The Catalyzed Conversion of Methane to Value-Added Products

The progress of direct, selective conversion of methane (CH4) to organic oxygenates utilizing heterogeneous and homogeneous processes conducted in the gaseous or liquid phase is reviewed. The most active catalytic systems are examined in terms of rates, longevity, and reaction mechanisms. Despite significant effort, the successful heterogeneous mimicking of selective CH4 oxidation performed in an enzymatic system (methane monooxygenase) is still elusive. Under mild reaction conditions, the reaction is too slow, and under harsh conditions, total oxidation prevails. Thus, of particular interest herein is the assessment of oxidation at intermediate temperatures to 1) reduce total oxidation and 2) obtain sufficient concentration of activated oxygen and CH4 species. Important operational parameters such as reaction conditions, catalyst preparation methods, and cofeeding of chemicals, which significantly affect the yield of desired products, are discussed. One particular system that is successful is the catalytic conversion of CH4 to methanol under mild conditions using nitrous oxide (N2O) with Fe-based catalysts or oxygen (O-2) with Cu-based catalysts. Special attention is paid to the controversy related to the identification of active sites, where the oxygen species suitable for CH4 oxidation are purportedly formed.