Highly efficient extraction of indole from model wash oil by using environmentally benign deep eutectic solvents

Indole is an important organic synthesis intermediate derived from the wash oil fraction in coal tar. Deep eutectic solvents (DESs) have been widely used in the separation field due to their unique properties such as easy preparation, low cost, green and high extraction efficiency, and reusability. In this study, five DESs, including tetramethylammonium chloride/ethylene glycol (TMAC/EG), triethylmethylammonium chloride/ethylene glycol (TEMAC/EG), tetraethylammonium chloride/ ethylene glycol (TEAC/EG), tetrapropylammonium chloride/ethylene glycol (TPAC/EG), tetrabutylammonium chloride /ethylene glycol (TBAC/EG) were prepared and used to extract indole from model wash oil. The results showed that TPAC/EG exhibited the best extraction ability under the same conditions. Then, the main experimental parameters such as mass ratio of DES to model wash oil, extraction temperature, extraction time, standing time and initial concentration of indole, were investigated when using TPAC/EG as extractant. In optimal conditions (25℃, 5 min, mass ratio 1:8), the extraction efficiency, selectivity and distribution coefficient for indole were 90.51%, 79.21% and 61.12, respectively. Moreover, removing neutral oil with petroleum ether, the purity of indole further reached 96.63%, which was much higher than that in the previous studies reported. In addition, accessible and environmentally friendly water instead of diethyl ether was used to regenerate the DES. Reused four times, the TPAC/EG extraction ability for indole almost did not change. Finally, the mechanism of extraction process was investigated by FT-IR analysis. This work provides a new route for extraction and recovery indole from wash oil.