Postsynthetic Modification of Core-Shell Magnetic Covalent Organic Frameworks for the Selective Removal of Mercury

Covalent Organic Framework Membranes: Synthesis Strategies and Separation Applications

Covalent organic frameworks (COFs) have emerged as highly promising materials for membrane separations due to their high porosity, tunable pore sizes, ordered crystalline structures, and exceptional chemical stability. With these features, COF membranes possess greater selectivity and permeability than conventional materials, making them the preferred choice in various fields, including membrane separations. Fascinating research endeavors have emerged encompassing fabrication strategies for COF-based membranes and their diverse separation applications. Hence, this review summarizes the latest advancements in COF synthesis, including COF powders and continuous COF-based membranes and their applications in separation membranes. Special consideration was given to regulation strategies for the performance optimization of COF membranes in separation applications, such as pore size, hydrophilicity/hydrophobicity, surface charge, crystallinity, and stability. Furthermore, applications of COF membranes in water treatment, metal ion separation, organic solvent nanofiltration, and gas separation are comprehensively reviewed. Finally, the research results and future prospects for the development of COF membranes are discussed. Future research may be focused on the following key directions: (1) single-crystal COF fabrication, (2) cost-effective membrane preparation, (3) subnanometer pore engineering, (4) advanced characterization techniques, and (5) AI-assisted development.

Covalent organic frameworks as superior adsorbents for the removal of toxic substances

Developing new materials capable of the safe and efficient removal of toxic substances has become a research hotspot in the field of materials science, as these toxic substances pose a serious threat to human health, both directly and indirectly. Covalent organic frameworks (COFs), as an emerging class of crystalline porous materials, have advantages such as large specific surface area, tunable pore size, designable structure, and good biocompatibility, which have been proven to be a superior adsorbent design platform for toxic substances capture. This review will summarize the synthesis methods of COFs and the properties and characteristics of typical toxicants, discuss the design strategies of COF-based adsorbents for the removal of toxic substances, and highlight the recent advancements in COF-based adsorbents as robust candidates for the efficient removal of various types of toxicants, such as animal toxins, microbial toxins, phytotoxins, environmental toxins, etc. The adsorption performance and related mechanisms of COF-based adsorbents for different types of toxic substances will be discussed. The complex host-guest interactions mainly include electrostatic, π-π interactions, hydrogen bonding, hydrophobic interactions, and molecular sieving effects. In addition, the adsorption performance of various COF-based adsorbents will be compared, and strategies such as reasonable adjustment of pore size, introduction of functionalities, and preparation of composite materials can effectively improve the adsorption efficiency of toxins. Finally, we also point out the challenges and future development directions that COFs may face in the field of toxicant removal. It is expected that this review will provide valuable insights into the application of COF-based adsorbents in the removal of toxicants and the development of new materials.

Thiolated non-conjugated nano polymer network for advanced mercury removal from water

Developing advanced adsorbents for selectively deducing mercury (Hg) in water to one billionth level is of great significance for public health and ecological security, but achieving the balance among efficiency, cost and environmental friendliness of adsorbents still faces enormous challenges. Herein, we present a high thiol content non-conjugated nano polymer network (PVB-SH) through simple microemulsion polymerization for efficient Hg ion (Hg(II)) removal. The PVB-SH is prepared by conventional commercial reagents and does not consume toxic organic solutions. This nano network reveals uniformly distributed nano sizes, leading to good accessibility of adsorption sites. The long and flexible polymer chains in the network allow two thiol sites to coordinate with one Hg(II), displaying significantly stronger binding than 1:1 coordination. Therefore, PVB-SH shows high affinity toward Hg(II) (Kd = 3.04 × 107 mL/g) and can selectively reduce Hg(II) in water to extremely low level of 0.14 μg/L, well below the safe limit of 2 μg/L. PVB-SH possesses excellent renewability (removal efficiency = 99.58 % after 10 regenerations), good resistance to various environmental factors (pH, ions and organic matter) and long-term stability in acid, alkali, and salt solutions. Impressively, PVB-SH is further made into a membrane by simple phase-inversion and can effectively purify 1592.4 L/m2 Hg(II) polluted drinking water before the breakthrough point of 2 μg/L. These results demonstrate the good practical potential of PVB-SH for decontamination of Hg from aqueous media.

Impact of Postsynthetic Modification on the Covalent Organic Framework (COF) Structures

Covalent organic frameworks (COFs) have emerged as a versatile class of materials owing to their well-defined crystalline structures and inherent porosity. In the realm of COFs, their appeal lies in their customizable nature, which can be further enhanced by incorporating diverse functionalities. Postsynthetic modifications (PSMs) emerge as a potent strategy, facilitating the introduction of desired functionalities postsynthesis. A significant challenge in PSM pertains to preserving the crystallinity and porosity of the COFs. In this study, we aim to investigate the intricate interplay between PSM strategies and the resulting crystalline and porous structures of the COFs. The investigation delves into the diverse methodologies employed in PSMs, to elucidate their distinct influences on the crystallinity and porosity of the COFs. Through a comprehensive analysis of recent advancements and case studies, the study highlights the intricate relationships among PSM parameters, including reaction conditions, precursor selection, and functional groups, and their impact on the structural features of COFs. By understanding how PSM strategies can fine-tune the crystalline and porous characteristics of COFs, researchers can harness this knowledge to design COFs with tailored properties for specific applications, contributing to the advancement of functional materials in diverse fields. This work not only deepens our understanding of COFs but also provides valuable insights into the broader realm of PSM strategies for other solid materials.

Host-guest mediated recognition and rapid extraction of Fusarium mycotoxins in cereals by nickel ferrite magnetic calix[4]arene-derived covalent organic framework fabricated in room-temperature

Fusarium mycotoxins are toxic secondary fungal metabolites and widely distributed in cereals. Herein, a nickel ferrite magnetic calix[4]arene-derived covalent organic framework (NiFe2O4@CX4-COF) was meticulously designed and synthesized using a room-temperature method for the enrichment of mycotoxins. The CX4-COF exhibited a porous crystalline network with an eclipsed AA-stacking configuration. The ingenious integration of NiFe2O4, supramolecular calix[4]arene and COF contributed to host-guest mediated recognition, size-selectivity and high adsorption capacity, rapidly reaching adsorption equilibrium within only 3 min. Simulation calculations revealed that the host-guest interaction, size effect and abundant binding sites facilitated synergistically recognize and capture mycotoxins. NiFe2O4@CX4-COF has successfully applied for simultaneous extraction and analysis of mycotoxins in cereals, achieving negligible matrix effects (-14% to 13%), high sensitivity (LODs of 0.003-0.014 μg/L) and satisfactory recoveries (74.4%-116%). This work provides a prospective platform for constructing tailored macrocycle-based COFs under mild conditions for precise recognition and accurate analysis of trace hazardous substances in food.

Co2+ coordination-assisted molecularly imprinted covalent organic framework for selective extraction of ochratoxin A

Covalent organic frameworks (COFs) are attractive materials for sample pretreatment due to their tunable structures and functions. However, the precise recognition of contaminant in complex environmental matrices by COFs remains challenging owing to their insufficient specific active sites. Herein, we report Co2+ coordination-assisted molecularly imprinted flexible COF (MI-COF@Co2+) for selective recognition of ochratoxin A (OTA). The MI-COF@Co2+ was prepared via one-step polymerization of 3,3-dihydroxybenzidine, 2,4,6-tris(4-formylphenoxy)- 1,3,5-triazine, Co2+ and template. The flexible units endowed COFs with the self-adaptable ability to regulate the molecular conformation and coordinate with Co2+ to locate the imprinted cavities. The coordination interaction greatly improved the adsorption capacity and selectivity of MI-COF@Co2+ for OTA. The prepared MI-COF@Co2+ was used as solid phase extraction adsorbent for high-performance liquid chromatography determination of OTA with the detection limit of 0.03 ng mL-1 and the relative standard deviation of < 2.5 %. In addition, this method permitted interference-free determination of OTA in real samples with recovery from 89.5 % to 102.8 %. This work provides a simple way to improve the selectivity of COFs for the determination of hazardous compounds in complex environments.

Core-Shell Gold Nanoparticles@Pd-Loaded Covalent Organic Framework for In Situ Surface-Enhanced Raman Spectroscopy Monitoring of Catalytic Reactions

A core-shell nanostructure of gold nanoparticles@covalent organic framework (COF) loaded with palladium nanoparticles (AuNPs@COF-PdNPs) was designed for the rapid monitoring of catalytic reactions with surface-enhanced Raman spectroscopy (SERS). The nanostructure was prepared by coating the COF layer on AuNPs and then in situ synthesizing PdNPs within the COF shell. With the respective SERS activity and catalytic performance of the AuNP core and COF-PdNPs shell, the nanostructure can be directly used in the SERS study of the catalytic reaction processes. It was shown that the confinement effect of COF resulted in the high dispersity of PdNPs and outstanding catalytic activity of AuNPs@COF-PdNPs, thus improving the reaction rate constant of the AuNPs@COF-PdNPs-catalyzed hydrogenation reduction by 10 times higher than that obtained with Au/Pd NPs. In addition, the COF layer can serve as a protective shell to make AuNPs@COF-PdNPs possess excellent reusability. Moreover, the loading of PdNPs within the COF layer was found to be in favor of avoiding intermediate products to achieve a high total conversion rate. AuNPs@COF-PdNPs also showed great catalytic activities toward the Suzuki-Miyaura coupling reaction. Taken together, the proposed core-shell nanostructure has great potential in monitoring and exploring catalytic processes and interfacial reactions.

High Sodium Ion Storage by Multifunctional Covalent Organic Frameworks for Sustainable Sodium Batteries

Rechargeable sodium batteries hold great promise for circumventing the increasing demand for lithium-ion batteries (LIBs) and the limited supply of lithium. However, efficient sodium ion storage remains a great impediment in this field. In this study, we report the designed synthesis of a multifunctional two-dimensional covalent organic framework featuring hexaazatrinaphthalene cores linked by imidazole moieties and demonstrate its effective performance in sodium ion storage. Benzimidazole-linked covalent organic framework (BCOF-1) was synthesized by a condensation reaction between hexaazatrinaphthalenehexamine (HATNHA) and terephthalaldehyde (TA) and exhibited a high theoretical specific capacity of 392 mA h g-1. BCOF-1 crystallizes, forming eclipsed AA stacking and mesoporous hexagonal one-dimensional channels with high surface area (840 m2 g-1), facilitating fast ionic mobility and charge transfer and enabling high-rate capability at high current rates. BCOF-1 exhibits pseudocapacitive-like behavior with a high specific capacity of 387 mA h g-1, an energy density of 302 W h kg-1 at 0.1 C, and a power density of 682 W kg-1 at 5 C. Our results demonstrate that redox-active COFs have the desired structural and electronic merits to advance the use of organic electrodes in sodium-ion storage toward sustainable and efficient batteries.

New sulfonated covalent organic framework for highly effective As(III) removal from water

The goal of taking out As(III) from water is to reduce the detriment that poisonous metals can do to people and nature. A substance that can absorb As(III), TFPOTDB-SO3H, was made by combining 2,5-diaminobenzenesulfonic acid and 2,4,6-tris-(4-formylphenoxy)-1,3,5-triazine in a reaction that joins molecules together. This substance can adsorb As(III) very well and has excellent qualities like being easy to use again, separate substances, and filter out liquids. At pH = 8 and at room temperature, TFPOTDB-SO3H adsorbed a lot of As(III). It achieved a removal rate of 97.1 % within 10 min and could adsorb up to 344.8 mg/g. A research was conducted to investigate the effect of co-existing anions on the elimination of arsenic. The findings indicated that the presence of anions had a minimal adverse impact, reducing As(III) uptake by approximately 1-7 %. The kinetics of the uptake process were found to be controlled by the quasi-second order kinetic model, while the Langmuir isotherm model validated that the mechanism for As(III) removal was monolayer chemisorption. According to the thermodynamic analysis, the adsorption process was endothermic and occurred spontaneously. Moreover, even after 4 successive adsorption-desorption cycles, the adsorbent preserved a substantial uptake productivity of 88.86 % for As(III). The results collectively indicate that TFPOTDB-SO3H holds considerable promise for the efficient adsorption and elimination of As(III) ions from wastewater.

Advanced Equilibrium Studies for the Synergetic Impact of Polyaniline on the Adsorption of Rhodamine B Dye by Polyaniline/Coal Composite

The synergetic improvement effect of the polyaniline (PANI) hybridization process on the adsorption of rhodamine B dye (RB) by PANI/coal hybrid material (PANI/C) has been evaluated using both traditional equilibrium modeling and advanced isotherm investigations. The composite was prepared by polymerizing polyaniline in the presence of coal fractions with a surface area of 27.7 m2/g. The PANI/C hybrid has an improved capacity to adsorb RB dye (423.5 mg/g) in comparison to coal particles (254.3 mg/g). The maintained increase in the elimination properties of PANI/C has been illustrated using the steric characteristics of active site density (Nm) as well as the total number of adsorbed RB on a single active site (n). However, the incorporation of PANI did not yield any substantial impact on the existing active sites' quantity, but the hybridization processes greatly influenced the selectivity and affinity of each active site, in addition to the aggregation characteristics of the dye as it interacts with the composite's surface. Whereas raw coal can only adsorb three molecules of RB, each active site throughout the PANI/C surface can adsorb approximately eight RB molecules. This is also evidence of RB dye adsorption in a vertical arrangement, which involves multimolecular processes. The Gaussian energy (4.01-5.59 kJ/mol) and adsorption energy (-4.34-4.68 kJ/mol) revealed the controllable impact of physical mechanisms. These mechanisms may include van der Waals forces, dipole-dipole interactions, and hydrogen bonds (<30 kJ/mol). The thermodynamic functions, such as enthalpy, internal energy, and entropy, that have been assessed provide evidence supporting the exothermic and spontaneous nature of the RB uptake processes by PANI/C.

Towards high-performance nonlinear optical materials through embedding a D-A system into β-ketoenamine-linked COFs

Two covalent organic framework (COF) films supported by a glass substrate were obtained by solvothermal reaction of an electron donor with electron acceptor 1,3,5-triformylbenzene (TF) or 2,4,6-triformylphloroglucinol (TFP), respectively. The TFP-BD film exhibits a nonlinear absorption coefficient of -3.01 × 105 cm GW-1. The TFP-BD film can aggregate electrons around the connected monomer through the D-A effect due to its highly polar and electronegative carbonyl oxygen atoms, thereby modulating the electronic structure of the COFs. This work provides a novel approach for the structural modulation of optical materials with strong nonlinearity.

Linkage Transformations in a Three-Dimensional Covalent Organic Framework for High-Capacity Adsorption of Perfluoroalkyl Substances

Despite their many advantages, covalent organic frameworks (COFs) built from three-dimensional monomers are synthetically difficult to functionalize. Herein, we provide a new synthetic approach to the functionalization of a three-dimensional covalent organic framework (COF-300) by using a series of solid-state linkage transformations. By reducing the imine linkages of the framework to amine linkages, we produced a more hydrolytically stable material and liberated a nucleophilic amino group, poised for further functionalization. We then treated the amine-linked COF with diverse electrophiles to generate a library of functionalized materials, which we tested for their ability to adsorb perfluoroalkyl substances (PFAS) from water. The framework functionalized with dimethylammonium groups, COF-300-dimethyl, adsorbed more than 250 mg of perfluorooctanoic acid (PFOA) per 1 g of COF, which represents an approximately 14,500-fold improvement over that of COF-300 and underscores the importance of electrostatic interactions to PFAS adsorption performance. This work provides a conceptually new approach to the design and synthesis of functional three-dimensional COFs.