Thermal Stability of Ammonium-Based Thiocyanate Protic Ionic Liquids

The thermal properties of a series of recently synthesized protic ionic liquids based on the thiocyanate anion [SCN] and mono-, di-, and trialkylammonium cations, namely, methylammonium ([MA]), n-decylammonium ([DecA]), 2-phenylethylammonium ([PhEA]), 2-aminoethylammonium ([EDA]), diethylammonium ([DEA]), di-n-butylammonium ([DBA]), and tri-n-butylammonium ([TBA]), are examined. According to differential scanning calorimetry data, [DEA][SCN] and [EDA][SCN] are room-temperature ionic liquids, [DecA][SCN], [DBA][SCN], [TBA][SCN], and [PhEA][SCN] melt below 323 K, and [MA][SCN] melts below 373 K. Thermogravimetric analysis shows that the thermal stability of the studied ionic liquids under dynamic heating conditions exhibits no general correlation with the number of substituents on the nitrogen atom or with the alkyl chain length. However, primary ammonium thiocyanates have a worse long-term thermal stability. They undergo chemical changes when kept for several hours at temperatures between 353 and 373 K, which is well below the mass loss temperature in dynamic experiments. Secondary and tertiary ammonium salts do not decompose even at the temperature at which they start to lose mass due to evaporation. The operating temperature windows for thiocyanate protic ionic liquids have been determined. Among the studied liquids, [DEA][SCN] can be used within the widest temperature range (from 266 to 480 K) in various applications that require long-term thermal stability.