Extraction of l-Tryptophan by Hydroxyl-Functionalized Ionic Liquids

In this work, several hydroxyl-functionalized ionic liquids (ILs) were synthesized and used to extract tryptophan from the aqueous phase. The experimental results indicated that the hydroxyl-functionalized ILs presented better extraction ability than the dialkylimidazolium-based ILs. The extraction efficiency was significantly influenced by the pH of the aqueous phase. Moreover, the cationic form of tryptophan preferred to transfer into the IL phase, thereby suggesting a cation-exchange extraction mechanism. Driving forces of the extraction processes included hydrogen bonding, hydrophobicity, π–π stacking, and the screening effect. Among these factors, hydrogen bonding was the main parameter, as suggested by the results of thermodynamic analysis. After extraction, [C1C6OHim]NTf2 could be reused by using the pH swing effect. These findings demonstrate that [C1C6OHim]NTf2 is a good alternative to conventional solvents for recovering tryptophan from the aqueous phase.