Elucidating the Molecular Mechanism of CO 2 Capture by Amino Acid Ionic Liquids.

Amino acid ionic liquids have received increasing attention as ideal candidates for the CO 2 chemisorption process. However, the underlying molecular mechanisms, especially those involving proton transfer, remain unclear. In this work, we elucidate the atomistic-level reaction mechanisms responsible for carbamate formation during CO 2 capture by amino acid ionic liquids through explicit ab initio molecular dynamics augmented by well-tempered metadynamics. The resulting ab initio free-energy sampling reveals a two-step reaction pathway in which a zwitterion, initially formed from the reaction between the anion of serine and CO 2 , undergoes a kinetically facile intermolecular proton transfer to the O atom of the COO - moiety in the nearby serine. Further analysis reveals that the significantly reduced free-energy barriers are attributed to enhanced intermolecular interaction between the zwitterion and serine, thus facilitating the kinetic favorability of the proton transfer, which governs the overall CO 2 capture mechanism. This work provides valuable insight into the important mechanistic and kinetic features of these reactions from explicit condensed phase ab initio MD free-energy sampling of the CO 2 capture process.