Formation of Silica Gels Composed of Micrometer-Sized Particles by

6 Formation of Silica Gels Composed Downloaded by UCSF LIB CKM RSCS MGMT on August 15, 2014 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0234.ch006

of Micrometer-Sized

Particles

by the Sol-Gel Method Hiromitsu Kozuka and Sumio Sakka Institute for Chemical Research, Kyoto University, U j i , Kyoto-Fu 611, Japan

Formation of silica gels composed of micrometer-sized particles from highly acidic solutions of tetramethoxysilane (TMOS) is described. In this method a limited amount of H O and a large amount of HCl are used in hydrolyzing TMOS. Detailed conditions and the mechanism of the formation of micrometer-sized particles in the solutions are discussed. Applications of the gels are also described. 2

SILICA GELS WITH CONTINUOUS LARGE PORES are ideal as precursors for

bulk silica glasses. Low-temperature synthesis of bulk silica glasses by the sol-gel method consists of hydrolysis and polycondensation of silicon alkoxide in a solution, gelation of the alkoxide solution, drying of the wet gel to remove residual liquid components, and heat treatment of the dried gel to remove the organic substances and for sintering. Because large, continuous pores reduce the capillary pressure on the gel framework during drying, synthesis of silica gel monoliths with large pores is desirable for the sol-gel process for the production of bulk glasses (1-5). A c i d or base is used as a catalyst for the hydrolysis of silicon alkoxide. A c i d or base with a mole ratio to alkoxide as low as 0.01 or less is usually enough for the catalytic effect. O n the other hand, water with a mole ratio to alkoxide as high as 10 or more is used for the complete hydrolysis of alkoxide. W e have found, however, that monolithic opaque gels composed of micrometer-sized silica particles can be prepared from tetramethoxysilane (TMOS) when a limited amount of water and a large amount of 0065-2393/94/0234-0129S08.00/0 © 1994 American Chemical Society

In The Colloid Chemistry of Silica; Bergna, H.; Advances in Chemistry; American Chemical Society: Washington, DC, 1994.

Downloaded by UCSF LIB CKM RSCS MGMT on August 15, 2014 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0234.ch006

130

T H E C O L L O I D C H E M I S T R Y O F SILICA

hydrochloric acid are used for the hydrolysis (6, 7). Because of the large particle size and the accordingly large pore size, the gels can be dried without cracking. The Stôber route (8) is a well-known method for providing submicrometer- or micrometer-sized silica particles by hydrolysis and condensation of silicon alkoxide; an excess of base and water is used i n the reaction. Compared with this method, ours has quite different reaction conditions, namely, the use of a limited amount of water and a large amount of acid. In contrast to the reaction of silicon alkoxide with a large amount of water in basic conditions, Sakka and Kamiya (9) noticed from the measurement of the intrinsic viscosity of silica sols that linear particles or polymers, not round particles, are formed with acidic conditions and the addition of a small amount of water. Therefore, the reaction conditions for this method for producing round micrometer-sized particles is new, and the mechanism of formation of round particles is of interest. In this chapter, detailed conditions for the formation of silica gels composed of micrometer-sized particles from highly acidic T M O S solutions are described, and the mechanism of the sol-gel reaction is discussed. Application of the gels is also discussed.

Conditions for the Formation of Micrometer-Sized Particles Process for M a k i n g Gels. Figure 1 shows the flow diagram for the sol-gel process for making gels. T M O S , methanol, 36% hydrochloric acid,

Si(OCH )4 CHaOH 3

H 2 O , HCI CHaOH

Transparent solution ^ Storage at 40°C Opaque sol

ι

Opaque gel Figure 1. Flow diagram of the sol-gel process for making silica gel monoliths composed of micrometer-sized particles.


6.

KOZUKA A N D SAKKA

Formation of Silica Gels by the Sol-Gel Method

131

and ion-exchanged water are used as the starting materials. Silicon alkoxide solutions of the desired mole ratios are prepared by mixing the reagents under vigorous stirring at room temperature. Fifty milliliters of the alkoxide solution is kept at 40 °C until gelation. The quantity of acid reported refers to hydrogen chloride (HCl), and that of water includes water from the hydrochloric acid solution. Effect of H 0 and H C l Content of the Starting Solutions. In the studies of gelation of T M O S solutions having mole ratios T M O S : H 2 O : H C l : C H 3 O H of 1:1.44-2.00:0.01-0.40:2, gels composed of larger particles were formed from the solutions with higher H C l content and lower H 2 O content (6). Figure 2 shows the scanning electron micrographs (SEMs) of the fracture surface of the dried gels. In the series of gels formed from the solutions with the same H 2 O content at H 2 O / T M O S = 1.53, larger particles i n the gel skeleton are visible i n the gels from the solutions with greater H C l concentrations. In the gel formed from the solution with H C l / T M O S = 0.40, particles 5 μπι i n diameter and connected at the neck are visible. Dependence of the particle size on the H C l content of the starting solution is shown i n Figure 3. As the particle size increases, the transparency of the resultant dried gel is lost, and as shown i n Figure 3, the bulk density of the gels decreases. Greater concentrations of H 2 O , however, decrease the particle size, as shown i n Figure 2d.


2

Effect of the Concentration of the Solutions. The structure of the gel also depends on the concentration of the solutions, that is, the amount of alcohol in the solutions. Figure 4 shows the SEMs of the fracture surface of the dried gels derived from various solutions. Micrometer-sized particles are visible i n the gel formed from the solution with C H 3 O H / T M O S = 3 (Fig. 4b) as well as that from the solution with C H 3 O H / T M O S = 2 (Fig. 2c). Solutions with smaller (CH3OH/TMOS = 0.5) and larger (CH3OH/TMOS = 5) C H 3 O H concentrations, however, resulted i n gels composed of fine particles (Fig. 4a and 4c). Effect of the K i n d of Alkoxides, Acids, and Alcohols. In the experiments on the gelation of solutions of tetramethoxysilane, tetraeth oxysilane, tetraisopropoxysilane, and tetra-n-butoxysilane, in which solu tions with H 0/alkoxide = 1.5, HCl/alkoxide = 0.40, and [alkoxide] = 2 mol/L with C H 3 O H as solvent were kept at 40 °C i n air-sealed flasks, an opaque gel formed only from the T M O S solution, whereas no gelation took place i n the solutions of the other alkoxides (7). Substitution of methanol by isopropyl or η-butyl alcohol also prevent ed formation of opaque monolithic gels composed of micrometer-sized particles (7). T M O S solutions with mole ratios T M O S : H 2 O : H C l : alcohol of 2



132

THE

C O L L O I D CHEMISTRY O F

SILICA

Figure 2. SEMs of dried gels formed from solutions with mole ratios TMOS:H2O:HCl: CH3OH of 1:1.53 or 2.00:0.01-0.40:2. Mole ratios were as follows: a, HCl/TMOS = 0.01 and H2O/TMOS = 1.53; h, HCl/TMOS = 0.25 and H2O/TMOS = 1.53; c, HCl/TMOS = 0.40 and H2O/TMOS = 1.53; and d, HCl/TMOS = 0.40 and H2O/TMOS = 2.00.

1:1.53:0.40:2 converted to opaque gel monoliths when methanol was used, whereas slightly opalescent gel fragments formed when isopropyl or η-butyl alcohol was used.

Mechanism

of the Formation

of Micrometer-Sized

Particles

S i N M R Spectra o f the Gels. Molecular structure of the poly merized species is compared for three kinds of materials: the opaque gel composed of micrometer-sized particles formed from a highly acidic TMOS solution with mole ratios T M O S : H 2 O : H C l : C H 3 O H of 1:1.53:0.40:2, the transparent gel derived from a weakly acidic T M O S solution with mole ratios T M O S : H 2 O : H C l : C H 3 O H of 1:1.53:0.01:2, and the silica particles formed from a solution with mole ratios T M O S : H 0 : N H 3 : C H O H of 1:30:5:171. The first and second gels corre29

2

3



6.

KOZUKA A N D SAKKA

Formation of Silica Gels by the Sol-Gel Method 133

0

02

0.1

0.3

0.4

0.5

HCl/TMOS

Figure 3. Dependence of particle size and gel bulk density on HCl/TMOS mole ratios. TMOS: H2O: HCl: CH OH is 1:1.53:0.01-0.40:2. 3

Figure 4. SEMs of the dried gels prepared from the solutions with mole ratios TMOS:H2O:HCl.CH3OHof1:1.53:0.40:^ wherex = 0.5 (a), 3 (b), and5 (c). spond to those shown in Figure 2e and 2a, respectively. The third one is made through a method similar to that of Stôber. Figure 5 shows the S i N M R spectra of these materials. There is less cross-linking in the gel from the highly acidic solution with a limited amount of water than in the silica particles from the highly basic solution with an excess of water; fewer Q species, Si atoms having four bridging oxygens, and more Q species, Si atoms having two bridging oxygens, are present in the gels from the acidic solutions. The extent of cross-linking of the polymerized species is totally different, although gels from both highly acidic and highly basic solutions have micrometer- or submicrometer-sized particles that can be seen microscopically. In contrast, the gels from the highly acidic and weakly acidic solutions have similar N M R spectra. This similarity indicates that the extent of crosslinking of the silica polymers from the highly acidic solution is as low as 29

4

2



134

T H E C O L L O I D CHEMISTRY O F SILICA

-80 -90

110-120 ppm

Figure 5. Silicon-29 NMR spectra of (a) a silica gel derived from a solution with molar composition TMOS: H2O: HCl: CH OH of 1:1.53:0.4:2, (b) a silica gel derived from a solution with molar composition TMOS: H2O: HCl: CH3OH of 1:1.53:0.01:2, and (c) silica particles derived from the solution with molar composition TMOS: H2O: NH : CH3OH of 1:30:5:171. 3

3

that from the weakly acidic solution. It would be possible to assume that the round particles formed from the highly acidic solution consist of linear polymers or particles, which have been proposed to be polymerized species in the weakly acidic solutions with a limited amount of water (9). The sol from the weakly acidic solution has spinnability, that is, gel fibers can be drawn from the sol, and the resultant gel shows no particulate microstructure, whereas the sol from the highly acidic solution has no spinnability (6) and has particulate structure. It is plausible to think that these differences in the microstructure of the gels and the rheological properties of the sols do not arise from different polymer structures but from different aggregation states of the polymerized species. The gels derived from the solutions with various C H 3 O H concentrations, which have different microstructures, as mentioned in the section "Effect of the K i n d of Alkoxides, Acids, and Alcohols", have almost the same S i N M R spectra as those in Figure 6. This similarity indicates that the different microstructures observed for the gels from the solutions with different C H 3 O H concentrations do not result from different structures of the polymers but rather from differences in the aggregation states of the polymers. 29



6.

KOZUKA A N D SAKKA


I

ι

I

ι

I

ι

I

ι

I

ι

1

ι

1

- 4 0 - 6 0 -80-100 -120-140 -160 ppm Figure 6. Silicon-29 NMR spectra of gels prepared from solutions with mole ratios TMOS:H 0:HCl: CH OH of1:1.53:0.40:x, where χ = 0.5, 2, 4, and 5. 2

3

Instability of Particles in Organic Liquids. Sol particles formed in the TMOS solutions with excess H C l and a limited amount of H 2 O are not stable in nonpolar organic solvents; a translucent sol derived from a solution with mole ratios TMOS: H 2 O : HCl: C H 3 O H of 1:1.53:0.40:2 be comes transparent when benzene is added. Thus, the particles, which are the source of the translucence of the sol, are composed of polymers or primary particles soluble in nonpolar organic solvents. Figure 7 shows SEMs of dried gels derived from the sols in which nonpolar benzene was added and polar methanol was added after the occurrence of translucence. Much finer microstructure is evident in the gel from the benzene-added sol, whereas micrometer-sized particles are seen in the gel from the methanol-added sol. Instability of Particles under Centrifugation. The sol particles, the source of the translucence of the sols, are not stable against centrifuga tion. Figure 8 shows the dried gel obtained by centrifuging a sol with mole ratios TMOS: H 2 O : HCl: C H 3 O H of 1:1.53:0.40:2 before the occurrence of opalescence. Round, closed pores are visible instead of particulate structure; this observation suggests that the micrometer-sized, round particles formed in the reaction are not stable against mechanical forces and may consist of weakly cross-linked, flexible polymers.



136

T H E C O L L O I D CHEMISTRY O F

SILICA

Figure 7. SEMS of the fracture surface of dried gels derived from solutions to which benzene was added (a) and methanol was added (b) after the occurrence of opalescence.

Figure 8. SEM of dried gel obtained with centrifugation. Mechanism of the Formation of Round Particles. Because of the small amount of water used in the hydrolysis reaction in the starting solutions that give large particles, a large number of unhydrolyzed alkoxy groups are left on the polymerized species. These alkoxy groups give rise to the lipophilic nature of the polymerized species. A larger portion of methanol may be protonated and the ionic nature of the solvent may increase when a larger amount of H C l or an acid with a larger dissociation constant is used; these conditions result in a decrease i n the solubility of the polymerized species in the solutions. In the starting solutions with a large amount of water, the polymerized species would be soluble in the solvent because of the larger number of hydroxyl groups. Acids with lower dissociation constants were assumed to decrease the number of protonated alcohols, which reduce the ionic nature of the solvent. Alkoxides with longer hydrocarbon chains than



6.

KOZUKA A N D SAKKA


methanol—which will release the alkoxy groups in the hydrolysis reaction to generate alcohols with longer chains than methanol—and alcohols other than methanol would increase the solubility of the polymerized species in the solvent. The amount of methanol in the solution will change the state of the aggregation of the polymerized species. The aggregation of the lipophilic polymerized species discussed here is also regarded as phase separation of the sol, i n which oil-like polymerized species are separated from the solutions. The change of the ratio of the solvent to the polymers is expected to change the degree of phase separation. Because of the lipophilic nature of the primary particles, translucent sols become transparent as a result of dissolution of the primary particles in the solvent when nonpolar organic solvents are added. Flexibility of the primary particles results in instability of the secondary particle, as seen in the centrifugation experiment. Changes in the gel structure by addition of organic compounds and by centrifugation and the formation of round particles are illustrated i n Figure 9.

Centrifugation

Formation of fnear polymers or particles

Formation of secondary particles by coagulation

m Gelation

Addition of benzene

Figure 9. Illustration of the changes in the gel structure caused by the addition of organic compounds and by centrifugation. Surface C h a n g e s i n the A m b i e n t A t m o s p h e r e . Large numbers of unhydrolyzed methoxy groups in the particles cause instability of the surface structure of the gel particles. Figure 10 shows SEMs of gels obtained from a T M O S solution and kept at 40 °C i n the ambient atmosphere for various times after gelation. Because the round particles are secondary particles, gels kept at this temperature for 2 days have rough


138

T H E C O L L O I D CHEMISTRY O F

SILICA


surfaces. The surfaces of the particles, however, become more smooth with time. Reaction of the unhydrolyzed methoxy groups with the vapor in the atmosphere takes place after gelation, and polycondensation at the hydrolyzed sites may lead to the smoothing of the particle surface.

Figure 10. SEMS of dried gels prepared from solutions with mole ratios TMOS: H2O: HCl: CH3OH of 1:1.53:0.40:2 kept at 40 °C in the ambient atmosphere for (a) 2 days, (b) 5 days, and (c) 13 days after gelation. Figure 11 shows the change of the specific surface area of the gel with time after gelation. Specific surface area is measured by the B r u nauer-Emmet-Teller (BET) method by using N 2 gas as the adsorptive agent. The surface area decreases with time; this observation corresponds to Figure 10, in which a rough particle surface turned smooth.

0

2

4

δ

8

10 12 14

Time (days)

Figure 11. Change of the specific surface area of a gel with time after gelation. The gel was obtained from a solution with molar ratios TMOS: H2O: HCl. CH3OH of1:1.53:0.40:2.


6.

KOZUKA A N D SAKKA


139


Applications As mentioned in the introduction, the most serious problem encountered in sol-gel process for making bulk silica glasses is the fracture of gels during drying. At the interface between the residual liquids and the pore walls, large capillary forces are generated, which cause cracking of the gel body. Preparation of gels having large pores is effective in preventing fracture, because the capillary force decreases as the pore radius increases. The method described in this chapter can produce such gels easily, as shown in Figure 12, where a crack-free gel plate of 21 cm X 17 cm X 0.9 cm prepared from a solution with mole ratios T M O S : H 2 O : H C l : C H 3 O H of 1:1.53:0.25:2 and dried at 40 °C for about 1 month is shown.

Figure 12. A gel plate and a rod obtained from a solution with mole ratios TMOS:H2O:HCl:CH3OH of 1:1.53:0.25:2 and dried at 40 °C for about 1 month. These gels can be used as the precursor for porous silica glass bodies as well as for pore-free bulk silica glasses. Conventionally, porous silica glasses have been made by preparing sodium borosilicate glasses by the melt-quench method, heating the resultant glasses for phase separation, and leaching the Na20- and B203-rich phase out of the phase-separated glasses (10). High-purity porous silica glasses should be produced by a simple method, however, i f silica gels are used as the precursor. Figure 13 shows a bulk porous silica glass made by the sol-gel method and an S E M of the fracture surface. The gel was prepared from a solution with mole ratios T M O S : H 2 O : H C l : C H 3 O H of 1:1.53:0.40:2 and dried at 40 °C for a few months. The dried gel was then heated at a rate of 0.5 °C/min to 1300 °C and kept at that temperature for 3 h. A crack-free porous glass was successfully obtained, and continuous pores more than 10 μτη in diameter are visible. The pore characteristics can be controlled not only by changing the composition of the starting solution but also by changing the heat



140

T H E C O L L O I D CHEMISTRY O F SILICA

Figure 13. Appearance (left) and SEM of the fracture surface (right) of a porous silica glass prepared by the sol-gel method. treatment conditions of the gels. Figure 14 shows pore-size distributions of a gel and its derivatives made with differing heat treatments. Because measurement was possible only at pressures higher than 1 atm (101 kPa), measurement of the pore-size distribution could not be made for pores larger than 7.5 μπι in radius. However, pores larger than 7.5 μπι i n radius are present i n the gel, and the pore volume decreases and the pore-size 15

Final cumulative pore volume

σι ε ο

-Gel - -800°C &WC —1000 C 1300 C lOOO'C^

10

e

e

1300 C-^$

i

e

g 0.5 1270'C, 3h 1

-1270 C, 3h e

10 100 1000 10000 Pore radius (nm)

Figure 14. Pore-size distribution curves of silica gel and gel-derived porous silica glasses. The gel was obtained from a solution with mole ratios TMOS: HsO: HCl: CH3OH of 1:1.53:0.40:2. The gel was dried at 40 °C for 7 days and heated at a rate of 0.5 °C/min to various temperatures.


6.

KOZUKA A N D SAKKA


141


distribution shifts to smaller radius when the upper heat treatment temperature is increased. Such porous silica glasses of micrometer-sized pores can serve as filters and enzyme and microbe supports. If the small pores on the particle surface or the roughness of the particle surface are designed to be retained after heat treatment, the resultant porous glasses can be used as filters that function as catalyst supports.

References 1. Rabinovich, E . M . ; Johnson, D . W., Jr.; MacChesney, J. B.; Vogel, Ε. M. J. Am.

Ceram. Soc. 1983, 66, 683. 2. Rabinovich, Ε. M. J. Mater. Sci. 1985, 20, 4295.

3. Scherer, G. W.; Luong, J. C. J. Non-Cryst. Solids 1984, 63, 163. 4. Prassas, M.; Phalippou, J.; Zarzycki, J. J. Mater. Sci. 1984, 19, 1656. 5. Toki, M.; Miyashita, S.; Takeuchi, T.; Kanbe, S.J.Non-Cryst. Solids 1988, 100, 479.

6. Kozuka, H.; Sakka, S. Chem. Mater. 1989, 1, 398. 7. Kozuka, H.; Yamaguchi, J.; Sakka, S. In Proceedings of the XVth International Congress on Glass; Mazurin, Ο. V . , E d . ; Nauka: Leningrad, U.S.S.R., 1989; V o l . 2a, p 32. 8. Stöber, W.; Fink, Α.; Bohm, Ε. J. Colloid Interface Sci. 1968, 26, 62.

9. Sakka S.; Kamiya, K. J. Non-Cryst. Solids 1982, 48, 31. 10. Nordberg, M . E. J. Am. Ceram. Soc. 1944, 27, 299. RECEIVED 1992.

for review December 19, 1990. ACCEPTED revised manuscript January 3,


Formation of Silica Gels Composed of Micrometer-Sized Particles by

Recommend Documents