Sorption uptake of hydrocarbons by molecular sieves with non-uniform 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 were 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 was derived. A strategy was developed to reduce the three-parameter problem to that with one parameter only, which was 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-hexane/MFI structure at 323K amounts to 5 x 10-10m2/s, which was a value independent of loading (as the product of the immobilization and mobilization rates was). It was impossible to interpret the measured uptake curves utilizing a model that encompasses intracrystalline diffusion only (i.e., 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 were lower by up to 3 orders of magnitude.

Sorbate Immobilization in Molecular Sieves. Rate-limiting Step for n-Hexane Uptake by Silicalite-I. A.Micke, M.Büllow, M.Kočiřík, P.Struve: Journal of Physical Chemistry, 1994, 98[47], 12337-44

Table 42

Diffusivity of C6 Hydrocarbons in Silicalite at 100C