Bis(μ-oxo) versus mono(μ-oxo)dicopper cores in a zeolite for converting methane to methanol: an in situ XAS and DFT investigation

Abstract

Dicopper species have been identified as the active sites in converting methane to methanol in Cu-zeolites. To understand the formation of these copper cores in mordenite, we used in situ time-resolved X-ray absorption spectroscopy during heat treatment. Significant dehydration enabled the reduction of the copper cores, after which molecular oxygen was cleaved. The activated oxygen bridged two copper atoms to make the reactive precursor for the activation of methane. Even though the active bridging oxygen was detected, the XAS data were unable to distinguish a bis(μ-oxo)dicopper core from a mono(μ-oxo)dicopper core since XAS measures the average structure of the total copper population and the sample contains a mixture of copper species. We therefore used DFT calculations to understand the energetics of the formation of the active copper species and found that if a copper dimer exists in a zeolite, the mono(μ-oxo)dicopper species is an energetically plausible structure. This is in contrast to molecular dicopper cores where the bis(μ-oxo)dicopper core is preferentially formed.