Absorption/desorption mechanism of carbon dioxide capture into amino acid ionic liquid aqueous solution

Amino acid ionic liquids (AAILs) are considered to be an efficient absorbent for CO2 capture with low viscosity and high absorption capacity in the aqueous solution. However, there is no literature reported regarding to the mechanism of CO2 capture into AAILs solution. To explain the reaction process, some researchers just referenced the zwitterion mechanism, which was proposed to depict the reaction among CO2 and alkanolamines. To disclose the reaction mechanism, the CO2 absorption/desorption process of tetramethylammonium glycinate ([N1111][Gly]) aqueous solution was investigated by using 13C Nuclear Magnetic Resonance. The results showed that the absorption/ desorption process of CO2 into [N1111][Gly] aqueous solution were very complicated. In the solution, [N1111][Gly] was completely dissociated to [N1111]+ and [Gly]-, and CO2 was initially captured in a carbamate form by [Gly]-. As CO2 continuously flowed into the reactor, the pH value of the solution decreased and the carbamate was hydrolyzed to glycine and HCO3-. At this stage, CO2 was mainly captured by hydration reaction. Finally, CO2 remaining in the solution shifted to bicarbonate and trace amount of carbamate. The regeneration of CO2-saturated [N1111][Gly] aqueous solution was the reverse process of absorption. Initially, the regeneration process was the thermolysis of HCO3- to release CO2, and part of HCO3- reacted with glycine to form carbamate, then the carbamate decomposed to [Gly]- and CO2.