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 TiO 2–SiO 2 xerogel, Ti-MCM-41, and a TiCl 4-modified HZSM-5 zeolite has been investigated. It was found that only the crystalline molecular sieve TS-1 and the TiCl 4 modified HZSM-5 zeolite were active, whereas the TiO 2–SiO 2 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 TiCl 4 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 H 2O and methanol. The oxirane ring opening reactions are catalyzed on acid sites. The H 2O 2 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 CH 3OH/H 2O ratio in the solvent as well as the reaction temperature on the title reaction and the regeneration of deactivated catalyst have also been investigated.