Recent Developments and Applications of Ionic Liquids in Gas Separation Membranes

Flue gas emissions and the harmful effects of these gases urge to separate and capture these unwanted gases. Ionic liquids due to negligible vapor pressure, thermal stability, and wide electrochemical stability have expanded its application in gas separations. A comprehensive overview of the recent developments and applications of ionic liquid membranes (ILMs) for gas separation is given. The three general classifications of ILMs, such as supported ionic liquid membranes (SILMs), ionic liquid polymeric membranes (ILPMs), and ionic liquid mixed‐matrix membranes (ILMMMs) along with their applications, for the separation of various mixed gases systems is discussed in detail. Furthermore, issues, challenges, computational study, and future perspectives for ILMs are also considered.     

Ionic Liquids as New Solvents for Textile Fiber Formation and Modification

Ionic liquids (ILs) with their special characteristics such as low melting point, high thermal stability, electrical conductivity, and good solubility in different chemical liquids, are widely used as new solvents for a large range of inorganic, organic, and polymeric materials. The development of ILs created a great untapped potential for commercial and academic applications to increase operating efficiencies of many chemical processes, including the processing of textiles. A summarizing introduction about ILs, their characterization, the general applications, and the major challenges is presented, followed by previous researches on using ILs for textile fiber formation and modification of their properties.     

Metathesis of Ionic Liquids: Continuous Ion Exchange by Donnan Dialysis

Ionic liquids (IL) are salts that have extraordinary low melting points below 100 °C. While only a very limited number of possible IL can be synthesized directly, the vast majority is prepared via the synthesis of a precursor IL with the desired cation and a subsequent anion exchange. This paper presents the continuous anion exchange by Donnan dialysis in aqueous solution with anion exchange membranes. The retention of the anion exchange membranes used in this work for inorganic and IL cations was found to be R > 99 %. The average integral transport coefficients were determined for the exchange of chloride/acetate, chloride/hydroxide and bromide/hydroxide for various IL and classical salts. The values of the integral average transport coefficients were found to be independent of the investigated counterions. Process modeling was applied to optimize the flow conditions to reach an anion exchange > 98 %. Ultrapure hydroxide solutions and acetates of common classes of IL cations were prepared with a conversion of the reactant anions of > 99 % and cationic impurity contents of < 1 %.     

离子液体在溶解方面的研究进展

离子液体作为一种新型的“绿色溶剂”,广泛应用于化学反应、萃取分离等化工过程。介绍了绿色溶剂——离子液体发展状况,详细讨论了离子液体在溶解方面的研究进展,包括离子液体在萃取分离方面的研究、离子液体作为反应过程中的溶剂以及离子液体作为高分子溶剂的研究。     

Charge Dynamics and Bending Actuation in Aquivion Membrane Swelled with Ionic Liquids

The actuation strain and speed of ionic electroactive polymer (EAP) actuators are mainly determined by the charge transport through the actuators and excess ion storage near the electrodes. We employ a recently developed theory on ion transport and storage to investigate the charge dynamics of short side chain Aquivion^® (Hyflon^®) membranes with different uptakes of ionic liquid (IL) 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMI-Tf). The results reveal the existence of a critical uptake of ionic liquids above which the membrane exhibits a high ionic conductivity (σ > 5 × 10⁻² mS/cm). Especially, we investigate the charge dynamics under voltages which are in the range for practical device operation (∼1 V and higher). The results show that the ionic conductivity, ionic mobility, and mobile ion concentration do not change with the applied voltage below 1 V (and for σ below 4 V). The results also show that bending actuation of the Aquivion membrane with 40wt% EMI-Tf is much larger than that of Nafion, indicating that the shorter flexible side chains improve the electromechanical coupling between the excess ions and the membrane backbones, while not affecting the actuation speed.     

Methanol-Water Solution Transport in Nafion Membranes with Different Cationic Forms

Measurements of liquid transport were made with a Nafion membrane at different cationic forms. The experimental data are used to estimate the alcohol permeability when the membrane is separating water and water-methanol solutions. The obtained permeability coefficient values were useful for analyzing the influence of the substituted cations on the transport process in the membranes. In the present article, the permeability coefficient of methanol in Nafion substituted by Na⁺, K⁺, Cs⁺, Mg²⁺, Ca²⁺, Ba²⁺, Fe²⁺, Fe³⁺, and Al³⁺ were reported at different methanol concentration values. The analysis of the results revealed that, in general, for ions with the same period in the periodic table, the alcohol permeability decreases with increasing the valence. In contrast, when ions with the same valence are compared, the alcohol permeability decreases when the atomic mass increases, with the exception of the Mg²⁺. As a general trend, similar alcohol permeability variation with the concentration is observed for all the cationic forms of the membrane. There is an initial increase in the permeability, and, when the methanol concentration in the solutions is about 60%, the permeability decreases with the alcohol concentration. However, in the case of trivalent ions, the methanol permeability decreases with the methanol concentration.     

离子液体及其应用进展

主要概述了离子液体及其发展概况,离子液体的种类特性以及合成方法.重点归纳了离子液体作为溶剂的优越性以及离子液体在化学反应、电化学、催化化学和分离纯化中的应用,并指出了离子液体在大规模工业应用方面存在的问题.     

功能型离子液体萃取氧化脱硫研究

合成了一系列含有不同阴离子的1-烷基-3-甲基咪唑型离子液体,以35%H2O2以及冰醋酸为氧化剂,分别考察了不同条件下离子液体对模拟油品和实际油品的脱硫效果。结果表明,离子液体阴离子的酸性以及阳离子烷基碳链的长度对脱硫效果具有显著影响,其中具有较长碳链的强酸性硫酸氢盐类离子液体在剂︰油︰氧化剂=1︰25︰1,30℃条件下对模拟油品与实际油品均具有较高的脱硫率,对模拟油品一次脱硫率在90%以上,对抚顺石化公司石油二厂汽油、柴油一次脱硫率在80%以上,其中汽油含硫量降至10 mg/kg左右,达到欧V标准,显示了非常好的工业应用前景。     

纳米TiO2增强阴离子导电膜中碱性离子液体的稳定性研究

首先，以N-甲基吡咯烷为原料，通过两步法合成碱性甲基吡咯烷离子液体（[DMPy]OH）。然后，通过物理掺杂方式在聚乙烯醇（PVA）膜基质中引入[DMPy]OH碱性离子液体和纳米二氧化钛（TiO2），最后引入戊二醛（GA）交联PVA，制得PVA-[DMPy][OH]-TiO2复合膜。利用纳米二氧化钛（Nano-TiO2）表面分布的未配位饱和的Ti4+、O2-与[DMPy]OH中阴阳离子间的自组装效应，以及戊二醛（GA）交联PVA形成半互穿结构来降低膜内碱性离子液体的流失率，增加膜内“阳离子活性点位”数量，加快OH-的迁移速率，提高电导率。借助X射线衍射(XRD)和Mapping测试证实了Nano-TiO2和[DMPy]OH间自组装作用的存在，同时对复合膜的机械性能、热稳定性能、电导率、耐碱性等指标进行测试。结果表明，该系列复合膜，形貌平整、均一；当离子液体添加量为25wt%，Nano-TiO2添加量为1wt%时，80℃下PVA-[DMPy][OH]-TiO2复合膜的电导率为2.78×10-3 S.cm-1，室温下浸渍于水中12h后，离子液体流失率为23.2%；与未添加Nano-TiO2的PVA-[DMPy][OH]膜相比，电导率提高了19.4%，离子液体流失率降幅约为34.3%；室温下，3M KOH中耐碱性测试120h后，电导率和初始值相比并无明显变化，表现出优异的耐碱稳定性。热稳定性测试结果表明，该膜分解温度高于200℃，具有良好的热稳定性。     

Ionic Liquids in Oxygen Compression

Ionic liquids are still a relatively new and very promising class of substances which have received increased research attention in recent years. Due to their unique physical and chemical properties these compounds are ideal for use in industrial applications and also offer a variety of very interesting possibilities in process technology, with a high potential of increasing performance and reducing investment and operating costs [1]. Ionic liquids are remarkable for their high chemical inertness and good lubricity. Therefore, one very promising application is the lubricated compression of oxygen as an alternative to the dry compression technique that has to be used because of the extremely high reactivity of pure oxygen with organic lubricating media. A screening of the relevant parameters including thermal stability, flammability, chemical inertness to pure oxygen, corrosiveness, tribological behavior and oxygen solubility was performed. Based on the results obtained, the most suitable ionic liquid was identified and used in a screw compressor setup that achieves a final pressure of 30 bar with a delivery volume of up to 200 Nm³/h.     

Modification of Nafion membranes by incorporation of cationic polymers for reduction of methanol permeability

A small amount of basic polymer was incorporated in the Nafion membrane. Compared with the re-cast Nafion membrane, the Nafion/basic polymer membrane reduced the methanol permeability considerably. The equilibrium water uptake and proton conductivity decreased, but the thermal and mechanical stability was enhanced with increasing concentration of basic polymer. These property changes were caused by formation of cation/anion complex between acidic Nafion and basic polymer molecules. The effects of the types and molecular weights of basic polymers on the methanol permeability and proton conductivity were not significant.     

High-resolution imaging of ion conductivity of Nafion membranes with electrochemical atomic force microscopy

Conductive electrochemical AFM images demonstrating the complex nature and structure of Nafion surface conductivity are presented. Nanoscale regions with high currents determining the overall total membrane current can be distinguished from majority domains with lower currents and non-conductive areas. The different conductive domains form ordered structures and show a specific dynamic behaviour. These observations were compared to the structural and electrical models in the literature. None of the models is able to explain all aspects of the current images. The existence of inverted micelles seems to be quite probable since the formation of agglomerates like chains and larger ordered clusters is clearly visible. This aspect is best described by the model of Schmidt-Rohr and Chen. In addition, the highly dynamic behaviour and distribution of conductive channels of Nafion leading to the formation of new current pathways also indicates the formation of different meso-phases with a high local fluctuation rate. The other discussed models also predict structural features which are in agreement with our observations like the formation of super-structures and agglomeration of fibers.The structural characterisation reflects the situation at or near the membrane surface and might differ from the bulk structure since the surface energy may have a large influence on the formation of structures during the membrane solidification process. The quite large dynamics of conductivity changes of Nafion reflected in the formation of new current pathways even at room temperature leads to the assumption that the internal structure of Nafion is subject to significant changes due to humidity and temperature variations. The local variation of individual structures may reflect the variation of concentration of hydrophilic and hydrophobic groups during membrane solidification. The minimization of surface free energy during a self-assembling process is essential for the formation of different phases and subsequent structures like chains, etc. as well as higher order clustering.     

A Review on Ionic Liquids-Based Membranes for Middle and High Temperature Polymer Electrolyte Membrane Fuel Cells (PEM FCs)

Today, the use of polymer electrolyte membranes (PEMs) possessing ionic liquids (ILs) in middle and high temperature polymer electrolyte membrane fuel cells (MT-PEMFCs and HT-PEMFCs) have been increased. ILs are the organic salts, and they are typically liquid at the temperature lower than 100 °C with high conductivity and thermal stability. The membranes containing ILs can conduct protons through the PEMs at elevated temperatures (more than 80 °C), unlike the Nafion-based membranes. A wide range of ILs have been identified, including chiral ILs, bio-ILs, basic ILs, energetic ILs, metallic ILs, and neutral ILs, that, from among them, functionalized ionic liquids (FILs) include a lot of ion exchange groups in their structure that improve and accelerate proton conduction through the polymeric membrane. In spite of positive features of using ILs, the leaching of ILs from the membranes during the operation of fuel cell is the main downside of these organic salts, which leads to reducing the performance of the membranes; however, there are some ways to diminish leaching from the membranes. The aim of this review is to provide an overview of these issues by evaluating key studies that have been undertaken in the last years in order to present objective and comprehensive updated information that presents the progress that has been made in this field. Significant information regarding the utilization of ILs in MT-PEMFCs and HT-PEMFCs, ILs structure, properties, and synthesis is given. Moreover, leaching of ILs as a challenging demerit and the possible methods to tackle this problem are approached in this paper. The present review will be of interest to chemists, electrochemists, environmentalists, and any other researchers working on sustainable energy production field.     

离子液体及其在电化学中的应用

离子液体即在室温或接近室温下呈液态的完全由离子构成的物质,作为环境友好和“可设计性”溶剂正在引起越来越多的重视。它具有熔点低、蒸汽压小、酸性可调及良好的溶解度、粘度和密度等特点。综述了离子液体的组成、分类、性质、制备和纯化,就离子液体在电池技术、电合成、电沉积、电容器等电化学方面的应用和研究进展加以阐述,并对该领域的研究前景作了展望。     

Study of lithiated Nafion ionomer for lithium batteries

Lithiated Nafion 112 ionomer was characterized by FT-IR spectroscopy, AC impedance, and cyclic voltammetry. The ionomer swollen with mixed solvents of propylene carbonate (PC) and ethylene carbonate shows ionic conductivity of 8.18×10^–5Scm^–1 at 25°C and good electrochemical stability to allow operation in Li/ionomer/LiCoO₂ cells. The discharge capacity of the first cycle is 126mAhg^–1. Significant capacity loss occurs during cycling due to the presence of PC. AC impedance shows that the passive layer formed at the Li/ionomer interface dominates the cycling performance of the cell.     

Nafion/PTFE Composite Membranes for Fuel Cell Applications

The composite membranes were prepared by impregnation of porous poly(tetrafluoroethylene) membranes with a 5 wt% Nafion solution. Scanning electron microscope micrographs of composite membranes show the surface and cross section of poly(tetrafluoroethylene) membranes were covered and filled with Nafion resin. Comparison of physical properties and fuel cell performance of composite membranes with those of Nafion membranes (DuPont Co) is presented. The composite membrane has better thermal stability and gas barrier property but worse ionic conductivity than Nafion membrane. Though the composite membrane has a lower conductivity than Nafion membrane, however, owing to the thinner thickness of composite membrane (in thickness of 20±5µm) than Nafion-115 (in thickness of 125µm) and Nafion-117 (in thickness of 175µm) membranes, the composite membrane has a shorter H⁺ ion transporting pathway and thus a higher conductance (conductance = conductivity/membrane thickness) than Nafion-115 and Nafion-117 membranes. Thus the composite membrane has a better fuel cell performance than Nafion-117 and Nafion-115 membranes. In this report, we show that our composite membrane has a fuel cell performance similar to Nafion-112 membrane (in thickness of 50µm).     

Kinetic Model of Two‐Phase Epoxidation with Ionic Liquids as Micellar Catalysts

The biphasic catalytic epoxidation of cyclooctene using the ionic liquid (IL) 1,2‐dimethyl‐3‐octyl‐imidazolium perrhenate ([OMMIM]ReO₄) as micellar catalyst and H₂O₂ as oxidant was investigated. Kinetic experiments were carried out in the intrinsic kinetic regime as proved by variation of stirring rate and temperature. Variation of catalyst concentration allowed for determination of the critical micellar concentration (CMC) of the catalytic IL. The effect of substrate concentrations on the reaction rate was also assessed. Based on the experiments, a kinetic model adapted from enzyme catalysis was proposed to account for the micellar reaction environment. The model takes into account the onset of micelle formation at the CMC. The application of the kinetic model illustrated the good agreement with the experimental data. The model will be applied to other micellar epoxidation reactions and for the design of an appropriate reaction setup in the future.     

Continuous Transesterification with Acidic Ionic Liquids as Homogeneous Catalysts

Ionic liquids (ILs) are recyclable acid catalysts for transesterification reactions. In the present study, different acidic ILs were examined in this reaction, with special focus on their recyclability. Furthermore, the IL‐catalyzed transesterification reaction was realized in continuous operation. A miniplant reactor with technically representative design and operating characteristics was used for this study. The applied rig has a volume of 5 L and an external thermosyphon reboiler. The miniplant reactor can be operated in batch and in continuous mode. ILs functionalized with a sulfonic acid group were found to be the most suitable IL catalysts for the transesterification reactions under investigation. Using these ILs, reaction rates as high as for H₂SO₄ could be realized. Moreover, the IL catalyst was demonstrated to be active for at least 1000 h of operation time.     

Solid polymer electrolytes V: microstructure and ionic conductivity of epoxide-crosslinked polyether networks doped with LiClO4

A crosslinked polyether network was prepared from poly(ethylene glycol) diglycidyl ether (PEGDE) cured with poly(propylene oxide) polyamine. Significant interactions between ions and polymer host have been observed for the crosslinked polyether network in the presence of LiClO₄ by means of FT-IR, DSC, TGA, and ⁷Li MAS solid-state NMR. Thermal stability and ionic conductivity of these complexes were also investigated by TGA and AC impedance measurements. The results of FT-IR, DSC, TGA and ⁷Li MAS solid-state NMR measurements indicate the formation of different types of complexes through the interaction of ions with different coordination sites of polymer electrolyte networks. The dependence of ionic conductivity was investigated as a function of temperature, LiClO₄ concentration and the molecular weight of polyether curing agents. It is observed that the behavior of ion transport follows the empirical Vogel-Tamman-Fulcher (VTF) type relationship for all the samples, implying the diffusion of charge carrier is assisted by the segmental motions of polymer chains. Moreover, the conductivity is also correlated with the interactions between ions and polymer host, and the maximum ionic conductivity occurs at the LiClO₄ concentration of [O]/[Li⁺]=15.     

A Method for Improving Ionic Conductivity of Nafion Membranes and its Application to PEMFC

In this paper, we show that proton conductivity and PEMFC performance of Nafion membranes prepared by solutions casting can be improved by aligning the side cahin ionic aggregations along the membrane thickness direction using an electric field. The nano-structures of Nafion membranes prepared by solutions casting with applying an electric field were investigated using transmission electron microscopy (TEM), which clearly shows fibril-like structures of Nafion molecular aggregations aligned along the electric field. The alignment of ionic aggregations along the membrane thickness direction causes linear and less curved proton transferring pathways across the membrane cross section and thus a higher proton conductivity and a better PEMFC performance.