Advantages and Limitations of Hydrogen Peroxide for Direct Oxidation of Methane to Methanol at Mono-Copper Active Sites in Cu-Exchanged Zeolites

The performance of the direct oxidation of methane to methanol (DMTM) is significantly influenced by the oxidant. It is still incredibly challenging to realize one-pot DMTM using dioxygen. So far, hydrogen peroxide is still the most frequently reported green oxidant for DMTM with a high selectivity for methanol. To achieve insights into the influence of oxidants on the DMTM performance, we computationally investigated the reaction mechanisms of DMTM using hydrogen peroxide at mono-copper sites in three kinds of Cu-exchanged zeolites with different sizes of the micropores. We identified the common advantages and limitations of hydrogen peroxide as the oxidant. In contrast to dioxygen, the O-O bond of hydrogen peroxide could be easily broken to produce reactive surface oxygen species, which enables the facile C-H bond activation of methane at a lower temperature. However, because of the radical-like process for C-H bond activation at mono-copper sites, it is kinetically challenging to actualize the preferential C-H bond activation of methane as compared to that of methanol. Moreover, the lower O-H bonding energy of hydrogen peroxide would result in the self-decomposition of hydrogen peroxide. Despite the bottlenecks, the kinetic analysis shows that it is still promising to improve catalysts to boost the performance of DMTM using hydrogen peroxide.