Molecular insights into CO₂ capture by Piperidinium amino acid ionic liquids: a combined DFT and MD approach

Amino acid-based ionic liquids (AAILs) have emerged as promising materials for CO₂ capture. In this work, we present a combined molecular dynamics (MD) and density functional theory (DFT) study of the CO₂ sorption behaviour in three piperidinium AAILs. DFT calculations were employed to investigate the chemisorption of CO₂ through two intramolecular proton transfer pathways leading to the formation of carbamate or carbamic acid. The results indicate that [Pip][Lys] and [Pip][Arg] proceed through the carbamate pathway, while [Pip][His] proceeds through the carbamic acid pathway. MD simulations were carried out to study the physicochemical properties and dynamics of CO₂ absorption in AAILs. The CO₂ molecules tend to accumulate at the AAIL/CO₂ interface prior to diffusing into the AAIL phase, following the order [Pip][His] > [Pip][Arg] > [Pip][Lys]. Upon CO₂ sorption, the lifetimes of hydrogen bonds between cation and anion decrease, leading to enhanced ion mobility and increased self-diffusion coefficients. The strongest anion-CO₂ interaction was found for the [Pip][His] system, while the fastest dynamics was observed for the [Pip][Lys] system.

Density functional theory calculations at the M06-2X/6-311++G(d,p) level were employed to examine interaction energy and CO₂ chemisorption mechanisms supported by Atoms-In-Molecules (AIM) analysis. Classical molecular dynamics simulations using the OPLS-AA force field were performed to investigate the physicochemical properties as well as the dynamics of CO₂ absorption in AAILs.