Kinetic stability of imidazolium cations and ionic liquids: A frontier molecular orbital approach

The effect of alkyl chain length on the thermal stability of 1-alkyl-3-methylimidazolium cation ([RMIm](+)), [RMIm](+)[Cl](-) ion-pair and [RMIm](+) [BF4](-) ion-pair has been assessed using thermogravimetric analysis and density functional theory calculations at B3LYP/6-311 + G(d,p) level. The HOMO-LUMO energy gap (Delta E*) calculated for [RMIm](+) ion showed a linear decreasing trend with increase in the number of carbon atoms (N-c) in the alkyl chain R which suggested the decreasing thermal stability of cations as size of R increases. Contrary to this, the Delta E* for [RMIm](+)[Cl](-) was in the range 4.08-4.09 eV and that of [RMIm](+) [BF4](-) was in the range 6.78-6.88 eV when N-c varied from C2 to C16 in the R. This implies that size of alkyl chain length has no significant role on the thermal stability of ion pairs. The decomposition temperature observed for [RMIm](+)[Cl](-) was in the range 284-296 degrees C and that of [RMIm](+) [BF4](-) was in the range 449-455 degrees C which agrees with the higher stability predicted for the latter using theoretical Delta E* analysis. A methodology based on the cation exchange of ionic liquids with sodium montmorillonite clay (MMT-Na+) was adopted to estimate thermal stability of [RMIm](+) for the first time which showed a decreasing trend in stability for all MMT-[RMIm](+) systems and this result agrees well with theoretical predictions. (C) 2018 Elsevier B.V. All rights reserved.