Chiral resolution by polysulfone-based membranes prepared via mussel-inspired chemistry

Reported herein is a chiral resolution polysulfone membrane prepared via mussel-inspired chemistry. Polysulfone membranes were modified with dopamine, which underwent in situ polymerization on the membrane substrate, and β-cyclodextrin was used as a chiral selector. The preparation conditions were optimized and the resultant membrane obtained under these conditions showed desirable water permeability (~ 24.0 L/(m2·h·bar)) and surface hydrophilicity (contact angle is lower than 37.1 ± 3.4°). The polydopamine layer exhibited desirable stability in a series of aqueous solutions with pH values ranging from 4.0-6.0 or in isopropanol for less than an hour (detachment ratio was lower than 1.2%). Characterization of the surface morphology and XPS elemental analysis revealed that the membrane surface was fully coated by polydopamine and a β-cyclodextrin monolayer formed on the surface of the polydopamine coating. The grafting density of β-cyclodextrin calculated from the XPS results was ~ 11.5 mg/m2. The optimal pH value for the resolution of D- or L-tryptophan feed solution was 5.90 and a low concentration of the feed solution provided a high resolution efficiency. The enantiomeric excess (e.e.) value of the membrane for Trp racemic mixture achieved to ~ 3.2% with the feed solution of tryptophan racemic mixture was 5 × 10− 5 mol/L and the operating pressure was 0.1 MPa. After 3 times of isopropanol-washed regeneration processes, the e.e. value was still stable around to ~ 3%. The mussel-inspired chemically modified membrane exhibited the same mechanical properties as the purely polysulfone-based membrane. The methods and results provided in this paper may facilitate the large-scale production of chiral resolution membranes or other chiral separation membranes with higher performance.