Method for preparing zif-8 supported nano-aucatalyst

Abstract

This invention provides a method for preparing ZIF-8 supported nano-Au catalyst, comprising the following steps: Step S1, pretreating ZIF-8 in an acid solution to obtain pretreated ZIF-8, and modifying the pretreated ZIF-8 in a solution containing an oxygen-containing functional group and / or a chelating agent to obtain a ZIF-8 support; Step S2, adding the ZIF-8 support to a chloroauric acid solution and adding an ammonium chloride solution to react, to obtain a nano-Au catalyst precursor supported on the ZIF-8 support; Step S3, thermally reducing the nano-Au catalyst precursor supported on the ZIF-8 support to obtain the ZIF-8 supported nano-Au catalyst.

Description

Technical Field

[0001] This invention relates to the field of catalyst materials technology, and more specifically, to a method for preparing a ZIF-8 supported nano-Au catalyst. Background Technology

[0002] Glucoic acid and its salts, such as zinc gluconate, are important chemical, pharmaceutical, and food raw materials and intermediates. Currently, the main industrial methods for synthesizing gluconic acid and its salts are bio-fermentation and heterogeneous catalytic oxidation. The former involves a complex process, demanding temperatures, numerous byproducts, and a long cycle, and the microbial composition affects product purity. Heterogeneous catalytic oxidation offers advantages such as a short cycle time, simple operation, easy product separation, and catalyst recycling. Currently, the catalysts used in the heterogeneous catalytic oxidation synthesis of gluconic acid and its salts are mainly supported noble metal catalysts such as Pt, Pd, and Au. In the selective catalytic oxidation of glucose, Au / C catalysts, compared to Pd catalysts, exhibit the same high conversion rate and selectivity. However, to achieve the same conversion rate, Au catalysts require a shorter time because they can selectively catalyze the oxidation of a hydroxyl or carbonyl group.

[0003] Traditional methods for preparing supported catalysts include impregnation-precipitation and thermal reduction. However, these methods often require adjusting the pH of the reaction system to 10.0 ± 0.1, then adding a strong reducing agent to prepare a gold sol, which is then loaded onto the support surface via physical adsorption; or the gold salt compound is physically adsorbed onto the support surface, and then reduced to elemental Au under an inert atmosphere using a thermal reduction atmosphere, such as H2 / CO. Therefore, precise control of the solution pH is difficult, which is not conducive to obtaining nano-Au particles with controllable particle size, thus hindering the efficient preparation of nano-Au catalysts.

[0004] A patent application with application number CN201410205120.5, publication date August 6, 2014, entitled "Activated Carbon-Supported Gold Nanocatalyst and Its Preparation and Application Method," discloses a method for preparing a gold nanocatalyst. In this technical solution, gold sol is prepared by first reducing chloroauric acid with a reducing agent. Activated carbon is pretreated with acid, and then the pretreated support is added to the gold sol solution system to obtain the activated carbon-supported gold nanocatalyst. However, the gold sol preparation process mentioned in this invention is complex and difficult to scale up for production. Furthermore, the reducing agent sodium borohydride (NaBH4) has strong reducing properties, leading to heterogeneous nucleation of nano-Au. Under the tendency of excessively high local concentrations of the reducing agent, nano-Au crystal nuclei undergo explosive nucleation, resulting in easy particle aggregation and crystal growth. The size of the Au particles obtained from the reduction preparation is difficult to control effectively.

[0005] Furthermore, the aforementioned patent involves pretreating the activated carbon with activated carbon acid to clean the surface and unclog the internal pores, followed by impregnation adsorption to load gold sol onto the support surface, thereby improving the selective oxidation efficiency of the catalyst. However, in this process, the loaded Au nanoparticles and the support are only bound by electrostatic adsorption, resulting in limited interaction between them. Consequently, migration of the Au nanoparticles cannot be prevented during use.

[0006] Given the aforementioned problems, it is necessary to develop an efficient preparation method for nano-Au catalysts that can precisely control the size of nano-Au particles. Summary of the Invention

[0007] The main objective of this invention is to provide a method for preparing nano-Au catalysts, thereby addressing the problems of easy aggregation and migration of nano-Au particles, difficulty in controlling the size of nano-Au particles, and poor performance of nano-Au catalysts in existing nano-Au catalyst preparation methods.

[0008] To achieve the above objectives, the present invention provides a method for preparing a ZIF-8 supported nano-Au catalyst, comprising the following steps:

[0009] Step S1: ZIF-8 is added to an acid solution for pretreatment to obtain pretreated ZIF-8. The pretreated ZIF-8 is then added to a solution containing a compound with oxygen-containing functional groups and / or a chelating agent for modification to obtain a ZIF-8 support.

[0010] Step S2: The ZIF-8 support is added to a chloroauric acid solution, and then an ammonium chloride solution is added to react and obtain a nano-Au catalyst precursor supported on the ZIF-8 support.

[0011] Step S3: The nano-Au catalyst precursor supported on the ZIF-8 support is thermally reduced to obtain the ZIF-8 supported nano-Au catalyst.

[0012] Furthermore, the acid solution includes solutions of one or more of hydrochloric acid, sulfuric acid, nitric acid, carbonic acid, phosphoric acid, and oxalic acid.

[0013] Furthermore, the compounds containing oxygen-containing functional groups include one or more of hydrogen peroxide, peracetic acid, ammonium persulfate, sodium peroxide, potassium peroxide, tert-butyl hydroperoxide, and cumene hydroperoxide, and the chelating agents include one or more of thiourea, sulfite, phosphoric acid, and arsenoic acid.

[0014] Furthermore, step S2 also includes adding a reducing agent solution and a dispersant solution during the reaction; the reducing agent includes one or more of tryptophan, ascorbic acid, ascorbate, citrate, formaldehyde, hydrazine hydrate, metal borohydride, sulfite, and ammonium chloride, and / or the dispersant includes one or more of gelatin, polyethylene glycol, polyvinyl alcohol, polyvinylpyrrolidone, methylcellulose, polyacrylic acid, sodium sulfosuccinate, sodium dodecyl sulfate, sodium citrate, and quaternary ammonium salts.

[0015] Furthermore, the solvent for the chloroauric acid solution is one or more of methanol, ethanol, propanol, ethylene glycol, isopropanol, N,N-dimethylformamide, N,N-dimethylacetamide, and diethylformamide.

[0016] Furthermore, the thermal reduction is carried out in an inert atmosphere environment, which includes one or more of argon, nitrogen, and helium.

[0017] Further, in step S1, the pretreatment is carried out at 20-50°C for 10-24 hours; and / or in step S3, the thermal reduction is carried out at 100-350°C for 2-5 hours.

[0018] Furthermore, the thermal reduction was carried out under the following conditions: the temperature was increased from 25℃ to 125℃ at a heating rate of 5-30℃ / min and held for 0.5-1.0h; then the temperature was increased from 125℃ to 185℃ at a heating rate of 5-30℃ / min and held for 0.5-1.0h; then the temperature was increased from 185℃ to 245℃ at a heating rate of 5-30℃ / min and held for 0.5-1.0h; finally, the temperature was increased from 245℃ to 350℃ at a heating rate of 5-30℃ / min and held for 1.0-3.0h.

[0019] Further, in step S1, the mass concentration of the acid solution is 10-30%, ZIF-8 is added to the acid solution at an amount of 1-10 mg / mL, and the mass concentration of the solution containing the oxygen-containing functional group compound and / or chelating agent is 10-30%; and / or in step S2, the concentration of the ammonium chloride solution is 1.5-50 mg / mL.

[0020] Furthermore, the concentration of the reducing agent solution is 1.5-50 mg / mL; and / or the concentration of the dispersant solution is 1.0-2.5 mg / mL; and the amount of ammonium chloride solution is 1.0-1.5 times the stoichiometric amount of the reaction.

[0021] The present invention provides a method for preparing ZIF-8 supported nano-Au catalysts. First, the ZIF-8 support is pretreated, and then modified using a solution of oxygen-containing functional groups (such as -OH, -C=O, -COOH, etc.) and / or chelating agents. Then, the support is impregnated with chloroaurate (AuCl4).- In a solution, after the chloroaurate solution completely wets the support, the soluble gold salt in the solution is converted into a thermally reducible gold salt compound. This gold salt compound is then thermally reduced and decomposed into elemental Au, which is then deposited onto the support as nano-Au particles. Precise control of the Au particle size allows for effective dispersion and loading of nano-Au particles onto the support. Using this method, the average particle size of the catalyst-supported Au particles is as low as 2.7 nm, making it suitable for the selective catalytic oxidation of glucose to prepare zinc gluconate. By pretreating the support with a solution of oxygen-containing functional groups (such as -OH, -C=O, -COOH, etc.) and / or chelating agents, and then loading Au particles onto the support, the binding force between Au particles and the support can be enhanced by the oxygen-containing functional groups and / or chelating agents (oxygen atoms fill the empty orbitals of gold ions and undergo coordination reactions with them), thus preventing the migration of Au particles in the catalyst. At the same time, the capillary aggregation initiated by the hierarchical porous structure of the support can also cause soluble chloroauric acid to wet the surface of the support. In the subsequent thermal reduction reaction, the spatial confinement effect of the hierarchical pores of the support can effectively improve the dispersion of nano-Au particles on the support surface and prevent the aggregation of Au particles. In addition, the oxygen-containing functional groups introduced by the ZIF-8 support can act as initiators in the selective catalytic oxidation of glucose, effectively improving the oxidation efficiency of the nano-Au catalyst in the selective oxidation of glucose. The method of the present invention has the advantages of low catalyst dosage, high catalytic conversion rate and selectivity, and effectively solves the problems of easy aggregation of nanoparticles, low catalytic activity and difficulty in precise control of nanoparticle size in the existing methods for preparing nano Au catalysts. Attached Figure Description

[0022] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0023] Figure 1 TEM image of the ZIF-8 supported nano-Au catalyst prepared in Example 1;

[0024] Figure 2 The specific surface area and pore size distribution of the ZIF-8 support nano-Au catalyst prepared in Example 1 are shown in the diagrams before and after ZIF-8 support pretreatment.

[0025] Figure 3 This is a size distribution diagram of the nano-Au particles in the ZIF-8 supported nano-Au catalyst prepared in Example 1;

[0026] Figure 4 The image shows the size distribution of the nano-Au particles in the supported nano-Au catalyst prepared in Comparative Example 3.

[0027] Figure 5 The image shows the XRD pattern of the ZIF-8 supported nano-Au catalyst prepared in Example 1.

[0028] Figure 6 The graph shows the catalytic conversion rate and selectivity of the ZIF-8 supported nano-Au catalyst prepared in Example 1. Detailed Implementation

[0029] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the embodiments.

[0030] As described in the background section, existing methods for preparing nano-Au catalysts suffer from problems such as easy aggregation and migration of nano-Au particles, difficulty in controlling the size of nano-Au particles, and poor catalyst performance. To address these technical problems, this application provides a method for preparing a ZIF-8 supported nano-Au catalyst, comprising the following steps:

[0031] Step S1: ZIF-8 is added to an acid solution for pretreatment to obtain pretreated ZIF-8. The pretreated ZIF-8 is then added to a solution containing a compound with oxygen-containing functional groups and / or a chelating agent for modification to obtain a ZIF-8 support.

[0032] Step S2: The ZIF-8 support is added to a chloroauric acid solution, and then an ammonium chloride solution is added to react and obtain a nano-Au catalyst precursor supported on the ZIF-8 support.

[0033] Step S3: The nano-Au catalyst precursor supported on the ZIF-8 support is thermally reduced to obtain the ZIF-8 supported nano-Au catalyst.

[0034] The present invention provides a method for preparing ZIF-8 supported nano-Au catalysts. First, the ZIF-8 support is pretreated, and then modified using a solution of oxygen-containing functional groups (such as -OH, -C=O, -COOH, etc.) and / or chelating agents. Then, the support is impregnated with chloroaurate (AuCl4). -In a solution, after the chloroaurate solution completely wets the support, the soluble gold salt in the solution is converted into a thermally reducible gold salt compound. This gold salt compound is then thermally reduced and decomposed into elemental Au, which is used to load nano-Au particles onto the support. Precise control of the Au particle size allows for effective dispersion and loading of nano-Au particles onto the support. Using this method, the average particle size of the catalyst-supported Au particles is as low as 2.7 nm, making it suitable for the selective catalytic oxidation of glucose to prepare zinc gluconate. By pretreating the support with a solution of oxygen-containing functional groups (such as -OH, -C=O, -COOH, etc.) and / or chelating agents, and then loading Au particles onto the support, the binding force between Au particles and the support can be enhanced by the oxygen-containing functional groups and / or chelating agents (oxygen atoms fill the empty orbitals of gold ions and undergo coordination reactions with them), thus preventing the migration of Au particles in the catalyst. At the same time, the capillary aggregation initiated by the hierarchical porous structure of the support can also cause soluble chloroauric acid to wet the surface of the support. In the subsequent thermal reduction reaction, the spatial confinement effect of the hierarchical pores of the support can effectively improve the dispersion of nano-Au particles on the support surface and prevent the aggregation of Au particles. In addition, the oxygen-containing functional groups introduced by the ZIF-8 support can act as initiators in the selective catalytic oxidation of glucose, effectively improving the oxidation efficiency of the nano-Au catalyst in the selective oxidation of glucose. The method of the present invention has the advantages of low catalyst dosage, high catalytic conversion rate and selectivity, and effectively solves the problems of easy aggregation of nanoparticles, low catalytic activity and difficulty in precise control of nanoparticle size in the existing methods for preparing nano Au catalysts.

[0035] ZIF-8 is a type of metal-organic framework (MOF) material. It is an inorganic metal-organic framework material synthesized at room temperature through the coordination of inorganic metal ions and organic matter with zinc nitrate hexahydrate and 2-methylimidazole. It has a high specific surface area (>1000 m²). 2ZIF-8 ( / g) can effectively capture and adsorb small molecules. Through subsequent heating pretreatment, ZIF-8 can be activated, removing small molecules and exposing metal active sites; therefore, after activation, the material exhibits high chemical activity. This invention selects ZIF-8 as a catalyst support, utilizing the hierarchical pore structure and high specific surface area of ​​MOF materials to introduce metal active centers onto a carbon-based substrate. Through high-temperature pyrolysis, a Zn / CN carbon-based material is obtained, which serves as a supported catalyst support, providing nucleation sites for nano-Au particles. Under the spatial physical confinement of the hierarchical pores (micropores and mesopores), the growth of Au nanocrystals can be effectively suppressed, achieving precise control of the nano-Au particle size. The Zn / CN carbon-based support material retains the characteristics of a large specific surface area and rich pore structure of the metal-organic framework. Under effective control of the Au nanocrystal size and spatial physical confinement, nano-Au particles can achieve high dispersion and uniform loading; thus achieving high utilization of active metal atoms in the catalyst and facilitating the transport and mass transfer of reactants and products, improving catalytic reaction efficiency. In addition, the catalyst of the present invention is applied to the catalytic conversion of glucose salts (e.g., zinc gluconate). Using ZIF-8 as the catalyst support avoids the introduction of new impurity metal elements and reduces the amount of precious metal Au, thus preparing Zn-Au into an alloy catalyst, which helps to reduce the catalyst preparation cost.

[0036] In step S1 of the method of this invention, ZIF-8 is pretreated with acid to clean its surface and open up its internal pore structure. Subsequently, ZIF-8 is modified with oxygen-containing functional groups and / or chelating agents. This pretreatment does not significantly damage the physical structure of the support, but mainly alters the chemical properties of the support surface (changing it from hydrophobic to hydrophilic). Furthermore, acid pretreatment cleans impurities from the support surface, increases the specific surface area and pore volume of the support, and shifts the isoelectric point of the support towards a lower pH value, thus facilitating the adsorption of chloroauric acid on the support surface. In the prior art, ZIF-8 is a tetrahedral material with a zeolite imidazole ester framework; moreover, the 2-methylimidazolium framework material in the Zn / NC (black product) precursor ZIF-8 is only soluble in water and ethanol, slightly soluble in cold benzene solvent, and thermodynamically unstable at high temperatures, with its porous structure prone to collapse. This reduces the specific surface area of ​​the catalyst and the interconnected pore structure, which is detrimental to the exposure of the active metal nanoparticles of the catalyst and the diffusion, transport, and mass transfer of the reaction medium and reaction products. Furthermore, due to the hydrophobic nature of ZIF-8 and the absence of functional groups on its surface, the interaction between it and the active metal nanoparticles of the catalyst is weak. A single porous structure is detrimental to the exposure of the catalyst's active sites and the diffusion, transport, and mass transfer of the reaction medium. In this invention, oxygen-containing functional groups and / or chelating agents (such as -OH, -C=O, -COOH, etc.) coordinate with the carbon support, functionalizing the carbon support surface. This facilitates the electrostatic adsorption of positively valent gold ions on the carbon surface of the support and allows oxygen atoms in the ligands to fill the empty orbitals of the gold ions, resulting in a coordination reaction and thus enhancing the interaction between the active metal catalyst and the support. Additionally, free oxygen-containing functional groups (such as -OH, -C=O, -COOH, etc.) in the solution are incorporated into the hierarchical pores of ZIF-8, which in turn benefits the subsequent selective catalytic oxidation of glucose. The free oxygen ions in the porous support layer can act as initiators, increasing the rate of gluconate catalytic oxidation.

[0037] In step S2 of the method of the present invention, chloroauric acid is converted into thermally decomposable gold salt compound ammonium chloroaurate under the action of ammonium chloride. Ammonium chloroaurate is loaded on the surface and inside the pores of the ZIF-8 hierarchical porous support. Through the characteristics of the support's pore structure and the thermally decomposable and reducible properties of the gold salt, the size of the gold nanocrystal nuclei can be controlled and loaded onto the support surface. The reaction equation occurring in step S2 is as follows:

[0038] HAuCl4+NH4Cl=NH4AuCl4↓+HCl

[0039] 2NH4AuCl4=2Au+N2+8HCl

[0040] This invention utilizes the phase transformation of gold compounds. Soluble chloroauric acid is first converted into thermally decomposable ammonium chloroaurate (a precipitate in the liquid phase) and deposited on the Zn / CN support surface. During subsequent thermal reduction, ammonium chloroaurate thermally decomposes into elemental gold. The physical spatial confinement of the hierarchical pores of the Zn / CN support effectively controls the size of the gold nanoparticles. Furthermore, during thermal reduction, Zn in the support forms an Au-Zn alloy with Au, reducing the amount of precious metal Au required and increasing the interaction between the active metal Au and the support. This improves the stability and lifetime of the catalyst during the catalytic reaction. In addition, the phase transformation of soluble gold salt compounds into thermally reducible gold salt compounds, followed by heat treatment, reduces gold ions to elemental gold and forms alloy compounds with Zn in the support during the reduction reaction. This reduces the amount of precious metal required while increasing the atomic utilization rate of Au in the catalytic reaction.

[0041] In a preferred embodiment, the acid solution includes one or more solutions selected from hydrochloric acid, sulfuric acid, nitric acid, carbonic acid, phosphoric acid, and oxalic acid.

[0042] Preferably, ZIF-8 is pretreated in nitric acid solution. Utilizing the inherent high specific surface area and hierarchical pore structure (micropores, mesopores) of ZIF-8, it can adsorb NO3 from the solution. - The carbon in ZIF-8 is modified with nitrogen, and CN doping is performed during subsequent heat treatment to prepare Zn / CN. This improves the stability of the Zn / CN support in acidic solutions and releases nitrogen oxides through thermal decomposition while retaining the porous structure of the support, which is beneficial for improving the efficiency of subsequent catalytic reactions.

[0043] In a preferred embodiment, the compound containing an oxygen-containing functional group includes one or more of hydrogen peroxide, peracetic acid, ammonium persulfate, sodium peroxide, potassium peroxide, tert-butyl hydroperoxide, and cumene hydroperoxide, and the chelating agent includes one or more of thiourea, sulfite, phosphoric acid, and arsenoic acid.

[0044] As described above, the oxygen-containing functional group modification in the method of this invention provides free oxygen-containing functional groups (such as hydroxyl groups) inside the pores of the support. This modifies the surface of the support, increasing its hydrophilicity, and alters the interaction force between the support and chloroauric acid, changing it from electrostatic adsorption to covalent bonding. This increases the interaction force between the support and the active metal ions, which is beneficial for improving the stability of the catalyst. Using a chelating agent to modify the support by adding active groups to the support surface allows these groups to chelate and adsorb free gold ions in the solution, enhancing the bonding between Au particles and the support.

[0045] In a preferred embodiment, step S2 further includes adding a reducing agent solution and a dispersant solution. The reducing agent includes one or more of tryptophan, ascorbic acid, ascorbate, citrate, formaldehyde, hydrazine hydrate, metal borohydride, sulfite, and ammonium chloride. The dispersant includes one or more of gelatin, polyethylene glycol, polyvinyl alcohol, polyvinylpyrrolidone, methylcellulose, polyacrylic acid, sodium sulfosuccinate, sodium dodecyl sulfate, sodium citrate, and quaternary ammonium salts.

[0046] In step S2, the chloroauric acid solution undergoes a phase transformation with gold compounds via ammonium chloride, generating a gold salt precipitate that is uniformly deposited on the Zn / CN support surface. This achieves a gold salt loading that matches the theoretical and actual values, resulting in high utilization of the gold salt. Simultaneously, the gold salt deposited on the Zn / CN support surface is thermally reduced to gold nanoparticles in subsequent steps. If the above reaction is incomplete, a reducing agent is added to reduce the residual chloroauric acid in the solution to gold nanoparticles, which are then loaded onto the support surface. This two-stage reduction process, as described above, allows for precise control of the gold nanoparticle size by regulating the nucleation-crystal growth of gold nanocrystals, while ensuring uniform dispersion and loading on the support surface, effectively exposing more active sites. Furthermore, the addition of an appropriate amount of dispersant promotes the dispersion of the gold compounds through the action of the dispersant's macromolecular chains, forming stable, highly dispersed Au nanoparticles. Additionally, the dispersant is chosen to decompose during thermal reduction, meaning it will not encapsulate the active metal on the support surface, thus ensuring high exposure of the active metal and its full participation in the catalytic reaction.

[0047] In a preferred embodiment, the solvent for the chloroauric acid solution is one or more of methanol, ethanol, propanol, ethylene glycol, isopropanol, N,N-dimethylformamide, N,N-dimethylacetamide, and diethylformamide.

[0048] Using the aforementioned organic solvent, the unsaturated zinc centers in the support bind to it to meet coordination requirements. After solvothermal treatment, solvent molecules are removed, exposing the active sites of zinc. During the catalytic reaction, these sites coordinate with the reactant gases, achieving gas adsorption and separation. Furthermore, the Zn / CN support containing unsaturated zinc sites provides nucleation sites for Au nanocrystals, inhibiting Au nanoparticle growth under spatial confinement and weakening the adsorption strength between the catalyst and poisoning molecules. Additionally, the selected organic solvent can contain nitrogen functional groups, allowing for the doping of carbon and nitrogen elements in the support. The coordination effect between nitrogen and carbon modifies the electronic structure of carbon, improving the corrosion resistance of the carbon-based support in acidic solutions and thus extending the lifespan of the nano-Au catalyst.

[0049] In a preferred embodiment, the thermal reduction is carried out in an inert atmosphere environment, which includes one or more of argon, nitrogen, and helium.

[0050] Inert gases prevent the negative interference of oxygen in the air on the oxidation of carbon and nitrogen elements in the support during heat treatment. At the same time, they prevent the gas from adsorbing onto the multi-level pores of the support and covering the active sites of the catalyst, thus reducing the atomic utilization rate of the active metal.

[0051] In a preferred embodiment, in step S1, the pretreatment is carried out at 20-50°C for 10-24 hours.

[0052] In a preferred embodiment, in step S3, the thermal reduction is carried out at 100-350°C for 2-5 hours.

[0053] In a preferred embodiment, the thermal reduction is carried out under the following conditions: the temperature is increased from 25°C to 125°C at a heating rate of 5-30°C / min, and held for 0.5-1.0 h; then the temperature is increased from 125°C to 185°C at a heating rate of 5-30°C / min, and held for 0.5-1.0 h; then the temperature is increased from 185°C to 245°C at a heating rate of 5-30°C / min, and held for 0.5-1.0 h; finally, the temperature is increased from 245°C to 350°C at a heating rate of 5-30°C / min, and held for 1.0-3.0 h.

[0054] In a preferred embodiment, in step S1, the mass concentration of the acid solution is 10-30%, ZIF-8 is added to the acid solution at an amount of 1-10 mg / mL, and the mass concentration of the solution containing the oxygen-containing functional group compound and / or chelating agent is 10-30%.

[0055] In a preferred embodiment, in step S1, the pretreated ZIF-8 is added to a solution containing an oxygen-containing functional group and / or a chelating agent at an amount of 5-10 mg / mL.

[0056] In a preferred embodiment, in step S2, the concentration of the ammonium chloride solution is 1.5-50 mg / mL, and the amount of ammonium chloride solution is 1.0-1.5 times the stoichiometric amount of the reaction.

[0057] In a preferred embodiment, the concentration of the dispersant solution is 1.0-2.5 mg / mL.

[0058] Using the above-described parameters such as temperature, time, and dosage in the method of this invention further facilitates the acquisition of ZIF-8 supported nano-Au catalysts with controllable nanoparticle size and high performance. Those skilled in the art, based on this invention, can further adjust the parameters in the method according to actual conditions and needs.

[0059] To ensure complete reaction of gold ions in the solution, the amount of reagent reacting with chloroauric acid must be 1.5-2.0 times or more of the stoichiometric amount. Therefore, even when the amount of ammonium chloride or reducing agent solution is 1.0-1.5 times the stoichiometric amount, free gold ions still exist in the solution. In this case, a second reduction can be performed by adding reducing agent or ammonium chloride solution to ensure complete reduction of gold ions to elemental gold. Furthermore, this two-stage reduction allows for precise control of the size of gold nanoparticles by regulating the nucleation and crystal growth of gold nanocrystals, while also ensuring uniform dispersion and exposure of more active sites on the carrier surface. Additionally, if a weak reducing agent (ascorbic acid, ascorbate, citrate) is used in the first reaction, a strong reducing agent (formaldehyde, hydrazine hydrate, metal borohydride, sulfite, ammonium chloride, tryptophan) can be used in the second reduction to ensure complete reduction of gold ions in the solution.

[0060] In a preferred embodiment, the preparation method of the ZIF-8 supported nano-Au catalyst includes the following steps:

[0061] Step S1: Add ZIF-8 to a nitric acid solution and pretreat the reaction at 20-50℃ for 10-24 hours. After filtration and washing until the pH of the filtrate is neutral, vacuum dry at 50-80℃ for 3-5 hours for later use. Grind the dried product and mix the powder obtained with H2O2 solution. Stir magnetically for 1-3 hours, filter, wash, and freeze dry to obtain the ZIF-8 carrier.

[0062] Step S2: Add the ZIF-8 carrier to an organic solvent solution of chloroauric acid and sonicate for 1-3 hours to ensure complete dispersion of the carrier in the solution; then slowly add ammonium chloride solution and stir magnetically for 1-3 hours to convert chloroauric acid into ammonium chloroaurate; then slowly add a mixture of reducing agent and dispersant to the reaction system for 2-30 minutes.

[0063] Step S3: After the above reaction is completed, the collected solid product is freeze-dried, ground, and then heat-treated at 100-350℃ for 2-5 hours under inert gas protection to thermally reduce and decompose ammonium chloroaurate into elemental Au, thereby obtaining a supported nano-Au catalyst.

[0064] Preferably, the particle size of the active metal Au particles in the catalyst is 2.0-3.0 nm.

[0065] In a preferred embodiment, the steps for synthesizing gluconic acid by oxidizing glucose using the ZIF-8 supported nano-Au catalyst prepared by the method of the present invention are as follows: Glucose is used as the reactant, with a solution concentration of 0.1-30%. The ZIF-8 supported Au catalyst is a selective oxidation catalyst, and the mass ratio of glucose to gold catalyst is 1-50. Under atmospheric pressure and stirring conditions, the oxidation reaction is carried out using oxygen, air, and hydrogen peroxide as oxidants at a reaction temperature of 30-80°C. During the reaction, 0.1-5 mol / L NaOH solution is continuously added dropwise using a peristaltic pump, and the pH of the solution system is adjusted to 8.0-10.0. The reaction time is 30-180 min. Sodium gluconate is then acidified, and zinc hydroxide solution is added dropwise for neutralization, thereby preparing the zinc gluconate product.

[0066] In a preferred embodiment, the synthesis steps of ZIF-8 used in the method of the present invention are as follows: Zn(NO3)2·6H2O and 2-methylimidazole are dissolved in N,N-dimethylformamide, and ultrasonically dispersed to obtain zinc nitrate solution and 2-methylimidazole solution. The 2-methylimidazole solution is then slowly poured into the zinc nitrate solution. The mixed solution is then transferred to a hydrothermal synthesis reactor lined with polytetrafluoroethylene. The mixture is hydrothermally reacted at 120°C in a forced-air drying oven for 24 hours. The mixture is then slowly cooled to room temperature, centrifuged, and washed 3-5 times with N,N-dimethylformamide solvent. After centrifugation, the mixture is vacuum dried at 50°C for 3 hours. The product is then ground and collected for later use.

[0067] Another aspect of the present invention provides a ZIF-8 supported nano-Au catalyst prepared by the method described above.

[0068] Another aspect of the present invention provides the use of the above-described ZIF-8 supported nano-Au catalyst for the selective catalytic oxidation of glucose to prepare gluconic acid and its salts.

[0069] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

[0070] Example 1

[0071] ZIF-8 was prepared by the following steps: 5.0 g of Zn(NO3)2·6H2O and 5.0 g of 2-methylimidazole were dissolved in 100 ml of N,N-dimethylformamide and ultrasonically dispersed to obtain zinc nitrate solution and 2-methylimidazole solution. The 2-methylimidazole solution was then slowly poured into the zinc nitrate solution. The mixed solution was then transferred to a hydrothermal synthesis reactor lined with polytetrafluoroethylene and hydrothermally reacted at 120°C in a forced-air drying oven for 24 h. The mixture was then slowly cooled to room temperature, centrifuged, and washed 3-5 times with N,N-dimethylformamide solvent. After centrifugation, the mixture was vacuum dried at 50°C for 3 h. The product was then ground and collected for later use.

[0072] ZIF-8 supported nano-Au catalyst was prepared using the following steps:

[0073] S1. 10.0 g of ZIF-8 carrier was added to 1000 mL of 30% dilute nitric acid and mixed evenly. The mixture was then pretreated at 50 °C for 10 h. After filtration and washing until the pH of the filtrate was neutral, the product was vacuum dried at 80 °C for 5 h. The dried product was ground and 5.0 g of the powder was weighed and added to 200 mL of 30% H2O2 solution. The mixture was magnetically stirred for 3 h and mixed evenly. After filtration and washing, the product was freeze-dried to obtain the ZIF-8 carrier.

[0074] S2. Add 5.0 g of the ZIF-8 carrier obtained in step S1 to 1000 mL of chloroauric acid / ethylene glycol solution with a concentration of 0.05 mg / mL, and sonicate for 3 h to ensure complete dispersion of the carrier in the solution; then, slowly add 15 mL of 25 mg / mL ammonium chloride solution, and magnetically stir for 3 h to convert the chloroaurate ions in the solution into ammonium chloroaurate and load it onto the surface and internal pore structure of the carrier. During the reaction, slowly add 5 mL of a mixture of tryptophan (5.0 mg / mL) and dispersant sodium citrate (1.0 mg / mL) to the above reaction system over a period of 10 min.

[0075] S3. After the above reaction is completed, the collected solid product is freeze-dried and ground, and then heat-treated at 300°C for 3 hours under nitrogen protection. Ammonium chloroaurate is thermally decomposed into elemental Au and uniformly deposited on the surface of the ZIF-8 support, thereby obtaining the ZIF-8 supported nano-Au catalyst.

[0076] Subsequently, using glucose as the reactant, with a solution concentration of 5% (1.76 g glucose dissolved in 30 mL of water), 0.18 g of nano-Au catalyst was weighed and added to the above solution system. The magnetic stirring speed was 500 r / min, the oxygen flow rate was 40 mL / min, the reaction temperature was 60℃, and 0.5 mol / L NaOH solution was continuously added dropwise using a peristaltic pump during the reaction. The pH of the solution system was adjusted to 10.0, and the reaction time was 180 min. Then, 1.0 mol / L H2SO4 was added dropwise to the sodium gluconate solution for acidification, and 0.5 mol / L Zn(OH)2 solution was added dropwise for neutralization to prepare zinc gluconate.

[0077] The TEM image of the catalyst prepared in this embodiment is as follows. Figure 1 As shown, from Figure 1 It can be seen that there is no obvious aggregation in the catalyst, and the nano-Au particles are uniformly distributed with an average particle size of 2.7 nm. The specific surface area and pore size distribution of ZIF-8 before and after step S1 in this embodiment are shown below. Figure 2 As shown in Figures (a) and (b), compare Figure 2 As shown in Figure (a), the adsorption-desorption curves of the pretreated and modified ZIF-8 support show that the adsorption isotherm type of the ZIF-8 support changes from type IV to type II after pretreatment and modification. The calculated specific surface areas of the ZIF-8 support after pretreatment and modification are 2404.9 m² and 2404.9 m², respectively. 2 / g and 2228.9m 2 / g; simultaneously from Figure 2 (b) It can be seen that ZIF-8 has a mesoporous structure with a size of 4.0-5.0 nm before pretreatment and modification. During the modification process, the mesoporous structure disappears due to the modification of the support by oxygen-containing functional groups (hydroxyl groups). The XRD pattern of the catalyst prepared in this example is shown below. Figure 5 As shown, according to the PDF (PDF-#04-0784) card search, the diffraction peaks at 2θ = 38.27°, 44.6°, 64.68° and 77.55°, 81.64° are attributed to the (111), (200), (220), (311), and (222) crystal planes of nano-Au, respectively. The results indicate that the (111) and (200) crystal planes are easily exposed during the formation of Au nanoparticles. The performance evaluation of the catalyst prepared in this example for the selective oxidation of glucose to prepare zinc gluconate is as follows: Figure 6 As shown, the catalytic conversion rate and selectivity of the catalyst are above 99%.

[0078] Example 2

[0079] ZIF-8 was prepared by the following steps: 5.0 g of Zn(NO3)2·6H2O and 5.0 g of 2-methylimidazole were dissolved in 100 mL of N,N-dimethylformamide and ultrasonically dispersed to obtain zinc nitrate solution and 2-methylimidazole solution. The 2-methylimidazole solution was then slowly poured into the zinc nitrate solution. The mixed solution was then transferred to a hydrothermal synthesis reactor lined with polytetrafluoroethylene and hydrothermally reacted at 120 °C for 24 h in a forced-air drying oven. The mixture was then slowly cooled to room temperature, centrifuged, and washed 3-5 times with N,N-dimethylformamide solvent. After centrifugation, the mixture was vacuum dried at 50 °C for 3 h. The product was then ground and collected for later use.

[0080] ZIF-8 supported nano-Au catalyst was prepared using the following steps:

[0081] S1. Add 10.0g of ZIF-8 carrier to 1000mL of a mixture of 30% dilute nitric acid and hydrochloric acid. After mixing evenly, pretreat the reaction at 50℃ for 10h. Then filter and wash until the pH of the filtrate is neutral, and vacuum dry at 80℃ for 5h. Grind the dried product, and weigh 5.0g of the powder obtained from the grinding and add it to 200mL of 30% thiourea solution. Stir magnetically for 3h to mix evenly, filter, wash, and freeze dry to obtain ZIF-8 carrier.

[0082] S2. Add 5.0 g of the ZIF-8 carrier obtained in step S1 to 1000 mL of chloroauric acid / ethylene glycol solution with a concentration of 0.05 mg / mL, and sonicate for 3 h to ensure complete dispersion of the carrier in the solution. Then, slowly add 15 mL of 20 mg / mL sodium sulfite solution and stir magnetically for 3 h to convert the chloroauric acid ions in the solution into nano-gold sol and load them onto the surface and internal pore structure of the carrier. During the reaction, slowly add 5 mL of a mixture of ammonium chloride (15.0 mg / mL) and dispersant polyvinylpyrrolidone (1.0 mg / mL) to the above reaction system over a period of 10 min.

[0083] S3. After the above reaction is completed, the collected solid product is freeze-dried and ground, and then heat-treated at 300°C for 3 hours under helium gas protection. The gold ammonium sulfite is thermally decomposed into elemental Au and deposited on the surface of the ZIF-8 support under the spatial physical confinement of the support, thereby obtaining the ZIF-8 supported nano Au catalyst.

[0084] Subsequently, using glucose as the reactant, with a solution concentration of 5% (1.76 g glucose dissolved in 30 mL of water), 0.18 g of nano-Au catalyst was weighed and added to the above solution system. The magnetic stirring speed was 500 r / min, the oxygen flow rate was 40 mL / min, the reaction temperature was 60℃, and 0.5 mol / L NaOH solution was continuously added dropwise using a peristaltic pump during the reaction. The pH of the solution system was adjusted to 10.0, and the reaction time was 180 min. Then, 1.0 mol / L H2SO4 was added dropwise to the sodium gluconate solution for acidification, and 0.5 mol / L Zn(OH)2 solution was added dropwise for neutralization to prepare zinc gluconate.

[0085] Example 3

[0086] The difference from Example 1 is that in step S2, no tryptophan solution is added.

[0087] Comparative Example 1

[0088] The difference from Example 1 is that H2O2 solution is not added in step S1.

[0089] Comparative Example 2

[0090] The difference from Example 1 is that step S1 is omitted, and in step S2, the carrier used is commercial activated carbon (specific surface area 2000 m²). 2 / g).

[0091] Comparative Example 3

[0092] The difference from Example 1 is that in step S2, ZIF-8 is not added to the chloroauric acid / ethylene glycol solution and then ammonium chloride solution. Instead, the same tryptophan reducing agent as in Example 1 is used to reduce chloroauric acid of the same concentration. The carrier ZIF-8 is then immersed in the reduced solution and magnetically stirred for 3 hours, so that the Au particles obtained from the reduction are deposited on the pore structure loaded on the surface and inside of the carrier ZIF-8.

[0093] The size distribution diagram of the catalyst material prepared by the above method is shown in the figure. Figure 4 As shown, compare it with Figure 3The size distribution of the catalysts prepared in Example 1 is compared with that of Example 1. The figure shows that the average particle size of Au particles in the catalyst is 3.4 nm, which is much larger than the 2.7 nm of Example 1. This is because: firstly, the reduction-then-loading method described above did not control the nucleation growth process of Au particles, resulting in larger Au particle sizes compared to the method in Example 1 where Au particles were grown in the porous structure of the support; secondly, the loading of Au particles in Comparative Example 3 relies on electrostatic adsorption between the Au particles and the support, thus the bonding force between the larger Au particles and the support is weaker. Therefore, the selective oxidation catalytic performance of the catalyst prepared in Comparative Example 3 is lower than that of Example 1.

[0094] Comparative Example 4

[0095] The difference from Example 1 is that step S1 was not performed; instead, ZIF-8 was directly added to the chloroauric acid / ethylene glycol solution to load Au particles.

[0096] The catalytic performance of the catalysts prepared in the above examples and comparative examples is shown in Table 1 below.

[0097] The methods for testing the conversion rate and selectivity of the catalyst are as follows:

[0098] The catalyst was used to selectively oxidize glucose. The residual sugar content in the solution after the reaction was determined by iodometric titration, and the conversion rate of the glucose catalytic oxidation reaction was calculated. The activity of the gold nanoparticle catalyst was evaluated by the glucose conversion rate. The conversion rate was calculated based on the amount of alkali added during the reaction. An automatic potentiometric titrator was used to detect the isostatic point (endpoint) by measuring the change in electrode potential (pH). During the measurement, a pH probe was inserted into the test solution. The added titrant reacted with the test solution to neutralize it, and the concentration of hydroxide ions in the test solution changed continuously, indicating a corresponding change in pH. A potential jump occurred near the isostatic point. The titration endpoint was determined by measuring the change in the electromotive force of the working cell.

[0099] The titrant used is a 0.5 mol / L NaOH solution. Each mole of alkali neutralizes one mole of gluconic acid to produce sodium gluconate. Therefore, the reaction conversion rate is calculated using the following formula:

[0100] Conversion rate % = 0.5 × V NaOH / (m 一水葡萄糖的质量 / M 一水葡萄糖的摩尔质量 )

[0101] Among them, V NaOH m is the volume of NaOH solution added in the reaction. 一水葡萄糖的质量 M represents the mass of glucose monohydrate added before the reaction. 一水葡萄糖的摩尔质量 It is 198.17 g / mol.

[0102] After centrifugation, the mass concentration of zinc gluconate was analyzed by liquid chromatography.

[0103] Calculate selectivity using the following formula:

[0104] Selectivity% = 100% × W1 / W2

[0105] Wherein, W1 is the mass concentration of zinc gluconate obtained from the liquid chromatography test sample solution, and W2 is the theoretical mass concentration of zinc gluconate in the product when both glucose conversion and glucose selectivity are 100%.

[0106] Table 1. Catalytic performance evaluation of catalysts in Comparative Examples 1 to 4 and Example 1

[0107] Catalyst selectivity (%) Conversion rate of gluconate (%) Example 1 99.1 99.7 Example 2 99.0 97.0 Example 3 98.0 95.0 Comparative Example 1 95.0 92.0 Comparative Example 2 93.2 90.1 Comparative Example 3 97.1 93.4 Comparative Example 4 95.8 92.1

[0108] As can be seen from the above description, the selectivity and glucose conversion rate of the catalysts prepared in Examples 1-3 are greater than those in Comparative Examples 1-4. This is because: during the preparation of the catalyst using the method of the present invention, when the ground powder is impregnated in H2O2 solution, the unstable H2O2 solution releases oxygen-containing functional groups. In the subsequent loading of Au particles, this facilitates the tight bonding between the Au particles and the support, ensuring the stability of the active metal Au particles loaded on the support. In addition, free oxygen-containing functional groups (such as -OH, -C=O, -COOH, etc.) in the solution are incorporated into the ZIF-8 hierarchical pores, which in turn facilitates the selective catalytic oxidation of glucose by the free oxygen ions in the support layer, which can act as initiators and increase the catalytic oxidation rate of gluconate. Another method of loading Au particles onto the support through a secondary reduction has two advantages. First, it utilizes the interaction between the oxygen-containing functional groups modified on the support and the active metal to inhibit the migration and aggregation of nano-Au particles. Second, it employs thermal reduction to control the reduction process of Au ions, limiting the growth of Au particles and effectively improving the dispersion of Au particles on the support surface. This reduces the agglomeration caused by Au particle migration during use and improves the atomic utilization rate of the active metal in the catalyst. Furthermore, comparing Examples 1 and 3 shows that adding a reducing agent for a second reduction during the reaction of chloroauric acid and ammonium chloride helps ensure the complete reaction of gold ions in the solution system and more precisely controls the size of the nano-gold, further improving the uniform dispersion of nano-gold on the support surface, thereby further improving the selectivity and conversion rate of the catalyst.

[0109] It should be noted that the terms "first," "second," etc., used in the specification and claims of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such terms can be used interchangeably where appropriate so that the embodiments of this application described herein can be implemented, for example, in a sequence other than those described herein.

[0110] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. The use of a ZIF-8 supported nano-Au catalyst for the selective catalytic oxidation of glucose to prepare gluconic acid or zinc gluconate, characterized in that, The preparation method of the ZIF-8 supported nano-Au catalyst includes the following steps: Step S1: ZIF-8 is added to an acid solution for pretreatment to obtain pretreated ZIF-8. The pretreated ZIF-8 is then added to a solution containing a compound with oxygen-containing functional groups and / or a chelating agent for modification to obtain a ZIF-8 support. Step S2: The ZIF-8 support is added to a chloroauric acid solution, and an ammonium chloride solution is added to react and convert the chloroauric acid into ammonium chloroaurate, thereby obtaining a nano-Au catalyst precursor supported on the ZIF-8 support. Step S3: Thermally reduce the nano-Au catalyst precursor supported on the ZIF-8 support to obtain the ZIF-8 supported nano-Au catalyst. The thermal reduction is carried out at 300-350℃ for 2-5 hours; The compound containing oxygen-containing functional groups includes one or more of hydrogen peroxide, peracetic acid, ammonium persulfate, sodium peroxide, potassium peroxide, tert-butyl hydroperoxide, and cumene hydroperoxide, and the chelating agent is thiourea.

2. The use of the ZIF-8 supported nano-Au catalyst according to claim 1 for the selective catalytic oxidation of glucose to prepare gluconic acid or zinc gluconate, characterized in that, The acid solution includes one or more of hydrochloric acid, sulfuric acid, nitric acid, carbonic acid, phosphoric acid, and oxalic acid.

3. The use of the ZIF-8 supported nano-Au catalyst according to claim 1 or 2 for the selective catalytic oxidation of glucose to prepare gluconic acid or zinc gluconate, characterized in that, Step S2 further includes adding a reducing agent solution and a dispersant solution during the reaction; the concentration of the reducing agent solution is 1.5-50 mg / mL; the concentration of the dispersant solution is 1.0-2.5 mg / mL; and the amount of ammonium chloride solution is 1.0-1.5 times the stoichiometry of the reaction; the reducing agent includes one or more of tryptophan, ascorbic acid, ascorbate, citrate, formaldehyde, hydrazine hydrate, metal borohydride, and sulfite, and / or the dispersant includes one or more of gelatin, polyethylene glycol, polyvinyl alcohol, polyvinylpyrrolidone, methylcellulose, polyacrylic acid, sodium sulfosuccinate, sodium dodecyl sulfate, and sodium citrate.

4. The use of the ZIF-8 supported nano-Au catalyst according to claim 1 or 2 for the selective catalytic oxidation of glucose to prepare gluconic acid or zinc gluconate, characterized in that, The solvent for the chloroauric acid solution is one or more of methanol, ethanol, propanol, ethylene glycol, isopropanol, N,N-dimethylformamide, N,N-dimethylacetamide, and diethylformamide.

5. The use of the ZIF-8 supported nano-Au catalyst according to claim 1 or 2 for the selective catalytic oxidation of glucose to prepare gluconic acid or zinc gluconate, characterized in that, The thermal reduction is carried out in an inert atmosphere environment, which includes one or more of argon, nitrogen, and helium.

6. The use of the ZIF-8 supported nano-Au catalyst according to claim 1 or 2 for the selective catalytic oxidation of glucose to prepare gluconic acid or zinc gluconate, characterized in that, In step S1, the pretreatment is carried out at 20-50°C for 10-24 hours.

7. The use of the ZIF-8 supported nano-Au catalyst according to claim 6 for the selective catalytic oxidation of glucose to prepare gluconic acid or zinc gluconate, characterized in that, The heat reduction is carried out under the following conditions: the temperature is increased from 25°C to 125°C at a heating rate of 5-30°C / min, and held for 0.5-1.0 h; then the temperature is increased from 125°C to 185°C at a heating rate of 5-30°C / min, and held for 0.5-1.0 h; then the temperature is increased from 185°C to 245°C at a heating rate of 5-30°C / min, and held for 0.5-1.0 h; finally, the temperature is increased from 245°C to 350°C at a heating rate of 5-30°C / min, and held for 1.0-3.0 h.

8. The use of the ZIF-8 supported nano-Au catalyst according to claim 1 or 2 for the selective catalytic oxidation of glucose to prepare gluconic acid or zinc gluconate, characterized in that: In step S1, the mass concentration of the acid solution is 10-30%, the ZIF-8 is added to the acid solution at a concentration of 1-10 mg / mL, and the mass concentration of the solution containing the oxygen-containing functional group compound and / or chelating agent is 10-30%; and / or In step S2, the concentration of the ammonium chloride solution is 1.5-50 mg / mL.

Images