MetalCenter-Dependent Selectivity Divergence in MN₄ Single-Atom Catalysts for Aerobic HMF Oxidation

Selective oxidation of carbohydrate-derived 5-hydroxymethylfurfural (HMF) to high-value chemicals is a critical pathway for sustainable development. However, the primary challenge is controlling selectivity, as the narrow energy gaps between intermediates make discriminating between specific functional groups difficult. Herein, two single-atom catalysts (SACs) with M-N₄ configurations were synthesized by varying the metal active center. The FeN₄ catalyst achieved 99.9% HMF conversion with 93.9% selectivity toward 2-formyl-5-furancarboxylic acid (FFCA) at 333.15 K. Conversely, the CoN₄ catalyst yielded 2,5-furandicarboxylic acid (FDCA) as the predominant product with comparable HMF conversion. Despite divergent product selectivities, the two catalysts displayed nearly identical TOFs for HMF consumption (∼18.2 h^-1). DFT calculations revealed that the metal center governs the adsorption/activation of the FFCA intermediate. Specifically, lower FFCA adsorption energy on FeN₄ inhibits dehydrogenation at the α-C position, while stronger adsorption on CoN₄ facilitates its subsequent oxidation to FDCA. Moreover, the catalysts showed distinct O₂ activation mechanisms. FeN₄ preferentially generates superoxide anions (O₂ ^•-) for HMF oxidation, while CoN₄ utilizes hydroxyl radicals (•OH) for FFCA oxidation to FDCA. This study demonstrates an effective strategy for controlled HMF oxidation, providing a theoretical basis for tailoring reaction selectivity through a mechanistic divergence driven by the intrinsic electronic structure of the metal center.