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 adju stment and salt addition was studied by SAXS, SEM, HR-TEM, BET-BJH, FTIR an d 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 wett ing with neutral water. Crystallization under hydrothermal conditions impro ved their wetting stability. pH adjustment and NaCl addition during hydroth ermal crystallization led to a further improvement. It was proposed, based on spectroscopic data, that the structure degradation during wetting is cau sed 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 interg rowth of curved hexagonal crystal rods, reduction of the surface area and p ore volume of the material forming a disordered pore structure with increas ed pore walls thickness. The water-stable Si-MCM-41 synthesized with pH adj ustment and salt addition was also insensitive to wetting with acidic aqueo us solution. However, treatment of this non-strained material with a basic solution at pH 7.8-8.9 resulted in silica leaching and a reduction in cryst allinity, the mean pore diameter and the pore volume. The rod shape and sur face area remained relatively unchanged after wetting at pH 7.8. (C) 1999 E lsevier Science B.V. All rights reserved.