Ceramic-supported thin PVA pervaporation membranes combining high flux and high selectivity; contradicting the flux-selectivity paradigm

Thin, high flux and highly selective cross-linked poly(vinyl)alcohol waterselective layers have been prepared on top of hollow fibre ceramic supports. The supports consist of an α-Al2O3 hollow fibre substrate and an intermediate γ-Al2O3 layer, which provides a sufficiently smooth surface for the deposition of ultra-thin PVA layers. Membranes have been characterised by SEM and pervaporation experiments. The thickness of the PVA layer formed on top of the γ-Al2O3 intermediate layer is in the order of 0.3-0.8 μm. No significant infiltration of PVA into the intermediate γ-Al2O3 layer can be observed. In the dehydration of 1-butanol (80 °C, 5 wt.% water) the membranes exhibit a high water flux (0.8-2.6 kg m−2 h−1), combined with a high separation factor (500-10,000). The values for the flux and separation factor exceed typical values obtained for cross-linked PVA membranes on polymeric supports. In the dehydration of 2-propanol and 1-butanol, a simultaneous increase in both water flux and separation factor is observed with increasing temperature or water concentration. This remarkable behaviour is in contrast to the trade-off generally observed for polymer membranes, i.e., an increase in flux is typically combined with a decrease in separation factor. A possible explanation for this behaviour is a low degree of three-dimensional swelling in the vicinity of the γ-Al2O3-PVA interface due to an enhanced structural stability. In the dehydration of ethanol and 1-propanol the traditional trade-off between flux and selectivity was observed.