CO2 uptake of zeolites Na-ZK-4 and NaK-ZK-4

https://doi.org/10.1021/acsomega.0c03749

“Zeolites Na-ZK-4 (1.8, 2.3, and 2.8) synthesized in this study were all highly crystalline (Figures S1–S3) with a cubic morphology (Figures S4 and S5) and had all high uptake of CO₂ at 273 K as is detailed in Table 1. The highest CO₂ uptake of 4.86 mmol/g (101 kPa, 273 K) was observed on zeolite Na-ZK-4 (2.3), but comparable levels were observed on zeolites Na-ZK-4 (1.8) (4.59 mmol/g) and Na-ZK-4 (2.8) (4.46 mmol/g). For comparison, zeolite Na-ZK-4 (1.3) and zeolite A (Si/Al = 1:1) showed a CO₂ uptake of 4.35 and 4.21 mmol/g, respectively. (26) Adsorbents with high capacity and selectivity for adsorption of CO₂ are currently being investigated for applications in adsorption-driven separation of CO₂ from the 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 the 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 CO₂ uptake (∼4 mmol/g, 101 kPa, 273 K) and a kinetically enhanced CO₂-over-N₂ selectivity. (23) We note here that the differences in the CO₂ uptake between these three samples could be related to the differences in the molecular weight of the different zeolites (because of the changes in the number of exchangeable cations). The CO₂ uptake (molecules) per unit cell is listed in Table S1. Unfortunately, the changes in the molecular weight of the zeolite alone could not explain the trend observed. We therefore speculate that the differences were probably also related to the crystallinity of zeolite Na-ZK-4 (the calcination could have affected the crystallinity of the zeolites differently). The comparably high CO₂ uptake on the Na-forms of zeolite ZK-4s was related to the absence of the K⁺ cations. K⁺ is larger and occupies more space, which reduced the pore volume being available for the CO₂ sorption. The CO₂ and N₂ adsorption isotherms for zeolite Na-ZK-4 (1.8, 2.3, and 2.8) recorded at 273 K are displayed in Figure 1. CO₂ and N₂ adsorption isotherms of all samples listed here can be found in the Supporting Information (Figures S7–S12).”

“In the structure of zeolites Na-ZK-4 (1.8, 2.3, and 2.8), there is one Na⁺ cation that is located close to the edge of the 8-ring window. (34,35) As a consequence, the small size of Na⁺ appears to allow for noticeable amounts of N₂ to diffuse through the window and be adsorbed within the pores of zeolites Na-ZK-4 (1.8–2.8). In this context, it is worthwhile noting that from a strictly geometric perspective, neither CO₂ nor N₂ would be able to diffuse through an 8-ring window unless there is some concerted movement of the cations. The initial slopes of the CO₂ adsorption isotherm (Henry’s law coefficient K_CO2—Table 1) were 0.244 mmol/g/kPa for Na-ZK-4 (1.8), 0.240 mmol/g/kPa for Na-ZK-4 (2.3), and 0.261 mmol/g/kPa for Na-ZK-4 (2.8). These values were significantly smaller than those for zeolite NaA (5.92 mol/g/kPa), and are likely to be related to the relatively large amount of chemisorption of CO₂ observed on the pure zeolite NaA. (36) The pressure- and temperature-dependent CO₂ physisorption isotherms are shown in the Supporting Information (Figures S13–S15). The isosteric heat of CO₂ physisorption calculated using the Clausius–Clapeyron equation is presented in Figure 1b.”