Photocatalytic Conversion of Methane to Ethanol at a Three-Phase Interface with Concentration-Matched Hydroxyl and Methyl Radicals

The direct oxidation of CH₄ to C₂H₅OH is attractive but challenging owing to the intricate processes involving carbon-chain growth and hydroxylation simultaneously. The inherent difficulty arises from the strong tendency of CH₄ to overoxidize in the commonly used pressurized powder suspension systems rich in reactive oxygen radicals (ROR), which are specifically designed for CH₄ concentration and activation. Meanwhile, the strong tendency of nucleophilic attack of potent ROR on the C-C bond of the resulting product C₂H₅OH ultimately leads to a higher selectivity for C1 oxygenates. This study addresses this multifaceted issue by designing a three-phase interface based on a hydrophilic floating Fe(III)-cross-linked macroporous alginate hydrogel film encapsulated with C₃N₄ [Fe(III)@ACN] to simultaneously enhance the accessibility of H₂O and CH₄ molecules to the active sites and species within the macroporous channel. The hydrophilic properties of Fe(III)@ACN allow the in situ production of H₂O₂ from C₃N₄ through the water oxidation reaction under irradiation. The concurrent photoinduced Fe(II) triggers Fenton reaction with H₂O₂ to produce ^•OH. The enhanced mass transfer of CH₄ at the three-phase interface ensures the efficient formation of ^•CH₃ by reacting with ^•OH, ultimately facilitating carbon-chain growth in the conversion pathway from CH₄ to CH₃OH and finally to C₂H₅OH with ^•CH₃ and ^•OH present in comparable concentrations. Thus, the Fe(III)@ACN catalyst exhibits a remarkable 96% selectivity for alcohol, achieving a 90% selectivity for C₂H₅OH in the alcohol products. The C₂H₅OH production rate reaches 171.7 μmol g^-1 h^-1 without the need for precious-metal additive.