SORBATE IMMOBILIZATION IN MOLECULAR-SIEVES - RATE-LIMITING STEP FOR N-HEXANE UPTAKE BY SILICALITE-I

Sorption uptake of hydrocarbons by molecular sieves with nonuniform mi cropore systems such as MFI-type zeolites may be governed by a complex of mechanisms instead of pure intracrystalline diffusion. In the part icular case of sorption kinetics of n-hexane on silicalite-I, processe s occur on the microcrystal level which comprise both Fickian diffusio n and sorbate immobilization/mobilization. The rate processes connecte d with the immobilization of the sorbing species are due to both geome trical constraints and differences in the interaction potential topolo gy between straight and sinusoidal channels within the zeolite crystal s. A full quantitative description of this complex transport phenomeno n has been derived. A strategy has been developed to reduce the three- parameter problem to that with one parameter only, which is the prereq uisite of a practical parameter-fitting procedure. In this way, rate c oefficients of the particular composite processes were calculated on t he basis of experimental uptake data. The latter were fitted by use of a Volterra integral equation technique. The coefficient of intracryst alline diffusion of the system n-hexane/MFI structure at 323 K amounts to 5 x 10(-10) m(2)/s, which is a value independent of loading (as th e product of the immobilization and-mobilization rates is). It is impo ssible to interpret the measured uptake curves utilizing a model that encompasses intracrystalline diffusion only (i.e., neglecting the pres ence of sorbate immobilization). Neglecting the strong deviation in up take curve shape by utilizing equations for pure intracrystalline diff usion (e.g., the method of statistical moments), diffusivities were ob tained that are lower by up to 3 orders of magnitude.