Ti complexes assembled HMS as effective catalysts for epoxidation of alkene

Abstract

The exchange reactions between Ti compounds and hexagonal mesoporous silicas (HMS) have been investigated in detail by means of UV-Vis diffuse reflection spectra, FT-IR, N2 volumetric adsorption, chemical and elemental analyses. It was found that the exchange of titanium tetraisoproxide (Ti(OiPr)4), and especially titanium tetrachloride (TiCl4) with the surface hydroxyls of the HMS easily occurred, this exchange interaction caused the distortion of the supports Si-O4 tetrahedron and resulted in the appearance of 960 cm−1 band in their IR spectra. The UV-Vis diffuse reflection spectra showed that the Ti-assembled HMS materials did not exist the bulk titania and had the very similar Ti-sites to the Ti-substituted HMS, indicating the high dispersed Ti species upon the surface of HMS. The surface area and pore volume of HMS after the Ti complexes grafting have been significantly reduced, further confirming the introduction of Ti complexes into the channels of HMS by chemically grafting. A chiral Ti tartrate complex (Ti(OiPr)4+DET) could be successfully grafted onto HMS through three assembled pathways. In catalyzing epoxidation of styrene and cyclohexene with tetrabutyl hydroperoxide (TBHP), the above described catalysts showed a higher catalytic activity and selectivity for the epoxides than those unloading Ti tartrate complexes (Ti(OiPr)4+DET) and the Ti-substituted HMS catalysts, among them, the HMS-Ti catalyst prepared with Ti(OiPr)4 at 393 K possessed the best catalytic activity, but the lowest epoxidative selectivity, which is mainly due to the Lewis acid sites of HMS-Ti for further catalyzing the rearrangement of the epoxides. The adjusting of d-(+)-diethyl (DET) to the Ti-sites anchored on the supports led to the decrease in the catalytic activity and the increase in the epoxidative selectivity. The obvious advantages of these assembled catalysts are that they can be recycled for many times without significant loss of activity. The assembled HMS catalysts with the chiral Ti tartrate complex were enantioselective for asymmetric epoxidation of styrene with TBHP, about 20∼32% enantiomeric excess (e.e.) of the epoxides could be obtained over these chiral Ti mesoporous catalysts.