Robust Covalent Organic Frameworks for Photosynthesis of H2O2: Advancements, Challenges and Strategies.

Since 2020, covalent organic frameworks (COFs) are emerging as robust catalysts for the photosynthesis of hydrogen peroxide (H2O2), benefiting from their distinct advantages. However, the current efficiency of H2O2 production and solar-to-chemical energy conversion efficiency (SCC) remain suboptimal due to various constraints in the reaction mechanism. Therefore, there is an imperative to propose efficiency improvement strategies to accelerate the development of this reaction system. This comprehensive review delineates recent advances, challenges, and strategies in utilizing COFs for photocatalytic H2O2 production. It explores the fundamentals and challenges (e.g., oxygen (O2) mass transfer rate, O2 adsorption capacity, response to sunlight, electron-hole separation efficiency, charge transfer efficiency, selectivity, and H2O2 desorption) associated with this process, as well as the advantages, applications, classification, and preparation strategies of COFs for this purpose. Various strategies to enhance the performance of COFs in H2O2 production are highlighted. The review aims to stimulate further advancements in utilizing COFs for photocatalytic H2O2 production and discusses potential prospects, challenges, and application areas in this field.

Protonated Z-scheme CdS-covalent organic framework heterojunction with highly efficient photocatalytic hydrogen evolution.

The low efficiency of photocatalytic hydrogen production from water is mainly suffer from limited light absorption, charge separation and water delivery to the active centers. Herein, an inorganic-organic Z-scheme heterojunction (CdS-COF-Ni) is constructed by in-situ growth of CdS nanosheets on the porphyrin-based covalent organic framework with nickel ions (COF-Ni) in the porphyrin centers. A built-in electric field is formed at the interface, which accelerates the separation and transfer of photogenerated charges. Moreover, through the surface protonation treatment in ascorbic acid (AC) solution, the hydrophilicity of the obtained composite is obviously increased and facilitates the transport of water molecules to the photocatalytic centers. Under the synergistic effect of the interfacial interaction and surface protonation treatment, the photocatalytic hydrogen production rate is optimized to be 18.23 mmol h-1 g-1 without adding any cocatalysts, which is 21 times that of CdS. After a series of photoelectrochemical measurements, in situ X-ray photoelectron spectroscopy (XPS) analysis, and density functional theory (DFT) calculations, it is found that the photocatalytic charge transfer pathway conforms to the Z-scheme mechanism, which not only greatly accelerates the separation and transfer of photogenerated charges, but also retains a high reduction capacity for water splitting. This work offers a good strategy for constructing highly efficient organic-inorganic heterojunctions for water splitting.

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.

Vinyl-bearing sp2 carbon-conjugated covalent organic framework composites for enhanced electrochemical performance in hydrogen evolution reaction and lithium-sulfur batteries.

Vinyl-bearing triazine-functionalized covalent organic frameworks (COFs) have emerged as promising materials for electrocatalysis and energy storage. Guided by density functional theory calculations, a vinyl-enriched COF (VCOF-1) featuring a donor-acceptor structure was synthesized based on the Knoevenagel reaction. Moreover, the VCOF-1@Ru without pyrolysis was obtained through chemical coordination interactions between VCOF-1 and RuCl3, exhibiting enhanced electrocatalytic performance in the hydrogen evolution reaction when exposed to 0.5 M H2SO4. The results demonstrated that the protonation of VCOF-1@Ru enhanced the electrical conductivity and accelerated the generation of H2 on the catalytically active site Ru. Additionally, VCOF-1@CNT with a tubular structure was prepared by uniformly wrapping VCOF-1 onto carbon nanotubes (CNTs) and using it as a cathode for lithium-sulfur batteries by chemically and physically encapsulating S. The enhanced performance of VCOF-1@CNT was attributed to the effective suppression of lithium polysulfide migration. This suppression was achieved through several mechanisms, including the inverse vulcanization of vinyl on VCOF-1@CNT, the enhancement of material conductivity, and the interaction between N in the materials and Li ions. This study demonstrated a strategy for enhancing material performance by precisely modulating the COF structure at the molecular level.

Synergistic Linker and Linkage of Covalent Organic Frameworks for Enhancing Gold Capture.

The tunable pore walls and skeletons render covalent organic frameworks (COFs) as promising absorbents for gold (Au) ion. However, most of these COFs suffered from low surface areas hindering binding sites exposed and weak binding interaction resulting in sluggish kinetic performance. In this study, COFs have been constructed with synergistic linker and linkage for high-efficiency Au capture. The designed COFs (PYTA-PZDH-COF and PYTA-BPDH-COF) with pyrazine or bipyridine as linkers showed high surface areas of 1692 and 2076 m2 g‒1, providing high exposed surface areas for Au capture. In addition, the Lewis basic nitrogen atoms from the linkers and linkages are easily hydronium, which enabled to fast trap Au via coulomb force. The PYTA-PZDH-COF and PYTA-BPDH-COF showed maximum Au capture capacities of 2314 and 1810 mg g-1, higher than other reported COFs. More importantly, PYTA-PZDH-COF are capable of rapid adsorption kinetics with achieving 95% of maximum binding capacity in 10 min. The theoretical calculation revealed that the nitrogen atoms in linkers and linkages from both COFs are simultaneously hydronium, and then the protonated PYTA-PZDH-COF are more easily binding the AuCl4 ‒, further accelerating the binding process. This study gives the a new insight to design COFs for ion capture.

Variation of Chemical Microenvironment of Pores in Hydrazone-Linked Covalent Organic Frameworks for Photosynthesis of H2O2.

Photocatalytic synthesis of H2O2 is an advantageous and ecologically sustainable alternative to the conventional anthraquinone process. However, achieving high conversion efficiency without sacrificial agents remains a challenge. In this study, two covalent organic frameworks (COF-O and COF-C) were prepared with identical skeletal structures but with their pore walls anchored to different alkyl chains. They were used to investigate the effect of the chemical microenvironment of pores on photocatalytic H2O2 production. Experimental results reveal a change of hydrophilicity in COF-O, leading to suppressed charge recombination, diminished charge transfer resistance, and accelerated interfacial electron transfer. An apparent quantum yield as high as 10.3% (λ = 420 nm) can be achieved with H2O and O2 through oxygen reduction reaction. This is among the highest ever reported for polymer photocatalysts. This study may provide a novel avenue for optimizing photocatalytic activity and selectivity in H2O2 generation.

A pH-dependent and charge selective covalent organic framework for removal of dyes from aqueous solutions.

A novel imine-linked COF is synthesized by the condensation of 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (TAPT) and 2-hydroxy-5-methoxyisophthalaldehyde (HMIPA) under solvothermal conditions. This COF adsorbs preferentially the neutral dye Neutral Red (NR) over the positively charged dye Methylene Blue (MB) at pH 7, and the negatively charged Methyl Orange (MO) over the positively charged Methylene Blue (MB) at pH 3. The maximum adsorption capacities (qe) obtained within very short times (11-60 min) under optimized conditions were 108, 185 and 429 mg.g-1 for the MB, MO, and NR dyes, respectively. These adsorptions obey the Langmuir isotherm and pseudo-second-order kinetics. The prepared TAPT-HMIPA-COF is used successfully for the removal of the dyes from real water and treated wastewater samples. The adsorption data, BET, FTIR, and zeta potential measurements show that the electrostatic, π-π stacking and hydrogen bond interactions are responsible for the adsorption of organic dyes on the surface of the prepared COF. Due to recyclability, high capacity and efficiency for the adsorption of positive, negative and neutral organic dyes, this COF can be considered promising for simultaneous removal of various dyes from aqueous solutions at adjusted pHs.

Wiring Covalent Organic Frameworks with Conducting Polymers.

Covalent organic frameworks are a class of crystalline porous polymers formed by linking organic units into periodically aligned skeletons and pores. Here we report a strategy for wiring these frameworks with conducting polymers via wall engineering and polymerization. We anchored each edge site with one pyrrole unit, which is densely packed along the z direction yet protruded from pore walls. This assembly enables the polymerization of pyrrole units to form polypyrrole and creates a new polypyrrole chain conformation. The resultant framework constitutes six single file polypyrrole chains in each pore and develop spatially segregated yet built-in single molecular wires with exceptional stable polarons. Hall effect measurements revealed that the materials are p-type semiconductors, increase conductivity by eight orders of magnitude compared to the pristine frameworks, and achieve a carrier mobility as large as 13.2 cm2 V-1 s-1. Our results open an avenue to π electronic frameworks by interlayer molecular wiring with conducting polymers.

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.

Ionic Covalent Organic Frameworks Consisting of Tetraborate Nodes and Flexible Linkers.

Covalent organic frameworks (COFs) have emerged as versatile materials with many applications, such as carbon capture, molecular separation, catalysis, and energy storage. Traditionally, flexible building blocks have been avoided due to their potential to disrupt ordered structures. Recent studies have demonstrated intriguing properties and enhanced structural diversity achievable with flexible components by judicious selection of building blocks. This study presents a novel series of ionic COFs (ICOFs) consisting of tetraborate nodes and flexible linkers. These ICOFs use borohydrides to irreversibly deprotonate the alcohol monomers to achieve a high polymerization degree. Structural analysis confirms the dia topologies. Reticulation is explored using various monomers and metal counter-ions. Also, these frameworks exhibit excellent stability in alcohols and coordinating solvents. The materials are tested as single-ion conductive solid-state electrolytes. ICOF-203-Li displays one of the lowest activation energies reported for ion conduction. This tetraborate chemistry is anticipated to facilitate further structural diversity and functionality in crystalline polymers.

Flexible Hydrazone-Linked Metal-Covalent Organic Frameworks with Copper Clusters for Efficient Electrocatalytic Oxygen Evolution Reaction.

Despite the challenges associated with the synthesis of flexible metal-covalent organic frameworks (MCOFs), these offer the unique advantage of maximizing the atomic utilization efficiency. However, the construction of flexible MCOFs with flexible building units or linkages has rarely been reported. In this study, novel flexible MCOFs are constructed using flexible building blocks and copper clusters with hydrazone linkages. The heterometallic frameworks (Cu, Co) are prepared through the hydrazone linkage coordination method and evaluated as catalysts for the oxygen evolution reaction (OER). Owing to the spatial separation and functional cooperation of the heterometallic MCOF catalysts, the as-synthesized MCOFs exhibited outstanding catalytic activities with an overpotential of 268.8 mV at 10 mA cm-2 for the OER in 1 M KOH, which is superior to those of the reported covalent organic frameworks (COFs)-based OER catalysts. Theoretical calculations further elucidated the synergistic effect of heterometallic active sites within the linkages and frameworks, contributing to the enhanced OER activity. This study thus introduces a novel approach to the fundamental design of flexible MCOF catalysts for the OER, emphasizing their enhanced atomic utilization efficiency.

Mechanochemical Synthesis of Boroxine-linked Covalent Organic Frameworks.

We report a rapid, room-temperature mechanochemical synthesis of 2- and 3-dimensional boroxine covalent organic frameworks (COFs), enabled by using trimethylboroxine as a dehydrating additive to overcome the hydrolytic sensitivity of boroxine-based COFs. The resulting COFs display high porosity and crystallinity, with COF-102 being the first example of a mechanochemically prepared 3D COF, exhibiting a surface area of ca. 2,500 m2 g-1. Mechanochemistry enabled a >20-fold reduction in solvent use and ~100-fold reduction in reaction time compared with solvothermal methods, providing target COFs quantitatively with no additional work-up besides vacuum drying. Real-time Raman spectroscopy permitted the first quantitative kinetic analysis of COF mechanosynthesis, while transferring the reaction design to Resonant Acoustic Mixing (RAM) enabled synthesis of multi-gram amounts of the target COFs (tested up to 10 g).

Biomimetic Phthalocyanine-Based Covalent Organic Frameworks with Tunable Pendant Groups for Electrocatalytic CO2 Reduction.

Electrocatalytic carbon dioxide reduction reaction (CO2RR) is an effective way of converting CO2 into value-added products using renewable energy, whose activity and selectivity can be in principle maneuvered by tuning the microenvironment near catalytic sites. Here, we demonstrate a strategy for tuning the microenvironment of CO2RR by learning from the natural chlorophyll and heme. Specifically, the conductive covalent organic frameworks (COFs) linked by piperazine serve as versatile supports for single-atom catalysts (SACs), and the pendant groups modified on the COFs can be readily tailored to offer different push-pull electronic effects for tunable microenvironment. As a result, while all the COFs exhibit high chemical structure stability under harsh conditions and good conductivity, the addition of -CH2NH2 can greatly enhance the activity and selectivity of CO2RR. As proven by experimental characterization and theoretical simulation, the electron-donating group (-CH2NH2) not only reduces the surface work function of COF, but also improves the adsorption energy of the key intermediate *COOH, compared with the COFs with electron-withdrawing groups (-CN, -COOH) near the active sites. This work provides insights into the microenvironment modulation of CO2RR electrocatalysts at the molecular level.

Assembly-Dissociation-Reconstruction Synthesis of Covalent Organic Framework Membranes with High Continuity for Efficient CO2 Separation.

Covalent organic frameworks (COFs), at the forefront of porous materials, hold tremendous potential in membrane separation; however, achieving high continuity in COF membranes remains crucial for efficient gas separation. Here, we present a unique approach termed assembly-dissociation-reconstruction for fabricating COF membranes tailored for CO2/N2 separation. A parent COF is designed from two-node aldehyde and three-node amine monomers and dissociated to high-aspect-ratio nanosheets. Subsequently, COF nanosheets are orderly reconstructed into a crack-free membrane by surface reaction under water evaporation. The membrane exhibits high crystallinity, open pores and a strong affinity for CO2 adsorption over N2, resulting in CO2 permeance exceeding 1060 GPU and CO2/N2 selectivity surpassing 30.6. The efficacy of this strategy offers valuable guidance for the precise fabrication of gas-separation membranes.

High-performance Piezoelectric Two-dimensional Covalent Organic Frameworks.

Organic piezoelectric nanogenerators (PENGs) are attractive in harvesting mechanical energy for various self-powering systems. However, their practical applications are severely restricted by their low output open circuit voltage. To address this issue, herein, we prepared two two-dimensional (2D) covalent organic frameworks (COFs, CityU-13 and CityU-14), functionalized with fluorinated alkyl chains for PENGs. The piezoelectricity of both COFs was evidenced by switchable polarization, characteristic butterfly amplitude loops, phase hysteresis loops, conspicuous surface potentials and high piezoelectric coefficient value (d33). The PENGs fabricated with COFs displayed highest output open circuit voltages (60 V for CityU-13 and 50 V for CityU-14) and delivered satisfactory short circuit current with an excellent stability of over 600 seconds. The superior open circuit voltages of CityU-13 and CityU-14 rank in top 1 and 2 among all reported organic materials-based PENGs.

Helicene Covalent Organic Frameworks for Robust Light Harvesting and Efficient Energy Transfers.

Helicenes represent a class of fascinating π compounds with fused yet folded backbones. Despite their broad structural diversity, harnessing helicenes to develop well-defined materials is still a formidable challenge. Here we report the synthesis of crystalline porous helicene materials by exploring helicenes to synthesize covalent 2D lattices and layered π frameworks. Topology-directed polymerization of [6]helicenes and porphyrin creates 2D covalent networks with alternate helicene-porphyrin alignment along the x and y directions at a 1.5-nm interval and develops [6]helicene frameworks through reversed anti-AA stack along the z direction to form segregated [6]helicene and porphyrin columnar π arrays. Notably, this π configuration enables the frameworks to be highly red luminescent with benchmark quantum yields. The [6]helicene frameworks trigger effieicnt intra-framework singlet-to-singlet state energy transfer from [6]helicene to porphyrin and facilitate intermolecular triplet-to-triplet state energy transfer from frameworks to molecular oxygen to produce reactive oxygen species, harvesting a wide range of photons from ultraviolet to near-infrared regions for light emitting and photo-to-chemical conversion. This study introduces a new family of extended frameworks, laying the groundwork for exploring well-defined helicene materials with unprecedented structures and functions.

Tris(triazolo)triazine-based Covalent Organic Frameworks for Efficiently Photocatalytic Hydrogen Peroxide Production.

Two-dimensional covalent organic frameworks (2D-COFs) have recently emerged as fascinating scaffolds for solar-to-chemical energy conversion because of their customizable structures and functionalities. Herein, two tris(triazolo)triazine-based COF materials (namely COF-JLU51 and COF-JLU52) featuring large surface area, high crystallinity, excellent stability and photoelectric properties were designed and constructed for the first time. Remarkably, COF-JLU51 gave an outstanding H2O2 production rate of over 4200 µmol g-1 h-1 with excellent reusability in pure water and O2 under one standard sun light, that higher than its isomorphic COF-JLU52 and most of the reported metal-free materials, owing to its superior generation, separation and transport of photogenerated carriers. Experimental and theoretical researches prove that the photocatalytic process undergoes a combination of indirect 2e- O2 reduction reaction (ORR) and 4e- H2O oxidation reaction (WOR). Specifically, an ultrahigh yield of 7624.7 µmol g-1 h-1 with apparent quantum yield of 18.2% for COF-JLU52 was achieved in a 1:1 ratio of benzyl alcohol and water system. This finding contributes novel, nitrogen-rich and high-quality tris(triazolo)triazine-based COF materials, and also designate their bright future in photocatalytic solar transformations.

Covalent Organic Frameworks for Electrocatalysis：Design, Applications, and Perspectives.

Covalent organic frameworks (COFs) are an innovative class of crystalline porous polymers composed of light elements such as C, N, O, etc., linked by covalent bonds. The distinctive properties of COFs, including designable building blocks, large specific surface area, tunable pore size, abundant active sites, and remarkable stability, have led their widespread applications in electrocatalysis. In recent years, COF-based electrocatalysts have made remarkable progress in various electrocatalytic fields, including the hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, nitrogen reduction reaction, nitrate reduction reaction, and carbon dioxide reduction reaction. This review begins with an introduction to the design and synthesis strategies employed for COF-based electrocatalysts. These strategies include heteroatom doping, metalation of COF and building monomers, encapsulation of active sites within COF pores, and the development of COF-based derived materials. Subsequently, a systematic overview of the recent advancements in the application of COF-based catalysts in electrocatalysis is presented. Finally, the review discusses the main challenges and outlines possible avenues for the future development of COF-based electrocatalysts.

Taming Two-Dimensional Polymerization by a Machine-Learning Discovered Crystallization Model.

Rapidly synthesizing high-quality two-dimensional covalent organic frameworks (2D COFs) is crucial to their practical applications. Here, we use a machine-learning approach that overcomes the challenges associated with bottom-up model derivation for the non-classical 2D COF crystallization processes. The resulting model, referred to as NEgen1, establishes correlations among the induction time, nucleation rate, growth rate, bond-forming rate constants, and common solution synthesis conditions for 2D COFs that grow by a nucleation-elongation mechanism. The results elucidate the detailed competition between the nucleation and growth dynamics in solution, which has been inappropriately described previously by classical, empirical models with assumptions invalid for 2D COF polymerization. By understanding the dynamic processes at play, the NEgen1 model reveals a simple strategy of gradually increasing monomer addition speed for growing large 2D COF crystals. This insight enables us to rapidly synthesize large COF-5 colloids, which could only be achieved previously by prolonged reaction times or by introducing chemical modulators. These results highlight the potential for systematically improving the crystal quality of 2D COFs, which has wide-reaching relevance for many of the applications for which 2D COFs are speculated to be valuable.

Asymmetric catalytic synthesis of chiral covalent organic framework composite (S)-DTP-COF@SiO2 for HPLC enantioseparations by normal-phase and reversed-phase chromatographic modes.

A novel CCOF core-shell composite material (S)-DTP-COF@SiO2 was prepared via asymmetric catalytic and in situ growth strategy. The prepared (S)-DTP-COF@SiO2 was utilized as separation medium for HPLC enantioseparation using normal-phase and reversed-phase chromatographic modes, which displays excellent chiral separation performance for alcohols, esters, ketones, and epoxides, etc. Compared with chiral commercial chromatographic columns (Chiralpak AD-H and Chiralcel OD-H columns) and some previously reported chiral CCOF@SiO2 (CC-MP CCTF@SiO2 and MDI-ß-CD-modified COF@SiO2)-packed columns, there are 4, 3, 13, and 15 tested racemic compounds that could not be resolved on the Chiralpak AD-H column, Chiralcel OD-H column, CC-MP CCTF@SiO2 column, and MDI-ß-CD-modified COF@SiO2 column, respectively, which indicates that the resolution effect of (S)-DTP-COF@SiO2-packed column can be complementary to the other ones. The effects of the analyte mass, column temperature, and mobile phase composition on the enantiomeric separation were investigated. The chiral column exhibits good reproducibility after multiple consecutive injections. The RSDs (n = 5) of the peak area and retention time were less than 1.5% for repetitive separation of 2-methoxy-2-phenylethanol and 1-phenyl-1-pentanol. The chiral core-shell composite (S)-DTP-COF@SiO2 exhibited good enantiomeric separation performance, which not only demonstrates its potential as a novel CSP material in HPLC but also expands the range of applications for chiral COFs.