Static and Dynamic Properties of the Nonframework Cations in Na-Mordenites Zeolite

Abstract

This work consists of exploring the effect of Si/Al ratio and water content on both the static and dynamic properties of the extra-framework cations in a typical zeolite system Na+-Mordenites. Atomistic simulations based on interatomic potentials and minimisation techniques have been used to determine the location of the cations as a function of the Si/Al ratio and the hydration level. Our calculations showed two different cation behaviours depending on the type of channels that they occupy, the positions of the cations in the main channels being substantially perturbed upon the sorption of water molecules whereas those of the cations located in the small side channels being only slightly shifted. By contrast, the populations of the cation sites have almost the same trend when the Si/Al ratio varies. We have then developed a Monte Carlo technique specialised for the simulation of rare events to study the activated cations motions in this zeolite. Mean activation barriers are obtained from minimum energy paths (MEP) calculated for lattices with various Si/Al ratio, by using a Metropolis algorithm. Finally, Molecular Dynamics (MD) simulations allowed us to compare the mobility of the cations in hydrated and dehydrated states and confirmed the results expected from the previous static calculations. This modelling has been successfully compared with experimental data obtained by dielectric relaxation spectroscopy (DRS) and provided a microscopic description of the static and dynamic properties of the extra-framework cations.