The reactions of CO2 into aqueous solutions of amine include the hydration of CO2 in the aqueous solution and the reaction of CO2 with the amine. The reactions discussed below are mainly responsible in determining the reaction rates of the absorption of CO2 into aqueous amine solution. Any reactions due to the presence of little impurity in APA, if any, is assumed to be negligible and does not contribute significantly to the data reported in the experiments.
Reactions of CO2 with APA
The kinetics of the primary and secondary amines with CO2 can be described using the zwitterionic mechanism.11,12 Primary and secondary amines reacts with CO2 to form zwitterion, which is subsequently deprotonated by any base present in the liquid: (1)…show more content… The structure of APA is shown in Figure 1. The chemistry of the CO2-APA reaction system is very complex since APA contains multiple amine functional groups. This gives rise to a large number of possible chemical reactions and formation of liquid phase species (e.g., mono-, di- and tri-carbamates corresponds to primary and secondary carbamates). Thus, it is hard to identify the most important reaction(s) and to exactly determine the effects of these reaction(s) on the overall absorption rate. However, a first estimation for these reactions can be made using the Brønsted dependency of the reactivity on the pKa. Pennyand Ritter16have shown that for many amines, a linear relation between the pKa value of an amine and the logarithm of the forward rate constants exists. In general, the difference between the first and second protonation constants for amines is very small.17 The first protonation constant represents either the primary amine or the secondary amine nitrogen (probably the former) while the second protonation constant represents the other of these two nitrogen atoms.17 If it is assumed that the Brønsted relation is also applicable to APA, the reaction of CO2 with the primary and secondary amine groups to form primary and secondary carbamates should be considered. Now, the possibility of formation of dicarbamate can be determined by the concentration of the main reactants APA and primary and secondary mono-carbamates. The conditions for the absorption of CO2 in APA solutions were selected in such a way as to ensure fast pseudo-first-order reaction regime. The interfacial concentration of APA in this regime is not noticeably decreased due to the reaction with CO2, and hence the concentration of the reaction product mono-carbamates will be small compared to the remaining APA concentration (even close to the gas-liquid interface) and, consequently, the