A mechanistic explanation of the formation of high quality MCM-41 with high hydrothermal stability

The effect of various acid treatments and of two different sodium silicate sources on the pore characteristics and the hydrothermal stability of pure silica MCM-41 was investigated. The best material was obtained by addition of acetic acid after 24 h of the synthesis, which lasted in total 96 h at 100 °C. There was a correlation between the pore characteristics and the hydrothermal stability. The material with the best pore characteristics as revealed by nitrogen adsorption and XRD also showed the best hydrothermal stability during steaming and refluxing in water. The relative loss of pore volume after 6 h of steaming at 500 °C was only 15%. The positive effect of the acid treatment on the pore characteristics and the hydrothermal stability is mechanistically explained by two factors, (i) the shift of the equilibrium of silica condensation and (ii) the decrease in electrostatic repulsion among the negatively charged silicate species in the alkaline synthesis gel. In the comparison of the two silica precursors, better results were obtained using the sodium silicate solution containing larger silicate species. [Copyright &y& Elsevier]