CO2 absorption behavior of azole-based protic ionic liquids: Influence of the alkalinity and physicochemical properties

Recently, chemical fixation of CO2 catalyzed by protic ionic liquids (PILs) has been successfully achieved, suggesting that CO2 absorption behavior plays an important role on CO2 activation and transformation. For this reason, the influence of the alkalinity and physicochemical properties on CO2 absorption behavior of four azole-based PILs, namely [DBNH][Pyr], [DBNH][Im], [DBUH][Pyr], and [DBUH][Im] was discussed in this work. The alkalinity of PILs was determined by potentiometric titration, and free space of PILs was evaluated by thermal expansion coefficient and refractive index. Solubility parameter of PILs was obtained from the activation energy for viscous flow. The results show that PILs with [Pyr]− exhibits stronger alkalinity and larger free space, which can help to improve CO2 capture capacity. Whereas PILs with larger cation size [DBUH]+ exerts higher viscosity, resulting in slower absorption rate of CO2. Furthermore, the CO2 absorption capacity is inversely proportional to the solubility parameter of PILs. The absorption mechanism proved by FT-IR and 13C NMR spectra indicates that [Pyr]− or [Im]− can react with CO2 to form carbamate. Accordingly, considering the CO2 capture capacity and absorption rate, [DBNH][Pyr] may be a good candidate for efficient CO2 capture and activation with low enthalpy of absorption of −38.2 kJ mol−1.