Membrane Separation in Organic Liquid: Technologies,Achievements, and Opportunities

Membrane technology is one of the most promising technologies for separation and purification that is routinely and commercially employed in aqueous solutions. In comparison, its applications in organic solvents are severely underdeveloped mainly due to the poor stability of traditional polymer membranes in organic solvents. The emerging materials such as crosslinked polymers, covalent organic frameworks, metal–organic frameworks, conjugated microporous polymers, carbon molecular sieves, and graphene provide the solutions to address this problem. The membranes constructed with these novel materials show outstanding separation performance in regard to both high selectivity and solvent permeability, greatly pushing forward utilization of membrane technology in organic media. Here, an overview of the most important organic mixtures that need to be separated, the major separation processes adopted nowadays in organic solvents, and the recent progress in new developed membranes is provided.     

Microporous Organic Materials for Membrane‐Based Gas Separation

Membrane materials with excellent selectivity and high permeability are crucial to efficient membrane gas separation. Microporous organic materials have evolved as an alternative candidate for fabricating membranes due to their inherent attributes, such as permanent porosity, high surface area, and good processability. Herein, a unique pore‐chemistry concept for the designed synthesis of microporous organic membranes, with an emphasis on the relationship between pore structures and membrane performances, is introduced. The latest advances in microporous organic materials for potential membrane application in gas separation of H₂, CO₂, O₂, and other industrially relevant gases are summarized. Representative examples of the recent progress in highly selective and permeable membranes are highlighted with some fundamental analyses from pore characteristics, followed by a brief perspective on future research directions.     

Electrochemically-aided membrane separation and catalytic processes

Electrochemically driven membrane separations and catalytic processes are interesting research areas which have, to date, received relatively little attention. Research into electrically-aided membrane separation and catalytic processes is currently being carried out in South Africa. The research objective is the development and characterisation of novel composite materials based on solid polyelectrolytes (SPE), containing nanoparticles of catalytically active metals, such as Pt, Ir or Pd, distributed within the polymeric matrices. An example of such an SPE matrix is a perfluorinated ion-exchange membrane. The novel composite materials (in fact, membranes) are both ionically and electronically conductive due to the presence of the metal nanoparticles. The application of potentials to conducting membranes results in the enhancement of catalytic activity as well as the selectivity of separations. The membranes based on SPE can be used for the catalytic processing of petrochemical mixtures, water treatment (disinfection, nitrate removal, etc.), oxygen, hydrogen and ozone generation, and electrically enhanced gas and vapour separations. In this paper Dr Dmitri Bessarabov briefly outlines the current status of the research, available technology and discusses challenges and possible applications.     

光催化TiO_2复合分离膜的制备技术

纳米TiO2复合分离膜集过滤、光催化降解、自清洁、防垢、提高膜通量等功能于一体,能有效解决传统膜分离系统存在的膜污染和膜通量衰减的问题,对发展新型的膜分离技术、开发纳米光催化新材料,以及在节水减排和中水回用等方面具有重要的应用价值。TiO2复合分离膜涉及TiO2纳米晶的合成和固载化工艺、多孔陶瓷载体的选择、烧结工艺及复合膜的稳定性。本文对目前国内外使用的TiO2复合分离膜的各种制备工艺和方法进行了归纳、总结和分析,并对其今后的发展和应用提供了建议。     

Clean production with membrane technology

 Membrane processes, because of unique, specialized, and cost-effective applications, have the potential of playing a significant role in preventing pollution from occurring in manufacturing plants. Opportunities are seen in resource recovery, species purification, and energy savings. Emerging technologies of membrane reactors that combine separation with reaction in one physical unit are important developments also. In this paper we discuss the scope of membrane technologies in industrial applications as well as hurdles that must be overcome. Received: 15 April 1998 / Accepted: 9 August 1998     

二维MXene膜的构筑及在水处理应用中的研究进展

MXene是一类新型的过渡金属碳/氮化物二维层状材料,通过选择性刻蚀掉MAX相中的A原子层制备而成。由于MXene具有特殊的微观结构和优异的理化性质,可用于高性能膜分离材料的构筑,并逐渐在废水处理和海水淡化等领域展现出良好的应用前景。但MXene在实际水处理过程中还存在不小的局限性。如MXene膜在水溶液中易发生溶胀现象,膜抗污染能力弱,分离机制也尚未有统一定论。本文归纳了近年来MXene纳米片、二维MXene膜材料的制备方法,例举了MXene膜的改性路径及其在水处理应用领域的最新报道,提出了MXene膜对水中污染物的分离机制,并对其当前急需解决的问题和未来的发展方向做出展望,为基于MXene高性能膜材料的设计和应用提供参考。      

Polycrystalline Advanced Microporous Framework Membranes for Efficient Separation of Small Molecules and Ions

Advanced porous framework membranes with excellent selectivity and high permeability of small molecules and ions are highly desirable for many important industrial separation applications. There has been significant progress in the fabrication of polycrystalline microporous framework membranes (PMFMs) in recent years, such as metal–organic framework and covalent organic framework membranes. These membranes possess small pore sizes, which are comparable to the kinetic diameter of small molecules and ions on the angstrom scale, very low thickness, down to tens to hundreds of nanometers, highly oriented crystalline structures, hybrid membrane structures, and specific functional groups for enhancing membrane selectivity and permeability. Recent advances in the fabrication methods of advanced PMFMs are summarized. Following this, four emerging separation applications of these advanced microporous framework membranes, including gas separation, water desalination, ion separation, and chiral separation, are highlighted and discussed in detail. Finally, a summary and some perspectives of future developments and challenges in this exciting research field are presented.     

A Self-Cleaning Heterostructured Membrane for Efficient Oil-in-Water Emulsion Separation with Stable Flux

Lack of clean water is a major global challenge. Membrane separation technology is an ideal choice for the treatment of industrial, domestic sewage owing to its low energy consumption and cost. However, membranes are highly susceptible to contamination, particularly during wastewater treatment, which has limited their practical applications in this field. Similarly, the flux of the membrane decreases with prolonged use due to its reduced interlayer spacing. Preparation of membranes with anticontamination properties and stable flux is the key to addressing this problem. In this study, a 2D heterostructure membrane with visible-light-driven self-cleaning performance is prepared via a self-assembly process. Notably, the addition of palygorskite increases the interlayer spacing of the graphene and heterojunction structures, which increases the flux of the membrane and avoids a decrease of the interlayer spacing of the membrane under pressure. The presence of a heterojunction with visible light catalytic properties effectively avoids membrane fouling and avoids a sharp decrease of the permeation flux. Importantly, the prepared 2D membrane has excellent separation performance for oil–water emulsions with both high flux and efficiency. These features suggest great potential for the prepared 2D membrane in wastewater treatment applications.     

Pervaporation developments

Membrane pervaporation offers economy, efficacy and simplicity and has gained widespread acceptance as a tool for the separation and recovery of liquid mixtures. It is currently identified mostly with dehydration of liquid hydrocarbons to yield high purity organics — most notably ethanol, iso-propanol and ethylene glycol — but new membranes are enabling the removal of solvents from water and potentially extending the process to any separation problem. The pervaporation technique is characterised by the imposition of a barrier layer (membrane) between a liquid and a gaseous phase, with mass transfer ocurring selectively across the barrier to the gas side (Figure 1). As different species permeate through the membrane at different rates, a substance at low concentration in the feed stream can be highly enriched in the permeate. Thus, separation occurs, with the efficacy of the separation effect being determined by the physico-chemical structure of the membrane.     

聚氧化乙烯气体分离膜的发展

聚氧化乙烯[poly(ethylene oxide),PEO]类膜材料含有大量与CO2有很强相互作用的醚氧基团,使得它具有很高的CO2/light gases(例如:H2、N2、CH4)溶解选择性,因此带来很高的CO2/light gases选择性.介绍了具有高溶解选择性CO2气体分离膜材料的筛选,重点叙述了PEO类膜材料的发展以及目前主要的PEO类膜材料的气体分离性能.当PEO含量达到足够高时,PEO类膜材料的CO2/light gases选择性大小基本相同,而它们的CO2透气性随着膜材料链段结构的不同而有较大不同.     

影响高分子合金膜内聚合物间相容性的主要因素

聚合物材料合金化是改善膜性能，拓宽膜材料使用范围的一种有效手段。聚合物间的相容性是影响合金分离膜结构与性能的重要因素。文中以二元合金体系为例，探讨了影响聚合物合金膜中聚合物间相容性的各种因素。     

Graphynes for Water Desalination and Gas Separation

Selective transport of mass through membranes, so‐called separation, is fundamental to many industrial applications, e.g., water desalination and gas separation. Graphynes, graphene analogs yet containing intrinsic uniformly distributed pores, are excellent candidates for highly permeable and selective membranes owing to their extreme thinness and high porosity. Graphynes exhibit computationally determined separation performance far beyond experimentally measured values of commercial state‐of‐the‐art polyamide membranes; they also offer advantages over other atomically thin membranes like porous graphene in terms of controllability in pore geometry. Here, recent progress in proof‐of‐concept computational research into various graphynes for water desalination and gas separation is discussed, and their theoretically predicted outstanding permeability and selectivity are highlighted. Challenges associated with the future development of graphyne‐based membranes are further analyzed, concentrating on controlled synthesis of graphyne, maintenance of high structural stability to withstand loading pressures, as well asthe demand for accurate computational characterization of separation performance. Finally, possible directions are discussed to align future efforts in order to push graphynes and other 2D material membranes toward practical separation applications.     

无机膜反应器的研究进展

膜反应器是具有反应与分离双重功能的单元集成设备,通过分离与反应的协同作用强化化学反应过程,已在加氢、脱氢、分解和氧化等苛刻反应中显示出优势。膜材料是决定膜反应器性能与应用的关键因素。重点从膜材料角度出发,介绍了致密无机膜反应器与多孔无机膜反应器的特点及发展。指出应开发高分离性能、抗污染、易于封装的膜材料,加强膜反应器质量和热量传递基本理论研究,以促进无机膜反应器早日大规模工业化应用。     

Conformation‐Controlled Molecular Sieving Effects for Membrane‐Based Propylene/Propane Separation

Membrane‐based separation is poised to reduce the operation cost of propylene/propane separation; however, identifying a suitable molecular sieve for membrane development is still an ongoing challenge. Here, the successful identification and use of a metal–organic framework (MOF) material as fillers, namely, the Zr‐fum‐ fcu ‐MOF possessing an optimal contracted triangular pore‐aperture driving the efficient diffusive separation of propylene from propane in mixed‐matrix membranes are reported. It is demonstrated that the fabricated hybrid membranes display a high propylene/propane separation performance, far beyond the current trade‐off limit of polymer membranes with excellent properties under industrial conditions. Most importantly, the mechanism behind the exceptional high propylene/propane selectivity is delineated by exploring theoretically the efficiency of sieving of different conformers of propane through the hypothesized triangular rigid pore‐aperture of Zr‐fum‐ fcu ‐MOF.     

Energy‐Efficient Oil–Water Separation of Biomimetic Copper Membrane with Multiscale Hierarchical Dendritic Structures

Membrane‐based materials with special surface wettability have been applied widely for the treatment of increasing industrial oily waste water, as well as frequent oil spill accidents. However, traditional technologies are energy‐intensive and limited, either by fouling or by the inability of a single membrane to separate all types of oil–water mixtures. Herein, a biomimetic monolayer copper membrane (BMCM), composed of multiscale hierarchical dendritic structures, is cleverly designed and successfully fabricated on steel mesh substrate. It not only possesses the ability of energy‐efficient oil–water separation but also excellent self‐recovery anti‐oil‐fouling properties (<150 s). The BMCM even keeps high separation efficiency (>93%) after ten‐time cycling tests. More importantly, it retains efficient oil–water separation capacity for five different oils. In fact, these advanced features are benefited by the synergistic effect of chemical compositions and physical structures, which is inspired by the typical nonwetting strategy of butterfly wing scales. The findings in this work may inspire a facile but effective strategy for repeatable and antipollution oil–water separation, which is more suitable for various applications under practical conditions, such as wastewater treatment, fuel purification, separation of commercially relevant oily water, and so forth.     

MXene Materials for Designing Advanced Separation Membranes

MXenes are emerging rapidly as a new family of multifunctional nanomaterials with prospective applications rivaling that of graphenes. Herein, a timely account of the design and performance evaluation of MXene-based membranes is provided. First, the preparation and physicochemical characteristics of MXenes are outlined, with a focus on exfoliation, dispersion stability, and processability, which are crucial factors for membrane fabrication. Then, different formats of MXene-based membranes in the literature are introduced, comprising pristine or intercalated nanolaminates and polymer-based nanocomposites. Next, the major membrane processes so far pursued by MXenes are evaluated, covering gas separation, wastewater treatment, desalination, and organic solvent purification. The potential utility of MXenes in phase inversion and interfacial polymerization, as well as layer-by-layer assembly for the preparation of nanocomposite membranes, is also critically discussed. Looking forward, exploiting the high electrical conductivity and catalytic activity of certain MXenes is put into perspective for niche applications that are not easily achievable by other nanomaterials. Furthermore, the benefits of simulation/modeling approaches for designing MXene-based membranes are exemplified. Overall, critical insights are provided for materials science and membrane communities to navigate better while exploring the potential of MXenes for developing advanced separation membranes.     

AIChE2011年会“膜与膜过程”分会报告概述

AIChE2011年会于2011年10月15～21日在美国明尼苏达州明尼阿波利斯市成功举办.简要介绍了AIChE2011年会中与"膜与膜过程"相关的分会报告的论文发表状况,并侧重介绍了膜技术用于水处理、气体分离、膜反应器和生物分离等方面的最新研究进展.     

膜材料的合金化改性

聚合物材料合金化是改善膜性能,拓宽膜材料使用范围的一种简便而有效的手段.文中对聚合物材料合金化对膜的一些物理化学性质的影响进行了探讨;并讨论了聚合物材料合金化对膜结构及膜的渗透性和选择性的影响.     

Advanced Covalent Organic Framework-Based Membranes for Recovery of Ionic Resources

Membrane technology has shown a viable potential in conversion of liquid-waste or high-salt streams to fresh waters and resources. However, the non-adjustability pore size of traditional membranes limits the application of ion capture due to their low selectivity for target ions. Recently, covalent organic frameworks (COFs) have become a promising candidate for construction of advanced ion separation membranes for ion resource recovery due to their low density, large surface area, tunable channel structure, and tailored functionality. This tutorial review aims to analyze and summarize the progress in understanding ion capture mechanisms, preparation processes, and applications of COF-based membranes. First, the design principles for target ion selectivity are illustrated in terms of theoretical simulation of ions transport in COFs, and key properties for ion selectivity of COFs and COF-based membranes. Next, the fabrication methods of diverse COF-based membranes are classified into pure COF membranes, COF continuous membranes, and COF mixed matrix membranes. Finally, current applications of COF-based membranes are highlighted: desalination, extraction, removal of toxic metal ions, radionuclides and lithium, and acid recovery. This review presents promising approaches for design, preparation, and application of COF-based membranes in ion selectivity for recovery of ionic resources.     

Development of CO2 Selective Poly(Ethylene Oxide)-Based Membranes: From Laboratory to Pilot Plant Scale

Membrane gas separation is one of the most promising technologies for the separation of carbon dioxide (CO₂) from various gas streams. One application of this technology is the treatment of flue gases from combustion processes for the purpose of carbon capture and storage. For this application, poly(ethylene oxide)-containing block copolymers such as Pebax^® or PolyActive™ polymer are well suited. The thin-film composite membrane that is considered in this overview employs PolyActive™ polymer as a selective layer material. The membrane shows excellent CO₂ permeances of up to 4 m³(STP)·(m²·h·bar)⁻¹ (1 bar = 10⁵ Pa) at a carbon dioxide/nitrogen (CO₂/N₂) selectivity exceeding 55 at ambient temperature. The membrane can be manufactured reproducibly on a pilot scale and mounted into flat-sheet membrane modules of different designs. The operating performance of these modules can be accurately predicted by specifically developed simulation tools, which employ single-gas permeation data as the only experimental input. The performance of membranes and modules was investigated in different pilot plant studies, in which flue gas and biogas were used as the feed gas streams. The investigated processes showed a stable separation performance, indicating the applicability of PolyActive™ polymer as a membrane material for industrial-scale gas processing.