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Molecular Dynamics Simulation Studies of Zeolite-A. IV. Structure and Dynamics of NH 4 + ions in a Rigid Dehydrated Zeolite-A

1996; Taylor & Francis; Volume: 17; Issue: 2 Linguagem: Inglês

10.1080/08927029608024100

1029-0435

Sang Gu Choi, Song Hi Lee,

Zeolite Catalysis and Synthesis

Abstract In recent papers [1–3] 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 NH4 + ions in a rigid dehydrated zeolite-A. During the preliminary equilibration, the unstable NH4(4) type ion (the 12th ion) is pushed down to near a more stable 6-ring position in the α-cage that is already associated with an NH4(1) type ion (the 1st) in the β-cage, which moves to another 6-ring position in the β-cage that is already associated with an NH4(2) type ion (the 7th) in the α-cage. Calculated x, y, and z coordinates of some NH4 + ions are in good agreement with those obtained from an X-ray diffraction experiment except that no NH4(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(1 – 1) and NH4(3) type ions are associated loosely with only one O (3) atom of the 6-ring and with only one O (1) atom of the 8-ring windows, respectively, while the NH4(1–2) and NH4(2) type ions are associated strongly with two or three O (3) atoms of the 6-ring windows in the α- and β-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 NH4(1–1), NH4(1–2), NH4(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 O atom hydrogen-bonded to the ion. Key Words: Zeolite-Adynamicsion positions