Development of Ion Speciation Plots for Three Promising Tertiary Amine-CO₂-H₂O Systems Using the pH Method and the ¹³C NMR Method

In this work, three promising tertiary amines - 1-(2-hydroxyethyl)piperidine (1-(2-HE)PP), 3-(diethylamino)-1,2-propanediol (DEA-1,2-PD), and N-(2-hydroxyethyl)pyrolidine (1-(2-HE)PRLD) - were experimentally studied and the results presented in terms of pK_a, the protonation calibration curves, and ion speciation plots. The pK_a of these amines were determined in the temperature range of 294-320 K. The protonation calibration curves for the three amines were also developed based on ¹³C NMR detection. In addition, ion (amine, amineH⁺, HCO₃^-, and CO₃^2-) speciation plots for the three tertiary amines were developed at the temperature of 298 K, at the amine concentration of 1.0 M, over the CO₂ loading ranges of 0-0.897 mol CO₂/mol amine for 1-(2-HE)PP, 0-0.915 mol CO₂/mol amine for DEA-1,2-PD, and 0-0.927 mol CO₂/mol amine for 1-(2-HE)PRLD by using the pH method, the ¹³C NMR method, and the pH + ¹³C NMR method. By comparing the ion (amine, amineH⁺, HCO₃^-, and CO₃^2-) concentrations of the three tertiary amines obtained from these three methods, it can be concluded that each of these three methods could be used to develop the ion speciation plots of amine-CO₂-H₂O systems, but there are some significant differences between them.