(592b) Thermal and Thermodynamic Properties of Ionic Liquids and Molten Salts with High Thermal Stability

Ionic liquids have attracted much attention in the past few decades because of their potential as novel process fluids. Their vanishingly low vapor pressures coupled with the ability to tune their properties through the full pallet of organic chemistry have made them promising alternative replacement solvents for volatile organic chemicals used as reaction and separation solvents. Additionally, their oft reported high thermal stability has made them potential candidates for high temperature reaction and separations solvents as well as heat transfer fluids. However, as many of the ionic liquids begin decomposing between 200 and 300°C, their use as high-temperature fluids is somewhat limited.

To this end, we have synthesized and studied a series of ionic liquids and molten salts that have shown thermal stability up to and above 300°C. We indicate molten salts in addition to ionic liquids as several of the salts have melting point above 100°C, the commonly used upper limit for the term ionic liquid. By judicious selection of cation and anion pairs, the thermal stability of these species can be significantly enhanced.

The species examined include a variety of cations, including peraryl phosphonium and sulfonium cations paired with several anions. In this work we present the melting points of these compounds, their solid and liquid heat capacities (measured via differential scanning calorimetry), as well as their thermal decomposition temperatures (measured via TGA). Additionally, we examine the relative contributions of enthalpy and entropy of fusion on the melting points of homologous series of the compounds. Furthermore, long-term, high-temperature stability studies of several of the species will be presented.

These initial studies will provide a basis by which to tailor additional ionic species and broaden the types of species available for use and study, and guide future work related to energy storage and high-temperature reactions in these non-volatile species.