A dispersion-corrected density-functional theory study of small molecules adsorbed in alkali-exchanged chabazites

Abstract

Recently, the removal of carbon dioxide from gas mixtures containing methane or nitrogen has received considerable attention, as it could help to reduce global carbon dioxide emissions. Both natural and synthetic zeolites could find use in adsorption-based carbon dioxide removal. In this contribution, the interaction of carbon dioxide, methane, and nitrogen with alkali-exchanged chabazites is studied computationally, using dispersion-corrected density-functional theory. All alkali metals from lithium to cesium are considered. Because the focus lies on a study of the interaction with a single cation and the surrounding framework, a very high Si/Al ratio is assumed in the model system used. Having determined the preferred cation site for each cation species, the interaction energies and equilibrium geometries for systems with one molecule äadsorbed at the cation are analysed. The relative contriäbutions of electrostatic and dispersion interactions are evaluated. Due to the complex interplay between cation-guest and framework-guest interactions, the evolution of the interaction energy on increasing atomic number of the cation is not monotonic. While the selectivity towards carbon dioxide cannot be inferred directly from the computations, estimations based on the difference in interaction energy reveal that K- and Rb-exchanged systems are expected to be most promising. The results are discussed in detail, establishing correlations with experimental results where possible.