Salt effect in post-synthesis hydrothermal treatment of MCM-41

Post-synthesis hydrothermal treatment of MCM-41 mesoporous silica provides a convenient method for pore expansion and silica wall thickening for improvement of its stability. The physical chemistry of the process is investigated by examining the effects of water content, salts and aluminum on pore expansion. A hydrothermal treatment at 150 °C in water or a salt solution leads to controlled pore expansion. The pore size and wall thickness vary with the kind of anion of the salt and their concentrations. The salt effect follows the well-known binding strength of the Hofmeister series of anion for the cationic surfactant, NO^-₃ > Br^- > Cl^- > SO^2-₄ ~ F^-. It is proposed that an equilibrium of distributing surfactants inside the MCM-41 channels and in solution controls the pore size and wall thickness upon varying the salts. The anion (X^-) binds with cationic surfactant molecules (S⁺) in solution to shift the equilibrium of surfactant/silicate binding leading to less surfactant and water in the pore, and hence less pore expansion. The effect of ammonia hydrothermal treatment is to shift the equilibrium to stronger surfactant/silicate binding and thus more pore expansion. At neutral condition, the wall thickness varies inversely with respect to the pore diameter. The wall thickness variation agrees with a model of elastic deformation of the wall silica materials at high temperature.