Sorption of Methane and Nitrogen on Cesium Exchanged Zeolite-X: Structure, Cation Position and Adsorption Relationship

The equilibrium adsorption study of methane and nitrogen on zeolite-X exchanged with different percentages of cesium ions was carried out using volumetric gas adsorption method. The dynamic breakthrough measurements were carried out using a fixed bed breakthrough reactor and binary (methane + nitrogen) gas mixture. The cesium ion exchanged zeolite samples were characterized by Brunauer–Emmett–Teller measurements, X-ray diffraction, scanning electron microscopy, and inductive coupled plasma-optical emission spectrophotometer analysis. Methane and nitrogen adsorption capacities depend on the percentage and position of cesium ions in the zeolite. The adsorption properties of ion exchanged zeolite were studied in correlation with cesium ion positions in the zeolite. Above 36% cesium ion exchange in NaX the methane adsorption capacity increases, while nitrogen adsorption capacity decreases. The cesium exchanged zeolite showed methane adsorption capacity of 21.1 molecules/(unit cell) and methane selectivity over nitrogen of 3.84 at 288 K, higher than that of zeolite NaX. The selectivity for methane over nitrogen was found to be in the order of Cs(84)­NaX > Cs(68)­NaX > Cs(53)­NaX > Cs(36)­NaX > NaX. All of the cesium exchanged zeolites showed nitrogen adsorption capacity less than that of NaX while, Cs(84)­NaX and Cs(68)­NaX showed methane adsorption capacity more than NaX. The adsorption isotherms were fitted using the Langmuir model equation and the virial equation. The methane stoichiometric adsorption capacity also increases on cesium ion exchange; NaX and Cs(80)­NaX showed stoichiometric methane adsorption capacities of 3.5 and 5.7 molecules/(unit cell), respectively. The stoichiometric adsorption capacity for methane increases with an increase in the partial pressure of methane in the gas mixture