Study of fast diffusion in zeolites using a higher harmonic frequency response method

Abstract

The high harmonic frequency response (FR) method has been applied for the first time to measure fast diffusion processes in zeolites, e.g. carbon dioxide/silicalite-1, hexane/beta-zeolite and hexane/NaX. The frequency range can be increased to 90 Hz when the ninth harmonic is taken in the Fourier transformation of the squarewave signals. The diffusivities of carbon dioxide in silicalite-1 and hexane in NaX measured by this frequency response method are in a good agreement with the corresponding values determined by PFG NMR. The diffusivities of carbon dioxide in silicalite-1 were found to be one order of magnitude larger than those in theta-1. The difference in diffusivity between these two 10-ring zeolites can be explained by the different diffusion mechanisms involved, i. e. the single-file diffusion for the one-dimensional theta-1 and the random jump model for the three-dimensional silicalite-1. The faster diffusion of n-hexane in NaX (12-ring windows link the supercages) than in beta-zeolite (three mutually intersected 12-ring channels) was also observed by this FR method.