Separation of indole by designed ionic liquids with dual functional chemical sites: Mechanism exploration and experimental validation

Abstract As a hopeful solvent, ionic liquids with tremendous combination of functional chemical sites in the cations and anions can make the separation process efficiently and selectively. Thus, to test the effect of the functional chemical sites in the cations and anions on extracting typical heterocyclic N-compound, indole was adopted as the representative compounds due to its high content and valuable application. In present work, NMP-based IL, [EtOHpyr][BuRA], with dual functional chemical sites was designed and employed to separate indole from diverse resources. The mechanism for the separation process was explored, the IL is evaluated to separate indole with the polarity analysis based on the COSMO-SAC model. Molecular dynamics simulation analysis contributed to understand the H-bonds interaction between indole and IL more indicative. The results demonstrated that the H-bonds between indole and dual functional chemical sites in [EtOHpyr][BuRA] served as the key driving force during the separation process. To validate the effect of functional chemical sites, [EtOHpyr][BF4], [Etpyr][BuRA] and [Etpyr][BF4] were also used for comparison. Furthermore, the influence of extraction of NMP-based ILs on indole, mass ratio (IL: model oil) and extraction temperature were investigated. Under the optimal conditions (25 ℃, mass ratio = 1:5), [EtOHpyr][BuRA] showed the best performance among those four ILs, which extraction efficiency can reach up to 95.15 wt%. The results agreed with the simulation results well. This work can provide an efficient designed ILs for the extraction of indole.