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

Abstract

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.

NMR Crystallography of Monovalent Cations in Inorganic Matrices: Na+ Siting and the Local Structure of Na+ Sites in Ferrierites

“…This procedure employs high-resolution 7 Li– 7 Li correlation MAS NMR spectroscopy coupled with periodic DFT calculations of the structure of Li + sites and subsequent DFT cluster computations of the 7 Li NMR shielding. Similarly, 23 Na solid-state NMR spectroscopy represents a powerful tool to identify the Na + siting in matrices of Si-rich zeolites. , The incorporation of Na + cations into diverse matrices has attracted particular attention because of its pivotal importance in various fields such as, for example, energy storage and CO 2 capture. − …”

“…17,18 The incorporation of Na + cations into diverse matrices has attracted particular attention because of its pivotal importance in various fields such as, for example, energy storage 19 and CO 2 capture. 20 In this paper, we demonstrate a new approach to the determination of the siting of Na + and the local structure of Na + extra-framework cationic sites in crystalline aluminosilicate matrices based on a combination of high-field and ultra-highfield MAS and MQMAS 23 Na NMR spectroscopy interpreted using predictions of the 23 Na NMR parameters obtained from periodic DFT calculations including extensive molecular dynamics (MD) simulations. The usage of high-field and ultra-high-field 23 Na solid-state NMR permits highly reliable evaluations of the 23 Na NMR parameters of Na + cations in various cationic sites.…”

“…20 In this paper, we demonstrate a new approach to the determination of the siting of Na + and the local structure of Na + extra-framework cationic sites in crystalline aluminosilicate matrices based on a combination of high-field and ultra-highfield MAS and MQMAS 23 Na NMR spectroscopy interpreted using predictions of the 23 Na NMR parameters obtained from periodic DFT calculations including extensive molecular dynamics (MD) simulations. The usage of high-field and ultra-high-field 23 Na solid-state NMR permits highly reliable evaluations of the 23 Na NMR parameters of Na + cations in various cationic sites. While 23 Na MAS NMR spectroscopy is used to identify Na + cations with a large quadrupolar broadening since the 23 Na NMR resonances of such cations are suppressed in 23 Na MQMAS NMR experiments, 23 Na MQMAS NMR experiments are employed to analyze close overlapping 23 Na NMR resonances with a smaller quadrupolar broadening.…”

“…The usage of high-field and ultra-high-field 23 Na solid-state NMR permits highly reliable evaluations of the 23 Na NMR parameters of Na + cations in various cationic sites. While 23 Na MAS NMR spectroscopy is used to identify Na + cations with a large quadrupolar broadening since the 23 Na NMR resonances of such cations are suppressed in 23 Na MQMAS NMR experiments, 23 Na MQMAS NMR experiments are employed to analyze close overlapping 23 Na NMR resonances with a smaller quadrupolar broadening. The results reveal that Na + cations can be ligated to oxygen atoms of nine distinct extra-framework cationic sites of which eight are occupied by Na + in one or more of the investigated samples.…”

“…The occupation of the Na + cationic sites is not controlled by their relative energies but by the kinetics of the Na + ion-exchange. 23 Na solid-state NMR spectroscopy alone can determine the ring forming the Na + site but not which T site is occupied by Al in that ring. The developed methodology represents a highly promising tool for the analysis of the Na + arrangements in zeolites and other crystalline matrices.…”