Porous texture of silica aerogels made with ionic liquids as gelation catalysts

Abstract

The effect of four ionic liquids on the porous texture of silica aerogels synthesized from mixed tetramethoxysilane and methyltrimethoxysilane and dried by the CO₂ supercritical method, was studied. Two of these ionic liquids were composed of BF₄ ⁻ anions while the other two included Cl⁻ anions. The synthesis of gels from ionic liquids did not require another acidic catalyst for silica hydrolysis, nor a basic catalyst for silica condensation. These aerogels were compared with traditional aerogels made according to a double step catalysis, which first involved hydrolysis with HCl followed by condensation with pH 9 Tris HCl buffer. Gel mass analysis and thermogravimetric data showed that, when the initial molar of ionic liquid to Si was 1.58, only ~2% (by mass) of the initial ionic liquids consisting of BF₄ ⁻ anions and ~10% (by mass) of ionic liquids containing Cl⁻ anions, remained in the aerogels after supercritical drying. Moreover, X-ray diffraction confirmed that in ionic liquids based on BF₄ ⁻ anions, evaporation of the volatile components before supercritical CO₂ drying led to the formation of regularly ordered mesopores.