This is examplified for an industrially relevant mixture, suggested by a chemical company. The mixture contains well-known phosphine ligands for homogenous transition metal catalysts, often used in fine chemistry. The preferred phospine ligand A is not oxidised, but in contact with air the oxidised versions B and C are formed. All three phosphines have similar sizes but their chemical affinity depends upon the oxidation degree. When filtrating such a mixture in isopropanol (IPA) using different nanofiltration A-membranes, differences in retentions appear, not achievable with polymeric benchmark membranes. With one type of A-membranes, the oxidised phosphine C can be preferentially permeated through the membrane (negative retention), while A and B are very well retained, allowing a clear separation. Changing the affinity of the A-membrane results in the opposite separation: phosphines A and B pass through the membrane, while C is retained. This example demonstrates the wide flexibility that A-membranes has to offer: membrane performance can be tuned by tailoring the solute-membrane affinity, resulting in affinity-based separations like in chromatography. Unlike chromatography, A-membrane separation is easily scalable, and continuous.
Similar type of affinity separations have been found, valuable for a wide variety of other applications. Further R&D is performed in different projects to elaborate and further develop this potential of A-membranes.