Synthesis method and application of central receptor type covalent organic framework

Abstract

The invention relates to the technical field of environmental protection, and provides a synthesis method and application of a central receptor type covalent organic framework. According to the method, 2, 4, 6-trihydroxy-1, 3, 5-benzenetricarboxaldehyde, McLeod acid and 2, 6-naphthylenediamine are taken as monomers, and a Mannich type polycondensation strategy is adopted, so that the central receptor type covalent organic framework Tp-ND-H is synthesized under the catalysis of benzotriazole-1-yl-oxytripyrrolidinyl phosphorus hexafluorophosphate. The Tp-ND-H has the advantages of being adjustable in energy level, high in exciton migration efficiency, good in structural stability and the like, is high in U (VI) photocatalytic reduction capacity, high in efficiency and good in selectivity, and has a good application prospect.

Description

Technical Field

[0001] This invention belongs to the field of environmental protection technology, and in particular relates to the synthesis method and application of center-receptor type covalent organic frameworks. Background Technology

[0002] Uranium is a key raw material for nuclear energy and plays a vital role in the global transition to a low-carbon energy system. However, the large amounts of radioactive and chemically toxic wastewater generated during uranium mining and refining pose a serious threat to the ecological environment and human health. Converting soluble and mobile U(VI) into the insoluble U(IV) form is one of the effective ways to mitigate uranium pollution (H. Yang, M. Hao, Y. Xie, X. Liu, Y. Liu, Z. Chen, X. Wang, GIN Waterhouse, S. Ma, Tuning local charge distribution in multicomponent covalent organic frameworks for dramatically enhanced photocatalytic uranium extraction. Angew. Chem. Int. Ed., 2023, 62, e202303129). Given the abundance of solar energy resources, the reduction of U(VI) using semiconductor-based photocatalysts has become a sustainable and promising method. Therefore, designing photocatalysts with well-aligned band structures is crucial.

[0003] Covalent organic frameworks (COFs), with their long-range π-conjugation, customizable pore environments, and permanent porosity, have become ideal materials for photocatalytic applications. Among various design strategies, donor-acceptor (DA) engineering is more effective in modulating the electronic structure of COF materials to enhance photon separation, charge transfer, and light absorption (R. Liu, Y. Chen, H. Yu, M. Položij, Y. Guo, TC Sum, T. Heine, D. Jiang, Linkage-engineered donor–acceptor covalent organic frameworks for optimal photosynthesis of hydrogen peroxide from water and air. Nat. Catal., 2024, 7, 195). Despite the significant advantages of the donor-acceptor strategy, structural imbalances between donor and acceptor components remain a limitation in COF materials. To date, most research has focused on developing electron-rich donor units, while the design of high-performance acceptor building blocks has lagged significantly (X. Cao, Z. Xu, R. Wang, J. Guo, W. Zhao, Y. Zhang, Z. Yao, Y. Guo, G. Long, X. Wan, Y. Chen, O, S, and N bridged atoms screening on 2D conjugated central units of high-performance acceptors. Adv. Mater., 2025, 37, 2503131). The development of COF-based photocatalysts still faces key bottlenecks such as limited acceptor unit availability and structural diversity.

[0004] Existing acceptor units are typically integrated spatially uniformly along the COF backbone. This uniform distribution creates an electronically symmetrical framework, which suppresses the formation of an internal electric field essential for driving the dissociation of photogenerated carriers and promoting long-distance charge transport. Without directional electronic asymmetry, photogenerated carriers tend to recombine rapidly, leading to low charge separation efficiency and limited photocatalytic activity (L. Hao, R. Shen, G. Liang, M. Kang, C. Huang, P. Zhang, X. Li, Precise local functionalization of covalent organic framework for efficient carrier separation in photocatalytic H2 evolution. Appl. Catal. B Environ. Energy, 2024, 348, 123837). The anisotropic electronic environment hinders the formation of directional charge transport paths, making it difficult to fully realize the potential of the DA strategy in COF photocatalysis.

[0005] Central acceptor covalent organic frameworks (COFs) enable precise spatial separation of electronic states and the construction of internal energy gradients, thereby addressing key issues such as exciton confinement, charge mobility, and energy level tunability. Mannig condensation polymerization can construct stable covalent bonds in one step and precisely anchor the acceptor unit at the framework center, achieving ordered energy level distribution and efficient electron trapping in central acceptor COFs, making it a preferred strategy for constructing high-performance photocatalytic systems. Currently, there are no reports on the construction of central acceptor covalent organic frameworks using Mannig condensation polymerization and its photocatalytic reduction of U(VI). Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a method for synthesizing and applying central acceptor-type covalent organic frameworks. The aim is to solve the problems of limited acceptor building units, low charge separation efficiency, and low photocatalytic activity in existing COF photocatalysts.

[0007] In a first aspect, the present invention provides a method for synthesizing a center-acceptor type covalent organic framework, using 2,4,6-trihydroxy-1,3,5-benzenetriformaldehyde, McFarland acid, and 2,6-naphthyldiamine as reactants, and synthesizing a center-acceptor type covalent organic framework via a Mannich polycondensation reaction catalyzed by benzotriazol-1-yl-oxytripyrrolidinephosphine hexafluorophosphate.

[0008] Further, the specific steps include: adding 2,4,6-trihydroxy-1,3,5-benzenetrialdehyde, McFarland acid, 2,6-naphthyldiamine, benzotriazol-1-yl-oxytripyrrolidinephosphine hexafluorophosphate, triethylamine, and anhydrous ethanol to a 20 mL Pyrex tube, sonicating at room temperature for 10 minutes to mix thoroughly, degassing through three freeze-pump-thaw cycles, flame-sealing the Pyrex tube, and reacting it in an oven at 80 °C for 72 h. After cooling to room temperature, the solid product is collected and washed with tetrahydrofuran, methanol, and acetone, respectively. The washed solid product is then vacuum-dried to prepare a center-acceptor type covalent organic framework.

[0009] Furthermore, the mass ratio of 2,4,6-trihydroxy-1,3,5-benzyltricarboxaldehyde, McFarland acid, and 2,6-naphthyldiamine is 16.8:34:13; the mass of benzotriazol-1-yl-oxytripyrrolidinephosphine hexafluorophosphate is 16 mg; and the volume ratio of triethylamine to anhydrous ethanol is 1:20.

[0010] Secondly, the present invention provides a central acceptor-type covalent organic framework, which is prepared by a method for synthesizing a central acceptor-type covalent organic framework.

[0011] Thirdly, this invention provides the application of a central acceptor-type covalent organic framework in the photocatalytic reduction of U(VI).

[0012] Furthermore, the central acceptor covalent organic framework was added to aqueous solutions containing different concentrations of U(VI), shaken with a constant temperature shaker and irradiated under UV / Vis light for 4 h. The suspension was then removed, filtered through a microporous membrane, and the filtrate was collected. The remaining U(VI) content in the filtrate was measured by inductively coupled plasma mass spectrometry (ICP-MS) to calculate the adsorption capacity and removal rate of U(VI) by the central acceptor covalent organic framework.

[0013] Furthermore, the concentration of U(VI) in aqueous solutions of different concentrations ranges from 0 to 600 mg / L.

[0014] Furthermore, before adding the central acceptor type covalent organic framework to aqueous solutions containing different concentrations of U(VI), the pH of the U(VI) aqueous solution is adjusted to 1.0-10.0 using nitric acid solution or sodium hydroxide solution.

[0015] Furthermore, the central acceptor covalent organic framework is able to selectively remove U(VI) in the presence of a variety of interfering ions, including Mg(II), Cd(II), Ni(II), Pb(II), Mn(II), Fe(II), Na(I), Al(III), Dy(III), Er(III), Yb(III), Tm(III), Tb(III), Sm(III), Pr(III), La(III) and Lu(III).

[0016] Furthermore, the central acceptor covalent organic framework exhibits an adsorption capacity of up to 2302 mg / g for U(VI); after 5 regeneration cycles, the removal rate of U(VI) still reaches 95%.

[0017] The present invention has the following beneficial effects: (1) The present invention synthesizes a central acceptor covalent organic framework in one step through the Mannich polycondensation reaction, which precisely anchors the acceptor unit to the center of the framework, which is beneficial to the ordered energy level distribution and efficient electron trapping of the central acceptor COFs, thereby improving the photocatalytic performance.

[0018] (2) The central acceptor type covalent organic framework prepared by the present invention has resonance-assisted hydrogen bonds of amide groups, which enhances the planarity and rigidity of the framework, strengthens electronic coupling, and has high structural stability.

[0019] (3) The central acceptor type covalent organic framework prepared by the present invention breaks the inherent electronic symmetry of the traditional donor-acceptor covalent organic framework, creates an anisotropic environment, and thus achieves efficient charge separation and directional transport.

[0020] (4) The central acceptor type covalent organic framework prepared by the present invention can achieve precise spatial separation of electronic states and construction of internal energy gradients, thereby solving key problems such as exciton confinement, charge mobility and energy level tunability.

[0021] (5) The central acceptor type covalent organic framework prepared in this invention has strong photocatalytic reduction ability of U(VI), high adsorption capacity and fast speed, and good selectivity.

[0022] (6) The central acceptor type covalent organic framework prepared by the present invention can be reused, which is conducive to economic conservation and sustainable development of the ecological environment and has good application prospects. Attached Figure Description

[0023] Exemplary embodiments of the present invention can be more fully understood by referring to the following figures: Figure 1 This is a schematic diagram of the synthesis of the central acceptor-type covalent organic framework Tp-ND-H in Example 1 of the present invention.

[0024] Figure 2 These are the Fourier transform infrared spectra of MA, Tp, ND, and Tp-ND-H in Embodiment 1 of the present invention.

[0025] Figure 3 These are experimental and simulated PXRD diagrams of Tp-ND-H in Embodiment 1 of the present invention.

[0026] Figure 4 These are the PXRD patterns and thermogravimetric analysis (TGA) diagrams of Tp-ND-H under different conditions in Embodiment 1 of the present invention, wherein:

[0027] Figure 4 In the figure, 'a' represents the PXRD plot of Tp-ND-H under different conditions;

[0028] Figure 4 b in the figure is the thermogravimetric analysis diagram of Tp-ND-H.

[0029] Figure 5 This is the adsorption kinetic diagram of Tp-ND-H on U(VI) in Example 2 of the present invention.

[0030] Figure 6 This is the adsorption isotherm diagram of U(VI) by Tp-ND-H in Example 2 of the present invention.

[0031] Figure 7 This is the selectivity diagram of Tp-ND-H for U(VI) in Embodiment 2 of the present invention.

[0032] Figure 8 This is a diagram illustrating the cyclic use of Tp-ND-H in Embodiment 3 of the present invention. Detailed Implementation

[0033] To make the technical problems, technical solutions, and beneficial effects of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. Unless otherwise specified, specific conditions in the embodiments are performed under conventional conditions or conditions recommended by the manufacturer. Reagents or instruments used, unless otherwise specified, are all commercially available conventional products.

[0034] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0035] This invention provides a method for synthesizing a center-acceptor type covalent organic framework. Using 2,4,6-trihydroxy-1,3,5-benzyltricarboxaldehyde, McFarland acid, and 2,6-naphthyldiamine as reactants, the center-acceptor type covalent organic framework is synthesized via Mannich polycondensation under the catalysis of benzotriazol-1-yl-oxytripyrrolidinylphosphine hexafluorophosphate.

[0036] In some embodiments, the process specifically includes the following steps: 2,4,6-trihydroxy-1,3,5-benzenetrialdehyde, McFarland acid, 2,6-naphthyldiamine, benzotriazol-1-yl-oxytripyrrolidinephosphine hexafluorophosphate, triethylamine, and anhydrous ethanol are added to a 20 mL Pyrex tube, and the mixture is sonicated at room temperature for 10 minutes to ensure homogeneity. After degassing through three freeze-pump-thaw cycles, the Pyrex tube is flame-sealed and placed in an oven at 80 °C for 72 h. After cooling to room temperature, the solid product is collected and washed with tetrahydrofuran, methanol, and acetone, respectively. The washed solid product is then vacuum-dried to prepare a center-acceptor type covalent organic framework.

[0037] In some embodiments, the mass ratio of 2,4,6-trihydroxy-1,3,5-benzyltricarboxaldehyde, McFarland acid, and 2,6-naphthyldiamine is 16.8:34:13; the mass of benzotriazol-1-yl-oxytripyrrolidinephosphine hexafluorophosphate is 16 mg; and the volume ratio of triethylamine to anhydrous ethanol is 1:20.

[0038] In some embodiments, the present invention provides a central receptor-type covalent organic framework, which is prepared by a method for synthesizing a central receptor-type covalent organic framework.

[0039] In some embodiments, the present invention provides the application of a central acceptor-type covalent organic framework in the photocatalytic reduction of U(VI).

[0040] In some embodiments, the central acceptor covalent organic framework was added to aqueous solutions containing different concentrations of U(VI), shaken with a constant temperature shaker and irradiated under ultraviolet / visible light for 4 h, the suspension was removed, filtered through a microporous membrane, the filtrate was collected, and the remaining U(VI) content in the filtrate was measured by inductively coupled plasma mass spectrometry to calculate the adsorption capacity and removal rate of the central acceptor covalent organic framework for U(VI).

[0041] In some embodiments, the concentration of U(VI) in aqueous solutions of different concentrations ranges from 0 to 600 mg / L.

[0042] In some embodiments, before adding the central acceptor covalent organic framework to an aqueous solution containing different concentrations of U(VI), the pH of the U(VI) aqueous solution is adjusted to 1.0-10.0 using nitric acid solution or sodium hydroxide solution.

[0043] In some embodiments, the central acceptor covalent organic framework is capable of selectively removing U(VI) in the presence of a variety of interfering ions, including Mg(II), Cd(II), Ni(II), Pb(II), Mn(II), Fe(II), Na(I), Al(III), Dy(III), Er(III), Yb(III), Tm(III), Tb(III), Sm(III), Pr(III), La(III) and Lu(III).

[0044] In some embodiments, the central acceptor covalent organic framework has an adsorption capacity of up to 2302 mg / g for U(VI); after 5 regeneration cycles, the removal rate of U(VI) is still 95%.

[0045] Example 1: Synthesis and characterization of the center-receptor type covalent organic framework Tp-ND-H 2,4,6-Trihydroxy-1,3,5-benzenetrialdehyde (Tp, 16.8 mg), McFarland acid (MA, 34 mg), 2,6-naphthyldiamine (ND, 13 mg), benzotriazol-1-yl-oxytripyrrolidinephosphine hexafluorophosphate (PyBOP, 16 mg), triethylamine (50 μL), and anhydrous ethanol (1 mL) were added to a 20 mL Pyrex tube and sonicated at room temperature for 10 minutes to ensure homogeneity. After degassing through three freeze-pump-thaw cycles, the Pyrex tube was flame-sealed and placed in an oven at 80 °C for 72 h. After cooling to room temperature, the solid product was collected and washed with tetrahydrofuran, methanol, and acetone, respectively. The washed solid product was then dried under vacuum at 60 °C for 8 h to prepare a center-acceptor covalent organic framework (Tp-ND-H).

[0046] A schematic diagram of the synthesis of the center-acceptor type covalent organic framework Tp-ND-H, as shown below. Figure 1 As shown.

[0047] In the Tp-ND-H topology, the lowest unoccupied molecular orbital is spatially confined within the central acceptor unit benzopyranone, while the highest occupied molecular orbital is delocalized in the surrounding donor region and the π system bridged by the amide, forming an intrinsic energy funnel. This enables precise spatial separation of electronic states and the construction of an internal energy gradient, thereby solving key problems such as exciton confinement, charge mobility, and energy level tunability.

[0048] Fourier transform infrared spectra of MA, Tp, ND, and Tp-ND-H, as shown Figure 2 As shown. The results show that in the infrared spectrum of Tp-ND-H, the characteristic absorption peak of the aldehyde group of Tp is (~1697 cm⁻¹). -1 The characteristic absorption peak of amino groups in ND (~3300 cm⁻¹) and ND. -1The significant decrease in Tp-ND-H indicates that the aldehyde and amino groups have reacted. The Tp-ND-H concentration is at ~1654 cm⁻¹. -1 Absorption peaks corresponding to the C=O vibration of the amide bond (-CONH-) and the carbonyl group in the benzopyranone core appear at 1610 and 1260 cm⁻¹. -1 Absorption peaks corresponding to the conjugated C=C and COC vibrations appeared at the points, respectively. These results indicate that a Mannich condensation reaction occurred between MA, Tp, and ND to prepare a central acceptor-type covalent organic framework.

[0049] Experimental and simulated PXRD plots of Tp-ND-H, such as Figure 3 As shown in the figure. The results show that the crystallinity of Tp-ND-H was characterized by X-ray powder diffraction (PXRD). The experimentally measured and simulated PXRD patterns of Tp-ND-H using Material Studio software show that Tp-ND-H exhibits diffraction peaks at 2θ = 5.37°, 9.15°, 10.55°, and 13.79°. Compared with the overlapping AA and offset AB stacking models, the experimental PXRD pattern of Tp-ND-H better conforms to the ABC stacking model, indicating that the Tp-ND-H synthesized using the method of this invention has good crystallinity.

[0050] PXRD patterns and thermogravimetric analysis (TGA) plots of Tp-ND-H under different conditions, such as Figure 4 As shown. By Figure 4 As shown in Figure a, after 24 hours of treatment with harsh conditions including 200 kGy γ-ray irradiation, dimethylformamide, 1 M sodium hydroxide, or 1 M hydrochloric acid, the PXRD pattern of Tp-ND-H remained almost unchanged, indicating that Tp-ND-H has good chemical stability. Furthermore, thermogravimetric analysis (TGA) showed that Tp-ND-H experienced almost no weight loss below 200 °C. Figure 4 (b) indicates that it has good thermal stability. The central acceptor type covalent organic framework prepared in this invention has resonance-assisted hydrogen bonds of amide groups, which enhances the planarity and rigidity of the framework, strengthens electronic coupling, and makes the framework structure highly stable.

[0051] Example 2: Photocatalytic reduction performance of Tp-ND-H on U(VI) (1) Adsorption kinetics of U(VI) by Tp-ND-H The effect of pH on the photocatalytic reduction of U(VI) by Tp-ND-H was investigated. The adsorption capacity gradually increased with increasing pH (1.0-5.0), reaching a maximum at pH 5.0. Further increases in pH (5.0-10.0) resulted in a decrease in adsorption capacity. Therefore, pH 5.0 was selected as the optimal pH value.

[0052] The pH of the U(VI) aqueous solution was adjusted to 5.0 using nitric acid or sodium hydroxide solution. 5 mg of Tp-ND-H was added to 25 mL of an aqueous solution containing 200 mg / L U(VI). The solution was shaken using a constant-temperature shaker and irradiated under UV / Vis light. At regular intervals, 0.5 mL of the suspension was collected and filtered through a 0.22 μm microporous membrane. The filtrate was collected, and the remaining U(VI) content in the filtrate was measured using inductively coupled plasma mass spectrometry (ICP-MS). The adsorption kinetics curve of U(VI) by Tp-ND-H was plotted. The adsorption capacity was calculated using the following formula: q t =(C o -C t ) / m×V; where: q t Adsorption capacity is expressed in mg / g; V is the solution volume in L; m is the amount of Tp-ND-H used in g; C o This is the initial concentration of U(VI), in mg / L; C t It is the equilibrium concentration of U(VI), in mg / L.

[0053] The adsorption kinetics of Tp-ND-H for U(VI) is shown in the figure below. Figure 5 As shown in the figure. The results show that Tp-ND-H adsorbs U(VI) rapidly, reaching equilibrium in just 10 minutes. The kinetic data fits well with the pseudo-second-order model, indicating that the adsorption process is mainly dominated by chemisorption between U(VI) and the binding sites of the Tp-ND-H framework.

[0054] (2) Thermodynamic study of adsorption of U(VI) by Tp-ND-H The pH of the U(VI) aqueous solution was adjusted to 5.0 using nitric acid or sodium hydroxide solution. 5 mg of Tp-ND-H was added to 25 mL of aqueous solutions containing different concentrations of U(VI) (0, 50, 100, 150, 200, 300, 400, 500, and 600 mg / L). The solutions were shaken using a constant-temperature shaker and irradiated under UV / Vis light for 4 h. 0.5 mL of the suspension was collected and filtered through a 0.22 μm microporous membrane. The filtrate was collected, and the remaining U(VI) content in the filtrate was measured using inductively coupled plasma mass spectrometry (ICP-MS). The adsorption capacity of Tp-ND-H for U(VI) was calculated, and the adsorption isotherm of Tp-ND-H for U(VI) was plotted.

[0055] The adsorption isotherm of Tp-ND-H for U(VI), as shown in the figure. Figure 6As shown in the figure. The results show that Tp-ND-H adsorbs up to 2302 mg / g of U(VI). Such a high adsorption capacity can be attributed to the highly conjugated π system of Tp-ND-H and the rigid framework provided by the naphthyl linker. These components synergistically enhance the interaction between U(VI) and Tp-ND-H and promote the efficient transport of substances within the pores.

[0056] (3) Study on the adsorption selectivity of Tp-ND-H for U(VI) Rare earth tailings wastewater contains a wide variety of metal ions, including Mg(II), Cd(II), Ni(II), Pb(II), Mn(II), Fe(II), Na(I), Al(III), Dy(III), Er(III), Yb(III), Tm(III), Tb(III), Sm(III), Pr(III), La(III), and Lu(III). These ions often interfere with the photocatalyst's effect on U(VI) by competitively occupying binding sites on the photocatalyst surface or altering the surface charge. Therefore, the influence of these coexisting metal ions on the photocatalytic reduction of U(VI) by Tp-ND-H was investigated. The pH of the wastewater containing U(VI) was adjusted to 5.0 using nitric acid or sodium hydroxide solution. 10 mg of Tp-ND-H was added to 30 mL of an aqueous solution containing 20 mg / L U(VI) and 100 mg / L interfering metal ions. The solution was shaken using a constant-temperature shaker and irradiated under UV / Vis light for 4 h. 0.5 mL of the suspension was collected and filtered through a 0.22 μm microporous membrane. The filtrate was collected, and the remaining U(VI) content in the filtrate was measured using inductively coupled plasma mass spectrometry (ICP-MS). The removal rate of U(VI) by Tp-ND-H was calculated, and a graph of the removal rate of U(VI) by Tp-ND-H was plotted. The removal rate was calculated using the following formula: Removal rate (%) = 100(C o -C t ) / C o In the formula: Removal rate (%) is the removal rate; C o This is the initial concentration of U(VI), in mg / L; C t It is the equilibrium concentration of U(VI), in mg / L.

[0057] The selectivity plot of Tp-ND-H for U(VI), as shown in the figure. Figure 7As shown in the figure. The results show that even in the presence of interfering ions such as Mg(II), Cd(II), Ni(II), Pb(II), Mn(II), Fe(II), Na(I), Al(III), Dy(III), Er(III), Yb(III), Tm(III), Tb(III), Sm(III), Pr(III), La(III) and Lu(III), the removal rate of U(VI) by Tp-ND-H is still higher than 90%, indicating that Tp-ND-H has good selectivity for U(VI).

[0058] Example 3: Recyclability of Tp-ND-H Tp-ND-H adsorbed with U(VI) was eluted with 0.1 M nitric acid solution, washed with anhydrous ethanol and water, and dried under vacuum at 120 °C overnight to obtain regenerated Tp-ND-H. The pH of the U(VI) aqueous solution was adjusted to 5.0 with nitric acid solution or sodium hydroxide solution. 5 mg of regenerated Tp-ND-H was added to 25 mL of an aqueous solution containing 200 mg / L U(VI). The solution was shaken with a constant temperature shaker and irradiated under UV / Vis light for 4 h. 0.5 mL of the suspension was collected and filtered through a 0.22 μm microporous membrane. The filtrate was collected, and the remaining U(VI) content in the filtrate was measured by inductively coupled plasma mass spectrometry (ICP-MS). The removal rate of U(VI) by Tp-ND-H was calculated, and a graph of the removal rate of U(VI) by Tp-ND-H was plotted.

[0059] The cyclic usage diagram of Tp-ND-H is as follows: Figure 8 As shown in the figure. The results show that after 5 adsorption / desorption cycles, the removal rate of U(VI) by Tp-ND-H still remains at 95%, indicating that the Tp-ND-H prepared by the method of this invention has good recyclability, which is conducive to economic conservation and sustainable development of the ecological environment.

[0060] In summary, this invention synthesizes a central acceptor covalent organic framework (COF) in one step via Mannich condensation reaction. This precisely anchors the acceptor unit at the framework center, which is beneficial for the ordered energy level distribution and efficient electron trapping of the central acceptor COF, thereby enhancing its photocatalytic performance. In its topological structure, the lowest unoccupied molecular orbital is spatially confined within the central acceptor unit benzopyranone, while the highest occupied molecular orbital is delocalized in the surrounding donor region and the π-system bridged by the amide, forming an intrinsic energy funnel that facilitates directional exciton migration and suppresses charge recombination. Furthermore, the introduction of the amide bond not only enhances the rigidity and planarity of the framework but also promotes the formation of resonance-assisted hydrogen bonds, contributing to electronic coupling and structural stability. The central acceptor covalent organic framework prepared by this invention can achieve precise spatial separation of electronic states and construct an internal energy gradient, thus solving key problems such as exciton confinement, charge mobility, and energy level tunability. It exhibits strong photocatalytic reduction ability, high efficiency, and good selectivity for U(VI), showing promising application prospects.

[0061] The above descriptions are merely several preferred embodiments of the present invention, and while the descriptions are relatively specific and detailed, they are not intended to limit the present invention. It should be noted that those skilled in the art can make various variations and modifications to the present invention, and any modifications, equivalent substitutions, and improvements made within the concept and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for synthesizing a center-receptor type covalent organic framework, characterized in that: Using 2,4,6-trihydroxy-1,3,5-benzyltricarboxaldehyde, McFarland acid, and 2,6-naphthyldiamine as reactants, a center-acceptor covalent organic framework was synthesized via Mannich polycondensation under the catalysis of benzotriazol-1-yl-oxytripyrrolidinylphosphine hexafluorophosphate.

2. The method for synthesizing a center-receptor type covalent organic framework as described in claim 1, characterized in that, Specifically, the following steps are included: 2,4,6-Trihydroxy-1,3,5-benzyltricarboxaldehyde, McFarland acid, 2,6-naphthyldiamine, benzotriazol-1-yl-oxytripyrrolidinylphosphine hexafluorophosphate, triethylamine and anhydrous ethanol were added to a 20 mL Pyrex tube and sonicated at room temperature for 10 minutes to mix them thoroughly. After three cycles of freezing-pumping-thawing for degassing, the Pyrex tube was flame-sealed and placed in an oven at 80 °C for 72 h. After cooling to room temperature, the solid product was collected. The solid products were washed with tetrahydrofuran, methanol and acetone respectively, and then dried under vacuum to prepare a center-acceptor type covalent organic framework.

3. The method for synthesizing a center-receptor type covalent organic framework as described in claim 2, characterized in that: The mass ratio of 2,4,6-trihydroxy-1,3,5-benzyltricarboxaldehyde, McFarland acid, and 2,6-naphthyldiamine was 16.8:34:13; the mass of benzotriazol-1-yl-oxytripyrrolidinephosphine hexafluorophosphate was 16 mg; and the volume ratio of triethylamine to anhydrous ethanol was 1:

20.

4. A central acceptor-type covalent organic framework, characterized by: It is prepared by the synthetic method of the central acceptor type covalent organic framework according to any one of claims 1-3.

5. The application of the central acceptor type covalent organic framework as described in claim 4 in the photocatalytic reduction of U(VI).

6. The application of the central acceptor-type covalent organic framework as described in claim 5 in the photocatalytic reduction of U(VI), characterized in that: The central acceptor covalent organic framework was added to aqueous solutions containing different concentrations of U(VI), shaken with a constant temperature shaker and irradiated under UV / Vis light for 4 h. The suspension was then removed, filtered through a microporous membrane, and the filtrate was collected. The remaining U(VI) content in the filtrate was measured by inductively coupled plasma mass spectrometry (ICP-MS) to calculate the adsorption capacity and removal rate of U(VI) by the central acceptor covalent organic framework.

7. The application of the central acceptor-type covalent organic framework as described in claim 6 in the photocatalytic reduction of U(VI), characterized in that: The concentration of U(VI) in aqueous solutions of different concentrations ranges from 0 to 600 mg / L.

8. The application of the central acceptor-type covalent organic framework as described in claim 7 in the photocatalytic reduction of U(VI), characterized in that: Before adding the central acceptor covalent organic framework to aqueous solutions containing different concentrations of U(VI), the pH of the U(VI) aqueous solution is adjusted to 1.0-10.0 using nitric acid solution or sodium hydroxide solution.

9. The application of the central acceptor-type covalent organic framework as described in claim 8 in the photocatalytic reduction of U(VI), characterized in that: The central acceptor covalent organic framework can selectively remove U(VI) in the presence of a variety of interfering ions, including Mg(II), Cd(II), Ni(II), Pb(II), Mn(II), Fe(II), Na(I), Al(III), Dy(III), Er(III), Yb(III), Tm(III), Tb(III), Sm(III), Pr(III), La(III) and Lu(III).

10. The application of the central acceptor-type covalent organic framework as described in claim 9 in the photocatalytic reduction of U(VI), characterized in that: The central acceptor covalent organic framework has an adsorption capacity of up to 2302 mg / g for U(VI); after 5 regeneration cycles, the removal rate of U(VI) is still 95%.

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