Innovative cycling reaction mechanisms of CO2 absorption in amino acid salt solvents

Abstract

Amino acid salt (AAS) solvents with equimolar base (used to obtain the salt) to amino acid (AA) have been well studied for CO2 absorption, however, very little is known about the reactions when more base is applied. This study determined the reaction mechanisms of AAS solvents with base to AA ratio over equimolar and the potential benefits of such solvents for CO2 absorption. The CO2 loading capacity was found to be dictated by the molar concentration of the base and was approximately half of the base applied. The reaction mechanisms were investigated based on the compositions of carbamate, carbonate/bicarbonate, and AA using 13C-quantitative nuclear magnetic resonance (13C qNMR). An innovative cycling reaction mechanism of CO2 absorption was proposed and experimentally confirmed for AAS solvents with base/AA > 1. Besides all steps of the current widely accepted Zwitterion mechanism, this reaction pathway also contains two cycles: 1) The extra base (i.e., OH−) may react with the protonated AA generated during CO2 absorption to form deprotonated AA which further absorbs CO2 to form more carbamate. 2) The carbamate, as a product of reaction cycle 1, undergoes hydrolysis to yield bicarbonate and deprotonated AA, which further absorbs CO2. As a result, high CO2 loadings were achieved. Compared to the use of alkaline solutions like KOH alone for CO2 absorption, AAS solvents with base to AA ratio over equimolar resulted in lower pH and temperature while maintaining high CO2 loading. These innovative cyclic mechanisms will enable advanced CO2 management approaches using AAS solvents with base/AA > 1.