COF-based Membranes Research Articles

Bio-inspired construction of ion conductive pathway in covalent organic framework membranes for efficient lithium extraction

Summary As the most sophisticated separation system, biological membranes have served as natural prototypes for the design of artificial membranes because they provide high permeability and solute selectivity, owing to the presence of specialized ion channels. However, developing stable and selective artificial ion channels remains a formidable challenge. Herein, we demonstrate the construction of lithium nanochannels in two-dimensional covalent organic frameworks (COFs) to create biomimetic membranes. Implanted lithiophilic oligoethers conferred specificity and facilitated Li+ diffusion along the pore pathway of the COF. The ion channel characteristics were indicated by reversal potential measurements, showing that the relative permeability decreased in the order Li+ > K+ > Na+ > Ca2+ > Mg2+. The Li+ transfer was enhanced, while other ions were obstructed, allowing for high selectivity and permeability. A Li+/Mg2+ separation factor of 64 was achieved, confirming high lithium affinity. This study may serve as a design principle to develop selective artificial membranes for effective ion separation.

Secondary growth of bi-layered covalent organic framework nanofilms with offset channels for desalination

Processing two-dimensional (2D) covalent organic frameworks (COFs) into nanofilms has gained widespread attention in water treatment. However, the designed synthesis of COF-based membranes for ion separations still remains a huge challenge though the strategies to produce COF molecular separation membranes have been greatly developed. Herein, we report the construction of bi-layered COF nanofilms for efficient desalination through a simple yet effective secondary growth. The suggested strategy is capable of easily regulating the growth of another COF layer on the pre-synthesized first layer, thus producing crystalline and defect-free bi-layered COF nanofilms with distinct laminated structures. Most importantly, the offset channels are involved at the interface of two nanofilms obtained from the first and secondary growth, which leads to a constricted effective aperture that significantly enhances the ion separation performance. Specifically, the bi-layered COF nanofilms composited with macroporous supports exhibit high rejection rates (~95.8%) to Na2SO4, observably surpassing the desalination performance of single-layered COF nanofilms (rejection<10%). This work not only develops an ingenious way to fabricate COF-on-COF nanofilms with narrowed channels by tailoring the laminated structure of 2D COFs at the film interface, but also starts an avenue for the adoption of COF platforms for membrane separations with an improved precision.

Covalent organic framework-based membranes for liquid separation

This review summarizes the synthesis and characterization methods of COF-based membranes in recent years and discusses their separation mechanism and application in liquid separation.

Multifunctional covalent organic framework (COF)-Based mixed matrix membranes for enhanced CO2 separation

Covalent organic frameworks (COFs) have stimulated an immense interest for membrane separation owing to their defined nanoscale channels, tailorable chemical functionality, and total organic backbone. However, the development of COF-based membranes for gas separation remains a great challenge due to the limited functional groups and relatively large pore size of the existing COFs. The rational design and appropriate modification of COFs are urgently demanded for more efficient CO2 separation. In this study, we propose a multi-function integration strategy for the design of COF fillers considering entire morphological structure, aperture adjustment and channel decoration, as well as interface optimization. The COF hollow microsphere with polyethylene glycol monomethyl ether (PEG) modification on both outer surface and inner channel wall was synthesized and filled into commercial Pebax polymer to fabricate mixed matrix membranes (MMMs). The hollow structure of COF fillers reduces the mass transport resistance. PEG functionalization provides channel wall with ethylene oxide groups and decreases the COF pore size for simultaneous enhancement of solubility selectivity and diffusion selectivity. Moreover, PEG chains on the outer surface improve the interface compatibility between COF fillers and Pebax matrix. The resulting MMMs exhibit superior CO2/CH4 separation performances, surpassing the 2008 Robeson's upper bound.

Enabling Covalent Organic Framework Nanofilms for Molecular Separation: Perforated Polymer-Assisted Transfer.

Covalent organic frameworks (COFs) with ordered arrays of sub-2 nm regular pores are drawing increasing attention in membrane separation, and it remains highly desirable for effective and controllable strategies to fabricate COF-based membranes. Herein, we demonstrate a perforated polymer-assisted transfer strategy enabling COF nanofilms for molecular separation. Solvothermal synthesis is used for the confined growth of TpPa, a stable, imine-linked COF, on the smooth surfaces of silicon substrates. Continuous, crystalline COF nanofilms are obtained, and their thicknesses are tunable in the range from a few tens to several hundred nanometers depending on monomer concentrations and reaction time. A block copolymer layer is coated on the COF nanofilms, which is then perforated to produce interconnected mesopores by the mechanism of selective swelling-induced pore generation. The perforated polymer coating functions as a protective but permeable layer enabling the easy transfer of the COF nanofilm onto porous substrates. Thus, we obtain a new type of composite membranes with the microporous COF nanofilm as the selective layer, sandwiched between the macroporous substrate and the mesoporous protective layer. The composite membranes exhibit good separation performances with water permeance up to ∼51 L m-2 h-1 bar-1 and high rejection rates to various dyes. This work demonstrates a new method to prepare COF-based membranes for molecular separation, and the invented perforated polymer-assisted transfer technology is expected to find applications in transferring other ultrathin materials to demanded substrates.

Covalent organic frameworks combined with graphene oxide to fabricate membranes for H2/CO2 separation

Covalent organic frameworks (COFs) are getting more attention by virtue of their intriguing architectures and properties. However, the obstacle for fabricating sequential COF-based membranes for gas separation has become a challenge. In this research, a series of TpPa COFs are successfully synthesized by a facile approach of mechanical grinding. To alleviate the dilemma of membrane formation, we introduce graphene oxide (GO) that owns two-dimensional flexibility to sufficiently assist the COFs to form lamellar membranes. By directly mixing the solutions of GO and COFs, robust and continue COFs/GO composite membranes are prepared without defects via the vacuum filtration. The thickness and the H2/CO2 selectivity of the membranes can be regulated by increasing the amount of GO. The optimal membrane obtained with a GO/COFs ratio of 33.33% (TpPa-1-30/GO-10) exhibits the H2/CO2 selectivity of 25.57 with H2 permeance of 1.067 × 10−6 mol·m−2·s−1·Pa−1. This work provides a simple, universal and applicable approach to prepare COF-based composite membranes for separation.

Growing covalent organic frameworks on porous substrates for molecule-sieving membranes with pores tunable from ultra- to nanofiltration

Crystalline imine-based covalent organic frameworks (COFs) have attracted great interest as next-generation water treatment materials owning to their numerous merits. The construction of COF layers with tunable pore sizes ranging from meso- to micropores, which can significantly broaden the membrane selectivity from ultra- (UF) to nanofiltration (NF), has remained unexplored. Herein, we demonstrate the controllable solvothermal synthesis of COF layers with adjustable pore sizes on anodic aluminum oxide (AAO) substrates. With rising synthesis duration the modulation on pore sizes of AAO supports can be achieved till the formation of continuous COF layers, leading to a gradually enhanced selectivity. Importantly, the synthesized microporous COFs shrink the pore sizes of AAO substrates and provide additional nanochannels for water transporting, eventuating in a ~2–9 times higher permeance than other reported UF membranes with comparable rejections. Once forming continuous COF layers, more rigid selectivities based on COF inherent nanochannels (~1.83 nm) can be realized with capacities of removing dyes from water or organic solvent. This work provides a facile methodology to construct COF-based membranes with broadly tunable separation performance, recommending the membranes for removing targeted molecules including proteins, nanoparticles, and dyes.

Covalent organic frameworks for membrane separation.

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.

Interfacial polymerization of covalent organic frameworks (COFs) on polymeric substrates for molecular separations

Covalent organic frameworks (COFs) represent a new family of porous polymers with highly ordered two or three-dimensional channels. Although numerous studies have been focused on the design and synthesis of COF in the form of powders, the development of COF-based separation membranes is still hampered by the challenges of COF particles agglomeration and harsh synthetic conditions. In this work, interfacial polymerization (IP) directly performed on polymeric substrates as employed in the traditional IP process of polyamide (PA) membranes is developed for the synthesis of COF-based membranes. With the moderate reaction rate between monomer pairs in corresponding aqueous and organic solutions, a conformal growth of COF crystallites directly composited with the polysulfone (PSF) ultrafiltration substrates can be realized within 1 min. The synthesis parameters including reaction time and precursor concentrations are optimized, and thus-synthesized COF/PSF membrane presents a stable rejection to dye (Congo red) of 99.5% with a high water permeance of up to 50 L m−2 h−1 bar−1, which is 2–10 times higher than that of many other membranes with similar rejection. This convenient IP process is expected to facilitate the up-scaling and real-world applications of COF-based membranes.

Highly water-selective membranes based on hollow covalent organic frameworks with fast transport pathways

Covalent organic frameworks (COFs) are attractive candidates for membrane separation owing to their well-ordered channels and facilely tailored functionalities. However, the development of COF-based membranes remains in its infancy. To establish novel hierarchical structures in COFs, in this study, a facile template-directed approach is exploited for constructing hollow COF nanospheres. An imine-linked COF TpBD, derived from 1,3,5-triformylphloroglucinol (Tp) and benzidine (BD), was selected as the building block due to its rich hydrophilic groups and high stability. The synthesized hollow TpBD (H-TpBD) nanospheres were introduced into sodium alginate (SA) matrices to fabricate water-selective hybrid membranes. The H-TpBD nanospheres confer the membranes rapid diffusion pathways and abundant interacting sites for water molecules, thus fast water-selective permeation through the membranes was achieved. The hybrid membranes exhibit the optimal performance with permeation flux of 2170 g m−2 h−1 and separation factor of 2099 when used for dehydrating 90 wt% ethanol aqueous solution at 76 °C. The effects of hollow structures of H-TpBD on membrane performance were investigated and the molecular transport mechanisms through the membranes were elucidated. Besides, the favorable compatibility between SA and H-TpBD endows the hybrid membranes with long-term stability.

Ultra-high selectivity COF-based membranes for biobutanol production

A novel membrane material consisting of hydrazone-linked COF-42 incorporated into polydimethylsiloxane (PDMS) exhibits ultra-high selectivity for n-butanol/water separation.