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

Abstract

Abstract The absorption of SO 2 and CO 2 on the ion pairs of trimethyl(ethyl)phosphonium tetrazole ([P 1112 ] + [Tetz] − ) and trimethyl(butyl)phosphonium tetrazole ([P 1114 ] + [Tetz] − ) is performed at the GGA/PW91 level. The ion pairs of [P 1112 ] + [Tetz] − and [P 1114 ] + [Tetz] − are both stable with four and five hydrogen bonds between the N in the anion and the H in the cations of the [P 1112 ] + and [P 1114 ] + , respectively. Each of the [P 1112 ] + [Tetz] − and [P 1114 ] + [Tetz] − can absorb three SO 2 (or CO 2 ) at most, one less than the SO 2 (or CO 2 ) absorbed on the isolated [Tetz] − . The ion pairs behave better in the absorption of the SO 2 than in the CO 2 based on the higher absorption energy of [P 1112 ] + [Tetz] − ·nSO 2 and [P 1114 ] + [Tetz] − ·nSO 2 than in the [P 1112 ] + [Tetz] − ·nCO 2 and [P 1114 ] + [Tetz] − ·nCO 2 , respectively. The frontier molecular orbital analysis shows that the absorbed SO 2 has larger overlap with the ion pairs than with the CO 2 , which corresponds with the absorption of SO 2 and CO 2 on the isolated anion of [Tetz] − . The transferred charge between the ion pairs of the [P 1112 ] + [Tetz] − (or [P 1114 ] + [Tetz] − ) and the absorbed SO 2 is much larger than that between the ion pairs and the CO 2 and this also proves that the ion pairs of [P 1112 ] + [Tetz] − and [P 1114 ] + [Tetz] − behave better in the absorption of the SO 2 than in the CO 2 . Our simulation study provides molecular insight into the interactions of the azole-based ionic liquids with multiple sites and the absorption of SO 2 and CO 2 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.