Photothermal direct methane conversion to formaldehyde at the gas-solid interface under ambient pressure

Photocatalytic direct oxidation of methane to C₁ oxygenates offers a green alternative to conventional energy-intensive and high-carbon-footprint multi-step processes. However, current batch-type gas–liquid–solid reaction systems under high-pressure conditions face critical challenges in real-time product separation and concentration for industrial implementation. Here, we demonstrate a continuous-flow gas–solid photothermal catalytic route for methane conversion to formaldehyde under ambient pressure, where the generated gas-phase formaldehyde can be easily collected by water absorption. The Ag single-atom modified ZnO photocatalyst achieves a formaldehyde production rate of 117.8 ± 1.7 μmol h⁻¹ with 71.2 ± 0.8% selectivity. Meanwhile, a highly concentrated formaldehyde solution (514.2 ± 33.7 µmol mL⁻¹, 1.54 ± 0.10 wt.%) is obtained through 12-hour water absorption, effectively overcoming the product enrichment barrier that plagues conventional batch reaction route. This study establishes a robust technological foundation for sustainable industrial-scale conversion of methane to value-added chemicals.