Production method for refined n-vinyl carboxylic acid amide

Abstract

[Problem] To provide a refinement method for producing refined N-vinyl carboxylic acid amides that exhibit good polymerization properties even if a conventional refinement step is used without a hydrogenation step. [Solution] A method of producing a refined N-vinyl carboxylic acid amide from a crude N-vinyl carboxylic acid amide, said method being characterized by including a distillation step and a crystallization step.

Description

Method for producing purified N-vinylcarboxylic acid amide

[0001] The present invention relates to a method for producing purified N-vinylcarboxylic acid amide.

[0002] N-vinylcarboxylic acid amide polymers derived from N-vinylcarboxylic acid amide 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, industrially, it is applied to binders, dispersants, adhesives, thickeners, flocculants, etc.

[0003] Many methods have been proposed for the production method of N-vinylcarboxylic acid amide. However, an additional step was required to remove N-butadienylacetamide, which is a polymerization inhibitor.

[0004] Patent Document 1 discloses a method for producing N-vinylcarboxylic acid amide, which has a step of controlling the content of unsaturated aldehydes in N-vinylcarboxylic acid amide to 20 mass ppm or less.

[0005] Patent Document 2 discloses a method for producing high-purity N-vinylformamide, which comprises adding water and an aromatic hydrocarbon to crude N-vinylformamide containing formamide as an impurity for extraction treatment to obtain an organic phase containing N-vinylformamide and an aqueous phase containing formamide, and then separating the two phases, and then recovering N-vinylformamide from the organic phase.

[0006] Patent Document 3 discloses a method for producing a highly polymerizable N-vinylcarboxylic acid amide monomer, which comprises subjecting crude N-(1-alkoxyethyl)carboxylic acid amide or crude ethylidenebiscarboxylic acid amide to thermal decomposition or catalytic decomposition to produce N-vinylcarboxylic acid amide, and controlling the N-vinylcarboxylic acid amide content in the crude N-(1-alkoxyethyl)carboxylic acid amide or crude ethylidenebiscarboxylic acid amide to 10 mass% or less.

[0007] International Publication No. 17 / 145569 JP 61-289069 JP 8-134029

[0008] However, the method described in Patent Document 1 uses a specialized and costly facility for thermal decomposition reactions in the synthesis of N-vinyl carboxylic acid amides, which incurs high construction and operating costs. Furthermore, it was necessary to reduce the N-1,3-butadienylacetamide, a by-product of the synthesis, through a hydrogenation process using a precious metal catalyst. These methods present significant management hurdles. The method described in Patent Document 2 uses aromatic hydrocarbons, making it difficult to adopt in modern times. The method described in Patent Document 3 requires a pressure crystallization process, which demands a large amount of thermal energy and involves significant capital investment. The present invention was made to solve the above problems and aims to provide a purification method for producing purified N-vinyl carboxylic acid amides with good polymerizability even when the hydrogenation process is omitted and a general-purpose purification process is used.

[0009] The present invention includes the following embodiments: [1] A method for producing a purified N-vinyl carboxylic acid amide from a crude N-vinyl carboxylic acid amide, comprising a distillation step and a crystallization step. [2] The method for producing a purified N-vinyl carboxylic acid amide according to [1], wherein the pressure of the distillation step is 0.01 kPa or more and 2 kPa or less. [3] The method for producing a purified N-vinyl carboxylic acid amide according to [1] or [2], wherein the N-vinyl carboxylic acid amide is N-vinylacetamide. [4] The method for producing a purified N-vinyl carboxylic acid amide according to [1] to [3], wherein the temperature of the distillation step is 60°C or more and 150°C or less. [5] The method for producing a purified N-vinyl carboxylic acid amide according to [1] to [4], wherein the solvent of the crystallization step comprises at least one selected from acetonitrile, methyl ethyl ketone, toluene, ethyl acetate, and 1,2-dimethoxyethane, and an aliphatic hydrocarbon having 6 to 7 carbon atoms. [6] The method for producing a purified N-vinyl carboxylic acid amide according to [1] to [5], wherein the aliphatic hydrocarbon having 6 to 7 carbon atoms is at least one selected from n-hexane, cyclohexane, n-heptane, cycloheptane, and methylcyclohexane. [7] The method for producing a purified N-vinyl carboxylic acid amide according to [1] to [6], wherein the starting temperature of the crystallization step is 30°C to 80°C and the ending temperature of the crystallization step is -20°C to 35°C. [8] The method for producing a purified N-vinyl carboxylic acid amide according to [1] to [7], wherein a salt is added in the distillation step. [9] The method for producing a purified N-vinyl carboxylic acid amide according to [8], wherein the salt is at least one selected from sodium bicarbonate and sodium carbonate.

[10] The method for producing a purified N-vinyl carboxylic acid amide according to [3] to [9], wherein N-vinylacetamide is produced from acetamide and vinyl acetate as raw materials.

[0010] According to the present invention, it is possible to provide a method for producing N-vinylcarboxylic acid amide with excellent polymerizability by omitting the hydrogenation step and using a general purification step.

[0011] The embodiments of the present invention will be described in detail below. However, the present invention is not limited to the embodiments shown below.

[0012] A method for producing purified N-vinyl carboxylic acid amide according to one embodiment of the present invention includes a distillation step and a crystallization step. In this specification, "crude N-vinyl carboxylic acid amide" refers to a product containing components other than N-vinyl carboxylic acid amide in an amount of 20 to 60% by mass relative to the total amount of N-vinyl carboxylic acid amide. It may also be called a crude N-vinyl carboxylic acid amide composition because it contains other components. In this embodiment, a compound containing crude N-vinyl carboxylic acid amide can be purchased, or recycled materials can be used as crude N-vinyl carboxylic acid amide. Furthermore, it may be produced by going through the crude N-vinyl carboxylic acid amide synthesis step shown below before the base addition step, or by going through an extraction step.

[0013] <Synthesis Process> The synthesis process is not particularly limited, but it is preferably a process of producing a compound containing crude N-vinyl carboxylic acid amide using a catalyst with a carboxylic acid amide and vinyl acetate as raw materials. The carboxylic acid amide is not limited, but acetamide is preferred. The N-vinyl carboxylic acid amide is not limited, but N-vinylamideacetamide is preferred.

[0014] The vinyl acetate / carboxylic acid amide is not particularly limited, but a molar ratio of 0.5 to 10.0 is preferred, more preferably 1.0 to 5.0, and even more preferably 1.5 to 3.0. Within this range, the yield of crude N-vinylcarboxylic acid amide is sufficiently high, which is preferable. In the case of N-vinylamide acetamide, acetamide and vinyl acetate are used.

[0015] The catalyst / carboxylic acid amide 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, 1,1,3,3-tetramethylguanidine, and even more preferably dimethylaminopyridine. Dimethylaminopyridine is preferred because it is easy to recover and reuse.

[0016] While a solvent may be used in the synthesis, solvent-free is preferred because it allows for the omission of subsequent solvent treatment steps. If a solvent is used, a polar solvent such as ethyl acetate is preferred. The carboxylic acid amide 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 or other means.

[0017] 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.

[0018] In embodiments of the present invention, low-boiling substances such as vinyl acetate that did not react may be removed, and the vinyl acetate and other substances are recovered and used as raw materials. The removal method is not particularly limited, but examples include distillation such as simple distillation and continuous distillation.

[0019] <Extraction Step> The extraction step in the embodiment of the present invention is a step of separating a compound containing crude N-vinylcarboxylic acid amide obtained in the synthesis step or acquired from the user into crude N-vinylcarboxylic acid amide and a catalyst using two types of immiscible solvents. The solvent is not particularly limited, but water and a solvent immiscible with water are preferred. Examples of solvents immiscible with water include ethyl acetate, hexane, and toluene, with ethyl acetate being preferred. 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.

[0020] Extraction is preferably carried out by repeatedly adding a solvent that is immiscible with water to the aqueous layer 1 to 10 times. In this operation, the crude N-vinylcarboxylic acid amide migrates to the organic layer. The solvent may be removed from the organic layer as appropriate. The method of removal is not particularly limited.

[0021] <Distillation Process> In the distillation process of this embodiment, crude N-vinylcarboxylic acid amide is distilled. The distillation temperature is not limited, but is preferably 60°C to 150°C, more preferably 65°C to 130°C, and even more preferably 70°C to 110°C. The distillation pressure is not limited, but is preferably 0.01 kPa to 3 kPa, more preferably 0.01 kPa to 2 kPa, and even more preferably 0.01 kPa to 1 kPa.

[0022] The distillation method is not limited, but examples include simple distillation and continuous distillation. The distillation method is not limited, but a method can also be used in which a liquid film of crude N-vinyl carboxylic acid amide is formed under reduced pressure and heated to leave behind and remove components with higher boiling points than N-vinyl carboxylic acid amide (hereinafter also referred to as the "thin film distillation method"). In the case of thin film distillation, the distillation temperature may be 60°C to 130°C, 65°C to 120°C, or 70°C to 110°C. In the case of thin film distillation, the distillation pressure may be 0.01 kPa to 3 kPa, 0.01 kPa to 2 kPa, or 0.01 kPa to 1 kPa.

[0023] It is preferable to add a salt before distillation. The salt is not particularly limited, but examples include sodium bicarbonate, potassium bicarbonate, sodium carbonate, and potassium carbonate, with sodium bicarbonate and sodium carbonate being more preferable. Adding a salt is preferable because it can suppress the decomposition of acetic acid during the distillation process. The distillation process removes unreacted products, solvent, salt, etc., and yields crude N-vinylcarboxylic acid amide.

[0024] <Crystallization Process> In the crystallization process of this embodiment, the crude N-vinyl carboxylic acid amide after the distillation process is crystallized. The crude N-vinyl carboxylic acid amide after the distillation process is dissolved in the presence of a crystallization solvent and a poor solvent, and then crystallized. The crude N-vinyl carboxylic acid amide monomer crystals after the crystallization process contain 88% by mass or more of the N-vinyl carboxylic acid amide monomer, preferably 90% by mass or more, and more preferably 92% by mass or more.

[0025] The crystallization solvent is not particularly limited, but at least one selected from acetonitrile, methyl ethyl ketone, toluene, ethyl acetate, and 1,2-dimethoxyethane is preferred. The mass of the crystallization solvent used is 0.01 to 0.5 in ratio to the mass of the N-vinylcarboxylic acid amide monomer crystals used, preferably 0.02 to 0.4, and more preferably 0.03 to 0.2.

[0026] Furthermore, while the poor solvent used is not particularly limited, aliphatic hydrocarbons having 6 to 7 carbon atoms are preferred. Preferred aliphatic hydrocarbons having 6 to 7 carbon atoms include n-hexane, cyclohexane, methylcyclohexane, n-heptane, and cycloheptane, with cyclohexane and methylcyclohexane being more preferred. Aliphatic hydrocarbons with 5 or fewer carbon atoms have low boiling points and are difficult to handle, while aliphatic hydrocarbons with 8 or more carbon atoms have high boiling points and may not be completely removed from the crystals. The mass of the aliphatic hydrocarbons having 6 to 7 carbon atoms is in a ratio of 0.5 to 3.0, preferably 0.5 to 2.0, relative to the mass of the N-vinylcarboxylic acid amide monomer crystals used. This ratio range is desirable because it allows for sufficient recrystallization of the N-vinylcarboxylic acid amide monomer, does not increase the amount of solvent used, and improves the volumetric efficiency of the crystallization apparatus.

[0027] The dissolution of N-vinylcarboxylic acid amide monomer crystals is not particularly limited as long as the temperature at which they can dissolve in the solvent is within limits. This temperature is the starting temperature for the crystallization process, and is preferably 30 to 80°C, more preferably 30 to 60°C, and even more preferably 35 to 45°C. This temperature range is desirable because the N-vinylcarboxylic acid amide dissolves sufficiently without denaturation.

[0028] Next, the dissolved N-vinylcarboxylic acid amide monomer is cooled to precipitate recrystallization. This cooling temperature is the end temperature of the crystallization process, and is preferably -20°C to 35°C, more preferably -15°C to 30°C, and even more preferably -10°C to 20°C. This temperature range is desirable because it keeps equipment costs low and provides a sufficient crystallization yield. The difference between the temperature at which the N-vinylcarboxylic acid amide monomer crystals are dissolved and the temperature at which the dissolved N-vinylcarboxylic acid amide monomer is cooled to precipitate recrystallization is preferably 1 to 100°C, more preferably 3 to 40°C, and even more preferably 15 to 30°C.

[0029] As an efficient method for separating the recrystallized N-vinylcarboxylic acid amide monomer precipitated in the crystallization process, separation by filtration is preferred. There are no restrictions on the filtration method, but from the viewpoint of separation from the crystallization mother liquor and productivity, centrifugal filtration or pressure filtration is preferred. Furthermore, rinsing after filtration is also preferred in order to improve the separation from the mother liquor. As the solvent used for rinsing, solvents selected from acetonitrile, methyl ethyl ketone, toluene, ethyl acetate, and 1,2-dimethoxyethane, and aliphatic hydrocarbons having 6 to 7 carbon atoms are preferred, similar to the crystallization solvent. The mass of the solvent selected from acetonitrile, methyl ethyl ketone, toluene, ethyl acetate, and 1,2-dimethoxyethane used for rinsing is preferably used in a ratio of 0.01 to 0.3, more preferably 0.01 to 0.2, and even more preferably 0.02 to 0.1, relative to the mass of the N-vinylcarboxylic acid amide monomer crystals recovered from distillation. Furthermore, the mass of the C6-C7 aliphatic hydrocarbon used in the rinsing process is preferably 0.1 to 3.0 in ratio to the mass of the N-vinylcarboxylic acid amide monomer crystals recovered from step (A), more preferably 0.1 to 2.0, and even more preferably 0.1 to 1.5. The crystallization step may be repeated multiple times.

[0030] The purified N-vinyl carboxylic acid amide obtained in the crystallization step preferably has an N-vinyl carboxylic acid amide purity of 90% by mass or more, more preferably 93% by mass or more, and even more preferably 95% by mass or more. The purified N-vinyl carboxylic acid amide obtained in the crystallization step preferably has an N-butadienylacetamide content of 20 ppm by mass or less, more preferably 15 ppm by mass or less, and even more preferably 10 ppm by mass or less. The purified N-vinyl carboxylic acid amide obtained in the crystallization step preferably has a sodium concentration of 10 ppm by mass or less, more preferably 5 ppm by mass or less, and even more preferably 1 ppm by mass or less.

[0031] 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.

[0032] <Measurement of N-vinylcarboxylic acid amide concentration> The concentration was quantified by GC analysis 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

[0033] <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

[0034] <Measurement of Sodium Ion Concentration> The sodium ion concentration was quantified by ion chromatography (IC) under the following conditions: Column: Shodex® YS-50, manufactured by Resonaq Corporation; Eluent: 4 mM methanesulfonic acid aqueous solution; Suppressor: CDRS600 4 mm; Column temperature: 35°C; Flow rate: 1.0 mL / min

[0035] <Polymerization Test> The polymerizability of the obtained N-vinylcarboxylic acid amide is evaluated by the polymerization test described below. [1] Prepare a 100 ml glass container equipped with a catalyst injection tube, nitrogen blowing tube, nitrogen exhaust tube, and thermometer. [2] Weigh 20 g of N-vinylcarboxylic acid amide and 58 g of deionized water into the glass container from [1]. [3] Heat to 30°C in a water bath while bubbling with 50 ccm of nitrogen gas. Allow nitrogen gas to flow until polymerization is complete. [4] As a polymerization initiator, dissolve 1.6 g of V-044 (azoimidazoline type manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) in 48.4 g of deionized water. [5] As a polymerization initiator, dissolve 4.0 g of V-50 (azoamidine type manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) in 46.0 g of deionized water. [6] After 1 hour of nitrogen gas aeration, 1 g of polymerization initiator [4] is added, followed by 1 g of polymerization initiator [5] using a syringe. [7] The glass container is removed from the water bath, the moisture on the glass surface is removed with paper, and then it is transferred to an insulated container to continue polymerization. [8] The polymerization temperature is monitored, and the time to reach the standard temperature peak (time from the injection of polymerization initiator to reaching the temperature peak) from the addition of polymerization initiator [6] is used as an indicator of polymerizability. A good polymer was judged to have a standard temperature peak time of less than 120 minutes.

[0036] [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. After removing the low-boiling components from this synthesis solution at 60°C / 2 kPaA, it was extracted with 990 g of ethyl acetate and 693 g of 7.5% by mass aqueous sodium carbonate solution. The aqueous layer was further repeated three times with 594 g of ethyl acetate and once with 396 g of ethyl acetate, and the ethyl acetate layers were washed together with 23% by mass sodium chloride and 2% by mass aqueous sodium carbonate solution. Then, ethyl acetate was removed by distillation at 40°C / 5 kPaA.

[0037] (Distillation process) 230 g of the obtained crude N-vinyl carboxylic acid amide solution (N-vinylacetamide concentration 67.3% by mass, sodium concentration 524 ppm by mass) was distilled at 100°C / 0.1 kPaA to obtain a fraction of 175 g. The N-vinylacetamide concentration in this distillate was 72% by mass, and the sodium concentration was 1.2 ppm by mass.

[0038] (Crystallization process) The fraction obtained after the distillation process was dissolved at 30°C with 146 g of methylcyclohexane (MCH) and 26 g of ethyl acetate (EA), and cooled to 5°C for 5 hours to crystallize. Solid-liquid separation was then performed using a centrifuge, and the mixture was further washed with 63 g of methylcyclohexane and 3 g of ethyl acetate (primary crystallization). 68 g of N-vinylacetamide crystals were obtained, and the N-vinylacetamide concentration was 97% by mass. The obtained crystals were further dissolved at 40°C with 73 g of methylcyclohexane and 13 g of ethyl acetate, and cooled to 5°C for 7 hours to crystallize. Solid-liquid separation was then performed using a centrifuge, and the mixture was further washed with 32 g of methylcyclohexane and 2 g of ethyl acetate (secondary crystallization).

[0039] As a result, 58 g of purified N-vinylacetamide crystals were obtained. The N-vinylacetamide concentration of the purified N-vinylacetamide crystals was 99.7% by mass, the N-butadienylacetamide concentration was 3 ppm by mass, and the sodium concentration was 0.3 ppm by mass. Polymerization tests of the obtained purified N-vinylacetamide showed a polymerization time of 105 minutes, and the liquid after polymerization was colorless and transparent.

[0040] [Example 2] (Distillation Process) In Example 1, 2.3 g of sodium carbonate was added to the still during distillation, and the process was carried out under the same conditions to obtain a crude N-vinyl carboxylic acid amide solution with a fraction of 187 g and an N-vinylacetamide concentration of 71.9% by mass.

[0041] (Crystallization process) Subsequently, 148 g of methylcyclohexane and 13 g of ethyl acetate were added, dissolved at 30°C, cooled and crystallized over 7 hours to -5°C, followed by solid-liquid separation using a centrifuge. Further washing was carried out using 63 g of methylcyclohexane and 3 g of ethyl acetate (primary crystallization). The obtained N-vinylacetamide crystals were 107 g, and the N-vinylacetamide concentration was 98% by mass. The obtained crystals were further dissolved by adding 117 g of methylcyclohexane and 21 g of ethyl acetate at 40°C, cooled and crystallized over 7 hours to 5°C, followed by solid-liquid separation using a centrifuge. Further washing was carried out using 50 g of methylcyclohexane and 3 g of ethyl acetate (secondary crystallization).

[0042] As a result, 96 g of purified N-vinylacetamide crystals were obtained. The purified N-vinylacetamide crystals had an N-vinylacetamide concentration of 99.5% by mass, an N-butadienylacetamide concentration of 4 ppm by mass, and a sodium concentration of 0.4 ppm by mass. The result of the polymerization test of the obtained purified N-vinylacetamide was 110 minutes, and the liquid properties after polymerization were colorless and transparent.

[0043] [Example 3] (Distillation process) The distillation process was carried out in the same manner as in Example 2 to obtain 190 g of a fraction and a crude N-vinylcarboxylic acid amide solution with an N-vinylacetamide concentration of 69% by mass.

[0044] (Crystallization process) Subsequently, 158 g of methylcyclohexane and 13 g of ethyl acetate were added, dissolved at 30°C, cooled and crystallized over 7 hours to -5°C, followed by solid-liquid separation using a centrifuge. Further washing was carried out using 63 g of methylcyclohexane and 3 g of ethyl acetate. The N-vinylacetamide crystals obtained at this time were 104 g, and the N-vinylacetamide concentration was 98% by mass (primary crystallization). The obtained crystals were redissolved by adding 113 g of methylcyclohexane and 20 g of ethyl acetate at 40°C, cooled and crystallized over 7 hours to 5°C, followed by solid-liquid separation using a centrifuge. Further washing was carried out using 49 g of methylcyclohexane and 3 g of ethyl acetate (primary crystallization).

[0045] As a result, 95 g of purified N-vinylacetamide was obtained. The purified N-vinylacetamide had a concentration of 99.5% by mass, an N-butadienylacetamide concentration of 4 ppm by mass, and a sodium concentration of 0.1 ppm by mass or less. Polymerization tests of the obtained purified N-vinylacetamide showed that polymerization took 100 minutes, and the liquid after polymerization was colorless and transparent.

[0046] [Example 4] (Distillation process) Distillation was carried out in the same manner as in Example 2, except that 712 g of a 5% by mass aqueous sodium carbonate solution was used during extraction, to obtain a crude N-vinyl carboxylic acid amide solution with a fraction of 178 g and an N-vinylacetamide concentration of 72% by mass. (Crystallization process) Then, 154 g of methylcyclohexane and 13 g of ethyl acetate were added, dissolved at 30°C, cooled to -5°C for 7 hours for crystallization, solid-liquid separation was performed using a centrifuge, and washing was further performed with 61 g of methylcyclohexane and 3 g of ethyl acetate. At this time, 102 g of N-vinylacetamide crystals were obtained, and the N-vinylacetamide concentration was 98% by mass (primary crystallization). The obtained N-vinylacetamide crystals were further dissolved at 40°C with 110 g of methylcyclohexane and 20 g of ethyl acetate, cooled to 5°C for 7 hours for crystallization, solid-liquid separation was performed using a centrifuge, and washing was further performed with 47 g of methylcyclohexane and 3 g of ethyl acetate (secondary crystallization).

[0047] As a result, 92 g of purified N-vinylacetamide crystals were obtained. The N-vinylacetamide concentration of the purified N-vinylacetamide crystals was 99.5% by mass, the N-butadienylacetamide concentration was 4 ppm by mass, and the sodium concentration was 0.1 ppm by mass or less. Polymerization tests of the obtained purified N-vinylacetamide took 110 minutes, and the liquid after polymerization was colorless and transparent.

[0048] [Example 5] (Distillation process) Similar to Example 1, ethyl acetate was distilled off, and the obtained crude N-vinylcarboxylic acid amide solution was subjected to thin-film distillation (vertical type (evaporation area 0.05 m2), external condenser, wiper rotation speed 200 rpm) at 90 °C / 0.2 kPaA and a raw material supply rate of 5 g / min to obtain 215 g of a distillate. The concentration of N-vinylacetamide in this distillate was 60.9% by mass, and the sodium concentration was 0.2 ppm by mass. (Crystallization process) Thereafter, 144 g of methylcyclohexane and 13 g of ethyl acetate were added, dissolved at 30 °C, cooled for crystallization over 7 hours to -5 °C, then solid-liquid separation was performed using a centrifuge, and further washing was carried out using 59 g of methylcyclohexane and 7 g of ethyl acetate. The N-vinylacetamide crystals obtained at this time were 98 g, and the N-vinylacetamide concentration was 98.8% by mass (primary crystallization). The obtained N-vinylacetamide crystals were further redissolved at 40 °C by adding 107 g of methylcyclohexane and 19 g of ethyl acetate, cooled for crystallization over 7 hours to 5 °C, then solid-liquid separation was performed using a centrifuge, and further washing was carried out using 44 g of methylcyclohexane and 5 g of ethyl acetate (secondary crystallization). As a result, 87 g of purified N-vinylacetamide crystals were obtained. The N-vinylacetamide concentration of the purified N-vinylacetamide crystals was 99.7% by mass, the N-butadienylacetamide concentration was 5 ppm by mass, and the sodium concentration was 0.1 ppm by mass or less. The result of the polymerization test of the obtained purified N-vinylacetamide was 110 minutes, and the liquid property after polymerization was colorless and transparent.

[0049] [Comparative Example 1] The process was carried out in the same manner as in Example 1, up to the washing stage. The ethyl acetate was removed by distillation at 74°C / 0.1 kPaA to obtain 186 g of crude N-vinylacetamide solution (N-vinylacetamide concentration 74% by mass). In Comparative Example 1, distillation of N-vinylacetamide was not performed. (Crystallization process) 108 g of methylcyclohexane and 10 g of ethyl acetate were added to this crude N-vinylacetamide solution, dissolved at 30°C, cooled to -5°C for 7 hours for crystallization, then solid-liquid separation was performed using a centrifuge, and further washing was performed with 94 g of methylcyclohexane and 5 g of ethyl acetate (primary crystallization). At this time, 103 g of N-vinylacetamide crystals were obtained, and the N-vinylacetamide concentration was 98.5% by mass. The obtained crystals were further dissolved at 40°C with 97 g of methylcyclohexane and 18 g of ethyl acetate, cooled to 5°C for 7 hours for crystallization, then solid-liquid separation was performed using a centrifuge, and further washing was performed with 42 g of methylcyclohexane and 2 g of ethyl acetate (secondary crystallization).

[0050] As a result, 89 g of purified N-vinylacetamide crystals were obtained. The N-vinylacetamide concentration of the purified N-vinylacetamide crystals was 99.4% by mass, the N-butadienylacetamide concentration was 59 ppm by mass, and the sodium concentration was 119 ppm by mass. Polymerization tests of the obtained N-vinylacetamide took 150 minutes, and the liquid after polymerization was pale yellow and transparent.

[0051] EDA: 1,1-Ethanediol acetate

[0052] In all of the examples, the purified N-vinylacetamide had a lower N-butadienylacetamide content than the comparative example. The metal content was reduced compared to the crude N-vinylacetamide. In the polymerization test, all samples showed good polymerization within the standard time (within 120 minutes), and were colorless and transparent in appearance. On the other hand, in the purified N-vinylacetamide of Comparative Example 1, the sodium and N-butadienylacetamide levels did not decrease sufficiently, the time to the polymerization peak in the polymerization test exceeded the standard, and the appearance was slightly yellow. This slightly yellow appearance suggests the presence of high molecular weight impurities containing unsaturated bonds. Furthermore, the concentrations of impurities such as N-butadienylacetamide and 1,1-ethanediol acetate were higher than in the examples after the crystallization process. These findings suggest that the desired N-vinylacetamide cannot be obtained by distillation alone.

Claims

1. A method for producing purified N-vinyl carboxylic acid amide from crude N-vinyl carboxylic acid amide, comprising a distillation step and a crystallization step.

2. The method for producing purified N-vinylcarboxylic acid amide according to claim 1, wherein the pressure in the distillation step is 0.01 kPa or more and 2 kPa or less.

3. A method for producing a purified N-vinylcarboxylic acid amide according to claim 1 or 2, wherein the N-vinylcarboxylic acid amide is N-vinylacetamide.

4. The method for producing purified N-vinylcarboxylic acid amide according to claim 1 or 2, wherein the temperature of the distillation step is 60°C or higher and 150°C or lower.

5. A method for producing purified N-vinylcarboxylic acid amide according to claim 1, wherein the solvent for the crystallization step comprises at least one selected from acetonitrile, methyl ethyl ketone, toluene, ethyl acetate, and 1,2-dimethoxyethane, and an aliphatic hydrocarbon having 6 to 7 carbon atoms.

6. The method for producing a purified N-vinyl carboxylic acid amide according to claim 5, wherein the aliphatic hydrocarbon having 6 to 7 carbon atoms is at least one selected from n-hexane, cyclohexane, n-heptane, cycloheptane, and methylcyclohexane.

7. The method for producing purified N-vinylcarboxylic acid amide according to claim 1, wherein the starting temperature of the crystallization step is 30°C to 80°C, and the ending temperature of the crystallization step is -20°C to 35°C.

8. The method for producing purified N-vinylcarboxylic acid amide according to claim 1, wherein a salt is added in the distillation step.

9. The method for producing purified N-vinylcarboxylic acid amide according to claim 8, wherein the salt is at least one selected from sodium bicarbonate and sodium carbonate.

10. The method for producing purified N-vinyl carboxylic acid amide according to claim 3, wherein crude N-vinylacetamide is produced using acetamide and vinyl acetate as raw materials.

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