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 [Cu3O3]2+, [Cu2O]2+, and two [Cu2O2]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) [Cu2O2]2+ moiety being the most stable and [Cu3O3]2+ the least stable. [Cu3O3]2+ and [Cu2O]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-(OCH3) intermediate, thus favoring over-oxidation. By contrast, Bro̷nsted acid sites provide similar levels of stabilization to the μ-(OCH3) 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.