Molecular simulation of water/alcohol mixtures' adsorption and diffusion in zeolite 4A membranes.

The COMPASS (condensed-phase optimized molecular potentials for atomistic simulation studies) force field with two sets of partial atomic charges of water was used to simulate adsorption and diffusion behavior of water/methanol and water/ethanol mixtures in zeolite 4A at 298 K. The adsorption of alcohol first increased and then decreased with increasing pressure, whereas the adsorption of water increased progressively until an adsorption equilibrium was reached. Both the adsorbed molecules and the zeolite framework were treated as a fully flexible model in MD simulations. The simulation results show that the effects of the size and steric hindrance of the diffusing molecules on diffusivity are significant. The diffusivity of water, methanol, and ethanol molecules decreases by 1 order of magnitude in the order of water > methanol > ethanol. The diffusivity of water molecules depends on the mass fraction and the partial charges of water in zeolite 4A. The ethanol and methanol molecules have restricted motion through the alpha-cages, whereas the water molecules can easily pass through the alpha-cages window at low feed alcohol concentrations. And the extent of hydrogen bonding increased with increasing water concentration.