Adsorption equilibrium and kinetic data were measured for the system p-ethyltoluene/Na,H-ZSM-5 zeolite at 343 K. The equilibrium data can be described by the superposition of two Langmuir isotherms. The kinetic data follow a complex mechanism of Fickian diffusion superimposed on the penetration of a surface barrier.The evaluation of the model parameters was carried out by simulation of sorption uptake curves including interactions of the system with both the apparatus and the intrinsic sorption kinetics. The dependence of fitted model parameters with respect to loading and pressure, respectively, is consistent with mechanistic understanding of the mobility within the crystal interface layer and in the crystal bulk.

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Sorption uptake of hydrocarbons by molecular sieves with nonuniform micropore systems such as MFI-type zeolites may be governed by a complex of mechanisms instead of pure intracrystalline diffusion. In the particular case of sorption kinetics of n-hexane on silicalite-I, processes occur on the microcrystal level which comprise both Fickian diffusion and sorbate immobilization/mobilization. The rate processes connected with the immobilization of the sorbing species are due to both geometrical constraints and differences in the interaction potential topology between straight and sinusoidal channels within the zeolite crystals. A full quantitative description of this complex transport phenomenon has been derived. A strategy has been developed to reduce the three-parameter problem to that with one parameter only, which is the prerequisite of a practical parameter-fitting procedure. In this way, rate coefficients of the particular composite processes were calculated on the basis of experimental uptake data. The latter were fitted by use of a Volterra integral equation technique.The coefficient of intracrystalline diffusion of the system n-hexaneMFI structure at 323 K amounts to 5 x m2/s, which is a value independent of loading (as the product of the immobilization and mobilization rates is). It is impossible to interpret the measured uptake curves utilizing a model that encompasses intracrystalline diffusion only (Le., neglecting the presence of sorbate immobilization). Neglecting the strong deviation in uptake curve shape by utilizing equations for pure intracrystalline diffusion (e.g., the method of statistical moments), diffusivities were obtained that are lower by up to 3 orders of magnitude.

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Recently, the single-step frequency-response method has been proposed as a new experimental technique for the detection of fast adsorption/desorption kinetics of gases on porous solids. In this technique, a microporous sorbent is subjected to a jump-like pressure change brought about by a very rapid volume compression/ expansion. By recording the gas pressure in the sorption system, sorption uptake/desorption kinetics can be followed. Volterra integral equations are shown to be a very useful tool for the treatment of the data obtained by this new type of diffusion experiment. The theoretically predicted features of the pressure vs. time curves are verified by a sorption uptake experiment of n-butane on silicalite.