Ma Changru, Pietrucci Fabio, Andreoni Wanda
Institut de Théorie des Phénomènes Physiques, Ecole Polytechnique Fédérale de Lausanne , Lausanne, Switzerland.
J Chem Theory Comput. 2015 Jul 14;11(7):3189-98. doi: 10.1021/acs.jctc.5b00379. Epub 2015 Jun 3.
Aqueous monoethanolamine (MEA) solution is commonly used for post-combustion carbon capture via chemical absorption. Extensive research has been carried out to characterize both uptake and release of carbon dioxide (CO2), with the aim of improving process performance. However, an intensive research is still needed on fundamental aspects of the key chemical reactions, to achieve a comprehensive understanding of the cyclic process at the microscopic level and a quantitative assessment. We present several ab initio simulations of MEA solutions at a concentration of 30 wt %-the current standard in the industry-and study the dynamics of key multistep chemical reactions, using the metadynamics technique. Pathways for the entire cycle are investigated and characterized in terms of related free-energy and enthalpy barriers, and of the accompanying variations in both structural and electronic properties. The results of this study lead us to propose, among competing processes, an unforeseen scenario in which the zwitterion acts as sn intermediate not only of CO2 uptake, in the form of carbamate, but also of its release. Rate-limiting steps are the formation of the zwitterion for the former and MEAH(+) deprotonation for the latter. Water is shown to play a multifaceted role, which is crucial in determining the development and the energetics of each step of the reactions. The level of comprehension here achieved for MEA should help defining a strategy for solvent optimization.
单乙醇胺（MEA）水溶液常用于通过化学吸收进行燃烧后碳捕获。为了提高工艺性能，已经开展了广泛的研究来表征二氧化碳（CO₂）的吸收和释放。然而，仍需要对关键化学反应的基本方面进行深入研究，以在微观层面全面理解循环过程并进行定量评估。我们对浓度为30 wt %（这是目前行业标准）的MEA溶液进行了几次从头算模拟，并使用元动力学技术研究关键多步化学反应的动力学。研究了整个循环的途径，并根据相关的自由能和焓垒以及结构和电子性质的伴随变化进行了表征。这项研究的结果使我们在竞争过程中提出了一种不可预见的情况，即两性离子不仅作为以氨基甲酸盐形式吸收CO₂的中间体，而且作为其释放的中间体。前者的限速步骤是两性离子的形成，后者的限速步骤是MEAH(+)的去质子化。水被证明起着多方面的作用，这对于确定反应各步骤的进展和能量学至关重要。此处对MEA所达到的理解水平应有助于确定溶剂优化策略。
