Reaction Mechanism of Monoethanolamine with CO₂ in Aqueous Solution from Molecular Modeling

Abstract

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.

Effect of Chain Topology of Polyethylenimine on Physisorption and Chemisorption of Carbon Dioxide

“…[11] It is difficult to study these reactions at the molecular level by experiment, whereas theoretical calculation has shown advantages in the study of reaction thermodynamics and kinetics. [29][30][31] Theoretical studies provideareliable and unbiased comparison of reaction energies, and allow the factorsm entioned above to be untangled. For example, detailed information such as the reactivity of amineg roups at different locations in ac hain can be distinguished.…”

Novel High Capacity Oligomers for Low Cost CO2 Capture

“…There is some debate in the literature about the exact details of the mechanism for the reaction between MEA and CO 2 . Several different mechanisms have been proposed, including a single-step reaction mechanism, 46 a two-step mechanism that proceeds via zwitterion formation, 59,63,64 and a two-step mechanism involving carbamic acid as an intermediate. 65 However, there is general agreement that Lewis acid-base interactions between amine and CO 2 are important in both carbamate and carbamic acid reactions.…”

“…58 In addition to promoters, a catalyst for CO 2 absorption, for example, carbonic anhydrase can be added. 59 It was shown that promotion mechanism for this reaction is similar for inorganic and organic promoters. 60 The reaction rate is controlled by the rate of proton abstraction from the zwitterion intermediate for high carbonate conversions.…”

“…A similar computational approach has successfully been used to calculate the kinetics of MEA-CO 2 reactions. 59 The enthalpy for each species in the liquid is calculated using the equation where H s is the enthalpy of species s in the liquid phase, H s (g) is enthalpy of species s in the gas phase, ZPE is the zero-point energy, and E sol is the solvation free energy of species s, which includes electrostatic interaction, cavitation, dispersion, and repulsion energies. [55][56][57] To account for the different conformations in a physically reasonable way, we generated a Boltzmann-weighted distribution to compute the conformational averaged energy of each species s:…”

Development of K and N based composite CO2 sorbents (KN) dried with a supercritical fluid

“…where K R1 and K R3 represent the equilibrium constants of R1 and R3, DG R1 ð % DG 0 R1 ¼ À36:6 kJ=molÞ [15] and DG R3 ð% DG 0 R3 ¼ À50:6 kJ=molÞ [24,25] are the Gibbs free energy changes of R1 and R3, respectively. E5, E6 and E7 indicate that the increase in temperature should negatively affect the CO 2 sorption capacities via both R1 and R3, and thus the total CO 2 capacity of SCFD-KN, as shown in the 50-80°C range in Fig.…”