Mechanistic Insights into the Aerobic Oxidation of Methane to Formaldehyde over Cu–Zeolite

Abstract

The catalytic cycle of methane (CH₄) oxidation mediated by Cu^II–OH species in the presence of molecular oxygen as an oxidant is discussed. The reaction pathways and energetics for the partial oxidation of CH₄ to formaldehyde on the AlO₄ surface of Cu–zeolite are computed and analyzed using the 3T cluster model at the B3LYP level of the theory. The presence of Cu^II–OH facilitates the C–H activation of CH₄. The activation energy for this process is calculated to be 27.0 kcal/mol. Subsequently, the oxygen in the system coordinates with the Cu atom, generating formaldehyde via the formation of Cu-oxyl species. The overall reaction for the partial oxidation of CH₄ is exothermic, with an energy change of 60.5 kcal/mol. Considering the activation energies of the catalytic cycle, the activation of the C–H bond of CH₄ and the cleavage of the O–O bond are comparable and are the rate-limiting processes for both. In addition, an alternative pathway begins with C–H bond cleavage by the copper-oxyl species, ultimately yielding formaldehyde. These findings suggest that Cu^II–OH can effectively induce C–H bond cleavage, which is crucial for informing design guidelines for zeolite catalysts. More interestingly, proton transfer, hydride transfer, and hydrogen-atom transfer are included in the catalytic cycle.