Thermal decomposition of ionic liquids

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. 

The determination of the right thermal decomposition temperature of an ionic liquid is not trivial. It is quite obvious from the above mentioned aspects that the thermal stability determined in a TGA experiment will be a strong function of the ionic liquid s quality, with many impurities significantly reducing the stability. Moreover, Ngo et al. [25] have shown that the thermal decomposition of ionic liquids, measured by TGA, varies depending on the sample pans used in some cases increased stabilization of up to 50 °C was obtained on changing from aluminum to alumina sample pans. Finally, and most importandy, the thermal decomposition of... 

Heym, F., Etzold, B.J.M., Kern, C., and Jess, A. (2011) Analysis of evaporation and thermal decomposition of ionic liquids by thermogravimetrical analysis at ambient pressure and high vacuum. Green Chem.(accepted). doi 10.1039/C0GC00876A... 

Figure 10 shows the steel covered with a layer of products of thermal decomposition of ionic liquid. In the presence of MBT that layer is more stable while in absence of MBT numerous pits are visible. 

Amariei, D., Courtheoux, L., Rossignol, S. Kapp>enstein, C. (2007). Catalytic and thermal decomposition of ionic liquid monopropellants using a dynamic reactor, Chem. Eng. Process., Vol. 46, No. 2, pp. 165-174... 

Kroon, M.C. Buij, W. Peters, C.J. Witkamp, G.J. (2006). Decomposition of ionic liquids in electrochemical processing. Green Chem., 8, 241-245, ISSN 1463-9262 Kroon, M.C. Buijs, W. Peters, C.J. Witkamp, G.-J. (2007). Quantum chemical aided prediction of the thermal decomposition mechanisms and temperatures of ionic liquids. Thermochim. Acta, 465,40-47, ISSN 0040-6031 Kumar, S. Ruth, W. Sprenger, B. Kragl, U. (2006). On the biodegradation of ionic liquid l-butyl-3-methylimidazolium tetrafluoroborate. Chim. Oggi, 24, 24-26 ISSN 0392-839X... 

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. 

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... 

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... 

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... 

The upper limit of the liquid range is usually bounded by the thermal decomposition temperature of the ionic liquid, since most ionic liquids exhibit extremely low vapor pressure to their respective decomposition temperatures. In contrast to molten salts, which form tight ion pairs in the vapor phase, the reduced Coulombic interactions between ions energetically restricts the ion pair formation required for volatilization of ILs, leading to low vapor pressures. This leads to high upper temperature limits, defined in many cases by decomposition of the IL rather than vaporization. However, it has also been demonstrated recently that certain, thermally very stable, ionic liquids do evaporate under harsh temperature and vacuum conditions [22]. It has even been demonstrated that mixtures of such ionic liquids can be separated by distillation. 

The deposition of metals has been observed in a number of catalytic processes in ionic liquids and this will be discussed in Section 6.3. This has encouraged the exploration of the possibility of the chemical deposition of metals from solutions in ionic liquids. For instance, lr(0) nanoparticles 2.3 0.4 run in diameter have been prepared by the reduction of [Ir(cod)Q]2 in [BMIMjfPFs] with H2 [24]. In a particularly elegant experiment, palladium nanoparticles were first formed by the thermal decomposition of paUadium(ii) acetate in the presence of triphenylphosphine (to give a regular particle size of 1 run) in [BMIM][Tf2N]. A silica aerogel support was then formed around the nanoparticles by adding (EtO)4Si and formic acid to the mixture [25],... 

The NHC complex (l-ethyl-3-methylimidazol-2-ylidene)silver(i) chloride is an ionic liquid, and was foimd to catalyze the ring-opening polymerization of lactide at elevated temperatures to give narrowly dispersed polylactide of predictable molecular weight [94]. Here, the ionic liquid is acting as a source of the NHC by the thermal decomposition of the silver imidazolylidene complex cation. 

Kroon MC, Buijs W, Peters CJ, Witkamp GJ (2007) Quantum chemical aided prediction of the thermal decomposition mechanisms and temperatures of ionic liquids. Thermochim Acta 465(l-2) 40-47 Lee YG, Chou TC (2004) Ionic liquid ethanol sensor. Biosens Bioelectron 20(1) 33 10... 

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