Use Of Covalent Organic Frameworks Research Articles

Structural Characteristics and Environmental Applications of Covalent Organic Frameworks

Covalent organic frameworks (COFs) are emerging crystalline polymeric materials with highly ordered intrinsic and uniform pores. Their synthesis involves reticular chemistry, which offers the freedom of choosing building precursors from a large bank with distinct geometries and functionalities. The pore sizes of COFs, as well as their geometry and functionalities, can be pre-designed, giving them an immense opportunity in various fields. In this mini-review, we will focus on the use of COFs in the removal of environmentally hazardous metal ions and chemicals through adsorption and separation. The review will introduce basic aspects of COFs and their advantages over other purification materials. Various fabrication strategies of COFs will be introduced in relation to the separation field. Finally, the challenges of COFs and their future perspectives in this field will be briefly outlined.

Study on the efficiency of a covalent organic framework as adsorbent for the screening of pharmaceuticals in estuary waters

Herein, we demonstrate, for the first time, that covalent organic frameworks (COFs) can be efficient adsorbents for the screening of pharmaceuticals in real water samples, obtaining highly representative data on their occurrence and avoiding the cost of carrying high volume samples and tedious and costly clean-up and preconcentration steps. Of the 23 pharmaceuticals found present in the water samples from the Tagus river estuary using state-of-the-art solid-phase extraction (SPE), 22 were also detected (adsorbed and recovered for analysis) using a COF as the adsorbent material with adsorption efficiency of over 80% for nearly all compounds. In specific cases, acidification of the water samples was identified to lead to a dramatic loss of adsorption efficiency, underlining the effect of sample pre-treatment on the results. The COF efficiently adsorbed (>80%) 19 pharmaceuticals without acid treatment of the sample, highlighting the potential of this class of materials for representative in situ passive adsorption of pharmaceuticals, making this material suitable for being used in water monitoring programs as a simple and cost-efficient sample preparation procedure. In the case of α-hydroxyalprazolam and diclofenac, the COF outperformed the SPE procedure in the recovery efficiency. Although further efforts should be made in tailoring the desorption of the pharmaceuticals from the COF by using different solvents or solvent mixtures, we propose COFs as convenient adsorbent for broad-scope screening and as an efficient adsorbent material to target specific classes of pharmaceuticals. To the best of our knowledge, this is the first study on the use of COFs for contaminant screening in real, naturally contaminated water samples.

Synthesis of nitrogen-containing covalent organic framework with reversible iodine capture capability

The use of covalent organic frameworks (COFs) to cope with environmental issues was promising but making them commercial and practical remains a challenge. In this work, we explored the crystallization of a stable nitrogen-containing covalent organic framework (TAPA-PDA COF) and used it as a porous platform for efficient and reversible volatile iodine capture. The TAPA-PDA COF, prepared by simple, rapid, low temperature reactions, possesses a high BET surface area of over 685 m 2 /g, good crystallinity, high thermal stability and ultrahigh capture of iodine vapor up to 5.09 g/g, which was significantly higher than most reported iodine adsorbents. Although the Raman spectroscopy confirmed the formation of polyiodide compounds through charge transfer between sorbents and iodine molecules, the TAPA-PDA COF still has the ability to be recycled five times while maintaining a high uptake capacity. This study provides promising strategy to solve the difficult problems of commercializing COFs and utilizing them for practical application. • The TAPA-PDA COF was constructed as a porous platform for efficient and reversible volatile iodine capture. • The TAPA-PDA COF demonstrated ultrahigh capture of iodine vapor up to 5.09 g/g. • The TAPA-PDA COF's capture of iodine was a hybrid of physisorption and chemisorption. • The simple, fast and low temperature preparation method provides promising strategy to solve the difficult problems of commercializing COFs.

Preparation of a two-dimensional azine-linked covalent organic framework-coated capillary and its application to the separation of nitrophenol environmental endocrine disruptors by open-tubular capillary electrochromatography

As a class of new porous crystalline materials, covalent organic frameworks (COFs) are attracting the attention of a large number of scientists. Because of their large specific surface area, low density, high stability, and tunable pore size, COFs have been widely applied in many fields, including analytical chemistry. Open-tubular capillary electrochromatography (OT-CEC) is a mode of capillary electrochromatography. In recent years, a variety of materials such as porous organic frameworks have been used as the stationary phase for OT-CEC to overcome the disadvantages of low phase ratio and column capacity, thereby improving the separation efficiency. However, there are a few reports on the use of COFs as the stationary phase to improve the separation efficiency of OT-CEC. Environmental endocrine disruptors (EEDs) are a large class of exogenous chemicals that can disturb the effect of the normal endocrine substances and adversely affect the endocrine and reproductive systems of human beings. Considering the widespread existence of EEDs and the disadvantages of existing detection methods (e. g., gas chromatography-mass spectrometry and high performance liquid chromatography-mass spectrometry), such as complicated operation and large sample consumption, it is necessary to develop new methods for the separation and determination of EEDs in complex samples. OT-CEC is a good choice in this regard because of its low sample dosage, simple operation, and high analytical speed. Accordingly, a two-dimensional azine-linked covalent organic framework (ACOF-1) with a large surface area and small pore size was synthesized according to the reference method. Then, an ACOF-1-coated capillary was fabricated using ACOF-1 as the stationary phase, via covalent bonding, and used as the separation channel to establish a new OT-CEC method for the separation and detection of nitrophenol EEDs. X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and scanning electron microscopy (SEM) were used to characterize the synthesized ACOF-1 and the ACOF-1-coated capillary. The peak pattern in the XRD spectrum confirmed the successful synthesis of ACOF-1. The absorption peaks in the FT-IR spectrum and the morphology of the inner wall seen in the SEM images also demonstrated that the ACOF-1-coated capillary was fabricated successfully. A series of experiments were carried out to investigate the effects of the organic additive (methanol) content, pH, and concentration of the borate buffer on the resolution, migration time, and peak shape. Based on the results, the optimal separation conditions for the four nitrophenol analytes were 15 mmol/L borate (pH 9.20) with 10% (v/v) methanol. Under the optimum conditions, 2-nitrophenol (2-NP), 4-nitrophenol (4-NP), 2,4-dinitrophenol (DNP), and 2,4,6-trinitrophenol (TNP) could be baseline separated within 20 min by the established OT-CEC method. The linear range for 2-NP and 4-NP was 10-500 mg/L, while that for DNP and TNP was 20-1000 mg/L. The determination coefficients (R2) were greater than 0.99. For the four analytes, the limits of detection and limits of quantitation were in the ranges of 0.13-0.23 mg/L and 0.45-0.60 mg/L, respectively. The intraday, interday, and column-to-column relative standard deviations (RSDs) of the migration time and peak area were less than 9.4%. These results revealed that the established method has good repeatability and high stability, thus being suitable for the separation and detection of nitrophenol EEDs. The mechanism studies revealed that the pore size of ACOF-1 was the main factor influencing the separation behavior of each analyte. This work demonstrated the feasibility of using capillary electrochromatography with COFs as the stationary phase for the separation and detection of EEDs. Future research will continue to focus on the preparation of COF-coated capillaries and their application to OT-CEC separation and determination of EEDs.

Metal–Organic Frameworks (MOFs) and Covalent Organic Frameworks (COFs) Applied to Photocatalytic Organic Transformations

Among the different alternatives for catalysis using metal–organic frameworks (MOFs) or covalent organic frameworks (COFs), photocatalysis has remarkably evolved during the last decade. Photocatalytic reticular materials allowed recyclability and easy separation of catalyst from the product, also reaching the activity and selectivity commonly observed for molecular systems. Recently, photocatalytic MOFs and COFs have been applied to synthetic applications in order to obtain organic molecules of different complexity. However, although a good number of works have been devoted to this issue, an updated comprehensive revision on this field is still needed. The aim of this review was to fill this gap covering the following three general aspects: (1) common strategies on the design of reticular photocatalytic materials, (2) a comprehensive discussion of the photocatalytic organic reactions achieved by the use of COFs and MOFs, and (3) some critical considerations highlighting directions that should be considered in order to make advances in the study of photocatalytic COFs and MOFs.

Three-dimensional electrochemical sensor with covalent organic framework decorated carbon nanotubes signal amplification for the detection of furazolidone

In this study, we developed a facile one-pot strategy for immobilizing a covalent organic framework (COF) on an amino-functionalized carbon nanotube (NH2-CNT) support at room temperature. The COF@NH2-CNT composite obtained possessed a high specific surface area (147.3 m2 g−1), and good conductivity when deposited on the glassy carbon electrode (GCE). The COF@NH2-CNT/GCE sensor offered excellent analytical performance for the detection of the nitrofuran antibacterial agent furazolidone. The sensor offered a wide range furazolidone detection range from 0.2 μM to 100 μM, with a low limit of detection of 7.75 × 10-8 M. The recovery test was performed in chicken and lamb samples with satisfactory recoveries ranged from 87.8%–126.9%. The furazolidone residues in beef and pork samples were determined using the COF@NH2-CNT/GCE sensor and high-performance liquid chromatography. There was no significant difference between the two methods. The ability of the COF to efficiently adsorb furazolidone was a key to high sensitivity and fast response of the sensor platform. Results encourage the wider use of COFs in electrochemical sensor development.

Adsorption characteristics and cooling/heating performance of COF-5

Adsorption-driven heat pumps (AHPs) powered by low-grade waste heat and solar energy are promising to reduce the energy consumption for cooling and heating. Seeking novel high-performing adsorbents of AHPs is the key to improve their cooling/heating performance. Very recently, the potential use of covalent-organic frameworks (COFs) adsorbents of AHPs has won increasing research interests. However, the adsorption cooling/heating performance of COFs under varying working conditions is still unexplored. In this study, COF-5 was synthesized and tested to obtain the adsorption isotherms of ethanol working fluid. For comparison, three commonly used metal-organic frameworks (MOFs), including Cu-BTC, MIL-101(Cr), and ZIF-8, were also investigated. The cooling/heating performance of COF-5/ethanol and MOF/ethanol working pairs was evaluated by mathematical modelling based on the basic thermodynamic cycle of AHPs. The results demonstrated that COF-5 with the high coefficient of performance for cooling/heating (COPC/COPH) under wide temperature lift (ΔTlift) outperformed MOFs, especially for cooling.

Fabricating Organic Nanotubes through Selective Disassembly of Two-Dimensional Covalent Organic Frameworks.

Covalent organic frameworks (COFs) are an emerging class of crystalline porous organic polymers with potential for innovative applications. Here we report the use of COFs as precursors for the fabrication of well-defined tubular nanomaterials. A proof-of-concept study is presented for the controllable fabrication of organic nanotubes through selective disassembly of two-dimensional heteropore COFs. Two dual-pore COFs are constructed based on orthogonal reactions. Each COF possesses two different kinds of pores, which are formed by linking all-hydrzaone-bonded nanopores with boroxines. Selectively hydrolyzing boroxine rings in the COFs while keeping hydrazone linkages untouched gives rise to organic nanotubes with diameters and shapes corresponding to the nanochannels of the COFs.

Energy transfer in covalent organic frameworks for visible-light-induced hydrogen evolution

We report herein the use of covalent organic frameworks (COFs) to facilitate the energy transfer from sensitizer to the active sites for efficient photocatalysis. The results indicate that the photocatalytic efficiency can be apparently enhanced by using the layered COFs. The visible-light-induced hydrogen evolution rate (10.4 mmol g−1 h−1) for Pd0/TpPa-1 sensitized by Eosin Y was 10 times higher than that of Pd/C. The enhanced photocatalytic H2-production activity could be originated from the improvement of the photogenerated electron transfer in conjugated COFs. The important role of COFs in facilitating the transfer of photogenerated electrons was verified by the transient photocurrent response and the luminescence analysis. This research highlights the use of COFs to investigate the energy transfer process.

Ball milling synthesis of covalent organic framework as a highly active photocatalyst for degradation of organic contaminants.

Facile, environmentally-friendly fabrication of high-yield and stable covalent organic framework (COF) materials has been a limitation to their large-scale production and application. In this work, ball milling was used to synthesize COF by mechanochemical reaction between 1,3,5-triformylphloroglucinol (Tp) and melamine (MA) at ambient temperature. Different routes (liquid-free, solvent-assisted and catalyst-assisted) and proportions of liquids (solvents or catalyst) were investigated. Two morphologies were obtained when various amounts of liquid were added during grinding. The two forms were interwoven thread-shaped and exfoliative thin ribbon-like structures. Further, visible-light photocatalytic (λ > 400 nm) performance and mechanism of the two structures of COFs were investigated. The exfoliative ribbon-like structures exhibited greater rates of photodegradation of phenol and retained 87.6% of initial photodegradation after being used four times. Addition of liquid catalyst not only improved crystallinity of the COF materials, but also enhanced rates of photocatalytic reactions. Photocatalytic activity of the exfoliative structure of TpMA synthesized by ball milling was comparable with that of the photocatalyst prepared by use of the solvothermal method, while time to prepare the catalyst was shortened by 36-fold and the amount of solvent used was reduced by 8-fold at room temperature. Mechanochemical synthesis is a promising potential tool for large-scale production of COFs, which will make greater use of COFs for degradation of pollutants.

The use of covalent organic frameworks as template for conductive polymer synthesis and their sensor applications

In this study, the synthesis of a mesoporous crystalline covalent organic framework (COF) based on melamine and 1,4-dibromo butane as COF-1,4 and its use as a template for in situ synthesis of conductive polymers such as poly(Aniline) (PANi) and poly(Pyrrole) (PPy) within the pores were reported. The synthesized COF-1,4/conductive polymer semi-Interpenetrating Network (semi-IPN) composites were characterized via FT-IR spectroscopy and thermogravimetric analyzer (TGA), and the conductivities of the prepared composites were determined with an electrometer. It was found that upon the in situ synthesis of the conductive polymers such as PANi and PPy within COF-1,4, the conductivity of bare COF-1,4 was increased 3 million-fold for COF-1,4/PANi, and 500 thousand-fold for COF-1,4/PPy, respectively. Furthermore, the potential sensor applications of these COF-1,4/conductive polymer semi-IPN composites were investigated against HCl and NH3, and methyl orange (MO), and methylene blue (MB) dyes in aqueous solutions. The sensor studies revealed that the conductivity of bare COF-1,4 increased 20 and 7K-folds upon 15 min exposure to HCl and NH3 gases, respectively. Interestingly, an eightfold decrease in the conductivity of COF-1,4/PANi was observed upon 15 min exposure to NH3 gas vapor at ambient conditions. Also, the conductivities of prepared COF-1,4 and its conductive polymer composites changed after treatment with MO and MB dyes suggesting other potential sensor applications of these porous materials.

Mechanically Shaped Two-Dimensional Covalent Organic Frameworks Reveal Crystallographic Alignment and Fast Li-Ion Conductivity.

Covalent organic frameworks (COFs) usually crystallize as insoluble powders, and their processing for suitable devices is thought to be limited. We demonstrate that COFs can be mechanically pressed into shaped objects having anisotropic ordering with preferred orientation between hk0 and 00l crystallographic planes. Five COFs with different functionality and symmetry exhibited similar crystallographic behavior and remarkable stability, indicating the generality of this processing. Pellets prepared from bulk COF powders impregnated with LiClO4 displayed room temperature conductivity up to 0.26 mS cm(-1) and high electrochemical stability. This outcome portends use of COFs as solid-state electrolytes in batteries.

Tunable Water and CO2 Sorption Properties in Isostructural Azine-Based Covalent Organic Frameworks through Polarity Engineering

The use of covalent organic frameworks (COFs) in environmental settings such as atmospheric water capture or CO2 separation under realistic pre- and post-combustion conditions is largely unexplored to date. Herein, we present two isostructural azine-linked COFs based on 1,3,5-triformyl benzene (AB-COF) and 1,3,5-triformylphloroglucinol (ATFG-COF) and hydrazine building units, respectively, whose sorption characteristics are precisely tunable by the rational design of the chemical nature of the pore walls. This effect is particularly pronounced for atmospheric water harvesting, which is explored for the first time using COFs as adsorbents. We demonstrate that the less polar AB-COF acts as a reversible water capture and release reservoir, featuring among the highest water vapor uptake capacity at low pressures reported to date (28 wt % at <0.3 p p0−1). Furthermore, we show tailored CO2 sorption characteristics of the COFs through polarity engineering, demonstrating high CO2 uptake at low pressures ( <1 bar)...