MOLECULAR-DYNAMICS SIMULATION STUDIES OF ZEOLITE-A .3. STRUCTURE AND DYNAMICS OF NA+ IONS AND WATER-MOLECULES IN A RIGID ZEOLITE-A

Molecular dynamics simulations have been performed on a rigid hydrated zeolite-A at 298.15 K using a simple Lennard-Jones potential plus Coulomb potential with Ewald summation technique to investigate the structure and dynamics of Na+ ions and water molecules in the rigid zeolite-A framework. The results of analysis show that eight Na-I type ions are kept at their sites of B-ring windows, characterized by nondiffusive vibrational motion, and that four Na-IV type ions show a diffusive mobility due to loose bindings to nearest framework atoms and due to full hydrations with alpha-cage water molecules. beta-cage water molecules form a distorted tetrahedron with lengths of edges in good agreement with those predicted in a X-ray diffraction experiment, and there is no clear evidence for the dodecahedral arrangement of 20 water molecules predicted in the X-ray diffraction experiment. The dynamical behavior of alpha- and beta-cage water is more diffusive than bulk water due to the lack of hydrogen bonds in the closed cages, and the rotational motions of water molecules in both alpha- and beta-cages are characterized by slower librational motions and faster rotational relaxations than bulk water. An MD simulation in the true cell shows very little sensitivity of the structure and dynamics of Na+ ions and water molecules to the size of the zeolite framework.