Methane Partial Oxidation over Cu2(μ-O)2+ and Cu3(μ-O)32+ Active Species in Large-Pore Zeolites

Copper-containing large-pore zeolites, such as Cu-mordenite (Cu-MOR) and Cu-omega (Cu-MAZ), oxidize methane to yield a high amount of methanol. Two distinct active centers in MOR zeolite, namely, Cu2(μ-O)2+ and Cu3(μ-O)32+, have been proposed and debated. In particular, the Cu2(μ-O)2+ species was experimentally found to be formed on two different Al pair sites with different reactivities toward methane. However, computational attempts based on density functional theory (DFT) have not been able to confirm them. Moreover, the full cycle of the reaction, which includes methane activation, water-assisted methanol desorption, and a second methane reaction with the active species, has not been well understood yet. In this study, we employed DFT calculations based on the Perdew, Burke, and Ernzerhof functional to reasonably calculate all activation energies involved in such a complete reaction over periodic systems of Cux(μ-O)y2+-MOR and -MAZ (x, y = 2, 1 and 3, 3) in the high-spin and low-spin states. We found two Al pair sites in MOR zeolite that form two distinct Cu2(μ-O)2+ structures able to cleave the C–H bond of methane with activation energies excellently comparable with the experimental values. Our computational results further suggest that the addition of a water molecule helps the reaction to reduce the high methanol desorption energies. We also show that two of the three bridging O atoms in Cu3(μ-O)32+-MOR and -MAZ significantly differ in reactivity toward methane.