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 N-2 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 I Na12-xKxI-A were prepared and studied with respect to the adsorption of CO2 N-2, 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 N-2 and CH4 was completely nulled already at relatively small contents of K. Of the studied samples, INa9K3I-A exhibited the highest CO2 over N-2/CH4 selectivities, with a(CO2/N-2 ) > 21 000 and a(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 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 alpha-cavities of INa12-xKxI-A. The adsorption of CH4 and N-2 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 a-cavities of INa12(_)x,KxI-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 CO(2 )molecules.

Zeolite |Na₁₂|-A is a commercial adsorbent, and its CO₂-over-N₂(CH₄) selectivity can be further enhanced kinetically by replacing Na⁺ in the 8-ring windows that control gas diffusion with large cations. In this study, samples of zeolite |Na_12–xK_x|-A with x = 0.0, 0.8, 2.0, and 3.0 were prepared, and the positions of adsorbed CO₂ molecules were determined using in situ neutron powder diffraction through profile refinement. Adsorbed CO₂ molecules were located at three different sites within the large α-cavities in the zeolite structure, revealing the interaction between the adsorbed CO₂ and the host framework. The number of CO₂ molecules at each site depends on CO₂ pressure and follows site-specific CO₂ isotherms described with a Langmuir model. Most of the CO₂ molecules in zeolite |Na_12–xK_x|-A bridge two cations at neighboring 8-ring sites. These are relatively weakly physisorbed, and therefore, most of the working capacity of CO₂ adsorption is related to this site. The CO₂ molecules at the second most populated site are coordinated to a cation in the 8-ring plane. Some of them seemed to form chemical bonds with the O atoms of the framework as carbonate-like species and acted as chemisorption. The remaining minor fraction of CO₂ is directly attracted by Na⁺ at the 6-rings. The different positioning of physisorbed CO₂ and the presence of chemisorbed CO₂ was confirmed by in situ infrared spectroscopy.

Raw biogas is a renewable fuel that can be upgraded into biomethane using several technologies that include adsorption-driven processes. We evaluated two adsorbents using a lab-sized pressure-vacuum-swing adsorption (PVSA) unit. Pellets of nano-sized zeolite A (bla421-A and INa10K21-A) were prepared together with a clay-based binder and sintered for 1-3 h at 450-650 degrees C. The pellets were subsequently PVSA tested to record the steady-state conditions and breakthrough of CO2. Numerous cycles were recorded with pellets that had been activated at 200 C under dynamic vacuum. The working capacity for CO2 removal exceeded 1.2 mol/kg at a raw-biogas pressure of 4 bar, and the nano-lbla40K21-A adsorbent had a low CH4 slip.