Manganese Catalyzed Partial Oxidation of Light Alkanes

The
catalytic partial oxidation of methane is achieved at low temperatures
(<200 °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 °C and under 25 psig of methane, product selectivity for
the mono-oxidized product methyl trifluoroacetate (MeTFA) is observed
to be >90% at ∼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 MnII by dioxygen, cleavage of a methane C–H
bond by a MnIII hydroxo intermediate, rebound of the methyl
radical to generate MeTFA, and finally regeneration of the starting
MnII complex. Also, this process is shown to be applicable
to the oxidation of ethane, favoring the mono-oxidized product ethyl
trifluoroacetate (EtTFA) and reaching ∼46% conversion.