Cu-exchanged zeolites have recently been found to be active for partial methane oxidation to methanol (PMO), but the activities of the catalysts are very low. Further advances require a fundamental molecular-level understanding of the structure and function of the active sites to design catalysts that activate O2 and selectively oxidize hydrocarbons. Dimeric Cu species have been proposed in literature to facilitate O2 and CH4 activation. In this project, we systematically investigate the stability and reactivity and predict the quantity of Cu dimers in Cu-SSZ-13 zeolite. Supercell density functional theory (DFT) calculations and thermodynamic analysis are applied to identify relevant species under PMO conditions. We consider the influence of local Al proximity on Cu dimer speciation, considering potential ligands (-O, -O2, -OH) and Cu formal charges from 1+ to 3+. We find the formation of Cu dimers is highly associated with framework Al proximity. In particular, at eight-membered ring (8MR) Al pair sites, a strong thermodynamic driving force to form O- and OH-bridged dimers from [CuOH]+ monomers is found. We subsequently predict Cu-SSZ-13 composition phase diagrams by predicting Al distributions based on Löwenstein’s rule (no Al-O-Al linkage) or rules derived from DFT calculations related to zeolite synthesis conditions (the use of organic and inorganic structure-directing agents). We find the quantity of Cu dimer sites depends on Si/Al, Al distribution rules and Cu/Al. Under Löwenstein’s rule, we predict a significant number of Cu dimer sites that exist over Si/Al ratio 3~40. We find the number of Cu dimer sites maximizes at Si:Al around 10, which qualitatively agrees with reported methanol productivity in Cu-SSZ-13. This work shows implications of finding optimal zeolite compositions and synthesis conditions for PMO reactions and eventually diversifying the strategies available to use the world’s abundant hydrocarbon resources.

Computational investigation and quantification of active Cu sites in Cu-SSZ-13 zeolite for selective activation of methane

ACS Fall 2021

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