Construction Of Covalent Organic Frameworks Research Articles

Azobenzene-Bridged Covalent Organic Frameworks Boosting Photocatalytic Hydrogen Peroxide Production from Alkaline Water: One Atom Makes a Significant Improvement.

Covalent organic frameworks (COFs) have been demonstrated as promising photocatalysts for hydrogen peroxide (H2O2) production. However, the construction of COFs with new active sites, high photoactivity, and wide-range light absorption for efficient H2O2 production remains challenging. Herein, we present the synthesis of a novel azobenzene-bridged 2D COF (COF-TPT-Azo) with excellent performance on photocatalytic H2O2 production under alkaline conditions. Notably, although COF-TPT-Azo differs by only one atom (-N=N- vs. -C=N-) from its corresponding imine-linked counterpart (COF-TPT-TPA), COF-TPT-Azo exhibits a significantly narrower band gap, enhanced charge transport, and prompted photoactivity. Remarkably, when employed as a metal-free photocatalyst, COF-TPT-Azo achieves a high photocatalytic H2O2 production rate up to 1498 μmol g-1 h-1 at pH = 11, which is 7.9 times higher than that of COF-TPT-TPA. Further density functional theory (DFT) calculations reveal that the -N=N- linkages are the active sites for photocatalysis. This work provides new prospects for developing high-performance COF-based photocatalysts.

Mesoporous Vinylene-Linked Covalent Organic Frameworks with Heteroatom-Tuned Crystallinity and Photocatalytic Behaviors.

Owing to its prominent π-delocalization and stability, vinylene linkage holds great merits in the construction of covalent organic frameworks (COFs) with promising semiconducting properties. However, carbon-carbon double bond formation reaction always exhibits relatively low reversibility, unfavorable for the formation of high crystalline frameworks through self-error correction and assembling processes. In this work, we report a heteroatom-tuned strategy to build up a series of two-dimensional (2D) vinylene-linked COFs by Knoevenagel condensation of an electron-deficient methylthiazolyl-based monomer with different triformyl substituted (hetero-)aromatic derivatives. The resulting COFs show high-quality periodic mesoporous structures with high surface areas. Embedding heteroatoms into the backbones enables significantly improving their crystallinity, and finely tailoring their semiconducting structures. Upon visible light stimulation, one of the as-prepared COFs with donor-π-acceptor structure could deliver a nearly seven-fold increase in the catalytic activity of hydrogen generation as compared with the other two. Meanwhile, in combination with high crystallinity and the matched conduction band energy level, such kind of COFs can be able to selectively generate singlet oxygen and superoxide radicals in a high ratio of up to 30:1, allowing for catalyzing aerobic thioanisole oxidation in distinctly tunable activities through the substituent electronic effect of the substrates.

Mesoporous Vinylene‐Linked Covalent Organic Frameworks with Heteroatom‐Tuned Crystallinity and Photocatalytic Behaviors

AbstractOwing to its prominent π‐delocalization and stability, vinylene linkage holds great merits in the construction of covalent organic frameworks (COFs) with promising semiconducting properties. However, carbon‐carbon double bond formation reaction always exhibits relatively low reversibility, unfavorable for the formation of high crystalline frameworks through self‐error correction and assembling processes. In this work, we report a heteroatom‐tuned strategy to build up a series of two‐dimensional (2D) vinylene‐linked COFs by Knoevenagel condensation of an electron‐deficient methylthiazolyl‐based monomer with different triformyl substituted (hetero−)aromatic derivatives. The resulting COFs show high‐quality periodic mesoporous structures with high surface areas. Embedding heteroatoms into the backbones enables significantly improving their crystallinity, and finely tailoring their semiconducting structures. Upon visible light stimulation, one of the as‐prepared COFs with donor‐π‐acceptor structure could deliver a nearly seven‐fold increase in the catalytic activity of hydrogen generation as compared with the other two. Meanwhile, in combination with high crystallinity and the matched conduction band energy level, such kind of COFs can be able to selectively generate singlet oxygen and superoxide radicals in a high ratio of up to 30 : 1, allowing for catalyzing aerobic thioanisole oxidation in distinctly tunable activities through the substituent electronic effect of the substrates.

Construction of Covalent organic frameworks with bex topology for efficient photocatalytic hydrogen production

Covalent organic frameworks (COFs) have become effective platforms for solar energy conversion and utilization. The rational design of the COF topology provides an effective strategy to expand the π-conjugation system and improve the photocatalytic activity. Herein, we have prepared two sub-stoichiometric pyrene-based 2D-COFs with the bex topology, HITMS-COF-10 and HITMS-COF-11, with high crystallinity and specific surface area. The HITMS-COF-11 exhibits an outstanding photocatalytic hydrogen evolution reaction (HER) rate of 25000 μmol g−1 h−1 in water under visible illumination (λ > 420 nm), demonstrating that the bex topological COFs could be one of the state-of-the-art photocatalyst candidates for hydrogen evolution and the importance of tuning the topological diversity at molecular levels for solar energy conversion and utilization.

Tailoring topology and bio-interactions of triazine frameworks

The construction of covalent organic frameworks with special geometery and optical properties is of high interest, due to their unique physicochemical and biological properties. In this work, we report on a new method for the construction of triazine frameworks with defined topologies using coordination chemistry. Ball milling and wet chemical reactions between cyanuric chloride and melamine were directed in spatial arrangements and opposite optical activity. Cobalt was used as a directing agent to drive reactions into special morphologies, optical properties and biological activity. The enantiorecognition ability of triazine frameworks that was manifested in their activities against bacteria, demonstrated a new way for the construction of materials with specific interactions at biointerfaces.

Construction of covalent organic framework and g-C3N4 heterojunction for photocatalytic degradation of tetracycline and photocatalytic production of hydrogen peroxide

Organic semiconductors, notably graphitic carbon nitride (g-C3N4, CN), are increasingly recognized as effective photocatalysts for environmental remediation and energy harvesting. In this study, we introduce a Van der Waals metal-free heterojunction (TTO/CN), composed of g-C3N4 and a vinyl-linked covalent organic framework (COF), fabricated via a simple self-assembly process. This novel heterojunction is adeptly utilized for the photocatalytic degradation of tetracycline (TC) in aqueous environments and for the generation of hydrogen peroxide (H2O2). The synthesized TTO/CN heterojunction demonstrates an expanded specific surface area and broadened absorption spectrum in the visible light range, coupled with enhanced efficiency in photogenerated carrier separation and transfer. These attributes collectively contribute to its remarkable photocatalytic performance. Notably, the TTO/CN-1 variant emerges as a highly efficient photocatalyst, exhibiting degradation rate constants for TC that surpass those of TTO-COF and CN by 2.0 and 6.2 times, respectively. Furthermore, TTO/CN-1 distinguishes itself as a dual-functional photocatalyst, showcasing exceptional activity in H2O2 production (745.3 μM·h−1, or 1863.3 μmol·g−1·h−1). This research heralds a potent strategy for the design and synthesis of van der Waals heterojunction photocatalysts, and the distinctive heterostructures of COF and CN hold significant promise for enhancing the performance and broadening the applicability of CN-based photocatalysts.

Constructing ordered and tunable extrinsic porosity in covalent organic frameworks via water-mediated soft-template strategy

As one of the most attractive methods for the synthesis of ordered hierarchically porous crystalline materials, the soft-template method has not appeared in covalent organic frameworks (COFs) due to the incompatibility of surfactant self-assembly and guided crystallization process of COF precursors in the organic phase. Herein, we connect the soft templates to the COF backbone through ionic bonds, avoiding their crystallization incompatibilities, thus introducing an additional ordered arrangement of soft templates into the anionic microporous COFs. The ion exchange method is used to remove the templates while maintaining the high crystallinity of COFs, resulting in the construction of COFs with ordered hierarchically micropores/mesopores, herein named OHMMCOFs (OHMMCOF-1 and OHMMCOF-2). OHMMCOFs exhibit significantly enhanced functional group accessibility and faster mass transfer rate. The extrinsic porosity can be adjusted by changing the template length, concentration, and ratio. Cationic guanidine-based COFs (OHMMCOF-3) are also constructed using the same method, which verifies the scalability of the soft-template strategy. This work provides a path for constructing ordered and tunable extrinsic porosity in COFs with greatly improved mass transfer efficiency and functional group accessibility.

Novel three-dimensional fibrous covalent organic frameworks constructed via silver amalgam bridging for efficient organic dye adsorption and removal.

The construction of covalent organic frameworks (COFs) with unique structures has great significance in exploring the structure-function relationship and extending their potential applications. Fibrous COFs have demonstrated superior performance in specific application scenarios owing to the distinctive three-dimensional (3D) structure. Herein, we report a facile strategy for the fabrication of 3D COF nanofiber by exploiting silver amalgam as a bridging agent to assemble one-dimensional-extended PA-COF modules into a tubular structure. Dimensions of the obtained 3D COF nanofiber were predicted by DFT calculations, and the nanofiber was endowed with the merits of favorable uniformity and high stability. Due to the enhanced exposure of conjugatable binding sites for dye retention offered by the novel 3D architecture, the PA-COF nanofiber exhibits fast adsorption (within 5 min) and superior adsorption capacity to various organic dyes, e.g., 1717 mg g-1 for methylene blue (MB) and 978.3 mg g-1 for methyl orange (MO). Moreover, the PA-COF nanofiber shows excellent reusability in dye adsorption, which makes it a potential medium for removing dye pollutants from wastewater. This work presents an effective strategy to construct COF materials with unique architecture and potential prospects in the fields of separation and wastewater treatment.

Construction of covalent organic frameworks via the Mannich reaction at room temperature for light-driven oxidative hydroxylation of arylboronic acids.

An increasing variety of organic reactions have been developed for the synthesis of more structurally stable and multifunctional COFs. Here, we report a class of β-ketamine linked covalent organic frameworks that were constructed through the CeCl3-catalyzed multi-component Mannich reaction at room temperature. And the TAD-COF obtained based on this method could significantly promote the light-driven oxidative hydroxylation of arylboronic acids.

Construction of covalent organic framework functionalized with carbene dual active sites for enhancing CO2 carboxylation conversion

It is challenging to drive the integral insertion of linear CO2 with thermodynamic stability and kinetic inertness as carboxylation reagents into C-H, O-H, and other chemical bonds. The known strategies mainly focus on unilaterally activating substrates as nucleophilic reagents, and the CO2 insertion process has to overcome high reaction energy barriers. Herein, this work reports copper carbene NHC-CuCl and CO2 adduct NHC-CO2 as dual active sites are introduced into the porous crystal covalent organic framework to effectively drive the simultaneous activation of substrate and CO2. The catalyst COF@NHC-CuCl/CO2 exhibited high space–time yield (STY = 71.9 h−1 at 40 °C and 36.7 h−1 at 25 °C, ambient pressure) in the carboxylative coupling of propargylic alcohols, which is superior to most of the cutting-edge heterogeneous catalysts. Experimental data and density functional theory calculation revealed that the carboxylation reaction involving NHC-CO2 and ordinary CO2 source follows different reaction mechanisms. The free energy barrier of the CO2 insertion process (TS:35.9 kcal/mol → 17.4 kcal/mol) is effectively reduced by NHC modulation of the CO2 bond energy parameters to facilitate the reaction. Due to the unique structural design, COF@NHC-CuCl/CO2 exhibited outstanding diffusion mass transfer, catalytic diversity and reusability. This work proposes a novel construction strategy of heterogeneous dual activation catalyst to provide a sustainable solution for efficiently producing value-added chemicals from CO2 under mild conditions.

Construction of Covalent Organic Frameworks with Partly Condensed Networks for Photocatalytic Hydrogen Evolution

Construction of Covalent Organic Frameworks with Partly Condensed Networks for Photocatalytic Hydrogen Evolution

Construction of covalent organic framework nanozymes with photo-enhanced hydrolase activities for colorimetric sensing of organophosphorus nerve agents

Construction of covalent organic frameworks (COFs)-based nanozymes is of great importance for the extensive applications in catalysis and sensing fields. In this work, a two-dimensional COF (DAFB-DCTP COF) was fabricated via Knoevenagel condensation reaction. The integration of catalytically active sites of pyridine groups into the donor-acceptor (D-A) conjugated skeleton endows DAFB-DCTP COF with both hydrolytic and photosensitive properties. The DAFB-DCTP COF can be utilized as an artificial enzyme with selective and photo-enhanced catalytic efficiency, facilitating its application in photocatalytic degradation of hydrolase substrates (p-nitrophenyl acetate, pNPA) by nucleophilic reaction and further realizing colorimetric detection of the nanozyme inhibitor of organophosphorus nerve agent (diethyl cyanophosphonate, DCNP). The distinct color changes could be distinguished by naked eyes even at a low DCNP concentration, and the versatile smartphone analysis featured with reliability and simplicity. For the first time, the COFs' intrinsic hydrolase activity depending on their structural characteristics was investigated in synergy with the photosensitive performance originating from their photoelectric features. The present contribution provides a promising direction towards construction of colorimetric sensing platform based on the regulation of COFs’ non-oxidoreductase activity under visible light irradiation.

PTSA-mediated interfacial catalytic polymerization of crystalline dense covalent organic framework membranes for enhanced desalination

Construction of covalent organic frameworks (COF) membranes for desalination applications was challenging, mainly due to the poor crystallinity and large intrinsic pore size of COFs. Herein, the p-toluenesulfonic acid (PTSA)-mediated interfacial catalytic polymerization (ICP) strategy was proposed to fabricate COF membranes with high crystallinity and sub-nano pores on the polysulfone substrate to enhance desalination and selectivity performance. The proposed PTSA-mediated ICP strategy facilitated the amorphous-to-crystalline transformation and regulated the stacking behavior of COF crystals. Via PTSA meditated ICP process, a ribbon-like crystal structure was observed on the membrane surface. The amorphous COF layers with a thickness of 122–412 nm transformed into crystalline COF layers with a thickness of approximately 245–390 nm, while the pore radius of the COF membranes decreased from 0.65-1.25 nm to 0.17–0.25 nm due to the multilayer stacking of COF crystals on the membrane surface. Meanwhile, the crystalline degree of these COF membranes increased from 21.45% to 73.95% with increasing PTSA concentration and ICP time. The optimal TpPa3%-15 membrane had a water permeability of approximately 3.74 L−1 m−2 h−1 bar−1 and substantial enhancement of Na2SO4 rejection from 19.7% to 90.4%. The 30-day duration of the cross-flow operation measurement demonstrated excellent stability and robust selectivity of SO42− and Cl− for the optimal TpPa3%-15 membrane. In addition, the optimal membrane exhibited excellent desalination performance for divalent ions in petrochemical wastewater and shale gas produced water than that of pristine membrane, rendering their applicability for industrial wastewater desalination. The mechanisms analysis verified that PTSA induced disorder multilayer stacking of crystalline COF crystals on the substrate surface, resulting in reduced pore sizes and increased desalination of the COF membranes. As the traditional IP process, this work proposed that PTSA mediated ICP process was a facile, scalable and time-saving approach for the construction of highly crystalline COF membranes with sub-nanometer pores to achieve enhanced desalination performance.

Interface engineering strategy construction of covalent organic framework for promoting highly reversible zinc metal

Zn-ion energy storage devices will play important roles in the future energy storage field. However, Zn-ion device development suffers significantly from adverse chemical reactions (dendrite formation, corrosion, and deformation) on the Zn anode surface. Zn dendrite formation, hydrogen evolution corrosion, and deformation combine to degrade Zn-ion devices. Zincophile modulation and protection using covalent organic frameworks (COF) inhibited dendritic growth by induced uniform Zn ion deposition, which also prevented chemical corrosion. The Zn@COF anode circulated stably for more than 1800 cycles even at high current density in symmetric cells and maintained a low and stable voltage hysteresis. This work explains the surface state of the Zn anode and provides information for further research.

Supramolecular mineralization strategy for engineering covalent organic frameworks with superior Zn-I2 battery performances

•A supramolecular mineralization strategy for constructing COFs is proposed •The strategy holds a generality by modulating different assembly blocks •PY-2PBA COF features strong I2 adsorption and accompanying increased conductivity •The engineered PY-2PBA COF delivers impressive performances in Zn-I2 batteries Construction of covalent organic frameworks (COFs) with fine-tuning pore structures and crystalline orientations at the nano-/meso-scale is vital for their potential applications, but common synthesis routes for COFs always result in the unpredictable nucleation growth and aggregation of small crystals with arbitrary orientations and geometries. Here, we develop a supramolecular mineralization strategy for controllable fabrication of semiconducting COF nanoarchitectures by pre-assembly and subsequent in situ covalent locking of supramolecules. This strategy allows us to change the dynamic covalent chemistry reaction path by reducing the entropy of the system, thus facilitating control over the growth and crystallization of COFs on demand (including 1D, 2D, etc.). The tailor-made COF can effectively suppress the shuttle effect and accelerate the conversion of polyiodide through strong intrinsic electron-pair induce interactions, thereby delivering an outstanding performance in zinc-iodine batteries. The studies create a link between supramolecular chemistry and polymer science for the controllable construction of COF and supramolecular materials. Construction of covalent organic frameworks (COFs) with fine-tuning pore structures and crystalline orientations at the nano-/meso-scale is vital for their potential applications, but common synthesis routes for COFs always result in the unpredictable nucleation growth and aggregation of small crystals with arbitrary orientations and geometries. Here, we develop a supramolecular mineralization strategy for controllable fabrication of semiconducting COF nanoarchitectures by pre-assembly and subsequent in situ covalent locking of supramolecules. This strategy allows us to change the dynamic covalent chemistry reaction path by reducing the entropy of the system, thus facilitating control over the growth and crystallization of COFs on demand (including 1D, 2D, etc.). The tailor-made COF can effectively suppress the shuttle effect and accelerate the conversion of polyiodide through strong intrinsic electron-pair induce interactions, thereby delivering an outstanding performance in zinc-iodine batteries. The studies create a link between supramolecular chemistry and polymer science for the controllable construction of COF and supramolecular materials.

Single Site Coordination Enabled Construction of Metal-Diketimine-Linked Covalent Organic Frameworks for Boosted Electrooxidation of Biomass Derivative.

Aromatic aldehydes are widely used for the construction of covalent organic frameworks (COFs). However, due to the high flexibility, high steric hindrance, and low reactivity, it remains challenging to synthesize COFs using ketones as building units, especially the highly flexible aliphatic ones. Here, the single nickel site coordination strategy is reported to lock the configurations of the highly flexible diketimine to transform discrete oligomers or amorphous polymers into highly crystalline nickel-diketimine-linked COFs (named as Ni-DKI-COFs). The strategy has been successfully extended to the synthesis of a series of Ni-DKI-COFs by the condensation of three flexible diketones with two tridentate amines. Thanks to the ABC stacking model with high amount and easily accessible single nickel (II) sites on their 1D channels, Ni-DKI-COFs are exploited as well-defined electrocatalyst platforms for efficiently electro-upgrading biomass-derived 5-Hydroxymethylfurfural (HMF) into value-added 2,5-furandicarboxylic acid (FDCA) with a 99.9% yield and a 99.5% faradaic efficiency as well as a high turnover frequency of 0.31s-1 .

Construction of Covalent Organic Frameworks via Multicomponent Reactions.

Multicomponent reactions (MCRs) combine at least three reactants to afford the desired product in a highly atom-economic way and are therefore viewed as efficient one-pot combinatorial synthesis tools allowing one to significantly boost molecular complexity and diversity. Nowadays, MCRs are no longer confined to organic synthesis and have found applications in materials chemistry. In particular, MCRs can be used to prepare covalent organic frameworks (COFs), which are crystalline porous materials assembled from organic monomers and exhibit a broad range of properties and applications. This synthetic approach retains the advantages of small-molecule MCRs, not only strengthening the skeletal robustness of COFs, but also providing additional driving forces for their crystallization, and has been used to prepare a series of robust COFs with diverse applications. The present perspective article provides the general background for MCRs, discusses the types of MCRs employed for COF synthesis to date, and addresses the related critical challenges and future perspectives to inspire the MCR-based design of new robust COFs and promote further progress in this emerging field.

Construction of covalent organic frameworks from nanospheres for Cu2+ detection

A strategy to construct hydrazine-linkage-functionalized COFs from the imine-linkage nanosphere via the linker exchange is provided.

Construction of covalent organic frameworks with alternating rigid and flexible units and their controlled release of active sites

Different from traditional covalent organic frameworks (COFs) with single structural mode, COFs constructed with unique alternating rigid and flexible structural elements can maximize the great advantages of COFs in both crystalline structure and application performance. Herein, a series of rigid & flexible COFs containing both rigid and flexible units were designed and synthesized. The rigid units can reduce the growth defects and maintain the regular crystalline structures. The flexible units endow COFs with abundant potential active sites that can be released through controlled dissociation reactions. Thus, the site-released multiple-site COFs exhibit significant improvement on adsorption performance towards mercury and uranium, with a maximum adsorption capacity increase of up to 61% compared to the original COFs. The strategy for the construction of rigid & flexible COFs and the controlled release of active sites points a new direction for the design and synthesis of COFs with multi-functional sites and outstanding application performance.

Modulated covalent organic frameworks with higher specific surface area for the ultrasensitive detection of polybrominated biphenyls

Controllable construction of covalent organic frameworks (COFs) is fascinating, and full elucidations on the relationships between their microstructures and adsorption performance for persistent organic pollutants (POPs) are imperative and desirable, yet challenging. Herein, the 2,5-dimethoxybenzaldehyde (DB) has been first employed as a modulator to construct COFs with higher specific surface area and enhanced hydrophobicity under the mild conditions, after competing with the building blocks of 2,5-dimethoxyterephaldehyde (DMTP) and 1,3,5-tris(4-aminophenyl)benzene (TAPB). These TAPB-DMTP-DB COFs have been further fabricated as solid phase microextraction (SPME) coatings with uniform morphology and comparative thickness, to investigate their enrichment capacities for POPs systematically. In particular, the DB-modulated COF demonstrated outstanding enrichment factors (4400–11360) for polybrominated biphenyls (PBBs). Moreover, ultrasensitive analytical method has been developed based on the DB-modulated COF-coated fiber and gas chromatography-mass spectrometry (GC–MS), with low limits of detection (0.04–0.28 ng/L), wide linear range (0.25–5000 ng/L), and satisfactory reproducibility, realizing the precise quantifications of ultra-trace PBBs in environmental aqueous samples.