A nitrogen-doped carbon supported Cu-Zn-Bi composite metal catalyst, a preparation method and application thereof

Abstract

The application provides a nitrogen-doped carbon supported Cu-Zn-Bi composite metal catalyst and a preparation method and application thereof, and belongs to the technical field of catalysts. The catalyst comprises a nitrogen-doped carbon skeleton and Cu-Zn-Bi nano composite metal particles supported on the nitrogen-doped carbon skeleton; the molar ratio of Cu, Zn and Bi in the Cu-Zn-Bi nano composite metal particles is 4-5:1:0.4; and the average particle size of the Cu-Zn-Bi nano composite metal particles is 5-8 nm. The pore channel limiting effect of the nitrogen-doped carbon skeleton can avoid metal particle agglomeration, reduce the average particle size of the Cu-Zn-Bi nano composite metal particles, improve the metal dispersion, expose more active sites, realize efficient catalysis under the conditions of 0.1 MPa normal pressure and 210 DEG C mild conditions, and significantly improve the aniline conversion rate.

Description

Technical Field

[0001] This invention relates to the field of catalyst technology, specifically to a nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst, its preparation method, and its application. Background Technology

[0002] N-Methylaniline is a class of fine chemicals with important applications. It is not only widely used in the synthesis of various high-performance oil additives, but also serves as a key intermediate in downstream petrochemical processes such as rubber processing and plastic synthesis to prepare polymer materials with special properties. The quality of its synthesis process directly affects the quality of related petrochemical products, production costs, and the sustainable development of the industry.

[0003] The main synthesis processes for N-methylaniline are divided into liquid-phase synthesis and gas-phase synthesis. Liquid-phase synthesis uses aniline as a raw material, methanol as an alkylating agent, and sulfuric acid as a catalyst. However, because this method cannot selectively produce N-methylaniline, has low yields, causes significant pollution, and leads to severe equipment corrosion, it has been gradually replaced by gas-phase synthesis. Gas-phase synthesis generally uses aniline and methanol as raw materials and a solid catalyst as the core, offering advantages such as atmospheric pressure and continuous reaction capability.

[0004] Currently, copper-oxygen mixed catalysts and copper-zinc-chromium metal oxide catalytic systems are commonly used in industry for the gas-phase synthesis of N-methylaniline. Although these solid catalysts have high catalytic activity, they require hydrogenation reduction pretreatment before use, i.e., reducing the inactive copper oxide in the catalyst system to elemental copper. This hydrogenation process carries certain safety risks. From an activity perspective, the active sites of metal oxide catalysts are easily deactivated by impurities and carbon deposits, leading to a rapid decline in aniline conversion over time. Furthermore, the reaction pathway is difficult to precisely control, and byproducts such as N,N-dimethylaniline, anisole, and tar are easily generated, reducing the yield of N-methylaniline and clogging reaction equipment pipelines, increasing equipment maintenance costs.

[0005] The catalyst disclosed in Chinese patent CN202510577852.5 uses copper, zinc, and chromium metal oxides as active components, γ-alumina as a support, and cerium-molybdenum bimetallic alloy as an additive. The system contains precious metal components, which increases the cost of preparation and application. The CuO-ZnO-metal X oxide catalyst disclosed in Chinese patent CN202410914646.4 (metal X is at least one of aluminum, chromium, lanthanum, and manganese) still requires hydrogenation pretreatment in the preparation of N-methylaniline, and the process safety risk problem has not been solved.

[0006] Furthermore, traditional catalysts exhibit poor dispersion when loaded with active components, leading to severe agglomeration of the active components and significantly hindering further improvements in catalyst activity and selectivity.

[0007] Therefore, developing a non-precious metal catalyst that is highly active, highly selective, stable, and environmentally friendly is of great significance for the industrial application of the gas-phase synthesis of N-methylaniline from aniline and methanol. Summary of the Invention

[0008] In view of this, the purpose of this invention is to provide a nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst, its preparation method, and its application. The catalyst provided by this invention exhibits excellent high-temperature stability, good anti-carbon deposition performance, high catalytic performance, and high N-methylaniline selectivity.

[0009] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst, comprising a nitrogen-doped carbon framework and Cu-Zn-Bi nanocomposite metal particles supported on the nitrogen-doped carbon framework; the molar ratio of Cu, Zn, and Bi in the Cu-Zn-Bi nanocomposite metal particles is 4~5:1:0.4. The average particle size of the Cu-Zn-Bi nanocomposite metal particles is 5~8 nm.

[0010] Preferably, the nitrogen-doped carbon framework includes pyrrole nitrogen sites and graphitic nitrogen sites.

[0011] Preferably, the nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst has a specific surface area of ​​300~320 m². 2 / g, pore volume 0.80~0.85cm³ 3 / g.

[0012] Preferably, the mass ratio of the nitrogen-doped carbon framework to the Cu-Zn-Bi nanocomposite metal particles is 7:2.7~3.3.

[0013] This invention provides a method for preparing the above-mentioned nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst, comprising the following steps; S1: Chitosan is mixed with acetic acid solution to obtain chitosan gel; S2: Mix the metal salt solution with the chitosan gel to carry out a coordination reaction to obtain a chitosan-metal composite gel; the metal salt solution includes copper salt, zinc salt and bismuth salt; S3: Mix urea with the chitosan-metal composite gel, and then allow it to stand, freeze and vacuum dry in sequence to obtain chitosan-metal-urea composite dry gel. S4: The chitosan-metal-urea composite dry gel is subjected to pyrolysis carbonization, hydrogen reduction activation, and oxygen-containing gas passivation in sequence to obtain a nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst.

[0014] Preferably, the mass ratio of chitosan to urea is 2:4.6~5.5, and the mass ratio of chitosan to metal salt dry weight is 20:7~11.

[0015] Preferably, the coordination reaction is carried out at a temperature of 26-35°C for 3-5 hours.

[0016] Preferably, the freezing temperature is -60 to -40°C, and the freezing time is 3 to 5 hours; The vacuum drying temperature is -85~-75℃, the time is 22~26h, and the pressure is <10Pa.

[0017] Preferably, the pyrolysis carbonization temperature is 650~750℃, and the holding time is 1.5~2.5h; the atmosphere for pyrolysis carbonization is argon and / or nitrogen. The hydrogen reduction activation temperature is 200~300℃, and the holding time is 1.5~2.5h; the hydrogen reduction activation atmosphere is an H2 / Ar mixed gas, and the volume ratio of H2 to Ar in the H2 / Ar mixed gas is 1:9; The oxygen-containing gas passivation temperature is <50℃, and the holding time is 0.5~1.5h; the oxygen-containing gas passivation atmosphere is an oxygen / argon mixture, and the oxygen volume content in the oxygen / argon mixture is 1%.

[0018] This invention provides the application of the above-mentioned nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst in the gas-phase synthesis of N-methylaniline from aniline and methanol.

[0019] This invention provides a nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst, comprising a nitrogen-doped carbon framework and Cu-Zn-Bi nanocomposite metal particles supported on the nitrogen-doped carbon framework; the molar ratio of Cu, Zn, and Bi in the Cu-Zn-Bi nanocomposite metal particles is 4~5:1:0.4; the average particle size of the Cu-Zn-Bi nanocomposite metal particles is 5~8 nm. This invention utilizes the pore confinement effect of the nitrogen-doped carbon framework to avoid metal particle agglomeration, reduce the average particle size of the Cu-Zn-Bi nanocomposite metal particles, improve metal dispersion, expose more active sites, and significantly improve aniline conversion.

[0020] The nitrogen-doped carbon framework of this invention contains pyrrole nitrogen sites and graphite nitrogen sites. The nitrogen-doped carbon framework containing the above-mentioned nitrogen sites can form a strong interaction with Cu-Zn-Bi nanocomposite metal particles, playing a role in chemical anchoring and electronic regulation of the composite metal particles, thereby inhibiting carbon deposition. The nitrogen-doped carbon framework has good structural rigidity and pore confinement effect, which can effectively prevent the migration and sintering of composite metal particles during the reaction process. The pyrrole nitrogen sites and graphite nitrogen sites can synergistically regulate the electronic structure and coordination environment of Cu-Zn-Bi nanocomposite metal particles, improve the catalyst's resistance to impurity poisoning, and help enhance the long-term stability of the catalyst.

[0021] This invention uses Cu as the core active center to activate aniline and methanol, Zn promotes the dehydrogenation of methanol to formaldehyde as an active methyl source, and Bi blocks the secondary methylation reaction of N-methylaniline, effectively inhibiting the formation of byproducts such as N,N-dimethylaniline, anisole, and tar. By controlling the molar ratio of Cu, Zn, and Bi, this invention can ensure a high conversion rate of aniline and a high selectivity for N-methylaniline. The generated N-methylaniline can be rapidly desorbed from the surface of the nitrogen-doped catalyst framework, avoiding side reactions and further improving selectivity.

[0022] This invention provides a method for preparing the above-mentioned nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst. In this invention, chitosan, acetic acid solution, and metal salt solution are coordinated and reacted, then mixed with urea, and subjected to static standing, freeze drying, pyrolysis carbonization, hydrogen reduction activation, and oxygen-containing gas passivation to obtain the nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst. This invention utilizes the amino-hydroxy bidentate coordination of chitosan to anchor metal ions. Acetic acid is used to provide an acidic environment for the coordination reaction between chitosan and metal ions, inhibiting bismuth salt precipitation and improving coordination uniformity. Urea acts as both a nitrogen source and a pore-forming agent, increasing the nitrogen doping amount of the carbon framework and constructing a hierarchical porous structure in situ, thereby improving the specific surface area and pore volume of the material. Freeze-drying maintains the three-dimensional network structure of the gel and prevents framework collapse. Pyrolysis and carbonization form a chitosan-derived nitrogen-doped carbon support. Hydrogen reduction activation reduces metal ions to elemental form and fuses them in situ to form Cu-Zn-Bi nanocomposite metal particles. Oxygen-containing gas passivation forms a thin and dense oxide layer on the surface of the composite metal, improving the stability of the catalyst during storage and use.

[0023] The present invention provides a nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst that directly forms an active composite metal phase through in-situ activation, eliminating the need for hydrogen reduction pretreatment and avoiding safety risks. The non-precious metal system significantly reduces costs and can efficiently catalyze under mild conditions of atmospheric pressure and 210°C, reducing energy consumption and equipment requirements while increasing the reaction space velocity, making it suitable for continuous industrial production. Detailed Implementation

[0024] This invention provides a nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst, comprising a nitrogen-doped carbon framework and Cu-Zn-Bi nanocomposite metal particles supported on the nitrogen-doped carbon framework. In this invention, the molar ratio of Cu, Zn, and Bi in the Cu-Zn-Bi nanocomposite metal particles is preferably 4~5:1:0.4, more preferably 4:1:0.4; the average particle size of the Cu-Zn-Bi nanocomposite metal particles is preferably 5~8 nm, more preferably 6~7 nm. This invention uses Cu as the core active center to activate aniline and methanol, Zn promotes the dehydrogenation of methanol to formaldehyde as an active methyl source, and Bi blocks the secondary methylation reaction of N-methylaniline, effectively inhibiting the formation of byproducts such as N,N-dimethylaniline, anisole, and tar. By controlling the Cu-Zn-Bi ratio, this invention ensures high conversion rate of aniline and high selectivity for N-methylaniline.

[0025] The nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst provided by this invention comprises a nitrogen-doped carbon framework. In this invention, the nitrogen-doped carbon framework preferably includes pyrrole nitrogen sites and graphitic nitrogen sites, and the ratio of pyrrole nitrogen to graphitic nitrogen in the nitrogen-doped carbon framework is preferably 1:2~2.3, more preferably 1:2.2~2.3. The nitrogen-doped carbon framework of this invention contains pyrrole nitrogen sites and graphitic nitrogen sites. These nitrogen sites can form strong interactions with Cu-Zn-Bi nanocomposite metal particles, playing a role in chemical anchoring and electronic regulation of the composite metal particles, thereby inhibiting carbon deposition. The pyrrole nitrogen sites and graphitic nitrogen sites can synergistically regulate the electronic structure and coordination environment of the Cu-Zn-Bi nanocomposite metal particles, improving the catalyst's resistance to impurity poisoning and enhancing the long-term stability of the catalyst.

[0026] In this invention, the specific surface area of ​​the nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst is preferably 300~320 m². 2 / g, more preferably 310~320m 2 / g; the pore volume of the nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst is preferably 0.8~0.85 cm³. 3 / g, more preferably 0.82~0.85cm 3 / g. In this invention, the mass ratio of the nitrogen-doped carbon framework to the Cu-Zn-Bi nanocomposite metal particles is preferably 7:2.7~3.3, more preferably 7:3. This invention utilizes the pore confinement effect of the nitrogen-doped carbon framework to avoid metal particle agglomeration, reduce the average particle size of the metal particles, improve metal dispersion, expose more active sites, and thus improve the aniline conversion rate.

[0027] This invention provides a method for preparing the above-mentioned nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst, comprising the following steps; S1: Chitosan is mixed with acetic acid solution to obtain chitosan gel; S2: Mix the metal salt solution with the chitosan gel to carry out a coordination reaction to obtain a chitosan-metal composite gel; the metal salt solution includes copper salt, zinc salt and bismuth salt; S3: Mix urea with the chitosan-metal composite gel, and then allow it to stand, freeze and vacuum dry in sequence to obtain chitosan-metal-urea composite dry gel. S4: The composite dry gel is subjected to pyrolysis carbonization, hydrogen reduction activation, and oxygen-containing gas passivation in sequence to obtain a nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst.

[0028] This invention involves mixing chitosan with an acetic acid solution to obtain a chitosan gel. In this invention, the molecular weight of the chitosan is preferably 50,000 to 150,000, more preferably 80,000, and the degree of deacetylation of the chitosan is preferably ≥95%; the concentration of the acetic acid solution is preferably 0.5 to 2 wt%, more preferably 1 wt%. In this invention, the mass ratio of the chitosan to the volume ratio of the acetic acid solution is preferably 2 g: 100 mL.

[0029] In this invention, the mixing method is preferably magnetic stirring, the mixing rate is preferably 200-400 r / min, more preferably 300 r / min; the mixing temperature is preferably 25-35℃, more preferably 30℃; and the mixing time is preferably 3-4 h, more preferably 3 h. This invention uses acetic acid to provide an acidic environment, allowing chitosan to form a flocculent, transparent gel in the acidic environment; the acidic environment can inhibit Bi... 2+ The formation of precipitate also facilitates subsequent Cu precipitation. 2+ Zn 2+ Bi 2+ The coordination complexation reaction with chitosan provides a suitable environment.

[0030] This invention involves mixing a metal salt solution with a chitosan gel to perform a coordination reaction, thereby obtaining a chitosan-metal composite gel. In this invention, the metal salt solution includes copper, zinc, and bismuth salts; the copper salt is preferably copper nitrate, the zinc salt is preferably zinc nitrate, and the bismuth salt is preferably bismuth nitrate. The mass ratio of chitosan to metal salt dry weight is 20:7~11, more preferably 20:8~10. In this invention, the mixing is preferably performed with magnetic stirring at a rate of 200~400 r / min, more preferably 300 r / min; the temperature of the coordination reaction is preferably 26~35℃, more preferably 30℃; and the reaction time is preferably 3~5 h, more preferably 4 h. This invention utilizes the bidentate coordination complexation between the amino and hydroxyl groups on the chitosan molecular chain and metal ions, resulting in highly uniform dispersion of metal ions within the three-dimensional network structure of chitosan, providing a basis for the in-situ transformation of metal particles during subsequent pyrolysis.

[0031] This invention involves mixing urea with the chitosan-metal composite gel, followed by sequential standing, freezing, and vacuum drying to obtain a chitosan-metal-urea composite dry gel. In this invention, the preferred mass ratio of chitosan to urea is 2:4.6~5.5, more preferably 2:5; the preferred mixing method is magnetic stirring, with a preferred mixing rate of 200~400 r / min, more preferably 300 r / min; the preferred mixing temperature is 26~35℃, more preferably 30℃; and the preferred mixing time is 1.5~2.5 h, more preferably 2 h. The preferred standing method is sealed standing, with a preferred standing time of 30 min. The urea of ​​this invention can decompose during subsequent pyrolysis, introducing nitrogen doping into the nitrogen-doped carbon framework, increasing the nitrogen content of the framework and the number of catalytically active sites; simultaneously, it releases gas, constructing a porous structure in situ, and improving the specific surface area and mass transfer performance of the material.

[0032] In this invention, the freezing temperature is preferably -60~-40℃, more preferably -50℃, and the freezing time is preferably 3~5h, more preferably 4h; the vacuum drying is preferably carried out in a low-temperature cold trap; the vacuum drying pressure is preferably <10Pa, the vacuum drying temperature is preferably -85~-75℃, more preferably -80℃, and the vacuum drying time is preferably 22~26h, more preferably 24h. In this invention, after obtaining the composite dry gel, a grinding operation is preferably performed, and the ground gel is preferably passed through a 40~60 mesh sieve, and after sieving, it is preferably placed in a desiccator for later use. This invention statically freezes and solidifies the composite gel at low temperature, then freezes and dries it under vacuum conditions, causing the water in the composite gel to sublimate and be removed in the form of ice crystals, resulting in a loose and porous dry gel. This avoids gel collapse and effectively preserves the three-dimensional network structure and pore characteristics in the gel, providing a pre-defined pore shape for urea pore formation during subsequent pyrolysis.

[0033] This invention involves sequentially subjecting the composite dry gel to pyrolysis carbonization, hydrogen reduction activation, and oxygen-containing gas passivation to obtain a nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst. In this invention, the pyrolysis carbonization temperature is preferably 650~750℃, more preferably 700℃; the holding time is preferably 1.5~2.5h, more preferably 2h; the heating rate is preferably 4~6℃ / min, more preferably 5℃ / min; the pyrolysis atmosphere is preferably argon and / or nitrogen, more preferably argon; and the pyrolysis is preferably completed by natural cooling to room temperature. As a specific embodiment of this invention, the pyrolysis is preferably carried out in a tube furnace; argon gas is preferably introduced for purging before pyrolysis, with an argon gas introduction rate preferably 50mL / min and an argon gas introduction time preferably 30min; and argon gas is preferably continuously introduced during pyrolysis. The use of an argon atmosphere during the pyrolysis process of this invention can prevent the chitosan from being oxidized during carbonization. In this invention, chitosan forms a porous carbon skeleton after pyrolysis and carbonization. Nitrogen elements generated from urea pyrolysis and nitrogen elements in chitosan are in situ doped into the carbon skeleton, forming nitrogen-containing active sites of pyrrole nitrogen and graphitic nitrogen. Urea in situ foaming forms a multi-level pore structure inside the carbon skeleton, increasing the specific surface area and pore volume of the material. At the same time, the high-temperature environment causes the metal components to nucleate and disperse in situ on the skeleton surface. The nitrogen doping sites can anchor the metal particles, inhibit the agglomeration of metal particles, and achieve uniform in situ dispersion of nanocomposite metal particles.

[0034] In this invention, the hydrogen reduction activation temperature is preferably 200~300℃, more preferably 250℃; the holding time is preferably 1.5~2.5h, more preferably 2h; the heating rate is preferably 4~6℃ / min, more preferably 5℃ / min; the atmosphere for hydrogen reduction activation is preferably a mixture of hydrogen and argon, and the volume ratio of hydrogen to argon is preferably 1:9.

[0035] In this invention, the oxygen-containing gas passivation temperature is preferably <50℃; the holding time is preferably 0.5~1.5h, more preferably 1h; the oxygen-containing gas passivation atmosphere is preferably a mixture of oxygen and argon, and the oxygen volume content is preferably 1%. As a specific embodiment of this invention, the activation and passivation are preferably carried out in a tube furnace. In this invention, after the oxygen-containing gas passivation is completed, it is preferable to further store the nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst in a desiccator to protect it from light. This invention uses hydrogen reduction activation to reduce metal ions to elemental form, and a high-temperature environment allows the three types of atoms to diffuse and fuse, forming Cu-Zn-Bi ternary composite metal nanoparticles in situ. Low-temperature oxygen-containing gas passivation causes a mild oxidation reaction on the surface of the composite metal nanoparticles, forming an extremely thin and dense oxide passivation layer. This avoids direct contact between the catalyst and oxygen in the air, preventing oxidation or structural collapse, and improves the stability of the nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst.

[0036] This invention also provides the application of the above-mentioned nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst in the gas-phase synthesis of N-methylaniline from aniline and methanol.

[0037] In this invention, during the gas-phase synthesis of N-methylaniline from aniline and methanol, the nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst is preferably packed with a mesh size of 40-60; the molar ratio of aniline to methanol is preferably 1:1-2, more preferably 1:1.4; the reaction temperature is preferably 190-250℃, more preferably 210℃; the reaction pressure is preferably atmospheric pressure; and the reaction space velocity is preferably 0.4-0.8 kg·h. -1 More preferably 0.6 kg·h -1 .

[0038] The technical solutions of this invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0039] Example 1 1. Take 2.0 g of chitosan (degree of deacetylation ≥ 95%, molecular weight 80000), slowly add it to 100.0 mL of 1.0 wt% acetic acid aqueous solution, place it on a constant temperature magnetic stirrer, and stir continuously for 3 h at a water bath temperature of 30℃ and a stirring speed of 300 r / min to obtain a uniform, transparent chitosan gel without flocculent precipitate. 2. Accurately weigh 0.668 g of copper nitrate trihydrate, 0.131 g of anhydrous zinc nitrate, and 0.134 g of bismuth nitrate pentahydrate according to the metal molar ratio Cu:Zn:Bi=4:1:0.4. Dissolve each ingredient separately in 10 mL of deionized water and mix them together. Add 1-2 drops of 0.1 mol / L dilute nitric acid to the bismuth nitrate to aid dissolution, and obtain the metal salt solution. 3. The metal salt solution was added dropwise to the obtained chitosan gel at a rate of 2 mL / min, and the mixture was magnetically stirred at 30℃ and 300 r / min for 4 h to obtain the chitosan-metal composite gel. 4. Add 5.0g of urea to the obtained chitosan-metal composite gel, and magnetically stir at 30℃ and 300r / min for 2h until the urea is completely dissolved. Seal and let stand for 30min to remove bubbles, and obtain chitosan-metal-urea composite gel; 5. Transfer the obtained composite gel to a polytetrafluoroethylene lyophilization bottle (sample volume ≤ 2 / 3 of the bottle body), place it in a -50℃ ultra-low temperature freezer for 4 hours until it is completely solidified; connect it to a vacuum freeze dryer and dry it for 24 hours at <10Pa and -80℃ cold trap until the ice crystals are completely sublimated to obtain a loose and porous dry gel; grind the dry gel and pass it through a 40~60 mesh sieve, then place it in a desiccator for later use; 6. Take 2.0g of the ground dry gel, spread it evenly on a quartz boat (thickness ≤5mm) and place it in the constant temperature zone of a tube furnace; purge the tube furnace with high-purity Ar gas at a flow rate of 50mL / min for 30min to remove all air; raise the temperature to 700℃ at a rate of 5℃ / min and hold it at that temperature for 2.0h, maintaining an Ar gas flow rate of 50mL / min throughout the process, and absorb the tail gas with alkaline solution; allow it to cool naturally to room temperature and collect the black solid powder as the precursor of the nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst; 7. Purge the tube furnace with a H2 / Ar mixed gas (volume ratio 1:9) at a flow rate of 30 mL / min for 20 min; raise the temperature to 250 °C at a heating rate of 5 °C / min and hold at that temperature for 2.0 h; close the H2 passage, maintain an Ar gas flow rate of 50 mL / min to cool to below 50 °C, and passivate with a 1% O2 / Ar mixed gas for 1 h; collect the black solid powder, seal it in a brown sample bottle and store it in a desiccator in the dark to obtain the nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst, denoted as PNC-CuZnBi.

[0040] Comparative Example 1 1. Take equimolar amounts of copper nitrate, zinc nitrate, and bismuth nitrate from Example 1, dissolve them directly in 100 mL of deionized water, and stir magnetically for 2 h to obtain an aqueous solution of nitrates; 2. Place the nitrate aqueous solution in a rotary evaporator and evaporate it to dryness under the conditions of 60°C in a water bath and a vacuum of -0.08MPa to obtain a solid metal salt. 3. Subsequent operations are the same as those described in steps 6 and 7 of Example 1, to obtain a supportless Cu-Zn-Bi catalyst.

[0041] Application Example 1 Metal particle size testing: Transmission electron microscopy (TEM, JEM-2100F, 200kV) was used. After the sample was ultrasonically dispersed in anhydrous ethanol for 30 min, it was dropped onto a carbon support film copper grid. After air drying, different field-of-view photographs were taken, and the average size of ≥200 particles was calculated using ImageJ software.

[0042] Catalytic performance test: A fixed-bed microreactor (8 mm inner diameter, 300 mm length) was used, loaded with 0.5 g of 40-60 mesh catalyst, mixed with an equal mass of quartz sand; the raw material ratio of aniline to methanol was 1:1.4 (molar ratio), the liquid feed rate was 0.15 mL / min, and the H2 carrier gas flow rate was 30 mL / min; the reaction temperature was 210℃, the pressure was 0.1 MPa, and the liquid hourly space velocity was 0.6 kg·h. -1 After 1 hour of stable operation, samples were taken; the products were detected by gas chromatograph (GC-2014, FID detector, SE-30 capillary column). Chromatographic conditions: column temperature 80℃ for 2 min, then increased to 200℃ at 10℃ / min and held for 5 min, injection port 250℃, detector 280℃, N2 carrier gas flow rate 1.0 mL / min, split ratio 50:1.

[0043] Cyclic stability test: Under the above catalytic performance test conditions, complete 10 catalytic cycles, measure the aniline conversion rate after each cycle, and calculate the activity retention rate.

[0044] The application effects of the catalysts obtained in Example 1 and Comparative Example 1 are shown in Table 1.

[0045] Table 1. Application effects of the catalysts obtained in Example 1 and Comparative Example 1

[0046] As shown in Table 1, the amino and hydroxyl groups of chitosan in Example 1 of the present invention can react with Cu. 2+ / Zn 2+ / Bi 2+ A stable bidentate coordination structure is formed, which effectively anchors metal ions and avoids metal particle agglomeration during pyrolysis, reducing the metal particle size from 25.8 nm to 6.2 nm. The improvement in metal dispersion significantly improves aniline conversion (by 43.8%) and N-methylaniline selectivity (by 17.1%). At the same time, the coordination effect and the confinement effect of the nitrogen-doped carbon skeleton greatly enhance the catalyst's cycle stability.

[0047] Example 2 The preparation method was completely consistent with that in Example 1, and a nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst was obtained.

[0048] Comparative Example 2 The preparation method was completely consistent with that of Comparative Example 1, and a supportless Cu-Zn-Bi catalyst was obtained.

[0049] Comparative Example 3 1. Take 2.0 g of MgAl-LDO (magnesium aluminum hydrotalcite, prepared according to the method disclosed in CN118878423), add it to 100 mL of deionized water, and sonicate for 30 min to obtain MgAl-LDO suspension; 2. Subsequent operations are the same as those described in steps 2 to 7 of Example 1, except that the chitosan gel in step 3 of Example 1 is replaced with the MgAl-LDO suspension obtained in step 1 of this comparative example; thus, a MgAl-LDO-supported Cu-Zn-Bi catalyst is obtained.

[0050] Comparative Example 4 1. Take 2.0g of ZSM-5 molecular sieve (Si / Al=25, 40~60 mesh), calcine at 550℃ for 4h, add it to 100mL of deionized water, and ultrasonically disperse for 30min to obtain a molecular sieve suspension; 2. Subsequent operations are the same as those described in steps 2 to 7 of Example 1, except that the chitosan gel in step 3 of Example 1 is replaced with the ZSM-5 molecular sieve suspension obtained in step 1 of this comparative example to obtain the ZSM-5 supported Cu-Zn-Bi catalyst.

[0051] Application Example 2 Metal dispersion was tested using the pulsed H2 chemisorption method. Sample pretreatment: Take 50 mg of each catalyst and place it in a quartz reaction tube. Heat to 300 °C at 10 °C / min under an Ar atmosphere at 50 mL / min, keep at the temperature for 2 h to remove impurities, and then cool to room temperature.

[0052] In-situ reduction: Introduce 30 mL / min of H2 / Ar mixed gas (1:9), raise the temperature to 250℃ at 5℃ / min, keep the temperature constant for 2 h to reduce metal ions, and purge with Ar gas for 30 min to remove residual H2.

[0053] Pulse adsorption: Cool to 50℃, inject 0.1 mL of pure H2 (≥99.999%) per pulse until the H2 peak area deviation is ≤±2% for 3 consecutive times, and record the total amount of adsorbed H2.

[0054] Calculation: Metal dispersion (%) = (Amount of adsorbed H2 substance × 2) / Total amount of metal substance in catalyst × 100%.

[0055] The catalytic performance and the 50-hour stability retention rate were determined according to the catalytic performance test method in Application Example 1. The stability test was conducted for a continuous reaction of 50 hours, with samples taken every 2 hours to detect the aniline conversion rate, and the activity retention rate after 50 hours was calculated.

[0056] The application effects of the catalysts obtained in Example 2 and Comparative Examples 2-4 are shown in Table 2.

[0057] Table 2. Application effects of the catalysts obtained in Example 2 and Comparative Examples 2-4

[0058] As shown in Table 2, the chitosan-derived nitrogen-carbon support, through the synergistic effect of coordination anchoring and nitrogen-doped porous carbon framework confinement, showed a 26% increase in metal dispersion and a 22.7% increase in 50-hour stability retention compared to the MgAl-LDO support, and all performance indicators were significantly improved compared to the ZSM-5 molecular sieve support. The unsupported catalyst had the lowest performance indicators due to severe metal particle agglomeration.

[0059] Example 3 The preparation method was completely consistent with that in Example 1, and a nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst was obtained.

[0060] Example 4 The preparation method is basically the same as in Example 1, except that the molar ratio of metal nitrate is replaced with Cu:Zn:Bi=5:1:0.4 to obtain a nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst.

[0061] Comparative Example 5 The preparation method is basically the same as in Example 1, except that the molar ratio of metal nitrate is replaced with Cu:Zn:Bi=3:1:0.4 to obtain a nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst.

[0062] Comparative Example 6 The preparation method is basically the same as in Example 1, except that the molar ratio of metal nitrate is replaced with Cu:Zn:Bi=4:0:0.4 to obtain a nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst.

[0063] Application Example 3 The catalytic performance was tested according to the catalytic performance test method in Application Example 1. The aniline conversion rate and the content of the byproduct dimethylaniline were measured, and the TOF value (conversion frequency) was calculated. The formula is: TOF = amount of aniline reacted / (amount of metal on the catalyst surface × reaction time).

[0064] The application effects of the catalysts obtained in Examples 3-4 and Comparative Examples 5-6 are shown in Table 3.

[0065] Table 3. Application effects of catalysts obtained in Examples 3-4 and Comparative Examples 5-6

[0066] As shown in Table 3, the introduction of Zn can promote the dehydrogenation of methanol to formaldehyde intermediate, providing an active methyl source for aniline methylation. The aniline conversion rate and TOF value of the Zn-free system (4:0:0.4) are significantly reduced, and the TOF value is reduced by 48.6% compared with the preferred ratio, which proves the effect of Zn on improving catalytic efficiency. Bi is a traditional metal element in non-N-methylaniline synthesis catalysts. It can effectively inhibit over-methylation reaction, reduce the content of by-product dimethylaniline, and maintain selectivity at a high level. When Cu:Zn:Bi=4:1:0.4, the aniline conversion rate and TOF value are the highest, the by-product content is the lowest, and the synergistic effect of each metal component is optimal.

[0067] Example 5 The preparation method was completely consistent with that in Example 1, and a nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst was obtained.

[0068] Comparative Example 7 The preparation method is basically the same as in Example 1, except that chitosan is replaced with an equal mass of activated carbon (specific surface area 1500 m²). 2 / g, pore size 2~50nm), to obtain activated carbon supported Cu-Zn-Bi composite metal catalyst.

[0069] Application Example 4 The catalytic performance was tested according to the catalytic performance test method in Application Example 1, with the reaction time extended to 100 h, and the aniline conversion rate was measured every 20 h. Carbon deposit test: Thermogravimetric analyzer (TGA, STA 449F5) was used. 10 mg of the catalyst after reaction was placed in an alumina crucible, with an empty crucible as a reference. The air atmosphere flow rate was 20 mL / min, and the temperature was increased to 800℃ at 10℃ / min. The initial mass at 50℃ and the residual mass at 800℃ were recorded. Formula: Carbon deposition (wt%) = (initial mass - residual mass) / initial mass × 100% - ash content (the ash content of the catalyst in this invention is ≤0.1%, which can be ignored).

[0070] Catalytic performance and carbon deposition were tested for Examples 5 and Comparative Example 7, and the results are shown in Table 4.

[0071] Table 4. Application effects of the catalysts obtained in Example 5 and Comparative Example 7

[0072] In this invention, the ratio of pyrrole nitrogen to graphite nitrogen in the nitrogen-doped carbon skeleton formed by chitosan pyrolysis is 1:2.3. As shown in Table 4, the nitrogen-doped carbon skeleton formed by chitosan pyrolysis in this invention forms a strong interaction with the metal particles, which can effectively inhibit the formation of surface carbon during the reaction. The amount of carbon deposit in Example 5 is only 25% of that in Comparative Example 7. The reduction in carbon deposit significantly improves the long-term stability of the catalyst. After 100 hours of reaction, the aniline conversion rate in Example 5 is still 89.5%, while that in Comparative Example 7 is 62.1%, proving that the nitrogen-doped carbon skeleton can significantly improve the catalyst's anti-carbon deposition performance and stability.

[0073] Example 6 The preparation method was completely consistent with that in Example 1, and a nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst was obtained.

[0074] Comparative Example 8 A basic copper carbonate-based composite catalyst was prepared according to the method disclosed in Example 1 of CN120022920A.

[0075] Comparative Example 9 CuO was prepared according to the method disclosed in Example 1 of CN119638577A. x ZnO y X composite oxide catalyst.

[0076] Comparative Example 10 An iridium catalyst combination was prepared according to the method disclosed in Example 1 of CN118878423A.

[0077] Application Example 5 Each catalyst was tested for catalytic performance under the optimal reaction conditions disclosed in its corresponding patent. The aniline conversion rate and N-methylaniline selectivity were determined using a unified catalytic performance testing method. The relative cost of each comparative sample was calculated with the cost of the catalyst of this invention as 1.0 as a benchmark.

[0078] The application effects of the catalysts obtained in Example 6 and Comparative Examples 8-10 are shown in Table 5.

[0079] Table 5. Application effects of the catalysts obtained in Example 6 and Comparative Examples 8-10

[0080] As shown in Table 5, the catalyst of this invention is a non-precious metal system. Under similar selectivity, the aniline conversion rate is higher than that of basic copper carbonate-based and CuO-based catalysts. x ZnO y X composite oxide catalysts increase reaction space velocity by 50%~100% and significantly improve catalytic efficiency; Compared with the combination of precious metal iridium catalysts, the catalyst of this invention significantly improves the aniline conversion rate, maintains the same selectivity, and costs only 1 / 25 of the former, greatly reducing the cost of industrial application and avoiding the risk of dependence on precious metal resources. The catalyst of this invention has a moderate reaction temperature, compared to basic copper carbonate-based and CuO catalysts. x ZnO y X-composite oxide catalysts can reduce temperature by 50-100°C, thereby reducing energy consumption and better meeting the requirements of green chemical development.

[0081] Example 7 The preparation method was completely consistent with that in Example 1, and a nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst was obtained.

[0082] Example 8 The preparation method is basically the same as in Example 1, except that the molecular weight of chitosan is 50,000, and a nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst is obtained.

[0083] Example 9 The preparation method is basically the same as in Example 1, except that the molecular weight of chitosan is 120,000, and a nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst is obtained.

[0084] Example 10 The preparation method is basically the same as in Example 1, except that the molecular weight of chitosan is 150,000, and a nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst is obtained.

[0085] Application Example 6 The metal dispersion was tested according to the test method in Application Example 2. The aniline conversion rate was tested according to the test method in Application Example 1. The stability retention rate after 50 hours was tested according to Application Example 2; The specific surface area and pore volume of the catalyst were determined using a specific surface area and pore volume analyzer (ASAP 2460) (N2 adsorption-desorption at 77K in liquid nitrogen; specific surface area was calculated by BET method; and pore volume was calculated by BJH method).

[0086] The application effects of the catalysts obtained in Examples 7-10 are shown in Table 6.

[0087] Table 6. Application effects of the catalysts obtained in Examples 7-10

[0088] Table 6 shows that chitosans with molecular weights in the range of 50,000 to 150,000 can form stable coordination structures with metal ions. After freeze-drying and pyrolysis, a nitrogen-doped porous carbon framework is constructed, with a catalyst metal dispersion ≥78%, aniline conversion ≥98%, 50-hour stability retention ≥94%, and specific surface area ≥300 m². 2 / g, pore volume ≥0.80cm 3 / g, all can achieve the core technical effects of the present invention; among them, 80,000~120,000 is the preferred molecular weight range, with better performance indicators, and 80,000 is the most preferred molecular weight, with the best catalytic performance.

[0089] The nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalysts prepared in Examples 1, 2, 3, 5, 6, and 7 of this invention were all prepared using the same raw materials and methods. The test data results for the corresponding performance tests are the average values ​​of multiple parallel tests. The numerical differences between different examples are normal experimental fluctuations during the testing process and do not change the core technical effects of the catalyst of this invention, such as high metal dispersion, high aniline conversion, high N-methylaniline selectivity, high stability, and low carbon deposition. They do not affect the repeatability and implementation value of the technical solution of this invention.

[0090] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst, characterized in that, It includes a nitrogen-doped carbon framework and Cu-Zn-Bi nanocomposite metal particles supported on the nitrogen-doped carbon framework; the molar ratio of Cu, Zn, and Bi in the Cu-Zn-Bi nanocomposite metal particles is 4~5:1:0.4; The average particle size of the Cu-Zn-Bi nanocomposite metal particles is 5~8 nm.

2. The nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst according to claim 1, characterized in that, The nitrogen-doped carbon framework includes pyrrole nitrogen sites and graphitic nitrogen sites.

3. The nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst according to claim 1, characterized in that, The nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst has a specific surface area of ​​300~320 m². 2 / g, pore volume 0.80~0.85cm³ 3 / g.

4. The nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst according to claim 1, characterized in that, The mass ratio of the nitrogen-doped carbon framework to the Cu-Zn-Bi nanocomposite metal particles is 7:2.7~3.

3.

5. The method for preparing the nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst according to any one of claims 1 to 4, characterized in that, Includes the following steps; S1: Chitosan is mixed with acetic acid solution to obtain chitosan gel; S2: Mix the metal salt solution with the chitosan gel to carry out a coordination reaction to obtain a chitosan-metal composite gel; the metal salt solution includes copper salt, zinc salt and bismuth salt; S3: Mix urea with the chitosan-metal composite gel, and then allow it to stand, freeze and vacuum dry in sequence to obtain chitosan-metal-urea composite dry gel. S4: The chitosan-metal-urea composite dry gel is subjected to pyrolysis carbonization, hydrogen reduction activation and oxygen-containing gas passivation in sequence to obtain a nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst.

6. The preparation method according to claim 5, characterized in that, The mass ratio of chitosan to urea is 2:4.6~5.5, and the mass ratio of chitosan to metal salt dry weight is 20:7~11.

7. The preparation method according to claim 5, characterized in that, The coordination reaction is carried out at a temperature of 26-35°C for 3-5 hours.

8. The preparation method according to claim 5, characterized in that, The freezing temperature is -60~-40℃, and the time is 3~5 hours; The vacuum drying temperature is -85~-75℃, the time is 22~26h, and the pressure is <10Pa.

9. The preparation method according to claim 5, characterized in that, The pyrolysis carbonization temperature is 600~750℃, and the holding time is 1.5~2.5h; the atmosphere for the pyrolysis carbonization is argon and / or nitrogen. The hydrogen reduction activation temperature is 200~300℃, and the holding time is 1.5~2.5h; the hydrogen reduction activation atmosphere is an H2 / Ar mixed gas, and the volume ratio of H2 to Ar in the H2 / Ar mixed gas is 1:9; The oxygen-containing gas passivation temperature is <50℃, and the holding time is 0.5~1.5h; the oxygen-containing gas passivation atmosphere is an oxygen / argon mixture, and the oxygen volume content in the oxygen / argon mixture is 1%.

10. The application of the nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst according to any one of claims 1 to 4 or the nitrogen-doped carbon-supported Cu-Zn-Bi composite metal catalyst prepared by the preparation method according to any one of claims 5 to 9 in the gas-phase synthesis of N-methylaniline from aniline and methanol.

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