Method for producing n-vinylacetamide

Abstract

The purpose of the present invention is to provide an inexpensive method for producing N-vinylacetamide without material loss.　This method for producing N-vinylacetamide by using acetamide and vinyl acetate as raw materials and using a catalyst is characterized by including, in the following order, a step for synthesizing N-vinylacetamide, a step for distilling off a residual low boiling-point substance, a step for extracting N-vinylacetamide, a step for distilling off aldehydes from the residue after separation of the N-vinylacetamide, a step for adding an alkali to the residue after distillation, a purification step for separating out the catalyst, and a step for drying the recovered catalyst.

Description

Method for producing N-vinylacetamide

[0001] The present invention relates to a method for producing N-vinylacetamide.

[0002] N-vinylacetamide polymers derived from N-vinylacetamide are water-soluble polymers and dissolve not only in water but also in polar solvents such as alcohol and dimethyl sulfoxide (DMSO). And since it is a nonionic polymer, it is not affected by salts or pH, and has high weather resistance, especially high stability to heat. Taking advantage of these properties, it is industrially applied to binders, dispersants, adhesives, thickeners, flocculants, etc.

[0003] Various catalysts have been proposed for use in the production of N-vinylacetamide. Patent Document 1 discloses a method for producing N-alkenylcarboxylic acid amide by reacting an alkenyl carboxylate with a carboxylic acid amide in the presence of a base. On the other hand, an efficient production method with less loss of raw materials has not been proposed conventionally.

[0004] Japanese Patent Laid-Open No. 8-59582

[0005] The raw materials containing the catalyst are preferably used without loss in order to reduce the production cost. Also, an expensive catalyst is preferably regenerated and reused. The present invention has been made to solve the above problems, and an object thereof is to provide a method for producing inexpensive N-vinylacetamide.

[0006] The present invention includes the following embodiments: [1] A method for producing N-vinylacetamide using acetamide and vinyl acetate as raw materials and a catalyst, characterized in that it includes, in this order: a synthesis step of N-vinylacetamide, a distillation step of residual low-boiling material, an extraction step of N-vinylacetamide, a distillation step of aldehydes from the residue after separation of N-vinylacetamide, an alkali addition step to the residue after distillation, a purification step to separate the catalyst, and a drying step of the recovered catalyst. [2] The method for producing N-vinylacetamide according to [1], further comprising the synthesis step of N-vinylacetamide using the recovered catalyst. [3] The method for producing N-vinylacetamide according to [1] or [2], wherein the pressure in the aldehyde distillation step is at or below atmospheric pressure. [4] The method for producing N-vinylacetamide according to [1] to [3], wherein the catalyst is dimethylaminopyridine. [5] The method for producing N-vinylacetamide according to [1] to [4], wherein the alkali used in the alkali addition step is at least one selected from sodium hydroxide and potassium hydroxide. [6] The method for producing N-vinylacetamide according to [1] to [5], wherein the purification step is crystallization and centrifugation. [7] The method for producing N-vinylacetamide according to [1] to [6], wherein the water content of the catalyst after the drying step is 0.01% by mass or more and 5.0% by mass or less. [8] The method for producing N-vinylacetamide according to [1] to [7], wherein the amount of aldehyde remaining after the aldehyde distillation step is 1.0% by mass or less. [9] The method for producing a regenerated catalyst, characterized in that, with respect to the catalyst used when producing N-vinylacetamide from acetamide and vinyl acetate as raw materials, the following steps are included in this order: distillation step of aldehyde produced as a by-product during the production of N-vinylacetamide, alkali addition step to the crude N-vinylacetamide after distillation, purification step to separate the catalyst, and drying step of the recovered catalyst.

[0007] According to the present invention, catalysts that previously suffered from high material loss can be reused, purification processes can be added to the series of steps, and a low-cost method for producing N-vinylcarboxylic acid amide can be provided.

[0008] This diagram shows a series of process flow charts for the manufacturing method according to the present invention.

[0009] The embodiments of the present invention will be described in detail below. However, the present invention is not limited to the embodiments shown below.

[0010] The manufacturing method of this embodiment is a method for producing N-vinylacetamide using acetamide and vinyl acetate as raw materials and a catalyst, characterized in that it includes the following steps in this order: a synthesis step of N-vinylacetamide, a distillation step of residual low-boiling material, an extraction step of N-vinylacetamide, a distillation step of aldehydes from the residue after separation of N-vinylacetamide, an alkali addition step to the residue after distillation, a purification step to separate the catalyst, and a drying step of the recovered catalyst. In other words, the alkali addition step is after the extraction step and before the purification step. A flowchart of the series is shown in Figure 1.

[0011] <Synthesis Process> The synthesis process of one embodiment of the present invention is a process of producing a compound containing N-vinylacetamide using acetamide and vinyl acetate as raw materials and a catalyst. The catalyst may be a recovered catalyst, or the catalyst may be recycled and reused in a series of processes. The vinyl acetate / acetamide ratio is not particularly limited, but is preferably 0.5 to 10.0 in molar ratio, more preferably 1.0 to 5.0, and even more preferably 1.5 to 3.0. Within this range, the yield of N-vinylacetamide is sufficiently high, which is preferable.

[0012] The catalyst / acetamide is not particularly limited, but is preferably 0.1 to 10.0 in molar ratio, more preferably 0.2 to 5.0, and even more preferably 0.4 to 3.0. Within this range, catalyst costs are kept down, which is preferable. The catalyst is not particularly limited, but is preferably an organic basic compound, more preferably dimethylaminopyridine, 1,8-diazabicyclo[5.4.0]undeca-7-ene, 1,5-diazabicyclo[4.3.0]-5-nonene, or 1,1,3,3-tetramethylguanidine, and even more preferably dimethylaminopyridine. Dimethylaminopyridine is preferred because it is easy to recover and reuse.

[0013] While a solvent may be used in the synthesis, a solvent-free approach is preferred because it eliminates the need to remove the solvent in subsequent steps. If a solvent is used, a polar solvent such as ethyl acetate is preferred. The acetamide and catalyst may be present in the reaction vessel in their entirety from the start of the reaction, or they may be added sequentially to the reaction vessel by dropwise addition.

[0014] The temperature of the synthesis process is not particularly limited, but is preferably 30°C to 150°C, more preferably 50°C to 120°C, and even more preferably 60°C to 90°C. The duration of the synthesis process is not particularly limited, but is preferably 4 hours to 50 hours, more preferably 6 hours to 30 hours, and even more preferably 8 hours to 24 hours.

[0015] <Distillation process to remove residual low-boiling material> In one embodiment of the present invention, the distillation process to remove residual low-boiling material is a process of distilling off and recovering vinyl acetate that remained unreacted during synthesis by distillation.

[0016] The temperature of the low-boiling removal step is not particularly limited, but is preferably 30°C to 90°C, more preferably 35°C to 80°C, and even more preferably 40°C to 70°C. The pressure of the low-boiling removal step is not particularly limited, but is preferably 0.1 kPaA to 10 kPaA, more preferably 0.1 kPaA to 8 kPaA, and even more preferably 0.1 kPaA to 5 kPaA. The distillation method is not limited, but examples include simple distillation and continuous distillation.

[0017] <Extraction Step> The extraction step of one embodiment of the present invention is a step of separating the reaction product containing N-vinylacetamide obtained in the synthesis step into N-vinylacetamide and a catalyst using two types of immiscible solvents. The solvents are not particularly limited, but water and solvents immiscible with water are preferred. Examples of solvents immiscible with water include ethyl acetate, hexane, and toluene, with ethyl acetate being preferred.

[0018] In addition to the solvent, it is also preferable to add a salt. The salt is not particularly limited, but examples include sodium chloride, sodium carbonate, and sodium bicarbonate. For extraction, it is preferable to add a solvent that is immiscible with water to the aqueous layer 1 to 10 times and repeat the operation.

[0019] The N-vinylacetamide synthesized in this operation migrates to the organic layer. The N-vinylacetamide that migrates to the organic layer is separated and purified by crystallization or recrystallization. For example, crystallization using known crystallization apparatus, or recrystallization solvents such as aromatic hydrocarbons, aliphatic hydrocarbons, alcohols, halogenated hydrocarbons, esters, and ethers can be used. There are no particular restrictions on the separation method, and known solid-liquid separation methods such as centrifugation and filtration can be used. If necessary, the recovered N-vinylacetamide is washed as appropriate and dried to obtain the final product. Meanwhile, the aqueous phase contains a catalyst, and this aqueous layer is treated in the next step, the aldehyde distillation step.

[0020] <Aldehyde Removal Process> In the aldehyde removal process of one embodiment of the present invention, distillation is performed on the aqueous phase of the extraction process to remove aldehydes such as acetaldehyde, which are low-boiling components. Since acetaldehyde has a strong solubility in water and tends to remain in the aqueous layer, a temperature higher than its boiling point is required. The distillation temperature is preferably 40°C to 160°C, more preferably 60°C to 120°C, and even more preferably 70°C to 100°C.

[0021] The distillation pressure is preferably reduced to atmospheric pressure or lower, more preferably 10 kPa (absolute pressure) to 80 kPa (absolute pressure), and even more preferably 20 kPa (absolute pressure) to 70 kPa. The residual liquid (high boiling point) component after aldehyde removal is used as the raw material for the next step. The acetaldehyde concentration after the aldehyde removal step is preferably 0.01% by mass to 5.0% by mass, more preferably 0.01% by mass to 1.0% by mass. If the acetaldehyde is within this range, the amount of by-products generated when alkali is added is small, which is preferable. The water content after the aldehyde removal step is preferably 40% by mass or more, more preferably 52% by mass or more, and even more preferably 55% by mass or more. 40% by mass is preferable because the yield of dimethylaminopyridine does not decrease even when recycling is performed. 52% by mass or more is preferable because sodium acetate does not accumulate in the recovered dimethylamidopyridine, allowing for repeated recycling.

[0022] <Alkali Addition Step> The alkali addition step in one embodiment of the present invention is a step of adding alkali to the aqueous phase after the aldehyde distillation step. The catalyst crystallizes upon alkali addition. The alkali is not particularly limited, but potassium hydroxide and sodium hydroxide are preferred. It is preferable to add the alkali as an aqueous solution.

[0023] The pH after the alkali addition step is preferably 9 to 14, more preferably 10 to 14, and even more preferably 11 to 14. The temperature during the alkali addition step is preferably 30°C to 95°C, more preferably 35°C to 80°C, and even more preferably 40°C to 60°C. This temperature range is preferable because the catalyst is less likely to precipitate. The alkali / catalyst molar ratio is preferably 1.0 to 10.0, more preferably 1.2 to 5.0, and even more preferably 1.5 to 3.0. The amount of catalyst can be determined by analysis using the method described in the examples.

[0024] <Purification Process> The purification process in one embodiment of the present invention is a process for purifying the catalyst. The operations for purifying the catalyst are not particularly limited, but include crystallization, precipitation, solid-liquid separation (filtration, centrifugation), etc. These may be performed individually or in combination. A combination of crystallization and centrifugation is preferred. The crystallization solvent is not particularly limited, but water is preferred because the solvent in the preceding step is water.

[0025] The starting temperature for crystallization is preferably 30°C to 90°C, more preferably 30°C to 80°C, and even more preferably 35°C to 55°C. The rate of temperature decrease during crystallization is preferably 2°C / hour to 20°C / hour, and more preferably 5°C / hour to 10°C / hour. The ending temperature for crystallization is preferably 1°C to 25°C, and more preferably 5°C to 20°C.

[0026] <Drying Process> The drying process in one embodiment of the present invention is a process of removing the solvent used during crystallization and the water introduced during the extraction process. The drying operation is not particularly limited as long as it is possible to remove the water. Examples include heating drying, air drying, and azeotropic dehydration with an organic solvent. After azeotropic dehydration with an organic solvent such as butyl acetate, the organic solvent may be introduced into the reaction if it does not interfere with the subsequent reaction.

[0027] In the case of distillation, the catalyst can be obtained as a high-boiling component, and water and solvent can be obtained as low-boiling components. The pressure is not particularly limited, and can be atmospheric pressure or reduced pressure. The moisture content of the catalyst after the drying process is not limited, but is preferably 0.01% by mass or more and 5.0% by mass or less, more preferably 0.01% by mass or more and 2.0% by mass or less, and even more preferably 0.01% by mass or more and 1.0% by mass or less.

[0028] The recovered catalyst obtained above can be used again as a catalyst in the synthesis of N-vinylacetamide. Even when reused as a catalyst in the synthesis process of N-vinylacetamide, N-vinylacetamide with the same polymerizability can be produced, and if repeated, continuous operation is possible. Therefore, N-vinylacetamide can be produced efficiently without material loss. Furthermore, according to the present invention, a method for producing a regenerated catalyst is also provided, characterized in that the catalyst used when producing N-vinylacetamide from acetamide and vinyl acetate as raw materials is subjected to the following steps in this order: a step of removing aldehydes produced as by-products during the production of N-vinylacetamide; a step of adding alkali to the crude N-vinylacetamide after removal; a purification step of separating the catalyst; and a drying step of the recovered catalyst.

[0029] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0030] <pH Measurement> pH was measured using a pH meter (F-70, manufactured by Horiba, Ltd.).

[0031] <Measurement of Catalyst, Acetaldehyde, and N-Vinylacetamide Concentrations> Quantitative analysis was performed using GC under the following conditions. Instrument: High-performance general-purpose gas chromatograph "GC-2014" (Shimadzu Corporation) Column: DB-WAX (φ0.25 mm × 30 m, Agilent Technologies) Carrier gas type: He Carrier gas flow rate: 1 mL / min Split ratio: 40 Column temperature: The heating program was set in the following order: 40°C (7 min) → heating (25°C / min) → 130°C (15 min) → heating (30°C / min) → 220°C (7 min) Injection temperature: 200°C Detector: Flame ionization detector (FID) Detector temperature: 230°C Internal standard: Diethylene glycol dimethyl ether

[0032] <Measurement of N-1,3-butadienylacetamide concentration> The concentration was quantified by high-performance liquid column chromatography (HPLC) under the following conditions: Column: Shodex® SIL-5B, manufactured by Resonaq Corporation; Eluent: Isopropyl alcohol (IPA) / N-hexane = 1 / 9 (mass ratio); Column temperature: 40°C; Flow rate: 1.0 mL / min; Detector: UV-Vis spectrometer, 254 nm

[0033] <Moisture content measurement> Measurement was performed by volumetric titration in accordance with JIS K0068-2001.

[0034] [Example 1] N-vinylacetamide was synthesized by reacting 200 g of acetamide, 583 g of vinyl acetate, and 207 g of 4-dimethylaminopyridine at 70°C for 16 hours. The yield of N-vinylacetamide (NVA) was 66% (synthesis step of N-vinylacetamide). The low-boiling components of this synthetic solution were removed by distillation at 60°C / 2 kPaA (distillation step of residual low-boiling components).

[0035] After removing the low-boiling components, 990 g of ethyl acetate and 693 g of a 7.5% by mass sodium carbonate aqueous solution were mixed with the synthesis solution. N-vinylacetamide was extracted into the organic layer of ethyl acetate and separated from the aqueous layer. The aqueous layer was then further mixed with 594 g of ethyl acetate three times and with 396 g of ethyl acetate once to extract N-vinylacetamide into the organic layer, and 1140 g of the aqueous layer was recovered (N-vinylacetamide extraction step). The acetaldehyde concentration in the aqueous layer at this time was 4.0% by mass.

[0036] Subsequently, the recovered aqueous layer was distilled off at 53°C / 12 kPaA to remove acetaldehyde, reducing the acetaldehyde concentration in the residual solution to 1.0% by mass (aldehyde distillation step). After the aldehydes were removed, 428 g of a 25% by mass aqueous sodium hydroxide solution was added at 25°C (alkali addition step).

[0037] The precipitated 4-dimethylaminopyridine (DMAP) wet crystals were recovered by filtration (catalyst purification step). The obtained solid was dried under reduced pressure (catalyst drying step). As a result, 175 g of 4-dimethylaminopyridine dry crystals with a purity of 90% by mass and a moisture content of 0.6% by mass were obtained. No unknown peaks were detected by GC analysis at retention times of 29 to 32 minutes. When N-vinylacetamide was synthesized using the recovered 4-dimethylaminopyridine, the N-vinylacetamide yield was 67.4%, and the N-1,3-butadienylacetamide content was 1,020 ppm.

[0038] [Example 2] The procedure was the same as in Example 1, except that the acetaldehyde removal step was carried out at 94°C / 60 kPaA. The acetaldehyde content in the residual solution after acetaldehyde removal was 0.01% by mass. The purity of the recovered 4-dimethylaminopyridine dry crystals was 87.8% by mass, the recovered amount was 252 g, and the moisture content was 0.5% by mass. No unknown peaks were detected in GC analysis at retention times of 29 to 32 minutes.

[0039] Reaction evaluation using recovered 4-dimethylaminopyridine showed an N-vinylacetamide yield of 64.4% and an N-1,3-butadienylacetamide content of 697 ppm.

[0040] [Example 3] In the alkali addition step, a 25% by mass aqueous sodium hydroxide solution was added at 70°C, and after cooling from 70°C to 10°C at a rate of 10°C per hour, the precipitated 4-dimethylaminopyridine was recovered by centrifugation. The procedure was the same as in Example 2, and 4-dimethylaminopyridine was recovered. Note that the solution obtained by centrifugation without filtration is labeled "crystallization" in Table 1.

[0041] The purity of the obtained 4-dimethylaminopyridine dry crystals was 95.2% by mass, the recovered amount was 209 g, and the moisture content was 1.1% by mass. No unidentified peaks were detected in GC analysis at retention times of 29 to 32 minutes. Reaction evaluation using the recovered 4-dimethylaminopyridine showed an N-vinylacetamide yield of 63.9% and an N-1,3-butadienylacetamide content of 613 ppm.

[0042] [Example 4] Except that an aqueous sodium hydroxide solution of 25% by mass was added at 50°C, followed by cooling from 50°C to 10°C at 5°C per hour, and the precipitated 4-dimethylaminopyridine was recovered by centrifugation, the procedure was the same as in Example 3, and 4-dimethylaminopyridine was recovered. The purity of the dried crystals of 4-dimethylaminopyridine was 97.8% by mass, the recovery amount was 203 g, and the water content was 1.4% by mass. No unknown peaks with a retention time of 29 minutes to 32 minutes were detected by GC analysis. The reaction evaluation using the recovered 4-dimethylaminopyridine showed an N-vinylacetamide yield of 64.4% and an N-1,3-butadienylacetamide content of 721 ppm.

[0043] [Example 5] Except that the acetaldehyde distillation step was carried out at 108°C / 101 kPaA and rinsing was performed using 35 g of vinyl acetate after centrifugation, the procedure was the same as in Example 4, and 4-dimethylaminopyridine was recovered. The purity of the dried crystals of the recovered 4-dimethylaminopyridine was 98.4% by mass, the recovery amount was 165 g, and the water content was 1.4% by mass. No unknown peaks with a retention time of 29 minutes to 32 minutes were detected by GC analysis. The reaction evaluation using the recovered 4-dimethylaminopyridine showed an N-vinylacetamide yield of 66.3% and an N-1,3-butadienylacetamide content of 831 ppm.

[0044] [Example 6] Except that rinsing was performed using 41 g of acetonitrile after centrifugation, the procedure was the same as in Example 5, and 4-dimethylaminopyridine was recovered. The purity of the dried crystals of the recovered 4-dimethylaminopyridine was 94.9% by mass, the recovery amount was 144 g, and the water content was 0.5% by mass. No unknown peaks with a retention time of 29 minutes to 32 minutes were detected by GC analysis. The reaction evaluation using the recovered 4-dimethylaminopyridine showed an N-vinylacetamide yield of 66.3% and an N-1,3-butadienylacetamide content of 883 ppm.

[0045] [Example 7] The acetaldehyde removal step was carried out at 108°C / 101 kPaA, a 25% by mass aqueous sodium hydroxide solution was added at 40°C, and the mixture was cooled from 40°C to 10°C at a rate of 5°C per hour. The precipitated 4-dimethylaminopyridine was recovered by centrifugation, except that the procedure was the same as in Example 4. The purity of the recovered 4-dimethylaminopyridine dry crystals was 97.1% by mass, the recovered amount was 202 g, and the moisture content was 1.1% by mass. No unknown peaks were detected in GC analysis at retention times of 29 to 32 minutes. Reaction evaluation using the recovered 4-dimethylaminopyridine showed an N-vinylacetamide yield of 65.1% and an N-1,3-butadienylacetamide content of 821 ppm.

[0046] [Example 8] The procedure was carried out in the same manner as in Example 4, except that the acetaldehyde removal step was performed at 85°C / 32 kPaA, and 4-dimethylaminopyridine was recovered. The purity of the recovered 4-dimethylaminopyridine dry crystals was 96.4% by mass, the recovered amount was 195 g, the moisture content was 0.3% by mass, and no unknown peaks were detected in GC analysis at retention times of 29 to 32 minutes. Reaction evaluation using the recovered 4-dimethylaminopyridine showed an N-vinylacetamide yield of 64.6% and an N-1,3-butadienylacetamide content of 874 ppm.

[0047] [Example 9] The procedure was carried out in the same manner as in Example 4, except that the acetaldehyde removal step was performed at 95°C / 65 kPaA, and 4-dimethylaminopyridine was recovered. The purity of the recovered 4-dimethylaminopyridine dry crystals was 97.7% by mass, the recovered amount was 199 g, and the moisture content was 0.7% by mass. No unknown peaks were detected in GC analysis at retention times of 29 to 32 minutes. Reaction evaluation using the recovered 4-dimethylaminopyridine showed an N-vinylacetamide yield of 64.7% and an N-1,3-butadienylacetamide content of 599 ppm.

[0048] [Example 10] 4-Dimethylaminopyridine was recovered in the same manner as in Example 4, except that the acetaldehyde distillation step was carried out at 108 °C / 101 kPaA. The purity of the recovered 4-dimethylaminopyridine dry crystals was 96.0% by mass, the recovery amount was 198 g, and the moisture content was 0.4% by mass. No unknown peak with a retention time of 29 to 32 minutes was detected by GC analysis. The reaction evaluation using the recovered 4-dimethylaminopyridine showed an N-vinylacetamide yield of 65.7% and an N-1,3-butadienylacetamide content of 762 ppm.

[0049] [Example 11] After the alkali addition step in the same manner as in Example 9, 197 g of the recovered 4-dimethylaminopyridine wet crystals had a purity of 94% by mass and a moisture content of 5% by mass. No unknown peak with a retention time of 29 to 32 minutes was detected by GC analysis. Then, the wet crystals were mixed with 374 g of butyl acetate and azeotropic dehydration was carried out at 80 °C / 20 kPaA. After confirming the distillation corresponding to the introduced moisture and that the moisture content in the flask was 0.3% by mass, 275 g of butyl acetate was distilled off.

[0050] Then, for the 286 g of the kettle residue, 200 g of acetamide, 583 g of vinyl acetate, and 23 g of 4-dimethylaminopyridine were added and the reaction was evaluated. As a result, the N-vinylacetamide yield was 67.7% and the NBA content was 928 ppm. Furthermore, the same operation was repeated 4 times to recover and reuse 4-dimethylaminopyridine.

[0051] 188 g of the 4-dimethylaminopyridine wet crystals recovered in the second time, with a purity of 93% by mass and a moisture content of 6% by mass, showed no unknown peak with a retention time of 29 to 32 minutes by GC analysis. The reaction evaluation after azeotropic dehydration showed an N-vinylacetamide yield of 65.9% and an NBA content of 902 ppm. 191 g of the 4-dimethylaminopyridine wet crystals recovered in the third time, with a purity of 91% by mass and a moisture content of 6% by mass, showed no unknown peak with a retention time of 29 to 32 minutes by GC analysis. The reaction evaluation after azeotropic dehydration showed an N-vinylacetamide yield of 65.7% and an NBA content of 653 ppm.

[0052] In the fourth extraction, 194 g of 4-dimethylaminopyridine wet crystals were recovered. Purity: 93% by mass. Moisture content: 6% by mass. GC analysis revealed no unidentified peaks between 29 and 32 minutes of retention. After azeotropic dehydration, the reaction evaluation showed an N-vinylacetamide yield of 65.2% and an NBA content of 691 ppm. In the fifth extraction, 189 g of 4-dimethylaminopyridine wet crystals were recovered. Purity: 94% by mass. Moisture content: 6% by mass. GC analysis revealed no unidentified peaks between 29 and 32 minutes of retention. After azeotropic dehydration, the reaction evaluation showed an N-vinylacetamide yield of 67.5% and an N-1,3-butadienylacetamide content of 1,021 ppm.

[0053] The obtained synthesis solution was subjected to distillation of low-boiling components and extraction with ethyl acetate in the same manner as in Example 1. The obtained ethyl acetate layer was washed with a 23% by mass sodium chloride + 2% by mass sodium carbonate aqueous solution. After that, ethyl acetate was removed by distillation at 40°C / 5 kPaA, and then 198 g of the N-vinylacetamide fraction was distilled off at 100°C / 0.1 kPaA. The concentration of N-vinylacetamide in this distillate was 68% by mass. 144 g of methylcyclohexane and 13 g of ethyl acetate were added to the obtained fraction, dissolved at 30°C, and cooled to -5°C for 7 hours for crystallization. Solid-liquid separation was then performed by centrifugation, and further washing was performed with 62 g of methylcyclohexane and 3 g of ethyl acetate. At this time, 96 g of N-vinylacetamide crystals were obtained, and the concentration of N-vinylacetamide was 98% by mass.

[0054] The obtained crystals were further dissolved at 40°C with 100 g of methylcyclohexane and 18 g of ethyl acetate, cooled to 5°C for 7 hours, and then crystallized. Solid-liquid separation was performed using a centrifuge, and the mixture was further washed with 43 g of methylcyclohexane and 2 g of ethyl acetate. At this time, 83 g of N-vinylacetamide crystals were obtained, and the N-vinylacetamide concentration was 99.7% by mass. The polymerization test result for the obtained N-vinylacetamide was 110 minutes.

[0055] [Examples 12-16] The procedure was carried out in the same manner as in Example 1, except that after adding a 25% by mass sodium hydroxide aqueous solution, pure water was added so that the water concentration was as described in Table 1, the water content at the time of crystallization.

[0056] [Example 17] The procedure was carried out in the same manner as in Example 1, except that after adding a 25% by mass sodium hydroxide aqueous solution, pure water was added so that the water concentration was as described in Table 1, Water content at the start of crystallization.

[0057] [Comparative Example 1] An aqueous layer was obtained in the same manner as in Example 1. The acetaldehyde concentration at this time was 4.5% by mass. To this aqueous layer, the acetaldehyde removal step was not performed, and 623 g of a 25% by mass aqueous sodium hydroxide solution was added at 25°C, and the mixture was extracted three times with 455 g of ethyl acetate. The ethyl acetate in the ethyl acetate layer after extraction was removed by vacuum distillation, and 254 g of 4-dimethylaminopyridine solid, with a purity of 75% by mass and a water content of 0.1% by mass, was recovered. GC analysis revealed multiple minute peaks at retention times of 29 to 32 minutes.

[0058] When N-vinylacetamide was synthesized using the recovered 4-dimethylaminopyridine, the yield of N-vinylacetamide was 60.2%, and the N-1,3-butadienylacetamide content was 1,035 ppm. Furthermore, after repeating the same procedure twice and recovering and reusing 4-dimethylaminopyridine again, the yield of N-vinylacetamide was 59.7% and the N-1,3-butadienylacetamide content was 1,937 ppm in the third operation, and the yield of N-vinylacetamide was 59.9% and the N-1,3-butadienylacetamide content was 2,077 ppm in the fourth operation.

[0059] In the same manner as in Example 11, N-vinylacetamide was purified to obtain 80 g of N-vinylacetamide crystals, with an N-vinylacetamide concentration of 99.7% by mass. The polymerization test result for the obtained N-vinylacetamide was 135 minutes.

[0060] [Comparative Example 2] The same procedure as in Comparative Example 1 was followed, except that the aqueous layer of the extraction step was subjected to an acetaldehyde distillation step at 70°C / 30 kPaA to reduce the acetaldehyde concentration to 0.9% by mass. 174 g of 4-dimethylaminopyridine solid, with a purity of 88% by mass and a water content of 0.1% by mass, was recovered. Multiple minute peaks were detected in GC analysis at retention times of 29 to 32 minutes. When N-vinylacetamide was synthesized using the recovered 4-dimethylaminopyridine, the N-vinylacetamide yield was 60.4%, and the N-1,3-butadienylacetamide content was 1,718 ppm.

[0061]

Claims

1. A method for producing N-vinylacetamide using acetamide and vinyl acetate as raw materials and a catalyst, characterized in that the method comprises the following steps in this order: a synthesis step of N-vinylacetamide, a distillation step of residual low-boiling material, an extraction step of N-vinylacetamide, a distillation step of aldehydes from the residue after separation of N-vinylacetamide, a step of alkali addition to the residue after distillation, a purification step to separate the catalyst, and a drying step of the recovered catalyst.

2. The method for producing N-vinylacetamide according to claim 1, further comprising a synthesis step of N-vinylacetamide using the recovered catalyst.

3. The method for producing N-vinylacetamide according to claim 1 or 2, wherein the pressure of the aldehyde removal step is at or below atmospheric pressure.

4. The method for producing N-vinylacetamide according to claim 1 or 2, wherein the catalyst is dimethylaminopyridine.

5. The method for producing N-vinylacetamide according to claim 1 or 2, wherein the alkali used in the alkali addition step is at least one selected from sodium hydroxide and potassium hydroxide.

6. The method for producing N-vinylacetamide according to claim 1 or 2, wherein the purification step is crystallization and centrifugation.

7. The method for producing N-vinylacetamide according to claim 1 or 2, wherein the moisture content of the catalyst after the drying step is 0.01% by mass or more and 5.0% by mass or less.

8. The method for producing N-vinylacetamide according to claim 1 or 2, wherein the amount of aldehyde remaining after the aldehyde distillation step is 1.0% by mass or less.

9. A method for producing a regenerated catalyst, characterized in that the catalyst used in the production of N-vinylacetamide using acetamide and vinyl acetate as raw materials comprises, in this order, a step of removing aldehydes produced as by-products during the production of N-vinylacetamide, a step of adding alkali to the crude N-vinylacetamide after removal of aldehydes, a purification step of separating the catalyst, and a drying step of the recovered catalyst.

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