Acid-Resistant Polyimide Ultrafiltration Membranes for Direct Separation of Lignin from Deep Eutectic Solvents

Direct separation of lignin from fully acidic deep eutectic solvent (DES) is challenging because of its high viscosity and acidity. In this study, three synthesized DES-resistant polyimides (PIs), namely, B-PI, 6F-PI, and P-PI based on 4′,6-diamidino-2-phenylindole (DAPI) with 4,4′-(4,4′-isopropylidenediphenoxy)bis-(phthalic anhydride) (BPADA), 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and pyromellitic dianhydride (PMDA), respectively, were used to prepare ultrafiltration (UF) membranes for the direct separation of sodium lignosulfonate (LS) from its DES (choline chloride:lactic acid) solutions for the first time. The morphology, pore size, surface property, degree of swelling, LS separation performance, and long-term stability of the resulting UF membranes were investigated in detail. The average pore sizes were in the range of 3.60–5.06 nm, and the LS rejections, as well as the DES fluxes, were 44.0–66.4% and 4.71–16.55 L·m –2 h –1, respectively, both higher than those from commercial ZrO 2 /TiO 2 membrane (25.6% and 1.91 L·m –2 h –1 ). Among them, the B-PI membrane had the best separation performance and excellent operating stability for 20 h due to its low fractional free volume and poor affinity with DES. Swelling and positive charges of membrane surface in the DES system decreased LS rejection. In addition, a higher degree of swelling and affinity with DES led to an increased DES flux when compared with the aqueous system. Herein the PI-based UF membranes are promising for the direct lignin separation process, especially in an acidic DES system.