Available online at www.sciencedirect.com Thermochimica Acta 465 (2007) 40–47 Quantum chemical aided prediction of the thermal decomposition mechanisms and temperatures of ionic liquids Maaike C. Kroon a,b , Wim Buijs c , Cor J. Peters a , Geert-Jan Witkamp b,∗ a Physical Chemistry and Molecular Thermodynamics, Department of Chemical Technology, Faculty of Applied Sciences, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands b Process Equipment, Department of Process & Energy, Faculty of Mechanical, Maritime and Materials Engineering, Delft University of Technology, Leeghwaterstraat 44, 2628 CA Delft, The Netherlands c Catalysis Engineering, Department of Chemical Technology, Faculty of Applied Sciences, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands Received 5 April 2007; received in revised form 22 August 2007; accepted 7 September 2007 Available online 19 September 2007 Abstract The long-term thermal stability of ionic liquids is of utmost importance for their industrial application. Although the thermal decomposition temperatures of various ionic liquids have been measured previously, experimental data on the thermal decomposition mechanisms and kinetics are scarce. It is desirable to develop quantitative chemical tools that can predict thermal decomposition mechanisms and temperatures (kinetics) of ionic liquids. In this work ab initio quantum chemical calculations (DFT-B3LYP) have been used to predict thermal decomposition mechanisms, temperatures and the activation energies of the thermal breakdown reactions. These quantum chemical calculations proved to be an excellent method to predict the thermal stability of various ionic liquids. © 2007 Published by Elsevier B.V. Keywords: Ionic liquids; Thermal stability; Decomposition mechanism and kinetics; Decomposition temperature; Quantum chemical calculations 1. Introduction Ionic liquids (ILs) have been described as potential environ- mentally benign replacements for volatile organic solvents in a variety of applications [1,2]. ILs are organic salts that have melt- ing points close to room temperature. Their most remarkable property is that the vapor pressure of ILs at room temperature is negligibly small. Therefore, ILs are non-volatile and non- flammable. Moreover, ILs have a wide liquid temperature range and a relatively high thermal and electrochemical stability. In principle, it is possible to tune the physical and chemical prop- erties of ILs by varying the nature of the anions and cations. In this way ILs can be made task-specific for a certain appli- cation. Applications include the usage of ILs as electrolytes in electrochemical devices [3–6], the usage as solvents in chemi- cal synthesis and catalysis [1,2,7–9] and separation technology ∗ Corresponding author. Tel.: +31 15 2783602; fax: +31 15 2786975. E-mail addresses: maaike.kroon@gmail.com (M.C. Kroon), G.J.Witkamp@3me.tudelft.nl (G.-J. Witkamp). [10,11] and the usage as lubricants or as heat-transfer fluids [12]. Because these applications often require prolonged operation at elevated temperatures, it is essential to know the long-term stability of ILs. The thermal stability of an IL is manifested by the height of the thermal decomposition temperature and has been extensively studied for a variety of ILs using thermogravimetric analysis (TGA) at a single linear heating rate (10–20 ◦ C/min) [13–25]. It was found that the decomposition temperature strongly depends on the IL structure. ILs with poorly proton-abstracting anions, such the bis(trifluoromethylsulfonyl)imide anion, are most stable to high-temperature decomposition (T decomp ≈ 420 ◦ C), whereas ILs with nucleophilic and highly proton-abstracting anions, such as halides, decompose at much lower temperatures (T decomp ≈ 270 ◦ C) [13–20]. The decomposition temperature of ILs also depends on the type of cation. For example, the imidazolium-based ILs appear to have a better thermal stabil- ity than the pyridinium-based and tetraalkylammonium-based ILs [13,21–23]. Methyl substitution on the 2-position of the imidazolium-cation enhances the thermal stability due to the removal of the acidic hydrogen [14,19,23,24]. The alkyl chain 0040-6031/$ – see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.tca.2007.09.003