Carbon dioxide must be removed from biogas or natural gas to obtain compressed or liquefied methane, and adsorption-driven isolation of CO2 could be improved by developing new adsorbents. Zeolite adsorbents can select CO2 over CH4, and the adsorption of CH4 on zeolite vertical bar Na12-xKx vertical bar-A is significantly lower for samples with a high K+ content, i.e., x > 2. Nevertheless, we show, using H-1 NMR experiments, that these zeolites adsorb CH4 after long equilibration times. Pulsed-field gradient NMR experiments indicated that in large crystals of zeolites vertical bar Na12-xKx vertical bar-A, the long-time diffusion coefficients of CH4 did not vary with x, and the upper limit of the mean-square displacement was about 1.5 mu m, irrespective of the diffusion time. Also for zeolite vertical bar Na-12 vertical bar-A samples of three different particle sizes (similar to 0.44, similar to 2.9, and similar to 10.6 mu m), the upper limit of the mean-square displacement of CH4 was 1.5 mu m and largely independent of the diffusion time. This similarity provided further evidence for an intracrystalline diffusion restriction for CH4 within the medium- and large-sized zeolite A crystals and possibly of clustering and close contact among the small zeolite A crystals. The upper limit of the long-time diffusion coefficient of adsorbed CH4 was (at 1 atm and 298 K) about 10(-10) m(2)/s irrespective of the size of the zeolite particle or the studied content of K+ in zeolites and vertical bar Na-12 vertical bar-A. The T-1 relaxation time for adsorbed CH4 on zeolites vertical bar Na12-xKx vertical bar-A with x > 2 was smaller than for those with x < 2, indicating that the short-time diffusion of CH4 was hindered.

Formation of CO32- and HCO3- species without the participation of the framework-bridging oxygen atoms (-O-) upon chemisorption of CO2 in zeolite vertical bar Na-12 vertical bar-A is revealed. The transfer of O and H atoms is very likely to have proceeded via the involvement of residual H2O or -OH groups. A combined study by the solid-state H-1 and C-13 MAS NMR, quantum chemical calculations, and in situ infrared spectroscopy showed that the chemisorption mainly occurred by the formation of HCO3-. However, at a low surface coverage of physisorbed and acidic CO2, a significant fraction of HCO3- was deprotonated and transformed into CO32-. We expect that a similar chemisorption of CO2 would occur for low-silica zeolites and other basic silicates of interest for capture of CO2 from the gas mixtures.

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.

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. 

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.