Improved hydrothermal stability of silica materials prepared from ethyl silicate 40

Abstract

Microporous silica materials with improved hydrothermal stability were synthesized through a sol–gel process using ethyl silicate 40 as the starting silica precursor. The effects of reactants for water (hydrolysing agent), acid (catalyst) and ethanol (solvent) ratios on the microstructure of the silica matrices and their hydrothermal stability under harsh conditions (550 °C, 75 mol% vapour, 20 h) were systematically studied. All the calcined silica matrices were microporous and the degree of microporosity was found to increase with decreasing water and acid ratios, or increasing ethanol ratio. The most hydrothermally stable matrix was obtained by promoting the water and acid ratios whilst decreasing the ethanol ratio. A strong correlation was found between the FTIR area ratio of silanol/siloxane vibrational peaks and the initial micropore volume, and this relationship revealed that the greatest pore volume loss (>70%) occurred in the xerogels possessing a high silanol/siloxane ratio (>0.16) and a high initial micropore volume percentage (>85%). SAXS data also revealed that the most robust, hydrothermally stable silica matrices are closely associated with the formation of a more open silica microstructure derived from thermal consolidation of larger silica particles.