Copper-Oxo Active Sites in the 8MR of Zeolite Mordenite: DFT Investigation of the Impact of Acid Sites on Methanol Yield and Selectivity
Olabisi Suleiman, Olajumoke Adeyiga, Dipak Panthi, Samuel O. Odoh
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.