Ideal CO2/Light Gas Separation Performance of Poly(vinylimidazolium) Membranes and Poly(vinylimidazolium)-Ionic Liquid Composite Films

Energy Applications of Ionic Liquids: Recent Developments and Future Prospects

Ionic liquids (ILs) consisting entirely of ions exhibit many fascinating and tunable properties, making them promising functional materials for a large number of energy-related applications. For example, ILs have been employed as electrolytes for electrochemical energy storage and conversion, as heat transfer fluids and phase-change materials for thermal energy transfer and storage, as solvents and/or catalysts for CO2 capture, CO2 conversion, biomass treatment and biofuel extraction, and as high-energy propellants for aerospace applications. This paper provides an extensive overview on the various energy applications of ILs and offers some thinking and viewpoints on the current challenges and emerging opportunities in each area. The basic fundamentals (structures and properties) of ILs are first introduced. Then, motivations and successful applications of ILs in the energy field are concisely outlined. Later, a detailed review of recent representative works in each area is provided. For each application, the role of ILs and their associated benefits are elaborated. Research trends and insights into the selection of ILs to achieve improved performance are analyzed as well. Challenges and future opportunities are pointed out before the paper is concluded.

The Influence of Polycation and Counter-Anion Nature on the Properties of Poly(ionic liquid)-Based Membranes for CO2 Separation

The current investigation is focused on the development of composite membranes based on polymeric ionic liquids (PILs) containing imidazolium and pyridinium polycations with various counterions, including hexafluorophosphate, tetrafluoroborate, and bis(trifluoromethylsulfonyl)imide. A combination of spectroscopic methods was used to identify the synthesized PILs and characterize their interaction with carbon dioxide. The density and surface free energy of polymers were performed by wettability measurements, and the results are in good agreement with the permeability and selectivity obtained within the gas transport tests. It was shown that the membranes with a selective layer based on PILs exhibit relatively high permeability with CO₂ and high ideal selectivity CO₂/CH₄ and CO₂/N₂. Additionally, it was found that the type of an anion significantly affects the performance of the obtained membranes, with the most pronounced effect from bis-triflimide-based polymers, showing the highest permeability coefficient. These results provide valuable insights into the design and optimization of PIL-based membranes for natural and flue gas treatment.

A Review on Ionic Liquid Gas Separation Membranes

Ionic liquids have attracted the attention of the industry and research community as versatile solvents with unique properties, such as ionic conductivity, low volatility, high solubility of gases and vapors, thermal stability, and the possibility to combine anions and cations to yield an almost endless list of different structures. These features open perspectives for numerous applications, such as the reaction medium for chemical synthesis, electrolytes for batteries, solvent for gas sorption processes, and also membranes for gas separation. In the search for better-performing membrane materials and membranes for gas and vapor separation, ionic liquids have been investigated extensively in the last decade and a half. This review gives a complete overview of the main developments in the field of ionic liquid membranes since their first introduction. It covers all different materials, membrane types, their preparation, pure and mixed gas transport properties, and examples of potential gas separation applications. Special systems will also be discussed, including facilitated transport membranes and mixed matrix membranes. The main strengths and weaknesses of the different membrane types will be discussed, subdividing them into supported ionic liquid membranes (SILMs), poly(ionic liquids) or polymerized ionic liquids (PILs), polymer/ionic liquid blends (physically or chemically cross-linked 'ion-gels'), and PIL/IL blends. Since membrane processes are advancing as an energy-efficient alternative to traditional separation processes, having shown promising results for complex new separation challenges like carbon capture as well, they may be the key to developing a more sustainable future society. In this light, this review presents the state-of-the-art of ionic liquid membranes, to analyze their potential in the gas separation processes of the future.

Poly[3-ethyl-1-vinyl-imidazolium] diethyl phosphate/Pebax® 1657 Composite Membranes and Their Gas Separation Performance

Poly(ionic liquid)s are an innovative class of materials with promising properties in gas separation processes that can be used to boost the neat polymer performances. Nevertheless, some of their properties such as stability and mechanical strength have to be improved to render them suitable as materials for industrial applications. This work explored, on the one hand, the possibility to improve gas transport and separation properties of the block copolymer Pebax® 1657 by blending it with poly[3-ethyl-1-vinyl-imidazolium] diethyl phosphate (PEVI-DEP). On the other hand, Pebax® 1657 served as a support for the PIL and provided mechanical resistance to the samples. Pebax® 1657/PEVI-DEP composite membranes containing 20, 40, and 60 wt.% of PEVI-DEP were cast from solutions of the right proportion of the two polymers in a water/ethanol mixture. The PEVI-DEP content affected both the morphology of the dense membranes and gas transport through the membranes. These changes were revealed by scanning electron microscopy (SEM), time-lag, and gravimetric sorption measurements. Pebax® 1657 and PEVI-DEP showed similar affinity towards CO2, and its uptake or solubility was not influenced by the amount of PIL in the membrane. Therefore, the addition of the PIL did not lead to improvements in the separation of CO2 from other gases. Importantly, PEVI-DEP (40 wt.%) incorporation affected and improved permeability and selectivity by more than 50% especially for the separation of light gases, e.g., H2/CH4 and H2/CO2, but higher PEVI-DEP concentrations lead to a decline in the transport properties.

Water Vapour Promotes CO2 Transport in Poly(ionic liquid)/Ionic Liquid-Based Thin-Film Composite Membranes Containing Zinc Salt for Flue Gas Treatment

A poly(ionic-liquid) (PIL) matrix can be altered by incorporating additives that will disrupt the polymer chain packing, such as an ionic liquid (IL) and inorganic salts to boost their exploitation as materials for membrane production to be used in CO2 capture. Herein, potential of PIL/IL/salt blends is investigated on the example of poly(diallyldimethyl ammonium) bis(trifluoromethylsulfonyl)imide (P[DADMA][Tf2N]) with N-butyl-N-methyl pyrrolidinium bis(trifluoromethylsulfonyl)imide ([Pyrr14][Tf2N]) and zinc di-bis(trifluoromethylsulfonyl)imide (Zn[Tf2N]2). Composite material with IL and a higher amount of Zn2+ showed an increase in the equilibrium CO2 sorption capacity to 2.77 cm3 (STP)cm −3 bar−1. Prepared blends were successfully processed into thick, dense membranes and thin-film composite membranes. Their CO2 separation efficiency was determined using ideal and mixed-gas feed (vol% CO2 = 50 , dry and with 90% relative humidity). The dominant role of solubility in the transport mechanism is confirmed by combining direct gravimetric sorption measurements and indirect estimations from time-lag experiments. The maximum incorporated amount of Zn2+ salts increased equilibrium solubility selectivity by at least 50% in comparison to the parent PIL. All materials showed increased CO2 permeance values by at least 30% in dry conditions, and 60% in humidified conditions when compared to the parent PIL; the performance of pure PIL remained unchanged upon addition of water vapor to the feed stream. Mixed-gas selectivities for all materials rose by 10% in humidified conditions when compared to dry feed experiments. Our results confirm that the addition of IL improves the performance of PIL-based composites due to lower stiffness of the membrane matrix. The addition of Zn2+-based salt had a marginal effect on CO2 separation efficiency, suggesting that the cation participates in the facilitated transport of CO2.

Synthesis and Characterization of Novel Nanoporous Gl-POSS-Branched Polymeric Gas Separation Membranes

Novel nanoporous Gl-POSS-branched polymers based on the macroinitiator of anionic nature, 2,4-toluene diisocyanate, and octaglycidyl polyhedral oligomeric silsesquioxane (Gl-POSS) were obtained as gas separation membranes. The synthesis of polymers was carried out using various loads of Gl-POSS. It was found that the main reaction proceeding with 2,4-toluene diisocyanate is the polyaddition, accompanied by the isocyanate groups opening of the carbonyl part. This unusual opening of isocyanate groups leads to the formation of coplanar acetal nature polyisocyanates (O-polyisocyanate). The terminal O-polyisocyanate links initiate the subsequent opening of the epoxide rings in Gl-POSS. As a result, Gl-POSS serves as a hard and bulky branching agent and creates the specific framing supramolecular structure, which leads to the formation of nanopores in the polymer, where the flexible polyether components are located inside the cavities. Thermal, mechanical, physical, and chemical properties of the obtained polymers were studied at various Gl-POSS contents in the polymer matrix. It was found that these polymers show high selectivity of gas transport properties for pure ammonia relative to nitrogen and hydrogen at ambient temperature. Measurements showed that the gas permeability coefficients and the values of ideal selectivity were in a non-additive dependence to the Gl-POSS content.

Influence of Anion Structure on Thermal, Mechanical and CO2 Solubility Properties of UV-Cross-Linked Poly(ethylene glycol) Diacrylate Iongels

Iongel-based CO₂ separation membranes were prepared by fast (< 1 min) UV-initiated polymerization of poly(ethylene glycol) diacrylate (PEGDA) in the presence of different ionic liquids (ILs) with the [C₂mim]⁺ cation and anions such as [TFSI]^-, [FSI]^-, [C(CN)₃]^- and [B(CN)₄]^-. The four ILs were completely miscible with the non-ionic PEGDA network. Transparent and free-standing iongels containing between 60 and 90 %wt of IL were obtained and characterized by diverse techniques (FTIR, TGA, DSC, DMTA, SEM, CO₂ solubility and pure gas permeability). The thermal and mechanical stability of the iongels, as well as CO₂ solubility, were found to be strictly dependent on the IL content and the anion's nature. The TGA results indicated that the iongels mostly follow the thermal profile of the respective neat ILs. The DMTA analysis revealed that the iongels based on fluorinated anions have higher storage modulus than those of cyano-functionalized anions. Conversely, the PEGDA-C(CN)₃ iongels presented the highest CO₂ solubility values ranging from 72 to 80 mmol/g. Single CO₂ permeabilities of 583 ± 29 Barrer and ideal CO₂/N₂ selectivities of 66 ± 3 were obtained with the PEGDA-70 C(CN)₃ iongel membrane. This work demonstrates that the combination of PEGDA with high contents of the best performing ILs is a promising and simple strategy, opening up new possibilities in the design of high-performance iongel membranes for CO₂ separation.

Self-healing behaviour of furan–maleimide poly(ionic liquid) covalent adaptable networks

Recovery of mechanical (tensile testing) and conductive (chronoamperometric cycling) properties was observed for PIL networks containing thermoreversible furan–maleimide crosslinks.

Reversible Ion-Conducting Switch in a Novel Single-Ion Supramolecular Hydrogel Enabled by Photoresponsive Host-Guest Molecular Recognition

A novel ion-conducting supramolecular hydrogel with reversible photoconductive properties in which the azobenzene motif, α-cyclodextrin (α-CD), and ionic liquid are grafted onto the gel matrix is reported. Host-guest interactions with different association constants between α-CD and azobenzene or the anionic part of the ionic liquid can be readily tuned by photoinduced trans-cis isomerization of the azobenzene unit. When irradiated by 365 nm light, α-CD prefers to form a complex with the anionic part of the ionic liquid, resulting in decreased ionic mobility and thus high resistance of the hydrogel. However, under 420 nm light irradiation, a more stable complex is again formed between α-CD and trans-azobenzene, thereby releasing the bound anions to regenerate the low-resistive hydrogel. As such, remote control of the ionic conductivity of the hydrogel is realized by simple host-guest chemistry. With the incorporation of a logic gate, this hydrogel is able to reversibly switch an electric circuit on and off by light irradiation with certain wavelengths. The concept of photoswitchable ionic conductivity of a hydrogel mediated by competitive molecular recognition is potentially promising toward the fabrication of optoelectronic devices and applications in bioelectronic technology.

Does poly(ionic liquid) modulate the non-covalent interactions of chicken egg white lysozyme? Elucidation of biomolecular interactions between biomolecules and macromolecular solvents

Stabilizing and destabilizing effects of different poly(ionic liquid) (PIL) concentrations on chicken egg white lysozyme as a reason for bimolecular interactions.

Towards Biohydrogen Separation Using Poly(Ionic Liquid)/Ionic Liquid Composite Membranes

Considering the high potential of hydrogen (H₂) as a clean energy carrier, the implementation of high performance and cost-effective biohydrogen (bioH₂) purification techniques is of vital importance, particularly in fuel cell applications. As membrane technology is a potentially energy-saving solution to obtain high-quality biohydrogen, the most promising poly(ionic liquid) (PIL)–ionic liquid (IL) composite membranes that had previously been studied by our group for CO₂/N₂ separation, containing pyrrolidinium-based PILs with fluorinated or cyano-functionalized anions, were chosen as the starting point to explore the potential of PIL–IL membranes for CO₂/H₂ separation. The CO₂ and H₂ permeation properties at the typical conditions of biohydrogen production (T = 308 K and 100 kPa of feed pressure) were measured and discussed. PIL–IL composites prepared with the [C(CN)₃]⁻ anion showed higher CO₂/H₂ selectivity than those containing the [NTf₂]⁻ anion. All the membranes revealed CO₂/H₂ separation performances above the upper bound for this specific separation, highlighting the composite incorporating 60 wt % of [C₂mim][C(CN)₃] IL.

Covalently Crosslinked 1,2,3-Triazolium-Containing Polyester Networks: Thermal, Mechanical, and Conductive Properties

Azide-alkyne "click" cyclization was used to prepare a series of polymerizable acetoacetate monomers containing a 1,2,3-trizolium ionic liquid group. The monomers were subsequently polymerized using base-catalyzed Michael addition chemistry, producing a series of covalently crosslinked 1,2,3-triazolium poly(ionic liquid) (TPIL) networks. Structure-activity relationships were conducted to gauge how synthetic variables, such as counteranion ([Br], [NO3], [BF4], [OTf], and [NTf2]), and crosslink density (acrylate/acetoacetate ratio) effected thermal, mechanical, and conductive properties. TPIL networks were found to exhibit ionic conductivities in the range of 10-6-10-9 S/cm (30 °C, 30% relative humidity), as determined from dielectric relaxation spectroscopy, despite their highly crosslinked nature. Temperature-dependent conductivities demonstrate a dependence on polymer glass transition, with free-ion concentrations impacted by various ions' Lewis acidity/basicity and ion mobilities impacted by freely mobile anion size.

New Pyridinium Type Poly(Ionic Liquids) as Membranes for CO₂ Separation

New pyridinium based PILs have been prepared by modification of their precursors based on high molecular weight aromatic polyethers bearing main chain pyridine units. The proposed methodology involves the conversion of the precursors to their ionic analogues via N-methylation reaction, followed by anion exchange methathesis reaction to result in PILs with the desirable anions (tetrafluoroborate and bis(trifluoromethylsulfonyl)imide). These PILs show excellent thermal stability, excellent mechanical properties, and most importantly can form very thin, free standing films with minimum thickness of 3 μm. As expected, the PIL containing the TFSI⁻ anion showed improved CO₂ and CH₄ permeabilities compared to its analogue containing the BF₄⁻. PIL-IL composites membranes have also been prepared using the same PIL and different percentages of pyridinium based IL where it was shown that the membrane with the highest IL weight percentage (45 wt %) showed the highest CO₂ permeability (11.8 Barrer) and a high CO₂/CH₄ ideal selectivity of 35 at room temperature.

Syntheses, characterizations and functions of cationic polyethers with imidazolium-based ionic liquid moieties

Cationic polyethers with ionic liquid groups are characterized with deliquescence, ionic conductivity and miscibility in ionic liquid.

Molecular Simulation of Ionic Polyimides and Composites with Ionic Liquids as Gas-Separation Membranes

Polyimides are at the forefront of advanced membrane materials for CO₂ capture and gas-purification processes. Recently, ionic polyimides (i-PIs) have been reported as a new class of condensation polymers that combine structural components of both ionic liquids (ILs) and polyimides through covalent linkages. In this study, we report CO₂ and CH₄ adsorption and structural analyses of an i-PI and an i-PI + IL composite containing [C₄mim][Tf₂N]. The combination of molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) simulations is used to compute the gas solubility and the adsorption performance with respect to the density, fractional free volume (FFV), and surface area of the materials. Our results highlight the polymer relaxation process and its correlation to the gas solubility. In particular, the surface area can provide meaningful guidance with respect to the gas solubility, and it tends to be a more sensitive indicator of the adsorption behavior versus only considering the system density and FFV. For instance, as the polymer continues to relax, the density, FFV, and pore-size distribution remain constant while the surface area can continue to increase, enabling more adsorption. Structural analyses are also conducted to identify the nature of the gas adsorption once the ionic liquid is added to the polymer. The presence of the IL significantly displaces the CO₂ molecules from the ligand nitrogen sites in the neat i-PI to the imidazolium rings in the i-PI + IL composite. However, the CH₄ molecules move from the imidazolium ring sites in the neat i-PI to the ligand nitrogen atoms in the i-PI + IL composite. These molecular details can provide critical information for the experimental design of highly selective i-PI materials as well as provide additional guidance for the interpretation of the simulated adsorption systems.

Catalytic Ionic-Liquid Membranes: The Convergence of Ionic-Liquid Catalysis and Ionic-Liquid Membrane Separation Technologies

Membrane technologies enable the facile separation of complex mixtures of gases, vapours, liquids and/or solids under mild conditions. Simultaneous chemical transformations can also be achieved in membranes by using catalytically active membrane materials or embedded catalysts, in so-called membrane reactors. A particular class of membranes containing or composed of ionic liquids (ILs) or polymeric ionic liquids (pILs) have recently emerged. These membranes often exhibit superior transport and separation properties to those of classical polymeric membranes. ILs and pILs have also been extensively studied as separation solvents, catalysts and co-catalysts in similar applications for which membranes are employed. In this review, after introducing ILs and their applications in catalysis, catalytic membranes and recent advances in membrane separation processes based on ILs are described. Finally, the nascent concept of catalytic IL membranes is highlighted, in which catalytically active ILs/pILs are incorporated into membrane technologies to act as a catalytic separation layer.