Measurements of Adsorption on Outer Surface of Zeolitee and their Influence on Evaluation of Intracrystalline Diffusivity

Abstract

Abstract The influence of the amount of hydrocarbons, which were adsorbed on the outer surface of ZSM-5 zeolite crystallites, on the evaluated intracrystalline diffusivity was investigated. Six kinds of hydrocarbons; n -hexane, n-heptane, n-octane, benzene, toluene and p-xylene, were used as adsorbates. The uptake curves of the amounts of these hydrocarbons adsorbed on high siliceous ZSM-5 zeolites with different sizes and shapes were measured. The amount adsorbed on the outer surface of zeolite crystallites and the intracrystalline diffusivities were evaluated from a theoretical equation and the obtained uptake curves. The magnitude of the amount adsorbed on the outer surface was found to be about 10 to 50% of the total amount adsorbed (zeolite crystal size: 0. 1 to 2 pm). The adsorption potential theory of Polanyi was found to well represent the adsorption isotherms considering only the outer surface of zeolite crystallite. The intrinsic uptake curve of the amount adsorbed within the zeolite crystallites was obtained by subtracting the amount adsorbed on the outer surface of the crystallite from the total amount adsorbed. Using the amount adsorbed within the crystallite, the intracrystalline diffusivities were reevaluated. These values were found to be about one-tenth of those evaluated from the uptake curves of the total amount adsorded