CH4-to-CH3OH on Mononuclear Cu(II) Sites Supported on Al2O3: Structure of Active Sites from Electron Paramagnetic Resonance

The selective conversion of methane to methanol remains one of the holy grails of chemistry, where Cu-exchanged zeolites have been shown to selectively convert methane to methanol under stepwise conditions. Over the years, several active sites have been proposed, ranging from mono-, di- to trimeric Cu(II). Herein, we report the formation of well-dispersed monomeric Cu(II) species supported on alumina using surface organometallic chemistry and their reactivity towards the selective and stepwise conversion of methane to methanol. Extensive studies using various transition alumina supports combined with spectroscopic characterization, in particular electron paramagnetic resonance (EPR), show that the active sites are associated with specific facets, which are typically found in gamma- and eta-alumina phase, and that their EPR signature can be attributed to species having a tri-coordinated [(Al 2 O)Cu II O(OH)] - ,T-shape geometry. Overall, the selective conversion of methane to methanol, a two-electron process, involve two of these isolated monomeric Cu(II) sites that play in concert.