The kinetics of thermal decomposition of 1-alkyl-3-methylimidazolium chloride ionic liquids under isothermal and non-isothermal conditions

Abstract

The thermal stability and decomposition kinetics of a series of 1-alkyl-3-methylimidazolium chloride ionic liquids with different alkyl chain lengths (ethyl, propyl, butyl, hexyl, and decyl) were investigated with isothermal and non-isothermal thermogravimetric analysis. Isothermal experiments were conducted at 100, 125, 150, 175, 200, and 225 °C in nitrogen atmosphere. Non-isothermal experiments were conducted from 25−650 °C with heating rates of 5, 10, 15, and 20 ℃/min. Under both heating modes, the ionic liquids with the longer alkyl chains were found to display lower thermal stability. The activation energies Ea determined from isothermal analysis ranged from 115-120 kJ/mol, except for the ionic liquid with the decyl chain, for which the value was higher (157 kJ/mol). The Ea determined by non-isothermal analysis were similar to the isothermal values for the ionic liquids with the ethyl and decyl chains, higher (144-154 kJ/mol) for the ionic liquids with the propyl and butyl chains, and lower (98 kJ/mol) for the ionic liquid with the hexyl chain. The pre-exponential factors ln(A) from isothermal analyses resulted in values in the range 25–29, which did not vary with reaction model. Non-isothermal analyses resulted in ln(A) values in the range 24–38, which were likewise not sensitive to reaction model. Among the models evaluated, the 3-dimensional reaction model was found to best represent both the isothermal and the non-isothermal data. No clear relationships were found between the alkyl chain length and the kinetic parameters.