New insights into the structure-activity relationship for CO2 capture by tertiary amines from the experimental and quantum chemical calculation perspectives

In comparison with primary and secondary amines, tertiary amines exhibit higher CO2 equilibrium and lower CO2 absorption heat, making them attractive for CO2 capture applications. However, the relationship between the structure of tertiary amines and their CO2 capture capacity needs to be further studied. This study aimed to investigate the structure–activity relationship between the molecular configuration of representative tertiary amines, and their CO2 equilibrium solubility, as well as CO2 absorption heat. The CO2 absorption heat of nine tertiary amines with distinct structures was measured using a high-precision microcalorimeter, while quantum chemical calculations were employed to explore this relationship. The reactivity of tertiary amines was assessed by visualizing the effects of electron density and steric hindrance by using natural population analysis and dual descriptor analysis, respectively. The findings revealed that the stronger electron-donating effect of ethyl groups than the methyl groups enhanced the reactivity of tertiary amines. Additionally, the presence of methyl groups on the side chain of tertiary amines promoted the reactivity. By contrast, the presence of too many hydroxyl groups resulted in steric hindrance thus reducing the reactivity of tertiary amines. Furthermore, interaction region indicators were employed to visually evaluate the influence of intramolecular hydrogen bonds on the reactivity of tertiary amines. The results show that the presence of intramolecular hydrogen bonds enhanced the equilibrium solubility of CO2 while facilitating the reduction of the CO2 absorption heat. The results obtained from this study are important for the selection and development of efficient tertiary amines that are suitable for industrial applications in CO2 capture.