Influence of Metal Identity in Amino Acid Functionalized ZIF-8 Nanozymes for Selective Methane Oxidation

The selective oxidation of methane to value-added oxygenates under mild aqueous conditions has garnered significant attention. However, the challenges in methane valorization stem from its inert C–H bond and the susceptibility of partially oxidized products to further oxidation. Inspired by the coordination-engineered environments found in metalloenzymes, two amino acid-functionalized zeolitic imidazolate frameworks (ZIF-8) containing either Fe or Cu biomimetic metal active sites (6.5 wt% metal) were developed to investigate how metal identity impacts methane oxidation. In this architecture, the ZIF-8 framework acts as a porous support, providing confinement and site isolation to the active sites, while amino acid ligands – histidine and aspartate – mimic biological coordination environments around Fe and Cu nanozymes. Collectively, these structural features regulate methane activation and the oxygenate formation. Catalytic testing was performed using H₂O₂ as the oxidant at 70 °C and 35 bar CH₄, and the resulting oxygenate products were quantified to evaluate catalyst activity and selectivity. We observed comparable methane conversions over the Fe- and Cu-based catalysts, reaching 31.2% and 29.6%, respectively. Despite the similar conversions, the metal species significantly influenced product distribution. Cu incorporation increased the methanol yield from 27.9 to 49.9 μmol gcat⁻¹ h⁻¹ and enhanced acetic acid formation nearly threefold, from 392 to 1097 μmol gcat⁻¹ h⁻¹, relative to the Fe counterpart. These results demonstrate that, under the investigated conditions, the choice of metal centre primarily governs oxygenate speciation rather than overall methane conversion. Compared to the Fe analogue, Cu nanozymes supported on amino-acid functionalized ZIF-8 exhibit a distinct selectivity pattern, favouring acetic acid formation as the dominant oxygenate while maintaining higher methanol productivity.