Theoretical exploration and experimental investigation of acidic deep eutectic solvents as green solvents to separate carbazole from model crude anthracene oil

Carbazole is a highly value-added compound derived from crude anthracene. Deep eutectic solvents (DESs) have been extensively used as promising green solvents for the separation of target compounds from mixture. In this work, 135 acidic DESs, formed by combining 9 quaternary ammonium salts with 15 common acids, were screened based on the conductor-like screening model for real solvents (COSMO-RS) model. The four top-ranked DESs, including tetraethylammonium chloride (TEAC)/2 acetic acid (AA), TEAC/2 propionic acid (PA), tetrapropylammonium chloride (TPAC)/2AA, and TPAC/2PA were identified as suitable extractants for carbazole separation based on their performance index (PI). Among them, TEAC/2AA had the best separation performance for carbazole due to its highest PI value (1448.19). Liquid–liquid extraction experiments were performed, and the results were in good accordance with those predicted by COSMO-RS calculations. Therefore, TEAC/2AA was determined as the most promising DES for the carbazole separation. The extraction conditions of TEAC/2AA were optimized. The extraction efficiency (EE) and selectivity (SE) of carbazole were 97.67 % and 66.64 %, respectively, under the optimal extraction conditions (25 ℃, 5 min of extraction time, DES-oil mass ratio of 1:5, initial carbazole concentration of 5000 ppm). Additionally, the deionized water was employed to regenerate the DES and recovery of carbazole. The recycling performance of promising DES was also investigated. Finally, quantum chemical calculations and Fourier Transform Infrared spectroscopy (FT-IR) analysis were conducted to explore the separation mechanism. The results indicated that the separation of carbazole by DESs were predominantly dominated by hydrogen bonds and van der Waals interaction. This work provided a valuable reference for the selection and design of DES extractants to separate other high value-added components from mixture.