CO2 capturing by self-assembled belt[14]pyridine encapsulated ionic liquid complexes: a DFT study†

Abstract

In the current study, CO₂ capturing ability of encapsulated ionic liquids (ENILs) i.e., tetramethylammonium chloride (TMACl), 1,3-dimethylimidazolium chloride (MIMCl), and methylpyridinium hexafluorophosphate (MPHP) encapsulated in self assembled belt[14]pyridine (BP) has been studied. The results show that strong van der Waals forces are involved in capturing of CO₂ by these encapsulated ionic liquids. Strong attractive forces arise from synergistic effect of ionic liquid (encapsulated) and atoms of belt. The interaction energies (E_int) ranging from −12.54 to −18.64 kcal mol⁻¹ reveal the capturing of CO₂ by these systems as thermodynamically feasible process. The type and strength of interactions between CO₂ and encapsulated ionic liquids is studied through QTAIM and NCI analyses. NCI analysis clearly shows that capturing of CO₂ is assisted by van der Waals forces between CO₂ and encapsulated ionic liquid complexes. The same feature is confirmed through QTAIM analysis as well. Natural bond orbital (NBO) analysis' results show the charge transfer between the fragments (encapsulated ionic liquids and CO₂) which is validated further through electron density differences (EDD) analysis. Overall, transfer of charge towards CO₂ from encapsulated ionic liquids is proved through the charge accumulation over CO₂ (i.e., blue isosurfaces on CO₂ molecules) through EDD analysis. The FMO analyses show the decrease in H–L gaps of encapsulated ionic liquids after CO₂ capturing. The successful charge transfer and reduction in H–L gap indicate better interaction in the designed systems thus revealing these systems as a potential candidates for CO₂ capturing. Overall, the best results for CO₂ capture i.e., the highest interaction energy, the lowest H–L gap, and the strongest forces of interactions are shown by methylpyridinium hexafluorophosphate (MPHP) encapsulated belt[14]pyridine (BP–MPHP) system. This is due to the larger anion of methylpyridinium hexafluorophosphate as compared to the other two encapsulated ionic liquids with Cl⁻ as anion which enables it to develop strong interactions with CO₂. The designed belt[14]pyridine based encapsulated ionic liquid systems are promising prospects with better CO₂ capture performance and represent a new entrant in the CO₂ capturing systems.