The search of promoters for silica condensation and rational synthesis of hydrothermally stable and well ordered mesoporous silica materials with high degree of silica condensation at conventional temperature

Abstract

Silica samples condensed at acidic, neutral and alkaline media in the presence of various inorganic anions have been carefully investigated by 29Si MAS NMR spectroscopy. The results show that SO 4 2 - and CO 3 2 - could effectively improve the degree of silica condensation under neutral and alkaline media. Accordingly, for example, it is designable to synthesize hydrothermally stable and well ordered hexagonal mesoporous SBA-15 with very high degree of silica condensation in the presence of sulfate and carbonate species under neutral condition. As we expected, hexagonal mesoporous silica materials in the presence of sulfate (SBA-15-S) and carbonate species (SBA-15-U) show high degree of silica condensation, giving Q 4/ Q 3 ratios at 6.3 and 4.3, which are much higher than that of conventional SBA-15 ( Q 4/( Q 3 + Q 2) ratio at 1.8) synthesized in the absence of sulfate and carbonate species. Very interestingly, hexagonal mesoporous SBA-15-S and SBA-15-U shows very high hydrothermal stability, compared with SBA-15. For example, after being treated in 100% steaming at 780 °C for 3 h, SBA-15-S still remains its mesostructure. In constrast, SBA-15 loses most of the mesostructure. Furthermore, heteroatoms-substituted mesoporous materials such as aluminosilicate (Al-SBA-15-U) could also be synthesized in the presence of urea under neutral condition, and Al-SBA-15-U exhibits extraordinary hydrothermal stability. The unique and novel route for syntheses of hydrothermally stable and well ordered mesoporous silica and aluminosilicate materials in the presence of sulfate and carbonate species would be very important for potential application of mesoporous silica (SBA-15) and aluminosilicate materials (Al-SBA-15) as industrially supports and acidic catalysts.