Micro-structural optimization of supported γ-alumina membranes

The micro-structural integrity of meso-porous inorganic membranes is of critical importance in their prolonged application. Inspection by transmission electron microscopy of state-of-the-art γ-alumina membrane cross-sections, made by focused ion beam milling, reveals structural defects which adversely affect performance, reproducibility, and lifetime. It is shown that the use of macro-porous supports with a smooth, defect-free deposition surface for membrane dip coating, in combination with purified precursor sols and clean processing, leads to major improvements in micro-structural homogeneity and properties of the membranes. Large particle contaminants/agglomerates can be effectively removed from precursor sols by high-speed centrifugation or ultrasound-assisted screening. While centrifugation is the most practical method for routine use, filter screening provides a more complete removal of agglomerates and larger particles (>80 nm Ø). Application of the purified membrane precursor sols results in membranes in which connected pore defects (pinholes) are no longer detectable for a membrane thickness of >500 nm thick. The almost complete absence of connected pinholes, obtained with improved membrane processing, is demonstrated by near-100% retention of CaCl₂ in aqueous solution by nano-filtration. The γ-alumina processing experience, collected in our group in the years 2002–2006, is accumulated in a 34-step processing protocol for γ-alumina membranes with a >500 nm single layer thickness.