Synthesis and characterization of primary alumina, titania and binary membranes

The synthesis and characterization of supported and non-supported membranes of γ-Al₂O₃, TiO₂ and their binary combinations are described. Non-supported γ-Al₂O₃ and TiO₂ membranes were prepared from colloidal dispersions (sols) of boehmite and titania, respectively. Stable binary sols were also prepared by mixing boehmite and titania sols under appropriate pH conditions, and these were used to prepare binary (γ-Al₂O₃/TiO₂) membranes. Supported γ-Al₂O₃, TiO₂ and binary membranes were made by the slip-casting process using the same sols. The membrane layers had, after calcination, a thickness of 3-6 μm depending on the dipping conditions, and an average pore diameter of ∼-3-4 nm with a narrow pore-size distribution. The structural transformation of titania from the anatase to the rutile phase was retarded by performing the hydrolysis in the presence of sulphate ions. Polyvinyl alcohol was added to the sols to strengthen the gel network during the drying and calcination process. This resulted in a less critical and more controllable membrane formation process. Multiple dipping (e.g. to repeat the dipping, drying and calcination steps) appeared to be a technique by which (i) defected layers could be repaired; (ii) thick membrane layers could be prepared; and (iii) multilayer membranes consisting of layers of different components could be prepared.