Development of Novel Liquidphase Oxidation System by Utilizing Characteristic of Oxide Clusters

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The Keggin-type di-iron-substituted silicotungstate, γ-SiW_10 {Fe(OH_2)}_2O_<38>^<6-> (I), was synthesized by the reaction of the lacunary [γ-SiW_<10>O_<36>]^<8-> with Fe(NO_3)_3 in an acidic aqueous solution and isolated as the tetra-n-butyl ammonium salt (TBA-I). It was characterized by various analyzes and the structure with the oxo-bridged di-iron site was clarified. TBA-I was stable and catalyzed selective oxidation of various alkenes and alkenes with hydrogen peroxide : Cyclohexane, adamantane, n-hexane, and n-pentane were catalytically oxidized. Even lower alkenes such as methane, ethane, propane, and n-butane were catalytically oxidized. It was remarkable that the efficiency of hydrogen peroxide utilization to oxygenated products reached up to ca. 100 % for the" oxidation of cyclopean and adamantine. Alkenes were mainly deoxidized with hydrogen peroxide. It was demonstrated that the TBA-I showed high turnover number of 135-147 for the oxidation of cyctohexane with 1 atm oxygen. The catalytic activity greatly depended on structures of iron centers and TBA-I showed the highest turnover number of 135-147 for the oxidation of cyclopean with 1 atm oxygen among iron and tri-transition-metal-substituted silicotungstates.