A green synthesis process of N, N-dimethylacrylamide based on fixed-bed catalysis and membrane separation

Abstract

This invention relates to the field of N,N-dimethylacrylamide preparation technology, specifically to a green synthesis process for N,N-dimethylacrylamide based on fixed-bed catalysis and membrane separation. The invention includes: firstly, preparing a polyetherimide composite membrane base; constructing a benzyl chloride initiating layer on the feed side of the polyetherimide selective layer; and sequentially grafting polyethylene glycol methyl ether methacrylate (300 number-average molecular weight), polyethylene glycol methyl ether methacrylate (500 number-average molecular weight), and sodium 4-vinylbenzenesulfonate to obtain a modified polyetherimide composite membrane; then, subjecting the reaction solution obtained after the fixed-bed catalytic reaction of methyl acrylate and anhydrous dimethylamine to cyclic pervaporation membrane separation and short-path light removal to obtain the N,N-dimethylacrylamide product. This invention helps reduce membrane surface polymerization fouling during the low-temperature de-alcoholization process of the fixed-bed outlet reaction solution and improves membrane flux retention and product color stability.

Description

Technical Field

[0001] This invention relates to the field of N,N-dimethylacrylamide preparation technology, specifically to a green synthesis process for N,N-dimethylacrylamide based on fixed-bed catalysis and membrane separation. Background Technology

[0002] N,N-Dimethylacrylamide is a class of functional monomers containing carbon-carbon double bonds and amide groups, which can be used to prepare water-soluble polymers, functional coatings, and fine chemical intermediates. Its subsequent applications are highly sensitive to product purity, color, and oligomer content; therefore, the synthesis process requires not only high conversion rates but also careful control of heating time and byproduct residues.

[0003] Existing preparation methods mainly include acyl chloride-amine hydrolysis and ester-amine hydrolysis. Acyl chloride-amine hydrolysis has high reactivity, but it easily produces acidic byproducts and inorganic salts, resulting in problems such as equipment corrosion, waste salt treatment, and heavy post-processing burdens. Ester-amine hydrolysis produces mainly alcohols as byproducts, making it more suitable for green production, but the reaction solution contains methanol, dimethylamine, acrylates, and N,N-dimethylacrylamide, which are difficult to separate.

[0004] Fixed-bed catalytic processes offer advantages such as continuous operation, easy catalyst separation, and controllable residence time. However, the effluent from the fixed-bed reactor still requires the removal of methanol and light components. If conventional vacuum distillation is used, N,N-dimethylacrylamide is prone to oligomerization, discoloration, or scaling under heating, amine components, and trace impurities, affecting the stability of the continuous process and the performance of the product.

[0005] Membrane separation can reduce the thermal risk of heat-sensitive monomers, but ordinary polyimide or polyetherimide membranes are prone to selective layer swelling, methanol selective decay, and membrane surface polymerization fouling in systems containing dimethylamine, methanol, and N,N-dimethylacrylamide. Existing monolithic hydrophilization or random grafting methods are difficult to simultaneously achieve low-temperature dehydration, membrane flux maintenance, and anti-deposition stability. Summary of the Invention

[0006] In view of this, the purpose of this invention is to propose a green synthesis process for N,N-dimethylacrylamide based on fixed-bed catalysis and membrane separation, in order to solve the problems of membrane swelling and fouling, insufficient methanol selectivity and flux retention during low-temperature de-alcoholization of the existing fixed-bed effluent.

[0007] To achieve the above objectives, this invention provides a green synthesis process for N,N-dimethylacrylamide based on fixed-bed catalysis and membrane separation, comprising the following steps: S1: Preparation of polyetherimide composite membrane base film; S2: Apply a benzyl chloride initiating layer grafting solution containing 4-vinylbenzyl chloride and benzophenone to the surface of the polyetherimide selective layer on one side, and perform surface grafting under ultraviolet light irradiation to obtain a polyetherimide composite film with a benzyl chloride initiating layer. S3: The polyetherimide selective layer side of the polyetherimide composite film with benzyl chloride initiation layer is first reacted with a first grafting solution containing polyethylene glycol methyl ether methacrylate, cuprous chloride and 2,2'-bipyridine with a number average molecular weight of 300, then reacted with a second grafting solution containing polyethylene glycol methyl ether methacrylate, cuprous chloride and 2,2'-bipyridine with a number average molecular weight of 500, and then reacted with a third grafting solution containing sodium 4-vinylbenzenesulfonate, cuprous chloride and 2,2'-bipyridine to obtain a modified polyetherimide composite film; S4: The synthesized hydrotalcite is shaped, dried and calcined to obtain a magnesium-aluminum composite oxide catalyst, which is then loaded into a fixed-bed reactor. S5: Methyl acrylate and anhydrous dimethylamine are continuously fed into a fixed-bed reactor containing the magnesium-aluminum composite oxide catalyst to carry out ammonolysis reaction, and the reaction liquid at the fixed-bed outlet is collected. S6: The fixed bed outlet reaction liquid is circulated through the modified polyetherimide composite membrane for pervaporation membrane separation to obtain permeate, and the permeate is subjected to short-path light removal to obtain N,N-dimethylacrylamide product. The polyetherimide composite film base film includes a polyester support film and a polyetherimide selective layer formed on one side of the polyester support film; By weight, the benzyl chloride initiator grafting solution in step S2 is obtained by mixing 7-9 parts of 4-vinylbenzyl chloride treated with the polymerization inhibitor removed, 0.15-0.25 parts of benzophenone and 91-93 parts of ethyl acetate and purging with nitrogen. By weight, the first grafting solution in step S3 is obtained by mixing 22-26 parts of polyethylene glycol methyl ether methacrylate with a number average molecular weight of 300 treated with a polymerization inhibitor, 53-57 parts of methanol, 19-23 parts of deionized water, 0.07-0.09 parts of cuprous chloride and 0.23-0.29 parts of 2,2'-bipyridine and purging with nitrogen. By weight, the second grafting solution in step S3 consists of 5-7 parts of polyethylene glycol methyl ether methacrylate with a number average molecular weight of 500 treated with a polymerization inhibitor removed, 16-20 parts of methanol, 5-7 parts of deionized water, 0.015-0.025 parts of cuprous chloride and 0.06-0.08 parts of 2,2'-bipyridine. By weight, the third grafting solution in step S3 consists of 7-9 parts sodium 4-vinylbenzenesulfonate, 43-47 parts deionized water, 43-47 parts methanol, 0.025-0.035 parts cuprous chloride, and 0.09-0.11 parts 2,2'-bipyridine.

[0008] Preferably, the preparation of the polyetherimide composite film base in step S1 includes: mixing polyetherimide resin and N-methylpyrrolidone to obtain a polyetherimide casting solution; the polyetherimide casting solution is coated onto one side of a polyester support film, and the wet film thickness is 25-35 μm.

[0009] Preferably, the polyester support film in step S1 consists of six circular polyester support films with a diameter of 47 mm, each with a geometric area of ​​17.3 cm². 2 The total geometric area is 104 cm². 2 After film formation and grafting modification, it is loaded into a flat-sheet membrane separation module with a total effective membrane area of ​​100 cm². 2 .

[0010] Preferably, the benzyl chloride initiator layer grafting solution in step S2 is applied to the surface of the polyetherimide selective layer with a wet film thickness of 20-30 μm.

[0011] Preferably, the ultraviolet irradiation in step S2 is irradiation with 365nm ultraviolet light for 7-9 minutes under nitrogen protection, with an ultraviolet light intensity of 18-22mW / cm². 2 .

[0012] Preferably, in step S3, the polyetherimide selective layer side of the polyetherimide composite film with the benzyl chloride initiating layer is in contact with the first grafting liquid and reacts at 28-32°C for 35-45 minutes.

[0013] Preferably, in step S3, the second grafting solution is added dropwise to the same reaction system within 4-6 minutes after the first grafting solution reaction is completed, and the reaction continues at 28-32°C for 18-22 minutes after the addition is complete.

[0014] Preferably, in step S3, after the second grafting solution reaction is completed, the membrane is immediately transferred into the third grafting solution that has been pre-purged with nitrogen for 25-35 minutes, so that the polyetherimide selective layer side comes into contact with the end grafting solution, and reacts at 33-37°C for 25-35 minutes.

[0015] Preferably, after the third grafting solution contact reaction in step S3 is completed, the membrane is immersed in a cleaning solution composed of disodium ethylenediaminetetraacetate dihydrate, methanol and deionized water, then rinsed with methanol, deionized water and methanol in sequence, and dried in nitrogen.

[0016] Before surface grafting, the polyethylene glycol methyl ether methacrylate with a number average molecular weight of 300, the polyethylene glycol methyl ether methacrylate with a number average molecular weight of 500, and 4-vinylbenzyl chloride need to be treated with a polymerization inhibitor through a neutral alumina short column.

[0017] Preferably, the preparation of the magnesium-aluminum composite oxide catalyst in step S4 includes: weighing synthetic hydrotalcite, methylcellulose and deionized water, mixing them evenly and then extruding them into shape, drying them at 110-130℃ for 5-7 hours, calcining them in air at 430-470℃ for 3-5 hours, and then sieving them after cooling to obtain magnesium-aluminum composite oxide catalyst particles.

[0018] Preferably, the weight ratio of the synthetic hydrotalcite and methylcellulose in step S4 is 90-110:1-3.

[0019] Preferably, the fixed-bed reaction in step S5 includes: weighing a magnesium-aluminum composite oxide catalyst and loading it into a conventional tubular fixed-bed reactor, activating it under a nitrogen atmosphere; weighing and mixing methyl acrylate and p-methoxyphenol; feeding anhydrous dimethylamine through a steel cylinder with back pressure control; the reaction temperature is 62-68℃; and the reaction pressure is 0.8-1.2MPa.

[0020] Preferably, the weight ratio of methyl acrylate, p-methoxyphenol, and anhydrous dimethylamine in step S5 is 480-520:0.05-0.07:260-290.

[0021] Preferably, the methyl acrylate feed rate in step S5 is 40-46 g / h, and the anhydrous dimethylamine feed rate is 22-26 g / h.

[0022] Preferably, during membrane separation in step S6, the feed-side temperature is 42-48℃, the feed-side pressure is 180-220kPa, the feed-side circulation flow rate is 55-65g / h, the permeate-side absolute pressure is 2-4kPa, and the permeate-side condensation temperature is -10-0℃.

[0023] The beneficial effects of this invention are: This invention concentrates the modification effect on the feed side of the polyetherimide composite membrane, which is beneficial for adjusting the mass transfer state of the membrane surface in the area where the reaction liquid first contacts the membrane at the outlet of the fixed bed, and reduces unnecessary swelling and mass transfer resistance inside the selective layer.

[0024] This invention utilizes the sequential grafting of polyethylene glycol methyl ether methacrylates of different molecular weights to give the membrane surface both alcohol-friendly transport and flexible buffering properties, which facilitates the preferential permeation of methanol and reduces the residence and accumulation of N,N-dimethylacrylamide near the membrane surface.

[0025] The present invention further introduces a sodium 4-vinylbenzenesulfonate end segment, which is beneficial to improve the stability of the membrane surface in the amino alcohol system, reduce the adhesion of double bond amide oligomers, and thus improve flux maintenance and product color control in the continuous membrane separation process.

[0026] Compared with existing technologies that rely on vacuum distillation or conventional organic membrane separation, this invention is more suitable for online dealcoholization, light component recovery, and low-temperature concentration in the continuous green synthesis of N,N-dimethylacrylamide. It can reduce the risk of polymerization discoloration caused by prolonged heating of heat-sensitive monomers and improve the stability of membrane separation operation and product purification efficiency. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0028] The sources and types of raw materials are as follows: Polyetherimide resin: SABIC's ULTEM 1000 is a non-reinforced polyetherimide resin with a glass transition temperature of 217°C; Polyester support film: Sterlitech polyester film, 47mm in diameter, 1.2μm in pore size, with nonwoven support; Synthetic hydrotalcite: Sigma-Aldrich, model 652288, linear formula Mg6Al2(CO3)(OH)16·4H2O.

[0029] Example 1: A green synthesis process for N,N-dimethylacrylamide based on fixed-bed catalysis and membrane separation, the specific steps of which are as follows: (1) Preparation of polyetherimide composite membrane base: Weigh 18g of polyetherimide resin and 82g of N-methylpyrrolidone, add them to a dry glass container with mechanical stirring, and stir at 60℃ for 12h to obtain a uniform and transparent polyetherimide casting solution; take 6 polyester support films, lay them flat and fix them on a clean glass plate, and use a doctor blade to scrape the polyetherimide casting solution onto one side of the polyester support film, controlling the wet film thickness to 30μm, then pre-dry at 60℃ for 30min, and then vacuum dry at 80℃ and 20kPa for 12h; after drying, immerse the film in 300g methanol, 300g deionized water and 300g methanol for 15min each, and then dry in nitrogen at 45℃ for 4h to obtain the polyetherimide composite film base film; (2) Treat the grafted monomers with polymerization inhibitors: Weigh 50g of polyethylene glycol methyl ether methacrylate with a number average molecular weight of 300, pass it through a short column packed with 100g of neutral alumina, collect the colorless effluent and place it in a brown bottle under nitrogen protection; weigh 20g of polyethylene glycol methyl ether methacrylate with a number average molecular weight of 500, pass it through a short column packed with 50g of neutral alumina, collect the colorless effluent and place it in a brown bottle under nitrogen protection; weigh 20g of 4-vinylbenzyl chloride, pass it through a short column packed with 50g of neutral alumina, collect the effluent and use it immediately in the next step; (3) Pre-coating: Weigh 8g of 4-vinylbenzyl chloride (treated with a polymerization inhibitor remover), 0.2g of benzophenone, and 92g of ethyl acetate. Mix them and purge with nitrogen for 30 minutes to obtain a benzyl chloride initiator layer grafting solution. Lay the polyetherimide composite film base obtained in step one flat on a polytetrafluoroethylene plate with the polyetherimide selective layer facing upwards and the polyester support layer facing downwards, ensuring that the polyester support layer does not directly contact the benzyl chloride initiator layer grafting solution. Uniformly coat the benzyl chloride initiator layer grafting solution onto the surface of the polyetherimide selective layer, controlling the wet film thickness to 25μm. Irradiate with 365nm ultraviolet light for 8 minutes under nitrogen protection, with the ultraviolet light intensity controlled at 20mW / cm². 2 After irradiation, the film was immediately rinsed three times with 300g ethyl acetate, then rinsed three times with 300g methanol, and dried in nitrogen at 35℃ for 2 hours to obtain a polyetherimide composite film with a benzyl chloride initiating layer. (4) First coat application: Weigh 24g of polyethylene glycol methyl ether methacrylate (300 number average molecular weight) treated with a polymerization inhibitor removal agent, 55g of methanol, 21g of deionized water, 80mg of cuprous chloride, and 260mg of 2,2'-bipyridine, and add them to a reaction vessel with a nitrogen inlet. After purging with nitrogen for 30 minutes, the first grafting solution is obtained. The polyetherimide composite membrane with a benzyl chloride initiating layer obtained in step three is brought into contact with the first grafting solution using a single-sided liquid film contact method, so that the polyetherimide selective layer side contacts the first grafting solution while the polyester support layer side remains unsubmerged. The reaction is carried out at 30°C for 40 minutes. During the reaction, nitrogen protection is maintained and slight agitation is performed to keep the membrane surface in a continuous liquid film state. After the reaction, no drying, no terminator is added, and the membrane is not exposed to air; it proceeds directly to the next step. (5) Second coat application: In the same reaction system after step four, a second grafting solution consisting of 6g of polyethylene glycol methyl ether methacrylate (number average molecular weight 500) treated with a polymerization inhibitor, 18g of methanol, 6g of deionized water, 20mg of cuprous chloride, and 70mg of 2,2'-bipyridine was slowly added dropwise along the edge of the membrane over 5 minutes. After the addition was complete, the reaction was continued at 30°C for 20 minutes, with nitrogen protection maintained throughout the reaction. After the reaction was completed, drying and air exposure were not performed, and the process proceeded directly to the next step. (6) Post-coating: Immediately after the reaction in step five, the membrane was transferred into a third grafting solution pre-purged with nitrogen for 30 minutes. The third grafting solution consisted of 8 g sodium 4-vinylbenzenesulfonate, 45 g deionized water, 45 g methanol, 30 mg cuprous chloride, and 100 mg 2,2'-bipyridine. The polyetherimide selective layer side was brought into contact with the third grafting solution, and the reaction was carried out at 35°C for 30 minutes. After the reaction, the membrane was immediately immersed in a cleaning solution consisting of 2 g disodium ethylenediaminetetraacetate dihydrate, 1400 g methanol, and 600 g deionized water for 30 minutes. Then, it was rinsed with 300 g methanol, 300 g deionized water, and 300 g methanol for 15 minutes each. Finally, it was dried in nitrogen at 40°C for 6 hours to obtain a modified polyetherimide composite membrane for membrane separation of the reaction liquid at the fixed bed outlet. (7) Preparation of magnesium-aluminum composite oxide catalyst for fixed bed: Weigh 100g of synthetic hydrotalcite, 2g of methylcellulose and 25g of deionized water, mix them evenly and extrude them into strips with a diameter of about 1mm. Dry them at 120℃ for 6h and then calcine them in air at 450℃ for 4h. After cooling, sieve them to obtain magnesium-aluminum composite oxide catalyst particles of 20-40 mesh. (8) Conduct fixed-bed catalytic synthesis reactions: 80g of magnesium-aluminum composite oxide catalyst was weighed and loaded into a conventional tubular fixed-bed reactor, activated at 120℃ under a nitrogen atmosphere for 2h, and then cooled to 65℃. 500g of methyl acrylate and 58mg of p-methoxyphenol were weighed, mixed, and placed in the first raw material tank. Anhydrous dimethylamine was fed through a steel cylinder with back pressure control, and 276g was cumulatively measured by a mass flow meter. The two raw materials were continuously fed into the fixed-bed reactor after being purged with nitrogen, with the methyl acrylate feed rate at 43g / h and the anhydrous dimethylamine feed rate at 24g / h. The reaction temperature was controlled at 65℃ and the reaction pressure was controlled at 1MPa. The reaction liquid in the first 60min after the start of the reaction was considered as a transition fraction and not included in the results. Subsequently, no less than 600g of the reaction liquid at the fixed bed outlet was collected. (9) Membrane separation of the reaction solution at the fixed bed outlet: Weigh 600g of the obtained fixed-bed outlet reaction solution, add 50mg of p-methoxyphenol, and cool to 45℃. Then, introduce the solution into a conventional flat-sheet membrane separation module containing the obtained modified polyetherimide composite membrane. Place six modified polyetherimide composite membranes into a conventional multi-sheet flat-sheet membrane separation module, making the total effective membrane area 100cm². 2Membrane separation is carried out using a circulating pervaporation method. The feed side temperature is controlled at 45℃, the feed side pressure is controlled at 200kPa, the feed side circulation flow rate is controlled at 60g / h, the permeate side absolute pressure is controlled at 3kPa, and the permeate side condensation temperature is controlled at -5℃. The residual liquid is returned to the feed tank for continued circulation until the mass of the residual liquid is approximately 500g. (10) Obtain N,N-dimethylacrylamide product: Weigh 500g of the obtained osmotic liquid, add 20mg of p-methoxyphenol, and perform conventional short-path desulfurization for 40min at 40℃ and 3kPa to remove residual dimethylamine, methanol and trace amounts of methyl acrylate to obtain N,N-dimethylacrylamide product.

[0030] The difference between Example 2 and Example 1 is as follows: In step (1), 16g of polyetherimide resin and 84g of N-methylpyrrolidone were weighed, the stirring temperature was 58℃, the stirring time was 12h, and the wet film thickness was controlled to be 28μm; in step (3), 7g of 4-vinylbenzyl chloride treated with the polymerization inhibitor removed, 150mg of benzophenone and 93g of ethyl acetate were weighed, the wet film thickness was controlled to be 22μm, and the ultraviolet light irradiation time was 7min; in step (4), 22g of polyethylene glycol methyl ether methacrylate with a number average molecular weight of 300 treated with the polymerization inhibitor removed, 56g of methanol, 22g of deionized water, 70mg of cuprous chloride and 230mg of 2,2'-bipyridine were weighed, and reacted at 28℃ for 35min; in step (5), the second grafting solution consisted of 5g of polyethylene glycol methyl ether methacrylate with a number average molecular weight of 500 treated with the polymerization inhibitor removed, 17g of methanol, 6g of deionized water, 15mg of cuprous chloride and 60mg of ethyl acetate. The solution was composed of 2,2'-bipyridine, with a dropping time of 4 min, followed by a reaction time of 18 min after dropping. In step (6), the end-graft solution consisted of 7 g sodium 4-vinylbenzenesulfonate, 43 g deionized water, 47 g methanol, 25 mg cuprous chloride, and 90 mg 2,2'-bipyridine. The reaction temperature was 33 °C, and the reaction time was 25 min. In step (8), the reaction temperature was controlled at 62 °C, the reaction pressure was controlled at 0.8 MPa, the methyl acrylate feed rate was 40 g / h, and the anhydrous dimethylamine feed rate was 22 g / h. In step (9), the membrane separation feed side temperature was controlled at 42 °C, the feed side pressure was controlled at 180 kPa, the feed side circulation flow rate was controlled at 55 g / h, the permeate side absolute pressure was controlled at 4 kPa, and the permeate side condensation temperature was controlled at 0 °C. The remaining conditions were the same as in Example 1.

[0031] The difference between Example 3 and Example 1 is as follows: In step (1), 20g of polyetherimide resin and 80g of N-methylpyrrolidone were weighed, the stirring temperature was 62℃, the stirring time was 12h, and the wet film thickness was controlled to be 32μm; in step (3), 9g of 4-vinylbenzyl chloride treated with the polymerization inhibitor removed, 250mg of benzophenone and 91g of ethyl acetate were weighed, the wet film thickness was controlled to be 28μm, and the ultraviolet light irradiation time was 9min; in step (4), 26g of polyethylene glycol methyl ether methacrylate with a number average molecular weight of 300 treated with the polymerization inhibitor removed, 53g of methanol, 19g of deionized water, 90mg of cuprous chloride and 290mg of 2,2'-bipyridine were weighed, and reacted at 32℃ for 45min; in step (5), the second grafting solution consisted of 7g of polyethylene glycol methyl ether methacrylate with a number average molecular weight of 500 treated with the polymerization inhibitor removed, 20g of methanol, 7g of deionized water, 25mg of cuprous chloride and 80mg of ethyl acetate. The solution was composed of 2,2'-bipyridine, with a dropping time of 6 min, followed by a 22 min reaction after dropping. In step (6), the end-graft solution consisted of 9 g sodium 4-vinylbenzenesulfonate, 47 g deionized water, 43 g methanol, 35 mg cuprous chloride, and 110 mg 2,2'-bipyridine. The reaction temperature was 37 °C, and the reaction time was 35 min. In step (8), the reaction temperature was controlled at 68 °C, the reaction pressure was controlled at 1.2 MPa, the methyl acrylate feed rate was 46 g / h, and the anhydrous dimethylamine feed rate was 26 g / h. In step (9), the membrane separation feed side temperature was controlled at 48 °C, the feed side pressure was controlled at 220 kPa, the feed side circulation flow rate was controlled at 65 g / h, the permeate side absolute pressure was controlled at 2 kPa, and the permeate side condensation temperature was controlled at -10 °C. The remaining conditions were the same as in Example 1.

[0032] The difference between Example 4 and Example 1 is as follows: In step (3), 8g of 4-vinylbenzyl chloride treated with the polymerization inhibitor removed, 200mg of benzophenone and 92g of ethyl acetate were weighed, the wet film thickness was controlled at 25μm, and the UV irradiation time was 8min; in step (4), 24g of polyethylene glycol methyl ether methacrylate with a number average molecular weight of 300 treated with the polymerization inhibitor removed, 55g of methanol, 21g of deionized water, 80mg of cuprous chloride and 260mg of 2,2'-bipyridine were weighed, and the reaction was carried out at 30℃ for 40min; in step (5), the second grafting solution consisted of 7g of polyethylene glycol methyl ether methacrylate with a number average molecular weight of 500 treated with the polymerization inhibitor removed, 18g of methanol, 6g of deionized water, 20mg of cuprous chloride and 70mg of 2,2'-bipyridine, and the dropwise addition time was 5min, and the reaction continued for 20min after the dropwise addition was completed; in step (6), the end grafting solution consisted of 7g of polyethylene glycol methyl ether methacrylate with a number average molecular weight of 500 treated with the polymerization inhibitor removed, 18g of methanol, 6g of deionized water, 20mg of cuprous chloride and 70mg of 2,2'-bipyridine, and the dropwise addition time was 5min, and the reaction continued for 20min after the dropwise addition was completed; The reaction mixture consisted of sodium 4-vinylbenzenesulfonate, 45 g deionized water, 45 g methanol, 30 mg cuprous chloride, and 100 mg 2,2'-bipyridine. The reaction temperature was 35 °C, and the reaction time was 30 min. The fixed-bed reaction conditions and membrane separation conditions in steps (8) and (9) were the same as in Example 1. All other conditions were the same as in Example 1.

[0033] The difference between Comparative Example 1 and Example 1 is that in step (3), the polyetherimide composite film base film is completely immersed in benzyl chloride initiating layer grafting solution and treated under the same ultraviolet light intensity and the same irradiation time, so that both the polyetherimide selective layer side and the polyester support layer side are in contact with benzyl chloride initiating layer grafting solution; the cleaning, drying and subsequent steps (4) to (10) after irradiation are the same as in Example 1.

[0034] The difference between Comparative Example 2 and Example 1 is that the polyetherimide composite film with benzyl chloride initiator layer obtained in step (3) is retained, but steps (4) to (6) are changed to one-step grafting. Specifically, 24g of polyethylene glycol methyl ether methacrylate with a number average molecular weight of 300 treated with a polymerization inhibitor, 6g of polyethylene glycol methyl ether methacrylate with a number average molecular weight of 500 treated with a polymerization inhibitor, 8g of sodium 4-vinylbenzenesulfonate, 118g of methanol, 72g of deionized water, 130mg of cuprous chloride and 430mg of 2,2'-bipyridine are weighed, mixed and nitrogen gas is introduced for 30min to obtain the one-step grafting solution. The polyetherimide selective layer side is brought into contact with the one-step grafting solution, and the reaction is carried out at 30°C for 70min, and then the temperature is raised to 35°C and the reaction is continued for 30min. The cleaning, drying and steps (7) to (10) after the reaction are completed are the same as in Example 1.

[0035] The difference between Comparative Example 3 and Example 1 is that the order of adding the two types of polyethylene glycol methyl ether methacrylate in steps (4) and (5) is reversed. Specifically, a grafting solution consisting of 6g of polyethylene glycol methyl ether methacrylate with a number average molecular weight of 500 treated with a polymerization inhibitor, 18g of methanol, 6g of deionized water, 20mg of cuprous chloride and 70mg of 2,2'-bipyridine is first reacted for 20min, and then a grafting solution consisting of 24g of polyethylene glycol methyl ether methacrylate with a number average molecular weight of 300 treated with a polymerization inhibitor, 55g of methanol, 21g of deionized water, 80mg of cuprous chloride and 260mg of 2,2'-bipyridine is added dropwise and the reaction continues for 40min. Steps (6) to (10) are the same as in Example 1.

[0036] The difference between Comparative Example 4 and Example 1 is that the 4-vinylbenzenesulfonate end grafting in step (6) is performed before step (4); specifically, after step (3), the polyetherimide selective layer is first brought into contact with a third grafting solution composed of 8g of 4-vinylbenzenesulfonate, 45g of deionized water, 45g of methanol, 30mg of cuprous chloride and 100mg of 2,2'-bipyridine, and reacted at 35°C for 30min; then, polyethylene glycol methyl ether methacrylate with a number average molecular weight of 300 and polyethylene glycol methyl ether methacrylate with a number average molecular weight of 500 are grafted according to steps (4) and (5) of Example 1, respectively; the subsequent steps are the same as in Example 1.

[0037] The difference between Comparative Example 5 and Example 1 is that in step (6), 8g of sodium 4-vinylbenzenesulfonate was replaced with 8g of polyethylene glycol methyl ether methacrylate with a number average molecular weight of 500 after treatment with the polymerization inhibitor removal agent. The 45g deionized water, 45g methanol, 30mg cuprous chloride and 100mg 2,2'-bipyridine in the third grafting solution, as well as the reaction temperature, reaction time, cleaning and drying conditions, remained unchanged. The other conditions were the same as in Example 1.

[0038] Sample source and test sample preparation: Examples 1 to 4 and Comparative Examples 1 to 5 were prepared according to the steps described therein, including modified polyetherimide composite membranes, fixed-bed outlet reaction solutions, and N,N-dimethylacrylamide products. At least three batches of parallel membranes were prepared for each sample group. After the corresponding modification steps were completed, the membranes used for intrinsic characterization were first rinsed with 300g methanol for 15min, then with 300g deionized water for 15min, then with 300g methanol for 15min, and then dried in nitrogen at 40°C for 6h. The membranes used for long-cycle membrane separation were taken out after 120h of continuous operation, quickly rinsed with 50g methanol for 5s to remove free liquid on the surface, and then dried in nitrogen at 40°C to constant weight. The samples used for catalyst characterization were taken from the 20-40 mesh magnesium-aluminum composite oxide catalyst particles obtained in step (7) of each example and comparative example, ground and passed through a 200-mesh sieve before testing. The samples used for product testing were taken from the N,N-dimethylacrylamide product obtained in step (10).

[0039] Water contact angle test: The wettability of the membrane surface was tested according to GB / T 30693-2014 "Measurement of the contact angle between plastic films and water". Three membranes from Examples 1 to 4 and Comparative Examples 1 to 5 were taken. The test surface was the feed side of the polyetherimide selective layer. The test environment temperature was 25°C and the relative humidity was 50%. 5 μL of deionized water was added to each test point. After the droplet contacted the membrane surface for 10 seconds, a photo was taken and the static water contact angle was calculated. Five different positions were tested for each membrane and the average value was taken.

[0040] Gas chromatography test: The conversion rate of methyl acrylate, the selectivity of N,N-dimethylacrylamide and the purity of N,N-dimethylacrylamide were determined according to GB / T 9722-2023 "General Rules for Gas Chromatography of Chemical Reagents". Take 0.5g of the fixed bed outlet reaction solution or the product obtained in step (10), add methanol solution containing n-butanol internal standard to dilute to 10g, mix well and inject 1μL; use flame ionization detector, capillary column specifications of 30m×0.32mm×0.25μm, injection port temperature of 220℃, detector temperature of 250℃, column temperature program of 40℃ for 3min, increase to 180℃ at 10℃ / min and hold for 5min, split ratio of 20:1. Based on the peak areas of methyl acrylate, N,N-dimethylacrylamide and by-products, combined with the internal standard correction factor, the conversion rate and selectivity were calculated.

[0041] Pervaporation membrane separation performance test: The test was conducted according to the membrane flux and separation coefficient test principle of GB / T 34243-2017 "Pervaporation Water-Permeable Membrane Performance Test Method". The test feed liquid was the fixed bed outlet reaction liquid obtained in step (8) of this invention. The membrane sheets obtained from Examples 1 to 4 and Comparative Examples 1 to 5 were loaded into a conventional multi-sheet flat membrane separation module of the same specification, with a total effective membrane area of ​​100 cm². 2Weigh 600g of the fixed-bed outlet reaction solution, add 50mg of p-methoxyphenol, and control the feed-side temperature at 45℃, feed-side pressure at 200kPa, feed-side circulation flow rate at 60g / h, permeate-side absolute pressure at 3kPa, and permeate-side condensation temperature at -5℃. After continuous operation for 6 hours, weigh the permeate mass and determine the composition of the permeate and feed solutions using gas chromatography. Membrane flux is calculated based on the permeate mass per unit membrane area and per unit time. The methanol to N,N-dimethylacrylamide separation factor is calculated based on the mass fraction ratio of methanol to N,N-dimethylacrylamide in the permeate and feed solutions.

[0042] Long-term membrane flux retention test: The membranes obtained from Examples 1 to 4 and Comparative Examples 1 to 5 were loaded into a conventional multi-sheet flat membrane separation module of the same specification and continuously fed under membrane separation conditions for 120 hours. The permeate mass and feed composition were recorded every 12 hours. The membrane flux retention rate was calculated as the ratio of the membrane flux at 120 hours to the initial membrane flux.

[0043] Product colorimetric test: The platinum cobalt colorimetric value of the N,N-dimethylacrylamide product obtained by step (10) of GB / T 605-2006 "General Method for Determination of Colorimetric Value of Chemical Reagents" was determined. Take 50 mL of each of the products obtained in Examples 1 to 4 and Comparative Examples 1 to 5, place them in a 50 mL colorimetric tube, and visually compare them with the same volume of platinum cobalt standard color scale at 25°C. Record the closest standard color number not exceeding the sample color.

[0044]

[0045] As shown in Table 1, Examples 1 to 4 constructed a shallow benzyl chloride initiation layer on the feed side of the polyetherimide composite membrane and sequentially grafted polyethylene glycol methyl ether methacrylate (300 number-average molecular weight), polyethylene glycol methyl ether methacrylate (500 number-average molecular weight), and sodium 4-vinylbenzenesulfonate. This enabled the membrane to maintain both preferential methanol permeation and long-term flux during the low-temperature separation of the reaction liquid at the fixed-bed outlet. Comparative Example 1 used an overall immersion method to form the initiation layer, but the grafted region diffused into the membrane, increasing the mass transfer resistance of the selective layer. Comparative Example 2 added all three grafted monomers simultaneously, making it difficult to form an ordered layered structure. Comparative Example 3 changed the order of the two polyether monomers, with the long-chain structure occupying the initiation site first, weakening the continuity of the short-chain alcohol-loving inner layer. Comparative Example 4 grafted sodium 4-vinylbenzenesulfonate in advance, causing the anionic aromatic sulfonate unit to occupy the shallow site too early, reducing methanol transfer efficiency. Comparative Example 5 replaced the anionic aromatic segment with an equal amount of long-chain polyether, retaining some permeability, but lacking external anti-deposition constraints. This demonstrates that the effect of the present invention does not stem from a single hydrophilic modification, but rather from the synergistic effect between shallow spatial confinement, sequential grafting of dimolecular-weight polyethers, and anionic aromatic segments.

[0046] In summary, this invention is applicable to the online de-alcoholization of the fixed-bed effluent reaction liquid, recovery of light components, and low-temperature concentration of products in the continuous green synthesis of N,N-dimethylacrylamide. It can reduce the risk of polymerization discoloration caused by high-temperature distillation dwell time and improve the stability of the membrane separation process and the product purification efficiency.

[0047] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of the different aspects of the invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. A green synthesis process for N,N-dimethylacrylamide based on fixed-bed catalysis and membrane separation, characterized in that, Includes the following steps: S1: Preparation of polyetherimide composite membrane base film; S2: Apply a benzyl chloride initiating layer grafting solution containing 4-vinylbenzyl chloride and benzophenone to the surface of the polyetherimide selective layer on one side, and perform surface grafting under ultraviolet light irradiation to obtain a polyetherimide composite film with a benzyl chloride initiating layer. S3: The polyetherimide selective layer side of the polyetherimide composite film with benzyl chloride initiation layer is first reacted with the first grafting liquid, then reacted with the second grafting liquid, and then reacted with the third grafting liquid to obtain the modified polyetherimide composite film. S4: The synthesized hydrotalcite is shaped, dried and calcined to obtain a magnesium-aluminum composite oxide catalyst, which is then loaded into a fixed-bed reactor. S5: Methyl acrylate and anhydrous dimethylamine are continuously fed into a fixed-bed reactor containing the magnesium-aluminum composite oxide catalyst to carry out ammonolysis reaction, and the reaction liquid at the fixed-bed outlet is collected. S6: The fixed bed outlet reaction liquid is circulated through the modified polyetherimide composite membrane for pervaporation membrane separation to obtain permeate, and the permeate is subjected to short-path light removal to obtain N,N-dimethylacrylamide product. The polyetherimide composite film base film in step S1 includes a polyester support film and a polyetherimide selective layer formed on one side of the polyester support film; The benzyl chloride initiator grafting solution in step S2 is obtained by mixing 7-9 parts of 4-vinylbenzyl chloride treated with the polymerization inhibitor removed, 0.15-0.25 parts of benzophenone and 91-93 parts of ethyl acetate and purging with nitrogen gas. By weight, in step S3, the first grafting solution is obtained by mixing 22-26 parts of polyethylene glycol methyl ether methacrylate with a number average molecular weight of 300 treated with a polymerization inhibitor, 53-57 parts of methanol, 19-23 parts of deionized water, 0.07-0.09 parts of cuprous chloride, and 0.23-0.29 parts of 2,2'-bipyridine under nitrogen purging; the second grafting solution is obtained by mixing 5-7 parts of polyethylene glycol methyl ether methacrylate with a number average molecular weight of 500 treated with a polymerization inhibitor. The third grafting solution consists of 16-20 parts of glycol methyl ether methacrylate, 5-7 parts of methanol, 0.015-0.025 parts of cuprous chloride, and 0.06-0.08 parts of 2,2'-bipyridine; the third grafting solution consists of 7-9 parts of sodium 4-vinylbenzenesulfonate, 43-47 parts of deionized water, 43-47 parts of methanol, 0.025-0.035 parts of cuprous chloride, and 0.09-0.11 parts of 2,2'-bipyridine.

2. The green synthesis process according to claim 1, characterized in that, The polyester support film mentioned in step S1 consists of six circular polyester support films with a diameter of 47 mm, each with a geometric area of ​​17.3 cm². 2 The total geometric area is 104 cm². 2 After film formation and grafting modification, it is loaded into a flat-sheet membrane separation module with a total effective membrane area of ​​100 cm². 2 .

3. The green synthesis process according to claim 1, characterized in that, The benzyl chloride initiator layer grafting solution described in step S2 is applied to the surface of the polyetherimide selective layer with a wet film thickness of 20-30 μm.

4. The green synthesis process according to claim 1, characterized in that, In step S3, the polyetherimide selective layer side of the polyetherimide composite membrane with the benzyl chloride initiating layer is contacted with the first grafting solution and reacted at 28-32°C for 35-45 min. The second grafting solution is added dropwise to the same reaction system within 4-6 min after the reaction of the first grafting solution is completed, and the reaction continues at 28-32°C for 18-22 min after the addition is completed. After the reaction of the second grafting solution is completed, the membrane is immediately transferred to the third grafting solution that has been pre-purged with nitrogen for 25-35 min, so that the polyetherimide selective layer side is in contact with the end-segment grafting solution, and reacted at 33-37°C for 25-35 min.

5. The green synthesis process according to claim 1, characterized in that, Before surface grafting, the polyethylene glycol methyl ether methacrylate with a number average molecular weight of 300, the polyethylene glycol methyl ether methacrylate with a number average molecular weight of 500, and 4-vinylbenzyl chloride need to be treated with a polymerization inhibitor through a neutral alumina short column.

6. The green synthesis process according to claim 1, characterized in that, The preparation of the magnesium-aluminum composite oxide catalyst in step S4 includes: weighing synthetic hydrotalcite, methylcellulose and deionized water, mixing them evenly and then extruding them into shape, drying them at 110-130℃ for 5-7 hours, calcining them in air at 430-470℃ for 3-5 hours, and then sieving them after cooling to obtain magnesium-aluminum composite oxide catalyst particles.

7. The green synthesis process according to claim 1, characterized in that, The fixed-bed reaction in step S5 includes: weighing a magnesium-aluminum composite oxide catalyst and loading it into a conventional tubular fixed-bed reactor, which is then activated under a nitrogen atmosphere; weighing and mixing methyl acrylate and p-methoxyphenol, and feeding anhydrous dimethylamine through a steel cylinder with back pressure control; the reaction temperature is 62-68℃ and the reaction pressure is 0.8-1.2MPa; the weight ratio of methyl acrylate, p-methoxyphenol, and anhydrous dimethylamine is 480-520:0.05-0.07:260-290.

8. The green synthesis process according to claim 1, characterized in that, In step S5, the methyl acrylate feed rate is 40-46 g / h, and the anhydrous dimethylamine feed rate is 22-26 g / h.

9. The green synthesis process according to claim 1, characterized in that, In step S6, the feed side temperature during membrane separation is 42-48℃, the feed side pressure is 180-220kPa, the feed side circulation flow rate is 55-65g / h, the permeate side absolute pressure is 2-4kPa, and the permeate side condensation temperature is -10-0℃.

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