Osmium-catalyzed selective oxidations of methane and ethane with hydrogen peroxide in aqueous medium
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Various transition metal chlorides including FeCl3, COCl2, RuCl3, RhCl3, PdCl2, OSCl3, IrCl3, H(2)PtC(l)6, CUCl2 and HAuCl4 were studied for the selective oxidations of methane and ethane with hydrogen peroxide in aqueous medium. Among the metal chlorides investigated, osmium(III) chloride (OSCl3) exhibited the highest turnover frequency (TOF) for the formation of organic oxygenates (mainly alcohols and aldehydes) from both methane and ethane. For the OsCl3-catalyzed oxidation of methane with hydrgen peroxide, methyl hydroperoxide was also formed together with methanol and formaldehyde. The effects of various kinetic factors on the catalytic behavior of the OsCl3-H2O2 system were investigated, and TOF values of 12 and 41 h(-1) could be obtained for oxygenate formation during the oxidations of methane and ethane, respectively. In the presence of OsCl3, NaCl0, NaCl0, or NaIO4 as oxidant was incapable of oxidizing methane and ethane to the corresponding oxygenates, and the use of tert-butyl hydroperoxide (TBHP) instead of H2O2 provided remarkably lower rates of formation of oxygenates. UV-Vis spectroscopic measurements suggested that OsCl3 was probably oxidized into an Os(IV) species by H2O2 in aqueous medium, and the Os(IV) species might be involved in the oxygenation of methane or ethane. The result that the conversions of both methane and ethane to oxygenates were suppressed by the addition of a radical scavenger suggested that the reactions proceeded via a radical pathway.