Propylene epoxidation with hydrogen peroxide catalyzed by molecular sieves containing framework titanium

Abstract

The catalytic epoxidation of propylene with aqueous hydrogen peroxide over four Ti-containing silicates, namely titanium silicalite (TS-1), a TiO2-SiO2 xerogel, Ti-MCM-41, and a TiCl4-modified HZSM-5 zeolite has been investigated. It was found that only the crystalline molecular sieve TS-1 and the TiCl4 modified HZSM-5 zeolite were active, whereas the TiO2-SiO2 xerogel and the mesoporous Ti-MCM-41 were almost inactive. The structure of these Ti-containing silica materials plays an important role in determining their catalytic performances. Shape-selectivity and the hydrophobic nature of the material are the two most important factors. In the presence of TS-1, propylene oxide was the predominant product, whereas propylene diol and its mono-methyl ethers were formed with a TiCl4 modified HZSM-5. It is suggested that the acidity of catalyst is a crucial factor in determining product selectivity. Propylene diol and the mono-methyl ethers are produced by further reactions of propylene oxide with H2O and methanol. The oxirane ring opening reactions are catalyzed on acid sites. The H2O2 turnover frequency based on the titanium content increased while the selectivity to propylene oxide decreased with an increase of the Si/Ti ratio in TS-1 catalyst. The influence of CH3OH/H2O ratio in the solvent as well as the reaction temperature on the title reaction and the regeneration of deactivated catalyst have also been investigated.