New Insights into CO₂ Absorption Mechanisms with Amino-Acid Ionic Liquids

The last decade saw an explosion of interest in using amine-functionalized materials for CO₂ capture and conversion, and it is of great importance to elucidate the relationship between the molecular structure of amine-functionalized materials and their CO₂ capacity. In this work, based on a new quantitative analysis method for the CO₂ absorption mechanism of amino-acid ionic liquids (ILs) and quantum chemical calculations, we show that the small difference in the local structure of amine groups in ILs could lead to much different CO₂ absorption mechanisms, which provides an opportunity for achieving higher CO₂ capacity by structure design. This work revealed that the actual CO₂ absorption mechanism by amino-acid ILs goes beyond the apparent CO₂/amine stoichiometry; a rigid ring structure around the amine group in ILs creates a unique electrostatic environment that inhibits the deprotonation of carbamic acid and enables actually equimolar CO₂/amine absorption.