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, CO₂ hydroxylation kinetics are sluggish due to a higher energy barrier associated with bending the linear CO₂ molecule before 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 reported previously. NMR and FTIR measurements, together with quantum-chemical calculations, are consistent with bicarbonate being the dominant stable species, while carbonate appears to revert to bicarbonate over time under the studied conditions. For instance, the addition of BMIM NTf₂ to a KOH-ethylene glycol mixture accelerates CO₂ hydroxylation by sixfold by lowering the activation energy (∼33%) without chemically binding CO₂, highlighting its catalytic role. Isotope-labeled ATR-FTIR experiments confirm enhanced CO₂ 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 CO₂ 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 anhydrous flue gas conditions, offering a tunable pathway for efficient CO₂ capture.