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Abstract:
A new method for studying diffusion and counter-diffusion in zeolites is presented. It is, in fact, a technique based on sorption kinetics. The rate of sorption and desorption is determined via the intensity change of typical sorbate IR bands with time. The variation in absorbance is monitored by a fast Fourier transform IR spectrometer. Experimental set-up and procedures are described in detail. The method is illustrated by measurements of the uptake of benzene and ethylbenzene into H-ZSM-5. From the sorption kinetics diffusivities for these single components are derived. In excellent agreement with literature data, a diffusion coefficient, D=5.7×10−10cm2·s−1, is obtained for the system benzene/H-ZSM-5 at 395 K and a coverage of n=0.2 mmol·g−1. Sorption of ethylbenzene into a zeolite sample preloaded with benzene is chosen as an example of counter-diffusion. The diffusivities in the case of counter-diffusion are decreased by 50 to 60% compared to the single-component diffusivities.
