Analyzing diffusion behaviors of methanol/water through MFI membranes by molecular simulation

The diffusion of methanol/water mixture through MFI-type zeolite (HZSM-5 and silicalite-1 in this work) membranes has been studied by NVT-ensemble equilibrium molecular dynamics (MD) techniques. We studied the influence of concentration and temperature on the diffusion behaviors of methanol and water after replacing aluminium atoms with silicon atoms in MFI-type zeolite. The diffusion coefficient of water first increased and then decreased with increasing water content. However, the diffusion coefficient of methanol decreased with increasing methanol content. The effect of concentration and temperature on the diffusion behavior of species in both single component and mixtures was also investigated by analysis of radial distribution function (RDF) and mean square displacement (MSD). The distribution of water in silicalite is almost the same as that in HZSM-5. There is an inter-water hydrogen bond formed when R(OH) is 1.75 Å. But the hydrogen peak in low-water concentration range in HZSM-5 is higher than in silicalite. There is an estimated maximum self-diffusion of water in HZSM-5, which agrees with the literature experimental observation.