Cobalt-doped silica membranes for pervaporation dehydration of ethanol/water solutions

Cobalt-doped silica (Co–SiO2) membranes were successfully fabricated by sol–gel processing. After measurement of single-gas permeation through the membranes, they were used in pervaporation (PV) experiments with ethanol feed concentrations ranging from 50 to 94 wt% and temperatures from 50 to75 °C. The membrane prepared by firing at 550 °C showed a He permeance of 2.6 × 10−6 mol/(m2 s Pa) with permeance ratios of 20 for He/N2 and 113,000 for He/SF6, respectively. In addition, a permeate water flux of 1.1 kg/(m2 h) with a separation factor of 2530 was obtained at 75 °C in 90 wt% ethanol. In PV dehydration experiments, the Co–SiO2 membranes showed a decrease in permeate flux and an increase in separation factor during the first several hours, then reached a steady state and subsequently remained constant. When the firing temperature of the top layer was increased from 350 to 550 °C, both the single-gas permeance and the PV flux decreased, while the gas permeance ratio (He/N2) and the PV separation factor increased, suggesting an increase in the density of the network structure. In the PV experiments, water and ethanol showed approximately the same permeances as that of single-gas permeation, suggesting the permeation mechanism of water and ethanol in PV was dominated by molecular sieving. Moreover, cobalt-doped silica membranes showed good stability in an aqueous environment during the long-term PV experiments, which lasted 150 days.