Intermolecular Interaction and Extraction Explorations for Separation of High-Boiling Neutral Nitrogen Compounds Using Biodegradable Ionic Liquids

High-boiling neutral nitrogen compounds (HBNNCs), such as carbazole and indole, which are mainly obtained from anthracene and wash oil, are widely applied in chemical and pharmaceutical industries. However, generally, in industry, HBNNCs are separated from anthracene and wash oil by alkali fusion and acid-washing methods, which can lead to a serious impact on the environment. Therefore, it is of significance to develop a sustainable technology to separate HBNNCs. In this work, four biodegradable tetramethylguanidine-based ionic liquids (TMG-based ILs) were synthesized as green solvents to extract HBNNCs. The TMG-based ILs have been synthesized using a one-step method and characterized. The intermolecular interactions between the ILs and HBNNCs were investigated using quantum chemical calculations and molecular dynamics simulations. The results showed that the IL tetramethylguanidine acetate ([TMG][Ac]) had the strongest interactions with carbazole and indole compared to the other TMG-based ILs. The extraction exploration for carbazole and indole using the IL [TMG][Ac] was performed, and the optimized extraction conditions were determined. The results showed that the highest removal efficiency of the IL [TMG][Ac] for separating carbazole and indole from the prepared representative anthracene and wash oil was 94.60 and 91.89%, respectively. Finally, the reusability of the IL [TMG][Ac] was validated. The results showed that the IL [TMG][Ac] can be recovered completely and reused.