A bi-zn bimetallic salen type covalent organic framework catalyst and a preparation method thereof

Abstract

This invention discloses a BiZn bimetallic Salen-type covalent organic framework catalyst and its preparation method. First, hexamethylenehexane octahydrate is reacted with 1,2,4,5-phenyltetramine tetrahydrochloride to prepare hexamethylenehexazatrinaphthalene building units. Then, hexamethylenehexazatrinaphthalene, 2-hydroxy-isophthalaldehyde, and bismuth salt are reacted with a Schiff base condensation reaction to obtain a Bi-Salen covalent organic framework precursor. Finally, a post-metallization reaction is performed to introduce zinc metal centers to obtain the BiZn bimetallic Salen-type covalent organic framework catalyst. This preparation method achieves high dispersion and local coordination environment regulation of bismuth and zinc bimetallic sites within the ordered channels of the covalent organic framework through a stepwise coordination strategy. The obtained catalyst has a stable framework structure and exhibits high Faradaic efficiency and excellent long-term electrolytic stability when used for the electrocatalytic reduction of carbon dioxide to formic acid. The preparation process is simple, mild, and reproducible.

Description

Technical Field

[0001] This invention relates to the field of electrocatalytic materials and new energy conversion technology, and in particular to a BiZn bimetallic Salen-type covalent organic framework catalyst and its preparation method. Background Technology

[0002] With the massive consumption of fossil fuels and the continuous increase in carbon dioxide emissions, the resulting energy and environmental problems have attracted widespread attention. Electrocatalytic carbon dioxide reduction can convert carbon dioxide into usable chemicals under relatively mild conditions and can be combined with renewable electricity, making it significant in the fields of carbon resource recycling and energy conversion. Among the various products of electrocatalytic carbon dioxide reduction, formic acid, as a typical two-electron transfer product, has a relatively simple reaction pathway and high chemical utilization value and energy storage application potential, making it one of the promising target products.

[0003] Among existing formic acid-based carbon dioxide reduction catalysts, bismuth-based materials have attracted attention due to their low hydrogen evolution activity, good formic acid formation selectivity, and relatively good environmental friendliness. Covalent organic frameworks (COF) offer advantages such as strong structural designability, tunable pore structure, and easy modification of functional sites, providing relatively well-defined coordination environments for metal active sites. Salen-type COFs, in particular, contain nitrogen-oxygen coordination sites, which can form relatively stable coordination structures with metal ions, providing a suitable framework platform for constructing well-defined metal catalytic centers. Therefore, introducing bismuth-zinc bimetallic sites into Salen-type COFs is expected to stabilize the bismuth active center while regulating the local coordination environment, thereby improving the interfacial reaction behavior of the catalyst.

[0004] However, existing bismuth-based catalysts still suffer from problems such as limited dispersion of active sites, difficulty in controlling the local coordination environment, high interfacial charge transfer resistance, and insufficient long-term electrolysis stability. Current technologies for constructing bismuth-zinc bimetallic sites within a Salen-type covalent organic framework and using them as electrocatalysts for the reduction of carbon dioxide to formic acid remain insufficient. Summary of the Invention

[0005] In view of the above problems, a BiZn bimetallic Salen-type covalent organic framework catalyst and its preparation method are proposed to overcome or at least partially solve the above problems, including: A method for preparing a BiZn bimetallic Salen-type covalent organic framework catalyst includes: S1. Cyclohexanehexane octahydrate and 1,2,4,5-phenyltetramine tetrahydrochloride were reacted to obtain hexamethylenehexaazatrinaphthalene; S2. Hexamethylenehexaazatrinaphthalene, 2-hydroxyisophthalaldehyde and bismuth salt were subjected to Schiff base condensation reaction to obtain Bi-Salen covalent organic framework. S3. The Bi-Salen covalent organic framework is subjected to a post-metallization reaction with zinc salt to obtain a BiZn bimetallic Salen-type covalent organic framework catalyst.

[0006] Optionally, in step S1, the reaction is carried out in a mixed solution of ethanol and acetic acid, with a volume ratio of ethanol to acetic acid of 4:1, a reaction temperature of 140℃, and a reaction time of 8h. After the reaction is completed, the mixture is centrifuged and washed with anhydrous diethyl ether to obtain hexamethylenehexaazatrinaphthalene.

[0007] Optionally, in step S2, the bismuth salt is bismuth acetate, and the Schiff base condensation reaction is carried out in a mixed solvent of 1,2-dichlorobenzene and n-butanol with a volume ratio of 1:1. A 6M aqueous acetic acid solution is added as a catalyst. After ultrasonic dispersion, the reaction system is subjected to liquid nitrogen cryogenic vacuum degassing treatment, then sealed and reacted at 120°C for 72 h. After the reaction is completed, the precipitate is collected, washed sequentially with N,N-dimethylformamide, acetone, methanol and dichloromethane, and dried under vacuum to obtain the Bi-Salen covalent organic framework.

[0008] Optionally, in step S3, the zinc salt is zinc acetate, and the subsequent metallization reaction is carried out in methanol. Specifically, the Bi-Salen covalent organic framework is dispersed in methanol to form a suspension, zinc acetate is dissolved in methanol and then added to the suspension, and the reaction is stirred at room temperature for 48 hours. After the reaction is completed, the product is separated, washed with methanol, and dried under vacuum to obtain the BiZn bimetallic Salen-type covalent organic framework catalyst.

[0009] A BiZn bimetallic Salen-type covalent organic framework catalyst is prepared by any of the above methods. The catalyst comprises a Salen-type covalent organic framework formed by the Schiff base condensation reaction of hexamethylenehexaazatrinaphthalene and 2-hydroxym-phthalaldehyde. The Salen-type covalent organic framework is coordinated with Bi metal centers and Zn metal centers.

[0010] Optionally, the Bi metal center and the Zn metal center are ionicly located in the Salen coordination cavity of the Salen-type covalent organic framework, and the Salen coordination cavity is provided by the imine bond and hydroxyl group of the Salen-type covalent organic framework.

[0011] Optionally, the catalyst is a covalent organic framework material formed by stacking layered structures.

[0012] Optionally, the catalyst is used in the electrocatalytic carbon dioxide reduction reaction, in which the catalyst serves as the cathode catalyst.

[0013] Optionally, the electrocatalytic carbon dioxide reduction reaction is carried out in a carbon dioxide-saturated 0.5M potassium bicarbonate electrolyte, and the main product of the electrocatalytic carbon dioxide reduction reaction is formic acid or formate.

[0014] Optionally, the catalyst is dispersed in a mixed solution of ethanol and perfluorosulfonic acid resin and drop-coated onto the surface of carbon cloth or carbon paper to form an electrode, with the catalyst loading on the electrode being 0.5 to 2 mg / cm³. 2 .

[0015] This invention pre-prepares HAHATN building blocks containing multiple ortho-amino groups, and then performs a Schiff base condensation reaction on hexamethylenehexaazatrinaphthalene, 2-hydroxyisophthalaldehyde, and bismuth salt in a one-pot process. Simultaneously, bismuth ions are in-situ anchored using N₂O₂ coordination cavities to obtain a Bi-Salen covalent organic framework. Zinc ions are then introduced via a post-metallization reaction to obtain a BiZn bimetallic Salen-type covalent organic framework catalyst. This preparation method achieves high dispersion of bismuth active centers within the ordered channels of the framework through a stepwise coordination strategy, overcoming the problem of easy aggregation of active components in traditional bismuth-based catalysts. The introduction of zinc ions further modulates the local coordination environment and electronic structure of bismuth sites, enhancing the catalyst surface's adsorption capacity for carbon dioxide reduction intermediates and effectively suppressing hydrogen evolution competition reactions. The resulting catalyst, with its two-dimensional conjugated framework and layered stacked structure of Salen-type covalent organic framework, provides an efficient electron transport channel and reduces interfacial charge transfer resistance. At the same time, the strong anchoring effect of multidentate coordination bonds on bimetallic sites ensures structural stability under long-term electrolysis conditions. It exhibits high formic acid Faraday efficiency and continuous operational stability in the electrocatalytic reduction of carbon dioxide to formic acid. Attached Figure Description

[0016] To more clearly illustrate the technical solution of the present invention, the accompanying drawings used in the description of the present invention will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 These are characterization diagrams of the morphology and elemental distribution of COFs prepared in the embodiments of the present invention; Figure 2 These are structural and chemical characterization diagrams of the BiZn-Salen COF prepared in the embodiments of the present invention; Figure 3 This is a graph showing the electrocatalytic carbon dioxide reduction performance of the BiZn-Salen COF prepared in the embodiments of the present invention; Figure 4These are test graphs of the electrochemical activity, interfacial charge transport, and stability of the BiZn-Salen COF prepared in the embodiments of the present invention. Figure 5 This is a flowchart illustrating the preparation method of a BiZn bimetallic Salen-type covalent organic framework catalyst provided in this embodiment of the invention. Detailed Implementation

[0018] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0019] Reference Figure 5 This invention provides a method for preparing a BiZn bimetallic Salen-type covalent organic framework catalyst, which may specifically include the following steps: S1. Cyclohexanehexane octahydrate and 1,2,4,5-phenyltetramine tetrahydrochloride were reacted to obtain hexamethylenehexaazatrinaphthalene; S2. Hexamethylenehexaazatrinaphthalene, 2-hydroxyisophthalaldehyde and bismuth salt were subjected to Schiff base condensation reaction to obtain Bi-Salen covalent organic framework. S3. The Bi-Salen covalent organic framework is subjected to a post-metallization reaction with zinc salt to obtain a BiZn bimetallic Salen-type covalent organic framework catalyst.

[0020] Specifically: Step S1: Cyclohexanehexane octahydrate and 1,2,4,5-phenyltetramine tetrahydrochloride are reacted to yield hexamethylenehexaazatrinaphthalene. Cyclohexanehexane octahydrate is a six-membered cyclic compound containing six ketone carbonyl groups. The eight water molecules in its molecular structure give it good solubility in polar solvents, which is beneficial for the uniform dispersion of the reactants in solution. 1,2,4,5-phenyltetramine tetrahydrochloride is an aromatic amine compound containing four ortho-amino groups. The tetrahydrochloride form ensures its stable chemical properties during storage and use. The two reactants are added to a mixed solvent of ethanol and acetic acid, and the reactant particles are fully dispersed in the liquid phase by ultrasonic treatment to form a homogeneous suspension. The principle of ultrasonic dispersion is to utilize the cavitation effect generated by ultrasound in the liquid medium, which causes solvent molecules to vibrate violently and impact the surface of the reactant particles, promoting full contact between the solid and liquid phases. The resulting suspension is transferred to a reaction vessel and heated under sealed conditions for a solvothermal reaction.

[0021] The chemical reaction principle of this step is an aldehyde-amine condensation reaction. In the acidic environment provided by acetic acid, the oxygen atom of the ketone carbonyl group in cyclohexanehexane octahydrate is protonated, increasing the electrophilicity of the carbonyl carbon atom. The lone pair of electrons on the ortho-amino nitrogen atom of 1,2,4,5-phenyltetramine tetrahydrochloride initiates a nucleophilic attack on the activated carbonyl carbon atom, generating a tetrahedral intermediate. This intermediate is unstable and, under acidic conditions, loses one molecule of water to form a carbon-nitrogen double bond, i.e., an imine bond. The cyclohexanehexane octahydrate molecule contains six ketone carbonyl groups, each of which can undergo the above condensation reaction sequentially with the ortho-amino group of 1,2,4,5-phenyltetramine. Multiple 1,2,4,5-phenyltetramine molecules condense with the same cyclohexanehexane molecule from different directions, gradually constructing a polycyclic fused aromatic skeleton containing six imine groups. Since all four amino groups of 1,2,4,5-phenyltetramine are located in the ortho position of the benzene ring, the imine bond is coplanar with the benzene ring after condensation, forming an extended conjugated system, and finally yielding the hexamethylenehexaazatrinaphthalene product.

[0022] After the reaction was complete, the reaction vessel was allowed to cool naturally to room temperature before the reaction mixture was removed and centrifuged. Centrifugation utilizes the density difference between the solid and liquid phases; under the influence of centrifugal force, the solid product settles to the bottom of the centrifuge tube, while the supernatant is removed by decanting. The resulting solid product was washed with anhydrous diethyl ether, a low-boiling-point organic solvent that effectively dissolves unreacted organic matter and byproducts. After washing, the product was dried to obtain a solid powder of hexamethylenehexaazatrinaphthalene. The molecular structure of this product retains several ortho-amino groups that did not participate in the first-step condensation, providing necessary reaction sites for the subsequent construction of the covalent organic framework.

[0023] Step S2: Hexamethylene hexaazatrinaphthalene, 2-hydroxyisophthalaldehyde and bismuth salt are subjected to Schiff base condensation reaction to obtain Bi-Salen covalent organic framework.

[0024] 2-Hydroxyisophthalaldehyde is a bifunctional aromatic aldehyde compound containing two aldehyde groups and one ortho-hydroxyl group. The two aldehyde groups are located at the meta position on the benzene ring, and the hydroxyl group is located at the ortho position of one of the aldehyde groups. This spatial arrangement allows it to form a polydentate coordination environment containing nitrogen and oxygen atoms after condensation with the ortho-amino group. Bismuth acetate can be used as the bismuth salt, as it has acceptable solubility in organic solvents and can release bismuth ions to participate in the coordination process in the reaction system. The hexamethylenehexaazatrinaphthalene, 2-hydroxyisophthalaldehyde, and bismuth salt obtained in step S1 are added to an organic mixed solvent, and an aqueous acetic acid solution is added as a catalyst. The mixture is ultrasonically dispersed to form a homogeneous suspension. The reaction system is then subjected to liquid nitrogen cryogenic vacuum degassing: the container containing the reaction mixture is immersed in liquid nitrogen, and the mixture solidifies at extremely low temperatures. At this point, the container is evacuated to remove oxygen and residual gases from the system, and then the container is sealed. This degassing operation prevents oxygen from initiating side reactions during the high-temperature reaction. After degassing, the sealed container is placed in a heating environment for a solvothermal reaction.

[0025] The chemical reaction principle of this step involves two parallel processes. The first process is a Schiff base condensation reaction: the ortho-amino group retained in the hexamethylene hexaazatrinaphthalene molecule condenses with the aldehyde group in the 2-hydroxyisophthalaldehyde molecule under the catalysis of acetic acid. The amino group attacks the carbon atom of the aldehyde group, and after nucleophilic addition, loses one molecule of water to form an imine bond. Since 2-hydroxyisophthalaldehyde contains two aldehyde groups, each aldehyde group can condense with the amino group from different hexamethylene hexaazatrinaphthalene molecules, thereby connecting the hexamethylene hexaazatrinaphthalene units through the bridging of 2-hydroxyisophthalaldehyde, gradually cross-linking and expanding into a covalent organic framework with a two-dimensional layered structure. The second process is an in-situ coordination reaction: while the condensation reaction is underway, the hydroxyl group adjacent to the aldehyde group in 2-hydroxyisophthalaldehyde forms a tetradentate coordination cavity of type N2O2 with the nitrogen atom of the condensed imine bond. Bismuth ions present in the reaction system enter the coordination cavity through electrostatic attraction, forming coordinate bonds with the lone pairs of electrons on nitrogen and oxygen atoms. This allows them to be in situ anchored to the ordered pore walls of the framework during covalent organic framework growth. This in-situ coordination strategy ensures that bismuth ions are uniformly dispersed and fixed simultaneously with framework formation, fundamentally avoiding the problem of random aggregation of metal components in traditional impregnation or post-treatment methods.

[0026] After the reaction was complete, the container was cooled to room temperature, the sealed container was opened, and the precipitate was collected. The precipitate was washed sequentially with N,N-dimethylformamide, acetone, methanol, and dichloromethane. N,N-dimethylformamide is a high-boiling-point polar aprotic solvent that can effectively dissolve uncondensed monomer molecules and low molecular weight oligomers. Acetone and methanol are miscible with residual acetic acid and polar organic impurities, gradually displacing the reaction solvent within the pores. Dichloromethane has a low boiling point and moderate polarity, allowing for the final cleaning of the pores. After multi-step solvent gradient washing, the product was dried under vacuum to remove physically adsorbed solvent molecules from the pores, yielding a Bi-Salen covalent organic framework solid.

[0027] Step S3: The Bi-Salen covalent organic framework is subjected to a post-metallization reaction with zinc salt to obtain a BiZn bimetallic Salen-type covalent organic framework catalyst.

[0028] Zinc acetate dihydrate can be used as the zinc salt, as it has good solubility in alcohol solvents and can provide zinc ions for coordination. The Bi-Salen covalent organic framework obtained in step S2 is dispersed in methanol to form a homogeneous suspension under stirring. Methanol, as a protic polar solvent, not only wets the pore surface of the covalent organic framework, but its small molecular size also facilitates the carrying of zinc ions into the pores of the framework. A zinc salt solution is also prepared by dissolving it in methanol. This zinc salt methanol solution is slowly added to the above suspension under continuous stirring. After the addition is complete, stirring continues at room temperature to carry out the post-metallization reaction.

[0029] The reaction principle of this step is as follows: In the Bi-Salen covalent organic framework obtained in step S2, the Salen coordination cavities are not entirely occupied by bismuth ions. During the condensation reaction of framework formation, due to steric hindrance and reaction kinetics, some coordination cavities fail to effectively coordinate with bismuth ions, and these vacant coordination cavities retain their coordination activity with metal ions. When the zinc salt methanol solution is added, zinc ions diffuse into the interlayer channels of the covalent organic framework under the drive of the concentration gradient. The zinc ions coordinate with the imine nitrogen atoms and hydroxyl oxygen atoms in the vacant Salen coordination cavities, and the lone pair electrons of the nitrogen and oxygen atoms enter the empty d orbitals of the zinc ions, forming stable coordinate bonds. The ionic radius and coordination geometry of the zinc ions allow them to adapt to the spatial constraints of the Salen cavities, thus being firmly anchored in the framework. Through the subsequent metallization step, the zinc ions are introduced into the framework, forming a bimetallic active site system together with the bismuth ions coordinated in step S2. The introduction of zinc ions not only increases the total number of metal active sites in the framework, but more importantly, it modulates the local electron density distribution of the bismuth active center through the electronic interaction between bimetallic sites, thereby optimizing the adsorption and activation characteristics of key reaction intermediates in the carbon dioxide reduction process of bismuth sites.

[0030] After the reaction, the solid product was separated from the liquid phase by centrifugation or filtration. The obtained solid product was washed repeatedly with methanol to remove free zinc ions and unreacted zinc acetate physically adsorbed on the product surface and in the pores. After washing, the product was dried under vacuum to remove the methanol solvent, yielding a BiZn bimetallic Salen-type covalent organic framework catalyst.

[0031] All solvothermal reactions involved in the above steps are carried out in a closed reactor with a polytetrafluoroethylene liner, which can withstand the required temperature and autogenous pressure. The solvents used for washing in each step are commercially available and do not require further purification before use. The intermediate and final products obtained in each step can be vacuum dried after washing to effectively remove residual solvent molecules from the pores at a temperature that does not cause product decomposition.

[0032] In one or more embodiments of the present invention, in step S1, the reaction is carried out in a mixed solution of ethanol and acetic acid, with a volume ratio of ethanol to acetic acid of 4:1, a reaction temperature of 140°C, and a reaction time of 8 h; after the reaction is completed, the mixture is centrifuged and washed with anhydrous diethyl ether to obtain hexamethylenehexaazatrinaphthalene.

[0033] In this step, the mixed solvent of ethanol and acetic acid serves as both the dispersion and reaction medium. Ethanol, as a polar solvent, ensures the complete dissolution or dispersion of cyclohexanehexone octhydrate and 1,2,4,5-phenyltetramine tetrahydrochloride in the liquid phase, while acetic acid provides the acidic catalytic environment required for the aldehyde-amine condensation reaction. The 4:1 volume ratio of ethanol to acetic acid is chosen based on the following considerations: too low an acetic acid content results in insufficient catalytic efficiency and a slow condensation rate; too high an acetic acid content may lead to an overly vigorous reaction, broadening the molecular weight distribution of the products. A reaction temperature of 140°C is suitable under the self-generated pressure of the solvent in the closed reactor. At this temperature, the thermal motion of reactant molecules intensifies, the collision frequency increases, and the condensation reaction proceeds at an acceptable rate. A reaction time of 8 hours is sufficient for the complete condensation of cyclohexanehexone octhydrate and 1,2,4,5-phenyltetramine tetrahydrochloride to form hexamethylenehexaazatrinaphthalene; a shorter time results in incomplete reaction and reduced product yield. After the reaction was completed, the reactor was allowed to cool naturally to room temperature. The reaction mixture was then transferred to centrifuge tubes for centrifugation. During centrifugation, the solid product accumulated at the bottom of the tube under centrifugal force, and the supernatant could be separated by decantation. The obtained solid product was washed with anhydrous diethyl ether. Anhydrous diethyl ether, as a low-boiling-point nonpolar solvent, has good solubility for organic impurities and evaporates rapidly in the air after washing, without introducing residual solvent. After drying, the washed product yielded a dark-colored powdery solid of hexamethylenehexaazatrinaphthalene, which could be directly used in the subsequent step S2.

[0034] In one or more embodiments of the present invention, in step S2, the bismuth salt is bismuth acetate, the Schiff base condensation reaction is carried out in a mixed solvent of 1,2-dichlorobenzene and n-butanol, the volume ratio of 1,2-dichlorobenzene to n-butanol is 1:1, and 6M acetic acid aqueous solution is added as a catalyst; after ultrasonic dispersion, the reaction system is subjected to liquid nitrogen cryogenic vacuum degassing treatment, then sealed and reacted at 120°C for 72 h; after the reaction is completed, the precipitate is collected, washed sequentially with N,N-dimethylformamide, acetone, methanol and dichloromethane, and dried under vacuum to obtain the Bi-Salen covalent organic framework.

[0035] The advantage of bismuth acetate as a bismuth source is that its organic acid ligands can be gradually replaced in the reaction system. Ligand exchange can occur between acetate and hydroxyl groups in the solvent or Salen coordination cavity, so that bismuth ions are orderly introduced into the coordination cavity formed during the condensation reaction.

[0036] A 1:1 volume ratio of 1,2-dichlorobenzene and n-butanol is a commonly used solvent combination in the synthesis of covalent organic frameworks. 1,2-Dichlorobenzene, as a high-boiling-point aromatic solvent, provides a favorable solubility environment for the expansion of rigid conjugated frameworks, while n-butanol, as a protic solvent, can regulate the reaction rate and facilitate the ordered assembly of the framework. Equal-volume mixing of the two yields a reaction medium with moderate polarity and solubility, which is beneficial for the reversible progress of Schiff base condensation reactions and for improving the crystallinity of the framework material.

[0037] A 6M aqueous acetic acid solution serves as the proton source for the catalyst. Choosing a concentration of 6M ensures sufficient catalytic activity while avoiding the adverse effects of introducing excessive water on the reversible equilibrium of the condensation reaction.

[0038] Ultrasonic dispersion enables hexamethylenehexaazatrinaphthalene, 2-hydroxyisophthalaldehyde, and bismuth acetate to form a homogeneous suspension in a mixed solvent, ensuring sufficient contact between the components and providing a uniform reaction microenvironment for the subsequent condensation reaction. Liquid nitrogen cryogenic vacuum degassing, achieved by solidifying the reaction system with liquid nitrogen followed by vacuuming, effectively removes dissolved oxygen and residual gases, avoiding oxygen-induced oxidation side reactions during high-temperature reactions.

[0039] After sealing, the reaction system was subjected to a solvothermal reaction at 120°C for 72 hours. This temperature was sufficient to overcome the activation energy of the Schiff base condensation reaction, allowing the reversible formation and error correction of imine bonds to continue. The 72-hour reaction time provided a guarantee for the full growth of the framework and the orderly arrangement of the structure.

[0040] After the reaction was completed, the mixture was cooled to room temperature, and the precipitate was collected and washed sequentially with various solvents. N,N-dimethylformamide could dissolve unreacted monomers and low molecular weight oligomers; acetone could remove residual acetic acid and high-boiling-point solvents; methanol could displace N,N-dimethylformamide and acetone in the pores; and dichloromethane, as the final washing solvent, could effectively clean the pores, and its low boiling point was beneficial for subsequent vacuum drying.

[0041] The washing solvents are used sequentially in descending order of polarity to gradually replace impurities within the product channels. Vacuum drying is performed under negative pressure, utilizing the low-pressure environment to lower the boiling point of the solvent, allowing physically adsorbed solvent molecules within the channels to desorb and be removed at a gentle temperature.

[0042] In one or more embodiments of the present invention, in step S3, the zinc salt is zinc acetate, and the subsequent metallization reaction is carried out in methanol. Specifically, Bi-Salen covalent organic framework is dispersed in methanol to form a suspension, zinc acetate is dissolved in methanol and added to the suspension, and the reaction is stirred at room temperature for 48 hours. After the reaction is completed, the product is separated, washed with methanol, and dried under vacuum to obtain BiZn bimetallic Salen-type covalent organic framework catalyst.

[0043] The advantage of zinc acetate dihydrate as a zinc source lies in the moderate binding force between the acetate ligand and zinc ions. In methanol, it can be gradually replaced by the coordination sites in the solvent molecules or framework, which is conducive to the smooth coordination reaction between zinc ions and Salen cavities.

[0044] Methanol serves as a solvent for post-metallization reactions, with the following advantages: its small molecular size allows it to carry zinc ions into the interlayer channels of the Bi-Salen covalent organic framework; its protonic nature enables it to wet the polar coordination sites on the inner walls of the framework channels, promoting contact between zinc ions and nitrogen and oxygen coordination atoms; and its good solubility for zinc acetate provides a homogeneous zinc ion solution.

[0045] The process of dispersing Bi-Salen covalent organic frameworks in methanol to form a suspension requires continuous stirring to prevent the framework material from stacking and agglomerating due to interlayer van der Waals forces. This ensures that the layered structure of the framework is fully unfolded, and the pores are exposed in the solution. Slowly adding the zinc acetate methanol solution to the suspension allows the zinc ion concentration to gradually increase in the reaction system, promoting the uniform diffusion of zinc ions into the framework pores and preventing non-specific adsorption of zinc ions on the framework surface due to excessively high local concentrations.

[0046] After the feed was completed, the reaction was continuously stirred at room temperature for 48 hours. Room temperature avoided potential disturbances to the framework structure or metal ion migration caused by high temperatures, and the 48-hour reaction time provided ample time for sufficient diffusion of zinc ions and the establishment of coordination equilibrium. After the post-metallization reaction was completed, the solid product was separated from the liquid phase by centrifugation or filtration. Washing the product with methanol effectively removed uncoordinated free zinc ions and residual zinc acetate physically adsorbed on the product surface and within the pores. The methanol washing operation was repeated several times to ensure thorough removal of impurities.

[0047] After washing, the catalyst was dried under vacuum to remove residual methanol molecules from the pores, yielding a BiZn bimetallic Salen-type covalent organic framework catalyst. In this catalyst, bismuth and zinc bimetallic sites coexist within the Salen coordination cavity. Zinc ions regulate the electronic structure of the bismuth active center through electronic interactions, thereby improving the catalyst's activity and product selectivity in the electrocatalytic carbon dioxide reduction reaction.

[0048] This invention also provides a BiZn bimetallic Salen-type covalent organic framework catalyst, prepared by any of the above methods. The catalyst comprises a Salen-type covalent organic framework formed by the Schiff base condensation reaction of hexamethylenehexaazatrinaphthalene and 2-hydroxym-phthalaldehyde, wherein the Salen-type covalent organic framework is coordinated with Bi metal centers and Zn metal centers.

[0049] In this system, the Bi metal center and the Zn metal center are ionicly located in the Salen coordination cavity of the Salen-type covalent organic framework. The Salen coordination cavity is provided by the imine bonds and hydroxyl groups of the Salen-type covalent organic framework, which together provide coordination sites.

[0050] The basic framework of this catalyst consists of alternating hexamethylenehexaazatrinaphthalene building units and 2-hydroxyisophthalaldehyde linking units connected by imine bonds. The hexamethylenehexaazatrinaphthalene molecule possesses a large conjugated plane, serving as nodes in the covalent organic framework. 2-hydroxyisophthalaldehyde acts as a linear bridging ligand, connecting these nodes and allowing the framework to extend in two dimensions, forming a layered structure. The layers are stacked in an orderly fashion along the vertical direction through van der Waals forces between aromatic rings, thus constructing a crystalline porous material with periodic channels. The imine bonds, acting as connecting bonds, not only impart chemical stability to the framework, but their nitrogen atoms also participate in the complexation of metal ions as coordinating atoms, which is the basis for the formation of Salen-type coordination cavities.

[0051] The Salen coordination cavity is a local structural unit with specific coordination function in a covalent organic framework, composed of the phenolic hydroxyl group of 2-hydroxyisophthalaldehyde and the nitrogen atom of the imine bond adjacent to the hydroxyl group. Spatially, the oxygen atom of the hydroxyl group and the nitrogen atom of the imine bond are located adjacent to each other on the benzene ring of 2-hydroxyisophthalaldehyde, with appropriate spacing, allowing them to simultaneously coordinate with the same metal ion, forming a tetradentate coordination configuration of type N₂O₂. Each 2-hydroxyisophthalaldehyde unit provides one oxygen coordinating atom and one nitrogen coordinating atom. Two adjacent 2-hydroxyisophthalaldehyde units each provide a pair of nitrogen-oxygen coordinating atoms, coordinating with the same metal ion from both sides to form a complete Salen coordination cavity. This coordination cavity has a planar quadrilateral geometry, with the metal ion located at the center of the cavity, forming coordinate bonds with the four coordinating atoms.

[0052] Bi and Zn metal centers exist in ionic form within the Salen coordination cavities, coordinating with the nitrogen atom of the imine bond and the oxygen atom of the hydroxyl group via coordinate bonds. Bismuth ions, being trivalent metal ions with a relatively large ionic radius, preferentially occupy positions with strong coordination fields in the Salen coordination cavities after coordination, forming stable complexes with high coordination numbers with the nitrogen and oxygen atoms. Zinc ions, being divalent metal ions with a relatively small ionic radius, exhibit good matching with the remaining coordination sites in the Salen coordination cavities. During post-metallization, zinc ions diffuse into the framework channels, coordinating with unoccupied Salen coordination cavities to form zinc-coordinated Salen complexes. Both bismuth and zinc ions are highly dispersed within the framework; direct bonding does not occur between metal sites, but rather electronic communication is achieved through the conjugated skeleton of the covalent organic framework. The electronic interaction between bismuth and zinc ions is transmitted through the conjugated system of the framework. This indirect electronic effect can modulate the outer electron density distribution of the bismuth active center, influencing its adsorption behavior for carbon dioxide molecules and reaction intermediates.

[0053] The pore structure of this catalyst originates from the periodic arrangement of a covalent organic framework. The condensation of hexamethylenetetranaphthalene and 2-hydroxyisophthalaldehyde forms regular hexagonal channels, the size of which is determined by the geometry of the building blocks. These channels open along the in-layer direction, providing pathways for mass transfer between reactant and product molecules. Metal active sites are anchored to the inner walls of the channels, allowing reactant molecules to directly contact the active sites for catalytic transformation upon entering the channels, while product molecules diffuse away from the active sites through the channels. This pore structure ensures full utilization of the active sites; compared to traditional supported catalysts where only surface atoms of the metal particles participate in the catalytic reaction, the highly dispersed atomic-level metal sites on the inner walls of the channels exhibit higher atomic utilization efficiency.

[0054] The coexistence of Bi and Zn metal centers in the catalyst produces a synergistic effect on catalytic performance. Bismuth ions, as the main active site for the electrocatalytic reduction of carbon dioxide to formic acid, possess the ability to adsorb the formate ion, a reaction intermediate. The introduction of zinc ions modulates the electronic structure of the bismuth site through the electronic interaction between the two metals, altering the electron density around the bismuth ion and optimizing the adsorption energy of the bismuth active center for the key reaction intermediate. This electronic regulation effect lowers the reaction energy barrier of the rate-determining step in the reduction of carbon dioxide to formic acid, while simultaneously weakening the adsorption capacity of the bismuth site for hydrogen atoms, suppressing the occurrence of hydrogen evolution side reactions, thereby improving the selectivity of the formic acid product. Furthermore, the occupancy of the Salen coordination cavity by zinc ions also reduces the possible changes in the coordination environment of bismuth ions during long-term electrolysis, helping to maintain the structural stability of the catalyst.

[0055] In one or more embodiments of the present invention, the catalyst is a covalent organic framework material formed by stacking layered structures.

[0056] The catalyst is a covalent organic framework material formed by stacked layered structures. This stacked layered structure originates from the alternating connection of hexamethylenehexaazatrinaphthalene building units and 2-hydroxyisophthalaldehyde linking units in a two-dimensional planar direction during the synthesis of the covalent organic framework. Within a single layer, hexamethylenehexaazatrinaphthalene acts as multiple connecting nodes, and 2-hydroxyisophthalaldehyde acts as a linear bridging unit, interconnected by imine bonds to form a periodically arranged two-dimensional network structure. The extension direction of the layers coincides with the large conjugated plane of hexamethylenehexaazatrinaphthalene, with the imine bonds and aromatic rings lying in the same plane, making the entire layer an extended conjugated system. The π electrons of the aromatic rings and imine bonds in the conjugated framework are delocalized within the layer plane, endowing the layers with in-plane electronic conductivity.

[0057] Multiple two-dimensional sheets are stacked in an ordered manner along the vertical direction by interlayer van der Waals forces. The driving force for stacking mainly comes from the π-π interactions between aromatic rings in adjacent sheets. The delocalized π electron clouds on the aromatic rings attract each other between the layers, causing the sheets to align parallel to each other in a face-to-face manner. In addition, the lone pairs of electrons of the nitrogen atoms in the imine bonds and the lone pairs of electrons of the oxygen atoms in the hydroxyl bonds on the sheets can also form dipole interactions between the layers, further enhancing the interlayer bonding force. This non-covalent interaction enables the stacking of multiple sheets to form a bulk covalent organic framework material, with the interlayer spacing determined by the van der Waals radius of the aromatic rings. The periodic arrangement of the stacked sheets also creates an ordered arrangement of channels in the direction perpendicular to the sheets. These channels extend along the stacking direction, providing pathways for molecular transport between the layers. In the bulk material formed by stacked sheets, intralayer covalent bonds and interlayer non-covalent bonds work together. The former maintains the integrity of the sheet structure, while the latter imparts structural stability to the material in the direction perpendicular to the sheets.

[0058] In the layered stacked structure, Salen coordination cavities are located on the inner walls of the pores of the two-dimensional sheets, with metal active sites anchored within the sheet framework. Each metal site is confined to a planar quadrilateral coordination environment composed of two adjacent 2-hydroxyisophthalaldehyde units. The metal ion is directly connected to the conjugated system of the sheet framework, and electrons can be transferred between the metal site and the sheet through the conjugated framework. The layered stacked structure ensures spatial isolation of the metal active sites, with adjacent active sites separated by a covalent organic framework, effectively preventing the migration or aggregation of metal ions during the catalytic reaction. Simultaneously, the interlayer stacking causes the active sites to form a columnar arrangement perpendicular to the sheet direction, allowing reactant molecules to approach the active sites from different directions through the pores.

[0059] The morphological characteristics of the stacked sheets can be characterized using electron microscopy. Scanning electron microscopy reveals that the material consists of nanoscale-thick sheet-like units stacked together, with relatively smooth surfaces and clearly visible edges. Transmission electron microscopy further distinguishes individual sheets or regions of a few stacked layers, which exhibit a semi-transparent thin-layer characteristic. The periodic ordered structure of the stacked sheets can be characterized by X-ray diffraction; the diffraction signals appearing at corresponding interlayer spacings in the diffraction pattern reflect the degree of order in the stacked sheets. This layered structure formed by the ordered stacking of two-dimensional sheets is a typical morphological feature of covalent organic framework materials, and the number of stacked sheets and the degree of stacking order are related to the synthesis conditions.

[0060] In one or more embodiments of the present invention, the catalyst is used in the electrocatalytic reduction of carbon dioxide, and in the electrocatalytic reduction of carbon dioxide, the catalyst serves as a cathode catalyst. The electrocatalytic reduction of carbon dioxide is carried out in a carbon dioxide-saturated 0.5M potassium bicarbonate electrolyte, and the main product of the electrocatalytic reduction of carbon dioxide is formic acid or formate.

[0061] In one or more embodiments of the present invention, the catalyst is dispersed in a mixed solution of ethanol and perfluorosulfonic acid resin and drop-coated onto the surface of carbon cloth or carbon paper to form an electrode, wherein the catalyst loading on the electrode is 0.5 to 2 mg / cm³. 2 .

[0062] In the electrode preparation process, BiZn bimetallic Salen-type covalent organic framework catalyst powder was dispersed in a mixed solution of ethanol and perfluorosulfonic acid resin, and then drop-coated onto the surface of carbon cloth or carbon paper to form an electrode. The catalyst loading on the electrode was 0.5 mg / cm³. 2 Up to 2 mg / cm 2Ethanol, as a dispersion medium, wets the surface of the catalyst powder, ensuring thorough dispersion of the catalyst particles in the liquid phase. The perfluorosulfonic acid resin solution, acting as a binder, forms an ion-conducting adhesive layer between the catalyst particles and the current collector substrate. Perfluorosulfonic acid resin is a perfluorinated polymer containing sulfonic acid groups. Its main chain is a polytetrafluoroethylene backbone, with sulfonic acid groups at the end of the side chains. This structure endows it with a hydrophobic main chain and hydrophilic ion conduction capabilities, providing a transport channel for hydrogen ions from the electrolytic liquid phase to the catalyst surface during electrolysis. The above mixture is ultrasonically treated to form a uniform catalyst dispersion. Ultrasonic treatment utilizes the cavitation effect generated by ultrasound in the liquid to break up the agglomeration between catalyst particles, ensuring uniform suspension of the particles in the dispersion medium. Carbon cloth and carbon paper, both woven or pressed from carbon fibers, possess a three-dimensional porous conductive structure with a large specific surface area, effectively loading the catalyst and providing channels for gas diffusion and electron conduction. After drop coating, the mixture is dried under natural conditions. After the ethanol evaporates, the perfluorosulfonic acid resin and catalyst particles form a uniform catalyst film layer on the surface of the carbon cloth or carbon paper, thus obtaining the working electrode.

[0063] The electrocatalytic carbon dioxide reduction reaction was carried out in a standard three-electrode H-type electrolytic cell. The H-type electrolytic cell consists of two electrolytic chambers separated by an ion-exchange membrane. The ion-exchange membrane conducts ions from the electrolyte while preventing cross-mixing of gaseous products from the cathode and anode chambers. A catalyst-loaded carbon cloth or carbon paper was used as the working electrode, a silver / silver chloride electrode as the reference electrode, and a platinum sheet or platinum wire as the counter electrode. The electrolyte was a 0.5 mol / L potassium bicarbonate aqueous solution. Potassium bicarbonate ionizes in water to generate potassium ions and bicarbonate ions, which participate in proton transfer during carbon dioxide reduction. The electrolyte volume in the cathode and anode chambers was the same, and the liquid level was kept consistent to avoid convection interference caused by level differences.

[0064] Before the formal electrolysis test, high-purity carbon dioxide gas was continuously introduced into the electrolyte in the cathode chamber for at least 25 minutes to ensure that the carbon dioxide reached saturation in the electrolyte. The solubility of carbon dioxide in aqueous solution is affected by temperature and pressure; under normal pressure and room temperature conditions, the saturation solubility is approximately 33 mmol / L. After carbon dioxide saturation, the large number of dissolved carbon dioxide molecules in the electrolyte provided sufficient reactants for the electrocatalytic reduction reaction on the electrode surface. In the background control test, inert argon gas was introduced into the electrolyte to purge the dissolved carbon dioxide, and the cathode current response under carbon dioxide-free conditions was measured. All test potentials were converted to potentials relative to the reversible hydrogen electrode to eliminate the influence of different reference electrodes and electrolyte conditions on the potential measurements, ensuring data comparability.

[0065] The working mechanism of the electrocatalytic carbon dioxide reduction reaction is as follows: Under negative bias applied to the cathode, the BiZn bimetallic Salen-type covalent organic framework catalyst on the working electrode surface acquires electrons from the current collector through the conjugated framework. These electrons are then transferred to the bismuth and zinc bimetallic active sites located in the Salen coordination cavities. Carbon dioxide molecules dissolved in the electrolyte diffuse through the electrolyte to the catalyst surface, enter the channels of the covalent organic framework, and are adsorbed and activated upon contact with the metal active sites. The adsorbed carbon dioxide molecule accepts an electron from the metal active site, generating a carbon dioxide radical anion intermediate. This step is the initial electron transfer step of the carbon dioxide reduction reaction. This intermediate undergoes a proton-coupled electron transfer process at the bismuth active site. After two electron transfers and one proton transfer, the carbon atom combines with a hydrogen atom from water to form a carbon-hydrogen bond. Simultaneously, the oxygen atom bound to the bismuth site continues to accept electrons and protons, ultimately generating formate ions, which desorb from the catalyst surface and enter the electrolyte. Formate ions combine with potassium ions or exist as free formate in potassium bicarbonate electrolyte, and formic acid can be obtained after acidification. The bismuth active site has a moderate adsorption energy for the formate intermediate, which is conducive to the desorption and release of the intermediate after its formation. This is the intrinsic reason for the high selectivity of bismuth-based catalysts for formic acid. The introduction of zinc ions reduces the electron density of the bismuth sites through electronic interactions between the bimetallic sites, weakens the adsorption capacity of the bismuth sites for hydrogen atoms, and inhibits the occurrence of hydrogen evolution side reactions, thereby improving the Faraday efficiency of the formic acid product.

[0066] In potentiostatic electrolysis testing, the working electrode is pre-activated at a cathode potential of -1V (relative to the reversible hydrogen electrode) to achieve a stable electrochemical state on the catalyst surface. After pre-activation, potentiostatic electrolysis is performed within the target potential range. After electrolysis, the gaseous products in the cathode chamber are qualitatively and quantitatively analyzed using gas chromatography. The gas chromatograph separates different gaseous components using a column, and the content of each component is detected by a thermal conductivity detector or flame ionization detector, allowing for the determination of the concentrations of gaseous products such as hydrogen and carbon monoxide. The liquid products are quantitatively analyzed using 1H NMR spectroscopy. The concentration of formate in the liquid phase is calculated by detecting the characteristic proton signal of formate ions and using the internal standard method. Based on the quantitative results of chromatography and NMR, combined with the total charge recorded during electrolysis, the Faraday efficiency of each product is calculated. The Faraday efficiency is defined as the ratio of the charge consumed to generate a specific product to the total charge passed during electrolysis, expressed as a percentage, and is a key indicator for evaluating the selectivity of a catalyst for the target product. Test results show that, under a potential condition of -1V (relative to the reversible hydrogen electrode), the Faradaic efficiency of the BiZn bimetallic Salen-type covalent organic framework catalyst for formic acid products can reach 92.8%. The current response and formic acid formation efficiency remain stable during continuous electrolysis, indicating that the catalyst has good selectivity for the target product and electrochemical stability.

[0067] This invention pre-prepares HAHATN building blocks containing multiple ortho-amino groups, and then performs a Schiff base condensation reaction on hexamethylenehexaazatrinaphthalene, 2-hydroxyisophthalaldehyde, and bismuth salt in a one-pot process. Simultaneously, bismuth ions are in-situ anchored using N₂O₂ coordination cavities to obtain a Bi-Salen covalent organic framework. Zinc ions are then introduced via a post-metallization reaction to obtain a BiZn bimetallic Salen-type covalent organic framework catalyst. This preparation method achieves high dispersion of bismuth active centers within the ordered channels of the framework through a stepwise coordination strategy, overcoming the problem of easy aggregation of active components in traditional bismuth-based catalysts. The introduction of zinc ions further modulates the local coordination environment and electronic structure of bismuth sites, enhancing the catalyst surface's adsorption capacity for carbon dioxide reduction intermediates and effectively suppressing hydrogen evolution competition reactions. The resulting catalyst, with its two-dimensional conjugated framework and layered stacked structure of Salen-type covalent organic framework, provides an efficient electron transport channel and reduces interfacial charge transfer resistance. At the same time, the strong anchoring effect of multidentate coordination bonds on bimetallic sites ensures structural stability under long-term electrolysis conditions. It exhibits high formic acid Faraday efficiency and continuous operational stability in the electrocatalytic reduction of carbon dioxide to formic acid.

[0068] The above is the overall concept of the present invention. For ease of understanding, the present invention also provides the following embodiments: Example 1: Preparation of HAHATN 112.5 mg of cyclohexanehexanone octahydrate and 322 mg of 1,2,4,5-phenyltetramine tetrahydrochloride were weighed and added to a mixed solution of ethanol and acetic acid, with a total volume of 16 mL and a volume ratio of ethanol to acetic acid of 4:1. The mixture was thoroughly sonicated to form a homogeneous suspension, which was then transferred to a reaction vessel lined with polytetrafluoroethylene (PTFE) and reacted at 140 °C for 8 h. After the reaction, the product was centrifuged and washed three times with anhydrous diethyl ether to obtain hexamethylenehexaazatrinaphthalene, i.e., HAHATN.

[0069] Example 2: Preparation of Salen COF 9.48 mg of HAHATN and 15.0 mg of 2-hydroxyisophthalaldehyde obtained in Example 1 were weighed and added to a mixed solvent of 1,2-dichlorobenzene and n-butanol, wherein the volume of 1,2-dichlorobenzene was 0.5 mL and the volume of n-butanol was 0.5 mL. Then, 0.1 mL of 6M acetic acid aqueous solution was added. The mixture was ultrasonically stirred to form a homogeneous suspension. Residual gases were removed by vacuum degassing assisted by liquid nitrogen freezing. After degassing, the reaction tube was sealed and reacted at 120°C for 72 h. After the reaction, the precipitate was collected and washed sequentially with N,N-dimethylformamide, acetone, methanol, and dichloromethane. The precipitate was then vacuum dried overnight to obtain a metal-free Salen-type covalent organic framework material, namely Salen COF.

[0070] Example 3: Preparation of Bi-Salen COF Weigh 9.48 mg of HAHATN, 15.0 mg of 2-hydroxyisophthalaldehyde, and 77.22 mg of bismuth acetate obtained in Example 1, and add them to a mixed solvent of 1,2-dichlorobenzene and n-butanol, wherein the volume of 1,2-dichlorobenzene is 0.5 mL and the volume of n-butanol is 0.5 mL, and then add 0.1 mL of 6M acetic acid aqueous solution. The above reaction mixture is ultrasonically dispersed for 15 min to ensure thorough mixing of all components; then, residual gas in the system is removed by vacuum degassing assisted by liquid nitrogen freezing. After degassing, the reaction tube is sealed and reacted at 120 °C for 72 h. After the reaction is complete, the resulting precipitate is collected and washed sequentially with N,N-dimethylformamide, acetone, methanol, and dichloromethane, and then vacuum dried overnight to obtain Bi-Salen COF.

[0071] Example 4: Preparation of BiZn-Salen COF 20 mg of Bi-Salen COF obtained in Example 3 was weighed and dispersed in 5 mL of methanol to form a homogeneous suspension under stirring. 11 mg of zinc acetate dihydrate was weighed and dissolved in 2 mL of methanol, and the resulting zinc acetate methanol solution was slowly added to the above Bi-Salen COF suspension. The resulting mixture was continuously stirred at room temperature for 48 h. After the reaction was complete, the product was separated from the system, washed three times with methanol, and then dried under vacuum for 12 h to obtain the BiZn bimetallic Salen-type covalent organic framework material, namely BiZn-Salen COF.

[0072] Comparative Example 1: Preparation of Zn-Salen COF The preparation steps of Zn-Salen COF are basically the same as those of Bi-Salen COF in Example 3, except that bismuth acetate is replaced with 21.9 mg of zinc acetate dihydrate. The other reaction conditions, washing methods and drying methods are the same as those in Example 3. The obtained product is denoted as Zn-Salen COF.

[0073] Example 5: Preparation of the working electrode 10 mg of catalyst sample was weighed and added to a mixed solution of 900 μL ethanol and 100 μL perfluorosulfonic acid resin solution. After ultrasonic treatment, a uniform catalyst dispersion was formed. This catalyst dispersion was then drop-coated onto the surface of carbon cloth (2 cm × 1 cm). After natural drying, it was used as the working electrode with a catalyst loading of 1 mg / cm². 2 The Salen COF obtained in Example 2, the Bi-Salen COF obtained in Example 3, the BiZn-Salen COF obtained in Example 4, and the Zn-Salen COF obtained in Comparative Example 1 were all prepared using the above method as working electrodes.

[0074] Example 6: Electrocatalytic carbon dioxide reduction performance test The electrocatalytic carbon dioxide reduction performance was tested in a two-chamber H-type electrolytic cell. The catalyst-modified carbon cloth prepared in Example 5 was used as the working electrode, the Ag / AgCl electrode as the reference electrode, and platinum wire or platinum sheet as the counter electrode. The electrolyte was a 0.5 mol / L potassium bicarbonate aqueous solution. Before the formal test, carbon dioxide gas was continuously introduced into the cathode electrolyte for at least 25 min to saturate it; during the background control test, argon gas was introduced into the electrolyte. All test potentials were converted to potentials relative to the reversible hydrogen electrode (RHE). The scan rate for the linear sweep voltammetry test was 10 mV / s. Before the potentiostatic electrolysis test, activation was performed at -1.0 V (vs. RHE) for 15 min, followed by a potentiostatic electrolysis test at the target potential, with a test potential range of -0.8 V to -1.2 V (vs. RHE). After electrolysis, the gaseous products were detected by gas chromatography, and the liquid products were quantitatively analyzed by proton nuclear magnetic resonance spectroscopy. The Faraday efficiency and partial current density of each product were also calculated.

[0075] Example 7: Characterization of material morphology and elemental distribution The Salen COF obtained in Example 2 and the BiZn-Salen COF obtained in Example 4 were characterized by scanning electron microscopy and transmission electron microscopy, and the results are as follows: Figure 1 As shown. Salen COF ( Figure 1 a) and BiZn-Salen COF ( Figure 1The scanning electron microscope images (b) all show a layered stacked morphology, indicating that the material retains the basic morphological characteristics of a covalent organic framework after the introduction of metal. Salen COF ( Figure 1 c) and BiZn-Salen COF ( Figure 1 Transmission electron microscopy images (d) further confirmed that both exhibit layered structural features. High-angle annular dark-field scanning transmission electron microscopy characterization of BiZn-Salen COF ( Figure 1 (e) Discretely distributed bright spots can be observed, indicating that the Bi species are highly dispersed. Energy-dispersive X-ray spectroscopy elemental distribution image ( Figure 1 f) shows that C, N, O, Zn and Bi elements are uniformly distributed in the material, indicating that Bi and Zn metal species were successfully introduced into the Salen-type covalent organic framework.

[0076] Example 8: Structural and Chemical State Characterization Fourier transform infrared spectroscopy was performed on HAHATN obtained in Example 1, Salen COF obtained in Example 2, and BiZn-Salen COF obtained in Example 4. The results are as follows: Figure 2 As shown in Figure a, the presence of imine bond-related characteristic absorption peaks in the obtained material indicates that the Schiff base condensation reaction was successfully carried out, forming a Salen-type covalent organic framework. X-ray photoelectron spectroscopy (XPS) was performed on BiZn-Salen COF, and the high-resolution O 1s spectrum is shown below. Figure 2 b) Shows the oxidation state associated with metal coordination, N 1s high-resolution spectrum ( Figure 2 c) Shows the nitrogen chemical state related to metal coordination, Bi 4f high-resolution spectrum ( Figure 2 d) The characteristic peaks corresponding to Bi³⁺ confirm that Bi metal species exist in the material as trivalent ions. Combined with the elemental distribution results of the energy-dispersive X-ray spectroscopy in Example 7, Zn element is uniformly distributed in BiZn-Salen COF, confirming the successful introduction of Zn metal species into the material.

[0077] Example 9: Results of Electrocatalytic Carbon Dioxide Reduction Performance Electrocatalytic carbon dioxide reduction tests were performed on the Salen COF obtained in Example 2, the Bi-Salen COF obtained in Example 3, the BiZn-Salen COF obtained in Example 4, and the Zn-Salen COF obtained in Comparative Example 1. The results are as follows: Figure 3 As shown. Under an argon atmosphere, the cathode current mainly originates from the hydrogen evolution reaction; under a carbon dioxide atmosphere, the current comes from the combined contribution of the carbon dioxide reduction reaction and the hydrogen evolution reaction. The current density difference between Salen COF and Zn-Salen COF under argon and carbon dioxide atmospheres is small ( Figure 3a) indicates that both have weak carbon dioxide reduction activity; the current density of Bi-Salen COF and BiZn-Salen COF is significantly increased under carbon dioxide atmosphere, indicating that the introduction of Bi sites is beneficial to improving the carbon dioxide reduction reaction activity.

[0078] The constant potential electrolysis test results showed that, within the range of -0.8V to -1.2V (vs. RHE), the main liquid phase product of the BiZn-Salen COF obtained in Example 4 was formic acid or formate, and the main gaseous product was hydrogen with a small amount of carbon monoxide. Under -1.0V (vs. RHE) conditions, the Faradaic efficiency of BiZn-Salen COF for formic acid or formate reached 92.8%. Figure 3 d), which is higher than the Faraday efficiency of Bi-Salen COF at the same potential ( Figure 3 c). At -1.2V (vs. RHE), its current density is -72.3 mA / cm². 2 Further calculations were made of the partial current density of each product in BiZn-Salen COF at different potentials. Figure 3 b) It can be seen that within the test potential range, the partial current density corresponding to formic acid or formate is higher than that corresponding to carbon monoxide and hydrogen, indicating that formic acid or formate is the main reduction product of this catalytic system, and BiZn-Salen COF has a good target product generation ability in the electrocatalytic reduction of carbon dioxide to formic acid.

[0079] Example 10: Electrochemical Activity, Interfacial Charge Transport and Stability Testing The double-layer capacitance of the catalyst was tested in the non-Radichrister interval using cyclic voltammetry. The test results show that ( Figure 4 a) The double-layer capacitances of Zn-Salen COF, Bi-Salen COF, and BiZn-Salen COF are 6.52 μF / cm. 2 24.78μF / cm 2 and 163.53 μF / cm 2 This indicates that BiZn-Salen COF has a larger effective reaction interface. Electrochemical impedance spectroscopy was used to test the interfacial charge transport performance of different catalysts, and the results are as follows: Figure 4 As shown in b, the impedance arc radius of BiZn-Salen COF is smaller than that of Bi-Salen COF and Zn-Salen COF, indicating that its interfacial charge transport resistance is lower. Tafel curves were plotted based on the partial current density of formic acid or formate. Figure 4 c) The Tafel slope of BiZn-Salen COF is 146.2 mV / dec, which is lower than that of Bi-Salen COF, indicating that it has better electrocatalytic reaction kinetics.

[0080] Long-term potentiostatic electrolysis of BiZn-Salen COF was performed at -1.0V (vs. RHE), while the change in the formic acid Faradaic efficiency was monitored. The test results are as follows: Figure 4 As shown in d, the current response remained basically stable during the continuous 8-hour electrolysis process, and the formic acid or formate Faraday efficiency remained above 85%, indicating that the material has good electrochemical stability.

[0081] The above provides a detailed description of a BiZn bimetallic Salen-type covalent organic framework catalyst and its preparation method. Specific examples have been used to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of the present invention. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A method for preparing a BiZn bimetallic Salen-type covalent organic framework catalyst, characterized in that, The method includes: S1. Cyclohexanehexane octahydrate and 1,2,4,5-phenyltetramine tetrahydrochloride were reacted to obtain hexamethylenehexaazatrinaphthalene; S2. The hexamethylenehexaazatrinaphthalene, 2-hydroxyisophthalaldehyde and bismuth salt are subjected to Schiff base condensation reaction to obtain the Bi-Salen covalent organic framework. S3. The Bi-Salen covalent organic framework is subjected to a post-metallization reaction with zinc salt to obtain the BiZn bimetallic Salen-type covalent organic framework catalyst.

2. The method of claim 1, wherein, In step S1, the reaction is carried out in a mixed solution of ethanol and acetic acid, with a volume ratio of ethanol to acetic acid of 4:1, a reaction temperature of 140°C, and a reaction time of 8 hours. After the reaction is completed, the mixture is centrifuged and washed with anhydrous diethyl ether to obtain the hexamethylenehexaazatrinaphthalene.

3. The method of claim 2, wherein, In step S2, the bismuth salt is bismuth acetate, and the Schiff base condensation reaction is carried out in a mixed solvent of 1,2-dichlorobenzene and n-butanol, with a volume ratio of 1:1 between 1,2-dichlorobenzene and n-butanol. A 6M aqueous acetic acid solution is added as a catalyst. After ultrasonic dispersion, the reaction system is subjected to liquid nitrogen cryogenic vacuum degassing treatment, then sealed and reacted at 120°C for 72 hours. After the reaction is completed, the precipitate is collected, washed sequentially with N,N-dimethylformamide, acetone, methanol and dichloromethane, and dried under vacuum to obtain the Bi-Salen covalent organic framework.

4. The method of claim 3, wherein, In step S3, the zinc salt is zinc acetate, and the post-metallization reaction is carried out in methanol. Specifically, the Bi-Salen covalent organic framework is dispersed in methanol to form a suspension, zinc acetate is dissolved in methanol and then added to the suspension, and the reaction is stirred at room temperature for 48 hours. After the reaction is completed, the product is separated, washed with methanol, and dried under vacuum to obtain the BiZn bimetallic Salen-type covalent organic framework catalyst.

5. A BiZn bimetallic Salen-type covalent organic framework catalyst characterized in that, The catalyst is prepared by the method according to any one of claims 1-4, and the catalyst comprises a Salen-type covalent organic framework formed by the Schiff base condensation reaction of hexamethylenehexaazatrinaphthalene and 2-hydroxyisophthalaldehyde, wherein the Salen-type covalent organic framework is coordinated with Bi metal centers and Zn metal centers.

6. The catalyst of claim 5, wherein The Bi metal center and the Zn metal center are ionicly located in the Salen coordination cavity of the Salen-type covalent organic framework, and the Salen coordination cavity is provided by the imine bond and hydroxyl group of the Salen-type covalent organic framework.

7. The catalyst according to claim 6, characterized in that, The catalyst is a covalent organic framework material formed by stacking layered structures.

8. The catalyst according to claim 7, characterized in that, The catalyst is used in the electrocatalytic carbon dioxide reduction reaction, in which the catalyst serves as the cathode catalyst.

9. The catalyst according to claim 8, characterized in that, The electrocatalytic carbon dioxide reduction reaction is carried out in a 0.5M potassium bicarbonate electrolyte saturated with carbon dioxide, and the main product of the electrocatalytic carbon dioxide reduction reaction is formic acid or formate.

10. The catalyst according to claim 9, characterized in that, The catalyst is dispersed in a mixed solution of ethanol and perfluorosulfonic acid resin and drop-coated onto the surface of carbon cloth or carbon paper to form an electrode. The catalyst loading on the electrode is 0.5 to 2 mg / cm³. 2 .

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