¹³C-NMR study of acid dissociation constant (pKa) effects on the CO₂ absorption and regeneration of aqueous tertiary alkanolamine-piperazine blends

We have studied the concentration changes of chemical species such as CO₃ ^2-/HCO₃ ^-, amine/protonated species, carbonate/protonated species, carbamates/protonated species, etc. in the course of CO₂ absorption and those after CO₂ release upon heating at 93 °C for 30 min for aqueous blends of piperazine (PZ) and each of 11 tertiary alkanolamines using ¹³C-NMR spectroscopy. The initial rates of CO₂ capture of the blends ranged between 0.125 and 0.167 mol/L min, which were contributed by the rapid formation of PZ monocarbamate. A positive linear correlation was found between the CO₂ release amounts of the blends upon heating and the pKa values of the tertiary alkanolamines. The CO₂ content captured as PZ mono- And biscarbamates decreased upon heating in the pKa range smaller than 9.34, whereas it increased upon heating in the pKa range higher than 9.55. A tertiary alkanolamine having the pKa smaller than 9.34 is promising as a blend amine with PZ.