Synthesis and properties of microporous sol–gel silica membranes

Unsupported silica membranes were prepared via sol–gel processing through acid-catalyzed hydrolysis and condensation of tetraethyl orthosilicate (TEOS). The possibility of controlling the pore sizes and porosities of silica membranes were investigated by varying processing parameters. The effect of the type and concentration of the acid catalyst and molar ratio water/alkoxide on the structure of the membranes was determined by measuring their structural properties. The membranes catalyzed only with nitric acid have a smaller pore structure, with larger pore-solid surface areas and solid densities than the membranes catalyzed with a combination of nitric acid and hydrofluoric acid. The average pore size increases for larger hydrofluoric acid concentrations, in a range from 1.1±0.1 to 22±0.9 nm. Silica membrane obtained with molar ratio water/alkoxide equal to 2 has a pore structure differing from the pore structures of the other membranes. The connectivity (ca. 1019 cm−3) and the diffusion coefficients (ca. 1.6±0.2×10−6 to 2.8±0.3×10−5 cm2/s) of the Cu in the pore structure of these silica membranes indicate the feasibility of using these materials in impregnation and separation processes.