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 CH₄ selective oxidation to CH₃OH over metal-exchanged zeolites. In the present work, we utilize density functional theory calculations to investigate several possible reactions following the CH₄ activation on the mono­(μ-O)­Cu₂^II, bis­(μ-O)­Cu₂^III, and bis­(μ-O)­Ni₂^III active sites anchored in the ZSM-5 zeolite framework. In the mono­(μ-O)­Cu₂ case, we found that a CH₃ ligand formed during the CH₄ activation is favorably oxidized to CH₃OH or C₂H₆O when H₂O or CH₃OH are, respectively, present on the reduced (CH₃)­O_F–Cu^I–OH–Cu^I site. Nonetheless, the reaction rates are predicted to be lower than the CH₄ activation, confirming the fact that the CH₃OH extraction step using steam requires a longer time. Similarly, although the bis­(μ-O)­Cu₂ active site is reported to easily form and desorb CH₃OH, the reduced Cu^II–O–Cu^II center is active to oxidize the formed CH₃OH to CH₂O with high exothermicity and reaction rate. The bis­(μ-O)­Ni₂ active 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.