Highly Porous Silica Glasses and Aerogels Made Easy: The Hypersaline Route

Abstract

Through the addition of inorganic salts as porogens during an alcohol-based sol-gel process high surface area silica glasses and aerogels with varying pore sizes are synthesized. In the presence of ZnCl2 and CaCl2 respectively, tetraethyl orthosilicate (TEOS) dissolved in ethanol hydrolyzes also in the absence of any acid or base catalysts, eventually forming high surface area monolithic silica networks. The porosity can be controlled by the gel ageing step as well as the salt nature. Two different gel drying conditions were investigated, i.e., room temperature (RT) air drying at ambient pressure and supercritical (sc) CO2 drying. While a reference solution which only contains ethanol and TEOS does not gel at all, the addition of ZnCl2 leads to mesoporous aerogels with high surface areas of 1400 m2 g−1 after sc CO2 drying. Interestingly, the utilization of CaCl2 however, produces aerogel-like materials of high porosity for both drying methods under preservation of the nano-and macroscopic features. Thereby, it is shown that inorganic salts can influence the structure formation during the sol-gel process where the type of salt controls the framework connectivity/stability and tunes the porosity towards smaller pore sizes than usually obtained. At the same time, the herein presented hypersaline aerogel-synthesis is very simple, fast and in the case of CaCl2 drying can even be performed in air