Covalent organic framework materials, their preparation methods and applications

Abstract

This invention provides a covalent organic framework material, its preparation method, and its applications. The invention selects 1,3,6,8-tetra(4-aminophenyl)pyrene as the central building block and achieves trapped states within covalent organic frameworks (COFs) by controlling the symmetry of the active site (benzothiadiazole, BT). Compared to its centrosymmetric counterpart, by breaking the symmetry of the central BT core, the density of trapped states in COFs can be significantly reduced, and the lifetime of shallow trapped states can be extended to 1160.02 picoseconds. This allows more photogenerated electrons to participate in the photocatalytic reaction, ultimately achieving significant photocatalytic reduction of uranium (VI) (>99%). This invention elucidates for the first time the crucial role of symmetry breaking in suppressing excited-state decay in COFs, providing profound insights into how to improve the photocatalytic performance of COFs in environmental remediation.

Description

Technical Field

[0001] This invention relates to the field of nuclear contaminant treatment technology, and in particular to a covalent organic framework material, its preparation method, and its application. Background Technology

[0002] During uranium mining, extraction, beneficiation, and tailings management processes release soluble hexavalent uranium (U(VI)) into aquatic environments through various pathways, including acid mine drainage, waste rock leaching, and accidental leaks. Unlike chemically inert pollutants, U(VI) is highly soluble, has strong migration capabilities, and is significantly toxic, allowing it to persist and spread in the environment for extended periods. The continuous accumulation of large amounts of soluble U(VI) in natural water bodies, coupled with the synergistic effect of its radiotoxicity and chemical toxicity, poses a persistent and irreversible threat to ecosystem function and human health.

[0003] To address the aforementioned challenges, photocatalytic reduction technology based on polymer semiconductors is considered a sustainable uranium recovery strategy due to its high efficiency, environmental friendliness, and recyclability. Among numerous candidate materials, covalent organic frameworks (COFs), with their regular pore structure, tunable electronic properties, and the ability to be precisely designed using organic precursors, have become a flexible platform material with broad application prospects in treating uranium-containing wastewater. However, a key limitation currently exists in the photocatalytic application of these crystalline porous polymers: their photocatalytic activity remains to be further improved due to the kinetic characteristics such as short lifetime of photogenerated carriers and high recombination rate.

[0004] The key to addressing these challenges lies in effectively utilizing the physical concept of "long-lived shallow trap states"—a transient process in which photogenerated carriers are briefly bound at shallow-energy local sites without recombination. These shallow trap states not only extend the lifetime of photogenerated carriers and maintain their ability to migrate to catalytically active sites, but also effectively delay unfavorable charge recombination processes, thereby significantly increasing the carrier concentration available for surface redox reactions. Existing research has shown that introducing structural asymmetry (such as defect engineering or doping with heteroatoms like B, S, N, and P) into photocatalysts can effectively break the electronic symmetry of materials, thereby inducing the formation of shallow trap states and modulating their photocatalytic performance. However, achieving precise control of shallow trap states still faces significant challenges, mainly because traditional modification methods often involve lattice distortion or uneven defect distribution, making it difficult to stably and controllably construct ideal shallow trap states.

[0005] The atomic-level structural designability of COF materials offers a new path to overcome the aforementioned limitations. Compared to traditional rigid semiconductors, the flexible framework and customizable organic units of COFs allow for the precise design at the molecular level to introduce asymmetric active sites while maintaining structural order, thereby modulating carrier relaxation paths and achieving the ordered construction of shallow trap states. However, research on the purposeful and controllable construction of long-lived shallow trap states in COFs based on symmetry breaking strategies is still in its early stages and requires further systematic exploration. Summary of the Invention

[0006] To address the problems mentioned in the background, this invention proposes a covalent organic framework material, its preparation method, and its applications. Based on a symmetry-breaking strategy, a series of covalent organic framework materials (COFs) with BT as the core connecting unit are constructed by controlling the symmetry of the benzothiadiazole (BT) core. This invention is the first to successfully induce long-lived shallow trap states in COFs materials through a symmetry-breaking strategy, achieving not only highly efficient photocatalytic reduction of uranium but also providing new theoretical basis and practical pathways for artificial photosynthesis and the design of related photocatalytic materials.

[0007] In a first aspect, the present invention provides a covalent organic framework material selected from at least one of BTCOF-Asy-1F, BTCOF-Sym-2F, and BTCOF-Sym-0F, wherein the structural unit formula of BTCOF-Sym-0F is as follows: Figure 16 As shown; the structural unit formula of BTCOF-Asy-1F is as follows Figure 17 As shown; the structural unit formula of BTCOF-Sym-2F is as follows Figure 18 As shown.

[0008] Secondly, the present invention provides a method for preparing a covalent organic framework material, comprising:

[0009]

[0010] The compound shown in formula (II) and the compound shown in formula (III) are mixed and dissolved in a solvent, and the reaction is carried out under vacuum conditions with acetic acid as a catalyst.

[0011] After the reaction is complete, centrifuge, wash, and dry to obtain the covalent organic framework material;

[0012] Among them, each X 1 and X 2 It can be hydrogen or F independently.

[0013] In some embodiments, the solvent is at least one of 1,1-dichlorobenzene and n-butanol.

[0014] In some embodiments, the molar ratio of the compound shown in formula (II) to the compound shown in formula (III) is (1~5):(1~5), preferably 5:1, 4:1, 3:1, 2:1, 1:1, 1:2, 1:3, 1:4, 1:5, and any two of the above values ​​constitute any one of the ranges.

[0015] In some embodiments, the reaction temperature is 100~150℃, preferably 100℃, 110℃, 120℃, 130℃, 140℃, 150℃, or any two of the above values ​​forming a range.

[0016] In some embodiments, the reaction time is 60 to 80 hours, preferably any one of the ranges of 60 hours, 65 hours, 70 hours, 72 hours, 75 hours, 80 hours, or any two of the above values.

[0017] In some embodiments, the molar ratio of the compound shown in formula (II) to acetic acid is 1:(10~50), preferably 1:10, 1:20, 1:30, 1:40, 1:50, and any two of the above values ​​forming a range.

[0018] In some embodiments, the drying is carried out under vacuum conditions, with a drying temperature of 30~60°C, preferably any one of any two values ​​from 30°C, 40°C, 50°C, 60°C, and above; and a drying time of 6~14 hours, preferably any one of any two values ​​from 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, 11 hours, 12 hours, 13 hours, 14 hours, and above.

[0019] In some embodiments, the washing includes: sequentially performing an acetone wash and a tetrahydrofuran wash; the tetrahydrofuran wash is performed 3 to 5 times, and the acetone wash is performed 3 to 5 times.

[0020] Thirdly, the present invention provides a material for uranium extraction, the material comprising the covalent organic framework material described in the present invention or the covalent organic framework material prepared by the preparation method described in the present invention.

[0021] Fourth aspect: The present invention provides a photocatalytic material, characterized in that the material includes the covalent organic framework material described in the present invention or the covalent organic framework material prepared by the preparation method described in the present invention.

[0022] Beneficial effects

[0023] (1) This invention provides a covalent organic framework material, specifically by introducing different numbers of electronegative fluorine atoms (BT_X, X = 0F, 2F, or 1F) into the BT unit to regulate its electron-deficient properties as an acceptor unit, thereby synthesizing COFs materials with a DA structure, named BTCOF-Sym-0F, BTCOF-Sym-2F, and BTCOF-Asy-1F, respectively. Among them, by replacing hydrogen atoms with fluorine atoms at the edge of the BT core, the original symmetry structure was successfully broken, resulting in BTCOF-Asy-1F with an asymmetric BT core. Compared with the centrosymmetric BTCOF-Sym-0F and BTCOF-Sym-2F, the asymmetric BTCOF-Asy-1F exhibits a significantly extended shallow electron trap lifetime (τ2 = 1160.02 ps), which is much higher than the 6.02 ps and 43.84 ps of the symmetric BTCOF-Sym-0F and BTCOF-Sym-2F, respectively. This extended trap lifetime effectively promotes the reaction of photogenerated electrons with O2 to generate superoxide radicals. This drives the efficient photocatalytic reduction of U(VI), achieving a removal rate of over 99% within 70 minutes.

[0024] (2) This invention selects 1,3,6,8-tetra(4-aminophenyl)pyrene (PPTA) as the central building block because it readily forms a highly crystalline framework with significant photoelectric properties. By modulating the symmetry of the active site (benzothiadiazole, BT), shallow trap states are achieved within covalent organic frameworks (COFs). Compared to its centrosymmetric counterpart, breaking the symmetry of the central BT core significantly reduces the trap state density in COFs, and extends the lifetime of the shallow trap states to 1160.02 picoseconds. This allows more photogenerated electrons to participate in the photocatalytic reaction, ultimately achieving significant photocatalytic reduction of uranium (VI) (>99%, within 70 minutes). This invention elucidates for the first time the crucial role of symmetry breaking in suppressing excited-state decay in COFs, providing profound insights into how to improve the photocatalytic performance of COFs in environmental remediation. Attached Figure Description

[0025] Figure 1 Experimental and simulated PXRD spectra of the covalent organic framework materials prepared in Examples 1-3 are shown. Figure 1 a) is BTCOF-Sym-0F. Figure 1 b) is BTCOF-Asy-1F. Figure 1 c) is BTCOF-Sym-2F.

[0026] Figure 2 FT-IR spectra of the covalent organic framework materials prepared in Examples 1-3.

[0027] Figure 3 Solid-state preparation of covalent organic framework materials prepared in Examples 1-3 13 C10 NMR spectrum.

[0028] Figure 4 Full-spectrum XPS images of the covalent organic framework materials prepared in Examples 1-3.

[0029] Figure 5 The nitrogen adsorption isotherm was measured at 77 K for the covalent organic framework materials prepared in Examples 1-3.

[0030] Figure 6 Pore ​​size distribution diagrams of the covalent organic framework materials prepared in Examples 1-3.

[0031] Figure 7 This is a SEM image of BTCOF-Sym-0F.

[0032] Figure 8 This is a SEM image of BTCOF-Asy-1F.

[0033] Figure 9 This is a SEM image of BTCOF-Sym-2F.

[0034] Figure 10 The UV-Vis diffuse reflectance spectra (UV-DRS) of the covalent organic framework materials prepared in Examples 1-3.

[0035] Figure 11 Tauc diagrams for the covalent organic framework materials prepared in Examples 1-3.

[0036] Figure 12 The images show femtosecond transient absorption spectra (fs-TAS) of the covalent organic framework materials prepared according to embodiments of the present invention. (a) Two-dimensional pseudo-color images of BTCOF-Sym-0F, (d) BTCOF-Asy-1F, and (g) BTCOF-Sym-2F; (b) femtosecond transient absorption (fs-TA) spectra of BTCOF-Sym-0F, (e) BTCOF-Asy-1F, and (h) BTCOF-Sym-2F at several representative detection delay times; and (c) kinetic decay curves of BTCOF-Sym-0F, (f) BTCOF-Asy-1F, and (i) BTCOF-Sym-2F detected at 540 nm.

[0037] Figure 13The following are photoelectric characterization images of the covalent organic framework materials prepared in the embodiments of the present invention. Among them, (a) trap state density spectrum; (b) steady-state PL spectrum; (c) transient PL spectrum; (d) transient photocurrent density; (e) transient open-circuit voltage decay (OCVD) test; (f) Nyquist plot.

[0038] Figure 14 The diagram shows the effect of pH on the covalent organic framework material prepared in the embodiments of the present invention.

[0039] Figure 15 Photocatalytic performance of BTCOF-Sym-0F, BTCOF-Asy-1F, and BTCOF-Sym-2F in uranium-bearing groundwater (U(VI) concentration 40.0 ppm, volume 50 mL). (a) Time course of U(VI) photocatalytic reduction; (b) Effect of BTCOF-Sym-0F, BTCOF-Asy-1F, and BTCOF-Sym-2F on UO2 under illumination. 2+ (c) Adsorption isotherms; (d) Comparison of uranium extraction performance of BTCOF-Asy-1F with reported materials in natural seawater; (e) Cycling performance of BTCOF-Asy-1F; (f) Effect of interfering cations; (c) Removal effect of BTCOF-Asy-1F on uranium from uranium mine wastewater samples.

[0040] Figure 16 The structural diagram of the structural unit of the covalent organic framework material BTCOF-Sym-0F prepared in Example 1.

[0041] Figure 17 The structural diagram of the structural unit of the covalent organic framework material BTCOF-Asy-1F prepared in Example 2.

[0042] Figure 18 The structural diagram of the structural unit of the covalent organic framework material BTCOF-Sym-2F prepared in Example 3.

[0043] Terminology Explanation

[0044] Certain embodiments of the invention will now be described in detail, examples of which are illustrated by the accompanying structural and chemical formulas. The invention is intended to encompass all alternatives, modifications, and equivalents, all of which are included within its scope. Those skilled in the art will recognize that many similar or equivalent methods and materials can be used to practice the invention. The invention is by no means limited to the methods and materials described herein. In the event that one or more of the incorporated documents, patents, and similar materials differ from or contradict this application (including, but not limited to, defined terminology, application of terminology, described techniques, etc.), this application shall prevail.

[0045] It should be further appreciated that certain features of the invention, for clarity, have been described in multiple independent embodiments, but may also be provided in combination in a single embodiment. Conversely, various features of the invention, for brevity, have been described in a single embodiment, but may also be provided individually or in any suitable sub-combination.

[0046] Unless otherwise stated, all technical terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art. All patents and publications related to this invention are incorporated herein by reference in their entirety.

[0047] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0048] In the following content, all numbers disclosed herein, whether or not they use words such as "approximately" or "about," are approximate values. The value of each number may vary by 1%, 2%, 5%, 7%, 8%, 10%, 15%, or 20%. Whenever a number with a value of N is disclosed, any numbers with values ​​of N+ / -1%, N+ / -2%, N+ / -3%, N+ / -5%, N+ / -7%, N+ / -8%, N+ / -10%, N+ / -15%, or N+ / -20% will be explicitly disclosed, where "+ / -" indicates addition or subtraction. Detailed Implementation

[0049] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. The specific embodiments described herein are for illustrative purposes only and are not intended to limit the invention in any way. Furthermore, descriptions of well-known structures and techniques are omitted in the following description to avoid unnecessarily obscuring the concepts of this disclosure. Such structures and techniques have also been described in many publications.

[0050] All reagents used in this invention can be purchased commercially or prepared by the methods described in this invention.

[0051] The solvents and chemicals used in the experiments were purchased from commercial sources and did not require further purification. Ultraviolet-visible absorption spectra were measured at room temperature using a PerkinElmer Lambda 100 spectrometer. Proton nuclear magnetic resonance ( 1 1H NMR) and carbon nuclear magnetic resonance ( 13 13C NMR) spectra were acquired using a Bruker AVANCE 300 system. Fourier transform infrared (FT-IR) spectra were measured using a Varian 1000 Fourier transform infrared spectrometer. Solid-state 13 13C cross-polarization magic angle spinning nuclear magnetic resonance (SS 13 13C CP / MAS NMR) spectra were recorded using a Bruker Avance II solid-state nuclear magnetic resonance spectrometer operating at a Larmor frequency of 300 MHz and equipped with a standard 4-mm magic angle spinning dual-resonance probe.

[0052] High-resolution transmission electron microscopy (HRTEM) images were obtained using a FEI Tecnai F20 instrument. Electron paramagnetic resonance (EPR) spectra were recorded using a Bruker EMX-plus spectrometer equipped with a nuclear magnetic resonance gaussmeter and a variable-temperature-controlled continuous-flow liquid nitrogen cryostat (Bruker B-VT 2000). Specific surface area and pore size distribution were determined by nitrogen adsorption-desorption measurements at 77 K using a Quantachrome Instruments Autosorb 1 analyzer. Samples were degassed under high vacuum at 150 °C for 24 h before analysis. The BET specific surface area was calculated using data points in the range 0 < P / P0 < 0.25 and fitted using a non-linear density functional theory equilibrium model. Pore size distribution and pore volume were calculated from the adsorption branch of the isotherm using quenched solid density functional theory (QSDFT, with nitrogen as the adsorbate and slit pore carbon as the model).

[0053] X-ray photoelectron spectroscopy (XPS) measurements were performed using an AXIS Ultra DLD instrument from Kratos Analytical, UK, with a monochromatic Al Kα X-ray source (photon energy 1487 eV). Transient photocurrent, open-circuit voltage decay curves, electrochemical impedance spectroscopy, linear sweep voltammetry, and Mott-Schottky measurements were all carried out using a Metrohm Autolab PGSTAT204 electrochemical workstation: a glassy carbon electrode coated with the polymer was used as the working electrode, a platinum wire electrode as the counter electrode, an Ag / AgCl (saturated KCl solution) electrode as the reference electrode, and a 0.2 M Na2SO4 aqueous solution as the electrolyte.

[0054] Working electrode preparation process: Indium tin oxide (ITO) conductive glass was ultrasonically cleaned in ethanol for 30 minutes and then dried at 353 K. The electrode boundary was protected with Scotch tape. 5 mg of sample was dispersed in 1 mL of N,N-dimethylformamide (DMF) and ultrasonically formed into a slurry. This slurry was coated onto the pretreated ITO glass, allowed to air dry, and then the tape was removed. Uncoated areas were encapsulated with epoxy resin. Photoluminescence (PL) spectroscopy was measured using an Edinburgh FLS-1000 fluorescence spectrometer with an Oxford Instruments cryostat at an excitation wavelength of 350 nm. The photoredox potential (PRP) scan rate was 100 mV / s. The test parameters for DMPO and TEMP solutions (0.1 M) were: scan frequency 9.05 GHz, central field strength 323 mT, scan width 100 G, microwave power 5 mW, and test temperature 293 K. All illumination experiments were performed using a continuous laser with a wavelength >400 nm.

[0055] Example 1: Synthesis of BTCOF-Sym-0F

[0056] A mixture containing 1,3,6,8-tetra(4-aminophenyl)pyrene (22.5 mg, 0.04 mmol) and 4,4'-(benzothiadiazole-4,7-diyl)dibenzaldehyde (27.4 mg, 0.08 mmol) was placed in a 10 mL Ambe reaction tube, and 2.0 mL of a 1,1-dichlorobenzene / n-butanol (v / v = 1:1) mixed solvent was added. Then, 0.2 mL of 6 M acetic acid was added as a catalyst. After degassing the system through three freeze-evacuation-thawing cycles, the tube was sealed under vacuum, and the reaction was carried out at 120°C for 72 hours. After the reaction, the solid product was obtained by centrifugation and washed sequentially with anhydrous acetone (3 × 5 mL) and anhydrous tetrahydrofuran (3 × 5 mL). The collected powder product was dried overnight under vacuum at 40°C, finally yielding a red solid BTCOF-Sym-0F (separation yield 94%), whose structural unit formula is as follows. Figure 16 As shown.

[0057] Example 2: Synthesis of BTCOF-Asy-1F

[0058] A mixture containing 1,3,6,8-tetra(4-aminophenyl)pyrene (22.5 mg, 0.04 mmol) and 4,4'-(5-fluorobenzo[c][1,2,5]thiadiazole-4,7-diyl)dibenzaldehyde (29.0 mg, 0.08 mmol) was placed in a 10 mL Ambe reaction tube, and 2.0 mL of a 1,1-dichlorobenzene / n-butanol (v / v = 1:1) mixed solvent was added. Then, 0.2 mL of 6 M acetic acid was added as a catalyst. After degassing the system through three freeze-evacuation-thawing cycles, the tube was sealed under vacuum, and the reaction was carried out at 120°C for 72 hours. After the reaction, the solid product was obtained by centrifugation and washed sequentially with anhydrous acetone (3 × 5 mL) and anhydrous tetrahydrofuran (3 × 5 mL). The collected powder product was dried overnight under vacuum at 40°C, finally yielding a red solid BTCOF-Asy-1F (separation yield 93%), whose structural unit formula is as follows. Figure 17 As shown.

[0059] Example 3: Synthesis of BTCOF-Sym-2F

[0060] A mixture containing 1,3,6,8-tetra(4-aminophenyl)pyrene (22.5 mg, 0.04 mmol) and 4,4'-(5,6-difluorobenzo[c][1,2,5]thiadiazole-4,7-diyl)dibenzaldehyde (30.4 mg, 0.08 mmol) was placed in a 10 mL Ambe reaction tube, and 2.0 mL of a 1,1-dichlorobenzene / n-butanol (v / v = 1:1) mixed solvent was added. Then, 0.2 mL of 6 M acetic acid was added as a catalyst. After degassing the system through three freeze-evacuation-thawing cycles, the tube was sealed under vacuum, and the reaction was carried out at 120°C for 72 hours. After the reaction, the solid product was obtained by centrifugation and washed sequentially with anhydrous acetone (3 × 5 mL) and anhydrous tetrahydrofuran (3 × 5 mL). The collected powder product was dried overnight under vacuum at 40°C, finally yielding a red solid BTCOF-Sym-2F (separation yield 95%), whose structural unit formula is as follows. Figure 18 As shown.

[0061] The structures of the covalent organic framework materials prepared in Examples 1-3 were verified, and the specific results are analyzed below:

[0062] like Figure 1As shown in a), b), and c), the crystallinity of the three COF materials was characterized by powder X-ray diffraction (PXRD). Specifically, their spectra all showed six distinct diffraction peaks: the strongest peak was located at 2.55°, and the remaining weaker signals were located at 3.75°, 5.36°, 8.18°, 10.84°, and 23.70°, respectively. These peaks were attributed to the (110), (200), (220), (330), (440), and (001) crystal planes, respectively. Among them, the diffraction peak near 23.70° corresponds to the (001) crystal plane generated by interlayer π-π stacking. The experimentally measured PXRD patterns were in high agreement with the AA-stacking simulation results of the three COFs. This high crystallinity likely enhances the π-electron interactions within the framework, thereby improving the charge transfer efficiency in the photocatalytic reduction of uranium (VI).

[0063] like Figure 2 As shown, Fourier transform infrared (FT-IR) spectroscopy confirmed the successful imine condensation reaction in all COFs, at ~1599 cm⁻¹. -1 C=N stretching vibration peaks were observed in BTCOF-Sym-0F, BTCOF-Sym-2F, and BTCOF-Asy-1F at approximately 831 cm⁻¹. Additionally, a peak was observed at approximately 831 cm⁻¹. -1 The SN stretching peak (from the BT unit) at 980 cm⁻¹ -1 The CF bond stretching peaks (observed in BTCOF-Sym-2F and BTCOF-Asy-1F) together confirm the formation of the framework.

[0064] like Figure 3 As shown, 13 C-Cross-polarized magic angle rotation ( 13 CP-MAS solid-state NMR spectroscopy revealed a characteristic peak observed at ~152 ppm, attributed to the imine carbon at position 1 (C=N), confirming the successful formation of the Schiff base polymer linkage. Signals in the 100-150 ppm range correspond to sp(s) within the aromatic framework. 2 Hybridized carbon. Notably, the chemical shift at position 2 originates from the C=N group in the benzothiadiazole (BT) structural unit, providing direct evidence for the successful incorporation of the BT structure into the polymer framework.

[0065] like Figure 4As shown, high-resolution X-ray photoelectron spectroscopy (XPS) analysis of BTCOF-Sym-0F, BTCOF-Sym-2F, and BTCOF-Asy-1F revealed similar characteristics in the N 1s, O 1s, S 2p, and F 1s spectra. Particularly noteworthy is the identification of three distinct chemical states after peak fitting of the N 1s spectrum, attributed to nitrogen in the benzothiadiazole (BT) unit, the terminal -NH2 group, and the nitrogen in the imine-linked (C=N) bond, respectively. This result is consistent with the successful construction of the benzothiadiazole (BT)-based imine framework in BTCOF-Sym-0F, BTCOF-Asy-1F, and BTCOF-Sym-2F.

[0066] like Figure 5 As shown, nitrogen adsorption experiments conducted at 77 K evaluated the permanent porosity of BTCOF-Sym-0F, BTCOF-Sym-2F, and BTCOF-Asy-1F. Notably, BTCOF-Sym-2F exhibited the highest specific surface area (1468.3 m²). 2 g -1 ), significantly exceeding BTCOF-Asy-1F (1130.5 m) 2 g -1 ) and BTCOF-Sym-0F (1074.0 m 2 g -1 Furthermore, the adsorption isotherms of these COFs exhibit typical Type IV characteristics, with a steep step in the P / P0 range of 0.05–0.20, confirming the presence of abundant mesopores in the material. Figure 6 Aperture distribution map combined with Figure 7-9 The SEM images fully demonstrate this point.

[0067] The study also examined the optical properties, band gap (Eg), and electronic structure of BTCOF-Sym-0F, BTCOF-Asy-1F, and BTCOF-Sym-2F. First, Figure 10 The UV-Vis diffuse reflectance spectra shown indicate that all COF materials exhibit band-edge exciton peaks beyond 700 nm, suggesting broad-spectrum absorption in the visible region. Subsequently, their optical band gaps were determined to be 2.11 eV (BTCOF-Sym-0F), 2.03 eV (BTCOF-Asy-1F), and 2.08 eV (BTCOF-Sym-2F) based on the Tauc plots. Figure 11 This indicates that they exhibit great potential in the photocatalytic removal of uranium (VI) under conditions where methanol is used as a hole trapping agent.

[0068] To investigate the effect of active site symmetry breaking strategies on the decay dynamics of excited-state charge carriers in BTCOF-Sym-0F, BTCOF-Asy-1F, and BTCOF-Sym-2F, this study used femtosecond transient absorption spectroscopy (fs-TAS) for analysis. Figure 12 By fitting the triple exponential equation to the time profile curve at a probe wavelength of 540 nm, it was found that the recombination process of free electrons and holes is usually too rapid, making it difficult for photogenerated electrons to migrate effectively. However, migratable photogenerated electrons can be trapped by shallow electron trap states, thus maintaining high mobility and driving force for photocatalytic reactions. Compared with the symmetric structure materials BTCOF-Sym-0F (0.61 ps) and BTCOF-Sym-2F (0.48 ps), the asymmetric structure BTCOF-Asy-1F exhibits a significantly longer τ1 lifetime (4.87 ps), which corresponds to the recombination process of free photogenerated electrons and holes. The slower decay kinetics help improve the utilization efficiency of active electrons and their transfer probability to electron acceptors. The lifetime component τ2 is related to the shallow electron trap states. The variation trend of the shallow electron trap state lifetime τ2 indicates that compared with BTCOF-Sym-0F (τ2=6.02 ps) and BTCOF-Sym-2F (τ2=43.84 ps), the shallow electron trap state lifetime in BTCOF-Asy-1F is significantly extended to 1160.02 ps, indicating that electron-hole recombination is effectively suppressed. Therefore, the longer active electron lifetime implies a higher photogenerated electron density, thereby promoting the photocatalytic activity under visible light irradiation. The third group of long lifetime time constants τ3 is related to deep trap states, which leads to charge carrier deactivation and thus reduces photocatalytic activity. The results show that the τ3 lifetime increases in the following order: BTCOF-Asy-1F (9.31 ps) < BTCOF-Sym-2F (11.70 ps) < BTCOF-Sym-0F (17.75 ps).

[0069] While the intrinsically deep trapped states of BTCOF-Sym-0F strongly confine the charge, leading to photocatalytic inertness and limiting electron transfer to surface reaction sites, BTCOF-Asy-1F exhibits a drastically different behavior. In contrast, its shallower trapped states allow for transient charge localization, preventing permanent recombination and thus promoting efficient electron migration. For example... Figure 13 As shown in a), the trap density of BTCOF-Asy-1F is (4.58-3.65)×10 17 cm -3 eV -1 Furthermore, the trap density decreases further within the energy level depth range of 0.2-0.8 eV; while the trap densities of BTCOF-Sym-0F and BTCOF-Sym-2F are (10.64-8.11)×10⁻⁶. 17cm -3 eV -1 and (8.50 - 4.79)×10 17 cm -3 eV -1 Obviously, the density of electronic trap states of BTCOF - Asy - 1F is significantly lower than that of the two symmetric - structure materials, indicating that breaking the symmetry of the active - site core can effectively inhibit the trap density and hinder charge recombination, and then realize the construction of shallow trap states in COFs through the symmetry breaking of the BT unit.

[0070] Given the crucial impact of shallow trap states on charge migration, the present invention explored the charge - transfer dynamics of photo - generated carriers through steady - state and transient photoluminescence (PL) spectra. The order of PL intensity is: BTCOF - Asy - 1F < BTCOF - Sym - 2F < BTCOF - Sym - 0F, indicating that the strategy of breaking the symmetry of active sites can effectively inhibit carrier recombination ( Figure 13 b)). This result is consistent with the carrier - lifetime test: the average lifetime τavg of BTCOF - Asy - 1F (4.00 ns) is longer than that of BTCOF - Sym - 2F (3.43 ns) and BTCOF - Sym - 0F (3.12 ns), further confirming that BTCOF - Asy - 1F has the optimal ability to inhibit photo - generated carrier recombination ( Figure 13 c)). The photocurrent intensity of BTCOF - Asy - 1F is higher than that of BTCOF - Sym - 0F and BTCOF - Sym - 2F, indicating that it has better photo - generated carrier generation ability ( Figure 13 d)).

[0071] To evaluate the kinetic behavior of photo - generated carriers, transient open - circuit voltage decay (OCVD) tests were carried out. The results show that the OCVD response of BTCOF - Asy - 1F is higher than that of the other two materials, BTCOF - Sym - 0F and BTCOF - Sym - 2F, indicating that its photo - generated carrier lifetime is longer ( Figure 13 e)). In addition, electrochemical impedance spectroscopy (EIS) analysis further shows that the charge - transfer resistance of BTCOF - Asy - 1F is significantly lower than that of BTCOF - Sym - 0F and BTCOF - Sym - 2F, once again verifying its improved charge - transfer efficiency ( Figure 13 f)). In summary, BTCOF - Asy - 1F has significant differences from symmetric - structure materials in terms of femtosecond transient absorption kinetics and photochemical behavior, proving that the strategy of breaking the symmetry of active sites can effectively optimize long - lifetime shallow trap states while enhancing the charge - transport ability of the material.

[0072] After completing structural and photoelectric characterization, this application evaluated the photocatalytic performance of BTCOF-Sym-0F, BTCOF-Asy-1F, and BTCOF-Sym-2F in uranium-containing groundwater (U(VI) concentration 40.0 ppm, volume 50 mL). CH3OH was used as a hole sacrificial agent, and the reaction was carried out in air. All systems reached adsorption equilibrium after stirring in the dark for 30 minutes.

[0073] Compared to BTCOF-Asy-1F and BTCOF-Sym-2F, BTCOF-Sym-0F exhibits stronger adsorption capacity, which may be attributed to its larger BET specific surface area, exposing more adsorption sites and enabling it to adsorb UO2²⁻. + Achieving approximately 20% removal of U(VI) Figure 15 a)). This adsorption mainly originates from the heteroatoms (S, N) in BTCOF-Sym-0F adsorbing UO2². + Adsorption and coordination effects.

[0074] In the photocatalytic stage, BTCOF-Asy-1F exhibited the highest U(VI) reduction efficiency (>99.0%), significantly better than BTCOF-Sym-0F (83.2%) and BTCOF-Sym-2F (90.2%), indicating that its excellent photocatalytic activity stems from the electronic structure optimization resulting from the breaking of symmetry at the active sites. Figure 15 a)).

[0075] In terms of adsorption performance, the U(VI) adsorption capacity of BTCOF-Asy-1F showed a concentration-dependent increase before saturation, eventually reaching 1490.3 mg·g⁻¹. -1 It was significantly higher than that of BTCOF-Sym-0F (904.2 mg·g⁻¹). -1 ) and BTCOF-Sym-2F (1080.3 mg·g -1 () Figure 15 Furthermore, compared to previously reported photocatalysts, BTCOF-Asy-1F ranks among the best performing materials in terms of U(VI) removal capacity. Figure 15 c) and as shown in Table 1). Figure 15 As shown in d), BTCOF-Asy-1F maintained stable photocatalytic activity in five consecutive cycles of testing, and its PXRD pattern was basically consistent before and after the reaction, indicating that the material has excellent structural stability and recycling potential.

[0076] Table 1 Comparison of photoreduction U(VI) performance between BTCOF-Asy-1F and other materials

[0077]

[0078] Example 4

[0079] To determine the optimal reaction pH for BTCOF-Asy-1F, its photocatalytic performance was evaluated within a pH range of 2.0 to 6.0. The results are as follows: Figure 14 As shown, at high H + At certain concentrations, key functional groups on the material surface are protonated and become positively charged, electrostatically repelling positively charged UO2. 2+ The presence of cations reduces adsorption capacity and overall photocatalytic removal efficiency. Furthermore, H... + Competitive adsorption on the BTCOF-Asy-1F surface consumes photogenerated electrons that could have been used for U(VI) photoreduction, further reducing photocatalytic activity.

[0080] Subsequently, by introducing multiple interfering cations (Na) + K + Yb 3+ VO 2+ Co 2+ Ni 2+ Mg 2+ Zn 2+ Al 3+ Co 3+ and Eu 3+ The initial concentration of BTCOF-Asy-1F was 10 times that of U(VI), and its photoreduction selectivity was evaluated. Although VO 2+ The presence of ions inhibits selectivity, but BTCOF-Asy-1F still maintains a U(VI) removal rate of up to 90% in the presence of all other ions, demonstrating strong anti-interference ability. Figure 15 e). VO 2+ The impact stems from its similar coordination mode to U(VI), indicating a need to design COF structural units with higher selectivity for U(VI) in the future. Notably, BTCOF-Asy-1F achieved a uranium removal efficiency exceeding 98.9% when treating actual uranium mining wastewater. Figure 15 These results confirm the significant practical value of BTCOF-Asy-1F in extracting uranium from complex radioactive wastewater. Therefore, BTCOF-Asy-1F represents a promising solution for addressing the pressing ecological challenge of uranium contamination in uranium mines and nuclear waste.

[0081] This study employed a symmetry-breaking strategy to achieve shallow trap states in COFs by modulating the symmetry of active sites. Compared to their symmetric counterparts BTCOF-Sym-0F and BTCOF-Sym-2F, BTCOF-Asy-1F, with its symmetry-breaking BT core, exhibited lower trap density and a longer shallow trap state lifetime (1160.02 ps). This enhancement allows more photogenerated electrons to participate in the photocatalytic reaction. Consequently, BTCOF-Asy-1F demonstrated superior performance in the photocatalytic removal of uranium (VI) from wastewater (removal rate >98% within 70 minutes), outperforming BTCOF-Sym-0F and BTCOF-Sym-2F, which possess symmetric acceptor units. This improved photocatalytic performance is attributed to the symmetry-breaking strategy implemented in the BT core structure. Overall, this study highlights symmetry-breaking modulation as an effective approach for developing highly efficient COF-based photocatalysts.

[0082] The method of this invention has been described through preferred embodiments. Those skilled in the art will readily be able to modify or appropriately alter and combine the methods and applications described herein within the scope, spirit, and context of this invention to implement and apply the technology of this invention. Those skilled in the art can refer to the content herein to appropriately improve process parameters. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included within the scope of this invention.

Claims

1. Use of a covalent organic framework material in a material for uranium extraction, characterized in that, The shallow trap state lifetime of the material is 1160.02 picoseconds, and the removal rate is more than 99% within 70 minutes when used for photocatalytic reduction of uranium (VI), and the structural unit of the covalent organic framework material has the structure shown as follows: BTCOF-Asy-1F.

2. Use according to claim 1, characterized in that, The preparation method of the covalent organic framework material comprises: The compound shown as formula (II) and the compound shown as formula (III) are mixed and dissolved in a solvent, and the reaction is carried out under the catalysis of acetic acid and in a vacuum condition; After the reaction is completed, centrifugation is carried out, and then acetone washing and tetrahydrofuran washing are carried out in sequence, and the covalent organic framework material BTCOF-Asy-1F is obtained after drying. wherein each X 1 independently is hydrogen; each X 2 independently is F; The solvent is a combination of 1,1-dichlorobenzene and n-butanol; the molar ratio of the compound shown as formula (II) and the compound shown as formula (III) is 1:

2.

3. Use according to claim 2, characterized in that, The temperature of the reaction is 100-150 DEG C. The reaction time is 60-80 hours.

4. Use according to claim 2, characterized in that, The molar ratio of the compound shown as formula (II) and acetic acid is 1:(10-50).

5. Use according to claim 2, characterized in that, The drying is carried out in a vacuum condition, the drying temperature is 30-60 DEG C, and the drying time is 6-14 hours. The tetrahydrofuran washing is carried out for 3-5 times, and the acetone washing is carried out for 3-5 times.

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