Some Problems of Zeolite Crystallization - Advances in Chemistry

2 Some Problems of Zeolite Crystallization

Downloaded by PENNSYLVANIA STATE UNIV on July 3, 2012 | http://pubs.acs.org Publication Date: August 1, 1974 | doi: 10.1021/ba-1971-0101.ch002

S. P. ZHDANOV I. V. Grebenshchikov Institute of Silicate Chemistry, Academy of Sciences USSR, Nab.Makarova, 2, Leningrad, V-164, USSR In some zeolite crystallization in heterogenous aluminosilicate systems, temperature is mainly a kinetic factor whereas the nature and composition of crystals are determined by concentrations and correlations of components. Studies of their chemical structure and the peculiarities of crystallization kinetics show that a solubility equilibrium exists between the solid and liquid phases of gels. Gel crystallization on heating is preceded by increased component concentrations in the liquid phase, which determines the composition of the zeolite crystals formed. Decreases in liquid component concentrations are compensated by dissolution of the solid phase of gels. This explains the linear rate of crystal growth during the first part of the crystallization process. Crystal size distribution for zeolite A and the linear rate of crystal growth indicate the autocatalysis of zeolite crystallization. ^ V w i n g to systematic studies b y B a r r e r i n zeolite synthesis w i t h the use of the a u t o c l a v e t e c h n i q u e (2-12),

m o r e t h a n 50 species of zeolites

d i f f e r i n g i n c o m p o s i t i o n a n d structure w e r e o b t a i n e d . T e m p e r a t u r e b o u n daries of z e o l i t e c r y s t a l l i z a t i o n w e r e d e t e r m i n e d o n t°-n Si0 /Al 0 ). 2

2

3

diagrams ( n

=

M a n y N a - , K - , a n d N a , K - z e o l i t e s , i n c l u d i n g zeolites A , X ,

a n d Y , w e r e s y n t h e s i z e d b y B r e c k a n d others (13-17, 22)

at l o w t e m p e r a -

tures w i t h o u t the a u t o c l a v e . C o n s i d e r a b l e progress a c h i e v e d i n the l o w - t e m p e r a t u r e synthesis of zeolites is c o n n e c t e d w i t h the use of h i g h l y r e a c t i v e materials i n the c o l l o i d - d i s p e r s e d state a l u m i n a gels, etc. ).

( aluminosilica

gels, a q u e o u s s i l i c a sols,

silica,

T h e use of these materials i n t h e m a j o r i t y of cases

facilitates synthesis, b u t at the s a m e t i m e it i n v o l v e s c e r t a i n difficulties. I n m a n y s u c h cases, not o n l y the k i n e t i c s of the process b u t also the a c t u a l final results a p p e a r to be d e p e n d e n t o n the c o m p l i c a t e d c h e m i c a l b e h a v i o r of these materials d u r i n g t h e i r p r e p a r a t i o n , i n the course of 20 In Molecular Sieve Zeolites-I; Flanigen, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.

2.

ZHDANOV

Problems

of Zeolite

21

Crystallization

c r y s t a l l i z a t i o n , or o n s t i r r i n g . I n i t i a l m a t e r i a l s c a n s i g n i f i c a n t l y influence the results of c r y s t a l l i z a t i o n at e l e v a t e d t e m p e r a t u r e s u n d e r c o n d i t i o n s of t h e a u t o c l a v e t e c h n i q u e (1, 8,19,

20, 23, 33).

T h e s e d e p e n d e n c e s as yet

are not u n d e r s t o o d , b u t m u s t be e x p l a i n e d i n terms of the p e c u l i a r i t i e s of the m e c h a n i s m a n d k i n e t i c s of zeolite c r y s t a l l i z a t i o n . Studies o n the m e c h a n i s m of z e o l i t e c r y s t a l l i z a t i o n h a v e

received

extensive a t t e n t i o n f r o m m a n y investigators (4, 16, 22, 24, 29, 30, 33, 39).

37,

H o w e v e r , the m a i n questions of t h e m e c h a n i s m of c r y s t a l l i z a t i o n

r e f e r r i n g to k i n e t i c s , s u c h as the autocatalysis of the c r y s t a l l i z a t i o n p r o c -


ess, a l k a l i n i t y effect o n the rate of c r y s t a l l i z a t i o n , the n a t u r e of the i n d u c t i o n p e r i o d , a n d seeding effects, h a v e b e e n insufficiently s t u d i e d .

The

q u e s t i o n of the role of s o l i d a n d l i q u i d phases i n the f o r m a t i o n of zeolite crystals i n the heterogenous a l u m i n o s i l i c a t e systems also is s t i l l b e i n g d i s cussed

(16,22,25,30,31,37,39).

D u r i n g recent years, n e w d a t a h a v e b e e n o b t a i n e d a n d p u b l i s h e d o n t h e influence of t e m p e r a t u r e a n d c o m p o s i t i o n of t h e i n i t i a l m i x t u r e s as w e l l as s e e d i n g a n d a d d i t i o n of c o m p l e x i n g reagents u p o n the k i n e t i c s a n d final results of the process of zeolite c r y s t a l l i z a t i o n (16,20,

23, 31, 32,

33,34,35,36,37,39,41). A c c o r d i n g to Refs. 6,19,

20, 23, 32, a n d 35, the t e m p e r a t u r e of zeolite

c r y s t a l l i z a t i o n c a n b e l o w e r e d s i g n i f i c a n t l y b y i n c r e a s i n g the a l k a l i n i t y of i n i t i a l m i x t u r e s . H o w e v e r , increases i n t e m p e r a t u r e a n d a l k a l i n i t y e q u a l l y influence the c o m p o n e n t

concentrations i n the l i q u i d phases of

hetero-

genous m i x t u r e s . T h e r e f o r e , the p r o b a b i l i t y of f o r m a t i o n of the zeolites i n s u c h cases is d e t e r m i n e d b y t h e c o n c e n t r a t i o n of c o m p o n e n t s t h a n b y the t e m p e r a t u r e .

rather

A t the constant t e m p e r a t u r e , the n a t u r e a n d

c o m p o s i t i o n of crystals are d e t e r m i n e d m a i n l y b y c h e m i c a l

factors—i.e.,

b y concentrations a n d correlations of components i n the systems (27, 35, 37, 39, 41)

a n d m o r e d i r e c t l y i n the l i q u i d phase (39).

31,

Therefore,

investigations i n t o the c h e m i s t r y a n d k i n e t i c s of zeolite c r y s t a l l i z a t i o n are i m p o r t a n t i n s t u d y i n g the f o r m a t i o n a n d g r o w t h of zeolite crystals i n heterogenous a l u m i n o s i l i c a t e systems.

U n f o r t u n a t e l y , the c o m p o s i t i o n of

the l i q u i d phase of s u c h a l u m i n o s i l i c a t e systems r e m a i n s insufficiently s t u d i e d i n c o n n e c t i o n w i t h zeolite c r y s t a l l i z a t i o n .

Chemical

Structure

of Alkali Aluminosilica

Gels

A l k a l i a l u m i n o s i l i c a gels f r o m w h i c h zeolites u s u a l l y are c r y s t a l l i z e d represent heterogenous

c o l l o i d a l systems consisting of l i q u i d a n d s o l i d

phases w h i c h s t r o n g l y differ i n t h e i r c h e m i c a l compositions.

I n those

cases w h e n a l u m i n o s i l i c a gels are o b t a i n e d u n d e r e q u a l c o n d i t i o n s a n d f r o m the same i n i t i a l m a t e r i a l s — f o r e x a m p l e , f r o m solutions of t h e s i l i cates a n d a l u m i n a t e s — t h e results of t h e i r c r y s t a l l i z a t i o n r e p r o d u c e w e l l ,

In Molecular Sieve Zeolites-I; Flanigen, E., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1974.

22

M O L E C U L A R SIEVE ZEOLITES

Table I.

1

Changes in Concentrations and Correlations of Components in Their General Compositions; Characteristics


Concentrations and Correlations in Initial Mixtures

a b c

Gel Samples

Si0 mole/I

V VIII 965 967 963 VII 467 III 966 196 660 104 105

0.133 0.184 0.239 0.270 0.620 0.710 0.770 0.470 0.930 1.340 1.210 2.140 2.530

2

Na-Al Si

SiOo Al 0

in Gel Skeleton

0.33 0.33 0.33 1.0 1.0 2.0 2.0 5.0 5.0 5.0 36.8 28.0 25.6

2.2 2.6 2.8 2.6 3.0 2.3 2.8 2.5 3.4 3.5 6.6

2

25.0 12.5 5.8 13.7 2.0 3.5 2.0 9.2 3.1 1.2 0.9 0.9 0.7* a

3

— 8.9

[ (Na + K ) - A l ] / S i . C h = K,Na-chabazite. E r = K,Na-erionite.

b e i n g d e t e r m i n e d o n l y b y g e l c o m p o s i t i o n at a g i v e n t e m p e r a t u r e 37-41).

(31,

C h a n g e s i n the c o n d i t i o n s of g e l p r e p a r a t i o n — t h a t is, the use of

s i l i c a sols i n s t e a d of the silicates, a g i n g of the o b t a i n e d gels at r o o m t e m p e r a t u r e , or s t i r r i n g — c a n l e a d to different results. T h e n a t u r e of c r y s t a l l i z i n g zeolites a n d k i n e t i c s of c r y s t a l l i z a t i o n i n s u c h cases are n o t d e t e r m i n e d o n l y b y the c o m m o n c o m p o s i t i o n of the system a n d b y t e m p e r a t u r e . T h e y p r o b a b l y d e p e n d o n the differences i n concentrations a n d correlations of c o m p o n e n t s i n different phases of gels, o n t h e i r d i s t r i b u t i o n a m o n g the phases—i.e.,

o n the c h e m i c a l structure of gels.

Therefore,

investigations into the c h e m i c a l structure of a l u m i n o s i l i c a gels i n terms of t h e i r c r y s t a l l i z a t i o n m a y a p p e a r u s e f u l for u n d e r s t a n d i n g the p e c u l i arities of f o r m a t i o n a n d g r o w t h of z e o l i t e crystals i n the

heterogenous

a l u m i n o s i l i c a t e systems. A n a l y s e s of the s o l i d phases of m a n y samples of a l u m i n o s i l i c a gels s h o w e d t h a t the S i / A l r a t i o i n the s o l i d phase of gels ( i n the g e l skeleton) a l w a y s exceeds 1, whereas the N a / A l r a t i o is close to 1; i.e., these r e l a tions i n the g e l skeleton are the same as those i n the c r y s t a l lattice of zeolites. It was c o n c l u d e d f r o m these results (41 ) that A l i n the skeleton of a l k a l i a l u m i n o s i l i c a gels is c o o r d i n a t e d f o u r - f o l d w i t h i n the

common

( S i , A l , 0 ) - f r a m e w o r k , whereas a l k a l i n e cations compensate excess n e g a t i v e charges of a l u m i n u m - o x y g e n t e t r a h e d r a . A n a l o g o u s conclusions w e r e d r a w n b y t h e authors (21)

w h o o b t a i n e d the same correlations of c o m -


2.

ZHDANOV

Problems

of Zeolite

23

Crystallization

in the L i q u i d and Solid Phases of Aluminosilica Gels with Changes of Zeolite Crystals Obtained Concentrations in Liquid


Al

(OHTi, mole/l 0.466 0.800 1.096 0.218 0.570 0.066 0.064 0.061 0.034 0.020 0.007 0.024 0.028

and Correlations Phase of Gels

Zeolite

Si0 , mole/l

[Al] [Si] X 10*

Si/Al X 10*

0.030 0.026 0.027 0.054 0.011 0.062 0.023 0.263 0.530 0.548 1.000 2.030 1.980

1.40 2.08 2.96 1.17 0.63 0.41 0.15 1.57 1.80 1.09 0.70 4.88 5.55

64 33 24 25 19 940 356 4300 15600 27400 143000 85000 71000

2

Crystals

SiOi AW

Type

3

A A A A A A A X X X Y Ch» Er

1.8

— 1.8

— 1.9

— 2.0

— 2.6 3.0 5.1 5.1 7.2

c

ponents i n the s o l i d phase for m a n y samples of a l u m i n o s i l i c a gels

of

different compositions. C o n s e q u e n t l y , the ( S i , A l , 0 ) - f r a m e w o r k of a l u m i n o s i l i c a gels c a n b e s i m i l a r to t h a t of zeolites, at least i n r e g a r d to the simplest s t r u c t u r a l elements f o r m i n g the n e t w o r k .

A p p a r e n t l y , the g e l skeleton consists of

m o r e c o m p l i c a t e d s t r u c t u r a l elements s u c h as the single a n d d o u b l e 4and 6-membered

rings and their combinations.

This can be

suggested,

for instance, f r o m the d a t a g i v e n b e l o w o n t h e compositions of e r i o n i t e - l i k e crystals ( p e r h a p s erionite +

offretite)

a n d the a l u m i n o s i l i c a t e skeleton

of gels y i e l d i n g these crystals. Na 0

K 0

A1 0

0.71 0.27 0.29

0.28 0.73 0.71

1.0 1.0 1.0

2

Initial mixture G e l skeleton Crystals

2

2

3

Si0

2

26 8.9 7.2

T h i s m a y i n d i c a t e that i n b o t h the ( Si,ΑΙ,Ο ) - f r a m e w o r k of K , N a - g e l s a n d c r y s t a l l a t t i c e of erionite, s i m i l a r s t r u c t u r a l elements are present, f o r m a t i o n of w h i c h r e q u i r e s the p a r t i c i p a t i o n of p o t a s s i u m ions.

Potas

s i u m ions w i l l p l a y the d o m i n a n t r o l e i n the erionite structure a n d i n the g e l skeleton o n p o t a s s i u m deficit i n the i n i t i a l m i x t u r e . S u c h s t r u c t u r a l elements c a n b e b u i l t f r o m 6 - m e m b e r e d

r i n g s of t e t r a h e d r a l i k e

c o l u m n s of erionite structure. P o t a s s i u m ions are b l o c k e d i n s i d e the ele ments of erionite s t r u c t u r e a n d o w i n g to this are n o n e x c h a n g e a b l e 40).


(37,

24


1

A n a l y t i c a l d a t a o n t h e c o m p o s i t i o n s of s o l i d a n d l i q u i d phases for different a l u m i n o s i l i c a gels are g i v e n i n T a b l e I. T h e d a t a g i v e n i n T a b l e I s h o w t h a t the d i s t r i b u t i o n of

components

b e t w e e n the l i q u i d a n d s o l i d phases of a l u m i n o s i l i c a gels appears to b e a c o m p l i c a t e d f u n c t i o n of

the initial

solution compositions.

A

general

t e n d e n c y exists for the increase i n S i / A l r a t i o i n b o t h t h e s o l i d phases of gels a n d z e o l i t e crystals w i t h i n c r e a s i n g S i 0 / A l 0 2

2

3

ratio ( =

n ) i n the

i n i t i a l m i x t u r e s . A t e q u a l n, the S i / A l r a t i o i n the s o l i d phase of

gels

increases w i t h d e c r e a s i n g c o n c e n t r a t i o n of t h e excess a l k a l i ( i n respect to A1 0 Downloaded by PENNSYLVANIA STATE UNIV on July 3, 2012 | http://pubs.acs.org Publication Date: August 1, 1974 | doi: 10.1021/ba-1971-0101.ch002

2

3

c o n t e n t ) ; i.e., w i t h decrease of N a - A l / S i 0

2

ratio. S u c h dependence

w a s s h o w n for zeolite crystals i n Ref. 37. T h i s d e p e n d e n c e c a n b e e x p l a i n e d i n terms of t h e p o l y c o n d e n s a t i o n m e c h a n i s m of f o r m a t i o n of a l u m i n o s i l i c a g e l skeleton f r o m h y d r o x o a l u m i n a t e ions a n d ( O H ) . 4

O T

Si(0")

m

Al(OH) " 4

silicate ions w i t h different

degrees of h y d r o x y l a t i o n . A t e q u a l correlations a n d concentrations of the silicate a n d a l u m i n a t e ions i n solutions, t h e a m o u n t of silicate ions p a r t i c i p a t i n g i n the r e a c t i o n of c o n d e n s a t i o n s h o u l d increase w i t h the g r o w t h of t h e i r h y d r o x y l a t i o n degree. T h e degree of h y d r o x y l a t i o n of the silicate ions i n a l k a l i n e solutions is d e t e r m i n e d b y E q u i l i b r i u m 1 a n d therefore it m u s t increase w i t h decrease i n N a O H / S i 0

2

ratio ( =

m)

and with

dilution. Si(OH)

4

+ m ( N a + + O H " ) ( O H ) _ 4

m

Si(0~Na ) +

m

+ mH 0 2

(1)

Increase i n the S i / A l r a t i o i n the crystals of s y n t h e t i c faujasites (37, 41)

a n d analcimes (35)

w i t h d e c r e a s i n g a l k a l i n i t y of i n i t i a l gels a n d

m i x t u r e s c a n b e e x p l a i n e d o n t h e basis of these considerations. G r o w t h of S i / A l r a t i o w i t h i n c r e a s i n g w a t e r content w a s o b s e r v e d i n R e f . 37 for zeolite crystals o b t a i n e d f r o m u s u a l N a - a n d K - a l u m i n o s i l i c a gels a n d i n R e f . 27 f o r zeolites o b t a i n e d f r o m gels w i t h

phosphate

additions. G e n e r a l d e p e n d e n c e s o b s e r v e d for t h e c h a n g e i n S i / A l r a t i o i n b o t h the a m o r p h o u s ( S i , A l , 0 ) - f r a m e w o r k of g e l skeleton a n d zeolite f r a m e works

(Table I)

suggest the same m e c h a n i s m of t h e i r f o r m a t i o n i n

solution. C o r r e l a t i o n s a n d concentrations of c o m p o n e n t s i n t h e l i q u i d phase of gels a p p e a r t o b e q u i t e different f r o m those i n t h e s o l i d phase a n d i n i t i a l m i x t u r e s . I t is essential that i n a l l cases b o t h the silicate a n d a l u m i n a t e ions are present i n the l i q u i d phase of gels, a n d t h e p r o d u c t of t h e i r c o n centrations is c o m p a r a t i v e l y constant at significant changes i n the absolute values f o r i o n concentrations of e a c h t y p e . T h i s c a n b e a n i n d i c a t i o n of the existence of a n e q u i l i b r i u m s i m i l a r to t h a t of d i s s o c i a t i o n or s o l u b i l i t y .


2.

ZHDANOV

Problems

of Zeolite

amorphous aluminosilicate

25

Crystallization a l u m i n a t e ions + silicate ions

(solid phase)

(solution) a l u m i n o s i l i c a t e ions (solution)

T h o u g h systems of this t y p e cannot b e r e g a r d e d as t h e r m o d y n a m i c a l l y true e q u i l i b r i u m states, the existence of q u a s i - e q u i l i b r i a b e t w e e n the s o l i d a n d l i q u i d phases of a l u m i n o s i l i c a gels is c o n f i r m e d b y g o o d r e p r o d u c i b i l i t y of t h e compositions of t h e i r s o l i d a n d l i q u i d phases i n Downloaded by PENNSYLVANIA STATE UNIV on July 3, 2012 | http://pubs.acs.org Publication Date: August 1, 1974 | doi: 10.1021/ba-1971-0101.ch002

r e p e a t e d syntheses. I n the case of gels p r e p a r e d f r o m solutions of the a l u m i n a t e s a n d s i l i c a sols, the concentrations a n d correlations of c o m p o n e n t s i n the l i q u i d phase a p p e a r to b e essentially different, as c o m p a r e d w i t h gels of the same compositions o b t a i n e d f r o m solutions of the silicates a n d a l u m i n a t e s (Table II). Table II. Gel samples 105 105 237 237

from from from from

silicate sol silicate sol

Concentrations in the L i q u i d Phase, M o l e / L Na 0 2

0.72 0.86 1.42 1.73

K0

Al 0

Si0

0.21 0.25

0.018 0.020 0.061 0.034

2.12 0.05 2.42 0.04

2

— —

2

3

2

Si0 /Al 0* 2

2

120 2.6 40 1.2

T h e compositions of l i q u i d phases w e r e a n a l y z e d at once after the g e l f o r m a t i o n . I n b o t h cases, the concentrations of s i l i c a i n t h e l i q u i d phases of gels o b t a i n e d f r o m s i l i c a sols a p p e a r e d to be v e r y l o w , a n d c o n s e q u e n t l y the S i 0

2

content i n the skeleton m u s t be h i g h e r . A l l c o l

l o i d a l s i l i c a i n s u c h gels is present i n t h e i r skeleton a n d is not f o u n d a n a l y t i c a l l y i n the l i q u i d phase. T h e skeleton of gels o b t a i n e d f r o m sols, u n l i k e the gels p r e p a r e d f r o m the silicate solutions, does not consist of c o n t i n u o u s ( S i , A l , 0 ) - f r a m e w o r k o n l y ; i t m u s t c o n t a i n i n c l u s i o n s of S i 0 c o l l o i d a l particles u n s t a b l e i n a l k a l i m e d i a .

2

Therefore, no e q u i l i b r i u m

exists b e t w e e n the s o l i d a n d l i q u i d phases of s u c h gels after t h e i r p r e p a r a t i o n . Differences i n c h e m i c a l structure of gels o b t a i n e d f r o m the silicate solutions a n d f r o m s i l i c a sols n a t u r a l l y s h o u l d affect t h e b e h a v i o r of gels d u r i n g t h e i r c r y s t a l l i z a t i o n a n d the final results as w e l l . T h u s , studies o n the c h e m i c a l structure of a l k a l i a l u m i n o s i l i c a gels u t i l i z e d i n zeolite synthesis r e v e a l a c o m p l i c a t e d d e p e n d e n c e i n t h e d i s t r i b u t i o n of c o m p o n e n t s b e t w e e n the s o l i d a n d l i q u i d phases. A t the same t i m e , i n the case of gels p r e p a r e d f r o m the h o m o g e n o u s

solutions, the

s t r u c t u r a l elements of d i s o r d e r e d ( Si,ΑΙ,Ο ) - n e t w o r k i n the g e l skeleton a n d those of the r e g u l a r ( S i , A l , 0 ) - f r a m e w o r k s i n zeolites p r o b a b l y are


26


similar.

T h e d i s t r i b u t i o n of c o m p o n e n t s

phases of gels d e p e n d s

1

b e t w e e n the l i q u i d a n d s o l i d

o n the m e t h o d of g e l p r e p a r a t i o n a n d i n i t i a l

materials u s e d . T h i s d e p e n d e n c e s h o u l d be m o r e e v i d e n t for heterogenous a l u m i n o silicate m i x t u r e s u t i l i z e d i n zeolite synthesis at e l e v a t e d temperatures. S t u d i e s of the l i q u i d phase c o m p o s i t i o n s of s u c h m i x t u r e s d i r e c t l y u n d e r the synthesis c o n d i t i o n s a p p e a r to b e e x t r e m e l y i m p o r t a n t for t h e e x p l a n a t i o n of the final results of c r y s t a l l i z a t i o n . A s seen f r o m the d a t a of T a b l e I, the n a t u r e a n d c o m p o s i t i o n of


z e o l i t e crystals f o r m e d f r o m a l u m i n o s i l i c a gels are closely c o n n e c t e d w i t h t h e compositions of t h e i r l i q u i d phases. Chemical

Changes in Aluminosilica

Course of Their

Gels in the

Crystallization

D a t a o n the compositions of l i q u i d a n d s o l i d phases of a l u m i n o s i l i c a gels g i v e n i n T a b l e I refer to the e q u i l i b r i u m state at r o o m t e m p e r a t u r e . C r y s t a l l i z a t i o n of gels proceeds,

as a r u l e , o n h e a t i n g ; therefore, i t is

essential to k n o w w h a t c h e m i c a l changes take p l a c e i n gels u n d e r these conditions.

T a b l e I I I shows t h e changes i n c o m p o s i t i o n of t h e l i q u i d

phase of gels after t h e i r h e a t i n g at 9 0 ° C for 4 hours. T h e s e d a t a s h o w that increases i n concentrations of a l l c o m p o n e n t s i n the l i q u i d phase of gels take p l a c e o n h e a t i n g . T h i s m a y b e c o n n e c t e d w i t h the g r o w t h of s o l u b i l i t y of the a l u m i n o s i l i c a t e skeleton w i t h i n c r e a s i n g t e m p e r a t u r e . C o n s e q u e n t l y , increases i n c o n c e n t r a t i o n of t h e silicate and

a l u m i n a t e ions i n the l i q u i d phase p r e c e d e t h e b e g i n n i n g of

gel

crystallization. C h a n g e s i n concentrations i n the l i q u i d phase of N a , K gels d u r i n g e r i o n i t e c r y s t a l l i z a t i o n are i l l u s t r a t e d b y F i g u r e 1, w h e r e the c o n t i n u o u s curves refer to the g e l o b t a i n e d f r o m the m i x t u r e s of silicate a n d a l u m i nate solutions, a n d the d a s h e d c u r v e represents the g e l p r e p a r e d f r o m s i l i c a sol. I n b o t h cases, the t o t a l c o m p o s i t i o n of i n i t i a l m i x t u r e s is the same (7.1 N a 0 -2.8 K 0 • A 1 0 2

2

2

3

· 25.6 S i 0

2

· 433 H 0 ) . 2

P o i n t s of the

axis of ordinates c o r r e s p o n d to the c o m p o n e n t concentrations i n t h e i n i t i a l mixtures. Table III.

Changes in Component

Concentrations

(Mole/L)

in

20°C

Gel Samples

Na 0

AW- '8

SiOt

[Al] [Si] X 10*

965 963 266 196 660

1.20 0.77 0.21 0.91 0.45

0.548 0.285 0.016 0.009 0.004

0.027 0.011 0.008 0.330 1.000

2.96 0.63 0.026 0.60 0.80

2


2.

Probtems of TLeolite

ZHDANOV

Crystallization

27

mot

ρ mol ^1000

9

4. T h e o r e t i c a l l y , at the constant l i n e a r g r o w t h rate of crystals a n d at the constant n u c l e a t i o n rate, η m u s t b e e q u a l to 4. T h e values of η > 4 o b t a i n e d f r o m t h e c a l c u l a t i o n s are c o n n e c t e d w i t h the g r o w t h of the n u c l e a t i o n rate d u r i n g the i n i t i a l a u t o c a t a l y t i c p e r i o d of c r y s t a l l i z a t i o n . S u c h a n increase i n the n u c l e a t i o n rate c a n b e e x p l a i n e d b y p o s t u l a t i n g t h a t n o t o n l y the a l u m i n o s i l i c a t e b l o c k s f o r m e d i n the l i q u i d phase b u t also t h e s i m i l a r b l o c k s w i t h o r d e r e d s t r u c t u r e o c c u r r i n g i n t h e g e l skeleton c a n be t h e n u c l e i of crystals. T h e n u m b e r of s u c h b l o c k s p a s s i n g into s o l u t i o n a n d c o m i n g o u t at the surface of g e l particles for a u n i t of t i m e m u s t increase w i t h i n c r e a s i n g d i s s o l u t i o n rate of the g e l skeleton d u r i n g the a u t o c a t a l y t i c stage of c r y s t a l l i z a t i o n .


34


oooo

ςρ


7 ο

1

EogZ--EogK+ntogt

CaEcutated vatues r y 4.3 κ,* 8 n _4.6

K -.0.0224

4.5

K =O.OOAA

R

n

3

=

2

3

o.io h

ο

0.2

0.4

Q6

0.&

10

Bog time(houfcs) Figure 6.

Kinetics of zeolite crystallization,

log Ζ vs. log t

1. Zeolite A, 100°C 2. Zeolite X, 100°C (Breck and Flanigen) 3. Mordenite, 300°C (Domine and Quobex) Temperature Dependence of the Rate of Crystal G r o w t h of Zeolite A.

T h e rate of zeolite c r y s t a l l i z a t i o n is s i g n i f i c a n t l y d e p e n d e n t o n t e m

perature.

Using

t h e temperature—time

d e p e n d e n c e of c r y s t a l l i z a t i o n ,

B r e c k a n d F l a n i g e n (16) estimated t h e a c t i v a t i o n energy o f t h e c r y s t a l l i z a t i o n process f o r zeolites A , X , a n d Y . T h e values o b t a i n e d a r e 11, 14, a n d 15 k c a l / m o l e , respectively.

Similarly, from kinetic data reported i n

R e f . 20, t h e a c t i v a t i o n e n e r g y o f m o r d e n i t e c r y s t a l l i z a t i o n c a n b e esti m a t e d at a b o u t 11 k c a l / m o l e . T h e values o b t a i n e d f o r a c t i v a t i o n energy are i m p o r t a n t as energetic characteristics f o r t h e z e o l i t e c r y s t a l l i z a t i o n process as a w h o l e , y e t t h e y give n o answer to t h e q u e s t i o n as to w h i c h of t h e p r o b a b l e stages o f t h e c r y s t a l l i z a t i o n process ( d i s s o l u t i o n o f t h e a m o r p h o u s phase, f o r m a t i o n of n u c l e i , o r c r y s t a l g r o w t h ) l i m i t s t h e rate of c r y s t a l l i z a t i o n . I n this l i g h t , studies o n t h e t e m p e r a t u r e d e p e n d e n c e of t h e rate o f zeolite c r y s t a l g r o w t h are o f interest. T h e influence of t e m p e r a t u r e o n t h e rate of c r y s t a l g r o w t h o f zeolite A is i l l u s t r a t e d b y the curves i n F i g u r e 2. A l i n e a r d e p e n d e n c e o f t h e l o g of t h e rate o f c r y s t a l g r o w t h aginst 1 / T ( F i g u r e 7 ) w a s o b t a i n e d f r o m the results g i v e n i n F i g u r e 2. T h e v a l u e of 10.5 k c a l / m o l e f o r a c t i v a t i o n e n e r g y of t h e zeolite A c r y s t a l g r o w t h w a s c a l c u l a t e d f r o m t h e slope of the s t r a i g h t l i n e i n F i g u r e 7. I t is i n g o o d agreement w i t h t h e v a l u e o f 11 k c a l / m o l e f o u n d f o r a c t i v a t i o n e n e r g y o f zeolite A c r y s t a l l i z a t i o n i n Ref. 16. T h e closeness o f t h e values f o r a c t i v a t i o n energy o f the c r y s t a l l i z a t i o n process as a w h o l e a n d f o r one of its stages, c r y s t a l g r o w t h , suggests that


2.

ZHDANOV

Problems

of Zeolite

Crystallization

35

i n this case t h e rate of c r y s t a l g r o w t h b u t n o t t h e rate of n u c l e a t i o n a n d that of d i s s o l u t i o n o r d i f f u s i o n l i m i t s t h e rate of c r y s t a l l i z a t i o n at t h e t e m p e r a t u r e of c r y s t a l l i z a t i o n . R e a l l y , t h e rate of n u c l e i f o r m a t i o n i n a l u m i n o s i l i c a gels m u s t b e v e r y h i g h a n d , as seen f r o m F i g u r e 4, i t i n creases d u r i n g c r y s t a l l i z a t i o n , a c c e l e r a t i n g the process.

T h e activation

energy of d i s s o l u t i o n a n d d i f f u s i o n i n solutions c a n b e e x p e c t e d

(28)

to

b e c o n s i d e r a b l y l o w e r t h a n the 10.5 k c a l / m o l e c a l c u l a t e d a b o v e

for

c r y s t a l g r o w t h . A c c o r d i n g to o u r p r e l i m i n a r y d a t a , the a c t i v a t i o n e n e r g y of d i s s o l u t i o n of a l u m i n o s i l i c a g e l s t u d i e d a b o v e ( F i g u r e 2 )

i n 0.5N


N a O H is a b o u t 5 k c a l / m o l e . H o w e v e r , at l o w a l k a l i concentrations i n gels, the r a t e of c r y s t a l l i z a t i o n m a y b e l i m i t e d also b y t h a t of d i s s o l u t i o n . T h e conclusions m a d e here c o n c e r n t h e c r y s t a l l i z a t i o n of a l u m i n o silica gels, b u t K e r r (24)

c a m e to analogous conclusions i n s t u d y i n g the

f o r m a t i o n of zeolite A f r o m t h e a m o r p h o u s a l u m i n o s i l i c a t e , t r e a t i n g i t w i t h s o d i u m h y d r o x i d e solution—i.e., u n d e r c o n d i t i o n s different

from

those of the u s u a l c r y s t a l l i z a t i o n of a l u m i n o s i l i c a gels.

3,5

3,0

2,5 26

V

2.8

2,9

$0

3,1

γ-10*

Figure 7. Log of the rate of crystal growth (A/h) vs. reciprocal of the absolute tem perature Effect of Alkalinity on the Rate of Crystallization. T h e causes of t h e effect of a l k a l i n i t y o n the rate of c r y s t a l l i z a t i o n h a v e n o t b e e n d i s cussed i n the l i t e r a t u r e . T h e authors (22)

only reported that crystalliza

t i o n is c a t a l y z e d b y excess a l k a l i . U s u a l l y , a n increase i n a l k a l i c o n c e n t r a t i o n i n a l u m i n o s i l i c a gels a n d a l u m i n o s i l i c a t e m i x t u r e s leads to a decrease i n b o t h the d u r a t i o n of c r y s t a l l i z a t i o n a n d c r y s t a l sizes (32, 38).

This can

be e x p l a i n e d b y the g r o w t h of the rate of n u c l e a t i o n w i t h i n c r e a s i n g a l k a l i c o n c e n t r a t i o n . T h e r e f o r e , a greater n u m b e r of n u c l e i s h o u l d f o r m d u r i n g crystallization i n a more alkaline medium.


36


1

Table V . Component Concentrations in the L i q u i d Phase of Gels ( M o l e / L ) and Rate of G r o w t h of Zeolite A Crystals at 9 0 ° C Gel Samples 266-1 266-2 266-3 266-4

NaOH 0.560 0.460 0.428 0.302

AW

[Al] [Si] X 10* Y

SiO*

Z

0.0266 0.0258 0.0208 0.0104

av

3.05 2.25 1.80 1.25

5.46 4.00 3.80 2.25

0.0103 0.0085 0.0092 0.0108

μ/Day


A s seen f r o m T a b l e I , t h e c h a n g e i n a l k a l i c o n c e n t r a t i o n i n gels causes c o m p l i c a t e d changes i n S i 0

2

a n d A 1 0 concentrations i n t h e l i q u i d 2

3

phase. L o w - a l k a l i a l u m i n o s i l i c a gels d o n o t c r y s t a l l i z e at a l l , e v e n at 90 ° C . T h i s i n d i c a t e s t h e existence of some m i n i m u m a l k a l i concentrations at w h i c h c r y s t a l l i z a t i o n at a g i v e n t e m p e r a t u r e becomes possible. F i g u r e 8 illustrates the u n u s u a l effect of changes i n a l k a l i c o n c e n t r a tions i n a l u m i n o s i l i c a gels o n the sizes of zeolite A crystals f o r m e d i n the 4 s a m p l e gels w i t h the same ratios of N a 0 : A 1 0 : S i 0 2

2

3

2

=

2.8 : 1 : 1.9

( a l k a l i c o n c e n t r a t i o n i n g e l w a s c h a n g e d b y d i l u t i o n ) . I n t h i s case, the increase i n a l k a l i c o n c e n t r a t i o n l e d to the g r o w t h of c r y s t a l size, n o t to its decrease as i n other cases

(38).

T o u n d e r s t a n d this a p p a r e n t d i s c r e p a n c y , the c o m p o s i t i o n s of l i q u i d phases of the other samples of gels w i t h the same oxide ratios w e r e i n v e s t i g a t e d together w i t h the rates of c r y s t a l g r o w t h .

T h e results o b

t a i n e d are g i v e n i n T a b l e V . T h e d a t a of T a b l e V s h o w t h a t the average rate of c r y s t a l g r o w t h at a g i v e n t e m p e r a t u r e is a p p r o x i m a t e l y a l i n e a r f u n c t i o n of the p r o d u c t of concentrations of the silicate a n d a l u m i n a t e ions i n the l i q u i d p h a s e of gel. C o n s e q u e n t l y , i n the case s h o w n i n F i g u r e 8, the c h a n g e i n c r y s t a l size c a n b e c a u s e d b y changes i n concentrations of o t h e r c o m p o n e n t s r a t h e r than a change i n alkali concentration. Seeding Effects. C r y s t a l l i z a t i o n of some zeolites ( scolecite, n a t r o l i t e ) p r o c e e d s o n l y i n the presence o f seeds (26, 36).

T h e m e c h a n i s m of seed

i n g is insufficiently c l e a r i n s u c h cases. B r e c k a n d F l a n i g e n (16, 22)

o b s e r v e d the a c c e l e r a t i o n of z e o l i t e A

c r y s t a l l i z a t i o n b y the a d d i t i o n of seeds i n a l u m i n o s i l i c a gels, b u t f o u n d no significant increase i n c r y s t a l size. T h i s w a s i n d i c a t e d also b y the authors (18). T r e a t i n g the a m o r p h o u s p o w d e r e d a l u m i n o s i l i c a t e w i t h N a O H s o l u t i o n u n d e r d y n a m i c c o n d i t i o n s , K e r r s h o w e d (24)

t h a t the use of z e o l i t e

A crystals as seeds i n the l i q u i d phase c o n s i d e r a b l y accelerates t h e c o n v e r s i o n of the a m o r p h o u s p h a s e i n t o the c r y s t a l l i n e one.

T o understand

t h e m e c h a n i s m of a l u m i n o s i l i c a g e l c r y s t a l l i z a t i o n , i t is essential to k n o w


2.

ZHDANOV

Problems

of Zeolite

37

Crystallization

w h e t h e r the seed g r o w t h occurs i n the course of c r y s t a l l i z a t i o n . A s f o l l o w s f r o m K e r r ' s d a t a o b t a i n e d u n d e r s p e c i a l c o n d i t i o n s of his e x p e r i ments, the seed crystals m u s t g r o w at t h e expense of s o l u b l e

components

of the l i q u i d phase, t h o u g h n o measurements of c r y s t a l size w e r e p e r f o r m e d i n his w o r k

(24).

S e e d i n g effects of different s y n t h e t i c zeolites o n the results of a l u m i nosilica gel crystallization have been investigated i n our laboratory b y S a m u l e v i c h a n d S h u b a e v a . T h e greater the a d d i t i o n of seed crystals, t h e less is the d u r a t i o n of c r y s t a l l i z a t i o n . D i r e c t measurements of c r y s t a l size


s h o w e d that the seed crystals g r o w d u r i n g c r y s t a l l i z a t i o n at a rate w h i c h differs o n l y s l i g h t l y f r o m t h a t of the g r o w t h of f o r m i n g crystals.

The

m a x i m u m size of zeolite A crystals o b t a i n e d f r o m gels w i t h o u t seeds w a s 19/x,, w h i l e the size of t h e seed crystals i n the next test of t h e same g e l a m o u n t e d to 36μ.

T h e s e results are of great i m p o r t a n c e f o r u n d e r s t a n d i n g

the m e c h a n i s m of zeolite c r y s t a l l i z a t i o n . On the Mechanism of Crystallization

of Aluminosilica

Gels

I n the m a j o r i t y of investigations o n zeolite synthesis, p a r t i c u l a r a t t e n t i o n w a s p a i d to the effect of temperatures o n c r y s t a l l i z a t i o n . R e c e n t w o r k s o n m o r d e n i t e c r y s t a l l i z a t i o n (20,

32)

indicate that temperature

1 to

Figure

8.

Dependence of crystal growth of zeolite A on alkali concentration in gel at 90° C


38


influences the k i n e t i c s of the process r a t h e r t h a n its final results. t e m p e r a t u r e of zeolite c r y s t a l l i z a t i o n c a n be s i g n i f i c a n t l y r e d u c e d

1

The either

b y increase i n a l k a l i n i t y of the heterogenous a l u m i n o s i l i c a t e m i x t u r e s or b y t h e use of m o r e r e a d i l y s o l u b l e i n i t i a l m a t e r i a l s (6, 20, 33, 3 4 ) .

Since

t h e increase i n b o t h t h e t e m p e r a t u r e a n d a l k a l i n i t y raises the s o l u b i l i t y of t h e s o l i d a l u m i n o s i l i c a t e phase, one c a n suggest t h a t t h e c o m p o s i t i o n

of

the l i q u i d phase i n s u c h heterogenous systems has a n essential effect o n c r y s t a l l i z a t i o n , yet there is a l a c k of i n f o r m a t i o n o n this q u e s t i o n i n the literature.


Investigations c a r r i e d o u t i n o u r l a b o r a t o r y (37,

39)

a n d results r e

p o r t e d here refer to a l u m i n o s i l i c a gels. N o c o m m o n o p i n i o n exists o n the m e c h a n i s m of c r y s t a l l i z a t i o n of a l u m i n o s i l i c a gels a n d the role o f t h e i r s o l i d a n d l i q u i d phases.

T h e authors (22,

29, 30)

suggest that zeolite

crystals are f o r m e d i n the s o l i d phase of gels. I n a d d i t i o n , i t is c o n s i d e r e d t h a t no d i s s o l u t i o n of the s o l i d p h a s e occurs d u r i n g c r y s t a l l i z a t i o n

(22)

a n d the c o m p o n e n t s of the l i q u i d p h a s e d o not d i r e c t l y p a r t i c i p a t e i n t h e f o r m a t i o n of crystals (29,

30).

T h e experiments o n the s u b s t i t u t i o n of l i q u i d phase i n a l u m i n o s i l i c a gels (37, 39, 41)

c l e a r l y s h o w e d that the c r y s t a l l i z a t i o n of a l u m i n o s i l i c a

gels cannot be c o n s i d e r e d as s i m p l e o r d e r i n g of the structure of t h e gel skeleton w i t h o u t a n y p a r t i c i p a t i o n of t h e l i q u i d phase a n d w i t h o u t a transport of the c o m p o n e n t s of t h e s o l i d p h a s e i n t o s o l u t i o n d u r i n g crystallization. I n o u r u n d e r s t a n d i n g (41),

b a s e d o n t h e results of i n v e s t i g a t i o n of

the c h e m i c a l structure of a l u m i n o s i l i c a gels, t h e n u c l e i of zeolite crystals b e g i n to f o r m i n the l i q u i d phase of gels or at t h e interface of g e l phases. T h e g r o w t h of c r y s t a l n u c l e i proceeds at the expense of a l u m i n o s i l i c a t e h y d r a t e d anions o c c u r r i n g i n t h e s o l u t i o n . T h e s e anions represent differ ent c o m b i n a t i o n s of ( S i , 0 ) - a n d ( ΑΙ,Ο )-tetrahedra, as i t w a s p o s t u l a t e d b y B a r r e r et al. (4). g r o w i n g crystals. compositions

T h e s e units c a n be the s t r u c t u r a l b l o c k s of

T h e i r compositions

of l i q u i d phases.

a n d structures are g i v e n b y

the the

T h e g r o w t h of crystals leads to the d i s

s o l v i n g of the s o l i d phase d u r i n g a l l the p e r i o d of c r y s t a l l i z a t i o n . T o e x p l a i n t h e a u t o c a t a l y t i c n a t u r e of a l u m i n o s i l i c a g e l c r y s t a l l i z a t i o n , it is a s s u m e d that the g e l skeleton, b e i n g x - r a y a m o r p h o u s

as a

w h o l e , contains together w i t h the d i s o r d e r e d (Si,ΑΙ,Ο)-network t h e s i m plest s t r u c t u r a l b l o c k s — f o r example, i n the f o r m of the single a n d d o u b l e 4- a n d 6 - m e m b e r e d rings of ( S i , 0 ) - a n d (ΑΙ,Ο)-tetrahedra. S t r u c t u r a l l y , these a l u m i n o s i l i c a t e b l o c k s are s i m i l a r to those of the zeolite f r a m e w o r k s b u t t h e y differ f r o m t h e m c h e m i c a l l y b y t h e presence of t e r m i n a l S i - O H , A l - O H , and S i - 0 " N a

+

(or generally S i - 0 " R ) +

groups w h i c h have

not

b e e n u s e d i n p o l y c o n d e n s a t i o n reactions of the f o r m a t i o n of gel skeleton o w i n g to the h i g h r e a c t i o n rate. T h e structure a n d S i / A l r a t i o of these


2.

ZHDANOV

Problems

of Zeolite

39

Crystallization

INITIAL ALUMINOSILICATE GEL

AMORPHOUS SOLID PHASE

LIQUID PHASE Hydrated species: îîaî 0H7 A1(0H)~, (HO) ^ SI(0)5. (Al,0,SI,CH)-anions· 0.1 V o i / - A V x I O - ^ 6 (anproxinately) y

n


2

Heating

Ο S i · Al ^ hydrated cations 1^Si:Al^5 (O-atoms not shown) CRYSTALLISING GEL

AMORPHOUS SOLID PHASE

t ,dissolving

LIQUID PHASE

increase of concentrations in the l i q u i d phase condensation ver. étions

autocatalytic growth of dissolution rate of amorphous phase

!

ACCUMULATION OF ZEOLITE CRYSTALS

time- fτον/th in size and number of nuclei ;

decrease of concentrations in the l i q u i d phase

Figure 9.

Schematic representation of aluminosilica

gel

crystallization

b l o c k s as w e l l as the c o m p o s i t i o n of the g e l skeleton as a w h o l e a n d that of the l i q u i d phase of g e l are set b y t h e c o m p o s i t i o n of the i n i t i a l m i x t u r e . T h e s o l i d a n d l i q u i d phases of a l u m i n o s i l i c a gels are c o n n e c t e d b y the s o l u b i l i t y e q u i l i b r i u m . O w i n g to this, b o t h the a l u m i n o s i l i c a t e

and

silicate ions are a l w a y s present i n the l i q u i d p h a s e of a l u m i n o s i l i c a gels (Table I)

a n d the e q u i l i b r i u m p r o d u c t c o n c e n t r a t i o n s of t h e ions

(the

s o l u b i l i t y p r o d u c t of a m o r p h o u s a l u m i n o s i l i c a t e ) d e p e n d o n the c o m p o s i t i o n of a m o r p h o u s a l u m i n o s i l i c a t e a n d t e m p e r a t u r e .

I n h e a t i n g t h e gels,

t h e i r s o l u b i l i t y increases a n d E q u i l i b r i u m 2 m o v e s to the r i g h t . T h i s leads to i n c r e a s e d c o n c e n t r a t i o n

of the silicate, a l u m i n a t e , a n d

ions i n the l i q u i d phase.

A s a result, t h e p r o b a b i l i t y of

aluminosilicate condensation


40


reactions b e t w e e n

the ions increases, g i v i n g rise to t h e f o r m a t i o n

p r i m a r y aluminosilicate blocks (4- a n d 6-membered

1

of

rings) and crystal

n u c l e i . T h e f o r m a t i o n a n d g r o w t h of c r y s t a l n u c l e i l e a d to the e x h a u s t i o n of l i q u i d phase i n the simplest silicate, a l u m i n a t e , a n d a l u m i n o s i l i c a t e ions, a n d the e q u i l i b r i u m state is r e a c h e d b y p e r m a n e n t d i s s o l v i n g of the s o l i d phase. B e c a u s e of the l o w e r s o l u b i l i t y of zeolite crystals i n c o m p a r i son w i t h t h a t of the a m o r p h o u s

a l u m i n o s i l i c a t e skeleton of gels

from

w h i c h t h e y are f o r m e d

I V ) , the c r y s t a l l i z a t i o n process

must

(Table

c o n t i n u e to c o m p l e t e d i s s o l u t i o n of the a m o r p h o u s

phase.


Closeness of the compositions b e t w e e n the s o l i d phase of gels a n d z e o l i t e crystals o b t a i n e d f r o m t h e m

(Table

I)

provides relative con-

stancy of c o m p o s i t i o n i n t h e l i q u i d phase d u r i n g c r y s t a l l i z a t i o n ; i.e., stab i l i t y of the c o n d i t i o n s of n u c l e a t i o n .

T h i s m a y e x p l a i n t h e ease of

o b t a i n i n g t h e p u r e zeolite phases i n t h e c r y s t a l l i z a t i o n of a l u m i n o s i l i c a gels. D u r i n g the d i s s o l v i n g of the a l u m i n o s i l i c a t e skeleton of gels, b o t h the s i m p l e s t silicate a n d a l u m i n a t e ions a n d a l u m i n o s i l i c a t e b l o c k s transp o r t i n t o the l i q u i d phase.

T h e s e b l o c k s m a y b e r e g a r d e d as p r e p a r e d

s t r u c t u r a l elements for g r o w i n g crystals a n d n u c l e i .

Consequently,

a

s i t u a t i o n develops w h e n s t r u c t u r a l elements of n u c l e i arise at t h e expense not o n l y of r e a c t i o n i n s o l u t i o n b u t also of the b r e a k d o w n of t h e gel skeleton d u r i n g its d i s s o l u t i o n . It has to l e a d to increases i n the rate of n u c l e a t i o n a n d q u a n t i t y of g r o w i n g n u c l e i a n d crystals ( F i g u r e 4 ) .

At

the constant rate of c r y s t a l g r o w t h at the expense of the components i n the l i q u i d phase ( F i g u r e 2 ) , this m u s t l e a d to a n increase i n the rate of d i s s o l u t i o n of the s o l i d phase—i.e., to the a u t o c a t a l y t i c a c c e l e r a t i o n of crystallization. S c h e m a t i c r e p r e s e n t a t i o n of the c r y s t a l l i z a t i o n of a l u m i n o s i l i c a gels a c c o r d i n g to o u r u n d e r s t a n d i n g of the process is g i v e n i n F i g u r e 9. A s the n u c l e i a n d crystals g r o w at the expense of components of the l i q u i d phase of gels, the c o m p o s i t i o n of crystals s h o u l d d e p e n d o n t h a t of the l i q u i d phase. T h i s is c o n f i r m e d b y t h e d a t a g i v e n i n T a b l e V I . Table V I . Dependence of S i / A l Ratio of Zeolite on the Composition of Gel Liquid Phase

Gel Samples 196-1 196-2 196-3 196-4 196-5

Concentrations in Liquid Phase, Mole/1000 Grams of Solution Na 0

Al 0

Si0

0.702 0.825 0.926 0.955 1.200

0.0124 0.0114 0.0105 0.0116 0.0202

0.397 0.304 0.220 0.174 0.117

2

2

3

2

SiQ Al O

2

2

z

32.0 26.6 21.0 15.0 5.8

Na Oex Si0 2

2

X

Si/Al in Zeolite X Crystals

1.75 2.68 4.17 5.42 10.05


1.63 1.38 1.33 1.15 1.07

2.

Problems

ZHDANOV

of Zeolite

41

Crystallization

It is essential to note that b o t h S i / A l r a t i o a n d N a O H content i n a l l gels u s e d w a s constant mole/l).

(Si/Al

=

2.5;

NaOH

concentration

=

2.61

T h e d a t a of T a b l e V I i n d i c a t e s that the g r o w t h of the S i / A l

r a t i o i n the crystals of synthetic faujasite is d i r e c t l y associated w i t h i n creases i n the S i 0

2

c o n c e n t r a t i o n a n d S i / A l r a t i o i n t h e l i q u i d phase of

the gels. As

we

concluded

above, the a c t i v a t i o n e n e r g y

of c r y s t a l l i z a t i o n

corresponds to t h a t of c r y s t a l g r o w t h . T h e a c t i v a t i o n energies of different zeolites are close.

F o r e x a m p l e , for m o r d e n i t e a n d z e o l i t e A , the c a l c u


l a t e d values m e n t i o n e d a b o v e are 10 to 11 k c a l / m o l e ; i.e., energy is e q u i v a l e n t to the e n e r g y of 2 h y d r o g e n bonds.

activation

It c a n b e c o n

n e c t e d w i t h the necessity of d e h y d r a t i o n of the silicate 0

OH \

Si

_ / Ο

H

/

\

\

/

\

\

/

OH

Ο

/ Η

and aluminate OH

OH

OH \ _ / ' Al /' \

OH

Η

/

Ο

\

Η

ions i n s o l u t i o n before the c o n d e n s a t i o n reactions b e t w e e n the ions c o u l d take p l a c e . M u c h e v i d e n c e c a n b e p r e s e n t e d i n f a v o r of the p r o p o s e d m e c h a n i s m of a l u m i n o s i l i c a g e l c r y s t a l l i z a t i o n , some of w h i c h w a s r e p o r t e d e a r l i e r (37, 39).

T h e most c o n v i n c i n g arguments s u p p o r t i n g this m e c h a n i s m are

the g r o w t h of seed crystals i n gels, d e p e n d e n c e of the rate of c r y s t a l g r o w t h u p o n the c o n c e n t r a t i o n of components

i n the l i q u i d phase, a n d

d e p e n d e n c e of the S i / A l r a t i o i n crystals o n the c o m p o s i t i o n of the l i q u i d phase. T h e p r o b a b i l i t y of c r y s t a l l i z a t i o n of a l u m i n o s i l i c a gels w i t h r e m o v a l of a c o n s i d e r a b l e p a r t of the l i q u i d phase ( w h i c h is suggested as the m a i n s u p p o r t i n g e v i d e n c e for the f o r m a t i o n of zeolite crystals w i t h o u t a n y p a r t i c i p a t i o n of l i q u i d p h a s e ) is q u i t e r e a l f r o m o u r p o i n t of v i e w . T h e e q u i l i b r i u m b e t w e e n the s o l i d a n d l i q u i d phases of gels does not d e p e n d o n the v o l u m e of s o l u t i o n b u t o n l y o n the c o m m o n of g e l a n d o n the t e m p e r a t u r e .

composition

So, r e m o v a l of some q u a n t i t y of l i q u i d

phase w i l l not l e a d to the c h a n g e i n e q u i l i b r i u m a n d s h o u l d n o t influence the c r y s t a l l i z a t i o n results of s u c h gels.


42

M O L E C U L A R

SIEVE

ZEOLITES

1

I n zeolite c r y s t a l l i z a t i o n f r o m t h e heterogenous a l u m i n o s i l i c a t e m i x tures, the d i s s o l v i n g of some q u a n t i t y of solids m u s t p r e c e d e t h e b e g i n n i n g of c r y s t a l l i z a t i o n ( i n d u c t i o n p e r i o d ) .


w e l l illustrated b y K e r r

F o r one specific case, i t w a s

(24).

Literature Cited (1) Ames, L. L., Am. Mineralogist 1963, 48, 1374. (2) Barrer, R. M.,J.Chem. Soc. 1948, 2158. (3) Barrer, R. M., Baynham, J. W., J. Chem. Soc. 1956, 2822. (4) Barrer, R. M., Baynham, J. W., Bultitude, F. W., Meier, W. M.,J.Chem. Soc. 1959, 195. (5) Barrer, R. M., Baynham, J. W., McCalum, N.,J.Chem. Soc. 1953, 4035. (6) Barrer, R. M., Cole, J. F., Sticher, H.,J.Chem.Soc.1968, A, 2475. (7) Barrer, R. M., Denny, P. J.,J.Chem.Soc.1961, 971. (8) Ibid., 1961, 983. (9) Barrer, R. M., MarshaU, D. J.,J.Chem. Soc. 1964, 485. (10) Barrer, R. M., McCalum, N.,J.Chem. Soc. 1953, 4029. (11) Barrer, R. M., White, E., J. Chem. Soc. 1951, 1267. (12) Ibid., 1952, 1561. (13) Breck, D. W.,J.Chem. Educ. 1964, 41, 678. (14) Breck, D. W., Eversole, W. G., Milton, R. M., J. Am. Chem. Soc. 1956, 78, 2338. (15) Breck, D. W., Eversole, W. G., Milton, R. M., Reed, T. B., Thomas, T. L.,J.Am. Chem.Soc.1956, 78, 5963. (16) Breck, D. W., Flanigen, Ε. M., "Molecular Sieves," p. 47, Society of the Chemical Industry, London, 1968. (17) Breck, D. W., Smith, J. V., Sci. Am. 1959, 200, 88. (18) Budnikov, P. P., Petrovykh, Ν. M., Zh. Prikl. Khim. 1965, 38, 10. (19) Coombs, D. S., Ellis, A. J., Fyfe, W. S., Taylor, A. M., Geochim. Cosmo chim. Acta 1959, 17, 53. (20) Domine, D., Quobex, J., "Molecular Sieves," p. 78, Society of the Chemi cal Industry, London, 1968. (21) Fahlke, B., Wieker, W., Thilo, Ε., Z. Anorg. Allgem. Chem. 1966, 34 82. (22) Flanigen, Ε. M., Breck, D. W., ACS, 137th Meeting, Cleveland, Ohio, 1960. (23) Hawkins, D. B., Mater. Res. Bull. 1967, 2, 951. (24) Kerr, G. T.,J.Phys. Chem. 1966, 70, 1047. (25) Ibid., 1968, 72, 1385. (26) Koizumi, M., Roy, R.,J.Geol. 1960, 68, 41. (27) Kuhl, G. H., "Molecular Sieves," p. 85, Society of the Chemical Industry, London, 1968. (28) Kuznetsov, V. Α., "Hydrothermal Synthesis of Crystals," p. 77, Nauka, Moscow, 1968. (29) Myrsky, Ya. V., Mitrofanov, M. G., Dorogochinski, Α. V., "New Adsorb ents—Molecular Sieves," Grosnyi, 1964. (30) Myrsky, Ya. V., Mitrofanov, M. G., Popkov, B. M., Bolotov, L. T., Ruchko, L. F., "Zeolites, Their Synthesis, Properties and Utilization," p. 192, Nauka, Moscow-Leningrad, 1965. (31) Ovsepyan, M. E., Zhdanov, S. P., Izv. Akad. Nauk SSSR, Ser. K 1965, 11. (32) Sand, L. B., "Molecular Sieves," p. 71, Society of the Chemical Industry, London, 1968.


2.

Problems

ZHDANOV

of Zeolite

Crystallization

43

(33) Senderov, Ε. E., "Geochemical Investigations on High Temperatures and Pressures," p. 173, Nauka, Moscow, 1965. (34) Senderov, Ε. E., Geochim. 1963, 820. (35) Senderov, Ε. E., "Zeolites, Their Synthesis, Properties and Utilization," p. 165, Nauka, Moscow-Leningrad, 1965. (36) Senderov, E. E., Khitarov, Ν. I., Geochim. 1966, 1398. (37) Zhdanov, S. P., "Molecular Sieves," p. 62, Society of the Chemical Indus try, London, 1968. (38) Zhdanov, S. P., Buntar, Ν. N., "Synthetic Zeolites," p. 105, Nauka, Moscow, 1962. (39) Zhdanov, S. P., Egorova, Ε. N., "Chemistry of Zeolites," Nauka, Lenin grad, 1968. (40) Zhdanov, S. P., Novikov, B. G., Izv. Akad. Nauk SSSR, Ser. Khim. Downloaded by PENNSYLVANIA STATE UNIV on July 3, 2012 | http://pubs.acs.org Publication Date: August 1, 1974 | doi: 10.1021/ba-1971-0101.ch002

1966,

44.

(41) Zhdanov, S. P., Samulevich, Ν. N., Egorova, Ε. N., "Zeolites, Their Syn thesis, Properties and Utilization," p. 129, Nauka, Moscow-Leningrad, 1965.

RECEIVED

February 20,

1970.

Discussion W. H. Flank (Houdry Laboratories, Marcus Hook, Pa. 19061): In crystallizing a given zeolite from a 2-phase nutrient system, the compo sition in the liquid phase, especially the effective hydroxyl ion concen tration, and the temperature must be controlled so that a balance is maintained between the ratio of the various silica and alumina species being obtained from dissolution of the solid phase and the ratio of these species already present in the liquid phase. Degree of supersaturation should also be controlled. Since the rate dependence of the various reac tion steps is not constant for all species, lack of such control may have a severe effect on the system. What was done to the gel samples in Table VI, which had constant Si/Al and Na/Si ratios, to produce the various ratios noted for the liquid phase? S. P. Zhdanov: The changes in the liquid phase composition were reached by diluting the initial silicate and aluminate solutions. The con centrations of NaOH in the initial mixtures were constant but not the Na/Si ratios. The constancy of NaOH concentration was reached by diluting the solution.


Some Problems of Zeolite Crystallization - Advances in Chemistry

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