Covalent organic frameworks (COFs), which are constructed from organic linkers, are a new class of crystalline porous materials comprising periodically extended and covalently bound network structures. The intrinsic structures and the tailorable organic linkers endow COFs with a low density, large surface area, tunable pore size and structure, and facilely-tailored functionality, attracting increasing interests in different fields including membrane separations. Exciting research activities ranging from fabrication strategies to separation applications of COF-based membranes have appeared. This review analyzes the synthesis and applications of diverse continuous/discontinuous COF membranes, such as COF-based mixed matrix membranes (MMMs), COF-based thin film nanocomposite (TFN) membranes, and free-standing COF films. Special attention was given to pore size, stability, hydrophilicity/hydrophobicity and surface charge of COFs in view of determining proper COFs for membrane fabrication, along with the approaches to fabricate COF-based membranes, such as blending, in situ growth, layer-by-layer stacking and interfacial polymerization (IP). Moreover, applications of COF-based membranes in gas separation, water treatment (deaslination and dye removal), organic solvent nanofiltration (OSN), pervaporation and fuel cell are disscussed. Finally, we illustrate the advantages and disadvantages of COF-based membranes through a comparison with MOF-based membranes, and the remaining challenges and future opportunities in this field.

Metal–organic frameworks (MOFs) are increasingly utilized for the design of molecular sieve membranes with unprecedented separation performance, mainly attributed to their nanopore structure and superior polymer affinity. Herein, UiO-66-NH₂ crystals were synthesized as functional nanofillers to prepare polyamide (PA) composite membranes. The aqueous solution containing piperazine and evenly dispersed UiO-66-NH₂ was transferred onto a membrane surface via vacuum filtration, followed by interfacial polymerization (IP) with trimesoyl chloride (TMC). Covalent bonding between the terminal amine groups of Zr-MOFs and TMC enables the stable immobilization of MOF crystals in the formed PA layer. The positioned MOF nanoaggregates that serve as robust nodes connected by the formed continuous striped film after IP reaction facilitate a rough, fishnet-like surface, which has a larger surface area in comparison to the nodular structure. This unique structure is beneficial for the creation of increased water transport channels, while not compromising the salt rejection. The best performing TFN membranes with a loading mass of 20.5 μg cm⁻² exhibited a favorable Na₂SO₄ rejection of 97.5%, and excellent water permeability (30.8 L m⁻² h⁻¹ bar⁻¹). This facile, controlled loading of hydrophilic MOFs via vacuum filtration allows fabrication of crumpled PA membranes with outstanding separation performance.

Various attempts are increasingly being made to decrease the thickness of polyamide (PA) nanofilms, in order to promote water transport. Despite optimization of interfacial polymerization (IP), it is challenging to prepare defect-free, ultrathin nanofilms in situ on top of membrane substrates. This issue is closely linked to the rapid, uncontrolled IP reaction, and the physicochemical properties of support materials. In this study, PA nanofilms less than 12 nm in thickness were fabricated at a free water–hexane interface, followed by directly transferring them onto polydopamine (PDA) coated polymer substrates via vacuum filtration. This method not only efficaciously controls the nanofilm thickness, but also largely reduces the monomer content compared to a traditional IP process. Manipulation of nanofilm thickness and structure through fine-tuning the monomer concentration enables the formation of smooth, ultrathin, and robust PA layers. Composite membranes with polypiperazine amide nanofilms exhibit a high water permeance (25.1 L m⁻² h⁻¹ bar⁻¹), and an excellent divalent salt rejection (R Na₂SO₄ = 99.1%). This more than doubles the permeance of a commercial nanofiltration (NF) membrane, although the Na₂SO₄ retention is generally below 98.5%. Importantly, the low NaCl retention endows the nanofilm composite membranes with a high mono/divalent salt selectivity (80.6). This facile, one-pot strategy provides a useful guideline for the construction of highly permeable TFC membranes for water purification.

With the rapid growth of the world population and water contamination, nanofiltration membranes with extraordinary permeance and high salt rejection are eminently desirable for addressing global water scarcity. Here, an innovative approach to design thin film composite (TFC) membranes with ultrahigh permeance via hydrogel assisted interfacial polymerization is reported. The hydrogel containing piperazine (PIP) monomers serving as an aqueous phase in interfacial polymerization enable homogeneous interfacial polymerization, reducing the diffusion rate of the PIP monomers, and providing mechanical strength to the thin polyamide (PA) selective layer. As a result, the synthesized advanced TFC membranes have ultrahigh high permeances of 52.8 and 62.9 L m⁻² h⁻¹ bar⁻¹ while maintaining satisfactorily high rejections of 96.4% and 93.5% for Na₂SO₄, demonstrating a desalination performance superior to the PA TFC nanofiltration membranes reported so far. Moreover, the mechanism behind the exceptional high permeance is delineated by exploring theoretical hydrogel assisted interfacial polymerization via simulation. Most importantly, the fabrication process of the hydrogel-TFC membranes is simple, enabling cost-efficient and scalable manufacturing.

The development of super-hydrophobic surfaces for water/oil separation has attracted much interest in fundamental research and industrial applications in recent years. This article proposes a facile method to fabricate super-hydrophobic surfaces on 3D printed polysulfone (PSU) membranes via the coating of candle soot. A 3D printed PSU membrane fabricated by selective laser sintering was applied for the oil/water separation and showed a different wettability on its top surface and bottom surface. The self-assembled candle soot loose network endowed the 3D printed PSU membrane with super-hydrophobicity with a water contact angle of 161° and a sliding angle of 5°, preserving an outstanding mechanical stability under sonication and chemical stability in 1 M HCl, 1 M NaOH, 1 M NaCl and hot water. Interestingly, this super-hydrophobic surface could dramatically switch to a super-oleophobic state after being prewetted by water. Ten cycles of switchable oil/water separation were performed, demonstrating a high oil/water separation stability of the dry candle soot coated PSU membrane and the water prewetted candle soot coated PSU membrane. Additionally, the effect of selective laser sintering processing parameters on the structure and performance and the influence of the immersion time on the deposition of candle soot were investigated. Overall, this study provides an efficient, simple and reliable fabrication method for super-hydrophobic surfaces with switchable wettability.