Role of cation and alkyl chain length on the extraction of phenol from aqueous solution using NTf2-based ionic liquids: Experimental and computational analysis

Abstract

In this study, the extraction behavior of several ionic liquids (ILs) containing the same anion, bis-(trifluoromethylsulfonyl)imide ([NTf2]) and a range of cations with different alkyl chain length are compared for the removal of phenol from aqueous solution. The [NTf2]− anion was selected due to its hydrophobic nature and low viscosity, which can contribute positively to mass transfer and to the removal efficiency of phenol, in combination with a range of imidazolium, pyrrolidinium and ammonium based cations. To study the effect of alkyl chain length on the extraction of phenol, the alkyl chain length attached to the imidazolium cation was varied from C2 to C12. The effect of process parameters such as IL to water volume ratio (VIL:Vw), phenol concentration, pH and extraction time were investigated. For a given alkyl chain length, the performance of the ILs followed the order: pyrrolidinium>imidazolium>ammonium. Increasing the alkyl chain length of the imidazolium cation resulted in decreasing removal efficiency of phenol. Around 89% removal of phenol was achieved in 3 min, at pH 2 and volume ratio (VIL: Vwater) of 1:3 at room temperature. The extraction of phenol into the ionic liquid phase was confirmed using Fourier Transform Infrared spectroscopy (FTIR). The interactions of phenol with the various studied ionic liquids were modeled using the Conductor like Screening Model for Real Solvents (COSMO-RS). The experimental results were explained based on hydrophobic-hydrophilic interactions between phenol and the ILs as revealed from the σ profiles.