Diffusion of water in zeolites naX and naY studied by quasi-Elastic neutron scattering and computer simulation

The diffusion of water in zeolites Na X and Y was investigated by quasi-elastic neutron scattering (QENS) and by a series of molecular dynamics simulations at different temperatures, for different water loadings. The largest measured values of the diffusion coefficients are one order of magnitude smaller than in bulk water and are slightly lower in zeolite X than in zeolite Y. The dependence of the calculated diffusion coefficients vs. the water content is very low at low loadings, diffusivity grows and reaches a maximum at intermediate loadings and finally decays to smaller values when the loading approaches saturation. The reproduction of the experimental diffusion coefficient and activation energies by molecular dynamics simulations are discussed. In spite of the use of empirical potential models and classical mechanics equations, the computed activation energies are close to the experiment and the diffusion coefficients follow the same trend as the experimental ones, are of the same order of magnitude, but are overestimated, in particular in zeolite Y. Nevertheless, from these results some suggestions about the details of the diffusion mechanism may be inferred.