Perspectives in Molecular Sieve Science - American Chemical Society

Chapter

14

Effect of Alkali Metal Cations on Silicate Structures in Aqueous Solution

Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: May 27, 1988 | doi: 10.1021/bk-1988-0368.ch014

Α. V. McCormick, A. T. Bell, and C. J . Radke Center for Advanced Materials, Lawrence Berkeley Laboratory, and Department of Chemical Engineering, University of California, Berkeley, CA 94720

29

By means of Si NMR spectroscopy, it is established that the distribution of silicate anions in alkaline silicate solutions is a moderate function of base composition. At a fixed SiO concentration and silicate ratio, the proportion of Si present in oligomeric and cage-like structures increases in progressing from Li to Cs hydroxide. This trend is ascribed to cation-silicate anion pairing and to a higher selectivity for ion pairing by large silicate anions as cation size increases. 2

I t i s w i d e l y b e l i e v e d t h a t t h e n u c l e a t i o n and growth o f z e o l i t e s o c c u r t h r o u g h t h e r e a c t i o n s o f d i s s o l v e d s i l i c a t e and a l u m i n o s i l i c a t e anions (1.2). S u p p o r t i n g t h i s view, s e v e r a l e x p e r i m e n t a l s t u d i e s have shown t h a t t h e s t r u c t u r e o f d i s s o l v e d s i l i c a t e s p e c i e s c a n influence the s t r u c t u r e of solid aluminosilicate intermediates present i n t h e g e l from which a z e o l i t e may form (3-5) . As a consequence, d e t e r m i n a t i o n o f t h e s t r u c t u r e and d i s t r i b u t i o n o f such anionic species has become a s u b j e c t of considerable interest. 29 Studies u t i l i z i n g S i NMR have r e v e a l e d a wide v a r i e t y o f s i l i c a t e s p e c i e s i n aqueous s o l u t i o n s o f s i l i c a a t h i g h pH (1.6-10). Anions resembling the secondary b u i l d i n g u n i t s o f z e o l i t e s have been o b s e r v e d and i t has been shown t h a t t h e c o n c e n t r a t i o n s o f these s p e c i e s , as w e l l as o t h e r s , a r e h i g h l y s e n s i t i v e t o t h e pH o f t h e s o l u t i o n (7.9). The s t r u c t u r e s o f a l u m i n o s i l i c a t e s a r e l e s s w e l l known t h a n t h o s e o f s i l i c a t e s b u t some i n f o r m a t i o n about t h e s i m p l e r 29 27 a l u m i n o s i l i c a t e s has been o b t a i n e d from S i and A l NMR (11.12). C a t i o n s i z e and charge a r e known t o i n f l u e n c e t h e s e l e c t i v i t y o f z e o l i t e synthesis (1). For instance, f a u j a s i t e synthesis i s quite s p e c i f i c t o sodium systems, whereas z e o l i t e L i s formed i n t h e presence o f potassium. The e f f e c t s o f c a t i o n c o m p o s i t i o n on t h e d i s t r i b u t i o n o f s i l i c a t e s p e c i e s has been examined by o n l y a few authors. Ray and Plaisted (13), using trimethylsilation/

0097-6156/88/0368-0222$06.00/0 © 1988 American Chemical Society

Flank and Whyte; Perspectives in Molecular Sieve Science ACS Symposium Series; American Chemical Society: Washington, DC, 1988.


14.

McCORMICK ET AL.

Effect of Alkali Metal Cations

223

chromatography, have found t h a t the p r o p o r t i o n o f s i l i c o n observed i n o l i g o m e r i c s p e c i e s i n c r e a s e s as the base i s changed from LiOH t o CsOH. D i f f e r e n c e s i n the d i s t r i b u t i o n o f s i l i c a t e s p e c i e s o f Na silicate and Κ silicate solutions can also be deduced from 29 d i f f e r e n c e s i n the h i g h r e s o l u t i o n S i NMR spectra of Κ silicates (8) and Na s i l i c a t e s ( 9 ) . The work p r e s e n t e d h e r e was aimed a t e s t a b l i s h i n g the i n f l u e n c e o f d i f f e r e n t a l k a l i m e t a l c a t i o n s on the d i s t r i b u t i o n o f s i l i c a t e a n i o n s i n aqueous s o l u t i o n s such as t h o s e u s e d i n z e o l i t e s y n t h e s i s . Changes i n the e x t e n t o f s i l i c a t e o l i g o m e r i z a t i o n were i d e n t i f i e d 29 using S i NMR s p e c t r o s c o p y . The r e s u l t s o b t a i n e d i n d i c a t e t h a t the i n f l u e n c e o f c a t i o n c o m p o s i t i o n i s e x p r e s s e d t h r o u g h the f o r m a t i o n o f cation-anion pairs. D i r e c t e v i d e n c e f o r the p r e s e n c e o f such i o n p a i r s has been o b t a i n e d u s i n g a l k a l i m e t a l c a t i o n NMR spectroscopy and i s p r e s e n t e d elsewhere ( 1 4 ) . EXPERIMENTAL A l k a l i m e t a l s i l i c a t e s o l u t i o n s were p r e p a r e d from Baker a n a l y z e d SiO^ g e l , r e a g e n t grade a l k a l i m e t a l h y d r o x i d e s , and d e i o n i z e d water. S t o c k s o l u t i o n s a t about 3 mol% SiC> , S i 0 / M 0 = 3 (M - a l k a l i m e t a l ) were aged i n p o l y p r o p y l e n e b o t t l e s f o r p e r i o d s o f s e v e r a l weeks, t o assure t h a t a l l the S i C ^ had dissolved. Samples were then formulated with extra water and base to achieve a desired composition. 29 Si NMR spectra were obtained with a Bruker AM-500 spectrometer a t 99.36 MHz. About 100 70° p u l s e s were u s e d w i t h r e l a x a t i o n d e l a y s o f about 5 times the s p i n - l a t t i c e r e l a x a t i o n time, T^. Values of T^ were estimated from inversion recovery experiments. R e l a x a t i o n times v a r i e d from 0.5 s f o r v e r y a l k a l i n e Na s i l i c a t e s t o 7 s f o r L i s i l i c a t e s . A l l s p e c t r a were r e c o r d e d a t room temperature. A g l a s s membrane e l e c t r o d e was u s e d t o measure the pH o f the s i l i c a t e solutions. A d s o r p t i o n o f the s m a l l e r a l k a l i m e t a l c a t i o n s , K, Na, and L i , caused an u n d e r e s t i m a t i o n o f the pH. The pH e r r o r r e a c h e d s e v e r a l pH u n i t s f o r the most a l k a l i n e L i s i l i c a t e samples. On the o t h e r hand, the e r r o r s f o r Na s i l i c a t e s were around 0.5 pH unit and the errors for larger c a t i o n s were r e l a t i v e l y small. C o r r e c t i o n s t o the measured pH were made by comparison o f the e l e c t r o d e response f o r the s i l i c a t e s o l u t i o n s w i t h t h a t f o r a l k a l i metal hydroxide solutions of known c o n c e n t r a t i o n . Since the h y d r o x i d e c o n t e n t o f the base s o l u t i o n s i s known, the i n t e r f e r e n c e from c a t i o n a d s o r p t i o n was c a l c u l a t e d and added t o the pH measured f o r the s i l i c a t e s o l u t i o n s ( 1 6 ) . 2

2

2

RESULTS 29 F i g u r e s 1, 2, and 3 show S i NMR s p e c t r a of a l k a l i metal s i l i c a t e s o l u t i o n s w i t h s i l i c a t e r a t i o s (R = [ S i 0 ] / [ M 0 ] ) o f R - 0.4, 1.5, and 2.0, r e s p e c t i v e l y . The t o t a l S i 0 c o n c e n t r a t i o n i s 1.75 mol% 2

2

9


224

PERSPECTIVES IN MOLECULAR SIEVE SCIENCE

R = 0.4

Li

1


0

^^^^^^

Cs

π

2

0

-

2

Γ "

1

-

4 S

sj

-6 (ppm)

-8

-10

-12

XBL 881-141

29 F i g u r e 1. S i NMR s p e c t r a metal s i l i c a t e s o l u t i o n s .

o f 1.75 mol% S i O

R=0.4


alkali

14. MCCORMICK ET AL.

225


R = 1.5

Li

Q Qf


Να

r , 2

R=2.0

alkali


14.

227


McCORMICK ET AL.

f o r R - 0.4, and 3.0 mol% f o r R = 1.5 and 2.0. Each peak i n t h e s e spectra represents a d i s t i n c t s i l i c o n environment. I n each f i g u r e , the c h e m i c a l s h i f t i s r e f e r e n c e d t o t h a t o f the monomer i n the L i s i l i c a t e s o l u t i o n (which appears a t 72 ppm r e l a t i v e t o TMS). The d i s t r i b u t i o n o f s i l i c o n c o n n e c t i v i t i e s was d e t e r m i n e d by i n t e g r a t i n g the

peaks

through

3

occurring

in

(7) .

results

The

the

regions of

designated

by

Q

1

in

Figures

these

c a l c u l a t i o n s are p r e s e n t e d i n 3 F i g u r e s 4 t h r o u g h 6. Because Q and Q peaks o c c u r i n o v e r l a p p i n g 2 3 r e g i o n s , the amount o f s i l i c o n p r e s e n t i n Q and Q environments i s shown as a s i n g l e b a r . The s i l i c a t e s o l u t i o n s a t R - 0.4 e x h i b i t o n l y t h r e e peaks, a s s i g n a b l e t o the monomer, dimer and c y c l i c t r i m e r ( 9 ) . The l i n e a r 2 t r i m e r i s r u l e d out because o f the absence o f Q resonances. The l a r g e r c a t i o n s produce h i g h e r c o n c e n t r a t i o n s o f the c y c l i c trimer t h a n do the s m a l l e r c a t i o n s . The L i s i l i c a t e spectrum i n F i g . 1 ought n o t be compared d i r e c t l y w i t h the o t h e r s p e c t r a , since a substantial amount o f l i t h i u m i n o s i l i c a t e p r e c i p i t a t e d from the solution. The r e s u l t i n g s u p e r n a t a n t s o l u t i o n was f o u n d t o have a c o m p o s i t i o n o f o n l y 0.03 mol% SiO^ and a s i l i c a t e r a t i o o f 0.02. The peaks i n the spectra of Figure 1 become progressively s h i e l d e d w i t h i n c r e a s i n g c a t i o n s i z e ; the monomer peak i s about 1.5 ppm f u r t h e r s h i e l d e d i n the Cs s i l i c a t e s o l u t i o n compared t o the Na s i l i c a t e solution. This trend i s r e a d i l y apparent only at low v a l u e s o f s i l i c a t e r a t i o and so i s n o t seen i n F i g s . 2 and 3. It is a l s o a p p a r e n t from F i g . 1 t h a t the f u l l w i d t h a t h a l f maximum (FWHM) o f the peaks i s a s t r o n g f u n c t i o n o f the c a t i o n . The FWHM i s l a r g e i n the p r e s e n c e o f i n t e r m e d i a t e s i z e c a t i o n s (Na and K) and i s s m a l l i n the p r e s e n c e o f L i and Cs. At R = 1.5 the s p e c t r a ^ d i s p l a y resonances i n a l l of the c o n n e c t i v i t y regions except Q ( -34 ppm). As the c a t i o n s i z e i n c r e a s e s , s i l i c o n i s d i s p l a c e d from lower c o n n e c t i v i t y s t a t e s to 3 3 states with Q and Q connectivity. A t R - 2.0, the amount o f S i 3 0 1 i n the Q environment i n c r e a s e s whereas the amounts i n Q , Q , and 2 Q environments d e c r e a s e w i t h i n c r e a s i n g c a t i o n s i z e . The p o r t i o n 2 3 o f S i i n the sum o f the Q and Q environments d e c r e a s e s , b u t i t i s 3 e v i d e n t from the s p e c t r a t h a t the s t r o n g Q r e s o n a n c e a t -18 ppm, a s s i g n a b l e to the p r i s m a t i c hexamer (8.), i n c r e a s e s w i t h c a t i o n s i z e . Hence we o b s e r v e t h a t the p o p u l a t i o n o f c a g e l i k e s p e c i e s i n c r e a s e s a t the expense o f s m a l l e r c h a i n s p e c i e s w i t h i n c r e a s i n g c a t i o n s i z e . To i n v e s t i g a t e whether the a d d i t i o n o f a n e u t r a l a l k a l i m e t a l s a l t causes a r e d i s t r i b u t i o n o f the s i l i c a t e s p e c i e s , a 1.75 mol% S i 0 2 , R = 0.4 Cs silicate s o l u t i o n was doped w i t h 46 wt% LiCl s o l u t i o n to a c h i e v e l i t h i u m c a t i o n mole f r a c t i o n s o f 0.42 and 0.59. The resulting connectivity distributions obtained from band i n t e g r a l s are shown i n F i g u r e 7. T h i s f i g u r e shows t h a t L i addition c a u s e s a moderate d e p o l y m e r i z a t i o n o f the s i l i c a t e s p e c i e s p r e s e n t i n the Cs s i l i c a t e s o l u t i o n . 2


1



228


F i g u r e 4. The i n f l u e n c e o f t h e c a t i o n on the d i s t r i b u t i o n o f S i among t h e c o n n e c t i v i t y s t a t e s a t t h e c o m p o s i t i o n 1.75 mol% SiO„, R=0.4.

Li

Να

Κ

Rb

Cs XBL

881-138

F i g u r e 5. The i n f l u e n c e o f t h e c a t i o n on the d i s t r i b u t i o n o f S i among t h e c o n n e c t i v i t y s t a t e s a t t h e c o m p o s i t i o n 3.0 mol% SiO R=1.5.



14.

McCORMICK ET

AL.


229

The pH o f the s i l i c a t e s o l u t i o n s w i t h R = 2.0 and R - 1.5 are given i n T a b l e 1. The l i s t e d values are c o r r e c t e d f o r c a t i o n interference. A c c u r a t e pH d e t e r m i n a t i o n s f o r L i s i l i c a t e s o l u t i o n s and s i l i c a t e s o l u t i o n s f o r which R = 0.4 c o u l d n o t be made because t h e e r r o r due t o c a t i o n i n t e r f e r e n c e was on the o r d e r o f 2 pH u n i t s . S i n c e c a t i o n - a d s o r p t i o n i n d u c e d e r r o r s i n pH measurements were s m a l l a t R — 2.0, the c o r r e c t e d pH v a l u e s f o r t h e s e s o l u t i o n s were u s e d t o e s t i m a t e the average charge p e r S i atom i n s o l u t i o n . This was done by s u b t r a c t i n g the OH c o n c e n t r a t i o n , d e t e r m i n e d from the pH, from the total anion concentration required to balance the concentration of cations i n s o l u t i o n and t h e n d i v i d i n g by the c o n c e n t r a t i o n o f d i s s o l v e d SiO^. (The pH v a l u e s f o r R - 1.5 a r e not considered p r e c i s e enough f o r t h i s c a l c u l a t i o n b u t a r e u s e f u l i n d i s c e r n i n g whether monotonie pH changes w i t h c a t i o n s i z e o c c u r . ) As shown i n T a b l e 1, t h e s e c a l c u l a t i o n s i n d i c a t e t h a t the average charge per silicon atom does not decrease as the cation size increases. DISCUSSION The results presented in Figures 1-3 demonstrate that the d i s t r i b u t i o n of s i l i c a t e anions i n a l k a l i n e s i l i c a t e s o l u t i o n s i s a f f e c t e d t o a moderate degree by the c h o i c e o f the a l k a l i m e t a l cation. I t i s e v i d e n t from F i g u r e s 4-6 t h a t the p r o p o r t i o n o f S i p r e s e n t i n s i n g l e and d o u b l e r i n g s i n c r e a s e s p r o g r e s s i n g from L i to Cs, i . e . , i n the d i r e c t i o n o f i n c r e a s i n g c a t i o n r a d i u s . Further d e m o n s t r a t i o n t h a t c a t i o n c o m p o s i t i o n i n f l u e n c e s the d i s t r i b u t i o n o f s i l i c a t e s t r u c t u r e s i s e v i d e n t from F i g u r e 7, which shows the e f f e c t s o f i n t r o d u c i n g L i C l to a Cs s i l i c a t e s o l u t i o n . With i n c r e a s i n g presence of L i c a t i o n s t h e r e i s a moderate r e d i s t r i b u t i o n o f the s i l i c a t e anions to^lower molecular weight s p e c i e s . F i g u r e 7 shows as w e l l t h a t when L i c o n s t i t u t e s 59% o f the c a t i o n s i n s o l u t i o n t h e r e i s a n o t a b l e d e c r e a s e i n the p r o p o r t i o n o f c y c l i c t r i m e r r e l a t i v e to dimer a n i o n s . The p r e s e n t r e s u l t s are c o n s i s t e n t w i t h t h o s e +

o f Ray and P l a i s t e d (13), who showed by means o f t r i m e t h y l s i l a t i o n / chromatography that the f r a c t i o n of Si present in oligomeric structures increases with increasing cation size. Also, in agreement w i t h the trends of the present work, these authors o b s e r v e d t h a t the a d d i t i o n o f KC1 t o a Na s i l i c a t e s o l u t i o n r e s u l t e d i n a modest i n c r e a s e i n the degree o f s i l i c a t e o l i g o m e r i z a t i o n . S i n c e pH i s known t o i n f l u e n c e s t r o n g l y the d i s t r i b u t i o n o f s i l i c a t e a n i o n s i n a l k a l i n e s o l u t i o n s ( 7 . 9 ) , one might e x p e c t the o b s e r v e d changes i n s i l i c a t e a n i o n d i s t r i b u t i o n w i t h c a t i o n s i z e t o be a r e s u l t o f changes i n pH. R e f e r e n c e t o T a b l e 1 shows, however, that for a fixed silicate ratio, the pH of the solutions is r e l a t i v e l y independent o f the base c o m p o s i t i o n and c e r t a i n l y does not increase with cation size, as might be expected from a consideration of hydroxide d i s s o c i a t i o n constants (17). This s u g g e s t s t h a t the i n f l u e n c e o f c a t i o n c o m p o s i t i o n i s a consequence o f d i r e c t c a t i o n - a n i o n i n t e r a c t i o n s , r a t h e r t h a n an e f f e c t o f pH. It is well known that cation-anion pairs can form in concentrated s o l u t i o n s o f i o n i c compounds ( 1 8 ) . The e x t e n t o f i o n



230


Li

Να

Κ

Rb

Cs XBL

881-139

F i g u r e 6. The i n f l u e n c e o f t h e c a t i o n on t h e d i s t r i b u t i o n o f S i among t h e c o n n e c t i v i t y s t a t e s a t t h e c o m p o s i t i o n 3.0 m o l % S i 0 , R=2.0. 2

T a b l e 1.

pH o f S i l i c a t e

Cation

pH 3 mol%

Na Κ Rb Cs

Si0 , 2

Charge/Si R=2 0 0 0 0 0

12.8 13.0 13.2 13.0 3 mol%

Na Κ Rb Cs

Solutions

Si0 , 2

14.1 13.6 13.7 13.9

R=l

46 46 45 47

5 _

-


MCCORMICK ET AL.


80r » mol% S i 0

2


70605040302010-

0-

0.42

0.59

L i C I / ( L i + Cs) XBL 881-140

F i g u r e 7. The i n f l u e n c e o f L i C l a d d i t i o n on the d i s t r i b u t i o n o f S i among t h e c o n n e c t i v i t y s t a t e s o f the Cs s i l i c a t e s o l u t i o n w i t h the c o m p o s i t i o n 1.75 mol% S i 0 , R=0.4. 0


231


232


concentrated s o l u t i o n s o f i o n i c compounds (18)· The e x t e n t o f i o n pair formation increases with increasing ion concentration, decreasing c a t i o n s i z e , and d e c r e a s i n g dielectric strength o f the s o l v e n t (18). K i n r a d e and Swaddle (19) have i n v o k e d t h e p r o x i m i t y of a l k a l i metal c a t i o n s t o s i l i c a t e s t o e x p l a i n anomalously s h o r t 29 Si spin lattice r e l a x a t i o n times. D i r e c t evidence o f p a i r i n g between a l k a l i m e t a l c a t i o n s and s i l i c a t e a n i o n s has r e c e n t l y been presented by McCormick e t a l . (14) u s i n g alkali metal NMR spectroscopy (14). The f o r m a t i o n o f c a t i o n - a n i o n p a i r s was f o u n d t o i n f l u e n c e b o t h t h e c h e m i c a l s h i f t and s p i n - r e l a x a t i o n time o f t h e cation. A d d i t i o n a l m a n i f e s t a t i o n s o f i o n p a i r i n g were o b s e r v e d i n s t u d i e s o f t h e s i l i c o n s p i n r e l a x a t i o n and exchange between s i l i c a t e a n i o n s (15.) . T h i s work r e v e a l e d t h a t c a t i o n - a n i o n c o n t a c t s a f f e c t the s i l i c o n v a l u e s , an e f f e c t a l s o seen i n F i g u r e 1. The way i n which cation-anion pairs might i n f l u e n c e the d i s t r i b u t i o n o f s i l i c a t e a n i o n s c a n be i l l u s t r a t e d w i t h t h e a i d o f r e a c t i o n s 1-5 below: 1.

M~

+ H 0 2

-

M

2.

M

+

M*

-

D"

3.

D"

+ HO

-

D

4.

M"

+

=

CM

5.

D"

+

-

CD

c* C

+

+ OH" +

H 0 2

+ OH*

Here M and D represent the a n i o n i c species Si(0H)^0 and ( H 0 ) ^ S i 0 S i ( 0 H ) 0 , r e s p e c t i v e l y , and M and D a r e t h e c o r r e s p o n d i n g n e u t r a l s p e c i e s S i ( O H ) ^ and ( H 0 ) S i 0 S i ( 0 H ) . C i s t h e c a t i o n and 2

3

3

CM and CD are cation-anion pairs formed with M and D , respectively. I t i s recognized that reactions 1-5 e x c l u d e t h e possibility t h a t M and D e x i s t i n m u l t i p l y , as w e l l as s i n g l y , ionized states; t o i n c l u d e such m u l t i p l y i o n i z e d s p e c i e s would complicate the subsequent discussion, without contributing additional insight. R e a c t i o n s 1-3 i n d i c a t e t h a t d i m e r i z a t i o n i s p r e f e r r e d a t low pH. I f one i n c l u d e s r e a c t i o n s f o r t h e f o r m a t i o n o f h i g h e r order o l i g o m e r s and t h e i r n e u t r a l i z a t i o n ( i . e . , t h e a n a l o g s o f r e a c t i o n 2 and 3 ) , then i t i s concluded that the extent of oligomerization i n c r e a s e s w i t h d e c r e a s i n g pH. T h i s t r e n d i s i n e x c e l l e n t agreement with the l i t e r a t u r e on silicate chemistry (7.9.20) and i s e x e m p l i f i e d by t h e r e s u l t s p r e s e n t e d i n F i g u r e s 1-3 f o r i n d i v i d u a l cations. R e a c t i o n s 4 and 5 r e p r e s e n t the formation of cation-anion pairs. I f the e q u i l i b r i u m constants f o r these r e a c t i o n s , and Κ,., are i d e n t i c a l , c a t i o n - a n i o n p a i r f o r m a t i o n w i l l have no i n f l u e n c e on the e x t e n t o f d i m e r i z a t i o n a t a f i x e d pH. On t h e o t h e r hand, i f K^/K^ > 1, t h e n t h e f o r m a t i o n o f dimers w i l l be enhanced. The q u e s t i o n t o be posed, then, i s what i s t h e e f f e c t o f c a t i o n s i z e on and K,., and more i m p o r t a n t l y , on K^/K^. Based on Bjerrum's model o f i o n p a i r i n g ( 1 8 ) , i t i s e x p e c t e d that the e q u i l i b r i u m constant f o r i o n p a i r formation o f a given



14.

McCORMICK ET AL.


233

anion with a c a t i o n decreases with i n c r e a s i n g c a t i o n s i z e . As a consequence and would b o t h be expected to decrease in magnitude as one p r o c e e d s from L i t o Cs . D i r e c t experimental c o n f i r m a t i o n o f t h i s t r e n d i s not a v a i l a b l e , b u t s u p p o r t f o r i t can be inferred from the following observations. First, the p r e c i p i t a t i o n o f the L i i n o s i l i c a t e s a l t a t s o l u t i o n c o m p o s i t i o n s (R — 0.4) which remain s t a b l e i n the presence of larger cations indicates that Li cations bind to small silicate a n i o n s more s t r o n g l y t h a n do l a r g e r c a t i o n s . Second, u s i n g a Born-Haber c y c l e and measured h e a t s o f f o r m a t i o n o f a l k a l i m e t a l o r t h o s i l i c a t e s (21) > we have e s t i m a t e d t h a t the h e a t o f f o r m a t i o n o f a L i OSi i o n p a i r i s 25.5 k c a l / m o l h i g h e r t h a n t h a t o f a Κ OSi p a i r i n the absence o f the m o d e r a t i n g e f f e c t s o f h y d r a t i o n . From t h i s we would i n f e r t h a t for L i i s l a r g e r than f o r Κ . F o r h a r d sphere i o n s , the i o n p a i r i n g e q u i l i b r i u m c o e f f i c i e n t i s roughly i n v e r s e l y p r o p o r t i o n a l to the d i s t a n c e between charge c e n t e r s i n the i o n p a i r . T h i s r e l a t i o n s h i p a l l o w s us t o p r e d i c t t h a t f o r h a r d s p h e r e s K^/K^ increases with increasing c a t i o n size. S i l i c a t e a n i o n s , though, cannot be t r e a t e d as s p h e r i c a l . Cations may a t t a c h a t s m a l l c h a r g e d s i t e s on the s u r f a c e o f the a n i o n , and as a consequence the d i s t a n c e between charge c e n t e r s i s an unknown f u n c t i o n o f the c a t i o n s i z e . The e f f e c t s o f c a t i o n s i z e on K^/K^ f o r s i l i c a t e a n i o n s cannot be determined directly, since the necessary thermochemical information i s unavailable. I n d i r e c t e v i d e n c e f o r an i n c r e a s e i n K,-/K^ w i t h i n c r e a s i n g c a t i o n s i z e can be drawn, however, from NMR observations o f the i n t e r a c t i o n s between a l k a l i m e t a l c a t i o n s and s i l i c a t e anions (14). These s t u d i e s i n d i c a t e t h a t o ^ i g o m e r i c a n i o n s s h i e l d the n u c l e u s o f Cs more e f f i c i e n t l y t h a n Na and t h a t Cs s i l i c a t e p a i r i n g e x h i b i t s higher s e l e c t i v i t y f o r l a r g e oligomers than does Na - s i l i c a t e p a i r i n g . Moreover, L i e b a u (22) has s u g g e s t e d t h a t large polarizable cations can b i n d more e f f e c t i v e l y than small c a t i o n s w i t h r i g i d r i n g and cage s i l i c a t e a n i o n s . Thus we a r e l e d t o p r o p o s e t h a t K^/K^ can i n c r e a s e w i t h c a t i o n s i z e t o cause the observed anion d i s t r i b u t i o n s h i f t to l a r g e r s i l i c a t e s . The i n f o r m a t i o n p r e s e n t e d i n T a b l e 1 i n d i c a t e s t h a t f o r a f i x e d s i l i c a c o n c e n t r a t i o n and s i l i c a t e r a t i o , the pH and thus the average n e g a t i v e charge p e r d i s s o l v e d S i atom i s n e a r l y i n d e p e n d e n t o f c a t i o n composition. S i n c e the e x t e n t o f s i l i c a t e o l i g o m e r i z a t i o n i n c r e a s e s p r o c e e d i n g from L i t o Cs , t h i s i m p l i e s t h a t the n e g a t i v e c h a r g e p e r s i l i c a t e anion increases with c a t i o n s i z e . Estimates of t h i s e f f e c t show t h a t a t R - 2.0, the average charge p e r a n i o n i n the C s - s i l i c a t e s o l u t i o n i s 8-10% g r e a t e r t h a n t h a t i n the L i - s i l i c a t e s o l u t i o n . The c o r r e s p o n d i n g i n c r e a s e i n average charge p e r a n i o n i s about 31% a t R 1.5. Further e v i d e n c e f o r the change i n average charge p e r anion w i t h i n c r e a s i n g c a t i o n s i z e can be drawn from the o b s e r v e d i n c r e a s e i n the chemical s h i f t ( i n c r e a s i n g s h i e l d i n g ) o f the peaks w i t h i n c r e a s i n g c a t i o n s i z e , shown i n F i g u r e 1. H a r r i s (8) has o b s e r v e d t h a t the s h i e l d i n g o f S i n u c l e i i n c r e a s e s w i t h s o l u t e concentration


234


at a constant silicate ratio. Since the pH (23) and the silicate distribution [9] remain roughly constant in such a case, the observed shift can be attributed to an increase in the charge on each anion. This suggests that the shielding of the Si nucleus increases in the order =SiOH < =SiO . Thus the shielding trend observed in Figure 1 is consistent with the theory that the silicate anions in the Cs-silicate solution have a greater number of ionized surface oxygens than in the Na-silicate solution.


CONCLUSIONS The distribution of silicate anions in alkaline silicate solutions changes with the composition of the base. At a fixed SiO^ concentration and silicate ratio, the proportion of Si present in high molecular weight and cage-like structures increases in progressing from Li to Cs hydroxide. This trend is ascribed to cation-silicate anion pairing and to the higher selectivity to ion pairing by large silicate anions as cation size increases. An elementary estimate of the energetics of ion pairing confirm that major differences in pairing equilibrium are to be expected as the cation size is changed. Increasing cation size also increases the degree of ionization of individual anion structures. These trends may contribute to the cation selectivity of zeolite synthesis.

Acknowledgment This work was supported by the Director, Office of Basic Energy Sciences, Material Science Division of the U.S. Department of Energy under contract DE-AC03-76SF00098 and by a grant from W.R. Grace and Company. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

Barrer, R. M. The Hydrothermal Chemistry of Zeolites; Academic Press: London, 1982. Zhdanov, S. P. In Molecular Sieves I, Advances in Chemistry Series No. 101; American Chemical Society: Washington, DC, 1971, p20. Derouane, E. Zeolites 1982, 2, 299. Engelhardt, G. Zeolites 1985, 5, 49. Wieker, W.: Fahlke, B. In Zeolites: Drzaj, H.; Hocevar, S.; Pejovnik, S., Eds.; Elsevier: New York, 1987; p 161. Marsmann, T. Ζ.Naturforschung 1974, 295, 495. Engelhardt, Z. An Allg. Chem. 1975, 418, 17. Harris, R. K.; Knight, C. T. G., J . Chem. Soc., Faraday 2 1983, 79, 1525 and 1539. McCormick, Α. V.; Bell, A. T.; Radke, C. J . Zeolites 1987, 7, 183. McCormick, Α. V.; Bell, A. T.; Radke, C. J., to be published. Dent Glasser, L. S.; Harvey, G. Proc. 6th Intl. Zeol. Conf.; Bisio. Α.; Olson, D.H., Eds.; Butterworth: Guildford, U.K., 1984; p 925.


14. McCORMICK ET AL.



12.

McCormick, Α. V.; Bell, A. T.; Radke, C. J. In New Develop ments in Zeolite Science andTechnology;Murakami, Y.; Iijima, Α.; Ward, J.W., Eds.; Studies in Surface Science and Catalysis Vol. 26; Elsevier: 1986; New York, p 247. 13. Ray, W.H.; Plaisted, R. J. J. Am. Chem. Soc. 1986, 108, 7159 14. McCormick, Α. V.; Bell, A. T.; Radke, C. J. to be published. 15. McCormick, Α. V.; Bell, A. T.; Radke, C. J . to be published. 16. Heydweiler, A. Ann. Phys. 1915, 48 (4), 688. 17. Gmelin Handbuch der Anorganische Chemie; Springer-Verlag: Berlin, 19 . 18. Bockris, J. O'M.; Reddy, Α. Κ. N. Modern Electrochemistry; Plenum: New York, 1970. 19. Kinrade, S. D.; Swaddle, T. W. J. Am. Chem. Soc.. 1986, 108, 7159. 20. Iler, R. K. The Chemistry of Silica; Wiley: New York, 1979. 21. McCormick, A.V. Ph.D. Thesis, University of California, Berkeley, CA 1987. 22. Liebau, F. The Structural Chemistry of the Silicates; Springer-Verlag: Berlin, 1986. 23. Weldes, H. H.; Lange, K. R. Ind. Eng. Chem. 1969, 61 (4), 29. RECEIVED February 3, 1988


235

Perspectives in Molecular Sieve Science - American Chemical Society

Recommend Documents