Significance Statement
CO2 is one of the greenhouse gases that cause the global climate change. Although many experimental and theoretical studies have been conducted on CO2 absorption in amine solvents, commonly used CO2 absorption solvent, the reaction mechanism is still unclear.
In this work, we investigated the reactions of CO2 with six different amines; monoethanol-amine (MEA), diethanolamine (DEA), 2-amino-aminoethylethanolamine (AEEA), 2-methyl-1-propanol (AMP), diethylenetetramine (DETA), and piperazine (PZ). The reactions were designed as 3 types: (1) reaction of CO2 and amine, (2) reaction of CO2 and amine in the presence of water, (3) reaction of CO2 and amine in the presence of additional amine. In three types of reaction model, amine nitrogen acts as a nucleophile that donates lone pair electrons to form a chemical bond with carbon of CO2. The additional molecules (water or amine) lowered the activation energy of the reaction of CO2 and amine. Many previous studies have focused on water catalyzed or non-catalyzed mechanisms. But the carbamate formation reaction mechanism with additional amine as a base, lowering energy barrier to level of experimental results, was confirmed to be more reasonable than the other models.
Moreover, the enhancement effect of CO2 absorption in blended amine solvents could be explained by additional amine model. Among various blended amine combination with MEA, MEA+PZ, MEA+AEEA, and MEA+DETA could improve the CO2 capturing capability compared with single MEA solvent, while AMP or DEA will decrease the performance of MEA according to the activation energy. Our result is in excellent agreement with previous experiment studies. Our work should be useful in choosing a proper blended amine solvent for better CO2 absorption. Highlights are listed as follows:
- The reaction of CO2 and amine solvent was analyzed by theoretical analysis.
- The reactions were designed with or without additional water or amine molecule.
- The additional amine model shows lowest activation energy.
- The additional amine model can describe the effect of blended amine solvents.
Journal Reference
International Journal of Greenhouse Gas Control, Volume 45, February 2016, Pages 181–188.
Sunkyung Kim, Hu Shi, Jin Yong Lee
Department of Chemistry, Sungkyunkwan University, 2066, Seobu-ro, Jangan-gu, Suwon-si, Gyeonggi-do, Republic of Korea
Abstract
The reaction mechanism for CO2 absorption in amine solvent was investigated by theoretical analysis. The reactants were CO2 and six amines, and the reactions were designed with or without additional water or amine molecule. These molecules increase the interaction between reactants and withdraw a proton from amino group of amine. From the additional amine model that shows lowest activation energy, zwitterionic and termolecular mechanisms seem to be suitable for CO2 capturing reaction in amine solvents. Moreover, the additional amine model can be applied to understand the enhancement effect of CO2 absorption in blended amine solvents. We report a new attempt that describes reaction mechanism in blended amine solvent by applying additional amine base. The results of CO2 capture ability were analogous to experimental observation. Comparing our results and previous QM/MM and ab initio MD calculations, it was found that the QM treatment including the reactants and surrounding water molecules would be very critical and the QM region should be properly selected large enough in QM/MM.
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