SO2-Induced Variations in the Viscosity of Ionic Liquids Investigated by in Situ Fourier Transform Infrared Spectroscopy and Simulation Calculations

Effective Absorption Mechanism of SO2 and NO2 in the Flue Gas by Ammonium-Bromide-Based Deep Eutectic Solvents

Simultaneous capture of SO₂ and NO _x from flue gas is critical for coal-fired power generation. In this study, environmentally friendly and high-performance deep eutectic solvents based on ethylene glycol and ammonium bromide were designed to capture SO₂ and NO₂ simultaneously. The SO₂ and NO₂ absorption performances and absorption mechanisms were systematically investigated by ¹H NMR and Fourier transform infrared (FT-IR) spectroscopy in combination with ab initio calculations using Gaussian software. The results showed that EG-TBAB DESs can absorb low concentrations of SO₂ and NO₂ from the flue gas simultaneously at low temperatures (≤50 °C). ¹H NMR, FT-IR, and simulation results indicate that SO₂ and NO₂ are absorbed by forming EG-TBAB-SO₂-NO₂ complexes, Br^- is the main active site for NO₂ absorption, and NO₂ is more active in an EG-TBAB-NO₂-SO₂ complex than SO₂. EG-TBAB DESs exhibit outstanding regeneration capability, and absorption capacities remain unchanged after five absorption-desorption cycles. The fundamental understanding of simultaneous capture of SO₂ and NO₂ from this study enables DES structures to be rationally designed for efficient and low-cost desulfurization and denitrification reagents.

Capture of Acidic Gases from Flue Gas by Deep Eutectic Solvents

Up to now, many kinds of deep eutectic solvents (DESs) were investigated for the capture of acidic gases from flue gases. In this review, non-functionalized and functionalized DESs, including binary and ternary DESs, for SO2, CO2 and NO capture, are summarized based on the mechanism of absorption, physical interaction or chemical reaction. New strategies for improving the absorption capacity are introduced in this review. For example, a third component can be introduced to form a ternary DES to suppress the increase in viscosity and improve the CO2 absorption capacity. DESs, synthesized with halogen salt hydrogen bond acceptors (HBAs) and functionalized hydrogen bond donors (HBDs), can be used for the absorption of SO2 and NO with high absorption capacities and low viscosities after absorption, due to physicochemical interaction between gases and DESs. Emphasis is given to introducing the absorption capacities of acidic gases in these DESs, the mechanism of the absorption, and the ways to enhance the absorption capacity.

SO2 absorption in pure ionic liquids: Solubility and functionalization

The SO₂ solubility in ionic liquids and absorption mechanisms with different functionalities, including ether, halide, carboxylate, dicarboxylate, thiocynate, phenol, amino, azole groups, etc., are presented in this review. Strategies of improving SO₂ capture with low binding energy and the separation performance from CO₂ are also concluded. Generally, moderate basicity is favourable for enhancing SO₂ capacity and the water (below 6 wt%) effect on absorption is indefinite but generally slight. Introducing electron-withdrawing substituents such as nitrile, halogen, aldehyde and carboxylic groups are proposed to decrease the chemical absorption enthalpy between ionic liquid and SO₂ in order to reduce regeneration power consumption. Although it is promising, the absorption enthalpy is still much higher than the physisorption performance especially of the ether-functionalized ones. The biocompatible choline-based, betaine-based, and amino acid ionic liquids have clear trends to be applied in SO₂ capture due to their biodegradability, nontoxicity and easy accessibility. Generally, comparing to the traditional solvents, ionic liquids have made great improvement in SO₂ capacity, however, the high viscosity and desorption energy are two main obstacles for SO₂ absorption and separation. Molecular simulations have been applied to reveal the absorption regimes involving the roles of basic functionalities and physical interactions especially the hydrogen bonds, which could be referred for structure designing of the available ionic liquids with readily fluid characteristics and absorption ability.

Liquid-liquid phase-change absorption of SO2 using N, N-dimethylcyclohexylamine as absorbent and liquid paraffin as solvent

In the present work, liquid-liquid phase-change absorption of SO₂ was investigated using N, N-dimethylcyclohexylamine (DMCHA) as an absorbent, and high boiling liquid paraffin (LP) as a solvent to reduce volatilization of the absorbent. The homogenous solution was split into two immiscible phases upon SO₂ loading. The phase-change mechanism was attributed to the polarity variation of DMCHA before and after absorption by forming the charge-transfer complex DMCHA·SO₂. The viscosity of the lower phase reached a maximum value of 24.5 mPa s at the absorption capacity of 1 mol SO₂/mol DMCHA, and the viscosity of the corresponding upper phase was 46.4 mPa s. Both are lower than the reported viscosity of most ionic liquids. This solution exhibited extremely high mass selectivity of SO₂/CO₂ with a value of 626. The mass absorption capacity was founded to be 1.19 g SO₂/g DMCHA at 1 atm, which is comparable with the highest reported mass absorption capacity. At low partial pressure, the absorption capacity still reached 0.78 g/g at 0.1 atm, 0.43 g/g at 0.02 atm and 0.27 g/g at 0.001 atm. Furthermore, DMCHA could be completely regenerated in 10 min via microwave heating. All the results indicated this phase-change solution is a promising candidate for SO₂ capture.

Simultaneous CO2 and SO2 capture by using ionic liquids: a theoretical approach

Density functional theory (DFT) methods were used to analyze the mechanism of interaction between acidic gases and ionic liquids based on the 1-ethyl-3-methylimidazolium cation coupled with five different anions.

Investigation of furoate-based ionic liquid as efficient SO2 absorbent

A series of furoate-based ionic liquids with good absorption and separation performance for SO₂ were reported.