w i n f a c t a r a p i d answer t o t h e p r o b l e m as i t was e x p e c t e d . At the p r e s e n t t i m e t h e p u b l i s h e d works p o i n t out t h e d i f f i c u l t y t o f i n d a d i r e c t and g e n e r a l c o r r e l a t i o n between a c i d i t y and a c t i v i t y . B e s i d e s t h e f a c t t h a t a c i d i t y s t u d i e s a r e p e r f o r med on f r e s h c a t a l y s t s w h i l e , o f t e n , c a t a l y t i c measurements r e f e r r e d t o aged m a t e r i a l s * some o t h e r p o i n t s w h i c h may a c c o u n t f o r t h i s d i f f i c u l t y w i l l be c o n s i d e r e d . §ZÎIËI_ÇE9Î9D_'! 9bility I n r e a c t i o n s s u c h as c r a c k i n g f o r instance i t i s g e n e r a l l y admitted that protons are a c t i v e centers ( 1 5 ) . The d i f f i c u l t i e s t h e n a r i s e f o r m t h e p r o t o n p r o p e r t i e s t h e m s e l v e s , p a r t i c u l a r l y t h e p r o t o n m o b i l i t y w h i c h depends on v a r i o u s f a c t o r s : z e o l i t e p r e t r e a t m e n t , adsorbed s p e c i e s , coverage, temper a t u r e . . . ( 1 8 , 3 6 - 3 8 ) . The p r o t o n m o t i o n i n v o l v e s two f a c t s . A t f i r s t p r o t o n s i n i n a c c e s s i b l e c a v i t i e s may be a t t r a c t e d i n l a r g e c a g e s . S e c o n d l y t h e jump f r e q u e n c y depends on t h e r e a c t a n t . Hence i t w i l l n o t be easy t o f i n d a b a s i c m o l e c u l e t o t i t r a t e a c i d i t y w h i c h r e a c t s , w i t h r e g a r d t o t h e s e two p a r a m e t e r s , i n t h e same way t h a n t h e r e a c t i n g m o l e c u l e . I n t h e c a s e o f h y d r o x y l s t u d i e s where no b a s i c m o l e c u l e i s i m p l i e d , c o r r e l a t i o n s have been o b s e r v e d b e t ween DH g r o u p s c o n c e n t r a t i o n and a c t i v i t y . However, R i c h a r d s o n (11) s u g g e s t s t h a t o n l y t h e more a c i d i c f r a c t i o n o f t h e h y d r o x y l s i s a c t i v e i n cumene c r a c k i n g . T h i s f r a c t i o n , up t o now n o t e a s i l y m e a s u r a b l e , w o u l d be i n c r e a s e d upon t h e i n f l u e n c e o f p a r a m e t e r s such as t h e i o n i c p o t e n t i a l o f t h e c a t i o n . S t u d y i n g a s e r i e s o f v a r i o u s hydrogen o r c a t i o n i c f o r m s o r Y z e o l i t e s , J a c o b s and a l . (15) a l s o showed t h a t o n l y a p a r t o f t h e h y d r o x y l s a c t as a c t i v e s i t e s . They a r e " s u p e r a c t i v e s i t e s " . The a u t h o r s e x t e n d t h e h y p o t h e s i s i n c o n s i d e r i n g t h a t due t o " t h e h i g h m o b i l i t y o f p r o t o n s i n the t e m p e r a t u r e range r e q u i r e d f o r c r a c k i n g , i t i s not p o s s i b l e to c o r r e l a t e the c r a c k i n g a c t i v i t y w i t h d e f i n i t e hydroxyl groups as o b s e r v e d i n t h e i n f r a r e d s p e c t r u m " . l


1

§_ïlkl_Number_of_Ô95î Sites W h i l e a c i d i t y measurements p r o v i d e v a l u e s c l o s e t o ÎÔ^O, io,2T s i t e s p e r g, i n agreement w i t h t h e number o f p o t e n t i a l a c i d s i t e s , a c t i v i t y s t u d i e s g i v e r e s u l t s n e a r t h e s e v a l u e s o n l y when c o m p l e t e p o i s o n i n g w i t h b a s e s a r e used ( 2 , 1 0 , 1 1 , 1 5 ) . The v a l u e s o b t a i n e d f r o m p o i s o n i n g o f t h e more a c t i ve s i t e s a r e 10^ t i m e s l o w e r (15) w h i l e t h o s e deduced f r o m k i n e t i c s t u d i e s a r e ^ 1 0 ^ t i m e s l o w e r ( 1 1 ) . Such d i f f e r e n c e s had a l r e a d y been r e p o r t e d f o r amorphous s i l i c a - a l u m i n a c a t a l y s t s . The q u e s t i o n a r i s e s as t o w h e t h e r t h e v a r i o u s measurements methods a r e q u e s t i o n n a b l e , o r , i f t h e number o f a c t i v e s i t e s b e i n g a c t u a l l y v e r y l o w , t h e v a l u e s g i v e n by p o i s o n n i n g must be r e g a r d e d as u p p e r l i mits only. v e

1

Β-I-c-_Comgetitiye_Adsorgtion on_Cation_and_Acid_Centers^ A d s o r p t i o n and c a t a l y t i c r e s u l t s s u g g e s t t h a t t h e r e i s a c o m p e t i -


38.

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Acidic

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t i v e a d s o r p t i o n o f a r e a c t a n t R on t h e u n c o v e r e d c a t i o n i c (C) and a c i d i c (A] s i t e s o f a z e o l i t e ( £ 4 ) . The f o l l o w i n g e q u i l i b r i u m w i t h t h e a d s o r b e d m o l e c u l e s RA and RC m i g h t d e s c r i b e t h e s y s t e m : A + C + 2 R J RA + RC [4] The t o t a l r a t e r o f t h e t r a n s f o r m a t i o n o f a r e a c t a n t i s t h e sum o f t h e r a t e s on a c i d and c a t i o n i c s i t e s ( r and r ç r e s p e c t i v e l y ] . I t may be w r i t t e n : r = r + r = Κ RA + K' RC [5] The h i g h e r t h e a d s o r p t i o n on c a t i o n s ( h i g h RC) t h e s m a l l e r w i l l be RA, hence a l s o t h e r a t e r on a c i d s i t e s . C a t a l y s i s on a c i d s i t e s may t h e n be dependent o f t h e a d s o r p t i o n on c a t i o n s . The t h r e e p o i n t s o f t h e s e g e n e r a l comments s u g g e s t t h a t i n order to reach a b e t t e r understanding of the c o r r e l a t i o n s a c i d i t y a c t i v i t y t h e d y n a m i c s o f t h e s y s t e m s h o u l d be c o n s i d e r e d i n a d d i t i o n t o t h e u s u a l " s t a t i c " p a r a m e t e r s used t o d e f i n e t h e a c i d c e n t e r s ( n a t u r e , s t r e n g t h , c o n c e n t r a t i o n , GH t y p e ) . T h i s c o n c l u s i o n i s r e i n f o r c e d when one c o n s i d e r t h e f a c i l i t i e s o f c a t i o n movements i n the s t r u c t u r e . A

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c


A

B-IIA c c e s s i b i l i t y o f S i t e s and C a v i t i e s . A g r o w i n g number o f f u n d a m e n t a l r e s e a r c h seems t o be d e v o t e d t o m o r d e n i t e c a t a l y s t s . Weeks e t a l . (840 p o i n t e d o u t t h a t amorphous a l u m i n a d e p o s i t s f o r m e d i n t h e c h a n n e l s upon h e a t i n g may c o n s i d e r a b l y m o d i f y c a t a l y t i c a c t i v i t y . S t e r i c e f f e c t s due t o t h e p o r e s i z e a r e n o t e d i n t - b u t y l a l c o h o l d e h y d r a t i o n ( 9 5 ) , benzene a l k y l a t i o n w i t h e t h y l e n e and p r o p y l e n e (9ΕΠ , t o l u e n e d i s p r o p o r t i o n a t i o n and o - x y l e n e i s o m e r i z a t i o n (75_) . Yashima e t a l . (75_) c o n c l u d e t h a t due t o s t e r i c hindrance, alkylbenzene r e a c t i o n s p a r a l l e l the p r o t o n i c a c i d i t y measured w i t h p y r i d i n e b u t n o t w i t h N H 3 . A c o r r e l a t i o n p y r i d i n e a c i d i t y and c a t a l y t i c a c t i v i t y i s a l s o o b s e r v e d w i t h c l i n o p t i l o l i t e i n b u t - 1 - e n e i s o m e r i z a t i o n ( 7 3 K Due t o t h e " c a g e e f f e c t " i n e r i o n i t e , the observed r a t e of r e a c t i o n p a r a l l e l s the r a t e of r e a c t a n t d i f f u s i o n through the z e o l i t e . This i m p l i e s a unique se l e c t i v i t y (97) (selectoforming). B-III- Proton Catalysed Reactions. A v e r y l a r g e number o f r e a c t i o n s have been d e s c r i b e d . I n many c a s e s t h e a c t i v i t y i s r e l a t e d t o h y d r o x y l g r o u p s ( 1 5 , 1 6 , 7 3 , 9 6 ) . W i t h NgHY z e o l i t e s Ward (96) showed t h a t t h e a c t i v i t y and a c i d i t y maxima o c c u r b e f o r e d e h y d r o x y l a t i o n , w h i c h d i f f e r s f r o m h y d r o g e n Y. The d i f f e r e n c e may a r i s e f r o m t h e g r e a t e r t h e r m a l s t a b i l i t y o f Hg m a t e r i a l s . I n d e p e n d e n t l y o f t h e h y d r o x y l s t a b i l i t y , J a c o b s e t a l . (_15_) c o n c l u d e t h a t s e v e r a l f a c t o r s ( L e w i s a c i d i t y , N a i o n s , e x t r a - l a t t i c e aluminum) a r e i n f l u e n c i n g t h e a c t i v i t y p e r h y d r o x y l g r o u p . The a c t i v i t y i n v a r i o u s r e a c t i o n s i s i n c r e a s e d i n t h e p r e s e n c e o f CO2 ( 3 , 9 9 , 1 0 0 ) . A l a r g e amount o f work has been done i n t o l u e n e d i s p r o p o r t i o n a t i o n by N i n a c h e v e t a l . ( 3 , 9 9 ) . S t u d y i n g i s o o c t a n e c r a c k i n g , N i r o d a t o s e t a l . (j34) r e l a t e t h e r e v e r s i b l e r i s e i n c a t a l y s i s t o t h e f o r m a t i o n o f p r o t o n i c h y d r o x y l s i n CO2 a t m o s p h e r e . As t o t h e s t r e n g t h o f s i t e s , J a c o b s e t a l . (15) showed t h a t u l t r a - a c t i v e h y d r o x y l s +


464

MOLECULAR

SIEVES—Π


a r e a c t i n g i n cumene c r a c k i n g and t o l u e n e d i s p r o p o r t i o n a t i o n . The s e c o n d r e a c t i o n r e q u i r e s h y d r o x y l g r o u p s w i t h h i g h e r a c i d i t y . Mos cou and Mone (46_) r e l a t e d t h e i n c r e a s e d c r a c k i n g s e l e c t i v i t y o f r a r e e a r t h z e o l i t e s upon s t e a m i n g t o t h e l o s s o f s t r o n g a c i d i t y . The h i g h o - x y l e n e i s o m e r i z a t i o n a c t i v i t y o f MgHY m a t e r i a l s w i t h low sodium l e v e l s i s r e l a t e d t o t h e h y p o t h e s i s o f v e r y s t r o n g a c i d i t y (^2). I t has been shown p r e v i o u s l y t h a t a c i d i t y g e n e r a t i o n may depend on s e v e r a l p a r a m e t e r s . A c c o r d i n g l y t h e c a t a l y s i s c o u l d a l s o be i n f l u e n c e d by t h e n a t u r e o f t h e c h a r g e and t h a t o f t h e c a r r i e r g a s . T h e r e may be f o r m a t i o n o f new a c i d c e n t e r s , d e h y d r o x y l a t i o n by r e a c t a n t s , c a t i o n m i g r a t i o n , change i n p r o t o n s t r e n g t h and l o c a t i o n . . . These f a c t o r s r e n d e r more d i f f i c u l t t h e u n d e r s tanding of c a t a l y t i c p r o p e r t i e s . B-IV- Other C a t a l y t i c S i t e s . The L e w i s a c i d i t y i s o f t e n i n v o l v e d i n c a t a l y t i c p r o c e s s e s . I t may i n c r e a s e t h e p r o t o n i c a c i d s t r e n g t h v i a an i n d u c t i v e e f f e c t (12) o r a c t as a c t i v e s i t e . The a l k y l a t i o n o f benzene w i t h e t h y l e n e o v e r LaY z e o l i t e s showed (101) t h a t benzene i s a d s o r b e d on L e w i s a c i d s i t e s t o f o r m a c h a r g e t r a n s f e r complex w h i l e e t h y l e n e i s a d s o r b e d on B r o n s t e d a c i d s i t e s t o form carbonium i o n s . In the c o n v e r s i o n of p r o p y l e n e t o a c e t y l e ne and b u t a d i e n e ( 1 0 2 ) , t h e c a t a l y t i c a c t i v i t y i s r e l a t e d t o t h e L e w i s a c i d i t y and a l s o depends on t h e c a t i o n s . O n l y few e x a m p l e s a r e known w h i c h i m p l y b a s i c s i t e s i n c a t a l y s i s w i t h z e o l i t e s . Ho w e v e r z e o l i t e s , l i k e amorphous s i l i c a s - a l u m i n a s , may c o n t a i n s u c h s i t e s . The b a s i c and a c i d i c s i t e s may t h e n a c t j o i n t l y o r c o n c u r r e n t l y . F o r example b a s i c c e n t e r s a r e i m p l i e d i n t o l u e n e a l k y l a t i o n w i t h m e t h a n o l and f o r m a l d e h y d e o v e r X and Y z e o l i t e s ( 1 0 3 ) . The s i d e c h a i n a l k y l a t i o n , p o i s o n e d by H C 1 g i v e s s t y r e n e and e t h y l benzene and i s r e l a t e d t o b a s i c i t y (K and Rb X z e o l i t e s ) . The r i n g a l k y l a t i o n g i v i n g x y l e n e , p o i s o n e d by a b a s i c r e a g e n t s u c h as a n i l i n e , i s r e l a t e d t o t h e weak a c i d i t y ( L i z e o l i t e s ) . The b a s i c s i t e s are a l s o i m p l i e d i n the decomposition of 2 - p r o p a n o l over a l k a l i c a t i o n exchanged Y z e o l i t e s ( 1 0 4 ) . Over L i o r Na z e o l i t e s t h e r e i s o n l y d e h y d r a t i o n w h i l e o v e r K, Rb and Cs s a m p l e s b o t h d e h y d r a t i o n and d e h y d r o g e n a t i o n o c c u r s . The d e h y d r a t i o n and d e h y d r o g e n a t i o n a r e s e l e c t i v e l y d e p r e s s e d by t h e a d d i t i o n o f a b a s i c r e a g e n t ( p y r i d i n e ) and an a c i d i c r e a g e n t ( p h e n o l ) r e s p e c t i v e l y . Hence K, Rb o r Cs z e o l i t e s have b o t h b a s i c and a c i d s i t e s . As t o h y d r o g e n a t i o n - d e h y d r o g e n a t i o n r e a c t i o n s , u n s a t u r a t e d compounds may be h y d r o g e n a t e d o v e r c a t i o n i c z e o l i t e s ( 1 0 5 , 1 0 6 ) . A l a r g e number o f p a t e n t s and p a p e r s have been p u b l i s h e d on t h e a c t i v i t y o f n o b l e - m e t a l c o n t a i n i n g z e o l i t e s ( 4 , 5 7 ) . Recent s t u d i e s are concerned w i t h the p r o p e r t i e s of unusual s m a l l agglomerates s t a b i l i z e d i n the c a ges o f Y z e o l i t e s ( 5 7 , 9 3 , 1 0 7 ) . The e l e c t r o n d e f i c i e n c y o f t h e c l u s t e r s i s s u g g e s t e d t o e x p l a i n t h e i r c a t a l y t i c p r o p e r t i e s and t h e i r b e h a v i o r t o w a r d p o i s o n s s u c h as N H 3 o r s u l f u r compounds ( 9 3 , 107,106). B-V- M i s c e l l a n e o u s C a t a l y t i c S t u d i e s . The l a r g e v a r i e t y o f r e a c t i o n s c a t a l y z e d by z e o l i t e s has a l r e a d y been d e s c r i b e d i n d e -


38.

BARTHOMEUF

Acidic

and Catalytic

Properties

of

Zeolites

465

t a i l s ( 1 - 5 ) . The more r e c e n t l y m e n t i o n n e d a r e r e p o r t e d h e r e . The n - h e x a n e d e h y d r o c y c l i s a t i o n a c t i v i t y o f Te Na X c a t a l y s t s has been s t u d i e d . The a c t i v e s i t e s has been w e l l d e s c r i b e d as a T e " c a t i o n i n t h e s u p e r c a g e , c o o r d i n a t e d w i t h Na i o n s [ 1 0 9 ] . N e a l e e t a l . (110) d e s c r i b e d t h e i n t e r e s t o f t h e s y n t h e s i s o f n - i s o p r o p y l i d e n e m e t h y l a m i n e o v e r ZnY z e o l i t e and a s c r i b e d t h e c a t a l y t i c a c t i v i t y to the Z n c a t i o n . P o n c e l e t e t a l . [111 ) r e p o r t t h e r e a c t i o n o f CO and NH3 o v e r CaY, NH4Y and F e X z e o l i t e s t o g i v e u r e a and a m i n o a c i d s w h i c h may be r e l a t e d t o o r i g i n o f l i f e . A v e r y i m p o r t a n t p r a c t i c a l r e a c t i o n i m p l i e s t h e methanol conversion t o higher c a r bon number h y d r o c a r b o n s o v e r ZSM-5 z e o l i t e ( 1 1 2 ) . The C hydrocar bon y i e l d i s m a x i m i z e d by i n c o r p o r a t i n g a l i m i t e d amount o f a B r o n s t e d o r L e w i s b a s e . B a r r e r e t a l . (113) f o r m e d p o l y m e r - z e o l i t e c o m p o s i t e s by t h e p o l y m e r i z a t i o n o f v a r i o u s v i n y l e t h e r s o v e r Hm o r d e n i t e and Y z e o l i t e . D i m e r i z a t i o n o f e t h y l e n e t o n - b u t e n e s p r o c e e d s s e l e c t i v e l y on Y z e o l i t e s c o n t a i n i n g z e r o v a l e n t N i and Rh h i g h l y d i s p e r s e d i n t h e f r a m e w o r k ( 1 1 4 ) . Over CrY h i g h p o l y m e r i z a t i o n o f e t h y l e n e o c c u r s . E t h y l e n e i s a l s o c o n v e r t e d t o C5 and h i g h e r h y d r o c a r b o n s o v e r a CaNaY z e o l i t e c o n t a i n i n g 1 % Pd i n t r o d u c e d by i o n exchange ( 1 1 5 ) . The c a t a l y t i c a c t i v i t y i s s h a r p l y r e d u c e d by t r e a t m e n t w i t h hydrogen o r h e l i u m . 2

2 +

2 +


5 +

B - V I - C a t a l y t i c P r o p e r t i e s o f A l u m i n u m - D e f i c i e n t , Deep-Bed o r U l t r a - S t a b l e Z e o l i t e s . Such s t u d i e s have been r e p o r t e d f o r f a u j a s i t e o r m o r d e n i t e m a t e r i a l s ( 4 , 5 1 ) . A c i d l e a c h e d m o r d e n i t e has a h i g h s e l e c t i v i t y f o r i s o m e r i z a t i o n and a low d e a c t i v a t i o n r a t e ( 1 1 6 ) . Over a l u m i n u m - d e f i c i e n t Y, Beaumont e t a l . (21) o b s e r v e d no change i n i s o o c t a n e c r a c k i n g i f aluminum l o s s i s l e s s t h a n ^ 35 %. B e y o n d , t h e c r a c k i n g r a t e i s d e c r e a s e d . The changes q u i t e p a r a l l e l t h e number o f s t r o n g a c i d s i t e s measured w i t h c o l o r e d i n d i c a t o r s (21) o r a t o m i c hydrogen f o r m e d u n d e r γ i r r a d i a t i o n ( 3 4 ) . J a c o b s e t a l . (15) f o u n d no good c o r r e l a t i o n between OH c o n c e n t r a t i o n and cracking or disproportionation a c t i v i t i e s f o r different stabilized c a t a l y s t s (deep-bed, u l t r a s t a b l e o r A l - d e f i c i e n t ) . In butene i s o m e r i z a t i o n t u r n o v e r numbers c a l c u l a t i o n s based on p o i s o n n i n g e x p e r i m e n t s i n d i c a t e t h a t h y d r o x y l g r o u p s a r e more h e t e r o g e n e o u s i n h y d r o l y z e d z e o l i t e s t h a n i n Y m a t e r i a l s ( 1 6 ) . Ward o b t a i n e d v e r y a c t i v e and s t a b l e c a t a l y s t s i n a r o m a t i c s i s 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 upon d e f i n e d t r e a t m e n t s o f NH4Y ( 1 1 7 ) . I t was m e n t i o n n e d above t h a t t h e a c i d i t y ( A - I V ) and OH (A-V) s t u d i e s o f s u c h m o d i f i e d z e o l i t e s made by v a r i o u s a u t h o r s d i d n o t g i v e e x a c t l y s i m i l a r r e s u l t s . D i s c r e p a n c i e s a r e a l s o observed i n c a t a l y t i c s t u d i e s . A v e r y c l e a r p a p e r was p r e s e n t e d on t h e c h e m i s t r y o f aluminum m i g r a t i o n and r e m o v a l i n Y z e o l i t e s (51_). I t seems t h a t i n f a c t s e v e r a l m o d i f y i n g e f f e c t s may o c c u r s i m u l t a n e o u s l y v e r y e a s i l y . Then s l i g h t c h a n g e s i n e x p e r i m e n t a l c o n d i t i o n s f r o m one l a b o r a t o r y t o t h e o t h e r may r e s u l t i n samples w i t h v a r i o u s a luminum atoms l o c a t i o n s . The c a t a l y t i c p r o p e r t i e s w i l l be c e r t a i n l y b e t t e r e x p l a i n e d when t h e p r e c i s e l o c a t i o n o f aluminum atoms w i l l be p o s s i b l e .


MOLECULAR


466

SIEVES—II

B - V I I - C a t a l y t i c A c t i v i t y o f T r a n s i t i o n M e t a l Complexes I o n s Zeolites. The c h e m i s t r y and c a t a l y t i c p r o p e r t i e s o f t r a n s i t i o n m e t a l c o m p l e x e s i n z e o l i t e s has r e c e n t l y r e c e i v e d c o n s i d e r a b l e a t t e n t i o n [ 1 1 8 , 1 1 9 ] . These c o m p l e x e s a p p e a r t o o f f e r new r e a c t i o n pathways w h i c h a r e i n many ways more s i m i l a r t o c a t a l y s i s by m e t a l c o m p l e x e s i n homogeneous m e d i a t h a n t o c o n v e n t i o n a l h e t e r o g e n e o u s c a t a l y s i s . R e a c t i o n s i m p l y i n g t h e c a t i o n s as a c t i v e c e n t e r s have been e a r l y m e n t i o n n e d [ 1 , 2 ] . I n o r d e r t o i n v e s t i g a t e t h e 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 of metal ions alone i t i s d e s i r a b l e to poison t h e a c i d s i t e s . R e a c t i o n s i m p l y i n g ammonia w h i c h may f o r m c o m p l e x e s w i t h some t r a n s i t i o n m e t a l c a t i o n s and p o i s o n t h e a c i d c e n t e r s have hence been e s p e c i a l l y s u c c e s s f u l . The f o r m a t i o n o f a n i l i n e f r o m ammonia and c h l o r o b e n z e n e was shown t o be c l o s e l y r e l a t e d t o t h e o r d e r o f e l e c t r o n e g a t i v i t y o r ammine complex f o r m a t i o n c o n s t a n t o f m e t a l c a t i o n s [ 1 2 0 ] . Over CuY z e o l i t e , t h e r i n g t r a n s f o r m a t i o n o f γ b u t y r o l a c t o n e and NH3 t o 2 - p y r r o l i d i n o n e c o u l d be r e l a t e d to the c o o r d i n a t i o n of the r e a c t a n t to the metal c a t i o n [121]. L u n s f o r d e t a l . [ 1 1 8 , 1 2 2 ] s t u d i e d t h e r o l e o f t h e c o p p e r ammonia complex [Cu[NH3]4]^ o x i d a t i o n r e a c t i o n o f ammonia by o x y g e n . I t was a l s o shown [ 1 1 8 , 1 2 3 ] t h a t [ C o [ N H ] N 0 ] within a Y t y p e z e o l i t e c a t a l y z e d t h e r e d u c t i o n o f NO by NH3 a t t e m p e r a t u r e s h i g h e r t h a n 50°C. I n t h e c y c l o t r i m e r i z a t i o n o f a c e t y l e n e o v e r Y z e o l i t e s i t was shown [124] t h a t t h e a c t i v e c a t i o n s a r e t h o s e w i t h an even number o f h a l f - f i l l e d d o r b i t a l s [ N i , C o , F e , C r ] . The d e c o m p o s i t i o n o f H2O2 has been u s e d t o s t u d y t h e a c t i v i t y o f v a r i o u s Y z e o l i t e s w i t h t r a n s i t i o n m e t a l i o n s [ 1 2 5 ] . The a c t i v i t y p a t t e r n i s c o r r e l a t e d w i t h the t r a n s i t i o n metal redox p o t e n t i a l s in basic solutions. λ

+

i

n

t

n

e

2 +

3

2 +

n

+

2 +

2 +

C - CONCLUSION I n t h e c a s e o f p r o t o n i c a c i d i t y , t h e d i f f i c u l t y t o f i n d good c o r r e l a t i o n s between a c i d i t y and c a t a l y t i c p r o p e r t i e s s u g g e s t t h a t t h e models u s e d f o r t h e c h a r a c t e r i z a t i o n o f t h e s i t e s may be t o o s i m p l e . The method c o n s i s t i n g i n t h e v e r y c a r e f u l l d e f i n i t i o n o f a c i d s i t e s has n o t been a b l e t o e x p l a i n a l l t h e c a t a l y t i c p r o p e r t i e s . Hence one can wonder w h e t h e r a more g l o b a l a p p r o a c h m i g h t n o t be c o n s i d e r e d . The l i t t é r a t u r e r e s u l t s s u g g e s t some r e m a r k s on t h e s u b j e c t . C o n s i d e r i n g t h e p r o p o s a l t h a t z e o l i t e s behave l i k e e l e c t r o l y t e s [50] one may s u p p o s e t h a t t h e e l e c t r o l y t e c o u l d be made o f t h e atoms o f t h e s o l i d f r a m e w o r k t h e m s e l v e s and o f t h e s p a c e s i n c a v i t i e s where i o n s s u c h as p r o t o n s and c a t i o n s a r e mov i n g . K e e p i n g i n mind t h e i d e a o f e l e c t r o l y t e some a n a l o g i e s b e t ween t h e p r o p e r t i e s o f a c i d s o l u t i o n s [126] and what i s known o f the p r o t o n i c a c i d i t y i n z e o l i t e s are very s t r i k i n g . A f i r s t e f f e c t w h i c h i s w e l l known i n a c i d s o l u t i o n s i s t h e change i n a c i d s t r e n g t h with the s u b s t i t u e n t s i n the a c i d s t r u c t u r e . I t i s already w e l l known i n s o l i d a c i d s t h a t f o r example CI o r F atoms i n t r o d u c e d i n aluminas or s i l i c a s - a l u m i n a s increase g r e a t l y t h e i r a c i d strength i . e . d e c r e a s e t h e s t r e n g t h o f t h e bond p r o t o n - s u r f a c e . Such examp l e s a r e not u s u a l i n z e o l i t e s but i t i s w e l l known t h a t p o l a r i z i n g



38.

BARTHOMEUF

Acidic

and Catalytic

Properties

of

Zeolites

467

c a t i o n s may i n c r e a s e t h e a c i d s t r e n g t h . The e x p l a n a t i o n s o f H i r s c h l e r [2) and R i c h a r d s o n ( 1 1 ) , f o r p o l y v a l e n t c a t i o n s , w h i c h i n v o l v e charge displacements a r e very s i m i l a r t o those i n v o l v e d i n s o l u t i o n s . A second example o f s t r u c t u r e e f f e c t i s r e l a t e d t o t h e a c i d s t r e n g t h o f p o l y a c i d s i n s o l u t i o n s . Due t o i n t e r a c t i o n s b e t ween n e g a t i v e l y c h a r g e d a n i o n s t h e s t r e n g t h o f a c i d i t y f u n c t i o n s d e c r e a s e as t h e s u c c e s s i v e a c i d i t i e s a r e i m p l i e d . A p p l i e d t o z e o l i t e t h i s c a n e x p l a i n on t h e same b a s i s t h e e x i s t e n c e o f a s c a l e of a c i d i t y s t r e n g t h s . Secondly s o l v e n t e f f e c t s modify g r e a t l y a c i d i t y s c a l e . T h i s i s due t o c h e m i c a l i n t e r a c t i o n s between b a s i c s o l v e n t s and t h e p r o t o n and f o r a p a r t t o e l e c t r o s t a t i c i n t e r a c t i o n s r e l a t e d to the d i e l e c t r i c constant of the solvents. Results on p r o t o n m o b i l i t y d e p i c t e d by F r e u d e e t a l . (1_8) and Mestdgah e t a l . (36) show t h a t t h e p r o t o n m o b i l i t y i s g r e a t l y dependent on t h e molecule carrying the proton. Besides the acid-base r e a c t i o n i m p l i e d , F r e u d e e t a l . (IjB) n o t e t h a t p y r i d i n e i n d u c e s a v e r y h i g h e r m o b i l i t y t h a n t o l u e n e . I t i s t h e same o r d e r t h a n t h e d i e l e c t r i c c o n s t a n t s . A c c o r d i n g l y one m i g h t s u g g e s t t h a t p y r i d i n e , b e s i d e s i t s b a s i c a c t i o n , i s a c t i n g as a d i e l e c t r i c which d e c r e a s e s t h e s t r e n g t h o f t h e bond z e o l i t e - H . Such a " s o l v e n t e f f e c t " m i g h t be i m p o r t a n t i n c a t a l y s i s d e p e n d i n g on t h e d i e l e c t r i c c o n s t a n t o f t h e r e a c t a n t and t h a t o f t h e " c o k e " d e p o s i t . T h i r d l y t h e p r o t o n m o b i l i t y j u s t m e n t i o n n e d and t h e c a t i o n movement a r e v e r y s i m i l a r t o i o n d i s p l a c e m e n t i n s o l u t i o n s . Of c o u r s e , e s p e c i a l l y f o r c a t i o n s , t h e u s u a l mean p a t h i s c e r t a i n l y l e s s t h a n i n t h e l i q u i d p h a s e . F o u r t h l y t h e a c i d p r o p e r t i e s o f s o l u t i o n s and k i n e t i c 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 a r e a l s o changed by s a l t e f f e c t s . The s a l t e f f e c t s l i n k e d to i o n i c strength of the s o l u t i o n modify the c o n c e n t r a t i o n of f r e e p r o t o n s i n s o l u t i o n . The q u e s t i o n a r i s e s a s t o w h e t h e r an i o n i c s t r e n g t h i n z e o l i t e c o u l d n o t be d e f i n e d and a s a l t e f f e c t c o u l d n o t be c o n s i d e r e d t o e x p l a i n a p e c u l i a r i n f l u e n c e o f c a t i o n c o n c e n t r a t i o n on a c t i v i t y r e s u l t s ( 9 4 ) . +

These t h r e e r e m a r k s s u g g e s t t h e h y p o t h e s i s t h a t z e o l i t e s may be c o n s i d e r e d as s o l i d s a c t i n g more o r l e s s l i k e p r o t o n i c p o l y a c i d s i n s o l u t i o n . In c o n t r a s t w i t h other s o l i d a c i d s , t h e protons, l i k e p r o t o n s i n s o l u t i o n s , may go i n s i d e and t h r o u g h t h e z e o l i t e f r a m e w o r k and be i n c l o s e c o n t a c t w i t h t h e s t r u c t u r e atoms. T h i s i s due t o t h e l a r g e v o i d s p a c e and t o t h e f a c t t h a t each f r a m e work atom i s a l s o a s u r f a c e atom. M o r e o v e r due t o t h e s m a l l s i z e of c a v i t i e s ( l e s s than usual pores i n oxides) t h e i n f l u e n c e of the w a l l s i s v e r y h i g h . A c c o r d i n g t o t h i s h y p o t h e s i s one s u g g e s t t h a t t h e a c i d p r o p e r t i e s and t h e a c i d c a t a l y s i s m i g h t t h e n be d e s c r i b e d u s i n g an a p p r o a c h deduced o f t h a t used f o r s o l u t i o n s . The v e r y f i ne d e s c r i p t i o n o f i n t e r a c t i o n s m o d i f y i n g t h e a c i d s i t e w h i c h have been p e r f o r m e d up t o now m i g h t be i n c l u d e d i n a c t i v i t y c o e f f i c i e n t s i n a manner s i m i l a r t o t h e c a s e o f a c t i v i t y c o e f f i c i e n t s i n i o n exchange i n z e o l i t e s . T h i s model c a n e x p l a i n on t h e one s i d e a l a r g e p a r t o f t h e a c i d p r o p e r t i e s o f z e o l i t e s and on t h e o t h e r s i de t h e d i f f i c u l t y t o c o r r e l a t e c a t a l y t i c r e s u l t s w i t h v e r y d e f i n e d a c i d i c s i t e s . The b a s i c p r o p e r t i e s o f some z e o l i t e s may a l s o be i n c l u d e d i n t h e m o d e l . M o r e o v e r , i n d e p e n d e n t l y o f t h e model p r e -


MOLECULAR

468

SIEVES—Π

s e n t e d , t h e r e s u l t s d e s c r i b e d h e r e shew t h a t z e o l i t e s c a n n o t be c o n s i d e r e d a s s t a t i c s y s t e m s b u t t h a t one s h o u l d t a k e i n t o a c c o u n t the dynamics o f t h e i r p r o p e r t i e s .


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38.

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38. 39.

40.

41. 42. 43. 44.

45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58.

BARTHOMEUF

Acidic

and Catalytic

Properties

of

Zeolites

469

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59. NACCACHE, C.M., BEN TAARIT, Υ . , J. Catal. (1971 ), 22, 171. 60. JACOBS, P . Α . , DE WILDE, W., SCHOONHEYDT, R . A . , UYTTERHOEVEN, J.B., BEYER, H . , J . C . S . Faraday I (1976), 72, 1221. 61. STEINBERG, K . H . , MINACHEV, Kh.M., BREMER, Η., DMITRIEV, R . V . , DETYUK , A . N . , Z. Chem (1975), 15, 372. 62. TSUTSUMI, K . , FUJI, S., TAKAHASHI, H., J . Catal.(1972), 24,8. 63. ANGELL, C . L . , HOWELL, M.V., J. Phys. Chem. (1970), 74, 2737. 64. MIRODATOS, C . , PICHAT, P . , BARTHOMEUF, D . , J. Phys. Chem. (1976), 80, 1335. 65. TURKEVICH, J., Catalysis Rev. (1967), 1, 1. 66. BEAUMONT, R., PICHAT, P . , BARTHOMEUF, D . , TRAMBOUZE, Υ . , Hightower, "Catalysis", 1, 343, North Holland Publish.Co., Amsterdam (1973). 67. BIELANSKI, Α . , DATKA, J., J . Catal. (1975), 37, 383. 68. RATOV, A . N . , KUBASOV, Α . Α . , TOPCHIEVA, K . V . , ROSOLOVSKAYA, E . N . , KALININ, V . P . , Kin. i Kat. (1973), 14., 1024. 69. ZUL'FUGAROV, Z . G . , ZUL'FUGAROVA, L. Sh., KAKHRAMANOVA, Ch.G., MURADOVA, S . A . , ANNAGIEV, M.Kh., ASKEROV, A . G . , DZHAFAROVA, E . M . , DZHAFAROVA, S., YUSIFOVA, Sh.A., Mech. Hydrocarbon React. Symp. (1975), 379. 70. TOPCHIEVA, K . V . , HUO SHI THUONG, Vestn. Moskov. Univ. Khim. (1974), 15, 239. 71. SCHERZER, J., BASS, J.L., J. Phys. Chem. (1975), 79, 1200. 72. BEN TAARIT, Y . , NACCACHE, C . , IMELIK, Β . , J . Chim. Phys. (1973), 70, 728. 73. DETREKOY, E.J., JACOBS, P . Α . , KALLO, D . , UYTTERHOEVEN, J.B., J . Catal. (1974), 32, 442. 74. FRANCO PARRA, C . , BALLIVET, D . , BARTHOMEUF, D . , J. Catal. (1975), 40, 52. 75. TATSUAKI YASHIMA, NOBUYOSHI HARA, J. Catal. (1972), 27, 329. 76. KARGE, H . , KLOSE, Κ., Z. Phys. Chem. (1973), 83, 100. 77. HUO SHI THUONG, TOPCHIEVA, K . V . , Dokl. Akad. Nauk. (1973), 211, 870. 78. THAKUR, D.K., WELLER, S.W., Adv. Chem. Ser. (1973), 121, 596. 79. JACOBS, P . Α . , UYTTERHOEVEN, J . , J . C . S . Faraday I (1973), 69, 359 and 379. 80. PERI, J.B., Hightower, "Catalysis", 1, 329, North Holland Publ. Co., Amsterdam, (1973). 81. WARD, J.W., J. Catal. (1975), 38, 351. 82. GUILLEUX, M . F . , DELAFOSSE, D . , J . C . S . Faraday I (1975), 71, 1777. 83. BIELANSKI, Α . , DATKA, J., J. Catal. (1974), 32, 183. 84. WEEKS, T.J., HILLERY, H . F . , BOLTON, A . P . , J . C . S . Faraday I (1975), 10, 2051. 85. KARGE, H . G . , Z. Phys. Chem. (1975), 95, 241. 86. SMITH, J . V . , Adv. Chem. Ser. (1971), 101, 171. 87. BAUR, W.H., Am. Mineralogist (1964), 49, 697. 88. LEITH, I . R . , KEMBALL, C . , LEACH, H . F . , J . C . S . Chem. Comm. (1971), 8, 407. 89. GALLEZOT, P . , BEN TAARIT, Y . , IMELIK, Β . , J. Catal. (1972), 26, 295.


38.

BARTHOMEUF

Acidic

and Catalytic

Properties

of

Zeolites


90.

All

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472

116. 117. 118. 119. 120. 121. 122.


123. 124. 125. 126.

MOLECULAR

SIEVES—II

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