Highly efficient lithium separation from sodium-rich mother liquor enabled by novel phenylazophenol derivatives

Six new phenylazophenol derivatives with tunable substituents were designed to address highly selective Li + /Na + separation. • Structure-performance analysis identifies typical phenol HS-5 as optimal extractants and establishes clear design rules. • The new HS-5/TOPO synergistic system achieves 95.3% single-stage extraction and 99.6% Li + recovery in two stages. • The mechanism underlying the high Li + /Na + selectivity and stability was elucidated by combined DFT and experimental analyses. As brines creasingly constitute source of lithium, the selective extraction of Li + from Na + -rich, high-salinity matrices has become a critical challenge in materials design. In this study, six phenylazophenol derivatives with systematically tunable were designed and synthesized to explore Li + /Na + separation from lithium-containing mother liquor, with structures by 1 H NMR and mass spectrometry. Systematic evaluation of electronic and steric substituent effects established clear structure–performance relationships, identifying 2-phenylazo- 4-trifluoromethylphenol (HS-4) and 4-methanesulfonyl-2-phenylazophenol (HS-5) as the most efficient extractants. A hetero-coordination synergistic extraction system based on HS-5 and trioctylphosphine oxide (TOPO) was further developed and optimized. The optimized system achieved a single-stage Li + extraction efficiency of 95.3%, a Li + /Na + separation factor of 293, and a stripping efficiency of 95.4%. Two-stage cross-flow extraction enabled 99.6% Li + recovery with only 9.3% Na + co-extraction, demonstrating that near-complete lithium removal can be realized with minimal stages. The HS-5/TOPO system retained 79.9% extraction efficiency over ten extraction-stripping cycles without significant degradation. Mechanistic investigations revealed a 1:2 Li + –ligand stoichiometry by Job's method, while DFT calculations showed that HS-5 coordinates to Li + in a bidentate manner through phenolate-O and azo-N, with two P O donors from TOPO completing the distorted tetrahedral coordination environment. These findings underscore the considerable potential of phenylazophenol-based systems for efficient lithium recovery from brines. Download: Download high-res image (209KB) Download: Download full-size image