Separation of CO2 from light gas mixtures using nanoporous silicoaluminophosphate sorbents: Effect of multiple-step ion exchange and adsorption mechanism via computational studies

Na+-SAPO-34 materials were partially exchanged with Sr2+ and Ba2+ cations in a multi-step fashion in an attempt to improve adsorptive properties and study the effect of ion exchange treatments on the morphological, textural and chemical properties. Characterization of the sorbents included scanning electron microscopy (SEM), X-ray diffraction (XRD), solid-state magic angle spinning nuclear magnetic resonance MAS-NMR energy-dispersive analysis by X-ray diffraction (EDAX), porosimetry, and pure component gas adsorption (sorbates: CO2, N2, O2 and CH4). Analysis of the textural and adsorptive properties of SAPO-34 as a function of the degree of exchange corroborated that the sorbents long-range order was preserved. MAS-NMR spectra revealed that the calcination process to remove the template from Na+-SAPO-34 materials produces a small fraction of extra-framework or amorphous species in the local structure, but no changes associated to the multi-stage ion exchange process were observed. Performance wise both the Sr2+ and Ba2+ sorbent variants showcase outstanding adsorption capacity and selectivity towards CO2, especially at low gas partial pressures. The multi-step ion exchange procedure was effective in increasing the barium content in the SAPO-34 unit cell, resulting in an increase in CO2 uptake. Both experimental and theoretical analyses suggest that Sr2+ and Ba2+ cations are located at exposed positions within the material framework (site II′), which should explain the resulting interaction with CO2 molecules and, therefore, high adsorption capacity. Furthermore, a Mulliken gross atomic charge study indicates that the interaction between CO2 and the adsorption sites is predominantly a bond of ionic character. In general, the results indicate that both sorbent materials are good alternatives for CO2 adsorption, especially for ultra-purification applications.