A process for the synthesis of 3-cyanopyridine based on mesoporous molecular sieves

Abstract

The application discloses a 3-cyanopyridine catalytic synthesis process based on a mesoporous molecular sieve, and belongs to the technical field of catalytic synthesis. The process takes 3-methylpyridine, ammonia and oxygen as main raw materials, and continuously synthesizes by relying on a composite mesoporous molecular sieve catalyst. The catalyst constructs a zirconium-cerium co-doped mesoporous silica framework in situ, improves the structural rigidity of the pore channel, and reduces the surface hydrophilicity. Meanwhile, in combination with the reduction and dissolution promotion of hypophosphorous acid and the strong complexation of phosphoric acid, vanadium sources are deeply penetrated into the mesoporous interior, so that the loss and failure of the active catalyst in a high-temperature hydrothermal environment are effectively avoided. Through the precise modification of the catalyst structure and the deep synergy of the continuous reaction conditions, the process realizes extremely high target product selectivity and excellent long-period anti-deactivation performance, and is suitable for large-scale industrialized continuous production of 3-cyanopyridine.

Description

Technical Field

[0001] This invention belongs to the field of catalytic synthesis technology, specifically a catalytic synthesis process for 3-cyanopyridine based on mesoporous molecular sieves. Background Technology

[0002] 3-Cyanopyridine is an important intermediate in the synthesis of nicotinic acid and its derivatives, among other fine chemical products. Currently, the mainstream industrial synthesis method is gas-phase ammonia oxidation. This process typically uses 3-methylpyridine, ammonia, and oxygen as the main raw materials, and involves continuous gas-phase catalytic conversion under the action of a solid catalyst. Traditional industrial catalysts often employ bulk metal oxides such as vanadium-titanium catalysts. These catalysts have a small specific surface area, resulting in limited exposure of catalytic active sites, reduced selectivity of the target product, and lower process yield. Furthermore, during the continuous catalytic reaction, a large amount of water is generated in the system and continuously vaporizes at high temperatures, creating a harsh hydrothermal environment. When the catalyst is in this aqueous system for a long time, it is prone to loss of surface active sites, leading to a significant decrease in the long-term operational stability and catalytic effect of the catalyst.

[0003] To address the structural defects of traditional bulk catalysts, and to effectively increase the specific surface area of ​​the catalyst and improve the dispersion of active sites, existing technologies typically attempt to introduce mesoporous molecular sieve materials with abundant pore structures as catalyst supports. Chinese patent application CN115364891A discloses a catalyst for synthesizing 3-cyanopyridine, its preparation method, and its application. Using titanium silicate molecular sieve powder as a base, a high specific surface area molecular sieve support is prepared by alkali treatment to expand pores and ion exchange modification. Oxalic acid is then added as a reducing agent and complexing agent to reduce vanadium pentoxide to a low-valence vanadium precursor solution in the liquid phase. The mesoporous molecular sieve-supported catalyst is then constructed through impregnation and high-temperature calcination.

[0004] In the above technical solution, the titanium-silicon molecular sieve framework after alkali treatment and pore expansion will have a large number of lattice defects. Under the impact of long-term high-temperature water vapor, irreversible desilication or detitanium hydrolysis will occur, which will cause its original mesoporous structure to collapse easily. The vanadium-based catalyst on the surface will also undergo severe hydration and loss, causing the conversion rate and selectivity of 3-cyanopyridine to drop sharply with the operating time, which cannot meet the needs of industrial long-cycle, high-yield continuous production. Summary of the Invention

[0005] The purpose of this invention is to provide a catalytic synthesis process for 3-cyanopyridine based on mesoporous molecular sieves. By constructing a composite catalyst with a high-rigidity support network and a stable catalytic active layer, the pore collapse phenomenon and loss of active components under harsh high-temperature hydrothermal conditions are effectively suppressed, greatly extending the long-term continuous operation life of the catalyst. It has excellent process stability and extremely high economic benefits, and is suitable for large-scale industrial continuous production.

[0006] The objective of this invention can be achieved through the following technical solutions:

[0007] A catalytic synthesis process for 3-cyanopyridine based on mesoporous molecular sieves includes the following steps:

[0008] Step 1: The composite mesoporous molecular sieve catalyst is loaded into a fixed-bed reactor. Under a nitrogen atmosphere, the fixed-bed reactor is heated to 380-420℃ and activated at a constant temperature for 1-2 hours. After activation, the temperature is lowered to 320-380℃ to complete the activation pretreatment of the catalyst.

[0009] Step 2: 3-Methylpyridine is added to the vaporization mixer via a micro-flow pump and vaporized at 200-240℃. Simultaneously, ammonia and oxygen are continuously introduced into the vaporization mixer at a molar ratio. The uniformly mixed gaseous material is then introduced into a fixed-bed reactor, where a continuous gas-phase ammonia oxidation reaction occurs at 320-380℃. Subsequently, the product is introduced into a gas-liquid separation cold trap for condensation and absorption, followed by solvent removal via rotary evaporation to obtain 3-cyanopyridine, thus completing the catalytic synthesis process of 3-cyanopyridine based on mesoporous molecular sieves.

[0010] Furthermore, the reactor heating rate is 15-20℃ / min.

[0011] Furthermore, the molar ratio of 3-methylpyridine, ammonia, and oxygen is 1:1.5-3:20.

[0012] Furthermore, the flow rate of the micro-flow pump is 0.02-0.04 mL / min.

[0013] Furthermore, the preparation process of the composite mesoporous molecular sieve catalyst is as follows:

[0014] The phosphorus-vanadium supported mesoporous molecular sieve precursor was placed in a tube furnace and heated to 420-460℃ at a heating rate of 1-2℃ / min under a nitrogen atmosphere. It was then calcined at a constant temperature for 4-6 hours, ground, and sieved to obtain the composite mesoporous molecular sieve catalyst.

[0015] Furthermore, the preparation process of the phosphorus-vanadium supported mesoporous molecular sieve precursor is as follows:

[0016] Vanadium pentoxide, 85 wt% phosphoric acid, 50 wt% hypophosphoric acid, and deionized water were placed in a reactor and reacted at 50-70℃ for 1-2 h. Cerium-zirconium-containing mesoporous molecular sieves were added and ultrasonically dispersed at the same temperature for 1-2 h, followed by stirring for 6-8 h. The reaction solution was evaporated to dryness and vacuum dried to constant weight to obtain a phosphorus-vanadium-supported mesoporous molecular sieve precursor.

[0017] Furthermore, the mass ratio of vanadium pentoxide, phosphoric acid, hypophosphoric acid, deionized water, and cerium-zirconium-containing mesoporous molecular sieve is 5-10:6-12:2-4:300-600:40-80.

[0018] Furthermore, the preparation process of cerium-zirconium mesoporous molecular sieves is as follows:

[0019] The zirconium-cerium co-doped silicon precursor was placed in a muffle furnace and heated to 520-560℃ at a heating rate of 1-2℃ / min in air atmosphere. It was then calcined at a constant temperature for 4-6 hours, ground, and sieved to obtain a cerium-zirconium mesoporous molecular sieve.

[0020] Furthermore, the preparation process of the zirconium-cerium co-doped silicon precursor is as follows:

[0021] Polyethylene oxide-polypropylene oxide-polyethylene oxide, sodium chloride, and deionized water were placed in a reaction vessel. 36wt% hydrochloric acid was added to adjust the pH of the reaction solution to 1-2. The mixture was stirred at 40-50℃ for 1-2 hours. Tetraethyl orthosilicate, zirconium oxychloride octahydrate, and cerium nitrate hexahydrate were then added. The reaction was continued at the same temperature for 20-24 hours. The temperature was then raised to 100-120℃ and the reaction continued for another 20-24 hours. The mixture was filtered, washed, and vacuum dried to constant weight to obtain the zirconium-cerium co-doped silicon precursor.

[0022] Furthermore, the ratio of polyethylene oxide-polypropylene oxide-polyethylene oxide, sodium chloride, deionized water, tetraethyl orthosilicate, zirconium oxychloride octahydrate, and cerium nitrate hexahydrate is 60-120g: 30-60g: 1-2L: 100-200g: 10-20g: 5-10g.

[0023] The beneficial effects of this invention are:

[0024] 1. In the catalytic synthesis process of 3-cyanopyridine based on mesoporous molecular sieves, this invention constructs an in-situ zirconium-cerium co-doped mesoporous silica framework structure. By introducing zirconium, which has strong coordination ability, and cerium, which has redox properties, zirconium ions provide structural rigidity to the framework due to their high coordination number, while cerium ions significantly consume the hydrophilic silanol groups on the surface through flexible electron transfer and oxygen vacancies, effectively reducing the hydrophilicity of the pore surface. Through this dual synergistic effect, the resistance of the mesoporous pore walls to water vapor impact is greatly enhanced. This allows the well-developed pore network inside the molecular sieve to not only significantly improve the exposure rate of vanadium-based active sites and the mass transfer accessibility of reactants, but also to effectively resist the harsh high-temperature hydrothermal environment in the continuous ammonia oxidation process, avoiding pore collapse, thus providing a solid solid catalytic foundation for the efficient synthesis of the product.

[0025] 2. In the catalytic synthesis process of 3-cyanopyridine based on mesoporous molecular sieves, this invention uses a composite mesoporous molecular sieve as a catalyst. The high specific surface area pore network constructed inside the catalyst by the zirconium-cerium mesoporous framework provides sufficient space for the loading and dispersion of vanadium-based active components, allowing the active components to be more uniformly distributed in the mesoporous internal region. At the same time, through the reducing properties of hypophosphoric acid and the complexing effect of phosphoric acid, the solid-phase vanadium source is efficiently dissociated and vanadium ions are chemically anchored to the zirconium-cerium mesoporous framework, promoting the efficient and uniform gas-phase ammonia oxidation reaction. Furthermore, thanks to the dual high-temperature rigidity defense of the phosphorus-oxygen network and the zirconium-cerium mesoporous framework, the loss of active sites under high-temperature hydrothermal environment is effectively reduced, thereby improving the reaction conversion efficiency and stabilizing the catalytic performance.

[0026] 3. In the catalytic synthesis process of 3-cyanopyridine based on mesoporous molecular sieves, the composite mesoporous molecular sieve catalyst reduces the hydrophilicity of the mesoporous molecular sieve framework, which can significantly promote the in-situ rapid desorption and exclusion of water, a byproduct continuously generated during the reaction, effectively weaken the competitive adsorption of water vapor on the catalytic active sites, and accelerate the reaction rate. At the same time, the stable redox microenvironment constructed by the composite mesoporous molecular sieve catalyst can alleviate local oxygen concentration fluctuations in the reaction system, inhibit the occurrence of side reactions, and ensure that the reaction system still has high catalytic activity and high product selectivity during long-term continuous operation, meeting the needs of long-term, high-yield continuous industrial production. Attached Figure Description

[0027] Figure 1 This is a flow chart of the catalytic synthesis process of 3-cyanopyridine based on mesoporous molecular sieves. Detailed Implementation

[0028] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. 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.

[0029] Example 1: This example provides a composite mesoporous molecular sieve catalyst for the catalytic synthesis of 3-cyanopyridine based on mesoporous molecular sieves, which is prepared through the following steps:

[0030] S1: 60g of polyethylene oxide-polypropylene oxide-polyethylene oxide, 30g of sodium chloride and 1L of deionized water were placed in a reaction vessel. 36wt% hydrochloric acid was added to adjust the pH of the reaction solution to 1. The reaction was stirred at 200r / min at 40℃ for 1h. 100g of tetraethyl orthosilicate, 10g of zirconium oxychloride octahydrate and 5g of cerium nitrate hexahydrate were added. The reaction was continued at the same temperature and stirring rate for 20h. The temperature was then raised to 100℃ and the reaction was continued for another 20h. After the reaction was completed, the mixture was cooled to room temperature, filtered, and the filter cake was washed twice with deionized water and anhydrous ethanol. The mixture was then vacuum dried at 60℃ to constant weight to obtain the zirconium-cerium co-doped silicon precursor.

[0031] S2: Place 100g of zirconium-cerium co-doped silicon precursor in a muffle furnace and heat it to 520℃ at a heating rate of 1℃ / min in an air atmosphere. Calcinate at this temperature for 4 hours. After calcination, cool to room temperature, grind, and sieve through a 100-mesh sieve to obtain a cerium-zirconium mesoporous molecular sieve.

[0032] S3: Place 5g vanadium pentoxide, 6g 85wt% phosphoric acid, 2g 50wt% hypophosphoric acid and 300mL deionized water in a reaction vessel and stir at 200r / min for 1h at 50℃. Add 40g cerium-zirconium mesoporous molecular sieve and ultrasonically disperse at the same temperature for 1h, and continue stirring for 6h. After the reaction is completed, evaporate the reaction solution to dryness and vacuum dry at 80℃ to constant weight to obtain the phosphorus-vanadium supported mesoporous molecular sieve precursor.

[0033] S4: 60g of phosphorus-vanadium supported mesoporous molecular sieve precursor was placed in a tube furnace and heated to 420℃ at a heating rate of 1℃ / min under a nitrogen atmosphere. It was then calcined at a constant temperature for 4h. After calcination, it was cooled to room temperature, ground, and sieved through a 40-mesh sieve to obtain the composite mesoporous molecular sieve catalyst.

[0034] Example 2: This example provides a composite mesoporous molecular sieve catalyst for the catalytic synthesis of 3-cyanopyridine based on mesoporous molecular sieves, which is prepared through the following steps:

[0035] S1: 90g of polyethylene oxide-polypropylene oxide-polyethylene oxide, 45g of sodium chloride and 1.5L of deionized water were placed in a reaction vessel. 36wt% hydrochloric acid was added to adjust the pH of the reaction solution to 1.5. The reaction was stirred at 250r / min at 45℃ for 1.5h. 150g of tetraethyl orthosilicate, 15g of zirconium oxychloride octahydrate and 8g of cerium nitrate hexahydrate were added. The reaction was continued at the same temperature and stirring rate for 22h. The temperature was then raised to 110℃ and the reaction was continued for another 22h. After the reaction was completed, the mixture was cooled to room temperature, filtered, and the filter cake was washed three times with deionized water and anhydrous ethanol. The mixture was then vacuum dried at 70℃ to constant weight to obtain the zirconium-cerium co-doped silicon precursor.

[0036] S2: 150g of zirconium-cerium co-doped silicon precursor was placed in a muffle furnace and heated to 540℃ at a heating rate of 1.5℃ / min in an air atmosphere. It was then calcined at a constant temperature for 5h. After calcination, it was cooled to room temperature, ground, and sieved through a 150-mesh sieve to obtain a cerium-zirconium mesoporous molecular sieve.

[0037] S3: Place 8g of vanadium pentoxide, 9g of 85wt% phosphoric acid, 3g of 50wt% hypophosphoric acid and 450mL of deionized water in a reaction vessel and stir at 250r / min for 1.5h at 60℃. Add 60g of cerium-zirconium mesoporous molecular sieve and ultrasonically disperse at the same temperature for 1.5h, and continue stirring for 7h. After the reaction is completed, evaporate the reaction solution to dryness and vacuum dry at 90℃ to constant weight to obtain the phosphorus-vanadium supported mesoporous molecular sieve precursor.

[0038] S4: 70g of phosphorus-vanadium supported mesoporous molecular sieve precursor was placed in a tube furnace and heated to 440℃ at a heating rate of 1.5℃ / min under a nitrogen atmosphere. It was then calcined at a constant temperature for 5h. After calcination, it was cooled to room temperature, ground, and sieved through a 50-mesh sieve to obtain the composite mesoporous molecular sieve catalyst.

[0039] Example 3: This example provides a composite mesoporous molecular sieve catalyst for the catalytic synthesis of 3-cyanopyridine based on mesoporous molecular sieves, which is prepared through the following steps:

[0040] S1: 120g of polyethylene oxide-polypropylene oxide-polyethylene oxide, 60g of sodium chloride and 2L of deionized water were placed in a reaction vessel. 36wt% hydrochloric acid was added to adjust the pH of the reaction solution to 2. The reaction was stirred at 300r / min at 50℃ for 2h. 200g of tetraethyl orthosilicate, 20g of zirconium oxychloride octahydrate and 10g of cerium nitrate hexahydrate were added. The reaction was continued at the same temperature and stirring rate for 24h. The temperature was then raised to 120℃ and the reaction was continued for another 24h. After the reaction was completed, the mixture was cooled to room temperature, filtered, and the filter cake was washed 4 times with deionized water and anhydrous ethanol. It was then vacuum dried at 80℃ to constant weight to obtain the zirconium-cerium co-doped silicon precursor.

[0041] S2: Place 200g of zirconium-cerium co-doped silicon precursor in a muffle furnace and heat it to 560℃ at a heating rate of 2℃ / min in an air atmosphere. Calcinate at this temperature for 6 hours. After calcination, cool to room temperature, grind, and sieve through a 200-mesh sieve to obtain a cerium-zirconium mesoporous molecular sieve.

[0042] S3: Place 10g vanadium pentoxide, 12g 85wt% phosphoric acid, 4g 50wt% hypophosphoric acid and 600mL deionized water in a reaction vessel and stir at 300r / min at 70℃ for 2h. Add 80g cerium-zirconium mesoporous molecular sieve and ultrasonically disperse at the same temperature for 2h, and continue stirring for 8h. After the reaction is completed, evaporate the reaction solution to dryness and vacuum dry at 100℃ to constant weight to obtain the phosphorus-vanadium supported mesoporous molecular sieve precursor.

[0043] S4: 80g of phosphorus-vanadium supported mesoporous molecular sieve precursor was placed in a tube furnace and heated to 460℃ at a heating rate of 2℃ / min under a nitrogen atmosphere. It was then calcined at a constant temperature for 6h. After calcination, it was cooled to room temperature, ground, and sieved through a 60-mesh sieve to obtain the composite mesoporous molecular sieve catalyst.

[0044] The composite mesoporous molecular sieve catalyst prepared in the above embodiments first uses a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer as a structure directing agent to induce hydrolysis and sol-gel self-assembly of tetraethyl orthosilicate in a strongly acidic system. Simultaneously, zirconium oxychloride and cerium nitrate are introduced in situ, and stable Si-O-Zr and Si-O-Ce heteroatom bonds are constructed in the silicon-oxygen network through a polycondensation reaction to obtain a zirconium-cerium co-doped silicon precursor. Subsequently, high-temperature calcination is performed in an air atmosphere to thermally remove the organic template agent and open abundant mesoporous channels, promoting deep cross-linking of the inorganic framework to obtain a cerium-zirconium mesoporous molecular sieve support. Then, in a liquid phase system, hypophosphite is used... The reducing properties of phosphoric acid cause vanadium pentoxide to be reduced and dissociated into highly soluble, low-valence vanadium oxide ions. At the same time, the strong complexing and coordination ability of phosphoric acid forms a stable vanadium-phosphorus oxide (VPO) structure with vanadium ions. Through ultrasonic dispersion and impregnation, the vanadium-phosphorus components are highly dispersed in the pores of the cerium-zirconium mesoporous molecular sieve, resulting in a phosphorus-vanadium supported mesoporous molecular sieve precursor. Finally, high-temperature calcination is carried out under a nitrogen atmosphere to promote the dehydration condensation and coordination complexation reactions between the vanadium-phosphorus active components and the Zr-OH, Ce-OH, and Si-OH hydroxyl sites on the pore walls of the support. This allows the vanadium-phosphorus active components to be firmly anchored on the molecular sieve framework in the form of covalent bonds, forming stable active centers, and thus obtaining a composite mesoporous molecular sieve catalyst.

[0045] Example 4: This example provides a catalytic synthesis process for 3-cyanopyridine based on mesoporous molecular sieves, such as... Figure 1 As shown, it includes the following steps:

[0046] Step 1: The composite mesoporous molecular sieve catalyst prepared in Example 1 is loaded into a fixed-bed reactor, nitrogen gas is introduced, the reactor is heated to 380°C at a heating rate of 15°C / min and activated at a constant temperature for 1 hour. After activation, the temperature is lowered to 320°C to complete the pretreatment of the catalyst.

[0047] Step 2: 3-Methylpyridine is added to a vaporization mixer at a constant flow rate of 0.02 mL / min using a micro-flow pump and vaporized at 200°C. Simultaneously, ammonia and oxygen are continuously introduced into the vaporization mixer in a specific ratio, controlling the molar ratio of 3-methylpyridine, ammonia, and oxygen in the mixed raw material gas to be 1:1.5:20. The uniformly mixed gaseous material is then introduced into a fixed-bed reactor, where a continuous gas-phase ammonia oxidation reaction occurs at 320°C. Subsequently, the product is introduced into a gas-liquid separation cold trap containing anhydrous ethanol for condensation and absorption. After collection, the resulting absorbent is placed in a rotary evaporator to remove the solvent, yielding 3-cyanopyridine, thus completing the catalytic synthesis process of 3-cyanopyridine based on mesoporous molecular sieves.

[0048] Example 5: This example provides a catalytic synthesis process for 3-cyanopyridine based on mesoporous molecular sieves, such as... Figure 1 As shown, it includes the following steps:

[0049] Step 1: The composite mesoporous molecular sieve catalyst prepared in Example 2 was loaded into a fixed-bed reactor, nitrogen gas was introduced, and the reactor was heated to 400°C at a heating rate of 18°C / min and activated at a constant temperature for 1.5 h. After activation, the temperature was reduced to 350°C to complete the pretreatment of the catalyst.

[0050] Step 2: 3-Methylpyridine is added to a vaporization mixer at a constant flow rate of 0.03 mL / min using a micro-flow pump and vaporized at 220°C. Simultaneously, ammonia and oxygen are continuously introduced into the vaporization mixer in a specific ratio, controlling the molar ratio of 3-methylpyridine, ammonia, and oxygen in the mixed raw material gas to be 1:2.3:20. The uniformly mixed gaseous material is then introduced into a fixed-bed reactor, where a continuous gas-phase ammonia oxidation reaction occurs at 350°C. Subsequently, the product is introduced into a gas-liquid separation cold trap containing anhydrous ethanol for condensation and absorption. After collection, the resulting absorbent is placed in a rotary evaporator to remove the solvent, yielding 3-cyanopyridine, thus completing the catalytic synthesis process of 3-cyanopyridine based on mesoporous molecular sieves.

[0051] Example 6: This example provides a catalytic synthesis process for 3-cyanopyridine based on mesoporous molecular sieves, such as... Figure 1 As shown, it includes the following steps:

[0052] Step 1: The composite mesoporous molecular sieve catalyst prepared in Example 3 was loaded into a fixed-bed reactor, nitrogen gas was introduced, and the reactor was heated to 420°C at a heating rate of 20°C / min and activated at a constant temperature for 2 hours. After activation, the temperature was reduced to 380°C to complete the pretreatment of the catalyst.

[0053] Step 2: 3-Methylpyridine is added to a vaporization mixer at a constant flow rate of 0.04 mL / min using a micro-flow pump and vaporized at 240 °C. Simultaneously, ammonia and oxygen are continuously introduced into the vaporization mixer in a specific ratio, controlling the molar ratio of 3-methylpyridine, ammonia, and oxygen in the mixed raw material gas to be 1:3:20. The uniformly mixed gaseous material is then introduced into a fixed-bed reactor, where a continuous gas-phase ammonia oxidation reaction occurs at 380 °C. Subsequently, the product is introduced into a gas-liquid separation cold trap containing anhydrous ethanol for condensation and absorption. After collection, the resulting absorbent is placed in a rotary evaporator to remove the solvent, yielding 3-cyanopyridine, thus completing the catalytic synthesis process of 3-cyanopyridine based on mesoporous molecular sieves.

[0054] Comparative Example 1: The difference from Example 2 is that a commercially available mesoporous molecular sieve was used instead of the cerium-zirconium-containing mesoporous molecular sieve prepared in step S2, while the other steps remained unchanged, and a composite mesoporous molecular sieve catalyst was prepared.

[0055] Comparative Example 2: The difference from Example 2 is that zirconium oxychloride octahydrate and cerium nitrate hexahydrate added in step S1 were replaced with equal amounts of zirconium oxychloride octahydrate, while the other steps remained unchanged, and a composite mesoporous molecular sieve catalyst was prepared.

[0056] Comparative Example 3: The difference from Example 2 is that an equal amount of cerium nitrate hexahydrate was used to replace the zirconium oxychloride octahydrate and cerium nitrate hexahydrate added in step S1, while the other steps remained unchanged, and a composite mesoporous molecular sieve catalyst was prepared.

[0057] Comparative Example 4: The difference from Example 2 is that the 85wt% phosphoric acid added in step S3 is removed, while the other steps remain unchanged, and a composite mesoporous molecular sieve catalyst is prepared.

[0058] The polyethylene oxide-polypropylene oxide-polyethylene oxide purchased in the above embodiments and comparative examples were produced by Shanghai Maclean Biochemical Technology Co., Ltd., with an average molecular weight of 5800 Da; the commercially available mesoporous molecular sieve was produced by Shanghai Aladdin Biochemical Technology Co., Ltd., and was SBA-15 type mesoporous silica with an average pore size of 6-10 nm.

[0059] The composite mesoporous molecular sieve catalysts prepared in Examples 1-3 and Comparative Examples 1-4 were used to simulate the catalytic synthesis process of 3-cyanopyridine based on mesoporous molecular sieves, and their catalytic performance was evaluated. The test results are shown in Table 1.

[0060] Experimental preparation: A fixed-bed continuous reaction apparatus commonly used in the laboratory was used to simulate the catalytic synthesis process of 3-methylpyridine. The catalyst sample prepared above was sieved through a 50-mesh sieve, and 1g was weighed and loaded into the fixed-bed reactor. The catalyst pretreatment was completed according to the catalytic synthesis process in Example 5, and it can then be used for catalytic performance testing.

[0061] Catalyst efficiency and long-term stability testing: Referring to the catalytic synthesis process in Example 5, 3-methylpyridine was added to a vaporization mixer at a constant flow rate of 0.03 mL / min using a micro-flow pump and vaporized at 220°C. Simultaneously, ammonia and oxygen were continuously introduced in a specific ratio, strictly controlling the molar ratio of 3-methylpyridine, ammonia, and oxygen in the mixed feed gas to be 1:2.3:20. The uniformly mixed gaseous material was continuously fed into a pretreated fixed-bed reactor, where a continuous gas-phase ammonia oxidation reaction occurred at 350°C. After 10 hours of reaction, samples were taken, and the contents of unreacted 3-methylpyridine and generated 3-cyanopyridine were determined using gas chromatography. The conversion rate of 3-methylpyridine, the selectivity of 3-cyanopyridine, and the yield were calculated.

[0062] To verify the long-term stability of the catalyst, the catalyst was kept running continuously for 200 hours. After 200 hours of reaction, the product was sampled and the conversion rate of 3-methylpyridine, the selectivity of 3-cyanopyridine, and the yield were calculated again. Under the same long-term reaction conditions, the higher the conversion rate and selectivity, and the smaller the data decay in the later stage of operation (200 hours), the higher the catalytic activity and the better the long-term stability of the catalyst.

[0063] Table 1 Catalytic Performance Test Table

[0064]

[0065] As shown in Table 1, the composite mesoporous molecular sieve catalysts prepared in Examples 1-3 exhibit significantly better catalytic performance than the comparative examples. Furthermore, they maintain good conversion and selectivity under continuous high-temperature hydrothermal reaction conditions for 200 hours. This indicates that the catalysts prepared in this invention provide excellent physical rigidity and hydrophobic barriers through the zirconium-cerium co-doped mesoporous framework in the catalytic synthesis process of 3-cyanopyridine. Through the complexation and anchoring effects of phosphate and the reducing and solubilizing effects of hypophosphoric acid, the vanadium source is converted into a more soluble and dispersible vanadium active component, which is uniformly dispersed and firmly anchored in the mesoporous network, forming stable and efficient active centers. This ensures that the catalyst maintains good structural stability under high-temperature hydrothermal conditions, guaranteeing long-term and high-yield operation of the entire process.

[0066] The conversion rate and target product yield of Comparative Example 1 both decreased significantly after long-term operation. This may be because the surface chemical environment of commercially available mesoporous molecular sieves is simple, and there is only a weak physical interaction between them and the supported vanadium and phosphorus components. Under high-temperature ammonia oxidation conditions, it is difficult to stably anchor the active components, which can easily lead to the migration, aggregation and loss of vanadium active species, resulting in a rapid decrease in the activity and selectivity of the catalyst.

[0067] The long-cycle yields of both Comparative Example 2 and Comparative Example 3 showed significant decreases to varying degrees. This is because the catalyst prepared in Comparative Example 2, which is doped with zirconium, has a certain physical rigidity structure, but may lack the surface hydrophobic regulation and oxygen storage capacity of cerium, making it easy for water vapor to be adsorbed on the inner wall of the pores, leading to the deactivation of active sites. On the other hand, the catalyst prepared in Comparative Example 3, which is doped with cerium, can maintain a certain degree of chemical stability, but may lack the rigid steric support provided by zirconium, and the framework will still partially hydrolyze and collapse, resulting in a decrease in process efficiency.

[0068] In Comparative Example 4, the selectivity decreased in the initial stage of the reaction, and the yield dropped significantly after 200 hours. This may be because the complexation regulation system of phosphoric acid was removed, making it difficult for the vanadium active center to form a stable coordination anchoring structure with the mesoporous framework matrix. There are many free vanadium species in the system. Under high-temperature ammonia oxidation reaction conditions, these free vanadium species are prone to migration, aggregation, and over-oxidation into high-valence vanadium species. The extremely strong oxidizing power may trigger side reactions such as deep oxidation and ring opening of 3-methylpyridine, resulting in a sharp decrease in the effective conversion and anti-deactivation performance of the composite catalyst.

[0069] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention.

Claims

1. A catalytic synthesis process for 3-cyanopyridine based on mesoporous molecular sieves, characterized in that, Includes the following steps: Step 1: The composite mesoporous molecular sieve catalyst is loaded into a fixed-bed reactor. Under a nitrogen atmosphere, the fixed-bed reactor is heated to 380-420℃ and activated at a constant temperature for 1-2 hours. After activation, the temperature is lowered to 320-380℃ to complete the activation pretreatment of the catalyst. Step 2: 3-Methylpyridine is added to the vaporization mixer via a micro-flow pump and vaporized at 200-240℃. Simultaneously, ammonia and oxygen are continuously introduced into the vaporization mixer at a molar ratio. The uniformly mixed gaseous material is then introduced into a fixed-bed reactor, where a continuous gas-phase ammonia oxidation reaction occurs at 320-380℃. Subsequently, the product is introduced into a gas-liquid separation cold trap for condensation and absorption, followed by solvent removal via rotary evaporation to obtain 3-cyanopyridine, thus completing the catalytic synthesis process of 3-cyanopyridine based on mesoporous molecular sieves.

2. The catalytic synthesis process of 3-cyanopyridine based on mesoporous molecular sieves according to claim 1, characterized in that, The composite mesoporous molecular sieve catalyst described in step one is prepared through the following steps: The phosphorus-vanadium supported mesoporous molecular sieve precursor was placed in a tube furnace and heated to 420-460℃ at a heating rate of 1-2℃ / min under a nitrogen atmosphere. It was then calcined at a constant temperature for 4-6 hours, ground, and sieved to obtain the composite mesoporous molecular sieve catalyst.

3. The catalytic synthesis process of 3-cyanopyridine based on mesoporous molecular sieves according to claim 2, characterized in that, The phosphorus-vanadium supported mesoporous molecular sieve precursor is prepared by the following steps: Vanadium pentoxide, 85 wt% phosphoric acid, 50 wt% hypophosphoric acid, and deionized water were placed in a reactor and reacted at 50-70℃ for 1-2 h. Cerium-zirconium-containing mesoporous molecular sieves were added and ultrasonically dispersed at the same temperature for 1-2 h, followed by stirring for 6-8 h. The reaction solution was evaporated to dryness and vacuum dried to constant weight to obtain a phosphorus-vanadium-supported mesoporous molecular sieve precursor.

4. The catalytic synthesis process of 3-cyanopyridine based on mesoporous molecular sieves according to claim 3, characterized in that, The mass ratio of vanadium pentoxide, phosphoric acid, hypophosphoric acid, deionized water and cerium-zirconium-containing mesoporous molecular sieve is 5-10:6-12:2-4:300-600:40-80.

5. The catalytic synthesis process of 3-cyanopyridine based on mesoporous molecular sieves according to claim 4, characterized in that, The cerium-zirconium-containing mesoporous molecular sieve is prepared by the following steps: The zirconium-cerium co-doped silicon precursor was placed in a muffle furnace and heated to 520-560℃ at a rate of 1-2℃ / min in air atmosphere. It was then calcined at a constant temperature for 4-6 hours, ground, and sieved to obtain a cerium-zirconium mesoporous molecular sieve.

6. The catalytic synthesis process of 3-cyanopyridine based on mesoporous molecular sieves according to claim 5, characterized in that, The zirconium-cerium co-doped silicon precursor is prepared by the following steps: Polyethylene oxide-polypropylene oxide-polyethylene oxide, sodium chloride, and deionized water were placed in a reaction vessel. 36wt% hydrochloric acid was added to adjust the pH of the reaction solution to 1-2. The mixture was stirred at 40-50℃ for 1-2 hours. Tetraethyl orthosilicate, zirconium oxychloride octahydrate, and cerium nitrate hexahydrate were then added. The reaction was continued at the same temperature for 20-24 hours. The temperature was then raised to 100-120℃ and the reaction continued for another 20-24 hours. The mixture was filtered, washed, and vacuum dried to constant weight to obtain the zirconium-cerium co-doped silicon precursor.

7. The catalytic synthesis process of 3-cyanopyridine based on mesoporous molecular sieves according to claim 6, characterized in that, The ratio of polyethylene oxide-polypropylene oxide-polyethylene oxide, sodium chloride, deionized water, tetraethyl orthosilicate, zirconium oxychloride octahydrate, and cerium nitrate hexahydrate is 60-120g: 30-60g: 1-2L: 100-200g: 10-20g: 5-10g.

8. The catalytic synthesis process of 3-cyanopyridine based on mesoporous molecular sieves according to claim 1, characterized in that, The heating rate of the fixed-bed reactor in step one is 15-20℃ / min.

9. The catalytic synthesis process of 3-cyanopyridine based on mesoporous molecular sieves according to claim 1, characterized in that, The flow rate of the micro-flow pump mentioned in step two is 0.02-0.04 mL / min.

10. The catalytic synthesis process of 3-cyanopyridine based on mesoporous molecular sieves according to claim 1, characterized in that, In step two, the molar ratio of 3-methylpyridine, ammonia, and oxygen is 1:1.5-3:20.

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