CH4 conversion to value added products: Potential, limitations and extensions of a single step heterogeneous catalysis

• CH4 single step (direct) heterogeneous catalytic activation reviewed.
• Single step oxygen catalyst or oxidizer pathways are kinetically limited.
• Sulfur and halogen based activation route and extensions discussed.

Natural gas is envisioned as a primary source of energy and hydrocarbons in the foreseeable future. Though shale gas has recently become abundant, it has two main concerns: its environmental impact and sustainable utilization. The former is the result of recent reports of natural gas emissions and flares into the environment, where it acts as a powerful greenhouse gas, whereas the latter is dictated by the need for efficient hydrocarbon utilization. Modern natural gas processing units that yield clean fuels and feedstock from methane, CH4, require extremely large capital investments and are not economical in remote natural gas extraction sites. Single step (direct), non-syngas based catalytic routes of CH4 conversion to value added products have not been competitive economically and need to be reevaluated in the light of shale gas availability. This perspective discusses general considerations for the desired hydrocarbon products, the thermodynamic limitations involved in a single step conversion of CH4 and heterogeneous catalytic routes based on high temperatures and oxide based catalysts. We then discuss other catalysts and methods of CH4 activation that have recently emerged and are conceptually different from metal oxide catalyst based routes, such as those using sulfur or halogens. Lastly, we discuss a possible route of CH4 monetization beyond the first reactive product (such as ethylene oligomerization into fuels), as well as currently explored photo(electro)chemical routes of CH4 activation.

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