MOLECULAR-DYNAMICS SIMULATION STUDIES OF ZEOLITE-A .2. STRUCTURE AND DYNAMICS OF CATIONS IN DEHYDRATED CA2-EXCHANGED ZEOLITE-A()

In a recent paper1 we reported molecular dynamics (MD) simulation of N a+ ions in a rigid dehydrated zeolite-A framework at several temperatu res using a simple Lennard-Jones potential plus Coulomb potential with Ewald summation to investigate the structure and dynamics of Na+ ions . In this paper the same technique is applied to study the structre an d dynamics of cations in dehydrated Ca2+-exchanged zeolite-A systems, Ca6-A and Ca4Na4-A. At 298.15 K the calculated positions of Ca2+ and N a+ ions are in poor agreement with those determined by X-ray diffracti on experiments, but this is reasonably explained by large repulsive in teractions between cations. A simple harmonic oscillation of Ca2+ ions in dehydrated Ca6-A zeolite and simultaneous random motions for the t he same kinds of cations in dehydrated Ca4Na4-A zeolite due to a posit ional symmetry are observed in their velocity auto-correlation functio ns and mean square displacements. MD simulations of the fictitious Ca8 -A zeolite confirm our result for the behavior of Ca2+ ions in dehydra ted Ca6-A zeolite.