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 (p<i>K</i>_a) of an amino acid salt is crucial for CO₂ capture, as it decreases with increasing absorption temperature. However, published p<i>K</i>_a values of amino acid salts have usually been determined at ambient temperatures. In this study, the p<i>K</i>_a 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 (Δ<i>H</i>_m⁰) and entropies (Δ<i>S</i>_m⁰) of the protonation reactions were also determined by the van’t Hoff equation. It was found that sarcosine can maintain a higher p<i>K</i>_a than the other amino acids studied at high temperatures. We also found that the CO₂ solubilities and overall mass-transfer coefficients of 5 <i>m</i>′ 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