Tetrachlorocobaltate-Catalyzed Methane Oxidation to Methyl Trifluoroacetate

In ongoing attempts to efficiently utilize abundant natural gas, there has been steady scientific and industrial interest in using an environmentally benign and inexpensive oxidant (dioxygen O-2) for the direct catalytic oxidation of methane to oxygenate products under mild conditions. Here, we report the homogeneous bis(tetramethylammonium) tetrachlorocobaltate ([Me4N](2)CoCl4)-catalyzed methane oxidation to methyl trifluoroacetate (MeTFA) with dioxygen O-2 in trifluoroacetic acid (HTFA) media. [Me4N](2)CoCl4 had the highest catalytic activity among previously reported homogeneous cobalt-based catalyst systems; the turnover of methane to MeTFA reached 8.26 mol(ester) mol(metal)(-1)h(-1) at 180 degrees C. Results suggest that the ionic form of the catalyst makes the Co species more soluble in the HTFA media; consequently, an active catalyst form, [CoTFA(x)Cl(y)](2-), can form very rapidly. Furthermore, chloride anions dissociated from CoCl42- appear to suppress oxidation of the solvent HTFA, thereby driving the reaction toward methane oxidation. The effects of reaction time, catalyst concentration, O-2 and methane pressure, and reaction temperature on MeTFA production were also investigated.