Counterdiffusion of p-xylene/benzene and p-xylene/o-xylene in silicalite studied by the zero-length column technique

This paper describes the extension of the zero-length column (ZLC) method of measuring intracrystalline diffusion to the study of counterdiffusion. Theoretical ZLC response curves are derived for two simple models considering chemical potential gradient as the driving force for transport with the intrinsic mobilities either constant or inversely dependent on total loading. Experimental response curves for benzene-p-xylene and p-xylene-benzene in silicalite are shown to be consistent with the first of these models. The response curves conform quantitatively to the response predicted from this model using the measured pure component diffusivities. However, the behavior of the benzene-o-xylene and p-xylene-o-xylene systems does not conform to the simple model. The retardation caused by o-xylene is substantially greater than that predicted from the single-component diffusivity data. A simple microdynamic explanation is suggested to account for this apparent anomaly.