A review of the recent progress in revealing the structures, formation, and reactivity of the active sites in Fe-, Co-, Ni- and Cu-exchanged zeolites as well as outlooks on future research challenges and opportunities is presented.

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.

Copper-containing large-pore zeolites, such as Cumordenite (Cu-MOR) and Cu-omega (Cu-MAZ), oxidize methane to yield a high amount of methanol. Two distinct active centers in MOR zeolite, namely, [Cu 2 (μ-O)] 2+ and [Cu 3 (μ-O) 3 ] 2+ , have been proposed and debated. In particular, the [Cu 2 (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 [Cu x (μ-O) y ] 2+ -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 [Cu 2 (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 [Cu 3 (μ-O) 3 ] 2+ -MOR and -MAZ significantly differ in reactivity toward methane.

Metal-exchanged zeolites are known to exhibit catalytic activity in the direct conversion of methane to methanol. The influence of different metals on this reaction has been theoretically investigated by using density functional theory (DFT) calculations on a periodic system of MO+-ZSM-5 zeolite (M = Fe, Co, Ni, Cu). The results indicate a high dependence of the reaction on the metals, where the reactivity toward C–H bond dissociation is predicted to increase in the order CoO+-ZSM-5 < NiO+-ZSM-5 < FeO+-ZSM-5 < CuO+-ZSM-5 and the selectivity of methanol is predicted to increase in the order FeO+-ZSM-5 < CoO+-ZSM-5 < NiO+-ZSM-5 < CuO+-ZSM-5. The role of ZSM-5 zeolite in the catalytic activity is also investigated by comparing our calculation results with those reported for the reaction by bare MO+ species in the gas phase. We found that the nanopores of ZSM-5 zeolite exert a confinement effect which destabilizes the adsorption of methane and lowers the activation energy for the C–H bond dissociation. In addition to the conversion of methane, we investigated the direct conversion of ethane to ethanol by FeO+-ZSM-5 and found that this reaction proceeds with a lower C–H bond activation energy and a higher product selectivity in comparison to the conversion of methane to methanol by the same catalyst.