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

There is an ongoing debate regarding the role of [CuO] 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 [CuO] is compared to that of the more ubiquitous [CuO]. Methane CH activation is associated with only the Cu/Cu redox couple in [CuO]. For [CuO], there is no basis for the Cu/Cu couple's participation at the density functional theory ground state. In [CuO], 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 Cu/Cu and oxyl/O redox couples. Based on this, the Cu 1s-edge XANES spectra of [CuO] and [CuO] should both have energy signatures of Cu → Cu reduction during methane activation. This is indeed what we obtained from the calculated XANES spectra. [CuO] and [CuO] 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 [CuO] will lead to formation of Cu sites. Importantly, for future quantitative XANES studies, involvement of O + e → O in [CuO] implies a disconnect between the overall reactivity and the number of electrons used in the Cu/Cu redox couple.