The conversion of methane to methanol is of significant economic and environmental importance, but this transformation remains a grand challenge due to the favored formation of over-oxidation products.^[1] In contrast, methane monooxygenases, which are enzymes based on Cu or Fe metal centers, are very selective for this reaction and have thus been an inspiration for catalyst design. Of the various systems studied, Cu-exchange zeolites have shown the greatest potential for the selective oxidation of methane to methanol. Stepwise reaction conditions are employed to avoid the intrinsic low yield associated with direct catalytic oxidation.^[2] However, despite years of research, the identity and nuclearity of the active site is still under debate.^[3]
In that context, surface organometallic chemistry (SOMC)^[4] combined with thermolytic molecular precursors (TMP)^[5] has emerged as a powerful approach to generate isolated metal sites with controlled nuclearity and oxidation state on oxide supports for a broad range of metals. Herein, we show that this approach allows generating on non-porous alumina support Cu(II) sites that convert methane into methanol with high selectivity (> 80%). Through combined X-ray absorption (XAS) and electron paramagnetic resonance (EPR) spectroscopies, we show that this reaction takes place on pairs of monomeric Cu(II) sites in close proximity.^[6] This discovery contrasts with the active centers proposed in Cu-exchanged zeolites, where µ²-Cu₂O₂ or [Cu₃O₃]²⁺ are proposed, but parallels the most recent findings on particulate methane monooxygenases (pMMO) where the presence of only mononuclear Cu centers are proposed.^[7]

Scheme for the partial oxidation of methane to methanol

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[6] Meyet et al. Angew. Chem. Int. Ed. 2019 – in press (doi: 10.1002/ange.201903802)
[7] Ross et al. Science 2019, 364, 6440