Abstract
A new means for generating silica gel has been developed. Simple two component systems comprising tetraalkoxysilanes and strong carboxylic acids such as formic acid can react rapidly to give transparent monolithic gels. The acid serves as solvent, water source, and catalyst for both hydrolysis and condensation. Water need not be present as an initial reactant; it is generated in situ during the reaction. The gelation reaction is at least two orders of magnitude faster than those conducted in conventional acid-catalyzed aqueous systems at comparable pH*. Kinetic evidence indicates a lowering of the activation energy of condensation reactions, believed to be associated with reaction of silyl carboxylates and silanol groups. Physical properties of the dry gels such as bulk density more closely resemble conventional acid-catalyzed gels than those associated with other rapidly gelling systems. Dry gels often exhibit porosity so fine that nitrogen (at 77°K) is not absorbed at significant rates. Independent evidence of porosity arises from comparison of skeletal and bulk densities, sample immersion in water and adsorption isotherms of CO2.
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Smith, D., Davis, P., and Brinker, C., Better Ceramics Through Chemistry IV, edited by Zelinski, B., Brinker, C., Clark, D., and Ulrich, D., (Materials Research Soc., v. 180, 1990), p. 235.
Room temperature gelation times can be as long as months at pH values near 2. See, for example, Ref. 12.
Pope, E. and Mackenzie, J., J. Non-Cryst. Solids 87, 185 (1986).
Coltrain, B., Melpolder, S., and Salvain, J., in Ultrastruct. Process. Adv. Mater., edited by Uhlmann, D. and Ulrich, D., (Wiley: New York, 1992), p. 69.
Sakka, S., Kozuka, H., and Kim, S.-H., Ultrastructure Processing of Advanced Ceramics edited by Mackenzie, J. and Ulrich, D., (Wiley: New York, 1988) p. 159.
Sanchez, C., Livage, J., Henry, M., and Babonneau, F., J. Non-Cryst. Solids 100, 65 (1988).
Sumrell, G. and Ham, G., J. Am. Chem. Soc. 78, 5573 (1956).
Aelion, R., Loebel, A., and Eirich, F., J. Am. Chem. Soc. 72, 5705 (1950).
Karmaker, R., De, G., Kundu, D., and Ganguli, D., J. Non-Cryst. Solids 135, 29 (1991).
That is, at comparable r and measured solution pH. a 3:1 molar mixture with TEOS shows an initial pH* of 1.3 evolving to a value of ca. 1.8 close to the gel point. 96% formic acid has a pH* of 0.7±0.4.
r is taken to be the mole ratio of the carboxylic acid to silane; any water present is not included in the calculation.
Gottardi, V., et al., J. Non-Cryst. Solids 63, 71. (1984).
Bechtold, M.F., Mahler, W., and Schunn, R., J. Polym. Sci., Polym. Chem. Ed. 18, 2823 (1980). These workers demonstrated that activation energy is a function of the molar composition of the system, with lower values at lower water: silane ratios. pH and the silica concentration are also likely influential on Arrhenius parameters. The value of 7.5 kcal/mole is extrapolated from the data provided in the paper.
Some allowance must be made for the powerful condensation-retarding effects of donor solvents such as THF when used to dissolve solid carboxylic acids.
See Ref. 4.
McFarlane, W. and Seaby, J., J. Chem. Soc. Perkin II 1972, 1563.
Emblem, H., Hargreaves, K., and Oxley, C., J. Appl. Chem. 18, 97 (1968).
Measurement made by D. Smith at the Univ. of New Mexico.
Measurement at the Du Pont Co. by L. Abrams.
Farenholtz, W., Smith, D., and Hua, D., J. Non-Cryst. Solids 144, 45 (1992).
Marsh, H. and Wynne-Jones, W., Carbon 1, 269 (1964).
Barnham, H. and Clark, L., J. Am. Chem. Soc. 73, 4638 (1951).
Vega, A. and Scherer, G., J. Non-Cryst. Solids 111, 153 (1989).
Densities were determined using a Mettler-Parr DMA 35 density meter, with digital display of both density and temperature.
Scherer, G., private communication.
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Sharp, K.G. A two-component, non-aqueous route to silica gel. J Sol-Gel Sci Technol 2, 35–41 (1994). https://doi.org/10.1007/BF00486210
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DOI: https://doi.org/10.1007/BF00486210