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

Abstract

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.