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

The catalytic epoxidation of propylene with aqueous hydrogen peroxide over four Ti-containing silicates, namely titanium silicalite (TS-I), 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-I 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(2)O and methanol. The oxirane ring opening reactions are catalyzed on acid sites. The H(2)O(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(3)OH/H(2)O ratio in the solvent as well as the reaction temperature on the title reaction and the regeneration of deactivated catalyst have also been investigated. (C) 1998 Elsevier Science B.V. All rights reserved.