Vaporisation and thermal decomposition of dialkylimidazolium halide ion ionic liquids

Lovelock, Kevin R. J.; Armstrong, James P.; Licence, Peter; Jones, Robert G.

Abstract

Vaporisation and liquid phase thermal decomposition, TD, of two halide ion ionic liquids, 1-octyl-3-methylimidazolium chloride, [C8C 1Im]Cl, and 1-octyl-3-methylimidazolium iodide, [C8C 1Im]I, are investigated using temperature programmed desorption (TPD) line of sight mass spectrometry (LOSMS) at ultra-high vacuum (UHV). The ability to use MS to distinguish between vaporisation and TD allows the thermodynamics/kinetics of both vaporisation and TD to be investigated within the same experiments. Vaporisation of both halide ion ionic liquids is demonstrated. For both [C8C1Im]Cl and [C8C 1Im]I the vapour is shown to be composed of neutral ion pairs (NIPs). The enthalpy of vaporisation at temperature T, ΔvapH T, was experimentally determined as ΔvapH 455 = 151 ± 10 kJ mol-1 for [C8C 1Im]Cl and ΔvapH480 = 149 ± 8 kJ mol-1 for [C8C1Im]I. Extrapolation of ΔvapHT to the reference temperature, 298 K, gave ΔvapH298 = 166 ± 10 kJ mol-1 for [C8C1Im]Cl and ΔvapH298 = 167 ± 8 kJ mol-1 for [C8C1Im]I, higher than most ΔvapH298 values measured to date for other [C8C1Im]+-containing ionic liquids. In addition, predictions of ΔvapH298 for other halide ion ionic liquids are made. Liquid phase TD is shown to proceed via nucleophilic substitution to give two sets of products: 1-octylimidazole and methylhalide, and 1-methylimidazole and 1-octylhalide. The activation energy of TD at a temperature T, Ea,TD,T, is measured for the nucleophilic substitution of [C8C1Im]I to give methyliodide; Ea,TD,480 = 136 ± 15 kJ mol-1. Ea,TD,T is measured for the nucleophilic substitution of [C8C1Im]Cl to give methylchloride; Ea,TD,455 = 132 ± 10 kJ mol-1. The fact that ΔvapHT and Ea,TD,T are the same (within error) for both ionic liquids is commented upon, and conclusions are drawn as to the thermal stability of these ionic liquids.

Citation

Lovelock, K. R. J., Armstrong, J. P., Licence, P., & Jones, R. G. (2014). Vaporisation and thermal decomposition of dialkylimidazolium halide ion ionic liquids. Physical Chemistry Chemical Physics, 16(4), 1339-1353. https://doi.org/10.1039/c3cp52950a

• Journal Article Type: Article
• Acceptance Date: Sep 11, 2013
• Online Publication Date: Sep 12, 2013
• Publication Date: Jan 28, 2014
• Deposit Date: Aug 8, 2018
• Journal: Physical Chemistry Chemical Physics
• Print ISSN: 1463-9076
• Electronic ISSN: 1463-9084
• Publisher: Royal Society of Chemistry
• Peer Reviewed: Peer Reviewed
• Volume: 16
• Issue: 4
• Pages: 1339-1353
• DOI: https://doi.org/10.1039/c3cp52950a
• Public URL: http://dx.doi.org/10.1039/C3CP52950A
• Publisher URL: https://pubs.rsc.org/en/content/articlelanding/2014/CP/C3CP52950A#!divAbstract