CO2 separation properties of a glassy aromatic polyimide composite membranes containing high-content 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquid

Healable, Recyclable, and Upcyclable Gel Membranes for Efficient Carbon Dioxide Separation

Ionic liquids (ILs) are prized for their selective dissolution of carbon dioxide (CO2), leading to their widespread use in ionogel membranes for gas separation. Despite their advantages, creating sustainable ionogel membranes with high IL contents poses challenges due to limited mechanical strength, leakage risks, and poor recyclability. Herein, we leverage copolymerized and supramolecularly bound ILs to develop ionogel membranes with high mechanical strength, zero leakage, and excellent self-healing and recycling capabilities. These membranes exhibit superior ideal selectivity for gas separation compared to other reported ionogel membranes, achieving a CO2/nitrogen selectivity of 61.7 and a CO2/methane selectivity of 24.6, coupled with an acceptable CO2 permeability of 186.4 Barrer. Additionally, these gas separation ionogel membranes can be upcycled into ionic skins for sensing applications, further enhancing their utility. This research outlines a strategic approach to molecularly engineer ionogel membranes, offering a promising pathway for developing sustainable, high-performance materials for advanced gas separation technologies.

Cellulose Acetate-Ionic Liquid Blends as Potential Polymers for Efficient CO2 Separation Membranes

CO2 capture, applied in CO2 separation from natural gas or in CO2/N2 separation from power plant flue gas streams, is of great importance for technical, economic, and environmental reasons. The latter seems important because CO2, as a greenhouse gas, is considered the main contributor to global warming. Using polymeric membranes for CO2 separation presents several advantages, such as low energy demand, small equipment volume, and the absence of liquid waste. In this study, two ionic liquids (ILs) were used for the preparation of cellulose acetate (CA)-IL blend membranes for potential CO2 capture applications, namely, 1-butyl-3-methylimidazolium hydrogen sulfate ([Bmim+][HSO4-]) and choline glycine ([Ch+]Gly-), as they present adequate CO2 dissolution ability. The first IL is commercially available, whereas the latter was synthesized by a novel route. Several composite membranes were prepared through the solvent casting technique and characterized by a variety of methods, including thermogravimetry, calorimetry, FTIR spectroscopy, and X-ray diffraction. The CO2 sorption in the composite membranes was experimentally measured using the mass loss analysis (MLA) technique. The results showed that the ILs strongly interacted with the C=O groups of CA, which exhibited high affinity with CO2. In the case of [Bmim+][HSO4-], a reduction in the available sites that allow strong intermolecular interactions with CO2 resulted in a decrease in CO2 sorption compared to that of pure CA. In the case of [Ch+]Gly-, the reduction was balanced out by the presence of specific groups in the IL, which presented high affinity with CO2. Thus, the CA-[Ch+]Gly- blend membranes exhibited increased CO2 sorption capability, in addition to other advantages such as non-toxicity and low cost.

Preparation of Polyimide/Ionic Liquid Hybrid Membrane for CO2/CH4 Separation

Imidazole ionic liquids (ILs) have good affinity and good solubility for carbon dioxide (CO2). Such ionic liquids, combined with polyimide membrane materials, can solve the problem that, today, CO2 is difficult to separate and recover. In this study, the ionic liquid (IL) of 1-ethyl-3-methylimidazolium tetrafluoroborate (IL1), 1-pentyl-3-methylimidazolium tetrafluoroborate (IL2), 1-octyl-3-methylimidazolium tetrafluoroborate (IL3), and 1-dodecylimidazolium tetrafluoroborate (IL4) with different contents were added to a polyimide matrix, and a series of polyimide membranes blended with ionic liquid were prepared using a high-speed mixer. The mechanical properties and gas separation permeability of the membranes were investigated. Among them, the selectivity of the PI/IL3 membrane for CO2/CH4 was 180.55, which was 2.5 times higher than the PI membrane, and its CO2 permeability was 16.25 Barrer, which exceeded the Robeson curve in 2008; the separation performance of the membrane was the best in this work.

Electro-casting for Superior Gas Separation Membrane Performance and Manufacturing

Gas separation polymer membranes play a pivotal role in various industrial processes including carbon capture and hydrogen production. However, the inherent trade-off between permeability and selectivity coupled with challenges in membrane manufacturing has hindered their widespread industrial deployment. To address the permselectivity challenges, researchers have explored increasingly complex polymers, composite systems, and other materials. In this study, we introduce a novel membrane manufacturing technique called "electro-casting" that not only enables efficient membrane fabrication but also enhances the trade-off of traditional polymer-based membranes. We fabricated cellulose acetate (CA) membranes embedded with 1-ethyl-3-methyl imidazolium via electro-casting and performed a comparative analysis of structural, morphological, and gas transport characteristics against membranes made via conventional casting techniques. We discovered that electro-casted membranes exhibited a unique crystalline structure, surface topology that induced a remarkable 200% improvement in CO2/N2 selectivity and a 110% increase in CO2/CH4 selectivity. The electric field generated during the manufacturing process played a crucial role in altering the supramolecular structure of the polymer, thereby increasing the separation properties of the membranes as well as their thermal and mechanical features. Electro-casting induced a polymer crystallization effect that disrupted the permeability-selectivity trade-off observed in conventional membranes, while producing highly stable membranes. Moreover, the simplicity of this manufacturing method and its significant impact on membrane properties have the potential to accelerate the deployment of gas separation membranes, facilitating the transition toward a NetZero chemical industry.

Enhancing CO2 separation from N2 mixtures using hydrophobic porous supports immobilized with tributyl-tetradecyl-phosphonium chloride [P44414][Cl]

The main obstacles in adopting solvent-based CO2 capture technology from power plant flue gases at the industrial scale are the energy requirements for solvent regeneration and their toxicity. These challenges can be overcome using new green and more stable ionic liquids (ILs) as solvents for post-combustion CO2 capture. In the current study, tributyl-tetradecyl-phosphonium chloride [P44414][Cl] as an IL, was immobilized on hydrophobic porous supports of polypropylene (PP), polyvinylidene fluoride (PVDF), and polytetrafluoroethylene (PTFE) at 298 ± 3 K and pressures up to 2 bar. The surface morphology indicated homogenous immobilization of the IL on the membrane support. Supported ionic liquid membranes (SILMs) were tested for CO2 permeability and CO2/N2 selectivity. None of the SILMs exhibited IL leaching up to 2 bar. The PTFE-based SILM performed better than other supports with minimum loss in water contact angle (WCA) and achieved good antiwetting with a maximum CO2 permeability and selectivity over N2 of 2300 ± 139 Barrer and 31.60 ± 2.4, respectively. This work achieves CO2 permeability about two-fold more than other works having CO2/N2 selectivity range of 25-35 in similar SILMs. The diffusivity of CO2 and N2 in [P44414][Cl] was measured as 3.64 ± 0.18 and 2.01 ± 0.09 [10-8 cm2 s-1] and CO2 and N2 solubility values were 9.79 ± 0.47 and 0.19 ± 0.001 [10-2 cm3(STP) cm-3 cmHg-1], respectively. The high values of Young's modulus and tensile strength of the PTFE support-based SILM (234 ± 12 MPa and 6.07 ± 0.31 MPa, respectively) indicated the long-term application of SILM in flue gas separation. The results indicated phosphonium chloride-based ILs could be better solvent candidates for CO2 removal from large volumes of flue gases than amine-based ILs.

Polyimide/Ionic Liquids Hybrid Membranes with NH3-Philic Channels for Ammonia-Based CO2 Separation Processes

An efficient separation technology involving ammonia (NH3) and carbon dioxide (CO2) is of great importance for achieving low-carbon economy, environmental protection, and resource utilization. However, directly separating NH3 and CO2 for ammonia-based CO2 capture processes is still a great challenge. Herein, we propose a new strategy for selective separation of NH3 and CO2 by functional hybrid membranes that integrate polyimide (PI) and ionic liquids (ILs). The incorporated protic IL [Bim][NTf2] is confined in the interchain segment of PI, which decreases the fractional free volume and narrows the gas transport channel, benefiting the high separation selectivity of hybrid membranes. At the same time, the confined IL also provides high NH3 affinity for transport channels, promoting NH3 selective and fast transport owing to strong hydrogen bonding interaction between [Bim][NTf2] and NH3 molecules. Thus, the optimal hybrid membrane exhibits an ultrahigh NH3/CO2 ideal selectivity of up to 159 at 30 °C without sacrificing permeability, which is 60 times higher than that of the neat PI membrane and superior to the state-of-the art reported values. Moreover, the introduction of [Bim][NTf2] also reduces the permeation active energy of NH3 and reverses the hybrid membrane toward "NH3 affinity", as understood by studying the effect of temperature. Also, NH3 molecules are much easier to transport at high temperature, showing great application potential in direct NH3/CO2 separation. Overall, this work provides a promising ultraselective membrane material for ammonia-based CO2 capture processes.

Environmental remediation through various composite membranes moieties: Performances and thermomechanical properties

Pollution harms ecosystems and poses a serious threat to human health around the world through direct or indirect effects on air, water, and land. The importance of remediating effluents is paramount to reducing environmental concerns. CO₂ emissions are removed efficiently and efficaciously with mixed matrix membranes (MMMs), which are viable replacements for less efficient and costly membranes. In the field of membrane technology, MMMs are advancing rapidly due to their good separation properties. The selection of filler to be incorporated in mixed matrix membranes is very considered very important. There has been considerable interest in MOFs, carbon nanotubes (CNTs), ionic liquids (ILs), carbon molecular sieves (CMSs), sulfonated fillers (SFs), and layered silicates (LSs) as inorganic fillers for improving the properties of mixed matrix membranes. These fillers promise superb results and long durability for mixed matrix membranes based on them. The purpose of this review is to review different fillers used in MMMs for improving separation properties, limitations, and thermomechanical properties for environmental control and remediation.

Preparation and gas separation performance of polyimide membranes endcapped with ionic liquid-type structures

A set of ionic liquid capped polyimide membranes was prepared using 1-aminoethyl-3-methylimidazolium hexafluorophosphate (IL1) and 1-aminopropylimidazolium bis(trifluoromethylsulfonyl)imine (IL2) as the terminal groups. The products’ molecular weights, mechanical properties, and separation permeability (CO2/CH4) were investigated. For CO2/CH4 separation, the selectivity of the ionic liquid capped polyimide membranes was higher than that of noncapped ones. Among them, the membrane synthesized by 4.4′- diaminodiphenyl ether and 4.4′-(hexafluoroisopropyl) diphthalic anhydride (6FDA) as monomer, with IL1 as terminal group, displayed the best selectivity. Its permeability was 7.47 Barrer and selectivity 102.42, which exceeded the 1991 Robeson curve. Polyimide membranes capped by ionic liquid showed high gas selectivity and good gas permeability as well as good physical and chemical properties. Consequently, it can be concluded that introducing an ionic liquid structure to polyimide chains could make attractive membrane materials for various gas separation and related applications.

Effect of Increased Ionic Liquid Uptake via Thermal Annealing on Mechanical Properties of Polyimide-Poly(ethylene glycol) Segmented Block Copolymer Membranes

Proton exchange membranes (PEMs) suffer performance degradation under certain conditions-temperatures greater than 80 °C, relative humidity less than 50%, and water retention less than 22%. Novel materials are needed that have improved water retention, stability at higher temperatures, flexibility, conductivity, and the ability to function at low humidity. This work focuses on polyimide-poly(ethylene glycol) (PI-PEG) segmented block copolymer (SBC) membranes with high conductivity and mechanical strength. Membranes were prepared with one of two ionic liquids (ILs), either ethylammonium nitrate (EAN) or propylammonium nitrate (PAN), incorporated within the membrane structure to enhance the proton exchange capability. Ionic liquid uptake capacities were compared for two different temperatures, 25 and 60 °C. Then, conductivities were measured for a series of combinations of undoped or doped unannealed and undoped or doped annealed membranes. Stress and strain tests were performed for unannealed and thermally annealed undoped membranes. Later, these experiments were repeated for doped unannealed and thermally annealed. Mechanical and conductivity data were interpreted in the context of prior small angle X-ray scattering (SAXS) studies on similar materials. We have shown that varying the compositions of polyimide-poly(ethylene glycol) (PI-PEG) SBCs allowed the morphology in the system to be tuned. Since polyimides (PI) are made from the condensation of dianhydrides and diamines, this was accomplished using components having different functional groups. Dianhydrides having either fluorinated or oxygenated functional groups and diamines having either fluorinated or oxygenated diamines were used as well as mixtures of these species. Changing the morphology by creating macrophase separation elevated the IL uptake capacities, and in turn, increased their conductivities by a factor of three or more compared to Nafion 115. The stiffness of the membranes synthesized in this work was comparable to Nafion 115 and, thus, sufficient for practical applications.

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.

CO2 capture using membrane contactors: a systematic literature review

With fossil fuel being the major source of energy, CO2 emission levels need to be reduced to a minimal amount namely from anthropogenic sources. Energy consumption is expected to rise by 48% in the next 30 years, and global warming is becoming an alarming issue which needs to be addressed on a thorough technical basis. Nonetheless, exploring CO2 capture using membrane contactor technology has shown great potential to be applied and utilised by industry to deal with post- and pre-combustion of CO2. A systematic review of the literature has been conducted to analyse and assess CO2 removal using membrane contactors for capturing techniques in industrial processes. The review began with a total of 2650 papers, which were obtained from three major databases, and then were excluded down to a final number of 525 papers following a defined set of criteria. The results showed that the use of hollow fibre membranes have demonstrated popularity, as well as the use of amine solvents for CO2 removal. This current systematic review in CO2 removal and capture is an important milestone in the synthesis of up to date research with the potential to serve as a benchmark databank for further research in similar areas of work. This study provides the first systematic enquiry in the evidence to research further sustainable methods to capture and separate CO2.

Organic Ionic Plastic Crystal-Based Composite Membranes for Light Gas Separation: The Impact of Varying Ion Type and Casting Method

The promise of organic ionic plastic crystals (OIPCs) for development of a novel type of gas separation membrane with competitive gas selectivity for CO₂ /N₂ was recently demonstrated. This work aimed to design more selective membranes by investigating a different type of OIPC and a new membrane preparation method. Two different OIPCs were solvent-cast or co-cast with poly(vinylidene difluoride) (PVDF), and their gas transport properties were compared. The first OIPC, methyl(diethyl)isobutylphosphonium hexafluorophosphate ([P_122i4 ][PF₆ ]), was previously studied using the co-cast method, and this was used as a benchmark. The second, N-methyl-N-ethylpyrrolidinium bis(fluorosulfonyl)imide ([C₂ mpyr][FSI]), was investigated for the first time for gas separation applications, achieving high selectivities (α CO 2 / N 2 >40). The thermophysical properties of the composites indicated that the co-casting method is a good way to fabricate solid, mechanically stable and durable membranes. Additionally, the enhanced molecular interactions indicated in OIPC/PVDF co-cast composites point to a new approach for synthesis of other highly selective OIPC-based membranes.

MOF-Based Membranes for Gas Separations

Metal-organic frameworks (MOFs) represent the largest known class of porous crystalline materials ever synthesized. Their narrow pore windows and nearly unlimited structural and chemical features have made these materials of significant interest for membrane-based gas separations. In this comprehensive review, we discuss opportunities and challenges related to the formation of pure MOF films and mixed-matrix membranes (MMMs). Common and emerging separation applications are identified, and membrane transport theory for MOFs is described and contextualized relative to the governing principles that describe transport in polymers. Additionally, cross-cutting research opportunities using advanced metrologies and computational techniques are reviewed. To quantify membrane performance, we introduce a simple membrane performance score that has been tabulated for all of the literature data compiled in this review. These data are reported on upper bound plots, revealing classes of MOF materials that consistently demonstrate promising separation performance. Recommendations are provided with the intent of identifying the most promising materials and directions for the field in terms of fundamental science and eventual deployment of MOF materials for commercial membrane-based gas separations.

Molecular Transport Behavior of CO2 in Ionic Polyimides and Ionic Liquid Composite Membrane Materials

Ionic polyimides (i-PI) are a new class of polymer materials that are very promising for CO₂ capture membranes, and recent experimental studies have demonstrated their enhanced separation performance with the addition of imidazolium-based ionic liquids (ILs). However, there is very little known about the molecular-level interactions in these systems, which give rise to interesting gas adsorption and diffusion characteristics. In this study, we use a combination of Monte Carlo and molecular dynamics simulations to analyze the equilibrium and transport properties of CO₂ molecules in the i-PI and i-PI + IL composite materials. The addition of several different common ILs are modeled, which have a plasticization effect on the i-PI, lowering the glass transition temperature (T_g). The solubility of CO₂ strongly correlates with the T_g, but the diffusion demonstrates more unpredictable behavior. At low concentrations, the IL has a blocking effect, leading to reduced diffusion rates. However, as the IL surpasses a threshold value, the relationship is inverted and the IL has a facilitating effect on the gas transport. This behavior is attributed to the simultaneous contributions of the increased i-PI plasticization at higher IL concentrations (facilitating gas hopping rates from cavity-to-cavity) and the increased IL continuity throughout the system, enabling more favorable transport pathways for CO₂ diffusion.

Molecular Modeling Investigations of Sorption and Diffusion of Small Molecules in Glassy Polymers

With a wide range of applications, from energy and environmental engineering, such as in gas separations and water purification, to biomedical engineering and packaging, glassy polymeric materials remain in the core of novel membrane and state-of the art barrier technologies. This review focuses on molecular simulation methodologies implemented for the study of sorption and diffusion of small molecules in dense glassy polymeric systems. Basic concepts are introduced and systematic methods for the generation of realistic polymer configurations are briefly presented. Challenges related to the long length and time scale phenomena that govern the permeation process in the glassy polymer matrix are described and molecular simulation approaches developed to address the multiscale problem at hand are discussed.

CO2-Philic [EMIM][Tf2N] Modified Silica in Mixed Matrix Membrane for High Performance CO2/CH4 Separation

Separation of carbon dioxide (CO2) from methane (CH4) using polymeric membranes is limited by trade-off between permeability and selectivity as depicted in Robeson curve. To overcome this challenge, this study develops membranes by incorporating silica particles (Si) modified with [EMIM][Tf2N] ionic liquid (IL) at different IL:Si ratio to achieve desirable membrane properties and gas separation performance. Results show that the IL:Si particle has been successfully prepared, indicated by the presence of fluorine and nitrogen elements, as observed via Fourier-Transform Infrared Spectroscopy (FTIR) and X-ray Photoelectron Spectrometer (XPS). Incorporation of the modified particles into membrane has given prominent effects on morphology and polymer chain flexibility. The mixed matrix membrane (MMM) cross-section morphology turns rougher in the presence of IL:Si during fracture due to higher loadings of silica particles and IL. Furthermore, the MMM becomes more flexible with IL presence due to IL-induced plasticization, independent of IL:Si ratio. The MMM with low IL content possesses CO2 permeance of 34.60 ± 0.26 GPU with CO2/CH4 selectivity of 85.10, which is far superior to a pure polycarbonate (PC) and PC-Sil membranes at 2 bar, which surpasses the Robeson Upper Bound. This higher CO2 selectivity is due to the presences of CO2-philic IL within the MMM system.

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.

Morphology and pervaporation performance of ionic liquid and waterborne polyurethane composite membranes

Blending an aromatic-selective ionic liquid (IL, namely 1-ethyl-3-methylimidazolium hexafluorophosphate, [emim][PF₆]) with waterborne polyurethane (WPU) enabled us to obtain [emim][PF₆]-modified waterborne polyurethane composite membranes. We characterized the structure and properties of the [emim][PF₆]/WPU composite membranes by ATR-FTIR, DSC, UV, SEM, EDX, swelling tests, and pervaporation testing. Characterization of the change in the morphology of the membranes in response to the IL loading indicated that a preferential interaction between the IL and soft segments of WPU was induced by hydrogen bonding. This interaction inhibited a potential interaction with benzene (Bz), which initially lowered the permeability. However, at high IL loading, the IL incorporation became ineffective owing to macrophase separation, which caused an increase in the permeability, as indicated by the SEM results. Swelling testing of the [emim][PF₆]/WPU composite membranes showed that the membranes exhibited preferential adsorption of Bz, and the swelling degree of the composite membranes in Bz solvent increased from 58% to 98% and remained almost constant in cyclohexane solvent as the IL content was increased. The [emim][PF₆]/WPU composite membranes enhanced the separation selectivity of Bz/Cy for an IL loading < 10 wt%. The best separation factor was 8.4, and the total flux was 0.19 kg (m² h)⁻¹ (50 wt% Bz/Cy mixtures at 50 °C) at w([emim][PF₆]) : w(WPU) = 10 : 100. In addition, the composite membrane exhibited excellent stability over long-term operation. These results demonstrated that the [emim][PF₆]/WPU composite membranes could be effective for separation of Bz/Cy mixtures by the pervaporation method.

Possible model of interactions in [emim][PF₆]/WPU composite membranes.

Functionalized ionic liquid membranes for CO2 separation

It is imperative to develop efficient, reversible and economic technologies for separating CO₂ which mainly comes from flue gas, natural gas and syngas.