Synthesis method of N-vinyl carboxylic acid amide

Abstract

The invention relates to a synthesis method of N-vinyl carboxylic acid amide. The method comprises the steps of catalytic loading system preparation, condensation reaction, membrane separation, dealcoholization reaction, crystallization purification and the like. A mixed system of a Fe3O4 (at) SiO2 (at) ZSM-5-SO3H solid acid catalyst and a compound stabilizer is adopted, and a tubular ZSM-5 molecular sieve membrane separation technology is combined, so that the problems of more side reactions, complex purification and the like of a traditional process are solved. The method has the advantages of high reaction efficiency, few byproducts, low energy consumption, recoverable catalyst, high product purity, suitability for polymerization reaction and the like, and overcomes the defects of the existing method.

Description

Technical Field

[0001] This invention belongs to the field of organic synthesis, specifically relating to a method for synthesizing N-vinylcarboxylic acid amide. Background Technology

[0002] N-Vinylcarboxylic amides are a class of bifunctional monomers possessing both highly reactive vinyl and amide groups, making them invaluable in polymer synthesis. Their vinyl groups can participate in free radical polymerization and photopolymerization, while the amide groups enhance interchain hydrogen bonding, improving the mechanical properties of the materials. By adjusting the R groups (such as methyl, ethyl, aryl, etc.), the polarity, solubility, thermal stability, and biocompatibility of the polymer can be optimized. N-Vinylcarboxylic amides are widely used as polymer monomers in the synthesis of functional polymers, finding applications in hydrogels, flocculants, medical dressings, drug carriers, industrial auxiliaries, and electronic chemicals.

[0003] The most common method for manufacturing N-vinylcarboxylic amides involves using carboxylic amides, acetaldehyde, and alcohols as intermediates to produce N-(1-alkoxyethyl)carboxylic amides, which are then thermally decomposed or catalytically cracked to synthesize the amide. Currently, the primary method for producing the intermediate N-(1-alkoxyethyl)carboxylic amide is the condensation of carboxylic amide, acetaldehyde, and alcohol under acid catalysis. This method is thermodynamically controlled, and the raw materials and intermediates are prone to reversible reactions, especially under acidic conditions. The intermediates are easily hydrolyzed into carboxylic amides and acetaldehyde acetals, leading to low yields and even initiating condensation polymerization to generate difficult-to-treat ethylene dicarboxylic amide solid impurities, thus reducing purification efficiency. Simultaneously, acetaldehyde is prone to aldol condensation under acidic or high-temperature conditions, generating unsaturated aldehyde byproducts such as crotonaldehyde. The residual aldehydes can then act as polymerization inhibitors in subsequent polymerization reactions, suppressing the polymerization process. CN108698981B points out the aforementioned problems and describes a method to achieve high polymerizability of N-vinylcarboxylic acid amide by adjusting its pH to 8.0–8.5 before distillation and using a dealcoholization reaction and distillation purification process to control the content of unsaturated aldehydes below 20 ppm by mass. However, this synthetic method requires multiple distillation steps to remove byproducts, and the pH control window is narrow. In actual production, if the reaction scale is large, pH differences exist in different regions, making precise control impossible.

[0004] Therefore, there is a need to develop a highly efficient, low-byproduct, and controllable method for synthesizing N-vinylcarboxylic acid amides to improve product purity and enhance subsequent polymerization performance, while also ensuring a simple process, reduced energy consumption, and environmental friendliness. Summary of the Invention

[0005] The purpose of this invention is to provide a method for synthesizing N-vinylcarboxylic amides, in order to solve the problems of numerous side reactions, complex product purification processes, and huge energy consumption and cost associated with existing methods for synthesizing N-vinylcarboxylic amides.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] This invention provides a method for synthesizing N-vinylcarboxylic acid amides, comprising the following steps:

[0008] (1) Preparation of catalyst system: Fe3O4@SiO2@ZSM-5-SO3H solid acid catalyst and phenylboronic acid-4-methoxypyridine composite stabilizer (BPA-MP) were mixed at a mass ratio of 1:0.1-0.5 to prepare catalytic support system;

[0009] (2) Condensation reaction: Carboxylic acid amide, acetaldehyde and the catalytic support system prepared in step 1 are reacted at 40-60℃ for 2-4 hours. The amount of catalytic support system is 1-5% of the total mass of the reactants.

[0010] (3) Reaction endpoint determination and separation: The acetaldehyde content in the reaction system was monitored by gas chromatography. When the acetaldehyde concentration dropped to below 1%, a tubular ZSM-5 molecular sieve membrane was used for separation. The membrane pore size was 0.2-1 nm and the operating pressure was 0.5±0.05 MPa. Unreacted acetaldehyde and methanol were separated, and the catalyst system was recovered at the same time to obtain the crude product of N-(1-alkoxyethyl)carboxylic acid amide.

[0011] (4) De-alcoholization reaction: The crude N-(1-alkoxyethyl)carboxylic acid amide obtained in step 3 is subjected to thermal decomposition reaction at 300-400℃ and 10-30kPa for 0.5-2 hours to obtain crude N-vinylcarboxylic acid amide.

[0012] (5) Purification treatment: The crude product obtained in step 4 is dissolved in an organic solvent and purified by crystallization under a gradient cooling condition of 40-10℃ to obtain a high-purity N-vinylcarboxylic acid amide product.

[0013] In one preferred embodiment, the Fe3O4@SiO2@ZSM-5-SO3H catalyst in step (1) is prepared by the following method: after coating Fe3O4 nanoparticles with SiO2, ZSM-5 molecular sieves are grown on their surface by hydrothermal method, and finally sulfonated with concentrated sulfuric acid.

[0014] In one preferred embodiment, the coating of Fe3O4 nanoparticles with SiO2 is specifically performed as follows: Fe3O4 magnetic nanoparticles with a particle size of 50-100 nm prepared by hydrothermal method are dispersed in an ethanol-water mixed solvent, wherein the volume ratio of ethanol to water is (3-8):1, and ultrasonic treatment is performed for 10-60 minutes to form a uniform suspension; under nitrogen protection, tetraethyl orthosilicate is added dropwise to the suspension, with a mass ratio of tetraethyl orthosilicate to Fe3O4 of 1:(1.5-2); simultaneously, ammonia water is added dropwise as a catalyst, the pH is controlled at 9.0-9.5, and the reaction is carried out at a constant temperature of 38-42℃ with stirring for 5-8 hours to form a dense amorphous SiO2 coating layer with a thickness of 10-15 nm on the surface of Fe3O4; after the reaction is completed, the product is separated by an external magnetic field, washed three times with ethanol and deionized water in sequence, and vacuum dried at 60℃ for 12-24 hours to obtain Fe3O4@SiO2 core-shell structured microspheres.

[0015] In one preferred embodiment, the ZSM-5 molecular sieve is grown on its surface by hydrothermal method as follows: a silicon source, an aluminum source, a template agent, and deionized water are mixed and stirred in a molar ratio of 1:(0.01-0.03):(0.2-0.3):30-50 to form a homogeneous silica sol; the obtained Fe3O4@SiO2 core-shell structured microspheres are mixed with the prepared silica sol in a mass ratio of 1:(3-8), transferred to a high-pressure reactor, and hydrothermally reacted at 170±2℃ for 36-48 hours to allow the ZSM-5 molecular sieve to grow directionally on the SiO2 surface, forming a ZSM-5 molecular sieve shell with a thickness of 50-80nm, thus obtaining the Fe3O4@SiO2@ZSM-5 precursor.

[0016] In one preferred embodiment, the sulfonation treatment with concentrated sulfuric acid specifically involves immersing the obtained Fe3O4@SiO2@ZSM-5 precursor in 98% concentrated sulfuric acid at a solid-liquid ratio of 1g:(8-12)mL and refluxing the reaction in an oil bath at 110℃-130℃ for 4-8h.

[0017] In one preferred embodiment, the molar ratio of carboxylic acid amide to acetaldehyde in step (2) is 1:1.1-1.3, the reactor is protected by nitrogen gas, and the oxygen content is ≤100ppm.

[0018] In one preferred embodiment, the tubular ZSM-5 molecular sieve membrane in step (3) adopts a hollow fiber structure with an inner diameter of 0.8±0.1mm and a membrane surface flow rate controlled at 1.5±0.2m / s.

[0019] In one preferred embodiment, the organic solvent in step (5) is one or more mixed solvents selected from methanol, isopropanol, toluene, or cyclohexane.

[0020] The present invention also provides the application of N-vinylcarboxylic amide obtained by the synthesis method according to the present invention as a polymer monomer in polymerization reactions.

[0021] Beneficial effects

[0022] This invention employs a catalytic support system formed by mixing a Fe3O4@SiO2@ZSM-5-SO3H solid acid catalyst with a phenylboronic acid-4-methoxypyridine composite stabilizer (BPA-MP) at a specific mass ratio. Compared to traditional acid catalysis, this catalytic support system with a specific structure exhibits excellent catalytic activity and selectivity, reducing intermediate hydrolysis and acetaldehyde self-polymerization caused by acidic conditions. Simultaneously, the solid acid catalyst provides acidic active sites (-SO3H), enabling synergistic effects with the molecular sieve to achieve high conversion rates under mild conditions (40-60℃) while avoiding high-temperature side reactions.

[0023] This invention utilizes tubular ZSM-5 molecular sieve membranes to replace traditional distillation separation. The ZSM-5 pore size allows for precise sieving of small molecules and intermediates, thereby removing unreacted aldehydes and byproduct alcohols in one step, preventing residual aldehydes from inhibiting subsequent polymerization. Furthermore, the physical sieving mechanism is unaffected by pH, overcoming the challenge of a narrow pH control window (8.0-8.5) in existing technologies.

[0024] The process of this invention is simple, the reaction is controllable, it can significantly reduce energy consumption costs, and ensure high product purity. The product is suitable for subsequent polymerization processes. Since the core of the Fe3O4@SiO2@ZSM-5-SO3H solid acid catalyst is magnetic Fe3O4, it can be magnetically separated and recovered, and the catalyst can be reused, reducing the synthesis cost of the catalyst. Attached Figure Description

[0025] Figure 1 Flowchart for the synthesis of N-vinylcarboxylic acid amide;

[0026] Figure 2 XRD diffraction comparison characterization of Fe3O4@SiO2@ZSM-5 precursor and Fe3O4@SiO2@ZSM-5-SO3H solid acid catalyst prepared in Example 1;

[0027] Figure 3 The image shows the transmission electron microscope (TEM) characterization of the Fe3O4@SiO2@ZSM-5-SO3H solid acid catalyst prepared in Example 1. Detailed Implementation

[0028] Although the invention has been described to a certain extent, it is apparent that appropriate variations can be made to the various conditions without departing from the spirit and scope of the invention. It is understood that the invention is not limited to the described embodiments, but falls within the scope of the claims, which include equivalent substitutions for each of the elements.

[0029] [Existing reaction pathway]

[0030] CN108698981B shows the synthetic route of N-vinylcarboxylic amide, where reaction 1 is the process of producing N-(1-alkoxyethyl)carboxylic amide as an intermediate from carboxylic amide, acetaldehyde and alcohol.

[0031] In the formula, R1 represents an alkyl group with 1 to 5 carbon atoms, R2 independently represents a hydrogen atom or an alkyl group with 1 to 5 carbon atoms, and R3 represents an alkyl group with 1 to 5 carbon atoms.

[0032] Reaction 1:

[0033]

[0034] Reaction 2 is an intermediate N-(1-alkoxyethyl)carboxylic acid amide condensed and de-alcoholized to obtain N-vinylcarboxylic acid amide.

[0035] Reaction 2:

[0036]

[0037] The equilibrium reactions shown in equations 3-7 yield various byproducts during the reaction process. For example, reactions 3, 5, and 6 all produce N,N'-methylenebiscarboxylic acid amides. These byproducts are mixed into the purified N-(1-alkoxyethyl)carboxylic acid amide fraction, further reacting with intermediates and reducing product yield. Simultaneously, the side reaction in reaction 7 introduces low concentrations of unsaturated aldehydes into the N-vinylcarboxylic acid amide, reducing its polymerizability.

[0038] Reaction 3:

[0039]

[0040] Reaction 4:

[0041]

[0042] Reaction 5:

[0043]

[0044] Reaction 6:

[0045]

[0046] Reaction 7:

[0047]

[0048] [Magnetic Solid Acid Catalyst System]

[0049] This invention selects Fe3O4@SiO2@ZSM-5-SO3H solid acid catalyst and phenylboronic acid-4-methoxypyridine composite stabilizer (BPA-MP) at a mass ratio of 1:0.1-0.5 to prepare a catalytic support system. The catalyst uses Fe3O4 magnetic material as the core, endowing the system with magnetic separation characteristics. The outer SiO2 isolation layer prevents the core from being corroded by the acidic environment; the ZSM-5 molecular sieve shell is hydrothermally synthesized on the silica surface, and its regular microporous structure provides a spatial confinement effect of 0.5-0.6 nm, which can effectively block the formation of macromolecular polycondensation byproducts (such as N,N'-methylenebiscarboxylate); the molecular sieve surface is sulfonated to introduce acidic sites, achieving high catalytic activity under mild conditions. Based on this, a phenylboronic acid-4-methoxypyridine composite stabilizer is introduced. This component dynamically and reversibly binds with acetaldehyde through the phenylboronic acid group, which significantly reduces the concentration of free acetaldehyde in the reaction system, thereby inhibiting the formation of byproducts such as crotonaldehyde by aldol condensation. At the same time, 4-methoxypyridine can precisely neutralize the excessively acidic sites of the catalyst locally, avoiding the reverse hydrolysis reaction of intermediates and the self-polymerization of acetaldehyde.

[0050] The Fe3O4@SiO2@ZSM-5-SO3H catalyst was prepared by coating SiO2 with Fe3O4 nanoparticles, growing ZSM-5 molecular sieves on its surface by hydrothermal method, and finally sulfonating with concentrated sulfuric acid.

[0051] The specific steps are as follows:

[0052] (1) SiO2 coating treatment of Fe3O4 nanoparticles:

[0053] Fe3O4 magnetic nanoparticles with a particle size of 50-100 nm prepared by hydrothermal method were dispersed in an ethanol-water mixed solvent with a volume ratio of ethanol to water of (3-8):1, and ultrasonically treated for 10-60 minutes to form a uniform suspension.

[0054] Under nitrogen protection, tetraethyl orthosilicate (TEOS) was added dropwise to the suspension at a mass ratio of 1:(1.5-2) to Fe3O4. Simultaneously, ammonia was added as a catalyst, and the pH was controlled at 9.0-9.5. The reaction was carried out at a constant temperature of 38-42℃ with stirring for 5-8 hours, resulting in a dense amorphous SiO2 coating layer with a thickness of 10-15 nm on the Fe3O4 surface.

[0055] After the reaction was completed, the product was separated by an external magnetic field, washed three times with ethanol and deionized water, and dried under vacuum at 60°C for 12-24 h to obtain Fe3O4@SiO2 core-shell structured microspheres.

[0056] The SiO2 layer can isolate the magnetic core from the acidic environment, preventing iron ions from dissolving and causing catalyst deactivation.

[0057] (2) Hydrothermal growth of ZSM-5 molecular sieve shell

[0058] A homogeneous silica sol is formed by mixing and stirring a silicon source (silica sol, in which the SiO2 content is 25-40 wt%), an aluminum source (sodium aluminate), a template agent (tetrapropylammonium hydroxide, TPAOH), and deionized water in a molar ratio of 1:(0.01-0.03):(0.2-0.3):30-50.

[0059] The Fe3O4@SiO2 core-shell structured microspheres obtained in step (1) are mixed with the silica sol prepared in step (2) at a mass ratio of 1:(3-8), and then transferred to a high-pressure reactor. The mixture is subjected to hydrothermal reaction at 170±2℃ for 36-48 hours to allow ZSM-5 molecular sieve to grow directionally on the SiO2 surface, forming a ZSM-5 molecular sieve shell with a thickness of 50-80nm.

[0060] After magnetic separation, the product was calcined at 500-550℃ for 5-8 hours to remove the template agent, yielding the Fe3O4@SiO2@ZSM-5 precursor with a micropore size of 0.5-0.6 nm.

[0061] (3) Sulfonation treatment introduces acidic sites

[0062] The Fe3O4@SiO2@ZSM-5 precursor obtained in step (2) was immersed in 98% concentrated sulfuric acid with a solid-liquid ratio of 1g:(8-12)mL and refluxed in an oil bath at 110℃-130℃ for 4-8h.

[0063] Sulfuric acid molecules diffuse through the molecular sieve channels to the surface and interior, reacting with silanol groups to generate sulfonic acid groups (-SO3H).

[0064] The prepared Fe3O4@SiO2@ZSM-5-SO3H solid acid catalyst was mixed with phenylboronic acid-4-methoxypyridine composite stabilizer (BPA-MP) at a mass ratio of 1:0.1-0.5 to prepare a catalytic support system. This mixing ratio balances catalytic activity with the stabilizer's inhibitory effect on side reactions.

[0065] [Condensation reaction]

[0066] The carboxylic acid amide, acetaldehyde, and the catalytic support system prepared in step 1 are reacted at 40-60℃ for 2-4 hours. The amount of the catalytic support system is 1-5% of the total mass of the reactants. The molar ratio of carboxylic acid amide to acetaldehyde is 1:1.1-1.3. The reaction vessel is purged with nitrogen for protection, and the oxygen content is ≤100ppm.

[0067] Under the specific catalyst system of this invention, the condensation reaction temperature is controlled within the range of 40-60℃, which can prevent acetaldehyde from undergoing aldol condensation to generate unsaturated aldehyde byproducts such as crotonaldehyde under high temperature conditions. At the same time, it can prevent the intermediate N-(1-alkoxyethyl)carboxylic acid amide from hydrolyzing or thermally decomposing due to high temperature, thus ensuring that the reaction is carried out under mild conditions.

[0068] The amount of catalytic support system is 1-5% of the total mass of reactants. Among them, Fe3O4@SiO2@ZSM-5-SO3H solid acid catalyst provides acidic active sites (-SO3H), which work synergistically with the microporous structure (pore size 0.5-0.6nm) of ZSM-5 molecular sieve to inhibit the formation of macromolecular byproducts (such as N,N'-methylene dicarboxylic acid amide) through spatial confinement effect, while achieving high-efficiency catalysis. The phenylboronic acid-4-methoxypyridine composite stabilizer reduces the concentration of free acetaldehyde to inhibit self-polymerization by dynamically and reversibly binding phenylboronic acid groups with acetaldehyde, while 4-methoxypyridine neutralizes the excessively acidic sites of the catalyst locally, avoiding reverse hydrolysis of intermediates.

[0069] The molar ratio of carboxylic amide to acetaldehyde is controlled at 1:1.1-1.3. Excess acetaldehyde can shift the reaction equilibrium towards the formation of intermediates, thereby increasing the conversion rate of carboxylic amide. At the same time, it avoids side reactions caused by excessive free aldehyde concentration in the system due to excessive acetaldehyde.

[0070] The reactor is filled with nitrogen for protection, and the oxygen content is ≤100ppm, so that the reaction is carried out in an inert environment to prevent acetaldehyde from being oxidized to produce impurities such as carboxylic acids, which would affect the purity of the product and the subsequent polymerization performance. At the same time, it avoids the oxidation and deterioration of carboxylic amides, ensuring the stability of the reaction system.

[0071] By monitoring the acetaldehyde content in the reaction system by gas chromatography, when the acetaldehyde concentration drops below 1%, it indicates that the condensation reaction between carboxylic amide and acetaldehyde has been basically completed. At this point, terminating the reaction can avoid the decomposition of intermediate N-(1-alkoxyethyl)carboxylic amide or the increase in the amount of by-products (such as ethylene dicarboxylic amide, unsaturated aldehydes) due to excessive reaction, thereby ensuring the yield and purity of intermediate.

[0072] Separation was performed using a tubular ZSM-5 molecular sieve membrane. A pore size of 0.2-1 nm effectively distinguished between small and large molecules; for example, acetaldehyde molecules have a diameter of approximately 0.4 nm, methanol approximately 0.38 nm, and the intermediate N-(1-alkoxyethyl)carboxylic acid amide molecules have a diameter of approximately 0.7-0.9 nm. Under low-pressure operating conditions of 0.5 ± 0.05 MPa, unreacted small molecules such as acetaldehyde and methanol could permeate through the membrane pores for separation, while intermediates and the catalytically supported system were retained.

[0073] The tubular ZSM-5 molecular sieve membrane employs a hollow fiber structure with an inner fiber diameter controlled at 0.8±0.1 mm, providing a large specific surface area and high liquid permeability. Combined with a membrane surface flow rate of 1.5±0.2 m / s, it ensures efficient permeation of small molecules while reducing membrane pore blockage through a cross-flow filtration mechanism, maintaining stable separation efficiency. This separation process eliminates the high-temperature energy consumption of traditional distillation, simultaneously removing unreacted raw materials through physical sieving. Furthermore, it is unaffected by the system's pH value, significantly simplifying the process and improving production stability, providing a low-impurity intermediate crude product for subsequent dealcoholization reactions.

[0074] [De-alcoholization reaction]

[0075] The crude N-(1-alkoxyethyl)carboxylic acid amide obtained in step 3 was subjected to thermal decomposition at 300-400℃ and 10-30 kPa for 0.5-2 hours to obtain the crude N-vinylcarboxylic acid amide. High temperature is required to provide the activation energy for dealcoholization, while low pressure promotes the rapid removal of volatiles such as methanol to drive the reaction forward. This process is an intramolecular dealcoholization reaction. Since the acidic catalyst and free aldehydes were removed by membrane separation in step 3, and the pH of the crude product is close to neutral, no additional pH control is needed to efficiently generate the crude N-vinylcarboxylic acid amide.

[0076] [Purification Process]

[0077] The crude product obtained in step 4 is dissolved in an organic solvent and purified by crystallization under a gradient cooling condition of 40-10℃ to obtain a high-purity N-vinylcarboxylic acid amide product. The organic solvent in step (5) is one or more mixed solvents selected from methanol, isopropanol, toluene, or cyclohexane. Utilizing the difference in solubility between the target product and impurities at different temperatures, the target product is gradually precipitated in crystal form by a gradient cooling of 40-10℃ at a rate of 5-10℃ / h. The low temperature environment reduces the solubility of the product, promoting crystallization, while the gradient cooling avoids disordered crystal growth or impurity encapsulation caused by a sudden drop in temperature, thereby improving the purity of the crystals. The selected solvent and product polarity are matched, and the dissolution-crystallization balance can be optimized by adjusting the ratio. Finally, a high-purity N-vinylcarboxylic acid amide product is obtained by filtration or centrifugation.

[0078] To further understand the present invention, preferred embodiments of the present invention are described below in conjunction with examples. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, and not for limiting the scope of the claims of the present invention.

[0079] Example 1:

[0080] 10g of Fe3O4 nanoparticles with an average particle size of 80nm were dispersed in 60ml of a mixed solvent of ethanol and water at a volume ratio of 5:1 and sonicated for 30 minutes. Under nitrogen protection, 5.7g of TEOS was added dropwise at a mass ratio of TEOS to Fe3O4 of 1:1.8, and ammonia was added dropwise to adjust the pH to 9.3. The mixture was stirred at 40℃ for 6 hours. After magnetic field separation, washing, and drying, Fe3O4@SiO2 microspheres were obtained. Subsequently, 10g of Fe3O4@SiO2 microspheres were reacted with silicon... 50g of silica sol, containing a silica source, aluminum source, template agent, and water in a molar ratio of 1:0.02:0.25:40, was mixed and hydrothermally reacted at 170℃ for 40 hours. After magnetic separation, it was calcined at 530℃ for 6 hours to obtain the Fe3O4@SiO2@ZSM-5 precursor. Finally, 10g of the obtained precursor was impregnated in 100ml of 98% concentrated sulfuric acid and refluxed in an oil bath at 120℃ for 6 hours to prepare the Fe3O4@SiO2@ZSM-5-SO3H solid acid catalyst. 10g of the obtained Fe3O4@SiO2@ZSM-5-SO3H solid acid catalyst was then used. Figure 2 XRD diffraction characterization patterns of Fe3O4@SiO2@ZSM-5 precursor and Fe3O4@SiO2@ZSM-5-SO3H solid acid catalyst are shown. It can be seen that obvious diffraction peaks appear at 2θ at approximately 30.1°, 35.5°, 43.1°, 53.4°, 57.0°, and 62.6°, which correspond to the (220), (311), (400), (422), (511), and (440) crystal planes of Fe3O4, respectively. Dense characteristic peaks appear at 23.1°, 23.9°, and 24.4°, which correspond to the crystal planes of ZSM-5 molecular sieve. There is also a broadened diffuse peak in the 2θ range of 20°-30°, which is a characteristic peak of amorphous SiO2. As can be seen from the figure, the introduction of the SO3H group did not produce any new characteristic diffraction peaks, and the positions of the peaks representing other structures did not change, indicating that the crystal form was not destroyed. However, the peak intensity decreased after the introduction compared to before the introduction, proving the introduction of the SO3H group.

[0081] Figure 3 This is a transmission electron microscope (TEM) characterization image of the Fe3O4@SiO2@ZSM-5-SO3H solid acid catalyst prepared in Example 1. The three-layer coating structure of Fe3O4@SiO2@ZSM-5-SO3H can be clearly seen.

[0082] Then, the obtained Fe3O4@SiO2@ZSM-5-SO3H solid acid catalyst was mixed with 3g of phenylboronic acid-4-methoxypyridine composite stabilizer to prepare a catalytic support system;

[0083] 71 g of N-vinylformamide carboxylic acid amide (1 mol) was mixed with 52.8 g of acetaldehyde (1.2 mol, approximately 66 mL), and 3.7 g of catalytic loading system was added. The mixture was reacted at 50 °C for 3 hours under conditions of nitrogen and oxygen content ≤100 ppm. The acetaldehyde concentration was monitored by gas chromatography until it was <1%. The catalyst and unreacted substances were separated and recovered using a tubular ZSM-5 molecular sieve membrane with a hollow fiber structure (0.8 mm inner diameter, 0.5 nm pore size, 0.5 MPa pressure, and 1.5 m / s flow rate) to obtain the crude intermediate product. The crude product was thermally decomposed at 350 °C and 20 kPa for 1 hour. The crude product was dissolved in methanol, and the mixture was purified by crystallization at a gradient cooling rate of 5 °C / h from 40 to 10 °C to obtain a high-purity N-vinylformamide product.

[0084] Example 2

[0085] Compared to Example 1, the amount of BPA-MP added was adjusted to 1g, while other conditions remained unchanged.

[0086] Example 3

[0087] Compared with Example 1, the amount of catalyst support system added was adjusted to 1.3g, while other conditions remained unchanged.

[0088] Example 4

[0089] Compared with Example 1, the conditions for the condensation reaction were adjusted to react at 60°C for 2 hours, and the amount of acetaldehyde added was 57.27 g (1.3 mol, about 71.6 mL), while other conditions remained unchanged.

[0090] Comparative Example 1

[0091] 71g of N-vinylformamide carboxylic acid amide (1mol) was mixed with 52.8g of acetaldehyde (1.2mol, about 66mL), and 5.4g of concentrated sulfuric acid was added. The mixture was reacted at 60℃ for 6 hours under conditions of nitrogen and oxygen content ≤100ppm. The acetaldehyde concentration was monitored by gas chromatography until it was <1%, and the pH of the reaction system was 2.5.

[0092] Sodium carbonate powder was added to the system to adjust the pH to between 8.0 and 8.5. Unreacted acetaldehyde and methanol were then separated by distillation at 21°C and 65°C under normal pressure using a conventional distillation apparatus to obtain crude N-(1-alkoxyethyl)carboxylic acid amide.

[0093] The crude product was thermally decomposed at 350℃ and 20kPa for 1 hour. The crude product was dissolved in methanol and subjected to a gradient cooling of 5℃ / h from 40 to 10℃. The product was then purified by crystallization to obtain a high-purity N-vinylcarboxylic acid amide product.

[0094] Comparative Example 2

[0095] Compared to Example 1, BPA-MP was not added, and other conditions remained unchanged.

[0096] Comparative Example 3

[0097] Compared with Example 1, the tubular ZSM-5 molecular sieve membrane was not used for separation. Instead, a conventional distillation apparatus was used to sequentially distill and separate unreacted acetaldehyde and methanol at atmospheric pressure and controlled temperatures of 21°C and 65°C, respectively, to obtain crude N-(1-alkoxyethyl)carboxylic acid amide product, while other conditions remained unchanged.

[0098] [Polymerization Test]

[0099] Prepare a 100ml glass container equipped with a catalyst injection tube, a nitrogen blowing tube, a nitrogen exhaust tube, and a thermometer. Weigh 20g of N-vinylacetamide and 58g of ion-exchanged water into the container. While bubbling with 50ccm nitrogen, heat the container to 30°C in a water bath and continue purging with nitrogen until the polymerization is complete.

[0100] 1.6g of V-044 (azoimidazoline type manufactured by Wako Pure Chemical Industries Co., Ltd.) as a polymerization initiator was dissolved in 48.4g of deionized water, and 4.0g of V-50 (azoamidine type manufactured by Wako Pure Chemical Industries Co., Ltd.) as a polymerization initiator was dissolved in 46.0g of deionized water.

[0101] One hour after nitrogen purging, 1g of dissolved V-044 polymerization initiator was added via syringe, followed by 1g of dissolved V-50 polymerization initiator.

[0102] Remove the glass container from the water bath, remove the moisture from the glass surface with paper, and then transfer it to an insulated container to continue polymerization. Monitor the polymerization temperature and use the time to reach the standard temperature peak from the time the polymerization initiator is added as an indicator of polymerization performance. The goal is to ensure that the standard temperature peak of the polymerized product reaches the standard temperature peak in less than 120 minutes.

[0103] [Yield Calculation]

[0104] The actual yield is the mass (g) of the N-vinylcarboxylate crystals obtained after final purification.

[0105] Yield (%) = Actual yield (g) / Theoretical yield (g) × 100%.

[0106] [Unsaturated aldehyde content]

[0107] The unsaturated aldehydes contained in the final product N-vinylacetamide were determined by GC / HPLC.

[0108] Table 1 shows the concentration, yield, and polymerizability test results of unsaturated aldehydes in N-vinylacetamide in each example and comparative example.

[0109]

[0110] Therefore, Examples 1-3 utilized the catalytic support system and membrane separation technology of the present invention, while Comparative Examples 1-3 used conventional sulfuric acid catalysis, stabilizer-free conditions, and membrane separation-free conditions, respectively. The data show that the concentration of unsaturated aldehydes in the Examples was significantly lower than that in the Comparative Examples. Furthermore, the catalytic system of the present invention achieved high conversion rates under mild conditions, reducing intermediate hydrolysis and side reactions, and improving yield.

[0111] In the polymerization tests, the standard temperature reached in the examples took approximately 100 minutes, while in Comparative Example 1 it took as long as 194 minutes, indicating that the product of the present invention has high purity, fewer byproducts, and superior polymerization performance. In traditional methods, high concentrations of unsaturated aldehydes act as polymerization inhibitors, significantly prolonging the polymerization time, while the present invention effectively removes these impurities through membrane separation.

[0112] Furthermore, compared to the longer condensation reaction time of Comparative Example 1, the examples also demonstrate that the catalytic support system of the present invention effectively shortens the reaction time.

[0113] It should be noted that this specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. A method for synthesizing N-vinylcarboxylic acid amide, characterized in that, Includes the following steps: (1) Preparation of catalyst system: Fe3O4@SiO2@ZSM-5-SO3H solid acid catalyst and phenylboronic acid-4-methoxypyridine composite stabilizer were mixed at a mass ratio of 1:0.1-0.5 to prepare catalytic support system; (2) Condensation reaction: Carboxylic acid amide, acetaldehyde and the catalytic support system prepared in step 1 are reacted at 40-60℃ for 2-4 hours. The amount of catalytic support system is 1-5% of the total mass of the reactants. (3) Reaction endpoint determination and separation: The acetaldehyde content in the reaction system was monitored by gas chromatography. When the acetaldehyde concentration dropped to below 1%, a tubular ZSM-5 molecular sieve membrane was used for separation. The membrane pore size was 0.2-1 nm and the operating pressure was 0.5±0.05 MPa. Unreacted acetaldehyde and methanol were separated, and the catalyst system was recovered at the same time to obtain the crude product of N-(1-alkoxyethyl)carboxylic acid amide. (4) De-alcoholization reaction: The crude N-(1-alkoxyethyl)carboxylic acid amide obtained in step 3 is subjected to thermal decomposition reaction at 300-400℃ and 10-30kPa for 0.5-2 hours to obtain crude N-vinylcarboxylic acid amide. (5) Purification treatment: The crude product obtained in step 4 is dissolved in an organic solvent and purified by crystallization under a gradient cooling condition of 40-10℃ to obtain a high-purity N-vinylcarboxylic acid amide product.

2. The synthesis method according to claim 1, characterized in that, The Fe3O4@SiO2@ZSM-5-SO3H catalyst in step (1) is prepared by the following method: Fe3O4 nanoparticles are coated with SiO2, ZSM-5 molecular sieves are grown on their surface by hydrothermal method, and finally sulfonated with concentrated sulfuric acid.

3. The synthesis method according to claim 2, characterized in that, The specific method of coating SiO2 with Fe3O4 nanoparticles is as follows: Fe3O4 magnetic nanoparticles with a particle size of 50-100nm prepared by hydrothermal method are dispersed in an ethanol-water mixed solvent, wherein the volume ratio of ethanol to water is (3-8):1, and ultrasonic treatment is performed for 10-60 minutes to form a uniform suspension. Under nitrogen protection, tetraethyl orthosilicate is added dropwise to the suspension at a mass ratio of 1:(1.5-2) to Fe3O4; ammonia is added dropwise as a catalyst, and the pH is controlled at 9.0-9.

5. The mixture is stirred at a constant temperature of 38-42℃ for 5-8 hours to form a dense amorphous SiO2 coating layer with a thickness of 10-15 nm on the surface of Fe3O4. After the reaction was completed, the product was separated by an external magnetic field, washed three times with ethanol and deionized water, and dried under vacuum at 60°C for 12-24 h to obtain Fe3O4@SiO2 core-shell structured microspheres.

4. The synthesis method according to claim 2, characterized in that, The ZSM-5 molecular sieve is grown on its surface by hydrothermal method as follows: silicon source, aluminum source, template agent and deionized water are mixed and stirred in a molar ratio of 1:(0.01-0.03):(0.2-0.3):30-50 to form a homogeneous silica sol; The obtained Fe3O4@SiO2 core-shell structured microspheres were mixed with the prepared silica sol at a mass ratio of 1:(3-8), transferred to a high-pressure reactor, and subjected to hydrothermal reaction at 170±2℃ for 36-48 hours to allow ZSM-5 molecular sieve to grow directionally on the SiO2 surface, forming a ZSM-5 molecular sieve shell with a thickness of 50-80nm, thus obtaining the Fe3O4@SiO2@ZSM-5 precursor.

5. The synthesis method according to claim 2, characterized in that, The sulfonation treatment with concentrated sulfuric acid specifically involves immersing the obtained Fe3O4@SiO2@ZSM-5 precursor in 98% concentrated sulfuric acid at a solid-liquid ratio of 1g:(8-12)mL and refluxing it in an oil bath at 110℃-130℃ for 4-8 hours.

6. The synthesis method according to claim 1, characterized in that, In step (2), the molar ratio of carboxylic acid amide to acetaldehyde is 1:1.1-1.3, the reactor is filled with nitrogen for protection, and the oxygen content is ≤100ppm.

7. The synthesis method according to claim 1, characterized in that, In step (3), the tubular ZSM-5 molecular sieve membrane adopts a hollow fiber structure with an inner diameter of 0.8±0.1mm and a membrane surface flow rate controlled at 1.5±0.2m / s.

8. The synthesis method according to claim 1, characterized in that, The organic solvent in step (5) is one or more mixed solvents selected from methanol, isopropanol, toluene, or cyclohexane.

9. The synthesis method according to claim 1, characterized in that, The cooling rate in step (5) is 5-10℃ / h.

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