Purification of Ionic Liquid Solvents in a Self-Optimizing, Continuous Microfluidic Process via Extraction of Metal Ions and Phase Separation

Industrial applications of ionic liquids (ILs)─solvents that can serve as green alternatives to volatile organic compounds─are often hampered by their high cost. Solvent recycling provides a feasible pathway to recover IL solvents to reduce lifecycle costs. Herein, we demonstrate a continuous microfluidic process to purify metal-ion-loaded IL solvents, wherein Fe(III) ions are extracted from a prototypical IL, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMIM-NTf2), to deionized (DI) water with subsequent membrane separation of the IL and aqueous phases. Inline analytical tools, design of experiment statistical optimization, and a self-optimizing, modified Nelder-Mead simplex algorithm facilitate locating the best parametric operating conditions to optimize both ion extraction and physical phase separation. This process was then adapted to a more challenging purification application: recovery of the IL trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethyl-pentyl)phosphinate (Cyphos 104) from the rare-earth metal Nd(III). This application demonstrated that optimized conditions obtained from a single stage could be applied across a multistage process. Together, these results demonstrate that statistical and inline optimization tools can be used to identify working parameters for different flow systems with a variety of governing fluid properties, for example, viscosity and interfacial tension.