Thermal stability ionic liquids

Eerreira AGM, Simoes PN, Ferreira AF (2012) Quaternary phosphonium-based ionic liquids thermal stability and heat capacity of the liquid phase. J Chem Thermodyn 45 16-27... 

The following discussion concerns the thermal liquidus ranges available in different ionic liquids, as functions of cation and anion structure and composition. In particular, those structural features of cation and anion that promote these properties (while providing other desirable, and sometimes conflicting characteristics of the liquid, such as low viscosity, chemical stability, etc.) and variations in liquidus ranges and stabilities are the focus of this chapter. 

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. 

Obviously, the use of a nonvolatile ionic liquid simplifies the distillative workup of volatile products, especially in comparison with the use of low-boiling solvents, where it may save the distillation of the solvent during product isolation. Moreover, common problems related to the formation of azeotropic mixtures of the volatile solvents and the product/by-products formed are avoided by use of a nonvolatile ionic liquid. In the Rh-catalyzed hydroformylation of 3-pentenoic acid methyl ester it was even found that the addition of ionic liquid was able to stabilize the homogeneous catalyst during the thermal stress of product distillation (Figure 5.2-1) [21]. This option may be especially attractive technically, due to the fact that the stabilizing effects could already be observed even with quite small amounts of added ionic liquid. 

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. 

Recently, room temperature ionic liquids (RT-ILs) have attracted much attention for their excellent properties, e.g., wide temperature range of liquid phase, ultra-low vapor pressure, chemical stability, potential as green solvents, and high heat capacities [64,65]. These properties make them good candidates for the use in many fields, such as thermal storage [66], electrochemical applications, homogeneous catalysis [67], dye sensitized solar cells [68], and lubricants [69,70]. 

Significant progress has been made in the application of ionic liquids (ILs) as alternative solvents to C02 capture because of their unique properties such as very low vapour pressure, a broad range of liquid temperatures, excellent thermal and chemical stabilities and selective dissolution of certain organic and inorganic materials. ILs are liquid organic salts at ambient conditions with a cationic part and an anionic part. 

There are many good reasons for applying ionic liquids as alternative solvents in transition metal catalysed reactions. Besides their very low vapour pressure and then-good thermal stability [33], an important advantage is the possibility of tuning then-solubility [34] and acidity/coordination properties [35] by varying the nature of the anions and cations systematically. 

We have also demonstrated that well-behaved quantized charging of gold MPCs is possible in air- and water-stable room-temperature ionic liquids, such as 1-hexyl-3-methylimidazolium tris(penta-fluoroethyl)-trifluorophosphate (HMImEEP), Fig. 30c, d [334, 335]. As ionic liquids have very attractive features, including nearzero vapor pressure, considerable thermal stability, and an electrochemical stability window that often exceeds 4 V, this demonstration is particularly significant from a technological point of view. 

N-halamines, 13 100-101 of ion-exchange resins, 14 401-402 of ionic liquids, 26 845-847 of olefin fibers, 11 229 of organic semiconductors, 22 209 of polyimides, 20 277 of silicones, 22 600 of Teflon AF, 18 340 Thermal stability tests, of organic peroxides, 18 491... 

So far, we have focused on the melting points and polarities of ionic liquids. Like conventional solvents, other properties such as viscosity and density are also very important when selecting a solvent for synthetic applications. Whilst this type of data is well known for other solvents, relatively little has been reported for ionic liquids. Table 4.6 lists available melting points, thermal stability, density, viscosity and conductivity data for the better studied ionic liquids. 

Imidazolium-based ionic liquids (ILs) have been used extensively as media for the formation and stabilization of transition-metal nanoparticles [14—17]. These 1,3-dialkylimidazolium salts (Figure 15.3) possess very interesting properhes they have a very low vapor pressure, they are nonflammable, have high thermal and electrochemical stabilities, and display different solubilities in organic solvents [18-20]. 

Heretofore, ionic liquids incorporating the 1,3-dialkylimidazolium cation have been preferred as they interact weakly with the anions and are more thermally stable than the quaternary ammonium cations. Recently, the physical properties of 1,2,3,4-tetraalkylimidazolium- and 1,3-dialkylimidazolium-containing ionic liquids in combination with various hydrophobic and hydrophilic anions have been systematically investigated (36,41). The melting point, thermal stability, density, viscosity, and other physical properties have been correlated with alkyl chain length of the imidazolium cation and the nature of the anion. The anion mainly determines water miscibility and has the most dramatic effect on the properties. An increase in the alkyl chain length of the cations from butyl to octyl, for example, increases the hydrophobicity and viscosity of the ionic liquid, whereas densities and surface tension values decrease, as expected. 

The cations in ionic liquids are generally bulky monovalent organics. The typical cations of ionic liquids, not including the familiar alkylammonium and alkylphosphonium ions, are shown in Fig. 2. It is primarily the cations, which account for the low melting points of ionic liquids. The dialkylimidazolium ions, such as 1-butyl-3-methyl imidazolium [BMIM], have been widely investigated because low-melting ionic liquids can be made readily from such cations and because of their thermal and chemical stability. 

An ionic liquid can be used as a pure solvent or as a co-solvent. An enzyme-ionic liquid system can be operated in a single phase or in multiple phases. Although most research has focused on enzymatic catalysis in ionic liquids, application to whole cell systems has also been reported (272). Besides searches for an alternative non-volatile and polar media with reduced water and orgamc solvents for biocatalysis, significant attention has been paid to the dispersion of enzymes and microorganisms in ionic liquids so that repeated use of the expensive biocatalysts can be realized. Another incentive for biocatalysis in ionic liquid media is to take advantage of the tunability of the solvent properties of the ionic liquids to achieve improved catalytic performance. Because biocatalysts are applied predominantly at lower temperatures (occasionally exceeding 100°C), thermal stability limitations of ionic liquids are typically not a concern. Instead, the solvent properties are most critical to the performance of biocatalysts. 

Promising developments of ionic liquids for biocatalysis reflect their enhanced thermal and operational stabilities, sometimes combined with high regio- or enantioselectivities. Ionic liquids are particularly attractive media for certain biotransformations of highly polar substrates, which cannot be performed in water owing to equilibrium limitations 297). 

In addition to their use as solvents, surfactants, and biocides, ionic liquids are attractive for use in catalytic reactions due to their ability to activate reactant molecules, the ease of separation from final products, thermal stability, solubility of gaseous reactants, among other properties. Amit Gujar and Mark White (Mississippi State Univ., USA) show, for example,... 

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. 

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

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.

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