Characterization and catalytic performance of methyl-grafted Ti-HMS catalysts prepared by the CVD method

Methyl-grafted Ti-HMS catalysts were prepared by the chemical vapor deposition (CVD) method using TiCl4 as titanium source and various organic silanes as silylating agents. The catalysts were characterized by XRD, N2-adsorption–desorption, FTIR, 29Si NMR, TGA, DR UV–vis, and evaluated by the epoxidation of propylene using cumene hydroperoxide (CHP) as oxidant. The results reveal that the silylated Ti-HMS catalysts are more active than the unsilylated Ti-HMS catalyst, and the methyl-grafted Ti-HMS catalyst silylated with hexamethyldisilazane (HMDSZ) exhibits better epoxidation performance than the analogues silylated with trimethylchlorosilane (TMCS) or dimethyldichorosilane (DMDCS). It is proposed that not only the form of T–O–SiMen (n = 2 or 3) but also the breakage of Ti–O–T (Si–O–Si or Si–O–Ti) are observed in silylation using Cl-containing silylating agents (TMCS or DMDCS), which decrease the hydrophobicity of Ti-HMS catalysts and destroy the tetracoordinated titanium species that responsible for the epoxidation performance. This phenomenon has not been observed in silylation with HMDSZ.