Cation Behavior in Faujasite Zeolites upon Water Adsorption: A Combination of Monte Carlo and Molecular Dynamics Simulations

Grand Canonical Monte Carlo and molecular dynamics simulations are employed to investigate the influence of water adsorption on the arrangements and the dynamics of the sodium cations in faujasites Na 56 Y and Na 96 X. The water adsorption provokes significant cation redistributions in Na 56 Y, while the partition of the cations among the different crystallographic sites is not affected upon the whole adsorption process in Na 96 X. The first water molecules in Na 56 Y are adsorbed both in the sodalite cage and in the supercage, that is, interacting with cations in SI′ and SIII′, respectively. In contrast, the first water molecules in Na 96 X are located within the supercage interacting with cations in SIII′ only. The cation dynamics are then explored. In Na 56 Y, only very local motion is observed for cations in sites SI′ whatever the water loading. At low and intermediate water loadings, the cations initially in SII and SIII′ present local displacements around their initial sites only whereas they move over much longer distances at high loading. Finally, due to a strong steric repulsion between cations in Na 96 X, the average cation mean square displacement for this system is always smaller than for Na 56 Y.