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

Abstract

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.

DFT Analysis of Methane C−H Activation and Over‐Oxidation by [Cu2O]2+ and [Cu2O2]2+ Sites in Zeolite Mordenite: Intra‐ versus Inter‐site Over‐Oxidation

“…This agrees quite well with our previous report. [40] We note also that Zhao et al found very high barriers for methane CÀ H activation trans-μ-1,2-peroxo dicopper sites in the 12MR of MOR. [16] The high barriers for methane activation by [Cu 2 O 2 ] 2 + species in MOR is in contrast to low barriers in the MFI framework.…”

“…Details of these periodic DFT calculations are like those from our previous reports. [40,41] Except where otherwise stated, the energies of the intermediates and transition state species are provided relative to the energies of the active site and gaseous methane, I. In I, the electronic energies from periodic DFT for the intermediate or transition state species is E pecie while those for the active site and methane are E acive ie and E CH4 , respectively.…”

“…The interconversion and relative stabilities of copper active sites have also been examined. [39,40] More recently, quantum-mechanical consideration of the formation mechanisms of copper active sites from hexaquo divalent copper ions, [Cu(H 2 O) 6 ] 2 + , have begun to emerge. [40,41] Focusing on the copper active sites, the impact of metal substitution (replacing copper centers by other metals) on methane reactivity and stability towards auto reduction have also been examined, mostly with theoretical analyses.…”

“…[39,40] More recently, quantum-mechanical consideration of the formation mechanisms of copper active sites from hexaquo divalent copper ions, [Cu(H 2 O) 6 ] 2 + , have begun to emerge. [40,41] Focusing on the copper active sites, the impact of metal substitution (replacing copper centers by other metals) on methane reactivity and stability towards auto reduction have also been examined, mostly with theoretical analyses. [26,28,33,[41][42][43] Similarly, quantum-mechanical descriptions of the spectroscopic (UV-Vis, NMR and XAS) properties of the copper active sites have also began to emerge, again belatedly after extensive experimental reports.…”

“…Quantum-mechanical investigations are not far behind in the description of the importance of BASs. [40,51] The thermodynamics of stabilizing the separated methyl group at BASs is comparable to stabilizing it at copper active sites. BASs thus provide an opportunity for the methyl group to avoid over-oxidation.…”