Electrospun nanofiber membrane of 14-crown-4 polyimide by in situ grafting diamino crown ether for highly selective Li+ adsorption and separation

The global demand for lithium is increasing rapidly with the blossom of the new energy field. It is imperative to develop a sustainable and efficient approach for recovering Li+ resources from salt-lake brine. In this study, diamino dibenzo-14-crown-4 ether (DAB) was chemically grafted on 14-crown-4 polyimide (14C4PI) by nucleophilic substitution to fabricate nanofiber membrane by electrospinning method. The crown ether loading of DAB-g-14C4PI membrane was increased from 1.38 up to 2.27 mmol/g, and the grafting degree reached 83.4 % by regulating the additive amount of DAB and the reaction time. The Li+ equilibrium adsorption capacity of the DAB-g-14C4PI nanofiber membrane was enlarged from 40.17 (14C4PI) to 60.71 mg/g. The adsorption for Li+ on the membrane was a monolayer chemical adsorption process proved by the adsorption kinetic and isotherm models. The separation factors of the membrane for Li+/Na+, Li+/K+, Li+/Mg2+ and Li+/Ca2+ were 69.24, 79.4, 37.46 and 67.53, respectively. DFT calculations indicated the raised Li+ adsorption capacity was also ascribed to the enhanced adsorption energy from −107.71 to −122.84 kcal/mol, owing to the relatively higher electric charge density (−0.478 vs −0.448 a.u.). In summary, this work put forward a novel method to fabricate a nanofiber membrane with a high crown ether loading for the extraction of lithium resources.