Offshore solar-driven methane valorization to chloromethane with seawater

Methane (CH4), a key component of combustible ice, is a plentiful yet largely underutilized ocean resource. Its extreme inertness and the complexity of its storage and transportation have severely hindered its use. Here, we present a tandem flow system that produces freshwater through seawater desalination, while the resulting concentrated seawater is rich in chlorine ions and serves as a source for CH4 chlorination over a floatable photocatalyst with a three-phase interface. The key lies in tailoring the interfacial acid–base microenvironment and electronic structure of Pt–TiO2 photocatalyst to steer the radical processes toward selective methane chlorination. ·OH radicals mediate C–H activation to generate ·CH3 radicals, which subsequently couple efficiently with interfacial ·Cl radicals to yield CH3Cl. The introduction of trace Lewis acids (e.g., Al3+) further enhances selectivity by regulating the local proton balance, suppressing OH− accumulation, and stabilizing key radical intermediates. Our system achieves 98.1% CH3Cl selectivity with production rates of 2.9 mmol g−1 h−1 in AlCl3 solution and 79.5 mmol m−2 h−1 under flow operation, demonstrating long-term stability over 15 days. This work establishes a general paradigm for controlling aqueous radical chemistry under mild conditions, and paves the way for the practical exploitation of offshore ocean resources.