Switching Photocatalytic Methane Oxidation Toward Ethanol by Tuning Spin States

Abstract

The selective oxidation of methane (CH4) has attractive potentials for mitigating global warming and producing value-added chemicals, whereas the efficient generation of multicarbon products such as ethanol remains challenging, due to the short lifetimes and high unpaired concentrations of reactive intermediates (like •OH, •CH3, •OCH3, and •CH2OH). Herein, we developed a medium-spin Zn–O–Fe(MS) catalyst with tunable Fe(III) spin states, which can efficiently photo-oxidize CH4 to ethanol at ambient conditions. The unpaired electrons in d orbitals of Fe sites allow for efficient adsorption of •OH, and the eg orbital occupancy enables stabilizing different key carbon-containing intermediates (•OCH3 versus •CH2OH). By balancing those two descriptors, the medium-spin Zn–O–Fe(MS) catalyst allows to achieve selective generation of •CH2OH intermediates with a moderate *OH coverage and subsequent coupling of •CH3 and •CH2OH to ethanol. With a light intensity of 100 mW·cm−2 and water as a weak oxidant, the Zn–O–Fe(MS) catalyst exhibited a peak photocatalytic CH4-to-ethanol yield of 372 µmol·g−1·h−1 without the use of photosensitizers or sacrificial reagents, which is ∼3 times higher than the previously best reported photocatalytic CH4 oxidation to ethanol under similar conditions. Our study suggests an attractive potential of converting CH4 to multicarbon products by modulating spin states of the catalytic sites.