(318c) An Electronic Structure Based Understanding of Amine-Carbon Dioxide Interactions for CO2 Capture

Gas scrubbing using amine solvents is one of the most promising first generation CO₂ capture technologies, due to the chemical and process knowledge gained through their long term use in natural gas. However amine scrubbing has  high parasitic energy costs due to  equipment heating, amine regeneration, and captured CO₂ compression costs.  Thus, minimizing this energy requirement is an obvious place to start when developing enhanced amines for CO₂ capture.  The energy associated with amine regeneration is a function of the CO₂ capacity of the amine as well as the reaction energy.  Early studies of amine-CO₂ interaction explained capacity and bond strength through two acid/base reactions to form carbamates (2:1 amine:CO₂ interaction) and bicarbonates(1:1 amine:CO₂ interaction), with bicarbonates favored for sterically hindered secondary amines and tertiary amines. The picture is not that simple, however, as significant variations in the equilibrium CO₂ capacity among structurally similar amines and amines of similar pH have been reported.  Understanding the origin of these variations in the experimental equilibrium capacity is the motivation behind this work.  At equilibrium, the capture capacity can be characterized by the Gibbs free energy of reaction.  Using Density Functional Theory (DFT) we studied the reactions of CO₂ with structurally similar amines and amines of similar pH to form carbamic acid, carbamate and bicarbonate ions, determined the reaction energies for each amine with CO₂ relate these energies to electronic structure properties of the amine. We show that the reaction energy associated with amine-bicarbonate formation correlates well with the amine electronegativity.  Additionally, we identify a means to manipulate amine electronegativity through functionalizing the amine with different functional groups. These results provide us with a fundamental understanding of one of the reactions critical to CO₂ capture and a means to control the reaction energy through modifying the amine itself.