Effect of catalyst used in the sol-gel process on the microstructure and adsorption/desorption performance of silica aerogels

Silica aerogels prepared by the sol-gel process are often used as solid desiccants in enthalpy wheels for dehumidifying ventilation air in air-conditioning systems. These hygroscopic materials have good moisture adsorption and desorption characteristics due to their porous structure. The current study is focused on the evaluation of the mass diffusivity of silica aerogels, which determines the rate at which a dehumidification process can be performed. The mass diffusivity of silica aerogels is affected by their porous structure which depends on the synthesis technique used to prepare the silica aerogels. The sol-gel process is used to prepared silica aerogels using various basic (ammonium hydroxide, sodium hydroxide, potassium hydroxide) and acidic (hydrofluoric acid, steric acid, hydrogen peroxide) catalysts with the same precipitator (tetramethyl orthosilicate - TMOS) and solvent (methanol). Scanning electron microscopy is used to analyze the microstructure of supercritically dried aerogels. The dynamic vapor sorption method is used to determine the effective mass diffusivity for the different silica aerogels. It is found that the mass diffusivity is related to the microstructure of silica aerogels, which depends on the catalysts used in the sol-gel process. In addition, a parametric study is conducted to determine the effect of relative humidity and temperature on the adsorption and desorption mass diffusivity.