The Synthesis And Catalytic Oxidation Study Of Polyoxometalates Immobilized On Layered Double Hydroxides

Polyoxometalates have received more and more attention in the area of catalysis, because they are economical, environmentally friendly, and their acidic and redox properties can be controlled at atomic or molecular levels, which make that polyoxometalates are capable of epoxidizing a broad of olefins.This paper report different epoxidation systems based on three types of polyoxometalates using hydrogen peroxide (30% aq.) as oxidant. The main progress and research results have been shown as follows:1. A halide-free reaction-controlled phase transfer catalytic system consisting of [CH3(CH3)17N(CH3)2]3PW4O32/H2O2/Ethyl acetate was developed, This system can be used for catalytic epoxidation of olefins and exhibits high conversion and selectivity. In an H2O/ethyl acetate biphasic system, catalyst I itself was not soluble in the ethyl acetate phase, but it dissolved slowly with the increase of reaction temperature and action of hydrogen peroxide and olefin, which subsequently led to homogeneous catalytic epoxidation of olefins. When the reaction stopped, with temperature dropping, catalyst I precipitated gradually. The catalyst could be easily recovered and reused after reaction. The fresh catalyst and used one were characterized by Raman, IR,31P MAS NMR and 31P NMR.2. We reported a new system based on silicon-tungsten acid salt, which was immobilized in hydrotalcite MgAl-SiWx. This system used hydrogen peroxide as oxidant and catalyzed the oxidation of benzaldehyde in solvent-free conditions. The system could be used repeatedly as a heterogeneous catalysis. There was no discernible loss in activity after four cycles.3. We synthesized a variety of catalytic systems based on hydrotalcite, which were used in a biphasic medium (H2O2/ethyl acetate). This new environmental friendly catalytic system, namely, "composite catalyst/H2O2/ethyl acetate "system, was used in the oxidation of benzyl sulfide, phenyl sulfide. A high conversion of 99% was obtained, and the conversion is not reduced after 5 cycles. However, the improving was necessary for olefin epoxidation.