Highly Efficient Separation of Phenolic Compounds from Oil Mixtures by Imidazolium-Based Dicationic Ionic Liquids via Forming Deep Eutectic Solvents

The separation of phenolic compounds from oil mixtures is an important step for further refining or applications of the oil mixtures. In this work, three imidazolium-based dicationic ionic liquids (DILs; 1,2-bis[N-(N′-methylimidazolium)]ethane dibromide, DIL1; 1,3-bis[N-(N′-methylimidazolium)]propane dibromide, DIL2; and 1,4-bis[N-(N′-methylimidazolium)]butane dibromide, DIL3) were synthesized and used to separate phenolic compounds from oil mixtures. The effects of time, temperature, DIL:phenol mole ratio, and initial phenol concentration on separation performance were investigated in detail. It was found that the removal efficiency of phenol followed the order DIL1 < DIL2 < DIL3. DIL3 showed the maximum phenol removal efficiency of 96.6% and the minimum ultimate phenol concentration of 3.9 g/dm3. Initial phenol concentration had little influence on the ultimate phenol concentration. Less than 0.4 in DIL:phenol mole ratio was needed to obtain the highest phenol removal efficiency, and the separation process could be completed within 5 min. These DILs could be reused without a decrease in the removal efficiency of phenol, and the properties of DILs did not change after four cycles. Also, DIL3 was demonstrated to separate phenolic compounds from real coal tar oil with the removal efficiency of 93.1%. Importantly, these DILs showed much smaller solubilities in oil mixtures than monocationic ILs. In addition, these DILs showed higher thermal stabilities than monocationic ILs, which provides a broader range of operation temperature than ever.