Carbon Dioxide\nOrigin during High-Yield Partial Oxidation\nof Methane to Protected Methyl Esters

Mn/TiO<sub>2</sub> catalysts are highly active for the\npartial\noxidation of methane using dioxygen in acidic media to form oxidation-protected\nmethyl esters. Nonetheless, the formation of carbon dioxide in these\nsystems remains significant. In this study, we used <sup>13</sup>C\nisotope-labeled methane to trace the origin of the carbon dioxide\nformation. These experiments showed that carbon dioxide primarily\nstems from acid degradation rather than product overoxidation in this\nsystem, confirming the high-yield methane partial oxidation of the\nesterification approach. Evaluation of activities and selectivities\nof the catalysts with different initial manganese distributions under\ndifferent methane partial pressures and reaction temperatures revealed\nthat ester selectivity within the liquid products remains nearly constant,\nwhile lower temperatures minimize the parasitic overoxidation of solvent.\nUsing this knowledge, we demonstrated that a Mn/TiO<sub>2</sub> catalyst\nsynthesized via coprecipitation can provide 90% ester selectivity\nat 21% methane conversion within 3 h at 215 °C, representing\nthe best reported heterogeneously catalyzed system for methane partial\noxidation with molecular oxygen. This study identifies the road ahead\nfor this chemistry, entailing further optimization of solid catalysts\nfor improved ester productivity and high product yields while curtailing\nundesired side reactions unrelated to methane conversion.