关键词: 多元胺型质子化离子液体, 二氧化碳, 分子间氢键作用, 密度泛函理论, 分子中原子理论

Abstract:

Three protic ionic liquids (PILs) composed of the same anion of [TFSA== (CF₃SO₂)₂N^-] and different cations of N-hexylammonium (HHexam⁺), monoprotic hexylethylenediaminium (HHexen⁺) and hexyldiethylenetriaminium (HHexdien⁺), [HHexam][TFSA], [HHexen][TFSA], and [HHexdien][TFSA] were studied for the absorption of CO₂. First, the more stable configurations of the three PILs were optimized via the M06-2X/6-311G(d,p) of the density functional theory. The results indicated that the stronger N—H···N-type hydrogen bonds were formed mainly between the N-atoms in the cations and the N-atoms in the anion of the PILs. Then, the configurations of PIL-nCO₂ were optimized. The N—H···O-type weak- or moderate-strength hydrogen bonds were formed mainly between the N—H bond in the cation of the PIL and the O atoms of CO₂. The results of the vibrational frequency of the N—H bond, and the electron density and the second-order perturbation energy of N—H···O showed that a single molecule of [HHexam][TFSA], [HHexen][TFSA] and [HHexdien][TFSA] was saturated when bonded with 2,3 and 4 CO₂ molecules, respectively. Meanwhile, the results calculated by COSMOtherm software found that the Henry constants (kPa) for CO₂ in the three PILs varied as 1.91×10⁴ for [HHexam][TFSA]>1.68×10⁴ for [HHexen][TFSA]>1.51×10⁴ for [HHexdien][TFSA], indicating that the solubility of CO₂ in the PILs followed the order of [HHexdien][TFSA] with three amino groups in the polar head > [HHexen][TFSA] with two amino groups > [HHexam][TFSA] with one amino group. These results suggested that the number of amino groups in the PIL structure had a significant effect on its ability to absorb CO₂. With increasing number of amino groups in the structure of PILs, its solubilization capacity for CO₂ increased.

Key words: protic ionic liquids with polyamines, carbon dioxide, intermolecular hydrogen bonding, density functional theory (DFT), atoms in molecules (AIM)