Molecular Dynamics Simulation Studies of Zeolite-A. IV. Structure and Dynamics of NH₄⁺ions in a Rigid Dehydrated Zeolite-A

Abstract

In recent papers C1-31 we reported molecular dynamics simulation studies of ions and water molecules adsorbed in a rigid zeolite-A framework using a simple Lennard-Jones potential plus Coulomb potential with Ewald summation to investigate the structure and dynamics of the adsorbates. In the present paper the same technique is applied to study the local structure and dynamics of NH: ions in a rigid dehydrated zeolite-A. During the preliminary equilibration, the unstable NH,(4) type ion (the 12th ion) is pushed down to near a more stable 6-ring position in the a-cage that is already associated with an NH,(l) type ion (the 1st) in the 8-cage, which moves to another 6-ring position in the 8-cage that is already associated with an NH,(2) type ion (the 7th) in the cc-cage. Calculated x, y, and z coordinates of some NHf ions are in good agreement with those obtained from an X-ray diffraction experiment except that no NH,(4) type ion is found and there are six NH4(2) type ions instead of 0.5 and 5.5 occupancy. The analyses of calculated interatomic distances and time correlation functions of these ions indicate that the NH4(l -1) and NH4(3) type ions are associated loosely with only one 0 (3) atom of the 6-ring and with only one O(1) atom of the 8-ring windows, respectively, while the NH,(l-2) and NH,(2) type ions are associated strongly with two or three 0 ( 3 ) atoms of the 6-ring windows in the a-and 8-cages, respectively. The analysis of hydrogen bond time correlation functions of these ions indicate that about one, two or three, three, and one hydrogen bond of each NH,(l-I), NH,(l-2), NH,(2) and NH4(3) type ion is kept for 1.4, 21, 75, and 1.4 ps, respectively, before breakup of the hydrogen bond occurs and significant exchange of 0 atom hydrogen-bonded to the ion.

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

“…The most interesting feature is a CH 3 NH 3 + ion at an opposite-4-ring site with no facing CH 3 NH 3 + ions through a 6-ring window. On the other hand, in the MD simulation of NH 4 + -exchanged zeolite-A, the results have shown that there are two facing NH 4 + ions through a 6-ring window with the disappearance of NH 4 + ion at an opposite-4-ring sites against an X-ray diffraction experimental result …”

“…The four kinds of time correlation functions for CH 3 NH 3 + show the dynamics of the ions which reflect the different structural arrangements of the ions well. The hydrogen bond time correlation functions of four types of CH 3 NH 3 + ions decay more rapidly than those of NH 4 + ions . From the analysis of the characteristic decay times for the H-bonds, 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.…”

“…In our previous studies, we have performed MD simulations of four different zeolite-A systems to investigate the structure and dynamics of adsorbates: rigid dehydrated zeolite-A, two rigid Ca 2+ -exchanged zeolite-A systems, rigid hydrated zeolite-A, and rigid NH 4 + -exchanged zeolite-A …”

“…In the fourth paper, we reported an MD simulation study of NH 4 + ions in rigid dehydrated zeolite-A at 298.15 K using the previously determined Lennard-Jones (LJ) potential parameters and electrostatic charges for the zeolite framework atoms 1 and the newly determined ones for the NH 4 + ion. During the preliminary equilibration, the unstable NH 4 (4) ion (a 12th ion) was pushed down to near a more stable 6-ring position in the α-cage that was already associated with an NH 4 (1) ion (a first ion) in the β-cage, which moved to another 6-ring position in the β-cage that was already associated with an NH 4 (2) ion (a seventh ion) in the α-cage.…”