Origin of Strong Acidity in Dealuminated Zeolite-Y - American

Chapter 1 Origin of Strong Acidity in Dealuminated Zeolite-Y Jack H. Lunsford

Downloaded by UNIV LAVAL on July 9, 2014 | http://pubs.acs.org Publication Date: January 23, 1991 | doi: 10.1021/bk-1991-0452.ch001

Department of Chemistry, Texas A&M University, College Station, TX 77843

Dealumination processes which leave residual extraframework aluminum in a Y-type zeolite result in a decrease in the overall number of Brönsted acid sites but an increase in the strength of the remaining acid sites. The net effect is an increase in activity for acid -catalyzed reactions up to a maximum at ca. 32 framework Al atoms per unit cell. A model for strong Brönsted acidity is proposed which includes (i) the presence of framework Al atoms that have no other Al atoms in a 4membered ring and (ii) complex Al cations in the βcages. The essential role of extraframework aluminum is evident from recent studies in which framework Al has been completely removed from zeolite-Y and by experiments on the related ZSM-20 zeolite. The commercial utilization of dealuminated Y-type zeolites has generated renewed interest in the origin of strong acidity in these materials. It is now evident that a carefully prepared H-Y zeolite, with its full complement of protons but with no extraf ramework Al, exhibits very little activity for acid-catalyzed reactions such as the dealkylation of cumene or the cracking of hexane (1.2) . If these zeolites are dealuminated to ca. 32 framework aluminum atoms per unit cell (Al /u.c), either by steaming or by treatment with silicon tetrachloride, their activities for these reactions increase remarkably. This framework Al concentration corresponds to a Si/Al ratio of 5.0 or a unit cell dimension, a , of 24.54 A. Even i f zeolites are not purposely dealuminated at the outset, they become dealuminated to an equilibrium level during their regeneration in a catalytic cracking unit (3). Early theoretical arguments of Dempsey (4) and also of Mikovsky and Marshall (5.) suggested that the acid strength was related to the Al distribution in the framework and that only A l atoms with no second-neighbor Al atoms in the 4-rings are responsible for strong Bronsted acidity. The number of aluminum atoms with 0, 1, 2 and 3 aluminum second neighbors in the 4-rings has been calculated by Beagley et al. (6), and the results are shown in Figure 1. The disordered unit cell, referred to in the figure caption, indicates that a random distribution of Si and Al atoms was assumed, with the only constraint being that an Al-O-Al linkage did not exist (Loewenstein's Rule). In the unique structures model it was assumed f

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0097-6156y91/0452-0001$06.00/0 © 1991 American Chemical Society In Fluid Catalytic Cracking II; Occelli, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

FLUID CATALYTIC CRACKING II: CONCEPTS

IN CATALYST DESIGN


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F i g u r e 1. D i s t r i b u t i o n o f t y p e s o f A l a t o m s i n f a u j a s i t e s : unique structures o f Cambridge group; ( — ) , d i s o r d e r e d cell.

In Fluid Catalytic Cracking II; Occelli, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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Strong Acidity in Dealuminated Zeolite-Y

that Al-Al repulsion across 4-rings or 6-rings resulted in energetically preferred structures. Thus, i f the h y p o t h e s i s of Dempsey and o f M i k o v s k y and M a r s h a l l ( h e r e i n r e f e r r e d t o as t h e DMM m o d e l ) i s c o r r e c t a n d i f one a s s u m e s t h e d i s t r i b u t i o n i n d i c a t e d b y t h e u n i q u e s t r u c t u r e s m o d e l , one w o u l d e x p e c t t h a t t h e n u m b e r o f s t r o n g l y a c i d i c p r o t o n s w o u l d i n c r e a s e t o a maximum l e v e l a t 32 A l / u . c . a n d w o u l d d e c r e a s e t o z e r o a t 64 A l / u . c . The d e a l u m i n a t i o n p r o c e s s , w h e t h e r b y s t e a m i n g o r b y t r e a t m e n t w i t h S i C l , not o n l y changes the A l d i s t r i b u t i o n but a l s o i n t r o d u c e s e x t r a f ramework A l i n t o the z e o l i t e c a v i t i e s . T h i s e x t r a f ramework A l , w h i c h may be p r e s e n t i n s e v e r a l f o r m s , a l s o h a s t h e p o t e n t i a l f o r i n c r e a s i n g o r d e c r e a s i n g b o t h t h e number and a c i d s t r e n g t h o f t h e protons. The p r o b l e m o f s e p a r a t i n g t h e e f f e c t s o f t h e A l a n d the v a r i o u s forms o f e x t r a f r a m e w o r k A l has been f o r m i d a b l e ; however, c o n s i d e r a b l e p r o g r e s s h a s b e e n made i n s e v e r a l l a b o r a t o r i e s . It is the p u r p o s e o f t h i s r e v i e w t o summarize t h e s e r e s u l t s and t o s u g g e s t a m o d e l o f s t r o n g a c i d i t y t h a t a p p e a r s t o f i t much o f t h e a v a i l a b l e data. The d a t a i n c l u d e c a t a l y t i c r e s u l t s o n f a u j a s i t e - t y p e z e o l i t e s p r e p a r e d by s e v e r a l methods, p o i s o n i n g s t u d i e s , and i n f r a r e d s t u d i e s of hydroxy1 groups. f

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E f f e c t o f Framework Aluminum

Concentration

E a r l y s t u d i e s i n our l a b o r a t o r y (1.2) f o c u s e d on the e f f e c t o f d e a l u m i n a t i o n , e f f e c t e d e i t h e r by s t e a m i n g o r by t r e a t m e n t w i t h S i C l , on t h e a c t i v i t y o f the z e o l i t e f o r t h r e e t e s t r e a c t i o n s : m e t h a n o l dehydration, cumene d e a l k y l a t i o n and hexane cracking. These r e a c t i o n s were chosen because they r e q u i r e i n c r e a s i n g a c i d s t r e n g t h , w i t h hexane c r a c k i n g b e i n g the most demanding. U n f o r t u n a t e l y , i t i s not p o s s i b l e ( a t l e a s t i n our hands) to prepare from a normal zeolite-Y c r y s t a l l i n e materials having A l values over the e n t i r e r a n g e b e t w e e n 0 a n d 54 A l / u . c . In p a r t i c u l a r , the z e o l i t e s i n the r a n g e f r o m a b o u t 32 t o 52 A l / u . c . a r e i n a c c e s s i b l e . Nevertheless, a s s h o w n b y t h e d a t a i n F i g u r e 2 o n cumene d e a l k y l a t i o n , t h e a c t i v i t y i n c r e a s e d i n an a l m o s t l i n e a r manner w i t h r e s p e c t t o t h e framework A l c o n c e n t r a t i o n up t o 32 A l / u . c . The DY550 s e r i e s a n d t h e DY-(Si/Al) s e r i e s w e r e p r e p a r e d b y d e a l u m i n a t i o n o f a NH Na-Y Z e o l i t e w i t h S i C l under d i f f e r e n t c o n d i t i o n s , f o l l o w e d by i o n exchange w i t h NH to a r e s i d u a l Na i o n c o n t e n t 5 c o m p o s i t i o n domain. More r e c e n t l y , F r i t z and L u n s f o r d (10) s t u d i e d t h e e f f e c t t h a t sodium p o i s o n i n g had on hexane c r a c k i n g o v e r a s e r i e s o f Y - t y p e z e o l i t e s t h a t had been m o d i f i e d by d e a l u m i n a t i o n w i t h S i C l o r by steaming. The r e s u l t s , d e p i c t e d i n F i g u r e 3, i n d i c a t e t h a t e a c h sodium i o n added back to the z e o l i t e e f f e c t i v e l y n e u t r a l i z e s the c a t a l y t i c a c t i v i t y p r e v i o u s l y t h o u g h t t o b e due t o f i v e B r o n s t e d a c i d sites. The a b s c i s s a i n F i g u r e 1 r e p r e s e n t s t h e n u m b e r o f f r a m e w o r k A l atoms w h i c h r e m a i n s u n p o i s o n e d a f t e r the a d d i t i o n o f sodium. For example, i f e a c h A l atom had a s t r o n g l y a c i d i c p r o t o n a s s o c i a t e d w i t h i t , t h e A - s e r i e s s a m p l e , w h i c h i n t h e p u r e f o r m h a d 15 A l / u . c . , w o u l d h a v e r e q u i r e d t h a t same n u m b e r N a ions to n e u t r a l i z e the a c i d i t y . I n f a c t , o n l y about 3 Na i o n s / u . c . ( A l - Na - 12) d e s t r o y e d the activity. B a s e d on NH p o i s o n i n g e x p e r i m e n t s , L o m b a r d o et al. (11) concluded that approximately 10% o f t h e p o t e n t i a l B r o n s t e d sites, c o u n t e d as A l i o n s , were i n v o l v e d i n the c a t a l y t i c d e c o m p o s i t i o n of neopentane. The e f f e c t o f s t e a m i n g a n d o f e x t e n s i v e p o i s o n i n g b y alkali m e t a l i o n s i s n o t l i m i t e d t o Y - t y p e z e o l i t e s , a s L a g o et al. (12) h a v e o b s e r v e d s i m i l a r p h e n o m e n a w i t h m i l d l y s t e a m e d H-ZSM-5. The a c t i v i t y f o r hexane c r a c k i n g i n c r e a s e d by about a f a c t o r o f f o u r upon m i l d steaming of the c a t a l y s t . S e l e c t i v e Cs p o i s o n i n g i n d i c a t e d t h a t t h e c o n c e n t r a t i o n o f a more a c t i v e s i t e i n t h e s t e a m e d s a m p l e was o n l y a b o u t 6% o f t h e t e t r a h e d r a l f r a m e w o r k a l u m i n u m . These s i t e s e x h i b i t e d a s p e c i f i c a c t i v i t y 45-75 t i m e s g r e a t e r t h a n t h a t o f a n o r m a l s i t e i n H-ZSM-5. Two p o s s i b l e mechanisms f o r the e f f e c t i v e n e s s o f the poisons have been p o s t u l a t e d . F i r s t , t h e p o i s o n may b e e f f e c t i v e t h r o u g h a l o n g range i n t e r a c t i o n . E v i d e n c e f o r t h i s h y p o t h e s i s i s g i v e n by D y e r a n d S i n g h ( 1 3 ) , who f o u n d t h a t K i o n s h a d a m u c h g r e a t e r e f f e c t on a c t i v i t y t h a n Na ions. T h i s was a t t r i b u t e d t o d i f f e r e n c e s i n c a t i o n i c r a d i i and e l e c t r o n e g a t i v i t i e s . A s e c o n d p o s s i b i l i t y i s t h a t even i n z e o l i t e s h a v i n g i s o l a t e d A l atoms, o n l y a f r a c t i o n o f the Bronsted s i t e s are a s s o c i a t e d w i t h strong a c i d i t y . Strong evidence f a v o r i n g the second p o s s i b i l i t y i s found i n the i n f r a r e d s t u d i e s to be d i s c u s s e d i n t h e n e x t s e c t i o n . f

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Hydroxyl

I n f r a r e d Bands

I n f r a r e d b a n d s o f d e a l u m i n a t e d z e o l i t e s i n t h e 0-H s t r e t c h i n g r e g i o n have been e x t e n s i v e l y s t u d i e d (14-17): however, emphasis w i l l be p l a c e d on the s t u d y by F r i t z and L u n s f o r d ( 1 0 ) , w h i c h d e s c r i b e s the e f f e c t o f Na on t h e s e i n f r a r e d bands. Perhaps the most i n f o r m a t i v e example i s the S i C l - t r e a t e d z e o l i t e ( s e r i e s A of F i g u r e 3 ) . The S i C l - t r e a t e d z e o l i t e s h a v e l e s s e x t r a f r a m e w o r k a l u m i n u m t h a n do t h e steamed samples; therefore, the infrared spectra i n the 0-H s t r e t c h i n g r e g i o n are l e s s complicated. The e f f e c t o f a d d e d N a on the i n f r a r e d s p e c t r a o f the S i C l t r e a t e d z e o l i t e i s s h o w n i n F i g u r e 4. At l e a s t f i v e major hydroxyl bands are apparent i n the s p e c t r a . T h e r e a r e two s i l a n o l b a n d s : one a t 3 7 4 5 cm , which r e s u l t s from t e r m i n a l h y d r o x a l groups i n the l a t t i c e ( 1 8 ) , a n d one a t 3 7 3 9 cm' , w h i c h i s p o s s i b l y due t o h y d r o x y l nests. The two b a n d s a t 3 6 3 5 a n d 3 5 6 0 cm" are a s s o c i a t e d w i t h the +

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LUNSFORD


F i g u r e 2. D e p e n d e n c e o f c a t a l y t i c a c t i v i t y f o r cumene c o n v e r s i o n a t 290°C o n f r a m e w o r k a l u m i n u m c o n t e n t : ( O ) DY550 s e r i e s , ( • ) D Y - ( S i / A l ) s e r i e s , ( • ) SDY, ( • ) HY.

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A l - No f

F i g u r e 3. The dependence o f h e x a n e c r a c k i n g a c t i v i t y o n t h e framework A l content f o r f o u r s e r i e s o f Na -poisoned c a t a l y s t s . E a c h Na* i s a s s u m e d t o p o i s o n o n e f r a m e w o r k A l a t o m . ( • ) A series, ( • ) B series, ( • ) C series, ( • ) D series. The s o l i d l i n e r e p r e s e n t s t h e t h e o r e t i c a l a c t i v i t y b a s e d on i s o l a t e d A l atoms ( 6 ) . R e p r o d u c e d w i t h p e r m i s s i o n f r o m R e f . 10. Copyright 1989 Academic Press. +


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IN CATALYST DESIGN


FLUID CATALYTIC CRACKING II: CONCEPTS

F i g u r e 4. Infrared spectra o f the hydroxyl region f o r a S i C l t r e a t e d z e o l i t e ( s e r i e s A, w i t h A l - 1 5 . 3 ) w i t h v a r i o u s a m o u n t s o f Na added as a p o i s o n : ( a ) 0.02 w t % N a , ( b ) 0.23 w t % N a , ( c ) 0.49 w t % N a . The o r d i n a t e i s a b s o r b a n c e i n a r b i t r a r y u n i t s . R e p r o d u c e d w i t h p e r m i s s i o n f r o m R e f . 1 0 . Copyright 1989 Academic Press. A

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t r a d i t i o n a l B r o n s t e d s i t e s f o u n d i n a n o r m a l H-Y z e o l i t e . The f i f t h b a n d , w h i c h i s t h e m o s t s e n s i t i v e t o N a a d d i t i o n , i s f o u n d a t 3602 cm" . T h i s b a n d d e c r e a s e d u p o n a d d i t i o n o f s m a l l a m o u n t s o f N a i n a manner t h a t p a r a l l e l e d t h e d e 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 i g u r e 3 ) , w h e r e a s t h e b a n d s a t 3 6 3 5 a n d 3 5 6 0 cm" were n o t significantly a f f e c t e d by t h e i n t r o d u c t i o n o f s m a l l amounts o f Na . More complete ion exchange w i t h Na would have r e s u l t e d i n a d e c r e a s e d a m p l i t u d e o f t h e 3 6 3 5 a n d 3 5 6 0 cm' bands. W i t h r e s p e c t t o the mechanism by w h i c h sodium p o i s o n s the a c i d i c z e o l i t e , i t i s important to note that at l e a s t three d i f f e r e n t types o f a c i d i c p r o t o n s a r e p r e s e n t , a n d t h a t one f o r m i s a f f e c t e d much more b y s o d i u m p o i s o n i n g t h a n a r e t h e o t h e r two. Thus, o n l y a f r a c t i o n o f the B r o n s t e d s i t e s i s s t r o n g l y a c i d i c , even i n z e o l i t e s h a v i n g o n l y t h e N(0) c o n f i g u r a t i o n i n the 4 - r i n g s ( F i g u r e 1 ) . The q u e s t i o n a r i s e s as t o what, i n a d d i t i o n t o t h e A l d i s t r i b u t i o n , i s essential for a strongly acidic site. S i n c e d e a l u m i n a t i o n by steaming o r by treatment w i t h S i C l generates extraframework aluminum i n v a r i o u s f o r m s , i t i s r e a s o n a b l e t o s u s p e c t t h a t one o f t h e s e t y p e s o f e x t r a f r a m e w o r k a l u m i n u m may b e i n v o l v e d i n t h e d e v e l o p m e n t of strong acidity. +

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Evidence f o r the Role of Extraframework

Aluminum

E v i d e n c e t h a t s o m e t h i n g more t h a n a p r o p e r A l d i s t r i b u t i o n i s n e e d e d t o c r e a t e s t r o n g a c i d i t y f i r s t came f r o m t h e e x p e r i m e n t s o f B e y e r l e i n et al. ( 7 ) . U s i n g t h e a c i d - c a t a l y z e d c o n v e r s i o n o f i s o b u t a n e as a measure o f s t r o n g a c i d i t y , they found t h a t a d e a l u m i n a t e d z e o l i t e prepared by treatment with ammonium hexafluorosilicate (AHF) e x h i b i t e d much l e s s c a r b o n i u m i o n a c t i v i t y t h a n m i g h t b e e x p e c t e d , b a s e d on t h e number o f A l atoms. This treatment leaves very l i t t l e extraframework A l i n the z e o l i t e . When t h e s a m p l e was mildly s t e a m e d , t h e a c t i v i t y became c o n s i d e r a b l y g r e a t e r . The authors c o n c l u d e d t h a t t h e e n h a n c e d a c i d i t y was a r e s u l t o f a s y n e r g i s m between the framework B r o n s t e d s i t e s and the L e w i s s i t e s a s s o c i a t e d w i t h e x t r a f r a m e w o r k aluminum. Experiments to further demonstrate the critical role of extraframework A l , or another p o l y v a l e n t c a t i o n , have r e c e n t l y been c a r r i e d out i n our l a b o r a t o r y (19.20). A series of faujasite-type z e o l i t e s was p r e p a r e d t h a t h a d A l c o n c e n t r a t i o n s b e t w e e n 21 a n d 54 p e r u.c. A t t h e l o w e n d o f t h e r a n g e , AHF was u s e d t o r e m o v e t h e framework A l , and an H-ZSM-20 z e o l i t e with 42 Al /u.c. was synthesized. ZSM-20 i s a n i n t e r g r o w t h o f t h e c u b i c faujasite s t r u c t u r e and t h e h e x a g o n a l v a r i a n t know as B r e c k ' s s t r u c t u r e s i x (BSS) ( 2 1 ) . Thus, i t i s a f a u j a s i t e - l i k e m a t e r i a l . The c a t a l y t i c a c t i v i t i e s o f these z e o l i t e s f o r hexane c r a c k i n g are compared i n F i g u r e 5 ( l o w e r d a t a s e t ) w i t h the a c t i v i t i e s o f z e o l i t e s p r e p a r e d by steaming o r by treatment w i t h S i C l (upper d a t a s e t ) . The s o l i d l i n e s r e p r e s e n t N(0) d i s t r i b u t i o n s . The s a m p l e s w i t h o u t e x t r a f r a m e w o r k A l e x h i b i t e d v e r y m o d e s t a c t i v i t y , e v e n t h o u g h some o f t h e m h a d a f a v o r a b l e N(0) c o n c e n t r a t i o n . The a m o u n t o f e x t r a f r a m e w o r k A l t h a t i s n e e d e d t o d e v e l o p a strongly acidic H-ZSM-20 z e o l i t e i s actually quite small, as i n d i c a t e d b y t h e r e s u l t s o f F i g u r e 6. D e a l u m i n a t i o n o f a n H-ZSM-20 z e o l i t e f r o m a n o r i g i n a l c o n c e n t r a t i o n o f 4 1 A l f / u . c . t o 31 A l / u . c . r e s u l t e d i n t h e maximum i n c r e a s e i n a c t i v i t y f o r h e x a n e c r a c k i n g . T h i s i n c r e a s e i s a l m o s t t h e m i r r o r image o f t h e d e c r e a s e s t h a t were n o t e d p r e v i o u s l y f o r N a p o i s o n i n g . The 10 A l a t o m s t h a t h a v e b e e n r e m o v e d f r o m t h e f r a m e w o r k c o r r e s p o n d t o a b o u t 1.3 A l p e r small cavity. As more A l atoms a r e removed, t h e a c t i v i t y d e c r e a s e d i n a manner c o n s i s t e n t w i t h t h e N(0) d i s t r i b u t i o n t h a t i s i n d i c a t e d by t h e s o l i d l i n e i n F i g u r e 6. F r i t z a n d L u n s f o r d ( 1 0 ) , a s w e l l a s L o m b a r d o et al. ( 1 1 ) , h a v e suggested that the e f f e c t i v e extraframework A l i s present i n the @ cages as c a t i o n s t h a t a r e b r i d g e - b o n d e d t h r o u g h oxygen atoms. In o r d e r t o t e s t t h i s h y p o t h e s i s , one w o u l d l i k e t o i o n - e x c h a n g e Al f

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FLUID CATALYTIC CRACKING II: CONCEPTS IN CATALYST DESIGN

Framework aluminum (Al/u.c.) F i g u r e 5. H e x a n e 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 o f f r a m e w o r k aluminum c o n t e n t : O , Y-type z e o l i t e dealuminated w i t h S i C l ; • , Y-type z e o l i t e prepared by steaming; A , Y-type z e o l i t e d e a l u m i n a t e d w i t h ammonium h e x a f l u o r o s i l i c a t e ; A , after La e x c h a n g e t o l e v e l o f maximum a c t i v i t y ; • , ZSM-20; g , a f t e r La e x c h a n g e t o l e v e l o f maximum a c t i v i t y ; v , as synthesized z e o l i t e Y; • , a f t e r e x c h a n g e t o l e v e l o f maximum a c t i v i t y ; HLa-X. Reproduced w i t h p e r m i s s i o n from Ref. 20. 4

3 +

3 +

Copyright 1990 Academic Press.

400°C

0

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20 30 40 Framework AI/U.C. by NMR

F i g u r e 6. D e p e n d e n c e o f h e x a n e c r a c k i n g a c t i v i t y a l u m i n u m c o n t e n t f o r s t e a m - d e a l u m i n a t e d H-ZSM-20.

50

60

on framework


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i n t o a s e r i e s o f f a u j a s i t e - t y p e z e o l i t e s h a v i n g a range o f A l v a l u e s , but t h i s i s probably not p o s s i b l e because A l hydrolyzes extensively. A s a n a l t e r n a t i v e , we h a v e i o n e x c h a n g e d L a ions i n t o the s e r i e s of z e o l i t e s d e s c r i b e d p r e v i o u s l y f o r t h e d a t a o f F i g u r e 5. Lanthanumexchanged zeolites are particularly attractive because of the s t r u c t u r a l i n f o r m a t i o n t h a t i s a v a i l a b l e on the l o c a t i o n o f the cations. When L a - Y z e o l i t e s a r e h e a t e d t o a p p r o x i m a t e l y 300°C, t h e h y d r a t i o n s p h e r e i s l o s t , a n d t h e l a n t h a n u m i o n s m i g r a t e i n t o t h e j3 cages where t h e y a r e p r e f e r e n t i a l l y a t t a c h e d t o s i t e s I ' and t a k e on an e f f e c t i v e c h a r g e o f +2.5 (22). Species such as [LaCQ^La] , [La ^ Q j j ^ T L a ] , and L a ( 0 H ) h a v e been proposed. There i s n e u t r o n d i f f r a c t i o n e v i d e n c e t o s u p p o r t b o t h t h e l o c a t i o n o f the L a i o n and t h e p r e s e n c e o f e x t r a f r a m e w o r k o x y g e n a n d h y d r o g e n i n t h e /? c a g e s f

3 +

3 +

2 +

A +

2 +

(23) . When t h e s e r i e s o f NH -Y o r NH -ZSM-20 z e o l i t e s h a v i n g 2 1 t o 54 Al a t o m s p e r u . c , a n d a l m o s t n o e x t r a f r a m e w o r k A l , was exchanged with La i o n s to v a r i o u s e x t e n t s , the a c t i v i t y f o r hexane c r a c k i n g f i r s t i n c r e a s e d and t h e n d e c r e a s e d w i t h the e x t e n t o f i o n exchange. The m a x i m a i n a c t i v i t y f o r e a c h z e o l i t e a r e p l o t t e d a s t h e m i d d l e s e t o f d a t a i n F i g u r e 5. Again, the r e s u l t s are reasonably w e l l a p p r o x i m a t e d b y t h e N ( 0 ) d i s t r i b u t i o n , w i t h d a t a now b e i n g a v a i l a b l e f o r t h e r a n g e o f 35 t o 54 A l / u . c . The HLa-X s a m p l e s w e r e a l m o s t completely i n a c t i v e , which i s c o n s i s t e n t w i t h the r e s u l t s expected from the N(0) d i s t r i b u t i o n . The s a m p l e s c o n t a i n i n g L a were l e s s a c t i v e than the samples dealuminated w i t h S i C l o r by steaming, but t h i s i s expected because the p o l a r i z i n g or i n d u c t i v e e f f e c t o f the smaller A l i o n i s much g r e a t e r t h a n t h a t o f t h e L a ion. These results support a model of strong acidity that involves the w i t h d r a w a l o f e l e c t r o n s f r o m s t r u c t u r a l 0-H bonds by polyvalent c a t i o n s t h a t are present i n the 0 cages. 4

A

f


3 +

f

3 +

A

3 +

Model f o r Strong

Acidity

The e v i d e n c e t o d a t e s u g g e s t s t h a t t h e d e v e l o p m e n t o f s t r o n g B r o n s t e d a c i d i t y i n Y-type z e o l i t e s r e q u i r e s b o t h i s o l a t e d A l * atoms and the p r e s e n c e o f e x t r a f r a m e w o r k a l u m i n u m . E x t r a f r a m e w o r k a l u m i n u m may, o f course, e x i s t i n s e v e r a l forms, i n c l u d i n g a b o e h m i t e - l i k e m a t e r i a l (24) and s e v e r a l i o n i c s p e c i e s , such as t h o s e d e s c r i b e d p r e v i o u s l y . I t seems u n l i k e l y t h a t p s e u d o - b o e h m i t e o r e v e n a l u m i n a i n t h e l a r g e c a v i t i e s w o u l d be c a p a b l e o f g e n e r a t i n g s t r o n g l y a c i d i c s i t e s i n t h e z e o l i t e framework, whereas through i n d u c t i v e e f f e c t s the i o n i c forms o f A l i n the & cages s h o u l d have a p r o f o u n d e f f e c t on t h e e l e c t r o n d i s t r i b u t i o n i n the framework. The e x t e n t o f t h e s e interactions c o u l d p r o f i t a b l y be e x p l o r e d by t h e o r e t i c a l c a l c u l a t i o n s . While the cationic forms of Al may promote the acid strength, they s i m u l t a n e o u s l y d e c r e a s e t h e number o f p r o t o n s n e e d e d t o m a i n t a i n charge n e u t r a l i t y . As a n u m e r i c a l e x a m p l e , c o n s i d e r a p a r t i a l l y d e a l u m i n a t e d H-Y z e o l i t e t h a t c o n t a i n s 32 A l / u . c , a l l o f w h i c h a r e i s o l a t e d , a n d 8 extraframework A l cations/u.c. T h i s example i s s i m i l a r t o the case o f t h e p a r t i a l l y d e a l u m i n a t e d H-ZSM-20 z e o l i t e i n F i g u r e 6 i f one assumes t h a t most o f the e x t r a f r a m e w o r k A l i s p r e s e n t i n t h e c a t i o n i c form. I f the e x t r a f r a m e w o r k A l i s complexed such t h a t each A l has an e q u i v a l e n t c h a r g e o f +2, 16 p r o t o n s w o u l d b e r e q u i r e d t o b a l a n c e t h e framework c h a r g e . Here i t i s assumed t h a t the c a t i o n i c e x t r a f r a m e w o r k f

A l i s p r e s e n t as [AI^QS^AI] . An e x a m i n a t i o n o f two r e p r e s e n t a t i v e 0-cages i n this zeolite reveals that one cage contains the [AICQ^AI] complex cation, whereas t h e o t h e r i s empty. I f the A l a t o m s a r e d i s t r i b u t e d s u c h t h a t t h e /?-cage w i t h t h e c o m p l e x c a t i o n h a s 6 A l f a t o m s a n d t h e o t h e r £-cage h a s 2, t h e c a g e w i t h t h e c o m p l e x c a t i o n would have 2 h i g h l y a c i d i c p r o t o n s , and the r e m a i n i n g cage would have 2 weakly a c i d i c p r o t o n s . Thus, f o r t h e 8 A l atoms t h e r e w o u l d be o n l y 2 h i g h l y a c i d i c p r o t o n s , w h i c h i s c o n s i s t e n t w i t h t h e A +

A +

f

f


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FLUID CATALYTIC CRACKING II: CONCEPTS IN CATALYST DESIGN +

Na -poisoning experiments. The presence of a larger number of weakly acidic protons is also consistent with the infrared results. As the number of A l / u . c . is increased, the probability of having isolated A l atoms decreases; thus, the activity would decrease. It is at the lower A l concentrations, however, that a simple s t a t i s t i c a l treatment f a i l s to predict the observed linear relationship between catalytic activity and A l concentration. For example, at 8 Alf/u.c. i t is d i f f i c u l t to imagine how there would be a significant concentration of strongly acidic centers. There must be fi-cages that contain at least 3 A l atoms and one A1(0H) cation in order for there to be a strongly acidic proton. But this proton may be even more acidic than in the case described previously.. Even within the more acidic materials, there may be subsets of strongly acidic sites. The net effect may be the observed linear relationship between catalytic activity and A l content. Clearly, more theoretical and experimental work is needed to test the v a l i d i t y of these models. f

f

f

f

2+

f

f


Acknowledgments The author is indebted to several associates who carried out many of the experiments described here and who were involved in the development of the concepts. These include Steven DeCanio, Paul F r i t z , S. J . Sohn, and Sun Yao. The research in our laboratory was supported in part by the U. S. Army Research Office and by the Regents of Texas A&M University through the AUF-sponsored Materials Science and Engineering Program. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

DeCanio, S. J.; Sohn, J. R.; F r i t z , P. O.; Lunsford, J . H. J . Catal. 1986, 101, 132. Sohn, J . R.; DeCanio, S. J.; F r i t z , P. O.; Lunsford; J . H. J . Phys. Chem. 1986, 90, 4847. Pine, L. A . ; Maher, P. J.; Wachter, W. A. J . Catal. 1984, 85, 466. Dempsey, E. J . Catal. 1974, 33, 497; 1975, 39, 155. Mikovsky, R. J.; Marshall, J. F. J . Catal. 1976, 44, 170. Beagley, B . ; Dwyer, J.; Fitch, F. R.; Mann, R.; Walters, J . J . Phys. Chem. 1984, 88, 1744. Beyerlein, R. A . ; McVicker, G. B.; Yacullo, L. N.; Ziemiak, J . J. J . Phys. Chem. 1988, 92, 1967. Haag, W. O.; Lago, R. M.; Weisz, P. B. Nature (London) 1984, 309, 589. Beyerlein, R. A . ; McVicker, G. B.; Yacullo, L. N . ; Ziemiak, J . J. ACS Meeting, Div. Petrol. Chem., New York, 1986; Vol. 31, p. 190. F r i t z , P. O.; Lunsford, J. H. J. Catal. 1989, 118, 85. Lombardo, E. A . ; Sill, G. A . ; Hall, W. K. J . Catal. 1989, 119. 426. Lago, R. M.; Haag, W. O.; Mikovsky, R. J.; Olson, D. H.; Hellring, S. D.; Schmitt, K. D.; Kerr, G. T. In Proceedings 7th I n t l . Zeo. Conf.: Murakami, Y . ; Iijima, A . ; Ward, J. W., Ed.; Kodansha L t d . : Tokyo, 1986; p. 677. Dyer, A . ; Singh, A. P. Zeolites 1988, 8, 242. Jacobs, P. A . ; Uytterhoeven, J . B. J . Chem. Soc. Faraday Trans. 1 1973, 2, 373. Anderson, M. A . ; Klinowski, J . Zeolites 1986, 6, 455. Lohse, U . ; Löffler, E . ; Hunger, M.; Stöchner, J., Patzelová, V. Zeolites 1987, 7, 11. Corma, A . ; Fornés , V . ; Perez-Pariente, J.; Sastre, E . ; Martens, J. A.; Jacobs, P. A. ACS Symp. Ser. 1988, 368, 555. Jacobs, P. A. Carboniogenic Activity of Zeolites; Elsevier: Amsterdam, 1977. Sun, Y . ; Lunsford, J . H. unpublished results. Carvajal, R.; Chu, P.-J.; Lunsford, J. II. J . Catal., in press.


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21. 22. 23. 24.


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Newsam, J. M . ; Treacy, M. M. J.; Vaughan, D. E. W.; Strohmaier, K. G . ; M o r t i e r , W. J. J . Chem. Soc., Chem. Commun. 1989, 493. Marynen, P . ; Maes, A.; Cremers, A. Z e o l i t e s 1984, 4, 287. Cheetham, A. K.; Eddy, M. M.; Thomas, J. M. J. Chem. Soc., Chem. Commun. 1984, 1337. Shannon, R. D . ; Gardner, K. H.; Staley, R. H.; Bergeret, G . ; G a l l e z o t , P . ; Auroux, A. J. Phys. Chem. 1985, 89, 4778.


R E C E I V E D September 17, 1990


Origin of Strong Acidity in Dealuminated Zeolite-Y - American

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