A Nonequilibrium Surface Barrier for Sorption Kinetics of p-Ethyltoluene on ZSM-5 Molecular Sieves

Abstract

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.

Internal Transport Resistances and their Influence on Diffusion in Zeolites as Traced by Microscopic Measuring Techniques

“…This result suggests the existence of strong transport barriers on the crystal surface (excluding the surface of the cracks), which mainly determine the rate of adsorption/desorption. The existence of transport barriers on the external surface of ZSM-5 crystals have already been reported in a number of papers (Kärger et al, 1991;Micke et al, 1994;Wloch, 2003). As demonstrated by Figs.…”

Transport of n‐paraffins in zeolite T

“…(iv) Nonequilibrium barrier (Micke et al, 1994). Adsorption at the surface may not be an instantaneous equilibrium process.…”

“…The external resistance is associated to nonequilibrium sorption conditions at the surface. Micke and coworkers (Micke et al, 1994) used Langmuir kinetics to model this phenomenon. Their model leads to a pseudo surface barrier at the particle surface, with k , being concentration-dependent:…”

Diffusion in zeolites: is surface resistance a critical parameter?

“…Bülow et al (1980) measured uptake curves of n-hexane in MgA zeolites and were able to dissociate intracrystalline and surface resistance from experiments with different crystal sizes. Micke et al (1994) showed that the uptake curves of p-ethyltoluene in Na, H-ZSM-5 cannot be correctly modelized without taking into account a surface barrier. After etching the surface of ZSM-5 crystals with fluorhydric acid, Wloch (2003) recorded an important increase of the mass transfer rate, thus confirming the existence of a surface barrier.…”