Postsynthesis Hydrothermal Restructuring of M41S Mesoporous Molecular Sieves in Water

Abstract

A high-quality Si−MCM-41 was prepared with the composition of SiO2, 0.2; TEAOH, 0.25; CTMABr, 35 in H2O using fumed silica as the silica source. The quality of the compounds was assessed using different analytical techniques such as XRD, N2 adsorption−desorption measurement, UV−vis absorption, FTIR, and 29Si MAS NMR. The quality of M41S mesoporous materials, synthesized using a single C16TMA+ surfactant, could be further improved by a postsynthesis hydrothermal treatment of the as-synthesized materials in water. In comparison with the ordinary preparation, the water-treated materials exhibited a higher long-distance order, a higher BET surface area, and a larger pore size. The postsynthesis water treatment method provided an easy and mild way to prepare high-quality M41S molecular sieves with the pore size controllable by altering either the treatment time or the temperature. The postsynthesis treatment also enhanced the stability, especially the hydrothermal stability, of Si−MCM-41. The 29Si MAS NMR results demonstrated that the water treatment promoted the wall polymerization or local atomic arrangement. It was suggested that the pH lowering of synthesis mixture by replacing the mother liquor with water played a crucial role in the restructuring toward a high-quality Si−MCM-41. The quality of Ti−MCM-41, Al−MCM-41, and Si−MCM-48 was also improved by a similar postsynthesis hydrothermal treatment.