Both carbamates and monoalkyl carbonates are involved in carbon dioxide capture by alkanolamines

Among the amines that have been systematically applied for CO2 capture, several alkanolamines have been proved to be very effective. Although the formation of carbamates - the reaction product of CO2 and the amine group - is recognized as the most important process, the behavior of a tertiary alkanolamine cannot be explained, because a carbamate cannot be formed. In such a case, the formation of a monoalkyl carbonate (MAC) - a hemiester of carbonic acid and the alcohol group of the alkanolamine - is the most important process. In the present study, we demonstrate that the MAC is formed not only for tertiary alkanolamines, but also for the simpler ones, and that this reaction takes place even in aqueous medium. The species were detected by 13C NMR, which allowed the estimation of the formation constant of MACs for choline, triethanolamine, diethanolamine, and monoethanolamine as 0.44, 2.6, 0.72, and 0.66, respectively. These values are similar to those ones for MACs of aliphatic alcohols and sugars. Although the formation constants are much higher for carbamates, MAC formation becomes a more significant process as the pH is lowered during CO2 capture. A brief review of previous works demonstrated that MACs could be detected in other experiments whether the authors would aware of this class of compounds.