Nuclear Magnetic Resonance Studies of CO₂ Absorption and Desorption in Aqueous Sodium Salt of Alanine

For the first time, speciation evolution in aqueous sodium salt of alanine (SSA) solution during both CO₂ absorption and desorption has been investigated via nuclear magnetic resonance (NMR) spectroscopy. Results suggest that amine, carbamate, and bicarbonate are the main species formed in the solvent system. During CO₂ absorption, deprotonated alanine (Ala) reacts with CO₂ first to form carbamate, which subsequently hydrolyzes into bicarbonate. At higher CO₂ loadings (∼0.6 mol/mol of Ala), it appears that bicarbonate is dominant. Interestingly, the carbamate concentration in the SSA solution increases first and then decreases slightly during CO₂ desorption, and the amount of carbamate after CO₂ desorption is more than that at the end of the CO₂ absorption, thus reducing the desorption efficiency. Furthermore, the species distributions have also been compared to those of the commercial monoethanolamine (MEA) absorbent, revealing that the main difference between SSA and MEA systems is the hydrolysis rate of carbamate. Determination of the species formed is a first step to understand the chemistry of the solvent system, which may facilitate developing more efficient and energy-saving solvents for CO₂ capture and sequestration.