CO2-Induced Displacement of Na+ and K+ in Zeolite |NaK|-A
Journal article, 2018
Adsorption technologies offer opportunities to remove CO2from 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 CO2from N2in flue gas and CO2from CH4in raw biogas can be further enhanced by replacing Na+with K+in the controlling pore window apertures. In this study, several compositions of |Na12-xKx|-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 CO2gradually decreased on an increasing content of K+, whereas the adsorption of N2and CH4was completely nulled already at relatively small contents of K+. Of the studied samples, |Na9K3|-A exhibited the highest CO2over N2/CH4selectivities, 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 CO2and possibly also the diffusion of CO2into the α-cavities of |Na12-xKx|-A. The adsorption of CH4and N2on 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 |Na12-xKx|-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 CO2molecules.