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

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 over-oxidation at the copper active sites of these zeolites. Using density functional theory (DFT) computations, we reported that tricopper [Cu3O3]2+ active sites can over-oxidize methane. However, the role of [Cu3O3]2+ sites in methane-to-methanol conversion remains under debate. Here, we examine methane over-oxidation by dicopper [Cu2O]2+ and [Cu2O2]2+ sites using DFT in zeolite mordenite (MOR). For [Cu2O2]2+, we considered the μ-(η2:η2) peroxo-, and bis(μ-oxo) motifs. These sites were considered in the eight-membered (8MR) ring of MOR. μ-(η2:η2) peroxo sites are unstable relative to the bis(μ-oxo) motif with a small interconversion barrier. Unlike [Cu2O]2+ which is active for methane C-H activation, [Cu2O2]2+ has a very large methane C-H activation barrier in the 8MR. Stabilization of methanol and methyl at unreacted dicopper sites however leads to over-oxidation via sequential hydrogen atom abstraction steps. For methanol, these are initiated by abstraction of the CH3 group, followed by OH and can proceed near 200 °C. Thus, for [Cu2O]2+ and [Cu2O2]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 [Cu3O3]2+ sites and the role of Bronsted acid sites.