Separation of CO2 from light gas mixtures using ion-exchanged silicoaluminophosphate nanoporous sorbents

Na+-SAPO-34 sorbents were ion-exchanged with several individual metal cations to study their effect on the adsorption of similar size light gases. Measurements of pure component adsorption equilibria, with emphasis on CO2, were performed at different temperatures (273-348 K) and pressures (< 1 atm). Adsorption isotherms for CO2 in Mn+-SAPO-34 materials displayed a nonlinear behavior and did not follow the typical pore-filling mechanism. In general, the overall adsorption performance of the exchanged materials increased as follows: Ce3+ < Ti3+ < Mg2+ < Ca2+ < Ag+ < Na+ < Sr2+. The strontium exchanged materials excelled at low-pressure ranges, exhibiting very sharp isotherm slopes at all temperatures. For example, the Sr2+-SAPO-34 sorbents were capable of removing as much as 2.8 wt % at a CO2 partial pressure of 10(-3) atm and room temperature. Isosteric heat of adsorption data confirmed that the Sr2+ species were responsible for the surface strong interaction, and therefore it is plausible to state that cations were occupying exposed sites (SII') in the material Chabazite cages. In addition, all divalent cations were found to interact more with the sorbate when compared to the other charged species. Adsorption isotherms for trivalent exchanged cations (Ce3+, Ti3+), on the other hand, showed what appears to be partial blockage of pore windows (occupancy of SIII), resulting in very low adsorption capacities. The surface interactions were analyzed according to electrostatic and nonspecific contributions. Due to strong ion-quadrupole interactions, all the sorbent materials exhibited higher affinity for CO2 over the other gases tested (i.e., CH4, H-2, N-2, and O-2). Mathematical modeling to estimate binary component adsorption performance during vacuum pressure adsorption (VPSA) corroborated that Sr(2+)SAPO-34 sorbents are by far the best option for CO2 removal from CH4 mixtures, especially at low concentrations.