Copper catalyzed selective methane oxidation to acetic acid using O2.

Abstract

The direct transformation of methane into C₂ oxygenates such as acetic acid selectively using molecular oxygen (O₂) is a significant challenge due to the chemical inertness of methane, the difficulty of methane C-H bond activation/C-C bond coupling and the thermodynamically favored over-oxidation. In this study, we have successfully developed a porous aluminium metal-organic framework (MOF)-supported single-site mono-copper(ii) hydroxyl catalyst [MIL-53(Al)-Cu(OH)], which is efficient in directly oxidizing methane to acetic acid in water at 175 °C with a remarkable selectivity using only O₂. This heterogeneous catalyst achieved an exceptional acetic acid productivity of 11 796 mmol_CH3CO2H mol_Cu ^-1 h^-1 in 9.3% methane conversion with 95% selectivity in the liquid phase and can be reused at least 6 times. Our experiments, along with computational studies and spectroscopic analyses, suggest a catalytic cycle involving the formation of a methyl radical (˙CH₃). The confinement of Cu-active sites within the porous MIL-53(Al) MOF facilitates C-C bond coupling, resulting in the efficient formation of acetic acid with excellent selectivity due to the internal mass transfer limitations. This work advances the development of efficient and chemoselective earth-abundant metal catalysts using MOFs for the direct transformation of methane into value-added products under mild and eco-friendly conditions.