A simple approach to hierarchical ceramic ultrafiltration membranes

Asymmetric alumina ultrafiltration membranes with a hierarchical structure have been fabricated using carboxylic acid surface stabilized alumina nanoparticles (alumoxanes). Pre-formed hollow α-alumina spheres (3 μm nominal diameter) were prepared, using 2, 5, or 8 wt.% alumoxane solutions, and suspended in an aqueous solution of acetic-alumoxane (A-alumoxane) nanoparticles. This suspension was contacted with an α-alumina support. Firing to 600 °C gave a defect-free alumina hierarchical membrane with a total thickness of ca. 2 μm. The flux and permeability for the membrane containing alumina spheres derived from 2 wt.% solution of A-alumoxane is comparable to the porous support, while those derived from 5 and 8 wt.% solution of alumoxane are similar to a “flat” alumoxane-derived membrane. An alternative route to increasing the flux and permeability of the asymmetric membranes was developed whereby colloidal polystyrene beads of either 0.75, 3, or 15 μm diameter, were suspended in an aqueous solution of either A-alumoxane (1 wt.%) or methoxy(ethoxyethoxy)acetic acid alumoxane (MEEA-alumoxane, 10 wt.%). The surface of an α-alumina support was dip coated in the polystyrene/alumoxane colloidal solution, dried then fired to 600 °C, resulting in an asymmetric alumina membrane with a hierarchical tertiary structure. As the polystyrene out-gasses during pyrolysis the top of the coated spheres burst, resulting in a macroporous membrane in which the ceramic walls have a pore size and hence MWCO defined by the alumina formed from the alumoxane nanoparticles (A-alumoxane versus MEEA-alumoxane) rather than the macroporous structure of the membrane itself. The permeabilities of these membranes are equivalent or better than the support.