DFT Analysis of Methane C-H Activation and Over-Oxidation by [Cu2 O]2+ and [Cu2 O2 ]2+ Sites in Zeolite Mordenite: Intra- versus Inter-site Over-Oxidation.
Chemphyschem
; 22(24): 2517-2525, 2021 12 13.
Article
em En
| MEDLINE
| ID: mdl-34519406
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 [Cu3 O3 ]2+ active sites can over-oxidize methane. However, the role of [Cu3 O3 ]2+ sites in methane-to-methanol conversion remains under debate. Here, we examine methane over-oxidation by dicopper [Cu2 O]2+ and [Cu2 O2 ]2+ sites using DFT in zeolite mordenite (MOR). For [Cu2 O2 ]2+ , we considered the µ-(η2η2 ) peroxo-, and bis(µ-oxo) motifs. These sites were considered in the eight-membered (8MR) ring of MOR. µ-(η2η2 ) peroxo sites are unstable relative to the bis(µ-oxo) motif with a small interconversion barrier. Unlike [Cu2 O]2+ which is active for methane C-H activation, [Cu2 O2 ]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 CH3 group, followed by OH and can proceed near 200 °C. Thus, for [Cu2 O]2+ and [Cu2 O2 ]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 [Cu3 O3 ]2+ sites and the role of Brønsted acid sites.
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1
Coleções:
01-internacional
Base de dados:
MEDLINE
Idioma:
En
Revista:
Chemphyschem
Assunto da revista:
BIOFISICA
/
QUIMICA
Ano de publicação:
2021
Tipo de documento:
Article
País de afiliação:
Estados Unidos