Mechanisms for direct methane conversion to oxygenates at low temperature

Methane's abundant and diverse reserves make it a promising feedstock for clean fuels and chemicals. To replace the current energy and capital intensive syngas-based indirect process, it is important to develop economically viable and environmentally responsible technology for converting CH₄ into value-added products, such as oxygenates. Direct catalytic conversion of CH₄ into oxygenates under mild conditions is a significant challenge within catalysis due to CH₄ being relatively inert and over-oxidation resulting in low selectivity. The development of high-efficiency and low-temperature C–H bond activation catalyst is the key to translation of the catalytic CH₄ conversion into an industrial implication. This review encapsulates typical catalysts for C₁ and C₂₊ oxygenates synthesis from CH₄, and compares their activities as well as corresponding homogeneous and heterogeneous reaction mechanisms. Also, the potential heterogeneous catalysts and new technologies to synthesize C₂₊ oxygenates from CH₄ and CO_x (x = 1,2) are highlighted. Moreover, future research directions in C₂₊ oxygenates synthesis, composite catalyst development, reaction mechanism analysis, and new reactor designs are discussed.