Hybrid Ionic Liquid Capsules for Rapid CO2 Capture

Ionic Liquid-Glycol Mixtures for Direct Air Capture of CO2: Decreased Viscosity and Mitigation of Evaporation Via Encapsulation

Herein we address the efficiency of the CO2 sorption of ionic liquids (IL) with hydrogen bond donors (e.g., glycols) added as viscosity modifiers and the impact of encapsulating them to limit sorbent evaporation under conditions for the direct air capture of CO2. Ethylene glycol, propylene glycol, 1,3-propanediol, and diethylene glycol were added to three different ILs: 1-ethyl-3-methylimidazolium 2-cyanopyrrolide ([EMIM][2-CNpyr]), 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]), and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]). Incorporation of the glycols decreased viscosity by an average of 51% compared to bulk IL. After encapsulation of the liquid mixtures using a soft template approach, thermogravimetric analysis revealed average reductions in volatility of 36 and 40% compared to the unencapsulated liquid mixtures, based on 1 h isothermal experiments at 25 and 55 °C, respectively. The encapsulated mixtures of [EMIM][2-CNpyr]/1,3-propanediol and [EMIM][2-CNpyr]/diethylene glycol exhibited the lowest volatility (0.0019 and 0.0002 mmol/h at 25 °C, respectively) and were further evaluated as CO2 absorption/desorption materials. Based on the capacity determined from breakthrough measurements, [EMIM][2-CNpyr]/1,3-propanediol had a lower transport limited absorption rate for CO2 sorption compared to [EMIM][2-CNpyr]/diethylene glycol with 0.08 and 0.03 mol CO2/kg sorbent, respectively; however, [EMIM][2-CNpyr]/diethylene glycol capsules exhibited higher absorptions capacity at ∼500 ppm of CO2 (0.66 compared to 0.47 mol of CO2/kg sorbent for [EMIM][2-CNpyr]/1,3-propanediol). These results show that glycols can be used to not only reduce IL viscosity while increasing physisorption sites for CO2 sorption, but also that encapsulation can be utilized to mitigate evaporation of volatile viscosity modifiers.

Ionic Liquids Hybridization for Carbon Dioxide Capture: A Review

CO2 absorption has been driven by the need for efficient and environmentally sustainable CO2 capture technologies. The development in the synthesis of ionic liquids (ILs) has attracted immense attention due to the possibility of obtaining compounds with designated properties. This allows ILs to be used in various applications including, but not limited to, biomass pretreatment, catalysis, additive in lubricants and dye-sensitive solar cell (DSSC). The utilization of ILs to capture carbon dioxide (CO2) is one of the most well-known processes in an effort to improve the quality of natural gas and to reduce the green gases emission. One of the key advantages of ILs relies on their low vapor pressure and high thermal stability properties. Unlike any other traditional solvents, ILs exhibit high solubility and selectivity towards CO2. Frequently studied ILs for CO2 absorption include imidazolium-based ILs such as [HMIM][Tf2N] and [BMIM][OAc], as well as ILs containing amine groups such as [Cho][Gly] and [C1ImPA][Gly]. Though ILs are being considered as alternative solvents for CO2 capture, their full potential is limited by their main drawback, namely, high viscosity. Therefore, the hybridization of ILs has been introduced as a means of optimizing the performance of ILs, given their promising potential in capturing CO2. The resulting hybrid materials are expected to exhibit various ranges of chemical and physical characteristics. This review presents the works on the hybridization of ILs with numerous materials including activated carbon (AC), cellulose, metal-organic framework (MOF) and commercial amines. The primary focus of this review is to present the latest innovative solutions aimed at tackling the challenges associated with IL viscosity and to explore the influences of ILs hybridization toward CO2 capture. In addition, the development and performance of ILs for CO2 capture were explored and discussed. Lastly, the challenges in ILs hybridization were also being addressed.

Formation of choline salts and dipolar ions for CO2 reactive eutectic solvents

Choline-based sorbents derived from imidazole (ImH), phenol (PhOH), pyrrole-2-carbonitrile (CNpyrH), and 1,2,4-triazole (TrzH) are developed for CO2 capture to enable alternative regeneration approaches over aqueous amines. During synthesis, the equilibrium between [Ch]+[OH]- and Ch± dipolar in water shifts to support the formation of Ch±ImH and Ch±PhOH in the presence of ImH and PhOH upon drying. In contrast, salts of [Ch]+[CNpyr]- and [Ch]+[Trz]- were obtained with CNpyrH and TrzH, as confirmed by NMR and FTIR spectroscopy. Density functional theory (DFT) calculations support a spontaneous proton transfer from CNpyrH and TrzH to Ch±, while they show an energy barrier in the case of ImH. These sorbents formed eutectic solvents upon mixing with ethylene glycol (EG) where deprotonation of EG and subsequent binding of CO2 contributed to capacities up to 3.56 mol CO2 kg-1 at 25 °C and 1 bar of CO2. The regenerability of the eutectic solvents was demonstrated by dielectric heating via microwaves (MWs) in support of renewable energy utilization. This study shows the impact of proton sharing on the CO2 capacity and regenerability of eutectic sorbents as molecular design guidance.

Physicochemical properties and density functional theory calculation of octahedral UiO-66 with Bis(Trifluoromethanesulfonyl)imide ionic liquids

In this study, the physicochemical properties and molecular interactions between zirconium-based metal-organic framework (UiO-66) and three different ionic liquids based on bis(trifluoromethanesulfonyl)imide anion (EMIM+, BMIM+ and OMIM+) was performed via a combined experimental and computational approach. The ionic liquid loaded UiO-66 or IL@UiO-66 was synthesized and characterized to understand the host-guest interaction. Density functional theory calculation was performed to analyse the electronic structure of IL@UiO-66 to provide molecular insight on the dominant interactions occurred in the hybrid material. Results showed that all ILs were successfully incorporated into the micropores of UiO-66. The 3D framework was retained even after loaded with ILs as analyzed from XRD pattern. FTIR spectrum reveals that interactions of ILs with UiO-66 influenced by the alkyl chain length of the cation. The anion has a profound affinity with the UiO-66 due to the presence of electronegative atoms. Phase transition study from DSC suggested that the incorporation of ILs has stabilized the framework of UiO-66 by shifting the endothermic peak to a higher state. These findings were further elaborated with DFT calculation. Geometrical optimizations confirmed the structural parameter changes of UiO-66 when loaded with ILs. These was mainly contributed by the non-covalent interactions which was confirmed by the reduced density gradient scattered plot. Another important findings are the strength of hydrogen bonding at the host-guest interface was influenced by the alkyl chain length. The molecular orbital analysis also shows that the size of alkyl chain influence the reactivity of the hybrid material. The present study provides fundamental insights on the molecular interaction of UiO-66 and ILs as a hybrid material, which can open new possibilities for advanced material for metal-organic framework applications in energy storage system, catalysis, gas storage and medicinal chemistry.

Ionic Liquid-in-Water Emulsions Stabilized by Molecular and Polymeric Surfactants

Ionic liquids have drawn notable attention for their unique solvent properties and use in applications such as batteries and chemical separations. While many ionic liquids are water-soluble, there are numerous examples of ionic liquids that are sufficiently hydrophobic to remain phase separated from water. However, relatively little is known about the stability and properties of ionic liquid-in-water emulsions. Here, we survey a series of ionic liquid-in-water emulsions stabilized by a range of ionic and nonionic molecular surfactants and polymers. To assess droplet stability and dynamics, we characterize the ionic liquid-surfactant interfacial tension, describe qualitative coarsening rates, and quantify droplet solubilization rate. In some instances, we observe unexpected spontaneous formation of complex double and triple emulsions. Our observations highlight approaches for ionic liquid emulsion formulation and provide insight into how to address challenges surrounding stabilization of ionic liquid-in-water droplets with molecular surfactants.

Impact of Shell Composition on Dye Uptake by Capsules of Ionic Liquid

Encapsulation of ionic liquids (ILs) has been shown to be an effective technique to overcome slow mass transfer rates and handling difficulties that stem from the high viscosity of bulk ILs. These systems commonly rely on diffusion of small molecules through the encapsulating material (shell), into the IL core, and thus the composition of the shell impacts uptake performance. Herein, we report the impact of polymer shell composition on the uptake of the small molecule dye methyl red from water by encapsulated IL. Capsules with core of 1-hexyl-3-methylimidazolium bis(trifluorosulfonyl)imide ([Hmim][TFSI]) were prepared by interfacial polymerization in emulsions stabilized by graphene oxide (GO) nanosheets; the use of different diamines and diisocyanates gave capsule shells with polyureas that were all aliphatic, aliphatic/aromatic, and aliphatic/polar aprotic. These capsules were then added to aqueous solutions of methyl red at different pH values, and migration of the dye into the capsules was monitored by UV-vis spectroscopy, compared to the capsule shell alone. Regardless of the polymer identity, similar extents of dye uptake were observed (>90% at pH = 2), yet capsules with shells containing polyureas with polar aprotic linkages took longer to reach completion. These studies indicate that small changes in capsule shell composition can lead to different performance in small molecule uptake, giving insight into how to tailor shell composition for specific applications, such as solvent remediation and gas uptake.

Pickering emulsions stabilized by metal-organic frameworks, graphitic carbon nitride and graphene oxide

Pickering emulsion is a heterogeneous system consisting of at least two immiscible liquids, which are stabilized by solid particles, in which organic solvent or water is dispersed into other phase in form of micrometre-sized droplets. Compared to traditional emulsions stabilized by surfactant, solids are cheap and can be easily separated and recycled by centrifugation or filtration after use. Moreover, the properties of Pickering emulsions can be adjusted by using different types of solid particles. Up to now, Pickering emulsions have been applied in a wide range of areas such as material science and catalysis. Here we review recent studies on Pickering emulsions stabilized by metal-organic framework, graphitic carbon nitride and graphene oxide.

Engineering encapsulated ionic liquids for next-generation applications

Ionic liquids (ILs) have attracted considerable attention in several sectors (from energy storage to catalysis, from drug delivery to separation media) owing to their attractive properties, such as high thermal stability, wide electrochemical window, and high ionic conductivity. However, their high viscosity and surface tension compared to conventional organic solvents can lead to unfavorable transport properties. To circumvent undesired kinetics effects limiting mass transfer, the discretization of ILs into small droplets has been proposed as a method to increase the effective surface area and the rates of mass transfer. In the present review paper, we summarize the different methods developed so far for encapsulating ILs in organic or inorganic shells and highlight characteristic features of each approach, while outlining potential applications. The remarkable tunability of ILs, which derives from the high number of anions and cations currently available as well as their permutations, combines with the possibility of tailoring the composition, size, dispersity, and properties (e.g., mechanical, transport) of the shell to provide a toolbox for rationally designing encapsulated ILs for next-generation applications, including carbon capture, energy storage devices, waste handling, and microreactors. We conclude this review with an outlook on potential applications that could benefit from the possibility of encapsulating ILs in organic and inorganic shells.

Encapsulated ionic liquids (ILs) are candidate materials for several applications owing to the attractive properties of ILs combined with the enhanced mass transfer rate obtained through the discretization of ILs in small capsules.

Soft nanoconfinement of ionic liquids in lyotropic liquid crystals

Nanoconfinement of ionic liquids (ILs) influences their physicochemical properties. In this study, we investigate the effect of soft nanoconfinement imposed by lyotropic liquid crystals (LLCs) on ILs. The LLC ion gels are obtained through self-assembly of a short chain block copolymer (BCP) of polyethylene-block-poly(ethylene oxide), PE-b-PEO, in ILs. The effect of confinement on the interaction of ions with PEO is investigated through electrochemical impedance spectroscopy (EIS) and carbon dioxide (CO₂) absorption measurements. The results show that the synergistic effect on the CO₂ absorption capacity of LLC ion gels takes place as a result of confinement. Formation of IL pathways through the LLC increases the CO₂ solubility, absorption capacity, and absorption rate. Increasing the concentration of block copolymer in the LLC structure enhances the dissociation of ILs and consequently lowers CO₂ absorption. Therefore, the competing effects of confinement and IL-PEO interaction control the properties of LLC ion gels.

Recent Developments in Solvent-Based Fluid Separations

The most important developments in solvent-based fluid separations, separations involving at least one fluid phase, are reviewed. After a brief introduction and discussion on general solvent trends observed in all fields of application, several specific fields are discussed. Important solvent trends include replacement of traditional molecular solvents by ionic liquids and deep eutectic solvents and, more recently, increasing discussion around bio-based solvents in some application fields. Furthermore, stimuli-responsive systems are discussed; the most significant developments in this field are seen for CO₂-switchable and redox-responsive solvents. Discussed fields of application include hydrocarbons separations, carbon capture, biorefineries, and metals separations. For all but the hydrocarbons separations, newly reported electrochemically mediated separations seem to offer exciting new windows of opportunities.

Inter-capsule fusion and capsule shell destruction using dynamic covalent polymers

Herein, capsule shells containing hindered urea bonds were prepared using interfacial polymerization in an oil-in-oil Pickering emulsion stabilized by functionalized graphene oxide nanosheets.

Simultaneous Removal of NO and SO2 from Flue Gas Using [Bmim]2FeCl4/Sulfolane Binary Mixtures

NO and SO2 are the major pollutants of coal combustion. As superior absorbents, ionic liquids are environmentally friendly, are reusable, and can clean flue gases, such as CO2, SO2, and NO x . However, NO and SO2 absorption with low concentration in flue gases under normal conditions is rarely studied. In this work, [Bmim]2FeCl4 was synthesized and mixed with sulfolane for NO and SO2 removal from flue gas. The investigated concentrations of NO and SO2 were 1100 and 2500 ppm, respectively, which are close to real fuel gas conditions. Results showed that 30 wt % [Bmim]2FeCl4/sulfolane mixture performed the best absorption behavior. The presence of SO2 could promote NO absorption by [Bmim]2FeCl4/sulfolane mixture. The 30 wt % [Bmim]2FeCl4/sulfolane mixture had removal efficiencies of 93.6 and 76.2% for NO and SO2, respectively. This mixture also showed great reusability for NO and SO2 after six cycles of absorption. Fourier transform infrared (FTIR) spectrum indicated that SO2 and NO removal by [Bmim]2FeCl4/sulfolane binary mixture was due to the chemical reaction between NO and [Bmim]2FeCl4 and the physical absorption between SO2 and sulfolane.

Efficient CO2 Separation of Multi-Permselective Mixed Matrix Membranes with a Unique Interfacial Structure Regulated by Mesoporous Nanosheets

A facile strategy to elevate gas separation performances of polymers is to introduce a versatile particle. In this study, the novel F-Ce nanosheets are synthesized, and then F-Ce is functionalized with 1-ethyl-3-methylimidazole thiocyanate (ionic liquids, ILs), obtaining multifunctional f-F-Ce nanosheets by the facile and environment-friendly methods. The multifunctional f-F-Ce nanosheets are incorporated into the Pebax (Pebax 1657) matrix to fabricate mixed matrix membranes (MMMs) for efficient CO₂ separation. The f-F-Ce nanosheets play versatile parts in elevating membrane gas separation performance. On the one hand, f-F-Ce tends to arrange horizontally and constructs a unique interfacial structure for cross-layer CO₂ transport in MMMs. On the other hand, the abundant mesopores from f-F-Ce construct high-speed CO₂ transport channels in MMMs and notably elevate the gas permeability. Moreover, the as-prepared MMMs separate CO₂ efficiently due to the comprehensive improvements of diffusivity selectivity, solubility selectivity, and reactivity selectivity. First, the high aspect ratio of f-F-Ce provides the tortuous pathways for gas transport and generates the rigid interface between the Pebax matrix and f-F-Ce nanosheets, increasing the diffusivity selectivity. Second, SCN^- groups from ILs show excellent affinity to CO₂, enhancing the solubility selectivity. Third, amine groups from ILs with abundant methylimidazole generate reversible reaction with CO₂ to elevate reactivity selectivity. Consequently, the f-F-Ce-doped MMMs display excellent CO₂ permeability and CO₂/CH₄ selectivity. In particular, the MMM incorporated with 8 wt % f-F-Ce displays a CO₂ permeability of 1823 Barrer and a CO₂/CH₄ selectivity of 35, overcoming the Robeson upper bound line (2008).

Polysulfone metal-activated carbon magnetic nanocomposites with enhanced CO2 capture

In the present study, polysulfone (PSF)-activated carbon nanocomposites were synthesized by a melt mixing technique. Here, 2 wt% activated carbon (CA, CA-Ni, and CA-Co) was used as filler, and effects on thermal, mechanical, magnetic, morphological, and carbon dioxide capture properties were studied. The pyrolysis of wood sawdust produced carbon materials activated by Co and/or Ni salt. The thermal degradation and the amount of metal in the carbon materials were investigated by thermogravimetric analysis. The maximum degradation temperature showed an improvement of up to 3 °C, while the initial degradation temperature decreased up to 4 °C with the addition of metal-activated carbons. The values of T g estimated by differential scanning calorimetry appear to be practically identical for pure PSF and its nanocomposites. The elasticity modulus of the nanocomposite shows an enhancement of 17% concerning the neat PSF. The water contact angle showed a decrease with the incorporation of the fillers, indicating the hydrophilic nature of the composite. The carbon dioxide sorption capacity of the nanocomposite showed an enhancement of almost 10% in contrast to neat PSF. Ferromagnetic behavior of the thermoplastic nanocomposite was observed with the introduction of 2.0 wt% metal-carbonized filler. The exceptional magnetic properties, for a thermoplastic material such as polysulfone, make it promising for various industrial applications.

Capsules of Reactive Ionic Liquids for Selective Capture of Carbon Dioxide at Low Concentrations

The task-specific ionic liquid (IL), 1-ethyl-3-methylimidazolium 2-cyanopyrolide ([EMIM][2-CNpyr]), was encapsulated with polyurea (PU) and graphene oxide (GO) sheets via a one-pot Pickering emulsion, and these capsules were used to scrub CO₂ (0-5000 ppm) from moist air. Up to 60 wt % of IL was achieved in the synthesized capsules, and we demonstrated comparable gravimetric CO₂ capacities to zeolites and enhanced absorption rates compared to those of bulk IL due to the increased gas/liquid surface-to-volume area. The reactive IL capsules show recyclability upon mild temperature increase compared to zeolites that are the conventional absorber materials for CO₂ scrubbing. The measured breakthrough curves in a fixed bed under 100% relative humidity establish the utility of reactive IL capsules as moisture-stable scrubber materials to separate CO₂ from air, outperforming zeolites owing to their higher selectivity. It is shown that thermal stability, CO₂ absorption capacity, and rate of uptake by IL capsules can be further modulated by incorporating low-viscosity and nonreactive ILs to the capsule core. This study demonstrates an alternative and facile approach for CO₂ scrubbing, where separation from gas mixtures with extremely low partial pressures of CO₂ is required.

Encapsulation of Ionic Liquids for Tailored Applications

This spotlight article highlights the favorable impact encapsulation of ionic liquids (ILs) has on multiple advanced applications. ILs are molten salts with many attractive properties such as negligible vapor pressure, good thermal stability, and high ionic conductivity; however, their widespread implementation in advanced applications is hampered by their relatively high viscosity, which makes them difficult to handle and results in slow mass transfer rates. The ability to encapsulate IL in a shell holds potential to impact many applications, including separations, gas sequestration, and energy storage and management, given that the capsule structure provides high surface area compared to that of bulk IL and also allows handling of the IL as a solid. Herein, we discuss encapsulation of ILs using different approaches and highlight the contributions from our lab in both capsule preparation and application. Specifically, we have developed the ability to use 2D carbon nanoparticle surfactants and interfacial polymerization to prepare capsules of IL using both IL-in-water and IL-in-oil Pickering emulsions as templates. This facile, one-step method to encapsulate ILs gives structures with beneficial performance in supercapacitors, separations, and CO₂ sequestration, as discussed herein. We conclude this spotlight with an outlook on how to improve upon these systems for next-generation applications.

The effect of nanoscale friction of mesoporous carbon supported ionic liquids on the mass transfer of CO2 adsorption

The nanofriction was linked with CO₂ mass transfer at ionic liquid–solid interfaces, where the smaller nanofriction accelerates the CO₂ adsorption.