Sorption and Diffusion of n-Heptane in 5A Zeolite

Abstract

Kinetic and equilibrium data for sorption of n-heptane in 5A zeolite are presented and discussed in relation to previously reported data for the C2–C4 hydrocarbons in the same zeolite.A satisfactory interpretation of the equilibrium data is obtained on the basis of a simple theoretical model in which it is assumed that there are, within each cavity of the zeolite lattice, two distinct and energetically different adsorption sites. These sites may correspond to the region close to the cavity wall in which adsorption is energetically favorable, giving rise to localized adsorption, and the central region of the cavity in which the sorbate is less strongly bound with greater rotational and translational freedom. The differential diffusivity increases strongly with sorbate concentration and, as with the lighter hydrocarbons, the form of this concentration dependence may be satisfactorily explained by considering the driving force for the transport process to be the gradient of chemical potential. The limiting diffusional activation energy for n-heptane is 7.5 kcal and this value is somewhat higher than the activation energy for the lighter paraffins.