Stability, of ionic liquids

Catalytic oxidation reactions in ionic liquids have been investigated only very recently. This is somewhat surprising in view of the well loiown oxidation stability of ionic liquids, from electrochemical studies [11], and the great commercial importance of oxidation reactions. Moreover, for oxidation reactions with oxygen, the nonvolatile nature of the ionic liquid is of real advantage for the safety of the reaction. While the application of volatile organic solvents may be restricted by the formation of explosive mixtures in the gas phase, this problem does not arise if a nonvolatile ionic liquid is used as the solvent. 

Figure 2. TGA analysis of supported Co(II)-ionic liquid catalysts (thermal stability of ionic liquid phase).

Further to their role as supporting electrolytes, the conductivity and electrochemical stability of ionic liquids clearly also allows them to be used as solvents for the electrochemical synthesis of conducting polymers, thereby impacting on the properties and performance of the polymers from the outset. Parameters such as the ionic liquid viscosity and conductivity, the high ionic concentration compared to conventional solvent/electrolyte systems, as well as the nature of the cation and... 

Typical low-temperature plasmas are usually only weakly ionized and quasineutral but are thermally in a non-equilibrium state, i.e. the different plasma species (molecules, atoms, ions and electrons) possess different kinetic energy distributions. Because of their small mass electrons acquire much more kinetic energy than atomic or molecular species and thus show an energy distribution which corresponds to a much higher temperature than in the case of much heavier particles. The stability of ionic liquids towards reduction by these hot electrons... 

Allen D, Baston G, Bradley AE, Gornam T, Haile A, Hamblett I, Hatter JE, Healey MJF, Hodgson B, Lewin R, Lovell KV, Newton B, Pitner WR, Rooney DW, Sanders D, Seddon KR, Sims HE, Thied RC. (2002) An investigation of the radiochemical stability of ionic liquids. Green Chem 4 152-158. 

The large miscibility gap observed for an ionic liquid mixed with an aliphatic compound (without the addition of water as in the case of NMP) can be directly used for the separation of aromatic from aliphatic hydrocarbons by liquid-liquid extraction. Besides the miscibility gap, there are other requirements necessary for a successful extractant, such as high selectivity, high capacity, a low solubility of the extractant in the raffinate phase, a simple separation of the extract and the raffinate phase, low viscosity, high chemical and thermal stability and a sufficient density difference. Nearly all these requirements are met by the ionic liquids that have so far been investigated. However, our present knowledge of the thermal and chemical stability of ionic liquids is limited. For example, for some ionic liquids (largely dependent on the anion), hydrolysis does occur. 

Paulechka Y U, Zaitsau D H, Kabo G J, et al. Vapor pressure and thermal stability of ionic liquid 1-butyl-3-methylimidazolium bis (trifluoromethylsulfonyl)amide. Thermochimica Acta. 2005. 439, 158-160. 

Minami I, Kamimura H, Mori S (2007) Thermo-oxidative stability of ionic liquids as lubricating fluids. J Synth Lubr 24 135-147... 

Sowmiah S, Srmivasadesikan V, Tseng MC, Chu YH (2009) On the chemical stabilities of ionic liquids. Molecules 14 3780-3813... 

Another, particularly restricting example of a recent paradigm change concerns the thermal stability of ionic liquids. We know today for certain that thermal stabilities of ionic liquids have been seriously overestimated in the past. Reported stabilities of over 400 °C in early papers [42] do not reflect long-term stabilities but have been obtained in TGA-onset measurements with steep heating ramps. In more recent times, the temperature limits for long-term stability have been found to be more than 100 °C lower than these earlier reported data for the same ionic liquids [43]. 

The densities of ionic liquids are also affected by the choice of the cation and anion. For imidazolium ionic liquids the density decreases slightly as the alkyl chain of the cation increases in length. In addition, the absoption of water by ionic liquids by increasing nitric acid concentration can also cause a decrease in the density (Giridhar et al. 2004). Regarding imidazolium-based ionic liquids, by increasing the alkyl chain it was found that the surface tension decreased due to the orientation of hydrocarbon tails on the surface (Kilaru et al. 2007). The vapour pressure of an ionic liquid is usually unmeasurable at room temperature. Studies on the thermal stability of ionic liquids showed that the type of the associated anion has the primary effect on the thermal stability of the ionic liquids (Holbrey and Seddon... 

Allen, D., Baston, G., Bradley, A. E., Gorman, T., Haile, A., Hamblett, I., et al. (2002). An investigation of the radiochemical stability of ionic liquids. Green Chemistry, 4, 152-158. Angelescu, D., Mercier, B., Siess, D., Schroeder, R. (2010). Microfluidic capillary separation and real-time spectroscopic analysis of specific components from multiphase mixtures. Analytical Chemistry, 82, 2412-2420. 

Seeberger, A., Andresen, A.K., and Jess, A. (2009) Prediction of long-term stability of ionic liquids at elevated temperatures by means of non-isothermal thermogravimetrical analysis. Phys. 

Thus, the goal of this chapter is to compile the main findings in the reduction of oxygen from a fundamental point of view, and its reaction mechanisms within ILs. The chapter will be divided according to families of ionic liquids depicted in Scheme 18.1 (e.g. imidazolium, pyrrolidinium, quaternary ammonium and phos-phonium) since differences in the physical properties, structure and stability of ionic liquids will affect the stability of the electrogenerated species of oxygen. 

The high thermal stability of ionic liquids provides considerable potential to utilize ionic liquids for CO2 capture applications. Most ionic liquids are stable to over 300 °C and therefore less likely to degrade via oxidation, to react with impurities or to be corrosive And because ionic liquids have negligible vapor pressure, this creates a possibility of ionic liquids regeneration over a wide range of temperatures and pressures. Thus, this offers a new opportunity for process optimization that is not achievable using traditional aqueous liquid capture media. 

The literature is full of misleading informations regarding the thermal stabilities of tetrafluoroborate ionic liquids. The results obtained by fast TGA lead to substantial overestimation of the temperature stability of ionic liquids [29-30]. Actually the long-term stability does not exceed 200 °C for most ionic liquids [21, 22, 31]. The metal ions produced in... 

Vapor Pressure and Thermal Stability of Ionic Liquid l-Butyl-3-methylimidazolium bis (trifluoromethy Isulfonyl) amide. 

Meine N, Benedito F, Rinaldi R (2010) Thermal stability of ionic liquids assessed by potentiometiic titration. Green Chem 12 1711—1714... 

Siedlecka, E.M. Czerwicka, M. Stolte, S. Stepnowski, R (2010). Stability of Ionic Liquids in Application Conditions. Curr. Org. Chem - in press, ISSN 1385-2728... 

---