Structural investigation of aqueous amine solutions for CO2 capture: CO2 loading, cyclic capacity, absorption–desorption rate, and pKa

Understanding the CO2 capture performance of individual amines based on their structure is essential for efficient CO2 capture. In this study, the effects of the molecular structure of amine on CO2 loading, cyclic capacity, absorption–desorption rate, and pKa were investigated, and promising candidates for CO2 capture, along with an amine blending strategy, were proposed. The results showed that electron-donating alkyl groups increased the CO2 loading by increasing the electron density and pKa of the amino groups, whereas the electron-withdrawing hydroxyl group exhibited the opposite effect. Thirty amines were tested and categorized into Group A, B, C, and D based on CO2 absorption rate and cyclic capacity. Amines in Group B had a slow absorption rate but high cyclic capacity and included sterically hindered primary or secondary amines (2-amino-2-methyl-1-propanol, 2-(isopropylamino)ethanol, and 2-(tert-butylamino)ethanol) and sterically hindered tertiary amines with one hydroxyl group on the β–carbon (1-dimethylamino-2-propanol, 1-(2-hydroxyethyl)piperidine and N,N-diethylethanolamine). The best-performing amines, Group A, were primarily multiamines (1-(2-hydroxyethyl)piperazine, 2-methylpiperazine, 2-(2-aminoethylamino)ethanol, N,N-dimethyl-1,3-propanediamine, N,N-diethyl-1,3-diaminopropane, hexamethylenediamine, diethylenetriamine, and 3,3'-diamno-N-methyldipropylamine) and included a sterically hindered heterocyclic secondary amine (2-piperidineethanol) with a hydroxyalkyl substituent on the α–carbon. Thus, amines in Group A are promising absorbents for CO2 capture, and blending amines in Group B with amines that have a high CO2 absorption rate (e.g., Group A) can also be a potential strategy for efficient CO2 capture.