Natural gas, consisting mainly of methane (CH₄), has a relatively low energy density at ambient conditions (~36 kJ∙L-1). Partial oxidation of CH4 to methanol (CH3OH) lifts the energy density to ~17 MJ∙L-1 and drives the production of numerous chemicals. In nature, it is achieved by methane monooxygenase with di-iron sites, which is extremely challenging to mimic in artificial systems due to the high dissociation energy of C−H bond in CH4 (439 kJ∙mol-1) and facile over-oxidation of CH3OH to CO and CO2. Here we report the direct photo-oxidation of CH₄ over mono-iron-hydroxyl sites immobilized within a metal-organic framework, PMOF-RuFe(OH). Under ambient and flow conditions using H2O and O2, CH4 is converted to CH₃OH with 100% selectivity and a time yield of 8.81±0.34 mmol∙gcat⁻¹∙h⁻¹ (versus 5.05 mmol∙gcat⁻1∙h⁻1 for methane monooxygenase). By using operando spectroscopic and modelling techniques, we find that confined mono-iron-hydroxyl sites bind CH4 by forming an [Fe−OH···CH4] intermediate, significantly lowering the activation barrier for C−H bonds. The confinement of mono-iron-hydroxyl sites in porous matrix demonstrates a strategy for C−H bond activation in CH4 to drive the direct photosynthesis of CH₃OH.
|Publication status||Accepted/In press - 5 May 2022|