Molecular dynamics studies of hydrated and dehydrated Na+-zeolite-4A

Molecular dynamics simulations of fully hydrated and dehydrated Na+-zeolite-4A with a mobile zeolite framework at 298 K and a steepest descent energy minimization simulation on the dehydrated zeolite have been performed. The optimized structure yields bond lengths, bond angles, and positions of sodium ions in very good agreement with published X-ray data. The simulations at 298 K confirm that the Na(1) and Na(3) ions oscillate about their mean positions and that migration is slow. The Na(2) ions, located in the oxygen eight-rings, are much more mobile and move out of the plane of the ring as well as within the plane. Most water molecules in the alpha-cages are found to migrate between preferred sites on the inside of the cage. The preferred sites form a polyhedron with 44 vertices, differing from the interpretation of experiment. On average, 2.5 water molecules are found up to 3 Angstrom from the alpha-cage center, with the remaining 21.75 water molecules at sites on the inside of the cage. Water molecules in the beta-cages remain in the cages for 300 ps at 298 K and occupy six octahedral preferred sites. The water molecules in beta-cages containing exactly four molecules are found to occupy planar sites.