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

Molecular dynamics simulations have been performed on a rigid hydrated zeolite-A at 298.15 K using a simple Lennard-Jones potential plus Cou lomb potential with Ewald summation technique to investigate the struc ture 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 a re kept at their sites of B-ring windows, characterized by nondiffusiv e vibrational motion, and that four Na-IV type ions show a diffusive m obility due to loose bindings to nearest framework atoms and due to fu ll hydrations with alpha-cage water molecules. beta-cage water molecul es form a distorted tetrahedron with lengths of edges in good agreemen t with those predicted in a X-ray diffraction experiment, and there is no clear evidence for the dodecahedral arrangement of 20 water molecu les predicted in the X-ray diffraction experiment. The dynamical behav ior of alpha- and beta-cage water is more diffusive than bulk water du e to the lack of hydrogen bonds in the closed cages, and the rotationa l motions of water molecules in both alpha- and beta-cages are charact erized 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 molecu les to the size of the zeolite framework.