¹³C NMR study on carbamate hydrolysis reactions in aqueous amine/CO₂ solutions

Carbamates of primary or secondary amines play a central role in amine CO₂ absorbents. The equilibrium constant of a carbamate hydrolysis reaction, K_ch, is important for evaluating the performance of absorbents. However, few K_ch values have been reported to date. We determined the K_ch values of normal amines, such as monoethanolamine (MEA), diethanolamine (DEA), and hindered amines, such as 2-amino-2-methyl-1-propanol (AMP) and 2-(isopropylamino)ethanol (IPAE), using a method involving a combination of ¹³C NMR spectroscopy and a chemical equilibrium analysis. The Gibbs energy changes derived from K_ch for AMP and IPAE at 25 °C were negative, whereas those for normal amines, MEA and DEA, were positive. These properties are characteristic of hindered amines. We determined the standard enthalpy and entropy changes of the carbamate hydrolysis reactions from the temperature dependence of the equilibrium constant. The equilibrium constants and standard enthalpies obtained using the present method will be valuable for evaluating the performance of amine CO₂ absorbents.