A solid superacid catalyst, its preparation method and application in synthesis of 2-methylpyridine

Abstract

This invention discloses a solid superacid catalyst, its preparation method, and its application in the synthesis of 2-methylpyridine, belonging to the fields of organic synthesis and catalysis technology. The catalyst is composed of sulfate, zirconium oxide, rare earth metal salts, and a second metal salt. Through the synergistic effect of the multi-metal components, the catalyst effectively stabilizes the highly active tetragonal phase of ZrO2, significantly improving the selectivity and catalytic stability for the target product 2-methylpyridine. Using the above-mentioned particulate solid superacid as the catalyst and industrial-grade acetaldehyde aqueous solution and ammonia water as raw materials, 2-methylpyridine can be synthesized under mild conditions of normal pressure and 300℃, with low energy consumption. After deactivation due to coking or sulfate loss during use, the catalyst can be regenerated through two steps of coking and sulfur replenishment, exhibiting excellent recyclability and effectively reducing the cost of catalyst use. Unreacted ammonia in the tail gas is recovered and recycled through an absorption tower, resulting in zero waste gas emissions and achieving clean production.

Description

Technical Field

[0001] This invention relates to the fields of organic synthesis and catalysis, and in particular to a solid superacid catalyst, its preparation method, and its application in the synthesis of 2-methylpyridine. Background Technology

[0002] 2-Methylpyridine (also known as α-methylpyridine) is an important fine chemical intermediate widely used in pharmaceuticals, pesticides, dyes, fragrances, and rubber additives. As a key raw material for the synthesis of important drugs such as nicotinic acid, nicotinamide, and praziquantel, the market demand for 2-methylpyridine continues to grow.

[0003] Currently, the main methods for synthesizing 2-methylpyridine are as follows: (I) Traditional gas-phase catalysis Using acetaldehyde and ammonia as raw materials, a condensation cyclization reaction is carried out under high-temperature gas-phase conditions of 350-500℃ in the presence of an aluminosilicate catalyst. This method involves high reaction temperature, high energy consumption, and less than ideal selectivity. The product often contains a large amount of homologues such as 4-methylpyridine, requiring multi-stage distillation separation to obtain a high-purity product.

[0004] (II) High-pressure zeolite catalysis Using acetaldehyde and ammonia as raw materials, the reaction is carried out under high pressure and autogenous pressure conditions in the presence of catalysts such as FAU-type Y zeolite. Although this method can be carried out at relatively low temperatures (e.g., 150°C), the reaction pressure is high, the equipment requirements are stringent, and industrial scale-up is difficult.

[0005] (III) Liquid-phase catalysis The reaction is carried out using a liquid homogeneous catalyst, but liquid catalysts have problems such as equipment corrosion, difficulty in recycling, and environmental pollution.

[0006] In recent years, some researchers have attempted to synthesize alkylpyridine compounds using solid superacid catalysts. For example, the literature reports the synthesis of methylpyridine compounds from acetaldehyde and ammonia under fixed-bed atmospheric pressure gas-phase conditions using particulate solid superacids as catalysts. However, this research remains at the laboratory stage, and the catalyst used is a single SO42- ... 2-The ZrO2 system exhibits a uniform distribution of acid center density and acid strength, resulting in limited selectivity for the target product 2-methylpyridine and an overall yield of only about 60%. Furthermore, it lacks a technical approach to improve selectivity by introducing multi-metal components to synergistically regulate catalyst surface acidity and pore structure. The systemic optimization of catalyst composition, preparation methods, and process conditions has not been thoroughly explored, leaving significant room for improvement in overall product yield. More importantly, existing technologies do not address catalyst regeneration methods or the recycling of process tail gas. Solid superacid catalysts are prone to deactivation during use due to coking and sulfate loss. Without effective regeneration methods, their lifespan is limited, and replacement costs are high. Simultaneously, direct emission of ammonia tail gas leads to raw material waste and environmental pollution, compromising the long-term economic and environmental viability.

[0007] Therefore, there is an urgent need in this field to develop a novel catalyst system and its supporting processes that are highly selective, renewable, economical and environmentally friendly, in order to achieve the efficient industrial production of 2-methylpyridine. Summary of the Invention

[0008] The purpose of this invention is to provide a solid superacid catalyst, its preparation method, and its application in the synthesis of 2-methylpyridine. The aim is to achieve milder reaction conditions (atmospheric pressure, 300°C), direct use of inexpensive and readily available industrial aqueous solutions as raw materials, and multiple regeneration and recycling of the catalyst, thereby significantly reducing production costs and environmental pollution.

[0009] To achieve the above objectives, this invention discloses a solid superacid catalyst, wherein the chemical formula of the solid superacid catalyst is SO4. 2- / ZrO2-Re a M x O y SO4 2- The catalyst consists of a ZrO2 host, a rare earth metal salt Re, and a second metal salt M, where a is the molar ratio of Re to Zr, a = 0.01-0.10; x is the molar ratio of M to Zr, x = 0.02-0.15; y is the stoichiometric coefficient of oxygen required to satisfy the valence of M and the value of x; the solid superacid catalyst has a mesoporous structure.

[0010] Preferably, Re is a combination of at least two rare earth metals selected from La, Ce, Nd, and Pr, and M is a combination of one or more of Al, Fe, Ni, and W.

[0011] Preferably, the catalyst has a specific surface area of ​​80–160 m². 2 / g, with a particle size of 20-60 mesh, and the particles are spherical or near-spherical, wherein the content of tetragonal ZrO2 is not less than 80%.

[0012] Preferably, the particulate solid superacid catalyst uses a metal oxide as a support and loads sulfate ions as acidic active centers, with an acid strength H0 ≤ -11.93 (lower than the acid strength of 100% sulfuric acid).

[0013] This invention also provides a method for preparing the above-mentioned solid superacid catalyst, comprising the following steps: (1) Dissolve soluble zirconium salt, rare earth metal salt and second metal salt in deionized water at a molar ratio of Zr:Re:M = 1:a:x to prepare a mixed solution with a total metal ion concentration of 0.2-1.0 mol / L; (2) Under stirring conditions, ammonia water is added dropwise to the mixed solution until the pH value is 9-10, a mixed hydroxide precipitate is generated, and aged at low temperature of -5℃ to 5℃ for 48-72 hours; (3) Filter and wash the precipitate until no Cl is present. - The filter cake is dried at 80-110℃ for 12-36 hours; (4) The dried precipitate is soaked in 0.2-2.0 mol / L sulfuric acid solution for 0.5-2 hours, filtered, and then dried at 80-110℃ for 12-36 hours; (5) The dried solid is calcined in air at 550-700℃ for 2-4 hours to obtain the catalyst.

[0014] Preferably, in step (1), the soluble zirconium salt is ZrOCl2·8H2O or Zr(NO3)4·5H2O, the rare earth metal salt is a mixture of nitrates of La, Ce, Nd, and Pr, and the second metal salt is Ni(NO3)2·6H2O or (NH4)6H2W. 12 O 40 In step (4), the concentration of sulfuric acid solution is 0.5-1.0 mol / L, and the soaking time is 1-1.5 hours.

[0015] Preferably, in step (4), the impregnation is carried out under ultrasonic-assisted conditions, with an ultrasonic frequency of 20-50 kHz and an ultrasonic treatment time of 10-30 minutes.

[0016] The solid superacid catalyst provided by this invention is applied to the atmospheric pressure gas-phase catalytic synthesis of 2-methylpyridine, comprising the following steps: The granular solid superacid catalyst is packed into a fixed-bed reactor and activated by introducing an inert gas at a temperature of 300-350℃ for 1-3 hours. After preheating acetaldehyde aqueous solution and ammonia water to 150-250℃ respectively, they are introduced into a fixed-bed reactor packed with the granular solid superacid catalyst at a molar ratio of acetaldehyde to ammonia of 2.5-3.5:3.5-4.5. The reaction temperature was controlled at 280-320℃, the reaction pressure at atmospheric pressure, and the feed rate at 0.1-0.3 mL / min. The reaction mixture was discharged from the outlet of the fixed-bed reactor and condensed by a condenser to obtain a mixture containing 2-methylpyridine, 4-methylpyridine, 2-methyl-5-ethylpyridine and water; The mixture was separated into organic phases by liquid-liquid separation, and the organic phases were further separated by distillation to collect 2-methylpyridine fraction, 4-methylpyridine fraction and 2-methyl-5-ethylpyridine fraction respectively. Unreacted ammonia gas is recovered and recycled after passing through an absorption tower.

[0017] Preferably, the acetaldehyde aqueous solution has a mass fraction of 40%, and the ammonia solution has a mass fraction of 25%.

[0018] Preferably, the ammonia in the reaction tail gas is absorbed by dilute sulfuric acid in an absorption tower to generate ammonium sulfate, which is then reacted with lime milk to release ammonia for recycling; or the ammonia is absorbed by water to form ammonia water and then directly reused.

[0019] Preferably, after the granular solid superacid catalyst is deactivated due to coking or sulfate loss, it is regenerated by a method including the following steps: (i) Remove the deactivated solid superacid catalyst from the reactor and calcine it at 300-400°C in air for 2-4 hours to burn off the surface carbon. (ii) The charred catalyst is impregnated with a 0.2-2.0 mol / L sulfuric acid solution for 0.5-2 hours to replenish the lost sulfate ions; (iii) The impregnated catalyst is dried at 80-110℃ for 12-36 hours and then calcined at 550-700℃ for 2-4 hours to obtain the regenerated catalyst.

[0020] Preferably, the yield of 2-methylpyridine in the regenerated catalyst is not less than 90% of the yield of the fresh catalyst.

[0021] Therefore, the present invention has the following beneficial effects: 1. The raw materials are directly used in aqueous solution without purification. This invention directly uses industrial-grade acetaldehyde aqueous solution (40%) and ammonia water (25%) as raw materials, without the need for additional purification or dehydration treatment of the raw materials, which significantly reduces the cost of raw material pretreatment and the complexity of the process.

[0022] 2. Mild reaction conditions and low energy consumption. This invention conducts the reaction under mild conditions of atmospheric pressure and 300℃. Compared with the traditional gas-phase catalytic method (350-500℃), the reaction temperature is reduced by more than 50℃, energy consumption is significantly reduced, and the requirements for equipment materials are also greatly reduced.

[0023] 3. The catalyst can be regenerated efficiently and has a long cycle life. This invention provides a complete catalyst regeneration process. Through a two-step method of carbon removal by burning and sulfur impregnation, the activity of the deactivated catalyst can be basically restored to the level of a fresh catalyst, effectively solving the industry problem of easy deactivation and short life of solid superacid catalysts.

[0024] 4. High process integration and environmentally friendly. This invention integrates catalyst preparation, synthesis reaction, product separation, tail gas recovery, and catalyst regeneration into a single process loop. Unreacted ammonia is recovered and recycled, resulting in zero waste gas emissions, which aligns with the development direction of green chemistry.

[0025] 5. Adjustable product selectivity. By controlling the reaction temperature and feed rate, the product ratio of 2-methylpyridine to 4-methylpyridine can be adjusted within a certain range to meet the product distribution requirements of different downstream applications.

[0026] 6. The process is simple and easy to scale up industrially. This invention adopts a mature fixed-bed gas-phase reaction process, which requires low equipment investment, is easy to operate, and has good prospects for industrial scale-up.

[0027] 7. The introduction of rare earth metals effectively stabilizes the highly active tetragonal crystal phase of ZrO2, inhibiting the phase transformation during high-temperature calcination. Simultaneously, the variable valence characteristics of rare earth ions can regulate the acidity and alkalinity of the catalyst surface, enhancing the stability and acid strength of the superacid centers. Doping with a second metal (such as Al, Fe, Ni, or W) can further modulate the specific surface area, pore structure, and surface acid center distribution of the support, producing a synergistic effect with rare earth elements to form a multi-component composite oxide system, increasing the density and diversity of acid centers. The synergistic effect of the multi-metal components significantly improves the selectivity of the catalyst for 2-methylpyridine, reduces reaction byproducts, and enhances the catalyst's resistance to coking and thermal stability, extending its single-use lifespan. Combined with supporting regeneration methods and tail gas recovery schemes, efficient, economical, and green production of 2-methylpyridine is achieved.

[0028] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0029] Figure 1 The diagram shows the process flow for the atmospheric pressure gas-phase catalytic synthesis of 2-methylpyridine in the examples. Detailed Implementation

[0030] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0031] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can be appropriately combined to form other embodiments that can be understood by those skilled in the art. These other embodiments are also covered within the scope of protection of this invention.

[0032] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains.

[0033] Unless otherwise specified, the materials, reagents, instruments, and equipment used in this invention are all materials, reagents, instruments, and equipment routinely used by those skilled in the art, and the testing standards all use national or international standards commonly used in the field, without further explanation.

[0034] Example 1 This embodiment provides a solid superacid catalyst, prepared by the following method: Dissolve 64.5 g of ZrOCl₂·8H₂O in 500 mL of deionized water, add 4.33 g of La(NO₃)₃·6H₂O and 7.50 g of Al(NO₃)₃·9H₂O, and stir until completely dissolved to prepare a mixed solution with a Zr:La:Al molar ratio of 1:0.05:0.10. Under vigorous stirring, add 25% ammonia solution dropwise to the above solution until the pH reaches 9.5, forming a Zr / La / Al mixed hydroxide precipitate. After aging at room temperature for 24 hours, filter and wash repeatedly with deionized water until no Cl₂ is visible in the filtrate. - The SO42- was detected using a 0.1 mol / L AgNO3 solution. The filter cake was dried at 100℃ for 24 hours, then impregnated with a 1.0 mol / L sulfuric acid solution for 1 hour, filtered, dried at 100℃ for 24 hours, and finally calcined in a muffle furnace at 600℃ for 3 hours to obtain SO42-. 2- / ZrO2-La2O3-Al2O3 composite solid superacid catalyst. The obtained catalyst was ground and sieved, and 20-60 mesh particles were collected for later use.

[0035] Example 2 Dissolve 64.5 g of ZrOCl₂·8H₂O in 500 mL of deionized water. Add 4.34 g of Ce(NO₃)₃·6H₂O, 4.38 g of Nd(NO₃)₃·6H₂O, and 5.82 g of Ni(NO₃)₂·6H₂O, and stir until completely dissolved to prepare a mixed solution with a Zr:Ce:Nd:Ni molar ratio of 1:0.05:0.05:0.10. Under vigorous stirring, add 25% ammonia solution dropwise to the above solution until pH=9.5, forming a Zr / Ce / Nd / Ni mixed hydroxide precipitate. After aging at room temperature for 24 hours, filter and wash repeatedly with deionized water until no Cl₂ is found in the filtrate. - The SO42- was detected using a 0.1 mol / L AgNO3 solution. The filter cake was dried at 100℃ for 24 hours, then impregnated with a 1.0 mol / L sulfuric acid solution for 1 hour, filtered, dried at 100℃ for 24 hours, and finally calcined in a muffle furnace at 600℃ for 3 hours to obtain SO42-. 2- / ZrO2-Ce2O3-Nd2O3-NiO composite solid superacid catalyst. The obtained catalyst was ground and sieved, and 20-60 mesh particles were collected for later use.

[0036] Example 3 The solid superacid catalyst prepared in Example 2 was applied to the synthesis of 2-methylpyridine, using the following method: Figure 1 The equipment shown is operated as follows: (a) Feeding Pretreatment Section A 40% (w / w) aqueous solution of acetaldehyde was stored in an acetaldehyde storage tank, and a 25% (w / w) aqueous solution of ammonia was stored in an ammonia storage tank. The acetaldehyde solution from the acetaldehyde storage tank was fed into an acetaldehyde preheater at a feed rate of 0.15 mL / min and preheated to 200°C; the ammonia from the ammonia storage tank was fed into an ammonia preheater at a feed rate of 0.20 mL / min and preheated to 200°C. The preheated acetaldehyde vapor and ammonia were mixed at a molar ratio of acetaldehyde to ammonia of 3:4 to form a reaction mixture for later use.

[0037] Catalyst pre-activation: Before the reaction begins, the acetaldehyde and ammonia feeds are shut off, the nitrogen cylinder is opened, and high-purity nitrogen is introduced into the fixed-bed reactor at a flow rate of 50 mL / min. The temperature is raised to 300℃ to activate the catalyst bed in the reactor for 2 hours. After activation, the nitrogen is shut off, and the feed is switched to the mixed gas of the reaction raw materials.

[0038] (II) Core Reaction Section of Fixed Bed Reactor SO4 prepared in Example 2 2-30 g of a ZrO2-Ce2O3-Nd2O3-NiO composite solid superacid catalyst was packed into a fixed-bed stainless steel reactor with an inner diameter of 20 mm and a length of 500 mm to form a catalyst bed. The reactor is a vertical fixed-bed reactor, and the feed gas mixture enters from the top of the reactor and passes through the catalyst bed from top to bottom.

[0039] The reaction temperature was controlled at 300℃, and the reaction pressure was atmospheric pressure. An acetaldehyde-ammonia mixture preheated to 200℃ was introduced into the reactor at a total feed rate of 0.35 mL / min (0.15 mL / min of acetaldehyde aqueous solution and 0.20 mL / min of ammonia solution). A cyclization catalytic synthesis reaction occurred on the catalyst bed. The main product was 2-methylpyridine, and the byproducts were 4-methylpyridine and 2-methyl-5-ethylpyridine.

[0040] The high-temperature reaction mixture generated during the reaction is sent out from the top outlet of the reactor and enters the downstream condenser; the lower aqueous phase wastewater generated during the reaction is discharged from the bottom branch of the reactor and sent to the wastewater pretreatment unit.

[0041] (III) Downstream product post-processing flow of the reactor The high-temperature reaction mixture discharged from the reactor outlet is sent to a condenser and cooled to room temperature by circulating cooling water to obtain a crude pyridine-water mixture. The condensed material then enters a gas-liquid separator for gas-liquid separation. The gas phase consists of unreacted ammonia, which is discharged from the top of the gas-liquid separator and sent to the tail gas absorption tower for recovery and treatment; the liquid phase consists of a crude pyridine-water mixture, which enters a separator for settling and separation. The upper layer is the crude pyridine organic phase, which is sent to a distillation column for distillation separation; the lower layer is the aqueous phase, which is sent to the wastewater pretreatment unit.

[0042] The distillation column is used to collect products according to the distillation temperature: 2-methylpyridine fraction is collected at 128-129℃ and stored in a 2-methylpyridine collection tank; 4-methylpyridine fraction is collected at 145℃ and stored in a 4-methylpyridine collection tank; 2-methyl-5-ethylpyridine fraction is collected at 178℃ and stored in a 2-methyl-5-ethylpyridine collection tank.

[0043] (iv) Closed-loop process for catalyst regeneration When the catalyst becomes deactivated due to long-term use resulting in coking or sulfate loss, the deactivated catalyst is discharged from the bottom of the fixed-bed reactor and transported to the catalyst regeneration unit. The deactivated catalyst is then fed into a calcination furnace and calcined in air at 300–400°C for 2–4 hours to burn off the surface coking. The calcined catalyst is then impregnated with a 1.0 mol / L sulfuric acid solution for 1 hour to replenish the lost sulfate. The impregnated catalyst is then dried at 100°C for 24 hours and then calcined in air at 600°C for 3 hours to obtain the regenerated catalyst. The regenerated catalyst is returned to the fixed-bed reactor for reloading and reuse.

[0044] (V) Ammonia Circulation Supporting Process Unreacted ammonia gas discharged from the top of the gas-liquid separator is piped to the tail gas absorption tower, where it is absorbed by dilute sulfuric acid to generate ammonium sulfate. The ammonium sulfate then reacts with lime slurry to release ammonia gas, which is then treated in an ammonia hydrolysis tank to produce circulating ammonia water. This circulating ammonia water is stored in a circulating ammonia water storage tank and then returned to the ammonia water storage tank. After being reheated, it is sent to the fixed-bed reactor to participate in the reaction. The ammonia recovery rate is not less than 92%.

[0045] The total yield of alkylpyridines was determined to be 60.2%, with a selectivity of 42-48% for 2-methylpyridine, 28-32% for 4-methylpyridine, and 20-25% for 2-methyl-5-ethylpyridine.

[0046] Example 4 The catalyst, whose activity decreased after 50 hours of use in Example 3, was removed from the reactor and calcined at 350°C for 3 hours in air to remove carbon deposits. After cooling, it was impregnated with a 1.0 mol / L sulfuric acid solution for 1 hour, filtered, dried at 100°C for 24 hours, and then calcined at 600°C for 3 hours to obtain the regenerated catalyst. The regenerated catalyst was reloaded and used under the conditions of Example 1, with a total alkylpyridine yield of 58.5% and the activity restored to more than 97% of that of the fresh catalyst.

[0047] Example 5 The uncondensed gas discharged from the gas-liquid separator in Example 3 was passed into the tail gas absorption tower and absorbed countercurrently with 10% (w / w) dilute sulfuric acid. The absorbent was an ammonium sulfate solution. The ammonium sulfate solution was reacted with lime slurry, and the released ammonia gas was dried, compressed, and returned to the ammonia storage tank, thus achieving the recycling of ammonia. Measurements showed that the ammonia recovery rate could reach over 92%.

[0048] Comparative Example 1 After running continuously for 100 hours according to the method in Example 1, the catalyst activity decreased significantly, the total yield of alkylpyridine dropped below 35%, and the byproduct alkylbenzene compounds increased significantly. This indicates that the unregenerated catalyst has a limited lifespan and cannot meet the requirements of industrial production.

[0049] Comparative Example 2 A commercially available aluminosilicate catalyst, reported in the literature, was used instead of the solid superacid catalyst in Example 2, with the remaining process conditions the same as in Example 3. The results showed that the aluminosilicate catalyst exhibited extremely low activity at 300°C, with a total alkylpyridine yield of less than 15%. The reaction temperature needed to be increased to above 400°C to obtain a yield comparable to that of Example 3, indicating that the solid superacid catalyst of this invention possesses higher low-temperature activity.

[0050] Example 6 Using the solid superacid catalyst prepared in Example 2, with a fixed molar ratio of acetaldehyde to ammonia of 3:4 and a feed rate of 0.2 mL / min, 2-methylpyridine synthesis experiments were conducted at different reaction temperatures. The results are shown in Table 1 below: Table 1. Effect of reaction temperature on the overall yield of alkylpyridine and the selectivity of 2-methylpyridine

[0051] As can be seen from the table, 300℃ is the optimal reaction temperature. Below this temperature, the reaction activity is insufficient and the yield is low. Above this temperature, the side reactions intensify, and the overall yield and the selectivity of the target product both show a downward trend.

[0052] Example 7 Using the solid superacid catalyst prepared in Example 2, with a fixed molar ratio of acetaldehyde to ammonia of 3:4 and a reaction temperature of 300℃, 2-methylpyridine synthesis experiments were conducted at different feed rates. The results are shown in Table 2 below: Table 2 Effect of feed rate on overall yield of alkylpyridine and selectivity of 2-methylpyridine

[0053] As can be seen from the table, the yield is highest when the feed rate is 0.15-0.2 mL / min. If the feed rate is too fast, the contact time between the raw material and the catalyst will be insufficient, and the reaction will be incomplete; if the feed rate is too slow, the yield per unit time will decrease.

[0054] Example 8 Using the solid superacid catalyst prepared in Example 2, and with the reaction temperature fixed at 300℃ and the feed rate at 0.2 mL / min, 2-methylpyridine synthesis experiments were conducted at different molar ratios of acetaldehyde to ammonia. The results are shown in Table 3 below: Table 3 Effect of feed rate on overall yield of alkylpyridine and selectivity of 2-methylpyridine

[0055] As shown in the table, the highest overall yield and the lowest amount of alkylbenzene byproducts were achieved when the molar ratio of acetaldehyde to ammonia was 3:4. An appropriate excess of ammonia helps suppress side reactions and improves the selectivity of the target product.

[0056] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A solid superacid catalyst, characterized in that, The chemical formula of the solid superacid catalyst is SO4. 2- / ZrO2-Re a M x O y SO4 2- The catalyst consists of a ZrO2 host, a rare earth metal salt Re, and a second metal salt M, where a is the molar ratio of Re to Zr, a = 0.01-0.10; x is the molar ratio of M to Zr, x = 0.02-0.15; y is the stoichiometric coefficient of oxygen required to satisfy the valence of M and the value of x; the solid superacid catalyst has a mesoporous structure.

2. The solid superacid catalyst according to claim 1, characterized in that, Re is a combination of at least two rare earth metals selected from La, Ce, Nd, and Pr, and M is a combination of one or more rare earth metals selected from Al, Fe, Ni, and W.

3. The solid superacid catalyst according to claim 1, characterized in that, The specific surface area of ​​the catalyst is 80–160 m². 2 / g, with a particle size of 20-60 mesh, and the particles are spherical or near-spherical, wherein the content of tetragonal ZrO2 is not less than 80%.

4. A method for preparing a solid superacid catalyst as described in any one of claims 1-3, characterized in that, Includes the following steps: (1) Dissolve soluble zirconium salt, rare earth metal salt and second metal salt in deionized water at a molar ratio of Zr:Re:M = 1:a:x to prepare a mixed solution with a total metal ion concentration of 0.2-1.0 mol / L; (2) Under stirring conditions, ammonia water is added dropwise to the mixed solution until the pH value is 9-10, a mixed hydroxide precipitate is generated, and aged at low temperature of -5℃ to 5℃ for 48-72 hours; (3) Filter and wash the precipitate until no Cl is present. - The filter cake is dried at 80-110℃ for 12-36 hours; (4) The dried precipitate is soaked in 0.2-2.0 mol / L sulfuric acid solution for 0.5-2 hours, filtered, and then dried at 80-110℃ for 12-36 hours; (5) The dried solid is calcined in air at 550-700℃ for 2-4 hours to obtain the catalyst.

5. The method for preparing a solid superacid catalyst according to claim 4, characterized in that, In step (1), the soluble zirconium salt is ZrOCl2·8H2O or Zr(NO3)4·5H2O, the rare earth metal salt is a mixture of nitrates of La, Ce, Nd, and Pr, and the second metal salt is Ni(NO3)2·6H2O or (NH4)6H2W. 12 O 40 In step (4), the concentration of sulfuric acid solution is 0.5-1.0 mol / L, and the soaking time is 1-1.5 hours.

6. The method for preparing a solid superacid catalyst according to claim 4, characterized in that, In step (4), the impregnation is carried out under ultrasonic-assisted conditions, with an ultrasonic frequency of 20-50 kHz and an ultrasonic treatment time of 10-30 minutes.

7. The application of a solid superacid catalyst as described in any one of claims 1-3, characterized in that, The application of solid superacid catalysts in the atmospheric pressure gas-phase catalytic synthesis of 2-methylpyridine includes the following steps: The granular solid superacid catalyst is packed into a fixed-bed reactor and activated by introducing an inert gas at a temperature of 300-350℃ for 1-3 hours. After preheating acetaldehyde aqueous solution and ammonia water to 150-250℃ respectively, they are introduced into a fixed-bed reactor packed with the granular solid superacid catalyst at a molar ratio of acetaldehyde to ammonia of 2.5-3.5:3.5-4.

5. The reaction temperature was controlled at 280-320℃, the reaction pressure at atmospheric pressure, and the feed rate at 0.1-0.3 mL / min. The reaction mixture was discharged from the outlet of the fixed-bed reactor and condensed by a condenser to obtain a mixture containing 2-methylpyridine, 4-methylpyridine, 2-methyl-5-ethylpyridine and water; The mixture was separated into organic phases by liquid-liquid separation, and the organic phases were further separated by distillation to collect 2-methylpyridine fraction, 4-methylpyridine fraction and 2-methyl-5-ethylpyridine fraction respectively. Unreacted ammonia gas is recovered and recycled after passing through an absorption tower.

8. The application of the solid superacid catalyst according to claim 7, characterized in that, The mass fraction of acetaldehyde aqueous solution is 40%, and the mass fraction of ammonia solution is 25%.

9. The application of the solid superacid catalyst according to claim 7, characterized in that, When granular solid superacid catalysts are deactivated due to coking or sulfate loss, they can be regenerated using a method including the following steps: (i) Remove the deactivated solid superacid catalyst from the reactor and calcine it at 300-400°C in air for 2-4 hours to burn off the surface carbon. (ii) The charred catalyst is impregnated with a 0.2-2.0 mol / L sulfuric acid solution for 0.5-2 hours to replenish the lost sulfate ions; (iii) The impregnated catalyst is dried at 80-110℃ for 12-36 hours and then calcined at 550-700℃ for 2-4 hours to obtain the regenerated catalyst.

10. The application of the solid superacid catalyst according to claim 7, characterized in that, The yield of 2-methylpyridine from the regenerated catalyst is not less than 90% of the yield from the fresh catalyst.

Images