In:
ChemPhysChem, Wiley, Vol. 22, No. 24 ( 2021-12-13), p. 2517-2525
Abstract:
Methane over‐oxidation by copper‐exchanged zeolites prevents realization of high‐yield catalytic conversion. However, there has been little description of the mechanism for methane over‐oxidation at the copper active sites of these zeolites. Using density functional theory (DFT) computations, we reported that tricopper [Cu 3 O 3 ] 2+ active sites can over‐oxidize methane. However, the role of [Cu 3 O 3 ] 2+ sites in methane‐to‐methanol conversion remains under debate. Here, we examine methane over‐oxidation by dicopper [Cu 2 O] 2+ and [Cu 2 O 2 ] 2+ sites using DFT in zeolite mordenite (MOR). For [Cu 2 O 2 ] 2+ , we considered the μ‐(η 2 :η 2 ) peroxo‐, and bis(μ‐oxo) motifs. These sites were considered in the eight‐membered (8MR) ring of MOR. μ‐(η 2 :η 2 ) peroxo sites are unstable relative to the bis(μ‐oxo) motif with a small interconversion barrier. Unlike [Cu 2 O] 2+ which is active for methane C−H activation, [Cu 2 O 2 ] 2+ has a very large methane C−H activation barrier in the 8MR. Stabilization of methanol and methyl at unreacted dicopper sites however leads to over‐oxidation via sequential hydrogen atom abstraction steps. For methanol, these are initiated by abstraction of the CH 3 group, followed by OH and can proceed near 200 °C. Thus, for [Cu 2 O] 2+ and [Cu 2 O 2 ] 2+ species, over‐oxidation is an inter‐site process. We discuss the implications of these findings for methanol selectivity, especially in comparison to the intra‐site process for [Cu 3 O 3 ] 2+ sites and the role of Brønsted acid sites.
Type of Medium:
Online Resource
ISSN:
1439-4235
,
1439-7641
DOI:
10.1002/cphc.202100580
Language:
English
Publisher:
Wiley
Publication Date:
2021
detail.hit.zdb_id:
2025223-7
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