Precise Modulation of Carbon Activity Sites in Metal-Free Covalent Organic Frameworks for Enhanced Oxygen Reduction Electrocatalysis.

Metal-free carbon-based materials have gained recognition as potential electrocatalysts for the oxygen reduction reaction (ORR) in new environmentally-friendly electrochemical energy conversion technologies. The presence of effective active centers is crucial for achieving productive ORR. In this study, we present the synthesis of two metal-free dibenzo[a,c]phenazine-based covalent organic frameworks (DBP-COFs), specifically JUC-650 and JUC-651, which serve as ORR electrocatalysts. Among them, JUC-650 demonstrates exceptional catalytic performance for ORR in alkaline electrolytes, exhibiting an onset potential of 0.90 V versus RHE and a half-wave potential of 0.72 V versus RHE. Consequently, JUC-650 stands out as one of the most outstanding metal-free COF-based ORR electrocatalysts report to date. Experimental investigations and density functional theory calculations confirm that modulation of the frameworks' electronic configuration allows for the reduction of adsorption energy at the Schiff-base carbon active sites, leading to more efficient ORR processes. Moreover, the DBP-COFs can be assembled as excellent air cathode catalysts for zinc-air batteries (ZAB), rivaling the performance of commercial Pt/C. This study provides valuable insights for the development of efficient metal-free organoelectrocatalysts through precise regulation of active site strategies.

Amphiphilic Covalent Organic Framework Nanoparticles for Pickering Emulsion Catalysis with Size Selectivity.

Exploiting advanced amphiphilic solid catalysts is crucial to the development of Pickering emulsion catalysis. Herein, covalent organic framework (COF) nanoparticles constructed with highly hydrophobic monomers as linkers were found to show superior amphiphilicity and they were then developed as a new class of solid emulsifiers for Pickering emulsion catalysis. Employing amphiphilic COFs as solid emulsifiers, Pickering emulsions with controllable emulsion type and droplet sizes were obtained. COF materials have also been demonstrated to serve as porous surface coatings to replace traditional surface modifications for stabilizing Pickering emulsions. After implanting Pd nanoparticles into amphiphilic COFs, the obtained catalyst displayed a 3.9 times higher catalytic efficiency than traditional amphiphilic solid catalysts with surface modifications in the biphasic oxidation reaction of alcohols. Such an enhanced activity was resulted from the high surface area and regular porous structure of COFs. More importantly, because of their tunable pore diameters, Pickering emulsion catalysis with remarkable size selectivity was achieved. This work is the first example that COFs were applied in Pickering emulsion catalysis, providing a platform for exploring new frontiers of Pickering emulsion catalysis.

Piezofluorochromism in Covalent Organic Frameworks: Pressure-induced Emission Enhancement and Blue-Shifted Emission.

Achieving enhanced or blue-shifted emission from piezochromic materials remains a major challenge. Covalent organic frameworks (COFs) are promising candidates for the development of piezochromic materials owing to their dynamic structures and adjustable optical properties, where the emission behaviors are not solely determined by the functional groups, but are also greatly influenced by the specific geometric arrangement. Nevertheless, this area remains relatively understudied. In this study, a successful synthesis of a series of bicarbazole-based COFs with varying topologies, dimensions, and linkages was conducted, followed by an investigation of their structural and emission properties under hydrostatic pressure generated by a diamond anvil cell. Consequently, these COFs exhibited distinct piezochromic behaviors, particularly a remarkable pressure-induced emission enhancement (PIEE) phenomenon with a 16-fold increase in fluorescence intensity from three-dimensional COFs, surpassing the performance of CPMs and most organic small molecules with PIEE behavior. On the contrary, three two-dimensional COFs with flexible structures exhibited rare blue-shifted emission, whereas the variants with rigid and conjugated structures showed common red-shifted and reduced emission. Mechanism research further revealed that these different piezochromic behaviors were primarily determined by interlayer distance and interaction.

Structural Modulation of Covalent Organic Frameworks for Efficient Hydrogen Peroxide Electrocatalysis.

The electrochemical production of hydrogen peroxide (H2O2) using metal-free catalysts has emerged as a viable and sustainable alternative to the conventional anthraquinone process. However, the precise architectural design of these electrocatalysts poses a significant challenge, requiring intricate structural engineering to optimize electron transfer during the oxygen reduction reaction (ORR). Herein, we introduce a novel design of covalent organic frameworks (COFs) that effectively shift the ORR from a four-electron to a more advantageous two-electron pathway. Notably, the JUC-660 COF, with strategically charge-modified benzyl moieties, achieved a continuous high H2O2 yield of over 1200 mmol g-1 h-1 for an impressive duration of over 85 hours in a flow cell setting, marking it as one of the most efficient metal-free and non-pyrolyzed H2O2 electrocatalysts reported to date. Theoretical computations alongside in-situ infrared spectroscopy indicate that JUC-660 markedly diminishes the adsorption of the OOH* intermediate, thereby steering the ORR towards the desired pathway. Furthermore, the versatility of JUC-660 was demonstrated through its application in the electro-Fenton reaction, where it efficiently and rapidly removed aqueous contaminants. This work delineates a pioneering approach to altering the ORR pathway, ultimately paving the way for the development of highly effective metal-free H2O2 electrocatalysts.

Three-Dimensional Covalent Organic Frameworks with Ultra-Large Pores for Highly Efficient Photocatalysis.

Benefiting from their unique structural merits, three-dimensional (3D) large-pore COF materials demonstrate high surface areas and interconnected large channels, which makes these materials promising in practical applications. Unfortunately, functionalization strategies and application research are still absent in these structures. To this end, a series of functional 3D stp-topologized COFs are designed based on porphyrin or metalloporphyrin moieties, named JUC-640-M (M = Co, Ni, or H). Interestingly, JUC-640-H exhibits a record-breaking low crystal density (0.106 cm3 g-1) among all crystalline materials, along with the largest interconnected pore size (4.6 nm) in 3D COFs, high surface area (2204 m2 g-1), and abundant exposed porphyrin moieties (0.845 mmol g-1). Inspired by the unique structural characteristics and photoelectrical performance, JUC-640-Co is utilized for the photoreduction of CO2 to CO and demonstrates a high CO production rate (15.1 mmol g-1 h-1), selectivity (94.4%), and stability. It should be noted that the CO production rate of JUC-640-Co has exceeded those of all reported COF-based materials. This work not only produces a series of novel 3D COFs with large channels but also provides a new guidance for the functionalization and applications of COFs.

Topological Isomerism in Three-Dimensional Covalent Organic Frameworks.

Although isomerism is a typical and significant phenomenon in organic chemistry, it is rarely found in covalent organic framework (COF) materials. Herein, for the first time, we report a controllable synthesis of topological isomers in three-dimensional COFs via a distinctive tetrahedral building unit under different solvents. Based on this strategy, both isomers with a dia or qtz net (termed JUC-620 and JUC-621) have been obtained, and their structures are determined by combining powder X-ray diffraction and transmission electron microscopy. Remarkably, these architectures show a distinct difference in their porous features; for example, JUC-621 with a qtz net exhibits permanent mesopores (up to ∼23 Å) and high surface area (∼2060 m2 g-1), which far surpasses those of JUC-620 with a dia net (pore size of ∼12 Å and surface area of 980 m2 g-1). Furthermore, mesoporous JUC-621 can remove dye molecules efficiently and achieves excellent iodine adsorption (up to 6.7 g g-1), which is 2.3 times that of microporous JUC-620 (∼2.9 g g-1). This work thus provides a new way for constructing COF isomers and promotes structural diversity and promising applications of COF materials.

Functional Regulation and Stability Engineering of Three-Dimensional Covalent Organic Frameworks.

ConspectusAs one of the most attractive members in the porous materials family, covalent organic frameworks (COFs) have been reported thousands of times since their first discovery in 2005, covering their design, synthesis, and applications. However, an overwhelming majority of these COFs are based on two-dimensional (2D) topologies while three-dimensional (3D) COFs are numbered fewer than 100 up to date. In fact, baring enhanced specific surface area, interconnected channels, well-exposed functional moieties, and highly adjustable structures, 3D COFs are often more competitive in various application fields like adsorption, separation, chemical sensing, and heterogeneous catalysis compared with their 2D counterparts. However, significant crystallization problems and poor chemical stabilities, which might be attributed to the highly void frameworks and the absence of π-π stacking, have raised severe limitations over the research and application of 3D COFs. To solve these problems, more elaborate synthesis regulations or more moderate functionalization conditions are required. More importantly, the strategies for enhancing chemical stabilities of 3D COFs are of vital importance for their further development and practical applications.In this Account, we review the design principles, functional approaches, and stability regulation methods toward functional 3D COFs. We begin the discussion with some essential elements in the construction of 3D COF structures, including topologies, interpenetrations, linkages, and synthetic methods. After that, we focus on several strategies for the functionalization of 3D COFs, including in situ approaches (utilizing in situ generated COF linkages as the active sites), bottom-up synthesis (embedding functional moieties from predesigned building blocks), and postsynthesis modification (covalent modification or metalation of pristine frameworks). At last, we highlight some approaches toward the durable amplification of 3D COFs, which is highly important for framework functionalization and practical application. This target could be achieved through not only the introduction of some extra strengthening force, such as hydrophobic effects, coulomb repulsion, and steric hindrance effects, but also the utilization of robust linkages, which could enhance the stability from material nature.Due to their high surface area, various interpenetrated channels, multifarious functionalities, and promising stabilities, 3D COFs demonstrated excellent performance and have great potential in a wide range of application fields including adsorption and separation, heterogeneous catalysis, energy storage, and so on. Although the development of these materials has been limited by serious crystallization problems and stability restriction, great efforts have been devoted by researchers in the past decade, and a mass of strategies have been developed in synthesis control, functionalization regulation, and stability enhancement for 3D COFs. We expect 3D COFs to be practically utilized in the future with further advances in the design, preparation, and functionalization of these materials.

Quantitative Assessment of Crystallinity and Stability in ß-Ketoenamine-Based Covalent Organic Frameworks.

The design and synthesis of covalent organic frameworks (COFs) with high chemical stability pose significant challenges for practical applications. Although a growing number of robust COFs have been developed and employed for a broad scope of applications, the assessment of COF stability has primarily relied on qualitative descriptions, lacking a rational and quantitative assessment. Herein, a novel assessment method is presented that enables visual and quantitative depiction of COF stability. By analyzing the PXRD patterns of chemically stable ß-ketoenamine-based COFs (KEA-COFs), two crystallinity-dependent parameters are identified, the relative intensity (I2θrel ) and the relative area (A2θrel ) of the main peak (2θ), which are expected to establish a standardized criterion for assessing COF crystallinity. Based on these parameters, the crystalline changes after stability tests can be visually presented, which provides a rational and quantitative assessment of their stability. This study not only demonstrates the remarkable chemical stability of KEA-COFs, but also provides valuable insights into the quantitative evaluation of COFs' crystallinity and stability.

Room-Temperature Preparation of Covalent Organic Framework Membrane for Nanofiltration.

The uniquely tunable nature of covalent organic frameworks (COFs), whose pore size and stability can be controlled by choosing diverse organic building blocks and linkage types, makes COFs potential candidates for the membrane separation. Therefore, the preparation of membranes with effective separation efficiency based on COFs has aroused great interest among researchers. Although solvothermal approach has been the most popular method for the preparation of COF membranes, fabricating COF membranes at room temperature (RT) will provide a simple and captivating strategy for separation membranes. Herein, a P-COF membrane on porous alumina substrate at RT, showing 99.7% rejection of rhodamine B and excellent water permeance up to 52 L m-2 h-1 bar-1 , which can effectively purify wastewater is successfully obtained. P-COF is directly grown on alumina to form the composite membrane, which enhances the mechanical strength of COF membrane and avoids the risk of damaging the membrane structure during the transfer process of self-standing membrane. Moreover, P-COF membrane is grown at RT, which is more energy efficient than the conventional solvothermal method. Thus, it is of great significance to obtain COF membranes with excellent nanofiltration performance in a simple and mild condition to alleviate environmental and energy concerns.

Three-Dimensional Covalent Organic Frameworks: From Synthesis to Applications.

Three-dimensional covalent organic frameworks (3D COFs) with spatially periodic networks demonstrate significant advantages over their 2D counterparts, including enhanced specific surface areas, interconnected channels, and more sufficiently exposed active sites. Nevertheless, research on these materials has met an impasse due to serious problems in crystallization and stability, which must be solved for practical applications. In this Minireview, we first summarize some strategies for preparing functional 3D COFs, including crystallization techniques and functionalization methods. Hereafter, applications of these functional materials are presented, covering adsorption, separation, catalysis, fluorescence, sensing, and batteries. Finally, the future challenges and perspectives for the development of 3D COFs are discussed.

Piezochromism in Dynamic Three-Dimensional Covalent Organic Frameworks.

Piezochromic materials with pressure-dependent photoluminescence tuning properties are important in many fields, such as mechanical sensors, security papers, and storage devices. Covalent organic frameworks (COFs), as an emerging class of crystalline porous materials (CPMs) with structural dynamics and tunable photophysical properties, are suitable for designing piezochromic materials, but there are few related studies. Herein, we report two dynamic three-dimensional COFs based on aggregation-induced emission (AIE) or aggregation-caused quenching (ACQ) chromophores, termed JUC-635 and JUC-636 (JUC=Jilin University China), and for the first time, study their piezochromic behavior by diamond anvil cell technique. Due to the various luminescent groups, JUC-635 has completely different solvatochromism and molecular aggregation behavior in the solvents. More importantly, JUC-635 with AIE effect exhibits a sustained fluorescence upon pressure increase (≈3 GPa), and reversible sensitivity with high-contrast emission differences (Δλem =187 nm) up to 12 GPa, superior to other CPMs reported so far. Therefore, this study will open a new gate to expand the potential applications of COFs as exceptional piezochromic materials in pressure sensing, barcoding, and signal switching.

Amorphous-to-Crystalline Transformation: General Synthesis of Hollow Structured Covalent Organic Frameworks with High Crystallinity.

Morphological control of covalent organic frameworks (COFs) is particularly interesting to boost their applications; however, it remains a grand challenge to prepare hollow structured COFs (HCOFs) with high crystallinity and uniform morphology. Herein, we report a versatile and efficient strategy of amorphous-to-crystalline transformation for the general and controllable fabrication of highly crystalline HCOFs. These HCOFs exhibited ultrahigh surface areas, radially oriented nanopore channels, quite uniform morphologies, and tunable particle sizes. Mechanistic studies revealed that H2O, acetic acid, and solvent played a crucial role in manipulating the hollowing process and crystallization process by regulating the dynamic imine exchange reaction. Our approach was demonstrated to be applicable to various amines and aldehydes, producing up to 10 kinds of HCOFs. Importantly, based on this methodology, we even constructed a library of unprecedented HCOFs including HCOFs with different pore structures, bowl-like HCOFs, cross-wrinkled COF nanocapsules, grain-assembled HCOFs, and hydrangea-like HCOFs. This strategy was also successfully applied to the fabrication of COF-based yolk-shell nanostructures with various functional interior cores. Furthermore, catalytically active metal nanoparticles were implanted into the hollow cavities of HCOFs with tunable pore diameters, forming attractive size-selective nanoreactors. The obtained metal@HCOFs catalysts showed enhanced catalytic activity and outstanding size-selectivity in hydrogenation of nitroarenes. This work highlights the significance of nucleation-growth kinetics of COFs in tuning their morphologies, structures, and applications.

Chemically Stable Guanidinium Covalent Organic Framework for the Efficient Capture of Low-Concentration Iodine at High Temperatures.

The capture of radioactive I2 vapor from nuclear waste under industrial operating conditions remains a challenging task, as the practical industrial conditions of high temperature (≥150 °C) and low I2 concentration (∼150 ppmv) are unfavorable for I2 adsorption. We report a novel guanidinium-based covalent organic framework (COF), termed TGDM, which can efficiently capture I2 under industrial operating conditions. At 150 °C and 150 ppmv I2, TGDM exhibits an I2 uptake of ∼30 wt %, which is significantly higher than that of the industrial silver-based adsorbents such as Ag@MOR (17 wt %) currently used in the nuclear fuel reprocessing industry. Characterization and theoretical calculations indicate that among the multiple types of adsorption sites in TGDM, only ionic sites can bond to I2 through strong Coulomb interactions under harsh conditions. The abundant ionic groups of TGDM account for its superior I2 capture performance compared to various benchmark adsorbents. In addition, TGDM exhibits exceptionally high chemical and thermal stabilities that fully meet the requirements of practical radioactive I2 capture (high-temperature, humid, and acidic environment) and differentiate it from other ionic COFs. Furthermore, TGDM has excellent recyclability and low cost, which are unavailable for the current industrial silver-based adsorbents. These advantages make TGDM a promising candidate for capturing I2 vapor during nuclear fuel reprocessing. This strategy of incorporating chemically stable ionic guanidine moieties in COF would stimulate the development of new adsorbents for I2 capture and related applications.

Self-Stirring Microcatalysts: Large-Scale, High-Throughput, and Controllable Preparation and Application.

Herein, we introduce a strategy to develop a kind of unprecedented microcatalyst, which owns self-stirring and catalytic performance based on pneumatic printing and magnetic field induction technology. A spindle-shaped microcatalyst based on metal-organic frameworks (MOFs) with a certain aspect ratio and size can be obtained by tuning the printing parameters and the intensity of the magnetic field. One nozzle can print 18 000 microcatalysts per hour, which provides a prerequisite for the realization of large-scale production in the industrial field. Furthermore, this strategy can be widely applied to a variety of other heterogeneous catalysts, such as mesoporous SiO2, zeolite, metallic oxide, and so on. To demonstrate the superiority of the printed catalyst, the series of printed microcatalysts were evaluated by various catalytic reactions including liquid-phase hydrogenation, microdroplet dye-fading, and photocatalytic degradation in microreactor, all of which exhibited excellent catalytic performance.

Design and Synthesis of a Zeolitic Organic Framework.

The development of novel zeolite-like materials with large channel windows and high stability is of importance but remains a tremendous challenge. Herein, we report the first example of a 3D covalent organic framework with zeolitic network, namely the zeolitic organic framework (ZOF). By combining two kinds of tetrahedral building blocks with fixed or relatively free bond angles, ZOF-1 with the zeolitic crb net has been successfully synthesized. Its structure was determined by the single-crystal 3D electron diffraction technique. Remarkably, ZOF-1 shows high chemical stability, large pore size (up to 16 Å), and excellent specific surface area (≈2785 m2 g-1 ), which is superior to its analogues with the same network, including traditional aluminosilicate zeolites and zeolitic imidazole frameworks. This study thus opens a new avenue to construct zeolite-like materials with pure organic frameworks and will promote their potential applications in adsorption and catalysis for macromolecules.

Three-Dimensional Triptycene-Functionalized Covalent Organic Frameworks with hea Net for Hydrogen Adsorption.

Owing to the finite building blocks and difficulty in structural identification, it remains a tremendous challenge to elaborately design and synthesize three-dimensional covalent organic frameworks (3D COFs) with predetermined topologies. Herein, we report the first two cases of 3D COFs with the non-interpenetrated hea net, termed JUC-596 and JUC-597, by using the combination of tetrahedral and triangular prism building units. Due to the presence of triptycene functional groups and fluorine atoms, JUC-596 exhibits an exceptional performance in the H2 adsorption up to 305 cm3 g-1 (or 2.72 wt%) at 77 K and 1 bar, which is higher than previous benchmarks from porous organic materials reported so far. Furthermore, the strong interaction between H2 and COF materials is verified through the DFT theoretical calculations. This work represents a captivating example of rational design of functional COFs based on a reticular chemistry guide and demonstrates its promising application in clean energy storage.

Gating Effects for Ion Transport in Three-Dimensional Functionalized Covalent Organic Frameworks.

The development of bioinspired nano/subnano-sized (<2 nm) ion channels is still considered a great challenge due to the difficulty in precisely controlling pore's internal structure and chemistry. Herein, for the first time, we report that three-dimensional functionalized covalent organic frameworks (COFs) can act as an effective nanofluidic platform for intelligent modulation of the ion transport. By strategic attachment of 12-crown-4 groups to the monomers as ion-driver door locks, we demonstrate that gating effects of functionalized COFs can be activated by lithium ions. The obtained materials exhibit an outstanding selective ion transmission performance with a high gating ratio (up to 23.6 for JUC-590), which is among the highest values in metal ion-activated solid-state nanochannels reported so far. Furthermore, JUC-590 offers high tunability, selectivity, and recyclability of ion transport proved by the experimental and simulated studies.

Maximized Green Photoluminescence in Tb-Based Metal-Organic Framework via Pressure-Treated Engineering.

Lanthanide metal-organic frameworks are of great interest in the development of photoluminescence (PL) materials owing to their structural tunability and intrinsic features of lanthanide elements. However, there exists some limitations arising from poor matching with metal ions, thereby exhibiting a weak ligand-to-metal energy transfer (LMET) process. Here we demonstrate a pressure-treated strategy for achieving high PL performance in green-emitting Tb(BTC)(H2 O)6 . The PL quantum yield of pressure-treated sample increased from 50.6 % to 90.4 %. We found that the enhanced hydrogen bonds locked the conjugated configuration formed by two planes of carboxyl group and benzene ring, enabling the promoted intersystem crossing to effectively drive LMET. Moreover, the optimized singlet and triplet states also validated the facilitated LMET process. This work opens the opportunity of structure optimization to improve PL performance in MOFs by pressure-treated engineering.

Three-Dimensional Triptycene-Based Covalent Organic Frameworks with ceq or acs Topology.

The growth of three-dimensional covalent organic frameworks (3D COFs) with new topologies is still considered as a great challenge due to limited availability of high-connectivity building units. Here we report the design and synthesis of 3D triptycene-based COFs, termed JUC-568 and JUC-569, following the deliberate symmetry-guided design principle. By combining a triangular prism (6-connected) node with a planar triangle (3-connected) or another triangular prism node, the targeted COFs adopt non-interpenetrated ceq or acs topology, respectively. Both materials show permanent porosity and impressive performance in the adsorption of CO2 (∼98 cm3/g at 273 K and 1 bar), CH4 (∼48 cm3/g at 273 K and 1 bar), and especially H2 (up to 274 cm3/g or 2.45 wt % at 77 K and 1 bar), which is highest among porous organic materials reported to date. This research thus provides a promising strategy for diversifying 3D COFs based on complex building blocks and promotes their potential applications in energy storage and environment-related fields.

3D Thioether-Based Covalent Organic Frameworks for Selective and Efficient Mercury Removal.

Developing functionalized 3D covalent organic frameworks (3D COFs) is critical to broaden their potential applications. However, the introduction of specific functionality in 3D COFs remains a great challenge because most of the functional groups are not compatible with the synthesis conditions. Herein, for the first time 3D thioether-based COFs (JUC-570 and JUC-571) for mercury (Hg2+ ) removal from aqueous solution is reported. These 3D thioether-based COFs prepared by the bottom-up approach display high Hg2+ uptakes (972 mg g-1 for JUC-570 and 970 mg g-1 for JUC-571 at pH = 5), fast adsorption kinetics (distribution coefficient Kd value of 2.29 × 107  mL g-1 for JUC-570 and 2.07 × 107  mL g-1 for JUC-571), and favorable selectivity. In particular, JUC-570 is periodically decorated with isopropyl groups around imine bonds that markedly improve its chemical stability and effectively prevent the pore collapse, and thus endows high Hg2+ adsorption capacity (619 mg g-1 ) and excellent cycle performance even at pH = 1. This study not only puts forward a new route to construct stable functionalized 3D COFs, but also promotes their potential applications in areas related to the environment.