Molecular simulation study on adsorption and diffusion behavior of ethanol/water molecules in NaA zeolite crystal

Molecular simulations have been performed on adsorption equilibria and diffusivities of pure and mixed-gases of ethanol/water in micropores of an NaA zeolite crystal by using the muVT-ensemble orientational-bias Monte Carlo and the NVT-ensemble equilibrium molecular dynamics techniques. The temperature and the total pressure were set at 378 K and 0.1 MPa respectively for comparison with the data of vapor permeation of a corresponding system. Adsorption equilibrium selectivity of water to ethanol was found to increase with an increasing ethanol composition in the gas phase, which was qualitatively in agreement with the experimental permselectivity, though the latter was three orders of magnitude higher than the first. The diffusivity of water in the zeolite was found to be in the region of 3.9-7.9 x 10(-10) m(2)/s while the ethanol molecules were confined in the alpha-cage and did not go through the window during a simulation period of 3 ns, which indicated that the diffusivity of ethanol might be much smaller than that of water. Therefore, it is concluded that the high permselectivity of water through an NaA zeolite membrane is ascribed to the cooperation of two factors: the high adsorption selectivity and the high diffusivity of water. It is also noted that the selective adsorption of water may be important for preventing the blockage by ethanol molecules at the entrance and inside of the membrane.