The effect of imidazolium and phosphonium ionic liquids on toluene absorption studied by a molecular simulation

Abstract Toluene absorption by the ion pairs of 1-butyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide ([Bmim][TFSI]), 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]) and tributyl(propyl)phosphonium tetrafluoroborate ([P4443][BF4]) is performed at the GGA/PW91 level using a density functional theory (DFT). The ion pairs of [Bmim][TFSI], [Bmim][BF4] and [P4443][BF4] are stable with five, five and six hydrogen bonds between the anions and cations, respectively. There is no hydrogen bond and just one C–H⋯π bond between toluene and [Bmim][TFSI], while two hydrogen bonds and three C–H⋯π bonds are formed between absorbed toluene and [Bmim][BF4] or [P4443][BF4]. That shows the interaction between the ion pairs of [Bmim][BF4] and [P4443][BF4] with toluene is stronger than that of [Bmim][TFSI]. Moreover, [P4443]+ has greater effect on the charge transfer of toluene than [Bmim]+ based on the higher electrostatic force with toluene, which indicates that [P4443][BF4] has better advantage on toluene absorption than [Bmim][BF4]. The absorption energy (Eabs, in kJ·mol-1) between toluene and ion pairs of [Bmim][TFSI], [Bmim][BF4] and [P4443][BF4] is −28.88 kJ mol−1, -34.13 kJ mol−1 and -39.38 kJ mol−1, respectively. The frontier molecular orbital (FMO) analysis indicates that absorption of toluene by ionic liquids is a physical process. The HOMO and LUMO of [P4443][BF4]·C7H8 are all localized in toluene, but the LUMO of [Bmim][BF4]·C7H8 is composed by the atoms of imidazolium ring in cation, which also shows that the interaction between toluene and [P4443][BF4] is stronger. In the simulation study, the molecular insight into mechanism of toluene absorption by imidazolium and phosphonium ionic liquids is provided by the comprehension including hydrogen bond, C–H⋯π bonds, electrostatic force and the frontier molecular orbital, which is fundamental for industrial application of ionic liquids in toluene separation.