Role of Alkyl Group in the Aromatic Extraction Using Pyridinium-Based Ionic Liquids

The performance of N-alkylpyridinium-based ionic liquids with a SCN anion (PylLs) was evaluated for the selective extraction of aromatics from aliphatic hydrocarbons. The aromatic extraction ability of PyILs was greatly enhanced by the presence of a methyl group on the pyridinium ring at the 3- or 4-position, whereas the solubility of the aromatics in the PyILs decreased with increasing the number of methyl groups on the benzene ring. The FT-IR studies revealed that the solubility of an aromatic compound in a PyIL is closely correlated with the degree of aromatic C H bending frequency shift observed during the dissolution of the aromatic compound in the PyIL: the larger the shift, the higher the solubility. The computational calculations on the dispersion interactions between aromatics and PyILs demonstrated that the anion aromatic interaction is much more important than the cation aromatic interaction in determining the aromatic solubility in PyILs, and such anion aromatic interaction can be enhanced by introducing a methyl group at the carbon atom of the pyridinium ring.