Infrared spectroscopic investigation of diffusion, co-diffusion and counter-diffusion of hydrocarbon molecules in zeolites

The basic principles of a Fourier Transform IR (FTIR) method are introduced for measurements of diffusivities (D(0)) in single-component diffusion, co- and counter-diffusion (in the case of binary mixtures) and related data (activation energy of diffusion, E,, heat of adsorption, Q) of systems with microporous sorbents. As microporous sorbents H-ZSM-5, H-mordenite, H,Na-ZSM-5, silicalite, Li-ZSM-5 and Na-ZSM-5 are employed. Adsorbates studied are benzene, ethylbenzene, p-xylene and pyridine. It is shown how the reliability of the FTIR method was checked and confirmed via comparison of the FTIR results derived for the system benzene/H-ZSM-5 with the large body of literature data reported for the same system but obtained with various independent techniques. Similarly, the FTIR results of the other systems are discussed in view of literature data, if available. In comparison to single-component diffusion, it is generally found via the FTIR method that the diffusivities in co- and counter-diffusion are lower by about 50%. It is observed that, in H-ZSM-5, D(0)(p-xylene) > D(0)(benzene), a result which is not a priori expected. The investigation of pyridine diffusion in H-mordenite and H-ZSM-5 was hampered by the strong interaction of the base with the Bronsted acid sites of the zeolite and the upper limitation of the temperature of the experiment (575 K); it could be studied, however, in the absence of strong acid sites, e.g. in the systems pyridine/silicalite or pyridine/Li-ZSM-5 and pyridine/Na-ZSM-5. (c) 2005 Academie des sciences. Published by Elsevier SAS. All rights reserved.