Recent interest in Cu-exchanged zeolite catalysts for methane hydroxylation has been broadened to small-pore Cu-zeolites such as Cu-SSZ-13 (Cu-CHA), Cu-SSZ-16 (Cu-AFX), and Cu-SSZ-39 (Cu-AEI), which were reported to produce more methanol per copper atom than the medium-pore Cu-ZSM-5 (Cu-MFI) and large-pore Cu-mordenite (Cu-MOR) zeolites do. To elucidate the nature of such fascinating catalytic activities, theoretical investigations based on density functional theory (DFT) were performed on the direct conversion of methane to methanol by [Cu2(μ-O)]2+-exchanged AEI, CHA, AFX, and MFI zeolites in periodic systems. DFT computational results show that the important activation energies for C-H bond dissociation by [Cu2(μ-O)]2+-AEI, -CHA, and -AFX zeolites are lower than those for [Cu2(μ-O)]2+-MFI zeolite. Moreover, the rate-determining methanol desorption and N2O decomposition by [2Cu]2+-AEI zeolite are also found to require low barriers, which renders [Cu2(μ-O)]2+-AEI zeolite highly active for the direct conversion of methane to methanol. Molecular orbital analyses show that AEI, CHA, AFX, and MFI zeolites exert similar confinement effects that stabilize the transition state for C-H bond cleavage. In addition, a decrease in the Cu-O-Cu angle, due to a change in the zeolite ring structure, lowers the acceptor orbital energy of [Cu2(μ-O)]2+-zeolite, which further stabilizes the transition state. We conclude that these two factors play important roles in the activation of methane.
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