MOLECULAR-DYNAMICS SIMULATION STUDIES OF ZEOLITE-A .5. STRUCTURE AND DYNAMICS OF CATIONS IN H-12-A AND (CH3NH3)(10)NA-2-A USING RIGID DEHYDRATED ZEOLITE-A FRAMEWORKS

In the present paper a molecular dynamics simulation technique is appl ied to study the local structure and dynamics of H+, CH3NH3+, and Naions in rigid dehydrated zeolite-A frameworks using a simple Lennard-J ones potential plus Coulomb potential with Ewald summation. In the H-1 2-A zeolite system, two structures appear, depending upon the choice o f the Lennard-Jones parameter, sigma, for the H+ ion. For the smaller values of sigma, the 12th H+ ion is located on one of the 8-ring windo w sites which are already occupied by three H+ ions; for the larger va lues of sigma, it is at one of the opposite-4-ring sites with the rema ining 11 H+ ions almost fixed near their initial positions. In the (CH 3NH3)(10)Na-2-A zeolite system, the main structural differences from a n X-ray crystallographic report are 4-fold: no facing CH3NH3+ ions thr ough a 6-ring window, two ions in the beta-cage, the appearance of a C H3NH3+ ion on one of the opposite-4-ring sites, and the lying of CH3NH 3+ ions on the planes of the 8-ring window sites. Four kinds of time c orrelation functions for the CH3NH3+ ions show the dynamics of the ion s, reflecting the different structural arrangements of the ions well. The analyses of hydrogen bond time correlation functions for the four nonequivalent CH3NH3+ ions indicate that about 0.8, 2.9, 0.6, 2.9, 1.6 , 0.9, 1.7, and 2.0 hydrogen bonds formed between I and O(2), I and O( 3), II and O(2), II and O(3), III and O(I), III and O(2), IV and O(I), and IV and O(3) are retained for 0.83, 1.89, 0.63, 1.61, 0.42, 0.31, 1.11, and 1.26 ps, respectively, before a breaking of the hydrogen bon d occurs, leading to a significant exchange of O atoms hydrogen-bonded to the ion.