| 30 wt% monoethanolamine(MEA) aqueous solution, as a benchmark solvent for CO2 capture, can efficiently absorb CO2. However, its reaction with CO2 is highly exothermic, which results in high energy consumption in the solvent regeneration step. Therefore, the design of novel and efficient amine solution, which can maintain absorption efficiency of MEA, and overcome the shortcoming of high regeneration energy cost, has become a hot topic. A sound understanding on the chemical processes involved in the capture of CO2 is considered to be important for rational design of novel amines. Previous studies only concerned about the effect of amine structures on reaction thermodynamics of amine with CO2. However, the changes in the molecular structure of amines, which lead to the change of reaction thermodynamics, may also change the reaction kinetics and competition of three typical pathways for the reaction of amines with CO2, and then in turn affect the absorption performance of amines. Therefore, amines reaction kinetics with CO2 have to be considered along with the reaction thermodynamics. Although the novel amine solution is still under development, some countries, such as Norway, the United States, Canada, have already planned to use the amine-based material to reduce CO2 emission. Given the possible large-scale implementation of amine-based material, lots of studies have been performed to assess the environmental risk caused by it. Here, we tend to investigate the effects of amine structures on their reaction kinetics with CO2 and formation mechanism of carcinogenic nitrosamines in CO2 capture, the contents are shown as follows:In terms of the reaction kinetics of amine with CO2, the substituted monoethanolamine(MEA) and diethanolamine(DEA) were selected as the model compounds. Quantum chemical methods were used to investigate the effects of amine structures on their reaction kinetics competition of three typical pathways. The computed Ea values for the substituted MEA with CO2 indicate that the formation of carbamate is the most favorable, followed by that to form bicarbonate, while the formation of carbamic acid is unfeasible for all the considered amines. Meanwhile, Ea values for all pathways negatively correlate with pKa of amines, and more importantly, the kinetic competition between carbamate and bicarbonate absorption pathways varies with pKa of amines, i.e. stronger basicity results in less difference in . In addition, Ea values are also effected by molecular structure of amines besides of pKa, i.e. intramolecular hydrogen bond have a great influence on Ea values. The results for secondary amines (substituted DEAs)) are similar to substituted MEAs.In terms of the formation mechanism of carcinogenic nitrosamines in CO2 capture, the piperazidine(PZ) was selected as the model compound, nitrite and dinitrogen trioxide were selected as nitrosating agent. The contribution of PZ species (protonated piperazine(PZH+), piperazine monocarbamate(PZCOO-), piperazine monocarbamic acid(PZCOOH), protonated piperazine monocarbamic acid(PZ(H+)COOH), piperazine dicarbamic acid(PZ(COOH)COOH), piperazine(carbamic) carbamic acid(PZ(COO-)COOH)) on N-nitrosopiperazine formation in CO2 capture has been studied. The reactions between different PZ speciation and NO2- are unfeasible due to high reaction energy barriers(>38.0kcal/mol). Therefore, the reaction initiated by NO2- contributes little to the formation of nitrosamines. For N2O3, all the energy barriers of nitrosation reactions between PZ speciation and N2O3 increase, compared with PZ. Although the reaction energy barriers are increased, PZ and PZCOO- still can form nitrosamines due to their relatively low barrier. In addition, we also considered the effect of CO2 on nitrosating agent. CO2 can react with NO2- to form ONOCO2-, however, it can’t react with N2O3. For the reaction initiated by ONOCO2-, the nitrosation reaction energy barriers of PZH+ and PZCOO- on-NH-site decrease, and the barrier of PZCOOH increases, compared with PZ. Therefore, parent PZ and PZ speciation are mainly nitrosated by N2O3 and ONOCO2- in the process of CO2 capture, and the PZ speciation has a great effect on the nitrosation reaction. |
