CO2-Assisted Back-Extraction Method for Ionic Liquid Biphasic Systems

Abstract

Ionic liquid (IL)-mediated extraction methods have drawn much attention due to their reduced environmental impact and improved extraction efficiencies compared to traditional separation methods. However, one long-standing challenge in the application of IL-based extraction processes is the difficulty in recovering high-boiling solutes from the IL phase due to the negligible vapor pressure of ILs and the strong interaction between solutes and ILs. In this study, a new CO2-assisted back-extraction method was developed for the recovery of high-boiling solutes from the IL phase based on the reversible chemical absorption and/or strong physical interaction of CO2 with ILs. We found the distribution coefficient (D) of the typical solute δ-tocopherol in the [P4444][Pro]/DMSO–hexane biphasic system significantly decreased from 10.38 to 2.42 after the introduction of CO2, which greatly reduced the required solvent consumption and the number of back-extractions required for tocopherol recovery. For example, the number of back-extractions required for the recovery of 90% of the tocopherols from the [P4444][Pro]/DMSO–hexane biphasic phase decreased from 24 to 5 with the aid of CO2 absorption, along with a decrease in solvent consumption by approximately 80%. The effects of the structure of the IL, the loading of CO2, the type of cosolvents, and the temperature of the CO2-assisted back-extraction processes were investigated in this study. This proof-of-concept study demonstrates the great potential of CO2-assisted back-extraction as a green method for the efficient recovery of high-boiling solutes from the IL phase.