Modelling CO₂ absorption in aqueous solutions of cholinium lysinate ionic liquid

Ionic liquids (ILs) with quaternary ammonium cations combined with biocompatible anions from renewable sources result in low-toxic, biocompatible, cost-efficient sorbent media that can efficiently capture carbon dioxide (CO₂). The understanding of the equilibrium and kinetics of CO₂absorption in these media is relevant for the design of new absorption processes in many application areas, such as CO₂ removal from post-combustion streams, biogas refinery waste gases, or confined spaces. Here CO₂absorption in an aqueous solution of cholinium lysinate IL is studied both theoretically, via mechanistic modelling, and experimentally in a membrane contactor operated in closed loop with online pH measurement and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) analysis of discrete sampling of the circulating aqueous IL solution. It is shown that both techniques are effective tools for CO₂quantification in the liquid phase. The IL yields an absorption capacity of 2.20 mol of CO₂ per mole of IL for an IL concentration of 2.13 M (or 50 wt% solution). A comprehensive model of chemisorption thermodynamics and absorption dynamics is proposed and validated experimentally. It provides not only the equilibrium constants of the reversible reactions of protonation of the amine groups and bicarbonate binding and overall mass-transfer coefficient based on liquid-phase concentrations, but is also the basis for a chemometric analysis of the experimental ATR-FTIR data. The potential use of ATR-FTIR as a monitoring tool of CO₂ in aqueous solutions of cholinium lysinate IL is also demonstrated.