Abstract
Adsorption isotherms of carbon dioxide (CO 2), methane (CH 4), and nitrogen (N 2) 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 CO 2 adsorption capacity was much higher than that of CH 4 and N 2. The selectivities of CO 2 over CH 4 and CO 2 over N 2 enhanced after sodium exchange. Also, the initial heat of adsorption data implied a stronger interaction of CO 2 molecules with Na + ions in Naβ. These results can be attributed to the larger electrostatic interaction of CO 2 with extraframework cations in zeolites. However, Naβ showed a decrease in the selectivity of CH 4 over N 2, which can be ascribed to the moderate affinity of N 2 with Naβ. The variation of isosteric heats of adsorption as a function of loading indicates that the adsorption of CO 2 in Naβ presents an energetically heterogeneous profile. On the contrary, the adsorption of CH 4 was found to be essentially homogeneous, which suggests the dispersion interaction between CH 4 and lattice oxygen atoms, and such interaction does not depend on the exchangeable cations of zeolite.
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