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-l-propanol (AMP) and 2-(isopropylamino)ethanol (IPAE), using a method involving a combination of C-13 NMR spectroscopy and a chemical equilibrium analysis. The Gibbs energy changes derived from K-ch for AMP and IPAE at 25 degrees 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.

Automated summary

The study determined the equilibrium constant K-ch values of carbamate hydrolysis reactions for both hindered amines and normal amines using a combination of C-13 NMR spectroscopy and chemical equilibrium analysis. The differences in properties between hindered amines and normal amines were revealed, with hindered amines showing negative Gibbs energy changes and normal amines showing positive changes. The obtained equilibrium constants and standard enthalpies will be useful for evaluating the performance of amine CO2 absorbents.