Selective Adsorption of CO2 on Zeolites NaK-ZK-4 with Si/AI of 1.8-2.8

• Published in: ACS omega 2020-10, Vol.5 (39), p.25371
• Main Authors: Cheung, Ocean, Bacsik, Zoltan, Fil, Nicolas, Krokidas, Panagiotis, Wardecki, Dariusz, Hedin, Niklas
• Format: Article
• Language: English
• ISSN: 2470-1343; 2470-1343
• DOI: 10.1021/acsomega.0c03749

Zeolites with appropriately narrow pore apertures can kinetically enhance the selective adsorption of CO2 over N-2. Here, we showed that the exchangeable cations (e.g., Na' or K) on zeolite ZK-4 play, an important role in the CO2 selectivity. Zeolites NaK ZK-4 with Si/Al = 1.8-2.8 had very high CO2 selectivity when an intermediate number of the exchangeable cations were K+ (the rest being Na+) Zeolites NaK ZK-4 with Si/Al = 1.8 had high CO, uptake capacity and very high CO2-over-N-2 selectivity (1190). Zeolite NaK ZK-4 with Si/A1 = 2.3 and 2.8 also had enhanced CO2 selectivity with an intermediate number of K cations. The high CO, selectivity was related to the I(' cation in the 8 -rings of the alpha-cage, together with Na+ cations in the 6 -ring, obstructing the diffusion of N2 throughout the zeolite. The positions of the K+ cation in the 8 -ring moved slightly (max 0.2 angstrom) toward the center of the a-cage upon the adsorption of CO2, as revealed by in situ X-ray diffraction. The CO2-over-N-2 selectivity was somewhat reduced when the number of K+ cations approached 100%. This was possibly due to the shift in the IC' cation positions in the 8 -ring when the number of Na+ was going toward 0%, glowing N, diffusion through the 8 -ring. According to 'm situ infrared spectroscopy, the amount of chemisorbed CO2 was reduced on zeolite ZK-4s with increasing Si/Al ratio. In the context of potential applications, a kinetically enhanced selection of CO2 could be relevant for applications in carbon capture and bio- and natural gas upgrading.