Chemical Control of Highly Porous Silica Xerogels: Physical Properties and Morphology

Highly porous silica xerogels were synthesized by the sol−gel process under atmospheric conditions. The silica alcogels were prepared by a two-step acid/base-catalyzed hydrolysis/condensation of tetraethoxysilane (TEOS), with a water:TEOS molar ratio of 4, in 2-propanol. The catalysts used in the two steps were HCl and NH 3, respectively, and the catalytic conditions were varied by changing the molar ratios HCl/TEOS (in the first step) and NH 3 /HCl (in the second step). After aging in an appropriate solution, the alcogels were washed and subcritically dried from 2-propanol, under atmospheric pressure. The resulting silica xerogels were characterized by the volume shrinkage upon drying, envelope density determinations, scanning electron microscopy, and nitrogen sorption isotherms. It is shown that by chemical control of the hydrolysis and condensation steps, through pH variation only, it is possible to obtain monolithic materials with properties similar to aerogels, stable under atmospheric conditions. Densities as low as 0.37 g cm - 3, corresponding to porosities of 80%, were obtained for xerogels with specific surface areas of ∼940 m 2 g - 1 and a bimodal pore structure, with a narrow mesopore size distribution (average pore diameter of 8 nm).