Molecular Sieves

13

Downloaded by UNIV OF NORTH CAROLINA on December 1, 2014 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0121.ch013

Infrared Studies of Aqueous Silicate Solutions WILLIAM C. BEARD

a

Union Carbide Corp., Tarrytown, Technical Center, Tarrytown, N.Y.

10591

Different zeolite phases nucleated when the silica source is varied may be related to the size and structure of silicate species in solution. Infrared spectroscopy proved successful as a new means of characterizing unaltered, concentrated silicate solutions, permitting direct correlation with behavior in zeolite synthesis. Silicate

solutions (15% SiO )with SiO /M O, M = Na, TMA 2

2

2

(tetramethylammonium) varying from 0.5 to ~100 were examined. All solutions showed an absorption band around 1000 cm-1. As the ratioSiO2/M2Oincreased the absorption maxima shifted from ~950 (dimer or monomer) to 1120 cm (molecular weight near one million). Solutions containing TMA had two peaks at ~1025 and 1120 cm , corresponding to a stable mixture of low (sodium metasilicate) and high (colloidal sol) molecular weight species, respectively. -1

-1

/

"|" he zeolite phase f o r m e d f r o m a g i v e n gel oxide m i x t u r e i s influenced b y 1

v a r i a b l e s s u c h as s i l i c a a n d a l u m i n a source, p H , w a t e r c o n t e n t , m i x i n g technique, aging, digestion time, a n d temperature. T o gain information o n t h e influence of one of these v a r i a b l e s — t h e silica s o u r c e — a means of s t u d y i n g aqueous s i l i c a t e solutions w a s sought. A n e w a p p r o a c h w a s t r i e d b y u s i n g i n f r a r e d a b s o r p t i o n spectroscopy t o detect differences i n silicate s o l u t i o n s of v a r y i n g s i l i c a concentrations, source, base c a t i o n , a n d s i l i c a - t o base r a t i o . T h i s a p p r o a c h h a s p r o v e d successful as a means of c h a r a c t e r i z i n g s i l i c a t e solutions. W i t h t h i s n e w t e c h n i q u e i t i s n o t necessary t o d i l u t e o r c h e m i c a l l y a l t e r t h e solutions i n a n y w a y before o r d u r i n g e x a m i n a t i o n , a n d t h u s a d i r e c t c o m p a r i s o n c a n b e m a d e between t h e observed s p e c t r a a n d t h e solutions used i n synthesis. D i l u t i o n of a silicate s o l u t i o n causes d e p o l y m e r i z a t i o n w h i c h leads t o a shift i n t h e e q u i l i b r i u m of species a

Present address: Department of Geology, Cleveland State University, Cleveland, Ohio 44115. 162 In Molecular Sieves; Meier, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

13.

163

Aqueous Silicate Solutions

BEARD

i n s o l u t i o n . A l s o , the p r e p a r a t i o n a n d r u n n i n g of t h e s a m p l e s p e c t r u m is r e l a t i v e l y easy a n d q u i c k . I n a s t u d y of the i n f r a r e d spectra of a b o u t 50 c r y s t a l l i n e silicates r e p r e s e n t i n g t h e sheet, c h a i n , d o u b l e t e t r a h e d r a l , a n d i s o l a t e d t e t r a h e d r a l groups of the silicate m i n e r a l s , S a k s e n a (1) f o u n d t h a t a l l silicates gave a s t r o n g b a n d near 1000 c m . T h e v a l u e s of t h e S i - 0 s t r e t c h i n g frequency observed i n m a n y of t h e silicate solutions are close t o Saksena's c a l c u l a t e d f r e q u e n c y for i s o l a t e d t e t r a h e d r a a n d higher t h a n the observed v a l u e s for the c r y s t a l l i n e o r t h o s i l i c a t e z i r c o n ( Z r S i 0 ) . T h e s o l u t i o n case w o u l d seem t o a p p r o a c h the s i t u a t i o n of t r u l y i s o l a t e d t e t r a h e d r a m o r e t h a n a c r y s t a l l a t t i c e where the " i s o l a t i o n " is b y v i r t u e of different cations ( Z r + ) a n d perhaps explains t h e better agreement w i t h his c a l c u l a t e d v a l u e . The c o m p a r i s o n between c r y s t a l l i n e v a r i e t i e s of silicate s t r u c t u r a l u n i t s c a n n o t be c a r r i e d too far since studies of i n f r a r e d s p e c t r a h a v e n o t been m a d e o n c r y s t a l l i n e silicates i n w h i c h t h e secondary cations are t h e same a n d a l l o w the effects of silicate t e t r a h e d r a l groupings o n the S i - 0 frequencies t o be isolated a n d evaluated. - 1


4

4

B e n e s i a n d Jones (2) r e p o r t e d a b r o a d 1 1 0 0 - c m b a n d i n t h e i r s t u d y of the w a t e r - s i l i c a gel s y s t e m . T h e y also assigned the 8 7 0 - c m b a n d t o a b e n d i n g v i b r a t i o n of S i O H groups a n d m e n t i o n a reference (3) w h i c h states t h a t silanols h a v e a s t r o n g a b s o r p t i o n b a n d i n t h e 8 3 0 - 8 8 0 - c m " r e g i o n . T h e presence of l i q u i d w a t e r has obscured a n y possible d e t e c t i o n of S i O H f r e q u e n c y i n t h e present i n v e s t i g a t i o n , b u t i t w o u l d p r o b a b l y be detectable if D 0 were to be used as a solvent i n place of w a t e r , because of i t s t r a n s p a r e n c y i n t h i s range of t h e I R s p e c t r u m . - 1

_ 1

1

2

F o r t n u m (4, 6), i n his R a m a n s p e c t r u m s t u d y of aqueous ions, o b served four d i s t i n c t lines a t 448, 607, 777, a n d 935 c m w h i c h he a t t r i b u t e d to the silicate i o n . H e adds t h a t a fifth l i n e is observed a t 1040 c m in solutions h a v i n g l i t t l e or no a d d e d s o d i u m h y d r o x i d e ; however, t h i s l i n e disappears i n solutions h a v i n g large a m o u n t s of s o d i u m h y d r o x i d e w i t h a n increase i n sharpness a n d i n t e n s i t y of t h e 777- a n d 9 3 5 - c m lines. H e says t h a t there are t w o species w h i c h c o u l d h a v e been present t o give rise to the 1 0 4 0 - c m - lines either h y d r o l y z e d silicate i o n , S i O ( O H ) ~ , or a dimer, H Si 07 ~". - 1

- 1

- 1

1

4

2

3

2

H o w e v e r , F o r t n u m concludes t h a t t h e 1040-cm"b y t h e d i m e r i c species of the silicate i o n . F o r t n u m ' s ranged from 3 % to 19.2% S i 0 w i t h S i 0 / N a 0 ratios p e r m i t t i n g a c o m p a r i s o n of his s p e c t r a w i t h those made tion. 1

2

2

2

l i n e is generated s i l i c a t e solutions f r o m 0.33 t o 1.0 i n this investiga-

Experimental A l t h o u g h t h e use of w a t e r as a solvent is u s u a l l y a v o i d e d i n i n f r a r e d spectroscopy, i t c a n be used to o b t a i n spectra of the dissolved substance i n the ranges 930-1580 a n d 1750-2930 c m . T h e silicate solutions were r u n as - 1

In Molecular Sieves; Meier, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

164

MOLECULAR SIEVES

t h i n , l i q u i d films between A g C l plates. W h e n the s o l u t i o n was v e r y caustic a n d a t t a c k e d t h e A g C l plates, t h i n sheets of p o l y e t h y l e n e were used t o c o n t a i n t h e l i q u i d . T h e A g C l plates a n d p o l y e t h y l e n e sheets were c o m p e n sated i n t h e other b e a m of t h e d o u b l e - b e a m spectrometer. W a t e r was generally n o t compensated unless w e w i s h e d t o i n t e n s i f y w e a k a b s o r p t i o n peaks. I n t h e l a t e r I R w o r k B a F plates were used t o c o n t a i n t h e silicate solutions, a n d t h i s is the p r e f e r r e d procedure. I n i t i a l l y , s e v e r a l solutions were m a d e u p w h i c h were assumed t o h a v e g r e a t l y differing silicate species sizes—e.g., s o d i u m m e t a s i l i c a t e (low m o l e c u l a r weight) a n d L u d o x o r N a l c o a g 1050 (colloidal silicas, v e r y h i g h m o l e c u l a r w e i g h t ) . T h e s e solutions were m a d e b y d i s s o l v i n g C a b - O - S i l i n s o d i u m h y d r o x i d e a n d t e t r a m e t h y l a m m o n i u m ( T M A ) h y d r o x i d e solutions t o t h e desired S i 0 / N a 2 0 o r ( T M A ) 0 ratios a n d S i 0 concentrations. S o m e solutions were m a d e b y m e r e l y d i l u t i n g c o m m e r c i a l l y a v a i l a b l e s i l i cate solutions, s u c h as N a l c o a g 1050 a n d D i a m o n d A l k a l i 40, w i t h w a t e r . T h e S K V N a ^ O = 1 solutions were m a d e f r o m s o d i u m metasilicate.


2

2

Table I.

2

2

Infrared Absorption Spectra of Silicate Solutions

Figure No.

Si0 /M 0

M

Si0

Si-0 Absorption Band(s), cm~

1A IB 1C ID IE IF 1G 1H 2A

3.25 3.25 —100 1.0 3.3 0.5 1.0 0.67 2.6

Na TMA Na Na Na Na TMA Na Na

15 15 15 15 15 15 10 15 13.3

1025,1150 (?) 1025,1120 1120 1000 1030,1090 (?) 920 (?),980 (?) 1010,1090 920 (?),985 1020

wt% 2

2

2

Silica Source

x

2B

2.6

Na

13.3

1090,1020

2C

2.6

Na

13.3

1090,1020

2D

2.6

Na

13.3

1020

3A

2.0

Na

20

1120,1025

3B

2.0

Na

20

1055,1025

3C

2.0

Na

20

1035,1015

3D

2.0

Na

20

1035,1010

Cab-O-Sil Cab-O-Sil Nalcoag 1050 Na Si0 -9H 0 Diamond Alkali 40 Cab-O-Sil Cab-O-Sil Cab-O-Sil Nalcoag 1050 + Na Si0 -9H 0 Nalcoag 1050 + Na Si0 -9H 0 Nalcoag 1050 + Na Si0 -9H 0 Nalcoag 1050 + NaSi0 -9H 0 Nalcoag 1050 + Na Si0 -9H 0 Nalcoag 1050 + Na Si0 -9H 0 Nalcoag 1050 + Na Si0 -9H 0 Nalcoag 1050 + Na Si0 .9H 0 2

3

2

2

3

2

2

3

2

2

3

2

3

2

2

3

2

2

3

2

2

3

2

2

3

2

T h e S i 0 / M 0 ( M = N a , T M A ) r a t i o v a r i e d f r o m 0.5 t o 3.25 w i t h t h e c o l l o i d a l silica N a l c o a g 1050 h a v i n g a S i 0 / N a 0 of a p p r o x i m a t e l y 100. A l l of t h e p r e l i m i n a r y solutions c o n t a i n e d 15% S i 0 except t h e S i 0 / T M A 0 = 1 s o l u t i o n w h i c h c o n t a i n e d 10% S i 0 . 2

2

2

2

2

2

2


2

13.


BEARD

165


800

1200

Figure 1. Infrared spectra of silicate solutions with different silica sources and bases (see Table I) Results A l l t h e silicate solutions e x a m i n e d show a b s o r p t i o n peaks a r o u n d 1000 cm (see T a b l e I a n d F i g u r e 1) c o r r e s p o n d i n g t o a n S i - 0 c o m b i n a t i o n s t r e t c h v i b r a t i o n (1). T h e f r e q u e n c y increases w i t h i n c r e a s i n g m o l e c u l a r w e i g h t of t h e silicate species. T h e r e is a shift of t h e p e a k t o h i g h e r w a v e numbers w i t h increasing S i 0 / M 0 . F o r i n s t a n c e , t h e 0.5 S i 0 / N a 0 s o l u t i o n ( F i g u r e I F ) shows a b r o a d peak centered a t a b o u t 950 c m ( a c t u a l l y t w o " p e a k s " at 920 (?) a n d 980 (?) c m " ) , whereas t h e 3.3 S i 0 / N a 0 ( F i g u r e I E ) has a peak a t a b o u t 1030 c m " . T h e N a l c o a g 1050 - 1

2

2

2

2

- 1

1

2

2

1


166

MOLECULAR SIEVES

( S i 0 / N a 0 ~ 100) has a p e a k a t 1120 c m (Figure 1C), corresponding t o a m o l e c u l a r w e i g h t near one m i l l i o n , e s t i m a t e d b y a n a l o g y t o L u d o x c o l l o i d a l sols w h i c h h a v e a p u b l i s h e d p a r t i c l e size of 12 πΐμ. A c c o r d i n g t o H e r (6), t h e m o l e c u l a r w e i g h t of a p a r t i c l e D τημ i n d i a m e t e r is ( 6 9 0 D ) . I n the solutions c o n t a i n i n g T M A ( F i g u r e I B , G ) , t w o peaks appear a t a b o u t 1025 a n d 1120 c m . F o l l o w i n g F o r t n u m ' s s p e c t r a l assignments, 2

- 1

2

3

- 1

t h e l o w m o l e c u l a r w e i g h t species present i n T M A - s i l i c a t e solutions (1025 c m " ) is m o s t p r o b a b l y a d i m e r . I n the 3.25 S i 0 / N a 0 s p e c t r u m ( F i g u r e 1 A ) there are also peaks a t b o t h 1025 a n d 1150 c m , b u t the second peak is n o t v e r y s t r o n g , a n d i t appears as a shoulder. A range of m o l e c u l a r species f r o m m o n o m e r o n u p t o large p o l y m e r s e x i s t i n g i n e q u i l i b r i u m w o u l d be expected i n these solutions w i t h t h e d i s t r i b u t i o n of sizes d e p e n d e n t u p o n t h e c o n c e n t r a t i o n of cations, s o d i u m or t e t r a m e t h y l a m m o n i u m . F r o m t h e t w o r a t h e r w e l l - d e f i n e d peaks of t h e T M A solutions as opposed t o t h e w e a k e r shoulder of t h e N a s o l u t i o n , one m i g h t h y p o t h e s i z e t h a t t h e T M A c a t i o n shifts t h e e q u i l i b r i u m of t h e s o l u t i o n t o a m i x t u r e of l o w a n d h i g h species ( N a S i 0 a n d N a l c o a g 1050 t y p e , r e s p e c t i v e l y ) . 1

2

2


- 1

2

3

T o check t h i s h y p o t h e s i s , a m i x t u r e of N a l c o a g 1050 a n d s o d i u m m e t a silicate solutions was m a d e , a n d I R s p e c t r a were r u n . T h e final s i l i c a t e s o l u t i o n m i x t u r e consisted of 5 g r a m s of N a l c o a g 1050, 7.1 grams of N a S i 0 - 9 H 0 , a n d 17.9 g r a m s of w a t e r t o give a final m i x t u r e of 13.3 w t % S i 0 a n d S i 0 / N a 0 = 2.6. A n I R s p e c t r u m was r u n of t h i s m i x t u r e after a p p r o x i m a t e l y six d a y s a g i n g ( F i g u r e 2 D ) . T h e p a t t e r n shows t w o peaks (1020 a n d 1090 c m " ) a t a p p r o x i m a t e l y the same positions as those of t h e T M A s o l u t i o n s , b u t t h e i r b r o a d c h a r a c t e r i n d i c a t e s a w i d e r range of h i g h a n d l o w m o l e c u l a r w e i g h t species a n d / o r m o r e i n t e r m e d i a t e f o r m s . In a d d i t i o n , t h e h i g h e r w a v e n u m b e r peak, c o r r e s p o n d i n g t o h i g h e r m o l e c u l a r w e i g h t species, is a p p r e c i a b l y l o w e r i n i n t e n s i t y a n d therefore c o n c e n t r a t i o n t h a n t h e p e a k c o r r e s p o n d i n g t o t h e l o w e r m o l e c u l a r w e i g h t species. I t was t h o u g h t t h a t perhaps t h e t w o peaks were a p p r o x i m a t e l y e q u a l i n t e n s i t y i n i t i a l l y a n d t h a t d e p o l y m e r i z a t i o n of t h e h i g h e r w e i g h t species o v e r t h e six d a y s a c c o u n t e d for the lower i n t e n s i t y of the 1 0 9 0 - c m peak. 2

3

2

2

2

2

-

1

- 1

N e x t , a series of I R s p e c t r a was m a d e o n the same m i x t u r e of N a l c o a g 1050 a n d s o d i u m m e t a s i l i c a t e as a b o v e t o d e t e r m i n e the effects of a g i n g (at r o o m t e m p e r a t u r e , u n a g i t a t e d ) o n the m o l e c u l a r w e i g h t d i s t r i b u t i o n . S p e c t r a were m a d e o n i n i t i a l l y m i x e d ( F i g u r e 2 A ) , 7-hour-aged ( F i g u r e 2 B ) , a n d 2 4 - h o u r - a g e d s o l u t i o n s ( F i g u r e 2 C ) . T h e spectra are s h o w n i n F i g u r e 2, w i t h m a x i m u m d e v e l o p m e n t of i n t e n s i t y of the 1 0 9 0 - c m peak - 1

i n t h e 7-hour-aged s o l u t i o n ( F i g u r e 2 B ) . A n o t h e r series of N a l c o a g 1 0 5 0 - s o d i u m m e t a s i l i c a t e solutions w a s ex a m i n e d b y I R s p e c t r o s c o p y t o observe the effect of S i 0 c o n c e n t r a t i o n o n molecular weight distribution. A 2 0 % S i 0 solution w i t h S i 0 / N a 0 = 2 w a s m a d e u p a n d s p l i t i n t o f o u r p a r t s for a t i m e s t u d y . I R s p e c t r a were r u n o n t h e i n i t i a l ( F i g u r e 3 A ) , 7-hour- ( F i g u r e 3 B ) , 24-hour- ( F i g u r e 3 C ) , 2

2

2


2


13.

BEARD

1200

167


1100

cm"'

1000

900

Figure 2. Infrared spectra of Nalcoag 1050-sodium metasilicate mixtures with different periods of aging, 13.8% Si0 2

Figure 3. Infrared spectra of Nal coag 1050-sodium metasilicate mix tures with different periods of aging, 20% Si0 2

a n d t h r e e - d a y - a g e d solutions ( F i g u r e 3 D ) . strong absorption i n the 1 0 0 0 - 1 0 5 0 - c m

- 1

A l l four solutions s h o w e d

region w i t h t h e i n i t i a l l y m i x e d

s o l u t i o n s h o w i n g a shoulder a t a b o u t 1120 c m

- 1

.

T h e o t h e r three s p e c t r a

s h o w t w o s m a l l peaks each r a n g i n g f r o m 1010 t o 1055 c m " . 1

Increasing

t h e S i 0 c o n c e n t r a t i o n decreases t h e c o n c e n t r a t i o n of h i g h e r w e i g h t species 2

as s h o w n b y t h e shoulder i n F i g u r e 3 a n d i n F i g u r e 2. Acknowledgment T h e a u t h o r t h a n k s U n i o n C a r b i d e C o r p . for p e r m i s s i o n t o p u b l i s h t h i s w o r k a n d acknowledges t h e i n i t i a l c o n t r i b u t i o n of Ε. M . F l a n i g e n i n


168

MOLECULAR SIEVES

applying infrared spectroscopy for determining size and structure of silicate and aluminate species in solution and assistance of E. R. Kellberg, R. W. Grose, J. J. Behen, Jr., and J. W. Mysliwiec of the Linde Research Labora tory of Union Carbide Corp.


Literature Cited 1. 2. 3. 4. 5. 6.

Sakesena, B. D., Trans. Faraday Soc. (1961) 57, 242. Benesi, Η. Α., Jones, A. C., J. Phys. Chem. (1959) 63, 179. Richards, R. E., Thompson, H. W., J. Chem. Soc. (1949) 124. Fortnum, D. H., Edwards, J.O.,J.Inorg. Nucl. Chem. (1956) 2, 264. Fortnum, D. H., Ph.D. Dissertation, Brown University, 1958. Iler, R. K., "The Colloid Chemistry of Silica and Silicates," Cornell Univer sity Press, Ithaca, Ν. Y., 1955.

RECEIVED

December 1, 1972.


Molecular Sieves

Recommend Documents