Theoretical insight and experimental exploration of indole extraction from wash oil with deep eutectic solvents

The extraction of indole from wash oil is of economic value and practical significance. In this study, deep eutectic solvents (DESs) are investigated to extract indole from wash oil. Initially, density functional theory calculations were used to explore the association of indole with aromatic compounds in wash oil, and the feasibility of indole extraction with a tetraethylammonium chloride/p-toluenesulfonic acid (TEAC/2TsOH) DES. Then, the influence of the temperature, stirring time, DESs dose, initial indole concentration, and amount of water added were investigated, followed by discussions on the comparison with other extractants and the performance of extracting of indole from real wash oil. The results show that indole associates with aromatic compounds in wash oil through π−π interactions, but that a large electrostatic interaction energy of −52.8 kJ/mol between indole and the DESs is the main driving force for the extraction process. Under optimal conditions, TEAC/2TsOH extracts more than 63.5 ± 3% of indole from wash oil, and its extraction capacity is superior to other extractants.