Mechanistic Insights into the Aerobic Oxidation of Methane to Formaldehyde over Cu–Zeolite

The catalytic cycle of methane (CH4) oxidation
mediated
by CuII–OH species in the presence of molecular
oxygen as an oxidant is discussed. The reaction pathways and energetics
for the partial oxidation of CH4 to formaldehyde on the
AlO4 surface of Cu–zeolite are computed and analyzed
using the 3T cluster model at the B3LYP level of the theory. The presence
of CuII–OH facilitates the C–H activation
of CH4. The activation energy for this process is calculated
to be 27.0 kcal/mol. Subsequently, the oxygen in the system coordinates
with the Cu atom, generating formaldehyde via the formation of Cu-oxyl
species. The overall reaction for the partial oxidation of CH4 is exothermic, with an energy change of 60.5 kcal/mol. Considering
the activation energies of the catalytic cycle, the activation of
the C–H bond of CH4 and the cleavage of the O–O
bond are comparable and are the rate-limiting processes for both.
In addition, an alternative pathway begins with C–H bond cleavage
by the copper-oxyl species, ultimately yielding formaldehyde. These
findings suggest that CuII–OH can effectively induce
C–H bond cleavage, which is crucial for informing design guidelines
for zeolite catalysts. More interestingly, proton transfer, hydride
transfer, and hydrogen-atom transfer are included in the catalytic
cycle.