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

The direct oxidation of CH 4 to C 2 H 5 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 CH 4 to overoxidize in the commonly used pressurized powder suspension systems rich in reactive oxygen radicals (ROR), which are specifically designed for CH 4 concentration and activation. Meanwhile, the strong tendency of nucleophilic attack of potent ROR on the C-C bond of the resulting product C 2 H 5 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 C 3 N 4 [Fe(III)@ACN] to simultaneously enhance the accessibility of H 2 O and CH 4 molecules to the active sites and species within the macroporous channel. The hydrophilic properties of Fe(III)@ACN allow the in situ production of H 2 O 2 from C 3 N 4 through the water oxidation reaction under irradiation. The concurrent photoinduced Fe(II) triggers Fenton reaction with H 2 O 2 to produce • OH. The enhanced mass transfer of CH 4 at the three-phase interface ensures the efficient formation of • CH 3 by reacting with • OH, ultimately facilitating carbon-chain growth in the conversion pathway from CH 4 to CH 3 OH and finally to C 2 H 5 OH with • CH 3 and • OH present in comparable concentrations. Thus, the Fe(III)@ACN catalyst exhibits a remarkable 96% selectivity for alcohol, achieving a 90% selectivity for C 2 H 5 OH in the alcohol products. The C 2 H 5 OH production rate reaches 171.7 μmol g -1 h -1 without the need for precious-metal additive.