Separation of Phenol from Oil and In Situ Oxidation of Phenol to p-Benzoquinone by Metal Complex Ionic Liquids

Phenol mainly separated from coal tar is a valuable raw material in industry. Ionic liquids and deep eutectic solvents are environmentally friendly extractants for phenol separation through hydrogen bond interaction. However, the strong interaction makes extractants difficult to regenerate. p-Benzoquinone is a high-value-added product, which can be produced by the selective oxidation of phenol, but the conversion and selectivity are not high. Herein, metal complex ionic liquids were synthesized and used for separation of phenol from oil and conversion of phenol to p-benzoquinone without the addition of catalysts. The ionic liquids can separate phenol efficiently with an extract rate of more than 99%, and the phenol in ionic liquids can be totally oxidized under mild conditions. This work reached a phenol conversion rate of 100% and a selectivity of up to 96.4% for p-benzoquinone under the conditions of methanol as a solvent, oxygen as an oxidant, an oxygen pressure of 3.30 MPa, a temperature of 70 °C, a reaction time of 60 min, and nphenol/n[Emim]2[CuCl4] = 2:1. The regenerated ionic liquid can be reused for both the separation and conversion of phenol with no obvious change. The reaction path and mechanism are speculated and verified. The results show that [CuCl4]2– plays the key role in both the separation and oxidization of phenol. This work opens up a promising direction for phenolic compound separation coupled with efficient conversion to high-value-added products.