Two Different Pathways from CO₂ to HCO₃⁻ in Aqueous Solutions of Arginine Sodium Salt

Abstract

Amino acids and their salts are amine compounds with great potential for the chemical absorption of CO2 via the amine method. The CO2 absorption properties of various amino acids and their salts have been previously reported. However, the detailed reaction mechanism of CO2 with basic amino acids in aqueous solutions has not yet been clarified. In this study, we focused on arginine (Arg), which has the highest basicity among the standard amino acids, and its monohydrochloride (ArgCl) and sodium (NaArg) salts for the dissolution of CO2 in the aqueous solutions of Arg, ArgCl, and NaArg. CO2 was chemically dissolved in aqueous Arg and NaArg solutions but not in the aqueous ArgCl solution. One‐dimensional 1H and 13C and two‐dimensional 1H‐13C heteronuclear multiple bond correlation nuclear magnetic resonance spectroscopy indicated that the final products in the aqueous Arg and NaArg solutions were protonated Arg and HCO3−. HCO3− was formed from both CO2‐bonded Arg/Arg− and CO32− intermediates in the aqueous NaArg solution but only from the CO2‐bonded Arg/Arg− intermediate in the aqueous Arg solution. In addition to their high environmental and biological compatibility, Arg and NaArg exhibit high CO2 absorption capacity, demonstrating their potential for practical use as naturally derived CO2 chemical absorbents.