Direct photo-oxidation of methane to methanol over a mono-iron hydroxyl site.

Natural gas, consisting mainly of methane (CH 4 ), has a relatively low energy density at ambient conditions (~36 kJ l -1 ). Partial oxidation of CH 4 to methanol (CH 3 OH) lifts the energy density to ~17 MJ l -1 and drives the production of numerous chemicals. In nature, this 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 the C-H bond in CH 4 (439 kJ mol -1 ) and facile over-oxidation of CH 3 OH to CO and CO 2 . Here we report the direct photo-oxidation of CH 4 over mono-iron hydroxyl sites immobilized within a metal-organic framework, PMOF-RuFe(OH). Under ambient and flow conditions in the presence of H 2 O and O 2 , CH 4 is converted to CH 3 OH with 100% selectivity and a time yield of 8.81 ± 0.34 mmol g cat -1  h -1 (versus 5.05 mmol g cat -1  h -1 for methane monooxygenase). By using operando spectroscopic and modelling techniques, we find that confined mono-iron hydroxyl sites bind CH 4 by forming an [Fe-OH···CH 4 ] intermediate, thus lowering the barrier for C-H bond activation. The confinement of mono-iron hydroxyl sites in a porous matrix demonstrates a strategy for C-H bond activation in CH 4 to drive the direct photosynthesis of CH 3 OH.