The extraction mechanism research for the separation of indole through the formation of deep eutectic solvents with quaternary ammonium salts

Indole is an important industrial substance that can be derived from coal tar. However, traditional acid-base separation method causes serious environmental problems. In this research, the indole separation abilities of different quaternary ammonium salts (QASs) from nitrogen-containing compounds and azeotrope systems were explored. Series of QASs with different substituent groups were investigated through examining the extraction efficiency, including tetraethylammonium chloride (TEAC), betaine hydrochloride (BHC), tetramethylammonium chloride (TMAC), tetraethylammonium bromide (TEAB), and benzyl trimethylammonium chloride (BTMAC). Among them, TEAC was found to be able to form deep eutectic solvents (DES) with indole. The effects of different experimental conditions such as extraction time, extraction temperature, and mole ratio were studied and optimum extraction conditions were obtained. Quantum calculation was employed to explore the extraction mechanism. σ-profiles and σ-surfaces of QASs ions and related compounds were derived from COSMO database and analyzed to explore their interaction forces. The interaction energies between QASs ions and related compounds composed by misfit, van der Waals (VDW) and hydrogen bond (HB) were calculated based on COSMO-RS theory. Using these methods, the separation possibilities of indole through DES formation by QASs can be speculated. Ultimately, the bond lengths and interaction energies between QASs and indole were calculated concretely using Material Studio. The above quantum calculation proved the existence of hydrogen bonds between indole and QASs and the deductions obtained from quantum calculation were consistent with the experimental results. According to the above analysis, the separation method through DES formation was proved to have a relatively high requirement for extractant. The mechanism research in this article provided essential reference for the selection of applicable extractant, and the prediction of the separation efficiency.