Epoxidation of Crotyl Alcohol Using Ti-Containing Heterogeneous Catalysts: Comments on the Loss of Ti by Leaching

The leaching of Ti from the redox molecular sieve TS-1 is compared and contrasted with Ti leaching from Ti–Alβ, Ti–MCM-41, and a Ti–xerogel under both batch and continuous flow reaction conditions for the oxidation of crotyl alcohol with hydrogen peroxide. The product distributions for the catalysts are similar, with epoxide being formed initially and, subsequently, the secondary reaction products of the triol and ether diols. Ti leaching is more pronounced under continuous flow conditions and the order of stability is TS-1 > Ti–Alβ > Ti–MCM-41 > Ti–xerogel, for both continuous flow and batch reaction conditions. TS-1 leaching is shown to be caused by the reaction of triol with TS-1 in the presence of hydrogen peroxide. A possible mechanism in which the triol byproduct chelates the Ti and breaks the Ti–O–Si framework bonds, leading in turn to the formation of a Ti species in solution, is proposed. The soluble Ti species is found to be an active homogeneous catalyst giving triol as the major product from crotyl alcohol. Under batch reaction conditions, it is possible that an equilibrium is established between the solution Ti species and Ti on the catalyst surface. This equilibrium is perturbed when continuous flow conditions are used, and this leads to increased Ti leaching.