Molecular Dynamics Studies of Hydrated and Dehydrated Na⁺-Zeolite-4A

Abstract

Molecular dynamics simulations of fully hydrated and dehydrated Na + -zeolite-4A with a mobile zeolite framework at 298 K and a steepest descent energy minimization simulation on the dehydrated zeolite have been performed. The optimized structure yields bond lengths, bond angles, and positions of sodium ions in very good agreement with published X-ray data. The simulations at 298 K confirm that the Na(1) and Na (3) ions oscillate about their mean positions and that migration is slow. The Na(2) ions, located in the oxygen eight-rings, are much more mobile and move out of the plane of the ring as well as within the plane. Most water molecules in the R-cages are found to migrate between preferred sites on the inside of the cage. The preferred sites form a polyhedron with 44 vertices, differing from the interpretation of experiment. On average, 2.5 water molecules are found up to 3 Å from the R-cage center, with the remaining 21.75 water molecules at sites on the inside of the cage. Water molecules in the -cages remain in the cages for 300 ps at 298 K and occupy six octahedral preferred sites. The water molecules in -cages containing exactly four molecules are found to occupy planar sites.

Water adsorption in hydrophilic zeolites: experiment and simulation

“…At low pressure, the water-zeolite interactions are dominant, while at high pressure the water-water interactions are more important 50 . This is also reflected in the fact that the structure of water at saturation conditions is very similar to the structure of bulk water 26 . The differences between simulation and experiment at high pressures can be attributed to the increase of the water dipole moment inside the zeolite when the loading increases.…”

Modeling local chemistry in the presence of collective phenomena.

“…A. Zeolite and Guest Molecule Models. Our zeolite model is identical to that used by Faux, et al [2][3][4] . One unit cell of zeolite 4A contains 96 Si, 96 Al, and 384 O atoms bridging the Si and Al.…”

“…To balance charge, 96 Na atoms are located in the structure as follows, 64 in sites Na(1), 24 in sites Na(2) and 8 in sites Na(3) corresponding to 6MR, 8MR and 4MR windows respectively. [2][3][4][8][9] The GEMC method allows for the insertion of molecules throughout the zeolite without regard to the physical diffusion pathways. Molecules of both NH 3 and CO 2 are therefore placed within the β-cages, even though this is unphysical.…”

“…The zeolite potential used in this work is that used by Faux and co-workers. [2][3][4] It contains two-body Coulombic, Lennard-Jones, and Buckingham terms. The form of the potential is:…”

“…Lee et al 1 and Faux and co-workers [2][3][4] have performed MD in hydrated zeolite 4A with fixed numbers of water molecules in order to observe their diffusion and locations in the zeolite cell. Koh and Jhon 5 obtained the positions and differential and integral heats of sorption of water in zeolite 4A using a theoretical approach.…”