77
Present
and
Future
Directions
in
Zeolite
Research
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W. F. AREY, JR. Esso Research Laboratories, Baton Rouge, La. 70821 Molecular sieve zeolites have become established as an area of scientific research and as commercial materials for use as sorbents and catalysts. Continuing studies on their synthesis, structure, and sorption properties will, undoubtedly, lead to broader application. In addition, crystalline zeolites offer one of the best vehicles for studying the fundamentals of heterogeneous catalysis. Several discoveries reported at this conference point toward new fields of investigation and potential commercial utility. These include phosphorus substitution into the silicon—aluminum framework, the structural modifications leading to "ultrastable" faujasite, and the catalytic properties of sodium mordenite.
/
T h e task of s u m m a r i z i n g the major results of the conference a n d s u g f
A
gesting the f u t u r e d i r e c t i o n of m o l e c u l a r sieve zeolites is n o t a n easy
one.
I n p a r t i c u l a r , i t is d i f f i c u l t to s u m m a r i z e the major results because
there w e r e so m a n y n o t e w o r t h y c o n t r i b u t i o n s .
O b v i o u s a n d substantial
progress is b e i n g m a d e i n a l l the major areas—i.e., synthesis,
structure
a n d m i n e r a l o g y , s o r p t i o n a n d d i f f u s i o n , a n d catalysis. B r e c k m a d e the f o l l o w i n g c o m m e n t
i n the i n t r o d u c t o r y l e c t u r e :
" T h e r e seems to b e a d i r e c t r e l a t i o n b e t w e e n the l e v e l of scientific interest a n d t h e area of m a j o r a p p l i c a t i o n . " I b e l i e v e that this m e e t i n g lends s u p p o r t to that statement.
T h e h i g h degree of representation of p e t r o
l e u m - o r i e n t e d research o r g a n i z a t i o n s is e v i d e n t . O n e of the major a p p l i c a t i o n s of c r y s t a l l i n e zeolites is as c a t a l y t i c materials f o r p e t r o l e u m r e f i n i n g processes.
A p r i m e use is i n c a t a l y t i c
c r a c k i n g , w h i c h is the major c o n v e r s i o n process
i n today's
refineries.
A n o t h e r g r o w i n g use is i n h y d r o c r a c k i n g , s u c h as the U n i c r a c k i n g — J H C process, w h i c h gives the refiner a d d e d f l e x i b i l i t y i n oils w h i c h c a n b e processed a n d p r o d u c t s w h i c h c a n b e m a d e . 451
In Molecular Sieve Zeolites-II; Flanigen, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
452
M O L E C U L A R SIEVE ZEOLITES
T o date, crystalline zeolite catalysts
have been
II
most effective i n
c a t a l y z i n g c a r b o n i u m i o n reactions s u c h as c a t a l y t i c c r a c k i n g a n d h y d r o c r a c k i n g . O t h e r c a r b o n i u m i o n reactions s u c h as a l k y l a t i o n a n d i s o m e r i z a t i o n also are c a t a l y z e d b y certain f o r m s o f zeolites.
I expect t o see
these a p p l i c a t i o n s e x p a n d — p r o v i d e d suitable catalyst c o m p o s i t i o n s a r e d e v e l o p e d to a l l o w e c o n o m i c a l l y v i a b l e processes.
Although X - and
Y - t y p e faujasite c a n be u s e d i n c a t a l y t i c c r a c k i n g a n d h y d r o c r a c k i n g , the Y - t y p e is p r e f e r r e d f o r p a r a f f f n - o l e f i n a l k y l a t i o n . Y - t y p e faujasite is suit Downloaded by UCSF LIB CKM RSCS MGMT on November 30, 2014 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0102.ch077
able for use i n h y d r o i s o m e r i z a t i o n catalysts, b u t synthetic m o r d e n i t e is also a p r o m i s i n g m a t e r i a l . O n the other h a n d , zeolite-based catalysts h a v e not d e v e l o p e d a n y significant interest i n other i m p o r t a n t processes s u c h as c a t a l y t i c r e f o r m i n g , hydrogénation
o f olefins a n d aromatics, h y d r o d e s u l f u r i z a t i o n a n d
h y d r o d e n i t r o g e n a t i o n . T h i s l a c k o f u t i l i t y is a t t r i b u t a b l e , i n part, to the h i g h c r a c k i n g a c t i v i t y e x h i b i t e d b y active z e o l i t e - b a s e d
catalysts.
A t present, the m a j o r interest lies i n catalysts b a s e d o n synthetic X a n d Y - t y p e faujasite a n d s y n t h e t i c m o r d e n i t e . With
this b a c k g r o u n d , I b e l i e v e
that
t h e f o l l o w i n g results
were
h i g h l i g h t s of the m e e t i n g : • T h e s u b s t i t u t i o n o f materials other t h a n s i l i c o n o r a l u m i n u m into the zeolite f r a m e w o r k — p a r t i c u l a r l y the p h o s p h o r u s s u b s t i t u t i o n r e p o r t e d b y F l a n i g e n a n d G r o s e . I t w a s , h o w e v e r , d i s a p p o i n t i n g that this was suc cessful w i t h o n l y one large-pore m a t e r i a l , n a m e l y , t y p e L zeolite. • T h e e l u c i d a t i o n o f t h e structures i n v o l v e d i n p r o d u c i n g u l t r a s t a b l e faujasite r e p o r t e d b y M a h e r , H u n t e r , a n d Scherzer. A l t h o u g h t h e d i s c o v e r y o f t h e ultrastable faujasite was r e p o r t e d at t h e 1967 L o n d o n m e e t i n g , these subsequent studies ( a n d w o r k b y G . T . K e r r ) are p o i n t i n g to a n u n e x p e c t e d m e t h o d o f a l t e r i n g the c r y s t a l l i n e f r a m e w o r k . • T h e h i g h a c t i v i t y o f s o d i u m m o r d e n i t e f o r c a t a l y z i n g benzene h y d r o génation as r e p o r t e d b y M i n a c h e v et al. T h i s result is so u n i q u e that a d d i t i o n a l i n v e s t i g a t i o n is w a r r a n t e d . It seems to m e that the above constitute discoveries w h i c h c o u l d l e a d to n e w a n d different c a t a l y t i c materials. T h e c o n t i n u i n g studies o n the m e c h a n i s m s i n v o l v e d i n z e o l i t e s y n thesis are l e a d i n g t o a better u n d e r s t a n d i n g o f the c o n t r o l l i n g factors i n this c o m p l i c a t e d r e a c t i o n system. B u t m o r e remains to be done. T h i s also h o l d s the p r o m i s e o f l e a d i n g to the synthesis o f n e w z e o l i t i c structures. T h e systematic investigations of the structure a n d s o r p t i o n p r o p e r t i e s of n a t u r a l a n d synthetic zeolites are c e r t a i n l y c e n t r a l to t h e b o d y o f k n o w l e d g e that w i l l a i d i n the f u t u r e research a n d u t i l i z a t i o n o f zeolites i n v a r i o u s possible w a y s . N o m e n c l a t u r e is i n f l u e n c e d b y structure. h a v e a better system of n o m e n c l a t u r e .
It w o u l d b e desirable to
H o w e v e r , the q u e s t i o n of just h o w
m u c h "fine s t r u c t u r e " needs to b e d e s c r i b e d m a y b e d i f f i c u l t t o answer.
In Molecular Sieve Zeolites-II; Flanigen, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
77.
ARE Y
Directions
in Zeolite
453
Research
M a n y n a t u r a l zeolites h a v e a v a r i a b l e c o m p o s i t i o n . I t m a y b e t h a t t h e degree o f S i - A l o r d e r - d i s o r d e r i n s y n t h e t i c zeolites m a y v a r y w i t h p r e p a ration procedures.
H o w e v e r , i t w o u l d b e of interest to d e t e r m i n e w h a t
effect t h e o r d e r - d i s o r d e r has o n a d s o r p t i v e a n d / o r c a t a l y t i c p r o p e r t i e s . S e v e r a l years a g o , i t w a s felt that c a t a l y t i c studies u s i n g c r y s t a l l i n e zeolites w o u l d d r a m a t i c a l l y a i d i n u n r a v e l i n g some of t h e mysteries o f catalysis.
T h i s w a s because t h e c r y s t a l l i n e zeolites h a d a m o r e u n i f o r m
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surface t h a n a m o r p h o u s materials a n d t h e structure c o u l d b e w e l l d e fined.
S o m e of t h e luster of this h o p e appears to b e f a d i n g s l i g h t l y . It
seems that t h e " a c t i v e sites" h a v e a s p e c t r u m o f intensities rather t h a n b e i n g of a u n i f o r m character.
N e v e r t h e l e s s , c r y s t a l l i n e zeolites s t i l l offer
one o f the best vehicles f o r s t u d y i n g t h e f u n d a m e n t a l s o f heterogeneous catalysis. C a t a l y s t s b a s e d o n m o l e c u l a r sieve zeolites w i l l c e r t a i n l y c o n t i n u e t o b e of m u c h interest i n p e t r o l e u m r e f i n i n g . I m p r o v e m e n t s c a n b e e x p e c t e d as t i m e goes o n . R a d i c a l l y different zeolite structures
offer t h e m o s t
c h a n c e of major i m p r o v e m e n t s . S e v e r a l authors have i n d i c a t e d t h e l i k e l i h o o d o f o b t a i n i n g r a d i c a l , c a r b a n i o n , a n d other n o n c a r b o n i u m i o n reac tions over zeolite catalysts.
P u r s u i t of this p o s s i b i l i t y m a y p r o v e f r u i t f u l .
Separations w e r e one o f t h e first uses of m o l e c u l a r sieve zeolites. A l t h o u g h some i s o - n o r m a l h y d r o c a r b o n separations plants are o p e r a t i n g , g r o w t h i n this area has n o t b e e n large.
Separations b a s e d o n i n h e r e n t
a d s o r p t i v e b e h a v i o r ( r a t h e r t h a n sieve a c t i o n ) interest.
are b e c o m i n g of m o r e
T h i s is t r u e n o t o n l y o f t h e s e p a r a t i o n o f d i m e t h y l n a p h t h a l e n e s
(as r e p o r t e d b y H e d g e at this m e e t i n g )
b u t also of x y l e n e
isomers.
Separations of this t y p e to p r o d u c e streams of interest i n c h e m i c a l s m a n u facture could grow. A s m e n t i o n e d b y B r e c k i n t h e i n t r o d u c t o r y lecture, m o l e c u l a r sieve zeolites s h o u l d find use i n v a r i o u s p o l l u t i o n c o n t r o l a p p l i c a t i o n s .
Both
t h e i r sorbent a n d c a t a l y t i c properties m a k e t h e m t e c h n i c a l l y f e a s i b l e as agents f o r v a r i o u s aspects of p o l l u t i o n c o n t r o l a n d abatement. I n s u m m a r y , c r y s t a l l i n e zeolites offer n o t o n l y u s e f u l articles o f c o m m e r c e b u t also a n i n t e r e s t i n g research area i n v o l v i n g m a n y different s c i entific d i s c i p l i n e s . U n d o u b t e d l y , t h e f u t u r e w i l l r e v e a l m o r e a b o u t these u n u s u a l materials, p e r h a p s e v e n i n w a y s n o t c l e a r l y foreseen at present. R E C E I V E D November 23,
1970.
In Molecular Sieve Zeolites-II; Flanigen, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.
