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Effects of temperature on formation of silica gel
Journal of Non-Crystalline Solids 82 (1986) 37-41 North-Holland, Amsterdam
EFFECTS OF TEMPERATURE
ON FORMATION
37
OF SILICA GEL
M a r y W. C O L B Y , A. O S A K A a n d J.D. M A C K E N Z I E Department of Materials Science and Engineering, University of California, Los Angeles, California 90024, USA
The gelation of solutions containing mixtures of tetramethoxysilane with methanol or tetraethoxysilane with ethanol, H20 and HCI or HF as a catalyst was studied at various temperatures in closed systems. Large differences in gelation rates were observed between HCI and HF at all temperatures. At 25, 50 and 70°C, the gelation times were 9.2, 1.25 and 0.30 h for HF and 380, 70 and 20 h for HCI, for the TEOS systems. The apparent activation energies for the two TEOS systems were almost identical. Apparent activation energies are attributed to the process of polymerization. This was justified by investigating the dependence of gelation time on the water concentration. The bulk density, apparent density and porosity were studied for the TEOS systems.
1. Introduction In recent years, the s o l - g e l process has been widely studied for m a n y oxide systems. By far the largest n u m b e r of investigations have been c o n c e r n e d with the p r e p a r a t i o n of silica via silicon alkoxides. It is generally agreed that the f o r m a t i o n of such oxide gels occurs t h r o u g h h y d r o l y s i s a n d then p o l y c o n d e n s a t i o n [1,2]. F r e q u e n t l y , the gelation process requires very long times at r o o m temperature. G e l a t i o n times longer than a week are not u n c o m m o n [3,4]. M o s t chemical reactions, especially those occurring in liquid solutions, are d e p e n d e n t on temperature. It is therefore s o m e w h a t surprising that there are very few studies of the effects of t e m p e r a t u r e on the s o l - g e l process for oxides. Bechtold et al. [4] have studied the f o r m a t i o n of silica gels at 4.5, 24.5 a n d 60.2°C. The gelation times were f o u n d to be highly d e p e n d e n t on t e m p e r a t u r e . Y a m a n e et al. [5] r e p o r t e d that the p o r e size d i s t r i b u t i o n for silica gels is d e p e n d e n t on the gelation temperature. L i m i t e d studies on the effects of t e m p e r a t u r e on the gelation of silica have also been r e p o r t e d b y others [6-11]. In this p a p e r we r e p o r t the effects of t e m p e r a t u r e on the f o r m a t i o n of silica gels derived from two alkoxides a n d two catalysts.
2. Experimental T e t r a m e t h o x y s i l a n e ( T M O S ) a n d t e t r a e t h o x y s i l a n e (TEOS) were o b t a i n e d f r o m A l d r i c h a n d A l f a chemical companies, respectively. A p a r t i c u l a r a l k o x i d e 0022-3093/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
38
M. W. Colby et al. / Effects of temperature on formation of silica gel
was mixed with its corresponding anhydrous alcohol, doubly-distilled water and catalyst in the molar ratio Si(OR)4 : ROH : H 2 0 : HX = 1 : 4 : 4 : 0.05. Thus for TMOS, methanol was used as the solvent and for TEOS, ethanol was the solvent. This selection was made to eliminate the possibility of ligand exchange between alkoxides and solvents. The two catalysts used were H F and HC1. For the T E O S - E t O H system, some additional experiments were carried out with different H 2 0 concentrations to ascertain the role of hydrolysis in the gelation process. The alkoxide and the solvent were mixed and taken to the desired temperature. The water and catalyst were separately mixed and also taken to the same temperature. The two solutions were then rapidly mixed in a sealed polypropylene container and kept at the reaction temperature for gelation and aging. Gelation time was arbitrarily chosen as that time when the solution viscosity reached 104 P. Reactions were studied from 0°C to 70°C. After gelation, the samples were kept at the same temperature in the sealed container for a further period equal to the gelation time for aging. The aged gels were then extracted, dried and heat-treated for bulk and apparent density measurements by the Archimedes method.
3. Results and discussions
Gelation is the result of chemical reactions. Our objective is to understand the influence of temperature on these reactions, namely, hydrolysis and condensation represented by: Si(OR)4 + 4 H 2 0 ~ Si(On)4 + 4ROH
(l)
2Si(OH), --, SiO 2 + 2H20.
(2)
It is difficult to separate completely hydrolysis from condensation during the entire course leading to gelation. As a first approximation, however, we assume hydrolysis to have been practically completed shortly after mixing so long as an adequate amount of water to complete the hydrolysis is present. Thus condensation should be the rate-determining process. It is understood that condensation which results in the progressive increase of viscosity is highly complex. In this study we have not measured the increase of viscosity continuously as a function of time. Rather, we only recorded tgel, the time at which the viscosity reaches 10 n P. Since tgel is the time over which the viscosity has increased from - 10 -1 P to 104 P, it can be considered as an averaged rate of gelation. Thus, 1 / t g e l = h exp( -
E*/RT)
(3)
where AS is the usual Arrhenius constant and E* is an "apparent" activation energy. In fig. 1, In t is shown as a function of 1/T. The plots are indeed linear. Plots (3) and (4) are nearly parallel for TEOS with H F and HC1 as catalysts respectively. The large difference of tge1 between H F and HCI previously
M. W. Colby et al. / Effects of temperature on formation of silica gel
soo
/
4
39
[TEOS HCl]
2 [TMOS HCI] 10o
3TEOS HF]
D 0
3 10--
F
1 [TMOS HF]
_o
O.1
2.6
J 2.8
I 30
I 3.2 1OOO/T
J 3.4
I 3,6
3.8
( K -1)
Fig. l. Plot of In (gelation time) versus I / T for Si(OR)4 : 4ROH :4H20:0.05 HX where (R = C2H5 and X = F or CI).
or CH 3
observed for room temperature is apparently maintained at higher as well as lower temperatures. The "apparent" activation energies of 14.6 and 13.2 k c a l / m o l for H F and HC1 are surprisingly similar. It appears that the effects of temperature are similar for H F and HC1. Plots (1) and (2) show that for TMOS, the effects of temperature are slightly different for H F and HC1 although E* is now significantly lower (9.5 and 11.8 kcal/mol., respectively). These higher values of E* for TEOS are possibly due to the differences in molecular volume and geometry between TEOS and TMOS. (The molecular volumes at 20°C are 222.7 cc and 147.5 cc, respectively). The magnitudes of
M. W.. Colby et al. / Effects of temperature on formation of silica gel
40
t#
5 O .C
4
4
E t"
3
._o @
!
1
m 0
2
a 4
Molar H20
m , 6
Rat io / T EOS
m
m i 8
Fig. 2. Plot o f g e l a t i o n t i m e versus m o l a r r a t i o o f w a t e r to T E O S f o r a c o n s t a n t v o l u m e series a t 7 0 ° C w i t h HF catalyst.
E* do not appear to be associated with Si-O bond energies. They seem to be more related to the transport of the condensing species. Bechtold et al. [4] reported similar values of tgel for TEOS with HC1 as catalyst. Their E* values are also comparable to ours. In fig. 2, tge~ for TEOS with H F as catalyst at 70°C is shown as a function of the H 2 0 / T E O S ratio. For complete hydrolysis, this ratio should be at least four. It is seen that when the ratio is less than four, tg~l can be very large. However, when the ratio is greater than four, tg~] is unchanged. Similar results were observed by Bechtold for TEOS at lower temperatures with HC1 as catalyst [4]. This then confirms that when adequate water is present, tg~l is probably governed primarily by condensation. Table 1 shows that after the TEOS-based gels were dried at 150°C, the bulk density and porosity were affected by gelation temperatures. For instance the porosity of gels made at 70°C with HF as catalyst was 63% compared to the value of 79% for 50°C. This obviously cannot be the direct result of the increased specific volume of the liquid mixture at the higher temperature. After all, from 50 to 70°C, the liquid mixture would have increased in specific
M.W. Colby et al. / Effects of temperature on formation of silica gel
41
Table 1 Bulk density, apparent density and porosity of gels heat-treated at 150°C for 24 h Gelation temp. (°C)
Bulk density (g/cm3)
Apparent density (g/cm3)
Porosity (%)
(A) HF TEOS 4 40 50 60 70
0.77 0.55 0.45 0.67 0.78
2.04 2.19 2.14 2.01 2.16
62 74 79 66 63
(B) HCl TEOS 25 40 50 60 70
1.34 1.45 1.46 1.56 1.55
2.18 2.18 2.01 2.10 2.09
38 33 27 25 26
v o l u m e b y some 1%. The variation of porosity with gelation temperature is difficult to explain for the results in table 1 because of the thermal history of the samples. F o r instance, after tgel, the samples were kept for a n o t h e r period of time equal to tgel. T h e n they were dried at 2 0 ° C for periods up to one m o n t h a n d then heat-treated at 150°C for 24 h. D u r i n g drying, a sample gelled at a higher temperature m a y shrink differently from one gelled at a lower temperature. Details of these changes at each step of the process are being investigated now. T a b l e 1 also shows the significant difference of the porosities of samples m a d e with H F a n d HC1 as catalysts. The authors are grateful to the Directorate of Chemical a n d Atmospheric Sciences, Air Force Office of Scientific Research for the support of this work. T h e experimental assistance of Ray Sibulkin a n d the advice of K.C. C h e n a n d E.J. Pope are m u c h appreciated.
References [1] [2] [3] [4]
H. SchmidL H. Scholze and A. Kaiser, J. Non-Cryst. Solids 63 (1984) 1. R. Aelion, A. Loebel and F. Eirich, J. Am. Chem. Soc. 72 (1950) 5705. M. Yamane, S. Inoue and A. Yasumori, J. Non-Cryst. Solids 63 (1984) 13. M.F. Bechtold, W. Mahler and R.A. Schunn, J. Polymer Sci: Polymer Chem. Ed. 18 (1980) 2823. [5] M. Yamane and S. Okano, Yogyo-Kyakai-Shi87 (1979) 434. [6] A.D. Bishop Jr and J.L. Bear, Thermochim. Acta 3 (1971) 399. [7] A.P. Brady, A.G. Brown and H. Huff, J. Colloid Sci. 8 (1953) 252. [8] C.B. Hurd and R.W. Barclay, J. Phys. Chem. 44 (1940) 847. [9] C.B. Hurd and A.J. Marotta, J. Am. Chem. Soc. 62 (1940) 2767. [10] S.P. Moulik and B.N. Ghosh, J. Indian Chem. Soc. 40 (1963) 907. [11] V.A. Vekhov, E.P. Dudnik and E.I. Ramyantseva, Russ. J. Inorg. Chem. 10 (1965) 1281.
EFFECTS OF TEMPERATURE
ON FORMATION
37
OF SILICA GEL
M a r y W. C O L B Y , A. O S A K A a n d J.D. M A C K E N Z I E Department of Materials Science and Engineering, University of California, Los Angeles, California 90024, USA
The gelation of solutions containing mixtures of tetramethoxysilane with methanol or tetraethoxysilane with ethanol, H20 and HCI or HF as a catalyst was studied at various temperatures in closed systems. Large differences in gelation rates were observed between HCI and HF at all temperatures. At 25, 50 and 70°C, the gelation times were 9.2, 1.25 and 0.30 h for HF and 380, 70 and 20 h for HCI, for the TEOS systems. The apparent activation energies for the two TEOS systems were almost identical. Apparent activation energies are attributed to the process of polymerization. This was justified by investigating the dependence of gelation time on the water concentration. The bulk density, apparent density and porosity were studied for the TEOS systems.
1. Introduction In recent years, the s o l - g e l process has been widely studied for m a n y oxide systems. By far the largest n u m b e r of investigations have been c o n c e r n e d with the p r e p a r a t i o n of silica via silicon alkoxides. It is generally agreed that the f o r m a t i o n of such oxide gels occurs t h r o u g h h y d r o l y s i s a n d then p o l y c o n d e n s a t i o n [1,2]. F r e q u e n t l y , the gelation process requires very long times at r o o m temperature. G e l a t i o n times longer than a week are not u n c o m m o n [3,4]. M o s t chemical reactions, especially those occurring in liquid solutions, are d e p e n d e n t on temperature. It is therefore s o m e w h a t surprising that there are very few studies of the effects of t e m p e r a t u r e on the s o l - g e l process for oxides. Bechtold et al. [4] have studied the f o r m a t i o n of silica gels at 4.5, 24.5 a n d 60.2°C. The gelation times were f o u n d to be highly d e p e n d e n t on t e m p e r a t u r e . Y a m a n e et al. [5] r e p o r t e d that the p o r e size d i s t r i b u t i o n for silica gels is d e p e n d e n t on the gelation temperature. L i m i t e d studies on the effects of t e m p e r a t u r e on the gelation of silica have also been r e p o r t e d b y others [6-11]. In this p a p e r we r e p o r t the effects of t e m p e r a t u r e on the f o r m a t i o n of silica gels derived from two alkoxides a n d two catalysts.
2. Experimental T e t r a m e t h o x y s i l a n e ( T M O S ) a n d t e t r a e t h o x y s i l a n e (TEOS) were o b t a i n e d f r o m A l d r i c h a n d A l f a chemical companies, respectively. A p a r t i c u l a r a l k o x i d e 0022-3093/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
38
M. W. Colby et al. / Effects of temperature on formation of silica gel
was mixed with its corresponding anhydrous alcohol, doubly-distilled water and catalyst in the molar ratio Si(OR)4 : ROH : H 2 0 : HX = 1 : 4 : 4 : 0.05. Thus for TMOS, methanol was used as the solvent and for TEOS, ethanol was the solvent. This selection was made to eliminate the possibility of ligand exchange between alkoxides and solvents. The two catalysts used were H F and HC1. For the T E O S - E t O H system, some additional experiments were carried out with different H 2 0 concentrations to ascertain the role of hydrolysis in the gelation process. The alkoxide and the solvent were mixed and taken to the desired temperature. The water and catalyst were separately mixed and also taken to the same temperature. The two solutions were then rapidly mixed in a sealed polypropylene container and kept at the reaction temperature for gelation and aging. Gelation time was arbitrarily chosen as that time when the solution viscosity reached 104 P. Reactions were studied from 0°C to 70°C. After gelation, the samples were kept at the same temperature in the sealed container for a further period equal to the gelation time for aging. The aged gels were then extracted, dried and heat-treated for bulk and apparent density measurements by the Archimedes method.
3. Results and discussions
Gelation is the result of chemical reactions. Our objective is to understand the influence of temperature on these reactions, namely, hydrolysis and condensation represented by: Si(OR)4 + 4 H 2 0 ~ Si(On)4 + 4ROH
(l)
2Si(OH), --, SiO 2 + 2H20.
(2)
It is difficult to separate completely hydrolysis from condensation during the entire course leading to gelation. As a first approximation, however, we assume hydrolysis to have been practically completed shortly after mixing so long as an adequate amount of water to complete the hydrolysis is present. Thus condensation should be the rate-determining process. It is understood that condensation which results in the progressive increase of viscosity is highly complex. In this study we have not measured the increase of viscosity continuously as a function of time. Rather, we only recorded tgel, the time at which the viscosity reaches 10 n P. Since tgel is the time over which the viscosity has increased from - 10 -1 P to 104 P, it can be considered as an averaged rate of gelation. Thus, 1 / t g e l = h exp( -
E*/RT)
(3)
where AS is the usual Arrhenius constant and E* is an "apparent" activation energy. In fig. 1, In t is shown as a function of 1/T. The plots are indeed linear. Plots (3) and (4) are nearly parallel for TEOS with H F and HC1 as catalysts respectively. The large difference of tge1 between H F and HCI previously
M. W. Colby et al. / Effects of temperature on formation of silica gel
soo
/
4
39
[TEOS HCl]
2 [TMOS HCI] 10o
3TEOS HF]
D 0
3 10--
F
1 [TMOS HF]
_o
O.1
2.6
J 2.8
I 30
I 3.2 1OOO/T
J 3.4
I 3,6
3.8
( K -1)
Fig. l. Plot of In (gelation time) versus I / T for Si(OR)4 : 4ROH :4H20:0.05 HX where (R = C2H5 and X = F or CI).
or CH 3
observed for room temperature is apparently maintained at higher as well as lower temperatures. The "apparent" activation energies of 14.6 and 13.2 k c a l / m o l for H F and HC1 are surprisingly similar. It appears that the effects of temperature are similar for H F and HC1. Plots (1) and (2) show that for TMOS, the effects of temperature are slightly different for H F and HC1 although E* is now significantly lower (9.5 and 11.8 kcal/mol., respectively). These higher values of E* for TEOS are possibly due to the differences in molecular volume and geometry between TEOS and TMOS. (The molecular volumes at 20°C are 222.7 cc and 147.5 cc, respectively). The magnitudes of
M. W.. Colby et al. / Effects of temperature on formation of silica gel
40
t#
5 O .C
4
4
E t"
3
._o @
!
1
m 0
2
a 4
Molar H20
m , 6
Rat io / T EOS
m
m i 8
Fig. 2. Plot o f g e l a t i o n t i m e versus m o l a r r a t i o o f w a t e r to T E O S f o r a c o n s t a n t v o l u m e series a t 7 0 ° C w i t h HF catalyst.
E* do not appear to be associated with Si-O bond energies. They seem to be more related to the transport of the condensing species. Bechtold et al. [4] reported similar values of tgel for TEOS with HC1 as catalyst. Their E* values are also comparable to ours. In fig. 2, tge~ for TEOS with H F as catalyst at 70°C is shown as a function of the H 2 0 / T E O S ratio. For complete hydrolysis, this ratio should be at least four. It is seen that when the ratio is less than four, tg~l can be very large. However, when the ratio is greater than four, tg~] is unchanged. Similar results were observed by Bechtold for TEOS at lower temperatures with HC1 as catalyst [4]. This then confirms that when adequate water is present, tg~l is probably governed primarily by condensation. Table 1 shows that after the TEOS-based gels were dried at 150°C, the bulk density and porosity were affected by gelation temperatures. For instance the porosity of gels made at 70°C with HF as catalyst was 63% compared to the value of 79% for 50°C. This obviously cannot be the direct result of the increased specific volume of the liquid mixture at the higher temperature. After all, from 50 to 70°C, the liquid mixture would have increased in specific
M.W. Colby et al. / Effects of temperature on formation of silica gel
41
Table 1 Bulk density, apparent density and porosity of gels heat-treated at 150°C for 24 h Gelation temp. (°C)
Bulk density (g/cm3)
Apparent density (g/cm3)
Porosity (%)
(A) HF TEOS 4 40 50 60 70
0.77 0.55 0.45 0.67 0.78
2.04 2.19 2.14 2.01 2.16
62 74 79 66 63
(B) HCl TEOS 25 40 50 60 70
1.34 1.45 1.46 1.56 1.55
2.18 2.18 2.01 2.10 2.09
38 33 27 25 26
v o l u m e b y some 1%. The variation of porosity with gelation temperature is difficult to explain for the results in table 1 because of the thermal history of the samples. F o r instance, after tgel, the samples were kept for a n o t h e r period of time equal to tgel. T h e n they were dried at 2 0 ° C for periods up to one m o n t h a n d then heat-treated at 150°C for 24 h. D u r i n g drying, a sample gelled at a higher temperature m a y shrink differently from one gelled at a lower temperature. Details of these changes at each step of the process are being investigated now. T a b l e 1 also shows the significant difference of the porosities of samples m a d e with H F a n d HC1 as catalysts. The authors are grateful to the Directorate of Chemical a n d Atmospheric Sciences, Air Force Office of Scientific Research for the support of this work. T h e experimental assistance of Ray Sibulkin a n d the advice of K.C. C h e n a n d E.J. Pope are m u c h appreciated.
References [1] [2] [3] [4]
H. SchmidL H. Scholze and A. Kaiser, J. Non-Cryst. Solids 63 (1984) 1. R. Aelion, A. Loebel and F. Eirich, J. Am. Chem. Soc. 72 (1950) 5705. M. Yamane, S. Inoue and A. Yasumori, J. Non-Cryst. Solids 63 (1984) 13. M.F. Bechtold, W. Mahler and R.A. Schunn, J. Polymer Sci: Polymer Chem. Ed. 18 (1980) 2823. [5] M. Yamane and S. Okano, Yogyo-Kyakai-Shi87 (1979) 434. [6] A.D. Bishop Jr and J.L. Bear, Thermochim. Acta 3 (1971) 399. [7] A.P. Brady, A.G. Brown and H. Huff, J. Colloid Sci. 8 (1953) 252. [8] C.B. Hurd and R.W. Barclay, J. Phys. Chem. 44 (1940) 847. [9] C.B. Hurd and A.J. Marotta, J. Am. Chem. Soc. 62 (1940) 2767. [10] S.P. Moulik and B.N. Ghosh, J. Indian Chem. Soc. 40 (1963) 907. [11] V.A. Vekhov, E.P. Dudnik and E.I. Ramyantseva, Russ. J. Inorg. Chem. 10 (1965) 1281.