Epoxidation of allylic alcohols on self-assembled polyoxometalates hosted in layered double hydroxides with aqueous H 2O 2 as oxidant

Abstract

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 H 2O 2 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% H 2O 2 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.