Comparative DFT analysis of CO₂ cycloaddition with ionic liquids in encapsulated and free states

The conversion of CO₂ into valuable products offers a promising route for both its mitigation and utilization. Ionic liquids (ILs), particularly in encapsulated forms (ENILs), have shown great potential for CO₂ capture, yet their application in CO₂ conversion remains underexplored. In this study, we present the first density functional theory (DFT) investigation of CO₂ conversion to cyclic carbonate using belt[14]pyridine-encapsulated tetramethylammonium chloride (BP-TMACl) in reaction with propylene oxide. The results reveal that encapsulation significantly reduces the activation barriers compared to unencapsulated TMACl. The energy barrier for propylene oxide ring-opening decreases from 34.95 kcal mol^-1 to 33.73 kcal mol^-1, while the second step-CO₂ insertion and cyclization-shows a more substantial reduction from 18.09 kcal mol^-1 to 10.00 kcal mol^-1. Non-covalent interaction (NCI) and quantum theory of atoms in molecules (QTAIM) analyses indicate that the confinement effect within the capsule stabilizes both reactants and transition states, lowering the energy difference and enhancing reaction feasibility. The overall reaction remains exergonic, with improved thermodynamic favorability for the encapsulated system. These findings demonstrate that encapsulated ILs, such as BP-TMACl, can significantly enhance CO₂ conversion efficiency, offering a more effective and economically viable approach for CO₂ utilization.