Wetting stability of Si-MCM-41 mesoporous material in neutral, acidic and basic aqueous solutions

The wetting stability of three calcined Si-MCM-41 materials synthesized at room temperature and under hydrothermal conditions with and without pH adjustment and salt addition was studied by SAXS, SEM, HR-TEM, BET-BJH, FTIR and NMR techniques. The stability was investigated after wetting with neutral, basic and acidic aqueous solutions and further recalcination. Calcined Si-MCM-41 materials synthesized at room temperature fully degraded under wetting with neutral water. Crystallization under hydrothermal conditions improved their wetting stability. pH adjustment and NaCl addition during hydrothermal crystallization led to a further improvement. It was proposed, based on spectroscopic data, that the structure degradation during wetting is caused by hydration of the siloxane structure at the wetting stage followed by siloxane hydrolysis-hydroxylation of strained Si(-OSi-)₄ units and their rearrangement-redehydroxylation during recalcination. This leads to intergrowth of curved hexagonal crystal rods, reduction of the surface area and pore volume of the material forming a disordered pore structure with increased pore walls thickness. The water-stable Si-MCM-41 synthesized with pH adjustment and salt addition was also insensitive to wetting with acidic aqueous solution. However, treatment of this non-strained material with a basic solution at pH7.8-8.9 resulted in silica leaching and a reduction in crystallinity, the mean pore diameter and the pore volume. The rod shape and surface area remained relatively unchanged after wetting at pH7.8.