The Influence of C–O–C and C–H Functionalities on Competitive Adsorption of Dimethyl Ether and Propane on FAU-Type X Zeolites

Abstract This study investigates the influence of ether (C–O–C) and aliphatic (C–H) functionalities on competitive adsorption in porous materials. Therefore, comparative isothermal adsorption experiments with dimethyl ether (DME) and propane (C3H8) were performed on FAU-type X zeolites (NaX and CaNaX) between −20 and 25 °C. Breakthrough curves measured by FTIR on fixed beds were used to derive both pure-component and binary mixture isotherms. Additionally, mixture equilibria were calculated using the Ideal Adsorbed Solution Theory (IAST). Due to its permanent dipole, DME exhibits significantly stronger cation–dipole interactions compared to nonpolar propane, resulting in higher loadings, especially at low concentrations. This effect is more significant on CaNaX, where divalent Ca2+ ions provide stronger electrostatic interaction sites. In contrast, propane adsorption is governed primarily by weaker dispersion and induced-dipole interactions, leading to lower affinities and capacities under the same conditions. In binary adsorption, a pronounced selectivity toward DME is observed for both zeolites across the entire temperature range. DME largely reaches its pure-component loadings, while propane is almost completely displaced. However, deviations occur on CaNaX at lower temperatures, where kinetic limitations prevent complete displacement of propane, resulting in residual propane loadings and reduced DME capacities. Mixture isotherms calculated using IAST qualitatively capture the strong selectivity and competitive behavior but show quantitative deviations from the experimental data, particularly at low loadings.