Site-Specific Interactions in CO2 Capture by Lysinate Anion and Role of Water Using Density Functional Theory

Abstract

The understanding of mechanism of reactions between the [Lys]− anion and CO2 is important for the optimization and design of salt mixtures and ionic liquids for facile CO2 capture. In this computational investigation, we employed density functional theory calculations to examine various reaction pathways associated with site-specific interactions possible in [Lys]−–CO2 and [Lys]−–H2O–CO2 complexes. The reaction mechanisms in each complex are characterized by energy parameters such as binding energy (BE), activation energy (Ea), and reaction energy (RE). The [Lys]−–CO2 interactions lead to the formation of three nonbonded (NB) complexes close to the near-carboxylate amine group (N1) and one NB complex close to the far-carboxylate amine group (N2). The N1 site reacts with CO2 with a small barrier of ∼1 kcal/mol to form a stable “carbamate–carboxylic acid” product. The formation of this product is due to an intramolecular proton transfer from the N1 amine site to the carboxylate group (COO–), in contrast to the intermolecular proton transfer for carbamic acid formation observed from the N2–CO2 reaction. The other two NB complexes show significant stability due to multiple-site-cooperative interactions of CO2 with the COO– group and N1 site. In [Lys]−–H2O–CO2 interactions, nine NB complexes are formed corresponding to different weak interactions. Among them, five NB complexes lead to reactions suggesting chemisorption, with four complexes forming a direct bicarbonate product and the remaining complex forming a carbamate–carboxylic acid product. The other four nonreactive complexes show notable stability due to the formation of multiple hydrogen bonds with the inclusion of water, which alludes to their possibility of occurrence in physisorption.