Several extra-framework copper-oxo clusters have been implicated in the conversion of methane to methanol by Cu-exchanged zeolites. The Cu-oxo clusters in the zeolite come in many forms with some of the most active being [Cu₃O₃]²⁺and [Cu₂O]²⁺ species. Previous investigations of the reaction mechanisms and the activity of the [Cu₃O₃]²⁺and [Cu₂O]²⁺ centers toward methane activation suggest the presence of characteristic Cu²⁺/Cu⁺redox couples. In other words, methane activation by [Cu₂O]²⁺is associated with reduction of Cu²⁺ to Cu⁺. For [Cu₃O₃]²⁺, the possibilities include a Cu³⁺/Cu²⁺ or O^-/O^2- redox couple. Using Density Functional Theory (DFT) and localized Orbital Bonding Analysis (LOBA) we investigated the nature of the redox couples present in the different Cu-oxo species. Using LOBA, we studied the oxidation state of Cu and O in the Cu-oxo clusters before and after methane hydroxylation. In addition, time-dependent density functional theory calculations were used to simulate the UV-Vis and X-ray K-edge spectra. Our findings reveal that methane activation in the dicopper system is associated with the Cu⁺/Cu²⁺redox couple while activation in the tricopper system is associated with a combination of Cu²⁺/Cu⁺ and O^-/O^2- redox couples. We use this information to describe previously reported relative empirical yields of CO, CO₂ and methanol.

Olajumoke Adeyiga

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