Optimizing solvent-resistant nanofiltration (SRNF) polyimide membrane preparation towards sustainable upscaling via casting from dimethyl isosorbide and crosslinking with polyethylenimine

Abstract

Membrane technology has emerged as a sustainable solution for separation processes. Solvent-resistant nanofiltration (SRNF) membranes attract attention by offering high selectivity in the separation of small molecules from organic media. This study investigates the implementation of green chemistry principles in the preparation of SRNF polyimide (PI) Matrimid® 5218 membranes through the use of a biobased solvent, dimethyl isosorbide (DMI), and a non-toxic crosslinker, hyperbranched polyethyleneimine (PEI, MW 800 Da). Four distinguished parameters, including crosslinker PEI concentration (2.5-10 w/v%), crosslinking time (2-30 min), casting temperature (25-50 °C), and coagulation bath temperature (25-50 °C), were examined. The resulting membranes were immersed in dimethylformamide, a good solvent for PI to determine gel content and qualify structural integrity. First, a feed made with a nonsolvent for PI, 2-methyltetrahydrofuran (MeTHF), and zinc tetraphenylporphyrin (ZTPP, MW 678 Da) as solute, was employed to explore the impact of varying preparation parameters on the performance of all membranes, including those with limited solvent stability. Two sets of favorable membrane formulations, each consisting of four membranes were selected for testing in a harsher medium (chloroform/ZTPP): one set with high MeTHF/ZTPP permeance/retention (high performance membranes; varied PEI concentration), and another set with high gel content (high stability membranes; varied crosslinking time, casting and coagulation bath temperature) were analyzed based on their characteristic parameters. Several membranes from both sets showed excellent performances, demonstrating the effectiveness of solvent DMI and crosslinker PEI as a more green system to prepare crosslinked PI membranes.