Selective Adsorption of CO(2) on Zeolites NaK-ZK-4 with Si/Al of 1.8–2.8

[Image: see text] 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.