Résumé
The design of metal-complexing copolymer architectures is essential to enable solvent-free recovery of critical metals, and of interest for a large number of applications. In this study, the lithium transport efficiency of benzo-12-crown-4-ether (B12C4) from various salts (LiNO3, LiOAc, Li2SO4 and Li2CO3) in supercritical carbon dioxide (scCO2) was investigated. Among these salts, only Li+ from LiNO3 was effectively complexed by B12C4 in scCO2. As both B12C4 and the [B12C4-Li]NO3 complex are poorly soluble in scCO2, a scCO2-philic gradient polymer, poly(B12C4 ethyl methacrylamide-grad-1,1,2,2-tetrahydroperfluorodecyl acrylate) [P(B12C4EMAAm-grad-FDA)] was synthesized by RAFT polymerization. In this copolymer, the FDA unit is CO2-philic, while B12C4EMAAm acts as a metal-complexing group. The solubility of the copolymer was determined by cloud point measurement and compared to that of a PFDA homopolymer. The lithium recovery yield from lithium nitrate, quantified by inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis, reached 83 % under supercritical conditions (40 °C and 250 bar) in the presence of a small amount of water (molar ratio [water]/[LiNO3]=7.6), whereas only 25 % was recovered under dry conditions. The Li+ transport efficiency of the copolymer was also evaluated in the presence of cobalt ions. Using a mixture of lithium nitrate and cobalt nitrate hexahydrate (molar ratio [B12C4EMAAm]:[Li]:[Co]=3.6:1:1), recovery yields of 46 % and 84 % for lithium and cobalt were obtained, respectively. Despite its lack of selectivity toward lithium, P(B12C4EMAAm-grad-FDA) demonstrates strong potential as a complexing ligand for both lithium and cobalt under scCO2 conditions.