Difference for the absorption of SO2 and CO2 on [Pnnnm][Tetz] (n = 1, m = 2, and 4) ionic liquids: A density functional theory investigation

The absorption of SO2 and CO2 on the ion pairs of trimethyl(ethyl)phosphonium tetrazole ([P1112]+[Tetz]−) and trimethyl(butyl)phosphonium tetrazole ([P1114]+[Tetz]−) is performed at the GGA/PW91 level. The ion pairs of [P1112]+[Tetz]− and [P1114]+[Tetz]− are both stable with four and five hydrogen bonds between the N in the anion and the H in the cations of the [P1112]+ and [P1114]+, respectively. Each of the [P1112]+[Tetz]− and [P1114]+[Tetz]− can absorb three SO2 (or CO2) at most, one less than the SO2 (or CO2) absorbed on the isolated [Tetz]−. The ion pairs behave better in the absorption of the SO2 than in the CO2 based on the higher absorption energy of [P1112]+[Tetz]−·nSO2 and [P1114]+[Tetz]−·nSO2 than in the [P1112]+[Tetz]−·nCO2 and [P1114]+[Tetz]−·nCO2, respectively. The frontier molecular orbital analysis shows that the absorbed SO2 has larger overlap with the ion pairs than with the CO2, which corresponds with the absorption of SO2 and CO2 on the isolated anion of [Tetz]−. The transferred charge between the ion pairs of the [P1112]+[Tetz]− (or [P1114]+[Tetz]−) and the absorbed SO2 is much larger than that between the ion pairs and the CO2 and this also proves that the ion pairs of [P1112]+[Tetz]− and [P1114]+[Tetz]− behave better in the absorption of the SO2 than in the CO2. Our simulation study provides molecular insight into the interactions of the azole-based ionic liquids with multiple sites and the absorption of SO2 and CO2 which are fundamental for understanding of absorption mechanisms in these azole-based IL applications which offer significant improvements over commonly used absorbents in industrial applications in acid gas separation.