| The incorporation of extraframework Sr2+ and Ba2+ cations onto silicoaluminophosphate (SAPO-34) zeolitic materials has been proven to influence their adsorptive properties. In this study, three approaches were employed to increase the cation content and, therefore, the material adsorption capacity: a multi-step liquid-phase ion exchange (LSIE), a novel coupled partial detemplation / solid-state ion exchange (PD-SSIE) and a coupled SSIE-LSIE. The use of multi-step LSIE resulted in an increase in cation content per unit cell, but the presence of solvated species and the interaction between the in-going cation and the available sites within the structure resulted in an equilibrium limitation. On the other hand, the introduction of Sr2+ and Ba2+ cations via SSIE was found to be heavily dependent on the nature of the starting material, the temperature of the solid-state reaction, and the amount of salt used (i.e. available sites). Among the different SSIE variants, the sample prepared via PD-SSIE exhibited the best CO2 uptake capacity at moderate gas partial pressures. Finally, the preparation of Sr2+-SAPO-34 and Ba2+-SAPO-34 via SSIE-LSIE resulted in materials with superior CO2 adsorption capacity at any pressure. Furthermore, experimental and theoretical calculations studies indicated that Sr2+ and Ba2+ cations were preferentially located on Site II' extra-framework positions, which allows for the development of stronger interactions with CO2 molecules. This was confirmed by 1H and 23Na MAS NMR studies, which allowed us to elucidate the presence and location of H+ and Na+ cations. This was essential to determine the amount and location of sites available for Sr2+ and Ba2+ ion exchange. |
