Abstract EN:

Zeolites possess unique features as gas adsorbents, gas separators, and catalysts. Adsorption properties are closely related to the localisation of non-framework cations and to their accessibility to adsorbed molecules. Knowledge of the location of these cations and of their interaction between both the zeolite framework and the adsorbates is a crucial point for understanding the surface properties. In fact, the cation forms with the oxygen of the framework a dipole, which reorientation is illustrated by a cation hopping process between two neighbouring equilibrium positions, separated by an energy barrier. The experimental approach based on the Dielectric Relaxation Spectroscopy (DRS) gives information on both the distribution of the cation among the different crystallographic sites and their de-trapping energy ΔE. In this work, we use DRS to investigate the effect of methanol and water adsorption on the dynamics and the localisation of sodium ions in faujasites, Mordenites and ZSM-5 sodic zeolites. We clearly show that the cation de-trapping energy depends on the site geometry, the Si/Al zeolite ratio and the presence of adsorbed molecules. It is then possible to discriminate the structurally different cations equilibrium position as a function the adsorbate loading. Moreover, combining DRS data with Molecular Dynamics (MD) calculations is fruitful in view of explaining the microscopic mechanisms of cation migrations and of adsorption