Hollow fiber membrane preparation with aluminum nitrate post-treatment for hydrogen separation

Asymmetry polysulfone hollow fibers were extruded using dry/wet phase inversion method. A novel aluminum nitrate post-treatment process was introduced aiming to enhance the hydrogen separation performance of polysulfone hollow fiber. This post-treatment process has successfully improved both permeability and ideal selectivities simultaneously – an ideal performance improvement. The hydrogen permeability of post-treated polysulfone hollow fiber increased more than twice as compared to as-spun polysulfone hollow fiber, while the ideal selectivity doubled. Results from EDX analysis suggested the presence of aluminum cluster on the surface of polysulfone hollow fiber membrane, and ATR-FTIR as well as DSC analyses shows properties modification of polymer. New O – H bonding in IR spectra, presence of dehydration temperature at approximately 136oC in thermogram, and the detection of aluminum element with element distribution images analysis were observed, suggesting that the aluminum cluster could be in the form of hexaaquaaluminum. During the post-treatment process, the ionic charge of highly protonated hexaaquaaluminum ion in salt solution initiated a dipole moment with polymer chain near surface, causing chain rearrangements, resulting in formation of crystallite phase brush-like polymer chain. FESEM analysis has evidenced the shift from nodular skin structure to distinct and thicker dense skin, supporting the rearrangement of polymer chain after post-treatment process. The posttreatment was then applied on different as-spun hollow fibers extruded with different extrusion parameters, where hollow fiber extruded with different inner coagulant compositions and flow rates, air gaps as well as draw ratio. The morphology and gas separation performance of these hollow fibers were studied and results showed that hollow fiber extruded using inner coagulant with moderate non-solvent strength at low flow rate, extruded through a moderate air gap length with low draw ratio has high hydrogen separation performance – positioned above the Robeson plot H2/N2 permeability/selectivity tradeoff line. Result also showed that hollow fiber extruded at high elongational stretch contributed by either long air gap or high draw ratio could potentially reduce the density of segregated chain-ends at surface of hollow fiber. This will result in a less effective post-treatment process, and subsequently decreases the hydrogen separation performance. This research has brought membrane technology a step closer towards the production of polymeric hollow fiber for ultrapure hydrogen separation.