Insight into the experiment and extraction mechanism for separating carbazole from anthracene oil with quaternary ammonium-based deep eutectic solvents

Carbazole is an irreplaceable basic organic chemical raw material and intermediate in industry. The separation of carbazole from anthracene oil by environmental benign solvents is important but still a challenge in chemical engineering. Deep eutectic solvents (DESs) as a sustainable green separation solvent have been proposed for the separation of carbazole from model anthracene oil. In this research, three quaternary ammonium-based DESs were prepared using ethylene glycol (EG) as hydrogen bond donor and tetrabutylammonium chloride (TBAC), tetrabutylammonium bromide or choline chloride as hydrogen bond acceptors. To explore their extraction performance of carbazole, the conductor-like screening model for real solvents (COSMO-RS) model was used to predict the activity coefficient at infinite dilution (γ∞) of carbazole in DESs, and the result indicated TBAC:EG (1:2) had the stronger extraction ability for carbazole due to the higher capacity at infinite dilution (C∞) value. Then, the separation performance of these three DESs was evaluated by experiments, and the experimental results were in good agreement with the COSMO-RS prediction results. The TBAC:EG (1:2) was determined as the most promising solvent. Additionally, the extraction conditions of TBAC:EG (1:2) were optimized, and the extraction efficiency, distribution coefficient and selectivity of carbazole could reach up to 85.74%, 30.18 and 66.10%, respectively. Moreover, the TBAC:EG (1:2) could be recycled by using environmentally friendly water as antisolvent. In addition, the separation performance of TBAC:EG (1:2) was also evaluated by real crude anthracene, the carbazole was obtained with purity and yield of 85.32%, 60.27%, respectively. Lastly, the extraction mechanism was elucidated by σ-profiles and interaction energy analysis. Theoretical calculation results showed that the main driving force for the extraction process was the hydrogen bonding ((N–H…Cl) and van der Waals interactions (C–H…O and C–H…π), which corresponding to the blue and green isosurfaces in IGMH analysis. This work presented a novel method for separating carbazole from crude anthracene oil, and will provide an important reference for the separation of other high value-added products from coal tar.