Studies on IR spectroscopy and quantum chemical calculation of chloroaluminate ionic liquids acidity

The acidity of chloroaluminate ionic liquids and the interaction mechanism of Lewis acid site Al₂Cl₇^- of chloroaluminate ionic liquid and pyridine were experimentally investigated by IR characterization by using pyridine as molecular probe and increasing pyridine adsorption, and theoretically confirmed by quantum chemical calculations at density functional theory (DFT) and ab initio levels. It was found that the anions, Al₂Cl₇^- and AlCl₄^-, which could withdraw lone pair electrons of pyridine, were characteristic of Lewis acid. Therefore, they displayed pyridine coordinated to Lewis acidic site using pyridine as probe. The acidity of Al₂Cl₇^- was found stronger than that of AlCl₄^- by analyzing TR absorption frequency, bond length and charge distribution. The mechanism of forming and evolvement of the Lewis acid site Al₂Cl₇^- of chloroaluminate ionic liquid was proposed. When the amount of pyridine is small, only the adsorption state of Py-Al₂Cl₇^- exists. The highly Lewis acidic adsorption state of Py-Al₂Cl₇^- complex was converted into Py-AlCl₄^- complex and Py-AlCl₄^- complex with increasing pyridine contents, leading to the changes in IR absorption spectra.