Reversible sulfur dioxide capture by amino acids containing a single amino group at low sulfur dioxide concentrations

SO₂, an air pollutant, is harmful to human health and causes air pollution; therefore, numerous studies have focused on the development of SO₂ control technologies. Although limestone- and ammonia-based absorbents have been widely used in wet desulfurization, they are difficult to regenerate and do not enable the recycling of SO₂, which is a useful resource. Recently, amino acids have attracted attention as reversible SO₂ absorbents because they are eco-friendly and have excellent reactivity with SO₂, as well as high regeneration performance. Glycine, L-alanine, β-alanine, 4-aminobutyric acid, 5-aminovaleric acid, and 6-aminohexanoic acid were analyzed to investigate the relationship between SO₂ absorption and the amino acid molecular structure using the simulated actual flue gas (200 ppmv SO₂ + 13% CO₂ in N₂ balance). The SO₂ absorption of amino acids (with the molecular structure of glycine and alkyl chains of various lengths) improved as the alkyl chain length increased, possibly owing to a decrease in the inductive effect in the molecular structure of the amino acid. Furthermore, ¹³C-nuclear magnetic resonance spectroscopy was conducted to analyze the SO₂ absorption reaction mechanism (including the possible generation of irreversible species), and experiments involving a number of consecutive absorption-desorption cycles were used to confirm the reusability of the amino acids. The tested amino acids exhibited higher cyclic capacities compared to those of deep eutectic solvents and ionic liquids reported in the literature, thereby exhibiting excellent potential as SO₂ absorbents. Thus, this study can guide the future design and development of eco-friendly SO₂ absorbents.