Full Spectroscopic Characterization of the Molecular Oxygen-Based Methane to Methanol Conversion over Open Fe(II) Sites in a Metal–Organic Framework

Iron-based enzymes
efficiently activate molecular oxygen to perform
the oxidation of methane to methanol (MTM), a reaction central to
the contemporary chemical industry. Conversely, a very limited number
of artificial catalysts have been devised to mimic this process. Herein,
we employ the MIL-100(Fe) metal–organic framework (MOF), a
material that exhibits isolated Fe sites, to accomplish the MTM conversion
using O2 as the oxidant under mild conditions. We apply
a diverse set of advanced operando X-ray techniques to unveil how
MIL-100(Fe) can act as a catalyst for direct MTM conversion. Single-phase
crystallinity and stability of the MOF under reaction conditions (200
or 100 °C, CH4 + O2) are confirmed by X-ray
diffraction measurements. X-ray absorption, emission, and resonant
inelastic scattering measurements show that thermal treatment above
200 °C generates Fe(II) sites that interact with O2 and CH4 to produce methanol. Experimental evidence-driven
density functional theory (DFT) calculations illustrate that the MTM
reaction involves the oxidation of the Fe(II) sites to Fe(III) via
a high-spin Fe(IV)O intermediate. Catalyst deactivation is
proposed to be caused by the escape of CH3• radicals from the relatively
large MOF pore cages, ultimately resulting in the formation of hydroxylated
triiron units, as proven by valence-to-core X-ray emission spectroscopy.
The O2-based MTM catalytic activity of MIL-100(Fe) in the
investigated conditions is demonstrated for two consecutive reaction
cycles, proving the MOF potential toward active site regeneration.
These findings will desirably lay the groundwork for the design of
improved MOF catalysts for the MTM conversion.