Insights into quaternary ammonium-based ionic liquids series with tetrafluoroborate anion for CO2 capture

Abstract

In this paper, the structures of quaternary ammonium ionic liquids (QAILs) composed of tetrafluoroborate anion and quaternary ammonium cations with different alkyl chains and their interactions with CO2 molecule were studied by density functional theory at the B3LYP-D3/6–311++G(d, p) level of theory. Geometries of isolate cations/anion, ionic pairs and QAILs-CO2 systems have been fully optimized. QTAIM and NCI analyses have also been performed to obtain the interaction information of fragments for QAILs-CO2 systems. NCI analysis also demonstrates that [BF4]− anion and quaternary ammonium cations actually have the synergistic effects in the CO2 capture process by QAILs. GKS-EDA method was firstly applied for incisive understanding of QAILs-CO2 systems from energetic perspectives; Moreover, we also took advantage of COSMO-RS calculation to obtain CO2 macroscopic solubility in QAILs, which is in good correspondence with the GKS-EDA results. Additionally, gas selectivity results calculated using COSMO-RS theory indicate that the selectivities of CO2/CO and CO2/CH4 decrease gradually as alkyl side chains of QAILs elongate in contrast to the increase of CO2/H2 selectivity. At last, molecular dynamics simulation was also conducted to obtain the dynamic transport property of CO2 in QAILs.