Diffusion of Water in Zeolites Na A and NaCa A: A Molecular Dynamics Simulation Study
Résumé
The behavior of water confined in nanoporous materials and, in particular, in zeolites is an important clue for understanding many various phenomena in geophysics, catalysis, and environmental science fields. In this study, the diffusion of water in zeolites Na A and NaCa A at three different loadings is studied by a classical molecular dynamics simulation technique. The results are in semiquantitative agreement with PFG NMR and QENS data, despite the good reproduction of structural and vibrational properties of the considered systems. Activation energies are in the range of those derived by the two experimental techniques. A detailed analysis of the simulated trajectories allows furnishing suggestions about the diffusion mechanism, involving a coordinated motion of water molecules and cations. Finally, the mobility of the cations, which depends strongly on the number of adsorbed water molecules and the nature of cations themselves, is investigated. The activation energies for the diffusion of the cations are too high to obtain quantitative results from MD simulations, but the observed motions can be of interest in view of the simulation of cation exchange in zeolites.