scattering (SAXS), mechanical testing, etc., have been used Silica aerogels were synthesized via the sol-gel polymerization to characterize silica aerogels (4-8). A few catalytic appliof tetramethylorthosilicate (TMOS) under basic conditions and cations of aerogels have been described in the literature (9). followed by supercritical drying with carbon dioxide. As a result Silica aerogels also have unique optical, acoustic, and therof characterization by nitrogen adsorption, the aerogels were mesomal properties. Since the porous structures minimize light porous materials with high surface areas and had few micropores.
scattering, the silica aerogels are transparent porous materi-
The experimental results suggested that the surface area and the als. The high porosity makes silica aerogels excellent insulapore radius of the aerogel were controlled by changing the mole tors with extremely low thermal conductivity (10). Using ratios of ammonia (NH 3 ) to TMOS in the sol-gel polymerization. the good optical property of silica aerogels, aerogel Cheren-On the other hand, the mesopore volume of the aerogel was independent of the [NH 3 ]/[TMOS] ratio. Adsorption isotherms of kov detectors have been studied (11). ethane, ethylene, propane, and propylene were measured on the It is well known that the catalysts used in the sol-gel aerogels. It was found that the [NH 3 ]/[TMOS] ratio influenced polymerization of TEOS or TMOS greatly influence physical the adsorption isotherms. α§ 1997 Academic Press
properties of silica gels (12-15). The polymerization pro-Key Words: silica aerogel; sol-gel polymerization; tetramethceeds according to the following two steps: the first is hydroylorthosilicate (TMOS); NH 3 {TMOS mole ratio; supercritical lysis of TEOS or TMOS and the second is condensation. For drying; mesoporous structure; hydrocarbon adsorption. example, hydrogen chloride (HCl) is used as a hydrolysis catalyst, and ammonia (NH 3 ) is used as a condensation catalyst. However, there are few works on the influence of the
Methods
sion electron microscopy (TEM), nuclear magnetic resonance (NMR), nitrogen (N 2 ) adsorption, small-angle X-ray Materials Preparation of wet silica gel. We synthesized wet