A highly-efficient lithium adsorptive separation membrane derived from a polyimide-containing dibenzo-14-crown-4 moiety

Crown ethers containing polymer membranes are considered as one of the most promising candidates for selective separation of Li+. Herein, a novel dibenzo- 14-crown-4 ether-based polyimide (Poly(DAB14C4-6FDA)) was synthesized by condensation polymerization between di(aminobenzo)-14-crown-4 (DAB14C4) and 4,4′-(hexafluoro-isopropylidene) diphthalic anhydride (6FDA). The resultant polyimide with a high molecular weight (63 kDa) was employed to fabricate microporous membranes by non-solvent induced phase separation using N,N-Dimethylformamide (DMF) as a solvent and water as coagulation bath. The pure-water permeance of the PI membrane (porosity of 80.5%) reached 580 L/m2 h (LMH) bar−1. Simultaneously, this membrane displayed excellent Li+ adsorption capacity (34.05 mg g−1) owing to the high crown ether content of 1.36 mmol g−1. The Li+ adsorption kinetics followed a pseudo-second-order adsorption equation and the adsorption isotherm data fitted a Langmuir model, indicating monolayer chemical adsorption. The selective separation factors of Li+ to Na+, K+, Mg2+, and Ca2+ were 45.6, 48.3, 23.5, and 41.2, respectively. Adsorption energy results obtained from density functional theory (DFT) calculations were consistent with the experimental results. The membrane also exhibited good adsorption repeatability and stability. The high performance of this novel membrane is ascribed to the sieving and chelating effects of the unique crown ether, and its porous structure.