Adsorption of N₂ and CH₄ by Ion-Exchanged Silicoaluminophosphate Nanoporous Sorbents:  Interaction with Monovalent, Divalent, and Trivalent Cations

Na-SAPO-34 sorbents were ion-exchanged with several cations (Ag⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ti³⁺, and Ce³⁺) to study their effect on the adsorption of similar size light gases, namely, nitrogen and methane. Measurements of adsorption equilibria were performed at different temperatures (273−348 K) and pressures (<1 atm). Isotherms for the functionalized SAPO-34 materials displayed a nonlinear behavior and, in many cases, did not follow the traditional pore-filling mechanism. For sorbents containing divalent cations, the nitrogen adsorption capacity increased with increasing cation size. Adsorption isotherms for trivalent exchanged cations showed what appears to be partial blockage of pore windows, resulting in low adsorption capacities for both sorbates. Analysis of isosteric heats of adsorption as a function of loading indicated that all sorbents exhibit a relatively low heterogeneity. The surface interactions were analyzed according to electrostatic and nonspecific contributions. It was concluded that the sorbents textural properties dominated the uptake process for both sorbates. Finally, diffusion time constants (D/L²) were estimated by fitting nitrogen and methane fractional uptake data, respectively, with different transport phenomenological models. The systematic analysis indicated that the SAPO-34 sorbent particles polydispersity plays a crucial role in diffusion transport.