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 CO$_2$-loaded aqueous solutions, including their deprotonation (p\textit{K$_a$}) and carbamate to bicarbonate reversion (p\textit{K$_c$}) 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 (H$_3$O$^+$) by matching to the single experimental (p\textit{K$_a$}) data point for the well--known MEA system at 298.15 K. We then used this FF to predict the p\textit{K$_a$} values for 76 other amines at 298.15 K and for all 77 amines at elevated temperatures. Additionally, we indirectly relate the H$_3$O$^+$ hydration free energy to that of H$^+$, and provide expressions for intrinsic hydration free energy and enthalpy of the proton. Using the derived H$_3$O$^+$ FF, we predicted the (p\textit{K$_a$}) values of a diverse set of alkanolamines with an overall AAD of less than 0.72 p\textit{K$_a$} units. Furthermore, the derived H$_3$O$^+$ force field 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 of different molecular fragments/groups on the stabilities of the carbamates.