Effect of inorganic salt addition during synthesis on pore structure and hydrothermal stability of mesoporous silica

Mesoporous materials were synthesized without and with the addition of salts such as NaCl and NH₄Cl and were characterized using different analytical techniques. The hydrothermal stability was investigated by water treating these mesoporous materials at 373 K for various times in closed bottles. Addition of salts resulted in transformation of mesophase, which depends strongly on the kinds and concentration of salts added. When added with NaCl, the mesoporous materials experienced phase transformations in the sequence MCM-41→KIT-1→MCM-41→KIT-1 →amorphous phase as the NaCl/surfactant molar ratio increased. However, addition of NH₄Cl led to the formation of disordered mesoporous materials. Mesoporous materials with high hydrothermal stability could be prepared by addition of a suitable amount of salts and the materials maintained their initial mesoporous structure after hydrothermal treatment at 373 K for at least 120 h in closed bottles. For mesoporous materials without salt addition or with addition of salts with unsuitable molar ratios, mesopores would collapse and the pore size distribution became very large after hydrothermal treatment.