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Phenols have wide applications and much commercial value, and they are obtained from oil mixtures by separation. However, the previous separation agents have low separation efficiency or corrosive halide ions or are difficult to be regenerated. In this work, we designed several tetraethylammonium amino acid (TAAA) ionic liquids (ILs) without corrosive halide ions and found that the ILs could separate phenols from oil mixtures with much higher extraction efficiency and could be regenerated using CO2. The effects of separation time, initial phenol content, TAAA type, water content in TAAA, and phenol type on separation were investigated. It has been found that TAAA can separate phenols with high separation efficiencies, and the maximum separation efficiency of phenol can reach up to 99.0% at a TAAA:phenol mole ratio of 0.60. Meanwhile, ultimate phenol contents can reach as low as 1.40 g/dm3. The initial phenol content almost has no influence on the ultimate phenol contents. For real coal tar oil mixtures, the separation efficiency of phenols can reach up to 98.6%. The TAAAs can be regenerated and reused without significant decreases in separation efficiency of phenols. The separation mechanism has also been proposed based on chemical reactions.

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.