Imidazolium-based deep eutectic solvents with multiple interaction sites for enhanced carbazole separation from...

Highly efficient separation of carbazole, a high-value added heterocyclic aromatic compound, from anthracene slag provides dual benefits for both resource recovery and environmental pollution control. However, current extraction methods, despite achieving high yields, suffer from low purity due to poor selectivity. In this study, six weakly basic bifunctionalized deep eutectic solvents (DESs) were designed and synthesized by combining two types of sulfonic acid compounds, benzenesulfonic acid (BSA) and p-toluenesulfonic acid (PTSA), with imidazole (Im), 2-methylimidazole (MIm), and 2-ethylimidazole (EIm). Experimental results revealed a positive correlation between extraction performance and the length of the substituent chain. Notably, DESs containing PTSA exhibited higher extraction efficiency (EE) compared to those with BSA. The optimal weakly basic DES, [3EIm:PTSA], achieved a carbazole EE of 84.3 %, with a partition coefficient (PC) of 6.2 and a selectivity coefficient (SC) of 22.1 at a DES-to-oil ratio of 1:5. A single-step extraction of anthracene slag yielded a carbazole purity of 86.7 % with a yield of 74.5 %. The DES maintained stability after five regeneration cycles using water as the anti-extractant. Proton nuclear magnetic resonance (¹H NMR) analysis and density functional theory (DFT) calculations revealed that the sulfonic acid group and imidazole ring in [3EIm:PTSA] form NH···O and NH···N double hydrogen bonds, collaborating to create specific recognition sites for selective carbazole separation. This work presents a sustainable and efficient strategy for the recovery of valuable heterocyclic compounds from hazardous industrial byproducts.