Copper-Oxo Active Sites for Methane C-H Activation in Zeolites: Molecular Understanding of Impact of Methane Hydroxylation on UV-Vis Spectra.

Here, we analyze changes in the optical spectra of activated copper-exchanged zeolites during methane activation with the Tamm-Dancoff time-dependent density functional theory, TDA-DFT, while using the ωB2PLYP functional. Two active sites, [Cu2O]2+ and [Cu3O3]2+, were studied. For [Cu2O]+, the 22 700 cm-1 peak is associated with μ-oxo 2p → Cu 3d/4s charge transfer. Of the [Cu2O]2+ methane C-H activation intermediates that we examined, only [Cu-O(H)(H)-Cu] and [Cu-O(H)(CH3)-Cu] have spectra that match experimental observations. After methane activation, the μ-oxo 2p orbitals lose two electrons and become hybridized with methanol C 2p orbitals and/or H 1s orbitals. The frontier unoccupied orbitals become more Cu 4s/4p Rydberg-like, reducing overlap with occupied orbitals. These effects cause the disappearance of the 22 700 cm-1 peak. For [Cu3O3]2+, the exact structures of the species formed after methane activation are unknown. Thus, we considered eight possible structures. Several of these provide a significant decrease in intensity near 23 000-38 000 cm-1, as seen experimentally. Notably, these species involve either rebound of the separated methyl to a μ-oxo atom or its remote stabilization at a Brønsted acid site in exchange for the acidic proton. These spectral changes are caused by the same mechanism seen in [Cu2O]2+ and are likely responsible for the observed reduced intensities near 23 000-38 000 cm-1. Thus, TDA-DFT calculations with ωB2PLYP provide a molecular-level understanding of the evolution of copper-oxo active sites during methane-to-methanol conversion.