Protonation constants and thermodynamic properties of amines for post combustion capture of CO2

The leading process for the post combustion capture (PCC) of CO2 from coal-fired power stations and hence reduction in greenhouse gases involves capture by aqueous amine solutions. Of the reactions that occur in solution, which include CO2 hydration, de-protonation of carbonic acid, amine protonation and carbamate formation, the protonation of the amine in the absorber and its subsequent de-protonation in the stripper involve the greatest enthalpy changes. In this study, protonation constants (reported as log10 Kprot) of selected series of primary, secondary and tertiary alkanolamines/amines over the temperature range 288–318 K are reported. Selected series studied involve primary, secondary and tertiary mono-, di- and tri-alkanolamines, secondary amines including heterocyclic species, and both –CH2OH and –CH2CH2OH substituted piperidines. van’t Hoff analyses have resulted in the standard molar enthalpies, ΔHmo, and molar entropies, ΔSmo, of protonation. Trends in ΔHmo are correlated with systematic changes in composition and structure of the selected series of amines/alkanolamines, while ΔHmo–ΔSmo plots generated linear correlations for the mono-, di-, and tri-alkanolamines, the –CH2OH and –CH2CH2OH substituted piperidines, and the alkylamines. These relationships provide a guide to the selection of an amine(s) solvent for CO2 capture, based on a greater difference in log10 Kprot between the absorber and stripper temperatures.

► Protonation constants for selected series of amines have been measured at 288–318 K.
► Standard molar enthalpies and entropies have been determined by van’t Hoff analyses.
► ΔHø–ΔSø plots give linear relationships for alkylamines and alkanolamines.
► Substituted piperidines form an isoequilibrium series (Tiso = 252 K).
► Such relationships provide a guide for the selection of amines for PCC applications.