Alumina and titania multilayer membranes for nanofiltration: preparation, characterization and chemical stability

The preparation and characterization of porous ceramic multilayer nanofiltration (NF) membranes is described. During preparation, special care was given to each sub-layer that forms a part of the multilayer configuration: the macroporous substrate, the membrane interlayers and the NF toplayers. High-quality macroporous supports are prepared from α-Al2O3. Three types of colloidal sol–gel derived mesoporous interlayers are considered: Al2O3, TiO2 and mixed Al2O3–TiO2. The active NF toplayer is a very thin and fine textured polymeric TiO2 toplayer. Optimized α-Al2O3/γ-Al2O3/anatase and α-Al2O3/anatase/anatase multilayer configurations show high retentions for relatively small organic molecules (molecular weight cut-off <200). Different membrane layers have very different crystallographic properties resulting in a considerably different chemical stability. Corrosion measurements showed that application of a multilayer configuration including weakly crystallized γ-Al2O3 layers is restricted to mild aqueous media (pH 3–11) or non-aqueous media (organic solvents). For NF applications in aqueous media with a lower or higher pH, the multilayer membrane composed of anatase on a α-Al2O3 support is to be preferred.