Highly efficient separation of indole from model wash oil using tetraethyl ammonium amino acid ionic liquids

Indole is an important chemical that can be directly obtained from wash oil mixtures. In this work, several tetraethyl ammonium amino acid (TAAA) ionic liquids (ILs) were designed to separate indole from model wash oil through forming hydrogen bonds. The effects of important factors, such as stirring time, TAAA type, indole concentration, temperature, and TAAA:indole mole ratio, on indole separation were investigated. It is found that these TAAAs can separate indole from model wash oil with separation efficacy up to 98.0% (for tetraethylammonium L-alanine ionic liquid), and the ultimate indole concentration is 1.6 g/dm3. This separation process takes less than 5 min. Also, for one kind of TAAA, the ultimate indole concentrations are almost constant despite different initial indole concentrations. The maximum distribution coefficients of indole are 200.9 at the studied conditions. We have also found that water content in TAAA shows a negative effect on indole separation, which inspired us to design a process for TAAA regeneration using water. After regeneration, TAAA can be reused, and the separation efficacy of indole is not significantly reduced. The FT-IR results show that there are hydrogen bonds between TAAAs and indole. Finally, we compared the TAAA method to other methods and demonstrated the advantages.