Adsorption of benzene mixtures on silicalite-1 and NaX zeolites

Adsorption of benzene/hydrocarbon liquid mixtures on silicalite-1 and NaX zeolites was measured by a volumetric/gravimetric method. The C₆-C₈ n-alkanes preferentially adsorb on silicalite-1 because of their higher heats of adsorption, and for n-C₆, its favorable molecular configuration. The adsorption selectivity, which is 1400 for n-C₆/benzene, decreases with increasing carbon number. In contrast, on NaX zeolite, benzene preferentially adsorbs over C₆ and C₇ n-alkanes (benzene/n-C₆ selectivity as high as 1150), because benzene has a higher heat of adsorption and it packs more efficiently (higher entropy) in the NaX super cages. Benzene also preferentially adsorbs over cyclohexane on NaX because of its higher heat of adsorption and higher entropy. The benzene/cyclohexane adsorption selectivity increased from 82 to ∼3000 as the benzene concentration in the liquid increased from 1.95% to 25.8 mol%. p-Xylene preferentially adsorbs over benzene on silicalite-1 over the entire concentration range, but the selectivity is only 1.3-8.8. In contrast, NaX selectively adsorbs benzene for benzene liquid concentrations less than 55 mol%, and selectively adsorbs p-xylene at higher concentrations. The benzene/p-xylene adsorption behavior on NaX zeolite is not predicted by ideal adsorbed solution theory.