Tuning the Basicity of Cyano-Containing Ionic Liquids to Improve SO2Capture through Cyano-Sulfur Interactions

Direct observation of arene⋯sulphur dioxide interaction: role of metal ions in electronic modulation for binding and activation

In the quest to understand biologically relevant interactions of environmentally detrimental SO2 with host molecules to modulate the electronic properties of the binding sites, we have directly observed the lone pair⋯π interaction between the aromatic ring and nucleophilic O of SO2 (3.11 Å), for the first time to the best of our knowledge, in addition to the interaction between electrophilic S of SO2 and metal-bound thiolate (2.63 Å).

Deoxygenation of chalcogen oxides EO2 (E = S, Se) with phospha-Wittig reagents

In here we present the deoxygenation of the chalcogen oxides EO₂ (E = S, Se) with R-P(PMe₃), so-called phospha-Wittig reagents. The reaction of DABSO (DABCO·2SO₂) with R-P(PMe₃) (R = Mes*, 2,4,6-tBu₃-C₆H₂; ^MesTer, 2,6-(2,4,6-Me₃-C₆H₂)₂-C₆H₃) resulted in the formation of thiadiphosphiranes (RP)₂S (1:R), while selenadiphosphiranes (RP)₂Se (2:R) were afforded with SeO₂, both accompanied by the formation of OPMe₃. Utilizing the sterically more encumbered ^DipTer-P(PMe₃) (^DipTer = 2,6-(2,6-iPr₂-C₆H₃)₂-C₆H₃) a different selectivity was observed and (^DipTerP)₂Se (2:DipTer) along with [Se(μ-P^DipTer)]₂ (3:DipTer) were isolated as the Se-containing species in the reaction with SeO₂. Interestingly, the reaction with DABSO (or with equimolar ratios of SeO₂ at elevated temperatures) gave rise to the formation of the OPMe₃-stabilized dioxophosphorane (phosphinidene dioxide) ^DipTerP(O)₂-OPMe₃ (4:DipTer) as the main product. This contrasting reactivity can be rationalized by two potential pathways in the reaction with EO₂: (i) a Wittig-type pathway and (ii) a pathway involving oxygenation of the phospha-Wittig reagents and release of SO. Thus, phospha-Wittig reagents are shown to be useful synthetic tools for the metal-free deoxygenation of EO₂ (E = S, Se).

Combined Superbase Ionic Liquid Approach to Separate CO2 from Flue Gas

Superbase ionic liquids (ILs) with a trihexyltetradecylphosphonium cation and a benzimidazolide ([P₆₆₆₁₄][Benzim]) or tetrazolide ([P₆₆₆₁₄][Tetz]) anion were investigated in a dual-IL system allowing the selective capture and separation of CO₂ and SO₂, respectively, under realistic gas concentrations. The results show that [P₆₆₆₁₄][Tetz] is capable of efficiently capturing SO₂ in preference to CO₂ and thus, in a stepwise separation process, protects [P₆₆₆₁₄][Benzim] from the negative effects of the highly acidic contaminant. This results in [P₆₆₆₁₄][Benzim] maintaining >53% of its original CO₂ uptake capacity after 30 absorption/desorption cycles in comparison to the 89% decrease observed after 11 cycles when [P₆₆₆₁₄][Tetz] was not present. Characterization of the ILs post exposure revealed that small amounts of SO₂ were irreversibly absorbed to the [Benzim]^- anion responsible for the decrease in CO₂ capacity. While optimization of this dual-IL system is required, this feasibility study demonstrates that [P₆₆₆₁₄][Tetz] is a suitable sorbent for reversibly capturing SO₂ and significantly extending the lifetime of [P₆₆₆₁₄][Benzim] for CO₂ uptake.

Regenerated Silk Nanofibers for Robust and Cyclic Adsorption-Desorption on Anionic Dyes

In recent years, adsorption-based membranes have been widely investigated to remove and separate textile pollutants. However, cyclic adsorption-desorption to reuse a single adsorbent and clear scientific evidence for the adsorption-desorption mechanism remains challenging. Herein, silk nanofibers were used to assess the adsorption potential for the typical anionic dyes from an aqueous medium, and they show great potential toward the removal of acid dyes from the aqueous solution with an adsorption rate of ∼98% in a 1 min interaction. Further, we measured the filtration proficiency of a silk nanofiber membrane in order to propose a continuous mechanism for the removal of acid blue dye, and a complete rejection was observed with a maximum permeability rate of ∼360 ± 5 L·m^-2·h^-1. The Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy studies demonstrate that this fast adsorption occurs due to multiple interactions between the dye molecule and the adsorbent substrate. The as-prepared material also shows remarkable results in desorption. A 50-time cycle exhibits complete adsorption and desorption ability, which not only facilitates high removal aptitude but also produces less solid waste than other conventional adsorbents. Additionally, fluorescent 2-bromo-2-methyl-propionic acid (abbreviated as EtOxPY)-silk nanofibers can facilitate to illustrate a clear adsorption and desorption mechanism. Therefore, the above-prescribed results make electrospun silk nanofibers a suitable choice for removing anionic dyes in real-time applications.

Theoretical prediction of the SO2 absorption by hollow silica based porous ionic liquids

As an acid gas, sulfur dioxide (SO₂) has caused serious pollution to the environment. Therefore, SO₂ capture is crucial. The silica-based porous ionic liquid possesses not only the porosity and high specific surface area of hollow silica, but also the fluidity of the liquid. The absorption mechanism of SO₂ absorption by porous ionic liquids through density functional theory (DFT) was systematically studied in this paper. First six kinds of absorption sites were predicted, and then various analyses such as structure, energy, and electrostatic potential analysis (ESP) were employed after optimization. The results show that SO₂ has the strongest adsorptive interaction between the canopy and the silica sphere. In addition, the main force between the porous ionic liquid and SO₂ is hydrogen bonding and π-hole bonding. Finally, by increasing the degree of polymerization of the canopy, that is, increasing the number of ether groups, will be beneficial to the absorption of SO₂.

Sulfur dioxide removal: An overview of regenerative flue gas desulfurization and factors affecting desulfurization capacity and sorbent regeneration

Numerous mitigation techniques have been incorporated to capture or remove SO₂ with flue gas desulfurization (FGD) being the most common method. Regenerative FGD method is advantageous over other methods due to high desulfurization efficiency, sorbent regenerability, and reduction in waste handling. The capital costs of regenerative methods are higher than those of commonly used once-through methods simply due to the inclusion of sorbent regeneration while operational and management costs depend on the operating hours and fuel composition. Regenerable sorbents like ionic liquids, deep eutectic solvents, ammonium halide solutions, alkyl-aniline solutions, amino acid solutions, activated carbons, mesoporous silica, zeolite, and metal-organic frameworks have been reported to successfully achieve high SO₂ removal. The presence of other gases in flue gas, e.g., O₂, CO₂, NOx, and water vapor, and the reaction temperature critically affect the sorption capacity and sorbent regenerability. To obtain optimal SO₂ removal performance, other parameters such as pH, inlet SO₂ concentration, and additives need to be adequately governed. Due to its high removal capacity, easy preparation, non-toxicity, and low regeneration temperature, the use of deep eutectic solvents is highly feasible for upscale utilization. Metal-organic frameworks demonstrated highest reported SO₂ removal capacity; however, it is not yet applicable at industrial level due to its high price, weak stability, and robust formulation.

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.

Exploration of tetra-branched multiple-site SO2 capture materials

Efficient exploration of the configuration space of the reaction complexes consisting of multi-branched structures and SO₂ molecules.

The role of cations in the interactions between anionic N-heterocycles and SO2

Our study shows that cation plays a more important role in the interactions between anionic N-heterocycles and SO2 than in the NHC-CO2 case. The adducts of NHC, SO2 and cation often exhibit multiple stable configurations with close energies rather than the only reported "CO2-sandwiched" planar NHC-CO2-cation structure. The structural diversity makes the models omitting cation inappropriate for predicting the SO2 capture products, which also leads to less clear trends of the cation effects than those observed in the CO2 case. The detailed cation effects are discussed in the text.

Designing tri-branched multiple-site SO2 capture materials

Tri-branched species with multiple isolated reactive sites are proposed for high and uniform SO₂ capture.

Computer-Assisted Design of Imidazolate-Based Ionic Liquids for Improving Sulfur Dioxide Capture, Carbon Dioxide Capture, and Sulfur Dioxide/Carbon Dioxide Selectivity

A new strategy involving the computer-assisted design of substituted imidazolate-based ionic liquids (ILs) through tuning the absorption enthalpy as well as the basicity of the ILs to improve SO₂ capture, CO₂ capture, and SO₂ /CO₂ selectivity was explored. The best substituted imidazolate-based ILs as absorbents for different applications were first predicted. During absorption, high SO₂ capacities up to ≈5.3 and 2.4 molSO2  mol_IL^-1 could be achieved by ILs with the methylimidazolate anions under 1.0 and 0.1 bar (1 bar=0.1 MPa), respectively, through tuning multiple N⋅⋅⋅S interactions between SO₂ and the N atoms in the imidazolate anion with different substituents. In addition, CO₂ capture by the imidazolate-based ILs could also be easily tuned through changing the substituents of the ILs, and 4-bromoimidazolate IL showed a high CO₂ capacity but a low absorption enthalpy. Furthermore, a high selectivity for SO₂ /CO₂ could be reached by IL with 4,5-dicyanoimidazolate anion owing to its high SO₂ capacity but low CO₂ capacity. The results put forward in this work are in good agreement with the predictions. Quantum-chemical calculations and FTIR and NMR spectroscopy analysis methods were used to discuss the SO₂ and CO₂ absorption mechanisms.

Preorganization and Cooperation for Highly Efficient and Reversible Capture of Low-Concentration CO2 by Ionic Liquids

A novel strategy based on the concept of preorganization and cooperation has been designed for a superior capacity to capture low-concentration CO₂ by imide-based ionic liquids. By using this strategy, for the first time, an extremely high gravimetric CO₂ capacity of up to 22 wt % (1.65 mol mol^-1 ) and excellent reversibility (16 cycles) have been achieved from 10 vol. % of CO₂ in N₂ when using an ionic liquid having a preorganized anion. Through a combination of quantum-chemical calculations and spectroscopic investigations, it is suggested that cooperative interactions between CO₂ and multiple active sites in the preorganized anion are the driving force for the superior CO₂ capacity and excellent reversibility.

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.

Understanding SO2 Capture by Ionic Liquids

Ionic liquids have generated interest for efficient SO2 absorption due to their low vapor pressure and versatility. In this work, a systematic investigation of the structure, thermodynamics, and dynamics of SO2 absorption by ionic liquids has been carried out through quantum chemical calculations and molecular dynamics (MD) simulations. MP2 level calculations of several ion pairs complexed with SO2 reveal its preferential interaction with the anion. Results of condensed phase MD simulations of SO2-IL mixtures manifested the essential role of both cations and anions in the solvation of SO2, where the solute is surrounded by the "cage" formed by the cations (primarily its alkyl tail) through dispersion interactions. These structural effects of gas absorption are substantiated by calculated Gibbs free energy of solvation; the dissolution is demonstrated to be enthalpy driven. The entropic loss of SO2 absorption in ionic liquids with a larger anion such as [NTf2](-) has been quantified and has been attributed to the conformational restriction of the anion imposed by its interaction with SO2. SO2 loading IL decreases its shear viscosity and enhances the electrical conductivity. This systematic study provides a molecular level understanding which can aid the design of task-specific ILs as electrolytes for efficient SO2 absorption.

Active chemisorption sites in functionalized ionic liquids for carbon capture

Carbon capture with site-containing ionic liquids is reviewed with particular attention on the activation and design of the interaction sites.

Improving SO2capture by basic ionic liquids in an acid gas mixture (10% vol SO2) through tethering a formyl group to the anions

Novel formyl-containing task-specific ILs could be used to improve SO₂capture under low SO₂partial pressure.

Highly efficient and reversible SO2 capture by halogenated carboxylate ionic liquids

Several halogenated carboxylate ionic liquids were developed for SO₂ capture. Both enhanced capacity, improved desorption, and reversibility of ionic liquids can be achieved via adding halogen sulfur interaction between halogen on the anion and SO₂.