Determination of the acidic structure and Lewis activity in an Fe-based ionic liquid with slope analysis of the quantitative pyridine-IR spectrum

Abstract

The acidic structure in an iron-based ionic liquid (Fe-IL) is significantly affected by the valent variation and solvent effect during the catalytic reaction. However, the magnetic property of Fe-IL limits the utilization of commonly used characterization methods that require a magnetic field for the structural analysis of Fe-IL. Herein, a quantitative pyridine-IR spectrum method was developed by analyzing the slope of straight lines (Slope_(X)) in IA_(X) ∼ n_(Py) relationship graph (IA_(X): integrated area of ν^PyH+ or ν^Py-L (cm⁻¹); n_(Py): the mole of added pyridine (mol)) to determine the structural forms of Brønsted and Lewis acid in Fe-IL, as well as the Lewis activity according to the Beer's law. Different Fe-IL systems, such as Fe-IL^aq (from FeCl₃·6H₂O), Fe-IL^non-aq (from non-aqueous FeCl₃) and Fe-IL^non-aq + HCl, were designed and synthesized with similar cationic and anionic structures, in which the Lewis acidity and Lewis activity are significantly influenced by Brønsted acidity. The results showed that the Brønsted acidity in Fe-IL^non-aq is lower than that in Fe-IL^aq, and Fe-IL^non-aq can be transferred into Fe-IL^aq after protonation. The Slope_(L) of FeCl₄⁻ has the value of 0.21–0.28 in Fe-IL^non-aq and Fe-IL^aq before protonation, and protonation could significantly decrease the value of Slope_(L) and Lewis activity, which is mainly caused by the steric hindrance of FeCl₄⁻ with the increased amount of the Brønsted acidic structure of Fe–Cl–HCl. In addition, the solvent effect of polyethylene glycol dimethyl ether (NHD), which is a commonly used organic solvent for the absorption of an acidic gas, on the acidic structure and Lewis activity of Fe-IL was also investigated. We found that relatively free HCl that is combined with the Fe–Cl bond in the Fe-IL^aq/NHD complex can be predicted by analyzing the slope of IA_(X) ∼ n_(Py), and the Lewis activity of FeCl₄⁻ decreases with increasing volume ratio of NHD. It is confirmed by density functional theory (DFT) calculation that the decreased Lewis activity of Fe-IL caused by the protonation and solvent effect is mainly attributed to the decreased electron-withdrawing ability of FeCl₄⁻ and the bond length of the Fe–Cl bond. Furthermore, DFT calculation and CV characterization further demonstrate the reliability of the developed quantitative pyridine-IR method by analyzing the slope of IA_(X) ∼ n_(Py).