Role of Ion Mobility in Molecular Sieving of CO2 over N2 with Zeolite NaKA

Classical molecular dynamics and Grand Canonical Monte Carlo simulations are carried out for sorbates (CO2 and N2) in zeolite NaKA using a universal type ab initio force field. By combining the results of these simulations, we reproduce the CO2 uptake as a function of the K+ content in the zeolite NaKA as measured experimentally by Liu et al. The experiment yielded an exceptionally high CO2-over-N2 selectivity of >200 at a specific K+/(K+ + Na+) ratio of 17 atom %. This high selectivity could be attributed to the nonlinear uptake dependency of the K+/(K+ + Na+) ratio measured for both CO2 and N2. Additionally, our simulations show a strong coupling between the self-diffusion of CO2 and the site-to-site jumping rate of the extra-framework cations. These results show that this nonlinear uptake dependency of CO2 is the result of molecular sieving. Following this, our simulations conclude that both thermodynamic and kinetic contributions must be taken into account when modeling the uptake of this and similar materials with the same functionalities.