Promoter-induced synthesis of MCM-41 type mesoporous materials including Ti- and V-MCM-41 and their catalytic properties in oxidation reactions

Abstract

The addition of small amounts of certain oxyanions of group VA, VIA and VIIA (such as nitrate, phosphate, arsenate, sulfate, bromate, iodate etc.) to the synthesis gel mixture of MCM-41 type mesoporous materials reduces the synthesis time considerably. The hydrothermal syntheses of Si-MCM-41, Al-MCM-41, Ti-MCM-41 and V-MCM-41 were carried out in the presence and absence of promoters by refluxing the gel mixture at atmospheric pressure. Studies on the effect of different promoters in the synthesis of Si-MCM-41 show different degrees of enhancement in the long-range ordering process. It has been observed that the higher the charge/radius ratio ( Z/ r, Å −1) of the central cation of the promoter, the higher is the ability of the promoter to speed up the synthesis. The concept of promoter-induced synthesis of MCM-41 type mesoporous materials is applicable not only to Si-MCM-41 but also to Al-MCM-41, Ti-MCM-41 and V-MCM-41. The materials were characterized by XRD, N 2 adsorption, diffuse reflectance UV–vis spectroscopy, FTIR, solid state MAS NMR, SEM, EDX and TEM. The XRD patterns and the quite high BET surface areas with isotherms of type IV of all the samples are typical for hexagonal MCM-41 type ordered mesoporous materials. EDX analysis, solid state 27 Al and 51 V MAS NMR spectra and diffuse reflectance UV–vis spectra of the M-MCM-41 (M = Ti and V) samples confirm the incorporation of Al, Ti and V into the framework. The catalytic properties of the two Ti-MCM-41 samples and the two V-MCM-41 samples, prepared in the presence and absence of a promoter, were compared by means of the epoxidation of cyclohexene and the hydroxylation of 1-naphthol in the presence of different peroxide sources. Comparable catalytic properties indicate that the samples, prepared in the presence and absence of a promoter, are similar in nature and the metal ions are present in the framework, presumably in a tetrahedral coordination.