(494e) Thermal Stability of Ionic Liquids Studied by TGA, DSC and GC/MS

Ionic liquids (ILs) are organic salts with low melting points (below 100°C). They are being studied for a variety of applications, including as solvents for reactions, as high temperature heat transfer fluids, and as solvents to separate CO₂ from flue gas. Thermal stability and decomposition are important properties, especially for applications with high temperature environments. This work focuses on the thermal stability of ionic liquids with phosphonium, imidazolium and pyridinium cations. Temperature ramped and isothermal experiments were performed using thermal gravimetric analysis (TGA). Glass transitions (T_g), cold crystallization temperatures (T_cc), melting points (T_m) and heat capacities were measured by differential scanning calorimetry (DSC). The decomposition products were analyzed by gas chromatography/mass spectrometry (GC/MS).

Dynamic and static decomposition experiments of ILs were performed with a Mettler Toledo TGA. The decomposition temperatures under two different gaseous environments (nitrogen and air to represent inert and oxidizing environments) were evaluated. For dynamic decomposition, one important descriptor of thermal stability is the decomposition onset temperature (T_onset) at a temperature ramp rate of 10 °C/min under a nitrogen environment. For the isothermal decomposition, all experiments were performed at temperature above 100 °C for 16 hours under nitrogen or air, the data was evaluated by using a pseudo zero-order reaction mechanism because the weight vs. time curve is linear, and then activation energy and T_1%/1day (the temperature at which 1% weight loss occurs in 1 day) were derived.

Glass transitions (T_g), cold crystallization temperature (T_cc), and melting point (T_m) were measured by DSC from Mettler Toledo. Many ILs tend to subcool easily to form glasses at very low temperatures rather than exhibit crystallization or melting transitions. T_g and T_m values are important for determining the lower limit of operating range where the IL is liquid, and T_onset can be regarded as the upper operating range since IL do not evaporate. Heat capacities can be important for evaluating ILs in heat transfer and thermal storage applications.

The decomposition mechanisms under an inert environment were investigated with a VARIAN Single Quadrupole GC/MS, including electron ionization (EI) and chemical ionization (CI). A chromatoprobe and temperature-programmed 1079 injector were used to introduce solid or large volume liquid samples. For tetra-alkylphosphonium ionic liquids, the two primary decomposition pathways involved are: 1) nucleophilic substitution reaction at the α carbon where a nucleophilic anion displaces a trialkylphosphine group and 2) β-elimination where the β-proton is abstracted by a base in concert with the expulsion of trialkylphosphine (Xie et al. Chem. Mater., Vol. 14, No. 11, 2002). For pyridinium and imidazolium ionic liquid, nucleophilic substitution reaction at alkyl group on the nitrogen atoms is the most likely mechanism for the pyrolysis process (W.H. Awad et al. Thermochimica Acta 409 (2004) 3?11).