Ionic liquids thermal decomposition

Ngo et al. [24] have shown that the thermal decomposition of ionic liquids, measured by TGA, varies depending on the sample pans used. Increased stabilization of up to 50 °C was obtained in some cases on changing from aluminium to alumina sample pans. 

Notwithstanding their very low vapor pressure, their good thermal stability (for thermal decomposition temperatures of several ionic liquids, see [11, 12]) and their wide operating range, the key property of ionic liquids is the potential to tune their physical and chemical properties by variation of the nature of the anions and cations. An illustration of their versatility is given by their exceptional solubility characteristics, which make them good candidates for multiphasic reactions (see Section 5.3.4). Their miscibility with water, for example, depends not only on the hydrophobicity of the cation, but also on the nature of the anion and on the temperature. 

Ionic liquids are characterised by the following three definition criteria. They consist entirely out of ions, they have melting points below 100 °C and they exhibit no detectable vapour pressure below the temperature of their thermal decomposition. As a consequence of these properties most ions forming ionic liquids display low charge densities resulting in low intermolecular interaction. Figure 7.1 displays some of the most common ions used so far for the formation of ionic liquids. 

In an ideal case, an ionic liquid dissolves the catalyst and displays a partial miscibility with the reactants under reaction conditions (giving a relatively high reaction rate) and negligible miscibility with the product (giving enhanced selectivity and yield). At the termination of the reaction, the product can be removed by simple decantation without the need to extract the catalyst. This mode of operation eliminates heating and therefore results in reduced loss of catalyst by thermal decomposition (/). 

They generally have reasonable thermal stability. While tetra-alkyl-ammonium salts have limited thermal stability, owing to decomposition via the Hoffmann elimination, [emim][BF4] is reportedly stable up to 300 °C and emim-(CF3S02)2N up to 400 °C. In other words many ionic liquids have liquid ranges of more than 300 °C, compared to the 100 °C liquid range of water. 

Kawasaki H, Yonezawa T, Nishimiu aK, ArakawaR (2007) Fabrication of submiUi-meter-sized gold plates from thermal decomposition of HAuCU in two-component ionic liquids. Chem Lett 36(8) 1038-1039... 

Figure 3.1-2 Thermal decomposition temperature ranges (in °C) for ionic liquids containing l-alkyl-3-methylimidazolium cations. The thermal stability of the ionic liquids depends on the nucleophilicity of the anion.

Amariei, D., Courtheoux, L., Rossignol, S., Batonneau, Y., Kappenstein, C., Ford, M., and Pillet, N., Influence of the fuel on the thermal and catalytic decompositions of ionic liquid monopropellants in 41st AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Tucson, AZ July 2005 (AIAA 2005-3980). 

Although ionic liquids do not boil it is useful to know the temperature range under which they can operate. For many ionic liquids, the onset of decomposition takes place at temperatures exceeding 250°C and often much higher/181 However, 1,3-dialkylimidazolium phosphates were found to slowly decompose below 200°C 19 and when the liquid is composed of less common cations or anions, the thermal stability may be considerably lower. Nevertheless, since the overwhelming majority of catalysed reactions take place at much lower temperatures, thermally induced solvent decomposition tends not to be a problem, although chemically induced decomposition can be a real concern For example, thermal decomposition of [C4Ciim][BF4] takes place a much lower temperature if nucleophiles are present/201... 

Room-temperature ionic liquids are the promising electrolytes for the electrodeposition of various metals because they have the merits of both organic electrolytes and high-temperature molten salts. Ionic liquids can be used in a wide temperature range, so temperatures can be elevated to accelerate such phenomena as nucleation, surface diffusion and crystallization associated with the electrodeposition of metals. In addition the process can be safely constracted because ionic liquids are neither flammable nor volatile if they are kept below the thermal decomposition temperature of the organic cations. 

Table 1 Melting points and thermal decomposition temperatures for a range of selected 1-alkyl-imidazolium ionic liquids. All temperatures in Kelvin, data taken from Fredlake et al. [123] except for superscript a values which are from Wilkes et al. [25]...

However, the use of basic anions is not the sole approach followed to obtaining basic ILs. Liquid salts bearing this additional property can be prepared also by incorporation of a basic center into the cation. This approach generally affords more thermally stable ILs than those based on basic anions, which frequently present relatively low decomposition temperatures. Basic ionic liquids bearing aliphatic or aromatic amines on the side chain(s) have been synthesized and, recently, some of these have been used as both the solvent and base for Heck, copper free Sonogashira, and for homocoupling reactions of terminal alkynes (Figure 4.2). 

An octanuclear europium cluster, formulated as [Eus(/i4-0)(/i3-OH)i2(/t-OTf)i4(OTf)2] (OTf = trifluoromethanesulfonate) was obtained from the thermal decomposition of europium triflate in ionic liquid... 

Ionic liquids usually have good thermal stability, although decomposition can occur at high temperatures. Vacuum pyrolysis studies of 1,3-dialkylimidazolium halide-based ionic liquids showed that the primary decomposition pathway was dealkylation (Scheme 7.3) [13]. This can be prevented by using non-nucleophilic... 

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