Effect of structure and composition on epoxidation of hexene catalyzed by microporous and mesoporous Ti-Si mixed oxides

A series of microporous titania-silica mixed oxides were characterized and tested as catalysts for the liquid-phase epoxidation of 1-hexene with t-butyl hydroperoxide. Results from Si-29 MAS NMR spectroscopy verified results from earlier characterization studies that indicated cohydrolysis of alkoxide precursors produced well-mixed oxide samples. The catalytic activity of the samples for hexene epoxidation at 353 K increased with increasing silica content. Since the fraction of tetrahedral Ti atoms in the samples also increased with silica content, the active site for the reaction is proposed to be a tetrahedrally coordinated Ti atom in a silica matrix. Polar solvents like water, acetone, and methanol inhibited the epoxidation reaction. To investigate the effect of pore size on activity, mesoporous Ti-Si mixed oxides analogous to MCM-41 were synthesized. The mesoporous samples were the most active and selective catalysts for epoxidation with TBHP, presumably due to the ease of access of the reactants to the active Ti sites. Results from EXAFS and UV reflectance spectroscopy indicated that Ti atoms in the mesoporous mixed oxides are tetrahedrally coordinated to oxygen atoms with the same Ti-O bond distance as TS-1. However, the activities of our mesoporous samples are orders of magnitude lower than that of TS-1 for hexene epoxidation with aqueous hydrogen peroxide. Lower hydrophobicity of a silica mesopore (2-4 mn) compared to a TS-1 micropore (0.6 nn) may account for the difference in activity observed in reactions with aqueous hydrogen peroxide. (C) 1996 Academic Press, Inc.