Direct Methane to Methanol Conversion: An Overview of Non-Syn Gas Catalytic Strategies

Direct methane to methanol conversion is a dream reaction in industrial chemistry, which takes inspiration from the biological methanol production catalysed by methane monooxygenase enzymes (MMOs). Over the years, extensive studies have been conducted on this topic by bioengineering the MMOs, and tailoring methods to isolate the MMOs in the active form. Similarly, remarkable achievements have been noted in other methane activation strategies such as the use of heterogeneous catalysts or molecular catalysts. In this review, we outline the methane metabolism performed by methanotrophs and detail the latest advancements in the active site structures and catalytic mechanisms of both types of MMOs. Also, recent progress in the bioinspired approaches using various heterogeneous catalysts, especially first-row transition metal zeolites and the mechanistic insights are discussed. In addition, studies using molecular complexes such as "Periana catalyst" for methane to methanol conversion through methyl ester formation in the presence of strong acids are also detailed. Compared to the progress noted in the metal zeolites-mediated methane activation field, the utilisation of molecular catalysts or MMOs for this application is still in its nascent phase and further research is required to overcome the limitations of these methods effectively.