Sulfur dioxide absorption characteristics of aqueous amino acid solutions

Insight into the interaction between amino acids and SO2: Detailed bonding modes

CONTEXT

Amino acids are a highly effective and environmentally friendly adsorbent for SO2. However, there has been no comprehensive study of the binding modes between amino acids and SO2 at the molecular level. In this paper, the binding modes of three amino acids (Asp, Lys, and Val) with SO2 are studied comprehensively and in detail using quantum chemical calculations. The results indicate that each amino acid has multiple binding modes: 22 for Asp, 49 for Lys, and 10 for Val. Both the amino and carboxyl groups in amino acids, as well as those in side chains, can serve as binding sites for chalcogen bonds. The binding energies range from - 6.42 to - 1.06 kcal/mol for Asp, - 12.43 to - 1.63 kcal/mol for Lys, and - 7.42 to - 0.60 kcal/mol for Val. Chalcogen and hydrogen bonds play a crucial role in the stronger binding modes. The chalcogen bond is the strongest when interacting with an amino group, with an adiabatic force constant of 0.475 mDyn/Å. Energy decomposition analysis indicates that the interaction is primarily electrostatic attraction, with the orbital and dispersive interactions dependent on the binding mode.

METHODS

Amino acids and complexes of amino acids with SO2 were used to do semi-empirical MD using Molclus combined with xtb at the GFN2 level. Optimization and frequency calculations of the structures were conducted using density-functional theory (DFT) B3LYP/6-311G* (with DFT-D3 correction). Single-point energy calculations were performed for all structures using DLPNO-CCSD(T)/aug-cc-pVTZ with tightPNO. Further analysis of the structures was conducted using ESP, AIM, IGMH, and sob-EDA to gain a deeper understanding of the interactions between amino acids and SO2.

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.

Palladium-Catalyzed Desulfonative Carbonylation of Thiosulfonates: Elimination of SO2 and Insertion of CO

A palladium-catalyzed desulfonative carbonylation of thiosulfonates has been explored. Without any additive, a series of S-aryl/alkyl benzenesulfonothioates were successfully transformed to thioesters in moderate to excellent yields by SO₂ extrusion and CO insertion under the pressure of 1 bar of CO. The solvent dimethylacetamide (DMAc) facilitated this desulfonative carbonylation due to its high absorbing ability of SO₂.