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 omega B2PLYP functional. Two active sites, [Cu2O](2+) and [Cu3O3](2+), were studied. For [Cu2O](+), the 22 700 cm(-1) peak is associated with mu-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 mu-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 mu-oxo atom or its remote stabilization at a Bronsted 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 omega B2PLYP provide a molecular-level understanding of the evolution of copper-oxo active sites during methane-to-methanol conversion.