K+ exchanged zeolite ZK-4 as a highly selective sorbent for CO2

Adsorbents with high capacity and selectivity for adsorption of CO ₂ are currently being investigated for applications in adsorption-driven separation of CO₂ from flue gas. An adsorbent with a particularly high CO₂-over-N₂ selectivity and high capacity was tested here. Zeolite ZK-4 (Si:Al ∼ 1.3:1), which had the same structure as zeolite A (LTA), showed a high CO₂ capacity of 4.85 mmol/g (273 K, 101 kPa) in its Na⁺ form. When approximately 26 at. % of the extraframework cations were exchanged for K⁺ (NaK-ZK-4), the material still adsorbed a large amount of CO₂ (4.35 mmol/g, 273 K, 101 kPa), but the N₂ uptake became negligible (<0.03 mmol/g, 273 K, 101 kPa). The majority of the CO₂ was physisorbed on zeolite ZK-4 as quantified by consecutive volumetric adsorption measurements. The rate of physisorption of CO₂ was fast, even for the highly selective sample. The molecular details of the sorption of CO₂ were revealed as well. Computer modeling (Monte Carlo, molecular dynamics simulations, and quantum chemical calculations) allowed us to partly predict the behavior of fully K ⁺ exchanged zeolite K-ZK-4 upon adsorption of CO₂ and N₂ for Si:Al ratios up to 4:1. Zeolite K-ZK-4 with Si:Al ratios below 2.5:1 restricted the diffusion of CO₂ and N₂ across the cages. These simulations could not probe the delicate details of the molecular sieving of CO₂ over N₂. Still, this study indicates that zeolites NaK-ZK-4 and K-ZK-4 could be appealing adsorbents with high CO ₂ uptake (∼4 mmol/g, 101 kPa, 273 K) and a kinetically enhanced CO₂-over-N₂ selectivity.