Switching Photocatalytic Methane Oxidation Toward Ethanol by Tuning Spin States

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 e_g 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.