Takayuki Higashii

Density functional theory study on carbon dioxide absorption into aqueous solutions of 2-amino-2-methyl-1-propanol using a continuum solvation model.

We used density functional theory (DFT) calculations with the latest continuum solvation model (SMD/IEF-PCM) to determine the mechanism of CO(2) absorption into aqueous solutions of 2-amino-2-methyl-1-propanol (AMP). Possible absorption process reactions were investigated by transition-state optimization and intrinsic reaction coordinate (IRC) calculations in the aqueous solution at the SMD/IEF-PCM/B3LYP/6-31G(d) and SMD/IEF-PCM/B3LYP/6-311++G(d,p) levels of theory to determine the absorption pathways. We show that the carbamate anion forms by a two-step reaction via a zwitterion intermediate, and this occurs faster than the formation of the bicarbonate anion. However, we also predict that the carbamate readily decomposes by a reverse reaction rather than by hydrolysis. As a result, the final product is dominated by the thermodynamically stable bicarbonate anion that forms from AMP, H(2)O, and CO(2) in a single-step termolecular reaction.

Ab initio study of CO2 capture mechanisms in aqueous monoethanolamine: reaction pathways for the direct interconversion of carbamate and bicarbonate.

Ab initio molecular orbital calculations combined with the polarizable continuum model (PCM) formalism have been carried out for a comprehensive understanding of the mechanism of carbon dioxide (CO2) absorption by aqueous amine solutions. CO2 is captured by amines to generate carbamates and bicarbonate. We have examined the direct interconversion pathways of these two species (collectively represented by a reversible hydrolysis of carbamate) with the prototypical amine, monoethanolamine (MEA). We evaluate both a concerted and a stepwise mechanism for the neutral hydrolysis of MEA carbamate. Large activation energies (ca. 36 kcal/mol) and lack of increase in catalytic efficiency with the inclusion of additional water molecules are predicted in both the mechanisms. We also examined the mechanism of alkaline hydrolysis of MEA carbamate at high concentrations of amine (high pH). The addition of OH(-) ion to carbamate anion was theoretically not allowed due to the reduction in the nucleophilicity of the former as a result of microsolvation. We propose an alternative pathway for hydrolysis: a proton transfer from protonated MEA to carbamate to generate the carbamic acid that initially undergoes a nucleophilic addition of OH(-) and subsequent low-barrier reactions leading to the formation of bicarbonate and free MEA. On the basis of the calculated activation energies, this pathway would be the most efficient route for the direct interconversion of carbamate and bicarbonate without the intermediacy of the free CO2, while the actual contributions will be dependent on the concentrations of protonated MEA and OH(-) ions.

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)mNH(CH2)nOH. 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 13C-NMR. The observed sensitivity to the alcohol chain length can be attributed to hydrogen bonding effects. The intramolecular hydrogen bonds of HN · · · HO, NH2+ · · · OH, and NCOO− · · · 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.

Experimental study into carbon dioxide solubility and species distribution in aqueous alkanolamine solutions

We investigated the solubility of CO2 in aqueous solutions of alkanolamines at 40C and 120C over CO2 partial pressures ranging from a few kPa to 100 kPa to evaluate the potential for CO2 capture from flue gas. CO2 capacities were compared between monoethanolamine, N-ethyl ethanolamine and N-isopropyl ethanolamine. Speciation analyses were conducted in the alkanolamine solutions at different CO2 loadings by accurate quantitative 13 C nuclear magnetic resonance spectroscopy. N-isopropyl ethanolamine showed a large capacity for CO2 because of the formation of bicarbonate. However, we also found that at a lower CO2 loading a significant amount of carbamate was present in the aqueous N-isopropyl ethanolamine solutions.