Process for the synthesis of pyridine bases based on MOFs derived catalysts

Abstract

This invention discloses a method for synthesizing pyridine bases based on MOF-derived catalysts, comprising the following steps: A Zn-based MOF precursor is generated by reacting Zn(NO3)2·6H2O and 2-methylimidazole to obtain pure ZIF-8 crystalline powder, which is then carbonized in a tubular muffle furnace to generate a Zn-N-C porous carbon catalyst, yielding the target product, the Zn-N-C catalyst; a precursor ZIF-67@Fe is generated by reacting Fe(NO3)2·6H2O and 2-methylimidazole and then carbonized in a tubular muffle furnace to obtain the target product, the Fe-N-C catalyst; gasified acetonitrile and ammonia are initially mixed and then introduced into the main reaction zone loaded with MOF-derived Zn-N-C or Fe-N-C catalysts for reaction. This invention utilizes MOF-derived Zn-N-C or Fe-N-C porous carbon supported catalysts, and through optimized ratios of MOF-derived catalysts and acetonitrile to ammonia, improves reaction conversion efficiency and reduces byproducts and pollution emissions through a green oxidation process.

Description

Technical Field

[0001] This invention relates to the fields of organic chemical synthesis and catalyst technology, and particularly to a method for synthesizing pyridine bases based on MOF-derived catalysts. Background Technology

[0002] Pyridine and its basic derivatives have wide applications in the pharmaceutical, pesticide, and chemical industries. The traditional Bönnemann method is a synthetic method that uses low-valence cobalt complexes and hydrogen peroxide or transition metal oxides as oxidants to directly construct the pyridine core by cyclizing three molecules of nitrile compounds in a [2+2+2] manner. However, conventional catalysts have limited activity, and the selectivity of pyridine bases is less than 50%, resulting in a large number of byproducts.

[0003] The Chichibabin synthesis is a classic method for pyridine synthesis, typically using aldehydes (or ketones) and ammonia as the main reactants. Amines can be used as alternative nitrogen sources for ammonia to efficiently construct the skeleton of pyridine and its derivatives. A condensation reaction occurs under high temperature and acidic catalytic conditions, directly generating a pyridine ring or its substituted derivatives. Nucleophilic addition occurs between the aldehyde (or ketone) and ammonia molecules, followed by dehydration to form an imine intermediate. The two imine intermediate molecules undergo further dehydration and condensation reactions, gradually forming a conjugated pyridine ring system. The Chichibabin synthesis usually requires high temperatures of 400-500℃ and acidic conditions, which may cause decomposition of some sensitive substrates, thus requiring certain heat resistance of the substrates. The reaction process generates a large amount of waste liquid, requiring wastewater treatment, increasing environmental pressure and treatment costs. Chinese Patent Publication No. CN113600228A discloses a method for preparing pyridine bases from ammonia and carbonyl compounds using molecular sieve catalysis, employing a novel molecular sieve catalyst aimed at improving the selectivity and yield of pyridine and methylpyridine. Although this patent improves the selectivity of pyridine base compounds through catalyst regeneration and fluidized bed technology, it suffers from problems such as high-temperature conditions (400℃), catalyst deactivation and carbon deposition, and difficulties in product separation. Especially at 200h... - ¹Up to 1800h - ¹ Under high space velocity conditions, the activity of the catalyst may decrease rapidly. To improve the economics and industrialization prospects of this process, further optimization of reaction conditions, catalyst selectivity, and stability is needed.

[0004] The Hantzsch pyridine synthesis process involves the condensation of simple hydrocarbons with ammonia, carbamates (or other reactive imine compounds) to generate polysubstituted dihydropyridines with functionalization potential, which are then oxidized to yield the pyridine ring. The Hantzsch method initially generates a 1,4-dihydropyridine intermediate, rather than directly yielding an aromatic pyridine ring. This means that an additional oxidation step is usually required to convert the 1,4-dihydropyridine to a fully aromatic pyridine. The oxidation of 1,4-dihydropyridine requires careful selection of the oxidant and reaction conditions. Chromium trichloride or potassium permanganate are commonly used as oxidants, but this introduces additional reagent costs and challenges in waste disposal.

[0005] Chinese patent CN104761486B discloses a method for synthesizing pyridine bases via glycerol amination. While this process offers the advantage of utilizing inexpensive glycerol to prepare high-value-added pyridine bases, it also presents several challenges due to the high-temperature reaction conditions: the method requires temperatures ranging from 420°C to 550°C. This not only increases energy consumption and production costs but also places higher demands on the heat resistance of the equipment. In industrial production, prolonged high-temperature operation may accelerate equipment aging and increase maintenance costs.

[0006] In summary, the following problems exist in the existing technology and need to be addressed:

[0007] 1. Insufficient catalyst activity and selectivity: In the traditional Bönnemann pyridine synthesis method, the catalyst has low activity and selectivity, resulting in low yield and the generation of a large number of by-products.

[0008] 2. Poor cycle stability of catalysts: Traditional catalysts suffer severe activity degradation after repeated use, failing to meet the needs of industrial production.

[0009] 3. Insufficient green and environmental protection: Traditional methods may use relatively dangerous oxidants such as organic oxidants or peroxides, which pose safety hazards.

[0010] 4. High energy consumption: Traditional pyridine synthesis methods often require high temperature conditions, resulting in high energy consumption. Summary of the Invention

[0011] The purpose of this invention is to provide a method for synthesizing pyridine bases based on MOFs-derived catalysts, which significantly improves the efficiency and selectivity of pyridine synthesis and provides an efficient and green pyridine base synthesis process to solve the problems mentioned in the background art.

[0012] To achieve the above objectives, the present invention provides the following technical solution:

[0013] The method for synthesizing pyridine bases based on MOF-derived catalysts includes the following steps:

[0014] Step 1: Dissolve component A in deionized water to generate Zn-based MOF precursor. Centrifuge to obtain solid precursor A, dry to obtain pure ZIF-8 crystal powder, put ZIF-8 precursor into a tube muffle furnace for carbonization to generate Zn-NC porous carbon catalyst. Wash the carbonization product with dilute hydrochloric acid and dry to obtain the target product Zn-NC catalyst.

[0015] Component B was dissolved in ultrapure water to form a suspension. After centrifugation, solid precursor B was obtained. Solid precursor B was washed with ultrapure water and rinsed with methanol and then dried to obtain precursor ZIF-67@Fe. Precursor ZIF-67@Fe was placed in a tube muffle furnace for carbonization and dried to obtain the target product Fe-NC catalyst.

[0016] Step 2: The vaporized acetonitrile and ammonia are transported to the premixing section. After the two gases are initially mixed in the premixing section, they enter the main reaction zone. The main reaction zone is filled with MOF-derived Zn-NC or Fe-NC catalyst. During the reaction, samples are taken every half hour and analyzed by high performance liquid chromatography to record the contents of pyridine, 2-methylpyridine and 3-methylpyridine.

[0017] Further, component A consists of Zn(NO3)2·6H2O and 2-methylimidazole, with a mass ratio of Zn(NO3)2·6H2O to 2-methylimidazole of 1:4.

[0018] Further, component B consists of Fe(NO3)2·6H2O and 2-methylimidazole, with a mass ratio of Fe(NO3)2·6H2O to 2-methylimidazole of 5:42.

[0019] Furthermore, the preparation process of the target product Zn-NC catalyst is as follows:

[0020] Zn(NO3)2·6H2O and 2-methylimidazole were dissolved in 100 mL of deionized water at a ratio of 1:4. The mixture was stirred at 25°C for 12 hours to generate a uniformly suspended Zn-based MOF precursor. The solid precursor in the suspension was separated by centrifugation to obtain solid precursor A, which was dried at 60°C for 24 hours to obtain pure ZIF-8 crystalline powder. The ZIF-8 precursor was placed in a tube furnace and heated to a certain temperature at a controlled heating rate under a nitrogen atmosphere, and held at that temperature for 2 hours to generate a Zn-NC porous carbon catalyst. The carbonization product was washed with dilute hydrochloric acid to remove residual zinc, and then dried at 120°C for 10 hours to obtain the target product, the Zn-NC catalyst.

[0021] Furthermore, the preparation process of the target product Fe-NC catalyst is as follows:

[0022] Fe(NO3)2·6H2O and 2-methylimidazole were dissolved in 90 mL of ultrapure water and stirred at room temperature for 12 h. After stirring, a uniform blue-brown suspension was formed. The suspension was centrifuged to obtain the desired solid precursor B. The solid precursor was then washed three times with ultrapure water to remove unreacted 2-methylimidazole. The solvent was removed by rinsing with methanol. After rinsing, the precursor was transferred to a vacuum drying oven and treated at 70 °C for 24 h to obtain the precursor ZIF-67@Fe. The precursor ZIF-67@Fe was placed in a tube muffle furnace and heated to a certain temperature at a certain heating rate under a nitrogen atmosphere. After maintaining the temperature for 3 hours, it was cooled to room temperature at a programmed rate of 12 °C / min to complete the carbonization process. The carbonized product was ultrasonically vibrated with dilute hydrochloric acid solution to remove residual impurities. Finally, after drying, the target product Fe-NC type catalyst was obtained.

[0023] Furthermore, the molar ratio of acetonitrile to ammonia is 3:1. The vaporized acetonitrile and ammonia are respectively transported to the premixing section through a precise metering device, and oxygen is introduced to promote the oxidation reaction. The vaporized acetonitrile enters the reaction tube through the outer pipe, and ammonia is injected through the inner sleeve.

[0024] Furthermore, the carbonization temperature in S1 is 750°C-950°C.

[0025] Furthermore, the heating rate is 5°C / min-15°C / min, and the atmosphere is nitrogen and other rare gases.

[0026] Furthermore, the molar ratio of acetonitrile, ammonia, and oxygen is 3:1:1 to 3:1:5.

[0027] Furthermore, the oxygen flow rate is 0.5-1 L / min, the reaction temperature is controlled at 200°C-250°C, the reaction pressure is 0.1-0.8 MPa, and the reaction time is 1-10 s.

[0028] Zn-NC catalysts can improve acetonitrile conversion, pyridine base selectivity, and pyridine base yield, mainly due to their unique structure and electronic properties:

[0029] 1. Improve acetonitrile conversion rate

[0030] Zn-NC catalysts provide highly efficient active sites, promoting the adsorption and activation of acetonitrile on the catalyst surface. Their main contributions include:

[0031] Lewis acidity of Zn species: As a Lewis acid center, Zn can enhance the polarization of acetonitrile, making it easier to activate and increasing the reaction rate.

[0032] N coordination effect: The Zn-N coordination structure helps stabilize Zn species, optimize the electronic structure of the catalyst, and thus improve catalytic activity.

[0033] Porous carbon support: Porous N-doped carbon not only provides a high specific surface area, which is beneficial to the diffusion of reactants, but also modulates the electronic structure and improves catalytic efficiency.

[0034] 2. Improve the selectivity of pyridine bases

[0035] The improved selectivity of pyridine bases mainly depends on the catalyst's ability to regulate the reaction pathway:

[0036] Regulatory role of Zn species: The Zn-NC structure can promote the coupling reaction of acetonitrile while reducing the formation of byproducts (such as amides or polymerization byproducts).

[0037] Electronic effect: N-doped carbon modulates the electronic environment of Zn species, making acetonitrile more likely to participate in cyclization reactions to form pyridine bases, rather than undergoing other side reactions.

[0038] Pore ​​effect: Appropriate pore structure can restrict the displacement of reaction intermediates, enabling the selective synthesis of pyridine bases at the optimal active site.

[0039] 3. Improve the yield of pyridine bases

[0040] The improvement in pyridine base yield is closely related to the stability and reusability of the catalyst:

[0041] Zn-NC structural stability: Zn obtains a stable structure through N coordination, avoiding the loss of Zn species and improving the long-term stability and recycling capacity of the catalyst.

[0042] The role of carbon support: Nitrogen-doped carbon enhances the catalyst's resistance to carbon deposition, reduces the catalyst deactivation rate, and improves the catalyst lifespan.

[0043] Optimized reaction environment: Zn-NC catalysts can achieve efficient catalysis under milder conditions, avoiding side reactions or catalyst structure damage caused by excessively high temperatures.

[0044] Compared with the prior art, the beneficial effects of the present invention are:

[0045] This invention utilizes MOF-derived Zn-NC or Fe-NC porous carbon-supported catalysts. Through optimized ratios of MOF-derived catalysts, acetonitrile, and ammonia, and by applying porous carbon support and acid washing post-treatment to the MOF-derived catalysts, the selectivity and conversion efficiency of pyridine are improved. The catalysts exhibit good cycle stability, allowing for repeated use and significantly reducing production costs. The porous structure and uniformly distributed active sites of the catalysts enhance the conversion efficiency of the reaction. Utilizing oxygen as a green oxidant reduces byproducts and pollution emissions through a green oxidation process. By using highly active MOF-derived catalysts, this invention enables efficient pyridine synthesis under relatively mild conditions, reducing the energy consumption required for the reaction. Detailed Implementation

[0046] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0047] Example 1: 10g of ZIF-8 precursor powder was placed in a tubular muffle furnace and heated to 800°C at a heating rate of 5°C / min under nitrogen protection, and held at this temperature for 2 hours. After carbonization, the powder was washed with dilute hydrochloric acid to remove incompletely carbonized impurities, and then dried at 120°C to obtain a Zn-NC catalyst, labeled Cat-A. The catalyst has a specific surface area of ​​750 m² / g and a pore size of approximately 12 Å, forming abundant Zn-N active sites, which helps to improve the synthesis efficiency of pyridine bases.

[0048] Example 2: 10 g of precursor ZIF-67@Fe powder was carbonized and heated to 800°C at a heating rate of 5°C / min under a nitrogen atmosphere, and held at that temperature for 3 hours. Subsequently, it was cooled to room temperature under a nitrogen atmosphere at a programmed cooling rate of 12°C / min to obtain the carbonized product. After carbonization, unreacted impurities were removed by washing with 0.5 mol / L dilute hydrochloric acid, and the product was dried to obtain the Fe-NC catalyst, labeled Cat-B. The catalyst had a specific surface area of ​​710 m² / g and a pore size of approximately 10 Å, forming abundant Fe-N active sites, suitable for promoting the oxidative coupling reaction of acetonitrile and ammonia.

[0049] Example 3: 150 g of Cat-A catalyst was placed in a fixed-bed reactor. Acetonitrile and ammonia, at a molar ratio of 3:1, were precisely metered and introduced into the premixing section of the reactor. Oxygen was introduced to assist the reaction (flow rate 0.5 L / min). The reaction temperature was set at 200°C, the pressure at 0.1 MPa, and the reaction time at 5 s. In the fixed-bed reactor, vaporized acetonitrile and ammonia were precisely metered and introduced into the catalyst bed. The reaction products were collected using a condensation system. The concentration of pyridine bases was analyzed by high-performance liquid chromatography (HPLC), and the purity and yield of the products were determined by external standard method. The conversion rate of acetonitrile reached 92.1%, and the yield of pyridine bases was 85.2%. The pyridine bases included pyridine, 2-methylpyridine, and 3-methylpyridine. The yield of pyridine was 63.1%, the yield of 2-methylpyridine was 18.6%, and the yield of 3-methylpyridine was 3.5%. The catalyst could be reused after simple cleaning and showed good stability.

[0050] Example 4: 150 g of Cat-B catalyst was placed in a fixed-bed reactor with an acetonitrile to ammonia molar ratio of 3:1. Oxygen was introduced to assist the reaction (flow rate 0.8 L / min), the temperature was set at 250°C, and the pressure at 0.3 MPa. The reaction time was 4 s. In the fixed-bed reactor, vaporized acetonitrile and ammonia were precisely metered and introduced into the catalyst bed, and the reaction products were collected using a condensation system. The concentration of pyridine base was analyzed by high-performance liquid chromatography (HPLC), and the purity and yield of the products were determined by external standard method. The conversion of acetonitrile reached 93.2%, and the yield of pyridine base was 87.1%. The pyridine base included pyridine, 2-methylpyridine, and 3-methylpyridine. The yield of pyridine was 63.2%, the yield of 2-methylpyridine was 20.6%, and the yield of 3-methylpyridine was 3.3%. This catalyst exhibited good cycling stability and is suitable for industrial scale-up.

[0051] Example 5: Using 5g of Cat-A catalyst, the temperature was set at 225°C and the pressure at 0.5 MPa. Acetonitrile and ammonia were introduced into the reactor at a molar ratio of 3:1, injected separately through external pipes, while oxygen was introduced into the internal pipe (flow rate 0.5 L / min). The reaction time was 5 s. In a fixed-bed reactor, vaporized acetonitrile and ammonia were precisely metered and introduced into the catalyst bed, and the reaction products were collected using a condensation system. The concentration of pyridine bases was analyzed by high-performance liquid chromatography (HPLC), and the purity and yield of the products were determined by external standard method. The conversion rate of acetonitrile reached 95.1%, and the yield of pyridine bases was 88.4%. The pyridine bases included pyridine, 2-methylpyridine, and 3-methylpyridine. The yield of pyridine was 66.1%, the yield of 2-methylpyridine was 18.3%, and the yield of 3-methylpyridine was 4.0%. The participation of oxygen improved the yield of pyridine bases.

[0052] Example 6: 5 g of Cat-B catalyst was placed in a fixed-bed reactor. The reaction temperature was controlled at 250°C, the pressure at 0.8 MPa, and the molar ratio of acetonitrile to ammonia was 3:1. Oxygen was introduced to assist the reaction (flow rate 0.5 L / min). The reaction time was 5 s. In the fixed-bed reactor, vaporized acetonitrile and ammonia were precisely metered and introduced into the catalyst bed. The reaction products were collected using a condensation system. The concentration of pyridine base was analyzed by high-performance liquid chromatography (HPLC), and the purity and yield of the products were determined by external standard method. The conversion of acetonitrile was increased to 96.1%, and the yield of pyridine base was 89.3%. The pyridine base included pyridine, 2-methylpyridine, and 3-methylpyridine. The yield of pyridine was 68.2%, the yield of 2-methylpyridine was 19.1%, and the yield of 3-methylpyridine was 2.0%. The Fe-NC catalyst exhibited excellent catalytic stability and high yield.

[0053] Example 7: Using 5g of Cat-A catalyst, the temperature was set at 250°C and the pressure at 0.5 MPa. Acetonitrile and ammonia were introduced into the reactor at a molar ratio of 3:1, injected separately through external pipes, while oxygen was introduced through the internal pipe (flow rate 0.5 L / min). The reaction time was 5 s. In a fixed-bed reactor, vaporized acetonitrile and ammonia were precisely metered and introduced into the catalyst bed, and the reaction products were collected using a condensation system. The concentration of pyridine bases was analyzed by high-performance liquid chromatography (HPLC), and the purity and yield of the products were determined by external standard method. The conversion of acetonitrile reached 96.6%, and the yield of pyridine bases was 89.6%. The pyridine bases included pyridine, 2-methylpyridine, and 3-methylpyridine. The yield of pyridine was 67.3%, the yield of 2-methylpyridine was 18.2%, and the yield of 3-methylpyridine was 4.1%. The participation of oxygen improved the yield of pyridine bases.

[0054] Example 8: Using 5g of Cat-A catalyst, the temperature was set at 250°C and the pressure at 0.5 MPa. Acetonitrile and ammonia were introduced into the reactor at a molar ratio of 3:1, injected separately through external pipes, while oxygen was introduced into the internal pipe (flow rate 0.5 L / min). The reaction time was 5 s. In a fixed-bed reactor, vaporized acetonitrile and ammonia were precisely metered and introduced into the catalyst bed, and the reaction products were collected using a condensation system. The concentration of pyridine bases was analyzed by high-performance liquid chromatography (HPLC), and the purity and yield of the products were determined by external standard method. The conversion of acetonitrile reached 95.9%, and the yield of pyridine bases was 88.5%. The pyridine bases included pyridine, 2-methylpyridine, and 3-methylpyridine. The yield of pyridine was 66.1%, the yield of 2-methylpyridine was 18.3%, and the yield of 3-methylpyridine was 4.1%. The participation of oxygen improved the yield of pyridine bases.

[0055] Example 9: Using 5g of Cat-A catalyst, the temperature was set at 250°C and the pressure at 0.1 MPa. Acetonitrile and ammonia were introduced into the reactor at a molar ratio of 3:1, injected separately through external pipes, while oxygen was introduced into the internal pipe (flow rate 1 L / min). The reaction time was 5 s. In a fixed-bed reactor, vaporized acetonitrile and ammonia were precisely metered and introduced into the catalyst bed, and the reaction products were collected using a condensation system. The concentration of pyridine bases was analyzed by high-performance liquid chromatography (HPLC), and the purity and yield of the products were determined by external standard method. The conversion rate of acetonitrile reached 96.8%, and the yield of pyridine bases was 89.2%. The pyridine bases included pyridine, 2-methylpyridine, and 3-methylpyridine. The yield of pyridine was 67.1%, the yield of 2-methylpyridine was 18.1%, and the yield of 3-methylpyridine was 4.0%. The participation of oxygen improved the yield of pyridine bases.

[0056] Example 10: 5 g of Cat-B catalyst was placed in a fixed-bed reactor. The reaction temperature was controlled at 220°C, the pressure at 0.8 MPa, and oxygen was introduced to assist the reaction (flow rate 0.8 L / min). The molar ratio of acetonitrile to ammonia was 3:1. The reaction time was 5 s. In the fixed-bed reactor, vaporized acetonitrile and ammonia were precisely metered and introduced into the catalyst bed. The reaction products were collected using a condensation system. The concentration of pyridine bases was analyzed by high-performance liquid chromatography (HPLC), and the purity and yield of the products were determined by external standard method. The conversion rate of acetonitrile was increased to 95.1%, and the yield of pyridine bases was 88.3%. Pyridine bases included pyridine, 2-methylpyridine, and 3-methylpyridine. The yield of pyridine was 67.1%, the yield of 2-methylpyridine was 19.1%, and the yield of 3-methylpyridine was 2.1%.

[0057] Example 11: 5 g of Cat-B catalyst was placed in a fixed-bed reactor. The reaction temperature was controlled at 250°C, the pressure at 0.8 MPa, and oxygen was introduced to assist the reaction (flow rate 0.8 L / min). The molar ratio of acetonitrile to ammonia was 3:1. The reaction time was 5 s. In the fixed-bed reactor, vaporized acetonitrile and ammonia were precisely metered and introduced into the catalyst bed. The reaction products were collected using a condensation system. The concentration of pyridine bases was analyzed by high-performance liquid chromatography (HPLC), and the purity and yield of the products were determined by external standard method. The conversion rate of acetonitrile was increased to 95.3%, and the yield of pyridine bases was 88.6%. Pyridine bases included pyridine, 2-methylpyridine, and 3-methylpyridine. The yield of pyridine was 65.8%, the yield of 2-methylpyridine was 18.6%, and the yield of 3-methylpyridine was 2.2%.

[0058] Example 12: 5 g of Cat-B catalyst was placed in a fixed-bed reactor. The reaction temperature was controlled at 250°C, the pressure at 0.8 MPa, and oxygen was introduced to assist the reaction (flow rate 1 L / min). The molar ratio of acetonitrile to ammonia was 3:1. The reaction time was 5 s. In the fixed-bed reactor, vaporized acetonitrile and ammonia were precisely metered and introduced into the catalyst bed. The reaction products were collected using a condensation system. The concentration of pyridine bases was analyzed by high-performance liquid chromatography (HPLC), and the purity and yield of the products were determined by external standard method. The conversion rate of acetonitrile was increased to 96.0%, and the yield of pyridine bases was 89.3%. The pyridine bases included pyridine, 2-methylpyridine, and 3-methylpyridine. The yield of pyridine was 67.9%, the yield of 2-methylpyridine was 19.8%, and the yield of 3-methylpyridine was 2.6%.

[0059] The Cat-A catalyst was used in five consecutive reactions. After each reaction, the catalyst was washed with deionized water, dried, and reused. In a fixed-bed reactor, vaporized acetonitrile and ammonia were precisely metered and introduced into the catalyst bed, and the reaction products were collected using a condensation system. The concentration of pyridine base was analyzed by high-performance liquid chromatography (HPLC), and the purity and yield of the products were determined by external standard method.

[0060] The cycling results of the Cat-A catalyst after 5 cycles are shown in Table 1 below.

[0061] Table 1. Cycling results of Cat-A catalyst

[0062] Loop count Acetonitrile conversion rate (%) Pyridine base selectivity (%) Pyridine base yield (%) 1 1st time 92.1 92.5 85.2 2 2nd time 95.1 93.0 88.4 3 3rd 96.6 92.7 89.6 4 4th 95.9 92.3 88.5 5 5th 96.8 92.1 89.2

[0063] Table 1 shows that the average conversion rate of acetonitrile was 95.3%, and the selectivity of pyridine base remained above 92%, indicating that the Zn-NC catalyst has good cycle stability. The reaction mechanism is that the Zn-NC catalyst provides highly efficient active sites, promoting the adsorption and activation of acetonitrile on the catalyst surface. The improved selectivity of pyridine base mainly depends on the catalyst's ability to regulate the reaction pathway: the Zn-NC structure can promote the coupling reaction of acetonitrile while reducing the formation of byproducts (such as amides or polymerization byproducts). The improved yield of pyridine base is closely related to the stability and reusability of the catalyst: Zn obtains a stable structure through N coordination, avoiding Zn species loss and improving the long-term stability and recyclability of the catalyst.

[0064] The cycling results of the Cat-B catalyst after 5 cycles are shown in Table 2 below.

[0065] Table 2. Cycling results of Cat-B catalyst

[0066] Loop count Acetonitrile conversion rate (%) Pyridine base selectivity (%) Pyridine base yield (%) 1 first 93.2 93.5 87.1 2 The second 96.1 93.0 89.3 3 The third 95.1 92.8 88.3 4 Fourth 95.3 93.0 88.6 5 Fifth 96.0 93.0 89.3

[0067] Table 2 shows that the Fe-NC catalyst was also subjected to cyclic catalysis, with an average acetonitrile conversion of 95.1% and a pyridine base selectivity maintained above 92%, indicating that the Fe-NC catalyst also exhibits good cycle stability. The Fe-NC catalyst, through its high-density and stable Fe–N active sites, excellent redox properties (Fe, as a transition metal, possesses excellent redox ability; during the reaction, Fe can participate in electron transfer, promoting the activation of reactants and the formation of key intermediates, thereby accelerating the conversion of acetonitrile), and the synergistic effect of the porous support, not only improves the acetonitrile conversion but also significantly enhances the pyridine base selectivity and the yield of the final product by effectively controlling the reaction pathway.

[0068] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for synthesizing pyridine bases based on MOF-derived catalysts, characterized in that, Includes the following steps: Step 1: Dissolve component A in deionized water to generate Zn-based MOF precursor. Centrifuge to obtain solid precursor A, dry to obtain pure ZIF-8 crystal powder, place the ZIF-8 precursor in a tube muffle furnace for carbonization, heat the carbonization temperature to 750°C-950°C at a heating rate of 5°C / min under nitrogen atmosphere, hold for 2 hours to generate Zn-NC porous carbon catalyst, wash the carbonization product with dilute hydrochloric acid, dry to obtain the target product Zn-NC catalyst; Component A consists of Zn(NO3)2·6H2O and 2-methylimidazole, with a mass ratio of Zn(NO3)2·6H2O to 2-methylimidazole of 1:

4. Step 2: Place 150g or 5g of Zn-NC catalyst in a fixed-bed reactor, introduce oxygen to assist the reaction at a flow rate of 0.5-1 L / min, and transport the vaporized acetonitrile and ammonia to the premixing section. After preliminary mixing in the premixing section, the two gases enter the main reaction zone. The molar ratio of acetonitrile, ammonia, and oxygen is 3:1:1-3:1:

5. The reaction temperature is controlled at 200°C-250°C, the reaction pressure is 0.1-0.8 MPa, and the reaction time is 4-5 s. The main reaction zone is filled with MOFs-derived Zn-NC catalyst. Samples are taken every half hour during the reaction, and high-performance liquid chromatography is used for analysis to record the contents of pyridine, 2-methylpyridine, and 3-methylpyridine.

2. The method for synthesizing pyridine bases based on MOFs-derived catalysts as described in claim 1, characterized in that, The specific preparation process of the target product Zn-NC catalyst is as follows: Zn(NO3)2·6H2O and 2-methylimidazole were dissolved in 100 mL of deionized water at a ratio of 1:4 and stirred at 25°C for 12 hours to generate a uniformly suspended Zn-based MOF precursor. The solid precursor in the suspension was separated by centrifugation to obtain solid precursor A, which was dried at 60°C for 24 hours to obtain pure ZIF-8 crystal powder. The ZIF-8 precursor was placed in a tube furnace to generate Zn-NC porous carbon catalyst. The carbonization product was washed with dilute hydrochloric acid to remove residual zinc, and then dried at 120°C for 10 hours to obtain the target product Zn-NC catalyst.

3. The method for synthesizing pyridine bases based on MOFs-derived catalysts as described in claim 1, characterized in that, The molar ratio of acetonitrile to ammonia is 3:

1. The vaporized acetonitrile and ammonia are respectively transported to the premixing section through a precise metering device, and oxygen is introduced to promote the oxidation reaction. The vaporized acetonitrile enters the reaction tube through the outer pipe, and ammonia is injected through the inner sleeve.