Observed and Calculated Silicate and Aluminosilicate Oligomer

Chapter 6

Observed and Calculated Silicate and Aluminosilicate Oligomer Concentrations in Alkaline Aqueous Solutions

Downloaded by PURDUE UNIV on May 18, 2016 | http://pubs.acs.org Publication Date: July 31, 1989 | doi: 10.1021/bk-1989-0398.ch006

P. Caullet and J. L. Guth Laboratoire de Matériaux Minéraux, Unité Associée au Centre National de la Recherche Scientifique No. 428, Ecole Nationale Supérieure de Chimie, 3 rue Alfred Werner, 68093 Mulhouse Cedex, France A model which enables one to calculate oligomer concentrations, provided that those of the monomeric Si(OH) and Al(OH) species are known, has been developed. The equilibrium constants needed are the product of a polymerization factor and of an ionization factor. The polymerization constants for the formation of a Si-O-Si or a Si-O-Al bond were estimated from solubility data. Whereas Al-OH groups do not ionize, the ionization constants of the silanol functions depend, probably in a very complicated way, on the precise structure of the oligomer. These constants were estimated partially on the assumption of existing analogies between polysilicic and other inorganic acids. The various constants used in the model were refined by comparison of the calculated concentrations with the available experimental data. The model was then applied to determine the influence of the chemical composition on the oligomer distribution . The found relationships agree with the usual observations in zeolite synthesis. 4

-

4

The study o f a l k a l i n e s i l i c a t e and a l u m i n o s i l i c a t e s o l u t i o n s i s o f fundamental importance f o r a b e t t e r u n d e r s t a n d i n g o f t h e mechanism of z e o l i t e s y n t h e s i s ( 1 , 2 ) . Real p r o g r e s s has been r e a l i z e d durina the l a s t f i f t e e n y e a r s thanks t o t h e use o f nen t e c h n i q u e s such as the t r i m e t h y l s i l y l a t i o n (3,4) of p o l y s i l i c i c acids ( f o l l o n e d by a c h r o m a t o g r a p h i c s e p a r a t i o n of t h e d e r i v a t i v e s ) and, even more, 29$! and A 1 NMR s p e c t r o s c o p y (5-11). The l a t t e r t e c h n i q u e s have enabled the i d e n t i f i c a t i o n of about t n e n t y t y p e s o f o l i g o m e r s i n silicate s o l u t i o n s and i n some c a s e s e s t i m a t e s o f t h e i r r e s p e c t i v e c o n c e n t r a tions. The i n v e s t i g a t i o n of a l k a l i n e a l u m i n o s i l i c a t e s o l u t i o n s i s 2 7

0097-6156/89/0398-0083$06.00/0 ο 1989 American Chemical Society Occelli and Robson; Zeolite Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

84

ZEOLITE SYNTHESIS

much l e s s advanced because t h e s e s o l u t i o n s a r e o n l y s t a b l e a t very low s i l i c a t e and a l u m i n a t e c o n c e n t r a t i o n s . The model p r e s e n t e d here a l l o w s a c a l c u l a t i o n o f the c o n c e n t r a t i o n s of a l l the i o n i z e d forms of any o l i g o m e r s p e c i e s from the conc e n t r a t i o n v a l u e s of S i ( O H ) and AH OH) 4" and from the pH v a l u e ( t h e simulation programs w r i t t e n i n BASIC a r e a v a i l a b l e from the authors). A communication d e s c r i b i n g the i n i t i a l development of this model appeared i n 1984 ( 1 2 ) . The f i r s t p a r t of the p r e s e n t paper i s r e l a t e d t o the alkaline silicate s o l u t i o n s and r e p r e s e n t s the main p o i n t of our work. The second p a r t c o n c e r n s the a l k a l i n e a l u m i n o s i l i c a t e s o l u t i o n s which were s t u d i e d l e s s e x t e n s i v e l y . 4


ALKALINE SILICATE SOLUTIONS It i s c l e a r l y e s t a b l i s h e d t h a t , i n t h e s e s o l u t i o n s , the s i l i c o n is in t e t r a h e d r a l c o o r d i n a t i o n (13). The s i m p l e s t form of silica in solution i s the m o n o s i l i c i c a c i d S i ( 0 H ) 4 . Polymerization results from the c o n d e n s a t i o n of two s i l a n o l groups with the e l i m i n a t i o n of a water molecule. A whole s e r i e s of s i l i c a t e anions, of various polymerization and i o n i z a t i o n degrees, a r e thus connected through dynamic e q u i l i b r i a . The e q u i l i b r i a are governed by the normal chemical parameters, namely the s i l i c a c o n c e n t r a t i o n , the pH and the c a t i o n type. A l l the s t u d i e s show t h a t , as a g e n e r a l r u l e , the f o r m a t i o n of more highly p o l y m e r i z e d s p e c i e s i s f a v o u r e d by decreasing pH at c o n s t a n t s i l i c a c o n c e n t r a t i o n and i n c r e a s i n g s i l i c a c o n c e n t r a t i o n a t constant M 2 O / S 1 O 2 r a t i o (14,15). Our model a p p l i e s t o sodium o r potassium s i l i c a t e s o l u t i o n s a t room temperature. A c c o r d i n g t o the few e x p e r i m e n t a l data available (16.17,18) t h e s e s o l u t i o n s show s i m i l a r behaviour. Four p o i n t s w i l l be s u c c e s s i v e l y developed : - the p r i n c i p l e s of the c a l c u l a t i o n of the o l i g o m e r s p e c i e s concentrations, - the c h o i c e of parameters, - comparison of c a l c u l a t e d and e x p e r i m e n t a l r e s u l t s , - s i l i c a distribution i n solution. PRINCIPLES Any o l i g o m e r s p e c i e s c o n t a i n i n g t s i l i c o n tetrahedra. £ bonding oxygens and b e a r i n g a n e g a t i v e charge i i s assumed t o be i n e q u i l i b r i u m w i t h the S i ( O H ) 4 monomeric s p e c i e s a c c o r d i n g to Kit ·

(a)

t Si(OH)

The

activity

4

+i OH" ^

Si O £ t

(

+ i )

OH'

1

[ ( t-A) - i ] 2

2

+ (

H

2

of each s p e c i e s , i . e. [ t, A, i ] . i s thus e x p r e s s e d [t,£,ij = K ^ i

[si(OH) ] 4

t

[OH"]

0

by

1

In o r d e r t o s i m p l i f y the c a l c u l a t i o n s , a c t i v i t y w i l l be r e p l a c e d by concentration. The e q u i l i b r i u m c o n s t a n t K ^ j i s c o n s i d e r e d t o be equal t o the product of a p o l y m e r i z a t i o n c o n s t a n t Kp and an i o n i z a t i o n constant

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

6. CAULLET AND GUTH

Silicate and Aluminosilicate Oligomer Concentrations

Ki. The pH o f t h e s o l u t i o n s i n the p r e s e n t study was determined by u s i n g a " h i g h - a l k a l i n i t y " g l a s s e l e c t r o d e , t a k i n g due p r e c a u t i o n s t o avoid carbonation of the s o l u t i o n s . Depending on the c a s e c o n s i d e r e d , the m o n o s i l i c i c acid concentration i s - e i t h e r independent of pH Hhen t h e s i l i c a t e s o l u t i o n i s i n e q u i l i brium w i t h t h e amorphous s i l i c a a c c o r d i n g t o S 1 O 2 + 2 H 2 0 ^ = ^ S i ( OH)4. i n Hhich case t h e chosen v a l u e i s c l o s e t o 2 χ 10~ mol. 1~ , as w i l l be seen below ; 3

1


- o r c a l c u l a t e d from t h e monomeric s i l i c a c o n c e n t r a t i o n i f a t e q u i l i b r i u m t h e system c o n t a i n s no s o l i d s i l i c a . T h i s i m p l i e s a know l e d g e o f t h e pH v a l u e and of the i o n i z a t i o n c o n s t a n t s o f Si(0H>4. CHOICE of PARAMETERS I o n i z a t i o n constants of Si(OH)4 As o n l y the pk, 2) the s i g n a l to n o i s e r a t i o becomes l e s s f a v o u r a b l e . Thus the measu rement of the i n t e n s i t y of each i n d i v i d u a l peak and c o n s e q u e n t l y the 1


6.

CAULLET AND GUTH


integration o f a l l S i peaks becomes l e s s p r e c i s e . Accordingly the experimental monomer c o n c e n t r a t i o n might be i n a c c u r a t e i n some c a ses. Indeed t h e observed v a r i a t i o n o f t h e monomer c o n c e n t r a t i o n versus pH ( F i g u r e 5 i n r e f e r e n c e 10) i s s u r p r i s i n g s i n c e t h e cons tancy o v e r an i n t e r v a l o f more than one pH u n i t (12.6 t o 11.54) i s r a t h e r unexpected.


T a b l e I I : E x p e r i m e n t a l (9) and c a l c u l a t e d c o n c e n t r a t i o n s o f mers i n a potassium s i l i c a t e s o l u t i o n (0.65 m o l . l pH = 1 2 . 53) (* v a l u e t a k e n as a b a s i s i n our computations)

Number t o f t e t r a h e d r a of the o l i g o m e r s

Experimental concentrations (mol. 1~ )

Calculated concentrations (mol. I " )

1

0.1

0.1*

1

2

χ 10~

2

2. 6 χ 1 0 "

2

3

2. 3 χ 1 0 "

2

4. 8 x 1 0 "

2

4

1.7 χ 1 0 ~

2

1

χ 10~

2

5

1. 5 χ 1 0 ~

2

0. 5 χ 1 0 "

2

6

2. 6 χ 1 0 ~

2

3. 2 x 1 0 ~

2

7

3. 5 χ 1 ( T

4. 2 x 1 0 "

3

8

4. 1 χ 1 0 ~

2. 7 x 1 0 "

3

3

oligo S1O2»

1

2

3

- 1

Taking i n t o account t h e s e remarks, o n l y t h e v a l u e s calculated by u s i n g o u r model f o r t h e t h r e e most a l k a l i n e s o l u t i o n s a r e compa r e d t o t h e c o r r e s p o n d i n g e x p e r i m e n t a l r e s u l t s ( T a b l e I I I ) . The gene ral e v o l u t i o n s w i t h pH a r e analogous and moreover agree w i t h those shown by H a r r i s e t a l . However, w h i l e t h e agreement remains a c c e p t a ble f o r t h e s m a l l o l i g o m e r s ( t < 4) i t becomes r a t h e r poor f o r t h e other o l i g o m e r s ( t > 4). These r e l a t i v e l y d i s a p p o i n t i n g r e s u l t s can probably be e x p l a i n e d by t h e f a c t t h a t t h e s o l u t i o n s studied here are much more c o n c e n t r a t e d i n s i l i c a than t h o s e o f H a r r i s e t a l . This favours t h e f o r m a t i o n o f more polymerized s p e c i e s , some o f which may be u n i d e n t i f i e d , and s t r e n g t h e n s t h e r e s e r v a t i o n s s t a t e d before c o n c e r n i n g t h e p r e c i s i o n of t h e e x p e r i m e n t a l concentrations of t h e s p e c i e s . As w e l l the assumption made i n o u r c a l c u l a t i o n s t h a t the a c t i v i t i e s and c o n c e n t r a t i o n s a r e i d e n t i c a l , i s l e s s justified


92

ZEOLITE SYNTHESIS

as the i o n i c s t r e n g t h of the s o l u t i o n s i n c r e a s e s . F i n a l l y the pH measurement of such very a l k a l i n e s o l u t i o n s becomes d i f f i c u l t .

T a b l e I I I : E x p e r i m e n t a l ( 1_0 ) and c a l c u l a t e d c o n c e n t r a t i o n s of o l i gomers i n a sodium s i l i c a t e s o l u t i o n (1.65 m o l . l S i 0 2 ) (* t : number of t e t r a h e d r a i n o l i g o m e r s ) (** v a l u e taken as a b a s i s i n our c a l c u l a t i o n s ) _ 1

pH = 12.60 t

pH = 13.30

exp. cone. c a l c . cone. 1


pH = 13.67

*

0. 08

0i. 08**

2

1. 4x10"~

3

exp. cone. c a l c . cone. exp. cone. 0. 11**

0. 11

8.. 4 x 1 0 "

2

6. 8 x 1 0 "

- 2

8.. 6 x 1 0 "

2

10. 6 x 1 0

3

4,. 9 x 1 0 "

2

2

2. 3 x 1 0 ~

2

1. 4 x 1 0 ~

2. 8x1 0~

2

2. 4 x 1 0 ~

2

4. 3 x 1 0 "

2

1. 1 x 1 0

4

4. 1 x 1 0 ~

2

4. 2x10~

3

3. 8 x 1 0 "

2

2. 1 x 1 0 "

5

6.. 2 x 1 0 "

2

1. 5x1 0~

7. 1 x 1 0 "

2

7x10

6

6.. 9x1 0"

2

2. 9 x 1 0 ~

3

4. 7 x 1 0 "

2

6x10"~

7x10"

4

8 12

= 0 4.. 7 x 1 0

- 3

3,. 4x10~

3

4

2. 8x10"" 2x10

- 7

4. 1 x 1 0

- 3

2x10~

3

1x10

5. 7x10

- 5

6x10~ 8x10

- 2

- 4

5. 4 x 1 0 "

4

1,. 1 x 1 0 "

2

6

6. 6x10"

3

3x10

- 5

- 7

2 . 5x10"

3

6x10

- 7

2x10~

1 2

s 0

In the f o l l o R i n g s e c t i o n t h e s t r u c t u r e of the s i l i c a t e mers R i l l be c o n s i d e r e d i n more d e t a i l .

- 2

2

5

3x10"

~ 0

2

2

1. 7 x 1 0 "

7

0. 30**

o. 30 :

2

3

c a l c . cone.

2x10-

11

oligo-

STRUCTURE of t h e SILICATE OLIGOMERS The e x p e r i m e n t a l l y identified s p e c i e s (9,10,11) a r e comprised of a r a t h e r s m a l l number of silicon tetrahedra ( t < 12). They a r e f o r i n s t a n c e l i n e a r ( t = 1 t o 4). c y clic ( t = 3 and 4), b i c y c l i c o l i g o m e r s ( t = 6,8 and 12) consisting of 2 i d e n t i c a l r i n g s . There are, i n a d d i t i o n , more complex species related t o the c y c l i c t r i m e r and t e t r a m e r and other polycyclic forms ( t = 7 and 8). The values g i v e n i n T a b l e IV and r e f e r r i n g t o the potassium silicate solution already considered before (paragraph entitled "Comparison R i t h the e x p e r i m e n t a l data o b t a i n e d by H a r r i s a t a l . ") shoR a good agreement between e x p e r i m e n t a l and c a l c u l a t e d c o n c e n t r a tions f o r the main o l i g o m e r s . S a t i s f a c t o r y agreement Ras found as H e l l f o r t h e o t h e r s p e c i e s not mentioned i n T a b l e IV. THO e s s e n t i a l o b s e r v a t i o n s Hhich a r e c o n f i r m e d by a n a l y s i s of the r e s u l t s of Mc Cormick e t a l . ( 1 0 ) can be deduced from t h i s t a b l e : - t h e c o n c e n t r a t i o n of s p e c i e s of a g i v e n type ( l i n e a r , c y c l i c or


6. CAULLET AND GUTH

Silicate andAluminosilicate Oligomer Concentrations

93

b i c y c l i c ) d i m i n i s h e s R i t h i n c r e a s i n g number t of t e t r a h e d r a , - f o r c o n s t a n t numbers of t e t r a h e d r a t h e c o n c e n t r a t i o n o f _ o l i g o m e r s i s l a r g e r f o r s t r o n g l y connected s p e c i e s ( b i c y c l i c R i t h Q = 3). As H e l l , the o r d e r of magnitude of the c o n c e n t r a t i o n s i s the same f o r trimers and t e t r a m e r s a c c o r d i n g t o whether they a r e l i n e a r or cy clic. Our c a l c u l a t i o n s seem t o shon t h a t the d i f f e r e n c e becomes larger f o r s p e c i e s R i t h h i g h e r p o l y m e r i z a t i o n degree ( t > 4), t h e f o r m a t i o n of l i n e a r o l i g o m e r s b e i n g f a v o u r e d r e l a t i v e t o t h e forma t i o n of c y c l i c s p e c i e s .

Table

IV : E x p e r i m e n t a l (9) and c a l c u l a t e d c o n c e n t r a t i o n s mers i n a p o t a s s i u m s i l i c a t e s o l u t i o n (0.65 mole S i 0 per l i t e r , pH=12. 53) (*t : number of t e t r a h e d r a i n o l i g o m e r s ) ( n. o. : non o b s e r v a b l e )

of

oligo


2

Linear species t

Cyclic

species

Bicyclic

species

A

exp. cone. 1

0. 1

2

2x10-2

3

1. 3x10"2

c a l c . cone.

exp. cone.

c a l c . cone.

exp. cone.

c a l c . cone.

-

-

-

-

2. 7x10-2

-

-

-

-

1. 3x10"2

ΙΟ"

-

-

0. 1

4

n. o.

6. 8x10"

3

5

n. o.

3. 4x10"

3

6

n. o.

1. 7x10"

8

n. o.

12

n. o.

3. 5x10"2

2

1. 9 x 1 0 "

3

n. o.

1x10"

4

3

n. o.

5. 4x10~

6

4. 1x10-

4

n. o.

1. 5x10"

8

2. 2x10"

5

n. o.

1. 3 x 1 0 "

6. 4x10"

3

5. 1x10~

1 3

-

3

1. 9x10"2

n. o.

1. 4 x 1 0 "

n. o.

6. 8x1 O"

3

6

In c o n c l u s i o n , i n s p i t e of some r e s e r v a t i o n s , i t i s p o s s i b l e Rith our c o m p u t a t i o n a l model t o account c o r r e c t l y f o r the general e v o l u t i o n s and i n most c a s e s the e s t i m a t e s a r e c l o s e t o e x p e r i m e n t a l v a l u e s . In t h e next p a r t our model R i l l be a p p l i e d , R i t h the r e q u i red m o d i f i c a t i o n s , t o t h e study of a l k a l i n e a l u m i n o s i l i c a t e s o l u tions. ALKALINE

SOLUTIONS CONTAINING SILICATE and ALUMINATE IONS

Generally the s o l u b i l i t y o f a l k a l i n e a l u m i n o s i l i c a t e s i n a l k a l i n e medium i s much l o n e r than t h a t of s i l i c a . T h i s i s r e l a t e d to the chemical b e h a v i o u r of aluminum. At pH l a r g e r than 11 this element e x i s t s s o l e l y as t e t r a h e d r a l A H O H ^ " i o n s ( 2 7 ) . The A1-0H f u n c t i o n s


94

ZEOLITE SYNTHESIS

are not i o n i z a b l e and remain a v a i l a b l e f o r p o l y m e r i z a t i o n , whatever the pH. E x p e r i m e n t a l data r e l a t i n g to the o l i g o m e r s p r e s e n t i n a l k a l i n e a l u m i n o s i l i c a t e s o l u t i o n s a r e t h e r e f o r e very s c a r c e , with p r a c t i c a l l y no q u a n t i t a t i v e i n f o r m a t i o n . The o n l y c l e a r l y s t a t e d qualitative information i s the apparent existence of a l u m i n o s i l i c a t e anions (15,28,29). Under such c o n d i t i o n s the a p p l i c a t i o n of our model c l a i m s o n l y here to p r e d i c t some g e n e r a l t r e n d s (30). Analogously to the c a l c u l a t i o n p r i n c i p l e used for silicate s p e c i e s , the a c t i v i t y of an a l u m i n o s i l i c a t e o l i q o m e r can be expressed by [ts. t a , £s. JU, i ] = Kg,s,£a, i [ S i ( OH) ]

t s


4

[Al(0H) -] 4

t a

[OH*]

i

ts and t a b e i n g r e s p e c t i v e l y the number of silicon and aluminum t e t r a h e d r a , £ s and £a b e i n g r e s p e c t i v e l y the number of Si-O-Si and Si-O-Al bonds. A l l assumptions made f o r the s i l i c a t e s o l u t i o n s are used here. T

h

u

s

K

£ s , £a. i

= KP

Ki=k ^s s

k a

£a

K i

.

F u r t h e r , the p o l y m e r i z a t i o n c o n s t a n t k ( r e f e r r i n g t o the formation of a s i l o x a n e bond) and the i o n i z a t i o n c o n s t a n t Ki are assumed not t o be m o d i f i e d by the presence of aluminum atoms. The k value, c o r r e s p o n d i n g to the f o r m a t i o n of a S i - O - A l bond, was estimated by i t e r a t i o n a c c o r d i n q to v a l u e s o r i g i n a t i n g from a semi-quant i t a t i v e e x p e r i m e n t a l study (29) and was found to be 35. The following r e s u l t s are based upon an example from t h i s study. The v a r i a t i o n of o l i g o m e r c o n c e n t r a t i o n s as a f u n c t i o n of p o l y merization degree and c o n n e c t i v i t y i s analogous to the variation o b s e r v e d f o r s i l i c a t e s o l u t i o n s . N e v e r t h e l e s s some s p e c i f i c f e a t u r e s become apparent. A l k a l i n e a l u m i n o s i l i c a t e s o l u t i o n s with a S i / A l r a t i o c l o s e to one, e x i s t o n l y at much lower c o n c e n t r a t i o n s than do s i l i c a t e s o l u t i o n s f o r a g i v e n pH. T h i s r e s u l t s d i r e c t l y from the g e n e r a l e x p r e s sion g i v i n g the a c t i v i t y of an oligomer. Indeed, as pH increases, the S i ( 0 H ) 4 c o n c e n t r a t i o n becomes very low whereas the AM OH)4" concentration remains p r a c t i c a l l y c o n s t a n t and a t a much h i g h e r l e v e l . A d i r e c t consequence of t h i s i s t h a t , f o r a q i v e n conformation, the most s t a b l e o l i g o m e r w i l l be the one c o n t a i n i n g the h i g h e s t number of aluminum atoms, p r o v i d e d i t i s c o n s i s t e n t w i t h Loewenstein' s r u l e (31). This trend becomes weaker as the pH d e c r e a s e s or the overall S i / A l r a t i o i n s o l u t i o n i n c r e a s e s , i n agreement with the usual o b s e r v a t i o n s made i n z e o l i t e s y n t h e s i s (32). These correlat i o n s a r e shown i n t a b l e 7 r e f e r r i n g to the c y c l i c tetramers. The numbers i n b r a c k e t s r e p r e s e n t the c o n c e n t r a t i o n r a t i o between the s p e c i e s c o n s i d e r e d and the p u r e l y s i l i c i c form. A last i m p o r t a n t f e a t u r e c o n c e r n s the b i c y c l i c species. Our c a l c u l a t i o n s show t h a t t h e i r f o r m a t i o n i s f a v o u r e d when both c o n s t i t u t i v e r i n g s c o n t a i n each an even number of t e t r a h e d r a . The concentration of o l i g o m e r s made of two f o u r - o r six-membered rings i s r e s p e c t i v e l y l a r g e r than the c o n c e n t r a t i o n of s p e c i e s c o n t a i n i n g two threeo r f i v e - membered r i n g s . T h i s b e h a v i o u r i s d i r e c t l y related to L o e w e n s t e i n ' s r u l e and can be r e l a t e d t o the f a c t t h a t the frames

a


6. CAULLETANDGUTH


work of a l u m i n a - r i c h z e o l i t e s , which a r e s y n t h e s i z e d medium. can be b u i l t i n most c a s e s with even-membered trahedra.

in alkaline r i n g s of t e -

Table V : C o n c e n t r a t i o n of the c y c l i c t e t r a m e r s in an a l k a l i n e aluminosilicate solution ( [monomeric s i l i c a ] + [AM OH) ~ ] = 0.02 m o l . l ) - 1

4

Monomeric s i l i c a / A H OH) ~ molar r a t i o


4

1

10

Composition of t h e species

C o n c e n t r a t i o n of the s p e c i e s (mol.l" ) pH = 13 pH = 12 1

4 S i , 0 Al

6. 7 x 1 0

( 1)

2. 6 x 1 0 "

8

(1)

3 Si, 1

Al

3. 7x10" ( 55)

5. 1x10~

6

( 200)

2 S i , 2 Al

2x10" ( 3020)

4 S i , 0 Al

7. 3x10~

3 Si. 1

4. 1x10~* ( 5. 5)

Al

2 S i . 2 Al

- 7

5

3

6

(1)

5

4

2. 2x10" ( 30)

95

4

9. 7x10" ( 37000) 2. 9 x 1 0 "

7

(1)

5. 6 x 1 0 ~

6

(19)

4

1. 1x10" ( 380)

CONCLUSION The model p r e s e n t e d here a l l o w s , i n p r i n c i p l e , the d e t e r m i n a t i o n of the c o n c e n t r a t i o n of any o l i g o m e r s p e c i e s i n an alkaline silicate s o l u t i o n p r o v i d e d t h a t the monomeric s i l i c a c o n c e n t r a t i o n and the pH are known. Due t o t h e i r g r e a t e r c o m p l e x i t y and e x p e r i m e n t a l difficulty, the i n v e s t i g a t i o n of a l u m i n o s i l i c a t e s o l u t i o n s corresponds only t o an e x t e n s i o n of t h i s model and i t was d e a l t w i t h i n a simp l i f i e d way. Several r e c e n t NMR s t u d i e s have a l l o w e d the i d e n t i f i c a t i o n of about twenty s i l i c a t e o l i g o m e r s and q u a n t i t a t i v e o r s e m i - q u a n t i t a t i ve estimates of t h e i r r e s p e c t i v e c o n c e n t r a t i o n s . In s p i t e of the p r e c a u t i o n s used by some authors, the measured c o n c e n t r a t i o n s and i n particular the e x p e r i m e n t a l monomer c o n c e n t r a t i o n ( u s e d as a basis i n our c a l c u l a t i o n s ) a r e p r o b a b l y not always very a c c u r a t e e x p e c i a l ly i n the c a s e of the l e s s a l k a l i n e o r the more c o n c e n t r a t e d (in silica) s o l u t i o n s . The use of an i n e r t s i l i c o n - c o n t a i n i n g i n t e r n a l s t a n d a r d c o u l d perhaps be u s e f u l . To t h i s l a c k of a c c u r a c y i s added the uncertainty of the pH measurement i n such strongly alkaline solutions. Moreover s e v e r a l assumptions made i n the s e t t i n g up of t h e c a l c u l a t i o n a l g o r i t h m f o r t h e i o n i z a t i o n c o n s t a n t s l e a d t o an a p p r o x i mate model. An improvement of our model would be t o take i n t o account i n a more s p e c i f i c way the p r e c i s e s t r u c t u r e of the o l i g o m e r s . It s h o u l d be added t h a t any p r e c i s e e x p e r i m e n t a l measurement of i o n i z a -


96

ZEOLITE SYNTHESIS


t i o n c o n s t a n t s , even i f o n l y those o f t h e monomer, would be very helpful. In s p i t e o f t h e s e problems, o u r model a l l o n s us t o account f o r the e x p e r i m e n t a l l y found e v o l u t i o n s . F u r t h e r , t h e aqreement between measured and computed v a l u e s i s g e n e r a l l y s a t i s f a c t o r y . The general t r e n d s c o n c e r n i n g the o l i g o m e r s present i n the solutions can be summarized as f o l l o w s . The o l i g o m e r s c o n s i s t o f a rather s m a l l number o f t e t r a h e d r a ( t < 12). F o r a g i v e n type (linear, c y c l i c , b i c y c l i c ) t h e c o n c e n t r a t i o n s o f t h e s p e c i e s d i m i n i s h as t increases and f o r a g i v e n t t h e f o r m a t i o n o f s t r o n g l y connected species i s favoured. Both t h e i n c r e a s e o f s i l i c a t e or aluminate c o n c e n t r a t i o n s and t h e d e c r e a s e of pH promote t h e f o r m a t i o n o f more polymerized species. In the a l u m i n o s i l i c a t e s o l u t i o n s the p r e f e r e n t i a l l y formed s p e c i e s a r e t h o s e w i t h t h e h i g h e s t p o s s i b l e number o f aluminum atoms. a l t h o u g h t h i s t r e n d becomes weaker as t h e pH i s decreased o r the o v e r a l l S i / A l r a t i o i n s o l u t i o n i s increased. LITERATURE CITED

1. 2. 3. 4. 5. 6. 7. 8. 9 . 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

R. M. Barrer, J. W. Baynham, F. W. Bultitude, F. M. Meier, J. Chem. Soc, 1959, 195. S. P. Zhdanov, Proc. 5th Int. Conf. on Zeolites, (Naples), Heyden, London, 1980, 75. C. W. Lentz, Inorg. Chem., 1964, 3, 574. L. S. Dent Glasser, E. E. Lachowski, J. Chem. Soc., Dalton Series, 1980, 393. G. Engelhardt, D. Zeigan, H. Jancke, D. Hoebbel, W. Hieker, Z. anorg. allg. Chem., 1975, 418, 17. S. Sjöberg, L. O. Ohman, N. Ingri, Acta. Chem. Scand. Ser. A. 1985, 39, 93. R. K. Harris, C. T. G. Knight, W. E. Hull, J. Am. Chem. Soc., 1981, 103, 1577. R. K. Harris, C. T. G. Knight, J. Chem. Soc., Faraday Trans. 2, 1983, 79, 1525. R. K. Harris, C. T. G. Knight, J. Chem. Soc., Faraday Trans. 2, 1983, 79, 1539. Α. V. Mc Cormick, A. T. Bell, C.J. Radke, Zeolites, 1987, 7, 183. C. T. G. Knight, J. Chem. Soc., Dalton Trans., 1988, 1457. J. L. Guth, P. Caullet, R. Wey, Studies in Surface Science and Catalysis 24, Elsevier, Amsterdam, 1985, 183. D. Fortnum, J. O. Edwards, J. Inorg. Nucl. Chem., 1956, 2, 264. R. K. Iler, The Chemistry of Silica, J. Wiley and Sons, New-York, 1979, chapter 2. G. Engelhardt, D. Michel, High-resolution solid-state NMR of silicates and zeolites, John Wiley and Sons, Chichester, 1987, chapter 3. C. T. G. Knight, Ph. D. Thesis, University of East Anglia, 1982. D. Barby, T. Griffiths, A. R. Jacques, D. Pawson, The modern inorganic chemicals Industry, Ed. Thompson, London, 1977, 320. Ν. H. Ray, R.J. Paisted. J. Chem. Soc., Dalton Trans., 1983, 475. N. Ingri, Nobel Symposium, 1977, 40, 3. D. D. Perrin, Ionisation constants of inorganic acids and bases in aqueous solutions, IUPAC Chemical Data Series n°29, Pergamon Press, Oxford, 2 edition, 1982. nd



6. CAULLET AND GUTH

Silicate and Aluminosilicate Oligomer Concentrations 97

21. E. Freund, Bull. Soc. Chim. Fr. , 1973, 2238. 22. J. G. Vail, Soluble Silicates, Reinhold, New-York, 1952. 23. L. W. Cary, B. H.W.S. de Jong, W. E. Dibbe Jr., Geochim. Cosmochim. Acta., 1982, 1317. 24. P. Schindler, H. R. Kamber, Helv. Chim. Acta., 1968, 51, 1781. 25. V. N. Belyakov, N. M. Soltinskii, D. N. Strazhesko, V. V. Strelko, Uhr. Khim. Zh., 1974, 40, 236. 26. D.N. Strazhesko, V. B. Strelko, V. N. Belyakov. S. C. Rubank, J. Chromatogr., 1974, 102, 191. 27. R. L. Moolenaar, J. C. Evans, L. D. Mac Keever, J. Phys. Chem., 1970, 74, 3629. 28. L. S. Dent Glasser, G. Harvey, J. Chem. Soc., Chem. Comm., 1984, 1250. 29. J. L. Guth, P. Caullet, P. Jacques, R. Wey, Bull. Soc. Chim. Fr., 1980, 121. 30. J. L. Guth. P. Caullet, Thesis, Mulhouse, France, 1983. 31. W. Loewenstein, Am. Mineral., 1954, 39, 92. 32. R. M. Barrer, Zeolites, 1981, 1, 130. RECEIVED December 22, 1988


Observed and Calculated Silicate and Aluminosilicate Oligomer

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