Extraction and multi-scale mechanism explorations for separating indole from coal tar via tetramethylguanidine-based ionic liquids

To reduce the impact of nitrogen-containing aromatic compounds on environment during utilization of low-temperature coal tar (LTCT), it is necessary to extract indole from LTCT. In this study, two kinds of tetramethylguanidine-based ionic liquids (ILs) 1,1,3,3-tetramethylguanidine perchlorate ([TMG][ClO4]) and 1,1,3,3-tetramethylguanidine lactate ([TMG][Lac]) were applied as the extractants to extract indole from the LTCT model oil. The ILs were synthesized by a one-step method and characterized by 1H NMR. The extraction performance of the ILs was evaluated using extraction efficiency (EE). The EE values are 95.2% for [TMG][Lac] and 81.9% for [TMG][ClO4] with the extraction time of 30 min and the mass ratio of ILs to model oil (mIL / moil) 0.1 at 298.15 K. The results showed that [TMG][Lac] had higher EE compared to [TMG][ClO4] with the same extraction conditions. Meanwhile, the interactions between the IL [TMG][Lac] and the nitrogen-containing aromatics in the LTCT model oil were studied using the molecular dynamics (MD) simulations and quantum chemical (QC) calculations. The results showed that the H-bond between indole and the ILs was the driving force in extracting indole from the LTCT model oil, which helps design the related ILs for separating the nitrogen-containing aromatic hydrocarbons from LTCT. [Display omitted] •Two TMG-based ILs were prepared as the extractants for separating indole.•[TMG][Lac] is a suitable extractant with better performance for extracting indole.•The extraction mechanism was explored by MD simulations and QC calculations.•[TMG][Lac] can be regenerated and reused without significant change in EE.