Epoxidation of allylic alcohols on self-assembled polyoxometalates hosted in layered double hydroxides with aqueous H2O2 as oxidant

摘要

A series of self-assembled polyoxometalate (POM) catalysts were directly immobilized into layered double hydroxides (LDH) by a selective ion-exchange method. Sandwich-type POM species are found to be more favorable than Keggin-type POM for the direct immobilization in LDH. because strict pH controlling is not needed and the LDH hosts can be kept intact. The resulting LDH-POM catalysts were evaluated in the epoxidation of allylic alcohols with aqueous H2O2 as the oxidant without using organic solvent. The heterogeneous LDH-POM catalysts show much higher epoxide selectivity than the corresponding homogeneous Na-POM catalysts, which can be attributed to the beneficial effect of basic LDH host on the suppression of the acid-catalyzed epoxide hydrolysis. The cooperation between the POM guest and the LDH host can achieve up to 99% selectivity of epoxide, 95% H2O2 efficiency and 37200 h(-1) TOF without the need of base additives and pH controlling, and the host-guest catalysts can be readily recycled with no apparent loss of catalytic performance. (C) 2008 Elsevier Inc. All rights reserved.