Carbon Dioxide Origin during High-Yield Partial Oxidation of Methane to Protected Methyl Esters

Mn/TiO2 catalysts are highly active for the partial oxidation of methane using dioxygen in acidic media to form oxidation-protected methyl esters. Nonetheless, the formation of carbon dioxide in these systems remains significant. In this study, we used 13C isotope-labeled methane to trace the origin of the carbon dioxide formation. These experiments showed that carbon dioxide primarily stems from acid degradation rather than product overoxidation in this system, confirming the high-yield methane partial oxidation of the esterification approach. Evaluation of activities and selectivities of the catalysts with different initial manganese distributions under different methane partial pressures and reaction temperatures revealed that ester selectivity within the liquid products remains nearly constant, while lower temperatures minimize the parasitic overoxidation of solvent. Using this knowledge, we demonstrated that a Mn/TiO2 catalyst synthesized via coprecipitation can provide 90% ester selectivity at 21% methane conversion within 3 h at 215 °C, representing the best reported heterogeneously catalyzed system for methane partial oxidation with molecular oxygen. This study identifies the road ahead for this chemistry, entailing further optimization of solid catalysts for improved ester productivity and high product yields while curtailing undesired side reactions unrelated to methane conversion.