Interrogating the chemical mechanism for carbon dioxide (CO2) reactivity in aqueous tertiary amine solutions: Can a simple parallel hydration reaction mechanism work?

In this study, the kinetics of CO2 absorption into tertiary amine methyldiethanolamine (MDEA) was studied intensively using both experimental and theoretical approaches. The experimental work measured the kinetics of CO2(aq) react with MDEA over a range of concentrations including dilute (0.5–10.0 mM) and concentrated (0.5–2.5 M) solutions via stopped‐flow spectrophotometric at 25.0°C. The resulting kinetic data has been theoretically evaluated using three chemical reaction mechanism variants including the complete base catalysis mechanism, the simplified base catalysis mechanism, and the parallel CO2 hydration mechanism. Results showed that the simplified base catalysis mechanism in the absence of the kinetic pathways for CO2 via hydroxide and water is insufficient to predict the measured kinetic data while the complete base catalysis mechanism and parallel hydration mechanisms can accurately predict the kinetics of CO2 absorption into MDEA solutions. In particular, the experimental data of reversibility of the hydration reactions at low pH was in poor agreement with the simplified model predicted results. Given the simplicity of the parallel hydration mechanism with no requirement for determination of rate constants of the base catalysis reaction unlike the complete base catalysis mechanism, the parallel hydration mechanism can be considered the simplest and preferred mechanism describing the reaction between tertiary amine MDEA and CO2. © 2022 Society of Chemical Industry and John Wiley & Sons, Ltd.