Diffusion, self-diffusion and counter-diffusion of benzene and p-xylene in silicalite

This paper reports the results of a detailed experimental study of transport diffusion and tracer diffusion of benzene and p-xylene in large crystals of silicalite. The measurements were carried out by the zero length column (ZLC) method. Benzene behaves as a textbook example of an idealized system in which the self-diffusivity is essentially independent of loading and coincides, in the low loading limit, with the limiting transport diffusivity. The behavior of p-xylene is more complex. The self-diffusivity is smaller than the (corrected) transport diffusivity by a factor of about 2.5-3.0, and this difference persists even as the zero loading limit is approached, implying a difference in mechanism between the self-diffusion and transport diffusion processes. A tentative explanation for the difference in behavior between benzene and p-xylene is suggested on the basis of the difference in the length of the molecule. (C) 2000 Elsevier Science B.V. All rights reserved.