Diffusion of propane in theta-1 and silicalite-1 zeolites

The diffusivities of propane in theta-1 have been investigated using the fr equency-response (FR) technique. A surface resistance has been observed in the FR spectra of propane in theta-1, which has been ascribed to the diffus ion of propane in theta-1 being controlled by a single-file diffusion mecha nism. Such a mechanism arises because the kinetic diameter of the propane m olecules is similar in size to the diameter of the one-dimensional channels in theta-1 and the impossibility for the propane molecules to pass over on e another inside the channels. The single-tile diffusion process results in diffusion coefficients of propane in theta-1 which are about two orders of magnitudes smaller than those of propane in silicalite-l which has channel s of similar diameter to those in theta-1. The activation energy for the di ffusion of propane in theta-1 is close to the value for the heat of adsorpt ion in this system and is more than twice as large as that for the propane/ silicalite-1 system. These energies support the assumption of single-file d iffusion and that the rate of adsorption and desorption of sorbate molecule s at the pore entrances of the theta-1 could be the rate-controlling step f or the mass transport of the sorbate molecules in the one-dimensional chann el network under the near-equilibrium conditions which exist with the FR te chnique. (C) 2000 Elsevier Science B.V. All rights reserved.