The chemical nature of CO2 adsorption in zeolite A

Sammanfattning: 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 CO32- 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 |Na12-xKx|-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 |Na12-xKx|-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.