Coby CLARKE1, Liem BUI-LE2, Patrick MORGAN1, Jason, P. HALLETT2, Licence PETER1
1University of Nottingham, Nottingham, United Kingdom
2Imperial College London, London, United Kingdom
One important advantage of ionic liquids (ILs) is their high thermal stabilities, which permit their use as liquid media at elevated temperatures (i.e., 100-200 °C). Thermal stability can be extended above 200 °C with dications, which push long-term thermal stability into higher temperature regions where heat transfer fluids and lubricants are needed. Practically all ionic liquids and heat transfer fluids stable at these temperatures have severely restricted structural possibilities and lack coordinating anions or functional groups. Thermal stability effectively incurs a tunability penalty, limiting function to render all liquids simple heat-stable fluids. Our work describes a new family of ionic liquids which have thermal stabilities that rival those of well-known thermally stable dications, with the added advantage of pyridine moieties capable of serving task-specific functions. For most ionic liquids, the only way to off-set their production is to recover, recycle, and reuse, which is unfortunately not practical for most functional ionic liquids because of their compromised stabilities that lead to the build-up of decomposition products over time. Hence, our thermally robust family of ionic liquids show that durability of functional ionic liquids is possible, which validates decades of research aimed toward exploiting the task-specific nature of ionic liquids as tunable, designer solvents. Since solvents can make up the bulk of a chemical process, thermally robust solvents can help prevent significant quantities of waste.
In this work, we present three structurally related series of dicationic pyridine salts (Figure 1),1,2 which have been characterized by a wide array of techniques to link thermal and electronic properties to structural variation. We show that phase transitions and thermal stabilities are significantly influenced by small structural changes, and we identify several new candidates for high-temperature-based applications. We show that the electron density, and therefore the electron donating ability, of the pyridine functional group can be controlled by structural variation of cations and anions. Dissolution of Zn(NTf2)2 in one functional dicationic IL (Figure 1b; [NTf2]-) is demonstrated, and the resulting solutions are characterised to demonstrate their liquid properties (DSC), thermal stabilities (TGA), and the coordination of the functional group to the metal centre (by XPS). This is the first example of a thermally stable, functional IL with the potential to reclaim the tuneable, task-specific nature of ILs at elevated temperatures.
1 C. J. Clarke, L. Bui-Le, J. P. Hallett and P. Licence, ACS Sustain. Chem. Eng., 2020, 8, 8762–8772.
2 C. J. Clarke, P. J. Morgan, J. P. Hallett and P. Licence, ACS Sustain. Chem. Eng., 2021, 9, 6224–6234.