Investigation of Physicochemical Properties of Lactam-Based Brønsted Acidic Ionic Liquids

Novel lactam-cation-based Brønsted acid ionic liquids (ILs) were prepared through a simple and atom-economic neutralization reaction between a lactam, such as caprolactam and butyrolactam, and a Brønsted acid, HX, where X is BF₄^-, CF₃COO^-, phCOO^-, ClCH₂COO^-, NO₃^-, or H₂PO₄^-. The density, viscosity, acidic scale, electrochemical window, temperature dependency of ionic conductivity, and thermal property of these ILs were measured and investigated in detail. The results show that protonated caprolactam tetrafluoroborate (CPBF) has a relatively strong acidity with −0.22 of Hammett acidic scale H₀ and caprolactam trifluoroacetate (CPTFA) and pyrrolidonium trifluoroacetate (PYTFA) ILs possess very low viscosities, that is, 28 cP and 11 cP, respectively. An investigation of thermal property showed that a wide liquid range (up to −90 °C), moderate thermal stability (up to 249 °C for 10% of decomposition), and complex polymorphism were observed in these ILs. In comparison to imidazolium-cation-based ILs, the lactam-cation-based Brønsted acid ILs have a relatively lower cost, lower toxicity, and comparable ion conductivity and heat storage density (more than 200 MJ/m³). They have wide applicable perspectives for fuel cell devices, thermal transfer fluids, and acid-catalyzed reaction media and catalysts as replacements of conventional inorganic acids.