Adeyiga Olajumoke, Suleiman Olabisi, Odoh Samuel O
Department of Chemistry, University of Nevada, Reno, 1664 North Virginia Street, Reno, Nevada 89557-0216, United States.
Inorg Chem. 2021 Jun 21;60(12):8489-8499. doi: 10.1021/acs.inorgchem.0c03510. Epub 2021 Jun 7.
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, [CuO] and [CuO], were studied. For [CuO], the 22 700 cm peak is associated with μ-oxo 2p → Cu 3d/4s charge transfer. Of the [CuO] methane C-H activation intermediates that we examined, only [Cu-O(H)(H)-Cu] and [Cu-O(H)(CH)-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 peak. For [CuO], 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, 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 [CuO] and are likely responsible for the observed reduced intensities near 23 000-38 000 cm. Thus, TDA-DFT calculations with ωB2PLYP provide a molecular-level understanding of the evolution of copper-oxo active sites during methane-to-methanol conversion.
在此，我们使用含时密度泛函理论（TDA-DFT）中的塔姆-丹科夫近似，并采用ωB2PLYP泛函，分析了甲烷活化过程中活性铜交换沸石的光谱变化。研究了两个活性位点，即[CuO]和[CuO]。对于[CuO]，22700 cm⁻¹的峰与μ-氧桥2p→Cu 3d/4s电荷转移相关。在我们研究的[CuO]甲烷C-H活化中间体中，只有[Cu-O(H)(H)-Cu]和[Cu-O(H)(CH)-Cu]的光谱与实验观测结果相符。甲烷活化后，μ-氧桥2p轨道失去两个电子，并与甲醇C 2p轨道和/或H 1s轨道杂化。前沿未占据轨道变得更像Cu 4s/4p里德堡轨道，减少了与占据轨道的重叠。这些效应导致22700 cm⁻¹的峰消失。对于[CuO]，甲烷活化后形成的物种的确切结构未知。因此，我们考虑了八种可能的结构。其中几种结构在23000 - 38000 cm⁻¹附近强度显著降低，这与实验观测结果一致。值得注意的是，这些物种涉及分离出的甲基反弹到μ-氧原子上，或者其在布朗斯特酸位点远程稳定化以交换酸性质子。这些光谱变化是由在[CuO]中观察到的相同机制引起的，并且可能是23000 - 38000 cm⁻¹附近观测到的强度降低的原因。因此，使用ωB2PLYP进行的TDA-DFT计算提供了对甲烷转化为甲醇过程中铜氧活性位点演化的分子水平理解。
