53
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Synthesis and Properties of Zeolite Omega Preparation and Use of Dual-Function Catalysts Based on Hydrogen-Omega J. F. COLE Shell Development Co., MTM Process Research and Development Laboratory, P. O. Box 100, Deer Park, Texas 77536 H. W. KOUWENHOVEN Koninklijke/Shell-Laboratorium Amsterdam, Badhuisweg 3, Amsterdam N., The Netherlands
A study is presented of the synthesis and properties of the novel synthetic zeolite omega. The synthesis variables and kinetics formation are discussed, as well as the ion exchange, sorption, and thermal properties. By decomposition of imbibed tetra methylammonium ions and exhaustive treatments of the zeolit with ammonium ions, a pure hydrogen form can be obtained which is a suitable substrate for the preparation of hydrocarbon conversion catalysts. Several catalysts were prepared an utilized to isomerize n-hexane, and to hydrocrack a heavy gas oil. Tn their pioneering work on zeolite synthesis, Barrer and Denny introduced the use of large organic cations to replace or partly replace the
A
a l k a l i m e t a l cations s u c h as N a w h i c h are n o r m a l l y present i n classical zeolite syntheses (1). I t was a r g u e d t h a t i f i n a zeolite c a v i t y several s o d i u m or other a l k a l i m e t a l i o n s c o u l d be r e p l a c e d b y one b u l k y organic c a t i o n of u n i t charge, the s i l i c o n - a l u m i n u m r a t i o of t h e zeolitic f r a m e w o r k w o u l d h a v e t o be c h a n g e d to preserve electrical n e u t r a l i t y , a n d i n f a c t w o u l d h a v e t o be increased. T h e a p p l i c a t i o n of t h i s a r g u m e n t i n p r a c t i c e l e d to t h e use of v a r i o u s t e t r a a l k y l a m m o n i u m ions i n zeolite synthesis. +
T h i s s p e c u l a t i o n was n o t o n l y c o m p l e t e l y v i n d i c a t e d i n t h e p r e p a r a t i o n of s i l i c a - r i c h forms of a v a r i e t y of k n o w n zeolites (2, 8) : i t w a s f o u n d t h a t direct use of organic cations i n syntheses p r o m o t e d u n u s u a l s t r u c t u r a l designs w h i c h l e d t o n o v e l zeolites. Z e o l i t e o m e g a (Ω) was one s u c h m a t e r i a l a n d was first s y n t h e s i z e d b y F l a n i g e n a n d K e l l b e r g (4). These 583 In Molecular Sieves; Meier, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.
584
MOLECULAR SIEVES
a u t h o r s gave s t r a i g h t f o r w a r d s y n t h e t i c routes, i n f o r m a t i o n a b o u t
the
s o r p t i v e properties a n d e s t i m a t e d the pore size to be a b o u t 1.1 n m .
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The Crystal Structure of Zeolite Omega F i g u r e 1 is a n i d e a l i z e d r e p r e s e n t a t i o n of t h e s t r u c t u r e p r o p o s e d for zeolite Ω b y B a r r e r a n d V i l l i g e r o n the basis of x - r a y p o w d e r d a t a (5). The intersections of the s t r a i g h t lines represent p o i n t a l u m i n u m or s i l i c o n a t o m s , each of w h i c h is b o n d e d t o four t e t r a h e d r a l l y disposed o x y g e n atoms.
Figure 1.
A schematic representation of the structure of zeolite omega.
T h e f r a m e w o r k is b u i l t of g m e l i n i t e cages w h i c h share t h e i r u p p e r a n d l o w e r s i x - r i n g faces a l o n g t h e c d i r e c t i o n . I n t h e (a, b) p l a n e , these cages are b r i d g e d b y oxygen a t o m s s u c h t h a t t h e e i g h t - r i n g w i n d o w s of adjacent cages face each other, as s h o w n i n F i g u r e 1. T h e r e s u l t i n g f r a m e w o r k has three i m p o r t a n t features. (1) A s y s t e m of a p p r o x i m a t e l y c y l i n d r i c a l channels r u n s p a r a l l e l t o c. These are b o u n d e d b y 12-membered r i n g s of S i - A 1 0 t e t r a h e d r a a n d h a v e a free d i a m e t e r of a b o u t 0.8 n m . T h e i r i n t e r i o r surfaces are c o m p o s e d of f o u r - a n d s i x - m e m b e r e d r i n g s of S1-AIO4 t e t r a h e d r a . T h i s means t h a t d i f f u s i o n i n the (a, b) p l a n e b e t w e e n a d j a c e n t channels is severely r e s t r i c t e d . (2) A s y s t e m of g m e l i n i t e cages, e a c h of free d i a m e t e r a r o u n d 0.6 n m , is l o c a t e d between adjacent c y l i n d r i c a l channels. (3) A m i n o r c h a n n e l s y s t e m b o u n d e d b y g r e a t l y d i s t o r t e d e i g h t m e m b e r e d rings r u n s p a r a l l e l t o c ( a n d hence t o the m a i n c h a n n e l system) l o c a t e d between adjacent g m e l i n i t e cages. T h e g m e l i n i t e cages o p e n o n t o t h i s c h a n n e l s y s t e m t h r o u g h e i g h t - m e m b e r e d r i n g w i n d o w s . Access t o t h e c o l u m n s of g m e l i n i t e cages is v e r y r e s t r i c t e d . I o n exchange o r s o r p t i o n o n sites w i t h i n these c o l u m n s m u s t necessarily i n v o l v e e x t r e m e l y t o r t u r o u s diffusion p a t h s . A l l these features are s h o w n i n F i g u r e 1. A n i m p o r t a n t 4
In Molecular Sieves; Meier, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.
53.
585
Zeolite Omega
COLE AND KOUWENHOVEN
p r o p e r t y of t h e present s t r u c t u r e is t h a t t h e m a i n c h a n n e l s y s t e m c a n n o t be b l o c k e d b y s t a c k i n g f a u l t s . Results and
Discussion
Synthesis
of
Zeolite
Omega.
SCOUTING
EXPERIMENTS.
In
pre
l i m i n a r y e x p e r i m e n t s a n u m b e r of a l u m i n o s i l i c a t e s t a r t i n g m a t e r i a l s of several c o m p o s i t i o n s were s t u d i e d .
T h e m o s t s a t i s f a c t o r y r e s u l t s were
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o b t a i n e d u s i n g K e t j e n or D a v i s o n fluid c r a c k i n g c a t a l y s t , or G e m b o
or
D a v i s o n s i l i c a a n d freshly p r e p a r e d s o d i u m a l u m i n a t e s o l u t i o n as sources of s i l i c a a n d a l u m i n a .
Some p r o p e r t i e s of these s t a r t i n g m a t e r i a l s are
given i n Table I. EFFECT
OF STOICHIOMETRY
OF REACTION
MIXTURE.
A
number
c o m p o s i t i o n s w h i c h gave p u r e zeolite Ω are c o l l e c t e d i n T a b l e I . m i x t u r e s h a v e been described u s i n g t h e general f o r m u l a p L i 0 ,
gNa 0,
2
r K 0 , z [ ( C H ) N ] 0 , A 1 0 , 2/Si0 , 2 U 0 . 2
3
4
2
2
3
d u c e d as t h e i r h y d r o x i d e s .
2
of
The 2
T h e a l k a l i n e ions were i n t r o
2
U n d e r the p r e v a i l i n g c o n d i t i o n s , a m o r p h o u s
p r o d u c t s are f o u n d i f t h e t o t a l base c o n c e n t r a t i o n (p + q + r + x) is m u c h l o w e r t h a n a b o u t 3.
F o r zeolite f o r m a t i o n t o o c c u r a t a l l u n d e r these
c o n d i t i o n s , h i g h e r r e a c t i o n t e m p e r a t u r e s h a v e t o be used.
A t a t o t a l base
c o n c e n t r a t i o n greater t h a n a b o u t 5, u n d e r t h e c o n d i t i o n s of t i m e a n d t e m p e r a t u r e a p p l i e d i n t h i s s t u d y , c r y s t a l l i n e phases s u c h as zeolite P , sodalite, a n d a n a l c i t e t e n d t o f o r m . A v a r i e t y c h a b a z i t e - g r o u p zeolites is o b t a i n e d a t p + and
100°C.
g + r + x = 4
A t a base c o m p o s i t i o n w h i c h y i e l d s p u r e Ω, i.e., ρ
