On the behavior of imidazolium versus pyrrolidinium ionic liquids as extractants of phenolic compounds from water: Experimental and computational analysis

Abstract

The main goal of this work is to compare the ability of aromatic and non-aromatic ionic liquids (ILs) as potential solvents to extract phenolic compounds from aqueous systems. Although these liquid salts have been widely studied in the separation of organic compounds, especially aromatic hydrocarbons such as benzene, toluene, ethylbenzene, and isomer xylenes, their application to separate phenols from wastewater is still much less widespread. For that reason, in this work, the extraction efficiency of phenolic compounds in molecular form (phenol, o-cresol, and resorcinol) from water using non-aromatic (1-hexyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, [HMpyr][NTf2]) and aromatic (1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [HMim][NTf2]) ILs was analyzed and discussed. Firstly, the optimal operating conditions (stirring and settling time, and phase volume ratio Vionic liquid/Vwater) were stablished and then other variables such as initial phenol concentration and temperature were also studied. This work was performed through equilibrium distribution studies and the tracking of the concentration of phenol was carried out by absorbance measurements using a UV/visible spectrophotometer. In order to provide a better understanding of the effect of the cation nature (aromatic and non-aromatic) as well as the role of the phenolic structure on the extraction ability of the ILs, the quantum chemical COSMO –RS method was used to seek an explanation in terms of molecular interactions between the solvents and the phenolic compounds. Overall results support that the aromatic nature of cations does not seem to be the predominant factor driving the extraction process, with hydrogen bonding significantly contributing to competitive solute-solvent interactions which promote the transfer of the phenolic compounds from the aqueous phase to the IL phase.