Preparation and application of covalent organic framework photocatalyst for photocatalytic CO2 reduction

Abstract

The invention discloses a preparation method of a covalent organic framework photocatalyst for photocatalytic reduction of CO2, which comprises the following steps: adding 4, 4 ', 4' '-(1, 3, 5-triazine-2, 4, 6-triyl) triphenylamine and 2, 5-dibromobenzene-1, 2, 4, 5-tetracarboxylic dianhydride into a reaction kettle, then adding 1-hydroxy-3-methylbenzene and isoquinoline, and uniformly mixing; and carrying out microwave heating on the reaction kettle, cooling to room temperature, filtering, collecting precipitate, washing, and then carrying out Soxhlet extraction and vacuum drying to obtain the covalent organic framework photocatalyst. A surface halogen atom modification strategy is designed and used for regulating and controlling electron distribution and light capture capacity of a donor-acceptor (D-A) in TzPm-COF, and results show that the strategy can effectively improve photocatalytic activity and selectivity of reduction of CO2 into CO, and can be applied to preparation of the TzPm-COF catalyst. A reasonable design strategy is provided for developing a functional modified COFs material for efficiently and selectively photocatalytic CO2 reduction.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of catalysts, and more particularly, relates to a preparation and application of a covalent organic framework photocatalyst for photocatalytic reduction of CO2. BACKGROUND

[0002] With the increasing consumption of global fossil fuels, addressing carbon dioxide emissions and mitigating climate change has become an urgent challenge. Photocatalytic reduction of carbon dioxide is an ideal way to achieve carbon neutral clean energy. An ideal photocatalyst should integrate photocatalytic activity, selectivity, durability and ease of use into the same material, while photocatalytic activity is usually measured by indicators such as quantum efficiency, yield, etc. Since the first synthesis of covalent organic frameworks (COFs) in 2005, COFs have been considered as an ideal photocatalytic platform due to their customizable pi-conjugated skeleton, ordered channels and adjustable functionality. In recent years, COF-based photocatalysts have been widely studied for the conversion of excess carbon dioxide into high-value fuels or chemicals. However, most COFs face problems such as rapid recombination of photo-generated carriers and short excited state lifetime, which severely limit their catalytic efficiency. Therefore, developing a new type of covalent organic framework photocatalyst with efficient electron-hole separation and longer excited state lifetime is crucial for achieving high-performance photocatalytic CO2 reduction. SUMMARY

[0003] An object of the present application is to solve at least the above problems and / or disadvantages and provide at least the advantages described later.

[0004] To achieve these objects and other advantages and in accordance with the purpose of the application, as embodied and broadly described herein, there is provided a method for preparing a covalent organic framework photocatalyst, characterized in that it comprises the following steps: Step one, 4,4',4''-(1,3,5-triazine-2,4,6-triazyl) triphenylamine (TAPTz) and 2,5-dibromobenzene-1,2,4,5-tetracarboxylic dianhydride (PMDCA-2Br) are added to a reaction kettle, and 1-hydroxy-3-methylbenzene and isoquinoline are added and mixed uniformly; Step two, the reaction kettle is subjected to microwave heating, and after cooling to room temperature, the precipitate is collected by filtration, washed, then subjected to Soxhlet extraction, and vacuum dried to obtain a covalent organic framework photocatalyst for photocatalytic reduction of CO2.

[0005] Preferably, in step one, the molar ratio of 4,4',4''-(1,3,5-triazine-2,4,6-triazyl) triphenylamine to 2,5-dibromobenzene-1,2,4,5-tetracarboxylic dianhydride is 1:1-2.

[0006] Preferably, in the step one, the amount ratio of 4,4',4''-(1,3,5-triazine-2,4,6-triyl) triphenylamine, 1-hydroxy-3-methylbenzene and isoquinoline is 1 mmol: 30-50 mL: 1-3 mL.

[0007] Preferably, in the step two, the temperature of microwave heating is 160℃, and the time is 5-20 min.

[0008] Preferably, in the step two, the washing is sequentially washing with N,N-dimethylformamide and tetrahydrofuran; and the Soxhlet extraction is carried out in tetrahydrofuran for 12-36 h.

[0009] Preferably, in the step two, the temperature of vacuum drying is 40-80℃, and the time is 6-24 h.

[0010] Preferably, the method further comprises post-treatment modification of the covalent organic framework photocatalyst for photocatalytic reduction of CO2 prepared in the step two, and the post-treatment modification method comprises: S21, mixing the covalent organic framework photocatalyst and nano-Fe2O3 into a solvent, stirring for 20-40 min, standing for 1-3 h, then heating to 60-80℃ and stirring until the solvent is completely evaporated, to obtain a pretreated covalent organic framework photocatalyst; S22, mixing the pretreated covalent organic framework photocatalyst, sodium citrate and zinc citrate into a solvent, ultrasonic dispersion for 0.5-2 h, stirring reflux at 50-70℃ for 2-4 h, then heating to 60-80℃ and stirring until the solvent is completely evaporated, and grinding to obtain a treated covalent organic framework photocatalyst; S23, heat-treating the treated covalent organic framework photocatalyst in an inert gas, and naturally cooling to room temperature to obtain a modified covalent organic framework photocatalyst.

[0011] Preferably, in the S21, the mass ratio of the covalent organic framework photocatalyst and nano-Fe2O3 is 1:0.01-0.1; and the mass-volume ratio of the covalent organic framework photocatalyst and the solvent is 1 g:20-60 mL.

[0012] Preferably, in the S22, the mass ratio of the pretreated covalent organic framework photocatalyst, sodium citrate and zinc citrate is 1:0.01-0.1:0.05-0.2; and the mass-volume ratio of the pretreated covalent organic framework photocatalyst and the solvent is 1 g:20-60 mL.

[0013] Preferably, the solvent is anhydrous ethanol.

[0014] Preferably, in the S23, the inert gas is one of argon and nitrogen; and the specific method of heat treatment is: heating to 200-300 DEG C at a heating rate of 5-15 DEG C / min and maintaining for 30-70 min.

[0015] The application of the covalent organic framework photocatalyst prepared by the preparation method to photocatalytic reduction of CO2.

[0016] The application at least has the following beneficial effects: the application designs a surface halogen atom modification strategy for regulating electron distribution and light capturing capacity of donor-acceptor (D-A) in TzPm-COF, and the results show that the strategy can effectively improve the photocatalytic activity and selectivity of CO2 reduction to CO. -1 h -1 , which is 2.7 times of that of TzPm-COF (57 μmol g -1 h -1 ), and has extremely high selectivity (99.4%). The introduction of bromine atoms promotes the directional electron transfer from TAPTz donor to PMDCA-2Br acceptor through the intrinsic polarization effect, thereby enhancing the adsorption and activation of CO2 at the oxygen site of the PMDCA unit, and providing a reasonable design strategy for developing functionalized modified COF materials for efficient and selective photocatalytic CO2 reduction.

[0017] In addition, the application also post-treats and modifies the obtained covalent organic framework photocatalyst, and further improves the photocatalytic CO2 reduction performance thereof. -1 , the CO generation rate (173 μmol g -1 h -1 ) thereof, the selectivity to CO in the CO2 reduction process reaches 99.8%, and the catalytic performance almost does not change (173 μmol g -1 h -1), which has excellent cycle stability. Among them, first, the nano Fe2O3 with catalytic activity is loaded on the surface or pores of TzPm-COF-2Br; then more active sites are introduced by adding zinc citrate, and it is found through experiments that Zn has a synergistic effect with Fe and Br, which can significantly improve the photocatalytic activity of the material; the dispersibility of the material is improved by adding sodium citrate, so that the components are uniformly dispersed, the citrate ions modify the surface of the covalent organic framework photocatalyst through chemical action, increase the adsorption active sites, and enhance the adsorption capacity of CO2; finally, heat treatment is carried out in an inert gas to obtain a uniform and stable covalent organic framework photocatalyst.

[0018] Other advantages, objects, and features of the present application will be apparent from the following specification, and will be appreciated by those skilled in the art. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 It is the synthesis route of TzPm-COF-2Br of the embodiment 1 of the present application; Figure 2 It is the SEM and EDS of TzPm-COF-2Br of the embodiment 1 of the present application and TzPm-COF of the comparative example 1; Figure 3 It is the FTIR spectrum (a), Raman spectrum (b), PXRD spectrum (c), nitrogen adsorption isotherm at 77 K, and pore size distribution of TzPm-COF-2Br (d) and TzPm-COF (e), C 1s XPS (f) of TzPm-COF-2Br of the embodiment 1 of the present application and TzPm-COF of the comparative example 1; Figure 4 It is the thermogravimetric curve (TG) and differential thermogravimetric curve (DTG) of TzPm-COF-2Br of the embodiment 1 of the present application and TzPm-COF of the comparative example 1; Figure 5 It is the ultraviolet-visible absorption spectrum of TzPm-COF-2Br of the embodiment 1 of the present application and TzPm-COF of the comparative example 1; Figure 6 It is the photocatalytic performance of TzPm-COF-2Br of the embodiment 1 of the present application and TzPm-COF of the comparative example 1 for reducing carbon dioxide into carbon monoxide and methane under visible light irradiation; Figure 7 It is the photocatalytic performance of TzPm-COF-2Br of the embodiment 1 of the present application for reducing carbon dioxide into carbon monoxide under different restricted conditions; Figure 8 It is the photocatalytic performance of TzPm-COF-2Br of the embodiment 1 of the present application for reducing carbon dioxide into carbon monoxide under different wavelengths. Figure 9 Cyclic performance of TzPm-COF-2Br of Example 1 of the present application for reduction of carbon dioxide to carbon monoxide (red) and methane (blue) under visible light irradiation. DETAILED DESCRIPTION

[0020] The present application will be further described in conjunction with the accompanying drawings, so that those skilled in the art can implement the present application according to the description and drawings.

[0021] It should be understood that the terms such as "have", "contain" and "include" used herein do not exclude the presence or addition of one or more other elements or combinations thereof.

[0022] In the following examples, the raw materials used include: 1-hydroxy-3-methylbenzene (AR, 99.5%), isoquinoline (AR, 97.0%), N,N-dimethylformamide (DMF, AR, 99.8%), tetrahydrofuran (THF, AR, 99.0%), 4,4',4''-(1,3,5-triazine-2,4,6-triyl)trianiline (TAPTz, AR, 95%), 2,5-dibromobenzene-1,2,4,5-tetracarboxylic dianhydride (PMDCA-2Br, AR, 95%) and 1,2,4,5-tetracarboxylic dianhydride (PMDCA, AR, 96%) are all purchased from Shanghai Aldrich Reagent Co., Ltd. All reagents are used directly without further purification.

[0023] Example 1 A method for preparing a covalent organic framework photocatalyst for photocatalytic reduction of CO2, comprising the following steps: Step one, TAPTz (177 mg, 0.5 mmol) and PMDCA-2Br (282 mg, 0.75 mmol) are added to a microwave reaction kettle at room temperature, then 1-hydroxy-3-methylbenzene (20 mL) and isoquinoline (1 mL) are added, and ultrasonic treatment is immediately performed to dissolve the reaction mixture; Step two, place the microwave reactor in the microwave oven, microwave heating to 160℃ and keep for 10 minutes, natural cooling to room temperature, filter the orange red precipitate obtained, and wash with DMF (20 mL) and THF (20 mL) in turn, then Soxhlet extraction in THF for 24 hours, dry the solid at 60℃ under vacuum for 12h, to obtain the covalent organic framework photocatalyst, recorded as TzPm-COF-2Br.

[0024] Example 2 The steps one to two of this example are consistent with example 1, the difference is that the covalent organic framework photocatalyst TzPm-COF-2Br prepared in step two is modified by post-treatment modification, and the post-treatment modification method comprises: S21, mix TzPm-COF-2Br and nano Fe2O3 according to the mass ratio of 1:0.05 into anhydrous ethanol, stir for 30 min, stand for 2h, then heat to 70℃ and stir until the solvent is completely evaporated, to obtain a pretreated covalent organic framework photocatalyst; wherein the mass volume ratio of TzPm-COF-2Br and anhydrous ethanol is 1g:40mL; S22, mix the pretreated covalent organic framework photocatalyst, sodium citrate and zinc citrate according to the mass ratio of 1:0.05:0.1 into anhydrous ethanol, ultrasonic dispersion for 1h, stir at 60℃ for 3h, then heat to 70℃ and stir until the solvent is completely evaporated, and grind to obtain a treated covalent organic framework photocatalyst; wherein the mass volume ratio of pretreated covalent organic framework photocatalyst and anhydrous ethanol is 1g:40mL; S23, put the treated covalent organic framework photocatalyst into a porcelain boat and transfer it into a tube furnace, heat to 250℃ at a heating rate of 10℃ / min under argon atmosphere and keep for 50min, and then naturally cool to room temperature to obtain a modified covalent organic framework photocatalyst, recorded as TzPm-COF-2Br-1.

[0025] Example 3 The steps one to two of this example are consistent with example 1, the difference is that the covalent organic framework photocatalyst TzPm-COF-2Br prepared in step two is modified by post-treatment modification, and the post-treatment modification method comprises: S21, mix TzPm-COF-2Br and nano Fe2O3 according to the mass ratio of 1:0.05 into anhydrous ethanol, stir for 30 min, stand for 2h, then heat to 70℃ and stir until the solvent is completely evaporated, to obtain a pretreated covalent organic framework photocatalyst; wherein the mass volume ratio of TzPm-COF-2Br and anhydrous ethanol is 1g:40mL; S22, the pretreated covalent organic framework photocatalyst is placed in a porcelain boat and transferred into a tube furnace, heated to 250 DEG C at a heating rate of 10 DEG C / min in an argon atmosphere and kept for 50 min, and naturally cooled to room temperature to obtain a modified covalent organic framework photocatalyst, denoted as TzPm-COF-2Br-2; Compared with Example 2, in the post-treatment modification method of Example 3, the sodium citrate and zinc citrate treatment steps are not performed.

[0026] Example 4 Steps one to two of this example are consistent with Example 1, except that the covalent organic framework photocatalyst TzPm-COF-2Br prepared in step two is subjected to post-treatment modification, and the post-treatment modification method comprises: S21, the covalent organic framework photocatalyst, sodium citrate and zinc citrate are mixed in anhydrous ethanol at a mass ratio of 1:0.05:0.1, ultrasonically dispersed for 1 h, stirred at 60 DEG C under reflux for 3 h, then heated to 70 DEG C and stirred until the solvent is completely evaporated, and ground to obtain a treated covalent organic framework photocatalyst; wherein the mass-volume ratio of TzPm-COF-2Br and anhydrous ethanol is 1 g:40 mL; S22, the treated covalent organic framework photocatalyst is placed in a porcelain boat and transferred into a tube furnace, heated to 250 DEG C at a heating rate of 10 DEG C / min in an argon atmosphere and kept for 50 min, and naturally cooled to room temperature to obtain a modified covalent organic framework photocatalyst, denoted as TzPm-COF-2Br-3; Compared with Example 2, in the post-treatment modification method of Example 4, the nano-Fe2O3 treatment step is not performed.

[0027] Example 5 Steps one to two of this example are consistent with Example 1, except that the covalent organic framework photocatalyst TzPm-COF-2Br prepared in step two is subjected to post-treatment modification, and the post-treatment modification method comprises: S21, the TzPm-COF-2Br and nano-Fe2O3 are mixed in anhydrous ethanol at a mass ratio of 1:0.05, stirred for 30 min, left to stand for 2 h, then heated to 70 DEG C and stirred until the solvent is completely evaporated to obtain a pretreated covalent organic framework photocatalyst; wherein the mass-volume ratio of TzPm-COF-2Br and anhydrous ethanol is 1 g:40 mL; S22, the pretreated covalent organic framework photocatalyst and zinc citrate were mixed in anhydrous ethanol at a mass ratio of 1:0.1, ultrasonic dispersion was carried out for 1 h, stirring reflux was carried out at 60 DEG C for 3 h, then heating to 70 DEG C and stirring until the solvent was completely evaporated, grinding to obtain a treated covalent organic framework photocatalyst; wherein the mass volume ratio of the pretreated covalent organic framework photocatalyst and anhydrous ethanol was 1 g:40 mL; S23, the treated covalent organic framework photocatalyst was placed in a porcelain boat and transferred into a tube furnace, heated to 250 DEG C at a heating rate of 10 DEG C / min in an argon atmosphere and kept for 50 min, and naturally cooled to room temperature to obtain a modified covalent organic framework photocatalyst, denoted as TzPm-COF-2Br-4. Compared with example 2, no sodium citrate was used in the post-treatment modification method of example 5.

[0028] Example 6 The steps one to two of this example were consistent with example 1, except that the covalent organic framework photocatalyst TzPm-COF-2Br prepared in step two was subjected to post-treatment modification, and the post-treatment modification method included: S21, TzPm-COF-2Br and nano Fe2O3 were mixed in anhydrous ethanol at a mass ratio of 1:0.05, stirring for 30 min, standing for 2 h, then heating to 70 DEG C and stirring until the solvent was completely evaporated, to obtain a pretreated covalent organic framework photocatalyst; wherein the mass volume ratio of TzPm-COF-2Br and anhydrous ethanol was 1 g:40 mL; S22, the pretreated covalent organic framework photocatalyst and sodium citrate were mixed in anhydrous ethanol at a mass ratio of 1:0.05, ultrasonic dispersion was carried out for 1 h, stirring reflux was carried out at 60 DEG C for 3 h, then heating to 70 DEG C and stirring until the solvent was completely evaporated, grinding to obtain a treated covalent organic framework photocatalyst; wherein the mass volume ratio of the pretreated covalent organic framework photocatalyst and anhydrous ethanol was 1 g:40 mL; S23, the treated covalent organic framework photocatalyst was placed in a porcelain boat and transferred into a tube furnace, heated to 250 DEG C at a heating rate of 10 DEG C / min in an argon atmosphere and kept for 50 min, and naturally cooled to room temperature to obtain a modified covalent organic framework photocatalyst, denoted as TzPm-COF-2Br-5. Compared with example 2, no zinc citrate was used in the post-treatment modification method of example 6.

[0029] Example 7 The steps one to two of this example were consistent with example 1, except that the covalent organic framework photocatalyst TzPm-COF-2Br prepared in step two was subjected to post-treatment modification, and the post-treatment modification method included: S21, TzPm-COF-2Br and nano-Fe2O3 were mixed according to a mass ratio of 1:0.05 and added to anhydrous ethanol, stirred for 30 min, and then placed for 2 h. Then, the mixture was heated to 70°C and stirred until the solvent was completely evaporated, to obtain a pretreated covalent organic framework photocatalyst; wherein the mass-volume ratio of TzPm-COF-2Br and anhydrous ethanol was 1 g:40 mL; S22, the pretreated covalent organic framework photocatalyst, sodium citrate and zinc citrate were mixed according to a mass ratio of 1:0.05:0.1 and added to anhydrous ethanol, and ultrasonic dispersion was performed for 1 h. The mixture was stirred at 60°C for 3 h, and then heated to 70°C and stirred until the solvent was completely evaporated. The treated covalent organic framework photocatalyst was obtained by grinding, and was recorded as TzPm-COF-2Br-6; wherein the mass-volume ratio of the pretreated covalent organic framework photocatalyst and anhydrous ethanol was 1 g:40 mL. Compared with Example 2, the post-treatment modification method of Example 7 does not perform a heat treatment step.

[0030] Comparative Example 1 In this comparative example, PMDCA-2Br was replaced by PMDCA, and the remaining steps were the same as those in Example 1, to obtain a covalent organic framework photocatalyst, which was recorded as TzPm-COF.

[0031] Comparative Example 2 In this comparative example, the covalent organic framework photocatalyst TzPm-COF prepared in Comparative Example 1 was subjected to post-treatment modification, and the post-treatment modification method was the same as that in Example 2, to obtain a covalent organic framework photocatalyst, which was recorded as TzPm-COF-1.

[0032] Characterization: Philips X’Pert Pro Super X-ray diffractometer (scanning rate of 5° / min) was used to collect X-ray diffraction (PXRD) patterns. TU-1901 spectrophotometer was used to record diffuse reflectance ultraviolet-visible spectra. Electrochemical experiments were performed on a CHI 660 electrochemical workstation under ambient conditions. Photocurrent density was collected by turning on and off a xenon lamp every 30 seconds in a 42.5 g / L sodium nitrate solution. X-ray photoelectron spectroscopy (XPS) analysis was performed using monochromatic Al Kα (1486.6 eV) as the X-ray source (Thermo Scientific K-Alpha X-ray photoelectron spectrometer). Fourier transform infrared spectroscopy (FTIR) measurements were performed using a Nexus 670 ThermoNicolet Fourier transform infrared spectrometer with KBr pellet method.

[0033] Photocatalytic activity of CO2 conversion: 2 mg of catalyst and 2 mL of deionized water were accurately weighed and mixed, and subjected to continuous sonication for 30 minutes to ensure complete dispersion. The mixture was uniformly coated onto a quartz membrane and dried in an oven at 60 °C. The prepared quartz membrane was then transferred to a 250 mL sealed quartz reactor, high-purity CO2 gas was introduced for 30 minutes, and 200 μL of H2O was added. A 300 W xenon lamp equipped with a 400 nm filter was used as the light source to simulate sunlight, and circulating cooling water was introduced to maintain a constant room temperature within the system. 3 mL of gaseous product was collected for analysis every 1 hour.

[0034] Figure 1 The synthetic route for TzPm-COF-2Br in Example 1 is shown below. Under microwave irradiation, 4,4',4''-(1,3,5-triazine-2,4,6-triyl)triphenylamine (TAPTz) underwent an imidization reaction with 2,5-dibromobenzene-1,2,4,5-tetracarboxylic acid dianhydride (PMDCA-2Br) to synthesize TzPm-COF-2Br. Compared to TzPm-COF, the introduction of Br atoms altered the overall charge distribution of the backbone, weakening the π-π stacking interactions between TzPm-COF-2Br molecules, leading to an increased twist angle and thus promoting the exposure of more active sites. The morphology of TzPm-COF-2Br and TzPm-COF was characterized using scanning electron microscopy (SEM). Figure 2 The results show that both TzPm-COF-2Br and TzPm-COF exhibit similar flower-like structures, indicating that the introduction of bromine-containing ligands did not change the morphology of the materials. Compared with TzPm-COF, the energy-dispersive spectroscopy (EDS) mapping of TzPm-COF-2Br shows a more uniform distribution of Br atoms, further confirming the successful synthesis of the material.

[0035] To analyze the composition of the prepared TzPm-COF-2Br, Fourier transform infrared spectroscopy and Raman spectroscopy were further performed. Figure 3 ab). For example Figure 3 As shown in a, TzPm-COF-2Br and TzPm-COF are at ~1777 cm⁻¹. -1 and ~1729 cm -1 The absorption peaks at these locations correspond to the asymmetric and symmetric vibrations of the C=O group in the imine ring, respectively. Meanwhile, the absorption peaks at ~1366 cm⁻¹ for TzPm-COF-2Br and TzPm-COF are at ~1366 cm⁻¹. -1 The absorption peak at [location] originates from the stretching vibration of the CNC bond within the imine ring. Additionally, the absorption peak corresponding to the amino group of the starting monomer TAPTz (3324 cm⁻¹) [is also present]. -1 and 3212 cm -1 The anhydride groups of PMDCA-2Br / PMDCA are at ~1771 cm⁻¹.-1 The absence of the absorption peak at that point indicates that the obtained product is a fully imine-crosslinked network. For example... Figure 3 As shown in b, the Raman spectra of both TzPm-COF and TzPm-COF-2Br are in the range of ~1370 cm⁻¹. -1 ~1606 cm -1 and ~1789 cm -1 Characteristic absorption bands were observed at the , which were attributed to C−N stretching vibration, C=C stretching vibration, and C=O stretching vibration, respectively. Both FTIR and Raman spectroscopy confirmed the successful synthesis of imine-crosslinked TzPm-COF-2Br.

[0036] To confirm the crystal structures of TzPm-COF-2Br and TzPm-COF, powder X-ray diffraction (PXRD) analysis was performed to assess the successful synthesis of these COFs. Figure 3 As shown in c, the PXRD patterns of TzPm-COF-2Br and TzPm-COF are completely identical, and highly consistent with the simulated PXRD patterns of the two-dimensional honeycomb structure. Subsequently, we further investigated the specific surface area and pore structure of TzPm-COF-2Br and TzPm-COF through nitrogen adsorption-desorption experiments. Figure 3 Based on the N2 adsorption-desorption isotherm, the BET specific surface areas of TzPm-COF-2Br and TzPm-COF were calculated to be approximately 447 m². 2 / g and 481m 2 / g. Furthermore, the pore size distributions of TzPm-COF-2Br and TzPm-COF, calculated using nonlocal density functional theory (NLDFT), are 3.61 nm and 3.59 nm, respectively. These experimental values ​​agree well with the theoretically predicted pore size of 37 Å (3.7 nm), confirming the consistency between the synthesized materials and their structural model. To further determine the stability of TzPm-COF-2Br, thermogravimetric analysis was employed. (For example...) Figure 4 As shown, the pyrolysis process of the samples can be divided into two stages. First, the weight change between 25-220℃ is mainly attributed to the evaporation of residual solvent and moisture in the material. Second, thermal decomposition occurs in the range of 410-830℃, with the maximum weight loss occurring between 500-800℃. TzPm-COF-2Br and TzPm-COF exhibit almost identical overall weight loss behavior in these stages. Further analysis of the differential thermogravimetric analysis (DTG) curves shows that the main thermal degradation peak temperatures of TzPm-COF and TzPm-COF-2Br are located at 615℃ and 626℃, respectively. The shift of the TzPm-COF-2Br degradation peak to higher temperatures indicates that the introduction of bromine-containing ligands enhances the thermal stability of the framework, which may be achieved by raising the energy barrier of thermal decomposition.

[0037] X-ray photoelectron spectroscopy was used to further determine and identify the surface elemental composition and electronic structure of TzPm-COF-2Br and TzPm-COF. Figure 3 As shown in f, the C 1s XPS spectrum of TzPm-COF-2Br is deconvolved into three distinct peaks with binding energies of approximately 284.8 eV, 286.4 eV, and 288.9 eV, respectively, corresponding to sp... 2 The hybrid aromatic carbon (C=C / CC), carbon in the C=N–C group, and the C=O bonding environment. Compared with TzPm-COF, the C 1s peak of TzPm-COF-2Br shifts to a higher binding energy, indicating that introducing Br atoms into the COF framework can change the electron distribution of the material, thereby potentially affecting the regulation of the electronic environment during CO2 photocatalytic reduction. The UV-Vis absorption spectra of TzPm-COF-2Br and TzPm-COF are shown below. Figure 6 As shown, compared with TzPm-COF, TzPm-COF-2Br exhibits stronger light absorption in the range of 200 nm to 800 nm, demonstrating superior light-harvesting ability.

[0038] The introduction of halogen atoms (Br) offers potential for enhancing photocatalytic CO2 reduction. The photocatalytic CO2 reduction performance of TzPm-COF and TzPm-COF-2Br was evaluated under simulated sunlight irradiation. Gas chromatography analysis confirmed that CO and CH4 were the only gaseous products, and no liquid products were detected. Compared with TzPm-COF, TzPm-COF-2Br exhibited superior photocatalytic CO2 reduction performance. Figure 6 The CO production reached 620 μmol g after 4 hours of reaction. -1 Its CO generation rate (155 μmolg) -1 h -1 The value is approximately TzPm-COF (57 μmol g). -1 h -1 The superior photocatalytic CO2 reduction performance of TzPm-COF-2Br highlights the importance of Br atoms in optimizing the electron distribution between donor and acceptor units within the COF framework. Furthermore, TzPm-COF-2Br exhibits significantly enhanced selectivity for CO formation during CO2 reduction, reaching 99.4%, far exceeding the 96.6% of unmodified TzPm-COF (where H2 is almost undetectable). This demonstrates that introducing Br atoms through PMDCA-2Br successfully improves the selectivity of the CO2 reduction pathway.

[0039] To further verify the source of CO in the photocatalytic CO2 reduction process, we conducted a control experiment. For example... Figure 7As shown, the amount of CO generated can be negligible under the condition of lacking catalyst, visible light, CO2 or H2O. This indicates that each component in the system is crucial, thus explicitly confirming that the photocatalytic reaction is the driving force for CO2 reduction. We measured the photocatalytic CO2 reduction rate of TzPm-COF-2Br under monochromatic light irradiation at 400 nm, 450 nm, 500 nm, 600 nm and 700 nm wavelengths (Figure 2b) Figure 8 ), and the observed photocatalytic activity is in good agreement with its optical absorption spectrum, indicating that light absorption is a key factor determining the photocatalytic efficiency. After five consecutive cycle tests (Figure 2c) Figure 9 ), the catalytic performance of TzPm-COF-2Br almost remains unchanged (155 μmol g -1 h -1 ), indicating its excellent cyclic stability in the reaction system.

[0040] The present application synthesizes D-A type COF rapidly (reaction time is only 10 minutes) by microwave assistance, constructs a bromine functionalized imine-based COF photocatalyst for efficient photocatalytic CO2 reduction. The designed TzPm-COF-2Br realizes a CO generation rate of 155 μmol g -1 h -1 in the visible light driven CO2 reduction reaction, which is about 2.7 times of TzPm-COF (57 μmol g -1 h -1 ). Experimental analysis shows that the introduction of bromine atom enhances the absorption of visible light by the catalyst, promotes the electron-hole separation in the CO2 reduction process, and promotes the directional charge transfer from TAPTz (donor) to PMDCA (acceptor). Theoretical calculation shows that the excellent photocatalytic activity of TzPm-COF-2Br is attributed to its higher charge separation efficiency and lower adsorption energy barrier. The present application demonstrates the rapid synthesis (only 10 minutes) of COF and confirms the regulation of Br atom in the COF-based photocatalyst, highlighting the great potential of this strategy in developing new efficient photocatalysts.

[0041] The present application also post-treats and modifies the obtained covalent organic framework photocatalyst to further improve its photocatalytic CO2 reduction performance. As shown in Table 1, the CO production of TzPm-COF-2Br-1 prepared in Example 2 is 692 μmol g -1 after 4h of reaction, the CO generation rate (173 μmol g -1 h -1 ), and the selectivity to CO in the CO2 reduction process reaches 99.8%. After five consecutive cycle tests, the catalytic performance of Examples 2-7 almost remains unchanged, all having excellent cyclic stability.

[0042] Table 1 While embodiments of the application have been disclosed in connection with the above specification and drawings this description is not intended to limit the scope of the application and many modifications, enhancements, alternatives, and variations will become apparent to those skilled in the art from this disclosure. Accordingly, it is expressly intended that the description should be regarded as exemplary only and that changes can be made without departing from the spirit and scope of the application, which is defined in the following claims.

Claims

1. A method for preparing a covalent organic framework photocatalyst for photocatalytic CO2 reduction, characterized in that, Includes the following steps: Step 1: Add 4,4',4''-(1,3,5-triazine-2,4,6-triyl)triphenylamine and 2,5-dibromobenzene-1,2,4,5-tetracarboxylic acid dianhydride to the reaction vessel, then add 1-hydroxy-3-methylbenzene and isoquinoline, and mix thoroughly. Step 2: Microwave heating is applied to the reaction vessel. After cooling to room temperature, the precipitate is collected by filtration, washed, and then subjected to Soxhlet extraction and vacuum drying to obtain a covalent organic framework photocatalyst for photocatalytic CO2 reduction.

2. The method for preparing a covalent organic framework photocatalyst for photocatalytic CO2 reduction as described in claim 1, characterized in that, In step one, the molar ratio of 4,4',4''-(1,3,5-triazine-2,4,6-triyl)triphenylamine and 2,5-dibromobenzene-1,2,4,5-tetracarboxylic acid dianhydride is 1:1~2.

3. The method for preparing a covalent organic framework photocatalyst for photocatalytic CO2 reduction as described in claim 1, characterized in that, In step one, the ratio of 4,4',4''-(1,3,5-triazine-2,4,6-triyl)triphenylamine, 1-hydroxy-3-methylbenzene, and isoquinoline is 1 mmol: 30~50 mL: 1~3 mL.

4. The method for preparing a covalent organic framework photocatalyst for photocatalytic CO2 reduction as described in claim 1, characterized in that, In step two, the microwave heating temperature is 160℃ and the time is 5~20min.

5. The method for preparing a covalent organic framework photocatalyst for photocatalytic CO2 reduction as described in claim 1, characterized in that, In step two, washing is performed sequentially with N,N-dimethylformamide and tetrahydrofuran; Soxhlet extraction is carried out in tetrahydrofuran for 12-36 hours.

6. The method for preparing a covalent organic framework photocatalyst for photocatalytic CO2 reduction as described in claim 1, characterized in that, In step two, the vacuum drying temperature is 40~80℃ and the time is 6~24h.

7. The method for preparing a covalent organic framework photocatalyst for photocatalytic CO2 reduction as described in claim 1, characterized in that, The method also includes post-treatment modification of the covalent organic framework photocatalyst for photocatalytic CO2 reduction prepared in step two, wherein the post-treatment modification method includes: S21. Mix the covalent organic framework photocatalyst and nano Fe2O3 into the solvent, stir for 20-40 min, let stand for 1-3 h, then heat to 60-80℃ and stir until the solvent is completely evaporated to obtain the pretreated covalent organic framework photocatalyst. S22. The pretreated covalent organic framework photocatalyst, sodium citrate and zinc citrate are mixed and added to the solvent, ultrasonically dispersed for 0.5-2 hours, stirred and refluxed at 50-70℃ for 2-4 hours, then heated to 60-80℃ and stirred until the solvent is completely evaporated, and ground to obtain the treated covalent organic framework photocatalyst. S23. The modified covalent organic framework photocatalyst is subjected to heat treatment in an inert gas and then naturally cooled to room temperature to obtain the modified covalent organic framework photocatalyst.

8. The method for preparing a covalent organic framework photocatalyst for photocatalytic CO2 reduction as described in claim 7, characterized in that, In S21, the mass ratio of covalent organic framework photocatalyst to nano Fe2O3 is 1:0.01~0.1; the mass-volume ratio of covalent organic framework photocatalyst to solvent is 1g:20~60mL. In step S22, the mass ratio of the pretreated covalent organic framework photocatalyst, sodium citrate, and zinc citrate is 1:0.01~0.1:0.05~0.2; the mass-volume ratio of the pretreated covalent organic framework photocatalyst and the solvent is 1g:20~60mL. The solvent is anhydrous ethanol.

9. The method for preparing a covalent organic framework photocatalyst for photocatalytic CO2 reduction as described in claim 7, characterized in that, In step S23, the inert gas is either argon or nitrogen; the specific method of heat treatment is to heat to 200-300℃ at a heating rate of 5-15℃ / min and hold at that temperature for 30-70min.

10. The application of a covalent organic framework photocatalyst prepared by the preparation method according to any one of claims 1-9 in the photocatalytic reduction of CO2.

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