Ionic Liquid Catalyzes Reactive CO2 capture

Abstract

CO2 hydroxylation is the underlying reaction for a wide range of reactive CO2 capture (RCC) techniques. Despite being thermodynamically favorable, the CO2 hydroxylation kinetics are sluggish due to a higher energy barrier associated with bending the linear CO2 molecule prior to nucleophilic attack by OH -. Here, we report a previously unrecognized catalytic effect in which ionic liquids (ILs) increase the rates of CO2 hydroxylation by pre-activating (or bending) CO2 locally around the IL. ILs are known for their higher CO2 solubilities due to their larger void fractions; however, their role in catalyzing the CO2 hydroxylation reaction, an important step in RCC, has not been previously reported. NMR, FTIR, and quantum-chemical calculations confirm that bicarbonate is the dominant stable species, with carbonate reverting to bicarbonate over time. For instance, the addition of BMIM NTf 2 in KOH-ethylene glycol mixture accelerates CO 2 hydroxylation by sixfold by lowering the activation energy (~33%) without chemically binding CO2, highlighting its catalytic role. Isotope-labeled ATR-FTIR experiments confirm enhanced CO 2 bending in ionic liquids through the appearance of a hot band, indicative of increased population of thermally accessible bent vibrational states. While low IL loadings enhance kinetics, higher concentrations hinder CO 2 mass transfer due to reduced interfacial tension. The nature of IL cation and anion strongly influences rates, with shorter alkyl chains and I⁻/OTf⁻ anions providing the highest activity. The system demonstrates reversibility and robustness under flue gas conditions, providing a tunable pathway for efficient CO2 capture.