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 cont rasted with Ti leaching from Ti-Al beta, Ti-MCM-41, and a Ti-xerogel under both batch and continuous flow reaction conditions for the oxidation of cro tyl alcohol with hydrogen peroxide. The product distributions for the catal ysts are similar, with epoxide being formed initially and, subsequently, th e secondary reaction products of the trial and ether dials. Ti leaching is more pronounced under continuous flow conditions and the order of stability is TS-1 > Ti-Al beta > Ti-MCM-41 > Ti-xerogel, for both continuous flow an d batch reaction conditions. TS-1 leaching is shown to be caused by the rea ction of trial with TS-1 in the presence of hydrogen peroxide. A possible m echanism in which the triol byproduct chelates the Ti and breaks the Ti-O-S i framework bonds, leading in turn to the formation of a Ti species in solu tion, is proposed. The soluble Ti species is found to be an active homogene ous catalyst giving triol as the major product from crotyl alcohol. Under b atch reaction conditions, it is possible that an equilibrium is established between the solution Ti species and Ti on the catalyst surface. This equil ibrium is perturbed when continuous flow conditions are used, and this lead s to increased Ti leaching. (C) 2001 academic Press.