Challenges and prospects for the photocatalytic liquefaction of methane into oxygenated hydrocarbons

The conversion of methane to fuel and value-added chemicals such as syngas and oxygenated hydrocarbons is an attractive proposition for the energy and chemical industry. The conventional technologies for methane conversion are energy-intensive, costly, and major sources of greenhouse gas emissions. Low-temperature direct methane valorization is an attractive energy solution to significantly reduce dependency on current commercial technologies. Amongst the wide portfolio of the direct methane conversion processes is the photocatalytic liquefaction of methane to oxygenated hydrocarbons, which utilizes solar radiation to stimulate methane activation for methanol production. The low activity, selectivity, and low efficiency of the photocatalytic route, compared to other well-established technologies for the conversion of methane, limited their wide-scale applications. Therefore, a review of the existing literature is required to give a perspective of this promising technology, to discuss challenges and to identify potential areas for research and development and ultimately contribute to promoting green methane conversion processes. This review aims to highlight the state-of-the-art and the progress achieved in the catalyst development studies of the photocatalytic conversion of methane to oxygenated hydrocarbons. A special focus is given to WO3, bismuth-based, and zeolite photocatalysts used in the direct photocatalytic conversion of methane under moderate conditions. The catalyst structure-property relationship, the effect of operating conditions on reactivity, and reaction mechanism studies are discussed to highlight the challenges and opportunities for future research work. A perspective and outlook of the direct methane liquefaction technology are presented which emphasize the potential areas for improvements of catalytic activity and selectivity.