Search

The selective separation of small molecules at the sub-nanometer scale has broad application prospects in the field, such as energy, catalysis, and separation. Conventional polymeric membrane materials (e.g., nanofiltration membranes) for sub-nanometer scale separations face challenges, such as inhomogeneous channel sizes and unstable pore structures. Combining polymers with metal–organic frameworks (MOFs), which possess uniform and intrinsic pore structures, may overcome this limitation. This combination has resulted in three distinct types of membranes: MOF polycrystalline membranes, mixed-matrix membranes (MMMs), and thin-film nanocomposite (TFN) membranes. However, their effectiveness is hindered by the limited regulation of the surface properties and growth of MOFs and their poor interfacial compatibility. The main issues in preparing MOF polycrystalline membranes are the uncontrollable growth of MOFs and the poor adhesion between MOFs and the substrate. Here, polymers could serve as a simple and precise tool for regulating the growth and surface functionalities of MOFs while enhancing their adhesion to the substrate. For MOF mixed-matrix membranes, the primary challenge is the poor interfacial compatibility between polymers and MOFs. Strategies for the mutual modification of MOFs and polymers to enhance their interfacial compatibility are introduced. For TFN membranes, the challenges include the difficulty in controlling the growth of the polymer selective layer and the performance limitations caused by the "trade-off" effect. MOFs can modulate the formation process of the polymer selective layer and establish transport channels within the polymer matrix to overcome the "trade-off" effect limitations. This review focuses on the mechanisms of synergistic construction of polymer–MOF membranes and their structure–nanofiltration performance relationships, which have not been sufficiently addressed in the past. [ABSTRACT FROM AUTHOR]

Composite membranes with asymmetric traits have gained attention in liquid separation, featuring gradient chemical and physical attributes that align or oppose mass transfer direction. Chemically asymmetric configurations harness internal driving forces to heighten separation efficiency, rendering them an appealing option for heightened separation efficiency and fouling prevention. Concurrently, the internal hierarchical structure differences within composite membranes—such as fiber‐based structural adjustments and the gradient density of functional layers—yield the dual benefits of effective liquid repelling and heightened transport efficiency. Unlike conventional phase‐change methods, electrospinning technology possesses advantages in constructing and governing composite fibrous membrane materials with asymmetric chemistry and hierarchical structures, driven by its adaptable stacking methodologies. Notably, the inherent pore structure of electrospun nanofibrous membranes emerges as a proven solution for minimizing transport resistance. In recent times, interest has surged in electrospun nanofibrous membranes endowed with internal asymmetric properties. However, the spotlight has predominantly graced Janus membranes, spotlighting opposite wettability on different sides, leaving other facets of asymmetric membrane enhancement somewhat underexplored. This comprehensive work unveils recent strides in design, fabrication, facilitated transport mechanisms, and real‐world liquid separation applications, all under the aegis of electrospun nanofiber membranes, each endowed with distinct asymmetric properties. [ABSTRACT FROM AUTHOR]

Isomer conversion and separation are crucial in the chemical, pharmaceutical, and food industries. However, this process remains challenging due to the reversed conversion of isomers and their similar boiling points and kinetic diameters. Herein, an enzyme‐catalyzed nanochannel membrane with a hierarchical structure is fabricated for selective and directional isomeric conversion and separation. The enzyme is embedded in a confined polymer network that shows high stability and retains 95% of its initial activity after 50 reaction cycles. Studying the transport mechanisms between isomers through the membrane shows that the hierarchical layers of the membrane impose unequal energy barriers to the forward and backward transport of products, promoting unidirectional diffusion of products and interrupting the equilibrium of the isomerization reaction to increase the conversion rate by 43%. Overall, this work opens a new avenue for the design of biocatalytic nanochannel membranes and the directional production and in situ separation of isomers. [ABSTRACT FROM AUTHOR]

BACKGROUND: Desalination membranes have been widely in demand for separating dyes and salts. However, dyes are easily adsorbed on/in the membranes, deteriorating membrane performance. Coupling photocatalysis and membrane separation might be a promising strategy for efficiently alleviating membrane fouling. Aiming at the defects of single catalyst, this work constructed photocatalytic membranes based on heterostructured Fe3O4‐doped g‐C3N4 (FeCN)/TiO2 catalysts by successively filtrating FeCN aqueous dispersion and titanium butoxide/isopropanol solution on the hydrolyzed polyacrylonitrile substrate. RESULTS: The H‐bonds among TiO2, FeCN and substrate lead to excellent load stability of the photoresponsive layer. Due to the low‐potential conduction band and high conductivity of Fe3O4 and the low‐potential conduction band of TiO2 in FeCN/TiO2, electrons can be more easily transferred to guarantee the high separation efficiency of electrons and holes. Hence the membrane not only exhibited superior permeability (104 L m−2 h−1 bar−1) and dye desalting performance (Coomassie brilliant blue R250 rejection of >98.80% and NaCl rejection of <7.00%), but also displayed highly efficient and stable dye photodegradation. CONCLUSION: The degradation efficiency surpassed that reported in the literature in comparable circumstances, and the recovery rate of permeability still reached 89.12% after three fouling–photodegradation cycles. This work provides new opportunities for designing high‐efficiency self‐cleaning membranes to solve membrane fouling based on low‐cost materials. © 2023 Society of Chemical Industry (SCI). [ABSTRACT FROM AUTHOR]

Molecular desalination is broadly used in chemical, food, and textile industries, which needs efficient and anti‐fouling separation technologies to reach this goal. Interfacial polymerization is one of the most promising routes to construct ultrahigh selective nanofiltration membranes. However, the irreversible hydrolysis of residual acyl chlorides makes Donnan charges of nascent films distribute unevenly which hinders fine molecular desalination and anti‐fouling. Here, we propose a pioneering solvation‐amination‐synergy strategy to synchronously inhibit the hydrolysis of residual acyl chlorides and promote their amination. The electroneutral nanofiltration membrane with high water permeance (13.2 L m−2 h−1 bar−1) is quantitatively fabricated that has superb anti‐fouling abilities and minimizes Donnan impacts on competitive ion penetrations, so it transmits Na2SO4 and NaCl while fully obstructs cationic or anionic dyes (< 500 Da). The ultrahigh molecule to ion selectivities outperform state‐of‐art nanofiltration membranes, which may provide a paradigm shift for scalable membrane fabrication for various industrial product desalination. [ABSTRACT FROM AUTHOR]

Nonpolar solvent separation is widely used in petroleum, chemical, food industries, but traditional separation methods consume intensive energy. State‐of‐art organic solvent nanofiltration membranes require complex modifications for nonpolar solvent transport. For the first time, we propose the concurrent modification of the surface, interface and support layer of dual‐layer membranes with three additives (perfluorodecylamine, fluoro substituted aromatic amine, silica nanoparticles) in a one‐step cocasting process. A delamination‐free dual‐layer membrane was obtained with a hierarchical hydrophobicity and transport channels. The novel designed structure elevated the pure n‐hexane permeance (28.75 L m−2 hr−1 bar−1) by 3 orders of magnitude with a high lecithin rejection (98.7%). This method of synergistically controlling the hierarchical structures and properties of dual‐layer membranes can significantly shorten the preparation process of high‐performance nonpolar solvent nanofiltration membranes. [ABSTRACT FROM AUTHOR]