Structural factors controlling thermal stability of imidazolium ionic liquids with 1-N-butyl-3-methylimidazolium cation on gamma-Al2O3

Abstract

Structural factors determining thermal stability limits of imidazolium ionic liquids (ILs) with 1-n-butyl-3-methylimidazolium, [BMIM](+), cation on one of the most commonly used metal-oxide support, gamma-Al2O3, were determined by thermogravimetric analysis complemented by infrared (IR) spectroscopy. IR results show that inter-ionic interaction strength in ILs increases as their anion structure varies in the following order: [NTf2](-) < [SbF6](-) < [BE4](-) < [TfO](-) < [OS](-) < [HSO4](-) < [TOS](-) < [DBP](-) < [OAc](-). TGA data illustrate a strong dependence of thermal stability limits on inter-ionic interactions. Thermal stability limits of both bulk and gamma-Al2O3-supported [BMIM](+)-based ILs increase with decreasing inter-ionic interaction strength. Thermal stability limit of IL with octyl sulfate anion was lower than that of analogous IL with hydrogen sulfate anion, because of its exceptionally large anion size. Moreover, the effect of gamma-Al2O3 on IL thermal stability conditions becomes dominant with decreasing inter-ionic interactions in ILs.