Stable Covalent Organic Frameworks Research Articles

Visible Light-Driven C(sp3)-H Carboxylation of Diverse Amines with CO2 into α-Amino Acids Using an Eco-Friendly and Reusable Covalent Organic Framework

Direct photochemical carboxylation of C(sp3)–H bonds with CO2 is an uphill task and it has attracted increasing attention. In the present study, we report an elegant strategy for visible-light-triggered C(sp3)-H carboxylation of amines with CO2 into α-amino acids using a stable crystalline polyimide-based covalent organic framework (PI-COF) as an efficient heterogeneous photocatalyst and NiO nanoparticles (NiO NPs) as a cocatalyst under ambient conditions (room temperature and atmospheric CO2 pressure). Diverse amino acids are produced in moderate-to-high yields. This methodology tolerates a range of functional groups and displays remarkable regioselectivity. Various drugs were effectively achieved using this light-assisted approach. The high chemical stability of the COF and its strong interactions with NiO NPs renders the catalytic system to be highly recyclable (i.e., over five times). More interestingly, this photoinduced carboxylation reaction occurred without the involvement of any sacrificial electron donors. Based on computational (DFT) investigations, a tentative mechanism revealed the formation of CO2 radical anion at the conduction band (CB) of the COF via single electron transfer mediated by NiO nanoparticles which combined with amine radical cation at the valence band of the COF to form α-amino acid.

Enhancing photocatalytic C-O/N coupling reaction over covalent organic frameworks by pull-push electron group regulation

Covalent organic frameworks (COFs) have emerged as novel materials in photocatalytic technology for green energy and chemicals production. However, exploring highly efficient and stable COFs based materials for photocatalytic aromatic ethers and amines synthesis is still urgent. Herein, olefin-linked COFs modified with or without –F/-OCH3 groups were constructed by Knoevenagel condensation reaction. In addition, Ni active sites were embedded in the frameworks through bipyridine units to afford highly efficient and repeatable photocatalytic C-O/N coupling reaction. Through this strategy, it was found that the -F group modification greatly elevated photogenerated carriers separation efficiency attributing to its strong electronegativity and electron-withdrawing properties. Photocatalytic experiments conveyed that sp2c-F-COFdpy-Ni displayed highest photocatalytic activity and catalyzed cross-coupling of aryl C-O/N both with high yield exceeding 99 %, and exhibited excellent substrate compatibility for aryl bromides substituted with electron-withdrawing/donating groups. This investigation provides new insights into structure design of COFs for photocatalytic organic transformation.

Novel Environmentally Friendly Covalent Organic Framework/Polylactic Acid Composite Material with High Chemical Stability for Sand-Control Material.

A new high-strength, thermally stable, and degradable covalent organic framework (COF) -modified polylactic acid fiber (PLA) material (COF-PLA) was constructed for reinforcing the PLA material, to be used to produce environmentally friendly sand barriers. The micrographs, structure, thermal stability, and photodegradation products of COF-PLA were investigated. The results indicated that the COF material was compatible with PLA, and that the COF-PLA material took on the merits of the COF, so that it had a more regular arrangement, smoother surface, and smaller size, and was more thermostable than PLA alone. The successful incorporation of the COF improved the thermal stability of PLA. The initial pyrolysis temperature of the COF-PLA material is 313.7 °C, higher than that of the PLA material at 297.5 °C. The photodegradation products of COF-PLA and PLA indicated that the COF and PLA materials were mixed in a complex manner. After photodegradation, the COF-PLA material can produce melamine molecules that can neutralize the lactic acid and CO2 produced by PLA, which can maintain the acid-base balance in sandy soil and is beneficial to plant growth. Therefore, COF-PLA degradation does not cause pollution, making it a promising sand-control material.

Synthesis of Pore‐Wall‐Modified Stable COF/TiO2 Heterostructures via Site‐Specific Nucleation for an Enhanced Photoreduction of Carbon Dioxide

Constructing stable heterostructures with appropriate active site architectures in covalent organic frameworks (COFs) can improve the active site accessibility and facilitate charge transfer, thereby increasing the catalytic efficiency. Herein, a pore-wall modification strategy is proposed to achieve regularly arranged TiO2 nanodots (≈1.82 nm) in the pores of COFs via site-specific nucleation. The site-specific nucleation strategy stabilizes the TiO2 nanodots as well as enables the controlled growth of TiO2 throughout the COFs' matrix. In a typical process, the pore wall is modified and site-specific nucleation is induced between the metal precursors and the organic walls of the COFs through a careful ligand selection, and the strongly bonded metal precursors drive the confined growth of ultrasmall TiO2 nanodots during the subsequent hydrolysis. This will result in remarkably improved surface reactions, owing to the superior catalytic activity of TiO2 nanodots functionalized to COFs through strong NTiO bonds. Furthermore, density functional theory studies reveal that pore-wall modification is beneficial for inducing strong interactions between the COF and TiO2 and results in a large energy transfer via the NTiO bonds. This work highlights the feasibility of developing stable COF and metal oxide based heterostructures via organic wall modifications to produce carbon fuels by artificial photosynthesis.

Thoughtful design of a covalent organic framework with tailor-made polarity and pore size for the enrichment of bisphenols and their derivatives: Extraction performance, adsorption mechanism and toxicity evaluation

A stable, reusable and cost-effective covalent organic framework (COF) with medium polarity was successfully decorated on Fe3O4. The Fe3O4@COF contained tailor-made polarity and pore size that fitted well with bisphenols and their derivatives (BPs). When coupling magnetic solid-phase extraction (MSPE) with high-performance liquid chromatography (HPLC) detection, the Fe3O4@COF featured efficient recognition and enrichment for BPs due to π-π stacking, C–H⋯π interactions, pore-filling effect, dispersion force and hydrophobic interactions. Under optimized conditions, calibration plots exhibited good linearity (5–1000 ng mL−1), and limits of detection (LOD) ranged from 0.15 to 0.39 ng mL−1. The method was successfully employed in quantifying BPs in authentic lake and river water samples with satisfactory recoveries ranging from 81.4% to 120%. Molecular dynamics simulation revealed extraction mechanisms, and a microscopic behavior related to the clustering property of the emerging brominated compounds was first discovered. Ecotoxicological assessments of target pollutants were conducted from multiple aspects, highlighting the harmfulness of the chemicals and the significance of the analytical method. The proposed methodology offered sensitive detection and quantification, which was beneficial for the timely tracking of the concentration, transportation and distribution of BPs to better explore their environmental behavior and tackle contamination problems in complex environmental matrices.

Construction of Stable Magnetic Vinylene-Linked Covalent Organic Frameworks for Efficient Extraction of Benzimidazole Fungicides.

Covalent organic frameworks (COFs) have attracted impressive interest in separation on aqueous media. Herein, we integrated the stable vinylene-linked COFs with magnetic nanosphere via the monomer-mediated in situ growth strategy to construct a crystalline Fe3O4@v-COF composite for enrichment and determination of benzimidazole fungicides (BZDs) from complex sample matrices. The Fe3O4@v-COF has a crystalline assembly, high surface area, porous character together with a well-defined core-shell structure, and serves as progressive pretreatment materials for magnetic solid phase extraction (MSPE) of BZDs. Adsorption mechanism studies revealed that the extended conjugated system and numerous polar cyan groups on v-COF provides abundant π-π and multiple hydrogen bonding sites, which are conducive to interact with BZDs collaboratively. Fe3O4@v-COF also displayed enrichment effects to various polar pollutions with conjugated structures and hydrogen-bonding sites. Fe3O4@v-COF-based MSPE-high-performance liquid chromatography exhibited the low limit of detection, wide linearity, and good precision. Moreover, Fe3O4@v-COF showed better stability, enhanced extraction performance, and more sustainable reusability in comparison with its imine-linked counterpart. This work proposes a feasible strategy on constructing the crystalline stable magnetic vinylene-linked COF composite for the determination of trace contaminants in complex food matrices.

Pore functionalization of cationic covalent organic frameworks membrane: A case towards acid recovery

Covalent organic frameworks (COFs), which are a new class of porous organic materials, are considered as potential candidates for separation membranes because of their regular channels and tailored functionality. The imine-linked COF was synthesized using reversible condensation reactions, resulting in its un-stabilization in the acid environment. Thus, it is a formidable challenge to develop stable COFs membranes for applications in harsh environments (e.g. strong acid). Herein, we demonstrated the in-situ construction of acid-stable COF separating layer (TMTATGCl) on the surface of the hydrophilic hydrolyzed polyacrylonitrile (HPAN) substrate via acid-catalyzed interfacial polymerization. Triaminoguanidine chloride (TGCl), a cationic monomer, was used to construct cationic nanochannels in the COF layer, conferring its screening feature toward cations, especially toward divalent cations. Besides, implanted methoxy groups could facilitate H+ diffusion along the pore pathway of the as-prepared COF. As a result, the methoxy functionalized cationic nanochannels of the DMTATGCl/HPAN enhanced H+ transfer, while other ions (e.g. Fe2+) were obstructed, allowing for high selectivity (S = 1248.9) and permeability (UH+ = 0.0118 m/h). Moreover, the membranes (TMTATGCl/HPAN) exhibited remarkable acid-stability, which maintained the high separation performance after 10 cycles under strong-acid conditions. This study provides an effective strategy for developing the ion channels membrane with specific interaction sites for effective ion separation.

Primary amide-functionalized cyclotricatechylene covalent organic frameworks membrane for efficient enrichment of melamine and its derivatives in migration solution of food contact materials.

A highly chemically stable primary amide-functionalized cyclotricatechylene covalent organic framework was synthesized by an irreversible reaction and a post-synthetic modification. It possessed a rod-like morphology and exhibited strong solvent stability owing to the polyether bonds. The material showed good adsorption performance for melamine and its derivatives and adsorption mechanism was investigated by molecular simulations. The adsorbent was coated on the nylon-66 membrane to prepare the enrichment membrane. Under optimized conditions, an in-syringe membrane-based extraction method, combined with ultra-high performance liquid chromatography-tandem mass spectrometry was developed for the analysis of melamine and six melamine derivatives in the migration solution. A good linearity was obtained with correlation coefficients ranging from 0.9924 to 0.9995. The limits of detection were 1-200ng/L and the limits of quantification were 3-500ng/L. This method was successfully applied to the migration solution of sushi bamboo rolling mats with spiked recoveries of 73.2%-115% and relative standard deviations of 0.9%-9.9%. This work shows a practical and perspective approach for the efficient enrichment of food contact material hazards.

Integrating Bifunctionality and Chemical Stability in Covalent Organic Frameworks via One-Pot Multicomponent Reactions for Solar-Driven H2O2 Production

Multicomponent reactions (MCRs) can be used to introduce different functionalities into highly stable covalent organic frameworks (COFs). In this work, the irreversible three-component Doebner reaction is utilized to synthesize four chemically stable quinoline-4-carboxylic acid DMCR-COFs (DMCR-1-3 and DMCR-1NH) equipped with an acid-base bifunctionality. These DMCR-COFs show superior photocatalytic H2O2 evolution (one of the most important industrial oxidants) compared to the imine COF analogue (Imine-1). This is achieved with sacrificial oxidants but also in pure water and under an oxygen or air atmosphere. Furthermore, the DMCR-COFs show high photostability, durability, and recyclability. MCR-COFs thus provide a viable materials' platform for solar to chemical energy conversion.

Nitrogen Abundant Polyimide Covalent Organic Frameworks as Nanotraps toward Uranium Sequestration

The design and synthesis of efficient and highly selective UO22+ nanotraps still pose difficulties in the field of radionuclide removal. Herein, a stable polyimide covalent organic framework (PI-COF-6) was rationally synthesized for targeted uranium adsorption through the irreversible imidization reaction between nitrogen-rich monomer 5,5′,5″-(1,3,5-triazine-2,4,6-triyl)tris(pyridin-2-amine) (TTPA) and pyromellitic dianhydride (PMDA). The PI-COF-6 showed satisfactory uranium adsorption performance with high capacity (424.5 mg g–1), fast kinetics (98.0% removal rate in 50 min), ultrahigh selectivity among multiple metal ions, excellent stability, and outstanding recyclability. XPS characterization and DFT calculations identified that the uranium adsorption of PI-COF-6 mainly relied on the N–N–O nanotraps which were rationally constructed by pyridine nitrogen in TTPA coupling with the tertiary amine nitrogen and carbonyl oxygen in the imide ring. Moreover, the results of high-resolution U 4f XPS spectroscopy revealed that part of U(VI) was reduced into U(IV). Hence, according to the design of rational nanotraps for targeted radioactive nuclides, highly effective adsorbents for practical application (rare earth mining waste remediation) were successfully achieved.

Integrated interfacial design of covalent organic framework photocatalysts to promote hydrogen evolution from water

Attempts to develop photocatalysts for hydrogen production from water usually result in low efficiency. Here we report the finding of photocatalysts by integrated interfacial design of stable covalent organic frameworks. We predesigned and constructed different molecular interfaces by fabricating ordered or amorphous π skeletons, installing ligating or non-ligating walls and engineering hydrophobic or hydrophilic pores. This systematic interfacial control over electron transfer, active site immobilisation and water transport enables to identify their distinct roles in the photocatalytic process. The frameworks, combined ordered π skeletons, ligating walls and hydrophilic channels, work under 300–1000 nm with non-noble metal co-catalyst and achieve a hydrogen evolution rate over 11 mmol g–1 h–1, a quantum yield of 3.6% at 600 nm and a three-order-of-magnitude-increased turnover frequency of 18.8 h–1 compared to those obtained with hydrophobic networks. This integrated interfacial design approach is a step towards designing solar-to-chemical energy conversion systems.

A Heterostructure Photoelectrode Based on Two-Dimensional Covalent Organic Framework Film Decorated TiO2 Nanotube Arrays for Enhanced Photoelectrochemical Hydrogen Generation.

The well-defined heterostructure of the photocathode is desirable for photoelectrochemically producing hydrogen from aqueous solutions. Herein, enhanced heterostructures were fabricated based on typical stable covalent organic framework (TpPa-1) films and TiO2 nanotube arrays (NTAs) as a proof-of-concept model to tune the photoelectrochemical (PEC) hydrogen generation by tailoring the photoelectrode microstructure and interfacial charge transport. Ultrathin TpPa-1 films were uniformly grown on the surface of TiO2 NTAs via a solvothermal condensation of building blocks by tuning the monomer concentration. The Pt1@TpPa-1/TiO2-NTAs photoelectrode with single-atom Pt1 as a co-catalyst demonstrated improved visible-light response, enhanced photoconductance, lower onset potential, and decreased Tafel slope value for hydrogen evolution. The hydrogen evolution rate of the Pt1@TpPa-1/TiO2-NTAs photoelectrode was five times that of Pt1@TpPa-1 under AM 1.5 simulated sunlight irradiation and the bias voltage of 0 V. A lower overpotential was recorded as 77 mV@10 mA cm-2 and a higher photocurrent density as 1.63 mA cm-2. The hydrogen evolution performance of Pt1@TpPa-1/TiO2-NTAs photoelectrodes may benefit from the well-matched band structures, effective charge separation, lower interfacial resistance, abundant interfacial microstructural sites, and surficial hydrophilicity. This work may raise a promising way to design an efficient PEC system for hydrogen evolution by tuning well-defined heterojunctions and interfacial microstructures.

Highly catalytic nanoenzyme of covalent organic framework loaded starch- surface-enhanced Raman scattering/absorption bi-mode peptide as biosensor for ultratrace determination of cadmium.

High affinity peptides (PTs) have been used in nanoanalysis, but there are no reports which combine PTs with a liquid crystal (LC) covalent organic framework (COF) supported soluble starch (SS) catalytic amplification system as a biosensor recognition element. In this study, a new, highly sensitive and selective bi-mode molecular biosensor has been developed for the determination of cadmium ion (Cd2+). Specifically, a highly catalytic and stable COF supported SS nanosol catalyst was fabricated such that a nanocatalytic indicator reaction system for HAuCl4-sodium formate was established based on surface-enhanced Raman scattering (SERS). The Au nanoparticles produced exhibited a surface plasmon resonance (SPR) absorption peak at 535 nm and a SERS peak at 1,615 cm-1. Combining the nanocatalytic amplification indicator system with the specific PTs reaction permitted a sensitive and selective SERS/absorption bi-mode platform to be developed for the determination of cadmium in rice. The linear range for SERS determination was 0.025-0.95 nmol/L and the detection limit (DL) was 0.012 nmol/L.

Stable and microporous covalent organic frameworks via weak interactions for gas uptake

Here, we reported two stable and micro-porous covalent organic frameworks for gas uptake.

Construction of highly-stable covalent organic framework with combined enol-imine and keto-enamine linkages.

A novel covalent organic framework (COF) (Tp-BI-COF) with combined ketimine-type enol-imine and keto-enamine linkages was prepared through a cascade of ketimine condensation followed by aldimine condensation and characterized by XRD, solid state 13C NMR, IR, TGA and BET. Tp-BI-COF showed high stability toward acid, organic solvent, and boiling water. The 2D COF exhibited photochromic properties after being irradiated with a xenon lamp. The stable COF, with aligned one-dimensional nanochannels, provided nitrogen sites on pore walls, which confine and stabilize the H3PO4 in the channel via hydrogen-bonding interactions. After loading with H3PO4, the material showed excellent anhydrous proton conductivity.

A series of polyoxometalate-based COF composites by one-pot mechanosynthesis of thioether to sulfone.

An effective combination of polyoxometalates (POMs) and porous materials is a feasible method to solve the homogeneity of POMs and synthesize extremely stable POM-based catalysts. Herein, by using simple mechanochemical synthesis, we fabricated a series of composites constructed by Keggin-POMs, p-phenylenediamine (Pa-1), and 1,3,5-triformylphloroglucinol (Tp), which in situ form a stable covalent organic framework (Keggin-POMs@TpPa-1). Notably, the different Keggin-POMs@TpPa-1 composites showed different catalytic effects on thioether oxidation reaction under mild conditions. From the comparison, the catalytic effect of PW12@TpPa-1 with its added amount of 27% H3PW12O40 is superior to that of other composites, whose catalytic efficiency can reach 99%. This study provides some inspiration for designing diverse POM-modified catalysts with outstanding stability and efficiency using COFs as supports.

Synthesis of a Highly Crystalline Amide‐Linked Covalent Organic Framework

Comprehensive SummaryThe dilemma for the synthesis of covalent organic frameworks (COFs) is crystallinity and stability. Herein, we present the synthesis of a single‐crystalline amide‐linked COF, and demonstrate its excellent chemical as well as thermal stabilities. This study will inspire the synthesis of a wide spectrum of highly crystalline and stable COFs, promote their structure‐property investigations and boost their applications in selective gas adsorption, storage and separation.

Templated Synthesis of 2D Polyimide Covalent Organic Framework for Rechargeable Sodium-Ion Batteries.

Covalent organic frameworks (COFs) hold great promise for electrochemical energy storage because of their high surface area, readily accessible redox-active sites, and environment-friendly chemical composition. In this study, the synthesis of a redox-active pyrene-containing polyimide COF (PICOF-1) by linker exchange using an imine-linked COF as a template is reported and its performance in sodium-ion batteries (SIBs) is demonstrated. The reported synthetic route based on linker exchange mitigates the challenges typically encountered with crystallizing chemically stable polyimide COFs from typical condensation reactions; thus, facilitating their rapid synthesis and purification. Using this approach, PICOF-1 exhibits high crystallinity with very low refinement parameters RP and RWP of 0.415% and 0.326%, respectively. PICOF-1 has a high Brunauer-Emmette-Teller (BET) surface area of 924m2 g-1 and well-defined one-dimentional (1D) channels of 2.46×1.90nm, which enable fast ion transport and charge transfer, reaching a capacity at 0.1C of almost nearly as its theoretical capacity and maintaining 99% Coulombic efficiency over 175 cycles at 0.3C. The study demonstrates that imine-linked COFs are effective templates for integrating carbonyl-rich polyimide moieties into high-surface COFs to advance electrochemical energy storage applications.

An integrated solid-state lithium-oxygen battery with highly stable anionic covalent organic frameworks electrolyte

<h2>Summary</h2> Rechargeable solid-state lithium-oxygen (Li-O₂) batteries are considered promising candidates for next-generation energy storage systems. However, the development of solid-state Li-O₂ batteries has been limited by the lack of solid-state electrolytes (SSEs) with high ionic conductivities and high stability toward air/metal Li. To address this challenge, we report the three-dimensional covalent organic framework (3D COF) CD-COF-Li with a prominent Li-ion conductivity of 2.7 × 10⁻³ S cm⁻¹ as the SSE for Li-O₂ batteries. The ion transport in the porous COFs is clarified through two-dimensional exchange nuclear magnetic resonance spectroscopy (2D-EXSY NMR), which provides a method at the forefront for understanding Li-ion transport in the molecular channel. Solid-state Li-O₂ batteries with CD-COF-Li deliver a high specific capacity (9,340 mAh g⁻¹) and a record cycling life (100 cycles). These findings pave the way for the application of COFs as superior Li-ion conductors in other next-generation solid-state energy storage systems.

Bithiophene-based COFs for silver ions detection and removal

Covalent organic frameworks (COFs) are promising in the detection and removal of toxic heavy metal ions from water. However, achieving detection and removal functions requires systematic control of stability, porosity, pore environment, and luminescence properties, which remains a challenge for most COFs materials. Herein, we develop a bithiophene-based stable COF, TAPA-BTDC, with dual functions of simultaneous detection and removal of silver ions from aqueous solution through elaborate structural design and control of the pore environment. The obtained TAPA-BTDC has good thermal stability and chemical stability, possesses high crystallinity and moderate specific surface area (396.83 m2/g), and contains dense bithiophene-S chelating sites on the pore walls. These characteristics work together to give TAPA-BTDC the dual function of selective detection and effective removal of silver ions from aqueous solution. This dual function can be attributed to the π-complexation between bithiophene-S chelating sites and silver ions, as verified by X-ray photoelectron spectroscopy. The charge transfer between bithiophene-S and silver ions alters the weak fluorescence emission caused by the π−π layered structure, enabling TAPA-BTDC to selectively detect silver ions through fluorescence enhancement, and even trace amounts of Ag+ (2.4 × 10−4 mmol/L) can cause observable fluorescence emission. Meanwhile, these bithiophene-S scattered across the pore wall also provide a large number of accessible chelating sites for TAPA-BTDC, enabling it to effectively remove Ag+ with a maximum adsorption capacity up to 398.61 mg/g, and TAPA-BTDC can be easily regenerated five times without loss of performance. The results of this study reveal the exceptional potential of COFs with reasonably tailor-made structure in dealing with heavy metal ion contamination.