Pervaporation of alcohol–water and dimethylformamide–water mixtures using hydrophilic zeolite NaA membranes: mechanisms and experimental results

Abstract Dehydration of solvents using hydrophilic polyvinylalcohol pervaporation membranes is a well-established technology. However, these polymeric membranes may not be suitable for applications involving high water concentrations or applications containing harsh solvents like dimethylformamide due to membrane stability problems and swelling effects. These applications are commonly encountered in the pharmaceutical industry. The recent development of solvent and temperature-resistant, hydrophilic zeolite NaA membranes has made it possible to overcome the above limitations of hydrophilic polymeric membranes. In this study, experiments were conducted with various alcohol–water (methanol–water, ethanol–water, isopropanol–water) mixtures and with dimethylformamide–water mixture over a wide range of temperatures (25–70°C) and solvent concentrations (0–100 wt.%). The total flux for ethanol–water mixture was found to vary from 2 to 0.05 kg/m 2 /h at 60°C as the feed solvent concentration was increased from 0 to 100 wt.%. The total flux for methanol–water and isopropanol–water mixtures was observed to vary from 2 to 0.15 and 2 to 0.21 kg/m 2 /h, respectively, as the alcohol concentration was changed from 0 to 100 wt.% . The total flux was also found to remain approximately constant up to 70 wt.% alcohol in the feed. Both water to ethanol and water to isopropanol separation factors were observed to lie between 1000 and 5000 over a wide range of solvent concentrations. The water to methanol separation factor was found to lie in the range of 500–1000. The total flux behavior was also found to be very similar for the other solvent–water mixtures such as acetone–water and ethyl acetate–water. The ionic Na + sites in the NaA zeolite matrix play a very important role in the water transport through the membrane. These sites act both as water sorption and transport sites. The surface diffusion of water occurs in an activated fashion on these sites. The precise micropore structure of the zeolite cage helps in a partial molecular sieving of the large solvent molecules leading to high separation factors. The zeolite membrane active layer may contain certain non-zeolitic interstitial pores with preferential water sorption. One of the reasons for the high hydrophilicity of zeolite NaA is the strong electrostatic interaction between ionic sites and the water molecule (due to its highly polar nature). A high degree of hydrophilicity of the zeolite membrane is suggested from a pure water sorption value of 0.6 g/g zeolite. The detailed interpretation of this result, however, requires consideration of both true zeolitic microcavity uptake as well as interstitially held water between crystallites. Any molecule highly polar in nature (indicated by a high dipole moment or dielectric constant) is expected to strongly interact with the ionic sites in the cage. In fact, this effect was indirectly observed for the dimethylformamide–water mixture. The water flux in this case, was found to be lower than that for alcohol–water mixtures thus indicating the possibility of competitive sorption of dimethylformamide molecules on the zeolite sites. A linear correlation was found to exist for the pure water flux through the membrane and the partial pressure driving force of the water. The other alcohol–water mixtures followed this trend reasonably well. The use of zeolite membranes for dehydration of complex solvent streams has been actually demonstrated by experiments with a synthetic ethyl acetate mixture. Thus, the use of inorganic zeolite membranes for difficult solvent separations seems to be very attractive.