Unexpectedly efficient absorption of low-concentration SO2 with phase-transition mechanism using deep eutectic solvent consisting of tetraethylammonium chloride and imidazole

Deep eutectic solvent-based blended membranes for ultra-super selective separation of SO2

SO2 is a major atmospheric pollutant leading to acid rain and smog. As a new generation of green solvents, deep eutectic solvents (DESs) have been widely investigated for gas capture. Nevertheless, studies on DES-based membranes for SO2 separation are yet minimal. Herein, we devised polymer/DES blended membranes comprising 1-butyl-3-methyl-imidazolium bromide ([Bmim]Br)/diethylene glycol (DEG) DES and poly (vinylidene fluoride) (PVDF), and these membranes were firstly used for selective separation of SO2 from N2 and CO2. The permeability of SO2 reaches up to 17480 Barrer (0.20 bar, 40 ºC) in PVDF/DES blended membrane containing 50 wt% of [Bmim]Br/DEG (2:1), with ultrahigh SO2/N2 and SO2/CO2 selectivity of 3690 and 211 obtained, respectively, far exceeding those in the state-of-the-art membranes reported in literature. The highly-reversible multi-site interaction between SO2 and [Bmim]Br/DEG DES was revealed by spectroscopic analysis. Furthermore, the PVDF/DES blended membrane was also able to efficiently and stably separate SO2/CO2/N2 (2.5/15/82.5%) mixed gas for at least 100 h. This work demonstrates for the first time that [Bmim]Br-based DESs are very efficient media for membrane separation of SO2. The easy preparation, low cost and high performance enable polymer/DES blended membranes to be promising candidates for flue gas desulfurization.

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.

Cation functional group effect on SO2 absorption in amino acid ionic liquids

Introduction: The effect of the functional group of the cation on SO2 acidic gas absorption by some designed amino acid ionic liquids (AAILs) was studied. Methods: An isolated pair of glycinate anion and pristine imidazolium-based cation, as well as decorated cation functionalized by hydroxyl (OH), amine (NH₂), carboxylic acid (COOH), methoxy (OCH₃), and acetate (CH₃COO) groups, were structurally optimized by density functional theory (DFT) using split-valence triple-zeta Pople basis set. Results and Discussion: The binding and Gibbs free energy (ΔG_int) values of SO₂ absorption show the AAIL functionalized by the COOH group is the most thermodynamically favorable green solvent and this functional group experiences the closest distance between anion and captured SO₂ and vice versa in the case of cation … SO₂ which may be the main reason for being the best absorbent; in addition, the highest net charge-transfer amount of SO₂ is observed. Comparing the non-covalent interaction of the systems demonstrates that the strongest hydrogen bond between captured gas and anion, as well as π-hole, and van der Waals (vdW) interaction play critical roles in gas absorption; besides, the COOH functional group decreases the steric effect while the CH₃COO functional group significantly increases steric effect after absorption that declines the hydrogen bond.

Homologue-paired liquids as special non-ionic deep eutectic solvents for efficient absorption of SO2

Low-viscous homologue-paired liquids (HPLs) are designed and employed as special non-ionic deep eutectic solvents for selective separation of SO₂ from CO₂ and N₂. The HPLs are found to have excellent inherent properties (e.g., low cost, volatility and viscosity), high absorption capacity, fast absorption rate, and moderate Lewis acid-base interaction with SO₂. Regeneration experiments are done to show their excellent recyclability, and industrial desulfurization is exemplified in a small column with suitable parameters to show their potential as SO₂ absorbents.