The post-activation reactions of methane (CH4) to methanol (CH3OH), formaldehyde (CH2O), and dimethyl ether (C2H6O) are crucial issues in the CH4selective oxidation to CH3OH over metal-exchanged zeolites. In the present work, we utilize density functional theory calculations to investigate several possible reactions following the CH4activation on the mono(μ-O)Cu2II, bis(μ-O)Cu2III, and bis(μ-O)Ni2IIIactive sites anchored in the ZSM-5 zeolite framework. In the mono(μ-O)Cu2case, we found that a CH3ligand formed during the CH4activation is favorably oxidized to CH3OH or C2H6O when H2O or CH3OH are, respectively, present on the reduced (CH3)OF-CuI-OH-CuIsite. Nonetheless, the reaction rates are predicted to be lower than the CH4activation, confirming the fact that the CH3OH extraction step using steam requires a longer time. Similarly, although the bis(μ-O)Cu2active site is reported to easily form and desorb CH3OH, the reduced CuII-O-CuIIcenter is active to oxidize the formed CH3OH to CH2O with high exothermicity and reaction rate. The bis(μ-O)Ni2active site, on the other hand, not only is reported to facilely form and desorb CH3OH but also is resistant to the overoxidation reaction forming CH2O, due to an early occupancy of the Ni δ* acceptor orbital at the H-CH2OH activation stage, resulting in a product-like (late) transition structure, where one of the Ni2+centers is already reduced to a highly unstable Ni+. This work provides insights into the reaction mechanisms and elaborates the importance of the CH3O formation to achieve high-selectivity CH3OH.
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