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

Natural gas, consisting mainly of methane (CH4), has a relatively low energy density at ambient conditions (-36 kJI(-1)). Partial oxidation of CH4 to methanol (CH3OH) lifts the energy density to similar to 17 MJI land 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 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 in the presence of H2O and O-2 , CH4 is converted to CH3OH 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 CH4 by forming an [Fe-OH-CH4] 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 CH4 to drive the direct photosynthesis of CH3OH.

Automated summary

This study presents a method to directly photo-oxidize CH4 for the synthesis of CH3OH using mono-iron hydroxyl sites immobilized within a metal-organic framework. By forming an [Fe-OH-CH4] intermediate, the activation energy for C-H bond cleavage is reduced. This research provides an effective strategy for the direct synthesis of CH3OH from CH4.