Mn/TiO₂ Catalysts for the High-Yield Partial Oxidation of Methane with Molecular Oxygen to a Methyl Ester

We synthesize and evaluate a diverse range of Mn/TiO₂ catalysts with varying physical and redox properties for the methane-to-methyl-ester reaction using molecular oxygen in a diluted acid medium (10 wt% trifluoroacetic acid). Despite initial differences in manganese distributions, nearly all catalysts are active under the reaction conditions, and the degree of activity is partially correlated to catalyst reducibility in hydrogen temperature programmed reduction and manganese K-edge X-ray absorption spectroscopy. Under methane-limited conditions, exceptionally high product yields of up to ca. 30% are obtained. Catalysts synthesized with a co-precipitation method demonstrate the highest productivity (up to 1440 µmol g_cat ^-1 h^-1) in methane-excess conditions, and their full activity can be restored in a subsequent reaction cycle upon a mild thermal treatment. Under high conversion conditions, the co-precipitated catalyst can achieve methane-based product yields of ca. 15%. The performance of Mn/TiO₂ catalysts using molecular oxygen under less corrosive conditions surpasses that of other heterogeneous catalysts.