CO2 as a Separation Switch for Ionic Liquid/Organic Mixtures

Abstract

A novel technique to separate ionic liquids from organic compounds is introduced which uses carbon dioxide to induce the formation of an ionic liquid-rich phase and an organic-rich liquid phase in mixtures of methanol and 3-butyl-1-methyl-imidazolium hexafluorophosphate ([C₄mim][PF₆]). If the temperature is above the critical temperature of CO₂ then the methanol-rich phase can become completely miscible with the CO₂-rich phase, and this new phase is completely ionic liquid-free. Since CO₂ is nonpolar, it is not equipped to solvate ions. As the CO₂ dissolves in the methanol/[C₄mim][PF₆] mixture, the solvent power of the CO₂-expanded liquid is significantly reduced, inducing the formation of the second liquid phase that is rich in ionic liquid. This presents a new way to recover products from ionic liquid mixtures and purify organic phases that have been contaminated with ionic liquid. Moreover, these results have important implications for reactions done in CO₂/ionic liquid biphasic mixtures.