Direct conversion of methane to methanol
using oxygen is experiencing
renewed interest owing to the availability of new natural gas resources.
Copper-exchanged zeolites such as mordenite and ZSM-5 have shown encouraging
results, and di- and tri-copper species have been suggested as active
sites. Recently, small eight-membered ring (8MR) zeolites including
SSZ-13, -16, and -39 have been shown to be active for methane oxidation,
but the active sites and reaction mechanisms in these 8MR zeolites
are not known. In this work, we use density functional theory (DFT)
calculations to systematically evaluate monocopper species as active
sites for the partial methane oxidation reaction in Cu-exchanged SSZ-13.
On the basis of kinetic and thermodynamic arguments, we suggest that
[CuIIOH]+ species in the 8MR are responsible
for the experimentally observed activity. Our results successfully
explain the available spectroscopic data and experimental observations
including (i) the necessity of water for methanol extraction and (ii)
the effect of Si/Al ratio on the catalyst activity. Monocopper species
have not yet been suggested as an active site for the partial methane
oxidation reaction, and our results suggest that [CuIIOH]+ active site may provide complementary routes for methane
activation in zeolites in addition to the known [Cu–O–Cu]2+ and Cu3O3 motifs.
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