Efficient separation of phenol from model oils using environmentally benign quaternary ammonium-based zwitterions via forming deep eutectic solvents

•Environmentally benign quaternary ammonium-based zwitterions were designed to form DESs.•Separation of phenol from oils via forming DES can avoid environmental pollution.•The zwitterions are free from halogen anion, avoiding its corrosion to steel equipment.•The maximum phenol extraction efficiency is 94.6% by using l-carnitine.•The zwitterions can be regenerated and reused without losses in extraction efficiency and extractant mass.

The separation of phenolic compounds from oil mixtures is widely used in the chemical industry. In this research, we found that two types of environmentally benign quaternary ammonium-based zwitterions, betaine and l-carnitine, could be used as new extractants for the separation of phenol from model oils by forming deep eutectic solvents (DESs). The effects of extraction time, temperature, mole ratio of zwitterion to phenol and initial phenol content on the efficiency of phenol separation were investigated. The results indicated that betaine and l-carnitine could form DESs with phenol, and the DESs were insoluble in model oils. Phenol in model oils could be extracted with extraction efficiencies up to 94.6% at an l-carnitine:phenol mole ratio of 0.4 (an l-carnitine:phenol mass ratio of 0.68) and 298.2 K. Phenol in DES could be recovered using an anti-solvent, and betaine and l-carnitine could be regenerated and reused for 4 cycles without obvious decreases in extraction efficiency and extractant mass, and they were not changed in their structures after reuse. The separation mechanism was investigated using FT-IR, which showed that betaine and l-carnitine formed DESs with phenol through hydrogen bonding.