DFT Analysis of Methane C−H Activation and Over‐Oxidation by [Cu₂O]²⁺ and [Cu₂O₂]²⁺ Sites in Zeolite Mordenite: Intra‐ versus Inter‐site Over‐Oxidation

Abstract

Methane over-oxidation by copper-exchanged zeolites prevents realization of high-yield catalytic conversion. However, there has been little description of the mechanism for methane overoxidation at the copper active sites of these zeolites. Using density functional theory (DFT) computations, we reported that tricopper [Cu 3 O 3 ] 2 + active sites can over-oxidize methane. However, the role of [Cu 3 O 3 ] 2 + sites in methane-to-methanol conversion remains under debate. Here, we examine methane over-oxidation by dicopper [Cu 2 O] 2 + and [Cu 2 O 2 ] 2 + sites using DFT in zeolite mordenite (MOR). For [Cu 2 O 2 ] 2 + , we considered the μ-(η 2 :η 2 ) peroxo-, and bis(μ-oxo) motifs. These sites were considered in the eight-membered (8MR) ring of MOR. μ-(η 2 :η 2 ) peroxo sites are unstable relative to the bis(μ-oxo) motif with a small interconversion barrier. Unlike [Cu 2 O] 2 + which is active for methane CÀ H activation, [Cu 2 O 2 ] 2 + has a very large methane CÀ H activation barrier in the 8MR. Stabilization of methanol and methyl at unreacted dicopper sites however leads to overoxidation via sequential hydrogen atom abstraction steps. For methanol, these are initiated by abstraction of the CH 3 group, followed by OH and can proceed near 200 °C. Thus, for [Cu 2 O] 2 + and [Cu 2 O 2 ] 2 + species, over-oxidation is an inter-site process. We discuss the implications of these findings for methanol selectivity, especially in comparison to the intra-site process for [Cu 3 O 3 ] 2 + sites and the role of Brønsted acid sites.

2-Pyridone-Directed CuII–Catalyzed General Method of C(sp2)-H Activation for C–S, C–Se, and C–N Cross-Coupling: Easy Access to Aryl Thioethers, Selenide Ethers, and Sulfonamides and DFT Study

“…This observation unequivocally indicates that the cleavage of the C–H bond is an irreversible process (Scheme ). Following these significant experimental observations, we conducted density functional theory (DFT) calculations to elucidate the operative reaction mechanism and various factors influencing different product selectivity observed experimentally. The free energy profiles depicting the initial C–H activation of benzamide derivative 1 and its subsequent coupling with 4-methoxythiophenol in DMSO are presented in Figure .…”