Methane-to-Methanol on Mononuclear Copper(II) Sites Supported on Al2 O3 : Structure of Active Sites from Electron Paramagnetic Resonance*

Abstract

The selective conversion of methane to methanol remains one of the holy grails of chemistry, where Cu-exchanged zeolites have been shown promote this reaction 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 γ- and η-alumina phase, and that their EPR signature can be attributed to species having a tri-coordinated [(Al₂ O)CuIIO(OH)]^- T-shape geometry. Overall, the selective conversion of methane to methanol, a two-electron process, involves two monomeric Cu^II sites that play in concert.

Keywords: EPR spectroscopy; alumina; copper; methane-to-methanol; surface organometallic chemistry.