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

摘要

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 ~(13)C-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 bis-carbamates decreased upon heating in the pKa range smaller than 9.34, whereas it increased upon heating in the pATa range higher than 9.55. A tertiary alkanolamine having the pKa smaller than 9.34 is promising as a blend amine with PZ.