DFT and COSMO-RS theoretical analysis of SO2 absorption by polyamines type ionic liquids

A computational study was conducted to investigate the absorption of sulfur dioxide (SO2) by protic ionic liquids (PILs) comprising the cations of N-hexylammonium ([HHexam]+ ), hexylethylenediaminium ([HHexen] + ), hexyldiethylenetriaminium ([HHexdien] + ), and the anion of TFSA[(CF3SO2)2N-]. The most stable configurations of PILs-nSO2 (n=1, 2, 3, 4, 5, 6) were determined by using density functional theory (DFT) at the M06-2X/6-311G(d, p) level. The results revealed that N—H…O type hydrogen bonding was formed between the polar N—H groups on the cations and the O atoms in SO2. The analysis of vibrational frequency, second-order perturbation energy, and electron density revealed that the maximum hydrogen bonding was achieved when [HHexam] [TFSA], [HHexen] [TFSA] and [HHexdien] [TFSA] combined with 3, 4, and 5 molecules of SO2, respectively, with maximum hydrogen bond energies of 57, 67, and 85 kJ/mol. The absence of further hydrogen bonding networks with SO2 suggests that SO2 absorption has reached saturation. The data also indicate that the SO2 absorption capacity increases with the number of amino groups in the PILs structure. COSMOtherm software calculations corroborate the findings, revealing an increasing trend for the SO2 solubilities from 5.0 to 5.3 to 6.2 mol in 1 mol PILs above, respectively. That is, as the number of amino groups in the cationic structure increases, its ability to absorb SO2 increases, which is basically consistent with the conclusions obtained from density functional theory calculations. © 2023 Chemical Industry Press. All rights reserved.