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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Ab initio study of CO_2 capture mechanisms in aqueous monoethanolamine: Reaction pathways for the direct interconversion of carbamate and bicarbonate
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Ab initio study of CO_2 capture mechanisms in aqueous monoethanolamine: Reaction pathways for the direct interconversion of carbamate and bicarbonate

机译：从头算研究单乙醇胺水溶液中的CO_2捕集机理：氨基甲酸酯和碳酸氢盐直接相互转化的反应途径

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摘要

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 (CO_2) absorption by aqueous amine solutions. CO_2 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 CO_2, while the actual contributions will be dependent on the concentrations of protonated MEA and OH- ions.
机译：从头开始进行分子轨道计算，并结合可极化连续体模型（PCM）形式，以全面了解胺水溶液吸收二氧化碳（CO_2）的机理。 CO_2被胺捕获，生成氨基甲酸酯和碳酸氢盐。我们已经检查了这两种物种与原型胺单乙醇胺（MEA）的直接相互转化途径（共同代表氨基甲酸酯的可逆水解）。我们评估MEA氨基甲酸酯的中性水解的协同和逐步机制。在这两种机理中均预测到大的活化能（约36 kcal / mol）和催化效率的提高以及其他水分子的引入。我们还检查了高浓度胺（高pH）下MEA氨基甲酸酯的碱性水解机理。理论上不允许向氨基甲酸酯阴离子中添加OH-离子，这是由于微溶剂化导致前者的亲核性降低。我们提出了一种水解的替代途径：质子从质子化MEA转移至氨基甲酸酯以生成氨基甲酸，该氨基甲酸最初经历OH〜-的亲核加成反应，随后发生低阻滞反应，导致形成碳酸氢根和游离MEA。根据计算出的活化能，该途径将是氨基甲酸酯和碳酸氢盐直接相互转化而无需游离CO_2中介的最有效途径，而实际贡献将取决于质子化MEA和OH-离子的浓度。

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