Structure Sensitivity in CH₄ Oxidation: Switching C₁ to C₂ Selectivity on Mn Single Atoms Versus Nanoparticles

The direct selective oxidation of methane (DSOM) to high value-added liquid oxygenates using only CH₄ and O₂ under mild conditions remains a formidable challenge in catalysis. Herein, we report the use of highly dispersed MnO_x/ZSM-5 catalysts, in the form of single atoms and nanoparticles, to steer the DSOM reaction toward C₁ and C₂ oxygenates, respectively. While the Mn single-atom catalysts (SACs) primarily produce C₁ products (HCOOH and CH₃OH) with a combined selectivity of ∼ 78.8%, the Mn nanoparticle (NP) catalysts exclusively produce liquid oxygenates with excellent stability, achieving a remarkable acetic acid selectivity of 81.8%. Various characterizations and density functional theory calculations reveal that the reaction path is governed by distinct methane activation mechanisms at different Mn sites. On SACs, CH₄ is selectively activated to *CH₃ at Mn-O sites, leading to C₁ oxygenates. In contrast, the multiple MnO_x sites on NPs promote further dehydrogenation to *CH₂ species, which subsequently couple into a key ethylene intermediate. This intermediate is then oxidized to acetic acid via a nonclassical pathway that circumvents CO carbonylation, thereby effectively suppressing overoxidation. This work provides fundamental insights into structure-sensitive methane activation and offers a practical route for the direct valorization of CH₄ into value-added oxygenates using a noble-metal-free catalytic system.