Anionic structural effect in liquid–liquid separation of phenol from model oil by choline carboxylate ionic liquids

Abstract Phenolic compounds exist in crude oil as pollutants, and their removal is vital important for the refining and further application of oils. In traditional separation approaches, strong acid and strong base have to be used to remove these compounds, which may cause serious environmental problems. In this work, 19 kinds of cholinium ionic liquids have been developed to separate phenol from model oil by liquid–liquid extraction. Structural effect of anions of the ionic liquids in the separation is systematically investigated. It is found that depending on the chemical structure of ionic liquids, phenol can be removed from toluene with single-step removal efficiency from 86 to 99% under optimal conditions. The type of substituent groups and the -CH 2 number between two carboxylates have obvious effect on the removal efficiency, and more hydrophilic ionic liquids have a stronger extraction performance for phenol. Furthermore, thermodynamic, 13 C NMR, 1 H NMR and density functional theory calculations have been performed to characterize the extraction process and to understand the extraction mechanism. It is shown that the extraction of phenol from oil to ionic liquid is a favorable process, and this process is mainly driven by enthalpy change. The formation of the hydrogen bond between anion of the ionic liquid and -OH of phenol is the main driving force for the extraction of phenol from oil to the ionic liquids.