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.