Mechanistic Insights and Structure-Efficiency Relationship of Amide Extractants in Lithium Extraction from High Mg/Li Ratio Salt Lake Brines

Amide compounds were employed as stable, environmentally friendly, and highly efficient extractants in hydrometallurgy, which exhibit considerable potential for lithium recovery from high Mg/Li ratio salt lake brines. To investigate the structure-efficiency relationship (SER) to inform the development of novel amide-based lithium extractants, 40 amide compounds were designed, synthesized, and characterized herein. The SER was determined via extraction efficiency evaluation under the optimized conditions for N,N-bis(2-ethylhexyl)acetamide (N523), and the extraction mechanism was clarified through integrated experimental and quantum calculation. The results demonstrate that most amides exhibit excellent lithium extraction efficiency and favorable Mg/Li separation performance. The maximum single-stage lithium extraction efficiency reaches as high as 93%. Molecular weight, structural rigidity, electron-withdrawing effect, and steric hindrance all act as key factors governing the SER within each series, among which the electron-withdrawing effect exerts the biggest influence. The existence of the [FeCl4]- anion in the extraction mixture generated by the representative N,N-dibenzylbenzamide (E25) was confirmed, and the first direct evidence for the integrated conjugated configuration of the amide group in the extraction complex was obtained via XPS analysis. Overall, this work delivers essential theoretical insights for designing and developing advanced amide extractants, promoting the high-efficiency utilization of lithium resources in salt lakes.