Computational investigation of carbon dioxide absorption in alkanolamine solutions

We investigated CO2 absorption in aqueous alkanolamine solutions using density functional theory with dielectric continuum solvation models (SMD/IEF-PCM and COSMO-RS). We varied the alkyl chain length (m = 2, 3, 4) and the alcohol chain length (n = 2, 3, 4) in the alkanolamine structures, H(CH2) (m) NH(CH2) (n) OH. Using the SMD/IEF-PCM/B3LYP/6-311++G(d,p) and COSMO-RS/BP/TZVP levels of theory, our calculations predict that the product of CO2 absorption (carbamate or bicarbonate) is strongly affected by the alcohol length but does not differ significantly by varying the alkyl chain length. This prediction was confirmed experimentally by C-13-NMR. The observed sensitivity to the alcohol chain length can be attributed to hydrogen bonding effects. The intramolecular hydrogen bonds of HN center dot center dot center dot HO, NH2 (+) center dot center dot center dot OH, and NCOO- center dot center dot center dot HO induce ring structure formation in neutral alkanolamines, protonated alkanolamines, and carbamate anions, respectively. The results from our studies demonstrate that intramolecular hydrogen bonds play a key role in CO2 absorption reactions in aqueous alkanolamine solutions.