Theoretical analysis of CO2 absorption by polyamines-TFSA type protic ionic liquids

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. © 2023 Chemical Industry Press. All rights reserved.