Effect of preparation method on leaching of Ti from the redox molecular sieve TS-1

Abstract

The leaching of Ti from the redox molecular sieve TS-1 when used as a catalyst for the oxidation of crotyl alcohol with hydrogen peroxide is described and discussed. The crystallisation time of the TS-1 is found to be the critical preparation parameter. TS-1 prepared using a 2 day crystallisation period leaches Ti, whereas TS-1 prepared using a 10 day crystallisation period is inert to leaching. Silanisation of the external surface of the 2 day preparation effectively stops the leaching of Ti, without significantly affecting the catalytic performance. Similarly, treatment of the 2 day TS-1 with sodium azide also minimises leaching of Ti but with a reduction in catalytic activity. The Ti leaching is shown to be caused by the reaction of a triol with TS-1 in the presence of hydrogen peroxide from the surface of the 2 day TS-1. A possible mechanism is proposed in which the triol by-product chelates the Ti, thereby breaking Ti–O–Si framework bonds and causing leaching to form a Ti species in solution. The reactivity of this Ti species in solution is modelled using the reaction of crotyl alcohol with hydrogen peroxide in the presence of titanyl acetylacetonate, and it is found that this effectively catalyses the formation of triol. Hence, it is concluded that, once initiated, Ti leaching will be catalysed by the reaction products of the solution Ti species that is formed.