Protonation Constants and Thermodynamic Properties of Amino Acid Salts for CO2 Capture at High Temperatures

Abstract

Amino acid salts have greater potential for CO2 capture at high temperatures than typical amine-based absorbents because of their low volatility, high absorption rate, and high oxidative stability. The protonation constant (pKa) of an amino acid salt is crucial for CO2 capture, as it decreases with increasing absorption temperature. However, published pKa values of amino acid salts have usually been determined at ambient temperatures. In this study, the pKa values of 11 amino acid salts were determined in the temperature range of 298–353 K using a potentiometric titration method. The standard-state molar enthalpies (ΔHm0) and entropies (ΔSm0) of the protonation reactions were also determined by the van’t Hoff equation. It was found that sarcosine can maintain a higher pKa than the other amino acids studied at high temperatures. We also found that the CO2 solubilities and overall mass-transfer coefficients of 5 m′ sarcosinate (moles of sarcosine per kilogram of solution) at 333–353 K are higher than those of 30% MEA at 313–353 K. These results show that some possible benefits can be produced from the use of sarcosine as a fast solvent for CO2 absorption at high temperatures. However, the pronotation reaction of sarcosine is the least exothermic among those of all amino acids studied. This could lead to a high regeneration energy consumption in the sarcosinate-based CO2 capture process