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The formation and reactivities of [Cu–O–M] 2+ species (M = Ti–Cu, Zr–Mo and Ru–Ag) in metal-exchanged zeolites, as well as stabilities of these species towards autoreduction by O 2 elimination are investigated with density functional theory. These species were investigated in zeolite mordenite in search of insights into active site formation mechanisms, the relationship between stability and reactivity as well as discovery of heterometallic species useful for isothermal methane-to-methanol conversion (MMC). Several [Cu–O–M] 2+ species (M = Ti–Cr and Zr–Mo) are substantially more stable than [Cu 2 O] 2+ . Other [Cu–O–M] 2+ species, (M = Mn–Ni and Ru–Ag) have similar formation energies to [Cu 2 O] 2+ , to within ±10 kcal mol −1 . Interestingly, only [Cu–O–Ag] 2+ is more active for methane activation than [Cu 2 O] 2+ . [Cu–O–Ag] 2+ is however more susceptible to O 2 elimination. By considering the formation energies, autoreduction, cost and activity towards the methane C–H bond, we can only conclude that [Cu 2 O] 2+ is best suited for MMC. Formation of [Cu 2 O] 2+ is initiated by proton transfer from aquo ligands to the framework and proceeds mostly via dehydration steps. Its μ-oxo bridge is formed via water-assisted condensation of two hydroxo groups. To evaluate the relationship between [Cu 2 O] 2+ and other active sites, we also examined the formation energies of other species. The formation energies follow the trend: isolated [Cu–OH] + < paired [Cu–OH] + < [Cu 2 O] 2+ < [Cu 3 O 3 ] 2+ . Inclusion of Gibbs free-energy corrections indicates activation temperatures of 257, 307 and 327 and 331 °C for isolated [Cu–OH] + , paired [Cu–OH] + , [Cu 2 O] 2+ and [Cu 3 O 3 ] 2+ , respectively. The provocative nature of the lower-than-expected activation temperature for isolated [Cu–OH] + species is discussed.
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Performance of density functional theory for describing hetero‐metallic active‐site motifs for methane‐to‐methanol conversion in metal‐exchanged zeolites
Methane‐to‐methanol conversion (MMC) can be facilitated with high methanol selectivities by copper‐exchanged zeolites. There are however two open questions regarding the use of these zeolites to facilitate the MMC process. The first concerns the possibility of operating the three cycles in the stepwise MMC process by these zeolites in an isothermal fashion. The second concerns the possibility of improving the methanol yields by systematic substitution of some copper centers in these active sites with other earth‐abundant transition metals. Quantum‐mechanical computations can be used to compare methane activation by copper oxide species and analogous mixed‐metal systems. To carry out such screening, it is important that we use theoretical methods that are accurate and computationally affordable for describing the properties of the hetero‐metallic catalytic species. We have examined the performance of 47 exchange‐correlation density functionals for predicting the relative spin‐state energies and chemical reactivities of six hetero‐metallic [M‐O‐Cu]2+and [M‐O2‐Cu]2+, (where MCo, Fe, and Ni), species by comparison with coupled cluster theory including iterative single, double excitations as well as perturbative treatment of triple excitations, CCSD(T). We also performed multireference calculations on some of these systems. We considered two types of reactions (hydrogen addition and oxygen addition) that are relevant to MMC. We recommend the use of τ‐HCTH and OLYP to determine the spin‐state energy splittings in the hetero‐metallic motifs. ωB97, ωB97X, ωB97X‐D3, and MN15 performed best for predicting the energies of the hydrogen and oxygen addition reactions. In contrast, local, and semilocal functionals do poorly for chemical reactivity. Using [Fe‐O‐Cu]2+as a test, we see that the nonlocal functionals perform well for the methane CH activation barrier. In contrast, the semilocal functionals perform rather poorly. © 2018 Wiley Periodicals, Inc.
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- Award ID(s):
- 1800387
- PAR ID:
- 10078600
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Journal of Computational Chemistry
- Volume:
- 39
- Issue:
- 32
- ISSN:
- 0192-8651
- Page Range / eLocation ID:
- p. 2667-2678
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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