Acidic Imidazolium-Based Deep Eutectic Solvents for Efficient One-Step Extraction of Quinoline and Indole from Coal Tar with Minimal Neutral Oil Entrainment

The presence of nitrogen compounds in coal tar significantly poisons hydrogenation catalysts and degrades the fuel quality. However, due to the different properties of basic and nonbasic nitrogen-containing compounds, the conventional extraction and denitrogenation process needs to be distributed, and there is inevitably coextraction of neutral oils, creating an intractable purification challenge. In this study, three acidic imidazolium-based deep eutectic solvents (DESs) were synthesized using p-toluenesulfonic acid (PTSA) as a hydrogen bond donor (HBD) and imidazole (Im), 2-methylimidazole (MIm), or 2-ethylimidazole (EIm) as hydrogen bond acceptors (HBAs). The extraction of quinoline and indole from model oils and coal tar was performed. The extraction efficiencies (EEs) were remarkable, achieving 100% for quinoline and 98.8% for indole at 35 °C with a DES-to-oil ratio of 1:5, along with minimal neutral oil entrainment (NE = 0.5%) using [Im:2PTSA]. Furthermore, it was found that the EE of indole when coextracted with quinoline surpassed that of indole extracted alone. DFT calculation and 1H NMR results revealed that extending the alkyl chain length in imidazoles enhances van der Waals interactions with neutral oil. Interestingly, the EE of [Im:2PTSA] for quinoline remained unchanged across multiple cycles, whereas a slight decrease was noted for indole. However, its efficiency consistently exceeded 90%, even after eight cycles. The application of [Im:2PTSA] to various coal tars demonstrated substantial denitrogenation efficiencies exceeding 60%, with performance variations directly correlated to feedstock composition. These findings underscore the critical importance of developing customized solvent systems tailored to specific oil characteristics for optimal nitrogen removal.