Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States.
J Phys Chem A. 2011 Jan 27;115(3):342-50. doi: 10.1021/jp1081627. Epub 2010 Dec 21.
Various amines have been considered as materials for chemical capture of CO(2) through liquid-phase reactions to form either carbamate or carbamic acid products. One of the main challenges in these CO(2)-amine reactions lies in tuning the heat of reaction to achieve the correct balance between the extent of reaction and the energy cost for regeneration. In this work, we use a computational approach to study the effect of substitution on the heats of reaction of monoethanolamine (MEA). We use ab initio methods at the MP2/aug-cc-pVDZ level, coupled with geometries generated from B3LYP/6-311++G(d,p) density functional theory along with the conductor-like polarizable continuum model to compute the heats of reaction. We consider two possible reaction products: carbamate, having a 2:1 amine:CO(2) reaction stoichiometry, and carbamic acid, having a 1:1 stoichiometry. We have considered CH(3), NH(2), OH, OCH(3), and F substitution groups at both the α- and β-carbon positions of MEA. We have experimentally measured heats of reaction for MEA and both α- and β-CH(3)-substituted MEA to test the predictions of our model. We find quantitative agreement between the predictions and experiments. We have also computed the relative basicities of the substituted amines and found that the heats of reaction for both carbamate and carbamic acid products are linearly correlated with the computed relative basicities. Weaker basicities result in less exothermic heats of reaction. Heats of reaction for carbamates are much more sensitive to changes in basicity than those for carbamic acids. This leads to a crossover in the heat of reaction so that carbamic acid formation becomes thermodynamically favored over carbamate formation for the weakest basicities. This provides a method for tuning the reaction stoichiometry from 2:1 to 1:1.
各种胺类已被认为是通过液相反应捕获 CO(2) 的材料，以形成氨基甲酸酯或氨基甲酸产物。这些 CO(2)-胺反应的主要挑战之一在于调节反应热，以在反应程度和再生能量成本之间取得正确的平衡。在这项工作中，我们使用计算方法研究取代对单乙醇胺（MEA）反应热的影响。我们使用 MP2/aug-cc-pVDZ 水平的从头算方法，与 B3LYP/6-311++G(d,p)密度泛函理论生成的几何形状相结合，并采用导体相似性极化连续模型来计算反应热。我们考虑了两种可能的反应产物：氨基甲酸酯，具有 2:1 的胺：CO(2)反应化学计量比，和氨基甲酸，具有 1:1 的化学计量比。我们已经考虑了在 MEA 的α-和β-碳原子位置上的 CH(3)、NH(2)、OH、OCH(3)和 F 取代基。我们已经实验测量了 MEA 和两种α-和β-CH(3)-取代 MEA 的反应热，以检验我们模型的预测。我们发现预测与实验之间存在定量一致性。我们还计算了取代胺的相对碱性，并发现氨基甲酸酯和氨基甲酸产物的反应热与计算得到的相对碱性呈线性相关。较弱的碱性导致反应热更不剧烈。氨基甲酸酯的反应热对碱性的变化更为敏感，而氨基甲酸的反应热则不然。这导致反应热的交叉，使得对于最弱的碱性，氨基甲酸的形成在热力学上比氨基甲酸酯的形成更有利。这提供了一种从 2:1 到 1:1 调节反应化学计量比的方法。
