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

The catalytic cycle of methane (CH4) oxidation mediated by CuII–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 CuII–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 CuII–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.