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

The direct oxidation of CH4 to C2 H5 OH 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 C2 H5 OH 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 C3 N4 [Fe(III)@ACN] to simultaneously enhance the accessibility of H2 O 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 H2 O2 from C3 N4 through the water oxidation reaction under irradiation. The concurrent photoinduced Fe(II) triggers Fenton reaction with H2 O2 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 CH3 OH and finally to C2 H5 OH 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 C2 H5 OH in the alcohol products. The C2 H5 OH production rate reaches 171.7 μmol g-1 h-1 without the need for precious-metal additive.