Recent advances on methane partial oxidation toward oxygenates under mild conditions

Developing a direct conversion route of methane toward oxygenates under milder conditions as a supplement to the two-step route via syngas holds high economic value and demonstrates great potential. The reactions are mostly performed below 200 °C and methane can achieve a 100% atom utilization efficiency. This review summarizes the efforts devoted to developing selective thermo-catalytic oxidation of methane to oxygenates, mainly CH3OH as well as HCHO, HCOOH, CH3COOH and C2H5OH in the past ten years. The intrinsic active site configurations and the catalytic mechanisms are disclosed within different categories of oxidants employed in the reaction system, including O2, H2O2, N2O and H2O. The specific role of H2O is also discussed. Additionally, perspectives on catalyst design and process innovation are presented. It is essential to synthesize catalysts with unitary and clear structures, despite their difficulty, to study the true structure-performance relationship, which in return provides an insight for the catalyst design. From a cost, reactivity and safety stand point, only O2 is deemed acceptable. Possible future directions of research include utilizing CO2 as oxidant or using ILs as solvent. Given the feature of direct methane conversion to methanol being a thermodynamically feasible yet kinetically unfavorable process, which exhibiting a conversion-selectivity tradeoff issue, diversifying innovation of both catalyst and reaction process would offer a solution.