Synthesis and properties of triethanolamine-based salts with mineral and organic acids as protic ionic liquids

Abstract

Seven triethanolammonium (tris(2-hydroethyl)ammonium)-based protic ionic liquids (PILs) and three molten salts were synthesized by proton transfer reaction from carboxylic and inorganic acids to triethanolamine (TEOA). The PILs were characterized by 1H NMR, 13C NMR, 1H/15N NMR and FT-IR spectroscopic methods. The thermal behavior of the synthesized PILs was studied, as well as the temperature dependence of their electric conductivity, viscosity, and their electrochemical stability window (ECW). The melting temperature (Tm) of the salts obtained in this work and of most of the TEOA salts with organic acids reported in literature is within the range of 313–373°C. Tm of most PILs with inorganic acids is higher than 100°C, besides, they have a narrow temperature range in liquid state. At the temperatures >90°C, triethanolammonium trifluoroacetate and nitrate have the highest electric conductivity values (~10−2Ohm−1∙cm−1). For the PILs synthesized in this work, just like for TEOA with sulfonic acids, a temperature increase leads to the ECW reduction. The triethanolammonium sulphate salt is the least sensitive to the temperature increase, while in the other PILs the temperature increase from 50°С to 120С makes the ECW 1.4–1.6 times narrower.