Covalent organic frameworks embedded in polystyrene membranes for ion sieving

Improving the ion sieving performance of MOF polycrystalline membranes based on interface modification

Metal-organic framework (MOF) membranes exhibit promising potential for high-precision molecule and ion sieving due to their uniform and tunable pore structures. Nevertheless, it remains a challenge to address interfacial compatibility to obtain a high-performance MOF membrane. This is because there is a weak interfacial interaction between the MOF and the substrate, which leads to non-selective areas. This work presents an interface-coating method to facilitate dense nucleation and rapid growth of MOF crystals on the substrate towards reinforcing interaction and eliminating defects. Polydopamine (PDA) and β-cyclodextrin (β-CD) were co-assembled on polyvinylidene fluoride (PVDF) substrates to modify the surface chemistry and enhance interfacial compatibility, thereby facilitating the dense growth of ZIF-8. The ZIF-8/PVDF membranes demonstrated an excellent K+ permeance of 0.33 mol m-2 h-1 and a K+/Mg2+ selectivity of 30.16. The simulation results indicate that the channel of Mg2+ through ZIF-8 must overcome a greater transport energy barrier than that of K+, resulting in a higher selectivity of K+/Mg2+.

Imine-Linked 3D Covalent Organic Framework Membrane Featuring Highly Charged Sub-1 nm Channels for Exceptional Lithium-Ion Sieving

Coupling ion exclusion and interaction screening within sub-nanoconfinement channels in novel porous material membranes hold great potential to realize highly efficient ion sieving, particularly for high-performance lithium-ion extraction. Diverse kinds of advanced membranes have been previously reported to realize this goal but with moderate performance and complex operations gained. Herein, these issues are circumvented by preparing the consecutive and intact imine-linked three-dimensional covalent organic framework (i.e., COF-300) membranes via a simple solvothermal approach and employing the intrinsically interconnected sub-1 nm one-dimensional channels for exceptional lithium-ion sieving. The synthesized membranes with highly charged angstrom scale channels of ≈0.78 nm achieve an excellent Li+ permeance (0.123 mol m-2 h-1) with an ultrahigh Li+/Mg2+ of 36 in the binary system. The experimental measurement and theoretical calculation reveal that a channel size right exactly between Li+ and Mg2+ enables restricted Mg2+ penetration. Meanwhile, the ion affinity interaction screening with imine groups further strengthens the fast Li+ permeability but severely suppresses the Mg2+ passage. In particular, the synthesized three-dimensional covalent organic framwork membranes also have a remarkable separation performance during a long-term operation test without sacrificing trade-off, demonstrating chemistry stability and mechanical integrity under the high-salinity aqueous environment.

Covalent Organic Framework Membranes with Regulated Orientation for Monovalent Cation Sieving

Continuous covalent organic framework (COF) thin membranes have garnered broad concern over the past few years due to their merits of low energy requirements, operational simplicity, ecofriendliness, and high separation efficiency in the application process. This study marks the first instance of fabricating two distinct, self-supporting COF membranes from identical building blocks through solvent modulation. Notably, the precision of the COF membrane's separation capabilities is substantially enhanced by altering the pore alignment from a random to a vertical orientation. Within these confined channels, the membrane with vertically aligned pores and micron-scale stacking thickness demonstrates rapid and selective transportation of Li+ ions over Na+ and K+ ions, achieving Li+/K+ and Li+/Na+ selectivity ratios of 38.7 and 7.2, respectively. This research not only reveals regulated orientation and layer stacking in COF membranes via strategic solvent selection but also offers a potent approach for developing membranes specialized in Li+ ion separation.

Covalent Organic Framework Membranes and Water Treatment

Covalent organic frameworks (COFs) are an emerging class of highly porous crystalline organic polymers comprised entirely of organic linkers connected by strong covalent bonds. Due to their excellent physicochemical properties (e.g., ordered structure, porosity, and stability), COFs are considered ideal materials for developing state-of-the-art separation membranes. In fact, significant advances have been made in the last six years regarding the fabrication and functionalization of COF membranes. In particular, COFs have been utilized to obtain thin-film, composite, and mixed matrix membranes that could achieve effective rejection (mostly above 80%) of organic dyes and model organic foulants (e.g., humic acid). COF-based membranes, especially those prepared by embedding into polyamide thin-films, obtained adequate rejection of salts in desalination applications. However, the claims of ordered structure and separation mechanisms remain unclear and debatable. In this perspective, we analyze critically the design and exploitation of COFs for membrane fabrication and their performance in water treatment applications. In addition, technological challenges associated with COF properties, fabrication methods, and treatment efficacy are highlighted to redirect future research efforts in realizing highly selective separation membranes for scale-up and industrial applications.

Function-Led Design of Covalent-Organic-Framework Membranes for Precise Ion Separation

Insufficient access to clean water and resources has emerged as one of the most pressing issues affecting people globally. Membrane-based ion separation has become a focal point of research for the generation of fresh water and the extraction of energy elements. This Review encapsulates recent advancements in the selective ion transport of covalent organic framework (COF) membranes, accomplished by strategically pairing diverse monomers to create membranes with various pore sizes and environments for specific purposes. We first discuss the merits of using COF materials as a basis for fabricating membranes for ion separation. We then explore the development of COF membranes in areas such as desalination, acid recovery, and energy element extraction, with a particular emphasis on the fundamental principles of membrane design. Lastly, we address both theoretical and practical challenges, as well as potential opportunities in the targeted design of ion-selective membranes. The goal of this Review is to stimulate future investigative efforts in this field, which is of significant scientific and strategic importance.

Continuous Covalent Organic Frameworks Membranes: From Preparation Strategies to Applications

Covalent organic frameworks (COFs) are porous crystalline polymeric materials formed by the covalent bonding of organic units. The abundant organic units library gives the COFs species diversity, easily tuned pore channels, and pore sizes. In addition, the periodic arrangement of organic units endows COFs regular and highly connected pore channels, which has led to the rapid development of COFs in membrane separations. Continuous defect-free and high crystallinity of COF membranes is the key to their application in separations, which is the most important issue to be addressed in the research. This review article describes the linkage types of covalent bonds, synthesis methods, and pore size regulation strategies of COFs materials. Further, the preparation strategies of continuous COFs membranes are highlighted, including layer-by-layer (LBL) stacking, in situ growth, interfacial polymerization (IP), and solvent casting. The applications in separation fields of continuous COFs membranes are also discussed, including gas separation, water treatment, organic solvent nanofiltration, ion conduction, and energy battery membranes. Finally, the research results are summarized and the future prospect for the development of COFs membranes are outlined. More attention may be paid to the large-scale preparation of COFs membranes and the development of conductive COFs membranes in future research.

Advanced Covalent Organic Framework-Based Membranes for Recovery of Ionic Resources

Membrane technology has shown a viable potential in conversion of liquid-waste or high-salt streams to fresh waters and resources. However, the non-adjustability pore size of traditional membranes limits the application of ion capture due to their low selectivity for target ions. Recently, covalent organic frameworks (COFs) have become a promising candidate for construction of advanced ion separation membranes for ion resource recovery due to their low density, large surface area, tunable channel structure, and tailored functionality. This tutorial review aims to analyze and summarize the progress in understanding ion capture mechanisms, preparation processes, and applications of COF-based membranes. First, the design principles for target ion selectivity are illustrated in terms of theoretical simulation of ions transport in COFs, and key properties for ion selectivity of COFs and COF-based membranes. Next, the fabrication methods of diverse COF-based membranes are classified into pure COF membranes, COF continuous membranes, and COF mixed matrix membranes. Finally, current applications of COF-based membranes are highlighted: desalination, extraction, removal of toxic metal ions, radionuclides and lithium, and acid recovery. This review presents promising approaches for design, preparation, and application of COF-based membranes in ion selectivity for recovery of ionic resources.

Mixed Matrix Membrane with Penetrating Subnanochannels: A Versatile Nanofluidic Platform for Selective Metal Ion Conduction

Biological ion channels penetrated through cell membrane form unique transport pathways for selective ionic conductance. Replicating the success of ion selectivity with mixed matrix membranes (MMMs) will enable new separation technologies but remains challenging. Herein, we report a soft substrate-assisted solution casting method to develop MMMs with penetrating subnanochannels for selective metal ion conduction. The MMMs are composed of penetrating Prussian white (PW) microcubes with subnanochannels in dense polyimide (PI) matrices, achieving selective monovalent metal ion conduction. The ion selectivity of K⁺ /Mg²⁺ is up to 14.0, and the ion conductance of K⁺ can reach 45.5 μS with the testing diameter of 5 mm, which can be further improved by increasing the testing area. Given the diversity of nanoporous materials and polymer matrices, we expect that the MMMs with penetrating subnanochannels could be developed into a versatile nanofluidic platform for various emerging applications.