Molecular Sieves - ACS Publications

19 Hydrogen Zeolite Y, Ultrastable Zeolite Y, and Aluminum-Deficient Zeolites Downloaded by UNIV OF MISSOURI COLUMBIA on May 16, 2013 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0121.ch019

G. T. KERR Mobil Research and Development Corp., Central Research Division, Princeton, N.J. 08540

During the past decade, the literature has become fraught with confusion regarding hydrogen, "decationized," and "decationated" zeolites and their various physical, chemical, and catalytic properties. A chronological review is presented of the development of understanding of the natures of true, normal hydrogen zeolites and the so-called ultrastable varieties, with particular emphasis on zeolite Y. A survey of the chemical, physical, and catalytic properties of these materials is presented together with a resuméof the state of knowledge of aluminumdeficient zeolites.

T p h e p h e n o m e n o n of base or c a t i o n exchange was first r e p o r t e d b y W a y i n 1850 (1). I n 1858, E i c h h o r n r e p o r t e d t h a t t r e a t m e n t of zeolites w i t h a l k a l i n e or n e u t r a l salt solutions r e s u l t e d i n cations b e i n g exchanged f r o m t h e zeolite a n d r e p l a c e d b y a n e q u i v a l e n t n u m b e r of cations f r o m s o l u t i o n (2). E i c h h o r n f o u n d t h a t t h e a n i o n i n s o l u t i o n p l a y e d a passive role i n t h i s exchange. Z o c h showed t h a t s t i l b i t e r e t a i n e d i t s c r y s t a l l i n i t y o n exchange, a n d he also showed t h a t e q u i l i b r i u m was a t t a i n e d between c a t i o n s i n s o l u t i o n a n d cations i n t h e zeolite (3). Some early workers attempted t o prepare zeolites c o n t a i n i n g h y d r o g e n ions (4~9), m o r e precisely h y d r o n i u m ions, b y exchange w i t h a c i d i c solutions, u s i n g acids or salts of s t r o n g acids a n d w e a k bases. I t appears t h a t i n a l l cases, a l u m i n u m was e x t r a c t e d f r o m t h e zeolites. T h e silicon-poor zeolites became a m o r p h o u s , b u t t h e m o r e s i l i c o n - r i c h zeolites a p p e a r e d c r y s t a l l i n e as j u d g e d f r o m o p t i c a l p r o p erties, p a r t i c u l a r l y birefringence. U n f o r t u n a t e l y , x - r a y d i f f r a c t i o n a n a l y sis was n o t a v a i l a b l e t o these w o r k e r s , a n d hence, i t is not a b s o l u t e l y c e r t a i n t h a t c r y s t a l l i n i t y , i n t h e m o d e r n sense, was i n fact r e t a i n e d . I n 1930 H e y s t a t e d , " t h e r e is no evidence for t h e existence of h y d r o g e n - z e o l i t e s " (10). D u r i n g t h e 1940's, m u c h effort was m a d e t o elucidate t h e n a t u r e of t h e a c t i v e sites i n a m o r p h o u s s i l i c a - a l u m i n a c r a c k i n g c a t a l y s t s (11). By 219 In Molecular Sieves; Meier, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

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1950, i t was generally agreed t h e sites were B r o n s t e d acids l o c a t e d at s u r faces a v a i l a b l e t o h y d r o c a r b o n molecules u n d e r g o i n g c r a c k i n g a n d t h e sites p r e s u m a b l y h a d t h e s t r u c t u r e s h o w n i n I .

A p p a r e n t l y most w o r k e r s

Si

I

H

Ο

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HO—Al—O—Si

I

Downloaded by UNIV OF MISSOURI COLUMBIA on May 16, 2013 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0121.ch019

Ο

I

Si

I t h e n v i s u a l i z e d t h e a m o r p h o u s c a t a l y s t t o be a h i g h l y porous m a t e r i a l , t h e s o l i d p o r t i o n s b e i n g r e l a t i v e l y dense a n d i m p e r v i o u s t o h y d r o c a r b o n s , a n d t h e a c t i v e sites at t h e s o l i d surfaces of t h e t y p e s h o w n i n I . T h e electro s t a t i c valence r u l e dictates these sites t o be as s t r o n g l y acidic as s u l f u r i c a c i d . T h i s m o d e l p r e c l u d e d c o o r d i n a t i o n of t e t r a h e d r a l a l u m i n u m t o four oxygens, each i n t u r n b e i n g b o n d e d t o a s i l i c o n . S u c h sites are v e r y s t r o n g l y acidic as is p e r c h l o r i c a c i d . N o t u n t i l t h e 1960's, w h e n consider able w o r k was u n d e r w a y o n h y d r o g e n zeolites, was i t generally recognized t h a t i t is t o p o l o g i c a l l y possible t o h a v e f o u r - c o o r d i n a t e a l u m i n u m b o n d e d t o four s i l o x y groups w i t h a l u m i n u m l o c a t e d at a surface a v a i l a b l e t o r e l a t i v e l y large molecules. I n v i e w of t o d a y ' s knowledge, i t seems i n e v i t a b l e t h a t such sites are c o m m o n i n a m o r p h o u s s i l i c a - a l u m i n a . Recent Developments in Hydrogen Zeolites with Emphasis on Zeolite Y B a r r e r first described t h e p r e p a r a t i o n of h y d r o g e n forms of zeolites b y o x i d a t i v e degradation of a m m o n i u m zeolites (12) 4NH Z + 30 4

2

4HZ + 6H 0 + 2N 2

2

(1)

B a r r e r showed these h y d r o g e n zeolites, m o r d e n i t e a n d c h a b a z i t e , t o be c r y s t a l l i n e u s i n g x - r a y diffraction, a n d s t a t e d , " H y d r o g e n zeolites are effectively c r y s t a l l i n e a l u m i n o s i l i c i c acids, t h e salts of w h i c h are t h e i r d i verse c a t i o n exchange p r o d u c t s . " S z y m a n s k i , S t a m i r e s , a n d L y n c h (13) used s i m p l e t h e r m a l d e c o m p o s i t i o n of a n a m m o n i u m zeolite X i n a n a t t e m p t t o prepare t h e h y d r o g e n zeolite N H X -> H X + N H 4

3

(2)

These workers c o n d u c t e d a n i n f r a r e d s t u d y o n t h e p r e s u m e d h y d r o g e n zeolite a n d concluded, q u i t e r i g h t l y , t h a t c o n s t i t u t i v e or c h e m i c a l w a t e r is lost f r o m t h e h y d r o g e n i o n c o n t a i n i n g s o l i d at t e m p e r a t u r e s a b o v e 4 0 0 ° C . I n v i e w of recent studies, however, i t is u n l i k e l y t h a t t h e zeolite

In Molecular Sieves; Meier, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

19.

KERR

Hydrogen Zeolite Y

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s t r u c t u r e was preserved d u r i n g t h e t h e r m a l d e c o m p o s i t i o n of a m m o n i u m zeolite X(l

4).

U y t t e r h o e v e n , C h r i s t n e r , a n d H a l l , i n a n elegant s t u d y of t h e t h e r m a l d e c o m p o s i t i o n p r o d u c t s of a m m o n i u m zeolite Y , p r o p o s e d a scheme t o e x p l a i n t h e loss of c h e m i c a l w a t e r f r o m t h e h y d r o g e n f o r m of t h e zeolite


(15).

T h e y assigned t h e t e r m s " d e c a t i o n a t e d " t o t h e h y d r o g e n zeolite a n d " d e h y d r o x y l a t e d " t o t h e a c i d a n h y d r i d e . I n retrospect, these designat i o n s are u n f o r t u n a t e ; o t h e r w o r k e r s h a d a l r e a d y c a l l e d t h e p r e s u m e d a c i d a n h y d r i d e t h e " d e c a t i o n i z e d " f o r m (16). T o d a y b o t h t e r m s are used i n d i s c r i m i n a t e l y , a n d t h e l i t e r a t u r e is u n c l e a r as t o w h a t p a r t i c u l a r substance a n y p a r t i c u l a r a u t h o r is referring. I n d e s c r i b i n g t h e a c i d a n h y d r i d e of h y d r o g e n zeolites, t h e t e r m " d e h y d r o x y l a t e d " is d e s c r i p t i v e i n t h e sense t h a t t h e c o n c e n t r a t i o n of h y d r o x y l groups is d i m i n i s h e d o n loss of c o n s t i t u t i v e w a t e r . H o w e v e r , t h e t e r m is s t o i c h i o m e t r i c a l l y i n correct since t h e zeolite loses h y d r o g e n ions s i m u l t a n e o u s l y w i t h h y d r o x i d e ions t o y i e l d w a t e r . A m o r e precise d e s i g n a t i o n is " d e h y d r o h y d r o x y l a t e d . " K e r r , C a t t a n a c h , a n d W u suggested t h e t e r m s " d e c a t i o n a t e d " a n d " d e c a t i o n i z e d " be d r o p p e d f r o m usage a n d t h a t h y d r o g e n or a c i d zeolites be c a l l e d s i m p l y t h e h y d r o g e n form—e.g., h y d r o g e n zeolite Y (17). The t e r m " d e h y d r o x y l a t e d " seems t o be used w i t h o u t confusion t h u s far, a n d i t s w i d e usage i n d i c a t e s i t w i l l p r o b a b l y persist. T h i s t e r m is n o w form a l l y used b y mineralogists. U y t t e r h o e v e n , C h r i s t n e r , a n d H a l l , i n t h e i r classic paper, were t h e first t o measure q u a n t i t a t i v e l y t h e p r o t o n i c c o n t e n t of h y d r o g e n z e o l i t e Y p r e p a r e d b y c a r e f u l c a l c i n a t i o n of t h e a m m o n i u m f o r m (IS). The measurements u t i l i z e d t h e r m o g r a v i m e t r y a n d d e u t e r i u m exchange of t h e a c i d f o r m . B e n e s i p u b l i s h e d t h e first t h e r m o g r a v i m e t r i c curves for t h e t h e r m a l decompositions of a m m o n i u m zeolite Y a n d a m m o n i u m m o r d e n i t e (18). These studies disclosed t h e t e m p e r a t u r e range o v e r w h i c h a m m o n i a was e v o l v e d f r o m t h e samples; moreover, t h e y i n d i c a t e d t h a t t h e q u a n t i t y of c h e m i c a l w a t e r lost f r o m t h e h y d r o g e n zeolite Y agreed w i t h t h e v a l u e calculated from the initial a m m o n i u m ion content. F o r the mordenite, B e n e s i f o u n d t h a t t h e t o t a l w e i g h t loss a b o v e 3 5 0 ° C agreed w i t h t h e s u m of t h e c a l c u l a t e d w e i g h t losses for a m m o n i a a n d c h e m i c a l w a t e r . Unlike zeolite Y , f r o m w h i c h a m m o n i a a n d c h e m i c a l w a t e r losses occur i n d i s t i n c t



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steps at l o w h e a t i n g rates, t h e r e is a l w a y s o v e r l a p of these losses f r o m t h e mordenite. L a t e r , C a t t a n a c h , W u , a n d V e n u t o d i d a n elaborate t h e r m o g r a v i m e t r i c s t u d y o n t h e c a l c i n a t i o n of a m m o n i u m zeolite Y a n d t h e r e s u l t i n g p r o d u c t s (19). T h e y f o u n d t h a t t h e h y d r o g e n zeolite r e a c t e d w i t h a n h y d r o u s a m m o n i a t o y i e l d a n a m m o n i u m zeolite i d e n t i c a l i n a m m o n i a c o n t e n t w i t h t h e i n i t i a l a m m o n i u m zeolite. F u r t h e r , these w o r k e r s r e p o r t e d t h a t after loss of c h e m i c a l w a t e r ( " d e h y d r o x y l a t i o n " a c c o r d i n g t o U y t t e r hoeven, C h r i s t n e r , a n d H a l l or " d e c a t i o n i z a t i o n " a c c o r d i n g t o R a b o , P i c k e r t , S t a m i r e s , a n d B o y l e ) t h e sample became a m o r p h o u s w h e n exposed t o m o i s t u r e . T h i s o b s e r v a t i o n conflicted w i t h t h e s t a t e m e n t of R a b o et al. (16) i n w h i c h t h e y emphasized t h e extreme s t a b i l i t y of t h e i r " d e c a t i o n i z e d " Y . T h e d a t a of C a t t a n a c h , W u , a n d V e n u t o p r o v e , bey o n d a n y d o u b t , t h a t t h e y o b t a i n e d t h e expected n o r m a l h y d r o g e n zeolite Y p r i o r t o t h e loss of c h e m i c a l w a t e r a b o v e 4 5 0 ° . R a b o et al. however, did not prove that the material from which they removed chemical water, was i n fact, t h e h y d r o g e n zeolite. T h e y p r o b a b l y prepared, u n k n o w n t o t h e m at t h e t i m e , t h e u l t r a s t a b l e zeolite described below. }

The Nature of Ultrastable Faujasite and Aluminum-Deficient

Zeolite Y

I n 1967, M c D a n i e l a n d M a h e r r e p o r t e d a h i g h l y stable m a t e r i a l , o b t a i n e d f r o m w h a t t h e y considered t o be a m m o n i u m zeolite Y , w h i c h t h e y c a l l e d " u l t r a s t a b l e " f a u j a s i t e (20). Since t h i s m a t e r i a l was o b t a i n e d b y c a l c i n i n g w h a t appeared t o be a m m o n i u m zeolite Y , i t was n o t u n r e a sonable t h e n t o assume t h a t t h i s was a n u n u s u a l l y stable f o r m of h y d r o g e n zeolite Y . T h e y f u r t h e r r e p o r t e d t h a t t h i s m a t e r i a l h a d a c o n t r a c t e d l a t t i c e c o m p a r e d w i t h t h e u s u a l h y d r o g e n zeolite, a n d i t also h a d a d i m i n i s h e d ion-exchange c a p a c i t y . T h e y i m p l i e d t h a t t h e s t a b i l i t y was t h e result of c a l c i n i n g a p a r t i a l l y a m m o n i u m - e x c h a n g e d s o d i u m zeolite Y p r i o r t o a final a m m o n i u m i o n exchange, w h e r e b y u l t i m a t e l y a l l s o d i u m ions were r e p l a c e d b y a m m o n i u m ions. A m b s a n d F l a n k , i n 1969, suggested t h a t t h e s t a b i l i t y of t h e u l t r a s t a b l e faujasite was t h e result of r e m o v i n g essentially a l l t h e s o d i u m i o n f r o m zeolite Y a n d r e p l a c i n g i t w i t h a m m o n i u m i o n (21). F r o m 1967 t o 1969, K e r r p u b l i s h e d a series of papers o n t h e q u e s t i o n of t h e r m a l a n d h y d r o t h e r m a l s t a b i l i t i e s of s o d i u m a n d h y d r o g e n zeolite Y (22-26). These studies i n d i c a t e d t h a t u p o n r e m o v a l of a b o u t o n e - t h i r d of t h e a l u m i n u m f r o m zeolite Y , u s i n g e t h y l e n e d i a m i n e t e t r a a c e t i c a c i d ( H E D T A ) , t h e t h e r m a l a n d h y d r o t h e r m a l s t a b i l i t i e s were m u c h enhanced. T h i s was observed for b o t h s o d i u m (23) a n d h y d r o g e n (25) f o r m s of t h e zeolite. T h e l a t t e r was p r e p a r e d b y careful c a l c i n a t i o n of a n a m m o n i u m zeolite f r o m w h i c h a b o u t 3 0 % of t h e a m m o n i u m a n d a l u m i n u m h a d been removed. K e r r also showed t h a t t h e t r u e or n o r m a l h y d r o g e n zeolite w i t h 4



19.

KERB

Hydrogen Zeolite Y

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r e l a t i v e l y poor t h e r m a l a n d h y d r o t h e r m a l s t a b i l i t y c o u l d be c o n v e r t e d t o a h i g h l y stable f o r m b y h e a t i n g t h e a c i d i n a closed r e a c t o r a t 8 0 0 ° C w h e r e b y t h e zeolite was assumed t o react w i t h t h e c h e m i c a l w a t e r (22). The s t a b i l i z e d zeolite c o n t a i n e d c a t i o n i c a l u m i n u m w h i c h c o u l d be r e m o v e d b y i o n exchange u s i n g s o d i u m h y d r o x i d e s o l u t i o n . M o r e r e c e n t l y , K e r r s t a t e d t h a t a zeolite Y , i n w h i c h o v e r 9 5 % of t h e s o d i u m ions h a d been exchanged b y a m m o n i u m ions ( w i t h o u t i n t e r m e d i a t e c a l c i n a t i o n ) , y i e l d e d t w o d i s t i n c t p r o d u c t s , d e p e n d i n g o n t h e g e o m e t r y of t h e a m m o n i u m z e o l i t e b e d d u r i n g c a l c i n a t i o n at 500° C i n a s t a t i c a t m o s p h e r e (26). B e d geome t r y w h i c h m a x i m i z e d r a p i d diffusion of gaseous p r o d u c t s f r o m t h e s a m p l e (shallow-bed c a l c i n a t i o n ) y i e l d e d t h e e x p e c t e d n o r m a l h y d r o g e n zeolite i n w h i c h one p r o t o n i c site was c r e a t e d for each a m m o n i u m i o n i n t h e i n i t i a l sample. C a l c i n a t i o n c o n d i t i o n s w h i c h i m p e d e d r e m o v a l of gaseous p r o d ucts f r o m t h e zeolite (deep-bed c a l c i n a t i o n ) y i e l d e d a p r o d u c t w h i c h a p p e a r e d t o be s i m i l a r i n m a n y respects t o t h e u l t r a s t a b l e f a u j a s i t e of M c D a n i e l a n d M a h e r (20). W a r d (27) r e p o r t e d a n i n f r a r e d s t u d y of u l t r a stable f a u j a s i t e p r e p a r e d a c c o r d i n g t o M c D a n i e l a n d M a h e r (20), a deepb e d c a l c i n e d a m m o n i u m zeolite Y sample a c c o r d i n g t o K e r r (26), a n d samples of a m m o n i u m zeolite Y c a l c i n e d i n a flowing s t e a m a t m o s p h e r e (2 p s i g at 500 a n d 6 5 0 ° C ) a c c o r d i n g t o H a n s f o r d (28). W a r d stated " t h a t a l l samples are s i m i l a r , at least i n gross properties, t o t h e " d e e p - b e d " samples of K e r r . " M a h e r , H u n t e r , a n d Scherzer (29) r e p o r t e d t h e results of x - r a y diffract i o n analyses o n t h e u l t r a s t a b l e faujasite p r e p a r e d b y t h e procedure of M c D a n i e l a n d M a h e r (20) together w i t h i n t e r m e d i a t e forms o b t a i n e d d u r i n g t h e process of p r e p a r i n g t h e final u l t r a s t a b l e f o r m . T h e y f o u n d t h e u l t r a s t a b l e f o r m h a d undergone loss of f r a m e w o r k a l u m i n u m t o a p p r o x i m a t e l y t h e same l e v e l as r e p o r t e d b y K e r r (26). M o r e o v e r , i n s u p p o r t of K e r r ' s findings, t h e y f o u n d t h a t a p a r t i a l l y exchanged a m m o n i u m zeolite Y ( 8 0 % a m m o n i u m a n d 2 0 % sodium) after c a l c i n a t i o n a t 5 4 0 ° C lost a l u m i n u m on treatment w i t h sodium hydroxide solution. Presumably the c a l c i n a t i o n was of t h e deep-bed t y p e ; t h e a u t h o r s d i d n o t describe t h e sample g e o m e t r y d u r i n g c a l c i n a t i o n . O n t h e basis of t h e i r x - r a y diffract i o n analyses, M a h e r , H u n t e r , a n d Scherzer offered a n e l a b o r a t e a n d d e t a i l e d m e c h a n i s m t o e x p l a i n t h e m o d e of f o r m a t i o n of several p r o p o s e d species i n t h e u l t r a s t a b l e faujasite. A d d i t i o n a l d a t a , perhaps i n c l u d i n g some c h e m i c a l studies, seem desirable t o s u p p o r t firmly t h e i r m e c h a n i s m . J a c o b s a n d U y t t e r h o e v e n s t u d i e d t h e n a t u r e of deep-bed c a l c i n e d a m m o n i u m zeolite Y a n d a l u m i n u m - d e f i c i e n t zeolite Y , t h e l a t t e r p r e p a r e d b y t h e H E D T A t e c h n i q u e (30). T h e y concluded t h a t the stability of u l t r a s t a b l e f a u j a s i t e was i m p a r t e d o n l y b y c a t i o n i c a l u m i n u m a n d t h a t a l u m i n u m deficiency i n itself d i d not c o n t r i b u t e t o s t a b i l i t y . T h e i r c o n c l u s i o n r e g a r d i n g a l u m i n u m deficiency was m a d e o n t h e basis of H4EDTA4


224

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t r e a t e d a m m o n i u m zeolite Y f r o m w h i c h a m a x i m u m of o n l y a b o u t 1 0 % of t h e a l u m i n u m h a d been r e m o v e d . K e r r p o i n t e d out t h a t a b o u t 2 5 3 5 % a l u m i n u m r e m o v a l r e s u l t e d i n increased s t a b i l i t y ; i t seems a p p a r e n t t h a t Jacobs a n d U y t t e r h o e v e n d i d not r e m o v e sufficient a l u m i n u m t o effect increased s t a b i l i t y . H o w e v e r , t h e i r suggestion t h a t c a t i o n i c a l u m i n u m increases s t a b i l i t y i n t h e u l t r a s t a b l e f a u j a s i t e is reasonable since i t is w e l l k n o w n t h a t m u l t i v a l e n t cations i n faujasites generally i m p a r t greater s t a b i l i t y t h a n m o n o v a l e n t cations. M o s t l i k e l y , t h e a l u m i n u m deficiency a n d t h e c a t i o n i c a l u m i n u m each c o n t r i b u t e t o t h e increased s t a b i l i t y of u l t r a s t a b l e faujasite. R e c e n t l y P e r i r e p o r t e d a s t u d y of u l t r a s t a b l e faujasites u s i n g p o t e n t i o m e t r i c t i t r a t i o n s , i n f r a r e d spectroscopy, a m m o n i a s o r p t i o n , a n d a c e t y l acetone e x t r a c t i o n of a l u m i n u m (31). H e concluded t h a t " d u r i n g format i o n of u l t r a s t a b l e f a u j a s i t e , A l m i g r a t e s f r o m t e t r a h e d r a l sites i n t h e a l u m i n o s i l i c a t e f r a m e w o r k t o c a t i o n positions outside t h e f r a m e w o r k . " P e r i f u r t h e r s t a t e d t h a t his studies " a l s o i n d i c a t e t h a t S i replaces t h e lost A l t h r o u g h r e c r y s t a l l i z a t i o n of t h e f r a m e w o r k . " Maher, Hunter, and Scherzer p r e v i o u s l y suggested m i g r a t i o n of s i l i c o n i n t o sites v a c a t e d b y a l u m i n u m (29). K e r r proposed t h a t upon a l u m i n u m removal using H E D T A t h e v a c a t e d a l u m i n u m sites are occupied b y four hydrogens as i n d i c a t e d i n r e a c t i o n 5 (23). F r o m t h e s t a n d p o i n t of f o r m a l charges, of course, t h e t r i v a l e n t a l u m i n u m is r e p l a c e d b y t h r e e protons, t h e f o u r t h p r o t o n b e i n g t h e o r i g i n a l h y d r o g e n of t h e silanol. K e r r l a t e r p r o p o s e d t h e loss of c o n s t i t u t i v e w a t e r f r o m these four s i l a n o l groups t o f o r m n e w S i O - S i bonds i n t h e f r a m e w o r k (25). T h i s suggestion has not b e e n v e r i f i e d ; moreover, t h e concept of some t y p e of " r e c r y s t a l l i z a t i o n " w h e r e b y a l l 192 t e t r a h e d r a l sites i n t h e f a u j a s i t e u n i t cell are occupied b y (Si + A l ) is a n i n t e r e s t i n g a n d l o g i c a l a l t e r n a t i v e . H o w e v e r , n o c o n v i n c i n g evidence has been presented t o resolve t h i s i m p o r t a n t q u e s t i o n of t h e n a t u r e of t h e a l u m i n u m - d e f i c i e n t sites. 4

The Three General Thermal Decomposition Products of Ammonium Zeolite Y T h r e e general categories of p r o d u c t s c a n be o b t a i n e d o n t h e t h e r m a l d e c o m p o s i t i o n of a m m o n i u m zeolite Y : (1) T h e e x p e c t e d t r u e or n o r m a l h y d r o g e n zeolite as i n d i c a t e d b y R e a c t i o n 4: N H Y -+ H Y + N H 4

3

(4)

(2) T h e a c i d a n h y d r i d e d e r i v e d f r o m t h e t r u e h y d r o g e n f o r m , g e n e r a l l y c a l l e d d e h y d r o x y l a t e d Y a n d a s s u m e d t o be f o r m e d a c c o r d i n g t o R e a c t i o n 3 of U y t t e r h o e v e n , C h r i s t n e r , a n d H a l l (15). (3) T h e u l t r a s t a b l e f o r m , first r e p o r t e d b y M c D a n i e l a n d M a h e r (20) a n d first c h e m i c a l l y c h a r a c t e r i z e d b y K e r r (22, 25, 26), w h o also suggested


19.

KERR

Hydrogen Zeolite Y

225

t h a t i t s f o r m a t i o n is t h e result of s a m p l e g e o m e t r y i n h e r e n t i n large-scale (100 grams) p r e p a r a t i o n s .


T h e c h e m i s t r y a n d s t r u c t u r e of t h e h y d r o g e n f o r m of zeolite Y h a v e been t h o r o u g h l y i n v e s t i g a t e d (32) a n d are n o t considered f u r t h e r . The s t r u c t u r e of t h e d e h y d r o x y l a t e d zeolite proposed b y U y t t e r h o e v e n , C h r i s t ner, a n d H a l l (15) r e m a i n s u n c h a n g e d . R e c e n t l y W a r d , o n t h e basis of i n f r a r e d studies, suggested t h a t t h i s f o r m m a y be a m o r p h o u s (27). The extreme i n s t a b i l i t y of d e h y d r o x y l a t e d zeolite Y t o m o i s t u r e c o m p l i c a t e s d e t a i l e d s t u d y (19). T h e e l u c i d a t i o n of t h e d e t a i l e d n a t u r e of t h i s m a t e r i a l lies i n t h e f u t u r e . A t present, c o m p l e t e l y d e h y d r o x y l a t e d Y is l i t t l e u n d e r s t o o d a n d presents a challenging v o i d i n our knowledge of t h e n a t u r e of a m m o n i u m zeolite Y t h e r m a l d e c o m p o s i t i o n p r o d u c t s . A m b s a n d F l a n k c o r r e c t l y observed t h a t v a r i a b l e s c a n be i n t r o d u c e d i n t o t h e c a l c i n a t i o n of a m m o n i u m Y so t h a t a " v a r i a b l e series of p r o d u c t s " c a n be o b t a i n e d (33). H o w e v e r , t h e r e is no d o u b t t h a t t h e n o r m a l h y d r o gen zeolite c a n be o b t a i n e d f r o m t h e a m m o n i u m f o r m b y c a r e f u l l y c o n t r o l l e d c a l c i n a t i o n . I n a d d i t i o n , c a r e f u l l y c o n t r o l l e d c a l c i n a t i o n of t h e a c i d y i e l d s t h e d e h y d r o x y l a t e d f o r m . T h e u l t r a s t a b l e f o r m , w h i c h c a n be p r e p a r e d b y a n u m b e r of procedures described below, differs d r a s t i c a l l y i n s t a b i l i t y a n d c o m p o s i t i o n f r o m t h e o t h e r t w o forms. T h a t i t m a y c o n t a i n some sites s i m i l a r t o , or perhaps i d e n t i c a l w i t h , sites i n t h e h y d r o g e n a n d d e h y d r o x y l a t e d forms cannot be r e f u t e d . U n q u e s t i o n a b l y , however, t h e u l t r a s t a b l e f o r m differs s i g n i f i c a n t l y f r o m t h e o t h e r t w o forms.

The Three General Methods for Preparing Ultrastable Faujasites (1) Direct Conversion of Ammonium Zeolite Y . T h e procedures of M c D a n i e l a n d M a h e r (20), H a n s f o r d (27), a n d K e r r (26) appear to h a v e i n c o m m o n r e a c t i o n c o n d i t i o n s w h i c h effect h y d r o l y s i s a n d r e m o v a l of a p o r t i o n of t h e t e t r a c o o r d i n a t e a l u m i n u m ions f r o m t h e f r a m e w o r k of t h e h y d r o g e n f o r m d u r i n g d e c o m p o s i t i o n of the a m m o n i u m f o r m at t e m p e r a tures of 4 0 0 ° C a n d above. (2) Direct Conversion of Hydrogen Zeolite Y . K e r r has r e p o r t e d t h a t t h e n o r m a l h y d r o g e n Y c a n be c o n v e r t e d d i r e c t l y to t h e u l t r a s t a b l e f o r m b y h e a t i n g i n a n i n e r t s t a t i c atmosphere at 700-800° C (22) or h e a t i n g i n a s t a t i c a m m o n i a a t m o s p h e r e a t 5 0 0 ° C (24). A t 700-800°C, chemical w a t e r is t h e r m a l l y l a b i l i z e d a n d is e n v i s i o n e d b y K e r r to effect h y d r o l y s i s of t h e a c i d zeolite. A t 500° C a m m o n i a l a b i l i z e s c h e m i c a l w a t e r w h e r e b y h y d r o l y s i s c a n occur. P e r h a p s a n o v e r l y simple m e c h a n i s m has been p r o p o s e d b y K e r r t o e x p l a i n t h e f o r m a t i o n of t h e n o n f r a m e w o r k a l u m i n u m f o u n d i n u l t r a s t a b l e faujasites p r e p a r e d b y t h e t w o m e t h o d s j u s t described (22,26).


226

MOLECULAR SIEVES

Si

Si

I

I

0

1 I O

ο

0

I

Si—0—Al

H

0 — S i — 0 + 3H 0

I O

H

2

—*

Si—OH


S i

(5

)

I

0—Al O

3

0

Si

I

H O — S i + A1(0H)

H

0—Si—0 + H

—•

A1(0H)3

O 0

1

0

- I

0—Al—0—Si—0

+

A1(0H)

2

+

+ H 0 2

(6)

o

0

(3) Controlled Calcination of Aluminum-Deficient Ammonium Zeolite Y.

K e r r showed that about one-third of the ammonium and aluminum

could be removed from ammonium Y using H E D T A (25).

Carefully

4

controlled calcination of this material (under conditions which yield the relatively unstable, normal hydrogen form from the normal ammonium form) yielded a hydrogen zeolite of very high stability.

K e r r proposed

the following reaction steps to explain the stability (23,25). 0 0 0—Al—5—Si—Ο 0

+ NH

4

+

+

0.5H EDTA 4

—>•

0 0

1

0—Al

A

0

I

0—Si—0

+

0.5(NH ) H EDTA 4

2

2

(7)

H O

T h e r e s u l t i n g h y d r o g e n sites are e n v i s i o n e d t o undergo h y d r o l y s i s a n d n e u t r a l i z a t i o n as s h o w n i n E q u a t i o n s 5 a n d 6, r e s p e c t i v e l y .

T h e n the

a l u m i n u m c a t i o n f o r m e d i n E q u a t i o n 6 is r e p l a c e d b y a m m o n i u m i o n A1(0H) + (NH ) H EDTA — N H + A1NH EDTA + 2H 0 (8) T h e t h r e e general procedures for p r e p a r i n g u l t r a s t a b l e zeolite Y h a v e one 2

+

effect i n c o m m o n :

4

2

2

4

+

4

r e m o v a l of a p o r t i o n , 2 0 - 3 5 % ,

2

of t h e t e t r a h e d r a l


19.

KERR

227

Hydrogen Zeolite Y

a l u m i n u m from the framework. creased s t a b i l i t y .

T h i s r e m o v a l appears

to impart i n

U n t i l our knowledge of t h e n a t u r e of t h e sites v a c a t e d

b y t h e a l u m i n u m is m o r e a d v a n c e d , we c a n o n l y c o n j e c t u r e as t o j u s t h o w the increased s t a b i l i t y arises.

Acidic Sites and Catalytic Cracking Properties of Some


Aluminum-Deficient Zeolites A n excellent comprehensive r e v i e w was p u b l i s h e d b y L e a c h o n t h e a p p l i c a t i o n of zeolites t o c a t a l y s i s (34)I t s coverage is m u c h broader t h a n t h a t a t t e m p t e d here. H o w e v e r , i n a d d i t i o n t o t h e h i g h l i g h t s of a l u m i n u m - d e f i c i e n t zeolites c o v e r e d b y L e a c h , recent significant findings w i l l be r e v i e w e d . K e r r , Plank, and Rosinski reported the preparation and catalytic properties of a l u m i n u m - d e f i c i e n t zeolite Y m a t e r i a l s (35). Topchieva a n d co-workers s t u d i e d t h e c a t a l y t i c properties of c a t i o n i c forms of a l u m i n u m - d e f i c i e n t Y zeolites, t h e a l u m i n u m deficiency b e i n g effected b y t h e H E D T A m e t h o d (36-40). T h e y f o u n d t h a t u p t o 5 0 % a l u m i n u m r e m o v a l increased b o t h s t a b i l i t y a n d cumene c r a c k i n g a c t i v i t y ; m a x i m u m a c t i v i t y was observed at t h e 5 0 % r e m o v a l l e v e l . Increased c a t a l y t i c c r a c k i n g a c t i v i t y was observed b y E b e r l y a n d K i m b e r l i n for mordenites f r o m w h i c h about 8 0 % a l u m i n u m h a d been r e m o v e d (41). W e i s s et al. r e m o v e d o v e r 9 9 % of t h e a l u m i n u m f r o m a h y d r o g e n m o r d e n i t e a n d f o u n d t h e zeolite r e t a i n e d c a t a l y t i c a c t i v i t y of t h e t y p e i n d u c e d b y B r o n s t e d acids (42). A l t h o u g h t h e i n i t i a l a c t i v i t y of t h i s m a t e r i a l was lower t h a n t h a t of m o r e a l u m i n u m - r i c h mordenites, t h e aging r a t e was m a r k e d l y r e d u c e d , a n d i n a r e l a t i v e l y short t i m e t h e a l u m i n u m - d e f i c i e n t c a t a l y s t was t h e most a c t i v e . 4

B e a u m o n t a n d B a r t h o m e u f r e c e n t l y s t u d i e d t h e effective a c i d i t y per B r o n s t e d a c i d site (a ) of zeolites X a n d Y a n d t h e i r a l u m i n u m - d e f i c i e n t d e r i v a t i v e s (43). T h e y f o u n d t h a t t h i s p a r a m e t e r increases m o n o t o n i c a l l y as t h e n u m b e r of anionic sites per u n i t cell decreases. I n a faujasite c o n t a i n i n g 96 a l u m i n u m t e t r a h e d r a per u n i t cell ( S i / A l = 1.0) a = 0 ; for a n a l u m i n u m - d e f i c i e n t faujasite c o n t a i n i n g 28 ± 3 a l u m i n u m s per u n i t cell ( S i / A l « 5.9) a = 1.0. T h e i r i m p o r t a n t finding t h a t t h e effective a c i d i t y per site increases as t h e c o n c e n t r a t i o n of sites decreases i n a c i d zeolites is analogous w i t h t h e effective acidities of t h e a c i d sites of d i c a r b o x y l i c acids, Η Ο Ο Ο - ( Ο Η ) - Ο Ο Ο Η : where χ = 0, K /K = 875 a n d where χ = 8, Κι/Κ = 9. A s t h e a c i d sites are m o v e d f a r t h e r a p a r t , t h e i r degrees of dissociation become more n e a r l y equal. A s t r i k i n g ex a m p l e of t h i s effect is e x h i b i t e d b y t h e hexabasic a c i d H [ C o W i 2 0 4 o ] . T i t r a t i o n of a n aqueous s o l u t i o n of t h i s a c i d w i t h N a O H s o l u t i o n gives o n l y one e n d p o i n t , a n d a l l six a c i d sites p a r t a k e e q u a l l y i n t h e n e u t r a l i z a t i o n r e a c t i o n (44)· T h e anionic sites of t h i s water-soluble f r a m e w o r k 0

0

0

2

ζ

x

2

2

6

n


MOLECULAR SIEVES

228


structure are sufficiently separated from one another so that each site be haves independently of its neighbors. The relationship between acid site density and effective acidity may account for the interesting observation of Hopkins that maximum cracking activity of n-hexane was obtained over a partially dehydroxylated hydrogen zeolite Y (45). While the normal hydrogen form would contain a greater overall concentration of acid sites, the partially dehydroxylated form may have a greater overall acid activity because of the increased effective acidity of the remaining sites. Literature Cited 1. Way, J. T., J. Roy. Agr. Soc. (1850) 11, 313; (1852) 13, 123; (1854) 15, 491. 2. Eichhorn, H., Ann. Phys. Chem. (1858) 105, 130. 3. Zoch, I., Chem. Erde (1915) 1, 219. 4. Rinne, F., Νeues Jahrb. Min. (1896) 1, 24. 5. Rinne, F., Νeues Jahrb. Min. (1897) 1, 40. 6. Rinne, F., Neues Jahrb. Min. (1897) 1, 28. 7. Rinne, F., Centr. Min. (1902) 594. 8. Daikuhara, G., Bull. Imp. Centr. Agr. Exp. Sta. Jap. (1914) 2, 1. 9. Kappen, H., Fischer, Β., Z. Pflanzen. Dungung Bod. A (1928) 12, 8. 10. Hey, M. H., Min. Mag. (1930) 22, 422. 11. For excellent reviews, see Tamele, M. W., Discuss. Faraday Soc. (1950) 8, 270; Milliken, T. H., Mills, G. Α., Oblad, A. G., Discuss. Faraday Soc. (1950) 8, 279. 12. Barrer, R. M., Nature (London) (1949) 164, 113. 13. Szymanski, Η. Α., Stamires, C. N., Lynch, G. R., J. Opt. Soc. Amer. (1960) 50, 1323. 14. Bolton, A. R., Lanewala, Μ. Α., J. Catal. (1970) 18, 154. 15. Uytterhoeven, J. B., Christner, L. G., Hall, W. K., J. Phys. Chem. (1965) 69, 2117. 16. Rabo, J. Α., Pickert, P. E., Stamires, D. N., Boyle, J. E., Act. Congr. Int. Catal. (1960) 2. 17. Kerr, G. T., Cattanach, J., Wu, E. L., J. Catal. (1969) 13, 114. 18. Benesi, Η. Α., J. Catal. (1967) 8, 368. 19. Cattanach, J., Wu, E. L., Venuto, P. B., J. Catal. (1968) 11, 342. 20. McDaniel, C. V., Maher, P. K., Conference on Molecular Sieves, Society of Chemical Industry, London, 1967. 21. Ambs, W. J., Flank, W. H., J. Catal. (1969) 14, 118. 22. Kerr, G. T., J. Phys. Chem. (1967) 71, 4155. 23. Kerr, G. T., J. Phys. Chem. (1968) 72, 2594. 24. Kerr, G. T., Shipman, G. F., J. Phys. Chem. (1968) 72, 3071. 25. Kerr, G. T., J. Phys. Chem. (1969) 73, 2780. 26. Kerr, G. T., J. Catal. (1969) 15, 200. 27. Ward, J. W., J. Catal. (1970) 18, 348. 28. Hansford, R. C., U. S. Patent 3,354,077 (1967). 29. Maher, P. K., Hunter, F. D., Scherzer, J., ADVAN. CHEM. SER. (1971) 101, 266. 30. Jacobs, P., Uytterhoeven, J. B., J. Catal. (1971) 22, 193. 31. Peri, J. B., "The Nature of Ultrastable Faujasite," Proc. Int. Congr. Catal. 5th (1972).



19. KERB

Hydrogen Zeolite Y

229

32. See Ref. 27 and previous publication of J. W. Ward; Refs. 15, 18, 19; Venuto, P. B., Hamilton, L. Α., Landis, P. S., Wise, J. J., J. Catal. (1966) 4, 81; Venuto, P. B., Hamilton, L. Α., Landis, P. S., J. Catal. (1966) 5, 484; Eberly, P. E., J. Phys. Chem. (1967) 71, 1717; Olson, D. H., Dempsey, E., J. Catal. (1969) 13, 221. 33. Ambs, W. J., Flank, W. H., J. Catal. (1970) 18, 238. 34. Leach, H. F. Annu. Rept. Progr. Chem. A (1971) 68, 195. 35. Kerr, G. T., Plank, C. J., Rosinski, E. J., U. S. Patent 3,442,795 (1969). 36. Topchieva, Κ. V., Thanh, H. C., Neftekhimiya (1969) 10, 525. 37. Topchieva, Κ. V., Thanh, H. C., Dokl. Akad. Nauk SSSR (1970) 193, 641. 38. Topchieva, Κ. V., Thanh, H. C., Kinet.Katal.(1970) 11, 490. 39. Topchieva, Κ. V., Rosolovskaya, Ε. N., Zh. Fiz. Khim. (1970) 44, 870. 40. Topchieva, Κ. V., Thuong, C. S., Dokl. Akad. Nauk SSSR (1971) 198, 141. 41. Eberly, P. E., Kimberlin, C. N., Ind. Eng. Chem., Prod. Res. Develop. (1970) 9, 335. 42. Bierenbaum, H. S., Chiramongkol, S., Weiss, A. H., J. Catal. (1971) 23, 61. 43. Beaumont, R., Barthomuef, D., J. Catal. (1972) 26, 218. 44. Simmons, V. E., Ph.D. Dissertation, Boston University, 1963. 45. Hopkins, P. D., J. Catal. (1968) 12, 325. RECEIVED November 22, 1972.


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