CO₂‑Induced Displacement of Na⁺ and K⁺ in Zeolite |NaK|‑A

Adsorption technologies offer opportunities to remove CO₂ from gas mixtures, and zeolite A has good properties that include a high capacity for the adsorption of CO₂. It has been argued that its abilities to separate CO₂ from N₂ in flue gas and CO₂ from CH₄ in raw biogas can be further enhanced by replacing Na⁺ with K⁺ in the controlling pore window apertures. In this study, several compositions of |Na_12–xK_x|-A were prepared and studied with respect to the adsorption of CO₂, N₂, and CH₄, and the detailed structural changes were induced by the adsorption of CO₂. The adsorption of CO₂ gradually decreased on an increasing content of K⁺, whereas the adsorption of N₂ and CH₄ was completely nulled already at relatively small contents of K⁺. Of the studied samples, |Na₉K₃|-A exhibited the highest CO₂ over N₂/CH₄ selectivities, with α­(CO₂/N₂) > 21 000 and α­(CO₂/CH₄) > 8000. For samples with and without adsorbed CO₂, analyses of powder X-ray diffraction (PXRD) data revealed that K⁺ preferred to substitute Na⁺ at the eight-ring sites. The Na⁺ ions at the six-ring sites were gradually replaced by K⁺ on an increasing K⁺ content, and these sites split into two positions on both sides of the six-ring mirror plane. It was observed that both the eight-ring and six-ring sites tailored the maximum adsorption capacity for CO₂ and possibly also the diffusion of CO₂ into the α-cavities of |Na_12–xK_x|-A. The adsorption of CH₄ and N₂ on the other hand appeared to be controlled by the K⁺ ions blocking the eight-ring windows. The in situ PXRD study revealed that the positions of the extra-framework cations were displaced into the α-cavities of |Na_12–xK_x|-A on the adsorption of CO₂. For samples with a low content of K⁺, the repositioning of the cations was consistent with a mutual attraction with the adsorbed CO₂ molecules.