Diffusion mechanism of hydrocarbons in zeolites—II. Analysis of experimental observations

Abstract The influence of the properties of molecule-zeolite systems on the diffusional and equilibrium behavior is experimentally investigated. Sixteen hydrocarbons, including paraffins, two aromatics and a naphthene, were studied in ZSM-5. The activation energy for diffusion is mainly responsible for the different diffusivities of various compounds. The diffusion coefficients of benzene, toluene, and 2-methylbutane in ZSM-5 are insensitive to the concentration up to about four molecules/unit cell, and the isotherms within this concentration range are of the Langmuir type. At higher concentrations, the diffusivities show strong concentration-dependent trends, and the Langmuir adsorption parameter sharply decreases. The predictions of the mechanistic models proposed in Part I agree, in general, with the experimental results of the diffusion measurements here, both with regard to the order of magnitude and the concentration dependence. Good agreement is also found for 5A between the theoretical predictions and literature data on the diffusion and equilibrium behavior.