Designing Ionic Liquids with Dual Lewis Basic Sites to Efficiently Separate Phenolic Compounds from Low-Temperature Coal Tar

Although separation of phenolic compounds from coal tar has a good practical application value in industry, the traditional separation method can result in serious environmental problems. In the present work, three ionic liquids (ILs) with dual Lewis basic sites were designed and synthesized by neutralizating 1,1,3,3-tetramethylguanidine (TMG) and corresponding acids ( L -proline, acetate acid, and tetrafluoroboric acid), which were employed to separate phenolic compounds from the model oil and coal tar. The basicity of TMG-based ILs were characterized by a probe molecule through 1 H NMR and quantum chemical calculations. Moreover, the influence of factors on the separation efficiency, such as stirring time, extraction temperature, and the ratio of ILs to model oil, was investigated in detail. The results showed that [TMG][BF 4 ] showed the best extraction efficiency of 98.2%, at a extraction temperature of 30 °C, stirring time of 35 min, and extraction ratio of 1.1 g/4 mL. After being separated, these ILs were recovered through back extraction. Furthermore, the separation mechanism was determined by analyzing the hydrogen bond and chemical bond for ILs and phenolic compounds via UV–vis, FT-IR, and quantum chemical calculations. Thus, TMG-based ILs can be effective in separating phenolic compounds from coal tar and as an alternative extractant in the future.