An Efficient Molecular Simulation Methodology for Chemical Reaction Equilibria in Electrolyte Solutions: Application to CO2 Reactive Absorption

Force-Field-Based Computational Study of the Thermodynamics of a Large Set of Aqueous Alkanolamine Solvents for Post-Combustion CO2 Capture

The ability to predict the thermodynamic properties of amine species in CO₂-loaded aqueous solutions, including their deprotonation (pK_a) and carbamate to bicarbonate reversion (pK_c) equilibrium constants and their corresponding standard reaction enthalpies, is of critical importance for the design of improved carbon capture solvents. In this study, we used isocoulombic forms of both reactions to determine these quantities for a large set of aqueous alkanolamine solvent systems. Our hybrid approach involves using classical molecular dynamics simulations with the general amber force field (GAFF) and semi-empirical AM1-BCC charges (GAFF/AM1-BCC) in the solution phase, combined with high-level composite quantum chemical ideal-gas calculations. We first determined a new force field (FF) for the hydronium ion (H₃O⁺) by matching to the single experimental pK_a data point for the well-known monoethanolamine system at 298.15 K. We then used this FF to predict the pK_a values for 76 other amines at 298.15 K and for all 77 amines at elevated temperatures. Additionally, we indirectly relate the H₃O⁺ hydration free energy to that of H⁺ and provide expressions for intrinsic hydration free energy and enthalpy of the proton. Using the derived H₃O⁺ FF, we predicted the pK_a values of a diverse set of alkanolamines with an overall average absolute deviation of less than 0.72 pK_a units. Furthermore, the derived H₃O⁺ FF is able to predict the protonation enthalpy of these amines when used with the GAFF. We also predicted the carbamate reversion constants of the primary and secondary amine species in the data set and their corresponding standard heats of reaction, which we compared with the scarcely available experimental data, which are often subject to significant uncertainty. Finally, we also described the influence of electronic and steric effects of different molecular fragments/groups on the stabilities of the carbamates.

Quantitative Kinetic Model of CO2 Absorption in Aqueous Tertiary Amine Solvents

Aqueous tertiary amine solutions are increasingly used in industrial CO₂ capture operations because they are more energy-efficient than primary or secondary amines and demonstrate higher CO₂ absorption capacity. Yet, tertiary amine solutions have a significant drawback in that they tend to have lower CO₂ absorption rates. To identify tertiary amines that absorb CO₂ faster, it would be efficacious to have a quantitative and predictive model of the rate-controlling processes. Despite numerous attempts to date, this goal has been elusive. The present computational approach achieves this goal by focusing on the reaction of CO₂ with OH^- forming HCO₃^-. The performance of the resulting model is demonstrated for a consistent experimental data set of the absorption rates of CO₂ for 24 different aqueous tertiary amine solvents. The key to the new model's success is the manner in which the free energy barrier for the reaction of CO₂ with OH^- is evaluated from the differences among the solvation free energies of CO₂, OH^-, and HCO₃^-, while the pK_a of the amines controls the concentration of OH^-. These solvation energies are obtained from molecular dynamics simulations. The experimental value of the free energy of reaction of CO₂ with pure water is combined with information about measured rates of absorption of CO₂ in an aqueous amine solvent in order to calibrate the absorption rate model. This model achieves a relative accuracy better than 0.1 kJ mol^-1 for the free energies of activation for CO₂ absorption in aqueous amine solutions and 0.07 g L^-1 min^-1 for the absorption rate of CO₂. Such high accuracies are necessary to predict the correct experimental ranking of CO₂ absorption rates, thus providing a quantitative approach of practical interest.