Physicochemical Properties of New Binary Protic Ionic Liquids Based on 2-Imidazolidone and Trifluoroacetic Acid

Abstract

We prepared a series of new binary protic ionic liquids (PILs) based on 2-imidazolidone and trifluoroacetic acid and determined their properties such as their viscosity, thermal and electrochemical properties. The results show that these PILs possess a wide liquid state temperature range, relatively good thermal stability, low viscosity and high ionic conductivity. The conductivity of the ionic liquid at a molar ratio of 3:7 was found to be 0.30 S/m and its viscosity was only 7.7 cP at room temperature. With an increase in temperature most of the samples gave better performance. The viscosity and the conductivity of the sample with a molar ratio of 5:5 was 6.1 cP and 0.32 S/m at 60 • C, respectively. Infrared spectroscopy showed that the organic molecules coordinate with the H + cation and CF 3 COO − anion through their polar groups (C=O and NH groups). These properties, therefore, result in these PILs being of interest for application in organic synthesis, especially as catalysts in acid catalytic reactions.Ionic liquids (ILs) possess several peculiar physicochemical properties such as negligible vapor pressure, high thermal stability, good solvation ability toward a wide range of catalysts and acceptable ionic conductivity, and their application in catalytic reactions, organic synthesis and electronic devices or fuel cells has been reported recently. [1][2][3][4][5][6] In particular, protic ionic liquids (PILs) consisting of combinations of Brønsted acids and bases are a new sub-class of ILs that have attracted an increasing amount of attention in many fields because of their similar properties to ILs. [7][8][9][10] The cations of PILs are normally pyridines (i.e, 2-methylpyridine), 11 imidazoliums (i.e, N-ethylimidazolium), 12, 13 pyrrolidiniums (i.e, 1-alkylpyrrolidine), 14-16 catenarian amines (i.e, methylamine) 17-20 or cyclic amines (i.e, caprolactam) 21 and the anions are Brønsted acids (i.e, HNO 3 ), and they are produced by proton transfer from a Brønsted acid to a Brønsted base. Greaves et al. 17 combined primary amine organic anions of the form RNH 3 and R(OH)NH 3 with organic anions of the form RCOO − and R(OH)COO − as well as inorganic anions to prepare a series of PILs. They found that a methylamine/HCOOH (MAF) composite had very low viscosity and high ionic conductivity. However, some problems impede the development and application of these compounds such as stability problems from condensation side reactions to form an amide during the preparation of MAF and strong water absorption of MAF. Lactams such as caprolactam and butyrolactam have been reported to react with Brønsted acids (HBF 4 , CF 3 COOH, PhCOOH, ClCH 2 COOH, HNO 3 or H 3 PO 4 ) through a simple and atom-economic neutralization reaction without a vice-reaction. Lactam-based PILs have a relatively low cost and low toxicity but their ionic conductivities are not satisfactory and are generally lower than those of primary amine-based PILs. 21 We have previously prepared acetamide-based PILs and we showed that the acylamino groups in aceta...