Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, USA.
J Phys Chem A. 2010 Nov 4;114(43):11844-52. doi: 10.1021/jp107516k.
We present a theoretical study of the reaction mechanism of monoethanolamine (MEA) with CO₂ in an aqueous solution. We have used molecular orbital reaction pathway calculations to compute reaction free energy landscapes for the reaction steps involved in the formation of carbamic acids and carbamates. We have used the conductor-like polarizable continuum model to calculate reactant, product, and transition state geometries and vibrational frequencies within density functional theory (DFT). We have also computed single point energies for all stationary structures using a coupled cluster approach with singles, doubles, and perturbational triple excitations using the DFT geometries. Our calculations indicate that a two-step reaction mechanism that proceeds via a zwitterion intermediate to form carbamate is the most favorable reaction channel. The first step, leading to formation of the zwitterion, is found to be rate-determining, and the activation free energies are 12.0 (10.2) and 11.3 (9.6) kcal/mol using Pauling (Bondi) radii within the CPCM model at the CCSD(T)/6-311++G(d,p) and CCSD(T)/6-311++G(2df,2p) levels of theory, respectively, using geometries and vibrational frequencies obtained at the B3LYP/6-311++G(d,p) level of theory. These results are in reasonable agreement with the experimental value of about 12 kcal/mol. The second step is an acid-base reaction between a zwitterion and MEA. We have developed a microkinetic model to estimate the effective reaction order at intermediate concentrations. Our model predicts an equilibrium concentration for the zwitterion on the order of 10⁻¹¹ mol/L, which explains why the existence of the zwitterion intermediate has never been detected experimentally. The effective reaction order from our model is close to unity, also in agreement with experiments. Complementary ab initio QM/MM molecular dynamics simulations with umbrella sampling have been carried out to determine the free energy profiles of zwitterion formation and proton transfer in solution; the results confirm that the formation of the zwitterion is rate-determining.
我们提出了一个关于单乙醇胺（MEA）与 CO₂在水溶液中反应机理的理论研究。我们使用分子轨道反应途径计算，计算了形成氨基甲酸和氨基甲酸酯的反应步骤的反应自由能景观。我们使用导体相似极化连续体模型，在密度泛函理论（DFT）中计算反应物、产物和过渡态的几何形状和振动频率。我们还使用耦合簇方法，用单电子、双电子和微扰三电子激发，用 DFT 几何结构对所有稳定结构进行单点能量计算。我们的计算表明，通过两性离子中间体形成氨基甲酸酯的两步反应机制是最有利的反应通道。第一步，导致两性离子的形成，被发现是速率决定步骤，活化自由能分别为 12.0（10.2）和 11.3（9.6）千卡/摩尔，使用 Pauling（Bondi）半径在 CPCM 模型中，在 CCSD（T）/6-311++G（d，p）和 CCSD（T）/6-311++G（2df，2p）理论水平，分别使用 B3LYP/6-311++G（d，p）水平的几何形状和振动频率获得。这些结果与约 12 千卡/摩尔的实验值吻合较好。第二步是两性离子与 MEA 之间的酸碱反应。我们开发了一个微动力学模型来估计中间浓度下的有效反应级数。我们的模型预测两性离子的平衡浓度约为 10⁻¹¹ mol/L，这解释了为什么两性离子中间体的存在从未在实验中被检测到。我们模型的有效反应级数接近 1，也与实验结果一致。我们还进行了互补的从头算 QM/MM 分子动力学模拟，带有伞状抽样，以确定溶液中两性离子形成和质子转移的自由能曲线；结果证实两性离子的形成是速率决定步骤。
