Silanol Groups and Properties of Silica Surfaces - ACS Symposium

11 S i l a n o l Groups and Properties of S i l i c a Surfaces

Downloaded by IOWA STATE UNIV on September 24, 2013 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/bk-1982-0194.ch011

J. J. FRIPIAT C.N.R.S. C.R.S.O.C.I., 45045 Orléans, France

This paper begins by reviewing briefly different techniques, namely precipitation of organic and inorganic compounds, oxidation of volatile silicic compounds and acid attack of magnesium silicates, for preparing porous and non porous silicagels with various surface coverages and distributions of silanol groups, and the effect of heating on these materials. Infrared spectroscopic investigations coupled with chemical specific reactions as well as with magnetic nuclear resonance (NMR) permit a characterization of these surfaces. The chemical properties of silanol groups and of silica surfaces are then studied from the point of view of adsorption processes, involving mainly water, methanol ammonia and amines. Proton interaction and exchange between silanol group and those reagents are studied using physical techniques and particularly pulse NMR. The proton exchange process is related to the acid properties of the silanol groups. Estherification reactions of silanol groups with chloro-alkyl silane, methanol and other reagents and the properties of the reaction products are examined. The reduction of silica surface at high temperature by the so-called spillover process leading to the formation of exposed silicon atoms and Si-H groups, is finally studied. The hydrolysis of soluble silicates or of silicon organic derivatives such as silicic ether in aqueous solution yields silicagels with variable but generally high, specific surface areas. 0097-6156/82/0194-0165$06.00/0 © 1982 American Chemical Society In Soluble Silicates; Falcone, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

166

SOLUBLE

SILICATES

For i n s t a n c e , as shown i n Table I, when 0.6 ml of Si(0C H^) i s mixed with 10 ml of water and t r e a t e d at 150°C under the c o r responding water pressure the p r e c i p i t a t e d s o l i d s outgassed at 100°C have s p e c i f i c s u r f a c e areas between 52 and 578 m /g accor ding to the i n i t i a l pH c o n d i t i o n s * 2

2

Table I - Nitrogen B.E.T. s p e c i f i c surface areas obtained by h y d r o l y z i n g S i ( 0 0 2 ^ ) 4 a t 150° under 6 atm water pressure (J_)


Hydrothermal treatment, aging (days) 2 8 16

Precipitation F i n a l pH

pH

2 2.5

3.5 3.6

5.5 4.5

8 5.2

578 520 365

345 309 155

58 78 52

112 70 115

T h i s i s an i l l u s t r a t i o n among many others of the importance of the aging time, and of the pH of h y d r o l y s i s on the f i n a l p o l y m e r i z a t i o n degree. S i l i c a g e l s are X-rays amorphous but the r a d i a l d i s t r i b u t i o n f u n c t i o n obtained according to the technique i n i t i a l l y proposed by Warren and c o l l a b o r a t o r s (2) r e v e a l s that i n the t e t r a h e d r a l u n i t SiO^ the Si-0 d i s t a n c e s are between 1.66 and 1.61 Â, e.g. i n the domain observed f o r c r y s t a l l i z e d s i l i c a t e s and that there i s some order i n the s t r u c t u r a l arrangement of the second c o o r d i n a t i o n s h e l l (J_) . The Si-0 d i s t a n c e w i t h i n a tetrahedron i s s l i g h t l y s h o r t e r i n gels prepared from S i ( O C 2 H 5 ) , at pH between 3 and 6 and somewhat l a r g e r at lower or higher pH. The S i j - 0 - S i 2 angle v a r i e s between 145 and 180° according to the p r e p a r a t i o n procedure ( 1 ) . In 3 c r i s t o b a l i t e t h i s angle i s 180° whereas i n α and 3 quartz and α c r i s t o b a l i t e i t ranges between 143° and 150°. Thus a s i l i c a g e l may be considered as formed of small u n i t s , i n v o l v i n g the second c o o r d i n a t i o n s h e l l , which are c h a r a c t e r i z e d by s t r u c t u r a l arrangements such as those found f o r these c r y s t a l l i n e solids. S i l i c a g e l s prepared from d i s t i l l e d SiiCK^H^)^ are of course q u i t e pure, the main impurity being N a ions from the NaOH s o l u t i o n used to adjust the pH. With respect to a c r y s t a l l i n e s i l i c a t e c o n t a i n i n g a dense network of s i l i c o n t e t r a h e d r a sharing corners, the main s t r u c t u r e breaking element i s the proton forming inner or e x t e r n a l s i l a n o l groups. In a d d i t i o n , h y d r a t i o n water may be^ r e t a i n e d by these groups because of the formation of S i - O H — 0 ^ hydrogen bonds. The s i l a n o l group may be thus considered as a structural defect. Numerous workers have t r i e d to measure the r e l a t i v e c o n t r i b u t i o n s of the inner and e x t e r n a l s i l a n o l group as w e l l as that of h y d r a t i o n water. I t i s not a simple problem because the amorphous nature of the g e l precludes the use of thermal methods such as +

In Soluble Silicates; Falcone, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

11.

Silanol

FRIPIAT

Groups

and

167

Properties

DTA or TGA. The h y d r a t i o n and c o n s t i t u t i o n a l water are l o s t i n an almost monotonous manner. Figure 1 shows an e a r l y attempt to make that type of d i s t i n c t i o n (3) u s i n g a combination of i n f r a r e d technique and chemical determinations. A l l r e s u l t s are expressed as OH i r r e s p e c t i v e of the simultaneous presence of h y d r a t i o n water and of s i l a n o l groups. The g e l i s the A e r o s i l Degussa obtained by flame p y r o l y s i s of S i C l ^ , I t s N 2 B.E.T. surface area amounts to 180 m /g. Curve 1 i s obtained from the weight l o s s . Curve 2 i s obtained using the r e a c t i o n of 0H s ( s i l a n o l or water) with LiCH^ or CH^Mgl, producing methane, whereas curve 3 i s the h y d r a t i o n water content deduced from the IR absorption bands i n the OH s t r e t c h i n g and the H 0 de formation r e g i o n s . The g e l was outgassed during 45 hrs at 25°C under a dynamic vacuum between 10~5 and 10~6 t o r r before these determinations were c a r r i e d out. The t o t a l OH content was about 2.9 10~ mole/g at 25°C and the e v o l u t i o n of the r a t i o of the surface to the t o t a l hydroxyl content i s shown i n Figure 2. To o b t a i n these r e s u l t s i t was assumed that the surface s i l a n o l s only react with the organometallic reagents. This example i l l u s t r a t e s the f a c t that the q u a n t i t a t i v e de termination of surface s i l a n o l groups r e q u i r e s a combination of d i f f e r e n t techniques, and yet i t r e q u i r e s hypothesis open to c r i t i c i s m s . According to F i g u r e 1, the surface d e n s i t y i n s i l a n o l s i s about 4 3 OH per nm . I t seems s t a b l e up to 300°C and i t s t a r t s decreasing above that temperature. A f t e r h e a t i n g between 600-700°C, the surface d e n s i t y reaches a value of about 1.5 (OH)/.nm . I t i s at t h i s dehydroxylation s t a t e that an i s o l a t e d OH s t r e t c h i n g v i b r a t i o n appears as a narrow band at 3740 cm" . At lower dehydra t i o n temperature but above 250°C, when most of the h y d r a t i o n water i s removed (see Figure 1, curve 3), the s i l a n o l s t r e t c h i n g band i s more complex because of c o n t r i b u t i o n s of i n t e r hydrogen bonds. The value which i s now g e n e r a l l y accepted ( 4 ) f o r surface d e n s i t y i n s i l a n o l s i s about 4.5 OH/nm . I t i s to the p r o p e r t i e s of the surface s i l a n o l s that t h i s c o n t r i b u t i o n i s devoted. f


2

3

2

%

2

1

2

D i s t r i b u t i o n of s i l a n o l groups on s i l i c a

surfaces

On the surface of amorphous s i l i c a g e l s , the d i s t r i b u t i o n of the s i l a n o l groups i s most probably random. T h i s means that there i s some p r o b a b i l i t y that any s i l a n o l group may have a near neigh bour s i l a n o l which might be bound to the same s i l i c o n or most probably, which i s l i n k e d to an adjacent s i l i c o n i n a =Si(0H)-0Si(OH)=arrangement. For instance on a deuterated A e r o s i l surface outgassed at 27°C, the s t r e t c h i n g 0D r e g i o n shows bands at 2760 cm" , 2665 cm" and a shoulder at 2573 cm""l. These bands correspond to OH v i b r a t i o n a l bands at 3740, 3607 and 3480 cm" r e s p e c t i v e l y . The 2573 cm~l band r e i n f o r c e s When D 0 i s p h y s i c a l l y adsorbed whereas the 2760 cm~l i n d i v i d u a l i z e d as a s i n g l e band upon outgassing at i n c r e a s i n g temperature. T h i s band i s , as s a i d before, due to i s o l a t e d deuterated s i l a n o l whereas the 2665 cm" 1

1

1

2

1



168

SOLUBLE

0

100 200

300 400 500

600

700 000

SILICATES

T[°C]

Figure 1. In abscissa: sample outgassing temperature under vacuum. Key: upper curve ( ), OH (gravimetric) content (including H O); curve 2 ( ), evolved CH for the reactions with either CH Li (O) or CH MgI (^); curve 2b, curve 2 corrected for physically adsorbed water; curve 2a; surface silanols content; lower curve (A) H 0 content (in OH) determined by IR spectroscopy. t

k

s

s

2

f 2oL of 0 Figure 2.

I

I I I 200 400

I

I 600

I

I

I

T[C]

Relative surface hydroxyl content as a function of outgassing ture.

tempera-


11.

Silanol

FRIPIAT

Groups

and

169

Properties

1

band (e.g. the 3607 cm"" OH band) may be t e n t a t i v e l y assigned to hydrogen bonded s i l a n o l s . Of course these v i b r a t i o n a l bands are not n e c e s s a r i l y those of surface s i l a n o l s s i n c e d e u t e r a t i o n may a f f e c t i n t e r n a l s i l a n o l s as w e l l . I t has been shown (5) that the r a t e s of i s o t o p i c exchange are d i f f e r e n t f o r i s o l a t e d and bridged s i l a n o l s but these k i n e t i c s data could not be used to c a l c u l a t e t h e i r r e s p e c t i v e c o n t r i b u t i o n s to the s i l a n o l s u r f a c e d e n s i t y . The second moment of the proton NMR resonance l i n e of s i l i c a g e l s from d i f f e r e n t o r i g i n has a l s o been proposed to o b t a i n more s i g n i f i c a n t data ( 6 ) . The c l a s s i c a l equation f o r the second moment M i s 2


M

2

= 3.56 1

ΙΟ"

4 6

2

Σ Σ rT? (gauss )

(1)

1 J

i

j where r . . i s the d i s t a n c e between protons i and j i n a volume that c o n t a i n s N protons. Any motion o c c u r i n g i n the domain of the pro ton n u c l e a r resonance frequency (^ 10 Hz) would reduce the second moment even i f the average d i s t a n c e remains constant. I f the pro tons were homogeneously spread on the s u r f a c e i n r i g i d p o s i t i o n s M should be 0.12 gauss f o r a s u r f a c e d e n s i t y of 4.4 proton/nm ^). J

8

2

2

2

In Table I I , the experimental second moments observed f o r va r i o u s s i l i c a g e l s are given as w e l l as the references t o the paper where a p a r t i c u l a r g e l has been c h a r a c t e r i z e d . The outgassing con d i t i o n s and the temperature dependence of M« are a l s o i n d i c a t e d . Table I I - Second moment of the proton NMR f o r v a r i o u s gels (6) Gel

Outgassing temperature (°C)

M

2

2

(gauss )

resonance

line

observed

Temperature dependence observed f o r M 2

Fibrous gel(6)

100

Aerogel

(8)

100

2.33

Constant -160°C.

from 20°C to

Xerogel

(9)

100

3.00

Constant -160°C.

from 140°C to

Davison

(JO)

500

0.51

Constant -210°C.

from 280°C to

(JLL>

unknown

0.55

unknown.

K

4

V a r i a b l e see Table I I I

10.7

The f i b r o u s g e l , w i t h the highest M was obtained by h y d r o l y z i n g completely asbestos c h r y s o t i l e i n a 6 Ν (50% water, 50% i s o p r o panol) HCl s o l u t i o n at 5 0 ° C In a l l cases, the experimental M are c o n s i d e r a b l y l a r g e r than that c a l c u l a t e d f o r an homogeneous d i s t r i b u t i o n . Because of the 1/r.. dependence of M , t h i s means 2

2

?


170

S O L U B L E SILICATES


that there are patches with higher concentrations i n s i l a n o l s . For the f i b r o u s g e l t h i s can be q u a l i t a t i v e l y explained by sketching the s t r u c t u r e of the h y d r o l y s i s product of the c h r y s o t i l e by the f o l l o w i n g arrangement, each Si-O-Mg bond being r e p l a c e d by one s i l a n o l group. OH OH

The approximate average d i s t a n c e between protons i n such an arrangement i s of the order of 2.3 A whereas f o r the Xerogel i t i s of the order of 3.2 A. I t may thus be expected that upon h e a t i n g the f i b r o u s g e l above 500°C, the number of i s o l a t e d OH w i l l be r a t h e r small since water molecules n u c l e a t e r e a d i l y from coupled OH's. A c t u a l l y no band above 3700 cm" appears i n the f i b r o u s g e l i n opposite to what i s observed f o r the Xerogel under the same c o n d i t i o n s . A l s o the second moment i s a p p r e c i a b l y temperature dependent i n the f i b r o u s g e l (Table I I I ) whereas i t i s p r a c t i c a l l y constant f o r the other gels (Table I I ) . The proton m o b i l i t y i s thus enhan ced by a more r e g u l a r and c l o s e packed d i s t r i b u t i o n of s i l a n o l s . However the NMR technique of measuring second moment or the obser v a t i o n of the OH i n f r a r e d bands as such do not allow to d i s t i n guish between i n t e r n a l and e x t e r n a l OH. 1

Table I I I - V a r i a t i o n of M with the measurement f o r the f i b r o u s s i l i c a g e l (6) 2

Τ

(°K)

temperature

2

S (Gauss ) 2

293

10.7

198

16.5

118

17.5

80

18.6

Measurements of the s u r f a c e d e n s i t y i n s i l a n o l s groups are founded on two types of technique i ) r e a c t i n g the weakly a c i d hydroxy1 group with an adequate reagent l i k e c h l o r o s i l a n e , aminos i l a n e , e t c ; or i i ) i n t e r a c t i n g the s u r f a c e OH with p h y s i c a l l y adsorbed molecule. In both cases u s i n g I.R. the m o d i f i c a t i o n i n the hydroxyl s t r e t c h i n g r e g i o n can by f o l l o w e d . The f i r s t type of method has been b r o a d l y used (12). The* second type has been l e s s popular s p e c i a l l y f o r non-polar molecules condensed at low temperature. The a d s o r p t i o n of rare gases 0 , N , CH^ on the s t r e t c h i n g band of i s o l a t e d s i l a n o l s produces frequency s h i f t s 2

2


11.

FRIPIAT

Silanol

Groups

and

Properties

171

1

between 8 and 43 cm" depending upon the p o l a r i z a b i l i t y o f the adsorbate (13). More r e c e n t l y , the s p e c t r o s c o p i c p r o p e r t i e s of these i s o l a t e d OH upon adsorption of weak hydrogen bond acceptor molecules, l i k e benzene, a c e t o n i t r i l e , e t c . were observed (14). The s h i f t s were o f course l a r g e r than those observed f o r non-polar adsorbates, ran ging from 87 t o 216 cm"" . From hydrogen bonding s t u d i e s i n s o l u t i o n , the frequency s h i f t s f o r two H bond donors R - HX and R X H i n t e r a c t i n g with v a r i o u s acceptors are o f t e n compared by p l o t t i n g the r e l a t i v e s h i f t frequency(Δν/v ) of one donor w i t h respect t o ( Δ ν / ν ) f o r the other. Such BHW p l o t s (Bellamy, Hallam and Williams) (15), are l i n e a r and q u i t e d i f f e r e n t Η-bond accep t o r s f i t onto the same s t r a i g h t l i n e when the proton b e a r i n g atoms i n both bonds are the same. Therefore the frequency s h i f t s observed f o r a r e f e r e n c e proton donor provide a u s e f u l s c a l e f o r p r e d i c t i n g the s h i f t s o f donors c o n t a i n i n g the same f u n c t i o n a l group. The slope i s an estimate of the r e l a t i v e Η bonding s t r e n g t h . With ρ-fluorophenol, f o r example, l i n e a r r e l a t i o n s h i p s have been observed (14) and by comparing the BHW slope and the p K o f v a r i o u s proton donors,the pK o f i s o l a t e d s i l a n o l groups i s determi ned t o be about 7. T h i s i s w e l l i n the range o f the values r e v i e wed by l i e r (4) (p.660). The studies performed on i s o l a t e d s i l a n o l s o f f e r the advantage of being r a t h e r simple t o interprète s i n c e most of these groups are on the e x t e r n a l surface a v a i l a b l e to the reagent (see F i g u r e 2 f o r instance) and the problem i s l e s s complicated than f o r surface bridged OH's. 1

f

f


0

&

It i s a matter f a c t that the problem of d i s t r i b u t i n g the surface hydroxyls i n t o two populations (bridged and i s o l a t e d surface s i l a n o l s ) has not y e t been s a t i s f a c t o r i l y s o l v e d . The techniques o f r e a c t i n g the surface with reagents forming r e a l chemical bonus may be expected t o change the o r i g i n a l surface s t r u c t u r e . Hence, even f o r a g e l heated at 800°C, thus bearing i s o l a t e d s i l a n o l s , over 40% S i C l ^ molecules r e a c t with two OH groups (16). The use o f diborane, f i r s t proposed by Shapiro and Weiss (17) i n 1953, f a i l e d t o lead t o unambiguous r e s u l t s s i n c e none o f the workers (18,20) who had followed by i n f r a r e d t h i s r e a c t i o n agree w i t h each other. A good example o f surface r e c o n s t r u c t i o n by r e a c t i n g the surface o f s i l i c a with a m i l d reagent (CHÔH) has been s t u d i e d i n d e t a i l (21). The methoxylation o f an A e r o g e l s u r f a c e p r e t r e a ted at 110°C i n vacuum was s t u d i e d between 150 and 190°C. I t was found that the r e a c t i o n proceeds not o n l y by es t e r i f i c a t i o n of the s i l a n o l group but a l s o through the opening o f the s i l o x a n e b r i d g e s , as f o l l o w s k

= SiOH + CH-OH

l

> Ξ Si-0-CH + H 0 r

(3)

?

k

2 Ξ Si-O-Si=+CH~0H — - — > Ξ Si-OH + = Si-0-CH

Q

(4)

The competition o f the two r e a c t i o n s i s evidenced by a maximum


172


i n the number o f s u r f a c e s i l a n o l s ( t i t r a t e d by LiCHg) d u r i n g the course o f the r e a c t i o n . The a n a l y s i s o f the experimental r e s u l t s showed t h a t k j / k - 3.0 a t 150°C and 1.5 a t 190°C. Thus, a t h i g h temperature the opening o f s i l o x a n e b r i d g e s c o n t r i b u t e s more e f f i c i e n t l y t o the methoxylation process. I t was a l s o shown i n t h i s work that the probable intermediate i n the r e a c t i o n process i s CHgOH . T h i s aspect w i l l be examined l a t e r . 2

2


Dynamics o f a d s o r p t i o n processes on s i l i c a g e l

surfaces

There have been many s t u d i e s concerned with t h e a d s o r p t i o n o f water on s i l i c a g e l s but i n order to study the dynamic aspects o f these processes, H 0 i s not the best s u i t e d molecule. Indeed pro ton exchange between the adsorbate and t h e s u r f a c e s i l a n o l s and s u r f a c e d i f f u s i o n occur simultaneously and these mechanisms cannot be separated e a s i l y . I t i s f o r t h i s reason that methanol was chosen, f o r the methyl group doesn't exchange w i t h s u r f a c e OH whereas the a l c o h o l i c OH does. By u s i n g CDÔH o r CHÔD and hyd r o x y l a t e d o r deuterated s u r f a c e s , i t i s p o s s i b l e by measuring the ^H o r ^H n u c l e a r resonance r e l a x a t i o n r a t e s , t o d i s t i n g u i s h b e t ween both kinds o f processes. The s p i n - l a t t i c e r e l a x a t i o n r a t e T J obtained by p u l s e n u c l e a r magnetic resonance i s the F o u r i e r transform o f the a u t o - c o r r e l a t i o n f u n c t i o n G ( T ) which d e s c r i b e s the e v o l u t i o n o f the system. 2

1

1

τ" -

G(T)COS ω τ d τ

I J

(4)

o

where G(T)

= < f(t)

f * ( t + τ) >

(5)

f contains the i n f o r m a t i o n about the motions. Random r e o r i e n t a t i o n or t r a n s l a t i o n a l jumps obey g e n e r a l l y the c o r r e l a t i o n f u n c t i o n : G

= < f(0)

(6) c where τ , the c o r r e l a t i o n time, d e f i n e s the time s c a l e o f the microscopic events which causes r e l a x a t i o n , ω i s the resonance frequency. The data obtained i n r e f e r e n c e s (8),(9) and (22) have been reviewed by F r i p i a t (23) and they w i l l be summarized here a f t e r . In order t o understand t h e experimental r e s u l t s , the s u r face h e t e r o g e n e i t y must be accounted f o r . T h i s i s u s u a l l y done by c o n s i d e r i n g a l o g normal d i s t r i b u t i o n o f c o r r e l a t i o n time P(T ) C

d î

f*(0) > exp

= 3" 7T 1

c

1 / 2

exp(-Z/3)

2

d Ζ

(7)

where Ζ = In τ /τ , 3 being the spreading c o e f f i c i e n t o f the d i s t r i b u t i o n f u n c t i o n and τ the average c o r r e l a t i o n time m τ - τ exp(H/RT) (8) ° where Η i s the average a c t i v a t i o n enthalpy o f some k i n d o f motion. m



11.

FRIPIAT

Silanol Groups and Properties

173

Since the adsorbent i s made of a c o l l e c t i o n of surfaces ran domly o r i e n t e d and confined w i t h i n an i n t r i c a t e network of pores, the approximation f o r i s o t r o p i c m o t i o n s ( r e l a t i o n s h i p 6) i s accep table. The a d s o r p t i o n of methanol has been studied f o r two g e l s . The f i r s t , c a l l e d Xerogel, i s c h a r a c t e r i z e d by pores w i t h an average diameter of 17.5 Â and the second c a l l e d Aerogel, contains pores smaller than 10 Â . In order to a s s i g n the c o r r e l a t i o n times to some d e f i n e d motion, information must be obtained about the magnitude of the l o c a l magnetic f i e l d a c t i n g on the proton and a r i s i n g e i t h e r from other protons i n the same, or from other molecules. In t h i s case the measurement of the proton second moment (the average quadratic l o c a l magnetic f i e l d ) allows one to a s s i g n the measured c o r r e l a t i o n time (s) to some defined motion(s), these motion(s) modulating the l o c a l f i e l d and provoking r e l a x a t i o n . In the Xerogel (X) , independently of the degree of coverage (θ) , the second moment at a temperature of the order of -140°C c o r r e s ponds to a molecule i n which the CH^ group i s already r e o r i e n t i n g r a p i d l y around the C« symmetry a x i s . By c o n t r a s t , at that tempe r a t u r e , there i s no f r e e r o t a t i o n of the CH^ group i n the A e r o g e l . When the l i n e a r r e l a t i o n s h i p s shown i n F i g u r e 3 are compared i t appears c l e a r l y that the average a c t i v a t i o n enthalpy (Equation 8) i s of a comparable magnitude i n the s i t u a t i o n s described by the Arrhenius p l o t s 2, 3 and 5 whereas f o r p l o t 4,(Aerogel), i t i s much l e s s . In s o l i d methanol the a c t i v a t i o n enthalpy f o r the r o t a t i o n i s 1.6 k c a l mole"l (29) whereas i n the l i q u i d s t a t e the a c t i v a t i o n enthalpy f o r d i f f u s i o n i s 3.2 k c a l m o l e ~ l . T h i s r e mark and a l s o what has been s a i d about the low-temperature values of the second moment suggest that c o r r e l a t i o n times 2, 3 and 5 i n F i g u r e 3 are those of t r a n s l a t i o n a l jumps, whereas c o r r e l a t i o n time 4 i s that of the methyl group r o t a t i o n . In the l a r g e r pores of Xerogel and i n the temperature range - 140° to + 50°C, the methanol would thus d i f f u s e w h i l e the methyl group i s r o t a t i n g freely. In the narrower pores of Aerogel (A) , and i n the same temperature range d i f f u s i o n would not occur. The thermal a c t i v a t i o n r e s u l t s i n a p r o g r e s s i v e l y f r e e r r o t a t i o n of the methyl group. In Aerogel at decreasing Θ, the methyl group r o t a t i o n becomes p r o g r e s s i v e l y hindered while i n Xerogel,as shown i n Figure 4,the t r a n s l a t i o n a l c o r r e l a t i o n time decreases w i t h Θ. The a c t i v a t i o n enthalpy f o r d i f f u s i o n obtained at d i f f e r e n t degrees of coverage i s shown i n the enclosure. I t increases from about 4 to about 6 k c a l mole" i n p a s s i n g from h a l f to the com p l e t e monolayer content and then i t decreases p r o g r e s s i v e l y toward the value obtained f o r the f r e e l i q u i d at θ > 2. T h i s i n d i c a t e s that the d i f f u s i o n a l motions are s t i l l i n f l u e n c e d by the surface f o r molecules i n the t h i r d l a y e r . 1


174

SOLUBLE


Î

< 2

6 4

2 6

?

»*

ό

-f-

? .11 10

2

/

/

4

3

5

1

6 ΟΟΟΟ/Τ)!*· ]

Figure 3. Correlation times observed at the coverage θ= 1.3 for various systems. Key: 1, H resonance in the CD OH-XOH system, β = 3 and Η = 5.4 kcal/mol; 2, H resonance in the CH OD-XOD system, g_ = 3.25 and H = 5.5 kcal/mol; 3, *H resonance in the same system, β = 4 and Η = 5.2 kcal/mol; 4, H resonance in the CH OH-AOH system, β = 0.8 and Η = 2.32 kcal/mol. X, Xerogel (aver age pore diameter: 17.5 A); A, Aerogel (average pore diameter < 10 A); ω, proton resonance frequency in the 14-kgauss field of the NMR instrument. 2

s

%

s

1

s


FRIPIÀT

Silanol

Groups

and

Properties


11.

Figure 4. Variation of the surface diffusion coefficient measured at three different^ degrees of coverage for the CH OD-XOD system. In enclosure: variation of H with respect to the degree of coverage. s


175

176


I t i s a l s o i n t e r e s t i n g t o p o i n t out that i n agreement w i t h de Boer (24), the a c t i v a t i o n enthalpy i s approximately h a l f the i s o s t e r i c heat o f a d s o r p t i o n obtained from q

2

s t

- - R T [(3£n p)/3T]

e

(9)

Indeed ( 8 ) , between θ = 0.7 and 6 = 1 , q • i n c r e a s e s from 10 t o 14 k c a l mole" and then i t decreases f o r 14 t o 12 k c a l mole" i n going from θ » 1 t o θ = 1.3. The molecular area o f methanol on the Xerogel and Aerogel surfaces i s about 25.5 A a t θ = 1. I f t h i s value i s considered as the q u a d r a t i c d i f f u s i o n a l jump, d i s t a n c e < t > and i f the surface d i f f u s i o n c o e f f i c i e n t i s approximated by 1

2


2

D - < l

2

>/6τ

m

(10)

then the s u r f a c e d i f f u s i o n c o e f f i c i e n t s shown by the s o l i d l i n e i n F i g u r e 5 are obtained f o r the Xerogel a t 25°C. Between the half-monolayer and the monolayer content a r a p i d i n c r e a s e i s observed. For Aerogel, the d i f f u s i o n c o e f f i c i e n t i s probably smaller than 10"10 cm s e c " s i n c e the t r a n s l a t i o n a l motion i s o u t s i d e the range o f o b s e r v a t i o n e.g., τ > 10"^ sec. By comparing the equations o f s t a t e f o r mobile and immobile f i l m s w i t h the i n f o r m a t i o n about the motions obtained by NMR, i t was shown (8) u s i n g the procedure proposed by Ross and O l i v i e r (25) that the equation f o r an immobile f i l m was f i t t e d by the adsorption data f o r Aerogel whereas the data obtained f o r Xerogel obeyed the equation f o r a mobile f i l m . Consider now the c o r r e l a t i o n time corresponding t o l i n e 1 i n F i g u r e 3. I t represents the c o r r e l a t i o n time obtained from the deuteron s p i n - l a t t i c e c o r r e l a t i o n time f o r the CDÔH - X OH sys-* terns a t three degrees of coverage : θ =* 0.8, 1.3, and 1.7, r e s p e c t i v e l y . I n that case there i s no i n f l u e n c e by the degree o f coverage. T h i s i s not s u r p r i s i n g because the quadrupole-inner e l e c t r i c a l f i e l d gradient i p t e r a c t i o n (the s o - c a l l e d quadrupole c o u p l i n g constant,(QCC),represents the main c o n t r i b u t i o n t o the deuterium n u c l e a r r e l a x a t i o n . In that case the c o r r e l a t i o n time has been assigned t o molecules tumbling w i t h i n a s u r f a c e p o t e n t i a l w e l l . Indeed, t h i s motion should imply an average a c t i v a t i o n enthalpy s i m i l a r t o that o f d i f f u s i o n e.g., that of breaking hydrogen bonds, but i t should be coverage independent s i n c e oppo s i t e t o d i f f u s i o n , i t does not i n c l u d e any cooperative e f f e c t . F i n a l l y i t i s i n t e r e s t i n g t o p o i n t out the good agreement between c o r r e l a t i o n times 2 and 3 i n F i g u r e 3. C o r r e l a t i o n time 3 has been computed from the d i f f u s i o n a l c o n t r i b u t i o n t o t h e proton s p i n - l a t t i c e r e l a x a t i o n time measured f o r the CDÔH - X OH system, a f t e r t h e proton exchange c o n t r i b u t i o n has been removed, whereas c o r r e l a t i o n time 2 has been obtained, i n a s t r a i g h t f o r ward manner, f o r the CH 0D-X-0D system. 1

q


FRIPIAT

Silanol

Groups

and

Properties


11.


177

178

SOLUBLE SILICATES

Proton exchange between s i l a n o l s and adsorbate

molecules

Although the s i l a n o l groups are weak a c i d , proton exchange may be observed by adsorbing NH^ f o r i n s t a n c e . T h i s process was studied simultaneously by IR spectroscopy and proton s p i n - l a t t i c e nuclear magnetic r e l a x a t i o n time measurements performed on Aerogel outgassed between 20°C and 200°C (26). Three deformation bands a t t r i b u t a b l e to the adsorbed species were detected at 1450 cm" (NH^), 1600 cm" (NH^) and at 1500 cm"" . The l a t t e r which becomes observable at degrees of coverage of the order or l a r g e r than the monolayer content f o r Aerogel outgassed at 120° or 200°C was t e n t a t i v e l y assigned tô a NH£ NH^ dimer. T h i s suggests that p r o ton may be t r a n s f e r r e d e a s i l y by t u n n e l i n g along the N - H — Ν bond. At the monolayer coverage, the r a t i o (NH^/NHo) was of the order of 30%. At t h i s degree of coverage the jump frequency was about 0.5 10 s e c " at 2 5 ° C T h i s v a l u e compares w e l l w i t h that dedu ced from the r a t e constant determined by C l u t t e r and Swift (27) for proton t r a n s f e r i n l i q u i d a c i d i f i e d ammonia and e x t r a p o l a t e d to 25°C : t h e i r r e s u l t s was 2 10^ s e c " f o r the same r a t i o NH^/NHg. On the Aerogel s u r f a c e the r a t e of t r a n s f e r i s of course somewhat reduced but s t i l l of the same order of magnitude. 1


1

9

1

1

1

As compared t o NH^, CHÔH i s a weaker base and i t was t h e r e fore i n t e r e s t i n g to i n v e s t i g a t e proton t r a n s f e r t between methanol and s i l i c a s u r f a c e . T h i s was performed (9) by combining the values obtained f o r the s p i n - l a t t i c e (Tj) and s p i n - s p i n ( T ) pro ton r e l a x a t i o n times f o r CD-OH adsorbed on Xerogel. In t h i s sys tem, two T were observed. The short one and the short T. c o n t r i b u t i o n t o the experimental Tj averaging the d i f f u s i o n and the proton exchange processes permitted the c o r r e l a t i o n time (τ ) of the proton exchange to be measured. I t was found that τ i s always higher than τ^, but the a c t i v a t i o n energies f o r the two mechanisms are approximately the same. T h i s again may be a n t i c i p a t e d s i n c e both d i f f u s i o n and proton exchange processes imply breaking hydrogen bonds. At 22° and f o r 0.8 < θ < 1.7, τ = (2.1 ± 1) 10"8 sec. T h i s value i s one or two orders of magnitude longer than the pseudo f i r s t - o r d e r constants τ = 4.5 χ Ι Ο sec and ^ 4.2 1 0 " sec determined (28) f o r proton exchange i n a c i d i f i e d methanol according t o the f o l l o w i n g processes 2

2

β

β

- 9

10

2

CH 0H + H 0 3

+

3

3

CH OH 3

2

+ H0

(11)

>

CH OH

2

+ CH 0H

2

Τ CH 0H + CH OH 3

3

1 2

Silanol Groups and Properties of Silica Surfaces - ACS Symposium

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