Molecular Dynamics Simulation Studies of Zeolite-A. 5. Structure and Dynamics of Cations in H₁₂-A and (CH₃NH₃)₁₀Na₂-A Using Rigid Dehydrated Zeolite-A Frameworks

Abstract

In the present paper a molecular dynamics simulation technique is applied to study the local structure and dynamics of H+, CH3NH3 +, and Na+ ions in rigid dehydrated zeolite-A frameworks using a simple Lennard-Jones potential plus Coulomb potential with Ewald summation. In the H12-A zeolite system, two structures appear, depending upon the choice of the Lennard-Jones parameter, σ, for the H+ ion. For the smaller values of σ, the 12th H+ ion is located on one of the 8-ring window sites which are already occupied by three H+ ions; for the larger values of σ, it is at one of the opposite-4-ring sites with the remaining 11 H+ ions almost fixed near their initial positions. In the (CH3NH3)10Na2-A zeolite system, the main structural differences from an X-ray crystallographic report are 4-fold: no facing CH3NH3 + ions through a 6-ring window, two ions in the β-cage, the appearance of a CH3NH3 + ion on one of the opposite-4-ring sites, and the lying of CH3NH3 + ions on the planes of the 8-ring window sites. Four kinds of time correlation functions for the CH3NH3 + ions show the dynamics of the ions, 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(1), III and O(2), IV and O(1), 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 bond occurs, leading to a significant exchange of O atoms hydrogen-bonded to the ion.

Crystal Structure of a Benzene Sorption Complex of Dehydrated Fully Ca2+-Exchanged Zeolite X

“…The eight Ca 2+ ions at Ca(2) which do not coordinate to benzene(1) are recessed only 0.24(1) Å into the supercage from their O(2) plane; compared to dehydrated Ca 46 -X, each of these Ca 2+ ions deviates 0.06 Å less from the plane of its three O(2) oxygens, to which it coordinates more trigonally. Three-coordinate Ca 2+ has remarkably high thermal motion at this position: this had been seen before crystallographically and is well established by molecular dynamics calculations 5 Structural Features of Ca 46 -X and Its Sorption Complexes dehydrated·30C 2 H 2 ·30C 2 H 4 ·28C 6 H 6 ·138H 2 S number of Ca 2+ ions at site I 16 16 16 16 16 number of Ca 2+ ions at site II 30 30 30 8, 22 30 frac.…”

“…Three-coordinate Ca 2+ has remarkably high thermal motion at this position: this had been seen before crystallographically 31 and is well established by molecular dynamics calculations. 32 The closest approach between benzene(1) and the zeolite framework involves the H(2) hydrogen atom and O(1) at a distance 2.94 Å, suggestive of a very weak electrostatic interaction. The smallest distance between adjacent benzene-(1) molecules is between hydrogen atoms, H(1)-H(2)′ ) 3.21 Å.…”