Abstract:

Methane, the most abundant constituent of natural gas, is a potential substitute for the dwindling petroleum resources for the chemical industry as a carbon-based feedstock. Over the last two decades, global research endeavors have focused on the development of more efficient and selective catalysts for the conversion of ubiquitous but inert methane. In addition, the transportation of gaseous methane in pipelines is unavoidably accompanied by leakage, and methane is recognized as a potent greenhouse gas (20 times more powerful than carbon dioxide per molecule). Thus, the conversion of methane into heavier derivatives is also of crucial environmental concern. Unfortunately, there is still a lack of economical and practical routes for methane conversion. Currently, the major route for methane conversion is the steam reforming of methane into synthetic gases, which is a multistep and energy-consuming route. Another option is to use photoenergy to drive the conversion of methane, which has significant advantages such as the capacity to minimize coking by running at room temperature. A promising approach to photocatalytic methane conversion is the photo-powered direct coupling or oxidation of methane to form ethane, methanol and hydrogen. The ethane or methanol produced can, in turn, be converted into ethene or liquid fuels through metathesis or dehydrogenation, respectively. Furthermore, the direct dehydrogenation of methane is the best way to produce clean H₂ energy from fossil fuels since methane has the highest H/C ratio among hydrocarbons. However, the methane conversion efficiency of previously reported photocatalysts is low. Furthermore, the wavelength of light used in previously reported photocatalytic systems usually needs to be less than 270 nm, which is beyond the range of the solar spectrum (wavelength λ > 290 nm) reaching the Earth's surface. To achieve substantial yield and selectivity, and to exploit solar energy effectively, the development of photocatalytic systems with distinctly higher activity, higher selectivity, and lower photon energy threshold is desired. Over the past decades, many efforts have been made to activate the strong C―H bond in methane by light at room temperature. Based on the current state of research on photocatalytic methane conversion, we have focused our review on the following aspects: non-oxidative coupling of methane, dehydroaromatization of methane, and total and partial oxidation of methane. Finally, we summarize the difference between photocatalysis and thermal catalysis in the methane conversion reaction.

Key words: Methane conversion, C―H bond activation, Photocatalysis, Non-oxidative dehydrogenation, Zeolites, Semiconductors