Kinetics and Mechanism of Crystal Growth of Zeolite Omega

Chapter 34

Kinetics and Mechanism of Crystal Growth of Zeolite Omega 1

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F. Fajula , S. Nicolas , F. Di Renzo , C. Gueguen , and F. Figuéras Downloaded by UNIV OF MICHIGAN ANN ARBOR on September 17, 2015 | http://pubs.acs.org Publication Date: July 31, 1989 | doi: 10.1021/bk-1989-0398.ch034

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Laboratoire de Chimie Organique Physique et Cinétique Chimique Appliquées, Unité Associée au Centre National de la Recherche Scientifique No. 418, Ecole Nationale Supérieure de Chimie de Montpellier, 8 Rue Ecole Normale, 34075 Montpellier Cedex, France Centre de Recherche ELF-France, BP 22, 69360 Saint Symphorien d'Ozon, France 2

The change in morphology of crystals of zeolite omega has been analyzed with respect to the evolution of the concentration of aluminium in the parent liquor. In agreement with the overlapping principle, crystal habit is determined by the faces with the lowest growing rate. As [Al] decreases, morphology evolves from spheroidal to euhedral hexagonal passing through intermediate cylinder-shaped crystals. The growth rate of the (001) face presents an activation energy of 23 kcal/mol and is proportional to [Al] . For the growth of the (hkO), surface the activation energy is 30 kcal/mol and the rate is proportional to [Al] . These differences are attributed to the occurrence of different growth mechanisms on the two kinds of faces. The modeling of the crystal growth and the continuous feeding of the system by an aluminate solution allowed modification of the final / size ratio. 0.8

1.6

The a p p l i c a t i o n o f t h e laws o f c r y s t a l growth t o t h e s y n t h e s i s o f z e o l i t e s i s s t i l l a l a r g e l y u n f u l f i l l e d t a s k . Most k i n e t i c s t u d i e s deal with the i n f l u e n c e o f the r e a c t i o n v a r i a b l e s (temperature, a l k a l i n i t y , c o m p o s i t i o n o f t h e o r i g i n a l medium) on t h e e v o l u t i o n o f the c r y s t a l l i z e d component (1 ). N u c l e a t i o n and c r y s t a l growth a r e g e n e r a l l y n o t s e p a r a t e d , b u t a r e d e s c r i b e d by a s i n g l e kinetic e q u a t i o n . The y i e l d v e r s u s time c u r v e s a r e i n d e e d v e r y u s e f u l f o r the m o d e l i n g o f z e o l i t e s y n t h e s i s ( 2 ) . However, they do n o t c o n t a i n q u a n t i t a t i v e i n f o r m a t i o n about t h e r a t e s o f c r y s t a l growth. E x p e r i m e n t a l methods have n o t been c o n d u c i v e t o c o n s i d e r a t i o n o f the s i z e o f t h e c r y s t a l s as t h e dependent v a r i a b l e i n k i n e t i c s . I n many media f o r example, i t i s n o t easy t o p a r t t h e growing c r y s t a l s from t h e p a r e n t amorphous phase. The f i r s t attempts t o pass from dm/dt r a t e e q u a t i o n s t o d l / d t ones (where m i s t h e mass o f z e o l i t e , 1 t h e s i z e o f t h e c r y s t a l s and t i s time) were based on t h e d e t e r m i n a t i o n o f average c r y s t a l s i z e s from t h e w i d t h o f X-ray d i f f r a c t i o n peaks ( 3 ) . Microscope measurements o f t h e i n c r e a s e o f t h e s i z e o f t h e l a r g e s t c r y s t a l s

0097-6156/89/0398-0493$06.00A) o 1989 American Chemical Society In Zeolite Synthesis; Occelli, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

ZEOLITE SYNTHESIS

Downloaded by UNIV OF MICHIGAN ANN ARBOR on September 17, 2015 | http://pubs.acs.org Publication Date: July 31, 1989 | doi: 10.1021/bk-1989-0398.ch034

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a l l o w e d Zhdanov and S a m u l e v i c h (4) t o e s t a b l i s h s i z e - v e r s u s - t i m e c u r v e s and t o a n a l y z e n u c l e a t i o n and c r y s t a l growth s e p a r a t e l y i n a s y n t h e s i s o f z e o l i t e Na-X. They i n t e r p r e t e d t h e s e d a t a by u s i n g an e q u a t i o n o f the type Z = l - e x p ( - k t ), where Ζ i s the degree of c o n v e r s i o n , k a c o n s t a n t , t time and η a v a l u e between 3 and 5. Such a r a t e f o r m u l a t i o n was a l s o p r o p o s e d by C i r i c (5) f o r z e o l i t e A c r y s t a l l i z a t i o n . I t i s known as the Kholmogorov e q u a t i o n and c o r r e s p o n d s f o r m a l l y t o t h a t f i r s t p r o p o s e d by Johnson and Mehl (6) and Avrami (7,8) f o r s o l i d i f i c a t i o n o f m e t a l s . The use o f a law t y p i c a l o f the s o l i d s t a t e t o d e s c r i b e the growth o f z e o l i t e s was p r o b a b l y s u g g e s t e d by the p r e s e n c e o f an amorphous g e l i n the s y n t h e s i s medium. The rearrangement o f t h i s phase t o form a z e o l i t e network has been a f a v o r e d t h e o r y u n t i l r e s e a r c h e r s showed t h a t z e o l i t e s can n u c l e a t e and grow d i r e c t l y from s o l u t i o n s f r e e o f suspended s o l i d s ( 9 ) . The assumption t h a t the c r y s t a l l i z a t i o n o f z e o l i t e s p r o c e e d s through the incorporation of growth units present in the s u r r o u n d i n g s o l u t i o n a l l o w s e l i m i n a t i o n o f time as independent v a r i a b l e and a d o p t i o n o f k i n e t i c e q u a t i o n s o f the t y p e r=kS (where r i s the r a t e and S r e p r e s e n t s the s u p e r s a t u r a t i o n ) , more r i c h i n p r e d i c t i n g power. At p r e s e n t the knowledge o f the n a t u r e o f the b a s i c u n i t s t a k i n g p a r t i n the growth p r o c e s s i s o n l y h y p o t h e t i c a l , but kinetic e v i d e n c e i s e x a c t l y what i s needed t o v e r i f y the s e l f - c o n s i s t e n c y of hypotheses about the mechanism o f growth o f zeolites. In s e a r c h i n g f o r such e v i d e n c e , the s y n t h e s i s o f z e o l i t e omega has been s t u d i e d i n d e t a i l ( 1 0 ) . T h i s paper a d d r e s s e s the e v o l u t i o n o f the c r y s t a l h a b i t d u r i n g the s y n t h e s i s and d i s c u s s e s i t i n r e l a t i o n to t h e change o f the aluminium c o n c e n t r a t i o n the r e a c t i o n medium. Experimental R e a c t o r and r e a g e n t s . The c r y s t a l l i z a t i o n o f z e o l i t e omega was c a r r i e d o u t i n a 1 l i t e r s t a i n l e s s s t e e l a u t o c l a v e equipped w i t h a thermocouple w e l l , an anchor-shaped s t i r r e r , an i n j e c t i o n p o r t and a s a m p l i n g o u t l e t . The l a t t e r was d e s i g n e d t o a v o i d a c c u m u l a t i o n o f m a t e r i a l i n s i d e s a m p l i n g p i p e and v a l v e s i n the c o u r s e o f the r e a c t i o n . The i n j e c t i o n p o r t was c o n n e c t e d t o a v o l u m e t r i c pump ( G i l s o n 302S), a s s u r i n g a c o n t r o l l e d f l o w o f s o l u t i o n . The r e a c t o r was h e a t e d by an e l e c t r i c a l f u r n a c e which was r e g u l a t e d (+1°C) t h r o u g h the thermocouple immersed i n the r e a c t i o n medium. In a l l experiments the h e a t i n g r a t e was 80°C/h and a s t i r r i n g r a t e o f 140 rpm was used. The r e a g e n t s were : sodium a l u m i n a t e ( A l 0 . N a 0 ) from C a r l o Erba, sodium hydroxide pellets (Normapur from Prolabo), tetramethylammonium hydroxide pentahydrate (TMA, Fluka purum g r a d e ) , non-porous s i l i c a ( Z e o s i l 175 MP from Rhone P o u l e n c , p o r e volume 0.08 ml/g, g r a i n s i z e 2-20 ym, H^0 6.5 wt%)^ k a o l i n i t e des C h a r e n t e s (from K a o l i n s d ' A r v o r , s u r f a c e a r e a 24 m /g, g r a i n s i z e 4-15 ym, wt% A l 19.3, S i 21.3, Fe 0.29, Ca 0.1, Mg 0.13, Κ 0.5, Na 300 ppm), and d e i o n i z e d water. 2

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In Zeolite Synthesis; Occelli, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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FAJULA ET Ah.

Crystal Growth of Zeolite Omega

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Methods. The crystallization of silicoaluminate mixtures into z e o l i t e omega i n the temperature range 105-130°C was p e r f o r m e d i n the p r e s e n c e o f a s t r u c t u r e - d i r e c t i n g m i x t u r e (SDM) (10,11). The method g i v e s minimum o v e r l a p between the n u c l e a t i o n and growth s t e p s as i n d i c a t e d by the v e r y homogeneous d i s t r i b u t i o n o f s i z e o f the c r y s t a l s i n the f i n a l p r o d u c t . The use o f k a o l i n i t e as the aluminium s o u r c e p r e s e n t s two main advantages ( 1 0 ) . F i r s t , the low solubility o f the clay under the crystallization conditions prevents the formation of a g e l . Second, under the low s u p e r s a t u r a t i o n l e v e l s a c h i e v e d , secondary n u c l e a t i o n i s h i n d e r e d . S y n t h e s e s . The s t r u c t u r e - d i r e c t i n g m i x t u r e had the composition 3.2Na 0. 0.8TMA 0. A l Og. 8.2Si0 160H 0. I t was prepared by a d d i n g the sodium a l u m i n a t e t o the sodium-TMA h y d r o x i d e s o l u t i o n . A f t e r s t i r r i n g f o r one hour, the s i l i c a was added. The f i n a l s l u r r y was t h o r o u g h l y mixed f o r 12 h and f i n a l l y t r a n s f e r r e d i n t o a s e a l e d p o l y p r o p y l e n e b o t t l e which was m a i n t a i n e d i n an oven a t 50°C f o r 20 days i n s t a t i c c o n d i t i o n s . The c r y s t a l l i z a t i o n mixture (500 ml) had the c o m p o s i t i o n 3.2Na 0. Α 1 0 · 8 . 6 S i 0 . 160H 0 and was o b t a i n e d by f i r s t p r e p a r i n g a s u s p e n s i o n o f k a o l i n i t e i n the sodium h y d r o x i d e s o l u t i o n . The SiO^/AlgO^ r a t i o was a d j u s t e d by a d d i t i o n o f the a p p r o p r i a t e amount of s i l i c a . A f t e r s t i r r i n g f o r 5 h, 25 t o 30 % i n volume o f aged SDM was added, a t room temperature, t o the f r e s h l y p r e p a r e d h y d r o g e l . The a u t o c l a v e was s e a l e d and h e a t e d under s t i r r i n g t o t h e d e s i r e d temperature. Zero time c o r r e s p o n d e d t o the time when the system r e a c h e d the c r y s t a l l i z a t i o n t e m p e r a t u r e . 2

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Sampling and c h a r a c t e r i z a t i o n . 10 ml samples, i n c l u d i n g l i q u i d and s o l i d phases, were withdrawn p e r i o d i c a l l y from the a u t o c l a v e d u r i n g the c r y s t a l l i z a t i o n . The s l u r r i e s were c o l l e c t e d i n 70 ml o f c o l d d e i o n i z e d water. The s o l i d f r a c t i o n was r e c o v e r e d by f i l t r a t i o n and washed f i r s t w i t h 250 ml o f water. The c l e a r s o l u t i o n , c o n t a i n i n g the d i l u t e d mother l i q u o r , was p r e s e r v e d f o r c h e m i c a l a n a l y s i s . A f t e r a d d i t i o n a l washing (up t o a pH o f 9) , the s o l i d was d r i e d a t 70°C i n a i r . X-ray powder d i f f r a c t i o n was used t o i d e n t i f y the phases p r e s e n t . The o n l y c r y s t a l l i n e phase formed d u r i n g the s y n t h e s i s was z e o l i t e omega. The degree o f c o n v e r s i o n o f k a o l i n i t e i n t o z e o l i t e was e s t i m a t e d by c a l i b r a t i n g the X-ray i n s t r u m e n t (CGR T h e t a 60, CuKa monochromated r a d i a t i o n ) w i t h p h y s i c a l m i x t u r e s o f the two components. Morphology and s i z e o f the c r y s t a l s 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 m i c r o s c o p y on a Cambridge S100 i n s t r u m e n t . Growth r a t e s o f d i f f e r e n t c r y s t a l f a c e s were c a l c u l a t e d from the r e l a t i o n 0.5 6 l / 6 t , where δ 1 r e p r e s e n t s the i n c r e a s e o f the s i z e o f the c r y s t a l i n the d i r e c t i o n normal t o the f a c e c o n s i d e r e d , i n the time interval, 6 t , between two c o n s e c u t i v e s a m p l i n g s . The synthesis p r o c e d u r e produced a narrow d i s t r i b u t i o n o f s i z e : the p r e c i s i o n i n the d e t e r m i n a t i o n o f the s i z e s was ±0.05 μιη. I d e n t i f i c a t i o n o f crystal o r i e n t a t i o n s was ascertained from e l e c t r o n diffraction p a t t e r n s r e c o r d e d w i t h a J e o l 200 CX m i c r o s c o p e . C h e m i c a l a n a l y s e s were p e r f o r m e d i n the S e r v i c e C e n t r a l d'Ana l y s e s , CNRS, i n S o l a i z e , by flame photometry.


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Results Phenomenology o f the c r y s t a l l i z a t i o n . The c o n v e r s i o n v e r s u s time c u r v e s o b t a i n e d a t t h r e e d i f f e r e n t t e m p e r a t u r e s a r e shown i n F i g u r e 1. With the s y n t h e s i s p r o c e d u r e used, the sigmoid curves were c h a r a c t e r i z e d by s h o r t e r i n d u c t i o n p e r i o d s t h a n the traditional method (11,12). As e x p e c t e d , t e m p e r a t u r e had a s t r o n g e f f e c t on the r a t e o f c r y s t a l l i z a t i o n . The o v e r a l l c r y s t a l l i z a t i o n r a t e s may be a p p r o x i m a t e d by the r e c i p r o c a l o f the t i m e s o f h a l f c o n v e r s i o n . From t h e s e v a l u e s an a p p a r e n t a c t i v a t i o n energy o f 22+1 kcal/mol was o b t a i n e d . With r e s p e c t t o l i t e r a t u r e d a t a , t h i s v a l u e exceeds t h a t r e p o r t e d , f o r i n s t a n c e , f o r z e o l i t e Na-X (1,4) b u t compares w e l l w i t h the 19.8 k c a l / m o l found f o r ZSM-11 ( 1 3 ) . A b e t t e r p i c t u r e o f the c r y s t a l l i z a t i o n phenomena was obtained from the m i c r o s c o p e e x a m i n a t i o n o f the c r y s t a l s a t d i f f e r e n t s t a g e s o f t h e i r development. The f i r s t c r y s t a l s o f z e o l i t e omega appeared as s p h e r e s w i t h d i a m e t e r s around 0.5-1 ym (Fig.2) . With the appearance o f (001) f a c e s , t h i s form e v o l v e d i n t o barrel-shaped p a r t i c l e s ( F i g . 3 ) . A t a l a t e r s t a g e o f the s y n t h e s e s , the h a b i t e v o l v e d t o e u h e d r a l hexagonal w i t h the development o f (100) faces (Fig.4). The o b s e r v e d e v o l u t i o n i n the morphology i s c o n s i s t e n t w i t h the o v e r l a p p i n g p r i n c i p l e , i . e . the c r y s t a l h a b i t i s d e t e r m i n e d by the f a c e s w i t h the l o w e s t growing r a t e ( 1 4 ) . The apperance o f l o w - i n d e x f a c e s c o r r e s p o n d e d t o the d e c r e a s e o f s u p e r s a t u r a t i o n r e l a t e d t o the consumption o f n u t r i e n t by the growing c r y s t a l s . In o r d e r t o r a t i o n a l i z e t h e s e e v o l u t i o n s , the c o m p o s i t i o n o f the s o l u t i o n s u r r o u n d i n g the c r y s t a l s has t o be examined. The growth o f z e o l i t e omega i s non-congruent i n the sense t h a t a l l elements o f the p a r e n t g e l a r e n o t i n c o r p o r a t e d q u a n t i t a t i v e l y i n the final c r y s t a l l i n e p r o d u c t . Moreover the formed z e o l i t e has a d i f f e r e n t composition t h a n the liquid. Both f e a t u r e s a r e comon t o most z e o l i t e s ( 1 , 9 ) . From our p r e v i o u s s t u d i e s (10-12) on the z e o l i t e omega s y n t h e s i s i t appears t h a t the element w i t h the h i g h e r y i e l d o f i n c o r p o r a t i o n i s aluminimun. Hence, we l o o k e d f o r c o r r e l a t i o n s between growth r a t e s and the c o n c e n t r a t i o n o f aluminium i n the liquid phase, as possible evidences f o r aluminium being the l i m i t i n g element i n the c r y s t a l l i z a t i o n . The change o f [ A l ] as a f u n c t i o n o f time i s shown i n F i g u r e 5 f o r an experiment c a r r i e d out a t 115°C. A f t e r about 10 h o u r s , the c o n c e n t r a t i o n dropped r a p i d l y as the z e o l i t e formed. When more e x p e r i m e n t s were c o n s i d e r e d , a good c o r r e l a t i o n c o u l d be established between the aluminium concentration and the v a r i a t i o n o f the c r y s t a l h a b i t . F i g u r e 6 r e p o r t s d a t a collected from seven crystallizations. Spherulites exhibiting high-index f a c e s e x i s t e d f o r the h i g h e r supersaturation levels. When the aluminium content became lower t h a n 4-5 mmol/1, (001) faces appeared, w h i l e (100) f a c e s formed o n l y below 2-2.5 mmol/1. The s i z e o f the c r y s t a l s i n the and d i r e c t i o n s a r e p l o t t e d as a f u n c t i o n o f time i n F i g u r e 7. I t appears t h a t the growth r a t e s i n the above d i r e c t i o n s do not follow parallel k i n e t i c s . I n the d i r e c t i o n , c r y s t a l growth o c c u r r e d during the whole c r y s t a l l i z a t i o n p r o c e s s w h i l e the development o f the c r y s t a l s i n the d i r e c t i o n s r a p i d l y d e c r e a s e d and s t o p p e d when (100) f a c e s became exposed. Thus the growth k i n e t i c s o f the (001)


34. FAJULA ET AL.


and (hkO) f a c e s have d i f f e r e n t d i s s o l v e d aluminium.

497

dependences on t h e c o n c e n t r a t i o n o f


Dependence o f t h e growth r a t e s on t h e aluminium i n s o l u t i o n . F i g u r e 8 shows t h e i n f l u e n c e o f t h e c o n c e n t r a t i o n o f aluminium (mmol/1) on the r a t e o f growth (nm/h) f o r t h e (001) and (hhO) f a c e s a t 115°C ( t h e aluminium c o n c e n t r a t i o n t a k e n i n t o a c c o u n t c o r r e s p o n d s t o t h e average value between two consecutive analyses). I f the experimental data are represented i n In r versus In [ A l ] p l o t s , s a t i s f a c t o r y l i n e a r c o r r e l a t i o n s a r e f o u n d . Growth r a t e s may t h e n be r e p r e s e n t e d as : r

Q 0 1

=k [Al]

0 , 8

w i t h k=5.5, 13 and 37.4 a t 105°C, 115°C and

130°C r e s p e c t i v e l y , Ε =23 k c a l / m o l , and 1

r

h k 0

and

=k[Al]

β

&

w i t h k=0.94, 3.94 and 11.7 a t 105°C, 115°C

130°C r e s p e c t i v e l y , E =30 &

kcal/mol.

Crystal habit modification. Several crystal habits have been r e p o r t e d i n t h e open and p a t e n t literature for zeolite omega. E l o n g a t e d h e x a g o n a l r o d s (15,16) o r f i b r e s (17) have been r e p o r t e d when t h e z e o l i t e r e s u l t e d from t h e r e c r y s t a l l i z a t i o n o f a n o t h e r z e o l i t e , Y (15) o r S ( 1 7 ) , o r o f c l a y s ( 1 6 ) . Moreover t h e n a t u r a l counterpart of zeolite omega, m a z z i t e , appears as b u n d l e s o f n e e d l e - s h a p e d p a r t i c l e s ( 1 8 ) . A l l t h e s e s o l i d s have been grown a t low s u p e r s a t u r a t i o n l e v e l s , hence under c o n d i t i o n s i n which our r e s u l t s show t h a t t h e growth i n t h e d i r e c t i o n p r e v a i l s on the growth normal t o t h e c - a x i s ( F i g . 8 ) . T h i s p r e f e r e n t i a l growth may h i n d e r many a p p l i c a t i o n s o f t h e zeolite omega, whose u n i d i m e n s i o n a l porosity, parallel t o the c - a x i s , p r e s e n t s d i f f u s i o n a l l i m i t a t i o n s which a r e f u n c t i o n s o f t h e elongation o f the c r y s t a l s . I n t h e case o f t h e s y n t h e s e s we r e p o r t e d i n F i g u r e 7, t h e f i n a l r a t i o between and < 100> s i z e was 2.5 t o 3.6. Knowledge o f t h e growth k i n e t i c s o f t h e d i f f e r e n t faces o f the z e o l i t e a l l o w e d us t o a t t r i b u t e t h e main i n c r e a s e in < 001 > s i z e t o t h e l o n g f i n a l p e r i o d o f growth a t aluminium concentration lower t h a n about 2 mmol/1 ( s e e F i g . 5 ) . Such a situation results from t h e need o f complete conversion of a s l i g h t l y s o l u b l e raw m a t e r i a l as a c l a y . Any p o s s i b l e way t o modify t h e f i n a l / s i z e r a t i o appeared t o r e q u i r e t h a t growth o f c r y s t a l s be c o n d u c t e d i n an optimum range o f s u p e r s a t u r a t i o n ( c o r r e s p o n d i n g t o a c o n c e n t r a t i o n o f aluminium between 2 and 4 mmol/1) w i t h o u t m o d i f i c a t i o n o f t h e initial conditions, which determine t h e number of crystals n u c l e a t e d . The p r a c t i c a l way i n which we r e a l i z e d t h i s o b j e c t i v e was t h e use o f an i n i t i a l s y n t h e s i s medium c o n t a i n i n g o n l y t h e aluminium p r e s e n t i n the s t r u c t u r e d i r e c t i n g mixture. Once t h e n u c l e a t i o n o c c u r r e d , an a l u m i n a t e s o l u t i o n was i n j e c t e d i n t h e a u t o c l a v e i n o r d e r t o m a i n t a i n t h e aluminium c o n c e n t r a t i o n a t t h e desired level. The e x p e c t e d increase i n growth surface was c a l c u l a t e d u s i n g t h e above k i n e t i c e q u a t i o n s , and t h e f l o w r a t e o f the i n j e c t e d s o l u t i o n was c o n t i n u o u s l y a d j u s t e d i n o r d e r t o b a l a n c e the i n c o r p o r a t i o n o f n u t r i e n t by t h e c r y s t a l s .


ZEOLITE SYNTHESIS


498

F i g u r e 2. M i x t u r e o f u n r e a c t e d k a o l i n i t e and o f s p h e r o i d s i n d e x f a c e s exposed) o f z e o l i t e omega o b t a i n e d i n t h e stages o f the c r y s t a l l i z a t i o n . Scale bar : 5 p .


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34. FAJULAETAK

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F i g u r e 3. K a o l i n i t e and b a r r e l - s h a p e d ((001) and (hkO) f a c e s exposed) c r y s t a l s o f z e o l i t e omega. (See t e x t ) . S c a l e b a r : 5 urn.

F i g u r e 4. Hexagonal p r i s m s ((001) and (100) f a c e s exposed) z e o l i t e omega and k a o l i n i t e . (See T e x t ) . S c a l e b a r : 5 Ajm.


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Time (hours) F i g u r e 5. V a r i a t i o n o f t h e c o n c e n t r a t i o n c r y s t a l l i z a t i o n s o l u t i o n a t 115°C.

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s i z e o f t h e c r y s t a l s o f z e o l i t e omega i s r e p o r t e d i n F i g u r e 10. C o n s i d e r i n g t h e aluminium c o n c e n t r a t i o n p l o t i n F i g u r e 9, i t c a n be observed that the f i r s t stages o f the synthesis closely p a r a l l e d e d t h e p a t t e r n o f t h e experiment r e p o r t e d i n F i g u r e 5. A f t e r 12 hours o f s y n t h e s i s a sharp r i s e o f aluminium c o n c e n t r a t i o n was o b s e r v e d i n c o r r e s p o n d e n c e w i t h t h e b e g i n n i n g o f t h e a l u m i n a t e f e e d i n g . The subsequent d e c r e a s e (15 h o u r s ) was r e l a t e d t o an i n c r e a s i n g consumption o f aluminium (due t o t h e i n c r e a s e o f t h e growth s u r f a c e ) n o t t o t a l l y b a l a n c e d by t h e i n c r e a s e o f t h e f l o w r a t e . As s u p e r s a t u r a t i o n d e c r e a s e d , t h e growth slowed down. When [ A l ] r e a c h e d t h e t h r e s h o l d o f 2 mmol/1, t h e (100) f a c e s formed and no f u r t h e r growth i n t h e d i r e c t i o n normal t o t h e c - a x i s was o b s e r v e d ( F i g . 1 0 ) . When t h e aluminium s u p p l y was c o n t i n u e d f o r a t l e a s t an hour a f t e r t h a t t h e e f f e c t i v e growth s u r f a c e has been l i m i t e d t o t h e (001) s u r f a c e s , a subsequent c o n c e n t r a t i o n peak was o b s e r v e d (25 h o u r s ) . The f i n a l r a t i o between t h e and t h e < hkO > s i z e o f t h e c r y s t a l s was about 1.6, s i g n i f i c a n t l y lower than t h e v a l u e o b s e r v e d i n c r y s t a l s o b t a i n e d by k a o l i n i t e c o n v e r s i o n . Discussion The n a t u r e o f t h e s i l i c o a l u m i n a t e u n i t s t a k i n g p a r t i n t h e z e o l i t e growth i s s t i l l unknown, d e s p i t e t h e amount o f e x p e r i m e n t a l data c o l l e c t e d about t h e s p e c i e s p r e s e n t i n t h e s o l u t i o n s from which d i f f e r e n t z e o l i t e s a r e formed (19-22). Some o f o u r r e s u l t s a r e p a r t i c u l a r l y s i g n i f i c a n t f o r this subject. 1) The r a t e s o f growth o f z e o l i t e omega c o r r e l a t e t o t h e conc e n t r a t i o n o f d i s s o l v e d aluminium. The use o f s o u r c e s o f aluminium as d i f f e r e n t as k a o l i n i t e , an aged s i l i c o a l u m i n a t e g e l and a sodium a l u m i n a t e s o l u t i o n does n o t a f f e c t t h e c o r r e l a t i o n . Thus as l o n g as aluminium i s c o n c e r n e d , no memory e f f e c t o f the t o p o l o g y o f t h e s o u r c e c a n be o b s e r v e d . T h i s argues f o r t h e f o r m a t i o n o f t h e growth u n i t s through r e a c t i o n s i n s o l u t i o n . 2) F o r z e o l i t e omega, aluminium appears t o be t h e l i m i t i n g e l e ment i n t h e f o r m a t i o n o f t h e growth u n i t s . The i n c o n g r u e n c e o f t h e s y n t h e s i s and t h e e x c e s s o f t h e elements o t h e r t h a n aluminium c o u l d a c c o u n t f o r t h i s r e s u l t . I n z e o l i t i c systems n o t p r e s e n t i n g these f e a t u r e s , t h e o c c u r e n c e o f such a s t r i c t r e l a t i o n s h i p c o u l d be harder to e s t a b l i s h . 3) The growth r a t e s o f t h e (001) f a c e and o f t h e l a t e r a l s u r f a c e o f t h e c y l i n d r i c a l c r y s t a l s (here d e s i g n e d as (hkO) f a c e s ) p r e s e n t a d i f f e r e n t energy o f a c t i v a t i o n and a d i f f e r e n t dependence on aluminium c o n c e n t r a t i o n . These r e s u l t s c o u l d s u p p o r t t h e h y p o t h e s i s o f d i f f e r e n t growth mechanisms on t h e d i f f e r e n t f a c e s .


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Time (hours) F i g u r e 9. S y n t h e s i s o f z e o l i t e omega a t 115°C w i t h e x t e r n a l s u p p l y o f aluminium. L e f t s c a l e : aluminium c o n c e n t r a t i o n i n t h e a u t o c l a v e s o l u t i o n . R i g h t s c a l e : r a t e o f aluminium s u p p l y .

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Time (hours) F i g u r e 10. S y n t h e s i s o f z e o l i t e omega a t 115°C w i t h e x t e r n a l s u p p l y o f aluminium. Change o f t h e c r y s t a l s i z e i n t h e (·) and < h k 0 > ( O ) d i r e c t i o n s .



504

ZEOLITE SYNTHESIS

The growth r a t e o f t h e f a c e (001) p r e s e n t ^ an a c t i v a t i o n energy o f 23 k c a l / m o l and i s p r o p o r t i o n a l t o [ Al] , a r e a c t i o n order near u n i t y . T h i s b e h a v i o r c o u l d be i n t e r p r e t e d as t h e p r o d u c t o f two n e a r - t o - l i n e a r dependences. On t h e one hand, there is a dependency between c o n c e n t r a t i o n o f growth u n i t s and c o n c e n t r a t i o n o f aluminium, and on t h e o t h e r hand, a dependency e x i s t s between r a t e o f growth and s u p e r s a t u r a t i o n . P e r s i s t e n t growth a t t h e l o w e s t c o n c e n t r a t i o n l e v e l s a n a l y z e d s u g g e s t s a v e r y low v a l u e f o r t h e s a t u r a t i o n c o n c e n t r a t i o n o f aluminium i n t h i s e x p e r i m e n t a l system. While f u l l y a r b i t r a r y , i t i s t e m p t i n g t o h y p o t h e s i z e t h a t some increase i n viscosity with i n c r e a s i n g aluminium concentration a l l o w s f o r t h e d e v i a t i o n from p e r f e c t l i n e a r i t y o f t h e growth rate-versus-concentration plot, a phenomenon well known by r e s e a r c h e r s f o n d o f c r y s t a l growth i n g e l s ( 2 3 ) . Growth on t h e l a t e r a l s u r f a c e o f t h e c y l i n d e r - l i k e c r y s t a l s p r e s e n t s gin a c t i v a t i o n energy o f 30 k c a l / m o l and i s p r o p o r t i o n a l t o [Al ] . Thus a s t e p p e d s u r f a c e ( S - f a c e , 24) w i t h no d e f i n e d crystallographic orientation like t h e (hkO) f a c e s presents a r e a c t i o n o r d e r n e a r l y double t h a n a f o r m a l l y f l a t f a c e ( F - f a c e , 24) l i k e (001). T h i s r e s u l t seems t o be i n c o m p a t i b l e w i t h t h e a c t i o n o f the same mechanism o f growth on t h e two k i n d s o f f a c e s . The dependency, h i g h e r t h a n f i r s t - o r d e r , on aluminium f o r t h e growth r a t e i n t h e d i r e c t i o n s c o u l d be e x p l a i n e d by t h e r e s u l t o f t h e p r o d u c t between two dependences : a first-order dependence o f growth rate on supersaturation and a neart o - s e c o n d - o r d e r dependence o f t h e growth u n i t s c o n c e n t r a t i o n on t h e aluminium c o n c e n t r a t i o n . I t would be s p e c u l a t i v e t o a t t r i b u t e such a r e s u l t t o a need, f o r growth, o f s i l i c o a l u m i n a t e s p e c i e s dimer o f t h e u n i t s r e s p o n s i b l e f o r growth on t h e (001) f a c e s . N e v e r t h e l e s s , such an i n t e r p r e t a t i o n c o u l d f i n d i n t u i t i v e support i n the topology o f the z e o l i t e omega framework ( 2 5 ) . W h i l e t h e < 001> growth c a n be e a s i l y v i s u a l i z e d as t h e i n c r e a s e i n l e n g t h o f p r e - e x i s t i n g c h a i n s o f five-members r i n g s , t h e growth must deal with the harder task o f completing twelve-members rings p r o b a b l y n o t i n c l u d i n g any o r g a n i c t e m p l a t e (11,26). T h i s k i n d o f d i f f i c u l t y c o u l d a l s o a c c o u n t f o r t h e o b s e r v e d beh a v i o r o f t h e (100) f a c e , w i t h growth so slow t h a t no s i z e i n c r e a s e normal t o the c-axis could be d e t e c t e d i n our experimental c o n d i t i o n s once t h e hexagonal p r i s m h a b i t was a t t a i n e d . Acknowledgments P a r t o f t h i s work has been c a r r i e d o u t i n t h e frame o f an EEC BRITE p r o j e c t (RI.1B.0133F). The a u t h o r s a r e g r a t e f u l t o G.Nabias f o r SEM measurements and R . D u t a r t r e f o r TEM s t u d i e s .

Literature cited 1. For a review, see Barrer, R.M. Hydrothermal Chemistry of Zeolites, Academic: London, 1982; p 133. 2. Thompson,R.W.; Dyer,A. Zeolites 1985, 5, 202. 3. Kacirek, H.; Lechert, H. J. Phys. Chem 1975, 79, 1589. 4. Zhdanov, S.P.; Samulevich, N.N. Proc. 5th Int. Conf. on Zeolites; Rees, L.V.C., Ed.; Heyden: London, 1980, p 75. 5. Ciric, J. J. Colloid and Interface Sci. 1968, 28, 315. 6. Johnson, W.A.; Mehl, R.F. Trans. AIME, 1939, 135, 416.



34. FAJULA ET AL. Crystal Growth of Zeolite Omega

505

7. Avrami, M. J. Chem. Phys. 1939, 7, 1103. 8. Avrami, M. J. Chem. Phys. 1940, 8, 212. 9. For a review, see Guth, J.L.; Gaullet, P. J. Chem. Phys. 1986, 83, 155. 10. Nicolas, S. Ph. D. Thesis, Université des Sciences et Techniques du Languedoc, Montpellier, 1987. 11. Nicolas, S.; Massiani, P.; Vera-Pacheco, M.; Fajula, F.; Figueras, F. Stud. Surf. Sci. and Catal. 1988, 37, 115. 12. Fajula, F.; Vera-Pacheco, M.; Figueras, F.Zeolites 1987, 7, 203. 13. Hou, L.Y.; Sand, L.B. Proc. 6th Int. Conf. on Zeolites; Olson, D.; Bisio, Α., Eds.; Butterworths: Guilford, 1984, p 887. 14. Strickland-Constable, R.F. Kinetics and Mechanism of Crystrallization, Academic : London, 1968, p.7. 15. Dwyer, F.G.; Chu, P.J. J. Catal. 1979, 59, 263. 16. Fajula, F.; Figueras, F.; Moudafi, L.; Vera-Pacheco, M.; Nicolas, S.; Dufresne, P.; Gueguen, C. French Pat. Appl. 8507772, 1985. 17. Araya, Α.; Barber, T.J.; Lowe, B.M.; Sinclair, D.M.; Varma, A. Zeolites 1984, 4, 263. 18. Galli, E.; Passaglia, E.; Pongiluppi, D.; Rinaldi, R. Contrib. Mineral. Petrol. 1974, 45, 99. 19. Guth, J.L.; Caullet, P.; Jacques, P.; Wey, R. Bull. Soc. Chim. 1980, 3, 121. 20. Dent Glasser, L.S.; Harvey, G. Proc. 6th Int. Conf. on Zeolites; Olson, D.; Bisio, Α., Eds.; Butterworth: Guilford, 1984, p 925. 21. Engelhardt, G.; Fahlke, B.; Magi, M.; Lippmaa, E. Zeolites 1985, 5, 49. 22. Boxhoorn, G.; Sudmeijer, O.; Van Kasteren, P.H.G. J. Chem. Soc. Chem. Comm. 1983, 1416. 23. Henixh H.K. Crystals in Gels and Liesegang Rings; Cambridge University: Cambridge, 1988, p 50. 24. Hartman, P. Growth of Crystals, Sheftal, N-N., Ed., Consultant Bureau : New York, London, 1969, Vol.7, p.3. 25. Rinaldi, R.; Pluth, J.J.; Smith, J.V. Acta Cryst. 1975, 31, 1603. 26. Aiello, R.; Barrer, R.M. J. Chem. Soc.(A) 1970, 1470. RECEIVED

December 22, 1988


Kinetics and Mechanism of Crystal Growth of Zeolite Omega

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