Roles of Zeolite Confinement and Cu–O–Cu Angle on the Direct Conversion of Methane to Methanol by [Cu₂(μ-O)]²⁺-Exchanged AEI, CHA, AFX, and MFI Zeolites

Abstract

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.

Comparative Study of Diverse Copper Zeolites for the Conversion of Methane into Methanol

“…Recently, some computational theoretical studies have reported that different metal–oxo active site geometries due to changes in zeolite ring‐structures lead to changes in reactivity related to the amount of methanol yield . The calculated activation energies for methane C−H bond dissociation by small‐pore zeolites are lower than those for medium‐pore zeolites . On the other hand, CuNa‐SSZ‐13H gave a considerably lower methanol yield despite the comparable copper content of each 8‐MB ring pore structure.…”

The Effect of the Active‐Site Structure on the Activity of Copper Mordenite in the Aerobic and Anaerobic Conversion of Methane into Methanol

“…Whereas the existence of smaller active sites has been suggested by natural enzymes and proven experimentally, the possibility of forming larger clusters has also been suggested theoretically based on thermodynamic stability . Copper monomers, on the other hand, are believed to require the formation of copper hydrides to facilitate methane oxidation, which is energetically unfavorable .…”

The Effect of the Active‐Site Structure on the Activity of Copper Mordenite in the Aerobic and Anaerobic Conversion of Methane into Methanol

“…Whereas the existence of smaller active sites has been suggested by natural enzymes [16] and proven experimentally, [2,[5][6][7][8] the possibility of forming larger clusters has also been suggested theoretically based on thermodynamic stability. [3,[17][18][19] Copper monomers,onthe other hand, are believed to require the formation of copper hydrides to facilitate methane oxidation, which is energetically unfavorable. [20] The intrinsic activities of copper species of different sizes are still as ubject of debate.F urthermore,r ecent reports suggest the possibility of forming am ixture of active sites, [2][3][4] and even give evidence to their dynamic structural behavior.…”