According to their low densities, low refractive indices, Silica-titania and silica-alumina aerogels were prepared by the small pore sizes, and good transparencies, much attention two-step hydrolysis of metal alkoxides, the sol-gel polymerization has been given to silica aerogels in recent years for use and the supercritical drying with carbon dioxide. As a result of in several applications: catalysts (5), transparent thermal characterization by nitrogen adsorption, the aerogels were mesoinsulators (6), Cherenkov detectors (7), monolithic precurporous materials with high surface areas and had few micropores.
sors to glasses (8).
The experimental results suggested that the surface area, the meso-Some of other inorganic aerogels as well as silica aeropore volume, and the pore radius of the aerogel depended on the titanium (Ti) or aluminum (Al) content. Adsorption isotherms of gels have been produced. Teichner et al. have reported ethane, ethylene, propane, and propylene were measured on the on Al 2 O 3 , ZrO 2 , MgO, Fe 3 O 4 , and TiO 2 aerogels ( 9, 10 ) . aerogels prepared. It was found that the adsorption capacities for Brinker et al. have studied on the synthesis and the structhese hydrocarbons were changed by mixing Ti or Al to the silica ture of borate based aerogels ( 11 ) . For aerogels of mixed aerogel. In the ethane-ethylene or propane-propylene system, the oxides, SiO 2 -Al 2 O 3 , Al 2 O 3 -MgO, TiO 2 -MgO, and adsorption selectivity of ethylene or propylene on the silica-titania ZrO 2 -MgO aerogels have been developed ( 9,10,12 ) .
aerogel was lower than that on the silica aerogel. On the other
The aerogels give good catalytic properties for activity, hand, the selectivity on the silica-alumina aerogel was higher than selectivity, and resistance to deactivation, and are interthat on the silica aerogel. Hence, the silica-alumina aerogel was esting for use in catalysis.
useful for separating ethylene from ethane or propylene from propane. ᭧ 1997 Academic Press
Although transmission electron microscopy (TEM), nu-Key Words: silica-titania aerogel; silica-alumina aerogel; sol-gel clear magnetic resonance (NMR), nitrogen (N 2 ) adsorption, polymerization; tetramethylorthosilicate (TMOS); supercritical small-angle X-ray scattering (SAXS), mechanical testing, drying; porous structure; hydrocarbon adsorption etc., have been used to characterize silica aerogels (13-17), porous structures of aerogels of mixed oxides have not been completely elucidated. The analysis on porous structures of