Connecting Microscopic Structures, Mesoscale Assemblies, and Macroscopic Architectures in 3D-Printed Hierarchical Porous Covalent Organic Framework Foams

Ionic Covalent Organic Frameworks for Energy Devices

Covalent organic frameworks (COFs) are a class of porous crystalline materials whose facile preparation, functionality, and modularity have led to their becoming powerful platforms for the development of molecular devices in many fields of (bio)engineering, such as energy storage, environmental remediation, drug delivery, and catalysis. In particular, ionic COFs (iCOFs) are highly useful for constructing energy devices, as their ionic functional groups can transport ions efficiently, and the nonlabile and highly ordered all-covalent pore structures of their backbones provide ideal pathways for long-term ionic transport under harsh electrochemical conditions. Here, current research progress on the use of iCOFs for energy devices, specifically lithium-based batteries and fuel cells, is reviewed in terms of iCOF backbone-design strategies, synthetic approaches, properties, engineering techniques, and applications. iCOFs are categorized as anionic COFs or cationic COFs, and how each of these types of iCOFs transport lithium ions, protons, or hydroxides is illustrated. Finally, the current challenges to and future opportunities for the utilization of iCOFs in energy devices are described. This review will therefore serve as a useful reference on state-of-the-art iCOF design and application strategies focusing on energy devices.

Two-Dimensional Polymers and Polymerizations

Synthetic chemists have developed robust methods to synthesize discrete molecules, linear and branched polymers, and disordered cross-linked networks. However, two-dimensional polymers (2DPs) prepared from designed monomers have been long missing from these capabilities, both as objects of chemical synthesis and in nature. Recently, new polymerization strategies and characterization methods have enabled the unambiguous realization of covalently linked macromolecular sheets. Here we review 2DPs and 2D polymerization methods. Three predominant 2D polymerization strategies have emerged to date, which produce 2DPs either as monolayers or multilayer assemblies. We discuss the fundamental understanding and scope of each of these approaches, including: the bond-forming reactions used, the synthetic diversity of 2DPs prepared, their multilayer stacking behaviors, nanoscale and mesoscale structures, and macroscale morphologies. Additionally, we describe the analytical tools currently available to characterize 2DPs in their various isolated forms. Finally, we review emergent 2DP properties and the potential applications of planar macromolecules. Throughout, we highlight achievements in 2D polymerization and identify opportunities for continued study.

Maximizing Electroactive Sites in a Three-Dimensional Covalent Organic Framework for Significantly Improved Carbon Dioxide Reduction Electrocatalysis

Synthesis of functional 3D COFs with irreversible bond is challenging. Herein, 3D imide-bonded COFs were constructed via the imidization reaction between phthalocyanine-based tetraanhydride and 1,3,5,7-tetra(4-aminophenyl)adamantine. These two 3D COFs are made up of interpenetrated pts networks according to powder X-ray diffraction and gas adsorption analyses. CoPc-PI-COF-3 doped with carbon black has been employed to fabricate the electrocatalytic cathode towards CO₂ reduction reaction within KHCO₃ aqueous solution, displaying the Faradaic efficiency of 88-96 % for the CO₂ -to-CO conversion at the voltage range of ca. -0.60 to -1.00 V (vs. RHE). In particular, the 3D porous structure of CoPc-PI-COF-3 enables the active electrocatalytic centers occupying 32.7 % of total cobalt-phthalocyanine subunits, thus giving a large current density (j_CO ) of -31.7 mA cm^-2 at -0.90 V. These two parameters are significantly improved than the excellent 2D COF analogue (CoPc-PI-COF-1, 5.1 % and -21.2 mA cm^-2 ).

Water-stable hydrazone-linked porous organic cages

Although porous organic cages (POCs), particularly imine-linked (C[double bond, length as m-dash]N) ones, have advanced significantly over the last few decades, the reversible nature of imine linkages makes them prone to hydrolysis and structural collapse, severely limiting their applications under moist or water conditions. Herein, seven water-stable hydrazone-linked (C[double bond, length as m-dash]N-N) POCs are prepared through a simple coupling of the same supramolecular tetraformylresorcin[4]arene cavitand with different dihydrazide linkers. Their structures are all determined by single-crystal X-ray crystallography, demonstrating rich structural diversity from the [2 + 4] lantern, [3 + 6] triangular prism, and unprecedented [4 + 8] square prism to the extra-large [6 + 12] octahedron. In addition, they respectively exhibit tunable window diameters and cavity volumes ranging from about 5.4 to 11.1 nm and 580 to 6800 ų. Moreover, their application in the water environment for pollutant removal was explored, indicating that they can effectively eliminate various types of contaminants from water, including radionuclide waste, toxic heavy metal ions, and organic micropollutants. This work demonstrates a convenient method for rationally constructing versatile robust POCs and presents their great application potentialities in water medium.

Self-assembly of copper nanoclusters: isomeric ligand effect on morphological evolution

Tailoring the hierarchical self-assembly of metal nanoclusters (NCs) is an emergent area of research owing to their precise structure and flexible surface environment. Herein, the morphological evolution from rods to platelets to ribbon-like structures through self-assembly of Cu₇ NCs is dictated by the positional isomerism of the surface capping ligand, dimethylbenzenethiol (DMBT). Besides cuprophilic interaction, the interplay between π-π stacking and agostic interaction (Cu⋯H-C) directs the inter-NC organization into different ordered architectures. The excited-state relaxation dynamics of the red phosphorescent assembled structures has been correlated with their compactness and the degree of bonding interactions present.

Capture of Toxic Oxoanions from Water Using Metal-Organic Frameworks

The effective capture of common water contaminants using metal-organic frameworks (MOFs) presents a remedy for current environmental concerns arising from the pollution of water sources. The crystalline porous nature of MOFs, their high internal surface area, and exceptional tunability make them suitable candidates for sequestration and removal of pollutants. However, the efficiency of capture depends largely on the nature of the interactions between the anions and the MOF. In this work, to elucidate the host-guest interactions involved in the capture of such pollutants, we explore three characteristically different MOFs: ZIF-8, iMOF-2c, and MOF-74. We demonstrate by ab initio electronic structure calculations the importance of exploiting qualitatively different binding modes for strong host-guest interactions available in the selected MOFs. Our simulations reveal the relative performance of neutral and cationic adsorbents while underscoring the importance of employing MOFs containing open metal sites for the efficient uptake of anions.

Structure-Processing-Property Relationships of 3D Printed Porous Polymeric Materials

Imparting porosity to 3D printed polymeric materials is an attractive option for producing lightweight, flexible, customizable objects such as sensors and garments. Although methods currently exist to introduce pores into 3D printed objects, little work has explored the structure-processing-property relationships of these materials. In this study, photopolymer/sacrificial paraffin filler composite inks were produced and printed by a direct ink writing (DIW) technique that leveraged paraffin particles as sacrificial viscosity modifiers in a matrix of commercial elastomer photocurable resin. After printing, paraffin was dissolved by immersion of the cured part in an organic solvent at elevated temperature, leaving behind a porous matrix. Rheometry experiments demonstrated that composites with between 40 and 70 wt % paraffin particles were able to be successfully 3D printed; thus, the porosity of printed objects can be varied from 43 to 73 vol %. Scanning electron microscopy images demonstrated that closed-cell porous structures formed at low porosity values, whereas open-cell structures formed at and above approximately 53 vol % porosity. Tensile tests revealed a decrease in elastic modulus as the porosity of the material was increased. These tests were simulated using finite element analysis (FEA), and it was found that the Neo-Hookean model was appropriate to represent the 3D printed porous material at lower and higher void fractions within a 75% strain, and the Ogden model also gave good predictions of porous material performance. The transition between closed- and open-cell behaviors occurred at 52.4 vol % porosity in the cubic representative volume elements used for FEA, which agreed with experimental findings that this transition occurred at approximately 53 vol % porosity. This work demonstrates that the tandem use of rheometry, FEA, and DIW enables the design of complex, tailorable 3D printed porous structures with desired mechanical performance.

Heteropore covalent organic framework-based composite membrane prepared by in situ growth on non-woven fabric for sample pretreatment of food non-targeted analysis

A heteropore covalent organic framework (COF)-based composite membrane material was prepared and proved to have a satisfactory effect on the pretreatment of vegetable samples. The composite membrane was fabricated by in situ growth of a dual-pore COF on the surface of polydopamine (PDA)-aminated non-woven (NW) fabric. Due to the difference in the strength of the interaction between the phytochromes/COF and the pesticides/COF, the removal of phytochromes and the recovery of pesticides can be achieved by adjusting the composition of the solution. Through a simple immersion or filtration operation, NW@PDA@COF composite membrane can quickly and almost completely remove interfering phytochromes in the samples. The recovery of pesticides was determined by HPLC-MS/MS, and the recovery efficiencies were 72.3~101.7% and 67.3~106.7% for immersion and filtration modes of five different vegetable samples, respectively; the RSD is between 1.1 and 19% (n = 3). The limits of detection and quantification for the 13 pesticides investigated were 0.08 μg·L^-1 and 0.23 μg·L^-1, respectively. A wide linear range of 1~1000 μg·L^-1 was observed with R² values from 0.9774 to 0.9998. The membrane can be repeatedly used for at least 10 times by using a facile elution treatment. Compared to other commonly used sample pretreatment materials, heteropore COF-based composite membrane is superior in terms of sorbent amount, treatment time, operation simplicity, and material reusability.

Chemical Conversion and Locking of the Imine Linkage: Enhancing the Functionality of Covalent Organic Frameworks

Imine-based covalent organic frameworks (COFs) are a widely studied class of functional, crystalline, and porous nanostructures which combine a relatively facile crystallization with tuneable compositions and porosities. However, the imine linkage constitutes an intrinsic limitation due to its reduced stability in harsh chemical conditions and its unsuitability for in-plane π-conjugation in COFs. Urgent solutions are therefore required in order to exploit the full potential of these materials, thereby enabling their technological application in electronics, sensing, and energy storage devices. In this context, the advent of a new generation of linkages derived from the chemical conversion and locking of the imine bond represents a cornerstone for the synthesis of new COFs. A marked increase in the framework robustness is in fact often combined with the incorporation of novel functionalities including, for some of these reactions, an extension of the in-plane π-conjugation. This Minireview describes the most enlightening examples of one-pot reactions and post-synthetic modifications towards the chemical locking of the imine bond in COFs.

Recent Advances in 3D Printing of Structured Materials for Adsorption and Catalysis Applications

Porous solids in the form of adsorbents and catalysts play a crucial role in various industrially important chemical, energy, and environmental processes. Formulating them into structured configurations is a key step toward their scale up and successful implementation at the industrial level. Additive manufacturing, also known as 3D printing, has emerged as an invaluable platform for shape engineering porous solids and fabricating scalable configurations for use in a wide variety of separation and reaction applications. However, formulating porous materials into self-standing configurations can dramatically affect their performance and consequently the efficiency of the process wherein they operate. Toward this end, various research groups around the world have investigated the formulation of porous adsorbents and catalysts into structured scaffolds with complex geometries that not only exhibit comparable or improved performance to that of their powder parents but also address the pressure drop and attrition issues of traditional configurations. In this comprehensive review, we summarize the recent advances and current challenges in the field of adsorption and catalysis to better guide the future directions in shape engineering solid materials with a better control on composition, structure, and properties of 3D-printed adsorbents and catalysts.

Preparative separation and purification of loliolide and epiloliolide from Ascophyllum nodosum using amine-based microporous organic polymer for solid phase extraction coupled with macroporous resin and prep-HPLC

Herein, we reported a novel approach for the preparative separation and purification of loliolide and epiloliolide from Ascophyllum nodosum. An amine-based microporous organic polymer (MOP) was used for the pretreatment of the nodosum extract via solid-phase extraction (SPE). The obtained extract was further purified using macroporous resin chromatography and preparative high-performance liquid chromatography (prep-HPLC). The loading and elution parameters of the MOP were evaluated using standard loliolide, and the optimized conditions were used during the SPE of the nodosum extract (37.5 g). After the pretreatment with MOP, the extract (2.79 g) was obtained and further purified using a D101 resin column followed by prep-HPLC. A pair of epimers were isolated and identified as loliolide (5.83 mg) and epiloliolide (2.50 mg) using high-resolution electrospray ionization tandem mass spectrometry (HRESI-MS), 1D- and 2D-nuclear magnetic resonance (NMR) spectroscopy. This study demonstrates the potential of MOPs in the separation and purification of monoterpenoids from complex plant samples.

Construction of Flexible Amine-linked Covalent Organic Frameworks by Catalysis and Reduction of Formic Acid via the Eschweiler-Clarke Reaction

Compared to the current mainstream rigid covalent organic frameworks (COFs) linked by imine bonds, flexible COFs have certain advantages of elasticity and self-adaptability, but their construction and application are greatly limited by the complexity in synthesis and difficulty in obtaining regular structure. Herein, we reported for the first time a series of flexible amine-linked COFs with high crystallinity synthesized by formic acid with unique catalytic and reductive bifunctional properties, rather than acetic acid, the most common catalyst for COF synthesis. The reaction mechanism was demonstrated to be a synchronous in situ reduction during the formation of imine bond. The flexibilities of the products endow them with accommodative adaptability to guest molecules, thus increasing the adsorption capacities for nitrogen and iodine by 27 % and 22 %, respectively. Impressively, a novel concept of flexibilization degree was proposed firstly, which provides an effective approach to rationally measure the flexibility of COFs.

Fabricating Nanosheets and Ratiometric Detection of 5-Fluorouracil by Covalent Organic Framework Hybrid Material

Covalent organic framework (COF) nanosheets (NSs) are a new member in the family of two-dimensional (2D) nanomaterials that received increasing attention. The ability to prepare COF NSs with rapid acquisition is of great importance to explore their distinctive properties and potential applications. Herein, we elaborate design a new COF hybrid material EB-TFP:Eu(BTA)₄ as a sensing platform. In the process of ratiometric fluorescence detection of 5-fluorouracil (5FU), an anticancer drug, we realize the preparation of COF NSs. Interaction occurs between 5FU and COF hybrid material, where the interlayer π-π stacking of COF was weakened, benefiting the exfoliation of bulk COF to acquire 2D COF NSs. This strategy provides not only a sensitive and selective 5FU sensor but also a significant inspiration for engineering 2D COF NSs.

Processing-Structure-Performance Relationships of Microporous Metal-Organic Polymers for Size-Selective Separations

Small-molecule impurities, such as N-nitrosodimethylamine (NDMA), have infiltrated the generic drug industry, leading to recalls in commonly prescribed blood pressure and stomach drugs in over 43 countries since 2018 and directly affecting tens of millions of patients. One promising strategy to remove small-molecule impurities like NDMA from drug molecules is by size exclusion, in which the contaminant is removed by selective adsorption onto a (micro)porous material due to its smaller size. However, current solution-phase size-exclusion separations are primarily limited by the throughput-selectivity trade-off. Here, we report a bioinspired solution to conquer these critical challenges by leveraging the assembly of atomically precise building blocks into hierarchically porous structures. We introduce a bottom-up approach to form micropores, mesopores, and macroscopic superstructures simultaneously using functionalized oxozirconium clusters as building blocks. Further, we leverage recent advances in photopolymerization to design macroscopic flow structures to mitigate backpressure. Based on these multiscale design principles, we engineer simple, inexpensive devices that are able to separate NDMA from contaminated drugs. Beyond this urgent model system, we expect this design strategy to open up hitherto unexplored avenues of nanomaterial superstructure fabrication for a range of size-exclusion purification strategies.

Three-dimensional graphene encapsulated Ag-ZnFe2O4 flower-like nanocomposites with enhanced photocatalytic degradation of enrofloxacin

Three-dimensional (3D) Ag–ZnFe₂O₄-reduced graphene oxide (rGO) was successfully synthesized using a hydrothermal and photo-reduction method, and the morphological differences of the materials were observed. Their photocatalytic activity was evaluated by photocatalytic degradation of enrofloxacin (ENR) under visible-light irradiation. The results indicated that Ag–ZnFe₂O₄–rGO exhibited superior photocatalytic properties and good stability. In this research, the enhancement of photocatalytic performance is mainly attributed to the electron channelization ability of rGO, which traps the photoexcited electrons of ZnFe₂O₄ on its π framework, and reduces the electron–hole recombination rate. Moreover, the high surface area of 3D pompon mum flower-like ZnFe₂O₄ provides more reactive sites. In addition, free radical capture and ESR experiments as well as pathway analysis of degradation also confirmed that superoxide radicals (˙O₂⁻) and photo-generated holes from Ag–ZnFe₂O₄–rGO were the main active species in the degradation progress of ENR.

Three-dimensional (3D) Ag–ZnFe₂O₄-reduced graphene oxide (rGO) was successfully synthesized using a hydrothermal and photo-reduction method, and the morphological differences of the materials were observed.

Covalent organic frameworks: an ideal platform for designing ordered materials and advanced applications

Covalent organic frameworks offer a molecular platform for integrating organic units into periodically ordered yet extended 2D and 3D polymers to create topologically well-defined polygonal lattices and built-in discrete micropores and/or mesopores.

Emerging new-generation hybrids based on covalent organic frameworks for industrial applications

This review highlights the advancement of COF hybrid-based materials for diverse industrial applications.

MOF/COF-based materials using 3D printing technology: applications in water treatment, gas removal, biomedical, and electronic industries

We have considered the newest outcomes in the uses of 3D-printed COF- and MOF-based materials for diverse applications.

Covalent organic frameworks (COFs) for electrochemical applications

Covalent organic frameworks are a class of extended crystalline organic materials that possess unique architectures with high surface areas and tuneable pore sizes. Since the first discovery of the topological frameworks in 2005, COFs have been applied as promising materials in diverse areas such as separation and purification, sensing or catalysis. Considering the need for renewable and clean energy production, many research efforts have recently focused on the application of porous materials for electrochemical energy storage and conversion. In this respect, considerable efforts have been devoted to the design and synthesis of COF-based materials for electrochemical applications, including electrodes and membranes for fuel cells, supercapacitors and batteries. This review article highlights the design principles and strategies for the synthesis of COFs with a special focus on their potential for electrochemical applications. Recently suggested hybrid COF materials or COFs with hierarchical porosity will be discussed, which can alleviate the most challenging drawback of COFs for these applications. Finally, the major challenges and future trends of COF materials in electrochemical applications are outlined.

Beyond Frameworks: Structuring Reticular Materials across Nano-, Meso-, and Bulk Regimes

Reticular materials are of high interest for diverse applications, ranging from catalysis and separation to gas storage and drug delivery. These open, extended frameworks can be tailored to the intended application through crystal-structure design. Implementing these materials in application settings, however, requires structuring beyond their lattices, to interface the functionality at the molecular level effectively with the macroscopic world. To overcome this barrier, efforts in expressing structural control across molecular, nano-, meso-, and bulk regimes is the essential next step. In this Review, we give an overview of recent advances in using self-assembly as well as externally controlled tools to manufacture reticular materials over all the length scales. We predict that major research advances in deploying these two approaches will facilitate the use of reticular materials in addressing major needs of society.