Covalent Organic Framework Membranes Research Articles

A 2D Soft Covalent Organic Framework Membrane Prepared viaaMolecular Bridge.

Highly flexible and robust self-standing covalent organic framework (COF) membranes with rapid preparation are important but technically challenging for achieving precise separation. Herein , a novel imine-based 2D soft covalent organic framework (SCOF) membrane with a large area of 226.9cm2 , via ingeniously selecting an aldehyde flexible linker and a trigonal building block, is reported. The soft 2D covalent organic framework membrane is rapidly formed (≈5min) based on the sodium dodecyl sulfate (SDS) molecular channel constructed at the water/dichloromethane (DCM) interface, which is the record-fast SCOF membrane formation and 72 times faster than that in the reported literature. MD simulation and DFT calculation elucidate that the dynamic, self-assembled SDS molecular channel facilitates faster and more homogeneous transfer of amine monomers in the bulk, thereby forming a soft 2D self-standing COF membrane with more uniform pores. The formed SCOF membrane exhibits superb sieving capability for small molecules, robustness in strong alkaline (5molL-1 NaOH), acid (0.1molL-1 HCl), and various organic solutions, and sufficient flexibility with a large curvature of 2000m-1 for membrane-based separation science and technology.

Pre-oxidation coupled with charged covalent organic framework membranes for highly efficient removal of organic chloramines precursors in algae-containing water treatment

Organic chloramines in water would pose both chemical and microbiological risks. It is essential to remove the precursors of organic chloramine (amino acids and decomposed peptides/proteins) to limit its formation in disinfection. In our work, nanofiltration was chosen to remove organic chloramines precursors. To solve the “trade-off” effect and low rejection of small molecules in algae organic matter, we synthesized a thin film composite (TFC) nanofiltration (NF) membrane with a crumpled polyamide (PA) layer via interfacial polymerization on polyacrylonitrile (PAN) composite support loaded with covalent organic framework (COF) nanoparticles (TpPa-SO3H). The obtained NF membrane (PA-TpPa-SO3H/PAN) increased the permeance from 10.2 to 28.2 L m−2 h−1 bar−1 and the amino acid rejection from 24% to 69% compared to the control NF membrane. The addition of TpPa-SO3H nanoparticles decreased the thickness of PA layers, increased the hydrophilicity of the membrane, and increased the transition energy barrier for amino acids transferring through the membrane, which was identified by scanning electron microscope, contact angle test, and density functional theory computations, respectively. Finally, pre-oxidation coupled with PA-TpPa-SO3H/PAN membrane nanofiltration on the limitation of organic chloramines formation was evaluated. We found that the combined application of KMnO4 pre-oxidation and PA-TpPa-SO3H/PAN membranes nanofiltration in algae-containing water treatment could minimize the formation of organic chloramines in subsequent chlorination and maintain a high flux during filtration. Our work provides an effective way for algae-containing water treatment and organic chloramines control.

Recovery of homogeneous photocatalysts by covalent organic framework membranes

Transition metal-based homogeneous photocatalysts offer a wealth of opportunities for organic synthesis. The most versatile ruthenium(II) and iridium(III) polypyridyl complexes, however, are among the rarest metal complexes. Moreover, immobilizing these precious catalysts for recycling is challenging as their opacity may obstruct light transmission. Recovery of homogeneous catalysts by conventional polymeric membranes is promising but limited, as the modulation of their pore structure and tolerance of polar organic solvents are challenging. Here, we report the effective recovery of homogeneous photocatalysts using covalent organic framework (COF) membranes. An array of COF membranes with tunable pore sizes and superior organic solvent resistance were prepared. Ruthenium and iridium photoredox catalysts were recycled for 10 cycles in various types of photochemical reactions, constantly achieving high catalytical performance, high recovery rates, and high permeance. We successfully recovered the photocatalysts at gram-scale. Furthermore, we demonstrated a cascade isolation of an iridium photocatalyst and purification of a small organic molecule product with COF membranes possessing different pore sizes. Our results indicate an intriguing potential to shift the paradigm of the pharmaceutical and fine chemical synthesis campaign.

Room-Temperature Synthesis of a COFs Membrane Via LBL Self-Assembly Strategy for Energy Harvesting.

The covalent organic frameworks (COFs) membrane with ordered and confined one-dimensional channel has been considered as a promising material to harvest the salinity gradient energy from the seawater and river water. However, the application of the COFs in the field of energy conversion still faces the challenges in membrane preparation. Herein, energy harvesting is achieved by taking advantage of a COFs membrane where TpDB-HPAN is synthesized via layer-by-layer self-assembly strategy at room temperature. The carboxy-rich TpDB COFs can be expediently assembled onto the substrate with an environmental-friendly method. The increased open-circuit voltage (Voc ) endows TpDB-HPAN membrane with a remarkable energy harvesting performance. More importantly, the application perspective is also illuminated by the cascade system. With the advantages of green synthesis, the TpDB-HPAN membrane can be considered as a low-cost and promising candidate for energy conversion.

Effects of linking group on the desalination performance of three-dimensional covalent organic framework membranes

The structure and the hydrophilicity can be tuned by adjusting the linking group of a covalent organic framework (COF), which significantly affects the desalination performance of COF membranes. In this work, molecular dynamics (MD) simulation was used to investigate the effect of different NPN-X membranes with the same elements and different linking groups on desalination performance. The results showed that NPN-1 and NPN-2 membrane had superior desalination performance. The water permeances of NPN-1 and NPN-2 membrane were two orders of magnitude higher than those of conventional reverse osmosis membranes with a salt rejection rate of 100 %. The water permeances of the membranes were in the following order: NPN-2 > NPN-1 > NPN-3, although the NPN-3 had the largest pore size. The nature of the linking group played a crucial role in water permeance, and the high hydrophilicity of the pore wall of the membranes improved the water permeance. Moreover, the interaction between NPN-1 and NPN-2 membranes and ions was stronger than that of the NPN-3 membrane. Additionally, the pore shapes of NPN-1 and NPN-2 were more slender than NPN-3. This study provides valuable information for the subsequent desalination membrane structure design.

Ingenious Artificial Leaf Based on Covalent Organic Framework Membranes for Boosting CO2 Photoreduction.

Covalent organic frameworks (COFs) hold the potential in converting CO2 with water into value-added fuels and O2 to save the deteriorating ecological environment. However, reaching high yield and selectivity is a grand challenge under metal-, photosensitizer-, or sacrificial reagent-free conditions. Here, inspired by microstructures of natural leaves, we designed triazine-based COF membranes with the integration of steady light-harvesting sites, efficient catalytic center, and fast charge/mass transfer configuration to fabricate a novel artificial leaf for the first time. Significantly, a record high CO yield of 1240 μmol g-1 in a 4 h reaction, approximately 100% selectivity, and a long lifespan (at least 16 cycles) were achieved under gas-solid conditions without using any metal, photosensitizer, or sacrificial reagent. Unlike the existing knowledge, the chemical structural unit of triazine-imide-triazine and the unique physical form of the COF membrane are predominant for such a remarkable photocatalysis. This work opens a new pathway to simulating photosynthesis in leaves and may motivate relevant research in the future.

Preliminary exploration of the dye/salt separation performance of 2D and 3D covalent organic frameworks/nylon 6 membranes prepared by layer-by-layer strategy

Covalent organic frameworks (COFs) had been widely studied as high-performance separation membrane materials because of their uniform pore size, high porosity and good stability. In this work, 1,3,5-tris(4-aminophenyl)benzene (TAPB) - 1,3,5-triformylphloroglucinol (Tp)-12/nylon 6 membrane, a two-dimensional (2D) COFs membrane, and 4-[tris(4-aminophenyl)methyl]aniline (TAPM) -Tp-6/nylon 6 membrane, a three-dimensional (3D) COFs membrane, were prepared by layer-by-layer strategy. During the experiment, these COFs/nylon 6 membranes showed good properties, such as high water permeance (4522 L m−2 h−1 bar−1 for TAPB-Tp-12/nylon 6 membrane, and 4840 L m−2 h−1 bar−1 for TAPM-Tp-6/nylon 6 membrane), excellent dye rejection (methylene blue rejection>99.7 %), low salt rejection (<3.4 %), good chemical stability and reusability, which might be due to the hydrophilicity and electrical properties of these materials and the support of nylon 6 skeleton. Moreover, the possible reasons for these differences between 2D COFs membrane and 3D COFs membrane were analysed by summarizing and comparing the data obtained from these COFs/nylon 6 membranes. Therefore, this work not only provided a universal method for the synthesis of different types of COFs membranes, but also supplied a reference for researchers in the same field to compare the performance of 2D and 3D COFs membranes.

Smart superwetting COF membrane for controllable oil/water separation

Covalent organic frameworks (COFs), as a kind of porous crystalline materials, have exhibited tremendous application prospects in various fields over the past few decades. Nonetheless, the employment of COFs in oil/water separation field was rarely reported. Herein, we prepared a robust and loose COF membrane using the collected COF nanofibers by filtration assembly method. The obtained COF membrane possesses unique superwettability, which endows the COF membrane with switchable transport performance. Consequently, the COF membrane can achieve the controllable and switchable separation of light oil/water mixture, heavy oil/water mixture, water-in-oil emulsion, and oil-in-water emulsion with high separation efficiency and permeance. This intelligent COF membrane with the underwater superoleophobic and underoil superhydrophobic properties will greatly enrich the application field of COFs and open up new prospects for designing more novel materials in controllable free oil/water mixture separation and on-demand oil/water emulsion separation.

Pore-in-Pore Engineering in a Covalent Organic Framework Membrane for Gas Separation

Covalent organic framework (COF) membranes have emerged as a promising candidate for energy-efficient separations, but the angstrom-precision control of the channel size in the subnanometer region remains a challenge that has so far restricted their potential for gas separation. Herein, we report an ultramicropore-in-nanopore concept of engineering matreshka-like pore-channels inside a COF membrane. In this concept, α-cyclodextrin (α-CD) is in situ encapsulated during the interfacial polymerization which presumably results in a linear assembly (LA) of α-CDs in the 1D nanochannels of COF. The LA-α-CD-in-TpPa-1 membrane shows a high H2 permeance (∼3000 GPU) together with an enhanced selectivity (>30) of H2 over CO2 and CH4 due to the formation of fast and selective H2-transport pathways. The overall performance for H2/CO2 and H2/CH4 separation transcends the Robeson upper bounds and ranks among the most powerful H2-selective membranes. The versatility of this strategy is demonstrated by synthesizing different types of LA-α-CD-in-COF membranes.

Donnan Effect-Engineered Covalent Organic Framework Membranes toward Size- and Charge-Based Precise Molecular Sieving.

Covalent organic frameworks (COFs), with ordered pores and well-defined topology, are ideal materials for nanofiltration (NF) membranes because of their capacity of transcending the permeance/selectivity trade-off predicament. However, most reported COF-based membranes are focused on separating molecules with different sizes, resulting in low selectivity to similar molecules with different charges. Here, the negatively charged COF layer was fabricated in situ on a microporous support for the separation of molecules with different sizes and charges. Ultrahigh water permeance (216.56 L m-2 h-1 bar-1) was obtained because of the ordered pores and excellent hydrophilicity, which exceeds that of most membranes with similar rejections. For the first time, we used multifarious dyes with different sizes and charges, for the investigation of the selectivity behavior caused by the Donnan effect and size exclusion. The obtained membranes represent superior rejections to negatively and neutrally charged dyes larger than 1.3 nm, while positively charged dyes with a size of 1.6 nm can pass through the membrane, resulting in the separation of negative/positive mixed dyes with similar molecular sizes. This strategy of combining the Donnan effect and size exclusion in nanoporous materials may evolve into a generic platform for sophisticated separation.

Solid-state synthesis of intrinsically proton-conducting covalent organic framework membrane

Ionic covalent organic framework (iCOF) membranes are attractive for a myriad of applications. Here, we report a mechanochemistry-based solid-state synthesis method to fabricate iCOF membranes with simplicity, scalability and sustainability. Time-dependent experiments reveal that the iCOF membrane formation process includes the structural transformation from disorder to order and the morphology change to the dense structure formed by the restacking of nanosheet-like crystallites simultaneously. The obtained free-standing iCOF membranes demonstrate high ion exchange capacity, mechanical strength and comparable proton conductivity, benefiting from the rigid crystalline framework structure and one-dimensional channels decorated with abundant proton-conducting sites. This work paves the way for synthesis of iCOF membranes in a simple and environmentally friendly manner.

Smart Solvent-Responsive Covalent Organic Framework Membranes with Self-regulating Pore Size

Covalent organic frameworks (COFs) are an emerging class of crystalline porous materials with low density, high porosity, and excellent physicochemical stability. Recently, smart COFs with special structures and functional groups have been widely concerned, which can respond to external stimuli such as pH, temperature, light, gas, etc. However, it is still a paramount challenge to fabricate smart COF membranes possessing the advantage of alternating the pore size to realize gradient separation of multiple organic pollutants. In this work, we fabricated a 2D amine-linked smart COF membrane coupling hardness with softness via interfacial polymerization under room temperature and atmospheric pressure conditions. Interestingly, these COF membranes show higher dye rejection performance in water than in acetone, and the pore size of smart COF membranes can be dynamically regulated by different solvents. Powder X-ray diffraction (PXRD) was used to identify the intrinsic structure change of this intriguing phenomenon. By serendipity, we found that the PXRD peak shifted to a lower degree when the membranes were immersed in acetone than in water, suggesting enlargement of the pore size due to flexible frameworks. The dynamic enlargement of the pore size was reversible when the solvent was switched from acetone to water. This study not only provides a frontier of smart COF membranes but also broadens potential applications of COF materials.

Ultrathin ionic COF Membrane via Polyelectrolyte‐Mediated Assembly for Efficient CO2 Separation

AbstractFabricating ultrathin covalent organic framework (COF) membranes toward high‐permeance molecular separations is highly desired yet challenging. Herein, a polyelectrolyte‐mediated assembly (PMA) strategy is developed to fabricate ultrathin ionic COF membrane for efficient CO2 separation. The PMA strategy allows a facile control over the assembly mode between polyethyleneimine (PEI) and TpPa‐SO3H, yielding PEI‐bridged ultra‐large COF nanosheets which are readily processed into COF membranes with thickness down to 8 nm. The resulting COF membranes exhibit a high CO2 permeances of 1371 GPU and CO2/N2 selectivity of 33 for stimulated flue gas. This PMA strategy may open up a new avenue to fabricating ultrathin 2D material membranes for diverse applications.

Covalent organic framework membrane with sub-nano pores for efficient desalination

Covalent organic frameworks (COFs) have emerged as advanced membrane materials for molecular separations, but it is rarely used for desalination due to the mismatch between the COFs pore size (1−5 nm) and the diameter of hydrated ions (<1 nm). Herein, we designed and fabricated an imide-linked TbTG COF membrane (COFM) with intrinsic sub-nano pores. The TbTG nanosheets were prepared by interfacial polymerization, and then self-assembled into COFMs on the porous PAN substrate. The sub-nano pore size (∼0.4 nm by simulation) conferred excellent size sieving capacity to the TbTG COFM with the molecular weight cut-off of only 180 Da, which was the lowest among COFMs ever reported. Moreover, the positive charge on the skeleton of TbTG COFM greatly fortified the Donnan effect. The TbTG COFM exhibits excellent salt rejections of 98.7% to Na2SO4, 99.1% to MgSO4, 99.5% to MgCl2 and 93.3% to NaCl, respectively. The TbTG COFM also maintained high salt rejections under both high salt concentration up to 10000 ppm and high pH value of 9.4, showing great potential in seawater desalination applications.

Covalent organic framework membranes for acid recovery: The effect of charges

Acid recovery has drawn an intensive interest due to the tremendous consumption of acid chemicals in various industry applications, which produce large amounts of acidic wastewater. For conventional anion exchange membranes (AEMs) via diffusion dialysis, the low selectivity between H+ and monovalent cations restricts the precise recovery of acid. Herein, we develop a self-standing COF (TpTAPA) membrane with high stability both in the acid and base solutions. The as-prepared membrane exhibits excellent separation performance for H+ over other monovalent and multivalent cations, where the selectivities of H+/K+, H+/Na+, H+/Li+, H+/Mg2+, and H+/Fe2+ are 136, 143, 145, 608, and 1499, respectively. Whereafter, the membrane nanochannels are decorated with different charges, where the positively charged Q-TpTAPA nanochannels are beneficial for H+ transport and the negatively charged S-TpTAPA nanochannels are adverse for the transport of H+ compared with the pristine TpTAPA membrane. However, both negatively and positively charged nanochannels can increase the leakage of multivalent cations, resulting in the decline of H+ selectivity. These significant findings shed light on the design of high-efficiency nanoporous membranes for exquisite acid recovery from complex feed system.

Spacer-Engineered Ionic Channels in Covalent Organic Framework Membranes toward Ultrafast Proton Transport.

Side-chain engineering of covalent organic frameworks as advanced ion conductors is a critical issue to be explored. Herein, ionic covalent organic framework membranes (iCOFMs) with spacer-engineered ionic channel are de novo designed and prepared. The ionic channels are decorated with side chains comprising spacers having different carbon chain lengths and the -SO3 H groups at the end. Attributed to the synergistic contribution from the spacers and the -SO3 H groups, the iCOFM with moderate-length spacer exhibit the highest through-plane proton conductivity of 889 mS cm-1 at 90°C.

Anti-scaling covalent organic framework membranes with custom-tailored nanochannels for efficient lithium extraction

Covalent organic framework membranes (COFMs) with well-defined nanochannels have infinite potential in efficient separation. However, unsuitable pore size and easily contaminated surface hamper the application of COFMs for ion separation. Herein, we report a custom-tailored nanochannels strategy for forming COF nanosheets (COFNs) with positively charged, narrowed and lithiophilic channels. The obtained COFNs can be easily assembled into free-defect and oriented COFMs via vacuum-assisted filtration method. Charged and narrowed channels augment entrance resistance of divalent cation, meanwhile, lithiophilic groups promote transport of Li+ in channels. Besides, positively charged surface disturbs the crystallization of scaling. Thus, the optimal COFM exhibited excellent water permeance of 32.1 L m−2 h−1 bar−1 and high Li+/Mg2+ separation factor of 30.2 in simulated salt-lake brine. After running in the gypsum solution, the anti-scaling COFMs showed low flux decline ratio (FDR) and the flux recovery ratio (FRR) was nearly 10% higher than NF90. This study may provide a facile regulation method for nanochannels and surface properties of COFMs, and the obtained membranes could be applied to efficient lithium extraction.

Pore functionalization of cationic covalent organic frameworks membrane: A case towards acid recovery

Covalent organic frameworks (COFs), which are a new class of porous organic materials, are considered as potential candidates for separation membranes because of their regular channels and tailored functionality. The imine-linked COF was synthesized using reversible condensation reactions, resulting in its un-stabilization in the acid environment. Thus, it is a formidable challenge to develop stable COFs membranes for applications in harsh environments (e.g. strong acid). Herein, we demonstrated the in-situ construction of acid-stable COF separating layer (TMTATGCl) on the surface of the hydrophilic hydrolyzed polyacrylonitrile (HPAN) substrate via acid-catalyzed interfacial polymerization. Triaminoguanidine chloride (TGCl), a cationic monomer, was used to construct cationic nanochannels in the COF layer, conferring its screening feature toward cations, especially toward divalent cations. Besides, implanted methoxy groups could facilitate H+ diffusion along the pore pathway of the as-prepared COF. As a result, the methoxy functionalized cationic nanochannels of the DMTATGCl/HPAN enhanced H+ transfer, while other ions (e.g. Fe2+) were obstructed, allowing for high selectivity (S = 1248.9) and permeability (UH+ = 0.0118 m/h). Moreover, the membranes (TMTATGCl/HPAN) exhibited remarkable acid-stability, which maintained the high separation performance after 10 cycles under strong-acid conditions. This study provides an effective strategy for developing the ion channels membrane with specific interaction sites for effective ion separation.

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.

Electrochemical Interfacial Polymerization toward Ultrathin COF Membranes for Brine Desalination

AbstractUltrathin 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.