Preparation parameters influencing the performance of SRNF membranes cast from polyimide solutions via SEPPI

Asymmetric nano-sized zeolite-filled polyimide membranes, prepared through the recently reported Solidification of Emulsified Polymer solutions via Phase Inversion (SEPPI) method, were applied in solvent resistant nanofiltration (SRNF). Thanks to the introduction of a short evaporation step prior to coagulation of the cast films, SEPPI allowed the synthesis of highly selective membranes with acceptable solvent fluxes in the separation of rose Bengal (RB) from 2-propanol (IPA). Addition to the casting solution of an organic suspension containing nano-sized silicalite-1 particles had a clear influence on membrane performance as a gradual transition from relatively open towards more dense membranes was observed. This transition coincided with a remarkable morphology change from finger-like to sponge-like membrane structures. An evaporation step of 90 s and an alternative post-treatment, involving solvent exchange coupled to mineral oil impregnation, allowed a significant performance increase of the most promising SEPPI membrane, with IPA permeances around 0.75 l m−2 h−1 bar−1 and RB rejections up to 99%. The performance of the optimized SEPPI membrane compared favourably to the best commercial SRNF membranes, not only in the RB/IPA separation but also in other solvent/solute systems, while its specific structure resulted in increased compaction resistances. Molecular weight cut-off (MWCO) measurements in IPA and hexane, using low molecular weight dyes and alkanes, respectively, revealed MWCO values around 380 and 730 g/mol for this membrane.