Theoretical and experimental exploration for efficient separation of carbazole from anthracene oil with quaternary ammonium salts via forming deep eutectic solvents

Carbazole is an important organic chemical raw material with high added value, mainly derived from crude anthracene. In this research, three quaternary ammonium salts (QASs), containing triethylmethylammonium chloride (TEMAC), tetraethylammonium chloride (TEAC) and tetrapropylammonium chloride (TPAC), were innovatively used to separate carbazole from model anthracene oil by forming deep eutectic solvents (DESs). The results indicated that TEAC showed the highest separation ability among three QASs. Afterwards, the influences of mass ratio of TEAC to model anthracene oil (MO), stirring time, extraction temperature and initial concentration on the separation effect of carbazole were investigated. Under the optimized conditions (25℃, 25 min, mass ratio 1:5), the extraction efficiency (EE), distribution coefficient (DI) and selectivity (SE) of carbazole could reach up to 91.25 %, 24.26 and 66.09 %, respectively. The purity and yield of carbazole product separated from model anthracene oil were 91.02 % and 76.89 %, respectively. Moreover, carbazole with purity of 90.42 % could be separated from real crude anthracene with TEAC under the above optimized conditions. In addition, environment friendly water was used to regenerate the TEAC. After recycled for five times, the extraction ability of TEAC for carbazole did not change obviously. Finally, the hydrogen bond (NH…Cl) and van der Waals interaction (CH…π) existed between QASs and carbazole were revealed by FT-IR analysis and quantum calculations. The green and novel method presented in this work has implications for the separation of other high value-added products from coal tar.