Efficient separation of phenolic compounds from model oil by the formation of choline derivative-based deep eutectic solvents

Phenolic compounds are an important class of compounds for organic chemical industry. In their traditional production processes, strong acid and base have to be used for the separation of phenolic compounds from oils, which often causes serious environmental problems. In this work, choline derivative salts [N 1,1, n C 2 OH]Cl ( n   =   1, 4, 6, 8) have been used as new extractants for the separation of 26 kinds of phenolic compounds from model oil (toluene) by forming deep eutectic solvents (DESs). It is found that only 16 kinds of phenolic compounds can be removed from toluene with single-step removal efficiency from 28.1% to 94.7%, strongly depending on the chemical structure of phenolic compounds. The nature, position and number of substituent groups in the phenolic compounds have a great effect on their removal efficiency. Far infrared spectroscopy and density functional theory calculations indicate that hydrogen bonds formed between –OH of the phenolic compounds and anion of the choline derivative salts is the main driving force for such DES-based separation. The correlation observed experimentally between structure of phenolic compounds and their removal efficiencies has been discussed from hydrogen bonding in DESs, electrostatic interaction of cation and anion of the choline salts, and solvation of the phenolic compounds and choline salts in the oil.