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

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