Encapsulated ionic liquids (ENILs): from continuous to discrete liquid phase

Advanced Materials for NH3 Capture: Interaction Sites and Transport Pathways

Ammonia (NH3) is a carbon-free, hydrogen-rich chemical related to global food safety, clean energy, and environmental protection. As an essential technology for meeting the requirements raised by such issues, NH3 capture has been intensively explored by researchers in both fundamental and applied fields. The four typical methods used are (1) solvent absorption by ionic liquids and their derivatives, (2) adsorption by porous solids, (3) ab-adsorption by porous liquids, and (4) membrane separation. Rooted in the development of advanced materials for NH3 capture, we conducted a coherent review of the design of different materials, mainly in the past 5 years, their interactions with NH3 molecules and construction of transport pathways, as well as the structure-property relationship, with specific examples discussed. Finally, the challenges in current research and future worthwhile directions for NH3 capture materials are proposed.

Ionic Liquids for the Separation of Fluorocarbon Refrigerant Mixtures

This review discusses the research being performed on ionic liquids for the separation of fluorocarbon refrigerant mixtures. Fluorocarbon refrigerants, invented in 1928 by Thomas Midgley Jr., are a unique class of working fluids that are used in a variety of applications including refrigeration. Fluorocarbon refrigerants can be categorized into four generations: chlorofluorocarbons, hydrochlorofluorocarbons, hydrofluorocarbons, and hydrofluoroolefins. Each generation of refrigerants solved a key problem from the previous generation; however, each new generation has relied on more complex mixtures that are often zeotropic, near azeotropic, or azeotropic. The complexity of the refrigerants used and the fact that many refrigerants form azeotropes when mixed makes handling the refrigerants at end of life extremely difficult. Today, less than 3% of refrigerants that enter the market are recycled. This is due to a lack of technology in the refrigerant reclaim market that would allow for these complex, azeotropic refrigerant mixtures to be separated into their components in order to be effectively reused, recycled, and if needed repurposed. As the market for recovering and reclaiming refrigerants continues to grow, there is a strong need for separation technology. Ionic liquids show promise for separating azeotropic refrigerant mixtures as an entrainer in extractive distillation process. Ionic liquids have been investigated with refrigerants for this application since the early 2000s. This review will provide a comprehensive summary of the physical property measurements, equations of state modeling, molecular simulations, separation techniques, and unique materials unitizing ionic liquids for the development of an ionic-liquid-based separation process for azeotropic refrigerant mixtures.

New water-based nanocapsules of poly(diallyldimethylammonium tetrafluoroborate)/ionic liquid for CO2 capture

Encapsulated ionic liquids as green solvents for CO₂ capture are reported in this work. We present a novel combination of water-based poly(ionic liquid) and imidazolium-based ionic liquids (Emim[X]). Poly(diallyldimethylammonium tetrafluoroborate)/Emim[X] capsules were developed for the first time using Nano Spray Dryer B-90. Capsules were characterized by FTIR, SEM/EDX, TEM, TGA, DSC, CO₂ sorption, and CO₂/N₂ selectivity, CO₂ sorption kinetic and recycling were also demonstrated. Comparing the capsules reported in this work, the combination of poly(diallyldimethylammonium tetrafluoroborate) and the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (P[DADMA]/BF₄) showed great potential for CO₂ capture and CO₂/N₂ separation, providing higher results (53.4 mg CO₂/g; CO₂/N₂ selectivity: 4.58).

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.

High load drug release systems based on carbon porous nanocapsule carriers. Ibuprofen case study

This work shows the application of carbon nanocapsules as carriers for sodium ibuprofen release. Hard templating was used to prepare spherical carbon nanocapsules (mean diameter and thick shell of 690 and 70 nm, respectively), exhibiting both micro and mesoporosity. For comparison purposes, a microporous commercial activated carbon and a home-made mesoporous CMK-3 were also studied. All carbons showed similar drug uptake, although microporous commercial carbon and nanocapsules showed higher uptake at low equilibrium concentration due to higher adsorption potential in micropores. Higher and faster release of sodium ibuprofen was observed for carbon nanocapsules at pH 1.8 and 7.4 for a starting load ca. 250 mg g-1. Subsequent loading of carbon nanocapsules by successive evaporation cycles led to a remarkable load of ca. 6010 mg g-1 thanks to sodium ibuprofen filling the internal void volume. In spite of the very high load a fast release was observed at pH 7.4, reaching a release of ca. 100% of the initial sodium ibuprofen load. However, a much slower and lower release was observed at pH 1.8. Thus, the system developed has interesting features for oral drug administration thanks to low toxicity of porous carbon, low release in gastric medium and important release in intestinal medium.

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.

Methanol-Promoted Oxidation of Nitrogen Oxide (NOx) by Encapsulated Ionic Liquids

The removal of nitrogen oxides (NO_x) has been extensively studied due to their harmful effects to health and environment. In this work, encapsulated ionic liquids (ENILs) are used as catalysts for the NO oxidation at humid conditions and low temperatures. Hollow carbon capsules (C_Cap) were first synthesized to contain different amounts of 1-butyl-3-methylimidazolium nitrate IL ([bmim][NO₃]), responsible for the catalytic oxidation. Then, the materials were characterized using different techniques, by analyzing microstructure, porosity, elemental composition, and thermal stability. The catalytic performance of ENIL materials was tested for NO conversion at different conditions. Thus, NO concentration was fixed at 2000 ppm at dry and humid conditions. Then, the methanol promotion of the reaction was demonstrated, increasing the NO conversion values in all cases, and the alcohol/water ratio was optimized. The temperature effect was studied as well, using the optimal conditions based on the previous measurements. The results reflect that humid conditions do not have a negative effect in terms of NO conversion when using ENILs, opposite behavior as observed for C_Cap and traditional catalysts studied before. The low amount of IL inside the material (40% in mass) was found to be the optimum for the task, reaching conversions of almost 45% in near industrial conditions of temperature and O₂ and H₂O concentrations in the flue gas with a GHSV of 10,000 h^-1.

Hybrid Ionic Liquid Capsules for Rapid CO2 Capture

The CO₂ absorption by ionic liquids (ILs) were enhanced by the use of hybrid capsules composed of a core of IL and shell of polyurea and alkylated graphene oxide (GO). These composite structures were synthesized using a Pickering emulsion as a template and capsules of two different ILs were prepared. The contribution of the encapsulated IL on the CO₂ absorption of the capsules is consistent with agitated neat IL, but with improved kinetics of absorption across different pressures. This novel materials design allows for CO₂ to be absorbed significantly faster compared to bulk IL and provides insight into improved carbon capture technologies.

Pickering Emulsion-Templated Encapsulation of Ionic Liquids for Contaminant Removal

Ionic liquids (ILs) have received attention for a diverse range of applications, but their liquid nature can make them difficult to handle and process and their high viscosities can lead to suboptimal performance. As such, encapsulated ILs are attractive for their ease of handling and high surface area and have potential for improved performance in energy storage, gas uptake, extractions, and so forth. Herein, we report a facile method to encapsulate a variety of ILs using Pickering emulsions as templates, graphene oxide (GO)-based nanosheets as particle surfactants, and interfacial polymerization for stabilization. The capsules contain up to 80% IL in the core, and the capsule shells are composed of polyurea and GO. We illustrate that capsules can be prepared from IL-in-water or IL-in-oil emulsions and explore the impact of monomer and IL identity, thereby accessing different compositions. The spherical, discrete capsules are characterized by optical microscopy, scanning electron microscopy, infrared spectroscopy, Raman spectroscopy, thermogravimetric analysis, and ¹H NMR spectroscopy. We illustrate the application of these IL capsules as a column material to remove phenol from oil, demonstrating ≥98% phenol removal after passage of >170 column volumes. This simple method to prepare capsules of IL will find widespread use across diverse applications.

Pickering Emulsion-Derived Liquid-Solid Hybrid Catalyst for Bridging Homogeneous and Heterogeneous Catalysis

We describe a novel method to prepare a liquid-solid hybrid catalyst via interfacial growth of a porous silica crust around Pickering emulsion droplets, which allowed us to overcome the current limitations of both homogeneous and heterogeneous catalysts. The inner micron-scaled liquid (for example, ionic liquids) pool of the resultant catalyst can host free homogeneous molecular catalysts or enzymes to create a true homogeneous catalysis environment. The porous silica crust of the hybrid catalyst has excellent stability, which makes it amenable to packing directly in fixed-bed reactors for continuous flow catalysis. As a proof of concept, the enzymatic kinetic resolution of racemic alcohols, Cr^III(salen) complex-catalyzed asymmetric ring opening of epoxides and Pd-catalyzed Tsuji-Trost allylic substitution reactions were used to verify the generality and versatility of our strategy for bridging homogeneous and heterogeneous catalysis. The hybrid catalyst-based continuous flow system exhibited a 1.6∼16-fold enhancement in activity relative to homogeneous counterparts even over 1500 h, and the afforded enantioselectivities were completely equal to those obtained in the homogeneous counterpart systems. Interestingly, the catalytic efficiency can be tuned through rational engineering of the porous crust and the dimensions of the liquid pool, resulting in features of an innovatively designed catalyst. This contribution provides a new method to design efficient catalysts that can bridge the conceptual and technical gaps between homogeneous and heterogeneous catalysis.

Ionic Liquid-Containing Pickering Emulsions Stabilized by Graphene Oxide-Based Surfactants

Emulsions stabilized by particles (i.e., Pickering emulsions) are complementary to those stabilized by small molecules or polymers and most commonly consist of oil droplets dispersed in a continuous water phase, with particles assembled at the fluid-fluid interface. New particle surfactants and different fluid-fluid interfaces are critical for developing next-generation systems for a number of advanced applications. Herein we report the preparation of IL-containing emulsions stabilized by graphene oxide (GO)-based nanoparticles using the IL [Bmim][PF₆]: GO nanosheets stabilize IL-in-water emulsions, and alkylated GO nanosheets (C₁₈-GO) stabilize IL-in-oil emulsions. The impact of particle concentration, fluid-fluid ratio, and addition of acid or base on emulsion formation and stability is studied, with distinct effects for the water and oil systems observed. We then illustrate the broad applicability of GO-based particle surfactants by preparing emulsions with different ILs and preparing inverted emulsions (water-in-IL and oil-in-IL emulsions). The latter systems were accessed by tuning the polarity of GO nanosheets by functionalization with a perfluorinated alkyl chain such that they were dispersible in IL. This work provides insight into the preparation of different IL-containing emulsions and lays a foundation for the architecture of dissimilar materials into composite systems.

Encapsulation of Ionic Liquids with an Aprotic Heterocyclic Anion (AHA-IL) for CO2 Capture: Preserving the Favorable Thermodynamics and Enhancing the Kinetics of Absorption

The performance of an ionic liquid with an aprotic heterocyclic anion (AHA-IL), trihexyl(tetradecyl)phosphonium 2-cyanopyrrolide ([P₆₆₆₁₄][2-CNPyr]), for CO₂ capture has been evaluated considering both the thermodynamics and the kinetics of the phenomena. Absorption gravimetric measurements of the gas-liquid equilibrium isotherms of CO₂-AHA-IL systems were carried out from 298 to 333 K and at pressures up to 15 bar, analyzing the role of both chemical and physical absorption phenomena in the overall CO₂ solubility in the AHA-IL, as has been done previously. In addition, the kinetics of the CO₂ chemical absorption process was evaluated by in situ Fourier transform infrared spectroscopy-attenuated total reflection, following the characteristic vibrational signals of the reactants and products over the reaction time. A chemical absorption model was used to describe the time-dependent concentration of species involved in the reactive absorption, obtaining kinetic parameters (such as chemical reaction kinetic constants and diffusion coefficients) as a function of temperatures and pressures. As expected, the results demonstrate that the CO₂ absorption rate is mass-transfer-controlled because of the relatively high viscosity of AHA-IL. The AHA-IL was encapsulated in a porous carbon sphere (Encapsulated Ionic Liquid, ENIL) to improve the kinetic performance of the AHA-IL for CO₂ capture. The newly synthesized AHA-ENIL material was evaluated as a CO₂ sorbent with gravimetric absorption measurements. AHA-ENIL systems preserve the good CO₂ absorption capacity of the AHA-IL but drastically enhance the CO₂ absorption rate because of the increased gas-liquid surface contact area achieved by solvent encapsulation.

Basicity Characterization of Imidazolyl Ionic Liquids and Their Application for Biomass Dissolution

Alkalinity determination is of crucial significance for the applications of basic ionic liquids with imidazolyl. In this work, the ionization constant pKb value and acid function H- values of ionic liquids synthesized were calculated by pH method and UV spectrum-Hammett method. The dissolution ratio of biomass in these ionic liquids was measured at different temperatures. Finally, the relationship between the alkalinity and structure of these ionic liquids was discussed, and the relationship between the alkalinity of ionic liquid and the dissolution mechanism biomass was also discussed. The results show that the basicity of carboxylate ionic liquids is determined mainly by their anions, whereas cations take some finely tuned roles. Furthermore, cations and anions are equally important and are involved in dissolution mechanisms.

Encapsulated Ionic Liquids for CO2 Capture: Using 1-Butyl-methylimidazolium Acetate for Quick and Reversible CO2 Chemical Absorption

The potential advantages of applying encapsulated ionic liquid (ENIL) to CO₂ capture by chemical absorption with 1-butyl-3-methylimidazolium acetate [bmim][acetate] are evaluated. The [bmim][acetate]-ENIL is a particle material with solid appearance and 70 % w/w in ionic liquid (IL). The performance of this material as CO₂ sorbent was evaluated by gravimetric and fixed-bed sorption experiments at different temperatures and CO₂ partial pressures. ENIL maintains the favourable thermodynamic properties of the neat IL regarding CO₂ absorption. Remarkably, a drastic increase of CO₂ sorption rates was achieved using ENIL, related to much higher contact area after discretization. In addition, experiments demonstrate reversibility of the chemical reaction and the efficient ENIL regeneration, mainly hindered by the unfavourable transport properties. The common drawback of ILs as CO₂ chemical absorbents (low absorption rate and difficulties in solvent regeneration) are overcome by using ENIL systems.

Ammonia capture from the gas phase by encapsulated ionic liquids (ENILs)

Encapsulated ionic liquids (ENILs) based on carbonaceous submicrocapsules were designed, synthesized and applied to the sorption of NH₃ from gas streams.

Isolating lignin from spent liquor of thermomechanical pulping process via adsorption

Wood chips are pretreated with steam prior to refining in the thermomechanical pulping process. The steam treatment dissolves part of lignin of wood chips in the spent liquor (SL) of this process, and subsequently the SL is sent to the wastewater system of the process. However, the lignin of SL can be used in the production of value-added chemicals, but it should first be separated from the SL in order to have a feasible downstream process. In this study, activated carbon (AC) was considered as an adsorbent to isolate lignin from SL. The results showed that the maximum adsorption of lignin on AC was 166 mg/g under the optimal conditions of pH 5.2, 30 degrees C and 3 h treatment. Furthermore, the separation of lignin from SL was improved from 45% to 60% by having a two-stage adsorption process at pH 5.2, which also reduced the turbidity and chemical oxygen demand of SL by 39% and 32%, respectively.