The climate changes are accelerated by increasing levels of carbon dioxide in the atmosphere connected to the fossil-fuel-based energy system. Substantial reforms of the system are needed immediately and could include the implementation of carbon capture and storage (CCS) technologies. Adsorption-driven CO₂ capture is one of the most promising post-combustion CO₂ capture techniques, which aim to remove CO₂ from N₂ in flue gas.

The nature of adsorption of CO₂ can vary. The process can act as physisorption with intermolecular interactions of the van der Waals type or as chemisorption with a significantly perturbed electronic structure of CO₂ and for example the formation of CO₃^2- and HCO₃^- species. The molecular details were elucidated by MAS NMR and IR studies for a zeolite, and the placement of adsorbed molecules was revealed by in situ diffraction data analysis.

Adsorption-driven processes can be implemented only if highly functional adsorbent materials have been developed. Zeolite A seems to be a promising candidate. This thesis broadly discussed the potential enhancement of the selectivity of CO₂ over N₂ and CH₄ by replacing Na⁺ with larger monovalent cation e.g. K⁺ in pore apertures of zeolite A. The positions of the extra-framework cations were analyzed by in situ X-ray diffraction using synchrotron light source. The cations were positioned at the 4- and 6-rings and the 8-ring apertures of the aluminosilicate framework of zeolite A. K⁺ was favored at the 8-ring sites, and this cation did also gradually substitute the 6-ring sites with and increasing x in |Na_12-xK_x|-A. Large cations did not fit the mirror plane of the 6-ring and were placed on both its sides. K⁺ at both positions, in 8-rings and 6-rings, seems to have tailored the size of pore openings.

The effective pore aperture size was shown to depend on the K⁺ content and to partition small CO₂ molecules from large N₂ and CH₄ because of, likely, differences in diffusivities. Various compositions of |Na_12-xK_x|-A demonstrated gradual decrease of CO₂ uptake with x and an exclusion of N₂ and CH₄ already for low x. Although already absorbed CO₂ molecules were revealed by in situ neutron diffraction to be coordinated mainly by the 8-ring cation or bridging adjacent 8-ring sites. Adsorbed CO₂ molecules displaced the cations into the a-cages and resulted in a slight contraction of the overall distribution of extra-framework cations upon the adsorption of CO₂.

The kinetically-enhanced separation of CO₂ from N₂/CH₄ seemed to be associated by a restrained diffusion also for the CO₂ molecules. This is problematic for pressure swing adsorption processes. However, it could potentially be addressed by the reduction of size of zeolite crystals to increase the extent of accessible porous space over limited time.