Chromatographic study of adsorption of n-alkanes on zeolites at high temperatures

Henry constants, adsorption enthalpies, and entropies of C5-C10 n-alkanes were determined using the chromatographic method on BEA*, TON, MOR, and FAU type zeolites in the temperature range 200-375 degrees C. On all zeolites, the Henry constants of the linear alkanes increase exponentially with the alkane chain length. Adsorption enthalpies and entropies are related to the pore geometry and have an opposite effect on the Henry adsorption constant: smaller pore sizes lead to a strong increase of the adsorption enthalpy with the carbon number, which is favorable for the adsorption equilibrium. However, this increase of -Delta H-0 with CN is accompanied by a high loss of degrees of freedom in small pores, leading to a strong increase of the adsorption entropy with chain length and counterbalancing the positive effect of the enthalpy on the Henry constants. On large-pore zeolites, the opposite effect is observed. The strongest increase of the Henry constants with the carbon number occurs on zeolites in which an intermediate increase of the adsorption enthalpy is accompanied by a moderate loss of degrees of freedom. For the zeolites studied, the rate at which the Henry constants increase with carbon number decreases in the order mordenite > beta > Y > CBV720 > ZSM-22 similar to CBV760.