Single Dispersion of Fe(H₂O)₂-Based Polyoxometalate on Polymeric Carbon Nitride for Biomimetic CH₄ Photooxidation

Direct methane conversion to value-added oxygenates under mild conditions with in-depth mechanism investigation has attracted wide interest. Inspired by methane monooxygenase, the K₉Na₂Fe(H₂O)₂{[γ-SiW₉O₃₄Fe(H₂O)]}₂·25H₂O polyoxometalate (Fe-POM) with well-defined Fe(H₂O)₂ sites is synthesized to clarify the key role of Fe species and their microenvironment toward CH₄ photooxidation. The Fe-POM can efficiently drive the conversion of CH₄ to HCOOH with a yield of 1570.0 µmol g_POM ^-1 and 95.8% selectivity at ambient conditions, much superior to that of [Fe(H₂O)SiW₁₁O₃₉]^5- with Fe(H₂O) active site, [Fe₂SiW₁₀O₃₈(OH)]₂ ^14- and [P₈W₄₈O₁₈₄Fe₁₆(OH)₂₈(H₂O)₄]^20- with multinuclear Fe-OH-Fe active sites. Single-dispersion of Fe-POM on polymeric carbon nitride (PCN) is facilely achieved to provide single-cluster functionalized PCN with well-defined Fe(H₂O)₂ site, the HCOOH yield can be improved to 5981.3 µmol g_POM ^-1. Systemic investigations demonstrate that the (WO)₄-Fe(H₂O)₂ can supply Fe═O active center for C-H activation via forming (WO)₄-Fe_a-O_t···CH₄ intermediate, similar to that for CH₄ oxidation in the monooxygenase. This work highlights a promising and facile strategy for single dispersion of ≈1-2 Å metal center with precise coordination microenvironment by uniformly anchoring nanoscale molecular clusters, which provides a well-defined model for in-depth mechanism research.