Application of nanoflower-like δ-manganese dioxide in catalytic preparation of azobenzene from aniline

Abstract

This invention relates to the application of nano-flower-like manganese dioxide in the catalytic oxidative coupling of aniline to azobenzene. Using nano-flower-like delta-crystalline manganese dioxide as a heterogeneous catalyst and toluene as a solvent, it catalyzes the aerobic oxidative coupling reaction of aniline in oxygen, achieving a one-step preparation of azobenzene. This catalyst possesses a unique spherical daisy-like structure composed of stacked nano-flowers, with abundant surface active sites. Under reaction conditions of 353 K and 1 MPa oxygen pressure, the aniline conversion rate reaches 99.9% after 10 h of reaction, and the selectivity of the target product, azobenzene, remains consistently above 99.9%. Furthermore, the catalyst can be recycled four times without significant decline in catalytic activity or selectivity. This invention offers mild catalytic reaction conditions, high product selectivity, and easy catalyst separation and recovery, demonstrating broad industrial application prospects in the large-scale green synthesis of azobenzene.

Description

Technical Field

[0001] This invention belongs to the field of organic catalytic synthesis technology, specifically relating to the application of nano-flower-like δ-crystalline manganese dioxide in the catalytic aerobic oxidative coupling reaction of aniline to prepare azobenzene. Background Technology

[0002] Azobenzenes are a class of aromatic compounds with a typical -N=N- double bond structure. As important fine chemical intermediates, they have wide applications in organic dyes, photoresponsive functional materials, drug synthesis, food additives, and liquid crystal materials. Among these, the unique cis-trans isomerism photoresponsive properties of azobenzene compounds give them irreplaceable application potential in cutting-edge fields such as molecular switches, information storage, and photoelectric devices, leading to continuously growing market demand.

[0003] Currently, the industrial synthesis of azobenzene mainly relies on the classic diazotization-coupling reaction route. This process uses aniline as a raw material, which is diazotized with a diazotizing reagent and then coupled with aromatic hydrocarbons to obtain the target product. However, this method has significant technical drawbacks: First, the reaction requires the use of large amounts of toxic and harmful reagents such as nitrites, and the reaction process generates a large amount of saline wastewater and organic waste, resulting in high costs for waste treatment and a heavy environmental burden, which does not meet the requirements of green chemical development; second, the reaction conditions are harsh, requiring low-temperature, strong acid systems, which causes severe corrosion to equipment, and the process steps are cumbersome, resulting in low production efficiency.

[0004] The oxidative coupling of aniline is a green synthetic route developed in recent years. Using oxygen or air as an oxidant, it achieves the one-step oxidative coupling of aniline to azobenzene, offering significant advantages such as high atom economy, clean oxidant, and simple process, making it a research hotspot in this field. The core of this reaction lies in the development of highly efficient catalysts. Currently reported catalytic systems can be divided into two main categories: noble metal and non-noble metal. While noble metal catalysts (such as Pd, Pt, and Au-based catalysts) exhibit high catalytic activity, their raw material costs are high, resources are scarce, and they are prone to over-oxidation, generating byproducts, making it difficult to meet the demands of low-cost industrial production. Existing non-noble metal catalysts (such as copper-based, cobalt-based, and vanadium-based oxides) generally suffer from insufficient catalytic activity, demanding reaction conditions (requiring high temperature and pressure, and the addition of alkaline promoters), poor selectivity for target products, poor catalyst stability, and difficulty in recycling, severely limiting the large-scale application of this green route.

[0005] Manganese oxides, as a class of inexpensive, readily available, and environmentally friendly non-precious metal oxides, have attracted widespread attention in the field of selective oxidation catalysis due to their rich valence state variations, excellent redox properties, and tunable crystal structures. Among them, manganese dioxide exists in various crystal forms, including α, β, γ, and δ. Different crystal forms exhibit significant differences in microstructure, surface active sites, and lattice oxygen migration capabilities, thus displaying distinct catalytic performances. δ-crystal manganese dioxide possesses a unique two-dimensional layered nanoflower-like structure, exposing more surface active sites and a larger reaction contact area, while also possessing abundant Mn. 3+ The active center and high-mobility lattice oxygen theoretically possess superior catalytic potential for the oxidative coupling reaction of aniline. However, existing δ-crystalline manganese dioxide is mostly prepared using batch processes such as hydrothermal methods and precipitation methods, which suffer from problems such as uneven product morphology, difficulty in scale-up production, and poor batch stability. Furthermore, there is a lack of systematic performance optimization and mechanism studies on its catalytic reaction system for the oxidative coupling of aniline to azobenzene.

[0006] Therefore, developing a nano-flower-like δ-crystalline manganese dioxide catalyst with simple preparation process, large-scale production capability, high catalytic activity, excellent selectivity and strong stability for the aerobic oxidative coupling synthesis of azobenzene with aniline is of great practical significance and industrial value for promoting the green upgrading of azobenzene production process and reducing production costs. Summary of the Invention

[0007] To address the problems of high catalyst cost, insufficient activity, poor selectivity, and difficulty in scaling up preparation processes in existing catalytic systems for the oxidative coupling of aniline to azobenzene, this invention aims to provide an application of nano-flower-like delta-crystalline manganese dioxide in the catalytic oxidative coupling reaction of aniline to azobenzene. This catalyst is prepared using a microfluidic process, exhibits uniform morphology, and can be mass-produced. It achieves efficient conversion of aniline and highly selective synthesis of azobenzene under mild conditions, demonstrating significant performance advantages and promising industrialization prospects.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: The core of this invention is to provide an application of nano-flower-like δ-manganese dioxide in the catalytic oxidative coupling reaction of aniline to prepare azobenzene.

[0009] Furthermore, using nano-flower-like delta-crystalline manganese dioxide as a heterogeneous catalyst and toluene as a reaction solvent, azobenzene was prepared in one step via the aerobic oxidative coupling reaction of aniline under an oxygen atmosphere. The specific reaction equation is as follows: Preferably, in this catalytic reaction, when the reaction temperature is 343~363 K, the oxygen pressure is 0.5~2 MPa, and the reaction time is 0.25~10 h, the conversion rate of aniline is 13.3%~99.9%, and the selectivity of the target product azobenzene is above 99.9%.

[0010] Preferably, in this catalytic reaction, when the reaction temperature is 353 K, the oxygen pressure is 1 MPa, and the reaction is carried out for 0.5 h, the aniline conversion rate is 37.42%, and when the reaction is carried out for 10 h, the aniline conversion rate can reach 99.9%, and the selectivity of the target product azobenzene is always maintained above 99.9%.

[0011] In the scheme of the present invention, the nanoflower-like δ-crystal form manganese dioxide is prepared by a microfluidic continuous process, and the specific steps are as follows: (1) Preheat the potassium permanganate solution and the manganese sulfate solution respectively to the temperature T, and control 80 °C ≤ T < 90 °C; (2) Introduce the preheated potassium permanganate solution and manganese sulfate solution simultaneously into the reactor of the microfluidic reaction device at a set flow rate, carry out a continuous reaction under constant temperature conditions, the reaction temperature is 80 °C ≤ T < 90 °C, and the reaction residence time is 1 h; (3) Collect the reaction product and let it stand for 24 h, after suction filtration, wash it with deionized water until the pH value of the filtrate is 7, place the filter cake in a constant-temperature drying oven at 80 °C for full drying, cool it to room temperature and then grind it, to obtain the nanoflower-like δ-crystal form manganese dioxide catalyst.

[0012] Preferably, in the step (1), the feeding concentration c1 of the potassium permanganate solution is controlled as 0 mol / L < c1 ≤ 1 mol / L, and the feeding concentration c2 of the manganese sulfate solution is controlled as 0 mol / L < c2 ≤ 1 mol / L.

[0013] Preferably, in the step (2), the feeding flow rate Q1 of the potassium permanganate solution is controlled as 60 g / min ≤ Q1 ≤ 80 g / min, and the feeding flow rate Q2 of the manganese sulfate solution is controlled as 70 g / min ≤ Q2 ≤ 120 g / min.

[0014] In the above preparation method, the microfluidic reaction device adopts the disclosed technical solution, for example, the invention patent with the authorized announcement number CN114956188B, which is a method for continuously preparing δ-crystal form manganese dioxide by microfluidics, and the present invention does not elaborate on this technical solution.

[0015] It is worth noting that this invention, through precise control of the microfluidic reaction temperature, can prepare manganese dioxide in three crystalline forms: δ, α, and γ. When the reaction temperature is controlled at 80 ℃ ≤ T < 90 ℃, the product is δ-crystalline manganese dioxide with a two-dimensional layered nanoflower-like structure; when the reaction temperature is controlled at 0 ℃ ≤ T < 80 ℃, the product is α-crystalline manganese dioxide with nanofiber-like structure; and when the reaction temperature is controlled at 90 ℃ ≤ T ≤ 150 ℃, the product is γ-crystalline manganese dioxide with a grain-like structure. The catalytic performance of the three crystalline manganese dioxide forms differs significantly: under the same reaction conditions, δ-MnO2 exhibits the highest conversion rate for aniline, 1.76 times that of α-MnO2 and 2.44 times that of γ-MnO2, respectively. All three crystalline forms show 100% selectivity for azobenzene. This performance difference stems from the inherent structure: the unique nanoflower-like layered structure of the δ-crystalline form exposes more active sites, resulting in a higher specific surface area and more abundant Mn. 3+ The active center, along with a significantly higher total number of acid and base sites on its surface compared to the α and γ crystal forms, can synergistically promote the adsorption and coupling processes of aniline, thus exhibiting optimal catalytic activity.

[0016] Under thermocatalytic reaction conditions, a large amount of Mn on the surface of the nano-flower-like δ-MnO2 catalyst 3+ The active sites and oxygen vacancies together constitute the active center of the catalytic reaction. The two-dimensional layered nanoflower structure of this catalyst can provide a high specific surface area, which can effectively enrich aniline molecules and oxygen molecules on the catalyst surface. During the reaction, the reactant adsorption process occurs first. The amino group of the aniline molecule adsorbs onto the Mn active sites on the surface of the δ-MnO2 catalyst using the lone pair electrons of the N atom, while oxygen molecules are activated on the catalyst surface. Subsequently, the adsorbed aniline undergoes a first-step dehydrogenation reaction with the help of the catalyst's lattice oxygen to generate a phenylhydroxylamine intermediate. This process consumes some lattice oxygen, further exposing oxygen vacancies on the catalyst surface. The generated phenylhydroxylamine intermediate then undergoes a second-step oxidation reaction under the action of surface active oxygen to generate a nitrosobenzene intermediate. Gas-phase activated oxygen simultaneously replenishes the oxygen source to the catalyst lattice, partially repairing the consumed lattice oxygen. Afterward, the nitrosobenzene intermediate generated in the system undergoes an intermolecular condensation reaction with another aniline molecule adsorbed on the catalyst surface to generate a condensation intermediate. The condensation intermediate further desorbs water molecules to generate the target product, azobenzene. Finally, the azobenzene product desorbs from the catalyst surface, allowing the Mn active sites and oxygen vacancies on the catalyst to be released again, completing the complete catalytic cycle and entering the next catalytic process. The δ-MnO2 catalyst surface is rich in Mn... 3+The catalytic sites, oxygen vacancies, and surface acid-base sites can work synergistically to complete the two-step oxidation of aniline and the subsequent condensation conversion process. The two-dimensional nanoflower layered structure can fully expose the catalytic active sites, accelerate the interfacial electron transfer rate and the mass transfer and diffusion efficiency of reactants and products, so that the catalytic system has both excellent catalytic activity and azobenzene product selectivity in the oxidation of aniline to azobenzene.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention uses a microfluidic continuous method to prepare nano-flower-like δ-crystal manganese dioxide. It uses inexpensive manganese salts as raw materials. The preparation process is simple and controllable, the product has uniform morphology and strong batch stability, and it can realize continuous mass production, which greatly reduces the cost of catalyst preparation. Moreover, the preparation process is green and environmentally friendly, with no toxic or harmful substances emitted, and it is easy to scale up to industrial scale.

[0018] 2. The nano-flower-like δ-crystal manganese dioxide prepared by this invention has excellent catalytic performance. It can achieve complete conversion of aniline under mild conditions of 353 K and 1 MPa oxygen, with a selectivity of up to 99.9% for the target product azobenzene. The reaction process does not require the addition of any alkaline additives, has few side reactions, and is easy to separate and purify the product.

[0019] 3. The catalytic reaction system of the present invention is a heterogeneous catalytic system. The catalyst and the reaction system are easy to separate and can be reused after simple filtration and recovery. After being recycled 4 times, the catalytic activity and selectivity do not decrease significantly, which further reduces the catalyst cost and operating energy consumption in the production process. Attached Figure Description

[0020] Figure 1 TEM images of three different crystal forms of MnO2 catalysts: (a) δ-MnO2, (b) α-MnO2, and (c) γ-MnO2.

[0021] Figure 2 This is a graph showing the process of catalytic performance of δ-MnO2 catalyst changing over time.

[0022] Figure 3 The figure shows the experimental performance of the stability of the δ-MnO2 catalyst. Detailed Implementation

[0023] The technical solution of the present invention will be further described below with specific embodiments, but the scope of protection of the present invention is not limited thereto. Although the steps in the present invention are arranged with reference numerals, they are not intended to limit the order of the steps. Unless the order of the steps is explicitly stated or the execution of a certain step requires other steps as a basis, the relative order of the steps can be adjusted. Unless otherwise specified, the chemical reagents and materials in the present invention are all purchased from the market or synthesized from raw materials purchased from the market.

[0024] In the examples of the present invention, the products prepared under different temperature conditions, namely the δ-type, α-type and γ-type manganese dioxide catalysts, are taken as examples to test the properties and parameters of the products and determine the optimal properties of the products.

[0025] The preparation steps of the nano-flower-like δ-crystal form manganese dioxide of the present invention are as follows: (1) Preheat the potassium permanganate solution and the manganese sulfate solution respectively to the temperature T, and control 80 °C≤T<90 °C; the concentration of the potassium permanganate solution is controlled to 0 mol / L<c1≤1 mol / L, and the concentration of the manganese sulfate solution is controlled to 0 mol / L<c2≤1 mol / L; (2) Feed the preheated potassium permanganate solution and manganese sulfate solution simultaneously into the reactor of the microfluidic reaction device at a set flow rate, and carry out a continuous reaction under constant temperature conditions, with the reaction temperature being 80 °C≤T<90 °C and the reaction residence time being 1 h; the feeding flow rate of the potassium permanganate solution is controlled to 60 g / min-80 g / min, and the feeding flow rate of the manganese sulfate solution is controlled to 70 g / min-120 g / min; (3) Collect the reaction product and let it stand for 24 h, wash it with deionized water after suction filtration until the pH value of the filtrate is 7, place the filter cake in a constant-temperature drying oven at 80 °C for full drying, cool it to room temperature and grind it, so as to obtain the nano-flower-like δ-crystal form manganese dioxide catalyst.

[0026] In addition, for the manganese dioxides of different crystal forms obtained in the subsequent examples, their specific preparation processes are all the same as the preparation process of Example 1, and the crystal form switching is realized only by adjusting the reaction temperature.

[0027] The aniline oxidative coupling reaction is carried out in a high-pressure reactor, and the specific operations are as follows: weigh 100 mg of the catalyst into the high-pressure reactor, measure 20 mL of the solvent and mix it with a set amount of aniline uniformly, then transfer the mixed solution into the high-pressure reactor, seal the high-pressure reactor and place it in a magnetic heating stirrer. First, introduce oxygen to check the air tightness of the high-pressure reactor; if there is no oxygen leakage, discharge the oxygen in the reactor, perform gas charging and discharging three times to ensure that there is no air in the reactor, and finally adjust the pressure of the high-pressure reactor to a set value. Adjust the rotating speed of the magnetic stir bar to the set rotating speed, and start timing when the temperature of the reactor rises to the set temperature. After the reaction is completed, take out the reactor, let its temperature drop to room temperature, then open the gas valve of the high-pressure reactor to discharge oxygen until the pressure is 0 MPa, and finally open the high-pressure reactor to take out the reaction solution. Extract and filter the reaction solution with a filter needle, then draw 0.4 μL of the filtrate from the filtrate with a micro syringe for gas chromatography, and conduct component analysis by gas chromatography.

[0028] The gas chromatography H2 flow rate was set to 50-80 mL / min, the air flow rate to 200-300 mL / min, the carrier gas flow rate to 10 mL / min, the injection temperature to 280.0 ℃, the initial column furnace temperature to 80 ℃ for 6 min, then increased to 180 ℃ at 10 ℃ / min for 5 min, then increased to 280 ℃ at 20 ℃ / min for 5 min, and the FID temperature to 280.0 ℃.

[0029] Example 1: Preparation of δ-MnO2 A 1 mol / L potassium permanganate solution and a 0.8 mol / L manganese sulfate solution were preheated to 85 °C. Then, the potassium permanganate solution was simultaneously added to the reactor of a microfluidic reactor at a feed rate of 60 g / min and the manganese sulfate solution at a feed rate of 100 g / min for 1 h. The reaction products were collected and allowed to stand for 24 h. After filtration, the products were washed with deionized water until the pH of the filtrate was 7. The filter cake was then placed in an 80 °C constant temperature drying oven for thorough drying. After cooling to room temperature, the cake was ground to obtain a nano-flower-like δ-crystalline manganese dioxide catalyst.

[0030] The performance of the δ-MnO2 catalyst prepared in Example 1 is as follows: Under the reaction conditions of 1 MPa oxygen pressure, 353 K temperature, 100 mg catalyst, 20 mL toluene and 4.0 mmol aniline, and 550 rpm stirring speed, the catalyst achieved a 37.4% conversion rate of aniline after 0.25 h of reaction, a 99.9% conversion rate after 10 h of reaction, and an azobenzene selectivity of over 99.9%, and can be recycled four times.

[0031] Example 2: Preparation of δ-MnO2 A 0.3 mol / L potassium permanganate solution and a 0.5 mol / L manganese sulfate solution were preheated to 85°C. Then, the potassium permanganate solution was simultaneously added to the reactor of a microfluidic reactor at a feed rate of 60 g / min and the manganese sulfate solution was simultaneously added at a feed rate of 100 g / min to obtain δ-MnO2.

[0032] Under reaction conditions of 1 MPa oxygen pressure, 353 K temperature, 100 mg catalyst, 20 mL toluene, and 4.0 mmol aniline, and a stirring speed of 550 rpm, the catalyst achieved a conversion rate of 83.3% for aniline and a selectivity of 100% for azobenzene after 10 h of reaction. The application of the α-MnO2 catalyst in this example followed the same reaction performance testing conditions and procedures as in Example 1.

[0033] Example 3: Preparation of α-MnO2 A 0.3 mol / L potassium permanganate solution and a 0.5 mol / L manganese sulfate solution were preheated to 60 °C. Then, the potassium permanganate solution was simultaneously added to the reactor of a microfluidic reactor at a feed rate of 60 g / min and the manganese sulfate solution was simultaneously added at a feed rate of 100 g / min to obtain α-MnO2.

[0034] Under reaction conditions of 1 MPa oxygen pressure, 353 K temperature, 100 mg catalyst, 20 mL toluene, and 4.0 mmol aniline, and a stirring speed of 550 rpm, the catalyst achieved a conversion rate of 80.3% for aniline and a selectivity of 100% for azobenzene after 10 h of reaction. The application of the α-MnO2 catalyst in this example followed the same reaction performance testing conditions and procedures as in Example 1.

[0035] Example 4: Preparation of γ-MnO2 A 0.3 mol / L potassium permanganate solution and a 0.5 mol / L manganese sulfate solution were preheated to 90°C. Then, the potassium permanganate solution was simultaneously added to the reactor of a microfluidic reactor at a feed rate of 65 g / min and the manganese sulfate solution was simultaneously added at a feed rate of 100 g / min to obtain γ-MnO2.

[0036] Under reaction conditions of 1 MPa oxygen pressure, 353 K temperature, 100 mg catalyst, 20 mL toluene, and 4.0 mmol aniline, and a stirring speed of 550 rpm, the catalyst achieved a 50.2% conversion rate for aniline and 100% selectivity for azobenzene after 10 h of reaction. The application of the γ-MnO2 catalyst in this example followed the same reaction performance testing conditions and procedures as in Example 1.

[0037] The catalysts used in Examples 5 to 11 below are all δ-MnO2 catalysts prepared in Example 1.

[0038] Example 5: The amount of aniline added was 4.0 mmol. Under reaction conditions of 1 MPa oxygen pressure, 353 K temperature, 100 mg δ-MnO2, 20 mL toluene, and 550 rpm stirring speed, the catalyst achieved a 37.4% conversion rate of aniline and a selectivity of over 99.9% for azobenzene after 0.25 h of reaction. The reaction performance testing conditions and procedures for the application of the δ-MnO2 catalyst were the same as in Example 1.

[0039] Example 6: The amount of aniline added was 2.0 mmol. Under reaction conditions of 1 MPa oxygen pressure, 353 K temperature, 100 mg δ-MnO2, 20 mL toluene, and 550 rpm stirring speed, the catalyst achieved a 25.6% conversion rate for aniline and a selectivity for azobenzene exceeding 99.9% after 0.25 h of reaction. The reaction performance testing conditions and procedures for the application of the δ-MnO2 catalyst were the same as in Example 1.

[0040] Example 7: The amount of aniline added was 8.0 mmol. Under reaction conditions of 1 MPa oxygen pressure, 353 K temperature, 100 mg δ-MnO2, 20 mL toluene, and 8.0 mmol aniline, and a stirring speed of 550 rpm, the catalyst achieved a 46.5% conversion rate for aniline and a selectivity for azobenzene exceeding 99.9% after 0.25 h of reaction. The reaction performance testing conditions and procedures for the application of the δ-MnO2 catalyst were the same as in Example 1.

[0041] Example 8: Reaction temperature was 343 K Under reaction conditions of 1 MPa oxygen pressure, 343 K temperature, 100 mg δ-MnO2, 20 mL toluene, and 4.0 mmol aniline, and a stirring speed of 550 rpm, the catalyst achieved a 24.1% conversion rate for aniline and a selectivity for azobenzene exceeding 99.9% after 0.25 h of reaction. The reaction performance testing conditions and procedures for the application of the δ-MnO2 catalyst were the same as in Example 1.

[0042] Example 9: Reaction temperature was 363K Under reaction conditions of 1 MPa oxygen pressure, 363 K temperature, 100 mg δ-MnO2, 20 mL toluene, and 4.0 mmol aniline, and a stirring speed of 550 rpm, the catalyst achieved a 44.3% conversion rate for aniline and a selectivity for azobenzene exceeding 99.9% after 0.25 h of reaction. The reaction performance testing conditions and procedures for the application of the δ-MnO2 catalyst were the same as in Example 1.

[0043] Example 10: Oxygen reaction pressure is 0.5 MPa Under reaction conditions of 0.5 MPa oxygen pressure, 353 K temperature, 100 mg MnO2, 20 mL toluene, 4.0 mmol aniline, and 550 rpm stirring speed, the catalyst achieved a 35.3% conversion rate for aniline and a selectivity for azobenzene exceeding 99.9% after 0.25 h of reaction. The reaction performance testing conditions and procedures for the δ-MnO2 catalyst were the same as in Example 1.

[0044] Example 11: Oxygen reaction pressure is 2 MPa Under reaction conditions of 2 MPa oxygen pressure, 353 K temperature, 100 mg δ-MnO2, 20 mL toluene, 4.0 mmol aniline, and 550 rpm stirring speed, the catalyst achieved a 39.3% conversion rate for aniline and a selectivity for azobenzene exceeding 99.9% after 0.25 h of reaction. The reaction performance testing conditions and procedures for the δ-MnO2 catalyst were the same as in Example 1.

[0045] Comparative Example 1 The blank experiment without catalyst was conducted under the same reaction conditions as in Example 1 without adding any catalyst. After 0.25 h of reaction, the aniline conversion rate was 0 and the azobenzene selectivity was 0.

[0046] Comparative Example 2 The catalytic performance of commercial δ-MnO2 was tested using commercially available δ-MnO2 (Shaanxi Zhiju New Material Technology Co., Ltd.) as the catalyst, under the same reaction conditions as in Example 1. The aniline conversion rate was 11.3% and the azobenzene selectivity was 99.9% after 0.25 h of reaction.

[0047] Comparative Example 3 The catalytic performance of δ-MnO2 prepared by the hydrothermal method was tested using δ-MnO2 prepared by the traditional hydrothermal method under the same reaction conditions as in Example 1. The aniline conversion rate was 21.7% and the azobenzene selectivity was 99.9% after 0.25 h of reaction.

[0048] Comparative Example 4 The catalytic performance under air atmosphere was tested. Using air instead of pure oxygen as the oxidant, the reaction was carried out under the same temperature and pressure conditions. The aniline conversion rate was 16.7% and the azobenzene selectivity was 99.9% after 0.25 h of reaction.

[0049] Figure 1 The images show transmission electron microscopy (TEM) images of three different crystalline forms of γ-MnO2 catalysts. As can be seen from the images, Figure 1 (a) The δ-MnO2 catalyst has a spherical daisy structure composed of stacked nanoflower-like structures. Figure 1 (b) The α-MnO2 catalyst exhibits a nanofiber structure. Figure 1 (c) The γ-MnO2 catalyst exhibits a grain-like structure. Morphologically, the δ-MnO2 catalyst possesses a spherical daisy-like structure composed of stacked nanoflower-like structures. Compared to α-MnO2 and γ-MnO2 catalysts, this structure exposes more active sites and increases the contact area between the catalyst and the substrate aniline and oxygen. Therefore, the catalytic activity of the δ-MnO2 catalyst is higher than that of the α-MnO2 and γ-MnO2 catalysts.

[0050] The catalytic performance of three manganese dioxide catalysts with different crystal forms for aniline under the same reaction conditions is as follows: Figure 2 As shown in Table 1.

[0051] Table 1 Catalytic performance of three MnO2 catalysts for aniline

[0052] The reaction conditions in Table 1 are: 4.0 mmol aniline, 100 mg catalyst, reaction temperature 353 K, oxygen pressure 1 MPa, 20 mL toluene, and reaction time 10 h.

[0053] Based on the data in Table 1 and Figure 2 Analysis of the catalytic effect curves showed that the δ-MnO2 catalyst could achieve complete catalytic conversion of aniline to azobenzene at a reaction time of 10 h, indicating that δ-MnO2 had the best performance under the same reaction conditions.

[0054] The experimental performance of the catalytic stability of three manganese dioxide catalysts with different crystal forms for aniline under the same reaction conditions is as follows: Figure 3 As shown in Table 2.

[0055] Table 2 Catalytic performance of three MnO2 catalysts for aniline

[0056] The reaction conditions in Table 2 are: 4.0 mmol aniline, 100 mg catalyst, reaction temperature 353 K, oxygen pressure 1 MPa, 20 mL toluene, and reaction time 0.25 h.

[0057] pass Figure 3 According to the data analysis in Table 2, the δ-MnO2 catalyst showed the best catalytic stability performance for aniline under the same reaction conditions. After four cycles of use, its activity and selectivity did not decrease significantly, and it was far superior to α-MMnO2 and γ-MnO2.

[0058] Table 3 shows the catalytic performance of δ-MnO2 catalyst for aniline under different temperature reaction conditions, as demonstrated by Examples 1, 8, and 9.

[0059] Table 3 Catalytic performance of δ-MnO2 on aniline at different temperatures

[0060] Analysis of the data in Table 3 shows that, under the same reaction conditions, the conversion rate of aniline by the δ-MnO2 catalyst continuously increases with increasing reaction temperature, while the selectivity for azobenzene remains at a high level. No significant over-oxidation side reactions are observed in the 343–363 K range, indicating that this catalyst possesses excellent selective catalytic ability. Based on the reaction rates at different temperatures, the apparent activation energy of this reaction is calculated to be 13.9 kJ / mol, which is far lower than most reported catalytic systems, indicating that this catalyst can effectively lower the reaction energy barrier and drive the reaction efficiently under mild conditions.

[0061] Data analysis from Examples 1, 10, and 11 showed that the initial reaction rate of the δ-MnO2 catalyst remained relatively stable within an oxygen pressure range of 0.5–2 MPa, with azobenzene selectivity consistently above 99.9%. Kinetic order fitting results indicated that the reaction order was approximately 0 under oxygen conditions, suggesting that oxygen concentration had no effect on the reaction rate.

[0062] Data analysis from Examples 1, 8, and 9 shows that the initial reaction rate increases approximately linearly with increasing aniline concentration, and the substrate reaction order is approximately 1.04. This indicates that the reaction is a first-order reaction with respect to substrate concentration and maintains excellent product selectivity even at high substrate concentrations, making it suitable for the needs of industrial high-concentration production.

[0063] The above description is only a partial embodiment of the present invention and is not intended to limit the present invention. Any equivalent changes and modifications made based on the content of this invention are within the protection scope of this invention.

Claims

1. Application of nano-flower-like delta-crystalline manganese dioxide in the catalytic preparation of azobenzene from aniline.

2. Use according to claim 1, characterized in that, Azobenzene was prepared by catalyzing the oxidative coupling reaction of aniline in an oxygen atmosphere using nano-flower-like delta-crystalline manganese dioxide as a catalyst and toluene as a solvent.

3. Use according to claim 2, characterized in that, In this catalytic reaction, when the reaction temperature is 343~363 K, the oxygen pressure is 0.5~2 MPa, and the reaction time is 0.25~10 h, the conversion rate of aniline is 13.3%~99.9%, and the selectivity of the target product azobenzene is above 99.9%.

4. Use according to claim 3, characterized in that, In this catalytic reaction, under the conditions of reaction temperature 353 K and oxygen pressure 1 MPa, the aniline conversion rate was 37.4% after 0.25 h of reaction and reached 99.9% after 10 h of reaction. The selectivity of the target product azobenzene remained above 99.9%.

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