Adsorption and diffusion of water, methanol, and ethanol in all-silica DD3R: Experiments and simulation

This study comprises both experimental measurement as well as molecular modeling of the adsorption of water, methanol, and ethanol on the hydrophobic all-silica decadodecasil 3R (DD3R) zeolite. The simulation data are compared with permeation data of these components, measured using a DD3R membrane under pervaporation conditions. The pure-component isotherms are measured by vapor-phase adsorption and are calculated by grand canonical Monte Carlo (GCMC) simulations. The simulations are conducted using molecular models from the literature, without any adjustments. The simulation results are in fair agreement with the experimental adsorption data. Computed mixture adsorption isotherms show that the water loading is significantly increased as compared to pure-component adsorption. The shape of the computed water isotherm changes from type II to type I when alcohol is present. The loading of both methanol and ethanol at low fugacities is slightly enhanced by the presence of water. The self-diffusivites are calculated using MD simulations. The self-diffusivity of water in DD3R is substantially larger than the diffusivites of both alcohols but is decreased in the presence of methanol or ethanol. The ethanol diffusivity is too low to be determined by conventional MD simulations. The flux calculated from the MD and GCMC results overpredicts the experimental values, but the predicted membrane selectivity corresponds well to the experimental data. The mechanism of separation in the dewatering of alcohols using a hydrophobic DD3R membrane is based on the high diffusivity of water as compared to that of the alcohols, where especially ethanol suffers from diffusion limitations.