Studies on acidity of chloroaluminate' ionic liquids using pyridine as infrared spectroscopic probe

Knowledge of the acidity of ionic liquid (ILs) is critically required for utilization of ionic liquid. The acidity of chloroaluminate ionic liquids was investigated by pyridine (Py) as infrared spectroscopic probe. It was found that when the anion molar composition x of ionic liquid varies within 0. 4-0. 5, ionic liquids exhibits weak Lewis acidity. Infrared (IR) absorption frequencies of ring stretching vibrations for pyridine on Lewis acid Al₂Cl ₇^-, A1C1₄^-, Al₂Cl ₆ and AlCl₃ were assigned, respectively. The frequency and peak area of IR absorption spectra could be indicative of the acidity strength of ionic liquids. In addition, the evolvement mechanism of the Lewis acid site of ionic liquid was studied on the basis of the method of adsorption incremental of pyridine. The results showed that the highly Lewis acidic adsorption state-Py-Al₂Cl₇^- complex was converted into Py-A1Cl₄^- complex and Py-AlCl₃ complex with increasing pyridine contents.