¹³C-NMR Study of Acid Dissociation Constant (pKa) Effects on the CO₂ Absorption and Regeneration of Aqueous Alkanolpiperidine

We have studied the formation rates of HCO₃^-/CO₃^2-, amine/protonated amine, carbamate/protonated carbamate, and carbonate/protonated carbonate in the course of CO₂ absorption at room temperature, and composition changes and CO₂ release upon heating at 93 °C for 30 min for aqueous solutions of 9 kinds of alkanolpiperidines using ¹³C-NMR spectroscopy. The pKa values of these alkanolpiperidines were measured. The initial CO₂ capture rates of alkanolpiperidines do not depend on their pKa values, which is quite different from general trends of aqueous amine absorbents. The CO₂ release amount of each alkanolpiperidine increased with decreasing pKa, as a general trend. The CO₂ release amounts of 1-HE-PD, 2-HM-1-M-PD, 3-HM-1-M-PD, and 4-HM-1-M-PD increased with increasing concentrations.