Amine neutralized amino acid as CO2 absorbents: A quantitative 13C NMR study

Amino acids, fully or partially neutralized with amines, are a new class of CO2 absorbents that have the possibility to combine the ionic character of amino acids with the better CO2 absorbing properties of the amines to form a more energy efficient solvent with improved environmental footprint. This work studies the effect of differences in pK(a) and structural properties between the amine and amino acid on the resulting active amino group in the blend. Equimolar mixtures of the amino acids, glycine, L-alanine, L-proline, taurine and L-serine, with alkanolamines, monoethanoamine (MEA) and 2-amino-2-methyl-1-propanol (AMP), were quantitatively studied by C-13 nuclear magnetic resonance spectroscopy (NMR) at 25 degrees C. The amino acids tested in the present work were chosen based on their solubility in water and on their relative basicity (pKa) to that of the amines. It is observed that in amino acid-MEA blended systems, carbamate formation from both MEA and the amino acids occur and that generally, the amine group with highest pKa value is most favored when neutralizing the amino acid carboxylic group. If this is the amino acid an amino acid zwitterion may be formed, or, if amine has the highest pKa value, an amine-amino acid complex may be formed, rendering the amino acid amine group active toward CO2 absorption. However, the experimental data show that not only the pKa value of the amino acid is important but that also steric effects play a role in determining the preferred carbamate forming position. In the amino acid-AMP systems almost all carbamate is formed on the amino acid amine group due to the steric hindrance in AMP. Generally, addition of amine to an aqueous amino acid solution increases the amino acid solubility. In this work solid precipitation was only observed at higher CO2 loadings for L-alanine blended with both MEA and AMP. NMR spectroscopy analysis showed that the precipitate consists mainly of amino acid and bicarbonate/carbonate with very small amounts of amine. (C) 2014 Elsevier Ltd. All rights reserved.