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

Direct separation of lignin fromfully acidic deep eutectic solvent(DES) is challenging because of its high viscosity and acidity. Inthis study, three synthesized DES-resistant polyimides (PIs), namely,B-PI, 6F-PI, and P-PI based on 4 & PRIME;,6-diamidino-2-phenylindole(DAPI) with 4,4 & PRIME;-(4,4 & PRIME;-isopropylidenediphenoxy)bis-(phthalicanhydride) (BPADA), 4,4 & PRIME;-(hexafluoroisopropylidene)diphthalicanhydride (6FDA) and pyromellitic dianhydride (PMDA), respectively,were used to prepare ultrafiltration (UF) membranes for the directseparation of sodium lignosulfonate (LS) from its DES (choline chloride:lacticacid) solutions for the first time. The morphology, pore size, surfaceproperty, degree of swelling, LS separation performance, and long-termstability of the resulting UF membranes were investigated in detail.The average pore sizes were in the range of 3.60-5.06 nm, andthe LS rejections, as well as the DES fluxes, were 44.0-66.4%and 4.71-16.55 L & BULL;m(-2) h(-1), respectively, both higher than those from commercial ZrO2/TiO2 membrane (25.6% and 1.91 L & BULL;m(-2) h(-1)). Among them, the B-PI membrane had the bestseparation performance and excellent operating stability for 20 hdue to its low fractional free volume and poor affinity with DES.Swelling and positive charges of membrane surface in the DES systemdecreased LS rejection. In addition, a higher degree of swelling andaffinity with DES led to an increased DES flux when compared withthe aqueous system. Herein the PI-based UF membranes are promisingfor the direct lignin separation process, especially in an acidicDES system.