“Small-pore zeolites have 8-ring pore apertures with similar dimensions as small gas molecules. These zeolites have been investigated as adsorbents for CO2 capture. (13−17) The cation-rich zeolite A may be relevant as they show promising CO2 adsorption properties and bear a low production cost. Specifically, zeolite A in its Na+ (4A), Mg2+ and Ca2+ (5A), and mix-cation NaK-A form have been studied for CO2 capture. (13,18−21) We have shown that zeolite NaK-A could effectively separate CO2 from N2 when the number of K+ cations was about ∼17 at. % of the total number of cations (the rest being Na+). (20,22−25) This K+-controlled selectivity was due to Na+ being selectively replaced by K+ in the 8-ring apertures in the α-cage. The K+ in the 8-ring can effectively hinder the diffusion of N2 in zeolite NaK-A when a sufficient number of pores are being blocked by K+.
We also found that the highly selective zeolite NaK-A adsorbent could rapidly take up CO
2 irrespectively of the particle size, which was indicative of transport resistance at the interfaces of small zeolite particles.
(23) We further showed that the related mix-cation zeolite NaK ZK-4 (Si/Al = 1.3) had similarly high CO
2 selectivity.
(26) This selectivity was also related to that K
+ selectively replaced Na
+ in the 8-ring apertures.
Other types of 8-ring zeolites have been studied with respect to their CO
2 adsorption capacity and CO
2 over N
2 selectivity. These studies have included investigations of the CO
2 adsorption on zeolites chabazite,
(27,28) ZK-5,
(29) RHO,
(15,30) ZSM-25,
(17,31) and other compositions.
(32,33) For many of these compositions, an enhanced CO
2 selectivity has been observed and related to thermally controlled gating/movement of cations in the zeolites or flexibility in the framework.
(16,30) In this study, we turned to zeolite NaK-ZK-4 with a high Si/Al ratio and studied its CO
2 adsorption and estimated its selectivity.”
