Optimal separation of phenol from model oils by forming deep eutectic solvents with quaternary ammonium salts

Various quaternary ammonium salts are used to separate phenol from model oil by forming deep eutectic solvents (DESs). The results indicate that the types of quaternary ammonium salt ions and the molecular symmetry of quaternary ammonium have obvious influence on recovery for phenol. Tetraethylammonium chloride (TEAC), which contains suitable ions and symmetry reveals highest phenol recovery in the model oil. The separation mechanism is discussed and the structures of DESs (TEAC/Phenol) are proved on Fourier transform infrared transform (FTIR). To recycle TEAC, diethyl ether is used as anti-solvent to precipitate TEAC from TEAC/Phenol system. The structure of regeneration TEAC is proved on FTIR and ¹H NMR. The recovery for phenol is decreased from 97.45% to 14.30% with the increase of regeneration times. The optimal separation condition was obtained by response surface methodology at 30 °C (separation temperature) for 40 min (separation time) with 3.75 g TEAC (mass of TEAC) in 10 mL model oil (phenol recovery: 99.3%). In contrast to the traditional methods to separate phenol, this proposed method avoids the use of alkalis and acid, and with less effluent.