The accuracy challenge of the DFT-based molecular assignment of<sup>13</sup>C MAS NMR characterization of surface intermediates in zeolite catalysis

Kolganov, Alexander A.; Gabrienko, Anton A.; Chernyshov, Ivan Yu.; Stepanov, Alexander G.; Pidko, Evgeny A.

doi:10.1039/d0cp04439c

Cited by 11 publications

(12 citation statements)

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“…We conclude that BASs are crucial for stabilizing the methyl group, away from over-oxidation at the copper-oxo sites. NMR spectroscopic measurements have been used to confirm location of surface methoxy groups at zeolite BASs. , Indeed, work by Kolganov et al and Sushkevich et al clearly indicate that the separated methyl group can be stabilized at BASs. ,, Sushkevich et al concluded that methoxy groups bound to BASs are one of the main products formed over Cu-MOR with an Si/Al ratio of 6. This is close to the ratio of 11 for our model systems.…”

Section: Resultsmentioning

confidence: 99%

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Copper-Oxo Active Sites in the 8MR of Zeolite Mordenite: DFT Investigation of the Impact of Acid Sites on Methanol Yield and Selectivity

et al. 2021

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There is significant interest in improving methanol yields from methane in copper-exchanged zeolites. Interestingly, zeolites with proton, H + , precursors provide greater methanol yields and selectivities than zeolites from sodium, Na + , precursors. There is however no quantitative description of the origins of these differences. Here, we use the density functional theory to probe differences in the properties of copper-oxo species in the 8-membered ring of zeolite mordenite, MOR. We focus only on [Cu 3 O 3 ] 2+ , [Cu 2 O] 2+ , and two [Cu 2 O 2 ] 2+ species in H-MOR and Na-MOR. Our calculations show that these sites are activated at 345−490 °C, with the bis(μ-oxo) dicopper(III) [Cu 2 O 2 ] 2+ moiety being the most stable and [Cu 3 O 3 ] 2+ the least stable. [Cu 3 O 3 ] 2+ and [Cu 2 O] 2+ are capable of activating methane at 200 °C, with similar C−H activation barriers in H-MOR and Na-MOR zeolites. The fate of the methyl group formed from methane C−H activation differentiates the Na-MOR and H-MOR zeolites.Crucially, we show that rebound of the methyl group to an active-site μ-oxo atom favors over-oxidation. Alternatively, the methyl group can be stabilized via exchange with H + /Na + located at remote aluminates. Exchange with Na + does not provide as much stabilization as the μ-(OCH 3 ) intermediate, thus favoring over-oxidation. By contrast, Bro̷ nsted acid sites provide similar levels of stabilization to the μ-(OCH 3 ) intermediate. This is a path to methanol, rather than over-oxidation products. The discrepancy in the stabilizations provided by Na + and H + -aluminate sites is rooted in the electronic structure.

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Section: Resultsmentioning

confidence: 99%

“…NMR spectroscopic measurements have been used to confirm location of surface methoxy groups at zeolite BASs. 46,47 44,48,49 Sushkevich et al concluded that methoxy groups bound to BASs are one of the main products formed over Cu-MOR with an Si/Al ratio of 6. This is close to the ratio of 11 for our model systems.…”

Section: Methane C−h Activation Bymentioning

confidence: 99%

Copper-Oxo Active Sites in the 8MR of Zeolite Mordenite: DFT Investigation of the Impact of Acid Sites on Methanol Yield and Selectivity

et al. 2021

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“…By providing a comprehensive computed data set for various alternative structures and geometries, a goal of establishing a good basis for a more rational assignment of the spectroscopic data is aimed at. 67 We have demonstrated that the local environment affects the stretching frequencies of both adsorbed NO and CO. 28 A decrease in the copper oxidation state, an increase in copper coordination, and a great proximity to the closely located atoms all tend to shift the CO frequency downward. The coordination number and the nature of the coordination environment have distinctive influence on the NO-stretching frequencies in Cu(II), which is in the range from 1850 to 2050 cm −1 .…”

Section: ■ Introductionmentioning

confidence: 86%

Mapping Vibrational Spectra to the Structures of Copper Species in Zeolites Based on Calculated Stretching Frequencies of Adsorbed Nitrogen and Carbon Monoxides

et al. 2021

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In this work, adsorption of nitrogen monoxide (NO) and carbon monoxide (CO) probe molecules on various copper sites in a range of zeolites is studied. The structures of copper sites, binding energies, and vibrational frequencies of adsorbed probe molecules are calculated using density functional theory (DFT). This allows mapping vibrational spectra regions to specific copper species as a function of the zeolite topology and Si/Al ratio. CO can adsorb on Cu + ions by forming monoand dicarbonyls or on copper ions bonded to methoxy species by forming methoxymonocarbonyls, which exhibit a blue shift in wavenumbers. The stretching frequencies of adsorbed NO generally increase in the following order: [

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“…In a complementary fashion, quantum‐mechanical computations have been used to study the MMC mechanism on these active sites. More recently, the spectroscopic properties of these active sites during MMC have been described with theoretical calculations [31–33] . Most of these computations have used cluster‐model or periodic density functional theory, DFT [34–36] .…”

Section: Introductionmentioning

confidence: 99%

Methane Over‐Oxidation by Extra‐Framework Copper‐Oxo Active Sites of Copper‐Exchanged Zeolites: Crucial Role of Traps for the Separated Methyl Group

Adeyiga

Odoh

2021

ChemPhysChem

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Copper‐exchanged zeolites are useful for stepwise conversion of methane to methanol at moderate temperatures. This process also generates some over‐oxidation products like CO and CO2. However, mechanistic pathways for methane over‐oxidation by copper‐oxo active sites in these zeolites have not been previously described. Adequate understanding of methane over‐oxidation is useful for developing systems with higher methanol yields and selectivities. Here, we use density functional theory (DFT) to examine methane over‐oxidation by [Cu3O3]2+ active sites in zeolite mordenite MOR. The methyl group formed after activation of a methane C−H bond can be stabilized at a μ‐oxo atom of the active site. This μ‐(O−CH3) intermediate can undergo sequential hydrogen atom abstractions till eventual formation of a copper‐monocarbonyl species. Adsorbed formaldehyde, water and formates are also formed during this process. The overall mechanistic path is exothermic, and all intermediate steps are facile at 200 °C. Release of CO from the copper‐monocarbonyl costs only 3.4 kcal/mol. Thus, for high methanol selectivities, the methyl group from the first hydrogen atom abstraction step must be stabilized away from copper‐oxo active sites. Indeed, it must be quickly trapped at an unreactive site (short diffusion lengths) while avoiding copper‐oxo species (large paths between active sites). This stabilization of the methyl group away from the active sites is central to the high methanol selectivities obtained with stepwise methane‐to‐methanol conversion.

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The accuracy challenge of the DFT-based molecular assignment of¹³C MAS NMR characterization of surface intermediates in zeolite catalysis

Abstract: The influence of the model and method choice on the DFT predicted 13C NMR chemical shifts of zeolite surface methoxide species has been systematically analyzed. Twelve 13C NMR chemical shift...

Cited by 11 publications

References 68 publications

Copper-Oxo Active Sites in the 8MR of Zeolite Mordenite: DFT Investigation of the Impact of Acid Sites on Methanol Yield and Selectivity

Copper-Oxo Active Sites in the 8MR of Zeolite Mordenite: DFT Investigation of the Impact of Acid Sites on Methanol Yield and Selectivity

Mapping Vibrational Spectra to the Structures of Copper Species in Zeolites Based on Calculated Stretching Frequencies of Adsorbed Nitrogen and Carbon Monoxides

Methane Over‐Oxidation by Extra‐Framework Copper‐Oxo Active Sites of Copper‐Exchanged Zeolites: Crucial Role of Traps for the Separated Methyl Group

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The accuracy challenge of the DFT-based molecular assignment of13C MAS NMR characterization of surface intermediates in zeolite catalysis

Abstract: The influence of the model and method choice on the DFT predicted 13C NMR chemical shifts of zeolite surface methoxide species has been systematically analyzed. Twelve 13C NMR chemical shift...

Cited by 11 publications

References 68 publications

Copper-Oxo Active Sites in the 8MR of Zeolite Mordenite: DFT Investigation of the Impact of Acid Sites on Methanol Yield and Selectivity

Copper-Oxo Active Sites in the 8MR of Zeolite Mordenite: DFT Investigation of the Impact of Acid Sites on Methanol Yield and Selectivity

Mapping Vibrational Spectra to the Structures of Copper Species in Zeolites Based on Calculated Stretching Frequencies of Adsorbed Nitrogen and Carbon Monoxides

Methane Over‐Oxidation by Extra‐Framework Copper‐Oxo Active Sites of Copper‐Exchanged Zeolites: Crucial Role of Traps for the Separated Methyl Group

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The accuracy challenge of the DFT-based molecular assignment of¹³C MAS NMR characterization of surface intermediates in zeolite catalysis