Distinct Behaviors of Cu- And Ni-ZSM-5 Zeolites toward the Post-activation Reactions of Methane

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)Cu₂^II, bis(μ-O)Cu₂^III, and bis(μ-O)Ni₂^IIIactive 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)O_F-Cu^I-OH-Cu^Isite. 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 Cu^II-O-Cu^IIcenter 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.