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

Abstract

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.

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

“…31−33 ωB2PLYP also captures the decrease in the intensities of the 30,000 and 38,000 cm −1 bands upon contact with methane. 34 This agrees very well with the experimental data. 22,35,36 These reports on computational evidence for the spectroscopic…”

“…For this, we employed a long-range corrected double hybrid density functional for Tamm–Dancoff time-dependent DFT, TDA-DFT, calculations . This functional is ωB2PLYP. − ωB2PLYP also captures the decrease in the intensities of the 30,000 and 38,000 cm –1 bands upon contact with methane . This agrees very well with the experimental data. ,, These reports on computational evidence for the spectroscopic properties of [Cu 3 O 3 ] 2+ are encouraging.…”

“…This active site has been extensively characterized in various zeolites. ,,,− As such, we can use it as a reference for comparing the [Cu 3 O 3 ] 2+ system. The cluster model, 1A , has been used by us previously. , This 47-atom model includes the complete first- and second-coordination spheres around the dicopper active site. After activating methane, there are several possible intermediate species that can be formed.…”

“…A cluster model for the [Cu 3 O 3 ] 2+ site in the eight-membered ring, 8MR, of MOR is shown in Figure . This model has been used previously by various workers. ,, In addition to [Cu 3 O 3 ] 2+ , 2A , we considered several possible intermediates formed after C – H activation. In some of these, the methyl group and H from methane are coordinated to two μ-oxo atoms at the active site.…”

“…Single-point calculations were performed on the optimized geometries at the ωB2LYP/cc-pVDZ, B3LYP/cc-pVDZ, ωB2LYP/cc-pVTZ, and B3LYP/cc-pVTZ levels. , B3LYP is often used to study transition metal complexes, while ωB2LYP provides a good description of the ground-state electronic structures of copper-oxo species. , Localized orbital bonding analyses, LOBA, were carried out on the converged self-consistent-field solutions in order to determine the formal oxidation states of the atoms in our cluster models . The threshold for performing LOBA was set to 46–50%.…”