Pervaporation characteristics of aqueous–organic solutions with microporous SiO2–ZrO2 membranes: Experimental study on separation mechanism

Five kinds of micro-porous SiO2–ZrO2 (ZrO2; 50 mol%) membranes of slightly different pore sizes less than 1 nm were prepared by the sol–gel techniques to study the pervaporation characteristics of some aqueous solutions of organic chemicals. Since no direct evaluation techniques to determine the membrane pore-sizes less than 1 nm have yet been established, the pore sizes of the membranes fabricated were estimated by observing the permeances of gases of various molecular diameters at 200 °C.Some pervaporation measurements of aqueous solutions of organic chemicals such as iso-propanol, ethanol and acetone have been done with the membranes fabricated in order to study the effects of the membrane pore size and the molecular interaction with the pore wall on the pervaporation performance. The pervaporation results for IPA/water mixture at 10 wt.% of water and at 75 °C showed a long time dependency of IPA and water fluxes. The IPA flux decreased drastically, while that of water decreased slightly, giving a large separation factor, larger than 15,000, after several hours of pervaporation with water fluxes of 120–300 mol/m2 h, for example.The observed concentration dependency of the IPA flux showed a maximum at around 48 mol% of IPA and then decreased with increasing IPA concentration in the feed. A similar behavior of the ethanol flux was observed for ethanol/water mixtures, but not for acetone/water mixtures. The acetone flux increased monotonously with the increase of acetone concentration in the feed.The pervaporation results obtained for the membranes of different pore sizes suggest that the separation performance depends largely on the pore size and also on the interaction between the molecules and the pore wall.