A study on reaction mechanism and kinetics of CO2 and MEA/DEA-tertiary amines in non-aqueous and water-lean solutions

The kinetic behavior of the reaction of CO2 with MEA-TREA in ethanol solution was studied using the stopped-flow technique. The results indicate that the zwitterion mechanism successfully correlates the experimentally measured pseudo first-order reaction constant (k0) and that the TREA participated in the deprotonation step of MEA-zwitterion as an alkaline substance after the formation of MEA-zwitterion. Then, the applicability of this proposed mechanism was investigated by using the reaction kinetics of CO2 with MEA-DMEA, MEA-DEEA, MEA-MDEA, DEA-TREA, DEA-DMEA blended amines in non-aqueous ethanol solution, as well as MEA-TREA, MEA-DMEA, DEA-TREA, DEA-DMEA blended amines in water-lean solutions. It was found that the mechanism of the reaction of CO2 with these mixed amines can be expressed by:k0=KA-Az[A]2+KA-Bz[A][B]+K1z[A]. The ethanol and water molecules which contribute to K1z can be used as proton transfer channels to accelerate proton transfer, but they do not directly participate in the reaction. In addition, the alkalinity and the steric hindrance effect controlled by the molecular structure of tertiary amines affected the reaction rate at the same time. The activation energies in MEA-EtOH and MEA-TREA-EtOH system were estimated by fitting the second-order rate constants to the Arrhenius expression, indicating that TREA can reduce the reaction activation energy and make the reaction more kinetically favorable.