Molecular dynamics simulations of hydrated zeolite 4A incorporating a mobile zeolite framework were performed at 298K for a range of hydration over a time scale of 0.5ns. The hydration energy of the water was found to be 57kJ/mol of water at full hydration, in excellent agreement with previous molecular dynamics simulations. The water molecules were found to be located at preferred sites on the inside of the α-cages, with additional water located within the volume of the α-cages. The latter were more mobile. The self-diffusion coefficient of the water molecules contained in the α-cages was found to increase from 2.7 x 10-10m2/s at 56 water molecules per unit cell to 6.8 x 10-10m2/s at 168 water molecules per unit cell. The diffusion coefficient was found to decrease close to full hydration, namely 224 water molecules per unit cell, due to the blocking effect of water molecules in the fully hydrated cages. The MD self-diffusion coefficient at full hydration was found to be approximately 3 times the experimental value, with evidence that a longer simulation will yield an improved agreement with experiment. The water self-diffusion coefficients were approximately an order of magnitude larger than the self-diffusion coefficients of the sodium ions, suggesting that the sodium ions were relatively strongly bonded to the zeolite cage and their primary effect was to hinder the motion of water between α-cages.

Molecular Dynamics Studies of Hydrated Zeolite 4A. D.A.Faux: Journal of Physical Chemistry B, 1999, 103[37], 7803-8

Table 1

Diffusivity of N2, CH4 and Kr in 5A Zeolite