A Two‐Dimensional Lamellar Membrane: MXene Nanosheet Stacks

Two‐dimensional (2D) materials are promising candidates for advanced water purification membranes. A new kind of lamellar membrane is based on a stack of 2D MXene nanosheets. Starting from compact Ti₃AlC₂, delaminated nanosheets of the composition Ti₃C₂T_x with the functional groups T (O, OH, and/or F) can be produced by etching and ultrasonication and stapled on a porous support by vacuum filtration. The MXene membrane supported on anodic aluminum oxide (AAO) substrate shows excellent water permeance (more than 1000 L m⁻² h⁻¹ bar⁻¹) and favorable rejection rate (over 90 %) for molecules with sizes larger than 2.5 nm. The water permeance through the MXene membrane is much higher than that of the most membranes with similar rejections. Long‐time operation also reveals the outstanding stability of the MXene membrane for water purification.     

Sub‐10 nm Polyamide Nanofiltration Membrane for Molecular Separation

To separate small molecules from the solvent with high permeability and selectivity, the membrane process is thought to be highly effective with much lower energy consumption when compared to the traditional thermal‐based separation process. To achieve high solvent permeance, a sub‐10 nm thick polyamide nanofiltration membrane was synthesized through interfacial polymerization of ethidium bromide (EtBr) and trimesoyl chloride (TMC). Thanks to the extremely low solubility of the EtBr monomer in the organic phase, the polymerization process was strictly limited at the interface of the water and hexane, leading to an ultrathin polyamide membrane with a thickness down to sub‐10 nm. When used in nanofiltration, these ultrathin membranes display ultrafast water permeation of 40 liter per square meter per hour per bar (L m^?2 h^?1 bar^?1), and a high Congo red rejection rate of 93 %. This work demonstrates a new route to synthesize ultrathin polyamide membranes by the traditional interfacial polymerization.     

Endowing g‐C3N4 Membranes with Superior Permeability and Stability by Using Acid Spacers

g‐C₃N₄ membranes were modulated by intercalating molecules with SO₃H and benzene moieties between layers. The intercalation molecules break up the tightly stacking structure of g‐C₃N₄ laminates successfully and accordingly the modified g‐C₃N₄ membranes give rise to two orders magnitude higher water permeances without sacrificing the separation efficiency. The sulfonated poly(2,6‐dimethyl‐1,4‐phenylene oxide) (SPPO)/g‐C₃N₄ with a thickness of 350 nm presents an exceptionally high water permeance of 8867 L h^?1 m^?2 bar^?1 and 100 % rejection towards methyl blue, while the original g‐C₃N₄ membrane with a thickness of 226 nm only exhibits a permeance of 60 L h^?1 m^?2 bar^?1. Simultaneously, SO₃H sites firmly anchor nitrogen with base functionality distributing onto g‐C₃N₄ through acid–base interactions. This enables the nanochannels of g‐C₃N₄ based membranes to be stabilized in acid, basic, and also high‐pressure environments for long periods.     

Ultrathin pH‐Sensitive Nanoporous Membranes for Superfast Size‐Selective Separation

Stimuli‐responsive nanoporous membranes have attracted increasing interest in various fields due to their abrupt changes of permeation/separation in response to the external environment. Here we report ultrathin pH‐sensitive nanoporous membranes that are easily fabricated by the self‐assembly of poly(acrylic acid) (PAA) in a metal hydroxide nanostrand solution. PAA‐adsorbed nanostrands (2.5–5.0 nm) and PAA‐Cu^II nanogels (2.0–2.5 nm) grow competitively during self‐assembly. The PAA‐adsorbed nanostrands are deposited on a porous support to fabricate ultrathin PAA membranes. The membranes display ultrafast water permeation and good rejection as well as significant pH‐sensitivity. The 28 nm‐thick membrane has a water flux decrease from 3740 to 1350 L m^?1 h^?1 bar^?1 (pH 2.0 to 7.0) with a sharp decrease at pH 5.0. This newly developed pH‐sensitive nanoporous membranes may find a wide range of applications such as controlled release and size‐ and charge‐selective separation.     

Self‐Assembled Asymmetric Block Copolymer Membranes: Bridging the Gap from Ultra‐ to Nanofiltration

The self‐assembly of block copolymers is an emerging strategy to produce isoporous ultrafiltration membranes. However, thus far, it has not been possible to bridge the gap from ultra‐ to nanofiltration and decrease the pore size of self‐assembled block copolymer membranes to below 5 nm without post‐treatment. It is now reported that the self‐assembly of blends of two chemically interacting copolymers can lead to highly porous membranes with pore diameters as small as 1.5 nm. The membrane containing an ultraporous, 60 nm thin separation layer can fully reject solutes with molecular weights of 600 g mol^?1 in aqueous solutions with a water flux that is more than one order of magnitude higher than the permeance of commercial nanofiltration membranes. Simulations of the membrane formation process by dissipative particle dynamics (DPD) were used to explain the dramatic observed pore size reduction combined with an increase in water flux.     

Ultrastable Imine‐Based Covalent Organic Frameworks for Sulfuric Acid Recovery: An Effect of Interlayer Hydrogen Bonding

A rapid and scalable synthesis of six new imine‐linked highly porous and crystalline COFs is presented that feature exceptionally high chemical stability in harsh environments including conc. H₂SO₄ (18 m ), conc. HCl (12 m ), and NaOH (9 m ). This is because of the presence of strong interlayer C?H???N hydrogen bonding among the individual layers, which provides significant steric hindrance and a hydrophobic environment around the imine (?C=N?) bonds, thus preventing their hydrolysis in such an abrasive environment. These COFs were further converted into porous, crystalline, self‐standing, and crack‐free COF membranes (COFMs) with extremely high chemical stability for their potential applications for sulfuric acid recovery. The as‐synthesized COFMs exhibit unprecedented permeance for acetonitrile (280 Lm^?2 h^?1 bar^?1) and acetone (260 Lm^?2 h^?1 bar^?1).     

Growth of High‐Quality,Thickness‐Reduced Zeolite Membranes towards N2/CH4 Separation Using High‐Aspect‐Ratio Seeds

Greatly improved zeolite membranes were prepared by using high‐aspect‐ratio zeolite seeds. Slice‐shaped seeds with a high aspect ratio (AR) facilitated growth of thinner continuous SAPO‐34 membranes of much higher quality. These membranes showed N₂ permeances as high as (2.87±0.15)×10^?7 mol m^?2 s^?1 Pa^?1 at 22 °C while maintaining a decent N₂/CH₄ selectivity (9–11.2 for equimolar mixture). On the basis of these thinner high‐quality SAPO‐34 membranes, fine‐tuning the local crystal structure by incorporating more silicon further increased the N₂ permeance by 1.4 times without sacrificing the N₂/CH₄ selectivity. We expect that application of large AR zeolite seeds might be a viable strategy to grow thin high‐quality zeolite membranes. In addition, fine‐tuning of the crystal structure by changing the crystal composition might be a feasible way for further improving the separating performance of high‐quality zeolite membranes.     

A Hollow Microtubular Triazine‐ and Benzobisoxazole‐Based Covalent Organic Framework Presenting Sponge‐Like Shells That Functions as a High‐Performance Supercapacitor

In this paper we report the construction of a hollow microtubular triazine‐ and benzobisoxazole‐based covalent organic framework (COF) presenting a sponge‐like shell through a template‐free [3+2] condensation of the planar molecules 2,4,6‐tris(4‐formylphenyl)triazine (TPT‐3CHO) and 2,5‐diaminohydroquinone dihydrochloride (DAHQ‐2HCl). The synthesized COF exhibited extremely high crystallinity, a high surface area (ca. 1855 m² g^?1), and ultrahigh thermal stability. Interestingly, a time‐dependent study of the formation of the hollow microtubular COF having a sponge‐like shell revealed a transformation from initial ribbon‐like crystallites into a hollow tubular structure, and confirmed that the hollow nature of the synthesized COF was controlled by inside‐out Ostwald ripening, while the non‐interaction of the crystallites on the outer surface was responsible for the sponge‐like surface of the tubules. This COF exhibited significant supercapacitor performance: a high specific capacitance of 256 F g^?1 at a current density of 0.5 A g^?1, excellent cycling stability (98.8 % capacitance retention over 1850 cycles), and a high energy density of 43 Wh kg^?1. Such hollow structural COFs with sponge‐like shells appear to have great potential for use as high‐performance supercapacitors in energy storage applications.     

Water Transport with Ultralow Friction through Partially Exfoliated g‐C3N4 Nanosheet Membranes with Self‐Supporting Spacers

Two‐dimensional (2D) graphitic carbon nitride (g‐C₃N₄) nanosheets show brilliant application potential in numerous fields. Herein, a membrane with artificial nanopores and self‐supporting spacers was fabricated by assembly of 2D g‐C₃N₄ nanosheets in a stack with elaborate structures. In water purification the g‐C₃N₄ membrane shows a better separation performance than commercial membranes. The g‐C₃N₄ membrane has a water permeance of 29 L m⁻² h⁻¹ bar⁻¹ and a rejection rate of 87 % for 3 nm molecules with a membrane thickness of 160 nm. The artificial nanopores in the g‐C₃N₄ nanosheets and the spacers between the partially exfoliated g‐C₃N₄ nanosheets provide nanochannels for water transport while bigger molecules are retained. The self‐supported nanochannels in the g‐C₃N₄ membrane are very stable and rigid enough to resist environmental challenges, such as changes to pH and pressure conditions. Permeation experiments and molecular dynamics simulations indicate that a novel nanofluidics phenomenon takes place, whereby water transport through the g‐C₃N₄ nanosheet membrane occurs with ultralow friction. The findings provide new understanding of fluidics in nanochannels and illuminate a fabrication method by which rigid nanochannels may be obtained for applications in complex or harsh environments.     

A Regularly Channeled Lamellar Membrane for Unparalleled Water and Organics Permeation

Lamellar membranes show exceptional molecular permeation properties of key importance for many applications. However, their design and development need the construction of regular and straight interlayer channels and the establishment of corresponding transport rate equation. The fabrication of a uniformly lamellar membrane is reported using double‐layered Ti₃C₂T_x MXenes as rigid building blocks. This membrane possesses ordered and straight 2 nm channels formed via a direct self‐stacking, in contrast to the conventional irregular ones from flexible sheets. Such channels permit precise molecular rejection and unparalleled molecular permeation. The permeance of water and organics by this membrane reached 2300 and 5000 L m^?2 h^?1 bar^?1, respectively. The molecular transfer mechanism in confined nanochannels, and the corresponding model equation are established, paving a way to nanoscale design of highly efficient channeled membranes for transport and separation applications.     

Synthesis and Characterization of Phenolphthalein‐Containing Sulfonated Poly(arylene ether phosphine oxide)s by Direct Polycondensation for Proton Exchange Membrane

A series of novel phenolphthalein‐containing sulfonated poly(arylene ether phosphine oxide)s (sPAEPP) with various sulfonation degrees were synthesized by direct polycondensation. The structure of sPAEPP was confirmed by ¹H‐NMR, ¹³C‐NMR, and IR spectroscopy. The high‐molecular weight of these polymers was determined by gel permeation chromatography (GPC). The transparent, tough, and flexible membranes could be achieved by solution casting. The macroscopic properties and microstructure of the obtained membranes were investigated in detail. The results showed that these sPAEPP membranes displayed excellent properties in terms of swelling, proton conductivity, and methanol permeability. For example, sPAEPP‐100 membrane exhibited an appropriate water uptake of 33.1%, a swelling ratio of only 11.7% (lower than 20.1% of Nafion 117), a proton conductivity of 0.11 S cm^?1 (similar to that of Nafion 117) at 80 °C, and a methanol permeability of 4.82 × 10^?7 cm² s^?1. Meanwhile, it also presented outstanding oxidative stability. Atomic force microscope (AFM) micrographs showed that the hydrophilic domains of the sPAEPP‐100 membrane formed connected and narrow ionic channels, which contributed to its high proton conductivity and good dimensional stability. As a result, sPAEPP‐100 membrane displays excellent application prospect for fuel cells. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 1097–1104     

A Uniformly Oriented MFI Membrane for Improved CO2 Separation

Membrane separation of CO₂ from natural gas, biogas, synthesis gas, and flu gas is a simple and energy‐efficient alternative to other separation techniques. But results for CO₂‐selective permeance have always been achieved by randomly oriented and thick zeolite membranes. Thin, oriented membranes have great potential to realize high‐flux and high‐selectivity separation of mixtures at low energy cost. We now report a facile method for preparing silica MFI membranes in fluoride media on a graded alumina support. In the resulting membrane straight channels are uniformly vertically aligned and the membrane has a thickness of 0.5 μm. The membrane showed a separation selectivity of 109 for CO₂/H₂ mixtures and a CO₂ permeance of 51×10^?7 mol m^?2 s^?1 Pa^?1 at ?35 °C, making it promising for practical CO₂ separation from mixtures.     

Nanoparticle Size‐Fractionation through Self‐Standing Porous Covalent Organic Framework Films

Covalent organic frameworks (COFs) have attracted attention due to their ordered pores leading to important industrial applications like storage and separation. Combined with their modular synthesis and pore engineering, COFs could become ideal candidates for nanoseparations. However, the fabrication of these microcrystalline powders as continuous, crack‐free, robust films remains a challenge. Herein, we report a simple, slow annealing strategy to construct centimeter‐scale COF films ( Tp‐Azo and Tp‐TTA ) with micrometer thickness. The as‐synthesized films are porous (SA_BET=2033 m² g^?1 for Tp‐Azo ) and chemically stable. These COFs have distinct size cut‐offs (ca. 2.7 and ca. 1.6 nm for Tp‐Azo and Tp‐TTA , respectively), which allow the size‐selective separation of gold nanoparticles. Unlike, other conventional membranes, the durable structure of the COF films allow for excellent recyclability (up to 4 consecutive cycles) and easy recovery of the gold nanoparticles from the solution.     

Xenon Recovery by DD3R Zeolite Membranes: Application in Anaesthetics

Xe is only produced by cryogenic distillation of air, and its availability is limited by the extremely low abundance. Therefore, Xe recovery after usage is the only way to guarantee sufficient supply and broad application. Herein we demonstrate DD3R zeolite as a benchmark membrane material for CO₂/Xe separation. The CO₂ permeance after an optimized membrane synthesis is one order magnitude higher than for conventional membranes and is less susceptible to water vapour. The overall membrane performance is dominated by diffusivity selectivity of CO₂ over Xe in DD3R zeolite membranes, whereby rigidity of the zeolite structure plays a key role. For relevant anaesthetic composition (<5 % CO₂) and condition (humid), CO₂ permeance and CO₂/Xe selectivity stabilized at 2.0×10^?8 mol m^?2 s^?1 Pa^?1 and 67, respectively, during long‐term operation (>320 h). This endows DD3R zeolite membranes great potential for on‐stream CO₂ removal from the Xe‐based closed‐circuit anesthesia system. The large cost reduction of up to 4 orders of magnitude by membrane Xe‐recycling (>99+%) allows the use of the precious Xe as anaesthetics gas a viable general option in surgery.     

Mussel‐inspired method to decorate commercial nanofiltration membrane for heavy metal ions removal

Nanofiltration (NF) membranes have been widely used for the treatment of electroplating, aerospace, textile, pharmaceutical, and other chemical industries. In this work, halloysite nanotubes (HNTs) were directly anchored on the surface of commercial nanofiltration (NF) membrane by dopamine modification following advantageous bio‐inspired methods. SEM and AFM images were used to characterize the HNTs decorated membrane surface in terms of surface morphology and roughness. Water contact angle (WCA) was employed in evidencing the incorporation of HNTs and dopamine in terms of hydrophilicity or hydrophobicity. Augmentation of HNTs was found to obviously enhance the hydrophilicity and surface roughness resulting in improved water permeability of membrane. More importantly, the rejection ratios of membrane also increased during the removal of heavy metal ions from wastewater. The permeability and Cu²⁺ rejection ratio of modified NF membrane were as high as 13.9 L·m^?2·h^?1·bar^?1 and 74.3%, respectively. Incorporation of HNTs was also found to enhance the anti‐fouling property and stability of membrane as evident from long‐term performance tests. The relative concentration of HNTs and dopamine on membrane surface was optimized by investigating the trade‐off between water permeability and rejection ratio.     

An Azine‐Linked Covalent Organic Framework: Synthesis,Characterization and Efficient Gas Storage

A azine‐linked covalent organic framework, COF‐JLU2, was designed and synthesized by condensation of hydrazine hydrate and 1,3,5‐triformylphloroglucinol under solvothermal conditions for the first time. The new covalent organic framework material combines permanent micropores, high crystallinity, good thermal and chemical stability, and abundant heteroatom activated sites in the skeleton. COF‐JLU2 possesses a moderate BET surface area of over 410 m² g^?1 with a pore volume of 0.56 cm³ g^?1. Specifically, COF‐JLU2 displays remarkable carbon dioxide uptake (up to 217 mg g^?1) and methane uptake (38 mg g^?1) at 273 K and 1 bar, as well as high CO₂/N₂ (77) selectivity. Furthermore, we further highlight that it exhibits a higher hydrogen storage capacity (16 mg g^?1) than those of reported COFs at 77 K and 1 bar.     

Non‐Interpenetrated Single‐Crystal Covalent Organic Frameworks

Growth of covalent organic frameworks (COFs) as single crystals is extremely challenging. Inaccessibility of open‐structured single‐crystal COFs prevents the exploration of structure‐oriented applications. Herein we report for the first time a non‐interpenetrated single‐crystal COF, LZU‐306, which possesses the open structure constructed exclusively via covalent assembly. With a high void volume of 80 %, LZU‐306 was applied to investigate the intrinsic dynamics of reticulated tetraphenylethylene (TPE) as the individual aggregation‐induced‐emission moiety. Solid‐state ²H NMR investigation has determined that the rotation of benzene rings in TPE, being the freest among the reported cases, is as fast as 1.0×10⁴ Hz at 203 K to 1.5×10⁷ Hz at 293 K. This research not only explores a new paradigm for single‐crystal growth of open frameworks, but also provides a unique matrix‐isolation platform to reticulate functional moieties into a well‐defined and isolated state.     

Microporous 3D aluminum MOF doped into chitosan‐based mixed matrix membranes for ethanol/water separation

An aluminum metal–organic framework (Al‐MOF), [Al(OH)(BPDC)] (DUT‐5; BPDC = Biphenyl‐4,4′‐dicarboxylate), was synthesized using solvothermal reactions. The high surface area and micropores (approximately 1.2 nm) of DUT‐5 were characterized using N₂ gas sorption measurements. The thermal stability of DUT‐5 and its phase purity were also investigated. The different amounts of DUT‐5 (0.1, 0.15, and 0.2 wt%) were successfully incorporated into the chitosan (CS) polymer to prepare a mixed matrix membrane (MMM) for the pervaporation of water/ethanol at 25°C. In particular, when 0.15 wt% of DUT‐5 was loaded, the DUT‐5@CS MMMs displayed excellent permeability and selectivity in ethanol/water separation. The results indicated that compared with pristine chitosan membranes, the flux of DUT‐5@CS membranes with 0.15 wt% loading significantly increased from 315 to 378 (g/m² h^?1) and the separation factor from 347 to 3,429. These promising results of the microporous Al‐MOF doped into chitosan MMMs reveal its good application potential for the bio‐ethanol separation processes.     

Formation of Ultrathin,Continuous Metal–Organic Framework Membranes on Flexible Polymer Substrates

Metal–organic framework (MOF) materials have an enormous potential in separation applications, but to realize their potential as semipermeable membranes they need to be assembled into thin continuous macroscopic films for fabrication into devices. By using a facile immersion technique, we prepared ultrathin, continuous zeolitic imidazolate framework (ZIF‐8) membranes on titania‐functionalized porous polymeric supports. The coherent ZIF‐8 layer was surprisingly flexible and adhered well to the support, and the composite membrane could sustain bending and elongation. The membranes exhibited molecular sieving behavior, close to the theoretical permeability of ZIF‐8, with hydrogen permeance up to 201×10^?7 mol m^?2 s^?1 Pa^?1 and an ideal H₂/CO₂ selectivity of 7:1. This approach offers significant opportunities to exploit the unique properties of MOFs in the fabrication of separation and sensing devices.     

Electrochemical Interfacial Polymerization toward Ultrathin COF Membranes for Brine Desalination

Ultrathin covalent organic framework (COF) membranes are urgently demanded in molecular/ionic separations. Herein, we reported an electrochemical interfacial polymerization strategy to fabricate ultrathin COF membranes with thickness of 85 nm, by actively manipulate self-healing effect and self-inhibiting effect. The resulting COF membrane exhibited superior performance in brine desalination with the permeation flux of 92 kg m⁻² h⁻¹ and the rejection of 99.96 %. Our electrochemical interfacial polymerization strategy enriches the fabrication approach of COF membranes and facilitates the rational design of ultrathin membranes.