Search

It is important to understand the mechanism of toluene absorption by phosphonium ionic liquids (PILs) for volatile organic compound removal with ionic liquids. In this work, the molecular structures of trimethyl(butyl)phosphonium tetrafluoroborate ([P 1114 [BF 4), trimethyl(butyl)phosphonium bis(trifluoromethylsulfonyl)imide ([P 1114 [Tf 2 N]), tributyl(propyl)phosphonium tetrafluoroborate ([P 4443 [BF 4) and tributyl(propyl)phosphonium bis(trifluoromethylsulfonyl)imide ([P 4443 [Tf 2 N]) were optimized by quantum chemical calculations at the GGA/PW91 level with the DNP basis set of density functional theory (DFT). The most stable configurations for toluene absorption by these PILs with multiple sites were also obtained by calculation under the same conditions. The main parameters for the most stable structure, data for the Mulliken charge and frontier molecular orbitals were further determined and discussed. It was found that the interactions of PILs with toluene are mainly composed of hydrogen bonds, C-H···π bond interactions and electrostatic attraction, and most of the calculated frontier molecular orbitals are concentrated on the toluene molecule. Additionally, analysis showed that [BF 4 − is inclined to form hydrogen bonds with cations and the methyl groups of toluene, and [Tf 2 N]− tends to attract toluene via electrostatic attraction. Investigation of the absorption energy with multiple sites indicated that PILs have great performance for toluene absorption. Analysis of the ionic size and geometric structure of the PILs demonstrated that the space available for accommodating toluene is an important condition for toluene absorption and is mainly provided by cations. In particular, PILs with cations having suitable long alkyl side chains show excellent capacity for toluene absorption. Moreover, NBO and AIM analyses confirmed the existence of weak closed-shell interactions (hydrogen bonds and C-H···π bonds) between toluene and PILs. The experimental results obtained for the static toluene absorption by the PILs provide a great supplement and confirmation of the calculations [Display omitted] • Structure of phosphonium cation is conducive for multiple sites absorption. • Space for accommodating toluene is mainly provided by phosphonium cation. • Suitable long alkyl chain is necessary for the high toluene absorption. • Static toluene absorption amount for [P 8883 [BF 4 is as high as 2840 mg/g. [ABSTRACT FROM AUTHOR]

Toluene absorption by the ion pairs of 1-butyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide ([Bmim][TFSI]), 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF 4) and tributyl(propyl)phosphonium tetrafluoroborate ([P 4443 [BF 4) is performed at the GGA/PW91 level using a density functional theory (DFT). The ion pairs of [Bmim][TFSI], [Bmim][BF 4 and [P 4443 [BF 4 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][BF 4 or [P 4443 [BF 4. That shows the interaction between the ion pairs of [Bmim][BF 4 and [P 4443 [BF 4 with toluene is stronger than that of [Bmim][TFSI]. Moreover, [P 4443 + has greater effect on the charge transfer of toluene than [Bmim]+ based on the higher electrostatic force with toluene, which indicates that [P 4443 [BF 4 has better advantage on toluene absorption than [Bmim][BF 4. The absorption energy (Eabs, in kJ·mol-1) between toluene and ion pairs of [Bmim][TFSI], [Bmim][BF 4 and [P 4443 [BF 4 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 [P 4443 [BF 4 ·C 7 H 8 are all localized in toluene, but the LUMO of [Bmim][BF 4 ·C 7 H 8 is composed by the atoms of imidazolium ring in cation, which also shows that the interaction between toluene and [P 4443 [BF 4 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. • [P 4443 + charge transfer to toluene is larger than [Bmim]+. • HOMO and LUMO of [P 4443 [BF 4 ·C 7 H 8 are all concentrated in toluene. • Hydrogen bond and C–H⋯π bond are all present in ILs absorption of toluene. [ABSTRACT FROM AUTHOR]