Adsorption separation of carbon dioxide, methane, and nitrogen on Hβ and Na-exchanged β-zeolite

Adsorption isotherms of carbon dioxide (CO₂), methane (CH₄), and nitrogen (N₂) on Hβ and sodium exchanged β-zeolite (Naβ) were volumetrically measured at 273 and 303 K. The results show that all isotherms were of Brunauer type I and well correlated with Langmuir-Freundlich model. After sodium ions exchange, the adsorption amounts of three adsorbates increased, while the increase magnitude of CO₂ adsorption capacity was much higher than that of CH₄ and N₂. The selectivities of CO₂ over CH₄ and CO₂ over N₂ enhanced after sodium exchange. Also, the initial heat of adsorption data implied a stronger interaction of CO₂ molecules with Na⁺ ions in Naβ. These results can be attributed to the larger electrostatic interaction of CO₂ with extraframework cations in zeolites. However, Naβ showed a decrease in the selectivity of CH₄ over N₂, which can be ascribed to the moderate affinity of N₂ with Naβ. The variation of isosteric heats of adsorption as a function of loading indicates that the adsorption of CO₂ in Naβ presents an energetically heterogeneous profile. On the contrary, the adsorption of CH₄ was found to be essentially homogeneous, which suggests the dispersion interaction between CH₄ and lattice oxygen atoms, and such interaction does not depend on the exchangeable cations of zeolite.