A 13C NMR study of the carbon dioxide absorption and desorption equilibria by aqueous 2-aminoethanol and N-methyl-substituted 2-aminoethanol

Abstract

The ¹³C NMR experimental study presented investigates the absorption of CO₂ by a series of primary, secondary and tertiary alkanolamines in aqueous solution. The absorption experiments were made at room temperature with four different amine concentrations in the range 0.167–0.667 M (1.01–5.88 wt%). As inferred by ¹³C NMR spectral analysis, the formation of carbamate increases with increasing amine concentration following the order secondary amine < primary amine. Moreover, it has been shown that carbamate reduces the CO₂ absorption efficiency. A considerable physical absorption (10–20%) contributes to the loading capacity of the amines and partially compensates for the yield of chemical capture, which turned out to be poorer than was expected theoretically. Quite unexpectedly, carbamate was also produced by an endothermic reaction during the thermal CO₂ desorption process which regenerated the amines (primary and secondary amines). In the case of the secondary amine 2-(methylamino)ethanol (MMEA), the amount of carbamate at the end of the desorption process is greater than the amount found at the end of the absorption step, thus reducing the desorption efficiency of the secondary amine in comparison to both primary and tertiary amines. Five cycles of absorption–desorption tests were carried out to verify the feasibility of regenerated amines for reuse. Our results indicate that absorption efficiency and loading capacity of the regenerated amine solutions remain essentially constant during the second to the fifth absorption–desorption experiments, but they both decrease slightly when compared to the initial amine.