Atomically dispersed metals within the ZSM-5 framework are promising catalysts for the liquid-phase oxidation of methane. In this work, we identified the location of Ir and Cu cations within the zeolite cage, elucidated the role of CO in forming active sites for C-H bond dissociation in methane, and the mechanism and trends in the formation of various oxygenates during methane oxidation in an aqueous medium with molecular oxygen on Ir-ZSM-5 and Cu-ZSM-5 catalysts, using detailed DFT simulations. In Ir-ZSM-5, CO remains as a spectator ligand at the Ir-O active site, responsible for the first C-H bond activation in methane, while in Cu-ZSM-5, it is a sacrificial ligand, undergoing oxidation to CO2 to facilitate the formation of the Cu-O active site for methane dissociation. The spectator CO ligand on Ir-ZSM-5 lowers the activation barrier for methane dissociation. Methanol, which forms from a radical rebound mechanism on Ir-ZSM-5, is likely to undergo further oxidation to formic acid via a formaldehyde intermediate, making it the most likely product. On Cu-ZSM-5, although the activation barrier to form the Cu-O active site is high, the methane dissociation barrier is extremely low, facilitating methanol formation in a radical rebound mechanism. The limited availability of the active sites for subsequent oxidation and the high activation barrier to form formaldehyde are likely to restrict formic acid formation on the Cu-ZSM-5 catalyst. Atomically dispersed Cu-ZSM-5 is a promising candidate for selective methane oxidation to methanol in an aqueous medium with molecular oxygen and CO co-feed.
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