Manganese Catalyzed Partial Oxidation of Light Alkanes

The catalytic partial oxidation of methane is achieved at low temperatures (<200 degrees C) using manganese oxides and manganese salts in mixtures of trifluoroacetic acid and trifluoroacetic anhydride. Dioxygen is used as the in situ terminal oxidant. For Mn oxides (e.g., MnO2, Mn2O3, and Mn3O4), we studied stoichiometric methane partial oxidation in HTFA (TFA = trifluoroacetate). Using a Mn trifluoroacetate salt, at 180 degrees C and under 25 psig of methane, product selectivity for the mono-oxidized product methyl trifluoroacetate (MeTFA) is observed to be >90% at similar to 35% methane conversion at approximately 6 turnovers. Under our catalytic methane oxidation reaction conditions, MeTFA is stable against overoxidation, which explains the likely high selectivity at conversions >15%. Using combined experimental studies and DFT calculations, a mechanism involving soluble and molecular Mn species in the catalytic cycle is proposed. The proposed reaction pathway involves initial activation of Mn-II by dioxygen, cleavage of a methane C-H bond by a Mn-II hydroxo intermediate, rebound of the methyl radical to generate MeTFA, and finally regeneration of the starting Mn-II complex Also, this process is shown to be applicable to the oxidation of ethane, favoring the mono-oxidized product ethyl trifluoroacetate (EtTFA) and reaching similar to 46% conversion.