Effect of Gravity on Silicalite Crystallization - ACS Symposium Series

Chapter 17

Effect of Gravity on Silicalite Crystallization 1

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David T. Hayhurst , Peter J. Melting , Wha Jung Kim , and William Bibbey 2

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Department of Chemical Engineering, Cleveland State University, Cleveland, OH 44115 Advanced Materials Center for the Commercial Development of Space, Battelle Columbus Division, 505 King Avenue, Columbus, OH 43201-2693

Downloaded by UNIV OF LEEDS on May 18, 2016 | http://pubs.acs.org Publication Date: July 31, 1989 | doi: 10.1021/bk-1989-0398.ch017

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Silicalite was synthesized from a batch composition of 2.55Na 0-5.OTPABr-100SiO -2800H O 2

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at 180°C and at times ranging from one to seven days. Autoclaves containing the synthesis mixture were centrifuged during reaction, providing an elevated gravitational force field. Tests were conducted at 30 and 50G and at normal gravity. For synthesis performed under elevated gravity, average and maximum crystal sizes were substantially greater than those measured under normal gravity. Average and maximum crystal lengths were 42 and 132µm for the 1G synthesis. For the elevated gravity crystallizations, two separate nucleations and growths were observed. For 50G, the first growth was rapid, producing 135µm s i l i c a l i t e crystals in two days. The second crystallization produced larger 200µm crystals in five days. The maximum crystal sizes measured were 190 and 300µm for the two crystallizations. Elevated gravity was also found to affect product yield. At normal gravity, crystal yields were less than 5%. For elevated gravity reactions, these yields increased to over 55% based on total s i l i c a in the i n i t i a l reaction mixture. A discussion of how elevated gravity affects nucleation, growth, yield and crystal size of silicalite is presented. The s y n t h e s i s o f l a r g e z e o l i t e c r y s t a l s has r e c e i v e d much a t t e n t i o n i n b o t h t h e open and p a t e n t l i t e r a t u r e . The f i r s t r e p o r t on t h e growth o f l a r g e c r y s t a l s o f t y p e s A, Ρ and X z e o l i t e s was by C i r i c i n 1967(1). Recent r e p o r t s have f o c u s e d on t h e p e n t a s i l z e o l i t e s , i n p a r t i c u l a r ZSM-5 and s i l i c a l i t e ( 2 - 2 0 ) . The l a r g e s t Z S M - 5 / s i l i c a l i t e c r y s t a l s a r e r e p o r t e d t o range up t o 420 um i n l e n g t h (Γ7,19). In each r e p o r t , i n v e s t i g a t o r s have v a r i e d and optimized the chemistry of the r e a c t i n g mixture i n order t o

0097-6156/89/0398-0233$06.00/0 ο 1989 American Chemical Society Occelli and Robson; Zeolite Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 1989.


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maximize t h e s i z e . The r e a c t i o n t e m p e r a t u r e and degree o f a g i t a t i o n have a l s o been c o n s i d e r e d as r e a c t i o n v a r i a b l e s . The e f f e c t o f g r a v i t y on c r y s t a l growth has r e c e n t l y come under s t u d y , p a r t i c u l a r l y i n t h e m i c r o - o r z e r o - g r a v i t y regime. Z e t t e r l u n d and F r e d r i k s s o n (22) s t u d i e d t h e growth o f i n d i u m a n t i m o n i d e from i t s m e l t under m i c r o g r a v i t y c o n d i t i o n s and f o r m u l a t e d m a t h e m a t i c a l e x p r e s s i o n s form t h e e f f e c t s o f g r a v i t y on f r e e l y growing c r y s t a l s . Rodot e t a l . (23) have s t u d i e d t h e e f f e c t s o f g r a v i t a t i o n a l f o r c e s up t o 5G on c r y s t a l s grown from t h e i r own m e l t (Bridgeman Method) and c o n c l u d e d t h a t t h e q u a l i t y o f t h e c r y s t a l s was s i m i l a r t o t h o s e grown i n space. S t u d i e s on t h e e f f e c t o f g r a v i t y on c r y s t a l s grown by t e c h n i q u e s o t h e r t h a n from a c r y s t a l m e l t a r e more l i m i t e d . Sand e t a l . (24) have p o s t u l a t e d t h a t t h e s y n t h e s i s o f z e o l i t e s i n m i c r o g r a v i t y would r e s u l t i n t h e f o r m a t i o n o f l a r g e r c r y s t a l s as p a r t i c l e s e t t l i n g and t h e r m a l c o n v e c t i v e c u r r e n t s would be a v o i d e d . Z e o l i t e n u c l e i s h o u l d remain suspended i n s o l u t i o n , growth would c o n t i n u e i n t h e n u t r i e n t - r i c h m o t h e r - l i q u o r and l a r g e r c r y s t a l s would form. Although m i c r o g r a v i t y e x p e r i m e n t s have not been p e r f o r m e d , c r y s t a l l i z a t i o n s conducted i n v i s c o u s s o l u t i o n s support t h i s hypothesis (24). As w i t h m i c r o - g r a v i t y , r e p o r t s on t h e e f f e c t s o f e l e v a t e d g r a v i t y on z e o l i t e c r y s t a l growth a r e a b s e n t from t h e l i t e r a t u r e . The a p p l i c a t i o n o f h i g h g r a v i t y d u r i n g c r y s t a l growth s t r a t i f i e s t h e s o l i d and l i q u i d r e a c t a n t s . N u c l e a t i o n and c r y s t a l growth o c c u r s a t the s o l i d - l i q u i d i n t e r f a c e i n c o n t r a s t t o a homogeneous g e l as i s p o s t u l a t e d f o r m i c r o g r a v i t y . In t h i s r e s e a r c h the e f f e c t o f a p p l y i n g an e x t e r n a l g r a v i t a t i o n a l f o r c e d u r i n g s y n t h e s i s was s t u d i e d . The h i g h g r a v i t y was a c h i e v e d by c e n t r i f u g i n g the r e a c t i o n v e s s e l s d u r i n g s y n t h e s i s . R e a c t i o n s were c a r r i e d out u s i n g a c e n t r i f u g e p l a c e d i n an oven. The e f f e c t s o f e l e v a t e d g r a v i t y on growth r a t e , c r y s t a l s i z e , y i e l d and morphology are presented. EXPERIMENTAL Synthesis The r e a c t a n t s u s e d i n t h i s s t u d y were a c o l l o i d i a l s i l i c a , Ludox AS-40 (Dupont), r e a g e n t - g r a d e tetrapropylammonium bromide ( A l d r i c h C h e m i c a l Co.) and a 50wt% sodium h y d r o x i d e s o l u t i o n ( M a l l i n c k r o d t Inc.). The r e a c t i o n m i x t u r e had t h e o x i d e f o r m u l a , 2.55Na 0-5.0TPABr-100Si0 -2800H 0. The mole r a t i o s o f t h e 2

2

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r e a c t a n t s a r e l i s t e d i n T a b l e 1. The s y n t h e s i s b a t c h was prepared u s i n g the f o l l o w i n g procedure. The c o l l o d i a l s i l i c a was weighed i n t o a t a r e d p l a s t i c beaker. To t h i s , weighed amounts o f water, sodium h y d r o x i d e s o l u t i o n and tetrapropylammonium bromide were added. The m i x t u r e was a g i t a t e d u n t i l a u n i f o r m g e l was obtained. R e a c t a n t s were t h e n r a p i d l y t r a n s f e r r e d i n t o 15-ml t e f l o n - l i n e d Morey-type a u t o c l a v e s . The v e s s e l s were s e a l e d , weighed and p l a c e d i n t o an e i g h t p o s i t i o n c e n t r i f u g e c o n t a i n e d w i t h i n a p r e h e a t e d f o r c e d c o n v e c t i o n oven s e t a t 180°C. Weighed v e s s e l s were c a r e f u l l y p l a c e d t o a c h i e v e t h e b e s t p o s s i b l e b a l a n c e o f the centrifuge. Procedure The c e n t r i f u g e was f a b r i c a t e d u s i n g an e i g h t p o s i t i o n sample b r a c k e t mounted w i t h i n a f o r c e d - c o n v e c t i o n oven. I n d i v i d u a l sample

Occelli and Robson; Zeolite Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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Effect ofGravity on Silicalite Crystallization

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T a b l e 1. M o l a r R a t i o s o f R e a c t i n g S p e c i e s Ratio

Species S i

V

A 1

H 0/Si0 2

OH"/SiO z + Na /SiOz + TPA / S i 0


*Alumina was aluminum may

00

2°3*

28.0

2

0.051 0.051 0.050

2

not added t o t h e r e a c t i o n m i x t u r e a l t h o u g h some be p r e s e n t as a t r a c e i m p u r i t y i n t h e r e a g e n t s .

mounts were machined t o s u p p o r t t h e 15-ml a u t o c l a v e s . The c e n t r i f u g e was r o t a t e d a t a p r e - d e t e r m i n e d speed u s i n g a h i g h t o r q u e L i g h t n i n g M i x e r motor mounted o u t s i d e t h e oven u s i n g g r a p h i t e b e a r i n g s . R o t a t i o n a l speed was m a i n t a i n e d w i t h a R e l i a n c e E l e c t r i c v a r i a b l e speed c o n t r o l l e r . Gravitational f o r c e s i n e x c e s s o f 100G were o b t a i n a b l e u s i n g t h i s c e n t r i f u g e , a l t h o u g h the p r e f e r r e d o p e r a t i n g range was 10 t o 50G. The c e n t r i f u g e was r o t a t e d a t p r e d e t e r m i n e d speeds y i e l d i n g different gravitational forces. A s e t speed was m a i n t a i n e d t h r o u g h o u t t h e experiment. At d a i l y i n t e r v a l s , the c e n t r i f u g e was s t o p p e d , and a v e s s e l was removed from t h e oven. The v e s s e l was r e p l a c e d w i t h a v e s s e l o f s i m i l a r weight ( l o a d e d w i t h w a t e r ) , t h e n t h e c r y s t a l l i z a t i o n was c o n t i n u e d . The p r o d u c t v e s s e l was quenched t o room temperature u s i n g c o l d t a p water. The a u t o c l a v e s were i m m e d i a t e l y opened, and t h e pH o f t h e mother l i q u o r was measured. The s o l i d p r o d u c t s were p l a c e d i n a Buchner f u n n e l , washed a minimum o f t h r e e t i m e s w i t h 100ml o f d i s t i l l e d water, and d r i e d o v e r n i g h t a t 110°C. P r o d u c t y i e l d was measured f o r a l l r u n s . The y i e l d was d e t e r m i n e d by emptying t h e e n t i r e c o n t e n t s o f t h e a u t o c l a v e i n t o a pyrex beaker. Sodium h y d r o x i d e s o l u t i o n (100ml, 0.1N) was added t o t h e b e a k e r , and t h e s l u r r y was h e a t e d t o b o i l i n g (100°C) f o r one hour t o c o m p l e t e l y d i s s o l v e t h e amorphous s o l i d s . The r e m a i n i n g p u r i f i e d s i l i c a l i t e c r y s t a l s were d r i e d a t 110°C. The weight o f s i l i c a i n i t i a l l y c h a r g e d t o t h e a u t o c l a v e was c a l c u l a t e d from t h e i n i t i a l c o l l o i d a l s i l i c a . The r a t i o o f t h e f i n a l weight o f p u r i f i e d s i l i c a l i t e c r y s t a l s t o t h e i n i t i a l weight o f s i l i c a c h a r g e d t o t h e a u t o c l a v e g i v e s t h e p r o d u c t y i e l d . Analysis Phase i d e n t i f i c a t i o n o f t h e p r o d u c t c r y s t a l s was p e r f o r m e d by powder x - r a y d i f f r a c t i o n , u s i n g radiation. The i n s t r u m e n t was a P h i l i p s Model PW1730/10 x - r a y g e n e r a t o r e q u i p p e d w i t h a PW 1050/70 v e r t i c a l goniometer. D i f f r a c t o g r a m s were measured f o r t h e as-synthesized product; that i s , c r y s t a l s plus g e l . C r y s t a l s i z e and morphology were d e t e r m i n e d by s c a n n i n g e l e c t r o n and o p t i c a l microscopy. The e l e c t r o n m i c r o s c o p e was an AMRay Model 1200B m i c r o s c o p e e q u i p p e d w i t h a KEVEX Model 7000 Energy x - r a y d i s p e r s i o n analyzer f o r q u a l i t a t i v e chemical a n a l y s i s . C

u

K

a

l



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The p a r t i c l e s i z e d i s t r i b u t i o n o f t h e s i l i c a l i t e c r y s t a l s was d e t e r m i n e d u s i n g o p t i c a l m i c r o s c o p y . To a n a l y z e s i z e , t h e a s - s y n t h e s i z e d samples ( c r y s t a l s p l u s g e l ) were d i s p e r s e d o n t o g l a s s s l i d e s from an a l c o h o l s u s p e n s i o n and t h e n examined under an o p t i c a l microscope u s i n g p o l a r i z e d l i g h t . Polarized light assists i n t h e s e p a r a t i o n o f u n r e a c t e d g e l and c r y s t a l s i n t h e image. The image was t h e n a c q u i r e d by a t e l e v i s i o n camera c o n n e c t e d t o a T r a c o N o r t h e r n 8502 Image a n a l y s i s system. The image was a n a l y z e d u s i n g the T r a c o r N o r t h e r n s o f t w a r e by f i r s t c o n v e r t i n g t h e image t o a b i n a r y ( b l a c k - w h i t e ) image, and t h e n t h e s o f t w a r e a n a l y z e s each p a r t i c l e i n the f i e l d . B e f o r e i n i t i a t i n g a t e s t , t h e system was c a l i b r a t e d by v i e w i n g s t a n d a r d p a r t i c l e s . Because o f a m b i g u i t i e s i n t r o d u c e d by t h e use o f p o l a r i z e d l i g h t , i t was n e c e s s a r y t o m a n u a l l y o u t l i n e many o f t h e p a r t i c l e s u s i n g a mouse. T h i s a l l o w s f o r t h e removal o f s p u r i o u s p a r t i c l e s , insuring a true accounting of a l l c r y s t a l s . The image a n a l y s i s s o f t w a r e uses t h e b i n a r y image t o i n d i v i d u a l l y a n a l y z e each p a r t i c l e (assuming a r e c t a n g u l a r g e o m e t r y ) , t h e r e b y o b t a i n i n g a l e n g t h and w i d t h f o r each c r y s t a l . To a s s u r e t h a t m e a n i n g f u l d a t a was b e i n g c o l l e c t e d , a minimum o f t h r e e image f i e l d s were c o l l e c t e d f o r each sample. RESULTS AND

DISCUSSION

The m i x t u r e u s e d i n t h e s t u d y was found t o p r o d u c e o n l y silicalite. C r y s t a l l i z a t i o n s were c a r r i e d o u t under normal g r a v i t y and a t 30 and 50G. Sample v e s s e l s were removed from t h e o v e n / c e n t r i f u g e assembly a t one day i n t e r v a l s and t e s t e d f o r a v e r a g e c r y s t a l l e n g t h , f o r t h e average l e n g t h o f t h e l a r g e s t 10% o f t h e c r y s t a l s , f o r c r y s t a l y i e l d and f o r t h e pH o f t h e mother liquor. R e s u l t s a r e shown i n F i g u r e s 1, 2, 4 and 5, r e s p e c t i v e l y . The v a l u e f o r average p a r t i c l e s i z e was d e t e r m i n e d u s i n g the p a r t i c l e s i z e d i s t r i b u t i o n system d e s c r i b e d e a r l i e r . Three r e p l i c a t e s i z e d i s t r i b u t i o n s were d e t e r m i n e d f o r each e x p e r i m e n t a l run. Each image was found t o have from twenty t o f o r t y c r y s t a l s . An average c r y s t a l l e n g t h was d e t e r m i n e d by u s i n g a l l t h e i n d i v i d u a l c r y s t a l l e n g t h s from t h e t h r e e r e p l i c a t e s ; t h a t i s , a v e r a g i n g a t o t a l o f 60 t o 120 c r y s t a l l e n g t h s . T h i s average c r y s t a l l e n g t h i s p l o t t e d i n F i g u r e 1 as a f u n c t i o n o f r e a c t i o n time. Under normal g r a v i t y , t r a c e amounts o f c r y s t a l l i z e d p r o d u c t , h a v i n g an average c r y s t a l l e n g t h o f 93um, appeared a f t e r one day. T h i s i n i t i a l growth o c c u r r e d h e t e r o g e n e o u s l y on t h e t e f l o n - l i n e d vessel walls. Few c r y s t a l s were o b s e r v e d t o form i n t h e b u l k g e l . A t l o n g e r t i m e s , s i l i c a l i t e was found t o c r y s t a l l i z e homogeneously i n t h e g e l . These c r y s t a l s a v e r a g e d 45 t o 60um i n l e n g t h . It is i n t e r e s t i n g t o note t h a t a t seven days l a r g e r s i l i c a l i t e c r y s t a l s were found t o form, some e x c e e d i n g lOOum i n l e n g t h . These r e s u l t s suggest a secondary c r y s t a l l i z a t i o n forming these l a r g e r c r y s t a l s . The average c r y s t a l l e n g t h s d e t e r m i n e d f o r t h e e l e v a t e d g r a v i t y e x p e r i m e n t s were s u b s t a n t i a l l y g r e a t e r t h a n t h o s e found f o r t h e 1G e x p e r i m e n t . A t 30 and 50G, t h e average c r y s t a l l e n g t h s were f o u n d t o be 160 and 156um r e s p e c t i v e l y as d e t e r m i n e d by a v e r a g i n g t h e mean c r y s t a l l e n g t h s f o r r e a c t i o n t i m e s o f two t o seven days. F o r t h e h i g h g r a v i t y r u n s , l a r g e c r y s t a l s formed i n one day. These c r y s t a l s were o f comparable l e n g t h t o t h e 1G r u n .



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W i t h i n c r e a s i n g r e a c t i o n t i m e s , average c r y s t a l l e n g t h i n c r e a s e d t o maximums o f 192 and 198um f o r t h e 30 and 50G e x p e r i m e n t s . In both e l e v a t e d g r a v i t y e x p e r i m e n t s t h e r e was an i n i t i a l f o r m a t i o n o f r e l a t i v e l y l a r g e c r y s t a l s f o l l o w e d two t o t h r e e days l a t e r by a second growth o f l a r g e r c r y s t a l s . These r e s u l t s s u g g e s t a d i s s o l u t i o n of the smaller c r y s t a l l i t e s providing nutrients f o r t h e c o n t i n u e d growth o f t h e l a r g e s t c r y s t a l s . Mean c r y s t a l s i z e s f o r t h e l a r g e s t 10% o f t h e c r y s t a l s were c a l c u l a t e d using c r y s t a l s i z e d i s t r i b u t i o n data. Results f o r normal g r a v i t y and f o r t h e h i g h g r a v i t y r u n s a r e p l o t t e d i n F i g u r e 2. The l a r g e s t c r y s t a l s were p r o d u c e d a t h i g h g r a v i t y d u r i n g t h e second phase o f c r y s t a l growth. The a v e r a g e s i z e o f the l a r g e s t 10% was 295 and 297um f o r t h e 30 and 50G r u n s . In the normal g r a v i t y r u n , some l a r g e 130um c r y s t a l s were o b s e r v e d f o r m i n g i n one day; a l t h o u g h as n o t e d p r e v i o u s l y , t h e s e c r y s t a l s n u c l e a t e d h e t e r o g e n e o u s l y on t h e w a l l s o f t h e t e f l o n a u t o c l a v e l i n e r . The l a r g e s t c r y s t a l s which formed homogeneously a t 1G r a n g e d from 50 t o 95um. A second growth, p r o d u c i n g l a r g e r lOOum c r y s t a l s , was found t o b e g i n a t seven days. A l t h o u g h F i g u r e s 1 and 2 q u a n t i f y t h e d i f f e r e n c e s i n average and maximum c r y s t a l l e n g t h s between normal and e l e v a t e d g r a v i t y s y n t h e s e s , t h e e f f e c t o f g r a v i t y on c r y s t a l s i z e i s b e s t demonstrated by a d i r e c t comparison. Optical m i c r o g r a p h s o f s i l i c a l i t e p r o d u c e d a f t e r f i v e days o f r e a c t i o n a t 1 and 30G a r e shown i n F i g u r e 3. Both m i c r o g r a p h s were t a k e n a t i d e n t i c a l m a g n i f i c a t i o n s o f 50X and c l e a r l y demonstrate t h e enhancement i n c r y s t a l s i z e c a u s e d by e l e v a t e d g r a v i t y . In a d d i t i o n t o i n c r e a s i n g c r y s t a l s i z e , t h e a p p l i c a t i o n o f e l e v a t e d g r a v i t y a l s o a f f e c t s c r y s t a l y i e l d . Y i e l d s were measured f o r t h e 1, 30, and 50G r u n s . R e s u l t s a r e p l o t t e d i n F i g u r e 4. F o r s y n t h e s i s c o n d u c t e d a t normal g r a v i t y , c r y s t a l y i e l d s n e v e r exceeded 4.3% o f t h e i n i t i a l c h a r g e o f s i l i c a . F o r t h e 30 and 50G p r e p a r a t i o n s , c r y s t a l y i e l d s o f 46.7 and 55.5 were measured. A t e l e v a t e d g r a v i t y , i t was o b s e r v e d t h a t c r y s t a l s formed i n a l a y e r d i r e c t l y above a dense l a y e r o f t h e amorphous s i l i c a g e l . With i n c r e a s i n g r e a c t i o n times, the c r y s t a l l a y e r t h i c k e n s while the s i l i c a g e l l a y e r d i m i n i s h e d . A t seven days, no s e p a r a t e amorphous s i l i c a l a y e r was p r e s e n t . The commercial s i g n i f i c a n c e o f enhanced p r o d u c t y i e l d u s i n g e l e v a t e d g r a v i t y i s o b v i o u s . The pH o f mother l i q u o r was measured f o r a l l t h r e e experimental runs. These d a t a a r e p l o t t e d on F i g u r e 5. The a l k a l i n i t y o f t h e r e a c t i n g s o l u t i o n was found t o d e c l i n e w i t h time. The pH o f t h e i n i t i a l m i x t u r e was 11.85 d e c l i n i n g t o l e s s t h a n 10.00 upon c o m p l e t i o n o f t h e c r y s t a l l i z a t i o n . The changes i n pH w i t h time were found t o e x h i b i t minima. F o r a l l t h r e e c a s e s , the l o w e s t pH v a l u e s were measured f o r t h e r e a c t i o n t i m e s which p r o d u c e d t h e l a r g e s t c r y s t a l s . Under normal g r a v i t y , minimum pH v a l u e s were n o t e d a t one and seven days. These t i m e s c o r r e s p o n d t o the f o r m a t i o n o f t h e two maximum c r y s t a l s i z e s . S i m i l a r trends are observed f o r the elevated g r a v i t y synthesis. Based on t h e c r y s t a l s i z e , y i e l d and pH d a t a , i t i s p o s s i b l e t o p o s t u l a t e a model f o r s i l i c a l i t e c r y s t a l l i z a t i o n i n a h i g h g r a v i t y environment. The p r i m a r y e f f e c t o f a p p l y i n g h i g h g r a v i t y during the c r y s t a l l i z a t i o n i s t o segregate the c o l l o d i a l s i l i c a


238


ZEOLITE SYNTHESIS

Figure 1.

Average c r y s t a l size of the 1, 30 and 50G

synthesis.




HAYHURST ET AL.

Figure 2.

Average size of largest 10% of c r y s t a l s synthesized at 1, 30 and 50G.


239

240

ZEOLITE SYNTHESIS

30G


1G

F i g u r e 3.

O p t i c a l m i c r o g r a p h o f c r y s t a l s p r o d u c e d a f t e r 5 days o f r e a c t i o n a t 1 and 30G ( m a g n i f i c a t i o n 50x).

ig

20 \-

F i g u r e 4.

Y i e l d o f c r y s t a l s s y n t h e s i z e d a t 1, 30 and


50G.



HAYHURST ET AL.


242

Z E O L I T E

SYNTHESIS


from t h e l i q u i d phase which c o n t a i n s b o t h t h e t e m p l a t e and t h e alkali. T h i s s e p a r a t i o n o f t h e s o l i d and l i q u i d p h a s e s r e q u i r e s t h a t d i s s o l u t i o n , n u c l e a t i o n and c r y s t a l l i z a t i o n o c c u r o n l y a t t h e i n t e r f a c e between t h e d e n s i f i e d s o l i d s i l i c a and t h e a l k a l i solution. T h i s l i m i t e d i n t e r f a c e r e d u c e s t h e number o f c r i t i c a l s i z e d n u c l e i which form. As c r y s t a l growth o c c u r s a t a l i m i t e d number o f s i t e s , l a r g e r c r y s t a l s a r e p r o d u c e d . In t h i s type o f c r y s t a l l i z a t i o n , t r u e s o l u t i o n phase c r y s t a l l i z a t i o n i s e x p e c t e d t o p r e d o m i n a t e . As n o t e d p r e v i o u s l y , c r y s t a l s were f o u n d t o form a l a y e r d i r e c t l y above t h e d e n s i f i e d s i l i c a l a y e r i n a l l e l e v a t e d gravity reactions. W i t h i n c r e a s e d r e a c t i o n time t h e c r y s t a l l a y e r i n c r e a s e s i n t h i c k n e s s w h i l e t h e s i l i c a g e l l a y e r d i m i n i s h e s . The r e a s o n s f o r a second c r y s t a l l i z a t i o n , and i n c r e a s e d p r o d u c t y i e l d , a r e l e s s o b v i o u s and r e q u i r e f u r t h e r i n v e s t i g a t i o n . CONCLUSIONS R e s u l t s r e p o r t e d i n t h i s s t u d y demonstrate t h a t t h e a p p l i c a t i o n o f an e x t e r n a l g r a v i t a t i o n a l f o r c e d u r i n g s y n t h e s i s p r o f o u n d l y a f f e c t s t h e c r y s t a l s i z e and p r o d u c t y i e l d o f s i l i c a l i t e . C r y s t a l s grown under h i g h g r a v i t y a r e s u b s t a n t i a l l y l a r g e r t h a n t h o s e formed under normal g r a v i t y . Correspondingly, product y i e l d i s enhanced w i t h e l e v a t e d g r a v i t y . A l t h o u g h t h e scope o f t h i s r e p o r t i s l i m i t e d , t h e changes i n s i l i c a l i t e c r y s t a l l i z a t o n due t o e l e v a t e d g r a v i t y appear t o r e s u l t from t h e s e p a r a t i o n o f t h e l i q u i d and s o l i d s i l i c a p h a s e s ; s u g g e s t i n g t h a t s o l u t i o n phase c r y s t a l l i z a t i o n may p r e d o m i n a t e . The commercial i m p l i c a t i o n s o f these r e s u l t s t o z e o l i t e producers are equally s i g n i f i c a n t . ACKNOWLEDGMENTS The a u t h o r s would l i k e t o acknowledge t h e work o f Mr. D a v i d W. E p p e r l y i n t h e d e s i g n and f a b r i c a t i o n o f t h e h i g h t e m p e r a t u r e c e n t r i f u g e / o v e n assembly. The f i n a n c i a l s u p p o r t o f NASA G r a n t NAGW-811 t h r o u g h B a t t e l l e ' s Advanced M a t e r i a l s C e n t e r f o r t h e Commercial Development o f Space i s a l s o g r a t e f u l l y acknowledged. LITERATURE CITED

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

Ciric, J., Science 1987, 155, 373. Nastro, A. and Sand, L.B., Zeolites 1983,3, 57. Mostowicz, R., and Sand, L.B., Zeolites 1983, 3, 219. Mostowicz, R., and Berak, J., Polish Pat. 135 293 (1982). Mostowicz, R. and Sand L.B., Zeolites 1982, 2, 143. Derouane, E.G. et a l . , Appl. Catal. 1981, 1, 201. Gabelica, Z., Derouane, E.G., and Blom, N., Appl. Catal. 1983, 5, 109. Gabelica, Z., Blom, N., and Derouane, E.G., Appl. Catal. 1983, 5, 227. Gabelica, Z., Derouane, E.G., and Blom, N., ACS Symp. Ser. 248, Am. Chem Soc., USA, 1984, p.219. Romannikov, V.N. et a l . , Zeolites 1983, 3, 311. Ghamami, M. and Sand, L.B., Zeolites 1983, 3, 155. Pelrine, B.P., US Pat. 4 100 262 (1978).



17. H A Y H U R S T E T ^

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13. Olson, D.H. and Valyocsik, E.W., Eur. Pat. Appl. 0 026 963 (1981). 14. Kuei-Jung, Chao et a l . , Zeolites 1986, 6, 35. 15. von Ballmoss, R. and Meier, W.M., Nature 1981, 289, 782. 16. Lermer, H. et a l . , Zeolites 1985, 5, 131. 17. Hayhurst, D.T. and Lee, J.C.,in New Developments in Zeolite Science and Technology (Eds. Y. Murakami, A. Iijima and J.W. Ward), Kodansha, Tokyo, and Elsevier, Amsterdam, 1986, p. 113. 18. Hou, L.Y., Sand, L.B. and Thompson, R.W. in New Developments in Zeolite Science and Technology (Eds. Y. Murakami, A. Iijima and J.W. Ward), Kodansha, Tokyo, and Elsevier, Amsterdam, 1986, p. 239. 19. Kornatowski, J. Zeolites 1988, 8, 77. 20. Chen, S.Z., Huddersman, K., Keir, D. and Rees, L.V.C., Zeolites 1988, 8, 106. 21. Guth, J.L., Kessler, H. and Wey, R. in New Developments in Zeolite Science and Technology (Eds. Y. Murakami, A. Iijima and J.W. Ward), Kodansha, Tokyo and Elsevier, Amsterdam, 1986, p 121. 22. Zetterlund, E.H. and Fredricsson, E., Material Letters, 1982, 1, p. 127. 23. Rodet, H., Regel, L.L, Ghamami,M., Videskii, I.V., J. Cryst. Growth, 1986, 79, p. 77. 24. Sand, L.B., Sacco, Α., Thompson, R.W. and Dixon, A.G., Zeolites, 1987, 7, p. 387. RECEIVED December 22, 1988


Effect of Gravity on Silicalite Crystallization - ACS Symposium Series

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