Sorbate Immobilization in Molecular Sieves. Rate-Limiting Step for n-Hexane Uptake by Silicalite-I

Abstract

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.

Positron Emission Profiling Study of the Diffusion of 3-Methylpentane in Silicalite-1

“…Frequency Response [8] and Square Wave [9] methods have been used to measure the decrease in the diffusivity for n-hexane in silicalite with loading, which is in agreement with the results obtained by Micke et al from the sorption uptake kinetics [10]. The nhexane diffusivity in silicalite-1 reported therein is independent of concentration, but a linear concentration dependence was found for the self-diffusivity.…”

Influence of isotherm inflection on diffusion in silicalite

“…has been used to describe the isotherm for n-hexane (Micke et al, 1994;Song and Rees, 1997). As can be seen from Fig.…”

Chain Length Effects of Linear Alkanes in Zeolite Ferrierite. 1. Sorption and 13C NMR Experiments

“…It has for example been reported that an immobilization term has to be considered to obtain a correct description of the n-hexane diffusion in ZSM-5. 11,12 In this work, we will study the adsorption of linear alkanes in ferrierite (FER), which has a pore structure consisting of intersecting channels and cages. We will focus on the possible existence of chain length effects in this system that will affect both the adsorption and diffusion properties.…”