Zeolites: Their Nucleation and Growth - ACS Symposium Series (ACS

Chapter 2

Zeolites: Their Nucleation and Growth

Downloaded by STANFORD UNIV GREEN LIBR on April 16, 2013 | http://pubs.acs.org Publication Date: July 31, 1989 | doi: 10.1021/bk-1989-0398.ch002

R. M. Barrer Chemistry Department, Imperial College, London SW7 2AZ, England

Zeolite synthesis proceeds via nucleation, which is a consequence of local fluctuations, small in extent but considerable in degree of departure from the mean for the solution, followed by spon taneous growth of nuclei exceeding a critical size. A physico-chemical basis for the critical size requirement has been described. There is evidence that chemical events rather than diffusion can govern subsequent linear growth of zeolite crystals. To succeed in synthesis it is essential during growth to stabilise the open structure by inclusion of quest molecules. This requirement has a thermo dynamic origin which has been developed and applied to formation of zeolites, porosils and AlPO's. The explanation of some experimentally observed features of zeolite synthesis follows from the treatment. A distinction is made between zeolitic stabilisers and nucleation templates. As t h e p o t e n t i a l i t i e s o f m i c r o p o r e s i n c r y s t a l s r a t h e r s l o w l y became r e a l i s e d t h e r e d e v e l o p e d an a r e a o f s y n t h e t i c c h e m i s t r y which has y i e l d e d a remarkable v a r i e t y o f microporous s t r u c t u r e s . Most a r e 3 - d i m e n s i o n a l 4-connected n e t s , o f which t h e numbers e n v i s a g e d v a s t l y exceed t h e numbers p r e p a r e d e x p e r i m e n t a l l y . A c c o r d i n g l y t h e s e a r c h f o r c h e m i c a l pathways t o new porous c r y s t a l s p r o c e e d s apace, both f o r i t s s c i e n t i f i c i n t e r e s t and i t s p o s s i b l e i n d u s t r i a l rewards. T h i s account w i l l emphasise s e v e r a l p h y s i c o - c h e m i c a l a s p e c t s o f s y n t h e s i s which h e l p i n u n d e r s t a n d i n g t h e "green f i n g e r s " a p p r o a c h o f much c u r r e n t r e s e a r c h on t h e f o r m a t i o n o f porous c r y s t a l s . Aluminate

and S i l i c a t e S o l u t i o n s

In t h e h i g h pH range needed f o r z e o l i t e s y n t h e s i s a l u m i n a t e s o l u t i o n s a r e r e l a t i v e l y s i m p l e i n t h a t t h e a n i o n s p r e s e n t a r e almost e x c l u s i v e l y A1(0H) On t h e o t h e r hand i n s i l i c a t e s o l u t i o n s around room

0097-6156/89A)398-0011$06.00/0 ο 1989 American Chemical Society In Zeolite Synthesis; Occelli, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.


12

ZEOLITE SYNTHESIS

t e m p e r a t u r e v a r i o u s a n i o n s c o - e x i s t a c c o r d i n g t o t h e r a t i o o f base t o s i l i c a , the n a t u r e o f t h e base, and t h e c o n c e n t r a t i o n . There has, on t h e o t h e r hand, been l i t t l e study o f s i l i c a t e s o l u t i o n s i n t h e t e m p e r a t u r e range most i m p o r t a n t f o r z e o l i t e s y n t h e s i s (up t o ~ 2 5 0 ° C ) . However K n i g h t e t a l (J_) made an i n t e r e s t i n g o b s e r v a t i o n on tetramethylammonium s i l i c a t e s o l u t i o n s 1 M i n S i and w i t h S i / N = 1/2. The room t e m p e r a t u r e e q u i l i b r i u m was p e r t u r b e d by ~30 s h e a t i n g t o 100°C, q u e n c h i n g i n l i q u i d n i t r o g e n and warming t o room temperature. F o r t h e f i r s t few minutes the NMR spectrum t h e n showed p r e d o m i n a n t l y monomeric s i l i c a t e a n i o n s w i t h s m a l l amounts o f dimer and t r i m e r . T r i g o n a l p r i s m a n i o n s next began t o appear f o l l o w e d a f t e r s e v e r a l hours by c u b i c a n i o n s . The l a t t e r i n c r e a s e d s t e a d i l y i n r e l a t i v e amount u n t i l a f t e r 16 days e q u i l i b r i u m was e s t ablished. The e x p e r i m e n t shows slow e q u i l i b r a t i o n a t room tempera t u r e , r a p i d p e r t u r b a t i o n o f t h i s e q u i l i b r i u m a t 100°C, and a p r i m a r i l y monomeric a n i o n i c c o n s t i t u t i o n a t 100°C. T h i s experiment s h o u l d be extended t o o t h e r systems s i n c e i t may have g e n e r a l i m p l i c a t i o n s f o r m o l e c u l a r mechanisms o f n u c l e a t i o n . T h i s would be e s p e c i a l l y true of the c l e a r a l u m i n o s i l i c a t e s o l u t i o n s r e f e r r e d t o below. Reaction

Mixtures

The u s u a l r e s u l t o f m i x i n g a l u m i n a t e and s i l i c a t e s o l u t i o n s i s t h e f o r m a t i o n o f a g e l , which may l a t e r s e p a r a t e i n t o a c l e a r s u p e r n a t a n t l i q u i d and a g e l . One may ask whether n u c l e a t i o n i s homoge neous ( i n s o l u t i o n ) o r h e t e r o g e n e o u s ( i n g e l ) . A p a r t i a l answer may be p r o v i d e d i f i t can be d e m o n s t r a t e d t h a t z e o l i t e s can grow from c l e a r s o l u t i o n s . Guth e t a l (2^) and Ueda e t a l have shown how such s o l u t i o n s can be p r e p a r e d . From c l e a r Na- a l u m i n o s i l i c a t e s o l u t i o n s Ueda e t a l have c r y s t a l l i s e d a n a l c i m e , s o d a l i t e h y d r a t e , m o r d e n i t e , f a u j a s i t e (Na-Y), z e o l i t e s ( g m e l i n i t e t y p e ) and z e o l i t e Ρ (gismondine t y p e ) , so t h a t homogeneous n u c l e a t i o n i s a t l e a s t p o s s i b l e . Where g e l i s p r e s e n t c r y s t a l s n u c l e a t e d homogeneously would, as growing c r y s t a l s , become enmeshed w i t h g e l so t h a t homogeneous and heterogeneous n u c l e a t i o n a r e d i f f i c u l t t o d i f f e r e n t i a t e i n g e l - c o n t a i n i n g media. Whether n u c l e a t i o n i s homogeneous, heterogeneous or both t h e r e i s s t r o n g e v i d e n c e t h a t i n subsequent c r y s t a l growth t h e g e l , i f p r e s e n t , p r o g r e s s i v e l y d i s s o l v e s and t h a t t h e d i s s o l v e d m a t e r i a l t h e n feeds t h e growing c r y s t a l s . This also applies i n successive trans f o r m a t i o n s such as (4^) : s o l u t i o n -+ Na-Y Na-S -> Na-P, because each c r o p o f c r y s t a l s i n the s u c c e s s i o n has i t s own s i z e range and morphology r a t h e r t h a n b e i n g a pseudomorph o f i t s p a r e n t , as would be t h e c a s e i n an i n t e r n a l s o l i d s t a t e t r a n s f o r m a t i o n . Z e o l i t e C r y s t a l l i s a t i o n from c l e a r S o l u t i o n s The c l e a r a l u m i n o s i l i c a t e s o l u t i o n s from which Ueda e t a l (j4) s t u d i e d c r y s t a l l i s a t i o n o f z e o l i t e s Y, S and Ρ were based on t h e composition range 10Na O.(0.35-0.55)A1 0 . ( 2 2 - 2 8 ) S i 0 . ( 2 5 0 - 3 0 0 ) H O . Figure 1 shows t h e r e g i o n i n which g e l and s o l u t i o n c o - e x i s t e a (cross-hatched), t h e r e g i o n of c l e a r s o l u t i o n , and the f o r m a t i o n f i e l d s o f t h e t h r e e z e o l i t e s from t h e c l e a r s o l u t i o n . 2

2

2

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


2.

BARRER


L

I

0.40

Figure

1.

ι

I

045

ι

I

13

;

050

C r y s t a l l i s a t i o n f i e l d s o f z e o l i t e s Y, S and Ρ a t 100°C from c l e a r a l u m i n o s i l i c a t e s o l u t i o n s . In t h e c r o s s h a t c h e d a r e a g e l and s o l u t i o n c o - e x i s t . (Reproduced w i t h p e r m i s s i o n from Ref. 4. C o p y r i g h t 1984 B u t t e r w o r t h s . )


14

ZEOLITE SYNTHESIS

For c r y s t a l l i s a t i o n o f Y t h e optimum c o m p o s i t i o n o f s o l u t i o n was 10Na O.O.45A1 0 26SiO .270H O. F a u j a s i t e ( z e o l i t e Y) appeared a f t e r ~12 h o u r s , and t h e c r y s t a l s were o f r a t h e r c o n s t a n t c o m p o s i t i o n , h a v i n g , o v e r a l l t h e e x p e r i m e n t s , r a t i o s SiO^/AlÔ between 5.1 and 5.6. Because t h e s e r a t i o s a r e so much g r e a t e r i n t h e p a r e n t s o l u t i o n s t h e A l has been s e l e c t i v e l y removed from s o l u t i o n and i t s c o n c e n t r a t i o n t h e r e i n drops much f a s t e r t h a n t h o s e o f Na and S i . When S i O /AlÔ^ i n s o l u t i o n had i n t h i s way r i s e n from 58 t o 73 z e o l i t e δ began t o appear and t h e y i e l d o f Y t o d e c r e a s e . When S i O / A l 0 had r e a c h e d about 102 z e o l i t e Ρ s t a r t e d t o form and t h e y i e l d of S Began t o d e c l i n e . 2

2

A l s o , i f t h e s o l u t i o n s had an i n i t i a l S i 0 / A l 0 r a t i o o f 73,S formed b u t no Y, w h i l e i f t h i s i n i t i a l r a t i o was 102,Ρ formed b u t no S. T h i s b e h a v i o u r s u g g e s t s c a u t i o n i n i n t e r p r e t i n g a l l c r y s t a l l i s a t i o n sequences as examples o f Ostwald's r u l e o f s u c c e s s i v e transformations. The r u l e s t a t e s t h a t i n a c r y s t a l l i s a t i o n sequence the new phases r e p l a c e each o t h e r i n t h e o r d e r o f a s t e p by s t e p d e s c e n t o f a l a d d e r o f i n c r e a s i n g thermodynamic s t a b i l i t y . An example i n a h y d r o t h e r m a l system i s {5): Amorphous S i O ^ cristobalite keatite quartz. The optimum check o f Ostwald's r u l e would, as i n t h e above sequence, i n v o l v e p a r e n t g e l and s u c c e s s i v e phases a l l o f t h e same c o m p o s i t i o n . T h i s c o n d i t i o n i s n o t met i n many c r y s t a l l i s a t i o n sequences.


2

2

N u c l e a t i o n and C r y s t a l Growth S t u d i e s o f Raman (2) and NMR (6) s p e c t r a o f s o l u t i o n s o f a l u m i n a t e s and s i l i c a t e s , and o f t h e i r m i x t u r e s under c o n d i t i o n s y i e l d i n g c l e a r a l u m i n o s i l i c a t e s o l u t i o n s , agree i n showing a t l e a s t p a r t i a l s u p p r e s s i o n o f t h e A l ( O H ) ^ i o n when s i l i c a t e a n i o n s a r e p r e s e n t , s u p p o r t i n g the view t h a t a l u m i n o s i l i c a t e a n i o n s form around room t e m p e r a t u r e . I t i s t h e n p o s s i b l e t o v i s u a l i s e how such a n i o n s c o u l d form more com p l e x u n i t s and germ n u c l e i . An example i s shown i n F i g u r e 2 i n which 4 - r i n g a n i o n s y i e l d t h e c u b i c u n i t found i n t h e z e o l i t e A framework, o r t h e d o u b l e c r a n k s h a f t c h a i n found i n f e l s p a r s o r i n p h i l l i p s i t e - h a r m o t o m e z e o l i t e s {!_). W h i l e a c t u a l c h e m i c a l e v e n t s i n v o l v e d i n n u c l e a t i o n and c r y s t a l growth a r e n o t known a p h e n o m e n o l o g i c a l t r e a t m e n t (8_) g i v e s some insight. W i l l a r d G i b b s (9_) c o n s i d e r e d p r o c e s s e s o f phase s e p a r a t i o n of two extreme k i n d s . In t h e f i r s t , f l u c t u a t i o n s i n c o n c e n t r a t i o n o c c u r which a r e minute i n volume b u t l a r g e i n e x t e n t o f d e p a r t u r e from t h e mean ( t h e c a s e o f b i n o d a l phase s e p a r a t i o n ) . In t h e second t h e volume o f t h e f l u c t u a t i o n i s l a r g e but t h e d e v i a t i o n from the mean f o r t h e s o l u t i o n i s minute ( r e s p o n s i b l e f o r s p i n o d a l phase separation). In n u c l e a t i o n o f z e o l i t e s one i s conerned o n l y w i t h fluctuations of the f i r s t kind. One may e n q u i r e what f a c t o r s oppose t h e immediate appearance o f v i a b l e n u c l e i growing s p o n t a n e o u s l y . Whether i n g e l o r s o l u t i o n a p o s i t i v e i n t e r f a c i a l f r e e energy term Δς^. a r i s e s which i s i n c r e a s i n g l y i m p o r t a n t r e l a t i v e t o o t h e r f r e e energy terms t h e l a r g e r t h e s u r f a c e t o volume r a t i o . In a r e s t r a i n i n g m a t r i x t h e germ, t h r o u g h m i s f i t , may a l s o produce a p o s i t i v e s t r a i n f r e e energy, Ag . Both t h e s e terms g r e a t l y r e d u c e t h e p r o b a b i l i t y o f a germ n u c l e u s becoming v i a b l e . The n e t f r e e energy o f f o r m a t i o n o f a germ



2.

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F i g u r e 2.


F o r m a t i o n from 4 - r i n g a n i o n s o f c u b i c u n i t s found i n z e o l i t e A and d o u b l e c r a n k s h a f t c h a i n s found i n f e l s p a r s and p h i l l i p s i t e - h a r m o t o m e z e o l i t e s . (Reproduced w i t h p e r m i s s i o n from Ref. 7. C o p y r i g h t 1982 Academic P r e s s . )


16

ZEOLITE SYNTHESIS

c o n s i s t i n g of j s t r u c t u r a l u n i t s of a p a r t i c u l a r k i n d i s t h e r e f o r e

AG Ag. = - — - j D N

+

J

Ag *

A

+

σ

Ag ^s

( 1 )

where AG i s t h e f r e e energy o f f o r m a t i o n o f a mole o f b u l k c r y s t a l , i . e . t h e amount o f c r y s t a l c o n t a i n i n g an Avogadro number, N^, o f t h e structural units. AG i s n e g a t i v e ^ i n s i g n . Ag i s proportional to t h e i n t e r f a c i a l a r e a , and so t o j , w h i l e Ag i s proportional to j. Thus, w i t h A = -AG/N , one has


A

Ag.. = - A j + B j

2

/

3

+ C j

(2)

where A, Β and C a r e p o s i t i v e c o e f f i c i e n t s . F o r s m a l l j , wjêçe t h e s u r f a c e t o volume r a t i o i s v e r y l a r g e , t h e p o s i t i v e term B j can be dominant. In s o l u t i o n o r g e l we e x p e c t C t o be z e r o o r s m a l l , i . e . A > C. A s J i n c r e a s e s t h e n e g a t i v e term - ( A - C ) j w i l l grow more r a p i d l y than B j . A c c o r d i n g l y , when Ag. i s p l o t t e d a g a i n s t j t h e c u r v e i n i t i a l l y has a p o s i t i v e s l o p e and Âg. i s p o s i t i v e , but a t some v a l u e o f j t h i s c u r v e p a s s e s t h r o u g h a maximum and t h e r e a f t e r d e c r e a s e s , as shown i n F i g u r e 3 (JO. A t t h e maximum dAg ./dj = 0 and so from E q u a t i o n 2 t h e v a l u e s o f j and o f Ag_. a t t h e maximum a r e 2

D_ = m

8B

3

3

. 27(A-C) 3

;

A

Ag

= ^

. (A-C) j , m

/ o l

(3)

2

Any n u c l e u s i n which j exceeds j w i l l add more l a t t i c e - f o r m i n g u n i t s w i t h a d e c r e a s e i n f r e e energy and t h e r e f o r e t e n d s t o grow s p o n t a n e o u s l y ; but any germ n u c l e u s i n which j i s l e s s t h a n j will l o s e l a t t i c e - f o r m i n g u n i t s w i t h a d e c r e a s e i n f r e e energy, anS w i l l t h e r e f o r e tend t o disappear. Even so, f l u c t u a t i o n s e n s u r e t h a t some germs e v e n t u a l l y r e a c h and c r o s s t h e s a d d l e p o i n t i n F i g u r e 2 and then grow s p o n t a n e o u s l y . As t h e t o t a l s u r f a c e a r e a o f growing c r y s t a l s i n c r e a s e s , and v a s t l y exceeds t h e t o t a l a r e a o f germ n u c l e i , c r y s t a l s w i l l dominate more and more s t r o n g l y o v e r f r e s h n u c l e i i n consuming t h e l a t t i c e forming u n i t s a v a i l a b l e . A c c o r d i n g l y the n u c l e a t i o n r a t e should b u i l d t o a maximum e a r l y i n t h e c u r v e o f y i e l d a g a i n s t t i m e , but w i l l t h e r e a f t e r , through the c o m p e t i t i o n w i t h c r y s t a l s f o r chemical n u t r i e n t s , t e n d t o decay towards z e r o . T h i s b e h a v i o u r was found by Zdhanov and Samuelevich (_1_0) from an a n a l y s i s o f t h e l i n e a r growth r a t e s o f i n d i v i d u a l c r y s t a l s o f Na-X and t h e s i z e d i s t r i b u t i o n o f the f i n a l crop of c r y s t a l s . A l s o , as t h e t o t a l a r e a o f growing c r y s t a l s i n c r e a s e s so does t h e r a t e a t which c h e m i c a l n u t r i e n t s a r e consumed. The s l o p e o f t h e c u r v e o f y i e l d a g a i n s t time t h e r e f o r e i n c r e a s e s . L a t e r , as t h e s u p p l y o f n u t r i e n t s becomes more and more exhausted, t h e s l o p e o f t h e c u r v e d e c r e a s e s t o z e r o . As a r e s u l t c u r v e s o f y i e l d v s . time a r e s i g m o i d i n c o n t o u r , as i l l u s t r a t e d i n F i g u r e 4 ( 1 1 ) . Two r e s u l t s o f p r a c t i c a l i n t e r e s t f o l l o w from t h e above reasoning. F i r s t l y , t h e s m a l l e r t h e number o f v i a b l e n u c l e i t h e l a r g e r w i l l be t h e average c r y s t a l l i t e s i z e i n t h e f i n a l c r o p o f c r y s t a l s . Secondly,


2.

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17


β;
' NaA10 ..3., o ( % ; .400H 0 .92 3. 0 8 ° 4 > NaA10 .,5. ,400H 0 o . ,78 3. 2 2 ° 4 ' NaA10 ..7.,5(Na ,400H 0 S i ,73 3. 2 7 V 2

3ι

2

2

0 ( N a

2

0

f o r Growth R a t e s on

Ε

H

S i

2

1 .53 .

49.4

H

S i

2

1 .78 .

51.5

H

S i

2

2,.20

59.0

2

2..54

65.3

H

Mineralisers Water and h y d r o x y l i o n a r e t h e c l a s s i c m i n e r a l i s e r s i n h y d r o t h e r m a l s y n t h e s i s , f i r s t l y because aqueous a l k a l i d i s s o l v e s amphoteric o x i d e s and so promotes m o b i l i t y and m i x i n g o f m o l e c u l a r and i o n i c s p e c i e s as a p r e - r e q u i s i t e f o r r e a c t i o n . A second v i t a l r o l e i s t h a t of m o l e c u l a r water which (see below) s t a b i l i s e s aluminous z e o l i t e s by f i l l i n g c h a n n e l s and c a v i t i e s . T h i s r o l e can be s h a r e d o r t a k e n o v e r by o r g a n i c m o l e c u l e s ( e . g . i n p o r o s i l s , s i l i c a - r i c h z e o l i t e s o r A l P O ' s ) , and by s a l t s (e.g. i n s c a p o l i t e s , s o d a l i t e and c a n c r i n i t e ) . S t a b i l i s i n g Porous C r y s t a l s ;

Host-Guest S o l u t i o n s

The z e o l i t e , p o r o s i l o r A1PO i s t h e " h o s t " and t h e z e o l i t i c component t h e "guest". The h o s t - g u e s t complex i s a s o l u t i o n , amenable a t e q u i l i b r i u m t o s o l u t i o n thermodynamics (J_4 ,J_5 ), and t h e h o s t - g u e s t r e l a t i o n s h i p t h e r e b y d e s c r i b e d i s one o f t h e most i m p o r t a n t i n t h e c h e m i s t r y o f porous c r y s t a l s because, w i t h o u t t h e z e o l i t i c g u e s t ,


2.

19


BARRER

m i c r o p o r o u s c r y s t a l s c o u l d not be s y n t h e s i s e d . The g u e s t i s r e q u i r e d o n l y d u r i n g s y n t h e s i s and i s t h e r e a f t e r removed b e f o r e t h e porous c r y s t a l s a r e put t o use. F o r p o r o s i l s t h e n a t u r a l c h o i c e o f t h e gram m o l e c u l e (or mole) i n h o s t - g u e s t s o l u t i o n s i s S i O ^ , and f o r c o m p a r a b i l i t y among a l l p o r o s i l s and z e o l i t e s t h e mole w i l l t h e r e f o r e be t a k e n as M ^ A l ^ S i ^ ^ _ w h e r e 0 < χ < 0.5 and M i s an e q u i v a l e n t o f c a t i o n s . χ

For AlPO's t h e n a t u r a l c h o i c e o f mole i s AlPO^. F o r a two component s o l u t i o n a t c o n s t a n t temperature t h e Gibbs-Duhem e q u a t i o n i s


n

H

d y

+

H

n

G

d y

=

G

V

d

P

( 4 )

where n and n denote numbers o f moles o f h o s t and q u e s t i n t h e s o l i d s o l u t i o n o f volume V a t t o t a l p r e s s u r e P. y and y are the r e s p e c t i v e c h e m i c a l p o t e n t i a l s o f h o s t and g u e s t i n t h e s o l u t i o n . E q u a t i o n 4 can be r e - a r r a n g e d , w i t h dy_ = RTdlna, and i n t e g r a t e d t o G give fl

Q

fl

Q

The a c t i v i t y , a, o f t h e g u e s t i s z e r o when t h e h o s t i s g u e s t - f r e e and has c h e m i c a l p o t e n t i a l i s t h u s t h e change i n c h e m i c a l p o t e n t i a l o f t h e h o s t due t o t h e i n c l u s i o n o f t h e 9^||^· ® i f r a c t i o n a l s a t u r a t i o n o f t h e h o s t by t h e g u e s t , V = n /n and η i | £ h e number o f moles o f g u e s t a t s a t u r a t i o n o f t h e h o s t , so that n Θ = n . / ^ volume o f t h e h o s t because t h e r i g i d h o s t framework r e n d e r s V v i r t u a l l y independent o f n . The t o t a l p r e s s u r e , P, i s o f t e n t h e vapour p r e s s u r e , p, o f t h e g u e s t , and t h e a c t i v i t y , a, can sometimes be r e p l a c e d by t h e r e l a t i v e vapour p r e s s u r e , x, o f t h e g u e s t , so t h a t s

t

n

e

Q

a

V

Q

Q

=

v

n

H

s

t

h

e

m

o

l

a

r

H

Q

Δ μ

Η

=

V

H

P

"

R T V

0

X

d

( 6 )

£*< / ) x

T h i s i n t e g r a l can be e v a l u a t e d g r a p h i c a l l y from t h e s o r p t i o n i s o t h e r m o f t h e g u e s t p l o t t e d as Θ/χ a g a i n s t x. From E q u a t i o n 6, Δ μ ^ ί ε seen t o c o n s i s t o f two p a r t s : a p o s i t i v e term Δμ = V P ; and a n e g a t i v e term Δ μ

2

= -RTV

Ί

jf^ (0/x)dx.

These two

parts w i l l

π

be e v a l u a t e d i n t u r n .

Table II g i v e s values of V and o f V P a t 100°C where Ρ = 1 atm. V i s based upon t h e number o f S i + A l atoms per 1000 A of z e o l i t e (16). A t 100°C f o r water Δμ = V P i s insignificant for a l l zeolites. However, i n a c l o s e d system above 100°C,P-p f o r water r i s e s rapidly. The s m a l l e s t and l a r g e s t m o l e c u l a r volumes a r e t h o s e o f b i k i t a i t e and f a u j a s i t e r e s p e c t i v e l y , and i n T a b l e I I I v a l u e s o f V p a r e t h e r e f o r e g i v e n f o r t h e s e two z e o l i t e s up t o 365°C n e g l e c t i n g any s m a l l change i n V w i t h t e m p e r a t u r e . V^p f o r a l l o t h e r z e o l i t e s l i e s between t h e v a l u e s f o r t h e s e two. A c c o r d i n g l y i n the temperature range most r e l e v a n t f o r z e o l i t e s y n t h e s i s , Δμ^ , i f water i s t h e g u e s t , w i l l not be l a r g e . However, i f , b y u s i n g an i n e r t p i s t o n f l u i d , one moves i n t o t h e k i l o b a r p r e s s u r e range t h e term V P w i l l have a major e f f e c t on Δμ . H

3

)

H

fl

fl


20

ZEOLITE SYNTHESIS

TABLE I I .

M o l a r Volumes, V , o f Z e o l i t e s and V P (100°C f o r Water a s G u e s t )

v

Zeolite


cm Bikitaite Li-ABW Analcime Ferrierite Dachiardite Mordenite Sodalite Hydrate Heulandite LTL Mazzite

V P

mol

J mol

29.8 31.7 32.4 34.0 34.8 35.0

3.03 3.21 3.28 3.45 3.53 3.55

35.0 35.4 36.7 37.4

3.55 3.59 3.72 3.79

v

Zeolite

H

H

a t one

cm

1

Merlinoite Phillipsite Erionite Offretite Paulingite Chabazite Gmelinite RHO LTA Faujasite

Atm

V P H

H

mol

37.6 38.1 38.6 38.9 38.9 41.3 41.3 42.1 46.7 47.4

J mol

1

3.82 3.88 3.92 3.94 3.94 4.18 4.18 4.29 4.73 4.83

We c o n s i d e r now t h e term Δ]^ . T a b l e IV g i v e s v a l u e s o f V i n Equation 6 f o r a v a r i e t y of h y d r o p h i l i c z e o l i t e s , according t o the u n i t c e l l c o m p o s i t i o n s g i v e n by M e i e r and O l s o n (JM6 ). To p r o c e e d f u r t h e r a s i m p l e model o f t h e h o s t - g u e s t s o l u t i o n w i l l be employed. A Model f o r H o s t - G u e s t S o l u t i o n s I n t r a c r y s t a l l i n e s o r p t i o n i s n o r m a l l y o f Type 1 i n B r u n a u e r ' s c l a s s i f i c a t i o n (_V7) and i s o t h e r m c o n t o u r s t h e r e f o r e resemble t h o s e a c c o r d i n g t o Langmuir's i s o t h e r m e q u a t i o n . T h i s can d e s c r i b e a c t u a l i s o t h e r m s w e l l enough (_1_8) t o be o f v a l u e i n p r e d i c t i n g , t h r o u g h E q u a t i o n s 5 o r 6, some f e a t u r e s o f z e o l i t e c h e m i s t r y . The maximum v a l u e o f t h e r e l a t i v e p r e s s u r e , x, i s u n i t y and t h i s v a l u e w i l l be c l o s e l y a p p r o a c h e d f o r t h e aqueous phase where the guest i s water. Then f o r Langmuir's i s o t h e r m Θ

=

Kx/(1

+ Kx)

(7)

Isotherm c o n t o u r s i n F i g u r e 5 show how l a r g e Κ must be t o g i v e r e c t a n g u l a r i s o t h e r m s l i k e t h o s e u s u a l l y o b s e r v e d f o r water i n z e o l i t e s and a l s o show some v a l u e s o f Θ a t χ = 1. W i t h E q u a t i o n 7, E q u a t i o n 6 integrates to Δμ

Η

= Δμ

ι

+ Δρ^

= V p + RTVln(1-0) fl

= V„p π

- RTVln(1+TCx)

(8)

The n e g a t i v e term, Δ μ ^ , becomes i n c r e a s i n g l y so t h e l a r g e r t h e v a l u e o f K, and hence t h e n e a r e r t h e v a l u e o f Θ a t χ = 1 i s t o u n i t y and t h e more r e c t a n g u l a r t h e i s o t h e r m .


2.

BARRER

TABLE I I I .

Zeolites: Their Nucleation and Growth V p f o r F a u j a s i t e and B i k i t a i t e Temperatures

i n Water* a t D i f f e r e n t

fl

T°C

p/atm

V p / J mol"


1.413 1.959 2.665 3.565 4.695 6.100 7.811 9.888 12.378 15.334 18.81 22.88 27.59 33.01 39.22 46.28 54.28 63.29 73.41 84.72 97.32 111.32 195.50 1 ,000 5,000

—

1

H

Bikitaite

Faujasite

110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 310 320 365

21

4.29 5.95 8.08 10.82 14.25 18.50 23.71 29.98 35.57 46.54 57.1 64.4 83.8 99.5 119.0 140.4 164.7 192.1 222.8 257.1 295.4 337.9 456.7 3,033 15,165

6.82 9.46 12.87 17.22 22.67 29.94 37.71 47.69 59.76 74.0 90.8 110.5 133.2 159.8 189.4 223.5 262.0 305.5 352.5 409.1 469.9 537.5 726.5 4,828 24,140

* Vapour p r e s s u r e s o f water from NBS/NRC Steam T a b l e s . Hemisphere P u b l i s h i n g Co., 1984. C o n v e r t e d from b a r s t o atm.

TABLE IV.

M o l e s o f Water p e r Mole o f Z e o l i t e (V)

Examples

Examples 0.33 0.40

Analcime, b i k i t a i t e Natrolite

0. 2 0, 75

0.50

Li-ABW, d a c h i a r d i t e f e r r i e r i t e , mordenite, yugawaralite

0, 77 0, 80 0, 91

5 8

°- 3 0.60 0.62 0.66

5

L

T

7 2

L

Thomsonite Brewsterite E p i s t i l b i t e , heulandite laumontite, s o d a l i t e hydrate

1

0.92 1.00 1.04 1.11 c 1.25 1

2

EAB Erionite, merlinoite, phillipsite Mazzite, o f f r e t i t e Edingtonite RHO Levynite Gismondine, g m e l i n i t e Paulingite Chabazite LTA Faujasite



22

ZEOLITE SYNTHESIS

X F i g u r e 5.

Isotherms o f Θ = Kx/(1 + K x ) , showing Θ a t χ = 1 and t h e changes i n c u r v a t u r e w i t h i n c r e a s i n g v a l u e s o f K.

T a b l e V g i v e s v a l u e s o f -à\x^/RTV f o r d i f f e r e n t values of Κ or of Θ a t χ = 1, a c c o r d i n g t o E q u a t i o n 8. F o r l a r g e Κ, Δμ dominates t h i s e q u a t i o n and t h e z e o l i t e i s t h e r e f o r e much s t a b i l i s e d r e l a t i v e to i t s guest-free s t a t e . Thus f o r f a u j a s i t e a t 100°C, Δμ = V ρ = 0.005 k J mol ( T a b l e I I ) and V = 1.25 ( T a b l e I V ) . The f i n a l two columns i n T a b l e V g i v e Δμ a t 100°C f o r d i f f e r e n t Κ when V = 1.25 TABLE V.

Κ

Δ μ i n k J mol a t χ = 1 f o r d i f f e r e n t Values o f Κ using t h e Langmuir Model 2

θ at χ = 1

-Δμ

-Δμ /κτν 2

V = 1 2 3 5 10 15 25 50 100 500 1000

0.500 0.6é 0.750 0.833 0.90909 0.93750 0.96154 0.98039 0.99010 0.99800 0.99900

0.6932 1.0986 1.3863 1.7918 2.3979 2.7726 3.2581 3.9318 4.6151 6.2166 6.9088

1.A

2.,687 4..259 5.,374 6..946 9..295 10..75 12,.63 15..24 17..89 24..10 26..78

a t 100°C V = 0.33 0.717 1 . 136 1 .433 1 .853 2.479 2.867 3.368 4.064 4.771 6.427 7.141

and 0.33, and so f o r t h e extremes among t h e v a l u e s o f V f o r z e o l i t e s (Table I V ) . One may c o n c l u d e from F i g u r e 5 and T a b l e s IV and V t h a t : (i) Because as z e o l i t e s become r i c h e r and r i c h e r i n s i l i c a t h e y a r e known t o become l e s s h y d r o p h i l i c and e v e n t u a l l y h y d r o p h o b i c , water w i l l i n c r e a s i n g l y l o s e i t s e f f e c t i v e n e s s as a s t a b i l i s e r because as Κ d e c l i n e s so does - Δ μ .


2.


BARRER

23

(ii) F o r s i l i c a - r i c h z e o l i t e s , p o r o s i l s and AlPO's water must t h e r e f o r e i n p a r t o r w h o l l y be r e p l a c e d by o t h e r g u e s t m o l e c u l e s which a r e more s t r o n g l y s o r b e d g i v i n g more r e c t a n g u l a r i s o t h e r m s t h a n water. These a r e u s u a l l y m u l t i a t o m i c o r g a n i c m o l e c u l e s . They o f t e n c o n t a i n a b a s i c group t o improve t h e i r s o l u b i l i t y i n water and h e l p m a i n t a i n a h i g h pH. (iii) The s t a b i l i s i n g r o l e i s a g e n e r a l one e x e r c i s e d by any g u e s t m o l e c u l e s a b l e t o e n t e r t h e porous c r y s t a l o r be i n c o r p o r a t e d d u r i n g i t s growth. T h i s a p p l i e s e q u a l l y t o permanent gases a t p r e s s u r e s h i g h enough t o g i v e s i g n i f i c a n t u p t a k e s . Thus c r y s t a l l i s a t i o n o f t h e p o r o s i l m e l a n o p h l o g i t e was e f f e c t e d f o r t h e f i r s t time a t 170°C under a p r e s s u r e o f 150 b a r o f C H (J_9 ). T a b l e V I g i v e s some o f t h e g u e s t m o l e c u l e s used t o a i d f o r m a t i o n o f ZSM-5 {20) and s e r v e s t o i l l u s t r a t e the generality of the e f f e c t .


4

TABLE V I .

Some G u e s t M o l e c u l e s

NPr OH NEt OH NPr NH à H NH OHC Β OHC,H*NH^ 3 6 2 4

NH +C H OH Glycerine n-CHJH (n-Cl)NH NH ( & C ) ÎNH 3

fa

0

The

2

5

Triethylenetetramine Diethylenetriamine / \ θ4 )* \ / Hexanediol Propylamine

HH

C(CH OH) Dipropylenetriamine

C H OH 2

u s e d t o a i d S y n t h e s i s o f ZSM-5 (20)

5

E f f e c t o f Temperature upon H o s t

Stabilisation

Temperature i n f l u e n c e s t h e vapour p r e s s u r e , p, and hence changes Δμ = V p as a l r e a d y c o n s i d e r e d ( T a b l e I I I ) . F o r t h e Langmuir moael Δ μ = - R T V i n M + Kx) i s a l s o dependent upon t e m p e r a t u r e . The e f f e c t i s r e a d i l y e v a l u a t e d assuming t h a t V i s independent o f Τ and that fl

2

Κ

=

K

q

exp(-AH/RT)

(9)

where Κ i s a c o n s t a n t and ΔΗ i s t h e h e a t o f w e t t i n g o f t h e w a t e r f r e e z e o l i t e by l i q u i d water (x = 1 ) . Relevant heats o f wetting f o r s e v e r a l z e o l i t e s i n d i f f e r e n t c a t i o n i c forms f e l l i n t h e range -6.8 t o -3.2 k c a l mol of l i q u i d water (2J_). C a l c u l a t i o n s o f Δη + Δμ a r e here made f o r ΔΗ =.,-6.0, -4.0 and -2.0 k c a l mol ( i . e . -Ί25.1, -16.7 and 8.37 k J mol ) t a k i n g Κ a t 100°C t o be 100, and a l s o 10. The r e s u l t s i n T a b l e V I I show t h a t Δμ^ becomes l e s s n e g a t i v e as t e m p e r a t u r e increases. F o r a g i v e n Κ t h e change i n Δμ i s l a r g e r t h e g r e a t e r t h e heat o f w e t t i n g . The c a l c u l a t i o n s l e a d one t o e x p e c t t h a t :


24

ZEOLITE SYNTHESIS

(i) As Τ i n c r e a s e s z e o l i t e s w i l l t e n d t o be r e p l a c e d a t e q u i l i b r i u m by denser phases, because, i n t h e r e l a t i o n Δμ = (μ - μ ), μ of the z e o l i t e s t a b i l i s e d by water may no l o n g e r be l e s s a t h i g h Τ t h a n t h e μ f o r dense phases ( f e l s p a r s , non-porous f e l s p a t h o i d s , s i l i c a t e s o r oxides). C o n v e r s e l y , as Τ i s lowered so t h a t Δμ^ becomes i n c r e a s i n g l y n e g a t i v e , z e o l i t e s w i l l i n c r e a s i n g l y r e p l a c e dense phases Η (ii) The more porous t h e h o s t t h e g r e a t e r one e x p e c t s i t s c h e m i c a l potential, μ when g u e s t - f r e e , t o become. The c r y s t a l l i s a t i o n o f t h e most porous z e o l i t e s w i l l t h e r e f o r e r e q u i r e u n u s u a l l y b i g n e g a t i v e v a l u e s o f Δ μ t o compensate f o r t h e l a r g e μ · Δμ i s seen i n T a b l e V I I t o become more n e g a t i v e t h e lower the t e m p e r a t u r e so t h a t t h e most porous c r y s t a l s w h i c h need t h e g r e a t e s t s t a b i l i s a t i o n s h o u l d form b e s t towards t h e low end o f t h e t e m p e r a t u r e range f o r z e o l i t e formation. ( μ

Zeolites: Their Nucleation and Growth - ACS Symposium Series (ACS

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