Methane C-H Activation by [Cu2O]2+ and [Cu3O3]2+ in Copper-Exchanged Zeolites: Computational Analysis of Redox Chemistry and X-ray Absorption Spectroscopy.

Abstract

There is an ongoing debate regarding the role of [Cu3O3]2+ in methane-to-methanol conversion by copper-exchanged zeolites. Here, we perform electronic structure analysis and localized orbital bonding analysis to probe the redox chemistry of its Cu and μ-oxo sites. Also, the X-ray absorption near-edge structure, XANES, of methane activation in [Cu3O3]2+ is compared to that of the more ubiquitous [Cu2O]2+. Methane C-H activation is associated with only the Cu2+/Cu+ redox couple in [Cu2O]2+. For [Cu3O3]2+, there is no basis for the Cu3+/Cu2+ couple's participation at the density functional theory ground state. In [Cu3O3]2+, there are many possible intrazeolite intermediates for methane activation. In the nine possibilities that we examined, methane activation is driven by a combination of the Cu2+/Cu+ and oxyl/O2- redox couples. Based on this, the Cu 1s-edge XANES spectra of [Cu2O]2+ and [Cu3O3]2+ should both have energy signatures of Cu2+ → Cu+ reduction during methane activation. This is indeed what we obtained from the calculated XANES spectra. [Cu2O]2+ and [Cu3O3]2+ intermediates with one Cu+ site are shifted by 0.9-1.7 eV, while those with two Cu+ sites are shifted by 3.0-4.2 eV. These are near a range of 2.5-3.2 eV observed experimentally after contacting methane with activated copper-exchanged zeolites. Thus, activation of methane by [Cu3O3]2+ will lead to formation of Cu+ sites. Importantly, for future quantitative XANES studies, involvement of O- + e- → O2- in [Cu3O3]2+ implies a disconnect between the overall reactivity and the number of electrons used in the Cu2+/Cu+ redox couple.